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

Am J Physiol Endocrinol Metab. 2008 Sep;295(3):E595-604. doi: 10.1152/ajpendo.90411.2008. Epub 2008 Jun 24.

Abstract

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 GSK3beta 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Amino Acids / pharmacology*
  • Blood Glucose / metabolism
  • Blotting, Western
  • Dose-Response Relationship, Drug
  • Gene Expression / drug effects
  • Humans
  • Hypoglycemic Agents / pharmacology*
  • Insulin / blood
  • Insulin / pharmacology*
  • Male
  • Muscle Proteins / metabolism*
  • Muscle, Skeletal / drug effects*
  • Muscle, Skeletal / metabolism*
  • Phosphorylation
  • Proteasome Endopeptidase Complex / metabolism
  • Protein Kinases / metabolism
  • RNA / biosynthesis
  • RNA, Messenger / biosynthesis
  • RNA, Messenger / genetics
  • Regional Blood Flow / physiology
  • Reverse Transcriptase Polymerase Chain Reaction
  • Ribosomal Protein S6 Kinases, 70-kDa / metabolism
  • Signal Transduction / drug effects*
  • TOR Serine-Threonine Kinases
  • Ubiquitin-Protein Ligase Complexes / metabolism*

Substances

  • Amino Acids
  • Blood Glucose
  • Hypoglycemic Agents
  • Insulin
  • Muscle Proteins
  • RNA, Messenger
  • RNA
  • Ubiquitin-Protein Ligase Complexes
  • Protein Kinases
  • MTOR protein, human
  • TOR Serine-Threonine Kinases
  • Ribosomal Protein S6 Kinases, 70-kDa
  • Proteasome Endopeptidase Complex