Factor Xa

Factor X is a vitamin K–dependent serine endopeptidase playing critical roles in normal hemostasis. Factor X is synthesized in the liver and becomes activated into factor Xa (FXa) by both the intrinsic and the extrinsic Xase complexes. FXa converts prothrombin into thrombin in the prothrombinase complex, which in turn cleaves fibrinogen into an insoluble fibrin clot. Besides a role in coagulation, FXa signaling is mainly mediated by the activation of PARs, which belong to G-protein coupled receptor family. Currently, four members of the PAR, PAR-1 through -4 have been identified. PAR-1, -3 and PAR-4 but not PAR-2 can be cleaved by thrombin whereas PAR-1, PAR-2 and PAR-4 can mediate FXa signaling. FXa exerts inflammatory responses partially through enhancing generation of an inflammatory signaling molecule, thrombin. FXa increases expression of inflammatory cytokines including IL-6, IL-8, and monocyte chemotactic protein-1 (MCP-1), as well as the expression of the adhesion molecules, E-selectin, intracellular adhesion molecule-1 (ICAM-1) and vascular-cell adhesion molecule-1 (VCAM-1) in human umbilical vein endothelial cells. Although the signaling function of FXa is mainly mediated through PARs, non-PAR cell receptors play pivotal roles in the potency and the specificity of FXa signaling. FXa is found in various malignant tissues. It can be expressed by tumor cells ectopically or delivered by stromal cells to the tumor microenvironment. FXa could be a novel diagnostic factor as well as a clinically invaluable post-treatment marker for fibrosis, atherosclerosis, and cancer.


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