Lack of contribution of covalent benzo[a]pyrene-7,8-quinone-DNA adducts in benzo[a]pyrene-induced mouse lung tumorigenesis

Catechins Controlled Bioavailability of Benzo[a]pyrene (B[α]P) from the Gastrointestinal Tract to the Brain towards Reducing Brain Toxicity Using the In Vitro Bio-Mimic System Coupled with Sequential Co-Cultures

The aim of the current study was to examine the preventive effect of green tea catechins on the transport of Benzo[a]pyrene (B[α]P) into the brain using an in vitro bio-mimic system coupled with sequential co-cultures. When 72 μM of catechins was pre-treated, cellular cytotoxicity induced by IC₅₀ of B[α]P in human liver hepatocellular carcinoma (HepG2) and human brain microvascular endothelial cells (HBMECs) was reduced by 27% and 26%, respectively. The cellular integrity measured in HBMECs, which was exposed to IC₅₀ of B[α]P, slowly decreased. However, the pre-treatment of catechins retained cellular integrity that was 1.14 times higher than with the absence of catechins. Co-consumption of catechins reduced not only the bio-accessibility of B[α]P in digestive fluid, but it also decreased absorption of B[α]P in human intestinal epithelial cells (Caco-2) with a HepG2 co-culture system. It was found that approximately a two times lower amount of B[α]P was transported via the blood-brain barrier (BBB) compared to only the B[α]P intake. These results are taken in conjunction with each other support that catechins could be able to prevent brain toxicity induced by B[α]P in the human body by limiting the bio-availability of B[α]P.

Genotoxicity of ortho-quinones: reactive oxygen species versus covalent modification

o-Quinones are formed metabolically from natural and synthetic estrogens as well as upon exposure to polycyclic aromatic hydrocarbons (PAH) and contribute to estrogen and PAH carcinogenesis by genotoxic mechanisms. These mechanisms include the production of reactive oxygen species to produce DNA strand breaks and oxidatively damaged nucleobases; and the formation of covalent depurinating and stable DNA adducts. Unrepaired DNA-lesions can lead to mutation in critical growth control genes and cellular transformation. The genotoxicity of the o-quinones is exacerbated by nuclear translocation of estrogen o-quinones by the estrogen receptor and by the nuclear translocation of PAH o-quinones by the aryl hydrocarbon receptor. The properties of o-quinones, their formation and detoxication mechanisms, quinone-mediated DNA lesions and their mutagenic properties support an important role in hormonal and chemical carcinogenesis.

Human aldo-keto reductases and the metabolic activation of polycyclic aromatic hydrocarbons

Aldo-keto reductases (AKRs) are promiscuous NAD(P)(H) dependent oxidoreductases implicated in the metabolic activation of polycyclic aromatic hydrocarbons (PAH). These enzymes catalyze the oxidation of non-K-region trans-dihydrodiols to the corresponding o-quinones with the concomitant production of reactive oxygen species (ROS). The PAH o-quinones are Michael acceptors and can form adducts but are also redox-active and enter into futile redox cycles to amplify ROS formation. Evidence exists to support this metabolic pathway in humans. The human recombinant AKR1A1 and AKR1C1–AKR1C4 enzymes all catalyze the oxidation of PAH trans-dihydrodiols to PAH o-quinones. Many human AKRs also catalyze the NADPH-dependent reduction of the o-quinone products to air-sensitive catechols, exacerbating ROS formation. Moreover, this pathway of PAH activation occurs in a panel of human lung cell lines, resulting in the production of ROS and oxidative DNA damage in the form of 8-oxo-2′-deoxyguanosine. Using stable-isotope dilution liquid chromatography tandem mass spectrometry, this pathway of benzo[a]pyrene (B[a]P) metabolism was found to contribute equally with the diol-epoxide pathway to the activation of this human carcinogen in human lung cells. Evaluation of the mutagenicity of anti-B[a]P-diol epoxide with B[a]P-7,8-dione on p53 showed that the o-quinone produced by AKRs was the more potent mutagen, provided that it was permitted to redox cycle, and that the mutations observed were G to T transversions, reminiscent of those observed in human lung cancer. It is concluded that there is sufficient evidence to support the role of human AKRs in the metabolic activation of PAH in human lung cell lines and that they may contribute to the causation of human lung cancer.

Identification of stable benzo[a]pyrene-7,8-dione-DNA adducts in human lung cells

Metabolic activation of the proximate carcinogen benzo[a]pyrene-7,8-trans-dihydrodiol (B[a]P-7,8-trans-dihydrodiol) by aldo-keto reductases (AKRs) leads to B[a]P-7,8-dione that is both electrophilic and redox-active. B[a]P-7,8-dione generates reactive oxygen species resulting in oxidative DNA damage in human lung cells. However, information on the formation of stable B[a]P-7,8-dione-DNA adducts in these cells is lacking. We studied stable DNA adduct formation of B[a]P-7,8-dione in human lung adenocarcinoma A549 cells, human bronchoalveolar H358 cells, and immortalized human bronchial epithelial HBEC-KT cells. After treatment with 2 μM B[a]P-7,8-dione, the cellular DNA was extracted from the cell pellets subjected to enzyme hydrolysis and subsequent analysis by LC-MS/MS. Several stable DNA adducts of B[a]P-7,8-dione were only detected in A549 and HBEC-KT cells. In A549 cells, the structures of stable B[a]P-7,8-dione-DNA adducts were identified as hydrated-B[a]P-7,8-dione-N²-2′-deoxyguanosine and hydrated-B[a]P-7,8-dione-N1-2′-deoxyguanosine. In HBEC-KT cells, the structures of stable B[a]P-7,8-dione-DNA adducts were identified as hydrated-B[a]P-7,8-dione-2′-deoxyadenosine, hydrated-B[a]P-7,8-dione-N1- or N3-2′-deoxyadenosine, and B[a]P-7,8-dione-N1- or N3-2′-deoxyadenosine. In each case, adduct structures were characterized by MSⁿ spectra. Adduct structures were also compared to those synthesized from reactions of B[a]P-7,8-dione with either deoxyribonucleosides or salmon testis DNA in vitro but were found to be different.

Metabolism and distribution of benzo[a]pyrene-7,8-dione (B[a]P-7,8-dione) in human lung cells by liquid chromatography tandem mass spectrometry: detection of an adenine B[a]P-7,8-dione adduct

Benzo[a]pyrene-7,8-dione (B[a]P-7,8-dione) is produced in human lung cells by the oxidation of (±)-B[a]P-7,8-trans-dihydrodiol, which is catalyzed by aldo-keto reductases (AKRs). However, information relevant to the cell-based metabolism of B[a]P-7,8-dione is lacking. We studied the metabolic fate of 2 μM 1,3-[(3)H(2)]-B[a]P-7,8-dione in human lung adenocarcinoma A549 cells, human bronchoalveolar H358 cells, and immortalized human bronchial epithelial HBEC-KT cells. In these three cell lines, 1,3-[(3)H(2)]-B[a]P-7,8-dione was rapidly consumed, and radioactivity was distributed between the organic and aqueous phase of ethyl acetate-extracted media, as well as in the cell lysate pellets. After acidification of the media, several metabolites of 1,3-[(3)H(2)]-B[a]P-7,8-dione were detected in the organic phase of the media by high performance liquid chromatography-ultraviolet-radioactivity monitoring (HPLC-UV-RAM). The structures of B[a]P-7,8-dione metabolites varied in the cell lines and were identified as B[a]P-7,8-dione conjugates with glutathione (GSH) and N-acetyl-l-cysteine (NAC), 8-O-monomethylated-catechol, catechol monosulfate, and monoglucuronide, and monohydroxylated-B[a]P-7,8-dione by liquid chromatography-tandem mass spectrometry (LC-MS/MS). We also obtained evidence for the first time for the formation of an adenine adduct of B[a]P-7,8-dione. Among these metabolites, the identity of the GSH-B[a]P-7,8-dione and the NAC-B[a]P-7,8-dione was further validated by comparison to authentic synthesized standards. The pathways of B[a]P-7,8-dione metabolism in the three human lung cell lines are formation of GSH and NAC conjugates, reduction to the catechol followed by phase II conjugation reactions leading to its detoxification, monohydroxylation, as well as formation of the adenine adduct.

Benzo(a)pyrene-induced pulmonary inflammation, edema, surfactant dysfunction, and injuries in rats: alleviation by farnesol

Benzo(a)pyrene (B(a)P) is a well-known environmental contaminant and carcinogen. Its sources include tobacco smoke, automobile exhaust, forest fire, and other combustion processes. Farnesol, an active principle of Vachellia farnesiana and other aromatic plants, possesses preventive properties against various toxicities. Present study was designed to estimate chemopreventive effects of farnesol against B(a)P-induced pulmonary injuries. To determine the protective effects of farnesol, it was administered orally at 2 doses (100 and 200 mg/kg body weight [b.w.]) once daily for 14 days. Rats were exposed intratracheally to B(a)P, 5 mg/kg b.w. on days 12 and 14, thereafter assessed for pulmonary toxicities 24 hours post last dose of B(a)P. B(a)P-induced edema, inflammation, oxidative stress, and consequent damages in lungs were assessed in terms of total protein, total cell count, nitric oxide (NO), lactate dehydrogenase (LDH), alkaline phosphatase, and in bronchoalveolar lavage fluid (BALF). B(a)P also reduced the levels of phospholipids (lung surfactants) in BALF. However, pretreatment with farnesol at both the doses significantly reduced the lung injuries and inflammatory responses. Farnesol also protected the levels of phospholipids to normal when compared with control. It also modified the activities of B(a)P metabolizing enzymes NADPH-cytochrome P450 reductase, microsomal epoxide hydrolase (mEH), and glutathione S-transferase (GST) in lung tissue of rats. Present findings suggest a prominent role of farnesol against B(a)P-induced lung inflammation, edema, surfactant dysfunction, and epithelial damages in Wistar rats. In conclusion, farnesol shows lung protection against B(a)P toxicities in Wistar rats.

Association between mutation spectra and stable and unstable DNA adduct profiles in Salmonella for benzo[a]pyrene and dibenzo[a,l]pyrene

Benzo[a]pyrene (BP) and dibenzo[a,l]pyrene (DBP) are two polycyclic aromatic hydrocarbons (PAHs) that exhibit distinctly different mutagenicity and carcinogenicity profiles. Although some studies show that these PAHs produce unstable DNA adducts, conflicting data and arguments have been presented regarding the relative roles of these unstable adducts versus stable adducts, as well as oxidative damage, in the mutagenesis and tumor-mutation spectra of these PAHs. However, no study has determined the mutation spectra along with the stable and unstable DNA adducts in the same system with both PAHs. Thus, we determined the mutagenic potencies and mutation spectra of BP and DBP in strains TA98, TA100 and TA104 of Salmonella, and we also measured the levels of abasic sites (aldehydic-site assay) and characterized the stable DNA adducts ((32)P-postlabeling/HPLC) induced by these PAHs in TA104. Our results for the mutation spectra and site specificity of stable adducts were consistent with those from other systems, showing that DBP was more mutagenic than BP in TA98 and TA100. The mutation spectra of DBP and BP were significantly different in TA98 and TA104, with 24% of the mutations induced by BP in TA98 being complex frameshifts, whereas DBP produced hardly any of these mutations. In TA104, BP produced primarily GC to TA transversions, whereas DBP produced primarily AT to TA transversions. The majority (96%) of stable adducts induced by BP were at guanine, whereas the majority (80%) induced by DBP were at adenine. Although BP induced abasic sites, DBP did not. Most importantly, the proportion of mutations induced by DBP at adenine and guanine paralleled the proportion of stable DNA adducts induced by DBP at adenine and guanine; however, this was not the case for BP. Our results leave open a possible role for unstable DNA adducts in the mutational specificity of BP but not for DBP.

Estrogen promotes benzo[a]pyrene-induced lung carcinogenesis through oxidative stress damage and cytochrome c-mediated caspase-3 activation pathways in female mice

Estrogen may contribute to the development of smoking-induced lung cancer in women. To test this hypothesis, an mouse model was used to investigate the effects of 17 beta-estradiol (E2) on benzo[a]pyrene (B[a]P)-induced lung carcinogenesis. We found that B[a]P could cause oxidative stress damage, upregulate mitochondrial cytochrome-c and caspase-3 expression, induce lung carcinogenesis in female mice, E2 promoted these effects of B[a]P while tamoxifen (TAM) inhibited this effects of E2. We conclude that E2 can promote the tumorigenic effects of B[a]P in female mice, and oxidative stress damage and activation of cytochrome-c-mediated caspase-3 pathway may be involved in this process.

Formation and repair of tobacco carcinogen-derived bulky DNA adducts

DNA adducts play a central role in chemical carcinogenesis. The analysis of formation and repair of smoking-related DNA adducts remains particularly challenging as both smokers and nonsmokers exposed to smoke are repetitively under attack from complex mixtures of carcinogens such as polycyclic aromatic hydrocarbons and N-nitrosamines. The bulky DNA adducts, which usually have complex structure, are particularly important because of their biological relevance. Several known cellular DNA repair pathways have been known to operate in human cells on specific types of bulky DNA adducts, for example, nucleotide excision repair, base excision repair, and direct reversal involving O⁶-alkylguanine DNA alkyltransferase or AlkB homologs. Understanding the mechanisms of adduct formation and repair processes is critical for the assessment of cancer risk resulting from exposure to cigarette smoke, and ultimately for developing strategies of cancer prevention. This paper highlights the recent progress made in the areas concerning formation and repair of bulky DNA adducts in the context of tobacco carcinogen-associated genotoxic and carcinogenic effects.