Formation of DNA adducts by microsomal and peroxidase activation of p-cresol: role of quinone methide in DNA adduct formation

We have investigated the activation of p-cresol to form DNA adducts using horseradish peroxidase, rat liver microsomes and MnO(2). In vitro activation of p-cresol with horseradish peroxidase produced six DNA adducts with a relative adduct level of 8.03+/-0.43 x 10(-7). The formation of DNA adducts by oxidation of p-cresol with horseradish peroxidase was inhibited 65 and 95% by the addition of either 250 or 500 microM ascorbic acid to the incubation. Activation of p-cresol with phenobarbital-induced rat liver microsomes with NADPH as the cofactor; resulted in the formation of a single DNA adduct with a relative adduct level of 0.28+/-0.08 x 10(-7). Similar incubations of p-cresol with microsomes and cumene hydroperoxide yielded three DNA adducts with a relative adduct level of 0.35+/-0.03 x 10(-7). p-Cresol was oxidized with MnO(2) to a quinone methide. Reaction of p-cresol (QM) with DNA produced five major adducts and a relative adduct level of 20.38+/-1.16 x 10(-7). DNA adducts 1,2 and 3 formed by activation of p-cresol with either horseradish peroxidase or microsomes, are the same as that produced by p-cresol (QM). This observation suggests that p-cresol is activated to a quinone methide intermediate by these activation systems. Incubation of deoxyguanosine-3'-phosphate with p-cresol (QM) resulted in a adduct pattern similar to that observed with DNA; suggesting that guanine is the principal site for modification. Taken together these results demonstrate that oxidation of p-cresol to the quinone methide intermediate results in the formation of DNA adducts. We propose that the DNA adducts formed by p-cresol may be used as molecular biomarkers of occupational exposure to toluene.

Induction of lacI mutations in Big Blue Rat-2 cells treated with 1-(2-hydroxyethyl)-1-nitrosourea: a model system for the analysis of mutagenic potential of the hydroxyethyl adducts produced by 1,3-bis (2-chloroethyl)-1-nitrosourea

We have investigated the genotoxic effects of 1-(2-hydroxyethyl)-1-nitrosourea (HENU). We have chosen this agent because of its demonstrated ability to produce N7-(2-hydroxyethyl) guanine (N7-HOEtG) and O(6)-(2-hydroxyethyl) 2'-deoxyguanosine (O(6)-HOEtdG); two of the DNA alkylation products produced by 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU). For these studies, we have used the Big Blue Rat-2 cell line that contains a lambda/lacI shuttle vector. Treatment of these cells with HENU produced a dose dependent increase in the levels of N7-HOEtG and O(6)-HOEtdG as quantified by HPLC with electrochemical detection. Treatment of Big Blue Rat-2 cells with either 0, 1 or 5mM HENU resulted in mutation frequencies of 7.2+/-2.2x10(-5), 45.2+/-2.9x10(-5) and 120.3+/-24.4x10(-5), respectively. Comparison of the mutation frequencies demonstrates that 1 and 5mM HENU treatments have increased the mutation frequency by 6- and 16-fold, respectively. This increase in mutation frequency was statistically significant (P<0.001). Sequence analysis of HENU-induced mutations have revealed primarily G:C-->A:T transitions (52%) and a significant number of A:T-->T:A transversions (16%). We propose that the observed G:C-->A:T transitions are produced by the DNA alkylation product O(6)-HOEtdG. These results suggest that the formation of O(6)-HOEtdG by BCNU treatment contributes to its observed mutagenic properties.

Levels of N7-(2-hydroxyethyl)guanine as a molecular dosimeter of drug delivery to human brain tumors

The level of N7-(2-hydroxyethyl)guanine (N7-HOEtG), one of the DNA alkylation products formed by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) treatment, was measured in human brain tumor samples by high performance liquid chromatography with electrochemical detection. The tumors from 6 recurrent chemotherapy-naive patients with recurrent glioblastoma multiforme were analyzed as controls. The mean level of N7-HOEtG in DNA of these specimens was 0.42 pmol/mg DNA. Samples were also obtained from a patient with a recurrent glioblastoma multiforme after direct intratumoral therapy with BCNU in ethanol (DTI-015). The levels of N7-HOEtG in the samples distal, medial, and adjacent to the site of injection were 0.8, 2.6, and 369.5 pmol/mg DNA, respectively. Comparison of the level of N7-HOEtG detected in the distal sample after injection with BCNU in ethanol with the mean level of the untreated samples indicated that it was not sufficiently different to be ruled out as a chance occurrence. Comparison of the levels of N7-HOEtG in the medial and adjacent brain tumor samples with the mean level of the control samples showed values that were approximately 6- and 879-fold higher. These results demonstrate that intratumoral administration of BCNU in ethanol produces significant levels of DNA alkylation and suggest that DNA adduct measurements provide a unique molecular dosimeter to evaluate delivery of alkylating agents to brain tumors.

Measurement of sister chromatid exchange induction in intracerebral brain tumors as a method for evaluation of therapeutic drug combinations

Rats with 9L or 9L-2 intracerebral brain tumors were treated with streptozotocin (STZ) and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) at various doses either as single agents or in combination. Treatment of rats bearing 9L or 9L-2 tumors with either STZ or BCNU produced a significant increase the level of sister chromatid exchanges (SCEs) (p < .001). Compared with 9L-2 tumors, 9L tumors were 7.8-fold more sensitive to the induction of SCEs by BCNU treatment. After combination treatments of STZ and BCNU, the number of SCEs observed in 9L tumors was additive but was synergistic in 9L-2 tumors. O6-alkylguanine DNA alkyltransferase activity (AGT) was measured in 9L and 9L-2 cells in vitro. No AGT activity was detected in 9L cells, however, a low level of activity was measured in 9L-2. In vitro, treatment of 9L-2 cells with STZ produced a dose dependent inhibition of AGT activity. We interpret, these results to suggest that STZ pretreatment potentiated the effects of BCNU in 9L-2 tumors by inhibiting AGT activity. The observations in this study suggest that measurement of SCE induction in intracerebral tumor models may provide a useful strategy for evaluating the effectiveness of new agents and potential therapeutic combinations for the treatment of gliomas.

Effect of cations on the formation of DNA alkylation products in DNA reacted with 1-(2-Chloroethyl)-1-nitrosourea

The purpose of this study was to examine the influence of cations on the formation of the individual DNA alkylation products derived from 1-(2-chloroethyl)-1-nitrosourea (CNU). Reaction of calf-thymus DNA with [(3)H]CNU in 10 mM triethanolamine buffer produced 13 DNA adducts. Seven of these adducts were identified as N7-(2-hydroxyethyl)guanine, N7-(2-chloroethyl)guanine, 1, 2-(diguan-7-yl)ethane, N1-(2-hydroxyethyl)-2-deoxyguanosine, 1-(N1-2-deoxyguanosinyl)-2-(N3-2-deoxycytidyl)ethane, O(6)-(2-hydroxyethyl)-2-deoxyguanosine, and phosphotriesters. The ratios of the individual products indicated that the chloroethyl and hydroxyethyl adducts are derived from different alkylating intermediates. The influence of cations on the formation of these DNA alkylation products was investigated by the addition of either NaCl, MgCl(2), or spermine. The results demonstrated that (1) the levels of DNA alkylation were inversely proportional to ionic strength, (2) the extent of inhibition was dependent on the alkylation product, and (3) the order of relative effectiveness of inhibition of DNA alkylation by these cations was as follows: spermine > Mg > Na. These results support a model whereby reactions which proceed via an S(N)2 mechanism are more sensitive to the effects of ionic strength than reactions which proceed via an S(N)1 mechanism. In 9L cells treated with CNU, the same alkylation products were formed as in purified DNA; however, the product distribution was different. We interpret this to indicate that within cells, cations modify the reaction of intermediates derived from CNU with DNA.

Oxidation of eugenol to form DNA adducts and 8-hydroxy-2'-deoxyguanosine: role of quinone methide derivative in DNA adduct formation

We have investigated the activation of eugenol to form DNA adducts and oxidative base damage. Treatment of myeloperoxidase containing HL-60 cells with eugenol, produced a dose-dependent formation of three DNA adducts as detected with P1-enhanced 32P-post-labeling. Incubation of HL-60 cells with the combination of 100 microM eugenol and 100 microM H2O2 potentiated the levels of DNA adduct in HL-60 cells by 14-fold, which suggests peroxidase activation in adduct formation. In vitro activation of eugenol with either horseradish peroxidase or myeloperoxidase and H2O2 produced three DNA adducts that were inhibited by the addition of either ascorbic acid or glutathione, by 66 and 90%, respectively. The DNA adducts formed in HL-60 cells treated with eugenol were the same as those formed by in vitro peroxidase activation. In addition to adduct formation, peroxidase activation of eugenol produced a 2- to 3-fold increase in the level of oxidative base damage. Eugenol quinone methide was prepared by Ag(I)oxide oxidation of eugenol. Peroxidase activation of eugenol gave a product that had the same UV spectrum as eugenol quinone methide, which suggests that it was one of the products. Reaction of eugenol quinone methide with either DNA or deoxyguanosine-3'-phosphate produced two principal adducts (2 and 4). When DNA adduct 2 formed by incubation of eugenol quinone methide with deoxyguanosine-3'-phosphate was compared with DNA 2 adduct formed in HL-60 cells treated with eugenol results demonstrated that they were the same. This suggests that eugenol quinone methide is one of the reactive intermediates leading to DNA adduct formation in cells. Activation of eugenol with 10 microM copper sulfate resulted in the production of one principal (2) and several minor adducts. DNA adduct 2 formed by activation of eugenol with copper sulfate was the same as DNA adduct 2 formed by either peroxidase activation of eugenol or by reactions with eugenol quinone methide, which indicates that the reactive intermediates generated by these activation systems were similar. Copper sulfate produced a 95-fold increase in the level of oxidative base damage, which was significantly inhibited by the addition of either bathocuproinedisulphonic acid or catalase. The formation of oxidative base damage was consistent with a Fenton reaction mechanism. Our results demonstrate that eugenol can be activated to form both DNA adducts and oxidative base damage. We propose that the formation of this DNA damage may contribute to the observed toxic properties of eugenol.

Formation of DNA adducts and oxidative base damage by copper mediated oxidation of dopamine and 6-hydroxydopamine

We have investigated the formation of DNA adducts and oxidative base damage produced by copper sulfate activation of dopamine and 6-hydroxydopamine. In the presence of 10 microM copper sulfate both 100 microM dopamine and 100 microM 6-hydroxydopamine formed three similar DNA adducts with relative adduct levels of 8.36 +/- 2.23 x 10(-8) and 7.98 +/- 2.53 x 10(-8), respectively. The levels of 8-hydroxy-2'-deoxyguanosine produced by these incubations were 5.2 +/- 0.03, 32.6 +/- 2.4, and 0.01 pmol/microg DNA for dopamine, 6-hydroxydopamine, and control incubations, respectively, representing a 520- to 3260-fold increase in the level of this base oxidation product. The use of specific chelators and catalase demonstrated that the reduction of Cu2+ to Cu1+ and the formation of a peroxide plays an important role in the activation of dopamine and 6-hydroxydopamine to form adducts and oxidative base damage. Our results suggest that the oxidation of dopamine by transition metals present in the brain may lead to the formation of both DNA adducts and oxidative base damage in dopaminergic cells. We propose that these processes may contribute to the observed loss of dopaminergic neurons in patients with Parkinson's disease.

Detection of N7-(2-hydroxyethyl)guanine adducts in DNA and 9L cells treated with 1-(2-chloroethyl)-1-nitrosourea

A sensitive analytical method, HPLC-ED, was developed for the measurement of N7-(2-hydroxyethyl)guanine (N7-HOEtG). A detection limit of 3.2 N7-HOEtG/10(8) nucleotides was obtained with this method. Linear dose response curves for the formation of N7-HOEtG were obtained following treatment of either calf thymus DNA or 9L cells with 1-(2-chloroethyl)-1-nitrosourea (CNU). Using HPLC-ED a significant increase in the level of N7-HOEtG could be detected in 9L cells following treatment with 5 microM CNU. Our study suggests that with this analytical method the formation of N7-HOEtG in the white blood cells of patients treated with chloroethylnitrosoureas may be determined.

Investigation of the DNA adducts formed in B6C3F1 mice treated with benzene: implications for molecular dosimetry

We have investigated the formation of DNA adducts in the bone marrow and white blood cells of male B6C3F1 mice treated with benzene using P1-enhanced 32P-postlabeling. No adducts were detected in the bone marrow of controls or mice treated with various doses of benzene once a day. After twice-daily treatment for 1 to 7 days with benzene, 440 mg/kg, one major (no. 1) and up to two minor DNA adducts were detected in both the bone marrow and white blood cells. The relative adduct levels in these cells ranged from 0.06 to 1.46 x 10(-7). a significant correlation (r2 = 0.95) between levels of adducts in bone marrow and white blood cells was observed. After a 7-day treatment with benzene, 440 mg/kg twice a day, the number of cells per femur decreased from 1.6 x 10(7) to 0.85 x 10(7), indicating myelotoxicity. In contrast, administration of benzene once a day produced only a small decrease in bone marrow cellularity. The observed induction of toxicity in bone marrow was paralleled by formation of DNA adducts. In vitro treatment of bone marrow with hydroquinone (HQ) for 24 hr produced the same DNA adducts as found after treatment of mice with benzene, suggesting that HQ is the principal metabolite of benzene leading to DNA adduct formation in vivo. Using P-postlabeling the principal DNA adduct formed in vivo was compared with N2-(4-hydroxyphenyl)-2'-deoxyguanosine-3'-phosphate. The results of this comparison demonstrated that the DNA adduct formed in vivo co-chromatographs with N2-(4-hydroxyphenyl)-2'-deoxyguanosine-3'-phosphate. These studies indicate that metabolic activation of benzene leads to the formation of DNA adducts in bone marrow and white blood cells and suggest that measurement of DNA adducts in white blood cells may be an indicator of biological effect following benzene exposure.

Activation of 4-hydroxytamoxifen and the tamoxifen derivative metabolite E by uterine peroxidase to form DNA adducts: comparison with DNA adducts formed in the uterus of Sprague-Dawley rats treated with tamoxifen

Daily intraperitoneal treatment of female Sprague-Dawley rats with either 5, 10 or 20 mg/kg tamoxifen (TAM) for 1 week increased the level of peroxidase activity in the uterus 2- to 10-fold compared to the control level. Using uterine extracts prepared from control and TAM treated animals, we investigated the activation of 4-hydroxytamoxifen (4-HO-TAM) and (E,Z)-1,2-diphenyl-1-(4-hydroxyphenyl)-but-1-ene (cis/trans-metabolite E) to form DNA adducts. Activation of 4-HO-TAM by uterine extracts prepared from either control or TAM-treated rats produced one major (a) and two minor DNA (b and c) adducts. A similar activation of cis/trans-metabolite E produced two adducts (d and e). There was good correlation between levels of uterine peroxidase activity and levels of DNA adducts formed by 4-HO-TAM and cis/trans-metabolite E. Activation of 4-HO-TAM and cis/trans-metabolite E with horseradish peroxidase (HRP) produced the same adducts as observed by activation with uterine extract. Treatment of Sprague-Dawley rats with 5 and 10 mg/kg for 7 days produced eleven DNA adducts in the liver with no adducts detected in the uterus. However, treatment of rats with 20 mg/kg of TAM for 7 days produced the same adduct pattern in the liver and also one major adduct (1) in the uterus with a relative adduct level of 6.4 - 4.1 x 10(-9). Tamoxifen-DNA adduct 1 detected both in the liver and in the uterus of treated rats was similar to adducts produced by activation of 4-HO-TAM with either uterine extract or HRP. The results of these studies suggest a general model whereby the tamoxifen metabolite 4-HO-TAM is further activated in the uterus by peroxidase enzymes to form DNA adducts.

Production of 8-hydroxy-2'-deoxguanosine in DNA by microsomal activation of tamoxifen and 4-hydroxytamoxifen

Using rat liver microsomal preparations, we have investigated the activation of the anti-estrogen compound tamoxifen (TAM) and its metabolite 4-hydroxytamoxifen (4-OH-TAM) to form 8-hydroxy-2'-deoxyguanosine (8-OH-dG) in DNA. When reduced nicotinamide adenine dinucleotide phosphate (NADPH) was used as a cofactor in microsomal activation of either TAM or 4-OH-TAM, the levels of 8-OH-dG were 3-fold higher than in microsomes plus cofactor only. In contrast, no significant increase in the level of 8-OH-dG was detected in DNA samples from microsomal activation of either TAM or 4-OH-TAM with cumene hydroperoxide as the cofactor. These results demonstrate that the microsomal activation of TAM and 4-OH-TAM to form 8-OH-dG is dependent upon the cofactor used. The addition of either EDTA or catalase to the activation system significantly decreased the formation of 8-OH-dG by TAM, but not by 4-OH-TAM. The presence of either sodium azide, superoxide dismutase or mannitol inhibited the formation of 8-OH-dG by both TAM and 4-OH-TAM. Taken together these findings indicate that microsomal activation of TAM and 4-OH-TAM with NADPH generates reactive oxygen species which result in the formation of 8-OH-dG. We propose that the formation of 8-OH-dG by TAM and its metabolites may contribute to the observed carcinogenic effects of TAM.

Role of hydrogen peroxide in the formation of DNA adducts in HL-60 cells treated with benzene metabolites

We have investigated the influence of peroxides on DNA adduct formation in HL-60 cells treated with polyphenolic metabolites of benzene. Treatment of HL-60 cells with 50 microM hydroquinone (HQ), 500 microM catechol (CAT) or 200 microM 1,2,4-benzenetriol (BT) resulted in adduct levels of 0.27, 0.21 and 0.21 x 10(-7), respectively. Addition of 50-250 microM H2O2 or 250 microM cumene hydroperoxide to HL-60 cells increased DNA adduct formation 2.7-10-fold following treatment with HQ or CAT but had no effect on adduct formation by BT. Treatment of HL-60 cells with the combinations of HQ plus either BT or phorbol myristate acetate (PMA) potentiated DNA adduct formation by 2.5-4-fold. Significant elevations of cellular H2O2 levels occurred after treatment of HL-60 cells with either PMA, CAT or BT. These results indicate that cellular levels of H2O2 regulate the peroxidase dependent activation of benzene metabolites to form DNA adducts.

Synthesis of N2-(4-hydroxyphenyl)-2'-deoxyguanosine 3'-phosphate: comparison by 32P-postlabeling with the DNA adduct formed in HL-60 cells treated with hydroquinone

A new adduct has been isolated from the reaction of guanosine 3'-phosphate and p-benzoquinone. The structure of this adduct has been determined as N2-(4-hydroxyphenyl)-guanosine 3'-phosphate. 32P-Postlabeling showed that this adduct is similar to the DNA adduct formed in HL-60 cells treated with hydroquinone. For comparison with the corresponding deoxyribonucleotide, a synthetic procedure was developed for the preparation of N2-substituted derivatives of 2'-deoxyguanosine 3'-phosphate. 2-Bromo-2'-deoxyinosine 3'-phosphate was synthesized with a combination of synthetic and enzymatic methods. Reaction of 2-bromo-2'-deoxyinosine 3'-phosphate with 4-hydroxyaniline gave N2-(4-hydroxyphenyl)-2'-deoxyguanosine 3'-phosphate. Using 32P-postlabeling, we compared this product with the DNA adduct produced in HL-60 cells treated with hydroquinone. The results of these studies suggest that the DNA adduct formed in HL-60 cells treated with hydroquinone corresponds to N2-(4-hydroxyphenyl)-2'-deoxyguanosine 3'-phosphate.

Detection of DNA adducts in the white blood cells of B6C3F1 mice treated with benzene

We have employed the P1-enhanced 32P-postlabeling procedure to detect the formation DNA of adducts in the white blood cells (WBC) of B6C3F1 mice treated by i.p. injection with benzene. Treatment twice a day with 440 mg/kg benzene for 1-7 days resulted in the formation of one major (adduct 1) and one minor (adduct 2) DNA adduct in the WBCs of mice. The same DNA adduct pattern was also found in the bone marrow (BM) of benzene treated mice. The relative adduct levels were dependent upon both benzene dose from 100-440 mg/kg and treatment time from 1 to 7 days. The relative adduct levels ranged between 0.11 and 1.33 adducts in 10(7) nucleotides for WBCs and 0.16-1.21 adducts in 10(7) nucleotides for BM. Following treatment with benzene, the levels of DNA adducts formed in WBCs were significantly correlated with the levels of DNA adducts formed in BM (r2 = 0.97, P < 0.001). Our results suggest that measurement of DNA adducts in WBCs may be an indicator of DNA adduct formation in BM following BZ exposure.

Reaction of N-(2-chloroethyl)-N-nitrosoureas with DNA: effect of buffers on DNA adduction, cross-linking, and cytotoxicity

N-(2-Chloroethyl)nitrosoureas (CNU) are clinically used anticancer drugs whose cytotoxicity is associated with the generation of DNA interstrand cross-links. While studying the sequence selectivity for a series of CNU, a dramatic increase in the formation of N7-alkyldeoxyguanosine was observed when Tris buffer was used rather than phosphate or cacodylate buffers. Moreover, the formation of N7-alkyldeoxyguanosine lesions continues in Tris long after all of the CNU has hydrolyzed. These effects are not seen with the monofunctional alkylating analogues, e.g., N-methyl- and N-(2-hydroxyethyl)-N-nitrosourea. In order to determine if the nature of the CNU-mediated DNA damage was altered by Tris, studies were initiated on the following: (1) alkylation of N7-G in end-labeled DNA restriction fragments; (2) covalent modification of DNA with [ethyl-3H]-N-(2-chloroethyl)-N-nitrosourea; and (3) cytotoxicity in L1210 cells. The data presented demonstrate that Tris increases the yield of the "normal" CNU monofunctional cross-linked adducts, i.e., N7-(2-hydroxyethyl)deoxyguanosine, N7-(2-chloroethyl)deoxyguanosine, O6-(2-chloroethyl)deoxyguanosine, and bifunctional adducts, i.e., 1-(deoxycytid-3-yl)-2-(deoxyguanosin-1-yl)ethane and 1,2-bis(deoxyguanosin-7-yl)ethane. In addition, CNU appears to react with Tris to give a long-lived alkylating intermediate that affords large amounts of DNA adducts not seen with CNU in the absence of Tris. However, in vivo toxicity of CNU in L1210 cells is not affected by the presence of Tris, indicating that the reaction pathway(s) responsible for cross-linking is not significantly sensitive to the nature of the buffer.

Tamoxifen metabolic activation: comparison of DNA adducts formed by microsomal and chemical activation of tamoxifen and 4-hydroxytamoxifen with DNA adducts formed in vivo

One of our laboratories recently showed by 32P-postlabeling that administration of tamoxifen to mice induces two groups of hepatic DNA adducts comprising two major spots, nos. 3 and 5, respectively. 4-Hydroxytamoxifen and alpha-hydroxytamoxifen appear to be the proximate metabolites of groups I and II adducts, respectively. The relative significance of these two adduct groups for tamoxifen carcinogenicity remains to be established. To determine the activation mechanism(s) of tamoxifen and 4-hydroxytamoxifen, in vivo adducts were compared by 32P-postlabeling with adducts generated by microsomal or chemical activation in vitro. Microsomal activation of 4-hydroxytamoxifen and tamoxifen, respectively, in the presence of DNA and cumene hydroperoxide, induced two adducts, which mapped similarly to the corresponding in vivo adduct spots 3 and 5. Chemical oxidation of 4-hydroxytamoxifen with silver(II) oxide, followed by incubation of the product(s) with DNA, elicited the formation of a major spot (Q1), while tamoxifen itself did not react. Rechromatographic analyses revealed that in vitro fractions 3 and Q1 (from 4-hydroxytamoxifen) matched the major in vivo group I adduct fraction 3, consistent with the hypothesis that 4-hydroxytamoxifen is a precursor for adduct fraction 3 in vivo. The in vitro adduct fraction 5 (from tamoxifen) was identical to that formed in vivo, indicating that the metabolic pathway for the formation of group II adducts did not involve 4-hydroxytamoxifen. In conclusion, the results support a model where primary metabolites of tamoxifen undergo secondary metabolism to form DNA adducts, which are detected in vivo after treatment with tamoxifen or 4-hydroxytamoxifen.

DNA adduct formation in the bone marrow of B6C3F1 mice treated with benzene

We used P1-enhanced 32P-postlabeling to investigate DNA adduct formation in the bone marrow of B6C3F1 mice treated intraperitoneally with benzene (BZ). No adducts were detected in the bone marrow of controls or mice treated with various doses of BZ once a day. After twice-daily treatment with BZ, 440 mg/kg, for 1 to 7 days, one major and two minor DNA adducts were detected. The relative adduct levels ranged from 0.06-1.46 x 10(-7). In vitro treatment of bone marrow from B6C3F1 mice with various doses of hydroquinone (HQ) for 24 h also produced three DNA adducts. These adducts were the same as those formed after in vivo treatment of bone marrow with BZ. Co-chromatography experiments indicated that the principal DNA adduct detected in the bone marrow of B6C3F1 mice was the same as that detected in HL-60 cells treated with HQ. This finding suggests that HQ may be the principal metabolite of BZ leading to DNA adduct formation in vivo. DNA adduct 2 corresponds to the DNA adduct formed in HL-60 cells treated with 1,2,4-benzenetriol. DNA adduct 3 remains unidentified. After a 7-day treatment with BZ, 440 mg/kg twice a day, the number of cells per femur decreased from 1.6 x 10(7) to 0.85 x 10(7), indicating myelotoxicity. In contrast, administration of BZ once a day produced only a small decrease in bone marrow cellularity. These studies demonstrate that metabolic activation of BZ leads to the formation of DNA adducts in the bone marrow. Further investigation is required to determine the role of DNA adducts and other forms of DNA damage in the myelotoxic effects of exposure to BZ.

Activation of the tamoxifen derivative metabolite E to form DNA adducts: comparison with the adducts formed by microsomal activation of tamoxifen

(Z)-1,2-Diphenyl-1-(4-hydroxyphenyl)but-1-ene (metabolite E) has been detected in the plasma of patients treated with tamoxifen. We therefore investigated whether the cis/trans isomers of metabolite E can be activated to form DNA adducts detected by 32P postlabeling. Microsomal activation of metabolite E produced two major (a and b) and up to six minor DNA adducts. Activation with horseradish peroxidase or silver(I)oxide produced the same adducts (a and b). Microsomal activation of tamoxifen produced one major (no. 6) and several minor DNA adducts. Rechromatography showed that adducts a and b formed by enzymatic and chemical activation of metabolite E were the same as adducts 9 and 4 produced by microsomal activation of tamoxifen. These results demonstrate that activation of metabolite E can lead to DNA adduct formation.

The benzene metabolite p-benzoquinone forms adducts with DNA bases that are excised by a repair activity from human cells that differs from an ethenoadenine glycosylase

Benzene is a ubitiquous human environment mental carcinogen. One of the major metabolites is hydroquinone, which is oxidized in vivo to give p-benzoquinone (p-BQ). Both metabolites are toxic to human cells. p-BQ reacts with DNA to form benzetheno adducts with deoxycytidine, deoxyadenosine, and deoxyguanosine. In this study we have synthesized the exocyclic compounds 3-hydroxy-3-N4-benzetheno-2'-deoxycytidine (p-BQ-dCyd) and 9-hydroxy-1,N6-benzetheno-2'-deoxyadenosine (p-BQ-dAdo), respectively, by reacting deoxycytidine and deoxyadenosine with p-BQ. These were converted to the phosphoamidites, which were then used to prepare site-specific oligonucleotides with either the p-BQ-dCyd or p-BQ-dAdo adduct (pbqC or pbqA in sequences) at two different defined positions. These oligonucleotides were efficiently nicked 5' to the adduct by partially purified HeLa cell extracts--the pbqC-containing oligomer more rapidly than the pbqA-containing oligomer. In contrast to the enzyme binding to derivatives produced by the vinyl chloride metabolite chloroacetaldehyde, the oligonucleotides up to 60-mer containing p-BQ adducts did not bind measurably to the same enzyme preparation in a gel retardation assay. Furthermore, there was no competition for the binding observed between oligonucleotides containing 1,N6-etheno A deoxyadenosine (1,N6-etheno-dAdo; epsilon A in sequences) and these oligomers containing either of the p-BQ adducts, even at 120-fold excess. When highly purified fast protein liquid chromatography (FPLC) enzyme fractions were obtained, there appeared to be two closely eluting nicking activities. One of these enzymes bound and cleaved the epsilon A-containing deoxyoligonucleotide. The other enzyme cleaved the pbqA- and pbqC-containing deoxyoligonucleotides. One additional unexpected fact was that bulk p-BQ-treated salmon sperm DNA did compete effectively with the epsilon A-containing oligonucleotide for protein binding. This raises the possibility that such DNA contains other, as-yet-uncharacterized adducts that are recognized by the same enzyme that recognizes the etheno adducts. In summary, we describe a previously undescribed human DNA repair activity, possibly a glycosylase, that excises from DNA pbqC and pbqA, exocyclic adducts resulting from reaction of deoxycytidine and deoxyadenosine with the benzene metabolite, p-BQ. This glycosylase activity is not identical to the one previously reported from this laboratory as excising the four etheno bases from DNA.

Microsomal and peroxidase activation of 4-hydroxy-tamoxifen to form DNA adducts: comparison with DNA adducts formed in Sprague-Dawley rats treated with tamoxifen

Using rat liver microsomal preparations and peroxidase enzymes, we have investigated the formation of DNA adducts by the antiestrogen compound tamoxifen (TAM) and its metabolite 4-hydroxy-tamoxifen (4-OH-TAM). When reduced nicotinamide-adenine dinucleotide phosphate (NADPH) was used as a cofactor in microsomal activation of either 4-OH-TAM or TAM, one DNA adduct and relative DNA adduct levels of 4.6 and 3.1 x 10(-8), respectively were detected by 32P-postlabeling. The DNA adduct produced by microsomal activation of 4-OH-TAM and TAM was the same. With cumene hydroperoxide (CuOOH) as the cofactor for the microsomal activation of either 4-OH-TAM or TAM, three to six DNA adducts were produced; the relative adduct levels were 8.0 and 20.6 x 10(-8), respectively. Comparison of the DNA adduct patterns produced by 4-OH-TAM and TAM showed that they were distinct. However one of the DNA adducts (a) produced by microsomal activation of 4-OH-TAM using CuOOH was the same as adduct a produced by microsomal activation of 4-OH-TAM with NADPH. Activation of 4-OH-TAM with horseradish peroxidase resulted in the formation of a single DNA adduct and a relative adduct level of 20.7 x 10(-8). Rechromatography analysis of this DNA adduct showed that it was identical to that produced by microsomal activation of 4-OH-TAM with NADPH and one of the adducts produced using CuOOH as the cofactor. Ten DNA adducts and a relative adduct level of 15.3 x 10(-8) were detected in the liver of female Sprague-Dawley rats treated daily with 20 mg/kg of TAM for 7 days. The DNA adduct pattern in the liver of the treated animals was similar to that produced by microsomal activation of TAM using CuOOH as the co-factor. The principal DNA adduct (no. 6) formed in the livers of rats treated with TAM was the same as the principal DNA adduct formed following microsomal activation of TAM using CuOOH as a cofactor. The DNA adduct formed following microsomal activation of either TAM or 4-OH-TAM using NADPH was also present as one of the adducts (1) formed in vivo following TAM treatment. These studies demonstrate that 4-OH-TAM can be activated to form DNA adducts and that it contributes to the formation of DNA adducts in the liver of rats treated with TAM.

32P-post-labeling detection of DNA adducts in mononuclear cells of workers occupationally exposed to styrene

32P-Post-labeling was used to analyze for the presence of DNA adducts in 47 workers exposed to styrene in a boat manufacturing facility. Individual airborne exposures measured several times over the course of 1 year ranged from 1 to 235 mg/m3 with a mean value of 65.6 mg/m3. Two adducts were detected in the DNA of mononuclear cells of these workers. The following levels of adducts were detected: adduct 1, range 0.6-102 x 10(-8) (mean 15.8 x 10(-8); adduct 2, range 0.1-70.9 x 10(-8) (mean 14.2 x 10(-8). Significant linear relationships were found between styrene exposure and both DNA adducts (adduct 2, r = 0.330, P = 0.012; adduct 1, r = 0.244, P = 0.049). Co-chromatography experiments identified DNA adduct 1 in the exposed samples as N2-(2-hydroxy-1-phenylethyl)-2'-deoxyguanosine-3',5'-bisphosphate. DNA adduct 2 remains unidentified. No significant linear relationships were observed between the level of DNA adducts and sister chromatid exchanges, possibly because of the poor precision of the 32P-post-labeling assay (the estimated coefficients of variation for adducts 1 and 2 were 2.54 and 1.96, respectively). These results demonstrate that occupational exposure to styrene results in the formation of DNA adducts in human mononuclear cells.

Synthesis of a 25 base oligonucleotide containing a styrene oxide modification at the O6 position of 2'-deoxyguanosine at a defined site and incorporation studies of the similarly modified 2'-deoxyguanosine-5'-triphosphate

A diastereomeric mixture of the regioisomers O6-(2-hydroxy-2-phenylethyl)-2'-deoxyguanosine (st6G, beta-isomer) and O6-(2-hydroxy-1-phenylethyl)-2'-deoxyguanosine (alpha-isomer) was site-specifically placed in a 25 base oligonucleotide template 5'-CCGCTAst6GCGGGTACCGAGCTCGAAT-3' using CED phosphoramidite chemistry. Using 32P-post-labeling we found the oligonucleotide to contain 95% of the beta-isomer and 5% of the alpha-isomer of st6G. st6G as the 3'-phosphate was found to be considerably more acid labile than O6-methyl-2'-deoxyguanosine-3'-phosphate, leading to dealkylation during oligonucleotide synthesis. The diastereomeric mixture of O6-(2-hydroxy-2-phenylethyl)-2'-deoxy-guanosine-5'-triphosphate (st6dGTP) was chemically synthesized and used as a substrate for the exonuclease-free Klenow fragment of Escherichia coli DNA polymerase I. This study demonstrated that st6dGTP could be incorporated opposite deoxycytidine and did not completely block replication.

The effects of incorporation of bromodeoxyuridine into mammalian DNA on the migration patterns of DNA fragments subjected to pulsed-field gel electrophoresis after X irradiation or cutting with a restriction enzyme

Incorporation of bromodeoxyuridine (BrdU) into DNA in both Chinese hamster ovary (CHO) cells and human melanoma (U1) cells reduced the rate of DNA migration in transverse alternating-field electrophoresis (TAFE) agarose gel optimized for separating molecules larger than 2 Mb. This "BrdU migration effect" was independent of the method of damaging the DNA; i.e., the effect was observed after irradiation of the cells or treatment of the plugs containing DNA with the restriction enzyme, Mlu-1, and similar results were found for CHO and U1 cells. However, when the amount of cutting of DNA was estimated from the amount of DNA migrating from the plugs, a difference between U1 cells and CHO cells was observed in that incorporation of BrdU enhanced the cutting of DNA by either irradiation or Mlu-1 digestion for U1 cells but not for CHO cells. Therefore, the "BrdU migration effect" could not be attributed to an increase in large molecules because of reduced cutting of the BrdU-labeled DNA. The decrease in migration rate during pulsed-field gel electrophoresis when BrdU replaces thymidine in the DNA is hypothesized to result from the increase in negative charge on BrdU-labeled DNA. An increase in electron charge density is expected to increase the reorientation time of BrdU-labeled DNA fragments during each voltage pulse due to an increase in the elongation length of the DNA caused by an increase in electronegativity.

DNA adduct formation by tamoxifen with rat and human liver microsomal activation systems

Using microsomal preparations from rat and human liver, we investigated the activation of the anti-estrogen compound tamoxifen (TMX) to form DNA adducts. Pretreatment of rats with phenobarbital increased DNA adduct formation by microsomal activation of TMX 3- to 6-fold, depending on the cofactors used. When reduced nicotinamide-adenine dinucleotide phosphate (NADPH) was used as a cofactor in human and rat microsomal activation systems, the relative DNA adduct levels were 2.9 and 5.2 x 10(-8) respectively and 1-3 TMX-DNA adducts were detected by 32P-postlabeling; DNA adduct 1 was the same in both microsomal systems. When cumene hydroperoxide (CuOOH) was used as a cofactor, activation of TMX produced four major DNA adducts and several minor DNA adducts in both rat and human liver microsomes; the relative adduct levels were 11.1 and 23.1 x 10(-8) respectively. TMX-DNA adducts 1, 4, 5 and 6 were similar in both human and rat microsomal systems with CuOOH as the cofactor. The TMX-DNA adducts formed with NADPH as the cofactor were clearly different from those formed with CuOOH as the cofactor, which implies that the metabolites leading to the individual DNA adducts were different. Addition of a P450 inhibitor, either n-octylamine or alpha-naphthylisothiocyanate, to the activation system reduced adduct formation by 70-93%. We propose that the TMX-DNA adducts formed with NADPH as the cofactor result from P450 acting as a mono-oxygenase, whereas the adducts formed with CuOOH as the cofactor result from P450 acting as a peroxidase. Our findings suggest that further studies may be required to establish the safety of TMX treatment of women for purposes other than chemotherapy.

Genotoxicity to human cells induced by air particulates isolated during the Kuwait oil fires

In an effort to examine the potential of exposure to soot from the 1991 oil fires in the Kuwait desert for inducing genetic effects we studied the in vitro genotoxicity of this material. Air particulates isolated near the Kuwait oil fires were studied using three assays. Dose-dependent increases were observed for both sister chromatid exchanges in human peripheral blood lymphocytes and mutation at the hprt locus in the metabolically competent human lymphoblast cell line AHH-1. Similar magnitudes of response were seen using these two assays when testing a standard air particulate sample which had been isolated from the Washington, DC, area. Using the 32P-postlabeling assay, no increase in DNA adduct formation was observed in AHH-1 cells treated with particulates isolated from sampling in Kuwait.

Peroxidase activation of hydroquinone results in the formation of DNA adducts in HL-60 cells, mouse bone marrow macrophages and human bone marrow

Metabolism of benzene results in the formation of multiple metabolites, including hydroquinone (HQ). HQ is a reducing co-substrate for peroxidase enzymes, and the resultant semiquinone and para-benzoquinone (p-BQ) may bind to DNA. The role of peroxidase activation in the formation of DNA adducts by benzene metabolites has not been established. In this study we investigated the role of peroxidase activation in the formation of DNA adducts by HQ and p-BQ in HL-60 cells, human bone marrow (HBM) cells, mouse bone marrow macrophages (MBMM) and the U-937 and Raji leukemia cell lines. Adduct formation was measured by P1-enhanced 32P-postlabeling; peroxidase activity was measured with a spectrophotometric assay. Treatment with p-BQ resulted in the formation of two DNA adducts in all of the cell lines. The DNA adducts were identical in all of the cells, however, the adduct level varied by 80-fold. Treatment with HQ produced one DNA adduct in HL-60 cells, HBM and MBMM; no adducts were detected in U-937 or Raji cells. The HQ-DNA adducts in the three cell lines were identical. The adduct level was highest in the HL-60 cells, followed by HBM and MBMM. There was a statistically significant correlation between peroxidase activity and the formation of HQ-DNA adducts. These results suggest that peroxidase-mediated metabolism is involved in the activation of HQ to form DNA adducts in mouse bone marrow and HBM.

Chemical synthesis and detection of the cross-link 1-[N3-(2'-deoxycytidyl)]-2-[N1-(2'-deoxyguanosinyl)]ethane in DNA reacted with 1-(2-chloroethyl)-1-nitrosourea

We have synthesized 1-[N3-(2'-deoxycytidyl)]-2-[N1-(2'-deoxyguanosinyl)]ethane and confirmed its structure by ultraviolet and high-resolution mass spectrometry. Treatment of calf thymus DNA with [3H](2-chloroethyl)-1-nitrosourea resulted in the formation of at least 13 DNA alkylation products that were separated by HPLC. 1-[N3-(2'-Deoxycytidyl)]-2-[N1-(2'-deoxyguanosinyl)]ethane was a minor product, accounting for 3.4% of the total DNA alkylation. The DNA cross-link 1,2-di-N7-guanylethane was formed to a similar extent (3.2%). Other minor alkylation products were O6-(2-hydroxyethyl)deoxyguanosine (1.5%) and N1-(2-hydroxyethyl)deoxyguanosine (3.8%). The principal alkylation products formed by 1-(2-chloroethyl)-1-nitrosourea (CNU) treatment of DNA were N7-(2-hydroxyethyl)guanine (36.4%), N7-(2-chloroethyl)guanine (14.6%), and phosphotriesters (26.1%). The development of analytical procedures to measure DNA alkylation products after treatment with CNU will allow us to investigate factors influencing their formation and repair.

Detection of dopamine--DNA adducts: potential role in Parkinson's disease

Oxidation of catecholamines may lead to the formation of o-semiquinones and o-quinones in catecholaminergic brain tissues, and these reactive molecules may form DNA or protein adducts. In this study, cultured cells were treated with dopamine (DA) for 24 h and 32P-postlabelling was used to detect DA-DNA adducts. In HL-60 cells, 250 microM DA induced 8.5 DNA adducts/10(8) nucleotides; adduct formation was dose-dependent up to 500 microM DA. Addition of H2O2 increased the relative adduct levels 7- to 13-fold, but no adducts were detected when DA and ascorbic acid were added simultaneously. In human glioblastoma cell lines U87, U251, SF-763 and SF-767, 1000 microM DA produced 0.98-2.31 adducts/10(8) nucleotides. These results suggest that the formation of DNA adducts by DA may contribute to the development of certain neurodegenerative diseases such as Parkinson's disease.

DNA crosslinking, sister chromatid exchange and cytotoxicity of N-2-chloroethylnitrosoureas tethered to minor groove binding peptides

Chloroethylnitrosoureas (CENU) are clinically important chemotherapeutic agents whose mechanism of action involves the formation of interstrand DNA crosslinks via an ethane bridge between N1-G and N3-C. CENU generally alkylate G at the N7- and O6-positions, with the latter lesion being the precursor to the interstrand crosslink. In previous studies, we reported the synthesis of CENU appended by a C2H4 linker to the N-terminus of DNA minor groove binding dipeptides (lex, information reading peptides) based on N-methylpyrrole-carboxamide subunits. Because of the dipeptide structure, these CENU-lex's react with DNA at adenines associated with lex equilibrium binding sites. No other CENU has been reported to yield A adducts. The biological evaluation of these CENU-lex's show that they are somewhat less cytotoxic than their simpler counterparts. In addition, in vitro studies show that the minor groove binding CENU-lex's afford a lower level of sister chromatid exchange (SCE) in 9L cells that are sensitive to CENU. There is no difference between CENU-lex in SCE induction in 9L-2 cells that are resistant to CENU. Formation of DNA interstrand crosslinks from the CENU-lex's is lower than for their nonaffinity binding analogs in low ionic strength buffer, but similar in the same buffer containing 200 mM NaCl. Salt inhibits crosslinking for all CENU, but distamycin, a competitive inhibitor of lex minor groove binding, uniquely enhances crosslinks for the CENU-lex's. These results are consistent with the novel minor groove adduction being a 'detoxification' pathway for the CENU-lex's since this lesion is formed at the expense of the cytotoxic major groove interstrand crosslink.

Restriction endonuclease recognition and southern hybridization of bromodeoxyuridine-substituted genomic DNA

Human glioma cell lines exposed to various concentrations of bromodeoxyuridine (BrdUrd) were studied to determine the effect of BrdUrd substitution on restriction endonuclease recognition and Southern hybridization of genomic DNA. BrdUrd substitution had no effect on the recognition of restriction endonucleases. When the exposure to BrdUrd was 2h or less and the BrdUrd substitution rate was less than 40%, there was no difference in the density of hybridized bands after Southern hybridization using human non-recombinant complementary DNA as a probe. Hybridization was suppressed significantly by exposures longer than 24 h or BrdUrd substitution rates greater than 40%. These results suggest that the BrdUrd substitution rate and the exposure time to BrdUrd influence the hybridization reaction by a DNA probe. Brief exposure (up to 2 h) to BrdUrd does not influence restriction endonuclease recognition or Southern hybridization of genomic DNA.

Investigation of benzene-DNA adducts and their detection in human bone marrow

We have examined DNA adduct formation in HL-60 cells and human bone marrow treated with either hydroquinone or p-benzoquinone and have found that these treatments produce the same DNA adduct in both cell types. The DNA adduct level from these treatments varied from 0.05 to 7.5 adducts per 10(7) nucleotides as a function of treatment time and concentration for both compounds. Reaction of calf thymus DNA with p-benzoquinone produced three adducts as detected by 32P-postlabeling. These adducts have been identified as (3'-hydroxy)-3,N4-benzetheno-2'-deoxycytidine-3'-phosphate; (3'-hydroxy)-1,N6-benzetheno-2'-deoxyadenosine-3'-phosphate; and (3'-hydroxy)-1,N2-benzetheno-2'-deoxyguanosine-3'-phosphate. The DNA adduct formed in HL-60 cells did not correspond to any of the principal adducts formed in DNA reacted with p-benzoquinone, suggesting that cellular environment modifies DNA adduct production by p-benzoquinone. These studies demonstrate that DNA adduct formation occurs in human bone marrow treated with benzene metabolites and suggest that P1-enhanced 32P-postlabeling may be used to detect DNA adducts resulting from benzene exposure.

Bis(hydroxyphenylethyl)deoxyguanosine adducts identified by [32P]-postlabeling and four-sector tandem mass spectrometry: unanticipated adducts formed upon treatment of DNA with styrene 7,8-oxide

Calf thymus DNA was incubated with [8-14C]styrene oxide in vitro, and six covalent xenobiotic-DNA adducts were detected using the [32P]-postlabeling procedure. Adducts 1-3 were purified by HPLC and identified as bis-substituted-2'-deoxyguanosine 3'-phosphate derivatives using four-sector tandem mass spectrometry. These adducts represented less than 2% of the total adducts detected by [14C]-radioactivity. Adducts 1-3 were also detected when styrene oxide was allowed to react with the mononucleotide, 2'-deoxyguanosine 3'-phosphate only. The elemental compositions of these adducts (C26H30N5O9P) were determined by measurement of their accurate masses by high-resolution mass spectrometry and revealed the unusual incorporation of 2 mol of hydroxyphenylethyl moieties. The structures of these bis(phenylethyl) adducts were established by interpretation of high-energy collision-induced dissociation (CID) mass spectra, together with UV/visible and fluorescence spectrophotometry as N2-(2-hydroxy-1-phenylethyl)-O6-(2-hydroxy-2-phenylethyl)-2'-deoxygua nos ine 3'-phosphate (adduct 1), N2-(2-hydroxy-1-phenylethyl)-O6-(2-hydroxy-1-phenylethyl)-2'-deoxygua nos ine 3'-phosphate (adduct 2), and N1,N2-bis(2-hydroxy-1-phenylethyl)-2'-deoxyguanosine 3'-phosphate (adduct 3). The other most abundant adducts were detected only by [14C]-radioactivity and represented approximately 65% of the total covalent binding. These were identified as depurinated N7-substituted guanines by tandem mass spectrometry and UV/visible spectroscopy. The combination of advanced techniques of mass spectrometry with the [32P]-postlabeling assay and spectroscopic techniques is a comprehensive strategy to assure complete structural identification of all xenobiotic-DNA adducts.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroxidative activation of o-phenylhydroquinone leads to the formation of DNA adducts in HL-60 cells

Using 32P-postlabeling we studied DNA adduct formation in HL-60 cells treated with the o-phenylphenol metabolites o-phenylhydroquinone (o-PHQ) and o-phenylbenzoquinone (o-PBQ). Treatment with 25-500 microM o-PHQ for 8 h produced one principal and three minor adducts with a relative distribution of 80, 10, 6 and 4%. The relative adduct levels from these treatments were 0.26-2.31 adducts/10(7) nucleotides. Treatment with 25-250 microM o-PBQ for 2 h resulted in a similar level of DNA modification and adduct distribution. Reaction of purified calf thymus DNA with o-PBQ produced one DNA adduct, which did not correspond to the major adduct produced in HL-60 cells. These results show that o-PHQ and o-PBQ can form DNA adducts. Peroxidase activation of o-phenylphenol may therefore play a role in the carcinogenic effect of this compound.

Potentiation of DNA adduct formation in HL-60 cells by combinations of benzene metabolites

Using P1 nuclease enhanced 32P postlabeling, we investigated DNA adduct formation in HL-60 promyelocytic leukemia cells treated with the benzene metabolites hydroquinone, catechol, and 1,2,4-benzenetriol. Comparison of the slopes of the dose-response curves showed that hydroquinone was 7-9 times more effective than 1,2,4,-benzenetriol and catechol at inducing DNA adducts. Comparison of hydroquinone with catechol showed a good correlation between adduct formation and cytotoxicity. Similar comparisons of hydroquinone and 1,2,4,-benzenetriol suggest that cellular processes in addition to DNA adduct formation contributed to cytotoxicity. In cells treated with the combination of hydroquinone and either catechol or 1,2,4,-benzenetriol, DNA adduct formation was 3-6 times greater than the sum of adduct formation produced by single-agent treatments. Treatment with hydroquinone and 1,2,4,-benzenetriol produced DNA adducts not detected after treatment with either metabolite alone. The synergistic interaction of benzene metabolites in the production of DNA adducts may play an important role in the genotoxic effects of benzene in vivo.

Identification of N2-substituted 2'-deoxyguanosine-3'-phosphate adducts detected by 32P-postlabeling of styrene-oxide-treated DNA

Styrene-7,8-oxide, a metabolite of the industrial chemical styrene, was reacted with calf thymus DNA. Six adducts were detected by 32P-postlabeling. The two diastereomers of N2-(2-hydroxy-1-phenylethyl)-2'-deoxyguanosine-3'-phosphate and the corresponding N-1 substituted compounds were isolated from the aqueous reaction mixture of 2'-deoxyguanosine-3'-phosphate and styrene-7,8-oxide (pH 10.5) and characterized by liquid secondary-ion and four-sector tandem mass spectrometry, ultraviolet, circular dichroism, and fluorescence spectrophotometry, and 32P-postlabeling. Co-chromatography of the DNA-styrene-7,8-oxide reaction products with the synthetic standards showed that adduct no. 6 arose as a result of aralkylation at the N2-exocyclic site of the guanine base. The recovery of the N2-adduct was dependent on the concentration of the solvent used during octadecylsilyl chromatography. These studies revealed that the N2-guanosine derivatives are the major products of the reaction of DNA and styrene-7,8-oxide in vitro detected by 32P-postlabeling.

Iododeoxyuridine incorporation and radiosensitization in three human tumor cell lines

Iododeoxyuridine is a halogenated pyrimidine and non-hypoxic cell radiosensitizer currently being used in clinical trials. The amount of radiosensitization by IdUrd is related to the amount of incorporation of the drug into a cell's DNA. These experiments were carried out in three human tumor cell lines (lung, glioma, and melanoma) in monolayer culture exposed to concentrations of IdUrd from 0.1-10 microM for one and three cell cycles before irradiation to determine incorporation and sensitization as a function of drug exposure. Except for the lung cell line, which required greater than 1 microM IdUrd, these cells demonstrate radiosensitization when exposed to 0.1 microM or greater of IdUrd. Maximum sensitization occurred at 10 microM IdUrd for all the cell lines at three cell cycles. The percent thymidine replacement by IdUrd increased with increasing concentrations, but was cell line dependent. Maximum percent replacement occurred at 10 microM at three cell cycles for all the cell lines: lung = 22.4%, glioma = 32.0%, and melanoma = 39.1%. The relationships between percent thymidine replacement and sensitization are not identical across these human tumor cell lines. If IdUrd is going to be a successful radiosensitizer in clinical trials, sustained plasma levels of 10 microM or greater for at least three cell cycles should be achieved during irradiation. This may be best accomplished with repeated short exposures to IdUrd (three cell cycles or approximately 4 days in these cell lines) every 1-2 weeks during radiation. Measurements of thymidine replacement in a tumor biopsy should be attempted prior to radiation to develop a predictive assay for radiosensitization.

Detection of DNA adducts in HL-60 cells treated with hydroquinone and p-benzoquinone by 32P-postlabeling

We have examined DNA adduct formation and cytotoxicity in HL-60 cells treated with either hydroquinone (HQ) or p-benzoquinone (p-BQ). Treatment of HL-60 cells with either HQ or p-BQ produced the same DNA adduct. The DNA adduct level varied from 0.05 to 10 adducts per 10(7) nucleotides as a function of treatment time and concentration for both compounds. To achieve the same DNA adduct level required higher concentrations and longer treatment times with HQ compared to p-BQ. p-BQ was also more cytotoxic to HL-60 cells than HQ. Reaction of calf thymus DNA with a p-BQ/HQ mixture produced five adducts as detected by 32P-postlabeling. Two isomers of (hydroxy)-1,N2-benzetheno-2'- deoxyguanosine-3'-phosphate were isolated from the reaction of 2'-deoxyguanosine-3'-phosphate with a p-BQ/HQ mixture and one of the isomers was identified as adduct no. 1 of the DNA reaction. The DNA adduct formed in HL-60 cells treated with HQ or p-BQ did not correspond to any of the principal adducts formed in DNA reacted with p-BQ/HQ. This result suggests that cellular mechanisms modify DNA adduct formation by HQ and p-BQ.

Positive therapeutic interaction between thiopurines and alkylating drugs in human glioma xenografts

We used human anaplastic glioma xenografts to evaluate the therapeutic efficacy of combinations of alkylating drugs, either 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), 1-(2-chloroethyl)-3-(2,5-dioxo-3-piperidyl)-1-nitrosourea (PCNU), or procarbazine, and thiopurines, either 6-mercaptopurine (6MP) or 6-thioguanine (6TG). Using growth delay as the endpoint in subcutaneous (s.c.) tumors and increased life span as the endpoint in intracranial (i.c.) tumors, we found that combinations of chloroethylnitrosoureas (CENUs) and thiopurines were significantly more active than either type of agent alone. In contrast, combinations of procarbazine and thiopurines were not significantly more active than procarbazine alone. The therapeutic potentiation of the CENU was greater when the latter was given on the 4th day of the thiopurine treatment cycle than when it was given on the 1st day. Characterization of the interaction between CENUs and thiopurines also revealed a supraadditive therapeutic response at higher BCNU doses in combination with 6TG. Interaction between the nitrosoureas and the thiopurines probably occurs in the guanine base of tumor DNA and has important therapeutic implications.

Molecular dosimetry of sister chromatid exchange induction in 9L cells treated with 6-thioguanine

The induction of sister chromatid exchanges (SCE) in 9L cells treated with 6-thioguanine (6-TG) has been investigated. A 24 h treatment with 0.2 microM 6-TG induced approximately 28 SCE/metaphase. The dose-response curve was linear at doses below 0.2 microM and had a slope of 139 SCE/metaphase/microM 6-TG. At concentrations of 0.023 to 1 microM, incorporation of 6-TG in DNA was linear with dose. The slope of the dose-response curve was 4135 mumol 6-TG/mol DNA/microM 6-TG. Comparison of these results with those obtained in our previous studies of the monofunctional alkylating agent ethylnitrosourea and the bifunctional alkylating agent 3-(4-amino-2-methyl-5-pyrimidinyl)methyl-1-(2-chloroethyl)-1-nitrosourea suggest that to accurately estimate the effectiveness of particular DNA modifications at inducing SCE, the extent of formation of these DNA modifications must be known.

Detection of 3'-hydroxy-1,N6-benzetheno-2'-deoxyadenosine 3'-phosphate by 32P postlabeling of DNA reacted with p-benzoquinone

2'-Deoxyadenosine 3'-phosphate was reacted with a mixture of p-benzoquinone (p-BQ) and hydroquinone in aqueous medium at pH 6, and the main product was isolated and characterized by 1H NMR spectroscopy, liquid secondary ion mass spectroscopy (LSIMS), and high-resolution direct chemical ionization mass spectroscopy (HRDCIMS). The structure of this covalent adduct was assigned as 3'-hydroxy-1,N6-benzetheno-2'-deoxyadenosine 3'-phosphate. Reaction of DNA with p-BQ produced three major adducts as detected by 32P postlabeling; the relative abundance was 1.1%, 22.4%, and 72.4%. Cochromatography of 32P-postlabeled 3'-hydroxy-1,N6-benzetheno-2'-deoxyadenosine 3',5'-bisphosphate with the 32P-postlabeled DNA-p-BQ reaction mixture established this compound, adduct 3, as the second most abundant product of the reaction.

3-Methyladenine DNA glycosylase activity in a glial cell line sensitive to the haloethylnitrosoureas in comparison with a resistant cell line

Extracts of a glial cell line (SF-126) which is sensitive to the cytotoxic effect of the haloethylnitrosoureas and of a cell line (SF-188) which is resistant to these agents have been tested for their ability to release methylated bases from a DNA substrate which has been modified with [3H]dimethyl sulfate. In comparison with the sensitive cell line, extracts from the resistant cell line have 2-3-fold higher enzymatic activity. High performance liquid chromatography profiles of the bases which are released by these extracts show that the activity is specific for 3-methyladenine, suggesting that the resistant cells contain elevated levels of 3-methyladenine DNA glycosylase. Previous studies have shown that these cells also contain elevated levels of O6-alkylguanine-DNA alkyl-transferase, suggesting that both enzyme activities may be involved in the resistance of this cell line to the haloethylnitrosoureas.

Molecular dosimetry for sister-chromatid exchange induction and cytotoxicity by monofunctional and bifunctional alkylating agents

The induction of sister-chromatid exchanges (SCEs) and cytotoxicity in 9L cells treated with monofunctional and bifunctional alkylating agents has been investigated. Three classes of monofunctional and bifunctional agents were studied: nitrosoureas, mustards and epoxides. Independent of class the bifunctional agents were 55-630-fold more effective at inducing SCEs and 300-2400-fold more effective at inducing cellular cytotoxicity than the corresponding monofunctional agents. Comparing the induction of SCEs and cytotoxicity by these agents showed that these two cellular responses to DNA damage are highly correlated. The extent of DNA alkylation in cells treated with 1-ethyl-1-nitrosourea (ENU) or 1-(2-chloro-ethyl)-1-nitrosourea (CNU) was similar indicating that the increased effectiveness of CNU to induce SCEs and cytotoxicity was not due to increased DNA alkylation. Molecular dosimetry calculations indicate that for CNU and ENU treatment of 9L cells there are 116 and 8500 alkylations per SCE induced and 2.6 x 10(4) and 4.6 x 10(6) alkylations at the dose required to reduce survival of 9L cells by 90%. Comparison of the DNA alkylation products produced by CNU and ENU treatment of 9L cells suggests that the formation of the intrastrand crosslink N7-bis(guanyl)ethane and the interstrand crosslink 1-(3-deoxycytidyl)-2-(1-deoxyguanosinyl)ethane by CNU is responsible for the increased effectiveness of CNU treatment at both induction of SCEs and cytotoxicity.

Molecular dosimetry of DNA adducts and sister chromatid exchanges in human lymphocytes treated with benzo[a]pyrene

We examined the relationship between benzo[a]pyrene-DNA adducts and sister chromatid exchanges (SCEs) in human lymphocytes. Cultures of isolated phytohemagglutinin (PHA)-stimulated lymphocytes from two normal donors were treated with 0.01-5.0 microM B[a]P from 24 to 72 h of culture. Using the highly sensitive 32P-postlabeling assay, we identified seven B[a]P-DNA adducts, one of which accounted for greater than 90% of the total DNA modifications. This adduct comigrated on polyethylenimine plates with the adduct produced by (+)-7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydro-benzo[a]pyrene. B[a]P-DNA adduct levels ranged from 0.02 to 8 adducts/10(7) nucleotides. SCE frequencies measured in parallel cultures ranged from 8 to 46 SCEs/cell. At the same B[a]P concentrations, B[a]P-induced SCE frequencies and B[a]P-DNA adduct levels were higher in lymphocytes from donor 1 than in lymphocytes from donor 2. There was a linear correlation between the number of B[a]P-DNA adducts and the number of SCEs induced; slopes of the linear regressions of induced SCEs on B[a]P-DNA adducts were similar for both donors. Our data suggest that SCE induction by B[a]P in human lymphocytes results from covalent DNA modification.

Detection of (3'-hydroxy)-3,N4-benzetheno-2'-deoxycytidine-3'-phosphate by 32P-postlabeling of DNA reacted with p-benzoquinone

Cytidine-3'-phosphate was reacted with p-benzoquinone under neutral aqueous conditions, and the fluorescent product formed was isolated and characterized. The structure of the covalent adduct was identified as (3'-hydroxy)-3,N4-benzetheno-cytidine-3'-phosphate by high-resolution MS and 1H NMR spectroscopy. A similar product was isolated from the reaction of 2'-deoxycytidine-3'-phosphate with a hydroquinone-p-benzoquinone mixture. 32P-Postlabeling of calf thymus DNA reacted with p-benzoquinone detected several adducts, the principal adduct being (3'-hydroxy)-3,N4-benzetheno-2'-deoxycytidine-3'-phosphate. Our studies demonstrate that the reaction of DNA with p-benzoquinone in vitro leads to multiple DNA adducts. 32P-Postlabeling may allow detection of benzene-DNA adducts in vivo.

Binding of benzo[a]pyrene to DNA by cytochrome P-450 catalyzed one-electron oxidation in rat liver microsomes and nuclei

To investigate whether cytochrome P-450 catalyzes the covalent binding of substrates to DNA by one-electron oxidation, the ability of both uninduced and 3-methylcholanthrene (MC) induced rat liver microsomes and nuclei to catalyze covalent binding of benzo[a]pyrene (BP) to DNA and formation of the labile adduct 7-(benzo[a]pyren-6-yl)guanine (BP-N7Gua) was investigated. This adduct arises from the reaction of the BP radical cation at C-6 with the nucleophilic N-7 of the guanine moiety. In the various systems studied, 1-9 times more BP-N7Gua adduct was isolated than the total amount of stable BP adducts in the DNA. The specific cytochrome P-450 inhibitor 2-[(4,6-dichloro-o-biphenyl)oxy]ethylamine hydrobromide (DPEA) reduced or eliminated BP metabolism, binding of BP to DNA, and formation of BP-N7Gua by cytochrome P-450 in both microsomes and nuclei. The effects of the antioxidants cysteine, glutathione, and p-methoxythiophenol were also investigated. Although cysteine had no effect on the microsome-catalyzed processes, glutathione and p-methoxythiophenol inhibited BP metabolism, binding of BP to DNA, and formation of BP-N7Gua by cytochrome P-450 in both microsomes and nuclei. The decreased levels of binding of BP to DNA in the presence of glutathione or p-methoxythiophenol are matched by decreased amounts of BP-N7Gua adduct and of stable BP-DNA adducts detected by the 32P-postlabeling technique. This study represents the first demonstration of cytochrome P-450 mediating covalent binding of substrates to DNA via one-electron oxidation and suggests that this enzyme can catalyze peroxidase-type electron-transfer reactions.

32P-postlabeling analysis of benzo[a]pyrene-DNA adducts formed in vitro and in vivo

Benzo[a]pyrene (BP) was bound to DNA by horseradish peroxidase, rat liver microsomes, and rat liver nuclei in vitro and in mouse skin in vivo. The BP-DNA adducts formed were analyzed by the 32P-postlabeling technique. Activation by microsomes and nuclei resulted in the detection of five adducts, including a major adduct (55%) which cochromatographed with the adduct (+/-)-10 beta-deoxyguanosin-N2-yl-7 beta, 8 alpha, 9 alpha-trihydroxy-7,8,9,10-tetrahydro-BP (BPDE-N2dG) formed by reaction of (+/-)-7 beta, 8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydro-BP (BPDE) with DNA or by microsomal activation of BP 7,8-dihydrodiol. Activation by horseradish peroxidase, which catalyzes one-electron oxidation, produced seven adducts, including a major one (30%) that coeluted with an adduct observed with microsomal (2%) and nuclear (14%) activation. The pattern of adducts formed in mouse skin treated with BP in vivo for 4 or 24 h contained four of the same adducts observed with nuclei or microsomes in vitro, and the predominant adduct detected (86%) was BPDE-N2dG. The adduct common to horseradish peroxidase, microsomes, and nuclei was also detected in mouse skin DNA (2%). These results demonstrate that multiple BP-DNA adducts are formed in these in vitro and in vivo systems and suggest that at least one adduct is formed in common in all of the systems. Thus, it appears that stable BP adducts can be formed in mouse skin DNA by both monooxygenation and one-electron oxidation.

O6-substituted-2'-deoxyguanosine-3'-phosphate adducts detected by 32P post-labeling of styrene oxide treated DNA

32P post-labeling of DNA reacted with styrene oxide resulted in the detection of six adducts. In order to determine which of these corresponded to modification at the O6 position of guanine, O6-substituted styrene oxide-deoxyguanosine-3'-monophosphate derivatives were synthesized. The two synthetic isomers were purified by HPLC and the structures were confirmed by mass spectrometry and 1H NMR. 32P post-labeling and co-chromatography with the DNA-styrene-7,8-oxide reaction products resulted in the assignment of adduct number 4 as O6-(2-hydroxy-2-phenylethyl)-2'--deoxyguanosine-3',5'-bisphosphate and adduct number 5 as O6-(2-hydroxy-1-phenylethyl)-2'-deoxyguanosine-3',5'-bisphosphate.