Sti 571 induces caspase 9 and 3 dependent apoptosis of bcr-abl positive cells in-vitro; augmentation of apoptotic responses by trail

STI571 is an abl-specific tyrosine kinase inhibitor that inhibits growth of bcr/abl+ cells. It is unclear if this is due to cell cycle arrest, apoptosis or both. To test the hypotheses that: (a) caspase mediated apoptosis is the dominant effect of STI571 in leukemic cells, and (b) that the apoptotic response is specific to bcr/abl+ cells, we sought to identify apoptotic pathways in bcr/abl+ and bcr/abl− cells in vitro. Using the bcr/abl+ cell line K562, the leukemic cell line MO7e, and MO7e cells transfected with bcr/abl (MO7p210), we quantified apoptosis using morphological and flow cytometric analyses of cells exposed to STI571. After 24 hours, P.I. staining revealed G1 arrest in K562 and MO7p210. By 84 hours, there was a substantial sub-G1 population with morphological signs of apoptosis. By 2 hours, a majority of cells expressed annexin-V. Caspase 3 activation increased over 72 hours. A caspase 9 inhibitor markedly inhibited the degree of caspase 3 activation seen at 48 hours. A caspase 8 inhibitor was less effective. MO7e cells were unaffected by STI571. We next sought to determine the potential synergistic effects of STI571 and TRAIL. TRAIL induces apoptosis by binding to death receptors via a pathway that is p53 independent. In K562 cells, TRAIL induced caspase 8 dependent, caspase 3 activation and annexin-V within 24 hours. TRAIL and STI571 had an additive effect with respect to induction of caspase 3 and annexin-V. We conclude: (a) STI571 induces G1 arrest followed by apoptosis in bcr/abl+ cells, and (b) because STI571 and TRAIL may induce apoptosis via different pathways and appear to have an additive anti-leukemic effect, the combination of these agents may have therapeutic potential.