Background and Purpose Oestrogen receptor alpha (ERα) binds to different ligand which can function as complete/partial oestrogen‐agonist or antagonist. This depends on the chemical structure of the ligands which modulates the transcriptional activity of the oestrogen‐responsive genes by altering the conformation of the liganded‐ERα complex. This study determined the molecular mechanism of oestrogen‐agonistic/antagonistic action of structurally similar ligands, bisphenol (BP) and bisphenol A (BPA) on cell proliferation and apoptosis of ERα + ve breast cancer cells. Experimental Approach DNA was measured to assess the proliferation and apoptosis of breast cancer cells. RT‐PCR and ChIP assays were performed to quantify the transcripts of TFF1 gene and recruitment of ERα and SRC3 at the promoter of TFF1 gene respectively. Molecular docking was used to delineate the binding modes of BP and BPA with the ERα. PCR‐based arrays were used to study the regulation of the apoptotic genes. Key Results BP and BPA induced the proliferation of breast cancer cells; however, unlike BPA, BP failed to induce apoptosis. BPA consistently acted as an agonist in our studies but BP exhibited mixed agonistic/antagonistic properties. Molecular docking revealed agonistic and antagonistic mode of binding for BPA and BP respectively. BPA treatment resembled E2 treatment in terms of PCR‐based regulation of apoptotic genes whereas BP was similar to 4OHT treatment. Conclusions and Implications The chemical structure of ERα ligand determines the agonistic or antagonistic biological responses by the virtue of their binding mode, conformation of the liganded‐ERα complex and the context of the cellular function.

Background X-box binding protein 1 (XBP1), a transcription factor involved in unfolded protein response, is also an estrogen-regulated gene and strongly correlates with estrogen receptor alpha (ERα) expression in breast cancers. We investigated the functional role of XBP1 in estrogen responsive breast and endometrial cancer cells as its functions are not fully understood. Materials and methods ERα positive breast (MCF7) and endometrial (ECC1) cancer cells were used to study XBP1 gene regulation by 17-β-estradiol (E2) and to investigate the role of XBP1 in E2-mediated growth using short interfering RNA. Quantitative real-time PCR and Western blot were used to assess RNA and protein levels. Recruitment of ERα and other cofactors at the promoter and enhancer region of the XBP1 gene was investigated by chromatin immunoprecipitation. Estrogen responsive element (ERE)-mediated transcriptional activity was evaluated by a luciferase reporter assay. Results E2 induced the transcription of XBP1 in both MCF7 and ECC1 cells. E2-dependent recruitment of ERα, steroid receptor coactivator (SRC)-1 and SRC-3, and RNA polymerase II were observed at the promoter and/or enhancer region of the XBP1 gene. Depletion of XBP1 markedly inhibited the E2-induced growth in MCF7 and ECC1 cells. However, ERE-mediated transcription was not altered in XBP1-overexpressing or XBP1-depleted MCF7 cells. Conclusion Our results confirm E2-induced transcription of XBP1 and demonstrate the crucial role of XBP1 in E2-induced growth of ERα positive breast and endometrial cancer cells without modulating the classical ERE-mediated transcription by ER. This knowledge creates new opportunities for therapeutic interventions.

Estrogen plays vital roles in human health and diseases. Estrogen mediates its actions almost entirely by binding to estrogen receptors (ER), alpha and beta which further function as transcription factors. Selective estrogen receptor modulators (SERMs) are synthetic molecules which bind to ER and can modulate its transcriptional capabilities in different ways in diverse estrogen target tissues. Tamoxifen, the prototypical SERM, is extensively used for targeted therapy of ER positive breast cancers and is also approved as the first chemo-preventive agent for lowering breast cancer incidence in high risk women. The therapeutic and preventive efficacy of tamoxifen was initially proven by series of experiments in the laboratory which laid the foundation of its clinical use. Unfortunately, use of tamoxifen is associated with de-novo and acquired resistance and some undesirable side effects. The molecular study of the resistance provides an opportunity to precisely understand the mechanism of SERM action which may further help in designing new and improved SERMs. Recent clinical studies reveal that another SERM, raloxifene, which is primarily used to treat post-menopausal osteoporosis, is as efficient as tamoxifen in preventing breast cancers with fewer side effects. Overall, these findings open a new horizon for SERMs as a class of drug which not only can be used for therapeutic and preventive purposes of breast cancers but also for various other diseases and disorders. Major efforts are therefore directed to make new SERMs with a better therapeutic profile and fewer side effects.

Estrogens and androgens influence many properties of breast cancer cells; hence, regulation of local estrogen and androgen levels by enzymes involved in steroid hormone biosynthesis and metabolism would impact signaling by these hormones in breast cancer cells. In this study, we show that the UDP-glucuronosyltransferase (UGT) enzyme UGT2B15, a member of the UGT family of phase II enzymes involved in the glucuronidation of steroids and xenobiotics, is a novel, estrogen-regulated gene in estrogen receptor (ER)-positive human breast cancer cells (MCF-7, BT474, T47D, and ZR-75). UGT2B15 is the only UGT2B enzyme up-regulated by estrogen, and marked estradiol stimulation of UGT2B15 mRNA levels is observed, in a time- and dose-dependent manner. UGT2B15 stimulation by estradiol is blocked by the antiestrogen ICI182,780, but not by the translational inhibitor cycloheximide, indicating that UGT2B15 is likely a primary transcriptional response mediated through the ER. UGT2B15 up-regulation is also evoked by other estrogens (propylpyrazoletriol, genistein) and by the androgen 5alpha-dihydrotestosterone working through the ER, but not by other steroid hormone receptor ligands. Western blot and immunocytochemical analyses with several UGT2B-specific antibodies we have designed and steroid glucuronidation assays indicate a large increase in both cellular UGT2B15 protein and enzyme activity after estrogen treatment. Due to the important role of UGT enzymes in forming conjugates between steroids and glucuronic acid, thereby inactivating them and targeting them for removal, the estrogen-induced up-regulation of UGT2B15 might have a significant moderating effect on estrogen and androgen concentrations, thereby reducing their signaling in breast cancer cells.

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