Numerous reports indicate that the platelet glycoprotein (GP) Ib-IX complex (GPIb-IX) binds directly to the potent platelet agonist thrombin and is important for promoting thrombin-induced platelet activation. However, how GPIb-IX contributes to thrombin-induced platelet activation is unclear. It has been suggested that thrombin binding to GPIb facilitates the cleavage, and thus activation, of the protease-activated receptors (PAR). Our data indicate that GPIb-IX promotes thrombin signaling through a GPIb-IX signaling mechanism. Pretreatment of human platelets with MPalphaC, an inhibitory peptide based on a critical 14-3-3 signaling protein binding site on the cytoplasmic domain of the GPIb alpha chain, inhibited thrombin-induced platelet activation. MPalphaC-treatment inhibited thrombin-induced activation of Rac1 and LIMK1, both of which are known to play essential roles in GPIb signaling. To more specifically determine the role of GPIb-IX, we reconstituted GPIb-IX-facilitated thrombin signaling in Chinese Hamster Ovary cells expressing PAR1. Thrombin induced signaling was significantly enhanced by GPIb-expression, and deletion of the cytoplasmic 14-3-3-binding domain of GPIb alpha abolished the stimulatory effect of GPIb on thrombin signaling. Furthermore, the role of GPIb-IX in promoting thrombin signaling requires Rac1, and GPIb-IX-dependent Rac1 activation and LIMK phosphorylation are abolished in delta 605 cells expressing a 14-3-3-binding defective mutant GPIb alpha. Taken together, these data suggest that the stimulatory role of GPIb in thrombin signaling requires a C-terminal 14-3-3-binding region which mediates activation of a Rac1/LIMK1 pathway that promotes thrombin signaling leading to platelet activation.
The Effects Of Protease-Activated Receptors 1 And 4 In Human Platelet Activation And Inflammation