Interplay between the nuclear receptor PXR and the uptake transporter OATP1A2 selectively enhances estrogen effects in breast cancer

摘要

The ligand-activated nuclear receptor PXR is known to play a role in the regulated expression of drug metabolizing enzymes and transporters. Recent studies suggest a potential clinically relevant role of PXR in breast cancer. However, the relevant pathway or target genes of PXR in breast cancer biology and progression have not yet been fully clarified.In this study, we show that mRNA expression of OATP1A2, a transporter capable of mediating the cellular uptake of estrogen metabolites, is nearly 10-fold greater in breast cancer compared to adjacent healthy breast tissues. Immunohistochemistry revealed exclusive expression of OATP1A2 in breast cancer tissue. Interestingly, treatment of breast cancer cells in vitro with the PXR agonist rifampin induced OATP1A2 expression in a time- and concentration-dependent manner. Consistent with a role as a hormone uptake transporter, induction of OATP1A2 was associated with increased uptake of estrone 3-sulfate. The rifampin response was abrogated after si-RNA targeting of PXR. We then identified a PXR response element in the human OATP1A2 promoter, located approximately 5.7 kb upstream of the transcription initiation site. The specificity of PXR-OATP1A2 promoter interaction was confirmed using chromatin immunoprecipitation. Importantly we utilized a novel potent and specific antagonist of PXR (A-792611) to demonstrate the reversal of the rifampin effect on the cellular uptake of E1S.These data provide important new insights into the interplay between a xenobiotic nuclear receptor PXR and OATP1A2 that could contribute to the pathogenesis of breast cancer and may also prove to be heretofore unrecognized targets for breast cancer treatment.