Differential induction of apoptosis by bor-tezomib and VP16 in bortezomib-resistant cells. (A) Loss of mitochondrial membrane potential (Δψ_m) after 24-hour exposure of THP1/WT cells and THP1/BTZ₂₀₀ cells to a concentration range of bortezomib. (B) Induction of apoptosis (annexin V–positive cells) in THP1/WT cells and THP1/BTZ₂₀₀ cells after 24-hour exposure to a concentration range of bortezomib. (C) A representative flow cytometric picture of apoptosis induction (annexinV/7-AAD staining) after 24-hour incubation of THP1/WT cells and THP1/BTZ₂₀₀ cells with 100 nM BTZ. (D) A representative flow cytometric picture of apoptosis induction (annexin-V/7-AAD staining) after 48-hour incubation of THP1/WT cells and THP1/BTZ₂₀₀ cells with 1 μM VP-16/etoposide. All results present the means (± SD) for 3 independent experiments.