Summary
Although plasmin can trigger strong platelet responses such as shape change and exocytosis
of internal granules, limited platelet aggregation is induced by this proteinase,
owing to its capacity to rapidly proteolyse secreted adhesive proteins. In this context,
we have investigated the state of activation of the fibrinogen receptor, the integrin
αIIbβ3, on platelets exposed to plasmin. Following incubation with plasmin at 37 °C, washing,
and resuspension, platelets exhibit a moderate, low-velocity aggregation when stirred
in the presence of fibrinogen. Optimum aggregability is observed when platelets have
been exposed to plasmin activity of ≈0.5 CU/ml for 20 min, and aggregation is insensitive
to the presence of antagonists such as prostaglandin (PG) E1 and apyrase. Plasmin-induced platelet aggregability is associated with the expression
of active fibrinogen receptors on the cell surface, which, using a l25I-fibrinogen binding assay, can be quantified to ≈2,300 molecules per platelet. Exposure of active αIIbβ3 receptors appears to depend partially, but not totally on a metabolic activation
and granule exocytosis at the time of incubation with plasmin. In contrast with a-thrombin,
plasmin-induced activation of αIIbβ3 is sustained and cannot be reversed by exposure of platelets to PGE1. Immunoblotting analysis of the receptor subunits shows no extensive proteolytic
modification of αIIbβ3 by plasmin, and only reveals a limited proteolysis of the aminoterminal domain of
the αIIb subunit. In addition to their capacity to aggregate in the presence of fibrinogen
alone, plasmin-treated platelets also show a potentiated aggregability in response
to low doses of ADP. Thus, plasmin has the potential to activate the platelet fibrinogen
receptor in such a way that it remains irreversibly available to fibrinogen on the
surface of nonaggregated cells, a feature that may participate to pathological states
of in vivo platelet hyperaggregability.
