Rapamycin does not affect post-absorptive protein metabolism in human skeletal muscle

Abstract

Administration of the mTORC1 inhibitor, rapamycin, to humans blocks the increase in skeletal muscle protein synthesis in response to resistance exercise or amino acid ingestion.

Objective: To determine whether rapamycin administration influences basal post-absorptive protein synthesis or breakdown in human skeletal muscle.

Materials/methods: Six young (26±2 years) subjects were studied during two separate trials, in which each trial was divided into two consecutive 2 h basal periods. The trials were identical except during one trial a single oral dose (16 mg) of rapamycin was administered immediately prior to the second basal period. Muscle biopsies were obtained from the vastus lateralis at 0, 2, and 4 h to examine protein synthesis, mTORC1 signaling, and markers of autophagy (LC3B-I and LC3B-II protein) associated with each 2 h basal period.

Results: During the Control trial, muscle protein synthesis, whole body protein breakdown (phenylalanine Ra), mTORC1 signaling, and markers of autophagy were similar between both basal periods (p>0.05). During the Rapamycin trial, these variables were similar to the Control trial (p>0.05) and were unaltered by rapamycin administration (p>0.05). Thus, post-absorptive muscle protein metabolism and mTORC1 signaling were not affected by rapamycin administration.

Conclusions: Short-term rapamycin administration may only impair protein synthesis in human skeletal muscle when combined with a stimulus such as resistance exercise or increased amino acid availability.

Trial registration: ClinicalTrials.gov NCT00891696.

Conflict of interest statement

Conflict of interest

The authors report no conflicts of interest.

Copyright © 2013 Elsevier Inc. All rights reserved.

Figures

Fig. 1

Schematic of the experimental trial. The experimental trial was designed to obtain measurements during two consecutive 2h basal post-absorptive periods, referred to as the initial basal period and experimental basal period, respectively. Each subject completed the experimental trial on two occasions (Control and Rapamycin) in a randomized, counterbalanced cross-over fashion. The experimental trial was identical for the Control and Rapamycin trials except that during one experimental trial subjects ingested 16mg of rapamycin (Rapamycin trial) at the start of the experimental basal period, whereas no rapamycin was ingested during the other experimental trial (Control trial).

Fig. 2

Mixed muscle protein fractional synthesis rate (FSR) in skeletal muscle (A) and whole body phenylalanine rate of appearance (Ra) (B) during the initial and experimental basal post-absorptive periods during the Control (CON) and Rapamycin (RAP) trials. To control for potential day-to-day variations in basal protein metabolism, both the CON and RAP trials included an initial basal period. However, during the RAP trial, subjects were administered 16mg of rapamycin at the start of the experimental basal period, whereas no rapamycin was ingested during the CON trial. Data are mean±SEM, n=6.

Fig. 3

Phosphorylation of mTOR at Ser2448 (A), S6K1 at Thr389 (B), and 4E-BP1 at Thr37/46 (C) during the initial and experimental basal post-absorptive periods during the Control (CON) and Rapamycin (RAP) trials. To control for potential day-to-day variations in basal protein metabolism, both the CON and RAP trials included an initial basal period. However, during the RAP trial, subjects were administered 16mg of rapamycin at the start of the experimental basal period, whereas no rapamycin was ingested during the CON trial. Data are mean±SEM, n=6.

Fig. 4

Protein expression of LC3B-I (A), LC3B-II (B), and the ratio of LC3B-II/LC3B-I protein (C) in skeletal muscle during the initial and experimental basal post-absorptive periods during the Control (CON) and Rapamycin (RAP) trials. To control for potential day-to-day variations in basal protein metabolism, both the CON and RAP trials included an initial basal period. However, during the RAP trial, subjects were administered 16mg of rapamycin at the start of the experimental basal period, whereas no rapamycin was ingested during the CON trial. Data are mean±SEM, n=6. #different from Initial Basal, main effect of time, p<0.05.