32P-postlabeling analysis of 1-nitropyrene-DNA adducts in female Sprague-Dawley rats

Genotoxic evaluation of an industrial effluent from an oil refinery using plant and animal bioassays

Polycyclic aromatic hydrocarbons (PAHs) are genotoxic chemicals commonly found in effluents from oil refineries. Bioassays using plants and cells cultures can be employed for assessing environmental safety and potential genotoxicity. In this study, the genotoxic potential of an oil refinery effluent was analyzed by means of micronucleus (MN) testing of Alium cepa, which revealed no effect after 24 h of treatment. On the other hand, primary lesions in the DNA of rat (Rattus norvegicus) hepatoma cells (HTC) were observed through comet assaying after only 2 h of exposure. On considering the capacity to detect DNA damage of a different nature and of these cells to metabolize xenobiotics, we suggest the association of the two bioassays with these cell types, plant (Allium cepa) and mammal (HTC) cells, for more accurately assessing genotoxicity in environmental samples.

Modulations of benzo[a]pyrene-induced DNA adduct, cyclin D1 and PCNA in oral tissue by 1,4-phenylenebis(methylene)selenocyanate

Tobacco smoking is an important cause of human oral squamous cell carcinoma (SCC). Tobacco smoke contains multiple carcinogens include polycyclic aromatic hydrocarbons typified by benzo[a]pyrene (B[a]P). Surgery is the conventional treatment approach for SCC, but it remains imperfect. However, chemoprevention is a plausible strategy and we had previously demonstrated that 1,4-phenylenebis(methylene)selenocyanate (p-XSC) significantly inhibited tongue tumors-induced by the synthetic 4-nitroquinoline-N-oxide (not present in tobacco smoke). In this study, we demonstrated that p-XSC is capable of inhibiting B[a]P-DNA adduct formation, cell proliferation, cyclin D1 expression in human oral cells in vitro. In addition, we showed that dietary p-XSC inhibits B[a]P-DNA adduct formation, cell proliferation and cyclin D1 protein expression in the mouse tongue in vivo. The results of this study are encouraging to further evaluate the chemopreventive efficacy of p-XSC initially against B[a]P-induced tongue tumors in mice and ultimately in the clinic.

Use of the single cell gel electrophoresis/comet assay for detecting DNA damage in aquatic (marine and freshwater) animals

The comet assay is a rapid, sensitive and inexpensive method for measuring DNA strand breaks. The comet assay has advantages over other DNA damage methods, such as sister chromatid exchange, alkali elution and micronucleus assay, because of its high sensitivity and that DNA strand breaks are determined in individual cells. This review describes a number of studies that used the comet assay to determine DNA strand breaks in aquatic animals exposed to genotoxicants both in vitro and in vivo, including assessment of DNA damage in aquatic animals collected from contaminated sites. One difficulty of using the comet assay in environmental work is that of comparing results from studies that used different methods, such as empirical scoring or comet tail lengths. There seems to be a consensus in more recent studies to use both the intensity of the tail and the length of the tail, i.e. DNA tail moment, percentage of DNA in the tail. The comet assay has been used to assess DNA repair and apoptosis in aquatic animals and modifications of the comet assay have allowed the detection of specific DNA lesions. There have been some recent studies to link DNA strand breaks in aquatic animals to effects on the immune system, reproduction, growth, and population dynamics. Further work is required before the comet assay can be used as a standard bio-indicator in aquatic environments, including standardization of methods (such as ASTM method E2186-02a) and measurements.

Biological fate of [14C]-1-nitropyrene in rats following intragastric administration

1-Nitropyrene (1-NP), a weak carcinogen associated with diesel exhaust particles, has previously been detected in workplace atmospheres with in-use diesel engines and in the general environment. In order to gain insight in its biological fate, a single dose of [14C]-1-NP (27.6 microCi, 750 mg/kg body weight, b.w.) was administered intragastrically to rats and the presence of metabolites in blood and tissue homogenates, and radioactivity associated with blood proteins and tissue DNA, were studied. Early peak levels of radioactivity observed in blood and tissue homogenates indicated a rapid absorption of [14C]-1-NP from the gastrointestinal tract. Metabolite patterns observed in plasma, liver and kidney homogenates strongly suggested an important role of the intestinal microflora in the enterohepatic recirculation, but not in nitroreduction of 1-NP prior to absorption from the gastrointestinal tract. This might explain the low levels of radioactivity associated with blood proteins, since 1-nitrosopyrene, a product of nitroreduction of 1-NP, is likely to be involved in protein binding. Levels of radioactivity associated with plasma proteins were approximately four times higher than the levels of radioactivity associated with hemoglobin (401.0 and 84.1 pmol/g protein per micromol 1-NP kg b.w., respectively, at 24 h). Maximal 25% of the associated radioactivity was released following mild alkaline hydrolysis of either hemoglobin or plasma proteins. 1-Aminopyrene was the only released compound after hydrolysis of hemoglobin. In addition to 1-aminopyrene, two more polar unidentified metabolites were detected following hydrolysis of plasma proteins. Association of radioactivity with DNA was highest in the liver at the first moments of observation (7.4 pmol 14C Eq./mg DNA per micromol 1-NP kg b.w.), but decreased rapidly to levels lower than observed for kidney DNA (max. 3.0 pmol 14C Eq./mg DNA per micromol 1-NP kg b.w. at 24 h). In lungs 8-50 times less radioactivity was associated with DNA than observed in the liver and kidneys. The results of this study show, that 1-NP undergoes an extensive and complex biotransformation in vivo, resulting in a variety of metabolites present in blood and tissue homogenates and a diversity of blood protein adducts. Concentrations of plasma metabolites, blood protein adducts and DNA adducts were rather low. In addition, previous studies also showed relatively low concentrations of metabolites present in urine. Therefore, sensitive and selective methods will be needed in order to evaluate the biological fate of 1-NP, associated with diesel exhaust particles, in humans.

Detection of DNA strand breaks in isolated mussel (Mytilus edulis L. ) digestive gland cells using the "Comet" assay

Isolated mussel (Mytilus edulis L.) digestive gland cells were analyzed using the single-cell gel electrophoresis or "comet" assay to assess the ability of potential aquatic contaminants to induce DNA strand breaks (SBs) and to investigate the potential application of this technique as part of an aquatic biomonitoring regime. Freshly prepared cell suspensions from digestive gland were exposed in vitro to hydrogen peroxide (H2O2, 0-200 microM), 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX, 0-200 microM), benzo[a]pyrene (BaP, 0-200 microM), 1-nitropyrene (1-NP, 0-250 microM) and nitrofurantoin (NF, 0-1000 microM) for 1 h in the dark at 15 degreesC in the presence of the DNA repair inhibitor cytosine-beta-D-arabinofuranoside (araC). DNA strand breakage was measured using the comet assay. There were significant concentration-dependent increases in the percentage of DNA in the comet tail (mean values+/-SD) for all doses compared with controls (P<0.05) with H2O2 (up to 61.4+/-5.1% at 100 microM), MX (up to 34. 3+/-2.2% at 200 microM), BaP (up to 24.7+/-5.1 at 100 microM), 1-NP (up to 54.7+/-5.0% at 200 microM), and NF (up to 68.1+/-4.5% at 500 microM). There was a decrease (P<0.05) in viability (eosin Y exclusion) of exposed compared with control cells at 200 microM H2O2 and BaP only. This study has demonstrated the potential of the comet assay to detect DNA strand breakage at subcytotoxic concentrations of a range of agents, some of which require metabolic activation. This may provide a sensitive, but nonspecific, molecular biomarker of genotoxicity.

Biological effect markers for exposure to carcinogenic compound and their relevance for risk assessment

In this review data are summarized on biomarkers that are used for biological effect monitoring of human populations exposed to genotoxic carcinogens. The biomarkers are DNA and protein adducts and cytogenetic effects. Most of these biomarkers are relevant for the process of carcinogenesis. Emphasis is on providing information on the properties of the biomarkers and on their relevance for predicting cancer risk. Overviews are presented of: (1) studies on effects of exposure in target tissues of human origin obtained by surgical biopsies or autopsies, (2) epidemiological studies on healthy (cancer-free) individuals, correlating the putative occupational, lifestyle or environmental exposure with increased levels of biomarkers in blood cells, and (3) studies with animal models on the relation between biomarkers and cancer. Finally, on the basis of epidemiological data the possibilities were explored to use biomarker data to estimate the risk of death due to cancer. For several biomarkers the increment of the cancer mortality risk was calculated on the basis of a lifetime doubling of the biomarker level.

Comparative metabolism and DNA binding of 1-, 2-, and 4-nitropyrene in rats

The metabolism and DNA binding studies of mono-NP isomers under identical conditions were conducted, as an initial investigation, in order to provide an understanding for the higher carcinogenic activity of 4-NP in the rat mammary gland. Urinary and fecal excretion patterns of 4-NP and 1-NP 24 h following administration to female CD rats (i.p.; 24 mg/kg body weight; 1.55 mCi/rat) were similar but higher than those of 2-NP. The identified metabolites were formed via nitroreduction and ring oxidation pathways. Neither the excretion patterns nor the nature of the metabolites readily explained why the mammary tumorigenic activity of these three isomers varied. Although overall levels of mono-NP bound to liver DNA did not account for the observed differences in the biological activity, further HPLC analysis of the liver DNA hydrolysates showed that only 4-NP had yielded putative multiple DNA adducts; none were detected in the case of 1-NP and 2-NP. 1-, 2-, and 4-NP were found to bind to mammary DNA at levels of 0.6, 0.3, and 2.1 pmol/mg DNA, respectively. The structure of DNA adducts in the mammary gland and in the liver of female CD rats following the i.p. administration of 4-NP has not been identified. Collectively, the results of this preliminary study indicate that the difference in levels of DNA binding in the mammary gland in vivo may reflect why 4-NP has higher carcinogenic activity.

Role of O-acetyltransferase in activation of oxidised metabolites of the genotoxic environmental pollutant 1-nitropyrene

The genotoxic environmental contaminant 1-nitropyrene is metabolised in mammalian systems by pathways more complex than the straightforward nitroreduction which accounts for most of its biological activity in bacteria. In order to evaluate the role of O-acetyltransferase (OAT) activity in generation of genotoxic intermediates from 1-nitropyrene, the mutagenicity of the major primary oxidised metabolites of 1-nitropyrene was characterised in the Ames Salmonella typhimurium plate incorporation assay with strain TA98, and with variants of TA98 deficient (TA98/1,8-DNP6) or enhanced (YG1024) in O-acetyltransferase. 1-Nitropyren-3-ol was more mutagenic in the absence than in the presence of S9, while 1-nitropyren-4-ol, 1-nitropyren-6-ol and 1-nitropyren-8-ol required S9 for maximum expression of mutagenicity. 1-Nitropyren-4-ol (176 rev/nmol without S9, 467 rev/nmol with S9 in TA98) and 1-nitropyren-6-ol (13 rev/nmol without S9, 266 rev/nmol with S9 in TA98) were overall the most potent nitropyrenol isomers assayed. 1-Acetamidopyren-8-ol and 1-acetamidopyrene 4,5-quinone were only minimally active. 1-Acetamidopyren-3-ol exhibited direct-acting mutagenicity. 1-Acetamidopyren-6-ol, previously shown to be a major contributor to mutagenicity in the urines of rats dosed with 1-nitropyrene (Ball et al., 1984b), was confirmed as a potent (359 rev/nmol) S9-dependent mutagen. Both the direct-acting and the S9-dependent mutagenicity of all the compounds studied was enhanced in the OAT-overproducing strain and much diminished (though not always entirely lost) in the OAT-deficient strain, showing that OAT amplifies expression of the genotoxicity of these compounds. 1-Acetamidopyren-6-ol required both S9 and OAT activity in order to exhibit any mutagenicity; this finding strongly implicates N-hydroxylation followed by O-esterification, as opposed to further S9-catalyzed ring oxidation, as a major route of activation for urinary metabolites of 1-nitropyrene.

Modulatory effects of polycyclic aromatic hydrocarbons on the mutagenicity of 1-nitropyrene: a structure-activity relationship study

Benzo[a]pyrene (B[a]P) is able to inhibit the mutagenicity of 1-nitropyrene (1-NP) through the reduction of nitroreductase activity and formation of adducts with DNA. The relationships between the chemical structure of 9 polycyclic aromatic hydrocarbons (PAHs) and antagonistic effects on the 1-NP-induced mutation were evaluated by the binary mixtures of 1-NP and PAHs with Salmonella typhimurium TA98 in the absence of S9 mix. Remarkably different antagonistic effects of 9 PAHs on the mutagenicity of 1-NP were observed. Among the tested PAHs, coronene demonstrates the most antagonistic potential followed by benzo[g,h,i]perylene (B[g,h,i]P), benzo[e]pyrene (B[e]P), dibenzo[a,h]pyrene (DB[a,h]P), benzo[a]pyrene (B[a]P) and pyrene. Naphthalene, anthracene, and chrysene had only minor inhibitory activity on the 1-NP mutagenicity. The modifying effects of PAHs on the nitroreductase activity of TA98 strains in the presence of 1-NP were further examined from the production of 1-AP. The statistical analytical data showed that the inhibitory effect of PAHs on the mutagenicity of 1-NP significantly correlated with their effects on the nitroreductase activity (r = -0.69, p < 0.05). In addition, the formation of 1-NP-DNA adducts of the binary mixtures of 1-NP and PAH was determined by the 32P-postlabeling method. The results indicated that the modulatory effects of PAHs on the formation of 1-NP-DNA adducts were correlated well with their antagonistic activity (r = -0.91, P < 0.01). From the above results, the relationships between the chemical structure of PAHs and the antagonistic effects on the 1-NP mutagenicity were revealed by the surface area and electronic parameters of PAHs. The planar molecular area of PAHs was more convincingly correlated with the antagonistic effect on the mutagenicity of 1-NP (r = -0.81, p < 0.01) than that with the difference in energy, delta E, between EHOMO and ELUMO (r = 0.69, p < 0.05). According to the above, two possible mechanisms are involved in the interactive effect of the binary mixtures: (1) a higher binding affinity with nitroreductase for PAHs having a large planar surface area; and (2) a high energy of interaction between 1-NP and PAHs with a low delta E might decrease the nitroreductive capability.

Emodin inhibits the mutagenicity and DNA adducts induced by 1-nitropyrene

Polygonum cuspidatum S. (PC) is frequently used as a laxative and an anticancer drug in Chinese medicine. The inhibitory effect of this herb and its component, emodin, on the direct-acting mutagenicity of 1-nitropyrene (1-NP) was examined using the Ames/microsomal test with Salmonella typhimurium TA98 and the genotoxicity of 1-NP was evaluated using the SOS chromotest with E. coli PQ37. Emodin and water extracts of PC markedly decreased the mutagenicity of 1-NP in a dose-dependent manner in both assay systems. Furthermore, emodin and the extracts of PC significantly inhibited the formation of 1-NP DNA adducts in S. typhimurium TA98 in the 32P-postlabeling study. The results suggest that PC extracts and emodin act as blocking and/or suppressing agents to reduce the direct-acting mutagenicity of 1-NP.

Human cell mutagenicity of mono- and dinitropyrenes in metabolically competent MCL-5 cells

Nitropyrenes are ubiquitous environmental pollutants that may pose a human health hazard because some are highly potent mutagens and carcinogens. The mutagenicity (trifluorothymidine resistance at the thymidine kinase locus) of 1-, 2-, and 4-nitropyrene (1-, 2-, and 4-NP), 1,3-, 1,6-, and 1,8-dinitropyrene (1,3-, 1,6-, and 1,8-DNP), and pyrene was assessed in a quantitative forward mutation assay using a metabolically competent line (MCL-5) of human B-lymphoblastoid cells. These cells contain endogenous cytochrome P450 activity (CYP1A1) and two plasmids that express cDNAs for four additional P450s (CYP1A2, CYP2A6, CYP2E1, CYP3A4) and microsomal epoxide hydrolase found in human liver. The major finding is that 2-NP and 1,3-DNP, both potent bacterial mutagens, were nonmutagenic in this assay. The following mutagenic potency series, expressed as the minimum detectable mutagen concentration (MDMC) in nmol/ml, was obtained: 1,6-DNP (0.8), 1,8-DNP (1.5), 4-NP (3.1), 1-NP (9.1), 2-NP (> 81), 1,3-DNP (> 86), pyrene (> 494). There was over an 11-fold difference between the most potent (1.6-DNP) and the least potent (1-NP) mutagen. 1,6-DNP was approximately twice as mutagenic as 1,8-DNP, which was almost twice as mutagenic as 4-NP, which, in turn was nearly three times as potent as 1-NP. This is the first report on the testing of 2-NP and 4-NP for mutagenicity in mammalian cell cultures. The human cell mutagenicity of these compounds was discussed in terms of potency series of nitropyrenes obtained from animal carcinogenicity experiments and other mammalian cell mutagenicity assays.

Prediction and monitoring of the carcinogenicity of polycyclic aromatic compounds (PACs)

Chemical carcinogenesis is a multistage process that includes initiation, promotion, and progression. Some carcinogenic PACs have been shown to activate proto-oncogenes and deactivate tumor-suppression genes in the carcinogenic process. The function of DNA repair processes appears to be changed in some cases by PACs. Many PACs are well known for their carcinogenic activity, but for this activity to be exerted, metabolic activation by microsomal enzymes must occur. The enzyme system responsible for PAC activation is the mixed-function oxidase system and, in particular, cytochrome P-450. In the case of PAHs, oxidation predominantly produces reactive diol-epoxides that can then be converted to carbonium ions as the reactive electrophiles that can then covalently bind to DNA. Regions of high activity exist in PAHs, namely, the "bay," "K," and "L" regions which are associated with pi electron distribution. The diol-epoxides can exist in either syn or anti forms, each of which has two enantiomers producing four stereoisomers in all. Energy considerations favor the formation of the anti form. Nitrogen-containing PACs can be metabolically activated in a manner similar to that for PAHs, or the nitrogen atom can be oxidized to form hydroxylamines. These reactive electrophiles can then form covalently bound DNA adducts. The monitoring of DNA adducts has been used in risk assessment for human exposure to PACs. This form of biomonitoring has advantages over the monitoring of external exposure or body levels of the chemicals in question. In the case of PACs, binding to DNA is an important step in the multistage carcinogenic process. The estimation of DNA adducts has been used in the monitoring of humans exposed to PAHs in a wide range of industrial situations. Recent research has shown a dose-response relationship between PAH adduct levels and human cancer, thus developing molecular epidemiology as a relevant science for the field of risk assessment. Techniques have been developed for the determination of DNA adducts and these include immunochemical, fluorescence spectroscopic, GC-MS, and 32P-postlabeling methods. The 32P-postlabeling assay is by far the most sensitive, with limits of detection being of the order of one adduct in 10(10) normal nucleotides. The use of HPLC for separation of adducted nucleotides in this postlabeling assay is becoming more common and gives better resolution of adducts than does the TLC technique used in the traditional assay. The detection of adducts on hemoglobin and other proteins has been used as a surrogate for DNA adduct estimation.(ABSTRACT TRUNCATED AT 400 WORDS)

Bacterial mutagenicity, metabolism, and DNA adduct formation by binary mixtures of benzo[a]pyrene and 1-nitropyrene

Air pollutants are a complex mixture containing polycyclic organic compounds. Among these are 1-NP and B[a]P, which are important contributors to the mutagenicity of diesel exhaust and airborne particulate matters. To investigate the interaction of a complex mixture of airborne mutagens, the mutagenicity of 1-NP was examined with S. typhimurium TA98 and TA98NR in the presence and absence of B[aP. B[a]P exhibited a more antagonistic effect on the mutagenicity of 1-NP in strain TA98 than in strain TA98NR. Also studied were (1) the inhibitory effects of B[a]P on the nitroreductive metabolism of 1-NP and (2) DNA adduct formation by 1-NP. Nitroreductase was associated with the metabolism of 1-NP, and was reduced in a dose-dependent manner in a binary mixture of 1-NP and B[a]P. HPLC analysis showed that the amounts of 1-AP and NAAP, the metabolites of 1-NP, were significantly decreased by the addition of B[a]P in mixtures. The results indicate that the antagonistic effect of B[a]P on the mutagenicity of 1-NP is mediated through altering its nitroreductive metabolism.

Metabolic activation of 1-nitropyrene to a mammalian cell mutagen and a carcinogen

1. The mutagenicity of 1-nitropyrene metabolites in Chinese hamster ovary (CHO) cells, in the absence of rat liver S9, decreased in the order 6-hydroxy-1-nitropyrene > 1-nitropyrene 9,10-oxide > 1-nitropyrene 4,5-oxide approximately 3-hydroxy-1-nitropyrene approximately 8-hydroxy-1-nitropyrene > 1-nitropyrene. The order of mutagenicity with rat liver S9 was 1-nitropyrene 4,5-oxide approximately 6-hydroxy-1-nitropyrene approximately 1-nitropyrene 9,10-oxide > 3-hydroxy-1-nitropyrene approximately 1-nitropyrene > 8-hydroxy-1-nitropyrene. 2. 1-Nitropyrene 4,5-oxide reacted with calf thymus DNA to give one or several closely related adducts. The same adducts were detected in CHO cells incubated with 1-nitropyrene 4,5-oxide. Inclusion of a nitroreductase, xanthine oxidase, in the incubations with calf thymus DNA resulted in the formation of an additional adduct identified as N-(deoxyguanosin-8-yl)-1-aminopyrene (dG-C8-AP). 3. 1-Nitropyrene 9,10-oxide reacted with calf thymus DNA to give an adduct pattern similar to that observed with 1-nitropyrene 4,5-oxide. Incubation of 1-nitropyrene 9,10-oxide with CHO cells resulted in the formation of the same adducts along with dG-C8-AP. 4. dG-C8-AP and N-(deoxyguanosin-8-yl)-1-amino-x-nitropyrene (x = 3, 6 or 8; dG-C8-ANP) were detected in injection site DNA from Sprague-Dawley rats treated with 1-nitropyrene. In mammary gland DNA, dG-C8-AP and an unidentified adduct were found. dG-C8-ANP was the only DNA adduct detected in the livers of newborn CD-1 mice and the lungs of A/J mice dosed with 1-nitropyrene.

32P-postlabeling of N-7, N2 and O6 2'-deoxyguanosine 3'-monophosphate adducts of styrene oxide

Adducts were prepared by reacting styrene oxide with 2-deoxyguanosine 3'-monophosphate (dGMP). Four isomeric N-7-, two diastereomeric N2- and three isomeric O6-adduct were isolated and characterized. The adducts were used as substrates in the 32P-postlabeling reaction. No phosphorylation products were seen with the N-7-alkylation products. One diastereomeric N2-adduct was labeled with 20% efficiency and the second with a markedly lower efficiency. Two of the three O6-adducts were labeled with 5% and the third with 10% labeling efficiency. The results suggest that large N-7-dGMP adducts are very poor substrates of T4 polynucleotide kinase. The diastereomeric products are labeled at different efficiencies indicating stereoselectivity in the kinase reaction.

Synthesis and stability of 2'-deoxyguanosine 3'-monophosphate adducts of dimethyl sulfate, ethylene oxide and styrene oxide

Deoxyguanosine 3'-monophosphate (dGMP) was alkylated at the 7-position by dimethyl sulfate, ethylene oxide and styrene oxide in aqueous media and glacial acetic acid, respectively, to yield reasonable quantities of the products, which were purified by HPLC. dGMP adducts are needed as standards for the 32P-postlabelling assay. The stability of the adducts was studied at 37 degrees and neutral pH. The half-lives of disappearance of 7-methyl-dGMP and the beta-isomers of the styrene oxide adducts were about 250 min; 7-hydroxy-ethyl-dGMP and the alpha-isomers of the styrene oxide adducts had respective half-lives of 340 and 440 min. In all cases the main degradation product was the corresponding guanine adduct. The results demonstrate considerable lability of the 7-alkylation products of dGMP which has to be taken into consideration in devising the 32P-postlabelling assay.