Formation and subsequent removal of O6-methylguanine from deoxyribonucleic acid in rat liver and kidney after small doses of dimethylnitrosamine

Nitrosamines crisis in pharmaceuticals - Insights on toxicological implications, root causes and risk assessment: A systematic review

The presence of N-nitroso compounds, particularly N-nitrosamines, in pharmaceutical products has raised global safety concerns due to their significant genotoxic and mutagenic effects. This systematic review investigates their toxicity in active pharmaceutical ingredients (APIs), drug products, and pharmaceutical excipients, along with novel analytical strategies for detection, root cause analysis, reformulation strategies, and regulatory guidelines for nitrosamines. This review emphasizes the molecular toxicity of N-nitroso compounds, focusing on genotoxic, mutagenic, carcinogenic, and other physiological effects. Additionally, it addresses the ongoing nitrosamine crisis, the development of nitrosamine-free products, and the importance of sensitive detection methods and precise risk evaluation. This comprehensive overview will aid molecular biologists, analytical scientists, formulation scientists in research and development sector, and researchers involved in management of nitrosamine-induced toxicity and promoting safer pharmaceutical products.

Interaction of N-nitrosamines with binuclear copper complexes for luminescent detection

Cu(I) from tetrakis(acetonitrile)copper(I) hexafluorophosphate ([Cu(MeCN)₄]PF₆) was complexed with five structurally related phosphines containing N-heterocycles. The interactions between the resulting complexes and some N-nitrosamines were studied using X-ray crystallography as well as emission spectroscopy. Upon complexation, three phosphine ligands bridge two Cu(I) centers to give paddlewheel type structures that displayed a range of emission wavelengths spanning the visible region. N-Nitrosodimethylamine (NDMA) was shown to coordinate to one of the two copper centers in some of the paddlewheel complexes in the solid state and this interaction also quenches their emissions in solution. The influence of the weakly coordinating anion on crystal and spectroscopic properties of one of the paddlewheel complexes was also examined using tetrakis(acetonitrile)copper(I) perchlorate ([Cu(MeCN)₄]ClO₄) as an alternative Cu(I) source. Similarly, copper(II) perchlorate hexahydrate (Cu(ClO₄)₂·6H₂O) was used for complexation to observe the impact of metal oxidation state on the two aforementioned properties. Lastly, the spectroscopic properties of the complex between Ph₂P(1-Isoquinoline) and Cu(I) was shown to exhibit solvent dependence when the counterion is ClO₄^-. These Cu(I) complexes are bench stable solids and may be useful materials for developing a fluorescence based detection method for N-nitrosamines.

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.

Excision of mutagenic replication-blocking lesions suppresses cancer but promotes cytotoxicity and lethality in nitrosamine-exposed mice

N-Nitrosodimethylamine (NDMA) is a DNA-methylating agent that has been discovered to contaminate water, food, and drugs. The alkyladenine DNA glycosylase (AAG) removes methylated bases to initiate the base excision repair (BER) pathway. To understand how gene-environment interactions impact disease susceptibility, we study Aag-knockout (Aag-/-) and Aag-overexpressing mice that harbor increased levels of either replication-blocking lesions (3-methyladenine [3MeA]) or strand breaks (BER intermediates), respectively. Remarkably, the disease outcome switches from cancer to lethality simply by changing AAG levels. To understand the underlying basis for this observation, we integrate a suite of molecular, cellular, and physiological analyses. We find that unrepaired 3MeA is somewhat toxic, but highly mutagenic (promoting cancer), whereas excess strand breaks are poorly mutagenic and highly toxic (suppressing cancer and promoting lethality). We demonstrate that the levels of a single DNA repair protein tip the balance between blocks and breaks and thus dictate the disease consequences of DNA damage.

Alkaline DNA Fragmentation in Vivo: Borderline or Negative Results Obtained Respectively with 7,12-Dimethylbenz[A]Anthracene and Benzo[A]Pyrene

Using the in vivo DNA damage alkaline elution assay, a satisfactory correlation with carcinogenicity in the same target organ has been previously shown for a variety of chemical agents. This work was intended to enlarge the exploration of the predictivity of this test. Benzo[a]pyrene (BP) was found negative for damage to liver DNA of mice and rats, and 7,12-dimethylbenz[a]anthracene (DMBA) negative for damage to liver and bone marrow DNA of mice and slightly positive for damage to mammary gland DNA of young female rats. The results were found to be correlated with the extension of DNA arylation in target organs in similar experimental conditions. From carcinogenicity data reported in the Survey of Compounds Which Have Been Tested for Carcinogenic Activity (vols. 1961-1973) BP and DMBA were both found to be essentially negative as liver carcinogens; however, DMBA was a potent carcinogen in inducing mammary tumors.

Critique of dose response in carcinogenesis

A few landmarks in the development of dose response in toxicology are presented, with an explanation of why dose should only be considered on a logarithmic scale. Examples are shown, illustrating that the current practice of labeling dose-response curves for carcinogenesis as supralinear, linear or sublinear, is meaningless unless the dose-response scales are defined. Since many reports labeling such curves as supralinear, linear, or sublinear are carried out with dose on a linear scale, the scientific significance of the shape of the curve is obscured. Examples of dose-response curves for carcinogenesis from 2-acetylaminofluorene, N-nitrosodiethylamine, aflatoxins, and radium are shown. In addition, more than 500 National Toxicology Program Technical Reports (NTP-TR) on carcinogenicity were examined; from this database, three groups of studies were selected. The first group consisted of those studies in which the lowest dose produced no tumors and the study had a positive dose response. The second group consisted of those studies with three or more doses, with a positive dose response producing tumors, but in which there were no tumors in the control group. The third group of more than 50 studies was from NTP-TR-00 to NTP-TR-52 that had only two data points with a positive dose response. These studies were all evaluated on the Rozman et al. scale, since it conforms to the laws of nature and allows evaluation of all doses. It was observed that virtually all of these NTP-TR carcinogenicity studies show a linear response when dose is on this logarithmic scale; a clear threshold for carcinogenicity is typical for nearly all of these chemicals. An exponential dose-response curve was a better fit for a few, but experimental error could account for this deviation from linearity. It is pointed out that there is strong experimental evidence that the mere presence of DNA adducts does not necessarily lead to tumor production. Hormesis probably applies to carcino-genesis and proof of this will require abandoning the no threshold concept. Experiments showing that cumulative dose is a better metric than daily dose may require reevaluating almost all carcinogenicity studies.

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.

Inter-individual variation in DNA repair capacity: a need for multi-pathway functional assays to promote translational DNA repair research

Why does a constant barrage of DNA damage lead to disease in some individuals, while others remain healthy? This article surveys current work addressing the implications of inter-individual variation in DNA repair capacity for human health, and discusses the status of DNA repair assays as potential clinical tools for personalized prevention or treatment of disease. In particular, we highlight research showing that there are significant inter-individual variations in DNA repair capacity (DRC), and that measuring these differences provides important biological insight regarding disease susceptibility and cancer treatment efficacy. We emphasize work showing that it is important to measure repair capacity in multiple pathways, and that functional assays are required to fill a gap left by genome wide association studies, global gene expression and proteomics. Finally, we discuss research that will be needed to overcome barriers that currently limit the use of DNA repair assays in the clinic.

Sustained overexpression of CYP1A1 and 1B1 and steady accumulation of DNA adducts by low-dose, continuous exposure to benzo[a]pyrene by polymeric implants

Many carcinogenesis and tumorigenesis studies reported in the past several decades have relied upon bolus dose(s) of test compounds to determine their DNA damage and carcinogenic potential. The high doses are far from the human scenario where exposure is almost always to low doses and for long duration. In this study, we report a novel polymeric implant system that provides continuous ("24/7") exposure to low doses using benzo[a]pyrene (BP) as a model carcinogen. Cylindrical implants (1 cm length, 3.2 mm diameter; 10 mg BP/100 mg implant) prepared from polycaprolactone:F68 (9:1) showed controlled release in vitro for long duration. To determine the rate of release and biochemical effects in vivo, groups of female Sprague-Dawley rats received either no treatment or subcutaneous sham or BP implants (1 cm, 10% load) and were euthanized after 6, 15, 30, and 180 days; the average dose of BP by the implant route was 16.7 ± 3 μg/rat. For comparison, rats were also treated with a single bolus dose of BP intraperitoneally (10 mg/rat) and euthanized at 6, 15, and 30 days. DNA adducts analyzed by (32)P-postlabeling in the lung and liver increased steadily with time with levels reaching 31 ± 3 and 17 ± 6 adducts/10(9) nucleotides, respectively, after 25 weeks; the adduct burden in the mammary tissue initially increased but then declined with time presumably due to high cell turn over. In contrast, the bolus dose treatment showed the highest DNA adduct levels after 6 days, followed by a steady decline. The steady accumulation of tissue DNA adducts in the implant groups corroborates the sustained overexpression of CYP1A1 and 1B1, the cytochrome P450s involved in the conversion of BP to its electrophilic metabolites. In contrast, the overexpression of CYP1A1 and 1B1 resulting from the bolus dose of BP lasted only for a few days. This is the first demonstration revealing that low-dose, continuous exposure to environmental polycyclic aromatic hydrocarbons such as BP can render sustained expression of CYPs and steady accumulation of tissue DNA adducts. On the basis of our recent study in which we showed the presence of 17β-estradiol in the lung, the sustained overexpression of CYP1A1 and 1B1 due to continuous exposure to BP may increase the susceptibility to estrogen-mediated carcinogenicity.

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.

Dose response relationship in anti-stress gene regulatory networks

To maintain a stable intracellular environment, cells utilize complex and specialized defense systems against a variety of external perturbations, such as electrophilic stress, heat shock, and hypoxia, etc. Irrespective of the type of stress, many adaptive mechanisms contributing to cellular homeostasis appear to operate through gene regulatory networks that are organized into negative feedback loops. In general, the degree of deviation of the controlled variables, such as electrophiles, misfolded proteins, and O₂, is first detected by specialized sensor molecules, then the signal is transduced to specific transcription factors. Transcription factors can regulate the expression of a suite of anti-stress genes, many of which encode enzymes functioning to counteract the perturbed variables. The objective of this study was to explore, using control theory and computational approaches, the theoretical basis that underlies the steady-state dose response relationship between cellular stressors and intracellular biochemical species (controlled variables, transcription factors, and gene products) in these gene regulatory networks. Our work indicated that the shape of dose response curves (linear, superlinear, or sublinear) depends on changes in the specific values of local response coefficients (gains) distributed in the feedback loop. Multimerization of anti-stress enzymes and transcription factors into homodimers, homotrimers, or even higher-order multimers, play a significant role in maintaining robust homeostasis. Moreover, our simulation noted that dose response curves for the controlled variables can transition sequentially through four distinct phases as stressor level increases: initial superlinear with lesser control, superlinear more highly controlled, linear uncontrolled, and sublinear catastrophic. Each phase relies on specific gain-changing events that come into play as stressor level increases. The low-dose region is intrinsically nonlinear, and depending on the level of local gains, presence of gain-changing events, and degree of feedforward gene activation, this region can appear as superlinear, sublinear, or even J-shaped. The general dose response transition proposed here was further examined in a complex anti-electrophilic stress pathway, which involves multiple genes, enzymes, and metabolic reactions. This work would help biologists and especially toxicologists to better assess and predict the cellular impact brought about by biological stressors.

To maintain a stable intracellular environment, cells are equipped with multiple specialized defense programs that are launched in response to various external chemical and physical stressors. These anti-stress mechanisms comprise primarily gene regulatory networks, and like many manmade control devices, such as thermostats and automobile cruise controls, they are often organized into negative feedback circuits. A quantitative understanding of how these control circuits operate in the cell can help us to assess and predict more accurately the cellular impacts brought about by perturbing stressors, such as environmental toxicants. Using control theory and computer simulations, we explored nature's design principle for anti-stress gene regulatory networks, and the manner in which cells respond and adapt to perturbations. We showed that cells can exploit multiple mechanisms, such as protein homodimerization, cooperative binding, and auto-regulation, to enhance the feedback loop gain, which, according to control theory, is a basic principle for effective perturbation resistance. We also illustrated that the steady-state dose response curve is likely to transition through multiple phases as stressor level increases, and that the low-dose region is inherently nonlinear. Our results challenge the common practice of linear extrapolation for evaluating the low-dose effect, and would lead to improved human health risk assessment for exposures to environmental toxicants.

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.

Formation and persistence of N7-methylguanine DNA adducts in the target pyloric tissue following chronic exposure to N-methyl-N'-nitro-N-nitrosoguanidine

Outbred 7-week old male Wistar rats were exposed for 21 days to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) via the drinking water and N7-methyl deoxyguanosine 3'-monophosphate (N7-MedGp) levels in DNA from the pyloric mucosa (target tissue) and white blood cells (wbc: non-target tissue) were determined by (32)P-postlabelling. Exposure to MNNG resulted in the non-linear, dose-related formation of N7-medGp in both tissues. Adduct levels in the pyloric mucosa were determined to be 1058, 5.4 and 1.1 μmole N7-medGp mole(-1) deoxyguanosine 3'-monophosphate (dGp) after exposure to 4.1, 0.62 and 0.006 mg MNNG kg(-1) day(-1) respectively whereas adduct levels in the wbc DNA were lower at 5.2, 0.52 and 0.68 μmoles N7-medGp mole(-1) dGp after exposure to 4.1, 0.62 and 0.062 mg MNNG kg(-1) day(-1) respectively. In addition, the persistence of N7-medGp was investigated. Loss of adduct occurred rapidly, with a decrease of 87 and 97% respectively in target tissue and wbc DNA by 48 h after cessation of 4.1 mg MNNG kg(-1) day(-1) exposure; 14 days post-MNNG treatment, however, N7-medGp was still detectable (0.46 μmole N7-medGp mole(-1) dGp) in pyloric mucosal DNA. The quantitation of N7-medGp after exposure to low doses of carcinogen, i.e. 0.006 mg MNNG kg(-1) day(-1), approaching environmentally relevant levels has not been previously reported, and indicates that the (32)P-postlabelling assay developed here possesses sufficient sensitivity to quantitate N7- medGp in human DNA arising from environmental exposure to methylating agents.

DNA adducts: biological markers of exposure and potential applications to risk assessment

DNA adducts have been investigated extensively during the past decade. This research has been advanced, in part, by the development of ultrasensitive analytical methods, such as 32P-postlabeling and mass spectrometry, that enable detection of DNA adducts at concentrations as low as one adduct per 10(9) to 10(10) normal nucleotides. Studies of mutations in activated oncogenes such as ras, inactivated tumor suppressor genes such as p53, and surrogate genes such as hprt provide linkage between DNA adducts and carcinogenesis. The measurement of DNA adducts, or molecular dosimetry, has important applications for cancer risk assessment. Cancer risk assessment currently involves estimating the probable effects of carcinogens in humans based on results of animal bioassays. Estimates of risk are then derived from mathematical models that fit data of tumor incidence at the high animal exposures and extrapolate to probable human exposures that may be orders of magnitude lower. Molecular dosimetry could extend the observable range of mechanistic data several orders of magnitude lower than can be achieved in carcinogenesis bioassays. This measurement also compensates automatically for individual and species differences in toxicokinetic factors, as well as any nonlinearities that affect the quantitative relationships between exposure and molecular dose. As a result, molecular dosimetry can provide a basis for conducting high- to low-dose, route-to-route, and interspecies extrapolations. The incorporation of such data into risk assessment promises to reduce uncertainties and produce more accurate estimates of risk compared to current methods.

Dose-response relationships for carcinogens

Biotransformation of chemical carcinogens involves both metabolic activation and detoxication. The molecular dose present on DNA as adducts represents a balance between these two pathways (formation) and DNA repair. All of these are enzymatic processes subject to saturation. When none of the pathways is saturated, linear molecular dosimetry is expected, whereas if metabolic activation is saturated, a supralinear response occurs. If detoxication or DNA repair is saturated, a sublinear response occurs. With chronic exposure, steady-state concentrations of DNA adducts develop and these follow the same patterns. With several alkylating agents, multiple adducts are formed. The extent of formation is chemically defined, but different DNA repair pathways can be involved for different adducts. By understanding the molecular dose and biology of each adduct and comparing these to the dose-response for tumor induction, it may be possible to identify the most appropriate biomarkers for risk assessment. Recently, endogenous DNA adducts identical to those induced by known human carcinogens have been identified. These endogenously formed adducts may play an important role in human carcinogenesis.

O6-alkyltransferase activity in normal and abnormal gastric mucosa

The activity of the DNA repair enzyme O6-alkyltransferase has been studied in a series of stomachs with abnormal gastric mucosa and the activities found compared with those in normal stomachs. Enzyme activities found in stomachs with the macroscopic abnormalities of gastric ulcer, duodenal ulcer or gastric cancer were not significantly different from normal. In those stomachs where there was histological evidence of chronic atrophic gastritis or intestinal metaplasia however enzyme activities (mean 398 fmole/mg) were significantly higher than normal (mean activity 228 fmole/mg activity P < 0.001). We speculate that the conditions which stimulate these histological changes also give rise to induction of O6-alkyltransferase.

What is an appropriate measure of exposure when testing drugs for carcinogenicity in rodents?

This discussion paper argues that in the carcinogenicity testing of drugs, biological measures of drug exposure may often be more relevant than the classical pharmacokinetic approaches applicable to reversible pharmacodynamic phenomena. Chemicals that produce tumors in rodents may do so (either directly or after bioactivation) by mechanisms involving inter alia a mutagenic, cytotoxic, or hormone-like effect. Such mechanisms may involve the formation of reactive metabolites of fleeting existence, and these are subject to the principles of irreversible pharmacokinetics. Examples are given of genotoxic and nongenotoxic substances for which the species and target site for tumor formation correlates not with plasma concentration, but with the amount metabolized and/or the rate of metabolism. Other compounds produce tumors in rodents, often in only one sex or species, in association with an exaggerated pharmacodynamic effect, with an increase in liver weight (by one of a diversity of mechanisms) or with hormonal or hormonal-like effects. In such cases the determinant of tumor formation is the degree of disturbance of homeostasis, not the plasma concentration of the parent substance. For drugs, the disturbance in question may have no relevance to the clinical use of the drug. Plasma concentrations of the parent substance are useful in assessing the proportion of an oral dose that is absorbed and the linearity of kinetics over the full dose range and for exploring the differences between dietary and gavage administration. They do not usually, however, provide information of direct relevance to assessment of "exposure" for the purposes of carcinogenic risk assessment.

Methylation of DNA guanine via the 1-carbon pool in dimethylnitrosamine-treated rats

Treatment of rats with radioactive methionine and nonradioactive dimethylnitrosamine resulted in the formation of radioactive 7-methylguanine in rat-liver DNA. By comparing the specific activity of administered [14C-Me]-dimethylnitrosamine to the specific activity of isolated 7-methylguanine it was determined that following 20 mg/kg dimethylnitrosamine DNA methylation via the 1-carbon pool may account for up to 30% of the total 7-methylguanine formed.

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.

Methylated purines formed in DNA by dimethylnitrosamine in rats previously exposed to hepatotoxic and hepatocarcinogenic regimes: effects on the repair of O6-methylguanine

Studies of mammalian systems for the repair of O6-methylguanine in DNA have revealed large differences in the capacities of tissues and cells to perform this function and in the case of rat liver it has been shown that the O6-methylguanine repair system can be stimulated by exposure to hepatotoxic and hepatocarcinogenic regimes. In this report an assessment is made of possible relationships between toxic liver injury, DNA synthesis, cell proliferation and DNA repair by treating Wistar rats with agents selected to provide differing degrees of liver involvement. The effects of long-term (20 week) treatments with acetylaminofluorene (15 mg/kg/day), quinoxaline 1,4-dioxide (10 mg/kg/day), 4-aminobiphenyl-HCl (15 mg/kg/day) and pronethalol (20 mg/kg/day) were assessed, using the same strain of animals in which the original toxicity and carcinogenicity data were obtained. Repair of O6-methylguanine produced in liver DNA by a low, non-toxic dose (2 mg/kg) of [14C]dimethylnitrosamine was increased 3-4-fold throughout the period of treatment with acetylaminofluorene, to a lesser extent by quinoxaline 1,4-dioxide and 4-aminophenyl-HCl and not at all in the case of pronethalol. No evidence was obtained to indicate a direct relationship between O6-methylguanine repair and either the induced hepatotoxicity or the ensuing increased rates of DNA synthesis which occur following exposure to these agents.

O6-methylguanine repair in liver cells in vivo: comparison between G1- and S-phase of the cell cycle

To compare the formation and persistence of alkylated DNA bases in the G1- and S-phase compartments in liver in vivo, regenerating rat liver was exposed to [14C]dimethylnitrosamine (0.57 mg/kg, IP injection) or N-[methyl14C]-N-nitrosourea (3.3 mg/kg, intraportal injection) during the G1 phase of the cell cycle (12 h after partial hepatectomy), or at 24 h after partial hepatectomy with 30% hepatocytes in DNA synthesis, or at 43 h after partial hepatectomy, 4 h after an hydroxyurea block from 14 to 39 h after operation with 80% hepatocytes in DNA synthesis. At 120 min after dimethylnitrosamine and 90 s, 5, 10, or 60 min after the intraportal pulse of N-methyl-N-nitrosourea the molar fractions of 7-methylguanine (7megua), O6-methylguanine (O6megua), and 3-methyladenine (3mead) and of metabolically labeled guanine were determined from DNA hydrolysates by Sephadex-G10 radiochromatography. After dimethylnitrosamine only minor differences were observed for 7megua formation in the three groups; the 3mead/7megua ratio remained constant irrespective of the number of cells in S phase. In contrast, the O6megua/7megua ratio revealed a loss of O6megua, the extent of which appeared proportional to the fraction of DNA-synthesizing cells in the liver. The rapid loss of O6megua in S-phase cells was confirmed after intraportal administration of N-methyl-N-nitrosourea. During the first 10 min after the methylnitrosourea pulse the O6megua/7megua ratio was constant in G1 cells and dropped from 90 s to 10 min by about 15% in liver containing 30% S-phase cells and by about 40% with 80% cells in DNA synthesis. DNA-synthesizing hepatocytes are apparently endowed with a higher O6megua DNA transferase activity than nonproliferating liver cells. The rapid, though exhaustible elimination of O6megua during S-phase might result in partial protection of DNA-synthesizing cells from base-mispairing and/or from hypomethylation at G-C sites.

DNA-purine methylation in hepatic chromatin following exposure to dimethylnitrosamine or methylnitrosourea

The investigations reported in this paper were designed to analyze the patterns of DNA-purine methylation in hepatic chromatin following in vivo exposure to the carcinogenic alkylating agents dimethylnitrosamine (DMN) or methylnitrosourea (MNU). Male Sprague-Dawley rats were exposed to [14C]DMN (8 mumoles, 1.0 microCi per mumole per 100 g) or [3H]MNU (15 mumoles, 10 microCi per mumole per 100 g) via gastric intubation. Hepatic chromatin was fractionated into portions having characteristics of template-active euchromatin (S2) and template-repressed heterochromatin (P2) by digestion with DNase II followed by MgCl2 precipitation. Specific DNA purines were identified at 24 hr post-intubation using an isocratic high pressure liquid chromatographic system. A qualitatively similar pattern of 7-methylguanine, O6-methylguanine, 1-methyladenine and 3-methyladenine alkylation was observed in DNA from total chromatin versus heterochromatin at 24 hr following exposure to either carcinogen. These assessments were made at times following carcinogen exposure which produced maximal quantitative differences in alkylation of euchromatin versus heterochromatin DNA. Similar patterns of DNA purine alkylation were observed in total chromatin and heterochromatin. These observations suggest that, once the reactive species is generated and access to chromatin DNA occurs, a similar pattern of DNA-purine alkylation is produced in different regions of the genome.

Implication of nonlinear kinetics on risk estimation in carcinogenesis

Efforts in estimating carcinogenic risk in humans from long-term exposure to chemical carcinogens have centered on the problem of low-dose extrapolation. For chemicals with metabolites that interact with DNA, it may be more meaningful to relate tumor response to the concentration of the DNA adducts in the target organ rather than to the applied dose. Many data suggest that the relation between tumor response and concentration of DNA adducts in the target organ may be linear. This implies that the nonlinearities of the dose-response curve for tumor induction may be due to the kinetic processes involved in the formation of carcinogen metabolite--DNA adducts. Of particular importance is the possibility that the kinetic processes may show a nonlinear "hockey-stick" like behavior which results from saturation of detoxification or DNA repair processes. The mathematical models typically used for low-dose extrapolation are shown potentially to overestimate risk by several orders of magnitude when nonlinear kinetics are present.

Role of O6-methylation in the initiation of GGTase-positive foci

The ability of seven methylating agents to form 7-methylguanine and O6-methylguanine was compared to their ability to initiate carcinogenesis as measured by the initiation of GGTase-positive foci. The seven methylating agents studied were methyl-N-nitroso-p-toluenesulfonamide (diazald), dimethylhydrazine (DMH), dimethylnitrosamine (DMN), dimethylsulfate (DMS), methyl methanesulfonate (MMS), methyl-N-nitro-N-nitrosoguanidine (MNNG) and methyl-N-nitrosourea (MNU). The DNA methylation and initiation of GGTase-positive foci was determined in partial hepatectomized rats. The formation of foci was promoted by 500 ppm sodium phenobarbital in the drinking water. While six of the seven compounds (DMH, DMN, DMS, MMS, MNNG and MNU) produced 7-methylguanine, only the four compounds (DMH, DMN, MNNG and MNU) that produced O6-methylguanine initiated GGTase-positive foci. The extent of O6-methylguanine produced by the methylating agents did not correspond with their potency to initiate GGTase-positive foci. Therefore, the initiation of GGTase-positive foci required the formation of O6-methylguanine. However, some sequential event altered the quantitative relationship of O6-methylguanine formation to the incidence of GGTase-positive foci.

Mechanistic considerations for carcinogenic risk estimation: chloroform

Chloroform has been reported to induce cancer in rodents after chronic administration of high doses by gavage. However, the interpretation of these findings is hampered by a lack of knowledge concerning the relative roles of genetic and nongenetic mechanisms in these bioassays. The present studies were carried out in male B6C3F1 mice in order to investigate the potential of chloroform to induce genetic damage and/or organ toxicity at the sites where tumors have been observed in the various bioassays. These studies revealed that carcinogenic doses of chloroform produced severe necrosis at the sites where tumors later developed. This was demonstrated by light microscopy as well as by determination of the cellular regeneration index following administration of 3H-thymidine. Noncarcinogenic doses of chloroform failed to induce these responses. In contrast, studies of DNA alkylation and DNA repair in vivo failed to give any indication that chloroform had produced the type of genetic alterations associated with known genotoxic chemicals. These data suggest that the primary mechanism of chloroform-induced carcinogenesis is nongenetic in nature. If the same mechanism predominates in man, there should be little to no carcinogenic risk associated with exposure to noncytotoxic levels of chloroform.

Removal of O6-methylguanine from DNA by human liver fractions

In in vitro assays using methylated DNAs as substrates, human liver fractions were shown to be able to catalyze the removal of O6-methylguanine. The amount of removal was proportional to the amount of protein added, and the loss of O6-methylguanine occurred with stoichiometric formation of guanine in the DNA and S-methylcysteine in protein. This indicates that human liver contains a protein similar to that previously found in bacteria exposed to alkylating agents. This protein acts as a transmethylase, transferring the intact methyl group from O6-methylguanine in DNA to a cysteine residue on that protein. A similar activity is present in rodent liver, but it was found that human liver was about 10 times more active in carrying out this reaction. In contrast, there was no difference between the human and rat liver extracts in catalyzing the loss of another methylation product, 7-methylguanine, from alkylated DNA. The liver is the organ most likely to be alkylated after exposure to exogenous potential alkylating agents such as dimethylnitrosamine. The present results show that human liver has a significant capacity to repair O6-methylguanine in DNA, which has been implicated as a critical product in carcinogenesis and mutagenesis.

Cell-specific differences in O6-alkylguanine DNA repair activity during continuous exposure to carcinogen

The activity of the alkyl acceptor protein (AAP) responsible for repair of DNA containing the promutagenic lesion O6-alkylguanine was determined in hepatocytes and nonparenchymal cells (NPC) obtained from livers of control rats and rats exposed to hepatocarcinogens that primarily induce vascular or hepatocellular neoplasms. Basal levels of AAP activity were found to be 4-5 times higher in hepatocytes than in NPC. Exposure to 1,2-dimethylhydrazine or diethylnitrosamine produced a 2- to 3-fold enhancement of this activity in hepatocytes after exposure for as little as 3 days. The enhanced hepatocyte activity persisted throughout a 28-day exposure to 1,2-dimethylhydrazine. In contrast, AAP activity in NPC was decreased during the first week of exposure to 1,2-dimethylhydrazine and subsequently returned to control levels. No enhancement of AAP was apparent in the NPC. These and related data suggest that enhancement of this activity in rat hepatocytes is a response to cell proliferation. In contrast, the data clearly demonstrate that neither increased cell replication nor the presence of O6-alkylguanine was capable of enhancing AAP activity in NPC. Cellular differences in the repair of O6-alkylguanine appear to be a critical mechanism responsible for cell specificity in chemical carcinogenesis by alkylating agents.

DNA glycosylases

Various DNA glycosylases exist, which initiate the first step in base-excision repair. A summary of the kinetic and physical characteristics of three classes of DNA glycosylases are presented here. The first class discussed, include glycosylases which recognize alkylated DNA. Various data from enzymes derived from both prokaryotic and eukaryotic sources is discussed. The second class deals with a glycosylase that recognizes and initiates the excision of pyrimidine dimers in DNA. To date, this enzyme has only been uncovered from two sources, Micrococcus luteus and the T4 bacteriophage of E. coli. The third class consists of the most studied of the glycosylases, the uracil-DNA glycosylase enzymes. Various characteristics are presented for the uracil-DNA glycosylases derived from various sources. Recent information from our laboratory is presented implicating that herpes simplex virus may mediate a uracil-DNA glycosylase activity in productivity infected cells.

Origin and formation of 1,7-dimethylguanosine in tRNA chemical and enzymatic methylation

tRNA chemical methylation: 1. 1,7-Dimethylguanosine was found in in vivo methylated tRNA from liver and kidney of rat after exposure to a low dose of dimethylnitrosamine (4 mg/kg body weight). 2. At 4 h after dimethylnitrosamine administration, the 1,7-dimethylguanosine: 7-methylguanine ratio (product ratio) for liver and kidney tRNA was 0.017 and 0.091, respectively. At 24 h after dimethylnitrosamine administration, the product ratio was lower in both hepatic and renal tRNA. 3. When dimethylnitrosamine was given in four separate daily injections, the product ratio in hepatic tRNA 4 h after the last dose was the same as for the same total dose given by a single injection, but in renal tRNA it was lower. No dialkyl compound was found in liver and kidney tRNA 24 h after the last multiple injection. tRNA enzymatic methylation: 1. Base analyses of Escherichia coli B tRNA methylated in vitro, by using S-adenosylmethionine as physiological methyl donor and enzyme preparations from liver and kidney of normal rat, indicated that 1,7-dimethylguanosine was also a product of enzymatic methylation. 2. The amount of 1,7-dimethylguanosine formed by kidney enzyme preparation was 3-times that produced by the liver extract. 3. A second type of enzymatic methylation assay where chemically methylated tRNA was used as substrate indicated that the 7-methylguanosine residues in the nucleic acid are not the substrate of the methylase activity forming the 1,7-dimethylguanosine moieties. Analogous data were obtained for the origin of 1,7-dimethylguanosine residues in tRNA chemical methylation by dimethyl sulphate.

Repair of O6-ethylguanine in DNA by a chromatin fraction from rat liver: transfer of the ethyl group to an acceptor protein

Incubation of O6-[3H]ethylguanine-containing DNA with a rat liver chromatin fraction resulted in a decrease in the O6-ethylguanine content of the DNA. Analysis of the products of this reaction showed that the ethyl group had been transferred from the O6-ethylguanine to a protein acceptor. When the incubation mixture was separated on a cesium chloride gradient, the radioactivity removed from O6-ethylguanine appeared in a low-density band. This material has been isolated and subjected to trypsin digestion and high-pressure liquid chromatography analysis; it was sensitive to trypsin and the digest contained new high-pressure liquid chromatography peaks characteristic of oligopeptides. Radioactive peaks from the trypsin digestion have been digested further to the amino acid level and have been shown to contain S-[3H]ethylcysteine. Thus, we conclude that the repair activity in rat liver chromatin removes the ethyl group from O6-ethylguanine and transfers it to a cysteine moiety contained in an acceptor protein.

A system in mouse liver for the repair of O6-methylguanine lesions in methylated DNA

An activity from mouse liver with catalyzes the disappearance of O6-methylguanine from DNA methylated with methylnitrosourea has been partially purified by ammonium sulfate fractionation and DNA-cellulose chromatography. The activity does not require divalent metal ions and is not affected by EDTA. It is specific for the repair of O6-methylguanine lesions and does not affect the removal of 7-methylguanine, 7-methyladenine or 3-methyladenine. The disappearance of O6-methylguanine is linear with respect to the concentration of protein and is dependent on incubation temperature. The kinetics and substrate dependence experiments suggest that the protein factor is product-inactivated. Amino acid analysis of hydrolysates of protein obtained after incubation of methylated DNA with the protein factor indicates the presence of radiolabeled S-methyl-L-cysteine, suggesting that during the repair of O6-methylguanine from methylated DNA, the methyl group is transferred to a sulfhydryl of a cysteine residue of a protein. This represents the first such demonstration in a mammalian system.