Acidosis-Sensing Glutamine Pump SNAT2 Determines Amino Acid Levels and Mammalian Target of Rapamycin Signalling to Protein Synthesis in L6 Muscle Cells

Wasting of lean tissue as a consequence of metabolic acidosis is a serious problem in patients with chronic renal failure. A possible contributor is inhibition by low pH of the System A (SNAT2) transporter, which carries the amino acid L-Glutamine into muscle cells. The aim of this study was to determine the effect of selective SNAT2 inhibition on intracellular amino acid profiles and amino acid-dependent signalling through mTOR in L6 skeletal muscle cells. Inhibition of SNAT2 with the selective competitive substrate methylaminoisobutyrate (MeAIB), or metabolic acidosis (pH 7.1), or silencing SNAT2 expression with small interfering RNAs, all depleted intracellular L-Glutamine. SNAT2 inhibition also indirectly depleted other amino acids whose intracellular concentrations are maintained by the L-Glutamine gradient across the plasma membrane, notably the anabolic amino acid L-Leu. Consequently SNAT2 inhibition strongly impaired signalling through mTOR to ribosomal protein (rp) S6 kinase, rp S6, and 4E-BP1, leading to impairment of protein synthesis comparable with that induced by Rapamycin. It is concluded that even though SNAT2 is only one of several L-Glutamine transporters in muscle, it may determine intracellular anabolic amino acid levels, regulating the amino acid signalling which affects protein mass, nucleotide/nucleic acid metabolism and cell growth.