Engagement of the plasma membrane receptor Fas can induce apoptosis of leuk
emic cells. Signaling through Fas requires the formation of a death-inducin
g signaling complex (DISC) that involves the cytoplasmic domain of Fas, the
adaptor molecule FADD/MORT-1, and procaspase-8, The present study investig
ated whether another caspase, known as procaspase-2L, played a role in Fas-
mediated cell death. A series of human leukemic variant cells was derived b
y stable transfection with a CASP2L antisense construct (CASP2L/AS), Specif
ic down-regulation of procaspase-2L decreased the sensitivity of these cell
s to Fas antibody (Ab, clone CH11), as determined by studying DNA fragmenta
tion, chromatin condensation, and externalization of phosphatidylserine on
the plasma membrane. In leukemic cells transfected with an empty vector, an
ti-fas Ab treatment activated caspase-8, decreased the expression of the BH
3 domain-only protein Bid, triggered the release of cytochrome c from the m
itochondria 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 n
o direct interaction between procaspase-2L and either Fas or FADD or procas
pase-8 was identified. Down-regulation of procaspase-2L inhibited anti-fas
Ab-mediated cleavage of c-FLIP (FLICE-inhibitory protein), a protein that i
nterferes with the formation of a functional DISC, These results suggest th
at the long isoform of caspase-2 plays a role in the Fas-mediated pathway t
o cell death by contributing to caspase-8 activation at the DISC level,(Blo
od. 2001;97:1835-1844) (C) 2001 by The American Society of Hematology.