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  • Title: Mediation of IGF-1-induced skeletal myotube hypertrophy by PI(3)K/Akt/mTOR and PI(3)K/Akt/GSK3 pathways.
    Author: Rommel C, Bodine SC, Clarke BA, Rossman R, Nunez L, Stitt TN, Yancopoulos GD, Glass DJ.
    Journal: Nat Cell Biol; 2001 Nov; 3(11):1009-13. PubMed ID: 11715022.
    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.
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