O4-ethyldeoxythymidine, but not O6-ethyldeoxyguanosine, accumulates in hepatocyte DNA of rats exposed continuously to diethylnitrosamine.

Abstract

In previous investigations into the mechanisms responsible for cell specificity in hepatocarcinogenesis, we have demonstrated that 06-methylguanine accumulates in the DNA of nonparenchymal cells (NPC) but is efficiently removed from hepatocellular DNA. 06-Alkylguanine may, therefore, be an important promutagenic lesion responsible for the induction of hepatic angiosarcomas after exposure to methylating agents, but other promutagenic DNA alkylation products-i.e., 04-alkylthymine-may be responsible for the initiation of hepatocellular carcinomas. F-344 male rats were provided drinking water containing diethylnitrosamine (DEN) at 40 ppm for 0, 2, 4, 8, 16, 28, 49, or is the hepatocarcinogenic effect of chronically administered alkylating agents in rodents (14, 15). Rat liver eliminates O6-alkylguanine from DNA more rapidly than any other rodent tissue. This removal is enhanced by chronic exposure to nitrosamines (16)(17)(18)(19) and during the proliferative response after partial hepatectomy (20, 21). More detailed investigations utilizing separated hepatocytes and nonparenchymal cells (NPC) indicated that rapid and enhanced removal of 06-alkylguanine were properties of hepatocytes only (22). Furthermore, these studies showed that 06-methylguanine accumulated in NPC during multiple or continuous exposure to either dimethylhydrazine (23-25) or dimethylnitrosamine (26) and that a pronounced mitogenic response in NPC was associated with exposure (26,27). Under exposure conditions leading to a high incidence of angiosarcomas, considerable amounts of promutagenic lesions thus accumulated in the DNA of replicating NPC. Contrary to NPC, hepatocytes removed 06-methylguanine with increasing efficiency as exposure continued such that the highest concentration of o methylguanine was detected in DNA after 1 day, but only 1/25th of this value was found after 2-4 weeks of exposure (23). When rats were exposed to diethynitrosamine (DEN) in the drinking water at 40 ppm, a regimen exclusively inducing hepatocellular carcinomas, no 0 -ethylguanine could be detected in hepatocytes by fluorescence after separation of the DNA bases by HPLC (2). In addition to the reaction for which it is named, 06-methylguanine-DNA methyltransferase activity also catalyzes the removal of ethyl residues from the 06 position of guanine (21,28)

The detection of alkylation damage in the DNA of human gastrointestinal tissues

“…In this study the mean values for hepatic 04-ethylthymine levels were significantly higher (4-5-fold) in patients with malignant disease (Huh et al, 1989). This may be a particularly appropriate lesion to follow in liver since animal tissues have shown that although 04-ethylthymine occurs initially at levels which are many fold lower than 06-MeG, it is repaired slowly, if at all, and so tends to accumulate (Swenberg et al, 1984). In the original study of alkylation damage in human DNA reported from Lin Xian, a district with high risk for oesophageal cancer in N. China (Umbenhauer et al, 1985), the DNA from subjects with oesophageal and gastric cancer which had detectable levels of O6-MeG were, on average, 3 x and 1+ times higher respectively, than those of the controls.…”

DNA methylation in rat tissues by a series of homologous aliphatic nitrosamines ranging from N-nitrosodimethylamine to N-nitrosomethyldodecylamine

“…The biological basis of organ-specific carcinogenesis is not yet fully understood, but several factors have been implicated; including distribution of the parent carcinogen, tissue-and cell-specific bioactivation, cell turnover and DNA repair (for review see refs [14][15][16][17][18][19][20]. The objective of the present study was to determine the extent to which aliphatic /V-nitrosomethylalkylamines methylate cellular DNA in various rat tissues.…”

The role of O6-methylguanine-DNA methyltransferase in cell survival, mutagenesis and carcinogenesis

“…C57/BL mice treated with MNU or ENU removed very little alkylphosphotriesters from the DNA of many organs including liver although O6-MT was present in C57/BL primary tissue and cells in culture (Yagi et al, 1984) and has been partially purified from the liver of this strain of mouse (Bogden et al, 1981). In rat liver O4-alkylThy was removed very slowly, with a half-life of 4 days or longer, and alkylphosphotriesters were essentially not repaired, while O6-alkylGua had a half-life of 13 h or less (O'Connor et al, 1973;Scherer et al, 1980;Singer et al, 1981;Swenberg et al, 1984). The difference in repair among the bases in rat DNA led to the remarkable situation in which O6-EtGua, which was formed 3-4 times more frequently than O4-EtThy by diethylnitrosamine (DEN), was found at 2% of 0 4-EtThy levels in rats continuously exposed to DEN for 10 weeks .…”