Down-Regulation of the Myoinositol Transporter SMIT by JAK2

Abstract

Background/Aims: Janus-activated kinase-2 JAK2 is activated by energy depletion and hyperosmotic shock and modifies the activity of several Na⁺ coupled transporters. The Na⁺ coupled osmolyte transporter SMIT (myoinositol transporter) is upregulated by osmotic shock and downregulated by energy depletion. The present study thus explored whether JAK2 contributes to the regulation of SMIT activity. Methods: To this end, cRNA encoding SMIT was injected into Xenopus oocytes with or without additional injection of cRNA encoding wild type JAK2, constitutively active ^V617FJAK2 or inactive ^K882EJAK2. Inositol-induced current (I_SMIT) was determined by dual electrode voltage clamp and taken as measure for electrogenic inositol transport. Results: No appreciable I_SMIT was observed in water injected oocytes. In SMIT expressing oocytes I_SMIT was significantly decreased by additional coexpression of JAK2 or ^V617FJAK2, but not by coexpression of ^K882EJAK2. According to kinetic analysis coexpression of JAK2 decreased maximal I_SMIT without significantly modifying the concentration required for halfmaximal I_SMIT (K_M). In oocytes expressing both, SMIT and JAK2, ISMIT was gradually increased by JAK2 inhibitor AG490 (40 µM). Disruption of carrier insertion with brefeldin A (5 µM) was followed by a decline of I_SMIT to a similar extent in Xenopus oocytes expressing SMIT with JAK2 and in Xenopus oocytes expressing SMIT alone, suggesting that JAK2 did not affect carrier stability in the cell membrane. Conclusion: JAK2 contributes to the regulation of the inositol transporter SMIT.