Fibroblast Growth Factor 23: A New Dimension to Diseases of Calcium-Phosphorus Metabolism

Traditionally, control of phosphorus in the body has been considered secondary to the tighter control of calcium by parathyroid hormone and vitamin D. However, over the past decade, substantial advances have been made in understanding the control of phosphorus by the so-called phosphatonin system, the lynchpin of which is fibroblast growth factor 23 (FGF23). FGF23 binds to the klotho/FGFR1c receptor complex in renal tubular epithelial cells, leading to upregulation of Na/P-i cotransporters and subsequent excretion of phosphorus from the body. In addition, FGF23 inhibits parathyroid hormone and the renal 1 alpha-hydroxylase enzyme, while it stimulates 24-hydroxylase, leading to decreased 1,25-dihydroxyvitamin D-3. FGF23 is intimately involved in the pathogenesis of a number of diseases, particularly the hereditary hypophosphatemic rickets group and chronic kidney disease, and is a target for the development of new treatments in human medicine. Little work has been done on FGF23 or the other phosphatonins in veterinary medicine, but increases in FGF23 are seen with chronic kidney disease in cats, and increased FGF23 expression has been found in soft tissue sarcomas in dogs.