Role of estrogens as promoters of hepatic neoplasia.

The administration of estrogens to humans has been associated with the development of benign and possibly malignant hepatocellular neoplasms. This study was designed to investigate the mechanism of this association. We present a rat model that demonstrates that stilbestrol (DES) and ethinyl estradiol promote the development of hepatic neoplasms in rats previously initiated with diethylnitrosamine (DEN). Eighty male and 12 female Fischer-344 rats were given a single intraperitoneal injection of either DEN (200 mg. per kg. of body weight) or saline. Beginning 4 weeks after injection, the rats were given an estrogen or corn oil twice weekly for up to 50 weeks. Treatments were stopped at the time of sacrifice or 11 to 29 weeks prior to sacrifice. Estrogen treatments included high dose DES (5 mg. per dose), low dose DES (0.5 mg. per dose), and ethinyl estradiol (0.2 mg. per dose). Male and female rats given both DEN and high dose DES developed grossly visible hepatic hyperplastic nodules (mean, 9.1 per liver) after 20 weeks of DES. Nodules also developed if the onset of DES treatment was delayed until 28 weeks after initiation. Rats given DEN alone or DES alone did not develop nodules after 20 weeks. Microscopic hyperplastic foci also developed in rats given DEN plus DES, DES alone, and DEN alone. The foci in rats given DES alone were largely reversible on cessation of estrogen therapy. Mitotic activity in foci and nodules was prominent during estrogen therapy but declined markedly after cessation of therapy. Similar promoting activity of ethinyl estradiol was observed. Low dose DES was not effective in promoting tumor formation. The data indicate that estrogens promote hepatic neoplasia, perhaps by increasing hepatocyte mitotic activity and thereby facilitating the evolution of previously initiated cells into neoplastic clones. Comparison with human disease should be made with caution, especially because the estrogenic dose administered was approximately 200-fold the usual contraceptive dose in humans. However, the analogy between this model and the development of human oral contraceptive-associated hepatic tumors is evident. It is possible that some women have latent "initiated" cells that divide faster than the surrounding hepatocytes in response to estrogens.