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Background: Fibrinogen-like protein 2 (FGL2) is a serine protease capable of converting prothrombin into thrombin (i.e., prothrombinase activity) while bypassing the classic coagulation cascade. It has been reported to be expressed by mononuclear blood cells and endothelial cells. There are multiple reports that FGL2 supports tumor development and metastasis. However, in the blood, the origin or functional significance has not been established.Objective: To identify the origin of the FGL2 prothrombinase, a malignancy related enzyme, in peripheral blood cells.Methods: Peripheral blood samples were collected in complete blood count tubes. Blood cells and platelets were thoroughly washed to produce plasma-free samples. Procoagulant activity was measured in the samples' extracts using a thrombin generation test or an adjusted prothrombin time (PT) test in plasma deficient factor X. Results were further supported by confocal microscopy, immunoprecipitation, flow cytometry, enzyme-linked immunosorbent assays and specific inhibition assays. Results: The pro-coagulant activity of FGL2 was detected in purified platelets but not in white blood cells. FGL2 protein was readily detected in platelets. Quiescent platelets were shown to store FGL2 protein in an active form. Upon activation, platelets secreted the active FGL2 into the milieu. Conclusions: Active FGL2 prothrombinase is stored in platelets. This suggests the involvement of platelets in malignancies. |
