The role of bone in phosphate metabolism.

Fibroblast growth factor 23 (FGF23) is a humoral factor that is produced by osteocytes and reduces serum phosphate and 1,25-dihydroxyvitamin D levels by acting on kidney through some FGF receptor and Klotho. Excessive action of FGF23 results in several hypophosphatemic diseases characterized by impaired renal tubular phosphate reabsorption. In contrast, deficient action of FGF23 causes familial hyperphosphatemic tumoral calcinosis with enhanced renal tubular phosphate reabsorption. In addition, FGF23 null mice also show hyperphosphatemia. The production and circulatory level of FGF23 seem to be tightly regulated while the detailed mechanism of this regulation remains to be clarified. These results indicate that FGF23 is a physiological factor working as a hormone produced by bone. The discovery of FGF23 has revealed the possibility that bone produces several humoral factors to communicate with other organs.