YAP1 increases organ size and expands undifferentiated progenitor cells

YAP1, the nuclear target of Hippo signaling, stimulates heart growth through cardiomyocyte proliferation but not hypertrophy

Heart growth is tightly controlled so that the heart reaches a predetermined size. Fetal heart growth occurs through cardiomyocyte proliferation, whereas postnatal heart growth involves primarily physiological cardiomyocyte hypertrophy. The Hippo kinase cascade is an important regulator of organ growth. A major target of this kinase cascade is YAP1, a transcriptional coactivator that is inactivated by Hippo kinase activity. Here, we used both genetic gain and loss of Yap1 function to investigate its role in regulating proliferative and physiologic hypertrophic heart growth. Fetal Yap1 inactivation caused marked, lethal myocardial hypoplasia and decreased cardiomyocyte proliferation, whereas fetal activation of YAP1 stimulated cardiomyocyte proliferation. Enhanced proliferation was particularly dramatic in trabecular cardiomyocytes that normally exit from the cell cycle. Remarkably, YAP1 activation was sufficient to stimulate proliferation of postnatal cardiomyocytes, both in culture and in the intact heart. A dominant negative peptide that blocked YAP1 binding to TEAD transcription factors inhibited YAP1 proliferative activity, indicating that this activity requires YAP1-TEAD interaction. Although Yap1 was a critical regulator of cardiomyocyte proliferation, it did not influence physiological hypertrophic growth of cardiomyocytes, because postnatal Yap1 gain or loss of function did not significantly alter cardiomyocyte size. These studies demonstrate that Yap1 is a crucial regulator of cardiomyocyte proliferation, cardiac morphogenesis, and myocardial trabeculation. Activation of Yap1 in postnatal cardiomyocytes may be a useful strategy to stimulate cardiomyocyte expansion in therapeutic myocardial regeneration.

Transcriptional analysis of pluripotency reveals the Hippo pathway as a barrier to reprogramming

Pluripotent stem cells are derived from culture of early embryos or the germline and can be induced by reprogramming of somatic cells. Barriers to reprogramming that stabilize the differentiated state and have tumor suppression functions are expected to exist. However, we have a limited understanding of what such barriers might be. To find novel barriers to reprogramming to pluripotency, we compared the transcriptional profiles of the mouse germline with pluripotent and somatic cells, in vivo and in vitro. There is a remarkable global expression of the transcriptional program for pluripotency in primordial germ cells (PGCs). We identify parallels between PGC reprogramming to pluripotency and human germ cell tumorigenesis, including the loss of LATS2, a tumor suppressor kinase of the Hippo pathway. We show that knockdown of LATS2 increases the efficiency of induction of pluripotency in human cells. LATS2 RNAi, unlike p53 RNAi, specifically enhances the generation of fully reprogrammed iPS cells without accelerating cell proliferation. We further show that LATS2 represses reprogramming in human cells by post-transcriptionally antagonizing TAZ but not YAP, two downstream effectors of the Hippo pathway. These results reveal transcriptional parallels between germ cell transformation and the generation of iPS cells and indicate that the Hippo pathway constitutes a barrier to cellular reprogramming.

A molecular mechanism that links Hippo signalling to the inhibition of Wnt/β-catenin signalling

The Hippo signalling pathway has emerged as a key regulator of organ size, tissue homeostasis, and patterning. Recent studies have shown that two effectors in this pathway, YAP/TAZ, modulate Wnt/β-catenin signalling through their interaction with β-catenin or Dishevelled, depending on biological contexts. Here, we identify a novel mechanism through which Hippo signalling inhibits Wnt/β-catenin signalling. We show that YAP and TAZ, the transcriptional co-activators in the Hippo pathway, suppress Wnt signalling without suppressing the stability of β-catenin but through preventing its nuclear translocation. Our results show that YAP/TAZ binds to β-catenin, thereby suppressing Wnt-target gene expression, and that the Hippo pathway-stimulated phosphorylation of YAP, which induces cytoplasmic translocation of YAP, is required for the YAP-mediated inhibition of Wnt/β-catenin signalling. We also find that downregulation of Hippo signalling correlates with upregulation of β-catenin signalling in colorectal cancers. Remarkably, our analysis demonstrates that phosphorylated YAP suppresses nuclear translocation of β-catenin by directly binding to it in the cytoplasm. These results provide a novel mechanism, in which Hippo signalling antagonizes Wnt signalling by regulating nuclear translocation of β-catenin.

Ablation of Rassf2 induces bone defects and subsequent haematopoietic anomalies in mice

RASSF2 belongs to the Ras-association domain family (RASSF) of proteins, which may be involved in the Hippo signalling pathway. However, the role of RASSF2 in vivo is unknown. Here, we show that Rassf2 knockout mice manifest a multisystemic phenotype including haematopoietic anomalies and defects in bone remodelling. Bone marrow (BM) transplantation showed that Rassf2(-/-) BM cells had a normal haematopoietic reconstitution activity, indicating no intrinsic haematopoietic defects. Notably, in vitro differentiation studies revealed that ablation of Rassf2 suppressed osteoblastogenesis but promoted osteoclastogenesis. Co-culture experiments showed that an intrinsic defect in osteoblast differentiation from Rassf2(-/-) osteoblast precursors likely leads to both haematopoiesis and osteoclast defects in Rassf2(-/-) mice. Moreover, Rassf2 deficiency resulted in hyperactivation of nuclear factor (NF)-κB during both osteoclast and osteoblast differentiation. RASSF2 associated with IκB kinase (IKK) α and β forms, and suppressed IKK activity. Introduction of either RASSF2 or a dominant-negative form of IKK into Rassf2(-/-) osteoclast or osteoblast precursors inhibited NF-κB hyperactivation and normalized osteoclast and osteoblast differentiation. These observations indicate that RASSF2 regulates osteoblast and osteoclast differentiation by inhibiting NF-κB signalling.

Hsp90 molecular chaperone inhibitors: are we there yet?

Heat shock protein (Hsp) 90 is an ATP-dependent molecular chaperone that is exploited by malignant cells to support activated oncoproteins, including many cancer-associated kinases and transcription factors, and it is essential for oncogenic transformation. Originally viewed with skepticism, Hsp90 inhibitors are now being actively pursued by the pharmaceutical industry, with 17 agents having entered clinical trials. Investigators established Hsp90's druggability using the natural products geldanamycin and radicicol, which mimic the unusual ATP structure adopted in the chaperone's N-terminal nucleotide-binding pocket and cause potent and selective blockade of ATP binding/hydrolysis, inhibit chaperone function, deplete oncogenic clients, and show antitumor activity. Preclinical data obtained with these natural products have heightened interest in Hsp90 as a drug target, and 17-allylamino-17-demethoxygeldanamycin (17-AAG, tanespimycin) has shown clinical activity (as defined by Response Evaluation Criteria in Solid Tumors) in HER2+ breast cancer. Many optimized synthetic, small-molecule Hsp90 inhibitors from diverse chemotypes are now in clinical trials. Here, we review the discovery and development of Hsp90 inhibitors and assess their potential. There has been significant learning from studies of the basic biology of Hsp90, as well as translational drug development involving this chaperone, enhanced by the use of Hsp90 inhibitors as chemical probes. Success will likely lie in treating cancers that are addicted to particular driver oncogene products (e.g., HER2, ALK, EGFR, and BRAF) that are sensitive Hsp90 clients, as well as malignancies (especially multiple myeloma) in which buffering of proteotoxic stress is critical for survival. We discuss approaches for enhancing the effectiveness of Hsp90 inhibitors and highlight new chaperone and stress-response pathway targets, including HSF1 and Hsp70.

Novel cis-regulatory modules control expression of the Hairy and Enhancer of Split-1 (HES1) transcription factor in myoblasts

The expression profile of a gene is controlled by DNA sequences called cis-regulatory modules (CRMs). CRMs can function over large genomic distances and can be located many kilobases away from their target promoters. hes1 is a key developmental gene that is overexpressed in certain cancers and is a primary target of NOTCH signaling. Despite this, analysis of hes1 transcriptional control has been limited solely to its promoter. Here, we identify seven conserved DNA sequence blocks, representing the hes1 promoter and six novel CRMs, within 57 kb upstream of the mouse hes1 gene. We identify 12 binding sites for the RBP-Jκ NOTCH effector and a single M-CAT motif within these regions. We validate RBP-Jκ and TEAD family occupancy in cells in culture and test the response of each of these CRMs to active NOTCH. We show that two regions, CRM5 and CRM7, function as enhancers, and four can repress transcription. A pair of RBP-Jκ motifs arranged in a tail-tail configuration in CRM5 and the M-CAT motif in CRM7 are necessary for enhancer function. Furthermore, these enhancers are occupied by transcriptional co-activators and loop onto the hes1 promoter within the endogenous hes1 locus. This work demonstrates the power of combining computational genomics and experimental methodologies to identify novel CRMs and characterize their function.

Connective tissue growth factor autocriny in human hepatocellular carcinoma: oncogenic role and regulation by epidermal growth factor receptor/yes-associated protein-mediated activation

The identification of molecular mechanisms involved in the maintenance of the transformed phenotype of hepatocellular carcinoma (HCC) cells is essential for the elucidation of therapeutic strategies. Here, we show that human HCC cells display an autocrine loop mediated by connective tissue growth factor (CTGF) that promotes DNA synthesis and cell survival. Expression of CTGF was stimulated by epidermal growth factor receptor (EGFR) ligands and was dependent on the expression of the transcriptional coactivator, Yes-associated protein (YAP). We identified elements in the CTGF gene proximal promoter that bound YAP-enclosing complexes and were responsible for basal and EGFR-stimulated CTGF expression. We also demonstrate that YAP expression can be up-regulated through EGFR activation not only in HCC cells, but also in primary human hepatocytes. CTGF contributed to HCC cell dedifferentiation, expression of inflammation-related genes involved in carcinogenesis, resistance toward doxorubicin, and in vivo HCC cell growth. Importantly, CTGF down-regulated tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor 2 expression and was involved in the reduced sensitivity of these cells toward TRAIL-mediated apoptosis.

CONCLUSION

We have identified autocrine CTGF as a novel determinant of HCC cells' neoplastic behavior. Expression of CTGF can be stimulated through the EGFR-signaling system in HCC cells in a novel cross-talk with the oncoprotein YAP. Moreover, to our knowledge, this is the first study that identifies a signaling mechanism triggering YAP gene expression in healthy and transformed liver parenchymal cells.

Yes-associated protein expression in head and neck squamous cell carcinoma nodal metastasis

Introduction

Yes-associated protein (YAP) is considered an oncogene found amplified in multiple tumors, including head and neck squamous cell carcinoma (HNSCC). However, the role for YAP expression in HNSCC is not understood. Based on the central role of YAP in the hippo pathway, we tested if YAP was associated with the stage of HNSCC progression and metastatic potential.

Methods

To determine the expression of YAP in human benign and HNSCC tissue specimens, immunohistochemical analyses were performed in whole tissue samples and tissue microarrays. The expression of YAP in tissues of microarray was first associated with clinic-pathologic factors and results verified in samples from whole tissue sections. To investigate the role of YAP and p63 in regulating HNSCC epithelial to mesenchymal transition, epithelial and mesenchymal markers were assayed in Fadu and SCC-25 cells, HNSCC cells with endogenously elevated YAP expression and siRNA-mediated expression knockdown.

Results

Analysis of human HNSCC tissues suggested YAP expression was elevated in tumors compared to benign tissues and specifically localized at the tumor invasive front (p value <0.05). But, indexed YAP expression was lower with greater tumor grade (p value  = 0.02). In contrast, p63 expression was primarily elevated in high-grade tumors. Interestingly, both YAP and p63 was strongly expressed at the tumor invasive front and in metastatic HNSCC. Strikingly, we demonstrated YAP expression in the primary HNSCC tumor was associated with nodal metastasis in univariate analysis (p value  = 0.02). However, the knockdown of YAP in Fadu and SCC-25 cell lines was not associated with changes in epithelial to mesenchymal transdifferentiation or p63 expression.

Conclusion

Together, YAP expression, in combination with p63 can facilitate identification of HNSCC tumors from hyperplastic and benign tissues and the metastatic function of YAP in HNSCC may not be a result of epithelia to mesenchymal transdifferentiation.

Yes-associated protein promotes tumour development in luminal epithelial derived breast cancer

Yes-associated protein (YAP) is inactivated by the tumour suppressing Hippo pathway. The YAP gene is amplified in human liver cancer, and promotes tumour growth. However, there are contrasting reports about its function in breast cancer. Studies have demonstrated both oncogenic or tumour suppressor functions. Our study aims to clarify the role of YAP in breast cancer. We investigated the expression of YAP in 69 cases of human breast cancer tissue by immunohistochemistry (IHC). The role of YAP on cell growth in vitro and tumourigenesis in vivo were evaluated. We found that YAP was expressed in 75.4% (52/69) of breast cancer samples; amongst these cases YAP was overexpressed in 29% (20/69). There was no YAP expression in the remainder (17/69) cases. Breast cancer cell lines in which YAP was either overexpressed or depleted confirmed that YAP markedly promotes cell proliferation. This was confirmed in vivo: overexpression of YAP enhanced tumour formation and growth, whereas downregulation of YAP decreased the tumour formation and growth. Our results suggest that YAP acts as an oncogene in a subtype of breast cancer.

Mst1 and Mst2 protein kinases restrain intestinal stem cell proliferation and colonic tumorigenesis by inhibition of Yes-associated protein (Yap) overabundance

Ablation of the kinases Mst1 and Mst2, orthologs of the Drosophila antiproliferative kinase Hippo, from mouse intestinal epithelium caused marked expansion of an undifferentiated stem cell compartment and loss of secretory cells throughout the small and large intestine. Although median survival of mice lacking intestinal Mst1/Mst2 is 13 wk, adenomas of the distal colon are common by this age. Diminished phosphorylation, enhanced abundance, and nuclear localization of the transcriptional coactivator Yes-associated protein 1 (Yap1) is evident in Mst1/Mst2-deficient intestinal epithelium, as is strong activation of β-catenin and Notch signaling. Although biallelic deletion of Yap1 from intestinal epithelium has little effect on intestinal development, inactivation of a single Yap1 allele reduces Yap1 polypeptide abundance to nearly wild-type levels and, despite the continued Yap hypophosphorylation and preferential nuclear localization, normalizes epithelial structure. Thus, supraphysiologic Yap polypeptide levels are necessary to drive intestinal stem cell proliferation. Yap is overexpressed in 68 of 71 human colon cancers and in at least 30 of 36 colon cancer-derived cell lines. In colon-derived cell lines where Yap is overabundant, its depletion strongly reduces β-catenin and Notch signaling and inhibits proliferation and survival. These findings demonstrate that Mst1 and Mst2 actively suppress Yap1 abundance and action in normal intestinal epithelium, an antiproliferative function that frequently is overcome in colon cancer through Yap1 polypeptide overabundance. The dispensability of Yap1 in normal intestinal homeostasis and its potent proliferative and prosurvival actions when overexpressed in colon cancer make it an attractive therapeutic target.

Emerging role for epithelial polarity proteins of the Crumbs family as potential tumor suppressors

Defects in apical-basal polarity regulation are associated with tissue overgrowth and tumorogenesis, yet the molecular mechanisms linking epithelial polarity regulators to hyperplasia or neoplasia remain elusive. In addition, exploration of the expression and function of the full complement of proteins required for the polarized architecture of epithelial cells in the context of cancer is awaited. This paper provides an overview of recent studies performed on Drosophila and vertebrates showing that apical polarity proteins of the Crumbs family act to repress tissue growth and epithelial to mesenchymal transition. Thus, these proteins emerge as potential tumor suppressors. Interestingly, analysis of the molecular function of Crumbs proteins reveals a function for these polarity regulators in junctional complexes stability and control of signaling pathways regulating proliferation and apoptosis. Thereby, these studies provide a molecular basis explaining how regulation of epithelial polarity is coupled to tumorogenesis.

KIBRA protein phosphorylation is regulated by mitotic kinase aurora and protein phosphatase 1

Recent genetic studies in Drosophila identified Kibra as a novel regulator of the Hippo pathway, which controls tissue growth and tumorigenesis by inhibiting cell proliferation and promoting apoptosis. The cellular function and regulation of human KIBRA remain largely unclear. Here, we show that KIBRA is a phosphoprotein and that phosphorylation of KIBRA is regulated in a cell cycle-dependent manner with the highest level of phosphorylated KIBRA detected in mitosis. We further demonstrate that the mitotic kinases Aurora-A and -B phosphorylate KIBRA both in vitro and in vivo. We identified the highly conserved Ser(539) as the primary phosphorylation site for Aurora kinases. Moreover, we found that wild-type, but not catalytically inactive, protein phosphatase 1 (PP1) associates with KIBRA. PP1 dephosphorylated Aurora-phosphorylated KIBRA. KIBRA depletion impaired the interaction between Aurora-A and PP1. We also show that KIBRA associates with neurofibromatosis type 2/Merlin in a Ser(539) phosphorylation-dependent manner. Phosphorylation of KIBRA on Ser(539) plays a role in mitotic progression. Our results suggest that KIBRA is a physiological substrate of Aurora kinases and reveal a new avenue between KIBRA/Hippo signaling and the mitotic machinery.

Integrating developmental signals: a Hippo in the (path)way

The Hippo pathway, a signaling cascade that controls cell cycle progression, apoptosis and cell differentiation, has emerged as a fundamental regulator of many physiological and pathological processes. Recent studies have revealed a complex network of interactions directing Hippo pathway activity, and have connected this pathway with other key signaling pathways. Such crosstalk has uncovered novel roles for Hippo signaling, including regulation of TGFβ/SMAD and WNT/β-catenin pathways. This review highlights some of the recent findings in the Hippo field with an emphasis on how the Hippo pathway is integrated with other pathways to mediate diverse processes.

Yes-associated protein 1 is widely expressed in human brain tumors and promotes glioblastoma growth

The hippo pathway and its downstream mediator yes-associated protein 1 (YAP1) regulate mammalian organ size in part through modulating progenitor cell numbers. YAP1 has also been implicated as an oncogene in multiple human cancers. Currently, little is known about the expression of YAP1 either in normal human brain tissue or in central nervous system neoplasms. We used immunohistochemistry to evaluate nuclear YAP1 expression in the fetal and normal adult human brains and in 264 brain tumors. YAP1 was expressed in fetal and adult brain regions known to harbor neural progenitor cells, but there was little YAP1 immunoreactivity in the adult cerebral cortex. YAP1 protein was also readily detected in the nuclei of human brain tumors. In medulloblastoma, the expression varied between histologic subtypes and was most prominent in nodular/desmoplastic tumors. In gliomas, it was frequently expressed in infiltrating astrocytomas and oligodendrogliomas but rarely in pilocytic astrocytomas. Using a loss-of-function approach, we show that YAP1 promoted growth of glioblastoma cell lines in vitro. High levels of YAP1 messenger RNA expression were associated with aggressive molecular subsets of glioblastoma and with a nonsignificant trend toward reduced mean survival in human astrocytoma patients. These findings suggest that YAP1 may play an important role in normal human brain development and that it could represent a new target in human brain tumors.

The Hippo pathway in organ size control, tissue regeneration and stem cell self-renewal

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.

Signaling networks in human hepatocarcinogenesis--novel aspects and therapeutic options

Hepatocellular carcinoma (HCC) represents one of the most common human malignancies with poor prognosis. Because therapeutic strategies are insufficient for most HCC patients, there is a great need to determine the central molecular mechanisms and pathways in order to derive novel targets for systemic therapy. There is vast evidence that not only the dysregulation of distinct signaling cascades, but also their interactions at different levels, affect tumor cell function. Through these interactions, the effects of pathways can be increased, and even new tumor-supporting qualities acquired that further facilitate HCC progression. Although several approaches for the modulation of these relevant pathways are under development, future therapeutic strategies should take into account that oncogenic stimuli cannot be understood in a monodimensional manner. In order to avoid escape mechanisms during therapy, strategies based on comprehensive knowledge of the interactive regulatory network in hepatocarcinogenesis are necessary.

Stem cell proliferation in the skin: alpha-catenin takes over the hippo pathway

Epithelial tissues in the human body undergo constant renewal. For example, the skin is regenerated continuously through the periodic proliferation of normally quiescent stem cells in the basal compartment of the skin. Proper balance between stem cell quiescence and proliferation is maintained over the lifetime of the organism to preserve pools of stem cells required to maintain and repair tissues. However, mechanisms controlling the rate of stem cell renewal are poorly understood. Additionally, whether deregulation of these mechanisms within epidermal stem cells leads to skin cancer is not known. The adherens junction component α-catenin has been identified as a regulator of epidermal stem cell proliferation and as a suppressor of skin cancer through its inhibition of Yap, a transcriptional effector of the Hippo growth control pathway. Understanding the pathways that regulate the proliferation of stem cells in the skin holds promise for reversing the aging process and tumor development.

Yki/YAP, Sd/TEAD and Hth/MEIS control tissue specification in the Drosophila eye disc epithelium

During animal development, accurate control of tissue specification and growth are critical to generate organisms of reproducible shape and size. The eye-antennal disc epithelium of Drosophila is a powerful model system to identify the signaling pathway and transcription factors that mediate and coordinate these processes. We show here that the Yorkie (Yki) pathway plays a major role in tissue specification within the developing fly eye disc epithelium at a time when organ primordia and regional identity domains are specified. RNAi-mediated inactivation of Yki, or its partner Scalloped (Sd), or increased activity of the upstream negative regulators of Yki cause a dramatic reorganization of the eye disc fate map leading to specification of the entire disc epithelium into retina. On the contrary, constitutive expression of Yki suppresses eye formation in a Sd-dependent fashion. We also show that knockdown of the transcription factor Homothorax (Hth), known to partner Yki in some developmental contexts, also induces an ectopic retina domain, that Yki and Scalloped regulate Hth expression, and that the gain-of-function activity of Yki is partially dependent on Hth. Our results support a critical role for Yki- and its partners Sd and Hth - in shaping the fate map of the eye epithelium independently of its universal role as a regulator of proliferation and survival.

The Hippo pathway controls polar cell fate through Notch signaling during Drosophila oogenesis

During Drosophila oogenesis, the somatic follicle cells form an epithelial layer surrounding the germline cells to form egg chambers. In this process, follicle cell precursors are specified into polar cells, stalk cells, and main-body follicle cells. Proper specification of these three cell types ensures correct egg chamber formation and polarization of the anterior-posterior axis of the germline cells. Multiple signaling cascades coordinate to control the follicle cell fate determination, including Notch, JAK/STAT, and Hedgehog signaling pathways. Here, we show that the Hippo pathway also participates in polar cell specification. Over-activation of yorkie (yki) leads to egg chamber fusion, possibly through attenuation of polar cell specification. Loss-of-function experiments using RNAi knockdown or generation of mutant clones by mitotic recombination demonstrates that reduction of yki expression promotes polar cell formation in a cell-autonomous manner. Consistently, polar cells mutant for hippo (hpo) or warts (wts) are not properly specified, leading to egg chamber fusion. Furthermore, Notch activity is increased in yki mutant cells and reduction of Notch activity suppresses polar cell formation in yki mutant clones. These results demonstrate that yki represses polar cell fate through Notch signaling. Collectively, our data reveal that the Hippo pathway controls polar cell specification. Through repressing Notch activity, Yki serves as a key repressor in specifying polar cells during Drosophila oogenesis.

Yes-associated protein promotes cell proliferation by activating Fos Related Activator-1 in oral squamous cell carcinoma

In our previous study, we established an in vitro cellular carcinogenesis model of oral squamous cell carcinoma (OSCC), including a human immortalized oral epithelial cell (HIOEC) and a cancerous cell line (HB96). Microarray analysis showed that the gene encoding Yes-associated protein (YAP) was significantly increased in HB96 cells compared with HIOEC cells. But the underlying mechanism of YAP on oncogenesis, especially its downstream targets, are still not clear. YAP expression in OSCC cell lines and tissue specimens were investigated by using real-time PCR, western blotting and immunohistochemistry staining. YAP put-back plasmid with four mutation sites after YAP-siRNA interference was constructed by site-directed mutagenesis. Cell growth and colony formation were observed after YAP-siRNA interference or YAP put-back again in CAL27 cells. YAP expression was increased in the cellular carcinogenesis models and the clinical samples from primary OSCC patients. Inhibition of YAP by siRNA interference in CAL27 cells significantly inhibited cell proliferation and colony formation in soft agar, but these abilities were rescued when YAP was put-back again. At the same time, Fos Related Activator-1 (Fra-1) was down-regulated when YAP was inhibited by siRNA interference while Fra-1 was rescued when YAP was put-back again. Immunohistochemistry results also indicated that higher levels of YAP were significantly associated with Fra-1 overexpression in OSCC clinical samples. YAP could promote cell proliferation by activating transcription factor Fra-1 in oral squamous cell carcinoma.

The growing role of the Hippo--NDR kinase signalling in neuronal development and disease

The nuclear Dbf2-realted (NDR) family members are highly conserved serine/threonine protein kinases that function in concert with the Hippo signalling pathway to play crucial roles in regulation of cell proliferation and survival in non-neuronal cells. Recent studies employing a range of animal models have implicated NDR kinases as regulators of multiple aspects of development in post-mitotic neurons including progenitor proliferation, fate specification and circuit formation, all of which are crucial for neuronal functions. This review summarizes the recent advances in our understanding of the neuronal functions of NDR kinases and discusses their association with neuronal diseases.

Yes-associated protein 65 (YAP) expands neural progenitors and regulates Pax3 expression in the neural plate border zone

Yes-associated protein 65 (YAP) contains multiple protein-protein interaction domains and functions as both a transcriptional co-activator and as a scaffolding protein. Mouse embryos lacking YAP did not survive past embryonic day 8.5 and showed signs of defective yolk sac vasculogenesis, chorioallantoic fusion, and anterior-posterior (A-P) axis elongation. Given that the YAP knockout mouse defects might be due in part to nutritional deficiencies, we sought to better characterize a role for YAP during early development using embryos that develop externally. YAP morpholino (MO)-mediated loss-of-function in both frog and fish resulted in incomplete epiboly at gastrulation and impaired axis formation, similar to the mouse phenotype. In frog, germ layer specific genes were expressed, but they were temporally delayed. YAP MO-mediated partial knockdown in frog allowed a shortened axis to form. YAP gain-of-function in Xenopus expanded the progenitor populations in the neural plate (sox2⁺) and neural plate border zone (pax3⁺), while inhibiting the expression of later markers of tissues derived from the neural plate border zone (neural crest, pre-placodal ectoderm, hatching gland), as well as epidermis and somitic muscle. YAP directly regulates pax3 expression via association with TEAD1 (N-TEF) at a highly conserved, previously undescribed, TEAD-binding site within the 5′ regulatory region of pax3. Structure/function analyses revealed that the PDZ-binding motif of YAP contributes to the inhibition of epidermal and somitic muscle differentiation, but a complete, intact YAP protein is required for expansion of the neural plate and neural plate border zone progenitor pools. These results provide a thorough analysis of YAP mediated gene expression changes in loss- and gain-of-function experiments. Furthermore, this is the first report to use YAP structure-function analyzes to determine which portion of YAP is involved in specific gene expression changes and the first to show direct in vivo evidence of YAP's role in regulating pax3 neural crest expression.

Yes-associated protein regulation of adaptive liver enlargement and hepatocellular carcinoma development in mice

The Hippo kinase cascade, a growth-suppressive pathway that ultimately antagonizes the transcriptional coactivator Yes-associated protein (YAP), has been shown in transgenic animals to orchestrate organ size regulation. The purpose of this study was to determine whether in non-genetically modified mice (1) the Hippo pathway is involved in the regulation of adaptive liver enlargement caused by the mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), an agonist of constitutive androstane receptor and (2) a dysregulation of this pathway occurs during the development of chemically induced hepatocellular carcinoma (HCC). We show that liver enlargement caused by TCPOBOP was associated with an increase of YAP protein levels that paralleled the increase in 2-bromodeoxyuridine incorporation. Interestingly, when a second dose of TCPOBOP was given to mice with enlarged livers, no further increases in liver mass or YAP protein levels were observed, suggesting that the Hippo pathway prevents further growth of the hyperplastic liver. Viral-mediated exogenous expression of active YAP in mouse livers was able to partially overcome the block of hepatocyte proliferation. We also show that HCCs developed in mice given diethylnitrosamine and then subjected to repeated treatments with TCPOBOP had increased levels of YAP that were associated with down-regulation of microRNA 375, which is known to control YAP expression, and with enhanced levels of alpha-fetoprotein and connective tissue growth factor, two target genes of YAP.

CONCLUSION

These results suggest that the Hippo pathway regulates adaptive liver enlargement and is probably inactivated in initiated cells that escape the suppressive constrain exerted on the surrounding normal tissue, thus allowing clonal expansion to HCC.

α-catenin is a tumor suppressor that controls cell accumulation by regulating the localization and activity of the transcriptional coactivator Yap1

The Hippo pathway regulates contact inhibition of cell proliferation and, ultimately, organ size in diverse multicellular organisms. Inactivation of the Hippo pathway promotes nuclear localization of the transcriptional coactivator Yap1, a Hippo pathway effector, and can cause cancer. Here, we show that deletion of αE (α epithelial) catenin in the hair follicle stem cell compartment resulted in the development of skin squamous cell carcinoma in mice. Tumor formation was accelerated by simultaneous deletion of αE-catenin and the tumor suppressor-encoding gene p53. A small interfering RNA screen revealed a functional connection between αE-catenin and Yap1. By interacting with Yap1, αE-catenin promoted its cytoplasmic localization, and Yap1 showed constitutive nuclear localization in αE-catenin-null cells. We also found an inverse correlation between αE-catenin abundance and Yap1 activation in human squamous cell carcinoma tumors. These findings identify αE-catenin as a tumor suppressor that inhibits Yap1 activity and sequesters it in the cytoplasm.

The human adenocarcinoma-associated gene, AGR2, induces expression of amphiregulin through Hippo pathway co-activator YAP1 activation

Anterior Gradient Homolog 2 (AGR2) is expressed by the normal intestine and by most human adenocarcinomas, including those derived from the esophagus, pancreas, lung, breast, ovary, and prostate. Xenografts of human adenocarcinoma cell lines in nude mice previously demonstrated that AGR2 supports tumor growth. In addition, AGR2 is able to induce in vitro a transformed phenotype in fibroblast and epithelial cell lines. The mechanism underlying the growth promoting effects of AGR2 is unknown. The present study shows that AGR2 induces expression of amphiregulin (AREG), a growth promoting EGFR ligand. Induced AREG expression in adenocarcinoma cells is able to rescue the transformed phenotype that is lost when AGR2 expression is reduced. Additional experiments demonstrate that AGR2 induction of AREG is mediated by activation of the Hippo signaling pathway co-activator, YAP1. Thus AGR2 promotes growth by regulating the Hippo and EGF receptor signaling pathways.

Cooperation between dE2F1 and Yki/Sd defines a distinct transcriptional program necessary to bypass cell cycle exit

The Hippo signaling pathway regulates organ size homeostasis, while its inactivation leads to severe hyperplasia in flies and mammals. The transcriptional coactivator Yorkie (Yki) mediates transcriptional output of the Hippo signaling. Yki lacks a DNA-binding domain and is recruited to its target promoters as a complex with DNA-binding proteins such as Scalloped (Sd). In spite of recent progress, an open question in the field is the mechanism through which the Yki/Sd transcriptional signature is defined. Here, we report that Yki/Sd synergizes with and requires the transcription factor dE2F1 to induce a specific transcriptional program necessary to bypass the cell cycle exit. We show that Yki/Sd and dE2F1 bind directly to the promoters of the Yki/Sd-dE2F1 shared target genes and activate their expression in a strong cooperative manner. Consistently, RBF, a negative regulator of dE2F1, negates this synergy and limits the overall level of expression of the Yki/Sd-dE2F1 target genes. Significantly, dE2F1 is needed for Yki/Sd-dependent full activation of these target genes, and a de2f1 mutation strongly blocks yki-induced proliferation in vivo. Thus, the Yki transcriptional program is determined through functional interactions with other transcription factors directly at target promoters. We suggest that such functional interactions would influence Yki activity and help diversify the transcriptional output of the Hippo pathway.

Yap1 acts downstream of α-catenin to control epidermal proliferation

During development and regeneration, proliferation of tissue-specific stem cells is tightly controlled to produce organs of a predetermined size. The molecular determinants of this process remain poorly understood. Here, we investigate the function of Yap1, the transcriptional effector of the Hippo signaling pathway, in skin biology. Using gain- and loss-of-function studies, we show that Yap1 is a critical modulator of epidermal stem cell proliferation and tissue expansion. Yap1 mediates this effect through interaction with TEAD transcription factors. Additionally, our studies reveal that α-catenin, a molecule previously implicated in tumor suppression and cell density sensing in the skin, is an upstream negative regulator of Yap1. α-catenin controls Yap1 activity and phosphorylation by modulating its interaction with 14-3-3 and the PP2A phosphatase. Together, these data identify Yap1 as a determinant of the proliferative capacity of epidermal stem cells and as an important effector of a "crowd control" molecular circuitry in mammalian skin.

The Hippo pathway in biological control and cancer development

The Hippo pathway is an evolutionally conserved protein kinase cascade involved in regulating organ size in vivo and cell contact inhibition in vitro by governing cell proliferation and apoptosis. Deregulation of the Hippo pathway is linked to cancer development. Its first core kinase Warts was identified in Drosophila more than 15 years ago, but it gained much attention when other core components of the pathway were identified 8 years later. Major discoveries of the pathway were made during past several years. The core kinase components Hippo, Salvador, Warts, and Mats in the fly and Mst1/2, WW45, Lats1/2, and Mob1 in mammals phosphorylate and inactivate downstream transcriptional co-activators Yorkie in the fly, Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) in mammals, respectively. Phosphorylated Yorkie, YAP, and TAZ are sequestered in the cytoplasm by interaction with 14-3-3 proteins. Here we review recent progresses of this pathway by focusing on how these proteins communicate with each other and how loss of regulation results in cancers.

Mammalian Hippo pathway: from development to cancer and beyond

The Hippo pathway was discovered as a signal transduction pathway that regulates organ size in Drosophila melanogaster. It is composed of three components: cell surface upstream regulators including cell adhesion molecules and cell polarity complexes; a kinase cascade comprising two serine-threonine kinases with regulators and adaptors; and a downstream target, a transcription coactivator. The coactivator mediates the transcription of cell proliferation-promoting and anti-apoptotic genes. The pathway negatively regulates the coactivator to restrict cell proliferation and to promote cell death. Thus, the pathway prevents tissue overgrowth and tumourigenesis. The framework of the pathway is conserved in mammals. A dysfunction of the pathway is frequently detected in human cancers and correlates with a poor prognosis. Recent works indicated that the Hippo pathway plays an important role in tissue homoeostasis through the regulation of stem cells, cell differentiation and tissue regeneration.

Wbp2 cooperates with Yorkie to drive tissue growth downstream of the Salvador-Warts-Hippo pathway

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.

The Hippo signaling pathway restricts the oncogenic potential of an intestinal regeneration program

Although a developmental role for Hippo signaling in organ size control is well appreciated, how this pathway functions in tissue regeneration is largely unknown. Here we address this issue using a dextran sodium sulfate (DSS)-induced colonic regeneration model. We find that regenerating crypts express elevated Yes-associated protein (YAP) levels. Inactivation of YAP causes no obvious intestinal defects under normal homeostasis, but severely impairs DSS-induced intestinal regeneration. Conversely, hyperactivation of YAP results in widespread early-onset polyp formation following DSS treatment. Thus, the YAP oncoprotein must be exquisitely controlled in tissue regeneration to allow compensatory proliferation and prevent the intrinsic oncogenic potential of a tissue regeneration program.

Angiomotin is a novel Hippo pathway component that inhibits YAP oncoprotein

The Yes-associated protein (YAP) is a transcription coactivator that plays a crucial role in organ size control by promoting cell proliferation and inhibiting apoptosis. The Hippo tumor suppressor pathway inhibits YAP through phosphorylation-induced cytoplasmic retention and degradation. Here we report a novel mechanism of YAP regulation by angiomotin (AMOT) family proteins via a direct interaction. Knockdown of AMOT family protein AMOTL2 in polarized Madin-Darby canine kidney (MDCK) cells leads to YAP activation, as indicated by decreased YAP tight junction localization, attenuated YAP phosphorylation, accumulation of nuclear YAP, and induction of YAP target gene expression. Transcriptional coactivator with PDZ-binding motif (TAZ), the YAP paralog, is also regulated by AMOT in a similar fashion. Furthermore, AMOTL2 knockdown results in loss of cell contact inhibition in a manner dependent on the functions of YAP and TAZ. Our results indicate a potential tumor-suppressing role of AMOT family proteins as components of the Hippo pathway, and demonstrate a novel mechanism of YAP and TAZ inhibition by AMOT-mediated tight junction localization. These observations provide a potential link between the Hippo pathway and cell contact inhibition.

Yes-associated protein (YAP) transcriptional coactivator functions in balancing growth and differentiation in skin

In mammals, skin begins as a single-layered epithelium, which, through a series of signals, either stratifies and differentiates to become epidermis or invaginates downward to make hair follicles (HFs). To achieve and maintain proper tissue architecture, keratinocytes must intricately balance growth and differentiation. Here, we uncover a critical and hitherto unappreciated role for Yes-associated protein (YAP), an evolutionarily conserved transcriptional coactivator with potent oncogenic potential. We show that YAP is highly expressed and nuclear in single-layered basal epidermal progenitors. Notably, nuclear YAP progressively declines with age and correlates with proliferative potential of epidermal progenitors. Shortly after initiation of HF morphogenesis, YAP translocates to the cytoplasm of differentiating cells. Through genetic analysis, we demonstrate a role for YAP in maintaining basal epidermal progenitors and regulating HF morphogenesis. YAP overexpression causes hair placodes to evaginate into epidermis rather than invaginate into dermis. YAP also expands basal epidermal progenitors, promotes proliferation, and inhibits terminal differentiation. In vitro gain-and-loss of function studies show that primary mouse keratinocytes (MKs) accelerate proliferation, suppress differentiation, and inhibit apoptosis when YAP is activated and reverse these features when YAP is inhibited. Finally, we identify Cyr61 as a target of YAP in MKs and demonstrate a requirement for TEA domain (TEAD) transcriptional factors to comediate YAP functions in MKs.

KIBRA regulates Hippo signaling activity via interactions with large tumor suppressor kinases

The Hippo pathway controls tissue growth and tumorigenesis by inhibiting cell proliferation and promoting apoptosis. Recent genetic studies in Drosophila identified Kibra as a novel regulator of Hippo signaling. Human KIBRA has been associated with memory performance and cell migration. However, it is unclear whether or how KIBRA is connected to the Hippo pathway in mammalian cells. Here, we show that KIBRA associates with and activates Lats (large tumor suppressor) 1 and 2 kinases by stimulating their phosphorylation on the hydrophobic motif. KIBRA overexpression stimulates the phosphorylation of Yes-associated protein (YAP), the Hippo pathway effector. Conversely, depletion of KIBRA by RNA interference reduces YAP phosphorylation. Furthermore, KIBRA stabilizes Lats2 by inhibiting its ubiquitination. We also found that KIBRA mRNA is induced by YAP overexpression in both murine and human cells, suggesting the evolutionary conservation of KIBRA as a transcriptional target of the Hippo signaling pathway. Thus, our study revealed a new connection between KIBRA and mammalian Hippo signaling.

Structural and functional insights into the TEAD-YAP complex in the Hippo signaling pathway

The control of organ size growth is one of the most fundamental aspects of life. In the past two decades, a highly conserved Hippo signaling pathway has been identified as a key molecular mechanism for governing organ size regulation. In the middle of this pathway is a kinase cascade that negatively regulates the downstream component Yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ)/Yorkie through phosphorylation. Phosphorylation of YAP/TAZ/Yorkie promotes its cytoplasmic localization, leads to cell apoptosis and restricts organ size overgrowth. When the Hippo pathway is inactivated, YAP/TAZ/Yorkie translocates into the nucleus to bind to the transcription enhancer factor (TEAD/TEF) family of transcriptional factors to promote cell growth and proliferation. In this review, we will focus on the structural and functional studies on the downstream transcription factor TEAD and its coactivator YAP.

Hippo signaling: growth control and beyond

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.

The Hippo tumor suppressor pathway regulates intestinal stem cell regeneration

Identification of the signaling pathways that control the proliferation of stem cells (SCs), and whether they act in a cell or non-cell autonomous manner, is key to our understanding of tissue homeostasis and cancer. In the adult Drosophila midgut, the Jun N-Terminal Kinase (JNK) pathway is activated in damaged enterocyte cells (ECs) following injury. This leads to the production of Upd cytokines from ECs, which in turn activate the Janus kinase (JAK)/Signal transducer and activator of transcription (STAT) pathway in Intestinal SCs (ISCs), stimulating their proliferation. In addition, the Hippo pathway has been recently implicated in the regulation of Upd production from the ECs. Here, we show that the Hippo pathway target, Yorkie (Yki), also plays a crucial and cell-autonomous role in ISCs. Activation of Yki in ISCs is sufficient to increase ISC proliferation, a process involving Yki target genes that promote division, survival and the Upd cytokines. We further show that prior to injury, Yki activity is constitutively repressed by the upstream Hippo pathway members Fat and Dachsous (Ds). These findings demonstrate a cell-autonomous role for the Hippo pathway in SCs, and have implications for understanding the role of this pathway in tumorigenesis and cancer stem cells.

BMI-1 suppresses contact inhibition and stabilizes YAP in Ewing sarcoma

The polycomb group family protein BMI-1 is overexpressed by and functions as an oncogene in many different human cancers. We have previously shown that BMI-1 promotes the tumorigenicity of Ewing sarcoma family tumors (ESFTs) and that this is mediated independently of CDKN2A repression. In this study, we have discovered that high levels of BMI-1 confer resistance to contact inhibition in ESFT cells. Using stable retroviral transduction, we evaluated the consequences of BMI-1 knockdown on the growth of CDKN2A wild-type and mutant ESFT cells in subconfluent and confluent conditions. Although knockdown of BMI-1 had no effect on proliferation in low-density cultures, at high cell densities it resulted in cell cycle arrest and death. The normal cell contact inhibition response is mediated, in large part, by the recently described Hippo pathway which functions to inhibit cell proliferation and promote cell death by inactivating the Yes-Associated Protein (YAP). Significantly, we found that YAP levels, activity and expression did not diminish in confluent ESFT cells that expressed high levels of BMI-1. In contrast, YAP expression and nuclear localization were reduced in confluent BMI-1 knockdown cells suggesting that silencing of BMI-1 restored contact inhibition by restoring normal activation of the Hippo-YAP growth-suppressor pathway. Importantly, knockdown of YAP in ESFT cells resulted in profound inhibition of cell proliferation and anchorage-independent colony formation suggesting that stabilization and continued expression of YAP is critical for ESFT growth and tumorigenicity. Together, these studies reveal a previously unrecognized link between BMI-1, contact inhibition and the Hippo-YAP pathway and suggest that resistance to contact inhibition in BMI-1 overexpressing cancer cells may be in part a result of Hippo inhibition and aberrant stabilization of YAP.

YAP is a candidate oncogene for esophageal squamous cell carcinoma

Yes-associated protein (YAP), the nuclear effector of the Hippo pathway, is a key regulator of organ size and a candidate human oncogene located at chromosome 11q22. Since we previously reported amplification of 11q22 region in esophageal squamous cell carcinoma (ESCC), in this study we focused on the clinical significance and biological functions of YAP in this tumor. Frequent overexpression of YAP protein was observed in ESCC cells including those with a robust amplicon at position 11q22. Overexpression of the YAP protein was frequently detected in primary tumors of ESCC as well. Patients with YAP-overexpressing tumors had a worse overall rate of survival than those with non-expressing tumors, and YAP positivity was independently associated with a worse outcome in the multivariate analysis. Further analyses in cells in which YAP was either overexpressed or depleted confirmed that cell proliferation was promoted in a YAP isoform-independent but YAP expression level-dependent manner. YAP depletion inhibited cell proliferation mainly in the G(0)-G(1) phase and induced an increase in CDKN1A/p21 transcription but a decrease in BIRC5/survivin transcription. Our results indicate that YAP is a putative oncogene in ESCC and it represents a potential diagnostic and therapeutic target.

The Salvador/Warts/Hippo pathway controls regenerative tissue growth in Drosophila melanogaster

During tissue regeneration, cell proliferation replaces missing structures to restore organ function. Regenerative potential differs greatly between organs and organisms; for example some amphibians can regrow entire limbs whereas mammals cannot. The process of regeneration relies on several signaling pathways that control developmental tissue growth, and implies the existence of organ size-control checkpoints that regulate both developmental, and regenerative, growth. Here we explore the role of one such checkpoint, the Salvador-Warts-Hippo pathway, in tissue regeneration. The Salvador-Warts-Hippo pathway limits tissue growth by repressing the Yorkie transcriptional co-activator. Several proteins serve as upstream modulators of this pathway including the atypical cadherins, Dachsous and Fat, whilst the atypical myosin, Dachs, functions downstream of Fat to activate Yorkie. Using Drosophila melanogaster imaginal discs we show that Salvador-Warts-Hippo pathway activity is repressed in regenerating tissue and that Yorkie is rate-limiting for regeneration of the developing wing. We show that regeneration is compromised in dachs mutant wing discs, but that proteins in addition to Fat and Dachs are likely to modulate Yorkie activity in regenerating cells. In conclusion our data reveal the importance of Yorkie hyperactivation for tissue regeneration and suggest that multiple upstream inputs, including Fat-Dachsous signaling, sense tissue damage and regulate Yorkie activity during regeneration of epithelial tissues.

Mst1/2 signalling to Yap: gatekeeper for liver size and tumour development

The mechanisms controlling mammalian organ size have long been a source of fascination for biologists. These controls are needed to both ensure the integrity of the body plan and to restrict inappropriate proliferation that could lead to cancer. Regulation of liver size is of particular interest inasmuch as this organ maintains the capacity for regeneration throughout life, and is able to regain precisely its original mass after partial surgical resection. Recent studies using genetically engineered mouse strains have shed new light on this problem; the Hippo signalling pathway, first elucidated as a regulator of organ size in Drosophila, has been identified as dominant determinant of liver growth. Defects in this pathway in mouse liver lead to sustained liver overgrowth and the eventual development of both major types of liver cancer, hepatocellular carcinoma and cholangiocarcinoma. In this review, we discuss the role of Hippo signalling in liver biology and the contribution of this pathway to liver cancer in humans.

Targeting YAP and Hippo signaling pathway in liver cancer

IMPORTANCE OF THE FIELD

The Hippo signaling pathway plays pivotal roles in controlling both cell growth and organ size, emerging as a new paradigm in tumor suppression. Yes-associated protein (YAP) functions as a potent transcription co-activator and is a major downstream target tightly regulated by the Hippo pathway. Inactivation of the Hippo signaling induces YAP-mediated activation of various target genes that functionally causes cellular proliferation and outgrowth of organ size. Recently, YAP has been implicated as a bona fide oncogene in solid tumors, but little is known about its exact molecular mechanism in carcinogenesis.

AREAS COVERED IN THIS REVIEW

We discuss the latest important findings in the Hippo signaling pathway and the possible means of developing potential cancer therapeutics by targeting multiple sites along the Hippo pathway.

WHAT THE READER WILL GAIN

An overview of the emerging roles of YAP and Hippo signaling in oncogenesis and the possible ways of developing cancer therapies against the pathway components, downstream targets or interconnected pathways.

TAKE HOME MESSAGE

YAP is a key oncogenic driver in liver carcinogenesis and deregulation of the Hippo pathway causes tumor formation and malignancy. Targeting YAP and cognate downstream signaling targets may have clinical utility in cancer therapies.

AXL receptor kinase is a mediator of YAP-dependent oncogenic functions in hepatocellular carcinoma

Yes-associated protein (YAP) is a downstream effector of the Hippo signaling pathway, which controls organ expansion and tissue development. We have recently defined the tumorigenic potential and clinical significance of the YAP1 oncogene in human hepatocellular carcinoma (HCC). The present study aims to define the tumorigenic properties of YAP in HCC and elucidate the related downstream signaling mechanism. In a gain-of-function study, we demonstrated that ectopic increased expression of YAP in the immortalized non-tumorigenic hepatocyte cell line MIHA confers tumorigenic and metastatic potentials, as evidenced by (1) enhanced aptitudes in cell viability, anchorage-independent growth, migration and invasion; (2) tumor formation in a xenograft mouse model; and (3) induction of HCC biomarker α-fetoprotein and activation of mitogen-activated protein kinase. Furthermore, we have identified AXL, a receptor tyrosine kinase, as a key downstream target that drives YAP-dependent oncogenic functions. RNAi-mediated knockdown of AXL expression decreased the ability of YAP-expressing MIHA cells and of the primary HCC cell line to proliferate and invade. These results indicate that AXL is a mediator of YAP-dependent oncogenic activities and implicates it as a potential therapeutic target for HCC.

The Hippo pathway regulates intestinal stem cell proliferation during Drosophila adult midgut regeneration

Intestinal stem cells (ISCs) in the adult Drosophila midgut proliferate to self-renew and to produce differentiating daughter cells that replace those lost as part of normal gut function. Intestinal stress induces the activation of Upd/Jak/Stat signalling, which promotes intestinal regeneration by inducing rapid stem cell proliferation. We have investigated the role of the Hippo (Hpo) pathway in the Drosophila intestine (midgut). Hpo pathway inactivation in either the ISCs or the differentiated enterocytes induces a phenotype similar to that observed under stress situations, including increased stem cell proliferation and expression of Jak/Stat pathway ligands. Hpo pathway targets are induced by stresses such as bacterial infection, suggesting that the Hpo pathway functions as a sensor of cellular stress in the differentiated cells of the midgut. In addition, Yki, the pro-growth transcription factor target of the Hpo pathway, is required in ISCs to drive the proliferative response to stress. Our results suggest that the Hpo pathway is a mediator of the regenerative response in the Drosophila midgut.

The hippo signaling pathway in development and cancer

First discovered in Drosophila, the Hippo signaling pathway is a conserved regulator of organ size. Central to this pathway is a kinase cascade leading from the tumor suppressor Hippo (Mst1 and Mst2 in mammals) to the oncoprotein Yki (YAP and TAZ in mammals), a transcriptional coactivator of target genes involved in cell proliferation and survival. Here, I review recent progress in elucidating the molecular mechanism and physiological function of Hippo signaling in Drosophila and mammals. These studies suggest that the core Hippo kinase cascade integrates multiple upstream inputs, enabling dynamic regulation of tissue homeostasis in animal development and physiology.

Expression patterns of ABCG2, Bmi-1, Oct-3/4, and Yap in the developing mouse incisor

Recent studies have demonstrated the existence of dental stem cells in the continuously growing tooth. However, much remains to be learned about the complex mechanism involving stem cells during tooth development. We determined the expression patterns of four stem cell markers ABCG2, Bmi-1, Oct-3/4, and Yap in the developing mouse incisors between embryonic day (E) 11 and postnatal day (PN) 20. ABCG2 was localized strongly in the perivascular region of the incisor mesenchyme from E11 to PN20, and in the odontoblasts from E18 to PN20. Bmi-1 was expressed in both the dental epithelium and mesenchyme from E11 to E14. The expression of Bmi-1 was noticeably reduced at E16, and was restricted to the apical bud from E16 to PN20. Oct-3/4 was localized in the nucleus of the cells in the superficial layer and stellate reticulum within the dental epithelium from E11 to E14 and in the apical bud from E16 to PN20. Meanwhile, once the ameloblasts and odontoblasts began to appear at E16, they expressed Oct-3/4 in the cytoplasm. Yap was expressed in most of the basal cells of the incisor dental epithelium from E11 to E14, but was expressed mainly in the transit-amplifying (TA) cells within the basal cell layer from E16 to PN20. The unique and overlapping expression patterns of ABCG2, Bmi-1, Oct-3/4, and Yap suggest the independent and interactive functions of the four stem cell markers in the developing mouse incisor.