[Disorders of phosphate metabolism]

Abstract

Serum phosphate is maintained within a certain range by intestinal phosphate absorption, renal phosphate handling, and dynamic equilibrium with the intracellular phosphate or phosphate in bone. Of these, renal phosphate handling is believed to be the main determinant of the serum phosphate level at least in a chronic state. Most of the phosphate filtered from the glomeruli is reabsorbed in proximal tubules through type 2a and 2c sodium-phosphate co-transporters. Therefore, chronic hypophosphatemia and hyperphosphatemia are usually caused by changes in renal phosphate handling. Several humoral factors, including parathyroid hormone and insulin-like growth factor-I, have been known to affect proximal tubular phosphate reabsorption. In addition, fibroblast growth factor 23 (FGF23) was shown to inhibit phosphate reabsorption by suppressing the expression of type 2a and 2c sodium-phosphate co-transporters. FGF23 also reduces the circulatory 1,25-dihydroxyvitamin D [1,25 (OH)2D] level. FGF23 is produced by bone, especially by osteocytes, and works in the kidney by binding to the Klotho-FGF receptor complex. It has been shown that excess actions of FGF23 cause several kinds of hypophosphatemic rickets/osteomalacia with impaired proximal tubular phosphate reabsorption and a rather low 1,25 (OH)2D level. In contrast, deficient actions of FGF23 result in familial hyperphosphatemic tumoral calcinosis with enhanced proximal tubular phosphate reabsorption and high 1,25 (OH)2D. These results indicate that FGF23 is a hormone regulating phosphate and vitamin D metabolism. In addition, several hypophosphatemic and hyperphosphatemic diseases can be classified as endocrine diseases caused by the aberrant actions of FGF23. It is possible that some drugs that modulate the action of FGF23 can be novel therapeutic measures for abnormal phosphate metabolism in the future.