Cytarabine-resistant cells express higher level of CD25 and are more sensitive to PIM kinase inhibition. (A) The effect of cytarabine on CD25 expression in cells. KG1 cells were treated with cytarabine at the indicated concentrations. The expression of CD25 was examined at the mRNA (i) and protein (ii) levels by quantitative reverse-transcription polymerase chain reaction and fluorescence-activated cell sorter, respectively. (B) Suppression of cytarabine-stimulated CD25 expression by PIM inhibitor. Cells were treated with compound C at the indicated concentrations in the presence of cytarabine (30 nM). CD25 expression was then examined by quantitative reverse-transcription polymerase chain reaction and fluorescence-activated cell sorter. (C) Fluorescence-activated cell sorter analysis of CD25 expression in the parental (i, iii) and cytarabine-resistant KG1 cell lines (ii, iii). KG1 cells were cultured in the presence of 30 nM cytarabine for 3 months. The survived cells were treated with cytarabine. The resistance was confirmed by the increased IC₅₀ value to cytarabine compared with that of the parental line. (D) CD25 mRNA expression in the cytarabine-resistant cell line. (E) Regulation of CD25 expression in the cytarabine-resistant line. KG1 cells were treated with either STAT5A/B siRNA or control siRNA for 72 hours, and CD25 mRNA levels were analyzed by quantitative reverse-transcription polymerase chain reaction (ii). STAT5 knockdown was confirmed by western blot (i). (F) Differential response of cytarabine-resistant line to cytarabine or PIM inhibitor. KG1 parental and cytarabine-resistance cell lines were treated with serial diluted (1:3) cytarabine or compound C. The cytarabine-resistant cell line is more sensitive to PIM kinase inhibition than the parental line (ii). IC₅₀ values are indicated.