NF-κB activation and polyubiquitin conjugation are required for pulmonary inflammation-induced diaphragm atrophy

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.