Fibrinogen is required for maintenance of platelet intracellular and cell-surface P-selectin expression

Abstract

Platelet P-selectin plays important roles in inflammation and contributes to thrombosis and hemostasis. Although it has been reported that von Willebrand factor (VWF) affects P-selectin expression on endothelial cells, little information is available regarding regulation of platelet P-selectin expression. Here, we first observed that P-selectin expression was significantly decreased on platelets of fibrinogen and VWF double-deficient mice. Subsequently, we identified this was due to fibrinogen deficiency. Impaired P-selectin expression on fibrinogen-deficient platelets was further confirmed in human hypofibrinogenemic patients. We demonstrated that this impairment is unlikely due to excessive P-selectin shedding, deficient fibrinogenmediated cell surface P-selectin binding, or impaired platelet granule release, but rather is due to decreased platelet P-selectin content. Fibrinogen transfusion completely recovered this impairment in fibrinogen-deficient (Fg ؊/؊ ) mice, and engagement of the C-terminus of the fibrinogen ␥ chain with ␤3 integrin was required for this process. Furthermore, Fg ؊/؊ platelets significantly increased Pselectin expression following transfusion into ␤3 integrin-deficient mice and when cultured with fibrinogen. These data suggest fibrinogen may play important roles in inflammation, thrombosis, and hemostasis via enhancement of platelet Pselectin expression. Since human fibrinogen levels vary significantly in normal and diseased populations, P-selectin as an activation marker on platelets should be used with caution. IntroductionPlatelets play a crucial role in both hemostasis and thrombosis. They also contribute to inflammation, including immune-mediated inflammation, 1-3 and the development of atherosclerosis. 4 At the site of vascular injury, particularly at high shear stress, the binding of the platelet GPIb complex to von Willebrand factor (VWF) on the injured vessel wall initiates platelet tethering and subsequent adhesion. Platelet aggregation is then mediated by interaction between platelet ␤3 integrin and fibrinogen (Fg), although Fgindependent platelet aggregation can also occur. [5][6][7] During platelet activation and degranulation, P-selectin translocates to the platelet surface. Through interaction with sulfatides 8 and the GPIb complex, 9 P-selectin may also contribute to the stabilization of platelet aggregates. Although Fg, VWF, and P-selectin are all important molecules that support hemostasis and thrombosis and are all involved in inflammation, their mutual impact on these processes is poorly understood. It is completely unknown whether Fg or VWF affects P-selectin expression on the platelet surface.P-selectin is a transmembrane protein belonging to the selectin family of leukocyte adhesion receptors. It is synthesized by megakaryocytes and endothelial cells and is subsequently stored in the ␣-granules of platelets 10 and the Weibel-Palade bodies of endothelial cells. 11 Upon agonist stimulation of platelets or endothelial cells, the granules may fuse with the plasma mem...

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