Elevated 8-hydroxydeoxyguanosine in hepatic DNA of rats following exposure to peroxisome proliferators: relationship to carcinogenesis and nuclear localization.

Increased oxidative DNA damage due to increased peroxisomal generation of H2O2 is a potential mechanism in the carcinogenicity of chemical peroxisome proliferators (PP) in rodent liver. In order to determine the relationship between carcinogenicity and peroxisome-dependent DNA damage, levels of DNA base oxidation were examined by comparing 8-hydroxydeoxyguanosine (8-OHdG) in DNA from unfractionated liver of male F344 rats following dietary exposure to PP [WY-14,643, 0.1% or 0.005%; di(2-ethylhexyl)phthalate (DEHP), 1.2%; clofibric acid, 0.5%] or phenobarbital (0.05%). Exposure-related increases in 8-OHdG were not observed at 3 or 11 weeks for any of the compounds fed. At 22 weeks, 8-OHdG was similarly elevated (2-3x) by WY-14,643 (0.1% and 0.005%) and clofibric acid (0.5%). These equivalent increases in 8-OHdG in DNA from unfractionated liver did not parallel the divergent carcinogenicity of these different dietary exposures in the present or previous studies. The potential oxidation of nuclear DNA was examined by comparing levels of 8-OHdG in DNA isolated from purified liver nuclei and unfractionated liver. Elevated levels of 8-OHdG were not detected in DNA isolated from nuclear fractions of livers from rats fed clofibric acid for 22 weeks, indicating the dependence of PP-induced oxidative DNA damage on extranuclear components of samples for DNA isolation. The absence of a quantitative relationship between PP-induced carcinogenicity and oxidative DNA base damage (as 8-OHdG), and the failure to localize this oxidative damage to nuclear DNA, suggest two possible conclusions: (1) quantitation of 8-OHdG, a specific and sensitive indicator of oxidative DNA damage, does not accurately reflect the potential peroxisomal H2O2-dependent DNA damage and carcinogenicity of PP exposure in rodents; (2) other hepatic responses may be more critical features of the mechanism of PP carcinogenicity.