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

Mol Pharmacol. 2016 Jan;89(1):14-26. doi: 10.1124/mol.115.100925. Epub 2015 Oct 20.


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 α (ERα)-estrogen response element (ERE)-DNA pull-down assays using HeLa nuclear extracts followed by mass spectrometry-immunoblotting analyses revealed that, upon TSEC treatment, ERα interacted with transcriptional repressors rather than coactivators. Therefore, the TSEC-mediated recruitment of transcriptional repressors suppresses ERα-mediated transcription in the breast and uterus. In addition, TSEC treatment also degraded ERα protein in uterine tissue and breast cancer cells, but not in bone cells. Interestingly, ERα-ERE-DNA pull-down assays also revealed that, upon TSEC treatment, ERα 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 ERα protein in endometrial and breast cells. In preclinical studies, these synergistic effects of TSEC on ERα 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 ERα and FBXO45-mediated degradation of the ERα protein.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Breast / drug effects
  • Breast / metabolism
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / metabolism
  • Endometriosis / drug therapy*
  • Endometriosis / metabolism
  • Endometrium / drug effects
  • Endometrium / metabolism
  • Estrogen Receptor alpha / antagonists & inhibitors*
  • Estrogen Receptor alpha / metabolism
  • Estrogens / pharmacology
  • Estrogens / therapeutic use
  • Estrogens, Conjugated (USP) / pharmacology*
  • Estrogens, Conjugated (USP) / therapeutic use*
  • Female
  • HeLa Cells
  • Humans
  • MCF-7 Cells
  • Mice
  • Mice, Inbred C57BL
  • Selective Estrogen Receptor Modulators / pharmacology
  • Selective Estrogen Receptor Modulators / therapeutic use


  • ESR1 protein, human
  • Estrogen Receptor alpha
  • Estrogens
  • Estrogens, Conjugated (USP)
  • Selective Estrogen Receptor Modulators