Chronic paraplegia-induced muscle atrophy downregulates the mTOR/S6K1 signaling pathway

J Appl Physiol (1985). 2008 Jan;104(1):27-33. doi: 10.1152/japplphysiol.00736.2007. Epub 2007 Sep 20.

Abstract

Ribosomal S6 kinase 1 (S6K1) is a downstream component of the mammalian target of rapamycin (mTOR) signaling pathway and plays a regulatory role in translation initiation, protein synthesis, and muscle hypertrophy. AMP-activated protein kinase (AMPK) is a cellular energy sensor, a negative regulator of mTOR, and an inhibitor of protein synthesis. The purpose of this study was to determine whether the hypertrophy/cell growth-associated mTOR pathway was downregulated during muscle atrophy associated with chronic paraplegia. Soleus muscle was collected from male Sprague-Dawley rats 10 wk following complete T(4)-T(5) spinal cord transection (paraplegic) and from sham-operated (control) rats. We utilized immunoprecipitation and Western blotting techniques to measure upstream [AMPK, Akt/protein kinase B (PKB)] and downstream components of the mTOR signaling pathway [mTOR, S6K1, SKAR, 4E-binding protein 1 (4E-BP1), and eukaryotic initiation factor (eIF) 4G and 2alpha]. Paraplegia was associated with significant soleus muscle atrophy (174 +/- 8 vs. 240 +/- 13 mg; P < 0.05). There was a reduction in phosphorylation of mTOR, S6K1, and eIF4G (P < 0.05) with no change in Akt/PKB or 4E-BP1 (P > 0.05). Total protein abundance of mTOR, S6K1, eIF2alpha, and Akt/PKB was decreased, and increased for SKAR (P < 0.05), whereas 4E-BP1 and eIF4G did not change (P > 0.05). S6K1 activity was significantly reduced in the paraplegic group (P < 0.05); however, AMPKalpha2 activity was not altered (3.5 +/- 0.4 vs. 3.7 +/- 0.5 pmol x mg(-1) x min(-1), control vs. paraplegic rats). We conclude that paraplegia-induced muscle atrophy in rats is associated with a general downregulation of the mTOR signaling pathway. Therefore, in addition to upregulation of atrophy signaling during muscle wasting, downregulation of muscle cell growth/hypertrophy-associated signaling appears to be an important component of long-term muscle loss.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • AMP-Activated Protein Kinases
  • Animals
  • Blotting, Western
  • Carrier Proteins / metabolism
  • Chronic Disease
  • Disease Models, Animal
  • Down-Regulation
  • Eukaryotic Initiation Factor-2 / metabolism
  • Eukaryotic Initiation Factor-4G / metabolism
  • Immunoprecipitation
  • Intracellular Signaling Peptides and Proteins
  • Male
  • Multienzyme Complexes / metabolism
  • Muscle, Skeletal / enzymology
  • Muscle, Skeletal / metabolism*
  • Muscle, Skeletal / pathology
  • Muscular Atrophy / enzymology
  • Muscular Atrophy / etiology
  • Muscular Atrophy / metabolism*
  • Muscular Atrophy / pathology
  • Paraplegia / complications*
  • Paraplegia / enzymology
  • Paraplegia / etiology
  • Paraplegia / metabolism
  • Paraplegia / pathology
  • Phosphoproteins / metabolism
  • Phosphorylation
  • Protein Kinases / metabolism*
  • Protein Serine-Threonine Kinases / metabolism
  • Proto-Oncogene Proteins c-akt / metabolism
  • Rats
  • Rats, Sprague-Dawley
  • Ribosomal Protein S6 Kinases / metabolism*
  • Signal Transduction*
  • Spinal Cord Injuries / complications*
  • Spinal Cord Injuries / enzymology
  • Spinal Cord Injuries / metabolism
  • Spinal Cord Injuries / pathology
  • TOR Serine-Threonine Kinases
  • Time Factors

Substances

  • Carrier Proteins
  • Eif4ebp1 protein, rat
  • Eukaryotic Initiation Factor-2
  • Eukaryotic Initiation Factor-4G
  • Intracellular Signaling Peptides and Proteins
  • Multienzyme Complexes
  • Phosphoproteins
  • Protein Kinases
  • mTOR protein, rat
  • Protein Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases
  • TOR Serine-Threonine Kinases
  • AMP-Activated Protein Kinases