The antiapoptotic effect of sustained BCR signaling is mediated by posttranscriptional regulation of Mcl-1. (A) CLL B cells were transfected with control or Mcl-1 siRNA and stimulated for 24 or 48 hours with immobilized anti-IgM antibodies. Protection from apoptosis was evaluated by PARP cleavage and Annexin V/PI staining. A total of 4 independent experiments are shown. (B) Comparison of changes in Mcl-1 mRNA and protein levels induced by sustained BCR engagement and sustained activation of Akt. CLL B cells from 5 patients were nucleofected with control or myr.Akt mRNA. Cells nucleofected with control mRNA were either left unstimulated or were stimulated with immobilized anti-IgM antibody. All samples were collected 24 hours after nucleofection and analyzed by immunoblotting (left panel) and by real-time RT-PCR. Mcl-1 mRNA and protein levels were normalized to actin and are expressed as fold change relative to corresponding levels in CLL cells transfected with control mRNA, which were set to 1 (dashed line).