RETINOID-X-RECEPTOR (RXR) LIGANDS ACTIVATE THE HUMAN 25-HYDROXYVITAMIN D-3-24-HYDROXYLASE PROMOTER VIA RXR HETERODIMER BINDING TO 2 VITAMIND-RESPONSIVE ELEMENTS AND ELICIT ADDITIVE EFFECTS WITH 1,25-DIHYDROXYVITAMN D-3

We have previously shown that RNA levels of kidney 25-hydroxyvitamin D -3-24-hydroxylase (24(OH)ase), a key metabolic enzyme for 1,25-dihydro xyvitamin D-3 (1,25(OH)(2)D-3), is up-regulated by retinoids in mice w ithin hours, Deletion analysis of similar to 5500 base pairs of the hu man 24(OH)ase promoter showed that the sequence between -316 and -142 contained the information necessary and sufficient for retinoid-induce d activation of the promoter, This region contains two previously defi ned 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)(2)D-3, while mutation of both VDREs essentially abolished the activity of the ligands via th e promoter, Heterologous promoter vectors driven by the VDREs were res ponsive to a retinoid X receptor (RXR)-selective ligand (LG100268), a retinoic acid receptor (RAR)-selective ligand (TTNPB), or 1,25(OH)(2)D -3, while combinations of LG100268 with either TTNPB or 1,25(OH)(2)D-3 resulted in additive increases in activity, Band shift analyses showe d 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 RX R led to retardation of each of the labeled probes, Treatment of nontr ansfected CV-1 cells with retinoids or 1,25(OH)(2)D-3 resulted in indu ction of 24(OH)ase RNA, and ligand combinations led to increased RNA l evels, These data imply that either dr both of the heterodimer partner s can be occupied with ligand to induce this enzyme, with dual recepto r occupation leading to increased activation.