Microsomal hydroxylation of 2- and 4-fluoroestradiol to catechol metabolites and their conversion to methyl ethers: catechol estrogens as possible mediators of hormonal carcinogenesis

Abstract

In male Syrian hamsters, an animal model for estradiol-induced carcinogenesis, 2-fluoroestradiol was not carcinogenic, whereas 4-fluoroestradiol induced kidney tumors after a prolonged latency period, compared with estradiol (100% tumor incidence), when the compounds were administered to hamsters in hormonally equipotent doses. Catechol estrogen metabolites have previously been postulated to mediate this estrogen-induced kidney carcinogenesis. To examine this proposed mechanism of tumor induction by estrogens, we investigated the conversion of 2- and 4-fluoroestradiol to catechol metabolites by kidney and liver microsomes of hamsters and the further conversion to methyl ethers by catechol-O-methyltransferase, and we compared the values with those obtained with nonfluorinated estrogens as substrates. The rates of conversion of 2-fluoroestradiol to 2-hydroxyestradiol and 2-fluoro-4-hydroxyestradiol by hepatic microsomes were 30-50% lower than corresponding rates with estradiol as substrate. With renal microsomes the rate of 4-hydroxylation was 10 times faster than that of estradiol, whereas 2-hydroxylation was at best marginal. With 4-fluoroestradiol as substrate the rate of 2-hydroxylation by hepatic microsomes was enhanced 5-fold, compared with values for estradiol, but 4-hydroxyestradiol formation was almost eliminated. In contrast, the conversion of this substrate to 4-fluoro-2-hydroxyestradiol by kidney microsomes occurred at a rate 15 times faster than 2-hydroxylation of estradiol, whereas 4-hydroxyestradiol formation proceeded at a rate of 315 pmol/mg of protein/min. Except for the decrease in both 2- and 4-hydroxylation of 2-fluoroestradiol by liver microsomes, fluorine substitution of estrogenic phenols enhanced microsome-mediated aromatic hydroxylation at sites unoccupied by substituents. At pH 7.5, the highest rates of catechol-O-methyltransferase-mediated methylation were observed with the catechol metabolites of 2-fluoroestradiol, 2-fluoro-4-hydroxyestradiol and 2-hydroxyestradiol (3780 and 2960 pmol/mg of protein/min, respectively). Lower rates were found with those of 4-fluoroestradiol, 4-fluoro-2-hydroxyestradiol and 4-hydroxyestradiol (1670 and 470 pmol/mg of protein/min, respectively). These data are consistent with the postulate that catechol metabolites of estrogens are reactive intermediates in estrogen-induced carcinogenesis. For the noncarcinogenic 2-fluoroestradiol, a high metabolic flux was observed through a pathway of renal 2-fluoro-4-hydroxyestradiol formation and further conversion to methyl ethers. This flux likely results in low steady state concentrations of catechol metabolites in kidneys of hamsters treated with this modified estrogen and therefore in its lack of carcinogenic activity. In contrast, the carcinogenic activity of 4-fluoroestradiol is consistent with its rapid conversion in the kidney to 2- and 4-hydroxylated metabolites and a less rapid methylation of these catechols.