title: Molecular brakes regulating mTORC1 activation in skeletal muscle following synergist ablation.
abstract: The goal of the current work was to profile positive (mTORC1 activation, autocrine/paracrine growth factors) and negative [AMPK, unfolded protein response (UPR)] pathways that might regulate overload-induced mTORC1 (mTOR complex 1) activation with the hypothesis that a number of negative regulators of mTORC1 will be engaged during a supraphysiological model of hypertrophy. To achieve this, mTORC1-IRS-1/2 signaling, BiP/CHOP/IRE1α, and AMPK activation were determined in rat plantaris muscle following synergist ablation (SA). SA resulted in significant increases in muscle mass of ~4% per day throughout the 21 days of the experiment. The expression of the insulin-like growth factors (IGF) were high throughout the 21st day of overload. However, IGF signaling was limited, since IRS-1 and -2 were undetectable in the overloaded muscle from day 3 to day 9. The decreases in IRS-1/2 protein were paralleled by increases in GRB10 Ser(501/503) and S6K1 Thr(389) phosphorylation, two mTORC1 targets that can destabilize IRS proteins. PKB Ser(473) phosphorylation was higher from 3-6 days, and this was associated with increased TSC2 Thr(939) phosphorylation. The phosphorylation of TSC2 (Thr1345) (an AMPK site) was also elevated, whereas phosphorylation at the other PKB site, Thr(1462), was unchanged at 6 days. In agreement with the phosphorylation of Thr(1345), SA led to activation of AMPKα1 during the initial growth phase, lasting the first 9 days before returning to baseline by day 12. The UPR markers CHOP and BiP were elevated over the first 12 days following ablation, whereas IRE1α levels decreased. These data suggest that during supraphysiological muscle loading at least three potential molecular brakes engage to downregulate mTORC1.
authors: Health and Exercise Sciences Research Group, University of Stirling, Stirling, United Kingdom; Division of Molecular Physiology, University of Dundee, Dundee, United Kingdom; d.l.hamilton@stir.ac.uk.
authors: D Lee
authors: DL
authors: Hamilton
authors: Neurobiology, Physiology, and Behavior, University of California Davis, Davis, California; and Exercise and Rehabilitation Sciences, University of Birmingham, Birmingham, United Kingdom.
authors: Andrew
authors: A
authors: Philp
authors: Division of Molecular Physiology, University of Dundee, Dundee, United Kingdom;
authors: Matthew G
authors: MG
authors: MacKenzie
authors: Neurobiology, Physiology, and Behavior, University of California Davis, Davis, California; and.
authors: Amy
authors: A
authors: Patton
authors: Division of Molecular Physiology, University of Dundee, Dundee, United Kingdom;
authors: Mhairi C
authors: MC
authors: Towler
authors: Health and Exercise Sciences Research Group, University of Stirling, Stirling, United Kingdom;
authors: Iain J
authors: IJ
authors: Gallagher
authors: Neurobiology, Physiology, and Behavior, University of California Davis, Davis, California; and.
authors: Sue C
authors: SC
authors: Bodine
authors: Neurobiology, Physiology, and Behavior, University of California Davis, Davis, California; and.
authors: Keith
authors: K
authors: Baar
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000378:metabolism; D055011:Ablation Techniques; D000818:Animals; D005260:Female; D051383:GRB10 Adaptor Protein / Q000378:metabolism; D006984:Hypertrophy / Q000235:genetics / Q000378:metabolism; D036341:Intercellular Signaling Peptides and Proteins / Q000494:pharmacology; D000076222:Mechanistic Target of Rapamycin Complex 1; D046912:Multiprotein Complexes / Q000378:metabolism*; D024510:Muscle Development / Q000187:drug effects / Q000235:genetics*; D018482:Muscle, Skeletal / Q000201:enzymology* / Q000473:pathology* / Q000502:physiology; D010766:Phosphorylation; D051381:Rats; D017208:Rats, Wistar; D019893:Ribosomal Protein S6 Kinases / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*; D016474:Weight-Bearing
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D046912:Multiprotein Complexes; D051383:GRB10 Adaptor Protein; D000076222:Mechanistic Target of Rapamycin Complex 1; D019893:Ribosomal Protein S6 Kinases; C526901:Rps6kb1 protein, rat; D058570:TOR Serine-Threonine Kinases; D055372:AMP-Activated Protein Kinases; C568305:Prkaa1 protein, rat
keywords: AMPK; S6K1; hypertrophy; mTORC1; skeletal muscle