title: Insulin-like growth factor-I inhibits dexamethasone-induced proteolysis in cultured L6 myotubes through PI3K/Akt/GSK-3beta and PI3K/Akt/mTOR-dependent mechanisms.
abstract: We and others reported previously that IGF-I inhibits dexamethasone-induced proteolysis in cultured L6 myotubes. Recent evidence suggests that this effect of IGF-I at least in part reflects PI3K/Akt-mediated inhibition of Foxo transcription factors. The potential role of other mechanisms, downstream of PI3K/Akt, is not well understood. Here we tested the hypothesis that PI3K/Akt-mediated inactivation of GSK-3beta and activation of mTOR contribute to the anabolic effects of IGF-I in dexamethasone-treated myotubes. Cultured L6 myotubes were treated with 1 microM dexamethasone in the absence or presence of 0.1 microg/ml of IGF-I and inhibitors of GSK-3beta and mTOR. Protein degradation was measured by determining the release of trichloroacetic acid soluble radioactivity from myotubes that had been prelabeled with (3)H-tyrosine for 48 h. IGF-I reduced basal protein breakdown rates and completely abolished the dexamethasone-induced increase in myotube proteolysis. These effects of IGF-I were associated with increased phosphorylation of Akt, GSK-3beta, and the mTOR downstream targets p70(S6K) and 4E-BP1. The PI3K inhibitor LY294002 and the mTOR inhibitor rapamycin reversed the anabolic effect of IGF-I in dexamethasone-treated myotubes. In addition, the GSK-3beta inhibitors LiCl and TDZD-8 reduced protein degradation in a similar fashion as IGF-I. Our results suggest that PI3K/Akt-mediated inactivation of GSK-3beta and activation of mTOR contribute to the anabolic effects of IGF-I in dexamethasone-treated myotubes.
authors.affiliation: Shriners Hospitals for Children, Cincinnati Burns Hospital, 3229 Burnet Avenue, Cincinnati, OH 45229, USA.
authors.forename: Bing-Guo
authors.initials: BG
authors.lastname: Li
authors.forename: Per-Olof
authors.initials: PO
authors.lastname: Hasselgren
authors.forename: Cheng-Hui
authors.initials: CH
authors.lastname: Fang
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002867:Chromones / Q000378:metabolism / Q000494:pharmacology; D003907:Dexamethasone / Q000378:metabolism / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005419:Flavonoids / Q000378:metabolism / Q000494:pharmacology; D038362:Glycogen Synthase Kinase 3 / Q000187:drug effects / Q000378:metabolism*; D000071679:Glycogen Synthase Kinase 3 beta; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology*; D018021:Lithium Chloride / Q000378:metabolism / Q000494:pharmacology; D020935:MAP Kinase Signaling System / Q000187:drug effects; D008954:Models, Biological; D009025:Morpholines / Q000378:metabolism / Q000494:pharmacology; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D009124:Muscle Proteins / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D011494:Protein Kinases / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D020123:Sirolimus / Q000378:metabolism / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D013830:Thiadiazoles / Q000378:metabolism / Q000494:pharmacology
chemical_list: D002867:Chromones; D003907:Dexamethasone; D005419:Flavonoids; D038362:Glycogen Synthase Kinase 3; D000071679:Glycogen Synthase Kinase 3 beta; D007334:Insulin-Like Growth Factor I; D018021:Lithium Chloride; D009025:Morpholines; D009124:Muscle Proteins; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D020123:Sirolimus; D058570:TOR Serine-Threonine Kinases; D013830:Thiadiazoles; C494356:4-benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C546842:MTOR protein, human; C000605506:GSK3B protein, human; C000605508:Gsk3b protein, rat; C093973:2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one