Endoxifen, 4-Hydroxytamoxifen and an Estrogenic Derivative Modulate Estrogen Receptor Complex Mediated Apoptosis in Breast Cancer

Estrogen therapy was used to treat advanced breast cancer in postmenopausal women for decades until the introduction of tamoxifen. Resistance to long-term estrogen deprivation (LTED) with tamoxifen and aromatase inhibitors used as a treatment of breast cancer inevitably occurs, but unexpectedly low-dose estrogen can cause regression of breast cancer and increase disease-free survival in some patients. This therapeutic effect is attributed to estrogen-induced apoptosis in LTED breast cancer. Here, we describe modulation of the estrogen receptor (ER) liganded with antiestrogens (endoxifen and 4-hydroxytamoxifen) and an estrogenic triphenylethylene (TPE), ethoxytriphenylethylene (EtOXTPE), on estrogen-induced apoptosis in LTED breast cancer cells. Our results show that the angular TPE estrogen (EtOXTPE) is able to induce the ER-mediated apoptosis only at a later time compared with planar estradiol in these cells. Using real-time polymerase chain reaction, chromatin immunoprecipitation, western blotting, molecular modeling, and X-ray crystallography techniques, we report novel conformations of the ER complex with an angular estrogen EtOXTPE and endoxifen. We propose that alteration of the conformation of the ER complexes, with changes in coactivator binding, governs estrogen-induced apoptosis through the protein kinase regulated by RNA-like endoplasmic reticulum kinase sensor system to trigger an unfolded protein response.