Chemical carcinogenesis in the nervous system. Preferential accumulation of O6-methylguanine in rat brain deoxyribonucleic acid during repetitive administration of N-methyl-N-nitrosourea

From cultivation to cancer: formation of N-nitrosamines and other carcinogens in smokeless tobacco and their mutagenic implications

Tobacco use is a major cause of preventable morbidity and mortality globally. Tobacco products, including smokeless tobacco (ST), generally contain tobacco-specific N-nitrosamines (TSNAs), such as N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-butanone (NNK), which are potent carcinogens that cause mutations in critical genes in human DNA. This review covers the series of biochemical and chemical transformations, related to TSNAs, leading from tobacco cultivation to cancer initiation. A key aim of this review is to provide a greater understanding of TSNAs: their precursors, the microbial and chemical mechanisms that contribute to their formation in ST, their mutagenicity leading to cancer due to ST use, and potential means of lowering TSNA levels in tobacco products. TSNAs are not present in harvested tobacco but can form due to nitrosating agents reacting with tobacco alkaloids present in tobacco during certain types of curing. TSNAs can also form during or following ST production when certain microorganisms perform nitrate metabolism, with dissimilatory nitrate reductases converting nitrate to nitrite that is then released into tobacco and reacts chemically with tobacco alkaloids. When ST usage occurs, TSNAs are absorbed and metabolized to reactive compounds that form DNA adducts leading to mutations in critical target genes, including the RAS oncogenes and the p53 tumor suppressor gene. DNA repair mechanisms remove most adducts induced by carcinogens, thus preventing many but not all mutations. Lastly, because TSNAs and other agents cause cancer, previously documented strategies for lowering their levels in ST products are discussed, including using tobacco with lower nornicotine levels, pasteurization and other means of eliminating microorganisms, omitting fermentation and fire-curing, refrigerating ST products, and including nitrite scavenging chemicals as ST ingredients.

Risk assessment of N-nitrosamines in food

EFSA was asked for a scientific opinion on the risks to public health related to the presence of N-nitrosamines (N-NAs) in food. The risk assessment was confined to those 10 carcinogenic N-NAs occurring in food (TCNAs), i.e. NDMA, NMEA, NDEA, NDPA, NDBA, NMA, NSAR, NMOR, NPIP and NPYR. N-NAs are genotoxic and induce liver tumours in rodents. The in vivo data available to derive potency factors are limited, and therefore, equal potency of TCNAs was assumed. The lower confidence limit of the benchmark dose at 10% (BMDL10) was 10 μg/kg body weight (bw) per day, derived from the incidence of rat liver tumours (benign and malignant) induced by NDEA and used in a margin of exposure (MOE) approach. Analytical results on the occurrence of N-NAs were extracted from the EFSA occurrence database (n = 2,817) and the literature (n = 4,003). Occurrence data were available for five food categories across TCNAs. Dietary exposure was assessed for two scenarios, excluding (scenario 1) and including (scenario 2) cooked unprocessed meat and fish. TCNAs exposure ranged from 0 to 208.9 ng/kg bw per day across surveys, age groups and scenarios. 'Meat and meat products' is the main food category contributing to TCNA exposure. MOEs ranged from 3,337 to 48 at the P95 exposure excluding some infant surveys with P95 exposure equal to zero. Two major uncertainties were (i) the high number of left censored data and (ii) the lack of data on important food categories. The CONTAM Panel concluded that the MOE for TCNAs at the P95 exposure is highly likely (98-100% certain) to be less than 10,000 for all age groups, which raises a health concern.

Metabolic Activation and DNA Interactions of Carcinogenic N-Nitrosamines to Which Humans Are Commonly Exposed

Carcinogenic N-nitrosamine contamination in certain drugs has recently caused great concern and the attention of regulatory agencies. These carcinogens-widely detectable in relatively low levels in food, water, cosmetics, and drugs-are well-established and powerful animal carcinogens. The electrophiles resulting from the cytochrome P450-mediated metabolism of N-nitrosamines can readily react with DNA and form covalent addition products (DNA adducts) that play a central role in carcinogenesis if not repaired. In this review, we aim to provide a comprehensive and updated review of progress on the metabolic activation and DNA interactions of 10 carcinogenic N-nitrosamines to which humans are commonly exposed. Certain DNA adducts such as O⁶-methylguanine with established miscoding properties play central roles in the cancer induction process, whereas others have been linked to the high incidence of certain types of cancers. We hope the data summarized here will help researchers gain a better understanding of the bioactivation and DNA interactions of these 10 carcinogenic N-nitrosamines and facilitate further research on their toxicologic and carcinogenic properties.

Genotoxic Damage During Brain Development Presages Prototypical Neurodegenerative Disease

Western Pacific Amyotrophic Lateral Sclerosis and Parkinsonism-Dementia Complex (ALS/PDC) is a disappearing prototypical neurodegenerative disorder (tau-dominated polyproteinopathy) linked with prior exposure to phytogenotoxins in cycad seed used for medicine and/or food. The principal cycad genotoxin, methylazoxymethanol (MAM), forms reactive carbon-centered ions that alkylate nucleic acids in fetal rodent brain and, depending on the timing of systemic administration, induces persistent developmental abnormalities of the cortex, hippocampus, cerebellum, and retina. Whereas administration of MAM prenatally or postnatally can produce animal models of epilepsy, schizophrenia or ataxia, administration to adult animals produces little effect on brain structure or function. The neurotoxic effects of MAM administered to rats during cortical brain development (specifically, gestation day 17) are used to model the histological, neurophysiological and behavioral deficits of human schizophrenia, a condition that may precede or follow clinical onset of motor neuron disease in subjects with sporadic ALS and ALS/PDC. While studies of migrants to and from communities impacted by ALS/PDC indicate the degenerative brain disorder may be acquired in juvenile and adult life, a proportion of indigenous cases shows neurodevelopmental aberrations in the cerebellum and retina consistent with MAM exposure in utero. MAM induces specific patterns of DNA damage and repair that associate with increased tau expression in primary rat neuronal cultures and with brain transcriptional changes that parallel those associated with human ALS and Alzheimer's disease. We examine MAM in relation to neurodevelopment, epigenetic modification, DNA damage/replicative stress, genomic instability, somatic mutation, cell-cycle reentry and cellular senescence. Since the majority of neurodegenerative disease lacks a solely inherited genetic basis, research is needed to explore the hypothesis that early-life exposure to genotoxic agents may trigger or promote molecular events that culminate in neurodegeneration.

Role of Hydrazine-Related Chemicals in Cancer and Neurodegenerative Disease

Hydrazine-related chemicals (HRCs) with carcinogenic and neurotoxic potential are found in certain mushrooms and plants used for food and in products employed in various industries, including aerospace. Their propensity to induce DNA damage (mostly O⁶-, N⁷- and 8-oxo-guanine lesions) resulting in multiple downstream effects is linked with both cancer and neurological disease. For cycling cells, unrepaired DNA damage leads to mutation and uncontrolled mitosis. By contrast, postmitotic neurons attempt to re-enter the cell cycle but undergo apoptosis or nonapoptotic cell death. Biomarkers of exposure to HRCs can be used to explore whether these substances are risk factors for sporadic amyotrophic laterals sclerosis and other noninherited neurodegenerative diseases, which is the focus of this paper.

Differential exposure to N-ethyl N-nitrosourea during pregnancy is relevant to the induction of glioma and PNSTs in the brain

Exposure to N-nitroso compounds (NOCs) during pregnancy has been associated with an increase in brain tumors in the progeny. This study investigated the brain tumorigenic effect of N-ethyl N-nitrosourea (ENU) after differential exposure of rats during pregnancy. Sprague Dawley rats were exposed to a single dose of ENU (80 mg/kg) in three different circumstances: 1) at first, second or third week of gestation; 2) at the 15th embryonic day (E15) in consecutive litters and 3) at E15 in three successive generations. Location and characterization of the offspring's brain tumors were performed by magnetic resonance imaging and histopathological studies. Finally, tumor incidence and latency and the animals' survival were recorded. ENU-exposure in the last two weeks of pregnancy induced intracranial tumors in over 70% of the offspring rats, these being mainly gliomas with some peripheral nerve sheath tumors (PNSTs). Tumors appeared in young adults; glioma-like small multifocal neoplasias converged on large glioblastomas in senescence and PNSTs in the sheath of the trigeminal nerve, extending to cover the brain convexity. ENU-exposure at E15 in subsequent pregnancies lead to an increase in glioma and PNST incidence. However, consecutive generational ENU-exposure (E15) decreased the animals' survival due to an early onset of both types of tumors. Moreover, PNST presented an inheritable component because progeny, which were not themselves exposed to ENU but whose progenitors were, developed PNSTs. Our results suggest that repeated exposure to ENU later in pregnancy and in successive generations favours the development of intracranial gliomas and PNSTs in the offspring.

Initiation/promotion bioassay in rat liver: use of gamma glutamyltranspeptidase-positive foci to indicate carcinogenic activity

Gamma Glutamyltranspeptidase (GGTase)-positive foci have been used to indicate activity in an initiation/promotion bioassay in rat liver. This rat liver foci bioassay has been proposed for inclusion in tier 2 of a three tier decision tree approach to carcinogenesis testing where it would function to confirm carcinogenic activity. The assay was sensitive to hepatocarcinogens and some non-hepatocarcinogens and was able to distinguish between tumor initiators and tumor promoters. The induction of GGTase-positive foci by methylating agents was associated with the formation of O6-methylguanine and not N-7 methylguanine, which would indicate a mutagenic origin for the foci. The foci once induced did not regress over the life time of the animal. Zonal induction of GGTase activity was induced by some promoters which confounded the scoring of foci incidence. The results to date indicate that the rat liver foci bioassay warrants further validation for inclusion in tier 2 and emphasizes the need to demonstrate the predictive and precursor relationship of GGTase-positive foci to cancer.

Chemical biology of mutagenesis and DNA repair: cellular responses to DNA alkylation

The reaction of DNA-damaging agents with the genome results in a plethora of lesions, commonly referred to as adducts. Adducts may cause DNA to mutate, they may represent the chemical precursors of lethal events and they can disrupt expression of genes. Determination of which adduct is responsible for each of these biological endpoints is difficult, but this task has been accomplished for some carcinogenic DNA-damaging agents. Here, we describe the respective contributions of specific DNA lesions to the biological effects of low molecular weight alkylating agents.

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.

Genetic pathways to primary and secondary glioblastoma

Glioblastoma is the most frequent and most malignant human brain tumor. The prognosis remains very poor, with most patients dying within 1 year after diagnosis. Primary and secondary glioblastoma constitute distinct disease subtypes, affecting patients of different age and developing through different genetic pathways. The majority of cases (>90%) are primary glioblastomas that develop rapidly de novo, without clinical or histological evidence of a less malignant precursor lesion. They affect mainly the elderly and are genetically characterized by loss of heterozygosity 10q (70% of cases), EGFR amplification (36%), p16(INK4a) deletion (31%), and PTEN mutations (25%). Secondary glioblastomas develop through progression from low-grade diffuse astrocytoma or anaplastic astrocytoma and manifest in younger patients. In the pathway to secondary glioblastoma, TP53 mutations are the most frequent and earliest detectable genetic alteration, already present in 60% of precursor low-grade astrocytomas. The mutation pattern is characterized by frequent G:C-->A:T mutations at CpG sites. During progression to glioblastoma, additional mutations accumulate, including loss of heterozygosity 10q25-qter ( approximately 70%), which is the most frequent genetic alteration in both primary and secondary glioblastomas. Primary and secondary glioblastomas also differ significantly in their pattern of promoter methylation and in expression profiles at RNA and protein levels. This has significant implications, particularly for the development of novel, targeted therapies, as discussed in this review.

MGMT: key node in the battle against genotoxicity, carcinogenicity and apoptosis induced by alkylating agents

O(6)-methylguanine-DNA methyltransferase (MGMT) plays a crucial role in the defense against alkylating agents that generate, among other lesions, O(6)-alkylguanine in DNA (collectively termed O(6)-alkylating agents [O(6)AA]). The defense is highly important, since O(6)AA are common environmental carcinogens, are formed endogenously during normal cellular metabolism and possibly inflammation, and are being used in cancer therapy. O(6)AA induced DNA damage is subject to repair, which is executed by MGMT, AlkB homologous proteins (ABH) and base excision repair (BER). Although this review focuses on MGMT, the mechanism of repair by ABH and BER will also be discussed. Experimental systems, in which MGMT has been modulated, revealed that O(6)-methylguanine (O(6)MeG) and O(6)-chloroethylguanine are major mutagenic, carcinogenic, recombinogenic, clastogenic and killing lesions. O(6)MeG-induced clastogenicity and cell death require MutS alpha-dependent mismatch repair (MMR), whereas O(6)-chloroethylguanine-induced killing occurs independently of MMR. Extensive DNA replication is required for O(6)MeG to provoke cytotoxicity. In MGMT depleted cells, O(6)MeG induces apoptosis almost exclusively, barely any necrosis, which is presumably due to the remarkable ability of secondarily formed DNA double-strand breaks (DSBs) to trigger apoptosis via ATM/ATR, Chk1, Chk2, p53 and p73. Depending on the cellular background, O(6)MeG activates both the death receptor and the mitochondrial apoptotic pathway. The inter-individual expression of MGMT in human lymphocytes is highly variable. Given the key role of MGMT in cellular defense, determination of MGMT activity could be useful for assessing a patient's drug sensitivity. MGMT is expressed at highly variable amounts in human tumors. In gliomas, a correlation was found between MGMT activity, MGMT promoter methylation and response to O(6)AA. Although the human MGMT gene is inducible by glucocorticoids and genotoxins such as radiation and alkylating agents, the role of this induction in the protection against carcinogens and the development of chemotherapeutic alkylating drug resistance are still unclear. Modulation of MGMT expression in tumors and normal tissue is currently being investigated as a possible strategy for improving cancer therapy.

Development of a novel site-specific mutagenesis assay using MALDI-ToF MS (SSMA-MS)

We have developed and validated a novel site-specific mutagenesis assay, termed SSMA-MS, which incorporates MALDI-ToF mass spectrometry (MALDI-MS) analysis as a means of determining the mutations induced by a single DNA adduct. The assay involves ligating an adducted deoxyoligonucleotide into supF containing pSP189 plasmid. The plasmid is transfected into human Ad293 kidney cells allowing replication and therefore repair or a mutagenic event to occur. Escherichia coli indicator bacteria are transformed with recovered plasmid and plasmids containing the insert are identified colormetrically, as they behave as frameshift mutations. The plasmid is then amplified and digested using a restriction cocktail of Mbo11 and Mnl1 to yield 12 bp deoxyoligonucleotides, which are characterized by MALDI-MS. MALDI-MS takes advantage of the difference in molecular weight between bases to identify any induced mutations. This analysis method therefore provides qualitative and quantitative information regarding the type and frequency of mutations induced. This assay was developed and validated using an O⁶-methyl-2′-deoxyguanosine adduct, which induced the expected GC→AT substitutions, when replicated in human or bacterial cells. This approach can be applied to the study of any DNA adduct in any biologically relevant gene sequence (e.g. p53) in human cells and would be particularly amenable to high-throughput analysis.

Genetic pathways to glioblastomas

Glioblastomas, the most frequent and malignant human brain tumors, may develop de novo (primary glioblastoma) or by progression from low-grade or anaplastic astrocytoma (secondary glioblastoma). These glioblastoma subtypes constitute distinct disease entities that affect patients of different ages and develop through different genetic pathways. Our recent population-based study in the Canton of Zürich, Switzerland, shows that primary glioblastomas develop in older patients (mean age, 62 years) and typically show LOH on chromosome 10q (69%) and other genetic alterations (EGFR amplification, TP53 mutations, p16INK4a deletion, and PTEN mutations) at frequencies of 24-34%. Secondary glioblastomas develop in younger patients (mean, 45 years) and frequently show TP53 mutations (65%) and LOH 10q (63%). Common to both primary and secondary glioblastoma is LOH on 10q, distal to the PTEN locus; a putative suppressor gene at 10q25-qter may be responsible for the glioblastoma phenotype. Of the TP53 point mutations in secondary glioblastomas, 57% were located in hotspot codons 248 and 273, while in primary glioblastomas, mutations were more widely distributed. Furthermore, G:C-->A:T mutations at CpG sites were more frequent in secondary than in primary glioblastomas (56% vs 30%). These data suggest that the TP53 mutations in these glioblastoma subtypes arise through different mechanisms. There is evidence that G:C-->A:T transition mutations at CpG sites in the TP53 gene are significantly more frequent in low-grade astrocytomas with promoter methylation of the O6-methylguanine-DNA methyltransferase (MGMT) gene than in those without methylation. This suggests that, in addition to deamination of 5-methylcytosine (the best known mechanism of formation of G:C-->A:T transitions at CpG sites), involvement of alkylating agents that produce O6-methylguanine or related adducts recognized by MGMT cannot be excluded in the pathway leading to secondary glioblastomas.

Epidemiology of brain tumors in childhood--a review

Malignant brain tumors are the leading cause of cancer death among children and the second most common type of pediatric cancer. Despite several decades of epidemiologic investigation, the etiology of childhood brain tumors (CBT) is still largely unknown. A few genetic syndromes and ionizing radiation are established risk factors. Many environmental exposures and infectious agents have been suspected of playing a role in the development of CBT. This review, based on a search of the medical literature through August 2003, summarizes the epidemiologic evidence to date. The types of exposures discussed include ionizing radiation, N-nitroso compounds (NOC), pesticides, tobacco smoke, electromagnetic frequencies (EMF), infectious agents, medications, and parental occupational exposures. We have chosen to focus on perinatal exposures and review some of the recent evidence indicating that such exposures may play a significant role in the causation of CBT. The scientific community is rapidly learning more about the molecular mechanisms by which carcinogenesis occurs and how the brain develops. We believe that advances in genetic and molecular biologic technology, including improved histologic subtyping of tumors, will be of huge importance in the future of epidemiologic research and will lead to a more comprehensive understanding of CBT etiology. We discuss some of the early findings using these technologies.

Effect of sequence context on O(6)-methylguanine repair and replication in vivo

Understanding the origins of mutational hotspots is complicated by the intertwining of several variables. The selective formation, repair, and replication of a DNA lesion, such as O(6)-methylguanine (m(6)G), can, in principle, be influenced by the surrounding nucleotide environment. A nearest-neighbor analysis was used to address the contribution of sequence context on m(6)G repair by the Escherichia coli methyltransferases Ada or Ogt, and on DNA polymerase infidelity in vivo. Sixteen M13 viral genomes with m(6)G flanked by all permutations of G, A, T, and C were constructed and individually transformed into repair-deficient and repair-proficient isogenic cell strains. The 16 genomes were introduced in duplicate into 5 different cellular backgrounds for a total of 160 independent experiments, for which mutations were scored using a recently developed assay. The Ada methyltransferase demonstrated strong 5' and 3' sequence-specific repair of m(6)G in vivo. The Ada 5' preference decreased in the general order: GXN > CXN > TXN > AXN (X = m(6)G, N = any base), while the Ada 3' preference decreased in the order: NX(T/C) > NX(G/A), with mutation frequencies (MFs) ranging from 35% to 90%. The Ogt methyltransferase provided MFs ranging from 10% to 25%. As was demonstrated by Ada, the Ogt methyltransferase repaired m(6)G poorly in an AXN context. When both methyltransferases were removed, the MF was nearly 100% for all sequence contexts, consistent with the view that the replicative DNA polymerase places T opposite m(6)G during replication irrespective of the local sequence environment.

Comparison of base-excision repair capacity in proliferating and differentiated PC 12 cells following acute challenge with dieldrin

Dieldrin, an organochlorine pesticide and known neurotoxicant, is ubiquitously distributed in the environment. Dieldrin depletes brain monoamines in some animal species and is toxic for dopaminergic neurons in vitro. Dieldrin interferes with mitochondrial electron transport and increases generation of superoxide anion. Reactive oxygen species have been shown to produce oxidative lesions to DNA bases, i.e., 8-hydroxy-2'-deoxyguanosine (8-oxodGuo). Accumulation of 8-oxodGuo has been shown to be promutagenic in proliferating cells, and can lead to degeneration in fully differentiated cells. The objective of this study was to determine the effects of dieldrin exposure on the activity of the enzyme responsible for removing 8-oxodGuo, OGG1, from undifferentiated (untreated with NGF) and differentiated (NGF-treated) PC 12 cells. Proliferating PC 12 cells exhibited a mild upregulation of glycosylase activity, reaching a maximum by 1 h and returning to baseline by 6 h. Differentiated (+) NGF cells showed a time-dependent decline in activity reaching a nadir at 3 h with a return towards baseline by 6 h. Levels of the damaged base, 8-oxodGuo, in the differentiated PC12 cells appeared to be regulated by the activity of OGG1. In contrast, levels of the damaged base in actively proliferating cells were independent of the OGG1 activity. This difference between actively dividing and differentiated cells in the regulation of base-excision repair and DNA damage accumulation explains, in part, the vulnerability of postmitotic neurons to oxidative stresses and neurotoxins.

In vivo repair during G1 of DNA lesions eliciting sister chromatid exchanges induced by methylnitrosourea or ethylnitrosourea in BrdU substituted or unsubstituted DNA in murine salivary gland cells

The difference in efficiency of methylnitrosourea (MNU) and ethylnitrosourea (ENU) to induce SCE in early or late G1 was determined in synchronized murine salivary gland cells in vivo, as a measure of the capacity of this tissue to repair the lesions involved in SCE formation during G1. The repair during G1 was determined by treating the cells in early or late G1. Treatment was in the first cycle (G1 before incorporation of 5-bromodeoxyuridine (BrdU)) or in G1 of the second cycle (after a single round of BrdU incorporation). It was observed that 50% of the lesions induced by MNU that elicit SCE are repaired during G1. BrdU incorporation into DNA increases the sensitivity of the cell to SCE induction by MNU nearly 40%; however under this circumstance a slightly lower SCE frequency was observed in the cells exposed to MNU at early G1, indicating that during G1 only few lesions are repaired. The ENU-induced DNA-lesions involved in SCE production are nearly 100% persistent along G1; besides, a slight but significantly higher SCE frequency was observed in cells exposed at early G1, suggesting the formation of SCE-inducing lesions during G1. BrdU incorporation to DNA sensitizes the cell to SCE induction by ENU, increasing the SCE frequency to nearly to a 40%, although these additional lesions involved in SCE induction seem to be susceptible to repair during G1.

Formation and removal of DNA adducts in Fischer-344 rats exposed to 2,4-diaminotoluene

32P-Postlabeling was used to examine DNA adduct formation and removal in Fischer-344 rats exposed to the animal carcinogen 2,4-diaminotoluene (DAT). Adduct formation and persistence were compared between target (liver and mammary gland) and non-target organs (kidney and lung) to determine if possible differences could explain the observed organ specificity of DAT induced carcinogenesis. The effects of different exposure conditions on DNA adduct formation and removal were also examined by varying the concentration and frequency of compound administration. DAT produced three distinct DNA adducts. Among the organs examined, DNA binding was highest in the liver, with levels approximately 10 times greater than that of the mammary gland and up to 50 times greater than of the two nontarget sites. Despite the large differences in the initial extent of adduct formation, the persistence of adducts among sites was not significantly different. In the liver, there were dose-dependent differences in DNA adduct formation, but adduct removal following different dosages did not vary significantly. The effects of multiple administration on DNA adduct formation and removal were examined by treating rats with 5 mg/kg DAT daily for 10 consecutive days. Adduct yields from multiple treatment were greater than from a single 50 mg/kg exposure. The persistence of adducts following multiple treatment was also greater than after an equivalent single exposure. The results demonstrated organ-specific and dose-dependent differences in initial extent of DNA adduct formation, but no differences in adduct persistence. However, the results did suggest that adduct formation and persistence may change with repeated administration of DAT.

Evidence that SCEs induced by mutagens do not occur at the same locus in successive cell cycles: lack of cancellation in three-way stained CHO chromosomes

An approach based on the synchronization of CHO cells after a first cell cycle incorporating a relatively low amount of bromodeoxyuridine (BrdUrd) into DNA, followed by mutagenic treatment and subsequent culture for second and third generations of BrdUrd incorporation for the scoring of sister chromatid exchanges (SCEs) per cell cycle in three-way differentially (TWD) stained chromosomes, has been used to investigate the possible cancellation of SCEs. Cancellation is expected to occur if two mutagen-induced SCEs occur at exactly the same site in subsequent rounds of replication. Lesions in DNA seem to persist and are able to induce SCE throughout two cell cycles after treatment with the three mutagens tested--mitomycin C (MMC), ethyl methanesulfonate (EMS) and ultraviolet (UV) light--though this latter agent was shown as only moderately persistent. Our results seem to indicate that SCEs induced by these mutagens do not take place at the same locus in successive cell generations, as assessed by a lack of SCE cancellation.

Estrous cycle modulation of O6-alkylguanine-DNA alkyltransferase expression in rat mammary epithelial cells

N-methyl-N-nitrosourea (MNU)-induced rat mammary tumor incidence and tumor number per rat, is directly correlated with an increase in the circulating level of estrogen(s) at the time of carcinogen administration and subsequent mammary epithelial O6-methylguanine content. We report that, expression of O6-alkyltransferase (AGT) is also regulated by reproductive hormones in a tissue specific manner. The level of mammary epithelial cell AGT activity on estrus (0.47 pmol/mg protein) and proestrus (0.32) was significantly higher than on metestrus (0.14) (P < 0.05, estrus vs. metestrus). However, no change was observed in liver AGT activity (0.52 pmol/mg protein). In contrast, the mean level of AGT protein was not significantly different between tumors from rats injected with MNU on different days of the estrous cycle. In conclusion, the different tumor biologies resulting from carcinogen injection on different days of the estrous cycle may be partially explained by variation in levels of DNA repair activity. However, the cells in the resulting tumors did not continue an obligatory differential expression of the AGT activity consistent with their stage of initiation.

In vivo fate of MMS-induced DNA lesions that elicit SCE

A previously reported in vivo protocol, which uses three-way differential staining (TWD) of sister chromatids, allows the screening of mutagen-induced sister-chromatid exchange (SCE) in each of the two cell divisions after mutagen treatment and also those occurring at apparently the same locus in both divisions. In the present work the effect of methyl methanesulfonate (MMS) was studied by means of this protocol. The results showed that MMS-induced DNA lesions that cause SCE are persistent. Some lesions were induced in the second division, as was inferred from the analysis of the response in single cells. The data also indicate that bromodeoxyuridine reduces DNA sensitivity to SCE induction by MMS.

DNA repair mechanisms in neurological diseases: facts and hypotheses

DNA repair mechanisms usually consist of a complex network of enzymatic reactions catalyzed by a large family of mutually interacting gene products. Thus deficiency, alteration or low levels of a single enzyme and/or of auxiliary proteins might impair a repair process. There are several indications suggesting that some enzymes involved both in DNA replication and repair are less abundant if not completely absent in stationary and non replicating cells. Postmitotic brain cell does not replicate its genome and has lower levels of several DNA repair enzymes. This could impair the DNA repair capacity and render the nervous system prone to the accumulation of DNA lesions. Some human diseases clearly characterized by a DNA repair deficiency, such as xeroderma pigmentosum, ataxia-telangiectasia and Cockayne syndrome, show neurodegeneration as one of the main clinical and pathological features. On the other hand there is evidence that some diseases characterized by primary neuronal degeneration (such as amyotrophic lateral sclerosis and Alzheimer disease) may have alterations in the DNA repair systems as well. DNA repair thus appears important to maintain the functional integrity of the nervous system and an accumulation of DNA damages in neurons as a result of impaired DNA repair mechanisms may lead to neuronal degenerations.

Renal transplacental carcinogenicity of 3,3-dimethyl-1-phenyltriazene in rats: relationship of renal mesenchymal tumor to congenital mesoblastic nephroma and intralobar nephrogenic rests

Exposure of rat embryos to 3,3-dimethyl-1-phenyltriazene (DMPT) results in numerous malformations, but the urogenital system is not affected. In contrast, exposure of rat fetuses to DMPT has been reported to result in renal neoplasms, which were not further classified. To better understand this discrepancy in organotropism of the teratogenic and transplacental carcinogenic processes, the present study was undertaken to characterize the neoplasms induced in rat fetuses exposed to DMPT in utero. Renal neoplasms and persistent mesenchyme were observed in 19.2 and 11.5%, respectively, of the offspring of rats treated with 1 mg DMPT/kg body weight intraperitoneally on gestation days 16, 18, and 20. The majority of these renal lesions were observed in females. The renal neoplasms were mixtures of various types of mesenchymal tissue derivatives including smooth muscle and fibrous connective tissue. These neoplasms would be classified as renal mesenchymal tumors in rats. Brain neoplasms (numerous types), compound odontomas, and micrognathism were observed predominantly in male offspring from the same group. This treatment also resulted in decreased body weights, increased incidence of sudden loss of body weight, tremors and ataxia, and hypoplastic testes. Exposure to single intraperitoneal doses of DMPT on gestation day 20 did not produce a classic dose-response pattern: Minimal effects were observed with 10 mg DMPT/kg (occasional renal mesenchymal tumors and brain neoplasms), marked effects were observed with 30 mg DMPT/kg (lower incidence rate of most of the alterations observed with 1 mg/kg on gestation days 16, 18, and 20), and no effects were observed with 60 mg DMPT/kg. DMPT administered intraperitoneally at 1 mg/kg body weight on gestation days 16, 18, and 20 is an animal model of transplacental chemically induced renal neoplasms, which provide lesions with similarities to both intralobar nephrogenic rests and congenital mesoblastic nephroma of humans. Why the kidney is a carcinogenic target and not a teratogenic target remains unknown.

DNA methylation in the digestive tract of F344 rats during chronic exposure to N-methyl-N-nitrosourea

The formation of O6-methyldeoxyguanosine (O6-MedGuo) was determined by an immuno-slot-blot assay in DNA of various tissues of F344 rats exposed to N-methyl-N-nitrosourea (MNU) in the drinking water at 400 ppm for 2 weeks. Although the pyloric region of the glandular stomach is a target organ under these experimental conditions, the extent of DNA methylation was highest in the forestomach (185 mumol O6-MedGuo/mol guanine). Fundus (91 mumol/mol guanine) and pylorus (105 mumol/mol guanine) of the glandular stomach, oesophagus (124 mumol/mol guanine) and duodenum (109 mumol/mol guanine) showed lower levels of O6-MedGuo but differed little between each other. Thus, no correlation was observed between target organ specificity and the extent of DNA methylation. This is in contrast to the gastric carcinogen, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), which preferentially alkylates DNA of the pylorus, the main site of induction of gastric carcinomas by this chemical. In contrast to MNU, the non-enzymic decomposition of MNNG is accelerated by thiol compounds (reduced glutathione, L-cysteine), which are present at much higher concentrations in the glandular stomach than in the forestomach and oesophagus. During chronic exposure to MNNG (80 ppm), mucosal cells immunoreactive to O6-MedGuo are limited to the luminal surface [Kobori et al. (1988) Carcinogenesis 9:2271-2274]. Although MNU (400 ppm) produced similar levels of O6-MedGuo in the pylorus, no cells containing methylpurines were detectable by immunohistochemistry, suggesting a more uniform methylation of mucosal cells by MNU than by MNNG. After a single oral dose of MNU (90 mg/kg) cells containing methyl-purines were unequivocally identified using antibodies to O6-MedGuo and the imidazole-ring-opened product of 7-methyldeoxyguanosine. In the gastric fundus, their distribution was similar to those methylated by exposure to MNNG, whereas the pyloric region contained immunoreactive cells also in the deeper mucosal layers. After a 2-week MNU treatment, the rate of cell proliferation, as determined by bromodeoxyuridine immunoreactivity, was only slightly enhanced in the oesophagus and in the fundus, but markedly in the forestomach and the pyloric region of the glandular stomach. It is concluded that the overall extent of DNA methylation, the distribution of alkylated cells within the mucosa and the proliferative response all contribute to the organ-specific carcinogenicity of MNU.

Fate of DNA lesions that elicit sister-chromatid exchanges

Using 3-way differential staining (TWD) of sister chromatids, the fate of DNA lesions involved in sister-chromatid exchange (SCE) formation was determined in murine bone marrow cells in vivo, after treatment with either mitomycin C (MMC) or cyclophosphamide (CP). Both MMC (2.6 mg/kg b.w.) and CP (7 mg/kg b.w.) induced an SCE frequency near the expected in the 2 subsequent cell divisions, but the frequency of SCE occurring at the same locus in successive cell divisions was substantially lower than expected. The results are compared with previous data obtained after exposure to gamma-rays. A model of SCE induction is proposed.

Characterization of N-nitrosourea-induced tumors of the nervous system; their prospective value for studies of neurocarcinogenesis and brain tumor therapy

Two decades of research with resorptive neurocarcinogens firmly established the high potency of methyl and ethylnitrosourea (MNU and ENU) as neurocarcinogens, particularly in rats. There are significant differences in susceptibility to these agents among species. There are also differences among age groups. Fetuses are between 50 to 100 times more susceptible than adult rats. One single iv inoculation of 20-50 mg/kg ENU into pregnant rats may produce neurogenic tumors in 100% of the offspring. The tumors produced by these compounds have been well characterized morphologically, biologically, biochemically and histochemically. Tumors produced by both compounds are mostly gliomas and neurinomas (Schwannomas), however, clear differences exist between ENU and MNU produced neoplasms. Transplacental exposure to ENU generally results in a high number of anaplastic neurinomas and mostly differentiated gliomas (astrocytomas, oligodendrogliomas or mixed gliomas). In contrast, multiple exposures of adult rats to MNU result in a moderate number of mostly differentiated neurinomas and a high number of anaplastic gliomas. Tumors usually start out as well differentiated oligodendrogliomas or astrocytomas. As they grow larger, they become more mixed and anaplastic. In contrast to spontaneous gliomas in old rats, MNU and ENU-induced astrocytomas can be readily identified with well established biomarkers such as the S100 protein and particularly GFAP (glial fibrillary acidic protein). Neurinomas are also strongly positive for S100 protein. No reliable markers exist for oligodendrogliomas. Neurogenic tumors induced by MNU or ENU, as well as derived cell lines and clones from such tumors, have been successfully used as models for neurocarcinogenesis and therapeutic screening.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanistic clues to the mutagenicity of alkylated DNA bases: a theoretical study

Experiment indicates that the N7-guanine site in DNA is not "promutagenic" (mutation-inducing) on alkylation, while the O6-guanine and O4-thymine sites are so. These differences in nucleic acid template activity are attributed to corresponding differences in acidity of the Watson-Crick hydrogen bonding protons. Mechanistic indicators for ease of Watson-Crick proton loss are calculated using molecular orbital theory for DNA bases alkylated at the N7-guanine, O6-guanine and O4-thymine sites. Their values point to a definite favouring of the proton loss for the O-alkylated bases compared to the N7-alkylguanines. This may suggest the possibility that, at biological pH, the O-alkylated bases deprotonate readily while the N7-alkylguanines do not, thus accounting for observed differences in promutagenicity and nucleic acid template activity.

Esophageal cancer and microelements

Epidemiological studies have identified dietary zinc deficiency, methylbenzylnitrosamine, and ethanol as factors strongly associated with an increased incidence of esophageal carcinoma in man. In addition, these studies have identified other trace elements which may also affect the incidence of esophageal carcinoma. Animal models have confirmed that dietary zinc deficiency increases the incidence of methylbenzylnitrosamine-induced esophageal carcinoma and that dietary zinc deficiency also increases the incidence of other dialkylnitrosamine-induced carcinomas. The dialkylnitrosamine carcinogens are activated by NADPH-dependent cytochrome P-450 enzymes in their target tissues. The activated methylbenzylnitrosamine methylates DNA, forming O6-methylguanine adducts. These O6-methylguanine adducts can lead to point mutations in DNA, and such mutations are known to be responsible for the induction of certain carcinogen-induced tumors. We have demonstrated that dietary zinc deficiency increased the cytochrome P-450-dependent microsomal metabolism of methylbenzylnitrosamine and dimethylnitrosamine, two members of this class of dialkylnitrosamine carcinogens, while the addition of zinc in vitro noncompetitively inhibits the microsomal metabolism of these carcinogens. We have also demonstrated that dietary zinc deficiency is associated with an increased formation of O6-methylguanine in the esophageal DNA of zinc-deficient animals treated with methylbenzylnitrosamine. This increased formation of the mutagenic DNA adduct O6-methylguanine may explain the increased incidence of dialkylnitrosamine-induced carcinomas observed with dietary zinc deficiency. Other trace elements, including molybdenum, selenium, and magnesium, may also alter the incidence of esophageal carcinoma, but studies of these elements are not as conclusive as the epidemiological and experimental studies linking dietary zinc deficiency with an increased incidence of human esophageal carcinoma.

Effect of nutritional zinc-deficiency on O6-alkylguanine-DNA-methyl-transferase activities in rat tissues

The effect of nutritional zinc-deficiency on the activities of O6-alkylguanine:DNA methyltransferase (AGT) in 9 rat tissues including liver, lung, kidney, spleen, brain, esophagus, forestomach, gastric-stomach and small intestine has been examined. Individual tissue extracts prepared from zinc-deficient and pair-fed, zinc-sufficient rats were incubated with N-[3H]methylnitrosourea-methylated calf thymus DNA for 1 h. The activities of AGT in these tissues were measured by two methods: (a) the transfer of the methyl group from O6-methylguanine in substrate DNA to AGT protein, and (b) the determination of the ratio of O6-methylguanine:7-methylguanine remaining in substrate DNA following incubation. AGT activities (expressed as fmol protein methylated/h per mg protein) were significantly reduced in the esophagus, spleen and lungs of zinc-deficient rats as compared to those in their corresponding zinc-sufficient counterparts. The ratio of O6-methylguanine:7-methylguanine was also reduced in the esophagus of the zinc-deficient rat. These results were consistent with our earlier findings that dietary zinc-deficiency enhances nitrosamine-induced esophageal carcinogenesis in rats.

Chronic ethanol consumption inhibits repair of dimethylnitrosamine-induced DNA alkylation

Chronic ethanol consumption causes a DNA repair deficiency. This was demonstrated in Sprague-Dawley rats injected with 14C-labeled dimethylnitrosamine after being pair-fed isocaloric, ethanol, or carbohydrate control diets for 4 weeks. Hepatic DNA was isolated from rats killed at intervals over a 36 hour period after administration of the nitrosamine and concentrations of alkylated guanine derivatives were measured. While N7-methylguanine was lost at equivalent rates from the DNA of both diet groups, 06methylguanine, a promutagenic lesion, persisted at higher levels for longer periods of time in the DNA from the alcohol-fed animals.

Studies on the mechanism of the acute and carcinogenic effects of N-nitrosodimethylamine on mink liver

Outbreaks of liver necrosis and liver hemangiosarcoma were detected in a mink breeding colony in Argentina. Analysis of the Minks' food revealed the presence of 2.6 ppm dimethylnitrosamine (NDMA) in it, apparently as a result of the addition of nitrite as preservative. Previous studies gave evidence of the particular susceptibility of minks to NDMA and other hepatic insults. We have determined several biochemical parameters known to correlate with NDMA hepatotoxic effects and compared them with those in rat liver. NDMA administration to both species resulted in the formation of reactive metabolites able to interact with liver DNA to give N7-methylguanine and O6-methylguanine adducts. Biotransformation of NDMA by liver slices to CO2 was significantly lower in the mink than in the rat, whereas the covalent binding (CB) to nucleic acids was slightly lower than in in the rat. Aminopyrine N-demethylase activity was also significantly less in mink than in rat liver. The CB of NDMA reactive metabolites to microsomal proteins was not significantly lower in mink as compared to the rat, and the same holds true for the biotransformation of NDMA to formaldehyde by microsomal preparations. Results suggest that the high susceptibility of minks to NDMA might be partially due to a decreased ability to detoxicate NDMA but also to a higher intrinsic susceptibility of their liver cells to a given chemical insult.

Estrous cycle modification of rat uterine DNA alkylation by N-methyl-N-nitrosourea

The present study was designed to determine whether the stage of the estrous cycle at the time of N-nitroso-N-methylurea (NMU) presentation altered DNA adduct formation and repair in the rat uterus. In uterus the rate of O6-methylguanine (O6-meGua) and 7-methylguanine (7-meGua) formation and the total yield of adducts was estrous cycle dependent. Uterine DNA from rats injected with NMU on diestrus formed O6-meGua and 7-meGua more rapidly and had significantly higher adduct levels than those rats injected on proestrus or estrus. Repair of O6-meGua and 7-meGua was also significantly faster between 1 and 24 h post-NMU in uterine DNA isolated from rats injected on diestrus compared to those injected on proestrus or estrus.

O6-methylguanine-DNA methyltransferase and alkylation of liver DNA in mice exposed to dimethylnitrosamine during dietary deficiency of essential amino acids

Male NMRI mice were fed a diet containing a complete mixture of amino acids or a mixture deficient in methionine-cysteine or lysine (30% of the control level) for a period of 6 days. During the feeding period all mice received dimethylnitrosamine in the drinking water ad libitum. The exposure averaged 1 mg dimethylnitrosamine/kg body weight and day. The concentration of O6-methylguanine-DNA methyltransferase was measured in liver extracts. It decreased significantly in the methionine-cysteine deficient mice. When DNA from the liver was analyzed for alkylated purine bases the mice received a single dose of 14C-labeled dimethylnitrosamine (0.5 or 1 mg/kg body weight) at 120 min before sacrifice. The concentration of O6-methylguanine increased significantly over the control level upon feeding the deficient diets and was restored to the concentration of the controls by refeeding lysine for 2 days following 6 days of lysine deficiency. The increased ratio of O6-methylguanine to N-7-methylguanine indicated that methylation of guanine in the N-7 position was not subject to variation by the intake of dimethylnitrosamine during the dietary deficiencies. The results demonstrate the requirement for a balanced composition of amino acids in the diet to maintain a sufficient concentration of O6-methylguanine-DNA methyltransferase in the cells and thus to permit efficient removal of the methyl group from the O-6 position of guanine in DNA after exposure to dimethylnitrosamine.

DNA damage in the central nervous system of rats after in vivo exposure to chemical carcinogens: correlation with the induction of brain tumors

The alkaline elution technique has been used to evaluate DNA damage in brain of rats treated with a single equimolar dose of 14 carcinogens of different chemical structure. A clear-cut increase of DNA elution rate, which is considered indicative of DNA fragmentation, was produced by 10 compounds known to induce the development of tumors in the rat central nervous system: N-nitroso-N-methylurea, N-nitroso-N-ethylurea, N-nitroso-N-butylurea, N-nitroso-N-methylurethane, methyl methanesulfonate, ethyl methanesulfonate, dimethyl sulfate, diethyl sulfate, 1,3-propansultone, and procarbazine. Similar amounts of DNA fragmentation were produced by both potent and weak brain carcinogens. In contrast, any significant increase of DNA elution rate was absent in rats treated with N-methyl-N'-nitro-N-nitrosoguanidine, N-nitrosodimethylamine, N-nitrosodiethylamine, and beta-propiolactone, all of which are devoid of carcinogenic activity for the rat central nervous system. These results suggest that the described in vivo brain DNA damage/alkaline elution assay deserves further studies on a wide number of carcinogens and noncarcinogens aimed to establish its possible usefulness for a qualitative preliminary assessment of the ability of a compound to induce neurogenic tumors.

Induction of rat liver O6-alkylguanine-DNA alkyltransferase by bleomycin

Alkyl adducts at the O6-position of guanine constitute promutagenic DNA lesions likely to be involved in the initiation of malignant transformation. They can be removed by a cellular acceptor protein termed O6-alkylguanine-DNA alkyltransferase (AT). In rat liver this repair enzyme can be induced by a variety of hepatotoxins, partial hepatectomy and X-irradiation. This paper describes a stimulation of the hepatic AT by treatment of rats with the radiomimetic agent, bleomycin. Induction of AT is dose-dependent up to 20 mg bleomycin/kg and appears to level off with higher doses. Enhancement of O6-meG repair is detectable within 24 h after a single i.p. injection. Maximum AT induction was reached after 6 days and amounted to 350% of the control levels. The enhancement of AT activity is not associated with acute liver injury and initially coincides with an inhibition of [3H]deoxythymidine incorporation into hepatic DNA. This indicates that AT induction in rat liver is not necessarily dependent on tissue necrosis with increased cell replication. Since bleomycin does not produce DNA lesions recognized and repaired by the AT, the hypothesis is entertained that AT induction by these agents is part of a concerted reaction to radiation-type DNA damage.

Comparative interactions of streptozotocin and chlorozotocin with DNA of an insulin-secreting cell line (RINr)

Both streptozotocin and chlorozotocin, the 2-chloroethyl analogue of streptozotocin, are diabetogenic chemicals in rodents. Although these chemicals are similar structurally, they appear to act on pancreatic B cells via different mechanisms. In studies here, damage and repair of DNA after exposure of an insulin-secreting cell line to streptozotocin and chlorozotocin were assessed by nucleoid sedimentation and alkaline elution. Equitoxic concentrations of streptozotocin and chlorozotocin caused significant single-strand breakage of DNA (p less than 0.005). These lesions were repaired in a time-dependent manner, with most repair completed by 24-h post-exposure to chemicals. Additionally, chlorozotocin caused DNA-DNA and DNA-protein crosslinks in insulinoma cells. When proteinase K was included in the crosslinking assay, a substantial proportion of the chlorozotocin-associated crosslinks proved to be DNA interstrand in nature. Analysis of the amount of interstrand crosslinking in insulinoma cells after exposure to chlorozotocin for 1 h showed that formation of interstrand crosslinks was slow. Increasing amounts appeared over a 24-h period. These results suggest that the formation of irreversible DNA interstrand crosslinks may be a critical factor in cytotoxicity and diabetogenicity caused by chlorozotocin.

Chemical dosimetry of ethyl nitrosourea in the mouse testis

[3H-Et]Nitrosourea was administered to male (101 X C3H) mice by i.p. injection at exposure levels of 10 mg/kg or 100 mg/kg. At intervals from 1 h to 6 days following treatment, the ratio of O6-ethylguanine to N7-ethylguanine in testis DNA averaged 1.13 following the 100 mg/kg exposure and 0.72 following the 10 mg/kg exposure. The amount of O6-ethylguanine recovered after the 100 mg/kg exposure was 40% greater than predicted from a linear extrapolation of the amount of O6-ethylguanine recovered after the 10 mg/kg exposure. We suggest that the high (100 mg/kg) exposure to ethyl nitrosourea results in depletion of the O6-alkylguanine acceptor protein within the testis and permits O6-ethylguanine to persist at higher levels than would be predicted from lower exposure data. W.L. Russell et al. (1982), W.L. Russell (1984) have found that specific-locus mutation frequencies induced in mouse spermatogonial stem cells are 5.8-fold greater after a single 100 mg/kg exposure to ethyl nitrosourea than after 10 weekly exposures to 10 mg/kg. The finding that the corresponding ratio for O6-ethylguanine formed in the testis is only 1.4 may be interpreted in a number of possible ways. If O6-ethylguanine is an important lesion for producing specific-locus mutations, then its formation in the stem cells must be at least 4-fold greater than that for the whole testis as the ENU exposure goes from 10 to 100 mg/kg: alternatively, the rate of repair of this lesion by the stem cells must decrease at least 4-fold relative to the average testicular cell. Other explanations for the difference in mutation response of the stem cells to acute vs. chronic ethyl nitrosourea-exposures include the possibility that other DNA lesions may be responsible for many of the mutations or that two hits on the DNA may be required to produce an effect.

Methionine-cysteine deficiency and alkylation of DNA in liver, kidney and lung of mice administered dimethylnitrosamine

The effect of methionine-cysteine deficiency on the methylation of DNA purines by dimethylnitrosamine metabolites was studied in subadult and adult mice. In liver, no dietary effect on the specific methylation of 7-methylguanine was observed, while that of 3-methyladenine decreased in the adult animals. The specific methylation of guanine in the 0(6)-position and the ratio of 0(6)-methylguanine to 7-methylguanine increased significantly after methionine-cysteine deficiency. Methylation in kidney decreased in subadult but increased in adult mice. In lung, the amount of 7-methylguanine was significantly elevated after methionine-cysteine deficiency in both the subadult and adult mice. The results demonstrate an increase in the specific methylation in liver of guanine in the 0(6)-position by the methionine-cysteine deficient diet, together with differences in the methylation pattern between organs of the two age groups.

Relationships between DNA adduct formation and carcinogenesis

An impressive array of evidence has been obtained during the past decade establishing correlations between specific DNA adducts and carcinogenesis. Many of the studies utilized organ specific differences in carcinogenesis to establish the correlations. More recently, we have investigated similar relationships between target and nontarget cell populations within the liver. Chronic exposure to methylating hepatocarcinogens predominantly induces hemangiosarcomas, whereas exposure to ethylating agents causes hepatocellular carcinomas. This cell specificity in carcinogenesis correlates well with the presence of promutagenic DNA adducts. In the case of methylating agents, the nonparenchymal cells accumulate O6-methylguanine whereas the hepatocytes do not. Exposure to ethylating agents leads to accumulation of O4-ethyldeoxythymidine, but not O6-ethyldeoxyguanosine in hepatocytes. These differences reflect the ability of the two cell populations to repair O6-alkylguanine and the extent of purine and pyrimidine alkylation with methylating and ethylating agents. Hepatocytes of rats exposed to diethylnitrosamine for 28 days have four to five times more promutagenic DNA adducts (O6-alkyldeoxyguanosine and O4-alkyldeoxythymidine) than hepatocytes of rats exposed to nearly equimolar doses of dimethylhydrazine. Both O6-methylguanine and O4-methyldeoxythymidine are rapidly repaired by rat hepatocytes, while only O6-ethyldeoxyguanosine is rapidly repaired. Studies comparing the relationship between the induction of gamma-glutamyl transpeptidase-positive foci, hepatocellular carcinoma and promutagenic lesions such as O4-ethyldeoxythymidine will be useful in understanding associations between the molecular dosimetry of DNA adducts, initiation, and progression of hepatocarcinogenesis.

Response of S. cerevisiae to N-methyl-N'-nitro-N-nitrosoguanidine: mutagenesis, survival and DDR gene expression

We have examined the effects of low concentrations of the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on survival and mutagenesis of logarithmically growing Saccharomyces cerevisiae. Pretreatment of cells with nontoxic and submutagenic concentrations of MNNG for several generations does not reduce the cytotoxic and mutagenic effects of exposure to a high concentration of drug. This lack of 'adaptation' in S. cerevisiae was further investigated biochemically and cell extracts prepared from pretreated cells were shown to be deficient in the ability to remove O6-methyl guanine from alkylated DNA. Moreover, we could not detect transfer of the methyl group from the DNA to a protein acceptor in yeast cell extracts suggesting that the level of O6-methyl guanine transferase is below 200 molecules/cell following pretreatment. Exposure of S. cerevisiae to mutagenic concentrations of MNNG stimulates transcription of at least three DNA damage responsive genes that also respond to UV-irradiation and 4-nitroquinoline-1-oxide treatment. These results support the contention that in Saccharomyces alkylation damage is processed by repair pathways that operate on a variety of lesions, and that one or more of these pathways is inducible.