Divergence of bacterial lipopolysaccharide pro-apoptotic signaling downstream of IRAK-1

The vascular endothelium is a key target of circulating bacterial lipopolysaccharide (LPS). LPS elicits a wide array of endothelial responses, including the up-regulation of cytokines, adhesion molecules, and tissue factor, many of which are dependent on NF-kappaB activation. In addition, LPS has been demonstrated to induce endothelial apoptosis both in vitro and in vivo. Although the mechanism by which LPS activates NF-kappaB has been well elucidated, the signaling pathway(s) involved in LPS-induced apoptosis remains unknown. Using a variety of dominant negative constructs, we have identified a role for MyD88 and interleukin-1 receptor-associated kinase-1 (IRAK-1) in mediating LPS pro-apoptotic signaling in human endothelial cells. We also demonstrate that LPS-induced endothelial NF-kappaB activation and apoptosis occur independent of one another. Together, these data suggest that the proximal signaling molecules involved in LPS-induced NF-kappaB activation have a requisite involvement in LPS-induced apoptosis and that the pathways leading to NF-kappaB activation and apoptosis diverge downstream of IRAK-1.