STI571-induced reduction of cell susceptibility to TRAIL is dependent on c-Abl. (A) HCT116 cells were transfected with c-Abl siRNA, and 24 h after transfection the intracellular c-Abl protein level was determined. In some experiments, cells were treated with TRAIL (50 ng/ml) and/or STI571 (0.3 μM) as indicated for another 24 h, and cell viability was determined. * p < 0.05, indicating a significant reduction in apoptosis by STI571. # p < 0.05, indicating a significant reduction of TRAIL-elicited apoptosis by c-Abl siRNA. (B) In some experiments under c-Abl siRNA treatment, JNK and p38 activation by TRAIL were determined. Data of immunoblotting were representative of three separate experiments with similar results.