The aryl hydrocarbon receptor interacts with estrogen receptor alpha and orphan receptors COUP-TFI and ERR alpha 1

The molecular mechanisms underlying the apparent "cross-talk" between estro gen receptor (ER)- and aryl-hydrocarbon receptor (AHR)-mediated activities are unknown. To determine how AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxi n (TCDD) may inhibit ER action and, conversely, to examine how 17-beta-estr adiol (E-2) affects AHR activity, we examined discrete activities of each r eceptor, i.e., protein-protein interactions, DNA binding, and transcription al activation. We report that AHR interacts directly with ER alpha, COUP-TF , and ERR alpha 1, in a ligand-specific manner in vitro. Unoccupied or beta -napthoflavone (beta-NF)-occupied AHR showed stronger interaction with ER a lpha, COUP-TF, and ERR alpha 1 than when AHR was occupied by the partial an tagonist cu-naphthoflavone (alpha-NF), indicating a role for ligand in AHR interaction with these proteins. We also report that AHR interacts with COU P-TF in transfected CV-1 cells. In contrast, the AHR nuclear translocator p rotein (ARNT) did not interact with COUP-TF, ERR alpha 1, or ER alpha. We n ext examined the interaction of either ER alpha or COUP-TF with a consensus xenobiotic response element (XRE). Purified ER alpha did not bind the cons ensus 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 MC F-7 human breast cancer cells, overexpression of COUP-TFI inhibited TCDD-ac tivated reporter gene activity from the CYP1A1 promoter. TCDD inhibited est radiol (E-2)-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 antiestr ogenic 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 c ompetition and through protein-protein interactions. (C) 2000 Academic Pres s.