Synergistic effects of HI and imatinib on K562 cells. (A-B) MTT assay. Isobologram plots for the effects of imatinib and CPX (A) or GC7 (B) on cellular cytotoxicity. Cells were incubated with CPX or GC7 plus imatinib for 48 hours. Proliferation was assessed with an MTT-based assay. Results were analyzed with Calcusyn software. (C-D) Apoptosis. Cotreatment with subapoptotic concentrations of imatinib and CPX (C) or GC7 (D), respectively, increased apoptosis in K562 cells. Apoptotic cell fractions were measured by PI-staining. Values (mean ± SD of 3 experiments) are depicted as bar graphs. (E) Phosphorylation. Compared with imatinib, no effects of HI on tyrosine, STAT5, or CRKL phosphorylation was observed in K562 cells. (F-G) Lentiviral delivery of siRNA directed against eIF5A in K562 cells. (F) RT-PCR analysis with total RNA from stable K562 cells was performed with a pair of primers amplifying an approximately 476-bp sequence of eIF5A. The mRNA level of eIF5A was normalized to the endogenous 18S rRNA level, as indicated. (G) eIF5A–specific siRNA sensitizes K562 cells to imatinib in the cell viability assay.