Methylation of DNA in various organs of C57B1 mice by a carcinogenic dose of N-methyl-N-nitrosourea and stabiltty of some methylation products up to 18 hours

Dietary restriction during murine development provides protection against MNU-induced mutations

The developmental stage is the most rapid period for the accumulation of somatic mutations. Epidemiological studies have also suggested a significant role of early life for cancer susceptibility, showing a protective effect of modest dietary restriction early in life. To determine if mutation rate, diet, and cancer risk are related, we have investigated the effect of dietary restriction on somatic mutations early in life. The diet of mouse dams was restricted during pregnancy and lactation by 10% from ad libitum control. F(1) pups (SWRxMutaMouse) were weaned at 3 weeks of age. Pups from dams that were on a restricted diet were kept under dietary restriction (40% until 5 weeks of age and then 20% until sacrifice). Only females from litters of seven or eight were used in this study. A portion of pups from both groups were treated with N-methyl-N-nitrosourea (MNU, 50mg/kg, i.p.) at 5 weeks of age and all mice were sacrificed at 10 weeks of age. The frequency of induced mutations was reduced by about 30% at the three loci studied, lacZ (P=0.028) and cII (P=0.042) and Dlb-1 (P=0.032) in the small intestine in the restricted group. A similar decrease in the lacZ mutant frequency was observed in the bone marrow, but the results did not reach statistical significance (P=0.074). Few differences in the lacZ mutant frequency were observed in the colon and the mammary epithelium, but variability of the mutant frequencies was such that an effect of similar magnitude could not be excluded statistically. Analysis of 47 cII mutants revealed that the majority of MNU-induced mutations were G:C to A:T transition at non-CpG sites, with no difference in the mutation spectrum between the two dietary groups.

Comparative mutagenicity of chemicals selected for test in the International Program on Chemical Safety's collaborative study on plant systems for the detection of environmental mutagens

A review has been made for the four compounds (maleic hydrazide, methyl nitrosourea, sodium azide, azidoglycerol) tested in the International Program on Chemical Safety's collaborative study on plant systems. Maleic hydrazide (MH) is a weak cytotoxic/mutagenic chemical in mammalian tissues and is classified as a class 4 chemical. In contrast, with few exceptions such as Arabidopsis, MH is a potent mutagen/clastogen in plant systems. The difference in its response between plant and animal tissue is likely due to differences in the way MH is metabolized. MH appears to be noncarcinogenic and has been given a negative NCI/NTP carcinogen rating. Methyl nitrosourea (MNU) is a toxic, mutagenic, radiomimetic, carcinogenic, and teratogenic chemical. It has been shown to be a mutagen in bacteria, fungi, Drosophila, higher plants, and animal cells both in vitro and in vivo. MNU is a clastogen in both animal and human cell cultures, plant root tips and cell cultures inducing both chromosome and chromatid aberrations as well as sister-chromatid exchanges. Carcinogenicity has been confirmed in numerous studies and involves the nervous system, intestine, kidney, stomach, bladder and uterus, in the rat, mouse, and hamster. MNU produces stage-specific teratogenic effects and also interferes with embryonic development. The experimental evidence that strongly indicates the mutagenic effects of MNU underlines the possible hazard of this compound to human beings. The experimental evidence for the stringent handling of this compound is clear. Sodium azide (NaN3) is cytotoxic in several animal and plant systems and functions by inhibiting protein synthesis and replicative DNA synthesis at low dosages. It is mutagenic in bacteria, higher plants and human cells and has been used as a positive control in some systems. In general, tests for clastogenicity have been negative or weakly positive. No evidence of carcinogenicity has been reported in a 2-year study seeking carcinogenic activity in male and female rats. Its advantages in comparison to other efficient mutagens are claimed to be a high production of gene mutations accompanied by a low frequency of chromosomal rearrangements and safer handling because of its nonclastogenic and noncarcinogenic action on humans. Azidoglycerol (AG) is a very potent mutagen in bacteria, yeast and higher plants including Arabidopsis and Tradescantia; however, it only slightly enhances the frequencies of recessive lethals in Drosophila. AG is at best a weak clastogen and is without effect in inducing chromosomal aberrations and SCEs in human peripheral lymphocytes in vitro. In microbial and plant systems, AG is considerably more potent than sodium azide in the maximal frequencies of mutation induced. In particular, in Saccharomyces cerevisae, AG is 3000-fold more mutagenic than sodium azide. Its carcinogenic and teratogenic properties are unknown.

Alkyltransferase transgenic mice: probes of chemical carcinogenesis

Transgenic mice expressing DNA-repair genes are an instructive model with which to study the protective role of DNA-repair pathways in both spontaneous and chemical carcinogenesis. Of particular interest in chemical carcinogenesis is the DNA-repair protein O6-alkylguanine-DNA alkyltransferase (alkyltransferase) which repairs O6-alkylguanine-DNA adducts. Transgenic mice carrying expression constructs for the alkyltransferase gene--either the human MGMT cDNA or the bacterial ada gene--express increased levels of alkyltransferase and have increased capacity to remove O6-methylguanine-DNA adducts. Protection from the DNA damaging effects of N-nitroso compounds occurs specifically in the cells and tissues in which the alkyltransferase transgene is expressed. For instance, mice carrying the PEPCKada construct have increased alkyltransferase in the liver and more rapid removal of O6methylguanine-DNA adducts. The protective effect is noted in hepatocytes, which express PEPCK-linked genes, not in nonparenchymal cells of the liver, which do not. Other tissues that express the transgene in the various models include the thymus, spleen, testes, muscle, stomach and brain. Mice expressing the human alkyltransferase in the thymus have a reduced incidence of thymic lymphomas following exposure to methyl nitrosourea (MNU), evidence of a role for this DNA-repair protein in protection from carcinogenesis due to N-nitroso compounds. Protection has also been observed in the induction of hepatic tumors by N-nitroso-dimethylamine (NDMA). These models will be used to identify whether overexpression of a single DNA-repair gene can block the carcinogenic process of N-nitroso compounds in many different tissues.

Age-related studies on the removal of 7-methylguanine from DNA of mouse kidney tissue following N-methyl-N-nitrosourea treatment

To investigate the effects of age on DNA repair of alkylation damage, C57BL/6NNia mice ranging from 9 months to 29 months of age were injected by the intraperitoneal route with single doses of N-methyl-N-nitrosourea (MNU). The rates of removal of 7-methylguanine (m7Gua) in nuclear DNA from kidney were determined at various intervals from 1 to 288 h after injection of either 25 mg or 50 mg MNU per kg body weight. Reversed phase HPLC with electrochemical detection was used to monitor adduct disappearance from DNA hydrolysates. The kinetics of m7Gua removal from DNA were at least biphasic. Evidence was obtained that there was a rapid removal of m7Gua occurring in the first 24 h after MNU administration, followed by a slow phase of removal with a t1/2 greater than 150 h. We assume that these two phases of m7Gua removal correspond to active repair of DNA by N-alkylglycosylases and to passive elimination via spontaneous hydrolysis, respectively. Young and old kidney tissues all exhibited significant repair of m7Gua (55-73% of the induced adducts were removed in the first 24 h), but a substantial fraction of m7Gua was removed slowly, indicating that there are methylated bases which were refractory to repair processes. At both doses of MNU studied, old tissues showed active repair of m7Gua that, within the limits of detection, had similar initial rates of removal as young tissues. However, old kidney did not remove this adduct with the same overall efficiency as young kidney. Therefore, the amount of m7Gua in the repair-resistant fraction was greater in the senescent tissues. The biochemical mechanisms responsible for the less efficient DNA repair in senescent kidney are not known, but we suggest that such differences are due in part to structural alterations in the chromatin.

The prevention of thymic lymphomas in transgenic mice by human O6-alkylguanine-DNA alkyltransferase

Nitrosoureas form O6-alkylguanine-DNA adducts that are converted to G to A transitions, the mutation found in the activated ras oncogenes of nitrosourea-induced mouse lymphomas and rat mammary tumors. These adducts are removed by the DNA repair protein O6-alkylguanine-DNA alkyltransferase. Transgenic mice that express the human homolog of this protein in the thymus were found to be protected from developing thymic lymphomas after exposure to N-methyl-N-nitrosourea. Thus, transgenic expression of a single human DNA repair gene is sufficient to block chemical carcinogenesis. The transduction of DNA repair genes in vivo may unravel mechanisms of carcinogenesis and provide therapeutic protection from known carcinogens.

Cholecystokinin inhibits DNA alkylation induced by N-nitrosobis (2-oxopropyl)amine (BOP) in hamster pancreas

Cholecystokinin (CCK) inhibits pancreatic cancer but not hepatic tumor induction by N-nitrosobis (2-oxopropyl) amine (BOP) in hamsters when administered with or shortly before BOP. In this study, we evaluated the capability of sulfated CCK-8 to inhibit DNA alkylation in the hamster pancreas. We examined the pattern of O6-methylguanine (G6-Me) and N7-methylguanine (G7-Me) in pancreatic ductal, acinar and liver tissues from Syrian hamsters treated with a single dose of BOP (20 mg/kg s.c.) and with five s.c. injections of CCK-8 (200 pM/kg, 30 min apart). The first CCK injection was given either 90 min before, or together, or 3 h after POP administration. The amount of G6-Me in liver DNA did not differ significantly. We observed a decrease of G7-Me in the liver of the group treated with CCK together with POP as compared to POP alone (P less than 0.005). Lower amounts of G6-Me were found in ductal preparations (P less than 0.01) of the animals treated with CCK before POP as compared to POP alone. CCK also modified the pattern of alkylation in the acinar tissue, but without a clear relationship with the timing of administration. The results suggest that the inhibitory effect of CCK-8 on pancreatic carcinogenicity of BOP could be related to its capability to modify DNA alkylation by yet unknown mechanisms.

Allylic compounds bind directly to DNA: investigation of the binding mechanisms in vitro

The in vitro binding of three allyl compounds, allyl methanesulphonate, allyl bromide and allyl chloride to DNA from salmon sperm was investigated. Kinetic DNA-alkylation measurements revealed significant differences between the strongly alkylating allyl methanesulphonate and allyl bromide with half-life values of 1.5 h and 8.1 h, respectively and the weakly alkylating allyl chloride with a half-life value of 360 h. Five alkylated nucleic bases, 3-allyladenine, N6-allyladenine, N2-allylguanine, 7-allylguanine and O6-allylguanine, could be identified after DNA-alkylation with all three allyl compounds.

Variables influencing DNA-binding in mouse liver

The suitability of certain mouse strains for carcinogenicity testing has been questioned. Some chemicals increase the incidence of liver tumors above a relatively high background, an effect not seen in rats. This raises the question whether species and tissue specific effects are involved which are reflected in the DNA binding of metabolites. DNA binding indices in mouse liver have been determined in only a few instances. They are comparable to those found for rat liver DNA with aniline, benzo(a)-pyrene, butadiene, dimethylnitrosamine, methylnitrosourea and they are lower in the mouse with aflatoxin B1, trans-4-acetylaminostilbene and 2-aminofluorene derivatives. The available data on DNA binding in mouse liver suggest that the same adducts are formed as in rats but that metabolism and repair are variables which can modify the extent of DNA damage. However, the extent of DNA binding does not always correlate with the susceptibility of this tissue to carcinogenesis. But mouse liver is no exception in this respect. It is concluded that the formation of mouse liver tumors in long term studies with genotoxic chemicals indicates tumor initiating potential. In contrast, there are other chemicals such as chlorinated hydrocarbon insecticides which do not bind to DNA to any extent and which are not genotoxic in common short term tests and yet give rise to liver tumors in mice but not in rats. Positive results in long term studies are suggested to indicate promoting properties of such compounds.

Interaction between 1,2-dichloroethane and tetraethylthiuram disulfide (disulfiram). II. Hepatotoxic manifestations with possible mechanism of action

The synergistic hepatotoxicity of dietary disulfiram (DSF) with 1,2-dichloroethane (DCE) subchronically administered by inhalation at three concentration levels (150, 300, and 450 ppm) was studied. The criteria for hepatotoxicity were treatment-related increases in serum activities of sorbitol dehydrogenase, 5'-nucleotidase, and alkaline phosphatase, and in liver-to-body weight ratios. DSF alone did not elicit these responses while DCE at the highest concentration level increased liver-to-body weight ratios and the activity of 5'-nucleotidase. Exposure to DSF alone decreased cytochrome P450 levels, but in combination with DCE, the decrement of cytochrome P450 was additive in a DCE concentration-dependent manner. However, depression of cytochrome P450 by DCE alone was not concentration dependent. Although DSF and DSF/DCE combination increased the activity of glutathione S-transferases (GSTs), both DSF and DCE singly and in combination increased the tissue levels of reduced glutathione (GSH). Evidence is presented showing that the potentiation of the hepatotoxicity of DCE observed in the presence of DSF may be due to an inhibition of microsomal mixed-function oxidase-mediated metabolism of DCE and to a compensatory increase in DCE metabolism to reactive metabolites generated by GST-mediated conjugation of DCE with GSH.

Modification of N-methyl-N-nitrosourea-induced urinary bladder carcinogenesis in rats following stimulation of urothelial proliferation by a partial cystectomy

The present experiments are concerned with the question whether stimulation of urothelial proliferation modifies tumor development in the urinary bladder. To induce proliferative activity of the urothelium a partial cystectomy (one-third resection of the bladder) was performed in female Wistar rats. N-Methyl-N-nitrosourea (MNU) was used as a carcinogen which acts directly on the urothelium without requiring metabolic activation. MNU was given as a single intravesicular dose of 5 mg/kg body weight via a urethral catheter. After an experimental period of 15 months rats with an intact quiescent bladder showed a tumor incidence of 32.6%. Rats having received MNU 45 h following partial cystectomy - when proliferative activity reached its peak - had developed bladder tumors with a frequency of 17.9%. Initial administration of MNU followed 24 h later by a one-third resection of the bladder resulted in a tumor incidence of only 8.8%. The histologic types of tumors induced proved to be similar to those found with other carcinogens. However, by contrast the majority of urothelial tumors were characterized by a squamous metaplasia. There was no substantial difference between the various histologic tumor types found in the resting and regenerating bladder. The mechanisms responsible for the observed inhibition of tumor development in the regenerating bladder are unknown. It is assumed that an increased capacity of the proliferating urothelial cells to repair carcinogen-induced DNA damage may play an important role.

The kinetics of in vivo sister chromatid exchange induction in mouse bone marrow cells by ethylnitrosourea and methylnitrosourea

Administration of ethylnitrosourea (ENU) (20, 25, 40, 50, and 60 mg/kg body weight) or methylnitrosourea (MNU) (25, 40, 50, 60, 75, and 80 mg/kg body weight) to male CD-1 mice 2 hr after sc implantation of a 5-bromo-2'-deoxyuridine (BrdUrd) pellet (55 mg) resulted in a dose-dependent increase in sister chromatid exchanges (SCE) in bone marrow cells. Treatment with ENU (50 mg/kg body weight) at several time points prior to BrdUrd implantation resulted in a multiphasic curve of SCE induction indicating at least two events that result in SCEs. Treatment with ENU at the time of BrdUrd implantation and post-BrdUrd reflected a similar mechanism apparent in the pre-BrdUrd curve. Treatment with MNU (50 mg/kg body weight) pre- and post-BrdUrd resulted in a linear monophasic curve of SCE induction in both the pre- and post-BrdUrd time periods. The overall MNU time-course curve resembled an inverted V function suggesting the mechanisms of SCE induction for ENU and MNU are different. These observations suggest that at least one explanation for the differences in the time courses for ENU and MNU SCE induction may result from a more persistent lesion being induced by ENU. In addition, these results indicate that in vivo SCE protocols which utilize a single acute chemical exposure at or near the time of BrdUrd labeling may not be useful for judging the relative activities of genotoxic agents.

DNA sequence has an effect on the extent and kinds of alkylation of DNA by a potent carcinogen

A system has been developed to study the effects of base sequence (neighboring bases) upon the alkylation of guanine (G) and adenine (A) bases in DNA. The study was performed on the synthetic polydeoxyribonucleotides, poly(dG).poly(dC), poly(dG-dC).poly(dG-dC), poly(dA).poly(dT), poly(dA-dT).poly(dA-dT), poly(dA-dC).poly(dG-dT), poly(dA-dG).poly(dC-dT), as well as calf thymus DNA. Each polynucleotide was treated with N-[3H]methyl-N-nitrosourea (MNU), depurinated, and the freed alkylpurines separated by HPLC and quantitated by liquid scintillation counting. The amounts of 3-methylguanine (3-MG), 7-MG, and O6-MG relative to guanine, and 3-methyladenine (3-MA) and 1-MA plus 7-MA relative to adenine, and also the O6-MG/7-MG ratios were highly reproducible for a given polynucleotide. Significant differences were found in the amounts of each of the methylpurines formed when compared among the six synthetic polynucleotides and DNA. This evidence is interpreted as an effect upon alkylation which is ultimately dependent upon the base sequence. These findings may have significance in defining the specificity of chemical carcinogens in terms of the susceptability to modification of nucleotide sequences such as those found in certain oncogenes.

O6 alkylguanine-DNA alkyltransferase activity in human myeloid cells

The association between alkylating agent exposure and acute nonlymphocytic leukemia in humans indicates that myeloid cells may be particularly susceptible to mutagenic damage. Alkylating agent mutagenesis is frequently mediated through formation and persistence of a particular DNA base adduct, O6alkylguanine, which preferentially mispairs with thymine rather than cytosine, leading to point mutations. O6alkylguanine is repaired by O6alkylguanine-DNA alkyltransferase (alkyltransferase), a protein that removes the adduct, leaving an intact guanine base in DNA. We measured alkyltransferase activity in myeloid precursors and compared it with levels in other cells and tissues. In peripheral blood granulocytes, monocytes, T lymphocytes, and B lymphocytes, there was an eightfold range of activity between individuals but only a twofold range in the mean activity between cell types. Normal donors maintained stable levels of alkyltransferase activity over time. In bone marrow T lymphocytes and myeloid precursors, there was an eightfold range of alkyltransferase activity between donors. Alkyltransferase activity in the two cell types was closely correlated in individual donors, r = 0.69, P less than 0.005, but was significantly higher in the T lymphocytes than the myeloid precursors, P less than 0.05. Liver contained the highest levels of alkyltransferase of all tissues tested. By comparison, small intestine contained 34%, colon 14%, T lymphocytes 11%, brain 11%, and myeloid precursors 6.6% of the activity found in liver. Thus, human myeloid precursors have low levels of O6alkylguanine-DNA alkyltransferase compared with other tissues. Low levels of this DNA repair protein may increase the susceptibility of myeloid precursors to malignant transformation after exposure to certain alkylating agents.

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.

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.

Reduced glutathione inhibits the alkylation by N-nitrosodimethylamine of liver DNA in vivo and microsomal fraction in vitro

The transfer of radioactivity from N-nitroso-[14C]dimethylamine to trichloroacetic acid precipitable macromolecules in the microsomal fraction of rat liver was investigated. This transfer was found to depend on N-nitrosodimethylamine being metabolized. Cytosolic fraction and cytosol enriched with reduced glutathione inhibited the binding of radioactivity to acid insoluble proteins. Depletion of glutathione in rat liver with diethylmaleate prior to i.v. administration of 10 mg N-nitroso-[14C]dimethylamine/kg led to an increase in O6-methylguanine and N-7-methylguanine in DNA. If rats were fed disulfiram for 6 days (2 g/kg feed), glutathione and glutathione S-transferase were enhanced, and the degree of methylation of guanine by N-nitrosodimethylamine was greatly reduced, as was the metabolism of N-nitrosodimethylamine in the intact animal. Fasting rats for 24 h did not change the N-nitrosodimethylamine-demethylase activity in vitro but greatly enhanced the methylation of guanine in vivo, while the glutathione content and glutathione S-transferase activity were not changed compared to fed animals.

Evaluation of the DNA-repair host-mediated assay. I. Induction of repairable DNA damage in E. coli cells recovered from liver, spleen, lungs, kidneys, and the blood stream of mice treated with methylating carcinogens

The DNA-repair host-mediated assay was further calibrated by determining the genotoxic activities of 4 methylating carcinogens, namely, dimethylnitrosamine (DMNA), 1,2-dimethylhydrazine (SDMH), methyl nitrosourea (MNU) and methyl methanesulphonate (MMS) in various organs of treated mice. The ranking of the animal-mediated genotoxic activities of the compounds was compared with that obtained in DNA repair assays performed in vitro. The differential survival of strain E. coli K-12/343/113 and of its DNA-repair-deficient derivatives recA, polA and uvrB/recA, served as a measure of genotoxic potency. In the in vitro assays and at equimolar exposure concentrations, MMS and MNU are the most active chemicals, followed by DMNA, which shows a slight genotoxic effect only in the presence of mouse liver homogenate; SDMH has no activity under these conditions. In the host-mediated assays, the order of genotoxic potency of the compounds was quite different: those carcinogens which require mammalian metabolic activation, namely, DMNA and SDMH, show strong effects in liver and blood, a lesser effect in the lungs and kidneys and the least effect in the spleen. The activity of MNU, a directly acting compound, is similar in all organs investigated, but it is clearly lower than that of DMNA and SDMH. MMS, also a directly acting carcinogen, causes some (barely significant) effect at the highest dose tested. A similar order of potency was observed when the compounds were tested in intrasanguineous host-mediated assays with gene mutation as an endpoint. DMNA and SDMH induce comparable frequencies of L-valine-resistant mutants in E. coli K-12/343/113 recovered from liver and spleen of treated mice, the effect in the liver being the strongest. MNU is mutagenic only at a higher dose, while MMS shows no effect. The results are discussed with respect to the literature data on organ-specific DNA adduct formation induced by the compounds. It is concluded that qualitatively there is a good correlation between the degree of genotoxic activity found in the DNA repair host-mediated assay and DNA adduct formation in the animal's own cells. This is exemplified by the finding that the relative order of genotoxic activity of the 4 methylating agents in bacteria recovered from various organs (DMNA approximately equal to SDMH greater than MNU greater than MMS) is reflected by the same order of magnitude in DNA alkylation in corresponding mammalian organs. Quantitatively, the indirectly acting agents DMNA and SDMH seem to induce fewer genotoxic effects in bacteria present in the liver than would be expected on the basis of DNA-adduct formation data.

Effects of disulfiram on the metabolism of nitrosodiethylamine during liver carcinogenesis

We have studied the effects of disulfiram (DSF) administration on the metabolism of nitrosodiethylamine (NDEA) in rats during acute and chronic administration. DSF was found to have the following effects during the course of carcinogenesis: (a) marked decrease in the exhalation of 14CO2 derived from 14C-NDEA; (b) reduction of the total levels of DNA and RNA ethylation in the liver. In acute experiments DSF caused an increase in the amount of NDEA in organs and in the urine. We suggest that inhibition of NDEA biotransformation and the subsequent decrease in the total level of DNA ethylation may prevent specific chemical interactions relevant to carcinogenesis.

Metabolism of, and DNA methylation by, N-nitrosomethylbenzylamine in chicken

The metabolism of 14C-nitrosomethylbenzylamine (NMBA) was studied in chicken. Following a single IV dose of 2 mg/kg, 14C-NMBA was cleared from the blood with a half-life of 3.8 min. At 10 min after administration 14C-NMBA was totally metabolized in the liver, whereas in the esophagus no measurable metabolic degradation had taken place. Maximum exhalation of radioactive CO2 occurred 1 h after IV administration of NMBA, and 11% of the total radioactivity had been exhaled as CO2 by 8 h. These results are compared with data on the metabolism of NMBA in the rat. The analysis of methylated bases in the DNA of different organs of chicken revealed that 7-me guanine was formed in all organs. The highest amount of 0(6)-me guanine was found in liver DNA, followed by kidney DNA. O6-me guanine was not detectable in any other organ. The O6-/7-me guanine ratio in DNA was calculated to be 0.05 and 0.02 for liver and 0.01 for kidneys.

Repair and replication of DNA in rat brain and liver during foetal and post-natal development, in relation to nitroso-alkyl-urea induced carcinogenesis

The susceptibility of rat brain to induction of cancer by N-nitroso-N-ethyl-urea (NEU) increases dramatically from a very low level in the 12 day foetus to a maximum at the time of birth, and then decreases with age. Liver tumors are rarely induced by a single treatment with NEU at any age. If induction of cancer by nitroso-alkyl-ureas depends on replication of DNA containing the mispairing base O6-alkylguanine, susceptibility would reflect the balance between the protective effect of removal of the base by repair mechanisms and the potentiating effect of cell replication. The capacity of tissues to remove O6-alkyl-guanine from DNA depends on the amount of alkyl acceptor protein (AAP) present. To study the concept that carcinogenesis results from replication of alkylated DNA, the AAP contents and relative rates of DNA replication were studied in brain and liver of rats at various stages of development, from the 12 day foetus to the 48 week old rat. Replication in liver was approximately ten times higher than in intra-cranial contents at each stage of development studies. The AAP content was higher in the 12 day foetal brain than later, decreasing to low levels as susceptibility to NEU increased until the time of birth, and then remaining low in the adult. The peak sensitivity of brain therefore corresponds to the time at which AAP content is low and rate of DNA replication is high. With liver, AAP levels are low in the foetus, although higher than in brain, and increase after birth. The higher level of AAP in the foetal liver compared with that of brain is possibly sufficient to explain why cancer is not induced in liver in spite of the high rate of DNA replication. The results are consistent with the concept that replication of alkylated DNA is an essential event in initiation. There may be no quantitative relationship between replication and repair and susceptibility to cancer on comparison of different tissues, owing to the fact that, at the cellular level, cancer is a rare event. The amount of mispairing at replication necessary to bring it about may depend on the detailed organisation of the genome, and hence on cell type.

Thermal melting of chromatin from the pancreas and liver of guinea pigs treated with 1-methyl-1-nitrosourea

The carcinogen 1-methyl-1-nitrosourea (MNU) can cause pancreatic cancer in guinea pigs. We have examined the relative damage produced by MNU treatment on the chromatin from pancreas and liver of these animals. Thermal denaturation of chromatin from guinea pig pancreas and liver was studied following parenteral administration of MNU in several doses. Estimates of single strand breakage were also obtained by examination of the fluorescence of intercalated ethidium bromide. Oligomeric chromatin melted with a main Tm at 78 degrees C, with additional components at 48 degrees C, 55 degrees C and 65 degrees C. Repetitive treatment with MNU at several doses between 20 mg/kg and 70 mg/kg produced destabilization of pancreatic chromatin as shown by a shift from 78 degrees C to lower melting components. The liver by contrast was relatively unaffected. In addition, pancreatic chromatin showed an increase in alkali-induced strand unwinding with MNU treatment, probably due to an increase in single strand breaks, while there was no change in this regard in the liver. The data indicate that the pancreas is more susceptible to damage by MNU than the liver.

DNA alkylation by mustard gas in yeast strains of different repair capacity

Reaction of the toxic and mutagenic alkylating agent mustard gas with DNA of the yeast Saccharomyces cerevisiae was analyzed qualitatively and quantitatively. Within the dose range tested (2 X 10(-5)-2 X 10(-3) M) DNA in vivo is alkylated dose-proportionally. DNA alkylation and relative distribution of purine derivatives are not influenced by the cell's sensitivity towards the mutagen. At LD37 (4.4 X 10(-4) M) the wild type contains 44 300 purine derivatives: 9200 3-alkyladenines (20%), 29600 7-alkylguanines (67%) and 5500 diguaninyl derivates (13%) per genome. In sensitive strains the number of derivates per genome at LD37 is reduced according to the dose reduction factor. Alkylation at the position O6 of guanine by mustard gas cannot be shown, the method's limit of detection being 0.3% amongst purine derivates.

Induction of bladder cancer in rats by fractionated intravesicular doses of N-methyl-N-nitrosourea

Experiments were conducted to determine the dose response of rat bladder urothelium to a range of different single and fractionated intravesicular doses of the carcinogen, N-methyl-N-nitrosourea (MNU). A dose-related response of bladder-tumour incidence to single graded doses of MNU was found, and a threshold does suitable for use of multistage carcinogenesis experiments was derived from these data. For any given total dose of MNU, the tumour incidence was greater if the MNU had been administered in several small fractions than if it had been administered in fewer larger ones. Extending the interval between doses did not reduce the tumour incidence. It is argued that these results support the multistage theory of carcinogenesis. The histopathology and cell-surface alterations which characterize the development of MNU-induced bladder cancer are described and the contribution of hyperplasia and calculi are discussed.

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.

Effect of disulfiram on the alkylation of rat liver DNA by nitrosodiethylamine

The extent of ethylation of guanine in 7 N and O 6 position in rat liver DNA was studied after chronic feeding with unlabelled nitrosodiethylamine followed by a single application of the 14C labelled drug. Disulfiram inhibits the alkylation of rat liver DNA. It was also shown that a discontinuous dosage of disulfiram protects against the alkylating effect of 14C nitrosodiethylamine only for a distinct period of time.

The interaction of chromatin with alkylating agents. The monofunctional action of bis(2-chloroethyl)methylamine

The reaction of L5178Y lymphoblast cell chromatin with the alkylating agent bis(2-chloroethyl)methylamine has been studied as a function of time, pH and reagent concentration. The reaction with DNA of chromatin from which the proteins were dissociated, as well as with purified calf thymus DNA, was studied in parallel. The extent of alkylation of DNA in intact chromatin was 4--5 times as much as in parallel free DNA samples; up to 4% of nucleotide base pairs were substituted. The extent of monofunctional substitution of the proteins was similar, on a weight basis, to that of DNA. Chromatographic analysis of the depurinated products showed that in chromatin, as in DNA, position N-7 of guanine is the major site of reaction. Up to 25% of the reaction products were guanines cross-linked as bis(2-guanin-7-yl-ethyl)methylamine, indicating a considerable degree of DNA-DNA cross linking. Column analysis shows that up to 40% of the nuclear proteins are cross-linked to DNA at 10 mM bis(2-chloroethyl)methylamine. The increased reactivity of intact chromatin is interpreted in terms of a conformational change in the position of the DNA bases when in the organized nucleohistone complex.

In vivo covalent binding of organic chemicals to DNA as a quantitative indicator in the process of chemical carcinogenesis

The covalent binding of chemical carcinogens to DNA of mammalian organs is expressed per unit dose, and a 'Covalent-Binding Index', CBI, is defined. CBI for various carcinogens span over 6 orders of magnitude. A similar range is observed for the carcinogenic potency in long-term bioassays on carcinogenicity. For the assessment of a risk from exposure to a carcinogen, the total DNA dmaage can be estimated if the actual dose is also accounted for. A detailed description is given for planning and performing a DNA-binding assay. A complete literature survey on DNA binding in vivo (83 compounds) is given with a calculation of CBI, where possible, 153 compounds are listed where a covalent binding to any biological macromolecule has been shown in vivo or in vitro. Recent, so far unpublished findings with aflatoxin M1, macromolecule-bound aflatoxin B1, diethylstilbestrol, and 1,2-epithiobutyronitrile are included. A comparison of CBI for rat-liver DNA with hepatocarcinogenic potency reveals a surprisingly good quantitative correlation. Refinements for a DNA-binding assay are proposed. Possibilites and limitations in the use of DNA binding in chemical carcinogenesis are discussed extensively.

The excision of N-methyl-N-nitrosourea-induced lesions from the DNA of Chinese hamster cells as measured by the loss of sites sensitive to an enzyme extract that excises 3-methylpurines but not O6-methylguanine

An enzyme extract from Micrococcus luteus excises 3-methyladenine and 3-methylguanine but not O6-methylguanine, 7-methylguanine, 1-methyladenine or 7-methyladenine from DNA reacted with N-methyl-N-nitrosourea. The extract was used to detect lesions in the DNA of Chinese hamster cells treated in culture with N-methyl-N-nitrosourea. It was concluded that 3-methyladenine is excised from these cells with a half-life of about 2.3 h.

The stability of methylated purines and of methylphosphotriesters in the DNA of V79 cells after treatment with N-methyl-N-nitrosourea

V79-379A cells growing in suspension culture were treated with N-methyl-N-nitrosourea at concentrations of 0.6 and 1.2 mM. After incubation for periods from 1 to 48 h DNA was isolated from the cells and the concentrations of 7-methylguanine, O6-methylguanine, 3-methyladenine and methyl phosphotriesters were determined. After correction for dilution resulting from DNA synthesis during the incubation it was found that no loss of O6-methylguanine or methylphosphotriesters occurred; 7-methylguanine disappeared with a half-life of 22 h and 3-methyladenine was detectable only immediately after the initial treatment. The results show that these cells eliminate 7-methylguanine and 3-methyladenine from DNA by a repair process but are unable to excise or repair O6-methylguanine or methyl phosphotriesters.

Effect of hypophysectomy on persistence of methylated purines in rat liver deoxyribonucleic acid after administration of dimethylnitrosamine

The formation of methylated purines in DNA following dimethylnitrosamine administration was studied in control and hypophysectomized rats. When given the same dose of this carcinogen (in mg/kg body weight) the formation of the major product 7-methylguanine and of the minor products 1-, 3- and 7-methyladenine and 3-methylguanine was slightly greater in the livers of hypophysectomized rats than in controls. The rate of loss of these products from the DNA was not affected by hypophysectomy. O6-Methylguanine levels were significantly greater in the hepatic DNA of hypophysectomized rats compared to controls after doses of dimethylnitrosamine ranging from 1 to 20 mg/kg. This difference was due to a slower rate of loss of this purine from the DNA in the hypothysectomized rats. Growth hormone treatment increased the rate of removal of O6-methylguanine in the hypophysectomized rats but did not restore the activity to that found in controls. The possible significance of these results in the induction of tumors by dimethylnitrosamine is discussed.

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.

The stability of methyl and ethyl phosphotriesters in DNA in vivo

C57BL male mice were injected with N-methyl-N-nitrosourea (MNUA) or N-ethyl-N-nitrosourea (ENUA) and the concentration of alkyl phosphotriesters in the DNA of lung, liver, brain, kidney, spleen and thymus determined from the extent of degradation induced in isolated DNA by alkali. The same total dose of reagent was given either as a single injection (i.p.) or by weekly injections carried out over 5-20 weeks. Methyl phosphotriesters induced in liver, lung and kidney by the single injection were lost with a half-life of about 7 days, in brain the loss was more rapid, t1/2 = 2-3 days. During the multiple injections the observed t1/2 was 16 days. Ethyl phosphotriesters formed in the DNA of lung, liver, kidney and brain were much more stable than the methyl derivatives, t1/2 = 10-15 weeks. Phosphotriesters formed in the DNA of spleen and thymus disappeared very quickly after the single injection presumably as a result of dilution due to DNA replication. No accumulation of phosphotriesters occurred in the DNA of these tissues during the multiple injections. The general pattern of the results suggests that phosphotriesters are not excised by cellular repair systems.

In vitro binding of beta-propiolactone to calf thymus DNA and mouse liver DNA to form 1-(2-carboxyethyl) adenine

In vitro reaction of beta-propiolactone (BPL) with calf thymus DNA and mouse liver DNA followed by acid (HCL) hydrolyses of the BPL-reacted DNA's resulted in the isolation of a new compound, 1-(2-carboxyethyl)-adenine (1-CEA). The structure was assigned on the basis of ultraviolet spectra at acidic, alkaline and neutral pH and electron impact and chemical ionization mass spectra as well as chemical synthesis of 1-CEA from BPL and 2'-adenosine-5'-monophosphoric acid. The only other compound previously isolated from the in vitro and in vivo reactions of BPL and DNA was 7-(2-carboxyethyl)guanine (7-CEG) which we also identified as a product of our in vitro reaction. Under the conditions used the main product of alkylation was 1-CEA and the ratios of the concentrations of 1-CEA to 7-CEG was approx 3 : 1. The possible effect of the formation of 1-CEA on the structure of DNA and its role in chemical carcinogenesis is discussed.

Excision of 7-bromomethylbenz[a]anthracene--DNA adducts in replicating mammalian cells

The excision of 7-bromomethylbenz[a]anthracene--DNA adducts was studied in two cell lines (HeLa S-3 and Chinese hamster V-79379A). In both cell lines, carcinogen-modified adenine residues were excised more readily than the modified guanine residues and the percentage of the total products excised decreased after treatment with higher concentrations of carcinogen. At the highest concentrations used in the Chinese hamster cells, neither DNA synthesis nor excision was detected. The lowest concentration used for these cells permitted almost 100% survival and all the DNA was replicated in a 30-h interval even though 50% of the initial damage was still present. The two- to threefold lower sensitivity of the Chinese hamster cells (compared with the Hela cells) to the carcinogen is attributed to this capacity for replication of DNA on a damaged template since the two cell lines' capacities for excision of the chemical damage were found to be comparable.

Excision of O6-methylguanine from DNA of various mouse tissues following a single injection of N-methyl-Nitrosourea

The persistence of O6-methylguanine produced by a single dose of N-methyl-N-nitrosourea (MNU) was determined in DNA of various murine tissues and compared with the location of tumours induced by MNU and related alkylating carcinogens in this species. A/J and C3HeB/FeJ mice received a single intravenous injection of MNU (10 mg/kg) and were killed at different time intervals ranging from 4 h to 7 days. The rate rate of loss of O6-methylguanine from brain DNA was considerably slower than from liver DNA; tumours have been found in both organs after administration of MNU and other alkylnitrosoureas. There was no difference in the rate of excision from cerebral DNA of A/J and C3HeB/FeJ mice, although these strains differ significantly in their susceptibility to the neurooncogenic effect of MNU and related carcinogens. Excision of O6-methylguanine from hepatic DNA was significantly slower in A/J than in C3HeB/FeJ mice; both strains habe been found to develop hepatic carcinomas following MNU administration. Seven days after the injection of 3H-MNU, O6-methylguanine concentrations were highest in brain and lung DNA, lowest in the liver, and intermediate in kidney, spleen, small intestine and stomach. The lung is a principal target organ for tumour induction by MNU and other carcinogens in mice; however, neural tumours are usually induced at a low incidence. The results obtained do not contradict the hypothesis that O6-alkylation of guanine in DNA is a critical event in the initiation of tumour induction by alkylating agents. However, the location of tumours produced in mice does not seem to depend solely on the formation and persistence of O6-alkylguanine in DNA.

Methylation of DNA in rat liver and intestine by dimethylnitrosamine and N-methylnitrosourea

A chromatographic procedure for improved separation of deoxyribonucleosides and methylated deoxyribonucleosides is described. DNA was isolated from liver and small intestine of rats treated with [14C]dimethylnitrosamine ([14C]DMN) or N-[3H]methyl-N-nitrosourea ([3H]MNU), and the purified DNA was hydrolyzed enzymatically. The deoxyribonucleosides were chromatographed on an Aminex A-6 cation exchange column at 37 degrees C with 0.4 M ammonium formate, pH 4.5, as eluant. In addition to showing the presence of the expected alkylated products, N7-methyldeoxyguanosine (determined as N7-methylguanine) and O6-methyldeoxyguanosine, several other minor methylated products were found in liver and intestinal DNA of rats treated with DMN or MNU. Two of these products are believed to be N3-methylthymidine and O4-methylthymidine.

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

The methylating agents methyl methanesulphonate (MMS) and N-methyl N'-nitro-N-nitrosoguanidine (MNNG), administered by single i.p. injection in mice failed to yield thymic lymphoma at doses around 60% of the LD50 values, in contrast to MNUA which gives a high yield of tumours by this route. Comparison of the tissue distribution and mode of DNA methylation by these agents showed a positive correlation with ability to methylate the 0-6 atom of guanine in DNA of the target tissues thymus and bone marrow and tumorigeneis. MMS gave a low yield of this product due to its relatively low Sn1 reactivity but was able to methylate DNA extensively at other sites in the target tissues and other organs examined. MNNG despite its ability to methylate 0-6 of guanine in DNA in vitro to the same relative extent as the potent carcinogen MNUA, methylated DNA of thymus and bone marrow to a very small extent in vivo but was able to methylate DNA in certain other tissues nearer the site of i.p. injection. These findings contrast with the general relatively extensive methylation of 0-6 of guanine in DNA of the target tissues and other organs by N-methyl-N-nitrosourea (MNUA).

An assay for phosphotriester formation in the reaction of alkylating agents with deoxyribosenucleic acid in vitro and in vivo

Preliminary studies in vitro using bacteriophage T7-DNA have shown that breaks formed in the DNA on the alkaline hydrolysis of apurinic sites and phosphotriesters can be distinguished from each other by measuring the extent of degradation of the DNA immediately after adding NaOH to 0.1 M and after incubating for 1 h in 0.5 M NaOH. This method has then been applied to the study of the formation and stability of phosphotriesters invivo. Methyl phosphotriesters formed in liver DNA following injection of mice with N-methyl-N-nitrosourea (MNUA) disappear with time (50% in 4-5 days). The concentration of ethyl phosphotriesters in liver DNA formed by injecting mice with N-ethyl-N-nitrosourea (ENUA) does not appear to decrease with time. Results of experiments on injecting methyl methane-sulphonate (MMS), ethyl methanesulphonate (EMS) and dimethyl sulphate (DMS) are also reported. The method described does not require the use of radioactively labelled reagents.

Some mechanisms operative in carcinogenesis a review

Multiple factors contribute to the development of neoplasia. Sometimes a single agency can bring about a tumour if it has many different effects, but at other times a tumour arises more insidiously due to a succession of events [240,241] which by themselves may be innocent. Alterations in the genome of the cell are at the fore-front of our interest because they can be brought about by most of the carcinogenic agents we know. The cell can repair some such alterations but both forward and destructive mutations do appear. The roles of cell proliferation, cell differentiation, the immune mechanism and carcinogen-activating enzymes are beginning to be understood. The effects of dose, route of administration, and of other agents given at the same time [242-245] must not be lost sight of. Other factors no doubt will be added as we begin to look at the structure and function of cell-surface membranes [246-248], at host susceptibility genetics [26, 249], and at the generation of carcinogens inside the body [250,251]. We are only beginning to understand carcinogenesis. In no single instance do we as yet know how a tumour comes about in full details of molecular biology. It is possible that fully rational treatment of cancer will not be possible until we have such an understanding. Once a tumour becomes independent of carcinogenic factors, it continues to develop in a bizarre fashion which makes its study and treatment by all means other than surgery difficult.