The approach to understanding aromatic hydrocarbon carcinogenesis. The central role of radical cations in metabolic activation

Carbocations from oxidized metabolites of benzo[a]anthracene: a computational study of their methylated and fluorinated derivatives and guanine adducts

Structure-reactivity relationships and substituent effects on carbocation stability in benzo[a]anthracene (BA) derivatives have been studied computationally at the B3LYP/6-31G and MP2/6-31G levels. Bay-region carbocations are formed by O-protonation of the 1,2-epoxides in barrierless processes. This process is energetically more favored as compared to carbocation generation via zwitterion formation/O-protonation, via single electron oxidation to generate a radical cation, or via benzylic hydroxylation. Relative carbocation stabilities were determined in the gas phase and in water as solvent (PCM method). Charge delocalization mode in the BA carbocation framework was deduced from NPA-derived changes in charges, and substitution by methyl or fluorine was studied at different positions selected on basis of the carbocation charge density. A bay-region methyl group produces structural distortion with consequent deviation from planarity of the aromatic system, which destabilizes the epoxide, favoring ring opening. Whereas fluorine substitution at sites bearing significant positive charge leads to carbocation stabilization by fluorine p-pi back-bonding, a fluorine atom at a ring position which presented negative charge density leads to inductive destabilization. Methylated derivatives are less sensitive to substituent effects as compared to the fluorinated analogues. Although the solvent decreases the exothermicity of the epoxide ring-opening reactions due to greater stabilization of the reactants, it provokes no changes in relative reactivities. Relative energies in the resulting bay-region carbocations are examined taking into account the available biological activity data on these compounds. In selected cases, quenching of bay-region carbocations was investigated by analyzing relative energies (in the gas phase and in water) and geometries of their guanine adducts formed via covalent bond formation with the exocyclic amino group and with the N-7.

The effect of melatonin on 7,12-dimethyl-benz[a]anthracene injury in comparison with vitamin E + selenium in mouse kidneys

The aims of this study were to observe the changes in antioxidative defense enzymes and renal morphology after 7,12-dimethyl-benz[a]anthracene (7,12-DMBA) administration in mice and to investigate the possible protective effects of melatonin against 7,12-DMBA-induced renal damage in comparison with vitamin E + selenium (vit E + Se). Forty female mice were divided into four groups: control, DMBA, DMBA + vit E + Se and DMBA + melatonin. In the DMBA group, mice were given injections of 7,12-DMBA (20 mg/kg). DMBA + vit E + Se group mice received injections of 7,12-DMBA + vit E + Se (20 mg/kg + 90 mg/kg + 1.8 microg/kg). In the melatonin group, mice were given injections of 7,12-DMBA + melatonin (20 mg/kg + 4.2 mg/kg). The experiment lasted for 21 days. Mice were killed and the kidneys were taken for enzyme analyses and histologic examination. Catalase (CAT) and glutathione peroxidase (GSH-Px) activities were found significantly decreased in the DMBA group and in the DMBA + vit E + Se group when compared with the control group (P < 0.05), whereas CAT and GSH-Px activities were found significantly elevated in the DMBA + melatonin group when compared with the control (P < 0.05) and the DMBA group (P < 0.01). Exposure to DMBA resulted in tubular alterations in renal cortex. Morphometric analysis revealed proximal and distal tubular damage (P < 0.05). These alterations were found to be prevented by melatonin but not with vit E + Se administration. These results reveal that melatonin stimulates CAT and GSH-Px activities and prevents renal injury better than vit E + Se combination in mice kidneys.

Quantification of benzo[a]pyrene diol epoxide DNA-adducts by stable isotope dilution liquid chromatography/tandem mass spectrometry

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants found in car exhausts, charbroiled food, and tobacco smoke. Three pathways for the metabolic activation of B[a]P to ultimate carcinogens have been proposed. The most widely accepted pathway involves cytochrome-P450 (CYP) 1A1- and/or 1B1-mediated formation of B[a]P-7,8-oxide, which undergoes epoxide hydrolase-mediated metabolism to the proximate carcinogen B[a]P-7,8-dihydro-7,8-diol. Further CYP1A1- and/or CYP1B1-mediated activation of the dihydrodiol results in the formation of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]PDE), the ultimate carcinogen. In previous studies, it was demonstrated that (+)-anti-B[a]PDE was the most potent tumorigen of the CYP-derived B[a]PDE diastereomers. We have developed a stable isotope dilution, liquid chromatography multiple reaction monitoring/mass spectrometry (LC-MRM/MS) assay for all eight (+/-)-anti-B[a]PDE-derived dGuo and dAdo DNA-adducts. The LC-MRM/MS assay was rigorously validated and used to show that (+)-anti-trans-B[a]PDE-dGuo was the major adduct formed when naked DNA and human bronchoalveolar adenocarcinoma H358 cells were treated with (+/-)-anti-B[a]PDE. The preference for DNA-adducts derived from (+)-anti-B[a]PDE was even more apparent in cellular DNA. Thus, the increased potency of (+)-anti-B[a]PDE as a tumorigen is most likely due its ability to preferentially form DNA-adducts when compared with (-)-anti-B[a]PDE. Also, the adduct profile suggests that this occurs by binding of (+)-anti-B[a]PDE to DNA in a manner that facilitates covalent binding to dGuo rather than dAdo residues.

Catalytic hydrogenation of polycyclic aromatic hydrocarbons over palladium/gamma-Al2O3 under mild conditions

As a prelude to the optimization of a continuous decontamination system, catalytic hydrogenations of selected tri-, tetra- and penta-cyclic aromatic hydrocarbon compounds over commercial alumina supported palladium were investigated under mild conditions ( approximately 90 degrees C/0.42 MPa H2) and interpreted in the light of reports from the literature. Acenaphthylene, acenaphthene, anthracene, phenanthrene, chrysene and benzo[alpha]pyrene were hydrogenated, virtually completely, to saturated polycyclic hydrocarbon compounds with no appreciable evidence of carbon-carbon bond rearrangement during equilibration. With comparable operating conditions, triphenylene was only partially hydrogenated; the central ring remained unsaturated. The effects of reaction temperature, time of equilibration and supporting gases on hydrogenation were evaluated. Whereas near-critical CO2 had no perceptible influence on rate/course of the reaction, nitrogen decreased the reaction rate somewhat.

Cytochrome P450 1B1 Is Required for 7,12-Dimethylbenz(a)-anthracene (DMBA) Induced Spleen Cell Immunotoxicity

7,12-Dimethylbenz(a)anthracene (DMBA) is a potent carcinogen that induces immunosuppression of both humoral and cell-mediated immunity in mice and other species. Previous studies have shown that CYP1B1 is required for bone marrow toxicity produced by DMBA in mice. Therefore, the purpose of these studies was to determine whether CYP1B1 was required for spleen cell immunotoxicity. Female C57BL/6N wild-type (WT) and CYP1B1 knockout (-/-) mice were treated with 0, 17, 50, or 150 mg/kg (cumulative dose) DMBA in corn oil by oral gavage once a day for five days. Several immunotoxicological assays were used to assess the effects of DMBA on systemic immunity. These included the in vitro T-dependent antibody response to sheep red blood cells (SRBC) measured using a direct plaque forming cell (PFC) assay, T- and B-cell mitogenesis induced by Con A and LPS, and nonspecific cell-mediated immunity was evaluated using an NK cytotoxicity assay. In addition, lymphocyte subpopulations were measured by flow cytometry using specific cell surface markers. Following five days of DMBA treatment, the body weights and spleen cell surface markers of the WT and CYP1B1 (-/-) mice showed no significant changes. A decrease in NK activity was found at the 50 mg/kg DMBA dose in WT mice, but not in the CYP1B1 (-/-) mice. Interestingly, at the 150 mg/kg dose of DMBA, CYP1B1 null mice had decreased NK activity, whereas WT mice did not. The SRBC PFC response demonstrated that the IgM antibody response was suppressed by DMBA in WT mice in a dose-dependent manner (significant at 50 and 150 mg/kg). However, there were no changes in the SRBC PFC responses in any DMBA test group in the CYP1B1 (-/-) mice. Similarly, while DMBA suppressed B- and T-cell mitogenesis at the 50 and 150 mg/kg dose levels in C57BL/6N WT mice, no effect was seen in CYP1B1 (-/-) mice. Thus, CYP1B1 appears to be critical for the immunosuppression of DMBA in mice, suggesting a role for bioreactive metabolites in the spleen cell immunotoxicity produced by DMBA.

Metabolic activation of polycyclic and heterocyclic aromatic hydrocarbons and DNA damage: a review

Polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic compounds (HACs) constitute a major class of chemical carcinogens present in the environment. These compounds require activation to electrophilic metabolites to exert their mutagenic or carcinogenic effects. There are three principal pathways currently proposed for metabolic activation of PAH and HAC: the pathway via bay region dihydrodiol epoxide by cytochrome P450 enzymes (CYPs), the pathway via radical cation by one-electron oxidation, and the ortho-quinone pathway by dihydrodiol dehydrogenase (DD). In addition to these major pathways, a brief description of a minor metabolic activation pathway, sulfonation, for PAHs that contain a primary benzylic alcoholic group or secondary hydroxyl group(s) is included in this review. The DNA damages caused through the reactive metabolites of PAH/HAC are described involving the DNA covalent binding to form stable or depurinating adducts, the formation of apurinic sites, and the oxidative damage. The review emphasizes the chemical/biochemical reactions involved in the metabolic processes and the chemical structures of metabolites and DNA adducts.

Identification and quantitation of benzo[a]pyrene-derived DNA adducts formed at low adduction level in mice lung tissue

The two major metabolic pathways of benzo[a]pyrene (BP) that lead to DNA lesions are monooxygenation that results in diolepoxides (BPDE) and one-electron oxidation that yields a BP radical cation. These pathways result in formation of stable and depurinating DNA adducts, respectively. Most in vivo animal studies with BP, however, have employed dosage/DNA adduct levels several orders of magnitude higher than the DNA damage level expected from environmentally relevant exposures. Presented are results of experiments in which A/J strain mice were intraperitoneally exposed to 50-microg/g doses of BP. It is shown that non-line-narrowed fluorescence and fluorescence line-narrowing spectroscopies possess the selectivity and sensitivity to distinguish between helix-external, base-stacked, and intercalated conformations of DNA-BPDE adducts formed in lung tissue. Concentrations measured by 32P postlabeling 2 and 3 days after intraperitoneal injection were 420-430 and 600-830 amol BPDE-type adducts per microg DNA. The external and base-stacked conformations are attributed mainly to (+)-trans-anti-BPDE-N2dG and the intercalated conformations to (+)-cis-anti adducts. A stable adduct derived from 9-OH-BP-4,5-epoxide was also detected at a concentration about a factor of 10 lower than the above concentrations. The DNA supernatants were analyzed for the presence of depurinating BP-derived adducts by capillary electrophoresis laser-induced fluorescence and high-performance liquid chromatography mass spectrometry.

Probing the Interaction of Benzo[a]pyrene Adducts and Metabolites with Monoclonal Antibodies Using Fluorescence Line-Narrowing Spectroscopy

A new approach for studying antibody-antigen interactions of DNA adducts and metabolites of polycyclic aromatic hydrocarbons (PAHs) is demonstrated in which fluorescence line-narrowing spectroscopy (FLNS) is used. It is based on the fact that in an FLN spectrum the relative intensities of the line-narrowed bands (that correspond to the excited-state vibrations) are, in general, strongly dependent on the local environment of the fluorophore. Information on the nature of the interactions can be obtained by comparing the FLN spectra of the antigen-antibody complexes to the spectra of the antigen in different types of solvents (H-bonding, aprotic, and pi-electron-containing solvent molecules) recorded under the same conditions. The antigens used were the DNA adduct 7-(benzo[a]pyren-6-yl)guanine (BP-6-N7Gua) and the metabolite (+)-trans-anti-7,8,9,10-benzo[a]pyrenetetrol (BP-tetrol) of benzo[a]pyrene; two monoclonal antibodies (MAbs) have been developed to selectively bind these compounds. It is shown that, for BP-tetrol, H-bonding solvents have a pronounced effect on the FLN spectra. The presence of pi electrons in the solvent molecules results in relatively small but still significant changes in the spectra. When BP-tetrol is bound to its MAb, however, neither of these effects is observed; its spectrum is very similar to the one obtained with an aprotic solvent, methylcyclohexane. Therefore, we can conclude that this MAb has an internal binding site in which the interaction with BP-tetrol is of a hydrophobic character. For BP-6-N7Gua, however, there is a strong effect of the presence of pi electrons in the solvent molecules. The FLN spectrum of this antigen bound to its MAb is very similar to its spectrum in acetone, indicating that pi-pi interactions play an important role in the binding.

Exposure to carbon nanotube material: assessment of nanotube cytotoxicity using human keratinocyte cells

Carbon nanotubes are new members of carbon allotropes similar to fullerenes and graphite. Because of their unique electrical, mechanical, and thermal properties, carbon nanotubes are important for novel applications in the electronics, aerospace, and computer industries. Exposure to graphite and carbon materials has been associated with increased incidence of skin diseases, such as carbon fiber dermatitis, hyperkeratosis, and naevi. We investigated adverse effects of single-wall carbon nanotubes (SWCNT) using a cell culture of immortalized human epidermal keratinocytes (HaCaT). After 18 h of exposure of HaCaT to SWCNT, oxidative stress and cellular toxicity were indicated by formation of free radicals, accumulation of peroxidative products, antioxidant depletion, and loss of cell viability. Exposure to SWCNT also resulted in ultrastructural and morphological changes in cultured skin cells. These data indicate that dermal exposure to unrefined SWCNT may lead to dermal toxicity due to accelerated oxidative stress in the skin of exposed workers.

Estrogen and its metabolites are carcinogenic agents in human breast epithelial cells

Estrogens play a crucial role in the development and evolution of human breast cancer. However, it is still unclear whether estrogens are carcinogenic to the human breast. There are three mechanisms that have been considered to be responsible for the carcinogenicity of estrogens: receptor-mediated hormonal activity, a cytochrome P450 (CYP)-mediated metabolic activation, which elicits direct genotoxic effects by increasing mutation rates, and the induction of aneuploidy by estrogen. To fully demonstrate that estrogens are carcinogenic in the human breast through one or more of the mechanisms explained above it will require an experimental system in which, estrogens by itself or one of the metabolites would induce transformation phenotypes indicative of neoplasia in HBEC in vitro and also induce genomic alterations similar to those observed in spontaneous malignancies. In order to mimic the intermittent exposure of HBEC to endogenous estrogens, MCF-10F cells that are ERalpha negative and ERbeta positive were first treated with 0, 0.007, 70 nM and 1 microM of 17beta-estradiol (E(2)), diethylstilbestrol (DES), benz(a)pyrene (BP), progesterone (P), 2-OH-E(2), 4-hydoxy estradiol (4-OH-E(2)) and 16-alpha-OH-E(2) at 72 h and 120 h post-plating. Treatment of HBEC with physiological doses of E(2), 2-OH-E(2), 4-OH-E(2) induce anchorage independent growth, colony formation in agar methocel, and reduced ductulogenic capacity in collagen gel, all phenotypes whose expression are indicative of neoplastic transformation, and that are induced by BP under the same culture conditions. The presence of ERbeta is the pathway used by E(2) to induce colony formation in agar methocel and loss of ductulogenic in collagen gel. This is supported by the fact that either tamoxifen or the pure antiestrogen ICI-182,780 (ICI) abrogated these phenotypes. However, the invasion phenotype, an important marker of tumorigenesis is not modified when the cells are treated in presence of tamoxifen or ICI, suggesting that other pathways may be involved. Although we cannot rule out the possibility, that 4-OH-E(2) may interact with other receptors still not identified, with the data presently available the direct effect of 4-OH-E(2) support the concept that metabolic activation of estrogens mediated by various cytochrome P450 complexes, generating through this pathway reactive intermediates that elicit direct genotoxic effects leading to transformation. This assumption was confirmed when we found that all the transformation phenotypes induced by 4-OH-E(2) were not abrogated when this compound was used in presence of the pure antiestrogen ICI. The novelty of these observations lies in the role of ERbeta in transformation and that this pathway can successfully bypassed by the estrogen metabolite 4-OH-E(2). Genomic DNA was analyzed for the detection of micro-satellite DNA polymorphism using 64 markers covering chromosomes (chr) 3, 11, 13 and 17. We have detected loss of heterozygosity (LOH) in ch13q12.2-12.3 (D13S893) and in ch17q21.1 (D17S800) in E(2), 2-OH-E(2), 4-OH-E(2), E(2) + ICI, E(2) + tamoxifen and BP-treated cells. LOH in ch17q21.1-21.2 (D17S806) was also observed in E(2), 4-OH-E(2), E(2)+ICI, E(2)+tamoxifen and BP-treated cells. MCF-10F cells treated with P or P+E(2) did not show LOH in the any of the markers studied. LOH was strongly associated with the invasion phenotype. Altogether our data indicate that E(2) and its metabolites induce in HBEC LOH in loci of chromosomes 13 and 17, that has been reported in primary breast cancer, that the changes are similar to those induced by the chemical carcinogen (BP) and that the genomic changes were not abrogated by antiestrogens.

Assessment of metabolites and AhR and CYP1A1 mRNA expression subsequent to prenatal exposure to inhaled benzo(a)pyrene

Few studies have focused on environmental aerosol contaminant, mechanistically-based, dose-related neurotoxicity with respect to development of the central nervous system. To fill this important data gap and to highlight possible mechanistic pathways, a study was undertaken to determine metabolite concentrations associated with the transplacental disposition of inhaled benzo(a)pyrene (B(a)P) and the resulting effects on the status of aryl hydrocarbon receptor (AhR), and cytochrome P450 1A1 (CYP1A1) mRNA in preweaning F1 generation animals. In this study, laparotomy on GD 8 was performed on timed-pregnant rats followed by dosing via nose-only exposure for 4h a day for 10 days (GD 11-GD 20) to three concentrations of a B(a)P: carbon black aerosol (25, 75 and 100 microg/m(3)). A dose-dependent decrease in birth index was observed in the B(a)P exposed group as compared to the controls (P<0.05). Analysis of cerebrocortical extracts from F1 generation pups revealed a dose-dependent (P<0.05) increase in total B(a)P metabolites. Analysis of cerebrocortical and hippocampal mRNA developmental expression profiles for AhR and CYP1A1 using 18sRNA as the internal standard, revealed that inhaled B(a)P upregulates AhR during the first postnatal month. The present study suggest that prenatal exposure to inhaled B(a)P upregulates hepatic aryl hydrocarbon receptor dependent mechanisms in the F1 generation. Hepatic upregulation of the aryl hydrocarbon receptor may modulate the potential for benzo(a)pyrene toxicity via the activation of cytochrome P450 and the subsequent deposition of lipophillic metabolites to developing central nervous system structures such as cerebral cortex and hippocampus.

Structural Differentiation of Diastereomeric Benzo[ghi]fluoranthene Adducts of Deoxyadenosine by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry and Postsource Decay

The product ion formation characteristics of four diastereomeric deoxyadenosine adducts formed by the reaction of the syn and anti diastereomers of trans-3,4-dihydroxy-5,5a-epoxy-3,4,5,5a-tetrahydrobenzo[ghi]fluoranthene are studied by matrix-assisted laser desorption ionization and postsource decay (PSD) to determine fragmentation pathways that may permit differentiation of their structures. The two adducts derived from each diol-epoxide with DNA differ in structure based on the cis/trans arrangement of the 3'-hydroxyl group on the benzo[ghi]fluoranthene (B[ghi]F) and the adenine base bound to the B[ghi]F 5a carbon. The two adduct diastereomers with the cis adenine-3'-hydroxyl configuration produce product ions at m/z 394 and m/z 510 formed by the loss of water that are not observed in the PSD spectra of the two trans isomers. The data suggest a mechanism of water loss that is initiated by a hydrogen-bonding interaction between the charge-bearing proton on the N1 atom and the 3'-hydroxyl oxygen on the polycyclic aromatic hydrocarbon (PAH). Fragmentation is initiated by the transfer of the adenine N1 proton from the nitrogen to the PAH 3'-hydroxyl oxygen and inductive cleavage of the C3-O(3) bond to form a benzylic carbocation on B[ghi]F. The proposed mechanism is supported by semiempirical molecular modeling calculations.

A proton-shuttle mechanism mediated by the porphyrin in benzene hydroxylation by cytochrome p450 enzymes

Benzene hydroxylation is a fundamental process in chemical catalysis. In nature, this reaction is catalyzed by the enzyme cytochrome P450 via oxygen transfer in a still debated mechanism of considerable complexity. The paper uses hybrid density functional calculations to elucidate the mechanisms by which benzene is converted to phenol, benzene oxide, and ketone, by the active species of the enzyme, the high-valent iron-oxo porphyrin species. The effects of the protein polarity and hydrogen-bonding donation to the active species are mimicked, as before (Ogliaro, F.; Cohen, S.; de Visser, S. P.; Shaik, S. J. Am. Chem. Soc. 2000, 122, 12892-12893). It is verified that the reaction does not proceed either by hydrogen abstraction or by initial electron transfer (Ortiz de Montellano, P. R. In Cytochrome P450: Structure, Mechanism and Biochemistry, 2nd ed.; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York, 1995; Chapter 8, pp 245-303). In accord with the latest experimental conclusions, the theoretical calculations show that the reactivity is an interplay of electrophilic and radicalar pathways, which involve an initial attack on the pi-system of the benzene to produce sigma-complexes (Korzekwa, K. R.; Swinney, D. C.; Trager, W. T. Biochemistry 1989, 28, 9019-9027). The dominant reaction channel is electrophilic and proceeds via the cationic sigma-complex,( 2)3, that involves an internal ion pair made from a cationic benzene moiety and an anionic iron porphyrin. The minor channel proceeds by intermediacy of the radical sigma-complex, (2)2, in which the benzene moiety is radicalar and the iron-porphyrin moiety is neutral. Ring closure in these intermediates produces the benzene oxide product ((2)4), which does not rearrange to phenol ((2)7) or cyclohexenone ((2)6). While such a rearrangement can occur post-enzymatically under physiological conditions by acid catalysis, the computations reveal a novel mechanism whereby the active species of the enzyme catalyzes directly the production of phenol and cyclohexenone. This enzymatic mechanism involves proton shuttles mediated by the porphyrin ring through the N-protonated intermediate, (2)5, which relays the proton either to the oxygen atom to form phenol ((2)7) or to the ortho-carbon atom to produce cyclohexenone product ((2)6). The formation of the phenol via this proton-shuttle mechanism will be competitive with the nonenzymatic conversion of benzene oxide to phenol by external acid catalysis. With the assumption that (2)5 is not fully thermalized, this novel mechanism would account also for the observation that there is a partial skeletal retention of the original hydrogen of the activated C-H bond, due to migration of the hydrogen from the site of hydroxylation to the adjacent carbon (so-called "NIH shift" (Jerina, D. M.; Daly, J. W. Science 1974, 185, 573-582)). Thus, in general, the computationally discovered mechanism of a porphyrin proton shuttle suggests thatthere is an enzymatic pathway that converts benzene directly to a phenol and ketone, in addition to nonenzymatic production of these species by conversion of arene oxide to phenol and ketone. The potential generality of protonated porphyrin intermediates in P450 chemistry is discussed in the light of the H/D exchange observed during some olefin epoxidation reactions (Groves, J. T.; Avaria-Neisser, G. E.; Fish, K. M.; Imachi, M.; Kuczkowski, R. J. Am. Chem. Soc. 1986, 108, 3837-3838) and the general observation of heme alkylation products (Kunze, K. L.; Mangold, B. L. K.; Wheeler, C.; Beilan, H. S.; Ortiz de Montellano, P. R. J. Biol. Chem. 1983, 258, 4202-4207). The competition, similarities, and differences between benzene oxidation viz. olefin epoxidation and alkanyl C-H hydroxylation are discussed, and comparison is made with relevant experimental and computational data. The dominance of low-spin reactivity in benzene hydroxylation viz. two-state reactivity (Shaik, S.; de Visser, S. P.; Ogliaro, F.; Schwarz, H.; Schröder, D. Curr. Opin. Chem. Biol. 2002, 6, 556-567) in olefin epoxidation and alkane hydroxylation is traced to the loss of benzene resonance energy during the bond activation step.

Ras transgene expression in TG-AC mice treated with benzo[a]pyrene

Transgene expression and skin tumorigenicity were investigated in transgenic TG-AC mice carrying the v-Ha-ras after treatment with benzo[a]pyrene (BP). Animals treated with 40 microg BP (x2/week/mouse) showed 100% tumor response after 25 weeks, as did 40% of the mice treated with 20 microg BP but 10 microg BP did not produce a tumor response. In the case of animals treated with 40 microg BP for 25 weeks, most of the tumors were proven to be carcinomas (80%, 4 out of 5 mice), and all tumors were shown to be positive in terms of transgene expression detected by in situ hybridization. These data suggest that BP was tumorigenic in a dose-dependent manner in TG-AC mice and that TG-AC mice were dependent on transgene expression during BP carcinogenesis.

Critical role of oxidative stress in estrogen-induced carcinogenesis

Mechanisms of estrogen-induced tumorigenesis in the target organ are not well understood. It has been suggested that oxidative stress resulting from metabolic activation of carcinogenic estrogens plays a critical role in estrogen-induced carcinogenesis. We tested this hypothesis by using an estrogen-induced hamster renal tumor model, a well established animal model of hormonal carcinogenesis. Hamsters were implanted with 17beta-estradiol (betaE2), 17alpha-estradiol (alphaE2), 17alpha-ethinylestradiol (alphaEE), menadione, a combination of alphaE2 and alphaEE, or a combination of alphaEE and menadione for 7 months. The group treated with betaE2 developed target organ specific kidney tumors. The kidneys of hamsters treated with alphaE2, alphaEE, or menadione alone did not show any gross evidence of tumor. Kidneys of hamsters treated with a combination of alphaE2 and alphaEE showed early signs of proliferation in the interstitial cells. Kidneys of hamsters treated with a combination of menadione and alphaEE showed foci of tumor with congested tubules and atrophic glomeruli. betaE2-treated tumor-bearing kidneys showed >2-fold increase in 8-iso-prostaglandin F(2alpha) (8-iso-PGF(2alpha)) levels compared with untreated controls. Kidneys of hamsters treated with a combination of menadione and alphaEE showed increased 8-iso-PGF(2alpha) levels compared with untreated controls, whereas no increase in 8-iso-PGF(2alpha) was detected in kidneys of alphaEE-treated group. A chemical known to produce oxidative stress or a potent estrogen with poor ability to produce oxidative stress, were nontumorigenic in hamsters, when given as single agents, but induced renal tumors, when given together. Thus, these data provide evidence that oxidant stress plays a crucial role in estrogen-induced carcinogenesis.

The combination of benzo[a]pyrene and ultraviolet A causes an in vivo time-related accumulation of DNA damage in mouse skin

Polycyclic aromatic hydrocarbons, including benzo[a]pyrene (BaP), are ubiquitous environmental carcinogens. BaP is metabolized in vivo to reactive intermediates that become covalently bound to DNA and form BaP-DNA adducts, an initial event in carcinogenesis. Ultraviolet A (UVA) synergizes with BaP to significantly enhance genetic damage and accelerate carcinogenic processes. This study was initiated to investigate in vivo cellular changes related to carcinogenesis induced by repeated exposures to BaP plus UVA. Simulated chronic exposure to an environmental carcinogen and sunlight was conducted through biweekly topical application of BaP followed 2 h later by UVA exposure over a 10 week period. BaP diol epoxide (BPDE)-DNA adducts were measured in vivo by immunohistochemistry using an anti-BPDE-DNA monoclonal antibody. Oxidative DNA damage was measured by the detection of 8-hydroxy-2'-deoxyguanosine (8-OHdG) formation using high-performance liquid chromatography. Alterations in the cell cycle that were relevant to carcinogenesis were revealed by changes in p53, as identified in vivo using a polyclonal anti-p53 antibody. We found that cells containing BPDE-DNA adducts and nuclear p53 expression significantly increased between 2 and 10 weeks of BaP-UVA treatment, whereas neither BPDE-DNA adducts nor significant changes in p53 were observed in untreated skin. Using regression analysis, oxidative 8-OHdG damage also showed a parallel increase over 2-10 weeks (r = 0.80). These results indicate that genetic damage caused by exposures to BaP plus UVA accumulates with time and increases the potential for inductive events leading to carcinogenesis and tumor formation.

Biomonitoring of polycyclic aromatic hydrocarbons in human urine

Measurement of polycyclic aromatic hydrocarbons (PAH) metabolites in human urine is the method of choice to determine occupational and/or environmental exposure of an individual to PAH, in particular, when multiple routes of exposure have to be taken into account. Requirements for methods of biomonitoring PAH metabolites in urine are presented. Studies using 1-hydroxypyrene or phenanthrene metabolites including its phenols and dihydrodiols are summarized. The role of these PAH metabolites as established biomarkers and also more recent developments of PAH biomonitoring are discussed.

Antioxidant functions of selected allium thiosulfinates and S-alk(en)yl-L-cysteine sulfoxides

Pure thiosulfinates, R-S(O)S-R (2), where R = Me (2a), Pr (2b), or All (2c), at levels up to 4 mM were not capable of scavenging hydrogen peroxide or superoxide anion. Relative to standard antioxidants (ascorbic acid, n-propyl gallate, butylated hydroxytoluene, Trolox, and reduced glutathione), these thiosulfinates were 1-3 orders of magnitude less efficient at reducing 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical, 0.5-2 orders of magnitude less efficient at quenching singlet oxygen, and about equally effective at scavenging hydroxyl radical. Generally, AllS(O)SAll (2c) was the most effective and PrS(O)SPr (2b) was the least effective thiosulfinate in these assays, except that MeS(O)SMe (2a) exhibited no quenching effect toward singlet oxygen. These thiosulfinates were also incapable at levels up to 0.1 mM (where they were toxic) of in vitro induction of quinone reductase (QR) in murine hepatoma (hepa 1c1c7) cells. However, S-1-propenyl-L-cysteine sulfoxide (isoalliin, 1a) and cycloalliin (3) induced QR in this system at 2 mM and 1 mM, respectively, although doubling of QR required levels of 10-15 mM.

A unified mechanism in the initiation of cancer

Estrogens are involved in the initiation of breast, prostate, and other kinds of human cancer. In this process, the endogenous estrogens, estrone and estradiol, are metabolized to 2-catechol estrogens (2-CE, major) and 4-CE (minor). If the 4-CEs are further oxidized to CE-3,4-quinones, they may react with DNA to form depurinating adducts at N-7 of guanine and N-3 of adenine, and generate apurinic sites. Similarly, the carcinogenic synthetic estrogen hexestrol, a hydrogenated derivative of diethylstilbestrol, is metabolized to its quinone, which reacts with DNA to form analogous depurinating adducts. This could be the primary critical event leading to oncogenic mutations and then initiation of cancer. Evidence supporting this hypothesis has been obtained from the human breast and animal models susceptible to estrogen-induced tumors, including the Syrian golden hamster kidney, ACI rat mammary gland, and Noble rat prostate. The oxidation of phenols to catechols and then to quinones is not only a mechanism of tumor initiation for natural and synthetic estrogens, but also for the leukemogen benzene. In fact, catechol, one of the metabolites of benzene, when oxidized to its quinone, reacts with DNA to form N7guanine and N3adenine depurinating adducts. Thus, a unifying mechanism, namely formation of catechol quinones and reaction with DNA, could initiate not only cancer by oxidation of specific endogenous estrogen metabolites, but also leukemia by oxidation of benzene.

Comparison of the chemopreventive potentials of melatonin and vitamin E plus selenium on 7,12-dimethylbenz[a]anthracene-induced inhibition of mouse liver antioxidant enzymes

Chemoprevention is a rapidly growing area of oncology that is identifying agents with a potentially preventive role in cancer. In this study, it was our goal to compare the chemopreventive effects of vitamin E plus selenium, and melatonin. Forty female mice were divided into four equal groups. The first group served as control. The second group had i.p. injections of 7,12-dimethylbenz[a]anthracene (DMBA) (20 mg/kg body weight) in corn oil for 21 days. The third group had the same procedure of DMBA injections as the second group and received vitamin E + selenium (90 microg + 1.8 microg/day), simultaneously. The fourth group had DMBA injections and melatonin (4.2 mg/kg body weight), simultaneously. DMBA alone caused significant inhibition of hepatic glutathione peroxidase (GSHPx), catalase (CAT), and superoxide dismutase (SOD) in the second group. In the third group, vitamin E + selenium restored DMBA-induced GSHPx inhibition significantly whereas CAT and SOD inhibition remained essentially unchanged. In the fourth group, melatonin not only significantly decreased DMBA-induced GSHPx inhibition but also fully reversed CAT and SOD inhibitions caused by DMBA. We speculate that melatonin alone provides better chemoprevention against DMBA-induced oxidative stress than the vitamin E+selenium combination.

Synergistic mechanisms in carcinogenesis by polycyclic aromatic hydrocarbons and by tobacco smoke: a bio-historical perspective with updates

B[a]P (benzo[a]pyrene) has been used as a prototype carcinogenic PAH since its isolation from coal tar in the 1930's. One of its diol epoxides, BPDE-2, is considered its ultimate carcinogen on the basis of its binding to DNA, mutagenicity and extreme pulmonary carcinogenicity in newborn mice. However, BPDE-1 has a similar binding to DNA and mutagenicity but it is not carcinogenic. In addition, BPDE-2 is a weak carcinogen relative to B[a]P when repeatedly applied to mouse skin, the conventional assay site. Its carcinogenicity is increased when applied once as an initiator followed repeatedly by a promoter. This indicates a major role for promotion in carcinogenesis by PAHs. Promotion itself is a 2-stage process, the second of which is selective propagation of the initiated cells. Persistent hyperplasia underlies selection by promoters. The non-carcinogenicity of BPDE-1 has yet to be resolved. PAHs have long been considered the main carcinogens of cigarette smoke but their concentration in the condensate is far too low to account by themselves for the production of skin tumors. The phenolic fraction does however have strong promotional activity when repeatedly applied to initiated mouse skin. Several constituents of cigarette smoke are co-carcinogenic when applied simultaneously with repeated applications of PAHs. Catechol is co-carcinogenic at concentrations found in the condensate. Since cigarette smoking involves protracted exposure to all the smoke constituents, co-carcinogenesis simulates its effects. Both procedures, however, indicate a major role for selection in carcinogenesis by cigarette smoke. That selection may operate on endogenous mutations as well as those induced by PAHs. There are indications that the nicotine-derived NNK which is a specific pulmonary carcinogen in animals contributes to smoking-induced lung cancer in man. Lung adenoma development by inhalation has been induced in mice by the gas phase of cigarette smoke. The role of selection has not been evaluated in either of these cases.

Efficient polycyclic aromatic hydrocarbons dihydroxylation in direct micellar systems

Optimization of whole-cell bioconversion of the polycyclic aromatic hydrocarbons (PAHs) anthracene, phenanthrene, and naphthalene to the enantiomerically pure corresponding cis-dihydroxydihydro derivatives by the Escherichia coli JM109 (pPS1778) recombinant strain, carrying the naphthalene dioxygenase and corresponding regulatory genes cloned from Pseudomonas fluorescens N3, in micellar systems, is presented. We show that direct microemulsion systems, where a nonionic surfactant such as 1.5% (v/v) Triton X-100 plus 0.6% to 1.0% (v/v) selected oils are able to solubilize the PAHs tested at relatively high concentrations (initial concentrations in the reaction medium > or =10 mM for naphthalene and phenanthrene and > or =2 mM for anthracene), and allow for more efficient substrate bioconversion. These media, while not affecting bacteria viability and performance, provide increased efficiency and final product yields (100% for naphthalene, >30% for anthracene, >60% for phenanthrene). The phase behavior of the direct microemulsion systems for the different substrates and oils utilized was monitored as a function of their volume fraction by light scattering experiments, and related to the bioconversion results. For anthracene and phenanthrene, the dihydroxylated products have an inhibitory effect on the conversion reactions, thus hindering complete turnover of the substrates. We ascertain that such inhibition is reversible because removal of the products formed allowed the process to start over at rates comparable to initial rates. To allow for complete conversion of the PAHs tested a stepwise or continuous separation of the product formed from the micellar reaction environment is being developed.

Oxidation of polychlorinated benzenes by genetically engineered CYP101 (cytochrome P450(cam))

Polychlorinated benzenes are recalcitrant environmental pollutants primarily because they are resistant to attack by dioxygenases commonly used by micro-organisms for the biodegradation of aromatic compounds. We have investigated the oxidation of polychlorinated benzenes by mutants of the haem mono-oxygenase CYP101 (cytochrome P450(cam)) from Pseudomonas putida with the aim of generating novel systems for their biodegradation. Wild-type CYP101 had low activity for the oxidation of dichlorobenzenes and trichlorobenzenes to the chlorophenols, but no products were detected for the heavily chlorinated benzenes. Increasing the active-site hydrophobicity with the Y96F mutation increased the activity up to 100-fold, and both pentachlorobenzene and hexachlorobenzene were oxidized slowly to pentachlorophenol. Decreasing the space available at the top of the active site with the F87W mutation to force the substrate to be bound closer to the haem resulted in a further 10-fold increase in activity with most substrates. Introducing the F98W mutation, also at the top of the active site, decreased the NADH-turnover rates but increased the coupling efficiencies, and > 90% coupling was observed for 1,3-dichlorobenzene and 1,3,5-trichlorobenzene with the F87W--Y96F--F98W mutant. The V247L mutation generally increased the NADH-turnover rates, and the F87W--Y96F--V247L mutant showed reasonably fast NADH turnover (229 min(-1)) with the highly insoluble pentachlorobenzene without the need for surfactants or organic cosolvents. As all chlorophenols are degraded by micro-organisms, novel biodegradation systems could be constructed in which CYP101 mutants convert the inert polychlorinated benzenes to the phenols, which are then readily degraded by natural pathways.

Estrogens as endogenous genotoxic agents--DNA adducts and mutations

Estrogens induce tumors in laboratory animals and have been associated with breast and uterine cancers in humans. In relation to the role of estrogens in the induction of cancer, we examine formation of DNA adducts by reactive electrophilic estrogen metabolites, formation of reactive oxygen species by estrogens and the resulting indirect DNA damage by these oxidants, and, finally, genomic and gene mutations induced by estrogens. Quinone intermediates derived by oxidation of the catechol estrogens 4-hydroxyestradiol or 4-hydroxyestrone may react with purine bases of DNA to form depurinating adducts that generate highly mutagenic apurinic sites. In contrast, quinones of 2-hydroxylated estrogens produce less harmful, stable DNA adducts. The catechol estrogen metabolites may also generate potentially mutagenic oxygen radicals by metabolic redox cycling or other mechanisms. Several types of indirect DNA damage are caused by estrogen-induced oxidants, such as oxidized DNA bases, DNA strand breakage, and adduct formation by reactive aldehydes derived from lipid hydroperoxides. Estradiol and the synthetic estrogen diethylstilbestrol also induce numerical and structural chromosomal aberrations and several types of gene mutations in cells in culture and in vivo. In conclusion, estrogens, including the natural hormones estradiol and estrone, must be considered genotoxic carcinogens on the basis of the evidence outlined in this chapter.

Impact of cellular metabolism on the biological effects of benzo[a]pyrene and related hydrocarbons

Polycyclic aromatic hydrocarbons are ubiquitous contaminants in the environment. Benzo[a]pyrene (BaP), a prototypical member of this class of chemicals, has been extensively studied for its toxic effects in laboratory animals and human populations. BaP toxicity is often mediated by oxidative metabolism to reactive intermediates that interact with macromolecules leading to alterations in target cell structure and function. More recent evidence suggests that disruption of cellular signaling pathways involved in the regulation of growth and differentiation contribute significantly to the toxicity of BaP and its metabolites. This review summarizes recent advances in our understanding of biological mechanisms of BaP toxicity at the molecular level, and the role of metabolic intermediates in carcinogenesis, atherogenesis, and teratogenesis.

Unaltered expression of multidrug resistance transporters in polycyclic aromatic hydrocarbon-resistant rat liver cells

Rat liver epithelial cells resistant to the chemical carcinogen 3MC, termed F258/3MC cells and generated by long-term exposure of parental F258 cells to the PAH, were characterized, especially with respect to expression of multidrug resistance transporters such as P-glycoprotein, MRP1 and MRP2. F258/3MC cells were found to be cross-resistant to other PAHs such as BP and dimethylbenz(a)anthracene but remained sensitive to known substrates of multidrug resistance efflux pumps such as doxorubicin and vincristine. They did not display either decreased cellular PAH accumulation or increased PAH efflux. In addition, P-glycoprotein and MRP2 mRNA levels were not, or only barely detected, in F258/3MC cells and in their parental counterparts whereas these PAH-resistant and sensitive cells showed closed levels of MRP1 mRNAs and activity. Moreover, P-gp- and MRP1-overexpressing cells were shown to display similar accumulation and efflux of BP than those found in P-gp- and MRP1-negative control cells. These data therefore suggest that multidrug resistance transporters do not contribute to PAH resistance in PAH-selected liver cells.

Peroxidases

The family of human peroxidases described includes myeloperoxidase, eosinophil peroxidase, uterine peroxidase, lactoperoxidase, salivary peroxidase, thyroid peroxidase and prostaglandin H1/2 synthases. The chemical identity of the peroxidase compound I and II oxidation states for the different peroxidases are compared. The identities of the distal and proximal amino acids of the catalytic site of each peroxidase are also compared. The gene characteristics and chromosomal location of the human peroxidase family have been tabulated and their molecular evolution discussed. Myeloperoxidase polymorphism and the mutations identified so far that affect myeloperoxidase activity and modulate their susceptibility to disease is described. The mechanisms for hypohalous and hypothiocyanate formation by the various peroxidases have been compared. The cellular function of the peroxidases and their hypohalites have been described as well as their inflammatory effects. The peroxidase catalysed cooxidation of drugs and xenobiotics that results in oxygen activation by redox cycling has been included. Low-density lipoprotein oxidation (initiation of atherosclerosis), chemical carcinogenesis, idiosyncratic drug reactions (e.g. agranulocytosis), liver necrosis or teratogenicity initiated by the cooxidation of endogenous substrates, plasma amino acids, drugs and xenobiotics catalysed by peroxidases or peroxidase containing cells have also been compared. Finally, peroxidase inhibitors currently in use for treating various diseases are described.

Monoclonal antibody--gold biosensor chips for detection of depurinating carcinogen--DNA adducts by fluorescence line-narrowing spectroscopy

A new direct readout methodology for detection and quantitation of fluorescent carcinogen-DNA adducts is described. It combines the binding specificity of an immobilized monoclonal antibody (MAb) with high-resolution, low-temperature fluorescence spectroscopy. The MAb, which is covalently bound to a gold surface via a chemisorbed disulfide coupling agent, binds the adduct of interest in an aqueous sample. Laser-induced fluorescence under nonline narrowing (FNLN) and line-narrowing (FLN) conditions was used to detect (benzo[a]pyren-6-yl)guanine (BP-6-N7Gua) bound to immobilized MAb. At room temperature, the BP-6-N7Gua fluorescence was not detected, most likely because of quenching by the gold surface and/or efficient dynamical quenching. However, fluorescence was observed at room temperature when the surface was covered with a thin layer of glycerol, and possible reasons for the fluorescence enhancement are considered. Lowering of the temperature to 77 K led to nearly an order of magnitude increase in fluorescence intensity. Highly structured FLN spectra obtained at 4.2 K allowed for definitive adduct identification. The potential of this methodology for risk assessments of individuals exposed to polycyclic aromatic hydrocarbons is discussed.

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.

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

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

Evaluation of fecal mutagenicity and colorectal cancer risk

Colorectal cancer is one of the most common internal malignancies in Western society. The cause of this disease appears to be multifactorial and involves genetic as well as environmental aspects. The human colon is continuously exposed to a complex mixture of compounds, which is either of direct dietary origin or the result of digestive, microbial and excretory processes. In order to establish the mutagenic burden of the colorectal mucosa, analysis of specific compounds in feces is usually preferred. Alternatively, the mutagenic potency of fecal extracts has been determined, but the interpretation of these more integrative measurements is hampered by methodological shortcomings. In this review, we focus on exposure of the large bowel to five different classes of fecal mutagens that have previously been related to colorectal cancer risk. These include heterocyclic aromatic amines (HCA) and polycyclic aromatic hydrocarbons (PAH), two exogenous factors that are predominantly ingested as pyrolysis products present in food and (partially) excreted in the feces. Additionally, we discuss N-nitroso-compounds, fecapentaenes and bile acids, all fecal constituents (mainly) of endogenous origin. The mutagenic and carcinogenic potency of the above mentioned compounds as well as their presence in feces, proposed mode of action and potential role in the initiation and promotion of human colorectal cancer are discussed. The combined results from in vitro and in vivo research unequivocally demonstrate that these classes of compounds comprise potent mutagens that induce many different forms of genetic damage and that particularly bile acids and fecapentaenes may also affect the carcinogenic process by epigenetic mechanisms. Large inter-individual differences in levels of exposures have been reported, including those in a range where considerable genetic damage can be expected based on evidence from animal studies. Particularly, however, exposure profiles of PAH and N-nitroso compounds (NOC) have to be more accurately established to come to a risk evaluation. Moreover, lack of human studies and inconsistency between epidemiological data make it impossible to describe colorectal cancer risk as a result of specific exposures in quantitative terms, or even to indicate the relative importance of the mutagens discussed. Particularly, the polymorphisms of genes involved in the metabolism of heterocyclic amines are important determinants of carcinogenic risk. However, the present knowledge of gene-environment interactions with regard to colorectal cancer risk is rather limited. We expect that the introduction of DNA chip technology in colorectal cancer epidemiology will offer new opportunities to identify combinations of exposures and genetic polymorphisms that relate to increased cancer risk. This knowledge will enable us to improve epidemiological study design and statistical power in future research.

8,9-dihydroxy-8,9-dihydrodibenzo[a,l]pyrene is a potent morphological cell-transforming agent in C3H10T(1)/(2)Cl8 mouse embryo fibroblasts in the absence of detectable stable covalent DNA adducts

The comparative genotoxic effects of racemic trans-8,9-dihydroxy-8, 9-dihydrodibenzo[a,l]pyrene (trans-DB[a,l]P-8,9-diol), the metabolic K-region dihydrodiol of dibenzo[a,l] pyrene (DB[a,l]P) (dibenzo[def, p]chrysene) and DB[a,l]P in transformable mouse embryo C3H10T(1)/(2)Cl8 (C3H10T(1)/(2)) fibroblasts was investigated. The C3H10T(1)/(2) mouse embryo morphological cell-transforming activities of these polycyclic aromatic hydrocarbons (PAHs) were assayed using concentration-response studies. At concentrations of 33 nM and above both trans-DB[a,l]P-8,9-diol and DB[a,l]P produced significant (and similar) numbers of type II and III foci per dish and numbers of dishes with type II and II foci. Concomitant cytotoxicity studies revealed a reduction in colony survival of approximately 25% up to 198 nM for both PAHs. DNA adducts of trans-DB[a,l]P-8,9-diol and DB[a,l]P in C3H10T(1)/(2) cells were analyzed by a (32)P-post-labeling TLC/HPLC method. No adducts were observed in the DNA of C3H10T(1)/(2) cells treated with trans-DB[a, l]P-8,9-diol at concentrations that induced morphological cell transformation. Under the same exposure and chromatographic conditions, DNA adducts of deoxyadenosine and deoxyguanosine derived from the fjord region anti-DB[a,l]P-11,12-diol-13,14-epoxide and syn-DB[a,l]P-11,12-diol-13,14-epoxide were observed in the DNA of DB[a,l]P-treated cells. These results indicate that trans-DB[a,l]P-8, 9-diol has intrinsic genotoxic activity equal to that of DB[a,l]P, based on morphological cell transformation of mouse embryo fibroblasts. The activity of trans-DB[a,l]P-8,9-diol is apparently not associated with the formation of observable stable covalent DNA adducts. These results suggest that under appropriate conditions, trans-DB[a,l]P-8,9-diol may serve as an intermediate in the genotoxicity of DB[a,l]P.

LacI mutation spectra following benzo[a]pyrene treatment of Big Blue mice

The mutation spectrum of the lacI gene from the liver of C57Bl6 Big Blue transgenic mice treated with benzo[a]pyrene (B[a]P) has been compared with the spectrum of spontaneous mutations observed in the liver of untreated Big Blue mice. Mice were treated with B[a]P for 3 days followed by a partial hepatectomy one day after the last injection. Liver tissue was removed for analysis at hepatectomy and, again, 3 days later at the time of sacrifice. Earlier, we reported that the lacI mutant frequency in these B[a]P-treated mice was elevated in the liver both at the time of hepatectomy and at sacrifice; however, a statistically significant increase in the mutant frequency was observed only at sacrifice. In this study, the DNA sequence spectra of lacI mutations observed in the liver of B[a]P-treated Big Blue mice at hepatectomy and at time of sacrifice were compared with each other and with the spectrum of spontaneous liver mutations. No differences were observed between the two B[a]P-treatment spectra. However, mutation frequencies of both GC-->TA and GC-->CG at the time of hepatectomy and at sacrifice were significantly elevated compared with the spontaneous frequency of these same transversions. Also, the frequency of AT-->TA transversions was significantly higher than the spontaneous frequency at the time of hepatectomy but not at sacrifice. The frequency of all other classes of mutations scored was not significantly different from the frequency of these same events in the spontaneous spectra. These data support the view that B[a]P treatment results in the induction of GC-->TA and GC-->CG transversions within 1 day of the last injection and they provide insights regarding the relative roles of benzo[a]pyrene-7,8-diol-9, 10-epoxide and radical cations of B[a]P in B[a]P-induced mutagenesis in vivo. Finally, these data provide evidence for B[a]P-induced mutagenesis under conditions where no statistical increase in mutant frequency could be shown.

On-line identification of depurinating DNA adducts in human urine by capillary electrophoresis--fluorescence line narrowing spectroscopy

The benzo[a]pyrene (BP)-derived 7-(benzo[a]pyren-6-yl)guanine (BP-6-N7Gua) depurinating one-electron oxidation adduct was identified in the urine extracts of coal-smoke-exposed humans for the first time. Urine samples were prepared by solid-phase extraction and reversed-phase high-performance liquid chromatography. Subsequently, the BP-6-N7Gua adduct was identified on-line with capillary electrophoresis-- fluorescence line narrowing spectroscopy (CE-FLNS) at 4.2 K. The daily excretion of BP-6-N7Gua in human urine of individuals exposed to coal smoke was approximately 226 pmol per micromol of creatinine. Due to the high level of excretion we propose that BP-6-N7Gua adducts found in urine could serve as effective biomarkers for risk assessment of BP exposure. The results demonstrate that CE-FLNS allows for on-line separation and DNA adducts identification in complex fluid extracts.

Involvement of p53 in X-ray induced intrachromosomal recombination in mice

The tumor suppressor gene Trp53 (also known as p53) is the most frequently mutated gene in human cancers. p53 is induced in response to DNA damage and effects a G(1) cell cycle arrest. It is believed that p53 plays a key role in maintaining genomic integrity following exposure to DNA-damaging agents. We determined the frequency of spontaneous and DNA damage-induced homologous intrachromosomal recombination in p53-deficient mouse embryos. Homologous intrachromosomal recombination events resulting in deletions at the pink eyed unstable (p(un)) locus result in reversion to the p gene. Reversions occurring in embryonic premelanocytes give rise to black spots on the gray fur of the offspring. Pregnant C57BL/6J p(un)/p(un) p53(+/-) mice were exposed to X-rays (1 Gy) or administered benzo¿apyrene (B¿aP; 30 or 150 mg/kg i.p.) 10 days after conception. Frequencies of spontaneous p(un) reversions in p53(-/-) and p53(+/-) animals were not significantly different compared with their wild-type littermates. X-ray treatment increased the recombination frequency in wild-type and p53(+/-), but surprisingly not in p53(-/-) offspring. In contrast, B¿aP treatment caused a dose-dependent increase in p(un) reversion frequencies in all three genotypes. Western blot analysis of embryos indicated that p53 protein levels increased approximately 3-fold following X-ray treatment, while B¿aP had no effect on p53 expression. These results are in agreement with the proposal that p53 is involved in the DNA damage response following X-ray exposure and suggest that X-ray-induced double-strand breaks are processed differently in p53(-/-) animals.

On-line identification of diastereomeric dibenzo[a,l]pyrene diol epoxide-derived deoxyadenosine adducts by capillary electrophoresis-fluorescence line-narrowing and non-line narrowing spectroscopy

A capillary electrophoretic method for the separation and on-line identification of closely related analytes using low-temperature fluorescence spectroscopy is reported for the eight diastereomeric deoxyadenosine (dA) adducts derived from dibenzo[a,l]pyrene diol epoxide (DB[a,l]PDE). Electrophoretic separation of stereoisomers was accomplished by application of a mixed surfactant buffer [dioctyl sulfosuccinate (DOSS) and Brij-S], which was below the critical micelle concentration (CMC) due to the high concentration (approximately 25%) of organic solvent. Addition of multiple surfactant additives to the separation buffer provided electrophoretic resolution, which was unattainable under single surfactant conditions. It is shown that the CE-separated analyte zones could be identified on-line via low-temperature (4.2 K) fluorescence non-line narrowing and fluorescence line-narrowing (FLN) spectroscopy. In addition, it was determined that in CE buffer trans-syn-,cis-syn- and cis-anti-DB[a,l]PDE-14-N6dA diastereomeric adducts exist mostly with the -dA and DB[a,l]P moiety in an "open"-type conformation while the trans-anti-DB[a,l]PDE-14-N6dA adducts exist in two different conformations whose relative distribution depends on matrix composition. The above conformations have also been revealed by selective laser excitation. Thus, the low-temperature methodology not only provides fingerprint structure via vibrationally resolved 4.2 K fluorescence spectra for adduct identification, but also provides conformational information on the spatial relationship of the carcinogen and dA moiety. These results, taken together with those for DB[a,l]P-DNA adducts formed in standard glasses and mouse epidermis exposed to DB[a,l]P, support our earlier findings that DB[a,l]P-derived adducts exist in different conformations [Jankowiak et al., Chem. Res. Toxicol. 11 (1998) 674]. Therefore, the combination of the separation power of CE and spectral selectivity of low-temperature fluorescence spectroscopy at NLN and FLN conditions provides a powerful methodology which should prove useful for identification of closely related DNA adducts formed at low levels in biological systems.

Redox regulation of c-Ha-ras and osteopontin signaling in vascular smooth muscle cells: implications in chemical atherogenesis

Reduction/oxidation (redox) reactions play a central role in the regulation of vascular cell functions. Recent studies in this laboratory have identified c-Ha-ras and osteopontin genes as critical molecular targets during oxidant-induced atherogenesis. This review focuses on the deregulation of gene transcription by redox-activated trans-acting factors after benzo(a)pyrene challenge and the modulation of extracellular matrix signaling in vascular smooth muscle cells by allylamine-induced oxidative injury. The induction of atherogenic vascular smooth muscle cell phenotypes by chemical injury exhibits remarkable parallels with those seen in other forms of atherogenesis.

Synthesis of (+/-)-trans-7,8-Dihydrodiol of 6-Fluoro-benzo[a]pyrene via Hydroxyl-Directed Regioselective Functionalization of Substituted Pyrene

Synthesis of (+/-)-trans-7,8-dihydroxy-6-fluoro-7,8-dihydrobenzo[a]pyrene, the metabolite from 6-fluoro-benzo[a]pyrene, is described. Position 6 of 7,8,9,10-tetrahydrobenzo[a]pyren-7-ol (1) was functionalized by bromination with N-bromosaccharin. Regioselectivity in the bromination is thought to derive from a substrate-reagent hydrogen bond. NMR evidence is offered to support this model. The 6-bromo derivative 2 was subjected to dehydration followed by bromine-lithium exchange. Quenching the lithio intermediate with NFSi afforded the 6-fluoro derivative 4. Prévost reaction on the 7,8 double bond resulted in the trans dibenzoate 5 (established by comparison to a cis derivative prepared by osmium tetroxide cis dihydroxylation). Introduction of the 9,10 double bond by a bromination-dehydrobromination procedure, followed by hydrolysis, gave racemic trans-7,8-dihydrodiol 7. Resolution of the enantiomers was achieved by chiral HPLC, and the absolute configurations of the early and late eluting isomers were determined through CD spectroscopy by comparison with the metabolically obtained (7R,8R)-dihydrodiol.

Tumor-initiating activity of the (+)-(S,S)- and (-)-(R,R)- enantiomers of trans-11,12-dihydroxy-11,12-dihydrodibenzo[a,l]pyrene in mouse skin

A single administration of enantiomerically pure 11,12-dihydrodiols of dibenzo[a,l]pyrene (DB[a,l]P) on the back of NMRI mice and subsequent chronic treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) (initiation/promotion assay) revealed strikingly different carcinogenic activities of both enantiomers. Tumor-initiating activity of (-)-(11R,12R)-DB[a,l]P-dihydrodiol, which is the metabolic precursor of the (-)-anti-(11R,12S)-dihydrodiol (13S,14R)-epoxide, was exceptionally higher than the corresponding effect of (+)-(11S,12S)-DB[a,l]P-dihydrodiol, the metabolic precursor of (+)-syn-(11S,12R)-dihydrodiol (13S,14R)-epoxide. After topical application of 10 nmol (-)-11,12-dihydrodiol and promotion with TPA twice weekly for a further 18 weeks 93% of treated animals exhibited four to five tumors. In contrast, no neoplasms were observed after treatment with 10 nmol (+)-11,12-dihydrodiol, whereas in the group exposed to 20 nmol of this enantiomer only 13% of mice developed neoplasms (0.1 tumors/survivor). For DB[a,l]P, considered as the most potent carcinogenic polycyclic aromatic hydrocarbon to date, stereoselective formation of (+)-syn- and (-)-anti-11,12-dihydrodiol 13,14-epoxides via the corresponding enantiomeric 11,12-dihydrodiols has been found to be the principal metabolic activation pathway leading to DNA adducts and mutagenicity. Our study demonstrates that the striking difference in carcinogenic activity in mouse skin of (+)-(11S,12S)- and (-)-(11R,12R)-DB[a,l]P-dihydrodiol convincingly reflects the different genotoxicity, i.e. DNA binding and mutagenicity, of both enantiomers observed earlier.

Methylcholanthrene causes increased thymocyte apoptosis

The polycyclic aromatic hydrocarbon (PAH), methylcholanthrene (MCA), is a well studied carcinogen and a teratogen. MCA and other PAH cause immune suppression of B cell and T cell responses in mice and MCA had been reported to induce thymus atrophy. Here we show that MCA treatment causes thymus atrophy in adrenalectomized mice and in C57BL/6 and DBA/2 mice which differ in aryl hydrocarbon receptor (AhR) expression. This indicates that MCA-mediated thymus atrophy is mediated, at least in part, by glucocorticoid hormone receptor- and aryl hydrocarbon receptor-independent mechanisms. Assay of thymocytes, both in situ and ex vivo, demonstrate that MCA induces thymocyte apoptosis. Apoptotic thymocytes can be found within or adjacent to thymic Mphi, suggesting rapid phagocytosis. Mice that are deficient in tumor necrosis factor-alpha receptor-1 or p53, or that overexpress bcl-2 are susceptible to MCA-mediated thymus atrophy.

Metabolism and activation of cyclopenta polycyclic aromatic hydrocarbons in liver tissue from rats and humans

The metabolism of radiolabelled benz(j)aceanthrylene (B(j)A) was studied by high performance liquid chromatography (HPLC) using suspensions of hepatocytes and liver microsomes from control- or Aroclor 1254 (PCB)-treated rats, or with human liver microsomes (five different donors) as activation systems. The major metabolites formed in hepatocytes were sulfate conjugates, indicating that sulfation is an important detoxication pathway for B(j)A. In incubations with B(j)A and rat or human liver microsomes, the major metabolite formed was B(j)A-1,2-diol. Studies with rat liver microsomes using antibodies (Ab) towards either P4501A1, 1A2 or 3A2, resulted in approximately 30% reduction in covalent binding with all Ab-using microsomes from control animals, whereas with microsomes from PCB-treated animals an 85% reduction was observed using Ab towards P4501A2, and only minor reductions were observed with 1A1 or 3A2. When compared to B(j)A and benzo(a)pyrene (B(a)P), benz(1)aceanthrylene (B(l)A) caused higher numbers of revertants in the Salmonella assay when plated with rat liver microsomes from control animals or human liver microsomes. The total DNA adduct levels in hepatocytes from control animals after 2 h exposure to 30 micrograms/ml (120 microM) B(j)A or B(l)A, as measured by the 32P-postlabelling technique, were 3.8 +/- 1.5 and 10.1 +/- 5.8 fmol/microgram DNA, respectively. PCB-treatment decreased the total level of B(j)A adducts slightly (1.8 +/- 0.5 fmol/microgram DNA), whereas in contrast the level of B(1)A adducts was increased (24.5 +/- 20.1 fmol/microgram DNA). The major DNA adduct formed in control hepatocytes exposed to B(j)A co-chromatographed with B(j)A-1,2-oxide, which also appeared to be the major adduct formed when rat or human liver microsomes were co-incubated with calf thymus DNA. The total DNA adduct levels in the modified calf thymus DNA after 30 min exposure to 30 micrograms/ml B(j)A, B(l)A or B(a)P using rat liver microsomes form control animals, were 3.6, 66.3 and 1.4 fmol/microgram DNA, respectively. These levels increased to 22.7, 93.3 and 7.4 fmol/microgram DNA, respectively, using microsomes from PCB-treated animals. With human liver microsomes, the total DNA adduct levels after exposure to B(j)A, B(l)A or B(a)P, ranged between 0.4-1.0, 0.3-4.3, and 0.1-0.3 fmol/microgram DNA, respectively. Overall, the present data supports the notion that oxidation at the cyclopenta-ring is an important activation pathway for B(j)A, and indicate that the activation mechanism for B(j)A is similar in rat and human liver tissue.

Effect of cancer chemopreventive agents on microsome-mediated DNA adduction of the breast carcinogen dibenzo[a,l]pyrene

Due to the large and expanding number of potential cancer chemopreventive agents, there is an increasing need for short term tests to study the efficacy and mechanisms of these agents. In this study, we have employed a microsome-mediated test system to study the effect of several suspected chemopreventive agents on the DNA adduct formation capacity of the potent mammary carcinogen, dibenzo[a,l]pyrene (DBP). Bioactivation of DBP by Aroclor 1254-induced rat liver microsomes in the presence of calf thymus DNA (300 microg/ml) resulted in the formation of one major and six other prominent DNA adducts (324 adducts/10(7) nucleotides). These adducts were previously determined to be deoxyadenosine (dA) and deoxyanosine (dG)-derivatives of both anti- and syn-DBP-11,12-diol-13,14-epoxides (DBPDE). Intervention with ellagic acid, chlorophyllin, benzyl isocyanate (BIC), oltipraz or genistein (150 microM) strongly diminished DBP-DNA adduction by > or = 75%. Linoleic acid, curcumin and butylated hydroxytoluene (BHT) also significantly inhibited DBP DNA adduction (26-46%) while N-acetylcysteine (NAC) had no effect. Moreover, nonenzymatic studies with anti- and syn-DBPDE isomers revealed that chlorophyllin, ellagic acid, BIC and BHT may be inhibiting DBP-DNA adduction in an enzymatic-independent manner since these agents diminished DBPDE-DNA adduction by 30-75%. Genistein, oltipraz and curcumin did not diminish DBPDE-DNA adduction and therefore most likely require the presence of the microsomal subcellular fraction to inhibit DBP-DNA adduction.

Formation of stable adducts and absence of depurinating DNA adducts in cells and DNA treated with the potent carcinogen dibenzo[a,l]pyrene or its diol epoxides

Polycyclic aromatic hydrocarbons (PAH) are widespread environmental contaminants, and some are potent carcinogens in rodents. Carcinogenic PAH are activated in cells to metabolites that react with DNA to form stable covalent DNA adducts. It has been proposed [Cavalieri, E. L. & Roger, E. G. (1995) Xenobiotica 25, 677-688] that unstable DNA adducts are also formed and that apurinic sites in the DNA resulting from unstable PAH adducts play a key role in the initiation of cancer. The potent carcinogen dibenzo[a,l]pyrene (DB[a, l]P) is activated in cells to (+)-syn- and (-)-anti-DB[a,l]P-11, 12-diol-13,14-epoxide (DB[a,l]PDE), which have been shown to form stable adducts with DNA. To evaluate the importance of unstable PAH adducts, we compared stable adduct formation to apurinic site formation. Stable DB[a,l]PDE adducts were determined by 33P-postlabeling and HPLC. To measure apurinic sites they were converted to strand breaks, and these were monitored by examining the integrity of a particular restriction fragment of the dihydrofolate reductase gene. The method easily detected apurinic sites resulting from methylation by treatment of cells or DNA with dimethyl sulfate or from reaction of DNA with DB[a,l]P in the presence of horseradish peroxidase. We estimate the method could detect 0.1 apurinic site in the 14-kb fragment examined. However, apurinic sites were below our limit of detection in DNA treated directly with (+)-syn- or (-)-anti-DB[a,l]PDE or in DNA from Chinese hamster ovary B11 cells so treated, although in these samples the frequency of stable adducts ranged from 3 to 10 per 14 kb. We also treated the human mammary carcinoma cell line MCF-7 with DB[a,l]P and again could not detect significant amounts of unstable adducts. These results indicate that the proportion of stable adducts formed by DB[a,l]P activated in cells and its diol epoxides is greater than 99% and suggest a predominant role for stable DNA adducts in the carcinogenic activity of DB[a,l]P.

Molecular origin of cancer: catechol estrogen-3,4-quinones as endogenous tumor initiators

Cancer is a disease that begins with mutation of critical genes: oncogenes and tumor suppressor genes. Our research on carcinogenic aromatic hydrocarbons indicates that depurinating hydrocarbon-DNA adducts generate oncogenic mutations found in mouse skin papillomas (Proc. Natl. Acad. Sci. USA 92:10422, 1995). These mutations arise by mis-replication of unrepaired apurinic sites derived from the loss of depurinating adducts. This relationship led us to postulate that oxidation of the carcinogenic 4-hydroxy catechol estrogens (CE) of estrone (E1) and estradiol (E2) to catechol estrogen-3,4-quinones (CE-3, 4-Q) results in electrophilic intermediates that covalently bind to DNA to form depurinating adducts. The resultant apurinic sites in critical genes can generate mutations that may initiate various human cancers. The noncarcinogenic 2-hydroxy CE are oxidized to CE-2,3-Q and form only stable DNA adducts. As reported here, the CE-3,4-Q were bound to DNA in vitro to form the depurinating adduct 4-OHE1(E2)-1(alpha,beta)-N7Gua at 59-213 micromol/mol DNA-phosphate whereas the level of stable adducts was 0.1 micromol/mol DNA-phosphate. In female Sprague-Dawley rats treated by intramammillary injection of E2-3,4-Q (200 nmol) at four mammary glands, the mammary tissue contained 2.3 micromol 4-OHE2-1(alpha, beta)-N7Gua/molDNA-phosphate. When 4-OHE1(E2) were activated by horseradish peroxidase, lactoperoxidase, or cytochrome P450, 87-440 micromol of 4-OHE1(E2)-1(alpha, beta)-N7Gua was formed. After treatment with 4-OHE2, rat mammary tissue contained 1.4 micromol of adduct/mol DNA-phosphate. In each case, the level of stable adducts was negligible. These results, complemented by other data, strongly support the hypothesis that CE-3,4-Q are endogenous tumor initiators.

Cigarette smoke radicals and the role of free radicals in chemical carcinogenicity

This article consists of two parts: a brief overview of the ways in which free radicals can be involved in chemical carcinogenesis, and a review of cigarette smoke chemistry. Carcinogenesis is generally agreed to involve at least three stages: initiation, promotion, and progression. It is suggested that radicals sometimes are involved in the initiation step, either in the oxidative activation of a procarcinogen (such as benzo[a]pyrene) to its carcinogenic form or in the binding of the carcinogenic species to DNA, or both. The fraction of initiation events that involve radicals, as opposed to two-electron steps, is not known, but radicals probably are involved in a substantial number, although probably not a majority, of cancer initiation reactions. Promotion always involves radicals, at least to some extent. Progression probably does not normally involve radicals. The second part of this article reviews the molecular mechanisms involved in cigarette-induced tumors, particularly by aqueous cigarette tar (ACT) extracts and by a model of these solutions, aged solutions of catechol. ACT solutions as well as aged solutions of catechol contain a quinone-hydroquinone-semiquinone system that can reduce oxygen to produce superoxide and hence hydrogen peroxide and the hydroxyl radical. Both the cigarette tar radical and the catechol-derived radical can penetrate viable cells, bind to DNA, and cause nicks.