VITAMIN-D-3 RETINOID-X-RECEPTOR DIMERIZATION, DNA-BINDING, AND TRANSACTIVATION ARE DIFFERENTIALLY AFFECTED BY ANALOGS OF 1,25-DIHYDROXYVITAMIN D-3

A number of analogs of 1,25-dihydroxyvitamin D-3 [1,25-(OH)(2)D-3] hav e been synthesized that act as more potent inducers of cellular differ entiation and inhibitors of cell growth than the natural ligand; at th e same time, many of the analogs have reduced hypercalcemic properties . This combination makes these compounds attractive candidates for cli nical use. The mechanism by which the analogs act, however, is unclear . Potentially, the analogs could be taken up more readily, be more slo wly catabolized, or have higher binding affinities for the vitamin D r eceptor (VDR). Analogs of 1,25-(OH)(2)D-3 could also differentially mo dulate one or more of the activities of VDR, namely dimerization, DNA binding, and/or transcriptional regulation. To directly examine this l atter possibility, we used a sensitive assay for the kinetics of dimer ization and DNA binding, surface plasmon resonance, and report here th at three 1,25-(OH)(2)D-3 analogs, 1,25-(OH)(2)-16-ene-23-yne-D-3, 1,25 -(OH)(2)-16-ene-23-yne-26,27-di homo-D-3, and 1,25-(OH)(2)-26,27-hexaf luoro-16-ene-23-yne-D-3, all confer distinct rate and equilibrium cons tants for VDR-retinoid X receptor heterodimerization and DNA binding t o a specific vitamin D response element relative to the natural ligand . In response to the hexafluoro analog, the apparent K-d for DNA bindi ng by VDR was significantly lower than that for 1,25-(OH)(2)D-3, and c orrespondingly, in vivo transactivation from a responsive reporter was greater. Interestingly, solution heterodimerization was not affected by this analog. These results suggest that vitamin D analogs do indeed confer biological effects by acting directly and differentially at th e level of VDR, and that specific vitamin D analogs can act on distinc t receptor functions.