Proteasome inhibitors rapidly up-regulate a protein specifically involved in ER stress–induced apoptosis. (A) The 8226/S, U266, and MM.1S human multiple myeloma cell lines were cultured for up to 24 hours in 50 μM tunicamycin (TM), 1 μM brefeldin A (BfA), or the indicated concentration of epoxomycin (EPOX) or bortezomib (BZ). At the indicated time points, an aliquot of cells was taken to assess cell viability by Annexin V–FITC and propidium iodide staining, and the remainder of the cells was used to prepare whole-cell lysates. Induction of GADD153 and actin expression were determined by Western blot analysis. Blots representative of at least 3 independent experiments are shown. (B) Five different human multiple myeloma cell lines were treated for 12 and 24 hours with 50 μM tunicamycin (TM), 1 μM brefeldin A (BfA), 30 μM melphalan (Mel), 250 nM staurosporine (STS), or the indicated concentration of the proteasome inhibitors epoxomycin (EPOX), PSI, or bortezomib (BZ). Western blot and cell viability analyses were performed as described above. The data are representative of at least 3 independent experiments.