UP-REGULATION OF BASE EXCISION-REPAIR CORRELATES WITH ENHANCED PROTECTION AGAINST A DNA-DAMAGING AGENT IN MOUSE-CELL LINES

DNA polymerase beta is required in mammalian cells for the predominant pathway of base excision repair involving single nucleotide gap filli ng DNA synthesis. Here we examine the relationship between oxidative s tress, cellular levels of DNA polymerase beta and base excision repair capacity in vitro, using mouse monocytes and either wild-type mouse f ibroblasts or those deleted of the DNA polymerase beta gene. Treatment with an oxidative stress-inducing agent such as hydrogen peroxide, 3- morpholinosydnonimine, xanthine/xanthine oxidase or lipopolysaccharide was found to increase the level of DNA polymerase beta in both monocy tes and fibroblasts, Base excision repair capacity in vitro, as measur ed in crude cell extracts, was also increased by lipopolysaccharide tr eatment in both cell types. In monocytes lipopolysaccharide-mediated u p-regulation of the base excision repair system correlated with increa sed resistance to the monofunctional DNA alkylating agent methyl metha nesulfonate. By making use of a quantitative PCR assay to detect lesio ns in genomic DNA we show that lipopolysaccharide treatment of fibrobl ast cells reduces the incidence of spontaneous DNA lesions. This effec t may be due to the enhanced DNA polymerase beta-dependent base excisi on repair capacity of the cells, because a similar decrease in DNA les ions was not observed in cells deficient in base excision repair by vi rtue of DNA polymerase beta gene deletion. Similarly, fibroblasts trea ted with lipopolysaccharide were more resistant to methyl methanesulfo nate than untreated cells. This effect was not observed in cells delet ed of the DNA polymerase beta gene. These results suggest that the DNA polymerase beta-dependent base excision repair pathway can be up-regu lated by oxidative stress-inducing agents in mouse cell lines.