Effects of combined treatment with SBHA and proteasome inhibitors on breast cancer cell growth. (A) MCF-7 and (B) MDA-MB-231 cells were treated with SBHA, Bortezomib, and MG-132 alone or in combination for 72 h and cell proliferation was assessed using the WST-8 assay. The concentration of Bortezomib and MG-132 was 8000- and 160-fold times that of SBHA, respectively. The proliferation of untreated control cells was considered as 100%. *P <0.05 vs. control; #P <0.05 vs. each reagent alone. (C) MCF-7 and (D) MDA-MB-231 cells were exposed to SBHA (40 μM), Bortezomib (5 nM), and MG-132 (250 nM), alone or in combination, for 72 h and plated onto 6-well plates at a density of 500 cells per well. Colonies were numbered after 14-day incubation. Left panel: Representative dishes of cells of each group stained with Giemsa. Right panel: Quantitation of colony formation. Colony formation rate was calculated as percentage of total seeded cells. 1-6: control, SBHA, Bortezomib, SBHA + Bortezomib, MG-132, and SBHA + MG-132 group, respectively. *P <0.05 vs. control; #P <0.05 vs. SBHA alone.