Characterization of 2 derived MCL cell lines resistant to bortezomib. (A) Dose-response of Jeko-1 and Z-138 parental, and JBR and ZBR derived cell lines exposed for 24 hours to bortezomib (bz) (*P < .04; **P < .003). (B) Jeko-1, JBR, Z-138, and ZBR cells were treated for 24 hours with 5μM BMS-345541 and 2μM obatoclax, and cytotoxicity was assessed by Annexin V/propidium iodide staining. Cytotoxic values are compared with untreated cells as the means ± SDs of triplicate experiments. (C) Jeko-1, JBR, Z-138, and ZBR cells were incubated with 15nM bortezomib for 8 hours, and the inhibition of 20S proteasome activity was analyzed as described in “Methods.” (D) Modulation of proteasome-degraded proteins, apoptotic regulators, and molecular chaperone levels in parental and resistant cells exposed to bortezomib. Total protein extracts from Jeko-1, JBR, Z-138, and ZBR cell lines incubated for 8 hours with 15nM bortezomib were analyzed by Western blotting. Membranes were probed with suitable antibodies, using ubiquitin conjugates and β-actin to normalize misfolded protein accumulation and protein loading, respectively. eIF2-p represents elongation initiation factor 2α phosphorylated.