Accumulation of 8-nitroguanine in human gastric epithelium induced by Helicobacter pylori infection

Oxidative and nitrative DNA damage as biomarker for carcinogenesis with special reference to inflammation

Reactive oxygen and nitrogen species are known to participate in a wide variety of human diseases. Oxidative DNAdamage is involved in chemical carcinogenesis and aging. Monocyclic chemicals induce mainly oxidative DNAdamage, whereas polycyclic chemicals can induce oxidative DNA damage in addition to DNA adduct formation. Recently, chronic infection and inflammation have been recognized as important factors for carcinogenesis. Nitrative DNA damage as well as oxidative DNA damage is induced in relation to inflammationrelated carcinogenesis. The authors examined the formation of 8-nitroguanine, a nitrative DNA lesion, in humans and animals under inflammatory conditions. An immunofluorescence labeling study demonstrated that 8-nitroguanine was strongly formed in gastric gland epithelial cells in gastritis patients with H. pylori infection, in hepatocytes in patients with hepatitis C, and in oral epithelium of patients with oral lichen planus. 8-Nitroguanine was also formed in colonic epithelial cells of model mice of inflammatory bowel diseases and patients with ulcerative colitis. Interestingly, 8-nitroguanine was formed at the sites of carcinogenesis regardless of etiology. Therefore, 8-nitroguanine could be used as a potential biomarker to evaluate the risk of inflammation- related carcinogenesis.

8-Nitroguanine formation in oral leukoplakia, a premalignant lesion

Oral leukoplakia is a premalignant lesion associated with development of oral cancer. To clarify the mechanism of development of oral carcinogenesis from leukoplakia, we examined DNA damage in oral epithelium of biopsy specimens of patients with leukoplakia by immunohistochemical methods. Histological changes, such as epithelial dysplasia and infiltration of inflammatory cells were observed in oral tissues of leukoplakia patients. A double immunofluorescence labeling study demonstrated that the accumulation of mutagenic 8-nitroguanine, an indicator of nitrative DNA damage, and 8-oxo-7,8-dihydro-2'-deoxyguanosine, an indicator of oxidative DNA damage, was apparently observed in the oral epithelium of patients with leukoplakia, whereas little or no immunoreactivity was observed in normal oral mucosa. Expression of inducible nitric oxide synthase (iNOS) was also observed in oral epithelium of leukoplakia patients. Immunoreactivity of 3-nitrotyrosine, an indicator of nitrative stress, was observed in oral epithelial cells and colocalized with 8-nitroguanine. Moreover, proliferating cell nuclear antigen and p53 were expressed in 8-nitroguanine-positive epithelial cells in the basal layer. These results suggest that iNOS-mediated nitrative stress contributes to development of oral carcinogenesis from leukoplakia through DNA damage as well as oxidative stress.

Analysis of urinary 8-nitroguanine, a marker of nitrative nucleic acid damage, by high-performance liquid chromatography-electrochemical detection coupled with immunoaffinity purification: association with cigarette smoking

We have developed an analytical method to quantitate urinary 8-nitroguanine, a product of nitrative nucleic acid damage caused by reactive nitrogen species such as peroxynitrite and nitrogen dioxide. 8-Nitroguanine was purified from human urine using immunoaffinity columns with an anti-8-nitroguanine antibody, followed by quantitation by high-performance liquid chromatography-electrochemical detection. Four sequential electrodes were used to (a) oxidize interfering compounds (+250 mV), (b) reduce nitrated bases (two online electrodes at -1000 mV), and (c) quantitate reduced derivatives (+150 mV). Using this system 8-nitroxanthine can also be detected, with the detection limits being 25 and 50 fmol/injection for 8-nitroguanine and 8-nitroxanthine, respectively. The method was used to analyze both adducts in the urine of smokers (n=12) and nonsmokers (n=17). We found that smokers excrete more 8-nitroguanine [median, 6.1 fmol/mg creatinine; interquartile range (IQR), 23.8] than nonsmokers (0; IQR, 0.90) (p=0.018), and although 8-nitroxanthine was detected in human urine, its level was not related to smoking status. This is the first report to show that 8-nitroguanine is present in human urine and the methodology developed can be used to study the pathogenic roles of this adduct in the etiology of cancers associated with cigarette smoking and inflammation.

Oxidative DNA damage in relation to neurotoxicity in the brain of mice exposed to arsenic at environmentally relevant levels

To clarify the association between oxidative DNA damage and the neurotoxicity of arsenic, the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) as an index of oxidative DNA damage in the brain was examined in mice fed with drinking water containing 1 or 2 ppm arsenic, using an HPLC-electrochemical detector and immunohistochemical method. 8-OHdG levels were significantly increased in the brain of mice given arsenic and its immunoreactivity was distributed in the cerebral and cerebellar cortexes. Cerebral cortex neurons and Purkinje cells in the cerebellar cortex showed degenerative changes in accordance with the distribution of 8-OHdG immunoreactivity. The levels of arsenic in this study were lower than those reported in epidemiological studies. Thus, we conclude that environmentally relevant levels of arsenic induce pathological changes through oxidative DNA damage in the brain tissues in vivo and that cerebral and cerebellar cortex neurons seem to be the major targets of arsenic neurotoxicity.

Peroxynitrite-induced oxidation and nitration products of guanine and 8-oxoguanine: structures and mechanisms of product formation

Peroxynitrite induces DNA base damage predominantly at guanine (G) and 8-oxoguanine (8-oxoG) nucleobases via oxidation reactions. Nitration products are also observed, consistent with the generation of radical intermediates that can recombine with the (.)NO(2) formed during peroxynitrite degradation. The neutral G radical, G(.), reacts with (.)NO(2) to yield 8-nitroguanine (8-nitroG) and 5-nitro-4-guanidinohydantoin (NI), while for 8-oxoG we have proposed a reactive guanidinylidene radical intermediate. The products generated during peroxynitrite-mediated 8-oxoG oxidation depend on oxidant flux, with dehydroguanidinohydantoin (DGh), 2,4,6-trioxo-[1,3,5]triazinane-1-carboxamidine (CAC) and NO(2)-DGh predominating at high fluxes and spiroiminodihydantoin (Sp), guanidinohydantoin (Gh) and 4-hydroxy-2,5-dioxo-imidazolidine-4-carboxylic acid (HICA) predominating at low fluxes. Both product sets are observed at intermediate fluxes. It is therefore important in model systems to ensure that the relative concentrations are well controlled to minimize competing reactions that may not be relevant in vivo. Increasingly sophisticated systems for modeling peroxynitrite production in vivo are being developed and these should help with predicting the products most likely to be formed in vivo. Together with the emerging information on the genotoxic and mutational characteristics of the individual oxidation products, it may be found that the extent of tissue damage, mutational spectra and, hence, cancer risk may change as a function of peroxynitrite fluxes as different product combinations predominate.

Nitrative stress through formation of 8-nitroguanosine: insights into microbial pathogenesis

Reactive oxygen and nitrogen species, respectively, mediate oxidative and nitrative stresses by means of oxidation and nitration of various biomolecules including proteins, lipids, and nucleic acids. We have observed nitric oxide (NO)-dependent formation of 8-nitroguanosine and 3-nitrotyrosine during microbial infection, and we determined that both 8-nitroguanosine and 3-nitrotyrosine are useful biomarkers of nitrative stress. Of importance, however, is the great difference in biological characteristics of these two nitrated compounds. 8-Nitroguanosine has unique biochemical and pharmacological properties such as redox activity and mutagenic potential, which 3-nitrotyrosine does not. In this review, we discuss the mechanism of nitrative stress occurring during microbial infections, with special emphasis on biological functions of 8-nitroguanosine formed via NO during the host response to pathogens. These findings provide insights into NO-mediated pathogenesis not only of viral infections but also of many other diseases.

Evaluation for safety of antioxidant chemopreventive agents

Antioxidants are considered as the most promising chemopreventive agents against various human cancers. However, some antioxidants play paradoxical roles, acting as "double-edged sword." A primary property of effective and acceptable chemopreventive agents should be freedom from toxic effects in healthy population. Miscarriage of the intervention by beta-carotene made us realize the necessity for evaluation of safety before recommending use of antioxidant supplements for chemoprevention. We have evaluated the safety of antioxidants on the basis of reactivity with DNA. Our results revealed that phytic acid, luteolin, and retinoic acid did not cause DNA damage under the experimental condition. Furthermore, phytic acid inhibited the formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine, an indicator of oxidative DNA damage, in cultured cells treated with a H(2)O(2)-generating system. Thus, it is expected that these chemopreventive agents can safely protect humans against cancer. On the other hand, some chemopreventive agents with prooxidant properties (alpha-tocopherol, quercetin, catechins, isothiocyanates, N-acetylcysteine) caused DNA damage via generation of reactive oxygen species in the presence of metal ions and endogenous reductants under some circumstances. Furthermore, other chemopreventive agents (beta-carotene, genistein, daidzein, propyl gallate, curcumin) exerted prooxidant properties after metabolic activation. Therefore, further studies on safety should be required when antioxidants are used for cancer prevention.

Nitrative and oxidative DNA damage in oral lichen planus in relation to human oral carcinogenesis

Oral lichen planus (OLP) is a chronic inflammatory disease, which has been clinically associated with development to oral cancer. A double immunofluorescence labeling study found that 8-nitroguanine and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) accumulated in oral epithelium in OLP and oral squamous cell carcinoma (OSCC) biopsy specimens, whereas little or no immunoreactivity was observed in normal oral mucosa. Colocalization of 8-nitroguanine and inducible nitric oxide synthase (iNOS) was found in oral epithelium of OLP and OSCC. Immunoreactivity of 3-nitrotyrosine, which is formed by protein tyrosine nitration and is considered to be a biochemical marker for inflammation, was also observed in oral epithelial cells and colocalized with 8-nitroguanine. Accumulation of p53 was more strongly observed in oral epithelium in OSCC than OLP, whereas there was no p53 accumulation in normal oral mucosa. Our findings demonstrate that iNOS-dependent DNA damage in OLP may lead to p53 accumulation in not only OLP but also OSCC. We conclude that the formation of potentially mutagenic DNA lesions including 8-nitroguanine and 8-oxodG may contribute to the development of oral cancer from OLP.

Accumulation of 8-nitroguanine in the liver of patients with chronic hepatitis C

BACKGROUND/AIMS

Nucleic acid damage by reactive nitrogen and oxygen species may contribute to inflammation-related carcinogenesis. To investigate the extent of nucleic acid damage in hepatitis C virus infection and its change after interferon treatment, we measured 8-nitroguanine and 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the liver of patients with chronic hepatitis C (CHC) before and after interferon therapy.

METHODS

Hepatic accumulation of 8-nitroguanine and 8-OHdG was immunohistochemically evaluated in 20 CHC patients and 7 control patients with non-alcoholic fatty liver.

RESULTS

Immunoreactivities of 8-nitroguanine and 8-OHdG were strongly detected in the liver from patients with CHC, but not in control livers. 8-Nitroguanine accumulation was found not only in infiltrating inflammatory cells, but also hepatocytes particularly in the periportal area. The accumulation of 8-nitroguanine and 8-OHdG increased with inflammatory grade (8-nitroguanine; P = 0.0019, 8-OHdG; P = 0.0009). In the sustained virological responder group after interferon therapy, 8-nitroguanine and 8-OHdG accumulation were markedly decreased in the liver (8-nitroguanine; P = 0.018, 8-OHdG; P = 0.018).

CONCLUSIONS

In this study, we demonstrated for the first time that 8-nitroguanine accumulated in the liver of patients with CHC. 8-Nitroguanine is a useful biomarker to evaluate the severity of HCV-induced chronic inflammation in relation to hepatocellular carcinoma.

Nitrative DNA damage in inflammation and its possible role in carcinogenesis

Chronic inflammation has long been recognized as a risk factor for human cancer at various sites. Examples include Helicobacter pylori-induced gastritis for gastric cancer, inflammatory bowel disease (ulcerative colitis and Crohn's disease) for colorectal cancer and chronic viral hepatitis for liver cancer. Here we review the role in carcinogenesis of nitrative damage to nucleic acids, DNA and RNA, which occurs during inflammation through the generation of reactive nitrogen species, such as peroxynitrite, nitroxyl, and nitrogen dioxide. Enhanced formation of 8-nitroguanine, representative of nitrative damage to nucleobases, has been detected in various inflammatory conditions. The biochemical nature of DNA damage mediated by reactive nitrogen species is discussed in relation to its possible involvement in mutations, genetic instability, and cell death. Better understanding of the mechanisms and role of such nitrative damage in chronic inflammation-associated human cancer is a necessary basis to develop new strategies for cancer prevention by modulating the process of inflammation.

Nitrative and oxidative DNA damage in intrahepatic cholangiocarcinoma patients in relation to tumor invasion

AIM: Nitrative and oxidative DNA damage such as 8-nitroguanine and 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxodG) formation has been implicated in initiation and/ or promotion of inflammation-mediated carcinogenesis. The aim of this study is to clarify whether these DNA lesions participate in the progression of intrahepatic cholangiocarcinoma.

METHODS: We investigated the relation of the formation of 8-nitroguanine and 8-oxodG and the expression of hypoxia-inducible factor-1α (HIF-1α) with tumor invasion in 37 patients with intra-hepatic cholangiocarcinoma.

RESULTS: Immunohistochemical analyses revealed that 8-nitroguanine and 8-oxodG formation occurred to a much greater extent in cancerous tissues than in non-cancerous tissues. HIF-1α could be detected in cancerous tissues in all patients, suggesting low oxygen tension in the tumors. HIF-1α expression was correlated with inducible nitric oxide synthase (iNOS) expression (r = 0.369 and P = 0.025) and 8-oxodG formation (r = 0.398 and P = 0.015). Double immunofluorescence study revealed that iNOS and HIF-1α co-localized in cancerous tissues. Notably, the formation of 8-oxodG was correlated significantly with lymphatic invasion (r = 0.386 and P = 0.018). Moreover, 8-nitroguanine and 8-oxodG in non-cancerous tissues were associated significantly with neural invasion (P = 0.042 and P = 0.026, respectively). These results suggest that reciprocal activation between HIF-1α and iNOS mediates persistent DNA damage, which induces tumor invasiveness via mutations, resulting in poor prognosis.

CONCLUSION: The formation of 8-nitroguanine and 8-oxodG plays an important role in multiple steps of genetic changes leading to tumor progression, including invasiveness.

Prevention of human cancer by modulation of chronic inflammatory processes

Chronic inflammation induced by biological, chemical and physical factors has been associated with increased risk of human cancer at various sites. Inflammation facilitates the initiation of normal cells and their growth and progression to malignancy through production of pro-inflammatory cytokines and diverse reactive oxygen and nitrogen species. These also activate signaling molecules involved in inflammation and carcinogenesis such as nuclear transcription factor (NF-kappaB), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). Several chemopreventive agents act through inhibition of signaling pathways (e.g. NF-kappaB), inhibition of oxidant-generating enzymes (e.g. iNOS) and mediators of inflammation (e.g. COX-2), scavenging reactive oxygen and nitrogen species, and modulation of xenobiotic-metabolizing enzymes (especially phase II enzyme induction). Some anti-inflammatory drugs have been tested in clinical trials to prevent human cancer at several sites. Better understanding of the molecular mechanisms by which chronic inflammation increases cancer risk will lead to further development of new strategies for cancer prevention at many sites.

Mechanism of site-specific DNA damage induced by ozone

Ozone has been shown to induce lung tumors in mice. The reactivity of ozone with DNA in an aqueous solution was investigated by a DNA sequencing technique using 32P-labeled DNA fragments. Ozone induced cleavages in the deoxyribose-phosphate backbone of double-stranded DNA, which were reduced by hydroxyl radical scavengers, suggesting the participation of hydroxyl radicals in the cleavages. The ozone-induced DNA cleavages were enhanced with piperidine treatment, which induces cleavages at sites of base modification, but the inhibitory effect of hydroxyl radical scavengers on the piperidine-induced cleavages was limited. Main piperidine-labile sites were guanine and thymine residues. Cleavages at some guanine and thymine residues after piperidine treatment became more predominant with denatured single-stranded DNA. Exposure of calf thymus DNA to ozone resulted in a dose-dependent increase of the 8-oxo-7,8-dihydro-2'-deoxyguanosine formation, which was partially inhibited by hydroxyl radical scavengers. ESR studies using 5,5-dimethylpyrroline-N-oxide (DMPO) showed that aqueous ozone produced the hydroxyl radical adduct of DMPO. In addition, the fluorescein-dependent chemiluminescence was detected during the decomposition of ozone in a buffer solution and the enhancing effect of D2O was observed, suggesting the formation of singlet oxygen. However, no or little enhancing effect of D2O on the ozone-induced DNA damage was observed. These results suggest that DNA backbone cleavages were caused by ozone via the production of hydroxyl radicals, while DNA base modifications were mainly caused by ozone itself and the participation of hydroxyl radicals and/or singlet oxygen in base modifications is small, if any. A possible link of ozone-induced DNA damage to inflammation-associated carcinogenesis as well as air pollution-related carcinogenesis is discussed.

Inducible nitric oxide synthase-dependent DNA damage in mouse model of inflammatory bowel disease

Increased cancer risk occurs in inflammatory bowel disease (IBD) undergoing long-term chronic inflammation. To evaluate whether inducible nitric oxide synthase (iNOS)-dependent DNA damage plays a role in the carcinogenic process triggered by IBD, we prepared a mouse model of IBD induced by transfer of CD45RBhighCD4+ T cells lacking regulatory T cells to female severe combined immunodeficiency (SCID) mice. CD45RBhighCD4+ T cells were isolated from mouse spleen after staining with fluorescein isothiocyanate (FITC)-conjugated anti-CD45RB monoclonal antibody, followed by anti-FITC-conjugated microbeads. This IBD mouse model showed that the bodyweight increased with aging to a lesser extent than non-treated controls, and that the intestine was shortened. Pathological findings of this mouse model, which showed severe inflammation in colon tissues, were similar to IBD patients. Double immunofluorescence technique revealed that both 8-nitroguanine and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) were formed mainly in epithelial cells of the IBD mouse model. 8-Nitroguanine was formed in most of 8-oxodG-immunoreactive nuclei of epithelial cells. iNOS, proliferating cell nuclear antigen and p53 protein were also expressed in the colon epithelium. These results indicate that nitrative DNA damage, as well as oxidative DNA damage, is induced in colon epithelial cells of the IBD mouse model followed by proliferation of these cells, which may contribute to colon carcinogenesis.

DNA damage and repair in Helicobacter pylori-infected gastric mucosa cells

Helicobacter pylori is a common human pathogen and its infection is believed to contribute to gastric cancer. Impaired DNA repair may fuel up cancer transformation by the accumulation of mutation and increased susceptibility to exogenous carcinogens. To evaluate the role of infection of H. pylori in DNA damage and repair we determined: (1) the level of endogenous basal, oxidative and alkylative DNA damage, and (2) the efficacy of removal of DNA damage induced by hydrogen peroxide and the antibiotic amoxicillin in the H. pylori-infected and non-infected GMCs. DNA damage and the efficacy of DNA repair were evaluated by the alkaline single cell gel electrophoresis (comet assay). Specific damage to the DNA bases were assayed with the DNA repair enzymes formamidopyrimidine-DNA glycosylase (Fpg) recognizing oxidized DNA bases and 3-methyladenine-DNA glycosylase II (AlkA) recognizing alkylated bases. The level of basal and oxidative DNA in the infected GMCs was higher than non-infected cells. H. pylori-infected GMCs displayed enhanced susceptibility to hydrogen peroxide than control cells. There was no difference between the efficacy of DNA repair in the infected and non-infected cells after treatment with hydrogen peroxide and amoxicillin. Our results indicate that H. pylori infection may be correlated with oxidative DNA damage in GMCs. Therefore, these features can be considered as a risk marker for gastric cancer associated with H. pylori infection and the comet assay may be applied to evaluate this marker.

Nitric Oxide-Induced Nitrative Stress Involved in Microbial Pathogenesis

The pathogenic mechanism of infections is a complicated but important scientific theme that is now attracting great attention because of its association with host-derived as well as microbial factors. Recent advances in free radical research revealed that reactive oxygen and nitrogen oxide species such as superoxide (O(2)(-)) and nitric oxide (NO) play a leading role in the pathogenesis of infections caused by viral pathogens including influenza virus and other RNA viruses. Although NO and O(2)(-) have antimicrobial activity against bacteria, fungi, and parasites, in some viral infections they have an opposite effect. This exacerbation caused by NO and O(2)(-) is mediated by reactive nitrogen oxides, for example, peroxynitrite (ONOO(-)), generated by reaction of NO with O(2)(-). These nitrogen oxides have strong oxidation and nitration potential and can modify biological molecules, thereby creating oxidative and nitrative stress that contributes to pathogenic processes during viral infection. Nitrative stress-mediated 8-nitroguanosine formation during influenza or Sendai virus infection has been the focus of enormous interest because it involves unique biochemical and pharmacological properties such as redox activity and mutagenic potential. In this review, we discuss the nature and impact of nitrative stress in viral infection, with emphasis on nitrative stress-mediated viral pathogenesis, which we have recently been investigating.