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.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
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Research Support, U.S. Gov't, P.H.S.
MeSH terms
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Animals
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Cells, Cultured
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Chromones / metabolism
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Chromones / pharmacology
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Dexamethasone / metabolism
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Dexamethasone / pharmacology*
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Dose-Response Relationship, Drug
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Flavonoids / metabolism
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Flavonoids / pharmacology
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Glycogen Synthase Kinase 3 / drug effects
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Glycogen Synthase Kinase 3 / metabolism*
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Glycogen Synthase Kinase 3 beta
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Humans
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Insulin-Like Growth Factor I / metabolism
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Insulin-Like Growth Factor I / pharmacology*
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Lithium Chloride / metabolism
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Lithium Chloride / pharmacology
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MAP Kinase Signaling System / drug effects
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Models, Biological
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Morpholines / metabolism
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Morpholines / pharmacology
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Muscle Fibers, Skeletal / drug effects
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Muscle Fibers, Skeletal / metabolism*
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Muscle Proteins / metabolism*
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Phosphatidylinositol 3-Kinases / metabolism*
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Protein Kinases / drug effects
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Protein Kinases / metabolism*
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Rats
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Sirolimus / metabolism
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Sirolimus / pharmacology
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TOR Serine-Threonine Kinases
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Thiadiazoles / metabolism
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Thiadiazoles / pharmacology
Substances
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4-benzyl-2-methyl-1,2,4-thiadiazolidine-3,5-dione
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Chromones
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Flavonoids
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Morpholines
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Muscle Proteins
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Thiadiazoles
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2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one
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Insulin-Like Growth Factor I
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Dexamethasone
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Protein Kinases
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MTOR protein, human
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GSK3B protein, human
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Glycogen Synthase Kinase 3 beta
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Gsk3b protein, rat
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TOR Serine-Threonine Kinases
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Glycogen Synthase Kinase 3
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Lithium Chloride
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2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one
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Sirolimus