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

Effect of partial hepatectomy on removal of O6-methylguanine from alkylated DNA by rat liver extracts

1. The activity of an enzyme catalysing the loss of O6-methylguanine from methylated DNA was increasing during liver regeneration after partial hepatectomy. Activity was increased 3-fold by 24h and was maximal (6-fold increase) over the period 48-72h after operation. 2. This activity could also be induced by chronic treatment with dimethylnitrosamine, but the maximal response amounted to a 2-3-fold change (with the greater effect in male rats) after 4-6 weeks of exposure to daily doses of 2 mg of dimethylnitrosamine/kg. 3. Neither partial hepatectomy nor treatment with dimethylnitrosamine increased the activities of two other enzymes repairing alkylated DNA, DNA (7-methylguanine-)glycosylase and DNA (3-methyladenine-)glycosylase. 4. These results therefore indicate that there is a selective induction of the O6-methylguanine removal system during hepatocyte proliferation. Since this product is known to lead to mutations and its persistence in DNA throughout cell replication has been implicated in tumour initiation, this induction may play a role in resistance to carcinogenesis by alkylating agents.

Liver and kidney nuclear RNA synthesis and modifications in dimethylnitrosamine-treated rats

RNA synthesis was measured in nuclei isolated from rat liver and kidney 22 h post injection of 30 mg dimethylnitrosamine/kg body weight. In nuclear preparations were shown by electron microscopy to consist of clean hepatocytes and the liver nuclei showed no apparent necrosis at that time. In vitro RNA synthesis and methylation were proportional to time and nuclear concentration, as well as dependent on exogenous nucleoside triphosphates and S-adenosylmethionine. 60-70% of the in vitro synthesis was inhibited by 1 microgram/ml alpha-amanitin. Total liver nuclear RNA synthesis was increased after dimethylnitrosamine exposure, but, unlike RNA synthesis in nuclei after partial hepatectomy, both alpha-amanitin-sensitive and -resistant synthesis were increased. Differences were found between dimethylnitrosamine-treated liver and kidney nuclear RNA synthesis which was sensitive to inhibition by 1-10 microgram/ml alpha-amanitin, presumably a product of RNA polymerase III. Nuclear RNA methylation with S-adenosylmethionine, which was dependent on new RNA synthesis, differed between dimethylnitrosamine-treated rat liver and kidney nuclei. The endogenous RNA methyl substituents labeled in vitro showed differences in levels of methylation of bases, the 2'-O position of ribose and caps in comparison between control and dimethylnitrosamine-treated nuclei from both liver and kidney. Significant differences were obtained in both nuclear RNA transcription and methylation in vitro between the two tissues in response to pretreatment of the rat in vito dimethylnitrosamine.

Possible formation of nitrosamine in guinea pigs following exposure to nitrogen dioxide and dimethylamine

The possibility of formation of nitrosamine was investigated in animals exposed to a combination of dimethylamine (DMA) and NO2. First, the distribution and covalent binding of DMA and dimethylnitrosamine (DMN) in rats and guinea pigs were determined. The apparent volume of distribution and biological half-life for [14C]-DMA or [14C]DMN did not reveal any species difference. In general, there were no marked differences in accumulation of radioactivity in tissues of guinea pigs and rats 4 h after the administration of DMA, while the guinea pig tissues showed higher accumulation after DMN administration. Nucleic acid fractions prepared from liver and lungs of both species following administration of DMN or DMA in vivo showed much higher covalent binding with DMN than with DMA. Furthermore, the covalent binding of DMN was found to be due to bioactivation, whereas the DMA binding was nonspecific. Since guinea pig liver showed a higher degree of covalent binding than rat liver, this species was used to investigate the possible increase in covalent binding in the presence of NO2 and DMA as a reflection of DMN formation. There was no evidence of enhancement of covalent binding when animals pretreated with [14C]-DMA were exposed for various lengths of time to different concentrations of NO2.

Alkylation of DNA and tissue specificity in nitrosamine carcinogenesis

A peculiarity of nitrosamines is the high degree of cell and organ specificity in inducing tumors. There is substantial evidence that the initiation of the carcinogenesis process by carcinogens of this group is linked to the metabolic competence of the target tissue or cell to convert these carcinogens into mutagenic metabolites and to the binding of those metabolites to cellular DNA. Alkylation occurs in the DNA at the N-1, N-3, and N-7 positions of adenine; the N-3, N-7, and O6 of guanine; the N-3, and O2 of cytosine; and the N-3, O4, and O2 of thymine; and the phosphate groups. The initial proportion of each DNA adduct depends upon the alkylating agent used. The various DNA adducts are lost to a variable extent from DNA in vivo by spontaneous release of bases and/or by specific DNA repair processes. Studies conducted in vitro and vivo indicate that alkylation at the oxygen atoms of DNA bases is more critical than alkylation at other positions in the mutagenesis and carcinogenesis induced by N-nitroso compounds. In particular, tissues in which tumors occur more frequently after a pulse dose of nitrosamine are those in which O6-alkylguanine persists longest in DNA, presumably resulting in an increased probability that a miscoding event (mutation) will take place during DNA synthesis. The more rapid removal of O6-methylguanine from the DNA of liver (as compared with extrahepatic tissues) of rats has been associated with the absence of tumor production in this organ by a single dose of dimethylnitrosamine; however, a significant incidence of liver tumors is observed if the same dose is given 24 hr after partial hepatectomy, and tumors are induced by such a dose of dimethylnitrosamine in the liver of hamsters, which has a low capacity to remove O6-methylguanine from its DNA. These data also indicate that the rate of disappearance of 7-methylguanine from the liver or extrahepatic tissues is independent of the dose of dimethylnitrosamine; whereas O6-methylguanine is lost from DNA more rapidly after a low dose of this nitrosamine. It has been shown that in liver the removal of O6-methylguanine but not other DNA adducts, from DNA can be affected by pretreating the animals with N-nitroso compounds. The modulation of DNA repair processes observed after a single dose and after chronic treatment with nitrosamines is discussed in relation to the tissue-specific carcinogenic effect of this group of carcinogens.

Alkylation of DNA and carcinogenicity of N-nitroso compounds

The possible mode of action of two classes of highly organ- and species-specific carcinogenic N-nitroso compounds, namely the N-nitrosamines and N-nitrosamides, is outlined in this review. Chemical or enzymatic conversion of these agents into active alkylating species is a prerequisite for their biological activity. Although these species are capable of reacting with all cellular macromolecules, there is good evidence that their ability to attack DNA is related to their carcinogenicity. This paper reviews the reasons and experimental support for the hypothesis that it is specifically alkylation of DNA at the O6 position of guanine that initiates malignant transformation, an important factor being the presence of an enzyme system capable of eliminating O6-alkylguanine from DNA. No such repair reaction appears to exist for this or other alkylation products when they are present in RNA. The activity of this system is enhanced by chronic administration of alkylating agents, and the role that this may play in carcinogenesis is discussed.

Formation and subsequent repair of alkylation lesions in tissues of rodents treated with nitrosamines

There is evidence that the formation of O6-alkylguanine in DNA may be an important reaction in the induction of tumors by dimethylnitrosamine and related carcinogens. The removal of this product from DNA may protect against carcinogenesis. The removal process has been studied both in vivo after administration of labelled dimethylnitrosamine and in vitro by incubation of isolated enzyme preparations with alkylated DNA. It has been found that: 1. Activity removing O6-alkylguanine from DNA is present in a number of tissues, but is most active in liver. 2. High doses of dimethylnitrosamine (20 mg/kg) inhibit removal of O6-methylguanine from DNA in vivo completely in rat kidney and hamster liver and partially in rat liver. This loss of activity was also seen in extracts prepared from these tissues assayed in vitro. 3. Chronic exposure to low levels of dimethylnitrosamine led to an increase in the activity of the hepatic enzyme removing O6-methylguanine. 4. Hypophysectomy or thyroidectomy decreased the activity of the hepatic removal system for O6-methylguanine and treatment with growth hormone or thyroxin increased it. 5. Dimethyl- and diethylnitrosamine are rapidly absorbed from the upper part of the small intestine (but not from the stomach). The degree of alkylation observed in the liver was independent of whether the carcinogen was administered orally or by i.v. injection, but at doses of 0.1 mg/kg or less, the reaction with the kidney was much lower after oral administration. Therefore, at low oral exposures to the carcinogen, the liver, which has the greatest capacity to remove O6-methylguanine from its DNA receives a relatively greater proportion of the alkylation. These results are discussed in terms of the degree to which the DNA repair system removing O6-alkylguanine might contribute towards a threshold dose and that variations in this activity might be reflected in the magnitude of this threshold.

Metabolism of O6-alkyldeoxyguanosines and their effect on removal of O6-methylguanine from rat liver DNA

O6-Methyldeoxyguanosine and O6-ethyldeoxyguanosine are weak inhibitors (of approximately equal potency) of the removal of O6-methylguanine from methylated DNA by a rat liver enzyme in vitro. When administered to rats, O6-ethyldeoxyguanosine retarded the removal from liver DNA of the O6-methylguanine which had been produced by pretreatment with dimethylnitrosamine, but the effect was short lived. O6-Methyldeoxyguanosine was much less effective. When cells in culture were grown in a medium containing radioactive O6-methylguanine or O6-methyldeoxyguanosine there was negligible incorporation of the methylated base into DNA, but substantial conversion to guanine which was incorporated. When these substances were injected into rats after partial hepatectomy, a very small incorporation of O6-methylguanine into DNA apparently occurred. Both O6-ethyldeoxyguanosine and O6-methyldeoxyguanosine were dealkylated by rat liver extracts, but the methylated derivative was metabolized much more rapidly. O6-Methylguanosine and O6-ethylguanosine were also dealkylated by rat liver extracts, but the corresponding bases were not attacked. This reaction was probably carried out by the adenosine deaminase in the extracts because it could be prevented by addition of erythro-9-(2-hydroxy-3-nonyl)adenine, a potent adenosine deaminase inhibitor, and could also be effected by purified calf intestinal adenosine deaminase. The Km for the demethylation of O6-methyldeoxyguanosine by calf intestinal adenosine deaminase was comparable to that for adenosine, whereas the Km for O6-ethyldeoxyguanosine was ten times greater. The V for O6-methyldeoxyguanosine was about 11% that for adenosine, but that for O6-ethyldeoxyguanosine was only 0.3%. The higher Km and the slower V for O6-ethyldeoxyguanosine may contribute to the slower dealkylation of this nucleoside by liver extracts and could account for its greater effect on slowing O6-methylguanine excision from DNA in vivo.

Pretreatment with acetylaminofluorene enhances the repair of O6-methylguanine in DNA

Epidemiological and experimental evidence suggests that the interplay of environmental factors may be particularly relevant to the induction of cancer in man. We have investigated the possible interrelationships, at the level of DNA repair, of two chemically different hepatocarcinogens administered to rats. Animals were pretreated for several weeks by inclusion of 2-acetylaminofluorene (AAF) in the diet and a study was made of the capacity of liver to repair lesions subsequently introduced into DNA by a pulse of dimethylnitrosamine (DMN). Of particular interest was the repair of adducts at the O6 position of guanine as this product has been implicated as a critical reaction site for carcinogenesis by the monofunctional alkylating agents. We show here that the capacity of liver to repair O6-methylguanine in DNA is enhanced by prolonged exposure of rats to the chemically unrelated agent, AAF.