Caspase-3-like activity determines the type of cell death following ionising radiation in MOLT-4 human leukaemia cells

Caspases, a family of cysteine proteases, play a central role in the pathwa ys leading to apoptosis, Recently, it has been reported that a broad spectr um inhibitor of caspases, the tripeptide Z-VAD-fmk, induced a switch from a poptosis to necrosis in dexamethasone-treated B lymphocytes and thymocytes. As such a cell death conversion could increase the efficiency of radiation therapy and in order to identify the caspases involved in this cell death transition, we investigated the effects of caspase-3-related proteases inhi bition in irradiated MOLT-4 cells. Cells were pretreated with Ac-DEVD-CHO, an inhibitor of caspase-3-like activity, and submitted to X-rays at doses r anging from 1 to 4 Gy. Our results show that the inhibition of caspase-3-li ke activity prevents completely the appearance of the classical hallmarks o f apoptosis such as internucleosomal DNA fragmentation or hypodiploid parti cles formation and partially the externalization of phosphatidylserine. How ever, this was not accompanied by any persistent increase in cell survival. Instead, irradiated cells treated by this inhibitor exhibited characterist ics of a necrotic cell death. Therefore, functional caspase-3-subfamily not only appears as key proteases in the execution of the apoptotic process, b ut their activity may also influence the type of cell death following an ex posure to ionizing radiation. (C) 2000 Cancer Research Campaign.