SENSITIVITY OF K562 AND HL-60 CELLS TO EDELFOSINE, AN ETHER LIPID DRUG, CORRELATES WITH PRODUCTION OF REACTIVE OXYGEN SPECIES

Edelfosine octadecyl-2-O-methyl-rac-glycero-3-phosphocholine; ET-18-OC H3), a membrane-targeting anticancer ether Lipid drug has been shown p reviously in vitro to be capable of initiating oxidative processes in cells, Here we study two human leukemia cell lines (HL-60 and K562) th at have different sensitivities to edelfosine; HL-60 cells are more se nsitive than K562 cells. To determine whether edelfosine alters the se nsitivity of these lines to an oxidative stress, cells were subjected to the oxidative stress of iron(II) plus ascorbate and then monitored for free radical formation, membrane integrity, and cytotoxicity, The HL-60 cell was sensitive to the ether lipid drug in clonogenic and dye exclusion assays; a lipid-derived free radical was generated by this sensitive cell in the presence of small amounts of Fe2+ and ascorbate as detected by electron paramagnetic resonance and the spin trap alpha -(4-pyridyl-1-oxide)-N-tert-butylnitrone, There was also simultaneous generation of an ascorbate-free radical, which has been shown to estim ate cellular oxidative flux. In contrast, the K562 cell was resistant to edelfosine cytotoxicity in all assays and did not generate either l ipid-derived or ascorbate-free radicals, Subcellular homogenates of th e HL-60 cell generated both radicals when exposed to the drug, but hom ogenates of K562 did not generate either, suggesting that differential drug uptake or intracellular drug localization is not the cause of th e difference in oxidation, Trypan blue uptake by the HL-60, but not th e K562 cells, measured under the same conditions as the oxidation expe riments, demonstrated a loss of membrane impermeability with similar t ime and concentration dependence, suggesting a causal relationship of membrane damage and radical generation. Complementary studies of HL-60 cell membrane integrity with propidium iodide impermeability and ligh t scatter using the flow cytometer showed a concentration dependence t hat was similar to radical generation. Biochemical studies of the fatt y acids of the HL-60 cell revealed more highly polyunsaturated lipids in the cells. Cellular antioxidant enzymes and vitamin E contents of t he two cell lines were similar. We conclude that there is a time- and concentration-dependent generation of important oxidations by the sens itive HL-60 cells exposed to the membrane-targeted ether lipid, but th e resistant K562 cells are oxidatively silent. This may be due in part to the differences in fatty acid polyunsaturation of the cellular mem branes. The difference in oxidative susceptibility could be the basis for drug resistance to this membrane-specific anticancer agent.