Relationships between DNA adduct formation and carcinogenesis

Chromosome aberrations cause tumorigenesis through chromosomal rearrangements in a hepatocarcinogenesis rat model

Chromosome aberrations (CAs), a genotoxic potential of carcinogens, are believed to contribute to tumorigenesis by chromosomal rearrangements through micronucleus formation. However, there is no direct evidence that proves the involvement of CAs in tumorigenesis in vivo. In the current study, we sought to clarify the involvement of CAs in chemical carcinogenesis using a rat model with a pure CA-inducer hepatocarcinogen, acetamide. Whole-genome analysis indicated that hepatic tumors induced by acetamide treatment for 26-30 weeks showed a broad range of copy number alterations in various chromosomes. In contrast, hepatic tumors induced by a typical mutagen (diethylnitrosamine) followed by a nonmutagen (phenobarbital) did not show such mutational patterns. Additionally, structural alterations such as translocations were observed more frequently in the acetamide-induced tumors. Moreover, most of the acetamide-induced tumors expressed c-Myc and/or MDM2 protein due to the copy number gain of each oncogene. These results suggest the occurrence of chromosomal rearrangements and subsequent oncogene amplification in the acetamide-induced tumors. Taken together, the results indicate that CAs are directly involved in tumorigenesis through chromosomal rearrangements in an acetamide-induced hepatocarcinogenesis rat model.

Direct and indirect effects of exposure to 900 MHz GSM radiofrequency electromagnetic fields on CHO cell line: Evidence of bystander effect by non-ionizing radiation

INTRODUCTION

The rapid rise in global concerns about the adverse health effects of exposure to radiofrequency radiation (RFR) generated by common devices such as mobile phones has prompted scientists to further investigate the biological effects of these environmental exposures. Non-targeted effects (NTEs) are responses which do not need a direct exposure to be expressed and are particularly significant at low energy radiations. Although NTEs of ionizing radiation are well documented, there are scarcely any studies on non-targeted responses such as bystander effect (BE) after exposure to non-ionizing radiation. The main goal of this research is to study possible RFR-induced BE.

MATERIAL AND METHODS

Chinese hamster ovary cells were exposed to 900 MHz GSM RFR at an average specific absorption rate (SAR) of 2 W/kg for 4, 12 and 24 hours (h). To generate a uniformly distributed electromagnetic field and avoid extraneous RF exposures a cavity was desined and used. Cell membrane permeability, cell redox activity, metabolic and mitotic cell death and DNA damages were analyzed. Then the most effective exposure durations and statistically significant altered parameters were chosen to assess the induction of BE through medium transfer procedure. Furthermore, intra and extra cellular reactive oxygen species (ROS) levels were measured to assess the molecular mechanism of BE induced by non-ionizing radiation.

RESULTS

No statistically significant alteration was found in cell membrane permeability, cell redox activity, metabolic cell activity and micronuclei (MN) frequency in the cells directly exposed to RFR for 4, 12, or 24 h. However, RFR exposure for 24 h caused a statistically significant decrease in clonogenic ability as well as a statistically significant increase in olive moment in both directly exposed and bystander cells which received media from RFR-exposed cells (conditioned culture medium; CCM). Exposure to RFR also statistically significant elevated both intra and extra cellular levels of ROS.

CONCLUSION

Our observation clearly indicated the induction of BE in cells treated with CCM. To our knowledge, this is the first report that a non-ionizing radiation (900 MHz GSM RFR) can induce bystander effect. As reported for ionizing radiation, our results proposed that ROS can be a potential molecule in indirect effect of RFR. On the other hand, we found the importance of ROS in direct effect of RFR but in different ways.

DNA modifications that do not cause gene mutations confer the potential for mutagenicity by combined treatment with food chemicals

Cell proliferation plays a key role in fixing mutations induced by DNA damage. We clarified whether this phenomenon occurred after combined treatment with chemicals in food. The effects of antibiotic flumequine (FL), a residue of veterinary medicinal products in foodstuffs, on mutagenicity in the liver were examined in mice treated with estragole (ES), a natural food flavouring compound. Gpt delta mice were orally administered 10 or 100 mg/kg/day ES and simultaneously fed a diet containing 0.4% FL for 4 weeks. Proliferating cell nuclear antigen-positive cells and cell cycle-related genes were additively increased in the livers of combined treatment groups as compared with high-dose ES or FL groups. Mutant frequencies (MFs) in gpt after cotreatment with low-dose ES and FL were significantly increased, although treatment with ES alone increased MFs only in the high-dose group. Sult1a1 mRNA levels were unchanged after FL treatment. Liquid chromatography with tandem-mass spectrometry analysis showed that FL did not affect the amount of ES-specific DNA adducts in the livers, indicating that FL treatment did not influence metabolic pathways of ES. Thus, enhancement of the mutagenic potential of a chemical by chemical-induced cell proliferation may occur as a result of the combined effects of chemicals in food.

Measurement of deoxyinosine adduct: Can it be a reliable tool to assess oxidative or nitrosative DNA damage?

Adenosine deaminases (ADA) are key enzymes that deaminate adenosine (A) or deoxyadenosine (dA) and produce inosine or deoxyinosine (dI), respectively. While ADA only deaminates free dA, reactive nitrogen species (RNS) or reactive oxygen species (ROS) deaminate adenine base on the DNA and leave dI, which is a pre-mutagenic lesion. Therefore, dI adduct in the genomic DNA has been considered a biomarker of DNA damage caused by RNS or by ROS. In the presented study, genomic DNA was isolated from frozen calf thymus in low or room temperature, with or without an addition of antioxidant. The number of dI in the DNA was measured using liquid chromatography-tandem mass spectrometry. While low temperature (LT) work-up with an addition of antioxidant in reagents helped to prevent artifactual formation of oxidative DNA lesions in the calf thymus DNA (CTD), it also significantly inhibited activities of proteinase, which in turn resulted in significant ADA contamination in the final DNA samples. ADA remained in LT-CTD completely deaminated most dA when the DNA was subjected to enzymatic hydrolysis to single nucleosides. The ADA contamination in the DNA was significantly reduced when DNA was isolated from pre-isolated nuclear fraction rather than from entire tissue homogenates. However, enzymes used for DNA hydrolysis were confirmed to contain significant amounts of ADA. Therefore, these enzymes would increase deamination of dA during DNA hydrolysis. Artifactual dI production by contaminated ADA was dramatically reduced by an addition of EHNA (erythro-9-(2-hydroxy-3-nonyl)adenine), which is a potent inhibitor of ADA. However, time- and temperature-dependent dI production from dA in phosphate buffer solution was observed. More importantly, TEMPO, an antioxidant commonly used to prevent DNA oxidation, was found to deaminate dA independent to ADA. Overall, these findings indicate that assay methods measuring dI or other dA DNA adducts in genomic DNA should be carefully validated to minimize artificial errors caused by dA deamination. Recommendations to overcome those technical challenges were discussed in this presentation.

Amelioration of Chemical-Induced Skin Carcinogenesis by Aegle marmelos, an Indian Medicinal Plant, Fruit Extract

Chemoprevention is a novel approach to study the anti-initiating and anti-tumor-promoting efficacy of medicinal plants and their active principles. The present study investigated the chemopreventive potential of Aegle marmelos fruit extract in 7,12-dimethylbenz[a]anthracene-induced skin carcinogenesis and its influence on oxidative stress and the antioxidant defense system. The oral administration of A marmelos at 100 mg/kg body weight/day during peri-initiational, postinitiational, and peri- & postinitiational phases of papillomagenesis showed significant reduction in tumor incidence, tumor yield, tumor burden, and cumulative number of papillomas when compared with carcinogen-treated control. The average latent period significantly increased (7.88 weeks; control group) to 9.45, 11.11, and 11.54 weeks in different A marmelos extract (AME) experimental groups. Enzyme analysis of skin and liver showed a significant elevation in antioxidant parameters such as superoxide dismutase, catalase, glutathione, and vitamin C in AME-treated groups when compared with the carcinogen-treated control. The elevated level of lipid peroxidation in the positive control was significantly inhibited by AME administration. These results indicate that AME has the potential to reduce chemical-induced skin papillomas by enhancing the antioxidant defense system.

Identification and characterization of 2'-deoxyadenosine adducts formed by isoprene monoepoxides in vitro

Isoprene, the 2-methyl analogue of 1,3-butadiene, is ubiquitous in the environment, with major contributions to total isoprene emissions stemming from natural processes despite the compound being a bulk industrial chemical. Additionally, isoprene is a combustion product and a major component in cigarette smoke. Isoprene has been classified as possibly carcinogenic to humans (group 2B) by IARC and as reasonably anticipated to be a human carcinogen by the National Toxicology Program. Isoprene, like butadiene, requires metabolic activation to reactive epoxides to exhibit its carcinogenic properties. The mode of action has been postulated to be that of a genotoxic carcinogen, with the formation of promutagenic DNA adducts being essential for mutagenesis and carcinogenesis. In rodents, isoprene-induced tumors show unique point mutations (A→T transversions) in the K-ras protooncogene at codon 61. Therefore, we investigated adducts formed after the reaction of 2'-deoxyadenosine (dAdo ) with the two monoepoxides of isoprene, 2-ethenyl-2-methyloxirane (IP-1,2-O) and propen-2-yloxirane (IP-3,4-O), under physiological conditions. The formation of N1-2'-deoxyinosine (N1-dIno) due to the deamination of N1-dAdo adducts was of particular interest, since N1-dIno adducts are suspected to have high mutagenic potential based on in vitro experiments. Major stable adducts were identified by HPLC, UV-spectroscopy, and LC-MS/MS and characterized by (1)H NMR and (1)H,(13)C HSQC and HMBC NMR experiments. Adducts of IP-1,2-O that were fully identified are R,S-C1-N(6)-dAdo, R-C2-N(6)-dAdo, and S-C2-N(6)-dAdo; adducts of IP-3,4-O are S-C3-N(6)-dAdo, R-C3-N(6)-dAdo, R,S-C4-N(6)-dAdo, S-C4-N1-dIno, R-C4-N1-dIno, R-C3-N1-dIno, S-C3-N1-dIno, and C3-N7-Ade. Both monoepoxides formed adducts on the terminal and internal oxirane carbons. This is the first study to describe adducts of isoprene monoepoxides with dAdo. Characterization of adducts formed by isoprene monoepoxides with deoxynucleosides and subsequently with DNA represent the first step toward evaluating their potential for being converted into a mutation or as biomarkers of isoprene metabolism and exposure.

Lung cancer: a biologically different disease in women?

Lung cancer is now the leading cancer killer of women, having surpassed breast cancer in 1987. Over 30,000 more US women are expected to die from lung cancer than from breast cancer annually. The vast majority of lung cancer cases are attributable to smoking, and smoking prevalence rates remain unacceptably high in US women. Mounting evidence suggests that there are significant differences in lung cancer between the sexes. Although the magnitude of the effect of smoking on the development of lung cancer may not be different, smoking appears to have an impact on the histology of lung cancer. Hormonal and biologic effects may play a role in lung cancer carcinogenesis, and may impact treatment response. A more thorough understanding of the biologically different aspects of lung cancer across different populations may lead to innovations in prevention and treatment.

The formation and biological significance of N7-guanine adducts

DNA alkylation or adduct formation occurs at nucleophilic sites in DNA, mainly the N7-position of guanine. Ever since identification of the first N7-guanine adduct, several hundred studies on DNA adducts have been reported. Major issues addressed include the relationships between N7-guanine adducts and exposure, mutagenesis, and other biological endpoints. It became quickly apparent that N7-guanine adducts are frequently formed, but may have minimal biological relevance, since they are chemically unstable and do not participate in Watson Crick base pairing. However, N7-guanine adducts have been shown to be excellent biomarkers for internal exposure to direct acting and metabolically activated carcinogens. Questions arise, however, regarding the biological significance of N7-guanine adducts that are readily formed, do not persist, and are not likely to be mutagenic. Thus, we set out to review the current literature to evaluate their formation and the mechanistic evidence for the involvement of N7-guanine adducts in mutagenesis or other biological processes. It was concluded that there is insufficient evidence that N7-guanine adducts can be used beyond confirmation of exposure to the target tissue and demonstration of the molecular dose. There is little to no evidence that N7-guanine adducts or their depurination product, apurinic sites, are the cause of mutations in cells and tissues, since increases in AP sites have not been shown unless toxicity is extant. However, more research is needed to define the extent of chemical depurination versus removal by DNA repair proteins. Interestingly, N7-guanine adducts are clearly present as endogenous background adducts and the endogenous background amounts appear to increase with age. Furthermore, the N7-guanine adducts have been shown to convert to ring opened lesions (FAPy), which are much more persistent and have higher mutagenic potency. Studies in humans are limited in sample size and differences between controls and study groups are small. Future investigations should involve human studies with larger numbers of individuals and analysis should include the corresponding ring opened FAPy derivatives.

Inhibition of rat testicular nuclear kinesins (krp2; KIFC5A) by acrylamide as a basis for establishing a genotoxicity threshold

Acrylamide is a toxic substance that induces a variety of cellular responses including neurotoxicity, male reproductive toxicity, tumorigenicity, clastogenicity, and DNA alkylation. Evidence is provided that inhibition of the microtubule motor protein kinesin is responsible for acrylamide-induced clastogenicity and aneuploidy. Two kinesin motors, KIFC5A and KRP2, which are responsible for spindle assembly and disassembly of kinetochore MT, respectively, are inhibited by acrylamide. The inhibitory concentration for a response is below the levels shown to adversely affect the cytogenetic parameters. The relative contribution of these inhibitions compared to DNA alkylation is considered. The implications of inhibition of these kinesins as the site of action of acrylamide with regard to risk assessment are substantial as this event will have a threshold and a safe level of acrylamide can be determined.

Women and lung cancer

OBJECTIVES

There is expanding evidence that lung cancer has a multitude of sex differences. This article will review these differences and discuss nursing implications for women with lung cancer.

DATA SOURCES

Research and journal articles.

CONCLUSION

Understanding the sex differences of lung cancer is essential for individualizing care for women with lung cancer.

IMPLICATIONS FOR NURSING PRACTICE

Nurses play an integral role throughout the trajectory of illness for women with lung cancer. Understanding the sex differences will help nurses to individualize the care for this population to meet their specific needs.

Lung cancer in women: exploring sex differences in susceptibility, biology, and therapeutic response

Src tyrosine kinases regulate a large number of important mechanisms in normal and cancerous cells, are overexpressed in a broad range of tumors including lung cancer, and thus represent a potential target for cancer therapy. Preclinical experiments indicate that small-molecule inhibitors of Src block tumor growth, metastasis, and angiogenesis. Phase I data from healthy volunteers also suggest that inhibitors of Src prevent bone resorption. Several phase II trials with small-molecule inhibitors of Src are under way or have been initiated in lung cancer and in other malignancies, as discussed herein.

Lung cancer in women

Lung cancer is the leading cause of cancer death in the United States and is responsible for 20,000 more deaths yearly in US women than breast cancer. Cigarette smoking is the major cause of lung cancer, and unfortunately, approximately 22 million US women smoke. Mounting evidence suggests that there are significant differences in lung cancer between the sexes. There is a difference in the histologic distribution of lung cancer, with glandular differentiation being more common in women. Genetic variation may account for differences in susceptibility, and hormonal and biologic factors may play a role in carcinogenesis. Lung cancer patients have few therapeutic options. A more thorough understanding of the heterogeneity of lung cancer across populations may lead to innovations in treatment and prevention strategies.

Gender and lung cancer

Accumulating data suggest that the risks for development of lung cancer are different in women compared with men. An increased susceptibility in women to the adverse effects of tobacco may be due to higher levels of DNA adducts, decreased DNA repair capacity, increased frequency of mutations in tumor suppressor genes, and hormonal differences. There are many sex and gender differences in lung cancer presentation, including a greater proportion of adenocarcinoma among women, a greater representation of women in cohorts of younger patients who have lung cancer, and women who do not smoke are more likely to be diagnosed with lung cancer than men. When guidelines for screening, preventive therapies, and treatment options for lung cancer are outlined these differences should be considered.

DNA adducts detected in human gastric mucosa

Human gastrointestinal neoplasms are mostly developed from the mucosa, not from the adjacent muscle layer. DNA adducts in the mucosa and adjacent muscle layer of the non-tumoral part of stomach from 19 patients with gastric neoplasms and from six newborns were analyzed by 32P-postlabeling, and then compared them with those of representative colon or small intestine sample. Five kinds of mucosa-specific DNA adducts (G1-5) were found in all of the adult stomach samples, but were entirely absent from the adjacent muscle layers and from the newborn stomachs. In addition, several common background adducts were also present in both the mucosa and muscle layer. G2 was the same DNA adduct as Si2 in the small intestine and C1 in the colon, and G3 was the same as Si1 in the small intestine. Thus, it was demonstrated that the mucosa of the stomach was exposed to DNA-reactive substances.

Versatile method employing basic techniques of genetic engineering to study the ability of low-molecular-weight compounds to bind covalently with DNA in cell-free systems

Numerous antitumor and carcinogenic compounds and free radicals are able to modify DNA by forming covalent bonds, mainly with nucleophilic centers in nucleobases. Such a binding is usually of utmost importance for the biological outcome. The level of DNA adducts formed by a given agent is in most cases extremely low; hence their detection is very difficult. Here we propose a simple approach, exploiting techniques widely used in genetic engineering, to demonstrate and characterize the covalent modification of a DNA fragment by any low-molecular-weight compound of interest in a cell-free system. The specifically designed, several-hundred-base-pairs-long double-stranded deoxyoligonucleotide (PCR amplified)--subject to modification--includes two restriction sites: one containing only GC base pairs recognized by restriction endonuclease MspI and the other including only AT base pairs recognized by restriction endonuclease Tru1I. The covalent modification of the restriction sites abolishes their recognition and thus cleavage by the endonucleases applied. The formation of DNA adducts is induced by incubating the oligonucleotide with increasing concentrations of a studied compound, in the appropriate activating system if required. Then, the modified oligonucleotide is submitted to digestion by the above-mentioned restriction endonucleases and the DNA fragments are separated by polyacrylamide gel electrophoresis. The inhibition of cleavage indicates the occurrence of covalent modification of the restriction site(s) while simultaneously pointing at the kind of base pairs involved in DNA adduct formation. The validation of the method was performed for two DNA binding antitumor compounds, cisplatin and CC-1065, which form adducts preferentially with guanine and adenine, respectively.

Methods for measuring DNA adducts and abasic sites II: methods for measurement of DNA adducts

This unit contains protocols for analyzing DNA adducts separated from the DNA backbone. HPLC is used to quantify total guanine or ribo- or deoxynucleotides as well as methods for analyzing specific adducts. These methods include HPLC with electrochemical detection, immunoaffininty chromatography to enrich for specific adducts, and gas and liquid chromatography in combination with HPLC and mass spectrometry.

DNA adducts and liver DNA replication in rats during chronic exposure to N-nitrosodimethylamine (NDMA) and their relationships to the dose-dependence of NDMA hepatocarcinogenesis

Exposure of rats to the hepatocarcinogen N-nitrosodimethylamine (NDMA) (0.2-2.64 ppm in the drinking water) for up to 180 days resulted in rapid accumulation of N7- and O6-methylguanine in liver and white blood cell DNA, maximum adduct levels being reached within 1-7 days, depending on the dose. The levels of both adducts remained constant up to treatment day 28, subsequently declining slowly to about 40% of maximal levels for the liver and 60% for white blood cells by day 180. In order to elucidate the role of DNA replication in NDMA hepatocarcinogenesis, changes in liver cell labeling index (LI) were also measured on treatment days 21, 120 and 180. Although the time- and dose-dependence of the observed effects were complex, a clear trend towards increased rates of hepatocyte LI, as indicated by BrdU incorporation, with increasing NDMA doses was evident, particularly above 1 ppm, a concentration above which NDMA hepatocarcinogenicity is known to increase sharply. In contrast, no increase in Kupffer cell DNA replication was found at any of the doses employed, in accordance with the low susceptibility of these cells to NDMA-induced carcinogenesis. No significant increase in the occurrence of necrotic or apoptotic cells was noted under the treatment conditions employed. These results suggest that, in addition to the accumulation of DNA damage, alterations in hepatocyte DNA replication during the chronic NDMA exposure may influence the dose-dependence of its carcinogenic efficacy.

High-performance liquid chromatographic method with electrochemical detection for the analysis of O6-methylguanine

An improved system consisting of a combination of high-performance liquid chromatographic methods with electrochemical detection for the separation and analysis of the DNA adduct O6-methylguanine (O6MG) has been developed. This adduct is produced by the interaction of methylating agents with DNA and induces mispairing in the DNA of the target cells. A good separation of modified from unmodified bases is first achieved with an HPLC system using a Partisil 10 SCX column and a salt gradient. A second HPLC step with electrochemical detection and a C18 column is used for farther separation and quantitation of O6-methylguanine. This method shows a linear response up to 15 pg of 06MG tested. The lowest amount detected was 0.5 pg of O6MG and is highly reproducible. This method is useful to study DNA damage as a product of cellular metabolism and its effects on the process of carcinogenesis.

DNA adduct formation and DNA strand breaks in green-lipped mussels (Perna viridis) exposed to benzo[a]pyrene: dose- and time-dependent relationships

Green-lipped mussels, Perna viridis, were exposed to 0, 0.3, 3 and 30 micrograms l-1 (nominal concentrations) B[a]P under laboratory conditions over a period of 24 days. Mussels were collected on day 0, 1, 3, 6, 12, 18 and 24, and the levels of DNA adducts and DNA strand breaks in their hepatopancreas tissues monitored. Mussels exposed to 0.3 and 3 micrograms l-1 B[a]P showed marked increases in strand breaks after 1 day of exposure. DNA strand break levels in these mussels remained high and significantly different from the control values until day 3 for the 0.3 microgram l-1 treatment group, and day 6 for the 3 micrograms l-1 treatment group. This was followed by a gradual reduction in strand breaks. After 12 days, the levels of both groups had returned to the same level as that of the control. No increase in DNA strand breaks was observable in mussels exposed to 30 micrograms l-1 B[a]P in the first 12 days of exposure, but a significant increase was observed from day 12 to day 24. Increasing B[a]P concentrations resulted in elevated DNA adduct levels after 3-6 days of exposure, but this pattern of dose-related increase disappeared after 12 days. These results indicate that a better understanding of the complex interactions between exposure levels and durations is crucially important before DNA adduct levels and DNA strand breaks in P. viridis can be used as effective biomarkers for monitoring genotoxicants in marine waters.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on initiation and promotion of GST-P-positive foci in rat liver: A quantitative analysis of experimental data using a stochastic model

We use a stochastic model describing initiation and clonal growth of altered cells to analyze data from an initiation-promotion hepatocarcinogenesis experiment in female Wistar rats. Starting at 7 weeks of age, the animals were treated for 10 days with the initiating agent diethylnitrosamine (DEN, 10 mg/kg body wt per day). After a 10-week resting period, the animals were treated either with corn oil or with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via biweekly sc injections of 1.4 microg/kg body wt of TCDD dissolved in corn oil. Groups of four or five animals were euthanized 3, 17, 31, 73, and 115 days after start of TCDD/corn oil treatment. The data analyzed consist of the number and sizes of GST-P-positive focal transections at various time points. By fitting the model to the data, we estimate the rates of initiation, cell division, and cell death during different time periods of the experiment. The model estimates of cell kinetic parameters are consistent with directly made experimental observations of cell division and cell death. The model predicts that DEN-induced initiation of GST-P-positive cells is highly protracted in controls and TCDD-treated animals alike. We also find that TCDD interferes with the normal rate at which cells with (DEN-inflicted) DNA damage are converted into cells expressing the GST-P-positive phenotype, suggesting a TCDD-mediated "acceleration" of the appearance of de novo GST-P-positive initiated cells from damaged precursor cells. Furthermore, the model predicts a significant reduction in the rate of apoptosis within the first 4 to 5 weeks of TCDD treatment, and after 10 weeks of TCDD treatment, but not in between.

Correlation between DNA or protein adducts and benzo[a]pyrene diol epoxide I-triglyceride adduct detected in vitro and in vivo

In this study, we demonstrated the in vitro and in vivo formation of carcinogen-lipid adduct and its correlation with DNA or protein adducts. The lipids from serum or hepatocyte membranes of Sprague-Dawley rats, human serum and standard major lipids were in vitro reacted with benzo[a]pyrene (B[a]P) and B[a]P metabolites. 7, 8-Dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene(BPDE-I), an ultimate carcinogenic form of B[a]P, was covalently bound to triglyceride (TG). BPDE-I-TG adducts isolated by thin-layer chromatography (TLC) were further detected by high-performance liquid chromatography. TGs, including triolein, tripalmitin and tristearin, showed positive reactions with BPDE-I. However, cholesterol, phospholipids (phosphatidylcholine, phosphatidyl-ethanolamine, phosphatidyl-inositol and sphingomyelin) and non-esterified fatty acids (palmitic acid, oleic acid, linoleic acid and stearic acid) did not react with BPDE-I. In addition, other B[a]P metabolites (B[a]P-phenols and -diols) did not react with TG. TG appeared to be the most reactive lipid yet studied with respect to its ability to form an adduct with BPDE-I. There was a clear-cut dose-related formation of [1,3-(3)H]BPDE-I-lipid adducts in vitro between TG and [1,3-(3)H]BPDE-I. In an animal study, BPDE-I-TG was also formed in the serum of rats orally treated with B[a]P (25 mg/rat). Also, obvious correlations between [(3)H]B[a]P related-biomolecule adducts (DNA or protein) or lipid damage and the BPDE-I-TG adducts were obtained in various tissues of mice i.p. treated with [(3)H]B[a]P. These data suggest that TG can form an adduct with BPDE-I, as do other macromolecules (DNA, RNA and protein). Therefore, a carcinogen-lipid adduct would be a useful biomarker for chemical carcinogenesis research and cancer risk assessment.

Quantitative determination of DNA adducts using liquid chromatography/electrospray ionization mass spectrometry and liquid chromatography/high-resolution inductively coupled plasma mass spectrometry

The quantitative determination of nucleotides from DNA modified by styrene oxide is described using a combination of inductively coupled plasma high-resolution mass spectrometry (ICP-HRMS) and electrospray ionization mass spectrometry (ESI-MS), both interfaced to reversed-phase high-performance liquid chromatography (HPLC). LC/ICP-MS (resolution > 1500 to discriminate against 15N16O+ and 14N16OH+) was employed to determine quantitatively the content of modified nucleotides in standard solutions based on the signal of phosphorus; phosphoric acid served as an internal standard. By means of the standard addition technique the sensitivity of the LC/ESI-MS approach was subsequently determined. Since a comparison of UV, ICP and ESI-MS data suggested that in ESI-MS the ionization efficiency of the adducts is identical within the error limits, quantitative determination of all adducts is possible. For LC/ESI-MS with single ion monitoring, the detection limit for styrene oxide adducts of nucleotides was determined to be 20 pg absolute or 14 modified in 10(8) unmodified nucleotides in a 5 micrograms DNA sample, which comes close to the best methods available for the detection of chemical modifications in DNA.

In vivo and in vitro antigenotoxic effect of nordihydroguaiaretic acid against SCEs induced by methyl methanesulfonate

Nordihydroguaiaretic acid (NDGA) is a phenolic lignan which has shown to cause a variety of actions potentially useful for human health; therefore, in this investigation we determined its capacity for inhibiting the rate of sister chromatid exchanges (SCEs) induced by methyl methanesulfonate (MMS). We tested the effect of 0.25, 0.50, 1.0, and 2.0 microM of NDGA on the damage exerted by 55 microM of MMS. Cultured human lymphocytes from two female donors were used for the experiment. The best result concerning its modulatory action was obtained with 1.0 microM of NDGA; with this dose the mean inhibitory index including both donors reached 68.2%. The values obtained for the mitotic and proliferative indexes were not significantly modified with respect to the basal data. We also used the mouse bone marrow in vivo system to evaluate the inhibitory effect of the chemical. In this study we tested 1.0, 6.0, and 11.0 mg/kg of NDGA intraperitoneally (i.p.) administered 1 h before an i.p. injection of MMS (40 mg/kg). The best inhibitory index in this model corresponded to the dose of 11 mg/kg of NDGA (86.9%). The mitotic index and the average generation time showed no significant variation with respect to the control data. Our study established that NDGA produces antigenotoxic action in mammalian cells in vitro and in vivo.

A carcinogenesis model describing mutational events at the DNA adduct level

A stochastic carcinogenesis model is proposed to describe a sequence of component mutational changes that constitute the G:C-->A:T base substitution. This paper provides the biological basis and mathematical formulation underlying the proposed model. In addition, the paper elaborates on a numerical approach for studying the cumulant functions, survival functions, and hazard functions of the model. Several numerical examples are given of potential applications of the model.

DNA adduct formation by hormonal steroids in vitro

We examined the binding of various steroid hormones to DNA in vitro by means of 32P-postlabeling. Seventeen steroid hormones and cholesterol (CS) were incubated with human liver DNA at 37 degrees C for 1 h under aerobic conditions in the absence of catalysis. The reaction mixtures were analyzed by the nuclease P-1 version of 32P-postlabeling. The results showed that cortexolone (CX), prednisolone (PS), cortisone (CN), cortisol (CL), tetrahydrocortisol (TC), corticosterone (CC), 11-deoxycorticosterone (DC), dexamethasone (DX), dihydrocortisol (DL), and aldosterone (AL) covalently bound with DNA. However, progesterone (PG), 17 alpha-hydroxyprogesterone (HG), estrone (E1), estradiol (E2), estriol (E3), testosterone (TS), cortol (CR) and the original compound for biosynthesis, CS, did not form adducts. In absence of DNA, the steroids themselves did not give rise to any spot on TLC under the same conditions. The dose-responses of DNA binding by DC, DL, CC, CL and CN were linear. The relative adduct labeling of reactive steroids at a concentration of 2 mM were as follows: 68.8 (CX), 53.2 (PS), 39.6 (CN), 29.9 (CL), 20.9 (TC), 12.9 (CC), 12.3 (DC), 7.5 (DX), 4.7 (DL), 1.2 (AL) adducts per 10(8) nucleotides. Reactive and nonreactive steroids were distinguishable by the presence or absence of the carbonyl group (-CO-CH2OH) at carbon seventeen (C17) of the cholesterol skeleton. This implies that the electrophilic carbonyl or a neighboring group perhaps involved in the formation of covalent bond with DNA. To investigate the nature of target base(s) of these DNA reactive steroids, mononucleotides of all four bases of DNA were reacted with CN, CL, CC and cochromatographed with the obtained spots of DNA reactions. The results of which stated that these steroids and guanine reaction gave the same spots as observed in DNA reaction, indicating guanine is the main target of these DNA reactive steroids. Hep G2 human hepatocellular carcinoma cells were used as an alternative model. Although nine steroids (CL, DL, TC, PS, DX, PG, E2, TX, CR) did not react with intracellular DNA under our experimental conditions, our findings suggested that some hormonal steroids can form covalent DNA adducts in vivo.

DNA damage and repair in somatic and germ cells in vivo

Alkylation-induced germ cell mutagenesis in the mouse versus Drosophila is compared based on data from forward mutation assays (specific-locus tests in the mouse and in Drosophila and multiple-locus assays in the latter species) but not including assays for structural chromosome aberrations. To facilitate comparisons between mouse and Drosophila, forward mutation test results have been grouped into three categories. Representatives of the first category are MMS (methyl methanesulfonate) and EO (ethylene oxide), alkylating agents with a high s value which predominantly react with ring nitrogens in DNA. ENU (N-ethyl-N-nitrosourea), MNU (N-methyl-N-nitrosourea), PRC (procarbazine), DEN (N-nitrosodiethylamine), and DMN (N-nitrosodimethylamine) belong to the second category. These agents have in common a considerable ability for modification at oxygens in DNA. Cross-linking agents (melphalan, chlorambucil, hexamethylphosphoramide) form the third category. The most unexpected, but encouraging outcome of this study is the identification of common features for three vastly different experimental indicators of genotoxicity: hereditary damage in Drosophila males, genetic damage in male mice, and tumors (TD50 estimates) in rodents. Based on the above three category classification scheme the following tentative conclusions are drawn. Monofunctional agents belonging to category 1, typified by MMS and EO, display genotoxic effects in male germ cell stages that have passed meiotic division. This phenomenon seems to be the consequence of a repair deficiency during spermiogenesis for a period of 3-4 days in Drosophila and 14 days in the mouse. We suggest that the reason for the high resistance of premeiotic stages, and the generally high TD50 estimates observed for this class in rodents, is the efficient error-free repair of N-alkylation damage. If we accept this hypothesis, then the increased carcinogenic potential in rodents, seen when comparing category 2 (ENU-type mutagens) to category 1 (MMS-type mutagens), along with the ability of category 2 genotoxins to induce genetic damage in premeiotic stages, must presumably be due to their enhanced ability for alkylations at oxygens in DNA; it is this property that actually distinguishes the two groups from each other. In contrast to category 1, examination of class 2 genotoxins (ENU and DEN) in premeiotic cells of Drosophila gave no indication for a significant role of germinal selection, and also removal by DNA repair was less dramatic compared to MMS. Thus category 2 mutagens are expected to display activity in a wide range of both post- and premeiotic germ cell stages. A number of these agents have been demonstrated to be among the most potent carcinogens in rodents.(ABSTRACT TRUNCATED AT 400 WORDS)

Tissue distribution of DNA adducts in pre-weanling BLU:Ha mice treated with a tumorigenic dose of fluoranthene

Fluoranthene (FA), a ubiquitous atmospheric pollutant, is tumorigenic to the lung when injected into BLU:Ha mice 1, 8, and 15 days after birth. DNA adducts were measured by a modified HPLC-32P-postlabeling method in target (lung) and non-target (liver, kidney, spleen plus thymus) organs of 16-day-old mice 24 h after the last treatment with a tumorigenic dose (3.5 mg/mouse) of FA. The anti-FADE adduct was the major DNA adduct and was present at levels ranging from 3.5 to 37 adducts per 10(8) nucleotides (106-1138 fmols/mg DNA) in all DNA samples from the organs of FA-treated mice. The lung contained anti-FADE adduct levels approximately 2, 2.5 and 5 times higher than those in kidney, liver, and spleen plus thymus, respectively. There was no significant difference between anti-FADE adduct levels in liver and kidney; however, adduct levels were significantly higher than those in spleen plus thymus. Significant variation was found among litters in anti-FADE adduct levels from lung, liver, and kidney, but not from spleen plus thymus. An unidentified peak was present in the adduct profile of the kidneys of treated but not control animals. Three additional unidentified radioactive peaks were present in adduct profiles of DNA from tissues of both treated and control animals, but their levels were significantly higher in the tissues of treated animals than in controls. Possible correlations between tumorigenic response and DNA adduct formation are discussed.

Prediction and monitoring of the carcinogenicity of polycyclic aromatic compounds (PACs)

Chemical carcinogenesis is a multistage process that includes initiation, promotion, and progression. Some carcinogenic PACs have been shown to activate proto-oncogenes and deactivate tumor-suppression genes in the carcinogenic process. The function of DNA repair processes appears to be changed in some cases by PACs. Many PACs are well known for their carcinogenic activity, but for this activity to be exerted, metabolic activation by microsomal enzymes must occur. The enzyme system responsible for PAC activation is the mixed-function oxidase system and, in particular, cytochrome P-450. In the case of PAHs, oxidation predominantly produces reactive diol-epoxides that can then be converted to carbonium ions as the reactive electrophiles that can then covalently bind to DNA. Regions of high activity exist in PAHs, namely, the "bay," "K," and "L" regions which are associated with pi electron distribution. The diol-epoxides can exist in either syn or anti forms, each of which has two enantiomers producing four stereoisomers in all. Energy considerations favor the formation of the anti form. Nitrogen-containing PACs can be metabolically activated in a manner similar to that for PAHs, or the nitrogen atom can be oxidized to form hydroxylamines. These reactive electrophiles can then form covalently bound DNA adducts. The monitoring of DNA adducts has been used in risk assessment for human exposure to PACs. This form of biomonitoring has advantages over the monitoring of external exposure or body levels of the chemicals in question. In the case of PACs, binding to DNA is an important step in the multistage carcinogenic process. The estimation of DNA adducts has been used in the monitoring of humans exposed to PAHs in a wide range of industrial situations. Recent research has shown a dose-response relationship between PAH adduct levels and human cancer, thus developing molecular epidemiology as a relevant science for the field of risk assessment. Techniques have been developed for the determination of DNA adducts and these include immunochemical, fluorescence spectroscopic, GC-MS, and 32P-postlabeling methods. The 32P-postlabeling assay is by far the most sensitive, with limits of detection being of the order of one adduct in 10(10) normal nucleotides. The use of HPLC for separation of adducted nucleotides in this postlabeling assay is becoming more common and gives better resolution of adducts than does the TLC technique used in the traditional assay. The detection of adducts on hemoglobin and other proteins has been used as a surrogate for DNA adduct estimation.(ABSTRACT TRUNCATED AT 400 WORDS)

Absence of formation of benzo[a]pyrene/DNA adducts in the cuttlefish (Sepia officinalis, Mollusca: Cephalopoda)

Benzo[a]pyrene (B[a]P) injected intramuscularly into the base of the arms of cuttlefish was released continuously from the injection site and removed from the organism. Only a portion of the compound accumulated in the body. Twenty-four hr after its injection, 75% of B[a]P applied in olive oil was removed from the cuttlefish, and 1.2% was found in the body outside the head, the site of injection. If the carcinogen was dissolved in dimethylformamide, the removal of B[a]P was slower, so that only 18% of the injected B[a]P was removed from the organism and 0.36% accumulated in the body outside the head 24 hr after injection. The high level of B[a]P in gills and hemolymph 4 hr after injection and the kinetics of the decrease of its concentration with time indicate that these two organs could be involved in the excretion of B[a]P from the body. The B[a]P/DNA adducts characteristic for vertebrates could not be demonstrated in gills, skin, brain, hepatopancreas, and lymphocytes of the cuttlefish 24 hr after injection of B[a]P. The dose of the carcinogen injected into the cuttlefish was 2-4 times higher than the dose resulting in the formation of a high level of B[a]P/DNA adducts in the vertebrates. A different metabolism of B[a]P in the tissue of cephalopods, compared to vertebrates, could be less favorable to the process leading to malignant transformation and could explain the absence from the literature of reports of tumors in cephalopods.

32P-postlabeling analysis of DNA adduct formation and persistence in English sole (Pleuronectes vetulus) exposed to benzo[a]pyrene and 7H-dibenzo[c,g]carbazole

The formation and persistence of benzo[a]pyrene (BaP)- and 7H-dibenzo[c,g]-carbazole (DBC)-DNA adducts in liver of English sole (Pleuronectes vetulus) were investigated. BaP is a putative hepatocarcinogen in English sole based on its ability to induce formation of preneoplastic foci, while DBC is a hepatocarcinogen in mammals but whose carcinogenicity in fish is not known. English sole liver was sampled from 2 h through 84 days after a single intermuscular injection of a BaP and DBC mixture (100 mumol of each/kg body wt.), and DNA adduct levels were measured by the nuclease P1 version of the 32P-postlabeling assay. The major BaP adducts detected were from binding of BaP-7,8-diol-9,10-epoxide to DNA, whereas multiple uncharacterized DBC-DNA adducts were detected. Total adduct levels for both BaP and DBC reached a maximum at 2 days post exposure. The levels of DBC-DNA adducts were greater than the levels of BaP adducts at all time points and increased more rapidly than did the levels of BaP-DNA adducts. The DBC to BaP adduct ratio was 33 +/- 8.8 at 2 h and declined to 4.2 +/- 0.48 by 12 h post exposure. From 2 to 28 days, the levels of both BaP and DBC adducts declined with apparent half-lives of 11 and 13 days, respectively. There was no apparent decline from 28 to 84 days in the levels of the remaining BaP or DBC adducts; these persistent adducts represented 32 and 36% of maximum levels, respectively. These results provide the first data on the kinetics of adduct formation and removal of a carcinogenic nitrogen-containing polycyclic aromatic compound in fish. The results showing greater binding and similar persistence of DBC-DNA adducts compared to BaP-DNA adducts suggest that DBC may be hepatotoxic and potentially carcinogenic in English sole. In a separate experiment, the effect of multiple doses of BaP (30 mumol/kg body wt.) on the levels of hepatic BaP-DNA adducts showed that adduct levels increased linearly (r = 0.815, P = 0.0007) with 5 successive doses administered at 2 day-intervals and sampled 2 days after the last dose. The persistence of both BaP-DNA and DBC-DNA adducts in liver, together with the increase in BaP-DNA adducts in English sole exposed to successive doses of BaP, suggest that hepatic xenobiotic-DNA adducts in English sole are molecular dosimeters of relatively longterm environmental exposure to genotoxic polycyclic aromatic compounds.

Differences in metabolic activation of dibenzo[a,e]fluoranthene characterized by 32P-postlabeling in two mouse fibroblast models

The formation of DNA adducts was investigated in mouse fibroblasts from two different tissues--embryos and adult lung--after incubation with dibenzo[a,e]fluoranthene (DBF) or its major proximate metabolites. The nuclease P1 modification of the 32P-postlabeling method was adapted for detection of DBF-DNA adducts. Quantitative and qualitative differences were observed in the metabolic activation mediated by the two cell types. DBF-DNA adducts generated three major spots reproducibly, and more than ten spots of medium or weak importance. The highest level of DNA binding occurred via the DBF-bay region vicinal dihydrodiol epoxide but with significant differences in the quantitative distribution of adducts. Striking qualitative differences were observed when lung fibroblasts were incubated with the DBF-pseudo bay region dihydrodiol (DBF-12,13-DHD). The spots representing adducts induced in embryo fibroblasts by DBF-3OH-12,13-DHD, a further metabolite of DBF-12,13-DHD, were totally absent from chromatograms of lung cells. These results show that both embryo and lung fibroblasts can activate DBF but that different cytochrome P-450 forms and substrate affinities are involved. The finding that different activation systems may be present in subcategories of the same tissue, may provide a partial explanation for the wide variations in sensitivity to carcinogens among species, organs and tissues.

Embryotoxicity induced by alkylating agents: 6. DNA adduct formation induced by methylnitrosourea in mouse embryos

Formation of DNA adducts in 11-day-old mouse embryos was studied by measuring the initial alkylation rates of the methylated purine bases 7-methylguanine, O6-methylguanine, and 3-methyladenine. In the first part of the studies the adduct rates were measured in the teratogenic dose range (ED10-ED90, 2.7-5.6 mg/kg). These results were compared with similar data obtained from studies with ethylmethanesulfonate and acetoxymethyl-methylnitrosamine. For the three investigated substances a correlation was found between the initial adduct rate of O6-alkylguanine in the DNA of the embryos and the teratogenic potency. In the second part of the study the rate of adduct formation was measured in the sub-teratogenic dose range. These data will be used for molecular dosimetry in a risk assessment of low doses.

Macromolecular adducts of ethylene oxide: a literature review and a time-course study on the formation of 7-(2-hydroxyethyl)guanine following exposures of rats by inhalation

The results of efforts to identify and quantify macromolecular adducts of ethylene oxide (ETO), to determine the source and significance of background levels of these adducts, and to generate molecular dosimetry data on these adducts are reviewed. A time-course study was conducted to investigate the formation and persistence of 7-(2-hydroxyethyl)guanine (7-HEG; Fig. 1) in various tissues of rats exposed to ETO by inhalation, providing information necessary for designing investigations on the molecular dosimetry of adducts of ETO. Male F344 rats were exposed 6 h/day for up to 4 weeks (5 days/wk) to 300 ppm ETO by inhalation. Another set of rats was exposed for 4 weeks to 300 ppm ETO, and then killed 1-10 days after cessation of exposures. DNA samples from control and treated rats were analyzed for 7-HEG using neutral thermal hydrolysis, HPLC separation, and fluorescence detection. The adduct was detectable in all tissues of treated rats following 1 day of ETO exposure and increased approximately linearly for 3-5 days before the rate of increase began to level off. Concentrations of 7-HEG were greatest in brain, but the extent of formation was similar in all tissues studied. The adduct disappeared slowly from DNA, with an apparent half-life of approx. 7 days. The shape of the formation curve and the in vivo half-life indicate that 7-HEG will approach steady-state concentrations in rat DNA by 28 days of ETO exposure. The similarity in 7-HEG formation in target and nontarget tissues indicates that the tissue specificity for tumor induction is due to factors in addition to DNA-adduct formation.

Distribution of methyl and ethyl adducts following alkylation with monofunctional alkylating agents

Alkylating agents, because of their ability to react directly with DNA either in vitro or in vivo, or following metabolic activation as in the case of the dialkylnitrosamines, have been used extensively in studying the mechanisms of mutagenicity and carcinogenicity. Their occurrence is widespread in the environment and human exposure from natural and pollutant sources is universal. Since most of these chemicals show varying degrees of both carcinogenicity and mutagenicity, and exhibit compound-specific binding patterns, they provide an excellent model for studying molecular dosimetry. Molecular dosimetry defines dose as the number of adducts bound per macromolecule and relates the binding of these adducts to the human mutagenic or carcinogenic response. This review complies DNA alkylation data for both methylating and ethylating agents in a variety of systems and discusses the role these alkylation products plays in molecular mutagenesis.

Long-term persistence of DNA alkylation in hamster tissues after N-nitrosobis(2-oxopropyl)amine

The persistence of 7- and O6-alkylation of guanine in DNA of cell nuclei of male Syrian hamster pancreas, liver, kidneys, lungs [target tissues of N-nitrosobis(2-oxopropyl)amine (BOP)] and salivary glands (nontarget tissue) was studied immunocytochemically 6 h, 1, 3, 7, 14, 28, and 56 days after a single s.c. injection of 20 mg BOP/kg. Conventional antisera raised against O6-methylguanine and imidazole-ring-opened 7-methyl-guanine were used. Persistent alkyl-specific staining was observed for up to 7 days (7-alkylguanine) or 56 days (O6-alkylguanine) in inter- and intralobular duct cells and centro-acinar cells of the pancreas, periportal hepatocytes and bile duct cells of the liver, cells of the proximal convoluted tubules of the renal cortex, and bronchiolar Clara and alveolar cells in the lungs. Both adducts disappeared from centrilobular liver cells within 1 day, from pancreatic acinar cells within 3 days, and from ducts and acini of the submandibular salivary glands within 14 days after BOP treatment. A high level of persistent O6-alkylation of guanine was related with a high tumor incidence only in case of the ductal/ductular system of the pancreas, the main target tissue of BOP-induced carcinogenesis. The relatively weak carcinogenicity of BOP in other tissues with long-term persistence of O6-alkylguanine in DNA indicates that the formation and persistence of DNA alkylation are not sufficient to account for the carcinogenic organotropism of BOP. Additional factors, such as cell proliferation, appropriate promoting stimuli and the (onco)genes critically involved, may be as important as the modification of DNA.

Quantitation of methylated hemoglobin via hydrolysis of methyl esters to yield methanol

Methyl esters formed by methylation of carboxylic acid groups on hemoglobin can be quantitated via GC/MS negative-ion chemical ionization analysis of the pentafluorobenzoate derivative of the methanol released from the hemoglobin upon base hydrolysis. Although the method requires extensive cleanup of extracting solvents because of the ubiquitous presence of trace levels of methanol, it is possible to quantitate adducted hemoglobin at levels as low as 4 ng per gram of hemoglobin using [13C, 2H3]methyl alcohol as the internal standard. In principle this method can be used in humans as a dosimeter for exposure to environmental methylating or alkylating agents.

Role of carcinogen-modified deoxynucleotide precursors in mutagenesis

Agents which damage or modify cellular DNA will generally also modify the nucleotide precursor pools, sometimes preferentially (Topal and Baker, 1982). There are at least two different ways that incorporation of modified (possibly promutagenic) nucleotides could, theoretically, make a significant contribution to the mutations induced by these agents. Modified bases may exhibit ambiguous base pairing and produce mutations during normal replication or they may induce secondary mutations as a result of processing subsequent to incorporation. There are important precedents for such possibilities. Classical studies on mutagenesis with prototype mutagens like 2-aminopurine (2-AP) and 5-bromouracil clearly show that mutations can occur by incorporation of deoxynucleotides of tautomeric or ionized (Sowers et al., 1987) bases into newly synthesized DNA (Ronen, 1979; Lasken and Goodman, 1984, Coulondre and Miller, 1977). 5-Hydroxymethyl-2'-deoxyuridine (HMdU), a product of oxidative DNA damage, can also be (re)incorporated into cellular DNA with both toxic and mutagenic consequences (Kaufman, 1987; Shirname-More et al., 1987). Furthermore, modified nucleotides may alter the pool sizes of the normal nucleotides and indirectly produce toxic and mutagenic effects. However, these effects are generally seen at high, nonphysiological, concentrations of the modified precursors and may not be relevant under physiological conditions. The relative importance of modified deoxynucleotide precursors in the production of mutations by alkylating and oxidative DNA-damaging agents is discussed.