Photoaffinity labeling of human retinoid X receptor beta (RXRbeta) with 9-cis-retinoic acid: identification of phytanic acid, docosahexaenoic acid, and lithocholic acid as ligands for RXRbeta.

We utilized [20-methyl-(3)H]-9-cis-retinoic acid ([(3)H]9-cis-RA) as a direct photoaffinity probe for the characterization of human recombinant retinoid X receptor beta protein (RXRbeta). The photoaffinity labeling was light- and concentration-dependent, saturable, and protected by unlabeled 9-cis-RA in a concentration-dependent manner, indicating that binding occurred in the RXR retinoid binding site. all-trans-Retinoic acid (atRA) did not affect labeling with the 9-cis derivative, confirming that atRA does not compete for the 9-cis-RA binding site. Several retinoid, fatty acid, and bile acid ligands were evaluated for their ability to recognize the 9-cis-RA binding site. Retinol, atRA glucuronide, 13-cis-RA, dolichol, 5,6-epoxy-RA, and vitamin D(3) did not compete for the 9-cis-RA binding site. However, the saturated diterpenoid phytanic acid (PA) and docosahexaenoic acid, which have been recently shown to activate the nuclear receptor, RXR, competed with 9-cis-RA labeling, showing high affinity for the 9-cis-RA binding site. Oleic acid, arachidonic acid, and butyric acid did not interact. However, the bile acid lithocholic acid competed efficiently with 9-cis-RA for the binding site. These data validated the photoaffinity assay as an excellent system for the identification and evaluation of ligands for RXR.