Downregulation of Peptide Transporters PEPT1 and PEPT2 by Oxidative Stress Responsive Kinase OSR1

Background/Aims: OSR1 (oxidative-stress-responsive kinase 1) participates in the regulation of renal tubular ion transport, cell volume and blood pressure. Whether OSR1 contributes to the regulation of organic solute transport remained; however, elusive. The present study thus explored the OSR1 sensitivity of the peptide transporters PEPT1 and PEPT2. Methods: cRNA encoding PEPT1 or PEPT2 were injected into Xenopus oocytes without or with additional injection of cRNA encoding wild-type OSR1, WNK1 insensitive inactive ^T185AOSR1, constitutively active ^T185EOSR1, and catalytically inactive ^D164AOSR1. Electrogenic peptide (glycine-glycine) transport was determined by dual electrode voltage clamp, the abundance of hemagglutinin-tagged PEPT2 (PEPT2-HA) by chemiluminescence. Results: In Xenopus oocytes injected with cRNA encoding PEPT1 or PEPT2, but not in oocytes injected with water, the dipeptide gly-gly (2 mM) generated an appreciable inward current (I_gly-gly). Coexpression of OSR1 significantly decreased I_gly-gly in both PEPT1 and PEPT2 expressing oocytes. The effect of OSR1 coexpression on I_gly-gly in PEPT1 expressing oocytes was mimicked by coexpression of ^T185EOSR1, but not of ^D164AOSR1 or ^T185AOSR1. Kinetic analysis revealed that coexpression of OSR1 decreased maximal I_gly-gly. OSR1 further decreased the PEPT2-HA protein abundance in the cell membrane. Conclusion: OSR1 has the capacity to downregulate the peptide transporters PEPT1 and PEPT2 by decreasing the carrier protein abundance in the cell membrane.