Early activation of mTORC1 signalling in response to mechanical overload is independent of phosphoinositide 3-kinase/Akt signalling

Non-technical summary Hypertrophy of skeletal muscle in response to resistance exercise is associated with significantly elevated rates of protein synthesis. The protein kinase mTORC1 has been shown to be a key signalling hub through which different anabolic factors (i.e. growth factors, nutrients and mechanical strain) contribute to the regulation of protein synthesis. In this study, we use an in vivo model of muscle hypertrophy to delineate the contribution of different input pathways regulating mTORC1. We found that the insulin/insulin like growth factor 1 pathway is not necessary for early activation of mTORC1 signalling but this probably occurs through activation of the ERK/TSC2 pathway. Knowledge of the key upstream pathways that modulate mTORC1 activity in vivo will provide the necessary foundation for the development of new therapeutic strategies for the maintenance of skeletal muscle mass.The mammalian target of rapamycin complex 1 (mTORC1) functions as a central integrator of a wide range of signals that modulate protein metabolism and cell growth. However, the contributions of individual pathways regulating mTORC1 activity in skeletal muscle are poorly defined. The purpose of this study was to determine the regulatory mechanisms that contribute to mTORC1 activation during mechanical overload-induced skeletal muscle hypertrophy. Consistent with previous studies, mechanical overload induced progressive hypertrophy of the plantaris muscle which was associated with significant increases in total RNA content and protein metabolism. mTORC1 was activated after a single day of overload as indicated by a significant increase in S6K1 phosphorylation at T389 and T421/S424. In contrast, Akt activity, as assessed by Akt phosphorylation status (T308 and S473), phosphorylation of direct downstream targets (glycogen synthase kinase 3 beta, proline-rich Akt substrate 40 kDa and tuberous sclerosis 2 (TSC2)) and a kinase assay, was not significantly increased until 2-3 days of overload. Inhibition of phosphoinositide 3-kinase (PI3K) activity by wortmannin was sufficient to block insulin-dependent signalling but did not prevent the early activation of mTORC1 in response to overload. We identified that the mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK)-dependent pathway was activated at day 1 after overload. In addition, a target of MEK/ERK signalling, phosphorylation of TSC2 at S664, was also increased at this early time point. These observations demonstrate that in vivo, mTORC1 activation at the early phase of mechanical overload in skeletal muscle occurs independently of PI3K/Akt signalling and provide evidence that the MEK/ERK pathway may contribute to mTORC1 activation through phosphorylation of TSC2.