Xu (33) reported that caspase 3 activation led to the cleavage of PARP1 and increased apoptosis, which is consistent with the results observed in the present study. combined with Olaparib, when compared with the control group. The alkaline comet assay demonstrated that ADR combined with Olaparib significantly upregulated the induction of the DNA damage response in ADR-resistant cells. Western blot analysis revealed that the protein expression of -H2A histone family member X, cleaved PARP, caspase 3 and cleaved caspase 3 was markedly enhanced, while the cell cycle-associated protein cyclin B1 was downregulated in K562/ADR cells following treatment with a combination Hexachlorophene of ADR and Olaparib. Similar synergistic cytotoxicity was observed in blood mononuclear cells, which were isolated from patients with chemotherapy-resistant leukemia. As Olaparib is available for clinical use, the results of the present study provide a rationale for the development of Olaparib combinational therapies for cases of ADR resistant leukemia. can also be achieved were dependent on the survival of the K562 and K562/ADR cells. According to the results of previous experiments by the authors, pre-treatment with ADR at 2 M consistently enhanced toxicity in K562 cell lines but not in K562/ADR cell lines (11). Therefore, 2 M ADR and 5 M Olaparib were selected for use in further experiments. Olaparib+ADR was capable of promoting Hexachlorophene ADR-mediated apoptosis in K562/ADR cells. Several CCN1 previous studies have reported that PARP1 inhibitors can exert synergistic inhibitory effects in tumors with various conventional chemotherapeutic agents, including doxorubicin (26), temozolomide (7) and oxaliplatin (27). The results of the present study demonstrated that treatment with Olaparib+ADR produced synergistic effects and revealed a significant increase in the sensitivity of ADR against K562/ADR cells. Cell cycle arrest at any phase will inhibit cell proliferation (28). The results revealed a synergistic effect in the treatment combination of ADR and Olaparib; combined treatment induced G2/M cell cycle arrest. In addition, the protein expression of Cyclin B1 was downregulated; the inhibition of cyclin B1 could lead to cell cycle arrest in the G2/M Hexachlorophene phase (29). In conclusion, these results suggested that the combined treatment of ADR and Olaparib may be more effective than monotherapy in treating ADR resistant leukemia. Histone Hexachlorophene H2AX serves a critical role in the regulation of DNA damage. H2AX phosphorylation is involved in DNA damage, as well as apoptosis in chronic myelogenous leukemia cells induced by imatinib (30). Olaparib+ADR induced more DNA damage than Olaparib alone in the present study. Olaparib may increase DNA damage induced by ADR by inhibiting DNA damage repair. To investigate the mechanism of PARP inhibitor re-sensitization in ADR resistant leukemia, the effect of Olaparib on apoptosis-associated proteins, such as cleaved caspase-3, caspase-3 (31), cleaved PARP (32) and PARP1 (33) was investigated. It was revealed that apoptosis induced the upregulation of caspase-3, cleaved caspase-3 and cleaved PARP protein expression, and downregulated PARP1 expression. Caspase-3 is responsible for cleaving specific cellular proteins during apoptosis (34). Hexachlorophene Cell death is accompanied by PARP cleavage, a caspase-3 substrate (35). Caspase-3 is the most active effector caspase in the intrinsic and extrinsic pathways where it is processed and activated by caspase-9 and caspase-8, respectively (36). A high level of caspase-3 activation and cleavage processing was observed in the present study following ADR and Olaparib treatment of drug resistant leukemia cells. PARP1 has a molecular weight of 113 kDa and is located in the nucleus (37). Following treatment with Olaparib+ADR, caspase-3 was activated and PARP1 was cleaved into its 89 kDa (cleaved PARP) and 24 kDa forms, therefore the level of full-length PARP1 (113 kDa) was significantly reduced. Xu (33) reported that caspase 3 activation resulted in the cleavage of PARP1 and increased apoptosis, which is consistent with the results observed in the present study. The results demonstrated drug synergism between the cells derived from patients with chemoresistant leukemia and the cultured cell lines, through analogous mechanisms. Therefore, PARP inhibitor re-sensitization of ADR resistant leukemia may be associated with the PARP1-mediated signaling pathway of caspase-dependent apoptosis. However, the apoptotic molecular mechanism of Olaparib requires further investigation. In conclusion, the present study provides evidence of a number of associated mechanisms,.