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

PURPOSE: The transcriptional cofactor yes-associated protein (YAP) is activated in primary liver cancers (PLCs). YAP is canonically inhibited through serine 127 (S127) phosphorylation via the Hippo pathway but activated by phosphorylation at tyrosine 357 (Y357) by SRC family kinases. Due to liver plasticity, PLCs can originate from different cell types. However, it is unknown whether YAP activation favors tumorigenesis from specific cells of origin or promotes lineage commitment during tumorigenesis. This study investigates how YAP Y357 phosphorylation influences tumor phenotype and cell of origin. METHODS: C57BL/6 mice underwent biliary transfection with myr-AKT and YAP S127A (YAP-S) or YAP S127A/Y357F (YAP-SY). RNA sequencing and pathway analysis were performed on tumors. To determine the cell of origin, tumors were generated in lineage-tracing mice by biliary transfection or hydrodynamic tail vein injection. Two tumor-derived cell lines were isolated and characterized. RESULTS: YAP-S mice developed more frequent intrahepatic cholangiocarcinoma (iCCA), whereas a shift to hepatocellular carcinoma (HCC) was identified in YAP-SY mice. Transcriptome analysis revealed differential activation of pathways linked to this phenotypic switch. Lineage tracing demonstrated iCCA originated from cholangiocytes and HCC from hepatocytes in the biliary transfection, whereas iCCA originated from hepatocytes in the hydrodynamic tail vein injection. Two novel, transplantable syngeneic cell lines with mixed HCC/iCCA features were established. Tumors and cell lines commonly exhibited differential TGF-β signaling. CONCLUSIONS: YAP Y357 phosphorylation modifies the tumor phenotype and the cell of origin in liver cancer models. Differential TGF-β signaling suggests a potential therapeutic avenue for iCCA and mixed tumors. CLINICAL TRIAL NUMBER: Not applicable. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13402-025-01133-x.