Identification of Novel Receptor Tyrosine Kinases Regulating the Hippo Pathway in Cancer Cells
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Several studies have shown that the Hippo pathway plays an important role in tissue growth, organ size control, and cell death. Deregulation of the Hippo pathway contributes to loss of cell contact inhibition and continuing cell proliferation, which is observed during tumorigenesis. Although many studies have been done to clarify the role of the Hippo pathway in various biological functions, currently the connection between the Hippo pathway and its potential upstream kinase regulators is not very clear. Since the overexpression and deregulation of receptor protein kinases (RTKs) have a pivotal role in many cancers, we hypothesize that RTKs may be involved in tumorigenesis by inhibiting the Hippo pathway. To test the hypothesis, we developed a biosensor based on split-luciferase complementation assay. To find new RTKs which regulate the Hippo pathway, gain of function and kinase-inhibitor screenings were performed. We found that the many of RTKs could regulate the Hippo signaling activity. We further characterized several of these novel relationships VEGFR, [TAM family members (TYRO3, AXL, METRK),RET, and FGFR family members (FGFR1 and FGFR2)] and found that the Hippo effectors YAP/TAZ are central mediators of the tumorigenic phenotypes (e.g., increased cell proliferation, transformation, increased cell motility, and angiogenesis) induced by these RTKs and their extracellular ligands (Gas6,GDNF, and FGF) through either PI3K or MAPK signaling pathway. Remarkably, we found three new tyrosine-phosphorylation sites on YAP, which regulate its function. Our studies indicate that some RTKs also regulate the Hippo pathway through interaction or phosphorylation of LATS, YAP, and TEAD. In conclusion, these findings highlight the pivotal role of the Hippo pathway in mediating RTK-MAPK/PI3K signalling and provide a compelling rationale for targeting YAP/TAZ in RTK-driven cancer therapies.