The co-mitogenic effects of various estrogens for TGF-α-induced DNA synthesis in cultured female rat hepatocytes

Abstract

The synthetic estrogens ethinyl estradiol (EE) and mestranol (M) are weak complete hepatic carcinogens and potent tumor promoters. In vivo, EE and M cause a rapid but transient increase in liver growth. However, studies in cultured female rat hepatocytes indicate that EE is not a strong complete hepatic mitogen but rather enhances epidermal growth factor (EGF)-induced DNA synthesis and is thus classified as a co-mitogen (Yager, J.D., Zurlo, J. and Ni, N. (1991) Proc. Soc. Exptl. Biol. Med., 198, 667–674). The endogenous estrogen 17β-estradiol (E 2) also exhibits co-mitogenic activity, enhancing the fraction of hepatocytes undergoing DNA synthesis induced by both EGF and transforming growth factor alpha (TGF-α) (Ni, N. and Yager, J.D. (1994) Hepatology, 19, 183–192). The objectives of the study reported here were: (1) to determine whether the co-mitogenic effects of EE and E 2 extend to other synthetic estrogens including mestranol and diethylstilbestrol, and to α-zearalanol, a natural product with estrogenic activity; (2) to compare the co-mitogenic effects of endogenous estrogens including E 2, estrone, estriol and the catechol metabolites 2- and 4-hydroxy-estradiol; and (3) to determine whether the conditioned medium from E 2-treated hepatocytes has co-mitogenic activity. Female rat hepatocytes in primary culture were exposed to the various estrogens ± TGF-α and DNA synthesis was determined by measuring [ 3H]thymidine incorporation into extracted DNA. The results show that the co-mitogenic effects previously observed with EE and E 2 also extend to all of these estrogens and to the E 2 catechol metabolites. Although the co-mitogenic potency of these estrogens does not correlate with their reported affinities to the estrogen receptor, their estrogenicity appears necessary since the non-estrogenic metabolite 2-methoxyestradiol lacks co-mitogenic activity. In addition, enhancement of TGF-α-induced DNA synthesis by conditioned medium from E 2-treated cells supports the notion that a metabolite mediates its co-mitogenic effect.