Background: Many previous reports have shown an age-related increase in DNA damage in hepatocytes. This change is thought to result from alterations in DNA repair capacity, DNA vulnerability to oxygen radicals, or both.
Experimental design: Single-cell gel electrophoresis (comet assay), which measures single-strand breaks/alkali-labile sites in the DNA of individual cells, was used to compare DNA repair capacity and vulnerability to damage in hepatocytes prepared from the livers of young (6 to 11 months) and old (26 to 29 months) male Fischer 344 rats. DNA damage was induced in a portion of the cells by exposure to 882 mM hydrogen peroxide. Comet images (migration distance of DNA (MDD) were assessed at the basal level (before exposure) and at 0, 1, 2, and 4 hours' incubation after exposure.
Results: The mean basal values of MDD (MV-MDD) and the proportion of hepatocytes with highly damaged DNA (P-HDD) were found to be significantly higher in advanced age. When cells were challenged with hydrogen peroxide, MV-MDD did not differ significantly between the two groups. Interestingly, however, cellular distribution analysis revealed a larger subpopulation of cells with high DNA vulnerability to hydrogen peroxide in rats of advanced age. Furthermore, both MV-MDD and P-HDD were restored almost to basal level at 2 hours incubation after exposure in cells from young rats, while neither recovered even at 4 hours incubation in cells from old rats. This contrast suggests that there is a significant age-related decline in DNA repair capacity.
Conclusions: The low repair capacity in advanced age depends on a subpopulation of cells showing high DNA vulnerability to hydrogen peroxide. Therefore, an increase in the proportion of hepatocytes exhibiting high vulnerability to oxygen radicals is more important than the deterioration in DNA repair capacity in the age-related increase in DNA damage.