Cathepsin B activity increases during GCDC-induced apoptosis of McNtcp.24 cells, and cathepsin B inhibitors reduce apoptosis. Cells were incubated in medium alone or with 50 μM GCDC in the absence or presence of 100 μM FA-fmk (throughout the experiment) or 0.1 μM CA-074-Me (pretreatment for 10 min before adding GCDC). Cathepsin B activity (a) was measured in cells after 2 h of incubation using the fluorogenic substrate VLK-CMAC as described in Methods. P < 0.01 for GCDC vs. control, GCDC plus FA-fmk, or GCDC plus CA-074-Me by ANOVA. Apoptosis (b) was quantitated after 2 h of incubation as described in Methods. P < 0.01 for GCDC vs. control, GCDC plus FA-fmk, or GCDC plus CA-074-Me by ANOVA. GCDC, glycochenodeoxycholate.