Transcriptional regulation by the Hippo signaling pathway in the liver

Development and maintenance of the hepatic phenotype is a tightly controlled process regulated by both master regulatory transcription factors and signaling pathways. Perturbations in these transcriptional networks are frequently seen in diseases such as liver cancer. The Hippo signaling pathway has been implicated in regulation of liver size and dysregulation of this pathway contributes to tumorigenesis. The primary mechanism of action of the Hippo pathway is to inhibit nuclear localization of the transcriptional co-regulator YAP, and thereby preventing YAP from binding to the TEAD family of transcription factors. Although it has been established that YAP plays a role in promoting cell proliferation, how it regulates its transcriptional targets in the liver have yet to be well-characterized. In this study, I show that YAP-overexpression in the adult mouse liver results in a shift from a mature hepatocyte to a hepatic progenitor-like gene expression pattern. Comparison of differentially expressed genes by RNA-seq revealed downregulation of hepatocyte metabolism genes and re-expression of hepatoblast genes, including Glypican-3 (Gpc3). Analysis of ChIP-seq data from both mouse liver and the human hepatoma cell line, HepG2, identified putative Gpc3 enhancers regulated by TEAD and HNF4a. I interrogated these regions using luciferase assays and identified important TEAD and HNF4a binding motifs necessary for transcriptional regulation. In addition, pathway analysis identified enrichment of the ERBB signaling pathway in the YAP-overexpressing liver. Examination of individual ERBB receptors identified upregulation of Her2 (Erbb2), which is normally enriched in hepatoblasts compared to hepatocytes. Analysis of HepG2 ChIP-seq data revealed a TEAD peak at the HER2 promoter. Using luciferase assays, I identified an important TEAD binding site contributing to transcriptional activity. Functionally, I found YAP to regulate EGF-induced HepG2 cell proliferation and PI3K-AKT signaling. This work explored novel mechanisms of gene regulation by YAP in the liver., I found that YAP activation results in re-expression of hepatic progenitor genes such as Gpc3 and Her2. Furthermore, I found the ERBB signaling pathway to be an important growth mediator downstream of YAP.Medicine, Faculty ofGraduat