Helicobacter pylori eradication reduced the incidence of gastric cancer, especially of the intestinal type

BACKGROUND: Although Helicobacter pylori infection is closely associated with gastric cancer development, follow-up studies after H. pylori eradication are still scarce. AIM: To clarify the cancer preventive effect of H. pylori eradication, with special attention to differences in effect according to histology. METHODS: Patients who underwent H. pylori eradication therapy and were followed-up endoscopically for at least 1 year were analysed. The incidence of gastric cancer and factors associated with cancer development were investigated. RESULTS: A total of 1807 patients were enrolled. Six of 1519 H. pylori eradicated and five of 288 persistent subjects developed gastric cancer. Four of the eradicated subjects developed the intestinal type and two the diffuse type, while four of the persistent subjects developed the intestinal type and one the diffuse type. Kaplan-Meier analysis indicated a significantly lower incidence in eradicated patients than in persistent patients. The incidence of intestinal type was significantly lower than in eradicated patients, while the diffuse type could not be evaluated because of the low incidence. CONCLUSIONS: Helicobacter pylori-eradicated patients had a reduced incidence of gastric cancer compared with H. pylori-persistent patients, particularly the intestinal type, suggesting that H. pylori is strongly associated with intestinal-type gastric cancer.     

Risks of interleukin-1 genetic polymorphisms and Helicobacter pylori infection in the development of gastric cancer

BACKGROUND: The host genetic factors that determine the clinical outcomes of Helicobacter pylori-infected individuals remain unclear. AIM: To elucidate the risks of host interleukin-1 (IL-1) genetic polymorphisms and H. pylori infection in the development of gastric cancer. METHODS: In a case-control study of 164 controls and 142 patients with gastric cancer, the IL-1B-511 biallelic polymorphisms and the IL-1RN penta-allelic variable number of tandem repeats were genotyped. RESULTS: The carriage of IL-1RN*2, male gender, old age and H. pylori infection independently increased the risk of gastric cancer, with odds ratios of 3.3 [95% confidence interval (CI), 1.4-7.7], 2.1 (95% CI, 1.2-3.8), 5.3 (95% CI, 3.1-9.0) and 2.2 (95% CI, 1.3-3.8), respectively. H. pylori-infected individuals who were carriers of IL-1RN*2 showed increased risks of both intestinal and diffuse types of gastric cancer, with odds ratios of 11.0 and 8.7, respectively. In addition, these individuals also had a higher score of intestinal metaplasia in the corpus than did uninfected non-carriers. CONCLUSIONS: This study is the first to verify IL-1RN*2 as an independent factor governing the development of gastric cancer in Asian individuals. A combination of H. pylori testing and host genotyping may target the eradication of H. pylori to high-risk individuals.     

Review article: intestinal metaplasia and gastric carcinogenesis

Gastric intestinal metaplasia, an intermediate step in Correa's cascade of gastric carcinogenesis, is generally regarded as a pre-malignant lesion. Epidemiological studies suggest that patients with intestinal metaplasia have more than a 10-fold increased risk of developing gastric cancer. Within the subclassification of intestinal metaplasia, incomplete or type III intestinal metaplasia appears to be associated with even higher malignant potential. The topographical distribution of intestinal metaplasia may also have prognostic implications. Certain genetic and epigenetic alterations have been demonstrated in gastric intestinal metaplasia which straddle into gastric cancer. These findings suggest that genetic changes occur early in the multistep gastric carcinogenesis process. Unlike Barrett's oesophagus and colonic polyp, which have well-defined surveillance guidelines, there is no widely accepted surveillance programme for gastric intestinal metaplasia. An annual surveillance programme may allow early detection of gastric cancer, which theoretically may improve survival. It remains elusive whether the treatment of Helicobacter pylori infection may reverse gastric intestinal metaplasia or reduce the subsequent risk of cancer development. Further controlled studies with longer follow-up are needed to resolve this controversy. The role of chemo-prophylactic agents, e.g. cyclo-oxygenase-2 inhibitor, should be investigated.     

Would eradication of Helicobacter pylori infection reduce the risk of gastric cancer?

This article reviews the data on the epidemiology of gastric cancer, to determine if treatment of an asymptomatic individual can be justified. It reviews retrospective and prospective case-control studies of gastric cancer in Italy and other countries. Mucosa-associated lymphoid tissue lymphoma is associated with Helicobacter pylori infection. The risk of noncardia gastric cancer is higher (4-fold or greater) in those with H. pylori infection. Although no studies have shown prevention following treatment, erradication of asymptomatic H. pylori infection in an individual in the age group 40 or lower may be expected to reduce the risk of gastric cancer.     

Would eradication of Helicobacter pylori infection reduce the risk of gastric cancer?

This article reviews the data on the epidemiology of gastric cancer, to determine if treatment of an asymptomatic individual can be justified. It reviews retrospective and prospective case-control studies of gastric cancer in Italy and other countries. Mucosa-associated lymphoid tissue lymphoma is associated with Helicobacter pylori infection. The risk of noncardia gastric cancer is higher (4-fold or greater) in those with H. pylori infection. Although no studies have shown prevention following treatment, eradication of asymptomatic H. pylori infection in an individual in the age group 40 or lower may be expected to reduce the risk of gastric cancer.     

Will eradication of Helicobacter pylori infection influence the risk of gastric cancer?

Gastric adenocarcinoma is a disease of high mortality and poor prognosis that is second only to lung cancer as a leading cause of cancer-related deaths worldwide. Although gastric cancer has a multifactorial etiology, infection with Helicobacter pylori is highly associated with its development. New information on bacterial and host genetics and results of epidemiologic studies suggest that better identification of individuals at high risk for gastric malignancy may be possible. Studies suggest that cure of H pylori infection may be associated with retardation of glandular atrophy and intestinal metaplasia but not reversal of dysplasia. Theoretically, it is attractive to believe that eradication of H pylori infection might prevent gastric cancer; however, studies supporting this hypothesis are not yet available. Public policy strategies for the identification of patients at risk for H pylori–related gastric malignancy are likely to be complex, but testing and treating for the infection earlier rather than later in life is anticipated to be the more beneficial approach.     

Targeting Helicobacter pylori in gastric carcinogenesis

Gastric infection by Helicobacter pylori is an important risk factor for the development of gastric cancer. Recent research has identified both bacterial and host factors related to increased gastric cancer risk, including virulence-associated genes located in the cytotoxin-associated gene pathogenicity island and the vacuolating toxin A exotoxin, as well as polymorphisms in key cytokines and cytokine receptors that mediate the host's gastric inflammatory response. Early randomized trials indicate that eradicating H. pylori with antibiotics may prevent gastric cancer, although the effects so far have been modest, and are probably confined to individuals who had not developed preneoplastic lesions at the time of eradication. Targeting H. pylori to prevent gastric cancer may be best achieved through vaccination, better understanding of the molecular pathogenesis of H. pylori-associated carcinogenesis and additional chemopreventive strategies.     

Helicobacter pylori and the risk and management of associated diseases: gastritis, ulcer disease, atrophic gastritis and gastric cancer

This review addresses the role of H. pylori and the effect of H. pylori eradication on gastritis, peptic ulcer disease, atrophic gastritis and gastric cancer. Specific emphasis is given to various factors that influence the clinical course of this infection. H. pylori induces chronic gastritis in virtually all infected subjects. This inflammation can lead to peptic ulceration and atrophic gastritis in a considerable number of infected subjects. A minority eventually develops gastric cancer. The risk of such complications depends upon the severity of gastritis, which is determined by various host- and bacteria-related factors. Among bacterial factors, most of the evidence addresses the cagA pathogenicity island, the presence of which has been associated with more severe gastritis, peptic ulceration, atrophic gastritis and gastric cancer. Among host factors, most of the evidence focuses on acid production in response to H. pylori infection. An increase in acid secretion limits H. pylori gastritis to the antrum at the risk of duodenal ulcer disease; a reduction allows more proximal inflammation at the risk of atrophic gastritis, gastric ulcer disease, and gastric cancer. Gastritis and atrophy negatively influence acid secretion. H. pylori eradication is required in peptic ulcer disease and may be advocated in patients on profound acid suppressive therapy; it has been shown to cure gastritis and prevent ulcer recurrence. Further study is required to determine the efficacy of H. pylori eradication in the primary and secondary prevention of atrophic gastritis and gastric cancer.     

Systematic review: Helicobacter pylori eradication for the prevention of gastric cancer

BACKGROUND: Helicobacter pylori is recognized as one of the most significant risk factors for gastric cancer, and H. pylori eradication has been proposed as a possible primary chemo-preventive strategy to reduce gastric cancer incidence. AIM: To evaluate the available evidence on the efficacy of H. pylori eradication in the prevention of gastric cancer. METHODS: Epidemiological, observational and interventional studies, as well as decisional models, were taken into account in this review. RESULTS: Large-scale epidemiological studies clearly link H. pylori infection with non-cardia gastric cancer. Current evidence suggests that, in a subpopulation of treated subjects, H. pylori eradication prevents the progression of preneoplastic lesions. Studies that have attempted to evaluate the effect of H. pylori eradication on the incidence of gastric cancer have not provided definitive answers. H. pylori eradication seems to reduce the incidence of gastric cancer in patients without baseline precancerous gastric lesions. Decisional models suggest that H. pylori screening could be cost-effective, but there is not yet sufficient evidence to support the setting up of a general screening programme. CONCLUSION: Helicobacter pylori eradication is a plausible intervention for gastric cancer prevention; however, it seems to be relevant in only a subset of subjects.     

Is Helicobacter pylori status relevant in the management of GORD?

There is growing interest in the relationship between H. pylori infection and gastro-oesophageal reflux disease (GORD). However, this relationship is complex, as yet not fully elucidated, and probably based on a multiplicity of factors. The prevalence of H. pylori infection in patients with GORD is similar, more often lower than in matched controls. There is a negative correlation between H. pylori infection and the severity of GORD. There are many hypothetical mechanisms by which H. pylori infection may protect from the development of GORD. Conversely, there are many possible mechanisms by which H. pylori infection could theoretically foster the GORD. Patients after H. pylori eradication may develop GORD, and this seems to suggest a protective role of H. pylori infection, but other possible explanations include weight gain after H. pylori eradication, changes in dietary habits and smoking, and pre-existing GORD. H. pylori infected patients treated by various acid-inhibiting therapies such as proton pump inhibitors (PPIs), H2-receptors antagonists (H2-RA) or vagotomy, have an increase of their corpus gastritis severity, both in the activity of inflammation and in the density of organisms. Long-term therapy of GORD in H. pylori infected may lead to rapid progression of atrophic gastritis intestinal metaplasia and dysplasia, and increase the risk of developing gastric cancer. More recently it has been shown that H. pylori infection may interfere with the acid suppressive therapies used for treating GORD. In our opinion the progression of gastritis depends on the threshold of acid output at which H. pylori can 'flourish'. Recently interest is growing on gastric transitional zones and Helicobacter ecology. Any decrease of acid secretion changes the behaviour of H. pylori: the activity of gastritis improves in the antrum, but it deteriorates in the body. During proton pump inhibitor treatment, H. pylori redistribution occurs within the stomach, from an antral to a corpus or fundus prevalent pattern; corpus-fundus gastritis, exacerbated by PPI therapy, may result both in a diminished acid secretion and gastro-oesophageal reflux. The interest in Barrett's oesophagus is growing due to the associated risk of adenocarcinoma. The literature seems to demonstrate that the prevalence of H. pylori infection of the stomach in Barrett's oesophagus patients is not different from that exhibited by controls, roughly one-third of the subjects. Intestinal metaplasia of the gastric cardia seems to be equally frequent in patients with and without GORD. Finally, it appears unlikely that a causal relationship exists between H. pylori infection and Barrett's-associated adenocarcinoma.     

Review article: Helicobacter pylori and gastric cancer--the clinicians'point of view

Although the incidence of gastric cancer has declined dramatically in Western countries, the most recent data from the International Agency for Research on Cancer show that it remains the second most common cancer worldwide and caused 628 000 deaths in 1990. The incidence and prevalence of gastric cancer are projected to increase over the next few decades in less developed countries as a result of the increased longevity of H. pylori-infected populations and improved therapies. Gastric carcinogenesis is a multistep and multifactorial process beginning with H. pylori-associated gastritis in most cases. H. pylori infection, together with other environmental factors and individual susceptibility, determine the final risk for the development of gastric cancer. The magnitude of H. pylori infection as a risk factor for gastric cancer in the published H. pylori and gastric cancer epidemiology studies may have been underestimated due to the inclusion of improperly selected controls. Eradication of the infection has been shown to prevent the occurrence of metachronous gastric cancer following endoscopic resection of early gastric cancer in a Japanese study. However, the generalization of this study to other populations is difficult because of the vast differences in the definition of gastric atrophy and early gastric cancer between Japanese and Western pathologists. Until an international consensus on the pathological diagnosis of gastric atrophy and early gastric cancer is reached, interpretation of studies performed in different countries remains difficult. Clinicians rely on the correct pathological diagnosis to guide the management of H. pylori infection-associated gastrointestinal diseases.     

DNA damage and oxidative stress induced by Helicobacter pylori in gastric epithelial cells: protection by vitamin C and sodium selenite

The direct effect of intact Helicobacter pylori on gastric epithelial cells SGC-7901 and the protection given by the antioxidants vitamin C and sodium selenite were studied. Incubation of SGC-7901 cells with H. pylori simultaneously caused a significant increase of DNA damage (DNA strand breakage and DNA fragmentation) and ROS formation, as well as a significant decrease of intracellular GSH content in a H. pylori multiplicity of infection (MOI) dependent manner in gastric cells. ROS formation was strongly positively correlated while GSH content was negatively correlated with DNA strand breakage and fragmentation, indicating that DNA damage may be mainly caused by H. pylori-induced oxidative stress in gastric cells. The antioxidants, vitamin C and sodium selenite, directly increased GSH content while diminishing ROS formation and DNA damage in H. pylori-infected SGC-7901 cells, indicating that vitamin C and sodium selenite can protect gastric cells against H. pylori damage. The protections by vitamin C and sodium selenite further demonstrated that DNA damage may be derived from oxidative stress in H. pylori-infected gastric cells. The results suggested that DNA damage caused by H. pylori-induced oxidative stress may be one important factor in the pathogenesis of H. pylori, and that vitamin C and sodium selenite may have a preventive or therapeutic role against H. pylori-associated gastric diseases.     

Chronic celecoxib users more often show regression of gastric intestinal metaplasia after Helicobacter pylori eradication

BACKGROUND AND AIM: To test whether the chronic users of celecoxib, a selective cyclo-oxygenase-2 inhibitor, had less Helicobacter pylori-related intestinal metaplasia or if such users' intestinal metaplasia could be prone to disappear after H. pylori eradication. METHODS: The study enrolled 150 chronic celecoxib users and 216 non-users who underwent pan-endoscopy to detect H. pylori infection and its related intestinal metaplasia. One hundred and three H. pylori-infected patients with intestinal metaplasia (43 chronic celecoxib users and 60 non-users) received anti-H. pylori therapy and completed the 12-month follow-up to survey the regression of intestinal metaplasia by mean intestinal metaplasia score. RESULTS: There were no differences in the prevalence of H. pylori-related intestinal metaplasia between the chronic celecoxib users and controls (P > 0.05). On the 12th month of follow-up, chronic celecoxib users had a lower mean intestinal metaplasia score (1.2 vs. 1.8, P < 0.005) and a higher regression rate of intestinal metaplasia (42% vs. 20%, P = 0.027) than non-users. CONCLUSIONS: With H. pylori infection, chronic celecoxib users still showed limited effects to decrease intestinal metaplasia. Nevertheless, celecoxib should be promising to assist H. pylori eradication for the control of gastric intestinal metaplasia and cancer risk.     

Helicobacter pylori, chronic gastritis and peptic ulcer.

H. pylori infection is strongly associated with chronic gastritis and is probably the main course of chronic inflammation in the gastric mucosa. Acquisition of the infection will lead to gastritis and the eradication of the bacterium results in healing and final cure of the gastritis. Chronic gastritis and H. pylori infection may occur in antrum and/or corpus, and will gradually result in atrophy of the underlying mucosa in a great number of affected persons. Correspondingly, impairments in several important functions of the gastric mucosa are consequences of the atrophy and inflammation. Hp infection and gastritis associate with important gastroduodenal diseases, such as peptic ulcer diseases and gastric cancer. There are reasons to suggest that the infection and subsequent gastritis precede these diseases, and that they are important risk factors for both disorders. In fact, peptic ulcer can be seen a late frequent complication of an H. pylori infection. In addition, the type and grade of gastritis can be used in prediction of the ulcer risk if adequate biopsy specimens are available for microscopy from both antrum and corpus mucosa (Table II). An association between chronic gastritis and Helicobacter pylori (Hp) infection is strong. Clearly more than 80% of cases with chronic gastritis are related to coexistent Hp infection. Furthermore, both Hp and gastritis are extremely common in patients with peptic ulcer supporting the view that they are causally related to the ulcer disease. Correspondingly, similar suggestions may also be presented about the links between Hp and gastritis in the pathogenesis of gastric cancer.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of cyclooxygenase-2 inhibition on human Helicobacter pylori gastritis: mechanisms underlying gastrointestinal safety and implications for cancer chemoprevention

Cyclooxygenase (COX)-2 expression and prostaglandin production is increased by Helicobacter pylori infection. Non-selective COX inhibitors reduce prostaglandins and mucosal proliferation in infected mucosa and may reduce gastric cancer risk, but ulceration precludes their use. COX-2 inhibitors cause fewer ulcers and may be chemopreventive. Physiological studies of COX-2 inhibitors in humans with H. pylori infection have not been performed. AIM: To study the impact of COX-2 specific inhibition on gastric prostaglandin levels, H. pylori gastritis and proliferation. METHODS: Twenty infected (eight males, 12 females; age 38 +/- 1.8) and six uninfected (four males, two females; age 36 +/- 3.5) healthy volunteers received rofecoxib 25 mg daily for 14 days. Endoscopic biopsies were evaluated for prostaglandin E2 (PGE2) content, gastritis and proliferation. RESULTS: Before drug therapy, compared to uninfected, H. pylori-infected subjects had significantly higher: (a) gastric mucosal PGE2 (pg/mg tissue) in the gastric body and antrum, (b) H. pylori score in body and antrum and (c) mid-gland proliferation index in antrum and body. The COX-2 inhibitor did not significantly affect PGE2 levels, gastritis scores or proliferation indices in the body or antrum in the H. pylori-positive or -negative subjects. CONCLUSION: The predominant source of increased gastric PGE2 in H. pylori infection appears to be COX-1-derived. In non-ulcerated H. pylori gastritis, COX-2 inhibition does not affect cellular proliferation. Rofecoxib's lack of effect on gastric prostaglandin levels and proliferation in H. pylori-infected mucosa may explain the absence of an increased ulcer risk among COX-2 inhibitor users with H. pylori infection. The lack of significant effect on intermediate biomarkers raises uncertainty regarding the potential of specific COX-2 inhibitors for chemoprevention of gastric cancer.     

胃黏膜病变组织COX-2蛋白表达的研究

目的：研究胃黏膜病变组织中环氧化酶－2（COX－2）蛋白的表达，探讨其与胃癌类型，分期，分化，转移及幽门螺杆菌（Hp)感染之间的内在关系。方法：外科手术中取30例胃癌者的癌组织，癌旁非癌组织各两块，石蜡包埋，切片HE染色作病理及免疫组织化学检查COX－2蛋白表达，Hp阳性感染由快速尿素酶试验结合病理染色／^14C尿素呼气试验而确立。结果：COX－2在胃癌中阳性表达率（66．7％），癌旁组织（26．7％），差异具有显著性（P＜0．01），其表达强度差异也有非常显著性意义（P＜0．001）。COX－2蛋白阳性表达率；进展期胃癌（76．0％）显著高于早期胃癌（20．0％）（P＜0．05）；有淋巴结转移（79．2％）显著高于无淋巴结转移（16．7％）（P＜0．05）；肠型胃癌（66．7％）与胃型胃癌（66．7％），低，未分化癌（80．0％）与高、中分化癌（57．1％），相比统计学上均无显著性差异（P＞0．05）。Hp阳性肠化或异型增生组COX－2的表达显著高于Hp阴性组（P＜0．05）。结论：COX－2蛋白在胃癌有较高水平的表达，它的异常表达多与胃癌的进展及淋巴结转移有关而与胃癌类型，病理学上分化程度关系上。COX－2基因在胃癌早期发生发展过程中可能起重要的作用，并与Hp感染有一定关系。     

Apoptosis and proliferation in Helicobacter pylori-associated gastric intestinal metaplasia

BACKGROUND: Imbalance between apoptosis and proliferation may be one of the mechanisms underlying H. pylori associated gastric carcinogenesis. AIM: To examine the cell kinetics of gastric intestinal metaplasia and the effect of H. pylori eradication. METHODS: Endoscopic gastric biopsies were obtained from 100 H. pylori-infected patients. Apoptosis was determined by triphosphate nick-end labelling (TUNEL) and apoptotic nuclei counting, whereas proliferation was assessed by Ki67 immunostaining. Gastric biopsies were repeated in a sub-group of intestinal metaplasia patients after H. pylori eradication. RESULTS: Antral apoptotic index was significantly lower in intestinal metaplasia than in non-intestinal metaplasia (0.19% vs. 0.51%; P < 0.0001) whereas the level of proliferation was comparable (28% vs. 22%, P=0.15). Serial antral biopsies taken from 14 intestinal metaplasia patients before and 1 year after H. pylori eradication showed a significant drop in proliferation in both intestinal metaplasia (50% vs. 12%, P < 0.001) and non-intestinal metaplasia area (47% vs. 9%, P < 0.001). A similar fall in apoptosis was detected in non-metaplastic region (0.58% vs. 0.38%, P < 0.001) but not in intestinal metaplasia (0.24% vs. 0.27%, P=0.56), resulting in a significant increase in the apoptosis/proliferation ratio (0.005-0.021; P=0.03). CONCLUSIONS: Dysregulation in apoptosis control of gastric intestinal metaplasia may contribute to gastric carcinogenesis, which may be retarded by clearance of H. pylori.     

Review article: gastric cancer and Helicobacter pylori

Gastric cancer is the second commonest cause of death from malignancy in the world. Its pathogenesis is comparatively well understood and its aetiology multifactorial. Non-cardia gastric cancer usually arises in a stomach that has been inflamed over a long period and where atrophy and intestinal metaplasia have supervened. The commonest cause of gastric inflammation is infection with Helicobacter pylori. Colonization with this organism increases the relative risk of developing this cancer by about six [Helicobacter and Cancer Collaborative Group. Gut 2001; 49: 347-53]. Its likelihood increases with the severity and extent of the gastritis. Severity is influenced by the virulence of the infecting organism, the genetics of the host, bile reflux, dietary factors and the presence of hypochlorhydria which influences the extent, as well as the severity, of the inflammation. The only predisposing factor which can easily be manipulated is H. pylori infection, which can be successfully treated in 80-90% of cases using a 1-week therapeutic regimen.     

Helicobacter pylori infection and gastric cancer: systematic review of the epidemiological studies

BACKGROUND: The published epidemiological studies of chronic Helicobacter pylori infection and gastric cancer yield conflicting results, so there is uncertainty as to whether any material association exists and, if so, how strong it is. AIM: To review these studies quantitatively. METHODS: A systematic review of sero-epidemiological studies published before 1998 of H. pylori and gastric cancer, as identified by computer-assisted literature searches of relevant journals, reference lists and discussions with authors. All relevant studies identified were included, subdivided by study design. The following was abstracted from published reports: adjusted odds ratio (or, in prospective studies, the risk ratio) and confidence interval, study design, type of controls, mean age, mean duration of follow-up, assay methods, location of study, and degree of adjustment for confounders. RESULTS: The 34 retrospective studies included in total 3300 gastric cancers, but their controls were of uncertain validity. The 10 'nested' case-control comparisons in prospective studies included in total only 800 gastric cancers, and combined analysis of them yielded a risk ratio of 2.5 (95% CI: 1.9-3.4; 2P < 0.00001) for gastric cancer in people seropositive for H. pylori antibodies. CONCLUSIONS: The prospective studies suggest that gastric cancer is 2 or 3 times as common in those chronically infected by H. pylori, but to help investigate causality, further observational studies are still needed, as are large-scale randomized trials of whether antibacterial regimens reduce the eventual incidence of gastric cancer.     

CagA protein of Helicobacter pylori: a hijacker of gastric epithelial cell signaling

Epidemiological study has shown strong correlation between the Helicobacter pylori (H. pylori) infection and gastric carcinogenesis. However, the mechanism by which H. pylori induces gastric carcinogenesis is not known. In this review, we focused on the product of cytotoxin-associated gene A (CagA), one of the important virulence factors of H. pylori. H. pylori injects CagA protein into the host gastric epithelial cells through its needle-like structure, type IV secretion system. Injected CagA hijacks physiological signal transduction and causes pathological cellular response such as increased cell proliferation, motility, apoptosis and morphological change through different mechanisms. H. pylori has been shown to produce reactive oxygen species (ROS) in infected gastric mucosa. Although the main source of ROS production is possibly host neutrophil, we propose novel source of ROS production in this review; CagA itself can induce ROS production in gastric epithelial cell. Excessive ROS production in gastric epithelial cells can cause DNA damage and thus might involve in gastric carcinogenesis. Understanding the molecular mechanism by which H. pylori-induced carcinogenesis is important for developing new strategies against gastric cancer.