Phosphaturic action of fibroblast growth factor 23 in Npt2 null mice.

In the present study, we evaluated the roles of type II and type III sodium-dependent P(i) cotransporters in fibroblast growth factor 23 (FGF23) activity by administering a vector encoding FGF23 with the R179Q mutation (FGF23M) to wild-type (WT) mice, Npt2a knockout (KO) mice, Npt2c KO mice, and Npt2a(-/-)Npt2c(-/-) mice (DKO mice). In Npt2a KO mice, FGF23M induced severe hypophosphatemia and markedly decreased the levels of Npt2c, type III Na-dependent P(i) transporter (PiT2) protein, and renal Na/P(i) transport activity. In contrast, in Npt2c KO mice, FGF23M decreased plasma phosphate levels comparable to those in FGF23M-injected WT mice. In DKO mice with severe hypophosphatemia, FGF23M administration did not induce an additional increase in urinary phosphate excretion. FGF23 administration significantly decreased intestinal Npt2b protein levels in WT mice but had no effect in Npt2a, Npt2c, and DKO mice, despite marked suppression of plasma 1,25(OH)(2)D(3) levels in all the mutant mice. The main findings were as follow: 1) FGF23-dependent phosphaturic activity in Npt2a KO mice is dependent on renal Npt2c and PiT-2 protein; 2) in DKO mice, renal P(i) reabsorption is not further decreased by FGF23M, but renal vitamin D synthesis is suppressed; and 3) downregulation of intestinal Npt2b may be mediated by a factor(s) other than 1,25(OH)(2)D(3). These findings suggest that Npt2a, Npt2c, and PiT-2 are necessary for the phosphaturic activity of FGF23. Thus complementary regulation of Npt2 family proteins may be involved in systemic P(i) homeostasis.

Correlation between hyperphosphatemia and type II Na-Pi cotransporter activity in klotho mice.

Recent studies have demonstrated that klotho protein plays a role in calcium/phosphate homeostasis. The goal of the present study was to investigate the regulation of Na-P(i) cotransporters in klotho mutant (kl/kl) mice. The kl/kl mice displayed hyperphosphatemia, high plasma 1,25(OH)(2)D(3) levels, increased activity of the renal and intestinal sodium-dependent P(i) cotransporters, and increased levels of the type IIa, type IIb, and type IIc transporter proteins compared with wild-type mice. Interestingly, transcript levels of the type IIa/type IIc transporter mRNA abundance, but not transcripts levels of type IIb transporter mRNA, were markedly decreased in kl/kl mice compared with wild-type mice. Furthermore, plasma fibroblast growth factor 23 (FGF23) levels were 150-fold higher in kl/kl mice than in wild-type mice. Feeding of a low-P(i) diet induced the expression of klotho protein and decreased plasma FGF23 levels in kl/kl mice, whereas colchicine treatment experiments revealed evidence of abnormal membrane trafficking of the type IIa transporter in kl/kl mice. Finally, feeding of a low-P(i) diet resulted in increased type IIa Na-P(i) cotransporter protein in the apical membrane in the wild-type mice, but not in kl/kl mice. These results indicate that hyperphosphatemia in klotho mice is due to dysregulation of expression and trafficking of the renal type IIa/IIc transporters rather than to intestinal P(i) uptake.