Abstract
Ligation of the Fas receptor induces death-inducing signaling complex (DISC) formation, caspase activation, and subsequent apoptotic death of several cell types. Epstein-Barr virus (EBV)-positive group III Burkitt’s lymphoma (BL) cell lines have a marked resistance to Fas-mediated apoptosis, although expressing each of the DISC components, Fas/ APO-1–associated death domain protein (FADD), and caspase-8 (FLICE/MACH/Mch5). The apoptotic pathway distal to the DISC is intact because ceramide analogs, staurosporine, and granzyme B activate caspase-3 and induce apoptosis. Fas resistance was not explained by the putative death-attenuating caspase-8 isoforms. However, while Fas-activated cytosolic extracts from sensitive cells were capable of processing both procaspase-8 and procaspase-3 into active subunit forms, resistant cell extracts did not possess either of these activities. Accordingly, reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed higher transcript levels for the FLICE-inhibitory protein (FLIPL) in resistant cells and the ratio of caspase-8 to FLIPLmeasured by competition RT-PCR analysis directly correlated with susceptibility to Fas-mediated apoptosis of all cell lines. In addition, modification of the caspase-8/FLIPL ratio by caspase-8 or FLIPL overexpression was able to alter the susceptibility status of the cell lines tested. Our results imply that the relative levels of caspase-8 and FLIPL are an important determinant of susceptibility to Fas-mediated apoptosis.
Smac mimetics and TRAIL cooperate to induce MLKL-dependent necroptosis in Burkitt's lymphoma cell lines