FIBULIN-1 MEDIATES PLATELET-ADHESION VIA A BRIDGE OF FIBRINOGEN

Fibulin-1 is a component of the extracellular matrix that surrounds va scular smooth muscle. This observation, along with the recent finding that fibulin-1 can bind fibrinogen (J Biol Chem 270:19458, 1995), prom pted investigation into the potential role of fibulin-1 as a thromboge nic 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. Platel ets free of plasma did not attach to fibulin-1 coated surfaces; howeve r, with the addition of fibrinogen, platelet adhesion to fibulin-1 too k place. When detergent extracts of platelets were subjected to fibuli n-1-Sepharose affinity chromatography, the integrin alpha(IIb)beta(3) was selected. Solid phase binding assays using purified components sho wed 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-c oated surfaces. Monoclonal alpha(IIb)beta(3), antibodies capable of bl ocking its interaction with fibrinogen completely blocked platelet adh esion to fibulin-1 in both whole blood perfusion and static adhesion a ssays. 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-incorpora ted fibulin-1 were used as adhesion substrates, platelet adhesion in t he presence of fibrinogen could be inhibited by 30% using antibodies t o fibulin-1. Following vascular injury, fibulin-1 present in the extra cellular 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 inc luding collagens I and IV and fibronectin that mediate platelet adhesi on via a plasma protein bridge. This bridging phenomenon may represent a general mechanism by which platelets interact with exposed subendot helial matrices following vascular injury. (C) 1996 by The American So ciety of Hematology.