Kinetics of plasmin activation of single chain urinary-type plasminogen activator (scu-PA) and demonstration of a high affinity interaction between scu-PA and plasminogen

Development of an updated assay for prekallikrein activator in albumin and immunoglobulin therapeutics

Background

Prekallikrein activator (PKA) is a contaminating enzyme found in therapeutic albumin and immunoglobulin products. The level is commonly measured using methods such as that defined by the European Pharmacopoeia (Ph Eur) with traceability to the WHO International Standard for PKA. This method generally works well, but problems are sometimes observed.

Materials and methods

A simplified one‐step method has been developed to replace the existing Ph Eur two‐step method which consists of kallikrein generation followed by kallikrein measurement using a chromogenic substrate. Analysis of data from the one‐stage method is simplified by the use of a dedicated online app.

Results

The one‐stage method was validated against the current Ph Eur method using batches of albumin and immunoglobulins. Problem batches of immunoglobulins were investigated using the one‐stage method. Improved methodology using true initial rate determinations and use of acid‐treated prekallikrein substrate (PKS) helped understand and reduce artefactual results.

Conclusions

The one‐stage method and associated app streamline real‐time determination of PKA and promote good principles of enzyme assays to limit substrate depletion, while also conserving expensive PKS. Blanking steps and reproducibility are simplified.

Fibrinogen counteracts the antiadhesive effect of fibrin-bound plasminogen by preventing its activation by adherent U937 monocytic cells

BACKGROUND

Fibrinogen and plasminogen strongly reduce adhesion of leukocytes and platelets to fibrin clots, highlighting a possible role for these plasma proteins in surface-mediated control of thrombus growth and stability. In particular, adsorption of fibrinogen on fibrin clots renders their surfaces non-adhesive, while the conversion of surface-bound plasminogen to plasmin by transiently adherent blood cells results in degradation of a superficial fibrin layer, leading to cell detachment. Although the mechanisms whereby these proteins exert their antiadhesive effects are different, the outcome is the same: the formation of a mechanically unstable surface that does not allow firm cell attachment.

OBJECTIVES

Since fibrin clots in circulation are exposed to both fibrinogen and plasminogen, their combined effect on adhesion of monocytic cells was examined.

METHODS

Fibrin gels were coated with plasminogen and its activation by adherent U937 monocytic cells in the presence of increasing concentrations of fibrinogen was examined by either measuring (125) I-labeled fibrin degradation products or plasmin amidolytic activity.

RESULTS

Unexpectedly, the antiadhesive effects of two fibrin binding proteins were not additive; in fact, in the presence of fibrinogen, the effect of plasminogen was strongly reduced. An investigation of the underlying mechanism revealed that fibrinogen prevented activation of fibrin-bound plasminogen by cells. Confocal microscopy showed that fibrinogen accumulates in a thin superficial layer of a clot, where it exerts its blocking effect on activation of plasminogen.

CONCLUSION

 The results point to a complex interplay between the fibrinogen- and plasminogen-dependent antiadhesive systems, which may contribute to the mechanisms that control the adhesiveness of a fibrin shell on the surface of hemostatic thrombi.

Monoclonal antibodies detect receptor-induced binding sites in Glu-plasminogen

When Glu-plasminogen binds to cells, its activation to plasmin is markedly enhanced compared with the reaction in solution, suggesting that Glu-plasminogen on cell surfaces adopts a conformation distinct from that in solution. However, direct evidence for such conformational changes has not been obtained. Therefore, we developed anti-plasminogen mAbs to test the hypothesis that Glu-plasminogen undergoes conformational changes on its interaction with cells. Six anti-plasminogen mAbs (recognizing 3 distinct epitopes) that preferentially recognized receptor-induced binding sites (RIBS) in Glu-plasminogen were obtained. The mAbs also preferentially recognized Glu-plasminogen bound to the C-terminal peptide of the plasminogen receptor, Plg-R(KT), and to fibrin, plasmin-treated fibrinogen, and Matrigel. We used trypsin proteolysis, immunoaffinity chromatography, and tandem mass spectrometry and identified Glu-plasminogen sequences containing epitopes recognized by the anti-plasminogen-RIBS mAbs: a linear epitope within a domain linking kringles 1 and 2; a nonlinear epitope contained within the kringle 5 domain and the latent protease domain; and a nonlinear epitope contained within the N-terminal peptide of Glu-plasminogen and the latent protease domain. Our results identify neoepitopes latent in soluble Glu-plasminogen that become available when Glu-plasminogen binds to cells and demonstrate that binding of Glu-plasminogen to cells induces a conformational change in Glu-plasminogen distinct from that of Lys-Pg.

A proposed reference method for plasminogen activators that enables calculation of enzyme activities in SI units

A method has been developed for accurately and precisely measuring the activity of a range of plasminogen activators (PAs) used as thrombolytic agents, including streptokinase, tissue plasminogen activator (tPA) and variants, and urokinase (uPA), both single and two chain forms. Plasminogen activation is monitored in a transparent, solid fibrin matrix but uses chromogenic substrate hydrolysis, rather than changes in fibrin, to quantitate the activity of PAs. The method has been tested in two recent international collaborative studies involving tPA and streptokinase where it has been shown to perform very well. Furthermore, the method is based on sound enzymological principles and once correction for the competitive inhibition of fibrin(ogen) is made, the generation of plasmin can be determined in molar terms and hence the activity of PAs can be expressed and compared in SI units (rate of increase in molar concentration of plasmin) as well as International Units. The assay is also arranged in such a way to reflect the behavior of PAs in vivo during thrombolytic therapy and it is shown that the specific activity of streptokinase and tPA in this system reflects plasmin generation capacity of these thrombolytics for doses given in infusions for treatment of myocardial infarction. The method would make a suitable reference method for PAs and provides a rigorous means of studying and modeling the enzymology of fibrinolysis and will be helpful in the rational design of third generation thrombolytic agents.

Plasminogen activator inhibitor-1 and the kidney

Plasminogen activator inhibitor-1 (PAI-1) is a serine protease inhibitor that was isolated 20 years ago. First recognized as an inhibitor of intravascular fibrinolysis, it is now evident that PAI-1 is a multifunctional protein with actions that may be dependent on or independent of its protease inhibitory effects. The latter often involve interactions between PAI-1 and vitronectin or the urokinase receptor. The protease-inhibitory actions of PAI-1 extend beyond fibrinolysis and include extracellular matrix turnover and activation of several proenzymes and latent growth factors. PAI-1 has been implicated in several renal pathogenetic processes, including thrombotic microangiopathies and proliferative and/or crescentic glomerulopathies. Most recently, it has become clear that PAI-1 also plays a pivotal role in progressive renal disease, both glomerulosclerosis and tubulointerstitial fibrosis. An active area of present research interest, untold stories are likely to be uncovered soon.

Characterization of a recombinant chimeric plasminogen activator with enhanced fibrin binding

A recombinant chimeric plasminogen activator (GHRP-scu-PA-32K), consisting of the tetrapeptide Gly-His-Arg-Pro fused to the N-terminus of the low-molecular single-chain urokinase-type plasminogen activator (Leu144-Leu411), was produced by expression in CHO cells. The stable expression cell line was selected for large-scale expression. The product was purified by antibody-Sepharose affinity chromatography with a recovery of 67%. The apparent molecular weight of purified GHRP-scu-PA-32K was 33 kDa according to SDS-PAGE. Its specific activity was 150000 IU/mg protein according to fibrin plate determination. The conversion of single-chain to two-chain molecules mediated by plasmin was comparable for GHRP-scu-PA-32K (K(m)=4.9 microM, k(2)=0.35 s(-1)) and scu-PA-32K. The activation of plasminogen by GHRP-scu-PA-32K (K(m)=1.02 microM, k(2)=0.0028 s(-1)) was also similar to that of scu-PA-32K. The fibrin binding of GHRP-scu-PA-32K was 2.5 times higher than that of scu-PA-32K at a fibrin concentration of 3.2 mg/ml. In contrast to scu-PA-32K in vitro 125I-fibrin-labeled plasma clot lysis, GHRP-scu-PA had a higher thrombolytic potency, whereas it depleted less fibrinogen in plasma. These results show that GHRP-scu-PA-32K as expected is a potential thrombolytic agent.

Differences between neonates and adults in the urokinase-plasminogen activator (u-PA) pathway of the fibrinolytic system

This study deals with plasminogen activation kinetics of fetal and adult Glu-plasminogen types 1 and 2 as well as fetal and adult Lys-plasminogen by urokinase in the presence and absence of the lysine analogues epsilon-amino-n-caproic acid (EACA) and tranexamic acid. In addition, activation kinetics of single-chain urokinase-plasminogen activator (scu-PA) by adult and fetal plasmin types were investigated in the absence and presence of soluble fibrin. All Lys-plasminogen isoforms were more readily activated by urokinase than their corresponding Glu-plasminogen types. No substantial differences of the catalytic constants of urokinase-catalyzed plasminogen activation could be obtained when all fetal plasminogen types were compared to the respective adult types. In the case of all Glu-plasminogen isoforms, EACA as well as tranexamic acid first stimulated the activation process and, at higher concentrations, showed inhibitory properties. Again, the relative ability of all fetal plasminogen types to interact with lysine analogues revealed no differences compared to the respective adult glycoforms. In the absence of soluble fibrin, the catalytic efficiency of scu-PA activation by plasmin was significantly lower for both fetal plasmin isoforms. However, there were no differences in catalytic efficiency between fetal and adult plasmin types in the presence of 4 microM soluble fibrin. In conclusion, no substantial differences exist in urokinase-catalyzed plasminogen activation between neonates and adults, which is in contrast to reported data on plasminogen activation by tissue-type plasminogen activator. In the absence of soluble fibrin, scu-PA activation by fetal plasmin is markedly slower than by adult plasmin. However, this is compensated when fibrin is added at a concentration that is close to the physiological fibrinogen concentration in plasma. It can be summarized that the differences in carbohydrate structures of fetal and adult plasminogen are not associated with major differences in the global function of this part of fibrinolysis, despite functional alterations of scu-PA activation.

Characterization of Cell-Associated Plasminogen Activation Catalyzed by Urokinase-Type Plasminogen Activator, but Independent of Urokinase Receptor (uPAR, CD87)

The 55-kD urokinase (uPA) receptor (uPAR, CD87) is capable of binding uPA and may be involved in regulating cell-associated plasminogen activation and pericellular proteolysis. While investigating the relationship between uPAR levels and plasmin generation, we found that uPA-catalyzed plasminogen activation is stimulated by cells which do not express uPAR. This uPAR-independent mechanism appears to be at least as effective in vitro as uPAR-dependent stimulation, such that stimulation on the order of 30-fold was observed, resulting from improvements in both apparent kcat and apparent Km. The mechanism depends on simultaneous binding of both uPA and plasminogen to the cell and requires the presence of the amino-terminal fragment (ATF), available in single chain and two chain high-molecular-weight uPA, but not low-molecular-weight uPA. Stimulation was observed in all leukemic cell lines investigated at similar optimum concentrations of 106to 107 cells/mL and may be more general. A mechanism is proposed whereby uPA can associate with binding sites on the cell surface of lower affinity, but higher capacity than uPAR, but these are sufficient to stimulate plasmin generation even at subphysiologic uPA concentrations. This mechanism is likely to operate under conditions commonly used for in vitro studies and may have some significance in vivo.

Characterization of Cell-Associated Plasminogen Activation Catalyzed by Urokinase-Type Plasminogen Activator, but Independent of Urokinase Receptor (uPAR, CD87)

The 55-kD urokinase (uPA) receptor (uPAR, CD87) is capable of binding uPA and may be involved in regulating cell-associated plasminogen activation and pericellular proteolysis. While investigating the relationship between uPAR levels and plasmin generation, we found that uPA-catalyzed plasminogen activation is stimulated by cells which do not express uPAR. This uPAR-independent mechanism appears to be at least as effective in vitro as uPAR-dependent stimulation, such that stimulation on the order of 30-fold was observed, resulting from improvements in both apparent kcat and apparent Km. The mechanism depends on simultaneous binding of both uPA and plasminogen to the cell and requires the presence of the amino-terminal fragment (ATF), available in single chain and two chain high-molecular-weight uPA, but not low-molecular-weight uPA. Stimulation was observed in all leukemic cell lines investigated at similar optimum concentrations of 106to 107 cells/mL and may be more general. A mechanism is proposed whereby uPA can associate with binding sites on the cell surface of lower affinity, but higher capacity than uPAR, but these are sufficient to stimulate plasmin generation even at subphysiologic uPA concentrations. This mechanism is likely to operate under conditions commonly used for in vitro studies and may have some significance in vivo.

Regulation of tissue plasminogen activator activity by cells. Domains responsible for binding and mechanism of stimulation

A number of cell types have previously been shown to bind tissue plasminogen activator (tPA), which in some cases can remain active on the cell surface resulting in enhanced plasminogen activation kinetics. We have investigated several cultured cell lines, U937, THP1, K562, Molt4, and Nalm6 and shown that they bind both tPA and plasminogen and are able to act as promoters of plasminogen activation in kinetic assays. To understand what structural features of tPA are involved in cell surface interactions, we performed kinetic assays with a range of tPA domain deletion mutants consisting of full-length glycosylated and nonglycosylated tPA (F-G-K1-K2-P), DeltaFtPA (G-K1-K2-P), K2-P tPA (BM 06.022 or Reteplase), and protease domain (P). Deletion variants were made in Escherichia coli and were nonglycosylated. Plasminogen activation rates were compared with and without cells, over a range of cell densities at physiological tPA concentrations, and produced maximum levels of stimulation up to 80-fold with full-length, glycosylated tPA. Stimulation for nonglycosylated full-length tPA dropped to 45-60% of this value. Loss of N-terminal domains as in DeltaFtPA and K2P resulted in a further loss of stimulation to 15-30% of the full-length glycosylated value. The protease domain alone was stimulated at very low levels of up to 2-fold. Thus, a number of different sites are involved in cell interactions especially within finger and kringle domains, which is similar to the regulation of tPA activity by fibrin. A model was developed to explain the mechanism of stimulation and compared with actual data collected with varying cell, plasminogen, or tPA concentrations and different tPA variants. Experimental data and model predictions were generally in good agreement and suggest that stimulation is well explained by the concentration of reactants by cells.

Regulation of single chain urokinase by small peptides

Whether single chain urokinase (scuPA) expresses intrinsic enzymatic activity continues to be a subject of controversy. We report that the activity of scuPA is enhanced by a small plasmin substrate, H-D-valyl-L-leucyl-L-lysine-p-nitroanilide diacetate (D-VLK-p), but not by a second plasmin substrate, H-D-norleucyl-hexahydrotyrosyl-lysine-p-nitroanilide diacetate (*L*YK-p). D-VLK-p had no effect on the activity of a plasmin insensitive scuPA variant (scuPA-glu158) indicating that native scuPA can be cleaved by plasmin even at saturating concentrations of D-VLK-P. In contrast, D-VLK-P inhibited the activity of the native and scuPA-glu158 complexed with soluble urokinase receptor. Further, D-VLK-p stimulated the enzymatic activity of low molecular weight scuPA (amino acids 144-410) suggesting that D-VLK-P interacts with a second, previously undescribed regulatory site in scuPA.

Characterization of single chain urokinase-type plasminogen activator with a novel amino-acid substitution in the kringle structure

ECV304 is a cell line established by a spontaneous transformation of endothelial cells of a human umbilical vein. It was shown that ECV304 secretes single chain urokinase-type plasminogen activator (scu-PA). A subclone, ECV304 clone 15, was obtained by acclimatization of parental clone to serum-free medium followed by limiting dilution. The clone was found to produce approximately five times as much scu-PA (approximately 20 IU/10(6) cells per day) as the parental clone after a 40 days' culture. Though the biochemical characteristics of the purified scu-PA were indistinguishable from those of the native scu-PA, it had a lower affinity for fibrin clots under the employed conditions. Molecular cloning of a cDNA encoding the scu-PA has identified a novel substitution from C to T in the nucleotide sequence encoding the kringle structure. The substitution resulted in an alteration from Pro (CCG) to Leu (CTG) at amino-acid position 121, which may be directly or indirectly involved in the decrease in the apparent affinity.

Analogs of human plasminogen that are labeled with fluorescence probes at the catalytic site of the zymogen. Preparation, characterization, and interaction with streptokinase

Fluorescent analogs of the proteinase zymogen, plasminogen (Pg), which are specifically inactivated and labeled at the catalytic site have been prepared and characterized as probes of the mechanisms of Pg activation. The active site induced non-proteolytically in Pg by streptokinase (SK) was inactivated stoichiometrically with the thioester peptide chloromethyl ketone. N alpha-[(acetylthio)acetyl]-(D-Phe)-Phe-Arg-CH2Cl; the thiol group generated subsequently on the incorporated inhibitor with NH2OH was quantitatively labeled with the fluorescence probe, 2-((4'-iodoacetamido)anilino)naphthalene-6-sulfonic acid; and the labeled Pg was separated from SK. Cleavage of labeled [Glu]Pg1 by urokinase-type plasminogen activator (uPA) was accompanied by a fluorescence enhancement (delta Fmax/Fo) of 2.0, and formation of 1% plasmin (Pm) activity. Comparison of labeled and native [Glu]Pg1 as uPA substrates showed that activation of labeled [Glu]Pg1 generated [Glu]Pm1 as the major product, while native [Glu]Pg1 was activated at a faster rate and produced [Lys]Pm1 because of concurrent proteolysis by plasmin. When a mixture of labeled and native Pg was activated, to include plasmin-feedback reactions, the zymogens were activated at equivalent rates. The lack of potential proteolytic activity of the Pg derivatives allowed their interactions with SK to be studied under equilibrium binding conditions. SK bound to labeled [Glu]Pg1, and [Lys]Pg1 with dissociation constants of 590 +/- 110 and 110 and 11 +/- 7 nM, and fluorescence enhancements of 3.1 +/- 0.1 and 1.6 +/- 0.1, respectively. Characterization of the interaction of SK with native [Glu]Pg1 by the use of labeled [Glu]Pg1 as a probe indicated a approximately 6-fold higher affinity of SK for the native Pg zymogen compared to the labeled Pg analog. Saturating levels of epsilon-aminocaproic acid reduced the affinity of SK for labeled [Glu]Pg1 by approximately 2-fold and lowered the fluorescence enhancement to 1.8 +/- 0.1, whereas the affinity of SK for labeled [Lys]Pg1 was reduced by approximately 98-fold with little effect on the enhancement. These results demonstrate that occupation of lysine binding sites modulates the affinity of SK for Pg and the changes in the environment of the catalytic site associated with SK-induced conformational activation. Together, these studies show that the labeled Pg derivatives behave as analogs of native Pg which report functionally significant changes in the environment of the catalytic site of the zymogen.

The urokinase/urokinase-receptor system and cancer invasion

u-PA binds with high affinity to its specific GPI-anchored receptor on the cell surface. The binding has at least two important consequences: (1) it enhances the rate of plasminogen activation on the cell surface; and (2) it focuses the u-PA proteolytic activity at the leading front of migrating cells. Several recent findings suggest that surface-bound u-PA is essential for the invasive ability of tumour cells, even if a picture is emerging indicating a concerted action with other proteases, like collagenases and cathepsin B (Kobayashi et al, 1992; Ossowski, 1992; Schmitt et al, 1992; (Danø et al, 1994). In some tumours, e.g. colon, breast and skin cancer, in situ hybridization studies have given an insight into the u-PA/u-PAR tumour biology showing a complex interplay between stromal and cancer cells Danø et al, 1994). u-PA, u-PAR, and PAI-1 tumour content are now well established prognostic factor in breast cancer. This body of knowledge could be used for theurapeutic purposes. For example, a large study with 671 patients has allowed the identification of node-negative patients which, according to their u-PA levels, would need adjuvant therapy (Foekens et al, 1992). Many other tumours, especially colorectal cancer, expect a direct clinical evaluation of u-PA, u-PAR and serpins as prognostic factors.

Plasminogen activators and plasminogen activator inhibitors: biochemical aspects

Although this chapter does not represent a historical review, it will be clear how the biochemistry of t-PA, u-PA, PAI-1 and PAI-2 has evolved and where we stand in 1994. While the functional activities of the proteins were recognized at least three to four decades ago, highly purified preparations became available around 1980. In the mid-eighties the cDNAs of the proteins were cloned, representing a major breakthrough in the biochemistry of the four proteins. Amino acid sequences were derived from the nucleotide sequences, homologies with other proteins were recognized and larger amounts of (recombinant) proteins became available for research. In addition, mutant proteins were prepared by recombinant DNA technology, enabling investigation of structure-function relationships. This report is mainly based on the latter studies. Detailed information about three-dimensional structures of the proteins and the mode of interaction with other macromolecules is still lacking. To obtain this information will be the goal for biochemists in the coming years.

Plasminogen activation by human keratinocytes: molecular pathways and cell-biological consequences

Keratinocytes are the major cellular constituent of stratified epithelia. Defects in these epithelia are re-epithelialized by keratinocytes migrating from the edge of the defect into the wound. The cells form a monolayer with subsequent differentiation into a multilayered epithelium. It is thought that plasminogen activation by migrating keratinocytes is an important event during re-epithelialization. In the present report we summarize the studies on plasminogen activation by human keratinocytes in vitro and in vivo. Under the aspect of pericellular proteolysis the discussion is focused on the molecular mechanisms of plasminogen activation at the keratinocyte surface and on the cell-biological consequences of pericellular plasmin formation. We describe a cell surface-associated pathway of plasminogen activation which crucially depends on cell surface receptors for (pro)-uPA and plasmin(ogen). uPA bound to its receptor converts cell-bound plasminogen into the active protease plasmin. Compared to plasminogen activation in solution, activation at the keratinocyte cell surface is accelerated by a factor of approx. 7-10, and the plasmin generated and bound at the cell surface is protected against its specific inhibitor alpha 2-antiplasmin. Plasmin thus provided in the pericellular space leads to detachment of cultured keratinocytes from the growth substratum. Plasmin interferes with the adhesion of keratinocytes to fibrin, but not with the adhesion to collagen type I. By demonstrating that keratinocytes of the epithelial outgrowth in healing skin wounds express uPA and the uPA-R and that plasmin(ogen) is colocalized with uPA and/or uPA-R, indirect evidence is provided that this pathway may be operative in vivo. In view of previous findings that plasminogen activation is also observed under certain pathologic conditions in the epidermis, we conclude that plasminogen activation by keratinocytes is rather related to tissue damage and subsequent repair mechanisms than to a specific pathologic situation.