Expression of BCR-ABL Does Not Inhibit Apoptosis In Vitro, on a B Lymphoblastoid Cell Line.

The bcr-abl oncogene is generated by the Philadelphia chromosome (Ph) translocation, fusing the BCR gene to the ABL gene and occurs mainly in two different forms. In chronic myelogenous leukemia (CML) a 210kDa Bcr-Abl protein is associated with proliferation and accumulation of myeloid cells and their precursors, whereas a 185kDa form is responsible for the pathogenesis of acute lymphocytic leukemia (ALL). Bcr-Abl not only induces cellular transformation but also regulates cell proliferation and confers resistance to a variety of apoptosis-inducing agents. Much attention has been focused on the development of novel therapies based on mechanistic understanding of Bcr-abl signaling in CML cells. However, we still do not fully understand how this molecule interferes with apoptotic machinery. The aim of this study was to investigate which step of the apoptotic machinery was most influenced by ectopic expression of Bcr-Abl myeloblastic (HL-60) and B lymphoblastic (SKW6.4) cell lines.

Ectopic expression of Bcr-Abl conferred resistance to apoptosis induced by staurosporine and anti-Fas in HL-60 cells, but not in SKW6.4 cells, although increased phosphotyrosine containing proteins could be easily observed in SKW6.4.Bcr-Abl cells. The protein levels of Bcl-x_L, Mcl-1 and Flip detected by western-blot were greater in HL-60.Bcr.Abl, when compared to wild type HL-60 cells. In contrast, the proapoptotic protein Bid was considerably reduced in HL-60. Bcr-abl cells. Two independent lines of SKW6.4.Bcr-Abl cells had a small reduction of in Bid levels, but no difference was observed in the expression of the anti-apoptotic proteins Bcl-x_L, Mcl-1 and Flip. At the mRNA level SKW6.4 and SKW.Bcr-Abl presented similar expression of Bcl-x_L, Mcl-1 and Flip. In addition a greater expression of anti-apoptotitc proteins A1 (IDV A1/actin=0.6) and Bcl-w (IDV Bcl-w/actin=1.6) was found in SKW6.4.Bcr-Abl compare to SKW (0.2 and 0.01, respectively) wild type. Interestingly, the upregulated levels of these anti-apoptotic genes were not sufficient to prevent apoptosis in these cells. Taken together these results suggest that Bcr-abl is not capable of protecting SKW6.4 cells from apoptosis induced by staurosporine and anti-Fas, perhaps because it interacts differently with the apoptotic machinery in B lymphoblasts compared to myeloblast cell lines. Further stydies are underway in our laboratory in order to better understand the Bcr-abl-mediated anti-apoptotic signaling in B lymphoblast cells.