Late apoptotic effects of taxanes on K562 erythroleukemia cells. Apoptosis is 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 characteristics on cytotoxic response to drugs. The present study dissects the response to taxanes of HL60 acute myelomonocytic leukemia and of K562 chronic myelogenous leukemia, in parallel over a 72-hr time-span. The kinetics of cytotoxicity following pulsed and continuous exposure to either taxol or taxotere showed a delayed response of K562 cells independently of dose and type of exposure. In K562 cells, apoptosis became evident at 48 hr and prominent 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 following treatment with taxanes. These observations indicate that early activation processes were responsible for apoptosis, but that the delay was determined by other factors. In addition, cell-free-system experiments excluded the presence of excess nuclear and/or cytoplasmic inhibitory factors and demonstrated that K562 cells possess a fully competent caspase system which can be readily activated. Processing of caspase-3 pro-enzyme was in fact increased by addition of cytochrome c. These results extend to taxol and taxotere the notion that drug-induced apoptosis is delayed upstream of caspase-3 activation in K562 cells, that such kinetics is independent of drug concentration and exposure time, and that it is linked to intrinsic cellular characteristics mapping between bcl-2 phosphorylation and cytochrome c release.