Complex formation of platelet thrombospondin with plasminogen. Modulation of activation by tissue activator.

Abstract

A bstract. Thrombospondin (TSP), a multifunctional a-granule glycoprotein of platelets, binds fibrinogen, fibronectin, heparin, and histidine-rich glycoprotein and thus may play an important role in regulating thrombotic influences at vessel surfaces. In this study we have demonstrated that purified human platelet TSP formed a complex with purified human plasminogen (Plg). Complex formation was detected by rocket immunoelectrophoresis of mixtures of the purified radiolabeled proteins. Significant complex formation of fluid-phase Plg with adsorbed TSP was also demonstrated by enzyme-linked immunosorbent assay (ELISA). The complex formation was specific, saturable, and inhibited by excess fluid-phase TSP, with an apparent KD of -35 nM. In both ELISA and rocket immunoelectrophoresis systems, complex formation was inhibited by 10 mM eamino-n-caproic acid, implying that there is a role for the lysine binding sites of Plg in mediating the interaction. TSP also formed a complex with plasmin as detected by ELISA but did not directly inhibit plasmin activity measured with a synthetic fluorometric substrate or with a '251-fibrin plate assay. TSP, when incubated with Plg before addition to '25I-fibrin plates significantly inhibited the generation of plasmin activity by tissue plasminogen activator (TPA) in a manner that was calcium dependent. A kinetic study of Plg activation by TPA in the presence of TSP demonstrated that Michaelis-Menten kinetics were followed and that TSP acted as a noncompetitive inhibitor. These studies support the hypothesis that TSP,

A comparison of the thrombolytic and hemorrhagic effects of tissue-type plasminogen activator and streptokinase in rabbits.

“…This hypothesis is supported by the results of recent studies that demonstrated that platelet thrombospondin combines with plasminogen and inhibits its activation. 28 The reason that bleeding was not induced by t-PA in a concentration that was able to produce approximately 35% lysis is less clear. It is possible that fibrin present in the hemostatic plug is more resistant to lysis by t-PA than fibrin in thrombi.…”

Calcium indirectly regulates immunochemical reactivity and functional activities of the N-domain of thrombospondin-1

“…Supramolecular assembly, covalent modifications, and limited proteolysis are known to modulate integrin binding activities of specific matrix proteins (reviewed in (Silverstein et al, 1984;Davis et al, 2000)). Some integrin ligands may also be subject to conformational regulation of activity similar to that described for their receptors.…”

Cartilage Oligomeric Matrix Protein (Thrombospondin-5) Is Expressed by Human Vascular Smooth Muscle Cells

“…3,4 After experimental arterial injury, TSP-1 mRNA expression is upregulated within hours. 5 Functionally, TSP-1 has been shown to (1) promote cell adhesion, proliferation, and migration, 2,6 (2) modulate angiogenesis, 7,8 (3) regulate plasmin production, 9 and (4) activate transforming growth factor (TGF)-␤1. 9,10 TSP-2 is similar to TSP-1 in terms of its molecular architecture, but it shows a different pattern of expression in developing organisms.…”