Abstract 984: High-level expression of YAP induces protumorigenic Notch signalling in human hepatocarcinogenesis

Abstract

Abstract The evolutionary conserved Hippo-pathway negatively regulates organ size control by phosphorylation and cytoplasmic retention of the transcriptional co-activator yes-associated protein (YAP). Recent studies demonstrated that deletion of essential Hippo-pathway constituents (e.g., Mst-1/2 and WW45) or overexpression of YAP lead to the development of liver cancer. However, the underlying molecular tumor-supporting mechanisms in carcinogenesis have not been defined so far. Overexpression and nuclear accumulation of YAP in nearly 70% of all human hepatocellular carcinomas (HCC) significantly correlated with tumor cell proliferation and dedifferentiation. In human HCC cell lines, siRNA-mediated inhibition of YAP significantly reduced tumor cell viability, and migration/invasion. Based on transcriptomic profiling approaches, the Notch ligand Jagged-1 (Jag-1) was identified as YAP-dependent target gene in HCC cells and in primary murine hepatocytes of transgenic animals expressing constitutively active YAPS127A. Inhibition of YAP reduced the protein levels of Jag-1, cleaved Notch receptor (NICD), and Hes-1, while YAP overexpression increased the amounts of all factors. As detected for YAP knock down, transfection of gene-specific siRNA targeting Jag-1 diminished HCC cell viability and migration. Overexpression and concomitant inhibition of Jag-1 abolished Hes-1 expression and YAP-induced HCC cell viability. By applying different mutant isoforms of YAP (e.g., YAPS127A and YAP5SA-delta-C - dominant negative isoform), TEAD4 but not TEAD1 was identified as the transcription factor required for YAP-dependent regulation of Jag-1 and Hes-1. Furthermore, the WNT/beta-catenin pathway, a putative inducer of Jag-1, did not influence the YAP-dependent modulation of Jag-1. Knock down experiments revealed Mst-2 and Lats-2 as negative regulators of YAP activity and Jag-1/Hes-1 expression. The amounts of YAP, Jag-1, and Hes-1 transcripts as well as proteins significantly correlated with each other in human HCC tissues. Most importantly, increased concentrations of all factors significantly associated with poor prognosis of HCC patients. These data demonstrate that high-level expression of YAP in HCC cells induces tumor growth and tumor cell dissemination in part through activation of the Jag-1/Notch pathway in a TEAD4-dependent and beta-catenin-independent manner. This regulatory cross-talk between Hippo- and Notch-signalling defines a group of HCC patients with poor overall survival. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 984. doi:1538-7445.AM2012-984