Early studies revealed that cigarette
smoke promotes
gastric cancer growth through the induction of
cyclooxygenase-2 (COX-2).
Nicotine, one of the active ingredients in cigarette
smoke, has detrimental effects in the stomach. To date, there is no direct evidence to validate the effect of
nicotine on gastric
tumor growth and its carcinogenic mechanism(s). We therefore investigated whether
nicotine could promote
tumor growth and neovascularization in vivo, and the
biological mechanism(s) in connection with the signaling cascade involving COX-2 and extracellular signal-regulated
protein kinase (ERK). Athymic nude mice, with
gastric cancer cells (AGS) orthotopically implanted into the gastric wall, treated with
nicotine (50 or 200 microg/ml) in their
drinking water for 3 months developed larger
tumor areas than mice in the control group.
Nicotine further increased proliferating cellular
nuclear antigen (
PCNA) staining and microvessel density by 70 and 30%, respectively, with concomitant activation of ERK phosphorylation, COX-2 and
vascular endothelial growth factor (
VEGF) expression in the
tumors. Intraperitoneal administration of a selective
COX-2 inhibitor (
SC-236, 2 mg/kg) prevented the
nicotine-induced
tumor growth and neovascularization dose-dependently. Consistent with our animal model, an in vitro study also demonstrated that incubation with
nicotine (50-200 microg/ml) for 5 h stimulated cell proliferation dose-dependently and increased COX-2 expression,
prostaglandin E(2) (
PGE(2)) and
VEGF release, as well as activation of ERK phosphorylation. Pre-treatment with specific
mitogen-activated protein kinase kinase (
MEK) inhibitors (
U0126 or
PD98059) attenuated COX-2 expression and subsequent
PGE(2) release by
nicotine. Furthermore, the stimulatory action of
nicotine on
cancer cell growth and
angiogenic factor VEGF production was suppressed by inhibitors of
MEK (
U0126) and COX-2 (SC-236). These findings reveal a direct promoting action of
nicotine on the growth of gastric
tumor and neovascularization through sequential activation of the ERK/COX-2/
VEGF signaling pathway, which can be targeted for
chemoprevention of
gastric cancer, particularly in cigarette smokers.