TRAIL-induced apoptosis in HCT116 cells is attenuated by STI571. (A) Cells were pretreated with zVAD (20 μM) for 30 min, followed by the addition of STI571 or TRAIL. After 24 h, cell viability was determined by the MTT assay. (B) After treatment with STI571 and/or TRAIL for 6 or 9 h, cells were labeled with PI, and the cell population with sub-G1 expression was determined by flow cytometry. (C) After treatment with STI571 (0.3 μM) and/or TRAIL (50 ng/ml) for 1 ~ 4 h, cell lysates were harvested for immunoblotting of caspase 3. * p < 0.05, indicating significant reduction of TRAIL-induced apoptosis by zVAD (A) and STI571 (B). Traces in (C) were representative of at least three separate experiments with similar results.