Involvement of intrinsic and extrinsic apoptotic pathways in cell death induced by TRAIL + HGF. A: MTT assay showing cytoprotection from specific inhibitors of caspase-8 (Z-IETD, 50 μM) and caspase-9 (Z-LEHD, 50 μM) against TRAIL + HGF induced apoptosis. DAOY cells were pre-incubated with HGF for 72 h and then treated with caspase inhibitors for 30 min before adding TRAIL. Z-IETD reversed cell death induced by TRAIL + HGF completely, whereas Z-LEHD protected ~80% of the cells from death. B. Western blot analysis of caspase-8, -9, -3 and Bid. Cells were treated with or without HGF (100 ng/ml) for 72 h before incubation with TRAIL (10 ng/ml) for another 24 h. TRAIL alone induced activation (cleavage) of caspase-3, 8, 9 and Bid. HGF enhanced the activation of caspases and Bid by TRAIL. Experiments were repeated three times (N = 9, * P < 0.001).