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Cytarabine induces monocytic differentiation via chk1 activation

Our recent research demonstrated that 5- aminoimidazol-4-carboxamide ribonucleoside (AICAr) inhibits UMP synthase and induces cellular differentiation via ataxia telangiectasia and RAD3- related (ATR)/checkpoint kinase 1 (Chk1)-mediated signaling pathway due to pyrimidine depletion, similarly to brequinar, a dihydroorotate dehydrogenase (DHODH) inhibitor. Cytarabine is a well-known chemotherapeutic which interferes with the process of DNA synthesis exerting not only cytotoxic effects, but also monocytic differentiation. Nevertheless, the mechanism responsible for cellular differentiation in response to cytarabine remains unclear. Therefore, this study is aimed to test for the role of Chk1 DNA- damage signaling pathway in differentiation of leukemia cells and to compare the effects of cytarabine to the effects of AICAr and brequinar. This study was conducted on human monocytic cell lines U937 and THP-1, as well as on nonadherent mononuclear cells from bone marrow samples of five acute myeloid leukemia (AML) patients. Cytarabine dose-dependently decreased cell viability and induced the expression of differentiation markers CD11b and CD64 in AML cell lines. Moreover, cytarabine dosedependently increased the level of activating Ser-345 phosphorylation of Chk1, and increased the level of inhibitory Thyr-15 phosphorylation of cyclin- dependent kinase 1 (Cdk1), a possibly important downstream target of Chk1 in cell cycle arrest. Torin2 and VE-821, pharmacological inhibitors of ATR/Chk1 signaling pathway, as well as transfection of U937 cells with siRNA targeting Chk1 reduced differentiative effects of cytarabine, AICAr and brequinar. Furthermore, cytarabine dose-dependently induced the expression of differentiation markers in two primary AML samples responsive to inhibitors of de novo pyrimidine synthesis. Therefore, our results suggest that cytarabine induces differentiation of AML cells by activating Chk1 and shares the same mechanism as pyrimidine synthesis inhibitors.

differentiation ; leukemia ; cytarabine

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