Combination of guanine arabinoside and Bcl-2 inhibitor YC137 overcomes the cytarabine resistance in HL-60 leukemia cell line

Cytarabine (ara-C) is the key agent for treating acute myeloid leukemia. After being transported into leukemic cells, ara-C is phosphorylated, by several Enzymes including deoxycytidine kinase (dCK), to ara-C triphosphate (ara-CTP), an active metabolite, and then incorporated into DNA, thereby inhibiting DNA synthesis. Therefore, the cytotoxicity of ara-C depends on the production of ara-CTP and the induction of Apoptosis. Here, we established a new ara-C-resistant acute myeloid leukemia cell line (HL-60/ara-C60) with dual resistance characteristics of the anti-antimetabolic character of decreased ara-CTP production and an increase in the antiapoptotic factors Bcl-2 and Bcl-xL. We further attempted to overcome resistance by augmenting ara-CTP production and stimulating Apoptosis. A relatively new nucleoside analog, 9-β-d-arabinofuranosylguanine (ara-G), and the small molecule Bcl-2 Antagonist YC137 were used for this purpose. HL-60/ara-C60 was 60-fold more ara-C-resistant than the parental HL-60 cells. HL-60/ara-C60 cells exhibited low dCK protein expression, which resulted in decreased ara-CTP production. HL-60/ara-C60 cells were also refractory to ara-C-induced Apoptosis due to overexpression of Bcl-2 and Bcl-xL. Combination treatment of ara-C with ara-G augmented the dCK protein level, thereby increasing ara-CTP production and subsequent cytotoxicity. Moreover, the combination of ara-C with YC137 produced a greater amount of Apoptosis than ara-C alone. Importantly, the three-drug combination of ara-C, ara-G and YC137 provided greater cytotoxicity than ara-C+ara-G or ara-C+YC137. These findings suggest possible combination strategies for overcoming ara-C resistance by augmenting ara-CTP production and reversing refractoriness against the induction of Apoptosis in ara-C resistant leukemic cells.