This study clearly demonstrates myofibrillar and not generic protein accretion in skeletal muscle following leucine supplementation, and suggests this involves pre-translational control of MyHC expression by leucine.

Loss of diaphragm muscle strength in inflammatory lung disease contributes to mortality and is associated with diaphragm fiber atrophy. Ubiquitin (Ub) 26S-proteasome system (UPS)-dependent protein breakdown, which mediates muscle atrophy in a number of physiological and pathological conditions, is elevated in diaphragm muscle of patients with chronic obstructive pulmonary disease. Nuclear factor kappa B (NF-κB), an essential regulator of many inflammatory processes, has been implicated in the regulation of poly-Ub conjugation of muscle proteins targeted for proteolysis by the UPS. Here, we test if NF-κB activation in diaphragm muscle and subsequent protein degradation by the UPS are required for pulmonary inflammation-induced diaphragm atrophy. Acute pulmonary inflammation was induced in mice by intratracheal lipopolysaccharide instillation. Fiber cross-sectional area, ex vivo tyrosine release, protein poly-Ub conjugation, and inflammatory signaling were determined in diaphragm muscle. The contribution of NF-κB or the UPS to diaphragm atrophy was assessed in mice with intact or genetically repressed NF-κB signaling or attenuated poly-Ub conjugation, respectively. Acute pulmonary inflammation resulted in diaphragm atrophy measured by reduced muscle fiber cross-sectional area. This was accompanied by diaphragm NF-κB activation, and proteolysis, measured by tyrosine release from the diaphragm. Poly-Ub conjugation was increased in diaphragm, as was the expression of muscle-specific E3 Ub ligases. Genetic suppression of poly-Ub conjugation prevented inflammation-induced diaphragm muscle atrophy, as did muscle-specific inhibition of NF-κB signaling. In conclusion, the present study is the first to demonstrate that diaphragm muscle atrophy, resulting from acute pulmonary inflammation, requires NF-κB activation and UPS-mediated protein degradation.

No abstract

Skeletal muscle cell lines have been applied to model muscle growth in vitro. In this study we investigated whether specific amino acids (AA), at concentrations as they appear post‐prandially, are sufficient to directly stimulate muscle specific and total protein accretion in vitro. Myoblasts were cultured in modified DMEM containing 1g/L glucose, 1% FBS and physiological AA levels. Supplementation during differentiation consisted of a mixture of AA at concentrations observed in plasma following consumption of 20g whey protein, which stimulated muscle protein synthetic rate (Pennings et al., 2010). The AA in the mixture were previously identified based on their capacity to stimulate myofibrillar protein accretion in differentiating myoblasts. In contrast to stimulation with any of the single AA, differentiation in the presence of the AA mixture increased creatine kinase activity, myosin heavy chain and myosin light chain 1 levels, as well as total protein accretion. These in vitro findings were in line with in vivo increases in protein synthesis rate and reveal synergistic effects of individual anabolic‐acting AA on general en muscle specific protein accretion.