Phosphatonins: physiological role and pathological changes.

Maintenance of proper serum phosphate concentrations is required for healthy life, and critical for normal skeletal development and integrity. Several hormones and regulatory factors such as vitamin D, parathyroid hormone (PTH), and the phosphatonins (FGF-23, sFRP-4, MEPE) among others, may play a role only in the long-term regulation of phosphorus homeostasis.FGF23 is part of a previously unrecognized hormonal bone-parathyroid-kidney axis. Its synthesis and secretion by osteocytes are positively regulated by 1,25(OH)2D and serum phosphorus and negatively by the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX), and the enzyme N-acetyl galactosamine trasferase 3 (PPGGalNacT3), encoded by GALnT3 gene, prevents its degradation.FGF23 requires Klotho protein as a coreceptor for high affinity binding to cognate FGF receptors (FGFRs). Mutations of any of FGF23, Klotho or GALnT3 genes can lead to a syndrome characterized by hyperphosphatemia, ectopic calcifications and recurrent long bone lesions with hyperostosis. Phosphatonin have been shown to be implicated in several common diseases involving kidney and mineral metabolism. FGF23 might also represent a promising putative marker for bone healing.