AbstractInfection with the Gram-negative bacterium Helicobacter pylori leads to different clinical and pathological outcomes in humans, including chronic gastritis, peptic ulcer disease and adenocarcinoma of the stomach. H. pylori-induced damage to gastric mucosal cells is controlled by bacterial virulence factors encoded by genes of the cag pathogenicity island, which trigger the inflammatory response of the host through the activation of nuclear factor κB-dependent gene transcription. Also, H. pylori infection impairs the processes of gastric mucosal healing through inhibition of epidermal growth factor receptor-dependent signal transduction pathways and induction of apoptosis. H. pylori infection may influence the progression from chronic gastritis to gastric adenocarcinoma by stimulating cell proliferation and growth factor expression, inhibiting apoptosis and increasing the DNA mutation rate of infected gastric mucosa.
Introduction Helicobacter pylori is a Gram-negative, microaerophilic, S-shaped bacterium that is free-living in the mucous layer of the human stomach; only a small proportion attaches to gastric epithelial cells without invading them. Colonization of the stomach by H. pylori induces infiltration of the lamina propria and epithelium with immunocytes and inflammatory cells, a condition referred to as chronic gastritis or chronic active gastritis. During the years or decades that follow the initial infection, chronic gastritis may remain asymptomatic or may evolve into more severe diseases, such as peptic ulcer or atrophic gastritis. In addition, infection with H. pylori increases the risk of developing gastric adenocarcinoma and mucosa-associated lymphoid tissue lymphoma ( Blaser and Parsonnet, 1994; Dunn et al., 1997; Covacci et al., 1999).
This microreview focuses on the molecular mechanisms of gastric epithelial cell response to H. pylori-induced cell damage and on experimental evidence supporting a role for H. pylori in gastric carcinogenesis. For an in-depth discussion of other aspects of H. pylori-induced gastroduodenal disease and virulence factors, the reader is referred to earlier reviews ( Blaser and Parsonnet, 1994; Dunn et al., 1997; Covacci et al., 1999).
Mechanisms of H. pylori-induced cell damage H. pylori-induced gastroduodenal disease depends on the inflammatory response of the host and on the production of specific virulence factors that cause damage to gastric epithelial cells and disruption of the gastric mucosal barrier, such as urease, responsible for ammonia generation, and the vacuolating cytotoxin VacA ( Dunn et al., 1997; Covacci et al., 1999).
Cytokines contribute to mucosal damage, either directly or indirectly, by mediating inflammatory response to H. pylori. The gastric mucosal levels of the proinflammatory cytokines interleukin 1β (IL-1β), IL-6, IL-8 and tumour necrosis factor (TNF-α) are increased in H. pylori-infected subjects ( Wilson et al., 1998). The local cytokine response to H. pylori infection is of the Th1 type, as interferon gamma (IFN-γ), but not IL-4, is upregulated ( Lindholm et al., 1998). Variation in the ability of H. pylori strains to trigger chemokines from gastric epithelium has been linked to the presence of genes in the cag (cytotoxin-associated gene) pathogenicity island (PAI) ( Covacci et al., 1999). The cag PAI is a 40 kb chromosomal DNA insertion at the end of the glutamate racemase gene containing 31 genes, which may encode a type IV secretion system for the export of virulence determinants. Transposon inactivation of several of the cag genes abolishes the induction of IL-8 expression, a potent chemotactic and activating factor for neutrophils, in gastric epithelial cells, thus confirming the role of cag genes in the induction of inflammation ( Covacci et al., 1999). Infection by cagA+ strains is associated with enhanced chemokine and cellular responses in vivo and an increased risk of peptic ulceration, gastric atrophy and gastric cancer ( Blaser et al., 1995; Peek et al., 1997).
H. pylori-mediated IL-8 secretion in gastric epithelial cells requires activation of the transcription nuclear factor κB (NF-κB), as H. pylori strains that fail to induce IL-8 secretion do not activate NF-κB ( Munzenmaier et al., 1997; Sharma et al., 1998). Moreover, the antioxidant curcumin, which inhibits NF-κB activation, also suppresses IL-8 induction by H. pylori ( Munzenmaier et al., 1997). Because bacterial suspensions and broth culture supernatants are equally potent in activating NF-κB, the authors concluded that NF-κB induction relies on a secreted/shed H. pylori product ( Munzenmaier et al., 1997). This as yet unidentified bacterial virulence factor is not produced by strains mutated in picB/cagE, a gene in the cag PAI that shows homology to the Bordetella pertussis toxin secretion protein and may thus also act as a transport protein ( Munzenmaier et al., 1997; Sharma et al., 1998). Additional studies demonstrate that H. pylori-dependent NF-κB induction in gastric epithelial cells requires genes of the cag PAI other than cagE, as isogenic strains carrying mutations in these loci no longer activate NF-κB ( Glocker et al., 1998). On the basis of this finding, it has been proposed that proteins encoded by the cag PAI genes form a multimeric structure on the H. pylori surface ( Covacci et al., 1999). This structure would be capable of eliciting a signal transduction cascade in gastric epithelial cells, leading to the activation of NF-κB. To support this hypothesis further, it has been shown recently that genes in the cag region control host protein tyrosine phosphorylation and that tyrosine phosphorylation of host proteins is required at least in part for IL-8 synthesis in gastric epithelial cells ( Segal et al., 1997). In addition, it has been found that virulence factors encoded by the cag PAI are responsible for the upregulation of cyclooxygenase-2 (COX-2) mRNA expression and prostaglandin E2 synthesis in gastric mucosal cells in culture ( Romano et al., 1998a). COX-2, but not COX-1, upregulation has also been reported in H. pylori gastritis in humans, and this may sustain inflammation ( Fu et al., 1999; Zarrilli et al., 1999).
Other mechanisms of H. pylori-dependent gastric tissue injury include high nitric oxide production by inducible nitric oxide synthase (iNOS), which is upregulated during inflammation ( Mannick et al., 1996; Fu et al., 1999), increased oxidative DNA damage ( Baik et al., 1996) and programmed cell death (apoptosis) of gastric epithelial cells ( Mannick et al., 1996; Jones et al., 1997; Wagner et al., 1997; Rudi et al., 1998). H. pylori-induced apoptosis can be potentiated by treatment of cultured gastric cells with TNF-α or a receptor activating CD95/APO-1/Fas antibody ( Wagner et al., 1997). Moreover, lymphocytes and gastric epithelial cells show increased expression of both CD95 receptor and CD95 ligand in H. pylori-associated chronic gastritis, and H. pylori-induced apoptosis of gastric epithelial cell lines is partially prevented by blocking CD95 ( Rudi et al., 1998). Because induction of the CD95 ligand and receptor system is obtained by incubating gastric cells with bacterial broth culture filtrates, it is possible to postulate that a soluble/shed bacterial product might be responsible for this effect ( Rudi et al., 1998). Taken together, these data suggest that H. pylori is able to stimulate apoptosis of gastric epithelial cells by at least two separate mechanisms: one involving the activation of TNF-α receptor by TNF-α; the other by activating the CD95 receptor and ligand system in gastric epithelial cells ( Fig. 1). It is also interesting to note that inflammatory stimuli activating the TNF-α receptor can positively regulate apoptosis directly through caspase activation or indirectly through sustained cytokine and chemokine synthesis ( Fig. 1). On the other hand, activation of TNF-α receptor might also inhibit apoptosis through NF-κB and c-Jun activation ( Fig. 1). The fact that death receptors trigger either proapoptotic or antiapoptotic stimuli depending on the activation of different intracellular signal transduction pathways has been demonstrated clearly in other experimental systems ( Ashkenazi and Dixit, 1998). Thus, the cell suicide programme of gastric epithelial cells is the result of the balance between survival and apoptotic stimuli.
