Activation and injury of endothelial cells by cytokines

Vascular endothelial cells (EC) are primary cellular targets for the actions of pro-inflammatory cytokines such as tumor necrosis factor (TNF). We have studied the signaling pathways used by TNF that lead to new gene expression (endothelial cell activation) or apoptosis (endothelial cell injury). Both responses are initiated by ligand binding to TNFR-I (the p55 receptor). TNF initiates transcription of the E-selectin gene by activation of the transcription factors NF-kappa B and c-Jun/ATF-2. NF-kappa B is activated following degradation of I kappa B alpha and I kappa B-beta. Activation of c-Jun/ATF-2 involves new c-Jun synthesis, and more importantly, phosphorylation of the amino terminus of c-Jun by Jun N-terminal kinase (JNK). Studies in transiently transfected human umbilical vein endothelial cells have revealed that NF-kappa B activation is initiated through the adaptor protein TRAF-2. The activation of JNK also depends upon TRAF-2 and probably involves a kinase cascade initiated by the small G proteins Rac-1 and/or cdc-42. Normally, TNF does not injure human EC. However, TNF can cause apoptosis of EC when cells are co-treated with either the protein synthesis inhibitor cycloheximide (CHX) or the lipid mediator ceramide (cer). The pathways leading to apoptosis following treatment with TNF + CHX and TNF + cer are different since only TNF + CHX is blocked by the caspase inhibitors crmA protein or the peptide zVAD.fmk while only TNF + cer is blocked by the anti apoptotic proteins Bcl-2, Bcl-XL or Al. Both pathways may be inhibited by the anti-apoptotic protein A-20. TNF does not cause the liberation of cer in EC, perhaps because of limited expression of neutral sphingomyelinase-activating adaptor protein FAN. These observations suggest that TNF normally acts as an activator of EC but may change from an activator to a killer of EC when combined with agents that release ceramide, such as u.v. irradiation or cytotoxic drugs, or with ceramide mimetics such as lipopolysaccharide. The activation and injury of endothelial cells induced by TNF and other proinflammatory cytokines may underlie the local effects of these mediators in vivo.