4-HPR induces apoptosis in BM metastatic neuroblasts. A-B) Dose-response and time-course analysis of 4-HPR-induced cell death in PTX and BM neuroblasts. Cell viabilities assessed by MTS analysis in PTX and BM neuroblasts treated with 5 or 10 μM 4-HPR for 48 hours (A) or with 10 μM 4-HPR for 1 to 4 days (B). The values reported are mean ± SD of three independent experiments. ** P < 0.01 indicates difference between PTX and BM neuroblasts. C) The increase of apoptotic cell death is correlated with caspase-8 re-expression in BM metastatic cells. Sub-G1 apoptotic cells were detected by the propidium staining method after stimulation for 72 hours with 2.5 or 5 μM 4-HPR. Mean values ± SD of 3 independent experiments are shown.** P< 0.01 indicates significant difference between PTX/BM cells and respective untreated cells.