The carcinogenic activity of ethinyl estrogens is determined by both their hormonal characteristics and their conversion to catechol metabolites.

Estrogens induce kidney tumors in Syrian hamsters. The mechanism of carcinogenesis is unknown and has been investigated in this study using a weak carcinogen, 17 alpha-ethinyl estradiol (EE), and a strongly carcinogenic estrogen, 17 alpha-ethinyl-11 beta-methoxyestradiol [moxestrol (MOX)]. We investigated rates of conversion of estrogens to catechol metabolites and rates of their methylation to methyl ethers in order to examine the hypothesis that catechol metabolites mediate estrogen-induced carcinogenesis. Rates of conversion of MOX to catechol metabolites by hamster liver or kidney cortex microsomes were 40-50% of those with estradiol as substrate. However, the rate of catechol-O-methyltransferase-catalyzed methylation of 2-hydroxy-MOX was the least of the catechol metabolites examined when incubated with cytosol of hamster kidney. In contrast, EE was converted to catechol metabolites by hamster liver and kidney microsomes at rates 25-35% of those obtained with estradiol. These catechol metabolites of EE were methylated by catechol-O-methyltransferase of hamster kidney cytosol at rates slightly lower than those observed with catechols of estradiol. The progesterone receptor binding of EE and MOX was investigated, because progesterone is known to inhibit estrogen-induced carcinogenesis in the hamster kidney. Neither estradiol nor MOX inhibited the binding of progesterone to its receptor in hamster kidney cytosol. However, in the presence of 20 nM EE, the binding affinity of radiolabeled progesterone to receptor was inhibited (increase in Kd from 0.98 nM in controls to 3.02 nM in the presence of EE). Maximum binding values (5.0 fmol/mg protein in controls and 6.0 fmol/mg protein in the presence of EE) were not significantly altered. These results support the hypothesis that estrogen-induced carcinogenesis is mediated by catechol estrogen metabolites. The carcinogenic estrogen MOX is converted to catechol metabolites at lower rates than estradiol, but their methylation may be sterically hindered by the 11 beta-methoxy substituent. In contrast, the rates of conversion of the weakly carcinogenic EE to catechol metabolites are low, whereas their methylation rates are only marginally lower than those of 2- and 4-hydroxyestradiol. The decreased capacity of EE to form catechol metabolites in conjunction with its partial progestin agonist activity in the target organ of hamsters may contribute to the low tumor incidence.