1,25-Dihydroxyvitamin D₃ Regulation of Fibroblast Growth Factor-23 Expression in Bone Cells: Evidence for Primary and Secondary Mechanisms Modulated by Leptin and Interleukin-6

Fibroblast growth factor-23 (FGF23) is a circulating hormone that acts to correct hyperphosphatemic states by inhibiting renal phosphate reabsorption and to prevent hypervitaminosis D by feedback repressing 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) biosynthesis. FGF23 gene expression in the osteoblast/osteocyte is induced by the nuclear vitamin D receptor (VDR) bound to 1,25(OH)₂D₃, but cycloheximide sensitivity of this induction suggests that it may occur largely via secondary mechanisms requiring cooperating transcription factors. We therefore sought to identify 1,25(OH)₂D₃-regulated transcription factors that might impact FGF23 expression. Although neither leptin nor interleukin-6 (IL-6) alone affects FGF23 expression, leptin treatment was found to potentiate 1,25(OH)₂D₃ upregulation of FGF23 in UMR-106 cells, whereas IL-6 treatment blunted this upregulation. Genomic analyses revealed conserved binding sites for STATs (signal transduction mediators of leptin and IL-6 action) along with transcription factor ETS1 in human and other mammalian FGF23 genes. Further, STAT3, STAT1, ETS1, and VDR mRNAs were induced in a dose-dependent manner by 1,25(OH)₂D₃ in UMR-106 cells. Bioinformatic analysis identified nine potential VDREs in a genomic interval containing human FGF23. Six of the putative VDREs were capable of mediating direct transcriptional activation of a heterologous reporter gene when bound by a 1,25(OH)₂D₃-liganded VDR complex. A model is proposed wherein 1,25(OH)₂D₃ upregulates FGF23 production directly via multiple VDREs and indirectly via induction of STAT3, ETS1, and VDR transcription factors that are then activated via cell surface and intracellular signaling to cooperate in the induction of FGF23 through DNA looping and generation of euchromatin architecture.