Generation and characterization of monoclonal antibodies against the N-terminus of alpha-2-antiplasmin

Around 70% of circulating alpha-2-antiplasmin (α2AP), the main natural plasmin inhibitor, is N-terminally cleaved between residues Pro12 and Asn13 by antiplasmin-cleaving enzyme. This converts native Met-α2AP into the more potent fibrinolysis inhibitor Asn-α2AP. The Arg6Trp (R6W) polymorphism affects the N-terminal cleavage rate of Met-α2AP in a purified system, with ~8-fold faster conversion of Met(R6)-α2AP than Met(W6)-α2AP. To date, assays to determine N-terminally intact Met-α2AP in plasma have been limited to an ELISA that only measures Met(R6)-α2AP. The aim of this study was to generate and characterize monoclonal antibodies (mAbs) against Met(R6)-α2AP, Met(W6)-α2AP and all α2AP forms (total-α2AP) in order to develop specific Met(R6)-α2AP and Met(W6)-α2AP ELISAs. Recombinant Met(R6)-α2AP, Met(W6)-α2AP and Asn-α2AP were expressed in Drosophila S2 cells. Using hybridoma technology, a panel of 25 mAbs was generated against a mixture of recombinant Met(R6)-α2AP and Met(W6)-α2AP. All mAbs were evaluated for their specific reactivity using the three recombinant α2APs in one-site non-competitive ELISAs. Three mAbs were selected to develop sandwich-type ELISAs. MA-AP37E2 and MA-AP34C4 were selected for their specific reactivity against Met(R6)-α2AP and Met(W6)-α2AP, respectively, and used for coating. MA-AP15D7 was selected for its reactivity against total-α2AP and used for detection. With the novel ELISAs we determined Met(R6)-α2AP and Met(W6)-α2AP levels in plasma samples and we showed that Met(R6)-α2AP was converted faster into Asn-α2AP than Met(W6)-α2AP in a plasma milieu. In conclusion, we developed two specific ELISAs for Met(R6)-α2AP and Met(W6)-α2AP, respectively, in plasma. This will enable us to determine N-terminal heterogeneity of α2AP in plasma samples.

The fibrinolysis inhibitor α2-antiplasmin restricts lymphatic remodelling and metastasis in a mouse model of cancer

Remodelling of lymphatic vessels in tumours facilitates metastasis to lymph nodes. The growth factors VEGF-C and VEGF-D are well known inducers of lymphatic remodelling and metastasis in cancer. They are initially produced as full-length proteins requiring proteolytic processing in order to bind VEGF receptors with high affinity and thereby promote lymphatic remodelling. The fibrinolytic protease plasmin promotes processing of VEGF-C and VEGF-D in vitro, but its role in processing them in cancer was unknown. Here we explore plasmin's role in proteolytically activating VEGF-D in vivo, and promoting lymphatic remodelling and metastasis in cancer, by co-expressing the plasmin inhibitor α₂-antiplasmin with VEGF-D in a mouse tumour model. We show that α₂-antiplasmin restricts activation of VEGF-D, enlargement of intra-tumoural lymphatics and occurrence of lymph node metastasis. Our findings indicate that the fibrinolytic system influences lymphatic remodelling in tumours which is consistent with previous clinicopathological observations correlating fibrinolytic components with cancer metastasis.

Plasmin and regulators of plasmin activity control the migratory capacity and adhesion of human T cells and dendritic cells by regulating cleavage of the chemokine CCL21

The homeostatic chemokine CCL21 has a pivotal role in lymphocyte homing and compartment localisation within the lymph node, and also affects adhesion between immune cells. The effects of CCL21 are modulated by its mode of presentation, with different cellular responses seen for surface-bound and soluble forms. Here we show that plasmin cleaves surface-bound CCL21 to release the C-terminal peptide responsible for CCL21 binding to glycosaminoglycans on the extracellular matrix and cell surfaces, thereby generating the soluble form. Loss of this anchoring peptide enabled the chemotactic activity of CCL21 and reduced cell tethering. Tissue plasminogen activator did not cleave CCL21 directly but enhanced CCL21 processing through generation of plasmin from plasminogen. The tissue plasminogen activator inhibitor neuroserpin prevented processing of CCL21 and blocked the effects of soluble CCL21 on cell migration. Similarly, the plasmin-specific inhibitor α2-antiplasmin inhibited CCL21-mediated migration of human T cells and dendritic cells and tethering of T cells to APCs. We conclude that the plasmin system proteins plasmin, tissue plasminogen activator and neuroserpin regulate CCL21 function in the immune system by controlling the balance of matrix- and cell-bound CCL21.

Plasminogen activation induced pericellular fibronectin proteolysis promotes fibroblast apoptosis

Apoptosis of fibroblasts/myofibroblasts is a critical event in the resolution of tissue repair responses; however, mechanisms for the regulation of (myo)fibroblast apoptosis/survival remain unclear. In this study, we demonstrate counter-regulatory interactions between the plasminogen activation system and transforming growth factor-beta1 (TGF-beta1) in the control of fibroblast apoptosis. Plasmin treatment induced fibroblast apoptosis in a time- and dose-dependent manner in association with proteolytic degradation of extracellular matrix proteins, as detected by the release of soluble fibronectin peptides. Plasminogen, which was activated to plasmin by fibroblasts, also induced fibronectin proteolysis and fibroblast apoptosis, both of which were blocked by alpha2-antiplasmin but not by inhibition of matrix metalloproteinase activity. TGF-beta1 protected fibroblasts from apoptosis induced by plasminogen but not from apoptosis induced by exogenous plasmin. The protection from plasminogen-induced apoptosis conferred by TGF-beta1 is associated with the up-regulation of plasminogen activator-1 (PAI-1) expression and inhibition of plasminogen activation. Moreover, lung fibroblasts from mice genetically deficient in PAI-1 lose the protective effect of TGF-beta1 against plasminogen-induced apoptosis. These findings support a novel role for the plasminogen activation system in the regulation of fibroblast apoptosis and a potential role of TGF-beta1/PAI-1 in promoting (myo)fibroblast survival in chronic fibrotic disorders.

Complete inhibition of fibrinolysis by sustained carboxypeptidase B activity: the role and requirement of plasmin inhibitors

BACKGROUND

The antifibrinolytic effect of activated thrombin-activatable fibrinolysis inhibitor (TAFIa) and carboxypeptidase B (CPB) displays threshold behavior. When CPB was used to simulate conditions mimicking continuous TAFIa activity, it affected the lysis of plasma clots differently to clots formed from a minimal fibrinolytic system comprising fibrinogen, plasminogen and alpha(2)-antiplasmin. Whereas CPB saturably prolonged clot lysis in the purified system, the effect of CPB did not appear saturable in plasma clots.

METHODS

To rationalize this difference, we investigated the effects of alpha(2)-antiplasmin, alpha(2)-macroglobulin, antithrombin and aprotinin on CPB-mediated antifibrinolysis.

RESULTS

CPB alone prolonged fibrinolysis in a saturable manner and the efficacy of CPB increased with decreasing tissue-type plasminogen activator (t-PA) concentration. The inhibitors by themselves did not halt fibrinolysis and the potency of each inhibitor in the absence of CPB mirrored their solution-phase plasmin inhibitory potentials: alpha(2)-antiplasmin approximately equal to aprotinin >> alpha(2)-macroglobulin >> antithrombin. With both CPB and inhibitor present, a synergistic effect was observed. The antifibrinolytic sensitivity to CPB was related to the plasmin inhibitory potential of the inhibitor.

CONCLUSIONS

Fibrinolysis could be completely inhibited by alpha(2)-antiplasmin, alpha(2)-macroglobulin and antithrombin, but not aprotinin, in the presence of CPB, and occurred only when the irreversible inhibitor or pool of inhibitors were in excess of plasminogen. Western blot analysis indicated that the CPB-mediated shutdown of fibrinolysis was a result of plasminogen consumption prior to clot lysis. The CPB concentration required for fibrinolytic shutdown was dependent on t-PA concentration and the inhibitory potential of the irreversible inhibitor pool.

The relative kinetics of clotting and lysis provide a biochemical rationale for the correlation between elevated fibrinogen and cardiovascular disease

BACKGROUND

Elevated plasma fibrinogen is a well known risk factor for cardiovascular disease. The mechanistic rationale for this is not known.

OBJECTIVES

These studies were carried out to determine the fibrinogen concentration dependencies of clotting and lysis times and thereby determine whether these times rationalize the correlation between an increased risk of cardiovascular disease and elevated plasma fibrinogen.

METHODS

The time courses of clot formation and lysis were measured by turbidity in systems comprising a) fibrinogen, thrombin and plasmin, or b) fibrinogen, thrombin, plasminogen and t-PA, or c) plasma, thrombin and t-PA. From the lysis times, k(cat) and K(m) values for plasmin action on fibrin were determined.

RESULTS

The time to clot increased linearly from 2.9 to 5.6 minutes as the fibrinogen concentration increased from 1 to 9 microM and did not increase further as the fibrinogen concentration was raised to 20 microM. In contrast, the clot lysis time increased linearly over the input fibrinogen concentration range of 2 to 20 microM. A similar linear trend was found in the two systems with t-PA and plasminogen. Apparent K(m) and k(cat) values for plasmin were 1.1 +/- 0.6 microM and 28 +/- 2 min(-1), respectively. K(m) values for plasmin in experiments initiated with t-PA and plasminogen were 1.6 +/- 0.2 microM in the purified system and 2.1 +/- 0.9 microM in plasma.

CONCLUSION

As the concentration of fibrinogen increases, especially above physiologic level, the balance between fibrinolysis and clotting shifts toward the latter, providing a rationale for the increased risk of cardiovascular disease associated with elevated fibrinogen.

Enzymatic properties of human kallikrein-related peptidase 12 (KLK12)

Human kallikrein-related peptidase 12 (KLK12) is a new member of the human tissue kallikrein family. Preliminary studies suggest that KLK12 is differentially expressed in breast cancer and may have potential use as a cancer biomarker. It has been predicted that KLK12 is a secreted serine protease. However, the enzymatic properties of this protein have not been reported so far. Here, we report the production of recombinant KLK12 and analyses of its enzymatic characteristics, including zymogen activation, substrate specificity, and regulation of its activity. KLK12 is secreted as an inactive pro-enzyme, which is able to autoactivate to gain enzymatic activity. Through screening of a panel of fluorogenic and chromogenic peptide substrates, we establish that active KLK12 possesses trypsin-like activity, cleaving peptide bonds after both arginine and lysine. Active KLK12 quickly loses its activity due to autodegradation, and its activity can also be rapidly inhibited by zinc ions and by alpha2-antiplasmin through covalent complex formation. Furthermore, we demonstrate that KLK12 is able to activate KLK11 zymogen in vitro. Our results indicate that KLK12 may participate in enzymatic cascades involving other kallikreins.

Blood inhibitory capacity toward exogenous plasmin

Stabilized, active plasmin is a novel thrombolytic for direct delivery to clots. Although it is known that protease inhibitors in plasma inhibit plasmin, the amount of plasmin that can be added to plasma/blood before free plasmin is observed is not clear. Determination of free plasmin activity in plasma using chromogenic substrates represents a challenge due to false-positive signals from plasmin entrapped by alpha2-macroglobulin. Size-exclusion chromatography was used to separate the plasmin-alpha2-macroglobulin complex from uninhibited, free plasmin. In this in-vitro study, exogenous plasmin is effectively inhibited up to 2.4 micromol/l after 5-min incubation with plasma at 37 degrees C. Initially, plasmin was consumed predominantly by alpha2-antiplasmin up to 1.2 micromol/l plasmin. Following exhaustion of alpha2-antiplasmin, plasmin was further consumed by alpha2-macroglobulin up to 2.4 micromol/l plasmin added to human plasma. Whole human blood was found to have an increased inhibitory capacity over that of plasma; free plasmin activity could be measured only above 3.8 micromol/l added plasmin. In conclusion, several mechanisms exist that control plasmin activity in human blood; in addition to alpha2-antiplasmin and alpha2-macroglobulin, blood cells contribute to the inhibition of exogenously administered plasmin. These in-vitro results indicate that doses of plasmin up to approximately 12 mg/kg in humans can be completely inactivated by blood.

Quantification of the effects of thrombin activatable fibrinolysis inhibitor and α2-antiplasmin on fibrinolysis in normal human plasma

Two major proteins that inhibit fibrinolysis include thrombin activatable fibrinolysis inhibitor (TAFI) and alpha2-antiplasmin. Our goal was to quantify the contribution of TAFI and alpha2-antiplasmin to antifibrinolytic defenses with thrombelastography. Plasma activated with tissue factor/kaolin was subjected to fibrinolysis with tissue-type plasminogen activator (100 U/ml). Prior to activation, TAFI activity was inhibited with either potato carboxypeptidase inhibitor (25 microg/ml) or an anti-TAFI antibody, and alpha2-antiplasmin activity was inhibited with an anti-alpha2-antiplasmin antibody. Data were collected for 30 min, with the time of onset and rate of fibrinolysis determined. Compared with uninhibited samples, TAFI inhibition significantly (P < 0.05) decreased the time of onset of fibrinolysis by 70% and increased the rate of lysis by 70%. There was no difference between potato carboxypeptidase inhibitor and anti-TAFI antibody inhibition. Inhibition of alpha2-antiplasmin resulted in a significantly (P < 0.05) decreased time of onset (85%) and increased the rate of lysis (557%) compared with uninhibited samples. Inhibition of alpha2-antiplasmin activity resulted in a significantly (P < 0.05) greater fibrinolytic response than TAFI inhibition. In conclusion, utilization of standard inhibitors and thrombelastography permitted quantification of the effects of TAFI and alpha2-antiplasmin on fibrinolysis in plasma. Future investigation of diseases involving hypofibrinolysis (e.g. left ventricular assist devices) could be conducted using this assay system.

Urokinase plasminogen activator released by alveolar epithelial cells modulates alveolar epithelial repair in vitro

Intra-alveolar fibrin is formed following lung injury and inflammation and may contribute to the development of pulmonary fibrosis. Fibrin turnover is altered in patients with pulmonary fibrosis, resulting in intra-alveolar fibrin accumulation, mainly due to decreased fibrinolysis. Alveolar type II epithelial cells (AEC) repair the injured alveolar epithelium by migrating over the provisional fibrin matrix. We hypothesized that repairing alveolar epithelial cells modulate the underlying fibrin matrix by release of fibrinolytic activity, and that the degree of fibrinolysis modulates alveolar epithelial repair on fibrin. To test this hypothesis we studied alveolar epithelial wound repair in vitro using a modified epithelial wound repair model with human A549 alveolar epithelial cells cultured on a fibrin matrix. In presence of the inflammatory cytokine interleukin-1beta, wounds increase by 800% in 24 hours mainly due to detachment of the cells, whereas in serum-free medium wound areas decreases by 22.4 +/- 5.2% (p < 0.01). Increased levels of D-dimer, FDP and uPA in the cell supernatant of IL-1beta-stimulated A549 epithelial cells indicate activation of fibrinolysis by activation of the plasmin system. In presence of low concentrations of fibrinolysis inhibitors, including specific blocking anti-uPA antibodies, alveolar epithelial repair in vitro was improved, whereas in presence of high concentrations of fibrinolysis inhibitors, a decrease was observed mainly due to decreased spreading and migration of cells. These findings suggest the existence of a fibrinolytic optimum at which alveolar epithelial repair in vitro is most efficient. In conclusion, uPA released by AEC alters alveolar epithelial repair in vitro by modulating the underlying fibrin matrix.

Plasminogen activation: a mediator of vascular smooth muscle cell apoptosis in atherosclerotic plaques

BACKGROUND

Apoptosis of vascular cells is considered to be a major determinant of atherosclerotic plaque vulnerability and potential rupture. Plasmin can be generated in atherosclerotic plaques and recent in vitro data suggest that plasminogen activation may trigger vascular smooth muscle cell (VSMC) apoptosis.

AIM

To determine whether plasminogen activation may induce aortic VSMC apoptosis ex vivo and in vivo.

METHODS AND RESULTS

Mice with single or combined deficiencies of apolipoprotein E (ApoE) and plasminogen activator inhibitor-1 (PAI-1) were used. Ex vivo incubation with plasminogen of isolated aortic tunica media from PAI-1-deficient mice induced plasminogen activation and VSMC apoptosis, which was inhibited by alpha2-antiplasmin. In vivo, levels of plasmin, active caspase 3 and VSMC apoptotic index were significantly higher in atherosclerotic aortas from mice with combined ApoE and PAI-1 deficiencies than in those from littermates with single ApoE deficiency. A parallel decrease in VSMC density was observed.

CONCLUSIONS

These data strongly suggest that plasminogen activation may contribute to VSMC apoptosis in atherosclerotic plaques.

Interaction of a plasminogen activator proteinase, LV-PA with human α2-macroglobulin

Lachesis venom plasminogen activator (LV-PA) is a 33-kDa serine proteinase isolated from bushmaster (Lachesis muta muta) snake venom, which activates the fibrinolytic system in vitro. This study has examined the effect of the plasma proteinase inhibitor alpha2-macroglobulin (alpha2-M) towards LV-PA and compares it with the effect on tissue type plasminogen activator (t-PA). The proteolytic activity of LV-PA alone or previously incubated with human plasminogen (Plg) on the large molecular mass protein substrates, dimethylcasein (DMC) and fibrinogen (Fg) was completely inhibited by human alpha2-M. However, the synthetic peptides Tos-Gly-Pro-Lys-pNA and H-D-Pro-Phe-Arg-pNA (S-2302) were hydrolyzed with almost no reduction in rate. At pH 7.4 and 37 degrees C the proteinase (0.15 microM over 15 min) interacted with alpha2-M, and each mole of alpha2-M bound 2 mol of enzyme. Sodium dodecyl sulfate gel electrophoresis of reduced samples showed that the interaction of alpha2-M with either LV-PA or t-PA preincubated with Plg resulted in the formation of approximately 90 kDa fragments and high molecular mass complexes (Mr 180 kDa), generated by the incubation mixture (LV-PA or t-PA) and Plg. The data suggest that LV-PA is a direct-type PA and its fibrinolytic effect can be reduced by alpha2-M in vivo.

The kinetics of plasmin inhibition by aprotinin in vivo

INTRODUCTION

The purpose of this study was to estimate, in patients undergoing cardiopulmonary bypass (CPB), the in vivo rates of tissue plasminogen activator (tPA) and plasminogen activator inhibitor 1 (PAI-1) secretion, plasmin generation, fibrin degradation, and plasmin inhibition by aprotinin versus antiplasmin.

MATERIALS AND METHODS

Estimates of in vivo rates were based on measured levels of tPA, PAI-1, antiplasmin, plasmin-antiplasmin complex (PAP), total aprotinin, plasmin-aprotinin complex and D-dimer, combined with a computer model of each patient's vascular system that continuously accounted for secretion, clearance, hemodilution, blood loss and transfusion. Plasmin regulation was studied in nine control patients undergoing CPB without aprotinin versus six patients treated with aprotinin.

RESULTS

In controls, plasmin-antiplasmin levels rose from a baseline of 3.0+/-0.9 to a peak of 8.1+/-2.7 nmol/L after CPB due to an average 44-fold rise in the plasmin generation rate. This rise in plasmin generation during CPB lead to increased fibrin degradation causing D-dimer levels to increase from a baseline of 1.2+/-0.6 to a peak of 9.7+/-4.4 nmol/L due to an average 74-fold rise in the D-dimer generation rate. During CPB in the aprotinin group, plasmin-antiplasmin levels dropped, plasmin-aprotinin complex levels rose, while D-dimer levels remained unchanged from baseline. Compared to controls, the aprotinin group showed similar rates of plasmin generation during CPB, but an 11-fold faster plasmin inhibition rate and a 10-fold lower D-dimer generation rate.

CONCLUSIONS

The rise in plasmin generation and fibrin degradation that occurs during standard CPB is suppressed by the addition of aprotinin, which returns the patient to near baseline fibrin degradation rates during CPB.

Reduction of excitotoxicity and associated leukocyte recruitment by a broad‐spectrum matrix metalloproteinase inhibitor

An important step in the cascade leading to neuronal cell death is degradation of laminin and other components of the brain extracellular matrix by microglia-derived proteases. Excitotoxic cell death of murine hippocampal neurones in vivo can be prevented by inhibitors of tissue plasminogen activator (tPA) or by inhibitors of plasmin. Plasmin is a potent activator of the matrix metalloproteinases (MMPs), which are made by resident and recruited leukocytes following CNS injury. In this study, we show, using Taqman RT-PCR, that MMP mRNAs, but not other calcium-dependent proteases such as calpain mRNAs, are acutely up-regulated after an excitotoxic challenge in vivo. alpha(2)-antiplasmin or BB-3103, a broad-spectrum inhibitor of the MMPs, co-injected with kainic acid into the striatum, inhibits excitotoxic cell death in the rat striatum, and reduces both the number of recruited macrophages and the size of the lesion. We also show that leukocyte populations differentially express MMPs, which may account, in part, for the expression profile we observe in the challenged brain. Our results show that inhibition of the MMPs in the rat will prevent kainic acid-induced cell death in the brain. These studies suggest that MMP inhibitors have therapeutic potential for use in stroke, and support the increasing evidence that microglial activation may contribute to neuronal cell death.

Inhibition of human pancreatic proteinases by human plasma α2-antiplasmin and antithrombin

Human plasma alpha1-antitrypsin inhibits human pancreatic trypsin, chymotrypsin and elastase, which are massively released into the blood stream during acute pancreatitis. To examine whether the plasma proteins of individuals with genetic deficiency of alpha1-antitrypsin are protected against the deleterious action of these enzymes by other inhibitors, we have tested their inhibition by alpha2-antiplasmin and antithrombin. We have determined the inhibition rate constants kass and calculated d(t), the in vivo inhibition time. Surprisingly, trypsin is inhibited faster by alpha2-antiplasmin [kass=2.5 x 10(6) M(-1)S(-1), d(t)=2.3 s] and antithrombin [kass=1.7 x 10(5) M(-1)s(-1), d(t)=5.8 s] than by alpha1-antitrypsin [d(t)=17 s or 116 s in alpha1-antitrypsin-sufficient or alpha1-antitrypsin-deficient individuals, respectively]. Low molecular weight heparin accelerates the inhibition of trypsin by antithrombin by a factor of 16 [d(t)=0.36 s]. Antithrombin and alpha2-antiplasmin are not physiological inhibitors of chymotrypsin and elastase. These enzymes are, however, physiologically inhibited by alpha1-antitrypsin and alpha1-antichymotrypsin even in alpha1-antitrypsin-deficient individuals. We conclude that (i) low molecular weight heparin may be helpful in the management of acute pancreatitis, and (ii) genetically determined alpha1-antitrypsin deficiency probably does not lead to a significantly increased risk of plasma protein degradation during this disease.

The effects of serine proteinase inhibitors on bone resorption in vitro

The aims of this study were to identify the role and sites of action of serine proteinases (SPs) in bone resorption, a process which involves a cascade of events, the central step of which is the removal of bone matrix by osteoclasts (OCs). This resorbing activity, however, is also determined by recruitment of new OCs to future resorption sites and removal of the osteoid layer by osteoblasts (OBs), which enables OCs to gain access to the underlying mineralized bone. The resorption systems we have studied consisted of (i) neonatal calvarial explants, (ii) isolated OCs cultured on ivory slices, (iii) mouse OBs cultured on either radiolabelled type I collagen films or bone-like matrix, (iv) bone marrow cultures to assess OC formation and (v) 17-day-old fetal mouse metatarsal bone rudiments to assess OC migration and fusion. Two separate SP inhibitors, aprotinin and alpha(2)-antiplasmin dose-dependently inhibited (45)Ca release from neonatal calvarial explants: aprotinin (10(-6) M) was the most effective SP inhibitor, producing a maximum inhibitory effect of 55.9%. Neither of the SP inhibitors influenced either OC formation or OC resorptive activity. In contrast, each SP inhibitor dose-dependently inhibited OB-mediated degradation of both type I collagen fibrils and non-mineralized bone matrix. In 17-day-old metatarsal explants aprotinin produced a 55% reduction in the migration of OCs from the periosteum to the mineralized matrix after 3 days in culture but after 6 days in culture aprotinin was without effect on OC migration. Primary mouse osteoblasts expressed mRNA for urokinase type plasminogen activator (uPA), tIssue type plasminogen activator (tPA), the type I receptor for uPA, plasminogen activator inhibitor types I and II and the broad spectrum serine proteinase inhibitor, protease nexin I. In situ hybridization demonstrated expression of tPA and uPA in osteoclasts disaggregated from 6-day-old mouse long bones. We propose that the regulation of these various enzyme systems within bone tIssue determines the sites where bone resorption will be initiated.

Inhibition of plasminogen activation protects against ganglion cell loss in a mouse model of retinal damage

PURPOSE

The mechanisms that trigger ganglion cell death in ischemic retinal diseases are not clearly understood. Using a mouse optic nerve ligation model, the objective of this study was to test the hypothesis that extracellular matrix (ECM) modulating plasminogen activators (PAs) potentiate ganglion cell loss.

METHODS

Optic nerve ligation was performed to initiate ganglion cell loss in the retina. Urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA) activity in retinal extracts was determined by plasminogen/fibrinogen zymography. Immunostaining and western blot analysis was performed to detect uPA and tPA proteins. Plasmin activity was determined by casein gel-zymography. Plasminogen and plasmin proteins were detected and quantified by western blotting. Morphology was assessed using hematoxylin and eosin stained retinal cross sections, and programmed cell death was monitored by an apoptotic assay. Laminin degradation in retinal extracts was assessed by western blot analysis.

RESULTS

Optic nerve ligation led to a transient increase in uPA and plasmin proteolytic activity in the retina. Urokinase inhibitor, amiloride, blocked uPA activity in retinal extracts. We found a correlation between the increased uPA activity, and conversion of zymogen plasminogen to active plasmin in retinal extracts with laminin degradation in the retina and apoptosis of ganglion cells. We found that by adding exogenous plasmin, in vitro, laminin present in control retinal extracts could be degraded in similar fashion. In addition, uPA or tPA failed to degrade laminin in control retinal extracts unless plasminogen was added, indicating that plasminogen activation is necessary for laminin degradation, in vitro. After intravitreal injection of plasmin inhibitor, alpha-2 antiplasmin, we found a significant protection against optic nerve ligation-induced ganglion cell loss.

CONCLUSIONS

Optic nerve ligation-induced plasmin(ogen) activation that precedes ganglion cell loss suggest that specific targeting of plasmin activity may have therapeutic potential in preventing ganglion cell loss in retinal diseases.

Influence of soluble fibrin on reaction kinetics of plasmin type 1 and type 2 with alpha2-antiplasmin

This study investigates, by slow binding kinetics methods, reaction kinetics of both plasmin types 1 and 2 with alpha -antiplasmin in the presence of increasing concentrations of either epsilon-amino-caproic acid (EACA) or soluble fibrin. All curves of plasmin-alpha -antiplasmin interaction followed the same pattern, indicating reversible slow binding inhibition with an initial loose complex and a following tight complex. Without soluble fibrin or EACA, differences between plasmin types 1 and 2 could be seen in the initial loose complex formation. The presence of increasing concentrations of EACA slowed down the first step of the reaction (without any effect on the second step), resulting in increasing values for K. Plasmin type 1 demonstrated a steep slope of K at an EACA concentration of 1 mmol/l. In plasmin type 2, the increase of K started at higher EACA concentrations. The value for K at a high EACA concentration (100 mmol/l) was the same for both plasmin types. In contrast to EACA, increasing concentrations of soluble fibrin slowed down both reaction steps. At high concentrations of soluble fibrin, the inhibitory effect of alpha -antiplasmin was almost completely abolished. Our data demonstrate that the effect of soluble fibrin and the lysine analogue EACA on plasmin-antiplasmin reactions are different and that the use of lysine analogues does not mimic fibrin in laboratory analyses of plasmin inhibition. In addition, our data indicate theoretical differences between plasmin type 1 and plasmin type 2, when used for local thrombolytic therapy.(2) (2) (i initial) (i initial) (i initial) (i initial) (2)

Different inhibitors of plasmin differentially affect angiostatin production and angiogenesis

Plasmin is a broad-spectrum serine proteinase, which is presumed to cleave many extracellular proteins and affect angiogenesis. In the present work, we studied the effect of two different inhibitors of plasmin (epsilon-aminocaproic and alpha(2)-antiplasmin) on angiogenesis in vivo using the chicken embryo chorioallantoic membrane assay, and in vitro using human umbilical vein endothelial cells. Epsilon-aminocaproic acid inhibited, while alpha(2)-antiplasmin induced, angiogenesis, as well as human umbilical vein endothelial cell proliferation, migration and tube formation on matrigel in a dose-dependent manner. Since plasmin has been implicated in the production of angiostatin, we studied the effect of the two plasmin inhibitors on angiostatin protein amounts in the chicken embryo chorioallantoic membrane. In this tissue, the 38- and 45-kDa isoforms of angiostatin are differentially affected by the two inhibitors: epsilon-aminocaproic acid increased, while alpha(2)-antiplasmin decreased the amounts of both isoforms. These data suggest that plasmin may have an antiangiogenic role in vivo through generation of angiostatin. Moreover, plasmin inhibitors differentially affect in vivo angiogenesis, depending on the mechanism by which they inhibit plasmin activity.

[Modifying effect of antiplasminogen monoclonal antibody IV-1c on human plasmin catalytic properties]

Antiplasminogen monoclonal antibody IV-1c (IV-1c) binds to Val 709-Gly 718 site of plasminogen (Pg) protease domain, which is far removed from the active site. Pg-IV-1c complex formation induces catalytic activity in proenzymes active site. Influence of IV-1c binding to plasmin (Pm) on Pm catalytic properties has not been investigated yet. Data on catalytic properties of Pm in equimolar Pm-IV-1c complex are presented. It was found that Pm and mini-Pm amidolytic and caseinolytic activity was twice as high as in Pm-IV-1c and mini-Pm-IV-1c complexes. 20 mM 6-AHA and 100 mM arginine did not influence this rise. The increase of amidolytic activity is connected with reduction of K(m) of S 2251 hydrolysis reaction for Pm and mini-Pm from 0.125 and 0.43 to 0.05 and 0.23 mM, correspondingly. Kcat remains almost the same. Fibrinolytic and fibrinogenolytic activity of Pm in Pm-IV-1c complex decreased to 20% of initial value alpha 2-Antiplasmin inhibited Pm activity in complex Pm-IV-1c by 80%. Pm-IV-1c complex did not activate free Pg, but activated equimolar Pg-IV-1c complex. Affinity of IV-1c to Pm and Pg was the same as C50 approximately 1.5 nM. Binding of Pm with IV-1c in a complex: a) leads to increase of Pm active site affinity to LMW substrates; b) causes steric hindrances for fibrin/fibrinogen access to Pm active site; c) proceeds with the same affinity for Pm and Pg, that indicates to invariable Val 709-Gly 718 site conformation after Pg transition in Pm.

Enhanced activation of bound plasminogen on Staphylococcus aureus by staphylokinase

Activation of plasminogen (plg) to plasmin by the staphylococcal activator, staphylokinase (SAK), is effectively regulated by the circulating inhibitor, alpha2-antiplasmin (alpha2AP). Here it is demonstrated that intact Staphylococcus aureus cells and solubilized staphylococcal cell wall proteins not only protected SAK-promoted plg activation against the inhibitory effect of alpha2AP but also enhanced the activation. The findings suggest that the surface-associated plg activation by SAK may have an important physiological function in helping staphylococci in tissue dissemination. Amino acid sequencing of tryptic peptides originating from the 59-, 56- and 43-kDa proteins, isolated as putative plg-binding proteins, identified them as staphylococcal inosine 5'-monophosphate dehydrogenase, alpha-enolase, and ribonucleotide reductase subunit 2, respectively.

Plasmin activity is required for myogenesis in vitro and skeletal muscle regeneration in vivo

Plasmin, the primary fibrinolytic enzyme, has a broad substrate spectrum and is implicated in biologic processes dependent upon proteolytic activity, such as tissue remodeling and cell migration. Active plasmin is generated from proteolytic cleavage of the zymogen plasminogen (Plg) by urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA). Here, we have investigated the role of plasmin in C2C12 myoblast fusion and differentiation in vitro, as well as in skeletal muscle regeneration in vivo, in wild-type and Plg-deficient mice. Wild-type mice completely repaired experimentally damaged skeletal muscle. In contrast, Plg(-/-) mice presented a severe regeneration defect with decreased recruitment of blood-derived monocytes and lymphocytes to the site of injury and persistent myotube degeneration. In addition, Plg-deficient mice accumulated fibrin in the degenerating muscle fibers; however, fibrinogen depletion of Plg-deficient mice resulted in a correction of the muscular regeneration defect. Because we found that uPA, but not tPA, was induced in skeletal muscle regeneration, and persistent fibrin deposition was also reproducible in uPA-deficient mice following injury, we propose that fibrinolysis by uPA-dependent plasmin activity plays a fundamental role in skeletal muscle regeneration. In summary, we identify plasmin as a critical component of the mammalian skeletal muscle regeneration process, possibly by preventing intramuscular fibrin accumulation and by contributing to the adequate inflammatory response after injury. Finally, we found that inhibition of plasmin activity with alpha2-antiplasmin resulted in decreased myoblast fusion and differentiation in vitro. Altogether, these studies demonstrate the requirement of plasmin during myogenesis in vitro and muscle regeneration in vivo.

Plasmin-independent gelatinase B (matrix metalloproteinase-9) release by monocytes under the influence of urokinase

It is shown that the release of matrix metalloproteinase-9 (gelatinase B) by THP-1 and U937 cells into conditioned media is increased under the action of recombinant single-chain urokinase. This effect is not accompanied by proteolytic activation of gelatinase B and is related to release of a pro-form of the enzyme. The action of urokinase on monocytes is time-dependent and becomes significant 12-24 h after the beginning of cell incubation. The dependence of the effect on the concentration of urokinase is characterized by half-maximum at about 20 nM and saturation at about 200 nM. The urokinase-induced gelatinase B release is not dependent on the action of plasmin because plasmin inhibitors aprotinin and alpha2-antiplasmin do not abolish this action. Additionally, tissue type plasminogen activator does not induce gelatinase B release by monocytes as observed under the action of urokinase. Nevertheless, the catalytic activity of urokinase participates in the development of the observed effect because it is significantly depressed by the natural urokinase inhibitor PAI-1. The effect of urokinase is completely abolished by actinomycin D and cycloheximide, indicating the participation of transcription and translation processes in its development.

Cellular pathology of atherosclerosis: smooth muscle cells prime cocultured endothelial cells for enhanced leukocyte adhesion

During the development of an atherosclerotic plaque, mononuclear leukocytes infiltrate the artery wall through vascular endothelial cells (ECs). At the same time, arterial smooth muscle cells (SMCs) change from the physiological contractile phenotype to the secretory phenotype and migrate into the plaque. We investigated whether secretory SMCs released cytokines that stimulated ECs in a manner leading to increased leukocyte recruitment and thus might accelerate atheroma formation. SMCs and ECs were established in coculture on the opposite sides of a porous membrane, and the cocultured cells were incorporated into a flow-based assay for studying leukocyte adhesion. We found that coculture primed ECs so that their response to the inflammatory cytokine tumor necrosis factor-alpha was amplified. ECs cocultured with SMCs supported greatly increased adhesion of flowing leukocytes and were sensitized to respond to tumor necrosis factor-alpha at concentrations 10 000 times lower than ECs cultured alone. In addition, coculture altered the endothelial selectin adhesion molecules used for leukocyte capture. EC priming was attributable to the cytokine transforming growth factor-beta(1), which was proteolytically activated to a biologically active form by the serine protease plasmin. These results suggest a new role for secretory SMCs in the development of atheromatous plaque. We propose that paracrine interaction between ECs and SMCs has the potential to amplify leukocyte recruitment to sites of atheroma and exacerbate the inflammatory processes believed to be at the heart of disease progression.

Like fibrin, (DD)E, the major degradation product of crosslinked fibrin, protects plasmin from inhibition by alpha2-antiplasmin

Plasmin generation is localized to the fibrin surface because tissue-type plasminogen activator (t-PA) and plasminogen bind to fibrin, an interaction that stimulates plasminogen activation over a hundred-fold. To ensure efficient fibrinolysis, plasmin bound to fibrin is protected from inhibition by alpha2-antiplasmin. (DD)E, a major soluble degradation product of cross-linked fibrin that is a potent stimulator of t-PA, compromises the fibrin-specificity of t-PA by promoting systemic activation of plasminogen. In this study we investigated whether (DD)E also protects plasmin from inhibition by alpha2-antiplasmin, facilitating degradation of this soluble t-PA effector. (DD)E and fibrin reduce the rate of plasmin inhibition by alpha2-antiplasmin by 5- and 10-fold, respectively. Kringle-dependent binding of plasmin to (DD)E and fibrin, with Kd values of 52 and 410 nM, respectively, contributes to the protective effect. When (DD)E is extensively degraded by plasmin, yielding uncomplexed fragment E and (DD), protection of plasmin from inhibition by alpha2-antiplasmin is attenuated. These studies indicate that (DD)E-bound plasmin, whose generation reflects the ability of (DD)E to stimulate plasminogen activation by t-PA, has the capacity to degrade (DD)E by virtue of its resistance to inhibition. This provides a mechanism to limit the concentration of (DD)E and maintain the fibrin-specificity of t-PA.

Effective lysis of model thrombi by a t-PA mutant (A473S) that is resistant to alpha2-antiplasmin

This study used two mutants of tissue-type plasminogen activator (t-PA) with resistance to inhibitors of fibrinolysis to define the contribution of plasminogen activator inhibitor (PAI)-1 and alpha2-antiplasmin (alpha2-AP) to the control of fibrin lysis. Wild-type t-PA was compared with KHRR296-299AAAA, which is resistant to PAI-1, and with A473S, which is resistant to alpha2-AP. We examined these forms of t-PA in model systems that are physiologically relevant. Neutralization of alpha2-AP was essential for lysis of plasma clots, irrespective of their platelet content, by either wild-type t-PA or KHRR296-299AAAA. In marked contrast, A473S lysed plasma clots without neutralization of alpha2-AP. Model thrombi, with structures similar to in vivo thrombi, were lysed slowly by wild-type t-PA; the rate and extent of lysis were enhanced by the addition of antibodies to alpha2-AP or PAI-1. A473S was more effective than wild-type t-PA without the addition of antibodies by virtue of its resistance to alpha2-AP. This resistance was remarkable, in that no complex formed between A473S t-PA and alpha2-AP, even after extended incubation, when 50% of wild-type t-PA could be converted to complex. Comparison of A473S and KHRR296-299AAAA mutants showed their similar effectiveness in lysis of platelet-rich model thrombi. Thus, PAI-1 and alpha2-AP contribute approximately equally to the inhibition of thrombus lysis. This study underlines the functional significance of alpha2-AP as a direct inhibitor of t-PA and further explains the basis of the accepted role of alpha2-AP as a regulator of fibrin persistence and thrombus resistance to lysis.

Modulation of tumor cell motility by plasmin

Using a pure chemotactic model, we investigated the effect of plasmin on tumor cell motility. In the presence of various extracellular matrix proteins, plasmin facilitated motility of human melanoma LOX and lung cancer Lu-99 cells. Laminin contributed most to the action of plasmin. The cell motility induced by plasmin and laminin was chemokinetic in nature and was almost completely suppressed by alpha2-antiplasmin. To further characterize the action of plasmin, various signal transduction kinase inhibitors were tried out. The results suggested that plasmin may modulate cell motility through protein kinase C and mitogen-activated protein kinase cascades in cooperation with laminin.

Binding and activation of human plasminogen by Mycobacterium tuberculosis

The first evidence of the interaction of Mycobacterium tuberculosis with the plasminogen system is herein reported. By FACScan analysis and affinity blotting, lysine-dependent binding of plasminogen to M. tuberculosis was demonstrated. The binding molecules were 30-, 60-, and 66-kDa proteins present in cell wall and soluble protein extracts. The activation of plasminogen, which occurred only in presence of fibrin and was not inhibited by the host serpin, alpha(2)-antiplasmin, was also demonstrated.

N-(4-hydroxyphenyl)retinamide activation of transforming growth factor-beta and induction of apoptosis in human breast cancer cells

N-(4 hydroxyphenyl)retinamide (4-HPR), a synthetic derivative of all-trans-retinoic acid, induces DNA synthesis arrest and apoptosis in human breast cancer cells in a dose- and time-dependent manner. MDA-MB-435 cells treated with 3 microM 4-HPR exhibited 58% and 75% DNA synthesis arrest after 1 and 2 days of treatment and 31%, 39%, 48%, and 56% apoptosis after 3, 4, 5, and 6 days of treatment, respectively. Conditioned media from 4-HPR-treated MDA-MB-435 cells contained 63 and 57 pg of active transforming growth factor-beta (TGF-beta) per 10(6) cells after 1 and 2 days of treatment, whereas conditioned media from control cells contained only 9 pg/10(6) cells. TGF-beta involvement in 4-HPR-induced apoptosis, but not DNA synthesis arrest, in MDA-MB-435 cells was demonstrated by 1) blockage of 4-HPR-induced apoptosis by 66-75% after treatment of cells with neutralizing antibodies to TGF-beta s, 2) blockage of 4-HPR-induced apoptosis by 64-67% after transient transfection of cells with antisense oligomers to TGF-beta 1 or TGF-beta type II receptor, 3) blockage of 4-HPR-induced apoptosis by approximately 50% after inhibition of latent TGF-beta activation, and 4) demonstration that human breast cancer cells (T47D) defective in TGF-beta signaling were refractive to 4-HPR-induced apoptosis. These data indicate that 4-HPR is a potent activator of TGF-beta and that TGF-beta participates in 4-HPR-induced apoptosis of human breast cancer cells.

Angiostatin inhibits endothelial and melanoma cellular invasion by blocking matrix-enhanced plasminogen activation

Angiostatin, a kringle-containing fragment of plasminogen, is a potent inhibitor of angiogenesis. The mechanism(s) responsible for the anti-angiogenic properties of angiostatin are unknown. We now report that human angiostatin blocks plasmin(ogen)-enhanced in vitro invasion of tissue plasminogen activator (t-PA)-producing endothelial and melanoma cells. Kinetic analyses demonstrated that angiostatin functions as a non-competitive inhibitor of extracellular-matrix (ECM)-enhanced, t-PA-catalysed plasminogen activation, with a Ki of 0.9+/-0.03 microM. This mechanism suggests that t-PA has a binding site for the inhibitor angiostatin, as well as for its substrate plasminogen that, when occupied, prevents ternary complex formation between t-PA, plasminogen and matrix protein. Direct binding experiments confirmed that angiostatin bound to t-PA with an apparent Kd [Kd(app)] of 6.7+/-0.7 nM, but did not bind with high affinity to ECM proteins. Together, these data suggest that angiostatin in the cellular micro-environment can inhibit matrix-enhanced plasminogen activation, resulting in reduced invasive activity, and suggest a biochemical mechanism whereby angiostatin-mediated regulation of plasmin formation could influence cellular migration and invasion.

Antiplasmin correlates to arterial reactivity in a healthy population of 35-year-old men and women

OBJECTIVE

To study whether haemostasis function variables correlate with endothelial function and other vasomotion characteristics of the brachial artery in a randomly selected healthy population of 35-year-old men and women.

DESIGN

Endothelial function was measured as flow mediated dilatation (FMD) of the brachial artery during reactive hyperaemia and the nonendothelial dependent dilatation after sublingual nitroglycerin (NTG) was administered. Haemostasis and fibrinolysis function were estimated by analysis of von Willebrand factor, plasminogen activator inhibitor-1, antiplasmin and fibrinogen.

SETTING

A general medicine research centre and a university hospital.

SUBJECTS

Randomly chosen men (n = 53) and women (n = 56).

RESULTS

Univariate correlation analysis showed significant correlations between haemostasis factors, conventional risk factors for cardiovascular disease and indices of vasomotion of the brachial artery. In multivariate analysis, with haemostasis variables and conventional risk factors included, antiplasmin was the strongest explanatory variable for FMD. When antiplasmin was removed from the analysis, the r-value dropped from 0.46 to 0.35. Antiplasmin also correlated with NTG-induced dilatation (positively) and brachial diameter at rest (negatively), albeit less consistently.

CONCLUSIONS

Antiplasmin correlates significantly and independently to FMD, reflecting endothelial function, and also to brachial artery diameter at rest and nitroglycerin-induced dilatation. In multivariate analysis these correlations of antiplasmin to arterial characteristics were stronger than for 'conventional' risk factors, such as smoking, blood pressure and serum cholesterol.

[Urokinase induces adhesion of monocytes to fibrinogen]

Urokinase activates adhesion of monocytic U937 cells to fibrinogen-coated surface. This effect is due to urokinase proteolytic activity and does not depend on the urokinase binding to its receptor.

The effects of alpha 2-antiplasmin complex and alpha 2-antiplasmin on the secretion of IgG and IgM by cultured human mononuclear cells

We tested alpha 2-antiplasmin (AP) and its complex with plasmin (PL) for their capacity to modulate spontaneous secretion of immunoglobulins (Ig) G and M by mononuclear cells (MNC). MNC were obtained from peripheral blood of 10 donors (5 males and 5 females) and cultured for 7 days in the presence of various concentrations of AP and AP-PL complex. The 18 hr incubation of MNC with AP-PL complex resulted in a dose-dependent increase in IgG and IgM secretion but the stimulatory effect was less significant than under conditions of incubation of MNC with the complex during a 7 day culture period. The AP-PL complex-induced stimulation occurred only in the cultures in which the dose of the complex was either close to or higher than the mean value of the complex levels in plasma of healthy donors. Elevation of IgG secretion in AP-PL complex-treated cultures from females was slightly higher than in those from males. No significant change in IgG and IgM secretion was observed in the presence of native AP.

Platelets and soluble fibrin promote plasminogen activation causing downregulation of platelet glycoprotein Ib/IX complexes: protection by aprotinin

Blood loss during and after open-heart surgery with cardiopulmonary bypass (CPB) is largely caused by platelet dysfunction. Previous studies indicate that plasmin can induce platelet dysfunction and affect primary hemostasis by proteolytic degradation and/or redistribution of essential platelet membrane glycoprotein complexes such as the glycoprotein Ib/IX complex. In this study, we present a model for plasmin generation localized on the platelet surface. Platelets treated with soluble fibrin or platelets in a mixture with soluble fibrin, t-PA, and plasminogen caused a significantly increased plasmin generation (p<0.01), dependent on t-PA, soluble fibrin, and platelet concentration. The plasmin generation resulted in a downregulation of platelet membrane glycoprotein Ib/IX glycoprotein complexes. Finally, we demonstrated that inhibitors of fibrinolysis, such as %2-antiplasmin, tranexamic acid, and aprotinin, can inhibit plasmin activity in the fluid phase. The downregulation of platelet glycoprotein Ib/IX complexes, however, was only prevented by aprotinin and not by alpha2-antiplasmin and tranexamic acid. These in vitro observations suggest a platelet localized activation of plasminogen, dependent on t-PA, enhanced by the presence of soluble fibrin. Since high concentrations of soluble fibrin and elevated levels of t-PA during CPB are observed, plasmin activity on the platelet surface during this period is anticipated. This plasmin activity reduces platelet metabolic functions and can be directed towards membrane glycoproteins such as glycoprotein Ib/IX complexes, thereby affecting hemostasis during and after CPB.

Effect of phenylglyoxal-modified alpha2-antiplasmin on urokinase-induced fibrinolysis

One of the functions of activated blood clotting factor XIII (FXIIIa) is the crosslinking of alpha2-antiplasmin (alpha2AP) to fibrin. This process results in localization and concentration of alpha2AP throughout fibrin, thereby making fibrin more resistant to digestion by plasmin. We reasoned that competition by chemically-modified inactive alpha2AP (mod alpha2AP) with native alpha2AP would diminish the resistance of fibrin to digestion by plasmin. Mod alpha2AP was prepared by treating native alpha2AP with an Arg-specific reagent, phenylglyoxal. An average of four of the total nineteen Arg residues in alpha2AP reacted with phenylglyoxal and resulted in complete loss of plasmin inhibitory activity; however, mod alpha2AP competed effectively with native alpha2AP for becoming crosslinked to fibrin by FXIIIa catalysis. In the presence of mod alpha2AP, urokinase (UK)-induced plasma clot lysis time shortened significantly. Mod alpha2AP enhanced UK-induced clot lysis in a whole blood system as shown by the similarities of rates of clot lysis for a mixture of 20 U/ml UK and 1.5 microM mod alpha2AP versus that induced by 100 U/ml UK without mod alpha2AP. Less fibrinogenolysis occurred in whole blood when mod alpha2AP was present since much lower UK concentrations were needed to achieve the same level of fibrinolysis than when only native alpha2AP was present. Our results indicate that mod alpha2AP enhances UK-induced fibrinolysis by competitive inhibition of factor XIIIa-mediated incorporation of native alpha2AP into fibrin.

Regulation of collagenolysis and cell death by plasmin within the formative stigma of preovulatory ovine follicles

Collagen breakdown and apoptotic cell death within the apex of the preovulatory ovine follicle are characteristic of impending rupture. It has been hypothesized that plasmin regulates these two responses by activating collagenases and tumour necrosis factor alpha (TNF-alpha), respectively. Apical plasmin bioactivity, collagenolysis, TNF-alpha-mediated fragmentation of cellular DNA (a marker of apoptosis), stigma formation, and follicular rupture in sheep were inhibited by intrafollicular injection of alpha 2-antiplasmin. Explants of follicular wall released hydroxyproline-containing peptides (degraded collagen) and bioactive TNF-alpha upon exposure to plasmin. These results indicate that plasmin has an essential intermediary role in the biomechanics of ovulatory ovarian tissue dissolution.

Alpha 2-antiplasmin causes thrombi to resist fibrinolysis induced by tissue plasminogen activator in experimental pulmonary embolism

BACKGROUND

In patients with pulmonary embolism, thrombi resist fibrinolysis induced by plasminogen activators. Because the molecular basis of this thrombus resistance is poorly understood, we used a potent inhibitor to examine the potential role of alpha 2-antiplasmin (alpha 2AP) in experimental pulmonary embolism.

METHODS AND RESULTS

Lysis of experimental pulmonary emboli was measured 4 hours after embolization in anesthetized ferrets. All animals received heparin (100 U/kg). Five experimental groups were studied: (1) no recombinant tissue plasminogen activator (rTPA); (2) rTPA at 1 mg/kg; (3) rTPA at 2 mg/kg; (4) rTPA at 1 mg/kg plus a control monoclonal antibody (MAb); and (5) rTPA at 1 mg/kg plus an alpha 2AP inhibitor (MAb 77A3). In comparison with ferrets receiving no rTPA (15.6 +/- 10.5% lysis, mean +/- SD), rTPA-treated groups showed significantly greater lysis (P < .01). Animals treated with rTPA and alpha 2AP inhibitor (56.2 +/- 4.7% lysis) showed significantly greater lysis than all other treatment groups, including ferrets treated with the same dose of rTPA alone (38.5 +/- 6.3%, P < .01), with twice the rTPA dose alone (45.0 +/- 6.5%, P < .05), or with a control MAb (35.2 +/- 4.6%, P < .01). The combination of rTPA treatment and alpha 2AP inhibition caused no consumption of fibrinogen.

CONCLUSIONS

Inhibition of alpha 2AP significantly amplified the lysis of experimental pulmonary emboli by rTPA without increasing fibrinogen consumption. These results suggest that alpha 2AP may play an important role in thrombus resistance in patients with venous thromboembolism.

Purification of enzymatically active kallikrein hK2 from human seminal plasma

Kallikrein hK2 is a member of the human glandular kallikrein family which includes prostate-specific antigen (PSA) and pancreatic-renal kallikrein. The purpose of this work was to isolate and characterize for the first time the enzymatically active form of the hK2 protein starting from the PCI-hK2 complex isolated from human seminal plasma (Deperthes, D., Chapdelaine, P., Tremblay, R.R., Brunet, C., Berton, J., Hébert, J., Lazure, C. and Dubé, J.Y. (1995) Biochim. Biophys. Acta 1245, 311-316). That complex was dissociated by an incubation at alkaline pH and final purification was achieved by C-18 reverse phase HPLC. The purified material contained a 27 kDa band by SDS gel electrophoresis and had the expected NH2-terminal amino acid sequence of hK2. It hydrolyzed synthetic chromogenic substrates containing esters of lysine and arginine but not of phenylalanine. Furthermore, hK2 formed molecular complexes with alpha 2 -antiplasmin, alpha 1-antichymotrypsin, antithrombin III and alpha 2-macroglobulin but not with alpha 1-antitrypsin. In conlusion, the new findings of the present paper are that the PCI-hK2 complex can be dissociated by mild procedures, that the free hK2 protein can be purified thereafter by standard HPLC procedures, that the recovered free hK2 is a trypsin-like enzyme and that it can form molecular complexes with many of the major serum proteinase inhibitors.

An extracellular proteolytic cascade promotes neuronal degeneration in the mouse hippocampus

Mice lacking the serine protease tissue plasminogen activator (tPA) are resistant to excitotoxin-mediated hippocampal neuronal degeneration. We have used genetic and cellular analyses to study the role of tPA in neuronal cell death. Mice deficient for the zymogen plasminogen, a known substrate for tPA, are also resistant to excitotoxins, implicating an extracellular proteolytic cascade in degeneration. The two known components of this cascade, tPA and plasminogen, are both synthesized in the mouse hippocampus. tPA mRNA and protein are present in neurons and microglia, whereas plasminogen mRNA and protein are found exclusively in neurons. tPA-deficient mice exhibit attenuated microglial activation as a reaction to neuronal injury. In contrast, the microglial response of plasminogen-deficient mice was comparable to that of wild-type mice, suggesting a tPA-mediated, plasminogen-independent pathway for activation of microglia. Infusion of inhibitors of the extracellular tPA/plasmin proteolytic cascade into the hippocampus protects neurons against excitotoxic injury, suggesting a novel strategy for intervening in neuronal degeneration.

Effects of fibrin and alpha2-antiplasmin on plasminogen activation by staphylokinase

Staphylokinase obtains plasminogen activating activity by forming a complex with plasminogen. Although the enzymatic activity of staphylokinase is enhanced by fibrin, how fibrin enhances enzymatic activity has not been determined yet. The effects of fibrin, or fibrinogen fragments, on the activation of plasminogen by staphylokinase was investigated using CNBr-digested fibrinogen fragments (FCB-2 and FCB-5) and plasmin-degraded cross-linked fibrin fragments ((DD)E complex, DD fragments and E fragments). Kinetic analysis of the activity of staphylokinase revealed that its plasminogen activating activity, which was expressed as kcat/Km, was enhanced by FCB-2 (10-fold) and FCB-5 (5-fold). These fibrin fragments caused 38-, 30-, and 8.5-fold increases in activity for the DD fragment, (DD)E complex and E fragment, respectively. Although alpha2-antiplasmin inhibited the activation of plasminogen by staphylokinase, FCB-2 abolished its inhibitory effects, and the plasminogen activating activity of staphylokinase was restored. The inhibitory effects of alpha2-antiplasmin on the activation of mini-plasminogen by staphylokinase were less than for Glu- or Lys-plasminogen, and the inhibitory effect of alpha2-antiplasmin was not altered by fibrin or EACA. These findings indicate that the staphylokinase/plasmin(ogen) complex reacts with fibrin even in the presence of alpha2-antiplasmin, and efficient plasminogen activation takes place on the surface of fibrin.

Plasminogen modulation of IL-1-stimulated degradation in bovine and human articular cartilage explants. The role of the endogenous inhibitors: PAI-1, alpha 2-antiplasmin, alpha 1-PI, alpha 2-macroglobulin and TIMP

The studies described here examine the involvement of the fibrinolytic cascade and its endogenous inhibitors in the regulation of activity of matrix metalloproteinases and cartilage degradation related to non-inflammatory joint disease, like osteoarthritis. An interleukin-1-induced model of degradation using [35S]-labeled bovine and human articular cartilage explants was utilized. One goal of these studies was to compare the responses of bovine and human articular cartilage. Degradation was not inhibited by alpha 1-PI, PAI-1, alpha 2-macroglobulin, alpha 2-antiplasmin or TIMP-2, when IL-1 alone was added. Addition of human plasminogen to bovine explants, at concentrations found in human synovial fluid, increased degradation by three to four-fold. Under these conditions, the degradation was inhibited effectively by all of the endogenous inhibitors tested, indicating the presence of a cascade where activated chondrocytes are a source of u-PA. Plasminogen activated by u-PA gives plasmin, which is known to further activate pro-stromelysin. Stromelysin is essential for activation of collegenase. Not only TIMP, but also inhibitors at earlier steps of activation like PAI-1, alpha 2-antiplasmin, alpha 1-PI and alpha 2-macroglobulin inhibited degradation, and could provide cartilage protection in vivo. An experiment with human articular cartilage explants showed that very little or no degradation occurred when human articular cartilage explants were stimulated with interleukin-1 alone. Addition of human plasminogen (at physiologically relevant concentrations) resulted in significant degradation, which was inhibited in the same manner as in bovine explants, by inhibitors of the fibrinolytic cascade and TIMP. TIMP is much more efficient in human explants, indicating the limited participation of human plasmin in the degradation of human cartilage. Although speculative, it is possible that in vivo, cartilage degradation could be promoted not only by TIMP/MMP imbalance, but also accelerated by decreased levels of certain serpins in synovial fluid (e.g. PAIs, alpha 2-antiplasmin and alpha 1-PI).

Plasmin regulates the activation of cell-associated latent TGF-beta 1 secreted by rat alveolar macrophages after in vivo bleomycin injury

Transforming growth factor beta s (TGF-beta s) are 25-kD multifunctional proteins that regulate inflammation and connective tissue synthesis. With rare exception TGF-beta 1 is secreted noncovalently bound to a latency-associated peptide (LAP) that renders the mature TGF-beta 1 biologically inactive. An important mechanism for the control of TGF-beta 1 action is the regulation of the post-translational processing that removes the LAP from the mature peptide and renders it biologically active. In a model of pulmonary inflammation and fibrosis induced by the antineoplastic antibiotic, bleomycin, we have demonstrated that explanted alveolar macrophages secrete progressively increasing quantities of a biologically active form of TGF-beta 1, the secretion of which was maximal 7 days after bleomycin administration. Thereafter, there was a rapid decline in the secretion of the active form of TGF-beta 1, whereas the latent form continued to be secreted in elevated quantities. Plasmin, a serine protease, was transiently generated by the same bleomycin-activated alveolar macrophages and paralleled the rise in active TGF-beta 1. When alpha 2-antiplasmin, an inhibitor of plasmin, was added to cultures of alveolar macrophages, the post-translational activation of L-TGF-beta 1, was totally abrogated. When plasmin was added to alveolar macrophages in culture, there was complete activation of the L-TGF-beta 1 that had been secreted during the culture period. However, there was no effect of plasmin on the same alveolar macrophage-derived L-TGF-beta 1 in cell-free conditioned media. Our findings suggest that the secretion of an active form of TGF-beta 1 by alveolar macrophages is regulated by the generation of plasmin and requires that the alveolar macrophages be present. Because the diminution of active TGF-beta 1 coincides with the resolution of inflammation, this suggests that the availability of plasmin regulates the biologically active form of TGF-beta 1, and thus, the inflammation seen after bleomycin-induced lung injury.

Effects of endothelial cells on activity of staphylokinase

Staphylokinase (SAK), produced by Staphylococcus aureus, induces fibrinolytic activity in circulation without systemic fibrinolytic activation. Since the effect of blood vessels on the activity of SAK has not yet been clarified, plasminogen activator (PA) activity of SAK in the presence or absence of endothelial cells was analyzed. The endothelial cells used in this experiment were of a cloned established cell line (TKM-33). In the expression of PA activity by SAK or streptokinase (SK), the kinetic constants revealed as Vmax/km were increased about 1.5-fold in the presence of endothelial cells. Furthermore, an initial lag phase which was observed during the plasminogen activation by SAK was markedly shortened in the presence of endothelial cells. In the case of SK, an initial lag phase was not observed in the absence or presence of endothelial cells. Although PA activity of SAK was inhibited by alpha 2-antiplasmin (alpha 2-AP), the inhibitory effect of alpha 2-AP in the presence of endothelial cells was weaker than in the absence of endothelial cells. The cyanogen bromide digested fibrinogen fragment-2 (FCB-2) distinctly enhanced the PA activity of SAK in the absence and the presence of endothelial cells. However, alpha 2-AP and FCB-2 did not cause a significant alteration of PA activity of SK even in the absence or presence of endothelial cells. These findings suggest that PA activity of SAK is enhanced by endothelial cells, but inhibited by alpha 2-AP. Moreover, PA activity of SAK is further enhanced by fibrin clot in the presence of endothelial cells.

Clearance of human factor XIa-inhibitor complexes in rats

The serpins C1 esterase inhibitor (C1Inh), antithrombin (AT), alpha 1-antitrypsin (alpha 1AT) and alpha 2-antiplasmin (alpha 2AP) are known inhibitors of coagulation factor XIa (FXIa). Although initial studies suggested alpha 1AT to be the main inhibitor of FXIa, we recently demonstrated C1Inh to be a predominant inhibitor of FXIa in vitro in human plasma. The present study was performed to investigate the plasma elimination kinetics of preformed human FXIa-FXIa inhibitor complexes injected in rats. The amounts of complexes remaining in circulation were measured using enzyme-linked immunosorbent assays. The plasma half-life time of clearance (t1/2) was 98 min for FXIa-alpha 1AT complexes, whereas it was considerably shorter, i.e. 19, 18 and 15 min for FXIa-C1Inh, FXIa-alpha 2AP and FXIa-AT complexes, respectively. Thus, due to this different plasma t1/2, preferentially FXIa-alpha 1AT complexes may be detected in clinical samples. Furthermore, measuring FXIa-FXIa inhibitor complexes in patient samples may not help to clarify the relative contribution of the individual serpins to inactivation of FXIa in vivo.

Modulation of contact system proteases by glycosaminoglycans. Selective enhancement of the inhibition of factor XIa

We investigated the influence of dextran sulfate, heparin, heparan sulfate, and dermatan sulfate on the inhibition of FXIa (where FXIa is activated factor XI, for example), FXIIa, and kallikrein by C1 inhibitor, alpha1-antitrypsin, alpha2-antiplasmin, and antithrombin III. The second-order rate constants for the inhibition of FXIa by C1 inhibitor, alpha1-antitrypsin, alpha2-antiplasmin, and antithrombin III, in the absence of glycosaminoglycans, were 1.8, 0.1, 0.43, and 0.32 x 10(3) M-1 s-1, respectively. The rate constants of the inactivation of FXIa by C1 inhibitor and by antithrombin III increased up to 117-fold in the presence of glycosaminoglycans. These data predicted that considering the plasma concentration of the inhibitors, C1 inhibitor would be the main inhibitor of FXIa in plasma in the presence of glycosaminoglycans. Results of experiments in which the formation of complexes between serine protease inhibitors and FXIa was studied in plasma agreed with this prediction. Glycosaminoglycans did not enhance the inhibition of alpha-FXIIa, beta-FXIIa, or kallikrein by C1 inhibitor. Thus, physiological glycosaminoglycans selectively enhance inhibition of FXIa without affecting the activity of FXIIa and kallikrein, suggesting that glycosaminoglycans may modulate the biological effects of contact activation, by inhibiting intrinsic coagulation without affecting the fibrinolytic potential of FXIIa/kallikrein.

Inhibition of Borrelia burgdorferi-bound fibrinolytic enzymes by alpha2-antiplasmin, PAI-1 and PAI-2

Lyme disease is caused by Borrelia burgdorferi. Human plasminogen and urokinase-type plasminogen activator bind to the surface of the spirochete where plasmin is generated. We have suggested that bound urokinase and plasminogen are utilized by the organism to disseminate. We tested whether the physiological inhibitors of urokinase, plasminogen activator inhibitor-1 and -2 (PAI-1, PAI-2), could regulate the activity of spirochete-bound urokinase. The k(ass) of PAI-1 and PAI-2 for bound urokinase were 1.3 x 10(6) M(-1)s(-1) and 6.9 x 10(4)M(-1)s(-1), respectively, whereas the k(ass) for free urokinase were 7.2 x 10(6) M(-1)s(-1) and 5.3 x 10(5) M(-1)s(-1), respectively. Plasmin associated with the spirochete was not inhibited by alpha2-antiplasmin. These results suggest that PAI-1, PAI-2 and alpha2 antiplasmin would not be efficient regulators of fibrinolytic protease activity on the Borrelial surface and would not pose a barrier to utilization of these enzymes for dissemination in the human host.

Mechanisms of physiological fibrinolysis

The fibrinolytic system comprises an inactive proenzyme, plasminogen, that is converted by plasminogen activators to the active enzyme, plasmin, which degrades fibrin. Two immunologically distinct plasminogen activators (PA) have been identified: tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). t-PA mediated plasminogen activation is mainly involved in the dissolution of fibrin in the circulation, whereas u-PA mediated plasminogen activation mainly plays a role in pericellular proteolysis. Plasminogen activation is regulated by specific molecular interactions between its main components, such as binding of plasminogen and t-PA to fibrin, or to specific cellular receptors resulting in enhanced plasminogen activation, inhibition of t-PA and u-PA by plasminogen activator inhibitors (PAI) and inhibition of plasmin by alpha 2-antiplasmin. Controlled synthesis and release of PAs and PAIs primarily from endothelial cells also contributes to the regulation of physiological fibrinolysis. The lysine binding sites situated in the kringle structures of plasminogen play a crucial role in the regulation of fibrinolysis by modulating its binding to fibrin and to cell surfaces, and by controlling the inhibition rate of plasmin by alpha 2-antiplasmin.

ECM degradation by cultured human mesangial cells is mediated by a PA/plasmin/MMP-2 cascade

We examined the role of the plasminogen activator/plasmin system in extracellular matrix (ECM) degradation by human mesangial cells cultured on thin films of 125I-labeled ECM (Matrigel). ECM degradation (release of 125I into the medium) was dependent on exogenous plasminogen, proportional to the number of mesangial cells and amount of plasminogen added, and coincident with the appearance of plasmin in the medium. ECM degradation was completely blocked (P < 0.001) by two plasmin inhibitors, alpha-2-antiplasmin (40 micrograms/ml) and aprotinin (216 KIU/ml), and partially reduced (-33 +/- 1.8%, P < 0.01) by TIMP-1 (40 micrograms/ml), a specific inhibitor of matrix metalloproteinases. Zymography of medium obtained from cells cultured in the absence of plasminogen revealed the presence of latent matrix metalloproteinase-2 (MMP-2) which was converted to a lower molecular weight, active form in the presence of mesangial cells and plasminogen. Northern analysis of poly A+RNA prepared from cultured human mesangial cells revealed mRNA for tissue-type plasminogen activator (tPA), urokinase-type plasminogen activator (uPA), plasminogen activator inhibitor-1 (PAI-1), and uPA receptor (uPAR). The presence of uPA protein in medium obtained from cultured human mesangial cells was demonstrated by Western blotting and ELISA which revealed a large molar excess of PAI-1 (1.2 +/- 0.1 x 10(-9) M) over uPA (1.2 +/- 0.1 x 10(-12) M) and tPA (0.19 +/- 0.04 x 10(-9) M). ECM degradation was reduced by a monoclonal antibody (MAb) against human tPA (-54 +/- 8.6%) or human uPA (-39 +/- 5.2%) compared to cells treated with identical amounts of non-specific monoclonal IgG (P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Hybrid peptide containing RGDF (Arg-Gly-Asp-Phe) coupled with the carboxy terminal part of alpha 2-antiplasmin capable of inhibiting platelet aggregation and promoting fibrinolysis

The aim of this study was to synthesize and investigate hybrid peptides which contain the RGD (Arg-Gly-Asp) sequence coupled with lysine residues in special arrangements (antiplasmin carboxyterminal peptide) in an effort to simultaneously inhibit platelet aggregation and promote fibrinolysis. The in vitro haemostatic modifying properties of the synthesized peptides were tested by ADP-induced platelet aggregation, plasmin-generation tests and fibrin-clot lysis assays. The hybrid peptide RGDFAP, composed of RGDF (Arg-Gly-Asp-Phe) coupled to a synthetic peptide residue of the carboxyterminal part of antiplasmin (AP26) inhibited platelet activation and increased plasmin generation and in vitro fibrin-clot lysis.

[A new thrombocyte aggregation-inhibiting and fibrinolysis-promoting synthetic molecule: RGDF (Arg-Gly-Asp-Phe) coupled with the carboxyterminal antiplasmin peptide]

OBJECTIVE

Synthesize such hybrid peptides, which contain the RGD (Arg-Gly-Asp) sequence coupled with another peptide containing lysine residues in special positions, to inhibit simultaneously platelet activation and promote fibrinolytic processes.

DESIGN

The in vitro haemostasis modifying properties of the synthesized peptides were tested with ADP induced platelet aggregation, in vitro plasmin generation tests and fibrin-clot lysis assays.

RESULTS AND CONCLUSIONS

RGDF (Arg-Gly-Asp-Phe) coupled with the carboxyterminal antiplasmin peptide (RGDFAP hybrid molecule) has a common concentration range for inhibiting platelet activation and increase plasmin generation along with accelerated in vitro fibrin-clot lysis.