Cell density-dependent mitogenic effect and -independent cellular handling of epidermal growth factor in primary...

Mitogenic effect and cellular handling of epidermal growth factor (EGF) were analyzed in primary cultured rat hepatocytes at several cell densities.

DNA synthesis, assessed by the incorporation of 125I-deoxyuridine, was accelerated by EGF at a low cell density while that stimulated by EGF was relatively low at the highest cell density, suggesting a cell density-dependent regulation of mitogenic response to EGF. An equilibrium binding study of 125I-EGF in the presence of various concentrations of unlabeled EGF at 0 degree C revealed that the dissociation constant (Kd) was 0.47-0.88 nM while the specific binding capacity (n) was 86-96 fmol/mg protein at each cell density. No significant difference was observed in the time profiles of the surface-bound, internalized, and degradation products of 125I-EGF, assessed per mg protein, between different cell densities. Based on a kinetic analysis of the time-profiles, the internalization rate constant and the degradation rate constant were found to be independent of cell density.

These results indicate that the cellular binding and disposition of EGF are not regulated by cell density, and that the cell density-dependence of the mitogenic effect cannot be attributed to differences in the affinity or capacity of the EGF receptor, internalization, or degradation of EGF. We speculate that the cell density-dependent mitogenic response may be accounted for by the difference in other factors such as the signal transduction processes induced by the receptor binding of EGF, or the translocation of a small fraction of the total EGF to hepatocyte nuclei.