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

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

Adducts formed by the food mutagen 2-amino-3-methylimidazo(4, 5-f ) quinoline induce frameshift mutations at hot spots through an SOS-independent pathway

The potency of 2-amino-3-methylimidazo(4, 5-f)quinoline (IQ) adducts to induce ?2, ?1 and +1 frameshift mutations has been determined on specific target DNA sequences, namely short runs of alternating GpC sequences and short runs of guanines. The genetic control of the mutational processes has been analyzed using different Escherichia coli mutants, affected either in the control or in the mutagenesis pathway of the SOS system. We have shown that IQ adducts induce very efficiently both ?1 and ?2 frameshift mutations in E. coli. Both types of deletion mutations are induced in bacteria without the need of SOS induction, indicating that no LexA-controlled functions, in particular the UmuDC proteins, are required for mutation fixation. We have also shown that the frequency of IQ-induced ?2 frameshift mutations in alternating GC sequences increases with the length of the repetition. The efficiency of IQ adducts to induce ?1 and ?2 frameshift mutations is similar to that of N‐2-acetylaminofluorene (AAF) adducts. Both chemicals are potent carcinogens which form covalent adducts at the C8 position of guanines. We suggest that in both cases the adduct-induced DNA structure allows the replication complex to perform a mutagenic bypass of the lesion by a slippage mechanism. However, in contrast to AAF-induced frameshift mutagenesis, IQ-induced frameshift mutagenesis is SOS-independent.     

Enhanced stimulation of chromosomal translocations by radiomimetic DNA damaging agents and camptothecin in Saccharomyces cerevisiae rad9 checkpoint mutants

Several chemical mutagens and carcinogens, including polycyclic aromatic hydrocarbons (PAHs) and nitrated PAHs, are adsorbed to the surface of diesel exhaust particles (DEP). DEP can induce formation of reactive oxygen species and cause oxidative DNA damage as well as bulky carcinogen DNA adducts. Lung tissue is a target organ for DEP induced cancer following inhalation. Recent studies have provided evidence that the lung is also a target organ for DNA damage and cancer after oral exposure to other complex mixtures of PAHs. The genotoxic effect of oral administration of DEP was investigated, in terms of markers of DNA damage, mutations and repair, in the lung of Big Blue^® rats fed a diet with 0, 0.2, 0.8, 2, 8, 20 or 80 mg DEP/kg feed for 21 days. There was no significant increase in the mutation frequency in the cII gene. However, an increase of DNA damage measured as DNA strand breaks (comet assay) and bulky DNA adducts (³²P post labeling) was observed. The level of DNA strand breaks increased significantly at all dose levels while the level of DNA adducts increased significantly only at the intermediate dose levels. Similarly, the number of oxidized DNA bases measured as endonuclease III and fapyguanine glycosylase (FPG) sensitive sites increased at the intermediate dose levels. The induction of DNA damage by DEP exposure did not increase the expression of the repair genes OGG1 and ERCC1 at the mRNA level. The present study indicates that the lung is a target organ for primary DNA damage following oral exposure to DEP. DNA damage was induced following exposure to relatively low levels of DEP, but under the conditions used in the present experiment DNA damage did not result in an increased mutation rate.     

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.     

Neutron radiation can activate K-ras via a point mutation in codon 146 and induces a different spectrum of ras mutations than does gamma radiation.

Neutron radiation is known to produce tumors in animals and cause cell transformation. We have developed a protocol to efficiently induce thymic lymphomas in RF/J mice by a single acute dose of neutron irradiation. Activated ras genes were detected in 17% (4 of 24) of the tumors analyzed. One of the tumors contained a K-ras gene activated by a point mutation in codon 146. Activating ras mutations at position 146 have not been previously detected in any known human or animal tumors. The spectrum of ras mutations detected in neutron radiation-induced thymic lymphomas was different from that seen in thymic lymphomas induced by gamma radiation in the same strain of mice. These results may have important implications for the mechanisms by which different types of radiation damage DNA.     

Adducts formed by the food mutagen 2-amino-3-methylimidazo(4, 5- f ) quinoline induce frameshift mutations at hot spots through an SOS-independent pathway

The potency of 2-amino-3-methylimidazo(4, 5-f)quinoline (IQ) adducts to induce −2, −1 and +1 frameshift mutations has been determined on specific target DNA sequences, namely short runs of alternating GpC sequences and short runs of guanines. The genetic control of the mutational processes has been analyzed using different Escherichia coli mutants, affected either in the control or in the mutagenesis pathway of the SOS system. We have shown that IQ adducts induce very efficiently both −1 and −2 frameshift mutations in E. coli. Both types of deletion mutations are induced in bacteria without the need of SOS induction, indicating that no LexA-controlled functions, in particular the UmuDC proteins, are required for mutation fixation. We have also shown that the frequency of IQ-induced −2 frameshift mutations in alternating GC sequences increases with the length of the repetition. The efficiency of IQ adducts to induce −1 and −2 frameshift mutations is similar to that of N‐2-acetylaminofluorene (AAF) adducts. Both chemicals are potent carcinogens which form covalent adducts at the C8 position of guanines. We suggest that in both cases the adduct-induced DNA structure allows the replication complex to perform a mutagenic bypass of the lesion by a slippage mechanism. However, in contrast to AAF-induced frameshift mutagenesis, IQ-induced frameshift mutagenesis is SOS-independent. Received: 13 June 1996 / Accepted: 24 September 1996     

DNA binding and mutation spectra of the carcinogen N-2-aminofluorene in Escherichia coli. A correlation between the conformation of the premutagenic lesion and the mutation specificity

When the chemical carcinogen N-2-acetylaminofluorene binds to DNA in vivo, two major adducts are formed, both at position C-8 of the guanine residue. One of these (the acetylaminofluorene adduct) retains the acetyl group, while the other (the aminofluorene adduct) is the corresponding deacetylated form. Unlike -AAF adducts, which trigger important structural changes of the DNA secondary structure (either the insertion-denaturation model or the induction of a Z-DNA structure, depending upon the local nucleotide sequence), -AF adducts bind to the C-8 of guanine residues without causing any major conformational change of the B-DNA structure. Well-defined adducts (either -AF or -AAF) can be formed in vitro by reacting DNA with either N-hydroxy-N-2-aminofluorene or N-acetoxy-N-2-acetylaminofluorene. Specific cleavage of the phosphodiester backbone at -AF adducts can be achieved by treating -AF-modified DNA in 1 M-piperidine at 90 degrees C. This observation led us to construct the spectrum for -AF binding to a defined DNA restriction fragment. It is found that only guanine residues react to form alkali-labile lesions and that the reactivity among the different guanines is similar. In a forward mutation assay, namely the inactivation of the tetracycline resistance gene, we found previously that more than 90% of mutations induced by -AAF adducts are frameshift mutations. Using the same assay, we show here that -AF adducts induce primarily base substitution mutations (85%), mainly of the G to T transversion type. There is therefore a strong correlation between the nature of the carcinogen-induced conformational change of the DNA structure and the corresponding mutation specificity. The -AF-induced base substitution mutations depend upon the umuC gene function(s). The data obtained in our forward mutation assay are compared to the data previously obtained in the histidine reversion assay (Ames test).     

Mutations in the 61st codon of the c-Ki-ras oncogene during transplacental lung tumor induction in mice and their difference in spontaneous and induced tumors]

DNA isolated from lung and liver tumor which were induced in CD-1 mice by transplacental treatment with 7,12-dimethylbenz(a)anthracene (DMBA) or developed spontaneously was analyzed for the presence of Ha- and Ki-ras oncogene codon 61 mutations. The A to T transversions at the second position of Ha-ras codon 61 were revealed only in liver tumors of DMBA-exposed animals, whereas only Ki-ras mutations occurred in both spontaneous and induced tumors of the lung. A to T mutations at the second position of Ki-ras codon 61 or non-identified yet mutations at the third position of the same codon were shown to be related to DMBA treatment. Thus both tissue and carcinogen specificity of ras oncogene activation was clearly demonstrated.     

Estrogen, DNA damage and mutations

Estrogen administration to rodents results in various types of DNA damage and ultimately leads to tumors in estrogen-responsive tissues. Yet these hormones have been classified as nonmutagenic, because they did not induce mutations in classical bacterial and mammalian mutation assays. In this review, we have discussed the induction by estrogens of DNA and chromosomal damage and of gene mutations, because the classical assays were designed to uncover mutations only at one specific locus and could not have detected other types of mutations or changes in other genes. Various types of estrogen-induced DNA damage include: (a) direct covalent binding of estrogen quinone metabolites to DNA; (b) enhancement of endogenous DNA adducts by chronic estrogen exposure of rodents; (c) free radical generation by metabolic redox cycling between quinone and hydroquinone forms of estrogens and free radical damage to DNA such as strand breakage, 8-hydroxylation of purine bases of DNA and lipid hydroperoxide-mediated DNA modification. Two different types of chromosomal damage have also been induced by estrogen in vivo and in cells in culture such as numerical chromosomal changes and also structural chromosomal aberrations. Gene mutations have been induced in several cell types in culture either by the parent estrogen or by reactive estrogen quinone metabolites. Furthermore, in estrogen-induced kidney tumors in hamsters, several mutations have been observed in the DNA polymerase beta gene mRNA. Estradiol also induces microsatellite instability in these kidney tumors and in premalignant kidney exposed to estradiol. Although this work is still ongoing, it can be concluded that estrogens are complete carcinogens capable of tumor initiation by mutation potentially in critical genes. The hormonal effects of estrogens may complete the development of tumors.     

Site-specific cleavage of MS2 RNA by a thermostable DNA-linked RNase H

A series of DNA-linked RNases H, in which the 15-mer DNA is cross-linked to the Thermus thermophilus RNase HI (TRNH) variants at positions 135, 136, 137 and 138, were constructed and analyzed for their abilities to cleave the complementary 15-mer RNA. Of these, that with the DNA adduct at position 135 most efficiently cleaved the RNA substrate, indicating that position 135 is the most appropriate cross-linking site among those examined. To examine whether DNA-linked RNase H also site-specifically cleaves a highly structured natural RNA, DNA-linked TRNHs with a series of DNA adducts varying in size at position 135 were constructed and analyzed for their abilities to cleave MS2 RNA. These DNA adducts were designed such that DNA-linked enzymes cleave MS2 RNA at a loop around residue 2790. Of the four DNA-linked TRNHs with the 8-, 12-, 16- and 20-mer DNA adducts, only that with the 16-mer DNA adduct efficiently and site-specifically cleaved MS2 RNA. Primer extension revealed that this DNA-linked TRNH cleaved MS2 RNA within the target sequence.     

Alternative pathways of polycyclic aromatic hydrocarbons activation: the formation of polar DNA adducts.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants, and some are potent carcinogens in rodents. Carcinogenic PAHs are activated in the cells to metabolites that react with DNA to form covalent adducts. For most PAHs the reactive, electrophilic species which bind to DNA, are bay-region diol-epoxides. Application of 32P-postlabeling to PAH-DNA adducts analysis revealed that for some PAHs the adduct profiles generated in model systems are more complex and include adducts which are more polar than those formed by classic bay-region diol-epoxides. This minireview summaries the information gained on typical representatives of polar PAH-DNA adducts. Formation of triol-epoxide-DNA adducts was proposed for chrysene and a non-alterant PAH, benzo[b]fluoranthene (B[b]F). 5-OH-B[b]F, the precursor of B[b]F triol-epoxide, was found to be a potent tumor initiator in mouse skin. For planar PAHs such as dibenzanthracenes the possibility of bis-diol epoxide-DNA adducts formation was suggested. The most comprehensive data were obtained for dibenz[a,j]anthracene (DB[a,j]A). This hydrocarbon when applied to SENCAR mouse skin forms up to 23 species of adducts, most of which are polar. Among these polar adducts seven were identified as derived from DB[aj]A-3,4-10,11-bis-diol. Analysis of tumor-initiating activity showed, however, that this proximate metabolite was inactive in this respect. In contrast, an excellent correlation was observed between levels of less polar DNA adducts (i.e. those derived from bay-region diolepoxides) and skin tumor initiating activity of DB[a,j]A. Thus, while triol-epoxides seems to be involved in tumor initiating activity of the parent compound, non alterant B[b]F, the significance of bis-diol epoxide-DNA adducts, at least those derived from DB[aj]A, is minor.     

Specificity of mutagenesis resulting from the induction of the SOS system in the absence of mutagenic treatment

Strains in which the E. coli SOS system is continuously induced, in the absence of mutagenic treatment, have been used to generate nonsense mutations in the lacl gene. The examination of over 600 independently occurring amber and ochre mutations reveals that inducing the SOS system stimulates specifically G:C----T:A and, to a lesser extent, A:T----T:A transversions. This specificity is similar to that seen for a number of carcinogens that form bulky adducts to DNA, such as benzo(a)pyrene diolepoxide (BPDE) and aflatoxin B1 (AFB1), and which are dependent on the SOS system to mutagenize bacteria. However, G:C----T:A transversions resulting from SOS induction alone display a unique site specificity. One possibility is that the SOS-induced mutations result from cryptic, spontaneous lesions, such as apurinic sites, which cannot induce the SOS system themselves but which can target mutations once the SOS system is induced.     

Mutagenesis by N-nitroso compounds: relationships to DNA adducts, DNA repair, and mutational efficiencies

The relationships between DNA alkylation, DNA repair and mutagenesis by N-nitroso compounds in Salmonella were examined. DNA adducts formed by treatment of the bacteria with N-nitroso compounds were monitored. Critical to the study was establishing which adducts led to mutations. Two methods were employed. In one, correlations in the dose-responses for adducts and mutagenesis were sought. For instance O⁶-methyl- and -ethyl-guanine, in contrast to other adducts, exhibited thresholds in their accumulation in Salmonella DNA, and mutagenesis at GC base pairs also exhibited the same threshold, suggesting a dependence of mutagenesis on the O⁶-alkyguanines. In the second method, mutagenesis induced by different mutagens with overlapping adduct spectra was compared. For example, EMS and ENU generate similar ratios of adenine adducts, but only ENU produces thymine adducts, and only ENU induced AT-GC and AT-CG base changes. These observations suggested that ethylthymines led to these mutations. Furthermore, it was found that these mutations were largely dependent on the presence of the plasmid, pKM101, indicating that error-prone repair activity contributes importantly in their processing to mutations. When DNA adducts by N-nitrosopyrrolidine were examined it was found that only one major adduct was detected in an excision-repair-deficient strain, and that this adduct was not present in a repair-proficient strain. Mutagenesis was also greatly reduced in the proficient strain, suggesting that mutagenesis was dependent on this adduct. From the relationships between premutagenic adducts levels adducts. This calculation assumed an average distribution of adducts and mutations and required knowledge of the target size and the types of mutations that could lead to phenotypic changes. For the unrepaired O⁶-methyl- and -ethyl-guanines, and the O-ethylthymines the mutational efficiencies were high (ca. 30–70%), but for the N-nitrosopyrrolidine adduct it was low (ca. 1%). Initial studies were carried out on the mutational specificities of two higher homologue N-nitroso compounds (the N-nitroso-N-propyl- and N-butyl-nitroguanidines) in uvrB/pKM101 strains. This class of nitroso compounds is known to form similar DNA adducts as ENU. Their specificities were similar to that of N-nitroso-N-ethylurea at a high dose except the fraction of mutations at AT base pairs was reduced. The fraction of GC-CG transversions was although low, increased. The mutational specificities of N-nitroso-N-methylurea and N-nitrosopyrrolidine were significantly different from the specificity of ENU as would be expected from their different adduct distributions.     

Polycyclic aromatic hydrocarbons and PAH-related DNA adducts

Investigations on the impact of chemicals on the environment and human health have led to the development of an exposome concept. The exposome refers to the totality of exposures received by a person during life, including exposures to life-style factors, from the prenatal period to death. The exposure to genotoxic chemicals and their reactive metabolites can induce chemical modifications of DNA, such as, for example, DNA adducts, which have been extensively studied and which play a key role in chemically induced carcinogenesis. Development of different methods for the identification of DNA adducts has led to adopting DNA adductomic approaches. The ability to simultaneously detect multiple PAH-derived DNA adducts may allow for the improved assessment of exposure, and offer a mechanistic insight into the carcinogenic process following exposure to PAH mixtures. The major advantage of measuring chemical-specific DNA adducts is the assessment of a biologically effective dose. This review provides information about the occurrence of the polycyclic aromatic hydrocarbons (PAHs) and their influence on human exposure and biological effects, including PAH-derived DNA adduct formation and repair processes. Selected methods used for determination of DNA adducts have been presented.     

Harvey murine sarcoma virus: influences of coding and noncoding sequences on cell transformation in vitro and oncogenicity in vivo.

The rat-derived Harvey murine sarcoma virus (Ha-MuSV) contains a transduced ras oncogene activated by two missense mutations and flanked by rat retroviruslike VL30 sequences. Ha-MuSV induces focal transformation of mouse NIH 3T3 cells in vitro and tumors (fibrosarcomas and splenic erythroleukemias) in newborn mice. We have used these two assays to study the contribution of coding and noncoding viral sequences to the biological activity of Ha-MuSV. A good correlation was found between the in vitro and in vivo assays. In several different isogenic Ha-MuSV variants, those with a rasH gene that had one or both of the Ha-MuSV missense mutations were much more active biologically than the corresponding proto-oncogene. A Ha-MuSV variant that encoded the proto-oncogene protein induced lymphoid leukemias (with thymomas), with a relatively long latent period, rather than the fibrosarcomas and erythroleukemias characteristic of Ha-MuSV with one or both missense mutations. A VL30-derived segment with enhancer activity was identified downstream from v-rasH. A mutant Ha-MuSV from which this 3' noncoding segment was deleted expressed lower levels of the wild-type viral protein, displayed impaired transforming activity in vitro, and induced lymphoid leukemias (with thymomas). 5' noncoding rat c-rasH sequences were found to increase the biological activity of the virus when substituted for the corresponding segment of v-rasH. We conclude that (i) the biological activity of Ha-MuSV can be influence significantly by noncoding sequences located outside the long terminal repeat as well as by coding sequences, (ii) VL30 sequences positively regulate the expression of v-rasH, (iii) relatively low biological levels of ras, whether resulting from low-level expression of wild type v-rasH or high-levels of ras proto-oncogene protein, induce a type of tumor that differs from tumors induced by high biological levels of ras, and (iv) the in vivo pathogenicity of the Ha-MuSV variants correlated with their transforming activity on NIH 3T3 cells.     

Differential repair of polycyclic aromatic hydrocarbon DNA adducts from an actively transcribed gene

Polycyclic aromatic hydrocarbons (PAHs) are carcinogens with varying potencies. These compounds are metabolized to diol epoxides that react to form DNA adducts. Nucleotide excision repair is a critical cellular defense against these bulky DNA adducts which, if not repaired, can lead to mutations and the initiation of cancer. The structural features of the PAH-adducts play a role in differential repair of these adducts by the global genomic repair subpathway of nucleotide excision repair. DNA adducts derived from the PAHs containing bay-regions are repaired more rapidly than adducts derived from PAHs containing fjord-regions. We have employed the host cell reactivation assay to examine the rate of repair of these adducts in an actively transcribing gene. The pGL3 plasmid containing a luciferase gene was damaged with diol epoxides of benzo[a]pyrene (B[a]P-DE), dibenzo[a,l]pyrene (DB[a,l]P-DE), benzo[g]chrysene (B[g]Ch-DE), and benzo[c]phenanthrene (B[c]Ph-DE). The plasmids were transfected into B-lymphocytes with normal repair capacity as well as lymphocytes derived from patients with the XP-A, XP-C and CS-B syndromes. We found that XPA cells were able to transcribe slowly past B[g]Ch-adducts but not the other PAHs. Using the amount of luciferase produced as a measure of DNA repair, we found that the relative rates of repair in the actively transcribing luciferase gene was B[a]P-DE > DB[a,l]P-DE, B[g]Ch-DE, >B[c]Ph-DE in repair proficient and XP-C cells. These results indicate that the abilities to transcribe past and to repair the PAH adducts are dependent on different structural features of the DNA adducts.     

DNA adducts formation and induction of apoptosis in rat liver epithelial 'stem-like' cells exposed to carcinogenic polycyclic aromatic hydrocarbons

The bipotent liver progenitor cells, so called oval cells, may participate at the early stages of hepatocarcinogenesis induced by chemical carcinogens. Unlike in mature parenchymal cells, little is known about formation of DNA adducts and other genotoxic events in oval cells. In the present study, we employed spontaneously immortalized rat liver WB-F344 cell line, which is an established in vitro model of oval cells, in order to study genotoxic effects of selected carcinogenic polycyclic aromatic hydrocarbons (PAHs). With exception of dibenzo[a,l]pyrene, and partly also benzo[g]chrysene and benz[a]anthracene, all other PAHs under the study induced high levels of CYP1A1 and CYP1B1 mRNA. In contrast, we observed distinct genotoxic and cytotoxic potencies of PAHs. Dibenzo[a,l]pyrene, and to a lesser extent also benzo[a]pyrene, benzo[g]chrysene and dibenzo[a,e]pyrene, formed high levels of DNA adducts. This was accompanied with accumulation of Ser-15 phosphorylated form of p53 protein and induction of apoptosis. Contrary to that, benz[a]anthracene, chrysene, benzo[b]fluoranthene and dibenzo[a,h]anthracene induced only low amounts of DNA adducts formation and minimal apoptosis, without exerting significant effects on p53 phosphorylation. Finally, we studied effects of 2,4,3',5'-tetramethoxystilbene and fluoranthene, inhibitors of CYP1B1 activity, which plays a central role in metabolic activation of dibenzo[a,l]pyrene. In a dose-dependent manner, both compounds inhibited apoptosis induced by dibenzo[a,l]pyrene, suggesting that it interferes with the metabolic activation of the latter one. The present data show that in model cell line sharing phenotypic properties with oval cells, PAHs can be efficiently metabolized to form ultimate genotoxic metabolites. Liver progenitor cells could be thus susceptible to this type of genotoxic insult, which makes WB-F344 cell line a useful tool for studies of genotoxic effects of organic contaminants in liver cells. Our results also suggest that, unlike in mature hepatocytes, CYP1B1 might be a primary enzyme responsible for formation of DNA adducts in liver progenitor cells.