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Inhibition of SNAT2 by Metabolic Acidosis Enhances Proteolysis in Skeletal Muscle

Kate Evans^*, Zeerak Nasim^*, Jeremy Brown^*, Emma Clapp, Amin Amin^*, Bin Yang^*, Terence P. Herbert and Alan Bevington^*

^* Department of Infection, Immunity and Inflammation, University of Leicester, John Walls Renal Unit, Leicester General Hospital, and Department of Cell Physiology and Pharmacology, University of Leicester, Maurice Shock Medical Sciences Building, Leicester, and School of Sport and Exercise Sciences, Loughborough University, Loughborough, United Kingdom

Correspondence: Dr. Alan Bevington, Department of Infection, Immunity and Inflammation, University of Leicester, John Walls Renal Unit, Leicester General Hospital, Leicester LE5 4PW, UK. Phone: +44-116-258-8041; Fax: +44-116-258-4764; E-mail: ab74{at}leicester.ac.uk

Received for publication October 16, 2007. Accepted for publication May 26, 2008.

Insulin resistance is a major cause of muscle wasting in patients with ESRD. Uremic metabolic acidosis impairs insulin signaling, which normally suppresses proteolysis. The low pH may inhibit the SNAT2 l-Glutamine (L-Gln) transporter, which controls protein synthesis via amino acid–dependent insulin signaling through mammalian target of rapamycin (mTOR). Whether SNAT2 also regulates signaling to pathways that control proteolysis is unknown. In this study, inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate or metabolic acidosis (pH 7.1) depleted intracellular L-Gln and stimulated proteolysis in cultured L6 myotubes. At pH 7.1, inhibition of the proteasome led to greater depletion of L-Gln, indicating that amino acids liberated by proteolysis sustain L-Gln levels when SNAT2 is inhibited by acidosis. Acidosis shifted the dose-response curve for suppression of proteolysis by insulin to the right, confirming that acid increases proteolysis by inducing insulin resistance. Blocking mTOR or phosphatidylinositol-3-kinase (PI3K) increased proteolysis, indicating that both signaling pathways are involved in its regulation. When both mTOR and PI3K were inhibited, methylaminoisobutyrate or acidosis did not stimulate proteolysis further. Moreover, partial silencing of SNAT2 expression in myotubes and myoblasts with small interfering RNA stimulated proteolysis and impaired insulin signaling through PI3K. In conclusion, SNAT2 not only regulates mTOR but also regulates proteolysis through PI3K and provides a link among acidosis, insulin resistance, and protein wasting in skeletal muscle cells.

Copyright © 2008 by the American Society of Nephrology. Online ISSN: 1533-3450 Print ISSN: 1046-6673