title: Insulin like growth factor-1-induced phosphorylation and altered distribution of tuberous sclerosis complex (TSC)1/TSC2 in C2C12 myotubes.
abstract: Insulin like growth factor-1 (IGF-1) is established as an anabolic factor that can induce skeletal muscle growth by activating the phosphoinositide 3-kinase/Akt/mammalian target of rapamycin (mTOR) pathway. Although this signaling pathway has been the subject of much study, the molecular mechanisms linking IGF-1 binding to mTOR activation remain poorly defined in muscle. The present study aimed to test the hypothesis that IGF-1 activation of mTOR in C2C12 myotubes requires a phosphorylation-dependent, altered distribution of the tuberous sclerosis complex (TSC)1/TSC2 complex from the membrane to the cytosol. We found that IGF-1 treatment does not affect complex formation between TSC1 and TSC2, but rather IGF-1 induces an altered distribution of the TSC1/TSC2 complex in C2C12 myotubes. In response to IGF-1 treatment, there was a relative redistribution of the TSC1/TSC2 complex, composed of TSC1 and phosphorylated TSC2, from the membrane to the cytosol. IGF-1-stimulated TSC1/TSC2 phosphorylation and redistribution were completely prevented by the phosphoinositide 3-kinase inhibitor wortmannin, but were not with the downstream mTOR inhibitor, rapamycin. When a nonphosphorylatable form of TSC2 (S939A) was overexpressed, phosphorylation-dependent binding of the scaffold protein 14-3-3 to TSC2 was diminished and no redistribution of the TSC1/TSC2 complex was observed after IGF-1 stimulation. These results indicate that TSC2 phosphorylation in response to IGF-1 treatment is necessary for the altered distribution of the TSC1/TSC2 complex to the cytosol. We suggest that this translocation is likely critical for mTOR activation by dissociating the interaction between the GTPase activating protein activity of the TSC1/TSC2 complex and its downstream target, Ras homolog enriched in brain.
authors.affiliation: Center for Muscle Biology, Department of Physiology, University of Kentucky, Lexington, USA.
authors.forename: Mitsunori
authors.initials: M
authors.lastname: Miyazaki
authors.forename: John J
authors.initials: JJ
authors.lastname: McCarthy
authors.forename: Karyn A
authors.initials: KA
authors.lastname: Esser
mesh_terms: D048948:14-3-3 Proteins / Q000378:metabolism; D000818:Animals; D002454:Cell Differentiation; D002460:Cell Line; D004789:Enzyme Activation; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D051379:Mice; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D000081082:Phosphoinositide-3 Kinase Inhibitors; D010766:Phosphorylation; D011485:Protein Binding; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D021381:Protein Transport; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D058570:TOR Serine-Threonine Kinases; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins / Q000378:metabolism*
chemical_list: D048948:14-3-3 Proteins; D007334:Insulin-Like Growth Factor I; D047908:Intracellular Signaling Peptides and Proteins; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D047428:Protein Kinase Inhibitors; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases; D000077004:Tuberous Sclerosis Complex 1 Protein; D000077005:Tuberous Sclerosis Complex 2 Protein; D025521:Tumor Suppressor Proteins; C000624651:Tsc1 protein, mouse; C000624654:Tsc2 protein, mouse; C546843:mTOR protein, mouse
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