Increased Nicotinamide Adenine Dinucleotide Phosphate Oxidase 4 Expression Mediates Intrinsic Airway Smooth Muscle Hypercontractility in Asthma

Abstract

Rationale: Asthma is characterized by disordered airway physiology as a consequence of increased airway smooth muscle contractility. The underlying cause of this hypercontractility is poorly understood. Objectives: We sought to investigate whether the burden of oxidative stress in airway smooth muscle in asthma is heightened and mediated by an intrinsic abnormality promoting hypercontractility. Methods: We examined the oxidative stress burden of airway smooth muscle in bronchial biopsies and primary cells from subjects with asthma and healthy controls. We determined the expression of targets implicated in the control of oxidative stress in airway smooth muscle and their role in contractility. Measurements and Main Results: We found that the oxidative stress burden in the airway smooth muscle in individuals with asthma is heightened and related to the degree of airflow obstruction and airway hyperresponsiveness. This was independent of the asthmatic environment as in vitro primary airway smooth muscle from individuals with asthma compared with healthy controls demonstrated increased oxidative stress-induced DNA damage together with an increased production of reactive oxygen species. Genome-wide microarray of primary airway smooth muscle identified increased messenger RNA expression in asthma of NADPH oxidase (NOX) subtype 4. This NOX4 overexpression in asthma was supported by quantitative polymerase chain reaction, confirmed at the protein level. Airway smooth muscle from individuals with asthma exhibited increased agonist-induced contraction. This was abrogated by NOX4 small interfering RNA knockdown and the pharmacological inhibitors diphenyleneiodonium and apocynin. Conclusions: Our findings support a critical role for NOX4 overexpression in asthma in the promotion of oxidative stress and consequent airway smooth muscle hypercontractility. This implicates NOX4 as a potential novel target for asthma therapy.Keywords: asthma; airway smooth muscle; airway hyperresponsiveness; NOX4; SOD2 Asthma affects over 300 million people worldwide. Importantly, asthma control is suboptimal in about half of sufferers, and 10% are refractory to current antiinflammatory and bronchodilator therapies (1, 2). There is therefore an urgent need for new therapies. Asthma is characterized by variable airflow obstruction and airway hyperresponsiveness as a consequence of increased airway smooth muscle contractility (1, 2). This heightened airway smooth muscle (ASM) contraction is considered to be largely due to the local asthmatic milieu with infiltration by inflammatory cells, in particular eosinophils and T cells, into the submucosa (2-4), to mast cells in the ASM bundle (4, 5), and to up-regulation of Th2 cytokines (6, 7).Interestingly, there is an increasing body of evidence that ASM is fundamentally altered in asthma compared with healthy controls, suggesting that abnormalities in these structural cells may also play a critical role in the development of the abnormal physiology in asthma and may contribute to the persistent airway ...

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