Activation of extracellular signal-regulated kinase (ERK) and cyclooxygenase-2 (COX-2) is required for nicotine-promoted gastric cancer growth

Abstract

Cigarette smoke is closely associated with high incidence rate of gastric cancer. Several evidences have revealed that cigarette smoke promoted gastric cancer growth through the induction of COX-2 expression. Among thousands of component that present in cigarette smoke, nicotine is the most likely constituent imposes the detrimental effects in the stomach. However, very little is known about the direct action of nicotine on the development of gastric cancer growth. The aim of this study was to delineate the role of nicotine in the carcinogenecity of gastric cancer and how COX-2 and MAPK are involved in this mechanistic pathway of gastric cancer growth. Human gastric cancer xenografts (AGS, human gastric carcinoma cell line) were pretreated with nicotine (200 μg/ml) prior to the inoculation of cells into the gastric wall of the stomach in nude mice and sacrificed after 3 months. This animal model was used to study the effect of nicotine on gastric tumor growth, and to develop an effective therapeutic agent for treating gastric cancer in smokers. In vivo study showed that tumor area in nicotine-treated group was increased by more than 40% than those without. Proliferation cellular nuclear antigen (PCNA)-staining was increased by 30% and 70% in the control and nicotine-treated groups when compared with the normal group without gastric tumor. Overexpression of COX-2, cPLA2 and phospho-ERK (extracellular signal-regulated kinase) were upregulated by nicotine in the gastric cancer tissues. In addition, nicotine stimulated the subsequent prostaglandin E2 (PGE2) release in the cancerous tissues. In vitro study also showed a consistency on the promoting effect of nicotine on gastric cancer growth, namely, it stimulated cell proliferation dose-dependently, with concomitant increased in COX-2 expression, PGE2 release and ERK phosphorylation in AGS cells. As demonstrated by Western blot analysis, mitogen-activated protein kinase (MAPK) is also involved in nicotine mediated signal transduction pathway. Pretreatment with U0126 (specific inhibitor of MAP kinase-1/2) or PD98059 (specific inhibitor of MAP kinase-1) attenuated COX-2 expression and also subsequent PGE2 release induced by nicotine. Moreover, SC-236 (specific COX-2 inhibitor) or MEK inhibitors significantly abolished nicotine-induced gastric cancer cell proliferation, implicating the promoting action of nicotine on gastric cancer tumor growth was via COX-2- and ERK-related pathways. These data obtained from this novel study revealed that nicotine would promote gastric cancer growth both in vivo and in vitro; and blocking the phosphorylation of ERK and COX-2 activity were required for the antiproliferative action of chemopreventive agents for gastric cancer patients.