Formation of DNA adducts and oxidative DNA damage in rats treated with 1,6-dinitropyrene

Nitrated Polycyclic Aromatic Hydrocarbon (Nitro-PAH) Signatures and Somatic Mutations in Diesel Exhaust-Exposed Bladder Tumors

BACKGROUND

Diesel exhaust is a complex mixture, including polycyclic aromatic hydrocarbons (PAH) and nitrated PAHs (nitro-PAH), many of which are potent mutagens and possible bladder carcinogens. To explore the association between diesel exposure and bladder carcinogenesis, we examined the relationship between exposure and somatic mutations and mutational signatures in bladder tumors.

METHODS

Targeted sequencing was conducted in bladder tumors from the New England Bladder Cancer Study. Using data on 797 cases and 1,418 controls, two-stage polytomous logistic regression was used to evaluate etiologic heterogeneity between bladder cancer subtypes and quantitative, lifetime estimates of respirable elemental carbon (REC), a surrogate for diesel exposure. Poisson regression was used to evaluate associations between REC and mutational signatures.

RESULTS

We observed significant heterogeneity in the diesel-bladder cancer risk relationship, with a strong positive association among cases with high-grade, nonmuscle invasive TP53-mutated tumors compared with controls [ORTop Tertile vs.Unexposed, 4.8; 95% confidence interval (CI), 2.2-10.5; Ptrend < 0.001; Pheterogeneity = 0.002]. In muscle-invasive tumors, we observed a positive association between diesel exposure and the nitro-PAH signatures of 1,6-dintropyrene (RR, 1.93; 95% CI, 1.28-2.92) and 3-nitrobenzoic acid (RR, 1.97; 95% CI, 1.33-2.92).

CONCLUSIONS

The relationship between diesel exhaust and bladder cancer was heterogeneous based on the presence of TP53 mutations in tumors, further supporting the link between PAH exposure and TP53 mutations in carcinogenesis. Future studies that can identify nitro-PAH signatures in exposed tumors are warranted to add human data supporting the link between diesel and bladder cancer.

IMPACT

This study provides additional insight into the etiology and possible mechanisms related to diesel exhaust-induced bladder cancer.

Nitroreduction: A Critical Metabolic Pathway for Drugs, Environmental Pollutants, and Explosives

Nitro group containing xenobiotics include drugs, cancer chemotherapeutic agents, carcinogens (e.g., nitroarenes and aristolochic acid) and explosives. The nitro group undergoes a six-electron reduction to form sequentially the nitroso-, N-hydroxylamino- and amino-functional groups. These reactions are catalyzed by nitroreductases which, rather than being enzymes with this sole function, are enzymes hijacked for their propensity to donate electrons to the nitro group either one at a time via a radical mechanism or two at time via the equivalent of a hydride transfer. These enzymes include: NADPH-dependent flavoenzymes (NADPH: P450 oxidoreductase, NAD(P)H-quinone oxidoreductase), P450 enzymes, oxidases (aldehyde oxidase, xanthine oxidase) and aldo-keto reductases. The hydroxylamino group once formed can undergo conjugation reactions with acetate or sulfate catalyzed by N-acetyltransferases or sulfotransferases, respectively, leading to the formation of intermediates containing a good leaving group which in turn can generate a nitrenium or carbenium ion for covalent DNA adduct formation. The intermediates in the reduction sequence are also prone to oxidation and produce reactive oxygen species. As a consequence, many nitro-containing xenobiotics can be genotoxic either by forming stable covalent adducts or by oxidatively damaging DNA. This review will focus on the general chemistry of nitroreduction, the enzymes responsible, the reduction of xenobiotic substrates, the regulation of nitroreductases, the ability of nitrocompounds to form DNA adducts and act as mutagens as well as some future directions.

The Effects of 1‐Nitropyrene on Oxidative DNA Damage and Expression of DNA Repair Enzymes

Nitropyrenes (NPs) present in diesel and gasoline emissions are mutagenic and carcinogenic in experimental animals. Nitro-reduction of 1-NP causes oxidative stress. It is unclear whether 8-hydroxydeoxyguanosine (8-OH-dG) is produced from 1-NP and whether it contributes to the carcinogenic activity of 1-NP. In this study, we measured the level of reactive oxygen species (ROS) in cultured human lung epithelial cells after exposure to 1-NP and the intracellular level of 8-OH-dG and expression level of the 8-OH-dG repair enzymes. As results, 1-NP induced the generation of 8-OH-dG via ROS, but 8-OH-dG repair enzymes prevented an increase of 8-OH-dG formation in cellular DNA of the A549 cell line below 250 microM of 1-NP. These data suggest that 1-NP can induce oxidative DNA damage by generation of ROS, which may play a role in the carcinogenesis induced by 1-NP. These data also suggest that individuals with impaired DNA repair enzymes might be more susceptible to lung cancer induced by 1-NP.

Oxidative DNA Damage Induced by Carcinogenic Dinitropyrenes in the Presence of P450 Reductase

Nitropyrenes are widespread in the environment due to mainly diesel engine emissions. Dinitropyrenes (DNPs), especially 1,8-dinitropyrene (1,8-DNP) and 1,6-dinitropyrene (1,6-DNP), are much more potent mutagens than other nitropyrenes. The carcinogenicity of 1,8-DNP and 1,6-DNP is stronger than 1,3-dinitropyrene (1,3-DNP). It is considered that adduct formation after metabolic activation plays an important role in the expression of carcinogenicity of nitropyrenes. However, Djuric et al. [(1993) Cancer Lett.] reported that oxidative DNA damage was also found as well as adduct formation in rats treated with 1,6-DNP. We investigated oxidative DNA damage by DNPs in the presence of NAD(P)H-cytochrome P450 reductase using 32P-5'-end-labeled DNA. After P450 reductase treatment, DNPs induced Cu(II)-mediated DNA damage in the presence of NAD(P)H. The intensity of DNA damage by 1,8-DNP or 1,6-DNP was stronger than 1,3-DNP. We also examined synthetic 1-nitro-8-nitrosopyrene (1,8-NNOP) and 1-nitro-6-nitrosopyrene (1,6-NNOP) as one of the metabolites of 1,8-DNP and 1,6-DNP, respectively, to find that 1,8-NNOP and 1,6-NNOP induced Cu(II)-mediated DNA damage in the presence of NAD(P)H but untreated DNPs did not. In both cases of P450 reductase-treated DNPs and NNOPs, catalase and a Cu(I) specific chelator attenuated DNA damage, indicating the involvement of H2O2 and Cu(I). Using a Clarke oxygen electrode, oxygen consumption by the reaction of NNOPs with NAD(P)H and Cu(II) was measured to find that NNOP was nonenzymatically reduced by NAD(P)H and that the addition of Cu(II) promoted the redox cycle. Therefore, these results suggest that DNPs are enzymatically reduced to NNOPs via nitro radical anion and that NNOPs are further reduced nonenzymatically by NAD(P)H. Subsequently, autoxidation of nitro radical anion and the reduced form of NNOP occurs, resulting in O2- generation and DNA damage. We conclude that oxidative DNA damage in addition to DNA adduct formation may play important roles in the carcinogenesis of DNPs via their metabolites.

Comparative tumorigenicity of the environmental pollutant 6-nitrochrysene and its metabolites in the rat mammary gland

Human exposure to the class of nitropolynuclear aromatic hydrocarbons is via inhalation and/or ingestion. Therefore, one of the goals of this study was to determine the propensity of the environmental contaminant 6-nitrochrysene (6-NC) for inducing mammary cancer following its oral administration to female CD rats. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), an established mammary carcinogen in the same animal model, was used as a positive control and trioctanoin as a negative control. Thirty-day-old female CD rats were gavaged once weekly for 8 weeks with 6-NC at 50, 25, or 12.5 micromol/rat or PhIP at 50 micromol/rat in 500 microL of trioctanoin. Twenty-three weeks after the last carcinogen administration, rats were decapitated, necropsied, and evaluated histologically. The most common mammary tumors were adenocarcinomas, followed by adenomas and fibroadenomas. The incidence and multiplicity (mean +/- standard deviation) of mammary adenocarcinomas induced by these two carcinogens at the highest dose (6-NC: 90%, 3.73 +/- 2.74; PhIP: 83%, 2.62 +/- 2.58) were significantly higher than those in control rats (10%, 0.10 +/- 0.31). However, there were no statistically significant differences between groups treated with 6-NC and PhIP or among groups receiving various doses of 6-NC. Following its metabolic activation, 6-NC is known to bind covalently to DNA; however, it remains to be determined whether it can also induce DNA base oxidation. Thus, employing the same route of administration, our studies revealed no effect of 6-NC on the basal level of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the mammary gland in tests at 6, 24, and 48 h after 6-NC treatment and at termination of the carcinogenesis assay in the normal, noninvolved tissue and in mammary tumors. This result suggests that covalent DNA binding of 6-NC metabolites is important in the induction of mammary cancer in rats. Therefore, the other goal of this study was to compare the tumorigenic activities of 6-NC and its metabolites in the rat mammary gland by intramammary administration. This route has also been used in our laboratory to induce mammary cancer in the rat by 6-NC and is employed here to avoid systemic effects and to determine the role of the mammary gland in the metabolic activation of 6-NC and its metabolites. Toward this end, a new method was developed to obtain ample materials of trans-1,2-dihydroxy-1,2-dihydro-6-aminochrysene (1,2-DHD-6-AC); other metabolites were synthesized as reported previously. On the basis of the results, the carcinogenic potency toward the mammary gland is ranked in the following order: 6-NC > 1,2-DHD-6-NC > 6-AC > 6-NCDE > 1,2-DHD-6-AC. Among the metabolites tested, 1,2-DHD-6-NC was the most potent carcinogen. It was significantly more active than its reduced product 1,2-DHD-6-AC. However, the potency of 1,2-DHD-6-NC was not significantly different from 6-AC, a metabolite derived from simple nitroreduction, or from 6-NCDE. Collectively, these results suggest that metabolites derived from both ring-oxidation and nitroreduction contribute to the overall carcinogenicity of 6-NC in the rat mammary gland.

DNA adducts from nitroreduction of 2,7-dinitrofluorene, a mammary gland carcinogen, catalyzed by rat liver or mammary gland cytosol

Nitrofluorenes are mutagenic and carcinogenic environmental pollutants arising chiefly from combustion of fossil fuels. Nitro aromatic compounds undergo nitroreduction to N-hydroxy arylamines that bind to DNA directly or after O-esterification. This study analyzes the DNA binding and adducts from the in vitro nitroreduction of 2,7-dinitrofluorene (2,7-diNF), a potent mammary carcinogen in the rat. Potential adduct(s) of 2,7-diNF was (were) generated by reduction of 2-nitroso-7-NF with ascorbate/H(+) in the presence of calf thymus DNA. The major adduct was characterized by HPLC/ESI/MS and (1)H NMR spectrometry as N-(deoxyguanosin-8-yl)-2-amino-7-NF, and a minor one was determined by HPLC/ESI/MS to be a deoxyadenosine adduct of 2-amino-7-NF. Products from enzymatic nitroreduction were monitored by HPLC and DNA adduct formation by (32)P-postlabeling. Xanthine oxidase/hypoxanthine-catalyzed nitroreduction of 2,7-diNF, 2-nitrofluorene (2-NF), and 1-nitropyrene (1-NP) yielded the respective amines to similar extents (30-50%). However, the level of the major adducts ( approximately 0.15/10(6) nucleotides) from 2-NF [N-(deoxyguanosin-8-yl)-2-aminofluorene] and 2,7-diNF [N-(deoxyguanosin-8-yl)-2-amino-7-NF] was < or = 2% that from 1-NP. In the presence of acetyl CoA, nitroreduction of 2-NF catalyzed by rat liver cytosol/NADH yielded the same adduct at a level of 2.2/10(6) nucleotides. Liver or mammary gland cytosol with acetyl CoA yielded mainly N-(deoxyguanosin-8-yl)-2-amino-7-NF from 2,7-diNF at >30 adducts/10(6) nucleotides, levels comparable to those from 1,6-dinitropyrene and 4- or 49-fold greater than the respective levels without acetyl CoA. Recovery of 2-nitroso-7-NF and 2-amino-7-NF from cytosol-catalyzed reduction of 2,7-diNF indicated nitroreduction and an N-hydroxy arylamine intermediate. Likewise, the presence of 2-acetylamino-7-NF indicated that reactivity with acyltransferase(s) was not prevented by the nitro group at C7. These data are consistent with activation of 2,7-diNF via nitroreduction to the N-hydroxy arylamine and acetyl CoA-dependent O-acetylation of the latter to bind to DNA. Enzymatic nitroreduction of 2,7-diNF was greatly enhanced by 9-oxidation. The nitroreduction of either 9-oxo-2,7-diNF or 9-hydroxy-2,7-diNF catalyzed by liver cytosol with acetyl CoA yielded two adducts (>2/10(6) nucleotides). Differences in the TLC migration of these adducts, compared to those from 2,7-diNF, and the lack of 2,7-diNF formation in the incubations suggested retention of the C9-oxidized groups. The relative ratios of the amine to amide from nitroreductions of 9-oxo-2,7-diNF and 2,7-diNF catalyzed by liver cytosol suggested that the 9-oxo group decreased reactivity with acyltransferase and, thus, the amount of N-acetoxy arylamine that binds to DNA. The mammary gland tumorigenicity of 2,7-diNF and the extent of its activation by the tumor target tissue shown herein suggest relevance of this environmental pollutant for breast cancer.

Dinitropyrenes induce gene mutations in multiple organs of the lambda/lacZ transgenic mouse (Muta Mouse)

Dinitropyrenes (DNPs), 1,3-, 1,6- and 1,8-dinitropyrene, are carcinogenic compounds found in diesel engine exhaust. DNPs are strongly mutagenic in the bacterial mutation assay (Ames test), mainly inducing frameshift type mutations. To assess mutagenicity of DNPs in vivo is important in evaluating their possible involvement in diesel exhaust-induced carcinogenesis in human. For this purpose, we used the lambda/lacZ transgenic mouse (Muta Mouse) to examine induction of mutations in multiple organs. A commercially available mixture of DNPs (1,3-, 1,6-, 1,8-, and unidentified isomer (s) with a content of 20.2, 30.4, 35.2, and 14.2%, respectively) was injected intragastrically at 200 and 400mg/kg once each week for 4 weeks. Seven days after the final treatment, liver, lung, colon, stomach, and bone marrow were collected for mutation analysis. The target transgene was recovered by the lambda packaging method and mutation of lacZ gene was analyzed by a positive selection with galE(-) E. coli. In order to determine the sequence alterations by DNPs, the mutagenicity of the lambda cII gene was also examined by the positive selection with hfl(-) E. coli. Since cII gene (294bp) is much smaller than the lacZ (3024bp), it facilitated the sequence analysis. Strongest increases in mutant frequencies (MFs) were observed in colon for both lacZ (7.5x10(-5) to 43.3x10(-5)) and cII (2.7x10(-5) to 22.5x10(-5)) gene. Three-four-fold increases were observed in stomach for both genes. A statistically significant increase in MFs was also evident in liver and lung for the lacZ gene, and in lung and bone marrow for the cII gene. The sequence alterations of the cII gene recovered from 37 mutants in the colon were compared with 50 mutants from untreated mice. Base substitution mutations predominated for both untreated (91%) and DNP-treated (84%) groups. The DNPs treatment increased the incidence of G:C to T:A transversion (2-43%) and decreased G:C to A:T transitions (70-22%). The G:C to T:A transversions, characteristic to DNPs treatment, is probably caused by the guanine-C8 adduct, which is known as a major DNA-adduct induced by DNPs, through an incorporation of adenine opposite the adduct ("A"-rule). The present study showed a relevant use of the cII gene as an additional target for mutagenesis in the Muta Mouse and revealed a mutagenic specificity of DNPs in vivo.

Oxidative DNA damage by a metabolite of carcinogenic 1-nitropyrene

Nitropyrenes are carcinogenic pollutants. Adduct formation following nitro-reduction is considered to be a major cause of nitropyrene-mediated DNA damage. We investigated the role of 1-nitrosopyrene, a metabolite of 1-nitropyrene, in causing oxidative DNA damage, using 32P-5'-end-labeled DNA. 1-Nitrosopyrene was found to facilitate Cu(II)-mediated DNA damage in the presence of NADH. Catalase and a Cu(I)-specific chelator attenuated DNA damage, indicating the involvement of H2O2 and Cu(I). Typical *OH scavenger did not have a significant effect. These results suggest that the main reactive species is probably a DNA-copper-hydroperoxo complex. We also measured 8-oxo-7,8-dihydro-2'-deoxyguanosine formation by 1-nitrosopyrene in the presence of Cu(II) and NADH, using an electrochemical detector coupled to a high-pressure liquid chromatograph. We conclude that oxidative DNA damage, in addition to DNA adduct formation, may play an important role in the carcinogenesis of nitropyrenes.

Inhibition of benzo[a]pyrene- and 1,6-dinitropyrene-DNA adduct formation in human mammary epithelial cells bydibenzoylmethane and sulforaphane

Numerous phytochemicals have been examined for their capacity to act as cancer chemopreventive agents. Dibenzoylmethane, a minor constituent of licorice and a compound structurally-related to curcumin, recently was identified as an effective inhibitor of chemically-induced rat mammary DNA-adduct formation and tumorigenesis (Carcinogenesis 19(1998)1039-1043). The present studies were conducted to examine the capacity of dibenzoylmethane to inhibit the formation of DNA adducts following exposure to benzo[a]pyrene (BP) and 1,6-dinitropyrene (1,6-DNP), and to stimulate the expression of glutathione-S-transferase (GST) and NAD(P)H-quinone reductase (QR) proteins in the human mammary epithelial cell line MCF-10F. In addition, the efficacy of dibenzoylmethane as an enzyme inducer and adduct inhibitor was compared with that of sulforaphane, a potent inducer of phase II detoxification enzymes and inhibitor of chemically-induced rat mammary tumorigenesis. Dibenzoylmethane at concentrations from 0.1 M to 2.0 microM inhibited BP-DNA adduct formation by 63 to 81%. Likewise, sulforaphane inhibited BP-DNA adduct formation by 68 to 80% over the same concentration range. DNA adduct formation following exposure to 1,6-DNP was significantly inhibited by 46 to 61% due to dibenzoylmethane treatment (0.1 to 2.0 microM) and 30 to 56% due to sulforaphane treatment at the same concentrations. The expression of QR and GSTP1-1 proteins were increased by 3 to 4-fold and 3 to 5-fold, respectively, for MCF-10F cells treated with sulforaphane (0.5-2.0 microM). Dibenzoylmethane treatment at the same concentrations did not induce GSTP1-1 expression and significantly stimulated QR expression only at the 2.0 microM concentration. These data indicate that human mammary epithelial MCF-10F cells can convert BP and 1,6-DNP to DNA-binding forms, and that DNA adduct formation can be inhibited by the phytochemicals dibenzoylmethane and sulforaphane. The inhibition of BP-DNA and 1, 6-DNP adduct formation by sulforaphane was associated with increases in QR and GST protein expression. The mechanisms underlying the capacity of dibenzoylmethane to inhibit BP-DNA and 1,6-DNP-DNA adduct formation could not be explained by changes in QR or GST expression and remain to be determined. Together these data suggest that dibenzoylmethane and sulforaphane warrant continued evaluation as breast cancer chemopreventive agents.

The effects of 1-nitropyrene, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 7,12-dimethylbenz[a]anthracene on 8-hydroxy-2'-deoxyguanosine levels in the rat mammary gland and modulation by dietary 1,4-phenylenebis(methylene) selenocyanate

Humans are exposed to 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) and 1-nitropyrene (1-NP) via several environmental sources and both are known mammary carcinogens in rodents, with the former being more potent (K. El-Bayoumy, Y.-H. Chae, P. Upadhyaya, A. Rivenson, K. Kurtzke, B. Reddy, S.S. Hecht, Comparative tumorigenicity of benzo[a]pyrene, 1-nitropyrene, and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine administered by gavage to female CD rats, Carcinogenesis 16 (1995) 431-434). Following their metabolic activation, both carcinogens are known to bind covalently to DNA. However, it remains to be determined whether these carcinogens can also induce DNA-base oxidation. Our goal was to determine the effects of PhIP and 1-NP on the levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG; a marker of oxidative DNA damage) in rat mammary glands and to evaluate the effect of the chemopreventive agent 1,4-phenylenebis(-methylene)selenocyanate (p-XSC) as an inhibitor of such damage. As an established potent mammary carcinogen, the synthetic 7,12-dimethylbenz[a]anthracene (DMBA) was included in this study. Female CD rats were fed a high-fat AIN-76A diet (23.5% corn oil) supplemented with p-XSC (10 ppm as selenium) or unsupplemented control diet for 1 week. At 50 days of age, each rat (12 rats/group) was gavaged with either PhIP (22 mg (100 micromol) per rat) or I-NP (20 mg (80 micromol) per rat) in trioctanoin (0.5 ml), DMBA (5 mg (20 micromol) per rat] in olive oil (0.2 ml), or the corresponding vehicle. Rats were sacrificed 6 and 24 h after carcinogen treatment (six rats per time point). Mammary fat pads were excised and DNA was isolated and enzymatically hydrolyzed. The hydrolysates were analyzed for 8-OHdG using HPLC with EC detection. PhIP significantly increased the levels of 8-OHdG by 83% after 6 h (P < 0.05), but the increase (47%) at the 24 h point was not significant. p-XSC alone had no effect on the levels of 8-OHdG. However, the elevation of 8-OHdG caused by PhIP at 6 h was significantly inhibited by p-XSC to levels similar to those measured in rats treated with the vehicle only (P < 0.05). p-XSC had no effect on PhIP-induced 8-OHdG at 24 h. I -NP had no effect on the levels of 8-OHdG at either time point. Levels of 8-OHdG were increased by 22% 6 h after DMBA administration and, significantly, rose to 84% at 24 h (P < 0.01); at either time point, this elevation was not inhibited by p-XSC. Although the mechanisms remain to be determined, to our knowledge, this is the first report demonstrating that PhIP and DMBA are capable of enhancing 8-OHdG levels in the rat mammary tissue in vivo.

Postlabeling: a sensitive method for studying DNA adducts and their role in carcinogenesis

The covalent binding of xenobiotics to DNA is an important trigger of the multistage process that leads to carcinogenesis. 32P-postlabeling represents a highly sensitive method for biomonitoring exposure to genotoxic agents and for cancer risk assessment; it is capable of detecting less than one DNA adduct per human genome. Recent improvements to the technique have shown that the resistance of adducted DNA to enzyme digestion may lead to an overestimation of the number of different adducts present in a sample.