8-Hydroxydeoxyguanosine causes death of human leukemia cells deficient in 8-oxoguanine glycosylase 1 activity by inducing apoptosis

Our previous study showed that KG-1, a human acute leukemia cell line, has mutational loss of 8-oxoguanine (8-hydroxyguanine; oh⁸Gua) glycosylase 1 (OGG1) activity and that its viability is severely affected by 8-hydroxydeoxyguanosine (8-oxodeoxyguanosine; oh⁸dG). In the present study, the nature of the killing action of oh⁸dG on KG-1 was investigated. Signs observed in oh⁸dG-treated KG-1 cells indicated that death was due to apoptosis, as demonstrated by: increased sub-G₁ hypodiploid (apoptotic) cells, DNA fragmentation, and apoptotic body formation; loss of mitochondrial transmembrane potential, the release of cytochrome c from mitochondria into the cytosol, and the down-regulation of bcl-2; and the activation of caspases 8, 9, and 3, and the efficient inhibition of the apoptotic process by caspases inhibitors. This apoptosis appears not to be associated with Fas/Fas ligand because the expressions of these proteins were unchanged. Apoptotic KG-1 cells showed a high concentration of oh⁸Gua in DNA. Moreover, the increased concentration of oh⁸Gua in DNA, and the apoptotic process were not suppressed by the antioxidant, N-acetylcysteine, and thus the process is independent of reactive oxygen species. Of the 18 cancer cell lines treated with oh⁸dG, 3 cell lines (H9, CEM-CM3, and Molt-4) were found to be committed to apoptosis, and all of these showed very low OGG1 activity and a marked increase in the concentration of oh⁸Gua in DNA. These observations indicate that in addition to its mutagenic action, oh⁸Gua in DNA disturbs cell viability by inducing apoptosis.