CELL DENSITY-DEPENDENT MITOGENIC EFFECT AND DENSITY-INDEPENDENT CELLULAR HANDLING OF EPIDERMAL GROWTH-FACTOR IN PRIMARY CULTURED RAT HEPATOCYTES

Aims: Mitogenic effect and cellular handling of epidermal growth facto r (EGF) were analyzed in primary cultured rat hepatocytes at several c ell densities. Methods/Results: DNA synthesis, assessed by the incorpo ration of I-125-deoxyuridine, was accelerated by EGF at a low cell den sity while that stimulated by EGF was relatively low at the highest ce ll density, suggesting a cell density-dependent regulation of mitogeni c response to EGF. An equilibrium binding study of I-125-EGF in the pr esence of various concentrations of unlabeled EGF at 0 degrees C revea led that the dissociation constant (K-d) was 0.47-0.88 nM while the sp ecific binding capacity (n) was 86-96 fmol/mg protein at each cell den sity. No significant difference was observed in the time profiles of t he surface-bound, internalized, and degradation products of I-125-EGF, assessed per mg protein, between different cell densities. Based on a kinetic analysis of the time-profiles, the internalization rate const ant and the degradation rate constant were found to be independent of cell density. Conclusions: These results indicate that the cellular bi nding and disposition of EGF are not regulated by cell density, and th at the cell density-dependence of the mitogenic effect cannot be attri buted to differences in the affinity or capacity of the EGF receptor, internalization, or degradation of EGF. We speculate that the cell den sity-dependent mitogenic response may be accounted for by the differen ce in other factors such as the signal transduction processes induced by the receptor binding of EGF, or the translocation of a small fracti on of the total EGF to hepatocyte nuclei.