Fibulin-1 mediates platelet adhesion via a bridge of fibrinogen.

Fibulin-1 is a component of the extracellular matrix that surrounds vascular smooth muscle. This observation, along with the recent finding that fibulin-1 can bind fibrinogen (J Biol Chem 270:19458, 1995), prompted investigation into the potential role of fibulin-1 as a thrombogenic agent. In perfusion chamber assays, platelets in whole blood under flow conditions attached and spread on surfaces coated with fibulin-1. This adhesion was completely blocked by fibulin-1 antibodies. Platelets free of plasma did not attach to fibulin-1 coated surfaces; however, with the addition of fibrinogen, platelet adhesion to fibulin-1 took place. When detergent extracts of platelets were subjected to fibulin-1-Sepharose affinity chromatography, the integrin alpha IIb beta 3 was selected. Solid phase binding assays using purified components showed that integrin alpha IIb beta 3 could not bind directly to fibulin-1 but in the presence of fibrinogen the integrin bound to fibulin-1-coated surfaces. Monoclonal alpha IIb beta 3 antibodies capable of blocking its interaction with fibrinogen completely blocked platelet adhesion to fibulin-1 in both whole blood perfusion and static adhesion assays. The results show that fibulin-1 can support platelet attachment via a bridge of fibrinogen to the platelet integrin alpha IIb beta 3. When fibroblast monolayers containing extracellular matrix-incorporated fibulin-1 were used as adhesion substrates, platelet adhesion in the presence of fibrinogen could be inhibited by 30% using antibodies to fibulin-1. Following vascular injury, fibulin-1 present in the extracellular matrix of the vessel wall may therefore interact with plasma fibrinogen and promote platelet adhesion, leading to the formation of a platelet plug. Thus, fibulin-1 joins the list of matrix proteins including collagens I and IV and fibronectin that mediate platelet adhesion via a plasma protein bridge. This bridging phenomenon may represent a general mechanism by which platelets interact with exposed subendothelial matrices following vascular injury.