Benzo[a]pyrene-7,8-diol-9,10-epoxide-DNA adducts and increased p53 protein in mouse skin

DNA adducts as link between in vitro and in vivo carcinogenicity - A case study with benzo[a]pyrene

To reduce the need for animal tests, in vitro assays are often used as alternative methods. To derive toxic doses for higher tier organisms from in vitro assay results, quantitative in vitro-in vivo extrapolation (qIVIVE) based on physiological-based toxicokinetic (PBTK) models is typically the preferred approach. Such PBTK models require many input parameters to address the route from dose to target site concentration. However, respective data is very often not available. Hence, our aim is to call attention to an alternative way to build a link between animal (in vivo) and cell-derived (in vitro) toxicity data. To this end, we selected the carcinogenic chemical benzo[a]pyrene (BaP) for our study. Our approach relates both in vitro assay and in vivo data to a main intermediate marker structure for carcinogenicity on the subcellular level - the BaP-DNA adduct BaP-7,8-dihydrodiol-9,10-epoxide-deoxyguanosine. Thus, BaP dose is directly linked to a measure of the toxicity-initiating event. We used Syrian hamster embryo (SHE) and Balb/c 3T3 cell transformation assay as in vitro data and compared these data to outcomes of in vivo carcinogenicity tests in rodents. In vitro and in vivo DNA adduct levels range within three orders of magnitude. Especially metabolic saturation at higher doses and interspecies variabilities are identified and critically discussed as possible sources of errors in our simplified approach. Finally, our study points out possible routes to overcome limitations of the envisaged approach in order to allow for a reliable qIVIVE in the future.

The intervention mechanism of folic acid for benzo(a)pyrene toxic effects in vitro and in vivo

Folic acid is a necessary micronutrient for normal human growth and development. Benzo(a)pyrene (BaP) is a ubiquitously distributed environmental pollutant and its metabolite, benzo(a)pyrene-diol-epoxide, is known to exert a strong teratogenic and carcinogenic effect on the body’s tissues and cells. The aim of this study was to investigate the mechanism by which folic acid can inhibit the toxic effects of BaP both in vivo and in vitro. We measured changes in 16HBE cell activity affected by the intervention of folic acid on BaP using the cell counting kit-8 assay and that of cell cycle distribution by flow cytometry. At the same time, we assessed the xeroderma pigmentosum group A, xeroderma pigmentosum group C, excision repair cross complementation group 1, cyclinD1, and CKD4 mRNAs, and their related protein expression both in mouse lung tissue and in 16HBE cells. In conclusion, the mechanisms by which this effect is mediated were not entirely elucidated by our study, possibly because folic acid antagonizes the toxic effects of BaP by upregulating the levels of excision repair cross complementation group 1, xeroderma pigmentosum group A, and xeroderma pigmentosum group C gene expression to improve the rate of DNA repair, in turn accelerating the speed of repair for DNA damage caused by BaP. Meanwhile, folic acid could restrain BaP-induced cyclinD1 protein expression, which could help cells return to their normal cell cycle.

Histopathological and Molecular Changes During Apoptosis Produced by 7H-Dibenzo[c,g]-Carbazole in Mouse Liver

The topical administration of 7H-dibenzo[c,g]carbazole (7H-DBC) at very low but repeated doses causes genotoxic effects such as DNA adduct formation and produces hepatocellular apoptosis in mouse liver. The purpose of this work was to investigate the alterations in gene expression and protein levels of biomarkers associated with the p53 pathway in mouse liver after exposure to cumulative low doses of 7H-DBC by skin paint applications. The compound was administered topically at the dose of 13.35 μg per animal every 2 days to give either 6, 8, 10, or 12 applications. Animals were sacrificed 48 hours after the different treatments. The apoptotic index increased with the number of applications, with a major proportion of apoptotic cells in the periportal areas. A significant increase of Bax mRNA and protein expression was observed after the 8th application whereas the expression of mRNA levels of Fas and p53 did not show significant differences between treated and control animals. Nuclear staining of p53 was detected in hepatocyte nuclei showing the activation of this protein. Later in the apoptosis process we observed the up-regulation of TGF- β1 in parenchymal cells. In addition to the induction of the p53 apoptosis pathway in vivo by 7H-DBC, we have observed molecular changes related to cell proliferation such as the overexpression of the antiapoptotic gene Bcl-2.

Dietary curcumin post-treatment enhances the disappearance of B(a)P-derived DNA adducts in mouse liver and lungs

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Effects of dietary curcumin on disappearance of DNA adducts were studied.

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Curcumin post-treatment enhanced the disappearance of BPDE-DNA adducts in liver/lungs.

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Curcumin post-treatment augmented B(a)P-induced apoptosis during 24–120 h.

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Curcumin post-treatment increased PCNA in B(a)P-treated tissues during 7–28 days.

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Curcumin-mediated increase in apoptosis and cell proliferation decreased DNA adducts.

To study the post-treatment effects of dietary curcumin on the levels of benzo(a)pyrene [B(a)P]-induced DNA adducts, mice were administered oil or B(a)P and randomized into 7 subgroups after 24 h. One of the subgroups from both the oil and B(a)P groups was killed at 24 h while the remaining 6 subgroups were shifted to powdered control or 0.05% curcumin diet and killed after 24, 72 and 120 h (experiment 1), and 7, 14, and 28 days (experiment 2). Quantitative comparisons of BPDE-DNA nuclear adducts (area and intensity) in immunohistochemically stained lungs and liver sections was carried out by IHC profiler. A time-dependent decrease in the levels of adducts in B(a)P-treated animals was further enhanced by curcumin exposure compared to the levels in time-matched controls. To assess the contribution of apoptosis and cell proliferation in observed curcumin-mediated enhanced decrease of BPDE-DNA adducts, comparative evaluation of apoptosis and cell proliferation markers was undertaken. Results suggested enhancement of B(a)P-induced apoptosis in liver and lungs by curcumin during 24–120 h while no such enhancement was observed at 7–28 days. Results suggest curcumin-mediated enhancement in apoptosis (experiment 1) and adduct dilution (experiment 2) to be the reason for the observed higher decrease of BPDE-DNA adducts.

DNA synthesis inhibition in response to benzo[a]pyrene dihydrodiol epoxide is associated with attenuation of p(34)cdc2: Role of p53

Our previous findings demonstrated that DNA damage by polynuclear aromatic hydrocarbons (PAHs) triggers a cellular protective response of growth inhibition (G1-S cell cycle arrest and inhibition of DNA synthesis) in human fibroblasts associated with accumulation of p53 protein, a growth-inhibitory transcription factor. Here, we report that BPDE (the ultimate carcinogenic metabolite of the PAH benzo[a]pyrene) treatment triggers a variable extent of inhibition of DNA synthesis/cell growth, which does not correspond to the extent of increased p53 accumulation. BPDE treatment of cells significantly attenuates expression of p(34)cdc2, a cell cycle activating protein. Although the role of cdc2 down-regulation in inhibition of cell cycle progression is well known, cdc2 down-regulation in response to cellular insult by PAHs has not been reported. Unlike p53 accumulation, there is a correspondence between DNA synthesis/cell growth inhibition and cdc2 down-regulation by BPDE. BPDE-induced cdc2 down-regulation is p53 dependent, although there is no correspondence between p53 accumulation and cdc2 down-regulation. BPDE-induced cdc2 down-regulation corresponded with accumulation of the cell cycle inhibitor protein p21 (transactivation product of p53). DNA synthesis/cell growth inhibition in response to DNA-damaging PAHs may involve down-regulation of cdc2 protein mediated by p53 activation (transactivation ability), and the extent of p53 accumulation is not the sole determining factor in this regard.

Phenolic fraction of tobacco smoke inhibits BPDE-induced apoptosis response and potentiates cell transformation: role of attenuation of p53 response

Polynuclear aromatic hydrocarbons (PAHs) present in tobacco smoke are regarded as chemical carcinogens. Previously, we observed that a weakly acidic phenolic fraction of tobacco smoke condensate (TSCPhFr), which is devoid of PAHs, significantly potentiates (±)-anti-BP-7,8-diol-9,10-epoxide (BPDE)-induced anchorage-independent cell growth of promotion-sensitive JB6 cell, indicating its tumor-promoting potential. In the present article, we report that further fractionation of phenolic components from TSCPhFr did not show any significant potentiation of BPDE-induced cell transformation by any of the HPLC-purified phenolic fractions, indicating several phenolic components as a whole are needed for observed activity. Although the tumor-promoting activity of weakly acidic phenolic fraction of tobacco smoke had been indicated long before, no studies have been pursued to understand the mechanism(s) underlying the tumor-promoting activity of TSCPhFr. We observed that BPDE, an ultimate carcinogenic metabolite of tobacco smoke carcinogen benzo[a]pyrene, elicits apoptosis induction, which is significantly inhibited by TSCPhFr. Increased cell transformation and decreased apoptosis by TSCPhFr were associated with attenuation of BPDE-induced p53 accumulation. JB6 cells transfected with p53 siRNA showed significantly less apoptosis induction by BPDE as compared to control cells. In p53 impaired cells (which are observed to have a faster growth rate as compared to normal cells), TSCPhFr has a practically negligible effect on apoptosis induction in response to BPDE. Also, in p53 null HCT116 p53(-/-) cells, BPDE-induced apoptosis is unresponsive to TSCPhFr. Inhibition of BPDE-induced NF-κB activation was also observed by us previously. Interestingly, treatment of cells with NF-κB-specific inhibitor IKK-NBD peptide showed no effect on BPDE-induced apoptosis, whereas TSCPhFr showed moderate inhibition of apoptosis in NF-κB inhibited cells as compared to control cells. Our observations indicate that attenuation of BPDE-induced p53 response has a role in apoptosis inhibition and increased cell transformation by TSCPhFr. These findings have implication with regard to the underlying mechanism of tumor-promoting activity of TSCPhFr in PAH-induced carcinogenesis. Although p53-mediated NF-κB activation has a role in apoptosis induction, the role of NF-κB in TSCPhFr-mediated potentiation of PAH-induced cell transformation is not clear from our studies.

Bitumen products induce skin cell apoptosis in chronically exposed road pavers

BACKGROUND

Worker's exposure to bitumen fumes, via inhalation and skin contamination, is related to adverse effects including an increased risk of lung, stomach and non-melanoma skin cancers and leukaemia. The two major mechanisms regulating apoptosis include the mitochondria-mediated intrinsic pathway and the extrinsic pathway induced by death signalling ligands. In a previous study, we showed activation of apoptosis-regulating proteins BAX and BCL-2 in road pavers chronically exposed to bitumen fumes. These molecules play a central role in activation of programmed cell death by the intrinsic pathway. In this study, we hypothesized that the apoptosis mechanism could be activated in the skin of road pavers chronically exposed to bitumen fumes also through the extrinsic pathway, via mediation by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and its death receptor, DR5.

METHODS

Skin punch biopsies from 16 occupationally exposed workers and a control group were processed immunohistochemically for TRAIL and DR5. Activation of programmed cell death was also investigated by expression of caspase-3 and visualization of DNA strand breaks.

RESULTS AND CONCLUSION

TRAIL, DR5 and caspase-3 overexpression and enhanced Terminal Deoxynucleotidyl Transferase Mediated dUTP Nick End Labeling (TUNEL) positivity were found in chronically bitumen-exposed skin, suggesting activation of apoptosis as a defence mechanism.

Overexpression of high mobility group (HMG) B1 and B2 proteins directly correlates with the progression of squamous cell carcinoma in skin

High mobility group B (HMGB) chromosomal proteins, which plays important role in cancer and inflammation, were followed at various stages of the squamous cell carcinoma of skin. Present results were analyzed by histopathology, BrdU assay, immunohistochemistry, western blot and RT-PCR, which indicate that at early stages of tumorigenesis, expression of HMGB (B1, B2), raised only by about 20%. However, the advanced (> or =12 weeks) tumors showed significant (> or =80%) increase in HMG levels. Using skin cancer model, we demonstrated that high levels of HMGB directly correlate with the extent of neoplastic changes, and it appears that HMGB is an effective stimulus for cell differentiation, tumor progression, and metastatic invasion.

Effects of the environmental mammary carcinogen 6-nitrochrysene on p53 and p21(Cip1) protein expression and cell cycle regulation in MCF-7 and MCF-10A cells

The environmental pollutant 6-nitrochrysene (6-NC) is a potent mammary carcinogen in rats; it is more potent than numerous classical mammary carcinogens such as benzo[a]pyrene (BaP). The mechanisms that account for the remarkable carcinogenicity of 6-NC remain elusive. Similar to BaP, 6-NC is also known to induce DNA damage in rodents and in human breast tissues. As an initial investigation, we reasoned that DNA damage induced by 6-NC may alter the expression of p53 protein in a manner that differs from other DNA damaging carcinogens (e.g. BaP). Using human breast adenocarcinoma MCF-7 cells and immortalized human mammary epithelial MCF-10A cells, we determined the effects of 6-NC on the expression of p53 protein and its direct downstream target cyclin-dependent kinase inhibitor p21(Cip1) as well as on the cell cycle progression. Western blot analysis demonstrated that treatments of MCF-7 and MCF-10A cells with 6-NC for 12, 24 or 48h did not increase the level of total p53 protein; however, an increase of p21(Cip1) protein and a commitment increase of G(1) phase were observed in MCF-10A cells but not in MCF-7 cells. Further studies using 1,2-dihydroxy-1,2-dihydro-6-hydroxylaminochrysene (1,2-DHD-6-NHOH-C), the putative ultimate genotoxic metabolite of 6-NC, was conducted and showed a significant induction of p53 (p<0.05) in MCF-7 cells; however, this effect was not evident in MCF-10A cells, indicating the varied DNA damage responses between the two cell lines. By contrast to numerous DNA damaging agents such as BaP which is known to stimulate p53 expression, the lack of p53 response by 6-NC imply the lack of protective functions mediated by p53 (e.g. DNA repair machinery) after exposure to 6-NC and this may, in part, account for its remarkable carcinogenicity in the mammary tissue.

Induction of P53, P21Waf1, orinithine decorboxylase activity, and DNA damage leading to cell-cycle arrest and apoptosis following topical application of repeated fish fried oil extract to mice

Repeated frying of food produces numerous carcinogens including polycyclic aromatic hydrocarbons (PAHs). Our prior studies have shown that repeated fish fried oil extract (RFFE) induces cytochrome P (CYP)-450 1A1/2 isozymes thereby causing increased generation of electrophilic reactive metabolites of PAHs and subsequent binding to DNA. In the present study, molecular events associated with DNA damage, apoptosis, and proliferation following topical exposure to RFFE have been investigated in mice. Single topical application of RFFE (500 microg) for 24-48 h caused significant DNA damage with Comet assay in terms of olive tail moment (OTM) (204-246%), tail DNA (253-293%), and tail length (172-195%). Overexpression of p53 and p21WAF1 proteins was observed in skin cells following single topical exposure of RFFE for 24-72 h, which was similar to that of benzo(a)pyrene (BP) exposure (24 h). Though RFFE and BP exposure separately, did not result in G(0)/G(1) arrest, but a significant increase in the proportion of cells in S-phase was observed. Apoptotic induction was noticed in skin cells, with maximum induction after 48 h of exposure to RFFE. Further, topical treatment of mice with RFFE (500 microg) for 6 h significantly increased orinithine decarboxylase (ODC) activity by 7.5-fold when compared to control. These results indicate that RFFE exposure caused ODC induction accompanied by increased levels of p53 and p21WAF1 proteins leading of apoptosis and delay of cells in S-phase thereby indicating the possible carcinogenic potential of RFFE.

Attenuation of BPDE-induced p53 accumulation by TPA is associated with a decrease in stability and phosphorylation of p53 and downregulation of NFkappaB activation: role of p38 MAP kinase

DNA damage caused by benzo[a]pyrene (B[a]P) or other polynuclear hydrocarbons (PAHs) induce p53 protein as a protective measure to eliminate the possibility of mutagenic fixation of the DNA damage. 12-O-tetradecanoylphorbol-13-acetate (TPA) inhibits p53 response induced by B[a]P and other DNA-damaging agents and may cause tumor promotion. The molecular mechanism of attenuation of B[a]P-induced p53 response by TPA is not known. We investigated the effect of TPA on p53 response in (+/-)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE)-treated mouse epidermal JB6(P(+)) Cl 41 cells. BPDE treatment induced p53 accumulation which was attenuated significantly by TPA. Cells treated with BPDE and TPA showed increased ratio of Mdm2 to p53 proteins in p53 immunoprecipitate and decreased p53 life span compared to BPDE-treated cells indicating p53 destabilization by TPA. TPA also inhibited BPDE-induced p53 phosphorylation at serine15. Activation of both ERKs and p38 MAPK by BPDE and attenuation of BPDE-induced p53 accumulation by U0126 or SB202190, specific inhibitor of MEK1/2 or p38 MAPK, indicate the role of ERKs and p38 MAPK in p53 accumulation. Interestingly, TPA potentiated BPDE-induced activation of ERKs whereas p38 MAPK activation was significantly inhibited by TPA, suggesting that inhibition of p38 MAPK is involved in p53 attenuation by TPA. Furthermore, SB202190 treatment caused decreased p53 stability and inhibition of phosphorylation of p53 at serine15 in BPDE-treated cells. We also observed that TPA or SB202190 attenuated BPDE-induced nuclear factor kappa B (NFkappaB) activation in JB6 Cl 41 cells harboring NFkappaB reporter plasmid. To our knowledge this is the first report that TPA inhibits chemical carcinogen-induced NFkappaB activation. Interference of TPA with BPDE-induced NFkappaB activation implicates abrogation of p53 function which has been discussed. Overall, our data suggest that abrogation of BPDE-induced p53 response and of NFkappaB activation by TPA is mediated by impairment of the signaling pathway involving p38 MAPK.

Effects of Cadmium(II) on (±)-anti-Benzo[a]pyrene-7,8-diol-9,10-epoxide-Induced DNA Damage Response in Human Fibroblasts and DNA Repair: A Possible Mechanism of Cadmium's Cogenotoxicity

Cadmium, a widespread environmental pollutant and a cigarette smoke constituent, enhances the genotoxicity of benzo[a]pyrene (BP). The mechanism(s) underlying the potentiation of BP-induced genotoxicity by Cd2+ is not clearly understood. Our studies of the effects of noncytotoxic concentrations of Cd2+ on the levels of p53 and p21 in (+/-)-anti-benzo[a]pyrene-7,8-diol-9,10-epoxide (BPDE)-treated human fibroblasts showed that Cd2+ decreased BPDE-induced p21 levels in a dose-dependent manner whereas p53 accumulation is attenuated only at higher noncytotoxic concentrations of cadmium. These findings suggest that both the activity and the accumulation of p53 in response of BPDE treatment are inhibited by Cd2+ although the possibility of p53-independent p21 transactivation cannot be ruled out. Exposure of synchronized human fibroblast cells to 0.5 microM of BPDE caused 72% of the cells remaining in G1 phase as compared to 52% in the case of untreated cells. Treatment of the cells with CdCl2 prior to exposing them to BPDE caused a decrease in the G1 population (72 to 54%) in a dose-dependent manner. An in vitro repair assay of BPDE-damaged pUC18 plasmid DNA using untreated and cadmium-treated nucleotide excision repair (NER) proficient HeLa extract showed that cadmium impaired the ability of HeLa cell extract to repair BPDE-damaged pUC18 DNA. Our findings indicate that cadmium not only inhibits NER pathway-dependent repair of BPDE-damaged DNA but also impairs p53 and p21 responses and overrides BPDE-induced G1-S cell cycle arrest. The effect of cadmium on these processes may explain, at least partly, the potentiating effect of the metal on the genotoxicity of BP.

Evaluation of skin tumors by magnetic resonance imaging

In vivo magnetic resonance imaging (MRI) is a powerful noninvasive technique in medical diagnosis; however, its application to analyze skin disorders is still at initial stages. To check whether MRI can be used as a noninvasive tool to analyze skin tumors, we carried out MRI of mice after treatment with benzo[a]pyrene (BP), a well known carcinogen. MRI was done on whole mice and was particularly focused on various layers and regions of interest of the skin: dermis, epidermis, and tumor. Initial MRIs of mice bearing skin tumors of 4, 8, 12, and 16 weeks after inducing BP clearly revealed the appearance of tumor. The MRIs of tumor-bearing mice with 20-week-old tumor development showed invasion to adjacent internal anatomic structures. The MRI data were in good agreement with the extent of cellular atypia and neoplastic changes that are typical of squamous cell carcinoma as noticed from the histopathologic findings. Therefore, MRI seems to have the potential to evaluate the tumor invasions equally well as that of histopathology or other clinical findings.

The combination of benzo[a]pyrene and ultraviolet A causes an in vivo time-related accumulation of DNA damage in mouse skin

Polycyclic aromatic hydrocarbons, including benzo[a]pyrene (BaP), are ubiquitous environmental carcinogens. BaP is metabolized in vivo to reactive intermediates that become covalently bound to DNA and form BaP-DNA adducts, an initial event in carcinogenesis. Ultraviolet A (UVA) synergizes with BaP to significantly enhance genetic damage and accelerate carcinogenic processes. This study was initiated to investigate in vivo cellular changes related to carcinogenesis induced by repeated exposures to BaP plus UVA. Simulated chronic exposure to an environmental carcinogen and sunlight was conducted through biweekly topical application of BaP followed 2 h later by UVA exposure over a 10 week period. BaP diol epoxide (BPDE)-DNA adducts were measured in vivo by immunohistochemistry using an anti-BPDE-DNA monoclonal antibody. Oxidative DNA damage was measured by the detection of 8-hydroxy-2'-deoxyguanosine (8-OHdG) formation using high-performance liquid chromatography. Alterations in the cell cycle that were relevant to carcinogenesis were revealed by changes in p53, as identified in vivo using a polyclonal anti-p53 antibody. We found that cells containing BPDE-DNA adducts and nuclear p53 expression significantly increased between 2 and 10 weeks of BaP-UVA treatment, whereas neither BPDE-DNA adducts nor significant changes in p53 were observed in untreated skin. Using regression analysis, oxidative 8-OHdG damage also showed a parallel increase over 2-10 weeks (r = 0.80). These results indicate that genetic damage caused by exposures to BaP plus UVA accumulates with time and increases the potential for inductive events leading to carcinogenesis and tumor formation.

TP53 mutations in workers exposed to occupational carcinogens

In some cases, evidence exists that exogenous carcinogenic exposures contribute to the mutation spectrum of the TP53 gene (p53) in human cancers. Although the clearest examples come from dietary and environmental sources, only a restricted number of papers have concentrated specifically on TP53 mutations in tumors from workers exposed to occupational carcinogens. In populations exposed to dietary aflatoxin B1 with liver cancer (AFB1) and ultraviolet (UV)-radiation with skin cancer, a single specific-looking TP53 mutation has been described in some of the tumors. Whether these fingerprints in the TP53 gene can be used to reveal an occupational etiology remains to be shown. In other cases, although differences in the TP53 mutation spectrum exist, they are more diffuse and difficult to interpret at this point. For instance, cigarette smoking seems to induce long-lasting molecular footprints in TP53. However, their use to rule out other occupational exposures as etiological factors in occupational cancers is still very questionable, especially due to the putative synergistic effects of cigarette smoke with other carcinogens. Although interesting implications of possible typical mutation spectra among cancers with other occupational etiologies exist, the data are scanty and await further development of TP53 mutation databases.

Novel stereoselective control over cis vs trans opening of benzo[c]phenanthrene 3,4-diol 1,2-epoxides by the exocyclic N(2)-amino group of deoxyguanosine in the presence of hexafluoropropan-2-ol

We describe a novel and efficient synthesis (62-84% yields) of the eight possible, diastereomerically pure, cis and trans, R and S O(6)-allyl-protected N(2)-dGuo phosphoramidite building blocks derived through cis and trans opening of (+/-)-3alpha,4beta-dihydroxy-1beta,2beta-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-1 (1)] and (+/-)-3alpha,4beta-dihydroxy-1alpha,2alpha-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-2 (2)] by hexafluoropropan-2-ol (HFP)-mediated addition of O(6)-allyl-3',5'-di-O-(tert-butyldimethylsilyl)-2'-deoxyguanosine (3) at C-1 of the epoxides. Simply changing the relative amount of HFP used in the reaction mixture can achieve a wide ratio of cis/trans addition products. Thus, the observed cis/trans adduct ratio for the reaction of DE-1 (1) in the presence of 5 equiv of 3 varied from 17/83 to 91/9 over the range of 5-532 equiv of HFP. The corresponding ratios for DE-2 (2) varied from 2/98 to 61/39 under the same set of conditions. When 1 or 2 was fused with a 20-fold excess of 3 at 140 degrees C in the absence of solvent HFP, almost exclusive trans addition (>95%) was observed for the both DEs. Through the use of varying amounts of HFP in the reaction mixture as described above, each of the eight possible phosphoramidite oligonucleotide building blocks (DE-1/DE-2, cis/trans, R/S) of the BcPh DE N(2)-dGuo adducts can be prepared in an efficient fashion. To rationalize the varying cis-to-trans ratio, we propose that the addition of 3 to 1 or 2 in the absence of solvent or in the presence of small amounts of HFP proceeds primarily via an S(N)2 mechanism to produce mainly trans-opened adducts. In contrast, increasing amounts of HFP promote increased participation of an S(N)1 mechanism involving a relatively stable carbocation with two possible conformations. One of these conformations reacts with 3 to give mostly trans adduct, while the other conformation reacts with 3 to give mostly cis adduct.

Comparison of anti-p53 antibodies in immunoblotting

Some of the most important tools to study p53 protein are various anti-p53 antibodies and immunological methods based on antibody-antigen reactions. Critical comments on the specificity and sensitivity of anti-p53 antibodies have been published. Four monoclonal and two polyclonal anti-p53 antibodies, four of them from two different sources, were compared for their ability to detect in immunoblotting the benzo(a)pyrene-induced p53 from C57BL/6 mouse skin and MCF-7 human breast carcinoma cells. Multiple extra bands were seen with most antibodies. A theoretical comparison of the equivalent epitopes of p53 homologues with the known epitopes of p53 antibodies indicated that the extra bands seen with most antibodies are not due to cross-reactivity with these homologues. A careful adjustment of antibody dilutions is needed for each application utilizing commercial p53 antibodies, regardless of the recommendations of the supplier.

Elevated levels of high-mobility-group chromosomal proteins, HMGA1, in murine skin carcinoma

The high-mobility-group, HMGA1 (formerly HMGI(Y)) chromosomal proteins are known to be involved in gene regulation and their high expression is associated with neoplastic transformation of cells and metastatic tumor progression. Here, we present our results on the expression of HMGA1 in murine skin carcinoma as detected by acid-urea electrophoresis, reverse-phase high-performance liquid chromatography and Western blot. The enhanced expression of HMGA1 proteins directly correlates with the extent of cellular atypia and neoplastic changes noticed in the histopathology of tumor and suggest a potential use of these proteins as marker for determining the grade of skin tumor.

Impact of GSTM1 on aromatic-DNA adducts and p53 accumulation in human skin and lymphocytes

The cellular response to DNA damage is often a p53-mediated cell cycle arrest to provide time for DNA repair or to direct damaged cells into apoptosis. In this study, the impact of glutathione-S-transferase M1 (GSTM1) on DNA damage and subsequent p53-protein accumulation was examined in lymphocytes of healthy volunteers in vitro exposed to benzo[a]pyrene-diol-epoxide (BPDE) and in skin of atopic eczema patients topically treated with coal tar. DNA adducts were determined by immunocytochemical staining (ICC) and 32P-postlabelling, p53 accumulation was studied by ICC and the GSTM1 genotype was assessed by polymerase chain reaction. In cultured lymphocytes treated with 2.5 microM BPDE for 18 h, increased levels of p53 were found, which were positively related to BPDE-DNA adduct levels assessed by ICC (rs = 0.66, P < 0.001) and 32P-postlabelling (rs = 0.56, P < 0.001) and appeared to be higher in GSTM1(-/-) than in GSTM1(+) subjects (P = 0.003). In skin biopsies of coal tar treated eczema patients, p53 levels were elevated in 7/10 patients and a correlation was observed between p53 and DNA adduct levels (rs = 0.50, P = 0.029). GSTM1(-/-) subjects contained higher levels of p53 in the stratum basale than GSTM1(+) individuals (P = 0.026), but no influence of GSTM1 on DNA adduct levels was observed. Thus, p53 accumulates in human skin and lymphocytes as a protective mechanism against polycyclic aromatic hydrocarbon induced DNA damage, and this is more pronounced in GSTM1(-/-) compared to GSTM1(+) individuals.

The effect of dibenzo[a,1]pyrene and benzo[a]pyrene on human diploid lung fibroblasts: the induction of DNA adducts, expression of p53 and p21(WAF1) proteins and cell cycle distribution

Polycyclic aromatic hydrocarbons (PAHs) present in ambient air are considered as potential human carcinogens, but the detailed mechanism of action is still unknown. Our aim was to study the in vitro effect of exposure to dibenzo[a,l]pyrene (DB[a,l]P), the most potent carcinogenic PAH ever tested, and benzo[a]pyrene (B[a]P) in a normal human diploid lung fibroblast cells (HEL) using multiple endpoints. DNA adduct levels were measured by 32P-postlabelling, the expression of p53 and p21(WAF1) proteins by western blotting and the cell cycle distribution by flow cytometry. For both PAHs, the DNA adduct formation was proportional to the time of exposure and dependent on the stage of cell growth in culture. DNA binding was detectable even at the lowest concentration used (24h exposure, 0.01 microM for both PAHs). The highest DNA adduct levels were observed after 24h of exposure in near-confluent cells (>90% of cells at G0/G1 phase), but DNA damage induced by DB[a,l]P was approximately 8-10 times higher at a concentration one order of magnitude lower as compared with B[a]P (for B[a]P at 1 microM and for DB[a,l]P at 0.1 microM: 237+/-107 and 2360+/-798 adducts/10(8) nucleotides, respectively). The induction of p53 and p21(WAF1) protein occurred subsequent to the induction of DNA adducts. The DNA adduct levels correlated with both p53 (R=0.832, P<0.001 and R=0.859, P<0.001, for DB[a,l]P and B[a]P, respectively) and p21(WAF1) levels (R=0.808, P<0.001 and R=0.797, P=0.001, for DB[a,l]P and B[a]P, respectively), regardless of the PAH exposure and the phase of cell growth. The results showed that a detectable increase of p53 and p21(WAF1) proteins (> or = 1.5-fold as compared with controls) requires a minimal DNA adduct level of approximately 200-250 adducts/10(8) nucleotides for both PAHs tested and suggest that the level of adducts rather than their structure triggers the p53 and p21(WAF1) responses. The cell cycle was altered after 12-16h of treatment, and after 24h of exposure to 0.1 microM DB[a,l]P in growing cells, there was approximately 24% increase in S phase cells accompanied by a decrease in G1 and G2/mitosis (G2/M) cells. Cell treatment with 1.0 microM B[a]P resulted in more subtle alterations. We conclude that DB[a,l]P, and to a lesser degree B[a]P, are able to induce DNA adducts as well as p53 and p21(WAF1) without eliciting G1 or G2/M arrests but rather an S phase delay/arrest. Whether the S phase delay observed in our study is beneficial for the survival of the cells remains to be further established.

New and highly efficient synthesis of cis- and trans-opened Benzo[a]pyrene 7,8-diol 9,10-epoxide adducts at the exocyclic N(2)-amino group of deoxyguanosine

We describe a new and facile method for the synthesis of both cis- and trans-opened N(2)-deoxyguanosine (dG) adducts of (+/-)-7alpha, 8beta-dihydoxy-9beta,10beta-epoxy-7,8,9,10-tetra hydrobenzo[a]pyrene and (+/-)-7alpha,8beta-dihydoxy-9alpha,10alpha -epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene at C-10. The key step in our approach is the direct coupling of O(6)-allyl-3', 5'-di-O-(tert-butyldimethylsilyl)-2'-deoxyguanosine with these epoxides followed by the separation of the mixtures of cis- and trans-diastereomers produced. Overall coupling yields ranged from 45 to 65%. Stereochemistry of addition of the N(2)-exocyclic amino group of dG (cis-trans, approximately 1:1) was assigned by NMR, and the absolute configuration of the dG adducts was unequivocally assigned by CD spectroscopy after separation of each individual diastereomer and cleavage of the allyl protecting group. A strong CD band at 279 nm in the O(6)-protected adduct was found to be diagnostic for configuration at C-10, with a negative band correlating with 10R configuration. The synthetic methodology described allows easy access to cis- and trans-opened N(2)-dG adducts which are valuable building blocks for the synthesis of adduct-containing oligonucleotides for physical and biochemical studies.

Epoxide hydrolase--polymorphism and role in toxicology

Microsomal epoxide hydrolase is a critical biotransformation enzyme that catalyzes the conversion of a broad array of xenobiotic epoxide substrates to more polar diol metabolites. The gene has been shown previously to exhibit polymorphism, including variation in the coding region leading to amino acid substitutions at positions 113 (Y/H) and 139 (H/R). To better evaluate the phenotype associated with the structural region genetic polymorphisms associated with mEH, we performed enzymatic analyses using purified mEH proteins that were expressed using a baculovirus system, or with microsomal preparations obtained from liver tissues that were derived from individuals with homozygous mEH allelic status. Benzo[a]pyrene-4, 5-oxide and cis-stilbene oxide were employed as substrates for the enzymatic determinations. Results obtained with the purified enzymes suggested that the reaction velocity catalyzed by the wild type (Y113/H139) protein was approximately two-fold greater than the corresponding velocities for the variant forms of the enzyme. However, when reaction rates were analyzed using human liver microsomal preparations, the maximal velocities generated among the variant mEH proteins were not statistically different. Collectively, these results indicate that the structural differences coded by the mEH genetic variants may have only modest impact on the enzyme's specific activity in vivo.

Involvement of p53 in X-ray induced intrachromosomal recombination in mice

The tumor suppressor gene Trp53 (also known as p53) is the most frequently mutated gene in human cancers. p53 is induced in response to DNA damage and effects a G(1) cell cycle arrest. It is believed that p53 plays a key role in maintaining genomic integrity following exposure to DNA-damaging agents. We determined the frequency of spontaneous and DNA damage-induced homologous intrachromosomal recombination in p53-deficient mouse embryos. Homologous intrachromosomal recombination events resulting in deletions at the pink eyed unstable (p(un)) locus result in reversion to the p gene. Reversions occurring in embryonic premelanocytes give rise to black spots on the gray fur of the offspring. Pregnant C57BL/6J p(un)/p(un) p53(+/-) mice were exposed to X-rays (1 Gy) or administered benzo¿apyrene (B¿aP; 30 or 150 mg/kg i.p.) 10 days after conception. Frequencies of spontaneous p(un) reversions in p53(-/-) and p53(+/-) animals were not significantly different compared with their wild-type littermates. X-ray treatment increased the recombination frequency in wild-type and p53(+/-), but surprisingly not in p53(-/-) offspring. In contrast, B¿aP treatment caused a dose-dependent increase in p(un) reversion frequencies in all three genotypes. Western blot analysis of embryos indicated that p53 protein levels increased approximately 3-fold following X-ray treatment, while B¿aP had no effect on p53 expression. These results are in agreement with the proposal that p53 is involved in the DNA damage response following X-ray exposure and suggest that X-ray-induced double-strand breaks are processed differently in p53(-/-) animals.

Expression of p53 protein and Ki-67 antigen in gingival hyperplasia induced by nifedipine and phenytoin

BACKGROUND

Although it has been thought that drug-induced gingival hyperplasia is not related to tumorigenesis, recent case reports have shown that squamous cell carcinomas may arise in gingival hyperplasia induced by cyclosporin and phenytoin. The possible implications between the pathogenesis of this disease and tumorigenesis have not been elucidated and remain to be studied.

METHODS

We immunohistochemically examined the expression of tumor-related markers such as p53 protein and Ki-67 antigen in 11 hyperplastic gingival tissues induced by nifedipine and phenytoin, as well as 5 control tissues using an avidin-biotin-peroxidase complex method.

RESULTS

Two specimens out of 4 nifedipine-induced and 4 out of 7 phenytoin-induced hyperplastic gingival tissues revealed the expression of p53 protein in the nuclei of epithelial cells, while no expression of p53 protein was observed in the epithelia of the 5 non-hyperplastic control tissues. The immunoreactions against p53 protein showed sporadic distribution in the suprabasal layers of hyperplastic epithelia. The mean percentage of epithelial cells expressing Ki-67 antigen in the hyperplastic gingival tissues was more than 10% higher than that in the controls. The expression of Ki-67 antigen was suppressed in the typical rete pegs deeply elongated into lamina propria of hyperplastic gingival tissues. Intense immunostaining of Ki-67 antigen was found in fibroblasts of hyperplastic gingival tissues, while that of the control tissues was negligible.

CONCLUSIONS

The expression of p53 protein in gingival hyperplasia suggests that the pathogenesis of this disease is involved with impaired DNA, while the growth arrest observed in the hyperplastic epithelia with typically elongated rete pegs as expressed with Ki-67 antigen may prevent the invasive expansion of epithelial cells undergoing DNA damage within gingival tissues and may consequently suppress tumorigenic progression.

The level of DNA modification by (+)-syn-(11S,12R,13S,14R)- and (-)-anti-(11R,12S,13S,14R)-dihydrodiol epoxides of dibenzo[a,l]pyrene determined the effect on the proteins p53 and p21WAF1 in the human mammary carcinoma cell line MCF-7

The polycyclic aromatic hydrocarbon (PAH) dibenzo[a,l]pyrene (DB[a,l]P), the most carcinogenic PAH tested in rodent bioassays, exerts its pathobiological activity via metabolic formation of electrophilically reactive DNA-binding fjord region (+)-syn-(11S,12R,13S,14R)- or (-)-anti-(11R,12S,13S,14R)-DB[a,l]P-dihydrodiol epoxides (DB[a,l]-PDEs). DB[a,l]P is metabolized to these DB[a,l]PDEs which bind to DNA in human mammary carcinoma MCF-7 cells. The molecular response of MCF-7 cells to DNA damage caused by DB[a,l]PDEs was investigated by analyzing effects on the expression of the tumor suppressor protein p53 and one of its target gene products, the cyclin-dependent kinase inhibitor p21WAF1. Treatment of MCF-7 cells with (+)-syn- and (-)-anti-DB[a,l]PDE at a concentration range of 0.001-0.1 microM resulted in DB[a,l]PDE-DNA adduct levels between 2 and 30, and 3 and 80 pmol/mg DNA, respectively, 8 h after exposure. (-)-anti-DB[a,l]PDE exhibited a higher binding efficiency that correlated with a significantly stronger p53 response at low concentrations of the dihydrodiol epoxides. The level of p53 increased by 6-8 h after treatment. The p21WAF1 protein amount exceeded control levels by 12 h and remained elevated for 96 h. At a dose of 0.01 microM (+)-syn-DB[a,l]PDE, an increase in p21WAF1 was observed in the absence of a detectable change in p53 levels. The results indicate that the increase in p53 induced by DB[a,l]PDEs in MCF-7 cells requires an adduct level of approximately 15 pmot/mg DNA and suggest that the level of adducts rather than the specific structure of the DB[a,l]PDE-DNA adduct formed triggers the p53 response. The PAH-DNA adduct level formed may determine whether p53 and p21VAF1 pathways respond, resulting in cell-cycle arrest, or fail to respond and increase the risk of mutation induction by these DNA lesions.

DNA adduct levels associated with p53 induction and delay of MCF-7 cells in S phase after exposure to benzo[g]chrysene dihydrodiol epoxide enantiomers

Optically active isomers of a mammary carcinogen, anti-benzo[g]chrysene 11, 12-dihydrodiol 13, 14-epoxide, react to different extents with DNA and generate DNA adducts that differ in their stereochemistry. In the study reported here, the effect of these two enantiomers on the progress of human breast carcinoma MCF-7 cells through the cell cycle was investigated. Each enantiomer caused the cells to accumulate in the S phase, but a higher dose of the benzo[g]chrysene 11S, 12R-dihydrodiol 13R, 14S-epoxide than of its enantiomer was required to induce this effect. Similarly, induction of p53 also required a higher dose of benzo[g]chrysene 11S, 12R-dihydrodiol 13R, 14S-epoxide. Postlabeling studies indicated that the latter enantiomer also caused less modification of MCF-7 cell DNA for a given level of exposure than did benzo[g]chrysene 11R, 12S-dihydrodiol 13S, 14R-epoxide. These results suggest that p53 induction and delay in the S phase are similarly related to DNA binding and that a level of binding of the order of 1 adduct per 10(5) nucleotides is associated with these effects.

p53 expression in skin carcinogenesis and its relationship to cell proliferation and tumour growth

The immunoreactivity of p53 protein was studied in relation to tumour development, histopathological characteristics, cell proliferation, and basement membrane organisation following the induction of skin carcinogenesis in tumour-sensitive and -resistant mouse strains by ultraviolet (UV) irradiation or 7,12-dimethylbenz(a)anthracene (DMBA). In non-neoplastic skin exposed to UV irradiation or DMBA, p53 immunoreactivity was observed in nearly 50% of the basal layer cells. These cells were morphologically and histochemically indistinguishable from the p53-negative cells, occurring similarly in the tumour-producing and the tumour-negative mouse strains and regardless of subsequent tumour formation. In induced epidermal hyperplasia and in benign tumours, p53-positive and proliferating cells constituted 40-50% of all cells in the basal layer, while superficial cells were p53 negative. In dysplastic epidermis, p53-positive cells and proliferating cells were seen in all cell layers. In the case of squamous cell carcinomas, p53-positive proliferating cells in differentiated neoplasms were localised close to the basement membrane and, more frequently, in border areas showing invasion and basement membrane destruction. In horn cysts, centrally located cells were non-proliferating and p53 negative. In moderately differentiated neoplasms, proliferating cells were located closer to the basement membrane, while p53-positive cells were distributed diffusely in the neoplasm. In poorly differentiated neoplasms, p53-positive cells were more common than proliferating cells and were arranged in a diffuse pattern. The results showed that the number and location of p53-positive cells depended upon histology, with a close relationship to tumour type and degree of malignancy, but not on the mode of induction, nor on the animal strain or the relationship to subsequent tumour formation.

p53 alterations in chemically induced hamster cheek-pouch lesions

To confirm that the hamster cheek-pouch carcinogenesis model reflects development of human squamous cell carcinoma (SCC), we determined if and when p53 mutations occur in the development of SCC in this model by using immunohistochemical staining and polymerase chain reaction (PCR)-single-strand conformation polymorphism (SSCP) analysis plus direct DNA sequencing. Twenty-four hamster cheek-pouches were treated with a solution of 0.5% 7,12-dimethylbenz[a]anthracene in mineral oil three times a week for 16 wk. The malignant endophytic and exophytic tumors induced with this protocol are preceded by a sequence of premalignant lesions such as hyperplasia with or without dysplasia and carcinoma in situ, similar to the development of this cancer in humans. For this study, p53 protein accumulation was evaluated by immunostaining of various hamster cheek-pouch exophytic and endophytic SCCs as well as flat dysplastic hyperplasia and carcinomas in situ. A moderate percentage (33.3%) of exophytic lesions and most endophytic carcinomas (90%) showed positive p53 staining. In addition we also found p53-positive staining in a number of preneoplastic lesions, including areas of focal hyperplasia, dysplastic hyperplasia, and carcinomas in situ. To determine whether the alterations in p53 staining were due to p53 gene mutation, we used PCR-SSCP analysis and direct sequencing. PCR products corresponding to exons 5a, 6, 7, and 8 from 40 tumors with the highest percentage of p53-stained cells were analyzed. We detected shifted bands in 17 lesions. Direct sequencing of eight selected shifted bands revealed four mutations, including two G-->T transversions in codons 216 (tumor #1) and 252 (tumor #2) and one G-->C transversion in codon 282 (tumor #3). Tumor #4 contained a frameshift mutation in codon 251. These mutations are consistent with those reported in many human cancers. Therefore, we concluded that in the hamster cheek-pouch model, p53 protein accumulation occurs frequently and early in carcinogenesis, as it does in human SCCs, and some of these p53 alterations are due to p53 gene mutations. These findings may help us better define the mechanisms of carcinogenesis in the hamster cheek-pouch model, and p53 alterations may be an early biomarker of progression for chemoprevention studies.

Expression of Mad, an antagonist of Myc oncoprotein function, in differentiating keratinocytes during tumorigenesis of the skin

The Myc oncoprotein is associated with cell proliferation and is often down-regulated during cell differentiation. The related Mad transcription factor, which antagonises Myc activity, is highly expressed in epidermal keratinocytes. Mad also inhibits cell proliferation in vitro. To study Mad expression in keratinocyte proliferation and differentiation, we have analysed Mad RNA expression in regenerating and hyperproliferative epidermal lesions and epidermal tumours of varying degrees of differentiation using the RNA in situ hybridisation and RNAase protection techniques. Mad was strongly expressed in differentiating suprabasal keratinocytes in healing dermal wounds and in benign hyperproliferative conditions, but also in squamous cell carcinomas, in which the keratinocytes retain their differentiation potential. However, Mad expression was lost in palisading basal carcinoma cells and poorly differentiated squamous cell carcinomas, which lacked the epithelial differentiation marker syndecan-1. We therefore suggest that Mad expression is closely associated with epithelial cell differentiation, and that this association is retained in epithelial tumours of the skin.

Detection and identification of benzo[a]pyrene diol epoxide adducts to DNA utilizing capillary electrophoresis-electrospray mass spectrometry

Capillary zone electrophoresis (CZE) coupled with negative ion electrospray mass spectrometry (ES-MS) is used for the detection and identification of adducts formed from the reaction of DNA with (+/-)-anti-7,8,9,10-tetrahydrobenzo[a]pyrene-7,8-diol 9,10-epoxide (BPDE),an active metabolite of benzo[a]pyrene (BaP). Results presented in this paper demonstrate low nanogram detection limits ( < 10 ng or < 15 pmol) for normal scan spectra and collision-induced dissociation spectra of the main nucleotide adduct formed from this reaction. (BPDE reacts predominantly with the exocyclic amino group of guanine.) Exploitation of selective reaction monitoring (SRM) produces detection limits in the low picogram range ( < 85 pg or < 130 fmol). The application of sample stacking significantly increases the concentration detection limit (to approximately 10(-8) M). Nucleotide adducts are negatively charged at most pHs and are therefore ideally suited to the stacking process used in this research. These techniques have been applied to the analysis of the adducts formed from the in vitro reaction of BPDE with DNA. In addition it is shown that CZE-ES-MS, combined with solid-phase sample cleanup, can detect adducts at levels of four adducts in 10(7) unmodified bases or less.

Demonstration of benzo(a)pyrene-induced DNA damage in mice by alkaline single cell gel electrophoresis: evidence for strand breaks in liver but not in lymphocytes and bone marrow

Alkaline single cell gel electrophoresis (also known as the 'comet assay') was used to measure DNA strand breaks and alkali-labile sites in peripheral lymphocytes, bone marrow and liver cells of C57BL/6 mice orally exposed to benzo(a)pyrene. Although this polycyclic aromatic hydrocarbon is a well-known genotoxic agent, little is known about to what extent it actually induces DNA strand breaks in peripheral lymphocytes and other tissues after in vivo exposure. Significant and dose-related damage was observed in liver cells after three days of exposure (lowest observed effect level being 3 x 100 mg benzo(a)pyrene/kg b.wt. No such damage could be observed in the lymphocytes and bone marrow cells even after administration of 3 x 150 mg benzo(a)pyrene/kg b.wt. The reference substance cyclophosphamide produced pronounced DNA damage in lymphocytes and bone marrow cells already in a single dose of 100 mg/kg b.wt. The present mouse study questions the usability of DNA strand breaks in peripheral lymphocytes as an indicator of benzo(a)pyrene-induced genotoxicity.