Ethinyl estradiol is a weak complete carcinogen and potent tumor promoter. In vivo, ethinyl estradiol causes a rapid but transient increase in liver growth, whereas in cultured female hepatocytes it enhances DNA synthesis induced by epidermal growth factor and is thus classified as a comitogen. The objectives of this study were to determine: (a) whether estradiol also has comitogenic activity; (b) whether the comitogenic effects of estrogen extend to complete hepatic mitogens other than epidermal growth factor and (c) whether inhibition of hepatocyte DNA synthesis by transforming growth factor‐β can be blocked by estradiol. Female rat hepatocytes in primary culture were exposed to estradiol with and without various growth factors, and we determined DNA synthesis by measuring [3H]thymidine incorporation into extracted DNA or by determining the nuclear labeling index by means of autoradiography. The results show that estradiol, like ethinyl estradiol, has comitogenic activity for epidermal growth factor, although it is somewhat less potent. Four complete hepatic mitogens showed different abilities to stimulate DNA synthesis, with hepatocyte growth factor > transforming growth factor‐alpha > epidermal growth factor > acidic fibroblast growth factor. Estradiol enhanced DNA synthesis occurring in response to all four of these complete hepatic mitogens. This finding suggests that the mechanism of estrogen comitogenesis may involve effects at a point where the different signal‐transduction pathways leading from the growth factors converge. The level of estrogen‐mediated enhancement of DNA synthesis was similar for all four mitogens, ranging from 1.5‐ to 2.2‐fold, depending on the experiment. Furthermore, determination of the nuclear labeling index showed that estrogen enhancement of DNA synthesis was associated with an increase in the percentage of hepatocytes that responded to the growth factors. Finally, estradiol did not specifically block the growth‐inhibitory effects of transforming growth factor‐ß. (Hepatology 1994;19:183–192).
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