Beyond Tumor Suppression: Senescence in Cancer Stemness and Tumor Dormancy

Here, we provide an overview of the importance of cellular fate in cancer as a group of diseases of abnormal cell growth. Tumor development and progression is a highly dynamic process, with several phases of evolution. The existing evidence about the origin and consequences of cancer cell fate specification (e.g., proliferation, senescence, stemness, dormancy, quiescence, and cell cycle re-entry) in the context of tumor formation and metastasis is discussed. The interplay between these dynamic tumor cell phenotypes, the microenvironment, and the immune system is also reviewed in relation to cancer. We focus on the role of senescence during cancer progression, with a special emphasis on its relationship with stemness and dormancy. Selective interventions on senescence and dormancy cell fates, including the specific targeting of cancer cell populations to prevent detrimental effects in aging and disease, are also reviewed. A new conceptual framework about the impact of synthetic lethal strategies by using senogenics and then senolytics is given, with the promise of future directions on innovative anticancer therapies.

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title: Cancer stem cells revisited
abstract: The cancer stem cell (CSC) concept was proposed four decades ago, and states that tumor growth, analogous to the renewal of healthy tissues, is fueled by small numbers of dedicated stem cells. It has gradually become clear that many tumors harbor CSCs in dedicated niches, and yet their identification and eradication has not been as obvious as was initially hoped. Recently developed lineage-tracing and cell-ablation strategies have provided insights into CSC plasticity, quiescence, renewal, and therapeutic response. Here we discuss new developments in the CSC field in relationship to changing insights into how normal stem cells maintain healthy tissues. Expectations in the field have become more realistic, and now, the first successes of therapies based on the CSC concept are emerging.

title: Oncogenic ras Provokes Premature Cell Senescence Associated with Accumulation of p53 and p16INK4a
abstract: Oncogenic ras can transform most immortal rodent cells to a tumorigenic state. However, transformation of primary cells by ras requires either a cooperating oncogene or the inactivation of tumor suppressors such as p53 or p16. Here we show that expression of oncogenic ras in primary human or rodent cells results in a permanent G1 arrest. The arrest induced by ras is accompanied by accumulation of p53 and p16, and is phenotypically indistinguishable from cellular senescence. Inactivation of either p53 or p16 prevents ras-induced arrest in rodent cells, and E1A achieves a similar effect in human cells. These observations suggest that the onset of cellular senescence does not simply reflect the accumulation of cell divisions, but can be prematurely activated in response to an oncogenic stimulus. Negation of ras-induced senescence may be relevant during multistep tumorigenesis.

title: Senescence of activated stellate cells limits liver fibrosis.
abstract: Cellular senescence acts as a potent mechanism of tumor suppression; however, its functional contribution to noncancer pathologies has not been examined. Here we show that senescent cells accumulate in murine livers treated to produce fibrosis, a precursor pathology to cirrhosis. The senescent cells are derived primarily from activated hepatic stellate cells, which initially proliferate in response to liver damage and produce the extracellular matrix deposited in the fibrotic scar. In mice lacking key senescence regulators, stellate cells continue to proliferate, leading to excessive liver fibrosis. Furthermore, senescent activated stellate cells exhibit gene expression profile consistent with cell-cycle exit, reduced secretion of extracellular matrix components, enhanced secretion of extracellular matrix-degrading enzymes, and enhanced immune surveillance. Accordingly natural killer cells preferentially kill senescent activated stellate cells in vitro and in vivo, thereby facilitating the resolution of fibrosis. Therefore, the senescence program limits the fibrogenic response to acute tissue damage.

title: The development of cell senescence
abstract: Cellular senescence was traditionally considered a stress response that protected the organism by limiting the proliferation of damaged and unwanted cells. However, the recent identification of developmentally-programmed cellular senescence during embryo development has changed our view of the process. There are now a number of examples of developmental senescence in evolutionary distant organisms ranging from mammals to fish, showing senescence at various sites during specific time windows of development. Developmental senescence shares many features with stress-induced senescence but also present some specific characteristics. The different examples of developmental senescence provide evidence of the diverse functions contributed by senescence and represent an opportunity to learn more about this process. Also, the existence of senescence during embryogenesis opens the possibility of identifying human developmental syndromes caused by alterations in this response. Studying in more detail this process will expand our understanding of cellular senescence and could offer new insights into the cause of human pathologies.

title: Senescence and aging: Causes, consequences, and therapeutic avenues
abstract: McHugh and Gil review the role of senescence in age-related diseases and how targeting senescence may improve health span and extend life span.

title: Tumor-initiating stem cells of squamous cell carcinomas and their control by TGF-β and integrin/focal adhesion kinase (FAK) signaling
abstract: Cancer stem cells (CSCs) sustain tumor growth through their ability to self-renew and to generate differentiated progeny. These functions endow CSCs with the potential to initiate secondary tumors bearing characteristics similar to those of the parent. Recently the hair follicle stem cell marker CD34 was used to purify a CSC-like cell population from early skin tumors arising from treatment with 7,12-dimethylbenz[α]anthracene/12-o-tetradecanoylphorbol-13-acetate, which typically generates benign papillomas that occasionally progress to squamous cell carcinomas (SCCs). In the present study, we identify and characterize CSCs purified from malignant SCCs. We show that SCCs contain two highly tumorigenic CSC populations that differ in CD34 levels but are enriched for integrins and coexist at the SCC–stroma interface. Intriguingly, whether CD34lo or CD34hi, α6hiβ1hi populations can initiate secondary tumors by serial limit-dilution transplantation assays, but α6loβ1lo populations cannot. Moreover, secondary tumors generated from a single CSC of either subtype contain both CD34lo and CD34hi α6hiβ1hiCSCs, indicating their nonhierarchical organization. Genomic profiling and hierarchical cluster analysis show that these two CSC subtypes share a molecular signature distinct from either the CD34− epidermal or the CD34hi hair follicle stem cell signature. Although closely related, α6hiβ1hiCD34lo and α6hiβ1hiCD34hi CSCs differ in cell-cycle gene expression and proliferation characteristics. Indeed, proliferation and expansion of α6hiβ1hiCD34hi CSCs is sensitive to whether they can initiate a TGF-β receptor II–mediated response to counterbalance elevated focal adhesion kinase-mediated integrin signaling within the tumor. Overall, the coexistence and interconvertibility of CSCs with differing sensitivities to their microenvironment pose challenges and opportunities for SCC cancer therapies.

title: Purification and characterization of mouse hematopoietic stem cells.
abstract: Mouse bone marrow hematopoietic stem cells were isolated with the use of a variety of phenotypic markers. These cells can proliferate and differentiate with approximately unit efficiency into myelomonocytic cells, B cells, or T cells. Thirty of these cells are sufficient to save 50 percent of lethally irradiated mice, and to reconstitute all blood cell types in the survivors.

title: The long-term repopulating subset of hematopoietic stem cells is deterministic and isolatable by phenotype.
abstract: The Thy-1.1loSca-1hiLin-/lo population, representing 0.05% of C57BL/Ka-Thy-1.1 bone marrow, is highly enriched for hematopoietic stem cells and includes all multipotent progenitors in this mouse strain; however, the functional reconstituting activity of this fraction is heterogeneous. Only around 25% of clonal reconstitutions by cells from this population are long term; remaining clones yield transient multilineage reconstitutions. By fractionating based on lineage marker expression, the Thy-1.1loSca-1hiLin-/lo population has been resolved into three subpopulations: Lin-Mac-1-CD4-; Lin-Mac-1loCD4-; and Mac-1loCD4lo. Of these, only the Lin-Mac-1-CD4- population is highly enriched for long-term reconstituting hematopoietic stem cells. A comparison of transient and long-term multipotent progenitors indicates that long-term progenitors have less CFU-S activity, are equally radioprotective, and are less frequently in cell cycle. The ability to predict the longevity of reconstitution based on lineage marker expression indicates that reconstitution potential is deterministic, not stochastic.

title: Dynamic Pluripotent Stem Cell States and Their Applications.
abstract: Embryonic pluripotency can be recapitulated in vitro by a spectrum of pluripotent stem cell states stabilized with different culture conditions. Their distinct spatiotemporal characteristics provide an unprecedented tool for the study of early human development. The newly unveiled ability of some stem cell types for crossing xeno-barriers will facilitate the generation of interspecies chimeric embryos from distant species, including humans. When combined with efficient zygote genome editing technologies, xenogeneic human pluripotent stem cells may also open new frontiers for regenerative medicine applications, including the possibility of generating human organs in animals via interspecies chimeric complementation.

title: Normal and neoplastic nonstem cells can spontaneously convert to a stem-like state
abstract: Current models of stem cell biology assume that normal and neoplastic stem cells reside at the apices of hierarchies and differentiate into nonstem progeny in a unidirectional manner. Here we identify a subpopulation of basal-like human mammary epithelial cells that departs from that assumption, spontaneously dedifferentiating into stem-like cells. Moreover, oncogenic transformation enhances the spontaneous conversion, so that nonstem cancer cells give rise to cancer stem cell (CSC)-like cells in vitro and in vivo. We further show that the differentiation state of normal cells-of-origin is a strong determinant of posttransformation behavior. These findings demonstrate that normal and CSC-like cells can arise de novo from more differentiated cell types and that hierarchical models of mammary stem cell biology should encompass bidirectional interconversions between stem and nonstem compartments. The observed plasticity may allow derivation of patient-specific adult stem cells without genetic manipulation and holds important implications for therapeutic strategies to eradicate cancer.

title: Hedgehog signals regulate multiple aspects of gastrointestinal development.
abstract: The gastrointestinal tract develops from the embryonic gut, which is composed of an endodermally derived epithelium surrounded by cells of mesodermal origin. Cell signaling between these two tissue layers appears to play a critical role in coordinating patterning and organogenesis of the gut and its derivatives. We have assessed the function of Sonic hedgehog and Indian hedgehog genes, which encode members of the Hedgehog family of cell signals. Both are expressed in gut endoderm, whereas target genes are expressed in discrete layers in the mesenchyme. It was unclear whether functional redundancy between the two genes would preclude a genetic analysis of the roles of Hedgehog signaling in the mouse gut. We show here that the mouse gut has both common and separate requirements for Sonic hedgehog and Indian hedgehog. Both Sonic hedgehog and Indian hedgehog mutant mice show reduced smooth muscle, gut malrotation and annular pancreas. Sonic hedgehog mutants display intestinal transformation of the stomach, duodenal stenosis (obstruction), abnormal innervation of the gut and imperforate anus. Indian hedgehog mutants show reduced epithelial stem cell proliferation and differentiation, together with features typical of Hirschsprung's disease (aganglionic colon). These results show that Hedgehog signals are essential for organogenesis of the mammalian gastrointestinal tract and suggest that mutations in members of this signaling pathway may be involved in human gastrointestinal malformations.

title: Induction of Pluripotent Stem Cells from Mouse Embryonic and Adult Fibroblast Cultures by Defined Factors
abstract: Differentiated cells can be reprogrammed to an embryonic-like state by transfer of nuclear contents into oocytes or by fusion with embryonic stem (ES) cells. Little is known about factors that induce this reprogramming. Here, we demonstrate induction of pluripotent stem cells from mouse embryonic or adult fibroblasts by introducing four factors, Oct3/4, Sox2, c-Myc, and Klf4, under ES cell culture conditions. Unexpectedly, Nanog was dispensable. These cells, which we designated iPS (induced pluripotent stem) cells, exhibit the morphology and growth properties of ES cells and express ES cell marker genes. Subcutaneous transplantation of iPS cells into nude mice resulted in tumors containing a variety of tissues from all three germ layers. Following injection into blastocysts, iPS cells contributed to mouse embryonic development. These data demonstrate that pluripotent stem cells can be directly generated from fibroblast cultures by the addition of only a few defined factors.

title: The Hedgehog and Wnt signalling pathways in cancer
abstract: The Wnt and Hedgehog (Hh) signalling pathways have long been known to direct growth and patterning during embryonic development. Recent evidence also implicates these pathways in the postembryonic regulation of stem-cell number in epithelia such as those of the skin and intestine, which undergo constant renewal. A pathological role for the Wnt and Hh pathways has emerged from studies showing a high frequency of specific human cancers associated with mutations that constitutively activate the transcriptional response of these pathways. This article focuses on Hh and Wnt signal transduction and reviews evidence suggesting that tumorigenesis associated with pathway activation may result from mis-specification of cells towards stem-cell or stem cell-like fates.

title: Reprogramming in vivo produces teratomas and iPS cells with totipotency features
abstract: Reprogramming of adult cells to generate induced pluripotent stem cells (iPS cells) has opened new therapeutic opportunities; however, little is known about the possibility of in vivo reprogramming within tissues. Here we show that transitory induction of the four factors Oct4, Sox2, Klf4 and c-Myc in mice results in teratomas emerging from multiple organs, implying that full reprogramming can occur in vivo. Analyses of the stomach, intestine, pancreas and kidney reveal groups of dedifferentiated cells that express the pluripotency marker NANOG, indicative of in situ reprogramming. By bone marrow transplantation, we demonstrate that haematopoietic cells can also be reprogrammed in vivo. Notably, reprogrammable mice present circulating iPS cells in the blood and, at the transcriptome level, these in vivo generated iPS cells are closer to embryonic stem cells (ES cells) than standard in vitro generated iPS cells. Moreover, in vivo iPS cells efficiently contribute to the trophectoderm lineage, suggesting that they achieve a more plastic or primitive state than ES cells. Finally, intraperitoneal injection of in vivo iPS cells generates embryo-like structures that express embryonic and extraembryonic markers. We conclude that reprogramming in vivo is feasible and confers totipotency features absent in standard iPS or ES cells. These discoveries could be relevant for future applications of reprogramming in regenerative medicine.

title: Tumor-initiating cells are rare in many human tumors.
abstract: Tumor-initiating cells (TICs) are defined by their ability to form tumors after xenotransplantation in immunodeficient mice and appear to be relatively rare in most human cancers. Recent data in melanoma indicate that the frequency of TICs increases dramatically via more permissive xenotransplantation conditions, raising the possibility that the true frequency of TICs has been greatly underestimated in most human tumors. We compared the growth of human pancreatic, non-small cell lung, and head and neck carcinomas in NOD/SCID and NSG mice. Although TIC frequency was detected up to 10-fold higher in NSG mice, it remained low (<1 in 2500 cells) in all cases. Moreover, aldehyde dehydrogenase-positive (ALDH(+)) and CD44(+)CD24(+) cells, phenotypically distinct cells enriched in TICs, were equally tumorigenic in NOD/SCID and NSG mice. Our findings demonstrate that TICs are rare in these cancers and that the identification of TICs and their frequency in other human malignancies should be validated via primary tumors and highly permissive xenotransplantation conditions.

title: Phenotypic heterogeneity of disseminated tumour cells is preset by primary tumour hypoxic microenvironments
abstract: Hypoxia is a poor-prognosis microenvironmental hallmark of solid tumours, but it is unclear how it influences the fate of disseminated tumour cells (DTCs) in target organs. Here we report that hypoxic HNSCC and breast primary tumour microenvironments displayed upregulation of key dormancy (NR2F1, DEC2, p27) and hypoxia (GLUT1, HIF1α) genes. Analysis of solitary DTCs in PDX and transgenic mice revealed that post-hypoxic DTCs were frequently NR2F1hi/DEC2hi/p27hi/TGFβ2hi and dormant. NR2F1 and HIF1α were required for p27 induction in post-hypoxic dormant DTCs, but these DTCs did not display GLUT1hi expression. Post-hypoxic DTCs evaded chemotherapy and, unlike ER− breast cancer cells, post-hypoxic ER+ breast cancer cells were more prone to enter NR2F1-dependent dormancy. We propose that primary tumour hypoxic microenvironments give rise to a subpopulation of dormant DTCs that evade therapy. These post-hypoxic dormant DTCs may be the source of disease relapse and poor prognosis associated with hypoxia.

title: Cell-of-Origin of Cancer versus Cancer Stem Cells: Assays and Interpretations.
abstract: A tumor originates from a normal cell that has undergone tumorigenic transformation as a result of genetic mutations. This transformed cell is the cell-of-origin for the tumor. In contrast, an established clinical tumor is sustained by subpopulations of self-renewing cancer cells operationally called cancer stem cells (CSC) that can generate, intraclonally, both tumorigenic and nontumorigenic cells. Identifying and characterizing tumor cell-of-origin and CSCs should help elucidate tumor cell heterogeneity, which, in turn, should help understand tumor cell responses to clinical treatments, drug resistance, tumor relapse, and metastatic spread. Both tumor transplantation and lineage-tracing assays have been helpful in characterizing these cancer cell populations, although each system has its strengths and caveats. In this article, we briefly review and summarize advantages and limitations of both assays in support of a combinatorial approach to accurately define the roles of both cancer-initiating and cancer-propagating cells. As an aside, we also wish to clarify the definitions of cancer cell-of-origin and CSCs, which are often interchangeably used by mistake.

title: Generation of Breast Cancer Stem Cells through Epithelial-Mesenchymal Transition
abstract: Recently, two novel concepts have emerged in cancer biology: the role of so-called “cancer stem cells” in tumor initiation, and the involvement of an epithelial-mesenchymal transition (EMT) in the metastatic dissemination of epithelial cancer cells. Using a mammary tumor progression model, we show that cells possessing both stem and tumorigenic characteristics of “cancer stem cells” can be derived from human mammary epithelial cells following the activation of the Ras-MAPK pathway. The acquisition of these stem and tumorigenic characters is driven by EMT induction.

title: Colon cancer stem cells dictate tumor growth and resist cell death by production of interleukin-4.
abstract: A novel paradigm in tumor biology suggests that cancer growth is driven by stem-like cells within a tumor. Here, we describe the identification and characterization of such cells from colon carcinomas using the stem cell marker CD133 that accounts around 2% of the cells in human colon cancer. The CD133(+) cells grow in vitro as undifferentiated tumor spheroids, and they are both necessary and sufficient to initiate tumor growth in immunodeficient mice. Xenografts resemble the original human tumor maintaining the rare subpopulation of tumorigenic CD133(+) cells. Further analysis revealed that the CD133(+) cells produce and utilize IL-4 to protect themselves from apoptosis. Consistently, treatment with IL-4Ralpha antagonist or anti-IL-4 neutralizing antibody strongly enhances the antitumor efficacy of standard chemotherapeutic drugs through selective sensitization of CD133(+) cells. Our data suggest that colon tumor growth is dictated by stem-like cells that are treatment resistant due to the autocrine production of IL-4.

title: Evidence for self-renewing lung cancer stem cells and their implications in tumor initiation, progression, and targeted therapy
abstract: The discovery of rare tumor cells with stem cell features first in leukemia and later in solid tumors has emerged as an important area in cancer research. It has been determined that these stem-like tumor cells, termed cancer stem cells, are the primary cellular component within a tumor that drives disease progression and metastasis. In addition to their stem-like ability to self-renew and differentiate, cancer stem cells are also enriched in cells resistant to conventional radiation therapy and to chemotherapy. The immediate implications of this new tumor growth paradigm not only require a re-evaluation of how tumors are initiated, but also on how tumors should be monitored and treated. However, despite the relatively rapid pace of cancer stem cell research in solid tumors such as breast, brain, and colon cancers, similar progress in lung cancer remains hampered in part due to an incomplete understanding of lung epithelial stem cell hierarchy and the complex heterogeneity of the disease. In this review, we provide a critical summary of what is known about the role of normal and malignant lung stem cells in tumor development, the progress in characterizing lung cancer stem cells and the potential for therapeutically targeting pathways of lung cancer stem cell self-renewal.

title: The intestinal stem cell signature identifies colorectal cancer stem cells and predicts disease relapse.
abstract: A frequent complication in colorectal cancer (CRC) is regeneration of the tumor after therapy. Here, we report that a gene signature specific for adult intestinal stem cells (ISCs) predicts disease relapse in CRC patients. ISCs are marked by high expression of the EphB2 receptor, which becomes gradually silenced as cells differentiate. Using EphB2 and the ISC marker Lgr5, we have FACS-purified and profiled mouse ISCs, crypt proliferative progenitors, and late transient amplifying cells to define a gene program specific for normal ISCs. Furthermore, we discovered that ISC-specific genes identify a stem-like cell population positioned at the bottom of tumor structures reminiscent of crypts. EphB2 sorted ISC-like tumor cells display robust tumor-initiating capacity in immunodeficient mice as well as long-term self-renewal potential. Taken together, our data suggest that the ISC program defines a cancer stem cell niche within colorectal tumors and plays a central role in CRC relapse.

title: Distinct populations of cancer stem cells determine tumor growth and metastatic activity in human pancreatic cancer.
abstract: Pancreatic adenocarcinoma is currently the fourth leading cause for cancer-related mortality. Stem cells have been implicated in pancreatic tumor growth, but the specific role of these cancer stem cells in tumor biology, including metastasis, is still uncertain. We found that human pancreatic cancer tissue contains cancer stem cells defined by CD133 expression that are exclusively tumorigenic and highly resistant to standard chemotherapy. In the invasive front of pancreatic tumors, a distinct subpopulation of CD133(+) CXCR4(+) cancer stem cells was identified that determines the metastatic phenotype of the individual tumor. Depletion of the cancer stem cell pool for these migrating cancer stem cells virtually abrogated the metastatic phenotype of pancreatic tumors without affecting their tumorigenic potential. In conclusion, we demonstrate that a subpopulation of migrating CD133(+) CXCR4(+) cancer stem cells is essential for tumor metastasis. Strategies aimed at modulating the SDF-1/CXCR4 axis may have important clinical applications to inhibit metastasis of cancer stem cells.

title: Identification of a population of blood circulating tumor cells from breast cancer patients that initiates metastasis in a xenograft assay
abstract: It has been hypothesized that carcinoma metastasis is initiated by a subpopulation of circulating tumor cells (CTCs) found in the blood of patients. However, although the presence of CTCs is an indicator of poor prognosis in several carcinoma entities, the existence and phenotype of metastasis-initiating cells (MICs) among CTCs has not been experimentally demonstrated. Here we developed a xenograft assay and used it to show that primary human luminal breast cancer CTCs contain MICs that give rise to bone, lung and liver metastases in mice. These MIC-containing CTC populations expressed EPCAM, CD44, CD47 and MET. In a small cohort of patients with metastases, the number of EPCAM+CD44+CD47+MET+ CTCs, but not of bulk EPCAM+ CTCs, correlated with lower overall survival and increased number of metastasic sites. These data describe functional circulating MICs and associated markers, which may aid the design of better tools to diagnose and treat metastatic breast cancer.

title: The serial cultivation of human diploid cell strains.
abstract: The isolation and characterization of 25 strains of human diploid fibroblasts derived from fetuses are described. Routine tissue culture techniques were employed. Other than maintenance of the diploid karyotype, ten other criteria serve to distinguish these strains from heteroploid cell lines. These include retention of sex chromatin, histotypical differentiation, inadaptability to suspended culture, non-malignant characteristics in vivo , finite limit of cultivation, similar virus spectrum to primary tissue, similar cell morphology to primary tissue, increased acid production compared to cell lines, retention of Coxsackie A9 receptor substance, and ease with which strains can be developed. Survival of cell strains at − 70 °C with retention of all characteristics insures an almost unlimited supply of any strain regardless of the fact that they degenerate after about 50 subcultivations and one year in culture. A consideration of the cause of the eventual degeneration of these strains leads to the hypothesis that non-cumulative external factors are excluded and that the phenomenon is attributable to intrinsic factors which are expressed as senescence at the cellular level. With these characteristics and their extremely broad virus spectrum, the use of diploid human cell strains for human virus vaccine production is suggested. In view of these observations a number of terms used by cell culturists are redefined.

title: A biomarker that identifies senescent human cells in culture and in aging skin in vivo.
abstract: Normal somatic cells invariably enter a state of irreversibly arrested growth and altered function after a finite number of divisions. This process, termed replicative senescence, is thought to be a tumor-suppressive mechanism and an underlying cause of aging. There is ample evidence that escape from senescence, or immortality, is important for malignant transformation. By contrast, the role of replicative senescence in organismic aging is controversial. Studies on cells cultured from donors of different ages, genetic backgrounds, or species suggest that senescence occurs in vivo and that organismic lifespan and cell replicative lifespan are under common genetic control. However, senescent cells cannot be distinguished from quiescent or terminally differentiated cells in tissues. Thus, evidence that senescent cells exist and accumulate with age in vivo is lacking. We show that several human cells express a beta-galactosidase, histochemically detectable at pH 6, upon senescence in culture. This marker was expressed by senescent, but not presenescent, fibroblasts and keratinocytes but was absent from quiescent fibroblasts and terminally differentiated keratinocytes. It was also absent from immortal cells but was induced by genetic manipulations that reversed immortality. In skin samples from human donors of different age, there was an age-dependent increase in this marker in dermal fibroblasts and epidermal keratinocytes. This marker provides in situ evidence that senescent cells may exist and accumulate with age in vivo.

title: BRAFE600-associated senescence-like cell cycle arrest of human naevi
abstract: Most normal mammalian cells have a finite lifespan, thought to constitute a protective mechanism against unlimited proliferation. This phenomenon, called senescence, is driven by telomere attrition, which triggers the induction of tumour suppressors including p16INK4a (ref. 5). In cultured cells, senescence can be elicited prematurely by oncogenes; however, whether such oncogene-induced senescence represents a physiological process has long been debated. Human naevi (moles) are benign tumours of melanocytes that frequently harbour oncogenic mutations (predominantly V600E, where valine is substituted for glutamic acid) in BRAF, a protein kinase and downstream effector of Ras. Nonetheless, naevi typically remain in a growth-arrested state for decades and only rarely progress into malignancy (melanoma). This raises the question of whether naevi undergo BRAFV600E-induced senescence. Here we show that sustained BRAFV600E expression in human melanocytes induces cell cycle arrest, which is accompanied by the induction of both p16INK4a and senescence-associated acidic β-galactosidase (SA-β-Gal) activity, a commonly used senescence marker. Validating these results in vivo, congenital naevi are invariably positive for SA-β-Gal, demonstrating the presence of this classical senescence-associated marker in a largely growth-arrested, neoplastic human lesion. In growth-arrested melanocytes, both in vitro and in situ, we observed a marked mosaic induction of p16INK4a, suggesting that factors other than p16INK4a contribute to protection against BRAFV600E-driven proliferation. Naevi do not appear to suffer from telomere attrition, arguing in favour of an active oncogene-driven senescence process, rather than a loss of replicative potential. Thus, both in vitro and in vivo, BRAFV600E-expressing melanocytes display classical hallmarks of senescence, suggesting that oncogene-induced senescence represents a genuine protective physiological process.

title: A senescence-like phenotype distinguishes tumor cells that undergo terminal proliferation arrest after exposure to anticancer agents.
abstract: Exposure of human tumor cell lines to different chemotherapeutic drugs, ionizing radiation, and differentiating agents induced morphological, enzymatic, and ploidy changes resembling replicative senescence of normal cells. Moderate doses of doxorubicin induced this senescence-like phenotype (SLP) in 11 of 14 tested cell lines derived from different types of human solid tumors, including all of the lines with wild-type p53 and half of p53-mutated cell lines. SLP induction seemed to be independent from mitotic cell death, the other major effect of drug treatment. Among cells that survived drug exposure, SLP markers distinguished those cells that became terminally growth-arrested within a small number of cell divisions from the cells that recovered and resumed proliferation. SLP induction in breast carcinoma cells treated with retinoids in vitro or in vivo was found to correlate with permanent growth inhibition under the conditions of minimal cytotoxicity, suggesting that this response may be particularly important for the antiproliferative effect of differentiating agents. The senescence-like program of terminal proliferation arrest may provide an important determinant of treatment outcome and a target for augmentation in cancer therapy.

title: Tumor cell senescence in cancer treatment.
abstract: Cell senescence is broadly defined as the physiological program of terminal growth arrest, which can be triggered by alterations of telomeres or by different forms of stress. Neoplastic transformation involves events that inhibit the program of senescence, and tumor cells were believed until recently to have lost the ability to senesce. It has now become apparent, however, that tumor cells can be readily induced to undergo senescence by genetic manipulations or by treatment with chemotherapeutic drugs, radiation, or differentiating agents. Treatment-induced senescence, which has both similarities with, and differences from, replicative senescence of normal cells, was shown to be one of the key determinants of tumor response to therapy in vitro and in vivo. Although senescent cells do not proliferate, they remain metabolically active and produce secreted proteins with both tumor-suppressing and tumor-promoting activities. Expression of tumor-promoting factors by senescent cells is mediated, at least in part, by senescence-associated cyclin-dependent kinase inhibitors such as p21(Waf1/Cip1/Sdi1). Clinical and preclinical studies indicate that expression of different biological classes of senescence-associated growth-regulatory genes in tumor cells has significant prognostic implications. Elucidation of the genes and regulatory mechanisms that determine different aspects of tumor senescence makes it possible to design new therapeutic approaches to improving the efficacy and to decreasing the side effects of cancer therapy.

title: Geriatric muscle stem cells switch reversible quiescence into senescence
abstract: Regeneration of skeletal muscle depends on a population of adult stem cells (satellite cells) that remain quiescent throughout life. Satellite cell regenerative functions decline with ageing. Here we report that geriatric satellite cells are incapable of maintaining their normal quiescent state in muscle homeostatic conditions, and that this irreversibly affects their intrinsic regenerative and self-renewal capacities. In geriatric mice, resting satellite cells lose reversible quiescence by switching to an irreversible pre-senescence state, caused by derepression of p16INK4a (also called Cdkn2a). On injury, these cells fail to activate and expand, undergoing accelerated entry into a full senescence state (geroconversion), even in a youthful environment. p16INK4a silencing in geriatric satellite cells restores quiescence and muscle regenerative functions. Our results demonstrate that maintenance of quiescence in adult life depends on the active repression of senescence pathways. As p16INK4a is dysregulated in human geriatric satellite cells, these findings provide the basis for stem-cell rejuvenation in sarcopenic muscles.

title: Stem cell senescence drives age-attenuated induction of pituitary tumours in mouse models of paediatric craniopharyngioma
abstract: Senescent cells may promote tumour progression through the activation of a senescence-associated secretory phenotype (SASP), whether these cells are capable of initiating tumourigenesis in vivo is not known. Expression of oncogenic β-catenin in Sox2+ young adult pituitary stem cells leads to formation of clusters of stem cells and induction of tumours resembling human adamantinomatous craniopharyngioma (ACP), derived from Sox2− cells in a paracrine manner. Here, we uncover the mechanisms underlying this paracrine tumourigenesis. We show that expression of oncogenic β-catenin in Hesx1+ embryonic precursors also results in stem cell clusters and paracrine tumours. We reveal that human and mouse clusters are analogous and share a common signature of senescence and SASP. Finally, we show that mice with reduced senescence and SASP responses exhibit decreased tumour-inducing potential. Together, we provide evidence that senescence and a stem cell-associated SASP drive cell transformation and tumour initiation in vivo in an age-dependent fashion.Senescent cells can promote tumour progression through the activation of a senescenceassociated secretory phenotype (SASP). Here, the authors show that SASP activation is associated with non-cell autonomous cell transformation and tumour initiation in an in vivo model of adamantinomatous craniopharyngioma.

title: Macrophage migration inhibitory factor rescues mesenchymal stem cells from doxorubicin-induced senescence though the PI3K-Akt signaling pathway.
abstract: Doxorubicin (DOXO), an anthracycline antibiotic, is a commonly used anticancer drug. Despite its widespread usage, the therapeutic effects of DOXO are limited by its cardiotoxicity. Mesenchymal stem cell (MSC)-based therapies have had positive outcomes in the treatment of DOXO-induced cardiac damage; however, DOXO exerts toxic effects on MSCs, decreasing the effectiveness of MSC therapy. Macrophage migration inhibitory factor (MIF) promotes MSC survival and rejuvenation, and thus is a promising candidate to protect MSCs against DOXO-induced injury. The present study revealed that DOXO induced the senescence of MSCs, resulting in decreased proliferation, viability and paracrine effects. However, pretreatment with MIF improved the proliferation rate, viability, paracrine function, telomere length and telomerase activity of MSCs. Furthermore, the results indicated that the molecular mechanism underlying the anti-senescent function of MIF involved the phosphatidylinositol 3-kinase (PI3K)-RAC-α serine/threonine-protein kinase (Akt) signaling pathway, which MIF activated. In agreement with this finding, silencing Akt was identified to abolish the anti-senescent effect of MIF. In addition, MIF decreased oxidative stress in MSCs, as revealed by the decreased production of reactive oxygen species and malondialdehyde, and the increased activity of superoxide dismutase. These results indicate that MIF can rescue MSCs from a state of DOXO-induced senescence by inhibiting oxidative stress and activating the PI3K-Akt signaling pathway. Thus, treatment with MIF may have an important therapeutic application for the rejuvenation of MSCs in patients with cancer being treated with DOXO.

title: DNA damage triggers a prolonged p53-dependent G1 arrest and long-term induction of Cip1 in normal human fibroblasts.
abstract: The tumor suppressor p53 is a cell cycle checkpoint protein that contributes to the preservation of genetic stability by mediating either a G1 arrest or apoptosis in response to DNA damage. Recent reports suggest that p53 causes growth arrest through transcriptional activation of the cyclin-dependent kinase (Cdk)-inhibitor Cip1. Here, we characterize the p53-dependent G1 arrest in several normal human diploid fibroblast (NDF) strains and p53-deficient cell lines treated with 0.1-6 Gy gamma radiation. DNA damage and cell cycle progression analyses showed that NDF entered a prolonged arrest state resembling senescence, even at low doses of radiation. This contrasts with the view that p53 ensures genetic stability by inducing a transient arrest to enable repair of DNA damage, as reported for some myeloid leukemia lines. Gamma radiation administered in early to mid-, but not late, G1 induced the arrest, suggesting that the p53 checkpoint is only active in G1 until cells commit to enter S phase at the G1 restriction point. A log-linear plot of the fraction of irradiated G0 cells able to enter S phase as a function of dose is consistent with single-hit kinetics. Cytogenetic analyses combined with radiation dosage data indicate that only one or a small number of unrepaired DNA breaks may be sufficient to cause arrest. The arrest also correlated with long-term elevations of p53 protein, Cip1 mRNA, and Cip1 protein. We propose that p53 helps maintain genetic stability in NDF by mediating a permanent cell cycle arrest through long-term induction of Cip1 when low amounts of unrepaired DNA damage are present in G1 before the restriction point.

title: Appearance of biomarkers of in vitro ageing after successive stimulation of WI-38 fibroblasts with IL-1alpha and TNF-alpha: senescence associated beta-galactosidase activity and morphotype transition.
abstract: Sublethal oxidative stresses increase the proportions of human fibroblasts positive for senescence associated beta-galactosidase activity and accelerate the transition in the fibroblast morphotypes characterising fibroblast ageing. Stimulation of fibroblasts with TNF-alpha or IL-1alpha transiently increases the production of reactive oxygen species (ROS) in human fibroblasts. Here we propose that repeated stimulation of WI-38 fibroblasts with TNF-alpha or IL-1alpha can generate enough ROS to accelerate the transition in the fibroblast morphotypes and increase the proportion of cells positive for senescence associated beta-galactosidase activity. The involvement of ROS is suggested by experiments where the stimulation of fibroblasts with TNF-alpha or IL-1alpha are performed in the presence of N-acetylcysteine which increases the intracellular antioxidant potential. It is proposed that the decrease in the proportions of morphotypes I and II, and the increase in the proportions of morphotypes III to VI observed after successive stimulation with TNF-alpha or IL1-alpha is attributed to an increased ROS production occurring during the stimulation.

title: A DNA damage checkpoint response in telomere-initiated senescence
abstract: Most human somatic cells can undergo only a limited number of population doublings in vitro. This exhaustion of proliferative potential, called senescence, can be triggered when telomeres—the ends of linear chromosomes—cannot fulfil their normal protective functions. Here we show that senescent human fibroblasts display molecular markers characteristic of cells bearing DNA double-strand breaks. These markers include nuclear foci of phosphorylated histone H2AX and their co-localization with DNA repair and DNA damage checkpoint factors such as 53BP1, MDC1 and NBS1. We also show that senescent cells contain activated forms of the DNA damage checkpoint kinases CHK1 and CHK2. Furthermore, by chromatin immunoprecipitation and whole-genome scanning approaches, we show that the chromosome ends of senescent cells directly contribute to the DNA damage response, and that uncapped telomeres directly associate with many, but not all, DNA damage response proteins. Finally, we show that inactivation of DNA damage checkpoint kinases in senescent cells can restore cell-cycle progression into S phase. Thus, we propose that telomere-initiated senescence reflects a DNA damage checkpoint response that is activated with a direct contribution from dysfunctional telomeres.

title: Both telomeric and non-telomeric DNA damage are determinants of mammalian cellular senescence
abstract: BackgroundCellular senescence is a state reached by normal mammalian cells after a finite number of cell divisions and is characterized by morphological and physiological changes including terminal cell-cycle arrest. The limits on cell division imposed by senescence may play an important role in both organismal aging and in preventing tumorigenesis. Cellular senescence and organismal aging are both accompanied by increased DNA damage, seen as the formation of γ-H2AX foci (γ-foci), which may be found on uncapped telomeres or at non-telomeric sites of DNA damage. However, the relative importance of telomere- and non-telomere-associated DNA damage to inducing senescence has never been demonstrated. Here we present a new approach to determine accurately the chromosomal location of γ-foci and quantify the number of telomeric versus non-telomeric γ-foci associated with senescence in both human and mouse cells. This approach enables researchers to obtain accurate values and to avoid various possible misestimates inherent in earlier methods.ResultsUsing combined immunofluorescence and telomere fluorescence in situ hybridization on metaphase chromosomes, we show that human cellular senescence is not solely determined by telomeric DNA damage. In addition, mouse cellular senescence is not solely determined by non-telomeric DNA damage. By comparing cells from different generations of telomerase-null mice with human cells, we show that cells from late generation telomerase-null mice, which have substantially short telomeres, contain mostly telomeric γ-foci. Most notably, we report that, as human and mouse cells approach senescence, all cells exhibit similar numbers of total γ-foci per cell, irrespective of chromosomal locations.ConclusionOur results suggest that the chromosome location of senescence-related γ-foci is determined by the telomere length rather than species differences per se. In addition, our data indicate that both telomeric and non-telomeric DNA damage responses play equivalent roles in signaling the initiation of cellular senescence and organismal aging. These data have important implications in the study of mechanisms to induce or delay cellular senescence in different species.

title: Senescence-Associated Secretory Phenotypes Reveal Cell-Nonautonomous Functions of Oncogenic RAS and the p53 Tumor Suppressor
abstract: Cellular senescence suppresses cancer by arresting cell proliferation, essentially permanently, in response to oncogenic stimuli, including genotoxic stress. We modified the use of antibody arrays to provide a quantitative assessment of factors secreted by senescent cells. We show that human cells induced to senesce by genotoxic stress secrete myriad factors associated with inflammation and malignancy. This senescence-associated secretory phenotype (SASP) developed slowly over several days and only after DNA damage of sufficient magnitude to induce senescence. Remarkably similar SASPs developed in normal fibroblasts, normal epithelial cells, and epithelial tumor cells after genotoxic stress in culture, and in epithelial tumor cells in vivo after treatment of prostate cancer patients with DNA-damaging chemotherapy. In cultured premalignant epithelial cells, SASPs induced an epithelial–mesenchyme transition and invasiveness, hallmarks of malignancy, by a paracrine mechanism that depended largely on the SASP factors interleukin (IL)-6 and IL-8. Strikingly, two manipulations markedly amplified, and accelerated development of, the SASPs: oncogenic RAS expression, which causes genotoxic stress and senescence in normal cells, and functional loss of the p53 tumor suppressor protein. Both loss of p53 and gain of oncogenic RAS also exacerbated the promalignant paracrine activities of the SASPs. Our findings define a central feature of genotoxic stress-induced senescence. Moreover, they suggest a cell-nonautonomous mechanism by which p53 can restrain, and oncogenic RAS can promote, the development of age-related cancer by altering the tissue microenvironment.

title: Chemokine Signaling via the CXCR2 Receptor Reinforces Senescence
abstract: Cells enter senescence, a state of stable proliferative arrest, in response to a variety of cellular stresses, including telomere erosion, DNA damage, and oncogenic signaling, which acts as a barrier against malignant transformation in vivo. To identify genes controlling senescence, we conducted an unbiased screen for small hairpin RNAs that extend the life span of primary human fibroblasts. Here, we report that knocking down the chemokine receptor CXCR2 (IL8RB) alleviates both replicative and oncogene-induced senescence (OIS) and diminishes the DNA-damage response. Conversely, ectopic expression of CXCR2 results in premature senescence via a p53-dependent mechanism. Cells undergoing OIS secrete multiple CXCR2-binding chemokines in a program that is regulated by the NF-kappaB and C/EBPbeta transcription factors and coordinately induce CXCR2 expression. CXCR2 upregulation is also observed in preneoplastic lesions in vivo. These results suggest that senescent cells activate a self-amplifying secretory network in which CXCR2-binding chemokines reinforce growth arrest.

title: Persistent DNA damage signaling triggers senescence-associated inflammatory cytokine secretion
abstract: Cellular senescence suppresses cancer by stably arresting the proliferation of damaged cells. Paradoxically, senescent cells also secrete factors that alter tissue microenvironments. The pathways regulating this secretion are unknown. We show that damaged human cells develop persistent chromatin lesions bearing hallmarks of DNA double-strand breaks (DSBs), which initiate increased secretion of inflammatory cytokines such as interleukin-6 (IL-6). Cytokine secretion occurred only after establishment of persistent DNA damage signalling, usually associated with senescence, not after transient DNA damage responses (DDRs). Initiation and maintenance of this cytokine response required the DDR proteins ATM, NBS1 and CHK2, but not the cell-cycle arrest enforcers p53 and pRb. ATM was also essential for IL-6 secretion during oncogene-induced senescence and by damaged cells that bypass senescence. Furthermore, DDR activity and IL-6 were elevated in human cancers, and ATM-depletion suppressed the ability of senescent cells to stimulate IL-6-dependent cancer cell invasiveness. Thus, in addition to orchestrating cell-cycle checkpoints and DNA repair, a new and important role of the DDR is to allow damaged cells to communicate their compromised state to the surrounding tissue.

title: Recurrent turnover of senescent cells during regeneration of a complex structure
abstract: Cellular senescence has been recently linked to the promotion of age-related pathologies, including a decline in regenerative capacity. While such capacity deteriorates with age in mammals, it remains intact in species such as salamanders, which have an extensive repertoire of regeneration and can undergo multiple episodes through their lifespan. Here we show that, surprisingly, there is a significant induction of cellular senescence during salamander limb regeneration, but that rapid and effective mechanisms of senescent cell clearance operate in normal and regenerating tissues. Furthermore, the number of senescent cells does not increase upon repetitive amputation or ageing, in contrast to mammals. Finally, we identify the macrophage as a critical player in this efficient senescent cell clearance mechanism. We propose that effective immunosurveillance of senescent cells in salamanders supports their ability to undergo regeneration throughout their lifespan.

title: An essential role for senescent cells in optimal wound healing through secretion of PDGF-AA.
abstract: Cellular senescence suppresses cancer by halting the growth of premalignant cells, yet the accumulation of senescent cells is thought to drive age-related pathology through a senescence-associated secretory phenotype (SASP), the function of which is unclear. To understand the physiological role(s) of the complex senescent phenotype, we generated a mouse model in which senescent cells can be visualized and eliminated in living animals. We show that senescent fibroblasts and endothelial cells appear very early in response to a cutaneous wound, where they accelerate wound closure by inducing myofibroblast differentiation through the secretion of platelet-derived growth factor AA (PDGF-AA). In two mouse models, topical treatment of senescence-free wounds with recombinant PDGF-AA rescued the delayed wound closure and lack of myofibroblast differentiation. These findings define a beneficial role for the SASP in tissue repair and help to explain why the SASP evolved.

title: The senescence-associated secretory phenotype induces cellular plasticity and tissue regeneration.
abstract: Senescence is a form of cell cycle arrest induced by stress such as DNA damage and oncogenes. However, while arrested, senescent cells secrete a variety of proteins collectively known as the senescence-associated secretory phenotype (SASP), which can reinforce the arrest and induce senescence in a paracrine manner. However, the SASP has also been shown to favor embryonic development, wound healing, and even tumor growth, suggesting more complex physiological roles than currently understood. Here we uncover timely new functions of the SASP in promoting a proregenerative response through the induction of cell plasticity and stemness. We show that primary mouse keratinocytes transiently exposed to the SASP exhibit increased expression of stem cell markers and regenerative capacity in vivo. However, prolonged exposure to the SASP causes a subsequent cell-intrinsic senescence arrest to counter the continued regenerative stimuli. Finally, by inducing senescence in single cells in vivo in the liver, we demonstrate that this activates tissue-specific expression of stem cell markers. Together, this work uncovers a primary and beneficial role for the SASP in promoting cell plasticity and tissue regeneration and introduces the concept that transient therapeutic delivery of senescent cells could be harnessed to drive tissue regeneration.

title: Tissue damage and senescence provide critical signals for cellular reprogramming in vivo
abstract: For cell reprogramming, context matters Differentiated cells in a culture dish can assume a new identity when manipulated to express four transcription factors. This “reprogramming” process has sparked interest because conceivably it could be harnessed as a therapeutic strategy for tissue regeneration. Mosteiro et al. used a mouse model to study the signals that promote cell reprogramming in vivo. They found that the factors that trigger reprogramming in vitro do the same in vivo; however, they also inflict cell damage. The damaged cells enter a state of senescence and begin secreting certain factors that promote reprogramming, including an inflammatory cytokine called interleukin-6. Thus, in the physiological setting, cell senescence may create a tissue context that favors reprogramming of neighboring cells. Science, this issue p. 10.1126/science.aaf4445 In mice, senescent cells created by tissue damage induce reprogramming of neighboring cells, enhancing tissue repair.  The ectopic expression of transcription factors OCT4, SOX2, KLF4, and cMYC (OSKM) enables reprogramming of adult differentiated cells into pluripotent cells, known as induced pluripotent stem cells (iPSCs), that are functionally equivalent to embryonic stem cells. Expression of OSKM in vivo leads to widespread cell dedifferentiation and reprogramming within tissues and eventually to the formation of teratomas (tumors arising from iPSCs). The molecular mechanisms operating during in vitro OSKM-driven reprogramming have been extensively characterized; however, little is known about in vivo reprogramming.  The process of OSKM reprogramming is inefficient both in vitro and in vivo. A number of cell-intrinsic barriers have been identified in vitro, most of which are activated by cellular damage and are particularly prominent in aged cells. Mechanistically, these cell-intrinsic barriers for reprogramming are primarily mediated by the tumor suppressors p53, p16INK4a, and ARF (the latter two are encoded by the Ink4a/Arf gene locus). In this work, we have investigated the effect of these tumor suppressors, cellular damage, and aging on in vivo reprogramming.  We found that the expression of OSKM in vivo not only triggers reprogramming of some cells but also inflicts extensive damage on many other cells, driving them into a state known as cellular senescence. Senescent cells are characterized by their inability to proliferate and by their secretion of inflammatory cytokines. We have observed a positive correlation between senescence and OSKM-driven reprogramming. For example, tissues lacking p16INK4a/ARF do not undergo senescence, and their ability to reprogram is severely compromised. By contrast, in tissues lacking p53, damage is rampant; this leads to maximal levels of senescence, exacerbated cytokine production, and increased in vivo reprogramming. To explore the connection between senescence and reprogramming, we manipulated these processes in vivo through pharmacological interventions. In particular, an increase in senescence produced by palbociclib (a drug that functionally mimics p16INK4a) results in higher levels of reprogramming. Conversely, a reduction in senescence achieved by navitoclax (a proapoptotic drug with selectivity against senescent cells) leads to decreased in vivo reprogramming. We found that the cross-talk between senescence and reprogramming is mediated by the cytokine-rich microenvironment associated with senescent cells. This is based, among other evidence, on the observation that pharmacological inhibition of NFκB, a major driver of cytokine production, reduces in vivo reprogramming. Analysis of the inflammatory cytokines produced by senescent cells, both in vivo and in vitro, led us to identify interleukin-6 (IL-6) as a critical secreted factor responsible for the ability of senescent cells to promote reprogramming. In support of this, blockade of IL-6 or its downstream kinase effector PIM potently reduced in vivo reprogramming. These observations can be recapitulated in vitro, where reprogramming efficiency is strongly enhanced by the presence of damaged cells or by the conditioned medium derived from damaged cells. Moreover, immunodepletion of IL-6 from the conditioned medium abolished reprogramming. Having established that senescence promotes reprogramming, we studied whether tissue injury leading to senescence has a positive effect on OSKM-driven reprogramming. In particular, we show that bleomycin-induced tissue damage strongly promotes reprogramming in the lung. Finally, aging, which is associated with higher levels of cellular senescence, also favors OSKM-driven reprogramming both in progeric and in physiologically aged mice.  The expression of OSKM in vivo triggers two different cellular outcomes: reprogramming in a small fraction of cells, and damage and senescence in many other cells. There is a strong positive association between these two processes, due to the fact that cellular senescence creates a tissue context that favors OSKM-driven reprogramming in neighboring cells. The positive effect of senescence on reprogramming is mediated by secreted factors, of which IL-6 is a key player. This also applies to tissue injury and aging, where there is an accumulation of senescent cells that send signals to surrounding cells to promote OSKM-driven dedifferentiation and reprogramming. A similar conceptual interplay may occur in physiological conditions, where damage-triggered senescence could induce cell dedifferentiation to promote tissue repair. Interplay between cellular senescence and OSKM-driven reprogramming. Expression of OSKM in vivo, apart from inducing the reprogramming of a small population of cells, also induces damage and senescence in many other cells. Senescent cells release factors that promote the reprogramming of neighboring cells, with IL-6 being a critical mediator. Tissue injury and aging, through the accumulation of senescent cells, favor in vivo reprogramming. Reprogramming of differentiated cells into pluripotent cells can occur in vivo, but the mechanisms involved remain to be elucidated. Senescence is a cellular response to damage, characterized by abundant production of cytokines and other secreted factors that, together with the recruitment of inflammatory cells, result in tissue remodeling. Here, we show that in vivo expression of the reprogramming factors OCT4, SOX2, KLF4, and cMYC (OSKM) in mice leads to senescence and reprogramming, both coexisting in close proximity. Genetic and pharmacological analyses indicate that OSKM-induced senescence requires the Ink4a/Arf locus and, through the production of the cytokine interleukin-6, creates a permissive tissue environment for in vivo reprogramming. Biological conditions linked to senescence, such as tissue injury or aging, favor in vivo reprogramming by OSKM. These observations may be relevant for tissue repair.

title: Injury-Induced Senescence Enables In Vivo Reprogramming in Skeletal Muscle.
abstract: In vivo reprogramming is a promising approach for tissue regeneration in response to injury. Several examples of in vivo reprogramming have been reported in a variety of lineages, but some including skeletal muscle have so far proven refractory. Here, we show that acute and chronic injury enables transcription-factor-mediated reprogramming in skeletal muscle. Lineage tracing indicates that this response frequently originates from Pax7+ muscle stem cells. Injury is associated with accumulation of senescent cells, and advanced aging or local irradiation further enhanced in vivo reprogramming, while selective elimination of senescent cells reduced reprogramming efficiency. The effect of senescence appears to be, at least in part, due to the release of interleukin 6 (IL-6), suggesting a potential link with the senescence-associated secretory phenotype. Collectively, our findings highlight a beneficial paracrine effect of injury-induced senescence on cellular plasticity, which will be important for devising strategies for reprogramming-based tissue repair.

title: The senescent microenvironment promotes the emergence of heterogeneous cancer stem-like cells.
abstract: There is a well-established association between aging and the onset of metastasis. Although the mechanisms through which age impinges upon the malignant phenotype remain uncharacterized, the role of a senescent microenvironment has been emphasized. We reported previously that human epithelial cells that undergo telomere-driven chromosome instability (T-CIN) display global microRNA (miR) deregulation and develop migration and invasion capacities. Here, we show that post-crisis cells are not able to form tumors unless a senescent microenvironment is provided. The characterization of cell lines established from such tumors revealed that these cells have acquired cell autonomous tumorigenicity, giving rise to heterogeneous tumors. Further experiments demonstrate that explanted cells, while displaying differences in cell differentiation markers, are all endowed of enhanced stem cell properties including self-renewal and multilineage differentiation capacity. Treatments of T-CIN+ cells with senescence-conditioned media induce sphere formation exclusively in cells with senescence-associated tumorigenicity, a capacity that depends on miR-145 repression. These results indicate that the senescent microenvironment, while promoting further transdifferentiations in cells with genome instability, is able to propel the progression of premalignant cells towards a malignant, cell stem-like state.

title: Senescence-associated secretory phenotype favors the emergence of cancer stem-like cells
abstract: The molecular mechanisms underlying cancer resistance remain elusive. One possible explanation is that cancer stem cells (CSCs) elude drug treatment, emerge and reproduce a tumor. Using multiple myeloma as a paradigm, we showed that cancer stem-like cells (CSLCs) appear after genotoxic stress because of their intrinsic properties. However, these properties do not drive the emergence of the CSLCs. Following genotoxic stress, remaining DNA damages lead to a senescence-associated secretory phenotype (SASP). Senescent cells, which are the non-CSLCs, secrete chemokines contributing to the emergence, maintenance and migration of CSLCs. Downregulation of checkpoint protein 2, a key player of SASP, significantly reduced the emergence of CSLCs. Our results unravel a novel molecular mechanism by which SASP might promote malignancy, underlining the dual role of senescence in tumorigenesis. This mechanism, based on mutual cooperation among tumor cells, illustrates how cancer may relapse; its targeting could represent new therapeutic opportunities.

title: SASP mediates chemoresistance and tumor-initiating-activity of mesothelioma cells
abstract: Here we show that pemetrexed-treated mesothelioma cells undergo accelerated senescence. This is characterized by the secretion of proinflammatory and mitogenic cytokines, reminiscent of an SASP (senescence-associated secretory phenotype). Conditioned media from senescent MPM (malignant pleural mesothelioma) cells trigger the emergence of EMT (epithelial-to-mesenchymal)-like, clonogenic and chemoresistant cell subpopulations, expressing high levels of ALDH (aldehyde dehydrogenase) activity (ALDHbright cells). We show by fluorescence-activated cell sorting of purified ALDHbright and ALDHlow cells, that both cell-autonomous and cell-non-autonomous mechanisms converge to maintain the SASP-induced, EMT-like cell subpopulations. Chemoresistant ALDHbright cells exist within primary MPM specimens and enrichment for ALDHbright cells correlates with an earlier tumor onset into NOD/SCID mice. We show that RASv12 expression induces SASP-like changes in untransformed human mesothelial cells, and that p53 ablation increases the effect of RASv12 expression. We identify STAT3 activation as a crucial event downstream to SASP signaling. In fact, small hairpin RNA-mediated ablation of STAT3 deeply attenuates the induction of EMT genes and the increase of ALDHbright cells induced by SASP-cytokines. This strongly affects the chemoresistance of MPM cells in vitro and leads to anticancer effects in vivo.

title: Senescence impairs successful reprogramming to pluripotent stem cells.
abstract: Somatic cells can be reprogrammed into induced pluripotent stem (iPS) cells by overexpressing combinations of factors such as Oct4, Sox2, Klf4, and c-Myc. Reprogramming is slow and stochastic, suggesting the existence of barriers limiting its efficiency. Here we identify senescence as one such barrier. Expression of the four reprogramming factors triggers senescence by up-regulating p53, p16(INK4a), and p21(CIP1). Induction of DNA damage response and chromatin remodeling of the INK4a/ARF locus are two of the mechanisms behind senescence induction. Crucially, ablation of different senescence effectors improves the efficiency of reprogramming, suggesting novel strategies for maximizing the generation of iPS cells.

title: Pluripotent stem cells escape from senescence-associated DNA methylation changes.
abstract: Pluripotent stem cells evade replicative senescence, whereas other primary cells lose their proliferation and differentiation potential after a limited number of cell divisions, and this is accompanied by specific senescence-associated DNA methylation (SA-DNAm) changes. Here, we investigate SA-DNAm changes in mesenchymal stromal cells (MSC) upon long-term culture, irradiation-induced senescence, immortalization, and reprogramming into induced pluripotent stem cells (iPSC) using high-density HumanMethylation450 BeadChips. SA-DNAm changes are highly reproducible and they are enriched in intergenic and nonpromoter regions of developmental genes. Furthermore, SA-hypomethylation in particular appears to be associated with H3K9me3, H3K27me3, and Polycomb-group 2 target genes. We demonstrate that ionizing irradiation, although associated with a senescence phenotype, does not affect SA-DNAm. Furthermore, overexpression of the catalytic subunit of the human telomerase (TERT) or conditional immortalization with a doxycycline-inducible system (TERT and SV40-TAg) result in telomere extension, but do not prevent SA-DNAm. In contrast, we demonstrate that reprogramming into iPSC prevents almost the entire set of SA-DNAm changes. Our results indicate that long-term culture is associated with an epigenetically controlled process that stalls cells in a particular functional state, whereas irradiation-induced senescence and immortalization are not causally related to this process. Absence of SA-DNAm in pluripotent cells may play a central role for their escape from cellular senescence.

title: Autophagy maintains stemness by preventing senescence.
abstract: During ageing, muscle stem-cell regenerative function declines. At advanced geriatric age, this decline is maximal owing to transition from a normal quiescence into an irreversible senescence state. How satellite cells maintain quiescence and avoid senescence until advanced age remains unknown. Here we report that basal autophagy is essential to maintain the stem-cell quiescent state in mice. Failure of autophagy in physiologically aged satellite cells or genetic impairment of autophagy in young cells causes entry into senescence by loss of proteostasis, increased mitochondrial dysfunction and oxidative stress, resulting in a decline in the function and number of satellite cells. Re-establishment of autophagy reverses senescence and restores regenerative functions in geriatric satellite cells. As autophagy also declines in human geriatric satellite cells, our findings reveal autophagy to be a decisive stem-cell-fate regulator, with implications for fostering muscle regeneration in sarcopenia.

title: Hypoxia induces senescence of bone marrow mesenchymal stem cells via altered gut microbiota
abstract: Systemic chronic hypoxia is a feature of many diseases and may influence the communication between bone marrow (BM) and gut microbiota. Here we analyse patients with cyanotic congenital heart disease (CCHD) who are experiencing chronic hypoxia and characterize the association between bone marrow mesenchymal stem cells (BMSCs) and gut microbiome under systemic hypoxia. We observe premature senescence of BMSCs and abnormal d-galactose accumulation in patients with CCHD. The hypoxia that these patients experience results in an altered diversity of gut microbial communities, with a remarkable decrease in the number of Lactobacilli and a noticeable reduction in the amount of enzyme-degraded d-galactose. Replenishing chronic hypoxic rats with Lactobacillus reduced the accumulation of d-galactose and restored the deficient BMSCs. Together, our findings show that chronic hypoxia predisposes BMSCs to premature senescence, which may be due to gut dysbiosis and thus induced d-galactose accumulation.Systemic chronic hypoxia is a feature of many diseases and may influence the communication between bone marrow and gut microbiota. Here, the authors show that chronic hypoxia predisposes bone marrow stem cells to premature senescence, which may be due to gut dysbiosis and gut microbiota-derived d-galactose accumulation.

title: TGF-β Signaling Accelerates Senescence of Human Bone-Derived CD271 and SSEA-4 Double-Positive Mesenchymal Stromal Cells
abstract: It is generally thought that the proliferative capacity and differentiation potential of somatic stem cells, including mesenchymal stromal/stem cells (MSCs) and hematopoietic stem cells, decline with age. We investigated the effects of aging on human bone-derived MSCs expressing CD271 and SSEA-4 (double-positive MSCs [DPMSCs]). The percentages of DPMSCs in bone tissue decreased significantly with age. The DPMSCs from elderly patients (old DPMSCs) showed cellular senescence, which was evidenced by low growth potential, high senescence-associated β-galactosidase activity, and elevated p16 and p21 CDK inhibitor levels. Moreover, old DPMSCs showed weak osteogenic differentiation potential and less hematopoiesis-supporting activity in comparison with young DPMSCs. Interestingly, the addition of transforming growth factor β2 (TGF-β2) induced cellular senescence in young DPMSCs. With the exception of the adipogenic differentiation potential, all of the aging phenomena observed in old DPMSCs were reversed by the addition of anti-TGF-β antibodies. These results suggest that, in part, old DPMSCs accelerate cellular senescence through TGF-β signaling.

title: Intrinsic tumour suppression
abstract: Mutations that drive uncontrolled cell-cycle progression are requisite events in tumorigenesis. But evolution has installed in the proliferative programmes of mammalian cells a variety of innate tumour-suppressive mechanisms that trigger apoptosis or senescence, should proliferation become aberrant. These contingent processes rely on a series of sensors and transducers that act in a coordinated network to target the machinery responsible for apoptosis and cell-cycle arrest at different points. Although oncogenic mutations that disable such networks can have profound and varied effects on tumour evolution, they may leave intact latent tumour-suppressive potential that can be harnessed therapeutically.

title: Crucial role of p53-dependent cellular senescence in suppression of Pten-deficient tumorigenesis
abstract: Cellular senescence has been theorized to oppose neoplastic transformation triggered by activation of oncogenic pathways in vitro, but the relevance of senescence in vivo has not been established. The PTEN and p53 tumour suppressors are among the most commonly inactivated or mutated genes in human cancer including prostate cancer. Although they are functionally distinct, reciprocal cooperation has been proposed, as PTEN is thought to regulate p53 stability, and p53 to enhance PTEN transcription. Here we show that conditional inactivation of Trp53 in the mouse prostate fails to produce a tumour phenotype, whereas complete Pten inactivation in the prostate triggers non-lethal invasive prostate cancer after long latency. Strikingly, combined inactivation of Pten and Trp53 elicits invasive prostate cancer as early as 2 weeks after puberty and is invariably lethal by 7 months of age. Importantly, acute Pten inactivation induces growth arrest through the p53-dependent cellular senescence pathway both in vitro and in vivo, which can be fully rescued by combined loss of Trp53. Furthermore, we detected evidence of cellular senescence in specimens from early-stage human prostate cancer. Our results demonstrate the relevance of cellular senescence in restricting tumorigenesis in vivo and support a model for cooperative tumour suppression in which p53 is an essential failsafe protein of Pten-deficient tumours.

title: Oncogene-induced senescence as an initial barrier in lymphoma development
abstract: Acute induction of oncogenic Ras provokes cellular senescence involving the retinoblastoma (Rb) pathway, but the tumour suppressive potential of senescence in vivo remains elusive. Recently, Rb-mediated silencing of growth-promoting genes by heterochromatin formation associated with methylation of histone H3 lysine 9 (H3K9me) was identified as a critical feature of cellular senescence, which may depend on the histone methyltransferase Suv39h1. Here we show that Eµ-N-Ras transgenic mice harbouring targeted heterozygous lesions at the Suv39h1, or the p53 locus for comparison, succumb to invasive T-cell lymphomas that lack expression of Suv39h1 or p53, respectively. By contrast, most N-Ras-transgenic wild-type (‘control’) animals develop a non-lymphoid neoplasia significantly later. Proliferation of primary lymphocytes is directly stalled by a Suv39h1-dependent, H3K9me-related senescent growth arrest in response to oncogenic Ras, thereby cancelling lymphomagenesis at an initial step. Suv39h1-deficient lymphoma cells grow rapidly but, unlike p53-deficient cells, remain highly susceptible to adriamycin-induced apoptosis. In contrast, only control, but not Suv39h1-deficient or p53-deficient, lymphomas senesce after drug therapy when apoptosis is blocked. These results identify H3K9me-mediated senescence as a novel Suv39h1-dependent tumour suppressor mechanism whose inactivation permits the formation of aggressive but apoptosis-competent lymphomas in response to oncogenic Ras.

title: Cellular senescence is an important mechanism of tumor regression upon c-Myc inactivation
abstract: Oncogene-induced senescence is an important mechanism by which normal cells are restrained from malignant transformation. Here we report that the suppression of the c-Myc (MYC) oncogene induces cellular senescence in diverse tumor types including lymphoma, osteosarcoma, and hepatocellular carcinoma. MYC inactivation was associated with prototypical markers of senescence, including acidic β-gal staining, induction of p16INK4a, and p15INK4b expression. Moreover, MYC inactivation induced global changes in chromatin structure associated with the marked reduction of histone H4 acetylation and increased histone H3 K9 methylation. Osteosarcomas engineered to be deficient in p16INK4a or Rb exhibited impaired senescence and failed to exhibit sustained tumor regression upon MYC inactivation. Similarly, only after lymphomas were repaired for p53 expression did MYC inactivation induce robust senescence and sustained tumor regression. The pharmacologic inhibition of signaling pathways implicated in oncogene-induced senescence including ATM/ATR and MAPK did not prevent senescence associated with MYC inactivation. Our results suggest that cellular senescence programs remain latently functional, even in established tumors, and can become reactivated, serving as a critical mechanism of oncogene addiction associated with MYC inactivation.

title: CD4(+) T cells contribute to the remodeling of the microenvironment required for sustained tumor regression upon oncogene inactivation.
abstract: Oncogene addiction is thought to occur cell autonomously. Immune effectors are implicated in the initiation and restraint of tumorigenesis, but their role in oncogene inactivation-mediated tumor regression is unclear. Here, we show that an intact immune system, specifically CD4(+) T cells, is required for the induction of cellular senescence, shutdown of angiogenesis, and chemokine expression resulting in sustained tumor regression upon inactivation of the MYC or BCR-ABL oncogenes in mouse models of T cell acute lymphoblastic lymphoma and pro-B cell leukemia, respectively. Moreover, immune effectors knocked out for thrombospondins failed to induce sustained tumor regression. Hence, CD4(+) T cells are required for the remodeling of the tumor microenvironment through the expression of chemokines, such as thrombospondins, in order to elicit oncogene addiction.

title: Uncoupling Oncogene-Induced Senescence (OIS) and DNA Damage Response (DDR) triggered by DNA hyper-replication: lessons from primary mouse embryo astrocytes (MEA)
abstract: Oncogene-induced senescence (OIS) is a complex process, in which activation of oncogenic signals during early tumorigenesis results in a high degree of DNA replication stress. The ensuing response to the DNA damage produces a permanent G1 arrest that prevents unlimited cell proliferation and lessens the development of tumours. However, despite the role of OIS in the proliferative arrest resulting from an activating oncogenic-lesion has obtained wide support, there is also evidence indicating that cells may overcome oncogene-induced senescence under some circumstances. In this study, we have investigated the possibility that some of the assumptions on the role of DNA damage response (DDR) in triggering OIS may depend on the fact that most of the available data were obtained in mouse embryo fibroblast. By comparing the degree of OIS observed in mouse embryo fibroblasts (MEF) and mouse embryo astrocytes (MEA) obtained from the same individuals we have demonstrated that, despite truthful activation of DDR in both cell types, significant levels of OIS were only detected in MEF. Therefore, this uncoupling between OIS and DDR observed in astrocytes supports the intriguingly possibility that OIS is not a widespread response mechanism to DDR.

title: Enhancing chemotherapy efficacy in Pten-deficient prostate tumors by activating the senescence-associated antitumor immunity.
abstract: Prosenescence therapy has recently emerged as a novel therapeutic approach for treating cancer. However, this concept is challenged by conflicting evidence showing that the senescence-associated secretory phenotype (SASP) of senescent tumor cells can have pro- as well as antitumorigenic effects. Herein, we report that, in Pten-null senescent tumors, activation of the Jak2/Stat3 pathway establishes an immunosuppressive tumor microenvironment that contributes to tumor growth and chemoresistance. Activation of the Jak2/Stat3 pathway in Pten-null tumors is sustained by the downregulation of the protein tyrosine phosphatase PTPN11/SHP2, providing evidence for the existence of a novel PTEN/SHP2 axis. Importantly, treatment with docetaxel in combination with a JAK2 inhibitor reprograms the SASP and improves the efficacy of docetaxel-induced senescence by triggering a strong antitumor immune response in Pten-null tumors. Altogether, these data demonstrate that immune surveillance of senescent tumor cells can be suppressed in specific genetic backgrounds but also evoked by pharmacological treatments.

title: Detecting Markers of Therapy-Induced Senescence in Cancer Cells.
abstract: Therapy-induced senescence (TIS), a lasting chemotherapy-evoked proliferative arrest of tumor cells, has gained increasing attention by cancer researchers because of its' profound biological implications, and by clinical oncologists due to its potential contribution to the long-term outcome of cancer patients post-treatment. Although both apoptosis and senescence represent therapy-inducible, ultimate cell-cycle exit programs, mediated via DNA damage response signaling, apoptotic cell death as the faster and often quantitatively more prominent tumor response has been in the scientific focus for decades. The more recently recognized TIS as another "safeguard" response of cancer cells that were never primed for or failed to execute apoptosis, not only reflects a more complex "arrest-plus-other features" cell-autonomous condition but produces non-cell-autonomous phenotypes at the tumor site, collectively impinging on tumor control and clinical outcome. Hence, TIS research is gaining pivotal interest from both a tumor biological and a therapeutic perspective, and the development of non-DNA damaging, senescence-evoking therapeutics is about to become a major research objective. In this chapter, we describe a well-characterized, genetically controlled TIS model system based on primary BCL2-expressing Eμ-myc transgenic lymphoma cells harboring defined genetic lesions and provide protocols for co-staining of either senescence-associated β-galactosidase (SA-β-gal) activity or trimethylated lysine 9 of histone H3 (H3K9me3) together with Ki67 to detect the senescent status of therapy-exposed cancer cells.

title: Therapy-induced senescence in cancer.
abstract: Cellular senescence is a response to nonlethal stress that results in persistent cytostasis with a distinct morphological and biochemical phenotype. The senescence phenotype, detected in tumors through the expression of mRNA and protein markers, can be generated in cancer cells lacking functional p53 and retinoblastoma protein. Current research suggests that therapy-induced senescence (TIS) represents a novel functional target that may improve cancer therapy. TIS can be induced in immortal and transformed cancer cells by selected anticancer compounds or radiation, and accumulating data indicate that TIS may produce reduced toxicity-related side effects and increased tumor-specific immune activity. This review examines the current status of TIS-regulated mechanisms, agents, and senescence biomarkers with the goal of encouraging further development of this approach to cancer therapy. Remaining hurdles include the lack of efficient senescence-inducing agents and incomplete biological data on tumor response. The identification of additional compounds and other targeted approaches to senescence induction will further the development of TIS in the clinical treatment of cancer.

title: Cellular Senescence Promotes Adverse Effects of Chemotherapy and Cancer Relapse.
abstract: Cellular senescence suppresses cancer by irreversibly arresting cell proliferation. Senescent cells acquire a proinflammatory senescence-associated secretory phenotype. Many genotoxic chemotherapies target proliferating cells nonspecifically, often with adverse reactions. In accord with prior work, we show that several chemotherapeutic drugs induce senescence of primary murine and human cells. Using a transgenic mouse that permits tracking and eliminating senescent cells, we show that therapy-induced senescent (TIS) cells persist and contribute to local and systemic inflammation. Eliminating TIS cells reduced several short- and long-term effects of the drugs, including bone marrow suppression, cardiac dysfunction, cancer recurrence, and physical activity and strength. Consistent with our findings in mice, the risk of chemotherapy-induced fatigue was significantly greater in humans with increased expression of a senescence marker in T cells prior to chemotherapy. These findings suggest that senescent cells can cause certain chemotherapy side effects, providing a new target to reduce the toxicity of anticancer treatments.



Many genotoxic chemotherapies have debilitating side effects and also induce cellular senescence in normal tissues. The senescent cells remain chronically present where they can promote local and systemic inflammation that causes or exacerbates many side effects of the chemotherapy. Cancer Discov; 7(2); 165-76.

title: TGF-β Family Signaling Pathways in Cellular Dormancy.
abstract: Individual cancer cells can switch, reversibly, to a non-proliferative dormant state, a process characterized by two principal stages: (i) establishment and maintenance, and (ii) the breaking of dormancy. This phenomenon is of clinical importance because dormant cells resist chemotherapy, and this can result in cancer relapse following years, if not decades, of clinical remission. Although the molecular mechanisms governing tumor cell dormancy have not been clearly delineated, accumulating evidence suggests that members of the transforming growth factor-β (TGF-β) family are integral. We summarize here recent findings which support the view that TGF-β family signaling pathways play a pivotal role in cellular dormancy, and discuss how affected cells could be therapeutically targeted to prevent cancer relapse.

title: Tumor cell escape from therapy‐induced senescence
abstract: &NA; H460 non‐small cell lung, HCT116 colon and 4T1 breast tumor cell lines induced into senescence by exposure to either etoposide or doxorubicin were able to recover proliferative capacity both in mass culture and when enriched for the senescence‐like phenotype by flow cytometry (based on &bgr;‐galactosidase staining and cell size, and a senescence‐associated reporter, BTG1‐RFP). Recovery was further established using both real‐time microscopy and High‐Speed Live‐Cell Interferometry (HSLCI) and was shown to be accompanied by the attenuation of the Senescence‐Associated Secretory Phenotype (SASP). Cells enriched for the senescence‐like phenotype were also capable of forming tumors when implanted in both immunodeficient and immunocompetent mice. As chemotherapy‐induced senescence has been identified in patient tumors, our results suggest that certain senescence‐like phenotypes may not reflect a terminal state of growth arrest, as cells that recover with self‐renewal capacity may ultimately contribute to disease recurrence.

title: Tumor Cell Escape from Therapy-Induced Senescence as a Model of Disease Recurrence after Dormancy.
abstract: Senescence, a durable form of growth arrest, represents a primary response to numerous anticancer therapies. Although the paradigm that senescence is "irreversible" has largely withstood the findings of tumor cell recovery from what has been termed "pseudo-senescence" or "senescence-like arrest," a review of the literature suggests that therapy-induced senescence in tumor cells is not obligatorily a permanent cell fate. Consequently, we propose that senescence represents one avenue whereby tumor cells evade the direct cytotoxic impact of therapy, thereby allowing for prolonged survival in a dormant state, with the potential to recover self-renewal capacity and contribute to disease recurrence.

title: Secretion of Vascular Endothelial Growth Factor by Primary Human Fibroblasts at Senescence*
abstract: Cellular senescence prevents the proliferation of cells at risk for neoplastic transformation. Nonetheless, the senescence response is thought to be antagonistically pleiotropic and thus contribute to aging phenotypes, including, ironically, late life cancers. The cancer-promoting activity of senescent cells is likely due to secreted molecules, the identity of which remains largely unknown. Here, we have shown that senescent fibroblasts, much more than presenescent fibroblasts, stimulate tumor vascularization in mice. Weakly malignant epithelial cells co-injected with senescent fibroblasts had larger and greater numbers of blood vessels compared with controls. Accordingly, increased vascular endothelial growth factor (VEGF) expression was a frequent characteristic of senescent human and mouse fibroblasts in culture. Importantly, conditioned medium from senescent fibroblasts, more than medium from presenescent cells, stimulates cultured human umbilical vein endothelial cells to invade a basement membrane, a hallmark of angiogenesis. Increased VEGF expression was specific to the senescent phenotype and increased whether senescence was induced by replicative exhaustion, overexpression of p16Ink4a, or overexpression of oncogenic RAS. The senescence-dependent increase in VEGF production was accompanied by very little increase in hypoxic-inducible (transcription) factor 1 α protein levels, and hypoxia further induced VEGF in senescent cells. This result suggests the rise in VEGF expression at senescence is not a hypoxic response. Our findings may in part explain why senescent cells stimulate tumorigenesis in vivo and support the idea that senescent cells may facilitate age-associated cancer development by secreting factors that promote malignant progression.

title: Senescent fibroblasts promote epithelial cell growth and tumorigenesis: A link between cancer and aging
abstract: Mammalian cells can respond to damage or stress by entering a state of arrested growth and altered function termed cellular senescence. Several lines of evidence suggest that the senescence response suppresses tumorigenesis. Cellular senescence is also thought to contribute to aging, but the mechanism is not well understood. We show that senescent human fibroblasts stimulate premalignant and malignant, but not normal, epithelial cells to proliferate in culture and form tumors in mice. In culture, the growth stimulation was evident when senescent cells comprised only 10% of the fibroblast population and was equally robust whether senescence was induced by replicative exhaustion, oncogenic RAS, p14ARF, or hydrogen peroxide. Moreover, it was due at least in part to soluble and insoluble factors secreted by senescent cells. In mice, senescent, much more than presenescent, fibroblasts caused premalignant and malignant epithelial cells to form tumors. Our findings suggest that, although cellular senescence suppresses tumorigenesis early in life, it may promote cancer in aged organisms, suggesting it is an example of evolutionary antagonistic pleiotropy.

title: Stromal Senescence By Prolonged CDK4/6 Inhibition Potentiates Tumor Growth
abstract: Senescent cells within the tumor microenvironment (TME) adopt a proinflammatory, senescence-associated secretory phenotype (SASP) that promotes cancer initiation, progression, and therapeutic resistance. Here, exposure to palbociclib (PD-0332991), a CDK4/6 inhibitor, induces senescence and a robust SASP in normal fibroblasts. Senescence caused by prolonged CDK4/6 inhibition is DNA damage–independent and associated with Mdm2 downregulation, whereas the SASP elicited by these cells is largely reliant upon NF-κB activation. Based upon these observations, it was hypothesized that the exposure of nontransformed stromal cells to PD-0332991 would promote tumor growth. Ongoing clinical trials of CDK4/6 inhibitors in melanoma prompted a validation of this hypothesis using a suite of genetically defined melanoma cells (i.e., Ras mutant, Braf mutant, and Ras/Braf wild-type). When cultured in the presence of CDK4/6i-induced senescent fibroblasts, melanoma cell lines exhibited genotype-dependent proliferative responses. However, in vivo, PD-0332991–treated fibroblasts enhanced the growth of all melanoma lines tested and promoted the recruitment of Gr-1–positive immune cells. These data indicate that prolonged CDK4/6 inhibitor treatment causes normal fibroblasts to enter senescence and adopt a robust SASP. Such senescent cells suppress the antitumor immune response and promote melanoma growth in immunocompetent, in vivo models.  The ability of prolonged CDK4/6 inhibitor treatment to induce cellular senescence and a robust SASP in primary cells may hinder therapeutic efficacy and promote long-term, gerontogenic consequences that should be considered in clinical trials aiming to treat melanoma and other cancer types.  237–49.

title: The gene expression program of prostate fibroblast senescence modulates neoplastic epithelial cell proliferation through paracrine mechanisms.
abstract: The greatest risk factor for developing carcinoma of the prostate is advanced age. Potential molecular and physiologic contributors to the frequency of cancer occurrence in older individuals include the accumulation of somatic mutations through defects in genome maintenance, epigenetic gene silencing, oxidative stress, loss of immune surveillance, telomere dysfunction, chronic inflammation, and alterations in tissue microenvironment. In this context, the process of prostate carcinogenesis can be influenced through interactions between intrinsic cellular alterations and the extrinsic microenvironment and macroenvironment, both of which change substantially as a consequence of aging. In this study, we sought to characterize the molecular alterations that occur during the process of prostate fibroblast senescence to identify factors in the aged tissue microenvironment capable of promoting the proliferation and potentially the neoplastic progression of prostate epithelium. We evaluated three mechanisms leading to cell senescence: oxidative stress, DNA damage, and replicative exhaustion. We identified a consistent program of gene expression that includes a subset of paracrine factors capable of influencing adjacent prostate epithelial growth. Both direct coculture and conditioned medium from senescent prostate fibroblasts stimulated epithelial cell proliferation, 3-fold and 2-fold, respectively. The paracrine-acting proteins fibroblast growth factor 7, hepatocyte growth factor, and amphiregulin (AREG) were elevated in the extracellular environment of senescent prostate fibroblasts. Exogenous AREG alone stimulated prostate epithelial cell growth, and neutralizing antibodies and small interfering RNA targeting AREG attenuated, but did not completely abrogate the growth-promoting effects of senescent fibroblast conditioned medium. These results support the concept that aging-related changes in the prostate microenvironment may contribute to the progression of prostate neoplasia.

title: Inducing and exploiting vulnerabilities for the treatment of liver cancer
abstract: Liver cancer remains difficult to treat, owing to a paucity of drugs that target critical dependencies1,2; broad-spectrum kinase inhibitors such as sorafenib provide only a modest benefit to patients with hepatocellular carcinoma3. The induction of senescence may represent a strategy for the treatment of cancer, especially when combined with a second drug that selectively eliminates senescent cancer cells (senolysis)4,5. Here, using a kinome-focused genetic screen, we show that pharmacological inhibition of the DNA-replication kinase CDC7 induces senescence selectively in liver cancer cells with mutations in TP53. A follow-up chemical screen identified the antidepressant sertraline as an agent that kills hepatocellular carcinoma cells that have been rendered senescent by inhibition of CDC7. Sertraline suppressed mTOR signalling, and selective drugs that target this pathway were highly effective in causing the apoptotic cell death of hepatocellular carcinoma cells treated with a CDC7 inhibitor. The feedback reactivation of mTOR signalling after its inhibition6 is blocked in cells that have been treated with a CDC7 inhibitor, which leads to the sustained inhibition of mTOR and cell death. Using multiple in vivo mouse models of liver cancer, we show that treatment with combined inhibition of of CDC7 and mTOR results in a marked reduction of tumour growth. Our data indicate that exploiting an induced vulnerability could be an effective treatment for liver cancer.CDC7 inhibition selectively induces senescence in hepatocellular carcinoma cells with TP53 mutations, which enables the selective apoptotic cell death of these senescent cells using inhibitors of mTOR signalling.

title: Therapy-Induced Cellular Senescence Induces Epithelial-to-Mesenchymal Transition and Increases Invasiveness in Rectal Cancer.
abstract: DNA damaging agents and ionizing radiation used in the therapy of human cancers can induce senescence of cancer cells. Senescent cells exhibit a secretory phenotype (senescence-associated secretome [SAS]) that can affect cancer cell behavior and, eventually, clinical prognosis. We assessed the effects of the SAS on the induction of epithelial-to-mesenchymal transition (EMT) in vitro and in clinical samples from patients with rectal cancer who had undergone neoadjuvant chemoradiotherapy (CRT).



Colorectal cancer cells (HCT 116) were induced into senescence by exposure to either 5-fluorouracil (5-FU) or doxorubicin. The senescent state was confirmed by staining for senescence-associated β-galactosidase (SA-β-Gal). The paracrine effects of SASs were assessed on proliferating HCT 116 cells. The quantified parameters were cell proliferation, invasive capacity, and induction of EMT. Senescence and EMT in clinical samples were assessed by the expression levels (reverse transcriptase-quantitative polymerase chain reaction) of genes related to senescence and EMT after laser-assisted microdissection of cancer cell clusters that stained either positive or negative for SA-β-Gal.



We have shown that cultured colon cancer cells induced into senescence by exposure to 5-FU exhibit a SAS capable of paracrine induction of EMT in colon and rectal cancer cell lines and increased cell invasion in vitro. Using laser-assisted microdissection, we found that in rectal cancer samples from patients treated with neoadjuvant CRT, tumor cell niches enriched for senescent cells bookmark regions of increased mRNA expression levels of EMT-related proteins (Slug, Snail, vimentin) compared with the nearby senescent-null tumor cell niches.



We have provided, first-hand, strongly suggestive evidence that senescent cancer cells emerging in the context of neoadjuvant CRT for rectal cancer influenced the tumor microenvironment by promoting EMT by way of short-range interactions.

title: Remodeling of the Collagen Matrix in Aging Skin Promotes Melanoma Metastasis and Affects Immune Cell Motility.
abstract: Physical changes in skin are among the most visible signs of aging. We found that young dermal fibroblasts secrete high levels of extracellular matrix (ECM) constituents, including proteoglycans, glycoproteins, and cartilage-linking proteins. The most abundantly secreted was HAPLN1, a hyaluronic and proteoglycan link protein. HAPLN1 was lost in aged fibroblasts, resulting in a more aligned ECM that promoted metastasis of melanoma cells.  HAPLN1 inhibited metastasis in an aged microenvironment, in 3-D skin reconstruction models, and in vivo. Intriguingly, aged fibroblast-derived matrices had the opposite effect on the migration of T cells, inhibiting their motility. HAPLN1 treatment of aged fibroblasts restored motility of mononuclear immune cells, while impeding that of polymorphonuclear immune cells, which in turn affected regulatory T-cell recruitment. These data suggest that although age-related physical changes in the ECM can promote tumor cell motility, they may adversely affect the motility of some immune cells, resulting in an overall change in the immune microenvironment. Understanding the physical changes in aging skin may provide avenues for more effective therapy for older patients with melanoma.  These data shed light on the mechanochemical interactions that occur between aged skin, tumor, and immune cell populations, which may affect tumor metastasis and immune cell infiltration, with implications for the efficacy of current therapies for melanoma.See related commentary by Marie and Merlino, p. 19.This article is highlighted in the In This Issue feature, p. 1.

title: Senescence induces a proangiogenic switch in human peritoneal mesothelial cells.
abstract: The incidence of cancers that metastasize to the peritoneum increases with age. Intraperitoneal cancer dissemination depends largely on angiogenesis and interactions with the peritoneal mesothelium. We assessed the proangiogenic potential of human peritoneal mesothelial cells. Conditioned media collected from these cells at senescence stimulated proliferation of endothelial cells to a significantly greater extent compared to media from early-passage cells. The effect was accompanied by a significantly increased release of proangiogenic mediators -- VEGF, CXCL1/GROalpha, CXCL8/IL-8, and CCL2/MCP-1. These results indicate that the senescent mesothelium exhibits increased angiogenic activity, which may contribute to accelerated intraperitoneal cancer progression in the aged.

title: Senescence-associated secretory phenotype contributes to pathological angiogenesis in retinopathy
abstract: Senescent cells encourage abnormal blood vessels and retinal disease. Old but deadly Diabetics often lose their sight, a result of poor blood supply to the retina. Oubaha and colleagues investigated how this oxygen deprivation leads to disease. They found that retinal cells do not simply die when oxygen-starved, rather they become senescent—a state in which the cells secrete a cocktail of undesirable molecules, in mice and humans. These cytokines drive more senescence, encourage abnormal blood vessel formation, and block healthy regeneration. Treatment of mice with a diabetes drug or a senescence inhibitor reduced retinal disease, suggesting that interrupting this process could be therapeutically beneficial. Pathological angiogenesis is the hallmark of diseases such as cancer and retinopathies. Although tissue hypoxia and inflammation are recognized as central drivers of vessel growth, relatively little is known about the process that bridges the two. In a mouse model of ischemic retinopathy, we found that hypoxic regions of the retina showed only modest rates of apoptosis despite severely compromised metabolic supply. Using transcriptomic analysis and inducible loss-of-function genetics, we demonstrated that ischemic retinal cells instead engage the endoplasmic reticulum stress inositol-requiring enzyme 1α (IRE1α) pathway that, through its endoribonuclease activity, induces a state of senescence in which cells adopt a senescence-associated secretory phenotype (SASP). We also detected SASP-associated cytokines (plasminogen activator inhibitor 1, interleukin-6, interleukin-8, and vascular endothelial growth factor) in the vitreous humor of patients suffering from proliferative diabetic retinopathy. Therapeutic inhibition of the SASP through intravitreal delivery of metformin or interference with effectors of senescence (semaphorin 3A or IRE1α) in mice reduced destructive retinal neovascularization in vivo. We conclude that the SASP contributes to pathological vessel growth, with ischemic retinal cells becoming prematurely senescent and secreting inflammatory cytokines that drive paracrine senescence, exacerbate destructive angiogenesis, and hinder reparative vascular regeneration. Reversal of this process may be therapeutically beneficial.

title: Senescent tumor cells lead the collective invasion in thyroid cancer
abstract: Cellular senescence has been perceived as a barrier against carcinogenesis. However, the senescence-associated secretory phenotype (SASP) of senescent cells can promote tumorigenesis. Here, we show senescent tumour cells are frequently present in the front region of collective invasion of papillary thyroid carcinoma (PTC), as well as lymphatic channels and metastatic foci of lymph nodes. In in vitro invasion analysis, senescent tumour cells exhibit high invasion ability as compared with non-senescent tumour cells through SASP expression. Collective invasion in PTC is led by senescent tumour cells characterized by generation of a C-X-C-motif ligand (CXCL)12 chemokine gradient in the front region. Furthermore, senescent cells increase the survival of cancer cells via CXCL12/CXCR4 signalling. An orthotopic xenograft in vivo model also shows higher lymphatic vessels involvement in the group co-transplanted with senescent cells and cancer cells. These findings suggest that senescent cells are actively involved in the collective invasion and metastasis of PTC.

title: Wnt5A promotes an adaptive, senescent‐like stress response, while continuing to drive invasion in melanoma cells
abstract: We have previously shown that Wnt5A drives invasion in melanoma. We have also shown that Wnt5A promotes resistance to therapy designed to target the BRAFV600E mutation in melanoma. Here, we show that melanomas characterized by high levels of Wnt5A respond to therapeutic stress by increasing p21 and expressing classical markers of senescence, including positivity for senescence‐associated β‐galactosidase (SA‐β‐gal), senescence‐associated heterochromatic foci (SAHF), H3K9Me chromatin marks, and PML bodies. We find that despite this, these cells retain their ability to migrate and invade. Further, despite the expression of classic markers of senescence such as SA‐β‐gal and SAHF, these Wnt5A‐high cells are able to colonize the lungs in in vivo tail vein colony‐forming assays. This clearly underscores the fact that these markers do not indicate true senescence in these cells, but instead an adaptive stress response that allows the cells to evade therapy and invade. Notably, silencing Wnt5A reduces expression of these markers and decreases invasiveness. The combined data point to Wnt5A as a master regulator of an adaptive stress response in melanoma, which may contribute to therapy resistance.

title: Senescent tumor cells building three-dimensional tumor clusters
abstract: Cellular senescence, a permanent cell-cycle arrest, is a common yet intriguing phenomenon, in which its beneficial significance for biological organisms has only begun to be explored. Among others, senescent cells are able to transform tissue structures around them. Tumor cells, whose hallmark is their ability to proliferate indefinitely, are not free from the phenomenon. Here, we report a remarkable observation where senescent cells in a dense mono-layer of breast cancer colony act as aggregating centers for non-senescent cells in their vicinity. Consequently, the senescent cells actively form localized 3D cell-clusters in a confluent 2D tumor layer. The biophysical mechanism underpinning the surprising phenomenon primarily involves mitotic cell-rounding, dynamic and differential cell attachments, and cellular chemotaxis. By incorporating these few biophysical factors, we were able to recapitulate the experimental observation via a cellular Potts Model.

title: Stromal-Initiated Changes in the Bone Promote Metastatic Niche Development.
abstract: More than 85% of advanced breast cancer patients suffer from metastatic bone lesions, yet the mechanisms that facilitate these metastases remain poorly understood. Recent studies suggest that tumor-derived factors initiate changes within the tumor microenvironment to facilitate metastasis. However, whether stromal-initiated changes are sufficient to drive increased metastasis in the bone remains an open question. Thus, we developed a model to induce reactive senescent osteoblasts and found that they increased breast cancer colonization of the bone. Analysis of senescent osteoblasts revealed that they failed to mineralize bone matrix and increased local osteoclastogenesis, the latter process being driven by the senescence-associated secretory phenotype factor, IL-6. Neutralization of IL-6 was sufficient to limit senescence-induced osteoclastogenesis and tumor cell localization to bone, thereby reducing tumor burden. Together, these data suggest that a reactive stromal compartment can condition the niche, in the absence of tumor-derived signals, to facilitate metastatic tumor growth in the bone.

title: Mechanisms of disseminated cancer cell dormancy: an awakening field
abstract: Metastases arise from residual disseminated tumour cells (DTCs). This can happen years after primary tumour treatment because residual tumour cells can enter dormancy and evade therapies. As the biology of minimal residual disease seems to diverge from that of proliferative lesions, understanding the underpinnings of this new cancer biology is key to prevent metastasis. Analysis of approximately 7 years of literature reveals a growing focus on tumour and normal stem cell quiescence, extracellular and stromal microenvironments, autophagy and epigenetics as mechanisms that dictate tumour cell dormancy. In this Review, we attempt to integrate this information and highlight both the weaknesses and the strengths in the field to provide a framework to understand and target this crucial step in cancer progression.

title: Tumor Dormancy and Relapse: From a Natural Byproduct of Evolution to a Disease State.
abstract: Species evolve by mutations and epigenetic changes acting on individuals in a population; tumors evolve by similar mechanisms at a cellular level in a tissue. This article reviews growing evidence about tumor dormancy and suggests that (i) cellular malignancy is a natural byproduct of evolutionary mechanisms, such as gene mutations and epigenetic modifications, which is manifested in the form of tumor dormancy in healthy individuals as well as in cancer survivors; (ii) cancer metastasis could be an early dissemination event that could occur during malignant dormancy even before primary cancer is clinically detectable; and (iii) chronic inflammation is a key factor in awakening dormant malignant cells at the primary site, leading to primary cancer development, and at distant sites, leading to advanced stage diseases. On the basis of this evidence, it is reasonable to propose that we are all cancer survivors rather than cancer-free individuals because of harboring dormant malignant cells in our organs. A better understanding of local and metastatic tumor dormancy could lead to novel cancer therapeutics for the prevention of cancer. Cancer Res; 77(10); 2564-9.

title: Models, mechanisms and clinical evidence for cancer dormancy
abstract: Patients with cancer can develop recurrent metastatic disease with latency periods that range from years even to decades. This pause can be explained by cancer dormancy, a stage in cancer progression in which residual disease is present but remains asymptomatic. Cancer dormancy is poorly understood, resulting in major shortcomings in our understanding of the full complexity of the disease. Here, I review experimental and clinical evidence that supports the existence of various mechanisms of cancer dormancy including angiogenic dormancy, cellular dormancy (G0–G1 arrest) and immunosurveillance. The advances in this field provide an emerging picture of how cancer dormancy can ensue and how it could be therapeutically targeted.

title: A multigenic program mediating breast cancer metastasis to bone.
abstract: We investigated the molecular basis for osteolytic bone metastasis by selecting human breast cancer cell line subpopulations with elevated metastatic activity and functionally validating genes that are overexpressed in these cells. These genes act cooperatively to cause osteolytic metastasis, and most of them encode secreted and cell surface proteins. Two of these genes, interleukin-11 and CTGF, encode osteolytic and angiogenic factors whose expression is further increased by the prometastatic cytokine TGF beta. Overexpression of this bone metastasis gene set is superimposed on a poor-prognosis gene expression signature already present in the parental breast cancer population, suggesting that metastasis requires a set of functions beyond those underlying the emergence of the primary tumor.

title: Metastatic cancer cell.
abstract: Metastasis is the result of cancer cell adaptation to a tissue microenvironment at a distance from the primary tumor. Metastatic cancer cells require properties that allow them not only to adapt to a foreign microenvironment but to subvert it in a way that is conducive to their continued proliferation and survival. Recent conceptual and technological advances have contributed to our understanding of the role of the host tissue stroma in promoting tumor cell growth and dissemination and have provided new insight into the genetic makeup of cancers with high metastatic proclivity.

title: Breast cancer recurrence dynamics following adjuvant CMF is consistent with tumor dormancy and mastectomy-driven acceleration of the metastatic process.
abstract: The aim of this study was to better understand human breast cancer biology by studying how the timing of metastasis following primary resection is affected by adjuvant CMF (cyclophoshamide, methotrexate, 5-fluorouracil) chemotherapy.



Discrete hazards of recurrence and recurrence risk reductions for treated patients relative to controls were analyzed for all patients enrolled in two separate randomized clinical trials [study 1 (386 women): no further treatment versus 12 cycles of CMF; study 2 (459 women): six versus 12 cycles of CMF] and a historical group (396 women: surgery alone) of axillary node-positive patients undergoing mastectomy.



(i) Nearly all CMF benefit occurs during the first 4 years following resection/chemotherapy. (ii) The CMF recurrence rate reduction is largely restricted to two specific spans. These temporally separate recurrence clusters occur during the first and third year of follow-up, while the second-year recurrences are weakly affected. (iii) Prolonging adjuvant treatment from 6 to 12 months partially alters this recurrence timing, without appreciably affecting the overall recurrence rate. (iv) These effects upon the dynamics of post-resection occurrence are menopausal status-independent.



At least two different therapeutically vulnerable proliferative events, resulting in clinical appearance of two metastasis temporally distinct clusters of post-resection cancer recurrence, apparently occur during the administration of adjuvant chemotherapy. Metastases that transpire outside of these temporal windows are refractory to adjuvant therapy. The dynamics of both post-treatment recurrence risk and CMF effectiveness are similar for both pre- and postmenopausal women, suggesting that post-resection mechanisms by which chemotherapy prevents metastases are similar, but of different magnitude in pre- and postmenopausal women. These findings are consistent with a metastasis model that includes tumor dormancy in specific micrometastatic phases (single cells and avascular foci) and with the acceleration of the metastatic process by the surgical resection of the primary breast cancer.

title: Emerging Topics on Disseminated Cancer Cell Dormancy and the Paradigm of Metastasis
abstract: Disseminated tumor cells (DTCs) are recognized as the seeds of metastasis. However, metastatic lesions can become symptomatic years or decades after primary tumor removal. This clinical finding suggests that DTCs are not immediately competent to initiate growth and can persist in a dormant state. Here we review recent data for three potential scenarios that could result in DTC dormancy: (a) The target organ microenvironment instructs DTCs to enter dormancy; (b) primary tumors pre-encode a dormancy signature that only becomes evident when DTCs enter target organs that produce dormancy-inducing cues; and (c) early dissemination spawns DTCs that, by virtue of being closely related to normal cells, would retain the capacity to respond to dormancy-instructing signals and enter dormancy in target organs. The literature supports the existence of these scenarios and provides insight into how to prevent metastasis. Importantly, cotargeting dormant and proliferative DTCs in stage IV cancer may also improve outcomes...

title: Autophagy promotes the survival of dormant breast cancer cells and metastatic tumour recurrence
abstract: Cancer recurrence after initial diagnosis and treatment is a major cause of breast cancer (BC) mortality, which results from the metastatic outbreak of dormant tumour cells. Alterations in the tumour microenvironment can trigger signalling pathways in dormant cells leading to their proliferation. However, processes involved in the initial and the long-term survival of disseminated dormant BC cells remain largely unknown. Here we show that autophagy is a critical mechanism for the survival of disseminated dormant BC cells. Pharmacologic or genetic inhibition of autophagy in dormant BC cells results in significantly decreased cell survival and metastatic burden in mouse and human 3D in vitro and in vivo preclinical models of dormancy. In vivo experiments identify autophagy gene autophagy-related 7 (ATG7) to be essential for autophagy activation. Mechanistically, inhibition of the autophagic flux in dormant BC cells leads to the accumulation of damaged mitochondria and reactive oxygen species (ROS), resulting in cell apoptosis.Highly metastatic dormant cancer cells contribute to breast cancer recurrence, but the underlying mechanism is less understood. Here, the authors show that dormant breast cancer cells depend on autophagy to ensure their long term survival and distant recurrence

title: NR2F1 stratifies dormant disseminated tumor cells in breast cancer patients
abstract: BackgroundThe presence of disseminated tumor cells (DTCs) in bone marrow (BM) is an independent prognostic factor in early breast cancer but does not uniformly predict outcome. Tumor cells can persist in a quiescent state over time, but clinical studies of markers predicting the awakening potential of DTCs are lacking. Recently, experiments have shown that NR2F1 (COUP-TF1) plays a key role in dormancy signaling.MethodsWe analyzed the NR2F1 expression in DTCs by double immunofluorescence (DIF) staining of extra cytospins prepared from 114 BM samples from 86 selected DTC-positive breast cancer patients. Samples collected at two or more time points were available for 24 patients. Fifteen samples were also analyzed for the proliferation marker Ki67.ResultsOf the patients with detectable DTCs by DIF, 27% had ≥ 50% NR2F1high DTCs, chosen a priori as the cut-off for “dormant profile” classification. All patients with systemic relapse within 12 months after BM aspiration carried ≤ 1% NR2F1high DTCs, including patients who transitioned from having NR2F1high-expressing DTCs in previous BM samples. Of the patients with serial samples, half of those with no relapse at follow-up had ≥ 50% NR2F1high DTCs in the last BM aspiration analyzed. Among the 18 relapse-free patients at the time of the last DTC-positive BM aspiration with no subsequent BM analysis performed, distant disease-free intervals were favorable for patients carrying ≥ 50% NR2F1high DTCs compared with those with predominantly NR2F1low DTCs (p = 0.007, log-rank). No survival difference was observed by classification according to Ki67-expressing DTCs (p = 0.520).ConclusionsOur study translates findings from basic biological analysis of DTC dormancy to the clinical situation and supports further clinical studies of NR2F1 as a marker of dormancy.

title: Genomic Profiling of Viable and Proliferative Micrometastatic Cells from Early-Stage Breast Cancer Patients
abstract: Metastases in distant organs are the major cause of death for cancer patients, and bone marrow is a prominent homing organ for early disseminated cancer cells. However, it remains still unclear which of these cells evolve into overt metastases. We therefore established a new approach based on the analysis of viable and proliferating cancer cells by single-cell comparative genomic hybridization.  The bone marrow of early-stage breast tumor patients (pN0M0) was screened for tumor cells by immunostaining. By applying special short-term culturing, we selected for viable and proliferative tumor cells. The short-term culturing allowed us to evaluate the proliferative potential of micrometastatic cells, which we had previously shown to represent an independent prognostic marker. We assessed genomic changes in single disseminated cancer cells by single-cell comparative genomic hybridization.  We found that these viable disseminated cancer cells already had a plethora of copy number changes in their genome. All of these cells showed chromosomal copy number changes with a substantial intercellular heterogeneity and differences to the matching primary tumors.  The established experimental strategy might pave the way for the identification of metastatic stem cells in cancer patients. Our preliminary results support the new concept that early disseminated cancer cells evolve independently from their primary tumor.

title: Obligate progression precedes lung adenocarcinoma dissemination.
abstract: Despite its clinical importance, very little is known about the natural history and molecular underpinnings of lung cancer dissemination and metastasis. Here, we used a genetically engineered mouse model of metastatic lung adenocarcinoma in which cancer cells are fluorescently marked to determine whether dissemination is an inherent ability or a major acquired phenotype during lung adenocarcinoma metastasis. We find very little evidence for dissemination from oncogenic KRAS-driven hyperplasias or most adenocarcinomas. p53 loss is insufficient to drive dissemination but rather enables rare cancer cells in a small fraction of primary adenocarcinomas to gain alterations that drive dissemination. Molecular characterization of disseminated tumor cells indicates that downregulation of the transcription factor Nkx2-1 precedes dissemination. Finally, we show that metastatic primary tumors possess a highly proliferative subpopulation of cells with characteristics matching those of disseminating cells. We propose that dissemination is a major hurdle during the natural course of lung adenocarcinoma metastasis.



Because of its aggressively metastatic nature, lung cancer is the top cancer killer of both men and women in the United States. We show that, unlike in other cancer types, lung cancer dissemination is a major initial barrier to metastasis. Our findings provide insight into the effect of p53 deficiency and downregulation of Nkx2-1 during lung adenocarcinoma progression.

title: Systemic spread is an early step in breast cancer.
abstract: It is widely accepted that metastasis is a late event in cancer progression. Here, however, we show that tumor cells can disseminate systemically from earliest epithelial alterations in HER-2 and PyMT transgenic mice and from ductal carcinoma in situ in women. Wild-type mice transplanted with single premalignant HER-2 transgenic glands displayed disseminated tumor cells and micrometastasis in bone marrow and lungs. The number of disseminated cancer cells and their karyotypic abnormalities were similar for small and large tumors in patients and mouse models. When activated by bone marrow transplantation into wild-type recipients, 80 early-disseminated cancer cells sufficed to induce lethal carcinosis. Therefore, release from dormancy of early-disseminated cancer cells may frequently account for metachronous metastasis.

title: Tumor cells disseminate early, but immunosurveillance limits metastatic outgrowth, in a mouse model of melanoma.
abstract: Although metastasis is the leading cause of cancer-related death, it is not clear why some patients with localized cancer develop metastatic disease after complete resection of their primary tumor. Such relapses have been attributed to tumor cells that disseminate early and remain dormant for prolonged periods of time; however, little is known about the control of these disseminated tumor cells. Here, we have used a spontaneous mouse model of melanoma to investigate tumor cell dissemination and immune control of metastatic outgrowth. Tumor cells were found to disseminate throughout the body early in development of the primary tumor, even before it became clinically detectable. The disseminated tumor cells remained dormant for varying periods of time depending on the tissue, resulting in staggered metastatic outgrowth. Dormancy in the lung was associated with reduced proliferation of the disseminated tumor cells relative to the primary tumor. This was mediated, at least in part, by cytostatic CD8+ T cells, since depletion of these cells resulted in faster outgrowth of visceral metastases. Our findings predict that immune responses favoring dormancy of disseminated tumor cells, which we propose to be the seed of subsequent macroscopic metastases, are essential for prolonging the survival of early stage cancer patients and suggest that therapeutic strategies designed to reinforce such immune responses may produce marked benefits in these patients.

title: Breast cancer and atypia among young and middle-aged women: a study of 110 medicolegal autopsies.
abstract: In 110 consecutive, medicolegal autopsies of young and middle-aged women (range 20-54 years) the breasts were examined by an extensive histopathologic method and by correlative specimen radiography. Malignancy was found in 22 women (20%) of which only one was known to have had clinical invasive breast cancer (IBC). At autopsy 2 women had IBC (2%), the remaining in situ carcinoma (in situ BC) of microfocal type (18%), i.e. 15 (14%) intraductal carcinomas (DCIS), 4 (3%) lobular carcinoma in situ (LCIS) and one (1%) both DCIS and LCIS. Forty-five per cent of the women with malignancy had multicentric and 41% had bilateral lesions. Forty-five per cent of all histologically confirmed malignant lesions were identified by specimen radiography. Adenosis, benign epithelial hyperplasia, papilloma and duct ectasia were positively associated with malignancy. In addition malignancy was significantly more frequent among women aged more than 40 years, with late age at first full-term pregnancy, with alcohol abuse and with steatosis or cirrhosis of the liver. The results suggest that clinically occult in situ BC are frequent in young and middle-aged women.

title: Early dissemination seeds metastasis in breast cancer
abstract: Accumulating data suggest that metastatic dissemination often occurs early during tumour formation, but the mechanisms of early metastatic spread have not yet been addressed. Here, by studying metastasis in a HER2-driven mouse breast cancer model, we show that progesterone-induced signalling triggers migration of cancer cells from early lesions shortly after HER2 activation, but promotes proliferation in advanced primary tumour cells. The switch from migration to proliferation was regulated by increased HER2 expression and tumour-cell density involving microRNA-mediated progesterone receptor downregulation, and was reversible. Cells from early, low-density lesions displayed more stemness features, migrated more and founded more metastases than cells from dense, advanced tumours. Notably, we found that at least 80% of metastases were derived from early disseminated cancer cells. Karyotypic and phenotypic analysis of human disseminated cancer cells and primary tumours corroborated the relevance of these findings for human metastatic dissemination.

title: Mammary carcinoma cell lines of high and low metastatic potential differ not in extravasation but in subsequent migration and growth
abstract: We examined the extravasation and subsequent migration and growth of murine mammary tumor cell lines (D2A1 and D2.OR) which differ in their metastatic ability in lung and liver, invasivenessin vitroand expression of the cysteine proteinase cathepsin L. In light of the differences in invasiveness and cathepsin L expression, we hypothesized that during hematogenous metastasis the two cell lines would differ primarily in their ability to extravasate. We usedin vivovideomicroscopy of mouse liver and chick embryo chorioallantoic membrane to examine the process and timing of extravasation and subsequent steps in metastasis for these cell lines. In contrast to our expectations, no differences were found between the cell lines in either the timing or mechanism of extravasation, at least 95% of cells having extravasated by 3 days after injection. However, after extravasation, the more metastatic and invasive D2A1 cells showed a greater ability to migrate to sites which favor tumor growth and to replicate to form micrometastases. These studies point to post-extravasation events (migration and growth) as being critical in metastasis formation.

title: Inhibition of metastatic outgrowth from single dormant tumor cells by targeting the cytoskeleton.
abstract: Metastatic breast cancer may emerge from latent tumor cells that remain dormant at disseminated sites for many years. Identifying mechanisms regulating the switch from dormancy to proliferative metastatic growth has been elusive due to the lack of experimental models of tumor cell dormancy. We characterized the in vitro growth characteristics of cells that exhibit either dormant (D2.0R, MCF-7, and K7M2AS1.46) or proliferative (D2A1, MDA-MB-231, and K7M2) metastatic behavior in vivo. Although these cells proliferate readily in two-dimensional culture, we show that when grown in three-dimensional matrix, distinct growth properties of the cells were revealed that correlate to their dormant or proliferative behavior at metastatic sites in vivo. In three-dimensional culture, cells with dormant behavior in vivo remained cell cycle arrested with elevated nuclear expression of p16 and p27. The transition from quiescence to proliferation of D2A1 cells was dependent on fibronectin production and signaling through integrin beta1, leading to cytoskeletal reorganization with filamentous actin (F-actin) stress fiber formation. We show that phosphorylation of myosin light chain (MLC) by MLC kinase (MLCK) through integrin beta1 is required for actin stress fiber formation and proliferative growth. Inhibition of integrin beta1 or MLCK prevents transition from a quiescent to proliferative state in vitro. Inhibition of MLCK significantly reduces metastatic outgrowth in vivo. These studies show that the switch from dormancy to metastatic growth may be regulated, in part, through epigenetic signaling from the microenvironment, leading to changes in the cytoskeletal architecture of dormant cells. Targeting this process may provide therapeutic strategies for inhibition of the dormant-to-proliferative metastatic switch.

title: Metastatic growth from dormant cells induced by a col-I-enriched fibrotic environment.
abstract: Breast cancer that recurs as metastatic disease many years after primary tumor resection and adjuvant therapy seems to arise from tumor cells that disseminated early in the course of disease but did not develop into clinically apparent lesions. These long-term surviving, disseminated tumor cells maintain a state of dormancy, but may be triggered to proliferate through largely unknown factors. We now show that the induction of fibrosis, associated with deposition of type I collagen (Col-I) in the in vivo metastatic microenvironment, induces dormant D2.0R cells to form proliferative metastatic lesions through beta1-integrin signaling. In vitro studies using a three-dimensional culture system modeling dormancy showed that Col-I induces quiescent D2.0R cells to proliferate through beta1-integrin activation of SRC and focal adhesion kinase, leading to extracellular signal-regulated kinase (ERK)-dependent myosin light chain phosphorylation by myosin light chain kinase and actin stress fiber formation. Blocking beta1-integrin, Src, ERK, or myosin light chain kinase by short hairpin RNA or pharmacologic approaches inhibited Col-I-induced activation of this signaling cascade, cytoskeletal reorganization, and proliferation. These findings show that fibrosis with Col-I enrichment at the metastatic site may be a critical determinant of cytoskeletal reorganization in dormant tumor cells, leading to their transition from dormancy to metastatic growth. Thus, inhibiting Col-I production, its interaction with beta1-integrin, and downstream signaling of beta1-integrin may be important strategies for preventing or treating recurrent metastatic disease.

title: Autocrine IGF1 Signaling Mediates Pancreatic Tumor Cell Dormancy in the Absence of Oncogenic Drivers.
abstract: Mutant KRAS and c-MYC are oncogenic drivers and rational therapeutic targets for the treatment of pancreatic cancer. Although tumor growth and homeostasis are largely dependent on these oncogenes, a few residual cancer cells are able to survive the ablation of mutant KRAS and c-MYC. By performing a genome-wide gene expression analysis of in vivo-derived bulk tumor cells and residual cancer cells lacking the expression of mutant KRAS or c-MYC, we have identified an increase in autocrine IGF1/AKT signaling as a common survival mechanism in dormant cancer cells. The pharmacological inhibition of IGF-1R reduces residual disease burden and cancer recurrence, suggesting that this molecular pathway is crucial for the survival of cancer cells in the absence of the primary oncogenic drivers.

title: Metastases in immune-mediated dormancy: a new opportunity for targeting cancer.
abstract: The aim of any anticancer treatment is to avoid, control, or eliminate disseminated tumor cells. Clinical and experimental evidence has revealed that metastases can remain in a latency state, that is, metastasis dormancy. Three mechanisms are thought to be involved in cancer dormancy: cellular dormancy, angiogenic dormancy, and immune-mediated dormancy. Here, we review the mechanisms and cells involved in immune-mediated cancer dormancy and discuss current and future immunotherapeutic strategies. Recent results indicate that the immune system can restrain disseminated cancer cells, promoting their permanent dormancy. CD8(+) T lymphocytes play a relevant role in maintaining immune equilibrium with metastatic dormant cells, and MHC class I surface expression on tumor cells may also be involved. Natural killer (NK) cells have an activator function that triggers a cytotoxic T lymphocyte (CTL) response. Furthermore, immune dormancy promotes cancer cell growth arrest and angiogenic control. Immunotherapeutic interventions in metastatic dormancy may help to control or eradicate cancer disease. Treatments that activate or increase the CTL immune response or reverse cancer cell-induced CTL immunosuppression might be useful to restrain or destroy metastatic cells. These objectives may be achieved by recovering or increasing MHC class I surface expression on cancer cells or even by activating NK cells. Immune-mediated metastasis dormancy provides an opportunity for targeting cancer in novel immune treatments.

title: Mechanisms Governing Metastatic Dormancy and Reactivation
abstract: Many cancer patients suffer from metastatic relapse several years after they have undergone radical surgery. Early cancer cell dissemination followed by a protracted period of dormancy potentially explains this prevalent clinical behavior. Increasing evidence suggests that the metastasis-initiating cells are cancer stem cells or revert to this functional state upon infiltrating a target organ. Their entry into dormancy and subsequent reactivation are governed by intrinsic programs and by contextual cues, which resemble those regulating the self-renewal capability of adult stem cells. In addition, metastatic cells undergoing reactivation are nursed by specialized extracellular matrix niches, which support positive signals, such as Wnt and Notch, and attenuate negative signals, such as BMP. In spite of significant remaining uncertainties, these findings provide a framework to understand the logic of metastatic dormancy and reactivation and open new avenues for therapeutic intervention.

title: Stem cell fate in cancer growth, progression and therapy resistance
abstract: Although we have come a long way in our understanding of the signals that drive cancer growth, and how these signals can be targeted, effective control of this disease remains a key scientific and medical challenge. The therapy resistance and relapse that are commonly seen are driven in large part by the inherent heterogeneity within cancers that allows drugs to effectively eliminate some, but not all, malignant cells. Here, we focus on the fundamental drivers of this heterogeneity by examining emerging evidence that shows that these traits are often controlled by the disruption of normal cell fate and aberrant adoption of stem cell signals. We discuss how undifferentiated cells are preferentially primed for transformation and often serve as the cell of origin for cancers. We also consider evidence showing that activation of stem cell programmes in cancers can lead to progression, therapy resistance and metastatic growth and that targeting these attributes may enable better control over a difficult disease.This Review discusses how cells with stem cell characteristics often serve as the tumour cell of origin, as they are preferentially primed for transformation. Furthermore, the activation of stem cell programmes can be crucial for promoting cancer progression and therapy resistance.

title: Characterization and functional analysis of a slow cycling stem cell-like subpopulation in pancreas adenocarcinoma
abstract: Evidence suggests that multiple tumors, including pancreatic adenocarcinoma, display heterogeneity in parameters that are critical for tumor formation, progression and metastasis. Understanding heterogeneity in solid tumors is increasingly providing a plethora of new diagnostic and therapeutic approaches. In this study, a particular focus was put on identifying a subpopulation of stem cell-like, slow cycling tumor cells in a pancreas adenocarcinoma cell lines. Using a label retention technique a subpopulation of slow cycling cells (DiI+/SCC) was identified and further evaluated in the BxPC-3 and Panc03.27 cell lines. These slowly cycling cells managed to retain the lipophilic labeling dye DiI, while the bulk of the cells (>94%) did not. The DiI+/SCC population, showed only a partial overlap with the CSC markers CD24+/CD44+, CD133+ and ALDH but they survived chemotherapeutic treatment, and were able to recreate the initial heterogeneous tumor cell population. DiI+/SCCs exhibited an increased invasive potential as compared with their non-label retaining, faster cycling cells (DiI−/FCC). They also had increased tumorigenic potential and morphological changes resembling cells that have undergone an epithelial to mesenchymal transition (EMT). Analysis of DiI+/SCC cells by real time PCR revealed a selective up-regulation of tell tale components of the Hedgehog/TGFβ pathways, as well as a down-regulation of EGFR, combined with a shift in crucial components implied in EMT. The presented findings offer an expanded mechanistic understanding that associates tumor initiating potential with cycling speed and EMT in pancreatic cancer cell lines.

title: Nonlinear Growth Kinetics of Breast Cancer Stem Cells: Implications for Cancer Stem Cell Targeted Therapy
abstract: Cancer stem cells (CSCs) have been identified in primary breast cancer tissues and cell lines. The CSC population varies widely among cancerous tissues and cell lines, and is often associated with aggressive breast cancers. Despite of intensive research, how the CSC population is regulated within a tumor is still not well understood so far. In this paper, we present a mathematical model to explore the growth kinetics of CSC population both in vitro and in vivo. Our mathematical models and supporting experiments suggest that there exist non-linear growth kinetics of CSCs and negative feedback mechanisms to control the balance between the population of CSCs and that of non-stem cancer cells. The model predictions can help us explain a few long-standing questions in the field of cancer stem cell research, and can be potentially used to predict the efficicacy of anti-cancer therapy.

title: Cancer stem cells: a step toward the cure.
abstract: This special issue of Journal of Clinical Oncology is devoted to the emerging field of cancer stem cells (CSCs). The goal of this article is to introduce general concepts of CSC biology to clinicians, and to provide a framework for their understanding the CSC-based approach to the development of novel diagnostics, therapeutics, and prevention strategies in oncology. This new approach differs from the traditional one because the latter is based on the long-held concept that tumor formation and growth are due to increased proliferation of cells in cancers compared with normal tissues. This traditional view is supported by molecular oncology research during the last 50 years, which showed that tumors undergo a series of genetic events (mutations) that result in the activation or overexpression of genes promoting proliferation (eg, oncogenes), the silencing of genes involved in inhibition of proliferation (eg, tumor suppressor genes), and the development of the ability of cancer cells to elude apoptosis. The consequence is the development and unchecked growth of tumors and their progression to metastases. Based on this traditional concept of tumor growth, it has been disappointing that therapies designed to kill proliferating cells often fail to cure cancer patients. Furthermore, during tumor development, tissues accumulate a series of mutations over years or even decades, but, because of tissue renewal, most cells in tissues are lost or eliminated in just a brief time, typically days or weeks, and with their loss, any mutations they have acquired would also be lost. Thus, new mechanisms need to be considered to explain human tumorigenesis. One explanation is that tumorigenic mutations occur in cells that are few in number but reside long term in tissues. But that is not sufficient to explain tumorigenesis. To be truly tumorigenic, this population of rare, mutated cells would have to self-renew, clonally expand, and acquire additional mutations. It is now widely believed that these long-lived, uncommon cells are tissue stem cells (SCs) or cells derived from them that acquire the ability to self-renew. When mutated, they can become CSCs. Both normal SCs and CSCs may account for only a small fraction of cells ( 1%) in any given tissue or tumor. By definition, SCs are self-renewing. Mounting evidence presented in the articles in this special issue suggests that CSCs initiate and drive tumor growth. This is nothing less than a paradigm shift. It is a shift to the view that, first, cancers originate in tissue stem or progenitor cells through dysregulation of the self-renewal process. Second, throughout tumorigenesis, CSCs drive tumor growth. Third, current chemotherapeutic agents and radiation therapy largely target proliferating and differentiated cells that form the bulk of the tumor but not the relatively quiescent CSCs. Also, SCs are drug resistant and CSCs appear to be so too. Thus, this quiescence and resistance may account for many treatment failures. Fourth, if this is the case, then the only effective way to treat cancer is to target the CSC population. Although this view may seem novel, the basic idea that SCs are the cells of clonal origin of malignancies has existed for at least several decades, if not longer, particularly for leukemias and teratomas. Early attempts—beginning in the 1970s—to directly measure SCs in tumors used several approaches. One was mousespleen colony assays (eg, injection of lymphoma cells into mice with the appearance of lymphoma colonies in the spleen). A second was the counting of metastatic lung colonies after intravenous inoculation of tumor cells into mice. A third was tumor colony formation in vitro. However, this latter method produced mixed results for human tumors. SCs have also been known for decades to radiation biologists. More recent evidence, as presented in the articles in this special issue, supports an SC origin for solid tumors as well. In the following paragraphs, we discuss the properties of normal SCs and CSCs, and their possible roles in carcinogenesis and in mediating tumor behavior.

title: Expansion of cancer stem cell pool initiates lung cancer recurrence before angiogenesis
abstract: Latent tumor cells are the crucial reason of tumor recurrence and the death of cancer patients. Preventing latent tumor relapse can prolong patients’ survival and have a long time surviving with latent tumor cells. Here, we describe a lung cancer suspensive tumor model in mouse and find that a high level of cancer stem cells undergoing asymmetric cell division in latent tumor is the key issue to reactivate a suspensive tumor. The results clearly delineate the state of latent tumor in vivo. A high level of serum IGF-1 can induce the suspensive-to-progressive tumor transition though promoting CSCs symmetric division, which illuminate a key checkpoint of cancer relapse before angiogenesis, highlighting a potential therapeutic target for preventing tumor recurrence. Angiogenesis is essential in the early stage of solid tumor recurrence, but how a suspensive tumor is reactivated before angiogenesis is mostly unknown. Herein, we stumble across an interesting phenomenon that s.c. xenografting human lung cancer tissues can awaken the s.c. suspensive tumor in nude mice. We further found that a high level of insulin-like growth factor 1 (IGF1) was mainly responsible for triggering the transition from suspensive tumor to progressive tumor in this model. The s.c. suspensive tumor is characterized with growth arrest, avascularity, and a steady-state level of proliferating and apoptotic cells. Intriguingly, CD133+ lung cancer stem cells (LCSCs) are highly enriched in suspensive tumor compared with progressive tumor. Mechanistically, high IGF1 initiates LCSCs self-renewal from asymmetry to symmetry via the activation of a PI3K/Akt/β-catenin axis. Next, the expansion of LCSC pool promotes angiogenesis by increasing the production of CXCL1 and PlGF in CD133+ LCSCs, which results in lung cancer recurrence. Clinically, a high level of serum IGF1 in lung cancer patients after orthotopic lung cancer resection as an unfavorable factor is strongly correlated with the high rate of recurrence and indicates an adverse progression-free survival. Vice versa, blocking IGF1 or CXCL1/PlGF with neutralizing antibodies can prevent the reactivation of a suspensive tumor induced by IGF1 stimulation in the mouse model. Collectively, the expansion of LCSC pool before angiogenesis induced by IGF1 is a key checkpoint during the initiation of cancer relapse, and targeting serum IGF1 may be a promising treatment for preventing recurrence in lung cancer patients.

title: The BMP Inhibitor Coco Reactivates Breast Cancer Cells at Lung Metastatic Sites
abstract: The mechanistic underpinnings of metastatic dormancy and reactivation are poorly understood. A gain-of-function cDNA screen reveals that Coco, a secreted antagonist of TGF-β ligands, induces dormant breast cancer cells to undergo reactivation in the lung. Mechanistic studies indicate that Coco exerts this effect by blocking lung-derived BMP ligands. Whereas Coco enhances the manifestation of traits associated with cancer stem cells, BMP signaling suppresses it. Coco induces a discrete gene expression signature, which is strongly associated with metastatic relapse to the lung, but not to the bone or brain in patients. Experiments in mouse models suggest that these latter organs contain niches devoid of bioactive BMP. These findings reveal that metastasis-initiating cells need to overcome organ-specific antimetastatic signals in order to undergo reactivation.

title: TGFβ2 dictates disseminated tumour cell fate in target organs through TGFβ-RIII and p38α/β signalling
abstract: In patients, non-proliferative disseminated tumour cells (DTCs) can persist in the bone marrow (BM) while other organs (such as lung) present growing metastasis. This suggested that the BM might be a metastasis ‘restrictive soil’ by encoding dormancy-inducing cues in DTCs. Here we show in a head and neck squamous cell carcinoma (HNSCC) model that strong and specific transforming growth factor-β2 (TGF-β2) signalling in the BM activates the MAPK p38α/β, inducing an (ERK/p38)low signalling ratio. This results in induction of DEC2/SHARP1 and p27, downregulation of cyclin-dependent kinase 4 (CDK4) and dormancy of malignant DTCs. TGF-β2-induced dormancy required TGF-β receptor-I (TGF-β-RI), TGF-β-RIII and SMAD1/5 activation to induce p27. In lungs, a metastasis ‘permissive soil’ with low TGF-β2 levels, DTC dormancy was short-lived and followed by metastatic growth. Importantly, systemic inhibition of TGF-β-RI or p38α/β activities awakened dormant DTCs, fuelling multi-organ metastasis. Our work reveals a ‘seed and soil’ mechanism where TGF-β2 and TGF-β-RIII signalling through p38α/β regulates DTC dormancy and defines restrictive (BM) and permissive (lung) microenvironments for HNSCC metastasis.

title: GAS6 Receptor Status Is Associated with Dormancy and Bone Metastatic Tumor Formation
abstract: Disseminated tumor cells (DTCs) are believed to lie dormant in the marrow before they can be activated to form metastases. How DTCs become dormant in the marrow and how dormant DTCs escape dormancy remains unclear. Recent work has shown that prostate cancer (PCa) cell lines express the growth-arrest specific 6 (GAS6) receptors Axl, Tyro3, and Mer, and become growth arrested in response to GAS6. We therefore hypothesized that GAS6 signaling regulates the proliferative activity of DTCs in the marrow. To explore this possibility, in vivo studies were performed where it was observed that when Tyro3 expression levels exceed Axl expression, the PCa cells exhibit rapid growth. When when Axl levels predominate, PCa cells remain largely quiescent. These findings suggest that a balance between the expression of Axl and Tyro3 is associated with a molecular switch between a dormant and a proliferative phenotype in PCa metastases.

title: Mechanism of early dissemination and metastasis in Her2+ mammary cancer
abstract: Metastasis is the leading cause of cancer-related deaths; metastatic lesions develop from disseminated cancer cells (DCCs) that can remain dormant. Metastasis-initiating cells are thought to originate from a subpopulation present in progressed, invasive tumours. However, DCCs detected in patients before the manifestation of breast-cancer metastasis contain fewer genetic abnormalities than primary tumours or than DCCs from patients with metastases. These findings, and those in pancreatic cancer and melanoma models, indicate that dissemination might occur during the early stages of tumour evolution. However, the mechanisms that might allow early disseminated cancer cells (eDCCs) to complete all steps of metastasis are unknown. Here we show that, in early lesions in mice and before any apparent primary tumour masses are detected, there is a sub-population of Her2+p-p38lop-Atf2loTwist1hiE-cadlo early cancer cells that is invasive and can spread to target organs. Intra-vital imaging and organoid studies of early lesions showed that Her2+ eDCC precursors invaded locally, intravasated and lodged in target organs. Her2+ eDCCs activated a Wnt-dependent epithelial–mesenchymal transition (EMT)-like dissemination program but without complete loss of the epithelial phenotype, which was reversed by Her2 or Wnt inhibition. Notably, although the majority of eDCCs were Twist1hiE-cadlo and dormant, they eventually initiated metastasis. Our work identifies a mechanism for early dissemination in which Her2 aberrantly activates a program similar to mammary ductal branching that generates eDCCs that are capable of forming metastasis after a dormancy phase.

title: Unresolved endoplasmic reticulum stress engenders immune-resistant, latent pancreatic cancer metastases
abstract: Chronic stress as a survival tactic Most patients with pancreatic ductal adenocarcinoma (PDA) develop liver metastases after surgical removal of their primary tumor. These metastases are thought to potentially arise from quiescent disseminated cancer cells, likely present at the time of surgery, which evade elimination by the immune system. Pommier et al. explored how these quiescent cells survive by analyzing mouse models and tissue samples from patients with PDA. They found that disseminated cancer cells do not express a cell surface molecule that triggers killing by T cells. This phenotypic feature is linked to their inability to resolve endoplasmic reticulum stress. When this stress is resolved, the disseminated cells begin proliferating and form metastases. Science, this issue p.  Chronic endoplasmic reticulum stress allows disseminated cancer cells that form metastases to evade immune control.  Pancreatic ductal adenocarcinoma (PDA) is the fourth most common cause of death from cancer worldwide and has a 5-year survival rate of 6%. Patients who have had their primary PDA surgically resected often develop metastatic disease, despite intra-operative examination of the liver confirming the absence of macrometastatic lesions. These observations lead to the conclusion that latent metastases, detectable only microscopically, were present in these patients and were responsible for the postoperative development of metastatic disease.  Latent metastases with the potential for outgrowth had been considered to be lesions in which cancer cell proliferation is balanced by immune-mediated cancer cell death, but a more recent explanation invokes quiescent, single disseminated cancer cells (DCCs). Single, nonreplicating DCCs have been observed in several cancer types, but whether quiescence is enforced by the microenvironment or is cancer cell–autonomous is not known. Immunity, both innate and adaptive, also is likely to have a role in the selection and/or maintenance of latent DCCs. This has long been suspected on the basis of the clinical observation of donor-derived cancer in immune-suppressed recipients of allografts. However, there is an unexplained paradox of immunity preventing the outgrowth of latent metastases but not eliminating them.  We studied the metastatic process in the context of an ongoing adaptive immune response because of the occurrence of cancer cell–specific immunity in human and mouse PDA. Livers from patients and mice with PDA contained single DCCs with an unusual phenotype of being negative for cytokeratin 19–negative (CK19) and major histocompatibility complex class I (MHCI). The absence of the expression of MHCI in DCCs and the occurrence of cancer-specific CD8+ T cells in the genetically engineered mouse model of PDA, and possibly in patients with PDA, suggested that DCCs may be selected by an anticancer immune response during the metastatic process. To investigate this hypothesis, we created a mouse model that would allow us to determine how DCCs develop, their relationship to metastatic latency, and the role of immunity. Intraportal injection of immunogenic PDA cells into preimmunized mice seeded livers only with single, nonreplicating DCCs lacking MHCI and CK19, whereas naïve recipients of PDA cells had macrometastases. We found that a subpopulation of PDA cells with the phenotype of DCCs was present in vitro and that those cells are the precursors of DCCs in vivo. We found that T cells select DCCs by eliminating MHCI+ proliferating cancer cells. To identify the cell-autonomous “switch” regulating the developmental state of the metastases, we preformed single-cell RNA sequencing of PDA cells with the DCC phenotype. This transcriptomic analysis demonstrated an endoplasmic reticulum (ER) stress response. Moreover, DCCs showed a lack of activation of the IRE1α (inositol-requiring enzyme 1α) pathway of the unfolded protein response, whereas the PERK (protein kinase RNA-like ER kinase) pathway was activated, suggesting that DCCs cannot resolve ER stress. Relieving ER stress pharmacologically with a chemical chaperone or genetically by overexpression of spliced XBP1, in combination with T cell depletion, stimulated outgrowth of macrometastatic lesions containing PDA cells expressing MHCI and CK19.  We find that a PDA-specific adaptive immune response selects DCCs, in which the ER stress response accounts for both quiescence and resistance to immune elimination. Accordingly, outgrowth of DCCs to macrometastases requires not only relief from the cancer cell–autonomous ER stress response, but also suppression of systemic immunity. Thus, the ER stress response is a cell-autonomous reaction that enables DCCs to escape immunity and establish latent metastases. Unresolved ER stress allows disseminated cancer cells to escape the T cell response. Quiescent cancer cells that exhibit unresolved ER stress lack expression of MHCI. They avoid killing by T cells and become latent disseminated cancer cells (DCCs). Resolution of ER stress allows DCCs to regain proliferative capacities and grow as overt metastases, only if the T cell response is disrupted, because they also regain the expression of MHCI. Ecad, E-cadherin. The majority of patients with pancreatic ductal adenocarcinoma (PDA) develop metastatic disease after resection of their primary tumor. We found that livers from patients and mice with PDA harbor single disseminated cancer cells (DCCs) lacking expression of cytokeratin 19 (CK19) and major histocompatibility complex class I (MHCI). We created a mouse model to determine how these DCCs develop. Intraportal injection of immunogenic PDA cells into preimmunized mice seeded livers only with single, nonreplicating DCCs that were CK19– and MHCI–. The DCCs exhibited an endoplasmic reticulum (ER) stress response but paradoxically lacked both inositol-requiring enzyme 1α activation and expression of the spliced form of transcription factor XBP1 (XBP1s). Inducible expression of XBP1s in DCCs, in combination with T cell depletion, stimulated the outgrowth of macrometastatic lesions that expressed CK19 and MHCI. Thus, unresolved ER stress enables DCCs to escape immunity and establish latent metastases.

title: Inflammation Triggers Zeb1-Dependent Escape from Tumor Latency.
abstract: The emergence of metastatic disease in cancer patients many years or decades after initial successful treatment of primary tumors is well documented but poorly understood at the molecular level. Recent studies have begun exploring the cell-intrinsic programs, causing disseminated tumor cells to enter latency and the cellular signals in the surrounding nonpermissive tissue microenvironment that maintain the latent state. However, relatively little is known about the mechanisms that enable disseminated tumor cells to escape cancer dormancy or tumor latency. We describe here an in vivo model of solitary metastatic latency in the lung parenchyma. The induction of a localized inflammation in the lungs, initiated by lipopolysaccharide treatment, triggers the awakening of these cells, which develop into macroscopic metastases. The escape from latency is dependent on the expression of Zeb1, a key regulator of the epithelial-to-mesenchymal transition (EMT). Furthermore, activation of the EMT program on its own, as orchestrated by Zeb1, is sufficient to incite metastatic outgrowth by causing carcinoma cells to enter stably into a metastasis-initiating cell state. Cancer Res; 76(23); 6778-84.

title: ERK1/2 and p38α/β Signaling in Tumor Cell Quiescence: Opportunities to Control Dormant Residual Disease
abstract: Systemic minimal residual disease after primary tumor treatment can remain asymptomatic for decades. This is thought to be due to the presence of dormant disseminated tumor cells (DTC) or micrometastases in different organs. DTCs lodged in brain, lungs, livers, and/or bone are a major clinical problem because they are the founders of metastasis, which ultimately kill cancer patients. The problem is further aggravated by our lack of understanding of DTC biology. In consequence, there are almost no rational therapies to prevent dormant DTCs from surviving and expanding. Several cancers, including melanoma as well as breast, prostate, and colorectal carcinomas, undergo dormant periods before metastatic recurrences develop. Here we review our experience in studying the cross-talk between ERK1/2 and p38α/β signaling in models of early cancer progression, dissemination, and DTC dormancy. We also provide some potential translational and clinical applications of these findings and describe how some currently used therapies might be useful to control dormant disease. Finally, we draw caution on the use of p38 inhibitors currently in clinical trials for different diseases as these may accelerate metastasis development.

title: Cancer cell dormancy: mechanisms and implications of cancer recurrence and metastasis
abstract: More recently, disease metastasis and relapse in many cancer patients several years (even some decades) after surgical remission are regarded as tumor dormancy. However, the knowledge of this phenomenon is cripplingly limited. Substantial quantities of reviews have summarized three main potential models that can be put forth to explain such process, including angiogenic dormancy, immunologic dormancy, and cellular dormancy. In this review, newly uncovered mechanisms governing cancer cell dormancy are discussed, with an emphasis on the cross talk between dormant cancer cells and their microenvironments. In addition, potential mechanisms of reactivation of these dormant cells in certain anatomic sites including lymph nodes and bone marrow are discussed. Molecular mechanism of cellular dormancy in head and neck cancer is also involved.

title: ATF6α-Rheb-mTOR signaling promotes survival of dormant tumor cells in vivo
abstract: The pathways that allow quiescent disseminated cancer cells to survive during prolonged dormancy periods are unknown. Here, we identify the transcription factor ATF6α as a pivotal survival factor for quiescent but not proliferative squamous carcinoma cells. ATF6α is essential for the adaptation of dormant cells to chemotherapy, nutritional stress, and, most importantly, the in vivo microenvironment. Mechanism analysis showed that MKK6 and p38α/β contribute to regulating nuclear translocation and transcriptional activation of ATF6α in dormant cancer cells. Downstream, ATF6α induces survival through the up-regulation of Rheb and activation of mTOR signaling independent of Akt. Down-regulation of ATF6α or Rheb reverted dormant tumor cell resistance to rapamycin and induced pronounced killing only of dormant cancer cells in vivo. Knocking down ATF6α also prolonged the survival of nude mice bearing dormant tumor cells. Targeting survival signaling by the ATF6α-Rheb-mTOR pathway in dormant tumor cells may favor the eradication of residual disease during dormancy periods.

title: Erk signaling suppresses embryonic stem cell self-renewal to specify endoderm.
abstract: Fgf signaling via Erk activation has been associated with both neural induction and the generation of a primed state for the differentiation of embryonic stem cells (ESCs) to all somatic lineages. To dissect the role of Erk in both ESC self-renewal and lineage specification, we explored the requirements for this pathway in various in vitro differentiation settings. A combination of pharmacological inhibition of Erk signaling and genetic loss of function reveal a role for Erk signaling in endodermal, but not neural differentiation. Neural differentiation occurs normally despite a complete block to Erk phosphorylation. In support of this, Erk activation in ESCs derepresses primitive endoderm (PrE) gene expression as a consequence of inhibiting the pluripotent/epiblast network. The early response to Erk activation correlates with functional PrE priming, whereas sustained Erk activity results in PrE differentiation. Taken together, our results suggest that Erk signaling suppresses pluripotent gene expression to enable endodermal differentiation.

title: SOX2 plays a critical role in EGFR-mediated self-renewal of human prostate cancer stem-like cells.
abstract: SOX2 is an essential transcription factor for stem cells and plays a role in tumorigenesis, however its role in prostate cancer stem cells (PCSCs) remains unclear. We report here a significant upregulation of SOX2 at both mRNA and protein levels in DU145 PCSCs propagated as suspension spheres in vitro. The expression of SOX2 in DU145 PCSCs is positively regulated by epidermal growth factor receptor (EGFR) signaling. Activation of EGFR signaling, following the addition of epidermal growth factor (EGF) or ectopic expression of a constitutively-active EGFR mutant (EGFRvIII), increased SOX2 expression and the self-renewal of DU145 PCSCs. Conversely, a small molecule EGFR inhibitor (AG1478) blocked EGFR activation, reduced SOX2 expression and inhibited PCSC self-renewal activity, implicating SOX2 in mediating EGFR-dependent self-renewal of PCSCs. In line with this notion, ectopic SOX2 expression enhanced EGF-induced self-renewal of DU145 PCSCs, while SOX2 knockdown reduced PCSC self-renewal with EGF treatment no longer capable of enhancing their propagation. Furthermore, SOX2 knockdown reduced the capacity of DU145 PCSCs to grow under anchorage-independent conditions. Finally, DU145 PCSCs generated xenograft tumors more aggressively with elevated levels of SOX2 expression compared to xenograft tumors derived from non-PCSCs. Collectively, we provide evidence that SOX2 plays a critical role in EGFR-mediated self-renewal of DU145 PCSCs.

title: Role of oncogenic K-Ras in cancer stem cell activation by aberrant Wnt/β-catenin signaling.
abstract: Adenomatous polyposis coli (APC) loss-of-function mutations and K-Ras gain-of-function mutations are common abnormalities that occur during the initiation and intermediate adenoma stages of colorectal tumorigenesis, respectively. However, little is known about the role these mutations play in cancer stem cells (CSCs) associated with colorectal cancer (CRC) tumorigenesis.



We analyzed tissue from CRC patients (n = 49) to determine whether K-Ras mutations contributed to CSC activation during colorectal tumorigenesis. DLD-1-K-Ras-WT and DLD-1-K-Ras-MT cells were cultured and evaluated for their ability to differentiate, form spheroids in vitro, and form tumors in vivo. Interaction between APC and K-Ras mutations in colorectal tumorigenesis was evaluated using APC (Min/+)/K-Ras (LA2) mice and DLD-1-K-Ras-WT and DLD-1-K-Ras-MT cell xenografts. (n = 4) Group differences were determined by Student t test. All statistical tests were two-sided.



The sphere-forming capability of DLD-1-K-Ras-MT cells was statistically significantly higher than that of DLD-1-K-Ras-WT cells (DLD-1-K-Ras-MT mean = 86.661 pixel, 95% confidence interval [CI] = 81.701 to 91.621 pixel; DLD-1-K-Ras-WT mean = 42.367 pixel, 95% CI = 36.467 to 48.267 pixel; P = .003). Moreover, both the size and weight of tumors from DLD-1-K-Ras-MT xenografts were markedly increased compared with tumors from DLD-1-K-Ras-WT cells. Expression of the CSC markers CD44, CD133, and CD166 was induced in intestinal tumors from APC (Min/+)/K-Ras (LA2)mice, but not K-Ras (LA2) mice, indicating that APC mutation is required for CSC activation by oncogenic K-Ras mutation.



K-Ras mutation activates CSCs, contributing to colorectal tumorigenesis and metastasis in CRC cells harboring APC mutations. Initial activation of β-catenin by APC loss and further enhancement through K-Ras mutation induces CD44, CD133, and CD166 expression.

title: MAPK pathway control of stem cell proliferation and differentiation in the embryonic pituitary provides insights into the pathogenesis of papillary craniopharyngioma
abstract: Despite the importance of the RAS-RAF-MAPK pathway in normal physiology and disease of numerous organs, its role during pituitary development and tumourigenesis remains largely unknown. Here, we show that the over-activation of the MAPK pathway, through conditional expression of the gain-of-function alleles BrafV600E and KrasG12D in the developing mouse pituitary, results in severe hyperplasia and abnormal morphogenesis of the gland by the end of gestation. Cell-lineage commitment and terminal differentiation are disrupted, leading to a significant reduction in numbers of most of the hormone-producing cells before birth, with the exception of corticotrophs. Of note, Sox2+ stem cells and clonogenic potential are drastically increased in the mutant pituitaries. Finally, we reveal that papillary craniopharyngioma (PCP), a benign human pituitary tumour harbouring BRAF p.V600E also contains Sox2+ cells with sustained proliferative capacity and disrupted pituitary differentiation. Together, our data demonstrate a crucial function of the MAPK pathway in controlling the balance between proliferation and differentiation of Sox2+ cells and suggest that persistent proliferative capacity of Sox2+ cells may underlie the pathogenesis of PCP.  Constitutive activation of the MAPK/ERK pathway causes pituitary hyperplasia, abnormal morphogenesis, and abnormal endocrine cell specification due to the sustained proliferation of the Sox2+ stem cell compartment.

title: Pivotal role for ROS activation of p38 MAPK in the control of differentiation and tumor-initiating capacity of glioma-initiating cells.
abstract: Reactive oxygen species (ROS) are involved in various aspects of cancer cell biology, yet their role in cancer stem cells (CSCs) has been poorly understood. In particular, it still remains unclear whether and how ROS control the self-renewal/differentiation process and the tumor-initiating capacity of CSCs. Here we show that ROS-mediated activation of p38 MAPK plays a pivotal role in the control of differentiation and tumor-initiating capacity of glioma-initiating cells (GICs) derived from human glioblastomas. Mechanistically, ROS triggered p38-dependent Bmi1 protein degradation and FoxO3 activation in GICs, which were shown to be responsible for the loss of their self-renewal capacity and differentiation, respectively. Thus, the results suggest that Bmi1 and FoxO3 govern distinct phases of transition from undifferentiated to fully differentiated cells. Furthermore, we also demonstrate in this study that oxidative stress deprives GICs of their tumor-initiating capacity through the activation of the ROS-p38 axis. As such, this is the first study to the best of our knowledge to delineate how ROS control self-renewal/differentiation and the tumor-initiating capacity of stem-like cancer cells. This study also suggests that targeting of the ROS-p38 axis could be a novel approach in the development of therapeutic strategies against gliomas, represented by glioblastoma.

title: miR-17-92/p38α Dysregulation Enhances Wnt Signaling and Selects Lgr6+ Cancer Stem-like Cells during Lung Adenocarcinoma Progression.
abstract: Defining the molecular and cellular roots of lung cancer relapse after initial treatment remains an imperative to improve survival. Here we report that the lung stem cell marker Lgr6 becomes enriched in non-small cell lung cancer (NSCLC) cells during malignant progression. Lgr6(+) NSCLC cells displayed self-renewal and differentiation properties along with a higher tumorigenic potential. Mechanistic investigations suggested that a defective repression of the miR-17-92 gene cluster was responsible for evolution of a selection for outgrowth of Lgr6(+) NSCLC cells. High levels of expression of miR-19 family members were found to target and downregulate levels of p38α kinase, providing a specific survival signal for Lgr6(+) cells as mediated by increased Wnt/ß-catenin activity. Our results identify a specific stem-like cell population in NSCLC with increased malignant potential, the elucidation of which may enable earlier prognosis and possibly the development of more effective targeted treatments. ; 4012-22.

title: The p38 signaling pathway mediates quiescence of glioma stem cells by regulating epidermal growth factor receptor trafficking
abstract: EGFR pathway is upregulated in malignant gliomas, and its downstream signaling is important for self-renewal of glioma cancer stem-like cells (GSC). p38 mitogen-activated protein kinase (MAPK) signaling, a stress-activated signaling cascade with suppressive and permissive effects on tumorigenesis, can promote internalization and ubiquitin ligase mediated degradation of EGFR. In this study, we investigated the role of p38 MAPK signaling on the self-renewal of GSCs with the hypothesis that inhibition may lead to enhanced self-renewal capacity by retention of EGFR. Inhibition of p38 MAPK pathway led to increase in EGFR expression but surprisingly, reduced proliferation. Additional functional evaluation revealed that p38 inhibition was associated with decrease in cell death and maintenance of undifferentiated state. Further probing the effect of p38 inhibition demonstrated attenuation of EGFR downstream signaling activity in spite of prolonged surface expression of the receptor. In vitro observations were confirmed in xenograft in vivo experiments. These data suggest that p38 MAPK control of EGFR signaling activity may alter GSC cell cycle state by regulating quiescence and passage into transit amplifying state.

title: Reciprocal Regulation of DUSP9 and DUSP16 Expression by HIF1 Controls ERK and p38 MAP Kinase Activity and Mediates Chemotherapy-Induced Breast Cancer Stem Cell Enrichment.
abstract: Triple-negative breast cancer (TNBC) has a poor prognosis due to its aggressive characteristics and lack of targeted therapies. Cytotoxic chemotherapy may reduce tumor bulk, but leaves residual disease due to the persistence of chemotherapy-resistant breast cancer stem cells (BCSC), which are critical for tumor recurrence and metastasis. Here, we demonstrate that hypoxia-inducible factor (HIF)-1-dependent regulation of mitogen-activated protein kinase (MAPK) signaling pathways contributes to chemotherapy-induced BCSC enrichment. Chemotherapy increased DUSP9 expression and decreased DUSP16 expression in a HIF1-dependent manner, leading to inhibition of ERK and activation of p38 signaling pathways, respectively. Inhibition of ERK caused transcriptional induction of the pluripotency factor Nanog through decreased inactivating phosphorylation of FoxO3, while activation of p38 stabilized Nanog and Klf4 mRNA through increased inactivating phosphorylation of RNA-binding protein ZFP36L1, both of which promoted specification of the BCSC phenotype. Inhibition of HIF1 or p38 signaling blocked chemotherapy-induced pluripotency factor expression and BCSC enrichment. These surprising results delineate a mechanism by which a transcription factor switches cells from ERK to p38 signaling in response to chemotherapy and suggest that therapeutic targeting of HIF1 or the p38 pathway in combination with chemotherapy will block BCSC enrichment and improve outcome in TNBC.Significance: These findings provide a molecular mechanism that may account for the increased relapse rate of women with TNBC who are treated with cytotoxic chemotherapy and suggest that combining chemotherapy with an inhibitor of HIF1 or p38 activity may increase patient survival.  4191-202.

title: A negative feedback signaling network underlies oncogene-induced senescence.
abstract: Oncogene-induced senescence functions to limit tumor development. However, a complete understanding of the signals that trigger this type of senescence is currently lacking. We found that mutations affecting NF1, Raf, and Ras induce a global negative feedback response that potently suppresses Ras and/or its effectors. Moreover, these signals promote senescence by inhibiting the Ras/PI3K pathway, which can impact the senescence machinery through HDM2 and FOXO. This negative feedback program is regulated in part by RasGEFs, Sprouty proteins, RasGAPs, and MKPs. Moreover, these signals function in vivo in benign human tumors. Thus, the ultimate response to the aberrant activation of the Ras pathway is a multifaceted negative feedback signaling network that terminates the oncogenic signal and participates in the senescence response.

title: Emerging roles of the p38 MAPK and PI3K/AKT/mTOR pathways in oncogene-induced senescence.
abstract: Oncogene-induced senescence (OIS) is a tumor-suppressing response that must be disrupted for cancer to develop. Mechanistic insights into OIS have begun to emerge. Activation of the p53/p21(WAF1) and/or p16(INK4A) tumor-suppressor pathways is essential for OIS. Moreover, the DNA damage response, chromatin remodeling, and senescence-associated secretory phenotype (SASP) are important for the initiation and maintenance of OIS. This review discusses recent advances in elucidating the mechanisms of OIS, focusing on the roles of the p38 mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K)/cellular homolog of murine thymoma virus AKT/mammalian target of rapamycin (mTOR) pathways. These studies indicate that OIS is mediated by an intricate signaling network. Further delineation of this network may lead to development of new cancer therapies targeting OIS.

title: TET2 controls chemoresistant slow-cycling cancer cell survival and tumor recurrence
abstract: Dormant or slow-cycling tumor cells can form a residual chemoresistant reservoir responsible for relapse in patients, years after curative surgery and adjuvant therapy. We have adapted the pulse-chase expression of H2BeGFP for labeling and isolating slow-cycling cancer cells (SCCCs). SCCCs showed cancer initiation potential and enhanced chemoresistance. Cells at this slow-cycling status presented a distinctive nongenetic and cell-autonomous gene expression profile shared across different tumor types. We identified TET2 epigenetic enzyme as a key factor controlling SCCC numbers, survival, and tumor recurrence. 5-Hydroxymethylcytosine (5hmC), generated by TET2 enzymatic activity, labeled the SCCC genome in carcinomas and was a predictive biomarker of relapse and survival in cancer patients. We have shown the enhanced chemoresistance of SCCCs and revealed 5hmC as a biomarker for their clinical identification and TET2 as a potential drug target for SCCC elimination that could extend patients’ survival.

title: Secreted Protein Acidic and Rich in Cysteine (SPARC) Mediates Metastatic Dormancy of Prostate Cancer in Bone *
abstract: Prostate cancer is known to frequently recur in bone; however, how dormant cells switch its phenotype leading to recurrent tumor remains poorly understood. We have isolated two syngeneic cell lines (indolent and aggressive) through in vivo selection by implanting PC3mm stem-like cells into tibial bones. We found that indolent cells retained the dormant phenotype, whereas aggressive cells grew rapidly in bone in vivo, and the growth rates of both cells in culture were similar, suggesting a role of the tumor microenvironment in the regulation of dormancy and recurrence. Indolent cells were found to secrete a high level of secreted protein acidic and rich in cysteine (SPARC), which significantly stimulated the expression of BMP7 in bone marrow stromal cells. The secreted BMP7 then kept cancer cells in a dormant state by inducing senescence, reducing “stemness,” and activating dormancy-associated p38 MAPK signaling and p21 expression in cancer cells. Importantly, we found that SPARC was epigenetically silenced in aggressive cells by promoter methylation, but 5-azacytidine treatment reactivated the expression. Furthermore, high SPARC promoter methylation negatively correlated with disease-free survival of prostate cancer patients. We also found that the COX2 inhibitor NS398 down-regulated DNMTs and increased expression of SPARC, which led to tumor growth suppression in bone in vivo. These findings suggest that SPARC plays a key role in maintaining the dormancy of prostate cancer cells in the bone microenvironment.

title: Cancer cells enter dormancy after cannibalizing mesenchymal stem/stromal cells (MSCs)
abstract: In many patients with cancer, some tumor cells tolerate conventional treatments and persist for years in an undetectable/dormant state, after which these same cells can mysteriously resume their growth and seed, almost invariably fatal, recurrent cancerous lesions. The therapeutic challenges of tumor dormancy and need to decode the underlying mechanisms involved are apparent. Here, we revealed that mesenchymal stem/stromal cells (MSCs), recognized determinants of breast cancer cell (BCC) behavior, were readily cannibalized by the BCCs they mingled with in 3D cocultures, a process that distinctly altered cancer cell phenotype, suppressed tumor formation, and supported tumor dormancy. Our discoveries provide original insight into the interactions between MSCs and cancer cells, with the potential to identify novel molecular targets for cancer therapy. Patients with breast cancer often develop malignant regrowth of residual drug-resistant dormant tumor cells years after primary treatment, a process defined as cancer relapse. Deciphering the causal basis of tumor dormancy therefore has obvious therapeutic significance. Because cancer cell behavior is strongly influenced by stromal cells, particularly the mesenchymal stem/stromal cells (MSCs) that are actively recruited into tumor-associated stroma, we assessed the impact of MSCs on breast cancer cell (BCC) dormancy. Using 3D cocultures to mimic the cellular interactions of an emerging tumor niche, we observed that MSCs sequentially surrounded the BCCs, promoted formation of cancer spheroids, and then were internalized/degraded through a process resembling the well-documented yet ill-defined clinical phenomenon of cancer cell cannibalism. This suspected feeding behavior was less appreciable in the presence of a rho kinase inhibitor and in 2D monolayer cocultures. Notably, cannibalism of MSCs enhanced survival of BCCs deprived of nutrients but suppressed their tumorigenicity, together suggesting the cancer cells entered dormancy. Transcriptome profiles revealed that the resulting BCCs acquired a unique molecular signature enriched in prosurvival factors and tumor suppressors, as well as inflammatory mediators that demarcate the secretome of senescent cells, also referred to as the senescence-associated secretory phenotype. Overall, our results provide intriguing evidence that cancer cells under duress enter dormancy after cannibalizing MSCs. Importantly, our practical 3D coculture model could provide a valuable tool to understand the antitumor activity of MSCs and cell cannibalism further, and therefore open new therapeutic avenues for the prevention of cancer recurrence.

title: Transforming growth factor beta triggers two independent-senescence programs in cancer cells.
abstract: Transforming growth factor-beta (TGF-beta)TG has been shown to play a multifunctional role in tumorigenesis. Here we demonstrate that TGF-beta induces a morphological change and expression of senescence-associated beta-galactosidase activity in the human lung adenocarcinoma cell line A549 cells within a week after the addition. These TGF-beta induced phenotypic changes are thought to characterize the rapid onset of senescence. When A549 cells were treated with TGF-beta, cell growth was not completely arrested, but the activity of telomerase was down regulated via transcriptional repression of telomerase reverse transcriptase, which led to a shortening of the telomere during long-term culture and finally resulted in replicative senescence. These results indicate that TGF-beta is able to induce a rapid senescence in A549 cells without significantly inhibiting cell growth and can further direct A549 cells to a replicative senescence state via the suppression of telomerase which culminates in telomere shortening. All these experimental results suggest that TGF-beta transmits several separate and independent signals to shift A549 cells back to a normal senescent cell.

title: Integrin Beta 3 Regulates Cellular Senescence by Activating the TGF-β Pathway
abstract: Cellular senescence is an important in vivo mechanism that prevents the propagation of damaged cells. However, the precise mechanisms regulating senescence are not well characterized. Here, we find that ITGB3 (integrin beta 3 or β3) is regulated by the Polycomb protein CBX7. β3 expression accelerates the onset of senescence in human primary fibroblasts by activating the transforming growth factor β (TGF-β) pathway in a cell-autonomous and non-cell-autonomous manner. β3 levels are dynamically increased during oncogene-induced senescence (OIS) through CBX7 Polycomb regulation, and downregulation of β3 levels overrides OIS and therapy-induced senescence (TIS), independently of its ligand-binding activity. Moreover, cilengitide, an αvβ3 antagonist, has the ability to block the senescence-associated secretory phenotype (SASP) without affecting proliferation. Finally, we show an increase in β3 levels in a subset of tissues during aging. Altogether, our data show that integrin β3 subunit is a marker and regulator of senescence.

title: Transforming growth factor‐beta induces senescence in hepatocellular carcinoma cells and inhibits tumor growth
abstract: Senescence induction could be used as an effective treatment for hepatocellular carcinoma (HCC). However, major senescence inducers (p53 and p16Ink4a) are frequently inactivated in these cancers. We tested whether transforming growth factor‐β (TGF‐β) could serve as a potential senescence inducer in HCC. First, we screened for HCC cell lines with intact TGF‐β signaling that leads to small mothers against decapentaplegic (Smad)‐targeted gene activation. Five cell lines met this condition, and all of them displayed a strong senescence response to TGF‐β1 (1‐5 ng/mL) treatment. Upon treatment, c‐myc was down‐regulated, p21Cip1 and p15Ink4b were up‐regulated, and cells were arrested at G1. The expression of p16Ink4a was not induced, and the senescence response was independent of p53 status. A short exposure of less than 1 minute was sufficient for a robust senescence response. Forced expression of p21Cip1 and p15Ink4b recapitulated TGF‐β1 effects. Senescence response was associated with reduced nicotinamide adenine dinucleotide phosphate oxidase 4 (Nox4) induction and intracellular reactive oxygen species (ROS) accumulation. The treatment of cells with the ROS scavenger N‐acetyl‐L‐cysteine, or silencing of the NOX4 gene, rescued p21Cip1 and p15Ink4b accumulation as well as the growth arrest in response to TGF‐β. Human HCC tumors raised in immunodeficient mice also displayed TGF‐β1–induced senescence. More importantly, peritumoral injection of TGF‐β1 (2 ng) at 4‐day intervals reduced tumor growth by more than 75%. In contrast, the deletion of TGF‐β receptor 2 abolished in vitro senescence response and greatly accelerated in vivo tumor growth.  TGF‐β induces p53‐independent and p16Ink4a‐independent, but Nox4‐dependent, p21Cip1‐dependent, p15Ink4b‐dependent, and ROS‐dependent senescence arrest in well‐differentiated HCC cells. Moreover, TGF‐β–induced senescence in vivo is associated with a strong antitumor response against HCC. HEPATOLOGY 2010

title: TGFβ2-induced senescence during early inner ear development
abstract: Embryonic development requires the coordinated regulation of apoptosis, survival, autophagy, proliferation and differentiation programs. Senescence has recently joined the cellular processes required to master development, in addition to its well-described roles in cancer and ageing. Here, we show that senescent cells are present in a highly regulated temporal pattern in the developing vertebrate inner ear, first, surrounding the otic pore and, later, in the otocyst at the endolymphatic duct. Cellular senescence is associated with areas of increased apoptosis and reduced proliferation consistent with the induction of the process when the endolymphatic duct is being formed. Modulation of senescence disrupts otic vesicle morphology. Transforming growth factor beta (TGFβ) signaling interacts with signaling pathways elicited by insulin-like growth factor type 1 (IGF-1) to jointly coordinate cellular dynamics required for morphogenesis and differentiation. Taken together, these results show that senescence is a natural occurring process essential for early inner ear development.

title: Axl is required for TGF-β2-induced dormancy of prostate cancer cells in the bone marrow
abstract: Disseminated prostate cancer (PCa) cells in the marrow survive for years without evidence of proliferation, while maintaining the capacity to develop into metastatic lesions. These dormant disseminated tumor cells (DTCs) may reside in close proximity to osteoblasts, while expressing high levels of Axl, one of the tyrosine kinase receptors for growth arrest specific 6 (Gas6). Yet how Axl regulates DTC proliferation in marrow remains undefined. Here, we explored the impact of the loss of Axl in PCa cells (PC3 and DU145) on the induction of their dormancy when they are co-cultured with a pre-osteoblastic cell line, MC3T3-E1. MC3T3-E1 cells dramatically decrease the proliferation of PCa cells, however this suppressive effect of osteoblasts is significantly reduced by the reduction of Axl expression in PCa cells. Interestingly, expression of both TGF-β and its receptors were regulated by Axl expression in PCa cells, while specific blockade of TGF-β signaling limited the ability of the osteoblasts to induce dormancy of PCa cells. Finally, we found that both Gas6 and Axl are required for TGF-β2-mediated cell growth suppression. Taken together, these data suggest that a loop between the Gas6/Axl axis and TGF-β2 signaling plays a significant role in the induction of PCa cell dormancy.

title: The Relationship Between Dormant Cancer Cells and Their Microenvironment.
abstract: The majority of cancer deaths are due to metastases that can occur years or decades after primary tumor diagnosis and treatment. Disseminated tumor cells (DTCs) surviving in a dormant state in target organs appear to explain the timing of this phenomenon. Knowledge on this process is important as it might provide a window of opportunity to prevent recurrences by eradicating dormant DTCs and/or by maintaining DTCs in a dormant state. Importantly, this research might offer markers of dormancy for early monitoring of metastatic relapse. However, our understanding of the mechanisms underlying the regulation of entry into and exit from dormancy is still limited and crippling any therapeutic opportunity. While cancer cell-intrinsic signaling pathways have been linked to dormancy regulation, it is likely that these pathways and the switch controlling reactivation from dormancy are regulated by microenvironmental cues. Here we review and discuss recent findings on how the microenvironment regulates cancer dormancy and raise new questions that may help advance the field.

title: SMAD signaling and redox imbalance cooperate to induce prostate cancer cell dormancy
abstract: Metastasis involves the dissemination of single or small clumps of cancer cells through blood or lymphatic vessels and their extravasation into distant organs. Despite the strong regulation of metastases development by a cell dormancy phenomenon, the dormant state of cancer cells remains poorly characterized due to the difficulty of in vivo studies. We have recently shown in vitro that clonogenicity of prostate cancer cells is regulated by a dormancy phenomenon that is strongly induced when cells are cultured both at low cell density and in a slightly hypertonic medium. Here, we characterized by RT-qPCR a genetic expression signature of this dormant state which combines the presence of both stemness and differentiation markers. We showed that both TFGβ/BMP signaling and redox imbalance are required for the full induction of this dormancy signature and cell quiescence. Moreover, reconstruction experiments showed that TFGβ/BMP signaling and redox imbalance are sufficient to generate a pattern of genetic expression displaying all characteristic features of the dormancy signature. Finally, we observed that low cell density was sufficient to activate TGFβ/BMP signaling and to generate a slight redox imbalance thus priming cells for dormancy that can be attained with a co-stimulus like hypertonicity, most likely through an increased redox imbalance. The identification of a dual regulation of dormancy provides a framework for the interpretation of previous reports showing a restricted ability of BMP signaling to regulate cancer cell dormancy in vivo and draws attention on the role of oxidative stress in the metastatic process.

title: TGF-β-Induced Quiescence Mediates Chemoresistance of Tumor-Propagating Cells in Squamous Cell Carcinoma.
abstract: Squamous cell carcinomas (SCCs) are heterogeneous tumors sustained by tumor-propagating cancer cells (TPCs). SCCs frequently resist chemotherapy through still unknown mechanisms. Here, we combine H2B-GFP-based pulse-chasing with cell-surface markers to distinguish quiescent from proliferative TPCs within SCCs. We find that quiescent TPCs resist DNA damage and exhibit increased tumorigenic potential in response to chemotherapy, whereas proliferative TPCs undergo apoptosis. Quiescence is regulated by TGF-β/SMAD signaling, which directly regulates cell-cycle gene transcription to control a reversible G1 cell-cycle arrest, independent of p21CIP function. Indeed, genetic or pharmacological TGF-β inhibition increases the susceptibility of TPCs to chemotherapy because it prevents entry into a quiescent state. These findings provide direct evidence that TPCs can reversibly enter a quiescent, chemoresistant state and thereby underscore the need for combinatorial approaches to improve treatment of chemotherapy-resistant SCCs.

title: Transforming growth factor beta-induced cell cycle arrest of human hematopoietic cells requires p57KIP2 up-regulation.
abstract: Transforming growth factor beta (TGFbeta) is one of few known negative regulators of hematopoiesis, yet the mechanisms by which it affects cell cycle arrest and stem cell quiescence are poorly understood. Induction of the cyclin-dependent kinase inhibitors, p15INK4b (p15) and p21WAF1 (p21) is important for TGFbeta-mediated cytostasis in epithelial cells but not in hematopoietic cells. Using primary human hematopoietic cells and microarray analysis, we identified p57KIP2 (p57) as the only cyclin-dependent kinase inhibitor induced by TGFbeta. Up-regulation of p57 mRNA and protein occurs before TGFbeta-induced G1 cell cycle arrest, requires transcription, and is mediated via a highly conserved region of the proximal p57 promoter. The up-regulation of p57 is essential for TGFbeta-induced cell cycle arrest in these cells, because two different small interfering RNAs that prevent p57 up-regulation block the cytostatic effects of TGFbeta on human hematopoietic cells. Reduction of basal p57 expression by this approach also allows hematopoietic cells to proliferate more readily in the absence of TGFbeta. p57 is a putative tumor suppressor gene whose expression is frequently silenced by promoter hypermethylation in hematologic malignancies. Our studies identify a molecular pathway by which TGFbeta mediates its cytostatic effects on human hematopoietic cells and suggests an explanation for the frequent silencing of p57 expression.

title: TGF-beta receptor mediated telomerase inhibition, telomere shortening and breast cancer cell senescence
abstract: ABSTRACTHuman telomerase reverse transcriptase (hTERT) plays a central role in telomere lengthening for continuous cell proliferation, but it remains unclear how extracellular cues regulate telomerase lengthening of telomeres. Here we report that the cytokine bone morphogenetic protein-7 (BMP7) induces the hTERT gene repression in a BMPRII receptor- and Smad3-dependent manner in human breast cancer cells. Chonic exposure of human breast cancer cells to BMP7 results in short telomeres, cell senescence and apoptosis. Mutation of the BMPRII receptor, but not TGFbRII, ACTRIIA or ACTRIIB receptor, inhibits BMP7-induced repression of the hTERT gene promoter activity, leading to increased telomerase activity, lengthened telomeres and continued cell proliferation. Expression of hTERT prevents BMP7-induced breast cancer cell senescence and apoptosis. Thus, our data suggest that BMP7 induces breast cancer cell aging by a mechanism involving BMPRII receptor- and Smad3-mediated repression of the hTERT gene.

title: Suppressing N-Myc downstream regulated gene 1 reactivates senescence signaling and inhibits tumor growth in hepatocellular carcinoma.
abstract: Hepatocellular carcinoma (HCC) is the fifth most lethal malignancy worldwide with no curative therapies. To discover potentially novel therapeutic targets for HCC, we previously studied the gene expression profiles of HCC patients and identified that significant upregulation of N-Myc downstream regulated gene 1 (NDRG1) is associated with more aggressive phenotypes and poorer overall survival of HCC patients. In this study, we further used a loss-of-function approach (RNA interference) to understand the role of NDRG1 in hepatocarcinogenesis. We found that suppression of NDRG1 significantly impaired HCC cell growth through inducing extensive cellular senescence of HCC cells both in vitro and in vivo, accompanied by cell cycle arrest at the G1 phase. The observed antitumor effects of NDRG1 suppression were correlated with activation of major senescence-associated signaling pathways, such as upregulation of tumor suppressors p53, p21 and p16, and decreased phosphorylated Rb. To obtain further insights into the clinical significance of NDRG1-modulated senescence in HCC patients, immunohistochemistry staining of 92 cases of HCC patients was done. We found that high NDRG1 expression (n = 66) is associated with low p21 (n = 82; P < 0.001) and low p16 (n = 86; P < 0.001) levels. In conclusion, this study demonstrated that NDRG1 is a potential therapeutic target for HCC because its suppression triggers senescence of HCC cells through activating glycogen synthase kinase-3β-p53 pathway, thereby inhibiting tumor progression.

title: BMP4-Smad Signaling Pathway Mediates Adriamycin-induced Premature Senescence in Lung Cancer Cells*
abstract: Cell senescence, an irreversible cell cycle arrest, reflects a safeguard program that limits the capacity of uncontrolled cell proliferation. Treatment of tumor cells with certain chemotherapeutic agents activates premature senescence to decrease the tumorigenecity. Here we show that sublethal concentrations of adriamycin could induce premature senescence in lung cancer cells. Adriamycin treatment resulted in the up-regulation of BMP4, which is underexpressed in NSCLC (non-small cell lung cancers). Moreover, the BMP4-Smad pathway played a key role in mediating adriamycin-induced senescence. Overexpression of BMP4 was able to induce premature senescence in lung cancer cells and this process required the participation of cyclin/cyclin-dependent kinase (cdk) inhibitors p16INK4a and p21WAF1/cip1. We also show that increases of p16INK4a and p21WAF1/cip1 expression in response to BMP4 were mediated by the Smad signaling pathway. Furthermore, our data revealed that p300 was recruited to P16INK4a and P21WAF1/cip1 promoters by Smad1/5/8 to induce the hyperacetylation of histones H3 and H4 at the promoters. The present study provides useful clues to the evaluation of the potentiality of BMP4 as a responsive molecular target for cancer chemotherapy.

title: BMP4 Mediates Oxidative Stress-induced Retinal Pigment Epithelial Cell Senescence and Is Overexpressed in Age-related Macular Degeneration*
abstract: The retinal pigment epithelium is a primary site of pathology in age-related macular degeneration. Oxidative stress and senescence are both thought to be important mediators of macular degeneration pathogenesis. We demonstrate here that bone morphogenetic protein-4 is highly expressed in the retinal pigment epithelium and adjacent extracellular matrix of patients with dry age-related macular degeneration. In vitro studies revealed that sublethal oxidative stress increased bone morphogenetic protein-4 expression in retinal pigment epithelial cells, and both bone morphogenetic protein-4 and persistent mild oxidative stress can induce retinal pigment epithelial cell senescence through p53-p21Cip1/WAF1-Rb pathway. We further demonstrate that bone morphogenetic protein-4 acts as a mediator in oxidative stress-induced senescence and that this mediator function is via Smad and the p38 signaling pathway to increase and activate p53 and p21Cip1/WAF1 and decrease phospho-Rb. Oxidative stress-induced senescence can be blocked by Chordin-like, an antagonist of bone morphogenetic protein-4, or SB203580, a phospho-p38 inhibitor. Our results suggest that oxidative stress and bone morphogenetic protein-4 may interact to promote retinal pigment epithelial cell senescence and that bone morphogenetic protein-4 may represent a novel therapeutic target to inhibit the progressive effects of oxidative stress and senescence in dry age-related macular degeneration.

title: A complex secretory program orchestrated by the inflammasome controls paracrine senescence
abstract: Oncogene-induced senescence (OIS) is crucial for tumour suppression. Senescent cells implement a complex pro-inflammatory response termed the senescence-associated secretory phenotype (SASP). The SASP reinforces senescence, activates immune surveillance and paradoxically also has pro-tumorigenic properties. Here, we present evidence that the SASP can also induce paracrine senescence in normal cells both in culture and in human and mouse models of OIS in vivo. Coupling quantitative proteomics with small-molecule screens, we identified multiple SASP components mediating paracrine senescence, including TGF-β family ligands, VEGF, CCL2 and CCL20. Amongst them, TGF-β ligands play a major role by regulating p15INK4b and p21CIP1. Expression of the SASP is controlled by inflammasome-mediated IL-1 signalling. The inflammasome and IL-1 signalling are activated in senescent cells and IL-1α expression can reproduce SASP activation, resulting in senescence. Our results demonstrate that the SASP can cause paracrine senescence and impact on tumour suppression and senescence in vivo.

title: TGFβ inhibition restores a regenerative response in acute liver injury by suppressing paracrine senescence
abstract: Inhibiting acute injury–induced senescence mediated by TGFβ signaling in regenerative epithelium improves liver regeneration. Setting liver regeneration free The liver is an excellent model of organ regeneration; however, regeneration may fail in a normal liver after acute severe injury such as acetaminophen poisoning. Bird and colleagues now show that a process that prevents proliferation termed senescence, which is classically associated with aging and carcinogenesis, inhibits the liver’s regenerative cells after acute injury. This senescence can be spread from cell to cell by the signaling molecule transforming growth factor–β (TGFβ). When TGFβ signaling was blocked during acetaminophen poisoning in mice, senescence was impeded, regeneration accelerated, and mouse survival increased. Therefore, targeting senescence induced by acute tissue injury is an attractive therapeutic approach to improve regeneration. Liver injury results in rapid regeneration through hepatocyte proliferation and hypertrophy. However, after acute severe injury, such as acetaminophen poisoning, effective regeneration may fail. We investigated how senescence may underlie this regenerative failure. In human acute liver disease, and murine models, p21-dependent hepatocellular senescence was proportionate to disease severity and was associated with impaired regeneration. In an acetaminophen injury mouse model, a transcriptional signature associated with the induction of paracrine senescence was observed within 24 hours and was followed by one of impaired proliferation. In mouse genetic models of hepatocyte injury and senescence, we observed transmission of senescence to local uninjured hepatocytes. Spread of senescence depended on macrophage-derived transforming growth factor–β1 (TGFβ1) ligand. In acetaminophen poisoning, inhibition of TGFβ receptor 1 (TGFβR1) improved mouse survival. TGFβR1 inhibition reduced senescence and enhanced liver regeneration even when delivered beyond the therapeutic window for treating acetaminophen poisoning. This mechanism, in which injury-induced senescence impairs liver regeneration, is an attractive therapeutic target for developing treatments for acute liver failure.

title: Cellular senescence mediates fibrotic pulmonary disease
abstract: Idiopathic pulmonary fibrosis (IPF) is a fatal disease characterized by interstitial remodelling, leading to compromised lung function. Cellular senescence markers are detectable within IPF lung tissue and senescent cell deletion rejuvenates pulmonary health in aged mice. Whether and how senescent cells regulate IPF or if their removal may be an efficacious intervention strategy is unknown. Here we demonstrate elevated abundance of senescence biomarkers in IPF lung, with p16 expression increasing with disease severity. We show that the secretome of senescent fibroblasts, which are selectively killed by a senolytic cocktail, dasatinib plus quercetin (DQ), is fibrogenic. Leveraging the bleomycin-injury IPF model, we demonstrate that early-intervention suicide-gene-mediated senescent cell ablation improves pulmonary function and physical health, although lung fibrosis is visibly unaltered. DQ treatment replicates benefits of transgenic clearance. Thus, our findings establish that fibrotic lung disease is mediated, in part, by senescent cells, which can be targeted to improve health and function.

title: TGF-β1 induces senescence of bone marrow mesenchymal stem cells via increase of mitochondrial ROS production
abstract: BackgroundBone marrow derived mesenchymal stem cells (bmMSCs) are multipotent cells that can differentiate into diverse cell types, including cardiomyocytes. BmMSC-based transplantation is capable of repairing acute and chronic myocardial infarction. Prior to the transplantation, MSCs are usually induced in vitro by biological reagents and chemicals for directional differentiation. Transforming growth factor beta (TGF-β) is one of the most commonly used biological reagents for induction of cardiomyocyte differentiation of bmMSCs. Previous studies have shown that TGF-β induces senescence in several cell types. However, whether TGF-β affects senescence of bmMSCs has not been elucidated. The goal of this study was to investigate the effect of TGF-β1 on senescence of bmMSCs and the underlying mechanisms.ResultsWe found that TGF-β1 increased activity of senescence-associated-galactosidase (SA-Gal) and production of mitochondrial reactive oxygen species (mtROS) in bmMSCs in a dose-dependent manner. TGF-β1 also significantly decreased expression of superoxide dismutase 2 (SOD2) and Id1, and increased expression of 4-Hydroxynonenal (4-HNE) subunits and p16 in bmMSCs in a dose-dependent manner. Pre-treatment with mtROS inhibitor acetyl-L-carnitine (ALCAR, 0.1 mM) significantly inhibited TGF-β1-induced mtROS production and SA-Gal activity.ConclusionTGF-β1 can induce senescence of bmMSCs, which at least partially depends on mtROS production.

title: Cancer dormancy and cell signaling: induction of p21(waf1) initiated by membrane IgM engagement increases survival of B lymphoma cells.
abstract: The p21(WAF1) (p21) cyclin-dependent kinase inhibitor plays a major role in regulating cell cycle arrest. It was recently reported that the p53-independent elevation of p21 protein levels is essential in mediating the G(1) arrest resulting from signal transduction events initiated by the crosslinking of membrane IgM on Daudi Burkitt lymphoma cells. Although the role of p21 in cell cycle regulation is well documented, there is little information concerning its role in antibody-mediated apoptosis. In the present study, we examined the involvement of p21 in the regulation of apoptosis by suppressing its induction in anti-IgM-treated Daudi cells through a p21 antisense expression construct approach. Reduction in induced p21 protein levels resulted in diminished G(1) arrest and increased apoptosis. The increased susceptibility to anti-IgM-mediated apoptosis was associated with increased caspase-3-like activity and poly-(ADP)ribose polymerase cleavage. These data suggest that p21 may directly interfere with the caspase cascade, thus playing a dual role in regulating both cell cycle progression and apoptosis.

title: Cloning of senescent cell-derived inhibitors of DNA synthesis using an expression screen.
abstract: We here describe a rapid and simple expression screen method that has allowed us to isolate cDNAs coding for inhibitors of DNA synthesis from senescent human diploid fibroblasts. The assay involved transient transcriptional overexpression of a gene product encoded by a cDNA in a proliferating cell, on the assumption that this would be sufficient to block DNA synthesis in a short-term assay using tritiated thymidine autoradiography. Three cDNAs, referred to as senescent cell-derived inhibitors (sdi), that exhibit DNA synthesis-inhibitory activity when introduced into young cycling cells, were successfully identified. Expression of one of the cDNAs, sdi1, increased 10- to 20-fold in senescent compared with young cells and the increase in RNA closely paralleled the onset of the senescent phenotype and loss of cell proliferation. sdi1 expression was also increased in young cells made nondividing (quiescent) by deprivation of growth factors or contact inhibition. Following serum stimulation, RNA levels of sdi1 in quiescent cells were initially increased, but then declined to low levels just prior to the entry of the cells into S phase. In contrast, RNA levels of sdi1 in senescent cells failed to decline, suggesting a role for this gene in maintaining the senescent phenotype. The sdi1 gene has been mapped to the p arm of chromosome 6.

title: VHL loss actuates a HIF-independent senescence programme mediated by Rb and p400
abstract: Germline von Hippel–Lindau tumour suppressor gene (VHL) mutations cause renal cell carcinomas, haemangioblastomas and phaeochromocytomas in humans. Mutations in VHL also occur in sporadic renal cell carcinomas. The protein encoded by VHL, VHL, is part of the ubiquitin ligase that downregulates the heterodimeric transcription factor Hif under well-oxygenated conditions. Here we show that acute VHL inactivation causes a senescent-like phenotype in vitro and in vivo. This phenotype was independent of p53 and Hif but dependent on the retinoblastoma protein (Rb) and the SWI2/SNF2 chromatin remodeller p400. Rb activation occurred through a decrease in Skp2 messenger RNA, which resulted in the upregulation of p27 in a Hif-independent fashion. Our results suggest that senescence induced by VHL inactivation is a tumour-suppressive mechanism that must be overcome to develop VHL-associated neoplasias.

title: Role of the p16 tumor suppressor gene in cancer.
abstract: Since its discovery as a CDKI (cyclin-dependent kinase inhibitor) in 1993, the tumor suppressor p16 (INK4A/MTS-1/CDKN2A) has gained widespread importance in cancer. The frequent mutations and deletions of p16 in human cancer cell lines first suggested an important role for p16 in carcinogenesis. This genetic evidence for a causal role was significantly strengthened by the observation that p16 was frequently inactivated in familial melanoma kindreds. Since then, a high frequency of p16 gene alterations were observed in many primary tumors. In human neoplasms, p16 is silenced in at least three ways: homozygous deletion, methylation of the promoter, and point mutation. The first two mechanisms comprise the majority of inactivation events in most primary tumors. Additionally, the loss of p16 may be an early event in cancer progression, because deletion of at least one copy is quite high in some premalignant lesions. p16 is a major target in carcinogenesis, rivaled in frequency only by the p53 tumor-suppressor gene. Its mechanism of action as a CDKI has been elegantly elucidated and involves binding to and inactivating the cyclin D-cyclin-dependent kinase 4 (or 6) complex, and thus renders the retinoblastoma protein inactive. This effect blocks the transcription of important cell-cycle regulatory proteins and results in cell-cycle arrest. Although p16 may be involved in cell senescence, the physiologic role of p16 is still unclear. Future work will focus on studies of the upstream events that lead to p16 expression and its mechanism of regulation, and perhaps lead to better therapeutic strategies that can improve the clinical course of many lethal cancers.

title: Preclinical characterization of abemaciclib in hormone receptor positive breast cancer.
abstract: Abemaciclib is an ATP-competitive, reversible kinase inhibitor selective for CDK4 and CDK6 that has shown antitumor activity as a single agent in hormone receptor positive (HR+) metastatic breast cancer in clinical trials. Here, we examined the mechanistic effects of abemaciclib treatment using in vitro and in vivo breast cancer models. Treatment of estrogen receptor positive (ER+) breast cancer cells with abemaciclib alone led to a decrease in phosphorylation of Rb, arrest at G1, and a decrease in cell proliferation. Moreover, abemaciclib exposure led to durable inhibition of pRb, TopoIIα expression and DNA synthesis, which were maintained after drug removal. Treatment of ER+ breast cancer cells also led to a senescence response as indicated by accumulation of β-galactosidase, formation of senescence-associated heterochromatin foci, and a decrease in FOXM1 positive cells. Continuous exposure to abemaciclib altered breast cancer cell metabolism and induced apoptosis. In a xenograft model of ER+ breast cancer, abemaciclib monotherapy caused regression of tumor growth. Overall these data indicate that abemaciclib is a CDK4 and CDK6 inhibitor that, as a single agent, blocks breast cancer cell progression, and upon longer treatment can lead to sustained antitumor effects through the induction of senescence, apoptosis, and alteration of cellular metabolism.

title: Thermal proteome profiling of breast cancer cells reveals proteasomal activation by CDK4/6 inhibitor palbociclib
abstract: Palbociclib is a CDK4/6 inhibitor approved for metastatic estrogen receptor positive breast cancer. In addition to G1 cell cycle arrest, palbociclib treatment results in cell senescence, a phenotype that are not readily explained by CDK4/6 inhibition. In order to identify a molecular mechanism responsible for palbociclib-induced senescence, we performed thermal proteome profiling of MCF7 breast cancer cells. In addition to affecting known CDK4/6 targets, palbociclib induces a thermal stabilization of the 20S proteasome, despite not directly binding to it. We further show that palbociclib treatment increases proteasome activity independently of the ubiquitin pathway. This leads to cellular senescence which can be counteracted by proteasome inhibitors. Palbociclib-induced proteasome activation and senescence is mediated by reduced proteasomal association of ECM29. Loss of ECM29 activates the proteasome, blocks cell proliferation and induces senescence. Finally, we find that ECM29 mRNA levels are predictive of relapse-free survival in breast cancer patients treated with endocrine therapy. In conclusion, thermal proteome profiling identifies the proteasome and ECM29 protein as mediators of palbociclib activity in breast cancer cells.

title: CDK4/6 and autophagy inhibitors synergistically induce senescence in Rb positive cytoplasmic cyclin E negative cancers
abstract: Deregulation of the cell cycle machinery is a hallmark of cancer. While CDK4/6 inhibitors are FDA approved (palbociclib) for treating advanced estrogen receptor-positive breast cancer, two major clinical challenges remain: (i) adverse events leading to therapy discontinuation and (ii) lack of reliable biomarkers. Here we report that breast cancer cells activate autophagy in response to palbociclib, and that the combination of autophagy and CDK4/6 inhibitors induces irreversible growth inhibition and senescence in vitro, and diminishes growth of cell line and patient-derived xenograft tumours in vivo. Furthermore, intact G1/S transition (Rb-positive and low-molecular-weight isoform of cyclin E (cytoplasmic)-negative) is a reliable prognostic biomarker in ER positive breast cancer patients, and predictive of preclinical sensitivity to this drug combination. Inhibition of CDK4/6 and autophagy is also synergistic in other solid cancers with an intact G1/S checkpoint, providing a novel and promising biomarker-driven combination therapeutic strategy to treat breast and other solid tumours.

title: Polyploidy road to therapy‐induced cellular senescence and escape
abstract: Therapy‐induced cellular senescence (TCS), characterized by prolonged cell cycle arrest, is an in vivo response of human cancers to chemotherapy and radiation. Unfortunately, TCS is reversible for a subset of senescent cells, leading to cellular reproliferation and ultimately tumor progression. This invariable consequence of TCS recapitulates the clinical treatment experience of patients with advanced cancer. We report the findings of a clinicopathological study in patients with locally advanced non‐small cell lung cancer demonstrating that marker of in vivo TCS following neoadjuvant therapy prognosticate adverse clinical outcome. In our efforts to elucidate key molecular pathways underlying TCS and cell cycle escape, we have previously shown that the deregulation of mitotic kinase Cdk1 and its downstream effectors are important mediators of survival and cell cycle reentry. We now report that aberrant expression of Cdk1 interferes with apoptosis and promotes the formation of polyploid senescent cells during TCS. These polyploid senescent cells represent important transition states through which escape preferentially occurs. The Cdk1 pathway is in part modulated differentially by p21 and p27 two members of the KIP cyclin‐dependent kinase inhibitor family during TCS. Altogether, these studies underscore the importance of TCS in cancer therapeutics.

title: The Cdk inhibitor p27 in human cancer: prognostic potential and relevance to anticancer therapy
abstract: The cyclin-dependent kinase (Cdk) inhibitor p27 (also known as KIP1) regulates cell proliferation, cell motility and apoptosis. Interestingly, the protein can exert both positive and negative functions on these processes. Diverse post-translational modifications determine the physiological role of p27. Phosphorylation regulates p27 binding to and inhibition of cyclin–Cdk complexes, its localization and its ubiquitin-mediated proteolysis. In cancers, p27 is inactivated through impaired synthesis, accelerated degradation and by mislocalization. Moreover, studies in several tumour types indicate that p27 expression levels have both prognostic and therapeutic implications.

title: A novel type of cellular senescence that can be enhanced in mouse models and human tumor xenografts to suppress prostate tumorigenesis.
abstract: Irreversible cell growth arrest, a process termed cellular senescence, is emerging as an intrinsic tumor suppressive mechanism. Oncogene-induced senescence is thought to be invariably preceded by hyperproliferation, aberrant replication, and activation of a DNA damage checkpoint response (DDR), rendering therapeutic enhancement of this process unsuitable for cancer treatment. We previously demonstrated in a mouse model of prostate cancer that inactivation of the tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (Pten) elicits a senescence response that opposes tumorigenesis. Here, we show that Pten-loss-induced cellular senescence (PICS) represents a senescence response that is distinct from oncogene-induced senescence and can be targeted for cancer therapy. Using mouse embryonic fibroblasts, we determined that PICS occurs rapidly after Pten inactivation, in the absence of cellular proliferation and DDR. Further, we found that PICS is associated with enhanced p53 translation. Consistent with these data, we showed that in mice p53-stabilizing drugs potentiated PICS and its tumor suppressive potential. Importantly, we demonstrated that pharmacological inhibition of PTEN drives senescence and inhibits tumorigenesis in vivo in a human xenograft model of prostate cancer. Taken together, our data identify a type of cellular senescence that can be triggered in nonproliferating cells in the absence of DNA damage, which we believe will be useful for developing a "pro-senescence" approach for cancer prevention and therapy.

title: DYRK1A protein kinase promotes quiescence and senescence through DREAM complex assembly.
abstract: In the absence of growth signals, cells exit the cell cycle and enter into G0 or quiescence. Alternatively, cells enter senescence in response to inappropriate growth signals such as oncogene expression. The molecular mechanisms required for cell cycle exit into quiescence or senescence are poorly understood. The DREAM (DP, RB [retinoblastoma], E2F, and MuvB) complex represses cell cycle-dependent genes during quiescence. DREAM contains p130, E2F4, DP1, and a stable core complex of five MuvB-like proteins: LIN9, LIN37, LIN52, LIN54, and RBBP4. In mammalian cells, the MuvB core dissociates from p130 upon entry into the cell cycle and binds to BMYB during S phase to activate the transcription of genes expressed late in the cell cycle. We used mass spectroscopic analysis to identify phosphorylation sites that regulate the switch of the MuvB core from BMYB to DREAM. Here we report that DYRK1A can specifically phosphorylate LIN52 on serine residue 28, and that this phosphorylation is required for DREAM assembly. Inhibiting DYRK1A activity or point mutation of LIN52 disrupts DREAM assembly and reduces the ability of cells to enter quiescence or undergo Ras-induced senescence. These data reveal an important role for DYRK1A in the regulation of DREAM activity and entry into quiescence.

title: Mirk/Dyrk1B maintains the viability of quiescent pancreatic cancer cells by reducing levels of reactive oxygen species.
abstract: The kinase Mirk/dyrk1B mediated the clonogenic growth of pancreatic cancer cells in earlier studies. It is now shown that Mirk levels increased 7-fold in SU86.86 pancreatic cancer cells when over a third of the cells were accumulated in a quiescent G(0) state, defined by Hoechst/Pyronin Y staining. Depletion of Mirk by a doxycycline-inducible short hairpin RNA increased the G(0) fraction to approximately 50%, suggesting that Mirk provided some function in G(0). Mirk reduced the levels of reactive oxygen species (ROS) in quiescent cultures of SU86.86 cells and of Panc1 cells by increasing transcription of the antioxidant genes ferroxidase, superoxide dismutase (SOD)2, and SOD3. These genes were functional antioxidant genes in pancreatic cancer cells because ectopic expression of SOD2 and ferroxidase in Mirk-depleted cells lowered ROS levels. Quiescent pancreatic cancer cells quickly lost viability when depleted of Mirk because of elevated ROS levels, exhibiting up to 4-fold less colony-forming activity and 4-fold less capability for dye exclusion. As a result, reduction of ROS by N-acetyl cysteine led to more viable cells. Mirk also destabilizated cyclin D1 and D3 in quiescent cells. Thus, quiescent pancreatic cancer cells depleted of Mirk became less viable because they were damaged by ROS, and had increased levels of G(1) cyclins to prime cells to escape quiescence.

title: The DREAM complex mediates GIST cell quiescence and is a novel therapeutic target to enhance imatinib-induced apoptosis.
abstract: Gastrointestinal stromal tumors (GIST) can be successfully treated with imatinib mesylate (Gleevec); however, complete remissions are rare and patients frequently achieve disease stabilization in the presence of residual tumor masses. The clinical observation that discontinuation of treatment can lead to tumor progression suggests that residual tumor cells are, in fact, quiescent and, therefore, able to re-enter the cell-division cycle. In line with this notion, we have previously shown that imatinib induces GIST cell quiescence in vitro through the APC(CDH1)-SKP2-p27(Kip1) signaling axis. Here, we provide evidence that imatinib induces GIST cell quiescence in vivo and that this process also involves the DREAM complex, a multisubunit complex that has recently been identified as an additional key regulator of quiescence. Importantly, inhibition of DREAM complex formation by depletion of the DREAM regulatory kinase DYRK1A or its target LIN52 was found to enhance imatinib-induced cell death. Our results show that imatinib induces apoptosis in a fraction of GIST cells while, at the same time, a subset of cells undergoes quiescence involving the DREAM complex. Inhibition of this process enhances imatinib-induced apoptosis, which opens the opportunity for future therapeutic interventions to target the DREAM complex for more efficient imatinib responses.

title: A Systematic Analysis of Negative Growth Control Implicates the DREAM Complex in Cancer Cell Dormancy
abstract: Epithelial ovarian cancer (EOC) generates multicellular aggregates called spheroids that detach from the primary tumor and disseminate through ascites. Spheroids possess a number of characteristics of tumor dormancy including withdrawal from the cell cycle and resistance to chemotherapeutics. This report systematically analyzes the effects of RNAi depletion of 21 genes that are known to contribute to negative regulation of the cell cycle in 10 ovarian cancer cell lines. Interestingly, spheroid cell viability was compromised by loss of some cyclin-dependent kinase inhibitors such as p57Kip2, as well as Dyrk1A, Lin52, and E2F5 in most cell lines tested. Many genes essential for EOC spheroid viability are pertinent to the mammalian DREAM repressor complex. Mechanistically, the data demonstrate that DREAM is assembled upon the induction of spheroid formation, which is dependent upon Dyrk1A. Loss of Dyrk1A results in retention of the b-Myb–MuvB complex, elevated expression of DREAM target genes, and increased DNA synthesis that is coincident with cell death. Inhibition of Dyrk1A activity using pharmacologic agents Harmine and INDY compromises viability of spheroids and blocks DREAM assembly. In addition, INDY treatment improves the response to carboplatin, suggesting this is a therapeutic target for EOC treatment.  Loss of negative growth control mechanisms in cancer dormancy lead to cell death and not proliferation, suggesting they are an attractive therapeutic approach.

title: Mechanistic understanding of the role of ATRX in senescence provides new insight for combinatorial therapies with CDK4 inhibitors.
abstract: Senescence is an irreversible form of growth arrest and is generally considered a favorable outcome of cancer therapies, yet little is known about the molecular events that distinguish this state from readily reversible growth arrest (i.e. quiescence). Recently, we discovered that during therapy induced senescence the chromatin remodeling protein α-thalassemia, mental retardation, X-linked (ATRX) represses Harvey rat sarcoma viral oncogene homolog (HRAS), and repression of HRAS is necessary to establish senescence, suggesting how new clinical combinations might be used to achieve durable senescence.

title: Itraconazole targets cell cycle heterogeneity in colorectal cancer
abstract: Cellular dormancy and heterogeneity in cell cycle length provide important explanations for treatment failure after adjuvant therapy with S-phase cytotoxics in colorectal cancer (CRC), yet the molecular control of the dormant versus cycling state remains unknown. We sought to understand the molecular features of dormant CRC cells to facilitate rationale identification of compounds to target both dormant and cycling tumor cells. Unexpectedly, we demonstrate that dormant CRC cells are differentiated, yet retain clonogenic capacity. Mouse organoid drug screening identifies that itraconazole generates spheroid collapse and loss of dormancy. Human CRC cell dormancy and tumor growth can also be perturbed by itraconazole, which is found to inhibit Wnt signaling through noncanonical hedgehog signaling. Preclinical validation shows itraconazole to be effective in multiple assays through Wnt inhibition, causing both cycling and dormant cells to switch to global senescence. These data provide preclinical evidence to support an early phase trial of itraconazole in CRC.

title: Metastasis prevention by targeting the dormant niche
abstract: Despite considerable advancements that shattered previously held dogmas about the metastatic cascade, the evolution of therapies to treat metastatic disease has not kept up. In this Opinion article, I argue that, rather than waiting for metastases to emerge before initiating treatment, it would be more effective to target metastatic seeds before they sprout. Specifically, I advocate directing therapies towards the niches that harbour dormant disseminated tumour cells to sensitize them to cytotoxic agents. Treatment sensitization, achieved by disrupting reservoirs of leukaemic stem cells and latent HIV, argues that this approach, although unconventional, could succeed in improving patient survival by delaying or even preventing metastasis.

title: Mesenchymal cell contributions to the stem cell niche.
abstract: Mesenchymal stromal cells (MSCs) are heterogeneous and primitive cells discovered first in the bone marrow (BM). They have putative roles in maintaining tissue homeostasis and are increasingly recognized as components of stem cell niches, which are best defined in the blood. The absence of in vivo MSC markers has limited our ability to track their behavior in vivo and draw comparisons with in vitro observations. Here we review the historical background of BM-MSCs, advances made in their prospective isolation, their developmental origin and contribution to maintaining subsets of hematopoietic cells, and how mesenchymal cells contribute to other stem cell niches.

title: Paracrine action accounts for marked protection of ischemic heart by Akt-modified mesenchymal stem cells
abstract: Paracrine action accounts for marked protection of ischemic heart by Akt-modified mesenchymal stem cells

title: A review of therapeutic effects of mesenchymal stem cell secretions and induction of secretory modification by different culture methods
abstract: The mesenchymal stem cell (MSC) is being broadly studied in clinical trials. Contrary to the early paradigm of cell replacement and differentiation as a therapeutic mechanism of action, evidence is mounting that the secretions of the cells are responsible for their therapeutic effects. These secretions include molecules and extracellular vesicles that have both local and distant effects. This review summarizes the up- and down-regulation of MSC anti-inflammatory, immune modulating, anti-tumor, and regenerative secretions resulting from different stimuli including: a) hypoxia, which increases the production of growth factors and anti-inflammatory molecules; b) pro-inflammatory stimuli that induce the secretion of immune modulating and anti-inflammatory factors; and c) 3 dimensional growth which up regulates the production of anti-cancer factors and anti-inflammatory molecules compared to monolayer culture. Finally we review in detail the most important factors present in conditioned medium of MSC that can be considered protagonists of MSC physiological effects including HGF, TGF-b, VEGF, TSG-6, PGE2 and galectins 1, and 9. We conclude that there is potential for the development of acellular therapeutic interventions for autoimmune, inflammatory, and malignant diseases and tissue regeneration from cellular secretions derived from MSCs cultured under the appropriate conditions.

title: Interferon‐γ does not break, but promotes the immunosuppressive capacity of adult human mesenchymal stem cells
abstract: The ability of mesenchymal stem cells (MSC) to suppress alloresponsiveness is poorly understood. Herein, an allogeneic mixed lymphocyte response was used as a model to investigate the mechanisms of MSC‐mediated immunomodulation. Human MSC are demonstrated to express the immunosuppressive cytokines hepatocyte growth factor (HGF), interleukin (IL)‐10 and transforming growth factor (TGF)‐β1 at concentrations that suppress alloresponses in vitro. MSC also express cyclooxygenase 1 and 2 and produce prostaglandin E2 constitutively. Blocking studies with indomethacin confirmed that prostaglandins contribute to MSC‐mediated allosuppression. The proinflammatory cytokine interferon (IFN)‐γ did not ablate MSC inhibition of alloantigen‐driven proliferation but up‐regulated HGF and TGF‐β1. IFN‐γ also induced expression of indoleamine 2,3, dioxygenase (IDO), involved in tryptophan catabolism. Use of an antagonist, 1‐methyl‐L‐tryptophan, restored alloresponsiveness and confirmed an IDO contribution to IFN‐γ‐induced immunomodulation by MSC. Addition of the tryptophan catabolite kynurenine to mixed lymphocyte reactions (MLR), blocked alloproliferation. These findings support a model where IDO exerts its effect through the local accumulation of tryptophan metabolites rather than through tryptophan depletion. Taken together, these data demonstrate that soluble factors, or products derived from MSC, modulate immune responses and suggest that MSC create an immunosuppressive microenvironment capable of modulating alloresponsiveness even in the presence of IFN‐γ.

title: Aggregation of human mesenchymal stromal cells (MSCs) into 3D spheroids enhances their antiinflammatory properties
abstract: Previous reports suggested that culture as 3D aggregates or as spheroids can increase the therapeutic potential of the adult stem/progenitor cells referred to as mesenchymal stem cells or multipotent mesenchymal stromal cells (MSCs). Here we used a hanging drop protocol to prepare human MSCs (hMSCs) as spheroids that maximally expressed TNFα stimulated gene/protein 6 (TSG-6), the antiinflammatory protein that was expressed at high levels by hMSCs trapped in the lung after i.v. infusion and that largely explained the beneficial effects of hMSCs in mice with myocardial infarcts. The properties of spheroid hMSCs were found to depend critically on the culture conditions. Under optimal conditions for expression of TSG-6, the hMSCs also expressed high levels of stanniocalcin-1, a protein with both antiinflammatory and antiapoptotic properties. In addition, they expressed high levels of three anticancer proteins: IL-24, TNFα-related apoptosis inducing ligand, and CD82. The spheroid hMSCs were more effective than hMSCs from adherent monolayer cultures in suppressing inflammatory responses in a coculture system with LPS-activated macrophages and in a mouse model for peritonitis. In addition, the spheroid hMSCs were about one-fourth the volume of hMSCs from adherent cultures. Apparently as a result, larger numbers of the cells trafficked through the lung after i.v. infusion and were recovered in spleen, liver, kidney, and heart. The data suggest that spheroid hMSCs may be more effective than hMSCs from adherent cultures in therapies for diseases characterized by sterile tissue injury and unresolved inflammation and for some cancers that are sensitive to antiinflammatory agents.

title: Human prostate cancer metastases target the hematopoietic stem cell niche to establish footholds in mouse bone marrow.
abstract: HSC homing, quiescence, and self-renewal depend on the bone marrow HSC niche. A large proportion of solid tumor metastases are bone metastases, known to usurp HSC homing pathways to establish footholds in the bone marrow. However, it is not clear whether tumors target the HSC niche during metastasis. Here we have shown in a mouse model of metastasis that human prostate cancer (PCa) cells directly compete with HSCs for occupancy of the mouse HSC niche. Importantly, increasing the niche size promoted metastasis, whereas decreasing the niche size compromised dissemination. Furthermore, disseminated PCa cells could be mobilized out of the niche and back into the circulation using HSC mobilization protocols. Finally, once in the niche, tumor cells reduced HSC numbers by driving their terminal differentiation. These data provide what we believe to be the first evidence that the HSC niche serves as a direct target for PCa during dissemination and plays a central role in bone metastases. Our work may lead to better understanding of the molecular events involved in bone metastases and new therapeutic avenues for an incurable disease.

title: AXL Is a Putative Tumor Suppressor and Dormancy Regulator in Prostate Cancer
abstract: Prostate cancer bone metastasis remains lethal and incurable, and often arises years after elimination of the primary tumor. It is unclear what underlies the decades-long clinical latency before recurrence, but evidence points to the existence of dormant residual tumor cells that disseminated before the primary tumor was eliminated. To design therapies to prevent progression of disseminated tumor cells (DTC) into lethal metastases, it is crucial to understand the mechanism(s) underlying this dormancy. The current study functionally validated our previous observation that implicated the GAS6/AXL axis in mediating DTC dormancy in the bone marrow. AXL-null and AXL-overexpressing prostate cancer cell lines were generated to determine if AXL was necessary and/or sufficient for dormancy. Characterization of these cells in vitro and using in vivo mouse models of DTC growth demonstrated that AXL was indeed sufficient to induce dormancy, but was unable to maintain it long-term and was not absolutely required for a dormancy period. Clinically, AXL expression correlated with longer survival in prostate cancer patients, and AXL was not expressed by cancer cells in primary or metastatic tissue. These data point to a tumor-suppressive role for AXL in prostate cancer, and future work is required to determine if AXL is expressed on human bone marrow DTCs.  The ability of AXL to initiate but not maintain dormancy, coupled with its dispensability, suggests that targeting AXL alone will not prevent lethal metastatic outgrowth, and likely a cooperative network of factors exists to mediate long-term cellular dormancy.

title: Gas6 Delays Senescence in Vascular Smooth Muscle Cells through the PI3K/ Akt/FoxO Signaling Pathway
abstract: Growth arrest-specific protein 6 (Gas6) is a cytokine that can be synthesized by a variety of cell types and secreted into the extracellular matrix. Previous studies have confirmed that Gas6 is involved in certain pathophysiological processes of the cardiovascular system through binding to its receptor, Axl. In the present study, we investigated the role of Gas6 in cellular senescence and explored the mechanisms underlying its activity.  We used vascular smooth muscle cells (VSMCs) to create two cellular senescence models, one for replicative senescence (RS) and one for induced senescence (IS), to test the hypothesis that Gas6 delays senescence.  Gas6-treated cells appear relatively younger compared with non-Gas6-treated cells. In particular, Gas6-treated cells displayed decreased staining for SA-β-Gal, fewer G1 phase cells, and decreased levels of p16INK4a and p21Cip1 expression; conversely, Gas6-treated cells displayed more S phase cells and significantly increased proliferation indexes. Furthermore, in both the IS and RS models with Gas6 treatment, the levels of PI3K, p-Akt, and p-FoxO3a decreased following Axl inhibition by R428; similarly, the levels of p-Akt and p-FoxO3a also decreased following PI3K inhibition by LY294002.  Gas6/Axl signaling is essential for delaying the cellular senescence process regulated by the PI3K/Akt/FoxO signaling pathway.

title: Testosterone delays vascular smooth muscle cell senescence and inhibits collagen synthesis via the Gas6/Axl signaling pathway
abstract: Testosterone deficiency is associated with a higher incidence of cardiovascular diseases in men. However, its effect on cell senescence, which plays a causal role in vascular aging, remains unclear. Here, we tested the hypothesis that testosterone alleviated vascular smooth muscle cell (VSMC) senescence and collagen synthesis via growth arrest-specific protein 6 (Gas6)/Axl- and Akt/FoxO1a-dependent pathways. Testosterone significantly ameliorated angiotensin II-induced VSMC senescence and collagen overexpression. In addition, testosterone inhibited angiotensin II-induced matrix metalloproteinase-2 (MMP-2) activity, which played a pivotal role in facilitating age-related collagen deposition. Testosterone increased the expression of tissue inhibitor of metalloproteinase-2 but decreased the expression of MMP-2 and membrane type-1 metalloproteinase which contributed to increase MMP-2 activity. The effects on VSMCs senescence and collagen synthesis were mediated by restoration of angiotensin II-induced downregulation of Gas6 and Axl expression and a subsequent reduction of Akt and FoxO1a phosphorylation. The effects of testosterone were reversed by a Gas6 blocker, Axl-Fc, and a specific inhibitor of Axl, R428. Treatment of VSMCs with PI3K inhibitor LY294002 abrogated the downregulating effect of testosterone on MMP-2 activity. Furthermore, when FoxO1a expression was silenced by using a specific siRNA, the inhibitory effect of testosterone on MMP-2 activity was revered as well, that indicated this process was Akt/FoxO1a dependence. Taken together, Gas6/Axl and Akt/FoxO1a were involved in protective effects of testosterone on VSMCs senescence and collagen synthesis. Our results provide a novel mechanism underlying the protective effect of testosterone on vascular aging and may serve as a theoretical basis for testosterone replacement therapy.

title: The senescence-associated secretory phenotype: the dark side of tumor suppression.
abstract: Cellular senescence is a tumor-suppressive mechanism that permanently arrests cells at risk for malignant transformation. However, accumulating evidence shows that senescent cells can have deleterious effects on the tissue microenvironment. The most significant of these effects is the acquisition of a senescence-associated secretory phenotype (SASP) that turns senescent fibroblasts into proinflammatory cells that have the ability to promote tumor progression.

title: Chemotherapy-induced senescent cancer cells engulf other cells to enhance their survival
abstract: The persistence of chemotherapy-induced senescent cancer cells is a barrier to achieving complete response. Tonnessen-Murray et al. reveal that these senescent cells acquire a phagocytic gene expression program and completely engulf and degrade whole cells in the lysosome, enhancing their survival.

title: Clonal neoantigens elicit T cell immunoreactivity and sensitivity to immune checkpoint blockade
abstract: The cellular ancestry of tumor antigens One contributing factor in antitumor immunity is the repertoire of neoantigens created by genetic mutations within tumor cells. Like the corresponding mutations, these neoantigens show intratumoral heterogeneity. Some are present in all tumor cells (clonal), and others are present in only a fraction of cells (subclonal). In a study of lung cancer and melanoma, McGranahan et al. found that a high burden of clonal tumor neoantigens correlated with improved patient survival, an increased presence of tumor-infiltrating lymphocytes, and a durable response to immunotherapy. Science, this issue p. 1463 Analysis of the cellular ancestry of tumor neoantigens can predict which are most likely to induce an immune response. As tumors grow, they acquire mutations, some of which create neoantigens that influence the response of patients to immune checkpoint inhibitors. We explored the impact of neoantigen intratumor heterogeneity (ITH) on antitumor immunity. Through integrated analysis of ITH and neoantigen burden, we demonstrate a relationship between clonal neoantigen burden and overall survival in primary lung adenocarcinomas. CD8+ tumor-infiltrating lymphocytes reactive to clonal neoantigens were identified in early-stage non–small cell lung cancer and expressed high levels of PD-1. Sensitivity to PD-1 and CTLA-4 blockade in patients with advanced NSCLC and melanoma was enhanced in tumors enriched for clonal neoantigens. T cells recognizing clonal neoantigens were detectable in patients with durable clinical benefit. Cytotoxic chemotherapy–induced subclonal neoantigens, contributing to an increased mutational load, were enriched in certain poor responders. These data suggest that neoantigen heterogeneity may influence immune surveillance and support therapeutic developments targeting clonal neoantigens.

title: A Single Dose of Neoadjuvant PD-1 Blockade Predicts Clinical Outcomes in Resectable Melanoma
abstract: Immunologic responses to anti-PD-1 therapy in melanoma patients occur rapidly with pharmacodynamic T cell responses detectable in blood by 3 weeks. It is unclear, however, whether these early blood-based observations translate to the tumor microenvironment. We conducted a study of neoadjuvant/adjuvant anti-PD-1 therapy in stage III/IV melanoma. We hypothesized that immune reinvigoration in the tumor would be detectable at 3 weeks and that this response would correlate with disease-free survival. We identified a rapid and potent anti-tumor response, with 8 of 27 patients experiencing a complete or major pathological response after a single dose of anti-PD-1, all of whom remain disease free. These rapid pathologic and clinical responses were associated with accumulation of exhausted CD8 T cells in the tumor at 3 weeks, with reinvigoration in the blood observed as early as 1 week. Transcriptional analysis demonstrated a pretreatment immune signature (neoadjuvant response signature) that was associated with clinical benefit. In contrast, patients with disease recurrence displayed mechanisms of resistance including immune suppression, mutational escape, and/or tumor evolution. Neoadjuvant anti-PD-1 treatment is effective in high-risk resectable stage III/IV melanoma. Pathological response and immunological analyses after a single neoadjuvant dose can be used to predict clinical outcome and to dissect underlying mechanisms in checkpoint blockade.Neoadjuvant PD-1 blockade in patients with resectable melanoma followed by adjuvant maintenance results in early immunological effects driving clinical benefit and reveals transcriptional and genomic mechanisms of response.

title: Chimeric antigen receptor-modified T cells in chronic lymphoid leukemia.
abstract: We designed a lentiviral vector expressing a chimeric antigen receptor with specificity for the B-cell antigen CD19, coupled with CD137 (a costimulatory receptor in T cells [4-1BB]) and CD3-zeta (a signal-transduction component of the T-cell antigen receptor) signaling domains. A low dose (approximately 1.5×10(5) cells per kilogram of body weight) of autologous chimeric antigen receptor-modified T cells reinfused into a patient with refractory chronic lymphocytic leukemia (CLL) expanded to a level that was more than 1000 times as high as the initial engraftment level in vivo, with delayed development of the tumor lysis syndrome and with complete remission. Apart from the tumor lysis syndrome, the only other grade 3/4 toxic effect related to chimeric antigen receptor T cells was lymphopenia. Engineered cells persisted at high levels for 6 months in the blood and bone marrow and continued to express the chimeric antigen receptor. A specific immune response was detected in the bone marrow, accompanied by loss of normal B cells and leukemia cells that express CD19. Remission was ongoing 10 months after treatment. Hypogammaglobulinemia was an expected chronic toxic effect.

title: Natural T cell autoreactivity to melanoma antigens: clonally expanded melanoma-antigen specific CD8 + memory T cells can be detected in healthy humans
abstract: We used four-color ImmunoSpot® assays, in conjunction with peptide pools that cover the sequence of tyrosinase (Tyr), melanoma-associated antigen A3 (MAGE-A3), melanocyte antigen/melanoma antigen recognized by T cells 1 (Melan-A/MART-1), glycoprotein 100 (gp100), and New York esophageal squamous cell carcinoma-1 (NY-ESO-1) to characterize the melanoma antigen (MA)-specific CD8 + cell repertoire in PBMC of 40 healthy human donors (HD). Tyr triggered interferon gamma (IFN-γ)-secreting CD8 + T cells in 25% of HD within 24 h of antigen stimulation ex vivo. MAGE-A3, Melan-A/MART-1, and gp100 also induced recall responses in 10%, 7.5%, and 2.5% of HD, respectively. At this time point, these CD8 + T cells did not yet produce GzB (granzyme B). However, they engaged in GzB production after 72 h of antigen stimulation. By this 72-h time point, 57.5% of the HD responded to at least one, and typically several, of the MA. A closer characterization of the Tyr-specific CD8 + T cell repertoire indicated that it was low-affinity, and to primarily entail a stem cell-like subpopulation. Collectively, our data reveal pre-existing endogenous T cell immunity against melanoma antigens in healthy donors, and analogous to natural autoantibodies, we have termed this “natural T cell autoreactivity”.

title: Senescence surveillance of pre-malignant hepatocytes limits liver cancer development
abstract: Upon the aberrant activation of oncogenes, normal cells can enter the cellular senescence program, a state of stable cell-cycle arrest, which represents an important barrier against tumour development in vivo. Senescent cells communicate with their environment by secreting various cytokines and growth factors, and it was reported that this ‘secretory phenotype’ can have pro- as well as anti-tumorigenic effects. Here we show that oncogene-induced senescence occurs in otherwise normal murine hepatocytes in vivo. Pre-malignant senescent hepatocytes secrete chemo- and cytokines and are subject to immune-mediated clearance (designated as ‘senescence surveillance’), which depends on an intact CD4+ T-cell-mediated adaptive immune response. Impaired immune surveillance of pre-malignant senescent hepatocytes results in the development of murine hepatocellular carcinomas (HCCs), thus showing that senescence surveillance is important for tumour suppression in vivo. In accordance with these observations, ras-specific Th1 lymphocytes could be detected in mice, in which oncogene-induced senescence had been triggered by hepatic expression of NrasG12V. We also found that CD4+ T cells require monocytes/macrophages to execute the clearance of senescent hepatocytes. Our study indicates that senescence surveillance represents an important extrinsic component of the senescence anti-tumour barrier, and illustrates how the cellular senescence program is involved in tumour immune surveillance by mounting specific immune responses against antigens expressed in pre-malignant senescent cells.

title: Distinct Functions of Senescence-Associated Immune Responses in Liver Tumor Surveillance and Tumor Progression.
abstract: Oncogene-induced senescence causes hepatocytes to secrete cytokines, which induce their immune-mediated clearance to prevent tumor initiation, a process termed "senescence surveillance." However, senescent hepatocytes give rise to hepatocellular carcinomas (HCCs), if the senescence program is bypassed or if senescent cells are not cleared. Here, we show context-specific roles for CCR2+ myeloid cells in liver cancer. Senescence surveillance requires the recruitment and maturation of CCR2+ myeloid cells, and CCR2 ablation caused outgrowth of HCC. In contrast, HCC cells block the maturation of recruited myeloid precursors, which, through NK cell inhibition, promote growth of murine HCC and worsen the prognosis and survival of human HCC patients. Thus, while senescent hepatocyte-secreted chemokines suppress liver cancer initiation, they may accelerate the growth of fully established HCC.

title: Senescent cells evade immune clearance via HLA-E-mediated NK and CD8+ T cell inhibition
abstract: Senescent cells accumulate in human tissues during ageing and contribute to age-related pathologies. The mechanisms responsible for their accumulation are unclear. Here we show that senescent dermal fibroblasts express the non-classical MHC molecule HLA-E, which interacts with the inhibitory receptor NKG2A expressed by NK and highly differentiated CD8+ T cells to inhibit immune responses against senescent cells. HLA-E expression is induced by senescence-associated secretary phenotype-related pro-inflammatory cytokines, and is regulated by p38 MAP kinase signalling in vitro. Consistently, HLA-E expression is increased on senescent cells in human skin sections from old individuals, when compared with those from young, and in human melanocytic nevi relative to normal skin. Lastly, blocking the interaction between HLA-E and NKG2A boosts immune responses against senescent cells in vitro. We thus propose that increased HLA-E expression contributes to persistence of senescent cells in tissues, thereby suggesting a new strategy for eliminating senescent cells during ageing.Senescent cells increase with ageing and may cause inflammatory conditions, but how this accumulation is mediated is still unclear. Here the authors show that senescent cells express HLA-E to suppress NKG2A-mediated natural killer and CD8 T cell activation to avoid targeted elimination, while blocking NKG2A helps promote immunity against senescent cells.

title: Targetable mechanisms driving immunoevasion of persistent senescent cells link chemotherapy-resistant cancer to aging.
abstract: Cellular senescence is a tumor suppressive mechanism that can paradoxically contribute to aging pathologies. Despite evidence of immune clearance in mouse models, it is not known how senescent cells (SnCs) persist and accumulate with age or in tumors in individuals. Here, we identify cooperative mechanisms that orchestrate the immunoevasion and persistence of normal and cancer human SnCs through extracellular targeting of natural killer receptor signaling. Damaged SnCs avoid immune recognition through MMPs-dependent shedding of NKG2D-ligands reinforced via paracrine suppression of NKG2D receptor-mediated immunosurveillance. These coordinated immunoediting processes are evident in residual, drug-resistant tumors from cohorts of >700 prostate and breast cancer patients treated with senescence-inducing genotoxic chemotherapies. Unlike in mice, these reversible senescence-subversion mechanisms are independent of p53/p16 and exacerbated in oncogenic RAS-induced senescence. Critically, the p16INK4A tumor suppressor can disengage the senescence growth arrest from the damage-associated immune senescence program, which is manifest in benign nevi lesions where indolent SnCs accumulate over time and preserve a non-pro-inflammatory tissue microenvironment maintaining NKG2D-mediated immunosurveillance. Our study shows how subpopulations of SnCs elude immunosurveillance, and reveals secretome-targeted therapeutic strategies to selectively eliminate -and restore the clearance of- the detrimental SnCs that actively persist after chemotherapy and accumulate at sites of aging pathologies.

title: Senescence associated secretory phenotype profile from primary lung mice fibroblasts depends on the senescence induction stimuli
abstract: Cellular senescence is a multifactorial phenomenon of growth arrest and distorted function, which has been recognized as an important feature during tumor suppression mechanisms and a contributor to aging. Senescent cells have an altered secretion pattern called Senescence-Associated Secretory Phenotype (SASP) that comprises a complex mix of factors including cytokines, growth factors, chemokines, and matrix metalloproteinases. SASP has been related with local inflammation that leads to cellular transformation and neurodegenerative diseases. Various pathways for senescence induction have been proposed; the most studied is replicative senescence due to telomere attrition called replicative senescence (RS). However, senescence can be prematurely achieved when cells are exposed to diverse stimuli such as oxidative stress (stress-induced premature senescence, SIPS) or proteasome inhibition (proteasome inhibition-induced premature senescence, PIIPS). SASP has been characterized in RS and SIPS but not in PIIPS. Hence, our aim was to determine SASP components in primary lung fibroblasts obtained from CD-1 mice induced to senescence by PIIPS and compare them to RS and SIPS. Our results showed important variations in the 62 cytokines analyzed, while SIPS and RS showed an increase in the secretion of most cytokines, and in PIIPS only 13 were incremented. Variations in glutathione-redox balance were also observed in SIPS and RS, and not in PIIPS. All senescence types SASP displayed a pro-inflammatory profile and increased proliferation in L929 mice fibroblasts exposed to SASP. However, the behavior observed was not exactly the same, suggesting that the senescence induction pathway might encompass dissimilar responses in adjacent cells and promote different outcomes.

title: A Proteomic Atlas of Senescence-Associated Secretomes for Aging Biomarker Development
abstract: The senescence-associated secretory phenotype (SASP) has recently emerged as both a driver of, and promising therapeutic target for, multiple age-related conditions, ranging from neurodegeneration to cancer. The complexity of the SASP, typically monitored by a few dozen secreted proteins, has been greatly underappreciated, and a small set of factors cannot explain the diverse phenotypes it produces in vivo. Here, we present ‘SASP Atlas’, a comprehensive proteomic database of soluble and exosome SASP factors originating from multiple senescence inducers and cell types. Each profile consists of hundreds of largely distinct proteins, but also includes a subset of proteins elevated in all SASPs. Based on our analyses, we propose several candidate biomarkers of cellular senescence, including GDF15, STC1 and SERPINs. This resource will facilitate identification of proteins that drive specific senescence-associated phenotypes and catalog potential senescence biomarkers to assess the burden, originating stimulus and tissue of senescent cells in vivo.

title: CDK4/6 inhibition triggers anti-tumor immunity
abstract: Cyclin-dependent kinases 4 and 6 (CDK4/6) are fundamental drivers of the cell cycle and are required for the initiation and progression of various malignancies. Pharmacological inhibitors of CDK4/6 have shown significant activity against several solid tumours. Their primary mechanism of action is thought to be the inhibition of phosphorylation of the retinoblastoma tumour suppressor, inducing G1 cell cycle arrest in tumour cells. Here we use mouse models of breast carcinoma and other solid tumours to show that selective CDK4/6 inhibitors not only induce tumour cell cycle arrest, but also promote anti-tumour immunity. We confirm this phenomenon through transcriptomic analysis of serial biopsies from a clinical trial of CDK4/6 inhibitor treatment for breast cancer. The enhanced anti-tumour immune response has two underpinnings. First, CDK4/6 inhibitors activate tumour cell expression of endogenous retroviral elements, thus increasing intracellular levels of double-stranded RNA. This in turn stimulates production of type III interferons and hence enhances tumour antigen presentation. Second, CDK4/6 inhibitors markedly suppress the proliferation of regulatory T cells. Mechanistically, the effects of CDK4/6 inhibitors both on tumour cells and on regulatory T cells are associated with reduced activity of the E2F target, DNA methyltransferase 1. Ultimately, these events promote cytotoxic T-cell-mediated clearance of tumour cells, which is further enhanced by the addition of immune checkpoint blockade. Our findings indicate that CDK4/6 inhibitors increase tumour immunogenicity and provide a rationale for new combination regimens comprising CDK4/6 inhibitors and immunotherapies as anti-cancer treatment.

title: Combined CDK4/6 and PI3Kα Inhibition Is Synergistic and Immunogenic in Triple-Negative Breast Cancer.
abstract: New treatments for triple-negative breast cancer (TNBC) are urgently needed. Despite there being little evidence of clinical activity as single-agent therapies, we show that dual blockade of PI3Kα and CDK4/6 is synergistically effective against multiple RB1-wild-type TNBC models. Combined PI3Kα and CDK4/6 inhibition significantly increased apoptosis, cell-cycle arrest, and tumor immunogenicity and generated immunogenic cell death in human TNBC cell lines. Combination treatment also significantly improved disease control in human xenograft models compared with either monotherapy. Combined PI3Kα and CDK4/6 inhibition significantly increased tumor-infiltrating T-cell activation and cytotoxicity and decreased the frequency of immunosuppressive myeloid-derived suppressor cells in a syngeneic TNBC mouse model. Notably, combined PI3Kα and CDK4/6 inhibition, along with inhibition of immune checkpoints PD-1 and CTLA-4, induced complete and durable regressions (>1 year) of established TNBC tumors in vivo Overall, our results illustrate convergent mechanisms of PI3Kα and CDK4/6 blockade on cell-cycle progression, DNA damage response, and immune-modulation and may provide a novel therapeutic approach for TNBC. ; 6340-52.

title: The CDK4/6 Inhibitor Abemaciclib Induces a T Cell Inflamed Tumor Microenvironment and Enhances the Efficacy of PD-L1 Checkpoint Blockade.
abstract: Abemaciclib, an inhibitor of cyclin dependent kinases 4 and 6 (CDK4/6), has recently been approved for the treatment of hormone receptor-positive breast cancer. In this study, we use murine syngeneic tumor models and in vitro assays to investigate the impact of abemaciclib on T cells, the tumor immune microenvironment and the ability to combine with anti-PD-L1 blockade. Abemaciclib monotherapy resulted in tumor growth delay that was associated with an increased T cell inflammatory signature in tumors. Combination with anti-PD-L1 therapy led to complete tumor regressions and immunological memory, accompanied by enhanced antigen presentation, a T cell inflamed phenotype, and enhanced cell cycle control. In vitro, treatment with abemaciclib resulted in increased activation of human T cells and upregulated expression of antigen presentation genes in MCF-7 breast cancer cells. These data collectively support the clinical investigation of the combination of abemaciclib with agents such as anti-PD-L1 that modulate T cell anti-tumor immunity.

title: Updated efficacy, safety, & PD-L1 status of patients with HR+, HER2- metastatic breast cancer administered abemaciclib plus pembrolizumab.
abstract: Abemaciclib is a selective inhibitor of CDK4 & 6 approved to treat HR+, HER2- metastatic breast cancer (MBC) patients (pts) as monotherapy and in combination with fulvestrant. In preclinical models, abemaciclib administered with anti-programmed death-ligand 1 (PD-L1) antibody therapy synergistically induced anti-tumor response and immunologic memory. A Phase I study (JPBJ, NCT02079636) of abemaciclib plus pembrolizumab (Merck & Co.), a programmed death receptor 1 (PD-1) antibody, demonstrated stable disease in 65% of pts with stage IV NSCLC along with a generally manageable safety profile.  JPCE is a multicenter, nonrandomized, open-label, Phase 1b study of abemaciclib plus pembrolizumab in pts with HR+, HER2- MBC or stage IV NSCLC. Key eligibility criteria for the MBC cohort were: HR+, HER2- MBC with 1 - 2 prior chemotherapy regimens, measurable disease, adequate organ function, ECOG PS ≤1, and no prior treatment with CDK4 & 6 or PD-1 & PD-L1 inhibitors. The primary objective was ...

title: Neutrophil extracellular traps produced during inflammation awaken dormant cancer cells in mice
abstract: Waking up in a trap Cancer patients who have undergone successful treatment can experience relapse of their disease years or even decades later. This is because cancer cells that have disseminated beyond the primary tumor site enter a state of dormancy, where they remain viable but not proliferating. Eventually, by mechanisms that are poorly understood, these clinically undetectable cells “wake up” and form actively growing metastases. Studying mouse models, Albrengues et al. found that sustained lung inflammation and the accompanying formation of neutrophil extracellular traps (NETs) could convert dormant cancer cells to aggressive lung metastases (see the Perspective by Aguirre-Ghiso). Awakening of these cells was associated with NET-mediated remodeling of the extracellular matrix and could be prevented by an antibody against the remodeled version of a matrix protein called laminin-111. Science, this issue p. eaao4227; see also p. 1314 In mice, neutrophil extracellular traps play a role in converting dormant cancer cells to actively growing metastases.  Most cancer patients die from cancer that recurs after spreading to a different tissue, rather than from their original tumor. After successful treatment of the original tumor, cancer cells that have disseminated to other sites can undergo dormancy, remaining viable but not proliferating. In breast, prostate, and other cancers, cancer cells can remain dormant and clinically undetectable for years and even decades before recurring, or awakening, as metastatic cancer. Little is known about what might initiate cancer awakening, and this in turn reduces our opportunities to prevent metastasis.  Epidemiological studies have suggested that inflammation is linked to a higher risk of breast cancer recurrence after a period of clinical dormancy. Smoking, which causes chronic lung inflammation, is also associated with a higher risk of recurrence. However, whether inflammation can cause awakening is not clear. Inflammatory cells, such as neutrophils, can provide many different signals that promote cancer progression. Neutrophils can kill harmful microorganisms by the release of neutrophil extracellular traps (NETs) into the extracellular space. NETs are scaffolds of DNA with associated cytotoxic proteins and proteases [e.g., neutrophil elastase (NE) and matrix metalloproteinase 9 (MMP9)]. NETs induced by bacteria or by cancer cells can promote metastasis, but the mechanism by which this occurs is not known. In this study, we tested whether NETs formed during lung inflammation could induce awakening.  We found that sustained experimental lung inflammation—induced by either tobacco smoke exposure or nasal instillation of lipopolysaccharide (LPS)—converted dormant cancer cells to aggressive lung metastases in mice. Both types of sustained inflammation also caused the formation of NETs. Inhibiting NET formation or digesting the NETs’ DNA scaffold prevented conversion of single disseminated cancer cells to growing metastases in mouse models of breast and prostate cancer. The NET DNA bound to the extracellular matrix (ECM) protein laminin, thus bringing two NET-associated proteases, NE and MMP9, to their substrate. This in turn facilitated a sequential cleavage of laminin, first by NE and then by MMP9. The NET-mediated proteolytic remodeling of laminin revealed an epitope that triggered proliferation of dormant cancer cells through integrin activation and FAK/ERK/MLCK/YAP signaling. We generated a blocking antibody against NET-remodeled laminin, and this antibody prevented or reduced tobacco smoke exposure– or LPS-induced inflammation from awakening dormant cancer cells in mice.  Our data implicate NETs and NET-mediated ECM remodeling as critical mediators of inflammation-induced awakening in mouse models of dormancy. We propose that NETs awaken cancer by concentrating neutrophil proteases at the ECM protein laminin, allowing for sequential proteolytic remodeling of laminin and leading to integrin-mediated signaling in the cancer cells. Our findings set the stage for epidemiological studies to test possible links among inflammation or smoking, NETs, and recurrence after dormancy in human patients. If such links can be established, we envision that approaches similar to the ones used in mouse models in our study could be used to target NETs and their downstream effectors to reduce the risk of cancer recurrence in human patients. NETs formed during lung inflammation awaken dormant cancer cells. Lung inflammation—caused, for instance, by tobacco smoke exposure—leads to NET formation. Two proteases on the NETs sequentially cleave the extracellular matrix protein laminin. This cleavage generates an epitope that activates integrin signaling and proliferation of the previously dormant cancer cells. Antibodies blocking the NET-generated laminin epitope prevent inflammation-induced awakening. Cancer cells from a primary tumor can disseminate to other tissues, remaining dormant and clinically undetectable for many years. Little is known about the cues that cause these dormant cells to awaken, resume proliferating, and develop into metastases. Studying mouse models, we found that sustained lung inflammation caused by tobacco smoke exposure or nasal instillation of lipopolysaccharide converted disseminated, dormant cancer cells to aggressively growing metastases. Sustained inflammation induced the formation of neutrophil extracellular traps (NETs), and these were required for awakening dormant cancer. Mechanistic analysis revealed that two NET-associated proteases, neutrophil elastase and matrix metalloproteinase 9, sequentially cleaved laminin. The proteolytically remodeled laminin induced proliferation of dormant cancer cells by activating integrin α3β1 signaling. Antibodies against NET-remodeled laminin prevented awakening of dormant cells. Therapies aimed at preventing dormant cell awakening could potentially prolong the survival of cancer patients.

title: Reawakening of dormant estrogen-dependent human breast cancer cells by bone marrow stroma secretory senescence
abstract: BackgroundDormant estrogen receptor positive (ER+) breast cancer micrometastases in the bone marrow survive adjuvant chemotherapy and recur stochastically for more than 20 years. We hypothesized that inflammatory cytokines produced by stromal injury can re-awaken dormant breast cancer cells.MethodsWe used an established in vitro dormancy model of Michigan Cancer Foundation-7 (MCF-7) breast cancer cells incubated at clonogenic density on fibronectin-coated plates to determine the effects of inflammatory cytokines on reactivation of dormant ER+ breast cancer cells. We measured induction of a mesenchymal phenotype, motility and the capacity to re-enter dormancy. We induced secretory senescence in murine stromal monolayers by oxidation, hypoxia and estrogen deprivation with hydrogen peroxide (H2O2), carbonyl-cyanide m-chlorophenylhydrazzone (CCCP) and Fulvestrant (ICI 182780), respectively, and determined the effects on growth of co-cultivated breast cancer cells.ResultsExogenous recombinant human (rh) interleukin (IL)-6, IL-8 or transforming growth factor β1 (TGFβ1) induced regrowth of dormant MCF-7 cells on fibronectin-coated plates. Dormant cells had decreased expression of E-cadherin and estrogen receptor α (ERα) and increased expression of N-cadherin and SNAI2 (SLUG). Cytokine or TGFβ1 treatment of dormant clones induced formation of growing clones, a mesenchymal appearance, increased motility and an impaired capacity to re-enter dormancy. Stromal injury induced secretion of IL-6, IL-8, upregulated tumor necrosis factor alpha (TNFα), activated TGFβ and stimulated the growth of co-cultivated MCF-7 cells. MCF-7 cells induced secretion of IL-6 and IL-8 by stroma in co-culture.ConclusionsDormant ER+ breast cancer cells have activated epithelial mesenchymal transition (EMT) gene expression programs and downregulated ERα but maintain a dormant epithelial phenotype. Stromal inflammation reactivates these cells, induces growth and a mesenchymal phenotype. Reactivated, growing cells have an impaired ability to re-enter dormancy. In turn, breast cancer cells co-cultured with stroma induce secretion of IL-6 and IL-8 by the stroma, creating a positive feedback loop.

title: Escape from therapy-induced accelerated cellular senescence in p53-null lung cancer cells and in human lung cancers.
abstract: Accelerated cellular senescence (ACS) has been described for tumor cells treated with chemotherapy and radiation. Following exposure to genotoxins, tumor cells undergo terminal growth arrest and adopt morphologic and marker features suggestive of cellular senescence. ACS is elicited by a variety of chemotherapeutic agents in the p53-null, p16-deficient human non-small cell H1299 carcinoma cells. After 10 to 21 days, infrequent ACS cells (1 in 10(6)) can bypass replicative arrest and reenter cell cycle. These cells express senescence markers and resemble the parental cells in their transcription profile. We show that these escaped H1299 cells overexpress the cyclin-dependent kinase Cdc2/Cdk1. The escape from ACS can be disrupted by Cdc2/Cdk1 kinase inhibitors or by knockdown of Cdc2/Cdk1 with small interfering RNA and can be promoted by expression of exogenous Cdc2/Cdk1. We also present evidence that ACS occurs in vivo in human lung cancer following induction chemotherapy. Viable tumors following chemotherapy also overexpress Cdc2/Cdk1. We propose that ACS is a mechanism of in vivo tumor response and that mechanisms aberrantly up-regulate Cdc2/Cdk1 promotes escape from the senescence pathway may be involved in a subset of tumors and likely accounts for tumor recurrence/progression.

title: Inflammaging: a new immune–metabolic viewpoint for age-related diseases
abstract: Ageing and age-related diseases share some basic mechanistic pillars that largely converge on inflammation. During ageing, chronic, sterile, low-grade inflammation — called inflammaging — develops, which contributes to the pathogenesis of age-related diseases. From an evolutionary perspective, a variety of stimuli sustain inflammaging, including pathogens (non-self), endogenous cell debris and misplaced molecules (self) and nutrients and gut microbiota (quasi-self). A limited number of receptors, whose degeneracy allows them to recognize many signals and to activate the innate immune responses, sense these stimuli. In this situation, metaflammation (the metabolic inflammation accompanying metabolic diseases) is thought to be the form of chronic inflammation that is driven by nutrient excess or overnutrition; metaflammation is characterized by the same mechanisms underpinning inflammaging. The gut microbiota has a central role in both metaflammation and inflammaging owing to its ability to release inflammatory products, contribute to circadian rhythms and crosstalk with other organs and systems. We argue that chronic diseases are not only the result of ageing and inflammaging; these diseases also accelerate the ageing process and can be considered a manifestation of accelerated ageing. Finally, we propose the use of new biomarkers (DNA methylation, glycomics, metabolomics and lipidomics) that are capable of assessing biological versus chronological age in metabolic diseases.Many mechanistic processes during ageing and age-related diseases cause inflammation. In this Review, the authors discuss the relationship between the immune and metabolic systems during ageing and age-related diseases and the potential use of new biomarkers capable of distinguishing between biological and chronological age in metabolic diseases. to geroscience, inflammation is one of the seven evolutionarily conserved mechanistic pillars of ageing that are shared by age-related diseases, including metabolic diseases.Inflammaging is the long-term result of the chronic physiological stimulation of the innate immune system, which can become damaging during ageing — a period of life largely unpredicted by evolution.Inflammaging is the by-product of the degeneracy of a few receptors that can sense a variety of non-self, self and quasi-self damage signals (or ‘garbage’) and activate the innate immune system.Inflammaging and metaflammation largely share the same molecular mechanisms, in which metaflammation can be conceptualized as a specific situation of chronic inflammation caused by nutrient excess.The gut microbiota has a central role in metaflammation and inflammaging, as it can release inflammatory products and contribute to the circadian rhythms and crosstalk with other organs and systems.Biomarkers of biological age, such as DNA methylation, glycomics, metabolomics and lipidomics, can be successfully applied to metabolic diseases.

title: The Achilles’ heel of senescent cells: from transcriptome to senolytic drugs
abstract: The healthspan of mice is enhanced by killing senescent cells using a transgenic suicide gene. Achieving the same using small molecules would have a tremendous impact on quality of life and the burden of age‐related chronic diseases. Here, we describe the rationale for identification and validation of a new class of drugs termed senolytics, which selectively kill senescent cells. By transcript analysis, we discovered increased expression of pro‐survival networks in senescent cells, consistent with their established resistance to apoptosis. Using siRNA to silence expression of key nodes of this network, including ephrins (EFNB1 or 3), PI3Kδ, p21, BCL‐xL, or plasminogen‐activated inhibitor‐2, killed senescent cells, but not proliferating or quiescent, differentiated cells. Drugs targeting these same factors selectively killed senescent cells. Dasatinib eliminated senescent human fat cell progenitors, while quercetin was more effective against senescent human endothelial cells and mouse BM‐MSCs. The combination of dasatinib and quercetin was effective in eliminating senescent MEFs. In vivo, this combination reduced senescent cell burden in chronologically aged, radiation‐exposed, and progeroid Ercc1−/Δ mice. In old mice, cardiac function and carotid vascular reactivity were improved 5 days after a single dose. Following irradiation of one limb in mice, a single dose led to improved exercise capacity for at least 7 months following drug treatment. Periodic drug administration extended healthspan in Ercc1−/∆ mice, delaying age‐related symptoms and pathology, osteoporosis, and loss of intervertebral disk proteoglycans. These results demonstrate the feasibility of selectively ablating senescent cells and the efficacy of senolytics for alleviating symptoms of frailty and extending healthspan.

title: Clearance of p16Ink4a-positive senescent cells delays ageing-associated disorders
abstract: Advanced age is the main risk factor for most chronic diseases and functional deficits in humans, but the fundamental mechanisms that drive ageing remain largely unknown, impeding the development of interventions that might delay or prevent age-related disorders and maximize healthy lifespan. Cellular senescence, which halts the proliferation of damaged or dysfunctional cells, is an important mechanism to constrain the malignant progression of tumour cells. Senescent cells accumulate in various tissues and organs with ageing and have been hypothesized to disrupt tissue structure and function because of the components they secrete. However, whether senescent cells are causally implicated in age-related dysfunction and whether their removal is beneficial has remained unknown. To address these fundamental questions, we made use of a biomarker for senescence, p16Ink4a, to design a novel transgene, INK-ATTAC, for inducible elimination of p16Ink4a-positive senescent cells upon administration of a drug. Here we show that in the BubR1 progeroid mouse background, INK-ATTAC removes p16Ink4a-positive senescent cells upon drug treatment. In tissues—such as adipose tissue, skeletal muscle and eye—in which p16Ink4a contributes to the acquisition of age-related pathologies, life-long removal of p16Ink4a-expressing cells delayed onset of these phenotypes. Furthermore, late-life clearance attenuated progression of already established age-related disorders. These data indicate that cellular senescence is causally implicated in generating age-related phenotypes and that removal of senescent cells can prevent or delay tissue dysfunction and extend healthspan.

title: Naturally occurring p16Ink4a-positive cells shorten healthy lifespan
abstract: Cellular senescence, a stress-induced irreversible growth arrest often characterized by expression of p16Ink4a (encoded by the Ink4a/Arf locus, also known as Cdkn2a) and a distinctive secretory phenotype, prevents the proliferation of preneoplastic cells and has beneficial roles in tissue remodelling during embryogenesis and wound healing. Senescent cells accumulate in various tissues and organs over time, and have been speculated to have a role in ageing. To explore the physiological relevance and consequences of naturally occurring senescent cells, here we use a previously established transgene, INK-ATTAC, to induce apoptosis in p16Ink4a-expressing cells of wild-type mice by injection of AP20187 twice a week starting at one year of age. We show that compared to vehicle alone, AP20187 treatment extended median lifespan in both male and female mice of two distinct genetic backgrounds. The clearance of p16Ink4a-positive cells delayed tumorigenesis and attenuated age-related deterioration of several organs without apparent side effects, including kidney, heart and fat, where clearance preserved the functionality of glomeruli, cardio-protective KATP channels and adipocytes, respectively. Thus, p16Ink4a-positive cells that accumulate during adulthood negatively influence lifespan and promote age-dependent changes in several organs, and their therapeutic removal may be an attractive approach to extend healthy lifespan.

title: Targeted Apoptosis of Senescent Cells Restores Tissue Homeostasis in Response to Chemotoxicity and Aging
abstract: The accumulation of irreparable cellular damage restricts healthspan after acute stress or natural aging. Senescent cells are thought to impair tissue function, and their genetic clearance can delay features of aging. Identifying how senescent cells avoid apoptosis allows for the prospective design of anti-senescence compounds to address whether homeostasis can also be restored. Here, we identify FOXO4 as a pivot in senescent cell viability. We designed a FOXO4 peptide that perturbs the FOXO4 interaction with p53. In senescent cells, this selectively causes p53 nuclear exclusion and cell-intrinsic apoptosis. Under conditions where it was well tolerated in vivo, this FOXO4 peptide neutralized doxorubicin-induced chemotoxicity. Moreover, it restored fitness, fur density, and renal function in both fast aging XpdTTD/TTD and naturally aged mice. Thus, therapeutic targeting of senescent cells is feasible under conditions where loss of health has already occurred, and in doing so tissue homeostasis can effectively be restored.

title: Clearance of senescent cells by ABT263 rejuvenates aged hematopoietic stem cells in mice
abstract: Senescent cells (SCs) accumulate with age and after genotoxic stress, such as total-body irradiation (TBI). Clearance of SCs in a progeroid mouse model using a transgenic approach delays several age-associated disorders, suggesting that SCs play a causative role in certain age-related pathologies. Thus, a 'senolytic' pharmacological agent that can selectively kill SCs holds promise for rejuvenating tissue stem cells and extending health span. To test this idea, we screened a collection of compounds and identified ABT263 (a specific inhibitor of the anti-apoptotic proteins BCL-2 and BCL-xL) as a potent senolytic drug. We show that ABT263 selectively kills SCs in culture in a cell type– and species-independent manner by inducing apoptosis. Oral administration of ABT263 to either sublethally irradiated or normally aged mice effectively depleted SCs, including senescent bone marrow hematopoietic stem cells (HSCs) and senescent muscle stem cells (MuSCs). Notably, this depletion mitigated TBI-induced premature aging of the hematopoietic system and rejuvenated the aged HSCs and MuSCs in normally aged mice. Our results demonstrate that selective clearance of SCs by a pharmacological agent is beneficial in part through its rejuvenation of aged tissue stem cells. Thus, senolytic drugs may represent a new class of radiation mitigators and anti-aging agents.

title: Local clearance of senescent cells attenuates the development of post-traumatic osteoarthritis and creates a pro-regenerative environment
abstract: Senescent cells (SnCs) accumulate in many vertebrate tissues with age and contribute to age-related pathologies, presumably through their secretion of factors contributing to the senescence-associated secretory phenotype (SASP). Removal of SnCs delays several pathologies and increases healthy lifespan. Aging and trauma are risk factors for the development of osteoarthritis (OA), a chronic disease characterized by degeneration of articular cartilage leading to pain and physical disability. Senescent chondrocytes are found in cartilage tissue isolated from patients undergoing joint replacement surgery, yet their role in disease pathogenesis is unknown. To test the idea that SnCs might play a causative role in OA, we used the p16-3MR transgenic mouse, which harbors a p16INK4a (Cdkn2a) promoter driving the expression of a fusion protein containing synthetic Renilla luciferase and monomeric red fluorescent protein domains, as well as a truncated form of herpes simplex virus 1 thymidine kinase (HSV-TK). This mouse strain allowed us to selectively follow and remove SnCs after anterior cruciate ligament transection (ACLT). We found that SnCs accumulated in the articular cartilage and synovium after ACLT, and selective elimination of these cells attenuated the development of post-traumatic OA, reduced pain and increased cartilage development. Intra-articular injection of a senolytic molecule that selectively killed SnCs validated these results in transgenic, non-transgenic and aged mice. Selective removal of the SnCs from in vitro cultures of chondrocytes isolated from patients with OA undergoing total knee replacement decreased expression of senescent and inflammatory markers while also increasing expression of cartilage tissue extracellular matrix proteins. Collectively, these findings support the use of SnCs as a therapeutic target for treating degenerative joint disease.

title: Synthetic lethal metabolic targeting of cellular senescence in cancer therapy
abstract: Activated oncogenes and anticancer chemotherapy induce cellular senescence, a terminal growth arrest of viable cells characterized by S-phase entry-blocking histone 3 lysine 9 trimethylation (H3K9me3). Although therapy-induced senescence (TIS) improves long-term outcomes, potentially harmful properties of senescent tumour cells make their quantitative elimination a therapeutic priority. Here we use the Eµ-myc transgenic mouse lymphoma model in which TIS depends on the H3K9 histone methyltransferase Suv39h1 to show the mechanism and therapeutic exploitation of senescence-related metabolic reprogramming in vitro and in vivo. After senescence-inducing chemotherapy, TIS-competent lymphomas but not TIS-incompetent Suv39h1– lymphomas show increased glucose utilization and much higher ATP production. We demonstrate that this is linked to massive proteotoxic stress, which is a consequence of the senescence-associated secretory phenotype (SASP) described previously. SASP-producing TIS cells exhibited endoplasmic reticulum stress, an unfolded protein response (UPR), and increased ubiquitination, thereby targeting toxic proteins for autophagy in an acutely energy-consuming fashion. Accordingly, TIS lymphomas, unlike senescence models that lack a strong SASP response, were more sensitive to blocking glucose utilization or autophagy, which led to their selective elimination through caspase-12- and caspase-3-mediated endoplasmic-reticulum-related apoptosis. Consequently, pharmacological targeting of these metabolic demands on TIS induction in vivo prompted tumour regression and improved treatment outcomes further. These findings unveil the hypercatabolic nature of TIS that is therapeutically exploitable by synthetic lethal metabolic targeting.

title: High-Throughput Functional Genetic and Compound Screens Identify Targets for Senescence Induction in Cancer.
abstract: Senescence is a proliferation arrest that can result from a variety of stresses. Cancer cells can also undergo senescence, but the stresses that provoke cancer cells to undergo senescence are unclear. Here, we use both functional genetic and compound screens in cancer cells harboring a reporter that is activated during senescence to find targets that induce senescence. We show that suppression of the SWI/SNF component SMARCB1 induces senescence in melanoma through strong activation of the MAP kinase pathway. From the compound screen, we identified multiple aurora kinase inhibitors as potent inducers of senescence in RAS mutant lung cancer. Senescent melanoma and lung cancer cells acquire sensitivity to the BCL2 family inhibitor ABT263. We propose a one-two punch approach for the treatment of cancer in which a drug is first used to induce senescence in cancer cells and a second drug is then used to kill senescent cancer cells.

title: Two-Step Senescence-Focused Cancer Therapies.
abstract: Damaged cells at risk of neoplastic transformation can be neutralized by apoptosis or engagement of the senescence program, which induces permanent cell-cycle arrest and a bioactive secretome that is implicated in tumor immunosurveillance. While from an evolutionary perspective senescence is beneficial in that it protects against malignancies, the accumulation of senescent cells in tissues and organs with aging and at sites of various pathologies is largely detrimental. Because induction of senescence in cancer cells is emerging as a therapeutic concept, it will be important to consider these detrimental effects, including tumor-promoting properties that may drive the formation of secondary tumors or cancer relapse. In this review we discuss the complex relationship between senescence and cancer, and highlight important considerations for therapeutics.

title: Senescence and senotherapeutics: a new field in cancer therapy
abstract: &NA; Cellular senescence is a stress response mechanism ensuring homeostasis. Its temporal activation during embryonic development or normal adult life is linked with beneficial properties. In contrast, persistent (chronic) senescence seems to exert detrimental effects fostering aging and age‐related disorders, such as cancer. Due to the lack of a reliable marker able to detect senescence in vivo, its precise impact in age‐related diseases is to a large extent still undetermined. A novel reagent termed GL13 (SenTraGorTM) that we developed, allowing senescence recognition in any type of biological material, emerges as a powerful tool to study the phenomenon of senescence in vivo. Exploiting the advantages of this novel methodological approach, scientists will be able to detect and connect senescence with aggressive behavior in human malignancies, such as tolerance to chemotherapy in classical Hodgkin Lymphoma and Langerhans Cell Histiocytosis. The latter depicts the importance of developing the new and rapidly expanding field of senotherapeutic agents targeting and driving to cell death senescent cells. We discuss in detail the current progress of this exciting area of senotherapeutics and suggest its future perspectives and applications.

title: Senolytics and senostatics as adjuvant tumour therapy
abstract: Cell senescence is a driver of ageing, frailty, age-associated disease and functional decline. In oncology, tumour cell senescence may contribute to the effect of adjuvant therapies, as it blocks tumour growth. However, this is frequently incomplete, and tumour cells that recover from senescence may gain a more stem-like state with increased proliferative potential. This might be exaggerated by the induction of senescence in the surrounding niche cells. Finally, senescence will spread through bystander effects, possibly overwhelming the capacity of the immune system to ablate senescent cells. This induces a persistent system-wide senescent cell accumulation, which we hypothesize is the cause for the premature frailty, multi-morbidity and increased mortality in cancer survivors. Senolytics, drugs that selectively kill senescent cells, have been developed recently and have been proposed as second-line adjuvant tumour therapy. Similarly, by blocking accelerated senescence following therapy, senolytics might prevent and potentially even revert premature frailty in cancer survivors. Adjuvant senostatic interventions, which suppress senescence-associated bystander signalling, might also have therapeutic potential. This becomes pertinent because treatments that are senostatic in vitro (e.g. dietary restriction mimetics) persistently reduce numbers of senescent cells in vivo, i.e. act as net senolytics in immunocompetent hosts.

title: A versatile drug delivery system targeting senescent cells
abstract: Senescent cells accumulate in multiple aging‐associated diseases, and eliminating these cells has recently emerged as a promising therapeutic approach. Here, we take advantage of the high lysosomal β‐galactosidase activity of senescent cells to design a drug delivery system based on the encapsulation of drugs with galacto‐oligosaccharides. We show that gal‐encapsulated fluorophores are preferentially released within senescent cells in mice. In a model of chemotherapy‐induced senescence, gal‐encapsulated cytotoxic drugs target senescent tumor cells and improve tumor xenograft regression in combination with palbociclib. Moreover, in a model of pulmonary fibrosis in mice, gal‐encapsulated cytotoxics target senescent cells, reducing collagen deposition and restoring pulmonary function. Finally, gal‐encapsulation reduces the toxic side effects of the cytotoxic drugs. Drug delivery into senescent cells opens new diagnostic and therapeutic applications for senescence‐associated disorders.

title: Exploiting interconnected synthetic lethal interactions between PARP inhibition and cancer cell reversible senescence
abstract: Senescence is a tumor suppression mechanism defined by stable proliferation arrest. Here we demonstrate that the known synthetic lethal interaction between poly(ADP-ribose) polymerase 1 inhibitors (PARPi) and DNA repair triggers p53-independent ovarian cancer cell senescence defined by senescence-associated phenotypic hallmarks including DNA-SCARS, inflammatory secretome, Bcl-XL-mediated apoptosis resistance, and proliferation restriction via Chk2 and p21 (CDKN1A). The concept of senescence as irreversible remains controversial and here we show that PARPi-senescent cells re-initiate proliferation upon drug withdrawal, potentially explaining the requirement for sustained PARPi therapy in the clinic. Importantly, PARPi-induced senescence renders ovarian and breast cancer cells transiently susceptible to second-phase synthetic lethal approaches targeting the senescence state using senolytic drugs. The combination of PARPi and a senolytic is effective in preclinical models of ovarian and breast cancer suggesting that coupling these synthetic lethalities provides a rational approach to their clinical use and may together be more effective in limiting resistance.Senescence induction is known to induce stable proliferation arrest. Here, the authors show that sustained PARP inhibition promotes a reversible p53-independent senescence, and that PARP inhibition is synthetic lethal when combined with senolytic agents in pre-clinical models of ovarian and breast cancer.

title: Identification and characterization of Cardiac Glycosides as senolytic compounds
abstract: Compounds with specific cytotoxic activity in senescent cells, or senolytics, support the causal involvement of senescence in aging and offer therapeutic interventions. Here we report the identification of Cardiac Glycosides (CGs) as a family of compounds with senolytic activity. CGs, by targeting the Na+/K+ATPase pump, cause a disbalanced electrochemical gradient within the cell causing depolarization and acidification. Senescent cells present a slightly depolarized plasma membrane and higher concentrations of H+, making them more susceptible to the action of CGs. These vulnerabilities can be exploited for therapeutic purposes as evidenced by the in vivo eradication of tumors xenografted in mice after treatment with the combination of a senogenic and a senolytic drug. The senolytic effect of CGs is also effective in the elimination of senescence-induced lung fibrosis. This experimental approach allows the identification of compounds with senolytic activity that could potentially be used to develop effective treatments against age-related diseases. Senolytic compounds have the ability to eliminate senescent cells from tissues and have been shown to be beneficial in various animal models of age-related diseases. Here the authors show that cardiac glycosides commonly used for heart diseases have senolytic properties in humanized mouse models of tumorigenesis and lung fibrosis.

title: Cardiac glycosides are broad-spectrum senolytics
abstract: Senescence is a cellular stress response that results in the stable arrest of old, damaged or pre-neoplastic cells. Oncogene-induced senescence is tumour suppressive but can also exacerbate tumorigenesis through the secretion of proinflammatory factors from senescent cells. Drugs that selectively kill senescent cells, termed ‘senolytics’, have proved beneficial in animal models of many age-associated diseases. In the present study, we show that the cardiac glycoside ouabain is a senolytic agent with broad activity. Senescent cells are sensitized to ouabain-induced apoptosis, a process mediated in part by induction of the proapoptotic Bcl-2 family protein NOXA. We demonstrate that cardiac glycosides synergize with anti-cancer drugs to kill tumour cells and eliminate senescent cells that accumulate after irradiation or in old mice. Ouabain also eliminates senescent pre-neoplastic cells. The findings of the present study suggest that cardiac glycosides may be effective anti-cancer drugs by acting through multiple mechanisms. Given the broad range of senescent cells targeted by cardiac glycosides, their use against age-related diseases warrants further exploration.In addition to having direct anti-cancer effects, the cardiac glycoside ouabain is shown to kill a broad range of senescent cells, thus suggesting that cardiac glycosides represent a novel class of senolytics.

title: Tumor cell senescence response produces aggressive variants
abstract: Tumors often respond favorably to initial chemotherapy but eventually relapse with drug resistance and increased metastatic potential. Cellular senescence is a major therapeutic outcome of cancer chemotherapy, which leads to tumor stasis or regression through immune clearance of senescent cells. However, senescent tumor cells have been shown to resume proliferation at low frequency. We found that subjecting arrested senescent tumor cells to cytotoxic treatments stimulates the clonogenic proliferation of remaining survivors. The senescence revertants showed a reduced rate of proliferation but increased migration and invasion potential in vitro, and increased tumorigenic potential in vivo. Gene expression profiling showed that the senescence revertants are distinct from both parental and senescent cells. A subset of senescence-activated genes remains active in the revertants. These genes are implicated in regulating cell motility, invasion, and metastasis, which may collectively contribute to the aggressiveness of the revertants. The findings suggest that although therapy-induced senescence has short-term benefits, the response also causes reprogramming of gene expression and activates invasion-related genes that accelerate tumor progression.

title: Senolytics in idiopathic pulmonary fibrosis: Results from a first-in-human, open-label, pilot study
abstract: Cellular senescence is a key mechanism that drives age-related diseases, but has yet to be targeted therapeutically in humans. Idiopathic pulmonary fibrosis (IPF) is a progressive, fatal cellular senescence-associated disease. Selectively ablating senescent cells using dasatinib plus quercetin (DQ) alleviates IPF-related dysfunction in bleomycin-administered mice.  A two-center, open-label study of intermittent DQ (D:100 mg/day, Q:1250 mg/day, three-days/week over three-weeks) was conducted in participants with IPF (n = 14) to evaluate feasibility of implementing a senolytic intervention. The primary endpoints were retention rates and completion rates for planned clinical assessments. Secondary endpoints were safety and change in functional and reported health measures. Associations with the senescence-associated secretory phenotype (SASP) were explored.  Fourteen patients with stable IPF were recruited. The retention rate was 100% with no DQ discontinuation; planned clinical assessments were complete in 13/14 participants. One serious adverse event was reported. Non-serious events were primarily mild-moderate, with respiratory symptoms (n = 16 total events), skin irritation/bruising (n = 14), and gastrointestinal discomfort (n = 12) being most frequent. Physical function evaluated as 6-min walk distance, 4-m gait speed, and chair-stands time was significantly and clinically-meaningfully improved (p < .05). Pulmonary function, clinical chemistries, frailty index (FI-LAB), and reported health were unchanged. DQ effects on circulat.ing SASP factors were inconclusive, but correlations were observed between change in function and change in SASP-related matrix-remodeling proteins, microRNAs, and pro-inflammatory cytokines (23/48 markers r ≥ 0.50).  Our first-in-humans open-label pilot supports study feasibility and provides initial evidence that senolytics may alleviate physical dysfunction in IPF, warranting evaluation of DQ in larger randomized controlled trials for senescence-related diseases.

title: Senolytics decrease senescent cells in humans: Preliminary report from a clinical trial of Dasatinib plus Quercetin in individuals with diabetic kidney disease
abstract: Senescent cells, which can release factors that cause inflammation and dysfunction, the senescence-associated secretory phenotype (SASP), accumulate with ageing and at etiological sites in multiple chronic diseases. Senolytics, including the combination of Dasatinib and Quercetin (D + Q), selectively eliminate senescent cells by transiently disabling pro-survival networks that defend them against their own apoptotic environment. In the first clinical trial of senolytics, D + Q improved physical function in patients with idiopathic pulmonary fibrosis (IPF), a fatal senescence-associated disease, but to date, no peer-reviewed study has directly demonstrated that senolytics decrease senescent cells in humans.  In an open label Phase 1 pilot study, we administered 3 days of oral D 100 mg and Q 1000 mg to subjects with diabetic kidney disease (N = 9; 68·7 ± 3·1 years old; 2 female; BMI:33·9 ± 2·3 kg/m2; eGFR:27·0 ± 2·1 mL/min/1·73m2). Adipose tissue, skin biopsies, and blood were collected before and 11 days after completing senolytic treatment. Senescent cell and macrophage/Langerhans cell markers and circulating SASP factors were assayed.  D + Q reduced adipose tissue senescent cell burden within 11 days, with decreases in p16INK4A-and p21CIP1-expressing cells, cells with senescence-associated β-galactosidase activity, and adipocyte progenitors with limited replicative potential. Adipose tissue macrophages, which are attracted, anchored, and activated by senescent cells, and crown-like structures were decreased. Skin epidermal p16INK4A+ and p21CIP1+ cells were reduced, as were circulating SASP factors, including IL-1α, IL-6, and MMPs-9 and −12.  “Hit-and-run” treatment with senolytics, which in the case of D + Q have elimination half-lives <11 h, significantly decreases senescent cell burden in humans. Fund NIH and Foundations.  Frailty, and Mesenchymal Stem Cell Functionality in Chronic Kidney Disease: Effect of Senolytic Agents.