Caspase-8 activation in BM metastatic neuroblasts. A-B) Cell viabilities assessed by MTS analysis in PTX cells (A) and in BM cells (B) pretreated with 50 μM of caspase inhibitor (zVDAVD-fmk, zIETD-fmk, zLEHD-fmk, zDEVD-fmk) for 1 hr, then treated with 10 μM 4HPR for 48 hrs. Data points show mean ± SD of 3 independent experiments. *P < 0.05, **P < 0.01 compared with control cells treated with 4-HPR only. C) Caspase-8 activity was measured using lysates from BM cells treated with 10 μM 4-HPR for 24 or 48 hrs. Activity was normalized to the activity in the untreated cells (control), and blocked by addition of 10 μM caspase-8 inhibitor (zIETD-fmk).