Phenotypic diversity and molecular mechanisms of airway smooth muscle proliferation in asthma

Chronic persistent asthma is characterized by poorly reversible airflow obstruction and airways inflammation and remodelling. Histopathological studies of airways removed at post mortem from patients with severe asthma reveal marked inflammatory and architectural changes associated with airway wall thickening. Increased airway smooth muscle content, occurring as a result of hyperplastic and/or hypertrophic growth, is believed to be one of the principal contributors to airway wall thickening. In recent years, significant advances have been made in elucidating the mediators and the intracellular pathways that regulate proliferation of airway smooth muscle. The contribution that smooth muscle makes to persistent airflow obstruction may not, however, be limited simply to its increased bulk within the airway wall. Interest is growing in the possibility that reversible phenotypic modulation and increased heterogeneity of airway smooth muscle function may also be a feature of the asthmatic airway. This review focuses on possible mechanisms controlling smooth muscle phenotype heterogeneity as well as on the mediators and intracellular pathways implicated in its cellular proliferation. Particular attention is paid to mechanisms involving activation of the extracellular signal regulated kinase-, protein kinase C- and phosphoinositide 3-kinase-dependent pathways, since these appear to be the major candidate second messenger pathways for G protein- and tyrosine kinase-coupled receptor-stimulated proliferation.