Quantitative estimation of the extent of alkylation of DNA following treatment of mammalian cells with non-radioactive alkylating agents

Methyl DNA Phosphate Adduct Formation in Rats Treated Chronically with 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone and Enantiomers of Its Metabolite 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanol

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a powerful lung carcinogen in animal models and is considered a causative factor for lung cancer in tobacco users. NNK is stereoselectively and reversibly metabolized to 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), which is also a lung carcinogen. Both NNK and NNAL undergo metabolic activation by α-hydroxylation on their methyl groups to form pyridyloxobutyl and pyridylhydroxybutyl DNA base and phosphate adducts, respectively. α-Hydroxylation also occurs on the α-methylene carbons of NNK and NNAL to produce methane diazohydroxide, which reacts with DNA to form methyl DNA base adducts. DNA adducts of NNK and NNAL are important in their mechanisms of carcinogenesis. In this study, we characterized and quantified methyl DNA phosphate adducts in the lung of rats treated with 5 ppm of NNK, (S)-NNAL, or (R)-NNAL in drinking water for 10, 30, 50, and 70 weeks, by using a novel liquid chromatography-nanoelectrospray ionization-high resolution tandem mass spectrometry method. A total of 23, 21, and 22 out of 32 possible methyl DNA phosphate adducts were detected in the lung tissues of rats treated with NNK, (S)-NNAL, and (R)-NNAL, respectively. Levels of the methyl DNA phosphate adducts were 2290-4510, 872-1120, and 763-1430 fmol/mg DNA, accounting for 15-38%, 8%, and 5-9% of the total measured DNA adducts in rats treated with NNK, (S)-NNAL, and (R)-NNAL, respectively. The methyl DNA phosphate adducts characterized in this study further enriched the diversity of DNA adducts formed by NNK and NNAL. These results provide important new data regarding NNK- and NNAL-derived DNA damage and new insights pertinent to future mechanistic and biomonitoring studies of NNK, NNAL, and other chemical methylating agents.

Comprehensive High-Resolution Mass Spectrometric Analysis of DNA Phosphate Adducts Formed by the Tobacco-Specific Lung Carcinogen 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK, 1) is a potent lung carcinogen in laboratory animals and is believed to play a key role in the development of lung cancer in smokers. Metabolic activation of NNK leads to the formation of pyridyloxobutyl DNA adducts, a critical step in its mechanism of carcinogenesis. In addition to DNA nucleobase adducts, DNA phosphate adducts can be formed by pyridyloxobutylation of the oxygen atoms of the internucleotidic phosphodiester linkages. We report the use of a liquid chromatography-nanoelectrospray ionization-high-resolution tandem mass spectrometry technique to characterize 30 novel pyridyloxobutyl DNA phosphate adducts in calf thymus DNA (CT-DNA) treated with 4-(acetoxymethylnitrosamino)-1-(3-pyridyl)-1-butanone (NNKOAc, 2), a regiochemically activated form of NNK. A (15)N3-labeled internal standard was synthesized for one of the most abundant phosphate adducts, dCp[4-oxo-4-(3-pyridyl)butyl]dC (CpopC), and this standard was used to quantify CpopC and to estimate the levels of other adducts in the NNKOAc-treated CT-DNA. Formation of DNA phosphate adducts by NNK in vivo was further investigated in rats treated with NNK acutely (0.1 mmol/kg once daily for 4 days by subcutaneous injection) and chronically (5 ppm in drinking water for 10, 30, 50, and 70 weeks). This study provides the first comprehensive structural identification and quantitation of a panel of DNA phosphate adducts of a structurally complex carcinogen and chemical support for future mechanistic studies of tobacco carcinogenesis in humans.

Creating context for the use of DNA adduct data in cancer risk assessment: II. Overview of methods of identification and quantitation of DNA damage

The formation of deoxyribonucleic acid (DNA) adducts can have important and adverse consequences for cellular and whole organism function. Available methods for identification of DNA damage and quantification of adducts are reviewed. Analyses can be performed on various samples including tissues, isolated cells, and intact or hydrolyzed (digested) DNA from a variety of biological samples of interest for monitoring in humans. Sensitivity and specificity are considered key factors for selecting the type of method for assessing DNA perturbation. The amount of DNA needed for analysis is dependent upon the method and ranges widely, from <1 microg to 3 mg. The methods discussed include the Comet assay, the ligation-mediated polymerase reaction, histochemical and immunologic methods, radiolabeled ((14)C- and (3)H-) binding, (32)P-postlabeling, and methods dependent on gas chromatography (GC) or high-performance liquid chromatography (HPLC) with detection by electron capture, electrochemical detection, single or tandem mass spectrometry, or accelerator mass spectrometry. Sensitivity is ranked, and ranges from approximately 1 adduct in 10(4) to 10(12) nucleotides. A brief overview of oxidatively generated DNA damage is also presented. Assay limitations are discussed along with issues that may have impact on the reliability of results, such as sample collection, processing, and storage. Although certain methodologies are mature, improving technology will continue to enhance the specificity and sensitivity of adduct analysis. Because limited guidance and recommendations exist for adduct analysis, this effort supports the HESI Committee goal of developing a framework for use of DNA adduct data in risk assessment.

Quantitation of DNA adduct of thymidylyl(3'-5')thymidine methyl phosphotriester by liquid chromatography/negative electrospray tandem mass spectrometry

A rapid and selective method based on liquid chromatography/electrospray tandem mass spectrometry (LC/ESI-MS/MS) has been developed for the direct quantitation of a methyl phosphotriester DNA adduct, thymidyl (3'-5') thymidine [dTp(Me)dT] from enzymatic hydrolysates of DNA (either in vitro DNA or in cell culture) treated with MNU (N-methyl-N-nitrosourea) or MMS (methyl methane sulfonate). The lower limit of quantitation was 2 ng/mL. Linearity of the calibration curve was greater than 0.999 from 2 to 1000 ng/mL. Intraday precision for four levels of quality controls ranged from 2.8 to 20.1%, and interday precision ranged from 2.9 to 5.6%. This method was used to quantify the levels of dTp(Me)dT in enzymatic hydrolysates of DNA obtained from a series of incubations of salmon testis DNA or mouse lymphoma cells with either MNU or MMS.

Induction and disappearance of DNA strand breaks in human peripheral blood lymphocytes and fibroblasts treated with methyl methanesulfonate

The induction and disappearance of DNA single-strand breaks (SSB) in human peripheral blood lymphocytes (PBL) and fibroblasts exposed to methyl methanesulfonate (MMS) were investigated by using the alkaline filter elution assay. In the two cell types, identical amounts of SSB were induced during a 45-min treatment with a given dose of MMS. In quiescent PBL only 9 +/- 4% (mean +/- SD) of the induced SSB had disappeared at 1 h after exposure, whereas in phytohemagglutinin-stimulated PBL, 23 +/- 12% disappeared within the same repair period. The percentage SSB disappearance in confluent fibroblasts was 25 +/- 2% at 1 h after exposure. As in PBL, the percentage SSB disappearance in fibroblasts appeared to be proliferation-dependent; actively dividing fibroblasts removed 50 +/- 12% of the MMS-induced SSB during the 1-h repair period. The accumulation of SSB in PBL, but not in fibroblasts, during MMS exposure in the presence of the excision-repair inhibitor 1-beta-D-arabinofuranosylcytosine indicated the utilization of different repair pathways in these two cell types. The generally lower rate of disappearance of MMS-induced SSB in PBL as compared to fibroblasts correlated with an increased loss of cell viability, measured by determining the incorporation of [3H]thymidine.

Apurinic site induction in the DNA of cells heated at hyperthermic temperatures

The induction of DNA damage in cells heated at hyperthermic (43-48 degrees C) temperatures was determined by alkaline filter elution and alkaline sucrose gradient-sedimentation analysis of cell DNA denatured at pH 13.0. A class of DNA lesion which converted to strand breaks during denaturation of DNA at pH 13.0 was produced randomly throughout the cell DNA at temperatures as low as 43 degrees C. Induction of this lesion occurred with a T0 of 90 and 10 min at 45 and 48 degrees C, respectively. We estimate that these pH 13.0-detectable DNA lesions are produced in the cell DNA with a frequency of approximately 75 and 660 per min of heating at 45 and 48 degrees C, respectively. Since the lesions were quantitatively converted to DNA strand breaks at pH 13.0 with a half-time of 30 min, or less, we suggest that these pH 13.0-detectable DNA lesions are heat-induced, abasic DNA sites. The induction of these lesions does not appear to be directly involved in the initial heat-induced inhibition of DNA synthesis. The presence of these lesions cannot be excluded as an explanation for the long-term inhibition of replicon initiated in heated cells.

Molecular dosimetry of DNA damage caused by alkylation. II. The induction and repair of different classes of single-strand breaks in cultured mammalian cells treated with ethylating agents

Cultured Chinese hamster ovary cells were treated with ethylating agents. DNA lesions giving rise to single-strand breaks (SSB) or alkali-labile sites were measured by elution through membrane filters at pH 12.0 and pH 12.6, and by centrifugation in alkaline sucrose gradients after 1 h and 21 h lysis in alkali. Two agents with different tendencies to ethylate preferentially either at N or O atoms were compared, namely N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) and diethyl sulphate (DES). The compounds differed greatly in their potency to induce lesions, but the ratios of SSB, measured with different methods after a treatment for 30 min, did not differ significantly. This suggested that the spectrum of lesions induced by the two compounds is very similar. However, when both agents were studied with alkaline elution at pH 12.0 after a short treatment time (5 min) only ENNG was found to induce rapidly-repairable SSB. Most of these were rejoined already within 5 min after treatment. These results suggest that rapidly-repairable lesions occurring in DNA after treatment of mammalian cells with ethylating agents are due mainly to alkylation at O-atoms.

Induction of single-strand breaks plus alkali-labile bonds by N-nitrosoureas in rat tissues in vivo: ethylnitrosourea versus benzylnitrosourea

Alkaline sucrose sedimentation procedures were used to quantitate the amount of single-strand breaks plus alkali-labile bonds (SSB + ALB) induced and repaired following a single intraperitoneal injection of the neurocarcinogen N-ethyl-N-nitrosourea (ENU) and its non-neurocarcinogenic analog N-benzyl-N-nitrosourea (BNU) in the brain, liver and kidney of female Sprague-Dawley rats. SSB + ALB were measured and used as an indicator of apurinic/apyrimidinic sites, phosphotriesters and in situ breaks. ENU induced a dose-dependent increase in the number of SSB + ALB at the doses studied (0, 0.39, 0.77, 1.54 mmoles/kg) in all 3 tissues. At 1 h postinjection with 0.77 mmoles/kg of these compounds there were 50-70% fewer breaks induced by BNU than ENU. The SSB + ALB induced by ENU persisted over a 7-day period, while those induced by BNU did not. Thus, these studies showed that 2 homologues of nitrosoureas, ENU and BNU, exhibited different potentials to induce and to persist SSB + ALB in vivo.

The induction of chromosomal damage in rat hepatocytes and lymphocytes. II. Alkylation damage and repair of rat-liver DNA after diethylnitrosamine, dimethylnitrosamine and ethyl methanesulphonate in relation to clastogenic effects

Rat-liver DNA alkylation by diethylnitrosamine (DEN), dimethylnitrosamine (DMN) and ethyl methanesulphonate (EMS) was studied in an attempt to relate chromosome-damaging effects of these agents (the formation of micronuclei in hepatocytes; see preceding paper) to specific alkylation patterns. No correlation was observed between the induction of micronuclei and liver DNA N-alkylation, measured as 3- and 7-alkyl-purines. O6-Alkylguanine is probably not involved in micronucleus induction because it is lost from DNA too rapidly to explain the much more persistent clastogenic effects. In contrast, both the initial amounts of alkylphosphotriesters and the persistencies of these products roughly paralleled the respective effects on micronucleus induction. The possible involvement of alkylphosphotriesters or other O-alkylation products of comparable stabilities is discussed. Results with DMN suggest that part of the primary DNA methylation damage is converted into a secondary (DNA) lesion and that both the primary and secondary lesion(s) contribute to the process of micronucleus formation.

The alkaline hydrolysis of phosphotriesters in alkylated mammalian DNA

Isolated DNA was alkylated with N-[14C]methyl-N-nitrosourea or N-[14C]ethyl-N-nitrosourea. Sedimentation analysis of the alkylated DNA before and after alkaline hydrolysis was used to determine the number of single-strand breaks introduced by hydrolysis of the triesters. Vacuum distillation from alkylated DNA solutions before and after alkaline hydrolysis was used to determine the numbers of triesters hydrolysing to the alcohol.

Molecular dosimetry of DNA damage caused by alkylation. I. Single-strand breaks induced by ethylating agents in cultured mammalian cells in relation to survival

Cultured Chinese hamster ovary cells were treated with ethylating agents. DNA lesions giving rise to single-strand breaks (ssb) or alkali-labile sites were measured by centrifugation in alkaline sucrose gradients after lysis in alkali. 4 agents with different tendencies to ethylate preferentially either at N or O atoms were compared, namely N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG), N-ethyl-N-nitrosourea (ENU), ethyl methanesulphonate (EMS) and diethyl sulphonate (DES). The compounds differed greatly in their potency to induce the lesions measured when compared on a molar basis, but comparison at equicytotoxic doses showed relatively small differences. Upon prolonged incubation of the DNA in alkali, the number of ssb increased considerably. DNA from untreated cells showed biphasic kinetics: slow ssb formation for about 10 h, then the rate increased and remained constant for up to 40 h. Treated cells showed an accelerated, dose-dependent linear generation of ssb for 10 h, followed by a short plateau; then ssb were formed again at a constant rate, somewhat higher than that in controls. Ssb formed in the initial phase are ascribed to phosphotriester hydrolysis, those after the plateau to unidentified causes. Zero intercepts appeared to be a measure of apurinic sites generated intracellularly. A 24-h repair period preceding lysis reduced the ENNG intercept, but not that of DES. Rapid degradation of DES during the 1-h treatment occurred, so most "apurinic-site lesions" were induced in the beginning of exposure and possibly were already repaired at the end. The types of lesion distinguished (reparable and non-reparable apurinic sites, phosphotriesters) appeared of little consequence for cell survival.