Activation of the Hippo-Yap pathway and c-Myc is known to be involved in human liver tumors, either hepatocellular carcinoma (HCC) and cholangiocarcinoma (CC). In liver-specific mouse knockouts of negative regulators of the Hippo-Yap pathway, such as Nf2 or Sav, both HCC and CC could have been induced, while the activation of c-Myc has been suggested to be involved in HCC development and activation of stem-like potential of hepatocytic tumor cells. Although an interaction of the Hippo-Yap pathway and c-Myc has been suggested in human HCC cell lines, the precise nature of the interaction in hepatocarcinogenesis is not clear. In the present study, we examined the roles of the Hippo-Yap pathway and c-Myc in mouse liver tumor models, in which, along with the active Akt (myrAkt) gene, the active Yap (YapS127A) gene and/or the c-Myc gene were introduced into hepatocytes in vivo through hydrodynamic tail vein injection and the Sleeping Beauty transposon-mediated somatic integration. Introduction of myrAkt alone induced multinodular liver tumors 9 months after transfection. The individual tumors showed various histologic features: HCC with ductular differentiation, HCC, and CC in order of frequency. Cotransfection of either YapS127A or Myc demonstrated tumor promoting activities with macroscopically evident liver tumors being developed after 6 to 8 weeks. The histology of tumors induced by myrAkt/YapS127A was CK19-positive ductule-forming adenocarcinoma reminiscent of CC. In contrast, the tumor induced by myrAkt/c-Myc showed typical histology of moderately differentiated HCC without CK19 expression. When all (myrAkt/YapS127A/c-Myc) were simultaneously introduced, marked hepatomegaly due to diffuse liver tumors was apparent even after 2 to 3 weeks. Histologically, the livers were replaced by poorly differentiated HCC or undifferentiated carcinoma. Although there were scattered foci of CK19-positive tumor cells, no apparent tubular structures were noted. RT-qPCR analyses revealed that the mRNA expression of genes involved in Notch signaling (Jag1, Hes1, and Hes2), Ctgf, and Tnf was significantly increased in myrAkt/YapS127A-induced tumors, but not in tumors induced by myrAkt/c-Myc or myrAkt/YapS127A/Myc. In conclusion, while both the Hippo-Yap pathway and c-Myc enhance hepatocarcinogenesis, the activation of the former induces ductular differentiation in hepatocytic tumors generating a CC-like phenotype, whereas the latter suppresses the transdifferentiation and maintains hepatocytic features.

Citation Format: Masahiro Yamamoto, Bing Xin, Xi Chen, Kiyonaga Fujii, Takako Ooshio, Yuji Nishikawa. Opposing effects of the Hippo-Yap pathway and c-Myc in phenotypic determination of mouse hepatocytic tumors induced by myrAkt. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 921. doi:10.1158/1538-7445.AM2015-921