Tissue distribution and mode of DNA methylation in mice by methyl methanesulphonate and N-methyl-N' -nitro-N-nitrosoguanidine: lack of thymic lymphoma induction and low extent of methylation of target tissue DNA at 0-6 of guanine

Guanine N7-alkylation in mice in vivo by metrifonate - discussion of possible genotoxic risk in mammals

Following intraperitoneal administration to male mice (strain AB Jena/Halle) of 14CH3-labelled metrifonate, 22 Ci/mol, in dosages of 0.48, 0.40 and 0.065 mmol/kg, DNA from liver and kidneys was analysed for 14C in N-7 methylguanine (7-MeG). The extent of methylation in liver was found to be maximal at 6 hrs after injection in amounts of 6-8 and 0.8 mumol 7-MeG/mol guanine for the high and the low dose, corresponding to a covalent binding index CBI 4-5. The half-life of excretion of 7-MeG was 5 hrs for the high and 15 hrs for the low dose. The extent of methylation at 0-6 of guanine was estimated to be around 0.002-0.01 mumol 0-6 MeG/mol guanine. Data from references concerning methyl methanesulfonate and dimethyl sulfate are compared with those of metrifonate and the genotoxic response of methylating and non-methylating metabolites is discussed.

Chemical induction of thymomas in AKR mice: interaction of chemical carcinogens and endogenous murine leukemia viruses. Comparison of N-methyl-N-nitrosourea and methyl methanesulphonate

The time course of development of thymic lymphoma, which occurs spontaneously in mice of the AKR strain, is accelerated by the methylating agents N-methyl-N-nitrosourea (MNU) and methyl methanesulphonate (MMS). Since MNU is a potent mutagen inducing G----A transition mutations and MMS a relatively weak mutagen, it was of interest to examine the genetic alterations associated with each class of the chemically induced tumors and to compare these alterations with those found in the spontaneous tumors. The same spectrum of genetic alterations was found for MMS-induced and spontaneous thymomas. Both showed rearrangements of c-myc and Pim-1 genes that appeared to result from integration of recombinant mink cytopathic focus-forming (MCF) proviruses but failed to reveal evidence for activation of ras oncogenes, either by DNA transfection experiments or by hybridization of DNA to specific oligonucleotide probes. Some alteration in c-myc and Pim-1 genes were also found in MNU-induced tumors, but, mainly, these involved integration of ecotropic-like rather than recombinant MCF viruses. Furthermore, MNU-induced tumors frequently (in 24% of thymomas) contained G----A transition mutations, activating the Ki-ras oncogene at codon 12 position 2. Another feature that distinguishes the MNU-induced tumors from those occurring in untreated and MMS-treated mice was the consistently high level of c-myc mRNA that occurred in the absence of c-myc gene rearrangement. Taken together, the data indicate that the mechanisms of development of tumors following treatment with MNU and MMS are distinct, and that the effect of MMS is probably to speed up the process of viral leukemogenesis.

Alteration of immune function in mice following carcinogen exposure

Treatment of mice with the direct-acting alkylating agent methyl methanesulfonate produced marked suppression of the humoral response to sheep erythrocytes and suppression of T cell responses to foreign antigens. These effects occurred without loss of spleen, thymus or body weight and in the absence of peripheral blood, splenic or bone marrow cytotoxicity. In comparison, exposure to urethan decreased spleen weights, number and viability of spleen cells, and numbers of circulating lymphocytes. Significant suppression of T cell mitogen responsiveness was observed at all dose levels of urethan. Thymus weights, proliferative responses to the B cell mitogen lipopolysaccharide and delayed hypersensitivity responses were decreased at the highest urethan dose. Cyclophosphamide treatment significantly depressed thymic weight, lymphoproliferative responses of T and B cells, antibody production and delayed hypersensitivity responses. These results suggest differential sensitivity in components of the host defense system to weak carcinogens.

Studies on the distribution of O6-methylguanine-DNA methyltransferase in rat

O6-Methylguanine-DNA methyltransferase, a DNA repair enzyme which transfers the methyl group of O6-methylguanine residue to a cysteinyl residue in the methyltransferase itself, was examined in rat organs by quantifying the S-methylcysteine formed in the methyl acceptor protein. Among the various organs examined, the spleen exhibited the highest enzyme specific activity followed by the thymus, liver, lung and testis. Brain had the lowest activity. The patterns of subcellular distribution of the methyltransferase in spleen and liver were different: while 75-80% of the activity was present in the nuclear fraction of the spleen, 54% of the activity in the liver was found in the nuclei and 35% in the cytosolic fraction. Forty-five and thirty-five percent of the total nuclear enzyme activity could be extracted with 1 M and 2 M NaCl solutions, respectively, indicating that the repair enzyme is not tightly bound to the nuclear matrix. When isolated nuclei were incubated with [methyl-3H]DNA substrate and subsequently fractionated into histone and non-histone protein fractions, over 90% of the radioactivity was coeluted on a Bio-Rex 70 column with the non-histone fraction and only a negligible amount of radioactivity was found to be associated with the histone fraction. The molecular mass of the [methyl-3H]methyltransferase in the non-histone fraction was determined to be 23,000, and its pI value was found to be 6.6 by two-dimensional polyacrylamide gel electrophoresis.

Urinary excretion of 3-methyladenine and 1-methylnicotinamide by rats, following administration of [methyl-14C]methyl methanesulphonate and comparison with administration of [14C]methionine or formate

Following i.p. injection of [methyl-14C]methyl methanesulphonate (MMS) into rats (100 mg/kg) 3-[14C]methyladenine was identified as a urinary product excreted mainly up to 24 h after treatment, the amount over this period being about 0.02 mumol 3-methyladenine. When [14C]MMS and L-[methyl-3H]methionine were injected together no methyl-3H-label was detected in 3-methyladenine, nor was this product detected following injection of [methyl-14C]methionine alone or of [14C]formate. Isotopically labelled 1-methylnicotinamide (1-meNmd) was detected following all the treatments listed, and as previously found by Chu and Lawley, 1-meNmd excretion was enhanced by MMS treatment as judged by increased excretion of 1-[3H]meNmd when [14C]MMS and [3H]methionine were given together. The extent of labelling of 1-meNmd was much lower following injection of [14C] formate, than that from methionine or MMS. The results showed that 3-methyladenine derived only from direct chemical methylation by MMS. They also support the previous suggestion that [methyl-14C]meNmd can result from direct methylation, with a maximal amount of about 3% of excreted meNmd deriving from this route. The possible utility of the methods described for monitoring in vivo alkylation is discussed.

Quantitation of chemically induced DNA strand breaks in human cells via an alkaline unwinding assay

DNA strand breaks induced in human CCRF-CEM cells by electrophilic chemicals (carcinogens/mutagens) can be readily quantitated via a facile alkaline unwinding assay. This procedure estimates the number of chemically induced DNA strand breaks on the basis of the percentage DNA converted from double-stranded to single-stranded form during an exposure to the alkaline unwinding conditions. The assay is based on the assumption that each strand break serves as a strand unwinding point during the alkaline denaturation. The extent of strand separation can be standardized with respect to the initial level of induced strand breaks by the use of X-rays, which produce known levels of DNA strand breaks per rad in mammalian cells. Subsequent to the alkaline exposure, the single- and double-stranded DNA were separated by use of thermostated hydroxylapatite columns (60 degrees C), and the DNA was quantitated via a fluorescence assay (Hoechst 33258 compound). A correlation was shown between mammalian DNA strand-breaking potential (as measured in this procedure) and the propensity of these chemicals to revert Salmonella typhimurium TA100.

DNA modification by chemical carcinogens

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

Induction of micronuclei in mouse fetal liver after exposure in utero to N-methyl-N'-nitro-N-nitrosoguanidine

The effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) on the induction of micronuclei were examined in mouse fetuses exposed in utero. By this study, MNNG was proved to be mutagenic in vivo. The frequency of micronucleated erythrocytes (MNEs) in fetal liver peaked at 18 h after a single intraperitoneal injection into pregnant mice on day 13 of gestation. Then, to examine the effects of administration routes on the induction of micronuclei, the chemical was given by various routes, and the percentage of MNEs (%MNEs) in fetuses were examined 18 h after treatment. The %MNEs after administration of MNNG intraperitoneally, subcutaneously, intravenously, and orally was 4.7, 1.9, 0.8 and 0.3, respectively. The control value was 0.3. In the intraperitoneally treated mice, %MNEs for fetuses in the uterine horn located nearer the injection site was higher than that in the other. In addition, in the intraperitoneally treated mice, there was a tendency for the higher %MNEs to occur in the fetuses located near the injection site. Together with the results on the distribution of MNNG in mice (Frei and Lawley, 1976), these findings suggest that MNNG might be inactivated in the maternal systemic circulation and that the agent which induces micronuclei might be distributed to the fetuses by diffusion.

A comparative study of guanine N7-alkylation in mice in vivo by the organophosphorus insecticides trichlorphon, dimethoate, phosmet and bromophos

Following intraperitoneal administration to male mice (strain AB Jena/Halle) of 14C-methyl-labelled trichlorphon, dimethoate, phosmet and bromophos, 10-20 Ci/mol, in dosages of 0.06-0.55 mmol/kg, DNA from liver and kidneys was analyzed for 14C in N-7 methylguanine (7-MeG). The extents of methylation were in the range of 5-10 mumol 7-MeG/mol guanine for trichlorphon and dimethoate and of 0.2-0.4 for phosmet and bromophos, for high doses, respectively Excretion half-lives of 7-MeG were differing between trichlorphon (5 hrs, high dose, and 15-17 hrs, low dose) and dimethoate (23-160 hrs, high dose). The extents of methylation at 0-6 of guanine were estimated to be around 0.01 mumol 0-6 MeG/mol guanine for high doses of organophosphates of sufficient water solubility. Factors associated with the partition of organophosphates in mammalian systems are useful for estimating DNA attack by organophosphates in mammals in vivo.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Dosimetry of genotoxic agents and dose-response relationships of their effects

Dose-response relationships and determination of dose of mutagens and carcinogens are summarized and discussed on the basis of conceptual and kinetic aspects. Different dose definitions may be referred to steps in the chain of events from exposure (or emission) to observed effects. A system is applied to show the influence of various processes on the kinetics of the transfers between consecutive steps. The same system illustrates processes influenced by protraction and fractionation of dose, synergists, comutagens/cocarcinogens, heritable factors, etc. The response at a given dose is expected to depend on the product of consecutive transfer functions. An application of general rules of chemical kinetics shows that when a chemical is introduced at a sufficiently low level, all processes affecting the transfers and therefore the transfer functions themselves become first-order, provided the induction status of enzymes and the cell-division rate remain constant. Under the same conditions, dose-response relationships are expected to be linear, i.e. without "safe" thresholds. However, present knowledge of the kinetics of repair at low levels of DNA damage and of the kinetics of induction of repair functions is not enough complete to be decisive. These considerations and the fact that observed dose-response data in some cases indicate the existence of thresholds but in others appear able to reject the threshold hypothesis lead to the conclusion that, generally, dose-response curves are most probably linear down to dose zero. However, certain mutagens/carcinogens give rise to lesions repaired so effectively that quasi-thresholds appear in certain subpopulations or organs.

The induction of urothelial hyperplasia by methyl methanesulphonate and ethyl methanesulphonate

The early and late morphological changes induced in rat bladder urothelium by intravesicular administration of the alkylating agents methyl methanesulphonate (MMS) and ethyl methanesulphonate (EMS) are described. In the short-term, both compounds produced dose-related toxic damage followed by a regenerative hyperplasia of the urothelium. At any given dose-level, the effects of MMS were more severe than those of EMS. Two years after administration of multiple doses of 2.5 mg MMS or 7.5 mg EMS the majority of animals had dose-related simple urothelial hyperplasias with occasional mild dysplasia. However, in three MMS-treated animals the hyperplasias had progressed to well-differentiated transitional-cell carcinomas. No bladder neoplasms were seen in EMS-treated animals. The urothelial response of the rat to MMS and EMS is discussed with reference to the known chemical reactivity of these compounds. It is concluded that EMS is a mitogen for the urothelium and that the few carcinomas which develop following topical exposure of the bladder to MMS do not necessarily reflect any initiating potential in this compound. Rather it is argued that the results are consistent with MMS acting as a promoter in cells which have either been previously initiated or which carry a latent oncogene.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

DNA-methylation by nitrosocimetidine and N-methyl-N-nitro-N-nitrosoguanidine in the intact rat

The ability of the nitroso derivative of the drug cimetidine to interact with cellular macromolecules in the intact rat was investigated. Radio-labelled nitrosocimetidine (NC) was shown to methylate DNA in a variety of tissues in the rat after oral administration. Radioactivity was also detected in the RNA and protein extracted from these same tissues. Methylation of DNA by the parent compound, cimetidine, was not detected in any of the tissues studied. For comparison, the DNA methylation produced by the carcinogen N-methyl-N-nitro-N-nitrosoguanidine (MNNG) dosed orally was measured. DNA alkylation by MNNG was found to be approx. 2-36 times greater than that produced by NC, varying with the tissues studied. The highest yield of DNA alkylation was found in the stomach for MNNG and the small intestine for nitrosocimetidine suggesting pharmacokinetic differences.

Binding of N-nitroso carcinogens in pancreatic tissue

Carcinoma of the pancreas can be produced in guinea pigs by the oral administration of 1-methyl-1-nitrosourea (MNU). Our present studies have examined the distribution of this compound in these animals following parenteral and oral administration and also its specific sites of binding within the pancreatic cell. Emphasis has been placed upon the comparison of the uptake and binding of MNU with the related compound 1-methyl-3-nitro-1-nitrosoguanidine (MNNG), which does not seem to be carcinogenic to the pancreas after oral administration. The distribution data indicate uptake into pancreas that is comparable to that of other organs. Binding data show both MNU and MNNG to extensively modify subcellular organelles and the DNA and proteins of chromatin, but that MNU does this to a somewhat greater extent than MNNG.

Mutagenicity of carcinogenic methylating agents is associated with a specific DNA modification

The carcinogenic potency of simple aliphatic alkylating agents such as the alkylnitrosamides and the alkylmethanesulphonates is positively correlated with their ability to alkylate the relatively weakly nucleophilic oxygen atoms in DNA, particularly the O6-atom of guanine. Differences in the spectrum of DNA alkylations produced by these agents can be rationalised on chemical grounds in that the electrophilic reactivity of the alkylating species determines the extent to which it will react at sites of weaker nucleophilicity. Alkylation of the more strongly nucleophilic ring nitrogen atoms of the purine bases, which is the main site of reaction with all these agents, appears to be much less important in alkylation carcinogenesis. O6-alkylation of guanine is likely to interfere with DNA base-pair hydrogen bonding and is possibly the major DNA modification responsible for the induction of GC yields AT transition mutations in bacteria and bacteriophage by alkylating agents. Here, we have studied the effects of three methylating agents of contrasting carcinogenic potency on mammalian (V79 Chinese hamster) cells in in culture. We report that the mutagenicity but not the cytotoxicity of each agent reflects its carcinogenicity and, furthermore, that the marked differences in mutagenicity are closely paralleled by differences in levels of O6-guanine methylation.

DNA synthesis with methylated poly(dC-dG) templates. Evidence for a competitive nature to miscoding by O(6)-methylguanine

The alternating copolymer poly(dC-dG) has been methylated with either dimethyl sulphate or N-methyl-N-nitrosourea and the levels of the various methylation products determined. In addition to the 3-methylcytosine, 3-methylguanine and 7-methylguanine (produced by both agents) reaction with N-methyl-N-nitrosourea also yielded easily detectable amounts of O(6)-methylguanine and phosphotriesters. These methylated polymers were then used as templates in an in vitro assay with Escherichia coli DNA polymerase I measuring the incorporation of complementary (dCMP and dGMP) and noncomplementary (dAMP and dTMP) nucleotides. When the dimethyl sulphate-methylated polymer was used as template there was virtually no detectable incorporation of non-complementary nucleotides indicating that no miscoding could be attributed to the presence of 3-methylcytosine, 3-methylguanine or 7-methylguanine. However, when the N-methyl-N-nitrosourea-methylated polymer was used as template there was a specific incorporation of dTMP but not of dAMP. The amount of dTMP incorporated was always less than the level of O(6)-methylguanine in the template and was found to vary with the relative concentrations of the deoxynucleoside 5'-triphosphates in the assay. As the amount of dCTP present in the assay was decreased the wrong incorporation of dTMP increased and approached the level that would have been expected for a one-to-one miscoding by O(6)-methylguanine as the concentration of dCTP approached zero. The results indicate that O(6)-methylguanine is capable of miscoding with a DNA polymerase but the miscoding is competitive with the normal incorporation of dCMP: when the 5'-triphosphate precursors are present in equal amounts approximately one O(6)-methylguanine in three miscodes leading to the incorporation of dTMP.

Alkylation of deoxyribonucleic acid in vivo in various organs of C57BL mice by the carcinogens N-methyl-N-nitrosourea, N-ethyl-N-nitrosourea and ethyl methanesulphonate in relation to induction of thymic lymphoma. Some applications of high-pressure liquid chromatography

1. Methods were developed for analysis of alkylpurines, O2-alkylcytosines, and representative phosphotriesters [alkyl derivatives of thymidylyl(3'-5')thymidine], in DNA alkylated in vivo, using high-pressure liquid chromatography. 2. The patterns of alkylation products in DNA in vivo at short times were closely similar to those found for reactions in vitro. Alkylation by the nitrosoureas was complete in vivo within 1 h, but with ethyl methanesulphonate was maximal at 2--4h. 3. The time course of persistence of alkylation products in vivo was determined for several tissues. In addition to the rapid loss of 3- and 7-alkyladenines reported previously for all tissues, a relatively rapid loss of O6-alkylguanines from DNA of liver was found which was more rapid at lower doses. In brain, lung and kidney, excision of O6-alkylguanine was much less marked, but was not entirely excluded by the data. In thymus, bone marrow and small bowel, all alkylated bases were lost with half-lives of 12--24h, at non-cytotoxic doses of alkylation. 4. No evidence for any marked excision of other minor products from alkylated DNA in vivo was found; thus 1-methyladenine, O2-ethylcytosine (found in appreciable amount only with N-ethyl-N-nitrosourea), 3-methylguanine, and dTp(Alk)dT persisted in alkylated DNA, including DNA of liver. 5. The induction of thymic lymphoma was determined over the range of single doses by intraperitoneal injection up to about 60% of the LD50 values, and related to the extent of alkylation of target tissues thymus and bone marrow. With N-methyl-N-nitrosourea over 90% tumour yield was attained at 60 mg/kg, and with N-ethyl-N-nitrosourea up to 52% at 240 mg/kg, but with ethyl methanesulphonate at up to 400 mg/kg only a few per cent of tumours were obtained. 6. The carcinogenic effectiveness of the agents was positively correlated with the extents of alkylation of guanine in DNA of target tissues at the O-6 atom. On the basis that at doses giving equal carcinogenic response these extents of alkylation would be equal, the chemical analyses showed that the ratio of equipotent doses to that for N-methyl-N-nitrosourea would be, for N-ethyl-N-nitrosourea, 5.3 for ethyl methanesulphonate about 21, and for methyl methanesulphonate [Frei & Lawley (1976) Chem.-Biol. Interact. 13, 215--222] about 144. These predictions were in reasonably good agreement with the observed dose-response data for these agents.