Involvement of growth factor receptor‐bound protein‐2 in rat hepatocyte growth

Growth factor receptor‐bound protein‐2 (GRB‐2) is a protein linking receptor tyrosine kinase and Sos (Son of Sevenlessgene; Ras GDP/GTP exchange protein), leading to activation of the Ras‐mitogen‐activated protein kinase (MAPK) cascade. So far, it remains unclear how GRB‐2 plays a role in signal transduction pathways evoked by hepatotrophic factors. This study was attempted to evaluate the involvement of GRB‐2 in signalling in rat hepatocyte growth. Using rat cultured hepatocytes stimulated by hepatotrophic factors and regenerating livers after partial hepatectomy (PH) we examined GRB‐2‐mediated linkage of hepatotrophic factor receptors to signal transducing molecules such as Sos or dynamin‐II by immunoprecipitation and western blot analysis. In primary cultured hepatocytes stimulated with hepatocyte growth factor (HGF) or epidermal growth factor (EGF), GRB‐2 linked HGF receptor or EGF receptor, respectively, to Sos which activated the mitogen‐activated protein kinase (MAPK) cascade. In contrast, in primary cultured hepatocytes stimulated with insulin, GRB‐2 linked insulin receptor substrate‐1 (IRS‐1) to dynamin‐II as well as Sos. In the early phase after PH, GRB‐2 activated the Ras‐MAPK cascade by linking HGF receptor, IRS‐1, or EGF receptor to Sos. In the late phase after PH, a complex of IRS‐1‐GRB‐2 associated with dynamin‐II, indicating that GRB‐2 may transduce signals from IRS‐1 to dynamin‐II. We conclude that GRB‐2 may play a role in transmitting signals from hepatotrophic factors to not only MAPK but also to other signalling pathways in hepatocyte growth.