Fibroblast growth factor 23--structure, function and role in kidney diseases.

The fibroblast growth factor (FGF) family comprises a number of polypeptides which share a common homology core region. FGF-23, produced by osteoblasts and osteocytes, belongs to the FGF-19 subfamily and serves as the main phosphatonine. Two forms of circulating FGF-23 are detectable in serum: full-length FGF-23--intact FGF-23 (iFGF-23), which is biologically active, and the inactive C-terminal FGF-23 (cFGF-23). FGF-23 with a coreceptor (Klotho protein) inhibits renal phosphate reabsorption and synthesis of calcitriol by reducing 1alpha-hydroxylase (CYP27B1) activity, reducing vitamin D-dependent phosphate intestinal absorption. High phosphorus intake, 1,25-dihydroxyvitamin D3 and PTH are the main stimuli for FGF-23 secretion. Impaired FGF-23 metabolism is involved in phosphate disturbances manifesting as rickets or osteomalacia or increased tissue calcinosis. FGF-23 may be also produced by some tumors leading to hypophosphatemia. Both cFGF-23 and iFGF-23 concentrations start to increase with mild impairment of the glomerular filtration rate in stage 2 or 3 of chronic kidney disease (CKD) as a consequence of the increased FGF-23 production. It seems that enhanced FGF-23 secretion may constitute a protective mechanism against enhanced phosphate accumulation in the early stages of CKD. However, it may lead to calcitriol deficiency and escalation of secondary hyperparathyroidism. Increased FGF-23 level is supposed to be an independent factor increasing mortality of CKD patients. There is ambiguous data if FGF-23 only reflects disturbances in calcium-phosphate metabolism or if it exerts a detrimental effect itself by diminishing calcitriol synthesis, inducing cell proliferation or acting through low-affinity, Klotho-independent receptors in the heart and endothelium. So far, little evidence supports direct FGF-23 toxicity.