Aryl Hydrocarbon Receptor Modulation of Estrogen Receptor α-Mediated Gene Regulation by a Multimeric Chromatin Complex Involving the Two Receptors and the Coregulator RIP140

Although crosstalk between aryl hydrocarbon receptor (AhR) and estrogen receptor alpha (ER alpha) is well established, the mechanistic basis and involvement of other proteins in this process are not known. Because we observed an enrichment of AhR-binding motifs in ER alpha-binding sites of many estradiol (E2)-regulated genes, we investigated how AhR might modulate ER alpha-mediated gene transcription in breast cancer cells. Gene regulations were categorized based on their pattern of stimulation by E2 and/or dioxin and were denoted E2-responsive, dioxin-responsive, or responsive to either ligand. ER alpha, AhR, aryl hydrocarbon receptor translocator, and receptor interacting protein 140 (RIP140) were recruited to gene regulatory regions in a gene-specific and E2/dioxin ligand-specific manner. Knockdown of AhR markedly increased the expression of ER alpha-mediated genes upon E2 treatment. This was not attributable to a change in ER alpha level, or recruitment of ER alpha, phosphoSer5-RNA Pol II, or several coregulators but rather was associated with greatly diminished recruitment of the coregulator RIP140 to gene regulatory sites. Changing the cellular level of RIP140 revealed coactivator or corepressor roles for this coregulator in E2- and dioxin-mediated gene regulation, the choice of which was determined by the presence or absence of ER alpha at gene regulatory sites. Coimmunoprecipitation and chromatin immunoprecipitation (ChIP)-reChIP studies documented that E2- or dioxin-promoted formation of a multimeric complex of ER alpha, AhR, and RIP140 at ER alpha-binding sites of genes regulated by either E2 or dioxin. Our findings highlight the importance of cross-regulation between AhR and ER alpha and a novel mechanism by which AhR controls, through modulating the recruitment of RIP140 to ER alpha-binding sites, the kinetics and magnitude of ER alpha-mediated gene stimulation.