Background Airway mucus hypersecretion is an important pathophysiological feature of chronic obstructive pulmonary disease,which is closely associated with cigarette smoking.However,the signal transduction pathway from the cell surface to the nucleus through which cigarette smoke causes upregulation of mucin gene expression is not well known.This study was designed to investigate the role of extracellular signal-regulated Kinase 1/2 (ERK 1/2) in airway mucus hypersecretion induced by cigarette smoke in rats.Methods A rat model of airway mucus hypersecretion was induced by exposure to cigarette smoke for 4 weeks.Rats exposed to inhalation of cigarette smoke or normal saline were given an intraperitoneal injection of U0126,a specific MEK1 kinase inhibitor,at doses of 0.25 mg/kg,0.5 mg/kg and 1 mg/kg for 14 days.Expression of MUC5AC mRNA and protein,ERK 1/2 and phosphorylated-ERK 1/2 (p-ERK 1/2) were detected by RT-PCR,immunohistochemistry and West blotting.Results Cigarette smoke significantly increased airway goblet cells metaplasia,induced the overexpression of MUG5AC mRNA and protein in bronchial epithelia,and increased the ratio of p-ERK 1/2 and ERK 1/2.U0126 significantly attentuated the expression of MUC5AC mRNA and protein induced by cigarette smoke (P ＜0.05).Moreover,there was a significant positive correlation between the ratio of p-ERK1/2 to ERK1/2 and the expression of MUC5AC mRNA and protein (P＜0.05).Conclusions Inhibition of ERK 1/2 by U0126 decreased the ratio of p-ERK 1/2 to ERK 1/2 and expression of MUC5AC mRNA and protein.ERK 1/2 may play an essential role in cigarette smoke-induced mucus hypersecretion in vivo.