Summary
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 α2-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 (1) and
in the early 1960’s a fibrinolytic activity, increasing with platelet concentration
in the experimental system, was shown (2, 3). In 1979, it was demonstrated that metabolically
active platelets were necessary for platelets to play a role in the fibrinolytic system
(4). More recently it was established by Plow and Collen (5) that the specific plasmin
inhibitor, α2-antiplasmin is a constituent of platelet α-granules.
In the present study, we investigated the fibrinolytic components and activity of
human megakaryocytes and platelets, using zymographic and immunochemical techniques.
We report here our observations that human megakaryocytes and platelets contain tissue
plasminogen (t-PA) which possesses fibrinolytic activity.
Keywords
Fibrinolysis - Immunoperoxidase - Megakaryocytes - Platelets - Immunoblotting
