This research article delves into the increased mass of airway smooth muscle (ASM) in asthma, its causes, and mechanisms, as well as studying the effects of certain drugs on this proliferation. It uses both in vivo and in vitro studies on different models, including equine and rat, for a comprehensive understanding.
Understanding the Growth of Airway Smooth Muscle
- The study starts by establishing that the increase in airway smooth muscle (ASM) mass is a significant feature in asthma’s airway wall remodeling process.
- Two categories of mediators are identified as responsible for the ASM proliferation. One category activates receptors coupled with intrinsic tyrosine kinase activity, and the other has its effects via receptors associated with heterotrimeric guanosine triphosphate-binding proteins.
- The key signaling pathways activated by these ASM growth triggers are primarily the extracellular signal-regulated kinase and the phosphoinositide 3′-kinase.
- Moreover, the research implies that ASM mass expansion might not just be about cell multiplication but also migration, with p38 mitogen-activated protein kinase and the p21-activated kinase 1 pathways being the major signaling mechanisms.
Empirical Evidence from Animal Models
- They introduce findings from an in vivo rat model that showed primed CD4(+) T cells can induce ASM and epithelial remodeling after allergen exposure.
- Hyperplasia or an abnormal increase in volume of a tissue or organ due to the increased rate of cell replication was also found in an equine model of asthma, which corroborates with the increased ASM mass.
- Furthermore, it proposes that a decreased apoptosis or cell death rate might contribute to the increase in airway smooth muscle.
Effects of Drugs on ASM
- The research then discusses the effects of drugs, specifically beta(2)-adrenergic receptor agonists (drugs that relax muscles of the airways, increasing airflow) and glucocorticoids (used for their anti-inflammatory effects), on the proliferation of ASM.
- Both types of drugs showed antiproliferative activity against a range of mitogens, but the sensitivity of mitogens varied.
- The study also found that cultivating ASM on type I collagen intensified the proliferative activity and hindered the suppressive effect of glucocorticoids. In contrast, beta(2)-agonists effects were only marginally impacted.
- Interestingly, there was no evidence suggesting that long-acting beta(2)-agonists are more effective than short-acting agonists, but persistent stimulation of the beta(2)-adrenergic receptor might assist in growth response suppression.
- Lastly, the maximum response of fluticasone propionate, a corticosteroid, against thrombin-induced proliferation was increased when combined with salmeterol, a long-acting beta2-adrenergic receptor agonist.