The oncogenic potential of YAP requires Y357 phosphorylation in cholangiocytes but not in hepatocytes

The transcriptional cofactor yes-associated protein (YAP) functions as a proto-oncogene and a key effector of the canonical Hippo pathway. Phosphorylation at serine 127 (S127) by core Hippo kinases inhibits YAP activity, while phosphorylation at tyrosine 357 (Y357) by SRC-family kinases promotes its activation. YAP activation has been implicated in many cancers, including primary liver cancers (PLCs). Owing to the inherent plasticity of the liver, PLCs may arise from different cells of origin. However, it is unknown whether the level of YAP activation favors liver tumorigenesis from specific cells of origin or instead promotes lineage commitment in a single cell of origin across the PLC spectrum. To assess the role of YAP tyrosine phosphorylation in liver tumorigenesis, C57BL/6 mice underwent biliary transfection with the Sleeping Beauty transposon system to deliver myr-AKT and YAP S127A (YAP-S) or YAP S127A/Y357F (YAP-SY). While YAP-S mice represent an established model of intrahepatic cholangiocarcinoma (iCCA), unexpectedly, a shift in tumor phenotype to hepatocellular carcinoma (HCC) was identified in YAP-SY mice. Transcriptome and pathway analysis revealed differential activation of cytokines and growth factors (e.g., TGFB1, TNF, IL1B, IL4, and INFG) and transcription factors (e.g., STAT3, PGR, AHR, STAT1, and NFKB) associated with this phenotypic switch. To determine the cell of origin, tumors were generated in lineage tracing mice (ROSA26mT/mG mice treated with AAV8-TBG-Cre), which demonstrated that HCC originated from hepatocytes and iCCA from cholangiocytes in the biliary transfection model. Finally, two tumor-derived cell lines with YAP-SY and constitutive AKT expression were isolated, representing a novel, transplantable murine syngeneic model with mixed features of HCC and iCCA.