We previously reported that agonist anti-TRAILR mAbs (HGS-ETR1) induced cell death of myeloma cells through both the extrinsic and intrinsic pathway. Although caspase 8 activation was fully required to initiate death signaling, it was unclear whether intrinsic pathway played or not a major role in apoptosis execution. In the current work, we evaluated the contribution of the intrinsic pathway in a large panel of HMCL (n=10). To address this question, we monitored mitochondrial depolarization and the contribution of caspase 9. Apoptosis and mitochondrial polarization were both evaluated by flow cytometry. Kinetic analysis of mitochondrial polarization (JC-1) shows that all mitochondria were fully depolarized after only 4h following HGS-ETR1 mAb addition: the depolarization started early, as soon as 1h. In good agreement with mitochondrial depolarization, we observed that specific inhibitor of caspase 9 activity (z-LEHD-fmk) always prevented at least 50% of death (and up to 90%). Our data show that in all cell lines evaluated, activation of the intrinsic pathway was required for cell death even with high concentrations of mAb. We previously showed that anti-TRAILR apoptosis was always associated with an early and massive Mcl-1 cleavage (
Disclosures: No relevant conflicts of interest to declare.