Mechanical stimuli regulate rapamycin-sensitive signalling by a phosphoinositide 3-kinase-, protein kinase B- and growth factor-independent mechanism

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.

Cellular mechanisms regulating protein synthesis and skeletal muscle hypertrophy in animals

“…The results of these experiments found that passive stretch was sufficient to induce increases in protein synthesis. Conditioned media experiments from these ex vivo stretch studies also suggested that the increase in protein synthesis seen with mechanical stretch oscillations was not due to release of growth factors from the stretched muscle (52,54). Taken together, the results of the in vitro/ex vivo studies support the findings in vivo and demonstrate that mechanical stretch/loading is sufficient to increase rates of protein synthesis.…”

“…To date, numerous studies have shown that mTOR plays a critical role in regulating protein synthesis and cell growth/ hypertrophy in skeletal muscle (11,36,54,74,86). Consistent with its role in regulation of protein synthesis, signaling through mTOR occurs in muscle after high force contractions/ mechanical loading and is maintained for many hours (18 -36 h) after a single bout of loading (10).…”

“…However, there is still very little known about the actual biochemical events that link the mechanical loading of a muscle/muscle fiber to mTOR signaling. Ex vivo muscle stretch and in vitro muscle cell stretch models of mechanical loading have defined that 1) passive mechanical loading is sufficient to activate mTOR signaling in skeletal muscle, 2) stretch-induced activation of mTOR occurs in a PI3K-independent manner, and 3) mechanical stretch can activate mTOR in the absence of amino acids or growth factors (49,50,54). Our lab has found that treatment of the myotubes with cytochalasin D to disrupt the microfilament system (49) will block stretch-induced signaling to mTOR even though insulin activation of mTOR was still intact.…”

“…Their models included both static stretch and stretch oscillations and clearly established biochemical links between mechanical stretch and regulation of protein synthesis (97-100). More recently, Hornberger et al (54) used an ex vivo system to test the contribution of mechanical strain oscillations on the regulation of protein synthesis in mammalian muscle. Mouse extensor digitorum longus (EDL) muscles were incubated in an organ bath and passively stretched 15% of resting length for up to 90 min.…”

Hypoxia and Resistance Exercise: A Comparison of Localized and Systemic Methods

“…Mechanical deformation of muscle fibres during contractile processes and stretching also stimulate intramuscular signalling pathways independently of hormones and growth factors [74]. In particular, mechanical disruptions activate the mammalian target of rapamycin (mTOR) pathway, which moderates the adaptive responses via translation initiation and muscle protein synthesis (MPS) [75].…”