BCL-2-INDEPENDENT BCR-ABL-MEDIATED RESISTANCE TO APOPTOSIS - PROTECTION IS CORRELATED WITH UP-REGULATION OF BCL-X(L)

Bcr-Abl is the molecule responsible for both the transformation phenot ype and the resistance to chemotherapeutic drugs found in chronic myel ogenous leukemia (CML) cells, Wild-type HL-60, a transformed pro-myelo cytic cell line, is very susceptible to apoptosis-inducing agents, We show here that expression of Bcr-Abl in HL-60 cells rendered them extr emely resistant to apoptosis induced by a wide variety of agents, The anti-apoptotic effect of Bcr-Abl was found to be independent of the ph ase of the cell cycle, Treatment with antisense oligonucleotides direc ted to bcr decreased the expression of the ectopic bcr-abl and restore d susceptibility to apoptosis, Double mutations affecting the autophos phorylation site and the phosphotyrosine-binding motif (FLVRES) have b een previously shown to impair the transforming activity of Bcr-Abl in fibroblasts and hematopoietic cells, however HL-60 cells expressing t his double mutant molecule exhibited the same level of resistance to a poptosis as those expressing the wild-type Bcr-Abl. Interestingly, wil d type and mutant Bcr-Abl induced in HL-60 cells a dramatic down regul ation 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 oligo nucleotides targeted to bcl-x downregulated the expression of Bcl-x(L) and increased the susceptibility of HL-60.Bcr-Abl cells to staurospor ine. Importantly, HL-60 cells overexpressing Bcl-x(L) showed higher ex pression of Bcl-x(L) but lower resistance to apoptosis when compared t o HL-60.Bcr-Abl cells. The results described here show that Bcr-Abl is a powerful mammalian anti-apoptotic molecule and can act independentl y of Bcl-2, Bcl-x(L), however, seems to participate in part in Bcr-Abl -mediated resistance to apoptosis in HL-60 cells.