Helicobacter pyloriInfection Induces Oxidative Stress and Programmed Cell Death in Human Gastric Epithelial Cells

Abstract

Helicobacter pylori infection is associated with altered gastric epithelial cell turnover. To evaluate the role of oxidative stress in cell death, gastric epithelial cells were exposed to various strains of H. pylori, inflammatory cytokines, and hydrogen peroxide in the absence or presence of antioxidant agents. Increased intracellular reactive oxygen species (ROS) were detected using a redox-sensitive fluorescent dye, a cytochrome c reduction assay, and measurements of glutathione. Apoptosis was evaluated by detecting DNA fragmentation and caspase activation. Infection with H. pylori or exposure of epithelial cells to hydrogen peroxide resulted in apoptosis and a dose-dependent increase in ROS generation that was enhanced by pretreatment with inflammatory cytokines. Basal levels of ROS were greater in epithelial cells isolated from gastric mucosal biopsy specimens from H. pylori-infected subjects than in cells from uninfected individuals. H. pylori strains bearing the cag pathogenicity island (PAI) induced higher levels of intracellular oxygen metabolites than isogenic cag PAI-deficient mutants. H. pylori infection and hydrogen peroxide exposure resulted in similar patterns of caspase 3 and 8 activation. Antioxidants inhibited both ROS generation and DNA fragmentation by H. pylori. These results indicate that bacterial factors and the host inflammatory response confer oxidative stress to the gastric epithelium during H. pylori infection that may lead to apoptosis.Helicobacter pylori infection has been implicated in the pathogenesis of gastritis, peptic ulcer disease, gastric carcinoma, and gastric lymphoma (10, 18), but the mechanisms leading from chronic active gastritis to other disease manifestations remain unclear. Various bacterial factors as well as the host response are believed to contribute to the outcome of infection with H. pylori (25). Strains bearing the cag pathogenicity island (PAI) (15), which includes the cagA gene, have been shown to be associated with increased gastric inflammation (57), increased bacterial load, and both peptic ulcer disease and gastric cancer (11). Increased induction of gastric epithelial cytokines that recruit and activate immune/inflammatory cells is also observed with these strains (17,41,55,67). Although the full functions of genes included in the cag PAI remain unclear, it is known that cag PAI-positive strains activate specific transcription factors and cell signaling pathways (37,41,52,55,66).Bacterial and host factors can damage the gastric mucosal barrier and lead to alterations of epithelial cell growth and differentiation. H. pylori infection is associated with increased cellular proliferation in vivo (13, 43) although most in vitro studies demonstrate bacterial inhibition of cell growth (76), suggesting that factors other than H. pylori regulate cell growth in the complex milieu of the infected gastric mucosa. Increased numbers of apoptotic cells are found in the gastric epithelium of infected patients (36,45,50,58), suggesting that induction of apoptosis may b...

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