Contractile cell apoptosis regulates airway smooth muscle remodeling in asthma

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Abstract Rationale Investigations on the mechanisms of airway smooth muscle remodeling, a prominent asthma feature contributing to its clinical manifestations and severity, have largely focused on its hyperplastic growth. Conversely, limited data and virtually no translational research have been produced on a plausible role of apoptosis in the homeostasis and remodeling of airway smooth muscle. Objectives We aimed at demonstrating an involvement of apoptosis, an essential regulator of organ structure and cell turnover, in the pathophysiology of airway smooth muscle remodeling in asthma. Methods Murine experimental asthma was modeled to analyze airway hyperresponsiveness, contractile tissue remodeling and apoptosis detection outcomes at early and late cutoffs, and under pharmacological inhibition of apoptosis by employing a caspase blocker. Clinical investigation followed through analyses on human bronchial biopsies. Results Airway hyperresponsiveness and contractile tissue remodeling were already established in early experimental asthma, and a subsequent upregulation of apoptosis limited the airway contractile tissue growth. Caspase inhibition elicited chaotic pulmonary mechanics and an unusual growth of airway smooth muscle that was structurally disorganized. In bronchial biopsies, airway smooth muscle increased from controls through subjects with intermittent and persistent moderate and severe asthma. Cleaved poly-ADP ribose polymerase (c-PARP, a byproduct of caspase activity) was increased in severe asthma. Conclusions Apoptosis is involved in airway contractile cell turnover and in shaping the size, structure and proper function of the airway smooth muscle layer. Apoptosis inhibitors may complicate concomitant asthma, whereas agents favouring airway contractile cell apoptosis may provide a novel pipeline of therapeutic development. Key messages How normal airway smooth muscle structure is preserved, and whether counteracting responses to remodeling are elicited in asthma, are outstanding questions not probed in vivo nor in the clinical setting. In this work, combined investigations on murine experimental asthma and human bronchial biopsies show that airway contractile cell apoptosis is involved in the homeostasis of airway smooth muscle, and apoptotic activity is upregulated as part of the remodeling process of this tissue in asthma. Apoptosis arises as a key regulator of the size and structure of the airway smooth muscle layer. This concept draws implications for clinical practice and drug development.

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