Retinoid X receptor α (RXRα) helix 12 plays an inhibitory role in the recruitment of the p160 co-activators by unliganded RXRα/retinoic acid receptor α heterodimers

Retinoid X receptor (RXR)/retinoic acid receptor (RAR) heterodimers control gene expression through recruitment of co-repressors or co-activators, depending on their hormone binding status. We show that the helix 12 of RXRα and RARα is critical for recruitment of the co-regulators and transcriptional regulation by RXRα, RARα, and RXRα/RARα. LG268, an RXR-specific agonist, was able to promote co-activator association with the heterodimers, but was unable to dissociate co-repressors. Reconstitution experiments in yeast demonstrated that LG268 was capable of activating transcription by RXRα/RARα through recruitment of the co-activator. We hypothesize that the inability to release co-repressors from RXRα/RARα is responsible for the inability of LG268 to activate RXRα/RARα heterodimers in mammalian cells. Deletion of RARα helix 12 (RXRα/ RARα Δ403) abolished both hormone-dependent dissociation from co-repressors and hormone-dependent association with co-activators. Deletion of RXRα helix 12 (RXRα Δ443/ RARα) resulted in a higher binding affinity for co-repressors. Unexpectedly, RXRα Δ443/RARαα also gained hormone-independent co-activator binding activity. Moreover, LG268 became an antagonist to RXRα Δ443/RARα heterodimers. These data suggest that the helix 12 of RXRα plays an inhibitory role in the recruitment of co-activators by unliganded RXRα/RARα.