HETERODIMERIC DNA-BINDING BY THE VITAMIN-D-RECEPTOR AND RETINOID-X-RECEPTORS IS ENHANCED BY 1,25-DIHYDROXYVITAMIN D-3 AND INHIBITED BY 9-CIS-RETINOIC ACID - EVIDENCE FOR ALLOSTERIC RECEPTOR INTERACTIONS

Gel mobility shift analysis was utilized to investigate the molecular function of 1 alpha,25-dihydroxyvitamin D-3 (1,25-(OH)(2)D-3) and 9-ci s-retinoic acid (9-cis-RA) ligands in the binding of the vitamin D rec eptor (VDR) and retinoid X receptor (RXR) to mouse osteopontin and rat osteocalcin vitamin D-response elements (VDREs). At physiological ion ic strength and reduced concentrations of expressed proteins, efficien t binding to either VDRE occurs as a VDR.RXR heterodimer, not as a VDR homodimer. 1,25-(OH)(2)D-3 dramatically enhances heterodimer-VDRE int eraction, whereas somewhat higher concentrations of 9-cis-RA inhibit t his association, perhaps related to the role of this retinoid in facil itating RXR homodimer formation. Interestingly, if VDR is occupied by 1,25-(OH)(2)D-3 prior to complexing with RXR, the resulting heterodime r is relatively resistant to dissociation and diversion to other pathw ays by 9-cis-RA. Therefore, a proposed molecular action of 1,25-(OH)(2 )D-3 is to generate an allosteric switch in VDR to a form that not onl y binds to the VDRE with high affinity and specificity as a heterodime r with RXR, but also interacts with the RXR partner to conformationall y restrict the action of its cognate ligand.