title: Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway.
abstract: The objectives of the present study were twofold: 1) to determine whether leucine is unique among the branched-chain amino acids (BCAA) in its ability to stimulate protein synthesis in skeletal muscle of food-deprived rats; and 2) to investigate whether changes in muscle protein synthesis after leucine administration involve a signaling pathway that includes the protein kinase mammalian target of rapamycin (mTOR). In the first set of experiments, food-deprived (18 h) male rats (200 g) were orally administered saline or 270 mg valine, isoleucine or leucine. In the second set of experiments, food-deprived rats were injected intravenously with rapamycin (0.75 mg/kg), a specific inhibitor of mTOR, before leucine administration. Only leucine stimulated protein synthesis in skeletal muscle above saline-treated controls (P: < 0.05). Furthermore, leucine was most effective among the BCAA at enhancing phosphorylation of eukaryotic initiation factor (eIF), 4E binding protein 1 (4E-BP1) and the 70-kDa ribosomal protein S6 kinase (S6K1). Leucine-dependent hyperphosphorylation of 4E-BP1 increased the availability of eIF4E to form the active eIF4G.eIF4E complex. To a lesser extent, isoleucine also enhanced phosphorylation of 4E-BP1 and S6K1. Rapamycin inhibited protein synthesis in both leucine-treated and food-deprived rats. Additionally, rapamycin prevented the stimulatory effects of leucine on eIF4E availability for binding eIF4G and inhibited leucine-dependent phosphorylation of S6K1. The data demonstrate that leucine is unique among the BCAA in its ability to stimulate protein synthesis in muscle of food-deprived rats. We show for the first time that leucine-dependent stimulation of translation initiation in vivo occurs via a rapamycin-sensitive pathway.
authors.affiliation: Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, PA 17033, USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Anthony
authors.forename: F
authors.initials: F
authors.lastname: Yoshizawa
authors.forename: T G
authors.initials: TG
authors.lastname: Anthony
authors.forename: T C
authors.initials: TC
authors.lastname: Vary
authors.forename: L S
authors.initials: LS
authors.lastname: Jefferson
authors.forename: S R
authors.initials: SR
authors.lastname: Kimball
mesh_terms: D000818:Animals; D002352:Carrier Proteins; D004591:Electrophoresis, Polyacrylamide Gel; D039561:Eukaryotic Initiation Factor-4E; D005502:Food; D005508:Food Deprivation; D047908:Intracellular Signaling Peptides and Proteins; D007532:Isoleucine / Q000494:pharmacology; D007930:Leucine / Q000494:pharmacology*; D008297:Male; D009124:Muscle Proteins / Q000096:biosynthesis* / Q000235:genetics; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism*; D010448:Peptide Initiation Factors / Q000378:metabolism; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D014176:Protein Biosynthesis / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D020123:Sirolimus / Q000494:pharmacology*; D014633:Valine / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D039561:Eukaryotic Initiation Factor-4E; D047908:Intracellular Signaling Peptides and Proteins; D009124:Muscle Proteins; D010448:Peptide Initiation Factors; D010750:Phosphoproteins; D007532:Isoleucine; D019893:Ribosomal Protein S6 Kinases; D007930:Leucine; D014633:Valine; D020123:Sirolimus