Immobilisation of monocytes to a solid support: a model for the study of ligand-binding interactions and plasminogen activation at the cell surface

Fibrinolytic cross-talk: a new mechanism for plasmin formation

Fibrinolysis and pericellular proteolysis depend on molecular coassembly of plasminogen and its activator on cell, fibrin, or matrix surfaces. We report here the existence of a fibrinolytic cross-talk mechanism bypassing the requirement for their molecular coassembly on the same surface. First, we demonstrate that, despite impaired binding of Glu-plasminogen to the cell membrane by epsilon-aminocaproic acid (epsilon-ACA) or by a lysine-binding site-specific mAb, plasmin is unexpectedly formed by cell-associated urokinase (uPA). Second, we show that Glu-plasminogen bound to carboxy-terminal lysine residues in platelets, fibrin, or extracellular matrix components (fibronectin, laminin) is transformed into plasmin by uPA expressed on monocytes or endothelial cell-derived microparticles but not by tissue-type plasminogen activator (tPA) expressed on neurons. A 2-fold increase in plasmin formation was observed over activation on the same surface. Altogether, these data indicate that cellular uPA but not tPA expressed by distinct cells is specifically involved in the recognition of conformational changes and activation of Glu-plasminogen bound to other biologic surfaces via a lysine-dependent mechanism. This uPA-driven cross-talk mechanism generates plasmin in situ with a high efficiency, thus highlighting its potential physiologic relevance in fibrinolysis and matrix proteolysis induced by inflammatory cells or cell-derived microparticles.

Plasminogen activation by blood monocytes and alveolar macrophages in primary pulmonary hypertension

The pathophysiology of primary pulmonary hypertension (PPH) remains poorly understood. Vascular wall remodeling and endothelial dysfunction reflected by modifications in plasma fibrinolytic proteins and von Willebrand factor have been well documented in PPH. We hypothesize that endothelial mediators, produced in excess in PPH patients, may stimulate migrating mononuclear cells and thereby modulate alveolar macrophage function; in particular, the plasminogen activation system. Components of the fibrinolytic system were therefore studied in plasma, blood monocytes and alveolar macrophages obtained from bronchoalveolar lavage in 10 patients with PPH and in four controls. Compared with controls, PPH patients had elevated plasma levels of tissue-type plasminogen activator (15.6 +/- 9.9 versus 5.5 +/- 3 ng/ml) and plasminogen activator inhibitor-1 (27.8 +/- 23 versus 16.4 +/- 12 ng/ml). In contrast, binding and activation of plasminogen by single-chain urokinase-type plasminogen activator (scu-PA) at the surface of blood monocytes and alveolar macrophages were not different from those of control values. Dissociation constants (K(d)) for binding of scu-PA and plasminogen to alveolar macrophages were similar in both PPH (4.7 +/- 1.5 and 0.88 +/- 0.3 micromol/l, respectively) and control (6.7 +/- 0.1 and 1.02 +/- 0.12 micromol/l, respectively) groups. These results indicate that in PPH patients the fibrinolytic activity of alveolar macrophages is normal, whereas endothelial fibrinolytic proteins are abnormally elevated in plasma.

Coordinate induction of collagenase-1, stromelysin-1 and urokinase plasminogen activator (uPA) by the 120-kDa cell-binding fibronectin fragment in fibrocartilaginous cells: uPA contributes to activation of procollagenase-1

Specific fibronectin (Fn) fragments found in synovial fluid of arthritic joints potentially contribute to the loss of cartilage proteoglycans by inducing matrix metalloproteinase (MMP) expression. However, whether or not the Fn fragment-modulated changes in expression of MMPs result in a net increase in matrix-degradative activity through alterations in the balance between MMP activation and inhibition has not been established. To understand the mechanisms by which proteolytic Fn fragments may contribute to joint degeneration, conditioned medium from fibrocartilaginous cells exposed to Fn, its 30-kDa fragment containing the collagen/gelatin-binding domain, its 120-kDa fragment containing the central cell-binding domain, and the RGD peptide were assayed for MMPs, and MMP activators and inhibitors. We found that the 120-kDa fragment of Fn (but not intact Fn), the 30-kDa fragment, and the RGD peptide, dose-dependently induced procollagenase-1 and prostromelysin-1 and decreased levels of the tissue inhibitor of metalloproteinases (TIMPs) -1 and -2. The alpha5beta1 integrin was implicated in the induction of collagenase by the 120-kDa Fn fragment, since collagenase induction was abrogated in the presence of blocking antibody to this integrin. Conditioned medium from cells exposed to the 120-kDa Fn fragment also demonstrated increased levels of the activated collagenase-1, which resulted in significantly elevated collagen degradative activity. That the urokinase plasminogen activator (uPA) was involved in the activation of procollagenase-1 was suggested by findings that the 120-kDa Fn fragment induced uPA coordinately with procollagenase-1, and the activation of procollagenase-1 was dose-dependently inhibited in the presence of plasminogen activator inhibitor-1. These data demonstrate that the 120-kDa cell-binding fragment of Fn induces a net increase in matrix-degradative activity in fibrocartilaginous cells by concomitantly inducing MMPs and their activator, uPA, while decreasing TIMPs.