The role of skeletal muscle mTOR in the regulation of mechanical load-induced growth

Chronic mechanical loading (CML) of skeletal muscle induces compensatory growth and the drug rapamycin has been reported to block this effect. Since rapamycin is considered to be a highly specific inhibitor of the mammalian target of rapamycin (mTOR), many have concluded that mTOR plays a key role in CML-induced growth regulatory events. However, rapamycin can exert mTOR-independent actions and systemic administration of rapamycin will inhibit mTOR signalling in all cells throughout the body. Thus, it is not clear if the growth inhibitory effects of rapamycin are actually due to the inhibition of mTOR signalling, and more specifically, the inhibition of mTOR signalling in skeletal muscle cells. To address this issue, transgenic mice with muscle specific expression of various rapamycin-resistant mutants of mTOR were employed. These mice enabled us to demonstrate that mTOR, within skeletal muscle cells, is the rapamycin-sensitive element that confers CML-induced hypertrophy, and mTOR kinase activity is necessary for this event. Surprisingly, CML also induced hyperplasia, but this occurred through a rapamycin-insensitive mechanism. Furthermore, CML was found to induce an increase in FoxO1 expression and PKB phosphorylation through a mechanism that was at least partially regulated by an mTOR kinase-dependent mechanism. Finally, CML stimulated ribosomal RNA accumulation and rapamycin partially inhibited this effect; however, the effect of rapamycin was exerted through a mechanism that was independent of mTOR in skeletal muscle cells. Overall, these results demonstrate that CML activates several growth regulatory events, but only a few (e. g. hypertrophy) are fully dependent on mTOR signalling within the skeletal muscle cells.