DOWN-REGULATION OF BCR-ABL IN K562 CELLS RESTORES SUSCEPTIBILITY TO APOPTOSIS - CHARACTERIZATION OF THE APOPTOTIC DEATH

We examined the susceptibility of a variety of human leukemic cell lin es to the induction of apoptosis. K562, a chronic myelogenous leukemic cell line which expresses the bcr-abl fusion gene, was found to be ex tremely resistant to apoptosis, irrespective of the inducing agent. Th is resistance can be attributed to the deregulated Abl kinase activity of bcr-abl, as downregulation of its expression using antisense oligo deoxynucleotides targeted to the beginning of the abl sequence in this chimeric gene rendered these cells susceptible to cytotoxic drug-indu ced apoptosis. Examination of the morphological and biochemical featur es of apoptosis in K562 cells revealed the typical membrane blebbing a nd chromatin condensation associated with this form of cell death. In situ TdT-mediated end labeling of the DNA revealed the presence of str and breaks in the treated cells and field inversion gel electrophoresi s revealed the presence of large 10-50 kb fragments. However there was an absence of oligonucleosomal DNA fragmentation, whether or not Bcr- Abl was expressed. Thus, while inhibition of expression of Bcr-Abl ren ders K562 cells susceptible to apoptosis, the absence of oligonucleoso mal DNA fragmentation in these cells is independent of the function of this molecule.