Abstract
BACKGROUND: In response to tissue damage or inflammation, adenosine-5'-triphosphate (ATP) is released into the extracellular compartment and has been demonstrated to augment inflammation via purinergic P2 receptors (P2Rs). Recently, ATP has been shown to be increased in the airways of COPD patients. In the present study, we examined the possible involvement of extracellular ATP in airway mucus hypersecretion during viral-induced COPD exacerbations. METHODS: The involvement of extracellular ATP in the release of a major airway mucin, MUC5AC, and its signal pathway was examined after stimulation with polyinosine-polycytidylic acid [poly(I:C)], a synthetic analog of dsRNA to mimic viral infection, and rhinovirus (RV) infection in NCI-H292 cells and differentiated airway epithelial cells from COPD patients. RESULTS: Treatment with poly(I:C) significantly increased the amount of extracellular ATP and induced MUC5AC release in NCI-H292 cells. Pre-treatment with a pannexin channel inhibitor, carbenoxolone (CBX), reduced the amount of extracellular ATP and suppressed MUC5AC release from poly(I:C)-treated cells. Pre-treatment with the P2R antagonist suramin significantly reduced the expression and release of MUC5AC. The inhibitory effects of CBX and suramin on the release of ATP and/or MUC5AC were replicated with RV infection. Pre-treatment with suramin also significantly reduced the expression and amount of extracellular EGFR ligands and the phosphorylation of EGFR and ERK in poly(I:C)-treated cells. In addition, pre-treatment with a P2Y2 receptor siRNA significantly suppressed the poly(I:C)-potentiated EGFR ligands and MUC5AC release. After poly(I:C) stimulation, the expression of MUC5AC in the differentiated cells from COPD patients was significantly higher than those from healthy subjects and the values of MUC5AC expression were inversely related with forced expiratory volume in 1 s (FEV1) % predicted. The inhibitory effects of CBX and suramin on poly(I:C)-potentiated MUC5AC expression were confirmed in differentiated airway epithelium from COPD patients. CONCLUSIONS: These results demonstrate that dsRNA induces the release of ATP via pannexin channel and that the extracellular ATP is involved in the expression and release of MUC5AC, mainly via P2Y2R, in an autocrine manner. Modulation of this pathway could be a therapeutic target for viral-induced mucus hypersecretion in COPD exacerbations.