Increase in 8-hydroxyguanine and its repair activity in the esophagi of rats given long-term ethanol and nutrition-deficient diet

Epidemiological studies have shown that an increased risk of esophageal cancer is associated with the chronic consumption of alcoholic beverages, although alcohol itself is not a carcinogen in animal models. Reactive oxygen species produced by the metabolism of ethanol or by chronic inflammation may play an important role in the carcinogenic process. In this study, we analyzed one type of oxidative DNA damage, 8-hydroxyguanine (8-OH-Gua), and its repair activity in the esophagus as indicators of cellular oxidative stress in rats given long-term ethanol and an autoclaved diet (nutrition-deficient diet). Three-week-old male Sprague-Dawley rats were fed an ethanol beverage whose concentration was increased from 12 to 70% over 20 weeks. When the concentration reached 50%, the diet of one group was changed from the regular diet to an autoclaved diet. At the feeding periods of 20, 25, 30, and 35 weeks, the rats were sacrificed and the 8-OH-Gua levels and repair activities within the esophagi were measured. After 30 weeks of ethanol- and autoclaved diet-feeding, significant increases of 8-OH-Gua and its repair activity were observed in the esophagi, but not in those of the ethanol- and normal diet-fed rats. This result indicates that the combined effects of long-term ethanol consumption and nutritional deficiency may be involved in inducing oxidative stress in the rat esophagus.

Analyses of oxidative DNA damage and its repair activity in the livers of 3'-methyl-4-dimethylaminoazobenzene-treated rodents

We measured the levels of 8-hydroxyguanine (8-OH-Gua) and its repair activity in the livers of the Donryu rat, the carcinogen-resistant DRH rat, and the ddy mouse, which were fed a 0.06% 3'-methyl-4-dimethylaminoazobenzene (3'-MeDAB)-containing diet. In a short-term rat experiment (maximum 2 months), 3'-MeDAB did not increase the 8-OH-Gua levels in the livers of the two rat strains, although it significantly increased the repair activity in only the Donryu rat liver at 1 and 2 months. After long-term 3'-MeDAB administration to the ddy mouse (8 months), the levels of 8-OH-Gua and its repair activity were increased in the liver by 3.6-fold and 1.6-fold, respectively. These experiments suggest that 3'-MeDAB increases 8-OH-Gua generation in rodent liver DNA and the 8-OH-Gua repair assay is a reliable marker of cellular oxidative stress induced by carcinogens.

Increased 8-hydroxyguanine in DNA and its repair activity in hamster and rat lung after intratracheal instillation of crocidolite asbestos

Asbestos and man-made-mineral fibers are known to increase one type of oxidative DNA damage, 8-hydroxyguanine (8-OH-(Gua), in vitro. In this study, we analyzed the 8-OH-Gua level in DNA and its repair activity after a single intratracheal instillation of fibers (crocidolite or glass) or saline to Syrian hamsters or Wistar rats. The 8-OH-Gua level was measured with a high-performance liquid chromatography-electrochemical detector (HPLC-ECD) system. The 8-OH-Gua repair enzyme activity was determined with an endonuclease nicking assay using a 32P-labeled or fluorescently labeled 22mer DNA that contains 8-OH-Gua at a specific position. A significant increase in the 8-OH-Gua level in the lung DNA was observed 1 day after the exposure to crocidolite, as compared to the saline control. The repair activity was increased significantly at 7 days. On the other hand, after exposure to glass fibers, little or no increase of these carcinogenicity indicators was detected. These assays of 8-OH-Gua and its repair activity in short-term animal experiments will be useful for evaluating the carcinogenicity of fibers. This is the first report of the increase of 8-OH-Gua and its repair activity in the animal lung after the instillation of asbestos fibers.

Infrequent mutations of the hOGG1 gene, that is involved in the excision of 8-hydroxyguanine in damaged DNA, in human gastric cancer

DNA glycosylase, encoded by the hOGG1 gene, repairs 8-hydroxyguanine (oh8Gua), which is an oxidatively damaged mutagenic base. To clarify whether the DNA repair activity of hOGG1 protein is involved in gastric carcinogenesis, we examined 9 gastric cancer cell lines and 35 primary gastric cancers for mutations and genetic polymorphisms of the hOGG1 gene by polymerase chain reaction-single strand conformation polymorphism analysis. A G-to-A transition was detected in a gastric cancer cell line, MKN1. This nucleotide change caused the conversion of the amino acid from Arg to His at codon 154, which is located in a domain highly conserved among human, mouse, and yeast OGG1 proteins. No mutation was detected in primary gastric cancers. We compared the distribution of the polymorphic alleles associated with enzymatic activity (hOGG1-Ser326 vs. hOGG1-Cys326) between 35 gastric cancer patients and 42 healthy individuals. Although the frequency of the Cys326 allele, associated with low enzymatic activity, in gastric cancer patients was a little higher than that in healthy individuals, the difference did not reach statistical significance. These results suggest that low hOGG1 activity due to mutations and genetic polymorphisms is involved in the development of only a small subset of gastric cancers.

Dose-dependent induction of 8-hydroxyguanine and preneoplastic foci in rat liver by a food-derived carcinogen, 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline, at low dose levels

Male F344 rats were administered 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in the diet at doses of 200, 50, 12.5, 3.2, 0.8, 0.2 and 0.05 ppm for six weeks, and partially hepatectomized 1 week after the beginning of MeIQx administration. Quantitative values for glutathione S-transferase placental form (GST-P)-positive foci in the liver were dose-dependently increased by the MeIQx treatment. 8-Hydroxyguanine (8-OHG) levels assessed after 1 week of dietary MeIQx administration were also dose-dependently increased, although the effect was no longer observed at the end of the treatment period. The correlation between numbers of GST-P-positive foci at week 6 and 8-OHG levels at week 1 was linear, values for both parameters being higher than the control levels even in the 0.8 ppm dose group. These findings indicate that, in addition to the previously reported MeIQx-DNA adduct formation, DNA modifications due to oxidative damage may play an important role in MeIQx liver carcinogenesis in rats.

Comparison of incorporation and extension of nucleotides in vitro opposite 8-hydroxyguanine (7,8-dihydro-8-oxoguanine) in hot spots of the c-Ha-ras gene

DNA templates with 8-hydroxyguanine (7,8-dihydro-8-oxoguanine, oh8Gua) at a site corresponding to the first or second position of codon 12 of the c-Ha-ras gene were prepared, and the nucleotides inserted opposite the modified base were compared. The Klenow fragment (KF) of Escherichia coli DNA polymerase I inserted C opposite oh8Gua at both positions. Taq DNA polymerase incorporated C and A opposite oh8Gua, and the ratio of C to A was higher at the first position than at the second position. DNA polymerase alpha (pol alpha) inserted A and C at the first position, and A at the second position of codon 12, indicating that the ratio of C to A was higher at the first position. Moreover, we studied the extensions of bases paired with oh8Gua by DNA polymerases with or without 3'-5' exonuclease activity. G and T opposite oh8Gua were removed, and subsequently C was inserted by KF. We found that an oh8Gua:A pair was recognized by the exonuclease activity of the enzyme and that A was partially substituted by C. On the other hand, pol alpha extended only C and A opposite oh8Gua. No difference was observed with oh8Gua at the two positions. These results indicate that the ratio of nucleotides incorporated opposite oh8Gua depends on the sequence context, while there is no particular difference in the extension of base pairs involving oh8Gua by DNA polymerases.

Different roles of 8-hydroxyguanine formation and 2-thiobarbituric acid-reacting substance generation in the early phase of liver carcinogenesis induced by a choline-deficient, L-amino acid-defined diet in rats

The present study was performed to assess the roles of hepatocellular oxidative damage to DNA and constituents other than DNA in rat liver carcinogenesis caused by a choline-deficient, L-amino acid-defined (CDAA) diet by examining the effects of the antioxidant N,N'-diphenyl-p-phenylenediamine (DPPD). The parameters used for cellular oxidative damage were the level of 8-hydroxy-guanine (8-OHGua) for DNA and that of 2-thiobarbituric acid-reacting substance (TBARS) for constituents other than DNA. A total of 40 male Fischer 344 rats, 6 weeks old, were fed the CDAA diet for 12 weeks with or without DPPD (0.05, 0.10 or 0.20%) or butylated hydroxytoluene (BHT, 0.25%). In the livers of the rats, the numbers and sizes of glutathione S-transferase (EC 2.5.1.18) placental form (GSTP)- and/or gamma-glutamyltransferase (GGT, EC 2.3.2.2)-positive lesions and levels of 8-OHGua and TBARS were determined. The GSTP-positive lesions of 0.08 mm2 or larger were all stained positively for GGT as well in cross-sectional area, whereas the smaller lesions were generally negative for GGT. DPPD and BHT reduced the size of the GSTP-positive lesions without affecting their total numbers. At the same time, they reduced TBARS generation without affecting 8-OHGua formation in DNA. The present results indicate that oxidative DNA damage (represented by 8-OHGua formation) and damage to constituents other than DNA (represented by TBARS generation) may play different roles in rat liver carcinogenesis caused by the CDAA diet; the former appears to be involved in the induction of phenotypically altered hepatocyte populations while the latter may be related to the growth of such populations.

Ubiquitous presence in mammalian cells of enzymatic activity specifically cleaving 8-hydroxyguanine-containing DNA

Here we report the finding of enzymatic activity that specifically cleaves DNA containing 8-hydroxyguanine (oh8Gua) residues in various mammalian cells. To detect this activity, we used a synthetic double-stranded DNA containing a single oh8Gua at a defined position as the substrate, and analyzed the products of enzymatic digestion by polyacrylamide gel electrophoresis. Two cleavage sites near the oh8Gua residue were detected with partially purified fractions from cow brain and rat liver, and also with preparations from all mammalian tissues examined. These results suggest that enzymatic activity for the removal of oh8Gua from DNA is widely distributed in mammalian cells.