Late apoptotic effects of taxanes on K562 erythroleukemia cells: Apoptosisis delayed upstream of caspase-3 activation

The efficacy of taxanes on human leukemia cells is the object of intensive in vitro investigation concerning the influence of cell-type-specific chara cteristics on cytotoxic response to drugs. The present study dissects the r esponse to taxanes of HL60 acute myelomonocytic leukemia and of K562 chroni c myelogenous leukemia, in parallel over a 72-hr time-span, The kinetics of cytotoxicity following pulsed and continuous exposure to either taxol or t axotere showed a delayed response of K562 cells independently of dose and t ype of exposure. In K562 cells, apoptosis became evident at 48 hr and promi nent at 72 hr of treatment. These events were mirrored by delayed kinetics of caspase-3 activation. Comparable microtubule targeting was demonstrated in HL60 and in K562 cell lines, as bcl-2 and raf-1 were phosphorylated foll owing treatment with taxanes. These observations indicate that early activa tion processes were responsible for apoptosis, but that the delay was deter mined by other factors, In addition, cell-free-system experiments excluded the presence of excess nuclear and/or cytoplasmic inhibitory factors and de monstrated that K562 cells possess a fully competent caspase system which c an be readily activated. Processing of caspase-3 pro-enzyme was in fact inc reased by addition of cytochrome c, These results extend to taxol and taxot ere the notion that drug-induced apoptosis is delayed upstream of caspase-3 activation in K562 cells, that such kinetics is independent of drug concen tration and exposure time, and that it is linked to intrinsic cellular char acteristics mapping between bcl-2 phosphorylation and cytochrome c release. Int. J. Cancer 85:527-533, 2000. (C) 2000 Wiley-Liss, Inc.