LACK OF CERAMIDE GENERATION IN TF-1 HUMAN MYELOID LEUKEMIC-CELLS RESISTANT TO IONIZING-RADIATION

The mechanism(s) by which ionizing radiation (IR) induces cell death i s of fundamental importance in understanding cell sensitivity and resi stance, Here we evaluated the response to IR of two subclones of the a utonomous human erythromyeloblastic cell line TF-1: TF-1-34 (which exp resses CD34) and TF-1-33 (which lacks CD34), In clonogenic assays, TF- 1-34 cells were found to be relatively less sensitive to IR compared t o TF-1-33 cells based on the Do determination (3.01 vs 1.56 Gy), Furth ermore, after IR at 12 Gy, TF-1-33 cell viability decreased by similar to 50% within 24 h, whereas TF-1-34 cell growth was unaffected during this time, Gradual loss of TF-1-34 cell viability was observed only a fter 48 h, Morphological and molecular analysis revealed that TF-1-33 cells died of apoptosis, and TF-1-34 cells of delayed reproductive cel l death. While IR produced comparable amounts of DNA double strand bre aks (DSB) in both cell lines, TF-1-34 retained DSB much longer than TF -1-33 suggesting that radioresistance and the defective apoptotic resp onse of TF-1-34 cells was not related to a higher DNA repair capacity, However, the lack of an apoptotic response in TF-1-34 was correlated to the absence of a sphingomyelin (SM)-ceramide (CER) signaling pathwa y, Indeed, IR triggered in TF-1-33 cells but not in TF-1-34. SM hydrol ysis (25% at 12 Gy) and CER generation (>50%) through the activation o f neutral but not acid sphingomyelinase. Synthetic cell permeate CER i nduced apoptosis in both TF-1-33 and TF-1-34 cells, This study indicat es that alterations of the SM-CER signaling pathway can significantly influence the cell death process as well as the survival of acute myel oid leukemia cells after IR exposure.