Multiple pathways of thrombin-induced platelet activation differentiated by desensitization and a thrombin exosite inhibitor

Recently a thrombin receptor with a unique mechanism of activation was cloned from a megakaryocyte-like cell line (Vu et al., Cell 64 :1057–1068, 1991). Thrombin cleaves a portion of this receptor creating a new N-terminus that acts as a “tethered-ligand” to activate the receptor. A thrombin receptor activating peptide (SFLLRNPNDKYEPF) homologous to the new N-terminus was shown to activate platelets. We synthesized this peptide and demonstrated that it desensitized platelets to activation by low concentrations of α-thrombin but not γ-thrombin. We also synthesized a thrombin exosite inhibitor (BMS 180742) that inhibited platelet aggregation induced by low, but not high, concentrations of α-thrombin. In contrast, a thrombin active site inhibitor, Nα-(2-naphthylsulfonyl-glycyl)-D,L-amidinophenylalanylpiperidide, competitively inhibited thrombin-induced platelet aggregation. We conclude that thrombin-induced platelet activation is mediated by at least two pathways: one activated by low concentrations of α-thrombin and blocked by a thrombin exosite inhibitor that appears to be coupled to the “tethered-ligand” thrombin receptor, and another that is stimulated by higher concentrations of α-thrombin and by γ-thrombin and does not require the thrombin exosite for activation. Both pathways are blocked by a thrombin active site inhibitor.