Rapamycin administration in humans blocks the contraction-induced increase in skeletal muscle protein synthesis

Abstract

Muscle protein synthesis and mTORC1 signalling are concurrently stimulated following muscle contraction in humans. In an effort to determine whether mTORC1 signalling is essential for regulating muscle protein synthesis in humans, we treated subjects with a potent mTORC1 inhibitor (rapamycin) prior to performing a series of high-intensity muscle contractions. Here we show that rapamycin treatment blocks the early (1-2 h) acute contraction-induced increase ( approximately 40%) in human muscle protein synthesis. In addition, several downstream components of the mTORC1 signalling pathway were also blunted or blocked by rapamycin. For instance, S6K1 phosphorylation (Thr421/Ser424) was increased post-exercise 6-fold in the control group while being unchanged with rapamycin treatment. Furthermore, eEF2 phosphorylation (Thr56) was reduced by approximately 25% post-exercise in the control group but phosphorylation following rapamycin treatment was unaltered, indicating that translation elongation was inhibited. Rapamycin administration prior to exercise also reduced the ability of raptor to associate with mTORC1 during post-exercise recovery. Surprisingly, rapamycin treatment prior to resistance exercise completely blocked the contraction-induced increase in the phosphorylation of ERK1/2 (Thr202/Tyr204) and blunted the increase in MNK1 (Thr197/202) phosphorylation. However, the phosphorylation of a known target of MNK1, eIF4E (Ser208), was similar in both groups (P > 0.05) which is consistent with the notion that rapamycin does not directly inhibit MAPK signalling. We conclude that mTORC1 signalling is, in part, playing a key role in regulating the contraction-induced stimulation of muscle protein synthesis in humans, while dual activation of mTORC1 and ERK1/2 stimulation may be required for full stimulation of human skeletal muscle protein synthesis.

Figures

Figure 1. Data represent the time course of rapamycin in the blood measured immediately after rapamycin consumption

PreEx is time before exercise while PostEx is time following exercise. *Significantly different from 1.5 h PreEx time period (P < 0.05).

Figure 2. Data represent mixed-muscle protein synthesis at basal and during the post-exercise period in control and rapamycin skeletal muscle

*Significantly different from basal (P < 0.05). #Significantly different from rapamycin group.

Figure 4. Data represent phosphorylation of mTOR at Ser2446 (A, control: n= 8; rapamycin: n= 7), raptor bound to mTOR (B, mTORC1; control: n= 6; rapamycin n= 7), S6K1 at Thr389 (C, control: n= 8; rapamycin: n= 7) and S6K1 at Thr421/Ser424 (D, control: n= 8; rapamycin: n= 7) in control and rapamycin skeletal muscle at basal, 1 h PostEx and 2 h PostEx

Data are expressed as fold change from basal (mean ±s.e.m.). *Significantly different from basal (P < 0.05).

Figure 3

Representative immunoblot protein images for control and rapamycin skeletal muscle samples at basal, 1 h PostEx and 2 h PostEx.

Figure 5. Data represent phosphorylation of 4E-BP1 at Thr37/46 (A, control: n= 8; rapamycin: n= 7), eEF2 at Thr56 (B, control: n= 8; rapamycin: n= 7), S6 at Ser240/244 (C, control: n= 8; rapamycin: n= 7) and S6 at Ser235/236 (D, control: n= 8; rapamycin: n= 7) in control and rapamycin skeletal muscle at basal, 1 h PostEx and 2 h PostEx

Data are expressed as fold change from basal (mean ±s.e.m.). *Significantly different from basal (P < 0.05). #Significantly different from rapamycin group (P < 0.05).

Figure 6. Data represent phosphorylation of ERK1/2 at Thr202/Tyr204 (A, control: n= 8; rapamycin: n= 7), MNK1 at Thr197/202 (B, control: n= 8; rapamycin: n= 7), eIF4E at Ser208 (C, control: n= 5; rapamycin: n= 6) and eIF4G at Ser1108 (D, control: n= 4; rapamycin: n= 7) in control and rapamycin skeletal muscle at basal, 1 h PostEx and 2 h PostEx

Data are expressed as fold change from basal (mean ±s.e.m.). *Significantly different from basal (P < 0.05). #Significantly different from rapamycin group (P < 0.05). **Main effect for time (P < 0.05).