Peroxisome Metabolism Pathway and EHHADH Expression are Downregulated in Macrophages in Neutrophilic Asthma

Neutrophilic asthma (NA) is associated with more severe symptoms and poor responsiveness to inhaled corticosteroid therapy. Macrophages are the most abundant immune cells in the airway, but the role of macrophages in NA pathogenesis has not been fully studied. We hypothesized that dysregulation of peroxisome metabolism in macrophages may drive NA.

We retrieved microarray datasets from the GEO Gene Expression Omnibus database by using induced sputum samples from eosinophilic and neutrophilic asthma patients as well as healthy controls. We identified key molecules in NA and validated the expression of the key genes in our cohort of asthma patients using quantitative polymerase chain reaction (PCR). Furthermore, immunofluorescence staining was performed to detect the expression and localization of the key molecule in bronchoalveolar lavage (BAL) cells from asthma patients and the murine model of neutrophilia-dominant allergic airway inflammation. The expression of the key molecule was also examined in mouse bone marrow-derived macrophages (BMDMs) by quantitative PCR and western blotting.

Enoyl-CoA hydratase and 3-hydroxyacyl CoA dehydrogenase (EHHADH), sterol carrier protein 2, and peroxisomal biogenesis factor 14 were identified as the key molecules and were downregulated in patients with NA or severe asthma. The peroxisomal fatty acid metabolism pathway was significantly downregulated in NA. In our cohort of asthma patients, the expression of EHHADH, a key enzyme of the peroxisomal fatty acid beta-oxidation, was significantly decreased in non-eosinophilic asthma patients and positively correlated with airflow limitation. EHHADH was primarily expressed in macrophages in BAL cells. EHHADH was downregulated in lipopolysaccharide (LPS)-induced M1-like macrophages in mouse BMDMs. Fenofibrate, an agonist of the peroxisome pathway, significantly inhibits LPS-induced macrophage M1 polarization.

EHHADH expression and the peroxisome metabolism pathway are downregulated in macrophages in patients with NA. This downregulation may contribute to macrophage M1 polarization and neutrophilic airway inflammation in asthma.