S-(3-Aminobenzanthron-2-yl)cysteine in the globin of rats as a novel type of adduct and possible biomarker of exposure to 3-nitrobenzanthrone, a potent environmental carcinogen.

3-Nitrobenzanthrone (3-NBA), a potent environmental mutagen and carcinogen, is known to be activated in vivo to 3-benzanthronylnitrenium ion which forms both NH and C2-bound adducts with DNA and also reacts with glutathione giving rise to urinary 3-aminobenzanthron-2-ylmercapturic acid. In this study, acid hydrolysate of globin from rats dosed intraperitoneally with 3-NBA was analysed by HPLC/MS to identify a novel type of cysteine adduct, 3-aminobenzanthron-2-ylcysteine (3-ABA-Cys), confirmed using a synthesised standard. The 3-ABA-Cys levels in globin peaked after single 3-NBA doses of 1 and 2 mg/kg on day 2 to attain 0.25 and 0.49 nmol/g globin, respectively, thereafter declining slowly to 70-80% of their maximum values during 15 days. After dosing rats for three consecutive days with 1 mg 3-NBA/kg a significant cumulation of 3-ABA-Cys in globin was observed. 3-ABA-Cys was also found in the plasma hydrolysate. Herein, after dosing with 1 and 2 mg 3-NBA/kg the adduct levels peaked on day 1 at 0.15 and 0.51 nmol/ml plasma, respectively, thereafter declining rapidly to undetectable levels on day 15. In addition, sulphinamide adducts were also found in the exposed rats, measured indirectly as 3-aminobenzanthrone (3-ABA) split off from globin by mild acid hydrolysis. Levels of both types of adducts in the globin samples parallelled very well with 3-ABA/3-ABA-Cys ratio being around 1:8. In conclusion, 3-ABA-Cys is the first example of arylnitrenium-cysteine adduct in globin representing a new promising class of biomarkers to assess cumulative exposures to aromatic amines, nitroaromatics and heteroaromatic amines.

Xenobiotic metabolism in differentiated human bronchial epithelial cells.

Differentiated human bronchial epithelial cells in air liquid interface cultures (ALI-PBEC) represent a promising alternative for inhalation studies with rodents as these 3D airway epithelial tissue cultures recapitulate the human airway in multiple aspects, including morphology, cell type composition, gene expression and xenobiotic metabolism. We performed a detailed longitudinal gene expression analysis during the differentiation of submerged primary human bronchial epithelial cells into ALI-PBEC to assess the reproducibility and inter-individual variability of changes in transcriptional activity during this process. We generated ALI-PBEC cultures from four donors and focussed our analysis on the expression levels of 362 genes involved in biotransformation, which are of primary importance for toxicological studies. Expression of various of these genes (e.g., GSTA1, ADH1C, ALDH1A1, CYP2B6, CYP2F1, CYP4B1, CYP4X1 and CYP4Z1) was elevated following the mucociliary differentiation of airway epithelial cells into a pseudo-stratified epithelial layer. Although a substantial number of genes were differentially expressed between donors, the differences in fold changes were generally small. Metabolic activity measurements applying a variety of different cytochrome p450 substrates indicated that epithelial cultures at the early stages of differentiation are incapable of biotransformation. In contrast, mature ALI-PBEC cultures were proficient in the metabolic conversion of a variety of substrates albeit with considerable variation between donors. In summary, our data indicate a distinct increase in biotransformation capacity during differentiation of PBECs at the air-liquid interface and that the generation of biotransformation competent ALI-PBEC cultures is a reproducible process with little variability between cultures derived from four different donors.

Numerical evaluation of bioaccumulation and depuration kinetics of PAHs in Mytilus galloprovincialis.

Polycyclic aromatic hydrocarbons (PAHs) are important organic pollutants in the aquatic environment due to their persistence and bioaccumulation potential both in organisms and in sediments. Benzo(a)anthracene (BaA) and phenanthrene (PHE), which are in the priority pollutant list of the U.S. EPA (Environmental Protection Agency), are selected as model compounds of the present study. Bioaccumulation and depuration experiments with local Mediterranean mussel species, Mytilus galloprovincialis were used as the basis of the study. Mussels were selected as bioindicator organisms due to their broad geographic distribution, immobility and low enzyme activity. Bioaccumulation and depuration kinetics of selected PAHs in Mytilus galloprovincialis were described using first order kinetic equations in a three compartment model. The compartments were defined as: (1) biota (mussel), (2) surrounding environment (seawater), and (3) algae (Phaeodactylum tricornutum) as food source of the mussels. Experimental study had been performed for three different concentrations. Middle concentration of the experimental data was used as the model input in order to represent other high and low concentrations of selected PAHs. Correlations of the experiment and model data revealed that they are in good agreement. Accumulation and depuration trend of PAHs in mussels regarding also the durations can be estimated effectively with the present study. Thus, this study can be evaluated as a supportive tool for risk assessment in addition to monitoring studies.

Bioaccessibility of nitro- and oxy-PAHs in fuel soot assessed by an in vitro digestive model with absorptive sink.

Ingestion of soot present in soil or other environmental particles is expected to be an important route of exposure to nitro and oxygenated derivatives of polycyclic aromatic hydrocarbons (PAHs). We measured the apparent bioaccessibility (Bapp) of native concentrations of 1-nitropyrene (1N-PYR), 9-fluorenone (9FLO), anthracene-9,10-dione (ATQ), benzo[a]anthracene-7,12-dione (BaAQ), and benzanthrone (BZO) in a composite fuel soot sample using a previously-developed in vitro human gastrointestinal model that includes silicone sheet as a third-phase absorptive sink. Along with Bapp, we determined the 24-h sheet-digestive fluid partition coefficient (Ks,24h), the soot residue-fluid distribution ratio of the labile sorbed fraction after digestion (Kr,lab), and the maximum possible (limiting) bioaccessibility, Blim. The Bapp of PAH derivatives was positively affected by the presence of the sheet due to mass-action removal of the sorbed compounds. In all cases Bapp increased with imposition of fed conditions. The enhancement of Bapp under fed conditions is due to increasingly favorable mass transfer of target compounds from soot to fluid (increasing bile acid concentration, or adding food lipids) or transfer from fluid to sheet (by raising small intestinal pH). Food lipids may also enhance Bapp by mobilizing contaminants from nonlabile to labile states of the soot. Compared to the parent PAH, the derivatives had larger Kr,lab, despite having lower partition coefficients to various hydrophobic reference phases including silicone sheet. The Blim of the derivatives under the default conditions of the model ranged from 65.5% to 34.4%, in the order, 1N-PYR > ATQ > 9FLO > BZO > BaAQ, with no significant correlation with hydrophobic parameters, nor consistent relationship with Blim of the parent PAH. Consistent with earlier experiments on a wider range of PAHs, the results suggest that a major determinant of bioaccessibility is the distribution of chemical between nonlabile and labile states in the original solid.

Activation and detoxification metabolism of urban air pollutants 2-nitrobenzanthrone and carcinogenic 3-nitrobenzanthrone by rat and mouse hepatic microsomes.

OBJECTIVES: 2-Nitrobenzanthrone (2-NBA) has recently been detected in ambient air particulate matter. Its isomer 3-nitrobenzanthrone (3-NBA) is a potent mutagen and suspected human carcinogen identified in diesel exhaust. Understanding which enzymes are involved in metabolism of these toxicants is important in the assessment of individual susceptibility. Here, metabolism of 2-NBA and 3-NBA by rat and mouse hepatic microsomes containing cytochromes P450 (CYPs), their reductase (NADPH:CYP reductase), and NADH:cytochrome b5 reductase was investigated under anaerobic and aerobic conditions. In addition, using the same microsomal systems, 2-NBA and 3-NBA were evaluated to be enzymatically activated under anaerobic conditions to species generating 2-NBA- and 3-NBA-derived DNA adducts. METHODS: High performance liquid chromatography (HPLC) with ultraviolet (UV) detection was employed for the separation and characterization of 2-NBA and 3-NBA metabolites formed by hepatic microsomes of rats and mice under the anaerobic and aerobic conditions. Microsomal systems isolated from the liver of the control (untreated) rats and rats pretreated with Sudan I, ß-naphthoflavone (ß-NF), phenobarbital (PB), ethanol and pregnenolon 16α-carbonitrile (PCN), the inducers of cytochromes P450 (CYP) 1A1, 1A1/2, 2B, 2E1 and 3A, respectively, were used in this study. Microsomes of mouse models, a control mouse line (wild-type, WT) and Hepatic Cytochrome P450 Reductase Null (HRN) mice with deleted gene of NADPH:CYP reductase in the liver, thus absenting this enzyme in their livers, were also employed. To detect and quantify the 2-NBA- and 3-NBA-derived DNA adducts, the 32P postlabeling technique was used. RESULTS: Both reductive metabolite of 3-NBA, 3-aminobenzanthrone (3-ABA), found to be formed predominantly under the anaerobic conditions, and two 3-NBA oxidative metabolites, whose structures have not yet been investigated, were formed by several microsomal systems used in the study. Whereas a 3-NBA reductive metabolite, 3-ABA, was found only in the microsomal systems of control rats, the rats treated with ß-NF and PB, and microsomes of WT and HRN mice, all hepatic microsomes tested in the study were capable of activating this carcinogen under the reductive conditions to form DNA adducts. A stability of a reactive intermediate of 3-NBA, N-hydroxy-3-aminobenzanthrone that is formed during 3-NBA reduction to 3-ABA, to form nitrenium (and/or carbenium) ions binding to DNA in individual microsomes as well as binding of these ions to proteins of these microsomes, might be the reasons explaining this phenomenon. In contrast to 3-NBA, its isomer 2-NBA was not metabolized by any of the used enzymatic systems both under the anaerobic and aerobic conditions. Likewise, no DNA adducts were detectable after reaction of 2-NBA in these systems with DNA. CONCLUSIONS: The results found in this study, the first report on the metabolism of 2-NBA and 3-NBA by rat and mouse hepatic microsomes demonstrate that 3-NBA, in contrast to 2-NBA, is reductively activated to form 3-NBA-derived DNA adducts by these enzymatic systems. NADPH:CYP reductase can be responsible for formation of these DNA adducts in rat livers, while NADH:cytochrome b5 reductase can contribute to this process in livers of HRN mice.

Carcinogenic 3-nitrobenzanthrone but not 2-nitrobenzanthrone is metabolised to an unusual mercapturic acid in rats.

3-Nitrobenzanthrone (3-NBA) is an extremely potent mutagen and suspect human carcinogen found in diesel exhaust. Its isomer 2-nitrobenzanthrone (2-NBA) has also been found in ambient air. These isomers differ in mutagenicity in Salmonella by 2-3 orders of magnitude. To identify their urinary metabolites and also to assess the assumed differences in their excretion, rats were dosed orally with 2mg/kg b.w. of either 2-NBA or 3-NBA. Their urine was collected for two consecutive days after dosage. Both LC-ESI-MS and GC-MS confirmed formation of the corresponding aminobenzanthrones (ABA). Excretion of these metabolites within the first day after dosing with 2- and 3-ABA amounted to 0.32±0.06 and 0.83±0.40% of the doses, respectively, while the excretion within the second day was by one order of magnitude lower. A novel mercapturic acid metabolite of 3-NBA was identified in urine by LC-ESI-MS as N-acetyl-S-(3-aminobenzanthron-2-yl)cysteine (3-ABA-MA) by comparison with the authentic standard. Its excretion amounted to 0.49±0.15 and 0.02±0.01% of dose within the first and second day after dosing, respectively. In contrast, no mercapturic acid was detected in the urine of rats dosed with 2-NBA. Observed difference in the mercapturic acid formation between 2- and 3-NBA is a new distinctive feature reflecting differences in the critical step of their metabolism, i.e., benzanthronylnitrenium ion formation that is intrinsically associated with biological activities of these two isomers. Moreover, 3-ABA-MA is a promising candidate biomarker of exposure to the carcinogenic 3-NBA.

Tissue-specific metabolic activation and mutagenicity of 3-nitrobenzanthrone in MutaMouse.

3-Nitrobenzanthrone (3-NBA) is a mutagen and suspected human carcinogen detected in diesel exhaust, airborne particulate matter, and urban soil. We investigated the tissue specific mutagenicity of 3-NBA at the lacZ locus of transgenic MutaMouse following acute single dose or 28-day repeated-dose oral administration. In the acute high dose (50 mg/kg) exposure, increased lacZ mutant frequency was observed in bone marrow and colonic epithelium, but not in liver and bladder. In the repeated-dose study, a dose-dependent increase in lacZ mutant frequency was observed in bone marrow and liver (2- and 4-fold increase above control), but not in lung or intestinal epithelium. In addition, a concentration-dependent increase in mutant frequency (8.5-fold above control) was observed for MutaMouse FE1 lung epithelial cells exposed in vitro. 1-Nitropyrene reductase, 3-NBA reductase, and acetyltransferase activities were measured in a variety of MutaMouse specimens in an effort to link metabolic activation and mutagenicity. High 3-NBA nitroreductase activities were observed in lung, liver, colon and bladder, and detectable N-acetyltransferase activities were found in all tissues except bone marrow. The relatively high 3-NBA nitroreductase activity in MutaMouse tissues, as compared with those in Salmonella TA98 and TA100, suggests that 3-NBA is readily reduced and activated in vivo. High 3-NBA nitroreductase levels in liver and colon are consistent with the elevated lacZ mutant frequency values, and previously noted inductions of hepatic DNA adducts. Despite an absence of induced lacZ mutations, the highest 3-NBA reductase activity was detected in lung. Further studies are warranted, especially following inhalation or intratracheal exposures.

Hepatic and immune biological effect assays in C57BL/6 mice to measure polycyclic aromatic hydrocarbon bioavailability under laboratory exposures with increasing environmental relevance.

BACKGROUND: Monitoring biological responses that are mediated via the aryl-hydrocarbon receptor (AhR) in animals exposed to environmental contaminants can indicate both the presence of chemicals that act through this biochemical pathway and whether these chemicals are bioavailable. OBJECTIVES: The use of an ex-situ method that incorporated biological responsiveness monitoring in mice for determining the presence of 'biologically active' hydrocarbons in contaminated soils was investigated. METHODS: The use of C57BL/6 as a test organism was validated by determining hepatic and immune responsiveness to two polyaromatic hydrocarbons (PAHs): 3,4 benz[a]pyrene (B[a]P) and 1,2 benz (a)anthracene (BA) administered via intraperitoneal (i.p.) injection. The responsiveness of mice exposed to soils spiked with hydrocarbons or ex situ exposures to soil removed from two contaminated sites was also investigated. RESULTS AND DISCUSSION: Mice that were exposed to B[a]P via i.p. injections showed a 14-fold increase in liver microsomal ethoxyresorufin O-deethylase (EROD) activity compared to the control group. In contrast EROD activity following BA exposure at the same level was not significantly enhanced. Mouse immune response was significantly inhibited in a dose-dependent manner by i.p. injections of B[a]P. No significant inhibition occurred with the same doses of BA. Following i.p. exposure, the retention of B[a]P in mouse carcasses was greater than BA. Mice exposed to clean soils spiked with environmentally relevant concentrations of B[a]P and BA failed to show any significantly different hepatic or immune responses. Carcass residue data indicated a limited uptake of PAH from the soil. In contrast, EROD activity in mice exposed (ex situ) to hydrocarbon-contaminated soils removed from a fuel-loading depot and decommissioned gas works was significantly enhanced (4- and 2-fold respectively). However, this increase in EROD activity did not appear to correlate with either soil or carcass PAH concentrations. CONCLUSIONS AND OUTLOOK: These results support the assumption that B[a]P has a higher affinity for the aryl hydrocarbon receptor (AhR) compared to BA. Soil parameters such as organic carbon content, structure and particle size distribution can modulate the bioavailability of contaminants to biological receptors. These factors are implicated in the lack of responsiveness demonstrated in the spiked soil experiments. However the responsiveness of EROD activity in mice exposed (ex situ) to soil contaminated with complex mixtures of hydrocarbon compounds confirms the potential usefulness of this model to determine the presence of 'biologically active' compounds in aged soils removed from contaminated sites.

DNA adduct and tumor formations in rats after intratracheal administration of the urban air pollutant 3-nitrobenzanthrone.

3-Nitrobenzanthrone (3-NBA) has been isolated from diesel exhaust and airborne particles and identified as a potent direct-acting mutagen in vitro and genotoxic agent in vivo. In order to evaluate the in vivo toxicity and carcinogenicity of 3-NBA in a situation corresponding to inhalation, a combined short-term and lifetime study with intratracheal (i.t.) instillation in female F344 rats was performed. DNA adduct formation, as a marker for the primary effect and analyzed by 32P-HPLC after single instillation, showed a few major DNA adducts and a rapid increase with a peak after 2 days, followed by a decline. No DNA adducts above the background level were observed after 16 days. The highest DNA adduct formation was observed in lung [approximately 250 DNA adducts/10(8) normal nucleotides (NN)] closely followed by kidney (approximately 200 DNA adducts/10(8) NN), whereas liver contained only 12% (approximately 30 DNA adducts/10(8) NN) of the levels of DNA adducts found in lung. In the tumor study, squamous cell carcinomas were found after 7-9 months in the high-dose group (total dose of 2.5 mg 3-NBA) and after 10-12 months in the low-dose group (total dose of 1.5 mg 3-NBA). The fraction of squamous cell carcinoma out of the total amount of tumors observed at the end of experiment at 18 months, corresponded to 3/16 and 11/16 in the low- and high-dose group, respectively. A single case of adenocarcinoma was also observed in each group. In the control group, no tumors were observed during the entire study of 18 months. In addition, a few cases of squamous metaplasia were also observed in the lung in both dose groups but not in the controls. In conclusion, 3-NBA forms DNA adducts in the lung immediately after i.t. administration but almost all DNA adducts were eliminated after 16 days. Tumor formation in two dose groups was observed in a dose-dependent manner with squamous cell carcinomas as the predominant tumor type at high exposure.

Environmental pollutant and potent mutagen 3-nitrobenzanthrone forms DNA adducts after reduction by NAD(P)H:quinone oxidoreductase and conjugation by acetyltransferases and sulfotransferases in human hepatic cytosols.

3-Nitrobenzanthrone (3-nitro-7H-benz[de]anthracen-7-one, 3-NBA) is a potent mutagen and suspected human carcinogen identified in diesel exhaust and air pollution. We compared the ability of human hepatic cytosolic samples to catalyze DNA adduct formation by 3-NBA. Using the (32)P-postlabeling method, we found that 12/12 hepatic cytosols activated 3-NBA to form multiple DNA adducts similar to those formed in vivo in rodents. By comparing 3-NBA-DNA adduct formation in the presence of cofactors of NAD(P)H:quinone oxidoreductase (NQO1) and xanthine oxidase, most of the reductive activation of 3-NBA in human hepatic cytosols was attributed to NQO1. Inhibition of adduct formation by dicoumarol, an NQO1 inhibitor, supported this finding and was confirmed with human recombinant NQO1. When cofactors of N,O-acetyltransferases (NAT) and sulfotransferases (SULT) were added to cytosolic samples, 3-NBA-DNA adduct formation increased 10- to 35-fold. Using human recombinant NQO1 and NATs or SULTs, we found that mainly NAT2, followed by SULT1A2, NAT1, and, to a lesser extent, SULT1A1 activate 3-NBA. We also evaluated the role of hepatic NADPH:cytochrome P450 oxidoreductase (POR) in the activation of 3-NBA in vivo by treating hepatic POR-null mice and wild-type littermates i.p. with 0.2 or 2 mg/kg body weight of 3-NBA. No difference in DNA binding was found in any tissue examined (liver, lung, kidney, bladder, and colon) between null and wild-type mice, indicating that 3-NBA is predominantly activated by cytosolic nitroreductases rather than microsomal POR. Collectively, these results show the role of human hepatic NQO1 to reduce 3-NBA to species being further activated by NATs and SULTs.

DNA adduct formation by the environmental contaminant 3-nitrobenzanthrone after intratracheal instillation in rats.

3-Nitrobenzanthrone (3-NBA) is an environmental pollutant and suspected human carcinogen found in emissions from diesel and gasoline engines and on the surface of ambient air particulate matter; human exposure to 3-NBA is likely to occur primarily via the respiratory tract. In our study female Sprague Dawley rats were treated by intratracheal instillation with a single dose of 0.2 or 2 mg/kg body weight of 3-NBA. Using the butanol enrichment version of the (32)P-postlabeling method, DNA adduct formation by 3-NBA 48 hr after intratracheal administration in different organs (lung, pancreas, kidney, urinary bladder, heart, small intestine and liver) and in blood was investigated. The same adduct pattern consisting of up to 5 DNA adduct spots was detected by thin layer chromatography in all tissues and blood and at both doses. Highest total adduct levels were found in lung and pancreas (350 +/- 139 and 620 +/- 370 adducts per 10(8) nucleotides for the high dose and 39 +/- 18 and 55 +/- 34 adducts per 10(8) nucleotides for the low dose, respectively) followed by kidney, urinary bladder, heart, small intestine and liver. Adduct levels were dose-dependent in all organs (approximately 10-fold difference between doses). It was demonstrated by high performance liquid chromatography (HPLC) that all 5 3-NBA-derived DNA adducts formed in rats after intratracheal instillation are identical to those formed by other routes of application and are, as previously shown, formed from reductive metabolites bound to purine bases. Although total adduct levels in the blood were much lower (41 +/- 27 and 9.5 +/- 1.9 adducts per 10(8) nucleotides for the high and low dose, respectively) than those found in the lung, they were related to dose and to the levels found in lung. These results show that uptake of 3-NBA by the lung induces high levels of specific DNA adducts in several organs of the rat and an identical adduct pattern in DNA from blood. Therefore, 3-NBA-DNA adducts present in the blood are useful biomarkers for exposure to 3-NBA and may help to assess the effective biological dose in humans exposed to it.

DNA adducts and mutagenic specificity of the ubiquitous environmental pollutant 3-nitrobenzanthrone in Muta Mouse.

3-nitrobenzanthrone (3-NBA) is an extremely potent mutagen in the Salmonella reversion assay and a suspected human carcinogen identified in diesel exhaust and in ambient airborne particulate matter. To evaluate the in vivo mutagenicity of 3-NBA, we analyzed the mutant frequency (MF) in the cII gene of various organs (lung, liver, kidney, bladder, colon, spleen, and testis) in lambda/lacZ transgenic mice (Muta Mouse) after intraperitoneal treatment with 3-NBA (25 mg/kg body weight injected once a week for 4 weeks). Increases in MF were found in colon, liver, and bladder, with 7.0-, 4.8-, and 4.1-fold increases above the control value, respectively, whereas no increase in MF was found in lung, kidney, spleen, and testis. Simultaneously, induction of micronuclei in peripheral blood reticulocytes was observed. The sequence alterations in the cII gene recovered from 41 liver mutants from 3-NBA-treated mice were compared with 32 spontaneous mutants from untreated mice. Base substitution mutations predominated for both the 3-NBA-treated (80%) and the untreated (81%) groups. However, the proportion of G:C-->T:A transversions in the mutants from 3-NBA-treated mice was higher (49% vs. 6%) and the proportion of G:C-->A:T transitions was lower than those from untreated mice (10% vs. 66%). The increase in MF in the liver was associated with strong DNA binding by 3-NBA, whereas in lung, in which there was no increase in MF, a low level of DNA binding was observed (268.0-282.7 vs. 8.8-15.9 adducts per 10(8) nucleotides). DNA adduct patterns with multiple adduct spots, qualitatively similar to those formed in vitro after activation of 3-NBA with nitroreductases and in vivo in rats, were observed in all tissues examined. Using high-pressure liquid cochromatographic analysis, we confirmed that all major 3-NBA-DNA adducts produced in vivo in mice are derived from reductive metabolites bound to purine bases (70-80% with deoxyguanosine and 20-30% with deoxyadenosine in liver). These results suggest that G:C-->T:A transversions induced by 3-NBA are caused by misreplication of adducted guanine residues through incorporation of adenine opposite the adduct (A-rule).

Metabolism of the polynuclear sulfur heterocycle benzo[b]phenanthro[2,3-d]thiophene by rodent liver microsomes: evidence for multiple pathways in the bioactivation of benzo[b]phenanthro[2,3-d]thiophene.

Benzo[b]phenanthro[2,3-d]thiophene (BPT), a thia analogue of dibenz[a,h]anthracene (DBA), is a carcinogenic environmental pollutant. We have examined the metabolism of BPT by rodent liver microsomes to investigate the mechanism by which BPT produces mutagenic and carcinogenic effects. Both rat and mouse liver microsomes biotransformed [G-(3)H]BPT to various metabolites including BPT 3,4-diol and BPT sulfoxide, which are significantly more mutagenic than the parent compound. Liver microsomes from both control mice and rats metabolize BPT at similar rates. Treatment of mice with P450 inducers DBA, 3-methylcholanthrene (3-MC), Aroclor 1254, and phenobarbital enhanced the rate of metabolism of BPT by 74-, 28-, 77-, and 6-fold, respectively. In comparison, the treatment of rats with DBA and 3-MC increased the rate of metabolism of BPT by 22- and 34-fold, respectively, suggesting that P450 enzymes responsible for the metabolism of BPT are enhanced to different extents in rats and mice by a similar class of compounds. In general, the liver microsomes from mice treated with DBA or 3-MC were more active than those from similarly treated rats in metabolizing BPT to its 3,4-diol, a precursor to the bay-region diol epoxide of BPT. BPT sulfone was a minor metabolite (if formed) in all cases. The liver microsomes from rats treated with DBA or 3-MC or from mice treated with PB produced a significant proportion of BPT sulfoxide (12-41%). In contrast, the liver microsomes from DBA- or 3-MC-treated mice formed BPT sulfoxide as a minor metabolite (<2%). These studies indicate that cytochrome P450 enzymes induced by PAHs (e.g., P450 1A1 and P450 1B1) and by PB (e.g., P450 2B1, 3A1, and/or 3A2) are involved in the metabolism of BPT to mutagenic BPT 3,4-diol and BPT sulfoxide, providing evidence for the first time that BPT and possibly other thia-PAHs are metabolically activated via the formation of both the dihydrodiol (and subsequently diol epoxide) and the sulfoxide.

Formation of benzo[a]pyrene and 7,12-dimethylbenz[a]anthracene adducts in vascular endothelia of cytochrome P4501A-induced chicken embryos.

Polycyclic aromatic hydrocarbons (PAHs) are widespread in the environment and birds may be exposed to PAHs via diet, from preening feathers contaminated with oil, or through contamination of the eggshell during embryo development. In the present study, tissue distribution and the cell-specific binding of two labeled PAHs, benzo[a]pyrene ([3H]BaP) and 7,12-dimethylbenz[a]anthracene ([3H]DMBA), were examined in chicken embryos exposed in ovo to CYP1A inducers. Tape-section autoradiograms revealed high concentrations of radioactivity in the bile, liver, kidneys, heart, and leptomeninges. Light microscopy autoradiography of solvent-extracted tissue slices showed a high and selective binding in endothelial cells in certain blood vessels in brain, heart, lung, and chest muscle. Binding was also observed in blood vessel endothelial cells in the chorioallantoic membrane (CAM), an extraembryonal tissue lining the eggshell. Endothelial binding was confirmed in CAM exposed in vitro, implying that tissue-binding metabolites were formed in situ. The CYP1A inhibitor ellipticine abolished bleeding in the target endothelial cells in CAM. It is thus concluded that blood vessel endothelia in various tissues in birds can bioactivate environmental contaminants and be targets for their toxicity. In view of its critical position beneath the shell, the CAM could be an important target for toxicants following external exposure in oviparous species.

Human enzymes involved in the metabolic activation of the environmental contaminant 3-nitrobenzanthrone: evidence for reductive activation by human NADPH:cytochrome p450 reductase.

Determining the capability of humans to metabolize the suspected carcinogen 3-nitrobenzanthrone (3-NBA) and understanding which human enzymes are involved in its activation are important in the assessment of individual susceptibility to this environmental contaminant found in diesel exhaust and ambient air pollution. We compared the ability of eight human hepatic microsomal samples to catalyze DNA adduct formation by 3-NBA. Using two enrichment procedures of the (32)P-postlabeling method, nuclease P1 digestion and butanol extraction, we found that all hepatic microsomes were competent to activate 3-NBA. DNA adduct patterns with multiple adducts, qualitatively similar to those found recently in vivo in rats, were observed. Additionally one major DNA adduct generated by human microsomes was detected. The role of specific cytochromes p450 (p450) and NADPH:p450 reductase in the human hepatic microsomal samples in 3-NBA activation was investigated by correlating the p450- and NADPH: p450 reductase-linked catalytic activities in each microsomal sample with the level of DNA adducts formed by the same microsomes. On the basis of this analysis, most of the hepatic microsomal activation of 3-NBA was attributed to NADPH: p450 reductase. Inhibition of DNA adduct formation in human liver microsomes by alpha-lipoic acid, an inhibitor of NADPH: p450 reductase, supported this finding. Using the purified rabbit enzyme and recombinant human NADPH: p450 reductase expressed in Chinese hamster V79 cells, we confirmed the participation of this enzyme in the formation of 3-NBA-derived DNA adducts. Moreover, essentially the same DNA adduct pattern found in microsomes was detected in metabolically competent human lymphoblastoid MCL-5 cells. The role of individual human recombinant p450s 1A1, 1A2, 1B1, 2A6, 2B6, 2D6, 2C9, 2E1, and 3A4 and of NADPH: p450 reductase in the metabolic activation of 3-NBA, catalyzing DNA adduct formation, was also examined using microsomes of baculovirus-transfected insect cells containing the recombinant enzymes (Supersomes). DNA adducts were observed in all Supersomes preparations, essentially similar to those found with human hepatic microsomes and in human cells. Of all of the recombinant human p450s, p450 2B6 and -2D6 were the most efficient to activate 3-NBA, followed by p450 1A1 and -1A2. These results demonstrate for the first time the potential of human NADPH: p450 reductase and recombinant p450s to contribute to the metabolic activation of 3-NBA by nitroreduction.

The metabolism of formyl-substituted benzanthracenes to hydroxymethyl metabolites in rat liver in vitro and in vivo.

Hydroxylation of benzylic methyl carbon atoms on drugs and carcinogenic polycyclic aromatic hydrocarbons (PAHs) forms benzylic alcohols. Many carcinogenic and mutagenic PAHs bear a primary or secondary benzylic hydroxyl group attached to the meso-region of the molecule. According to the unified theory, PAHs bearing a benzylic hydroxyl group are proximate carcinogenic metabolites. This paper demonstrates that carcinogenic benz[a]anthracenes bearing a formyl group at the meso-region undergo enzymatic reductive metabolism to the corresponding carcinogenic benzylic alcohol in vitro and in vivo. The unified theory would then predict sulfuric acid esterification of such benzylic alcohols as the final common step in their metabolic activation to generate ultimate electrophilic benzylic carbocations. Finally, oxidative metabolism of 7-formylbenz[a]anthracenes gives rise to corresponding carboxylic acids and other oxygenated metabolites that are carcinogenically inert. Thus, oxidative metabolism of meso-region formyl compounds represents an avenue for the elimination of the carcinogen in a detoxified form.

DNA adduct formation by the ubiquitous environmental pollutant 3-nitrobenzanthrone and its metabolites in rats.

Diesel exhaust is known to induce tumours in animals and is suspected of being carcinogenic in humans. Of the compounds found in diesel exhaust, 3-nitrobenzanthrone (3-NBA) is an extremely potent mutagen and suspected human carcinogen forming multiple DNA adducts in vitro. 3-Aminobenzanthrone (3-ABA), 3-acetylaminobenzanthrone (3-Ac-ABA), and N-acetyl-N-hydroxy-3-aminobenzanthrone (N-Ac-N-OH-ABA) were identified as 3-NBA metabolites. In order to gain insight into the pathways of metabolic activation leading to 3-NBA-derived DNA adducts we treated Wistar rats intraperitoneally with 2mg/kg body weight of 3-NBA, 3-ABA, 3-Ac-ABA, or N-Ac-N-OH-ABA and compared DNA adducts present in different organs. With each compound either four or five DNA adduct spots were detected by TLC in all tissues examined (lung, liver, kidney, heart, pancreas, and colon) using the nuclease P1 or butanol enrichment version of the 32P-postlabelling method, respectively. Using HPLC co-chromatographic analysis we showed that all major 3-NBA-DNA adducts produced in vivo in rats are derived from reductive metabolites bound to purine bases and lack an N-acetyl group. Our results indicate that 3-NBA metabolites (3-ABA, 3-Ac-ABA and N-Ac-N-OH-ABA) undergo several biotransformations and that N-hydroxy-3-aminobenzanthrone (N-OH-ABA) appears to be the common intermediate in 3-NBA-derived DNA adduct formation. Therefore, 3-NBA-DNA adducts are useful biomarkers for exposure to 3-NBA and its metabolites and may help to identify enzymes involved in their metabolic activation.

Metabolic activation of the environmental contaminant 3-nitrobenzanthrone by human acetyltransferases and sulfotransferase.

3-Nitrobenzanthrone (3-NBA) an extremely potent mutagen and suspected human carcinogen identified in diesel exhaust and in airborne particulate matter was shown to form multiple DNA adducts in vitro and in vivo in rats. In order to investigate whether human N,O-acetyltransferases (NATs) and sulfotransferases (SULTs) contribute to the metabolic activation of 3-NBA we used a panel of newly constructed Chinese hamster lung fibroblast V79MZ derived cell lines expressing human NAT1, human NAT2 or human SULT1A1, as well as TA1538-derived Salmonella typhimurium strains expressing human NAT1 (DJ400) or human NAT2 (DJ460) and determined DNA binding and mutagenicity. The formation of 3-NBA-derived DNA adducts was analysed by (32)P-postlabelling after exposing V79 cells to 0.01 micro M 3-NBA or 0.1 micro M N-acetyl-N-hydroxy-3-aminobenzanthrone (N-Ac-N-OH-ABA), a potential metabolite of 3-NBA. Similarly up to four major and two minor adducts were detectable for both compounds, the major ones being identical to those detected previously in DNA from rats treated with 3-NBA. Comparison of DNA binding between different V79MZ derived cells revealed that human NAT2 and, to a lesser extent, human NAT1 and human SULT1A1, contribute to the genotoxic potential of 3-NBA and N-Ac-N-OH-ABA to form DNA adducts. However, the extent of DNA binding by 3-NBA was higher in almost all V79 cells at a 10-fold lower concentration than by N-Ac-N-OH-ABA, suggesting that N-Ac-N-OH-ABA is not a major intermediate in the formation of 3-NBA-derived adducts. 3-NBA showed a 3.8-fold and 16.8-fold higher mutagenic activity in Salmonella strains expressing human NAT1 and human NAT2, respectively, than in the acetyltransferase-deficient strain, whereas N-Ac-N-OH-ABA was only clearly (but weakly) mutagenic in Salmonella DJ460 expressing human NAT2. This finding suggests that N-Ac-N-OH-ABA is not a major reactive metabolite responsible for the high mutagenic potency of 3-NBA in Salmonella. Collectively our results indicate that O-acetylation and O-sulfonation by human NATs and SULTs may contribute significantly to the high mutagenic and genotoxic potential of 3-NBA. Moreover, the yet-unidentified four major 3-NBA-derived adducts may be DNA adducts without an N-acetyl group.

DNA adduct formation by the ubiquitous environmental contaminant 3-nitrobenzanthrone in rats determined by (32)P-postlabeling.

Diesel exhaust is known to induce tumors in animals and is suspected of being carcinogenic in humans. Of the compounds found in diesel exhaust and in airborne particulate matter, 3-nitrobenzanthrone (3-NBA), is a particularly powerful mutagen. We investigated the capacity of 3-NBA to form DNA adducts in vivo that could be used as agent-specific biomarkers of exposure. Female Sprague-Dawley rats were treated orally with 2 mg/kg body weight of 3-NBA, and DNA from various organs was analyzed by (32)P-postlabeling. High levels of 3-NBA-specific adducts were detectable in all organs. Both enrichment versions nuclease P1 digestion and n-butanol extraction resulted in patterns consisting of either 3 or 4 adducts remarkably similar in all tissues examined. The highest level of DNA adducts was found in the small intestine (38 adducts per 10(8) nucleotides) followed by forestomach, glandular stomach, kidney, liver, lung and bladder. To provide information on the nature of the adducts formed in vivo in rats, DNA adducts were cochromatographed in 2 independent systems with standardized deoxyguanosine adducts and deoxyadenosine adducts produced by reaction of 3-NBA in the presence of xanthine oxidase with deoxyribonucleoside 3'-monophosphates in vitro. In both systems, each of the rat adducts comigrated either with a deoxyguanosine or a deoxyadenosine-derived 3-NBA adduct. Our results demonstrate that 3-NBA binds covalently to DNA after metabolic activation, forming multiple DNA adducts in vivo, all of which are products derived from reductive metabolites bound to the purine bases (deoxyguanosine 60% and deoxyadenosine 40%).

Cytochrome P450-dependent binding of 7,12-dimethylbenz[a]anthracene (DMBA) and benzo[a]pyrene (B[a]P) in murine heart, lung, and liver endothelial cells.

Autoradiography was used to investigate the cellular sites of irreversible binding of 3H-labelled 7,12-dimethylbenz[a]anthracene (DMBA) and benzo[a]pyrene (B[a]P) in mice. Autoradiograms obtained from solvent-extracted tape-sections revealed an even distribution of DMBA- and B[a]P-derived radioactivity in control mice lacking sites of selective binding in the tissues. In mice pretreated with a cytochrome P4501A (CYP1A) inducer, beta-naphthoflavone (BNF) or 3,3',4,4', 5-pentachlorobiphenyl (PCB 126), a noticeable accumulation of bound radioactivity was observed in the pulmonary alveolar region. Increased labelling was also observed in heart tissue of induced mice. As demonstrated by microautoradiography of tissues from CYP1A-induced mice treated with 3H-DMBA or 3H-B[a]P in vivo, irreversible binding in lung tissue was present in endothelial cells of arteries and veins, in the alveolar septal walls, and in type 2 pneumocytes. In heart tissue, binding was confined to endothelial cells of arteries, capillaries and veins. In liver, binding was found in the hepatocytes as well as in endothelial cells of the portal veins, whereas no binding was seen in endothelial cells of the sinusoids, central veins, or arteries. These findings were confirmed in vitro using 3H-DMBA-exposed precision-cut slices, indicating that reactive intermediates of DMBA and B(a)P were formed in situ. The addition of the CYP1A inhibitor ellipticine abolished binding in the target endothelial cells. Increased endothelial binding in the lungs and liver of CYP1A-induced mice was concomitant with increased 7-ethoxyresorufin O-deethylase (EROD) and DMBA hydroxylase activity. In heart, endothelial binding was positively correlated with EROD, but not with DMBA hydroxylase. The results suggest that endothelial cells may be targets for CYP-dependent activation of such toxicants as polycyclic aromatic hydrocarbons. Consequently, the possibility that chemically induced endothelial dysfunction is a risk factor in the aetiology of cardiovascular disease demands consideration.