Effect of fibroblast growth factor-23 on phosphate transport in proximal tubules

BACKGROUND: Fibroblast growth factor-23 (FGF-23) has been implicated in the renal phosphate wasting in tumor-induced osteomalacia, X-linked hypophosphatemia, and autosomal-dominant hypophosphatemic rickets. METHODS: In this in vitro microperfusion study we examined if FGF23R176Q, a stable mutant of FGF-23, impairs phosphate transport in rabbit proximal convoluted and proximal straight tubules perfused in vitro. We also examined if heparin, a molecule that is known to facilitate binding of FGFs to their receptor was necessary for the action of FGF23R176Q on transport. RESULTS: In the presence of heparin, FGF23R176Q reduced phosphate transport from 10.8 +/- 2.0 to 9.9 +/- 1.9 pmol/mm/min in proximal convoluted tubules and 1.0 +/- 0.2 to 0.8 +/- 0.2 pmol/mm/min in proximal straight tubules (both P < 0.05). There was no effect of FGF23R176Q in the absence of heparin. Incubation of finely minced mouse renal cortical tissue in tissue culture media for 3 hours resulted in a reduction in brush border membrane vesicles (BBMV) sodium-dependent phosphate transport (NaPi-2A) protein abundance in the presence but not in the absence of heparin. CONCLUSION: These data demonstrate that the inhibition of phosphate transport by FGF23R176Q in vitro requires heparin. The action of FGF23R176Q is associated with a reduction in BBMV NaPi-2A protein abundance.