Oxidation of DNA bases by tumor promoter-activated processes

Evidence has accumulated showing that active oxygen species participate in at least one stage of tumor promotion. Tumor promoters can induce various types of cells to undergo processes that result in formation of active oxygen species. They stimulate polymorphonuclear leukocytes (PMNs) to undergo an oxidative burst that is characterized by rapid formation of .O2- and H2O2. We find that in vitro formation of H2O2 by tumor promoter-activated PMNs correlates with their in vivo first-stage promoting activity. Moreover, two thymidine derivatives are formed in DNA coincubated with tumor promoter-stimulated PMNs: 5-hydroxymethyl-2'-deoxyuridine (HMdU) and thymidine glycol (dTG). The amounts of HMdU and dTG formed correlate with the first-stage tumor-promoting potencies of the agents used for PMN stimulation and with the amount of H2O2 generated. We find that HMdU is also formed in the DNA of HeLa cells coincubated with 12-O-tetradecanoylphorbol-13-acetate (TPA)-activated PMNs, with the amount of HMdU being proportional to that of TPA used. Even in the absence of PMNs, HMdU is increasingly formed in cellular DNA with increased TPA concentration, although at much lower levels than in the presence of PMNs. When rat liver microsomes are incubated with benzo[a]pyrene (BaP), a complete carcinogen, H2O2 is also generated. Production of H2O2 increases linearly with increasing concentrations of BaP. Furthermore, HMdU is formed in DNA exposed to BaP-treated microsomes, and its formation is inhibited by catalase. These results suggest that carcinogen-induced processes generating H2O2 are associated with the first-stage promoting activity of complete carcinogens.