Evidence of protein biochemistry for the involvement of the intrinsic, extrinsic, and ER stress-mediated apoptosis. (A) Caspase-8 activity assays. Caspase-8 inhibitor Z-IETD-FMK was added into culture medium 30 minutes before drug treatment, as recommended by the manufacturer (R&D Systems). Data were obtained from 3 independent experiments. (B) Western blot analysis of caspases and marker molecules involved in the intrinsic, extrinsic, and ER stress-mediated apoptosis. Specific antibodies were obtained from commercial sources (see “Materials and methods”) and reacted with Western blots of cell lysates of the drug treatment series as indicated.