Cooperative Activation of Gene Expression by Agonists and Antagonists Mediated by Estrogen Receptor Heteroligand Dimer Complexes

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 activate 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. Coimmunoprecipitation experiments confirmed interaction between the agonist-bound and antagonist-bound receptors. This cooperative activation of gene expression was enhanced by steroid receptor coactivator 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, these findings broaden our understanding of the complex relationship between ER agonist and antagonist, and suggest a novel model by which cell and tissue selective effects of antiestrogens may be achieved.