Measurement of O6-ethyldeoxyguanosine and N-(deoxyguanosin-8-yl)-N-acetyl-2-aminofluorene in DNA by high-sensitive enzyme immunoassays

What is the significance of increases in background levels of carcinogen-derived protein and DNA adducts? Some considerations for incremental risk assessment

Improvements in analytical methodology have led to the detection and quantification of 'background' levels of a number of DNA and protein adducts. Many of these adducts are derived from 'low molecular weight' reactive species which may be generated during normal physiological processes, metabolic pathways or inflammatory processes. The adducts have been detected using gas chromatography-mass spectrometry, HPLC in combination with various detection systems, 32P-postlabelling and immunoassay methods. The reliability and accuracy of many widely used methods for adduct measurements are discussed with reference to several examples where human data is available, namely 4-aminobiphenyl, malondialdehyde, methylating agents, ethylene oxide and hydroxyl radical damage. The accurate and specific quantitation of 'background' levels of damage is essential if reliable estimates of increases in risk associated with incremental increases in exposure to exogenous agents are to be calculated. In experimental studies using low dose exposures to carcinogens, such as N-nitrosodimethylamine, adduct levels in liver correlate closely with tumour incidence. In all likelihood, such relationships need to be established for each exposure and, in order to be relevant to human risk assessment, need to take into account factors such as DNA repair and mutagenic efficiency. Finally, in order to estimate the increase in cancer attributable to a given level of external exposure, it is clearly important to establish background levels of corresponding DNA damage so that the scale of the incremental increase can be calculated.

Monoclonal antibodies to thymidine glycol generated by different immunization techniques

Monoclonal antibodies specific for thymidine glycol in oxidized DNA were generated by immunization with thymidine glycol monophosphate (TMP-OH) or thymidine glycol (T-OH), respectively, conjugated to keyhole limpet hemocyanin (KLH) or thyreoglobulin (TG). Forty-five clones (TMP-OH) and 70 clones (T-OH) were examined upon antibody production in ELISA. Four clones secreting IgG1, kappa, were characterized further. In several studies the antibodies derived from the immunization with TMP-OH were inhibited by various inhibitors. In descending order of effectiveness, they were thymidine glycol monophosphate (TMP-OH), thymidine glycol (T-OH), thymidine monophosphate (TMP), and thymidine (Thn). After immunization with T-OH, antibodies were inhibited in following order: T-OH > TMP-OH > TMP > Thn. Inhibition by the bases thymine, adenine, cytosine, and guanine were negligible. In ISB (Immuno Slot Blot) performed with OsO4-treated DNA (Poly-[dA-dT]) and amount of 70 fmol thymidine glycol was detectable. DNA had to be irradiated at a level of at least 20 Gy to detect any damage in ELISA but at a lower level of irradiation (10 Gy) in ISB by one of these antibodies, TPS-1. The antibodies obtained after immunization with hapten-protein are therefore capable of the detection of low frequency lesions in DNA generated by free radicals after radiation or oxidative stress.

Fifty years of research on N-acetyl-2-aminofluorene, one of the most versatile compounds in experimental cancer research

It is just about 50 years since the publication of the report on the toxicity and carcinogenicity of the potent carcinogen N-acetyl-2-aminofluorene (AAF). In 1940 very few reports on the carcinogenic activity of chemical compounds in experimental animals were available. The discovery of pure chemicals as carcinogens, such as AAF, azo dyes and benzo[a]pyrene, provided cancer researchers with a number of tools whereby the progressive changes involved in the induction of cancer could be studied in experimental systems. Contrary to the results with other carcinogens then known, AAF induced numerous types of tumors, but not at the site of application. This finding stimulated a great deal of interest in its use as an experimental carcinogen to study its metabolic fate and mechanism of action. During the following years an ever increasing number of reports appeared on the carcinogenicity of AAF in various species, on its metabolic fate, on the interaction of reactive metabolites with nucleic acids and proteins, and on its mutagenic activity. Particularly studies on the metabolism of AAF and the interaction with nucleic acids have contributed appreciably to our understanding of the mechanism of action of aromatic amines and also of other chemical carcinogens. It can be expected that AAF and its derivatives will continue to be used for specific applications in experimental cancer research. One of the most recent achievements is the preparation of site-specific AAF- and aminofluorene-modified DNA sequences for mutagenesis studies.

Immunocytochemical analysis of O6-alkylguanine shows tissue specific formation in and removal from esophageal and liver DNA in rats treated with methylbenzylnitrosamine, dimethylnitrosamine, diethylnitrosamine and ethylnitrosourea

The formation and repair of carcinogen-DNA adducts in esophagus and liver of rats treated with a single i.p. dose of methylbenzylnitrosamine (MBN), dimethylnitrosamine (DMN), diethylnitrosamine (DEN) or ethylnitrosourea (ENU) has been studied using peroxidase immunocytochemistry to visualize O6-alkylguanine in DNA of individual cells. After MBN O6-methylguanine (O6-MeG) specific nuclear staining was only present in the target tissue for tumor induction, the esophageal epithelium. Part of the adducts persisted for at least 72 h. No O6-MeG could be detected in liver. DEN, a carcinogen in liver and esophagus, led to DNA modification of esophageal epithelial cells, and liver parenchymal and non-parenchymal (Kupffer and sinusoidal) cells of the centrilobular area. O6-EtG was removed within 72 h from both liver cell populations. A similar distribution of adduct (O6-MeG) formation was observed in liver after the hepatocarcinogen DMN, but this nitrosamine did not detectably modify esophageal cells. O6-MeG persisted in Kupffer and especially sinusoidal lining cells of liver, consistent with the induction of sarcomas by DMN. The relatively unspecific, directly alkylating carcinogen ENU modified DNA of all cell types to a similar extent. A qualitative correlation was obtained between the tissue specific ability to induce tumors and the formation of O6-alkylguanine (O6-alkylG). Our experiments support the hypothesis that DNA modification is necessary for the initiation of carcinogenesis by chemical carcinogens, and that a low capacity to repair promutagenic lesions, like O6-alkylG, potentiates this process.

Application of new techniques for the detection of carcinogen adducts to human population monitoring

Several techniques have recently been developed for the detection and quantitation of carcinogen-DNA or -protein adducts without the requirement for radioactive carcinogens. These assays can be used to detect adducts in animals or cultured cells exposed to test compounds or in humans exposed to environmental carcinogens. Immunologic, 32P-postlabeling and fluorescence techniques, used on human samples for DNA adduct measurement, are reviewed here. Methods for the detection of carcinogen-protein adducts on human samples are also summarized.

Polyclonal antibodies to DNA modified with 4-nitroquinoline 1-oxide: application for the detection of 4-nitroquinoline 1-oxide-DNA adducts in vivo

Antibodies against 4-nitroquinoline 1-oxide (4NQO) adducts were elicited in rabbits immunized with 4NQO-modified DNA complexed with methylated bovine serum albumin. In enzyme-linked immunosorbent assay (ELISA), the antibodies could recognize either denatured or native 4NQO-modified DNA, but not unmodified DNA, DNA modified with other carcinogens or free 4NQO derivative. Modification levels as low as 5 mumol of adduct per one mole DNA nucleotide (5 adducts/10(6) nucleotides) can be easily detected by the competitive ELISA. Indirect immunofluorescence staining by anti 4NQO-DNA antibody indicated that the antibodies bound specifically to the nuclei of normal human skin fibroblast cells treated with 4NQO. The intensity of fluorescence was proportional to the dose of 4NQO used to treat the cells, and the fluorescence-positive cells could be detected after treatment with 0.25 microM 4NQO (which resulted in the formation of 10(4) adducts per cell). Applying the competitive ELISA to the quantitation of DNA-adducts in rats treated with 4NQO, it was confirmed that the sensitivity of immunochemical assays was equivalent to that of isotopic assays. These methods should be helpful in studies on the formation of adducts and their removal in cells and tissues.

The organ-specific induction of DNA adducts in 2-acetylaminofluorene-treated rats, studied by means of a sensitive immunochemical method

Exposure of cells to chemical carcinogens and mutagens may result in the formation of DNA adducts, which can give rise to mutations in the genome and to cellular transformation. Methods to measure DNA-adduct formation may be useful for 'biomonitoring', to establish exposure of laboratory animals or humans to DNA-damaging agents. For such purposes, immunochemical methods appear to be suitable, because they allow sensitive detection and quantification of DNA adducts in small amounts of sample in a non-radiolabelled form. We have worked out optimal conditions for the detection of DNA adducts by means of competitive enzyme-linked immunosorbent assay (ELISA). This technique involves interaction of soluble antigen, immobilized antigen and antibody. It appeared that the sensitivity of the competitive assay can be improved by lowering the amount of immobilized antigen, adsorbed to the wall of the plastic reaction vessel. On the basis of these observations, suitable conditions were selected for a sensitive quantitative assay of adducts in DNA isolated from various organs of rats, treated (p.o.) with the liver carcinogen 2-acetylaminofluorene (2-AAF). Under the conditions of these experiments, the available rabbit antiserum recognizes the guanosine-AAF adduct with high specificity. A time- and dose-dependent induction of AAF adducts could be measured in liver DNA from exposed rats, whereas the amount of adducts in DNA from spleen and nucleated blood cells remained below the detection limit (1 adduct/10(8) nucleotides). The implications of these findings with respect to the relevance of blood cell biomonitoring for target cell exposure are discussed.

DNA modification by chemical carcinogens

The chemistry and molecular biology of DNA adducts is only one part of the carcinogenic process. Many other factors will determine whether a particular chemical will exert a carcinogenic effect. For example, the size of particles upon which a carcinogenic may be adsorbed will influence whether or not, and if so where, deposition within the lung will occur. The simultaneous exposure to several different agents may enhance or inhibit the metabolism of a chemical to its ultimate carcinogenic form (Rice et al., 1984; Smolarek and Baird, 1984). The ultimate carcinogenic metabolites may be influenced in their ability to react with DNA by a number of factors such as internal levels of detoxifying enzymes, the presence of other metabolic intermediates such as glutathione with which they could react either enzymatically or non-enzymatically, and the state of DNA which is probably most heavily influenced by whether or not the cell is undergoing replication or particular sequences being expressed. Replicating forks have been shown to be more extensively modified than other areas of DNA. Another critical factor which can influence the final outcome of the DNA damage is whether or not the modifications can be repaired. If this occurs with high fidelity and the cell has not previously undergone replication then the effect of the damage by the carcinogen is likely to be minimal. The major area in which progress is needed is an understanding of what this damage really does to the cell such that after an additional period of time, which may be as long as twenty or more years, these prior events are expressed and cell proliferation occurs. Clearly additional stimulatory factors, for example tumor promoting agents such as the phorbol esters or phenobarbital, are often needed. After such prolonged periods it seems likely that the DNA adducts would no longer be present. However, the way in which their earlier presence is remembered is not clear. Simple mutations do not explain all the characteristics of tumor progression and, when it occurs, regression. Even if a specific site mutation does occur then its expression must be under other types of control. Any explanation of the action of DNA modification at the molecular level also requires that account be taken of the diverse nature of the DNA adducts from simple modifications such as methylation to bulkier adducts such as benzo[a]pyrene, aflatoxin or aromatic amines.(ABSTRACT TRUNCATED AT 400 WORDS)

Hybridization of N-acetoxy-N-acetyl-2-aminofluorene-labelled RNA to Q-banded metaphase chromosomes

This paper describes improvements of a recently developed immunocytochemical method for the detection of specific polynucleotide sequences within chromosomes, as well as conditions by which this method can be combined with chromosome banding. The immunocytochemical method involves modification of polynucleotide probes with N-acetoxy-N-acetyl-2-aminofluorene (AAAF)2), and hybridization of the modified probes with metaphase chromosomes; the hybrids are made visible immunocytochemically by means of an antiserum which recognizes AAAF-induced polynucleotide modifications. We have sorted out conditions which allow a high sensitivity of hybrid detection by the above procedure, in combination with chromosome banding. The best results are obtained if the ABC-technique is used for the visualization of the hybrids; the lower limit of detection is estimated to be a sequence of about 7000 nucleotides. The method can be combined with Q-banding of chromosomes, if this is performed not more than 1 day prior to hybridization, and if excitation of the Q-banded chromosomes is kept to a minimum.

Immuno-slot-blot: a highly sensitive immunoassay for the quantitation of carcinogen-modified nucleosides in DNA

We have established a highly sensitive immuno-slot-blot (ISB) procedure that can be routinely applied for detection and quantitation of any heat- or alkali-stable structural DNA modification (caused by carcinogens or mutagens, for example) for which a specific (monoclonal) antibody (MAB) is available. The essential step in this assay is the immobilization on nitrocellulose filters of the structurally modified DNA in its single-stranded form. The immobilized DNA is first reacted with an MAB specifically directed against a particular modified DNA component (e.g., an alkyldeoxynucleoside), and thereafter with a second antibody directed against the first one. The second antibody can be either labeled with 125I or linked to an enzyme complex capable of eliciting a color reaction with a suitable substrate. The sensitivity of the ISB is demonstrated for two different alkyldeoxynucleosides, O6-ethyldeoxyguanosine (O6-EtdGuo) and O4-ethyldeoxythymidine (O4-EtdThd), both of which are produced in cellular DNA exposed to the alkylating N-nitroso carcinogen N-ethyl-N-nitrosourea and both of which represent DNA lesions miscoding during DNA replication and transcription. Using anti-(O6-EtdGuo) and anti-(O4-EtdThd) MABs, respectively, O6-EtdGuo and O4-EtdThd are detected at levels as low as greater than or equal to 0.3 X 10(-15) mol O6-EtdGuo/3 micrograms DNA (O6-EtdGuo/deoxyguanosine molar ratio in DNA, greater than or equal to 2 X 10(-7) ) and greater than or equal to 0.1 X 10(-15) mol of O4-EtdThd/3 micrograms DNA (O4-EtdThd/deoxythymidine molar ratio in DNA, greater than or equal to 4 X 10(-8) ).

Enzyme immunoassay of benzo[a]pyrene conjugated to DNA, RNA and microsomal proteins using a monoclonal antibody

An enzyme immunoassay for the detection of benzo[a]pyrene covalently conjugated to macromolecules has been developed. The monoclonal antibody, raised through in vitro immunization reacted with benzo[a]pyrene metabolites bound to DNA, RNA and proteins. The lower detection limit for the assay was 1 pmol for benzo[a]pyrene bound to DNA or RNA, and 5 pmol when bound to protein.

Methodologies for the determination of various genetic effects in permeable strains of E. coli K-12 differing in DNA repair capacity. Quantification of DNA adduct formation, experiments with organ homogenates and hepatocytes, and animal-mediated assays

Derivatives of E. coli K-12 strain 343/113 differing in DNA repair capacity, in permeability to large molecules, and in some metabolizing activities (nitroreductase, glutathione), were constructed for the quantitative determination of the induction of various genetic effects, such as forward and back mutations, lysogenic induction of prophage lambda, and repairable DNA damage. These E. coli strains can be used in assay procedures which allow variation and control over several experimental conditions, such as oxygen tension, time, pH, temperature of incubation and growth phase of the indicator cells. Methods are described for the simultaneous determination of genetic effects and of DNA-adduct formation during mutagen treatment, i.e. by using radio-labeled compounds or by means of an enzyme-linked immunosorbent assay (ELISA). Mammalian biotransformation of xenobiotics can be investigated by including various fractions of mammalian organs in the system. Examples of the relative effectiveness of the activating potential of S9, S100 and isolated hepatocytes for dialkylnitrosamines and other carcinogens are presented. Host-mediated assays, finally, are described which, in addition to gene mutations, can also be used for the determination of repairable DNA damage in bacteria present in different organs, including the liver, spleen, lungs, kidneys, pancreas, and the blood stream of chemically treated mice. It is concluded that quantitative tests in vitro for assessment of induced mutagenic spectrum and genotoxic potency, combined with the host-mediated assay as a monitor, in vivo, of genotoxic factors present in various organs of animals, may become useful in the assessment of genotoxic (and possibly tumor-initiating) properties of chemicals for which long-term in-vivo mutagenicity and/or carcinogenicity data are not yet available.

The use of carcinogen-DNA adduct antisera for quantitation and localization of genomic damage in animal models and the human population

The use of antibodies to detect chemical carcinogen-induced DNA damage involves quantitative determination and morphological localization utilizing antisera specific for carcinogen-DNA adducts. In recent years a large number of polyclonal and monoclonal antisera have been produced against individual adducts and modified DNAs with addition products varying in structure from ethyl and methyl groups to aromatic amines, polycyclic aromatic hydrocarbons, aflatoxins, and platinum-ammine complexes. The quantitative assays developed through the use of these antisera are able to detect attomole (10(-18) M) adduct concentrations, corresponding to one adduct in 10(8) nucleotides or a few hundreds of modifications per mammalian cell genome. This review focuses on data generated during the past 3 yr utilizing this immunotechnology as a tool to probe mechanisms of carcinogen-DNA interactions in various model systems and in the human population. Areas discussed in depth include quantitative and morphological studies involving detection of 2-acetylaminofluorene-DNA adducts in rat liver, O6-ethyl and O6-methyl deoxyguanosine adducts in rat brain, benzo[a]pyrene-DNA adducts in mouse skin and cis-diamminedichloroplatinum (II)-DNA adducts in peripheral nucleated blood cells of cancer patients.