The conformation of the estrogen receptor directs estrogen-induced apoptosis in breast cancer: A hypothesis

Background: Estrogens are classified as type I (planar) and type II (angular) based on their structures. In this study, we used triphenylethylenes (TPEs) compounds related to 4-hydroxytamoxifen 4OHT to address the hypothesis that the conformation of the liganded estrogen receptor (ERα) can dictate the E2-induced apoptosis of the ER+ breast cancer cells. Materials and methods: ERα positive MCF7:5C cells were used to study apoptosis induced by E2, 4OHT and TPEs. Growth and apoptosis assays were used to evaluate apoptosis and the ability to reverse E2-induced apoptosis. ERα protein was measured by Western blotting to investigate the destruction of ERα by TPEs in MCF7 cells. Chromatin immunoprecipitation (ChIP) assays were performed to study the in vivo recruitment of ERα and SRC3 at classical E2-responsive promoter TFF1 (PS2) by TPEs. Molecular modeling was used to predict the binding mode of the TPE to the ERα. Results: TPEs were not only unable to induce efficient apoptosis in MCF7:5C cells but also reversed the E2-induced apoptosis similar to 4OHT. Furthermore, the TPEs and 4OHT did not reduce the ERα protein levels unlike E2. ChIP assay confirmed very weak recruitment of SRC3 despite modest recruitment of ERα in the presence of TPEs. Moleular modeling suggests that TPE would bind in antagonistic mode with ERα. Conclusion: Our results advances the hypothesis that the TPE liganded ERα complex structurally resembles the 4OHT bound ERα and cannot efficiently recruit co-activator SRC3. As a result, the TPE complex cannot induce apoptosis of ER + breast cancer cells, although it can cause growth of the breast cancer cells. The conformation of the estrogen-ER complex differentially controls growth and apoptosis.