Oxidative DNA damage in tissues of pregnant female mice and fetuses caused by the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

Regulation of Nrf2/ARE Pathway by Dietary Flavonoids: A Friend or Foe for Cancer Management?

The nuclear factor erythroid 2-related factor 2 (Nrf2)/antioxidant response element (ARE) pathway is an important cell signaling mechanism in maintaining redox homeostasis in humans. The role of dietary flavonoids in activating Nrf2/ARE in relation to cancer chemoprevention or cancer promotion is not well established. Here we summarize the dual effects of flavonoids in cancer chemoprevention and cancer promotion with respect to the regulation of the Nrf2/ARE pathway, while underlying the possible cellular mechanisms. Luteolin, apigenin, quercetin, myricetin, rutin, naringenin, epicatechin, and genistein activate the Nrf2/ARE pathway in both normal and cancer cells. The hormetic effect of flavonoids has been observed due to their antioxidant or prooxidant activity, depending on the concentrations. Reported in vitro and in vivo investigations suggest that the activation of the Nrf2/ARE pathway by either endogenous or exogenous stimuli under normal physiological conditions contributes to redox homeostasis, which may provide a mechanism for cancer chemoprevention. However, some flavonoids, such as luteolin, apigenin, myricetin, quercetin, naringenin, epicatechin, genistein, and daidzein, at low concentrations (1.5 to 20 µM) facilitate cancer cell growth and proliferation in vitro. Paradoxically, some flavonoids, including luteolin, apigenin, and chrysin, inhibit the Nrf2/ARE pathway in vitro. Therefore, even though flavonoids play a major role in cancer chemoprevention, due to their possible inducement of cancer cell growth, the effects of dietary flavonoids on cancer pathophysiology in patients or appropriate experimental animal models should be investigated systematically.

Metabolites of Tobacco- and E-Cigarette-Related Nitrosamines Can Drive Cu2+-Mediated DNA Oxidation

Nitrosamine metabolites resulting from cigarette smoking and E-cigarette (E-cig) vaping cause DNA damage that can lead to genotoxicity. While DNA adducts of metabolites of nitrosamines 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N-nitrosonornicotine (NNN) are well-known tobacco-related cancer biomarkers, only a few studies implicate NNN and NNK in DNA oxidation in humans. NNK and NNN were found in the urine of E-cigarette users who never smoked cigarettes. This paper proposes the first chemical pathways of DNA oxidation driven by NNK and NNN metabolites in redox reactions with Cu2+ and NADPH leading to reactive oxygen species (ROS). A microfluidic array with thin films of DNA and metabolic enzymes that make metabolites of NNN and NNK in the presence of Cu2+ and NADPH was used to estimate relative rates of DNA oxidation. Detection by electrochemiluminescence (ECL) employed a new ECL dye [Os(tpy-benz-COOH)2]2+ that is selective for and sensitive to the primary DNA oxidation product 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxodG) in DNA. Enzyme-DNA films on magnetic beads were used to produce nitrosamine metabolites that enter ROS-forming redox cycles with Cu2+ and NADPH, and liquid chromatography-mass spectrometry (LC-MS) was used to quantify 8-oxodG and identify metabolites. ROS were detected by optical sensors. Metabolites of NNK and NNN + Cu2+ + NADPH generated relatively high rates of DNA oxidation. Lung is the exposure route in smoking and vaping, human lung tissue contains Cu2+ and NADPH, and lung microsomal enzymes gave the highest rates of DNA oxidation in this study. Also, E-cigarette vapor contains 6-fold more copper than that in cigarette smoke, which could exacerbate DNA oxidation.

Cigarette smoke inhalation aggravates diabetic kidney injury in rats

Diabetic kidney disease (DKD) is the leading cause of end-stage kidney disease. Epidemiological studies have demonstrated that cigarette smoke or nicotine is a risk factor for the progression of chronic kidney injury. The present study analyzed the kidney toxicity of cigarette smoke in experimental rats with DKD. Experimental diabetes was induced in 7-week-old Sprague-Dawley rats by a single intraperitoneal injection of streptozotocin (60 mg kg^-1). Four weeks after the induction of diabetes, rats were exposed to cigarette smoke (200 μg L^-1), 4 h daily, and 5 days per week for 4 weeks. Cigarette smoke did not affect the levels of plasma glucose, hemoglobin A1c, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol or non-esterified fatty acids in both control and diabetic rats under the experimental conditions. Cigarette smoke, however, significantly increased diabetes-induced glomerular hypertrophy and urinary kidney injury molecule-1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) excretion, suggesting exacerbation of diabetic kidney injury. Cigarette smoke promoted macrophage infiltration and fibrosis in the diabetic kidney. As expected, cigarette smoke increased oxidative stress in both control and diabetic rats. These data demonstrated that four weeks of exposure to cigarette smoke aggravated the progression of DKD in rats.

Betel quid containing safrole enhances metabolic activation of tobacco specific 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)

Cigarette smoking (CS) and betel quid (BQ) chewing are two known risk factors that have synergistic potential for the enhancing the development of oral squamous cell carcinoma (OSCC) in Taiwan. Most mutagens and carcinogens are metabolically activated by cytochrome P450 (CYP450) to exert their mutagenicity or carcinogenicity. Previous studies have shown that metabolic activation of the tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), by CYP2A6 activity determines NNK-induced carcinogenesis. In addition, safrole affects cytochrome P450 activity in rodents. However, the effect of BQ safrole on the metabolism of tobacco-specific NNK and its carcinogenicity remains elusive. This study demonstrates that safrole (1 mg/kg/d) induced CYP2A6 activity, reduced urinary 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL) levels, and increased NNK-induced DNA damage, including N7-methylguanine, 8-OH-deoxyguanosine and DNA strand breaks in a Syrian golden hamster model. Furthermore, altered NNK metabolism and increased NNK-induced DNA damage were also observed in healthy subjects with CS and BQ chewing histories compared to healthy subjects with CS histories. In conclusion, BQ containing safrole induced tobacco-specific NNK metabolic activation, resulting in higher NNK-induced genotoxicity. This study provides valuable insight into the synergistic mechanisms of CS- and BQ-induced OSCC.

Tobacco nitrosamines as culprits in disease: mechanisms reviewed

The link between tobacco abuse and cancer is well-established. However, emerging data indicate that toxins in tobacco smoke cause cellular injury due to enhanced toxic/metabolic effects of metabolites, disruption of intracellular signaling mechanisms, and formation of DNA, protein, and lipid adducts that impair function and promote oxidative stress and inflammation. These effects of smoking, which are largely non-carcinogenic, can be produced by tobacco-specific nitrosamines and their metabolites. These factors could account for the increased rates of neurodegeneration and insulin resistance diseases among smokers. Herein, we review nicotine and tobacco-specific nitrosamine metabolism, mechanisms of adduct formation, DNA damage, mutagenesis, and potential mechanisms of disease.

Bcl2 inhibition of mitochondrial DNA repair

Background

Accumulation of mitochondrial DNA (mtDNA) damage could enhance the frequency of mitochondrial mutations and promote a variety of mitochondria-related diseases, including cancer. However, the mechanism(s) involved are not fully understood.

Methods

Quantitative extended length PCR was used to compare mtDNA and nDNA damage in human lung H1299 cells expressing WT Bcl2 or vector-only control. mtAPE1 endonuclease activity was analyzed by AP oligonucleotide assay. mtDNA mutation was measured by single molecule PCR. Subcellular localization of Bcl2 and APE1 was analyzed by subcellular fractionation.

Results

Bcl2, an anti-apoptotic molecule and oncoprotein, effectively inhibits the endonuclease activity of mitochondrial APE1 (mtAPE1), leading to significant retardation of mtDNA repair and enhanced frequency of mtDNA mutations following exposure of cells to hydrogen peroxide (H₂O₂) or nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, a carcinogen in cigarette smoke). Inversely, depletion of endogenous Bcl2 by RNA interference increases mtAPE1 endonuclease activity leading to accelerated mtDNA repair and decreased mtDNA mutation. Higher levels of mtAPE1 were observed in human lung cancer cells than in normal human bronchial epithelial cells (i.e. BEAS-2B). Bcl2 partially co-localizes with APE1 in the mitochondria of human lung cancer cells. Bcl2 directly interacts with mtAPE1 via its BH domains. Removal of any of the BH domains from Bcl2 abolishes Bcl2’s capacity to interact with mtAPE1 as well as its inhibitory effects on mtAPE1 activity and mtDNA repair.

Conclusions

Based our findings, we propose that Bcl2 suppression of mtDNA repair occurs through direct interaction with mtAPE1 and inhibition of its endonuclease activity in mitochondria, which may contribute to enhanced mtDNA mutations and carcinogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1594-1) contains supplementary material, which is available to authorized users.

Thirdhand smoke causes DNA damage in human cells

Exposure to thirdhand smoke (THS) is a newly described health risk. Evidence supports its widespread presence in indoor environments. However, its genotoxic potential, a critical aspect in risk assessment, is virtually untested. An important characteristic of THS is its ability to undergo chemical transformations during aging periods, as demonstrated in a recent study showing that sorbed nicotine reacts with the indoor pollutant nitrous acid (HONO) to form tobacco-specific nitrosamines (TSNAs) such as 4-(methylnitrosamino)-4-(3-pyridyl)butanal (NNA) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). The goal of this study was to assess the genotoxicity of THS in human cell lines using two in vitro assays. THS was generated in laboratory systems that simulated short (acute)- and long (chronic)-term exposures. Analysis by liquid chromatography-tandem mass spectrometry quantified TSNAs and common tobacco alkaloids in extracts of THS that had sorbed onto cellulose substrates. Exposure of human HepG2 cells to either acute or chronic THS for 24h resulted in significant increases in DNA strand breaks in the alkaline Comet assay. Cell cultures exposed to NNA alone showed significantly higher levels of DNA damage in the same assay. NNA is absent in freshly emitted secondhand smoke, but it is the main TSNA formed in THS when nicotine reacts with HONO long after smoking takes place. The long amplicon-quantitative PCR assay quantified significantly higher levels of oxidative DNA damage in hypoxanthine phosphoribosyltransferase 1 (HPRT) and polymerase β (POLB) genes of cultured human cells exposed to chronic THS for 24h compared with untreated cells, suggesting that THS exposure is related to increased oxidative stress and could be an important contributing factor in THS-mediated toxicity. The findings of this study demonstrate for the first time that exposure to THS is genotoxic in human cell lines.

In utero-initiated cancer: the role of reactive oxygen species

It is becoming more evident that not only can drugs and environmental chemicals interfere with normal fetal development by causing structural malformations, such as limb defects, but that xenobiotic exposure during development can also cause biochemical and functional abnormalities that may ultimately lead to cancer later on in life. Fetal toxicity may be partly mediated by the embryonic bioactivation of xenobiotics to free radical intermediates that can lead to oxidative stress and potentially lead, in some cases, to carcinogenesis. Using a number of examples, this review will focus on the role of reactive oxygen species (ROS) in the mechanisms pertaining to in utero initiated cancers.

Bcl2 suppresses DNA repair by enhancing c-Myc transcriptional activity

Bcl2 and c-Myc are two major oncogenic proteins that can functionally promote DNA damage, genetic instability, and tumorigenesis. However, the mechanism(s) remains unclear. Nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is the most potent carcinogen contained in cigarette smoke that induces cellular DNA damage. Here we found that Bcl2 potently suppresses the repair of NNK-induced abasic sites of DNA lesions in association with increased c-Myc transcriptional activity. The Bcl2 BH4 domain (amino acids 6-31) was found to bind directly to c-Myc MBII domain (amino acids 106-143), and this interaction is required for Bcl2 to enhance c-Myc transcriptional activity and inhibit DNA repair. In addition to mitochondria, Bcl2 is also expressed in the nucleus, where it co-localizes with c-Myc. Expression of nuclear-targeted Bcl2 enhances c-Myc transcriptional activity with suppression of DNA repair but fails to prolong cell survival. Depletion of c-Myc expression from cells overexpressing Bcl2 significantly accelerates the repair of NNK-induced DNA damage, indicating that c-Myc may be essential for the Bcl2 effect on DNA repair. It is known that apurinic/apyrimidinic endonuclease (APE1) plays a crucial role in the repair of abasic sites of DNA lesions. That overexpression of Bcl2 results in up-regulation of c-Myc and down-regulation of APE1 suggests APE1 may function as the downstream target of Bcl2/c-Myc in the DNA repair machinery. Thus, Bcl2, in addition to its survival function, may also suppress DNA repair in a novel mechanism involving c-Myc and APE1, which may lead to an accumulation of DNA damage in living cells, genetic instability, and tumorigenesis.

Genotoxicity of tobacco smoke and tobacco smoke condensate: a review

This report reviews the literature on the genotoxicity of mainstream tobacco smoke and cigarette smoke condensate (CSC) published since 1985. CSC is genotoxic in nearly all systems in which it has been tested, with the base/neutral fractions being the most mutagenic. In rodents, cigarette smoke induces sister chromatid exchanges (SCEs) and micronuclei in bone marrow and lung cells. In humans, newborns of smoking mothers have elevated frequencies of HPRT mutants, translocations, and DNA strand breaks. Sperm of smokers have elevated frequencies of aneuploidy, DNA adducts, strand breaks, and oxidative damage. Smoking also produces mutagenic cervical mucus, micronuclei in cervical epithelial cells, and genotoxic amniotic fluid. These data suggest that tobacco smoke may be a human germ-cell mutagen. Tobacco smoke produces mutagenic urine, and it is a human somatic-cell mutagen, producing HPRT mutations, SCEs, microsatellite instability, and DNA damage in a variety of tissues. Of the 11 organ sites at which smoking causes cancer in humans, smoking-associated genotoxic effects have been found in all eight that have been examined thus far: oral/nasal, esophagus, pharynx/larynx, lung, pancreas, myeoloid organs, bladder/ureter, uterine cervix. Lung tumors of smokers contain a high frequency and unique spectrum of TP53 and KRAS mutations, reflective of the PAH (and possibly other) compounds in the smoke. Further studies are needed to clarify the modulation of the genotoxicity of tobacco smoke by various genetic polymorphisms. These data support a model of tobacco smoke carcinogenesis in which the components of tobacco smoke induce mutations that accumulate in a field of tissue that, through selection, drive the carcinogenic process. Most of the data reviewed here are from studies of human smokers. Thus, their relevance to humans cannot be denied, and their explanatory powers not easily dismissed. Tobacco smoke is now the most extreme example of a systemic human mutagen.

Carcinogen-specific targeting of chromosome 12 for loss of heterozygosity in mouse lung adenocarcinomas: implications for chromosome instability and tumor progression

Genotoxic carcinogens exert their tumorigenic effects in part by inducing genomic instability. We recently showed that loss of heterozygosity (LOH) on chromosome 12 associates significantly with the induction of chromosome instability (CIN) by the likely human lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and vinyl carbamate (VC) during mouse lung carcinogenesis. Here, we demonstrate the carcinogen specificity of this event and its effect on lung tumor evolution. LOH on chromosome 12 was observed in 45% of NNK-induced, 59% of VC-induced, 58% of aflatoxin B1 (AFB1)-induced, 14% of N-ethyl-N-nitrosourea (ENU)-induced and 12% of spontaneous lung adenocarcinomas. The frequency of LOH in each of the carcinogen-induced groups, except ENU, was significantly higher than in the spontaneous group (P<0.001). Deletion mapping revealed four potential candidate regions of 1-4 centiMorgans suspected to contain targeted tumor suppressor genes, with at least one expected to have a role in CIN. The relationship between LOH on chromosome 12 and additional chromosomal alterations occurring during lung tumor progression was also examined. LOH on chromosomes 1 and 14 were moderately frequent during malignant progression in tumors from all treatment groups, occurring in 21-35 and 18-33% of tumors. However, these alterations showed significant concurrence with LOH on chromosome 12 in VC-, NNK- and AFB1-induced tumors (P<0.05). The results suggest that a carcinogen-selective mechanism of lung cancer induction involves the frequent inactivation of genes on chromosome 12, including a stability gene that evidently promotes the evolutionary selection of additional chromosomal alterations during malignant progression.

Genomic and transcriptional alterations in mouse fetus liver after transplacental exposure to cigarette smoke

The transplacental exposure of fetuses to maternal cigarette smoke may increase the risk of developmental impairments, congenital diseases, and childhood cancer. The whole-body exposure of Swiss mice to environmental cigarette smoke (ECS) during pregnancy decreased the number of fetuses per dam, placenta weight, and fetus weight. ECS increased DNA adducts, oxidative nucleotide alterations, and cytogenetic damage in fetus liver. Evaluation by cDNA array of 746 genes showed that 61 of them were expressed in fetus liver under basal conditions. The oral administration of N-acetylcysteine (NAC) during pregnancy enhanced the expression of three genes only, including two glutathione S-transferases and alpha1-antitrypsin precursor, whose deficiency plays a pathogenetic role in congenital emphysema. Transplacental ECS upregulated the expression of 116 genes involved in metabolism, response to oxidative stress, DNA and protein repair, and signal transduction. NAC inhibited the ECS-related genetic damage and upregulation of most genes. ECS stimulated pro-apoptotic genes and genes downregulating the cell cycle, which may justify growth impairments in the developing fetus. Thus, both genetic and epigenetic mechanisms were modulated by ECS. Moreover, hypoxia-related genes and several oncogenes and receptors involved in proliferation and differentiation of leukocytes were induced in the fetal liver, which also bears hematopoietic functions.

The protective role of selenium on genetic damage and on cancer

Collectively, results from epidemiologic studies, laboratory bioassays, and human clinical intervention trials clearly support a protective role of selenium against cancer development. Several hypotheses have been proposed to explain these observations. Increased genomic instability, either inherent or induced by exogenous agents (mutagens or carcinogens), has been considered as a primary event leading to neoplastic transformation. This report deals specifically with the evidence for a role of selenium in the inhibition of carcinogen-induced covalent DNA adduct formation and retardation of oxidative damage to DNA, lipids and proteins, and for modulating cellular and molecular events that are critical in cell growth inhibition and in the multi-step carcinogenesis process. At present, the bulk of our knowledge on the role of selenium on genetic stability is based primarily on animal data and from studies conducted in in vitro systems. Studies performed in vitro showed that the dose and form of selenium compounds are critical factors with regard to cellular responses. Inorganic (at doses up to 10microM) and organic selenium compounds (at doses equal to or greater than 10microM) elicit distinctly different cellular responses. The recommended daily allowance (RDA) is 50-70 microgramSe per day for healthy adults; with 40 microgramSe as minimum requirement. Less than 11 microgramSe will definitely put people at risk of deficiency that would be expected to cause genetic damage. Daily doses of 100-200 microgramSe inhibited genetic damage and cancer development in humans. About 400 microgramSe per day is considered an upper limit. Clearly, doses above the RDA are needed to inhibit genetic damage and cancer. However, it has been hypothesized that the intake of excessive doses of selenium may cause oxidative damage, leading to genomic instability. The use of a cocktail consisting of selenium, and other vitamins and minerals appears to be a promising approach to inhibit genetic damage and the development of cancer. It is the author's recommendation that development of mechanism-based hypotheses that can be tested in pilot studies in different populations prior to a large-scale clinical trial in humans, is of paramount importance in order to better understand the role of selenium on genetic stability and cancer.

Characterization of enzymes participating in carbonyl reduction of 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) in human placenta

4-Methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) has been identified as one of the strongest nitrosamine carcinogens in tobacco products in all species tested. Carbonyl reduction to 4-methylnitrosamino-1-(3-pyridyl)-1-butanol (NNAL) followed by glucuronosylation is considered to be the main detoxification pathway in humans. In previous investigations, we have identified a microsomal NNK carbonyl reductase as being identical to 11beta-hydroxysteroid dehydrogenase 1, a member of the short-chain dehydrogenase/reductase (SDR) superfamily. Recently, we provided evidence that carbonyl reduction of NNK does also take place in cytosol from mouse and human liver and lung. In human liver cytosol, carbonyl reductase, a SDR enzyme, and AKR1C1, AKR1C2 and AKR1C4 from the aldo-keto reductase (AKR) superfamily were demonstrated to be responsible for NNK reduction. Since NNK and/or its metabolites can diffuse through the placenta and reach fetal tissues, we now investigated NNK carbonyl reduction in the cytosolic fraction of human placenta in addition to that in microsomes. Concluding from the sensitivity to menadione, ethacrynic acid, rutin and quercitrin as specific inhibitors, mainly carbonyl reductase (EC 1.1.1.184) seems to perform this reaction in human placenta cytosol. The presence of carbonyl reductase was confirmed by RT-PCR. This is the first report to provide evidence that NNAL formation in placenta is mediated by carbonyl reductase.

Strain-dependent lung tumor formation in mice transplacentally exposed to 3-methylcholanthrene and post-natally exposed to butylated hydroxytoluene

The carcinogenic effects of in utero exposure to 3-methylcholanthrene (MC) have been demonstrated in the tumor-resistant C57BL/6 (B6) and DBA (D2) strains of mice. In this study, we determined the effects of in utero exposure to MC in BALB/c mice, a strain which demonstrates greater susceptibility to lung tumor induction, and compared our findings with those previously found in [D2xB6D2F(1)]F(2) mice. In addition, we assessed the molecular pathogenesis of the chemically induced tumors and examined the effects of the putative lung tumor promoter butylated hydroxytoluene (BHT) in BALB/c mice. BALB/c mice were treated on day 17 of gestation with 5, 15 or 45 mg/kg MC and 6 weeks after birth with BHT for 6 consecutive weeks. Mice were killed at 6 months of age. Ki-ras, p16Ink4a and p19ARF gene loci were amplified from paraffin-embedded lung tumor tissue and screened for the presence of point mutations via allele-specific oligonucleotide hybridization and single strand conformation polymorphism (SSCP) analyses. Ki-ras point mutations were found in 56% (20/36) of BALB/c lung tumors, with 33% (2/6) of the hyperplasias, 58% (10/19) of the adenomas and 73% (8/11) of the carcinomas exhibiting point mutations at this gene locus. Similar incidences of Ki-ras mutations were previously found following transplacental exposure of [D2xB6D2F(1)]F(2) mice to MC and treatment of adult A/J mice with urethane. Interestingly, a strain-dependent difference was observed in the mutational spectrum. Sixty-two and 38% of the lung lesions in BALB/c mice exhibited G-->C and G-->T transversions, respectively, in contrast to the 13 and 84% incidences previously observed in [D2xB6D2F(1)]F(2) mice. SSCP analysis of the tumor suppressor gene p16Ink4a showed a 6% incidence of point mutations, consistent with that found in [D2xB6D2F(1)]F(2) mice. No mutations were found in exon 1beta of the p19ARF gene of either strain. BHT, a lung tumor promoter in adult mice, had no statistically significant effects on either tumor incidence, tumor multiplicity or the mutational spectrum produced in the Ki-ras gene by in utero MC treatment. However, though not significant, there was an observable trend in increased tumor multiplicity in mice co-treated with BHT. These data demonstrate the transplacental carcinogenic effect of MC in BALB/c mice and show that mutagenic damage to Ki-ras is a critical early event mediating murine lung tumorigenesis in both the tumor-sensitive and tumor-resistant strains. Unlike what occurs when adult BALB/c mice are treated with MC, BHT does not appear to significantly promote the formation of lung tumors following transplacental exposure to MC, possibly due to the rapid growth and cell proliferation in the developing organism. Strain-dependent differences in the Ki-ras mutational spectrum may be associated with their differential susceptibility to lung tumor initiation.

Higher activity of 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase (8-oxo-dGTPase) coincides with lower background levels of 8-oxo-2'-deoxyguanosine in DNA of fetal compared with maternal mouse organs

Mammalian homologues of Escherichia coli MutT, a protein having 8-oxo-2'-deoxyguanosine 5'-triphosphate pyrophosphohydrolase (8-oxo-dGTPase) activity, are thought to play the same role in preventing the incorporation of promutagenic 8-oxo-2'-deoxyguanosine (8-oxo-dG) into DNA. One could thus expect that higher activity of 8-oxo-dGTPase should correlate with a lower background level of 8-oxo-dG in nuclear DNA. During transplacental carcinogenesis experiments, in control healthy Swiss mice on day 18 of gestation we found consistently lower levels of 8-oxo-dG in DNA in fetal livers and lungs (1.74+/-0.04 SE and 1.49+/-0.08 SE 8-oxo-dG/10(5) dG, respectively; pooled organs of fetuses of 8 dams) as compared with maternal organs (3.05+/-0.20 SE and 3.08+/-0.17 SE 8-oxo-dG/10(5) dG, respectively; n = 8). The 8-oxo-dGTPase activity determination in the same organs revealed that the lower levels of 8-oxo-dG in fetal DNA did, indeed, coincide with higher 8-oxo-dGTPase activity (48.8+/-2.6 SE and 52.5+/-2.5 SE U/mg protein in livers and lungs, respectively); and vice versa, higher 8-oxo-dG levels in DNA of maternal organs were associated with lower levels of 8-oxo-dGTPase activity (24.3+/-1.3 SE and 4.7+/-0.6 SE U/mg protein, as above). Without excluding other reasons for the relatively low 8-oxo-dG background in DNA of fetal tissues (e.g., higher level of antioxidants and antioxidative enzymes; more efficient DNA repair), this inverse relationship may support or at least does not contradict the concept of a guardian role of 8-oxo-dGTPase against 8-oxo-dGTP mutagenicity in mammalian cells.

DNA adduct formation from tobacco-specific N-nitrosamines

Tobacco-specific N-nitrosamines are a group of carcinogens derived from the tobacco alkaloids. They are likely causative factors for cancers of the lung, esophagus, pancreas, and oral cavity in people who use tobacco products. The most carcinogenic tobacco-specific nitrosamines in laboratory animals are 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), and N'-nitrosonornicotine (NNN). DNA adduct formation from NNK and NNN has been studied extensively and is reviewed here. NNK is metabolically activated by cytochromes P450 to intermediates which methylate and pyridyloxobutylate DNA. The resulting adducts have been detected in cells and tissues susceptible to NNK carcinogenesis in rodents. The methylation and pyridyloxobutylation pathways are both important in carcinogenesis by NNK. NNK also induces single strand breaks and increases levels of 8-oxodeoxyguanosine in DNA of treated animals. NNAL, which like NNK is a potent pulmonary carcinogen, is also metabolically activated to methylating and pyridyloxobutylating intermediates. NNN pyridyloxobutylates DNA in its rat target tissues, esophagus and nasal mucosa. Methyl and pyridyloxobutyl DNA adducts are detected in human tissues. The methyl adducts most likely result in part from exposure of smokers to NNK, but these adducts are also detected in non-smokers. Some of the methyl adducts detected in non-smokers may be due to environmental tobacco smoke exposure. There are also potential dietary and endogenous sources of these adducts. Pyridyloxobutyl DNA adducts in human tissues result mainly from exposure to tobacco-specific N-nitrosamines. In laboratory animals, DNA adduct formation and carcinogenicity of tobacco-specific N-nitrosamines are closely correlated in many instances, and it is likely that similar relationships will hold in humans.

Experimental study of oxidative DNA damage

Animal experiments allow the study of oxidative DNA damage in target organs and the elucidation of dose-response relationships of carcinogenic and other harmful chemicals and conditions as well as the study of interactions of several factors. So far the effects of more than 50 different chemical compounds have been studied in animal experiments mainly in rats and mice, and generally with measurement of 8-oxodG with HPLC-EC. A large number of well-known carcinogens induce 8-oxodG formation in liver and/or kidneys. Moreover several animal studies have shown a close relationship between induction of dative DNA damage and tumour formation. In principle the level of oxidative DNA damage in an organ or cell may be studied by measurement of modified bases in extracted DNA by immunohistochemical visualisation, and from assays of strand breakage before and after treatment with repair enzymes. However, this level is a balance between the rates of damage and repair. Until the repair rates and capacity can be adequately assessed the rate of damage can only be estimated from the urinary excretion of repair products albeit only as an average of the entire body. A number of model compounds have been used to induce oxidative DNA damage in experimental animals. The hepatocarcinogen 2-nitropropane induces up to 10-fold increases in 8-oxodG levels in rat liver DNA. The level of 8-oxodG is also increased in kidneys and bone marrow but not in the testis. By means of 2-nitropropane we have shown correspondence between the increases in 8-oxodG in target organs and the urinary excretion of 8-oxodG and between 8-oxodG formation and the comet assay in bone marrow as well potent preventive effects of extracts of Brussels sprouts. Others have shown similar effects of green tea extracts and its components. Drawbacks of the use of 2-nitropropane as a model for oxidative DNA damage relate particularly to formation of 8-aminoguanine derivatives that may interfere with HPLC-EC assays and have unknown consequences. Other model compounds for induction of oxidative DNA damage, such as ferric nitriloacetate, iron dextran, potassium bromate and paraquat, are less potent and/or more organ specific. Inflammation and activation of an inflammatory response by phorbol esters or E. coli lipopolysaccharide (LPS) induce oxidative DNA damage in many target cells and enhance benzene-induced DNA damage in mouse bone marrow. Experimental studies provide powerful tools to investigate agents inducing and preventing oxidative damage to DNA and its role in carcinogenesis. So far, most animal experiments have concerned 8-oxodG and determination of additional damaged bases should be employed. An ideal animal model for prevention of oxidative DNA damage has yet to he developed.

Methodological considerations and factors affecting 8-hydroxy-2-deoxyguanosine analysis

Oxidative stress is related to a number of diseases due to the formation of reactive oxygen species (ROS). There are also several substances found in the occupational environment or as life style related situations that generates ROS. A stable biomarker for oxidative stress on DNA is 8-hydroxy-2'-deoxyguanosine (8-OH-dG). A potential problem in the work-up and analysis of 8-OH-dG is oxidation of dG with false high levels as a result of analysis. This paper summarizes and discusses some of the critical moments in terms of auto-oxidation. The removal of transition metals, low temperatures, absence of isotopes (or 2'-deoxyguanosine) and incubation times are all important factors. Removal of oxygen is complicated while the problem is reduced if a nitroxide (TEMPO) is added during work-up. Certain reducing agents and enzymes could be critical if added during work-up. The application of the 32p-HPLC method to analyze 8-OH-dG is discussed. The 32P-HPLC method is suitable for 8-OH-dG analysis and avoids several factors that oxidizes dG by removal of dG before addition of isotopes. Factors of crucial importance (columns, eluents, gradients and detection of 32p) for the analysis of 8-OH-dG are commented upon and certain recommendations are made to make it possible to apply the 32P-HPLC methodology for this type of analysis.