β2-glycoprotein I inhibits vascular endothelial growth factor and basic fibroblast growth factor induced angiogenesis through its amino terminal domain

Summary.  Background:  Beta-2 glycoprotein I (β₂GPI) is a plasma glycoprotein which interacts with various proteins of the coagulation and fibrinolysis system. β₂GPI has recently been shown to have anti-angiogenic properties. Objectives:  We undertook this study to investigate the specific domain of β₂GPI involved in the anti-angiogenic function and its effect on downstream signaling of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). Methods:  Various preparations of β₂GPI were used on human umbilical vein endothelial cells (HUVECs) in the absence or presence of VEGF and bFGF. The effect on HUVECs' proliferation, migration and tubule formation in Matrigel matrix was investigated. The effect of β₂GPI on the mRNA expression of VEGF receptors and phosphorylation of signaling molecules was also studied. Results:  β₂GPI is shown in this study to be an anti-angiogenic molecule in vitro by inhibiting VEGF and bFGF-induced proliferation, migration and papillary-like tubule formation of HUVECs. This inhibition was achieved by native, proteolytically clipped and domain deletion mutants, domain I-IV (DI-IV) but not domain II-V (DII-V) of β₂GPI. Native β₂GPI was found to downregulate the expression of the VEGF receptor KDR/Flk-1 on endothelial cells and to block the phosphorylation of VEGF's downstream effector molecules in the MAPK/ERK and PI3K/Akt/GSK3β pathways. Conclusions:  These results indicate that β₂GPI has anti-angiogenic functions which depend on the presence of domain I. This anti-angiogenic activity may have important implications for the therapeutic manipulation of angiogenesis in various disease states.