Blockade of the extrinsic apoptotic pathway diminished LCL/LBH–induced apoptosis. (A) MM.1S cells were exposed to the indicated concentrations of LCL ± LBH for 24 hours. Immunoblotting analysis was then performed to monitor levels of caspase-8 and cleaved caspase-3. (B) Bortezomib-naive U266 cells or bortezomib-resistant PS-R cells were exposed to the indicated concentrations of TL ± MS for 24 hours. Immunoblotting analysis was then performed to monitor levels of caspase-8. (C and D) U266/dominant-negative caspase-8 (DN-C8) and U266/dominant-negative FADD (DN-FADD) cells were established by stably transfecting human dominant-negative caspase-8 complementary DNA or DN-FADD complementary DNA. Immunoblotting analysis was then performed to monitor levels of GFP and FADD. A black border indicates that the blots are cut from the same membrane and same exposure time. α-Tubulin or β-actin was assayed to ensure equivalent loading and transfer. Cells were treated with LCL ± LBH for 24 hours, after which the percentage of 7-AAD⁺ cells was determined by flow cytometry. *P < .05, **P < .01. All experiments were repeated three times. CF, cleavage fragment.