Determination of benzo[a]pyrene- and 7,12-dimethylbenz[a]anthracene-DNA adducts formed in rat mammary glands

Both 7,12-dimethylbenz[a]anthracene (DMBA) and benzo[a]pyrene (BP) are carcinogenic in the rat mammary gland. The depurinating and stable adducts of DMBA and BP formed in vitro and in mouse skin were previously identified and quantitated. Identification and quantitation of the depurinating and stable DNA adducts of DMBA and identification of the depurinating adducts of BP formed in rat mammary glands in the 24 h after intramammillary injection of DMBA or BP are reported in this paper. The depurinating adducts of DMBA, which constitute 52% of all adducts detected, are DMBA bound at the 12-methyl group to the N-7 of adenine (Ade) or guanine (Gua), namely, 7-methylbenz[a]anthracene (MBA)-12-CH2-N7Ade (39%) and 7-MBA-12-CH2-N7Gua (13%). All of the stable adducts were formed from the diol epoxide(s) of DMBA. Depurinating adducts of BP with guanine, namely, 8-(BP-6-yl)-guanine (BP-6-C8Gua) and BP-6-N7Gua, were identified in rat mammary glands treated with BP. The major stable adduct, formed via the diol epoxide pathway, BP-diol epoxide-10-N2dG, accounted for over 64% of all the stable adducts. Three other BP-DNA stable adducts remain unidentified. Thus, rat mammary cells form depurinating adducts of DMBA and BP predominantly via their radical cations and stable adducts via the diol epoxides.

Conformational studies of stereoisomeric tetrols derived from syn- and anti-dibenzo[a,l]pyrene diol epoxides

An understanding of the conformational behavior of the stereoisomeric tetrols at the 11,12,13,14-positions of dibenzo[a,l]pyrene (DB[a,l]P) is essential for the spectroscopic identification of DNA adducts derived from the biologically highly active fjord region syn- and anti-DB[a,l]P-11,12-diol 13,14-epoxides. Conformational effects are expected to play an important role in DNA-DB[a,l]P diol epoxide reactivity, base-sequence specificity, and conformation dependent repair. The results of conformational studies on trans-anti-, cis-anti-, and cis-syn-DB[a,l]P tetrol isomers are presented and compared to the results obtained previously for trans-syn-DB[a,l]P tetrol (Carcinogenesis 17, 829-837, 1996). Molecular mechanics, dynamical simulations, and semiempirical calculations of electronic transitions are used to interpret the low-temperature fluorescence spectra and 1H NMR data. Molecular dynamics simulations (in vacuo) identified two conformers (I and II) for each of the tetrol isomers; in all conformations the aromatic ring system is severely distorted. Fluorescence line-narrowing (FLN) spectroscopy identified two distinct conformational species for the trans-anti isomer, one occurring in ethanol and the other occurring in a glycerol/water matrix. The corresponding structures are assigned based on the S1<--S0 transition energies calculated for conformers I and II, respectively. 1H NMR spectroscopy confirmed the structure of conformer I at room temperature. In contrast to trans-syn-DB[a,l]P tetrol (where the major conformation was identified as a boat structure), both conformations of trans-anti-DB[a,l]P tetrol feature a half-chair structure for the cyclohexenyl ring with different orientations of the hydroxyl groups. For cis-anti- and cis-syn-DB[a,l]P tetrols, only a single conformer is detected by FLN spectroscopy. The NMR results for the latter appear to be most consistent with a mixture of two half-chair conformers I and II, while for the cis-anti isomer a flattened, boatlike conformation was observed. The generally good agreement between the NMR coupling constants and those estimated theoretically indicates that these structures should serve as good starting points for spectroscopic or computational studies of DNA adducts derived from DB[a,l]P diol epoxides.

Skin and liver diseases induced in flounder (Platichthys flesus) after long-term exposure to contaminated sediments in large-scale mesocosms

Disease development in flounder (Platichthys flesus) was studied over a period of 3 years in three large mesocosms (40 m x 40 m x 3 m). Two of the mesocosms contained clean sand and the third, sharing a common water circulation with one of the clean-sand mesocosms, was stocked with contaminated dredged spoil. In this way, one of the clean-sand mesocosms was indirectly polluted via the water phase, and analysis of contaminant concentrations in sediments and flounder tissues showed that it had a status intermediate between the other two. Random samples of the flounder populations from the indirectly polluted and reference mesocosms were examined every 2 months for epidermal diseases (lymphocystis, skin ulcers, fin rot) and then released. In addition, every 6 months, random samples of fish from all three mesocosms were sacrificed for histological and chemical investigation. With regard to the development of epidermal disease, the results showed little difference between the reference mesocosm and the indirectly polluted mesocosm, with the exception that lymphocystis was significantly elevated in the indirectly polluted mesocosm. Although pollution may be a risk factor in the etiology of this disease, such a relationship would probably be obscured under field conditions due to variation arising from other factors. Histopathological analysis of the livers revealed in total four cases of hepatocellular adenoma (1.5% of sampled population) in fish from the polluted mesocosms, the first occurring after 2.5 years of exposure in fish from the indirectly polluted mesocosm. Furthermore, several other liver lesions, including foci of cellular alteration and hydropic vacuolated lesions, developed during the course of the experiment before tumor formation was apparent. Prevalences of these conditions were very much lower in the reference mesocosm than in the two polluted mesocosms. Densities of melanomacrophage centers in the liver showed a similar trend. The findings clearly indicate that long-term exposure to chemically contaminated dredged spoil can induce liver neoplasia and other liver lesions in flounder at contaminant levels comparable to those found in the natural environment.

Depurinating and stable benzo[a]pyrene-DNA adducts formed in isolated rat liver nuclei

Polycyclic aromatic hydrocarbons are bound to DNA by two major pathways, one-electron oxidation and monooxygenation, to form adducts that are stable in DNA under normal conditions of isolation and depurinating adducts that are released from DNA by cleavage of the bond between the purine base and deoxyribose. Isolated rat liver nuclei have been used as an in vitro model for studying covalent binding of aromatic hydrocarbons to DNA, but the depurinating adducts formed by nuclei have not been identified or compared to those formed by the more commonly used rat liver microsomes. To examine the profiles of stable and depurinating adducts, nuclei from the livers of 3-methylcholanthrene-induced male MRC Wistar rats were incubated with [3H]benzo[a]pyrene (BP) and NADPH. Three depurinating adducts, 8-(BP-6-yl)Gua, 7-(BP-6-yl)Gua, and 7-(BP-6-yl)Ade, were obtained from the nuclei, as seen previously with rat liver microsomes or in mouse skin. The profile of stable adducts analyzed by the 32P-postlabeling method was qualitatively similar to that found in the microsomal activation of BP or in mouse skin treated with BP. Low-temperature fluorescence studies of the nuclear DNA revealed the presence of stable BP adducts originating from syn- and anti-BP diol epoxide.

Expanded analysis of benzo[a]pyrene-DNA adducts formed in vitro and in mouse skin: their significance in tumor initiation

This paper reports expanded analyses of benzo[a]pyrene (BP)-DNA adducts formed in vitro by activation with horseradish peroxidase (HRP) or 3-methylcholanthrene-induced rat liver microsomes and in vivo in mouse skin. The adducts formed by BP are compared to those formed by BP-7,8-dihydrodiol and anti-BP diol epoxide (BPDE). First, activation of BP by HRP produced 61% depurinating adducts: 7-(benzo[a]pyrene-6-yl)guanine (BP-6-N7Gua), BP-6-C8Gua, BP-6-N7Ade, and the newly identified BP-6-N3Ade. As a standard, the last adduct was synthesized along with BP-6-N1Ade by electrochemical oxidation of BP in the presence of adenine. Second, identification and quantitation of BP-DNA adducts formed by microsomal activation of BP showed 68% depurinating adducts: BP-6-N7Ade, BP-6-N7Gua, BP-6-C8Gua, BPDE-10-N7Ade, and the newly detected BPDE-10-N7Gua. The stable adducts were mostly BPDE-10-N2dG (26%), with 6% unidentified. BPDE-10-N7Ade and BPDE-10-N7Gua were the depurinating adducts identified after microsomal activation of BP-7, 8-dihydrodiol or direct reaction of anti-BPDE with DNA. In both cases, the predominant adduct was BPDE-10-N2dG (90% and 96%, respectively). Third, when mouse skin was treated with BP for 4 h, 71% of the total adducts were the depurinating adducts BP-6-N7Gua, BP-6-C8Gua, BP-6-N7Ade, and small amounts of BPDE-10-N7Ade and BPDE-10-N7Gua. These newly detected depurinating diol epoxide adducts were found in larger amounts when mouse skin was treated with BP-7,8-dihydrodiol or anti-BPDE. The stable adduct BPDE-10-N2dG was predominant, especially with anti-BPDE. Comparison of the profiles of DNA adducts formed by BP, BP-7,8-dihydrodiol, and anti-BPDE with their carcinogenic potency indicates that tumor initiation correlates with the levels of depurinating adducts, but not with stable adducts. Furthermore, the levels of depurinating adducts of BP correlate with mutations in the Harvey-ras oncogene in DNA isolated from mouse skin papillomas initiated by this compound [Chakravarti et al. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 10422-10426]. The depurinating adducts formed by BP in mouse skin appear to be the key adducts leading to tumor initiation.

Conformational studies of depurinating DNA adducts from syn-dibenzo[a,l]pyrene diolepoxide

One of the major DNA adducts from the extremely potent aromatic carcinogen dibenzo[a,l]pyrene (DB[a,l]P) is the depurinating adduct syn-DB[a,l]P diolepoxide-14-N7Ade. Low-temperature fluorescence spectra of this adduct (and related derivaties bound to N3-adenine and N7-guanine) showed two distinct (0,0) origin bands with different excited-state vibrational frequencies, as measured by means of fluorescence line narrowing spectroscopy. The relative intensity of the two origin bands was solvent dependent. The hypothesis that this phenomenon could be due to a conformational equilibrium was tested using molecular mechanics, dynamical simulations and semi-empirical quantum-mechanical calculations. The hydrolyzed metabolite DB[a,l]P tetraol was also studied for comparison. It was found that the syn-DB[a,l]P diolepoxide-14-N7Ade adduct is formed via trans addition to the epoxide. Exploration of the conformational space indeed produced two potential energy minima; both corresponding to structures in which the aromatic ring system is severely distorted. In conformation I the proximity of the distal ring forces the adenine base into a pseudo-axial position and the cyclohexenyl ring adopts a half-boat structure. In conformation II the distal ring is bent in the opposite direction, allowing the cyclohexenyl ring to adopt a half-chair structure with the base in a pseudo-equatorial position, partially stacked over the distal ring. THe difference in (0,0) transition energy calculated for the two conformers agrees very well with the spectroscopic data, and the relative orientations of the hydrogens bound to the cyclohexenyl ring in the major (half-boat) conformation I are in full agreement with the experimentally observed proton NMR coupling constants.

Conformational studies of the (+)-trans, (-)-trans, (+)-cis, and (-)-cis adducts of anti-benzo[a]pyrene diolepoxide to N2-dG in duplex oligonucleotides using polyacrylamide gel electrophoresis and low-temperature fluorescence spectroscopy

Using polyacrylamide gel electrophoresis (PAGE) and low-temperature, laser-induced fluorescence line narrowing (FLN) and non-line narrowing (NLN) spectroscopic methods, the conformational characteristics of stereochemically defined and site-specific adducts derived from the binding of 7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (anti-BPDE, a metabolite of the environmental carcinogen benzo[a]pyrene), to DNA were studied. The focus of these studies was on the four stereochemically distinct anti-BPDE modified duplexes 5'-d(CCATCGCTACC).(GGTAGCGATGG), where G denotes the lesion site derived from trans or cis addition of the exocyclic amino group of guanine to the C10 position of either (+) or (-)-anti-BPDE. PAGE experiments under non-denaturing conditions showed that the (+)-trans adduct causes a significantly greater retardation in the electrophoretic mobility than the other three adducts, probably the result of important adduct-induced distortions of the duplex structure. Low-temperature fluorescence studies in frozen aqueous buffer matrices showed that the (+)-trans adduct adopts primarily an external conformation with only minor interactions with the helix, but a smaller fraction (approximately 25%) appears to exists in a partially base-stacked conformation. The (-)-trans adduct exists almost exclusively (approximately 97%) in an external conformation. Both cis adducts were found to be intercalated; strong electron-phonon coupling observed in their FLN spectra provided additional evidence for significant pi-pi stacking interactions between the pyrenyl residues and the bases. FLN spectroscopy is shown to be suitable for distinguishing between trans and cis adducts, but lesions with either (+)- or (-)-trans, or (+)- or (-)-cis stereochemical characteristics showed very similar vibrational patterns.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation and persistence of benzo[a]pyrene-DNA adducts in mouse epidermis in vivo: importance of adduct conformation

The formation and repair of benzo[a]pyrene diol epoxide-N2-deoxyguanosine adducts (BPDE-N2-dG) in DNA isolated from the skin of mice treated topically with benzo[a]pyrene (BP) was studied by 32P-postlabeling and by low-temperature fluorescence spectroscopy under low resolution and under high resolution fluorescence line narrowing (FLN) conditions. In agreement with earlier studies, total BP-DNA binding reached a maximum at 24 h after treatment (dose: 1 mumol/mouse), then declined rapidly until 4 days after treatment and much more slowly thereafter. An HPLC method was developed which resolved the 32P-postlabeled (-)-trans- from (-)-cis-anti-BPDE-N2-dG, and (+)-trans-from (+)-cis-anti-BPDE-N2-dG. High performance liquid chromatography analysis of the major TLC adduct spot (containing > 80% of the total adducts) obtained by postlabeling BP-modified mouse skin DNA showed that it consisted of a major component that coeluted with (-)-cis-/(+)-trans-anti-BPDE-N2-dG and a minor component that coeluted with (-)-trans-/(+)-cis-anti-BPDE-N2-dG and that the minor component was repaired at a slower rate than the major component. Low-temperature fluorescence spectroscopy of the intact DNA identified the major adduct as (+)-trans-anti-BPDE-N2-dG and the minor adduct fraction consisted mainly of (+)-cis-anti-BPDE-N2-dG. In agreement with the 32P-postlabeling results it was observed by fluorescence spectroscopy that the (+)-cis-adducts were repaired more slowly than most other adducts. Moreover, the (+)-trans-adducts exhibited a broad distribution of base-stacked, partially base-stacked and helix-external conformations. Mouse skin DNA samples obtained at early timepoints (2-8 h) after treatment with BP contained substantially more of the 'external' adducts, while samples at later timepoints (24-48 h) contained relatively more adducts in the base-stacked conformation, indicating also that the latter adducts are repaired less readily than the former. The possible biological significance of these novel observations of conformation-dependent rates of DNA adduct repair and their possible dependence on DNA sequence, are discussed.

Identification and quantitation of dibenzo[a,l]pyrene--DNA adducts formed by rat liver microsomes in vitro: preponderance of depurinating adducts

Dibenzo[a,l]pyrene (DB[a,l]P) is the most potent carcinogen known among aromatic hydrocarbons. DB[a,l]P-11,12-dihydrodiol, precursor to the bay-region diol epoxide, is slightly less carcinogenic than the parent compound. DB[a,l]P and its 11,12-dihydrodiol were covalently bound to DNA by cytochrome P-450 in 3-methylcholanthrene-induced rat liver microsomes, and DB[a,l]P was also bound to DNA by horseradish peroxidase. The "stable" (remaining intact in DNA under normal conditions of purification) and "depurinating" (released from DNA by cleavage of the glycosidic link between the purine base and deoxyribose) adducts were identified and quantified. Stable adducts were analyzed by the 32P-postlabeling technique. Depurinating adducts were identified by comparison of their retention times with those of standard adducts on HPLC in two solvent systems. Confirmation of their identity was obtained by means of fluorescence line-narrowing spectroscopy. When DB[a,l]P was activated by horseradish peroxidase, the depurinating adducts 3-(DB[a,l]P-10-yl)adenine (DB[a,l]P-10-N3Ade, 33%), 7-(DB[a,l]P-10-yl)adenine (DB[a,l]P-10-N7Ade, 27%), and 7-DB[a,l]P-10-yl)guanine (DB[a,l]P-10-N7Gua, 5%) were formed. Unidentified stable adducts comprised the remaining 35% of the detected adducts. When DB[a,l]P was activated by microsomes, the one-electron oxidation depurinating adducts DB[a,l]P-10-N3Ade (28%), DB[a,l]P-10-N7Ade (14%), DB[a,l]P-10-N7Gua (2%), and DB[a,l]P-10-C8Gua (6%), as well as the diol epoxide depurinating adducts (+/-)-syn-DB[a,l]P-diol epoxide (DE)-14-N7Ade (31%) and (+/-)-anti-DB[a,l]PDE-14-N7Gua (3%), were formed. Stable adducts predominantly formed via the DB[a,l]PDE pathway represented 16% of the adducts detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Flanking base effects on the structural conformation of the (+)-trans-anti-benzo[a]pyrene diolepoxide adduct to N2-dG in sequence-defined oligonucleotides

Conformations of the trans adduct of (+)-anti-benzo[a]pyrene -7,8-dihydrodiol-9,10-epoxide (BPDE) to N2-guanine, the major stable DNA adduct of the environmental carcinogen benzo[a]pyrene, were studied as a function of flanking bases in single-stranded and in double-stranded oligonucleotides. Three 11mer oligonucleotides d(CTAT-G1G2G3TATC) were synthesized containing the (+)-trans-anti-BPDE adduct at one specific guanine of the GGG sequence (a known mutational hot spot). Polyacrylamide gel electrophoresis of the three single-stranded oligonucleotides showed that the adduct bound to G2 or G3 (5'-flanking base guanine) caused significantly stronger retardation than the same adduct bound to G1 (5'-flanking base thymine). The strength of the carcinogen-base interaction was reflected in the spectroscopic properties of the pyrenyl moiety. Low temperature fluorescence measurements under line-narrowing (FLN) or non-line-narrowing (NLN) conditions showed that in single-stranded form the adduct at G2 or G3 (5'-flanking base guanine) adopts a conformation with strong interaction with the bases. This was also observed for the same adduct at the sequence AGA. In contrast, the (+)-trans-anti-BPDE adduct with a 5'-flanking thymine exists in a primarily helix-external conformation. Similar differences were observed in the double-stranded oligonucleotides: the adducts at G2 and G3 were found to exist in similar conformational equilibria, again with significant carcinogen-base interactions, while the adduct at G1 showed a predominantly external conformation. The nature of the 3'-flanking base appeared to have little influence on the conformational equilibrium of the (+)-trans-anti-BPDE-guanine adduct. The results could provide insight into the mutational specificity and flanking base effects observed for (+)-anti-BPDE.

Trace analysis of 3-hydroxy benzo[a]pyrene in urine for the biomonitoring of human exposure to polycyclic aromatic hydrocarbons

Determination of benzo[a]pyrene (BaP) metabolites in urine can provide direct insight into recent exposure to BaP integrated from all uptake routes. In order to detect 3-OH BaP in human urine after exposure to BaP at the workplace, extremely sensitive methods need to be developed. In this paper, a new extraction method is presented, and two laser-based fluorescence techniques are evaluated. Using HPLC with laser-induced fluorescence detection, a detection limit of 8 ng/L was obtained. With laser-excited Shpol'skii spectrometry after chemical derivatization, 3-OH BaP could be detected at even a 0.5-ng/L concentration. In a pilot study, urine samples from coke-oven workers and from occupationally nonexposed control persons were analyzed. In the control samples, the average 3-OH BaP concentration was 8.3 ng/L; the 3-OH BaP concentrations were found to be highly correlated (r2 = 0.89) with urinary 1-OH pyrene, a widely used biomarker for polycyclic aromatic hydrocarbon (PAH) exposure. Significantly elevated 1-OH pyrene concentrations were measured in urine samples from coke-oven workers, but in most samples a corresponding increase of 3-OH BaP was not observed. Possible explanations for this discrepancy are discussed.