Our most recent results in this project include: (1)EGFR and c-Met are involved in both activation and differentiation of hepatic progenitor cell (HPC) . However, the underlying mechanisms have not been elucidated. Here we have addressed the impact of EGFR and c-Met signaling on the differentiation of HPC into heptatocytic and biliary epithelial lineages using clonally derived progenitor cell lines from EGFRfl/fl and Met fl/fl conditional knockout mice.Precise control of lineage commitment and maintenance of stem/progenitor cells is crucial for regeneration of diseased liver. Mechanistic studies on the signaling pathways governing adult hepatic progenitor cells (HPC) fate proved difficult due to their rare frequency, complexity of overlapping signaling pathways, and lack of appropriate model systems. We have developed a robust culture system that permitted expansion and genetic manipulation of cells capable of multi-lineage differentiation in vitro and in vivo to examine the individual roles of HGF/Met and EGF/Egfr, two key signal transduction systems in normal and diseased liver, in HPC self-renewal and binary cell fate decision. By employing loss-of-function and rescue experiments in vitro, we showed that both receptors collaborate to increase the self-renewal of HPC through activation of ERK pathway. Met was a strong inducer of hepatocyte differentiation by activating AKT and STAT3. Conversely, Egfr selectively induced Notch1 to promote cholangiocyte specification and branching morphogenesis while concomitantly suppressing hepatocyte commitment. Furthermore, unlike the deleterious effects of c-Met deletion, the liver-specific conditional loss of Egfr facilitated rather than suppressed HPC-mediated liver regeneration by switching HPC differentiation towards hepatocyte lineage. These data provide new insight into the mechanisms regulating the stemness properties of adult HPCs and reveal a previously unrecognized link between Egfr and Notch1 in directing cholangiocyte differentiation. Redox signaling is emerging as an essential mechanism in the regulation of biological activities of the cell. The HGF/c-Met signaling pathway has been implicated as a key regulator of the cellular redox homeostasis and oxidative stress. We previously demonstrated that genetic deletion of c-Met in hepatocytes disrupts redox homeostasis by a mechanism involving NADPH oxidase. Here, we addressed the mechanism(s) of NADPH oxidase regulation by HGF/c-Met signaling in primary mouse hepatocytes and its relevance. HGF induced a biphasic mechanism of NADPH oxidase regulation. The first phase employed the rapid increase in production of ROS as signaling effectors to activate the Nrf2-mediated protective response resulting in up-regulation of the antioxidant proteins, such as NAD(P)H quinone oxidoreductase and gamma-glutamylcysteine synthetase. The second phase operated under a prolonged HGF exposure, caused a suppression of the NADPH oxidase components, including NOX2, NOX4, p22 and p67, and was able to abrogate the TGF beta-induced ROS production and improve cell viability. We conclude that HGF/c-Met induces a Nrf2-mediated protective response by a double mechanism driven by NADPH oxidase.
