Stimulation of Na+ coupled phosphate transporter NaPiIIa by janus kinase JAK2

Abstract

BackgroundNa+ coupled phosphate transporter NaPiIIa is the main carrier accomplishing phosphate transport across the apical cell membrane of proximal renal tubules and thus renal tubular phosphate reabsorption. The carrier is regulated by a wide variety of hormones and cellular signaling molecules. Hormones stimulating renal tubular phosphate transport and thus leading to hyperphosphatemia include growth hormone. Signaling of growth hormone involves activation of janus-activated kinase-2 JAK2, which has previously been shown to participate in the regulation of several Na+ coupled transporters. Experiments exploring the effect of JAK2 on phosphate transport have, however, never been reported. The present study thus addressed the effect of JAK2 on NaPiIIa. MethodscRNA encoding NaPiIIa was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild type JAK2, the gain of function mutant JAK2V617F or inactive JAK2K882E. Phosphate-induced current (INaPi) reflecting electrogenic phosphate transport was determined by two electrode voltage clamp. Moreover, NaPiIIa protein abundance in the cell membrane was determined by chemiluminescence. ResultsNo appreciable INaPi was observed in water injected oocytes or in oocytes expressing JAK2 alone. In NaPiIIa expressing oocytes INaPi was significantly increased by additional expression of JAK2 or JAK2V617F, but not by coexpression of JAK2K882E. In oocytes expressing both, NaPiIIa and JAK2, INaPi was gradually decreased by JAK2 inhibitor AG490 (40μM). Coexpression of NaPiIIa and JAK2 or JAK2V617F, but not of JAK2K882E increased NaPiIIa protein abundance in the cell membrane. Disruption of carrier protein insertion with Brefeldin A (5μM) was followed by a decline of INaPi to a similar extent in Xenopus oocytes expressing NaPiIIa with JAK2 and in Xenopus oocytes expressing NaPiIIa alone, suggesting that JAK2 did not affect carrier stability in the cell membrane. ConclusionJAK2 contributes to the regulation of phosphate transporter NaPiIIa.