SELECTIVE EFFECTS OF LIGANDS ON VITAMIN-D-3-RECEPTOR-MEDIATED AND RETINOID-X-RECEPTOR-MEDIATED GENE ACTIVATION IN-VIVO

Steroid/nuclear hormone receptors are ligand-regulated transcription f actors that play key roles in cell regulation, differentiation, and on cogenesis. Many nuclear receptors, including the human 1,25-dihydroxyv itamin D-3 receptor (VDR), bind cooperatively to DNA either as homodim ers or as heterodimers with the 9-cis retinoic acid (RA) receptor (ret inoid X receptor [RXR]), We have previously reported that the ligands for VDR and RXR can differentially modulate the affinity of the recept ors' interaction with DNA in vitro, primarily by modulating the dimeri zation status of these receptors, These experiments suggested a comple x interaction between VDR and RXR and their respective ligands on indu cible target genes in vivo, To examine these effects in cells, we used a transient-transfection strategy whereby we simultaneously introduce d two different reporter plasmids that are selectively inducible by ea ch ligand, Although VDR can bind as a homodimer to the osteopontin gen e vitamin D response element, we find that a RXR-VDR heterodimer must be the transactivating species from the element in vivo, since RXR enh ances and 9-cis RA and other RXR-specific ligands attenuate this induc tion, Conversely, when VDR is overexpressed, vitamin D-3 attenuates 9- cis RA induction from an RXR-responsive element, These effects, howeve r, appear to be very sensitive to both the relative ratios of the two receptors and their respective target elements, Functional RXR-VDR com plexes are strictly dependent on their DNA-binding polarity, Chimeric versions of VDR and RXR were also constructed to examine the putative activities of homodimeric receptors; a VDR chimera can transactivate i n the absence of RXR, demonstrating that VDR has intrinsic transactiva tion properties, Taken together, these results establish a complex, se nsitive cross talk in vivo between two ligands and their receptors tha t signal through two distinct endocrine pathways.