Cannabinoid receptor 1 and acute resistance exercise – In vivo and in vitro studies in human skeletal muscle

Aim This study aimed to determine whether Cannabinoid receptor 1 (CB1) is involved in mammalian target of rapamycin (mTOR) signaling and skeletal muscle protein synthesis. Methods This study used human vastus lateralis skeletal muscle biopsies obtained before and after a resistance exercise (RE) bout in young men (n = 18). The signaling mechanisms were studied in vitro in human myotubes. Protein expression was determined by Western blot and confocal microscopy, and gene expression by quantitative PCR. Protein synthesis was measured in vitro using puromycin-based SuNSET technique. Results In human skeletal muscle, an anabolic stimulus in the form of RE down-regulated CB1 expression. The negative change in CB1 expression was associated with increased phosphorylation of mTOR signaling proteins. In vitro, CB1 antagonist AM251 induced phosphorylation of mTOR downstream targets, ribosomal protein S6 kinase (S6K1), S6 and eukaryotic initiation factor 4E binding protein (4E-BP1) in human myotubes. These effects were ERK1/2-dependent and insensitive to mTOR inhibitor, rapamycin. Compared to AM251 treatment alone, inhibition of ERK1/2 by UO126 in the presence of AM251 decreased phosphorylation of S6K1, S6 and and 4E-BP1 at Thr37/46. AM251 increased protein synthesis in cultured human myotubes, which was not rapamycin-sensitive but was ERK1/2-dependent. Conclusions Our results indicate that RE down-regulates CB1 expression. Inhibition of CB1 signaling increases skeletal muscle anabolic signaling down-stream of mTOR and protein synthesis through ERK1/2. Our study may provide base for the development of CB1-blocking drugs to treat or prevent muscle wasting.