Differential removal of DNA adducts derived from anti-diol epoxides of dibenzo[a,l]pyrene and benzo[a]pyrene in human cells

The polycyclic aromatic hydrocarbons (PAHs) dibenzo[a,l]pyrene (DBP) and benzo[a]pyrene (BP) are widespread environmental contaminants and potent carcinogens. The fjord-region DBP is considerably more carcinogenic than the bay-region BP. This fact can be ascribed to differences in DNA binding efficiency of their ultimate carcinogenic diol epoxide (DE) intermediates, differences in structural features of the DNA adducts, and differences in DNA adduct recognition and the subsequent lesion removal by nucleotide excision repair (NER). We have compared the formation and removal of adducts as a function of time formed by the carcinogenic metabolites (-)-anti-DBPDE and (+)-anti-BPDE in A549 human epithelial lung carcinoma cells. Cells were exposed to 0.1 or 1.0 microM (-)-anti-DBPDE and (+)-anti-BPDE, respectively. Adducts were measured at various post-treatment times (up to 6 h) by enzymatic DNA hydrolysis and a HPLC procedure that allows monitoring of all cis- and trans-nucleoside adducts of dA and dG. Treatment with 0.1 microM (-)-anti-DBPDE resulted in an initial increase of adducts to a maximal level of 144 pmol adducts/mg of DNA after 1 h of incubation. This was followed by an apparent, although not statistically significant, slow removal of adducts. After 6 h of incubation, at least 80% seems to remain. In cells treated with 1.0 microM (+)-anti-BPDE, the maximal level of 140 pmol adducts/mg of DNA was reached within 20 min of exposure. The formation was followed by an initial rapid decline in the adduct level (1.54 pmol adducts/mg of DNA/min) and a later statistically significant slower rate (0.14 pmol adducts/mg of DNA/min) of adduct removal. After 1 h of incubation, about 45% of the adducts are removed followed by 75% at 6 h. The biphasic pattern of BPDE removal has been observed previously in mammalian cells and, at least in part, may reflect the action of transcription-coupled repair (TCR) and the subsequent global genomic repair (GGR). Comparing the rate of removal of adducts derived from BPDE with those of DBPDE, the latter are obviously more refractory to the NER-coupled repair than the former. Furthermore, the apparent resistance of adducts from DBPDE to be eliminated may reflect the ability of such adducts to escape recognition and/or the subsequent removal by the NER machinery. Further analysis of DNA adduct distribution as a function of incubation time reveals that the dA/dG adduct ratio for BPDE was independent of time (4% dA, 96% dG), whereas the corresponding ratio for DBPDE was significantly increased from 2.9 (74% dA, 26% dG) at 20 min to 4.0 (80% dA, 20% dG) after 6 h of incubation. The results presented here on DNA adduct removal in mammalian cells are in part consistent with recent results on NER-coupled activity on bay- and fjord-region DE-modified oligonucleotides in vitro and further substantiate the hypothesis that the high carcinogenicity of the nonplanar PAHs arise from the ability of the preferentially formed dA adducts to escape recognition by surveillance systems and the subsequent NER-coupled lesion removal.

AN INVESTIGATION OF HUMAN AND RAT LIVER MICROSOMAL MYCOPHENOLIC ACID GLUCURONIDATION: EVIDENCE FOR A PRINCIPAL ROLE OF UGT1A ENZYMES AND SPECIES DIFFERENCES IN UGT1A SPECIFICITY

Mycophenolic acid (MPA; 1,3-dihydro-4-hydroxy-6-methoxy-7-methyl-3-oxo-5-isobenzylfuranyl)-4-methyl-4-hexenoate), the active metabolite of the immunosuppressant prodrug, mycophenolate mofetil, undergoes glucuronidation to its 7-O-glucuronide as a primary route of metabolism. Because differences in glucuronidation may influence the efficacy and/or toxicity of MPA, we investigated the MPA UDP-glucuronosyltransferase (UGT) activities of human liver microsomes (HLMs) and rat liver microsomes with the goal of identifying UGTs responsible for MPA catalysis. HLMs (n = 23) exhibited higher average MPA glucuronidation rates (14.7 versus 6.0 nmol/mg/min, respectively, p < 0.001) and higher apparent affinity for MPA (K(m) = 0.082 mM versus 0.20 mM, p < 0.001) compared with rat liver microsomes. MPA UGT activities were reduced >80% in liver microsomes from Gunn rats. To identify the active enzymes, human and rat UGT1A enzymes were screened for MPA-glucuronidating activity. UGT1A9 was the only human liver-expressed UGT1A enzyme with significant activity and exhibited both high affinity (K(m) = 0.077 mM) and high activity (V(max) = 28 nmol x min(-1) x mg(-1)). Spearman correlation analyses revealed a stronger relationship between HLM MPA UGT activities and 1A9-like content (r(2) = 0.79) relative to 1A1 (r(2) = 0.20), 1A4-like (r(2) = 0.22), and 1A6 (r(2) = 0.41) protein. A different profile was observed for rat with three active liver-expressed UGT1A enzymes: 1A1 (medium affinity/capacity), 1A6 (low affinity/medium capacity), and 1A7 (high affinity/capacity). Our data suggest that UGT1A enzymes are the major contributors to hepatic MPA metabolism in both species, but 1A9 is dominant in human, whereas 1A1 and 1A7 are likely the principal mediators in control rat liver. This information should be useful for interpretation of MPA pharmacokinetic and toxicity data in clinical and animal studies.

Polycyclic aromatic hydrocarbons induce an inflammatory atherosclerotic plaque phenotype irrespective of their DNA binding properties

Although it has been demonstrated that carcinogenic environmental polycyclic aromatic hydrocarbons (PAHs) cause progression of atherosclerosis, the underlying mechanism remains unclear. In the present study, we aimed to investigate whether DNA binding events are critically involved in the progression of PAH-mediated atherogenesis. Apolipoprotein E knockout mice were orally (24 wk, once/wk) exposed to 5 mg/kg benzo[a]pyrene (B[a]P), or its nonmutagenic, noncarcinogenic structural isoform benzo[e]pyrene (B[e]P). 32P-postlabeling of lung tissue confirmed the presence of promutagenic PAH-DNA adducts in B[a]P-exposed animals, whereas in B[e]P-exposed and vehicle control animals, these adducts were undetectable. Morphometrical analysis showed that both B[a]P and B[e]P caused an increase in plaque size, whereas location or number of plaques was unaffected. Immunohistochemistry revealed no differences in oxidative DNA damage (8-OHdG) or apoptosis in the plaques. Also plasma lipoprotein levels remained unchanged after PAH-exposure. However, T lymphocytes were increased > or =2-fold in the plaques of B[a]P- and B[e]P-exposed animals. Additionally, B[a]P and to a lesser extent B[e]P exposure resulted in increased TGFbeta protein levels in the plaques, that was mainly localized in the plaque macrophages. In vitro studies using the murine macrophage like RAW264.7 cells showed that inhibition of TGFbeta resulted in decreased tumor necrosis factor (TNF) alpha release, suggesting that enhanced TGFbeta expression in the plaque macrophages contributes to the proinflammatory effects in the vessel wall. In general, this inflammatory reaction in the plaques appeared to be a local response since peripheral blood cell composition (T cells, B cells, granulocytes, and macrophages) was not changed upon PAH exposure. In conclusion, we showed that both B[a]P and B[e]P cause progression of atherosclerosis, irrespective of their DNA binding properties. Moreover, our data revealed a possible novel mechanism of PAH-mediated atherogenesis, which likely involves a TGFbeta-mediated local inflammatory reaction in the vessel wall.

In vitro inhibition of human hepatic and cDNA-expressed sulfotransferase activity with 3-hydroxybenzo[a]pyrene by polychlorobiphenylols

Sulfonation is a major phase II biotransformation reaction. In this study, we found that several polychlorobiphenylols (OH-PCBs) inhibited the sulfonation of 3-hydroxybenzo[a]pyrene (3-OH-BaP) by human liver cytosol and some cDNA-expressed sulfotransferases. At concentrations > 0.15 microM, 3-OH-BaP inhibited its own sulfonation in cytosol fractions that were genotyped for SULT1A1 variants, as well as with expressed SULT1A1*1, SULT1A1*2, and SULT1E1, but not with SULT1A3 or SULT1B1. The inhibition fit a two-substrate kinetic model. We examined the effects of OH-PCBs on the sulfonation of 0.1 or 1.0 microM 3-OH-BaP, noninhibitory and inhibitory substrate concentrations, respectively. At the lower 3-OH-BaP concentration, OH-PCBs with a 3-chloro-4-hydroxy substitution pattern were more potent inhibitors of cytosolic sulfotransferase activity [with concentrations that produced 50% inhibition (IC50) between 0.33 and 1.1 microM] than were OH-PCBs with a 3,5-dichloro-4-hydroxy substitution pattern, which had IC50 values from 1.3 to 6.7 microM. We found similar results with expressed SULT1A1*1 and SULT1A1*2. The OH-PCBs were considerably less potent inhibitors when assay tubes contained 1.0 microM 3-OH-BaP. The inhibition mechanism was noncompetitive, and our results suggested that the OH-PCBs competed with 3-OH-BaP at an inhibitory site on the enzyme. The OH-PCBs tested inhibited sulfonation of 3-OH-BaP by SULT1E1, but the order of inhibitory potency was different than for SULT1A1. SULT1E1 inhibitory potency correlated with the dihedral angle of the OH-PCBs. The OH-PCBs tested were generally poor inhibitors of SULT1A3- and SULT1B1-dependent activity with 3-OH-BaP. These findings demonstrate an interaction between potentially toxic hydroxylated metabolites of PCBs and polycyclic aromatic hydrocarbons, which could result in reduced clearance by sulfonation.

Role of cell signaling in B[a]P-induced apoptosis: characterization of unspecific effects of cell signaling inhibitors and apoptotic effects of B[a]P metabolites

Here we show that several cell signaling inhibitors have effect on cyp1a1 expression and the metabolism of benzo[a]pyrene (B[a]P) in Hepa1c1c7 cells. The CYP1A1 inhibitor alpha-naphthoflavone (alpha-NF), the p53 inhibitor pifithrin-alpha (PFT-alpha), the ERK inhibitors PD98059 and U0126, and the p38 MAPK inhibitors SB202190 and PD169316 induced the expression and level of cyp1a1 protein. On the other hand, during the first h the inhibitors appeared to reduce the metabolism of B[a]P as measured by the generation of tetrols and by covalent binding of B[a]P to macromolecules. In contrast, the phosphatidylinositol-3 (PI-3) kinase inhibitor wortmannin, had neither an effect on the cyp1a1 expression nor the B[a]P-metabolism. In order to avoid these unspecific effects, we characterized the mechanisms involved in the apoptotic effects of B[a]P-metabolites. B[a]P and the B[a]P-metabolites B[a]P-7,8-DHD and BPDE-I induced apoptosis, whereas B[a]P-4,5-DHD had no effect. B[a]P, B[a]P-7,8-DHD and BPDE-I induced an accumulation and phosphorylation of p53, while the Bcl-2 proteins Bcl-xl, Bad and Bid were down-regulated. Interestingly, the levels of anti-apoptotic phospho-Bad were up-regulated in response to B[a]P as well as to B[a]P-7,8-DHD and BPDE-I. Both p38 MAPK and JNK were activated, but the p38 MAPK inhibitors were not able to inhibit BPDE-I-induced apoptosis. PFT-alpha reduced the BPDE-I-induced apoptosis, while both the PI-3 kinase inhibitor and the ERK inhibitors increased the apoptosis in combination with BPDE-I. BPDE-I also triggered apoptosis in primary cultures of rat lung cells. In conclusion, often used cell signaling inhibitors both enhanced the expression and the level of cyp1a1 and more directly acted as inhibitors of cyp1a1 metabolism of B[a]P. However, studies with the B[a]P-metabolite BPDE-I supported the previous suggestion that p53 has a role in the pro-apoptotic signaling pathway induced by B[a]P. Furthermore, these studies also show that the reactive metabolites of B[a]P induce the anti-apoptotic signals, Akt and ERK. Neither the induction nor the activity of p38 MAPK and JNK seems to be of major importance for the B[a]P-induced apoptosis.

Mutagenesis studies of the major benzo[a]pyrene N2-dG adduct in a 5'-TG versus a 5'-UG sequence: removal of the methyl group causes a modest decrease in the [G->T/G->A] mutational ratio

The potent mutagen/carcinogen benzo[a]pyrene (B[a]P) is metabolically activated to (+)-anti-B[a]PDE, which induces a full spectrum of mutations primarily at the G:C base pairs (e.g. GC-->TA, GC-->AT, etc.). Each of these mutations can be induced by its major adduct [+ta]-B[a]P-N(2)-dG, where DNA sequence context appears to influence both the quantitative and qualitative pattern of mutagenesis. We noted previously that 5'-TG sequences tend to have a higher fraction of G-->T mutations for both [+ta]-B[a]P-N(2)-dG and (+)-anti-B[a]PDE in comparison with 5'-CG, 5'-GG or 5'-AG sequences. To investigate a possible structural element for this trend, the role (if any) of the methyl group on the 5'-T is considered. Using adduct site-specific means, the [G-->T/G-->A] mutational ratio for [+ta]-B[a]P-N(2)-dG is determined to be approximately 1.08 in a 5'-TGT sequence, and approximately 0.60 in a 5'-UGT sequence. (G-->C mutations are minor.) Although this modest approximately 1.8-fold decrease in [G-->T/G-->A] ratio is statistically significant (P = 0.03), it suggests that the methyl group on the 5'-T is not the main reason why a 5'-T tends to enhance G-->T mutations. This study was prompted by an adduct conformational hypothesis, which predicted that the removal of the methyl group in a 5'-TG sequence would lower the fraction of G-->T mutations; however, the approximately 1.8-fold decrease is too small to do additional experiments to assess whether this conformational hypothesis, or other hypotheses, are the true cause of the decrease, which is discussed in this paper.

Dibenzo[a,l]pyrene induced DNA adduct formation in lung tissue in vivo

Polycyclic aromatic hydrocarbons (PAHs) are environmental carcinogens present in the atmosphere from combustion sources such as cigarette smoke, diesel exhaust, residential heating processes, and industrial coke production. To date, dibenzo[a,l]pyrene (DBP) has been found to be the strongest tumor-initiating PAH ever tested in rodent skin and mammary tumor models. Here we show for the first time that systemic exposure to DBP causes DNA damage in mouse lung tissue. C57BL/6 mice were gavaged with 1, 5 or 20 mg DBP/kg body weight, daily for 10 days. Toxicity of DBP was revealed by a decrease in body and organ weight of mice while no apparent cell death was observed on P815 mastocytoma cells (allograft model) in vitro. However, treatment of P815 cells in vitro with the ultimate carcinogenic metabolite of DBP, the fjord region (-)-anti-11,12-diol 13,14-epoxide [(-)-anti-DBPDE], resulted in the total loss of cell viability. Lungs from the animals were removed and subjected to DBP-DNA adduct analysis. A dose dependent adduct formation was revealed by 33P-postlabeling analysis of DNA from lung tissue. The majority of DNA adducts formed in lungs of mice after systemic exposure to DBP were contributed by (-)-anti-DBPDE. The data from this in vivo model are consistent with previous metabolic activation results obtained with DBP in human cells in culture.

Two fish bile reference materials certified for PAH metabolites

Environmental pollution with polycyclic aromatic hydrocarbons (PAHs) from pyrogenic or petrochemical sources is a matter of concern in many aquatic ecosystems. For field monitoring purposes, PAH metabolite levels in fish bile samples can be used as biomarkers of exposure, but previously there were no reference materials for analytical method validation or quality control. In this paper we describe the preparation and certification of two fish bile materials. BCR 720 is a pure bile material from sediment-exposed flounder (Platichthys flesus) and BCR 721 is a pure bile material from oil-exposed plaice (Pleuronectes platessa). Homogeneity and stability tests were carried out and a group of 12 expert laboratories was assembled for the certification measurements. The methods applied were liquid chromatography with fluorescence detection or gas chromatography with mass spectrometric detection. In BCR 720 three PAH metabolites were certified: 1-hydroxyphenanthrene, 1-hydroxypyrene and 3-hydroxybenzo[a]pyrene. In BCR 721 1-hydroxypyrene was certified. The fourth target compound, 2-hydroxynaphthalene, could not be certified, but the results obtained are listed. The major findings from earlier feasibility studies and recommendations for the optimal use of the reference materials are also included.

A role for DNA polymerase V in G --> T mutations from the major benzo[a]pyrene N2-dG adduct when studied in a 5'-TGT sequence in E. coli

Benzo[a]pyrene (B[a]P), a potent mutagen/carcinogen, is metabolically activated to (+)-anti-B[a]PDE, which induces a full spectrum of mutations (e.g. GC --> TA, GC --> AT, etc.) principally via its major adduct [+ta]-B[a]P-N2-dG. Recent findings suggest that different lesion bypass DNA polymerases may be involved in different mutagenic pathways, which is the subject of this report. [+ta]-B[a]P-N2-dG built into a plasmid in a 5'-TGT sequence gives approximately equal numbers of G --> T and G --> A mutations when host E. coli are UV irradiated prior to transformation, so this sequence context was chosen to investigate what DNA polymerases are involved in G --> T versus G --> A mutations. G --> T mutations decline (>10-fold) if E. coli either are not UV-irradiated or are deficient in DNA polymerase V ((delta)umuD/C), demonstrating a role for damage-inducible DNA Pol V in a G --> T pathway. G --> T mutations are not affected by transformation into E. coli deficient in either DNA polymerases II or IV. While the work herein was in progress, Lenne-Samuel et al. [Mol. Microbiol. 38 (2000) 299] built the same adduct into a plasmid in a 5'-GGA sequence, and showed that the frequency of G --> T mutations was similar in UV-irradiated and unirradiated host E. coli cells, suggesting no involvement by damage-inducible, lesion bypass DNA polymerases (i.e., not II, IV or V); furthermore, a role for DNA Pol V was explicitly ruled out. The easiest way to reconcile the findings of Lenne-Samuel et al. with the findings herein is if two G --> T mutagenic pathways exist for [+ta]-B[a]P-N2-dG, where sequence context dictates which pathway is followed. In contrast to the G --> T mutations, herein G --> A mutations from [+ta]-B[a]P-N2-dG in the 5'-TGT sequence context are shown not to be affected by UV-irradiation of host E. coli, and are not dependent on DNA Pol V, or Pol II, Pol IV, or the damage-inducible, but SOS-independent UVM system. Published studies, however, have shown that G --> A mutations are usually enhanced by UV-irradiation of host E. coli prior to the introduction of plasmids either site-specifically modified with [+ta]-B[a]P-N2-dG or randomly adducted with (+)-anti-B[a]PDE; both findings imply the involvement of a lesion-bypass DNA polymerase. These disparate results suggest the existence of two G --> A mutagenic pathways for [+ta]-B[a]P-N2-dG as well, although confirmation of this awaits further study. In conclusion, a comparison between the evidence presented herein and published findings suggests the existence of two distinct mutagenic pathways for both G --> T and G --> A mutations from [+ta]-B[a]P-N2-dG, where in each case one pathway is not damage-inducible and not dependent on a lesion-bypass DNA polymerase, while the second pathway is damage-inducible and dependent on a lesion-bypass DNA polymerase. Furthermore, DNA sequence context appears to dictate which pathway (as defined by the involvement of different DNA polymerases) is followed in each case.

DNA Damage, Repair, and Mutation Induction by (+)-Synand (−)-Anti-Dibenzo[a,l]Pyrene-11,12-Diol-13,14-Epoxides in Mouse Cells

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental carcinogens. PAHs are classified into bay and fjord region compounds according to structural differences in the molecule region where enzymatic epoxidation occurs. Dibenzo[a,l]pyrene (DB[a,l]P), one of the fjord region compounds, has been demonstrated to be the most carcinogenic PAH known to date. DB[a,l]P is activated to fjord region (+)-syn and (-)-anti-11,12-dihydroxy-13,14-epoxy-11,12,13,14-tetrahydrodibenzo[a,l]pyrene (DB[a,l]PDE) metabolites. In this study, we analyzed mutagenesis induced by (+)-syn- and (-)-anti-DB[a,l]PDE at the cII transgene in Big-Blue mouse cells. The mutant frequency of untreated cells (background level) was 6.53 x 10(-5). This level increased 3.7-fold for 20 nmol/L, 5.3-fold for 50 nmol/L, and 7.9-fold for 100 nmol/L (+)-syn-DB[a,l]PDE, respectively. In the case of (-)-anti-DB[a,l]PDE it increased 4.5-fold for 20 nmol/L, 6.7-fold for 50 nmol/L, and 10.6-fold for 100 nmol/L, respectively, indicating that (-)-anti-DB[a,l]PDE is slightly more mutagenic than (+)-syn-DB[a,l]PDE. The mutational spectra of (+)-syn- and (-)-anti-DB[a,l]PDE were quite similar except for several hotspots, specific for either (+)-syn-DB[a,l]PDE or (-)-anti-DB[a,l]PDE. The most frequently induced mutations were A to T transversions, which were 43.9% for (+)-syn- and 38.8% for (-)-anti-DB[a,l]PDE. In addition, G to T transversions were induced significantly, at frequencies of 18.5% by (+)-syn- and 18.1% by (-)-anti-DB[a,l]PDE. Using UvrABC cleavage and ligation-mediated PCR or the terminal transferase-dependent PCR method, we have determined DB[a,l]PDE-DNA adduct formation sites and repair rates in carcinogen-exposed cells. The mutation hotspots coincided with sites of strong adduct formation, but not all of the adduct hotspots were mutational hotspots. Slow adduct removal occurred for both (+)-syn- and (-)-anti-DB[a,l]PDE adducts over a time period of up to 72 hours. The data suggest that, although the (-)-anti-isomer is slightly more mutagenic, DNA adducts of both DB[a,l]PDE stereoisomers may have similar biological properties. We discuss the implications of these findings for human cancer mutagenesis.

Degradation of benzo[a]pyrene by Mycobacterium vanbaalenii PYR-1

Metabolism of the environmental pollutant benzo[a]pyrene in the bacterium Mycobacterium vanbaalenii PYR-1 was examined. This organism initially oxidized benzo[a]pyrene with dioxygenases and monooxygenases at C-4,5, C-9,10, and C-11,12. The metabolites were separated by reversed-phase high-performance liquid chromatography (HPLC) and characterized by UV-visible, mass, nuclear magnetic resonance, and circular dichroism spectral analyses. The major intermediates of benzo[a]pyrene metabolism that had accumulated in the culture media after 96 h of incubation were cis-4,5-dihydro-4,5-dihydroxybenzo[a]pyrene (benzo[a]pyrene cis-4,5-dihydrodiol), cis-11,12-dihydro-11,12-dihydroxybenzo[a]pyrene (benzo[a]pyrene cis-11,12-dihydrodiol), trans-11,12-dihydro-11,12-dihydroxybenzo[a]pyrene (benzo[a]pyrene trans-11,12-dihydrodiol), 10-oxabenzo[def]chrysen-9-one, and hydroxymethoxy and dimethoxy derivatives of benzo[a]pyrene. The ortho-ring fission products 4-formylchrysene-5-carboxylic acid and 4,5-chrysene-dicarboxylic acid and a monocarboxylated chrysene product were formed when replacement culture experiments were conducted with benzo[a]pyrene cis-4,5-dihydrodiol. Chiral stationary-phase HPLC analysis of the dihydrodiols indicated that benzo[a]pyrene cis-4,5-dihydrodiol had 30% 4S,5R and 70% 4R,5S absolute stereochemistry. Benzo[a]pyrene cis-11,12-dihydrodiol adopted an 11S,12R conformation with 100% optical purity. The enantiomeric composition of benzo[a]pyrene trans-11,12-dihydrodiol was an equal mixture of 11S,12S and 11R,12R molecules. The results of this study, in conjunction with those of previously reported studies, extend the pathways proposed for the bacterial metabolism of benzo[a]pyrene. Our study also provides evidence of the stereo- and regioselectivity of the oxygenases that catalyze the metabolism of benzo[a]pyrene in M. vanbaalenii PYR-1.

Binding of 3-hydroxybenzo[a]pyrene to bovine hemoglobin and albumin

Previous studies examined the bioavailability and first-pass biotransformation of 3-hydroxy[(3)H]benzo[a]pyrene ([(3)H]-3-OHBaP) in an isolated perfused catfish intestinal model. This work showed that 3-OHBaP, or a metabolite formed in intestine, bound covalently to blood protein. In this study, the blood adducts were characterized in vitro by incubating bovine ferric hemoglobin or albumin with [(3)H]-3OHBaP under various conditions. Incubation of 2 microM [(3)H]-3-OHBaP with hemoglobin for 1 h resulted in 7.49 pmol bound/mg protein, while albumin binding was 1.37 pmol/mg protein. Mild acid hydrolysis released only 5% of the radioactivity from 3-OHBaP-hemoglobin adducts. After gel filtration, the 3-OHBaP-hemoglobin adducts were examined by HPLC analysis. A single peak of radioactivity was detected at the same retention time as the heme component of hemoglobin. Unbound 3-OHBaP was oxidized to BaP-3,6-dione during incubation with ferric hemoglobin. Treatment of hemoglobin with ascorbic acid decreased the formation of hemoglobin adducts by 33%, while hydrogen peroxide treatment increased adduct formation by 44%. Incubation of [(3)H]-BaP-3-beta-D-glucuronide (BaP-3G) with hemoglobin and beta-glucuronidase resulted in greater binding to hemoglobin than incubation with [(3)H]-3-OHBaP alone. The hemoglobin adduct obtained from [(3)H]-BaP-3G also co-migrated with heme. These results indicate that an oxidative process is involved in formation of the heme adduct and that 3-OHBaP or BaP-3G might be a precursor of the bound metabolite.

Effects of chlorophyllin on transport of dibenzo(a, l)pyrene, 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine, and aflatoxin B1 across Caco-2 cell monolayers

Chlorophyllin (CHL) is a sodium copper derivative of chlorophyll that is capable of forming strong non-covalent complexes with several known carcinogens. Antimutagenic and anticarcinogenic effects, including reduced DNA adduct and tumor formation have been demonstrated for CHL against aflatoxin B(1) (AFB(1)), dibenzo(a,l)pyrene (DBP) and 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine (PhIP). Alterations in uptake and/or metabolism of planar molecules with at least partial ring structure have been proposed as mechanisms of action for CHL chemoprevention. The Caco-2 cell model of intestinal epithelial transport was used to evaluate the absorption of 1 microM DBP, AFB(1) and PhIP across cell monolayers in the presence of 0, 1, 10, and 100 microM CHL. No significant differences were observed in the permeability (P(e)) of DBP and AFB(1) from the basolateral-to-apical (BL --> AP) compared to apical-to-basolateral (AP --> BL) compartments for DBP and AFB(1), however, the P(e) of PhIP from BL --> AP, 1.26 x 10(5) +/- 2.10 x 10(6) cm/s, was significantly higher than AP --> BL, 5.83 x 10(6) +/- 7.56 x 10(7) cm/s, (P<0.001) suggesting an active efflux pathway. Transport of DBP from AP --> BL compartments was significantly reduced at all CHL concentrations (P<0.05). AP --> BL transport of AFB(1) was significantly reduced by the addition of 100 microM CHL (P<0.05) while 1 microM or 10 microM CHL had no effect. Complexation studies revealed a higher binding affinity (K(b)) for DBP to CHL compared to AFB(1) to CHL in transport buffer. AP --> BL transport of PhIP, which has a lower binding affinity for CHL than AFB(1) or DBP, was not significantly altered by the addition of CHL. These data suggest that the transport of AFB(1) and DBP can be inhibited by CHL, which supports a model of direct binding in the intestinal tract of CHL to these carcinogens with resultant reduction of bioavailability as one mechanism of action as a cancer chemopreventive agent.

A Gene Cluster Encoding Resistomycin Biosynthesis in Streptomyces resistomycificus; Exploring Polyketide Cyclization beyond Linear and Angucyclic Patterns

Resistomycin is a pentacyclic polyketide metabolite of Streptomyces resistomycificus that exhibits a variety of pharmacologically relevant properties. While virtually all bacterial aromatic polyketides can be grouped into linear and angular polyphenols, resistomycin has a unique "discoid" ring system. We have successfully identified the entire gene cluster encoding resistomycin biosynthesis by heterologously expressing a pooled cosmid library and screening for the fluorescence of the metabolite produced. The rem gene cluster exhibits several unusual features of the type II PKS involved, most remarkably a putative MCAT with highest homology to AT domains from modular PKSs. In addition, we provide the first insight into the molecular basis of a unique mode of cyclization giving rise to a discoid polyketide.

Nucleotide incorporation by human DNA polymerase gamma opposite benzo[a]pyrene and benzo[c]phenanthrene diol epoxide adducts of deoxyguanosine and deoxyadenosine

Mitochondria are major cellular targets of benzo[a]pyrene (BaP), a known carcinogen that also inhibits mitochondrial proliferation. Here, we report for the first time the effect of site-specific N2-deoxyguanosine (dG) and N6-deoxyadenosine (dA) adducts derived from BaP 7,8-diol 9,10-epoxide (BaP DE) and dA adducts from benzo[c]phenanthrene 3,4-diol 1,2-epoxide (BcPh DE) on DNA replication by exonuclease-deficient human mitochondrial DNA polymerase (pol gamma) with and without the p55 processivity subunit. The catalytic subunit alone primarily misincorporated dAMP and dGMP opposite the BaP DE-dG adducts, and incorporated the correct dTMP as well as the incorrect dAMP opposite the DE-dA adducts derived from both BaP and BcPh. In the presence of p55 the polymerase incorporated all four nucleotides and catalyzed limited translesion synthesis past BaP DE-dG adducts but not past BaP or BcPh DE-dA adducts. Thus, all these adducts cause erroneous purine incorporation and significant blockage of further primer elongation. Purine misincorporation by pol gamma opposite the BaP DE-dG adducts resembles that observed with the Y family pol eta. Blockage of translesion synthesis by these DE adducts is consistent with known BaP inhibition of mitochondrial (mt)DNA synthesis and suggests that continued exposure to BaP reduces mtDNA copy number, increasing the opportunity for repopulation with pre-existing mutant mtDNA and a resultant risk of mitochondrial genetic diseases.

Role of cytochrome P4501B1 in benzo[a]pyrene bioactivation to DNA-binding metabolites in mouse vascular smooth muscle cells: evidence from 32P-postlabeling for formation of 3-hydroxybenzo[a]pyrene and benzo[a]pyrene-3,6-quinone as major proximate genotoxic intermediates

Benzo[a]pyrene (BP), a polycylic aromatic hydrocarbon (PAH), is a potent atherogen and carcinogen in laboratory animals. Since genotoxic mechanisms may contribute to the development of atherosclerosis by PAHs, we have tested the hypotheses that: 1) BP induces DNA adducts in mouse aortic smooth muscle cells (SMCs); 2) 3-hydroxybenzo[a]pyrene (3-OH-BP) and benzo[a]pyrene-3,6-quinone (BPQ) are proximate genotoxic metabolites; and 3) cytochrome P4501B1 (CYP1B1) mediates the activation of BP and its metabolites to ultimate genotoxic intermediates. Cultured mouse aortic SMCs were treated with BP, 3-OH-BP, or BPQ for 24 h, and DNA adduct formation was analyzed by (32)P-postlabeling. In some experiments, cells were pretreated with the CYP1B1 inhibitor 1-ethynylpyrene (EP) prior to exposure to BP or its metabolites. BP, 3-OH-BP, and BPQ induced formation of several DNA adducts that were not observed in dimethylsulfoxide-treated cells. Re- and cochromatography experiments indicated that 3-OH-BP and BPQ were proximate genotoxic metabolites of BP. DNA adduct formation was strongly inhibited by EP, a specific inhibitor of CYP1B1. BP treatment of SMCs resulted in induction of aryl hydrocarbon hydroxylase (AHH) activity and CYP1B1, but not CYP1A1, apoprotein. EP also blocked AHH induction by BP. In conclusion, the results of this study support the hypothesis that in SMCs, which are target sites for the development of atherosclerosis, the major bioactivation pathway of BP entails CYP1B1-mediated formation of the 3-OH-BP and BPQ, which are proximate genotoxic metabolites that may in turn get transformed to ultimate DNA-binding metabolites, which may contribute to atherogenesis by PAHs.

Fluorobenzo[a]pyrenes as probes of the mechanism of cytochrome P450-catalyzed oxygen transfer in aromatic oxygenations

Fluoro substitution of benzo[a]pyrene (BP) has been very useful in determining the mechanism of cytochrome P450-catalyzed oxygen transfer in the formation of 6-hydroxyBP (6-OHBP) and its resulting BP 1,6-, 3,6-, and 6,12-diones. We report here the metabolism of 1-FBP and 3-FBP, and PM3 calculations of charge densities and bond orders in the neutral molecules and radical cations of BP, 1-FBP, 3-FBP, and 6-FBP, to determine the mechanism of oxygen transfer for the formation of BP metabolites. 1-FBP and 3-FBP were metabolized by rat liver microsomes. The products were analyzed by HPLC and identified by NMR. Formation of BP 1,6-dione and BP 3,6-dione from 1-FBP and 3-FBP, respectively, can only occur by removal of the fluoro ion from C-1 and C-3, respectively, via one-electron oxidation of the substrate. The combined metabolic and theoretical studies reveal the mechanism of oxygen transfer in the P450-catalyzed formation of BP metabolites. Initial abstraction of a pi electron from BP by the [Fe(4+)=O](+)(*) of cytochrome P450 affords BP(+)(*). This is followed by oxygen transfer to the most electropositive carbon atoms, C-6, C-1, and C-3, with formation of 6-OHBP (and its quinones), 1-OHBP, and 3-OHBP, respectively, or the most electropositive 4,5-, 7,8-, and 9,10- double bonds, with formation of BP 4,5-, 7,8-, or 9,10-oxide.

Degradation of straight-chain aliphatic and high-molecular-weight polycyclic aromatic hydrocarbons by a strain of Mycobacterium austroafricanum

AIMS

Our goal was to characterize a newly isolated strain of Mycobacterium austroafricanum, obtained from manufactured gas plant (MGP) site soil and designated GTI-23, with respect to its ability to degrade polycyclic aromatic hydrocarbons (PAHs).

METHODS AND RESULTS

GTI-23 is capable of growth on phenanthrene, fluoranthene, or pyrene as a sole source of carbon and energy; it also extensively mineralizes the latter two in liquid culture and is capable of extensive degradation of fluorene and benzo[a]pyrene, although this does not lead in either of these cases to mineralization. Supplementation of benzo[a]pyrene-containing cultures with phenanthrene had no significant effect on benzo[a]pyrene degradation; however, this process was substantially inhibited by the addition of pyrene. Extensive and rapid mineralization of pyrene by GTI-23 was also observed in pyrene-amended soil.

CONCLUSIONS

Strain GTI-23 shows considerable ability to mineralize a range of polycyclic aromatic hydrocarbons, both in liquid and soil environments. In this regard, GTI-23 differs markedly from the type strain of Myco. austroafricanum (ATCC 33464); the latter isolate displayed no (or very limited) mineralization of any tested PAH (phenanthrene, fluoranthene or pyrene). When grown in liquid culture, GTI-23 was also found to be capable of growing on and mineralizing two aliphatic hydrocarbons (dodecane and hexadecane).

SIGNIFICANCE AND IMPACT OF THE STUDY

These findings indicate that this isolate of Myco. austroafricanum may be useful for bioremediation of soils contaminated with complex mixtures of aromatic and aliphatic hydrocarbons.

Residues 207, 216, and 221 and the catalytic activity of mGSTA1-1 and mGSTA2-2 toward benzo[a]pyrene-(7R,8S)-diol-(9S,10R)-epoxide

Murine class alpha glutathione S-transferase subunit types A2 (mGSTA2-2) and A1 (mGSTA1-1) have high catalytic efficiency for glutathione (GSH) conjugation of the ultimate carcinogenic metabolite of benzo[a]pyrene, (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene, [(+)-anti-BPDE]. Only 10 residues differ between the sequences of mGSTA1-1 and 2-2. However, the catalytic efficiency of mGSTA1-1 for GSH conjugation of (+)-anti-BPDE is >3-fold higher as compared with mGSTA2-2. The crystal structure of mGSTA1-1 in complex with the GSH conjugate of (+)-anti-7,8-dihydroxy-9,10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (GSBpd) reveals that R216 and I221 in the last helix play important roles in catalysis [Gu, Y., Singh, S. V., and Ji, X. (2000) Biochemistry 39, 12552-12557]. The crystal structure of mGSTA2-2 in complex with GSBpd has been determined, which reveals a different binding mode of GSBpd. Comparison of the two structures suggests that residues 207 and 221 are responsible for the different binding mode of GSBpd and therefore contribute to the distinct catalytic efficiency of the two isozymes.

Xenobiotic conjugation systems in deer compared with cattle and rat

The ability of cattle and deer liver to catalyse xenobiotic conjugation reactions was investigated and compared with that of the rat. Marked differences in the activity of these enzymes were noted between the domestic animals and rats. Hepatic microsomal epoxide hydrolase activity in cattle and deer, determined using benzo[a]pyrene 4,5-oxide as substrate, was nearly twice that of the rat. In contrast, glutathione S-transferase activity in hepatic cytosol, determined with 1-chloro-2,4-dinitrobenzene as substrate, was significantly lower in the cattle and deer. When 1,2-dichloro-4-nitrobenzene served as the accepting substrate, no activity was detectable in the cattle and deer. Similarly, glutathione reductase activity and total glutathione levels were markedly lower in the cattle and deer compared with the rat. Cytosolic sulfotransferase activity, monitored using 2-naphthol as substrate, was higher in cattle compared with the rat. Finally, microsomal UDP-glucuronosyl transferase activity, determined using 1-napththol as substrate, did not differ significantly among the three species.

Stable expression of rat sulfotransferase 1B1 in V79 cells: activation of benzylic alcohols to mutagens

We have constructed Chinese hamster V79-derived cell lines (V79-rSULT1B1-A and -B) that express rat sulfotransferase 1B1 (rSULT1B1). Sulfotransferase activity towards 1-naphthol was 1020 +/- 220 pmol/min/mg cytosolic protein in V79-rSULT1B1-A cells and 57 +/- 9 pmol/ min/mg in V79-rSULT1B1-B cells. These activities were similar over 100 population doublings and at varying cell densities. Immunostaining indicated a cytoplasmatic localization of rSULT1B1. Expression usually was homogeneous within colonies but showed some variation between colonies. The level of rSULT1B1 protein in V79-rSULT1B1-B cells was similar to that in rat liver but higher than in colon mucosa. The cytotoxicity of the benzylic alcohols 4H-cyclopenta[def]chrysen-4-ol and 6-hydroxymethylbenzo-[a]pyrene was enhanced >100-fold in V79-rSULT1B1-A cells compared with SULT-deficient cells (V79p). Likewise, these compounds showed mutagenic effects (at the hprt locus) in V79-rSULT1B1-A cells starting at a concentration of 0.02 and 0.01 micro M, respectively, but were inactive in V79p cells even at a concentration of 1 micro M. The cell line with the lower expression level, V79-rSULT1B1-B, showed only marginal toxification of the compounds investigated, indicating an important role of the expression level in the test system. A thoroughly characterized mammalian cell system, including positive controls, is now available for studying rSULT1B1-mediated bioactivation of promutagens and protoxicants.

Effect of polycyclic aromatic hydrocarbons on the pallial fluid buffering capacity of the marine mussel, Mytilus galloprovincialis

Pallial fluid buffering capacity of the sea mussel Mytilus galloprovincialis was investigated to establish the potential of this biological parameter to serve as a biomarker. Four groups of 15 animals were used in a 72-h toxicity test. Group 1, the aerobic control group, was placed in a filtered aerated natural seawater aquarium. Groups 2-4 were subjected to hypoxic conditions by removal from water after animals were injected with a single dose of the following: group 2 (anaerobic control) was administered 10 microl of UV-treated filtered natural seawater; group 3 (anaerobic solvent control) was injected with 10 microl of acetonitrile and group 4 (PAH exposed group) with 10 microl of 2 mM anthracene. Pallial fluid was taken from all the animals following seawater immersion or air exposure. Pallial fluid from each individual was extracted, adjusted to pH 5.0 and titrated with NaOH until reaching pH 6.0. The buffering capacity index (beta), defined as the amount of mu equivalents of NaOH needed to change in one unit the pH of a 5-ml sample of pallial fluid, was calculated for each group. Values were: for group 1, beta = 3.17 (+/- 0.782); for group 2, beta=15.713 (+/-2.992); group 3 was beta=18.124 (+/-2.288); and group 4 was beta=28.109 (+/-11.398). The statistically significant increase (P<0.05) in group 4 compared with the other groups indicates that the buffering capacity index (beta) is a worthy biological parameter to be further explored as a biomarker for ecotoxicological monitoring programs. The increase in buffering capacity is discussed and a biochemical link between anaerobic metabolism and the exposure to PAH is suggested to explain changes of this biological parameter.

[Cloning of differentially expressed cDNA sequences involved in malignant transformation induced by benzo(a)pyrene metabolite dihydroxyepoxy benzo pyrene]

OBJECTIVE

To clone differentially expressed cDNA sequences involved in malignant transformation induced by benzo(a)pyrene metabolite dihydroxyepoxy benzo pyrene (BPDE).

METHOD

The malignant transformation of human bronchial epithelial cell line 16HBE induced by BPDE in vitro was used as a model for comparing gene expression between the transformed cells and controls. cDNA representational difference analysis (cDNA-RDA) was performed to isolate differentially expressed cDNA fragment in transformed cells. The cDNA fragments were ligated to pGEM-T vector and transformed into JM109 bacteria. The plasmid DNA were sequenced and compared with data in GenBank by BLASTN.

RESULTS

Five cDNA sequences were found to be novel ones and were registered in dbest database, which assigned accession numbers in GenBank are BG354691, BG354692, BG354693, BG354694 and BG354695, respectively. Eight of the remaining cDNA sequences showed sequence homology to those previously reported such as ribosomal protein S23, MLN137, ACTN4, transforming growth factor and G protein gene.

CONCLUSIONS

These 13 genes may be involved in BPDE-induced malignant transformation, but their biological characteristics and functions are left to further studies.

Hydroxylated polychlorinated biphenyls as inhibitors of the sulfation and glucuronidation of 3-hydroxy-benzo[a]pyrene

Polychlorinated biphenyls (PCBs) can be metabolized by cytochromes P450 to hydroxylated biotransformation products. In mammalian studies, some of the hydroxylated products have been shown to be strong inhibitors of steroid sulfotransferases. As a part of ongoing research into the bioavailability of environmental pollutants in catfish intestine, we investigated the effects of a series of hydroxylated PCBs (OH-PCBs) on two conjugating enzymes, phenol-type sulfotransferase and glucuronosyltransferase. We incubated cytosolic and microsomal samples prepared from intestinal mucosa with 3-hydroxy-benzo[a]pyrene and appropriate cosubstrates and measured the effect of OH-PCBs on the formation of BaP-3-glucuronide and BaP-3-sulfate. We used PCBs with 4, 5, and 6 chlorine substitutions and the phenolic group in the ortho, meta, and para positions. OH-PCBs with the phenolic group in the ortho position were weak inhibitors of sulfotransferase; the median inhibitory concentration (IC50) ranged from 330 to 526 microM. When the phenol group was in the meta or para position, the IC50 was much lower (17.8-44.3 microM). The OH-PCBs were more potent inhibitors of glucuronosyltransferase, with IC50s ranging from 1.2 to 36.4 microM. The position of the phenolic group was not related to the inhibitory potency: the two weakest inhibitors of sulfotransferase, with the phenolic group in the ortho position, were 100 times more potent as inhibitors of glucuronosyltransferase. Inhibition of glucuronosyltransferase by low concentrations of OH-PCBs has not been reported before and may have important consequences for the bioavailability, bioaccumulation, and toxicity of other phenolic environmental contaminants.

Microsomal activation of dibenzo[def,mno]chrysene (anthanthrene), a hexacyclic aromatic hydrocarbon without a bay-region, to mutagenic metabolites

Metabolically formed dihydrodiol epoxides in the bay-region of polycyclic aromatic hydrocarbons are thought to be responsible for the genotoxic properties of these environmental pollutants. The hexacyclic aromatic hydrocarbon dibenzo[def,mno]chrysene (anthanthrene), although lacking this structural feature, was found to exhibit considerable bacterial mutagenicity in histidine-dependent strains TA97, TA98, TA100, and TA104 of S. typhimurium in the range of 18-40 his(+)-revertant colonies/nmol after metabolic activation with the hepatic postmitochondrial fraction of Sprague-Dawley rats treated with Aroclor 1254. This mutagenic effect amounted to 44-84% of the values determined with benzo[a]pyrene under the same conditions. The specific mutagenicity of anthanthrene in strain TA100 obtained with the cell fraction of untreated animals was 6 his(+)-revertant colonies/nmol and increased 2.7-fold after treatment with phenobarbital and 4.5-fold after treatment with 3-methylcholanthrene. To elucidate the metabolic pathways leading to genotoxic metabolites, the microsomal biotransformation of anthanthrene was investigated. A combination of chromatographic, spectroscopic, and biochemical methods allowed the identification of the trans-4,5-dihydrodiol, 4,5-oxide, 4,5-, 1,6-, 3,6-, and 6,12-quinones, and 1- and 3-phenols. Furthermore, two diphenols derived from the 3-phenol, possibly the 3,6 and 3,9 positional isomers, as well as two phenol dihydrodiols were isolated. Three pathways of microsomal biotransformation of anthanthrene could be distinguished: The K-region metabolites are formed via pathway I dominated by monooxygenases of the P450 1B subfamily. On pathway II the polynuclear quinones of anthanthrene are formed. Pathway III is preferentially catalyzed by monooxygenases of the P450 1A subfamily and leads to the mono- and diphenols of anthanthrene. The K-region oxide and the 3-phenol are the only metabolites of anthanthrene with strong intrinsic mutagenicity, qualifying them as ultimate mutagens or their precursors. From the intrinsic mutagenicity of these two metabolites and their metabolic formation, the maximal mutagenic effect was calculated. This demonstrates the dominating role of pathway III in the mutagenicity of anthanthrene under conditions where it exhibits the strongest bacterial mutagenicity.

Glutathione conjugation and DNA adduct formation of dibenzo[a,l]pyrene and benzo[a]pyrene diol epoxides in V79 cells stably expressing different human glutathione transferases

Mammalian V79 cells stably expressing human glutathione transferase (GST) A1-1, M1-1, and P1-1 (the allelic variant with Val105 and Ala114) have been constructed and characterized. The cells have been used to study the capacity of individual GST isoenzymes in conjunction with GSH to detoxify diol epoxides from dibenzo[a,l]pyrene (DBPDE), the most carcinogenic polycyclic aromatic hydrocarbon (PAH) identified so far, and diol epoxides from benzo[a]pyrene (BPDE). The relationship between GSH-conjugation and DNA adduct-formation has been investigated as well as factors governing the accessibility of lipophilic diol epoxide substrates for the soluble GSTs in the cells. Relative to control cells, those expressing GSTA1-1 showed the highest rate (about 50-fold increase) to perform GSH-conjugation of (-)-anti-DBPDE (R-absolute configuration at the benzylic oxirane carbon in the fjord-region) followed by GSTM1-1 (25-fold increase) and GSTP1-1 (10-fold increase). GSTA1-1 was found to be strongly inhibited when expressed in cells (10% of fully functional protein). Taking this factor into account, the rates of conjugation found in the cells fairly well reflected the order of catalytic efficiencies (k(cat)/K(m)) obtained with the pure enzymes. Increased GSH conjugation of (-)-anti-DBPDE was associated with a reduction in DNA adduct formation. GSTA1-1 inhibited the formation of adducts more than 6-fold and GSTM1-1 and GSTP1-1 about 2-fold. With (+)-anti-BPDE, GSTP1-1-expressing cells demonstrated a substantially higher rate of GSH-conjugate formation than cells with GSTA1-1 and GSTM1-1 cells (33- and 10-fold increase, respectively). Relative to control cells, GSTM1-1 was found to inhibit DNA adduct formation of (+)-anti-BPDE most effectively followed by GSTP1-1 and GSTA1-1 (12-, 4-, and 3-fold, respectively). Values of k(cat)/K(m) and estimated oil/water partition coefficients of DBPDE and BPDE were used to calculate the concentration of free diol epoxides in solution and expected rates of GSH conjugate formation in cells, and these theoretical results were compared with the observed ones. With the highly reactive (+)-anti-BPDE, 1-2% of the expected activity was observed, whereas the corresponding values for the less reactive (-)-anti-DBPDE were up to 13%. The most obvious explanations for the low observed rate with (+)-anti-BPDE are rapid and competing reactions such as hydrolysis and/or more unspecific chemical and physical reactions with cellular constituents (proteins, lipids, nucleic acids, etc.). In addition, the difference between the theoretical and observed rates may also reflect participation of factors such as macromolecular crowding and reduced rates of diffusion, factors expected to further restrict the accessibility of GST and the diol epoxides in the intact cell.

Effect of soluble and particulate nickel compounds on the formation and repair of stable benzo[a]pyrene DNA adducts in human lung cells

Nickel compounds are well-known human carcinogens, but the underlying mechanisms are still not fully understood. Even though only weakly mutagenic, nickel chloride has been shown previously to impair the repair of UV-induced DNA damage as well as oxidative DNA damage. However, the carcinogenic potential depends largely on solubility, with poorly water-soluble nickel subsulfide and nickel oxide being strong carcinogens. Within the present study we investigated the effects of particulate black NiO and soluble NiCl(2) on the induction and removal of stable DNA adducts formed by benzo[a]pyrene (B[a]P) measured by a highly sensitive high performance liquid chromatography (HPLC)/fluorescence assay. With respect to adduct formation, NiO but not NiCl(2) reduced the generation of DNA lesions by approximately 30%. Regarding their repair, in the absence of nickel compounds, most lesions were removed within 24 h; nevertheless, between 20 and 35% of induced adducts remained even 48 h after treatment. NiCl(2) and NiO reduced the removal of adducts in a dose-dependent manner. Thus, 100 microM NiCl(2) led to approximately 80% residual repair capacity; after 500 microM the repair was reduced to approximately 36%. Also, even at the completely non-cytotoxic concentration of 0.5 microg/cm(2) black NiO, lesion removal was reduced to approximately 35% of control and to 15% at 2.0 microg/cm(2). Furthermore, both nickel compounds increased the benzo[a]pyrene-7,8-diol 9,10-epoxide (BPDE)-induced cytotoxicity. Taken together, our results indicate that the nucleotide excision repair pathway is affected in general by water-soluble and particulate nickel compounds and provide further evidence that DNA repair inhibition may be one predominant mechanism in nickel-induced carcinogenicity.

Fluorescence measurements of DNA-bound metabolites of benzo(a)pyrene derivatives with different carcinogenic effects

(+/-)-trans-dihydroxy-7,8-dihydrobenzo(a)pyrene (BP-7,8-diol) and 9-hydroxybenzo(a)pyrene (9-OH-BP) were metabolized by rat liver microsomes in the presence of calf thymus DNA, resulting in preferential DNA binding of fluorescent (+)-anti-BP-7,8-diol-9,10-epoxide (BPDE) and 9-OH-BP-4,5-epoxide, respectively. When the DNA is denatured the fluorescence intensities of the bound metabolites change in a characteristic manner. Fluorescence decay measurements show that the intensity changes are due to changes in lifetimes of the excited states. Model substances for the bound metabolites were studied in solvents of different polarity. We found that the fluorescence changes observed after denaturation of the DNA may be explained as solvent polarity effects, so that denaturation forces the bound metabolites from a more hydrophobic environment to a hydrophilic one. Fluorescence depolarization studies as a function of temperature in combination with previous linear dichroism studies show that both BPDE and 9-OH-BP-4,5-epoxide form rigidly associated complexes with native DNA.

Stable expression of rat dihydrodiol dehydrogenase (AKR1C9) in human breast MCF-7 cells results in the formation of PAH-o-quinones and enzyme mediated cell death

Dihydrodiol dehydrogenase members of the aldo-keto reductase (AKR) superfamily have been implicated in the metabolic activation of PAH trans-dihydrodiols because they convert these proximate carcinogens to reactive and redox-active o-quinones. In this study, rat liver 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (AKR1C9) was stably transfected into human breast carcinoma (MCF-7) cells, which represent a null-environment for AKR expression, to detect the formation of PAH o-quinones in a cellular context and the cellular consequences of o-quinone formation. The heterologous transfected cells expressed AKR1C9 mRNA and protein. Immunotitration of the enzyme activity indicated that the expressed protein constituted 1.0% of the soluble protein. The specific activity of the expressed enzyme was also comparable to that observed in rat liver cytosol. The transfectants were found to convert (+/-)-trans-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (B[a]P-7,8-diol) to benzo[a]pyrene-7,8-dione (BPQ). The identity of this metabolite was confirmed by co-chromatography and by UV-Vis diode-array spectrometry. This conversion was not evident in mock-transfected cells. The cytotoxic consequences of BPQ formation was also examined. Transfectants exposed to 1 microM B[a]P-7,8-diol revealed that cytotoxicity, as measured by lactate dehydrogenase (LDH) release, occurred over the time course of o-quinone formation leading to 77% of the cellular LDH being released by 16 h. AKR1C9 inhibitors blocked the B[a]P-7,8-diol dependent cytotoxicity indicating that it was mediated by the enzymatically formed BPQ. These data indicate that high stable constitutive expression of AKR1C9 will result in B[a]P-7,8-diol mediated cytotoxicity due to the formation of unconjugated BPQ.

Metabolism of carcinogenic 2-hydroxybenz

The rates of metabolism of the carcinogenic 2-hydroxybenzo[a]pyrene (2-OH-B[a]P) and the non-carcinogenic 3- and 9-hydroxybenzo[a]pyrenes in cultured cell systems have been studied and compared. While 70-80% of the non-carcinogens are converted to water-soluble derivatives by hamster embryo fibroblasts in 24 h, carcinogenic 2-OH-B[a]P is metabolized at a slower rate (45% in 24 h), comparable to that for the parent hydrocarbon, benzo[a]pyrene (B[a]P). Analysis of extracellular organic solvent-soluble metabolites of 2-OH-B[a]P in cultured hamster embryo fibroblasts, using h.p.l.c., indicates the presence of a single major metabolite, which has been identified by mass spectroscopy as a dihydroxy derivative of B[a]P. At least one additional major organic solvent-soluble metabolite is formed in cultures of either mouse epidermal epithelial cells or human foreskin fibroblasts, indicating a different balance of metabolic pathways in these cell systems. The greater persistence of carcinogenic 2-OH-B[a]P in cells and its higher concentration in the cell cytoplasm compared with the non-carcinogenic phenols may be related to its relatively high biological activity. Differences in metabolism of 2-OH-B[a]P in several cultured cell systems indicate the importance of an appropriate choice of activating system in understanding the relationship between metabolism and carcinogenesis.

Fluorescence spectral evidence that benzo[a]pyrene is activated by metabolism in mouse skin to a diol-epoxide and a phenol-epoxide

Hydrolysates of DNA that had been isolated from mouse skin treated with 3H-labelled benzo[a]pyrene were subjected to chromatography on Sephadex LH20. Two major products were eluted in the region expected for deoxyribonucleoside-hydrocarbon adducts and these were purified further by h.p.l.c. The fluorescence emission and excitation spectra of one of the adducts were identical to that of the adduct obtained from DNA that was treated with BP-7,8-diol 9,10-oxide (r-7,t-8-dihydroxy-t-9,10-oxy-7,8,9,10-tetrahydrobenzo[a] pyrene). The fluorescence emission and excitation spectra of the other adducts were identical to the published spectra of 9-OHBP-4,5-diol (4,5-dihydro-4,5,9-trihydroxy-benzo[a]pyrene) and of the deoxyribonucleoside-hydrocarbon adduct obtained from DNA that had been incubated with 9-OHBP (9-hydroxybenzo[a] pyrene) in the presence of a rat-liver microsomal system. The metabolic activation of benzo[a]pyrene in mouse skin, a target tissue for carcinogenesis by this hydrocarbon, thus appears to involve the formation of adducts derived from both BP-7,8-diol 9,10-oxide and 9-OHBP 4,5-oxide (9-hydroxybenzo-[a]pyrene 4,5-oxide), although quantitatively, the adduct derived from 9-OHBP 4,5-oxide is a minor product.

Effects of the (-)-anti-11R,12S-dihydrodiol 13S,14R-epoxide of dibenzo

The tumor suppressor protein p53 plays an important role in recognition of DNA damage and induction of subsequent cell cycle arrest. One of its target genes encodes the protein p21(WAF1), which is involved in mediation of growth arrest after DNA damage has occurred. Dibenzo[a,l]pyrene (DB[a,l]P) is a polycyclic aromatic hydrocarbon which is an exceptionally potent carcinogen. A reactive secondary metabolite of DB[a,l]P, the fjord region (-)-anti-11R,12S-dihydrodiol 13R,14S-epoxide [(-)-anti-DB[a,l]PDE] was used to investigate DNA damage via adduct formation and cell cycle arrest in human diploid fibroblast cell cultures (HDF). Synchronous HDF were exposed to increasing concentrations (0.014, 0.028 and 0.07 microM) of (-)-anti-DB[a,l]PDE and at 1, 12, 24 and 42 h after treatment cell pellets were analyzed for DNA adduct formation and cell cycle arrest. Exposure of HDF to 0.07 microM (-)-anti-DB[a,l]PDE caused a total DNA binding level of 113 pmol adducts/mg DNA (42 h after treatment). G(1) arrest was induced by this treatment, with 91% of the cells remaining in G(1) phase compared with the solvent-treated control cultures (50%) as analyzed by propidium iodide staining and flow cytometry. Further investigation of the percentage of cells in S phase by 5-bromo-2'-deoxyuridine incorporation confirmed the G(1) arrest in HDF treated with 0.07 microM (-)-anti-DB[a,l]PDE, with only 1.5% of the cells moving into S phase compared with 39% in the control 42 h after treatment. Induction of p53 and p21(WAF1) was demonstrated by western blot analysis.

1,2-dithiole-3-thione and its structural analogue oltipraz are potent inhibitors of dibenz

Dithiolethiones are currently one of the most promising classes of cancer chemopreventive agents that exhibit antitumorigenic properties at numerous organ sites against several classes of carcinogens. In the current study, we examined the effects of 2 dithiolethiones, 1,2-dithiole-3-thione (D3T) and its structural analogue oltipraz, on DNA adduction induced by the potent mammary carcinogen dibenzo-[a,l]pyrene (DBP) in vivo. Female Sprague-Dawley rats were provided dietary D3T and oltipraz (500 ppm each) for I week followed by a single intragastric dose of DBP (8 micromol/kg body weight) and killed 5 days later. D3T inhibited DBP-DNA adduction from 78% to 82% in all tissues examined, while oltipraz was equally effective in the lung and liver but less effective in the mammary glands, inhibiting DBP-DNA adduction by nearly 60%. These data coupled with their broad anti-tumor specificity support the use of D3T and oltipraz as cancer-preventive agents in clinical trials.

Position-specific trapping of topoisomerase I-DNA cleavage complexes by intercalated benzo[a]- pyrene diol epoxide adducts at the 6-amino group of adenine

DNA topoisomerase I (top1) is the target of potent anticancer agents, including camptothecins and DNA intercalators, which reversibly stabilize (trap) top1 catalytic intermediates (cleavage complexes). The aim of the present study was to define the structural relationship between the site(s) of covalently bound intercalating agents, whose solution conformations in DNA are known, and the site(s) of top1 cleavage. Two diastereomeric pairs of oligonucleotide 22-mers, derived from a sequence used to determine the crystal structure of top1-DNA complexes, were synthesized. One pair contained either a trans-opened 10R- or 10S-benzo[a]pyrene 7, 8-diol 9,10-epoxide adduct at the N(6)-amino group of a central 2'-deoxyadenosine residue in the scissile strand, and the other pair contained the same two adducts in the nonscissile strand. These adducts were derived from the (+)-(7R,8S,9S,10R)- and (-)-(7S,8R,9R, 10S)-7,8-diol 9,10-epoxides in which the benzylic 7-hydroxyl group and the epoxide oxygen are trans. On the basis of analogy with known solution conformations of duplex oligonucleotides containing these adducts, we conclude that top1 cleavage complexes are trapped when the hydrocarbon adduct is intercalated between the base pairs flanking a preexisting top1 cleavage site, or between the base pairs immediately downstream (3' relative to the scissile strand) from this site. We propose a model with the +1 base rotated out of the duplex, and in which the intercalated adduct prevents religation of the corresponding nucleotide at the 5' end of the cleaved DNA. These results suggest mechanisms whereby intercalating agents interfere with the normal function of human top1.

Synthesis and structure determination of 6-methylbenzo[a]pyrene-deoxyribonucleoside adducts and their identification and quantitation in vitro and in mouse skin

Activation of the moderate carcinogen 6-methylbenzo[a]pyrene (6-CH(3)BP) by one-electron oxidation to form DNA adducts was studied. Iodine oxidation of 6-CH(3)BP in the presence of dGuo produces BP-6-CH(2)-N(2)dGuo, BP-6-CH(2)-N7Gua and a mixture of 6-CH(3)BP-(1&3)-N7Gua, whereas in the presence of Ade the adducts BP-6-CH(2)-N1Ade, BP-6-CH(2)-N3Ade, BP-6-CH(2)-N7Ade and 6-CH(3)BP-(1&3)-N1Ade are obtained. Furthermore, for the first time an aromatic hydrocarbon radical cation afforded an adduct with dThd, the stable adduct BP-6-CH(2)-N3dThd. Formation of these adducts indicates that the 6-CH(3)BP radical cation has charge localized at the 6, 1 and 3 position. When 6-CH(3)BP was activated by horseradish peroxidase in the presence of DNA, two depurinating adducts were identified, BP-6-CH(2)-N7Gua (48%) and 6-CH(3)BP-(1&3)-N7Gua (23%), with 29% unidentified stable adducts. In the binding of 6-CH(3)BP catalyzed by rat liver microsomes, the same two depurinating adducts, BP-6-CH(2)-N7Gua (22%) and 6-CH(3)BP-(1&3)-N7Gua (10%), were identified, with 68% unidentified stable adducts. In 6-CH(3)BP-treated mouse skin, the two depurinating adducts, BP-6-CH(2)-N7Gua and 6-CH(3)BP-(1&3)-N7Gua, were identified. Although quantitation of these two adducts was not possible due to coelution of metabolites on HPLC, they appeared to be the major adducts found in mouse skin. These results show that 6-CH(3)BP forms depurinating adducts only with the guanine base of DNA, both in vitro and in mouse skin. The weaker reactivity of 6-CH(3)BP radical cation vs. BP radical cation could account for the weaker tumor-initiating activity of 6-CH(3)BP in comparison to that of BP.

Lipid peroxidation, antioxidant enzymes, and benzo[a]pyrene-quinones in the blood of rats treated with benzo[a]pyrene

The lipid peroxidation (as malondialdehyde, MDA), activities of superoxide dismutase (SOD) and catalase (CAT), and benzo[a]pyrene (BaP) metabolites were investigated in sera and erythrocytes of male Sprague-Dawley rats treated with BaP (20 mg per rat). MDA levels were significantly increased in sera (16.98+/-3.29 nmol/ml serum, P<0.05) 12 h after BaP treatment and persisted up to 96 h (13.80+/-1. 65 nmol/ml serum, P<0.05), but no significant change in NIDA levels was observed in erythrocytes. SOD and CAT activities were significantly increased in erythrocytes shortly after BaP exposure, and they were slightly decreased in sera, indicating an inverse correlation between lipid peroxidation and antioxidant enzyme activity. BaP and BaP-quinones (BaP-1,6-quinone and BaP-3,6-quinone) were measured in sera during the study period. A rapid increase of unmetabolized BaP was observed in sera (41.27+/-4.14 pmol/ml serum) 3 h after BaP treatment, reaching a peak at 6 h (48.56+/-4.62 pmol/ml serum) followed by a sharp decrease. Formation of the BaP-1, 6-quinone and BaP-3,6-quinone started in sera 3 h after BaP treatment, reached a peak at 24 h (7.23+/-1.02 pmol/ml serum) and 12 h (9.20+/-0.98 pmol/ml serum), respectively, and then decreased gradually. The time-dependent pattern of serum lipid peroxidation and the level of erythrocyte antioxidant enzymes were shown to be related to the concentrations of the BaP-quinone metabolites. These results suggest that BaP treatment, probably via the formation of BaP-quinones, oxidatively altered lipids and antioxidant enzymes in the blood, and might be associated with BaP-related vascular toxicity including carcinogenesis.

DNA adducts in hematopoietic tissues and blood of the mummichog (Fundulus heteroclitus) from a creosote-contaminated site in the Elizabeth River, Virginia

Hydrophobic DNA adducts were examined in liver, anterior kidney, spleen, and blood of tumor-prone mummichog (Fundulus heterclitus) from the creosote-contaminated Atlantic Wood (AW) site (Elizabeth River, Virginia). DNA adducts eluted in a diagonal radioactive zone, characteristic of polycyclic aromatic hydrocarbon exposure, in all examined tissues of AW fish. Mummichog demonstrated significantly higher levels of DNA adducts in spleen (394 +/- 109 nmol adducts/mol nucleotides) than in liver (201 +/- 77 nmol adducts/mol nucleotides) or anterior kidney (211 +/- 68 nmol adducts/mol nucleotides; P = 0.036). The levels of DNA adducts in the pooled blood (pool of four) were 142 nmol adducts/mol nucleotides. DNA adducts were not detected in the liver, anterior kidney, spleen and blood of fish collected from the reference site (< 2 nmol adducts/mol nucleotides). The high levels of DNA adducts detected in tissues of AW mummichog may be linked to the increased cancer incidence and immunosuppression in this population.

Microbial degradation and detoxification of high molecular weight polycyclic aromatic hydrocarbons by Stenotrophomonas maltophilia strain VUN 10,003

The ability of Stenotrophomonas maltophilia strain VUN 10,003 to degrade and detoxify high molecular weight polycyclic aromatic hydrocarbons (PAHs) was evaluated in a basal liquid medium. Using high cell density inocula of strain VUN 10,003, the concentration of pyrene, fluoranthene, benz[a]anthracene, benzo[a]pyrene, dibenz[a, h]anthracene and coronene decreased by 98, 45, 26, 22, 22 and 55% over periods ranging from 5 to 42 d. When a PAH mixture containing three- to seven-ring compounds was used, degradation of both low and high molecular weight compounds occurred concurrently. Mutagenicity assays (Ames Test) demonstrated a decrease in the mutagenic potential of dichloromethane culture extracts from all cultures containing single PAH over the incubation period, corresponding to the decrease in the concentration of the PAH. These observations indicate that strain VUN 10,003 could be used for the detoxification of PAH-contaminated wastes.

Enhanced sensitivity to anti-benzo(a)pyrene-diol-epoxide DNA damage correlates with decreased global genomic repair attributable to abrogated p53 function in human cells

DNA damage from exposure to environmental chemical carcinogens and failure of repair systems to eliminate these lesions from the genome are considered as the crucial initial steps in the development of various human malignancies. Many cellular proteins are known to play vital roles to overcome the effects of DNA damage. Among such proteins, p53 is known to respond to DNA damage by accumulating in the nucleus and inhibiting cell cycle progression to facilitate DNA repair and the maintenance of genomic stability. In this study, we have investigated the role of p53 protein in modulating nucleotide excision repair of anti-benzo-(a)pyrene-diol-epoxide (BPDE)-DNA adducts and related effects using human fibroblasts with normal (p53-WT) and altered p53 protein (p53Mut and p53-Null). Interestingly, irrespective of the presence or absence of p53, the anti-BPDE dose-dependent p21 protein induction response was qualitatively comparable in all of the three cell lines. However, cells with defective p53 function were deficient for the removal of anti-BPDE-DNA adducts from the overall genome compared to cells with wild-type p53 activity. Strand-specific repair analysis within the individual strands of the p53 gene revealed decreased repair of adducts from the nontranscribed strand in p53-Mut and p53-Null cells. However, the repair of the transcribed strand appeared to be identical in all of the three cell lines. Furthermore, p53-Mut and p53-Null cells were more sensitive than p53-WT cells and displayed increased levels of anti-BPDE-induced apoptosis. Thus, wild-type p53 is required for the efficient global genomic repair of anti-BPDE-induced DNA adducts from the overall genome, but not for transcription-coupled repair of actively transcribed genes. These findings indicate that inefficient DNA repair of potentially cytotoxic and mutagenic lesions from the nontranscribed strand due to the loss of p53, but not the loss of p21, function might be responsible for enhanced cytotoxicity and apoptosis in human cells upon DNA damage.

Correlation between DNA or protein adducts and benzo[a]pyrene diol epoxide I-triglyceride adduct detected in vitro and in vivo

In this study, we demonstrated the in vitro and in vivo formation of carcinogen-lipid adduct and its correlation with DNA or protein adducts. The lipids from serum or hepatocyte membranes of Sprague-Dawley rats, human serum and standard major lipids were in vitro reacted with benzo[a]pyrene (B[a]P) and B[a]P metabolites. 7, 8-Dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene(BPDE-I), an ultimate carcinogenic form of B[a]P, was covalently bound to triglyceride (TG). BPDE-I-TG adducts isolated by thin-layer chromatography (TLC) were further detected by high-performance liquid chromatography. TGs, including triolein, tripalmitin and tristearin, showed positive reactions with BPDE-I. However, cholesterol, phospholipids (phosphatidylcholine, phosphatidyl-ethanolamine, phosphatidyl-inositol and sphingomyelin) and non-esterified fatty acids (palmitic acid, oleic acid, linoleic acid and stearic acid) did not react with BPDE-I. In addition, other B[a]P metabolites (B[a]P-phenols and -diols) did not react with TG. TG appeared to be the most reactive lipid yet studied with respect to its ability to form an adduct with BPDE-I. There was a clear-cut dose-related formation of [1,3-(3)H]BPDE-I-lipid adducts in vitro between TG and [1,3-(3)H]BPDE-I. In an animal study, BPDE-I-TG was also formed in the serum of rats orally treated with B[a]P (25 mg/rat). Also, obvious correlations between [(3)H]B[a]P related-biomolecule adducts (DNA or protein) or lipid damage and the BPDE-I-TG adducts were obtained in various tissues of mice i.p. treated with [(3)H]B[a]P. These data suggest that TG can form an adduct with BPDE-I, as do other macromolecules (DNA, RNA and protein). Therefore, a carcinogen-lipid adduct would be a useful biomarker for chemical carcinogenesis research and cancer risk assessment.

Exposure-route-dependent DNA adduct formation by polycyclic aromatic hydrocarbons

Understanding the kinetics of aromatic-DNA adducts in target tissues and white blood cells (WBC) would enhance the applicability of DNA adducts in WBC as surrogate source of DNA in biomonitoring studies. In the present study, rats were acutely exposed to benzo[a]pyrene (B[a]P; 10 mg/kg body wt) via intratracheal (i.t.), dermal and oral administration. DNA adducts were analyzed in relevant target organs and WBC by nuclease P1 enriched (32)P-post-labeling at 1, 2, 4, 11 and 21 days after exposure. Additionally, the internal dose was assessed by measurement of urinary excretion of 3-hydroxy-B[a]P (3-OH-B[a]P). Total B[a]P-DNA adduct levels in WBC were highest after i.t. and oral administration, whereas DNA adducts were hardly detectable after dermal exposure. Highest adduct levels were reached at 2 days after exposure. In lung tissue, DNA adduct levels reached maximal values at 2 days and were highest after i.t., oral and dermal exposure, respectively. DNA adduct levels were significantly lower in WBC as compared with lung. Nonetheless, overall B[a]P-DNA adduct levels in WBC were significantly correlated with those in lung. In target organs, highest DNA adduct levels were observed in skin after topical application, and lowest in stomach after oral administration of B[a]P. Furthermore, DNA adduct levels in WBC were correlated with DNA adduct levels in skin after dermal exposure and stomach after oral administration of B[a]P. Two-fold higher levels of 3-OH-B[a]P were excreted after i.t. administration of B[a]P as compared with dermal or oral exposure. Urinary 3-OH-B[a]P concentrations were correlated with DNA adduct levels at the site of B[a]P application. Overall, it can be concluded that aromatic-DNA adduct levels in WBC can be applied as a surrogate source of DNA for the site of application of B[a]P and reflect binding to lung DNA, independently of the exposure route.

Formation of stable DNA adducts and apurinic sites upon metabolic activation of bay and fjord region polycyclic aromatic hydrocarbons in human cell cultures

Carcinogenic polycyclic aromatic hydrocarbons (PAH), such as benzo[a]pyrene (B[a]P), 7,12-dimethylbenz[a]anthracene (DMBA), and dibenzo[a,l]pyrene (DB[a,l]P), are metabolically activated to electrophilically reactive bay or fjord region diol epoxides that bind to the exocyclic amino groups of purine bases in DNA to form stable adducts. In addition, it has been reported that these PAH can be enzymatically oxidized to yield radical cations that form apurinic (AP) sites in DNA via depurinating adducts. The formation of stable adducts and AP sites in DNA of human cells exposed to PAH was examined in cytochrome P450 (P450)-expressing mammary carcinoma MCF-7 cells and in leukemia HL-60 cells, which display a high peroxidase but no P450-mediated activity, after exposure to these PAH. Stable DNA adducts were assessed by (33)P-postlabeling/HPLC analysis, and the induction of AP sites in DNA was analyzed by an aldehyde reactive probe (ARP) and a slot blot method. After exposure for 4 h, the levels of stable DNA adducts were comparable in MCF-7 cells treated with B[a]P and DMBA, but significantly lower than those observed in MCF-7 cells treated with the stronger carcinogen DB[a,l]P. While the levels of stable adducts increased more than 10-fold (B[a]P and DMBA) or 100-fold (DB[a,l]P) after exposure for 24 h, the levels of AP sites remained low after both treatment periods. Thus, the levels of stable adducts were approximately 5-fold higher than the levels of AP sites after treatment with B[a]P or DMBA and more than 100-fold higher in cells exposed to DB[a,l]P for 24 h. None of these carcinogenic PAH formed detectable levels of stable DNA adducts or AP sites in HL-60 cells. The results demonstrate that metabolic activation of B[a]P, DMBA, and DB[a,l]P is catalyzed by P450 enzymes leading to diol epoxides that form predominantly stable DNA adducts but only low levels of AP sites.

HPLC analysis of benzo[a]pyrene-albumin adducts in benzo[a]pyrene exposed rats. Detection of cis-tetrols arising from hydrolysis of adducts of anti- and syn-BPDE-III with proteins

Quantitation of protein-benzo[a]pyrene adducts represent a more sensitive analysis method than quantitation of benzo[a]pyrene-DNA adducts. By accurate analysis of benzo[a]pyrene-protein adducts several different molecular adduct forms can be studied. Male Wistar rats were injected i.p. with benzo[a]pyrene, and serum albumin was isolated and subjected to acid hydrolysis at 90 degrees C for 3 h. The hydrolysate was analyzed by HPLC with fluorescence detection. The HPLC profiles obtained after albumin hydrolysis from benzo[a]pyrene exposed animals were compared to similar HPLC profiles from in vitro adducted bovine serum albumin (BSA) and direct hydrolysis of both r-10,t-9-dihydrodiol-c-7,8-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (syn-BPDE-III) and r-10,t-9-t-dihydrodiol-t-7,8-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE-III). After acid hydrolysis of albumin from benzo[a]pyrene exposed rats, 6 fluorescent peaks were separated. Four of the peaks were isomers of benzo[a]pyrene-tetrahydrotetrols, (+/-)-benzo[a]pyrene-r-7,t-8,9,10-tetrahydrotetrol, (+/-)-benzo[a]pyrene-r-7,t-8,9,c-10-tetrahydrotetrol, (+/-)-benzo[a]pyrene-r-7,t-8,c-9,t-10-tetrahydrotetrol and (+/-)-benzo[a]pyrene-r-7,t-8,c-9,10-tetrahydrotetrol. In addition we found two fluorescent peaks, named X1 and X2 with retention times similar to the benzo[a]pyrene-tetrols. The unknown fluorescent peaks reacted similar to the four known tetrols in both dose response experiments and time course experiments. Fluorescent material with retention times equal to X1 and X2 were found after acid hydrolysis of syn-BPDE-III and anti-BPDE-III in acid and in hydrolysates from BSA treated in vitro with syn-BPDE-III and anti-BPDE-III. The ratio X1/X2 was relatively constant indicating epimerization equilibrium between these to species. Synchronous fluorescence analysis of fractions containing X1 or X2 from both in vivo and in vitro experiments showed fluorescence spectra characteristic of benzo[a]pyrene tetrols using a wavelength difference of 34 nm.

A quantitative comparison of dibenzo[a,l]pyrene-DNA adduct formation by recombinant human cytochrome P450 microsomes

Dibenzo[a,l]pyrene (DB[a,l]P), an extremely potent environmental carcinogen, is metabolically activated in mammalian cells and microsomes through the fjord-region dihydrodiol, trans-DB[a,l]P-11, 12-diol, to syn- and anti-DB[a,l]P-11,12-diol-13,14-epoxides (syn- and anti-DB[a,l]PDEs). The role of seven individual recombinant human cytochrome P450s (1A1, 1A2, 1B1, 2B6, 2C9, 2E1, and 3A4) in the metabolic activation of DB[a,l]P and formation of DNA adducts was examined by using (32)P postlabeling, thin-layer chromatography, and high-pressure liquid chromatography. We found that, in the presence of epoxide hydrolase, only P450 1A1 and P450 1B1 catalyzed the formation of DB[a,l]PDE-DNA adducts and several unidentified polar adducts. Human P450 1A1 catalyzed the formation of DB[a, l]PDE-DNA adducts and unidentified polar adducts at rates threefold and 17-fold greater than did human P450 1B1 (256 fmol/h/nmol P450 versus 90 fmol/h/nmol P450 and 132 fmol/h/nmol P450 versus 8 fmol/h/nmol P450, respectively). P450 1A1 DNA adducts were derived from both anti- and syn-DB[a,l]PDE at rates of 73 fmol/h/nmol P450 and 51 fmol/h/nmol P450, respectively. P450 1B1 produced adducts derived from anti-DB[a,l]PDE at a rate of 82 fmol/h/nmol, whereas only a small number of adducts were derived from syn-DB[a,l]PDE (0.4 fmol/h/nmol). These results demonstrated the potential of human P450 1A1 and P450 1B1 to contribute to the metabolic activation and carcinogenicity of DB[a,l]P and provided additional evidence that human P450 1A1 and 1B1 differ in their stereospecific activation of DB[a,l]P. Mol. Carcinog. 26:74-82, 1999. Published 1999 Wiley-Liss, Inc.

Amino acid substitutions at positions 207 and 221 contribute to catalytic differences between murine glutathione S-transferase Al-1 and A2-2 toward (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene

We have previously identified a novel Alpha class murine glutathione (GSH) S-transferase isoenzyme (designated mGSTAl-2) which is exceptionally efficient in catalyzing the GSH conjugation of (+)-anti-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-anti-BPDE], the ultimate carcinogen of widespread environmental pollutant benzo[a]pyrene. Furthermore, we have demonstrated that the Al-type subunit of this isoenzyme is significantly more active toward (+)-anti-BPDE than the other subunit (mGSTA2). To establish the basis for catalytic differences between mGSTAl and mGSTA2, which differ in their primary structures by 10 amino acids [distributed in three sections (I-III) as clusters of two (residues 65 and 95), three (residues 157, 162, and 169), and five (residues 207, 213, 218, 221, and 222) amino acids], three chimeric enzymes were expressed and tested for their activity toward (+)-anti-BPDE. These studies revealed that amino acid substitution(s) in section III determined the high catalytic activity of mGSTAl. Molecular modeling studies suggested that amino acid substitutions at positions 207 and/or 221, but not at positions 213, 218, and 222, may be responsible for such a difference. To test this possibility, amino acids at positions 207 and 221 of mGSTAl were mutated with the equivalent residues of mGSTA2. Kinetic analysis of the wild type and the mutant enzymes revealed that both methionine-207 and isoleucine-221 are critical for higher activity of mGSTA1-1 toward (+)-anti-BPDE compared with that of mGSTA2-2.

Mineralization of benzo[a]pyrene by Marasmiellus troyanus, a mushroom isolated from a toxic waste site

Mycelia from the mushroom Marasmiellus troyanus were grown in the presence of radiolabeled benzo[a]pyrene in liquid culture. After 15 days, 8.1% of the label from M. troyanus cultures was recovered in CO2 as compared to 1.1% for Phanerochaete chrysosporium and 0.2% for Aspergillus niger. M. troyanus efficiently transformed B[a]P into water soluble metabolites with 64% of the label recovered in the water soluble fraction as compared to 11.7% for P. chrysosporium and 4.1% for A. niger. Glucuronic acid and sulfate conjugates of B[a]P were identified from the aqueous fraction of cultures of M. troyanus, after 17 days.