Involvement of the "tethered-ligand" receptor in thrombin inhibition of platelet adenylate cyclase.

Thrombin is thought to activate platelets through multiple signaling pathways. Recently a new thrombin receptor was identified (Vu et al., Cell 64:1057-1068, 1991) that recognizes alpha-thrombin's anion-binding exosite. Thrombin cleaves this receptor generating a new N-terminal ("tethered-ligand") that activates the receptor. We report here that this receptor is involved in alpha-thrombin inhibition of platelet adenylate cyclase, a process thought mediated by thrombin's high-affinity pathway. In gel-filtered human platelets, iloprost-stimulated cAMP levels were lowered by alpha- and zeta-thrombin addition and, to a much lesser extent, by gamma-thrombin. The alpha- and zeta-thrombin mediated decreases in cAMP were prevented by the thrombin anion-binding exosite inhibitor, BMS 180742, implying that binding to thrombin's anion-binding exosite was required. The iloprost-stimulated increase in cAMP was also reversed (in a concentration-dependent fashion) by a peptide mimicking the new N-terminal of the "tethered-ligand" thrombin receptor (SFLLRNPNDKYEPF). In broken cell preparations, platelet adenylate cyclase activity was also inhibited by SFLLRNPNDKYEPF (but not by a similar peptide used as a control, FSLLRNPNDKYEPF). These results support the hypothesis that thrombin inhibition of platelet adenylate cyclase activity is mediated, at least in part, via the "tethered-ligand" receptor. Moreover, this data is consistent with the "tethered-ligand" receptor mediating the high affinity actions of alpha-thrombin.