The Dual Estrogen Receptor α Inhibitory Effects of the Tissue-Selective Estrogen Complex for Endometrial and Breast Safety

The conjugated estrogen/bazedoxifene tissue-selective estrogen complex (TSEC) is designed to minimize the undesirable effects of estrogen in the uterus and breast tissues and to allow the beneficial effects of estrogen in other estrogen-target tissues, such as the bone and brain. However, the molecular mechanism underlying endometrial and breast safety during TSEC use is not fully understood. Estrogen receptor alpha (ER alpha)-estrogen response element (ERE)-DNA pull-down assays using HeLa nuclear extracts followed by mass spectrometry-immunoblotting analyses revealed that, upon TSEC treatment, ER alpha interacted with transcriptional repressors rather than coactivators. Therefore, the TSEC-mediated recruitment of transcriptional repressors suppresses ER alpha-mediated transcription in the breast and uterus. In addition, TSEC treatment also degraded ER alpha protein in uterine tissue and breast cancer cells, but not in bone cells. Interestingly, ER alpha-ERE-DNA pull-down assays also revealed that, upon TSEC treatment, ER alpha interacted with the F-box protein 45 (FBXO45) E3 ubiquitin ligase. The loss-of- and gain-of-FBXO45 function analyses indicated that FBXO45 is involved in TSEC-mediated degradation of the ER alpha protein in endometrial and breast cells. In preclinical studies, these synergistic effects of TSEC on ER alpha inhibition also suppressed the estrogen-dependent progression of endometriosis. Therefore, the endometrial and breast safety effects of TSEC are associated with synergy between the selective recruitment of transcriptional repressors to ER alpha and FBXO45-mediated degradation of the ER alpha protein.