Benzo[a]pyrene Diol Epoxide Research Articles

Elevation of Cellular BPDE Uptake by Human Cells: A Possible Factor Contributing to Co-Carcinogenicity by Arsenite

BackgroundArsenite (iAsIII) can promote mutagenicity and carcinogenicity of other carcinogens. Considerable attention has focused on interference with DNA repair by inorganic arsenic, especially the nucleotide excision repair (NER) pathway, whereas less is known about the effect of arsenic on the induction of DNA damage by other agents.ObjectivesWe examined how arsenic modulates DNA damage by other chemicals.MethodsWe used an NER-deficient cell line to dissect DNA damage induction from DNA repair and to examine the effects of iAsIII on the formation of benzo[a]pyrene diol epoxide (BPDE)–DNA adducts.ResultsWe found that pretreatment with iAsIII at subtoxic concentrations (10 μM) led to enhanced formation of BPDE–DNA adducts. Reduced glutathione levels, glutathione S-transferase activity and chromatin accessibility were also measured after iAsIII treatment, but none of these factors appeared to account for the enhanced formation of DNA adducts. However, we found that pretreatment with iAsIII increased the cellular uptake of BPDE in a dose-dependent manner.ConclusionsOur results suggest that iAsIII enhanced the formation of BPDE–DNA adducts by increasing the cellular uptake of BPDE. Therefore, the ability of arsenic to increase the bioavailability of other carcinogens may contribute to arsenic co-carcinogenicity.

Comparative in vitro metabolism of benzo[ a]pyrene by recombinant zebrafish CYP1A and liver microsomes from β-naphthoflavone-treated rainbow trout

The zebrafish ( Danio rerio) is a sensitive non-mammalian model used for studying polycyclic aromatic hydrocarbon (PAH)-induced chemical carcinogenesis. The susceptibility of zebrafish to PAH-induced carcinogenesis may be related to the ability of the zebrafish P450s to bioactivate these procarcinogens. As a part of our overall effort to identify the various P450 enzymes that are involved in the activation and detoxification of PAHs in zebrafish, therefore, we have examined the ability of recombinant zebrafish CYP1A (zCYP1A) expressed in yeast to metabolize BaP in vitro. Comparison studies also were conducted with liver microsomes from β-naphthoflavone (BNF)-treated rainbow trout ( Oncorhynchus mykiss). Results demonstrated that the trout liver microsomes were almost twice as active as zCYP1A in oxidizing BaP, with V max values of 1.7 and 0.94 nmol/min/nmol P450 for trout and zebrafish preparations, respectively. Like trout CYP1A1, cDNA-expressed zCYP1A was found to oxidize BaP to phenols, quinones and diols (BaP-7,8-diol and BaP-9,10-diol) in the presence of exogenous human microsomal epoxide hydrolase (hEH). BaP-7,8-diol is the precursor of the ultimate carcinogen, BaP-7,8-diol-9,10-epoxide (BaPDE). The ability of zCYP1A to bioactivate BaP was confirmed by the formation of DNA adducts when calf thymus DNA was added to the incubation mixture. BaP-DNA binding was enhanced by the addition of hEH to the incubation mixture. HPLC analysis of the [ 33P]-postlabeled DNA adducts showed the formation of at least four adducts mediated by both zCYP1A and trout liver microsomes, and one of these adducts co-migrated with BaPDE-dG in HPLC analysis. The addition of hEH to the incubation mixture decreased the formation of BaPDE-dG by zCYP1A and by trout liver microsomes while increasing the formation of an unidentified DNA adduct in the case of zCYP1A. zCYP1A also mediated the binding of BaP to protein, providing further evidence that this enzyme is capable of oxidizing BaP to reactive metabolites that bind to macromolecules. It thus appears that zCYP1A may play an important role in BaP-induced carcinogenesis in the zebrafish model by catalyzing the sequential formation of the ultimate diol epoxide carcinogenic metabolite of BaP.

Malondialdehyde, a major endogenous lipid peroxidation product, sensitizes human cells to UV- and BPDE-induced killing and mutagenesis through inhibition of nucleotide excision repair

Aldehydes are ubiquitous contaminants in the human environment. Intracellular aldehydes are mainly derived from the metabolism of polyunsaturated fatty acids and from lipid peroxidation, which is significantly elevated under oxidative stress conditions. Oxidative stress has long been suspected to be involved in many disease processes, including carcinogenesis, neurodegeneration and aging, but its mechanisms are largely unknown. Aldehydes are reactive not only toward nucleic acids but also to many amino acids, and these aldehyde–protein interactions have been suspected of affecting many cellular functions, including DNA repair. To test this possibility we determined the effect of malondialdehyde (MDA), one of the most abundant intracellular aldehyde, on ultraviolet (UV) light- and benzo( a)pyrene diol epoxide (BPDE)-induced cytotoxicity and mutagenesis in human cells. We found that MDA treatment greatly sensitized cells to both UV- and BPDE-induced cell killing and that, MDA pre-treatment significantly enhanced UV-induced mutagenesis. Using in vitro DNA repair synthesis and host cell reactivation assays we found that MDA treatment of cells greatly inhibited nucleotide excision repair for both and UV light- and BPDE-induced DNA damage. Further experiments raise the possibility that the inhibitory effect on nucleotide excision repair is mainly caused by the direct interaction of MDA with cellular repair proteins. Together these results strongly suggest that intracellular aldehydes play an important role in oxidative stress-related mutagenesis and carcinogenesis through their inhibitory effect on DNA repair mechanisms as well as on induction of DNA damage.

PYK2 mediates anti-apoptotic AKT signaling in response to benzo[a]pyrene diol epoxide in mammary epithelial cells

Polycyclic aromatic hydrocarbons, such as benzo[a]pyrene (BaP), are known mammary carcinogens in rodents and may be involved in human breast cancer. The carcinogenicity of BaP has been partially attributed to the formation of the BaP diol epoxide (BPDE), which has been shown to stably bind DNA and act as an initiator. BaP is a complete carcinogen, but the mechanisms for tumor promotion are less well characterized. Previous studies have demonstrated that BPDE enhanced anti-apoptotic signaling through Akt; however, mechanisms for Akt activation by BPDE are not well defined. In the current studies, we found that BPDE increased intracellular Ca2+ concentration in the human mammary epithelial cell line MCF-10A. A peak in Ca2+ concentration at 20 min was followed by increased phosphorylation of Pyk2 at Tyr881 and increased total tyrosine phosphorylation of the epidermal growth factor receptor (EGFR). Consistent with activation of the EGFR, Akt and ERK1/2 phosphorylation was detected in MCF-10A cells treated with BPDE. Pharmacological methods to prevent Ca2+ elevation and EGFR activity, and small-interfering RNA against Pyk2, prevented Akt phosphorylation by BPDE, which suggested that Ca2+, Pyk2 and EGFR activation lay upstream of Akt. In addition, we found that BPDE increased p53 activity and apoptosis in MCF-10A; however, transient transfection of constitutively active Akt attenuated both BPDE-dependent apoptosis and p53 activity. In contrast, apoptosis was enhanced by inhibitors of phosphatidyl inositol 3-kinase (PI3-K). This work demonstrates a novel mechanism for Akt activation by BPDE that occurs through increased Ca2+ concentration, and implicates Ca2+, Pyk2, EGFR and Akt as a potential pathway by which BPDE can inhibit apoptosis and act as a promoter of carcinogenesis.

Rad18 Regulates DNA Polymerase κ and Is Required for Recovery from S-Phase Checkpoint-Mediated Arrest

We have investigated mechanisms that recruit the translesion synthesis (TLS) DNA polymerase Polkappa to stalled replication forks. The DNA polymerase processivity factor PCNA is monoubiquitinated and interacts with Polkappa in cells treated with the bulky adduct-forming genotoxin benzo[a]pyrene dihydrodiol epoxide (BPDE). A monoubiquitination-defective mutant form of PCNA fails to interact with Polkappa. Small interfering RNA-mediated downregulation of the E3 ligase Rad18 inhibits BPDE-induced PCNA ubiquitination and association between PCNA and Polkappa. Conversely, overexpressed Rad18 induces PCNA ubiquitination and association between PCNA and Polkappa in a DNA damage-independent manner. Therefore, association of Polkappa with PCNA is regulated by Rad18-mediated PCNA ubiquitination. Cells from Rad18(-/-) transgenic mice show defective recovery from BPDE-induced S-phase checkpoints. In Rad18(-/-) cells, BPDE induces elevated and persistent activation of checkpoint kinases, indicating persistently stalled forks due to defective TLS. Rad18-deficient cells show reduced viability after BPDE challenge compared with wild-type cells (but survival after hydroxyurea or ionizing radiation treatment is unaffected by Rad18 deficiency). Inhibition of RPA/ATR/Chk1-mediated S-phase checkpoint signaling partially inhibited BPDE-induced PCNA ubiquitination and prevented interactions between PCNA and Polkappa. Taken together, our results indicate that ATR/Chk1 signaling is required for Rad18-mediated PCNA monoubiquitination. Recruitment of Polkappa to ubiquitinated PCNA enables lesion bypass and eliminates stalled forks, thereby attenuating the S-phase checkpoint.

Flow cytometric detection of tandem repeat mutations induced by various chemical classes

To facilitate detection of genotoxicity from environmental mutagen exposure, we generated an in vitro enhanced green fluorescence protein (EGFP) reactivation assay that quickly and effectively detects frameshift mutations in tandem repeat sequences (TRS). Two murine cell lines, C3H10T1/2 and mismatch repair deficient MC2a, were stably transfected with EGFP reporter plasmids in which the EGFP constructs contain TRS that put the EGFP sequence out of frame. These included several 2, 3, 4, 5 and 6 bp repeat sequences, a control non-repetitive sequence and a human gene sequence containing a 4 bp repeat motif. Transfected cultures were exposed to five model mutagens and carcinogens: hydrogen peroxide (H 2O 2), 12- O-tetradecanoyl-phorbol-13-acetate (TPA), benzo- a-pyrene-diol-epoxide (BPDE), ethyl nitrosourea (ENU), 9-aminoacridine (9AA) and two controls: acetone and ethanol. Frameshift mutations resulted in green fluorescent revertants, as determined by flow cytometry, and were confirmed, for 9AA treatments, by sequencing. All five treatments with model agents induced statistically significant sequence- and exposure-dependent responses in MC2a cells and a negative response with the two negative control treatments, acetone and ethanol. Similar responses were seen in a smaller panel of treatments and plasmids in C3H10T1/2 cells. The mutation frequencies were higher in cells transfected with the plasmids containing TRS than those harbouring the control construct lacking repeats. The highest mutation frequencies were observed with H 2O 2 and 9AA treatments, yielding up to a 50-fold difference between vehicle and highest concentration treatment. ENU, BPDE, and to a lesser extent TPA treatments, also showed a statistically significant exposure response. Results from these experiments reveal that the assay responds robustly to various classes of mutagenic substances, as well as to rodent carcinogens that are inactive in conventional mutation assays, and that responses are not linked to cytotoxicity. This assay is a promising approach for detecting chemically induced frameshifts within certain DNA sequences of interest, but further characterization and validation are required prior to general use in genotoxicity screening.

Stereochemistry and Position-Dependent Effects of Carcinogens on TATA/TBP Binding

The TATA-box binding protein (TBP) is required by eukaryotic RNA polymerases to bind to the TATA box, an eight-basepair DNA promoter element, to initiate transcription. Carcinogen adducts that bind to the TATA box can hamper this important process. Benzo[a]pyrene (BP) is a representative chemical carcinogen that can be metabolically converted to highly reactive benzo[a]pyrene diol epoxides (BPDE), which in turn can form chemically stereoisomeric BP-DNA adducts. Depending on the TATA-bound adduct’s location and stereochemistry, TATA/TBP binding can be decreased or increased. Our previous study interpreted the location-dependent effect in terms of conformational freedom and major-groove space available to BP. Here we further explore specific structural changes of the TATA/TBP complex to help interpret the stereochemical effect in terms of the flexibility of the TATA bases that frame the intercalated adduct. Thermodynamic analyses using molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) yield large standard deviations, which make the computed binding free energies the same within the error bars and point to current limitations of free energy calculations of large and highly charged systems like DNA/protein complexes.

Bladder Cancer Predisposition: A Multigenic Approach to DNA-Repair and Cell-Cycle–Control Genes

The candidate-gene approach in association studies of polygenic diseases has often yielded conflicting results. In this hospital-based case-control study with 696 white patients newly diagnosed with bladder cancer and 629 unaffected white controls, we applied a multigenic approach to examine the associations with bladder cancer risk of a comprehensive panel of 44 selected polymorphisms in two pathways, DNA repair and cell-cycle control, and to evaluate higher-order gene-gene interactions, using classification and regression tree (CART) analysis. Individually, only XPD Asp312Asn, RAG1 Lys820Arg, and a p53 intronic SNP exhibited statistically significant main effects. However, we found a significant gene-dosage effect for increasing numbers of potential high-risk alleles in DNA-repair and cell-cycle pathways separately and combined. For the nucleotide-excision repair pathway, compared with the referent group (fewer than four adverse alleles), individuals with four (odds ratio [OR] = 1.52, 95% CI 1.05-2.20), five to six (OR = 1.81, 95% CI 1.31-2.50), and seven or more adverse alleles (OR = 2.50, 95% CI 1.69-3.70) had increasingly elevated risks of bladder cancer (P for trend <.001). Each additional adverse allele was associated with a 1.21-fold increase in risk (95% CI 1.12-1.29). For the combined analysis of DNA-repair and cell-cycle SNPs, compared with the referent group (<13 adverse alleles), the ORs for individuals with 13-15, 16-17, and >or=18 adverse alleles were 1.22 (95% CI 0.84-1.76), 1.57 (95% CI 1.05-2.35), and 1.77 (95% CI 1.19-2.63), respectively (P for trend = .002). Each additional high-risk allele was associated with a 1.07-fold significant increase in risk. In addition, we found that smoking had a significant multiplicative interaction with SNPs in the combined DNA-repair and cell-cycle-control pathways (P<.01). All genetic effects were evident only in "ever smokers" (persons who had smoked >or=100 cigarettes) and not in "never smokers." A cross-validation statistical method developed in this study confirmed the above observations. CART analysis revealed potential higher-order gene-gene and gene-smoking interactions and categorized a few higher-risk subgroups for bladder cancer. Moreover, subgroups identified with higher cancer risk also exhibited higher levels of induced genetic damage than did subgroups with lower risk. There was a significant trend of higher numbers of bleomycin- and benzo[a]pyrene diol-epoxide (BPDE)-induced chromatid breaks (by mutagen-sensitivity assay) and DNA damage (by comet assay) for individuals in higher-risk subgroups among cases of bladder cancer in smokers. The P for the trend was .0348 for bleomycin-induced chromosome breaks, .0036 for BPDE-induced chromosome breaks, and .0397 for BPDE-induced DNA damage, indicating that these higher-order gene-gene and gene-smoking interactions included SNPs that modulated repair and resulted in diminished DNA-repair capacity. Thus, genotype/phenotype analyses support findings from CART analyses. This is the first comprehensive study to use a multigenic analysis for bladder cancer, and the data suggest that individuals with a higher number of genetic variations in DNA-repair and cell-cycle-control genes are at an increased risk for bladder cancer, confirming the importance of taking a multigenic pathway-based approach to risk assessment.

Inhibition of Werner Syndrome Helicase Activity by Benzo[a]pyrene Diol Epoxide Adducts Can Be Overcome by Replication Protein A

RecQ helicases are believed to function in repairing replication forks stalled by DNA damage and may also play a role in the intra-S-phase checkpoint, which delays the replication of damaged DNA, thus permitting repair to occur. Since little is known regarding the effects of DNA damage on RecQ helicases, and because the replication and recombination defects in Werner syndrome cells may reflect abnormal processing of damaged DNA associated with the replication fork, we examined the effects of specific bulky, covalent adducts at N(6) of deoxyadenosine (dA) or N(2) of deoxyguanosine (dG) on Werner (WRN) syndrome helicase activity. The adducts are derived from the optically active 7,8-diol 9,10-epoxide (DE) metabolites of the carcinogen benzo[a]pyrene (BaP). The results demonstrate that WRN helicase activity is inhibited in a strand-specific manner by BaP DE-dG adducts only when on the translocating strand. These adducts either occupy the minor groove without significant perturbation of DNA structure (trans adducts) or cause base displacement at the adduct site (cis adducts). In contrast, helicase activity is only mildly affected by intercalating BaP DE-dA adducts that locally perturb DNA double helical structure. This differs from our previous observation that intercalating dA adducts derived from benzo[c]phenanthrene (BcPh) DEs inhibit WRN activity in a strand- and stereospecific manner. Partial unwinding of the DNA helix at BaP DE-dA adduct sites may make such adducted DNAs more susceptible to the action of helicase than DNA containing the corresponding BcPh DE-dA adducts, which cause little or no destabilization of duplex DNA. The single-stranded DNA binding protein RPA, an auxiliary factor for WRN helicase, enabled the DNA unwinding enzyme to overcome inhibition by either the trans-R or cis-R BaP DE-dG adduct, suggesting that WRN and RPA may function together to unwind duplex DNA harboring specific covalent adducts that otherwise block WRN helicase acting alone.

Synthesis and Characterization of Enantiomeric anti-2-Fluorobenzo[a]pyrene-7,8-dihydrodiol-9,10-epoxides and Their 2‘-Deoxyguanosine and Oligodeoxynucleotide Adducts

Benzo[a]pyrene diol epoxides (BPDEs) are the ultimate carcinogenic species of benzo[a]pyrene, a prototype polycyclic aromatic hydrocarbon (PAH). BPDE-modified DNA duplexes can adopt multiple conformations depending on the nature of the modified bases, the stereochemistry at the location of the covalent linkage, and the sequence context surrounding the lesion site. In this paper, we describe the preparation of enantiomeric 2-fluoro-BPDEs, trans-(7R,8S)-dihydroxy-(9S,10R)- and trans-(7S,8R)-(9R,10S)-epoxy-7,8,9,10-tetrtahydro-2-fluorobenzo[a]pyrene (22 and 23, respectively), as models for probing the BPDE-induced conformational heterogeneity. The multistep synthesis of the target diol epoxides described herein entails regiospecific succinoylation of 2-fluoropyrene, followed by a ring closure, regio- and stereospecific construction of the 7,8-dihydrodiol functionality, and a subsequent meta-chloroperbenzoic acid-mediated epoxidation. Stereoselectivity was achieved by using Jacobsen chiral catalysts, which produced greater than approximately 90% enantiomeric excess. Absolute configurations at the C(7,8) carbons of the FBP derivatives were determined by comparison of the circular dichroism (CD) spectra with those reported for the BP analogues. Analysis of the 3J(7,8) vicinal coupling constants, CD shape, and charge density calculations all indicated that the prepared anti-FBPDEs preferentially adopt the pseudo-diequatorial C(7,8) diol conformation. Hydrolysis of anti-FBPDEs produced a 9:1 ratio of trans- to cis-opened tetraols. Reactions of each of the anti-FBPDEs with deoxyguanosine 5'-monophosphate produced predominantly trans-anti-N2-dG as the major adducts. Analogous reactions with two 11-mer oligodeoxynucleotides (5'-CCATXGCTACC-3' where X = dT, dC) gave FBP-modified oligodeoxynucleotides with structures that were characterized by enzyme digest/HPLC and electrospray ionization time-of-flight mass spectrometry data. The oligonucleotide adducts were annealed with the appropriate sequences to form fully complementary duplexes [(5'-CCATXGCTACC-3')(5'-GGTAGCYATGG-3'), G = FBP-N2-dG adduct, X = dT, Y = dA in duplex I; X = dC, Y = dG in duplex II] for CD and UV melting studies. The results of the present study were consistent with those reported previously for BPDE-modified duplexes in the same sequence contexts and support the utility of FBPDEs as useful structural probes.

Effects of Chemical Carcinogens and Physicochemical Factors on the UV Spectrophotometric Determination of DNA

The ultraviolet (UV) absorbance ratio of 260/280 nm has been used as an indicator of DNA purity. However, the A260/A280 ratio may be beyond the normal range (1.8–1.9) due to physicochemical alterations produced by pH and temperature, and carcinogenic chemical modification. When the pH of the DNA solution buffer increased from 3 to 11, the A260/A280 ratio changed significantly from 1.5 to 2.2 in mixtures of DNA bases [A:T:C:G = 28.5:28.5: 21.5:21.5, i.e., (A + T)/(all four bases) = 57%, expressed as mole percent], of deoxyribonucleosides (adenosine:thymidine:cytidine:guanosine= 28.5:28.5:21.5:21.5, as mole percent), or of deoxyribonucleotides (dAMP:dTMP:dGMP:dCMP = 28.5:28.5:21.5:21.5, as mole percent) examined. The A260/A280 ratio increased with RNA contamination and exceeded 1.9 when RNA concentration was >30%, as mole percent. In contrast, the A260/A280 ratio was linearly reduced by increasing the protein concentration. Phenol (>0.02%) contamination also reduced the A260/A280 ratio to below 1.8. Benzo[a]pyrene diol epoxide (BPDE), a reactive carcinogen metabolite of benzo[a]pyrene (BaP), decreased the A260/A280 ratio correlated with the degree to which it modified the DNA. These results suggest that the UV A260/A280 ratio is significantly affected by pH and the presence of contaminating species of macromolecules and chemicals.

Reaction between Ellagic Acid and an Ultimate Carcinogen

The reaction coordinate between a typical ultimate carcinogen benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE) and ellagic acid, a proven chemopreventive agent active against cancers caused by polycyclic aromatic hydrocarbons (PAHs), was examined by density functional theory (DFT) and semiempirical MO calculations, and activation energy was calculated. The effect of a polar environment was included using Tomasi and the Langevin dipoles methods. The calculated BPDE/ellagic acid reaction free energy of activation is found to be in decent agreement with experimental data [Sayer, J. M. et al. J. Am. Chem. Soc. 1982, 104, 5562-5564]. This work sheds light on the mechanism of action of ellagic acid. Quantum chemical calculations of this kind are valuable for the design of ellagic acid derivatives with even lower activation energy and increased reactivity toward ultimate carcinogens as well as controlled reactivity toward DNA.

Predictors of esophageal cancer risk: Assessment of susceptibility to DNA damage using comet assay

Individuals' susceptibility to DNA damage could be identified by mutagen-challenged assays. We tested the hypothesis that susceptibility to DNA damage, measured by comet assay, may be associated with increased esophageal cancer (EC) risk. We recruited 102 subjects with previously untreated EC and 112 healthy controls. Baseline (untreated), benzo[a]pyrene diol epoxide (BPDE)-induced, and gamma-radiation-induced DNA damage were quantified by the Olive tail moment parameter. The mean tail moment was significantly higher in cases than in controls at baseline (case vs. control: 2.6 vs. 1.9, P < 0.01), after BPDE induction (case vs. control: 3.8 vs. 2.7, P < 0.01), and after gamma-radiation-induction (case vs. control: 5.0 vs. 3.8, P < 0.01). When data were dichotomized with the median values in the controls, a significantly increased risk for EC was observed for high baseline tail moment [odds ratio (OR) = 5.7, 95% confidence interval (CI) = 2.9-11.4], high BPDE-induced tail moment (OR = 5.8, 95% CI = 2.9-11.8), and high gamma-radiation-induced tail moment (OR = 4.6, 95% CI = 2.4-8.8). Further, the association between DNA damage and EC was stronger in never smokers than in ever smokers. Compared with subjects not sensitive to both mutagens, individuals sensitive to only one mutagen showed a 3.4-fold risk for EC and those sensitive to both mutagens showed an 8.7-fold risk for EC. Thus, we conclude that susceptibility to DNA damage as assessed by comet assay might help identify individuals with high EC risk.

Characterization of a novel protein that specifically binds to DNA modified by N-acetoxy-acetylaminofluorene and cis-diamminedichloroplatinum.

Proteins recognizing DNA damaged by the chemical carcinogen N-acetoxy-acetylaminofluorene (AAAF) were analyzed in nuclear extracts from rat tissues, using a 36 bp oligonucleotide as a substrate and electrophoretic mobility shift and Southwestern blot assays. One major damage-recognizing protein was detected, whose amount was estimated as at least 10(5) copies per cell. Levels of this protein were similar in extracts from brain, kidney and liver, but much lower in extracts from testis. The affinity of the detected protein for DNA damaged by AAAF was about 70-fold higher than for undamaged DNA. DNA damaged by cis-diamminedichloroplatinum (cis-DDP), benzo(a)pyrene diolepoxide (BPDE) or UV-radiation also bound this protein with an increased affinity, the former more strongly and the latter two more weakly as compared to AAAF-damaged DNA. The detected AAAF/DDP-damaged-DNA-binding (AAAF/DDP-DDB) protein had a molecular mass of about 25 kDa and was distinct from histone H1 or HMGB proteins, which are known to have a high affinity for cis-DDP-damaged DNA. The level of this damage-recognizing protein was not affected in rats treated with the carcinogen 2-acetylaminofluorene. The activity of an AAAF/DDP-DDB protein could also be detected in extracts from mouse liver cells but not from the Hep2G human hepatocellular carcinoma.

DNA Polymerase κ Is Specifically Required for Recovery from the Benzo[a]pyrene-Dihydrodiol Epoxide (BPDE)-induced S-phase Checkpoint

Previously we identified an intra-S-phase cell cycle checkpoint elicited by the DNA-damaging carcinogen benzo[a]pyrene-dihydrodiol epoxide (BPDE). Here we have investigated the roles of lesion bypass DNA polymerases polkappa and poleta in the BPDE-induced S-phase checkpoint. BPDE treatment induced the re-localization of an ectopically expressed green fluorescent protein-polkappa fusion protein to nuclear foci containing sites of active DNA synthesis in human lung carcinoma H1299 cells. In contrast, a similarly expressed yellow fluorescent protein-poleta fusion protein showed a constitutive nuclear focal distribution at replication forks (in the same cells) that was unchanged in response to BPDE. BPDE-induced formation of green fluorescent protein-polkappa nuclear foci was temporally coincident with checkpoint-mediated S-phase arrest. Unlike "wild-type" cells, Polk(-/-) mouse embryonic fibroblasts (MEFs) failed to recover from BPDE-induced S-phase arrest, while exhibiting normal recovery from S-phase arrest induced by ionizing radiation and hydroxyurea. XPV fibroblasts lacking poleta showed a normal S-phase checkpoint response to BPDE (but failed to recover from the UV light-induced S-phase checkpoint), in sharp contrast to Polk(-/-) MEFs. The persistent S-phase arrest in BPDE-treated Polk(-/-) cells was associated with increased levels of histone gammaH2AX (a marker of DNA double-strand breaks (DSBs)) and activation of the DSB-responsive kinases ATM and Chk2. These data suggest that in the absence of polkappa, replication forks stall at sites of damage and collapse and generate DSBs. Therefore, we conclude that the trans-lesion synthesis enzyme polkappa is specifically required for normal recovery from the BPDE-induced S-phase checkpoint.

Headliners: Breast Cancer: Repair of DNA Damage Differs Between Sisters With and Without Breast Cancer

Kennedy DO, Agrawal M, Shen J, Terry MB, Zhang FF, Senie RT, Motykiewicz G, Santella RM. 2005. DNA repair capacity of lymphoblastoid cell lines from sisters discordant for breast cancer. J Natl Cancer Inst 97:127–132. Like many types of cancer, breast cancer results from complex interactions of genetic predisposition and environmental exposures. Individuals differ in both their unique genetic makeup and the exposures that they encounter throughout their lifetimes. Damage to DNA is known to be a critical early step in the development of cancer; unrepaired damage leads to alterations in cellular functions. Therefore, individual differences in DNA repair capacity may influence the risk of developing cancer. NIEHS grantee Regina M. Santella and colleagues at Columbia University in New York recently tested this hypothesis by analyzing DNA repair capacity in pairs of sisters, one of whom had breast cancer and one of whom did not. The researchers used cell culture lines for 158 breast cancer patients and 154 control sisters obtained from the Metropolitan New York Registry of Breast Cancer Families. The cell cultures were treated with benzo[a]pyrene diolepoxide (BPDE), a DNA-damaging agent, then either harvested immediately or washed and cultured for four hours to allow DNA repair. The team then measured the number of BPDE–DNA adducts to determine the extent of DNA damage and repair capacity. The DNA repair capacity of the breast cancer patients proved to be significantly lower than that of their sisters without breast cancer. When the Columbia researchers divided DNA repair capacity into quartiles, they observed a threefold higher cancer risk among the women with the lowest repair capacity compared with those women with the highest capacity. This study supports the theory that deficits in DNA repair capacity are associated with higher susceptibility of breast cancer development. The finding may point to a valuable marker to identify women who are at high risk for the disease, especially among families with high incidence of breast cancer. Further research is necessary to determine if therapeutic interventions could improve DNA repair capacity.

Phenotypic Anchoring of Global Gene Expression Profiles Induced by N-Hydroxy-4-acetylaminobiphenyl and Benzo[a]pyrene Diol Epoxide Reveals Correlations between Expression Profiles and Mechanism of Toxicity

The goal of this study was to compare changes in gene expression induced by exposure to different carcinogens and to anchor these changes to the induced levels of toxicity and mutagenesis. The human TK6 lymphoblastoid cell line was used as an in vitro model system, and reactive metabolites of two human carcinogens, benzo[a]pyrene and 4-aminobiphenyl, were used as model compounds. We first determined the toxicity of the model compounds N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) and benzo[a]pyrene diol epoxide (BPDE) in TK6 cells. BPDE was about 1000-fold more toxic and mutagenic than N-OH-AABP in TK6 cells on a molar basis. We next treated cells with three doses of each compound that resulted in low, medium, and high toxicities (5, 15, and 40%) and harvested cells at different times after exposure. Using comparable levels of toxicity as the phenotypic anchor, we compared the patterns of gene expression induced by each reactive metabolite using printed cDNA microarrays comprising approximately 18,000 human gene/EST sequences. The microarray data from the N-OH-AABP and BPDE treatment groups were compared using self-organizing map clustering algorithms, as well as a statistical regression modeling approach. While subsets of genes indicative of a generalized stress response [Hsp 40 homologue (DNAJ), Hsp70, Hsp105, and Hsp 125] were detected after exposure to both compounds at all concentrations, there were also many differentially regulated genes, including phase I xenobiotic metabolism [e.g., glutathione transferase omega (GSTTLp28) and antioxidant enzymes (Apxl)]. Other differentially regulated genes included those encoding proteins involved in all major DNA repair pathways, including excision repair (e.g., ERCC5), mismatch repair (e.g., MLH3), damage specific DNA binding protein (e.g., DDB2), and cisplatin resistance-associated overexpressed protein (LUC7A, CRA). Differences in the transcriptional response of TK6 cells to N-OH-AABP or BPDE exposure may explain the dramatic differences in the toxicity and mutagenicity of these human carcinogens.

Interplay between Human High Mobility Group Protein 1 and Replication Protein A on Psoralen-Cross-linked DNA

Human high mobility group box (HMGB) 1 and -2 proteins are highly conserved and abundant chromosomal proteins that regulate chromatin structure and DNA metabolism. HMGB proteins bind preferentially to DNA that is bent or underwound and to DNA damaged by agents such as cisplatin, UVC radiation, and benzo[a]pyrenediol epoxide (BPDE). Binding of HMGB1 to DNA adducts is thought to inhibit nucleotide excision repair (NER), leading to cell death, but the biological roles of these proteins remain obscure. We have used psoralen-modified triplex-forming oligonucleotides (TFOs) to direct a psoralen-DNA interstrand cross-link (ICL) to a specific site to determine the effect of HMGB proteins on recognition of these lesions. Our results reveal that human HMGB1 (but not HMGB2) binds with high affinity and specificity to psoralen ICLs, and interacts with the essential NER protein, replication protein A (RPA), at these lesions. RPA, shown previously to bind tightly to these lesions, also binds in the presence of HMGB1, without displacing HMGB1. A discrete ternary complex is formed, containing HMGB1, RPA, and psoralen-damaged DNA. Thus, HMGB1 has the ability to recognize ICLs, can cooperate with RPA in doing so, and likely modulates their repair by the NER machinery. The abundance of HMGB1 suggests that it may play an important role in determining the sensitivity of cells to DNA damage under physiological, experimental, and therapeutic conditions.

Alleviation of benzo[a]pyrene-diolepoxide-DNA damage in human lung carcinoma by glutathione S-transferase M2

Cellular detoxification is important for the routine removal of environmental and dietary carcinogens. Glutathione S-transferases (GST) are major cellular phase II detoxification enzymes. MRC-5 cells have been found to exhibit significantly higher GST activity than human H1355 cells. This study investigates whether GST-M2 activity acts as a critical determinant of the target dose of carcinogenic benzo[a]pyrene-diolepoxide (BPDE) and whether it has an effect on MDM2 splicing in the two cell lines. We used RT-PCR to clone Mu-class GST cDNA. Two forms of GST coming from the cell lines were characterized as GST-M2 (from MRC-5 cells) and GST-M4 (from H1355 cells). Nested-PCR showed that BPDE-induced MDM2 splicing had occurred in the H1355 cell line but not in normal MRC-5 cells. Furthermore, using nested-PCR and competitive ELISA, we found that in H1355 cells modified to stably overexpress GST-M2, splicing was abolished and BPDE adducts appeared in low abundance. In conclusion, exogenously overexpressed GST-M2 was effective in reducing BPDE-induced DNA damage in H1355 cells. The catalytic activity of GST-M2 may play an important future role in lowering the incidence of BPDE-induced DNA damage.

Measuring DNA Repair Capacity: Small Steps

DNA repair capacity is a system of defenses designed to protect the integrity of the genome. The difficulty in studying DNA repair capacity in relationship to cancer etiology stems from the costs inherent in conducting whole cell assays, which represent an integrated response to DNA damage, and from the lack of specificity of the current genetic assays, which generally focus on the role of single nucleotide polymorphisms. Studies of DNA repair capacity and cancer etiology require careful consideration of design, potential bias and confounding, and, importantly, assessment of assay variability, both intra-individual and inter-individual, as well as biologic plausibility (1). Design issues center on the difficulty of selecting appropriate subjects for study. A major problem with almost all studies of DNA repair capacity and its role in cancer etiology is that susceptible individuals should be identified before the development of cancer, rather than after cancer has been diagnosed and treated. This is because the cancer itself and likely its treatment may result in observations slanted toward lower DNA repair capacity among case patients who may have been treated with radiation or chemicals. Therapies may modify DNA repair capacity, and, in fact, are designed to knock out the DNA repair capacity of the tumor. Potential biases and confounding in studies of DNA repair capacity can result from several factors: an inability to identify all relevant exposures; the inducibility of some DNA repair genes (1), which can occur as a result of exposure to many different agents; and those that represent biologic cross reactivity, age, dietary habits, and exposure to pro-oxidants (2). Measurement of assay variability is critical for assessing the true, underlying risk estimates. Because DNA repair capacity may be modified by environmental exposures—which could be useful as we consider the potential of DNA repair capacity to be a target for intervention (3) and a potential therapeutic target (4)—it is critical to understand the interand intra-individual variability in any particular set of assays. In this issue of the Journal, Kennedy et al. (5) report on the assay they developed for DNA repair capacity that takes advantage of the adducts induced by the carcinogenic metabolite of benzo[a]pyrene, benzo[a]pyrene diolepoxide (BPDE). BPDE adducts are repaired by the nucleotide excision repair pathway, which has been implicated along with the homologous recombination and the non-homologous end joining repair pathways in BRCA1/2 associated breast cancer. Kennedy et al. found that this measure of DNA repair capacity was lower in breast cancer case patients than control subjects. This study overcomes some previous limitations of case– control studies of breast cancer by using a robust study design, using a functional and specific characterization of nucleotide excision repair, BPDE, and controlling for daily assay variation using cell lines. Although some limitations are present, the findings should lead to renewed interest in DNA repair capacity as a risk factor for cancer and suggest new avenues for investigation. A major strength of the study is the family-based design using sisters from the same families that are discordant for breast cancer. The authors report that not only do younger breast cancer patients have statistically significantly lower DNA repair capacity than their sisters without breast cancer, but that also in multivariable models, breast cancer patients have a dose response increase in the odds of breast cancer with decreasing DNA repair capacity, ranging from odds of 1.23 (95% CI 0.57 to 2.65) to 2.38 (95% CI 1.17 to 4.86) to 2.99 (95% CI 1.45 to 6.17) (Ptrend .002). The potential confounder body mass index (BMI) was statistically significantly different in simple comparisons between case patients and control subjects. At a BMI equal to or greater than 25, indicative of overweight and obesity, case patients were statistically significantly more likely to have lower DNA repair capacity than control subjects. To our knowledge, this is the first evidence that overweight/obesity may affect DNA repair capacity. The critical issue of whether breast cancer patients have experienced therapy—either radiation or chemotherapy—has, unfortunately, not been mentioned or controlled for in this study. Radiation therapy, and possibly chemotherapy, appears to lead to decreased DNA repair capacity (6–8). Kennedy et al. (5) measured the daily variation in two cell lines, one DNA repair deficient and one DNA repair proficient. We would also be interested in the repeatability and variation of these assays on the individual lymphoblastoid cell lines over time. There is also the