The hepatocyte growth factor and its receptor c-Met direct a pleiotropic signal transduction pathway that controls cell survival. We previously demonstrated that mice lacking c-Met (Met-KO) in hepatocytes were hypersensitive to Fas-induced liver injury. We have used primary hepatocytes isolated from Met-KO and control (Cre-Ctrl) mice to address more directly the protective effects of c-Met signaling. Loss of c-Met function increased sensitivity to Fas-mediated apoptosis. Hepatocyte growth factor suppressed apoptosis in Cre-Ctrl but not Met-KO hepatocytes concurrently with up-regulation of NF-kappaB and major antiapoptotic proteins Bcl-2 and Bcl-xL. Intriguingly, Met-KO hepatocytes exhibited intrinsic activation of NF-kappaBas well as Bcl-2 and Bcl-xL. Furthermore, unchallenged Met-KO cells displayed oxidative stress as evidenced by overproduction of reactive oxygen species, which was associated with greater NADPH and Rac1 activities, was blocked by the known NADPH oxidase inhibitors, and was paralleled by increased lipid peroxidation and reduced glutathione (GSH) content. N-Acetylcysteine, an antioxidant and GSH precursor, significantly reduced Jo2-induced cell death. Conversely, the GSH-depleting agent buthionine sulfoximine completely abolished the protective effects of N-acetylcysteine in Met-KO hepatocytes. We conclude that genetic inactivation of c-Met in mouse hepatocytes caused defects in redox regulation, which may account for the increased sensitivity to Fas-induced apoptosis and adaptive up-regulation of NF-kappaB survival signaling. Our data provide evidence that intact c-Met signaling is a critical factor in the protection against excessive generation of endogenous reactive oxygen species. HGF/c-Met supports a pleiotrophic signal transduction pathway that controls stem cell homeostasis. We have directly addressed the role of c-Met in stem cell-mediated liver regeneration by utilizing mice harboring c-met floxed alleles and Alb-cre or Mx-cre transgenes. To activate stem (oval) cell compartment, we used a model of chronic liver injury induced by diet containing the porphyrinogenic agent, 3, 5-diethocarbonyl-1,4-dihydrocollidine (DDC). Inactivation of c-met was achieved specifically in hepatocytes (Alb-Met) or in hepatocytes and other liver cells (Mx -Met) allowing us to distinguish between adaptive versus acute effects of c-met deletion and examine the significance of cross-talk between different cell types. Lack of c-met expression was restricted to hepatocytes in Alb-Met mice, and was confirmed in both hepatocytes and oval/ductular cells in Mx-Met mice by PCR analysis of isolated hepatocytes and FACS sorted EpCam-positive oval/ductular cells. The phenotype of Mx-Met mice was identical albeit more severe than that of Alb-Met mice as determined by biochemical and morphological data. Differently from c-met f/f mice, the c-met -/- mice did not show a compensatory increase in liver mass due to imbalance between hepatocyte proliferation and apoptosis. Both mice showed remarkably high bile acid levels and died from severe cholestasis within 4-6 wk after the start of DDC diet indicating an essential role for c-Met in protection against bile acid toxicity. Significantly, loss of c-Met signaling had a detrimental effect on stem cell activation and differentiation. There was a considerable reduction in the number of immature oval/ductular cells and almost complete lack of their migration and subsequent hepatocytic differentiation as demonstrated by confocal microscopy and double immunostaining using antibodies directed against oval cells (A6 and EpCam) and HNF4. Instead, all c-met -/- livers exhibited more severe portal fibrosis and bile duct proliferation confined to portal tracts. The number of collagen-producing stellate cells as defined by Thy1 staining and FACS analysis was comparable in Met knockout and CTRL mice. However, c-met -/- livers expressed significantly less MMP9 as judged by western blotting and in situ zymography implying that lack of c-Met signaling in hepatocytes disrupts the balance between extracellular matrix production and degradation and thus affects stem cell microenvironment. These studies establish requirement for c-met in controlling expansion and differentiation of hepatic stem cells and support its crucial role in liver regeneration and repair.
| Kaposi-Novak, Pal; Lee, Ju-Seog; Gomez-Quiroz, Luis et al. (2006) Met-regulated expression signature defines a subset of human hepatocellular carcinomas with poor prognosis and aggressive phenotype. J Clin Invest 116:1582-95 |
