Disassociation between the effects of amino acids and insulin on signaling, ubiquitin ligases, and protein turnover in human muscle

• Published in: American journal of physiology: endocrinology and metabolism Vol. 295; no. 3; pp. E595 - E604
• Main Authors: Greenhaff, P. L, Karagounis, L. G, Peirce, N, Simpson, E. J, Hazell, M, Layfield, R, Wackerhage, H, Smith, K, Atherton, P, Selby, A, Rennie, M. J
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 01.09.2008
• Subjects: Adult; Amino acids; Amino Acids - pharmacology; Biochemistry; Blood Glucose - metabolism; Blotting, Western; Dose-Response Relationship, Drug; Enzymes; Gene Expression - drug effects; Human subjects; Humans; Hypoglycemic Agents - pharmacology; Insulin; Insulin - blood; Insulin - pharmacology; Male; Muscle Proteins - metabolism; Muscle, Skeletal - drug effects; Muscle, Skeletal - metabolism; Muscles; Muscular system; Phosphorylation; Proteasome Endopeptidase Complex - metabolism; Protein Kinases - metabolism; Protein synthesis; Proteins; Regional Blood Flow - physiology; Reverse Transcriptase Polymerase Chain Reaction; Ribosomal Protein S6 Kinases, 70-kDa - metabolism; RNA - biosynthesis; RNA, Messenger - biosynthesis; RNA, Messenger - genetics; Signal Transduction - drug effects; TOR Serine-Threonine Kinases; Ubiquitin-Protein Ligase Complexes - metabolism
• DOI: 10.1152/ajpendo.90411.2008
• ISSN: 0193-1849

1 Centre for Integrated Systems Biology and Medicine, 2 School of Biomedical Sciences, and 3 School of Graduate Entry Medicine and Health, University of Nottingham, Nottingham; and 4 Institute of Medical Sciences, University of Aberdeen, Aberdeen, United Kingdom Submitted 1 May 2008 ; accepted in final form 21 June 2008 We determined the effects of intravenous infusion of amino acids (AA) at serum insulin of 5, 30, 72, and 167 mU/l on anabolic signaling, expression of ubiquitin-proteasome components, and protein turnover in muscles of healthy young men. Tripling AA availability at 5 mU/l insulin doubled incorporation of [1- 13 C]leucine [i.e., muscle protein synthesis (MPS), P < 0.01] without affecting the rate of leg protein breakdown (LPB; appearance of d 5 -phenylalanine). While keeping AA availability constant, increasing insulin to 30 mU/l halved LPB ( P < 0.05) without further inhibition at higher doses, whereas rates of MPS were identical to that at 5 mU/l insulin. The phosphorylation of PKB Ser 473 and p70 S6k Thr 389 increased concomitantly with insulin, but whereas raising insulin to 30 mU/l increased the phosphorylation of mTOR Ser 2448 , 4E-BP1 Thr 37/46 , or GSK3β Ser 9 and decreased that of eEF2 Thr 56 , higher insulin doses to 72 and 167 mU/l did not augment these latter responses. MAFbx and proteasome C2 subunit proteins declined as insulin increased, with MuRF-1 expression largely unchanged. Thus increasing AA and insulin availability causes changes in anabolic signaling and amounts of enzymes of the ubiquitin-proteasome pathway, which cannot be easily reconciled with observed effects on MPS or LPB. muscle protein synthesis; muscle protein breakdown Address for reprint requests and other correspondence: P. Greenhaff, School of Biomedical Sciences, Centre for Integrated Systems Biology and Medicine, Univ. of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK (e-mail: paul.greenhaff{at}nottingham.ac.uk )