Stabilization of a safety-catch in caspase-3 in plasma cells. (A) Immunoblotting on whole cell extracts prepared from undifferentiated (d0) and plasmacytic (d3) I.29μ⁺ cells that had been treated for 9 hours with 10μM PAC-1 or DMSO (control). Images are from the same blot. (B) Fold increase in apoptotic loss of ΔψM in undifferentiated (d0) and plasmacytic (d3) I.29μ⁺ cells after treatment with PAC-1 for 9 hours (mean and SEM of 2 independent experiments). P values refer to the comparison between PAC-1 treated and control cells, and statistically significant differences between means are indicated by an asterisk. (C) Apoptosis (positive staining with annexinV-FITC and/or propidium iodide) after 14 hours of treatment with PAC-1 in primary resting B and plasma cells. To account for different baseline viabilities, relative cell death (percent dead cells/untreated control) was expressed as the difference in cell death in the absence (A) and presence (P) of a drug, normalized by the cell death achievable in the control cells [(P - A)/(100 - A) × 100]. Mean and SEM of 2 independent experiments. (D) Immunoblotting of whole cell extracts from splenic B cells 3 days after in vitro activation with LPS and freshly isolated in vitro–differentiated plasma cells. (E) Immunoblotting of whole cell extracts from the indicated human (KMS-11, RPMI8226, U266, OPM-2) and mouse (MPC-11) myeloma cell lines that had been treated with PAC-1 for 14 hours.