Previous studies demonstrated that
ataxia telangiectasia mutated- and Rad3-related (ATR)
kinase and its downstream target
checkpoint kinase 1 (Chk1) facilitate survival of cells treated with
nucleoside analogs and other replication inhibitors. Recent results also demonstrated that Chk1 is depleted when cells are treated with
heat shock protein 90 (Hsp90) inhibitor
17-allylamino-17-demethoxygeldanamycin (17-AAG). The present study examined the effects of
17-AAG and its major metabolite,
17-aminogeldanamycin (17-AG), on Chk1 levels and cellular responses to
cytarabine in human
acute myelogenous leukemia (AML) cell lines and clinical isolates.
Cytarabine, at concentrations as low as 30 nM, caused activating phosphorylation of Chk1, loss of the
phosphatase Cdc25A, and S-phase slowing. Conversely, treatment with 100 to 300 nM
17-AAG for 24 hours caused Chk1 depletion that was accompanied by diminished
cytarabine-induced S-phase accumulation, decreased Cdc25A degradation, and enhanced cytotoxicity as measured by inhibition of colony formation and induction of apoptosis. Additional studies demonstrated that small inhibitory
RNA (
siRNA) depletion of Chk1 also sensitized cells to
cytarabine, whereas disruption of the
phosphatidylinositol 3-kinase (PI3k) signaling pathway, which is also blocked by Hsp90 inhibition, did not. Collectively, these results suggest that treatment with
17-AAG might represent a means of reversing checkpoint-mediated
cytarabine resistance in AML.