Genomic actions of 1,25-dihydroxyvitamin D₃

Recent studies have identified a heterodimer of the vitamin D receptor (VDR) and the retinoid X receptor (RXR) as the active complex for mediating positive transcriptional effects of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], the active hormonal form of vitamin D. The VDR-RXR heterodimer has been shown to bind to direct repeat vitamin D-responsive elements (VDREs) upstream of positively controlled genes in the target tissues for vitamin D, including bone (osteocalcin, osteopontin, and β₃ integrin), kidney (24-hydroxylase) and intestine (calbindin). Residues that participate in heterodimer formation have been identified in the C-terminal hormone-binding domain by analysis of VDR mutants. The role of the 1,25(OH)₂D₃ ligand in transcriptional activation by the VDR-RXR heterodimer is not entirely clear, but studies of two natural VDR mutants suggest that the binding of both hormone and RXR are required to induce a receptor conformation that is competent to activate transcription. A final level of complexity is added by recent observations that VDR is modified by phosphorylation. Thus, the VDR-mediated action of 1,25(OH)₂D₃ is now known to involve multiple factors that may provide a conceptual basis for future understanding of the tissue-specific genomic effects of 1,25(OH)₂D₃.