Production of oxidative DNA damage during the metabolic activation of benzo[a]pyrene in human mammary epithelial cells correlates with cell killing.

We have studied the generation of reactive oxygen species during the metabolism of a carcinogen, benzo[a]pyrene, by human mammary epithelial cells. We have quantitated the production of one type of oxidative DNA damage, thymine glycols, by using a monoclonal antibody specific to this base modification. Thymine glycols were produced in DNA in a dose-dependent manner after exposure of human mammary epithelial cells to benzo[a]pyrene. The number of thymine glycols formed in the DNA was similar to that of 7,8-dihydroxy-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene covalently bound to the DNA. Exposure of cells to the carcinogen in the presence of superoxide dismutase, which reduces superoxide anions, inhibited the production of thymine glycols and increased cell survival but had little effect on adduct formation. At equitoxic doses, approximately equal to 10-fold more thymine glycols were formed after exposure to benzo[a]pyrene than to gamma-irradiation. Thymine glycols, produced by either agent, were efficiently removed from the DNA of the cells. Since thymine glycols represent only a portion of the oxidative damage possibly produced, our results indicate that the total amount of oxidative damage induced during the exposure of human mammary epithelial cells to benzo[a]pyrene greatly exceeds the amount produced by direct adduct formation and that this indirect damage plays an important role in the cytotoxicity of benzo[a]pyrene.