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

DNA repair systems in early and persistent hepatocyte nodules in the rat

The repair of three DNA lesions, namely O6-methylguanine, 7-methylguanine, and 3-methyladenine, was investigated within early and persistent hepatocyte nodules generated in Fischer 344 rats by a modified Solt-Farber procedure (diethylnitrosamine initiation followed by a 2-acetylaminofluorene/CCl4 cycle). The O6-methylguanine-DNA methyltransferase concentration within both hepatocyte nodule types was always higher than that found in age-matched controls (normal, initiated-only and promoted-only livers). As far as 3-methyladenine and 7-methylguanine-DNA glycosylases are concerned, the early hepatocyte nodules showed far higher activities for both enzymes than were found in the controls, whereas in the persistent ones they underwent a significant decrease. In conclusion hepatocyte nodules are endowed with a high DNA repair activity, which is partly adaptive, partly constitutive; along with others, such a defence mechanism could allow transformed cells to resist many cytotoxic drugs.

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

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

Enhanced O6-methylguanine-DNA alkyltransferase activity in rat liver nodules

The progression of chemical carcinogen induced hepatic lesions in the rat from altered enzyme foci to hepatocyte nodules and ultimately to hepatocellular carcinoma appears to be related to the evolution of new cell populations within these hepatic lesions. Initiator-promoter-initiator experiments in rat liver models using alkylating agents as the second genotoxic compound suggest accelerated progression toward malignancy could be the result of mutations caused by O6-alkylguanine formation in the DNA of preneoplastic liver cells. Since mutation frequency is believed to be related not only to the extent of O6-alkylguanine formation in DNA, but also to the rate of O6-alkylguanine repair, we measured the activity of the enzyme which repairs O6-alkylguanine, O6-alkylguanine-DNA alkyltransferase, in rat liver nodules. The activity of O6-methylguanine-DNA alkyltransferase in extracts of rat liver nodules 15 and 25 weeks post-initiation was approximately 1.4- and 1.8-fold greater, respectively, than comparable extracts from untreated-control and promoter only-treated rat liver tissues. Thus, the enhanced progression toward malignancy caused by treatment of rats bearing carcinogen-induced preneoplastic hepatic lesions with alkylating agents cannot be explained by a generalized deficiency of O6-alkylguanine-DNA alkyltransferase in hepatic preneoplastic lesions as a whole. These studies show that the distinctive xenobiotic resistant phenotype of rat hepatocyte nodules includes, in addition to the previously documented alterations in xenobiotic activation and detoxification enzyme activities, enhanced activity of a specific DNA repair system.