Abstract 16050: Selective Inhibition of Outside-In Signaling for Platelet Integrin alphaIIbbeta3 as a Novel Anti-Thrombotic Strategy Without Adverse Effect of Bleeding
Background: Anti-platelet drugs have been successfully used to treat thrombotic diseases. The platelet integrin alphaIIbbeta3 is the major adhesion receptor mediating platelet adhesion and aggregation, and thus plays critical roles in thrombosis and hemostasis. Therefore, antagonists of alphaIIbbeta3 (such as abciximab, eptifibatide, and tirofiban) are the most potent anti-platelet drugs in clinical use. However, integrin antagonists all have the adverse effect of bleeding, which can be life-threatening and limits their clinical use.
Hypothesis: Integrins transmit signals bidirectionally. Inside-out signaling induces activation of the ligand binding function of integrin and thus the formation of primary thrombus. Ligand binding to integrins induces integrin outside-in signaling, which greatly amplifies thrombus size, and is critically important in occlusive thrombus formation. Therefore, we hypothesize that selective inhibition of integrin outside-in signaling inhibits occlusive thrombosis but not hemostasis.
Results: We have recently shown that integrin outside-in signaling requires the binding of Galpha13 (Science 2010). Here we show that Galpha13 and talin bind to mutually exclusive but distinct sites in the cytoplasmic domain of integrin beta3, and their bindings occur in opposite waves, which switch the directions and consequences of integrin signaling. Importantly, while talin binding mediates inside-out signaling and integrin activation, Galpha13 binding selectively mediates early phase outside-in signaling. We identified the Galpha13 binding site and developed a peptide-based inhibitor, mP6, that selectively disrupts integrin outside-in signaling without affecting integrin activation. mP6 diminished second wave platelet aggregation and platelet spreading. Importantly, mP6 inhibited arterial thrombosis in laser- or FeCl3-induced thrombosis models in vivo, similar to eptifibatide. In contrast to eptifibatide that dramatically prolonged bleeding, mP6 had no effect on bleeding time.
Conclusions: We have discovered a novel switch mechanism that regulates the direction of integrin signaling and, based on this mechanism, we designed a potent new anti-thrombotic that does not cause bleeding.
- © 2013 by American Heart Association, Inc.