The glycoprotein Ibα-von Willebrand factor interaction induces platelet apoptosis

Background: The interaction of glycoprotein (GP) Ib alpha with von Willebrand factor (VWF) initiates platelet adhesion, and simultaneously triggers intracellular signaling cascades leading to platelet aggregation and thrombus formation. Some of the signaling events are similar to those occurring during apoptosis, however, it is still unclear whether platelet apoptosis is induced by the GPIb alpha-VWF interaction. Objectives: To investigate whether the GPIb alpha-VWF interaction induces platelet apoptosis and the role of 14-3-3 zeta in apoptotic signaling. Methods: Apoptotic events were assessed in platelets or Chinese hamster ovary (CHO) cells expressing wild-type (1b9) or mutant GPIb-IX interacting with VWF by flow cytometry or western blotting. Results: Ristocetin-induced GPIb alpha-VWF interaction elicited apoptotic events in platelets, including phosphatidylserine exposure, elevations of Bax and Bak, gelsolin cleavage, and depolarization of mitochondrial inner transmembrane potential. Apoptotic events were also elicited in platelets exposed to pathologic shear stresses in the presence of VWF; however, the shear-induced apoptosis was eliminated by the anti-GPIb alpha antibody AK2. Furthermore, apoptotic events occurred in 1b9 cells stimulated with VWF and ristocetin, but were significantly diminished in two CHO cell lines expressing mutant GPIb-IX with GPIb alpha truncated at residue 551 or a serine-to-alanine mutation at the 14-3-3 zeta-binding site in GPIb alpha. Conclusions: This study demonstrates that the GPIb alpha-VWF interaction induces apoptotic events in platelets, and that the association of 14-3-3 zeta with the cytoplasmic domain of GPIb alpha is essential for apoptotic signaling. This finding may suggest a novel mechanism for platelet clearance or some thrombocytopenic diseases.