CML and apoptosis: the ceramide pathway

In patients with Chronic Myeloid Leukemia (CML), the neoplastic (Bcr-Abl+) progenitors are characterised by an increased proliferative activity. These cells appear to become resistant to apoptosis following growth factor withdraw. We demonstrate that despite this property, Bcr-Abl transformed cells (primitive hematopoietic progenitors or cell lines) remains sensitive to apoptosis induced by Ceramides analogues. This effect is dose dependent and occurs faster in transformed cells as compared to their normal counterparts. In addition to the classical features of apoptosis, we observed that Ceramide-treated CML cells display a rapid and sequential activation of the Bcr-Abl and PI3 kinases. We then demonstrated the role of the Bcr-Abl kinase activity in the accelerated response observed in CML cells treated by Ceramide. The PI3 kinase seems to be partly involved in the accelerated Phosphatidyl-Serine exposure observed in Bcr-Abl transformed cells. Finally, we observed that Ceramide-induced apoptosis does not seem to implicate a Bcl2 protein modulation. Taken together these results support the hypothesis of at least two independent signaling pathways initiating programmed cell death: one will be involved in apoptosis mediated by signals such as cytokine-starving is blocked by the Bcr-Abl fusion protein while the other one initiated by Ceramide is accelerated by the Bcr-Abl protein.