Mitogen-independent DNA synthesis by fetal rat hepatocytes in primary culture

We have identified conditions under which late gestation fetal rat hepatocytes in primary culture proliferate in the absence of serum, polypeptide growth factors, or insulin. Fetal hepatocytes, cultured in defined minimal essential medium (MEM) with hydrocortisone, synthesized DNA within the first 6 h after plating and for up to 72 h. Rates of thymidine incorporation into DNA by fetal hepatocytes exceeded peak rates seen with adult rat hepatocytes. The latter were quiescent following isolation, with DNA synthesis only occurring after 48 h exposure to insulin plus epidermal growth factor. Although they exhibited a high rate of DNA synthesis, the fetal hepatocytes retained sensitivity to added mitogens; DNA synthesis was stimulated three- to fourfold by subnanomolar concentrations of TGF-alpha. Fetal hepatocytes also were sensitive to the growth inhibitory effects of TGF-beta at concentrations below 10 pM. Finally, ontogenic changes in serum- and mitogen-independent fetal hepatocyte growth were observed, with declining rates of DNA synthesis as term approached. We speculate that the ability of fetal rat hepatocytes to synthesize DNA independent of added serum or mitogens may coincide with a proliferative in vivo phenotype.