Hepatocyte growth factor/c‐met signaling is required for stem‐cell–mediated liver regeneration in mice

It is now well recognized that adult liver contains a stem cell compartment that can be activated under conditions of severe liver injury to give rise to both hepatocytic and biliary epithelial cell (BEC) lineages1-4. Hepatic stem cells (HSC) are thought to reside within the terminal bile ductules (Hering Canals) located at the interface between parenchyma and biliary tracts. Upon activation, HSC give rise to oval cells which form a network of proliferating brunching ducts that migrate into parenchyma where they finally differentiate into hepatocytes5-7. Numerous molecular factors and cell types contribute to HSC activation either directly or indirectly8-10. We and others have established that oval cell expansion requires a close cooperation with accompanying stellate cells which provides hepatocyte growth factor (HGF) and also promote pericellular collagen deposition thus creating a microenvironment supporting growth of expanding progenitor cells11-16. HGF was originally characterized as a potent mitogen for mature hepatocytes17. All biological effects of HGF are mediated by a single tyrosine kinase receptor c-Met18-19. Gene knockout studies have shown that both HGF and c-Met are absolutely required for survival including liver development20-21. The unique property of c-Met signaling is activation of a complex biological program supporting morphogenesis, mitogenesis and motogenesis (also referred to as ”invasive growth”)22-23.The program operates during embryogenesis, tissue regeneration and cancer metastasis, and controls proliferation, branching morphogenesis, survival, migration through ECM, and differentiation. Previously, we have shown that HGF/c-Met signaling promotes activation and early expansion of oval cells following severe liver injury in an AAF+PH rat model12. However, the molecular mechanisms supporting adult stem cell activation are not well understood, and knowledge about the role of HGF/c-Met pathway in this process is still limited. Recently, we and others have provided direct genetic evidence for the essential role of HGF/c-Met in hepatocyte-mediated liver regeneration24-26. Here, we analyzed the contribution of the c-Met signaling pathway in stem cell-mediated liver regeneration by utilizing liver-specific c-Met conditional knockout mice. To gain insight in the intricate nature of epithelial-mesenchymal cross-talk that defines the stem cell behavior, inactivation of c-Met was achieved either in epithelial cell lineages (c-Metfl/fl; Alb-Cre+/-) or in various subsets of liver cells including stromal cells (c-Metfl/fl; Mx1-Cre+/-) by crossing c- Metfl/fl mice with transgenic mice expressing Cre-recombinase under the control of constitutively active albumin promoter or ubiquitous interferon-inducible Mx1 promoter. 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 have been described previously27-28. Our results show that the absence of c-Met caused severe damage both to hepatocytes and biliary epithelium, disrupted the balance between extracellular matrix production and degradation and prevented stem cell-mediated liver regeneration. Consequently, our study establishes the HGF/c-Met signaling pathway as an essential component of the hepatic regenerative capability.