FTY720 increased production of apoptosis-inducing lipid sphingosine. (A) Five million RNK-16 cells were treated with 10μM FTY720 for 6, 16, and 24 hours then were extracted to obtain lipids; extracts were then identified and quantitated by tandem mass spectrometry. Each sample had 4 replicates. *P < .05 and **P < .005 indicate significant differences versus control (Student t test). (B) PBMCs from 3 individual chronic NK-cell leukemia patients (nos. 8, 9, 10) were treated with 5μM FTY720, 5μM sphingosine, or DMSO for 6 and 24 hours then cells were assayed for apoptosis by flow cytometry. (C) RNK-16 cells were treated with 2.5, 5.0, 10.0μM FTY720 or sphinginosine (SPH) for 48 hours, then cells were assayed for apoptosis by flow cytometry. *P < .05, **P < .005, and ***P < .0005 indicate significant differences versus control (Student t test).