The aryl hydrocarbon receptor interacts with estrogen receptor alpha and orphan receptors COUP-TFI and ERRα1

The molecular mechanisms underlying the apparent 'cross-talk' between estrogen receptor (ER)- and aryl-hydrocarbon receptor (AHR)-mediated activities are unknown. To determine how AHR ligand 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) may inhibit ER action and, conversely, to examine how 17-β-estradiol (E₂) affects AHR activity, we examined discrete activities of each receptor, i.e., protein-protein interactions, DNA binding, and transcriptional activation. We report that AHR interacts directly with ERα, COUP-TF, and ERRα1, in a ligand-specific manner in vitro. Unoccupied or β-napthoflavone (β-NF)-occupied AHR showed stronger interaction with ERα, COUP-TF, and ERRα1 than when AHR was occupied by the partial antagonist α-naphthoflavone (α-NF), indicating a role for ligand in AHR interaction with these proteins. We also report that AHR interacts with COUP- TF in transfected CV-1 cells. In contrast, the AHR nuclear translocator protein (ARNT) did not interact with COUPTF, ERRα1, or ERα. We next examined the interaction of either ERα or COUP-TF with a consensus xenobiotic response element (XRE). Purified ERα did not bind the consensus XRE, but COUP-TFI bound the consensus XRE, suggesting a role for COUP-TF as a AHR/ARNT competitor for XRE binding. In transiently transfected MCF-7 human breast cancer cells, overexpression of COUP-TFI inhibited TCDD-activated reporter gene activity from the CYP1A1 promoter. TCDD inhibited estradiol (E₂)-activated reporter gene activity from a consensus ERE and from the EREs in the pS2 and Fos genes, and COUP-TFI did not block the antiestrogenic activity of TCDD. The specific interaction of COUP-TF with XREs and AHR together with the inhibition of TCDD-induced gene expression by COUP-TF suggests that COUP-TF may regulate AHR action both by direct DNA binding competition and through protein-protein interactions.