Cellular FLICE-inhibitory protein splice variants inhibit different steps of caspase-8 activation at the CD95 death-inducing signaling complex.
Upon stimulation, CD95 (APO-1/Fas) recruits the adapter molecule FADD/MORT1, procaspase-8, and the cellular FLICE-inhibitory proteins (c-FLIP) into the death-inducing signaling complex (DISC). According to the induced proximity model, procaspase-8 is activated in the DISC in an autoproteolytic manner by two subsequent cleavage steps. c-FLIP proteins exist as a long (c-FLIP(L)) and a short (c-FLIP(S)) splice variant, both of them capable of protecting cells from death receptor-mediated apoptosis. In stably transfected BJAB cells, both c-FLIP(S) and c-FLIP(L) block procaspase-8 activation at the DISC. However, cleavage is blocked at different steps. c-FLIP(L) allows the first cleavage step of procaspase-8, leading to the generation of the p10 subunit. In contrast, c-FLIP(S) completely inhibits cleavage of procaspase-8. Interestingly, p43-c-FLIP(L) lacking the p12 subunit also prevents cleavage of procaspase-8. In contrast, a nonprocessable mutant of c-FLIP(L) allows the first cleavage of procaspase-8. In conclusion, both c-FLIP proteins prevent caspase-8 activation at different levels of procaspase-8 processing at the DISC. Our results indicate that c-FLIP(L) induces a conformation of procaspase-8 that allows partial but not complete proteolytical processing, whereas in contrast c-FLIP(S) even prevents partial procaspase-8 activation at the DISC.
Tumor Immunology Program, German Cancer Research Center, Im Neuenheimer Feld 280, 69120 Heidelberg, Germany.
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Krueger
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Schmitz
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Baumann
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Krammer
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Kirchhoff
D000818:Animals; D017209:Apoptosis / Q000502:physiology; D001483:Base Sequence; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; D002352:Carrier Proteins / Q000235:genetics* / Q000502:physiology; D053181:Caspase 8; D053453:Caspase 9; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism; D002460:Cell Line; D017931:DNA Primers; D004789:Enzyme Activation; D006868:Hydrolysis; D047908:Intracellular Signaling Peptides and Proteins; D051379:Mice; D015398:Signal Transduction; D019014:fas Receptor / Q000235:genetics* / Q000502:physiology
D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; D002352:Carrier Proteins; D061945:Caspase Inhibitors; C506387:Cflar protein, mouse; D017931:DNA Primers; D047908:Intracellular Signaling Peptides and Proteins; D019014:fas Receptor; C505526:Casp8 protein, mouse; C506434:Casp9 protein, mouse; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases