Abstract

In asthma and chronic obstructive pulmonary disease (COPD) multiple mediators act on Gα_q-linked G protein coupled receptors (GPCRs) to cause bronchoconstriction. However, acting on the airway epithelium, such mediators may also elicit inflammatory responses. In human bronchial epithelial BEAS-2B cells, regulator of G protein signalling (RGS) 2 mRNA and protein was synergistically induced in response to combinations of long-acting β₂-adrenoceptor agonist (LABA) (salmeterol, formoterol) plus glucocorticoid (dexamethasone, fluticasone propionate, budesonide). Equivalent responses occurred in primary human bronchial epithelial cells. Concentrations of glucocorticoid plus LABA required to induce RGS2 expression in BEAS-2B cells were consistent with the levels achieved therapeutically in the lungs. As RGS2 is a GTPase-activating protein that switches off Gα_q, intracellular free calcium ([Ca²⁺]_i) flux was used as a surrogate of responses induced by histamine, methacholine and the thromboxane receptor agonist, U46619. This was significantly attenuated by salmeterol plus dexamethasone pre-treatment, or RGS2 over-expression, and the protective effect of salmeterol plus dexamethasone was abolished by RGS2 RNA silencing. While methacholine and U46619 induced IL-8 release and this was inhibited by RGS2 over-expression, the repression of U46619-induced IL-8 release by salmeterol plus dexamethasone was unaffected by RGS2 knockdown. Given a role for Gα_q-mediated pathways in inducing IL-8 release, we propose that RGS2 acts redundantly with other effector processes to repress IL-8 expression. Thus, RGS2 expression is a novel effector mechanism, in the airway epithelium, that is induced by glucocorticoid/LABA combinations. This could contribute to the efficacy of glucocorticoid/LABA combinations in asthma and COPD.