Tumour Cell Secretome in Chemoresistance and Tumour Recurrence
Authors: Emma C. Madden, Adrienne M. Gorman, Susan E. Logue, Afshin Samali
Journal: Trends in cancer (2020)

Chemotherapy is a common treatment for patients with cancer. Tumour resistance to chemotherapy (chemoresistance) results in tumour regrowth and nonresponsiveness to further chemotherapeutic challenge.Cancer cells release protumorigenic factors, termed the TCS, to prevent chemotherapy-dependent cytotoxicity.Chemotherapy exposure can also change the types and abundance of components in the TCS, a phenomenon known as therapy-induced TCS.The TCS and therapy-induced TCS promote tumour recurrence by enabling cancer cell survival and the expansion of CSCs, and preventing antitumour immunity. Chemoresistance is a major factor driving tumour relapse and the high rates of cancer-related deaths. Understanding how cancer cells overcome chemotherapy-induced cell death is critical in promoting patient survival. One emerging mechanism of chemoresistance is the tumour cell secretome (TCS), an array of protumorigenic factors released by tumour cells. Chemotherapy exposure can also alter the composition of the TCS, known as therapy-induced TCS, and can promote tumour relapse and the formation of an immunosuppressive tumour microenvironment (TME). Here, we outline how the TCS can protect cancer cells from chemotherapy-induced cell death. We also highlight recent evidence describing how therapy-induced TCS can impact cancer stem cell (CSC) expansion and tumour-associated immune cells to enable tumour regrowth and antitumour immunity. Chemoresistance is a major factor driving tumour relapse and the high rates of cancer-related deaths. Understanding how cancer cells overcome chemotherapy-induced cell death is critical in promoting patient survival. One emerging mechanism of chemoresistance is the tumour cell secretome (TCS), an array of protumorigenic factors released by tumour cells. Chemotherapy exposure can also alter the composition of the TCS, known as therapy-induced TCS, and can promote tumour relapse and the formation of an immunosuppressive tumour microenvironment (TME). Here, we outline how the TCS can protect cancer cells from chemotherapy-induced cell death. We also highlight recent evidence describing how therapy-induced TCS can impact cancer stem cell (CSC) expansion and tumour-associated immune cells to enable tumour regrowth and antitumour immunity. Chemotherapy is a common and successful therapeutic strategy for many patients with cancer as a neoadjuvant or adjuvant treatment to surgery. However, a significant number of patients relapse months or years later with a tumour that is resistant to further chemotherapeutic challenge, a feature known as chemoresistance. Many relapsing tumours are also associated with distant metastasis, which is the leading cause of cancer-related deaths [1.Chaffer C.L. Weinberg R.A. 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Cancer. 2013; 13: 714-726Crossref PubMed Scopus (2212) Google Scholar]. Several key cellular processes are involved in the development of chemoresistance, including drug activation or inactivation, DNA damage repair, modifications to drug targets, enhanced drug efflux, inactivation of apoptosis machinery, and increased autophagy [3.Holohan C. et al.Cancer drug resistance: an evolving paradigm.Nat. Rev. Cancer. 2013; 13: 714-726Crossref PubMed Scopus (2212) Google Scholar,4.Cree I.A. Charlton P. Molecular chess? Hallmarks of anti-cancer drug resistance.BMC Cancer. 2017; 17: 1-8Crossref PubMed Scopus (55) Google Scholar]. Tumour heterogeneity also contributes to a resistant phenotype due to the presence of genetically diverse clones within a tumour. Each of these populations may respond differently to treatment, resulting in the emergence of drug resistant clones. CSCs are a small subpopulation of cancer cells found within the tumour that are another cellular source of chemoresistance. These cells have similar characteristics to nontransformed stem cells, such as the ability to self-renew, express the embryonic factors Oct4, Sox2, and Nanog, and are capable of differentiating into committed tumour cells. Due to their inherent quiescent state, CSCs can evade the actions of drugs that target rapidly proliferating cells, and can limit drug toxicity due to increased expression of aldehyde dehydrogenase (ALDH), drug efflux pumps, and prosurvival proteins [5.Dagogo-Jack I. Shaw A.T. Tumour heterogeneity and resistance to cancer therapies.Nat. Rev. Clin. Oncol. 2018; 15: 81-94Crossref PubMed Scopus (480) Google Scholar,6.Barbato L. et al.Cancer stem cells and targeted therapies.Cells. 2019; 8: 926Crossref Google Scholar]. Ultimately, these characteristics allow CSCs to repopulate the tumour, resulting in tumour relapse and chemoresistance. In addition to these mechanisms, the tumour establishes the TME to promote tumour progression and chemoresistance. Secreted factors from tumour cells, also known as the TCS, enable the recruitment of various cell types required to form the TME, such as mesenchymal stromal cells, immune cells, and vascular endothelial cells, among others (Box 1). The TCS is defined as the collection of soluble proteins and insoluble vesicles secreted by the cancer cell into the extracellular space [7.Paltridge J.L. et al.The secretome in cancer progression.Biochim. Biophys. Acta Proteins Proteomics. 2013; 1834: 2233-2241Crossref Scopus (54) Google Scholar]. The TCS is critical for cell–cell communication and is utilised to influence the function of cells in the TME and shape the premetastatic niche [8.Karagiannis G.S. et al.Cancer secretomics reveal pathophysiological pathways in cancer molecular oncology.Mol. Oncol. 2010; 4: 496-510Crossref PubMed Scopus (97) Google Scholar]. Emerging evidence is establishing the TCS as a major contributor to the development of chemoresistance. In this review, we discuss the recent literature describing how components of the TCS confer resistance to cancer cells to prevent chemotherapy-induced cell death. We also describe important emerging evidence showing that chemotherapy can rewire the TCS to confer tumour cell resistance and how this further impacts cells within the TME to promote tumour regrowth post therapy.Box 1Cellular Components of the Tumour MicroenvironmentShaping of the TME during early stages of tumorigenesis supports essential processes required for cancer progression. Bone marrow-endothelial progenitors and nearby vascular endothelial cells are recruited to the tumour and stimulated to sprout new blood vessels (angiogenesis) by tumour-secreted factors, such as VEGF and basic fibroblast growth factor (bFGF) [132.Weis S.M. Cheresh D.A. Tumor angiogenesis: molecular pathways and therapeutic targets.Nat. Med. 2011; 17: 1359-1370Crossref PubMed Scopus (1109) Google Scholar]. This process is not only critical for supplying the tumour with oxygen and essential nutrients for growth, but also provides an avenue for circulating immune cells and bone marrow-derived cells to reach the TME. Stromal cells, such as mesenchymal stromal cells (MSCs), are recruited from the bone marrow through various chemokines, such as TGF-β, C-X-C motif chemokine ligand 12 (CXCL12), and platelet-derived growth factor (PDGF) and have been demonstrated to promote CSC expansion, immune cell recruitment, and metastasis [133.Chaturvedi P. et al.Hypoxia-inducible factor-dependent signaling between triple-negative breast cancer cells and mesenchymal stem cells promotes macrophage recruitment.Proc. Natl. Acad. Sci. U. S. 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Med. 2018; 215: 3075-3093Crossref PubMed Scopus (57) Google Scholar, 137.Goetz J.G. et al.Biomechanical remodeling of the microenvironment by stromal caveolin-1 favors tumor invasion and metastasis.Cell. 2011; 146: 148-163Abstract Full Text Full Text PDF PubMed Scopus (430) Google Scholar, 138.Feig C. et al.Targeting CXCL12 from FAP-expressing carcinoma-associated fibroblasts synergizes with anti-PD-L1 immunotherapy in pancreatic cancer.Proc. Natl. Acad. Sci. U. S. A. 2013; 110: 20212-20217Crossref PubMed Scopus (671) Google Scholar, 139.Lotti F. et al.Chemotherapy activates cancer-associated fibroblasts to maintain colorectal cancer-initiating cells by IL-17A.J. Exp. Med. 2013; 210: 2851Crossref PubMed Scopus (166) Google Scholar]. Immune cells, such as monocytes, TAMs, MDSCs, tumour-associated neutrophils (TANs), dendritic cells, natural killer (NK) cells, B cells, cluster of differentiation (CD) 4+ and CD8+ T cells, and Tregs all form the tumour immune microenvironment in various combinations, in both solid and haematological cancers. Tumour-associated immune cells promote tumour growth and metastasis and sculpt an immunosuppressive environment to enable tumour-immune evasion. For example, CTLs have been observed to be excluded from the tumour due to a surrounding 'border' of TAMs, hypothetically preventing CTL infiltration [140.Beatty G.L. et al.Exclusion of T cells from pancreatic carcinomas in mice is regulated by Ly6Clow F4/80+ extratumoral macrophages.Gastroenterology. 2015; 149: 201-210Abstract Full Text Full Text PDF PubMed Google Scholar]. By contrast, other cancers can be infiltrated with CTLs, but these CTLs are 'exhausted' due to interactions with immunosuppressive programmed death ligand 1 (PDL1+) tumour cells and leukocytes within the tumour, resulting in defective immune mediated-cancer cell killing [141.Iwai Y. et al.Involvement of PD-L1 on tumor cells in the escape from host immune system and tumor immunotherapy by PD-L1 blockade.Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 12293-12297Crossref PubMed Scopus (1671) Google Scholar]. Overall, these various cell types promote cell survival under chemotherapy and evade antitumour immunity by supporting an immunosuppressive TME. Shaping of the TME during early stages of tumorigenesis supports essential processes required for cancer progression. Bone marrow-endothelial progenitors and nearby vascular endothelial cells are recruited to the tumour and stimulated to sprout new blood vessels (angiogenesis) by tumour-secreted factors, such as VEGF and basic fibroblast growth factor (bFGF) [132.Weis S.M. Cheresh D.A. Tumor angiogenesis: molecular pathways and therapeutic targets.Nat. Med. 2011; 17: 1359-1370Crossref PubMed Scopus (1109) Google Scholar]. This process is not only critical for supplying the tumour with oxygen and essential nutrients for growth, but also provides an avenue for circulating immune cells and bone marrow-derived cells to reach the TME. Stromal cells, such as mesenchymal stromal cells (MSCs), are recruited from the bone marrow through various chemokines, such as TGF-β, C-X-C motif chemokine ligand 12 (CXCL12), and platelet-derived growth factor (PDGF) and have been demonstrated to promote CSC expansion, immune cell recruitment, and metastasis [133.Chaturvedi P. et al.Hypoxia-inducible factor-dependent signaling between triple-negative breast cancer cells and mesenchymal stem cells promotes macrophage recruitment.Proc. Natl. Acad. Sci. U. S. A. 2014; 111: E2120-E2129Crossref PubMed Scopus (108) Google Scholar, 134.Luo J. et al.Infiltrating bone marrow mesenchymal stem cells increase prostate cancer stem cell population and metastatic ability via secreting cytokines to suppress androgen receptor signaling.Oncogene. 2014; 33: 2768-2778Crossref PubMed Scopus (63) Google Scholar, 135.Ren G. et al.CCR2-dependent recruitment of macrophages by tumor-educated mesenchymal stromal cells promotes tumor development and is mimicked by TNFα.Cell Stem Cell. 2012; 11: 812-824Abstract Full Text Full Text PDF PubMed Scopus (173) Google Scholar]. Tumour-MSCs can also differentiate into CAFs, which aid in ECM deposition and remodelling, and support tumour growth, metastasis, and chemoresistance [136.Raz Y. et al.Bone marrow-derived fibroblasts are a functionally distinct stromal cell population in breast cancer.J. Exp. Med. 2018; 215: 3075-3093Crossref PubMed Scopus (57) Google Scholar, 137.Goetz J.G. et al.Biomechanical remodeling of the microenvironment by stromal caveolin-1 favors tumor invasion and metastasis.Cell. 2011; 146: 148-163Abstract Full Text Full Text PDF PubMed Scopus (430) Google Scholar, 138.Feig C. et al.Targeting CXCL12 from FAP-expressing carcinoma-associated fibroblasts synergizes with anti-PD-L1 immunotherapy in pancreatic cancer.Proc. Natl. Acad. Sci. U. S. A. 2013; 110: 20212-20217Crossref PubMed Scopus (671) Google Scholar, 139.Lotti F. et al.Chemotherapy activates cancer-associated fibroblasts to maintain colorectal cancer-initiating cells by IL-17A.J. Exp. Med. 2013; 210: 2851Crossref PubMed Scopus (166) Google Scholar]. Immune cells, such as monocytes, TAMs, MDSCs, tumour-associated neutrophils (TANs), dendritic cells, natural killer (NK) cells, B cells, cluster of differentiation (CD) 4+ and CD8+ T cells, and Tregs all form the tumour immune microenvironment in various combinations, in both solid and haematological cancers. Tumour-associated immune cells promote tumour growth and metastasis and sculpt an immunosuppressive environment to enable tumour-immune evasion. For example, CTLs have been observed to be excluded from the tumour due to a surrounding 'border' of TAMs, hypothetically preventing CTL infiltration [140.Beatty G.L. et al.Exclusion of T cells from pancreatic carcinomas in mice is regulated by Ly6Clow F4/80+ extratumoral macrophages.Gastroenterology. 2015; 149: 201-210Abstract Full Text Full Text PDF PubMed Google Scholar]. By contrast, other cancers can be infiltrated with CTLs, but these CTLs are 'exhausted' due to interactions with immunosuppressive programmed death ligand 1 (PDL1+) tumour cells and leukocytes within the tumour, resulting in defective immune mediated-cancer cell killing [141.Iwai Y. et al.Involvement of PD-L1 on tumor cells in the escape from host immune system and tumor immunotherapy by PD-L1 blockade.Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 12293-12297Crossref PubMed Scopus (1671) Google Scholar]. Overall, these various cell types promote cell survival under chemotherapy and evade antitumour immunity by supporting an immunosuppressive TME. The healthy secretome comprises cytokines, growth factors, hormones, coagulation factors, enzymes, glycoproteins, and extracellular vesicles (EVs). A finely tuned secretome is essential for maintaining physiological homeostasis and represents a large fraction of the proteins synthesised in highly secretory tissues; for example, 70% of the transcripts in the pancreas encode secretory proteins [9.Uhlén M. et al.Tissue-based map of the human proteome.Science. 2015; 347: 1260419Crossref PubMed Google Scholar]. For secretory proteins to reach the plasma membrane, an N-terminal signal sequence is generated during protein synthesis to target them to the classical secretory pathway [10.Blobel G. Dobberstein B. Transfer of proteins across membranes: I. Presence of proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-bound ribosomes of murine myeloma.J. Cell Biol. 1975; 67: 835-851Crossref PubMed Google Scholar,11.Farhan H. Rabouille C. Signalling to and from the secretory pathway.J. Cell Sci. 2011; 124: 171-180Crossref PubMed Scopus (79) Google Scholar]. 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For example, a recent study demonstrated altered expression of tumour secretome genes across 32 cancer types compared with adjacent-normal tissue or healthy tissue from the same donor organ [16.Robinson J.L. et al.A systematic investigation of the malignant functions and diagnostic potential of the cancer secretome.Cell Rep. 2019; 26: 2622-2635Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. The most commonly changed genes encoding components of the TCS tend to be downregulated compared with adjacent normal or healthy tissue, and are implicated in tumour suppression and inhibition of cell–cell and/or cell–matrix interactions. Increased TCS genes are less commonly shared across cancer types, but those that are encode extracellular matrix (ECM) structural proteins and modifiers, signalling proteins, such as receptors, and promoters of vascularisation [16.Robinson J.L. et al.A systematic investigation of the malignant functions and diagnostic potential of the cancer secretome.Cell Rep. 2019; 26: 2622-2635Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. Cancers that exhibited an overall reduction in the expression of tissue-specific secretory genes also had reduced unfolded protein response (UPR) activation, which is an adaptive mechanism used to reduce the protein processing burden in the endoplasmic reticulum (ER), the entry point of the secretory pathway [17.Almanza A. et al.Endoplasmic reticulum stress signalling - from basic mechanisms to clinical applications.FEBS J. 2019; 286: 241-278Crossref PubMed Scopus (138) Google Scholar]. For some cancer types, a general reduction in tissue-specific secretome transcripts may be a way to limit secretory stress to allow cancer cells to survive. For other cancer types, such as multiple myeloma, there may be a global reduction in secretory transcripts, but a disproportionate increase in a minority of transcripts, such as immunoglobulins, leading to UPR activation (reviewed in [18.Nikesitch N. et al.Endoplasmic reticulum stress in the development of multiple myeloma and drug resistance.Clin. Transl. Immunol. 2018; 7e1007Crossref PubMed Scopus (12) Google Scholar]). Taken together, this not only highlights the need for cancer cells to have a functioning secretory pathway to ensure cell survival and growth, but also demonstrates the potential of targeting the secretory pathway as an anticancer therapy. The most abundant components of the TCS are cytokines, growth factors, enzymes, glycoproteins, and EVs. Since the 1990s, the contribution of the cytokine family in mediating tumour relapse has been extensively studied. Early studies demonstrated that expression of the interleukin (IL) family of cytokines, such as IL-6 and IL-8, strongly correlated with tumour recurrence and poor responsiveness to therapy in various cancers [19.Zhang G. Adachi I. Serum interleukin-6 levels correlate to tumor progression and prognosis in metastatic breast carcinoma.Anticancer Res. 1999; 19: 1427-1432PubMed Google Scholar,20.Yuan A. et al.Interleukin-8 messenger ribonucleic acid expression correlates with tumor progression, tumor angiogenesis, patient survival, and timing of relapse in non-small-cell lung cancer.Am. J. Respir. Crit. Care Med. 2000; 162: 1957-1963Crossref PubMed Google Scholar]. 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For example, BMP-7 protects oral squamous cancer cells from cetuximab through SMAD1, -5, and -8 signalling processes [31.Yin J. et al.Inhibition of BMP signaling overcomes acquired resistance to cetuximab in oral squamous cell carcinomas.Cancer Lett. 2018; 414: 181-189Crossref PubMed Scopus (0) Google Scholar], and BMP–BMP receptor signalling confers resistance to EGFR-mutated lung squamous cancer cells through the Akt/mTOR signalling kinase p70S6K [32.Wang Z. et al.Activation of the BMP-BMPR pathway conferred resistance to EGFR-TKIs in lung squamous cell carcinoma patients with EGFR mutations.Proc. Natl. Acad. Sci. U. S. A. 2015; 112: 9990-9995Crossref PubMed Scopus (24) Google Scholar]. This highlights the potential of stratifying squamous cancers based on BMP levels to predict patients who would benefit from anti-EGFR therapy. Secreted enzymes not only degrade the ECM to enable the migration and invasion of tumour cells, but can also free trapped secreted factors within the ECM, further enhancing the actions of the secretome in protumorigenic processes. Matrix metalloproteinase proteins (MMPs) are key proteases involved in ECM degradation and aid in tumour cell migration and invasion, angiogenesis, and immune evasion [33.Gialeli C. et al.Roles of matrix metalloproteinases in cancer progression and their pharmacological targeting.FEBS J. 2011; 278: 16-27Crossref PubMed Scopus (973) Google Scholar]. 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Although the mechanism of TIMP1-mediated chemoresistance was not fully explored in this model, it is potentially through mediators downstream of IL-6 signalling, such as STAT3 or NF-κB. Glycosylated proteins, or glycoproteins, are proteins with a covalently linked carbohydrate (glycan) and are abundantly found in the TCS. The stress-induced chaperone clusterin is a glycoprotein required for correct protein folding, protein homeostasis, and cell death mechanisms, and is implicated in many disease states, including Alzheimer's disease and cancer [41.Wilson M.R. Zoubeidi A. Clusterin as a therapeutic target.Expert Opin. Ther. Targets. 2017; 21: 201-213Crossre