Induction of UDP-Glucuronosyltransferase 2B15 Gene Expression by the Major Active Metabolites of Tamoxifen, 4-Hydroxytamoxifen and Endoxifen, in Breast Cancer Cells

We previously reported upregulation of UGT2B15 by 17 beta-estradiol in breast cancer MCF7 cells via binding of the estrogen receptor alpha (ER alpha) to an estrogen response unit (ERU) in the proximal UGT2B15 promoter. In the present study, we show that this ER alpha-mediated upregulation was significantly reduced by two ER antagonists (fulvestrant and raloxifene) but was not affected by a third ER antagonist, 4-hydroxytamoxifen (4-OHTAM), a major active tamoxifen (TAM) metabolite. Furthermore, we found that, similar to 17 beta-estradiol, 4-OHTAM and endoxifen (another major active TAM metabolite) elevated UGT2B15 mRNA levels, and that this stimulation was significantly abrogated by fulvestrant. Further experiments using 4-OHTAM revealed a critical role for ER alpha in this regulation. Specifically; knockdown of ER alpha expression by anti-ER alpha small interfering RNA reduced the 4-OHTAM-mediated induction of UGT2B15 expression; 4-OHTAM activated the wild-type but not the ERU-mutated UGT2B15 promoter; and chromatin immunoprecipitation assays showed increased ER alpha occupancy at the UGT2B15 ERU in MCF7 cells upon exposure to 4-OHTAM. Together, these data indicate that both 17 beta-estradiol and the antiestrogen 4-OHTAM upregulate UGT2B15 in MCF7 cells via the same ER alpha-signaling pathway. This is consistent with previous observations that both 17 beta-estradiol and TAM upregulate a common set of genes in MCF7 cells via the ER-signaling pathway. As 4-OHTAM is a UGT2B15 substrate, the upregulation of UGT2B15 by 4-OHTAM in target breast cancer cells is likely to enhance local metabolism and inactivation of 4-OHTAM within the tumor. This represents a potential mechanism that may reduce TAM therapeutic efficacy or even contribute to the development of acquired TAM resistance.