Mouse Lung DNA Research Articles

Identification of a Novel Methylation Site (CpG 388) in the Promoter/Enhancer Region of Cytochrome P450 (CYP)1B1: Possible Role of Promoter Methylation in Attenuation of CYP1B1 expression and DNA adducts by Omega 3‐ Fatty Acids in Mouse Lung In Vivo

3‐Methylcholanthrene (MC) and benzo[a]pyrene (BP) are polycyclic aromatic hydrocarbons (PAHs) carcinogens. Cytochrome P450 (CYP)1B1 enzymes plays a key role in the activation of PAHs to carcinogenic metabolites, which initiate carcinogenesis by binding covalently to DNA, and the adducts, if not repaired, could lead to tumorigenesis. In this study we tested the hypothesis that pre‐treatment of mice with omega‐3‐fatty acids, i.e. [eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) will lead to attenuation of PAH‐mediated pulmonary DNA adduct formation in part by inhibiting CYP1B1. Twelve week old male and female A/J mice received EPA (60 mg/kg) and DHA (40 mg/kg) from day 1 to day 24. Control mice were treated with vehicle corn oil. On day 3, mice were treated with BP (40 μmol/kg) or 3‐methylcholanthrene (MC, 40 μmol/kg) by i.p. In the short‐term experiment (DNA adduct studies), 3–5 mice from each group were terminated at day 10 (7 days after BP or MC treatment). EPA/DHA significantly suppressed formation of BP‐DNA and MC‐DNA adducts in lung and liver of both male and female mice. CYP1B1 expression in lung at protein and mRNA levels was induced significantly by PAHs, but was suppressed by about 70% in the lungs of EPA/DHA‐treated mice. We tested the hypothesis that PAHs in part induce CYP1B1 by methylating specific CpG sites on the negative regulatory elements (NREs) of the promoter. Mouse lung DNA samples were subjected to bisulfite deamination, Methylation‐Specific PCR (MSP), and a sequencing primer‐flanking nested PCR. ClustalW multiple sequence alignment determined the methylation state of a total of 103 putative CpG sites on the CYP1B1 promoter/enhancer region, which included 3 putative CpG islands. We found that mice treated with BP resulted in partially methylated CpG 388 (83%) in lung DNAs, while co‐administration with EPA/DHA significantly suppressed the methylation of CpG 388 (to 33%). These results suggest that PAHs induce CYP1B1 in part by inducing methylation of CpG 388, which might be putative NREs of CYP1B1. We hypothesize that suppression of methylation by EPA/DHA at CpG 388 leads to restoration of negative regulation, leading to repression of CYP1B1 expression, which in turn leads to inhibition of PAH carcinogenesis. We further verified the role of CpG 388 by mutating it in a luficerase reporter plasmid containing mouse CYP1B1 promoter/enhancer. Preliminary results in transiently transfected H441 lung cell line indicated that both wild type and mutant CYP1B1 promoters were activated about 2.5 fold by BP, and mutation of CpG 384 decreased both basal and inducible promoter activities. Further studies could lead to CYP1B1 as an important molecular target for the prevention and/or treatment of lung carcinogenesis by PAHs in humans.Support or Funding InformationThis work was supported in part by NIEHS grant R01ES029382 to BM.

Abstract 4645: Identification of two methylation sites (CpG 388 and CpG 1664) in the promoter/enhancer region of cytochrome P450 (CYP)1B1: Possible role of promoter methylation in attenuation of CYP1B1 expression and DNA adducts by omega 3- fatty acids in mouse lung in vivo

Abstract 3-Methylcholanthrene (MC) and benzo[a]pyrene (BP) are polycyclic aromatic hydrocarbons (PAHs) carcinogens. Cytochrome P450 (CYP)1B1 enzymes plays a key role in the activation of PAHs to carcinogenic metabolites, which initiate carcinogenesis by binding covalently to DNA, and the adducts, if not repaired, could lead to tumorigenesis. In this study we tested the hypothesis that pre-treatment of mice with omega-3-fatty acids, i.e. [eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) will lead to attenuation of PAH-mediated pulmonary DNA adduct formation in part by inhibiting CYP1B1. Twelve week old male and female A/J mice received EPA (60 mg/kg) and DHA (40 mg/kg) from day 1 to day 24. Control mice were treated with vehicle corn oil. On day 3, mice were treated with BP (40 µmol/kg) or 3-methylcholanthrene (MC, 40 µmol/kg) by i.p. In the short-term experiment (DNA adduct studies), 3-5 mice from each group were terminated at day 10 (7 days after BP or MC treatment). EPA/DHA significantly suppressed formation of BP-DNA and MC-DNA adducts in lung and liver of both male and female mice. CYP1B1 expression in lung at protein and mRNA levels was induced significantly by PAHs, but was suppressed by about 70% in the lungs of EPA/DHA-treated mice. We tested the hypothesis that PAHs in part induce CYP1B1 by methylating specific CpG sites in the negative regulatory elements of the promoter. Mouse lung DNA samples were subjected to bisulfite deamination, Methylation-Specific PCR (MSP), and a sequencing primer-flanking nested PCR. ClustalW multiple sequence alignment determined the methylation state of a total of 103 putative CpG sites in the CYP1B1 promoter/enhancer region, which included 3 putative CpG islands. We found that CpG 1664, which is located in a putative CpG island, was methylated by BP, and the extent of methylation was decreased by co-treatment with BP and EPA/DHA. BP partially methylated CpG 388 (83%), while EPA/DHA-treated animals showed significantly decreased methylation (33%) at this site. These results suggest that PAHs induce CYP1B1 in part by inducing methylation of CpG 1664 and CpG 388, which might be putative negative regulatory elements of CYP1B1. We hypothesize that suppression of methylation by EPA/DHA at sites CpG 388 and 1664 leads to repression of CYP1B1 expression, which in turn leads to inhibition of PAH carcinogenesis. This is the first report that links in vivo CYP1B1 promoter methylation to modulation of carcinogenesis by PAHs. Further studies could lead to CYP1B1 as an important molecular target for the prevention and/or treatment of lung carcinogenesis by PAHs in humans. Note: This abstract was not presented at the meeting. Citation Format: Bhagavatula Moorthy, Weiwu Jiang, Lihua Wang, Sudha R. Kondraganti, Guodong Zhou, Chun Chu. Identification of two methylation sites (CpG 388 and CpG 1664) in the promoter/enhancer region of cytochrome P450 (CYP)1B1: Possible role of promoter methylation in attenuation of CYP1B1 expression and DNA adducts by omega 3- fatty acids in mouse lung in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4645.

Quantification of Kras mutant fraction in the lung DNA of mice exposed to aerosolized particulate vanadium pentoxide by inhalation

This study investigated whether Kras mutation is an early event in the development of lung tumors induced by inhalation of particulate vanadium pentoxide (VP) aerosols. A National Toxicology Program tumor bioassay of inhaled particulate VP aerosols established that VP-induced alveolar/bronchiolar carcinomas of the B6C3F1 mouse lung carried Kras mutations at a higher frequency than observed in spontaneous mouse lung tumors. Therefore, this study sought to: (1) characterize any Kras mutational response with respect to VP exposure concentration, and (2) investigate the possibility that amplification of preexisting Kras mutation is an early event in VP-induced mouse lung tumorigenesis. Male Big Blue B6C3F1 mice (6 mice/group) were exposed to aerosolized particulate VP by inhalation, 6h/day, 5 days/week for 4 or 8 weeks, using VP exposure concentrations of 0, 0.1, and 1mg/m3. The levels of two different Kras codon 12 mutations [GGT→GAT (G12D) and GGT→GTT (G12V)] were measured in lung DNAs by Allele-specific Competitive Blocker PCR (ACB–PCR). For both exposure concentrations (0.1 and 1.0mg/m3) and both time points (4 and 8 weeks), the mutant fractions observed in VP-exposed mice were not significantly different from the concurrent controls. Given that 8 weeks of inhalation of a tumorigenic concentration of particulate aerosols of VP did not result in a significant change in levels of lung Kras mutation, the data do not support either a direct genotoxic effect of VP on Kras or early amplification of preexisting mutation as being involved in the genesis of VP-induced mouse lung tumors under the exposure conditions used. Rather, the data suggest that accumulation of Kras mutation occurs later with chronic VP exposure and is likely not an early event in VP-induced mouse lung carcinogenesis.

In vivo treatment with aflatoxin B1 increases DNA oxidation, base excision repair activity and 8-oxoguanine DNA glycosylase 1 levels in mouse lung

Carcinogenicity of the mycotoxin aflatoxin B1 (AFB1), which is produced by Aspergillus fungi, is associated with bioactivation of AFB1 to AFB1-8,9-exo-epoxide and formation of DNA adducts. However, AFB1 also causes 8-hydroxy-2′-deoxyguanosine (8-OHdG) formation in mouse lung DNA, suggesting that oxidative DNA damage may also contribute to AFB1 carcinogenicity. The oxidative DNA damage 5-hydroxy-2′-deoxycytidine (5-OHdC) may also contribute to AFB1 carcinogenicity. The objective of the present study was to determine the effect of treatment of mice with AFB1 on pulmonary and hepatic: 8-OHdG and 5-OHdC levels; base excision repair (BER, which repairs oxidative DNA damage) activities; and on levels of 8-oxoguanine DNA glycosylase (OGG1, the rate-limiting enzyme in the BER of 8-OHdG). Female A/J mice were treated with vehicle (dimethyl sulfoxide) or 50mg/kg AFB1 ip. Oxidative DNA damage was measured using HPLC with electrochemical detection, BER activity was assessed using an in vitro assay that employs a substrate plasmid DNA with 8-OHdG lesions, and OGG1 protein levels were determined by immunoblotting. Two hours post treatment, AFB1 increased 8-OHdG levels in mouse lung DNA by approximately 69% relative to control (p<0.05), but did not alter 8-OHdG levels in liver or 5-OHdC levels in lung or liver (p>0.05). AFB1 treatment also increased BER activity in mouse lung by approximately 87% (p<0.05) but did not affect hepatic BER activity (p>0.05). Levels of OGG1 immunoreactive protein were increased in both lung (20%) and liver (60%) (p<0.05). These results are consistent with oxidative DNA damage contributing to the carcinogenicity of AFB1 in this model.

DNA adducts in rats and mice following exposure to [4- [formula omitted]]-1,2-epoxy-3-butene and to [2,3- [formula omitted]]-1,3-butadiene

1,3-Butadiene (BD) is a major industrial chemical and a rodent carcinogen, with mice being much more susceptible than rats. Oxidative metabolism of BD, leading to the DNA-reactive epoxides 1,2-epoxy-3-butene (BMO), 1,2-epoxy-3,4-butanediol (EBD) and 1,2:3,4-diepoxybutane (DEB), is greater in mice than rats. In the present study the DNA adduct profiles in liver and lungs of rats and mice were determined following exposure to BMO and to BD since these profiles may provide qualitative and quantitative information on the DNA-reactive metabolites in target tissues. Adducts detected in vivo were identified by comparison with the products formed from the reaction of the individual epoxides with 2′-deoxyguanosine (dG). In rats and mice exposed to [4- 14 C ]-BMO (1–50 mg/kg, i.p.), DNA adduct profiles were similar in liver and lung with N 7-(2-hydroxy-3-butenyl)guanine ( G1) and N 7-(1-(hydroxymethyl)-2-propenyl)guanine ( G2) as major adducts and N 7-2,3,4-trihydroxybutylguanine ( G4) as minor adduct. In rats and mice exposed to 200 ppm [2,3- 14 C ]-BD by nose-only inhalation for 6 h, G4 was the major adduct in liver, lung and testes while G1 and G2 were only minor adducts. Another N 7-trihydroxybutylguanine adduct ( G3), which could not unambiguously be identified but is either another isomer of N 7-2,3,4-trihydroxybutylguanine or, more likely, N 7-(1-hydroxymethyl-2,3-dihydroxypropyl)guanine, was present at low concentrations in liver and lung DNA of mice, but absent in rats. The evidence indicates that the major DNA adduct formed in liver, lung and testes following in vivo exposure to BD is G4, which is formed from EBD, and not from DEB.

Dose responses for DNA adduct formation in tissues of rats and mice exposed by inhalation to low concentrations of 1,3-[2,3- [formula omitted]]-butadiene

Male Sprague–Dawley rats and B6C3F1 mice were exposed to either a single 6 h or a multiple (5) daily (6 h) nose-only dose of 1,3-[2,3- 14 C ]-butadiene at exposure concentrations of nominally 1, 5 or 20 ppm. The aim was to compare the results with those from a similar previous study at 200 ppm. DNA isolated from liver, lung and testis of exposed rats and mice was analysed for the presence of butadiene related adducts, especially the N7-guanine adducts. Total radioactivity present in the DNA from liver, lung and testis was quantified and indicated more covalent binding of radioactivity for mouse tissue DNA than rat tissue DNA. Following release of the depurinating DNA adducts by neutral thermal hydrolysis, the liberated depurinated DNA adducts were measured by reverse phase HPLC coupled with liquid scintillation counting. The guanine adduct G4, assigned as N7-(2,3,4-trihydroxybutyl)- guanine, was the major adduct measured in liver, lung and testis DNA in both rats and mice. Higher levels of G4 were detected in all mouse tissues compared with rat tissue. The dose–response relationship for the formation of adduct G4 was approximately linear for all tissues studied for both rats and mice exposed in the 1–20 ppm range. The formation of G4 in liver tissue was about three times more effective for mouse than rat in this exposure range. Average levels of adduct G4 measured in liver DNA of rats and mice exposed to 5×6 h 1, 5 and 20 ppm 1,3-[2,3- 14 C ]-butadiene were, respectively, for rats: 0.79±0.30, 2.90±1.19, 16.35±4.8 adducts/10 8 nucleotides and for mice: 2.23±0.71, 12.24±2.15, 48.63±12.61 adducts/10 8 nucleotides. For lung DNA the corresponding values were for rats: 1.02±0.44, 3.12±1.06, 17.02±4.07 adducts/10 8 nucleotides, and for mice: 3.28±0.32, 14.04±1.55, 42.47±13.12 adducts/10 8 nucleotides. Limited comparative data showed that the levels of adduct G4 formed in liver and lung DNA of mice exposed to a single exposure to butadiene in the present 20 ppm study and earlier 200 ppm study were approximately directly proportional across dose, but this was not observed in the case of rats. From the available evidence it is most likely that adduct G4 was formed from a specific isomer of the diol-epoxide metabolite, 3,4-epoxy-1,2-butanediol rather than the diepoxide, 1,2,3,4-diepoxybutane. Another adduct G3, possibly a diastereomer of N7-(2,3,4-trihydroxybutyl)-guanine or most likely the regioisomer N7-(1-hydroxymethyl-2,3-dihydroxypropyl)-guanine, was also detected in DNA of mouse tissues but was essentially absent in DNA from rat tissue. Qualitatively similar profiles of adducts were observed following exposures to butadiene in the present 20 ppm study and the previous 200 ppm study. Overall the DNA adduct levels measured in tissues of both rats and mice were very low. The differences in the profiles and quantity of adducts seen between mice and rats were considered insufficient to explain the large difference in carcinogenic potency of butadiene to mice compared with rats.

Stereospecific deuterium substitution attenuates the tumorigenicity and metabolism of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK).

Stereochemical determinants of the tumorigenicity and metabolism of the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were investigated using the stereospecifically deuterated isotopomers (4R)-[4-(2)H(1)]NNK and (4S)-[4-(2)H(1)]NNK. Upon ip administration to groups of 20 female A/J mice, NNK and (4S)-[4-(2)H(1)]NNK exhibited similar lung tumorigenicity at three different doses, whereas (4R)-[4-(2)H(1)]NNK was 2-fold less tumorigenic at all three doses. In a parallel experiment, levels of O(6)-methylguanine and 7-methylguanine were 2-fold lower in lung DNA of mice treated with (4R)-[4-(2)H(1)]NNK than in mice treated with NNK or (4S)-[4-(2)H(1)]NNK. To corroborate these in vivo data, the in vitro metabolism of these compounds was investigated using A/J mouse lung microsomes and Spodoptera frugiperda (Sf9)-expressed mouse cytochrome p450s 2A4 and 2A5. Kinetic isotope effects on the apparent V(max) ((D)V) for the product of NNK 4-hydroxylation, OPB, were 2.7 +/- 0.2 and 2.8 +/- 0.4 when (4R)- and (4S)-[4-(2)H(1)]NNK were incubated with mouse lung microsomes, respectively. The (D)V values for OPB formation were 3.2 +/- 0.2 and 2.2 +/- 0.2 when (4R)-[4-(2)H(1)]NNK was the substrate for p2A4 and 2A5, respectively, whereas they were 1.3 +/- 0.1 and 1.1 +/- 0.1 when (4S)-[4-(2)H(1)]NNK was the substrate for these respective enzymes. Analysis of an OPB derivative (10) for deuterium content by LC/MS confirmed the results from the kinetic assays and indicated that p450s 2A4 and 2A5 preferentially abstract the pro-R 4-hydrogen of NNK. The results obtained using Sf9-expressed p450s provide a rationale for the differences observed in the lung tumor and DNA adduct experiments, namely, that the attenuated tumorigenicity of (4R)-[4-(2)H(1)]NNK relative to (4S)-[4-(2)H(1)]NNK is due to prochiral selectivity during p450-catalyzed metabolic activation.

Statistical calculation of detection limits for DNA adducts using the [formula omitted]-postlabeling assay with a standard addition procedure

The 32 P -postlabeling assay is widely used for the analysis of DNA adducts. Some adducts can be detected with very high sensitivity but quantification can be unreliable, particularly if it is based only on comparison with unmodified nucleotides (relative adduct labeling, RAL values). Furthermore, guidelines to calculate detection limits for adduct concentrations are lacking. This is particularly important for human biomonitoring studies of environmental exposures, where a low adduct level can remain undetected. Reports of null results of toxicity studies should always include a limit of detection, indicating the effect magnitude that would have produced, with a given probability of false negative (type II error), a statistically significant increase (type I error). Here, we report on a procedure based on t-statistics to calculate two types of detection limits, the “critical level (CL)” and the “detection level (DL)”. The first is the size of the difference between exposed and controls required to achieve statistical significance. The second is the size of the difference that will be detected with a chosen probability of a false negative. For the degrees of freedom (d.f.) to be used for the t-values, a general formula is given so that different standard deviations and group sizes of control and exposed groups can be handled. A sample calculation of the whole procedure is shown, using the null data for the formation of a particular adduct in lung DNA of styrene-treated mice, analyzed by 32 P -postlabeling. The procedure takes into account: (i) TLC-specific background radioactivity; (ii) variability within the control and exposed groups; and (iii) confidence limits for the factor to convert 32 P -radioactivity to amounts of adduct. The latter step incorporates the variance of the differences between the samples and replicates spiked with adduct standard. A statement such as follows is the result: the concentration of the α-isomer adduct of styrene 7,8-oxide at the O 6-position of guanine in mouse lung DNA would have to be at least 12 adducts per 10 8 nucleotides to be detected in the given experiment on a 5% level (type I error), with a probability of 5% to miss an existing effect (type II error).

Mechanisms of chronic disease causation by nutritional factors and tobacco products and their prevention by tea polyphenols

The beverage tea, from the top leaves of the plant Camellia sinensis is one of the most widely used beverages in the world, second only to water. Black and green tea have mostly similar actions. The active components are polyphenols, mainly epigallocatechin gallate in green tea, and the tea leaf polyphenol oxidase mediated oxidation to oolong and black tea, yielding other polyphenols, theaflavin and thearubigins. There is 40−50 mg caffeine in a 160-ml cup of tea. The chemopreventive effects of tea depend on: (1) its action as an antioxidant; (2) the specific induction of detoxifying enzymes; (3) its molecular regulatory functions on cellular growth, development and apoptosis; and (4) a selective improvement in the function of the intestinal bacterial flora. The oxidation of LDL cholesterol, associated with a risk for atherosclerosis and heart disease, is inhibited by tea. Many of cancers are caused by lifestyle elements. One is cigarette and tobacco use, leading to cancer in the oral cavity, esophagus and lung, inhibited by tea. Mice administered a tobacco nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), developed significantly fewer lung tumors than controls when given green tea or its major polyphenol, epigallocatechin gallate (EGCG). Tea suppressed the formation of 8-hydroxydeoxyguanosine (8-OHdG), a marker of oxidative DNA damage, in the lung DNA of mice given NNK. Gastric cancer, caused by a combination of Helicobacter pylori and salted foods, is lower in tea drinkers. Western nutritionally-linked cancers of the breast, colon, prostate and pancreas can be inhibited by tea. The formation of genotoxic carcinogens for these target organs during the cooking of meats, heterocyclic amines, and their effects were decreased by tea. Tea inhibited the formation of reactive oxygen species and radicals and induced cytochromes P450 1A1, 1A2 and 2B1, and glucuronosyl transferase. The higher formation of glucuronides represents an important mechanism in detoxification. The developmental aspects and growth of cancers through promotion are decreased by tea. The regular use of a widely available, tasty, inexpensive beverage, tea, has displayed valuable preventive properties in chronic human diseases.

Lung Carcinogenesis by Diesel Exhaust Particles and the Carcinogenic Mechanism Via Active Oxygens

In an experiment to clarify the involvement of oxygen radicals in lung carcinogenesis induced by diesel exhaust particles (DEP), we found that there is a strong relation between lung tumor response and formation of 8-hydroxydeoxyguanosine (8-OHdG) in lung DNA of mice administered DEP by repeated intratracheal instillation. Repeated intratracheal instillation of DEP also induced the activity of cytochrome P-450 reductase in the lungs as a representative enzyme of superoxide generation, and two types of nitric oxide (NO) synthase, cNOS and iNOS, in the lungs. On the other hand, activities of CuZn-superoxide dismutase (SOD) and Mn-SOD antioxidant enzymes were depressed by the instillation of DEP. These results suggest that generation of superoxide, hydroxyI radical, and nitric oxide are increased in epithelial cells in airways, and that the increased superoxide and nitric oxide react very easily to produce peroxynitrite (ONOO−). The peroxynitrite also produce hydroxyI radical. The hydroxyl radical may play an important role in carcinogenesis by DEP.

LUNG CARCINOGENESIS BY DIESEL EXHAUST PARTICLES AND THE CARCINOGENIC MECHANISM VIA ACTIVE OXYGENS

In an experiment to clarify the involvement of oxygen radicals in lung carcinogenesis induced by diesel exhaust particles (DEP), we found that there is a strong relation between lung tumor response and formation of 8-hydroxydeoxyguanosine (8-OHdG) in lung DNA of mice administered DEP by repeated intratracheal instillation. Repeated intratracheal instillation of DEP also induced the activity of cytochrome P-450 reductase in the lungs as a representative enzyme of superoxide generation, and two types of nitric oxide (NO) synthase, cNOS and iNOS, in the lungs. On the other hand, activities of CuZn-superoxide dismutase (SOD) and Mn-SOD antioxidant enzymes were depressed by the instillation of DEP.These results suggest that generation of superoxide, hydroxyl radical, and nitric oxide are increased in epithelial cells in airways, and that the increased superoxide and nitric oxide react very easily to produce peroxynitrite (ONOO) . The peroxynitrite also produce hydroxyl radical. The hydroxyl radical may play an important role in carcinogenesis by DEP.

Effects of dietary 1,4-phenylenebis(methylene)selenocyanate on 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced DNA adduct formation in lung and liver of A/J mice and F344 rats.

1,4-Phenylenebis(methylene)selenocyanate (p-XSC) was tested for its ability to inhibit DNA adduct formation induced by the tobacco-specific N-nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) in the liver and lung of A/J mice and F344 rats. Dietary p-XSC, providing a dose of 5 p.p.m. selenium, significantly inhibited the formation of 7-methylguanine (7-mGua) induced by a single i.p. injection of 10 mumol of NNK(12.8% inhibition at 4 h and 19.9% at 96 h) and O6-methylguanine (O6-mGua) (16.5% at 4 h and 34.8% at 96 h) in the liver of A/J mice. Dietary supplements of p-XSC providing 15 p.p.m. of selenium reduced the levels of 7-mGua by 17.3% (4 h) and 33.6% (96 h). The formation of O6-mGua was inhibited by 69.5% (4th) and 73.8 (96h). In A/J mouse lung DNA the most significant reduction was observed in levels of O6-mGua. Dietary p-XSC at 5 p.p.m. as selenium inhibited the formation of this adduct by 73.1% (4 h). Ninety-six hours after NNK injection, and at both time points with p-XSC providing 15 p.p.m. selenium, O6-mGua was not detected. Although levels of 7- mGua in mouse lung DNA were also reduced, this was significant only 4 h after carcinogen administration. In general, selenite at a5 p.p.m. as selenium had no significant effect on the levels of these lesions; however, it inhibited O6-mGua in the liver only 4 h after NNK administration. These effects may explain why there is chemopreventive activity for p-XSC, but not for selenite, in NNK-induced lung carcinogenesis in A/J mice. Moreover, these findings raised our interest in determining the potential chemopreventive activity of p-XSC against NNK-induced lung adenocarcinomas in male F344 rats by first determining its effects on NNK-induced DNA methylation in the lungs of rats. Diet supplemented with 10 p.p.m. selenium as p-XSC did indeed inhibit the formation of adducts in pulmonary DNA of F344 rats treated with four consecutive injections of 81 mg/kg of NNK. Statistically significant inhibition of O6-mGua formation was observed 4 h after carcinogen treatment in both pulmonary (49.1% inhibition) and hepatic (39.8%) DNA. Statistically significant inhibition of 7-mGua formation was also measured in lung DNA isolated 24 h after the last NNK injection (45.0%) and in liver DNA 4 h after carcinogen treatment (31.8%). These results suggest that p-XSC would also inhibit induction of lung adenocarcinoma in male F344 rats by NNK.

Detection of 1,N6-ethenodeoxyadenosine and 3,N4-ethenodeoxycytidine by immunoaffinity/32P-postlabelling in liver and lung DNA of mice treated with ethyl carbamate (urethane) or its metabolites.

The capacity of the chemical carcinogen ethyl carbamate (EC, urethane) and its metabolites vinyl carbamate (VC) and vinyl carbamate epoxide (VCO) to form ethenobases was studied in liver and lung DNA of 12-day-old and adult CD-1, B6C3F1, C3H/HeJ and C57BL/6J mice. Following single and multiple doses of EC, VC or VCO, the formation of 1,N6-ethenodeoxyadenosine (epsilon dA) and 3,N4-ethenodeoxycytidine (epsilon dC) was quantified by an immunoaffinity chromatography/32P-postlabelling technique. Both etheno adducts were detected in untreated control DNA samples from liver and lung in the range of 2-15 adducts/10(9) parent nucleotides. Following five repeated injections of 250 or 280 nmol/g body wt VC to adult mice, 51, 57 and 78 epsilon dA/10(9) dA and 28, 42 and 42 epsilon dC/10(9) dC (means of duplicate analyses) were detected in liver DNA of CD-1, C3H/HeJ and C57BL/6J mice respectively. In lung DNA of these VC-treated mice, the levels were 87, 49 and 58 (epsilon dA/10(9) dA) and 64, 39 and 43 (epsilon dC/10(9) dC) respectively. Under similar dose regimens, lower levels of etheno adducts were detected in B6C3F1 mice. Etheno-DNA adducts were also formed in liver and lung upon treatment with EC in adult mice, but at 3-fold lower levels as compared with VC. In 12-day-old C3H/HeJ and C57BL/6J mice, 2- to 3-fold higher etheno adduct levels were detected in liver DNA, when compared with adults, upon a single treatment with 250 nmol/g body wt VC, suggesting that young animals are more susceptible to adduct formation. Combined analysis of adduct formation in adult CD-1, C3H/HeJ and C57BL/6J mice at the higher dose showed a statistically significant increase in etheno adduct formation in the order EC > VC. The results demonstrate that EC and its activated intermediates bind to liver and lung DNA to form epsilon dA and epsilon dC, and the differences in DNA binding further support the hypothesis that metabolic activation of EC to VC is involved. Preliminary data also suggest that background levels of epsilon dA and epsilon dC in DNA are affected by the type of diet given to the animals.

Formation of an oxidative DNA damage, 8-hydroxydeoxyguanosine, in mouse lung DNA after intratracheal instillation of diesel exhaust particles and effects of high dietary fat and beta-carotene on this process.

Diesel exhaust particles (DEP) cause tumors in the respiratory tracts of experimental animals. It was previously shown that DEP produced superoxide and hydroxyl radical. To examine whether oxygen radicals are involved in mouse lung tumorigenesis induced by DEP, formation of an oxidative DNA damage, 8-hydroxydeoxyguanosine (oh8dG), by DEP was investigated. Furthermore, the role of high dietary fat and beta-carotene on this process was studied. After intratracheal instillation of DEP, a significant increase of oh8dG in mouse lung DNA was observed. High dietary fat enhanced the formation of oh8dG in lung DNA. Intake of beta-carotene suppressed the formation of oh8dG in lung DNA, but the protective effect of beta-carotene against this process was not statistically significant. These results suggest that formation of oh8dG in lung DNA was induced by oxygen free radicals produced by DEP. Thus, it is possible that oh8dG is a promutagenic lesion in DEP-induced lung tumorigenesis in mice and high dietary fat enhances this process through the generation of oh8dG in mouse lung DNA.

DNA adducts of the ubiquitous environmental contaminant cyclopenta[cd]pyrene.

The bioactivation of cyclopenta[cd]pyrene (CPP) was investigated to determine the major DNA adduct-forming metabolite(s) of this widespread environmental contaminant and suspect carcinogen. DNA adducts were analyzed by 32P-postlabeling. Four major and at least seven minor adducts formed when CPP was incubated with calf thymus DNA in the presence of rat liver microsomal systems. P450 subfamilies IA and IIB both activated CPP as microsomes from either phenobarbital- or beta-naphthoflavone-treated rats produced quantitatively similar and qualitatively identical adducts. When the epoxide hydrolase inhibitors, 1,1,1-trichloropropene-2,3-oxide or cyclohexene oxide were added to the incubations, binding increased 2.5- to 4-fold, suggesting epoxidation as a mechanism of adduct formation in vitro. Sprague-Dawley rats were killed 1, 3, 7, 18, 45 and 80 days postdosing i.p. with 50 mg/kg CPP. In all tissues analyzed, four major and several minor qualitatively identical adducts were produced. Binding was highest and most persistent in lung followed by heart, white blood cells (WBCs) and liver. CPP adducts were detectable at doses from 1 microgram/kg to 50 mg/kg. Rat lung DNA adducts were cochromatographed with standardized deoxyguanosine and deoxyadenosine adducts produced by reaction of CPP-3,4-epoxide in vitro. All rat lung adducts comigrated with the deoxyguanosine adducts but one was clearly deoxyadenosine derived. Mouse skin DNA adducts from NIH Swiss mice and mouse lung DNA adducts from B6C3F1 mice were also analyzed. All adducts from either mouse tissue comigrated with rat lung DNA adducts, suggesting CPP-3,4-epoxide was also the major DNA adduct-forming species in the mouse. CPP-3,4-epoxide has been suggested to be the key mediator of the biological activities of CPP. Evidence presented here strongly suggests CPP-3,4-epoxide as the major adduct-forming species of CPP as catalyzed in vitro by rat liver preparations known to mediate the mutagenic activation of CPP, in the rat in vivo, and in mouse skin and lung, two tissues with known sensitivity to CPP tumorigenicity.

Inhibition of tobacco-specific nitrosamine-induced lung tumorigenesis in A/J mice by green tea and its major polyphenol as antioxidants : Xu Y, Ho C-T, Amin SG, Han C, Chung F-L. American Health Foundation, Valhalla, NY 10595. Cancer Res 1992;52:3875–3879

In this study we examined the effects of green tea and its major components, (-)-epigallocatechin gallate (EGCG) and caffeine, on the tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung tumorigenesis in A/J mice. We also studied the effects of green tea and EGCG on O6-methylguanine and 8-hydroxydeoxyguanosine (8-OH-dGuo) formation in lung tissues caused by NNK treatment. Mice were given 2% tea, 560 ppm EGCG, or 1120 ppm caffeine in drinking water for 13 weeks. During this time, NNK (11.65 mg/kg body weight) was administered by gavage three times weekly for 10 weeks from weeks 3 to 12. The bioassay was terminated 6 weeks after the last NNK treatment. Mice treated with NNK developed 22.5 lung adenomas per mouse, whereas NNK-treated mice that drank green tea or EGCG as drinking water developed only 12.2 (P less than 0.01) and 16.1 (P less than 0.05) tumors per mouse, respectively. Mice that drank green tea or caffeine solution showed lower body weight gains, although little difference in water and diet consumption was noted in these groups. While green tea and EGCG exerted little effect on the formation of O6-methylguanine, a critical DNA lesion in NNK lung tumorigenesis, both treatments suppressed the increase of 8-OH-dGuo levels in mouse lung DNA. The inhibition of 8-OH-dGuo formation in lung DNA by green tea and EGCG is consistent with their ability to inhibit lung tumorigenesis by NNK. Because 8-OH-dGuo is a DNA lesion caused by oxidative damage, these results suggest that the mechanism of inhibition by green tea and EGCG in NNK-induced lung tumorigenesis is due at least partly to their antioxidant properties.

Damage and repair of mouse lung DNA induced by 1-nitropyrene

The induction of DNA single-strand breaks (SSB), the repair of SSB, and the role of cell turnover in the removal of SSB were determined in mouse lung following intratracheal instillation of 1-nitropyrene (1-NP). Cellular DNA was prelabeled with [ 3H]thymidine in neonate and adult mice. 1-NP was administered to labeled neonate mice after they became adults and to the adult mice that were labeled when adults. 1-NP induced a dose-related increase in SSB (10–22 times control rate) as early as 2 h after 1-NP administration. The mice labeled as neonates were more sensitive to SSB induction by 1-NP and had a faster repair rate than the mice labeled when adults. By one week after 1-NP administration, the levels of SSB in both groups of mice were similar to controls. The half-lives for DNA turnover in mice prelabeled as neonates and in mice prelabeled when adults were ∼22 and ∼9 days, respectively, at the time of 1-NP treatments. These data indicate that both highly proliferative cell populations and populations with lower rates of proliferation are amenable to 1-NP-induced DNA lesions. The rate of cell DNA turnover suggests that active DNA repair processes are involved in the removal of DNA lesions. The slower rate of DNA repair coupled with a relatively high rate of cell division in the rapid proliferative cells suggest that these cells may be involved in the induction of cancer in animals after 1-NP administration.

Binding of nitropyrenes and benzo[ a]pyrene to mouse lung deoxyribonucleic acid after pretreatment with inducing agents

In assessing the biological effects of exposure to a complex chemical mixture, it is important to determine how the behavior of one compound may be influenced by the presence of other compounds in the mixture. In this study the effect of pre-exposure to an organic extract of diesel exhaust or to selected compounds in diesel exhaust on the binding of diesel exhaust compounds to DNA was determined. The amount of radiolabel covalently bound to mouse lung DNA following intratracheal administration of radiolabeled benzo[ a]pyrene (BaP), 1-nitropyrene, 1,3,6-trinitropyrene, or a mixture of dinitropyrene was determined following pretreatment with benzo[ a]pyrene, 1-nitropyrene, and diesel exhaust extract. Male CD-1 mice, 15–18 weeks of age, received 10 mg/kg radiolabeled putative inducing agents by intratracheal instillation and, after 24 hr, 0.03 to 1.2 mg/kg radiolabeled putative DNA binding agents. Lung DNA was extracted, and covalent binding was quantitated by liquid scintillation spectroscopy. 1-Nitropyrene was a potent lung DNA binding agent in the absence of inducing agents [Covalent Binding Index (CBI) = 970] and was extremely potent after benzo[a]pyrene pretreatment (CBI = 21,540, comparable to the CBI for aflatoxin B 1). Similar results were obtained for DNA binding of dinitropyrene and trinitropyrene with and without BaP pretreatment. DNA binding of BaP was lower (CBI = 40) and less inducible (BaP-pretreatment CBI = 230). Pretreatment with diesel extract caused an elevation in the binding of benzo[ a]pyrene but little or no elevation in the binding of the nitropyrenes. Pretreatment with 1-nitropyrene did not increase significantly DNA binding of any of the agents tested. These results indicate that nitropyrenes bind readily to lung DNA and this binding may be increased in the presence of respirable mixtures, especially those containing inducing agents such as BaP.

Effect of aryl hydrocarbon hydroxylase induction on the in vivo covalent binding of 1-nitropyrene, benzo[ a]pyrene, 2-aminoanthracene, and phenanthridone to mouse lung deoxyribonucleic acid

The effect of aryl hydrocarbon hydroxylase induction on the covalent binding of 1-nitropyrene (1-NP), benzo[ a]pyrene (BaP), 2-aminoanthracene (2-AA), and phenanthridone (PNDO) to mouse lung DNA was investigated. Cytochrome P-450-dependent monooxygenases were induced in mouse lung by intratracheal instillation of BaP, Aroclor-1254, or coal gas condensate (CGC) 24 hr before instillation of [ 3H]BaP, [ 3H]-2-AA, [ 14C]-1-NP, or [ 14C]PNDO. All inducing agents increased the amount of radioactivity of [ 3H]BaP, [ 3H]-2-AA, and [ 14C]-1-NP or metabolites bound to DNA. However, pretreatment with BaP resulted in the highest amounts of radiolabels covalently bound to DNA. At 4 hr after instillation of radiolabels in BaP-induced mice, the amounts of [ 3H]BaP, [ 3H]-2-AA, and [ 14C]-1-NP bound to DNA were increased 5.4−, 5.2−, and 160-fold above that of control levels; the amount of 1-NP bound to DNA was fifty times higher than the amount bound by BaP. Labeled compounds were still bound to DNA 1 week after administration. [ 14C]PNDO was not bound to DNA in uninduced or induced mice. Based on the amount of labeled compounds bound to DNA, pretreatment of mice with BaP and CGC induced enzymes with similar specificities; however, enzymes induced by Aroclor were less effective in the metabolism of labeled compounds to DNA-bound products. These data show that specific cytochrome P-450-dependent monooxygenases are inducible in mouse lung and suggest that pre-exposure to inducing agents may be important in the potential toxicity to proximal tissues in direct contact with inhaled xenobiotics.

Effect of aluminum chloride on binding of 4-hydroxyamino-quinoline 1-oxide to nucleic acids.

Effect of aluminium chloride on the binding of carcinogenic 4-hydroxyaminoquinoline 1-oxide (4-HAQO) with mouse lung DNA, RNA, and various homopolyribonucleotides was examined in vitro, in the presence of seryl-AMP. Mouse lung DNA, RNA, or homopolyribonucleotide [poly(A), poly(G), poly(I), poly(X), poly(C), or poly(U)] was pretreated with aluminium chloride in an ice bath and the binding with 4-HAQO was examined. Binding with DNA, RNA, poly(A), and poly(G) was markedly inhibited, and their binding rates were 46%, 56%, 53%, and 18% of that of the control, respectively. Binding with poly(C) and poly(U) was hardly different from that of the control. Consequently, effect of aluminium chloride in inhibiting the binding of 4-HAQO with mouse lung DNA and RNA is assumed to be due to the inhibition of its binding with guanine. Effect of various metals (Mg2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, and Fe3+) on the binding of 4-HAQO with mouse lung DNA was examined and it was found that aluminium chloride had the strongest inhibitory effect, followed by copper and zinc. Trivalent iron showed hardly any inhibition.