BCR/ABL-positive leukemia cells accumulated more replication-dependent DNA doublestrand breaks (DSBs) than normal counterparts after treatment with cisplatin and mitomycin C (MMC, as assessed by pulse field gel electrophoresis (PFGE) and neutral comet assay. In addition, leukemia cells could repair these lesions more efficiently than normal cells and eventually survive genotoxic treatment. Elevated levels of drug-induced DSBs in leukemia cells were associated with higher activity of ATR kinase, and enhanced phosphorylation of histone H2AX on serine 139 (γ-H2AX). γ-H2AX eventually started to disappear in BCR/ABL cells, while continued to increase in parental cells. In addition, the expression and ATR-mediated phosphorylation of Chk1 kinase on serine 345 were often more abundant in BCR/ ABL-positive leukemia cells than normal counterparts after genotoxic treatment. Inhibition of ATR kinase by caffeine but not Chk1 kinase by indolocarbazole inhibitor, SB218078 sensitized BCR/ABL leukemia cells to MMC in a short-term survival assay. Nevertheless, both caffeine and SB218078 enhanced the genotoxic effect of MMC in a long-term clonogenic assay. This effect was associated with the abrogation of transient accumulation of leukemia cells in S and G 2 /M cell cycle phases after drug treatment. In conclusion, ATR -Chk1 axis was strongly activated in BCR/ABL-positive cells and contributed to the resistance to DNA cross-linking agents causing numerous replication-dependent DSBs.

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Zinc plays an essential role in a wide range of cellular processes, including defense against free radicals and maintaining genomic stability. The presence of zinc in some proteins is fundamental for their functioning as transcription factors. Little is known about interaction between zinc and DNA, which can be important in light of reports on the role of zinc in cancer transformation and sometimes contradictory character of these reports. In the present study we studied cyto- and genotoxicity of zinc sulfate (ZnSO(4)) in normal human lymphocytes and human myelogenous leukemia K562 cancer cells in the presence of zinc and hydrogen peroxide (H(2)O(2)). Zinc at concentrations from the range 10-1000 microM decreased the viability of cancer cells and this effect, especially for low concentrations of the element, was much more pronounced than in normal cells. Zinc did not induce DNA damage in normal cells, but did so in cancer cells. We observed a key difference between the action of zinc in normal and cancer cells in the presence of H(2)O(2), since the element exerted a protective effect against cyto- and geno-toxic action of H(2)O(2) in the former, whereas it increased such action in the latter. Zinc inhibited the repair of DNA damage induced by H(2)O(2) in cancer cells. The results suggest that zinc may protect normal cells against DNA-damaging action and increase this action in cancer cells, which indicates the dual action of this element in dependency of target cells and can be useful in cancer therapy.

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