SiRNA-mediated silencing of proteasome β5-subunit induction restores bortezomib sensitivity and induces apoptosis in bortezomib-resistant cells. (A) β5-siRNA–induced silencing of β5-protein expression in THP1/BTZ_(-100) and THP1/BTZ₂₀₀ cells after 24-hour preincubation with transfection medium (DF2), control siRNA constructs (nontarget/GAPDH), and a β5-specific siRNA construct (transduction efficiency: > 85% based on siGlo⁺ cells by flow cytometry). After transfection, cells were incubated (24-48 hours) with the indicated concentrations of bortezomib. Protein expressions of α/β-core and β-actin are shown as controls. A representative picture of 3 separate experiments is depicted. (B) Bortezomib-induced apoptosis in THP1/WT, THP1/BTZ_(-100), and THP1/BTZ₂₀₀ cells after siRNA-induced prevention of induction of β5-subunit expression as described in panel A. Bor-tezomib exposure time: 24 hours. Bortezomib concentrations used: 25 nM for THP1/WT and THP1/BTZ_(-100) cells. Means of 4 separate experiments (± SD) are shown. *P < .01. (C) Bortezomib-induced cell growth inhibition of THP1/BTZ_(-100) cells after prevention of induction of β5-subunit expression by siRNA silencing as described in panel A and 24-hour exposure to 25 nM bortezomib. Means of 3 separate experiments (± SD) are shown. *P < .01.