Human microvascular endothelial cells adhere to thrombospondin-1 via an RGD/CSVTCG domain independent mechanism

Thrombospondin-1 (TSP-1), 450-kDa glycoprotein secreted by platelets and endothelial cells at sites of tissue injury or inflammation, plays an important role in angiogenesis, inflammation, and vascular occlusive skin diseases. Many of the physiologic and pathologic activities of TSP-1 are dependent upon its interactions with endothelial cells. To better understand the basis of these activities, we examined the mechanisms mediating the binding of human dermal microvascular endothelial cells (HDMEC) to immobilized TSP-1. HDMEC bound to but did not spread on TSP-1 in a concentration-dependent manner. Monoclonal antibodies (MoAbs) which recognize two purported TSP-1 binding proteins, CS36 and the alphav integrin chain, or TSP-1-derived peptides CGRGDS and CSVTCG, alone or in combination with heparin, did not inhibit HDMEC adhesion to immobilized TSP-1. Furthermore, CSVTCG-ovalbumin conjugates failed to support HDMEC adhesion. Although RGD-containing peptides immobilized on plastic wells supported HDMEC binding, they also induced cell spreading not characteristic of cell binding to TSP-1 and binding was inhibited by free RGD peptide. Two MoAbs against different domains of TSP-1 (A 4.1 and C 6.1) failed to block HDMEC binding to TSP-1, but both MoAbs inhibited G361 human melanoma cell binding to TSP-1 by 60%. Acid treatment of TSP-1 almost completely abrogated its ability to support HDMEC binding, while acid treatment inhibited G361 binding by 50%. However, either antibody completely abrogated G-361 cell binding to acid-treated TSP-1. These data demonstrate that HDMEC bind to immobilized TSP-1 in an RGD- and CSVTCG-independent manner via an acid labile epitope(s) which recognized via a receptor or receptors distinct from CD36 or alphavbeta3 integrin receptor.