Inhibitory effect of bortezomib on 26S proteasome system in AML cells. (A) Accumulation of polyubiquitinated proteins after bortezomib treatment. MV4-11 cells were treated with 60 nM bortezomib or 2.5 μM decitabine for the indicated time points. (B) Bortezomib (60 nM) exposure resulted in reduction of proteasome activity compared with untreated control. Error bars represent SD.