Background: The interaction of glycoprotein (GP) Iba 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 GPIba-VWF interaction. Objectives: To investigate whether the GPIba-VWF interaction induces platelet apoptosis and the role of 14-3-3f 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 GPIba-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-GPIba antibody AK2. Furthermore, apoptotic events occurred in 1b9 cells stimulated withVWFand ristocetin, but were significantly diminished in two CHO cell lines expressing mutant GPIb-IX with GPIba truncated at residue 551 or a serine-to-alanine mutation at the 14-3-3fbinding site in GPIba. Conclusions: This study demonstrates that the GPIba-VWF interaction induces apoptotic events in platelets, and that the association of 14-3-3f with the cytoplasmic domain of GPIba is essential for apoptotic signaling. This finding may suggest a novel mechanism for platelet clearance or some thrombocytopenic diseases.

There are evidence that both a disintegrin and metalloproteinase 17 (ADAM17) and calpain are involved in platelet glycoprotein (GP)Iba ectodomain cleavage. However, the relationship between the two enzymes in the shedding process remains unclear. Here we show that calcium ionophore A23187-and a-thrombin-induced GPIba shedding is completely inhibited by the metalloproteinase inhibitor GM6001, whereas it is only partially inhibited by calpain inhibitors. Calpain activator dibucaine-induced GPIba shedding was completely inhibited by both metalloproteinase and calpain inhibitors. On the other hand, calpain inhibitors did not inhibit GPIba shedding induced by the reagents that specifically activate ADAM17. Furthermore, A23187-induced GPIba shedding in Chinese hamster ovary cells expressing wildtype or mutant GPIb-IX was also partially inhibited by calpain inhibitors and almost completely inhibited by GM6001. Therefore, these data indicate that calpain plays an important role in the regulation of ADAM17-dependent GPIba ectodomain shedding in both platelets and nucleated cells.