Unique pathway of thrombin-induced platelet aggregation mediated by glycoprotein Ib

Thrombin plays a central role in normal and abnormal hemostatic processes. It is assumed that alpha -thrombin activates platelets by hydrolyzing the p rotease-activated receptor (PAR)-1, thereby exposing a new N-terminal seque nce, a tethered ligand, which initiates a cascade of molecular reactions le ading to thrombus formation. This process involves cross-linking of adjacen t platelets mediated by the interaction of activated glycoprotein (GP) IIb/ IIIa with distinct amino acid sequences, LG-GAKQAGDV and/or RGD, at each en d of dimeric fibrinogen molecules. We demonstrate here the existence of a s econd alpha -thrombin-induced platelet-activating pathway, dependent on GP Ib, which does not require hydrolysis of a substrate receptor, utilizes pol ymerizing fibrin instead of fibrinogen, and can be inhibited by the Fab fra gment of the monoclonal antibody LJIb-10 bound to the GP Ib thrombin-bindin g site or by the cobra venom metalloproteinase, mocarhagin, that hydrolyzes the extracellular portion of GP Ib. This alternative alpha -thrombin pathw ay is observed when PAR-1 or GP IIb/ IIIa is inhibited. The recognition sit es involved in the cross-linking of polymerizing fibrin and surface integri ns via the GP Ib pathway are different from those associated with fibrinoge n, This pathway is insensitive to RGDS and anti-GP IIb/IIIa antibodies but reactive with a mutant fibrinogen, gamma 407, with a deletion of the gamma -chain sequence, AGDV, The reaction is not due to simple trapping of platel ets by the fibrin clot, since ligand binding, signal transduction, and seco nd messenger formation are required. The GP Ib pathway is accompanied by mo bilization of internal calcium and the platelet release reaction. This latt er aspect is not observed with ristocetin-induced GP m-von Willebrand facto r agglutination nor with GP Ib-von Willebrand factor-polymerizing fibrin tr apping of platelets. Human platelets also respond to gamma -thrombin, an au toproteolytic product of cu-thrombin, through PAR-4, Go-activation of the G P Ib, PAR-1, and PAR-4 pathways elicit synergistic responses. The presence of the GP Ib pathway may explain why anti-alpha -thrombin/anti-platelet reg imens fail to completely abrogate thrombosis/restenosis in the cardiac pati ent.