Oval cells (hepatic stem cell progeny) originate from stem cells which reside within the terminal bile ductules located at the interface between the parenchyma and the bile tree and have the potential to differentiate into hepatocytes and biliary epithelial cells (BEC). In response to severe hepatocyte injury, oval cells form a system of proliferating branching ducts that move inside the parenchyma where they finally differentiate towards hepatocytic lineage. Multiple molecular factors and cell types contribute to the process of adult stem cell activation. We and others have established that oval cell infiltration of hepatic tissue occurs in a close conjunction with accompanying stellate cells which provide HGF, and also promote pericellular collagen deposition, thus creating a microenvironment supporting growth of expanding ductular cells. The goal of our study was to establish the role for c-Met in different phases of stem cell activation and interaction of c-Met and EGFR signaling pathways by utilizing conditional knock-out mouse models for both receptors. To activate oval cells, we used a model of chronic liver injury induced by diet containing the porphyrinogenic agent 3, 5-diethocarbonyl-1,4-dihydrocollidine (DDC), which was introduced into the field by our previous work. Our previous work has established that HGF/c-Met signaling plays a pivotal role in regulating the onset of S phase following partial hepatectomy (PH). In this study, we used Metfl/fl;Alb-Cre+/- conditional knockout mice to determine the effects of c-Met dysfunction in hepatocytes on kinetics of liver regeneration. The priming events appeared to be intact in Metfl/fl;Alb-Cre+/- livers. Up-regulation of stress response (MAFK, IKBZ, SOCS3) and early growth response (c-Myc, c-Jun, c-Fos, DUSP1) genes as assessed by RT-qPCR and/or microarray profiling was unchanged. This was consistent with an early induction of MAPK/Erk and STAT3. However, after a successful completion of the first round of DNA replication, c-Met deficient hepatocytes were blocked in early/mid G2 phase as shown by staining with phosphorylated form of histone H3. Furthermore, loss of c-Met in hepatocytes diminished the subsequent G1/S progression and delayed liver recovery after partial hepatectomy. Upstream signaling pathways involved in the blockage of G2/M transition included lack of persistent Erk1/2 activation and inability to up-regulate the levels of Cdk1, Plk1, Aurora A and B, and Mad2 along with a defective histone 3 phosphorylation and lack of chromatin condensation. Continuous supplementation with EGF in vitro increased proliferation of Metfl/fl;Alb-Cre+/- primary hepatocytes and partially restored expression levels of mitotic cell cycle regulators albeit to a lesser degree as compared to control cultures. Conclusion/Significance. From these results we conclude that our results assign a novel non-redundant function for HGF/c-Met signaling in regulation of G2/M gene expression program via maintaining a persistent Erk1/2 activation throughout liver regeneration. The results from our study aimed at elucidating the role of c-Met and EGFR, the tyrosine kinase receptors for the most potent liver mitogens (HGF and EGF, respectively), in hepatic progenitor cell biology include: To investigate the role of c-Met and/or EGFR in self-renewal, we monitored the sphere-forming capacity of oval cells in vitro. HGF and EGF had a similar impact of the size and number of spheres generated by oval cells with intact Met and EGFR signaling. Combined treatment with HGF+EGF produced a further increase in sphere growth whereas knockdown of c-Met and EGFR signaling completely abolished the sphere-forming ability of oval cells. The HGF+EGFR treatment also significantly increased and prolonged phosphorylation of each receptor as well as activation of ERK, Gab1, and PLC-gamma downstream signaling suggesting that a cross-talk between c-Met and EGFR contributes to self-renewal of oval cells. Conversely, in the differentiation assays, HGF alone was more effective than EGF in inducing oval cell differentiation along hepatocytic and biliary epithelial cell lineage. This was consistent with a stronger and more persistent Stat3 and Akt phosphorylation following HGF treatment as compared to that caused by EGF or combination of EGF+HGF. From these results we conclude that c-Met and EGFR have similar potency in maintaining self-renewing proliferation of hepatic progenitor cells which can be further enhanced by simultaneous activation of both receptors. However, c-Met is a major driving force of oval cell differentiation towards both hepatic epithelial cell lineages through sustaining a long term Stat3 and AKT signaling.
