Effects of kringles derived from human plasminogen on fibrinolysis in vitro

Thermoresponsive copolymer decorated surface enables controlling the adsorption of a target protein in plasma

The control of protein/surface interactions by external stimuli is often required in bioapplications such as bioseparation and biosensors. Although regulation of protein adsorption has been achieved on the surfaces modified with stimuli-responsive polymers, controlled protein adsorption is still challenging for a target protein in a multiprotein system. The present study developed a concept of surface design for the controlled adsorption of a specific protein from plasma by combining a thermoresponsive polymer with an affinity ligand on the surface. In this regard, a polyurethane (PU) surface was modified with the copolymer of N-isopropylacrylamide (NIPAAm) and a ε-lysine-containing monomer (LysMA). ε-Lysine is a specific ligand for plasminogen that was used as the model "target protein" in this study. The PU-P(NIPAAm-co-Lys) surfaces exhibited distinct thermoresponsivity of plasminogen adsorption from plasma with a larger quantity adsorbed at 37 °C than at 23 °C. By contrast, the surfaces showed a low level of adsorption for other plasma proteins at both temperatures. In addition, plasminogen adsorbed on a PU-P(NIPAAm-co-Lys) surface could be partly desorbed by lowering the temperature, and the activity of plasminogen adsorbed was well preserved. We believe that the concept developed in this study can be extended to other proteins by combining PNIPAAm and specific ligands with affinities for the proteins of interest.

Antiangiogenic kringles derived from human plasminogen and apolipoprotein(a) inhibit fibrinolysis through a mechanism that requires a functional lysine-binding site

Many proteins in the fibrinolysis pathway contain antiangiogenic kringle domains. Owing to the high degree of homology between kringle domains, there has been a safety concern that antiangiogenic kringles could interact with common kringle proteins during fibrinolysis leading to adverse effects in vivo. To address this issue, we investigated the effects of several antiangiogenic kringle proteins including angiostatin, apolipoprotein(a) kringles IV(9)-IV(10)-V (LK68), apolipoprotein(a) kringle V (rhLK8) and a derivative of rhLK8 mutated to produce a functional lysine-binding site (Lys-rhLK8) on the entire fibrinolytic process in vitro and analyzed the role of lysine binding. Angiostatin, LK68 and Lys-rhLK8 increased clot lysis time in a dose-dependent manner, inhibited tissue-type plasminogen activator-mediated plasminogen activation on a thrombin-modified fibrinogen (TMF) surface, showed binding to TMF and significantly decreased the amount of plasminogen bound to TMF. The inhibition of fibrinolysis by these proteins appears to be dependent on their functional lysine-binding sites. However, rhLK8 had no effect on these processes owing to an inability to bind lysine. Collectively, these results indicate that antiangiogenic kringles without lysine binding sites might be safer with respect to physiological fibrinolysis than lysine-binding antiangiogenic kringles. However, the clinical significance of these findings will require further validation in vivo.

Inhibition of fibrinolysis by lipoprotein(a)

A high plasma concentration of lipoprotein Lp(a) is now considered to be a major and independent risk factor for cerebro- and cardiovascular atherothrombosis. The mechanism by which Lp(a) may favour this pathological state may be related to its particular structure, a plasminogen-like glycoprotein, apo(a), that is disulfide linked to the apo B100 of an atherogenic LDL-like particle. Apo(a) exists in several isoforms defined by a variable number of copies of plasminogen-like kringle 4 and single copies of kringle 5 and the catalytic region. At least one of the plasminogen-like kringle 4 copies present in apo(a) (kringle IV type 10) contains a lysine binding site (LBS) that is similar to that of plasminogen. This structure allows binding of these proteins to fibrin and cell membranes. Plasminogen thus bound is cleaved at Arg561-Val562 by plasminogen activators and transformed into plasmin. This mechanism ensures fibrinolysis and pericellular proteolysis. In apo(a) a Ser-Ile substitution at the Arg-Val plasminogen activation cleavage site prevents its transformation into a plasmin-like enzyme. Because of this structural/functional homology and enzymatic difference, Lp(a) may compete with plasminogen for binding to lysine residues and impair, thereby, fibrinolysis and pericellular proteolysis. High concentrations of Lp(a) in plasma may, therefore, represent a potential source of antifibrinolytic activity. Indeed, we have recently shown that during the course of the nephrotic syndrome the amount of plasminogen bound and plasmin formed at the surface of fibrin are directly related to in vivo variations in the circulating concentration of Lp(a) (Arterioscler. Thromb. Vasc. Biol., 2000, 20: 575-584; Thromb. Haemost., 1999, 82: 121-127). This antifibrinolytic effect is primarily defined by the size of the apo(a) polymorphs, which show heterogeneity in their fibrin-binding activity--only small size isoforms display high affinity binding to fibrin (Biochemistry, 1995, 34: 13353-13358). Thus, in heterozygous subjects the amount of Lp(a) or plasminogen bound to fibrin is a function of the affinity of each of the apo(a) isoforms and of their concentration relative to each other and to plasminogen. The real risk factor is, therefore, the Lp(a) subpopulation with high affinity for fibrin. According to this concept, some Lp(a) phenotypes may not be related to atherothrombosis and, therefore, high Lp(a) in some individuals might not represent a risk factor for cardiovascular disease. In agreement with these data, it has been recently reported that Lp(a) particles containing low molecular mass apo(a) emerged as one of the leading risk conditions in advanced stenotic atherosclerosis (Circulation, 1999, 100: 1154-1160). The predictive value of high Lp(a) as a risk factor, therefore, depends on the relative concentration of Lp(a) particles containing small apo(a) isoforms with the highest affinity for fibrin. Within this context, the development of agents able to selectively neutralise the antifibrinolytic activity of Lp(a), offers new perspectives in the prevention and treatment of the cardiovascular risk associated with high concentrations of thrombogenic Lp(a).

Lysine-derivatized polyurethane as a clot lysing surface: conversion of adsorbed plasminogen to plasmin and clot lysis in vitro

Polyurethane surfaces to which lysine residues are immobilized by photochemical methods are proposed as a basis for clot lysing surfaces. The lysines are attached in such a way that the epsilon-amino and carboxyl groups are free. We showed previously that these surfaces, when placed in contact with plasma, adsorb only plasminogen and virtually no other proteins (McClung et al., J. Biomed. Mater. Res. 49 (2000) 409). In this communication, data based on a chromogenic substrate assay are presented showing that plasminogen adsorbed to these surfaces is readily converted to plasmin in the presence of tissue-plasminogen activator (t-PA). Moreover, the rate of activation on the surface is considerably greater than in solution. Experiments demonstrating the ability of these surfaces to dissolve fibrin clots are also reported. Surfaces exposed to plasma and then to t-PA were placed in citrated plasma. On recalcification, clotting was initiated, but the incipient clots were soon dissolved. On control surfaces (no lysine or lysine in which the epsilon-amino groups were not available) coagulation continued until a stable clot was formed. Similar observations were made when the plasma/t-PA exposed surfaces were placed in a pure fibrinogen solution and thrombin was added.

No grip, no growth: the conceptual basis of excessive proteolysis in the treatment of cancer

The formation of new bloodvessels, called angiogenesis, is critical for a tumour to grow beyond a few mm(3) in size. A provisional matrix promotes endothelial cell adhesion, migration, proliferation and survival. Synthesis and degradation of this matrix closely resemble processes that occur during coagulation and fibrinolysis. Degradation of the matrix and fibrinolysis are tightly controlled and balanced by stimulators and inhibitors of the plasminogen activation system. Here we give an overview of these processes during tumour progression. We postulate a novel way to inhibit angiogenesis by removal of the matrix through specific and localised overstimulation of the plasminogen activation system.

Structural basis for the pathophysiology of lipoprotein(a) in the athero-thrombotic process

Lipoprotein Lp(a) is a major and independent genetic risk factor for atherosclerosis and cardiovascular disease. The essential difference between Lp(a) and low density lipoproteins (LDL) is apolipoprotein apo(a), a glycoprotein structurally similar to plasminogen, the precursor of plasmin, the fibrinolytic enzyme. This structural homology endows Lp(a) with the capacity to bind to fibrin and to membrane proteins of endothelial cells and monocytes, and thereby to inhibit plasminogen binding and plasmin generation. The inhibition of plasmin generation and the accumulation of Lp(a) on the surface of fibrin and cell membranes favor fibrin and cholesterol deposition at sites of vascular injury. Moreover, insufficient activation of TGF-beta due to low plasmin activity may result in migration and proliferation of smooth muscle cells into the vascular intima. These mechanisms may constitute the basis of the athero-thrombogenic mode of action of Lp(a). It is currently accepted that this effect of Lp(a) is linked to its concentration in plasma. An inverse relationship between Lp(a) concentration and apo(a) isoform size, which is under genetic control, has been documented. Recently, it has been shown that inhibition of plasminogen binding to fibrin by apo(a) is also inversely associated with isoform size. Specific point mutations may also affect the lysine-binding function of apo(a). These results support the existence of functional heterogeneity in apolipoprotein(a) isoforms and suggest that the predictive value of Lp(a) as a risk factor for vascular occlusive disease would depend on the relative concentration of the isoform with the highest affinity for fibrin.

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.

The plasminogen-plasmin system in malignancy

The study of the plasminogen-plasmin system has, in the past, contributed much to the understanding of fibrinolysis and thrombolysis. Attention is now focused on the role of the components of this system in many biologic functions. Findings of uPA, its receptor and its inhibitor in many tumor tissues and tumor cell lines, strongly implicate their involvement in tumor invasion, tumor cell proliferation and metastasis. The characteristics of the plasminogen activators, the uPA receptor and the plasminogen activator inhibitors as well as their expression and regulation in tumors and tumor cell lines are reviewed.

6-aminohexanoate and chloride ion in the activation by urokinase of porcine plasminogens

The rate of activation by urokinase of porcine plasminogen is accelerated by 6-aminohexanoate, although the maximally enhanced rate is 10-fold less than that of human plasminogen without the amino acid. 6-Aminohexanoate facilitates only activation of native porcine plasminogen (asp-plasminogen), but has no effect on activation of des-kringle1-4-plasminogen. Sodium chloride, on the other hand, inhibits activation by urokinase of both porcine asp-plasminogen and des-kringle1-4-plasminogen. It is concluded that 6-aminohexanoate exerts its effect via kringle1-4 domains of plasminogen, whereas Cl- acts, at least in part, through effects on the kringle5 or proteinase domains.

Binding site of alpha 2-plasmin inhibitor to plasminogen

Peptide T-11, a carboxyl terminal tryptic fragment of alpha 2-plasmin inhibitor, inhibits the reversible first step of the reaction between plasmin and alpha 2-plasmin inhibitor. To elucidate which amino-acid residues played a important role in the inhibitory activity of peptide T-11, we prepared the various synthetic derivatives of peptide T-11 and determined the peptide concentration that inhibited the apparent rate constant of the reaction between plasmin and alpha 2-plasmin inhibitor by 50% (IC50). Peptide III, which lacked the residues Gly-1 to Pro-7 of peptide I (peptide T-11), had a strong inhibitory activity, like peptide I (IC50: peptide I, 7 microM; peptide III, 13 microM). The peptides that lacked the Leu-9 and Lys-10 or Lys-26 of peptide III showed much weaker activity, and the loss or amidation of the C-terminal lysine of peptide III also markedly reduced the inhibitory activity. Peptide III competitively inhibited the binding of [14C]tranexamic acid to kringle 1 + 2 + 3 (K1-3) and kringle 4 (K4) in a binding assay performed by the gel-diffusion method. The respective dissociation constants (Kd) of peptide III for K1-3 and K4 were 0.85 microM and 35.2 microM. These data suggest that the amino residue of Lys-10 and the carboxylic acid of Lys-26 in peptide T-11 play crucial roles in the ionic binding of alpha 2-plasmin inhibitor to the tranexamic acid-binding site (lysine-binding site) of plasminogen. Peptide T-11: H-G-D-K-L-F-G-P-D-L-K-L-V-P-P-M-E-E-D-Y-P-Q-F-G-S-P-K-OH.