Skeletal muscles adapt to changes in their workload by regulating fibre size by unknown mechanisms. The roles of two signalling pathways implicated in muscle hypertrophy on the basis of findings in vitro, Akt/mTOR (mammalian target of rapamycin) and calcineurin/NFAT (nuclear factor of activated T cells), were investigated in several models of skeletal muscle hypertrophy and atrophy in vivo. The Akt/mTOR pathway was upregulated during hypertrophy and downregulated during muscle atrophy. Furthermore, rapamycin, a selective blocker of mTOR, blocked hypertrophy in all models tested, without causing atrophy in control muscles. In contrast, the calcineurin pathway was not activated during hypertrophy in vivo, and inhibitors of calcineurin, cyclosporin A and FK506 did not blunt hypertrophy. Finally, genetic activation of the Akt/mTOR pathway was sufficient to cause hypertrophy and prevent atrophy in vivo, whereas genetic blockade of this pathway blocked hypertrophy in vivo. We conclude that the activation of the Akt/mTOR pathway and its downstream targets, p70S6K and PHAS-1/4E-BP1, is requisitely involved in regulating skeletal muscle fibre size, and that activation of the Akt/mTOR pathway can oppose muscle atrophy induced by disuse.

Skeletal muscle size depends upon a dynamic balance between anabolic (or hypertrophic) and catabolic (or atrophic) processes. Previously, no link between the molecular mediators of atrophy and hypertrophy had been reported. We demonstrate a hierarchy between the signals which mediate hypertrophy and those which mediate atrophy: the IGF-1/PI3K/Akt pathway, which has been shown to induce hypertrophy, prevents induction of requisite atrophy mediators, namely the muscle-specific ubiquitin ligases MAFbx and MuRF1. Moreover, the mechanism for this inhibition involves Akt-mediated inhibition of the FoxO family of transcription factors; a mutant form of FOXO1, which prevents Akt phosphorylation, thereby prevents Akt-mediated inhibition of MuRF1 and MAFbx upregulation. Our study thus defines a previously uncharacterized function for Akt, which has important therapeutic relevance: Akt is not only capable of activating prosynthetic pathways, as previously demonstrated, but is simultaneously and dominantly able to suppress catabolic pathways, allowing it to prevent glucocorticoid and denervation-induced muscle atrophy.

Skeletal muscle atrophy is a severe morbidity caused by a variety of conditions, including cachexia, cancer, AIDS, prolonged bedrest, and diabetes. One strategy in the treatment of atrophy is to induce the pathways normally leading to skeletal muscle hypertrophy. The pathways that are sufficient to induce hypertrophy in skeletal muscle have been the subject of some controversy. We describe here the use of a novel method to produce a transgenic mouse in which a constitutively active form of Akt can be inducibly expressed in adult skeletal muscle and thereby demonstrate that acute activation of Akt is sufficient to induce rapid and significant skeletal muscle hypertrophy in vivo, accompanied by activation of the downstream Akt/p70S6 kinase protein synthesis pathway. Upon induction of Akt in skeletal muscle, there was also a significant decrease in adipose tissue. These findings suggest that pharmacologic approaches directed toward activating Akt will be useful in inducing skeletal muscle hypertrophy and that an increase in lean muscle mass is sufficient to decrease fat storage.

Genetic ablation of Inppl1, which encodes SHIP2 (SH2-domain containing inositol 5-phosphatase 2), was previously reported to induce severe insulin sensitivity, leading to early postnatal death. In the previous study, the targeting construct left the first eighteen exons encoding Inppl1 intact, generating a Inppl1(EX19-28-/-) mouse, and apparently also deleted a second gene, Phox2a. We report a new SHIP2 knockout (Inppl1(-/-)) targeted to the translation-initiating ATG, which is null for Inppl1 mRNA and protein. Inppl1(-/-) mice are viable, have normal glucose and insulin levels, and normal insulin and glucose tolerances. The Inppl1(-/-) mice are, however, highly resistant to weight gain when placed on a high-fat diet. These results suggest that inhibition of SHIP2 would be useful in the effort to ameliorate diet-induced obesity, but call into question a dominant role of SHIP2 in modulating glucose homeostasis.

Clenbuterol and other beta2-adrenergic agonists are effective at inducing muscle growth and attenuating muscle atrophy through unknown mechanisms. This study tested the hypothesis that clenbuterol-induced growth and muscle sparing is mediated through the activation of Akt and mammalian target of rapamycin (mTOR) signaling pathways. Clenbuterol was administered to normal weight-bearing adult rats to examine the growth-inducing effects and to adult rats undergoing muscle atrophy as the result of hindlimb suspension or denervation to examine the muscle-sparing effects. The pharmacological inhibitor rapamycin was administered in combination with clenbuterol in vivo to determine whether activation of mTOR was involved in mediating the effects of clenbuterol. Clenbuterol administration increased the phosphorylation status of PKB/Akt, S6 kinase 1/p70(s6k), and eukaryotic initiation factor 4E binding protein 1/PHAS-1. Clenbuterol treatment induced growth by 27-41% in normal rats and attenuated muscle loss during hindlimb suspension by 10-20%. Rapamycin treatment resulted in a 37-97% suppression of clenbuterol-induced growth and a 100% reduction of the muscle-sparing effect. In contrast, rapamycin was unable to block the muscle-sparing effects of clenbuterol after denervation. Clenbuterol was also shown to suppress the expression of the MuRF1 and MAFbx transcripts in muscles from normal, denervated, and hindlimb-suspended rats. These results demonstrate that the effects of clenbuterol are mediated, in part, through the activation of Akt and mTOR signaling pathways.