The Role of Osteoprotegerin and Tnf-related Apoptosis Inducing Ligand in Human Microvascular Endothelial Cell Survival

Endothelial cell survival and anti-apoptotic pathways, including those stimulated by extracellular matrix, are critical regulators of vasculogenesis, angiogenesis, endothelial repair and shear-stress-induced endothelial activation. One of these pathways is mediated by αvβ3 integrin ligation, downstream activation of nuclear factor-kappa B (NF-κB), and subsequent upregulation of osteoprotegerin (OPG). In this study, the mechanism by which OPG protects endothelial cells from death was examined. Serumstarved human microvascular endothelial cells (HMEC) plated on the αvβ3 ligand, osteopontin (OPN), were protected from cell death. Immunoprecipitation experiments indicated that OPG formed a complex with TNF-related apoptosis-inducing ligand (TRAIL) in HMEC under these conditions. Furthermore, inhibitors of TRAIL including recombinant soluble TRAIL receptors and a neutralizing antibody against TRAIL blocked apoptosis of serum-starved HMEC plated on the non-integrin attachment factor, poly-D-lysine (PDL). While TRAIL was unable to induce apoptosis in HMEC plated on OPN, the addition of recombinant TRAIL did increase the percentage of apoptotic HMEC plated on PDL. This evidence indicates that OPG blocks endothelial cell apoptosis through binding TRAIL and preventing its interaction with death-inducing TRAIL-receptors Introduction Endothelial apoptosis is an important regulator of angiogenesis, vasculogenesis, vascular pruning and shear stress-induced endothelial activation (Dimmeler et al., 1996; Dimmeler and Zeiher, 2000). Angiogenesis, the formation of capillaries from preexistent