Vitamin D Receptor Mediates a Myriad of Biological Actions Dependent on Its 1,25-Dihydroxyvitamin D Ligand: Distinct Regulatory Themes Revealed by Induction of Klotho and Fibroblast Growth Factor-23.

The hormonal vitamin D metabolite, 1,25-dihydroxyvitamin D [1,25(OH)2D], produced in kidney, acts in numerous end organs via the nuclear vitamin D receptor (VDR) to trigger molecular events that orchestrate bone mineral homeostasis. VDR is a ligand-controlled transcription factor that obligatorily heterodimerizes with retinoid X receptor (RXR) to target vitamin D responsive elements (VDREs) in the vicinity of vitamin D-regulated genes. Circulating 1,25(OH)2D concentrations are governed by PTH, an inducer of renal D-hormone biosynthesis catalyzed by CYP27B1 that functions as the key player in a calcemic endocrine circuit, and by fibroblast growth factor-23 (FGF23), a repressor of the CYP27B1 renal enzyme, creating a hypophosphatemic endocrine loop. 1,25(OH)2D/VDR-RXR acts in kidney to induce Klotho (a phosphaturic coreceptor for FGF23) to correct hyperphosphatemia, NPT2a/c to correct hypophosphatemia, and TRPV5 and CaBP28k to enhance calcium reabsorption. 1,25(OH)2D-liganded VDR-RXR functions in osteoblasts/osteocytes by augmenting RANK-ligand expression to paracrine signal osteoclastic bone resorption, while simultaneously inducing FGF23, SPP1, BGLP, LRP5, ANK1, ENPP1, and TNAP, and conversely repressing RUNX2 and PHEX expression, effecting localized control of mineralization to sculpt the skeleton. Herein, we document the history of 1,25(OH)2D/VDR and summarize recent advances in characterizing their physiology, biochemistry, and mechanism of action by highlighting two examples of 1,25(OH)2D/VDR molecular function. The first is VDR-mediated primary induction of Klotho mRNA by 1,25(OH)2D in kidney via a mechanism initiated by the docking of liganded VDR-RXR on a VDRE at -35 kb in the mouse Klotho gene. In contrast, the secondary induction of FGF23 by 1,25(OH)2D in bone is proposed to involve rapid nongenomic action of 1,25(OH)2D/VDR to acutely activate PI3K, in turn signaling the induction of MZF1, a transcription factor that, in cooperation with c-ets1-P, binds to an enhancer element centered at -263 bp in the promoter-proximal region of the mouse fgf23 gene. Chronically, 1,25(OH)2D-induced osteopontin apparently potentiates MZF1.