Modulation of fibrin cofactor activity in plasminogen activation

Impact of Bradykinin Generation During Thrombolysis in Ischemic Stroke

Ischemic stroke is one of the leading causes of death and disability worldwide. Current medical management in the acute phase is based on the activation of the fibrinolytic cascade by intravenous injection of a plasminogen activator (such as tissue-type plasminogen activator, tPA) that promotes restauration of the cerebral blood flow and improves stroke outcome. Unfortunately, the use of tPA is associated with deleterious effects such as hemorrhagic transformation, symptomatic brain edema, and angioedema, which limit the efficacy of this therapeutic strategy. Preclinical and clinical evidence suggests that intravenous thrombolysis generates large amounts of bradykinin, a peptide with potent pro-inflammatory, and pro-edematous effects. This tPA-triggered generation of bradykinin could participate in the deleterious effects of thrombolysis and is a potential target to improve neurological outcome in tPA-treated patients. The present review aims at summarizing current evidence linking thrombolysis, bradykinin generation, and neurovascular damage.

Inhibition of plasminogen activation by apo(a): role of carboxyl-terminal lysines and identification of inhibitory domains in apo(a)

Apo(a), the distinguishing protein component of lipoprotein(a) [Lp(a)], exhibits sequence similarity to plasminogen and can inhibit binding of plasminogen to cell surfaces. Plasmin generated on the surface of vascular cells plays a role in cell migration and proliferation, two of the fibroproliferative inflammatory events that underlie atherosclerosis. The ability of apo(a) to inhibit pericellular plasminogen activation on vascular cells was therefore evaluated. Two isoforms of apo(a), 12K and 17K, were found to significantly decrease tissue-type plasminogen activator-mediated plasminogen activation on human umbilical vein endothelial cells (HUVECs) and THP-1 monocytes and macrophages. Lp(a) purified from human plasma decreased plasminogen activation on THP-1 monocytes and HUVECs but not on THP-1 macrophages. Removal of kringle V or the strong lysine binding site in kringle IV10 completely abolished the inhibitory effect of apo(a). Treatment with carboxypeptidase B to assess the roles of carboxyl-terminal lysines in cellular receptors leads in most cases to decreases in plasminogen activation as well as plasminogen and apo(a) binding; however, inhibition of plasminogen activation by apo(a) was unaffected. Our findings directly demonstrate that apo(a) inhibits pericellular plasminogen activation in all three cell types, although binding of apo(a) to cell-surface receptors containing carboxyl-terminal lysines does not appear to play a major role in the inhibition mechanism.

Regulation of fibrinolysis by C-terminal lysines operates through plasminogen and plasmin but not tissue-type plasminogen activator

BACKGROUND

Binding of tissue-type plasminogen (Pgn) activator (t-PA) and Pgn to fibrin regulates plasmin generation, but there is no consistent, quantitative understanding of the individual contribution of t-PA finger and kringle 2 domains to the regulation of fibrinolysis. Kringle domains bind to lysines in fibrin, and this interaction can be studied by competition with lysine analogs and removal of C-terminal lysines by carboxypeptidase B (CPB).

METHODS

High-throughput, precise clot lysis assays incorporating the lysine analog tranexamic acid (TA) or CPB and genetically engineered variants of t-PA were performed. In particular, wild-type (WT) t-PA (F-G-K1-K2-P) and a domain-switched variant K1K1t-PA (F-G-K1-K1-P) that lacks kringle 2 but retains normal t-PA structure were compared to probe the importance of fibrin lysine binding by t-PA kringle 2.

RESULTS

WT t-PA showed higher rates of fibrinolysis than K1K1t-PA, but the inhibitory effects of TA or CPB were very similar for WT t-PA and the variant t-PA (< 10% difference). Urokinase plasminogen activator (u-PA)-catalyzed fibrinolysis was also inhibited by TA, even though Pgn activation could be stimulated. Fibrin treated with factor XIIIa (FXIIIa) generates crosslinked degradation products, but these did not affect the results obtained with WT t-PA and K1K1t-PA.

CONCLUSIONS

t-PA kringle 2 has a minor role in the initial interaction of t-PA and fibrin, but stimulation of fibrinolysis by C-terminal lysines (or inhibition by carboxypeptidases or TA) operates through Pgn and plasmin binding, not through t-PA. This is also true when fibrin is crosslinked by treatment with FXIIIa.

3-Mercaptopropionic acids as efficacious inhibitors of activated thrombin activatable fibrinolysis inhibitor (TAFIa)

A novel series of cyclic potent, selective, small molecule, thiol-based inhibitors of activated thrombin activatable fibrinolysis inhibitor (TAFIa) and the crystal structures of TAFIa inhibitors bound to porcine pancreatic carboxypeptidase B are described. Three series of cyclic arginine and lysine mimetic inhibitors vary significantly in their selectivity against other human basic carboxypeptidases, carboxypeptidase N and carboxypeptidase B. (-)2a displays TAFIa IC50 = 3 nM and 600-fold selectivity against CPN. Inhibition of TAFIa with (rac)2a resulted in dose dependent acceleration of human plasma clot lysis in vitro and was efficacious as an adjunct to tPA in an in vivo rabbit jugular vein thrombolysis model.

The role of β2-glycoprotein I (β2GPI) in the activation of plasminogen

Beta2-glycoprotein I (beta2GPI) is a glycoprotein of unknown physiological function. It is the main target antigen for antiphospholipid antibodies in patients with antiphospholipid syndrome (APS). beta2GPI binds with high affinity to the atherogenic lipoprotein Lp(a) which shares structural homology with plasminogen, a key molecule in the fibrinolytic system. Impaired fibrinolysis has been described in APS. The present work reports the interaction between beta2GPI and Glu-Plasminogen which may explain the recently described proteolytic effect of plasmin on beta2GPI. In the process of Glu-Plasminogen activation, we found an increase in plasmin generation both at fibrin and cellular surface level as a function of the concentration of beta2GPI added, suggesting an important role as a cofactor in the trimolecular complex beta2GPI-Plasminogen-tPA. This phenomenon represents a novel regulatory step both in the positive feedback mechanism for extrinsic fibrinolysis and in antithrombotic regulation. IgG anti-beta2GPI antibodies recognized the beta2GPI at the endothelial surface inducing its activation with an increase of ICAM-I and a decrease in the expression of thrombomodulin favoring a pro-thrombotic state in the vascular endothelium. The interference in the plasmin conversion by anti-beta2GPI antibodies could generate thrombosis as observed in APS.

Development of a fast kinetic method for the determination of carboxypeptidase U (TAFIa) using C-terminal arginine containing peptides as substrate

Carboxypeptidase U (CPU, TAFIa) is a novel determinant of the fibrinolytic rate. It circulates in blood as an inactive zymogen, procarboxypeptidase U, which is activated during the process of coagulation and fibrinolysis. CPU has a very short half-life at 37 degrees C. Its intrinsic instability complicates the determination of kinetic parameters of different substrates using an endpoint method. We developed a fast kinetic assay for measuring continuously the release of the C-terminal arginine by CPU independent of the nature of the substrate peptide used, allowing us to perform substrate specificity studies of CPU. This method uses arginine kinase, pyruvate kinase, and lactate dehydrogenase as auxiliary enzymes. The CPU activities measured using this kinetic assay were in the range of 97-103% of those determined with our HPLC-assisted reference assay, and the obtained K(m) and k(cat) values for hippuryl-l-arginine and bradykinin were in good accordance with those described in the literature. As expected, no arginine cleaving was seen using dipeptides and peptide substrates with a proline in the penultimate position. The presented kinetic assay enables the fast screening of substrates with a C-terminal arginine and is a valuable new tool for the kinetic evaluation of both synthetic and physiological substrates of CPU.

β-Amyloid (Aβ) causes detachment of N1E-115 neuroblastoma cells by acting as a scaffold for cell-associated plasminogen activation

A major component of neuritic plaques in brain tissue of Alzheimer's disease patients is the beta-amyloid peptide (Abeta). Accumulation of Abeta has been associated with increased neuronal cell death and cognitive decline. We have previously shown that amyloid peptides like Abeta bind tissue-type plasminogen activator (tPA) and stimulate plasmin production. Here we investigated how Abeta regulates plasmin formation by N1E-115 neuroblastoma cells and the effects of Abeta-mediated plasmin formation on cell attachment and cell survival. We find that Abeta induces excessive cell-associated plasmin generation that causes cell detachment. Cell detachment is inhibited by carboxypeptidase B (CPB), an enzyme that blocks plasmin formation by cleaving off C-terminal lysine residues. Plasmin and CPB control Abeta-induced cell detachment independently of direct effects on cell viability. Abeta40 as well as oligomeric and fibrillar forms of Abeta42 stimulated tPA-mediated plasminogen activation and cell detachment. Our results suggest that plasmin-mediated cell detachment could contribute to the pathological effects of Abeta in diseased brain.

Arthritis is linked to local and systemic activation of coagulation and fibrinolysis pathways

BACKGROUND

Activation of coagulation and fibrinolysis play a role in the pathophysiology of experimental arthritis.

OBJECTIVE

To determine the extent of activation of the coagulation and fibrinolytic pathways in different joint diseases in humans and to ascertain the factors that may influence fibrin deposition within the joint.

METHODS

Plasma from normal subjects (controls, n= 21) and plasma and synovial fluid samples from patients with rheumatoid arthritis (RA; n = 64), osteoarthritis (OA; n = 29), spondyloarthropathy (SpA; n = 22) and crystal arthritis (CA; n = 25) were analyzed for the levels of TF (tissue factor) and tissue factor pathway inhibitor (TFPI) activities, thrombin-antithrombin III (TAT) complexes, and F1 + 2 (thrombin fragment), fibrin d-dimer and thrombin-activated fibrinolysis inhibitor (TAFI) antigenic levels. The measurements were analyzed by pairwise correlation with each other as well as with standard parameters of inflammation [C-reactive protein (CRP), joint leukocyte count]. Inter-group comparisons were performed to look for disease-specific differences.

RESULTS

Compared with healthy controls, patients with joint diseases had higher levels of TAT, F1 + 2 and d-dimers in their plasma. In the synovial fluid, TF activity, TAT, d-dimers, and TAFI were significantly higher in inflammatory arthritides than in OA. The levels were highest in RA patients. In the plasma, TF activity was correlated with TAT and d-dimer levels with CRP, TFPI, and TAT. In the synovial fluid, TF activity correlated with plasma CRP levels, synovial fluid leukocyte count, and synovial TAT and TAFI levels. In addition, synovial d-dimers correlated with CRP, and synovial TAFI levels were correlated with synovial F1 + 2 and TAT.

CONCLUSIONS

Activation of the coagulation and fibrinolytic cascades in the joint and in the circulation is evident in both inflammatory and degenerative joint diseases. Within the joint, inflammatory mechanisms leading to TF-mediated activation of the coagulation pathway and subsequent fibrin deposition is the most likely explanation for the observed findings. In the plasma, the link between inflammation (CRP increase) and TF activation is weak, and a non-TF-mediated mechanism of coagulation activation could explain these findings. RA is characterized by significantly higher levels of TAT in the synovial fluid and plasma than other arthritides. Although fibrinolytic activity is linked to inflammation, the increased amounts of TAFI in the joint, particularly in RA, may explain why fibrin formation is so prominent in this condition compared with other joint diseases.

Impaired healing of cutaneous wounds and colonic anastomoses in mice lacking thrombin-activatable fibrinolysis inhibitor

Plasmin and other components of the plasminogen activation system play an important role in tissue repair by regulating extracellular matrix remodeling, including fibrin degradation. Thrombin-activatable fibrinolysis inhibitor (TAFI) is a procarboxypeptidase that, after activation, can attenuate plasmin-mediated fibrin degradation by removing the C-terminal lysine residues from fibrin, which play a role in the binding and activation of plasminogen. To test the hypothesis that TAFI is an important determinant in the control of tissue repair, we investigated the effect of TAFI deficiency on the healing of cutaneous wounds and colonic anastomoses. Histological examination revealed inappropriate organization of skin wound closure in the TAFI knockout mice, including an altered pattern of epithelial migration. The time required to completely heal the cutaneous wounds was slightly delayed in TAFI-deficient mice. Healing of colonic anastomoses was also impaired, as reflected by decreased strength of the tissue at the site of the suture, and by bleeding complications in 3 of 14 animals. Together, these abnormalities resulted in increased mortality in TAFI-deficient mice after colonic anastomoses. Although our study shows that tissue repair, including re-epithelialization and scar formation, occurs in TAFI-deficient mice, TAFI appears to be important for appropriate organization of the healing process.

Mutations in the substrate binding site of thrombin-activatable fibrinolysis inhibitor (TAFI) alter its substrate specificity

Thrombin-activable fibrinolysis inhibitor (TAFI) is a zymogen that inhibits the amplification of plasmin production when converted to its active form (TAFIa). TAFI is structurally very similar to pancreatic procarboxypeptidase B. TAFI also shares high homology in zinc binding and catalytic sites with the second basic carboxypeptidase present in plasma, carboxypeptidase N. We investigated the effects of altering residues involved in substrate specificity to understand how they contribute to the enzymatic differences between TAFI and carboxypeptidase N. We expressed wild type TAFI and binding site mutants in 293 cells. Recombinant proteins were purified and characterized for their activation and enzymatic activity as well as functional activity. Although the thrombin/thrombomodulin complex activated all the mutants, carboxypeptidase B activity of the activated mutants against hippuryl-arginine was reduced. Potato carboxypeptidase inhibitor inhibited the residual activity of the mutants. The functional activity of the mutants in a plasma clot lysis assay correlated with their chromogenic activity. The effect of the mutations on other substrates depended on the particular mutation, with some of the mutants possessing more activity against hippuryl-His-leucine than wild type TAFIa. Thus mutations in residues around the substrate binding site of TAFI resulted in altered C-terminal substrate specificity.

Inhibition of plasminogen activation by lipoprotein(a): critical domains in apolipoprotein(a) and mechanism of inhibition on fibrin and degraded fibrin surfaces

Similarity between the apolipoprotein(a) (apo(a)) moiety of lipoprotein(a) (Lp(a)) and plasminogen suggests a potentially important link between atherosclerosis and thrombosis. Lp(a) may interfere with tissue plasminogen activator (tPA)-mediated plasminogen activation in fibrinolysis, thereby generating a hypercoagulable state in vivo. A fluorescence-based system was employed to study the effect of apo(a) on plasminogen activation in the presence of native fibrin and degraded fibrin cofactors and in the absence of positive feedback reactions catalyzed by plasmin. Human Lp(a) and a physiologically relevant, 17-kringle recombinant apo(a) species exhibited strong inhibition with both cofactors. A variant lacking the protease domain also exhibited strong inhibition, indicating that the apo(a)-plasminogen binding interaction mediated by the apo(a) protease domain does not ultimately inhibit plasminogen activation. A variant in which the strong lysine-binding site in kringle IV type 10 had been abolished exhibited substantially reduced inhibition whereas another lacking the kringle V domain showed no inhibition. Amino-terminal truncation mutants of apo(a) also revealed that additional sequences within kringle IV types 1-4 are required for maximal inhibition. To investigate the inhibition mechanism, the concentrations of plasminogen, cofactor, and a 12-kringle recombinant apo(a) species were systematically varied. Kinetics for both cofactors conformed to a single, equilibrium template model in which apo(a) can interact with all three fibrinolytic components and predicts the formation of ternary (cofactor, tPA, and plasminogen) and quaternary (cofactor, tPA, plasminogen, and apo(a)) catalytic complexes. The latter complex exhibits a reduced turnover number, thereby accounting for inhibition of plasminogen activation in the presence of apo(a)/Lp(a).