EVIDENCE THAT TUMOR-NECROSIS-FACTOR TRIGGERS APOPTOSIS IN HUMAN ENDOTHELIAL-CELLS BY INTERLEUKIN-1-CONVERTING ENZYME-LIKE PROTEASE-DEPENDENT AND PROTEASE-INDEPENDENT PATHWAYS

Cultured human endothelial cells (EC) resist tumor necrosis factor (TN F)-mediated apoptosis. However, the combination of TNF and the protein synthesis inhibitor cycloheximide (CHX) induces apoptosis in up to 50 % of EC within 24 hours. TNF + CHX killing is effectively blocked by t ransfected CrmA protein or treatment with Z-VAD.fmk peptide-both inhib itors of interleukin-1-converting enzyme-like proteases-but not by tra nsfected antiapoptotic proteins Bcl-2, Bcl-XL, or A1. C6-ceramide (cer ) can also sensitize EC to TNF-induced apoptosis. TNF + cer killing, w hich can affect more than 50% of EC, is not effectively inhibited by C rmA or Z-VAD.fmk, but can be readily blocked by Bcl-2, Bcl-XL, or A1. Both TNF + CHX and TNF + cer killing are induced by a TNF mutein that only interacts with the type 1 TNF receptor, and both responses can be inhibited by the antiapoptotic protein A20. These data suggest that T NF activates two biochemically separable pathways of EC injury.