| In this study, estrogen metabolism by UDP-glucuronosyltransferases (UGTs) in human breast tissues and cells were investigated. Using in vitro glucuronidation activity assays, we obtained results, which indicate that seven human UGTs are involved in conjugating native estrogens and/or their 2- and 4- hydroxylated metabolites. UGT1A10 conjugated estrogens with the highest activities, while UGT2B7 conjugated the genotoxic 4-OH-E 1 and E3 with high enzymatic activity. In addition to these findings, identification of crucial amino acids, Phe90 and Phe93, which are responsible for the binding of estrogens within the estrogen binding site of human UGT1A10, were shown. Demonstrating that UGT1A10 is the major UGT involved in conjugation of the estrogens studied and UGT2B7 in conjugating genotoxic 4-OH-E1, their expression was characterized on the levels of mRNA, protein, and enzymatic activity, using real-time PCR, Western blot, and glucurondiation activity assays in human breast tissues. UGT1A10 and UGT2B7 were highly expressed in the breast. All breast tissue specimens displayed significant inter-individual differences inUGT1A10 and UGT2B7 mRNA expression and enzymatic activity. Further characterization of UGT1A10 and UGT2B7 in breast carcinomas and normal breast tissues from African American and Caucasian women was evaluated. UGT1A10 mRNA expression was significantly down-regulated in breast carcinomas as compared to normal breast tissues from African American women. In Caucasian women, UGT2B7 mRNA expression was significantly down-regulated in breast carcinomas as compared to normal breast tissues. Finally, UGT1A10 was identified for the first time as a potential estrogen target gene that might be regulated by E2 via ER in MCF-7 ER-positive breast cancer cells. The antiestrogen, ICI 182,780, blocked the mRNA expression of UGT1A10, indicating that the regulation of UGT1A10 by E2 might be mediated through the ER. Therefore, UGT1A10 might have a significant moderating effect on estrogen concentrations, thereby impacting E2 signaling in estrogen-responsive breast cancer cells. The findings from this study emphasize the importance of UGT1A10 in estrogen conjugation and in general detoxification pathways and suggest that UGT1A10 might play a critical role in protecting breast tissues from the carcinogenic effects of estrogens. |
