Background: β 2 -adrenoceptor agonists are widely used to treat asthma because of their bronchial-dilation effects. However, a recent study describing a side effect of aggravating eosinophilic inflammation in the mouse airway epithelia by β 2 -adrenoceptor agonists could impact the future clinical use of these bronchodilators. We previously reported that isoprenaline, via the apical and basolateral β 2 -adrenoceptor, induced Cl - secretion by activating cyclic AMP (cAMP)-dependent pathways in human bronchial epithelia. Despite these results, whether and how the β 2 -adrenoceptor-mediated cAMP-dependent pathway contributes to pro-inflammatory cytokine release in human bronchial epithelia remains poorly understood.
Methods: We investigated β 2 -adrenoceptor-mediated signaling pathways involved in the production of two pro-inflammatory cytokines, interleukin (IL)-6 and IL-8, in 16HBE14o- human bronchial epithelia. The effects of isoprenaline or formoterol were assessed in the presence of protein kinase A (PKA), exchange protein directly activated by cAMP (EPAC), Src, and extracellular signal-regulated protein kinase (ERK)1/2 inhibitors. The involvement of b-arrestin2 was examined using siRNA knockdown.
Results: Both isoprenaline and formoterol (both β 2 agonists) induced IL-6, but not IL-8, release, which could be inhibited by ICI 118551 (β 2 antagonist). The PKA-specific inhibitor, H89, partially inhibited IL-6 release. Another intracellular cAMP receptor, EPAC, was not involved in IL-6 release. Isoprenaline-mediated IL-6 secretion was attenuated by dasatinib, a Src inhibitor, and PD98059, an ERK1/2 inhibitor. Isoprenaline treatment also led to ERK1/2 phosphorylation. In addition, knockdown of β-arrestin2 by siRNA specifically suppressed cytokine release when a high concentration of isoprenaline (1 mM) was used.
Conclusion: Our results suggest that activation of the β 2 -adrenoceptor in 16HBE14o- cells stimulated the PKA/Src/ERK1/2 and/or β-arrestin2 signaling pathways, leading to IL-6 release. Therefore, our data reveal that β 2 -adrenoceptor signaling plays a role in the immune regulation of human airway epithelia.