Ashburn S P, Han X, Liehr J G
Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston 77555-1031.
Mol Pharmacol. 1993 Apr;43(4):534-41.
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.
在叙利亚雄性仓鼠（一种雌二醇诱导致癌作用的动物模型）中，当以激素等效剂量给仓鼠施用化合物时，2-氟雌二醇无致癌性，而4-氟雌二醇在较长潜伏期后可诱发肾肿瘤，与之相比，雌二醇（肿瘤发生率为100%）。儿茶酚雌激素代谢产物此前被推测介导这种雌激素诱导的肾致癌作用。为研究雌激素诱导肿瘤的这一假定机制，我们研究了仓鼠肾和肝微粒体将2-氟雌二醇和4-氟雌二醇转化为儿茶酚代谢产物以及儿茶酚-O-甲基转移酶将其进一步转化为甲醚的过程，并将这些值与以非氟化雌激素为底物时获得的值进行比较。肝微粒体将2-氟雌二醇转化为2-羟基雌二醇和2-氟-4-羟基雌二醇的速率比以雌二醇为底物时的相应速率低30 - 50%。对于肾微粒体，4-羟基化速率比雌二醇快10倍，而2-羟基化最多处于边缘水平。以4-氟雌二醇为底物时，肝微粒体的2-羟基化速率比雌二醇的值提高了5倍，但4-羟基雌二醇的形成几乎被消除。相反，肾微粒体将该底物转化为4-氟-2-羟基雌二醇的速率比雌二醇的2-羟基化速率快15倍，而4-羟基雌二醇的形成速率为315 pmol/mg蛋白质/分钟。除了肝微粒体对2-氟雌二醇的2-和4-羟基化均减少外，雌激素酚类的氟取代增强了微粒体介导的在未被取代基占据位点的芳香族羟基化。在pH 7.5时，观察到2-氟雌二醇、2-氟-4-羟基雌二醇和2-羟基雌二醇的儿茶酚代谢产物的儿茶酚-O-甲基转移酶介导的甲基化速率最高（分别为3780和2960 pmol/mg蛋白质/分钟）。4-氟雌二醇、4-氟-2-羟基雌二醇和4-羟基雌二醇的甲基化速率较低（分别为1670和470 pmol/mg蛋白质/分钟）。这些数据与雌激素的儿茶酚代谢产物是雌激素诱导致癌作用中的反应性中间体这一假设一致。对于无致癌性的2-氟雌二醇，通过肾形成2-氟-4-羟基雌二醇并进一步转化为甲醚的途径观察到高代谢通量。这种通量可能导致用这种修饰雌激素处理的仓鼠肾脏中儿茶酚代谢产物的稳态浓度较低，因此其缺乏致癌活性。相反，4-氟雌二醇的致癌活性与其在肾脏中快速转化为2-和4-羟基化代谢产物以及这些儿茶酚的甲基化较慢一致。
