Submitted to: Biochemical Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/19/2004
Publication Date: 6/15/2004
Citation: Hornberger, T.A., Stuppard, R., Conley, K.E., Fedele, M.J., Fiorotto, M.L., Chin, E.R., Esser, K.A. 2004. Mechanical stimuli regulate rapamycin-sensitive signalling by a phosphoinositide 3-kinase-, protein kinase B- and growth factor-independent mechanism. Biochemical Journal. 380(Pt 3):795-804. Interpretive Summary: Skeletal muscles undergo hypertrophy with resistance exercise. However, we do not understand how the mechanical activity of the muscles is translated into a change in growth rate. The goal of this study was to determine if the mechanically stimulated muscle growth required the production of local growth factors, IGF-I in particular. In this study, muscles were studied in vitro where they were stimulated mechanically. The mechanical stimulation increased the rate of muscle protein synthesis, and this required activation of a molecule named mTOR. Using pharmacological inhibitors and transgenic mice, we showed that the activation of mTOR did not require the production of growth factors by the muscle as was generally thought to be the case. Additionally, these studies showed that the activation of mTOR was not by the canonical PI3-kinase/Akt pathway used by some growth factors. Thus, these studies showed that growth factors and mechanical stimuli make muscles grow by different mechanisms.
Technical Abstract: In response to growth factors, mTOR (mammalian target of rapamycin) has been identified as a central component of the signalling pathways that control the translational machinery and cell growth. Signalling through mTOR has also been shown to be necessary for the mechanical load-induced growth of cardiac and skeletal muscles. Although the mechanisms involved for mechanically induced activation of mTOR are not known, it has been suggested that activation of PI3K (phosphoinositide 3-kinase) and protein kinase B (Akt), via the release of locally acting growth factors, underlies this process. In the present study, we show that mechanically stimulating (passive stretch) the skeletal muscle "ex vivo" results in the activation of mTOR-dependent signalling events. The activation of mTOR-dependent signalling events was necessary for an increase in translational efficiency, demonstrating the physiological significance of this pathway. Using pharmacological inhibitors, we show that activation of mTOR-dependent signalling occurs through a PI3K-independent pathway. Consistent with these results, mechanically induced signalling through mTOR was not disrupted in muscles from Akt1-/- mice. In addition, "ex vivo" co-incubation experiments, along with "in vitro" conditioned-media experiments, demonstrate that a mechanically induced release of locally acting autocrine/paracrine growth factors was not sufficient for the activation of the mTOR pathway. Taken together, our results demonstrate that mechanical stimuli can activate the mTOR pathway independent of PI3K/Akt1 and locally acting growth factors. Thus mechanical stimuli and growth factors provide distinct inputs through which mTOR co-ordinates an increase in the translational efficiency.