Helicobacter pylori and H2O2 increase AP endonuclease-1/redox factor-1 expression in human gastric epithelial cells

Helicobacter pylori and Gastric Cancer: Pathogenetic Mechanisms

Gastric cancer is the sixth most commonly diagnosed cancer and the fourth leading cause of cancer death worldwide. Helicobacter pylori (H. pylori) is one of the main risk factors for this type of neoplasia. Carcinogenetic mechanisms associated with H. pylori are based, on the one hand, on the onset of chronic inflammation and, on the other hand, on bacterial-specific virulence factors that can damage the DNA of gastric epithelial cells and promote genomic instability. Here, we review and discuss the major pathogenetic mechanisms by which H. pylori infection contributes to the onset and development of gastric cancer.

A Novel Hypomorphic Apex1 Mouse Model Implicates Apurinic/Apyrimidinic Endonuclease 1 in Oxidative DNA Damage Repair in Gastric Epithelial Cells

Aims: Though best known for its role in oxidative DNA damage repair, apurinic/apyrimidinic endonuclease 1 (APE1) is a multifunctional protein that regulates multiple host responses during oxidative stress, including the reductive activation of transcription factors. As knockout of the APE1-encoding gene, Apex1, is embryonically lethal, we sought to create a viable model with generalized inhibition of APE1 expression. Results: A hypomorphic (HM) mouse with decreased APE1 expression throughout the body was generated using a construct containing a neomycin resistance (NeoR) cassette knocked into the Apex1 site. Offspring were assessed for APE1 expression, breeding efficiency, and morphology with a focused examination of DNA damage in the stomach. Heterozygotic breeding pairs yielded 50% fewer HM mice than predicted by Mendelian genetics. APE1 expression was reduced up to 90% in the lungs, heart, stomach, and spleen. The HM offspring were typically smaller, and most had a malformed tail. Oxidative DNA damage was increased spontaneously in the stomachs of HM mice. Further, all changes were reversed when the NeoR cassette was removed. Primary gastric epithelial cells from HM mice differentiated more quickly and had more evidence of oxidative DNA damage after stimulation with Helicobacter pylori or a chemical carcinogen than control lines from wildtype mice. Innovation: A HM mouse with decreased APE1 expression throughout the body was generated and extensively characterized. Conclusion: The results suggest that HM mice enable studies of APE1's multiple functions throughout the body. The detailed characterization of the stomach showed that gastric epithelial cells from HM were more susceptible to DNA damage. Antioxid. Redox Signal. 38, 183-197.

APE1/Ref-1 Role in Inflammation and Immune Response

Apurinic/apyrimidinic endonuclease 1/redox effector factor 1 (APE1/Ref-1) is a multifunctional enzyme that is essential for maintaining cellular homeostasis. APE1 is the major apurinic/apyrimidinic endonuclease in the base excision repair pathway and acts as a redox-dependent regulator of several transcription factors, including NF-κB, AP-1, HIF-1α, and STAT3. These functions render APE1 vital to regulating cell signaling, senescence, and inflammatory pathways. In addition to regulating cytokine and chemokine expression through activation of redox sensitive transcription factors, APE1 participates in other critical processes in the immune response, including production of reactive oxygen species and class switch recombination. Furthermore, through participation in active chromatin demethylation, the repair function of APE1 also regulates transcription of some genes, including cytokines such as TNFα. The multiple functions of APE1 make it an essential regulator of the pathogenesis of several diseases, including cancer and neurological disorders. Therefore, APE1 inhibitors have therapeutic potential. APE1 is highly expressed in the central nervous system (CNS) and participates in tissue homeostasis, and its roles in neurodegenerative and neuroinflammatory diseases have been elucidated. This review discusses known roles of APE1 in innate and adaptive immunity, especially in the CNS, recent evidence of a role in the extracellular environment, and the therapeutic potential of APE1 inhibitors in infectious/immune diseases.

Escherichia coli induces DNA repair enzymes to protect itself from low-grade hydrogen peroxide stress

E. coli responds to hydrogen peroxide (H₂ O₂ ) by inducing defenses that protect H₂ O₂ -sensitive enzymes. DNA is believed to be another important target of oxidation, and E. coli contains enzymes that can repair oxidative lesions in vitro. However, those enzymes are not known to be induced by H₂ O₂ , and experiments have indicated that they are not necessary for the cell to withstand natural (low-micromolar) concentrations. In this study we used H₂ O₂ -scavenging mutants to impose controlled doses of H₂ O₂ for extended time. Transcriptomic analysis revealed that in the presence of 1 µM cytoplasmic H₂ O₂ , the OxyR transcription factor induced xthA, encoding exonuclease III. The xthA mutants survived a conventional 15-minute exposure to even 100 times this level of H₂ O₂ . However, when these mutants were exposed to 1 µM H₂ O₂ for hours, they accumulated DNA lesions, failed to propagate, and eventually died. Although endonuclease III (nth) was not induced, nth mutants struggled to grow. Low-grade H₂ O₂ stress also activated the SOS regulon, and when this induction was blocked, cell replication stopped. Collectively, these data indicate that physiological levels of H₂ O₂ are a real threat to DNA, and the engagement of the base-excision-repair and SOS systems is necessary to enable propagation during protracted stress.

Impact of Helicobacter pylori Infection and Its Major Virulence Factor CagA on DNA Damage Repair

Helicobacter pylori infection induces a plethora of DNA damages. Gastric epithelial cells, in order to maintain genomic integrity, require an integrous DNA damage repair (DDR) machinery, which, however, is reported to be modulated by the infection. CagA is a major H. pylori virulence factor, associated with increased risk for gastric carcinogenesis. Its pathogenic activity is partly regulated by phosphorylation on EPIYA motifs. Our aim was to identify effects of H. pylori infection and CagA on DDR, investigating the transcriptome of AGS cells, infected with wild-type, ΔCagA and EPIYA-phosphorylation-defective strains. Upon RNA-Seq-based transcriptomic analysis, we observed that a notable number of DDR genes were found deregulated during the infection, potentially resulting to base excision repair and mismatch repair compromise and an intricate deregulation of nucleotide excision repair, homologous recombination and non-homologous end-joining. Transcriptome observations were further investigated on the protein expression level, utilizing infections of AGS and GES-1 cells. We observed that CagA contributed to the downregulation of Nth Like DNA Glycosylase 1 (NTHL1), MutY DNA Glycosylase (MUTYH), Flap Structure-Specific Endonuclease 1 (FEN1), RAD51 Recombinase, DNA Polymerase Delta Catalytic Subunit (POLD1), and DNA Ligase 1 (LIG1) and, contrary to transcriptome results, Apurinic/Apyrimidinic Endodeoxyribonuclease 1 (APE1) upregulation. Our study accentuates the role of CagA as a significant contributor of H. pylori infection-mediated DDR modulation, potentially disrupting the balance between DNA damage and repair, thus favoring genomic instability and carcinogenesis.

Crosstalk Between DNA Damage and Inflammation in the Multiple Steps of Gastric Carcinogenesis

Over the last years, intensive investigations in molecular biology and cell physiology extended tremendously the knowledge about the association of inflammation and cancer. In frame of this paradigm, the human pathogen Helicobacter pylori triggers gastritis and gastric ulcer disease, and contributes to the development of gastric cancer. Mechanisms, by which the bacteria-induced inflammation in gastric mucosa leads to intestinal metaplasia and carcinoma, are represented in this review. An altered cell-signaling response and increased production of free radicals by epithelial and immune cells account for the accumulation of DNA damage in gastric mucosa, if infection stays untreated. Host genetics and environmental factors, especially diet, can accelerate the process, which offers the opportunity of intervention based on a balanced nutrition. It is supposed that inflammation might influence stem- or progenitor cells in gastric tissue predisposing for metaplasia or tumor relapse. Herein, DNA is strongly mutated and labile, which restricts therapy options. Thus, the understanding of the mechanisms that underlie gastric carcinogenesis will be of preeminent importance for the development of strategies for screening and early detection. As most gastric cancer patients face late-stage disease with a poor overall survival, the development of multi-targeted therapeutic intervention strategies is a major challenge for the future.

Analysis of key genes and signaling pathways involved in Helicobacter pylori-associated gastric cancer based on The Cancer Genome Atlas database and RNA sequencing data

BACKGROUND

Helicobacter pylori (H. pylori) infection is associated with the development of gastric cancer, although the mechanism is unclear. Herein, this study aimed to clarify the key genes and signaling pathways involved in H. pylori pathogenesis based on The Cancer Genome Atlas (TCGA) database and RNA sequencing analysis.

MATERIALS AND METHODS

Forty-nine gastric cancer samples (16 with H. pylori and 33 without H. pylori) and 35 cancer-adjacent normal samples from TCGA database were analyzed by bioinformatics. The differentially expressed genes between H. pylori-positive and H. pylori-negative patients were verified in 18 gastric cancer (GC) samples (9 with H. pylori and 9 without H. pylori), which were analyzed using RNA sequencing. Survival analysis was carried out to explore associations between the differentially expressed genes and prognosis. Bioinformatics analysis was performed to determine the signaling pathways associated with H. pylori.

RESULTS

The baseline level of clinical features from TCGA database and RNA sequencing showed no differences between the H. pylori-positive and H. pylori-negative GC groups (P > 0.05). TP53 was shown to be upregulated in the H. pylori-positive group in both TCGA database and RNA sequencing data, which also showed higher expression in the GC tissues than in adjacent normal tissues (P < 0.05). CCDC151, CHRNB2, GMPR2, HDGFRP2, and VSTM2L were shown to be downregulated in the H. pylori-positive group by both TCGA database and RNA sequencing, which also showed lower expression in the GC tissues than in adjacent normal tissues (P < 0.05). GC patients with low expression levels of HDGFRP2 had a poor prognosis (P < 0.05). Thirty-three signaling pathways and 10 biological processes were found to be positively associated with H. pylori infection (P < 0.05, FDR < 0.05).

CONCLUSIONS

These results indicate that some genes (TP53, CCDC151, CHRNB2, GMPR2, HDGFRP2, VSTM2L) and previously unidentified signaling pathways (eg, the Hippo signaling pathway) might play an important role in H. pylori-associated GC.

Molecular Mechanisms of H. pylori-Induced DNA Double-Strand Breaks

Infections contribute to carcinogenesis through inflammation-related mechanisms. H. pylori infection is a significant risk factor for gastric carcinogenesis. However, the molecular mechanism by which H. pylori infection contributes to carcinogenesis has not been fully elucidated. H. pylori-associated chronic inflammation is linked to genomic instability via reactive oxygen and nitrogen species (RONS). In this article, we summarize the current knowledge of H. pylori-induced double strand breaks (DSBs). Furthermore, we provide mechanistic insight into how processing of oxidative DNA damage via base excision repair (BER) leads to DSBs. We review recent studies on how H. pylori infection triggers NF-κB/inducible NO synthase (iNOS) versus NF-κB/nucleotide excision repair (NER) axis-mediated DSBs to drive genomic instability. This review discusses current research findings that are related to mechanisms of DSBs and repair during H. pylori infection.

Testin and filamin-C downregulation by acetylated Siah2 increases invasiveness of Helicobacter pylori-infected gastric cancer cells

Helicobacter pylori is the strongest known risk-factor for gastric cancer. However, its role in gastric cancer metastasis remains unclear. Previously we have reported that H. pylori promotes gastric cancer invasiveness by stabilizing the E3 ubiquitin ligase Siah2 which is mediated by Siah2 acetylation at Lys 139 (K¹³⁹) residue. Here we identify that cell adhesion-related proteins testin (TES) and filamin-C (FLN-C) interact with Siah2 and get proteasomally degraded. The efficiency of TES and FLN-C degradation is significantly potentiated by K¹³⁹-acetylated Siah2 (ac-K¹³⁹ Siah2) in infected gastric cancer cells (GCCs). ac-Siah2-mediated downregulation of TES and FLN-C disrupts filopodia structures but promotes lamellipodia formation and enhances invasiveness and migration of infected GCCs. Since H. felis-infected mice as well as human gastric cancer biopsy samples also show high level of ac-K¹³⁹ Siah2 and downregulated TES and FLN-C, we believe that acetylation of Siah2 is an important checkpoint that can be useful for therapeutic intervention.

The Pivotal Role of DNA Repair in Infection Mediated-Inflammation and Cancer

Pathogenic and commensal microbes induce various levels of inflammation and metabolic disease in the host. Inflammation caused by infection leads to increased production of reactive oxygen species (ROS) and subsequent oxidative DNA damage. These in turn cause further inflammation and exacerbation of DNA damage, and pose a risk for cancer development. Helicobacter pylori-mediated inflammation has been implicated in gastric cancer in many previously established studies, and Fusobacterium nucleatum presence has been observed with greater intensity in colorectal cancer patients. Despite ambiguity in the exact mechanism, infection-mediated inflammation may have a link to cancer development through an accumulation of potentially mutagenic DNA damage in surrounding cells. The multiple DNA repair pathways such as base excision, nucleotide excision, and mismatch repair that are employed by cells are vital in the abatement of accumulated mutations that can lead to carcinogenesis. For this reason, understanding the role of DNA repair as an important cellular mechanism in combatting the development of cancer will be essential to characterizing the effect of infection on DNA repair proteins and to identifying early cancer biomarkers that may be targeted for cancer therapies and treatments.

Exploiting the Ref-1-APE1 node in cancer signaling and other diseases: from bench to clinic

Reduction-oxidation factor 1-apurinic/apyrimidinic endonuclease (Ref-1/APE1) is a critical node in tumor cells, both as a redox regulator of transcription factor activation and as part of the DNA damage response. As a redox signaling protein, Ref-1/APE1 enhances the transcriptional activity of STAT3, HIF-1α, nuclear factor kappa B, and other transcription factors to promote growth, migration, and survival in tumor cells as well as inflammation and angiogenesis in the tumor microenvironment. Ref-1/APE1 is activated in a variety of cancers, including prostate, colon, pancreatic, ovarian, lung and leukemias, leading to increased aggressiveness. Transcription factors downstream of Ref-1/APE1 are key contributors to many cancers, and Ref-1/APE1 redox signaling inhibition slows growth and progression in a number of tumor types. Ref-1/APE1 inhibition is also highly effective when paired with other drugs, including standard-of-care therapies and therapies targeting pathways affected by Ref-1/APE1 redox signaling. Additionally, Ref-1/APE1 plays a role in a variety of other indications, such as retinopathy, inflammation, and neuropathy. In this review, we discuss the functional consequences of activation of the Ref-1/APE1 node in cancer and other diseases, as well as potential therapies targeting Ref-1/APE1 and related pathways in relevant diseases. APX3330, a novel oral anticancer agent and the first drug to target Ref-1/APE1 for cancer is entering clinical trials and will be explored in various cancers and other diseases bringing bench discoveries to the clinic.

Membrane-bound β-catenin degradation is enhanced by ETS2-mediated Siah1 induction in Helicobacter pylori-infected gastric cancer cells

β-catenin has two different cellular functions: intercellular adhesion and transcriptional activity. The E3 ubiquitin ligase Siah1 causes ubiquitin-mediated degradation of the cytosolic β-catenin and therefore, impairs nuclear translocation and oncogenic function of β-catenin. However, the effect of Siah1 on the cell membrane bound β-catenin has not been studied. In this study, we identified that the carcinogenic bacterium H. pylori increased ETS2 transcription factor-mediated Siah1 protein expression in gastric cancer cells (GCCs) MKN45, AGS and Kato III. Siah1 protein level was also noticeably higher in gastric adenocarcinoma biopsy samples as compared to non-cancerous gastric epithelia. Siah1 knockdown significantly decreased invasiveness and migration of H. pylori-infected GCCs. Although, Siah1 could not increase degradation of the cytosolic β-catenin and its nuclear translocation, it enhanced degradation of the membrane-bound β-catenin in the infected GCCs. This loss of membrane-bound pool of β-catenin was not associated with the proteasomal degradation of E-cadherin. Thus, this work delineated the role of Siah1 in increasing invasiveness of H. pylori-infected GCCs.

Redox signaling in the gastrointestinal tract

Redox signaling regulates physiological self-renewal, proliferation, migration and differentiation in gastrointestinal epithelium by modulating Wnt/β-catenin and Notch signaling pathways mainly through NADPH oxidases (NOXs). In the intestine, intracellular and extracellular thiol redox status modulates the proliferative potential of epithelial cells. Furthermore, commensal bacteria contribute to intestine epithelial homeostasis through NOX1- and dual oxidase 2-derived reactive oxygen species (ROS). The loss of redox homeostasis is involved in the pathogenesis and development of a wide diversity of gastrointestinal disorders, such as Barrett's esophagus, esophageal adenocarcinoma, peptic ulcer, gastric cancer, ischemic intestinal injury, celiac disease, inflammatory bowel disease and colorectal cancer. The overproduction of superoxide anion together with inactivation of superoxide dismutase are involved in the pathogenesis of Barrett's esophagus and its transformation to adenocarcinoma. In Helicobacter pylori-induced peptic ulcer, oxidative stress derived from the leukocyte infiltrate and NOX1 aggravates mucosal damage, especially in HspB+ strains that downregulate Nrf2. In celiac disease, oxidative stress mediates most of the cytotoxic effects induced by gluten peptides and increases transglutaminase levels, whereas nitrosative stress contributes to the impairment of tight junctions. Progression of inflammatory bowel disease relies on the balance between pro-inflammatory redox-sensitive pathways, such as NLRP3 inflammasome and NF-κB, and the adaptive up-regulation of Mn superoxide dismutase and glutathione peroxidase 2. In colorectal cancer, redox signaling exhibits two Janus faces: On the one hand, NOX1 up-regulation and derived hydrogen peroxide enhance Wnt/β-catenin and Notch proliferating pathways; on the other hand, ROS may disrupt tumor progression through different pro-apoptotic mechanisms. In conclusion, redox signaling plays a critical role in the physiology and pathophysiology of gastrointestinal tract.

Helicobacter pylori infection and gastric carcinoma: Not all the strains and patients are alike

Gastric carcinoma (GC) develops in only 1%-3% of Helicobacter pylori (H. pylori) infected people. The role in GC formation of the bacterial genotypes, gene polymorphisms and host's factors may therefore be important. The risk of GC is enhanced when individuals are infected by strains expressing the oncoprotein CagA, in particular if CagA has a high number of repeats containing the EPIYA sequence in its C'-terminal variable region or particular amino acid sequences flank the EPIYA motifs. H. pylori infection triggers an inflammatory response characterised by an increased secretion of some chemokines by immunocytes and colonised gastric epithelial cells; these molecules are especially constituted by proteins composing the interleukin-1beta (IL-1β) group and tumour necrosis factor-alpha (TNF-α). Polymorphisms in the promoter regions of genes encoding these molecules, could account for high concentrations of IL-1β and TNF-α in the gastric mucosa, which may cause hypochlorhydria and eventually GC. Inconsistent results have been attained with other haplotypes of inflammatory and anti-inflammatory cytokines. Genomic mechanisms of GC development are mainly based on chromosomal or microsatellite instability (MSI) and deregulation of signalling transduction pathways. H. pylori infection may induce DNA instability and breaks of double-strand DNA in gastric mucocytes. Different H. pylori strains seem to differently increase the risk of cancer development run by the host. Certain H. pylori genotypes (such as the cagA positive) induce high degrees of chronic inflammation and determine an increase of mutagenesis rate, oxidative-stress, mismatch repair mechanisms, down-regulation of base excision and genetic instability, as well as generation of reactive oxygen species that modulate apoptosis; these phenomena may end to trigger or concur to GC development.

Regulation of Rac1 and Reactive Oxygen Species Production in Response to Infection of Gastrointestinal Epithelia

Generation of reactive oxygen species (ROS) during infection is an immediate host defense leading to microbial killing. APE1 is a multifunctional protein induced by ROS and after induction, protects against ROS-mediated DNA damage. Rac1 and NAPDH oxidase (Nox1) are important contributors of ROS generation following infection and associated with gastrointestinal epithelial injury. The purpose of this study was to determine if APE1 regulates the function of Rac1 and Nox1 during oxidative stress. Gastric or colonic epithelial cells (wild-type or with suppressed APE1) were infected with Helicobacter pylori or Salmonella enterica and assessed for Rac1 and NADPH oxidase-dependent superoxide production. Rac1 and APE1 interactions were measured by co-immunoprecipitation, confocal microscopy and proximity ligation assay (PLA) in cell lines or in biopsy specimens. Significantly greater levels of ROS were produced by APE1-deficient human gastric and colonic cell lines and primary gastric epithelial cells compared to control cells after infection with either gastric or enteric pathogens. H. pylori activated Rac1 and Nox1 in all cell types, but activation was higher in APE1 suppressed cells. APE1 overexpression decreased H. pylori-induced ROS generation, Rac1 activation, and Nox1 expression. We determined that the effects of APE1 were mediated through its N-terminal lysine residues interacting with Rac1, leading to inhibition of Nox1 expression and ROS generation. APE1 is a negative regulator of oxidative stress in the gastrointestinal epithelium during bacterial infection by modulating Rac1 and Nox1. Our results implicate APE1 in novel molecular interactions that regulate early stress responses elicited by microbial infections.

Helicobacter pylori infection of the gastric mucosa is largely lifelong leading to continued stimulation of immune cells. This results in the generation of reactive oxygen species (ROS) which are produced to kill bacteria, but at the same time ROS regulate cellular events in the host. However, prolonged generation of ROS has been implicated in damage of DNA, which ultimately could lead to the development of cancer. We studied a molecule known as APE-1 in gastric and intestinal cells, which is activated upon encounter of ROS. Our results show that APE1 limits the production of ROS in cells that form the lining of the gastrointestinal tract. APE1 regulates ROS production by inhibiting activation of the molecule Rac1. Inhibition of ROS production by APE1 occurred after infection of gastric cells with Helicobacter pylori and after Salmonella infection of intestinal cells. These data demonstrate that APE1 inhibits production of ROS in cells that line the inside of the digestive tract.

Pathobiology of Helicobacter pylori-Induced Gastric Cancer

Colonization of the human stomach by Helicobacter pylori and its role in causing gastric cancer is one of the richest examples of a complex relationship among human cells, microbes, and their environment. It is also a puzzle of enormous medical importance given the incidence and lethality of gastric cancer worldwide. We review recent findings that have changed how we view these relationships and affected the direction of gastric cancer research. For example, recent data have indicated that subtle mismatches between host and microbe genetic traits greatly affect the risk of gastric cancer. The ability of H pylori and its oncoprotein CagA to reprogram epithelial cells and activate properties of stemness show the sophisticated relationship between H pylori and progenitor cells in the gastric mucosa. The observation that cell-associated H pylori can colonize the gastric glands and directly affect precursor and stem cells supports these observations. The ability to mimic these interactions in human gastric organoid cultures as well as animal models will allow investigators to more fully unravel the extent of H pylori control on the renewing gastric epithelium. Finally, our realization that external environmental factors, such as dietary components and essential micronutrients, as well as the gastrointestinal microbiota, can change the balance between H pylori's activity as a commensal or a pathogen has provided direction to studies aimed at defining the full carcinogenic potential of this organism.

Accumulation of abasic sites induces genomic instability in normal human gastric epithelial cells during Helicobacter pylori infection

Helicobacter pylori infection of the human stomach is associated with inflammation that leads to the release of reactive oxygen and nitrogen species (RONs), eliciting DNA damage in host cells. Unrepaired DNA damage leads to genomic instability that is associated with cancer. Base excision repair (BER) is critical to maintain genomic stability during RONs-induced DNA damage, but little is known about its role in processing DNA damage associated with H. pylori infection of normal gastric epithelial cells. Here, we show that upon H. pylori infection, abasic (AP) sites accumulate and lead to increased levels of double-stranded DNA breaks (DSBs). In contrast, downregulation of the OGG1 DNA glycosylase decreases the levels of both AP sites and DSBs during H. pylori infection. Processing of AP sites during different phases of the cell cycle leads to an elevation in the levels of DSBs. Therefore, the induction of oxidative DNA damage by H. pylori and subsequent processing by BER in normal gastric epithelial cells has the potential to lead to genomic instability that may have a role in the development of gastric cancer. Our results are consistent with the interpretation that precise coordination of BER processing of DNA damage is critical for the maintenance of genomic stability.

Oxidative stress: an essential factor in the pathogenesis of gastrointestinal mucosal diseases

Reactive oxygen species (ROS) are generated as by-products of normal cellular metabolic activities. Superoxide dismutase, glutathione peroxidase, and catalase are the enzymes involved in protecting cells from the damaging effects of ROS. ROS are produced in response to ultraviolet radiation, cigarette smoking, alcohol, nonsteroidal anti-inflammatory drugs, ischemia-reperfusion injury, chronic infections, and inflammatory disorders. Disruption of normal cellular homeostasis by redox signaling may result in cardiovascular, neurodegenerative diseases and cancer. ROS are produced within the gastrointestinal (GI) tract, but their roles in pathophysiology and disease pathogenesis have not been well studied. Despite the protective barrier provided by the mucosa, ingested materials and microbial pathogens can induce oxidative injury and GI inflammatory responses involving the epithelium and immune/inflammatory cells. The pathogenesis of various GI diseases including peptic ulcers, gastrointestinal cancers, and inflammatory bowel disease is in part due to oxidative stress. Unraveling the signaling events initiated at the cellular level by oxidative free radicals as well as the physiological responses to such stress is important to better understand disease pathogenesis and to develop new therapies to manage a variety of conditions for which current therapies are not always sufficient.

At the Bench: Helicobacter pylori, dysregulated host responses, DNA damage, and gastric cancer

Helicobacter pylori infection is the strongest known risk factor for the development of gastric cancer. Given that ∼50% of the global population is infected with this pathogen, there is great impetus to elucidate underlying causes that mediate progression from infection to cancer. Recent evidence suggests that H. pylori-induced chronic inflammation and oxidative stress create an environment conducive to DNA damage and tissue injury. DNA damage leads to genetic instability and eventually, neoplastic transformation. Pathogen-encoded virulence factors induce a robust but futile immune response and alter host pathways that lower the threshold for carcinogenesis, including DNA damage repair, polyamine synthesis and catabolism, antioxidant responses, and cytokine production. Collectively, such dysregulation creates a protumorigenic microenvironment within the stomach. This review seeks to address each of these aspects of H. pylori infection and to call attention to areas of particular interest within this field of research. This review also seeks to prioritize areas of translational research related to H. pylori-induced gastric cancer based on insights garnered from basic research in this field. See related review by Dalal and Moss, At the Bedside: H. pylori, dysregulated host responses, DNA damage, and gastric cancer.

Impact of bacterial infections on aging and cancer: impairment of DNA repair and mitochondrial function of host cells

The commensal floras that inhabit the gastrointestinal tract play critical roles in immune responses, energy metabolism, and even cancer prevention. Pathogenic and out of place commensal bacteria, can however have detrimental effects on the host, by introducing genomic instability and mitochondrial dysfunction, which are hallmarks of both aging and cancer. Helicobacter pylori and Enterococcus faecalis are bacteria of the gastrointestinal tract that have been demonstrated to affect these two hallmarks. These, and other bacteria, have been shown to decrease the transcription and translation of essential DNA repair subunits of major DNA repair pathways and increase production of reactive oxygen species (ROS). Defects in DNA repair cause mutations and genomic instability and are found in several cancers as well as in progeroid syndromes. This review describes our contemporary view on how bacterial infections impact DNA repair and damage, and the consequence on the mitochondrial and nuclear genomes. We argue that in the gastrointestinal tract, these mechanisms can contribute to tumorigenesis as well as cellular aging of the digestive system.

Brain angiogenesis inhibitor 1 is expressed by gastric phagocytes during infection with Helicobacter pylori and mediates the recognition and engulfment of human apoptotic gastric epithelial cells

After Helicobacter pylori infection in humans, gastric epithelial cells (GECs) undergo apoptosis due to stimulation by the bacteria or inflammatory cytokines. In this study, we assessed the expression and function of brain angiogenesis inhibitor 1 (BAI1) in the engulfment of apoptotic GECs using human tissue and cells. After induction of apoptosis by H. pylori or camptothecin, there was a 5-fold increase in the binding of apoptotic GECs to THP-1 cells or peripheral blood monocyte-derived macrophages as assayed by confocal microscopy or conventional and imaging flow cytometry. Binding was impaired 95% by pretreating apoptotic cells with annexin V, underscoring the requirement for phosphatidylserine recognition. The phosphatidylserine receptor BAI1 was expressed in human gastric biopsy specimens and gastric phagocytes. To confirm the role of BAI1 in apoptotic cell clearance, the functional domain of BAI1 was used as a competitive inhibitor or BAI1 expression was inhibited by small interfering RNA. Both approaches decreased binding and engulfment >40%. Exposing THP-1 cells to apoptotic cells inhibited IL-6 production from 1340 to <364 pg/ml; however, this decrease was independent of phagocytosis. We conclude that recognition of apoptotic cells by BAI1 contributes to their clearance in the human gastric mucosa and this is associated with anti-inflammatory effects.

H(2)O(2) production in species of the Lactobacillus acidophilus group: a central role for a novel NADH-dependent flavin reductase

Hydrogen peroxide production is a well-known trait of many bacterial species associated with the human body. In the presence of oxygen, the probiotic lactic acid bacterium Lactobacillus johnsonii NCC 533 excretes up to 1 mM H(2)O(2), inducing growth stagnation and cell death. Disruption of genes commonly assumed to be involved in H(2)O(2) production (e.g., pyruvate oxidase, NADH oxidase, and lactate oxidase) did not affect this. Here we describe the purification of a novel NADH-dependent flavin reductase encoded by two highly similar genes (LJ_0548 and LJ_0549) that are conserved in lactobacilli belonging to the Lactobacillus acidophilus group. The genes are predicted to encode two 20-kDa proteins containing flavin mononucleotide (FMN) reductase conserved domains. Reductase activity requires FMN, flavin adenine dinucleotide (FAD), or riboflavin and is specific for NADH and not NADPH. The Km for FMN is 30 ± 8 μM, in accordance with its proposed in vivo role in H(2)O(2) production. Deletion of the encoding genes in L. johnsonii led to a 40-fold reduction of hydrogen peroxide formation. H(2)O(2) production in this mutant could only be restored by in trans complementation of both genes. Our work identifies a novel, conserved NADH-dependent flavin reductase that is prominently involved in H(2)O(2) production in L. johnsonii.

In vivo and in vitro evidence for placental DNA damage in preeclampsia

Preeclampsia (PE) is an idiopathic multisystem disease affecting 5–7% of pregnant women. Placental oxidative stress is a characteristic feature of PE and occurs when the production of reactive oxygen species (ROS) within the placenta overwhelms the intrinsic anti-oxidant defenses. We hypothesize that excessive oxidative DNA damage at the fetal-maternal interface coupled with a defective DNA damage/repair response is causally related to PE. Here we demonstrate that γH2AX (a sensitive marker of DNA damage) is expressed in the maternal decidua but not trophoblast of normal placentas, and that expression is significantly higher in PE placental tissues in vivo. Using primary in vitro cultures of maternal decidual stromal cells (DSCs) and fetal cytotrophoblast cells (CTs), we show an increase in γH2AX foci in DSCs cultured with vs without H₂O₂ (70.6% vs 11.6%; P<0.0001) or under hypoxia-reperfusion vs normoxia (20- vs 3-fold; P = 0.01); no foci were seen in CTs. We further demonstrate that Base Excision Repair (BER) intermediates are significantly increased in DSCs (not CTs) under these same conditions. Our data show that DNA damage is significantly more common in PE placentas, and that this DNA damage is localized to the maternal and not fetal side of the placenta. CTs may be selectively resistant to DNA damage in an effort to protect the fetus.

Lipid-based signaling modulates DNA repair response and survival against Klebsiella pneumoniae infection in host cells and in mice

Klebsiella pneumoniae causes serious infections in the urinary tract, respiratory tract, and blood. Lipid rafts, also known as membrane microdomains, have been linked to the pathogenesis of bacterial infection. However, whether lipid rafts affect K. pneumoniae internalization into host cells remains unknown. Here, we show for the first time that K. pneumoniae was internalized into lung cells by activating lipid rafts. Disrupting lipid rafts by methyl-β-cyclodextrin inhibited pathogen internalization, impairing host defense. A deficient mutant of capsule polysaccharide (CPS) showed a higher internalization rate than a wild-type strain, indicating that CPS may inhibit bacterial entry to host cells. Furthermore, lipid rafts may affect the function of extracellular regulated kinase (ERK)-1/2, and knocking down ERK1/2 via short, interfering RNA increased apoptosis in both alveolar macrophages and epithelial cells after infection. To gain insights into bacterial pathogenesis, we evaluated the impact of lipid rafts on DNA integrity, and showed that raft aggregates also affect DNA damage and DNA repair responses (i.e., 8-oxoguanine DNA glycosylase [Ogg1]) through the regulation of reactive oxygen species. Importantly, cells overexpressing Ogg1 demonstrated reduced cytotoxicity during bacterial infection. Taken together, these results suggest that lipid rafts may modulate bacterial internalization, thereby affecting DNA damage and repair, which is critical to host defense against K. pneumoniae.

Is the oxidative DNA damage level of human lymphocyte correlated with the antioxidant capacity of serum or the base excision repair activity of lymphocyte?

A random screening of human blood samples from 24 individuals of nonsmoker was conducted to examine the correlation between the oxidative DNA damage level of lymphocytes and the antioxidant capacity of serum or the base excision repair (BER) activity of lymphocytes. The oxidative DNA damage level was measured with comet assay containing Fpg/Endo III cleavage, and the BER activity was estimated with a modified comet assay including nuclear extract of lymphocytes for enzymatic cleavage. Antioxidant capacity was determined with trolox equivalent antioxidant capacity assay. We found that though the endogenous DNA oxidation levels varied among the individuals, each individual level appeared to be steady for at least 1 month. Our results indicate that the oxidative DNA damage level is insignificantly or weakly correlated with antioxidant capacity or BER activity, respectively. However, lymphocytes from carriers of Helicobacter pylori (HP) or Hepatitis B virus (HBV) tend to give higher levels of oxidative DNA damage (P < 0.05). Though sera of this group of individuals show no particular tendency with reduced antioxidant capacity, the respective BER activities of lymphocytes are lower in average (P < 0.05). Thus, reduction of repair activity may be associated with the genotoxic effect of HP or HBV infection.

Chronic inflammation and oxidative stress: the smoking gun for Helicobacter pylori-induced gastric cancer?

Helicobacter pylori is the leading risk factor associated with gastric carcinogenesis. H. pylori leads to chronic inflammation because of the failure of the host to eradicate the infection. Chronic inflammation leads to oxidative stress, deriving from immune cells and from within gastric epithelial cells. This is a main contributor to DNA damage, apoptosis and neoplastic transformation. Both pathogen and host factors directly contribute to oxidative stress, including H. pylori virulence factors, and pathways involving DNA damage and repair, polyamine synthesis and metabolism, and oxidative stress response. Our laboratory has recently uncovered a mechanism by which polyamine oxidation by spermine oxidase causes H 2O 2 release, DNA damage and apoptosis. Our studies indicate novel targets for therapeutic intervention and risk assessment in H. pylori-induced gastric cancer. More studies addressing the many potential contributors to oxidative stress, chronic inflammation, and gastric carcinogenesis are essential for development of therapeutics and identification of gastric cancer biomarkers.

Helicobacter pylori infection affects mitochondrial function and DNA repair, thus, mediating genetic instability in gastric cells

Helicobacter pylori infection is an important factor for the development of atrophic gastritis and gastric carcinogenesis. However, the mechanisms explaining the effects of H. pylori infection are not fully elucidated. H. pylori infection is known to induce genetic instability in both nuclear and mitochondrial DNA of gastric epithelial cells. The mutagenic effect of H. pylori infection on nuclear DNA is known to be a consequence, in part, of a down-regulation of expression and activity of major DNA repair pathways. In this study, we demonstrate that H. pylori infection of gastric adenocarcinoma cells causes mtDNA mutations and a decrease of mtDNA content. Consequently, we show a decrease of respiration coupled ATP turnover and respiratory capacity and accordingly a lower level and activity of complex I of the electron transport chain. We wanted to investigate if the increased mutational load in the mitochondrial genome was caused by down-regulation of mitochondrial DNA repair pathways. We lowered the expression of APE-1 and YB-1, which are believed to be involved in mitochondrial base excision repair and mismatch repair. Our results suggest that both APE-1 and YB-1 are involved in mtDNA repair during H. pylori infection, furthermore, the results demonstrate that multiple DNA repair activities are involved in protecting mtDNA during infection.

Catechins and Sialic Acid Attenuate Helicobacter pylori-Triggered Epithelial Caspase-1 Activity and Eradicate Helicobacter pylori Infection

The inflammasome/caspase-1 signaling pathway in immune cells plays a critical role in bacterial pathogenesis; however, the regulation of this pathway in the gastric epithelium during Helicobacter pylori infection is yet to be elucidated. Here, we investigated the effect of catechins (CAs), sialic acid (SA), or combination of CA and SA (CASA) on H. pylori-induced caspase-1-mediated epithelial damage, as well as H. pylori colonization in vitro (AGS cells) and in vivo (BALB/c mice). Our results indicate that the activity of caspase-1 and the expression of its downstream substrate IL-1β were upregulated in H. pylori-infected AGS cells. In addition, we observed increased oxidative stress, NADPH oxidase gp91phox, CD68, caspase-1/IL-1β, and apoptosis, but decreased autophagy, in the gastric mucosa of H. pylori-infected mice. We have further demonstrated that treatment with CASA led to synergistic anti-H. pylori activity and was more effective than treatment with CA or SA alone. In particular, treatment with CASA for 10 days eradicated H. pylori infection in up to 95% of H. pylori-infected mice. Taken together, we suggest that the pathogenesis of H. pylori involves a gastric epithelial inflammasome/caspase-1 signaling pathway, and our results show that CASA was able to attenuate this pathway and effectively eradicate H. pylori infection.

Epithelial-mesenchymal transition during oncogenic transformation induced by hexavalent chromium involves reactive oxygen species-dependent mechanism in lung epithelial cells

Hexavalent chromium [Cr(VI)] is an important human carcinogen associated with pulmonary diseases and lung cancer. Exposure to Cr(VI) induces DNA damage, cell morphological change and malignant transformation in human lung epithelial cells. Despite extensive studies, the molecular mechanisms remain elusive, it is also not known if Cr(VI)-induced transformation might accompany with invasive properties to facilitate metastasis. We aimed to study Cr(VI)-induced epithelial-mesenchymal transition (EMT) and invasion during oncogenic transformation in lung epithelial cells. The results showed that Cr(VI) at low doses represses E-cadherin mRNA and protein expression, enhances mesenchymal marker vimentin expression and transforms the epithelial cell into fibroblastoid morphology. Cr(VI) also increases cell invasion and promotes colony formation. Further studies indicated that Cr(VI) uses multiple mechanisms to repress E-cadherin expression, including activation of E-cadherin repressors such as Slug, ZEB1, KLF8 and enhancement the binding of HDAC1 in E-cadherin gene promoter, but DNA methylation is not responsible for the loss of E-cadherin. Catalase reduces Cr(VI)-induced E-cadherin and vimentin protein expression, attenuates cell invasion in matrigel and colony formation on soft agar. These results demonstrate that exposure to a common human carcinogen, Cr(VI), induces EMT and invasion during oncogenic transformation in lung epithelial cells and implicate in cancer metastasis and prevention.

Genetic aspects of gastric cancer instability

Unravelling the molecular mechanisms underlying gastric carcinogenesis is one of the major challenges in cancer genomics. Gastric cancer is a very complex and heterogeneous disease, and although much has been learned about the different genetic changes that eventually lead to its development, the detailed mechanisms still remain unclear. Malignant transformation of gastric cells is the consequence of a multistep process involving different genetic and epigenetic changes in numerous genes in combination with host genetic background and environmental factors. The majority of gastric adenocarcinomas are characterized by genetic instability, either microsatellite instability (MSI) or chromosomal instability (CIN). It is believed that chromosome destabilizations occur early in tumour progression. This review summarizes the most common genetic alterations leading to instability in sporadic gastric cancers and its consequences.

Dietary vitamin A intake and incidence of gastric cancer in a general Japanese population: the Hisayama Study

BACKGROUND

The results of prospective studies examining the association between dietary vitamin A intake and the risk of gastric cancer have often been conflicting. The objective of this study was to investigate this issue in a general Japanese population.

METHODS

A total of 2,467 community-dwelling Japanese subjects aged 40 years or older were followed up prospectively for 14 years. Dietary vitamin A intake was estimated using a semiquantitative food frequency method.

RESULTS

During the follow-up period, gastric cancer developed in 93 subjects. The age- and sex-adjusted incidence of gastric cancer rose progressively with increasing levels of dietary vitamin A intake: at 2.2, 3.0, 3.8, and 4.5 per 1,000 person-years for quartile groups defined by dietary vitamin A intake levels of <639, 639-837, 838-1,061, and >1,061 μg retinol equivalents (RE)/day, respectively (P for trend <0.01). The risk of gastric cancer was significantly higher in the fourth quartile than in the first one even after multivariate adjustment [hazard ratio (HR) = 1.47, 95% confidence interval (CI) = 0.70-3.09, P = 0.30 for the second quartile; HR = 1.85, 95% CI = 0.82-4.18, P = 0.14 for the third quartile; HR = 2.96, 95% CI = 1.12-7.80, P = 0.03 for the fourth quartile]. Comparable effects of vitamin A intake were observed irrespective of the location or histological type of gastric cancer. The HR for gastric cancer increased significantly only in subjects with a combination of high vitamin A intake (>1,061 μg RE/day) and Helicobacter pylori infection.

CONCLUSIONS

Our findings suggest that dietary vitamin A intake is clearly associated with the risk of gastric cancer in the general Japanese population.

Effect of quercetin on inflammatory gene expression in mice liver in vivo - role of redox factor 1, miRNA-122 and miRNA-125b

The anti-inflammatory properties of the flavonol quercetin have been intensively investigated using in vitro cell systems and are to a great extent reflected by changes in the expression of inflammatory markers. However, information relating to the degree at which quercetin affects inflammatory gene expression in vivo is limited. Recently, micro RNAs (miRNAs) have been identified as powerful post-transcriptional gene regulators. The effect of quercetin on miRNA regulation in vivo is largely unknown. Laboratory mice were fed for six weeks with control or quercetin enriched high fat diets and biomarkers of inflammation as well as hepatic levels of miRNAs previously involved in inflammation (miR-125b) and lipid metabolism (miR-122) were determined. We found lower mRNA steady state levels of the inflammatory genes interleukin 6, C-reactive protein, monocyte chemoattractant protein 1, and acyloxyacyl hydrolase in quercetin fed mice. In addition we found evidence for an involvement of redox factor 1, a modulator of nuclear factor κB signalling, on the attenuation of inflammatory gene expression mediated by dietary quercetin. Furthermore, the results demonstrate that hepatic miR-122 and miR-125b concentrations were increased by dietary quercetin supplementation and may therefore contribute to the gene-regulatory activity of quercetin in vivo.

Differential expression of MYC in H. pylori-related intestinal and diffuse gastric tumors

Evidence suggests that the carcinogenic process guided by Helicobacter pylori is related to the expression of cell cycle and apoptosis proteins as BCL-2, BAX, and MYC. However, the literature is conflicting regarding the expression frequency in the histological subtypes and did not consider cagA gene presence. To investigate the expression of these proteins considering the histological subtypes of gastric cancer associated with H. pylori (cagA), a total of 89 cases were used. H. pylori infection and cagA status were determined by PCR. Immunodetection was performed for MYC, BCL-2, and BAX proteins. H. pylori was found in 95.5% of the patients, among them, 65.8% were cagA(+). Nuclear MYC was detected in 36.4%, BAX in 55.7%, while BCl-2 in just 5%. Nuclear MYC staining was significantly lower in the intestinal than diffuse subtype (p = 0.008) and was related with the presence of H. pylori cagA(+). Additionally, most of the few cases cytoplasmic MYC positive were in the intestinal subtype. In diffuse tumors, although most nuclear MYC positive cases were cagA(+), it was not significant. No difference was observed between BCL-2 or BAX expression considering the presence of cagA gene in the histological subtypes. It seems that MYC could be relevant for the diffuse tumorigenic pathway associated with H. pylori and possibly influenced by the presence of cagA gene, while in intestinal tumors, the tumorigenic pathway does not occur through the MYC expression.

Helicobacter pylori activation of PARP-1: usurping a versatile regulator of host cellular health

Chronic infection of the human stomach by Helicobacter pylori is an important risk factor for gastric cancer. H. pylori produces a cache of virulence factors that promote colonization and persistence, which, in turn, contributes to a robust inflammatory response at the host-pathogen interface. Recently, we reported that H. pylori activates the abundant nuclear regulator poly(ADP-ribose) polymerase (PARP)-1, resulting in the production of the catabolite poly(ADP-ribose) (PAR). PARP-1 is emerging as a key player in establishing homeostasis at the host-pathogen interface. In this article, we summarize the discovery of H. pylori-dependent PARP-1 activation, and discuss potential roles for PARP-1 in H. pylori-mediated gastric disease. In light of the remarkable successes that have reported for treating inflammatory disorders and cancers with PARP-1 inhibitors, we discuss the prospects of targeting PARP-1 for treatment of H. pylori-associated gastric disease.

Mechanism of hypoxia-inducible factor 1 alpha-mediated Mcl1 regulation in Helicobacter pylori-infected human gastric epithelium

Hypoxia-inducible factor 1 (HIF1) consists of a hypoxia-inducible α subunit and a constitutively expressed β subunit. Reactive oxygen species (ROS) induced by Helicobacter pylori stabilize HIF1α in the human gastric epithelium in normoxia. HIF1α plays crucial role in carcinogenesis and has been associated with malignant progression of gastric cancer. Several genes contain functional hypoxia-response elements (HREs) in their promoters including Bcl2 family member, Mcl1. Cellular ratios of antiapoptotic oncogenic protein, Mcl1, and tumor suppressor proapoptotic protein, Noxa, determine cell fate by regulating normal cellular growth, cell death and oncogenic processes. The aim of the present study was to examine the mechanism of HIF1α induction in the H. pylori-infected gastric epithelium to better understand disease pathogenesis by H. pylori relevant to gastric carcinogenesis. Our data showed that the dose-dependent increase in HIF1α in H. pylori-infected gastric epithelia is mediated by induction of a ROS-inducible protein, apurinic/apyrimidinic endonuclease 1 (APE1), and an enhanced interaction of APE1 with the transcriptional coactivator p300. Surprisingly, with accumulation of HIF1α, further transcriptional activation of mcl1 was not observed. We identified a HIF-binding site (HBS) in the hif1α promoter and showed that increased HIF1α expression, whether H. pylori-induced or hypoxia-mimetic agent, CoCl(2)-induced, resulted in enhanced HIF1α binding to its own promoter. This resulted in a transcriptionally inactive hif1α promoter since hif1α HBS lacks HIF ancillary sequence (HAS) required for HIF1 transcriptional activity. We conclude that enhanced binding of "nonfunctional" HIF1α to hif1α promoter and limiting availability of p300 in the cell serves as checkpoints for uncontrolled HIF1α activity.

Host DNA repair proteins in response to Pseudomonas aeruginosa in lung epithelial cells and in mice

Although DNA repair proteins in bacteria are critical for pathogens' genome stability and for subverting the host defense, the role of host DNA repair proteins in response to bacterial infection is poorly defined. Here, we demonstrate, for the first time, that infection with the Gram-negative bacterium Pseudomonas aeruginosa significantly altered the expression and enzymatic activity of 8-oxoguanine DNA glycosylase (OGG1) in lung epithelial cells. Downregulation of OGG1 by a small interfering RNA strategy resulted in severe DNA damage and cell death. In addition, acetylation of OGG1 is required for host responses to bacterial genotoxicity, as mutations of OGG1 acetylation sites increased Cockayne syndrome group B (CSB) protein expression. These results also indicate that CSB may be involved in DNA repair activity during infection. Furthermore, OGG1 knockout mice exhibited increased lung injury after infection with P. aeruginosa, as demonstrated by higher myeloperoxidase activity and lipid peroxidation. Together, our studies indicate that P. aeruginosa infection induces significant DNA damage in host cells and that DNA repair proteins play a critical role in the host response to P. aeruginosa infection, serving as promising targets for the treatment of this condition and perhaps more broadly Gram-negative bacterial infections.

Dual regulation by apurinic/apyrimidinic endonuclease-1 inhibits gastric epithelial cell apoptosis during Helicobacter pylori infection

Human apurinic/apyrimidinic endonuclease-1 (APE-1), a key enzyme involved in repair of oxidative DNA base damage, is an important transcriptional coregulator. We previously reported that Helicobacter pylori infection induces apoptosis and increases APE-1 expression in human gastric epithelial cells (GEC). Although both the DNA repair activity and the acetylation-mediated transcriptional regulation of APE-1 are required to prevent cell death, the mechanisms of APE-1-mediated inhibition of infection-induced apoptosis are unclear. Here, we show that short hairpin RNA-mediated stable suppression of APE-1 results in increased apoptosis in GEC after H. pylori infection. We show that programmed cell death involves both the caspase-9-mediated mitochondrial pathway and the caspase-8-dependent extrinsic pathway by measuring different markers for both the pathways. Overexpression of wild-type APE-1 in APE-1-suppressed GEC reduced apoptosis after infection; however, overexpression of the DNA repair mutant or the nonacetylable mutant of APE-1 alone was unable to reduce apoptosis, suggesting that both DNA repair and acetylation functions of APE-1 modulate programmed cell death. We show for the first time that the DNA repair activity of APE-1 inhibits the mitochondrial pathway, whereas the acetylation function inhibits the extrinsic pathway during H. pylori infection. Thus, our findings establish that the two different functions of APE-1 differentially regulate the intrinsic and the extrinsic pathway of H. pylori-mediated GEC apoptosis. As proapoptotic and antiapoptotic mechanisms determine the development and progression of gastritis, gastric ulceration, and gastric cancer, this dual regulatory role of APE-1 represents one of the important molecular strategies by H. pylori to sustain chronic infection.

Helicobacter pylori infection generates genetic instability in gastric cells

The discovery that Helicobacter pylori is associated with gastric cancer has led to numerous studies that investigate the mechanisms by which H. pylori induces carcinogenesis. Gastric cancer shows genetic instability both in nuclear and mitochondrial DNA, besides impairment of important DNA repair pathways. As such, this review highlights the consequences of H. pylori infection on the integrity of DNA in the host cells. By down-regulating major DNA repair pathways, H. pylori infection has the potential to generate mutations. In addition, H. pylori infection can induce direct changes on the DNA of the host, such as oxidative damage, methylation, chromosomal instability, microsatellite instability, and mutations. Interestingly, H. pylori infection generates genetic instability in nuclear and mitochondrial DNA. Based on the reviewed literature we conclude that H. pylori infection promotes gastric carcinogenesis by at least three different mechanisms: (1) a combination of increased endogenous DNA damage and decreased repair activities, (2) induction of mutations in the mitochondrial DNA, and (3) generation of a transient mutator phenotype that induces mutations in the nuclear genome.

Helicobacter pylori infection induces oxidative DNA damage in human gastric epithelial cell line GES-1 and human gastric cancer cell line SGC-7901

AIM: To investigate whether Helicobacter pylori (H. pylori) infection can induce oxidative DNA damage in human gastric epithelial cell line GES-1 and human gastric cancer cell line SGC-7901.

METHODS: H. pylori was co-cultured with GES-1 and SGC-7901 cells, respectively. The morphological changes of GES-1 and SGC-7901 cells between before and after co-culture were observed. The 8-OHdG expression in the two types of cells was detected by laser scanning confocal microscopy.

RESULTS: H. pylori infection induced oxidative DNA damage in both GES-1 and SGC-7901 cells. The expression levels of 8-OHdG in GES-1 and SGC-7901 cells co-cultured with H. pylori were significantly higher than those in control cells (64.9396 ± 17.8142 vs 32.3010 ± 7.3620 and 102.8344 ± 30.2632 vs 77.1336 ± 32.3223, respectively; both P = 0.000). The extent of 8-OHdG upregulation in GES-1 cells co-cultured with H. pylori was significantly higher than that in SGC-7901 cells co-cultured with H. pylori.

CONCLUSION: H. pylori infection induces oxidative DNA damage in both GES-1 and SGC-7901 cells. This result supports the hypothesis that H. pylori induced-oxidative DNA damage plays a pivotal role in the development of gastric carcinoma in patients with chronic gastritis. GES-1 cell line is superior to SGC-7901 cell line in the study of oxidative damage induced by H. pylori.

Acetylation of apurinic/apyrimidinic endonuclease-1 regulates Helicobacter pylori-mediated gastric epithelial cell apoptosis

BACKGROUND & AIMS

Helicobacter pylori-induced gastric epithelial cell (GEC) apoptosis is a complex process that includes activation of the tumor suppressor p53. p53-mediated apoptosis involves p53 activation, bax transcription, and cytochrome c release from mitochondria. Apurinic/apyrimidinic endonuclease-1 (APE-1) regulates transcriptional activity of p53, and H pylori induce APE-1 expression in human GECs. H pylori infection increases intracellular calcium ion concentration [Ca2+]i of GECs, which induces APE-1 acetylation. We investigated the effects of H pylori infection and APE-1 acetylation on GEC apoptosis.

METHODS

AGS cells (wild-type or with suppressed APE-1), KATO III cells, and cells isolated from gastric biopsy specimens were infected with H pylori. Effects were examined by immunoblotting, real-time reverse-transcription polymerase chain reaction, immunoprecipitation, immunofluorescence microscopy, chromatin immunoprecipitation, mobility shift, DNA binding, and luciferase assays.

RESULTS

H pylori infection increased [Ca2+]i and acetylation of APE-1 in GECs, but the acetylation status of APE-1 did not affect the transcriptional activity of p53. In GECs, expression of a form of APE-1 that could not be acetylated increased total and mitochondrial levels of Bax and induced release of cytochrome c and fragmentation of DNA; expression of wild-type APE-1 reduced these apoptotic events. We identified a negative calcium response element in the human bax promoter and found that poly (adenosine diphosphate-ribose) polymerase 1 recruited the acetylated APE-1/histone deacetylase-1 repressor complex to bax nCaRE.

CONCLUSIONS

H pylori-mediated acetylation of APE-1 suppresses Bax expression; this prevents p53-mediated apoptosis when H pylori infect GECs.

A2A adenosine receptor (AR) activation inhibits pro-inflammatory cytokine production by human CD4+ helper T cells and regulates Helicobacter-induced gastritis and bacterial persistence

Helicobacter pylori causes a lifelong infection and provides a model of bacterial adaptation and persistent colonization. Adenosine is an anti-inflammatory mediator that limits tissue damage during inflammation. We studied the role of adenosine in the T-cell-mediated regulation of gastritis and bacterial persistence. After 4 h of activation, human T helper (Th) cells increased A(2A) adenosine receptor (A(2A)AR) mRNA level (sevenfold). A(2A)AR was the predominant subtype expressed in resting and stimulated gastric or peripheral Th cells. Stimulation with ATL313, an A(2A)AR agonist, increased cyclic AMP (cAMP) accumulation and reduced interleukin-2 (IL-2) production by 20-50%. ATL313 also attenuated tumor necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) production, which was inhibited by an A(2A)AR antagonist. Infection of IL-10-deficient mice with H. pylori is cleared spontaneously due to the marked inflammation. Administration of ATL313 during infection reduced gastritis and pro-inflammatory cytokine responses while bacterial load increased. In contrast, infection of A(2A)AR-deficient mice enhanced gastritis. Thus, A(2A)AR limits the pro-inflammatory effects of Th cells and favor chronic Helicobacter infection.

Helicobacter pylori infection induces genetic instability of nuclear and mitochondrial DNA in gastric cells

PURPOSE

Helicobacter pylori is a major cause of gastric carcinoma. To investigate a possible link between bacterial infection and genetic instability of the host genome, we examined the effect of H. pylori infection on known cellular repair pathways in vitro and in vivo. Moreover, various types of genetic instabilities in the nuclear and mitochondrial DNA (mtDNA) were examined.

EXPERIMENTAL DESIGN

We observed the effects of H. pylori infection on a gastric cell line (AGS), on C57BL/6 mice, and on individuals with chronic gastritis. In AGS cells, the effect of H. pylori infection on base excision repair and mismatch repair (MMR) was analyzed by reverse transcription-PCR, Western blot, and activity assays. In mice, MMR expression was analyzed by reverse transcription-PCR and the CA repeat instabilities were examined by Mutation Detection Enhancement gel electrophoresis. Mutation spectra in AGS cells and chronic gastritis tissue were determined by PCR, single-stranded conformation polymorphism, and sequencing. H. pylori vacA and cagA genotyping was determined by multiplex PCR and reverse hybridization.

RESULTS

Following H. pylori infection, the activity and expression of base excision repair and MMR are down-regulated both in vitro and in vivo. Moreover, H. pylori induces genomic instability in nuclear CA repeats in mice and in mtDNA of AGS cells and chronic gastritis tissue, and this effect in mtDNA is associated with bacterial virulence.

CONCLUSIONS

Our results suggest that H. pylori impairs central DNA repair mechanisms, inducing a transient mutator phenotype, rendering gastric epithelial cells vulnerable to the accumulation of genetic instability and thus contributing to gastric carcinogenesis in infected individuals.

Tumor necrosis factor (TNF)-alpha-induced IL-8 expression in gastric epithelial cells: role of reactive oxygen species and AP endonuclease-1/redox factor (Ref)-1

TNF-alpha contributes to oxidative stress via induction of reactive oxygen species (ROS) and pro-inflammatory cytokines. The molecular basis of this is not well understood but it is partly mediated through the inducible expression of IL-8. As redox factor-1 (Ref-1), is an important mediator of redox-regulated gene expression we investigated whether ROS and Ref-1 modulate TNF-alpha-induced IL-8 expression in human gastric epithelial cells. We found that TNF-alpha treatment of AGS cells enhanced nuclear expression of Ref-1 and potently induced IL-8 expression. Overexpression of Ref-1 enhanced IL-8 gene transcription at baseline and after TNF-alpha treatment whereas Ref-1 suppression and antioxidant treatment inhibited TNF-alpha-stimulated IL-8 expression. TNF-alpha-mediated enhancement of other pro-inflammatory chemokines like MIP-3 alpha and Gro-alpha was also regulated by Ref-1. Although TNF-alpha increased DNA binding activity of Ref-1-regulated transcription factors, AP-1 and NF-kappaB, to the IL-8 promoter, promoter activity was mainly mediated by NF-kappaB binding. Silencing of Ref-1 in AGS cells inhibited basal and TNF-alpha-induced AP-1 and NF-kappaB DNA binding activity, but not their nuclear accumulation. Collectively, we provide the first mechanistic evidence of Ref-1 involvement in TNF-alpha-mediated, redox-sensitive induction of IL-8 and other chemokines in human gastric mucosa. This has implications for understanding the pathogenesis of gastrointestinal inflammatory disorders.

Transcriptional regulatory functions of mammalian AP-endonuclease (APE1/Ref-1), an essential multifunctional protein

The mammalian AP-endonuclease (APE1/Ref-1) plays a central role in the repair of oxidized and alkylated bases in mammalian genomes via the base excision repair (BER) pathway. However, APE1, unlike its E. coli prototype Xth, has two unique and apparently distinct transcriptional regulatory activities. APE1 functions as a redox effector factor (Ref-1) for several transcription factors including AP-1, HIF1-alpha, and p53. APE1 was also identified as a direct trans-acting factor for repressing human parathyroid hormone (PTH) and renin genes by binding to the negative calcium-response element (nCaRE) in their promoters. We have characterized APE1's post-translational modification, namely, acetylation which modulates its transcriptional regulatory function. Furthermore, stable interaction of APE1 with several other trans-acting factors including HIF-1alpha, STAT3, YB-1, HDAC1, and CBP/p300 and formation of distinct trans-acting complexes support APE1's direct regulatory function for diverse genes. Multiple functions of mammalian APE1, both in DNA repair and gene regulation, warrant extensive analysis of its own regulation and dissection of the mechanisms. In this review, we have discussed APE1's own regulation and its role as a transcriptional coactivator or corepressor by both redox-dependent and redox-independent (acetylation-mediated) mechanisms, and explore the potential utility of targeting these functions for enhancing drug sensitivity of cancer cells.

Dynamic compartmentalization of base excision repair proteins in response to nuclear and mitochondrial oxidative stress

DNAs harbored in both nuclei and mitochondria of eukaryotic cells are subject to continuous oxidative damage resulting from normal metabolic activities or environmental insults. Oxidative DNA damage is primarily reversed by the base excision repair (BER) pathway, initiated by N-glycosylase apurinic/apyrimidinic (AP) lyase proteins. To execute an appropriate repair response, BER components must be distributed to accommodate levels of genotoxic stress that may vary considerably between nuclei and mitochondria, depending on the growth state and stress environment of the cell. Numerous examples exist where cells respond to signals, resulting in relocalization of proteins involved in key biological transactions. To address whether such dynamic localization contributes to efficient organelle-specific DNA repair, we determined the intracellular localization of the Saccharomyces cerevisiae N-glycosylase/AP lyases, Ntg1 and Ntg2, in response to nuclear and mitochondrial oxidative stress. Fluorescence microscopy revealed that Ntg1 is differentially localized to nuclei and mitochondria, likely in response to the oxidative DNA damage status of the organelle. Sumoylation is associated with targeting of Ntg1 to nuclei containing oxidative DNA damage. These studies demonstrate that trafficking of DNA repair proteins to organelles containing high levels of oxidative DNA damage may be a central point for regulating BER in response to oxidative stress.

Implications of apurinic/apyrimidinic endonuclease in reactive oxygen signaling response after cisplatin treatment of dorsal root ganglion neurons

Peripheral neuropathy is one of the major side effects of the anticancer drug cisplatin. Although previous work suggests that this neuropathy correlates with formation of DNA adducts in sensory neurons, growing evidence suggests that cisplatin also increases the generation of reactive oxygen species (ROS), which could cause DNA damage. Apurinic/apyrimidinic endonuclease/redox factor-1 (Ape1/Ref-1) is a multifunctional protein involved in DNA base excision repair of oxidative DNA damage and in redox regulation of a number of transcription factors. Therefore, we asked whether altering Ape1 functions would influence cisplatin-induced neurotoxicity. Sensory neurons in culture were exposed to cisplatin for 24 hours and several end points of toxicity were measured, including production of ROS, cell death, apoptosis, and release of the immunoreactive calcitonin gene-related peptide (iCGRP). Reducing expression of Ape1 in neuronal cultures using small interfering RNA (siRNA) enhances cisplatin-induced cell killing, apoptosis, ROS generation, and cisplatin-induced reduction in iCGRP release. Overexpressing wild-type Ape1 attenuates all the toxic effects of cisplatin in cells containing normal endogenous levels of Ape1 and in cells with reduced Ape1 levels after Ape1siRNA treatment. Overexpressing the redox deficient/repair competent C65-Ape1 provides partial rescue, whereas the repair-deficient Ape1 (N226A + R177A) does not protect neurons from cisplatin toxicity. We also observe an increase in phosphorylation of p53 after a decrease in Ape1 levels in sensory neuronal cultures. These results strongly support the notion that Ape1 is a potential translational target such that protecting Ape1 levels and particularly its DNA repair function could reduce peripheral neuropathy in patients undergoing cisplatin treatment.