Comparison of cell proliferation and proteasome inhibition in MCF-7 and MDA-MB-231 cells treated with different compounds. MCF-7 and MDA-MB-231 cells were treated with each indicated compound at 40 μM for 24 h, followed by MTT assay (A), the chymotrysin-like activity assay (B), and Western blot analysis of polyubiquitinated proteins, PARP, IκB-α (C, D). Molecular weight of intact PARP is 116 kDa and the cleaved PARP fragment is 85 kDa. Molecular weight of IκB-α is 37 kDa. β-actin was used as a loading control. Columns, mean+/−SE of three independent experiments; bars, SD.