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Tissue plasminogen activator in human megakaryocytes and platelets: immunocytochemical localization, immunoblotting
1Laboratoire d'Hémostase et'INSERM U 152, Hôpital Cochin, Paris, France.
Abstract:
Two approaches were used to identify and characterize the presence of tissue plasminogen activator (t-PA) in megakaryocytes and platelets. We investigated the fibrinolytic activity of human megakaryocytes (MK) and platelets. The presence of t-PA antigen in megakaryocytes and platelets was demonstrated using immunocytochemical techniques and polyclonal or monoclonal antibodies specific for t-PA. When cells were applied to fibrin plates, lysis zones developed around isolated human megakaryocytes, whereas no fibrinolytic activity appeared when either intact washed platelets or platelet lysate were deposited. After SDS-PAGE of platelet and MK extracts (Triton X-100) immunoblotting and peroxidase staining identified t-PA antigen in several bands. Zymographic analysis of SDS-PAGE carried out on fibrin film overlays identified one or two zones corresponding to free or complexed t-PA. These results indicate that t-PA is present in platelets as well as in the precursor cells, however, in platelets, t-PA may not be immediately available for plasminogen activation and fibrin degradation. From our findings and from previous work of others, it appears that platelets may either activate or inhibit the fibrinolytic system. Therefore the conditions of plasminogen activation by platelet t-PA and plasmin inhibition by platelet alpha 2-antiplasmin or other inhibitors have to be precised before the role of platelets in clot dissolution is understood. The physiological role of platelets in fibrinolysis and clot dissolution remains unclear. In 1953, the antifibrinolytic activity of blood platelets was demonstrated and in the early 1960's a fibrinolytic activity, increasing with platelet concentration in the experimental system, was shown.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
Tissue plasminogen activator (t-PA) is present in megakaryocytes and platelets, but its availability for fibrinolysis in platelets requires further investigation. Platelets
Area of Science:
- Hematology
- Biochemistry
- Molecular Biology
Background:
- The role of platelets in hemostasis and thrombosis is well-established.
- Their involvement in fibrinolysis, the breakdown of blood clots, is complex and debated.
- Megakaryocytes, the precursors of platelets, also warrant investigation for fibrinolytic components.
Purpose of the Study:
- To identify and characterize tissue plasminogen activator (t-PA) in human megakaryocytes and platelets.
- To investigate the fibrinolytic activity of these cells.
- To elucidate the potential role of platelet t-PA in clot dissolution.
Main Methods:
- Immunocytochemical techniques using specific antibodies to detect t-PA antigen.
- Fibrin plate assays to assess cellular fibrinolytic activity.
- SDS-PAGE, immunoblotting, and zymography to analyze t-PA presence and activity in cell extracts.
Main Results:
- t-PA antigen was detected in both megakaryocytes and platelets via immunocytochemistry.
- Isolated megakaryocytes exhibited fibrinolytic activity on fibrin plates, unlike platelets.
- SDS-PAGE and zymography confirmed the presence of t-PA in platelet and megakaryocyte extracts, suggesting it may be complexed or less accessible in platelets.
Conclusions:
- t-PA is present in megakaryocytes and platelets, but its functional availability in platelets for fibrinolysis is questionable.
- Platelets may have a dual role in modulating the fibrinolytic system.
- Further research is needed to clarify the precise conditions of plasminogen activation and plasmin inhibition by platelet components to understand their role in clot dissolution.