Modulation of caspase-8 and FLICE-inhibitory protein expression as a potential mechanism of Epstein-Barr virus tumorigenesis in Burkitt's lymphoma

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, althoug h expressing each of the DISC components, Fas/ APO-1-associated death domai n protein (FADD), and caspase-8 (FLICE/MACH/Mch5). The apoptotic pathway di stal to the DISC is intact because ceramide analogs, staurosporine, and gra nzyme B activate caspase-3 and induce apoptosis. Fas resistance was not exp lained by the putative death-attenuating caspase-8 isoforms. However, while Fas-activated cytosolic extracts from sensitive cells were capable of proc essing both procaspase-8 and procaspase-3 into active subunit forms, resist ant cell extracts did not possess either of these activities. Accordingly, reverse transcriptase-polymerase chain reaction (RT-PCR) analysis showed hi gher transcript levels for the FLICE-inhibitory protein (FLIPL) in resistan t cells and the ratio of caspase-8 to FLIPL measured 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 b y caspase-8 or FLIPL overexpression was able to alter the susceptibility st atus of the cell lines tested. Our results imply that the relative levels o f caspase-8 and FLIPL are an important determinant of susceptibility to Fas -mediated apoptosis. (C) 1999 by The American Society of Hematology.