Characterization of the binding of plasminogen to fibrin surfaces: the role of carboxy-terminal lysines

Augmented pulse-spray thrombolysis with tPA by early pulsed intrathrombic plasminogen enrichment

PURPOSE

This study was designed to evaluate the efficacy of plasminogen enrichment of subacute thrombus in further accelerating pulse-spray pharmacomechanical thrombolysis (PSPMT) with urokinase (UK) or tissue plasminogen activator (tPA) in a rabbit model.

MATERIALS AND METHODS

With use of a subacute rabbit inferior vena cava (IVC) thrombosis model, 78 rabbits were divided into eight groups according to the agents used for thrombolysis: (i) controls (IVC thrombosis, no lysis performed), (ii) pulse-spray thrombolysis with saline only, (iii) PSPMT with UK, (iv) PSPMT with UK, plus interim pulse-spray plasminogen enrichment after 14 minutes, (v) pulse-spray plasminogen enrichment, followed at 10 minutes by PSPMT with UK, (vi) PSPMT with tPA, (vii) PSPMT with tPA, plus interim plasminogen enrichment, and (viii) pulse-spray plasminogen enrichment, followed at 10 minutes by PSPMT with tPA.

RESULTS

Intrathrombic pulsed injection of glu-plasminogen after 14 minutes of tPA PSPMT demonstrated significant augmentation of lysis (approximately 31% decrease in residual thrombus) compared with tPA alone (P = .006). Lysis was not augmented significantly when plasminogen was sprayed into thrombus before tPA, or before or after UK.

CONCLUSION

Plasminogen enrichment of thrombus after onset of PSPMT with tPA significantly accelerated thrombolysis in a subacute in vivo rabbit model. A clinical trial of this method may be warranted.

Enzymatic and chemical modifications of lipoprotein(a) selectively alter its lysine-binding functions

The pathogenicity of lipoprotein(a) [Lp(a)] as a risk factor for cardiovascular disease may depend upon its lysine binding sites (LBS) which impart unique functions to Lp(a) not shared with low density lipoprotein. Biologically relevant modifications of Lp(a) were tested for alterations of LBS activity using two previously described functional assays, a LBS-Lp(a) immunoassay and a lysine-Sepharose bead assay. In the LBS-Lp(a) immunoassay, minimal changes in the LBS activity of Lp(a) were observed after modification with lipoprotein lipase, sphingomyelinase, or phospholipase C. In contrast, a significant (p<0.003) increase in the LBS activity of Lp(a) occurred after phospholipase A2 (PLA2) treatment, and this increase was confirmed using the lysine-Sepharose bead assay. The increase depended upon the release of fatty acids from Lp(a) by PLA2. A decrease in the LBS activity of Lp(a) occurred after oxidation of Lp(a) with 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) (44% decrease), but CuSO4 oxidation increased LBS activity (210%). N-acetylcysteine (NAC) treatment of Lp(a) decreased (48%) LBS activity while homocysteine treatment had no (89%) effect. Thus, modification of phospholipids and protein moieties can alter the LBS-activity of Lp(a). Such enzymatic and chemical modifications may contribute to the variability in LBS function of Lp(a) seen within the population.

PROACT: a phase II randomized trial of recombinant pro-urokinase by direct arterial delivery in acute middle cerebral artery stroke. PROACT Investigators. Prolyse in Acute Cerebral Thromboembolism

BACKGROUND AND PURPOSE

To test the safety and recanalization efficacy of intra-arterial local delivery of plasminogen activators in acute ischemic stroke, a randomized trial of recombinant pro-urokinase (rpro-UK) versus placebo was undertaken in patients with angiographically documented proximal middle cerebral artery occlusion.

METHODS

After exclusion of intracranial hemorrhage by CT scan, patients with abrupt onset of symptoms of focal ischemia likely to receive treatment within 6 hours who satisfied all clinical eligibility criteria underwent carotid angiography. Patients displaying Thrombolysis in Acute Myocardial Infarction grade 0 or 1 occlusion of the M1 or M2 middle cerebral artery were randomized 2:1 to receive rpro-UK (6 mg) or placebo over 120 minutes into the proximal thrombus face. All patients received intravenous heparin. Recanalization efficacy was assessed at the end of the 2-hour infusion, and intracerebral hemorrhage causing neurological deterioration was assessed at 24 hours.

RESULTS

Of 105 patients who underwent angiography, 59 were excluded from randomization. Among the 46 patients randomized, 40 were treated with rpro-UK (n=26) or placebo (n=14) a median of 5.5 hours from symptom onset. Recanalization was significantly associated with rpro-UK (2P=.017). Hemorrhagic transformation causing neurological deterioration within 24 hours of treatment occurred in 15.4% of the rpro-UK-treated patients and 7.1% of the placebo-treated patients (2P=.64). Both recanalization and hemorrhage frequencies were influenced by heparin dose.

CONCLUSIONS

Intra-arterial local rpro-UK infusion was associated with superior recanalization in acute thrombotic/ thromboembolic stroke compared with placebo. In this regimen, heparin dose influenced hemorrhage frequency and recanalization. Although symptomatic hemorrhage remains a concern, this study suggests that recanalization is enhanced with rpro-UK and heparin.

Lipoprotein(a) isoforms display differences in affinity for plasminogen-like binding to human mononuclear cells

Binding of lipoprotein(a) (Lp(a)) to membrane proteins of the monocyte-macrophage cell lineage may be an important event in atheroma formation. Since Lp(a) with distinct apolipoprotein(a) (apo(a)) isoforms may show differences in their affinity with regard to fibrin binding, the existence of such a functional behavior in the interaction of apo(a) in Lp(a) with these cells was explored using the monocytic cell line THP-1. Lp(a) preparations containing small size apo(a) isoforms (M(r) = 450,000 to 550,000) and high molecular mass isoforms (M(r) > or = 700,000) were purified from plasmas containing > 0.35 g/L of Lp(a) obtained from subjects (n = 14) with cardiovascular atherosclerotic disease. Binding of plasminogen to THP-1 cells was performed using the method of radioisotopic dilution. For binding of Lp(a) to cells, the THP-1 monocytic cells were incubated with varying concentrations of the different Lp(a) preparations; cells were then washed and the amount of Lp(a) bound was detected with a radiolabeled polyclonal antibody directed against apo(a). Binding due to kringle interactions with lysine residues was calculated by subtracting from the total bound the amount of Lp(a) bound (approximately 10%) in the presence of 6-aminohexanoic acid. Analysis of data with the Langmuir equation indicated identical and independent (non-interacting) sites and allowed evaluation of the Kd. Binding isotherms of small size isoforms showed saturation and a high affinity (Kd = 25.8 +/- 19 nmol/L) relative to that of plasminogen (Kd = 1750 +/- 760 nmol/L). A similar difference (Kd = 17.5 +/- 7.9 nmol/L versus Kd = 600 +/- 220 nmol/L) was found when binding experiments were performed with a fibrin surface. In contrast, binding isotherms of the high molecular mass isoforms did not show saturation at the highest Lp(a) concentrations used, thus indicating a lower affinity. In conclusion, these results show that apo(a) isoforms may display polymorphism-linked functional heterogeneity with regard to cell binding, which may explain the higher association with cardiovascular risk of small size isoforms. These qualitative differences in the binding of apo(a) isoforms to fibrin or cells may modulate the cardiovascular risk associated with high levels of Lp(a).

A Novel Degradation Pathway of Tissue Factor Pathway Inhibitor: Incorporation Into Fibrin Clot and Degradation by Thrombin

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor with three tandem inhibitory domains (K1, K2, and K3) that regulates the initial reactions of the extrinsic blood coagulation pathway through K1 and K2. In the present study, the effect of thrombin on TFPI in a purified system was first examined using recombinant TFPI from Chinese hamster ovary (CHO) cells. TFPI was inactivated by thrombin with cleavage of three peptide bonds, Lys 254-Thr 255 in the C-terminal basic region, Arg 107-Gly 108 (reactive site toward factor Xa in K2), and Lys 86-Thr 87 between K1 and K2. Then, degradation of radiolabeled TFPI by thrombin was examined in two systems: (1) mixed with plasma and then tissue factor (TF ) and calcium ion, and (2) mixed with fibrinogen and then thrombin. TFPI degradation was detected in serum from normal plasma and more extensively from antithrombin (AT)-depleted plasma by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Significant radioactivity was found in the clot after coagulation of the plasma, which decreased after 20 hours' incubation. These changes were more prominent in AT-depleted plasma than in normal plasma. When TFPI lacking the C-terminal basic region was used instead of full-length TFPI, most of the radioactivity was found in serum rather than in fibrin clots. Incorporation of TFPI into the fibrin clot was prevented by a synthetic C-terminal peptide of TFPI. Similar results were obtained after mixing radiolabeled TFPI with fibrinogen and then thrombin in the presence of calcium ion or EDTA. These results demonstrate a novel degradation pathway of TFPI, ie, incorporation into fibrin via the C-terminal basic region and degradation by thrombin (possibly fibrin-bound thrombin).

Complement C5b-9 increases plasminogen binding and activation on human endothelial cells

Deposition of the terminal complement proteins (C5b-9) on human endothelial cells can result in cell lysis or nonlytic alterations of cell function including procoagulant responses. Because regulation of fibrinolysis is a central endothelial function and because C9 contains a carboxyl-terminal lysine similar to other proteins that bind and facilitate activation of plasminogen (PG), the effects of complement injury on PG binding and activation on these cells were investigated. Activation of complement through deposition of C5b67 complexes on endothelial cells resulted in a small increase (approximately 20%) in PG binding. Incorporation of C8 into C5b-8 resulted in no further increase in binding; however, specific 125I-PG binding was increased by approximately 100% after C5b-9 deposition. Moreover, PG was found to bind specifically to C7 and C9. The PG bound to endothelial cells after C5b-9 deposition was readily activated by tissue-type plasminogen activator (TPA). In a cell-free system, complement C9 and a synthetic peptide composed of the 20 carboxyl-terminal amino acids of C9 enhanced PG activation by TPA. Removal of the carboxyl-terminal lysine of C9 abolished the enhancement of PG activation without diminishing PG binding. We conclude that membrane C9 may comprise a binding site for PG and serve to enhance activation of this zymogen by TPA. These findings suggest that immune injury to the endothelium may enhance both the fibrin-generating and fibrinolytic capacity of the vessel wall.

Cell-surface cytokeratin 8 is the major plasminogen receptor on breast cancer cells and is required for the accelerated activation of cell-associated plasminogen by tissue-type plasminogen activator

Cytokeratin 8 (CK 8) has been identified on the external surfaces of viable, unpermeabilized epithelial cells (Hembrough, T. A., Vasudevan, J., Allietta, M. M., Glass, W. F., and Gonias, S. L. (1995) J. Cell Sci. 108, 1071-1082). In this study, we demonstrated that CK 8 is the major plasminogen-binding protein in plasma membrane fractions isolated from three breast cancer cell lines, BT20, MCF-7, and MDA-MB-157. To assess the function of CK 8 as a plasminogen receptor, monoclonal antibody 1E8 was raised against the carboxyl-terminal 12 amino acids of CK 8. The 1E8 epitope was present on the external surfaces of breast cancer cells, as determined by immunofluorescence microscopy. 125I-1E8 bound to MCF-7 cells; the maximum binding capacity (1.5 x 10(6) sites per cell) was comparable with that determined for plasminogen. When MCF-7 cells were incubated with Fab fragments of 1E8, specific 125I-plasminogen binding was decreased up to 82%. Specific plasminogen binding was decreased up to 67%, even when the unbound 1E8 Fab was removed by washing the cells prior to adding 125I-plasminogen. Preincubation with 1E8 Fab decreased plasminogen binding to BT20 and MDA-MB-157 cells, although to a lesser extent than with MCF-7 cells. Plasminogen activation by tissue-type plasminogen activator was greatly accelerated, due to a large decrease in Km, when the plasminogen was bound to MCF-7 cells. Pretreatment with 1E8 Fab decreased the rate of plasminogen activation by up to 83%, implicating CK 8 in the MCF-7 cell-accelerated reaction. These studies identify cell-surface CK 8 as a major plasminogen receptor in breast cancer cells and as a required component for the rapid activation of cell-associated plasminogen by tissue-type plasminogen activator.

Cytokeratin 8 released by breast carcinoma cells in vitro binds plasminogen and tissue-type plasminogen activator and promotes plasminogen activation

Cell-surface activation of plasminogen may be important in diseases that involve cellular migration, including atherosclerosis and tumour invasion/metastasis. Cytokeratin 8 (CK 8) has been identified as a plasminogen-binding protein expressed on the external surfaces of hepatocytes and breast carcinoma cells [Hembrough, Vasudevan, Allietta, Glass and Gonias (1995) J. Cell Sci. 108, 1071-1082]. In this investigation, we demonstrate that a soluble form of CK 8 is released into the culture medium of breast cancer cell lines. The released CK 8 is in the form of variably sized polymers that bind plasminogen and promote the activation of [Glu1]plasminogen and [Lys78]plasminogen by single-chain tissue-type plasminogen activator (sct-PA). To assess the mechanism by which CK 8 promotes plasminogen activation, CK 8 was purified from rat hepatocytes and immobilized in microtitre plates. Immobilized CK 8 bound 125I-plasminogen and 125I-sct-PA in a specific and saturable manner. The KDs were 160 +/- 40 nM and 250 +/- 48 nM, respectively. Activation of plasminogen bound to immobilized CK 8 was accelerated compared with plasminogen in solution, as determined using a coupled-substrate fluorescence assay and SDS/PAGE. The ability of CK 8 to promote plasminogen activation may be important in the pericellular spaces surrounding breast cancer cells and at the cell surface.

Evidence that the fibrinogen binding domain of Apo(a) is outside the lysine binding site of kringle IV-10: a study involving naturally occurring lysine binding defective lipoprotein(a) phenotypes

It is now established that the lysine binding site (LBS) of apo(a) kringle IV-10, and particularly Trp72, plays a dominant role in the binding of lipoprotein(a) [Lp(a)] to lysine. To determine the role of the LBS in the binding of Lp(a) to fibrinogen, we examined the binding to plasmin-modified (PM) fibrinogen of human and rhesus monkey Lp(a) species classified as either Lys' or Lys- based on their capacity to bind lysine Sepharose and to have Trp or Arg, respectively, in position 72 of the LBS of kringle IV-10. We also examined the free apo(a)s obtained by subjecting their corresponding parent Lp(a)s to a mild reductive procedure developed in our laboratory. Our results show that both Lyst and Lys- Lp(a)s and their derived apo(a)s, bound to PM-fibrinogen with similar affinities (Kds: 33-100 nM), whereas the B(max) values were threefold higher for apo(a)s. Both the lysine analog epsilon-aminocaproic acid and L-proline inhibited the binding of Lp(a) and apo(a) to PM fibrinogen. We conclude that the LBS of kringle IV-10 is not involved in this process and that apo(a) binds to PM-fibrinogen via a lysine-proline-sensitive domain located outside the LBS and largely masked by the interaction of apo(a) with apoB100. The significant difference in the PM fibrinogen binding capacity also suggests that apo(a) may have a comparatively higher athero-thrombogenic potential than parent Lp(a).

The complete cDNA sequence encoding plasminogen from the European hedgehog (Erinaceus europaeus)

Using a PCR-based strategy, we have determined the complete cDNA sequence encoding hedgehog plasminogen (Plg). The 2700-nucleotide cDNA (corresponding to a 2.9-kb liver-derived transcript) encodes an open reading frame of 811 amino acids which shares 74-76% identity with Plg characterized from mouse, human and rhesus monkey. Residues corresponding to the catalytic triad, tPA-cleavage site, as well as seven of the eight lysine-binding residues in kringle IV are conserved in the hedgehog. However, potential N-linked glycosylation sites which have been reported in human and rhesus Plg are not present in analogous positions in the hedgehog Plg sequence.

Inducible carboxypeptidase activity. A role in clot lysis in vivo

BACKGROUND

An inducible carboxypeptidase activity in human plasma delays tissue-type plasminogen activator (TPA)-induced clot lysis in vitro. We investigated whether carboxypeptidase activity is induced in vivo during thrombosis and thrombolytic therapy in a canine model of myocardial infarction.

METHODS AND RESULTS

By use of synthetic substrate assays, dog plasma was shown to contain an inducible carboxypeptidase activity that is efficiently inhibited by potato carboxypeptidase inhibitor. This inhibitor accelerates TPA-mediated clot lysis in vitro by an average of 27% (n = 5, P = .046). Analysis of the inducible carboxypeptidase activity in plasma samples of dogs with electrically induced thrombosis of the circumflex coronary artery treated with TPA revealed that (1) inducible carboxypeptidase activity is increased during thrombosis (8.7 +/- 2.0 U/L, P < .013) and thrombolytic therapy (9.9 +/- 1.8 U/L, P < .024) compared with baseline (3.2 +/- 2.0 U/L); (2) thrombosis is a prerequisite of carboxypeptidase induction during and after TPA infusion, since carboxypeptidase levels were lower in dogs without a coronary thrombus; and (3) a significant positive correlation (r = .6, P < .0069) of carboxypeptidase activity with time to restoration of blood flow was observed.

CONCLUSIONS

These data indicate that carboxypeptidase activity is induced in vivo and may influence thrombolysis.

Lys and fibrinogen binding of wild-type (Trp72) and mutant (Arg72) human apo(a) kringle IV-10 expressed in E coli and CHO cells

In a previous study, we identified a lysine (Lys)-binding-defective form of human lipoprotein(a) and attributed this defect to the presence of a Trp72-->Arg mutation in apolipoprotein(a) [apo(a)] kringle IV-10. To document this relationship, we expressed both wild-type (wt) and mutant (mut) forms of kringle IV-10 in Escherichia coli (nonglycosylated form) and Chinese hamster ovary (CHO) cells (glycosylated form). The Arg72 mut was prepared by introducing the T-->A mutation in apo(a) kringle IV-10 amplified from human liver mRNA by the reverse-transcriptase polymerase chain reaction technique. All expressed kringles were tested for their ability to bind Lys and plasmin-modified fibrinogen (PM-fibrinogen). wt kringle IV-10 expressed in both E coli and CHO cells bound to Lys-Sepharose with comparable affinity. In contrast, the Arg72 mut expressed in both systems exhibited no Lys-binding capacity. Moreover, the wt kringle IV-10 expressed in both systems bound to PM-fibrinogen and exhibited two binding components, one Lys mediated (inhibitable by epsilon-amino-n-caproic acid) and one Lys insensitive, occurring in about the same proportions. Only the latter type of binding was present in the Arg72 mut expressed in E coli. We conclude that kringle IV-10 of human apo(a) has Lys- and PM-fibrinogen-binding capacities that are independent of glycosylation and require the presence of Trp72, one of the seven amino acids that constitute the Lys-binding site of kringle IV-10. Our results also show that the binding of kringle IV-10 to PM- fibrinogen is more complex than that to Lys, in that the former requires an additional binding site or sites outside the Lys-binding site.

Rearrangements of the fibrin network and spatial distribution of fibrinolytic components during plasma clot lysis. Study with confocal microscopy

Binding of components of the fibrinolytic system to fibrin is important for the regulation of fibrinolysis. In this study, decomposition of the fibrin network and binding of plasminogen and plasminogen activators (PAs) to fibrin during lysis of a plasma clot were investigated with confocal microscopy using fluorescein-labeled preparations of fibrinogen, plasminogen, tissue-type PA (t-PA), and two-chain urokinase-type PA (tcu-PA). Lysis induced by PAs present throughout the plasma clot was accompanied by a gradual loss of fibrin content of fibers and by accumulation of plasminogen onto the fibers. Two sequential phases could be distinguished: a phase of prelysis, during which the fibrin network remained immobile, and a phase of final lysis, during which fibers moved with a tendency to shrink and eventually disappeared. The two phases occurred simultaneously but in different locations when lysis was induced by PAs present in the plasma surrounding the clot. The zone of final lysis was located within a 5-8 microns superficial layer, where fibers were mobile, a surface-associated fibrin agglomerates appeared. Plasminogen accumulated in these agglomerates up to 30-fold as compared with its concentration in the outer plasma. t-PA was also highly concentrated in the agglomerates, and tcu-PA bound to them slightly. The zone of prelysis, where plasminogen was moderately accumulated on the immobile fibers, was located deeper in the clot. This zone was much thinner in the case of t-PA-induced lysis than in the case of tcu-PA-induced lysis, reflecting the difference in penetration of the two PAs into the clot. We conclude that under conditions of diffusional transport of fibrinolytic enzymes from outside a plasma clot, extensive lysis is spatially restricted to a zone not exceeding 5-8 microns from the clot surface. In this zone the structure of the fibrin network undergoes significant changes, and strikingly high accumulation of fibrinolytic components takes place.

Plasma carboxypeptidases as regulators of the plasminogen system

Carboxy-terminal lysine residues on the surface of cells and fibrin bind plasminogen and control its activation. Since plasma contains basic carboxypeptidases, which remove carboxy-terminal lysines from protein substrates, we investigated if these enzymes are involved in the regulation of plasminogen binding sites. Plasma reduced plasminogen binding to cells, and this effect could be ascribed to the activity of the plasma carboxypeptidases. Purified carboxypeptidase N, which is constitutively active, and plasma carboxypeptidase B, which circulates as a zymogen, were both capable of significantly reducing plasminogen binding to cells. Dose titration experiments verified that plasma concentrations of either carboxypeptidase were sufficient to maximally affect plasminogen binding to cells. Furthermore, plasma carboxypeptidase B, but not carboxypeptidase N, reduced the rate of whole blood clot lysis induced by tissue-type plasminogen activator. These findings establish that plasma carboxypeptidases can modulate plasminogen binding to cells and control the rate of fibrinolysis. These functions delineate a novel role for the plasma carboxypeptidases in the regulation of the plasminogen system.

Fibrinolytic mechanism, biochemistry, and preclinical pharmacology of recombinant prourokinase

The purpose of this review is to provide a biochemical characterization of recombinant prourokinase (r-ProUK [ABT-187]), including a description of its clot-specific fibrinolytic mechanism. In addition, the preclinical data will be briefly reviewed, demonstrating the efficacy of r-ProUK as a potent therapeutic plasminogen activator. r-ProUK was purified to homogeneity from the culture medium of SP2/0 mouse hybridoma cells. The fibrinolytic potency of r-ProUK was characterized by both in vitro clot lysis experiments in human plasma and a canine femoral artery thrombosis model. In the in vitro clot lysis system, with use of clots prepared from fresh frozen human plasma, r-ProUK exhibits a lag phase to the onset of lysis and a concentration-dependent threshold effect due to the presence of the inhibitors alpha 2-antiplasmin and plasminogen activator inhibitor (PAI-1). Effective clot lysis can be achieved without degradation of the fibrinogen in the surrounding plasma. Over a dose range of 50,000-220,000 IU, the canine femoral artery thrombosis model shows a dose-dependent relationship for r-ProUK and effective clot lysis. The lytic activity of r-ProUK is significantly enhanced in this model by the concomitant administration of heparin as an adjunctive agent for thrombolytic treatment. Fibrinogen, plasminogen, and alpha 2-antiplasmin levels in the systemic circulation were unaltered during the 30-minute infusion period and a 4-hour observation period, in which 85% lysis was achieved with r-ProUK (100,000 IU) and heparin. Moreover, restoration of blood flow in the previously fully occluded femoral artery was achieved within minutes of the start of the r-ProUK infusion. The experimental findings presented here are consistent with the clot-specific fibrinolytic mechanism of r-ProUK. Effective clot lysis can be achieved without alteration of the systemic coagulation and fibrinolytic parameters.

Superficial accumulation of plasminogen during plasma clot lysis

BACKGROUND

Binding of plasminogen to partially degraded fibrin is an important step in fibrinolysis, influencing its rate and fibrin specificity. Little is known about the spatial distribution of plasminogen and of plasminogen-binding sites inside thrombi during lysis. In the present study, we investigated this problem, which is important for a better understanding of the local regulation of fibrinolysis and the rate-limiting factors of therapeutic thrombolysis.

METHODS AND RESULTS

An experimental system was used that allowed continuous visualization and quantification by fluorescence microscopy of the spatial distribution of fluorescein-labeled plasminogen inside and outside model thrombi. Strong superficial accumulation of plasminogen was observed during lysis of a plasma clot induced by tissue-type or urokinase-type plasminogen activators in the surrounding plasma. A distinctly visible plasminogen-accumulating shell moved continuously with the reducing surface of the clot. The accumulation decreased in conditions of exhaustive activation of plasminogen in the outer plasma. It was found in a purified system that a thin superficial layer (approximately 50 microns wide) of a plasmin-treated fibrin clot exposes about 2.5 plasminogen-binding sites per fibrin monomer with a Kd of 2.2 mumol/L. At a physiological concentration of plasminogen (1.5 mumol/L) in the outer medium, plasminogen was concentrated about 10-fold in this layer. The binding was dose-dependently inhibited by epsilon-aminocaproic acid.

CONCLUSIONS

We conclude that the generation of potent surface-associated plasminogen-binding sites during thrombolysis results in a strikingly high plasminogen concentration at the dynamically changing surface of a lysing clot. The necessity of a continuous plasminogen supply from the plasma supports the use of fibrin-specific and plasminogen-sparing agents for thrombolytic therapy.

Serum antibodies to distinct epitopes of the tissue-type plasminogen activator (t-PA) in patients with systemic lupus erythematosus

Defective fibrinolysis due to decreased tissue-type plasminogen activator (t-PA) activity is a well-established finding in patients with systemic lupus erythematosus (SLE). The possibility that this decrease in t-PA activity may be related to the presence of autoantibodies directed against t-PA, and the possible role of these autoantibodies in the pathophysiology of fibrinolysis in SLE, were investigated. Serum samples from 115 SLE patients and 63 normal volunteers were analyzed for the presence of such antibodies. The search for antibodies to t-PA was performed by means of several systems, allowing for the identification of epitopes presented in different conformational physical states of t-PA: free or associated to its inhibitor (PAI-1) in plasma, specifically bound to fibrin surface, or passively adsorbed to solid supports. Antibodies in variable amounts were detected by all systems used; however, t-PA activity was not inhibited by the IgG fraction of the positive sera in a fibrin-agar fibrinolysis system. Moreover, the demonstration of serum anti-t-PA antibodies was not associated with clinical or laboratory abnormalities related to vasoocclusive episodes. These results indicate that, as in the case of other autoantibodies, their detection in serum does not imply their direct participation in the pathophysiology of thrombosis in SLE.

Identification of two functionally distinct lysine-binding sites in kringle 37 and in kringles 32-36 of human apolipoprotein(a)

The well documented association between high plasma levels of lipoprotein(a) (Lp(a)) and cardiovascular disease might be mediated by the lysine binding of apolipoprotein(a) (apo(a)), the plasminogen-like, multikringle glycoprotein in Lp(a). We employed a mutational analysis to localize the lysine-binding domains within human apo(a). Recombinant apo(a) (r-apo(a)) with 17 plasminogen kringle IV-like domains, one plasminogen kringle V-like domain, and a protease domain or mutants thereof were expressed in the human hepatocarcinoma cell line HepG2. The lysine binding of plasma Lp(a) and r-apo(a) in the culture supernatants of transfected HepG2 cells was analyzed by lysine-Sepharose affinity chromatography. Wild type recombinant Lp(a) (r-Lp(a)) revealed lysine binding in the range observed for human plasma Lp(a). A single accessible lysine binding site in Lp(a) is indicated by a complete loss of lysine binding observed for r-Lp(a) species that contain either a truncated r-apo(a) lacking kringle IV-37, kringle V, and the protease or a point-mutated r-apo(a) with a Trp-4174-->Arg substitution in the putative lysine-binding pocket of kringle IV-37. Evidence is also presented for additional lysine-binding sites within kringles 32-36 of apo(a) that are masked in Lp(a) as indicated by an increased lysine binding for the point mutant (Cys-4057-->Ser), which is unable to assemble into particles. An important role of these lysine-binding site(s) for Lp(a) assembly is suggested by a decreased assembly efficiency for deletion mutants lacking either kringle 32 or kringles 32-35.

A cytokeratin 8-like protein with plasminogen-binding activity is present on the external surfaces of hepatocytes, HepG2 cells and breast carcinoma cell lines

Plasminogen binding to cell surfaces may be important for tumor invasion and other processes that involve cellular migration. In this investigation, the principal plasminogen-binding protein was identified in the plasma membrane fraction of rat hepatocytes. The protein had an apparent mass of 59 kDa, was insoluble in a spectrum of detergents, and was identical to cytokeratin 8 (CK 8) as determined by sequence analysis of nine amino acids at the N terminus of two cyanogen bromide fragments. The 59 kDa protein bound CK 8-specific antibody in western blot analyses. These studies demonstrate that CK 8 or a CK 8-like protein binds plasminogen. Given this newly determined and potentially important CK 8 function, immunofluorescence and immunoelectron microscopy studies were performed to determine whether CK 8 may be present on the external surfaces of unpermeabilized, viable hepatocytes. All of the cells in each preparation were immunopositive with two separate CK 8-specific antibodies. A punctate pattern of immunofluorescence was detected on the cell surface with approximately even intensity from cell to cell. By immunoelectron microscopy, CK 8 was preferentially associated with microvilli. In order to determine whether other epithelial cells express cell-surface CK 8, immunofluorescence and immunoelectron microscopy studies were performed with HepG2 hepatocellular carcinoma cells and with BT20 and MCF-7 breast carcinoma cells. The pattern of antigen expression was equivalent with each cell type and comparable to that observed with hepatocytes. These studies support the hypothesis that CK 8 is associated with the external cell surface where it may express important proteinase receptor function.

1H-NMR assignments and secondary structure of human plasminogen kringle 1

The 1H-NMR spectrum of the kringle 1 domain of human plasminogen complexed with 6-aminohexanoic acid, an antifibrinolytic drug, has been assigned. Elements of secondary structure have been identified on the basis of sequential, medium and long-range dipolar interactions, back-bone amide spin-spin couplings (3JHN-H alpha) and 1H-2H exchange rates. The kringle contains scarcely any repetitive secondary structure: eight reverse turns and two short beta-sheets. These comprise 40% and 12% of the domain, respectively. No alpha-helix was found. An aromatic cluster formed by His31, Phe36, Trp62, Phe64, Tyr72 and Tyr74 is indicated by several inter-residue Overhauser connectivities. Contacts between the methyl groups of Leu46 and the side chains of Phe36, Trp62 and Trp25 are observed. A second hydrophobic cluster formed by Tyr9, Ile77 and Leu78 is also indicated. A comparison of secondary structure elements among plasminogen kringles 1 and 4 and tissue-type plasminogen activator kringle 2 suggests that there is variability in the position and number of reverse turns on going from one kringle to another; however, the beta-sheets are conserved among the homologs.

Inhibitory effects of lysine analogues on t-PA induced whole blood clot lysis

The lysine analogues epsilon-aminocaproic acid (EACA) and trans-4-amino-methyl cyclohexane carboxylic acid (AMCA) are used to prevent excessive bleeding in patients with coagulopathies, such as hemophilia and thrombocytopenia, or in those who have received tissue plasminogen activator (t-PA). However, their relative efficacy in inhibiting lysis of clots that have been formed in the presence of exogenous t-PA or that have been formed and then exposed to exogenous t-PA has not been well characterized. The present study utilized blood from normal volunteers and 125I-fibrinogen in a dilute whole blood clot assay to determine the relative concentrations of lysine analogues required for inhibition of clot lysis induced by exogenous t-PA. AMCA (0.06 mM) and EACA (0.6 mM) were effective in prolonging clot lysis if (1) whole blood clots were formed and then exposed to a lysine analogue and exogenous t-PA or if (2) whole blood clots were formed in the presence of exogenous t-PA and a lysine analogue. However, their inhibitory effect was markedly reduced if clots were formed in the presence of t-PA and then exposed to either of the lysine analogues. The analogues did not inhibit the initial binding of t-PA to fibrin. They did inhibit binding of plasminogen to fibrin as well as the activation of plasminogen by t-PA in the absence of fibrin. The data suggest that lysine analogues, even at low concentrations, reduce the rate of t-PA induced whole blood clot lysis by several mechanisms.

Study of tissue-type plasminogen activator binding sites on fibrin using distinct fragments of fibrinogen

It is well established that tissue-type plasminogen activator (t-PA) binds to the D region of fibrin(ogen) and that two distinct CNBr fragments of fibrinogen (FCB), FCB-2 and FCB-5, comprising parts of this region, stimulate plasminogen activation by t-PA. In the present work, ligand-binding studies were performed to characterize the interactions between t-PA and the corresponding fibrin regions using a well defined model of a fibrin surface and both FCB-2 and FCB-5 in liquid and solid phase. Binding isotherms showed a characteristic Langmuir adsorption saturation profile. The dissociation constants determined for the binding of t-PA to immobilized FCB-2 (Kd = 0.70 +/- 0.10 nM) and FCB-5 (Kd = 0.47 +/- 0.08 nM) were of the same order of magnitude as the Kd for fibrin binding (Kd = 1 +/- 0.2 nM). The specificity of the binding was demonstrated by the ability of soluble FCB-2 and FCB-5 to inhibit t-PA binding to solid-phase fibrin (Ki = 3.3 microM and 6.4 microM, respectively). The binding of t-PA to fibrin and to immobilized FCB-2 was partially inhibited by the lysine analogue 6-aminohexanoic acid (Ki = 123 +/- 47 microM and 364 microM, respectively) but was not modified by carboxypeptidase B, thus indicating involvement of internal lysine residues. Removal of lysine residues by treatment with, successively, plasmin and carboxypeptidase B, produced only a partial inhibition of t-PA binding, thus confirming the existence of both a lysine-dependent and a lysine-independent mechanism of binding of t-PA to both fibrin and FCB-2. In contrast, the binding of t-PA to FCB-5 was not significantly affected by 6-aminohexanoic acid. Altogether, these data indicate that the mechanism of binding of t-PA to fibrin involves mainly a lysine-independent interaction with the D region which is contributed by sequences present in FCB-5 and FCB-2; contribution to binding by a lysine-dependent interaction was detected only in FCB-2 and is probably of minor relevance as suggested by the limited effect of 6-aminohexanoic acid.

Polymorphic forms of Lp(a) with different structural and functional properties: cold-induced self-association and binding to fibrin and lysine-Sepharose

Two different Lp(a) polymorphs were isolated from the same individual and shown to have important differences both in their solution properties and in interaction with lysine Sepharose and fibrin. One Lp(a) particle (d-Lp(a)) with a large apo(a) isoform had a density of 1.087 g/ml and a molecular weight of 3.17 million, while the other Lp(a) particle with a small apo(a) isoform having a mobility faster than that of apoB was larger and had a molecular weight of 3.75 million and a density of 1.054 g/ml. D-Lp(a) underwent cold-induced self-association and also had a higher affinity for lysine Sepharose, whereas the other Lp(a) polymorph did not. Both Lp(a) particles bound fibrin via two different binding sites, one of which involved fibrin lysine residues which are also recognized by plasminogen. Lysine-mediated binding of d-Lp(a) by fibrin was ten times stronger than that of the other Lp(a) particle, whereas non-lysine-mediated binding of either Lp(a) species by fibrin was of equal strength. At saturation, 80% of d-Lp(a) bound fibrin at sites that did not involve lysine residues, whereas only 33% of the other Lp(a) polymorph bound to these sites. These findings indicate that the binding of Lp(a) to fibrin is more complex than previously thought and imposes another layer of difficulty on our understanding of how Lp(a) regulates and/or impairs fibrinolysis.

Overview on fibrinolysis: plasminogen activation pathways on fibrin and cell surfaces

Plasminogen activation at the surface of fibrin or of cell membranes is a sophisticated specialized system for localized extracellular proteolysis implicated in a large variety of biological functions (fibrinolysis, cell migration and extracellular matrix degradation). Assembly of plasminogen and/or activators at specific binding sites induces conformational changes that make accessible the scissile peptide bond of plasminogen and exposes the active centre of the tissue-type plasminogen activator. The mechanism of activation by pro-urokinase, a second type of activator that binds to cell membrane but not to fibrin, is far from being understood. It may be able, however, in contrast to urokinase, to specifically activate plasminogen bound to partially degraded fibrin. An extremely low Km and high catalytic rate are characteristic of the process of activation at surfaces. In contrast, activation in liquid phase by tissue-type plasminogen activator proceeds at an extremely low catalytic rate. The initiation and amplification of plasminogen activation depend on specific interactions between the modular constitutive units of these proteins and binding sites present on cell or fibrin surfaces. Thus, the most important mechanism for the acceleration of fibrinolysis and pericellular proteolysis is the unveiling of carboxy-terminal lysine residues on these surfaces, to which plasminogen may bind. Since plasminogen bound to carboxy-terminal lysines of progressively degraded fibrin or membranes is readily transformed into plasmin by fibrin-bound t-PA, this mechanism represents the most important pathway for the acceleration and amplification of fibrinolysis. Alpha-2-antiplasmin, by inhibiting plasmin release from surfaces, regulates the extent and rate of this process but has no effect on fibrin-bound or membrane-bound plasmin. Lipoprotein(a), a particle possessing a plasminogen-like apolipoprotein, apo(a), may interfere with this mechanism by inhibiting the specific binding of plasminogen to lysine residues in membrane or fibrin surfaces.

Effects of lipoprotein(a) on the binding of plasminogen to fibrin and its activation by fibrin-bound tissue-type plasminogen activator

Molecular assembly of plasminogen and tissue-type plasminogen activator (t-PA) at the surface of fibrin results in the generation of fibrin-bound plasmin and thereby in the dissolution of a clot. This mechanism is triggered by specific interactions of intra-chain surface lysine residues in fibrin with the kringle domains of plasminogen, and is further amplified via the interaction of plasminogen kringles with the carboxy-terminal lysine residues of fibrin that are exposed by plasmin cleavage. By virtue of its marked homology with plasminogen, apo(a), the specific apolipoprotein component of Lp(a), may bind to the lysine sites available for plasminogen on the surface of fibrin and thereby interfere with the fibrinolytic process. A sensitive solid-phase fibrin system, which allows the study of plasminogen activation at the plasma fibrin interface and makes feasible the analysis of products bound to fibrin, has been used to investigate the effects of Lp(a) on the binding of plasminogen and its activation by fibrin-bound t-PA. Plasma samples from human subjects with high levels of Lp(a) were studied. We have established that Lp(a) binds to the fibrin surface and thereby competes with plasminogen (Ki = 44 nM) so as to inhibit its activation. We have further shown that Lp(a) blocks specifically carboxy-terminal lysine residues on the surface of fibrin. To further explore the role of apo(a) on the Lp(a) fibrin interactions, we have performed ligand-binding studies using a recombinant form of apo(a) that contains 17 kringle 4-like units. We have shown that recombinant apo(a) binds specifically to fibrin (Kd = 26 +/- 8 nM, Bmax = 26 +/- 2 fmol/well) and that this binding increases upon treatment of the fibrin surface with plasmin (Kd = 8 +/- 4 nM, Bmax = 115 +/- 14 fmol/well). Altogether, our results indicate clearly that binding of native Lp(a) through this mechanism may impair clot lysis and may favor the accumulation of cholesterol in thrombi at sites of vascular injury.

Molecular assembly of plasminogen and tissue-type plasminogen activator on an evolving fibrin surface

A well characterized model of an intact and a degraded surface of fibrin that represents the states of fibrin during the initiation and the progression of fibrinolysis was used to quantitatively characterize the molecular interplay between tissue-type plasminogen activator (t-PA), plasminogen and fibrin. The molecular assembly of t-PA and plasminogen on these surfaces was investigated using combinations of proteins that preclude complications due to side reactions caused by generated plasmin: native plasminogen with di-isopropylphosphofluoridate-inactivated t-PA, and a recombinant human plasminogen with the active-site Ser741 mutagenized to Ala which renders the catalytic site inactive. Under these conditions, neither the affinity nor the maximal number of binding sites for plasminogen were modified by the presence of t-PA, indicating that binding sites for plasminogen pre-exist in intact fibrin and are not dependent on the presence of t-PA. In contrast, when plasminogen activation is allowed, increasing binding of plasminogen to the progressively degraded fibrin surface is directly correlated (r = 0.98) to the appearance of the fibrin E-fragment as shown using a monoclonal antibody (FDP-14) that has its epitope in the E domain of fibrin. t-PA was shown to bind with a high affinity to both the intact (Kd = 3.3 +/- 0.6 nM) and the degraded surface of fibrin (Kd = 1.2 +/- 0.4 nM). Binding of t-PA to carboxy-terminal lysine residues of degraded fibrin was shown to be efficiently competed by physiological concentrations of plasminogen (2 microM), indicating that the affinity of t-PA for these residues was lower than that of plasminogen (Kd = 0.66 +/- 0.22 microM) and unrelated to the high affinity of t-PA for specific binding sites on intact fibrin. These data confirm and establish that the generation of carboxy-terminal lysine residues on fibrin during ongoing fibrinolysis, and the binding of plasminogen to these sites, is an important pathway in the acceleration of clot dissolution.

Specific targeting with poly(ethylene glycol)-modified liposomes: coupling of homing devices to the ends of the polymeric chains combines effective target binding with long circulation times

One possibility for bringing drugs to their specific targets is to use the drug-laden liposomes that have been made target-specific by the attachment of appropriate proteins. Such 'directed' proteoliposomes and most other particles are rapidly removed from the bloodstream, however, by the mononuclear phagocytes in the liver and spleen. This causes suboptimal drug accumulation at the target site. Coating the liposome surface with poly(ethylene glycol) (PEG) may prolong the circulation time of liposomes. Using plasminogen as a homing device we have shown that the PEG-modified liposomes with such a homing device coupled to the ends of the long PEG chains may combine long vesicle circulation times in the blood with high target binding capability. The PEG-coated proteoliposomes with homing devices attached at the very bilayer surface, on the contrary, are longlived but have only little or no capability to bind to their targets.

Plasminogen binding by alpha 2-antiplasmin and histidine-rich glycoprotein does not inhibit plasminogen activation at the surface of fibrin

alpha 2-antiplasmin (alpha 2-AP) exerts its inhibitory effect on fibrinolysis by rapidly inhibiting the plasmin evolved; in addition, it has been suggested that interference with the binding of plasminogen to fibrin, a function shared with histidine-rich glycoprotein (HRGP), may also be significant in inhibition of fibrinolysis. To elucidate if plasminogen binding by these two alpha 2-globulins may decrease the generation of plasmin by tissue-type plasminogen activator (t-PA) at the surface of fibrin, a system mimicking the fibrin/plasma interface was used. Attempts were made to differentiate the plasminogen binding from the plasmin inhibitory function of alpha 2-AP. The activation of human Glu-plasminogen (native plasminogen with NH2-terminal glutamic acid) by fibrin-bound t-PA was performed in a plasma environment using either normal plasma, alpha 2-AP- or HRGP-depleted plasmas supplemented with increasing amounts of the lacking protein, or in a reconstituted system with purified plasminogen and various concentrations of alpha 2-AP and HRGP. The activation of Glu-plasminogen in alpha 2-AP-depleted plasma containing a normal concentration of HRGP produced a time-dependent increase in the generation of plasmin. The addition of 1 microM-alpha 2-AP to this plasma prevented the formation of Lys-derivatives and produced a marked decrease (42%) in the number of plasminogen-binding sites. In contrast, the addition of 1.5 microM-HRGP to HRGP-depleted plasma containing a normal amount of alpha 2-AP produced only a modest (17%) decrease in the amount of plasmin(ogen) bound. Moreover, in a purified system the amount of plasminogen-binding sites and thereby of plasmin generated at the surface of fibrin in the presence of both alpha-2 globulins was similar to the amount generated in the presence of alpha 2-AP alone. These results indicate clearly that the formation of reversible complexes between plasminogen and alpha 2-AP does not interfere with the binding and activation of plasminogen at the fibrin surface. In contrast, the inhibition of plasmin by alpha 2-AP decreases importantly the number of plasminogen-binding sites (carboxyl-terminal lysines) and inhibits thereby the accelerated phase of fibrinolysis. It can be concluded that interference of the binding of plasminogen to fibrin by alpha 2-AP during plasminogen activation, does not play a significant role in inhibition of fibrinolysis, and that the plasminogen-binding effect of HRGP, if any, is obscured by the important inhibitory effect of alpha 2-AP.

Antifibrinolytic activities of alpha-N-acetyl-L-lysine methyl ester, epsilon-aminocaproic acid, and tranexamic acid. Importance of kringle interactions and active site inhibition

alpha-N-acetyl-L-lysine methyl ester (NALME) is a lysine analogue that reportedly binds to low-affinity lysine binding sites in plasmin(ogen) and miniplasmin(ogen). In the studies presented here, we show that NALME has antifibrinolytic activity; however, unlike the therapeutic agents epsilon-amino-n-caproic acid (epsilon ACA) and tranexamic acid (TEA), the activity of NALME is based on inhibition of the plasmin active site. NALME (0.1-10 mM) significantly inhibited the amidase activity of plasmin, miniplasmin, and streptokinase-plasmin complex without affecting alpha-thrombin or tissue plasminogen activator. epsilon ACA and TEA (0.1-10 mM) did not affect the amidase activity of plasmin or miniplasmin. A kinetic analysis showed that NALME is a competitive inhibitor of D-Val-L-Lys-p-nitroanilide HCl (S-2251) hydrolysis by plasmin; NALME binding to plasmin completely prevented S-2251 binding. The Kl for the plasmin-NALME interaction was 0.4 mM. epsilon ACA and TEA inhibited fibrin monomer digestion by plasmin and miniplasmin without binding to the active site of either enzyme. This result suggests that epsilon ACA and TEA function as antifibrinolytics by disrupting the noncovalent association of fibrin monomer with a domain common to both plasmin and miniplasmin (probably kringle 5). NALME inhibited fibrin monomer digestion principally by decreasing amidase activity. NALME was the only lysine analogue that prevented fragment X formation; TEA and epsilon ACA primarily inhibited the formation of fragments Y and D. When plasmin was incubated simultaneously with alpha 2-antiplasmin and alpha 2-macroglobulin, epsilon ACA increased the fraction of plasmin reacting with alpha 2-macroglobulin; NALME had no effect on the plasmin distribution.(ABSTRACT TRUNCATED AT 250 WORDS)

Hedgehog lipoprotein(a) is a modulator of activation of plasminogen at the fibrin surface. An in vitro study

Lipoprotein(a) (Lp[a]), a highly atherogenic lipoprotein particle, is the prominent apolipoprotein B-containing lipoprotein in the hedgehog (Laplaud PM et al, J Lipid Res 1988;29:1157-1170). In the present work, we studied the consequences of the structural homology between the specific Lp(a) glycoprotein, apoprotein(a), and plasminogen on the generation of plasmin by fibrin-bound tissue-type plasminogen activator. The activation of plasminogen was initiated by adding either native plasma or Lp(a)-free plasma supplemented with the equivalent of 0.25 mg/ml of either purified Lp(a) or albumin to a surface of fibrin prepared on micortitration plates and to which human tissue-type plasminogen activator was specifically bound. With the Lp(a)-free plasma, an increase in the binding and activation of plasminogen as a function of time was observed. In contrast, in the presence of Lp(a) (i.e., native plasma or the reconstituted system), a significant decrease in the binding of plasmin(ogen) (approximately 60%) was obtained. These data indicate that hedgehog Lp(a) interferes with the binding and activation of plasminogen at the fibrin surface and may thereby behave as a factor regulating the extent of fibrin deposition. These results support our previous data indicating that high levels of Lp(a) may have antifibrinolytic effects in humans (Rouy D et al, Arterioscler Thromb 1991;11:629-638), are in agreement with the observation that Lp(a) is a risk factor for atherosclerotic disease, and provide further support to the view of Lp(a) as a link between atherosclerosis and thrombosis.