Involvement of E3 Ubiquitin ligases in Cigarette Smoke associated muscle catabolism

Epidemiological studies have identified tobacco smoking as a risk factor for sarcopenia, the age related loss of muscle mass and strength. Clinical, in vivo and in vitro studies have revealed that cigarette smoke (CS) induces skeletal muscle damage due to impaired muscle metabolism, increased inflammation and oxidative stress and activation of various intracellular signaling pathways. In order to investigate the cellular mechanisms by which CS leads to muscle catabolism, C2 myotubes were exposed to different levels of whole vapor phase CS. Myotube diameters, muscle proteins degradation and signaling pathways activation in response to CS exposure were examined by microscopy, Western blot and real time quantitative PCR. Exposure of C2 myotubes to CS caused a reduction in myotube diameters and degradation of the main contractile proteins: myosin heavy chain and actin proteins in a time- and dose-dependent manner. CS exposure to C2 myotubes also resulted in p38 MAPK phosphorylation, which led to up-regulation of the muscle specific E3 ubiquitin ligase enzymes: MAFbx/Atrogin-1 and MuRF1. Inhibition of p38 MAPK by SB203580 prevented both CS associated degradation of myosin heavy chain and up-regulation of the above E3 ubiquitin ligases. In addition, C2 exposure to CS resulted in IkBα degradation and NFkB activation which led to up-regulation of MuRF1 but not MAFbx/Atrogin-1.

Our results demonstrate that vapor phase CS exposure to skeletal myotubes activates the p38 MAPK pathway leading to skeletal muscle cell damage and muscle protein breakdown mediated by muscle specific E3 ubiquitin ligases. However, MAFbx/Atrogin-1 is activated directly through p38 pathway while MuRF1 is also activated by p38 but through activation of NFkB leading to up-regulation of MuRF1. Our findings provide a possible molecular mechanism for the catabolic effects of CS in skeletal muscle.