Cell density-dependent regulation of hepatocyte growth factor receptor on adult rat hepatocytes in primary culture

The proliferative potential of hepatocytes in the normal intact liver is highly suppressed, but they proliferate actively after liver injury. In this study, adult rat hepatocytes in primary culture were used to study mechanisms controlling hepatocyte growth in liver regeneration. DNA synthesis in hepatocytes cultured at a low cell density was highly stimulated in response to hepatocyte growth factor (HGF), but this stimulatory effect was not so obvious in hepatocytes cultured at a high cell density. In close parallel to the potency of DNA synthesis, the amounts of ¹²⁵I-HGF specifically bound to hepatocytes cultured at a low cell density were much greater than in high cell density culture. Scatchard plots revealed that change in the specific binding of ¹²⁵I-HGF was due to change in the number of high-affinity HGF receptors, but without change in the K_d values. Affinity cross-linking of the HGF receptor with ¹²⁵I-HGF confirmed the higher expression of HGF receptor on hepatocytes cultured at a lower cell density. Since there was no significant change in the expression of c-met mRNA in hepatocytes cultured at different cell densities, the number of cell surface HGF receptors is probably regulated by post-transcriptional mechanisms. We also found that the rates of HGF-induced down-regulation and recovery of HGF receptors on hepatocytes cultured at a low cell density were much faster than in cases of a high cell density. These results suggest that hepatocytes in the injured liver, where the cell-cell contact is loosened, as it is when cells are cultured at a low cell density, express a greater number of HGF receptors, which internalize in response to HGF, whereas hepatocytes in the intact liver, where the cell-cell contact is tight, as it is for cells cultured at a high cell density, express a relatively low number of HGF receptors which have lost the ability to internalize and respond to HGF.