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This Article Full Text Full Text (PDF) All Versions of this Article: 586/6/1539    most recent jphysiol.2007.146191v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Klaus, F. Articles by Boehmer, C. PubMed PubMed Citation Articles by Klaus, F. Articles by Boehmer, C. Related Collections Cellular

¹ Department of Physiology I, University of Tübingen, Germany
² Department of Molecular Biology, University of Duisburg - Essen, Germany

Mechanisms of regulatory cell volume increase following cell shrinkage include accumulation of organic osmolytes such as betaine, taurine, sorbitol, glycerophosphorylcholine (GPC) and myo-inositol. Myo-inositol is taken up by the sodium-myo-inositol-transporter SMIT1 (SLC5A3) expressed in a wide variety of cell types. Hypertonicity induces the transcription of the SMIT1 gene upon binding of the transcription factor tonicity enhancer binding protein (TonEBP) to tonicity responsive enhancers (TonE) in the SMIT1 promoter region. However, little is known about post-translational regulation of the carrier protein. In this study we show that SMIT1 is modulated by the serum- and glucocorticoid-inducible kinase SGK1, a protein genomically up-regulated by hypertonicity. As demonstrated by two-electrode voltage-clamp in the Xenopus oocyte expression system, SMIT1-mediated myo-inositol-induced currents are up-regulated by coexpression of wild type SGK1 and constitutively active ^S422DSGK1 but not by inactive ^K127NSGK1. The increase in SMIT1 activity is due to an elevated cell surface expression of the carrier while its kinetic properties remain unaffected. According to the decay of SMIT1 activity in the presence of brefeldin A, SGK1 stabilizes the SMIT1 protein in the plasma membrane. The SGK isoforms SGK2, SGK3 and the closely related protein kinase B (PKB) are similarly capable of activating SMIT1 activity. SMIT1-mediated currents are decreased by coexpression of the ubiquitin-ligase Nedd4-2, an effect counteracted by additional coexpression of SGK1. In conclusion, the present observations disclose SGK isoforms and protein kinase B as novel regulators of SMIT1 activity.

(Received 4 October 2007; accepted after revision 17 January 2008; first published online 17 January 2008)
Corresponding author F. Lang: Physiologisches Institut der Universität Tübingen, Gmelinstr. 5, D-72076 Tübingen, Germany. Email: florian.lang{at}uni-tuebingen.de