Metabolic inactivation of estrogens in breast tissue by UDP-glucuronosyltransferase enzymes: an overview

The breast tissue is the site of major metabolic conversions of estradiol (E-2) mediated by specific cytochromes P450 hydroxylations and methylation by catechol-O-methyltransferase. In addition to E-2 itself, recent findings highlight the significance of 4-hydroxylated estrogen metabolites as chemical mediators and their link to breast cancer development and progression, whereas, in opposition, 2-methoxylated estrogens appear to be protective. Recent data also indicate that breast tissue possesses enzymatic machinery to inactivate and eliminate E-2 and its oxidized and methoxylated metabolites through conjugation catalyzed by UDP-glucuronosyltransferases (UGTs), which involves the covalent addition of glucuronic acid. In opposition to other metabolic pathways of estrogen, the UGT-mediated process leads to the formation of glucuronides that are devoid of biologic activity and are readily excreted from the tissue into the circulation. This review addresses the most recent findings on the identification of UGT enzymes that are responsible for the glucuronidation of E-2 and its metabolites, and evidence regarding their potential role in breast cancer.