Abstract

The ether phospholipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (ET-18-OCH₃; edelfosine) is a potent inducer of apoptosis in human tumor cells. We show that ET-18-OCH₃-induced apoptosis is associated with activation of the c-Jun NH₂-terminal kinase (JNK) signaling. The addition of ET-18-OCH₃ to distinct human leukemic cells (HL-60, U937, and Jurkat), which undergo rapid apoptosis on treatment with ET-18-OCH₃, induced a dramatic and sustained increase in the of c-jun mRNA level that was associated with activation of activator protein-1 transcription factor. We found that ET-18-OCH₃ induced a persistent activation of JNK in HL-60 cells that was detected before the onset of apoptosis, the latter being assessed by DNA fragmentation and by the appearance of phosphatidylserine on the external leaflet of the plasma membrane. The inductions of JNK after HL-60 monocyte/macrophage differentiation and ET-18-OCH₃-mediated apoptosis were distinguished by the different activation patterns, transient versus persistent, respectively. ET-18-OCH₃ analogues unable to induce apoptosis failed to activate JNK. ET-18-OCH₃-dependent JNK activation was not detected in K562 cells, which did not undergo apoptosis on treatment with ET-18-OCH₃. Phorbol myristate acetate inhibited both ET-18-OCH₃-induced apoptosis and sustained JNK activation; thus, persistent JNK activation by ET-18-OCH₃ is associated with the capacity of this ether phospholipid to induce apoptosis. Furthermore, antisense oligonucleotides directed against c-jun blocked ET-18-OCH₃-induced apoptosis, indicating a role for c-Jun in this apoptotic response. These data indicate that JNK activation and c-Jun are involved in the induction of apoptosis by ET-18-OCH₃.