ACTIVATION OF APOPTOSIS BY APO-2 LIGAND IS INDEPENDENT OF FADD BUT BLOCKED BY CRMA

A new member of the tumor necrosis factor (TNF) cytokine family, desig nated Apo-2 ligand (Apo-2L) [1] or TRAIL [2], has been shown recently to induce apoptosis in various tumor cell lines; however, its biologic al role is unknown. Here, we show that Apo-PL activated apoptosis in T -cell-enriched cultures of peripheral blood lymphocytes stimulated by interleukin-2 (IL-2), but not in unstimulated cells. This finding sugg ests that, like Fas/Apo-1 ligand and TNF [3-5], Apo-2L may play a role in regulating post-stimulation apoptosis of mature lymphocytes. Studi es on the mechanism of Apo-2L action demonstrated marked membrane bleb bing, a hallmark of apoptosis, within a few minutes of the addition of Apo-2L to tumor cells, Ectopic expression of a dominant negative muta nt of FADD, a cytoplasmic protein that mediates death signalling by Fa s/Apo-1 and by TNF receptor type 1 (TNFR1) [6-9], inhibited the induct ion of apoptosis by anti-Fas/Apo-1 antibody, but had little effect on Apo-2L function, In contrast, expression of CrmA, a cowpox virus-deriv ed inhibitor of the Ced-3-like proteases ICE [10] and CPP32/Yama [11,1 2], blocked the induction of apoptosis by either Apo-2L or anti-Fas/Ap o-1 antibody These results suggest that Apo-PL activates a rapid, FADD -independent pathway to trigger a cell-death programme that requires t he function of cysteine proteases such as ICE or CPP32/Yama.