Retinoid X receptor (RXR) ligands activate the human 25-hydroxyvitamin D3-24-hydroxylase promoter via RXR heterodimer binding to two vitamin D-responsive elements and elicit additive effects with 1,25-dihydroxyvitamin D3. 
We have previously shown that RNA levels of kidney 25-hydroxyvitamin D3-24-hydroxylase (24(OH)ase), a key metabolic enzyme for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), is up-regulated by retinoids in mice within hours.  Deletion analysis of approximately 5500 base pairs of the human 24(OH)ase promoter showed that the sequence between -316 and -142 contained the information necessary and sufficient for retinoid-induced activation of the promoter.  This region contains two previously defined vitamin D-responsive elements (VDREs) at -294 to -274 and -174 to -151.  Mutation of either VDRE diminished responsiveness of the -316 to -22 promoter sequence to retinoids or 1,25(OH)2D3, while mutation of both VDREs essentially abolished the activity of the ligands via the promoter.  Heterologous promoter vectors driven by the VDREs were responsive to a retinoid X receptor (RXR)-selective ligand (LG100268), a retinoic acid receptor (RAR)-selective ligand (TTNPB), or 1,25(OH)2D3, while combinations of LG100268 with either TTNPB or 1,25(OH)2D3 resulted in additive increases in activity.  Band shift analyses showed that vitamin D receptor, RAR, or RXR alone did not bind to the VDREs; however, the combination of either vitamin D receptor or RAR with RXR led to retardation of each of the labeled probes.  Treatment of nontransfected CV-1 cells with retinoids or 1,25(OH)2D3 resulted in induction of 24(OH)ase RNA, and ligand combinations led to increased RNA levels.  These data imply that either or both of the heterodimer partners can be occupied with ligand to induce this enzyme, with dual receptor occupation leading to increased activation.