Biotransformation of trans-4,5-dihydroxy-4,5-dihydrobenzo[a]pyrene to benzo[a]pyrene bis-diols and DNA adducts by induced rat liver microsomes

Benzo[a]pyrene-7,8-quinone-3'-mononucleotide adduct standards for 32P postlabeling analyses: detection of benzo[a]pyrene-7,8-quinone-calf thymus DNA adducts

Benzo[a]pyrene-7,8-quinone (BPQ) is one of the reactive metabolites of the widely distributed archetypal polycyclic aromatic hydrocarbon, benzo[a]pyrene (B[a]P). The formation of BPQ from B[a]P through trans-7,8-dihydroxy-7,8-dihydroB[a]P by the mediation of aldo-keto reductases and its role in the genotoxicity and carcinogenesis of B[a]P currently are under extensive investigation. Toxicity pathways related to BPQ are believed to include both stable and unstable (depurinating) DNA adduct formation as well as reactive oxygen species. We previously reported the complete characterization of four novel stable BPQ-deoxyguanosine (dG) and two BPQ-deoxyadenosine (dA) adducts (Balu et al., Chem. Res. Toxicol. 17 (2004) 827-838). However, the identification of BPQ-DNA adducts by 32P postlabeling methods from in vitro and in vivo exposures required 3'-monophosphate derivatives of BPQ-dG, BPQ-dA, and BPQ-deoxycytidine (dC) as standards. Therefore, in the current study, BPQ adducts of dGMP(3'), dAMP(3'), and dCMP(3') were prepared. The syntheses of the BPQ-3'-mononucleotide standards were carried out in a manner similar to that reported previously for the nucleoside analogs. Reaction products were characterized by UV, LC/MS analyses, and one- and two-dimensional NMR techniques. The spectral studies indicated that all adducts existed as diastereomeric mixtures. Furthermore, the structural identities of the novel BPQ-dGMP, BPQ-dAMP, and BPQ-dCMP adducts were confirmed by acid phosphatase dephosphorylation of the BPQ-nucleotide adducts to the corresponding known BPQ-nucleoside adduct standards. The BPQ-dGMP, BPQ-dAMP, and BPQ-dCMP adduct standards were used in 32P postlabeling studies to identify BPQ adducts formed in vitro with calf thymus DNA and DNA homopolymers. 32P postlabeling analysis revealed the formation of 8 major and at least 10 minor calf thymus DNA adducts. Of these BPQ-DNA adducts, the following were identified: 1 BPQ-dGMP adduct, 2 BPQ-dAMP adducts, and 3 BPQ-dCMP adducts. This study represents the first reported example of the characterization of stable BPQ-DNA adducts in isolated mammalian DNA and is expected to contribute significantly to the future BPQ-DNA adduct studies in vivo and thereby to the contribution of BPQ in B[a]P carcinogenesis.

Microsomal Biotransformation of Benzo[ghi]perylene, a Mutagenic Polycyclic Aromatic Hydrocarbon without a “Classic” Bay Region

Carcinogenic polycyclic aromatic hydrocarbons (PAH), e.g., benzo[a]pyrene (BaP), possess a bay region comprising an ortho-fused benzene ring. Benzo[ghi]perylene (BghiP) represents the group of PAHs lacking such a "classic" bay region and hence cannot be metabolically converted like BaP to bay region dihydrodiol epoxides considered as ultimate mutagenic and carcinogenic metabolites of PAH. BghiP exhibits bacterial mutagenicity in strains TA98 (1.3 his(+)-revertant colonies/nmol) and TA100 (4.3 his(+)-revertant colonies/nmol) of Salmonella typhimurium after metabolic activation by the postmitochondrial hepatic fraction of CD rats treated with 3-methylcholanthrene. Inhibition of microsomal epoxide hydrolase (mEH) with 1,1,1-trichloro-2-propene oxide raised the bacterial mutagenicity of BghiP in TA98 almost 4-fold indicating arene oxides as ultimate mutagens. To confirm this assumption, the biotransformation of BghiP was elucidated. Incubation of BghiP with liver microsomes of CD rats treated with Aroclor 1254 yielded 17 ethyl acetate extractable metabolic products. Twelve metabolites were identified by a combination of chromatographic, spectroscopic, and biochemical methods. The microsomal biotransformation of BghiP proceeds by two pathways: Pathway I starts with the monooxygenase attack at the 7-position leading to the 7-phenol, which is transformed to the 7,8- and 7,10-diphenols followed by oxidation to the 7,8- and 7,10-quinones. On pathway II, the K regions of BghiP are successively converted to arene oxides yielding the indirectly identified 3,4-oxide and the 3,4,11,12-bisoxides. Enzymatic hydrolysis of the 3,4-oxide leads to the trans-3,4-dihydrodiol, which is oxidized to the 3,4-quinone. Similarly, the trans-3,4-trans-11,12-bisdihydrodiols and the trans-3,4-dihydrodiol 11,12-quinone are generated from the 3,4,11,12-bisoxides. The trans-3,4-dihydrodiol and the trans-3,4-trans-11,12-bisdihydrodiols are preferentially formed as R,R and R,R,R,R enantiomers, respectively. The intrinsic bacterial mutagenicity of the 3,4,11,12-bisoxides is rather low and hardly explains the strong increase in bacterial mutagenicity of BghiP after inhibition of mEH. Thus, we believe that the 3,4-oxide plays a more important role as the ultimate mutagenic metabolite of BghiP.

Identification and Characterization of Novel Stable Deoxyguanosine and Deoxyadenosine Adducts of Benzo[a]pyrene-7,8-quinone from Reactions at Physiological pH

Benzo[a]pyrene (B[a]P) is an archetypal member of the family of polycyclic aromatic hydrocarbons (PAHs) and is a widely distributed environmental pollutant. B[a]P is known to induce cancer in animals, and B[a]P-containing complex mixtures are human carcinogens. B[a]P exerts its genotoxic and carcinogenic effects through metabolic activation forming reactive intermediates that damage DNA. DNA adduction by B[a]P is a complex phenomenon that involves the formation of both stable and unstable (depurinating) adducts. One pathway by which B[a]P can mediate genotoxicity is through the enzymatic formation of B[a]P-7,8-quinone (BPQ) from B[a]P-7,8-diol by members of the aldo-keto-reductase (AKR) family. Once formed, BPQ can act as a reactive Michael acceptor that can alkylate cellular nucleophiles including DNA and peptides. Earlier studies have reported on the formation of stable and depurinating adducts from the reaction of BPQ with DNA and nucleosides, respectively. However, the syntheses and characterization of the stable adducts from these interactions have not been addressed. In this study, the reactivity of BPQ toward 2'-deoxyguanosine (dG) and 2'-deoxyadenosine (dA) nucleosides under physiological pH conditions is examined. The identification and characterization of six novel BPQ-nucleoside adducts obtained from the reaction of BPQ and dG or dA in a mixture of phosphate buffer and dimethylformamide are reported. The structures of these adducts were determined by ultraviolet spectroscopy, electrospray mass spectrometry, and NMR experiments including (1)H, (13)C, two-dimensional COSY, one-dimensional NOE, ROESY, HMQC, HSQC, and HMBC. The reaction of BPQ with dG afforded four unique Michael addition products: two diastereomers of 8-N(1),9-N(2)-deoxyguanosyl-8,10-dihydroxy-9,10-dihydrobenzo[a]pyren-7(8H)-one (BPQ-dG(1,2)) and two diastereomers of 10-(N(2)-deoxyguanosyl)-9,10-dihydro-9-hydroxybenzo[a]pyrene-7,8-dione (BPQ-dG(3,4)). The BPQ-dG(1,2)( )()adducts suggest a 1,6-Michael addition reaction of dG, an oxidation of the hydroquinone to the quinone, a 1,4-Michael addition of water, and an internal cyclization. The BPQ-dG(3,4)( )()adducts suggest a 1,4-Michael addition reaction of dG, an oxidation of the hydroquinone to the quinone, and a 1,6-Michael addition of water. Under similar but extended reaction conditions, the reaction of BPQ with dA produced only one diastereomeric pair of adducts identified as 8-N(6),10-N(1)-deoxyadenosyl-8,9-dihydroxy-9,10-dihydrobenzo[a]pyren-7(8H)-one (BPQ-dA(1,2)). The BPQ-dA(1,2)( )()adducts suggest a 1,4-Michael addition reaction of dA, an oxidation of the hydroquinone to the quinone, a 1,6-Michael addition of water, and an internal cyclization. As considerable efforts have been placed in documenting the genotoxic effects of BPQ, this first report of the identification and characterization of these stable adducts of BPQ formed under physiological pH conditions is expected to contribute significantly to the area of BPQ-mediated genotoxicity and carcinogenesis.

Differential c-myc expression profiles in normal human bronchial epithelial cells following treatment with benzo[a]pyrene, benzo[a]pyrene-4,5 epoxide, and benzo[a]pyrene-7,8-9,10 diol epoxide

Bronchial epithelial cells are often exposed to airborne mutagens that have the potential to induce genetic changes involved in the development of lung cancer. Although lung tumors often display alterations in the expression of oncogenes and/or tumor suppressor genes, the role of specific chemicals and/or metabolites in causing these alterations is not well defined. The polycyclic aromatic hydrocarbon (PAH) benzo[a]pyrene (B[a]P), a by-product of combustion, is a prevalent airborne environmental mutagen and a constituent of cigarette smoke. The primary objective of this study was to compare the effect of B[a]P and two of its reactive metabolites, benzo[a]pyrene diol epoxide (BPDE or bay region epoxide) and benzo[a]pyrene-4,5-dihydroepoxide (BPE or K-region epoxide), on expression of the proto-oncogene c-myc in normal human bronchial epithelial (NHBE) cells using a quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) method. Changes in c-myc gene expression were compared with DNA adduct formation, growth inhibition, and cell-cycle progression as determined by (32)P-postlabelling, neutral red (NR), and flow cytometric analyses, respectively. None of the three test compounds altered the levels of 18S ribosomal RNA or beta-actin at the concentrations evaluated for c-myc expression, indicating that nonspecific changes in gene expression induced by cytotoxicity, for example, were not present at the concentrations evaluated. Cells exposed to B[a]P exhibited a dose-dependent increase in c-myc expression; conversely, a dose-dependent decrease in c-myc expression was observed following BPDE exposure. A marginal but concentration-dependent increase in c-myc mRNA levels was observed following exposure to the K-region epoxide. Our results demonstrated that, although B[a]P and its metabolites alter c-myc expression, the parent compound and its metabolites produce unequal and contrasting effects on the expression of this gene.

Comparison of the genotoxic activities of the K-region dihydrodiol of benzo[a]pyrene with benzo[a]pyrene in mammalian cells: morphological cell transformation; DNA damage; and stable covalent DNA adducts

Benzo[a]pyrene (B[a]P) is the most thoroughly studied polycyclic aromatic hydrocarbon (PAH). Many mechanisms have been suggested to explain its carcinogenic activity, yet many questions still remain. K-region dihydrodiols of PAHs are metabolic intermediates depending on the specific cytochrome P450 and had been thought to be detoxification products. However, K-region dihydrodiols of several PAHs have recently been shown to morphologically transform mouse embryo C3H10T1/2CL8 cells (C3H10T1/2 cells). Because K-region dihydrodiols are not metabolically formed from PAHs by C3H10T1/2 cells, these cells provide a useful tool to independently study the mechanisms of action of PAHs and their K-region dihydrodiols. Here, we compare the morphological cell transforming, DNA damaging, and DNA adducting activities of the K-region dihydrodiol of B[a]P, trans-B[a]P-4,5-diol with B[a]P. Both trans-B[a]P-4,5-diol and B[a]P morphologically transformed C3H10T1/2 cells by producing both Types II and III transformed foci. The morphological cell transforming and cytotoxicity dose response curves for trans-B[a]P-4,5-diol and B[a]P were indistinguishable. Since morphological cell transformation is strongly associated with mutation and/or larger scale DNA damage in C3H10T1/2 cells, the identification of DNA damage induced in these cells by trans-B[a]P-4,5-diol was sought. Both trans-B[a]P-4,5-diol and B[a]P exhibited significant DNA damaging activity without significant concurrent cytotoxicity using the comet assay, but with different dose responses and comet tail distributions. DNA adduct patterns from C3H10T1/2 cells were examined after trans-B[a]P-4,5-diol or B[a]P treatment using 32P-postlabeling techniques and improved TLC elution systems designed to separate polar DNA adducts. While B[a]P treatment produced one major DNA adduct identified as anti-trans-B[a]P-7,8-diol-9,10-epoxide-deoxyguanosine, no stable covalent DNA adducts were detected in the DNA of trans-B[a]P-4,5-diol-treated cells. In summary, this study provides evidence for the DNA damaging and morphological cell transforming activities of the K-region dihydrodiol of B[a]P, in the absence of covalent stable DNA adducts. While trans-B[a]P-4,5-diol and B[a]P both induce morphological cell transformation, their activities as DNA damaging agents differ, both qualitatively and quantitatively. In concert with the morphological cell transformation activities of other K-region dihydrodiols of PAHs, these data suggest a new mechanism/pathway for the morphological cell transforming activities of B[a]P and its metabolites.