title: Chronic paraplegia-induced muscle atrophy downregulates the mTOR/S6K1 signaling pathway.
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.
authors: Department of Physical Therapy, University of Texas Medical Branch, Galveston, Texas 77555-1144, USA.
authors: Hans C
authors: HC
authors: Dreyer
authors: Erin L
authors: EL
authors: Glynn
authors: Heidi L
authors: HL
authors: Lujan
authors: Christopher S
authors: CS
authors: Fry
authors: Stephen E
authors: SE
authors: DiCarlo
authors: Blake B
authors: BB
authors: Rasmussen
mesh_terms: D055372:AMP-Activated Protein Kinases; D000818:Animals; D015153:Blotting, Western; D002352:Carrier Proteins / Q000378:metabolism; D002908:Chronic Disease; D004195:Disease Models, Animal; D015536:Down-Regulation; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism; D039603:Eukaryotic Initiation Factor-4G / Q000378:metabolism; D047468:Immunoprecipitation; D047908:Intracellular Signaling Peptides and Proteins; D008297:Male; D009097:Multienzyme Complexes / Q000378:metabolism; D018482:Muscle, Skeletal / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology; D009133:Muscular Atrophy / Q000201:enzymology / Q000209:etiology / Q000378:metabolism* / Q000473:pathology; D010264:Paraplegia / Q000150:complications* / Q000201:enzymology / Q000209:etiology / Q000378:metabolism / Q000473:pathology; D010750:Phosphoproteins / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism*; D015398:Signal Transduction; D013119:Spinal Cord Injuries / Q000150:complications* / Q000201:enzymology / Q000378:metabolism / Q000473:pathology; D058570:TOR Serine-Threonine Kinases; D013997:Time Factors
chemical_list: D002352:Carrier Proteins; C499526:Eif4ebp1 protein, rat; D015852:Eukaryotic Initiation Factor-2; D039603:Eukaryotic Initiation Factor-4G; D047908:Intracellular Signaling Peptides and Proteins; D009097:Multienzyme Complexes; D010750:Phosphoproteins; D011494:Protein Kinases; C546845:mTOR protein, rat; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D019893:Ribosomal Protein S6 Kinases; D058570:TOR Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases