Effect of purified, soluble urokinase receptor on the plasminogen-prourokinase activation system

Association of the Circulating Supar Levels with Inflammation, Fibrinolysis, and Outcome in Severe Burn Patients

BACKGROUND

Hyperfibrinolysis and pro/anti-inflammatory imbalance usually occur in the early stage of severe burns. Soluble urokinase-type plasminogen activator receptor (suPAR) is involved in fibrinolysis and inflammation. To date, the levels of circulating suPAR in non-survivors with severe burns remain unknown. This study aimed to investigate the early association between circulating suPAR levels and biomarkers of fibrinolysis, pro/anti-inflammatory, and prognosis.

METHODS

Sixty-four consecutive Chinese patients with severe burns and 26 healthy volunteers were enrolled in a prospective observational cohort. Clinical characteristics and laboratory data were collected prospectively. Blood samples were collected at 48 h post-burn, and suPAR and biomarkers of pro/anti-inflammatory and fibrinolysis were detected by enzyme-linked immunosorbent assays. Important indicators between non-survivors and survivors were compared. Linear regression analysis was performed to screen variables associated with suPAR. Logistic regression analysis and receiver operating characteristic curve (ROC) analysis were performed to evaluate the prognostic value of suPAR.

RESULT

Compared with the control group, the circulating suPAR levels in the survivors (P < 0.001) and non-survivors (P = 0.017) were higher. Compared with survivors, non-survivors had lower circulating suPAR levels at 48 h post-burn, and they showed a higher degree of fibrinolysis (higher D-dimer) and a lower TNF-α/IL-10 ratio. According to linear regression analysis, the variables independently associated with a lower suPAR level were lower platelet factor 4 (PF-4), urokinase-type plasminogen activator (uPA), and TNF-α/IL-10 levels and a higher D-dimer level. Logistic regression and ROC analyses indicated that a suPAR level ≤ 4.70 μg/L was independently associated with 30-day mortality.

CONCLUSION

Low circulating suPAR levels at 48 h post-burn in severe burn patients may reflect decreased TNF-α/IL-10 ratio and increased hyperfibrinolysis. suPAR can predict 30-day mortality in patients with severe burn.

The Contribution of the Urokinase Plasminogen Activator and the Urokinase Receptor to Pleural and Parenchymal Lung Injury and Repair: A Narrative Review

Pleural and parenchymal lung injury have long been characterized by acute inflammation and pathologic tissue reorganization, when severe. Although transitional matrix deposition is a normal part of the injury response, unresolved fibrin deposition can lead to pleural loculation and scarification of affected areas. Within this review, we present a brief discussion of the fibrinolytic pathway, its components, and their contribution to injury progression. We review how local derangements of fibrinolysis, resulting from increased coagulation and reduced plasminogen activator activity, promote extravascular fibrin deposition. Further, we describe how pleural mesothelial cells contribute to lung scarring via the acquisition of a profibrotic phenotype. We also discuss soluble uPAR, a recently identified biomarker of pleural injury, and its diagnostic value in the grading of pleural effusions. Finally, we provide an in-depth discussion on the clinical importance of single-chain urokinase plasminogen activator (uPA) for the treatment of loculated pleural collections.

Increased soluble urokinase plasminogen activator receptor (suPAR) is associated with thrombosis and inhibition of plasmin generation in paroxysmal nocturnal hemoglobinuria (PNH) patients

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired genetic disorder of the bone marrow that produces intravascular hemolysis, proclivity to venous thrombosis, and hematopoietic failure. Mutation in the PIG-A gene of a hematopoietic stem cell abrogates synthesis of glycosylphosphoinositol (GPI) anchors and expression of all GPI-anchored proteins on the surface of progeny erythrocytes, leukocytes, and platelets. Urokinase plasminogen activator receptor (uPAR), a GPI-linked protein expressed on neutrophils, mediates endogenous thrombolysis through a urokinase-dependent mechanism. Here we show that membrane GPI-anchored uPAR is decreased or absent on granulocytes and platelets of patients with PNH, while soluble uPAR (suPAR) levels are increased in patients' plasma. Serum suPAR concentrations correlated with the number of GPI-negative neutrophils and were highest in patients who later develop thrombosis. In vitro, suPAR is released from PNH hematopoietic cells and from platelets upon activation, suggesting that these cells are the probable source of plasma suPAR in the absence of GPI anchor synthesis and trafficking of uPAR to the cell membrane. In vitro, the addition of recombinant suPAR results in a dose-dependent decrease in the activity of single-chain urokinase. We hypothesized that suPAR, prevents the interaction of urokinase with membrane-anchored uPAR on residual normal cells.

Plasticity in urokinase-type plasminogen activator receptor (uPAR) display in colon cancer yields metastable subpopulations oscillating in cell surface uPAR density--implications in tumor progression

It is becoming increasingly clear that tumor growth and progression is not entirely due to genetic aberrations but also reflective of tumor cell plasticity. It follows therefore that proteins contributing to tumor progression oscillate in their expression a contention yet to be shown. Because the urokinase-type plasminogen activator receptor (uPAR) promotes tumor growth and invasion, we determined whether its expression is itself plastic. In fluorescence-activated cell sorting (FACS), three independent colon cancer clonal populations revealed the expected Gaussian distribution for cell surface uPAR display. However, subcloning of cells collected from the trailing edge of the FACS yielded subpopulations, displaying low cell surface uPAR number. Importantly, these subclones spontaneously reverted to cells enriched in uPAR display, indicating a metastable phenotype. uPAR display plasticity was associated with divergent in vivo behavior with weak tumor growth and progression segregating with receptor deficiency. Mechanistically, reduced uPAR display reflected not repressed gene expression but a switch in uPAR protein trafficking from membrane insertion to shedding. To our knowledge, this is the first demonstration that uPAR cell surface density is oscillatory and we propose that such an event might well contribute to tumor progression.

The urokinase receptor (uPAR) and the uPAR-associated protein (uPARAP/Endo180): membrane proteins engaged in matrix turnover during tissue remodeling

The breakdown of the barriers formed by extracellular matrix proteins is a pre-requisite for all processes of tissue remodeling. Matrix degradation reactions take part in specific physiological events in the healthy organism but also represent a crucial step in cancer invasion. These degradation processes involve a highly organized interplay between proteases and their cellular binding sites as well as specific substrates and internalization receptors. This review article is focused on two components, the urokinase plasminogen activator receptor (uPAR) and the uPAR-associated protein (uPARAP, also designated Endo180), that are considered crucially engaged in matrix degradation. uPAR and uPARAP have highly diverse functions, but on certain cell types they interact with each other in a process that is still incompletely understood. uPAR is a glycosyl-phosphatidylinositol-anchored glycoprotein on the surface of various cell types that serves to bind the urokinase plasminogen activator and localize the activation reactions in the proteolytic cascade system of plasminogen activation. uPARAP is an integral membrane protein with a pronounced role in the internalization of collagen for intracellular degradation. Both receptors have additional functions that are currently being unraveled. The present discussion of uPAR and uPARAP is centered on their protein structure and molecular and cellular function.

The pro-urokinase plasminogen-activation system in the presence of serpin-type inhibitors and the urokinase receptor: rescue of activity through reciprocal pro-enzyme activation

The reciprocal pro-enzyme activation system of plasmin, urokinase-type plasminogen activator (uPA) and their respective zymogens is a potent mechanism in the generation of extracellular proteolytic activity. Plasminogen activator inhibitor type 1 (PAI-1) acts as a negative regulator. This system is complicated by a poorly understood intrinsic reactivity of the uPA pro-enzyme (pro-uPA) before proteolytic activation, directed against both plasminogen and PAI-1. We have studied the integrated activation mechanism under the repression of PAI-1 in a purified system. A covalent reaction between pro-uPA and PAI-1 was positively demonstrated but the reaction of PAI-1 with two-chain uPA was found to be at least 1000-fold faster. However, in spite of this very fast inhibition, two-chain uPA still became the dominant plasminogen activator when plasminogen was incubated with pro-uPA and PAI-1. The activity pattern observed under these conditions revealed an initial lag phase, followed by a continuous generation of minute amounts of active two-chain uPA, this uPA having a short lifetime before inhibition but still succeeding to generate new plasmin activity, thus preventing a complete inactivation of the feedback system. This property of the activation system was retained even in the simultaneous presence of PAI-1 and alpha(2)-antiplasmin. Addition of soluble uPA receptor to the system did not change the role of pro-uPA and the same pattern was observed when pro-uPA was bound to the uPA receptor on U937 cells. The present mechanism maintains the system at standby level and may be triggered to increased activity without the need for an external initiating event.

Peptide-derived antagonists of the urokinase receptor. affinity maturation by combinatorial chemistry, identification of functional epitopes, and inhibitory effect on cancer cell intravasation

The high-affinity interaction between urokinase-type plasminogen activator (uPA) and its glycolipid-anchored receptor (uPAR) plays an important role in pericellular plasminogen activation. Since proteolytic degradation of the extracellular matrix has an established role in tumor invasion and metastasis, the uPA-uPAR interaction represents a potential target for therapeutic intervention. By affinity maturation using combinatorial chemistry we have now developed and characterized a 9-mer, linear peptide antagonist of the uPA-uPAR interaction demonstrating specific, high-affinity binding to human uPAR (K(d) approximately 0.4 nM). Studies by surface plasmon resonance reveal that the off-rate for this receptor-peptide complex is comparable to that measured for the natural protein ligand, uPA. The functional epitope on human uPAR for this antagonist has been delineated by site-directed mutagenesis, and its assignment to loop 3 of uPAR domain III (Met(246), His(249), His(251), and Phe(256)) corroborates data previously obtained by photoaffinity labeling and provides a molecular explanation for the extreme selectivity observed for the antagonist toward human compared to mouse, monkey, and hamster uPAR. When human HEp-3 cancer cells were inoculated in the presence of this peptide antagonist, a specific inhibition of cancer cell intravasation was observed in a chicken chorioallantoic membrane assay. These data imply that design of small organic molecules mimicking the binding determinants of this 9-mer peptide antagonist may have a potential application in combination therapy for certain types of cancer.

Polymorphonuclear leucocytes mediate endogenous thrombus lysis via a u-PA-dependent mechanism

Many human thrombi lyse spontaneously without the administration of lytic drugs and cause no clinical symptoms. The mechanisms by which this occurs are incompletely understood. We found that model thrombi prepared from whole human blood in a Chandler loop also exhibited significant spontaneous lysis. Lysis was inhibited by chemical protease inhibitors, consistent with proteolysis resulting primarily from serine proteases, with a small contribution from matrix metalloproteinases. Whole blood was fractionated into platelet-rich plasma and cell populations. Significant spontaneous lysis was observed in platelet-rich thrombi enriched with polymorphonuclear leucocytes (PMNs), whereas mononuclear cells (MCs) and erythrocytes did not contribute to lysis. Incorporation of antibodies to urokinase (u-PA) and its receptor u-PAR neutralized a large proportion of the activity. Incubation of plasma with PMNs generated free u-PA activity, which was also detectable in model thrombi and in vivo human thrombi. Purified neutrophils, free of eosinophils, generated activity identical to PMNs. Smaller contributions to lysis by tissue-type plasminogen activator (t-PA), elastase and cathepsin G were also identified. These findings suggest a major role for circulating PMNs in endogenous thrombus lysis.

Plasminogen activation by pro-urokinase in complex with its receptor--dependence on a tripeptide (Spectrozyme plasmin)

The intrinsic activity of single-chain pro-urinary-type plasminogen activator (pro-uPA) and whether its receptor (uPAR) potentiates this activity remains controversial. In this report, the pro-uPA/uPAR-(1-281)-peptide complex in solution is shown to have equivalent plasminogen-activator activity to that of active two-chain uPA (tc-uPA). However, the activity of the complex was dependent on a synthetic tripeptide, Spectrozyme plasmin (Spl, H-D-2-aminohexanoic acid(Ahx)-hexatyrosyl-lysine-p-nitroanilide), which can also be used as a chromogenic substrate for plasmin. Furthermore, this activity could be completely suppressed by commonly used carrier proteins and detergents. The pro-uPA/uPAR-(1-281)-peptide complex at 1 nM displayed similar activity to that of tc-uPA for either [Glu1]plasminogen or [Lys77]plasminogen in chromogenic assays with Spl present as the plasmin substrate. When assayed with another plasmin substrate, S2251, the pro-uPA/uPAR-(1-281)-peptide complex was unable to activate plasminogen. The pro-uPA/uPAR-(1-281)-peptide complex and tc-uPA also showed a similar extent of plasminogen activation as measured by SDS/PAGE, when incubated with plasminogen and Spl in the presence of 100 micro M aprotinin, and plasminogen activation by pro-uPA alone was also stimulated in the presence of Spl in this assay. Activation of plasminogen by the pro-uPA/uPAR-(1-281)-peptide strictly required the presence of Spl, and pro-uPA remained in single-chain form during these assays. This activity of the pro-uPA/uPAR-(1-281)-peptide complex but not that of tc-uPA was completely inhibited by human serum albumin, bovine serum albumin, Tween-80, Triton X-100, and Pluronic-F68. Taken together, the data indicates that uPAR-(1-281)-peptide itself is not sufficient to augment pro-uPA activity and the presence of an effector molecule (e.g. Spl) is required to elicit the full plasminogen-activator activity of the pro-uPA/uPAR-(1-281)-peptide complex. It remains to be seen whether there is a physiological counterpart to this phenomenon.

Kinetics of reciprocal pro-urokinase/plasminogen activation--stimulation by a template formed by the urokinase receptor bound to poly(D-lysine)

The two zymogens, plasminogen and pro-urokinase plasminogen activator (pro-uPA), constitute a system of reciprocal activation, since plasmin, generated by uPA-catalysed plasminogen activation, can activate pro-uPA to uPA. Two such zymogens, when mixed, will undergo autocatalytic, reciprocal activation resulting in generation of proteolytic activity. As an example of reciprocal zymogen activation, the plasminogen/pro-uPA system was analysed in terms of a kinetic model which describes the progression in activated enzymes. This model gave a detailed description of the progress curves in plasmin and uPA. It accounted for the effects of varying the concentration of the zymogens, and also for the effects of plasmin substrates and inhibitors in the reaction mixture. The model assumes non-significant zymogen activity. It did not, however, exclude that a very low initial proteolytic activity, accounting for maximally 0.01% of that obtained when pro-uPA is fully activated, could be attributed to a genuine pro-uPA activity. Binding of the uPA receptor (uPAR) to pro-uPA/uPA might affect separate steps of the reciprocal activation reaction, or it might induce a significant pro-uPA activity. To distinguish between these possibilities the effect of a recombinant soluble (residues 1-277) form of uPAR, uPAR-(1-277)-peptide, on reciprocal pro-uPA/plasminogen activation was studied. uPAR-(1-277)-peptide attenuated reciprocal zymogen activation, and the results suggested that this was due to a decreased accessibility of the pro-uPA/uPAR-(1-277)-peptide complex to activation by plasmin. The uPAR-(1-277)-peptide in the presence of poly(D-lysine) caused a 20-fold enhancement of reciprocal zymogen activation. Kinetic analysis of separate activation steps revealed that this was due to a threefold stimulation of plasminogen activation by uPA/uPAR-(1-277)-peptide combined with a sixfold stimulation of plasmin's activation of pro-uPA/uPAR-(1-277)-peptide. The results suggested that poly(D-lysine) provided a template for a catalytically favourable interaction between plasminogen/plasmin and the uPAR-(1-277)-peptide complex with pro-uPA/uPA. There was no indication of a significant uPAR-(1-277)-peptide-induced enhancement of pro-uPA activity.