ICE family protease activation in CLL apoptosis. (A) Cleavage of PARP. Cells were incubated for 16 hours in the absence or presence of 10 μmol/L methylprednislone with or without 25 mmol/L zAPFcmk or 25 mmol/L zVADfmk, and PARP integrity was assessed by immunoblotting. Arrows indicate positions of intact PARP (p116) and the 85 kD caspase-generated PARP fragment (p85). Results from one experiment (patient 22) representative of four independent replicates with different patient isolates. (B) Measurement of caspase activity. Cells were incubated for the times indicated in the absence or presence of 10 μmol/L methylprednisolone, and hydrolysis of the ICE protease substrate DEVD-AMC was measured in a spectrofluorimeter as described in Materials and Methods. Results from two representative patient isolates (not included in Table 1) analyzed on the same day. Levels of DNA fragmentation were measured in parallel (16 hours): (□), patient A, control — 2%; (▪), MPS — 30%; (○), patient B, control — 9%; (•), MPS — 16%. Similar results were observed in three other patient isolates.