Abstract
Skeletal muscle is composed of multinucleated fibres, formed after the differentiation and fusion of myoblast precursors. Skeletal muscle atrophy and hypertrophy refer to changes in the diameter of these pre-existing muscle fibres. The prevention of atrophy would provide an obvious clinical benefit; insulin-like growth factor 1 (IGF-1) is a promising anti-atrophy agent because of its ability to promote hypertrophy. However, the signalling pathways by which IGF-1 promotes hypertrophy remain unclear, with roles suggested for both the calcineurin/NFAT (nuclear factor of activated T cells) pathway and the PtdIns-3-OH kinase (PI(3)K)/Akt pathway. Here we employ a battery of approaches to examine these pathways during the hypertrophic response of cultured myotubes to IGF-1. We report that Akt promotes hypertrophy by activating downstream signalling pathways previously implicated in activating protein synthesis: the pathways downstream of mammalian target of rapamycin (mTOR) and the pathway activated by phosphorylating and thereby inhibiting glycogen synthase kinase 3 (GSK3). In contrast, in addition to demonstrating that calcineurin does not mediate IGF-1-induced hypertrophy, we show that IGF-1 unexpectedly acts via Akt to antagonize calcineurin signalling during myotube hypertrophy.
MeSH terms
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Adaptor Proteins, Signal Transducing
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Animals
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Calcineurin / metabolism
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Calcium-Calmodulin-Dependent Protein Kinases / antagonists & inhibitors
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Calcium-Calmodulin-Dependent Protein Kinases / metabolism*
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Carrier Proteins / metabolism
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Cell Cycle Proteins
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Cell Differentiation
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Cell Line
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Eukaryotic Initiation Factors
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Glycogen Synthase Kinase 3
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Glycogen Synthase Kinases
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Insulin-Like Growth Factor I / metabolism*
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Insulin-Like Growth Factor I / pharmacology
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Mice
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Muscle, Skeletal / cytology
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Muscle, Skeletal / metabolism*
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Muscular Atrophy / metabolism*
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Phosphatidylinositol 3-Kinases / metabolism
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Phosphoinositide-3 Kinase Inhibitors*
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Phosphoproteins / metabolism
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Phosphorylation
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Protein Kinase Inhibitors
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Protein Kinases / metabolism*
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Protein Serine-Threonine Kinases*
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Proto-Oncogene Proteins / antagonists & inhibitors
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Proto-Oncogene Proteins / metabolism*
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Proto-Oncogene Proteins c-akt
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Ribosomal Protein S6 Kinases / metabolism
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Signal Transduction*
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TOR Serine-Threonine Kinases
Substances
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Adaptor Proteins, Signal Transducing
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Carrier Proteins
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Cell Cycle Proteins
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Eif4ebp1 protein, mouse
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Eukaryotic Initiation Factors
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Phosphoinositide-3 Kinase Inhibitors
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Phosphoproteins
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Protein Kinase Inhibitors
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Proto-Oncogene Proteins
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Insulin-Like Growth Factor I
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Protein Kinases
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mTOR protein, mouse
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Glycogen Synthase Kinases
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Protein Serine-Threonine Kinases
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Proto-Oncogene Proteins c-akt
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Ribosomal Protein S6 Kinases
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TOR Serine-Threonine Kinases
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Calcium-Calmodulin-Dependent Protein Kinases
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Glycogen Synthase Kinase 3
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Calcineurin