Increasing proteasome workload through ER stress sensitizes MMCs to PIs. Pharmacologic ER stressors increase proteasome workload and PI-induced cytotoxicity in MMCs. (A) FACS analysis of apoptosis upon treatment with tunicamycin (Tm; 2.5 μg/mL) or bortezomib (Btz; 10 nM for U266 and 1 nM for MM.1S), alone or together for 48 hours. Top panels show the level of XBP-1 splicing after 24 hours (u and s for unspliced and spliced, respectively). (B) Twenty-four–hour treatment with Tm (2.5 μg/mL) and Btz (5 nM) synergistically causes ER stress (XBP-1 splicing; top panel) and accumulation of Ub^G76V-GFP (bottom overlay FACS histogram) in engineered U266 cells. (C) Forty-eight–hour treatment with Tm and Btz triggers synergistic death of engineered U266 cells. Cell death was assessed by modifications of physical parameters by FACS. One representative experiment is shown.