1,25-dihydroxyvitamin D3/VDR-mediated induction of FGF23 as well as transcriptional control of other bone anabolic and catabolic genes that orchestrate the regulation of phosphate and calcium mineral metabolism

1.25-Dihydroxyvitamin D-3 (1,25D) is known primarily as a regulator of calcium, but 1,251) also promotes phosphate absorption from intestine, reabsorption from kidney, and bone mineral resorption. FGF23 is a newly discovered phosphaturic hormone that, like PTH, lowers serum phosphate by inhibiting renal reabsorption via Npt2a. We show that 1,251) strongly upregulates FGF23 in bone. FGF23 then represses 1 alpha-OHase activity in kidney, thus preventing spiraling induction of FGF23 by 1,251). We also report that LRP5, Runx2, TRPV6, and Npt2c, all anabolic toward bone, and RANKL, which is catabolic, are transcriptionally regulated by 1,251). This coordinated regulation together with that of FGF23 and PTH allows 1,251) to play a central role in maintaining calcium and phosphate homeostasis and bone metabolism. In the cases of LRP5. Runx2. TRPV6. and Npt2c we show that transcriptional regulation results at least in part from direct binding of VDR near the relevant gene promoter. Finally, because 1,251) induces FGF23, and FGF23 in turn represses 1,251) synthesis, a reciprocal relationship is established with FGF23 indirectly curtailing 1,25D-mediated intestinal absorption and counterbalancing renal reabsorption of phosphate. This newly revealed FGF23/1,25D/Pi axis is comparable in significance to phosphate and bone metabolism as the PTH/1,25D/Ca axis is to calcium homeostasis. (c) 2007 Elsevier Ltd. All rights reserved.