The Second Extracellular Loop of the Thromboxane Receptor Plays an Important Role in Platelet Activation

Abstract

In efforts to more effectively target pathways for antithrombotic activity, there is interest in utilizing antibodies targeting ligand binding domains for various platelet receptors. To this end, we hypothesize that an antibody (designated C-EL2Ab), targeting the second extracellular loop (C-EL2) of the thromboxane receptor (TPR), which contains the ligand binding domain, exhibits antagonistic activity against platelet TPRs. Our results show that C-EL2Ab inhibited TPR-mediated aggregation (triggered by the agonists U46619 and arachidonic acid) in vitro in both human and murine platelets, in a dose-dependent fashion. On the other hand, control experiments revealed that C-EL2Ab lacks the ability to inhibit aggregation stimulated by ADP. Consistent with the notion of receptor antagonism, ligand binding studies demonstrated that C-EL2Ab has the capacity to completely displace the radiolabeled TPR antagonist H3-SQ29,548. In addition, separate control studies showed that normal rabbit IgG or a TPR-specific Ab that targets the first EL, were devoid of effects on aggregation regardless of the agonist used. Taken together, these findings indicate that: the C-EL2 of TPR plays a crucial role in platelet activation, and hence may serve as a target for therapeutic interventions; and the C-EL2Ab has the potential to be used as an antiplatelet agent.