Summary
Thrombospondin (TSP) is a major platelet secretory glycoprotein. Earlier studies of
various investigators demonstrated that TSP is the endogenous platelet lectin and
is responsible for the hemagglutinating activity expressed on formaldehyde-fixed thrombin-treated
platelets. The direct effect of highly purified TSP on thrombin-induced platelet aggregation
was studied. It was observed that aggregation of gel-filtered platelets induced by
low concentrations of thrombin (≤0.05 U/ml) was progressively inhibited by increasing
concentrations of exogenous TSP (≥60 μg/ml). However, inhibition of platelet aggregation
by TSP was not observed when higher than 0.1 U/ml thrombin was used to activate platelets.
To exclude the possibility that TSP inhibits platelet aggregation by affecting thrombin
activation of platelets, three different approaches were utilized. First, by using
a chromogenic substrate assay it was shown that TSP does not inhibit the proteolytic
activity of thrombin. Second, thromboxane B2 synthesis by thrombin-stimulated platelets was not affected by exogenous TSP. Finally,
electron microscopy of thrombin-induced platelet aggregates showed that platelets
were activated by thrombin regardless of the presence or absence of exogenous TSP.
The results indicate that high concentrations of exogenous TSP (≥60 μg/ml) directly
interfere with interplatelet recognition among thrombin-activated platelets. This
inhibitory effect of TSP can be neutralized by anti-TSP Fab. In addition, anti-TSP
Fab directly inhibits platelet aggregation induced by a low (0.02 U/ml) but not by
a high (0.1 U/ml) concentration of thrombin. In conclusion, our findings demonstrate
that TSP is functionally important for platelet aggregation induced by low (≤0.05
U/ml) but not high (≥0.1 U/ml) concentrations of thrombin. High concentrations of
exogenous TSP may univalently saturate all its platelet binding sites consequently
interfering with TSP-crosslinking of thrombin-activated platelets.
Key words
Thrombospondin - Thrombin-induced platelet aggregation
