High-intensity muscle contraction-mediated increases in Akt1 and Akt2 phosphorylation do not contribute to mTORC1 activation and muscle protein synthesis

Abstract

High-intensity muscle contraction (HiMC) is known to induce muscle protein synthesis, a process in which mechanistic target of rapamycin (mTOR) is reported to play a critical role. However, the mechanistic details have not been completely elucidated. Here, we investigated whether Akt plays a role in regulating HiMC-induced mTORC1 activation and muscle protein synthesis using a rodent model of resistance exercise and MK2206 (an Akt kinase inhibitor). The right gastrocnemius muscle of male C57BL/6J mice aged 10 wk was isometrically contracted via percutaneous electrical stimulation (100 Hz, 5 sets of 10 3-s contractions, 7-s rest between contractions, and 3-min rest between sets), while the left gastrocnemius muscle served as a control. Vehicle or MK2206 was injected intraperitoneally 6 h before contraction. MK2206 inhibited both resting and HiMC-induced phosphorylation of Akt1 Ser-473 and Akt2 Ser-474. MK2206 also inhibited the resting phosphorylation of p70S6K and 4E-BP1, which are downstream targets of mTORC1; however, it did not inhibit the HiMC-induced increase in phosphorylation of these targets. Similarly, MK2206 inhibited the resting muscle protein synthesis, but not the resistance exercise-induced muscle protein synthesis. On the basis of these observations, we conclude that although Akt2 regulates resting mTORC1 activity and muscle protein synthesis, HiMC-induced increases in mTORC1 activity and muscle protein synthesis are Akt-independent processes. NEW & NOTEWORTHY Akt is well known to be an upstream regulator of mechanistic target of rapamycin (mTOR) and has three isoforms in mammals, namely, Akt1, Akt2, and Akt3. We found that high-intensity muscle contraction (HiMC) increases Akt1 and Akt2 phosphorylation; however, HiMC-induced increases in mTORC1 activity and muscle protein synthesis are Akt-independent processes.

Effect of mechanistic/mammalian target of rapamycin complex 1 on mitochondrial dynamics during skeletal muscle hypertrophy

“…In an attempt to further elucidate the underlying regulation of muscle hypertrophy, we investigated the activation status of molecules in mTORC1 signaling pathways and determined that rapamycin administration inhibited the functional overload-induced phosphorylation levels of p70S6 K (Thr389), rpS6, and the total protein expression levels of 4EBP1. Since these results were consistent with previous studies (Maruyama et al, 2020;Ogasawara et al, 2017Ogasawara et al, , 2020Ogasawara & Suginohara., 2018;Takegaki et al, 2017), rapamycin administration may have inhibited mTORC1 signaling activation and subsequent muscle protein synthesis induced by functional overload.…”

Transient activation of mTORC1 signaling in skeletal muscle is independent of Akt1 regulation

“…These results are consistent with a previous study reporting that the overload-induced expression of IGF-1 mRNA in the rat's skeletal muscle was increased from 48 hr and at the later time point following the initiation of the workload (Adams et al, 1985). Recent studies from several independent laboratories have supported the idea that muscle contraction-or mechanical load-induced activation of mTORC1 signaling is not mediated by IGF-1/PI3K/Akt pathway, especially during a relatively early stage of muscle hypertrophic response (Goodman et al, 2010;Hamilton et al, 2014;Ito et al, 2013;Maruyama et al, 1985;Miyazaki et al, 2011;Philp et al, 2011). These notions suggest the presence of potential mediators that convert acute mechanical stimulation into the activation of mTORC1 signaling in the skeletal muscle.…”

“…Recent studies from several independent laboratories have supported the idea that muscle contraction‐ or mechanical load‐induced activation of mTORC1 signaling is not mediated by IGF‐1/PI3K/Akt pathway, especially during a relatively early stage of muscle hypertrophic response (Goodman et al., 2010; Hamilton et al., 2014; Ito et al., 2013; Maruyama et al., 1985; Miyazaki et al., 2011; Philp et al., 2011). These notions suggest the presence of potential mediators that convert acute mechanical stimulation into the activation of mTORC1 signaling in the skeletal muscle.…”

Rapamycin and mTORC2 inhibition synergistically reduce contraction‐stimulated muscle protein synthesis

“…Rapamycin (1.5 mg/kg) or vehicle control (equal volume of DMSO) was injected intraperitoneally 1 h before muscle contraction in the morning (between 06.00 and 08.00 h). Under 2% isoflurane anaesthesia, the right gastrocnemius muscle was contracted isometrically by fixing the sole of the foot at a 90°angle using percutaneous electrical stimulation (100 Hz, five sets of ten 3-s contractions, 7 s rest between contractions, 3 min rest between sets), as described previously (Maruyama et al 2020). The voltage and stimulation frequency were adjusted to produce maximum isometric tension (Zhao et al 2017;Ashida et al 2018).…”