Esther Yee

Endothelial cell death induced by tumor necrosis factor-alpha is inhibited by the Bcl-2 family member, A1.

Endothelial cells play a central role in the inflammatory process. Tumor necrosis factor-α (TNF) is a multifunctional cytokine which elicits many of the inflammatory responses of endothelial cells. While TNF directly causes apoptosis of tumor cells and virally infected cells, normal cells are generally resistant. However, most resistant cells, including human endothelial cells, can be rendered susceptible to TNF by inhibiting RNA or protein synthesis. This finding suggests that TNF provides a cell survival signal in addition to a death signal. We have previously cloned a human Bcl-2 homologue, A1, and shown that it is specifically induced by proinflammatory cytokines but not by endothelial growth factors. In this study, we show that retroviral-mediated transfer of the A1 cDNA to a human microvascular endothelial cell line provides protection against cell death initiated by TNF in the presence of actinomycin D. The induction of A1 by TNF in this system is mediated via a protein kinase C pathway. Since TNF signaling has also been shown to proceed via ceramides, we tested whether exogenous ceramides could induce A1. Our findings indicate that ceramides do not induce A1 but do up-regulate c-jun and induce endothelial death. Ceramide-activated endothelial death is also inhibited by A1, suggesting that TNF may initiate divergent survival and death pathways via separate lipid second messengers.

NF-κB Activation Is Required for Human Endothelial Survival during Exposure to Tumor Necrosis Factor-α but Not to Interleukin-1β or Lipopolysaccharide

In the presence of a protein synthesis inhibitor, cycloheximide, tumor necrosis factor-α (TNF-α), interleukin 1-β (IL-1β), or lipopolysaccharide (LPS) induces human umbilical vein endothelial cells (HUVECs) to undergo apoptosis, suggesting that constitutive or inducible cytoprotective pathways are required for cell survival. We studied the correlation between nuclear factor-κB (NF-κB) activation and cell death induced by TNF-α, IL-1β, or LPS. Adenovirus-mediated overexpression of a dominant-negative IκBα (inhibitor of κB) mutant blocked NF-κB activation by gel shift assay and blocked induction of vascular cell adhesion molecule-1 protein by TNF-α, IL-1β, and LPS, a NF-κB-dependent response. In cells overexpressing the IκBα mutant, TNF-α induced cell death, whereas IL-1β or LPS did not. We conclude that cell survival following TNF-α stimulation is NF-κB-dependent but that a constitutive or inducible NF-κB-independent pathway(s) protects IL-1β- or LPS-treated HUVECs from cell death.

Interleukin‐4 induces endothelial vascular cell adhesion molecule‐1 (VCAM‐1) by an NF‐κb‐independent mechanism

While all features of the inflammatory response induced by IL‐1 are not observed following IL‐4 stimulation, suboptimal concentrations both cytokines result in synergistic VCAM‐1 expression in HUVEC. We have shown that, while IL‐1 stimulated HUVEC express GM‐CSF, tissue factor and VCAM‐1, only VCAM‐1 is detectable after exposure to IL‐4. While kB was found essential for both basal and IL‐1‐mediated activity of VCAM‐1, IL‐4 induction was kB‐independent. Inducible kB‐binding proteins were identified in IL‐1‐, but not IL‐4‐stimulated nuclear extracts. Our results indicate that IL‐4 exerts its transcriptional effects on the VCAM‐1 gene through element(s) which do not require kB.