Environmental Toxicology Program, Department of Community and Environmental Medicine, University of California at Irvine, Irvine, California 926971820
Received April 6, 1999; accepted September 27, 1999
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ABSTRACT |
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Key Words: guanine adducts; aflatoxin B1 (AFB1); AFB1-8,9-epoxide; N-acetoxy-acetylaminofluorine (N-AcO-AAF); dimethylsulfate (DMS); mixtures.
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INTRODUCTION |
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There is considerable evidence that adduct formation is non-random and occurs with considerable specificity (Hemminki, 1993; Warpehoski and Hurley, 1988
). DNA sequence-context is important in influencing the specificity of adduct formation, and neighboring nucleotides can significantly influence the reactivity of guanine toward genotoxins. GC-rich regions and runs of contiguous guanines in the genome have a higher probability of being targeted by genotoxins (Mattes et al., 1986
; 1988
; Said and Shank, 1991
). Additionally, 5-methylcytosine in CpG sites can modulate the distribution of guanine adduct formation by N-methyl-N-nitrosourea (MNU; Mathison et al., 1993
; Ross et al., 1999
), benzo(a)pyrene diolepoxide (BPDE; Denissenko et al., 1997
), N-AcO-AAF and AFB1-8,9-epoxide (Chen et al., 1998
). In vitro experiments have described enhanced binding of BPDE to guanines in H-ras codons 12 and 13 (Dittrich and Krugh, 1991
) as well as to guanines in p53 codons 157, 248, and 273 (Denissenko et al., 1996
) and preferential binding of AFB1-8,9-epoxide to guanines in the p53 codon 249 (Puisieux et al., 1991
). Mutational hot spots found in the p53 gene in human lung (Denissenko et al., 1996
) and liver (Hsu et al., 1991
) tumors correlated with the sequence-specific reactivity of the etiologically linked chemical carcinogens. Therefore, genotoxins (singly or in combination) may modify DNA bases in close proximity to one another and this may have important physiological effects, e.g., in DNA damage recognition and repair mechanisms (Bridges and Brown, 1992
; Jones et al., 1998
).
Drugs (Bailly et al., 1996; Chaires, 1985
; Walker et al., 1985a
,b
) and genotoxins such as N-hydroxy-AAF and 4-nitroquinoline-1-oxide (Winkle and Krugh, 1981
) and AFB1 (Stone et al., 1988
), non-covalently bind with a high degree of cooperativity to a small minority of high affinity sites on the DNA lattice. The precovalent binding affinity of these agents can dictate the ultimate specificity of covalent modification of the genome (Warpehoski and Hurley, 1988
). "Clustering" of ligands within a localized region of the genome may occur and high affinity binding sites for different ligands could overlap one another. This overlapping could be biologically relevant, particularly since the cooperative phase of binding isotherms described for several drugs and carcinogens (Bailly et al., 1996
; Chaires, 1985
; Rosenberg et al., 1986
; Stone et al, 1988
; Walker et al, 1985a
,b
; Winkle and Krugh, 1981
) saturate at low levels of bound ligand. This may apply to environmental exposures to genotoxins.
Previously, we have shown that prior treatment of DNA with both small (MNU) and bulky (AFB1, AAF, benzo(a)pyrene) genotoxins inhibited the formation of DNA adducts after subsequent treatment of the adducted DNA with a second genotoxin of different chemical structure (Said et al., 1995; Said and Shank, 1991
); this effect is not due to saturation of available guanines by the initial carcinogen treatment (Said et al., 1995
). These binding alterations could be caused by (a) steric effects due to the initial DNA adduct, (b) conformational changes within the DNA polymer induced by the pre-existing adduct, and/or (c) decreases in the nucleophilicity of reactive centers in the DNA molecule (e.g., the N7 position of guanine). These possible mechanisms are addressed in the current experiments that investigated the effect of pre-existing AAF adducts on the reactivity of neighboring guanine bases (target guanines) toward both small and bulky carcinogens.
In the present study, defined sequences of DNA containing a single adduct were used to determine what effect the adduct had on the reactivity of neighboring target guanines. This allowed control of adduct distribution, site-selectivity, and neighboring base content within the sequence of DNA such that modulations of target guanine reactivity could be directly attributed to the presence of initial adduct and not to nearest-neighbor effects, saturation of available sites by the initial carcinogen, or other uninterpretable effects.
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MATERIALS AND METHODS |
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Preparation of DNA oligomers containing site-specific Gua-C8-AAF-and AF-adducts.
Single and double stranded oligomers (~50100 µg of 10m0T, 10m1T, 10m3T, 10m0C, and 10m3C) were treated with a molar excess of N-AcO-AAF in 10 mM sodium citrate buffer, pH 7.2 (total volume of 200 µl), incubated up to 12 h at 37°C, recovered free of unreacted carcinogen and purified by reversed-phase HPLC (described below). Aliquots were taken to determine oligomer yields and for 5'-end labeling (~250500 ng). AAF adducts were converted to AF adducts by treating oligomers (10m0C.1AAF or 10m0C.2AAF) with basic mercaptoethanol (Zhou and Romano, 1993).
Characterization of site-specific AAF adducts in DNA oligomers.
Oligonucleotide constructs (Fig. 1) were characterized by 3 different methods. First, oligomers containing AAF adducts were initially characterized by showing that their UV spectra agreed with literature reports (Koehl et al., 1989
; Kriek et al., 1967
). Second, approximately 500 ng of each purified oligomer was 5' end-labeled with [
-32P]ATP and T4 kinase and analyzed by a modification of the Maxam-Gilbert (1977) piperidine method to determine adduct position. Third, selected oligonucleotides containing site-specific AAF or AF adducts and standards of dG-C8-AAF or dG-C8-AF were hydrolyzed in either anhydrous trifluoroacetic acid or 0.1 M HCl to liberate free Gua-C8-AAF or Gua-C8-AF adducts. Hydrolysates were analyzed by reversed-phase HPLC and ultraviolet detection. Identification was made by co-elution with authentic standards of Gua-C8-AAF or Gua-C8-AF.
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Reactions of oligomers with chemical carcinogens.
End-labeled oligonucleotides (single- or double-stranded) containing site-specific adducts (~60 ng) were reacted with DMS (final conc. 10 or 53 mM) or AFB1-8,9-epoxide (10 or 100 µM) in 10 mM sodium citrate buffer, pH 7.2 (final volume of 200 µl per reaction) and the final solvent concentration was 10% (v/v). Single-stranded DNA was reacted at 37°C for 30 min and double-stranded DNA at 4°C for 60 min. Reactions were stopped by either ethanol precipitation or by organic extraction. Positions of adducts in the oligonucleotides were determined by Maxam-Gilbert analysis using 1 M piperidine (90°C) for 30 min or 8 h. The extent of reaction at the target guanine (G1 or G2) was determined by densitometry using the freeware program NIH Image version 1.60.
HPLC analysis of products derived from reaction of ds10m0T, ds10m0T.1AAF, and ds10m0T.2AAF with AFB1-8,9-epoxide.
Non-radiolabeled oligonucleotides 10m0T, 10m0T.1AAF, and 10m0T.2AAF (1 µg of each) were individually mixed with a 3-fold molar excess of its complementary strand, and annealed. The duplex DNA was reacted with AFB1-8,9-epoxide exactly as described above for the radiolabeled oligomers (100 µM final concentration). Following extraction of the reaction mix with ethyl acetate, the aqueous layers were analyzed by reversed-phase HPLC to determine the extent of reaction at the target guanine by the epoxide. Eluted oligomers were detected by UV absorption at 260 nm.
Chemical probe analysis of DNA conformation.
Conformation of duplex oligonucleotides, 10m0C:*10m0G and 10m0C.1AAF:*10m0G (*denotes 5'-32P labeled oligonucleotide) was analyzed by the chemical probe method of Johnston and Rich (1985) using hydroxylamine and diethylpyrocarbonate.
In vitro treatment of calf thymus DNA with combinations of N-AcO-AAF and AFB1-8,9-epoxide.
Pretreatment of DNA with N-AcO-AAF was followed by reaction with AFB1-8,9-epoxide. Calf thymus DNA (0.9 mg/ml) in 10 mM sodium citrate buffer, pH 7.2 (total volume 200 µl) was reacted (in triplicate) in the presence of variable amounts of N-AcO-AAF (250500 µM) or vehicle at 37°C for various times up to 21 h to create increasing levels of AAF adducted DNA. Following initial treatment with N-AcO-AAF, each sample was extracted with ethyl acetate 34 times and the DNA was precipitated with ethanol. Following centrifugation, the DNA pellet was washed with 100% ethanol and dried. After suspension in 200 µl 10mM sodium citrate buffer, pH 7.2, each sample was pre-incubated at 37°C for 10 min followed by treatment with a final concentration of 100 µM AFB1-8,9-epoxide. Reaction mixtures were incubated for 30 min at 37°C followed by organic solvent extraction and ethanol precipitation as above. After washing with ethanol and drying the pellets for 510 min in a Speed-Vac centrifuge, the DNA was resuspended in 180 µl of water and heated for 510 min at 37°C to ensure complete dissolution. Samples were subjected to mild acid hydrolysis (0.1 N HCl, 70°C, 45 min) to liberate both Gua-C8-AAF and Gua-N7-AFB1 adducts from the DNA. Hydrolysates were cooled, filtered (0.45 µm) and analyzed by reversed-phase HPLC (Supelco LC-18, 5 µm, 250 x 4.6 mm, i.d., column; Supelco, Bellefonte, PA). Purines were eluted with 5% methanol in 0.01 M potassium phosphate, pH 7.0 for 5 min, followed by a linear gradient of 580% (v/v) methanol (in 0.01 M potassium phosphate buffer, pH 7.0) over 30 min and then holding at 80% methanol for 10 min; the flow rate was 1.0 ml/min. Eluted compounds were detected by ultraviolet absorbance (260 nm).
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RESULTS |
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Gua-C8-AAF influences alkylation by AFB1-8,9-epoxide.
Reaction of AFB1-8,9-epoxide (10 and 100 µM) with the target guanine in 10m0T.1AAF or 10m0T.2AAF was inhibited in both double- and single-stranded DNA (Fig. 3; data not shown for single-stranded DNA). The inhibition was significantly more pronounced in double-stranded DNA than single-stranded. Figure 3A
shows results obtained when piperidine incubation lasted for 30 min instead of 8 h. In this instance, cleavage of DNA at sites modified with AAF adducts (lanes 49) was barely detectable since AAF adducts do not destabilize guanine to the same degree as AFB1 adducts (Johnson, et al., 1987
); however, cleavage of DNA modified with AFB1 did occur (lanes 13). Attenuation of AFB1 epoxide binding due to the presence of an AAF adduct is clearly evident (lanes 49). Densitometry of autoradiograms of polyacrylamide gels showed that reactivity of the target guanine in 10mOT.1AAF (i.e., G2) towards 100 µM AFB1-8,9-epoxide was reduced ~4-fold when compared with the same guanine in the unadducted control oligomer (10m0T) (Fig. 3B
, compare lanes 5 and 3; inset) following 8 h piperidine incubation. Similarly, the reactivity of the target guanine in 10m0T.2AAF (i.e., G1) towards AFB1-8,9-epoxide (at the same concentration) was decreased about 19-fold when compared to its control guanine (Fig. 3B
, inset).
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Increasing distance between Gua-C8-AAF adduct and target guanine.
Double-stranded oligonucleotides, containing AAF adducts at G1 or G2 with intervening nucleotides (either 1 or 3) between the site-specific adduct and the target guanine, were also reacted with AFB1-8,9-epoxide. The effect of the AAF adduct on the formation of the AFB1 adduct decreased with increasing separation of the target guanine (Fig. 6). The reactivity of the target guanine towards AFB1-8,9-epoxide (100 µM final concentration) was moderately attenuated, ~3-fold in 10m1T (both isomers gave similar results; gel data not shown) and ~1.5-fold in 10m3T (both isomers gave similar results; gel data not shown), when compared to the strong attenuation that occurs when the AAF adduct was located immediately adjacent to the target guanine (~4-fold for 10m0T.1AAF and ~19-fold for 10m0T.2AAF). Because of the strong negative influence of the 5' and 3' nearest neighboring thymidines surrounding the target guanine in 10m3T oligos and because of unexplained high background that was consistently found after piperidine treatment of control 10m3T, another double-stranded oligonucleotide was constructed (10m3C; see Fig. 1
). This oligonucleotide contained cytosines as the intervening nucleotides between the site-specific AAF adduct and the target guanine. When this oligomer was annealed with its complement and reacted with AFB1-8,9-epoxide, results similar to the inhibition seen with AAF-modified 10m3T oligomers were observed (an ~1.5-fold decrease) (Fig. 6
). It therefore appeared that the reactivity of the target guanine towards AFB1-8,9-epoxide increased as a function of the number of intervening nucleotides between the target guanine and the AAF adduct.
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DISCUSSION |
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Bailly and coworkers (1997) found in three different restriction fragments that prior treatment of DNA with actinomycin caused alterations in the sequence specificity of bleomycinFe-mediated cleavages, including the disappearance of cleavage sites and appearance of new sites. The authors argued that the results provided evidence of "mutual interference" between two individual small molecules binding with DNA.
Misra and coworkers (1983) used combinations of intercalative agents and AFB1 to study the mechanisms of sequence-specific binding to DNA by AFB1-8,9-epoxide; simultaneous treatment of a 174-bp restriction fragment derived from pBR322 with [3H]-AFB1 (activated by chloroperoxybenzoic acid) and either various non-reactive AFB1 structural analogues or ethidium bromide led to a decreased level of AFB1 covalently bound to DNA, and a reduced sequence-specificity of AFB1 reactivity. These observations were thought to be due to competitive non-covalent binding by the non-reactive ligands and/or to alterations in helix conformation due to the binding of intercalative agents.
The findings reported here describing attenuation or blockage of reaction by AFB1 epoxide on guanine when located immediately adjacent to a dG-C8-AAF adduct, on the same sequence of DNA, are similar to those of Kobertz and coworkers (1997), who showed that the presence of an intercalation inhibitor (a thymidine-benzofuran derivative) on the non-target strand, at an intercalation site immediately 5' to a guanine on the target strand, protected the guanine from reaction with AFB1-epoxide (due to prevention of epoxide intercalation). The reactivity of the epoxide shifted to another intercalation site and reacted with the N7 atom of guanine in a stereospecific manner.
An initial AAF adduct, but not an AF adduct, inhibits Gua-N7-AFB1 formation at the target guanine in oligonucleotides.
The efficient alkylation by AFB1-8,9-epoxide of G2 in ds10m0T oligomers (which did not contain AAF adducts) concurs with the reported sequence-specificity of aflatoxin adduction (Benasutti et al., 1988). 5'-GG*T closely follows 5'-GG*G as the most reactive sequence in DNA towards AFB1-8,9-epoxide (G* = targeted guanine). The nearest neighboring bases to a target guanine (5'-XG*Z-3') that best predict reactivity towards AFB1-8,9-epoxide are described as follows, 5'-side (i.e, X-position): G > C > A > T and 3'-side (i.e., Z-position): G > T > C > A (Benasutti, 1988).
Steric influences and conformational alterations in the DNA polymer, mediated by the site-specific AAF adduct, were initially thought to be probable causes of the described AFB1 adduction alterations. The bulky C8-acetylaminofluorene adduct destabilizes the normal anti conformation of guanine in DNA in favor of the syn conformation, whereas, C8-aminofluorene adducts do not (Evans et al., 1980). Hydrogen bonding of Gua and Cyt is disrupted because the guanine moiety is displaced out of the double helix, into the major groove, while the acetylaminofluorene moiety is inserted into the helix with some evidence of base-fluorene stacking interactions, which can contribute to structural stabilization (O'Handley et al., 1993
). The initial intercalation of AFB1-epoxide may be reduced by either (or a combination of) (i) local denaturation of DNA, caused by the AAF adduct, or (ii) steric interference which physically blocks the AFB1-epoxide from effectively intercalating. The fact that alkylation of the target guanine by the epoxide increases as it is placed 2 and 4 nucleotides away from the site-specific AAF adduct supports both possibilities. The distortion of local DNA conformation in ds10m0C.1AAF, caused by the presence of the AAF adduct, was confirmed with the chemical probes, hydroxylamine and diethylpyrocarbonate, and is consistent with reports describing AAF-induced perturbations of DNA conformation cited above. It was also observed that the reactivities of single-stranded AAF-adducted 10m1T and 10m3T towards AFB1 epoxide were not attenuated to the same degree as their corresponding double stranded oligonucleotides (data not shown). This would also support a conformational rationale for binding alterations. Additionally, the observation that a bulky AF adduct does not lead to significant changes in AFB1-8,9-epoxide binding to a neighboring guanine, while an AAF adduct does, suggests that conformation may be a more important parameter than steric hindrance.
There are two possible reasons for the apparent contradiction in data that describes a block in the reaction of AFB1-epoxide with guanine in the sequence context 5'-TG1G2(AAF)-3', while there appears no block in the context 5'-CG1G2(AAF)3'. First, the difference may be related to the enhanced neighboring effect C has, relative to T, when found 5' to a target G. The sequence-context 5'-CGG is ~4-fold more reactive than 5'-TGG towards AFB1-epoxide (Benasutti et al., 1988). The positive influence of the 5' C neighboring the target guanine may override the negative influence of the 3' G(AAF) adduct. Second, the difference may lie in the thermal stability of the 2 oligonucleotides. Double-stranded 10m0C.2AAF is GC-rich and presumably has a higher melting temperature than 10m0T.2AAF. The decreased stability of the 10m0T.2AAF oligomer could lead to increased single-strandedness with a subsequent reduction in the concentration of intercalation sites 5' to the target guanine and subsequent reduced covalent adduct formation. Therefore, perturbations in DNA conformation, induced by an AAF adduct, may be dependent upon the sequence-context in which it is found.
It is therefore a reasonable assumption, with the model oligonucleotides used in the present study, that the efficiency of initial non-covalent binding of AFB1-8,9-epoxide with DNA substrate will determine the rate of Gua-N7-AFB1 formation. This non-covalent binding serves to concentrate the reactive epoxide from the bulk solution by intercalation of the planar portion of the aflatoxin moiety above the 5' face of the target guanine (Raney et al., 1990; 1993
). The molecule is therefore oriented for efficient backside attack by N7-guanine on the exo-stereoisomer of the epoxide (via an SN2-type mechanism) to form the covalently bonded adduct (Iyer et al., 1994
). Therefore, the inhibition of N7-AFB1 formation on the target guanine in oligonucleotides containing site-specific AAF adducts may result, firstly, from a reduction in the rate of non-covalent association, thereby decreasing the effective concentration of intercalated non-covalent AFB1-epoxide complex. It then follows that less covalently bound AFB1 would be formed at the target guanine. Secondly, the attenuated adduction by AFB1-epoxide is probably not due to perturbations in the "molecular electrostatic potential" (i.e., nucleophilicity) (Pullman and Pullman, 1981
) of the target N7 guanine atom, because there were no observable changes in the reactivity of the SN2-type carcinogen DMS with AAF-adducted oligomers as compared to control oligomers. DMS is a small methylating agent that was used to probe the nucleophilicity of the target N7-guanine.
HPLC analysis of calf thymus DNA treated sequentially with carcinogens.
In order to confirm the observations in the oligomer model in which the presence of an AAF adduct contributed to formation of a subsequent adduct by AFB1, calf thymus DNA was treated with the 2 carcinogens in each order of sequence. Prior modification of calf thymus DNA with N-AcO-AAF caused a reduction in the number of AFB1 epoxide-derived Gua-N7-AFB1 adducts that was dependent upon the initial levels of AAF adducts. However, when the order of carcinogens was reversed, no effect was observed.
It is unlikely that the decrease in the number of AFB1 adducts was due to saturation of available binding sites by AAF molecules, as the maximum level of AAF adduction involved <7% of the total guanine bases. Favored binding sites along the DNA lattice for both AAF and AFB1 with a potential for overlap (or close interaction) of these binding sites is possible. The presence of an AAF adduct at its high affinity site could effectively occupy the high affinity site for AFB1-epoxide, although global levels of AAF adducts may be relatively low. However, the molecular details of such an interaction are not apparent and were investigated further by using oligonucleotides containing site-specific adducts.
In conclusion, non-additivity between 2 chemical carcinogens at the level of DNA adduct formation has been demonstrated and may help understand similar interactions demonstrated in an in vivo system in which pretreatment of Salmonella typhimurium with AAF, induced alone (Said et al, 1999). These results should be useful for development of risk assessment models, in consideration of mixtures to chemical carcinogens.
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ACKNOWLEDGMENTS |
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NOTES |
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REFERENCES |
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Bailly, C., Kenani, A., and Waring, M. J. (1997). Altered cleavage of DNA sequences by bleomycin and its deglycosylated derivative in the presence of actinomycin. Nucleic Acids Res. 25, 15161522.
Bailly, C., Suh, D., Waring, M. J., and Chaires, J. B. (1998). Binding of daunomycin to diamminopurine- and/or inosine-substituted DNA. Biochemistry 37, 10331045.[ISI][Medline]
Belguise-Valladier, P., and Fuchs, R. P. P. (1991). Strong sequence-dependent polymorphism in adduct-induced DNA structure: Analysis of single N-2-acetylaminofluorene residues bound within the NarI mutation hot spot. Biochemistry 30, 1009110100.[ISI][Medline]
Benasutti, M., Ejadi, S., Whitlow, M. D., and Loechler, E. L. (1988). Mapping the binding site of aflatoxin B1 in DNA: Systematic analysis of the reactivity of aflatoxin B1 with guanines in different DNA sequences. Biochemistry 27, 472481.[ISI][Medline]
Bridges, B. A., and Brown, G. M. (1992). Mutagenic DNA repair in Escherichia coli XXI. A stable SOS-inducing signal persisting after excision repair of ultraviolet damage. Mutat. Res. 270, 13544.[ISI][Medline]
Chaires, J. B. (1985). Long-range allosteric effects on the B to Z equilibrium by daunomycin. Biochemistry 24, 74797486.[ISI][Medline]
Chen, J. X., Zheng, Y., West, M., and Tang, M. S. (1998). Carcinogens preferentially bind at methylated CpG in the p53 mutational hot spots. Cancer Res. 58, 20702075.[Abstract]
Denissenko, M. F., Chen, J. X., Tang, M. S., and Pfeifer, G. P. (1997). Cytosine methylation determines hot spots of DNA damage in the human p53 gene. Proc. Natl. Acad. Sci. USA 94, 38933898.
Denissenko, M. F., Pao, A., Tang, M. S., and Pfeifer, G. P. (1996). Preferential formation of benzo[a]pyrene adducts at lung cancer mutational hot spots in p53. Science 274, 430432.
Dittrich, K. A. and Krugh, T. R. (1991). Mapping of (+/-)-anti-benz[a]pyrene diol epoxide adducts to human c-Ha-ras1 proto-oncogene. Chem. Res. Toxicol. 4, 277281.[ISI][Medline]
Essigmann, J. M., Croy, R. G., Nadzan, A. M., Busby, Jr., Reinhold, V.N., Buchi, G., and Wogan, G. N. (1977). Structural identification of the major DNA adduct formed by aflatoxin B1 in vitro. Proc. Natl. Acad. Sci. USA 74, 18701874.[Abstract]
Evans, F. E., Miller, D. W., and Beland, F. A. (1980). Sensitivity of the conformation of deoxyguanosine to binding at the C-8 position by N-acetylated and unacetylated 2-aminofluorene. Carcinogenesis 1, 955959.[ISI][Medline]
Hemminki, K. (1993). DNA adducts, mutations, and cancer. Carcinogenesis 14, 20072012.[Abstract]
Hsu, I. C., Metcalf, R. A., Sun, T., Welsh, J. A., Wang, N. J., and Harris, C. C. (1991). Mutational hot spot in the p53 gene in human hepatocellular carcinomas. Nature 350, 427428.[ISI][Medline]
Iyer, R., and Harris, T. M. (1993). Preparation of aflatoxin B1 8,9-epoxide using m-chloroperbenzoic acid. Chem. Res. Toxicol. 6, 313316.[ISI][Medline]
Iyer, R. S., Coles, B. F., Raney, K. D., Thier, R., Guengerich, F. P., and Harris, T. M. (1994). DNA adduction by the potent carcinogen aflatoxin B1: Mechanistic studies. J. Am. Chem. Soc. 116, 16031609.[ISI]
Johnson, D. L., Reid, T. M., Lee, M. S., King, C. M., and Romano, L. J. (1987). Chemical stability of oligonucleotides containing the acetylated and deacetylated adducts of the carcinogen N-2-acetylaminofluorene. Carcinogenesis 8, 619623.[Abstract]
Johnston, B. H., and Rich, A. (1985). Chemical probes of DNA conformation: Detection of Z-DNA at nucleotide resolution. Cell 42, 713724.[ISI][Medline]
Jones, W. R., Johnston, D. S., and Stone, M. P. (1998). Refined structure of the doubly intercalated d[TAT(AFB1)GCATA]2 aflatoxin B1 adduct. Chem. Res. Toxicol. 11, 873881.[ISI][Medline]
Kobertz,W. R., Wang, D., Wogan, G. N., and Essigmann, J. M. (1997). An intercalation inhibitor altering the target selectivity of DNA damaging agents: Synthesis of site-specific aflatoxin B1 adducts in a p53 mutational hot spot. Proc. Natl. Acad. Sci. USA 94, 95799584.
Koehl, P., Burnouf, D., and Fuchs, R. P. P. (1989). Construction of plasmids containing a unique acetylaminofluorene adduct located within a mutation hot spot. J. Mol. Biol. 207, 355364.[ISI][Medline]
Kriek, E., Miller, J. A., Juhl, U., and Miller, E. C. (1967). 8-(N2-Fluorenylacetamido)guanosine, an arylamidation reaction product of guanosine and the carcinogen N-acetoxy-N-2-fluorenylacetamide in neutral solution. Biochemistry 6, 177182.[ISI][Medline]
Mathison, B. H., Said, B., and Shank, R. C. (1993). Effect of 5-methylcytosine as a neighboring base on methylation of DNA guanine by N-methyl-N-nitrosourea. Carcinogenesis 14, 323327.[Abstract]
Mattes, W. B., Hartley, J. A., and Kohn, K. W. (1986). DNA sequence selectivity of guanine-N7 alkylation by nitrogen mustards. Nucleic Acids Res. 14, 29712987.[Abstract]
Mattes, W. B., Hartley, J. A., Kohn, K. W., and Matheson, D. W. (1988). GC-rich regions in genomes as targets for DNA alkylation. Carcinogenesis 9, 20652072.[Abstract]
Maxam, A. M., and Gilbert, W. (1977). Sequencing end-labeled DNA with base-specific chemical cleavages. Methods Enzymol. 65, 499560.
Misra, R. P., Muench, K. F., and Humayun, M. Z. (1983). Covalent and noncovalent interactions of aflatoxin with defined deoxyribonucleic acid sequences. Biochemistry 22, 33513359.[ISI][Medline]
O'Handley, S. F., Sanford, D. G., Xu, R., Lester, C. C., Hingerty, B. E., Broyde, S., and Krugh, T. R. (1993). Structural characterization of an N-acetyl-2-aminofluorene (AAF) modified DNA oligomer by NMR, energy minimization, and molecular dynamics. Biochemistry 32, 24812497.[ISI][Medline]
Puisieux, A., Lim, S., Groopman, J., and Ozturk, M. (1991). Selective targeting of p53 gene mutational hot spots in human cancers by etiologically defined carcinogens. Cancer Res. 51, 61856189.[Abstract]
Pullman, A., and Pullman, B. (1981). Molecular electrostatic potential of the nucleic acids. Q. Rev. Biophys. 14, 289380.[ISI][Medline]
Raney, K. D., Gopalakrishnan, S., Byrd, S., Stone, M. P., and Harris, T. M. (1990). Alteration of the aflatoxin cyclopentenone ring to a -lactone reduces intercalation with DNA and decreases formation of guanine N7 adducts by aflatoxin epoxides. Chem. Res. Toxicol. 3, 254261.[ISI][Medline]
Raney, V. M., Harris, T. M., and Stone, M. P. (1993). DNA conformation mediates aflatoxin B1-DNA binding and the formation of guanine N7 adducts by aflatoxin B1 8,9-exo-epoxide. Chem. Res. Toxicol. 6, 6468.[ISI][Medline]
Rosenberg, L. S., Carvlin, M. J., and Krugh, T. R. (1986). The antitumor agent mitoxantrone binds cooperatively to DNA: Evidence for heterogeneity in DNA conformation. Biochemistry 25, 10021008.[ISI][Medline]
Ross, M. K., Mathison, B. H., Said, B., and Shank, R. C. (1999). 5-Methylcytosine in CpG sites and the reactivity of nearest neighboring guanines toward the carcinogen aflatoxin B1-8,9-epoxide. Biochem. Biophys. Res. Commun. 254, 114119.[ISI][Medline]
Said, B., Ross, M. K., Hamade, A. K., Matsumoto, D. C., and Shank, R. C. (1999). DNA-damaging effects of genotoxins in mixture: 1. Non-additive effects of aflatoxin B1 and N-acetylaminofluorene on their mutagenicity in Salmonella typhimurium. Toxicol. Sci. 52, 226231.[Abstract]
Said, B., Ross, M. K., Salib, T., and Shank, R. C. (1995). Modulation of DNA adduct formation by successive exposures of DNA to small and bulky chemical carcinogens. Carcinogenesis 16, 30573062.[Abstract]
Said, B., and Shank, R. C. (1991). Nearest-neighbor effects on carcinogen binding to guanine runs in DNA. Nucleic Acids Res. 19, 13111316.[Abstract]
Stone, M. P., Gopalakrishnan, S., Harris, T. M., and Graves, D. E. (1988). Carcinogen-nucleic acid interactions: Equilibrium binding studies of aflatoxins B1 and B2 with DNA and the oligodeoxynucleotide d(ATGCAT)2. J. Biomol. Struct. Dyn. 5, 10251041.[ISI][Medline]
Suh, M., Ariese, F., Small, G. J., Jackowiak, R., Liu, T. M., and Geacintov, N. E. (1995). Conformational studies of the (+)-trans, (-)-trans, (+)-cis, and (-)-cis adducts of anti-benzo[a]pyrene diol epoxide to N2-dG in duplex oligonucleotides using polyacrylamide gel electrophoresis and low-temperature fluorescence spectroscopy. Biophys. Chem. 56, 281296.[ISI][Medline]
Tullius, T. D., and Lippard, S. J. (1982). Ethidium bromide changes the nuclease-sensitive DNA binding sites of the antitumor drug cis-diaminedichloroplatinum (II). Proc. Natl. Acad. Sci. USA 79, 34893492.[Abstract]
Walker, G. T., Stone, M. P., and Krugh, T. R. (1985a). Ethidium binding to left-handed (Z) DNAs results in regions of right-handed DNA at the intercalation site. Biochemistry 24, 74627471.[ISI][Medline]
Walker, G. T., Stone, M. P., and Krugh, T. R. (1985b). Interaction of drugs with Z-DNA: Cooperative binding of actinomycin D or actinomine to the left-handed forms of poly(dG-dC), poly(dG-dC), and poly(dG-m5dC).Poly(dG-m5dC) reverses the conformation of the helix. Biochemistry 24, 74717479.[ISI][Medline]
Warpehoski, M. A., and Hurley, L. H. (1988). Sequence selectivity of DNA covalent modification. Chem. Res. Toxicol. 1, 315333.[ISI][Medline]
Winkle, S. A., and Krugh, T. R. (1981). Equilibrium binding of carcinogens and antitumor antibiotics to DNA: Site-selectivity, cooperativity, allosterism. Nucleic Acids Res. 9, 31753186.[Abstract]
Zhou, Y., and Romano, L. J. (1993). Solid-phase synthesis of oligonucleotides containing site-specific N-(2'-deoxyguanosin-8-yl)-2-(acetylamino)fluorene adducts using 9-flourenylmethoxycarbonyl as the base-protecting group. Biochemistry 32, 1404314052.[ISI][Medline]