Incorporation of Vitronectin into Fibrin Clots

Abstract

19788 -19794). The current studies were undertaken to further examine the interactions between vitronectin and fibrin(ogen). Comparison of vitronectin levels in plasma with those in serum indicates that ϳ20% of plasma vitronectin is incorporated into the clot. When the time course of biotinylated-vitronectin incorporation into clots formed from 125 I-fibrinogen is monitored, vitronectin incorporation into the clot parallels that of fibrinogen in the absence or presence of activated factor XIII. Vitronectin binds specifically to fibrin matrices with an estimated K d of ϳ0.6 M. Additional vitronectin subunits are assembled on fibrin-bound vitronectin multimers through self-association. Confocal microscopy of fibrin clots reveals the globular vitronectin aggregates anchored at intervals along the fibrin fibrils. This periodicity raised the possibility that vitronectin interacts with the ␥A/␥ variant of fibrin(ogen) that represents about 10% of total fibrinogen. In support of this concept, the vitronectin which contaminates fibrinogen preparations co-purifies with the ␥A/␥ fibrinogen fraction, and clots formed from ␥A/␥ fibrinogen preferentially bind vitronectin. These studies reveal that vitronectin associates with fibrin during coagulation, and may thereby modulate hemostasis and inflammation.Vitronectin is a multifunctional plasma glycoprotein that participates in the regulation of coagulation, fibrinolysis, and the complement cascade (reviewed in Refs. 1 and 2). Vitronectin also regulates cell adhesion and pericellular proteolysis on surfaces of cells and extracellular matrices (1-5). Like fibrinogen, vitronectin is found in plasma at micromolar concentrations (6), and is stored in megakaryocyte and platelet ␣-granules (7-9). In plasma, vitronectin circulates as a native, monomeric form that is a mixture of 72-kDa single-chain and two-chain disulfide-linked species (10 -12). Under normal conditions, less than 3% of plasma vitronectin is comprised of more reactive oligomeric forms that display enhanced affinity for heparin or heparin-like molecules, and for the conformationsensitive monoclonal antibody 8E6 (10 -12).During acute phase response, plasma vitronectin levels increase (6), with a relative increase in the percentage of oligomeric vitronectin (13). Levels of the oligomeric forms of vitronectin in serum relative to plasma also increase; indicating the process of coagulation alters vitronectin structure and function (10 -12, 14). The altered, oligomeric form of vitronectin is generated, at least in part, by interactions with other plasma proteins such as thrombin-antithrombin complexes (11, 12, 14, 15, and complement C5b-9 complexes (11, 12, 16).A portion of vitronectin in plasma (17,18), and in platelets is complexed with type 1 plasminogen activator inhibitor (PAI-1) 1 (8,9,19), an interaction that induces the formation of higher order complexes (4,5,20,21) and influences the structure and function of both PAI-1 and vitronectin. Thus, when bound to vitronectin, PAI-1 is stabilized in its active conforma...

The Solution Structure of the N-terminal Domain of Human Vitronectin

“…In a concurrent study from this laboratory, two of the disulfide bonds of SMB were determined by biochemical methods and mass spectrometry. 2 This work revealed one disulfide bridge as 5-9 and another as 25-39. These findings were in agreement with the initial calculations, so the two experimentally determined disulfides were imposed to refine the structure of the SMB domain, and two possible alternatives were then pursued separately in the structure determination.…”

“…Human vitronectin is a glycoprotein found in the circulation, where it contributes to hemostasis by regulating blood coagulation and fibrinolysis (1,2). Vitronectin is also found in the ECM, 1 where it plays important roles in cell adhesion, pericellular proteolysis, tissue invasion, angiogenesis, and metastasis (3)(4)(5)(6)(7).…”

A Deletion Mutant of Vitronectin Lacking the Somatomedin B Domain Exhibits Residual Plasminogen Activator Inhibitor-1-binding Activity

“…Other work from our laboratory extends these results and defines the complementary binding site within PAI-1, a necessary feature that must exist for an ordered mechanism of binding and assembly of higher order complexes to be operable. 3 Furthermore, we have used use pre-steady state kinetics to evaluate role of these dual sites in the mechanism of binding. 4 Results from the kinetic measurements demonstrate two-step binding of PAI-1 to vitronectin.…”

“…2 Vitronectin competes for heparin binding with both thrombin and antithrombin, effectively reducing the anticoagulant ability of heparin (2). Because PAI-1 inhibits plasminogen activators that convert plasminogen into the protease plasmin, vitronectin acts to prevent fibrinolysis by prolonging the functional lifespan of PAI-1 and localizing it to fibrin clots (3).…”

Advancing Biomarker Science in the 21st Century

“…Globular vitronectin multimers assembled on fibrin strands attract plasma vitronectin, which, in turn, becomes anchored to the developing thrombus, potentially at increased rates with specific fibrinogen chain variants. 4 In addition, plasma vitronectin, once incorporated within developing fibrin thrombi, attenuates fibrinolysis through mediation of a somatomedin B domain-directed plasminogen activator inhibitor-1-fibrin interaction. Vitronectin receptors (␣ v ␤ 3 ) on vascular endothelial cells may also be involved directly with platelet binding by means of von Willebrand factor strings under high shear stress.…”