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17β-Estradiol (E2) and estrone (E1), two major endogenous estrogens, are metabolized to catechol estrogens (CEs, i.e., 2- and 4-hydroxy-E2/E1) in humans. The CEs can be further oxidized to produce electrophilic CE quinines/semiquinones and other free radicals, and these reactive intermediates can cause damage to various cellular components and thus may contribute to hormonal carcinogenesis. In humans, CEs are effectively deactivated by the catechol-O-methyltransferase (COMT)-mediated O-methylation coupled with enzymatic glucuronidation and/or sulfation. UGT2B7 is a human UDP-glucuronosyltransferase that has a very high affinity (low Km) for the glucuronidation of 4-hydroxy-E1/E2, and was reported to be at a reduced level in invasive human breast cancer. As an extension of our recent studies which showed that the chlorinated derivatives of bisphenol A (BPA) have considerable estrogenic activity in vitro and in vivo (Takemura H. et al., Toxicology 2005, in press), we further examined in this study the modulating effects of BPA and two of its chlorinated derivatives (3-ClBPA and 3,3’-diClBPA) on the O-methylation and glucurondiation of CEs in vitro catalyzed by human liver cytosolic COMT and human UGT2B7 microsomes, respectively. We found that BPA and its chlorinated derivatives had little or no effect on the COMT-mediated O-methylation of CEs in vitro. However, 3-ClBPA inhibited the human UGT2B7-mediated glucuronidation of 4-hydroxy-E1 in a concentration-dependent manner (1-100 nM). Furthermore, we also studied the effects of BPA and its derivatives on the levels of the UGT2B7 protein in cultured MCF-7 human breast cancer cells by using Western blot analysis. We found that treatment of MCF-7 cells with 10 μM BPA for 5-6 days suppressed the expression of UGT2B7 in cultured MCF-7 cells, but BPA and 3,3’-diClBPA had little or no effect on the expression of this enzyme. The results of our present study showed that 3-ClBPA, but not BPA or 3,3’-diClBPA, inhibited the human UGT2B7-mediated glucuronidation of 4-hydroxyestrogens in vitro, and it also inhibited the expression of this conjugating enzyme in cultured MCF-7 cells. The implication of these interesting observations in human mammary carcinogenesis remains to be determined. [Supported by a Grant-in-Aid for Scientific Research (No.14580570), and also in part, by the 21st Century Center of Excellence Program from the Ministry of Education, Culture, Sports, Science and Technology of Japan.]

[Proc Amer Assoc Cancer Res, Volume 46, 2005]