Blood, 1 November 2001, Vol. 98, No. 9, pp. 2800-2807
NEOPLASIA
Caspase 8 activation independent of Fas (CD95/APO-1) signaling
may mediate killing of B-chronic lymphocytic leukemia cells by
cytotoxic drugs or 
radiation
Dylan T. Jones,
Kanagasabai Ganeshaguru,
Andres E. Virchis,
Najeem I. Folarin,
Mark W. Lowdell,
Atul B. Mehta,
H. Grant Prentice,
A. Victor Hoffbrand, and
R. Gitendra Wickremasinghe
From the Department of Hematology, Royal Free and
University College Medical School, Royal Free Campus, London, United
Kingdom.
Ligation of the cell-surface Fas molecule by its ligand (Fas-L) or
agonistic anti-Fas monoclonal antibodies results in the cleavage and
activation of the cysteine protease procaspase 8 followed by the
activation of procaspase 3 and by apoptosis. In some leukemia cell
lines, cytotoxic drugs induce expression of Fas-L, which may contribute
to cell killing through the ligation of Fas. The involvement of Fas,
Fas-L, and caspase 8 was studied in the killing of B-cell chronic
lymphocytic leukemia (B-CLL) cells by chlorambucil,
fludarabine, or 
radiation. Spontaneous apoptosis was observed at
24-hour incubation, with additional apoptosis induced by each of the
cytotoxic treatments. Although Fas mRNA expression was elevated after
exposure to chlorambucil, fludarabine, or radiation, Fas protein
levels only increased after irradiation. Therefore, Fas expression may
be regulated by multiple mechanisms that allow the translation of Fas
mRNA only in response to restricted cytotoxic stimuli. None of the cytotoxic stimuli studied here induced Fas-L expression. An agonistic anti-Fas monoclonal antibody (CH-11) did not significantly augment apoptosis induction by any of the death stimuli. A Fas-blocking antibody (ZB4) did not inhibit spontaneous, chlorambucil-,
fludarabine-, or radiation-induced apoptosis. However, procaspase 8 processing was induced by all cytotoxic stimuli. These data suggest
that the Fas/Fas-L signaling system does not play a major role in the induction of apoptosis in B-CLL cells treated with cytotoxic drugs or
radiation. However, Fas-independent activation of caspase 8 may play a
crucial role in the regulation of apoptosis in these cells.
