Characterization of common UGT1A8, UGT1A9, and UGT2B7 variants with different capacities to inactivate mutagenic 4-hydroxylated metabolites of estradiol and estrone

The oxidative metabolism of estrone (E-1) and estradiol (E-2) to form carcinogenic 4-hydroxy-catecholestrogens (4-OHCE) is associated with uterine and breast carcinogenesis. In this study, we conducted functional analyses of genetic variants in the UDP-glucuronosyltransferase UGT1A8, UGT1A9, and UGT2B7 enzymes primarily involved in the inactivation of 4-OHCEs. Compared with UGT2B7*2 ((HY)-Y-268), UGT2B7*1 exhibited a 2-fold lower efficiency (intrinsic clearance) at conjugating 4-hydroxyestrone and 4-hydroxyestradiol at positions 3 and 4 caused by altered capacities (V-max) and affinities (K). The -79 G>A promoter variation, characterizing the UGT2B7*2g haplotype, leads to a 50% reduction of transcription (P < 0.00.1) in human endometrial carcinoma-113 cells. Furthermore, a > 12-fold decreased intrinsic clearance of the *1 proteins was induced by selected amino acid substitutions in UGT1A8 (*3 (CY)-Y-277) and UGT1A9 (*3 (MT)-T-33). Frequencies of the low-activity alleles in Caucasians were 45% for UGT2B7*1, 5% for the -79A promoter variant, 1.2% for UGT1A8*3, and 2.2% for UGT1A9*3. Supporting a protective role in two organs sensitive to 4-OHCE-induced damages, the expression of UGT enzymes was shown by immunohistochemistry in normal breast and endometrial tissues and confirmed by Western blotting in a subset of samples. Altogether, findings suggest that specific polymorphisms in UGT genes may modulate the exposure to carcinogenic metabolites of E-2 and potentially lead to an altered risk of breast and endometrial cancers in women carrying the variant alleles.