Osteo-renal cross-talk and phosphate metabolism by the FGF23-Klotho system

Phosphate is widely distributed in the body and an adequate balance is required for maintaining essential cellular and organ functions. Dysregulation of phosphate balance, either in the form of hypophosphatemia or hyperphosphatemia can induce disorders ranging from rickets/osteomalacia to cardiovascular calcification. A physiologic phosphate balance is delicately maintained by multiorgan cross-talks among the intestine, kidney, and bone. Sodium-dependent phosphate (Na/Pi) cotransporters present in the intestine and kidney play a major role in phosphate absorption and reabsorption, according to the body's demand. Some of the calcium regulating factors, including parathyroid hormone and vitamin D can influence the activities of Na/Pi cotransporters, and thereby can affect phosphate balance. In addition, molecular analysis of the unexplained hypophosphatemic diseases, including autosomal-dominant hypophosphatemic rickets and tumor-induced osteomalacia has led to the identification of fibroblast growth factor 23 (FGF23). Subsequent studies have documented that bone-derived FGF23 and kidney-derived klotho can form an endocrine network to control urinary phosphate excretion. Studies have also documented negative effect of FGF23/klotho system on vitamin D metabolism and Na/Pi cotransporter activities. This article will summarize how the FGF23/klotho system might influence systemic phosphate metabolism, and consequences of its abnormal regulation will be briefly described.