Polycyclic Aromatic Hydrocarbons Carcinogenesis Research Articles

Oxidative DNA damage induced by benz[a]anthracene metabolites via redox cycles of quinone and unique non-quinone.

Benz[a]anthracene (BA) is one of the most abundant polycyclic aromatic hydrocarbons (PAHs) that are ubiquitous environmental pollutants. PAH carcinogenesis is explained by DNA adduct formation by PAH diol epoxide and oxidative DNA damage by PAH o-quinone. Benz[a]anthracene-trans-3,4-dihydrodiol (BA-3,4-dihydrodiol) is a minor metabolite but shows higher mutagenicity and tumorigenicity than parent BA. We confirmed that a BA o-quinone type metabolite, benz[a]anthracene-3,4-dione (BA-3,4-dione), induced oxidative DNA damage in the presence of cytochrome P450 reductase. Interestingly, we found that BA-3,4-dihydrodiol nonenzymatically caused Cu(II)-mediated DNA damage including 8-oxo-7,8-dihydro-2'-deoxyguanosine formation and the addition of NADH enhanced DNA damage. BA-3,4-dihydrodiol induced a double-base lesion of C and G at the 5'-ACG-3' sequence complementary to codon 273 of the human p53 tumor suppressor gene, which is known as a hotspot. The DNA damage was inhibited by catalase and bathocuproine, indicating the involvement of H(2)O(2) and Cu(I). Time-of-flight mass spectroscopic study suggested that BA-3,4-dihydrodiol undergoes Cu(II)-mediated autoxidation leading to the formation of its hydroxylated form of BA-3,4-dihydrodiol, capable of causing oxidative DNA damage. It is noteworthy that BA-3,4-dihydrodiol can nonenzymatically induce DNA damage more efficiently than BA-3,4-dione with metabolic activation. In conclusion, oxidative DNA damage induced by BA-3,4-dihydrodiol not only via quinone-type redox cycle but also via a new type of redox cycle participates in the expression of carcinogenicity of BA and BA-3,4-dihydrodiol.

A QSAR Model of PAHs Carcinogenesis Based on Thermodynamic Stabilities of Biactive Sites.

Polycyclic aromatic hydrocarbons (PAHs) have been adopted to study the carcinogenesis of chemicals experimentally and theoretically. A model of carcinogenic activity of 48 PAHs was obtained based on the calculated relative thermodynamic stabilities of epoxide and carbonium intermediates of the PAHs. Using both epoxyl-energy and cationized-energy of two active sites, the model reasonably predicts the carcinogenic activity of these PAHs and shows a good ability to distinguish between carcinogenic and noncarcinogenic PAHs. Furthermore, the model suggests that double active sites and their distance characteristics are important factors in the chemical carcinogenesis of PAHs. The physical meaning of the energies corresponding addition reactions with DNA is also discussed.

A QSAR model of PAHs carcinogenesis based on thermodynamic stabilities of biactive sites.

Polycyclic aromatic hydrocarbons (PAHs) have been adopted to study the carcinogenesis of chemicals experimentally and theoretically. A model of carcinogenic activity of 48 PAHs was obtained based on the calculated relative thermodynamic stabilities of epoxide and carbonium intermediates of the PAHs. Using both epoxyl-energy and cationized-energy of two active sites, the model reasonably predicts the carcinogenic activity of these PAHs and shows a good ability to distinguish between carcinogenic and noncarcinogenic PAHs. Furthermore, the model suggests that double active sites and their distance characteristics are important factors in the chemical carcinogenesis of PAHs. The physical meaning of the energies corresponding addition reactions with DNA is also discussed.

Discovery and analysis of hepatocellular carcinoma genes using cDNA microarrays.

Microarray analysis on a genomic scale was used to profile changes in gene expression accompanying hepatocellular carcinoma. Gene expression profiles of liver tissues from twelve hepatocellular carcinoma samples relative to the gene expression profile of the normal liver tissue were analyzed using 4096 chips and 12800 chips. The results of microarray experiments were verified by the Northern blot technique. A group of 1,820 genes with altered expression were identified in more than 50% of the patients examined. This highly concordant expression profile included human genes encoding proteins involved in the function of peroxisomes, serum control, polycyclic aromatic hydrocarbon carcinogenesis, cell growth and differentiation, metastasis, the function of the immune system, apoptosis, and remodeling of the cytoskeleton. The newly identified genes afford a quantitative view of the changes that accompany liver cancer at the genomic level, enable deeper insights into the molecular basis of disease, and provide an extensive list of potential early-onset molecular markers for improved diagnosis.

Induction of CYP1A1. The AhR/DRE paradigm: transcription, receptor regulation, and expanding biological roles.

The CYP1A1 gene encodes microsomal cytochrome P4501A1 that catalyzes the metabolism of many xenobiotics, including the oxygenation of polycyclic aromatic hydrocarbons (PAH). Induction of CYP1A1 enhances the metabolism of PAHs, and therefore, represents an adaptive response to chemical exposure in mammalian cells. Mechanistic studies reveal an AhR/DRE paradigm for the induction, which involves activation of the aryl hydrocarbon receptor (AhR) by an agonist, dimerization of AhR with the Ah recceptor nuclear translocator (Arnt), followed by binding of the AhR/Arnt heterodimer to the dioxin-responsive enhancer (DRE) and transcription of the gene. The AhR mediated transcription is tightly regulated through, at least, two mechanisms: (a) the cytoplasmic AhR interacts with hsp90 and an immunophilin chaperone AIP for proper folding and receptivity, and (b) the agonist-activated, nuclear AhR is degraded through the ubiquitin-26S proteasome mediated protein turnover, such that the transcription by AhR is controlled at a physiologically adequate level. In addition to CYP1A1 induction, AhR mediates a broad range of biological responses to CYP1A1 inducers, typified by the environmental contaminant dioxin, via modulating gene expression. Thus, mechanistic studies of CYP1A1 induction have provided insights into P450 induction, PAH carcinogenesis, dioxin action, AhR function, and receptor-mediated mammalian gene expression.

Detoxication of carcinogenic fjord-region diol epoxides of polycyclic aromatic hydrocarbons by glutathione transferase P1-1 variants and glutathione

Epidemiological studies suggest that individuals differing in the expression of allelic variants of the human glutathione transferase (GST) Pi gene differ in susceptibility to chemical carcinogens such as polycyclic aromatic hydrocarbons (PAH). This study reports the catalytic efficiencies ( k cat/ K m) of two naturally occurring variants, GSTP1-1/I-105 and GSTP1-1/V-105, towards a series of fjord-region diol epoxides representing potent biologically active PAH metabolites, and two GSTP1-1 mutants with Ala 105 and Trp 105 in the active site. The results indicate that individuals who are homozygous for the allele encoding GSTP1-1/V-105 might be more susceptible to PAH carcinogenesis due to other reasons than a reduced capacity for detoxifying diol epoxides.

Mechanisms Of Carcinogenesis of Polycyclic Aromatic Hydrocarbons

Abstract Recent investigations into the mechanisms of carcinogenesis of polycyclic aromatic hydrocarbons (PAHs) are discussed with major emphasis on the diol epoxide pathway. These investigations are in three principal areas: (1) enantiospecific synthesis of dihydrodiol and diol epoxide metabolites; (2) sitespecific synthesis of PAH-oligonucleotide adducts; and (3) studies of bis−dihydrodiol (i.e. tetrahydrotetraol) and tetraol monoepoxide metabolites.

Suppression of Hydroxysteroid Sulfotransferase-a Gene Expression by 3-Methylcholanthrene

The hydroxysteroid sulfotransferases (HSTs) are phase II drug metabolizing enzymes which play a role in xenobiotic detoxication, carcinogen activation, and intratissue hormone modulation. Rat liver contains three principal HST enzymes. Of these, HST-a is most abundant in female rat liver. The phase I drug metabolizing enzyme cytochrome P450 1A1 (CYP1A1) provides a major pathway for polycyclic aromatic hydrocarbon (PAH) carcinogen activation, whereas HST-a, through enzymatic sulfation, offers an alternative pathway for PAH carcinogen activation. Transcription of the rat hepatic CYP1A1 gene is induced in response to treatment with PAH carcinogen 3-methylcholanthrene (3-MC). In view of the potential importance of enzymatic sulfation to the complex process of PAH carcinogenesis, the effect of 3-MC on HST-a gene expression was investigated. Groups of prepubescent (aged ≍22-30 days) and young adult female (aged ≍69-84 days) Sprague-Dawley rats were injected ip with corn oil vehicle or with 3-MC (80 mg/kg). After fasting for 24 hr, rats were terminated and hepatic HST-a gene expression was analyzed using slot blot, Northern blot, and Western blot analyses. Initial Northern blot and slot blot analyses demonstrated a substantial suppression of rat hepatic HST-a mRNA in both prepubescent and young adult female rats. Subsequent Northern blot analyses on liver tissue isolated from female rats aged ≍55 days confirmed CYP1A1 mRNA induction in 3-MC-treated animals and demonstrated a dose-dependent suppression of HST-a mRNA expression of ≍25, 50, and 75% in rats treated with 10, 25, and 80 mg/kg 3-MC ip, respectively. In contrast, HST-a protein levels remained unaltered 24 hr after 3-MC treatment. A time-course study revealed that hepatic HST-a mRNA levels returned to control levels by 36 hr following 3-MC treatment. These results suggest that xenobiotics such as 3-MC modulate HST-a mRNA expression and that HST-a mRNA suppression after 3-MC treatment may occur at the level of transcription.

The mechanism of adduct formation between reduced flavins and arene epoxides

The mechanism of nucleophilic epoxide opening by reduced flavins, a potentially relevant transformation to the carcinogenesis of polycyclic aromatic hydrocarbons, was investigated. In the spontaneous epoxide hydrolysis pH region, the reaction pathway between the epoxide and the dihydroflavin is bimolecular epoxide opening. Chromatographic analysis of the reaction of (±)-1a,2,3,7b-tetrahydro-(1aα,2α,3β,7bα)-naphth[1,2-b]oxirene-2,3-diol (1) with 5,10-dihydro-1,8,10- trimethylbenzo[g]pteridine-2,4(1H,3H)-dione (LFH 2 , dihydrolumiflavin) in 9/1 (v/v) aqueous pH 1.86 Tris bufferdiorane yielded the flavin N(5) adduct 8 as the only major product

Role of Radical Cations in Aromatic Hydrocarbon Carcinogenesis

Carcinogenic activation of polycyclic aromatic hydrocarbons (PAH) involves two main pathways: one-electron oxidation and monooxygenation. One-electron oxidation produces PAH radical cations, which can react with cellular nucleophiles. Results from biochemical and biological experiments indicate that only PAH with ionization potentials below ca. 7.35 eV can be metabolically activated by one-electron oxidation. In addition, the radical cations of carcinogenic PAH must have relatively high charge localization to react effectively with macromolecules in target cells. Metabolic formation of PAH quinones proceeds through radical cation intermediates. Binding of benzo[a]pyrene (BP) to mouse skin DNA occurs predominantly at C-6, the position of highest charge localization in the BP radical cation, and binding of 6-methyl BP to DNA in mouse skin yields a major adduct with the 6-methyl group bound to the 2-amino group of deoxyguanosine. Studies of carcinogenicity by direct application of PAH to rat mammary gland indicate that only PAH with ionization potentials low enough for activation by one-electron oxidation produce tumors in this target tissue. These constitute some of the results which provide evidence for the involvement of one-electron oxidation in PAH carcinogenesis.

Role of radical cations in aromatic hydrocarbon carcinogenesis.

Carcinogenic activation of polycyclic aromatic hydrocarbons (PAH) involves two main pathways: one-electron oxidation and monooxygenation. One-electron oxidation produces PAH radical cations, which can react with cellular nucleophiles. Results from biochemical and biological experiments indicate that only PAH with ionization potentials below ca. 7.35 eV can be metabolically activated by one-electron oxidation. In addition, the radical cations of carcinogenic PAH must have relatively high charge localization to react effectively with macromolecules in target cells. Metabolic formation of PAH quinones proceeds through radical cation intermediates. Binding of benzo[a]pyrene (BP) to mouse skin DNA occurs predominantly at C-6, the position of highest charge localization in the BP radical cation, and binding of 6-methyl BP to DNA in mouse skin yields a major adduct with the 6-methyl group bound to the 2-amino group of deoxyguanosine. Studies of carcinogenicity by direct application of PAH to rat mammary gland indicate that only PAH with ionization potentials low enough for activation by one-electron oxidation produce tumors in this target tissue. These constitute some of the results which provide evidence for the involvement of one-electron oxidation in PAH carcinogenesis.

Differences in benzo(a)pyrene metabolism between cultured human and murine bronchial cells after pre-treatment with benz(a)anthracene

Primary cultures of human and murine (strain C3Hz) bronchial epithelial cells were pretreated with benz(a)anthracene (BA) (10 μM). 16 h later the formation of phenolic as well as dihydrodiol metabolites of benzo(a)pyrene (BP) was measured. Whereas murine cultures showed enhanced metabolism towards both phenolic and dihydrodiol compounds, in the human cultures only phenolic BP-metabolites were increased. In view of their precursor role in the formation of biologically active diol-epoxides, formation of dihydrodiol-derivatives can be considered as a key factor in determining susceptibility to polycyclic aromatic hydrocarbon (PAH)-induced carcinogenesis. Therefore the observations of this study indicate that animal model systems for PAH carcinogenesis in man have to be selected on the basis of comparable metabolite patterns.

Differential ability of Ambystoma tigrinum hepatic microsomes to produce mutagenic metabolites from polycyclic aromatic hydrocarbons and aromatic amines

A number of carcinogenic aromatic amines when activated by liver microsomes from a salamander, Ambystoma tigrinum, are mutagenic for Salmonella tester strains sensitive to frameshift mutagens. However, 2 polycyclic aromatic hydrocarbons (PAH) (benzo[ a]pyrene (BaP) and perylene) that are rendered mutagenic by mammalian microsomes are not activated by Ambystoma mixed-function oxidases. BaP was chosen for study because it is a well-known environmental carcinogen; perylene, an isomer of BaP, has been implicated as an etiological agent in cutaneous neoplasia in Ambystoma. These results support the observation that amphibians are quite resistant to PAH carcinogenesis and suggest that aromatic amines may be more appropriate model carcinogens.

Separation of ten benzo(a)pyrene phenols by recycle high pressure liquid chromatography and identification of four phenols as metabolites

Elucidation of estrogen carcinogenesis required a few fundamental discoveries made by studying the mechanism of carcinogenesis of polycyclic aromatic hydrocarbons (PAH). The two major mechanisms of metabolic activation of PAH involve formation of radical cations and diol epoxides as ultimate carcinogenic metabolites. These intermediates react with DNA to yield two types of adducts: stable adducts that remain in DNA unless removed by repair and depurinating adducts that are lost from DNA by cleavage of the glycosyl bond between the purine base and deoxyribose. The potent carcinogenic PAH benzo[a]pyrene, dibenzo[a,l]pyrene, 7,12-dimethylbenz[a]anthracene and 3-methylcholanthrene predominantly form depurinating DNA adducts, leaving apurinic sites in the DNA that generate cancer-initiating mutations. This was discovered by correlation between the depurinating adducts formed in mouse skin by treatment with benzo[a]pyrene, dibenzo[a,l]pyrene or 7,12-dimethylbenz[a]anthracene and the site of mutations in the Harvey-ras oncogene in mouse skin papillomas initiated by one of these PAH.By applying some of these fundamental discoveries in PAH studies to estrogen carcinogenesis, the natural estrogens estrone (E1) and estradiol (E2) were found to be mutagenic and carcinogenic through formation of the depurinating estrogen–DNA adducts 4-OHE1(E2)-1-N3Ade and 4-OHE1(E2)-1-N7Gua. These adducts are generated by reaction of catechol estrogen quinones with DNA, analogously to the DNA adducts obtained from the catechol quinones of benzene, naphthalene, and the synthetic estrogens diethylstilbestrol and hexestrol. This is a weak mechanism of cancer initiation.Normally, estrogen metabolism is balanced and few estrogen–DNA adducts are formed. When estrogen metabolism becomes unbalanced, more catechol estrogen quinones are generated, resulting in higher levels of estrogen–DNA adducts, which can be used as biomarkers of unbalanced estrogen metabolism and, thus, cancer risk.The ratio of estrogen–DNA adducts to estrogen metabolites and conjugates has repeatedly been found to be significantly higher in women at high risk for breast cancer, compared to women at normal risk. These results indicate that formation of estrogen–DNA adducts is a critical factor in the etiology of breast cancer. Significantly higher adduct ratios have been observed in women with breast, thyroid or ovarian cancer. In the women with ovarian cancer, single nucleotide polymorphisms in the genes for two enzymes involved in estrogen metabolism indicate risk for ovarian cancer. When polymorphisms produce high activity cytochrome P450 1B1, an activating enzyme, and low activity catechol-O-methyltransferase, a protective enzyme, in the same woman, she is almost six times more likely to have ovarian cancer. These results indicate that formation of estrogen–DNA adducts is a critical factor in the etiology of ovarian cancer.Significantly higher ratios of estrogen–DNA adducts to estrogen metabolites and conjugates have also been observed in men with prostate cancer or non-Hodgkin lymphoma, compared to healthy men without cancer. These results also support a critical role of estrogen–DNA adducts in the initiation of cancer.Starting from the perspective that unbalanced estrogen metabolism can lead to increased formation of catechol estrogen quinones, their reaction with DNA to form adducts, and generation of cancer-initiating mutations, inhibition of estrogen–DNA adduct formation would be an effective approach to preventing a variety of human cancers. The dietary supplements resveratrol and N-acetylcysteine can act as preventing cancer agents by keeping estrogen metabolism balanced. These two compounds can reduce the formation of catechol estrogen quinones and/or their reaction with DNA. Therefore, resveratrol and N-acetylcysteine provide a widely applicable, inexpensive approach to preventing many of the prevalent types of human cancer.