Abstract
Estrogen receptor (ER) antagonists are generally thought to inhibit estrogen action through competitive inhibition resulting in receptor binding to antagonist rather than agonist. However, microarray analyses reveal a group of genes for which ER agonist and antagonist cooperatively regulate expression, suggesting additional models of combined agonist/antagonist action must exist. In conjunction with a chimeric reporter gene and two modified ERs, one [ERα(GSCKV)] with a mutation in the DNA binding domain and the other (ERα-G521R) with a ligand binding specificity mutation, we herein demonstrate that ER agonist and antagonist cooperatively active gene expression through an ER heteroligand dimer complex (ER-HLD) consisting of one subunit of the receptor dimer bound to agonist and another occupied by antagonist. Co-immunoprecipitation experiments confirmed interaction between the agonist-bound and antagonist-bound receptors. This cooperative activation of gene expression was enhanced by SRC-3 coactivator and required each ligand-bound subunit of the dimer to bind to DNA, as well as both activation function-1 domains for maximal transcriptional activity. Ligand combinations able to induce ER-HLD transcriptional activity include the agonists 17β-estradiol or conjugated estrogens with the antagonists tamoxifen, raloxifene, bazedoxifene or fulvestrant. Moreover, ER-HLD can activate transcription in the context of a natural promoter. Taken together, this finding broadens our understanding of the complex relationship between ER agonist and antagonist, and suggests a novel model by which cell and tissue selective effects of antiestrogens may be achieved.
- Sex hormones
- Mutagenesis/Chimeric approaches
- Regulation of gene expression
- Regulation - transcriptional
- Structure/function/mechanism
- Transcription targets
- Endocrine cells
- Received December 4, 2012.
- Revision received March 4, 2013.
- Accepted March 4, 2013.
- The American Society for Pharmacology and Experimental Therapeutics