Defective Resensitization in Human Airway Smooth Muscle Cells Evokes β-Adrenergic Receptor Dysfunction in Severe Asthma

beta(2)-adrenergic receptor (beta(2)AR) agonists (beta(2)-agonist) are the most commonly used therapy for acute relief in asthma, but chronic use of these bronchodilators paradoxically exacerbates airway hyper-responsiveness. Activation of beta ARs by beta-agonist leads to desensitization (inactivation) by phosphorylation through G-protein coupled receptor kinases (GRKs) which mediate beta-arrestin binding and beta AR internalization. Resensitization occurs by dephosphorylation of the endosomal beta ARs which recycle back to the plasma membrane as agonist-ready receptors. To determine whether the loss in beta-agonist response in asthma is due to altered beta AR desensitization and/or resensitization, we used primary human airway smooth muscle cells (HASMCs) isolated from the lungs of non-asthmatic and fatal-asthmatic subjects. Asthmatic HASMCs have diminished adenylyl cyclase activity and cAMP response to beta-agonist as compared to non-asthmatic HASMCs. Confocal microscopy showed significant accumulation of phosphorylated beta(2)ARs in asthmatic HASMCs. Systematic analysis of desensitization components including GRKs and beta-arrestin showed no appreciable differences between asthmatic and non-asthmatic HASMCs. However, asthmatic HASMC showed significant increase in PI3K gamma activity and was associated with reduction in PP2A activity. Since reduction in PP2A activity could alter receptor resensitization, endosomal fractions were isolated to assess the agonist ready beta(2)ARs as a measure of resensitization. Despite significant accumulation of beta(2)ARs in the endosomes of asthmatic HASMCs, endosomal beta(2)ARs cannot robustly activate adenylyl cyclase. Furthermore, endosomes from asthmatic HASMCs are associated with significant increase in PI3K gamma and reduced PP2A activity that inhibits beta(2)AR resensitization. Our study shows that resensitization, a process considered to be a homeostasis maintaining passive process is inhibited in asthmatic HASMCs contributing to beta(2)AR dysfunction which may underlie asthma pathophysiology and loss in asthma control.