β-Klotho as a Negative Regulator of the Peptide Transporters PEPT1 and PEPT2

Abstract

Background/Aims: β-Klotho, a transmembrane protein expressed in several tissues including the brain and the kidney, is critically important for inhibition of 1,25(OH) 2 D 3 formation by FGF23. The extracellular domain of Klotho protein could be cleaved off, thus being released into blood or cerebrospinal fluid. Soluble klotho is a β-glucuronidase participating in the regulation of several ion channels and carriers. The present study explored the effect of β-Klotho protein on the peptide transporters PEPT1 and PEPT2. Methods: cRNA encoding PEPT1 or PEPT2 was injected into Xenopus laevis oocytes and glycine-glycine (2 mM)-induced inward current (I Gly ) taken as measure of glycine-glycine transport. Measurements were made without or with prior 24 h treatment with soluble β-Klotho protein (30 ng/ml) in the absence and presence of β-glucuronidase inhibitor D-saccharic acid 1,4-lactone monohydrate (DSAL,10 μM). Ussing chamber experiments were employed to determine electrogenic peptide transport across intestinal epithelia of klotho deficient (kl -/- ) and corresponding wild type (kl +/+ ) mice. Results: I Gly was observed in PEPT1 and in PEPT2 expressing oocytes but not in water injected oocytes. In both, PEPT1 and PEPT2 expressing oocytes I Gly was significantly decreased by treatment with soluble β-Klotho protein. As shown for PEPT1, β-klotho protein decreased significantly the maximal transport rate without significantly modifying the affinity of the carrier. The effect of β-Klotho on PEPT1 was reversed by DSAL. Intestinal I Gly was significantly larger in kl -/- than in kl +/+ mice. Conclusion: β-Klotho participates in the regulation of the peptide transporters PEPT1 and PEPT2.