BCR-ABL fails to inhibit apoptosis in U937 myelomonocytic cells expressinga carboxyl-terminal truncated Stat5

Recent experimental data suggest that one of the major effects of BCR-ABL g ene expression in hematopoietic cells is the inhibition of apoptosis. Altho ugh the exact mechanisms of this phenomenon are not clear, it is thought to be related to the fact that BCR-ABL induces several signalling pathways al so activated by growth factors. In order to determine the anti-apoptotic ro le of BCR-ABL in a hematopoietic cell line and to by-pass the influence of cytokine-dependence, BCR-ABL gene was expressed in the autonomously growing myelomonocytic U937 cell line using retroviral vectors. There was no resis tance to apoptosis induced by either serum deprivation or different doses o f etoposide in any U937 clones expressing BCR-ABL protein. In addition to s erum deprivation and etoposide, BCR-ABL-expressing clones were not protecte d from apoptosis induced by TNF, ceramide-C2 and FAS-cross-linking. BCL2 ex pression was absent in U937 cells and BAX levels were identical between Neo and BCR-ABL clones. To further investigate the mechanisms of this phenomen on, band-shift assays were performed to detect activation of STAT molecules . No constitutive activation of STATs was detected in either NeoR or BCR-AB L-U937 cells, although both IFN-gamma and GM-CSF activated STAT1 and STAT5, respectively, with similar kinetics in both NeoR and BCR-ABL-U937 cells. I n addition, the GM-CSF-induced-STAT5 activation was found to be weakened in all clones expressing BCR-A-BL. In both control NeoR and BCR-ABL-transfect ed clones, band-shift assays revealed the presence of an abnormal truncated STAT5 recognized only by an anti- N-terminal but not by an anti-C-Terminal STAT5 antibody. These findings suggest a possible link between the absence of anti-apoptotic potential of BCR-ABL and abnormalities of the STAT5 path way, including, absence of constitutive activation of STAT5, inhibition of GM-CSF-induced STAT5 activation and expression of a carboxyl-terminal-trunc ated STAT5.