Identifying tumor suppressors in genetic mosaics: the Drosophila lats gene encodes a putative protein kinase

PHLPP1 regulates contact inhibition by dephosphorylating Mst1 at the inhibitory site

Contact inhibition has been largely elusive despite that a loss of contact inhibition is a critical event for cancer development and progression. Here, we report that PHLPP1 is a binding protein for Mst1 and it modulates the Hippo pathway by dephosphorylating Mst1 at the inhibitory Thr(387) of Mst1. Yap1 was localized predominantly in the nucleus but marginally in the cytoplasm in HeLa cells under sparse conditions, whereas the functional protein was more directed to sequestration in the cytoplasm under dense environments. Furthermore, loss of PHLPP1 resulted in a failure of the apoptotic control. It is interesting that down-regulated expression of PHLPP1 appears to mimic the loss of contact inhibition, a hallmark of cancer.

LATS2 suppresses oncogenic Wnt signaling by disrupting β-catenin/BCL9 interaction

Abnormal activation of Wnt/β-catenin-mediated transcription is associated with a variety of human cancers. Here, we report that LATS2 inhibits oncogenic Wnt/β-catenin-mediated transcription by disrupting the β-catenin/BCL9 interaction. LATS2 directly interacts with β-catenin and is present on Wnt target gene promoters. Mechanistically, LATS2 inhibits the interaction between BCL9 and β-catenin and subsequent recruitment of BCL9, independent of LATS2 kinase activity. LATS2 is downregulated and inversely correlated with the levels of Wnt target genes in human colorectal cancers. Moreover, nocodazole, an antimicrotubule drug, potently induces LATS2 to suppress tumor growth in vivo by targeting β-catenin/BCL9. Our results suggest that LATS2 is not only a key tumor suppressor in human cancer but may also be an important target for anticancer therapy.

The two faces of Hippo: targeting the Hippo pathway for regenerative medicine and cancer treatment

The Hippo signalling pathway is an emerging growth control and tumour suppressor pathway that regulates cell proliferation and stem cell functions. Defects in Hippo signalling and hyperactivation of its downstream effectors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) contribute to the development of cancer, which suggests that pharmacological inhibition of YAP and TAZ activity may be an effective anticancer strategy. Conversely, YAP and TAZ can also have beneficial roles in stimulating tissue repair and regeneration following injury, so their activation may be therapeutically useful in these contexts. A complex network of intracellular and extracellular signalling pathways that modulate YAP and TAZ activities have recently been identified. Here, we review the regulation of the Hippo signalling pathway, its functions in normal homeostasis and disease, and recent progress in the identification of small-molecule pathway modulators.

Role of extracellular matrix and YAP/TAZ in cell fate determination

The mechanical signals transduced from cellular microenvironment can regulate cell shape and affect cell fate determination. However, how these mechanical signals are transduced to regulate biological processes of cells has remained elusive. Recent studies had elucidated a novel mechanism through which the interactions between mechanical signals from extracellular matrix and cell behavior regulation converged on the function of core components in Hippo signaling pathway, including YAP and TAZ in mammals. Moreover, several very recent studies have found a new crosstalk between Wnt and Hippo signaling in the regulation of cell fate determination. Such mechanism may explain how mechanical signals from microenvironment can regulate cell behavior and determine cell fate.

Differential proliferation rates generate patterns of mechanical tension that orient tissue growth

Orientation of cell divisions is a key mechanism of tissue morphogenesis. In the growing Drosophila wing imaginal disc epithelium, most of the cell divisions in the central wing pouch are oriented along the proximal-distal (P-D) axis by the Dachsous-Fat-Dachs planar polarity pathway. However, cells at the periphery of the wing pouch instead tend to orient their divisions perpendicular to the P-D axis despite strong Dachs polarization. Here, we show that these circumferential divisions are oriented by circumferential mechanical forces that influence cell shapes and thus orient the mitotic spindle. We propose that this circumferential pattern of force is not generated locally by polarized constriction of individual epithelial cells. Instead, these forces emerge as a global tension pattern that appears to originate from differential rates of cell proliferation within the wing pouch. Accordingly, we show that localized overgrowth is sufficient to induce neighbouring cell stretching and reorientation of cell division. Our results suggest that patterned rates of cell proliferation can influence tissue mechanics and thus determine the orientation of cell divisions and tissue shape.

Alcohol interacts with genetic alteration of the Hippo tumor suppressor pathway to modulate tissue growth in Drosophila

Alcohol-mediated cancers represent more than 3.5% of cancer-related deaths, yet how alcohol promotes cancer is a major open question. Using Drosophila, we identified novel interactions between dietary ethanol and loss of tumor suppressor components of the Hippo Pathway. The Hippo Pathway suppresses tumors in flies and mammals by inactivating transcriptional co-activator Yorkie, and the spectrum of cancers associated with impaired Hippo signaling overlaps strikingly with those associated with alcohol. Therefore, our findings may implicate loss of Hippo Pathway tumor suppression in alcohol-mediated cancers. Ethanol enhanced overgrowth from loss of the expanded, hippo, or warts tumor suppressors but, surprisingly, not from over-expressing the yorkie oncogene. We propose that in parallel to Yorkie-dependent overgrowth, impairing Hippo signaling in the presence of alcohol may promote overgrowth via additional alcohol-relevant targets. We also identified interactions between alcohol and Hippo Pathway over-activation. We propose that exceeding certain thresholds of alcohol exposure activates Hippo signaling to maintain proper growth control and prevent alcohol-mediated mis-patterning and tissue overgrowth.

Defining the protein-protein interaction network of the human hippo pathway

The Hippo pathway, which is conserved from Drosophila to mammals, has been recognized as a tumor suppressor signaling pathway governing cell proliferation and apoptosis, two key events involved in organ size control and tumorigenesis. Although several upstream regulators, the conserved kinase cascade and key downstream effectors including nuclear transcriptional factors have been defined, the global organization of this signaling pathway is not been fully understood. Thus, we conducted a proteomic analysis of human Hippo pathway, which revealed the involvement of an extensive protein-protein interaction network in this pathway. The mass spectrometry data were deposited to ProteomeXchange with identifier PXD000415. Our data suggest that 550 interactions within 343 unique protein components constitute the central protein-protein interaction landscape of human Hippo pathway. Our study provides a glimpse into the global organization of Hippo pathway, reveals previously unknown interactions within this pathway, and uncovers new potential components involved in the regulation of this pathway. Understanding these interactions will help us further dissect the Hippo signaling-pathway and extend our knowledge of organ size control.

Phosphorylation of angiomotin by Lats1/2 kinases inhibits F-actin binding, cell migration, and angiogenesis

The Hippo tumor suppressor pathway plays important roles in organ size control through Lats1/2 mediated phosphorylation of the YAP/TAZ transcription co-activators. However, YAP/TAZ independent functions of the Hippo pathway are largely unknown. Here we report a novel role of the Hippo pathway in angiogenesis. Angiomotin p130 isoform (AMOTp130) is phosphorylated on a conserved HXRXXS motif by Lats1/2 downstream of GPCR signaling. Phosphorylation disrupts AMOT interaction with F-actin and correlates with reduced F-actin stress fibers and focal adhesions. Furthermore, phosphorylation of AMOT by Lats1/2 inhibits endothelial cell migration in vitro and angiogenesis in zebrafish embryos in vivo. Thus AMOT is a direct substrate of Lats1/2 mediating functions of the Hippo pathway in endothelial cell migration and angiogenesis.

Evidence for a tumor suppressor role for the large tumor suppressor genes LATS1 and LATS2 in human cancer

The role of Large tumor suppressor LATS/Warts in human cancer is not clearly understood. Here we show that hLATS1/2 cancer mutations affect their expression and kinase activity. hLATS1/2 mutants exhibit a decreased activity in inhibiting YAP and tissue growth. Therefore, hLATS1/2 alleles from human cancer can be loss-of-function mutations.

Mapping of Mcs30, a new mammary carcinoma susceptibility quantitative trait locus (QTL30) on rat chromosome 12: identification of fry as a candidate Mcs gene

Rat strains differ dramatically in their susceptibility to mammary carcinogenesis. On the assumption that susceptibility genes are conserved across mammalian species and hence inform human carcinogenesis, numerous investigators have used genetic linkage studies in rats to identify genes responsible for differential susceptibility to carcinogenesis. Using a genetic backcross between the resistant Copenhagen (Cop) and susceptible Fischer 344 (F344) strains, we mapped a novel mammary carcinoma susceptibility (Mcs30) locus to the centromeric region on chromosome 12 (LOD score of ∼8.6 at the D12Rat59 marker). The Mcs30 locus comprises approximately 12 Mbp on the long arm of rat RNO12 whose synteny is conserved on human chromosome 13q12 to 13q13. After analyzing numerous genes comprising this locus, we identified Fry, the rat ortholog of the furry gene of Drosophila melanogaster, as a candidate Mcs gene. We cloned and determined the complete nucleotide sequence of the 13 kbp Fry mRNA. Sequence analysis indicated that the Fry gene was highly conserved across evolution, with 90% similarity of the predicted amino acid sequence among eutherian mammals. Comparison of the Fry sequence in the Cop and F344 strains identified two non-synonymous single nucleotide polymorphisms (SNPs), one of which creates a putative, de novo phosphorylation site. Further analysis showed that the expression of the Fry gene is reduced in a majority of rat mammary tumors. Our results also suggested that FRY activity was reduced in human breast carcinoma cell lines as a result of reduced levels or mutation. This study is the first to identify the Fry gene as a candidate Mcs gene. Our data suggest that the SNPs within the Fry gene contribute to the genetic susceptibility of the F344 rat strain to mammary carcinogenesis. These results provide the foundation for analyzing the role of the human FRY gene in cancer susceptibility and progression.

Opposite feedbacks in the Hippo pathway for growth control and neural fate

Signaling pathways are reused for multiple purposes in plant and animal development. The Hippo pathway in mammals and Drosophila coordinates proliferation and apoptosis via the coactivator and oncoprotein YAP/Yorkie (Yki), which is homeostatically regulated through negative feedback. In the Drosophila eye, cross-repression between the Hippo pathway kinase LATS/Warts (Wts) and growth regulator Melted generates mutually exclusive photoreceptor subtypes. Here, we show that this all-or-nothing neuronal differentiation results from Hippo pathway positive feedback: Yki both represses its negative regulator, warts, and promotes its positive regulator, melted. This postmitotic Hippo network behavior relies on a tissue-restricted transcription factor network-including a conserved Otx/Orthodenticle-Nrl/Traffic Jam feedforward module-that allows Warts-Yki-Melted to operate as a bistable switch. Altering feedback architecture provides an efficient mechanism to co-opt conserved signaling networks for diverse purposes in development and evolution.

Mst1 and Mst2 kinases: regulations and diseases

The Hippo signaling pathway has emerged as a critical regulator for organ size control. The serine/threonine protein kinases Mst1 and Mst2, mammalian homologs of the Hippo kinase from Drosophila, play the central roles in the Hippo pathway controlling the cell proliferation, differentiation, and apoptosis during development. Mst1/2 can be activated by cellular stressors and the activation of Mst1/2 might enforce a feedback stimulation system to regulate oxidant levels through several mechanisms, in which regulation of cellular redox state might represent a tumor suppressor function of Mst1/2. As in Drosophila, murine Mst1/Mst2, in a redundant manner, negatively regulate the Yorkie ortholog YAP in multiple organs, although considerable diversification in the pathway composition and regulation is observed in some of them. Generally, loss of both Mst1 and Mst2 results in hyperproliferation and tumorigenesis that can be largely negated by the reduction or elimination of YAP. The Hippo pathway integrates with other signaling pathways e.g. Wnt and Notch pathways and coordinates with them to impact on the tumor pathogenesis and development. Furthermore, Mst1/2 kinases also act as an important regulator in immune cell activation, adhesion, migration, growth, and apoptosis. This review will focus on the recent updates on those aspects for the roles of Mst1/2 kinases.

Protein kinase A activates the Hippo pathway to modulate cell proliferation and differentiation

The Hippo tumor suppressor pathway plays an important role in tissue homeostasis that ensures development of functional organs at proper size. The YAP transcription coactivator is a major effector of the Hippo pathway and is phosphorylated and inactivated by the Hippo pathway kinases Lats1/2. It has recently been shown that YAP activity is regulated by G-protein-coupled receptor signaling. Here we demonstrate that cyclic adenosine monophosphate (cAMP), a second messenger downstream from Gαs-coupled receptors, acts through protein kinase A (PKA) and Rho GTPases to stimulate Lats kinases and YAP phosphorylation. We also show that inactivation of YAP is crucial for PKA-induced adipogenesis. In addition, PKA activation in Drosophila inhibits the expression of Yorki (Yki, a YAP ortholog) target genes involved in cell proliferation and death. Taken together, our study demonstrates that Hippo-YAP is a key signaling branch of cAMP and PKA and reveals new insight into mechanisms of PKA in regulating a broad range of cellular functions.

Par-1 regulates tissue growth by influencing hippo phosphorylation status and hippo-salvador association

The evolutionarily conserved Hippo (Hpo) signaling pathway plays a pivotal role in organ size control by balancing cell proliferation and cell death. Here, we reported the identification of Par-1 as a regulator of the Hpo signaling pathway using a gain-of-function EP screen in Drosophila melanogaster. Overexpression of Par-1 elevated Yorkie activity, resulting in increased Hpo target gene expression and tissue overgrowth, while loss of Par-1 diminished Hpo target gene expression and reduced organ size. We demonstrated that par-1 functioned downstream of fat and expanded and upstream of hpo and salvador (sav). In addition, we also found that Par-1 physically interacted with Hpo and Sav and regulated the phosphorylation of Hpo at Ser30 to restrict its activity. Par-1 also inhibited the association of Hpo and Sav, resulting in Sav dephosphorylation and destabilization. Furthermore, we provided evidence that Par-1-induced Hpo regulation is conserved in mammalian cells. Taken together, our findings identified Par-1 as a novel component of the Hpo signaling network.

The BTB-zinc finger transcription factor abrupt acts as an epithelial oncogene in Drosophila melanogaster through maintaining a progenitor-like cell state

The capacity of tumour cells to maintain continual overgrowth potential has been linked to the commandeering of normal self-renewal pathways. Using an epithelial cancer model in Drosophila melanogaster, we carried out an overexpression screen for oncogenes capable of cooperating with the loss of the epithelial apico-basal cell polarity regulator, scribbled (scrib), and identified the cell fate regulator, Abrupt, a BTB-zinc finger protein. Abrupt overexpression alone is insufficient to transform cells, but in cooperation with scrib loss of function, Abrupt promotes the formation of massive tumours in the eye/antennal disc. The steroid hormone receptor coactivator, Taiman (a homologue of SRC3/AIB1), is known to associate with Abrupt, and Taiman overexpression also drives tumour formation in cooperation with the loss of Scrib. Expression arrays and ChIP-Seq indicates that Abrupt overexpression represses a large number of genes, including steroid hormone-response genes and multiple cell fate regulators, thereby maintaining cells within an epithelial progenitor-like state. The progenitor-like state is characterised by the failure to express the conserved Eyes absent/Dachshund regulatory complex in the eye disc, and in the antennal disc by the failure to express cell fate regulators that define the temporal elaboration of the appendage along the proximo-distal axis downstream of Distalless. Loss of scrib promotes cooperation with Abrupt through impaired Hippo signalling, which is required and sufficient for cooperative overgrowth with Abrupt, and JNK (Jun kinase) signalling, which is required for tumour cell migration/invasion but not overgrowth. These results thus identify a novel cooperating oncogene, identify mammalian family members of which are also known oncogenes, and demonstrate that epithelial tumours in Drosophila can be characterised by the maintenance of a progenitor-like state.

Cancer is a multigenic process, involving cooperative interactions between oncogenes or tumour suppressors. In this study, in a genetic screen in the vinegar fly, Drosophila melanogaster, for genes that cooperate with a mutation in the cell polarity (shape) regulator, scribbled (scrib), we identify a novel cooperative oncogene, abrupt. Expression of abrupt in scrib mutant tissue in the developing eye/antennal epithelium results in overgrown invasive tumours. abrupt encodes a BTB-zinc finger transcription factor, which has homology to several cancer-causing proteins in humans, such as BCL6. Analysis of the Abrupt targets and misexpressed genes in abrupt expressing-tissue and abrupt-expressing scrib mutant tumours, revealed cell fate regulators as a major class of targets. Thus, our results reveal that deregulation of multiple cell fate factors by Abrupt expression in the context of polarity disruption is associated with a progenitor-like cell state and the formation of overgrown invasive tumours. Our findings suggest that defective polarity may also be a critical factor in BTB-zinc finger-driven human cancers, and warrants further investigation into this issue.

The neuronal transcription factor erect wing regulates specification and maintenance of Drosophila R8 photoreceptor subtypes

Signaling pathways are often re-used during development in surprisingly different ways. The Hippo tumor suppressor pathway is best understood for its role in the control of growth. The pathway is also used in a very different context, in the Drosophila eye for the robust specification of R8 photoreceptor neuron subtypes, which complete their terminal differentiation by expressing light-sensing Rhodopsin (Rh) proteins. A double negative feedback loop between the Warts kinase of the Hippo pathway and the PH-domain growth regulator Melted regulates the choice between 'pale' R8 (pR8) fate defined by Rh5 expression and 'yellow' R8 (yR8) fate characterized by Rh6 expression. Here, we show that the gene encoding the homolog of human Nuclear respiratory factor 1, erect wing (ewg), is autonomously required to inhibit warts expression and to promote melted expression to specify pR8 subtype fate and induce Rh5. ewg mutants express Rh6 in most R8s due to ectopic warts expression. Further, ewg is continuously required to maintain repression of Rh6 in pR8s in aging flies. Our work shows that Ewg is a critical factor for the stable down-regulation of Hippo pathway activity to determine neuronal subtype fates. Neural-enriched factors, such as Ewg, may generally contribute to the contextual re-use of signaling pathways in post-mitotic neurons.

What do mechanotransduction, Hippo, Wnt, and TGFβ have in common? YAP and TAZ as key orchestrating molecules in ocular health and disease

Cells in vivo are exposed to a complex signaling environment. Biochemical signaling modalities, such as secreted proteins, specific extracellular matrix domains and ion fluxes certainly compose an important set of regulatory signals to cells. However, these signals are not exerted in isolation, but rather in concert with biophysical cues of the surrounding tissue, such as stiffness and topography. In this review, we attempt to highlight the biophysical attributes of ocular tissues and their influence on cellular behavior. Additionally, we introduce the proteins YAP and TAZ as targets of biophysical and biochemical signaling and important agonists and antagonists of numerous signaling pathways, including TGFβ and Wnt. We frame the discussion around this extensive signaling crosstalk, which allows YAP and TAZ to act as orchestrating molecules, capable of integrating biophysical and biochemical cues into a broad cellular response. Finally, while we draw on research from various fields to provide a full picture of YAP and TAZ, we attempt to highlight the intersections with vision science and the exciting work that has already been performed.

The Hippo pathway polarizes the actin cytoskeleton during collective migration of Drosophila border cells

Collective migration of Drosophila border cells depends on a dynamic actin cytoskeleton that is highly polarized such that it concentrates around the outer rim of the migrating cluster of cells. How the actin cytoskeleton becomes polarized in these cells to enable collective movement remains unknown. Here we show that the Hippo signaling pathway links determinants of cell polarity to polarization of the actin cytoskeleton in border cells. Upstream Hippo pathway components localize to contacts between border cells inside the cluster and signal through the Hippo and Warts kinases to polarize actin and promote border cell migration. Phosphorylation of the transcriptional coactivator Yorkie (Yki)/YAP by Warts does not mediate the function of this pathway in promoting border cell migration, but rather provides negative feedback to limit the speed of migration. Instead, Warts phosphorylates and inhibits the actin regulator Ena to activate F-actin Capping protein activity on inner membranes and thereby restricts F-actin polymerization mainly to the outer rim of the migrating cluster.

On the genetic control of planar growth during tissue morphogenesis in plants

Tissue morphogenesis requires extensive intercellular communication. Plant organs are composites of distinct radial cell layers. A typical layer, such as the epidermis, is propagated by stereotypic anticlinal cell divisions. It is presently unclear what mechanisms coordinate cell divisions relative to the plane of a layer, resulting in planar growth and maintenance of the layer structure. Failure in the regulation of coordinated growth across a tissue may result in spatially restricted abnormal growth and the formation of a tumor-like protrusion. Therefore, one way to approach planar growth control is to look for genetic mutants that exhibit localized tumor-like outgrowths. Interestingly, plants appear to have evolved quite robust genetic mechanisms that govern these aspects of tissue morphogenesis. Here we provide a short summary of the current knowledge about the genetics of tumor formation in plants and relate it to the known control of coordinated cell behavior within a tissue layer. We further portray the integuments of Arabidopsis thaliana as an excellent model system to study the regulation of planar growth. The value of examining this process in integuments was established by the recent identification of the Arabidopsis AGC VIII kinase UNICORN as a novel growth suppressor involved in the regulation of planar growth and the inhibition of localized ectopic growth in integuments and other floral organs. An emerging insight is that misregulation of central determinants of adaxial-abaxial tissue polarity can lead to the formation of spatially restricted multicellular outgrowths in several tissues. Thus, there may exist a link between the mechanisms regulating adaxial-abaxial tissue polarity and planar growth in plants.

BMP4 is a novel transcriptional target and mediator of mammary cell migration downstream of the Hippo pathway component TAZ

Since the metastatic progression of cancers is often fatal with limited treatment options, understanding the mechanism of metastasis is imperative for designing novel and targeted therapies. TAZ has been identified as a novel oncogene in both breast and lung cancers and is inhibited by the Hippo signaling pathway. In this study we provide convincing evidence that overexpression of TAZ in a mammary epithelial cell line, MCF10A, leads to enhanced cell migration - a fundamental characteristic of the metastatic progression of cancers. In addition, we identified the secreted growth factor BMP4 as a mediator of TAZ-induced cell migration. TAZ induces BMP4 transcription through the TEAD family of transcription factors, which mediate BMP4 promoter activation through binding to TEAD response element 1 (TRE1). Importantly, BMP4 activation by TAZ also enhances signaling downstream of TAZ, in particular, promoting Smad1/5 intracellular signaling. Functionally, short hairpin RNA-mediated knockdown of BMP4 rescued TAZ-induced cell migration. Our findings have identified a novel TAZ/TEAD/BMP4 signaling axis responsible for cell migration, with future implications in the development of targeted therapeutics for metastatic breast cancers.

The Drosophila Cadherin Fat regulates tissue size and planar cell polarity through different domains

The Drosophila Cadherin Fat (Ft) has been identified as a crucial regulator of tissue size and Planar Cell Polarity (PCP). However, the precise mechanism by which Ft regulates these processes remains unclear. In order to advance our understanding of the action of Ft, we have sought to identify the crucial Ft effector domains. Here we report that a small region of the Ft cytoplasmic domain (H2 region) is both necessary and sufficient, when membrane localized, to support viability and prevent tissue overgrowth. Interestingly, the H2 region is dispensable for regulating PCP signaling, whereas the mutant Ft lacking the H2 region is fully capable of directing PCP. This result suggests that Ft's roles in PCP signaling and tissue size control are separable, and each can be carried out independently. Surprisingly, the crucial regions of Ft identified in our structure-function study do not overlap with the previously reported interaction regions with Atrophin, Dco, or Lowfat.

Histone acetylation-mediated regulation of the Hippo pathway

The Hippo pathway is a signaling cascade recently found to play a key role in tumorigenesis therefore understanding the mechanisms that regulate it should open new opportunities for cancer treatment. Available data indicate that this pathway is controlled by signals from cell-cell junctions however the potential role of nuclear regulation has not yet been described. Here we set out to verify this possibility and define putative mechanism(s) by which it might occur. By using a luciferase reporter of the Hippo pathway, we measured the effects of different nuclear targeting drugs and found that chromatin-modifying agents, and to a lesser extent certain DNA damaging drugs, strongly induced activity of the reporter. This effect was not mediated by upstream core components (i.e. Mst, Lats) of the Hippo pathway, but through enhanced levels of the Hippo transducer TAZ. Investigation of the underlying mechanism led to the finding that cancer cell exposure to histone deacetylase inhibitors induced secretion of growth factors and cytokines, which in turn activate Akt and inhibit the GSK3 beta associated protein degradation complex in drug-affected as well as in their neighboring cells. Consequently, expression of EMT genes, cell migration and resistance to therapy were induced. These processes were suppressed by using pyrvinium, a recently described small molecule activator of the GSK 3 beta associated degradation complex. Overall, these findings shed light on a previously unrecognized phenomenon by which certain anti-cancer agents may paradoxically promote tumor progression by facilitating stabilization of the Hippo transducer TAZ and inducing cancer cell migration and resistance to therapy. Pharmacological targeting of the GSK3 beta associated degradation complex may thus represent a unique approach to treat cancer.

The Hippo pathway regulates stem cells during homeostasis and regeneration of the flatworm Macrostomum lignano

The Hippo pathway orchestrates activity of stem cells during development and tissue regeneration and is crucial for controlling organ size. However, roles of the Hippo pathway in highly regenerative organisms, such as flatworms, are unknown. Here we show that knockdown of the Hippo pathway core genes in the flatworm Macrostomum lignano affects tissue homeostasis and causes formation of outgrowths through hyperproliferation of stem cells (neoblasts), and leads to disruption of allometric scaling during regeneration and increased size of regenerated parts. We further show that Yap, the downstream effector of the Hippo pathway, is a potential neoblast marker gene, as it is expressed in dividing cells in M. lignano and is essential for neoblast self-renewal. The phenotypes we observe in M. lignano upon knockdown of the Hippo pathway core genes and Yap are consistent with the known functions of the pathway in other model organisms and demonstrate that the Hippo pathway is functionally conserved between flatworms and mammals. This work establishes M. lignano as a productive model for investigation of the Hippo pathway.

Drosophila ste-20 family protein kinase, hippo, modulates fat cell proliferation

Background

Evolutionarily conserved Hippo (Hpo) pathway plays a pivotal role in the control of organ size. Although the Hpo pathway regulates proliferation of a variety of epidermal cells, its function in non-ectoderm-derived cells is largely unknown.

Methodology/Principal Findings

Through methods including fat quantification assays, starvation assays, in vivo labeling assays, we show that overexpression of Hpo in Drosophila melanogaster fat body restricts Drosophila body growth and reduces fat storage through regulation of adipocyte proliferation rather than through influencing the size of fat cells and lipid metabolism, whereas compromising Hpo activity results in weight gain and greater fat storage. Furthermore, we provide evidence that Yorkie (Yki, a transcriptional coactivator that functions in the Hpo pathway) antagonizes Hpo to modulate fat storage in Drosophila.

Conclusions/Significance

Our findings specify a role of Hpo in controlling mesoderm-derived cell proliferation. The observed anti-obesity effects of Hpo may indicate great potential for its utilization in anti-obesity therapeutics.

The Hippo kinase promotes Scalloped cytoplasmic localization independently of Warts in a CRM1/Exportin1-dependent manner in Drosophila

Scalloped (SD) is a transcription factor characterized by a TEA/ATTS DNA binding domain. To activate transcription, SD must interact with its coactivators, including Yorkie (YKI) or Vestigial (VG). YKI is the downstream effector of the Hippo signaling pathway that plays a key role in the control of tissue growth. The core components of this pathway are two kinases, Hippo (HPO) and Warts (WTS), which negatively regulate the activity of the SD/YKI complex, retaining YKI in the cytoplasm. We previously showed that HPO kinase can also reduce SD/VG transcriptional activity in Drosophila S2 cells. We further investigated the relationship between the SD/VG complex and the Hippo pathway. We show here that HPO overexpression suppresses overgrowth induced by SD/VG in vivo during Drosophila development. Using S2 cells, we show that HPO promotes the translocation of SD to the cytoplasm in a CRM1-dependent manner, thereby inhibiting the induction of SD/VG target genes. Using RNAi-mediated depletion of yki and a mutant SD protein unable to interact with YKI, we demonstrate that HPO regulates SD localization independently of YKI. This function requires HPO kinase activity, yet surprisingly, not its downstream effector kinase WTS. Taken together, these observations reveal a new and unexpected role of HPO kinase in the regulation of a transcription factor independently of YKI.

The Hippo pathway and human cancer

The Hippo pathway controls organ size in diverse species, whereas pathway deregulation can induce tumours in model organisms and occurs in a broad range of human carcinomas, including lung, colorectal, ovarian and liver cancer. Despite this, somatic or germline mutations in Hippo pathway genes are uncommon, with only the upstream pathway gene neurofibromin 2 (NF2) recognized as a bona fide tumour suppressor gene. In this Review, we appraise the evidence for the Hippo pathway as a cancer signalling network, and discuss cancer-relevant biological functions, potential mechanisms by which Hippo pathway activity is altered in cancer and emerging therapeutic strategies.

The E3 ubiquitin ligase Itch regulates tumor suppressor protein RASSF5/NORE1 stability in an acetylation-dependent manner

Ras association (RalGDS/AF-6) domain family member RASSF5 is a non-enzymatic RAS effector super family protein, known to be involved in cell growth regulation. Expression of RASSF5 is found to be extinguished by promoter hypermethylation in different human cancers, and its ectopic expression suppresses cell proliferation and tumorigenicity. Interestingly, this role in tumorigenesis has been confounded by the fact that regulation at molecular level remains unclear and many transformed cells actually display elevated RASSF5 expression. Here, we demonstrate that E3 ubiquitin ligase Itch is a unique binding partner of RASSF5. Itch can interact with PPxY motif in RASSF5 both in vivo and in vitro through its WW domains. Importantly, the overexpression of Itch induces RASSF5 degradation by poly-ubiquitination via 26S proteasome pathway. In addition, our results indicate that the elevated levels of RASSF5 found in tumor cells due to acetylation, which restricts its binding to Itch and results in a more stable inert protein. Inhibition of RASSF5 acetylation permits its interaction with Itch and provokes proteasomal degradation. These data suggest that apart from promoter methylation, hyperacetylation could also be downregulating RASSF5 function in different human cancer. Finally, results from functional assays suggest that the overexpression of wild type, not the ligase activity defective Itch negatively regulate RASSF5-mediated G1 phase transition of cell cycle as well as apoptosis, suggesting that Itch alone is sufficient to alter RASSF5 function. Collectively, the present investigation identifies a HECT class E3 ubiquitin ligase Itch as a unique negative regulator of RASSF5, and suggests the possibility that acetylation as a potential therapeutic target for human cancer.

Regulation of cytoskeletal organization and junctional remodeling by the atypical cadherin Fat

The atypical cadherin Fat is a conserved regulator of planar cell polarity, but the mechanisms by which Fat controls cell shape and tissue structure are not well understood. Here, we show that Fat is required for the planar polarized organization of actin denticle precursors, adherens junction proteins and microtubules in the epidermis of the late Drosophila embryo. In wild-type embryos, spatially regulated cell-shape changes and rearrangements organize cells into highly aligned columns. Junctional remodeling is suppressed at dorsal and ventral cell boundaries, where adherens junction proteins accumulate. By contrast, adherens junction proteins fail to accumulate to the wild-type extent and all cell boundaries are equally engaged in junctional remodeling in fat mutants. The effects of loss of Fat on cell shape and junctional localization, but not its role in denticle organization, are recapitulated by mutations in Expanded, an upstream regulator of the conserved Hippo pathway, and mutations in Hippo and Warts, two kinases in the Hippo kinase cascade. However, the cell shape and planar polarity defects in fat mutants are not suppressed by removing the transcriptional co-activator Yorkie, suggesting that these roles of Fat are independent of Yorkie-mediated transcription. The effects of Fat on cell shape, junctional remodeling and microtubule localization are recapitulated by expression of activated Notch. These results demonstrate that cell shape, junctional localization and cytoskeletal planar polarity in the Drosophila embryo are regulated by a common signal provided by the atypical cadherin Fat and suggest that Fat influences tissue organization through its role in polarized junctional remodeling.

The Hippo pathway: regulators and regulations

Control of cell number is crucial in animal development and tissue homeostasis, and its dysregulation may result in tumor formation or organ degeneration. The Hippo pathway in both Drosophila and mammals regulates cell number by modulating cell proliferation, cell death, and cell differentiation. Recently, numerous upstream components involved in the Hippo pathway have been identified, such as cell polarity, mechanotransduction, and G-protein-coupled receptor (GPCR) signaling. Actin cytoskeleton or cellular tension appears to be the master mediator that integrates and transmits upstream signals to the core Hippo signaling cascade. Here, we review regulatory mechanisms of the Hippo pathway and discuss potential implications involved in different physiological and pathological conditions.

Genome-wide association of Yorkie with chromatin and chromatin-remodeling complexes

The Hippo pathway regulates growth through the transcriptional coactivator Yorkie, but how Yorkie promotes transcription remains poorly understood. We address this by characterizing Yorkie's association with chromatin and by identifying nuclear partners that effect transcriptional activation. Coimmunoprecipitation and mass spectrometry identify GAGA factor (GAF), the Brahma complex, and the Mediator complex as Yorkie-associated nuclear protein complexes. All three are required for Yorkie's transcriptional activation of downstream genes, and GAF and the Brahma complex subunit Moira interact directly with Yorkie. Genome-wide chromatin-binding experiments identify thousands of Yorkie sites, most of which are associated with elevated transcription, based on genome-wide analysis of messenger RNA and histone H3K4Me3 modification. Chromatin binding also supports extensive functional overlap between Yorkie and GAF. Our studies suggest a widespread role for Yorkie as a regulator of transcription and identify recruitment of the chromatin-modifying GAF protein and BRM complex as a molecular mechanism for transcriptional activation by Yorkie.

Mask proteins are cofactors of Yorkie/YAP in the Hippo pathway

The Hippo signaling pathway acts via the Yorkie (Yki)/Yes-associated protein (YAP) transcriptional coactivator family to control tissue growth in both Drosophila and mammals [1-3]. Yki/YAP drives tissue growth by activating target gene transcription, but how it does so remains unclear. Here we identify Mask as a novel cofactor for Yki/YAP. We show that Drosophila Mask forms a complex with Yki and its binding partner, Scalloped (Sd), on target-gene promoters and is essential for Yki to drive transcription of target genes and tissue growth. Furthermore, the stability and subcellular localization of both Mask and Yki is coregulated in response to various stimuli. Finally, Mask proteins are functionally conserved between Drosophila and humans and are coexpressed with YAP in a wide variety of human stem/progenitor cells and tumors.

Hippo activation through homodimerization and membrane association for growth inhibition and organ size control

Hippo (Hpo) signaling plays a critical role in restricting tissue growth and organ size in both invertebrate and vertebrate animals. However, how the Hpo kinase is regulated during development has not been clearly understood. Using a Bimolecular Fluorescence Complementation assay, we have investigated the functional significance of Hpo homo-dimer formation and subcellular localization in living cells. We found that Hpo dimerization and membrane association are critical for its activation in growth inhibition. As dimerization facilitates Hpo to access its binding partner, Hpo kinases in the homo-dimer trans-phosphorylate each other to increase their enzymatic activity. Moreover, loss- and gain-of-function studies indicate that upstream regulators, Expanded, Merlin and Kibra, play a critical role in promoting Hpo dimerization as well as association to the cortical F-actin beneath the plasma membrane. Enforced Hpo localization to the plasma membrane increases Hpo dimerization and activity. Therefore, homo-dimerization and plasma membrane association are two important mechanisms for Hpo activation in growth control during animal development.

The effort to make mosaic analysis a household tool

The analysis of genetic mosaics, in which an animal carries populations of cells with differing genotypes, is a powerful tool for understanding developmental and cell biology. In 1990, we set out to improve the methods used to make genetic mosaics in Drosophila by taking advantage of recently developed approaches for genome engineering. These efforts led to the work described in our 1993 Development paper.

Scribble acts in the Drosophila fat-hippo pathway to regulate warts activity

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.

A non-cell-autonomous tumor suppressor role for Stat in eliminating oncogenic scribble cells

Elucidating signaling events between tumor cells and their microenvironment is a major challenge in understanding cancer development. Drosophila melanogaster has emerged as an important tool for dissecting the genetic circuits tumors depend on because their imaginal discs, simple epithelia present in the larva, can be genetically manipulated to serve as models to study cancer mechanisms. Imaginal disc cells mutant for the tumor-suppressor gene scribble (scrib) lose apical-basal polarity and have the potential to form large neoplastic tumors. Interestingly, when scrib mutant (scrib(-)) cells are surrounded by normal cells the scrib(-) population is eliminated. However, the signals and mechanisms that cause the elimination of clones of scrib(-) cells are poorly understood. Here, we analyzed the role of Stat, a component of the JAK/STAT signaling pathway, in tissues with clones of scrib(-) cells. We found that Stat activity is required in normal cells for the elimination of neighboring scrib(-) cells. Importantly, these competitive defects of stat mutant cells are not simply due to defects in cell proliferation because even stat(-) cells manipulated to hyperproliferate are unable to eliminate scrib(-) cells. These data identify Stat activity as a critical determinant of whether or not a tissue can eliminate abnormal cells and provide an important step forward in understanding the complex network of signals operating in and around tumorigenic cells.

Quit your YAPing: a new target for cancer therapy

The Hippo pathway is an evolutionarily conserved signaling module that plays multiple roles in embryonic development. Components of the pathway, which includes a kinase cascade and a downstream complex composed of YAP and TEAD transcription factors, are dysregulated in a significant fraction of human cancers. In this issue of Genes & Development, Liu-Chittenden and colleagues (pp. 1300-1305) use genetic and pharmacological means to disrupt the active YAP-TEAD complex. As this intervention impedes tumorigenesis in the liver with no apparent effect on normal liver homeostasis, the work paves the way for the development of new strategies to target this pervasive oncogenic pathway.

Decreased expression of LATS1 is correlated with the progression and prognosis of glioma

Background

LATS1 is a tumor suppressor genes implicated in the pathogenesis of certain types of tumors, but its role is not known in human glioma.

Methods

Using real-time PCR and immunohistochemistry, we detected the mRNA and protein expression of LATS1 in glioma. The effect of LATS1 on cell growth and invasion were investigated.

Results

We found that mRNA and protein of LATS1 expression is significantly downregulated in glioma compared with normal control brain tissues. Furthermore, reduced LATS1 expression was markedly negatively correlated with WHO grade and KPS (p<0.001 and p<0.001) in glioma patients. Patients with lower LATS1 expression had a significantly shorter overall survival time than did patients with higher LATS1 expression. Multivariate analysis suggested that the level of LATS1 expression was an independent prognostic indicator (p<0.001) for the survival of patients with glioma. Forced expression of LATS1 in glioma U251 cells not only significantly suppressed cell growth, migration and invasion, but retarded cell cycle progression from G2/M to G1 in vitro. Finally, we found that overexpressed LATS1 markedly inhibited the expression level of cell cycle factor CCNA1.

Conclusion

These results indicate that LATS1 is an important candidate tumor suppressor and its downregulated expression may contribute to glioma progression.

KIBRA regulates aurora kinase activity and is required for precise chromosome alignment during mitosis

The Hippo pathway controls organ size and tumorigenesis by inhibiting cell proliferation and promoting apoptosis. KIBRA was recently identified as a novel regulator of the Hippo pathway. Several of the components of the Hippo pathway are important regulators of mitosis-related cell cycle events. We recently reported that KIBRA is phosphorylated by the mitotic kinases Aurora-A and -B. However, the role KIBRA plays in mitosis has not been established. Here, we show that KIBRA activates the Aurora kinases and is required for full activation of Aurora kinases during mitosis. KIBRA also promotes the phosphorylation of large tumor suppressor 2 (Lats2) on Ser(83) by activating Aurora-A, which controls Lats2 centrosome localization. However, Aurora-A is not required for KIBRA to associate with Lats2. We also found that Lats2 inhibits the Aurora-mediated phosphorylation of KIBRA on Ser(539), probably via regulating protein phosphatase 1. Consistent with playing a role in mitosis, siRNA-mediated knockdown of KIBRA causes mitotic abnormalities, including defects of spindle and centrosome formation and chromosome misalignment. We propose that the KIBRA-Aurora-Lats2 protein complexes form a novel axis that regulates precise mitosis.

Transduction of mechanical and cytoskeletal cues by YAP and TAZ

The physical and mechanical properties of the cellular microenvironment regulate cell shape and can strongly influence cell fate. How mechanical cues are sensed and transduced to regulate gene expression has long remained elusive. Recently, cues from the extracellular matrix, cell adhesion sites, cell shape and the actomyosin cytoskeleton were found to converge on the regulation of the downstream effectors of the Hippo pathway YAP (Yes-associated protein) and TAZ (transcriptional co-activator with PDZ-binding motif) in vertebrates and Yorkie in flies. This convergence may explain how mechanical signals can direct normal and pathological cell behaviour.

Protein kinases of the Hippo pathway: regulation and substrates

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.

Homeodomain-interacting protein kinase regulates Hippo pathway-dependent tissue growth

The Salvador-Warts-Hippo (SWH) pathway is an evolutionarily conserved regulator of tissue growth that is deregulated in human cancer. Upstream SWH pathway components convey signals from neighboring cells via a core kinase cassette to the transcription coactivator Yorkie (Yki). Yki controls tissue growth by modulating activity of transcription factors including Scalloped (Sd). To date, five SWH pathway kinases have been identified, but large-scale phosphoproteome studies suggest that unidentified SWH pathway kinases exist. To identify such kinases, we performed an RNA interference screen and isolated homeodomain-interacting protein kinase (Hipk). Unlike previously identified SWH pathway kinases, Hipk is unique in its ability to promote, rather than repress, Yki activity and does so in parallel to the Yki-repressive kinase, Warts (Wts). Hipk is required for basal Yki activity and is likely to regulate Yki function by promoting its accumulation in the nucleus. Like many SWH pathway proteins, Hipk's function is evolutionarily conserved as its closest human homolog, HIPK2, promotes activity of the Yki ortholog YAP in a kinase-dependent fashion. Further, HIPK2 promotes YAP abundance, suggesting that the mechanism by which HIPK2 regulates YAP has diverged in mammals.

The Hippo signaling pathway and stem cell biology

Stem cell (SC) activity fluctuates throughout an organism's lifetime to maintain homeostatic conditions in all tissues. As animals develop and age, their organs must remodel and regenerate themselves in response to environmental and physiological demands. Recently, the highly conserved Hippo signaling pathway, discovered in Drosophila melanogaster, has been implicated as a key regulator of organ size control across species. Deregulation is associated with substantial overgrowth phenotypes and eventual onset of cancer in various tissues. Importantly, emerging evidence suggests that the Hippo pathway can modulate its effects on tissue size by the direct regulation of SC proliferation and maintenance. These findings provide an attractive model for how this pathway might communicate physiological needs for growth to tissue-specific SC pools. In this review, we summarize the current and emerging data linking Hippo signaling to SC function.

The YAP and TAZ transcription co-activators: key downstream effectors of the mammalian Hippo pathway

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 [1,2]. 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.

Organ size control by Hippo and TOR pathways

The determination of final organ size is a highly coordinated and complex process that relies on the precise regulation of cell number and/or cell size. Perturbation of organ size control contributes to many human diseases, including hypertrophy, degenerative diseases, and cancer. Hippo and TOR are among the key signaling pathways involved in the regulation of organ size through their respective functions in the regulation of cell number and cell size. Here, we review the general mechanisms that regulate organ growth, describe how Hippo and TOR control key aspects of growth, and discuss recent findings that highlight a possible coordination between Hippo and TOR in organ size regulation.

Hippo signalling controls Dronc activity to regulate organ size in Drosophila

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.

The Hippo pathway regulates stem cell proliferation, self-renewal, and differentiation

Stem cells and progenitor cells are the cells of origin for multi-cellular organisms and organs. They play key roles during development and their dysregulation gives rise to human diseases such as cancer. The recent development of induced pluripotent stem cell (iPSC) technology which converts somatic cells to stem-like cells holds great promise for regenerative medicine. Nevertheless, the understanding of proliferation, differentiation, and self-renewal of stem cells and organ-specific progenitor cells is far from clear. Recently, the Hippo pathway was demonstrated to play important roles in these processes. The Hippo pathway is a newly established signaling pathway with critical functions in limiting organ size and suppressing tumorigenesis. This pathway was first found to inhibit cell proliferation and promote apoptosis, therefore regulating cell number and organ size in both Drosophila and mammals. However, in several organs, disturbance of the pathway leads to specific expansion of the progenitor cell compartment and manipulation of the pathway in embryonic stem cells strongly affects their self-renewal and differentiation. In this review, we summarize current observations on roles of the Hippo pathway in different types of stem cells and discuss how these findings changed our view on the Hippo pathway in organ development and tumorigenesis.

Integration of intercellular signaling through the Hippo pathway

Metazoan cells are exposed to a multitude of signals, which they integrate to determine appropriate developmental or physiological responses. Although the Hippo pathway was only discovered recently, and our knowledge of Hippo signal transduction is far from complete, a wealth of interconnections amongst Hippo and other signaling pathways have already been identified. Hippo signaling is particularly important for growth control, and I describe how integration of Hippo and other pathways contributes to regulation of organ growth. Molecular links between Hippo signaling and other signal transduction pathways are summarized. Different types of mechanisms for signal integration are described, and examples of how the complex interconnections between pathways are used to guide developmental and physiological growth responses are discussed. Features of Hippo signaling appear to make it particularly well suited to signal integration, including its responsiveness to cell-cell contact and the mediation of its transcriptional output by transcriptional co-activator proteins that can interact with transcription factors of other pathways.