Histidine-rich glycoprotein modulates the anti-angiogenic effects of vasculostatin

Brain angiogenesis inhibitor 1 (BAI1) is a transmembrane protein expressed on glial cells within the brain. Its expression is dramatically down-regulated in many glioblastomas, consistent with its functional ability to inhibit angiogenesis and tumor growth in vivo. We have shown that the soluble anti-angiogenic domain of BAI1 (termed Vstat120) requires CD36, a cell surface glycoprotein expressed on microvascular endothelial cells (MVECs), for it to elicit an anti-angiogenic response. We now report that Vstat120 binding to CD36 on MVECs activates a caspase-mediated pro-apoptotic pathway, and this effect is abrogated by histidine-rich glycoprotein (HRGP). HRGP is a circulating glycoprotein previously shown to function as a CD36 decoy to promote angiogenesis in the presence of thrombospondin-1 or -2. Data here show that Vstat120 specifically binds HRGP. Under favorable MVEC growth conditions this interaction allows chemotactic-directed migration as well as endothelial tube formation to persist in in vitro cellular systems, and increased tumor growth in vivo as demonstrated in both subcutaneous and orthotopic brain tumor models, concomitant with an increase in tumor vascularity. Finally, we show that HRGP expression is increased in human brain cancers, with the protein heavily localized to the basement membrane of the tumors. These data help define a novel angiogenic axis that could be exploited for the treatment of human cancers and other diseases where excess angiogenesis occurs.