The BCAA, leucine, stimulates protein synthesis in skeletal muscle in part through enhanced initiation of mRNA translation. However, understanding how leucine regulates protein synthesis remains elusive. The intent of the present investigation was to examine the effect of leucine, independent of other regulatory agents, on key events in translation initiation in skeletal muscle and to elucidate the extent to which signaling through the mammalian target of rapamycin (mTOR) accounts for the effect of the amino acid on protein synthesis. Hindlimb preparations from postabsorptive rats were perfused with medium containing food-deprived (1X) or superphysiologic (10X) concentrations of leucine with all other amino acids at 1 X concentration. Protein synthesis was significantly greater in both gastrocnemius and soleus perfused with 10X compared with 1X leucine. The stimulatory effects of leucine on protein synthesis were unaffected by a Specific inhibitor of PI3-kinase (LY 294002). Moreover, signaling through mTOR, as monitored by the phosphorylation status of eukaryotic initiation factor (elF)4E binding protein-1 (4E-BP1) or the 70-kDa ribosomal protein S6 kinase (S6K1), was not further enhanced by 1 OX compared with 1 X leucine. However, binding of elF4E to elF4G and eIF4G(Ser-1 108) phosphorylation in the elF4E immunoprecipitate were enhanced as was elF4G(Ser-1 108) phosphorylation in the total tissue extract after perfusion with medium containing 1 OX leucine. Collectively, these observations illustrate an experimental model whereby leucine in the absence of other regulatory agents stimulates elF4-E - elF4G assembly and protein synthesis directly in skeletal muscle, possibly by augmenting phosphorylation of elF4G through a signaling pathway independent of mTOR.
