Effects of the Liver Tumor Promoter Ethinyl Estradiol on Epidermal Growth Factor-induced DNA Synthesis and Epidermal Growth Factor Receptor Levels in Cultured Rat Hepatocytes1

The objective of this study was to determine whether DNA synthesis induced in the livers of female rats treated with ethinyl estradiol (EE) was due to direct effects of this synthetic estrogen on hepatocytes. Hepatocytes, obtained by collagenase perfusion from female Lewis rats, were cultured in serum-free medium containing low or no phenol red and supplemented with insulin, transferrin, and selenium. When present at 10-15 MMfor the initial 30 h of culture, EE caused a subsequent 2-2.7fold increase in hepatocyte DNA synthesis. Pretreatment of the hepato cytes with EE during the first 30 h of culture caused an EE concentrationdependent enhancement of their subsequent DNA synthetic response to epidermal growth factor (EOF). Pretreatment with EE shifted the EGF dose-response curve, causing a dramatic enhancement of the response to EGE beginning at 2 ng EGF/ml. The response to a saturating (25 ng/ml) dose of EGF was also greatly enhanced. Determination of the effect of EE on hepatocyte surface EGF receptors revealed that the increased responsiveness of DNA synthesis to EGF was accompanied by a twofold increase in EGF receptor number per cell. These results indicate that EE has direct, growth-related effects on hepatocytes which may contribute to liver growth induced in vivo by this tumor promoter.