Control of tissue growth and cell transformation by the Salvador/Warts/Hippo pathway. PLoS One. 2012;7:e31994. [PMID: 22359650 DOI: 10.1371/journal.pone.0031994]

The Hippo pathway in cancer: YAP/TAZ and TEAD as therapeutic targets in cancer

Tumorigenesis is a highly complex process, involving many interrelated and cross-acting signalling pathways. One such pathway that has garnered much attention in the field of cancer research over the last decade is the Hippo signalling pathway. Consisting of two antagonistic modules, the pathway plays an integral role in both tumour suppressive and oncogenic processes, generally via regulation of a diverse set of genes involved in a range of biological functions. This review discusses the history of the pathway within the context of cancer and explores some of the most recent discoveries as to how this critical transducer of cellular signalling can influence cancer progression. A special focus is on the various recent efforts to therapeutically target the key effectors of the pathway in both preclinical and clinical settings.

The regulation of Yorkie, YAP and TAZ: new insights into the Hippo pathway

The Hippo pathway is a highly conserved signalling pathway that regulates multiple biological processes, including organ size control and cell fate. Since its discovery, genetic and biochemical studies have elucidated several key signalling steps important for pathway activation and deactivation. In recent years, technical advances in microscopy and genome modification have allowed new insights into Hippo signalling to be revealed. These studies have highlighted that the nuclear-cytoplasmic shuttling behaviour of the Hippo pathway transcriptional co-activators Yorkie, YAP and TAZ is far more dynamic than previously appreciated, and YAP and TAZ are also regulated by liquid-liquid phase separation. Here, we review our current understanding of Yorkie, YAP and TAZ regulation, with a focus on recent microscopy-based studies.

Epidermal YAP2-5SA-ΔC Drives β-Catenin Activation to Promote Keratinocyte Proliferation in Mouse Skin In Vivo

The epidermis is a highly regenerative tissue. YAP is a pivotal regulator of stem/progenitor cells in tissue regeneration, including in the epidermis. The molecular mechanisms downstream of YAP that activate epidermal cell proliferation remain largely unknown. We found that YAP and β-catenin co-localize in the nuclei of keratinocytes in the regenerating epidermis in vivo and in proliferating HaCaT keratinocytes in vitro. Inactivation of YAP in HaCaT keratinocytes resulted in reduced activated β-catenin and reduced keratinocyte numbers in vitro. In addition, we found that in the hyperplastic epidermis of YAP2-5SA-ΔC mice, the mutant YAP2-5SA-ΔC protein was predominantly localized in the keratinocyte nuclei and caused increased expression of activated nuclear β-catenin. Accordingly, β-catenin transcriptional activity was elevated in the skin of live YAP2-5SA-ΔC/TOPFLASH mice. Lastly, loss of β-catenin in basal keratinocytes of YAP2-5SA-ΔC/K14-creERT/CtnnB1^-/- mice resulted in reduced proliferation of basal keratinocytes and a striking rescue of the hyperplastic abnormalities. Taken together, our work shows that YAP2-5SA-ΔC drives β-catenin activity to promote basal keratinocyte proliferation in the mouse skin in vivo. Our data shine new light on the etiology of regenerative dermatological disorders and other human diseases that display increased YAP and β-catenin activity.

Splice variant insertions in the C-terminus impairs YAP's transactivation domain

The yes-associated protein (YAP) is a key effector of the mammalian Hippo signaling pathway. YAP has eight known alternately spliced isoforms and these are widely expressed across multiple tissues. Variable effects have been ascribed to different YAP isoforms by inducing their expression in cells, but whether these differences are due to variability in the transcriptional potency of individual YAP isoforms has not been addressed. Indeed a systematic comparison of the transcriptional potencies of YAP isoforms has not been done. To address this, using overexpression and transcriptional reporter analyses we investigated the transcriptional activities of several human YAP isoforms and determined the effects of the splice variant insertions within the transactivation domain on its transcriptional potency. Utilising full-length coding sequence constructs we determined that the number of WW domains and disruption of the leucine zipper motif within YAP’s transactivation domain both contribute to transcriptional activity. Notably, disruption of YAP’s leucine zipper had a greater effect on transcriptional activity than the absence of the second WW domain. Using GAL4-YAP transcriptional activation domain fusion proteins we found that disruption of the leucine zipper significantly decreased YAP’s transcriptional activity in several cell lines. Our data indicates that expression of different YAP isoforms with varying transcriptional potencies may enable fine control of Hippo pathway signaling. Furthermore the specific isoform being utilised should be taken into consideration when interpreting published data or when designing experiments to ascribe YAP’s function.

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Transcriptional activities of yes-associated protein (YAP) isoforms were compared.

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YAP’s WW domains and leucine zipper motif both contribute to transcriptional activity.

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Absence of YAP’s second WW domain weakens transcriptional potency.

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Disruption of YAP’s leucine zipper weakens the transactivation domain (TAD).

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Potency of the TAD from YAP α, β, γ, δ isoforms is cell-context dependent.

YAP is closely correlated with castration-resistant prostate cancer, and downregulation of YAP reduces proliferation and induces apoptosis of PC-3 cells

Yes-associated protein 65 (YAP65) has been implicated as an oncogene, and its expression is increased in human cancer. Previous studies have demonstrated that alterations in YAP activity may result in tumourigenesis of the prostate. With androgen deprivation therapies becoming progressively ineffective, often leading to life‑threatening androgen‑resistant prostate cancer (CRPC). The present study aimed to analyse the role of YAP in prostate cancer (PCa), particularly in CRPC. YAP protein was detected using immunohistochemistry and western blot analysis in different prostatic tissues. In addition, three specific RNA interference vectors targeting the human YAP gene were synthesised, and PC‑3 cells with a stable inhibition of YAP were obtained by transfection. MTT, flow cytometry, reverse transcription‑quantitative polymerase chain reaction and western blot assays were used to analyse the effects of YAP inhibition on the proliferation and apoptosis of PC‑3 cells. The frequency of cells that were positive for YAP protein in PCa (78.13%) was significantly higher, compared with para‑PCa (26.67%; P=0.007) and benign prostatic hyperplasia (0%; P=0.002). The frequency of cells, which were positive for the expression of YAP exhibited a positive correlation (P=0.008) with the Gleason score, the tumour‑node‑metastasis staging (P=0.033) and the level of prostate specific antigens (P=0.0032) in PCa. The proliferative capacity of the transfected group was significantly lower, compared with the negative control group (P=0.022). The cell‑cycle of the transfected group was arrested in the G1 stage, which was detected using flow cytometry, and there was a significant increase in the apoptosis of cells in the transfected group (P=0.002). The mRNA and protein levels of TEA domain family member 1 were inhibited in the transfected group (P=0.001 and P=0.00, respectively). Therefore, it was concluded that gene transcription and protein expression of YAP may be involved in the development of PCa, particularly CRPC, and may be a novel biomarker for investigation of the occurrence and progression of CRPC. However, the mechanism underlying the modulation of YAP in CRPC remains to be fully elucidated.

Molecular pathogenesis and molecular targeted agents for primary liver cancer

The treatment of primary liver cancer (PLC) is still challenging in China nowadays, and it fundamentally depends on the systematic knowledge of hepatocarcinogenesis. In recent years, the gradual clarification of the molecular pathogenesis of PLC has brought new opportunities and challenges to its treatment. Molecular targeted agents, including tyrosine kinase inhibitors and monoclonal antibodies, have appeared and developed rapidly since the mechanisms were elucidated. These agents have gradually became a preferred choice of treatment of PLC and represents the future trend. In this paper, we will review the molecular pathogenesis of PLC and the targeted agents.

YAP-mediated induction of monoacylglycerol lipase restrains oncogenic transformation

The Hippo pathway is an evolutionarily conserved regulator of normal and oncogenic growth. Engagement of Hippo pathway signaling results in the inactivation of the transcriptional coactivator YAP by preventing its nuclear entry. The mechanisms underlying the oncogenic properties of YAP remain incompletely understood. Here we find that although the transactivation (TA) domain of YAP mediates YAP-dependent gene expression, it serves as an inhibitor of YAP-mediated anchorage-independent growth. We identify monoacylglycerol lipase (MAGL) as a YAP transcriptional target and an inhibitor of anchorage-dependent cell growth. Significantly, knockdown of MAGL expression leads to the augmentation of YAP-dependent cell transformation. Our results identify MAGL as a transcriptional target of YAP that restrains YAP-mediated cellular transformation.

Yorkie promotes transcription by recruiting a histone methyltransferase complex

Hippo signaling limits organ growth by inhibiting the transcriptional coactivator Yorkie. Despite the key role of Yorkie in both normal and oncogenic growth, the mechanism by which it activates transcription has not been defined. We report that Yorkie binding to chromatin correlates with histone H3K4 methylation and is sufficient to locally increase it. We show that Yorkie can recruit a histone methyltransferase complex through binding between WW domains of Yorkie and PPxY sequence motifs of NcoA6, a subunit of the Trithorax-related (Trr) methyltransferase complex. Cell culture and in vivo assays establish that this recruitment of NcoA6 contributes to Yorkie's ability to activate transcription. Mammalian NcoA6, a subunit of Trr-homologous methyltransferase complexes, can similarly interact with Yorkie's mammalian homolog YAP. Our results implicate direct recruitment of a histone methyltransferase complex as central to transcriptional activation by Yorkie, linking the control of cell proliferation by Hippo signaling to chromatin modification.

KRAS and YAP1 converge to regulate EMT and tumor survival

Cancer cells that express oncogenic alleles of RAS typically require sustained expression of the mutant allele for survival, but the molecular basis of this oncogene dependency remains incompletely understood. To identify genes that can functionally substitute for oncogenic RAS, we systematically expressed 15,294 open reading frames in a human KRAS-dependent colon cancer cell line engineered to express an inducible KRAS-specific shRNA. We found 147 genes that promoted survival upon KRAS suppression. In particular, the transcriptional coactivator YAP1 rescued cell viability in KRAS-dependent cells upon suppression of KRAS and was required for KRAS-induced cell transformation. Acquired resistance to Kras suppression in a Kras-driven murine lung cancer model also involved increased YAP1 signaling. KRAS and YAP1 converge on the transcription factor FOS and activate a transcriptional program involved in regulating the epithelial-mesenchymal transition (EMT). Together, these findings implicate transcriptional regulation of EMT by YAP1 as a significant component of oncogenic RAS signaling.

A TEAD1/p65 complex regulates the eutherian-conserved MnSOD intronic enhancer, eRNA transcription and the innate immune response

Manganese superoxide dismutase (MnSOD), a critical anti-oxidant enzyme, detoxifies the mitochondrial-derived reactive oxygen species, superoxide, elicited through normal respiration or the inflammatory response. Proinflammatory stimuli induce MnSOD gene expression through a eutherian-conserved, intronic enhancer element. We identified two prototypic enhancer binding proteins, TEAD1 and p65, that when co-expressed induce MnSOD expression comparable to pro-inflammatory stimuli. TEAD1 causes the nuclear sequestration of p65 leading to a novel TEAD1/p65 complex that associates with the intronic enhancer and is necessary for cytokine induction of MnSOD. Unlike typical NF-κB-responsive genes, the induction of MnSOD does not involve p50. Beyond MnSOD, the TEAD1/p65 complex regulates a subset of genes controlling the innate immune response that were previously viewed as solely NF-κB-dependent. We also identified an enhancer-derived RNA (eRNA) that is induced by either proinflammatory stimuli or the TEAD1/p65 complex, potentially linking the intronic enhancer to intra- and interchromosomal gene regulation through the inducible eRNA.

Yes-associated protein (YAP) increases chemosensitivity of hepatocellular carcinoma cells by modulation of p53

The yes-associated protein (YAP) transcription co-activator has been reported either as an oncogene candidate or a tumor suppressor. Liver tissue chips revealed that about 51.4% human hepatocellular carcinoma (HCC) samples express YAP and 32.9% HCC samples express phosphorylated YAP. In this study, we found that chemotherapy increased YAP protein expression and nuclear translocation in HepG2 cells, as well as p53 protein expression and nuclear translocation. However, little is known about YAP functions during chemotherapy. Our results show that overexpression of YAP increases chemosensitivity of HepG2 cells during chemotherapy. Dominant negative transfection of Flag-S94A (TEAD binding domain mutant) or Flag-W1W2 (WW domain mutant) to HepG2 cells decreases p53 expression/ nuclear translocation and chemosensitivity when compared with control HepG2 cells. Furthermore, rescue transfection of Flag-5SA-S94A or Flag-5SA-W1W2, respectively to HepG2 cells regains p53 expression/nuclear translocation and chemosensitivity. These results indicate that YAP promotes chemosensitivity by modulating p53 during chemotherapy and both TEAD and WW binding domains are required for YAP-mediated p53 function. ChIP assay results also indicated that YAP binds directly to the p53 promoter to improve its expression. In addition, p53 could positively feedback YAP expression through binding to the YAP promoter. Taken together, our current data indicate that YAP functions as a tumor suppressor that enhances apoptosis by modulating p53 during chemotherapy.

Spinal expression of Hippo signaling components YAP and TAZ following peripheral nerve injury in rats

Previous studies have shown that the morphology and number of cells in the spinal cord dorsal horn could change following peripheral nerve injury and that the Hippo signaling pathway plays an important role in cell growth, proliferation, apoptosis, and dendritic remolding. In the present study, we examined whether the expression of YAP and TAZ, two critical components regulated by Hippo signaling, in the spinal cord dorsal horn would be altered by chronic constriction sciatic nerve injury (CCI). We found that (1) YAP was mainly expressed on CGRP- and IB4-immunoreactive primary afferent nerve terminals without noticeable expression on glial cells, whereas TAZ was mainly expressed on spinal cord second order neurons as well as microglia; (2) upregulation of YAP and TAZ expression followed two distinct temporal patterns after CCI, such that the highest expression of YAP and TAZ was on day 14 and day 1 after CCI, respectively; (3) there were also unique topographic patterns of YAP and TAZ distribution in the spinal cord dorsal horn consistent with their distinctive association with primary afferents and second order neurons; (4) changes in the YAP expression were selectively induced by CCI but not CFA-induced hindpaw inflammation; and (5) the number of nuclear profiles of TAZ expression was significantly increased after CCI, indicating translocation of TAZ from the cytoplasma to nucleus. These findings indicate that peripheral nerve injury induced time-dependent and region-specific changes in the spinal YAP and TAZ expression. A role for Hippo signaling in synaptic and structural plasticity is discussed in relation to the cellular mechanism of neuropathic pain.

Yap controls stem/progenitor cell proliferation in the mouse postnatal epidermis

Tissue renewal is an ongoing process in the epithelium of the skin. We have begun to examine the genetic mechanisms that control stem/progenitor cell activation in the postnatal epidermis. The conserved Hippo pathway regulates stem cell turnover in arthropods through to vertebrates. Here we show that its downstream effector, yes-associated protein (YAP), is active in the stem/progenitor cells of the postnatal epidermis. Overexpression of a C-terminally truncated YAP mutant in the basal epidermis of transgenic mice caused marked expansion of epidermal stem/progenitor cell populations. Our data suggest that the C-terminus of YAP controls the balance between stem/progenitor cell proliferation and differentiation in the postnatal interfollicular epidermis. We conclude that YAP functions as a molecular switch of stem/progenitor cell activation in the epidermis. Moreover, our results highlight YAP as a possible therapeutic target for diseases such as skin cancer, psoriasis, and epidermolysis bullosa.