Caspase 8 activation independent of Fas (CD95/APO-1) signaling may mediatekilling of B-chronic lymphocytic leukemia cells by cytotoxic drugs or gamma radiation

Ligation of the cell-surface Fas molecule by its ligand (Fas-L) or agonisti c 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 ex pression of Fas-L, which may contribute to cell killing through the ligatio n of Fas. The involvement of Fas, Fas-L, and caspase 8 was studied in the k illing of B-cell chronic lymphocytic leukemia (B-CLL) cells by chlorambucil , fludarabine, or gamma radiation. Spontaneous apoptosis was observed at 24 -hour incubation, with additional apoptosis induced by each of the cytotoxi c treatments. Although Fas mRNA expression was elevated after exposure to c hlorambucil, fludarabine, or gamma radiation, Fas protein levels only incre ased after irradiation. Therefore, Fas expression may be regulated by multi ple mechanisms that allow the translation of Fas mRNA only in response to r estricted cytotoxic stimuli. None of the cytotoxic stimuli studied here ind uced Fas-L expression. An agonistic anti-Fas monoclonal antibody (CH-11) di d 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 proces sing 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 apopt osis in B-CLL cells treated with cytotoxic drugs or radiation. However, Fas -independent activation of caspase 8 may play a crucial role In the regulat ion of apoptosis in these cells. (C) 2001 by The American Society of Hemato logy.