Nrf2 Deficiency Exacerbates the Decline in Swallowing and Respiratory Muscle Mass and Function in Mice with Aspiration Pneumonia

Aspiration pneumonia exacerbates swallowing and respiratory muscle atrophy. It induces respiratory muscle atrophy through three steps: proinflammatory cytokine production, caspase-3 and calpain, and then ubiquitin-proteasome activations. In addition, autophagy induces swallowing muscle atrophy. <i>Nrf2</i> is the central detoxifying and antioxidant gene whose function in aspiration pneumonia is unclear. We explored the role of Nrf2 in aspiration pneumonia by examining swallowing and respiratory muscle mass and function using wild-type and <i>Nrf2</i>-knockout mice. Pepsin and lipopolysaccharide aspiration challenges caused aspiration pneumonia. The swallowing (digastric muscles) and respiratory (diaphragm) muscles were isolated. Quantitative RT-PCR and Western blotting were used to assess their proteolysis cascade. Pathological and videofluoroscopic examinations evaluated atrophy and swallowing function, respectively. <i>Nrf2</i>-knockouts showed exacerbated aspiration pneumonia compared with wild-types. <i>Nrf2</i>-knockouts exhibited more persistent and intense proinflammatory cytokine elevation than wild-types. In both mice, the challenge activated calpains and caspase-3 in the diaphragm but not in the digastric muscles. The digastric muscles showed extended autophagy activation in <i>Nrf2</i>-knockouts compared to wild-types. The diaphragms exhibited autophagy activation only in <i>Nrf2</i>-knockouts. <i>Nrf2</i>-knockouts showed worsened muscle atrophies and swallowing function compared with wild-types. Thus, activation of Nrf2 may alleviate inflammation, muscle atrophy, and function in aspiration pneumonia, a major health problem for the aging population, and may become a therapeutic target.