Identification and characterization of a novel benzo[a]pyrene-derived DNA adduct

Benzo[a]pyrene-Induced Genotoxicity in Rats Is Affected by Co-Exposure to Sudan I by Altering the Expression of Biotransformation Enzymes

The environmental pollutant benzo[a]pyrene (BaP) is a human carcinogen that reacts with DNA after metabolic activation catalysed by cytochromes P450 (CYP) 1A1 and 1B1 together with microsomal epoxide hydrolase. The azo dye Sudan I is a potent inducer of CYP1A1/2. Here, Wistar rats were either treated with single doses of BaP (150 mg/kg bw) or Sudan I (50 mg/kg bw) alone or with both compounds in combination to explore BaP-derived DNA adduct formation in vivo. Using ³²P-postlabelling, DNA adducts generated by BaP-7,8-dihydrodiol-9,10-epoxide were found in livers of rats treated with BaP alone or co-exposed to Sudan I. During co-exposure to Sudan I prior to BaP treatment, BaP-DNA adduct levels increased 2.1-fold in comparison to BaP treatment alone. Similarly, hepatic microsomes isolated from rats exposed to Sudan I prior to BaP treatment were also the most effective in generating DNA adducts in vitro with the activated metabolites BaP-7,8-dihydrodiol or BaP-9-ol as intermediates. DNA adduct formation correlated with changes in the expression and/or enzyme activities of CYP1A1, 1A2 and 1B1 in hepatic microsomes. Thus, BaP genotoxicity in rats in vivo appears to be related to the enhanced expression and/or activity of hepatic CYP1A1/2 and 1B1 caused by exposure of rats to the studied compounds. Our results indicate that the industrially employed azo dye Sudan I potentiates the genotoxicity of the human carcinogen BaP, and exposure to both substances at the same time seems to be hazardous to humans.

Effects of catechin hydrate in benzo[a]pyrene-induced lung toxicity: roles of oxidative stress, apoptosis, and DNA damage

The major sources for human exposure to Benzo [a] pyrene (B[a]P) are contaminated food, water, and inhalation of polycyclic aromatic hydrocarbon. B[a]P is a well-known human genotoxic carcinogen (IARC Group 1). It has a tumorigenic potential in virtually all in vivo experimental animal model systems. The study aimed to evaluate the effect of catechin hydrate (CH) against B [a] P-induced toxicity in the lung of adult albino rats. Thirty-six adult male albino rats (150-200 g) were divided into six groups, three control groups, and three experimental groups: B[a] P-treated group, (CH)-treated group, and B[a] P+(CH)-treated group. At the end of the fourth week of the study, blood samples and lung tissues were obtained for the biochemical and genotoxicity, RT-PCR, histopathological, and immunohistochemical investigations, respectively. Our results clarified that B[a] P exposure caused a subsequent decrease in the activities of antioxidant enzymes (SOD, CAT), and conversely (MDA) levels elevated markedly. Also, B[a] P induced DNA damages and activated the apoptotic pathway, presented by upregulated Bax, caspase-3, and downregulated Bcl-2 gens. However, treatment with CH increased antioxidant enzymes as well as regulated apoptosis. Discernible histological changes in the lung also supported the protective effects of CH. These findings suggested that CH is an effective natural product that attenuates Benzo [a] pyrene-induced lung toxicity.

Assessment of 9-OH- and 7,8-diol-benzo[a]pyrene in Blood as Potent Markers of Cognitive Impairment Related to benzo[a]pyrene Exposure: An Animal Model Study

The potent neurotoxicity of benzo[a]pyrene (B[a]P) has been suggested to be a susceptibility factor accelerating the onset of brain tumours and the emergence of neurobehavioural disturbances. B[a]P has been shown to be neurotoxic, acting directly on both the central and peripheral nervous systems, as well as indirectly via peripheral organs like liver and gut. By using a realistic B[a]P exposure scenario (0.02-200 mg/kg/day, 10 days) in mice, we elucidated brain-specific B[a]P metabolism and at identified hydroxylated B[a]P metabolites in serum which could be used as markers of cognitive impairment. Repeated oral administration of B[a]P led to, at the doses of 20 and 200 mg/kg/day, significant overexpression of Cyp1a1/Cyp1b1 in 2 out of the 3 brain regions considered, thereby suggesting the ability of the brain to metabolize B[a]P itself. At the same doses, mice exhibited a reduction in anxiety in both the elevated plus maze and the hole board apparatus. Concomitantly, B[a]P triggered dose-dependent changes in Nmda subunit expression (Nr1 and Nr2a/Nr2b) in areas involved in cognition. We detected 9-OH-B[a]P and 7,8-diol-B[a]P in serum at the level for which cognitive impairment was observed. We suggest that these metabolites may, in the future be exploited as potent biomarkers of B[a]P-induced cognitive impairments.

Protective Effects of Myricetin on Benzo[a]pyrene-Induced 8-Hydroxy-2'-Deoxyguanosine and BPDE-DNA Adduct

Benzo[a]pyrene (B[a]P), a group 1 carcinogen, induces mutagenic DNA adducts. Myricetin is present in many natural foods with diverse biological activities, such as anti-oxidative and anti-cancer activities. The aim of this study was to investigate the protective effects of myricetin against B[a]P-induced toxicity. Treatment of B[a]P induced cytotoxicity on HepG2 cells, whereas co-treatment of myricetin with B[a]P reduced the formation of the B[a]P-7,8-dihydrodiol-9,10-epoxide (BPDE)-DNA adduct, which recovered cell viability. Furthermore, we found a protective effect of myricetin against B[a]P-induced genotoxicity in rats, via myricetin-induced inhibition of 8-hydroxy-2′-deoxyguanosine (8-OHdG) and BPDE-DNA adduct formation in the liver, kidney, colon, and stomach tissue. This inhibition was more prominent in the liver than in other tissues. Correspondingly, myricetin regulated the phase I and II enzymes that inhibit B[a]P metabolism and B[a]P metabolites conjugated with DNA by reducing and inducing CYP1A1 and glutathione S-transferase (GST) expression, respectively. Taken together, this showed that myricetin attenuated B[a]P-induced genotoxicity via regulation of phase I and II enzymes. Our results suggest that myricetin is anti-genotoxic, and prevents oxidative DNA damage and BPDE-DNA adduct formation via regulation of phase I and II enzymes.

Exposure to endocrine disruptors 17alpha-ethinylestradiol and estradiol influences cytochrome P450 1A1-mediated genotoxicity of benzo[a]pyrene and expression of this enzyme in rats

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17α-ethinylestradiol (EE2) and estradiol affect genotoxicity of benzo[a]pyrene (BaP) in rats.

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Cytochrome P450 (CYP) 1A1 and 1B1 are induced in rats by BaP but not EE2 and estradiol.

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Exposure of rats to EE2, estradiol and BaP decreased BaP-DNA adduct formation in vivo.

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The decrease results from inhibition of CYP1A1-mediated BaP activation by EE2 and estradiol.

Endocrine disruptors (EDs) are compounds that interfere with the balance of the endocrine system by mimicking or antagonising the effects of endogenous hormones, by altering the synthesis and metabolism of natural hormones, or by modifying hormone receptor levels. The synthetic estrogen 17α-ethinylestradiol (EE2) and the environmental carcinogen benzo[a]pyrene (BaP) are exogenous EDs whereas the estrogenic hormone 17β-estradiol is a natural endogenous ED. Although the biological effects of these individual EDs have partially been studied previously, their toxicity when acting in combination has not yet been investigated. Here we treated Wistar rats with BaP, EE2 and estradiol alone or in combination and studied the influence of EE2 and estradiol on: (i) the expression of cytochrome P450 (CYP) 1A1 and 1B1 in rat liver on the transcriptional and translational levels; (ii) the inducibility of these CYP enzymes by BaP in this rat organ; (iii) the formation of BaP-DNA adducts in rat liver in vivo; and (iv) the generation of BaP-induced DNA adducts after activation of BaP with hepatic microsomes of rats exposed to BaP, EE2 and estradiol and with recombinant rat CYP1A1 in vitro. BaP acted as a strong and moderate inducer of CYP1A1 and 1B1 in rat liver, respectively, whereas EE2 or estradiol alone had no effect on the expression of these enzymes. However, when EE2 was administered to rats together with BaP, it significantly decreased the potency of BaP to induce CYP1A1 and 1B1 gene expression. For EE2, but not estradiol, this also correlated with a reduction of BaP-induced CYP1A1 enzyme activity in rat hepatic microsomes. Further, while EE2 and estradiol did not form covalent adducts with DNA, they affected BaP-derived DNA adduct formations in vivo and in vitro. The observed decrease in BaP-DNA adduct levels in rat liver in vivo resulted from the inhibition of CYP1A1-mediated BaP bioactivation by EE2 and estradiol. Our results indicate that BaP genotoxicity mediated through its activation by CYP1A1 in rats in vivo is modulated by estradiol and its synthetic derivative EE2.

Cytochrome b 5 impacts on cytochrome P450-mediated metabolism of benzo[a]pyrene and its DNA adduct formation: studies in hepatic cytochrome b 5 /P450 reductase null (HBRN) mice

Benzo[a]pyrene (BaP) is an environmental pollutant that, based on evidence largely from in vitro studies, exerts its genotoxic effects after metabolic activation by cytochrome P450s. In the present study, Hepatic Reductase Null (HRN) and Hepatic Cytochrome b 5 /P450 Reductase Null (HBRN) mice have been used to study the role of P450s in the metabolic activation of BaP in vivo. In HRN mice, cytochrome P450 oxidoreductase (POR), the electron donor to P450, is deleted specifically in hepatocytes. In HBRN mice the microsomal haemoprotein cytochrome b 5 , which can also act as an electron donor from cytochrome b 5 reductase to P450s, is also deleted in the liver. Wild-type (WT), HRN and HBRN mice were treated by i.p. injection with 125 mg/kg body weight BaP for 24 h. Hepatic microsomal fractions were isolated from BaP-treated and untreated mice. In vitro incubations carried out with BaP-pretreated microsomal fractions, BaP and DNA resulted in significantly higher BaP-DNA adduct formation with WT microsomal fractions compared to those from HRN or HBRN mice. Adduct formation (i.e. 10-(deoxyguanosin-N2-yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydro-BaP [dG-N2-BPDE]) correlated with observed CYP1A activity and metabolite formation (i.e. BaP-7,8-dihydrodiol) when NADPH or NADH was used as enzymatic cofactors. BaP-DNA adduct levels (i.e. dG-N2-BPDE) in vivo were significantly higher (~ sevenfold) in liver of HRN mice than WT mice while no significant difference in adduct formation was observed in liver between HBRN and WT mice. Our results demonstrate that POR and cytochrome b 5 both modulate P450-mediated activation of BaP in vitro. However, hepatic P450 enzymes in vivo appear to be more important for BaP detoxification than its activation.

Comparison of human cytochrome P450 1A1-catalysed oxidation of benzo[a]pyrene in prokaryotic and eukaryotic expression systems

ABSTRACT

Cytochrome P450 (CYP) 1A1 is the most important enzyme activating and detoxifying the human carcinogen benzo[a]pyrene (BaP). In the previous studies, we had shown that not only the canonic NADPH:CYP oxidoreductase (POR) can act as electron donor but also cytochrome b5 and its reductase, NADH:cytochrome b5 reductase. Here, we studied the role of the expression system used on the metabolites generated and the levels of DNA adducts formed by activated BaP. We used an eukaryotic and a prokaryotic cellular system (Supersomes, microsomes isolated from insect cells, and Bactosomes, a membrane fraction of Escherichia coli, each transfected with cDNA of human CYP1A1 and POR). These were reconstituted with cytochrome b5 with and without NADH:cytochrome b5 reductase. We evaluated the effectiveness of each cofactor, NADPH and NADH, to mediate BaP metabolism. We found that both systems differ in catalysing the reactions activating and detoxifying BaP. Two BaP-derived DNA adducts were generated by the CYP1A1-Supersomes, both in the presence of NADPH and NADH, whereas NADPH but not NADH was able to support this reaction in the CYP1A1-Bactosomes. Seven BaP metabolites were found in Supersomes with NADPH or NADH, whereas NADPH but not NADH was able to generate five BaP metabolites in Bactosomes. Our study demonstrates different catalytic efficiencies of CYP1A1 expressed in prokaryotic and eukaryotic cells in BaP bioactivation indicating some limitations in the use of E. coli cells for such studies.

GRAPHICAL ABSTRACT

NADH:Cytochrome b5 Reductase and Cytochrome b5 Can Act as Sole Electron Donors to Human Cytochrome P450 1A1-Mediated Oxidation and DNA Adduct Formation by Benzo[a]pyrene

Benzo[a]pyrene (BaP) is a human carcinogen that covalently binds to DNA after activation by cytochrome P450 (P450). Here, we investigated whether NADH:cytochrome b5 reductase (CBR) in the presence of cytochrome b5 can act as sole electron donor to human P450 1A1 during BaP oxidation and replace the canonical NADPH:cytochrome P450 reductase (POR) system. We also studied the efficiencies of the coenzymes of these reductases, NADPH as a coenzyme of POR, and NADH as a coenzyme of CBR, to mediate BaP oxidation. Two systems containing human P450 1A1 were utilized: human recombinant P450 1A1 expressed with POR, CBR, epoxide hydrolase, and cytochrome b5 in Supersomes and human recombinant P450 1A1 reconstituted with POR and/or with CBR and cytochrome b5 in liposomes. BaP-9,10-dihydrodiol, BaP-7,8-dihydrodiol, BaP-1,6-dione, BaP-3,6-dione, BaP-9-ol, BaP-3-ol, a metabolite of unknown structure, and two BaP-DNA adducts were generated by the P450 1A1-Supersomes system, both in the presence of NADPH and in the presence of NADH. The major BaP-DNA adduct detected by (32)P-postlabeling was characterized as 10-(deoxyguanosin-N(2)-yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydro-BaP (assigned adduct 1), while the minor adduct is probably a guanine adduct derived from 9-hydroxy-BaP-4,5-epoxide (assigned adduct 2). BaP-3-ol as the major metabolite, BaP-9-ol, BaP-1,6-dione, BaP-3,6-dione, an unknown metabolite, and adduct 2 were observed in the system using P450 1A1 reconstituted with POR plus NADPH. When P450 1A1 was reconstituted with CBR and cytochrome b5 plus NADH, BaP-3-ol was the predominant metabolite too, and an adduct 2 was also generated. Our results demonstrate that the NADH/cytochrome b5/CBR system can act as the sole electron donor both for the first and second reduction of P450 1A1 during the oxidation of BaP in vitro. They suggest that NADH-dependent CBR can replace NADPH-dependent POR in the P450 1A1-catalyzed metabolism of BaP.

NADPH- and NADH-dependent metabolism of and DNA adduct formation by benzo[a]pyrene catalyzed with rat hepatic microsomes and cytochrome P450 1A1

ABSTRACT

Benzo[a]pyrene (BaP) is a human carcinogen that covalently binds to DNA after metabolic activation by cytochrome P450 (CYP) enzymes. Here we investigated the efficiencies of rat hepatic microsomes and rat recombinant CYP1A1 expressed with its reductase, NADPH:CYP oxidoreductase (POR), NADH:cytochrome b5 reductase, epoxide hydrolase and/or cytochrome b5 in Supersomes™ to metabolize this carcinogen. We also studied the effectiveness of coenzymes of two of the microsomal reductases, NADPH as a coenzyme of POR, and NADH as a coenzyme of NADH:cytochrome b5 reductase, to mediate BaP metabolism in these systems. Up to eight BaP metabolites and two DNA adducts were generated by the systems, both in the presence of NADPH and NADH. Among BaP metabolites, BaP-9,10-dihydrodiol, BaP-4,5-dihydrodiol, BaP-7,8-dihydrodiol, BaP-1,6-dione, BaP-3,6-dione, BaP-9-ol, BaP-3-ol, and a metabolite of unknown structure were formed by hepatic microsomes and rat CYP1A1. One of two DNA adducts formed by examined enzymatic systems (rat hepatic microsomes and rat CYP1A1) was characterized to be 10-(deoxyguanosin-N2-yl)-7,8,9-trihydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene (dG-N2-BPDE), while another adduct has similar chromatographic properties on polyethylaneimine-cellulose thin layer chromatography to a guanine adduct derived from reaction with 9-hydroxy-BaP-4,5-oxide. In the presence of either of the reductase cofactors tested, NADPH or NADH, cytochrome b5 stimulated CYP1A1-mediated formation of both BaP-DNA adducts. The results demonstrate that NADH can act as a sole electron donor for both the first and the second reduction of CYP1A1 during its reaction cycle catalyzing oxidation of BaP, and suggest that the NADH:cytochrome b5 reductase as the NADH-dependent reductase might substitute POR in this enzymatic system.

GRAPHICAL ABSTRACT

Comparison of toxicogenomics and traditional approaches to inform mode of action and points of departure in human health risk assessment of benzo[a]pyrene in drinking water

Toxicogenomics is proposed to be a useful tool in human health risk assessment. However, a systematic comparison of traditional risk assessment approaches with those applying toxicogenomics has never been done. We conducted a case study to evaluate the utility of toxicogenomics in the risk assessment of benzo[a]pyrene (BaP), a well-studied carcinogen, for drinking water exposures. Our study was intended to compare methodologies, not to evaluate drinking water safety. We compared traditional (RA1), genomics-informed (RA2) and genomics-only (RA3) approaches. RA2 and RA3 applied toxicogenomics data from human cell cultures and mice exposed to BaP to determine if these data could provide insight into BaP's mode of action (MOA) and derive tissue-specific points of departure (POD). Our global gene expression analysis supported that BaP is genotoxic in mice and allowed the development of a detailed MOA. Toxicogenomics analysis in human lymphoblastoid TK6 cells demonstrated a high degree of consistency in perturbed pathways with animal tissues. Quantitatively, the PODs for traditional and transcriptional approaches were similar (liver 1.2 vs. 1.0 mg/kg-bw/day; lungs 0.8 vs. 3.7 mg/kg-bw/day; forestomach 0.5 vs. 7.4 mg/kg-bw/day). RA3, which applied toxicogenomics in the absence of apical toxicology data, demonstrates that this approach provides useful information in data-poor situations. Overall, our study supports the use of toxicogenomics as a relatively fast and cost-effective tool for hazard identification, preliminary evaluation of potential carcinogens, and carcinogenic potency, in addition to identifying current limitations and practical questions for future work.

Punicalagin and ellagic acid demonstrate antimutagenic activity and inhibition of benzo[a]pyrene induced DNA adducts

Punicalagin (PC) is an ellagitannin found in the fruit peel of Punica granatum. We have demonstrated antioxidant and antigenotoxic properties of Punica granatum and showed that PC and ellagic acid (EA) are its major constituents. In this study, we demonstrate the antimutagenic potential, inhibition of BP-induced DNA damage, and antiproliferative activity of PC and EA. Incubation of BP with rat liver microsomes, appropriate cofactors, and DNA in the presence of vehicle or PC and EA showed significant inhibition of the resultant DNA adducts, with essentially complete inhibition (97%) at 40 μM by PC and 77% inhibition by EA. Antimutagenicity was tested by Ames test. PC and EA dose-dependently and markedly antagonized the effect of tested mutagens, sodium azide, methyl methanesulfonate, benzo[a]pyrene, and 2-aminoflourine, with maximum inhibition of mutagenicity up to 90 percent. Almost all the doses tested (50–500 μM) exhibited significant antimutagenicity. A profound antiproliferative effect on human lung cancer cells was also shown with PC and EA. Together, our data show that PC and EA are pomegranate bioactives responsible for inhibition of BP-induced DNA adducts and strong antimutagenic, antiproliferative activities. However, these compounds are to be evaluated in suitable animal model to assess their therapeutic efficacy against cancer.

Cytochrome b5 and epoxide hydrolase contribute to benzo[a]pyrene-DNA adduct formation catalyzed by cytochrome P450 1A1 under low NADPH:P450 oxidoreductase conditions

In previous studies we had administered benzo[a]pyrene (BaP) to genetically engineered mice (HRN) which do not express NADPH:cytochrome P450 oxidoreductase (POR) in hepatocytes and observed higher DNA adduct levels in livers of these mice than in wild-type mice. To elucidate the reason for this unexpected finding we have used two different settings for in vitro incubations; hepatic microsomes from control and BaP-pretreated HRN mice and reconstituted systems with cytochrome P450 1A1 (CYP1A1), POR, cytochrome b5, and epoxide hydrolase (mEH) in different ratios. In microsomes from BaP-pretreated mice, in which Cyp1a1 was induced, higher levels of BaP metabolites were formed, mainly of BaP-7,8-dihydrodiol. At a low POR:CYP1A1 ratio of 0.05:1 in the reconstituted system, the amounts of BaP diones and BaP-9-ol formed were essentially the same as at an equimolar ratio, but formation of BaP-3-ol was ∼ 1.6-fold higher. Only after addition of mEH were BaP dihydrodiols found. Two BaP-DNA adducts were formed in the presence of mEH, but only one when CYP1A1 and POR were present alone. At a ratio of POR:CYP1A1 of 0.05:1, addition of cytochrome b5 increased CYP1A1-mediated BaP oxidation to most of its metabolites indicating that cytochrome b5 participates in the electron transfer from NADPH to CYP1A1 required for enzyme activity of this CYP. BaP-9-ol was formed even by CYP1A1 reconstituted with cytochrome b5 without POR. Our results suggest that in livers of HRN mice Cyp1a1, cytochrome b5 and mEH can effectively activate BaP to DNA binding species, even in the presence of very low amounts of POR.

Protective role of cytochrome P450 1A1 (CYP1A1) against benzo[a]pyrene-induced toxicity in mouse aorta

Benzo[a]pyrene (BaP) is an environmental pollutant produced by combustive processes, such as cigarette smoke and coke ovens, and is implicated in the pathogenesis of atherosclerosis. Cytochrome P450 1A1 (CYP1A1) plays a role in both metabolic activation and detoxication of BaP in a context-dependent manner. The role of CYP1A1 in BaP-induced toxicity in aorta remains unknown. First, we fed Apoe⁻/⁻ mice an atherogenic diet plus BaP and found that oral BaP-enhanced atherosclerosis is associated with increased reactive oxygen species (ROS) and inflammatory markers, such as plasma tumor necrosis factor levels and aortic mRNA expression of vascular endothelial growth factor A (Vegfa). We next examined the effect of an atherogenic diet plus BaP on ROS and inflammatory markers in Cyp1a1⁻/⁻ mice. Although this treatment was not sufficient to induce atherosclerotic lesions in Cyp1a1⁻/⁻ mice, plasma antioxidant levels were decreased in Cyp1a1⁻/⁻ mice even in the absence of BaP treatment. The atherogenic diet plus BaP effectively elevated plasma ROS levels and expression of atherosclerosis-related genes, specifically Vegfa, in Cyp1a1⁻/⁻ mice compared with wild-type mice. BaP treatment increased Vegfa mRNA levels in mouse embryonic fibroblasts from Cyp1a1⁻/⁻ mice but not from wild-type mice. BaP-induced DNA adduct formation was increased in the aorta of Cyp1a1⁻/⁻ mice, but not wild-type or Apoe⁻/⁻ mice, and the atherogenic diet decreased BaP-induced DNA adducts in Cyp1a1⁻/⁻ mice compared with mice on a control diet. These data suggest that ROS production contributes to BaP-exacerbated atherosclerosis and that CYP1A1 plays a protective role against oral BaP toxicity in aorta.

The relationship between DNA adduct formation by benzo[a]pyrene and expression of its activation enzyme cytochrome P450 1A1 in rat

Benzo[a]pyrene (BaP) is a human carcinogen requiring metabolic activation prior to reaction with DNA. Cytochrome P450 (CYP) 1A1 is the most important hepatic and intestinal enzyme in both BaP activation and detoxification. CYP1A2 is also capable of oxidizing BaP, but to a lesser extent. The induction of CYP1A1/2 by BaP and/or β-naphthoflavone in liver and small intestine of rats was investigated. Both BaP and β-naphthoflavone induced CYP1A expression and increased enzyme activities in both organs. Moreover, the induction of CYP1A enzyme activities resulted in an increase in formation of BaP-DNA adducts detected by (32)P-postlabeling in rat liver and in the distal part of small intestine in vivo. The increases in CYP1A enzyme activity were also associated with bioactivation of BaP and elevated BaP-DNA adduct levels in ex vivo incubations of microsomes of both organs with DNA and BaP. These findings indicate a stimulating effect of both compounds on BaP-induced carcinogenesis.

The analysis of DNA adducts: the transition from (32)P-postlabeling to mass spectrometry

The technique of (32)P-postlabeling, which was introduced in 1982 for the analysis of DNA adducts, has long been the method of choice for in vivo studies because of its high sensitivity as it requires only <10μg DNA to achieve the detection of 1 adduct in 10(10) normal bases. (32)P-postlabeling has therefore been utilized in numerous human and animal studies of DNA adduct formation. Like all techniques (32)P-postlabeling does have several disadvantages including the use of radioactive phosphorus, lack of internal standards, and perhaps most significantly does not provide any structural information for positive identification of unknown adducts, a shortcoming that could significantly hamper progress in the field. Structural methods have since been developed to allow for positive identification of DNA adducts, but to this day, the same level of sensitivity and low sample requirements provided by (32)P-postlabeling have not been matched. In this mini review we will discuss the (32)P-postlabeling method and chronicle the transition to mass spectrometry via the hyphenation of gas chromatography, capillary electrophoresis, and ultimately liquid chromatography which, some 30years later, is only just starting to approach the sensitivity and low sample requirements of (32)P-postlabeling. This paper focuses on the detection of bulky carcinogen-DNA adducts, with no mention of oxidative damage or small alkylating agents. This is because the (32)P-postlabeling assay is most compatible with bulky DNA adducts. This will also allow a more comprehensive focus on a subject that has been our particular interest since 1990.

Controlled-release systemic delivery - a new concept in cancer chemoprevention

Many chemopreventive agents have encountered bioavailability issues in pre-clinical/clinical studies despite high oral doses. We report here a new concept utilizing polycaprolactone implants embedded with test compounds to obtain controlled systemic delivery, circumventing oral bioavailability issues and reducing the total administered dose. Compounds were released from the implants in vitro dose dependently and for long durations (months), which correlated with in vivo release. Polymeric implants of curcumin significantly inhibited tissue DNA adducts following the treatment of rats with benzo[a]pyrene, with the total administered dose being substantially lower than typical oral doses. A comparison of bioavailability of curcumin given by implants showed significantly higher levels of curcumin in the plasma, liver and brain 30 days after treatment compared with the dietary route. Withaferin A implants resulted in a nearly 60% inhibition of lung cancer A549 cell xenografts, but no inhibition occurred when the same total dose was administered intraperitoneally. More than 15 phytochemicals have been tested successfully by this formulation. Together, our data indicate that this novel implant-delivery system circumvents oral bioavailability issues, provides continuous delivery for long durations and lowers the total administered dose, eliciting both chemopreventive/chemotherapeutic activities. This would also allow the assessment of activity of minor constituents and synthetic metabolites, which otherwise remain uninvestigated in vivo.

Enhanced activity of punicalagin delivered via polymeric implants against benzo[a]pyrene-induced DNA adducts

We investigated the effect of punicalagin (PC) on benzo[a]pyrene (BP)-induced DNA adducts in vitro and in vivo. Incubation of BP (1 μM) with rat liver microsomes, appropriate co-factors and DNA in the presence of vehicle or punicalagin (1-40 μM) showed dose-dependent inhibition of the resultant DNA adducts, with essentially complete (97%) inhibition at 40 μM. However, PC failed to inhibit anti-BPDE-induced DNA adducts when tested in an in vitro non-microsomal system, suggesting that the inhibition of the microsomal BP-DNA adducts occurred due to inhibition of P450 1A1 by PC. To determine its efficacy in vivo, female S/D rats were administered punicalagin via the diet (1500 ppm; approximately 19 mg/day/animal) or subcutaneous polymeric implants (two 2-cm, 200mg with 20% drug load; 40 mg PC/implant) and then treated with continuous low-dose of BP by a subcutaneous polymeric implant (2 cm, 200mg with 10% load; 20mg BP/implant) and euthanized after 10 days. Analysis of the lung DNA by (32)P-postlabeling showed significant (60%; p=0.029) inhibition of DNA adducts by PC administered via the implants; the dietary route showed modest (34%) but statistically insignificant inhibition. Furthermore, total PC administered by implants was approximately 38-fold lower compared with the dietary route. Analysis of the lung microsomes showed significant inhibition of cytochrome P450 1A1 activity and induction of glutathione. Release of PC from the implants was found to be biphasic starting with a burst release, followed by a gradual decline. Ultra performance liquid chromatography analysis showed no detectable PC in the plasma but its hydrolyzed product, ellagic acid was readily detected. The plasma concentration of ellagic acid was over two orders of magnitude higher (589 ± 78 ng/mL) in the implant group compared with diet (4.36 ± 0.83 ng/mL). Together, our data show that delivery of PC by implants can reduce its effective dose substantially, and that the inhibition of DNA adducts in vivo occurred presumably due to the conversion of PC to ellagic acid.

Sustained overexpression of CYP1A1 and 1B1 and steady accumulation of DNA adducts by low-dose, continuous exposure to benzo[a]pyrene by polymeric implants

Many carcinogenesis and tumorigenesis studies reported in the past several decades have relied upon bolus dose(s) of test compounds to determine their DNA damage and carcinogenic potential. The high doses are far from the human scenario where exposure is almost always to low doses and for long duration. In this study, we report a novel polymeric implant system that provides continuous ("24/7") exposure to low doses using benzo[a]pyrene (BP) as a model carcinogen. Cylindrical implants (1 cm length, 3.2 mm diameter; 10 mg BP/100 mg implant) prepared from polycaprolactone:F68 (9:1) showed controlled release in vitro for long duration. To determine the rate of release and biochemical effects in vivo, groups of female Sprague-Dawley rats received either no treatment or subcutaneous sham or BP implants (1 cm, 10% load) and were euthanized after 6, 15, 30, and 180 days; the average dose of BP by the implant route was 16.7 ± 3 μg/rat. For comparison, rats were also treated with a single bolus dose of BP intraperitoneally (10 mg/rat) and euthanized at 6, 15, and 30 days. DNA adducts analyzed by (32)P-postlabeling in the lung and liver increased steadily with time with levels reaching 31 ± 3 and 17 ± 6 adducts/10(9) nucleotides, respectively, after 25 weeks; the adduct burden in the mammary tissue initially increased but then declined with time presumably due to high cell turn over. In contrast, the bolus dose treatment showed the highest DNA adduct levels after 6 days, followed by a steady decline. The steady accumulation of tissue DNA adducts in the implant groups corroborates the sustained overexpression of CYP1A1 and 1B1, the cytochrome P450s involved in the conversion of BP to its electrophilic metabolites. In contrast, the overexpression of CYP1A1 and 1B1 resulting from the bolus dose of BP lasted only for a few days. This is the first demonstration revealing that low-dose, continuous exposure to environmental polycyclic aromatic hydrocarbons such as BP can render sustained expression of CYPs and steady accumulation of tissue DNA adducts. On the basis of our recent study in which we showed the presence of 17β-estradiol in the lung, the sustained overexpression of CYP1A1 and 1B1 due to continuous exposure to BP may increase the susceptibility to estrogen-mediated carcinogenicity.

Colorectal polyp type and the association with charred meat consumption, smoking, and microsomal epoxide hydrolase polymorphisms

We determined the association between charred meat consumption, cigarette smoking, microsomal epoxide hydrolase (mEH) polymorphisms (rs1051740 and rs2234922), and colorectal adenomas and hyperplastic polyps (HPs) and explored gene-environment interactions. Men and women with colorectal adenomas (n = 519), HPs (n = 691), or concurrently with both types of polyps (n = 227) and polyp-free controls (n = 772) receiving a colonoscopy from December 2004 to September 2007 were recruited. Participants completed telephone interviews and provided buccal cell samples; genotyping of mEH was completed using Taqman assays. We conducted polytomous regression and calculated odd ratios (OR) and 95% confidence intervals. Interactions were evaluated using Wald chi-square tests. Consumption of >3 servings of charred meat per week was associated with distal HPs (OR = 2.0, 1.2-3.4) but not adenomas nor either type of proximal polyp. Heavy cigarette smoking (≥ 22 pack-years) was associated with an increased risk for colorectal adenomas (OR = 1.7, 95% CI: 1.2-2.4), HPs (OR = 2.4, 95% CI: 1.7-3.3), and both types (OR = 2.8, 95% CI: 1.8-4.3) with the strongest association for distal polyps. There was no association between mEH genotype and colorectal polyps, nor were any statistically significant gene-environment interactions identified. Future investigation of BaP exposure and colorectal neoplasia should analyze whether associations are dependent upon anatomic location.

Sustained systemic delivery of green tea polyphenols by polymeric implants significantly diminishes benzo[a]pyrene-induced DNA adducts

The polyphenolics in green tea are believed to be the bioactive components. However, poor bioavailability following ingestion limits their efficacy in vivo. In this study, polyphenon E (poly E), a standardized green tea extract, was administered by sustained-release polycaprolactone implants (two, 2-cm implants; 20% drug load) grafted subcutaneously or via drinking water (0.8% w/v) to female S/D rats. Animals were treated with continuous low dose of benzo[a]pyrene (BP) via subcutaneous polymeric implants (2 cm; 10% load) and euthanized after 1 and 4 weeks. Analysis of lung DNA by (32)P-postlabeling resulted in a statistically significant reduction (50%; p = 0.023) of BP-induced DNA adducts in the implant group; however, only a modest (34%) but statistically insignificant reduction occurred in the drinking water group at 1 week. The implant delivery system also showed significant reduction (35%; p = 0.044) of the known BP diolepoxide-derived DNA adduct after 4 weeks. Notably, the total dose of poly E administered was >100-fold lower in the implant group than the drinking water group (15.7 versus 1,632 mg, respectively). Analysis of selected phase I, phase II, and nucleotide excision repair enzymes at both mRNA and protein levels showed no significant modulation by poly E, suggesting that the reduction in the BP-induced DNA adducts occurred presumably due to known scavenging of the antidiolepoxide of BP by the poly E catechins. In conclusion, our study demonstrated that sustained systemic delivery of poly E significantly reduced BP-induced DNA adducts in spite of its poor bioavailability following oral administration.

Effect of green tea catechins and hydrolyzable tannins on benzo[a]pyrene-induced DNA adducts and structure-activity relationship

Green tea catechins and hydrolyzable tannins are gaining increasing attention as chemopreventive agents. However, their mechanism of action is poorly understood. We investigated the effects of four green tea catechins and two hydrolyzable tannins on microsome-induced benzo[a]pyrene (BP)-DNA adducts and the possible structure-activity relationship. BP (1 microM) was incubated with rat liver microsomes and DNA in the presence of the test compound (1-200 microM) or vehicle. The purified DNA was analyzed by (32)P-postlabeling. The inhibitory activity of the catechins was in the following descending order: epigallocatechin gallate (IC(50) = 16 microM) > epicatechin gallate (24 microM) > epigallocatechin (146 microM) > epicatechin (462 microM), suggesting a correlation between the number of adjacent aromatic hydroxyl groups in the molecular structure and their potencies. Tannic acid (IC(50) = 4 microM) and pentagalloglucose (IC(50) = 26 microM) elicited as much DNA adduct inhibitory activity as the catechins or higher presumably due to the presence of more functional hydroxyl groups. To determine if the activity of these compounds was due to direct interaction of phenolic groups with electrophilic metabolite(s) of BP, DNA was incubated with anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (anti-BPDE) (0.5 microM) in the presence of test compounds (200 microM) or vehicle. Significant inhibition of DNA adduct formation was found (tannic acid > pentagalloglucose > epigallocatechin gallate > epicatechin gallate). This notion was confirmed by analysis of the reaction products of anti-BPDE with the catechins and pentagalloglucose by electrospray ionization mass spectrometry and liquid chromatography-mass spectrometry. In conclusion, our data demonstrate that green tea catechins and the hydrolyzable tannins are highly effective in inhibiting BP-DNA adduct formation at least, in part, due to direct interaction of adjacent hydroxyl groups in their structures and that the activity is higher with an increasing number of functional hydroxyl groups.

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

Benzo[a]pyrene (B[a]P) is a potent human and rodent lung carcinogen. This activity has been ascribed in part to the formation of anti-trans-7,8-dihydroxy-7,8-dihydroB[a]P-9,10-epoxide (BPDE)-DNA adducts. Other carcinogenic mechanisms have been proposed: (1) the induction of apurinic sites from radical cation processes, and (2) the metabolic formation of B[a]P-7,8-quinone (BPQ) that can form covalent DNA adducts or reactive oxygen species which can damage DNA. The studies presented here sought to examine the role of stable BPQ-DNA adducts in B[a]P-induced mouse lung tumorigenesis. Male strain A/J mice were injected intraperitoneally once with BPQ or trans-7,8-dihydroxy-7,8-dihydroB[a]P (BP-7,8-diol) at 30, 10, 3, or 0mg/kg. Lungs and livers were harvested after 24h, the DNA extracted and subjected to (32)P-postlabeling analysis. Additional groups of mice were dosed once with BPQ or BP-7,8-diol each at 30 mg/kg and tissues harvested 48 and 72 h later, or with B[a]P (50mg/kg, a tumorigenic dose) and tissues harvested 72 h later. No BPQ or any other DNA adducts were observed in lung or liver tissues 24, 48, or 72 h after the treatment with 30 mg/kg BPQ. BP-7,8-diol gave BPDE-DNA adducts at all time points in both tissues and B[a]P treatment gave BPDE-DNA adducts in the lung. In each case, no BPQ-DNA adducts were detected. Mouse body weights significantly decreased over time after BPQ or BP-7,8-diol treatments suggesting that systemic toxicity was induced by both agents. Model studies with BPQ and N-acetylcysteine suggested that BPQ is rapidly inactivated by sulfhydryl-containing compounds and not available for DNA adduction. We conclude that under these treatment conditions BPQ does not form stable covalent DNA adducts in the lungs or livers of strain A/J mice, suggesting that stable BPQ-covalent adducts are not a part of the complex of mechanisms involved in B[a]P-induced mouse lung tumorigenesis.

Detection of DNA damage in fish Oreochromis mossambicus induced by co-exposure to phenanthrene and nitrite by ESI-MS/MS

BACKGROUND, AIM, AND SCOPE

Mutagenic nitrated polycyclic aromatic hydrocarbons (nitro-PAHs) have been known to arise in the environment through direct emissions from combustion sources and nitration of PAHs, primarily in the atmosphere. In the marine environment, PAHs are one of the classic anthropogenic organic pollutants, while nitrite (NO(2)(-)) is produced naturally via various biological processes like imbalance in nitrification/denitrification or eutrophication and subsequent oxygen depletion from an oversupply of nutrients. In this paper, we report the formation of PAH-DNA adducts in fish contaminated with PAHs and exposed to NO(2)(-) in the ambient water. Electrospray ionization tandem mass spectrometric (ESI-MS/MS) analysis of the bile of the euryhaline fish Oreochromis mossambicus exposed simultaneously to field relevant sublethal concentrations of phenanthrene and NO(2)(-) and collision-induced dissociation of selected ions revealed the presence of DNA-PAH adducts. The present study indicates that, although several high sensitivity techniques have been developed for the analysis of PAH derived DNA adducts, MS/MS has emerged as a powerful tool in the detection and structure elucidation of DNA adducts.

MATERIALS AND METHODS

Juvenile O. mossambicus from a local estuarine fish farm were used with increasing frequency for carcinogenicity testing and comparative cancer research. The fish were exposed to the alkylating agent phenanthrene in the presence of NO(2)(-). Composite untreated bile samples after dilution with methanol: water (1:1; v/v) were analyzed by ESI-MS.

RESULTS

Several adducts could be evidenced in the bile by MS/MS. Deoxyadenosine/deoxyguanosine having a mass in the range of 450-650 amu is detected. In addition, a segment of modified dinucleotide with a mass that corresponds to a dimer consisting of a modified guanosine and a normal guanosine has also been identified in the bile.

DISCUSSION

The formation of certain types of DNA adducts is a crucial step in the induction of cancer and a primary stage in mutagenesis. Phenanthrene injected by i.p. route led to the transformation of phenanthrene to N-formyl amino phenanthrene-N(6)-deoxyadenosine adduct, whereas the fish co-exposed to phenanthrene and ambient nitrite metabolizes PAH to mono-, di- as well as trinitro derivatives, which then react with DNA leading to the formation of mainly modified guanosine and adenosine adducts. In the present investigation, dinitrophenanthrene diol epoxide (DNPDE) adduct with guanosine (m/z 587) seems to be the dominant adduct in the mixture, and its presence is shown first as a comparatively less stable adduct, which decomposes to give a more stable N(2) adduct (m/z 567).

CONCLUSIONS

MS/MS has proved to be useful in the rapid determination and discrimination of structurally different phenanthrene/derivatives DNA adducts in a complex mixture of fish bile co-exposed to phenanthrene and nitrite. However, the nature of metabolites formed is likely determined by the route of PAH administration, and there is a need to further define the early biochemical events of carcinogenesis in these species.

RECOMMENDATIONS AND PERSPECTIVES

DNA adduct analysis in fish bile offers a promising approach to study the risk of potentiation of anthropogenic chemicals into genotoxic compounds in the presence of nitrite in the marine environment. We believe this is the first report on the formation of DNA-phenanthrene adducts on co-exposure of the fish to PAH and nitrite.

Potential use of DNA adducts to detect mutagenic compounds in soil

In this study, three different soils with contrasting features, spiked with 300 mg benzo[a]pyrene (BaP)/kg dry soil, were incubated at 20 degrees C and 60% water holding capacity for 540 days. At different time points, BaP and DNA were extracted and quantified, and DNA adducts were quantified by (32)P-postlabelling. After 540 days incubation, 69.3, 81.6 and 83.2% of initial BaP added remained in Cruden Bay, Boyndie and Insch soils, respectively. Meanwhile, a significantly different amount of DNA-BaP adducts were found in the three soils exposed to BaP over time. The work demonstrates the concept that DNA adducts can be detected on DNA extracted from soil. Results suggest the technique is not able to directly reflect bioavailability of BaP transformation products. However, this new method provides a potential way to detect mutagenic compounds in contaminated soil and to assess the outcomes of soil remediation.

Detection of DNA Damage Induced by Hydroquinone and Catechol Using an Electrochemical DNA Biosensor

DNA damage induced by hydroquinone and catechol was detected by an electrochemical method. Calf thymus DNA was immobilized onto the surface of a pretreated glassy carbon electrode (GCE(ox)) to form a DNA/GCE(ox) modified electrode. Then the DNA/GCE(ox) was incubated in acetate buffer solution containing hydroquinone or catechol at a constant potential for the desired time. Differential pulse voltammetric experiments were then performed. The anodic peaks corresponding to the oxidation of guanisine and adenosine on the electrode could be observed on voltammetric curves. The experimental results showed that DNA damage could be detected using electrochemical DNA biosensors. The extent of DNA damage could be electrochemically recognized via the change of the anodic peak current. DNA damage induced by hydroquinone was greater than that by catechol. The response conditions were optimized with respect to DNA concentration, pH, ionic strength, and other variables.

Species-specific testicular and hepatic microsomal metabolism of benzo(a)pyrene, an ubiquitous toxicant and endocrine disruptor

Information on the metabolism of the environmental toxicant, benzo(a)pyrene (BaP) in the male reproductive system is crucial for understanding BaP-induced infertility. Microsomes were isolated from the liver and testes of rat, mouse, hamster, ram, boar, bull, and monkey and incubated with BaP. Post-incubation, samples were extracted with ethyl acetate and analyzed for BaP/metabolites by reverse-phase HPLC with fluorescence detection. A great variation among species to metabolize BaP was observed. The rodent testicular microsomes produced higher proportions of BaP 4,5-diol and 9,10-diol than did boar, ram, bull, and monkey. On the other hand, hepatic microsomes from higher mammals converted a greater proportion of BaP to 3-hydroxy and 9-hydroxy BaP, the detoxification products of BaP. Given the ability of BaP 7-8-diol 9, 10-epoxide, 3-, and 9-hydroxy BaP to bind with DNA and form adducts, there is a likelihood of risk arising from the accumulation of BaP metabolites in testicular tissues. These metabolites may interfere with the formation and function of gametes, eventually contributing to infertility.

Comparison of immunnological and genotoxicological parameters in automobile emission inspectors exposed to polycyclic aromatic hydrocarbons

In this study, we investigated the immunotoxicities of polycyclic aromatic hydrocarbons in 54 automobile emission inspectors and in 84 control subjects, and evaluated associations between immunological and genotoxicological parameters. Specific surface antigens of peripheral lymphocytes, namely, CD3, CD4, CD8, CD19, and CD69 were subjected to measure immune status in automobile emission inspectors and control subjects. T-and B-cells showed no significant differences between automobile emission inspectors and control subjects (p=0.740 and 0.395). In addition, the ratio of T helper cells to T cytotoxic cells was not deferent (p=0.144). However, T-cell activation was found to be significantly higher in automobile emission inspectors (p=0.041), but not B-cell activation. The levels of two cytokines (IL-4 an INF-γ) and four immunoglobulins (IgA, IgE, IgG, and IgM) were also determined in automobile emission inspectors and control subjects. All immunoglobulin types were lower in automobile emission inspectors, but this was significant only for IgG (0.047). In addition, the levels of two cytokines, IL-4 and INF-γ, were also higher in automobile emission inspectors, though this was not significant. DNA damage in mononuclear and polynuclear lymphocytes and in the level of urinary metabolites, 1-OHP and 2-naphthol, were evaluated in automobile emission inspectors and in control subjects and significant differences were found between the two groups. Examinations of urinary metabolites, DNA damage, and immunological parameters, including leukocyte subpopulations, immunoglobulins, and cytokines, showed that the cytokines levels were associated with the levels of two urinary metabolites, 1-OHP and 2-naphthol.

Analysis of nucleic acid constituents by on-line capillary electrophoresis-mass spectrometry

This review is focused on the capillary electrophoresis-mass spectrometric (CE-MS) analysis of nucleic acid constituents in the broadest sense, going from nucleotides and adducted nucleotides over nucleoside analogues to oligonucleotides. These nucleic acid constituents play an important role in a variety of biochemical processes. Hence, their isolation, identification, and quantification will undoubtedly help reveal the process of life and disease mechanisms, such as carcinogenesis, and can also be useful for antitumor and antiviral drug research to provide valuable information about mechanism of action, pharmacokinetics, pharmacodynamics, toxicity, therapeutic drug level monitoring, and quality control related to this substance class. Fundamental investigations into their structure, the search for modifications, the occurrence and biochemical impact of structural variation amongst others, are therefore of great value. In view of the related bioanalytical procedures, the coupling of CE to MS has emerged as a powerful tool for the analysis of the complex mixtures of nucleic acid constituents: CE confers rapid analysis and efficient resolution, while MS provides high selectivity and sensitivity with structural characterization of minute amounts of compound. After an introduction about the biochemical and analytical perspectives on the nucleic acid constituents, the different modes of CE used in this field of research as well as the relevant CE-MS interfaces and the difficulties associated with quantitative CE-MS are briefly discussed. A large section is finally devoted to field-oriented applications.

Effects of benzo(a)pyrene on protein expression in Jurkat T-cells

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants of air, water and soil, and are produced by the incomplete combustion of organic materials. The International Agency for Research on Cancer has characterized PAHs as carcinogens. In this study, we investigated the effects of benzo(a)pyrene (B(a)P), which is the most carcinogenic member of the PAHs, on Jurkat cell protein by proteomic analysis. Jurkat cells were treated with various concentrations of B(a)P (0, 2.5, 5, 10, 20 or 40 microM) for 24 or 48 h and 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium and lactate dehydrogenase assays were carried out to determine cytotoxicity and a Comet assay was used to determinate genotoxicity. The cytotoxicity assays showed that 2.5 microM of B(a)P was the maximal concentration that did not cause any toxicity, but nevertheless, at this level B(a)P produced significant DNA damage in Jurkat cells at 48 h. Proteomic analysis using three different pI ranges and large two-dimensional gel electrophoresis showed 3427 protein spots. A total of 46 (13 up- and 33 down-regulated) proteins were identified as biomarkers of B(a)P and showed dose-dependent expressions in Jurkat T-cell line exposed to B(a)P. Of these, 27 protein spots were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Two functionally differentiated protein groups were found. The protein group involving apoptosis and tumor suppression were found to be up-regulated, and B(a)P down-regulated enzyme was involved in energy metabolism, DNA synthesis and in cell structure and motility.

The effect of dietary restriction on PhIP-induced mutation in the distal colon and B[a]P- and ENU-induced mutation in the liver of the rat

A reduction in dietary intake has been shown to significantly increase the lifespan of rodents, lower the incidence of tumors, and reduce DNA damage. The objective of this study was to determine whether dietary restriction (DR) reduced the frequency of mutation induced by two environmentally relevant metabolically activated mutagens and one direct-acting mutagen in the lacI transgene of male and female Big BlueR rats. Both male and female rats were maintained on either an ad libitum (AL) or a 40%-reduced diet for 22 wk. The mutagenicity of a 100-mg/kg intraperitoneal injection with 2-amino-1-methyl-6-pheny-imidazo[4,5-b] pyridine (PhIP), benzo[a]pyrene (B[a]P), and N-ethyl-N- nitrosourea (ENU) was determined in the colon or liver. The results indicated that DR did not significantly alter the PhIP-induced mutant frequency in male or female colons. DR completely prevented mutagenicity induced by B[a]P in the female liver (2.6 +/- 0.6 10(-5) vs 10.9 +/- 5.8 10(-5) in AL females), yet increased the induced frequency in male livers (16.3 +/- 3.7 10(-5) vs 10.6 +/- 1.5 10(-5) in AL male livers). Although there was no difference in mutation frequency in the liver between AL and DR females treated with ENU, there was approximately a 40% decrease in induced frequency in DR males compared with AL males. These results indicate that a reduction in dietary intake has no preventive effect against PhIP-induced mutation in the colon, but has sex-dependent protective effects against B[a]P- and ENU-induced mutation in the liver.

Epoxide hydrolase and CYP2C9 polymorphisms, cigarette smoking, and risk of colorectal carcinoma in the Nurses' Health Study and the Physicians' Health Study

Microsomal epoxide hydrolase (mEH) and cytochrome P450 2C9 (CYP2C9) are involved in the bioactivation and detoxification of polycyclic aromatic hydrocarbons (PAHs) derived from tobacco smoke. Two coding-region mEH variants (Tyr113His, His139Arg) and CYP2C9 variants (Arg144Cys, Ile359Leu) have been described and affect enzyme specific activity. We investigated these polymorphisms and tested interactions with smoking in relationship to risk of colorectal carcinoma in two case-control studies nested in the Nurses' Health Study (NHS) and Physicians' Health Study (PHS) cohorts. mEH Tyr113His and His139Arg polymorphisms were not associated with the risk of cancer among 197 incident cases and 490 controls from the NHS. Among 273 incident cases and 453 controls from the PHS, carrying one or two copies of the 'rapid' 139Arg allele was associated with a significantly reduced risk of colorectal cancer (OR=0.70, 95% CI 0.49--0.99) when compared with His139 wild-type individuals. Risk of colorectal cancer was significantly reduced among men carrying the CYP2C9 *1/*2 genotype (OR=0.62, 95% CI 0.42--0.92) or at least one CYP2C9 variant allele (OR=0.72, 95% CI 0.52--1.00) when compared with *1/*1 wild-type individuals. For women, carrying at least one variant CYP2C9 allele was inversely associated with the risk of colorectal cancer (OR=0.85, 95% CI, 0.57--1.27) when compared with *1/*1 wild-type individuals. No statistically significant genotype-smoking or gene-gene interactions were found in this study. Our results indicate that individuals exposed to tobacco carcinogens were at increased risk of colorectal cancer and that overall risk is related to mEH and CYP2C9 genotype, although the results were not consistent between men and women.

DNA adduction by polychlorinated biphenyls: adducts derived from hepatic microsomal activation and from synthetic metabolites

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants and complete carcinogens in rodents. Metabolism of lower chlorinated congeners with rat liver microsomes was investigated in earlier studies and DNA adduction was also reported. The current study was designed to compare DNA adducts formed after bioactivation of PCBs with rat, mouse and human hepatic microsomes, and to investigate the role of quinoid PCB metabolites in DNA adduct formation. Eight congeners ranging from mono- to hexachlorinated biphenyls were tested. Metabolites obtained through microsomal bioactivation as well as synthetic quinoid metabolites of 4-monochlorobiphenyl (4-CB) were incubated with calf-thymus DNA (CT-DNA), and the resulting adducts were analyzed by the 32P-post-labelling method. DNA adducts were formed with mono- di- and tri-chlorinated congeners, but not with higher chlorinated congeners. Similar adduct patterns were observed for 2-monochlorobiphenyl (2-CB) activated with hepatic microsomes from rat, mouse and human, while 4-CB, 3,4-dichlorobiphenyl (3,4-CB) and 3,4,5-trichlorobiphenyl (3,4,5-CB) showed similar patterns for two out of the three microsomal systems tested. 4,4' -trichlorobiphenyl (4,4' -CB) showed different adduct patterns in all microsomal systems. Higher adduct levels were obtained with the rodent microsomes compared with human microsomes and were related to higher cytochrome P450 activity. When adducts derived from microsomal activation of 4-CB were compared by co-chromatography with those derived from the incubation of DNA with synthetic 2-(4' -chlorophenyl)-1,4-benzoquinone (4-BQ), one adduct co-migrated in three different chromatography systems. This study demonstrates that rodents as well as human hepatic enzymes metabolize lower chlorinated biphenyl congeners to reactive intermediates that form DNA adducts in vitro and shows that the para-quinone metabolites of PCBs are, in part, involved in direct DNA adduction.