Regulation of benzo[a]pyrene-mediated DNA- and glutathione-adduct formation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in human lung cells

Long-Term Storage Study of the Certified 1R6F Reference Cigarette

The first certified reference cigarette, 1R6F, was produced by the Center for Tobacco Reference Products at the University of Kentucky in 2015 and certified in 2016. 1R6F reference cigarettes have been stored at -20 °C since they were manufactured. 1R6F has been widely used as a control cigarette or a monitor for nonclinical investigational purposes in tobacco product analysis and scientific research. However, there is little published data to demonstrate the stability of the 1R6F cigarette. In this paper, we report the results of a long-term storage study of the 1R6F cigarette tobacco filler and the resulting mainstream smoke. 1R6F cigarettes were stored under different conditions (room temperature, refrigerator (4 °C), and freezer (-20 °C)) for 3 years since April 2017. The constituents in the cigarette tobacco filler (oven volatiles, nicotine, N'-nitrosornicotine (NNN), and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK)) and the mainstream smoke (nicotine, NNN, NNK, benzo[α]pyrene, carbon monoxide, total particulate matter) were analyzed. Some physical parameters (resistance to draw and ventilation) were also measured. Analysis of our data showed that no significant differences in these major constituents were detected after storage of the 1R6F cigarette at -20 °C for 3 years.

Cigarette smoking enhances the metabolic activation of the polycyclic aromatic hydrocarbon phenanthrene in humans

Although it is well established that human cytochrome P450 1 family enzymes are induced by cigarette smoking through activation of the Ah receptor, it is not known whether this leads to increased metabolic activation or detoxification of carcinogenic polycyclic aromatic hydrocarbons (PAH), which are present in cigarette smoke and the general environment. We gave oral doses of deuterated phenanthrene ([D10]Phe), a non-carcinogenic surrogate of carcinogenic PAH such as benzo[a]pyrene, to smokers (N = 170, 1 or 10 μg doses) and non-smokers (N = 57, 1 μg dose). Bioactivation products (dihydrodiol and tetraol) and detoxification products (phenols) of [D10]Phe were determined in 6-h urine to obtain a comprehensive metabolic profile. Cigarette smoking increased the bioactivation of [D10]Phe and decreased its detoxification resulting in significantly different metabolic patterns between smokers and non-smokers (P < 0.01), consistent with increased cancer risk in smokers. The Phe bioactivation ratios ([D10]PheT/total [D9]OHPhe) were significantly higher (2.3 (P < 0.01) to 4.8 (P < 0.001) fold) in smokers than non-smokers. With solid human in vivo evidence, our results for the first time demonstrate that cigarette smoking enhances the metabolic activation of Phe, structurally representative of carcinogenic PAH, in humans, strongly supporting their causal role in cancers caused by smoking. The results suggest potential new methods for identifying smokers who could be at particularly high risk for cancer.

Human Family 1-4 cytochrome P450 enzymes involved in the metabolic activation of xenobiotic and physiological chemicals: an update

This is an overview of the metabolic activation of drugs, natural products, physiological compounds, and general chemicals by the catalytic activity of cytochrome P450 enzymes belonging to Families 1-4. The data were collected from > 5152 references. The total number of data entries of reactions catalyzed by P450s Families 1-4 was 7696 of which 1121 (~ 15%) were defined as bioactivation reactions of different degrees. The data were divided into groups of General Chemicals, Drugs, Natural Products, and Physiological Compounds, presented in tabular form. The metabolism and bioactivation of selected examples of each group are discussed. In most of the cases, the metabolites are directly toxic chemicals reacting with cell macromolecules, but in some cases the metabolites formed are not direct toxicants but participate as substrates in succeeding metabolic reactions (e.g., conjugation reactions), the products of which are final toxicants. We identified a high level of activation for three groups of compounds (General Chemicals, Drugs, and Natural Products) yielding activated metabolites and the generally low participation of Physiological Compounds in bioactivation reactions. In the group of General Chemicals, P450 enzymes 1A1, 1A2, and 1B1 dominate in the formation of activated metabolites. Drugs are mostly activated by the enzyme P450 3A4, and Natural Products by P450s 1A2, 2E1, and 3A4. Physiological Compounds showed no clearly dominant enzyme, but the highest numbers of activations are attributed to P450 1A, 1B1, and 3A enzymes. The results thus show, perhaps not surprisingly, that Physiological Compounds are infrequent substrates in bioactivation reactions catalyzed by P450 enzyme Families 1-4, with the exception of estrogens and arachidonic acid. The results thus provide information on the enzymes that activate specific groups of chemicals to toxic metabolites.

New insights of CYP1A in endogenous metabolism: a focus on single nucleotide polymorphisms and diseases

Cytochrome P450 1A (CYP1A), one of the major CYP subfamily in humans, not only metabolizes xenobiotics including clinical drugs and pollutants in the environment, but also mediates the biotransformation of important endogenous substances. In particular, some single nucleotide polymorphisms (SNPs) for CYP1A genes may affect the metabolic ability of endogenous substances, leading to some physiological or pathological changes in humans. This review first summarizes the metabolism of endogenous substances by CYP1A, and then introduces the research progress of CYP1A SNPs, especially the research related to human diseases. Finally, the relationship between SNPs and diseases is discussed. In addition, potential animal models for CYP1A gene editing are summarized. In conclusion, CYP1A plays an important role in maintaining the health in the body.

Benzo[ a]pyrene Induction of Glutathione S-Transferases: An Activity-Based Protein Profiling Investigation

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants generated from combustion of carbon-based matter. Upon ingestion, these molecules can be bioactivated by cytochrome P450 monooxygenases to oxidized toxic metabolites. Some of these metabolites are potent carcinogens that can form irreversible adducts with DNA and other biological macromolecules. Conjugative enzymes, such as glutathione S-transferases or UDP-glucuronosyltransferases, are responsible for the detoxification and/or facilitate the elimination of these carcinogens. While responses to PAH exposures have been extensively studied for the bioactivating cytochrome P450 enzymes, much less is known regarding the response of glutathione S-transferases in mammalian systems. In this study, we investigated the expression and activity responses of murine hepatic glutathione S-transferases to benzo[ a]pyrene exposure using global proteomics and activity-based protein profiling for chemoproteomics, respectively. Using this approach, we identified several enzymes exhibiting increased activity including GSTA2, M1, M2, M4, M6, and P1. The activity of one GST enzyme, GSTA4, was found to be downregulated with increasing B[ a]P dose. Activity responses of several of these enzymes were identified as being expression-independent when comparing global and activity-based data sets, possibly alluding to as of yet unknown regulatory post-translational mechanisms.

Modulation of benzo[a]pyrene-DNA adduct formation by CYP1 inducer and inhibitor

Benzo[a]pyrene (BaP) is a well-studied pro-carcinogen that is metabolically activated by cytochrome P450 enzymes. Cytochrome P4501A1 (CYP1A1) has been considered to play a central role in the activation step, which is essential for the formation of DNA adducts. This enzyme is strongly induced by many different chemical agents, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which binds to the aryl hydrocarbon receptor (AhR). Therefore, AhR activators are suspected to have the potential to aggravate the toxicity of BaP through the induction of CYP1A1. Besides, CYP1A1 inhibitors, including its substrates, are estimated to have preventive effects against BaP toxicity. However, strangely, increased hepatic BaP–DNA adduct levels have been reported in Cyp1a1 knockout mice. Moreover, numerous reports describe that concomitant treatment of AhR activators reduced BaP–DNA adduct formation. In an experiment using several human cell lines, TCDD had diverse modulatory effects on BaP–DNA adducts, both enhancing and inhibiting their formation. In this review, we focus on the factors that could influence the BaP–DNA adduct formation. To interpret these complicated outcomes, we propose a hypothesis that CYP1A1 is a key enzyme for both generation and reduction of (±)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE), the major carcinogenic intermediate of BaP. Conversely, CYP1B1 is thought to contribute only to the metabolic activation of BaP related to carcinogenesis.

Aryl morpholino triazenes inhibit cytochrome P450 1A1 and 1B1

Many cytochrome P450 1A1 and 1B1 (CYP1A1 and CYP1B1) inhibitors, such as resveratrol, have planar, hydrophobic, aromatic rings in their structure and exhibit anti-cancer activity. Aryl morpholino triazenes have similar structural features and in addition contain a triazene unit consisting of three consecutive, conjugated nitrogen atoms. Several aryl morpholino triazenes, including 4-[(E)-2-(3,4,5-trimethoxyphenyl)diazenyl]-morpholine (2), were prepared from a reaction involving morpholine and a diazonium ion produced from different aniline derivatives, such as 3,4,5-trimethoxyaniline. The aryl morpholino triazenes were then screened at 100μM for their ability to inhibit CYP1A1 and CYP1B1 using ethoxyresorufin as the substrate. Triazenes that inhibited the enzymes to less than 80% of the uninhibited enzyme activity were assayed to determine their IC50 value. Compound 2 was the only triazene to inhibit both CYP1A1 and CYP1B1 to the same degree as resveratrol with IC50 values of 10μM and 18μM, respectively. Compounds 3 and 6 selectively inhibited CYP1B1 over CYP1A1 with IC values of 2μM and 7μM, respectively. Thus, aryl morpholino triazenes are a new class of compounds that can inhibit CYP1A1 and CYP1B1 and potentially prevent cancer.

Stabilization of membrane bound ATPases and lipid peroxidation by carotenoids from Chlorococcum humicola in Benzo(a)pyrene induced toxicity

OBJECTIVE

To identify the alteration of the membrane potential and the effect of carotenoid extracts from Chlorococcum humicola (C. humicola) on membrane bound ATPases and lipid peroxidation.

METHODS

The total carotenoids were extracted from C. humicola. Four groups of Swiss albino mice were treated as control, Benzo(a)pyrene [B(a)P], total carotenoids, B(a)P + total carotenoids respectively for a period of 60 days. Membrane lipid peroxidation and ATPases (Total ATPases, Ca(2+)- ATPases, Mg(2+) - ATPases, Na(+)K(+) - ATPase) were determined in lung, liver and erythrocyte samples.

RESULTS

The activity of total ATPase was found to be significantly increased in the B(a)P treated liver and lung tissue. Erythrocyte membrane also showed higher ATPase activity which was significantly reverted on total carotenoid treatment.

CONCLUSIONS

It can be concluded that the changes in membrane potential favour the functional deterioration of physiological system. The overall findings demonstrates that the animals post treated with carotenoid extract from C. humicola may maintains the alterations in membrane bound ATPase and lipid peroxidation in tissues against the carcinogenic chemical and hence aid in establishing the membrane potential action. Therefore C. humicola can be further extended to exploits its possible application for various health benefits as neutraceuticals and food additives.

Analysis of the AHR gene proximal promoter GGGGC-repeat polymorphism in lung, breast, and colon cancer

The aryl hydrocarbon receptor (AhR) regulates expression of numerous genes, including those of the CYP1 gene family. With the goal of determining factors that control AHR gene expression, our studies are focused on the role of the short tandem repeat polymorphism, (GGGGC)n, located in the proximal promoter of the human AHR gene. When luciferase constructs containing varying GGGGC repeats were transfected into cancer cell lines derived from the lung, colon, and breast, the number of GGGGC repeats affected AHR promoter activity. The number of GGGGC repeats was determined in DNA from 327 humans and from 38 samples representing 5 species of non-human primates. In chimpanzees and 3 species of macaques, only (GGGGC)2 alleles were observed; however, in western gorilla, (GGGGC)n alleles with n=2, 4, 5, 6, 7, and 8 were identified. In all human populations examined, the frequency of (GGGGC)n was n=4>5≫2, 6. When frequencies of the (GGGGC)n alleles in DNA from patients with lung, colon, or breast cancer were evaluated, the occurrence of (GGGGC)2 was found to be 8-fold more frequent among lung cancer patients in comparison with its incidence in the general population, as represented by New York State neonates. Analysis of matched tumor and non-tumor DNA samples from the same individuals provided no evidence of microsatellite instability. These studies indicate that the (GGGGC)n short tandem repeats are inherited, and that the (GGGGC)2 allele in the AHR proximal promoter region should be further investigated with regard to its potential association with lung cancer susceptibility.

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.

Quantitation of enantiomers of r-7,t-8,9,c-10-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]-pyrene in human urine: evidence supporting metabolic activation of benzo[a]pyrene via the bay region diol epoxide

Benzo[a]pyrene (BaP), a potent polycyclic aromatic hydrocarbon carcinogen, is widely distributed in the human environment. All humans are exposed to BaP through the diet and contact with the general environment; cigarette smokers have higher exposure. An important pathway of BaP metabolism proceeds through formation of diol epoxides including the 'bay region diol epoxide' 7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [BaP-(7R,8S)-diol-(9S,10R)-epoxide] and the 'reverse diol epoxide' 9S,10R-dihydroxy-7R,8S-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene [BaP-(9S,10R)-diol-(7R,8S)-epoxide]. The bay region diol epoxide is considered a major ultimate carcinogen of BaP based on studies in cell culture and laboratory animals, but the available data in humans are less convincing. The bay region diol epoxide and the reverse diol epoxide react with H2O to produce enantiomeric BaP-tetraols that are excreted in the urine. We used chiral stationary-phase high-performance liquid chromatography and gas chromatography-negative ion chemical ionisation-tandem mass spectrometry to quantify these enantiomeric BaP-tetraols in the urine of 25 smokers and 25 non-smokers. The results demonstrated that the BaP-tetraol enantiomer representing the carcinogenic bay region diol epoxide pathway accounted for 68±6% (range 56-81%) of total BaP-tetraol in smokers and 64±6% (range 46-78%) in non-smokers. Levels of the major BaP-tetraol enantiomer decreased by 75% in smokers who quit smoking. These data provide convincing evidence in support of the bay region diol epoxide mechanism of BaP carcinogenesis in humans.

Recent developments in DNA adduct analysis by mass spectrometry: a tool for exposure biomonitoring and identification of hazard for environmental pollutants

DNA adducts represent an important category of biomarkers for detection and exposure surveillance of potential carcinogenic and genotoxic chemicals in the environment. Sensitive and specific analytical methods are required to detect and differentiate low levels of adducts from native DNA from in vivo exposure. In addition to biomonitoring of environmental pollutants, analytical methods have been developed for structural identification of adducts which provides fundamental information for determining the toxic pathway of hazardous chemicals. In order to achieve the required sensitivity, mass spectrometry has been increasingly utilized to quantify adducts at low levels as well as to obtain structural information. Furthermore, separation techniques such as chromatography and capillary electrophoresis can be coupled to mass spectrometry to increase the selectivity. This review will provide an overview of advances in detection of adducted and modified DNA by mass spectrometry with a focus on the analysis of nucleosides since 2007. Instrument advances, sample and instrument considerations, and recent applications will be summarized in the context of hazard assessment. Finally, advances in biomonitoring applying mass spectrometry will be highlighted. Most importantly, the usefulness of DNA adducts measurement and detection will be comprehensively discussed as a tool for assessment of in vitro and in vivo exposure to environmental pollutants.

Polymorphisms in DNA repair genes of XRCC1, XPA, XPC, XPD and associations with lung cancer risk in Chinese people

BACKGROUND

The carcinogenic chemicals and reactive oxygen species in tobacco can result in DNA damage. DNA repair genes play an important role in maintaining genome integrity. Genetic polymorphisms of DNA repair genes and smoking may contribute to susceptibility of lung cancer.

METHODS

In this hospital-based case-control study, we investigated the relationship between 13 tagging single nucleotide polymorphisms (SNPs) in base excision repair pathway and nucleotide excision repair pathway genes, smoking, and lung cancer susceptibility. Thirteen tag SNPs were genotyped in 265 lung cancer patients and 301 healthy controls. Logistic regression and multifactor dimensionality reduction method were applied to explore the association and high-order gene-gene and gene-smoking interaction.

RESULTS

In single tag SNP analysis, XPA rs2808668, XPC rs2733533, and XPD rs1799787 were significantly associated with lung cancer susceptibility. Joint effects analysis of XPA rs2808668, XPC rs2733533 and XPD rs1799787 showed that there was an increased risk of lung cancer with increasing numbers of risk alleles. Haplotype analysis showed that XRCC1 (rs25487, rs1799782, rs3213334) GCC had a positive association with lung cancer. Analysis of gene-gene and gene-smoking interaction by multifactor dimensionality reduction showed that a positive interaction existed between the four genes and smoking. The two-factor model, including XPC rs2755333 and smoking, had the best prediction ability for lung cancer. Compared with the C/C genotype of XPC rs2733533 and no smoking, the combination of genotype A carriers with XPC rs2733533 and heavy smokers (≥30 pack-year) had a 13.32-fold risk of lung cancer.

CONCLUSION

Our results suggest multiple genetic variants in multiple DNA repair genes may jointly contribute to lung cancer risk through gene-gene and gene-smoking interactions.

Systems toxicology approach to understand the kinetics of benzo(a)pyrene uptake, biotransformation, and DNA adduct formation in a liver cell model

Cell-based models are important for deriving mechanistic information about stress response pathways that have evolved to protect cells from toxic insult, such as exposure to environmental pollutants. One determinant of the stress response is the amount of chemical entering the cell and the cell's ability to detoxify and remove the chemical. If the stress response is overwhelmed, an adverse outcome will ensue. It was the goal of our study to quantify uptake and elimination rates of benzo(a)pyrene (BaP), a ubiquitous environmental pollutant, in a murine liver cell line. We evaluated the kinetic behavior in the context of BaP uptake, biotransformation, DNA adduct formation and repair along with the transcriptional and cell proliferation response. A low (50 nM) and a high (5 μM) BaP concentration were chosen in order to differentiate the role of exposure concentration in the time-resolved interaction of BaP with cells. While rates of uptake and the initial transcriptional response were similar for both BaP concentrations, cells exposed to 50 nM BaP completely recovered from exposure within 24 h, whereas cells exposed to 5 μM BaP did not. Biotransformation proceeded faster on 50 nM BaP, and the few DNA adducts formed were completely repaired after transient cell cycle arrest. In contrast, DNA adducts greatly accumulated in cells exposed to 5 μM BaP, despite significant biotransformation; complete cell cycle arrest and toxicity evolved. On the basis of the kinetic rate constants and cellular response, we conclude that at least short-term, pulsed exposures to 50 nM BaP, which we consider environmentally relevant, can be handled by cells without adverse outcome. Further studies are needed to determine the ability of cells to recover from repeated exposure. Our study emphasizes the importance of quantifying chemical uptake and fate in cell models to differentiate a stress response from an adverse outcome for better risk assessment.

The extreme variety of genotoxic response to benzo[a]pyrene in three different human cell lines from three different organs

Polycyclic aromatic hydrocarbons (PAHs) are associated with occupational exposure and urban atmospheric pollution. Determination of the genotoxic properties of these compounds is thus of outmost importance. For this purpose a variety of cellular models have been widely used. Reliable results can however only be obtained with models reflecting the specific sensitivity of different organs towards PAHs. In this work, we compared the response to benzo[a]pyrene in cell lines from human lungs (A549) and bladder (T24); two important target organs for PAHs-induced cancer. Human hepatocytes (HepG2) were used as a reference, although liver is not a concern for PAHs carcinogenesis. Adducts arising from the ultimate diol-epoxide metabolite of B[a]P, BPDE, were found to be produced in a dose-dependent manner in HepG2. BPDE DNA adducts were not detected in T24 and in A549 their formation was found to be most efficient at the lowest concentration studied (0.2 µM). These results are probably explained by differences in induction and activity of phase I metabolization enzymes, as well as by proteins eliminating the B[a]P epoxide in A549. In addition to BPDE adducts, oxidative DNA damage, namely strand breaks and oxidized purines were measured and found to be produced only in minute amounts in all three cell lines. In summary, our results emphasize the large differences in the response of cells originating from different organs. Our data also point out the importance of carefully selecting the doses used in in vitro toxicological experiments. The example of A549 shows that working at high doses may lead to an underestimation of the risk. Finally, the choice of method for evaluating genotoxicity appears to be of crucial importance. The comet assay does not seem to be the best method for a compound like B[a]P which induces stable adducts causing limited oxidative damage.

The Red Flour Beetle Tribolium castaneum as a Model to Monitor Food Safety and Functionality

: Food quality is a fundamental issue all over the world. There are two major requirements to provide the highest quality of food: having the lowest reachable concentrations of health-threatening ingredients or contaminants and having the optimal concentrations of health-improving functional ingredients. Often, the boundaries of both requirements are blurred, as might be best exemplified by nutraceuticals (enriched food products invented to prevent or even treat diseases), for which undesirable side effects have been reported sometimes. Accordingly, there is an increasing need for reliable methods to screen for health effects of wanted or unwanted ingredients in a complex food matrix before more complex model organisms or human probands become involved. In this chapter, we present the red flour beetle Tribolium castaneum as a model organism to screen for effects of complex foods on healthspan or lifespan by assessing the survival of the beetles under heat stress at 42 °C after feeding different diets. There is a higher genetic homology between T. castaneum and humans when compared to other invertebrate models, such as Drosophila melanogaster or Caenorhabditis elegans. Therefore, the red flour beetle appears as an interesting model to study interactions between genes and food ingredients, with relevance for stress resistance and lifespan. In that context, we provide data showing reduced lifespans of the beetles when the food-relevant contaminant benz(a)pyrene is added to the flour they were fed on, whereas a lifespan extension was observed in beetles fed on flour enriched with an extract of red wine.

Contributions of human enzymes in carcinogen metabolism

Considerable support exists for the roles of metabolism in modulating the carcinogenic properties of chemicals. In particular, many of these compounds are pro-carcinogens that require activation to electrophilic forms to exert genotoxic effects. We systematically analyzed the existing literature on the metabolism of carcinogens by human enzymes, which has been developed largely in the past 25 years. The metabolism and especially bioactivation of carcinogens are dominated by cytochrome P450 enzymes (66% of bioactivations). Within this group, six P450s--1A1, 1A2, 1B1, 2A6, 2E1, and 3A4--accounted for 77% of the P450 activation reactions. The roles of these P450s can be compared with those estimated for drug metabolism and should be considered in issues involving enzyme induction, chemoprevention, molecular epidemiology, interindividual variations, and risk assessment.

Multidrug resistance protein (MRP) 4 attenuates benzo[a]pyrene-mediated DNA-adduct formation in human bronchoalveolar H358 cells

Multi-drug resistance protein (MRP) 4, an ATP-binding cassette (ABC) transporter, has broad substrate specificity. It facilitates the transport of bile salt conjugates, conjugated steroids, nucleoside analogs, eicosanoids, and cardiovascular drugs. Recent studies in liver carcinoma cells and hepatocytes showed that MRP4 expression is regulated by the aryl hydrocarbon receptor (AhR) and nuclear factor E2-related factor 2 (Nrf2). The AhR has particular importance in the lung and is most commonly associated with the up-regulation of cytochrome P-450 (CYP)-mediated metabolism of benzo[a]pyrene (B[a]P) to reactive intermediates. Treatment of H358, human bronchoalveolar, cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or (-)-benzo[a]pyrene-7,8-dihydro-7,8-diol (B[a]P-7,8-dihydrodiol), the proximate carcinogen of B[a]P, revealed that MRP4 expression was increased compared to control. This suggested that MRP4 expression might contribute to the paradoxical decrease in (+)-7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene-2'-deoxyguanosine ((+)-anti-trans-B[a]PDE-dGuo) DNA-adducts observed in TCDD-treated H358 cells. We have now found that decreased MRP4 expression induced by a short hairpin RNA (shRNA), or chemical inhibition with probenecid, increased (+)-anti-trans-B[a]PDE-dGuo formation in cells treated with (-)-B[a]P-7,8-dihydrodiol, but not the ultimate carcinogen (+)-anti-trans-B[a]PDE. Thus, up-regulation of MRP4 increased cellular efflux of (-)-B[a]P-7,8-dihydrodiol, which attenuated DNA-adduct formation. This is the first report identifying a specific MRP efflux transporter that decreases DNA damage arising from an environmental carcinogen.