Hippo Signaling in Cancer: Lessons From Drosophila Models

Hippo pathway was initially identified through genetic screens for genes regulating organ size in fruitflies. Recent studies have highlighted the role of Hippo signaling as a key regulator of homeostasis, and in tumorigenesis. Hippo pathway is comprised of genes that act as tumor suppressor genes like hippo (hpo) and warts (wts), and oncogenes like yorkie (yki). YAP and TAZ are two related mammalian homologs of Drosophila Yki that act as effectors of the Hippo pathway. Hippo signaling deficiency can cause YAP- or TAZ-dependent oncogene addiction for cancer cells. YAP and TAZ are often activated in human malignant cancers. These transcriptional regulators may initiate tumorigenic changes in solid tumors by inducing cancer stem cells and proliferation, culminating in metastasis and chemo-resistance. Given the complex mechanisms (e.g., of the cancer microenvironment, and the extrinsic and intrinsic cues) that overpower YAP/TAZ inhibition, the molecular roles of the Hippo pathway in tumor growth and progression remain poorly defined. Here we review recent findings from studies in whole animal model organism like Drosophila on the role of Hippo signaling regarding its connection to inflammation, tumor microenvironment, and other oncogenic signaling in cancer growth and progression.

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title: Understanding resistance to targeted cancer drugs through loss of function genetic screens.
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title: The Hippo pathway in organ size control, tissue regeneration and stem cell self-renewal
abstract: Precise control of organ size is crucial during animal development and regeneration. In Drosophila and mammals, studies over the past decade have uncovered a critical role for the Hippo tumour-suppressor pathway in the regulation of organ size. Dysregulation of this pathway leads to massive overgrowth of tissue. The Hippo signalling pathway is highly conserved and limits organ size by phosphorylating and inhibiting the transcription co-activators YAP and TAZ in mammals and Yki in Drosophila, key regulators of proliferation and apoptosis. The Hippo pathway also has a critical role in the self-renewal and expansion of stem cells and tissue-specific progenitor cells, and has important functions in tissue regeneration. Emerging evidence shows that the Hippo pathway is regulated by cell polarity, cell adhesion and cell junction proteins. In this review we summarize current understanding of the composition and regulation of the Hippo pathway, and discuss how cell polarity and cell adhesion proteins inform the role of this pathway in organ size control and regeneration.

title: Dissecting tumour heterogeneity in flies: genetic basis of interclonal oncogenic cooperation.
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title: The Hippo-YAP pathway: new connections between regulation of organ size and cancer.
abstract: The control of organ size is a basic biological question. In the past several years, the Hippo signaling pathway has been delineated and shown to be crucial in control of organ size in both Drosophila and mammals. Acting downstream of the Hippo pathway is the Yki/YAP/TAZ transcription co-activators. In mammalian cells, the Hippo pathway kinase cascade inhibits YAP and its paralog TAZ by phosphorylation and promotion of their cytoplasmic localization. The TEAD family transcription factors have recently been identified as evolutionarily conserved key mediators of YAP biological functions. yap is a candidate oncogene, and several other components of the Hippo pathway are tumor suppressors. Dysregulation of the Hippo pathway contributes to the loss of contact inhibition observed in cancer cells. Therefore, the Hippo-YAP pathway connects the regulation of organ size and tumorigenesis.

title: Upstream paths for Hippo signaling in Drosophila organ development
abstract: Organ growth is fundamental to animal development. One of major mechanisms for growth control is mediated by the conserved Hippo signaling pathway initially identified in Drosophila. The core of this pathway in Drosophila consists of a cascade of protein kinases Hippo and Warts that negatively regulate transcriptional coactivator Yorkie (Yki). Activation of Yki promotes cell survival and proliferation to induce organ growth. A key issue in Hippo signaling is to understand how core kinase cascade is activated. Activation of Hippo kinase cascade is regulated in the upstream by at least two transmembrane proteins Crumbs and Fat that act in parallel. These membrane proteins interact with additional factors such as FERM-domain proteins Expanded and Merlin to modulate subcellular localization and function of the Hippo kinase cascade. Hippo signaling is also influenced by cytoskeletal networks and cell tension in epithelia of developing organs. These upstream events in the regulation of Hippo signaling are only partially understood. This review focuses on our current understanding of some upstream processes involved in Hippo signaling in developing Drosophila organs.

title: The history and regulatory mechanism of the Hippo pathway
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title: Hippo signaling: growth control and beyond
abstract: The Hippo pathway has emerged as a conserved signaling pathway that is essential for the proper regulation of organ growth in Drosophila and vertebrates. Although the mechanisms of signal transduction of the core kinases Hippo/Mst and Warts/Lats are relatively well understood, less is known about the upstream inputs of the pathway and about the downstream cellular and developmental outputs. Here, we review recently discovered mechanisms that contribute to the dynamic regulation of Hippo signaling during Drosophila and vertebrate development. We also discuss the expanding diversity of Hippo signaling functions during development, discoveries that shed light on a complex regulatory system and provide exciting new insights into the elusive mechanisms that regulate organ growth and regeneration.

title: Wingless/Wnt signaling in Drosophila: The pattern and the pathway
abstract: Wnt signaling generates pattern in all animal embryos, from flies and worms to humans, and promotes the undifferentiated, proliferative state critical for stem cells in adult tissues. Inappropriate Wnt pathway activation is the major cause of colorectal cancers, a leading cause of cancer death in humans. Although this pathway has been studied extensively for years, large gaps remain in our understanding of how it switches on and off, and how its activation changes cellular behaviors. Much of what is known about the pathway comes from genetic studies in Drosophila, where a single Wnt molecule, encoded by wingless (wg), directs an array of cell‐fate decisions similar to those made by the combined activities of all 19 Wnt family members in vertebrates. Although Wg specifies fate in many tissues, including the brain, limbs, and major organs, the fly embryonic epidermis has proven to be a very powerful system for dissecting pathway activity. It is a simple, accessible tissue, with a pattern that is highly sensitive to small changes in Wg pathway activity. This review discusses what we have learned about Wnt signaling from studying mutations that disrupt epidermal pattern in the fly embryo, highlights recent advances and controversies in the field, and sets these issues in the context of questions that remain about how this essential signaling pathway functions.  2013.

title: The Drosophila tumor suppressor gene warts encodes a homolog of human myotonic dystrophy kinase and is required for the control of cell shape and proliferation.
abstract: Homozygous loss of the warts (wts) gene of Drosophila, caused by mitotic recombination in somatic cells, leads to the formation of cell clones that are fragmented, rounded, and greatly overgrown compared with normal controls. Therefore, the gene is required for the control of the amount and direction of cell proliferation as well as for normal morphogenesis. The absence of wts function also results in apical hypertrophy of imaginal disc epithelial cells. Secretion of cuticle over and between the domed apical surfaces of these cells leads to a honeycomb-like structure and gives the superficial wart-like phenotype of mitotic clones on the adult. One wts allele allows survival of homozygotes to the late larval stage, and these larvae show extensive imaginal disc overgrowth. Because of the excess growth and abnormalities of differentiation that follow homozygous loss, we consider wts to be a tumor suppressor gene. The wts gene is defined by the breakpoints of overlapping deficiencies in the right telomeric region of chromosome 3, region 100A, and by lethal P-element insertions and excisions. It encodes a protein kinase that is most similar to human myotonic dystrophy kinase, the Neurospora cot-1 protein kinase, two cell-cycle regulated kinases of yeast, and several putative kinases from plants. These proteins define a new subfamily of protein kinases that are closely related to but distinct from the cyclic AMP-dependent kinases. Although myotonic dystrophy is defined by a neuromuscular disorder, it is sometimes associated with multiple pilomatrixomas, which are otherwise rare epithelial tumors, and with other tumors including neurofibromas and parathyroid adenomas. Our results raise the possibility that homozygous loss of the myotonic dystrophy kinase may contribute to the development of these tumors.

title: Identifying tumor suppressors in genetic mosaics: the Drosophila lats gene encodes a putative protein kinase.
abstract: We have identified recessive overproliferation mutations by screening and examining clones of mutant cells in genetic mosaics of the fruitfly Drosophila melanogaster. This type of screen provides a powerful approach for identifying and studying potential tumor suppressors. One of the identified genes, lats, has been cloned and encodes a putative protein kinase that shares high levels of sequence similarity with three proteins in budding yeast and Neurospora that are involved in regulation of the cell cycle and growth. Mutations in lats cause dramatic overproliferation phenotypes and various developmental defects in both mosaic animals and homozygous mutants.

title: Shar-pei mediates cell proliferation arrest during imaginal disc growth in Drosophila
abstract: During animal development, organ size is determined primarily by the amount of cell proliferation, which must be tightly regulated to ensure the generation of properly proportioned organs. However, little is known about the molecular pathways that direct cells to stop proliferating when an organ has attained its proper size. We have identified mutations in a novel gene, shar-pei, that is required for proper termination of cell proliferation during Drosophila imaginal disc development. Clones of shar-pei mutant cells in imaginal discs produce enlarged tissues containing more cells of normal size. We show that this phenotype is the result of both increased cell proliferation and reduced apoptosis. Hence, shar-pei restricts cell proliferation and promotes apoptosis. By contrast, shar-pei is not required for cell differentiation and pattern formation of adult tissue. Shar-pei is also not required for cell cycle exit during terminal differentiation, indicating that the mechanisms directing cell proliferation arrest during organ growth are distinct from those directing cell cycle exit during terminal differentiation. shar-pei encodes a WW-domain-containing protein that has homologs in worms, mice and humans, suggesting that mechanisms of organ growth control are evolutionarily conserved.

title: salvador Promotes Both Cell Cycle Exit and Apoptosis in Drosophila and Is Mutated in Human Cancer Cell Lines
abstract: The number of cells in an organism is determined by regulating both cell proliferation and cell death. Relatively few mechanisms have been identified that can modulate both of these processes. In a screen for Drosophila mutations that result in tissue overgrowth, we identified salvador (sav), a gene that promotes both cell cycle exit and cell death. Elevated Cyclin E and DIAP1 levels are found in mutant cells, resulting in delayed cell cycle exit and impaired apoptosis. Salvador contains two WW domains and binds to the Warts (or LATS) protein kinase. The human ortholog of salvador (hWW45) is mutated in three cancer cell lines. Thus, salvador restricts cell numbers in vivo by functioning as a dual regulator of cell proliferation and apoptosis.

title: The Drosophila Mst Ortholog, hippo, Restricts Growth and Cell Proliferation and Promotes Apoptosis
abstract: Establishing and maintaining homeostasis is critical to the well-being of an organism and is determined by the balance of cell proliferation and death. Two genes that function together to regulate growth, proliferation, and apoptosis in Drosophila are warts (wts), encoding a serine/threonine kinase, and salvador (sav), encoding a WW domain containing Wts-interacting protein. However, the mechanisms by which sav and wts regulate growth and apoptosis are not well understood. Here, we describe mutations in hippo (hpo), which encodes a protein kinase most related to mammalian Mst1 and Mst2. Like wts and sav, hpo mutations result in increased tissue growth and impaired apoptosis characterized by elevated levels of the cell cycle regulator cyclin E and apoptosis inhibitor DIAP1. Hpo, Sav, and Wts interact physically and functionally, and regulate DIAP1 levels, likely by Hpo-mediated phosphorylation and subsequent degradation. Thus, Hpo links Sav and Wts to a key regulator of apoptosis.

title: The Drosophila Ste20 family kinase dMST functions as a tumor suppressor by restricting cell proliferation and promoting apoptosis.
abstract: In a genetic screen for mutations that restrict cell growth and organ size, we identified a new tumor suppressor gene, dMST, which encodes the Drosophila homolog of the mammalian Ste20 kinase family members MST1 and MST2. Loss-of-function mutations in dMST result in overgrown tissues containing more cells of normal size. dMST mutant cells exhibit elevated levels of Cyclin E and DIAP1, increased cell growth and proliferation, and impaired apoptosis. dMST forms a complex with Sav and Wts, two tumor suppressors also implicated in regulating both cell proliferation and apoptosis, suggesting that they act in common pathways.

title: The Salvador partner Hippo promotes apoptosis and cell-cycle exit in Drosophila
abstract: Tissue growth during animal development is tightly controlled so that the organism can develop harmoniously. The salvador (sav) gene, which encodes a scaffold protein, has been shown to restrict cell number by coordinating cell-cycle exit and apoptosis during Drosophila development. Here we identify Hippo (Hpo), the Drosophila orthologue of the mammalian MST1 and MST2 serine/threonine kinases, as a partner of Sav. Loss of hpo function leads to sav-like phenotypes, whereas gain of hpo function results in the opposite phenotype. Whereas Sav and Hpo normally restrict cellular quantities of the Drosophila inhibitor of apoptosis protein DIAP1, overexpression of Hpo destabilizes DIAP1 in cell culture. We show that DIAP1 is phosphorylated in a Hpo-dependent manner in S2 cells and that Hpo can phosphorylate DIAP1 in vitro. Thus, Hpo may promote apoptosis by reducing cellular amounts of DIAP1. In addition, we show that Sav is an unstable protein that is stabilized by Hpo. We propose that Hpo and Sav function together to restrict tissue growth in vivo.

title: Hippo promotes proliferation arrest and apoptosis in the Salvador/Warts pathway
abstract: Proliferation and apoptosis must be precisely regulated to form organs with appropriate cell numbers and to avoid tumour growth. Here we show that Hippo (Hpo), the Drosophila homologue of the mammalian Ste20-like kinases, MST1/2, promotes proper termination of cell proliferation and stimulates apoptosis during development. hpo mutant tissues are larger than normal because mutant cells continue to proliferate beyond normal tissue size and are resistant to apoptotic stimuli that usually eliminate extra cells. Hpo negatively regulates expression of Cyclin E to restrict cell proliferation, downregulates the Drosophila inhibitor of apoptosis protein DIAP1, and induces the proapoptotic gene head involution defective (hid) to promote apoptosis. The mutant phenotypes of hpo are similar to those of warts (wts), which encodes a serine/threonine kinase of the myotonic dystrophy protein kinase family, and salvador (sav), which encodes a WW domain protein that binds to Wts. We find that Sav binds to a regulatory domain of Hpo that is essential for its function, indicating that Hpo acts together with Sav and Wts in a signalling module that coordinately regulates cell proliferation and apoptosis.

title: hippo Encodes a Ste-20 Family Protein Kinase that Restricts Cell Proliferation and Promotes Apoptosis in Conjunction with salvador and warts
abstract: The coordination between cell proliferation and cell death is essential to maintain homeostasis within multicellular organisms. The mechanisms underlying this regulation are yet to be completely understood. Here, we report the identification of hippo (hpo) as a gene that regulates both cell proliferation and cell death in Drosophila. hpo encodes a Ste-20 family protein kinase that binds to and phosphorylates the tumor suppressor protein Salvador (Sav), which is known to interact with the Warts (Wts) protein kinase. Loss of hpo results in elevated transcription of the cell cycle regulator cyclin E and the cell-death inhibitor diap1, leading to increased proliferation and reduced apoptosis. Further, we show that hpo, sav, and wts define a pathway that regulates diap1 at the transcriptional level. A human homolog of hpo completely rescues the overgrowth phenotype of Drosophila hpo mutants, suggesting that hpo might play a conserved role for growth control in mammals.

title: Cloning and Characterization of a Human Protein Kinase with Homology to Ste20 (*)
abstract: A human protein kinase (termed MST1) has been cloned and characterized. The MST1 catalytic domain is most homologous to Ste20 and other Ste20-like kinases (62-65% similar). MST1 is expressed ubiquitously, and the MST1 protein is present in all human cell lines examined. Biochemical characterization of MST1 catalytic activity demonstrates that it is a serine/threonine kinase, and that it can phosphorylate an exogenous substrate as well as itself in an in vitro kinase assay. Further characterization of the protein indicates MST1 activity increases approximately 3-4-fold upon treatment with PP2A, suggesting that MST1 is negatively regulated by phosphorylation. MST1 activity decreases approximately 2-fold upon treatment with epidermal growth factor; however, overexpression of MST1 does not affect extracellular signal-regulated kinase-1 and −2 activation. MST1 is unaffected by heat shock or high osmolarity, indicating that it is not involved in the stress-activated or high osmolarity glycerol signal transduction pathways. Thus MST1, although homologous to a member of a yeast MAPK cascade, is not involved in the regulation of a known mammalian MAPK pathway and potentially regulates a novel signaling cascade.

title: The Ste20 group kinases as regulators of MAP kinase cascades.
abstract: Ste20p (sterile 20 protein) is a putative yeast mitogen-activated protein kinase kinase kinase kinase (MAP4K) involved in the mating pathway. Its homologs in mammals, Drosophila, Caenorhabditis elegans and other organisms make up a large emerging group of protein kinases including 28 members in human. The Ste20 group kinases are further divided into the p21-activated kinase (PAK) and germinal center kinase (GCK) families. They are characterized by the presence of a conserved kinase domain and a noncatalytic region of great structural diversity that enables the kinases to interact with various signaling molecules and regulatory proteins of the cytoskeleton. This review describes the phylogenetic relationships of the Ste20 group kinases based on discussions with many researchers in this field. With the newly established phylogenetic relationships, crucial arguments can be advanced regarding the functions of these kinases as upstream activators of the MAPK pathways and possible activity as MAP4Ks. Their involvement in apoptosis, morphogenesis and cytoskeletal rearrangements is also discussed.

title: From Cell Structure to Transcription: Hippo Forges a New Path
abstract: The control of cell number during animal development is a longstanding puzzle. Recent studies in the fruit fly Drosophila melanogaster have defined a new signaling pathway that restricts cell proliferation in differentiating epithelia. The cytoskeletal proteins Merlin and Expanded, which play a role in cell adhesion and structure, control the activation of the Hippo/Salvador kinase complex, which in turn activates the Warts/Mats kinase complex. Warts/Mats kinase phosphorylates and inhibits Yorkie, a transcriptional coactivator that positively regulates cell growth, survival, and proliferation. This conserved signaling pathway contains several tumor-suppressor genes and regulates the contact inhibition of proliferation in cultured cells.

title: The Hippo Signaling Pathway Coordinately Regulates Cell Proliferation and Apoptosis by Inactivating Yorkie, the Drosophila Homolog of YAP
abstract: Coordination between cell proliferation and cell death is essential to maintain homeostasis in multicellular organisms. In Drosophila, these two processes are regulated by a pathway involving the Ste20-like kinase Hippo (Hpo) and the NDR family kinase Warts (Wts; also called Lats). Hpo phosphorylates and activates Wts, which in turn, through unknown mechanisms, negatively regulates the transcription of cell-cycle and cell-death regulators such as cycE and diap1. Here we identify Yorkie (Yki), the Drosophila ortholog of the mammalian transcriptional coactivator yes-associated protein (YAP), as a missing link between Wts and transcriptional regulation. Yki is required for normal tissue growth and diap1 transcription and is phosphorylated and inactivated by Wts. Overexpression of yki phenocopies loss-of-function mutations of hpo or wts, including elevated transcription of cycE and diap1, increased proliferation, defective apoptosis, and tissue overgrowth. Thus, Yki is a critical target of the Wts/Lats protein kinase and a potential oncogene.

title: Yes-associated protein (YAP65) is a proline-rich phosphoprotein that binds to the SH3 domain of the Yes proto-oncogene product.
abstract: Yes belongs to the Src family of protein-tyrosine kinases. In order to understand molecular aspects of its signaling, we decided to isolate proteins that bind to the modulatory region of the Yes molecule. By generating anti-idiotypic antibodies against the aminoterminal domain of the Yes protein, we have identified, characterized and cloned a cDNA for a novel protein that binds to the Src homology domain 3 (SH3) of the Yes proto-oncogene product. The protein is of 65 kiloDalton (kDa) molecular mass, it is phosphorylated in vivo on serine and is particularly rich in proline. We named it YAP65 for Yes-Associated Protein of 65 kDa. Within the YAP65 sequence, we identified a motif, PVKQPPPLAP, similar to that found in proteins that bind to the SH3 domain of the Abl kinase. Competition assays with synthetic peptides showed the involvement of the predicted proline-rich sequence in binding between YAP65 and the Yes kinase. The YAP65 protein was also shown to bind to other signaling molecules that contain SH3 domains including Nck, Crk and Src. At lower stoichiometry, YAP65 was also shown to bind to the SH3 domains of Abl and guanosine triphosphatase activating protein (GAP). Further characterization of YAP65 should illuminate Yes signaling pathways and could also identify a novel link between protein-tyrosine and serine kinases.

title: TAZ: a novel transcriptional co‐activator regulated by interactions with 14‐3‐3 and PDZ domain proteins
abstract: The highly conserved and ubiquitously expressed 14‐3‐3 proteins regulate differentiation, cell cycle progression and apoptosis by binding intracellular phosphoproteins involved in signal transduction. By screening in vitro translated cDNA pools for the ability to bind 14‐3‐3, we identified a novel transcriptional co‐activator, TAZ (transcriptional co‐activator with PDZ‐binding motif) as a 14‐3‐3‐binding molecule. TAZ shares homology with Yes‐associated protein (YAP), contains a WW domain and functions as a transcriptional co‐activator by binding to the PPXY motif present on transcription factors. 14‐3‐3 binding requires TAZ phosphorylation on a single serine residue, resulting in the inhibition of TAZ transcriptional co‐activation through 14‐3‐3‐mediated nuclear export. The C‐terminus of TAZ contains a highly conserved PDZ‐binding motif that localizes TAZ into discrete nuclear foci and is essential for TAZ‐stimulated gene transcription. TAZ uses this same motif to bind the PDZ domain‐containing protein NHERF‐2, a molecule that tethers plasma membrane ion channels and receptors to cytoskeletal actin. TAZ may link events at the plasma membrane and cytoskeleton to nuclear transcription in a manner that can be regulated by 14‐3‐3.

title: Identification of Novel Human WW Domain-containing Proteins by Cloning of Ligand Targets*
abstract: A recently described protein module consisting of 35–40 semiconserved residues, termed the WW domain, has been identified in a number of diverse proteins including dystrophin and Yes-associated protein (YAP). Two putative ligands of YAP, termed WBP-1 and WBP-2, have been found previously to contain several short peptide regions consisting of PPPPY residues (PY motif) that mediate binding to the WW domain of YAP. Although the function(s) of the WW domain remain to be elucidated, these observations strongly support a role for the WW domain in protein-protein interactions. Here we report the isolation of three novel human cDNAs encoding a total of nine WW domains, using a newly developed approach termed COLT (cloningof ligand targets), in which the rapid cloning of modular protein domains is accomplished by screening cDNA expression libraries with specific peptide ligands. Two of the new genes identified appear to be members of a family of proteins, including Rsp5 and Nedd-4, which have ubiquitin-protein ligase activity. In addition, we demonstrate that peptides corresponding to PY and PY-like motifs present in several known signaling or regulatory proteins, including RasGAP, AP-2, p53BP-2 (p53-bindingprotein-2), interleukin-6 receptor-α, chloride channel CLCN5, and epithelial sodium channel ENaC, can selectively bind to certain of these novel WW domains.

title: YAP, TAZ, and Yorkie: a conserved family of signal-responsive transcriptional coregulators in animal development and human disease.
abstract: How extracellular cues are transduced to the nucleus is a fundamental issue in biology. The paralogous WW-domain proteins YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif; also known as WWTR1, for WW-domain containing transcription regulator 1) constitute a pair of transducers linking cytoplasmic signaling events to transcriptional regulation in the nucleus. A cascade composed of mammalian Ste20-like (MST) and large tumor suppressor (LATS) kinases directs multisite phosphorylation, promotes 14-3-3 binding, and hinders nuclear import of YAP and TAZ, thereby inhibiting their transcriptional coactivator and growth-promoting activities. A similar cascade regulates the trafficking and function of Yorkie, the fly orthologue of YAP. Mammalian YAP and TAZ are expressed in various tissues and serve as coregulators for transcriptional enhancer factors (TEFs; also referred to as TEADs, for TEA-domain proteins), runt-domain transcription factors (Runxs), peroxisome proliferator-activated receptor gamma (PPARgamma), T-box transcription factor 5 (Tbx5), and several others. YAP and TAZ play distinct roles during mouse development. Both, and their upstream regulators, are intimately linked to tumorigenesis and other pathogenic processes. Here, we review studies on this family of signal-responsive transcriptional coregulators and emphasize how relative sequence conservation predicates their function and regulation, to provide a conceptual framework for organizing available information and seeking new knowledge about these signal transducers.

title: Newcomers to the WW Domain-Mediated Network of the Hippo Tumor Suppressor Pathway.
abstract: The Hippo tumor suppressor pathway regulates the size of organs by controlling 2 opposing processes: proliferation and apoptosis. The pathway was originally defined in Drosophila, but it is well conserved in mammals. One of the unique features of Hippo signaling is the unusually wide occurrence of WW domains and its cognate PPxY ligand motifs within components of this pathway. Recently, it was proposed that the prevalence of WW domain-mediated complexes in the Hippo signaling pathway should facilitate its molecular analysis and help in the identification of new components of the Hippo-centered network. Indeed, several new members of the Hippo pathway, which form functional complexes with WW domains of YAP and TAZ effectors, were recently described. We focus here on 2 families of such proteins, angiomotins and SMADs, plus 1 regulatory factor, WBP-2, which together shed new light on the rapidly expanding Hippo network. Since the Hippo pathway acts as a tumor suppressor pathway, the complexes described here, which assemble on WW domains of YAP and TAZ, represent potential targets of cancer therapy.

title: Physical Interaction with Yes-associated Protein Enhances p73 Transcriptional Activity*
abstract: Specific protein-protein interactions are involved in a large number of cellular processes and are mainly mediated by structurally and functionally defined domains. Here we report that the nuclear phosphoprotein p73 can engage in a physical association with the Yes-associated protein (YAP). This association occurs under physiological conditions as shown by reciprocal co-immunoprecipitation of complexes from lysates of P19 cells. The WW domain of YAP and the PPPPY motif of p73 are directly involved in the association. Furthermore, as required for ligands to group I WW domains, the terminal tyrosine (Y) of the PPPPYmotif of p73 was shown to be essential for the association with YAP. Unlike p73α, p73β, and p63α, which bind to YAP, the endogenous as well as exogenously expressed wild-type p53 (wt-p53) and the p73γ isoform do not interact with YAP. Indeed, we documented that YAP interacts only with those members of the p53 family that have a well conserved PPXY motif, a target sequence for WW domains. Overexpression of YAP causes an increase of p73α transcriptional activity. Differential interaction of YAP with members of the p53 family may provide a molecular explanation for their functional divergence in signaling.

title: The YAP and TAZ transcription co-activators: key downstream effectors of the mammalian Hippo pathway.
abstract: The Hippo signaling pathway was initially defined by genetic studies in Drosophila to regulate tissue growth and organ size [1,2]. This pathway is highly conserved in mammals and dysregulation of the Hippo pathway has been implicated in human cancer. Although the exact extracellular signal that controls the Hippo pathway is currently unknown, compelling evidence supports a critical role of the Hippo pathway in cell contact inhibition, which is a property commonly lost in cancer cells. Many molecules, such as the merlin tumor suppressor protein, have been identified as regulating the activity of the core Hippo pathway components . Acting downstream are two key transcription co-activators, YAP and TAZ, which mediate the major gene regulation and biological functions of the Hippo pathway. This article will focus on the physiological function and molecular regulation of YAP/TAZ and its Drosophila homolog Yki.

title: The Hippo Signaling Network and Its Biological Functions.
abstract: Hippo signaling is an evolutionarily conserved network that has a central role in regulating cell proliferation and cell fate to control organ growth and regeneration. It promotes activation of the LATS kinases, which control gene expression by inhibiting the activity of the transcriptional coactivator proteins YAP and TAZ in mammals and Yorkie in Drosophila. Diverse upstream inputs, including both biochemical cues and biomechanical cues, regulate Hippo signaling and enable it to have a key role as a sensor of cells' physical environment and an integrator of growth control signals. Several components of this pathway localize to cell-cell junctions and contribute to regulation of Hippo signaling by cell polarity, cell contacts, and the cytoskeleton. Downregulation of Hippo signaling promotes uncontrolled cell proliferation, impairs differentiation, and is associated with cancer. We review the current understanding of Hippo signaling and highlight progress in the elucidation of its regulatory mechanisms and biological functions.

title: Elucidation of a Universal Size-Control Mechanism in Drosophila and Mammals
abstract: Coordination of cell proliferation and cell death is essential to attain proper organ size during development and for maintaining tissue homeostasis throughout postnatal life. In Drosophila, these two processes are orchestrated by the Hippo kinase cascade, a growth-suppressive pathway that ultimately antagonizes the transcriptional coactivator Yorkie (Yki). Here we demonstrate that a single phosphorylation site in Yki mediates the growth-suppressive output of the Hippo pathway. Hippo-mediated phosphorylation inactivates Yki by excluding it from the nucleus, whereas loss of Hippo signaling leads to nuclear accumulation and therefore increased Yki activity. We further delineate a mammalian Hippo signaling pathway that culminates in the phosphorylation of YAP, the mammalian homolog of Yki. Using a conditional YAP transgenic mouse model, we demonstrate that the mammalian Hippo pathway is a potent regulator of organ size, and that its dysregulation leads to tumorigenesis. These results uncover a universal size-control mechanism in metazoan.

title: TEAD mediates YAP-dependent gene induction and growth control.
abstract: The YAP transcription coactivator has been implicated as an oncogene and is amplified in human cancers. Recent studies have established that YAP is phosphorylated and inhibited by the Hippo tumor suppressor pathway. Here we demonstrate that the TEAD family transcription factors are essential in mediating YAP-dependent gene expression. TEAD is also required for YAP-induced cell growth, oncogenic transformation, and epithelial-mesenchymal transition. CTGF is identified as a direct YAP target gene important for cell growth. Moreover, the functional relationship between YAP and TEAD is conserved in Drosophila Yki (the YAP homolog) and Scalloped (the TEAD homolog). Our study reveals TEAD as a new component in the Hippo pathway playing essential roles in mediating biological functions of YAP.

title: WW domain-mediated interaction with Wbp2 is important for the oncogenic property of TAZ
abstract: The transcriptional co-activators YAP and TAZ are downstream targets inhibited by the Hippo tumor suppressor pathway. YAP and TAZ both possess WW domains, which are important protein–protein interaction modules that mediate interaction with proline-rich motifs, most commonly PPXY. The WW domains of YAP have complex regulatory roles as exemplified by recent reports showing that they can positively or negatively influence YAP activity in a cell and context-specific manner. In this study, we show that the WW domain of TAZ is important for it to transform both MCF10A and NIH3T3 cells and to activate transcription of ITGB2 but not CTGF, as introducing point mutations into the WW domain of TAZ (WWm) abolished its transforming and transcription-promoting ability. Using a proteomic approach, we discovered potential regulatory proteins that interact with TAZ WW domain and identified Wbp2. The interaction of Wbp2 with TAZ is dependent on the WW domain of TAZ and the PPXY-containing C-terminal region of Wbp2. Knockdown of endogenous Wbp2 suppresses, whereas overexpression of Wbp2 enhances, TAZ-driven transformation. Forced interaction of WWm with Wbp2 by direct C-terminal fusion of full-length Wbp2 or its TAZ-interacting C-terminal domain restored the transforming and transcription-promoting ability of TAZ. These results suggest that the WW domain-mediated interaction with Wbp2 promotes the transforming ability of TAZ.

title: WW domain interactions regulate the Hippo tumor suppressor pathway
abstract: The Hippo kinase pathway is emerging as a conserved signaling pathway that is essential for organ growth and tumorigenesis in Drosophila and mammalians. Although the signaling of the core kinases is relatively well understood, less is known about the upstream inputs, downstream outputs and regulation of the whole cascade. Enrichment of the Hippo pathway components with WW domains and their cognate proline-rich interacting motifs provides a versatile platform for further understanding the mechanisms that regulate organ growth and tumorigenesis. Here, we review recently discovered mechanisms of WW domain-mediated interactions that contribute to the regulation of the Hippo signaling pathway in tumorigenesis. We further discuss new insights and future directions on the emerging role of such regulation.

title: Mechanisms of Hippo pathway regulation.
abstract: The Hippo pathway was initially identified in Drosophila melanogaster screens for tissue growth two decades ago and has been a subject extensively studied in both Drosophila and mammals in the last several years. The core of the Hippo pathway consists of a kinase cascade, transcription coactivators, and DNA-binding partners. Recent studies have expanded the Hippo pathway as a complex signaling network with >30 components. This pathway is regulated by intrinsic cell machineries, such as cell-cell contact, cell polarity, and actin cytoskeleton, as well as a wide range of signals, including cellular energy status, mechanical cues, and hormonal signals that act through G-protein-coupled receptors. The major functions of the Hippo pathway have been defined to restrict tissue growth in adults and modulate cell proliferation, differentiation, and migration in developing organs. Furthermore, dysregulation of the Hippo pathway leads to aberrant cell growth and neoplasia. In this review, we focus on recent developments in our understanding of the molecular actions of the core Hippo kinase cascade and discuss key open questions in the regulation and function of the Hippo pathway.

title: Control of Cell Proliferation and Apoptosis by Mob as Tumor Suppressor, Mats
abstract: Appropriate cell number and organ size in a multicellular organism are determined by coordinated cell growth, proliferation, and apoptosis. Disruption of these processes can cause cancer. Recent studies have identified the Large tumor suppressor (Lats)/Warts (Wts) protein kinase as a key component of a pathway that controls the coordination between cell proliferation and apoptosis. Here we describe growth inhibitory functions for a Mob superfamily protein, termed Mats (Mob as tumor suppressor), in Drosophila. Loss of Mats function results in increased cell proliferation, defective apoptosis, and induction of tissue overgrowth. We show that mats and wts function in a common pathway. Mats physically associates with Wts to stimulate the catalytic activity of the Wts kinase. A human Mats ortholog (Mats1) can rescue the lethality associated with loss of Mats function in Drosophila. As Mats1 is mutated in human tumors, Mats-mediated growth inhibition and tumor suppression is likely conserved in humans.

title: Mob as tumor suppressor is activated by Hippo kinase for growth inhibition in Drosophila
abstract: Tissue growth and organ size are determined by coordinated cell proliferation and apoptosis in development. Recent studies have demonstrated that Hippo (Hpo) signaling plays a crucial role in coordinating these processes by restricting cell proliferation and promoting apoptosis. Here we provide evidence that the Mob as tumor suppressor protein, Mats, functions as a key component of the Hpo signaling pathway. We found that Mats associates with Hpo in a protein complex and is a target of the Hpo serine/threonine protein kinase. Mats phosphorylation by Hpo increases its affinity with Warts (Wts)/large tumor suppressor (Lats) serine/threonine protein kinase and ability to upregulate Wts catalytic activity to target downstream molecules such as Yorkie (Yki). Consistently, our epistatic analysis suggests that mats acts downstream of hpo. Coexpression analysis indicated that Mats can indeed potentiate Hpo‐mediated growth inhibition in vivo. Our results support a model in which Mats is activated by Hpo through phosphorylation for growth inhibition, and this regulatory mechanism is conserved from flies to mammals.

title: Inactivation of YAP oncoprotein by the Hippo pathway is involved in cell contact inhibition and tissue growth control.
abstract: The Hippo pathway plays a key role in organ size control by regulating cell proliferation and apoptosis in Drosophila. Although recent genetic studies have shown that the Hippo pathway is regulated by the NF2 and Fat tumor suppressors, the physiological regulations of this pathway are unknown. Here we show that in mammalian cells, the transcription coactivator YAP (Yes-associated protein), is inhibited by cell density via the Hippo pathway. Phosphorylation by the Lats tumor suppressor kinase leads to cytoplasmic translocation and inactivation of the YAP oncoprotein. Furthermore, attenuation of this phosphorylation of YAP or Yorkie (Yki), the Drosophila homolog of YAP, potentiates their growth-promoting function in vivo. Moreover, YAP overexpression regulates gene expression in a manner opposite to cell density, and is able to overcome cell contact inhibition. Inhibition of YAP function restores contact inhibition in a human cancer cell line bearing deletion of Salvador (Sav), a Hippo pathway component. Interestingly, we observed that YAP protein is elevated and nuclear localized in some human liver and prostate cancers. Our observations demonstrate that YAP plays a key role in the Hippo pathway to control cell proliferation in response to cell contact.

title: Mst2 and Lats Kinases Regulate Apoptotic Function of Yes Kinase-associated Protein (YAP)*
abstract: The Hippo pathway in Drosophila controls the size and shape of organs. In the fly, activation of this pathway conveys growth-inhibitory signals and promotes apoptosis in epithelial cells. We “reconstituted” the Hippo pathway in a human epithelial cell line and showed that, in contrast to flies, the activation of this pathway results in anti-apoptotic signals. We have shown that in human embryonic kidney (HEK) 293 cells, the complex formation between transcriptional co-activators YAPs (Yes kinase-associated proteins) and Lats kinases requires the intact WW domains of YAPs, as well as intact Pro-Pro-AA-Tyr (where AA is any amino acid) motifs in Lats kinases. These kinases cooperate with the upstream Mst2 kinase to phosphorylate YAPs at Ser-127. Overexpression of YAP2 in HEK293 cells promoted apoptosis, whereas the Mst2/Lats1-induced phosphorylation of YAP partially rescued the cells from apoptotic death. Apoptotic signaling of YAP2 was mediated via stabilization of p73, which formed a complex with YAP2. All components of the Hippo pathway that we studied were localized in the cytoplasm, with the exception of YAP, which also localized in the nucleus. The localization of YAP2 in the nucleus was negatively controlled by the Lats1 kinase. Our apoptotic “readout” of the Hippo pathway in embryonic kidney cells represents a useful experimental system for the identification of the putative upstream receptor, membrane protein, or extracellular factor that initiates an entire signaling cascade and ultimately controls the size of organs.

title: Protein kinases of the Hippo pathway: regulation and substrates.
abstract: The "Hippo" signaling pathway has emerged as a major regulator of cell proliferation and survival in metazoans. The pathway, as delineated by genetic and biochemical studies in Drosophila, consists of a kinase cascade regulated by cell-cell contact and cell polarity that inhibits the transcriptional coactivator Yorkie and its proliferative, anti-differentiation, antiapoptotic transcriptional program. The core pathway components are the GC kinase Hippo, which phosphorylates the noncatalytic polypeptide Mats/Mob1 and, with the assistance of the scaffold protein Salvador, phosphorylates the ndr-family kinase Lats. In turn phospho-Lats, after binding to phospho-Mats, autoactivates and phosphorylates Yorkie, resulting in its nuclear exit. Hippo also uses the scaffold protein Furry and a different Mob protein to control another ndr-like kinase, the morphogenetic regulator Tricornered. Architecturally homologous kinase cascades consisting of a GC kinase, a Mob protein, a scaffolding polypeptide and an ndr-like kinase are well described in yeast; in Saccharomyces cerevisiae, e.g., the MEN pathway promotes mitotic exit whereas the RAM network, using a different GC kinase, Mob protein, scaffold and ndr-like kinase, regulates cell polarity and morphogenesis. In mammals, the Hippo orthologs Mst1 and Mst2 utilize the Salvador ortholog WW45/Sav1 and other scaffolds to regulate the kinases Lats1/Lats2 and ndr1/ndr2. As in Drosophila, murine Mst1/Mst2, in a redundant manner, negatively regulate the Yorkie ortholog YAP in the epithelial cells of the liver and gut; loss of both Mst1 and Mst2 results in hyperproliferation and tumorigenesis that can be largely negated by reduction or elimination of YAP. Despite this conservation, considerable diversification in pathway composition and regulation is already evident; in skin, e.g., YAP phosphorylation is independent of Mst1Mst2 and Lats1Lats2. Moreover, in lymphoid cells, Mst1/Mst2, under the control of the Rap1 GTPase and independent of YAP, promotes integrin clustering, actin remodeling and motility while restraining the proliferation of naïve T cells. This review will summarize current knowledge of the structure and regulation of the kinases Hippo/Mst1&2, their noncatalytic binding partners, Salvador and the Rassf polypeptides, and their major substrates Warts/Lats1&2, Trc/ndr1&2, Mats/Mob1 and FOXO.

title: MOBKL1A/MOBKL1B Phosphorylation by MST1 and MST2 Inhibits Cell Proliferation
abstract: MST1 and MST2 are the mammalian Ste20-related protein kinases most closely related to Drosophila Hippo, a major regulator of cell proliferation and survival during development. Overexpression of MST1 or MST2 in mammalian cells is proapototic; however, little is known concerning the physiologic regulation of the endogenous MST1/MST2 kinases, their role in mammalian cell proliferation, or the identity of the MST1/MST2 substrates critical to proliferative regulation.



We show that MST1 and MST2 activity increases during mitosis, especially in nocodazole-arrested mitotic cells, where these kinases exhibit both an increase in both abundance and activation. MST1 and MST2 also can be activated nonphysiologically by okadaic acid or H2O2. The MOBKL1A and MOBKL1B polypeptides, homologs of the Drosophila MATS polypeptide, are identified as preferred MST1/MST2 substrates in vitro and are phosphorylated in cells in an MST1/MST2-dependent manner in mitosis and in response to okadaic acid or H2O2. MST1/MST2-catalyzed MOBKL1A/MOBKL1B phosphorylation alters the ability of MOBKL1A/MOBKL1B to bind and regulate downstream targets such as the NDR-family protein kinases. Thus, MOBKL1A/MOBKL1B phosphorylation in cells promotes MOBKL1A/MOBKL1B binding to the LATS1 kinase and enables H2O2-stimulated LATS1 activation loop phosphorylation. Most importantly, replacement of endogenous MOBKL1A/MOBKL1B by a nonphosphorylatable mutant is sufficient to accelerate cell proliferation substantially by speeding progression through G1/S as well as mitotic exit.



These results establish that MST1 and MST2 are activated in mitosis and catalyze the mitotic phosphorylation of MOBKL1A/MOBKL1B. MOBKL1A/MOBKL1B phosphorylation, in turn, is sufficient to inhibit proliferation through actions at several points in the cell cycle.

title: Tumor Suppressor LATS1 Is a Negative Regulator of Oncogene YAP*
abstract: LATS (large tumor suppressor) or warts is a Ser/Thr kinase that belongs to the Ndr/LATS subfamily of AGC (protein kinase A/PKG/PKC) kinases. It is a tumor suppressor gene originally isolated from Drosophila and recently isolated from mice and humans. Drosophila or mice mutant for LATS develop tumors in various tissues. Recent studies in Drosophila demonstrate that LATS is a central player of an emerging tumor suppressor pathway called the Hippo-LATS/Warts pathway that suppresses tumor growth by regulating cell proliferation, cell growth, and cell death. Although tremendous progress has been made toward understanding the roles of LATS in tumorigenesis, the kinase substrates of LATS or downstream target proteins mediating LATS function remain largely unknown. In this study, we have provided convincing evidence that the LATS1 tumor suppressor can bind to and phosphorylate transcription regulator and oncogene YAP in vitro and in vivo. We have also identified HX(R/H/K)XX(S/T) as the consensus phosphorylation sequence for LATS/Ndr kinase substrates. Significantly, we have discovered that LATS1 inactivates YAP oncogenic function by suppressing its transcription regulation of cellular genes via sequestration of YAP in the cytoplasm after phosphorylation of YAP. Finally, by using microarray analysis, we have also identified many oncogenes or tumor suppressor genes up-regulated or down-regulated by YAP. These research findings will have profound impacts on our understanding of the molecular mechanism of the LATS tumor suppressor and the emerging Hippo-LATS/Warts pathway.

title: In vivo regulation of Yorkie phosphorylation and localization
abstract: Yorkie (Yki), a transcription factor of the Fat and Hippo signaling pathways, is negatively regulated by the Warts kinase. Here, we use Phos-tag gels to characterize Warts-dependent phosphorylation of Yki in vivo, and show that Warts promotes phosphorylation of Yki at multiple sites. We also show that Warts inhibits Yki nuclear localization in vivo, and can promote binding of Yki to 14-3-3 proteins in cultured cells. In vivo assessment of the influence of individual upstream regulators of Warts reveals that some mutants (e.g. fat) have only partial effects on Yki phosphorylation, and weak effects on Yki localization, whereas other genotypes (e.g. ex fat double mutants) have stronger effects on both Yki phosphorylation and localization. We also identify serine 168 as a critical site through which negative regulation of Yki by Warts-mediated phosphorylation occurs, but find that this site is not sufficient to explain effects of Hippo signaling on Yki in vivo. These results identify modulation of subcellular localization as a mechanism of Yki regulation, and establish that this regulation occurs in vivo through multiple sites of Warts-dependent phosphorylation on Yki.

title: Hippo signaling regulates Yorkie nuclear localization and activity through 14-3-3 dependent and independent mechanisms.
abstract: The Hippo (Hpo) signaling pathway controls cell growth, proliferation and apoptosis in both Drosophila and vertebrates. In Drosophila, Hpo signaling regulates gene expression by inhibiting a transcription complex consisting of the transcriptional coactivator Yorkie (Yki) and the TEAD/TEF family of transcription factor Scalloped (Sd). Here we provide genetic evidence that both isoforms of 14-3-3, 14-3-3varepsilon and 14-3-3zeta, regulate Yki activity through modulating its subcellular localization. Inactivation of 14-3-3 by RNAi or genetic mutations enhanced whereas overexpression of 14-3-3 suppressed tissue overgrowth induced by Yki overexpression. Loss of 14-3-3 resulted in the accumulation of Yki in the nucleus. We found that regulation of Yki by 14-3-3 was mediated by phosphorylation of Yki at S168. In addition, we found that Hpo signaling also inhibited Yki nuclear localization and activity by phosphorylating Yki at S111 and S250, and this inhibition appears to be independent of 14-3-3. Finally, we provided evidence that Hpo signaling restricted Yki nuclear localization depending on CRM1-mediated nuclear export.

title: Dmp53 Activates the Hippo Pathway to Promote Cell Death in Response to DNA Damage
abstract: Developmental and environmental signals control a precise program of growth, proliferation, and cell death. This program ensures that animals reach, but do not exceed, their typical size . Understanding how cells sense the limits of tissue size and respond accordingly by exiting the cell cycle or undergoing apoptosis has important implications for both developmental and cancer biology. The Hippo (Hpo) pathway comprises the kinases Hpo and Warts/Lats (Wts), the adaptors Salvador (Sav) and Mob1 as a tumor suppressor (Mats), the cytoskeletal proteins Expanded and Merlin, and the transcriptional cofactor Yorkie (Yki) . This pathway has been shown to restrict cell division and promote apoptosis. The caspase repressor DIAP1 appears to be a primary target of the Hpo pathway in cell-death control. Firstly, Hpo promotes DIAP1 phosphorylation, likely decreasing its stability. Secondly, Wts phosphorylates and inactivates Yki, decreasing DIAP1 transcription. Although we understand some of the events downstream of the Hpo kinase, its mode of activation remains mysterious. Here, we show that Hpo can be activated by Ionizing Radiations (IR) in a Dmp53 (Drosophila melanogaster p53)-dependent manner and that Hpo is required (though not absolutely) for the cell death response elicited by IR or Dmp53 ectopic expression.

title: The Drosophila RASSF Homolog Antagonizes the Hippo Pathway
abstract: Correct organ size is determined by the balance between cell death and proliferation. Perturbation of this delicate balance leads to cancer formation . Hippo (Hpo), the Drosophila ortholog of MST1 and MST2 (Mammalian Sterile 20-like 1 and 2) is a key regulator of a signaling pathway that controls both cell death and proliferation 2, 3. This pathway is so far composed of two Band 4.1 proteins, Expanded (Ex) and Merlin (Mer), two serine/threonine kinases, Hpo and Warts (Wts), the scaffold proteins Salvador (Sav) and Mats, and the transcriptional coactivator Yorkie (Yki). It has been proposed that Ex and Mer act upstream of Hpo, which in turn phosphorylates and activates Wts. Wts phosphorylates Yki and thus inhibits its activity and reduces expression of Yki target genes such as the caspase inhibitor DIAP1 and the micro RNA bantam4, 5, 6. However, the mechanisms leading to Hpo activation are still poorly understood. In mammalian cells, members of the Ras association family (RASSF) of tumor suppressors have been shown to bind to MST1 and modulate its activity [7]. In this study, we show that the Drosophila RASSF ortholog (dRASSF) restricts Hpo activity by competing with Sav for binding to Hpo. In addition, we observe that dRASSF also possesses a tumor-suppressor function.

title: Salvador Protein Is a Tumor Suppressor Effector of RASSF1A with Hippo Pathway-independent Functions*
abstract: The RASSF1A tumor suppressor binds and activates proapoptotic MST kinases. The Salvador adaptor protein couples MST kinases to the LATS kinases to form the hippo pathway. Upon activation by RASSF1A, LATS1 phosphorylates the transcriptional regulator YAP, which binds to p73 and activates its proapoptotic effects. However, although serving as an adaptor for MST and LATS, Salvador can also bind RASSF1A. The functional role of the RASSF1A/Salvador interaction is unclear. Although Salvador is a novel tumor suppressor in Drosophila and mice, its role in human systems remains largely unknown. Here we show that Salvador promotes apoptosis in human cells and that Salvador inactivation deregulates the cell cycle and enhances the transformed phenotype. Moreover, we show that although the salvador gene is seldom mutated or epigenetically inactivated in human cancers, it is frequently down-regulated posttranscriptionally. Surprisingly, we also find that although RASSF1A requires the presence of Salvador for full apoptotic activity and to activate p73, this effect does not require a direct interaction of RASSF1A with MST kinases or the activation of the hippo pathway. Thus, we confirm a role for Salvador as a human tumor suppressor and RASSF1A effector and show that Salvador allows RASSF1A to modulate p73 independently of the hippo pathway.

title: Ajuba LIM Proteins Are Negative Regulators of the Hippo Signaling Pathway
abstract: The mammalian Ajuba LIM proteins (Ajuba, LIMD1, and WTIP) are adaptor proteins that exhibit the potential to communicate cell adhesive events with nuclear responses to remodel epithelia. Determining their role in vivo, however, has been challenging due to overlapping tissue expression and functional redundancy. Thus, we turned to Drosophila, where a single gene, CG11063 or djub, exists. Drosophila lacking the djub gene or depleted of dJub by RNA interference identify djub as an essential gene for development and a novel regulator of epithelial organ size as a component of the conserved Hippo (Hpo) pathway, which has been implicated in both tissue size control and cancer development. djub-deficient tissues were small and had decreased cell numbers as a result of increased apoptosis and decreased proliferation, due to downregulation of DIAP1 and cyclin E. This phenocopies tissues deficient for Yorkie (Yki), the downstream target of the Hippo pathway. djub genetically interacts with the Hippo pathway, and epistasis suggests that djub lies downstream of hpo. In mammalian and Drosophila cells, Ajuba LIM proteins/dJub interact with LATS/Warts (Wts) and WW45/Sav to inhibit phosphorylation of YAP/Yki. This work describes a novel role for the Ajuba LIM proteins as negative regulators of the Hippo signaling pathway.

title: Zyxin Links Fat Signaling to the Hippo Pathway
abstract: Using genetic and molecular analyses, the authors identify Zyx as a positive regulator of Hippo signaling and characterize its role within the pathway.

title: JNK signaling is converted from anti- to pro-tumor pathway by Ras-mediated switch of Warts activity.
abstract: The c-Jun N-terminal kinase (JNK) pathway is a dual-functional oncogenic signaling that exerts both anti- and pro-tumor activities. However, the mechanism by which JNK switches its oncogenic roles depending on different cellular contexts has been elusive. Here, using the Drosophila genetics, we show that hyperactive Ras acts as a signaling switch that converts JNK's role from anti- to pro-tumor signaling through the regulation of Hippo signaling activity. In the normal epithelium, JNK signaling antagonizes the Hippo pathway effector Yorkie (Yki) through elevation of Warts activity, thereby suppressing tissue growth. In contrast, in the presence of hyperactive Ras, JNK signaling enhances Yki activation by accumulating F-actin through the activity of the LIM domain protein Ajuba, thereby promoting tissue growth. We also find that the epidermal growth factor receptor (EGFR) signaling uses this Ras-mediated conversion of JNK signaling to promote tissue growth. Our observations suggest that Ras-mediated switch of the JNK pathway from anti- to pro-tumor signaling could play crucial roles in tumorigenesis as well as in normal development.

title: Zyxin Antagonizes the FERM Protein Expanded to Couple F-Actin and Yorkie-Dependent Organ Growth
abstract: Coordinated multicellular growth during development is achieved by the sensing of spatial and nutritional boundaries. The conserved Hippo (Hpo) signaling pathway has been proposed to restrict tissue growth by perceiving mechanical constraints through actin cytoskeleton networks. The actin-associated LIM proteins Zyxin (Zyx) and Ajuba (Jub) have been linked to the control of tissue growth via regulation of Hpo signaling, but the study of Zyx has been hampered by a lack of genetic tools.



We generated a zyx mutant in Drosophila using TALEN endonucleases and used this to show that Zyx antagonizes the FERM-domain protein Expanded (Ex) to control tissue growth, eye differentiation, and F-actin accumulation. Zyx membrane targeting promotes the interaction between the transcriptional co-activator Yorkie (Yki) and the transcription factor Scalloped (Sd), leading to activation of Yki target gene expression and promoting tissue growth. Finally, we show that Zyx's growth-promoting function is dependent on its interaction with the actin-associated protein Enabled (Ena) via a conserved LPPPP motif and is antagonized by Capping Protein (CP).



Our results show that Zyx is a functional antagonist of Ex in growth control and establish a link between actin filament polymerization and Yki activity.

title: Tao-1 phosphorylates Hippo/MST kinases to regulate the Hippo-Salvador-Warts tumor suppressor pathway.
abstract: Recent studies have shown that the Hippo-Salvador-Warts (HSW) pathway restrains tissue growth by phosphorylating and inactivating the oncoprotein Yorkie. How growth-suppressive signals are transduced upstream of Hippo remains unclear. We show that the Sterile 20 family kinase, Tao-1, directly phosphorylates T195 in the Hippo activation loop and that, like other HSW pathway genes, Tao-1 functions to restrict cell proliferation in developing imaginal epithelia. This relationship appears to be evolutionarily conserved, because mammalian Tao-1 similarly affects MST kinases. In S2 cells, Tao-1 mediates the effects of the upstream HSW components Merlin and Expanded, consistent with the idea that Tao-1 functions in tissues to regulate Hippo phosphorylation. These results demonstrate that one family of Ste20 kinases can activate another and identify Tao-1 as a component of the regulatory network controlling HSW pathway signaling, and therefore tissue growth, during development.

title: The sterile 20-like kinase Tao-1 controls tissue growth by regulating the Salvador-Warts-Hippo pathway.
abstract: The Salvador-Warts-Hippo (SWH) pathway is a complex signaling network that controls both developmental and regenerative tissue growth. Using a genetic screen in Drosophila melanogaster, we identified the sterile 20-like kinase, Tao-1, as an SWH pathway member. Tao-1 controls various biological phenomena, including microtubule dynamics, animal behavior, and brain development. Here we describe a role for Tao-1 as a regulator of epithelial tissue growth that modulates activity of the core SWH pathway kinase cassette. Tao-1 functions together with Hippo to activate Warts-mediated repression of Yorkie. Tao-1's ability to control SWH pathway activity is evolutionarily conserved because human TAO1 can suppress activity of the Yorkie ortholog, YAP. Human TAO1 controls SWH pathway activity by phosphorylating, and activating, the Hippo ortholog, MST2. Given that SWH pathway activity is subverted in many human cancers, our findings identify human TAO kinases as potential tumor suppressor genes.

title: Regulators of mammalian Hippo pathway in cancer.
abstract: Hippo pathway, originally discovered in Drosophila, is responsible for organ size control. The pathway is conserved in mammals and has a significant role in restraining cancer development. Regulating the Hippo pathway thus represents a potential therapeutic approach to treat cancer, which however requires deep understanding of the targeted pathway. Despite our limited knowledge on the pathway, there are increasing discoveries of new molecules that regulate and modulate the Hippo downstream signaling particularly in various solid malignancies, from extracellular stimuli or via pathway crosstalk. Herein, we discuss the roles of newly identified and key regulators that connect with core components (MST1/2, LATS1/2, SAV1, and MOB1) and downstream effector (YAP) in the Hippo pathway having an important role in cancer development and progression. Understanding of the mammalian Hippo pathway regulation may shed new insights to allow us selecting the right oncogenic targets and designing effective drugs for cancer treatments.

title: The Hippo pathway effector proteins YAP and TAZ have both distinct and overlapping functions in the cell
abstract: The Hippo pathway plays an important role in regulating tissue homeostasis, and its effectors, the transcriptional co-activators Yes-associated protein (YAP) and WW domain–containing transcription regulator 1 (WWTR1 or TAZ), are responsible for mediating the vast majority of its physiological functions. Although YAP and TAZ are thought to be largely redundant and similarly regulated by Hippo signaling, they have developmental, structural, and physiological differences that suggest they may differ in their regulation and downstream functions. To better understand the functions of YAP and TAZ in the Hippo pathway, using CRISPR/Cas9, we generated YAP KO, TAZ KO, and YAP/TAZ KO cell lines in HEK293A cells. We evaluated them in response to many environmental conditions and stimuli and used RNA-Seq to compare their transcriptional profiles. We found that YAP inactivation has a greater effect on cellular physiology (namely, cell spreading, volume, granularity, glucose uptake, proliferation, and migration) than TAZ inactivation. However, functional redundancy between YAP and TAZ was also observed. In summary, our findings confirm that the Hippo pathway effectors YAP and TAZ are master regulators for multiple cellular processes but also reveal that YAP has a stronger influence than TAZ.

title: The Maguk protein, Pals1, functions as an adapter, linking mammalian homologues of Crumbs and Discs Lost
abstract: Membrane-associated guanylate kinase (Maguk) proteins are scaffold proteins that contain PSD-95–Discs Large–zona occludens-1 (PDZ), Src homology 3, and guanylate kinase domains. A subset of Maguk proteins, such as mLin-2 and protein associated with Lin-7 (Pals)1, also contain two L27 domains: an L27C domain that binds mLin-7 and an L27N domain of unknown function. Here, we demonstrate that the L27N domain targets Pals1 to tight junctions by binding to a PDZ domain protein, Pals1-associated tight junction (PATJ) protein, via a unique Maguk recruitment domain. PATJ is a homologue of Drosophila Discs Lost, a protein that is crucial for epithelial polarity and that exists in a complex with the apical polarity determinant, Crumbs. PATJ and a human Crumbs homologue, CRB1, colocalize with Pals1 to tight junctions, and CRB1 interacts with PATJ albeit indirectly via binding the Pals1 PDZ domain. In agreement, we find that a Drosophila homologue of Pals1 participates in identical interactions with Drosophila Crumbs and Discs Lost. This Drosophila Pals1 homologue has been demonstrated recently to represent Stardust, a crucial polarity gene in Drosophila. Thus, our data identifies a new multiprotein complex that appears to be evolutionarily conserved and likely plays an important role in protein targeting and cell polarity.

title: Scribble protein domain mapping reveals a multistep localization mechanism and domains necessary for establishing cortical polarity
abstract: The Drosophila tumor suppressor protein Scribble is required for epithelial polarity, neuroblast polarity, neuroblast spindle asymmetry and limiting cell proliferation. It is a member of the newly described LAP protein family, containing 16 leucine rich repeats (LRRs), four PDZ domains and an extensive carboxyl-terminal (CT) domain. LRR and PDZ domains mediate protein-protein interactions, but little is know about their function within LAP family proteins. We have determined the role of the LRR, PDZ and CT domains for Scribble localization in neuroblasts and epithelia, and for Scribble function in neuroblasts. We found that the LRR and PDZ domains are both required for proper targeting of Scribble to septate junctions in epithelia; that the LRR domain is necessary and sufficient for cortical localization in mitotic neuroblasts, and that the PDZ2 domain is required for efficient cortical and apical localization of Scribble in neuroblasts. In addition, we show that the LRR domain is sufficient to target Miranda protein to the neuroblast cortex, but that LRR+PDZ will exclude Miranda from the cortex. Our results highlight the importance of both LRR and PDZ domains for the proper localization and function of Scribble in neuroblasts.

title: Regulation of the MST1 kinase by autophosphorylation, by the growth inhibitory proteins, RASSF1 and NORE1, and by Ras.
abstract: MST1 (mammalian Sterile20-like 1) and MST2 are closely related Class II GC (protein Ser/Thr) kinases that initiate apoptosis when transiently overexpressed in mammalian cells. In the present study, we show that recombinant MST1/2 undergo a robust autoactivation in vitro, mediated by an intramolecular autophosphorylation of a single site  on the activation loop of an MST dimer. Endogenous full-length MST1 is activated by a variety of stressful stimuli, accompanied by the secondary appearance of a 36 kDa Thr183-phosphorylated, caspase-cleaved catalytic fragment. Recombinant MST1 exhibits only 2-5% activation during transient expression; endogenous MST1 in the cycling HeLa or KB cells has a similar low fractional activation, but 2 h incubation with okadaic acid (1 mM) results in 100% activation. Endogenous MST1 immunoprecipitated from KB cells is specifically associated with substoichiometric amounts of the growth inhibitory polypeptides RASSF1A and NORE1A (novel Ras effector 1A; a Ras-GTP-binding protein). Co-expression of RASSF1A, RASSF1C, NORE1A and NORE1B with MST1 markedly suppresses MST1(Thr183) phosphorylation in vivo and abolishes the ability of MST1 to undergo Mg-ATP-mediated autoactivation in vitro; direct addition of purified NORE1A in vitro also inhibits MST1 activation. In contrast, co-transfection of MST1 with NORE1A modified by the addition of a C-terminal CAAX motif results in a substantial increase in MST1(Thr183) phosphorylation, as does fusion of a myristoylation motif directly on to the MST1 N-terminus. Moreover, MST1 polypeptides, bound via wild-type NORE1A to Ras(G12V) (where G12V stands for Gly12Val), exhibit higher Thr183 phosphorylation compared with MST1 bound to NORE1A alone. Nevertheless, serum stimulation of KB cells does not detectably increase the activation state of endogenous MST1 or MST2 despite promoting the recruitment of the endogenous NORE1-MST1 complex to endogenous Ras. We propose that the NORE1/RASSF1 polypeptides, in addition to their role in maintaining the low activity of MST1 in vivo, direct MST1 to sites of activation and perhaps co-localization with endogenous substrates.

title: Tight Junction Protein Par6 Interacts with an Evolutionarily Conserved Region in the Amino Terminus of PALS1/Stardust*
abstract: Tight junctions are the structures in mammalian epithelial cells that separate the apical and basolateral membranes and may also be important in the establishment of cell polarity. Two evolutionarily conserved multiprotein complexes, Crumbs-PALS1 (Stardust)-PATJ and Cdc42-Par6-Par3-atypical protein kinase C, have been implicated in the assembly of tight junctions and in polarization of Drosophila melanogaster epithelia. These two complexes have been linked physically and functionally by an interaction between PALS1 and Par6. Here we identify an evolutionarily conserved region in the amino terminus of PALS1 as the Par6 binding site and identify valine and aspartic acid residues in this region as essential for interacting with the PDZ domain of Par6. We have also characterized, in more detail, the amino terminus of Drosophila Stardust and demonstrate that the interaction mechanism between Stardust and Drosophila Par6 is evolutionarily conserved. Par6 interferes with PATJ in binding PALS1, and these two interactions do not appear to function synergistically. Taken together, these results define the molecular mechanisms linking two conserved polarity complexes.

title: Association of mammalian sterile twenty kinases, Mst1 and Mst2, with hSalvador via C‐terminal coiled‐coil domains, leads to its stabilization and phosphorylation
abstract: Genetic screens in Drosophila have revealed that the serine/threonine kinase Hippo (Hpo) and the scaffold protein Salvador participate in a pathway that controls cell proliferation and apoptosis. Hpo most closely resembles the pro‐apoptotic mammalian sterile20 kinases 1 and 2 (Mst1 and 2), and Salvador (Sav) has a human orthologue hSav (also called hWW45). Here we show that Mst and hSav heterodimerize in an interaction requiring the conserved C‐terminal coiled‐coil domains of both proteins. hSav was also able to homodimerize, but this did not require its coiled‐coil domain. Coexpression of Mst and hSav led to phosphorylation of hSav and also increased its abundance. In vitro phosphorylation experiments indicate that the phosphorylation of Sav by Mst is direct. The stabilizing effect of Mst was much greater on N‐terminally truncated hSav mutants, as long as they retained the ability to bind Mst. Mst mutants that lacked the C‐terminal coiled‐coil domain and were unable to bind to hSav, also failed to stabilize or phosphorylate hSav, whereas catalytically inactive Mst mutants that retained the ability to bind to hSav were still able to increase its abundance, although they were no longer able to phosphorylate hSav. Together these results show that hSav can bind to, and be phosphorylated by, Mst, and that the stabilizing effect of Mst on hSav requires its interaction with hSav but is probably not due to phosphorylation of hSav by Mst.

title: The conserved c2 domain protein lethal (2) giant discs regulates protein trafficking in Drosophila.
abstract: Drosophila sensory organ precursor (SOP) cells undergo several rounds of asymmetric cell division to generate the four different cell types that make up external sensory organs. Establishment of different fates among daughter cells of the SOP relies on differential regulation of the Notch pathway. Here, we identify the protein Lethal (2) giant discs (Lgd) as a critical regulator of Notch signaling in the SOP lineage. We show that lgd encodes a conserved C2 domain protein that binds to phospholipids present on early endosomes. When Lgd function is compromised, Notch and other transmembrane proteins accumulate in enlarged early endosomal compartments. These enlarged endosomes are positive for Rab5 and Hrs, a protein involved in trafficking into the degradative pathway. Our experiments suggest that Lgd is a critical regulator of endocytosis that is not present in yeast and acts in the degradative pathway after Hrs.

title: The Fat Cadherin Acts through the Hippo Tumor-Suppressor Pathway to Regulate Tissue Size
abstract: The Hippo tumor-suppressor pathway has emerged as a key signaling pathway that controls tissue size in Drosophila. Merlin, the Drosophila homolog of the human Neurofibromatosis type-2 (NF2) tumor-suppressor gene, and the related protein Expanded are the most upstream components of the Hippo pathway identified so far. However, components acting upstream of Expanded and Merlin, such as transmembrane receptors, have not yet been identified.



Here, we report that the protocadherin Fat acts as an upstream component in the Hippo pathway. Fat is a known tumor-suppressor gene in Drosophila, and fat mutants have severely overgrown imaginal discs. We found that the overgrowth phenotypes of fat mutants are similar to those of mutants in Hippo pathway components: fat mutant cells continued to proliferate after wild-type cells stopped proliferating, and fat mutant cells deregulated Hippo target genes such as cyclin E and diap1. Fat acts genetically and biochemically upstream of other Hippo pathway components such as Expanded, the Hippo and Warts kinases, and the transcriptional coactivator Yorkie. Fat is required for the stability of Expanded and its localization to the plasma membrane. In contrast, Fat is not required for Merlin localization, and Fat and Merlin act in parallel in growth regulation.



Taken together, our data identify a cell-surface molecule that may act as a receptor of the Hippo signaling pathway.

title: Drosophila lowfat, a novel modulator of Fat signaling
abstract: The Fat-Hippo-Warts signaling network regulates both transcription and planar cell polarity. Despite its crucial importance to the normal control of growth and planar polarity, we have only a limited understanding of the mechanisms that regulate Fat. We report here the identification of a conserved cytoplasmic protein, Lowfat (Lft), as a modulator of Fat signaling. Drosophila Lft, and its human homologs LIX1 and LIX1-like, bind to the cytoplasmic domains of the Fat ligand Dachsous, the receptor protein Fat, and its human homolog FAT4. Lft protein can localize to the sub-apical membrane in disc cells, and this membrane localization is influenced by Fat and Dachsous. Lft expression is normally upregulated along the dorsoventral boundary of the developing wing, and is responsible for elevated levels of Fat protein there. Levels of Fat and Dachsous protein are reduced in lft mutant cells, and can be increased by overexpression of Lft. lft mutant animals exhibit a wing phenotype similar to that of animals with weak alleles of fat, and lft interacts genetically with both fat and dachsous. These studies identify Lft as a novel component of the Fat signaling pathway, and the Lft-mediated elevation of Fat levels as a mechanism for modulating Fat signaling.

title: The RASSF proteins in cancer; from epigenetic silencing to functional characterization.
abstract: The Ras-Association Domain Family (RASSF) comprises ten members, termed RASSF1 to RASSF10. RASSF1 to RASSF6 harbor a C-terminal Ras-association (RA) domain and RASSF7 to RASSF10 contain an N-terminal RA domain. Interestingly, it was observed that in various tumor types distinct RASSFs transcripts (e.g. RASSF1A and RASSF2A) are missing due to hypermethylation of their CpG island promoter. Since methylation of the RASSF1A promoter is described as an early and frequent event in tumorigenesis, RASSF1A could serve as a useful diagnostic marker in cancer screens. RASSFs are implicated in various cellular mechanisms including apoptosis, cell cycle control and microtubule stabilization, though little is known about the underlying mechanisms. Tumor suppressing functions were reported for several members. Here we review the current literature on RASSF members focusing on structural, functional and epigenetic aspects. Characterizing the cellular mechanisms that regulate the signaling pathways RASSFs are involved in, could lead to a deeper understanding of tumor development and, furthermore, to new strategies in cancer treatment.

title: An autoinhibitory effect of the homothorax domain of Meis2
abstract: Myeloid ecotropic insertion site (Meis)2 is a homeodomain protein containing a conserved homothorax (Hth) domain that is present in all Meis and Prep family proteins and in the Drosophila Hth protein. The Hth domain mediates interaction with Pbx homeodomain proteins, allowing for efficient DNA binding. Here we show that, like Meis1, Meis2 has a strong C‐terminal transcriptional activation domain, which is required for full activation of transcription by homeodomain protein complexes composed of Meis2 and Pbx1. We also show that the activity of the activation domain is inhibited by the Hth domain, and that this autoinhibition can be partially relieved by the interaction of Pbx1 with the Hth domain of Meis2. Targeting of the Hth domain to DNA suggests that it is not a portable trans‐acting repression domain. However, the Hth domain can inhibit a linked activation domain, and this inhibition is not limited to the Meis2 activation domain. Database searching reveals that the Meis3.2 splice variant, which is found in several vertebrate species, disrupts the Hth domain by removing 17 codons from the 5′‐end of exon 6. We show that the equivalent deletion in Meis2 derepresses the C‐terminal activation domain and weakens interaction with Pbx1. This work suggests that the transcriptional activity of all members of the Meis/Prep Hth protein family is subject to autoinhibition by their Hth domains, and that the Meis3.2 splice variant encodes a protein that bypasses this autoinhibitory effect.

title: Kibra functions as a tumor suppressor protein that regulates Hippo signaling in conjunction with Merlin and Expanded.
abstract: The Hippo signaling pathway regulates organ size and tissue homeostasis from Drosophila to mammals. Central to this pathway is a kinase cascade wherein Hippo (Hpo), in complex with Salvador (Sav), phosphorylates and activates Warts (Wts), which in turn phosphorylates and inactivates the Yorkie (Yki) oncoprotein, known as the YAP coactivator in mammalian cells. The FERM domain proteins Merlin (Mer) and Expanded (Ex) are upstream components that regulate Hpo activity through unknown mechanisms. Here we identify Kibra as another upstream component of the Hippo signaling pathway. We show that Kibra functions together with Mer and Ex in a protein complex localized to the apical domain of epithelial cells, and that this protein complex regulates the Hippo kinase cascade via direct binding to Hpo and Sav. These results shed light on the mechanism of Ex and Mer function and implicate Kibra as a potential tumor suppressor with relevance to neurofibromatosis.

title: MOB control: reviewing a conserved family of kinase regulators.
abstract: The family of Mps One binder (MOB) co-activator proteins is highly conserved from yeast to man. At least two different MOB proteins have been identified in every eukaryote analysed to date. Initially, yeast genetics revealed essential roles for Mob1p and Mob2p in the regulation of mitotic exit and cell morphogenesis. Studies in flies then showed that dMOB1/MATS is a core component of Hippo signalling. Loss of dMOB1 resulted in increased cell proliferation and decreased cell death, suggesting that MOB1 acts as tumour suppressor protein. Recent work focused primarily on mammalian cells has shown how hMOB1 can regulate NDR/LATS kinases, a function that can to be counteracted by hMOB2. Here we summarise and discuss our current knowledge of this emerging protein family, with emphasis on subcellular localisation, protein-protein interactions and biological functions in apoptosis, mitosis, morphogenesis, cell proliferation and centrosome duplication.

title: A novel partner of Scalloped regulates Hippo signaling via antagonizing Scalloped-Yorkie activity
abstract: The Hippo (Hpo) pathway controls tissue growth and organ size by regulating the activity of transcriptional co-activator Yorkie (Yki), which associates with transcription factor Scalloped (Sd) in the nucleus to promote downstream target gene expression. Here we identify a novel protein Sd-Binding-Protein (SdBP)/Tgi, which directly competes with Yki for binding to Sd through its TDU domains and inhibits the Sd-Yki transcriptional activity. We also find that SdBP retains Yki in the nucleus through the association with Yki WW domains via its PPXY motifs. Collectively, we identify SdBP as a novel component of the Hpo pathway, negatively regulating the transcriptional activity of Sd-Yki to restrict tissue growth.

title: Biochemical Large-Scale Interaction Analysis of Murine Olfactory Receptors and Associated Signaling Proteins with Post-Synaptic Density 95, Drosophila Discs Large, Zona-Occludens 1 (PDZ) Domains*
abstract: G protein-coupled receptors (GPCRs) constitute the largest family among mammalian membrane proteins and are capable of initiating numerous essential signaling cascades. Various GPCR-mediated pathways are organized into protein microdomains that can be orchestrated and regulated through scaffolding proteins, such as PSD-95/discs-large/ZO1 (PDZ) domain proteins. However, detailed binding characteristics of PDZ–GPCR interactions remain elusive because these interactions seem to be more complex than previously thought. To address this issue, we analyzed binding modalities using our established model system. This system includes the 13 individual PDZ domains of the multiple PDZ domain protein 1 (MUPP1; the largest PDZ protein), a broad range of murine olfactory receptors (a multifaceted gene cluster within the family of GPCRs), and associated olfactory signaling proteins. These proteins were analyzed in a large-scale peptide microarray approach and continuative interaction studies. As a result, we demonstrate that canonical binding motifs were not overrepresented among the interaction partners of MUPP1. Furthermore, C-terminal phosphorylation and distinct amino acid replacements abolished PDZ binding promiscuity. In addition to the described in vitro experiments, we identified new interaction partners within the murine olfactory epithelium using pull-down-based interactomics and could verify the partners through co-immunoprecipitation. In summary, the present study provides important insight into the complexity of the binding characteristics of PDZ–GPCR interactions based on olfactory signaling proteins, which could identify novel clinical targets for GPCR-associated diseases in the future.

title: Localization and Function of Pals1-associated Tight Junction Protein in Drosophila Is Regulated by Two Distinct Apical Complexes*
abstract: Pals1-associated tight junction protein (PATJ) is an important regulator of Myosin-driven morphogenetic processes.  PATJ associates with two different apical complexes and accomplishes its function by a redundancy of its protein interaction domains.  The correct targeting of PATJ to distinct apical domains is important for its function in Drosophila development.  Learning how adaptor proteins can be regulated by protein targeting to distinct cellular compartments. The transmembrane protein Crumbs (Crb) and its intracellular adaptor protein Pals1 (Stardust, Sdt in Drosophila) play a crucial role in the establishment and maintenance of apical-basal polarity in epithelial cells in various organisms. In contrast, the multiple PDZ domain-containing protein Pals1-associated tight junction protein (PATJ), which has been described to form a complex with Crb/Sdt, is not essential for apical basal polarity or for the stability of the Crb/Sdt complex in the Drosophila epidermis. Here we show that, in the embryonic epidermis, Sdt is essential for the correct subcellular localization of PATJ in differentiated epithelial cells but not during cellularization. Consistently, the L27 domain of PATJ is crucial for the correct localization and function of the protein. Our data further indicate that the four PDZ domains of PATJ function, to a large extent, in redundancy, regulating the function of the protein. Interestingly, the PATJ-Sdt heterodimer is not only recruited to the apical cell-cell contacts by binding to Crb but depends on functional Bazooka (Baz). However, biochemical experiments show that PATJ associates with both complexes, the Baz-Sdt and the Crb-Sdt complex, in the mature epithelium of the embryonic epidermis, suggesting a role of these two complexes for the function of PATJ during the development of Drosophila.

title: AJUBA LIM Proteins Limit Hippo Activity in Proliferating Cells by Sequestering the Hippo Core Kinase Complex in the Cytosol
abstract: The Hippo pathway controls organ growth and is implicated in cancer development. Whether and how Hippo pathway activity is limited to sustain or initiate cell growth when needed is not understood. The members of the AJUBA family of LIM proteins are negative regulators of the Hippo pathway. In mammalian epithelial cells, we found that AJUBA LIM proteins limit Hippo regulation of YAP, in proliferating cells only, by sequestering a cytosolic Hippo kinase complex in which LATS kinase is inhibited. At the plasma membranes of growth-arrested cells, AJUBA LIM proteins do not inhibit or associate with the Hippo kinase complex. The ability of AJUBA LIM proteins to inhibit YAP regulation by Hippo and to associate with the kinase complex directly correlate with their capacity to limit Hippo signaling during Drosophila wing development. AJUBA LIM proteins did not influence YAP activity in response to cell-extrinsic or cell-intrinsic mechanical signals. Thus, AJUBA LIM proteins limit Hippo pathway activity in contexts where cell proliferation is needed.

title: Domain-specific functions of Stardust in Drosophila embryonic development
abstract: In Drosophila, the adaptor protein Stardust is essential for the stabilization of the polarity determinant Crumbs in various epithelial tissues, including the embryonic epidermis, the follicular epithelium and photoreceptor cells of the compound eye. In turn, Stardust recruits another adaptor protein, PATJ, to the subapical region to support adherens junction formation and morphogenetic events. Moreover, Stardust binds to Lin-7, which is dispensable in epithelial cells but functions in postsynaptic vesicle fusion. Finally, Stardust has been reported to bind directly to PAR-6, thereby linking the Crumbs–Stardust–PATJ complex to the PAR-6/aPKC complex. PAR-6 and aPKC are also capable of directly binding Bazooka (the Drosophila homologue of PAR-3) to form the PAR/aPKC complex, which is essential for apical–basal polarity and cell–cell contact formation in most epithelia. However, little is known about the physiological relevance of these interactions in the embryonic epidermis of Drosophila in vivo. Thus, we performed a structure–function analysis of the annotated domains with GFP-tagged Stardust and evaluated the localization and function of the mutant proteins in epithelial cells of the embryonic epidermis. The data presented here confirm a crucial role of the PDZ domain in binding Crumbs and recruiting the protein to the subapical region. However, the isolated PDZ domain is not capable of being recruited to the cortex, and the SH3 domain is essential to support the binding to Crumbs. Notably, the conserved N-terminal regions (ECR1 and ECR2) are not crucial for epithelial polarity. Finally, the GUK domain plays an important role for the protein's function, which is not directly linked to Crumbs stabilization, and the L27N domain is essential for epithelial polarization independently of recruiting PATJ.

title: Structures of the L27 Domain of Disc Large Homologue 1 Protein Illustrate a Self-Assembly Module.
abstract: Disc large 1 (Dlg1) proteins, members of the MAGUK protein family, are linked to cell polarity via their participation in multiprotein assemblies. At their N-termini, Dlg1 proteins contain a L27 domain. Typically, the L27 domains participate in the formation of obligate hetero-oligomers with the L27 domains from their cognate partners. Among the MAGUKs, Dlg1 proteins exist as homo-oligomers, and the oligomerization is solely dependent on the L27 domain. Here we provide biochemical and structural evidence of homodimerization via the L27 domain of Dlg1 from Drosophila melanogaster. The structure reveals that the core of the dimer is formed by a distinctive six-helix assembly, involving all three conserved helices from each subunit (monomer). The homodimer interface is extended by the C-terminal tail of the L27 domain of Dlg1, which forms a two-stranded antiparallel β-sheet. The structure reconciles and provides a structural context for a large body of available mutational data. From our analyses, we conclude that the observed L27 homodimerization is most likely a feature unique to the Dlg1 orthologs within the MAGUK family.

title: A coordinated phosphorylation by Lats and CK1 regulates YAP stability through SCF(beta-TRCP).
abstract: The Yes-associated protein (YAP) transcription coactivator is a key regulator of organ size and a candidate human oncogene. YAP is inhibited by the Hippo pathway kinase cascade, at least in part via phosphorylation of Ser 127, which results in YAP 14-3-3 binding and cytoplasmic retention. Here we report that YAP is phosphorylated by Lats on all of the five consensus HXRXXS motifs. Phosphorylation of Ser 381 in one of them primes YAP for subsequent phosphorylation by CK1delta/epsilon in a phosphodegron. The phosphorylated phosphodegron then recruits the SCF(beta-TRCP) E3 ubiquitin ligase, which catalyzes YAP ubiquitination, ultimately leading to YAP degradation. The phosphodegron-mediated degradation and the Ser 127 phosphorylation-dependent translocation coordinately suppress YAP oncogenic activity. Our study identified CK1delta/epsilon as new regulators of YAP and uncovered an intricate mechanism of YAP regulation by the Hippo pathway via both S127 phosphorylation-mediated spatial regulation (nuclear-cytoplasmic shuttling) and the phosphodegron-mediated temporal regulation (degradation).

title: Processing and phosphorylation of the Fat receptor
abstract: The Drosophila tumor suppressors fat and discs overgrown (dco) function within an intercellular signaling pathway that controls growth and polarity. fat encodes a transmembrane receptor, but post-translational regulation of Fat has not been described. We show here that Fat is subject to a constitutive proteolytic processing, such that most or all cell surface Fat comprises a heterodimer of stably associated N- and C-terminal fragments. The cytoplasmic domain of Fat is phosphorylated, and this phosphorylation is promoted by the Fat ligand Dachsous. dco encodes a kinase that influences Fat signaling, and Dco is able to promote the phosphorylation of the Fat intracellular domain in cultured cells and in vivo. Evaluation of dco mutants indicates that they affect Fat's influence on growth and gene expression but not its influence on planar cell polarity. Our observations identify processing and phosphorylation as post-translational modifications of Fat, correlate the phosphorylation of Fat with its activation by Dachsous in the Fat-Warts pathway, and enhance our understanding of the requirement for Dco in Fat signaling.

title: Phosphorylation of the Tumor Suppressor Fat Is Regulated by Its Ligand Dachsous and the Kinase Discs Overgrown
abstract: The Drosophila tumor suppressor gene fat encodes a large cadherin that regulates growth and a form of tissue organization known as planar cell polarity (PCP). Fat regulates growth via the Hippo kinase pathway, which controls expression of genes promoting cell proliferation and inhibiting apoptosis (reviewed in). The Hippo pathway is highly conserved and is implicated in the regulation of mammalian growth and cancer development. Genetic studies suggest that Fat activity is regulated by binding to another large cadherin, Dachsous (Ds). The tumor suppressor discs overgrown (dco)/Casein Kinase I delta/epsilon also regulates Hippo activity and PCP. The biochemical nature of how Fat, Ds, and Dco interact to regulate these pathways is poorly understood. Here we demonstrate that Fat is cleaved to generate 450 kDa and 110 kDa fragments (Fat(450) and Fat(110)). Fat(110) contains the cytoplasmic and transmembrane domain. The cytoplasmic domain of Fat binds Dco and is phosphorylated by Dco at multiple sites. Importantly, we show Fat forms cis-dimers and that Fat phosphorylation is regulated by Dachsous and Dco in vivo. We propose that Ds regulates Dco-dependent phosphorylation of Fat and Fat-associated proteins to control Fat signaling in growth and PCP.

title: Regulation of Yki/Yap subcellular localization and Hpo signaling by a nuclear kinase PRP4K
abstract: Hippo (Hpo) signaling pathway controls tissue growth by regulating the subcellular localization of Yorkie (Yki)/Yap via a cytoplasmic kinase cassette containing an upstream kinase Hpo/MST1/2 and a downstream kinase Warts (Wts)/Lats1/2. Here we show that PRP4K, a kinase involved in mRNA splicing, phosphorylates Yki/Yap in the nucleus to prevent its nuclear accumulation and restrict Hpo pathway target gene expression. PRP4K inactivation accelerates whereas excessive PRP4K inhibits Yki-driven tissue overgrowth. PRP4K phosphorylates a subset of Wts/Lats1/2 sites on Yki/Yap to inhibit the binding of Yki/Yap to the Scalloped (Sd)/TEAD transcription factor and exclude Yki/Yap nuclear localization depending on nuclear export. Furthermore, PRP4K inhibits proliferation and invasiveness of cultured breast cancer cells and its high expression correlates with good prognosis in breast cancer patients. Our study unravels an unanticipated layer of Hpo pathway regulation and suggests that PRP4K-mediated Yki/Yap phosphorylation in the nucleus provides a fail-safe mechanism to restrict aberrant pathway activation.The Hippo signaling pathway controls tissue growth by regulating the subcellular localization of Yorkie /Yap. Here the authors show that PRP4K, a kinase involved in mRNA splicing, phosphorylates Yki/Yap in the nucleus, which prevents its nuclear accumulation and inhibits Hippo signaling.

title: Dynamic Fluctuations in Subcellular Localization of the Hippo Pathway Effector Yorkie In Vivo
abstract: The Hippo pathway is an evolutionarily conserved signaling network that integrates diverse cues to control organ size and cell fate. The central downstream pathway protein in Drosophila is the transcriptional co-activator Yorkie (YAP and TAZ in humans), which regulates gene expression with the Scalloped/TEA domain family member (TEAD) transcription factors [1-8]. A central regulatory step in the Hippo pathway is phosphorylation of Yorkie by the NDR family kinase Warts, which promotes Yorkie cytoplasmic localization by stimulating association with 14-3-3 proteins [9-12]. Numerous reports have purported a static model of Hippo signaling whereby, upon Hippo activation, Yorkie/YAP/TAZ become cytoplasmic and therefore inactive, and upon Hippo repression, Yorkie/YAP/TAZ transit to the nucleus and are active. However, we have little appreciation for the dynamics of Yorkie/YAP/TAZ subcellular localization because most studies have been performed in fixed cells and tissues. To address this, we used live multiphoton microscopy to investigate the dynamics of an endogenously tagged Yorkie-Venus protein in growing epithelial organs. We found that the majority of Yorkie rapidly traffics between the cytoplasm and nucleus, rather than being statically localized in either compartment. In addition, discrete cell populations within the same organ display different rates of Yorkie nucleo-cytoplasmic shuttling. By assessing Yorkie dynamics in warts mutant tissue, we found that the Hippo pathway regulates Yorkie subcellular distribution by regulating its rate of nuclear import. Furthermore, Yorkie's localization fluctuates dramatically throughout the cell cycle, being predominantly cytoplasmic during interphase and, unexpectedly, chromatin enriched during mitosis. Yorkie's association with mitotic chromatin is Scalloped dependent, suggesting a potential role in mitotic bookmarking.

title: The tyrosine kinase c-Abl regulates p73 in apoptotic response to cisplatin-induced DNA damage
abstract: Cancer chemotherapeutic agents such as cisplatin exert their cytotoxic effect by inducing DNA damage and activating programmed cell death (apoptosis). The tumour-suppressor protein p53 is an important activator of apoptosis. Although p53-deficient cancer cells are less responsive to chemotherapy, their resistance is not complete, which suggests that other apoptotic pathways may exist. A p53 -related gene, p73, which encodes several proteins as a result of alternative splicing,, can also induce apoptosis. Here we show that the amount of p73 protein in the cell is increased by cisplatin. This induction of p73 is not seen in cells unable to carry out mismatch repair and in which the nuclear enzyme c-Abl tyrosine kinase is not activated by cisplatin. The half-life of p73 is prolonged by cisplatin and by co-expression with c-Abl tyrosine kinase; the apoptosis-inducing function of p73 is also enhanced by the c-Abl kinase. Mouse embryo fibroblasts deficient in mismatch repair or in c-Abl do not upregulate p73 and are more resistant to killing by cisplatin. Our results indicate that c-Abl and p73 are components of a mismatch-repair-dependent apoptosis pathway which contributes to cisplatin-induced cytotoxicity.

title: A Regulatory Circuit Controlling Itch-mediated p73 Degradation by Runx*
abstract: The members of the tumor suppressor p53 family are under tight regulation by distinct ubiquitin-protein isopeptide (E3) ligases. The level of p73 is regulated by the E3 ligase Itch. Itch levels are sharply reduced in response to DNA damage with concomitant p73 accumulation and activation. The mechanism of controlling Itch level is not known. We show that the Itch promoter is a target of the transcription activator Runx. Yes-associated protein (Yap1) is a shared transcription co-activator of Runx and p73. Under normal conditions, the Runx-Yap1 complex binds the Itch promoter and supports its transcription and p73 degradation. In response to DNA damage, Yap1 is phosphorylated by c-Abl at the position Tyr-357. The modified Yap1 does not co-activate Runx in supporting Itch transcription. The subsequent reduction in the Itch level gives rise to p73 accumulation. These results demonstrate how Yap1 supports degradation of p73 via Runx and how it plays an opposite role in response to DNA damage.

title: c-Abl forces YAP to switch sides
abstract: Cancer research has been significantly accelerated by viewing cancer as a functional collision between 2 dichotomous sets of genes: oncogenes and tumor suppressors. Signaling pathways turn oncogenes and tumor suppressors on and off to dictate cell fate decisions. We contend that signaling also dictates opposing behaviors of a given effector.

title: Modularity in the Hippo signaling pathway.
abstract: Metazoans have evolved several pathways to regulate the size of organs and ultimately that of organisms. One such pathway is known as Salvador-Warts-Hippo, or simply Hippo. Research on the Hippo pathway has grown exponentially during the past 8 years, revealing a complex signaling network. Intriguingly, within this complexity, there are levels of modularity. One level of modularity is represented by the unusually wide occurrence of the WW module in the Hippo core kinase cassette, the upstream regulatory components and the downstream nuclear proteins. We suggest that the prevalence of WW domain-mediated complexes in the Hippo pathway should facilitate its molecular analysis and aid prediction of new pathway components.

title: The transcriptional coactivator Yes-associated protein drives p73 gene-target specificity in response to DNA Damage.
abstract: The transcriptional coactivator Yes-associated protein (YAP) has been shown to interact with and to enhance p73-dependent apoptosis in response to DNA damage. Here, we show that YAP requires the promyelocytic leukemia gene (PML) and nuclear body localization to coactivate p73. YAP imparts selectivity to p73 by promoting the activation of a subset of p53 and/or p73 target promoters. Endogenous p73, YAP, and p300 proteins are concomitantly recruited onto the regulatory regions of the apoptotic target gene p53AIP1 only when cells are exposed to apoptotic conditions. Silencing of YAP by specific siRNA impairs p300 recruitment and reduces histone acetylation on the p53AIP1 target gene, resulting in delayed or reduced apoptosis mediated by p73. We also found that YAP contributes to the DNA damage-induced accumulation of p73 and potentiates the p300-mediated acetylation of p73. Altogether, our findings identify YAP as a key determinant of p73 gene targeting in response to DNA damage.

title: Apical junctions and growth control in Drosophila.
abstract: Recent studies have revealed unexpected links between cell polarity and proliferation, suggesting that the polarized organization of cells is necessary to regulate growth. Drosophila melanogaster is a genetically simple model that is especially suited for the study of polarity and growth control, as polarized tissues undergo a well-defined pattern of proliferation and differentiation during the development. In addition, genetic studies have identified a number of tumor suppressor genes, which later studies have shown to be associated with junctions, or in the regulation of junctional proteins. We will explore in this review the links between growth and apical junction proteins in the regulation of growth control in Drosophila.

title: Wbp2 cooperates with Yorkie to drive tissue growth downstream of the Salvador–Warts–Hippo pathway
abstract: The Salvador–Warts–Hippo (SWH) pathway is a key controller of tissue growth in both flies and mammals, and deregulation of pathway activity contributes to tumour formation. The SWH pathway regulates cell growth, proliferation and apoptosis by restricting activity of the Yorkie transcriptional co-activator protein. The proteins that function together with Yorkie to drive transcription and tissue growth are beginning to be revealed and include the Scalloped (Sd), Teashirt (Tsh) and Homothorax (Hth) transcription factors. In this study, we define Wbp2 as a promoter of Yorkie-dependent growth of Drosophila melanogaster tissues. Mammalian WBP2 was previously identified as a protein that interacts with the mammalian Yorkie homologue, Yes-associated protein. WBP2 has been shown to enhance steroid hormone-dependent transcription in cultured cells but its in vivo function has remained obscure. We show that D. melanogaster Wbp2 interacts with Yorkie in a WW domain- and PY motif-dependent manner and that Wbp2 can enhance Yorkie's transcriptional co-activator properties. In vivo, Wbp2 is required for growth of the D. melanogaster wing, and reduction of Wbp2 expression suppresses overgrowth of tissues that lack the warts growth-suppressive gene. Collectively, these studies define an important role for Wbp2 as a downstream component of the SWH tissue growth-control pathway.

title: SCALLOPED Interacts with YORKIE, the Nuclear Effector of the Hippo Tumor-Suppressor Pathway in Drosophila
abstract: In Drosophila, SCALLOPED (SD) belongs to a family of evolutionarily conserved proteins characterized by the presence of a TEA/ATTS DNA-binding domain [1, 2]. SD physically interacts with the product of the vestigial (vg) gene, where the dimer functions as a master gene controlling wing formation [3, 4]. The VG-SD dimer activates the transcription of several specific wing genes, including sd and vg themselves [5, 6]. The dimer drives cell-cycle progression by inducing expression of the dE2F1 transcription factor [7], which regulates genes involved in DNA replication and cell-cycle progression. Recently, YORKIE (YKI) was identified as a transcriptional coactivator that is the downstream effector of the Hippo signaling pathway, which controls cell proliferation and apoptosis in Drosophila[8]. We identified SD as a partner for YKI. We show that interaction between YKI and SD increases SD transcriptional activity both ex vivo in Drosophila S2 cells and in vivo in Drosophila wing discs and promotes YKI nuclear localization. We also show that YKI overexpression induces vg and dE2F1 expression and that proliferation induced by YKI or by a dominant-negative form of FAT in wing disc is significantly reduced in a sd hypomorphic mutant context. Contrary to YKI, SD is not required in all imaginal tissues. This indicates that YKI-SD interaction acts in a tissue-specific fashion and that other YKI partners must exist.

title: The TEAD/TEF family protein Scalloped mediates transcriptional output of the Hippo growth-regulatory pathway.
abstract: The Hippo (Hpo) kinase cascade restricts tissue growth by inactivating the transcriptional coactivator Yorkie (Yki), which regulates the expression of target genes such as the cell death inhibitor diap1 by unknown mechanisms. Here we identify the TEAD/TEF family protein Scalloped (Sd) as a DNA-binding transcription factor that partners with Yki to mediate the transcriptional output of the Hpo growth-regulatory pathway. The diap1 (th) locus harbors a minimal Sd-binding Hpo Responsive Element (HRE) that mediates transcriptional regulation by the Hpo pathway. Sd binds directly to Yki, and a Yki missense mutation that abrogates Sd-Yki binding also inactivates Yki function in vivo. We further demonstrate that sd is required for yki-induced tissue overgrowth and target gene expression, and that sd activity is conserved in its mammalian homolog. Our results uncover a heretofore missing link in the Hpo signaling pathway and provide a glimpse of the molecular events on a Hpo-responsive enhancer element.

title: The TEAD/TEF family of transcription factor Scalloped mediates Hippo signaling in organ size control.
abstract: The Hippo (Hpo) signaling pathway governs cell growth, proliferation, and apoptosis by controlling key regulatory genes that execute these processes; however, the transcription factor of the pathway has remained elusive. Here we provide evidence that the TEAD/TEF family transcription factor Scalloped (Sd) acts together with the coactivator Yorkie (Yki) to regulate Hpo pathway-responsive genes. Sd and Yki form a transcriptional complex whose activity is inhibited by Hpo signaling. Sd overexpression enhances, whereas its inactivation suppresses, tissue overgrowth caused by Yki overexpression or tumor suppressor mutations in the Hpo pathway. Inactivation of Sd diminishes Hpo target gene expression and reduces organ size, whereas a constitutively active Sd promotes tissue overgrowth. Sd promotes Yki nuclear localization, whereas Hpo signaling retains Yki in the cytoplasm by phosphorylating Yki at S168. Finally, Sd recruits Yki to the enhancer of the pathway-responsive gene diap1, suggesting that diap1 is a direct transcriptional target of the Hpo pathway.

title: Nuclear CDKs Drive Smad Transcriptional Activation and Turnover in BMP and TGF-β Pathways
abstract: TGF-beta and BMP receptor kinases activate Smad transcription factors by C-terminal phosphorylation. We have identified a subsequent agonist-induced phosphorylation that plays a central dual role in Smad transcriptional activation and turnover. As receptor-activated Smads form transcriptional complexes, they are phosphorylated at an interdomain linker region by CDK8 and CDK9, which are components of transcriptional mediator and elongation complexes. These phosphorylations promote Smad transcriptional action, which in the case of Smad1 is mediated by the recruitment of YAP to the phosphorylated linker sites. An effector of the highly conserved Hippo organ size control pathway, YAP supports Smad1-dependent transcription and is required for BMP suppression of neural differentiation of mouse embryonic stem cells. The phosphorylated linker is ultimately recognized by specific ubiquitin ligases, leading to proteasome-mediated turnover of activated Smad proteins. Thus, nuclear CDK8/9 drive a cycle of Smad utilization and disposal that is an integral part of canonical BMP and TGF-beta pathways.

title: Transcription factor choice in the Hippo signaling pathway: homothorax and yorkie regulation of the microRNA bantam in the progenitor domain of the Drosophila eye imaginal disc.
abstract: The accurate control of cell proliferation and survival is critical for animal development. The Hippo tumor suppressor pathway regulates both of these parameters by controlling the nuclear availability of the transcriptional coactivator Yorkie (Yki), which regulates downstream target genes together with Scalloped (Sd), a DNA-binding protein. Here we provide evidence that Yki can also regulate target genes in conjunction with Homothorax (Hth) and Teashirt (Tsh), two DNA-binding transcription factors expressed in the uncommitted progenitor cells of the Drosophila eye imaginal disc. Clonal analyses demonstrate that Hth and Tsh promote cell proliferation and protect eye progenitor cells from apoptosis. Genetic epistasis experiments suggest that Hth and Tsh execute these functions with Yki, in part by up-regulating the microRNA bantam. A physical interaction between Hth and Yki can be detected in cell culture, and we show that Hth and Yki are bound to a DNA sequence approximately 14 kb upstream of the bantam hairpin in eye imaginal disc cells, arguing that this regulation is direct. These data suggest that the Hippo pathway uses different DNA-binding transcription factors depending on the cellular context. In the eye disc, Hth and Tsh provide spatial information to this pathway, promoting cell proliferation and survival in the progenitor domain.

title: Cooperative regulation of growth by Yorkie and Mad through bantam.
abstract: The Dpp and Fat-Hippo signaling pathways both regulate growth in Drosophila. Dpp is a BMP family ligand and acts via a Smad family DNA-binding transcription factor, Mad. Fat-Hippo signaling acts via a non-DNA-binding transcriptional coactivator protein, Yorkie. Here, we show that these pathways are directly interlinked. They act synergistically to promote growth, in part via regulation of the microRNA gene bantam, and their ability to promote growth is mutually dependent. Yorkie and Mad physically bind each other, and we identify a 410 bp minimal enhancer of bantam that responds to Yorkie:Mad in vivo and in cultured cells, and show that both Yorkie and Mad associate with this enhancer in vivo. Our results indicate that in promoting the growth of Drosophila tissues, Fat-Hippo and Dpp signaling contribute distinct subunits of a shared transcriptional activation complex, Yorkie:Mad.

title: TGF-beta signaling, Smads, and tumor suppressors.
abstract: The transforming growth factor-beta (TGF-beta) superfamily is used throughout animal development for regulating the growth and patterning of many tissue types. During the past few years, rapid progress has been made in deciphering how TGF-beta signals are transduced from outside the cell to the nucleus. This progress is based on biochemical studies in vertebrate systems and a combination of genetic studies in Drosophila and Caenorhabditis elegans. These studies have identified a novel family of signaling proteins, the Smad family. Smads can act positively and be phosphorylated by TGF-beta-like receptors or can act negatively and prevent activation of the positively acting group. The positively acting Smads translocate to the nucleus, bind DNA, and act as transcriptional activators. Thus, genetic and biochemical studies suggest a very simple signaling pathway, in which Smads are the primary downstream participant.

title: Yes-associated protein (YAP65) interacts with Smad7 and potentiates its inhibitory activity against TGF-beta/Smad signaling
abstract: HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Yes-associated protein (YAP65) interacts with Smad7 and potentiates its inhibitory activity against TGF-beta/Smad signaling. Olivier Ferrigno, François Lallemand, Franck Verrecchia, Sébastien l’Hoste, Jacques Camonis, Azeddine Atfi, Alain Mauviel

title: TAZ controls Smad nucleocytoplasmic shuttling and regulates human embryonic stem-cell self-renewal
abstract: Transforming growth fazctor-β (TGFβ) family members regulate many developmental and pathological events through Smad transcriptional modulators. How nuclear accumulation of Smad is coupled to the transcriptional machinery is poorly understood. Here we demonstrate that in response to TGFβ stimulation the transcriptional regulator TAZ binds heteromeric Smad2/3–4 complexes and is recruited to TGFβ response elements. In human embryonic stem cells TAZ is required to maintain self-renewal markers and loss of TAZ leads to inhibition of TGFβ signalling and differentiation into a neuroectoderm lineage. In the absence of TAZ, Smad2/3–4 complexes fail to accumulate in the nucleus and activate transcription. Furthermore, TAZ, which itself engages in shuttling, dominantly controls Smad nucleocytoplasmic localization and can be retained in the nucleus by transcriptional co-factors such as ARC105, a component of the Mediator complex. TAZ thus defines a hierarchical system regulating Smad nuclear accumulation and coupling to the transcriptional machinery.

title: Validating upstream regulators of Yorkie activity in Hippo signaling through scalloped-based genetic epistasis
abstract: Genetic studies in Drosophila have been instrumental in characterizing the Hippo pathway, which converges on the co-activator Yorkie to regulate target gene transcription. A routinely used strategy to interrogate upstream regulators of Yorkie involves the examination of selected Hippo target genes upon loss or gain of function of a suspected pathway regulator. A caveat with this strategy is that aberrant expression of a given Hippo target per se does not distinguish whether it is caused by changes in Yorkie or Yorkie-independent inputs converging on the same target gene. Building on previous findings that the DNA-binding transcription factor Scalloped mediates both Yorkie overexpression and loss-of-function phenotypes yet is itself dispensable for normal eye development, we describe a simple strategy to distinguish these possibilities by analyzing double-mutant clones of scalloped and a suspected Yorkie regulator. We provide proof of principle that this strategy can be used effectively to validate canonical Yorkie regulators and to exclude proteins that impact target expression independent of Yorkie. The described methodology and reagents should facilitate efforts to assess the expanding repertoire of proteins implicated in regulation of Yorkie activity.  Analysis of double-mutant clones of sd and putative Yki regulators provides a simple and efficient strategy to validate upstream regulators of Yki activity in Hippo signaling.

title: Mechanical strain regulates the Hippo pathway in Drosophila
abstract: Animal cells are thought to sense mechanical forces via the transcriptional co-activators YAP (or YAP1) and TAZ (or WWTR1), the sole Drosophila homolog of which is named Yorkie (Yki). In mammalian cells in culture, artificial mechanical forces induce nuclear translocation of YAP and TAZ. Here, we show that physiological mechanical strain can also drive nuclear localisation of Yki and activation of Yki target genes in the Drosophila follicular epithelium. Mechanical strain activates Yki by stretching the apical domain, reducing the concentration of apical Crumbs, Expanded, Kibra and Merlin, and reducing apical Hippo kinase dimerisation.  Hippo kinase to induce ectopic activation in the cytoplasm is sufficient to prevent Yki nuclear localisation even in flattened follicle cells. Conversely, blocking Hippo signalling in warts clones causes Yki nuclear localisation even in columnar follicle cells. We find no evidence for involvement of other pathways, such as Src42A kinase, in regulation of Yki. Finally, our results in follicle cells appear generally applicable to other tissues, as nuclear translocation of Yki is also readily detectable in other flattened epithelial cells such as the peripodial epithelium of the wing imaginal disc, where it promotes cell flattening.  Drosophila Yorkie can sense physiological mechanical strain forces via the canonical Hippo pathway in different epithelial tissues.

title: Recruitment of Jub by α-catenin promotes Yki activity and Drosophila wing growth
abstract: The Hippo signaling network controls organ growth through YAP family transcription factors, including the Drosophila Yorkie protein. YAP activity is responsive to both biochemical and biomechanical cues, with one key input being tension within the F-actin cytoskeleton. Several potential mechanisms for the biomechanical regulation of YAP proteins have been described, including tension-dependent recruitment of Ajuba family proteins, which inhibit kinases that inactivate YAP proteins, to adherens junctions. Here, we investigate the mechanism by which the Drosophila Ajuba family protein Jub is recruited to adherens junctions, and the contribution of this recruitment to the regulation of Yorkie. We identify α-catenin as the mechanotransducer responsible for tension-dependent recruitment of Jub by identifying a region of α-catenin that associates with Jub, and by identifying a region, which when deleted, allows constitutive, tension-independent recruitment of Jub. We also show that increased Jub recruitment to α-catenin is associated with increased Yorkie activity and wing growth, even in the absence of increased cytoskeletal tension. Our observations establish α-catenin as a multi-functional mechanotransducer and confirm Jub recruitment to α-catenin as a key contributor to biomechanical regulation of Hippo signaling.  Identification of α-catenin as the mechanotransducer that recruits Jub to junctions under tension; this recruitment of Jub is sufficient to increase Yorkie activity.

title: Yorkie Functions at the Cell Cortex to Promote Myosin Activation in a Non-transcriptional Manner.
abstract: The Hippo signaling pathway is an evolutionarily conserved mechanism that controls organ size in animals. Yorkie is well known as a transcriptional co-activator that functions downstream of the Hippo pathway to positively regulate transcription of genes that promote tissue growth. Recent studies have shown that increased myosin activity activates both Yorkie and its vertebrate orthologue YAP, resulting in increased nuclear localization and tissue growth. Here we show that Yorkie also can accumulate at the cell cortex in the apical junctional region. Moreover, Yorkie functions at the cortex to promote activation of myosin through a myosin regulatory light chain kinase, Stretchin-Mlck. This Yorkie function is not dependent on its transcriptional activity and is required for larval and adult tissues to achieve appropriate size. Based on these results, we suggest that Yorkie functions in a feedforward "amplifier" loop that promotes myosin activation, and thereby greater Yorkie activity, in response to tension.

title: The Hippo tumor-suppressor pathway regulates apical-domain size in parallel to tissue growth
abstract: The Hippo tumor-suppressor pathway controls tissue growth in Drosophila and mammals by regulating cell proliferation and apoptosis. The Hippo pathway includes the Fat cadherin, a transmembrane protein, which acts upstream of several other components that form a kinase cascade that culminates in the regulation of gene expression through the transcriptional coactivator Yorkie (Yki). Our previous work in Drosophila indicated that Merlin (Mer) and Expanded (Ex) are members of the Hippo pathway and act upstream of the Hippo kinase. In contrast to this model, it was suggested that Mer and Ex primarily regulate membrane dynamics and receptor trafficking, thereby affecting Hippo pathway activity only indirectly. Here, we examined the effects of Mer, Ex and the Hippo pathway on the size of the apical membrane and on apical-basal polarity complexes. We found that mer;ex double mutant imaginal disc cells have significantly increased levels of apical membrane determinants, such as Crb, aPKC and Patj. These phenotypes were shared with mutations in other Hippo pathway components and required Yki, indicating that Mer and Ex signal through the Hippo pathway. Interestingly, however, whereas Crb was required for the accumulation of other apical proteins and for the expansion of the apical domain observed in Hippo pathway mutants, its elimination did not significantly reverse the overgrowth phenotype of warts mutant cells. Therefore, Hippo signaling regulates cell polarity complexes in addition to and independently of its growth control function in imaginal disc cells.

title: The apical-basal cell polarity determinant Crumbs regulates Hippo signaling in Drosophila
abstract: Defects in apical-basal cell polarity and abnormal expression of cell polarity determinants are often associated with cancer in vertebrates. In Drosophila, abnormal expression of apical-basal determinants can cause neoplastic phenotypes, including loss of cell polarity and overproliferation. However, the pathways through which apical-basal polarity determinants affect growth are poorly understood. Here, we investigated the mechanism by which the apical determinant Crumbs (Crb) affects growth in Drosophila imaginal discs. Overexpression of Crb causes severe overproliferation, and we found that loss of Crb similarly results in overgrowth of imaginal discs. Crb gain and loss of function caused defects in Hippo signaling, a key signaling pathway that controls tissue growth in Drosophila and mammals. Manipulation of Crb levels caused the up-regulation of Hippo target genes, genetically interacted with known Hippo pathway components, and required Yorkie, a transcriptional coactivator that acts downstream in the Hippo pathway, for target gene induction and overgrowth. Interestingly, Crb regulates growth and cell polarity through different motifs in its intracellular domain. A juxtamembrane FERM domain-binding motif is responsible for growth regulation and induction of Hippo target gene expression, whereas Crb uses a PDZ-binding motif to form a complex with other polarity factors. The Hippo pathway component Expanded, an apically localized adaptor protein, is mislocalized in both crb mutant cells and Crb overexpressing tissues, whereas the other Hippo pathway components, Fat and Merlin, are unaffected. Taken together, our data show that Crb regulates growth through Hippo signaling, and thus identify Crb as a previously undescribed upstream input into the Hippo pathway.

title: Transcriptional output of the Salvador/warts/hippo pathway is controlled in distinct fashions in Drosophila melanogaster and mammalian cell lines.
abstract: The Salvador/Warts/Hippo (SWH) pathway is an important modulator of organ size, and deregulation of pathway activity can lead to cancer. Several SWH pathway components are mutated or expressed at altered levels in different human tumors including NF2, LATS1, LATS2, SAV1, and YAP. The SWH pathway regulates tissue growth by restricting the activity of the transcriptional coactivator protein known as Yorkie (Yki) in Drosophila melanogaster and Yes-associated protein (YAP) in mammals. Yki/YAP drives tissue growth in partnership with the Scalloped (Sd)/TEAD1-4 transcription factors. Yki/YAP also possesses two WW domains, which contact several proteins that have been suggested to either promote or inhibit the ability of Yki to induce transcription. To investigate the regulatory role of the Yki/YAP WW domains, we analyzed the functional consequence of mutating these domains. WW domain mutant YAP promoted transformation and migration of breast epithelial cells with increased potency, suggesting that WW domains mediate the inhibitory regulation of YAP in these cells. By contrast, the WW domains were required for YAP to promote NIH-3T3 cell transformation and for the ability of Yki to drive tissue growth in D. melanogaster and optimally activate Sd. This shows that Yki/YAP WW domains have distinct regulatory roles in different cell types and implies the existence of proteins that promote tissue growth in collaboration with Yki and Sd.

title: The WW domain protein Kibra acts upstream of Hippo in Drosophila.
abstract: The conserved Hippo kinase pathway plays a pivotal role in organ size control and tumor suppression by restricting proliferation and promoting apoptosis. Whereas the function of the core kinase cascade, consisting of the serine/threonine kinases Hippo and Warts, in phosphorylating and thereby inactivating the transcriptional coactivator Yorkie is well established, much less is known about the upstream events that regulate Hippo signaling activity. The FERM domain proteins Expanded and Merlin appear to represent two different signaling branches that feed into the Hippo pathway. Signaling by the atypical cadherin Fat may act via Expanded, but how Merlin is regulated has remained elusive. Here, we show that the WW domain protein Kibra is a Hippo signaling component upstream of Hippo and Merlin. Kibra acts synergistically with Expanded, and it physically interacts with Merlin. Thus, Kibra predominantly acts in the Merlin branch upstream of the core kinase cascade to regulate Hippo signaling.

title: Kibra Is a Regulator of the Salvador/Warts/Hippo Signaling Network
abstract: The Salvador (Sav)/Warts (Wts)/Hippo (Hpo) (SWH) network controls tissue growth by inhibiting cell proliferation and promoting apoptosis. The core of the pathway consists of a MST and LATS family kinase cascade that ultimately phosphorylates and inactivates the YAP/Yorkie (Yki) transcription coactivator. The FERM domain proteins Merlin (Mer) and Expanded (Ex) represent one mode of upstream regulation controlling pathway activity. Here, we identify Kibra as a member of the SWH network. Kibra, which colocalizes and associates with Mer and Ex, also promotes the Mer/Ex association. Furthermore, the Kibra/Mer association is conserved in human cells. Finally, Kibra complexes with Wts and kibra depletion in tissue culture cells induces a marked reduction in Yki phosphorylation without affecting the Yki/Wts interaction. We suggest that Kibra is part of an apical scaffold that promotes SWH pathway activity.

title: Lgl, aPKC, and Crumbs Regulate the Salvador/Warts/Hippo Pathway through Two Distinct Mechanisms
abstract: The Drosophila neoplastic tumor suppressor Lethal (2) giant larvae (Lgl) controls apicobasal cell polarity and proliferation. We have previously shown that lgl(-) clones in the developing eye exhibit ectopic proliferation and suppress apoptosis without affecting apicobasal cell polarity. Ectopic expression of the apical polarity regulators atypical protein kinase C (aPKC) and Crumbs also leads to increased cell proliferation and/or survival. Here we investigate how these cell polarity regulators control proliferation and survival.



We report that depletion of lgl in eye epithelial tissue, where polarity is maintained, results in upregulation of targets of the Salvador/Warts/Hippo (SWH) tumor suppressor pathway. Consistent with this, the SWH pathway transcriptional coactivator Yorkie is hyperactivated in Lgl-deficient tissue and is rate limiting for lgl(-) phenotypes. Overexpression of the apical polarity regulators Crumbs or aPKC also leads to ectopic expression of SWH pathway targets without affecting polarity. We show that Lgl depletion or aPKC overexpression results in comislocalization of Hippo and Ras-associated domain family protein (RASSF), consistent with RASSF's ability to block Hippo activation by Salvador. In contrast, Crumbs overexpression leads to mislocalization of Expanded away from the apical cortex, which is predicted to deregulate the pathway.



Collectively, our data reveal that the cell polarity regulators Lgl, aPKC, and Crumbs regulate the SWH pathway by two distinct pathways: Lgl acts antagonistically to aPKC to regulate Hippo and RASSF localization, whereas Crumbs regulates Expanded localization. Thus, our study implicates Lgl, aPKC, and Crumbs as regulators of tissue growth via the SWH pathway.

title: The apical transmembrane protein Crumbs functions as a tumor suppressor that regulates Hippo signaling by binding to Expanded
abstract: The Hippo signaling pathway regulates organ size and tissue homeostasis from Drosophila to mammals. At the core of the Hippo pathway is a kinase cascade extending from the Hippo (Hpo) tumor suppressor to the Yorkie (Yki) oncoprotein. The Hippo kinase cascade, in turn, is regulated by apical membrane-associated proteins such as the FERM domain proteins Merlin and Expanded (Ex), and the WW- and C2-domain protein Kibra. How these apical proteins are themselves regulated remains poorly understood. Here, we identify the transmembrane protein Crumbs (Crb), a determinant of epithelial apical-basal polarity in Drosophila embryos, as an upstream component of the Hippo pathway in imaginal disk growth control. Loss of Crb leads to tissue overgrowth and target gene expression characteristic of defective Hippo signaling. Crb directly binds to Ex through its juxtamembrane FERM-binding motif (FBM). Loss of Crb or mutation of its FBM leads to mislocalization of Ex to basolateral domain of imaginal disk epithelial cells. These results shed light on the mechanism of Ex regulation and provide a molecular link between apical-basal polarity and tissue growth. Furthermore, our studies implicate Crb as a putative cell surface receptor for Hippo signaling by uncovering a transmembrane protein that directly binds to an apical component of the Hippo pathway.

title: Crumbs Regulates Salvador/Warts/Hippo Signaling in Drosophila via the FERM-Domain Protein Expanded
abstract: Altered expression of apicobasal polarity factors is associated with cancer in vertebrates and tissue overgrowth in invertebrates, yet the mechanisms by which these factors affect growth-regulatory pathways are not well defined. We have tested the basis of an overgrowth phenotype driven by the Drosophila protein Crumbs (Crb), which nucleates an apical membrane complex that functionally interacts with the Par6/Par3/aPKC and Scrib/Dlg/Lgl apicobasal polarity complexes.



We find that Crb-driven growth is dependent upon the Salvador/Warts/Hippo (SWH) pathway and its transcriptional effector Yorkie (Yki). Expression of the Crb intracellular domain elevates Yki activity, and this correlates in tissues and cultured cells with loss of Expanded (Ex), an apically localized SWH component that inhibits Yki. Reciprocally, loss of crb elevates Ex levels, although this excess Ex does not concentrate to its normal location at apical junctions. The Ex-regulatory domain of Crb maps to the juxtamembrane FERM-binding motif (JM), a cytoskeletal interaction domain distinct from the PDZ-binding motif (PBM) through which Crb binds polarity factors. Expression of Crb-JM drives Yki activity and organ growth with little effect on tissue architecture, while Crb-PBM reciprocally produces tissue architectural defects without significant effect on Yki activity.



These studies identify Crb as a novel SWH regulator via JM-dependent effects on Ex levels and localization and suggest that discrete domains within Crb may allow it to integrate junctional polarity signals with a conserved growth pathway.

title: The Crumbs complex couples cell density sensing to Hippo-dependent control of the TGF-β-SMAD pathway.
abstract: The Hippo pathway senses cell density information to control tissue growth by regulating the localization of the transcriptional regulators TAZ and YAP (TAZ/YAP). TAZ/YAP also regulate TGF-β-SMAD signaling, but whether this role is linked to cell density sensing is unknown. Here we demonstrate that TAZ/YAP dictate the localization of active SMAD complexes in response to cell density-mediated formation of polarity complexes. In high-density cell cultures, the Hippo pathway drives cytoplasmic localization of TAZ/YAP, which sequesters SMAD complexes, thereby suppressing TGF-β signaling. We show that during mouse embryogenesis, this is reflected by differences in TAZ/YAP localization, which define regions of active SMAD2/3 complexes. Interfering with TAZ/YAP phosphorylation drives nuclear accumulation of TAZ/YAP and SMAD2/3. Furthermore, we demonstrate that the Crumbs polarity complex interacts with TAZ/YAP, which relays cell density information by promoting TAZ/YAP phosphorylation, cytoplasmic retention, and suppressed TGF-β signaling. Accordingly, disruption of the Crumbs complex enhances TGF-β signaling and predisposes cells to TGF-β-mediated epithelial-to-mesenchymal transitions.

title: Fat Cadherin Modulates Organ Size in Drosophila via the Salvador/Warts/Hippo Signaling Pathway
abstract: The atypical Fat cadherin has long been known to control cell proliferation and organ size in Drosophila, but the mechanism by which Fat controls these processes has remained elusive. A newly emerging signaling pathway that controls organ size during development is the Salvador/Warts/Hippo pathway.



Here we demonstrate that Fat limits organ size by modulating activity of the Salvador/Warts/Hippo pathway. ft interacts genetically with positive and negative regulators of this pathway, and tissue lacking fat closely phenocopies tissue deficient for genes that normally promote Salvador/Warts/Hippo pathway activity. Cells lacking fat grow and proliferate more quickly than their wild-type counterparts and exhibit delayed cell-cycle exit as a result of elevated expression of Cyclin E. fat mutant cells display partial insensitivity to normal developmental apoptosis cues and express increased levels of the anti-apoptotic DIAP1 protein. Collectively, these defects lead to increased organ size and organism lethality in fat mutant animals. Fat modulates Salvador/Warts/Hippo pathway activity by promoting abundance and localization of Expanded protein at the apical membrane of epithelial tissues.



Fat restricts organ size during Drosophila development via the Salvador/Warts/Hippo pathway. These studies aid our understanding of developmental organ size control and have implications for human hyperproliferative disorders, such as cancers.

title: Delineation of a Fat tumor suppressor pathway
abstract: Recent studies in Drosophila melanogaster of the protocadherins Dachsous and Fat suggest that they act as ligand and receptor, respectively, for an intercellular signaling pathway that influences tissue polarity, growth and gene expression, but the basis for signaling downstream of Fat has remained unclear. Here, we characterize functional relationships among D. melanogaster tumor suppressors and identify the kinases Discs overgrown and Warts as components of a Fat signaling pathway. fat, discs overgrown and warts regulate a common set of downstream genes in multiple tissues. Genetic experiments position the action of discs overgrown upstream of the Fat pathway component dachs, whereas warts acts downstream of dachs. Warts protein coprecipitates with Dachs, and Warts protein levels are influenced by fat, dachs and discs overgrown in vivo, consistent with its placement as a downstream component of the pathway. The tumor suppressors Merlin, expanded, hippo, salvador and mob as tumor suppressor also share multiple Fat pathway phenotypes but regulate Warts activity independently. Our results functionally link what had been four disparate groups of D. melanogaster tumor suppressors, establish a basic framework for Fat signaling from receptor to transcription factor and implicate Warts as an integrator of multiple growth control signals.

title: The Tumor-Suppressor Gene fat Controls Tissue Growth Upstream of Expanded in the Hippo Signaling Pathway
abstract: The tight control of cell proliferation and cell death is essential to normal tissue development, and the loss of this control is a hallmark of cancers. Cell growth and cell death are coordinately regulated during development by the Hippo signaling pathway. The Hippo pathway consists of the Ste20 family kinase Hippo, the WW adaptor protein Salvador, and the NDR kinase Warts. Loss of Hippo signaling in Drosophila leads to enhanced cell proliferation and decreased apoptosis, resulting in massive tissue overgrowth through increased expression of targets such as Cyclin E and Diap1. The cytoskeletal proteins Merlin and Expanded colocalize at apical junctions and function redundantly upstream of Hippo. It is not clear how they regulate growth or how they are localized to apical junctions.



We find that another Drosophila tumor-suppressor gene, the atypical cadherin fat, regulates both cell proliferation and cell death in developing imaginal discs. Loss of fat leads to increased Cyclin E and Diap1 expression, phenocopying loss of Hippo signaling. Ft can regulate Hippo phosphorylation, a measure of its activation, in tissue culture. Importantly, fat is needed for normal localization of Expanded at apical junctions in vivo. Genetic-epistasis experiments place fat with expanded in the Hippo pathway.



Together, these data suggest that Fat functions as a cell-surface receptor for the Expanded branch of the conserved Hippo growth control pathway.

title: Hippo signalling controls Dronc activity to regulate organ size in Drosophila
abstract: The Hippo pathway controls organ size by multiple mechanisms that ultimately regulate the transcriptional co-activator Yorkie (Yki). Downregulation of Hippo signalling leads to tissue overgrowths due to Yki-mediated activation of target genes, whereas overexpression of the pathway triggers apoptosis in developing tissues. However, the mechanism underlying cell death induced by Hippo (Hpo)-activation is not understood. We found that overexpression of Hpo leads to induction of Dronc (Drosophila Caspase-9 homologue) expression and downregulation of dronc can suppress/block Hpo-mediated apoptosis. Furthermore, upregulation of Dronc activity strongly suppressed the overgrowth caused by Yki overexpression thereby suggesting that Hippo signalling restricts Dronc activity. Hippo-mediated cell death requires the activity of the initiator caspase Dronc. Loss-of-function of dronc in genetic mosaics leads to cell survival and increased cell proliferation in imaginal discs. dronc is transcriptionally suppressed in Yki overexpressing cells or cells mutant for other Hippo pathway components like warts (wts). We propose that Dronc is a transcriptional target of the Hippo signalling pathway. The Hippo–Dronc connection has implications in control of overall organ size and other growth regulatory mechanisms like compensatory proliferation and cell competition.

title: Regulation of Hippo signaling by Jun kinase signaling during compensatory cell proliferation and regeneration, and in neoplastic tumors.
abstract: When cells undergo apoptosis, they can stimulate the proliferation of nearby cells, a process referred to as compensatory cell proliferation. The stimulation of proliferation in response to tissue damage or removal is also central to epimorphic regeneration. The Hippo signaling pathway has emerged as an important regulator of growth during normal development and oncogenesis from Drosophila to humans. Here we show that induction of apoptosis in the Drosophila wing imaginal disc stimulates activation of the Hippo pathway transcription factor Yorkie in surviving and nearby cells, and that Yorkie is required for the ability of the wing to regenerate after genetic ablation of the wing primordia. Induction of apoptosis activates Yorkie through the Jun kinase pathway, and direct activation of Jun kinase signaling also promotes Yorkie activation in the wing disc. We also show that depletion of neoplastic tumor suppressor genes, including lethal giant larvae and discs large, or activation of aPKC, activates Yorkie through Jun kinase signaling, and that Jun kinase activation is necessary, but not sufficient, for the disruption of apical-basal polarity associated with loss of lethal giant larvae. Our observations identify Jnk signaling as a modulator of Hippo pathway activity in wing imaginal discs, and implicate Yorkie activation in compensatory cell proliferation and disc regeneration.

title: Warts and Yorkie Mediate Intestinal Regeneration by Influencing Stem Cell Proliferation
abstract: Homeostasis in the Drosophila midgut is maintained by stem cells [1, 2]. The intestinal epithelium contains two types of differentiated cells that are lost and replenished: enteroendocrine (EE) cells and enterocytes (ECs). Intestinal stem cells (ISCs) are the only cells in the adult midgut that proliferate [3, 4], and ISC divisions give rise to an ISC and an enteroblast (EB), which differentiates into an EC or an EE cell [3-5]. If the midgut epithelium is damaged, then ISC proliferation increases [6-12]. Damaged ECs express secreted ligands (Unpaired proteins) that activate Jak-Stat signaling in ISCs and EBs to promote their proliferation and differentiation [7, 9, 13, 14]. We show that the Hippo pathway components Warts and Yorkie mediate a transition from low- to high-level ISC proliferation to facilitate regeneration. The Hippo pathway regulates growth in diverse organisms and has been linked to cancer [15, 16]. Yorkie is activated in ECs in response to tissue damage or activation of the damage-sensing Jnk pathway. Activation of Yorkie promotes expression of unpaired genes and triggers a nonautonomous increase in ISC proliferation. Our observations uncover a role for Hippo pathway components in regulating stem cell proliferation and intestinal regeneration.

title: scribble mutants promote aPKC and JNK-dependent epithelial neoplasia independently of Crumbs
abstract: BackgroundMetastatic neoplasias are characterized by excessive cell proliferation and disruptions to apico-basal cell polarity and tissue architecture. Understanding how alterations in cell polarity can impact upon tumour development is, therefore, a central issue in cancer biology. The Drosophila gene scribble (scrib) encodes a PDZ-domain scaffolding protein that regulates cell polarity and acts as a tumour suppressor in flies. Increasing evidence also implicates the loss of human Scrib in cancer. In this report, we investigate how loss of Scrib promotes epithelial tumourigenesis in Drosophila, both alone and in cooperation with oncogenic mutations.ResultsWe find that genetically distinct atypical protein kinase C (aPKC)-dependent and Jun N-terminal kinase (JNK)-dependent alterations in scrib mutants drive epithelial tumourigenesis. First, we show that over-expression of the apical cell polarity determinants Crumbs (Crb) or aPKC induces similar cell morphology defects and over-proliferation phenotypes as scrib loss-of-function. However, the morphological and proliferative defects in scrib mutants are independent of Crb function, and instead can be rescued by a dominant negative (kinase dead) aPKC transgene. Secondly, we demonstrate that loss of Scrib promotes oncogene-mediated transformation through both aPKC and JNK-dependent pathways. JNK normally promotes apoptosis of scrib mutant cells. However, in cooperation with oncogenic activated Ras or Notch signalling, JNK becomes an essential driver of tumour overgrowth and invasion. aPKC-dependent signalling in scrib mutants cooperates with JNK to significantly enhance oncogene-mediated tumour overgrowth.ConclusionThese results demonstrate distinct aPKC and JNK-dependent pathways through which loss of Scrib promotes tumourigenesis in Drosophila. This is likely to have a direct relevance to the way in which human Scrib can similarly restrain an oncogene-mediated transformation and, more generally, on how the outcome of oncogenic signalling can be profoundly perturbed by defects in apico-basal epithelial cell polarity.

title: Scribble participates in Hippo signaling and is required for normal zebrafish pronephros development
abstract: Spatial organization of cells and their appendages is controlled by the planar cell polarity pathway, a signaling cascade initiated by the protocadherin Fat in Drosophila. Vertebrates express 4 Fat molecules, Fat1–4. We found that depletion of Fat1 caused cyst formation in the zebrafish pronephros. Knockdown of the PDZ domain containing the adaptor protein Scribble intensified the cyst-promoting phenotype of Fat1 depletion, suggesting that Fat1 and Scribble act in overlapping signaling cascades during zebrafish pronephros development. Supporting the genetic interaction with Fat1, Scribble recognized the PDZ-binding site of Fat1. Depletion of Yes-associated protein 1 (YAP1), a transcriptional co-activator inhibited by Hippo signaling, ameliorated the cyst formation in Fat1-deficient zebrafish, whereas Scribble inhibited the YAP1-induced cyst formation. Thus, reduced Hippo signaling and subsequent YAP1 disinhibition seem to play a role in the development of pronephric cysts after depletion of Fat1 or Scribble. We hypothesize that Hippo signaling is required for normal pronephros development in zebrafish and that Scribble is a candidate link between Fat and the Hippo signaling cascade in vertebrates.

title: Loss of Cell Adhesion Increases Tumorigenic Potential of Polarity Deficient Scribble Mutant Cells
abstract: Epithelial polarity genes are important for maintaining tissue architecture, and regulating growth. The Drosophila neoplastic tumor suppressor gene scribble (scrib) belongs to the basolateral polarity complex. Loss of scrib results in disruption of its growth regulatory functions, and downregulation or mislocalization of Scrib is correlated to tumor growth. Somatic scribble mutant cells (scrib-) surrounded by wild-type cells undergo apoptosis, which can be prevented by introduction of secondary mutations that provide a growth advantage. Using genetic tools in Drosophila, we analyzed the phenotypic effects of loss of scrib in different growth promoting backgrounds. We investigated if a central mechanism that regulates cell adhesion governs the growth and invasive potential of scrib mutant cells. Here we show that increased proliferation, and survival abilities of scrib- cells in different genetic backgrounds affect their differentiation, and intercellular adhesion. Further, loss of scrib is sufficient to cause reduced cell survival, activation of the JNK pathway and a mild reduction of cell adhesion. Our data show that for scrib cells to induce aggressive tumor growth characterized by loss of differentiation, cell adhesion, increased proliferation and invasion, cooperative interactions that derail signaling pathways play an essential role in the mechanisms leading to tumorigenesis. Thus, our study provides new insights on the effects of loss of scrib and the modification of these effects via cooperative interactions that enhance the overall tumorigenic potential of scrib deficient cells.

title: Scribble Acts in the Drosophila Fat-Hippo Pathway to Regulate Warts Activity
abstract: Epithelial cells are the major cell-type for all organs in multicellular organisms. In order to achieve correct organ size, epithelial tissues need mechanisms that limit their proliferation, and protect tissues from damage caused by defective epithelial cells. Recently, the Hippo signaling pathway has emerged as a major mechanism that orchestrates epithelial development. Hippo signaling is required for cells to stop proliferation as in the absence of Hippo signaling tissues continue to proliferate and produce overgrown organs or tumors. Studies in Drosophila have led the way in providing a framework for how Hippo alters the pattern of gene transcription in target cells, leading to changes in cell proliferation, survival, and other behaviors. Scribble (Scrib) belongs to a class of neoplastic tumor suppressor genes that are required to establish apical-basal cell polarity. The disruption of apical-basal polarity leads to uncontrolled cell proliferation of epithelial cells. The interaction of apical basal polarity genes with the Hippo pathway has been an area of intense investigation. Loss of scrib has been known to affect Hippo pathway targets, however, its functions in the Hippo pathway still remain largely unknown. We investigated the interactions of Scrib with the Hippo pathway. We present data suggesting that Drosophila scrib acts downstream of the Fat (Ft) receptor, and requires Hippo signaling for its growth regulatory functions. We show that Ft requires Scrib to interact with Expanded (Ex) and Dachs (D), and for regulating Warts (Wts) levels and stability, thus placing Scrib in the Hippo pathway network.

title: A dp53/JNK-dependant feedback amplification loop is essential for the apoptotic response to stress in Drosophila
abstract: Programmed cell death (apoptosis) is a conserved process aimed to eliminate unwanted cells. The key molecules are a group of proteases called caspases that cleave vital proteins, which leads to the death of cells. In Drosophila, the apoptotic pathway is usually represented as a cascade of events in which an initial stimulus activates one or more of the proapoptotic genes (hid, rpr, grim), which in turn activate caspases. In stress-induced apoptosis, the dp53 (Drosophila p53) gene and the Jun N-terminal kinase (JNK) pathway function upstream in the activation of the proapoptotic genes. Here we demonstrate that dp53 and JNK also function downstream of proapoptotic genes and the initiator caspase Dronc (Drosophila NEDD2-like caspase) and that they establish a feedback loop that amplifies the initial apoptotic stimulus. This loop plays a critical role in the apoptotic response because in its absence there is a dramatic decrease in the amount of cell death after a pulse of the proapoptotic proteins Hid and Rpr. Thus, our results indicate that stress-induced apoptosis in Drosophila is dependant on an amplification loop mediated by dp53 and JNK. Furthermore, they also demonstrate a mechanism of mutual activation of proapoptotic genes.

title: The Hippo pathway acts via p53 and microRNAs to control proliferation and proapoptotic gene expression during tissue growth
abstract: The Hippo pathway has a central role in coordinating tissue growth and apoptosis. Mutations that compromise Hippo pathway activity cause tissue overgrowth and have been causally linked to cancer. In Drosophila, the transcriptional coactivator Yorkie mediates Hippo pathway activity to control the expression of cyclin E and Myc to promote cell proliferation, as well as the expression of bantam miRNA and DIAP1 to inhibit cell death. Here we present evidence that the Hippo pathway acts via Yorkie and p53 to control the expression of the proapoptotic gene reaper. Yorkie further mediates reaper levels post-transcriptionally through regulation of members of the miR-2 microRNA family to prevent apoptosis. These findings provide evidence that the Hippo pathway acts via several distinct routes to limit proliferation-induced apoptosis.

title: Drosophila miR2 induces pseudo-polysomes and inhibits translation initiation
abstract: MicroRNAs (miRs) inhibit protein synthesis by mechanisms that are as yet unresolved. We developed a cell-free system from Drosophila melanogaster embryos that faithfully recapitulates miR2-mediated translational control by means of the 3′ untranslated region of the D. melanogaster reaper messenger RNA. Here we show that miR2 inhibits translation initiation without affecting mRNA stability. Surprisingly, miR2 induces the formation of dense (heavier than 80S) miRNPs (‘pseudo-polysomes’) even when polyribosome formation and 60S ribosomal subunit joining are blocked. An mRNA bearing an ApppG instead of an m7GpppG cap structure escapes the miR2-mediated translational block. These results directly show the inhibition of m7GpppG cap-mediated translation initiation as the mechanism of miR2 function, and uncover pseudo-polysomal messenger ribonucleoprotein assemblies that may help to explain earlier findings.

title: bantam Encodes a Developmentally Regulated microRNA that Controls Cell Proliferation and Regulates the Proapoptotic Gene hid in Drosophila
abstract: Cell proliferation, cell death, and pattern formation are coordinated in animal development. Although many proteins that control cell proliferation and apoptosis have been identified, the means by which these effectors are linked to the patterning machinery remain poorly understood. Here, we report that the bantam gene of Drosophila encodes a 21 nucleotide microRNA that promotes tissue growth. bantam expression is temporally and spatially regulated in response to patterning cues. bantam microRNA simultaneously stimulates cell proliferation and prevents apoptosis. We identify the pro-apoptotic gene hid as a target for regulation by bantam miRNA, providing an explanation for bantam's anti-apoptotic activity.

title: The bantam MicroRNA Is a Target of the Hippo Tumor-Suppressor Pathway
abstract: The Hippo tumor-suppressor pathway has emerged as a key signaling pathway that controls tissue size in Drosophila. Hippo signaling restricts tissue size by promoting apoptosis and cell-cycle arrest, and animals carrying clones of cells mutant for hippo develop severely overgrown adult structures. The Hippo pathway is thought to exert its effects by modulating gene expression through the phosphorylation of the transcriptional coactivator Yorkie. However, how Yorkie regulates growth, and thus the identities of downstream target genes that mediate the effects of Hippo signaling, are largely unknown.



Here, we report that the bantam microRNA is a downstream target of the Hippo signaling pathway. In common with Hippo signaling, the bantam microRNA controls tissue size by regulating cell proliferation and apoptosis. We found that hippo mutant cells had elevated levels of bantam activity and that bantam was required for Yorkie-driven overgrowth. Additionally, overexpression of bantam was sufficient to rescue growth defects of yorkie mutant cells and to suppress the cell death induced by Hippo hyperactivation. Hippo regulates bantam independently of cyclin E and diap1, two other Hippo targets, and overexpression of bantam mimics overgrowth phenotypes of hippo mutant cells.



Our data indicate that bantam is an essential target of the Hippo signaling pathway to regulate cell proliferation, cell death, and thus tissue size.

title: The Hippo Pathway Regulates the bantam microRNA to Control Cell Proliferation and Apoptosis in Drosophila
abstract: The Hippo signaling pathway acts upon the Yorkie transcriptional activator to control tissue growth in Drosophila. Activated Yorkie drives growth by stimulating cell proliferation and inhibiting apoptosis, but how it achieves this is not understood. Yorkie is known to activate Cyclin E (CycE) and the apoptosis inhibitor DIAP1. However, overexpression of these targets is not sufficient to cause tissue overgrowth. Here we show that Yorkie also activates expression of the bantam microRNA, a known regulator of both proliferation and apoptosis. bantam overexpression mimics Yorkie activation while loss of bantam function slows the rate of cell proliferation. bantam is necessary for Yorkie-induced overproliferation and bantam overexpression is sufficient to rescue survival and proliferation of yorkie mutant cells. Finally, we show that bantam levels are regulated during both developmentally programmed proliferation arrest and apoptosis. In summary, the results show that the Hippo pathway regulates expression of bantam to control tissue growth in Drosophila.

title: Cell detachment activates the Hippo pathway via cytoskeleton reorganization to induce anoikis.
abstract: Cell attachment to the extracellular matrix (ECM) is crucial to cell physiology such as polarity, motility, and proliferation. In normal cells, loss of attachment to the ECM induces a specific type of apoptosis, termed anoikis. Resistance to anoikis in cancer cells promotes their survival in circulation and dispersion to distant anatomic sites, leading to tumor metastasis. The Yes-associated protein (YAP) transcription coactivator is a human oncogene and a key regulator of organ size. The Hippo tumor suppressor pathway phosphorylates and inhibits YAP. However, little is known about the signals that regulate the Hippo pathway. Here we report that through cytoskeleton reorganization, cell detachment activates the Hippo pathway kinases Lats1/2 and leads to YAP phosphorylation and inhibition. The detachment-induced YAP inactivation is required for anoikis in nontransformed cells, whereas in cancer cells with deregulation of the Hippo pathway, knockdown of YAP and TAZ restores anoikis. Furthermore, we provided evidence that Lats1/2 expression level is indeed significantly down-regulated in metastatic prostate cancer. Our findings provide a novel connection between cell attachment and anoikis through the Hippo pathway and have important implications in cancer therapeutics.

title: Giant and duplicated imaginal discs in a new lethal mutant of Drosophila melanogaster.
abstract: An account is given of a new mutant,lethal (2) giant discs (l(2)gd; 2–42.7 ± 0.5) ofDrosophila melanogaster, in which the imaginal discs grow to far beyond their normal size during an extended larval period (Figs. 2–6). The extent of the delay in puparium formation in this strain can be modified by changing the culture conditions . When the larvae are crowded, puparium formation is delayed for up to 9 days, and during this time the imaginal discs grow extensively and acquire an abnormal morphology. Under noncrowded conditions, the delay in puparium formation is usually less than 1 day, and the imaginal discs are correspondingly less abnormal. The time of death (effective lethal phase) is also dependent on culture conditions. Alll(2)gd homozygotes form puparia and pupal cuticle, but there are various defects in eversion of the discs. When the larvae are grown under crowded conditions, death follows shortly after disc eversion; hence no adult cuticle is secreted. In noncrowded cultures, however, about half of the mutant animals are able to produce some adult structures (Figs. 7 and 8). There are characteristic defects in the adult structures produced in these animals, which are also produced byl(2)gd imaginal discs transplanted into wild-type host larvae for metamorphosis (Figs. 9–12). The cuticular hair patterns are normal, but the number of bristles is reduced to about 20% of the number in wild type, and the number of bracts and of sensilla is reduced even more. Those bristles which are produced are usually normal in appearance. Normal aristae are produced, but the ungius is usually missing from the claw organ. Tarsi produced by the leg disc have only two segments instead of five, and appear to have excessive amounts of intersegmental membrane  In certain imaginal discs, the extensive growth during the lengthened larval period seems to occur preferentially in particular regions. This is reflected in the size of the corresponding structures produced after metamorphosis in wild-type hosts . In the second and third leg discs, the region of maximal growth is in the thoracic anlage, and this growth is followed by the appearance of a secondary involution and folding in this region . Leg discs which contain such secondary folds give rise to duplicated legs after metamorphosis in a wild-type host (Figs. 13 and 14), hence the new involution and folding corresponds with a real duplication of anlagen in the disc. The details of the duplication process are discussed; it is suggested that the production of these secondary anlagen resembles the normal development of the leg disc more than it resembles other examples of pattern duplication in imaginal discs. There appears to be a population of cells in the leg disc ofl(2)gd which have special regulative properties. l(2)gd tissue has the capacity for transdetermination, although the frequency is lower than in wild-type tissue.

title: Malignant neoplasms of genetic origin in Drosophila melanogaster.
abstract: Malignant neoplasms that develop in 12 recessive-lethal, larval mutants of Drosophila melanogaster are discussed. These mutations affect the adult optic neuroblasts and ganglion-mother cells in the larval brain, the imaginal discs, and the hematopoietic organs. The malignant neoplasms exhibit fast, autonomous growth, loss of the capacity for differentiation, increased mobility and invasiveness, lethality in situ and after transplantation, and histological, fine structural, and karyotypic abnormalities. Intermediate neoplasms are also found. These combine both benign and malignant qualities. They grow in a noninvasive, compact fashion, typical of benign tumors, yet they also exhibit malignant qualities such as fast, autonomous, and lethal growth, loss of differentiation capacity, changes in cellular morphology, and lethal growth after transplantation into wild-type hosts. Thus Drosophila and vertebrate neoplasms show striking similarities.

title: Mosaic analysis with a repressible cell marker (MARCM) for Drosophila neural development
abstract: We have modified an FLP/FRT-based genetic mosaic system to label either neurons derived from a common progenitor or isolated single neurons, in the Drosophila CNS. These uniquely labeled neurons can also be made homozygous for a mutation of interest within an otherwise phenotypically wild-type brain. Using this new mosaic system, not only can normal brain development be described with unprecedented single cell resolution, but also the underlying molecular mechanisms can be investigated by identifying genes that are required for these developmental processes.

title: scribble mutants cooperate with oncogenic Ras or Notch to cause neoplastic overgrowth in Drosophila
abstract: Cancer is a multistep process involving cooperation between oncogenic or tumor suppressor mutations and interactions between the tumor and surrounding normal tissue. Here we present the first description of cooperative tumorigenesis in Drosophila, by using a system that mimics the development of tumors in mammals. We have used the MARCM system to generate mutant clones of the apical–basal cell polarity tumor suppressor gene, scribble, in the context of normal tissue. We show that scribble mutant clones in the eye disc exhibit ectopic expression of cyclin E and ectopic cell cycles, but do not overgrow due to increased cell death mediated by the JNK pathway and the surrounding wild‐type tissue. In contrast, when oncogenic Ras or Notch is expressed within the scribble mutant clones, cell death is prevented and neoplastic tumors develop. This demonstrates, for the first time in Drosophila, that activated alleles of Ras and Notch can act as cooperating oncogenes in the development of epithelial tumors, and highlights the importance of epithelial polarity regulators in restraining oncogenes and preventing tumor formation.

title: A Genetic Screen in Drosophila for Metastatic Behavior
abstract: A genetic screen was designed in Drosophila to interrogate its genome for mutations sufficient to cause noninvasive tumors of the eye disc to invade neighboring or distant tissues. We found that cooperation between oncogenic RasV12 expression and inactivation of any one of a number of genes affecting cell polarity leads to metastatic behavior, including basement membrane degradation, loss of E-cadherin expression, migration, invasion, and secondary tumor formation. Inactivation of these cell polarity genes cannot drive metastatic behavior alone or in combination with other tumor-initiating alterations. These findings suggest that the oncogenic background of tissues makes a distinct contribution toward metastatic development.

title: A Feed-Forward Circuit Linking Wingless, Fat-Dachsous Signaling, and the Warts-Hippo Pathway to Drosophila Wing Growth
abstract: The secreted morphogen Wingless promotes Drosophila wing growth by fueling a wave front of Fat-Dachsous signaling that recruits new cells into the wing primordium.

title: Ajuba Family Proteins Link JNK to Hippo Signaling
abstract: An evolutionarily conserved family of proteins links pathways to promote cell proliferation in response to cell death or injury.  Sun and Irvine found that the Ajuba family of proteins enabled JNK to stimulate the activity of Yki and YAP. In Drosophila, the growth of tumors dependent on JNK and Yki activation was reduced by ablation of Jub (also known as Ajuba LIM protein). In mammalian cells, JNK activation decreased the activity of LATS1, a kinase that phosphorylates and inhibits YAP, and enhanced the association of LATS1 with various Ajuba family proteins in a manner dependent on phosphorylation of the Ajuba proteins. These results suggest a model in which JNK-mediated phosphorylation of Ajuba proteins inhibits the Hippo pathway and thus stimulates the transcriptional activation of genes that promote proliferation. Wounding, apoptosis, or infection can trigger a proliferative response in neighboring cells to replace damaged tissue. Studies in Drosophila have implicated c-Jun amino-terminal kinase (JNK)–dependent activation of Yorkie (Yki) as essential to regeneration-associated growth, as well as growth associated with neoplastic tumors. Yki is a transcriptional coactivator that is inhibited by Hippo signaling, a conserved pathway that regulates growth. We identified a conserved mechanism by which JNK regulated Hippo signaling. Genetic studies in Drosophila identified Jub (also known as Ajuba LIM protein) as required for JNK-mediated activation of Yki and showed that Jub contributed to wing regeneration after wounding and to tumor growth. Biochemical studies revealed that JNK promoted the phosphorylation of Ajuba family proteins in both Drosophila and mammalian cells. Binding studies in mammalian cells indicated that JNK increased binding between the Ajuba family proteins LIMD1 or WTIP and LATS1, a kinase within the Hippo pathway that inhibits the Yki homolog YAP. Moreover, JNK promoted binding of LIMD1 and LATS1 through direct phosphorylation of LIMD1. These results identify Ajuba family proteins as a conserved link between JNK and Hippo signaling, and imply that JNK increases Yki and YAP activity by promoting the binding of Ajuba family proteins to Warts and LATS.

title: Interplay among Drosophila transcription factors Ets21c, Fos and Ftz-F1 drives JNK-mediated tumor malignancy
abstract: Cancer initiation and maintenance of the transformed cell state depend on altered cellular signaling and aberrant activities of transcription factors (TFs) that drive pathological gene expression in response to cooperating genetic lesions. Deciphering the roles of interacting TFs is therefore central to understanding carcinogenesis and for designing cancer therapies. Here, we use an unbiased genomic approach to define a TF network that triggers an abnormal gene expression program promoting malignancy of clonal tumors, generated in Drosophila imaginal disc epithelium by gain of oncogenic Ras (RasV12) and loss of the tumor suppressor Scribble (scrib1). We show that malignant transformation of the rasV12scrib1 tumors requires TFs of distinct families, namely the bZIP protein Fos, the ETS-domain factor Ets21c and the nuclear receptor Ftz-F1, all acting downstream of Jun-N-terminal kinase (JNK). Depleting any of the three TFs improves viability of tumor-bearing larvae, and this positive effect can be enhanced further by their combined removal. Although both Fos and Ftz-F1 synergistically contribute to rasV12scrib1 tumor invasiveness, only Fos is required for JNK-induced differentiation defects and Matrix metalloprotease (MMP1) upregulation. In contrast, the Fos-dimerizing partner Jun is dispensable for JNK to exert its effects in rasV12scrib1 tumors. Interestingly, Ets21c and Ftz-F1 are transcriptionally induced in these tumors in a JNK- and Fos-dependent manner, thereby demonstrating a hierarchy within the tripartite TF network, with Fos acting as the most upstream JNK effector. Of the three TFs, only Ets21c can efficiently substitute for loss of polarity and cooperate with RasV12 in inducing malignant clones that, like rasV12scrib1 tumors, invade other tissues and overexpress MMP1 and the Drosophila insulin-like peptide 8 (Dilp8). While rasV12ets21c tumors require JNK for invasiveness, the JNK activity is dispensable for their growth. In conclusion, our study delineates both unique and overlapping functions of distinct TFs that cooperatively promote aberrant expression of target genes, leading to malignant tumor phenotypes.  This study provides genetic evidence that malignancy driven by oncogenic Ras and loss of polarity requires transcription factors of three distinct protein families, acting in synergy downstream of JNK signaling.

title: An Ectopic Network of Transcription Factors Regulated by Hippo Signaling Drives Growth and Invasion of a Malignant Tumor Model
abstract: Cancer cells have abnormal gene expression profiles; however, to what degree these are chaotic or driven by structured gene regulatory networks is often not known. Here we studied a model of Ras-driven invasive tumorigenesis in Drosophila epithelial tissues and combined in vivo genetics with next-generation sequencing and computational modeling to decipher the regulatory logic of tumor cells. Surprisingly, we discovered that the bulk of the tumor-specific gene expression is controlled by an ectopic network of a few transcription factors that are overexpressed and/or hyperactivated in tumor cells. These factors are Stat, AP-1, the bHLH proteins Myc and AP-4, the nuclear hormone receptor Ftz-f1, the nuclear receptor coactivator Taiman/SRC3, and Mef2. Notably, many of these transcription factors also are hyperactivated in human tumors. Bioinformatic analysis predicted that these factors directly regulate the majority of the tumor-specific gene expression, that they are interconnected by extensive cross-regulation, and that they show a high degree of co-regulation of target genes. Indeed, the factors of this network were required in multiple epithelia for tumor growth and invasiveness, and knockdown of several factors caused a reversion of the tumor-specific expression profile but had no observable effect on normal tissues. We further found that the Hippo pathway effector Yorkie was strongly activated in tumor cells and initiated cellular reprogramming by activating several transcription factors of this network. Thus, modeling regulatory networks identified an ectopic and ordered network of master regulators that control a large part of tumor cell-specific gene expression.

title: Non-cell-autonomous induction of tissue overgrowth by JNK/Ras cooperation in a Drosophila tumor model.
abstract: The role of c-Jun N-terminal kinase (JNK) signaling in cancer is enigmatic, and both tumor-promoting and tumor-suppressing functions have been ascribed to JNK pathway components. We have used the Drosophila eye to investigate the function of the JNK pathway in three different tumor models of increasing malignancy. Benign lesions caused by loss of the neoplastic tumor suppressor gene scribble can efficiently be eliminated by JNK-induced apoptosis. In such a scenario, the eye reverts to a wild-type phenotype, indicating that the JNK pathway prevents tumor formation. The situation changes in the case of aggressive tissue overgrowth, which can be induced by oncogenic activation of the Ras/Raf pathway in the eye, or in malignant invasive tumors resulting when Raf activation is combined with loss of scribble. The growth of these more aggressive tumor types is significantly, yet incompletely, suppressed by JNK-mediated apoptosis. Remarkably, oncogenic Raf and JNK cooperate in these tumors, to induce massive hyperplasia in adjacent wild-type tissue. Thus, depending on the genetic context, JNK signaling can eradicate tumors by removing premalignant cells, or promote aberrant overgrowth in tissues surrounding primary lesions.

title: Cell Competition Drives the Growth of Intestinal Adenomas in Drosophila
abstract: Tumor-host interactions play an increasingly recognized role in modulating tumor growth. Thus, understanding the nature and impact of this complex bidirectional communication is key to identifying successful anti-cancer strategies. It has been proposed that tumor cells compete with and kill neighboring host tissue to clear space that they can expand into; however, this has not been demonstrated experimentally. Here we use the adult fly intestine to investigate the existence and characterize the role of competitive tumor-host interactions. We show that APC−/−-driven intestinal adenomas compete with and kill surrounding cells, causing host tissue attrition. Importantly, we demonstrate that preventing cell competition, by expressing apoptosis inhibitors, restores host tissue growth and contains adenoma expansion, indicating that cell competition is essential for tumor growth. We further show that JNK signaling is activated inside the tumor and in nearby tissue and is required for both tumor growth and cell competition. Lastly, we find that APC−/− cells display higher Yorkie (YAP) activity than host cells and that this promotes tumor growth, in part via cell competition. Crucially, we find that relative, rather than absolute, Hippo activity determines adenoma growth. Overall, our data indicate that the intrinsic over-proliferative capacity of APC−/− cells is not uncontrolled and can be constrained by host tissues if cell competition is inhibited, suggesting novel possible therapeutic approaches.

title: Hippo pathway effector Yap is an ovarian cancer oncogene.
abstract: The Hippo pathway regulates organ size and tumorigenesis in Drosophila and mammals and is altered in a variety of human cancers, yet it remains unclear if the Hippo pathway is of prognostic significance to cancer patients. Here we show that the key targets of Hippo signaling, transcriptional coactivators Yki and Yap, play a conserved role in promoting ovarian cancer in flies and humans, respectively. Whereas studies linking Yap to cancer in other tissues have focused on overall Yap levels, we show for the first time that subcellular levels of Yap show an exceptionally strong association with poor patient survival. Specifically, high levels of nuclear Yap (nYap), or low levels of cytoplasmic phosphorylated Yap (cpYap), associated with poor survival from ovarian cancer. Patients with both high nYap and low cpYap had ∼50% lower 5-year survival, and this combination is an independent prognostic marker for survival, with an exceptionally high hazard ratio of 7.8. We find that Yap2 is the predominantly expressed Yap isoform in both the ovarian surface epithelium (OSE) and epithelial ovarian cancers. Overexpression of Yap2 and phosphorylation-defective Yap2-5SA in immortalized OSE cells resulted in increased cell proliferation, resistance to cisplatin-induced apoptosis, faster cell migration, and anchorage-independent growth, whereas Yap knockdown resulted in increased sensitivity to cisplatin-induced apoptosis. Findings argue that the Hippo signaling pathway defines an important pathway in progression of ovarian cancer.

title: Niche Appropriation by Drosophila Intestinal Stem Cell Tumors
abstract: Mutations that inhibit differentiation in stem cell lineages are a common early step in cancer development, but precisely how a loss of differentiation initiates tumorigenesis is unclear. We investigated Drosophila intestinal stem cell (ISC) tumours generated by suppressing Notch (N) signalling, which blocks differentiation. Notch-defective ISCs require stress-induced divisions for tumour initiation and an autocrine EGFR ligand, Spitz, during early tumour growth. On achieving a critical mass these tumours displace surrounding enterocytes, competing with them for basement membrane space and causing their detachment, extrusion and apoptosis. This loss of epithelial integrity induces JNK and Yki/YAP activity in enterocytes and, consequently, their expression of stress-dependent cytokines (Upd2, Upd3). These paracrine signals, normally used within the stem cell niche to trigger regeneration, propel tumour growth without the need for secondary mutations in growth signalling pathways. The appropriation of niche signalling by differentiation-defective stem cells may be a common mechanism of early tumorigenesis.

title: A Genetic Screen Reveals an Unexpected Role for Yorkie Signaling in JAK/STAT-Dependent Hematopoietic Malignancies in Drosophila melanogaster
abstract: A gain-of-function mutation in the tyrosine kinase JAK2 (JAK2V617F) causes human myeloproliferative neoplasms (MPNs). These patients present with high numbers of myeloid lineage cells and have numerous complications. Since current MPN therapies are not curative, there is a need to find new regulators and targets of Janus kinase/Signal transducer and activator of transcription (JAK/STAT) signaling that may represent additional clinical interventions . Drosophila melanogaster offers a low complexity model to study MPNs as JAK/STAT signaling is simplified with only one JAK [Hopscotch (Hop)] and one STAT (Stat92E). hopTumorous-lethal (Tum-l) is a gain-of-function mutation that causes dramatic expansion of myeloid cells, which then form lethal melanotic tumors. Through an F1 deficiency (Df) screen, we identified 11 suppressors and 35 enhancers of melanotic tumors in hopTum-l animals. Dfs that uncover the Hippo (Hpo) pathway genes expanded (ex) and warts (wts) strongly enhanced the hopTum-l tumor burden, as did mutations in ex, wts, and other Hpo pathway genes. Target genes of the Hpo pathway effector Yorkie (Yki) were significantly upregulated in hopTum-l blood cells, indicating that Yki signaling was increased. Ectopic hematopoietic activation of Yki in otherwise wild-type animals increased hemocyte proliferation but did not induce melanotic tumors. However, hematopoietic depletion of Yki significantly reduced the hopTum-l tumor burden, demonstrating that Yki is required for melanotic tumors in this background. These results support a model in which elevated Yki signaling increases the number of hemocytes, which become melanotic tumors as a result of elevated JAK/STAT signaling.

title: Oncogenic cooperation between Yorkie and the conserved microRNA miR-8 in the wing disc of Drosophila
abstract: Tissue growth has to be carefully controlled to generate well-functioning organs. MicroRNAs are small non-coding RNAs that modulate the activity of target genes and play a pivotal role in animal development. Understanding the functions of microRNAs in development requires the identification of their target genes. Here, we find that miR-8, a conserved microRNA in the miR-200 family, controls tissue growth and homeostasis in the Drosophila wing imaginal disc. Upregulation of miR-8 causes the repression of Yorkie, the effector of the Hippo pathway in Drosophila, and reduces tissue size. Remarkably, co-expression of Yorkie and miR-8 causes the formation of neoplastic tumors. We show that upregulation of miR-8 represses the growth inhibitor brinker, and depletion of brinker cooperates with Yorkie in the formation of neoplastic tumors. Hence, miR-8 modulates a positive growth regulator, Yorkie, and a negative growth regulator, brinker. Deregulation of this network can result in the loss of tissue homeostasis and the formation of tumors.  miR-8 controls growth and tumorigenesis in the Drosophila wing epithelium.

title: Transformed Drosophila Cells Evade Diet-Mediated Insulin Resistance through Wingless Signaling
abstract: The risk of specific cancers increases in patients with metabolic dysfunction, including obesity and diabetes. Here, we use Drosophila as a model to explore the effects of diet on tumor progression. Feeding Drosophila a diet high in carbohydrates was previously demonstrated to direct metabolic dysfunction, including hyperglycemia, hyperinsulinemia, and insulin resistance. We demonstrate that high dietary sugar also converts Ras/Src-transformed tissue from localized growths to aggressive tumors with emergent metastases. Whereas most tissues displayed insulin resistance, Ras/Src tumors retained insulin pathway sensitivity, increased the ability to import glucose, and resisted apoptosis. High dietary sugar increased canonical Wingless/Wnt pathway activity, which upregulated insulin receptor gene expression to promote insulin sensitivity. The result is a feed-forward circuit that amplified diet-mediated malignant phenotypes within Ras/Src-transformed tumors. By targeting multiple steps in this circuit with rationally applied drug combinations, we demonstrate the potential of combinatorial drug intervention to treat diet-enhanced malignant tumors.

title: Salt-inducible kinases mediate nutrient-sensing to link dietary sugar and tumorigenesis in Drosophila
abstract: Cancer cells demand excessive nutrients to support their proliferation but how cancer cells sense and promote growth in the nutrient favorable conditions remain incompletely understood. Epidemiological studies have indicated that obesity is a risk factor for various types of cancers. Feeding Drosophila a high dietary sugar was previously demonstrated to not only direct metabolic defects including obesity and organismal insulin resistance, but also transform Ras/Src-activated cells into aggressive tumors. Here we demonstrate that Ras/Src-activated cells are sensitive to perturbations in the Hippo signaling pathway. We provide evidence that nutritional cues activate Salt-inducible kinase, leading to Hippo pathway downregulation in Ras/Src-activated cells. The result is Yorkie-dependent increase in Wingless signaling, a key mediator that promotes diet-enhanced Ras/Src-tumorigenesis in an otherwise insulin-resistant environment. Through this mechanism, Ras/Src-activated cells are positioned to efficiently respond to nutritional signals and ensure tumor growth upon nutrient rich condition including obesity.

title: ARRDC3 suppresses colorectal cancer progression through destabilizing the oncoprotein YAP
abstract: Although colorectal cancer (CRC) is a prevalent malignancy of the digestive system, the underlying mechanisms of CRC tumorigenesis are still elusive. Arrestin‐related domain‐containing protein‐3 (ARRDC3) has been reported to promote lysosome‐mediated protein degradation. In the present study, we find that the expression of ARRDC3 is downregulated in CRC specimens. Mechanistically, we reveal that ARRDC3 binds and decreases expression of the oncoprotein YAP, the cotranscription factor of the Hippo pathway. The regulation of the Hippo pathway by ARRDC3 is conserved from Drosophila to mammals. Furthermore, we demonstrate that ARRDC3 plays an anti‐oncogenic role in CRC progression by promoting YAP degradation. Finally, we show that ARRDC3 increases the sensitivity of CRC cells toward chemotherapeutic drugs. Taken together, our findings point to ARRDC3 as a potential target for CRC treatment.

title: Molecular profiling of nonalcoholic fatty liver disease-associated hepatocellular carcinoma using SB transposon mutagenesis
abstract: Nonalcoholic fatty liver disease (NAFLD) is the fastest rising cause of hepatocellular carcinoma (HCC) in Western countries; however, the molecular mechanisms driving NAFLD-HCC remain elusive. Using Sleeping Beauty transposon mutagenesis in two mouse models of NAFLD-HCC, we identified hundreds of NAFLD-HCC candidate cancer genes that were enriched in pathways often associated with NAFLD and HCC. We also showed that Sav1, which functions in the Hippo signaling pathway and was the most frequently mutated gene identified by SB in both screens, prevents progression of steatohepatitis and subsequent HCC development in coordination with PI3K signaling via suppression of Yap, a downstream effector of the Hippo pathway. Our forward genetic screens have thus identified pathways and genes driving the development of NAFLD-HCC. Nonalcoholic fatty liver disease (NAFLD) is the fastest rising cause of hepatocellular carcinoma (HCC) in Western countries; however, the molecular mechanisms that cause NAFLD-HCC remain elusive. To identify molecular drivers of NAFLD-HCC, we performed Sleeping Beauty (SB) transposon mutagenesis screens in liver-specific Pten knockout and in high-fat diet-fed mice, which are murine models of NAFLD-HCC. SB mutagenesis accelerated liver tumor formation in both models and identified 588 and 376 candidate cancer genes (CCGs), respectively; 257 CCGs were common to both screens and were enriched in signaling pathways known to be important for human HCC. Comparison of these CCGs with those identified in a previous SB screen of hepatitis B virus-induced HCC identified a core set of 141 CCGs that were mutated in all screens. Forty-one CCGs appeared specific for NAFLD-HCC, including Sav1, a component of the Hippo signaling pathway and the most frequently mutated gene identified in both NAFLD-HCC screens. Liver-specific deletion of Sav1 was found to promote hepatic lipid accumulation, apoptosis, and fibrogenesis, leading to the acceleration of hepatocarcinogenesis in liver-specific Pten mutant mice. Sav1/Pten double-mutant livers also showed a striking up-regulation of markers of liver progenitor cells (LPCs), along with synergistic activation of Yap, which is a major downstream effector of Hippo signaling. Lastly, Yap activation, in combination with Pten inactivation, was found to accelerate cell growth and sphere formation of LPCs in vitro and induce their malignant transformation in allografts. Our forward genetic screens in mice have thus identified pathways and genes driving the development of NAFLD-HCC.

title: Hippo pathway coactivators Yap and Taz are required to coordinate mammalian liver regeneration
abstract: The mammalian liver has a remarkable capacity for repair following injury. Removal of up to two-third of liver mass results in a series of events that include extracellular matrix remodeling, coordinated hepatic cell cycle re-entry, restoration of liver mass and tissue remodeling to return the damaged liver to its normal state. Although there has been considerable advancement of our knowledge concerning the regenerative capacity of the mammalian liver, many outstanding questions remaining, such as: how does the regenerating liver stop proliferating when appropriate mass is restored and how do these mechanisms relate to normal regulation of organ size during development? Hippo pathway has been proposed to be central in mediating both events: organ size control during development and following regeneration. In this report, we examined the role of Yap and Taz, key components of the Hippo pathway in liver organ size regulation, both in the context of development and homeostasis. Our studies reveal that contrary to the current paradigms that Yap/Taz are not required for developmental regulation of liver size but are required for proper liver regeneration. In livers depleted of Yap and Taz, liver mass is elevated in neonates and adults. However, Yap/Taz-depleted livers exhibit profound defects in liver regeneration, including an inability to restore liver mass and to properly coordinate cell cycle entry. Taken together, our results highlight requirements for the Hippo pathway during liver regeneration and indicate that there are additional pathways that cooperate with Hippo signaling to control liver size during development and in the adult.

title: The transcriptional coactivator TAZ regulates mesenchymal differentiation in malignant glioma.
abstract: Recent molecular classification of glioblastoma (GBM) has shown that patients with a mesenchymal (MES) gene expression signature exhibit poor overall survival and treatment resistance. Using regulatory network analysis of available expression microarray data sets of GBM, including The Cancer Genome Atlas (TCGA), we identified the transcriptional coactivator with PDZ-binding motif (TAZ), to be highly associated with the MES network. TAZ expression was lower in proneural (PN) GBMs and lower-grade gliomas, which correlated with CpG island hypermethylation of the TAZ promoter compared with MES GBMs. Silencing of TAZ in MES glioma stem cells (GSCs) decreased expression of MES markers, invasion, self-renewal, and tumor formation. Conversely, overexpression of TAZ in PN GSCs as well as murine neural stem cells (NSCs) induced MES marker expression and aberrant osteoblastic and chondrocytic differentiation in a TEAD-dependent fashion. Using chromatin immunoprecipitation (ChIP), we show that TAZ is directly recruited to a majority of MES gene promoters in a complex with TEAD2. The coexpression of TAZ, but not a mutated form of TAZ that lacks TEAD binding, with platelet-derived growth factor-B (PDGF-B) resulted in high-grade tumors with MES features in a murine model of glioma. Our studies uncover a direct role for TAZ and TEAD in driving the MES differentiation of malignant glioma.

title: Intercellular Cooperation and Competition in Brain Cancers: Lessons From Drosophila and Human Studies
abstract: Glioblastoma (GBM) is a primary brain cancer with an extremely poor prognosis. GBM tumors contain heterogeneous cellular components, including a small subpopulation of tumor cells termed glioma stem cells (GSCs). GSCs are characterized as chemotherapy‐ and radiotherapy‐resistant cells with prominent tumorigenic ability. Studies in Drosophila cancer models demonstrated that interclonal cooperation and signaling from apoptotic clones provokes aggressive growth of neighboring tumorigenic clones, via compensatory proliferation or apoptosis induced proliferation. Mechanistically, these aggressive tumors depend on activation of Jun‐N‐terminal kinase (upstream of c‐JUN), and Drosophila Wnt (Wg) in the apoptotic clones. Consistent with these nonmammalian studies, data from several mammalian studies have shown that c‐JUN and Wnt are hyperactivated in aggressive tumors (including GBM). However, it remains elusive whether compensatory proliferation is an evolutionarily conserved mechanism in cancers. In the present report, we summarize recent studies in Drosophila models and mammalian models (e.g., xenografts of human cancer cells into small animals) to elucidate the intercellular interactions between the apoptosis‐prone cancer cells (e.g., non‐GSCs) and the hyperproliferative cancer cells (e.g., GSCs). These evolving investigations will yield insights about molecular signaling interactions in the context of post‐therapeutic phenotypic changes in human cancers. Furthermore, these studies are likely to revise our understanding of the genetic changes and post‐therapeutic cell‐cell interactions, which is a vital area of cancer biology with wide applications to many cancer types in humans.

title: Phenotypic Plasticity of Invasive Edge Glioma Stem-like Cells in Response to Ionizing Radiation
abstract: Unresectable glioblastoma (GBM) cells in the invading tumor edge can act as seeds for recurrence. The molecular and phenotypic properties of these cells remain elusive. Here, we report that the invading edge and tumor core have two distinct types of glioma stem-like cells (GSCs) that resemble proneural (PN) and mesenchymal (MES) subtypes, respectively. Upon exposure to ionizing radiation (IR), GSCs, initially enriched for a CD133+ PN signature, transition to a CD109+ MES subtype in a C/EBP-β-dependent manner. Our gene expression analysis of paired cohorts of patients with primary and recurrent GBMs identified a CD133-to-CD109 shift in tumors with an MES recurrence. Patient-derived CD133−/CD109+ cells are highly enriched with clonogenic, tumor-initiating, and radiation-resistant properties, and silencing CD109 significantly inhibits these phenotypes. We also report a conserved regulation of YAP/TAZ pathways by CD109 that could be a therapeutic target in GBM.

title: Loss of the Drosophila cell polarity regulator Scribbled promotes epithelial tissue overgrowth and cooperation with oncogenic Ras-Raf through impaired Hippo pathway signaling
abstract: BackgroundEpithelial neoplasias are associated with alterations in cell polarity and excessive cell proliferation, yet how these neoplastic properties are related to one another is still poorly understood. The study of Drosophila genes that function as neoplastic tumor suppressors by regulating both of these properties has significant potential to clarify this relationship.ResultsHere we show in Drosophila that loss of Scribbled (Scrib), a cell polarity regulator and neoplastic tumor suppressor, results in impaired Hippo pathway signaling in the epithelial tissues of both the eye and wing imaginal disc. scrib mutant tissue overgrowth, but not the loss of cell polarity, is dependent upon defective Hippo signaling and can be rescued by knockdown of either the TEAD/TEF family transcription factor Scalloped or the transcriptional coactivator Yorkie in the eye disc, or reducing levels of Yorkie in the wing disc. Furthermore, loss of Scrib sensitizes tissue to transformation by oncogenic Ras-Raf signaling, and Yorkie-Scalloped activity is required to promote this cooperative tumor overgrowth. The inhibition of Hippo signaling in scrib mutant eye disc clones is not dependent upon JNK activity, but can be significantly rescued by reducing aPKC kinase activity, and ectopic aPKC activity is sufficient to impair Hippo signaling in the eye disc, even when JNK signaling is blocked. In contrast, warts mutant overgrowth does not require aPKC activity. Moreover, reducing endogenous levels of aPKC or increasing Scrib or Lethal giant larvae levels does not promote increased Hippo signaling, suggesting that aPKC activity is not normally rate limiting for Hippo pathway activity. Epistasis experiments suggest that Hippo pathway inhibition in scrib mutants occurs, at least in part, downstream or in parallel to both the Expanded and Fat arms of Hippo pathway regulation.ConclusionsLoss of Scrib promotes Yorkie/Scalloped-dependent epithelial tissue overgrowth, and this is also important for driving cooperative tumor overgrowth with oncogenic Ras-Raf signaling. Whether this is also the case in human cancers now warrants investigation since the cell polarity function of Scrib and its capacity to restrain oncogene-mediated transformation, as well as the tissue growth control function of the Hippo pathway, are conserved in mammals.

title: Oncogenic Signaling Pathways in The Cancer Genome Atlas
abstract: Genetic alterations in signaling pathways that control cell-cycle progression, apoptosis, and cell growth are common hallmarks of cancer, but the extent, mechanisms, and co-occurrence of alterations in these pathways differ between individual tumors and tumor types. Using mutations, copy-number changes, mRNA expression, gene fusions and DNA methylation in 9,125 tumors profiled by The Cancer Genome Atlas (TCGA), we analyzed the mechanisms and patterns of somatic alterations in ten canonical pathways: cell cycle, Hippo, Myc, Notch, Nrf2, PI-3-Kinase/Akt, RTK-RAS, TGFβ signaling, p53 and β-catenin/Wnt. We charted the detailed landscape of pathway alterations in 33 cancer types, stratified into 64 subtypes, and identified patterns of co-occurrence and mutual exclusivity. Eighty-nine percent of tumors had at least one driver alteration in these pathways, and 57% percent of tumors had at least one alteration potentially targetable by currently available drugs. Thirty percent of tumors had multiple targetable alterations, indicating opportunities for combination therapy.

title: K‐ras Mutation Subtypes in NSCLC and Associated Co‐occuring Mutations in Other Oncogenic Pathways
abstract: Although KRAS mutations in NSCLC have been considered mutually exclusive driver mutations for a long time, there is now growing evidence that KRAS‐mutated NSCLC represents a genetically heterogeneous subgroup. We sought to determine genetic heterogeneity with respect to cancer‐related co‐mutations and their correlation with different KRAS mutation subtypes.  Diagnostic samples from 4507 patients with NSCLC were analyzed by next‐generation sequencing by using a panel of 14 genes and, in a subset of patients, fluorescence in situ hybridization. Next‐generation sequencing with an extended panel of 14 additional genes was performed in 101 patients. Molecular data were correlated with clinical data. Whole‐exome sequencing was performed in two patients.  We identified 1078 patients with KRAS mutations, of whom 53.5% had at least one additional mutation. Different KRAS mutation subtypes showed different patterns of co‐occurring mutations. Besides mutations in tumor protein p53 gene (TP53) (39.4%), serine/threonine kinase 11 gene (STK11) (19.8%), kelch like ECH associated protein 1 gene (KEAP1) (12.9%), and ATM serine/threonine kinase gene (ATM) (11.9%), as well as MNNG HOS Transforming gene (MET) amplifications (15.4%) and erb‐b2 receptor tyrosine kinase 2 gene (ERBB2) amplifications (13.8%, exclusively in G12C), we found rare co‐occurrence of targetable mutations in EGFR (1.2%) and BRAF (1.2%). Whole‐exome sequencing of two patients with co‐occurring phosphatidylinositol‐4,5‐bisphosphate 3‐kinase catalytic subunit alpha gene (PIK3CA) mutation revealed clonality of mutated KRAS in one patient and subclonality in the second, suggesting different evolutionary backgrounds.  KRAS‐mutated NSCLC represents a genetically heterogeneous subgroup with a high frequency of  mutations in cancer‐associated pathways, partly associated with distinct KRAS mutation subtypes. This diversity might have implications for understanding the variability of treatment outcome in KRAS‐mutated NSCLC and for future trial design.

title: Hippo Reprograms the Transcriptional Response to Ras Signaling.
abstract: Hyperactivating mutations in Ras signaling are hallmarks of carcinomas. Ras signaling mediates cell fate decisions as well as proliferation during development. It is not known what dictates whether Ras signaling drives differentiation versus proliferation. Here we show that the Hippo pathway is critical for this decision. Loss of Hippo switches Ras activation from promoting cellular differentiation to aggressive cellular proliferation. Transcriptome analysis combined with genetic tests show that this excessive proliferation depends on the synergistic induction of Ras target genes. Using ChIP-nexus, we find that Hippo signaling keeps Ras targets in check by directly regulating the expression of two key downstream transcription factors of Ras signaling: the ETS-domain transcription factor Pointed and the repressor Capicua. Our results highlight how independent signaling pathways can impinge on each other at the level of transcription factors, thereby providing a safety mechanism to keep proliferation in check under normal developmental conditions.

title: The Tumor Suppressor CIC Directly Regulates MAPK Pathway Genes via Histone Deacetylation.
abstract: Oligodendrogliomas are brain tumors accounting for approximately 10% of all central nervous system cancers. CIC is a transcription factor that is mutated in most patients with oligodendrogliomas; these mutations are believed to be a key oncogenic event in such cancers. Analysis of the Drosophila melanogaster ortholog of CIC, Capicua, indicates that CIC loss phenocopies activation of the EGFR/RAS/MAPK pathway, and studies in mammalian cells have demonstrated a role for CIC in repressing the transcription of the PEA3 subfamily of ETS transcription factors. Here, we address the mechanism by which CIC represses transcription and assess the functional consequences of CIC inactivation. Genome-wide binding patterns of CIC in several cell types revealed that CIC target genes were enriched for MAPK effector genes involved in cell-cycle regulation and proliferation. CIC binding to target genes was abolished by high MAPK activity, which led to their transcriptional activation. CIC interacted with the SIN3 deacetylation complex and, based on our results, we suggest that CIC functions as a transcriptional repressor through the recruitment of histone deacetylases. Independent single amino acid substitutions found in oligodendrogliomas prevented CIC from binding its target genes. Taken together, our results show that CIC is a transcriptional repressor of genes regulated by MAPK signaling, and that ablation of CIC function leads to increased histone acetylation levels and transcription at these genes, ultimately fueling mitogen-independent tumor growth. Inactivation of CIC inhibits its direct repression of MAPK pathway genes, leading to their increased expression and mitogen-independent growth.

title: Loss of Cell Polarity Drives Tumor Growth and Invasion through JNK Activation in Drosophila
abstract: Apparent defects in cell polarity are often seen in human cancer. However, the underlying mechanisms of how cell polarity disruption contributes to tumor progression are unknown. Here, using a Drosophila genetic model for Ras-induced tumor progression, we show a molecular link between loss of cell polarity and tumor malignancy. Mutation of different apicobasal polarity genes activates c-Jun N-terminal kinase (JNK) signaling and downregulates the E-cadherin/beta-catenin adhesion complex, both of which are necessary and sufficient to cause oncogenic Ras(V12)-induced benign tumors in the developing eye to exhibit metastatic behavior. Furthermore, activated JNK and Ras signaling cooperate in promoting tumor growth cell autonomously, as JNK signaling switches its proapoptotic role to a progrowth effect in the presence of oncogenic Ras. Our finding that such context-dependent alterations promote both tumor growth and metastatic behavior suggests that metastasis-promoting mutations may be selected for based primarily on their growth-promoting capabilities. Similar oncogenic cooperation mediated through these evolutionarily conserved signaling pathways could contribute to human cancer progression.

title: JNK‐ and Fos‐regulated Mmp1 expression cooperates with Ras to induce invasive tumors in Drosophila
abstract: Loss of the epithelial polarity gene scribble in clones of Drosophila imaginal disc cells can cooperate with Ras signaling to induce malignant tumors. Such mutant tissue overproliferates, resists apoptosis, leaves its place of origin and invades other organs, ultimately causing lethality. We show that increased Jun N‐terminal kinase (JNK) signaling resulting from the loss of scribble promotes the movement of transformed cells to secondary sites. This effect requires Fos‐dependent transcriptional activation of a matrix metalloprotease gene mmp1 downstream of JNK. Expression of the Mmp inhibitor Timp or Mmp RNAi knockdown suppresses cell invasiveness. The proinvasive function of the JNK pathway is revealed in a tumor context when active Ras signaling prevents the apoptotic response to JNK activity as it occurs in nontransformed cells. Based on these results, we present a model that explains the oncogenic cooperation between JNK and Ras, and describes how aberrant regulation of cell survival, proliferation and mobilization cooperate to incite malignant tumor formation.

title: Drosophila endocytic neoplastic tumor suppressor genes regulate Sav/Wts/Hpo signaling and the c-Jun N-terminal kinase pathway
abstract: Genetic screens in the fruit fly Drosophila melanogaster have identified a class of neoplastic tumor suppressor genes (endocytic nTSGs), which encode proteins that localize to endosomes and facilitate the trafficking of membrane-bound receptors and adhesion molecules into the degradative lysosome. Loss of endocytic nTSGs transforms imaginal disc epithelia into highly proliferative, invasive tissues that fail to differentiate and display defects in cellular apicobasal polarity, adhesion and tissue architecture. As vertebrate homologs of some Drosophila nTSGs are linked to tumor formation, identifying molecular changes in signaling associated with nTSG loss could inform understanding of neoplastic transformation in vertebrates. Here we show that mutations in genes that act at multiple steps of the endolysosomal pathway lead to autonomous activation of the Sav/Wts/Hpo (SWH) transcriptional effector Yki (YAP/TAZ in vertebrates) and the Jun N-terminal kinase (JNK), which is known to promote Yki activity in cells with disrupted polarity. Yki and JNK activity are elevated by mutations at multiple steps in the endolysosomal pathway including mutations in the AP-2σ gene, which encodes a component of the AP-2 adaptor complex that recruits cargoes into clathrin-coated pits for subsequent internalization. Moreover, reduction of JNK activity can decrease elevated Yki-signaling caused by altered endocytosis. These studies reveal a broad requirement for components of the endocytic pathway in regulating SWH and JNK outputs, and place Drosophila endocytic nTSGs into a network that involving two major signaling pathways implicated in oncogenesis.

title: Influence of Fat-Hippo and Notch signaling on the proliferation and differentiation of Drosophila optic neuroepithelia
abstract: The Drosophila optic lobe develops from neuroepithelial cells, which function as symmetrically dividing neural progenitors. We describe here a role for the Fat-Hippo pathway in controlling the growth and differentiation of Drosophila optic neuroepithelia. Mutation of tumor suppressor genes within the pathway, or expression of activated Yorkie, promotes overgrowth of neuroepithelial cells and delays or blocks their differentiation; mutation of yorkie inhibits growth and accelerates differentiation. Neuroblasts and other neural cells, by contrast, appear unaffected by Yorkie activation. Neuroepithelial cells undergo a cell cycle arrest before converting to neuroblasts; this cell cycle arrest is regulated by Fat-Hippo signaling. Combinations of cell cycle regulators, including E2f1 and CyclinD, delay neuroepithelial differentiation, and Fat-Hippo signaling delays differentiation in part through E2f1. We also characterize roles for Jak-Stat and Notch signaling. Our studies establish that the progression of neuroepithelial cells to neuroblasts is regulated by Notch signaling, and suggest a model in which Fat-Hippo and Jak-Stat signaling influence differentiation by their acceleration of cell cycle progression and consequent impairment of Delta accumulation, thereby modulating Notch signaling. This characterization of Fat-Hippo signaling in neuroepithelial growth and differentiation also provides insights into the potential roles of Yes-associated protein in vertebrate neural development and medullablastoma.

title: Notch Signaling Activates Yorkie Non-Cell Autonomously in Drosophila
abstract: In Drosophila imaginal epithelia, cells mutant for the endocytic neoplastic tumor suppressor gene vps25 stimulate nearby untransformed cells to express Drosophila Inhibitor-of-Apoptosis-Protein-1 (DIAP-1), conferring resistance to apoptosis non-cell autonomously. Here, we show that the non-cell autonomous induction of DIAP-1 is mediated by Yorkie, the conserved downstream effector of Hippo signaling. The non-cell autonomous induction of Yorkie is due to Notch signaling from vps25 mutant cells. Moreover, activated Notch in normal cells is sufficient to induce non-cell autonomous Yorkie activity in wing imaginal discs. Our data identify a novel mechanism by which Notch promotes cell survival non-cell autonomously and by which neoplastic tumor cells generate a supportive microenvironment for tumor growth.

title: Tumor suppressor properties of the ESCRT-II complex component Vps25 in Drosophila.
abstract: We have found that the Drosophila gene vps25 possesses several properties of a tumor suppressor. First, vps25 mutant cells activate Notch and Dpp receptor signaling, inducing ectopic organizers in developing eyes and limbs and consequent overproliferation of both mutant and nearby wild-type cells. Second, as the mutant cells proliferate, they lose their epithelial organization and undergo apoptosis. Strikingly, when apoptosis of mutant cells is blocked, tumor-like overgrowths are formed that are capable of metastasis. vps25 encodes a component of the ESCRT-II complex, which sorts membrane proteins into multivesicular bodies during endocytic trafficking to the lysosome. Activation of Notch and Dpp receptor signaling in mutant cells results from an endocytic blockage that causes accumulation of these receptors and other signaling components in endosomes. These results highlight the importance of endocytic trafficking in regulating signaling and epithelial organization and suggest a possible role for ESCRT components in human cancer.

title: vps25 mosaics display non-autonomous cell survival and overgrowth, and autonomous apoptosis
abstract: Appropriate cell-cell signaling is crucial for proper tissue homeostasis. Protein sorting of cell surface receptors at the early endosome is important for both the delivery of the signal and the inactivation of the receptor, and its alteration can cause malignancies including cancer. In a genetic screen for suppressors of the pro-apoptotic gene hid in Drosophila, we identified two alleles of vps25, a component of the ESCRT machinery required for protein sorting at the early endosome. Paradoxically, although vps25 mosaics were identified as suppressors of hid-induced apoptosis, vps25 mutant cells die. However, we provide evidence that a non-autonomous increase of Diap1 protein levels, an inhibitor of apoptosis, accounts for the suppression of hid. Furthermore, before they die, vps25 mutant clones trigger non-autonomous proliferation through a failure to downregulate Notch signaling, which activates the mitogenic JAK/STAT pathway. Hid and JNK contribute to apoptosis of vps25 mutant cells. Inhibition of cell death in vps25 clones causes dramatic overgrowth phenotypes. In addition, Hippo signaling is increased in vps25 clones, and hippo mutants block apoptosis in vps25 clones. In summary, the phenotypic analysis of vps25 mutants highlights the importance of receptor downregulation by endosomal protein sorting for appropriate tissue homeostasis, and may serve as a model for human cancer.

title: Kicking it up a Notch for the best in show: Scalloped leads Yorkie into the haematopoietic arena
abstract: Maintenance and differentiation of progenitor cells is essential for proper organ development and adaptation to environmental stress and injury. In Drosophila melanogaster, the haematopietic system serves as an ideal model for interrogating the function of signaling pathways required for progenitor maintenance and cell fate determination. Here we focus on the role of the Hippo pathway effectors Yorkie and Scalloped in mediating and facilitating Notch signaling-mediated lineage specification in the lymph gland, the primary center for haematopoiesis within the developing larva. We discuss the regulatory mechanisms which promote Notch activity during normal haematopoiesis and its modulation during immune challenge conditions. We provide additional evidence establishing the hierarchy of signaling events during crystal cell formation, highlighting the relationship between Yorkie, Scalloped and Lozenge, while expanding on the role of Yorkie in promoting hemocyte survival and the developmental regulation of Notch and its ligand, Serrate, within the lymph gland. Finally, we propose additional areas of exploration that may provide mechanistic insight into the environmental and non-cell autonomous regulation of cell fate in the blood system.

title: Warts Is Required for PI3K-Regulated Growth Arrest, Autophagy, and Autophagic Cell Death in Drosophila
abstract: Cell growth arrest and autophagy are required for autophagic cell death in Drosophila. Maintenance of growth by expression of either activated Ras, Dp110, or Akt is sufficient to inhibit autophagy and cell death in Drosophila salivary glands, but the mechanism that controls growth arrest is unknown. Although the Warts (Wts) tumor suppressor is a critical regulator of tissue growth in animals, it is not clear how this signaling pathway controls cell growth.



Here, we show that genes in the Wts pathway are required for salivary gland degradation and that wts mutants have defects in cell growth arrest, caspase activity, and autophagy. Expression of Atg1, a regulator of autophagy, in salivary glands is sufficient to rescue wts mutant salivary gland destruction. Surprisingly, expression of Yorkie (Yki) and Scalloped (Sd) in salivary glands fails to phenocopy wts mutants. By contrast, misexpression of the Yki target bantam was able to inhibit salivary gland cell death, even though mutations in bantam fail to suppress the wts mutant salivary gland-persistence phenotype. Significantly, wts mutant salivary glands possess altered phosphoinositide signaling, and decreased function of the class I PI3K-pathway genes chico and TOR suppressed wts defects in cell death.



Although we have previously shown that salivary gland degradation requires genes in the Wts pathway, this study provides the first evidence that Wts influences autophagy. Our data indicate that the Wts-pathway components Yki, Sd, and bantam fail to function in salivary glands and that Wts regulates salivary gland cell death in a PI3K-dependent manner.

title: TOR Signaling in Growth and Metabolism
abstract: The target of rapamycin (TOR) is a conserved Ser/Thr kinase that regulates cell growth and metabolism in response to environmental cues. Here, highlighting contributions from studies in model organisms, we review mammalian TOR complexes and the signaling branches they mediate. TOR is part of two distinct multiprotein complexes, TOR complex 1 (TORC1), which is sensitive to rapamycin, and TORC2, which is not. The physiological consequences of mammalian TORC1 dysregulation suggest that inhibitors of mammalian TOR may be useful in the treatment of cancer, cardiovascular disease, autoimmunity, and metabolic disorders.

title: Mechanisms of growth and homeostasis in the Drosophila wing.
abstract: Animal shape and size is controlled with amazing precision during development. External factors such as nutrient availability and crowding can alter overall animal size, but individual body parts scale reproducibly to match the body even with challenges from a changing environment. How is such precision achieved? Here, we review selected research from the last few years in Drosophila--arguably the premier genetic model for the study of animal growth--that sheds light on how body and tissue size are regulated by forces intrinsic to individual organs. We focus on two topics currently under intense study: the influence of pattern regulators on organ and tissue growth and the role of local competitive interactions between cells in tissue homeostasis and final size.

title: YAP mediates crosstalk between the Hippo and PI(3)K–TOR pathways by suppressing PTEN via miR-29
abstract: Organ development is a complex process governed by the interplay of several signalling pathways that have critical functions in the regulation of cell growth and proliferation. Over the past years, the Hippo pathway has emerged as a key regulator of organ size. Perturbation of this pathway has been shown to play important roles in tumorigenesis. YAP, the main downstream target of the mammalian Hippo pathway, promotes organ growth, yet the underlying molecular mechanism of this regulation remains unclear. Here we provide evidence that YAP activates the mammalian target of rapamycin (mTOR), a major regulator of cell growth. We have identified the tumour suppressor PTEN, an upstream negative regulator of mTOR, as a critical mediator of YAP in mTOR regulation. We demonstrate that YAP downregulates PTEN by inducing miR-29 to inhibit PTEN translation. Last, we show that PI(3)K–mTOR is a pathway modulated by YAP to regulate cell size, tissue growth and hyperplasia. Our studies reveal a functional link between Hippo and PI(3)K–mTOR, providing a molecular basis for the coordination of these two pathways in organ size regulation.

title: An innate immune response of blood cells to tumors and tissue damage in Drosophila
abstract: Studies in mice and humans have demonstrated a role for the immune system in preventing the growth of tumors. Deciphering the mechanisms involved in the immune response to tumors is essential to our understanding of immune recognition and cancer progression. Here we report an innate immune response to tumors in Drosophila melanogaster. We found that circulating blood cells, termed hemocytes, adhere to tumors upon detection of basement membrane disruption, and subsequently counter their growth. Basement membrane components are remarkably conserved throughout the animal kingdom, providing a unique structure for the immune system to sense tissue integrity. Further, we show that tissue damage activates JNK signaling in both tumors and aseptic wounds, causing expression of JAK/STAT-activating cytokines. Cytokine secretion from the injured tissue is amplified into a systemic response through the induction of additional cytokine expression in the hemocytes and the fat body, resulting in hemocyte proliferation. Our findings reveal common mechanisms in the response to tumors and wounds in flies. A similar innate reaction may underlie the response to tumors and tissue damage in vertebrates and humans.

title: Interaction between RasV12 and scribble clones induces tumour growth and invasion
abstract: Human tumours have a large degree of cellular and genetic heterogeneity. Complex cell interactions in the tumour and its microenvironment are thought to have an important role in tumorigenesis and cancer progression. Furthermore, cooperation between oncogenic genetic lesions is required for tumour development; however, it is not known how cell interactions contribute to oncogenic cooperation. The genetic techniques available in the fruitfly Drosophila melanogaster allow analysis of the behaviour of cells with distinct mutations, making this the ideal model organism with which to study cell interactions and oncogenic cooperation. In Drosophila eye-antennal discs, cooperation between the oncogenic protein RasV12 (ref. 5) and loss-of-function mutations in the conserved tumour suppressor scribbled (scrib) gives rise to metastatic tumours that display many characteristics observed in human cancers. Here we show that clones of cells bearing different mutations can cooperate to promote tumour growth and invasion in Drosophila. We found that the RasV12 and scrib- mutations can also cause tumours when they affect different adjacent epithelial cells. We show that this interaction between RasV12 and scrib- clones involves JNK signalling propagation and JNK-induced upregulation of JAK/STAT-activating cytokines, a compensatory growth mechanism for tissue homeostasis. The development of RasV12 tumours can also be triggered by tissue damage, a stress condition that activates JNK signalling. Given the conservation of the pathways examined here, similar cooperative mechanisms could have a role in the development of human cancers.

title: Mitochondrial defect drives non-autonomous tumour progression through Hippo signalling in Drosophila
abstract: Mitochondrial respiratory function is frequently impaired in human cancers. However, the mechanisms by which mitochondrial dysfunction contributes to tumour progression remain elusive. Here we show in Drosophila imaginal epithelium that defects in mitochondrial function potently induce tumour progression of surrounding tissue in conjunction with oncogenic Ras. Our data show that Ras activation and mitochondrial dysfunction cooperatively stimulate production of reactive oxygen species, which causes activation of c-Jun amino (N)-terminal kinase (JNK) signalling. JNK cooperates with oncogenic Ras to inactivate the Hippo pathway, leading to upregulation of its targets Unpaired (an interleukin-6 homologue) and Wingless (a Wnt homologue). Mitochondrial dysfunction in Ras-activated cells further cooperates with Ras signalling in neighbouring cells with normal mitochondrial function, causing benign tumours to exhibit metastatic behaviour. Our findings provide a mechanistic basis for interclonal tumour progression driven by mitochondrial dysfunction and oncogenic Ras.

title: Apical deficiency triggers JNK-dependent apoptosis in the embryonic epidermis of Drosophila
abstract: Epithelial homeostasis and the avoidance of diseases such as cancer require the elimination of defective cells by apoptosis. Here, we investigate how loss of apical determinants triggers apoptosis in the embryonic epidermis of Drosophila. Transcriptional profiling and in situ hybridisation show that JNK signalling is upregulated in mutants lacking Crumbs or other apical determinants. This leads to transcriptional activation of the pro-apoptotic gene reaper and to apoptosis. Suppression of JNK signalling by overexpression of Puckered, a feedback inhibitor of the pathway, prevents reaper upregulation and apoptosis. Moreover, removal of endogenous Puckered leads to ectopic reaper expression. Importantly, disruption of the basolateral domain in the embryonic epidermis does not trigger JNK signalling or apoptosis. We suggest that apical, not basolateral, integrity could be intrinsically required for the survival of epithelial cells. In apically deficient embryos, JNK signalling is activated throughout the epidermis. Yet, in the dorsal region, reaper expression is not activated and cells survive. One characteristic of these surviving cells is that they retain discernible adherens junctions despite the apical deficit. We suggest that junctional integrity could restrain the pro-apoptotic influence of JNK signalling.

title: Wingless modulates activator protein-1-mediated tumor invasion
abstract: Metastasis begins with a subset of local tumor cells acquiring the potential to invade into surrounding tissues, and remains to be a major obstacle for cancer treatments. More than 90% of cancer patients died from tumor metastasis, instead of primary tumor growth. The canonical Wnt/β-catenin pathway plays essential roles in promoting tumor formation, yet its function in regulating tumor metastasis and the underlying mechanisms remain controversial. Here we employed well-established Drosophila tumor models to investigate the regulating mechanism of Wingless (Wg) pathway in tumor invasion. Our results showed that Wg signaling is necessary and sufficient for cell polarity disruption-induced cell migration and molecular changes reminiscent of epithelial-mesenchymal transition (EMT). Moreover, reducing Wg signaling suppressed lgl−/−/RasV12-induced tumor invasion, and cooperation between Arm and RasV12 is sufficient to induce tumor invasion. Mechanistically, we found that cell polarity disruption activates JNK signaling, which in turn upregulate wg expression through transcription factor activator protein-1 (AP-1). We identified a consensus AP-1 binding site located in the 2nd intron of wg, and confirmed that it is essential for AP-1 induced wg transcription both in vitro and in vivo. Lastly, we confirmed that the transcriptional activation of WNT by AP-1 is conserved in human cancer cells. These evidences reveal a positive role of Wnt/β-catenin pathway in tumor invasion, and provide a conserved mechanism that connects JNK and Wnt signaling in regulating tumor progression.

title: Src controls tumorigenesis via JNK‐dependent regulation of the Hippo pathway in Drosophila
abstract: Cell–cell interactions within the tumour microenvironment have crucial roles in epithelial tumorigenesis. Using Drosophila genetics, we show that the oncoprotein Src controls tumour microenvironment by Jun N‐terminal kinase (JNK)‐dependent regulation of the Hippo pathway. Clones of cells with elevated Src expression activate the Rac‐Diaphanous and Ras‐mitogen‐activated protein kinase (MAPK) pathways, which cooperatively induce F‐actin accumulation, thereby leading to activation of the Hippo pathway effector Yorkie (Yki). Simultaneously, Src activates the JNK pathway, which antagonizes the autonomous Yki activity and causes propagation of Yki activity to neighbouring cells, resulting in the overgrowth of surrounding tissue. Our data provide a mechanism to explain how oncogenic mutations regulate tumour microenvironment through cell–cell communication.

title: The conserved misshapen-warts-Yorkie pathway acts in enteroblasts to regulate intestinal stem cells in Drosophila.
abstract: Similar to the mammalian intestine, the Drosophila adult midgut has resident stem cells that support growth and regeneration. How the niche regulates intestinal stem cell activity in both mammals and flies is not well understood. Here, we show that the conserved germinal center protein kinase Misshapen restricts intestinal stem cell division by repressing the expression of the JAK-STAT pathway ligand Upd3 in differentiating enteroblasts. Misshapen, a distant relative to the prototypic Warts activating kinase Hippo, interacts with and activates Warts to negatively regulate the activity of Yorkie and the expression of Upd3. The mammalian Misshapen homolog MAP4K4 similarly interacts with LATS (Warts homolog) and promotes inhibition of YAP (Yorkie homolog). Together, this work reveals that the Misshapen-Warts-Yorkie pathway acts in enteroblasts to control niche signaling to intestinal stem cells. These findings also provide a model in which to study requirements for MAP4K4-related kinases in MST1/2-independent regulation of LATS and YAP.

title: Identification of Happyhour/MAP4K as Alternative Hpo/Mst-like Kinases in the Hippo Kinase Cascade.
abstract: In Drosophila and mammals, the canonical Hippo kinase cascade is mediated by Hpo/Mst acting through the intermediary kinase Wts/Lats to phosphorylate the transcriptional coactivator Yki/YAP/TAZ. Despite recent reports linking Yki/YAP/TAZ activity to the actin cytoskeleton, the underlying mechanisms are poorly understood and/or controversial. Using Drosophila imaginal discs as an in vivo model, we show that Wts, but not Hpo, is genetically indispensable for cytoskeleton-mediated subcellular localization of Yki. Through a systematic screen, we identify the Ste-20 kinase Happyhour (Hppy) and its mammalian counterpart MAP4K1/2/3/5 as an alternative kinase that phosphorylates the hydrophobic motif of Wts/Lats in a similar manner as Hpo/Mst. Consistent with their redundant function as activating kinases of Wts/Lats, combined loss of Hpo/Mst and Hppy/MAP4K abolishes cytoskeleton-mediated regulation of Yki/YAP subcellular localization, as well as YAP cytoplasmic translocation induced by contact inhibition. These Hpo/Mst-like kinases provide an expanded view of the Hippo kinase cascade in development and physiology.

title: Drosophila Schip1 Links Expanded and Tao-1 to Regulate Hippo Signaling.
abstract: Regulation of organ size is essential in animal development, and Hippo (Hpo) signaling is a major conserved mechanism for controlling organ growth. In Drosophila, Hpo and Warts kinases are core components of this pathway and function as tumor suppressors by inhibiting Yorkie (Yki). Expanded (Ex) is a regulator of the Hpo activity, but how they are linked is unknown. Here, we show that Schip1, a Drosophila homolog of the mammalian Schwannomin interacting protein 1 (SCHIP1), provides a link between Ex and Hpo. Ex is required for apical localization of Schip1 in imaginal discs. Schip1 is necessary for promoting membrane localization and phosphorylation of Hpo by recruiting the Hpo kinase Tao-1. Taking these findings together, we conclude that Schip1 directly links Ex to Hpo signaling by recruiting Tao-1. This study provides insights into the mechanism of Tao-1 regulation and a potential growth control function for SCHIP1 in mammals.