Characterization of Cellular Binding Sites and Interactive Regions within Reactants Required for Enhancement of Plasminogen Activation by tPA on the Surface of Leukocytic Cells

Plasminogen and tPA bind to a common set of binding sites on nucleated cells. To assess the functional consequences of cellular binding, we have measured the kinetic changes induced by plasminogen activation by tPA on cell surfaces. These studies were carried out with U937 and THP-1 monocytoid cells, with Raji, Nalm6 and Molt4 lymphoid cells and with peripheral blood monocytes and neutrophils. The interactions of plasminogen and tPA with cells induced an increase in the rate of plasmin generation which depended upon the cell concentration. With saturating amounts of U937 monocytoid cells (1.25 × 105/ml) the rate of plasmin generation was 0.39 nM.s-1 versus 0.07 and 0.09 nM.s-1 without cells or without tPA, respectively. The catalytic efficiency of Glu- or Lys-plasminogen activation by tPA increased by 7.2- and 24.2-fold, respectively. These changes were induced by a 72-242-fold reduction in the Km of these interactions which was in the range of 0.3-0.9 µM. These values are below the plasminogen concentration in plasma (1-2 µM). Moreover, we provide new data indicating that 1) only a specific subset of plasminogen binding sites, i.e. molecules exposing carboxyl terminal lysines on the cell surface, promotes plasminogen activation on cells; 2) the first four kringles of plasminogen and the finger of tPA are critical for enhanced plasmin generation on cell surfaces; 3) the simultaneous co-localization of tPA with plasminogen on cell surfaces is required for enhanced plasminogen activation; 4) modulation of plasminogen/tPA receptor expression induces concomitant modulation of the stimulatory effects of cells on plasminogen activation and 5) in a direct comparison, the mechanism by which cells and fibrin fragments accelerate plasminogen activation are similar but not identical. These data suggest that modulation of plasminogen/tPA binding sites permits local and efficient generation of plasmin on cell surfaces.

Novel Role for Matrix Metalloproteinase 9 in Modulation of Cholesterol Metabolism

Background

The development of atherosclerosis is strongly linked to disorders of cholesterol metabolism. Matrix metalloproteinases (MMPs) are dysregulated in patients and animal models with atherosclerosis. Whether systemic MMP activity influences cholesterol metabolism is unknown.

Methods and Results

We examined MMP‐9–deficient (Mmp9^−/−) mice and found them to have abnormal lipid gene transcriptional responses to dietary cholesterol supplementation. As opposed to Mmp9^+/+ (wild‐type) mice, Mmp9^−/− mice failed to decrease the hepatic expression of sterol regulatory element binding protein 2 pathway genes, which control hepatic cholesterol biosynthesis and uptake. Furthermore, Mmp9^−/− mice failed to increase the expression of genes encoding the rate‐limiting enzymes in biliary cholesterol excretion (eg, Cyp7a and Cyp27a). In contrast, MMP‐9 deficiency did not impair intestinal cholesterol absorption, as shown by the ¹⁴C‐cholesterol and ³H‐sitostanol absorption assay. Similar to our earlier study on Mmp2^−/− mice, we observed that Mmp9^−/− mice had elevated plasma secreted phospholipase A₂ activity. Pharmacological inhibition of systemic circulating secreted phospholipase A₂ activity (with varespladib) partially normalized the hepatic transcriptional responses to dietary cholesterol in Mmp9^−/− mice. Functional studies with mice deficient in other MMPs suggested an important role for the MMP system, as a whole, in modulation of cholesterol metabolism.

Conclusions

Our results show that MMP‐9 modulates cholesterol metabolism, at least in part, through a novel MMP‐9–plasma secreted phospholipase A₂ axis that affects the hepatic transcriptional responses to dietary cholesterol. Furthermore, the data suggest that dysregulation of the MMP system can result in metabolic disorder, which could lead to atherosclerosis and coronary heart disease.

ADAMTS13 deficiency promotes microthrombosis in a murine model of diet-induced liver steatosis

ADAMTS13 cleaves ultralarge multimeric von Willebrand Factor (VWF), thereby preventing formation of platelet-rich microthrombi. ADAMTS13 is mainly produced by hepatic stellate cells, and numerous studies have suggested a functional role of ADAMTS13 in the pathogenesis of liver diseases. The aim of our study was to investigate a potential role of ADAMTS13 in formation of hepatic microthrombi and development of non-alcoholic steatohepatitis (NASH), and furthermore to evaluate whether plasmin can compensate for the absence of ADAMTS13 in removal of thrombi. Therefore, we used a model of high-fat diet-induced steatosis in Adamts13 deficient (Adamts13^-/-) and wild-type (WT) control mice. Microthrombi were more abundant in the liver of obese Adamts13^-/- as compared to obese WT or to lean Adamts13^-/- mice. Obese Adamts13^-/- mice displayed lower platelet counts and higher prevalence of ultra-large VWF multimers. Hepatic plasmin-α2-antiplasmin complex levels were comparable for obese WT and Adamts13^-/- mice and were lower for lean Adamts13^-/- than WT mice, not supporting marked activation of the fibrinolytic system. High fat diet feeding, as compared to normal chow, resulted in enhanced liver triglyceride levels for both genotypes (p < 0.0001) and steatosis (p < 0.0001 for WT mice, p = 0.002 for Adamts13^-/- mice) without differences between the genotypes. Expression of markers of inflammation, oxidative stress, steatosis and fibrosis was affected by diet, but not by genotype. Thus, our data confirm that obesity promotes NASH, but do not support a detrimental role of ADAMTS13 in its development. However, Adamts13 deficiency in obese mice promotes hepatic microthrombosis, whereas a compensatory role of plasmin in removal of microthrombi in the absence of ADAMTS13 could not be demonstrated.

Bacterial pathogens activate plasminogen to breach tissue barriers and escape from innate immunity

Both coagulation and fibrinolysis are tightly connected with the innate immune system. Infection and inflammation cause profound alterations in the otherwise well-controlled balance between coagulation and fibrinolysis. Many pathogenic bacteria directly exploit the host's hemostatic system to increase their virulence. Here, we review the capacity of bacteria to activate plasminogen. The resulting proteolytic activity allows them to breach tissue barriers and evade innate immune defense, thus promoting bacterial spreading. Yersinia pestis, streptococci of group A, C and G and Staphylococcus aureus produce a specific bacterial plasminogen activator. Moreover, surface plasminogen receptors play an established role in pneumococcal, borrelial and group B streptococcal infections. This review summarizes the mechanisms of bacterial activation of host plasminogen and the role of the fibrinolytic system in infections caused by these pathogens.

Plasminogen activation by staphylokinase enhances local spreading of S. aureus in skin infections

BACKGROUND

Staphylococcus aureus (S. aureus) is a frequent cause of skin and soft tissue infections. A unique feature of S. aureus is the combined presence of coagulases that trigger fibrin formation and of the plasminogen activator staphylokinase (SAK). Whereas the importance of fibrin generation for S. aureus virulence has been established, the role of SAK remains unclear. We studied the role of plasminogen activation by SAK in a skin infection model in mice and evaluated the impact of alpha-2-antiplasmin (α2AP) deficiency on the spreading and proteolytic activity of S. aureus skin infections. The species-selectivity of SAK was overcome by adenoviral expression of human plasminogen. Bacterial spread and density was assessed non-invasively by imaging the bioluminescence of S. aureus Xen36.

RESULTS

SAK-mediated plasmin activity increased the local invasiveness of S. aureus, leading to larger lesions with skin disruption as well as decreased bacterial clearance by the host. Even though fibrin and bacterial surfaces protected SAK-mediated plasmin activity from inhibition by α2AP, the deficiency of α2AP resulted in increased bacterial spreading. SAK-mediated plasmin also induced secondary activation of gelatinases, shown both in vitro and in lesions from the in vivo model.

CONCLUSION

SAK contributes to the phenotype of S. aureus skin infections by enhancing bacterial spreading as a result of fibrinolytic and proteolytic activation.

Differential effects of a gelatinase inhibitor on adipocyte differentiation and adipose tissue development

(1) A potential role for the gelatinases in adipocyte differentiation in vitro and adipose tissue development in vivo was investigated using the gelatinase inhibitor tolylsam ((R)-3-methyl-2-[4-(3-p-tolyl-[1,2,4]oxadiazol-5-yl)-benzenesulphonylamino]-butyric acid). (2) Differentiation of murine 3T3-F442A preadipocytes (12 days after reaching confluence) into mature adipocytes in vitro was promoted in the presence of tolylsam (10-100 μmol/L). (3) De novo development of fat tissue in nude mice injected with preadipocytes and kept on a high-fat diet was significantly impaired following treatment with tolylsam (100 mg/kg per day for 4 weeks). (4) Adipose tissue development in matrix metalloproteinase (MMP)-2 deficient mice, kept on a high-fat diet, was significantly impaired following administration of tolylsam (100 mg/kg per day for 15 weeks). This was associated with markedly enhanced metabolic rate. (5) Treatment of MMP-2-deficient mice with tolylsam (100 mg/kg per day, 15 weeks) was associated with the preservation of collagen and a reduction in blood vessel size in adipose tissues in vivo. (6) Furthermore, plasma levels of triglycerides and free fatty acids were reduced by tolylsam treatment of MMP-2-deficient mice (100 mg/kg per day, 15 weeks), whereas nutrient adsorption in the intestine was not affected. (7) The results of the present study indicate that tolylsam promotes preadipocyte differentiation in vitro, but impairs adipose tissue development in vivo.

Tissue-type plasminogen activator is an extracellular mediator of Purkinje cell damage and altered gait

Purkinje neurons are a sensitive and specialised cell type important for fine motor movement and coordination. Purkinje cell damage manifests as motor incoordination and ataxia - a prominent feature of many human disorders including spinocerebellar ataxia and Huntington's disease. A correlation between Purkinje degeneration and excess cerebellar levels of tissue-type plasminogen activator (tPA) has been observed in multiple genetically-distinct models of ataxia. Here we show that Purkinje loss in a mouse model of Huntington's disease also correlates with a 200% increase in cerebellar tPA activity. That elevated tPA levels arise in a variety of ataxia models suggests that tPA is a common mediator of Purkinje damage. To address the specific contribution of tPA to cerebellar dysfunction we studied the T4 mice line that overexpresses murine tPA in postnatal neurons through the Thy1.2 gene promoter, which directs preferential expression to Purkinje cells within the cerebellum. Here we show that T4 mice develop signs of cerebellar damage within 10 weeks of birth including atrophy of Purkinje cell soma and dendrites, astrogliosis, reduced molecular layer volume and altered gait. In contrast, T4 mice displayed no evidence of microgliosis, nor any changes in interneuron density, nor alteration in the cerebellar granular neuron layer. Thus, excess tPA levels may be sufficient to cause targeted Purkinje cell degeneration and ataxia. We propose that elevated cerebellar tPA levels exert a common pathway of Purkinje cell damage. Therapeutically lowering cerebellar tPA levels may represent a novel means of preserving Purkinje cell integrity and motor coordination across a wide range of neurodegenerative diseases.

Plasmin-dependent proteolysis of tissue factor pathway inhibitor in a mouse model of endotoxemia

BACKGROUND

The development of a procoagulant state in sepsis, owing to aberrant expression of tissue factor (TF) and a sharp decrease in the level of its major inhibitor, TF pathway inhibitor (TFPI), could lead to microthrombotic organ failure. The mechanism for the decline in TFPI activity in the lung could involve plasmin-mediated cleavage of the inhibitor.

OBJECTIVE

To investigate the effect of plasmin generation on lung-associated TFPI activity, in normal conditions and during infusion of endotoxin (lipopolysaccharide [LPS]) in mice.

METHODS

Plasmin generation and TFPI activity were assayed in the lungs of mice deficient in tissue-type plasminogen (Plg) activator (t-PA) or Plg, at 2 h after LPS or saline injection.

RESULTS

The sharp loss of lung-associated TFPI activity at 2 h after LPS challenge paralleled the abrupt increase in plasmin generation. TFPI activity was significantly retained in both t-PA(-/-) and Plg(-/-) mice, which are unable to generate plasmin.

CONCLUSION

The increased plasmin generation during the early stages of sepsis could cleave/inactivate TFPI and thus lead to thrombotic complications.

Host-derived plasminogen activator inhibitor-1 (PAI-1) concentration is critical for in vivo tumoral angiogenesis and growth

Plasminogen activator inhibitor type 1 (PAI-1) plays a key role in tumor progression and is believed to control proteolytic activity and cell migration during angiogenesis. We report here that host PAI-1, at physiological concentration, promotes in vivo tumor invasion and angiogenesis. In sharp contrast, inhibition of tumor vascularization was observed when PAI-1 was produced at supraphysiologic levels, either by host cells (transgenic mice overexpressing PAI-1) or by tumor cells (after transfection with murine PAI-1 cDNA). This study provides for the first time in vivo evidence for a dose-dependent effect of PAI-1 on tumor angiogenesis. Of great interest is the finding that PAI-1 produced by tumor cells, even at high concentration, did not overcome the absence of PAI-1 in the host, emphasizing the importance of the cellular source of PAI-1.

Characterization of cellular binding sites and interactive regions within reactants required for enhancement of plasminogen activation by tPA on the surface of leukocytic cells

Plasminogen and tPA bind to a common set of binding sites on nucleated cells. To assess the functional consequences of cellular binding, we have measured the kinetic changes induced by plasminogen activation by tPA on cell surfaces. These studies were carried out with U937 and THP-1 monocytoid cells, with Raji, Nalm6 and Molt4 lymphoid cells and with peripheral blood monocytes and neutrophils. The interactions of plasminogen and tPA with cells induced an increase in the rate of plasmin generation which depended upon the cell concentration. With saturating amounts of U937 monocytoid cells (1.25 x 10(5)/ml) the rate of plasmin generation was 0.39 nM.s-1 versus 0.07 and 0.09 nM.s-1 without cells or without tPA, respectively. The catalytic efficiency of Glu- or Lys-plasminogen activation by tPA increased by 7.2- and 24.2-fold, respectively. These changes were induced by a 72-242-fold reduction in the Km of these interactions which was in the range of 0.3-0.9 microM. These values are below the plasminogen concentration in plasma (1-2 microM). Moreover, we provide new data indicating that 1) only a specific subset of plasminogen binding sites, i.e. molecules exposing carboxyl terminal lysines on the cell surface, promotes plasminogen activation on cells; 2) the first four kringles of plasminogen and the finger of tPA are critical for enhanced plasmin generation on cell surfaces; 3) the simultaneous co-localization of tPA with plasminogen on cell surfaces is required for enhanced plasminogen activation; 4) modulation of plasminogen/tPA receptor expression induces concomitant modulation of the stimulatory effects of cells on plasminogen activation and 5) in a direct comparison, the mechanism by which cells and fibrin fragments accelerate plasminogen activation are similar but not identical. These data suggest that modulation of plasminogen/tPA binding sites permits local and efficient generation of plasmin on cell surfaces.

Construction and characterization of a functional chimeric murine-human antibody directed against human fibrin fragment-D dimer

Fibrin-directed monoclonal antibodies may be clinically useful for in vitro thrombus imaging and for the targeting of fibrinolytic agents to blood clots. One such murine monoclonal antibody, (mAb-15C5), raised against the fragment-D dimer epitope of cross-linked human fibrin, was previously characterized [Holvoet, P., Stassen, J. M., Hashimoto, Y., Spriggs, D., Devos, P. & Collen, D. (1989) Thromb. Haemostasis 61, 307-313] has recently been cloned and expressed [Vandamme, A.-M., Bulens, F., Bernar, H., Nelles, L., Lijnen, H. R. & Collen, D. (1990) Eur. J. Biochem. 192, 767-775]. In order to reduce the immunogenicity of the murine mAb-15C5 in man, we have now constructed a murine--human chimera of mAb-15C5, by substituting the cDNA sequences encoding the constant regions of the murine kappa light chain and gamma 1 heavy chain by the corresponding human genomic sequences. Both chimeric murine--human Ig chains were cloned into two separately selectable expression vectors, which were contransfected into Chinese hamster ovary (CHO) cells. Murine--human chimeric mAb-15C5 (mAb-15C5Hu) was purified from the conditioned medium of selected cell lines by chromatography on Zn-chelating Sepharose, protein-A-Sepharose and on insolubilized antigen (fragment-D dimer), with a final yield of 29 micrograms/l and a recovery of 33%. SDS/PAGE without reduction revealed a homogeneous band with a mobility similar to that of natural mAb-15C5, whereas after reduction, both the heavy and the light chains had slightly slower mobilities than their natural counterparts. Expression in the presence of tunicamycin suggested that the differences in gamma 1-chain mobility were due to different N-glycosylation patterns. Immunoblotting of proteins from SDS gels showed immunological reactivity of recombinant mAb-15C5Hu with goat anti-(human IgG) IgG and of recombinant and natural murine mAb-15C5 with goat anti-(mouse IgG) IgG. Competitive binding revealed a comparable affinity of recombinant murine mAb-15C5, recombinant mAb-15C5Hu and natural mAb-15C5, for fragment-D dimer, indicating that recombinant mAb-15C5Hu was obtained in a functionally intact form. Thus, mAb-15C5Hu may constitute a useful alternative to mAb-15C5 for in vivo use in man.

Construction and characterization of a recombinant murine monoclonal antibody directed against human fibrin fragment-D dimer

cDNA libraries in lambda phage were generated from the murine hybridoma secreting mAb-15C5, a monoclonal antibody directed against fragment-D dimer of crosslinked human fibrin [Holvoet et al. (1989) Thromb. Haemostasis 61, 307-313], and clones encoding fragments of the heavy (gamma 1) and the light (kappa) chain were isolated. The kappa-chain cDNA was reconstructed from two overlapping clones encoding 20 amino acids of signal sequence and the 214 amino acids of the mature protein chain. The gamma 1-chain cDNA was reconstructed from the mAb-15C5 kappa-chain signal sequence, the mAb-15C5 gamma 1 variable-domain coding sequence and murine gamma 1-gene and gamma 1-chain cDNA fragments encoding the constant domains. These cDNAs were expressed in Chinese hamster ovary cells, selected cell lines were scaled up in roller bottle culture, and recombinant mAb-15C5 was purified from the conditioned medium by chromatography on Zn-chelate - Sepharose, protein-A - Sepharose and insolubilized fragment-D dimer, with a yield of 50 micrograms/l and a recovery of 20%. SDS-gel electrophoresis without reduction revealed a homogeneous band, and after reduction a light-chain band with identical and a heavy-chained band with a somewhat slower mobility than that of the natural mAb-15C5. Competitive binding revealed a comparable affinity of natural and recombinant mAb-15C5 for fibrin fragment-D dimer. Thus recombinant mAb-15C5, obtained by co-expression of the reconstructed cDNAs of the kappa and gamma 1 chain in Chinese hamster ovary cells, has very similar properties to natural mAb-15C5. These recombinant mAb-15C5 cDNAs may be useful for the construction of a humanized monoclonal antibody for thrombus imaging, and for targeting of thrombolytic agents to fibrin.