Cathepsin inhibitor I (CI) amplifies drug-induced apoptosis in HL-60 cells. (A) HL-60 cells were treated with 2 μM camptothecin only or in the presence of 20 μM calpeptin (calpain inhibitor), or 20 μM cathepsin inhibitor I (CI) or 20 μM ALLM (inhibits calpain and cathepsin) for the indicated times. (B) HL-60 cells were treated with 20 μM cisplatin, or 50 μM etoposide or 0.5 μg/mL adriamycin alone for 20 hours or in the presence of inhibitor. Control cells were left untreated or treated with either the same dose of inhibitor only or a corresponding volume of dimethyl sulfoxide (DMSO). Proteins were extracted and subjected to SDS-PAGE and Western blotting with human-specific anti-Bax antibody, anti-Bcl-2 antibody, and anti-actin antibody for loading control. Cell viabilities were measured by trypan blue exclusion method. Error bars indicate means ± SD (n = 3).