The loss of Hippo signaling is a key prognostic factor in predicting cancer patient survival and sensitivity to chemotherapeutic drugs. Cancer onset and/or development is generally associated with Hippo pathway inactivation or YAP activation, which is caused mainly by epigenetic changes (e.g. methylation of MST or LATS). In addition, genetic mutations (e.g. germline or somatic NF2 mutations) and YAP amplification (somatic) have also been reported. However, mutations in the Hippo pathway genes themselves are uncommon. For a detailed review of Hippo pathway alterations in various human cancers, please refer to Harvey et al.
Tumor-related roles of YAP
YAP can act as an oncogene or a TSG, depending on the cellular context and YAP's interacting partners. Conditional YAP activation in the liver reversibly increases liver size, and YAP dysregulation leads to hepatocellular carcinomas (HCCs). Conversely, YAP inactivation leads to loss of hepatocytes and bile duct cells.[14, 15]
In mice, Camargo et al. reported that conditional YAP activation in the intestine expanded multipotent progenitor cells and resulted in early onset of polyps following DSS treatment. However, Barry et al. found that transgenic YAP expression reduced WNT target gene expression and induced rapid destruction of intestinal crypts, while loss of YAP led to excessive WNT signaling causing hyperplasia, intestinal stem cell expansion, and formation of ectopic crypts and microadenomas. Barry et al. therefore speculated that cytoplasmic YAP may normally bind to DVL proteins and dampen WNT signaling. The discrepancy between these two reports requires resolution, since either increased nuclear YAP or decreased cytoplasmic YAP heightens the risk of colon cancer. In a third report, Rosenbluh et al. showed that YAP and the transcription factor TBX5 form a complex with β-catenin. Phosphorylation of YAP(Thr357) by the tyrosine kinase YES1 induces localization of this complex to the promoters of the anti-apoptotic genes BCL2L1 and BIRC5, inducing the transformation and survival of β-catenin-driven tumor cells. YAP is thus a key driver of β-catenin-related intestinal cancers.
Two transgenic mouse strains overexpressing YAP in the skin have been described.[8, 18] Both strains initially show expanded basal epidermal progenitors that fail to undergo normal terminal differentiation, and later develop squamous cell carcinomas (SCCs). These mutants have a severe defect in hair follicle (HF) formation that may be due to impaired planar cell polarity caused by strong expression of YAP before HF morphogenesis.
Crosstalk between Hippo and other tumor-related pathways
YAP/TAZ binds to phosphorylated R-SMADs,[19, 20] connecting the Hippo pathway to TGF-β- and BMPR-related signaling (Fig. 4). In general, nuclear YAP/TAZ enhances SMAD activity in the nucleus, while cytoplasmic YAP/TAZ restricts SMAD nuclear accumulation. TGF-β-SMAD activation induced by low cell density depends on nuclear YAP/TAZ, while YAP/TAZ-induced stem cell self-renewal depends on nuclear SMADs.[19, 20] Thus, Hippo signaling may underlie the opposing roles of the TGF-β pathway in early versus late stages of cancer. TGF-β inhibits cell-cycle progression during tumor initiation but subsequently promotes cancer cell proliferation, EMT, and metastasis. Normally-polarized epithelial cells possess cytoplasmic YAP/TAZ, which restricts TGF-β-induced SMAD activity. In contrast, cells that have lost Crumbs complex function exhibit impaired cell polarity, elevated nuclear YAP/TAZ, increased nuclear SMADs, and enhanced sensitivity to TGF-β ligand. EMT and the transformed phenotype are then promoted. Crosstalk between Hippo signaling and the TGF-β-SMADs pathway may thus explain the differential functions of TGF-β as a tumor progresses.
Figure 4. Crosstalk between the Hippo pathway and tumor-related signaling pathways. The Hippo pathway is influenced by and interacts with components of several tumor-related pathways, including the transforming growth factor-β (TGF-β)/BMP, WNT/β-catenin, PI3K/Akt, SHH, and NOTCH pathways, as indicated. Please see main text for details.
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The Hippo pathway also regulates WNT-induced signaling.[21, 22] YAP/TAZ is recruited to TCF/LEF binding sites together with β-catenin. Heart-specific conditional deletion of SAV1, MST1/2 or LATS2 in mice increases nuclear YAP activity, which promotes WNT/β-catenin signaling and triggers cardiomyocyte proliferation. Conversely, WNT signaling enhances YAP mRNA expression. Nuclear accumulation of β-catenin is augmented by nuclear TAZ via LATS inhibition. Phosphorylated YAP/TAZ also inhibits Disheveled (DVL), promoting β-catenin degradation and dampening WNT signaling.[16, 21] Thus, nuclear YAP/TAZ enhances WNT activity, whereas cytoplasmic YAP/TAZ restricts it.
SHH pathway activation leads to YAP mRNA expression, followed by YAP protein stabilization and nuclear accumulation. SHH also increases TEAD1 and IRS1, and induces IRS1 nuclear translocation. YAP, TEAD1 and IRS1 form a nuclear complex that regulates GLI2 and GLI1 expression, enhancing cell-cycle progression. In mice, YAP overexpression results in hyperactivation of SHH signaling, causing a neuronal differentiation defect in primary cortical progenitors. In humans, increased YAP occurs in medulloblastomas with aberrant SHH signaling, suggesting that therapeutic targeting of YAP might eliminate medulloblastoma recurrence.
AKT inhibits apoptosis by inactivating MST1/2 via phosphorylation at Thr120/117. In addition, AKT-mediated MST2 (Thr117) phosphorylation reduces MST2 binding to RASSF1A while increasing its binding to RAF1, again inhibiting MST2 activity. AKT also phosphorylates MST1 (Thr387), which reduces MST1 activity and decreases its cleavage in response to H2O2. Reciprocal regulation exists, since MST1/2 overexpression in a prostate cancer cell line inhibits AKT activity. However, AKT-mediated YAP (Ser127) phosphorylation promotes YAP localization in the cytoplasm and attenuation of p73-mediated apoptosis. Thus, AKT's effects on YAP localization are cell context-dependent.
MST1/2 deficiency in murine intestinal epithelium decreases YAP phosphorylation and activates NOTCH and β-catenin signaling. Jagged-1 (NOTCH ligand) is a YAP target in hepatocytes, and Hippo pathway activation in these cells inhibits their proliferation and survival. Conversely, overexpression of constitutively active YAP upregulates Jagged-1 and increases hepatocyte proliferation. Jagged-1 induction is required for YAP binding to TEAD4 after MST stimulation. Accordingly, γ-secretase inhibitors suppress intestinal dysplasia caused by YAP overexpression in mice. Thus, YAP and NOTCH may be therapeutic targets for gastrointestinal cancer.