title: Down-regulation of the myoinositol transporter SMIT by JAK2.
abstract: OBJECTIVE
  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 (V617F)JAK2 or inactive (K882E)JAK2. 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 (V617F)JAK2, but not by coexpression of (K882E)JAK2. 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.
  
  
  CONCLUSIONS
  JAK2 contributes to the regulation of the inositol transporter SMIT.
authors.affiliation: Department of Physiology I, University of Tübingen, Tübingen, Germany.
authors.forename: Zohreh
authors.initials: Z
authors.lastname: Hosseinzadeh
authors.forename: Shefalee K
authors.initials: SK
authors.lastname: Bhavsar
authors.forename: Florian
authors.initials: F
authors.lastname: Lang
mesh_terms: D000818:Animals; D001692:Biological Transport; D002478:Cells, Cultured; D015536:Down-Regulation; D006360:Heat-Shock Proteins / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D007294:Inositol / Q000378:metabolism; D053614:Janus Kinase 2 / Q000037:antagonists & inhibitors / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials; D055550:Protein Stability; D027981:Symporters / Q000235:genetics* / Q000378:metabolism; D020032:Tyrphostins / Q000494:pharmacology; D014981:Xenopus
chemical_list: D006360:Heat-Shock Proteins; D027981:Symporters; D020032:Tyrphostins; C095512:alpha-cyano-(3,4-dihydroxy)-N-benzylcinnamide; C073884:SLC5A3 protein, human; D007294:Inositol; C507924:JAK2 protein, human; D053614:Janus Kinase 2