Bcl-2-independent Bcr-Abl-mediated resistance to apoptosis:: protection is correlated with up regulation of Bcl-xL

Bcr-Abl is the molecule responsible for both the transformation phenotype and the resistance to chemotherapeutic drugs found in chronic myelogenous leukemia (CML) cells, Wild-type HL-60, a transformed pro-myelocytic cell line, is very susceptible to apoptosis-inducing agents, We show here that expression of Bcr-Abl in HL-60 cells rendered them extremely resistant to apoptosis induced by a wide variety of agents, The anti-apoptotic effect of Bcr-Abl was found to be independent of the phase of the cell cycle, Treatment with antisense oligonucleotides directed to bcr decreased the expression of the ectopic bcr-abl and restored susceptibility to apoptosis, Double mutations affecting the autophosphorylation site and the phosphotyrosine-binding motif (FLVRES) have been previously shown to impair the transforming activity of Bcr-Abl in fibroblasts and hematopoietic cells, however HL-60 cells expressing this double mutant molecule exhibited the same level of resistance to apoptosis as those expressing the wild-type Bcr-Abl. Interestingly, wild type and mutant Bcr-Abl induced in HL-60 cells a dramatic down regulation of Bcl-2 and increased the levels of Bcl-x(L). The level of Bar did not change in response to the presence of Bcr-Abl, Antisense oligonucleotides targeted to bcl-x downregulated the expression of Bcl-x(L) and increased the susceptibility of HL-60.Bcr-Abl cells to staurosporine. Importantly, HL-60 cells overexpressing Bcl-x(L) showed higher expression of Bcl-x(L) but lower resistance to apoptosis when compared to HL-60.Bcr-Abl cells. The results described here show that Bcr-Abl is a powerful mammalian anti-apoptotic molecule and can act independently of Bcl-2, Bcl-x(L), however, seems to participate in part in Bcr-Abl-mediated resistance to apoptosis in HL-60 cells.