Evidence for a direct hepatotrophic role for insulin in the fetal rat: implications for the impaired hepatic growth seen in fetal growth retardation.

Perturbations of fetal growth produce parallel but disproportionate changes in fetal liver growth that correlate with circulating fetal insulin concentration. We have studied the effects of insulin and two hepatotrophic factors, transforming growth factor-alpha (TGF alpha) and hepatocyte growth factor (HGF), on DNA synthesis by fetal and adult rat hepatocytes in primary culture. Using serum-free Minimum Essential Medium, fetal hepatocytes synthesized DNA without growth factors, unlike adult hepatocytes. Insulin augmented fetal hepatocyte DNA synthesis after 16-24 h in culture. In contrast, TGF alpha or HGF maximally stimulated fetal hepatocyte DNA synthesis after 40 h in culture. Insulin and TGF alpha were not synergistic in stimulating fetal hepatocyte DNA synthesis, but were synergistic in their action on adult hepatocytes. Brief (10-min) exposure of fetal hepatocytes to TGF alpha or HGF, but not insulin, activated mitogen-activated protein kinases 4-fold. Prolonged (24-h) exposure to TGF alpha or HGF abolished the ability of either to activate mitogen-activated protein kinases, whereas insulin had no effect. Maternal fasting for 48 h before isolation and culturing of fetal hepatocytes abolished the in vitro stimulation of DNA synthesis by insulin without affecting TGF alpha action. We conclude that insulin has growth-promoting actions on fetal hepatocytes that are distinct and independent from those of TGF alpha of HGF.