The physiological function of vitamin D which maintains phosphate homeostasis

Inorganic phosphate (Pi) plays essential roles in many biological processes. Blood Pi level should be maintained at a constant range because hypophosphatemia causes rickets/osteomalacia, whereas hyperphosphatemia causes cardiovascular events in chronic kidney disease (CKD) or hemodialysis patients. As a phosphaturic hormone, parathyroid hormone (PTH) or fibroblast growth factor (FGF) 23 strongly suppresses Pi reabsorption. 1,25-Dihydroxyvitamin D［1,25(OH) ₂D］ up-regulates Pi absorption in intestines, and regulates urinary Pi excretion through PTH or FGF23. <br> Activated FGF23 signal leads urinary Pi excretion, and hypophosphatemic rickets (FGF23-related hypophosphatemic rickets). Rickets characterized as vitamin D deficiency also associates with impaired renal Pi reabsorption in juvenile stage in addition to low Pi absorption in intestines. Now, burosumab (FGF23 neutralizing antibody) has a beneficent effect on FGF23-related hypophosphatemic rickets/osteomalacia. The antibody increases renal Pi reabsorption and 1,25(OH) ₂D production. On the other hand, treatment with vitamin D alone in an animal model of X-linked hypophosphatemic rickets can potentially increase renal Pi reabsorption, despite of excess FGF23, and improve hypophosphatemia and bone phenotypes. <br> CKD patients often accompany with mineral bone disorder (CKD-MBD) such as hyperparathyroidism, cardiovascular calcification, and osteodystrophy. Prevention and treatment for hyperphosphatemia or hypovitaminosis D would improve their QOL or mortality. <br> Thus, renal Pi reabsorption is important for bone and mineral homeostasis. We focus on the metabolism of Pi and vitamin D to understand the molecular mechanism for the onset and prevention of diseases associated with Pi and vitamin D.