Involvement of caspase-2 long isoform in Fas-mediated cell death of human leukemic cells

Engagement of the plasma membrane receptor Fas can induce apoptosis of leukemic cells. Signaling through Fas requires the formation of a death-inducing signaling complex (DISC) that involves the cytoplasmic domain of Fas, the adaptor molecule FADD/MORT-1, and procaspase-8, The present study investigated whether another caspase, known as procaspase-2L, played a role in Fas-mediated cell death. A series of human leukemic variant cells was derived by stable transfection with a CASP2L antisense construct (CASP2L/AS), Specific down-regulation of procaspase-2L decreased the sensitivity of these cells to Fas antibody (Ab, clone CH11), as determined by studying DNA fragmentation, chromatin condensation, and externalization of phosphatidylserine on the plasma membrane. In leukemic cells transfected with an empty vector, anti-fas Ab treatment activated caspase-8, decreased the expression of the BH3 domain-only protein Bid, triggered the release of cytochrome c from the mitochondria to the cytosol, and activated caspase-3, All these events could not be observed when CASP2L/AS cells were similarly treated with anti-Fas Abs, CASP2L/AS transfection did not inhibit the formation of the DISC and no direct interaction between procaspase-2L and either Fas or FADD or procaspase-8 was identified. Down-regulation of procaspase-2L inhibited anti-fas Ab-mediated cleavage of c-FLIP (FLICE-inhibitory protein), a protein that interferes with the formation of a functional DISC, These results suggest that the long isoform of caspase-2 plays a role in the Fas-mediated pathway to cell death by contributing to caspase-8 activation at the DISC level,(Blood. 2001;97:1835-1844) (C) 2001 by The American Society of Hematology.