Differential regulation of muscle protein turnover in response to emphysema and acute pulmonary inflammation

Background: Exacerbations in COPD are often accompanied by pulmonary and systemic inflammation, and associated with increased susceptibility to and prevalence of weight loss and muscle wasting. Muscle mass loss during disease exacerbations may contribute to emphysema-associated muscle atrophy. However, whether pulmonary inflammation in presence of emphysema differentially affects skeletal muscle, including protein synthesis and degradation signaling pathways has not previously been addressed. The aims of this study were to 1) develop a mouse model of disease exacerbation-associated muscle wasting, 2) evaluate whether emphysema and muscle wasting can be monitored non-invasively and 3) assess alterations in muscle protein turnover regulation.

Methods: Emphysema was induced by three, weekly intra-tracheal (IT) elastase (E) or vehicle control (vc) instillations, followed by one single IT-LPS bolus (L) or vc instillation to mimic pulmonary inflammation-driven disease exacerbation. Consequently, four experimental groups were defined: vc/vc ('C'), E/vc ('E'), vc/LPS ('L'), E/LPS ('E + L'). Using micro cone-beam CT-scans, emphysema development and muscle mass changes were monitored, and correlated to muscle weight 48 h after LPS instillation. Protein turnover signaling was assessed in muscle tissue collected 24 h post LPS instillation.

Results: Micro-CT imaging correlated strongly with established invasive measurements of emphysema and muscle atrophy. Pulmonary inflammation following LPS instillation developed irrespective of emphysema and body and muscle weight were similarly reduced in the 'L' and 'E + L' groups. Accordingly, mRNA and protein expression levels of genes of the ubiquitin-proteasome pathway (UPS) and the autophagy-lysosomal pathway (ALP) were upregulated in skeletal muscle following IT-LPS ('L' and 'E + L'). In contrast, mTOR signaling, which controls ALP and protein synthesis, was reduced by pulmonary inflammation ('L' and 'E + L') as well as emphysema as a single insult ('E') compared to control.

Conclusion: Changes in lung tissue density and muscle mass can be monitored non-invasively to evaluate emphysema and muscle atrophy longitudinally. Acute loss of muscle mass evoked by pulmonary inflammation is similar in control and emphysematous mice. Although muscle atrophy cues in response to pulmonary inflammation are not altered by emphysema, emphysema itself affects protein synthesis and ALP signaling, which may interfere with muscle mass recovery and impair maintenance of muscle mass in emphysema.