Background: HNF4␣ is a key factor regulating hepatocyte differentiation and liver-specific functions. Results: Acute disruption of HNF4␣ in the adult liver causes rapid hepatocyte proliferation through direct and indirect control of multiple pathways. Conclusion: HNF4␣ is necessary to maintain the differentiated state of hepatocytes in adult liver. Significance: HNF4␣ expression maintains normal liver function, and its loss stimulates hepatocytes proliferation, possibly leading to cancer.

The concept that cellular terminal differentiation is stably maintained once development is complete has been questioned by numerous observations showing that differentiated epithelium may undergo an epithelial-to-mesenchymal transition (EMT) program. EMT and the reverse process, mesenchymal-to-epithelial transition (MET), are typical events of development, tissue repair, and tumor progression. In this study, we aimed to clarify the molecular mechanisms underlying these phenotypic conversions in hepatocytes. Hepatocyte nuclear factor 4a (HNF4a) was overexpressed in different hepatocyte cell lines and the resulting gene expression profile was determined by real-time quantitative polymerase chain reaction. HNF4a recruitment on promoters of both mesenchymal and EMT regulator genes was determined by way of electrophoretic mobility shift assay and chromatin immunoprecipitation. The effect of HNF4a depletion was assessed in silenced cells and in the context of the whole liver of HNF4 knockout animals. Our results identified key EMT regulators and mesenchymal genes as new targets of HNF4a. HNF4a, in cooperation with its target HNF1a, directly inhibits transcription of the EMT master regulatory genes Snail, Slug, and HMGA2 and of several mesenchymal markers. HNF4a-mediated repression of EMT genes induces MET in hepatomas, and its silencing triggers the mesenchymal program in differentiated hepatocytes both in cell culture and in the whole liver. Conclusion: The pivotal role of HNF4a in the induction and maintenance of hepatocyte differentiation should also be ascribed to its capacity to continuously repress the mesenchymal program; thus, both HNF4a activator and repressor functions are necessary for the identity of hepatocytes. (HEPATOLOGY 2011;53:2063-2074 E pithelial-to-mesenchymal transition (EMT) is the process by which polarized cells of the epithelium lose cell-cell connections and acquire the mesenchymal characteristics of motility and invasiveness. The reverse process, mesenchymal-to-epithelial transition (MET), often occurs at a site secondary to the original EMT population. The dynamic EMT/MET processes are essential for embryonic development and wound repair and initiate the pathological states of fibrosis and metastatic cancer.