Combination of guanine arabinoside and Bcl-2 inhibitor YC137 overcome 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 apoptosis inhibitor YC137 were used for this purpose. The established subclone HL-60/ara-C60 was 60-fold more ara-C-resistant than the parental HL-60 cells. HL-60/ara-C60 cells exhibited low capacity for 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 the antiapoptotic proteins 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 the Bcl-2 antagonist YC137 produced greater apoptosis than ara-C alone. Though the combination of ara-C with two drugs could only partially overcome ara-C resistance in ara-C-resistant cells, the three-drug combination of ara-C, ara-G and YC137 substantially overcame resistance. 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