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1 Molecular Nutrition, Institution of Health Biosciences,The University of Tokushima Graduate School, Tokushima, Japan
2 Tokushima, Japan; Molecular Nutrition, Institution of Health Biosciences,The University of Tokushima Graduate School, Tokushima, Japan
3 Department of Pathology and Tumor Biology, Graduate School of Medicine, Kyoto University, Japan
* To whom correspondence should be addressed. E-mail: miyamoto{at}nutr.med.tokushima-u.ac.jp.
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/Pi cotransporters in klotho mutant (kl/kl) mice. The kl/kl mice displayed hyperphosphatemia, high plasma 1,25(OH)2D3 levels, increased activity of the renal and intestinal sodium-dependent Pi cotransporters, and increased levels of the type IIa, type IIb, and type IIc transporter proteins when 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 when compared with wild-type mice. Further, plasma fibroblast growth factor 23 (FGF23) levels were 150-fold higher in kl/kl mice than in wild-type mice. Feeding of a low Pi diet induced the expression of klotho protein and decreased plasma FGF23 levels in kl/kl mice, while colchicine treatment experiments revealed evidence of abnormal membrane trafficking of the type IIa transporter in kl/kl mice. Finally, feeding of a low Pi diet resulted in increased type IIa Na/Pi 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 due to intestinal Pi uptake.
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