β₂-Adrenoceptor signaling is required for the development of an asthma phenotype in a murine model

Chronic regular use of β₂-adrenoceptor (β₂- AR) agonists in asthma is associated with a loss of disease control and increased risk of death. Conversely, we have found that administration of β₂-AR inverse agonists results in attenuation of the asthma phenotype in an allergen-driven murine model. Besides antagonizing agonist-induced signaling and reducing signaling by empty receptors,β-AR inverse agonists can also activate signaling by novel pathways. To determine the mechanism of the β-AR inverse agonists, we compared the asthma phenotype in β₂-AR-null and wild-type mice. Antigen challenge of β₂-AR-null mice produced results similar to what was observed with chronic β₂-AR inverse agonist treatment, namely, reductions in mucous metaplasia, airway hyperresponsiveness (AHR), and inflammatory cells in the lungs. These results indicate that the effects of β₂-AR inverse agonists are caused by inhibition of β₂-AR signaling rather than by the induction of novel signaling pathways. Chronic administration of alprenolol, a β-blocker without inverse agonist properties, did not attenuate the asthma phenotype, suggesting that it is signaling by empty receptors, rather than agonist-induced β₂-AR signaling, that supports the asthma phenotype. In conclusion, our results demonstrate that, in a murine model of asthma, β₂-AR signaling is required for the full development of three cardinal features of asthma: mucous metaplasia, AHR, and the presence of inflammatory cells in the lungs.