Benzo[alpha]pyrene diol epoxide I binds to DNA at replication forks

CYP2 C9 polymorphism among patients with oral squamous cell carcinoma and its role in altering the metabolism of benzo[a]pyrene

OBJECTIVES

The aim of this study was to evaluate the prevalence of CYP2 C9 polymorphism among healthy controls and patients with oral squamous cell carcinoma (OSCC) and to analyze the risk of disease development. We also investigated the interaction between CYP2 C9 wild type and the polymorphic variants with benzo[a]pyrene by using molecular docking analysis.

STUDY DESIGN

The study included 46 patients with OSCC and 46 controls. Amplification of the genomic DNA was done by using allele-specific polymerase chain reaction and then analyzed by using agarose gel electrophoresis. Molecular docking was then carried out to determine the interaction of CYP2 C9*1, CYP2 C9*2, and CYP2 C9*3 with benzo[a]pyrene.

RESULTS

In the OSCC group, CYP2 C9*2 and CYP2 C9*3 polymorphisms were 17.4% and 15.2%, respectively, and in the control group, they were 8.7% and 6.5%, respectively. The OSCC group showed a statistically significant (P = .043) increase in the prevalence of CYP2 C9 polymorphic variants compared with the control group. The docking analysis showed benzo[a]pyrene to bind specifically to the altered single nucleotide catalytic site in the polymorphic CYP2 C9*3 enzyme.

CONCLUSIONS

This study demonstrates that functionally important CYP2 C9 polymorphism exists among patients with OSCC, with a modest increase in the risk of disease development in those individuals who acquire these poor metabolizing variants. The modified docking of CYP2 C9*3 with benzo[a]pyrene signifies altered metabolism in vivo.

PARP1 protects from benzo[a]pyrene diol epoxide-induced replication stress and mutagenicity

Poly(ADP-ribosyl)ation (PARylation) is a complex and reversible posttranslational modification catalyzed by poly(ADP-ribose)polymerases (PARPs), which orchestrates protein function and subcellular localization. The function of PARP1 in genotoxic stress response upon induction of oxidative DNA lesions and strand breaks is firmly established, but its role in the response to chemical-induced, bulky DNA adducts is understood incompletely. To address the role of PARP1 in the response to bulky DNA adducts, we treated human cancer cells with benzo[a]pyrene 7,8-dihydrodiol-9,10-epoxide (BPDE), which represents the active metabolite of the environmental carcinogen benzo[a]pyrene [B(a)P], in nanomolar to low micromolar concentrations. Using a highly sensitive LC-MS/MS method, we revealed that BPDE induces cellular PAR formation in a time- and dose-dependent manner. Consistently, PARP1 activity significantly contributed to BPDE-induced genotoxic stress response. On one hand, PARP1 ablation rescued BPDE-induced NAD⁺ depletion and protected cells from BPDE-induced short-term toxicity. On the other hand, strong sensitization effects of PARP inhibition and PARP1 ablation were observed in long-term clonogenic survival assays. Furthermore, PARP1 ablation significantly affected BPDE-induced S- and G2-phase transitions. Together, these results point towards unresolved BPDE-DNA lesions triggering replicative stress. In line with this, BPDE exposure resulted in enhanced formation and persistence of DNA double-strand breaks in PARP1-deficient cells as evaluated by microscopic co-localization studies of 53BP1 and γH2A.X foci. Consistently, an HPRT mutation assay revealed that PARP inhibition potentiated the mutagenicity of BPDE. In conclusion, this study demonstrates a profound role of PARylation in BPDE-induced genotoxic stress response with significant functional consequences and potential relevance with regard to B[a]P-induced cancer risks.

Cellular deficiency of Werner syndrome protein or RECQ1 promotes genotoxic potential of hydroquinone and benzo[a]pyrene exposure

The 5 known RecQ helicases in humans (RECQ1, BLM, WRN, RECQL4, and RECQ5) have demonstrated roles in diverse genome maintenance mechanisms but their functions in safeguarding the genome from environmental toxicants are poorly understood. Here, we have evaluated a potential role of WRN (mutated in Werner syndrome) and RECQ1 (the most abundant homolog of WRN) in hydroquinone (HQ)- and benzo[a]pyrene (BaP)-induced genotoxicity. Silencing of WRN or RECQ1 expression in HeLa cells increased their sensitivity to HQ and BaP but elicited distinct DNA damage response. The RECQ1-depleted cells exhibited increased replication protein A phosphorylation, Chk1 activation, and DNA double-strand breaks (DSBs) as compared to control or WRN-depleted cells following exposure to BaP treatment. The BaP-induced DSBs in RECQ1-depleted cells were dependent on DNA-dependent protein kinase activity. Notably, loss of WRN in RECQ1-depleted cells ameliorated BaP toxicity. Collectively, our results provide first indication of nonredundant participation of WRN and RECQ1 in protection from the potentially carcinogenic effects of BaP and HQ.

Exposure to polycyclic aromatic hydrocarbons: bulky DNA adducts and cellular responses

Environmental and dietary carcinogens such as polycyclic aromatic hydrocarbons (PAHs) have been intensively studied for decades. Although the genotoxicity of these compounds is well characterized (i.e., formation of bulky PAH-DNA adducts), molecular details on the DNA damage response triggered by PAHs in cells and tissues remain to be clarified. The conversion of hazardous PAHs into carcinogenic intermediates depends on enzyme-catalyzed biotransformation. Certain cytochrome P450-dependent monooxygenases (CYPs) play a pivotal role in PAH metabolism. In particular, CYP1A1 and 1B1 catalyze oxidation of PAHs toward primary epoxide species that can further be converted into multiple follow-up products, both nonenzymatically and enzymatically. Distinct functions between these major CYP enzymes have only been appreciated since transgenic animal models had been derived. Electrophilic PAH metabolites are capable of forming stable DNA adducts or to promote depurination at damaged nucleotide sites. During the following DNA replication cycle, bulky PAH-DNA adducts may be converted into mutations, thereby affecting hot spot sites in regulatory important genes such as Ras, p53, and others. Depending on the degree of DNA distortion and cell cycle progression, PAH-DNA adducts trigger nucleotide excision repair (NER) and various DNA damage responses that might include TP53-dependent apoptosis in certain cell types. In fact, cellular responses to bulky PAH-DNA damage are complex because distinct signaling branches such as ATM/ATR, NER, TP53, but also MAP kinases, interact and cooperate to determine the overall outcome to cellular injuries initiated by PAH-DNA adducts. Further, PAHs and other xenobiotics can also confer DNA damage via an alternative route of metabolic activation, which leads to the generation of PAH semiquinone radicals and reactive oxygen species (ROS). One-electron oxidations mediated by peroxidases or other enzymes can result in PAH radical cations that mainly form unstable DNA adducts subjected to depurination. In addition, generation of ROS can also trigger multiple cellular signaling pathways not directly related to mutagenic or cytotoxic effects, including those mediated by NFκB, SAPK/JNK, and p38. In recent years, it became clear that PAHs may also be involved in inflammatory diseases, autoimmune disorders, or atherosclerosis. Further research is under way to better characterize the significance of such newly recognized systemic effects of PAHs and to reconsider risk assessment for human health.

Temporal and functional analysis of DNA replicated in early S phase

In summary, recently developed technologies have begun to draw back the curtain of mystery that obscures some of the basic mechanisms of DNA replication at multiple levels. Studies using extended DNA and chromatin fiber techniques have proven valuable for identifying the location of origins of replication at specific genomic sites and determining their temporal order of replication, for identifying and quantifying sites of DNA damage and localizing chromatin proteins in relation to sites of DNA replication. The future potential of these methods include further discoveries in functional genomics and contributions to the elucidation of the histone code. Such studies could prove very valuable in studies of the mechanisms of cancer development, aging, and other processes of disordered genomic functioning.

Abasic sites preferentially form at regions undergoing DNA replication

We investigated whether apurinic/apyrimidinic (AP/abasic) sites were more frequent in regions of DNA replication in cells and whether their number increased during oxidative stress. DNA fiber spreading and fluorescent immunostaining were used to detect areas of DNA replication and sites of AP lesions in extended DNA fibers. The distribution of AP sites was determined in DNA fibers from vertebrate cells maintained under normal culture conditions or stressed with exogenous H(2)O(2). AP lesions per unit length were enumerated in bulk DNA or at replication sites. The background density of AP sites in DNA fibers was 5.4 AP sites/10(6) nt, while newly replicated DNA contained 12.9 AP sites/10(6) nt. In cells exposed to 20 μM H(2)O(2), AP sites in newly replicated DNA increased to 20.8/10(6) nt. Determinations of AP site density in bulk DNA by fiber analysis or standard slot blot assays agreed to within 10%. Our findings show that the fiber assay not only accurately determines the frequency of AP sites but also shows their distribution. They also reveal that there is increased susceptibility to oxidative damage in DNA regions undergoing replication, which may explain the previously observed clustering of AP sites.

Early replication and the apoptotic pathway

In higher eukaryotes there is a link between time of replication and transcription. It is generally accepted that genes that are actively transcribed are replicated in the first half of S phase while inactive genes replicate in the second half of S phase. We have recently reported that in normal human fibroblasts there are some functionally related genes that replicate at the same time in S phase. This had been previously reported for functionally related genes that are located in clusters, for example the alpha- and beta-globin complexes. We have shown, however, that this also occurs with some functionally related genes that are not organized in a cluster, but rather are distributed throughout the genome. For example, using GOstat analysis of data from our and other groups, we found an overrepresentation of genes involved in the apoptotic process among sequences that are replicated very early (approximately in the first hour of S phase) in both fibroblasts and lymphoblastoid cells. This finding leads us to question how and why the replication of genes in the apoptotic pathway is temporally organized in this manner. Here we discuss the possible explanations and implications of this observation.

The role of polycyclic aromatic hydrocarbon-DNA adducts in inducing mutations in mouse skin

Polycyclic aromatic hydrocarbons (PAH) form stable and depurinating DNA adducts in mouse skin to induce preneoplastic mutations. Some mutations transform cells, which then clonally expand to establish tumors. Strong clues about the mutagenic mechanism can be obtained if the PAH-DNA adducts can be correlated with both preneoplastic and tumor mutations. To this end, we studied mutagenesis in PAH-treated early preneoplastic skin (1 day after exposure) and in the induced papillomas in SENCAR mice. Papillomas were studied by PCR amplification of the H-ras gene and sequencing. For benzo[a]pyrene (BP), BP-7,8-dihydrodiol (BPDHD), 7,12-dimethylbenz[a]anthracene (DMBA) and dibenzo[a,l]pyrene (DB[a,l]P), the codon 13 (GGC to GTC) and codon 61 (CAA to CTA) mutations in papillomas corresponded to the relative levels of Gua and Ade-depurinating adducts, despite BP and BPDHD forming significant amounts of stable DNA adducts. Such a relationship was expected for DMBA and DB[a,l]P, as they formed primarily depurinating adducts. These results suggest that depurinating adducts play a major role in forming the tumorigenic mutations. To validate this correlation, preneoplastic skin mutations were studied by cloning H-ras PCR products and sequencing individual clones. DMBA- and DB[a,l]P-treated skin showed primarily A.T to G.C mutations, which correlated with the high ratio of the Ade/Gua-depurinating adducts. Incubation of skin DNA with T.G-DNA glycosylase eliminated most of these A.T to G.C mutations, indicating that they existed as G.T heteroduplexes, as would be expected if they were formed by errors in the repair of abasic sites generated by the depurinating adducts. BP and its metabolites induced mainly G.C to T.A mutations in preneoplastic skin. However, PCR over unrepaired anti-BPDE-N(2)dG adducts can generate similar mutations as artifacts of the study protocol, making it difficult to establish an adduct-mutation correlation for determining which BP-DNA adducts induce the early preneoplastic mutations. In conclusion, this study suggests that depurinating adducts play a major role in PAH mutagenesis.

Srcasm overexpression in psoriasis-insights into pathogenesis

BACKGROUND

Psoriasis is a prevalent, chronic cutaneous disorder associated with a T-cell lymphocytic infiltrate and altered keratinocyte growth. Some of the molecular features of enhanced keratinocyte growth include increased growth factor receptor activation leading to enhanced cellular tyrosine kinase activity. Receptor tyrosine kinases, including the epidermal growth factor (EGF) receptor, are important regulators of keratinocyte growth, and increased activity of this receptor has been detected in psoriasis. A recently discovered, novel regulator of Src tyrosine kinases, termed Src-activating and signaling molecule (Srcasm), has been shown to modulate EGF signaling and promote differentiation in human keratinocytes. Given the properties of Srcasm, it would be of interest to characterize its expression in psoriasis. In this study, the levels of Srcasm mRNA and protein are characterized, and the relationship of these experimental observations to the psoriasis pathogenesis is discussed.

METHODS

The levels of Srcasm mRNA were determined by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) on RNA isolated from unremarkable and lesional patient tissue. These data were supplemented by performing radioactive in situ hybridization on formalin-fixed biopsy specimens of psoriatic lesions and unremarkable epidermis. Expression of Srcasm protein was evaluated by protein immunohistochemistry and Western blotting of protein lysates derived from patient samples.

RESULTS

All experimental modalities show that levels of Srcasm mRNA and protein were elevated in psoriatic lesions compared to unremarkable epidermis.

CONCLUSIONS

Increased levels of Srcasm mRNA and protein are seen in psoriasis. Given what is known regarding Srcasm function, increased levels of this molecule in keratinocytes may represent a cell compensatory mechanism that is primed to re-establish a physiologic differentiation program.

Evidence that a burst of DNA depurination in SENCAR mouse skin induces error-prone repair and forms mutations in the H-ras gene

Treatment of SENCAR mouse skin with dibenzo[a,l]pyrene results in abundant formation of abasic sites that undergo error-prone excision repair, forming oncogenic H-ras mutations in the early preneoplastic period. To examine whether the abundance of abasic sites causes repair infidelity, we treated SENCAR mouse skin with estradiol-3,4-quinone (E(2)-3,4-Q) and determined adduct levels 1 h after treatment, as well as mutation spectra in the H-ras gene between 6 h and 3 days after treatment. E(2)-3,4-Q formed predominantly (> or =99%) the rapidly-depurinating 4-hydroxy estradiol (4-OHE(2))-1-N3Ade adduct and the slower-depurinating 4-OHE(2)-1-N7Gua adduct. Between 6 h and 3 days, E(2)-3,4-Q induced abundant A to G mutations in H-ras DNA, frequently in the context of a 3'-G residue. Using a T.G-DNA glycosylase (TDG)-PCR assay, we determined that the early A to G mutations (6 and 12 h) were in the form of G.T heteroduplexes, suggesting misrepair at A-specific depurination sites. Since G-specific mutations were infrequent in the spectra, it appears that the slow rate of depurination of the N7Gua adducts during active repair may not generate a threshold level of G-specific abasic sites to affect repair fidelity. These results also suggest that E(2)-3,4-Q, a suspected endogenous carcinogen, is a genotoxic compound and could cause mutations.

Evidence that error-prone DNA repair converts dibenzo[a,l]pyrene-induced depurinating lesions into mutations: formation, clonal proliferation and regression of initiated cells carrying H-ras oncogene mutations in early preneoplasia

Initiation of skin tumors in mice is associated with the formation of oncogenic mutations in the H-ras gene. Mice treated on the dorsal skin with the potent polycyclic aromatic hydrocarbon (PAH) carcinogen dibenzo[a,l]pyrene (DB[a,l]P) form papillomas carrying the H-ras codon 61 (CAA to CTA) mutations. These mutations are induced in early preneoplastic skin within 1 day after DB[a,l]P treatment (Oncogene 16 (1998) 3203-3210) and appear to be related to DB[a,l]P-Ade-depurinating adducts (Proc. Natl. Acad. Sci. U. S. A. 92 (1995) 10422-10426). The rapid kinetics of mutation induction suggests that abasic sites generated from base depurination may undergo error-prone excision repair in pre-S-phase cells to induce these mutations. Analysis of mutations in the H-ras exon 1 and 2 region in DB[a,l]P-treated early preneoplastic skin indicated great changes in mutation spectra in the preneoplastic period. The initial spectra contained abundant A-->G mutations, which frequently occurred 3' to a putative conserved sequence (TGN-doublet). These mutations appeared to be induced initially as mismatched (G.T) heteroduplexes and then converted into double-stranded mutations by one round of replication. Unlike the A-->G mutations found in DB[a, l]P-treated skin (which forms 99% depurinating adducts), A-->G mutations found in anti-DB[a,l]P-diol epoxide-treated skin (forms 97% stable adducts) did not appear to be G.T heteroduplexes. These results, therefore, suggest that under these conditions, the repair errors occurred only from abasic sites but not from stable adducts. Initiated cells carrying specific oncogenic mutations, formed presumably by misrepair, underwent rapid clonal expansion and regression (transient clonoplasia). The multiplication of initiated stem cells during transient clonoplasia may be a factor determining the tumor-initiating potential of some PAH carcinogens.

Cell cycle control, checkpoint mechanisms, and genotoxic stress

The ability of cells to maintain genomic integrity is vital for cell survival and proliferation. Lack of fidelity in DNA replication and maintenance can result in deleterious mutations leading to cell death or, in multicellular organisms, cancer. The purpose of this review is to discuss the known signal transduction pathways that regulate cell cycle progression and the mechanisms cells employ to insure DNA stability in the face of genotoxic stress. In particular, we focus on mammalian cell cycle checkpoint functions, their role in maintaining DNA stability during the cell cycle following exposure to genotoxic agents, and the gene products that act in checkpoint function signal transduction cascades. Key transitions in the cell cycle are regulated by the activities of various protein kinase complexes composed of cyclin and cyclin-dependent kinase (Cdk) molecules. Surveillance control mechanisms that check to ensure proper completion of early events and cellular integrity before initiation of subsequent events in cell cycle progression are referred to as cell cycle checkpoints and can generate a transient delay that provides the cell more time to repair damage before progressing to the next phase of the cycle. A variety of cellular responses are elicited that function in checkpoint signaling to inhibit cyclin/Cdk activities. These responses include the p53-dependent and p53-independent induction of Cdk inhibitors and the p53-independent inhibitory phosphorylation of Cdk molecules themselves. Eliciting proper G1, S, and G2 checkpoint responses to double-strand DNA breaks requires the function of the Ataxia telangiectasia mutated gene product. Several human heritable cancer-prone syndromes known to alter DNA stability have been found to have defects in checkpoint surveillance pathways. Exposures to several common sources of genotoxic stress, including oxidative stress, ionizing radiation, UV radiation, and the genotoxic compound benzo[a]pyrene, elicit cell cycle checkpoint responses that show both similarities and differences in their molecular signaling.

Repair analysis of promutagenic (+)-anti-BPDE DNA adduct in transcriptionally active sequences of plasmid DNA in Escherichia coli

The extent of formation and repair of promutagenic (+)-anti-BPDE-N2-dG in transcriptionally active thymidine kinase (tk) gene insert and vector DNA fragments was assessed in the (+)-anti-BPDE treated plasmid p220-tk within the Escherichia coli hosts of varying repair potential. Polyclonal antibody (BP1), specific for (+)-anti-BPDE DNA adduct, was utilized for quantitative estimation of this bulky lesion in nanograms amounts of membrane transblotted DNA fragments. A carcinogen dose-dependent quantitative antibody binding response, due to selective recognition of the major (+)-anti-BPDE adduct, was seen with various DNA fragments separated by gel electrophoresis. The sensitivity of the immunodetection at 0.2 fmol (+)-anti-BPDE DNA adduct, allowed a linear detection in the range of modification level of 0.64 x 10(-7) to 86 x 10(-7) adducts per nucleotide in plasmid DNA. Based on this sensitivity, detection of 0.07 and 0.46 (+)-anti-BPDE DNA adducts in respective tk and vector DNA fragments was achieved upon immunoanalysis of the in vitro modified DNA. Adduct concentration dependent antibody binding was independent of size of the vector or insert fragments. Antibody binding response, to DNA modified in vivo, was dependent upon the dose of (+/-)-anti-BPDE to plasmid DNA replicating within bacterial hosts. The repair of (+)-anti-BPDE DNA adducts was determined as the loss of antibody binding sites in the specific fragments of plasmid DNA within host E. coli. About 50% of the initial DNA damage was repaired from the individual fragments during 15 min post-incubation in the repair-proficient (wild-type) E. coli cells. Complete adduct removal occurred in approx. 60 min of post-incubation period. A significant (91%) decrease in the survival of mutant (uvrA- recA-) cells was observed at 4 microM (+/-)-anti-BPDE treatment without any reduction in the colony forming units in the wild-type cells. On the contrary, no repair was seen in the excision repair-deficient (uvrA-) E. coli cells. The results indicate (1) the selectivity of the immunological method and the unique ability of the (+)-anti-BPDE specific antibodies to monitor the direct loss of this promutagenic base lesion from the in vivo modified DNA (2) the role of host excision repair pathway in efficient removal of adducts from bacterial genome determines the survival of the bacterial cells and (3) the repair of (+)-anti-BPDE DNA adducts in episomally replicating, transcriptionally active sequences occur at a rapid rate presumably due to the ease of accessibility of repair enzymes to lesions within DNA.

Preferential DNA damage in the p53 gene by benzo[a]pyrene metabolites in cytochrome P4501A1-expressing xeroderma pigmentosum group A cells

Gene-specific DNA damage levels were determined by quantitative polymerase chain reaction (QPCR) after treating cytochrome P450 (CYP) 1A1-expressing xeroderma pigmentosum fibroblasts with [3H]benzo[a]pyrene-trans-7,8-dihydrodiol ([3H]BPD) or [3H]benzo[a]pyrene-trans-7,8-dihydrodiol-9,10-epoxide ([3H]BPDE). DNA damage in the p53 gene (which is transcriptionally active) and the beta-globin gene (which is transcriptionally inactive) was measured in cells treated with [3H](+/-)-anti-BPDE, [3H](+/-)-BPD, and [3H](-)-BPD. DNA adduct formation in the genome overall was determined by measuring the incorporation of 3H into DNA. DNA damage in a p53 gene fragment (exons 8-9, 445 bp) was readily detected by QPCR. DNA damage was either not detected or much reduced in a similarly sized target in the beta-globin gene (exons 1-2, 551 bp). At equivalent levels of genomic DNA adducts, BPD treatment induced more damage in the p53 gene than BPDE treatment did. The lesion frequencies in the p53 and beta-globin genes in purified DNA treated with BPDE in vitro were the same, indicating that there was no sequence-specific basis for preferential lesion formation in the p53 gene in treated cells. DNA damage in both the p53 and beta-globin genes showed a dose response to [3H](-)-BPD. The frequency of BPD-induced lesions in the p53 gene was sixfold to sevenfold greater than in the beta-globin gene and 200- to 300-fold greater than in bulk DNA. The BPD-induced lesion frequency in the beta-globin gene was 30- to 50-fold greater than in bulk DNA. The data indicate that the distribution of BPDE-induced DNA lesions is dramatically nonrandom and suggest that the nonrandomness is governed by DNA sequence composition, chromatin structure, and dose rate.

A possible role in chemical carcinogenesis for epigenetic, heritable changes in gene expression

Although genetic changes are clearly important in the initiation of carcinogenesis, there is reason to think that epigenetic changes may also play a role in the process. A key feature of carcinogenesis is the long latency between exposure to carcinogenic insults and the appearance of malignancy. Thus, if epigenetic changes are to be involved, they must somehow be inherited at each cell division without the continued presence of the carcinogen. I propose that self-perpetuating changes in patterns of gene expression are a plausible mechanism for an epigenetic component of carcinogenesis. Networks of transcription factors that regulate each other's and their own expression are known to control important developmental processes, particularly the determination of entire cell lineages. An inherent property of many such autoregulatory networks is the existence of two very distinct, stable steady-states, defined in terms of the concentration of each transcription factor in the network. In this report, I present a model in which an acute carcinogen exposure is postulated to shift such a network from one steady-state to the other, effectively turning on or off the expression of at least one of the genes. Because of the autoregulatory nature of the network, this new steady-state is stably inherited at each cell division. Such changes in gene expression may ultimately contribute to the malignant phenotype if the regulatory network affects genes important in cell-cycle checkpoints, maintenance of genome stability, signal transduction, or other processes that are altered in tumor cells.

Relationship between benzo(a)pyrene-DNA adducts and somatic mutation and recombination in Drosophila melanogaster

The evaluation of the relationship between the dose to DNA of a mutagen/carcinogen and in vivo somatic cell mutagenesis may provide information on the mechanisms leading to induced mutational events. This can be achieved, for example, by coupling test systems that permit the detection of somatic mutation and recombination on the basis of phenotypic changes in cuticular structures of Drosophila melanogaster, with methods for the quantitation of carcinogen-DNA adducts such as the 32P-postlabeling technique. In this article, we evaluate the quantitative relationship between BaP-DNA adduct formation, determined by 32P-postlabeling, and the induction of mutant cells in the wing marker version of the somatic mutation and recombination test (SMART) in Drosophila melanogaster. The total single clones in the trans-heterozygous mwh/flr3 flies show a linear relationship with the BaP-DNA adduct levels, suggesting a single hit mechanism for the genetic damage giving rise to this type of clones. In contrast, the twin clones (which are of recombinational origin) display a linear-quadratic relationship with the adduct levels, suggesting that multiple hits may be involved in generating these clones. The total single clones in the mwh/TM3, Ser flies (in which mitotic recombination is suppressed) show a logarithmic relationship with the adduct levels. The discussion of these data in terms of the pathways that may be involved in the repair of the BaP-DNA adducts leads to the suggestion that in Drosophila melanogaster the repair of Bap metabolite-DNA adducts in somatic cells may proceed, in large part, via post-replicative recombinational repair.

Comparison of effects of direct-acting DNA methylating and ethylating agents on inducible gene expression in vivo

Our laboratory is interested in whether chemical carcinogen-induced DNA damage is non-randomly distributed in the genome, i.e., "targeted," at the level of individual genes. As one means of investigating this, we have examined whether carcinogen treatment differentially alters the expression of specific genes in vivo. In this study, we have compared the effects of four direct-acting simple alkylating agents (methyl methanesulfonate, ethyl methanesulfonate, methylnitrosourea, and ethylnitrosourea) on the steady-state mRNA expression of a model inducible gene, phosphoenolpyruvate carboxykinase (PEPCK), using the chick embryo as a simple in vivo test system. We observed no effect of any of these four carcinogens on the steady-state mRNA expression of the constitutively expressed beta-actin, transferrin, or albumin genes in chick embryo liver following a single dose of carcinogen. In contrast, these same treatments significantly altered both the basal and inducible expression of the glucocorticoid-inducible PEPCK gene. These results support the hypothesis that inducible gene expression is a target for the effects of chemical carcinogens in vivo. In addition, the direction, magnitude, and time course of these effects were agent-specific. Qualitative and quantitative differences in effects between the methylating and ethylating agents and between the methanesulfonates and nitrosoureas were correlated with differences in their specific patterns of DNA adduct formation, suggesting that different DNA lesions have different effects on inducible gene expression.

Immunological probing of induction and repair of 8-methoxypsoralen photoadducts in DNA from Fanconi anemia and normal human fibroblasts: quantitative analysis by electron microscopy

A direct method of immuno-electron microscopy has been employed to simultaneously determine 8-methoxypsoralen (8-MOP) photoinduced monoadducts (MA) and interstrand cross-links (CL), their relative localization along the DNA molecule, and their removal. It has been applied to DNA from cultures of Fanconi anemia (FA) fibroblasts (complementation groups A and D), and of normal human fibroblasts, following treatment by 8-MOP and 365 nm radiation. The immuno-reaction with monoclonal antibody 8G1 was performed on DNA extracted from the cells just after photoreaction, or after a 24 h repair period, and then denatured. Furan-side MA and also a significant proportion of pyrone-side MA were very efficiently immuno-detected. Only 1-2% CL were IgG-labeled. This is why CL were directly visualized and quantified on denatured DNA from the same cellular samples as used for immuno-detection. Results demonstrate that FA group A cells are not only impaired in the incision of CL, but also of MA. The response of FA group D cells is intermediate between normal and FA group A cells.

Preferential alteration of inducible gene expression in vivo by carcinogens that induce bulky DNA lesions

Our laboratory is interested in whether chemical carcinogen-induced DNA damage is nonrandomly distributed in the genome, i.e., "targeted," at the level of individual genes. To examine this, we have been investigating whether carcinogen treatment in vivo differentially alters the expression of specific genes. In this study, we examined the effects of four model carcinogens that induce bulky lesions in DNA--benzo[a]pyrene (B[a]P), aflatoxin B1 (AFB1), 7,12-dimethylbenz[a]anthracene (DMBA), and 2-acetylaminofluorene (AAF)--on the steady-state mRNA expression of several constitutive and drug-inducible genes in vivo. We specifically tested the hypothesis that carcinogen-induced DNA damage is preferentially targeted to inducible genes relative to constitutively expressed genes using the chick embryo as a simple in vivo test system. In summary, the four carcinogens had no effect on the steady-state mRNA expression of constitutively expressed beta-actin, transferrin, or albumin genes over a 24-h period after a single dose of each carcinogen. In contrast, each of these same treatments significantly altered the mRNA expression of two glutethimide-inducible genes, ALA synthase and CYP2H1. Both the basal expression of these genes and their drug-inducible expression was altered. B[a]P and AFB1 had similar effects on expression of the two inducible genes and caused similar levels of covalent adducts in total DNA, even though the administered doses differed by 30-fold. B[a]P binding to DNA, and the basal expression of CYP2H1 were similar in liver and lung. However, B[a]P significantly altered basal CYP2H1 mRNA expression in liver, a tissue in which this gene is highly inducible by glutethimide, and had no effect on basal CYP2H1 mRNA expression in lung, a tissue in which this gene is not drug-inducible. These data support the hypothesis that inducible gene expression is a target for carcinogen-induced DNA damage in vivo.

The use of rabbit polyclonal antibodies for the isolation of carcinogen-adducted DNA by immunoprecipitation

Polyclonal rabbit antibodies elicited against DNA with high levels of (+/-) 7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE-I) adducts were used to isolate DNA fragments modified by this carcinogen. DNA treated in vitro with different concentrations of BPDE-I was used as substrate in double-antibody immunoprecipitation reactions. The IgG fraction from immune rabbit serum (primary antibody) was reacted with single-stranded plasmid DNA bearing BPDE-I adducts, and the complexes were immunoprecipitated using goat antirabbit-IgG as secondary antibody. DNA was isolated from the immunoprecipitated pellet, blotted onto nitrocellulose or nylon, and hybridized with 32P-labeled sequences homologous to a fragment of the plasmid DNA used in the assay. The recovery of both DNA and adducts in the immunoprecipitated pellet increased with the level of carcinogen adduction of the DNA. The immunoprecipitation reaction appeared to be more efficient for fragments of DNA containing a high number of adducts. The amount of 32P-hybridizing material recovered by immunoprecipitation was virtually identical to the amount added to the reaction in DNA samples that contained three adducts per 10(3) nucleotides.

Localization of DNA adducts induced by N-acetoxy-N-2-acetylaminofluorene in Chinese hamster ovary cells using electron microscopy and colloidal gold

DNA adduct induction by N-acetoxy-N-2-acetylaminofluorene (N-Ac-AAF) has been investigated in Chinese hamster ovary (CHO) cells using immunoelectron microscopy. The major RNA and DNA adducts, N-(guanosin-8-yl)-2-aminofluorene (G-C8-AF) and N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF), were localized with a rabbit anti-G-C8-AF antiserum and colloidal gold cytochemistry. Appropriate controls, including incubation of untreated cells with normal rabbit serum and immunogen-absorbed serum, demonstrated that colloidal gold deposits were indicative of the presence of adducts. The localization of gold particles in close association with nuclear chromatin revealed high concentration of adducts in DNA and RNA of nuclei. Morphometric evaluation of adduct formation in organelles of from different carcinogen exposures showed that 85-88% of total adducts were concentrated in nuclei. DNA adducts remaining in nuclei after RNAse treatment appeared to concentrate in heterochromatic areas, and these areas contained 59% of bound gold particles by morphometry. A total of 137-178 particles were found in nuclei of treated cells vs. 15-26 in the surrounding cytoplasm. Treated cells incubated with normal rabbit serum or specific adduct-absorbed serum showed 19-34 particles for all cellular compartments.

Chromosome site-specific immunohistochemical detection of DNA adducts in N-acetoxy-2-acetylaminofluorene--exposed Chinese hamster ovary cells

In these studies a polyclonal antiserum elicited against a carcinogen-DNA adduct was used to explore the localization of DNA adducts in metaphase chromosomes of cultured cells. Morphological visualization of the adduct N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF) in Chinese hamster ovary (CHO) cells exposed to the direct-acting carcinogen N-acetoxy-2-acetylaminofluorene (N-Ac-AAF) was accomplished by indirect immunofluorescence with an anti-G-C8-AF antiserum. At the same time the pattern of chromosomal DNA replication was determined by replicative incorporation of bromodeoxyuridine (BrdUrd) and chromosomal staining with anti-BrdUrd. Visualization of DNA in chromosomes was accomplished with Hoechst 33258 dye. When synchronized CHO cells were exposed to N-Ac-AAF for 0.5 h during early S phase, the chromosomal pattern of dG-C8-AF adduct formation was not random. Metaphase chromosome spreads from cells exposed to N-Ac-AAF in different experiments contained certain chromosome regions that had a consistently high adduct concentration. The regions of high DNA damage corresponded to the regions active in DNA synthesis when BrdUrd and the carcinogen were given simultaneously in early S phase. In addition, the patterns of high adduct concentration and replicative synthesis shifted when the carcinogen and BrdUrd were given simultaneously during late S phase. Thus, the stage of cell cycle in which adducts are induced is an important factor in the specific location of the highest concentrations of this type of DNA lesion.

Preferential modification of GC boxes by benzo[a]pyrene-7,8-diol-9,10-epoxide

The distribution of binding sites for the ultimate carcinogen anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE-l) in the 5' region of the Chinese hamster ovary aprt gene has been determined. A plasmid (pGAL) containing the entire hamster aprt gene including the 3' and 5' flanking regions was inserted into the BamHI site of the multiple cloning site of pGEM so that the T7 promoter was 5' to the aprt gene. In vitro transcription of BPDE-I-modified pGAL, using the T7 RNA polymerase, revealed two prominent transcriptional stop sites. One of these sites was located in the first exon of the aprt gene, whereas the second transcriptional stop was located approximately 150 bp upstream from the translational start site. This latter region contains two perfect GC-box consensus sequences that are potential Sp1 binding sites. Using a specific laser cutting technique to map BPDE-I DNA binding sites in the 5' flanking region of the aprt gene, we found that the DNA region containing the GC-box consensus sequences was indeed a hot spot for BPDE-I modification.