Glucose-mediated inactivation of AMP-activated protein kinase reduces the levels of L-type amino acid transporter 1 mRNA in C2C12 cells.

Branched chain amino acids (BCAAs) have protective effects against muscle atrophy. Although plasma BCAA concentrations are higher in patients with diabetes than in healthy subjects, diabetes is related to sarcopenia. We hypothesized that high glucose concentration reduces the quantity of BCAA transporters, and consequently, the effects of BCAAs are diminished despite their high levels. We examined whether glucose reduces the expression of L-type amino acid transporter 1 (LAT1), which transports neutral amino acids, including BCAA, in C2C12 myocytes. Glucose reduced LAT1 mRNA level by 80% in the C2C12 cells, compared with that in the glucose-free control cells. Regarding LAT1-related transporters, glucose also reduced the level of sodium-dependent neutral amino acid transporter 2 mRNA, but not that of 4F2 heavy chain. Although fructose reduced LAT1 mRNA levels, 2-deoxyglucose exhibited low effectiveness in reducing LAT1 mRNA level; galactose and mannitol had no effect. These results suggest a relationship between ATP produced during glycolysis and LAT1 mRNA levels. In fact, the AMP-activated protein kinase (AMPK) inhibitor dorsomorphin reduced LAT1 mRNA levels in the absence of glucose, whereas the AMPK activator 5-Aminoimidazole-4-carboxamide-1-β-d-ribofuranoside increased LAT1 mRNA levels even in the presence of glucose. Consistent with these findings, glucose reduced the levels of phospho-AMPKα (Thr172) compared with that in the glucose-free control. These findings indicate that glucose inactivates AMPK, leading to a reduction in LAT1 mRNA levels in the C2C12 cells. This glucose-induced reduction in LAT1 expression may explain the unresponsiveness to BCAA in the patients with diabetes.