The Principal Site of Action of the β2-Adrenoceptor in Promoting the Asthma Phenotype Is Airway Epithelium

Long P Nguyen1,2, Michael J Tuvim1,2, Jose M Gonzalez1,3, Craig J Daily1,3, John C McGrath1,3, Brian J Knoll1, Burton F Dickey1,2, Richard A Bond1. 1University of Houston, Department of Pharmacological and Pharmaceutical Sciences, Houston, TX 77204-5037, USA, 2MD Anderson Cancer Center, Department of Pulmonary Medicine, Houston, TX 77030, USA, 3University of Glasgow, Department of Integrated Biology, Glasgow, G12 8QQ, UK.

We have found previously that the β2-adrenoceptor (β2AR) promotes the asthma phenotype, as mice with genetic deletion of the receptor or chronic inactivation of the by β2AR inverse agonists results in attenuation of mucous metaplasia, eosinophilic inflammation, and airway hyperresponsiveness to methacholine in a murine allergen-driven asthma model (ovalbumin sensitization and challenge). Here we address the cellular site of action of the β2AR using murine models, and find seven pieces of evidence that the airway epithelium plays a central role (n = 4 to 6 mice for each group described below). 1 – Labeling with the fluorescent βAR ligand BODIPY-TMR-CGP 12177 shows that β2ARs are more highly expressed in airway epithelial cells than any other cell type within the lungs. 2 – Direct instillation of the pro-inflammatory cytokine IL-13 (1.25 µg) onto the airway epithelium of mice with genetic deletion of the β2AR, which bypasses defects in leukocyte function due to loss of β2AR activity in those cells, fails to rescue the attenuation of mucous metaplasia (P<0.05), suggesting that β2AR in epithelial cells themselves promote IL-13-dependent mucous metaplasia. 3 – Direct instillation of the β2AR inverse agonist nadolol (1.25 µg twice daily) onto the airway epithelium recapitulates the effects on mucous metaplasia and airway inflammation of nadolol administered systemically (at 250 ppm in chow). 4 – Direct instillation of the β2AR agonist formoterol (20 µg twice daily) onto the airway epithelium induces an increase in IL-13-induced mucous metaplasia and airway inflammation. 5 – Transgenic expression of the β2AR in airway secretory cells of mice with genetic deletion of the β2AR rescues most of the attenuation of IL-13-induced mucous metaplasia (85%) and airway inflammation (P<0.05). 6 – Transgenic overexpression of β2AR in airway secretory cells of wild-type mice induces a significant increase in IL-13-induced mucous metaplasia and airway inflammation. 7 – Instillation of IL-13 onto cultured human airway epithelial cells fails to induce upregulation of the expression of MUC5AC unless the β2AR is activated by agonist (P<0.05). Together, these results indicate that activity of endogenous β2AR on airway epithelial cells activated by endogenous β2AR agonists is required for full expression of the asthma phenotype induced by IL-13, and that a gain-of-function of epithelial β2AR induced by transgenic overexpression or persistent activation with an agonist of high intrinsic efficacy results in augmentation of the asthma phenotype induced by IL-13. These results may help explain the β-paradox whereby the chronic administration of β2AR agonists worsens asthma control despite improving airflow, and have implications for the chronic treatment of asthma.