Complex formation of platelet thrombospondin with plasminogen. Modulation of activation by tissue activator

Thrombospondin-1 Restricts Interleukin-36γ-Mediated Neutrophilic Inflammation during Pseudomonas aeruginosa Pulmonary Infection

Interleukin-36γ (IL-36γ), a member of the IL-1 cytokine superfamily, amplifies lung inflammation and impairs host defense during acute pulmonary Pseudomonas aeruginosa infection. To be fully active, IL-36γ is cleaved at its N-terminal region by proteases such as neutrophil elastase (NE) and cathepsin S (CatS). However, it remains unclear whether limiting extracellular proteolysis restrains the inflammatory cascade triggered by IL-36γ during P. aeruginosa infection. Thrombospondin-1 (TSP-1) is a matricellular protein with inhibitory activity against NE and the pathogen-secreted Pseudomonas elastase LasB-both proteases implicated in amplifying inflammation. We hypothesized that TSP-1 tempers the inflammatory response during lung P. aeruginosa infection by inhibiting the proteolytic environment required for IL-36γ activation. Compared to wild-type (WT) mice, TSP-1-deficient (Thbs1-/-) mice exhibited a hyperinflammatory response in the lungs during P. aeruginosa infection, with increased cytokine production and an unrestrained extracellular proteolytic environment characterized by higher free NE and LasB, but not CatS activity. LasB cleaved IL-36γ proximally to M19 at a cleavage site distinct from those generated by NE and CatS, which cleave IL-36γ proximally to Y16 and S18, respectively. N-terminal truncation experiments in silico predicted that the M19 and the S18 isoforms bind the IL-36R complex almost identically. IL-36γ neutralization ameliorated the hyperinflammatory response and improved lung immunity in Thbs1-/- mice during P. aeruginosa infection. Moreover, administration of cleaved IL-36γ induced cytokine production and neutrophil recruitment and activation that was accentuated in Thbs1-/- mice lungs. Collectively, our data show that TSP-1 regulates lung neutrophilic inflammation and facilitates host defense by restraining the extracellular proteolytic environment required for IL-36γ activation.IMPORTANCEPseudomonas aeruginosa pulmonary infection can lead to exaggerated neutrophilic inflammation and tissue destruction, yet host factors that regulate the neutrophilic response are not fully known. IL-36γ is a proinflammatory cytokine that dramatically increases in bioactivity following N-terminal processing by proteases. Here, we demonstrate that thrombospondin-1, a host matricellular protein, limits N-terminal processing of IL-36γ by neutrophil elastase and the Pseudomonas aeruginosa-secreted protease LasB. Thrombospondin-1-deficient mice (Thbs1-/-) exhibit a hyperinflammatory response following infection. Whereas IL-36γ neutralization reduces inflammatory cytokine production, limits neutrophil activation, and improves host defense in Thbs1-/- mice, cleaved IL-36γ administration amplifies neutrophilic inflammation in Thbs1-/- mice. Our findings indicate that thrombospondin-1 guards against feed-forward neutrophilic inflammation mediated by IL-36γ in the lung by restraining the extracellular proteolytic environment.

Thrombospondin-1 promotes hemostasis through modulation of cAMP signaling in blood platelets

Thrombospondin-1 (TSP-1) is released by platelets upon activation and can increase platelet activation, but its role in hemostasis in vivo is unclear. We show that TSP-1 is a critical mediator of hemostasis that promotes platelet activation by modulating inhibitory cyclic adenosine monophosphate (cAMP) signaling. Genetic deletion of TSP-1 did not affect platelet activation in vitro, but in vivo models of hemostasis and thrombosis showed that TSP-1-deficient mice had prolonged bleeding, defective thrombosis, and increased sensitivity to the prostacyclin mimetic iloprost. Adoptive transfer of wild-type (WT) but not TSP-1-/- platelets ameliorated the thrombotic phenotype, suggesting a key role for platelet-derived TSP-1. In functional assays, TSP-1-deficient platelets showed an increased sensitivity to cAMP signaling, inhibition of platelet aggregation, and arrest under flow by prostacyclin (PGI2). Plasma swap experiments showed that plasma TSP-1 did not correct PGI2 hypersensitivity in TSP-1-/- platelets. By contrast, incubation of TSP-1-/- platelets with releasates from WT platelets or purified TSP-1, but not releasates from TSP-1-/- platelets, reduced the inhibitory effects of PGI2. Activation of WT platelets resulted in diminished cAMP accumulation and downstream signaling, which was associated with increased activity of the cAMP hydrolyzing enzyme phosphodiesterase 3A (PDE3A). PDE3A activity and cAMP accumulation were unaffected in platelets from TSP-1-/- mice. Platelets deficient in CD36, a TSP-1 receptor, showed increased sensitivity to PGI2/cAMP signaling and diminished PDE3A activity, which was unaffected by platelet-derived or purified TSP-1. This scenario suggests that the release of TSP-1 regulates hemostasis in vivo through modulation of platelet cAMP signaling at sites of vascular injury.

Plasmin-Induced Activation of Human Platelets Is Modulated by Thrombospondin-1, Bona Fide Misfolded Proteins and Thiol Isomerases

Inflammatory processes are triggered by the fibrinolytic enzyme plasmin. Tissue-type plasminogen activator, which cleaves plasminogen to plasmin, can be activated by the cross-β-structure of misfolded proteins. Misfolded protein aggregates also represent substrates for plasmin, promoting their degradation, and are potent platelet agonists. However, the regulation of plasmin-mediated platelet activation by misfolded proteins and vice versa is incompletely understood. In this study, we hypothesize that plasmin acts as potent agonist of human platelets in vitro after short-term incubation at room temperature, and that the response to thrombospondin-1 and the bona fide misfolded proteins Eap and SCN^--denatured IgG interfere with plasmin, thereby modulating platelet activation. Plasmin dose-dependently induced CD62P surface expression on, and binding of fibrinogen to, human platelets in the absence/presence of plasma and in citrated whole blood, as analyzed by flow cytometry. Thrombospondin-1 pre-incubated with plasmin enhanced these plasmin-induced platelet responses at low concentration and diminished them at higher dose. Platelet fibrinogen binding was dose-dependently induced by the C-terminal thrombospondin-1 peptide RFYVVMWK, Eap or NaSCN-treated IgG, but diminished in the presence of plasmin. Blocking enzymatically catalyzed thiol-isomerization decreased plasmin-induced platelet responses, suggesting that plasmin activates platelets in a thiol-dependent manner. Thrombospondin-1, depending on the concentration, may act as cofactor or inhibitor of plasmin-induced platelet activation, and plasmin blocks platelet activation induced by misfolded proteins and vice versa, which might be of clinical relevance.

Thrombospondin 1 (THBS1) Promotes Follicular Angiogenesis, Luteinization, and Ovulation in Primates

Angiogenesis is essential to both ovulation and the formation of the corpus luteum. The thrombospondin (THBS) family of glycoproteins plays diverse roles in regulation of angiogenesis, but the role of these vascular regulators in ovulation and luteinization remain to be elucidated. Using the cynomolgus macaque as a model for human ovulation, we demonstrated that levels of THBS1 mRNA and protein in preovulatory follicle granulosa cells increased after the ovulatory gonadotropin surge, with peak levels just before the expected time of ovulation. THBS1 treatment of monkey ovarian microvascular endothelial cells in vitro stimulated migration, proliferation, and capillary sprout formation, consistent with a pro-angiogenic action of THBS1. Injection of an anti-THBS1 antibody into monkey preovulatory follicles reduced rates of follicle rupture and oocyte release in response to an ovulatory gonadotropin stimulus when compared with control IgG-injected follicles. Interestingly, two of three oocytes from anti-THBS1 antibody injected follicles were germinal vesicle intact, indicating that meiosis failed to resume as anticipated. Follicles injected with anti-THBS1 antibody also showed reduced granulosa cell layer expansion, endothelial cell invasion, and capillary formation when compared to control IgG-injected follicles. Overall, these findings support a critical role for THBS1 in follicular angiogenesis, with implications for both successful ovulation and corpus luteum formation.

Hantavirus Infection Suppresses Thrombospondin-1 Expression in Cultured Endothelial Cells in a Strain-Specific Manner

Hantavirus infection is associated with two frequently fatal diseases in humans: Hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). The pathogenesis of hantavirus infection is complex and not fully understood; however, it is believed to involve virus-induced hyperinflammatory immune responses. Thrombospondin-1 (THBS1) is a large homotrimeric protein that plays a putative role in regulating blood homeostasis. Hyperresponsiveness to inflammatory stimuli has also been associated with defects in the THBS1 gene. Our data suggest that hantavirus infection of human umbilical cord vein endothelial cells (HUVEC) suppress the accumulation of THBS1 in the extracellular matrix. Additionally, this suppression is dependent on virus replication, implying a direct mechanism of action. Our data also imply that the pathogenic Andes and Hantaan strains inhibit THBS1 expression while the non-pathogenic Prospect Hill strain showed little inhibition. These observations suggest that a dysregulation of THBS1 may contribute to the pathogenesis of hantavirus infection.

Extracellular proteolysis of reelin by tissue plasminogen activator following synaptic potentiation

The secreted glycoprotein reelin plays an indispensable role in neuronal migration during development and in regulating adult synaptic functions. The upstream mechanisms responsible for initiating and regulating the duration and magnitude of reelin signaling are largely unknown. Here we report that reelin is cleaved between EGF-like repeats 6-7 (R6-7) by tissue plasminogen activator (tPA) under cell-free conditions. No changes were detected in the level of reelin and its fragments in the brains of tPA knockouts, implying that other unknown proteases are responsible for generating reelin fragments found constitutively in the adult brain. Induction of NMDAR-independent long-term potentiation with the potassium channel blocker tetraethylammonium chloride (TEA-Cl) led to a specific up-regulation of reelin processing at R6-7 in wild-type mice. In contrast, no changes in reelin expression and processing were observed in tPA knockouts following TEA-Cl treatment. These results demonstrate that synaptic potentiation results in tPA-dependent reelin processing and suggest that extracellular proteolysis of reelin may regulate reelin signaling in the adult brain.

Decoy plasminogen receptor containing a selective Kunitz-inhibitory domain

Kunitz domain 1 (KD1) of tissue factor pathway inhibitor-2 in which P2' residue Leu17 (bovine pancreatic trypsin inhibitor numbering) is mutated to Arg selectively inhibits the active site of plasmin with ∼5-fold improved affinity. Thrombin cleavage (24 h extended incubation at a 1:50 enzyme-to-substrate ratio) of the KD1 mutant (Leu17Arg) yielded a smaller molecule containing the intact Kunitz domain with no detectable change in the active-site inhibitory function. The N-terminal sequencing and MALDI-TOF/ESI data revealed that the starting molecule has a C-terminal valine (KD1L17R-VT), whereas the smaller molecule has a C-terminal lysine (KD1L17R-KT). Because KD1L17R-KT has C-terminal lysine, we examined whether it could serve as a decoy receptor for plasminogen/plasmin. Such a molecule might inhibit plasminogen activation as well as the active site of generated plasmin. In surface plasmon resonance experiments, tissue plasminogen activator (tPA) and Glu-plasminogen bound to KD1L17R-KT (Kd ∼ 0.2 to 0.3 μM) but not to KD1L17R-VT. Furthermore, KD1L17R-KT inhibited tPA-induced plasma clot fibrinolysis more efficiently than KD1L17R-VT. Additionally, compared to ε-aminocaproic acid KD1L17R-KT was more effective in reducing blood loss in a mouse liver-laceration injury model, where the fibrinolytic system is activated. In further experiments, the micro(μ)-plasmin-KD1L17R-KT complex inhibited urokinase-induced plasminogen activation on phorbol-12-myristate-13-acetate-stimulated U937 monocyte-like cells, whereas the μ-plasmin-KD1L17R-VT complex failed to inhibit this process. In conclusion, KD1L17R-KT inhibits the active site of plasmin as well as acts as a decoy receptor for the kringle domain(s) of plasminogen/plasmin; hence, it limits both plasmin generation and activity. With its dual function, KD1L17R-KT could serve as a preferred agent for controlling plasminogen activation in pathological processes.

Current understanding of the thrombospondin-1 interactome

The multifaceted action of thrombospondin-1 (TSP-1) depends on its ability to physically interact with different ligands, including structural components of the extracellular matrix, other matricellular proteins, cell receptors, growth factors, cytokines and proteases. Through this network, TSP-1 regulates the ligand activity, availability and structure, ultimately tuning the cell response to environmental stimuli in a context-dependent manner, contributing to physiological and pathological processes. Complete mapping of the TSP-1 interactome is needed to understand its diverse functions and to lay the basis for the rational design of TSP-1-based therapeutic approaches. So far, large-scale approaches to identify TSP-1 ligands have been rarely used, but many interactions have been identified in small-scale studies in defined biological systems. This review, based on information from protein interaction databases and the literature, illustrates current knowledge of the TSP-1 interactome map.

The plasminogen system in regulating stem cell mobilization

The treatment of patients with hematopoietic progenitor and stem cells (HPSCs) to reconstitute hematopoiesis after myeloablative therapy or to repair ischemia after myocardial infarction has significantly improved clinical outcomes. Successful blood or bone marrow transplants require a sufficient number of HPSCs capable of homing to the injured site to regenerate tissue. Granulocyte-colony stimulating factor (G-CSF) is widely used clinically for stem cell mobilization. However, in some patients the response is poor, thus a better understanding of the mechanisms underlying G-CSF-regulated stem cell mobilization is needed. The pasminogen (Plg) system is the primary fibrinolytic pathway responsible for clot dissolution after thrombosis. Recent evidence suggests that Plg plays a pivotal role in stem cell mobilization from the bone marrow to the peripheral circulation, particularly in HPSC mobilization in response to G-CSF. This paper will discuss the potential mechanisms by which the Plg system regulates stem cell mobilization, focusing on stepwise proteolysis and signal transduction during HPSC egress from their bone marrow niche. Clear elucidation of the underlying mechanisms may lead to the development of new Plg-based therapeutic strategies to improve stem cell mobilization in treating hematological and cardiovascular diseases.

The effect of plasmin on platelet function

There is controversy in the literature regarding the effects of plasmin on human platelets. We have studied the effects of plasmin on platelet glycoproteins, aggregation, shape change and secretion and found them to be dependent on experimental conditions: (a) Plasmin's effects on human platelets are only seen in gel-filtered platelets (GFP), presumably because in platelet rich plasma plasmin is bound to a2-antiplasmin; (b) in GFP to which fibrinogen has been added, platelet function remains intact; and (c) in the absence of fibrinogen, the effect of plasmin on GFP depends on whether stirring is performed or not. With stirring, platelets undergo shape change, secretion and aggregation in response to added plasmin. Aggregation is much stronger when CaCl(z) 1 mM is added. Without stirring, preincubation of GFP with plasmin leads to inhibition of platelet aggregation induced by subsequent platelet stimuli (thrombin, collagen, ristocetin or U46619). We have demonstrated that plasmin is a true platelet activating agent, in the sense that it induces platelet shape change and secretion. Plasmin will induce aggregation when added to stirred GFP. This may be because stirring protects glycoprotein (GP) IIbIIIa bound fibrinogen from being degradated by plasmin. When added to unstirred GFP, GP IIbIIIa bound fibrinogen may be readily accessible to degradation by plasmin, which may then behave like a platelet inhibitor.

Effects of altered plasminogen activator inhibitor-1 expression on cardiovascular disease

Plasminogen Activator Inhibitor-1 (PAI-1) is a multifunctional protein with the ability to not only regulate fibrinolysis through inhibition of plasminogen activation, but also cell signaling events which have direct downstream effects on cell function. Elevated plasma levels of this protein have been shown to have profound effects on the development and progression of cardiovascular diseases. However, results from a number of studies, especially those using PAI-1 deficient mouse models, have demonstrated that its function is ambiguous, with evidence of both preventing and enhancing various disease states. A number of lifestyle changes and pharmacological reagents have been identified that can regulate PAI-1 levels or function. Those reagents that target function are focused on its ability to regulate plasmin formation, and have been studied in in vivo models of thrombosis. Further investigations involving regulation of cell function could potentially resolve paradoxical issues associated with the function of this protein in regulating cardiovascular disease.

The role of thrombospondin 1 on intestinal inflammation and carcinogenesis

Crohn’s disease and ulcerative colitis are inflammatory bowel diseases (IBD) quite common in the United States and other Western countries. Patients suffering IBD are at greater risk of developing colorectal adenocarcinoma than the general population. Both, the adenomacarcinoma and the inflammation-carcinogenesis processes are characterized by active angiogenesis. Recent studies also have shown that anti-angiogenesis might be a novel therapeutic approach for IBD. Thrombospondin 1 (TSP1) is an extracellular protein well known for its anti-angiogenic properties. TSP1 also has key functions in inflammation, which is assumed to be the primary cause for carcinogenesis in IBD. This review is focused on the role of TSP1 in colorectal carcinogenesis. The therapeutic effects of TSP derived-peptides on inhibiting the inflammation-carcinogenesis progression are also discussed.

Thrombospondin-1 as a Paradigm for the Development of Antiangiogenic Agents Endowed with Multiple Mechanisms of Action

Uncontrolled neovascularization occurs in several angiogenesis-dependent diseases, including cancer. Neovascularization is tightly controlled by the balance between angiogenic growth factors and antiangiogenic agents. The various natural angiogenesis inhibitors identified so far affect neovascularization by different mechanisms of action. Thrombospondin-1 (TSP-1) is a matricellular modular glycoprotein that acts as a powerful endogenous inhibitor of angiogenesis. It acts both indirectly, by sequestering angiogenic growth factors and effectors in the extracellular environment, and directly, by inducing an antiangiogenic program in endothelial cells following engagement of specific receptors including CD36, CD47, integrins and proteoglycans (all involved in angiogenesis ). In view of its central, multifaceted role in angiogenesis, TSP-1 has served as a source of antiangiogenic tools, including TSP-1 fragments, synthetic peptides and peptidomimetics, gene therapy strategies, and agents that up-regulate TSP-1 expression. This review discusses TSP-1-based inhibitors of angiogenesis, their mechanisms of action and therapeutic potential, drawing our experience with angiogenic growth factor-interacting TSP-1 peptides, and the possibility of exploiting them to design novel antiangiogenic agents.

Calcium indirectly regulates immunochemical reactivity and functional activities of the N-domain of thrombospondin-1

Conformational changes induced in thrombospondin-1 by removal of calcium regulate interactions with some ligands of its N-modules. Because calcium binds primarily to elements of the C-terminal signature domain of thrombospondin-1, which are distant from the N-modules, such regulation was unexpected. To clarify the mechanism for this regulation, we compared ligand binding to the N-modules of thrombospondin-1 in the full-length protein and recombinant trimeric thrombospondin-1 truncated prior to the signature domain. Three monoclonal antibodies were identified that recognize the N-modules, two of which exhibit calcium-dependent binding to native thrombospondin-1 but not to the truncated trimeric protein. These antibodies or calcium selectively modulate interactions of fibronectin, heparin, sulfatide, alpha3beta1 integrin, tumor necrosis factor-alpha-stimulated gene-6 protein, and, to a lesser extent, alpha4beta1 integrin with native thrombospondin-1 but not with the truncated protein. These results indicate connectivity between calcium binding sites in the C-terminal signature domain and the N-modules of thrombospondin-1 that regulates ligand binding and functional activities of the N-modules.

Nanoscale intracellular organization and functional architecture mediating cellular behavior

Cells function based on a complex set of interactions that control pathways resulting in ultimate cell fates including proliferation, differentiation, and apoptosis. The inter-workings of this immensely dense network of intracellular molecules are influenced by more than random protein and nucleic acid distribution where their interactions culminate in distinct cellular function. By probing the design of these biological systems from an engineering perspective, researchers can gain great insight that will aid in building and utilizing systems that are on this size scale where traditional large-scale rules may fail to apply. The organized interaction and gradient distribution in intracellular space imply a structural architecture that modulates cellular processes by influencing biochemical interactions including transport and binding-reactions. One significant structure that plays a role in this modulation is the cell cytoskeleton. Here, we discuss the cytoskeleton as a central and integrating functional structure in influencing cell processes and we describe technology useful for probing this structure. We explain the nanometer scale science of cytoskeletal structure with respect to intracellular organization, mechanotransduction, cytoskeletal-associated proteins, and motor molecules, as well as nano- and microtechnologies that are applicable for experimental studies of the cytoskeleton. This biological architecture of the cytoskeleton influences molecular, cellular, and physiological processes through structured multimodular and hierarchical principles centered on these functional filaments. Through investigating these organic systems that have evolved over billions of years, understanding in biology, engineering, and nanometer-scaled science will be advanced.

Thrombospondin 1, thrombospondin 2 and the eye

Thrombospondin 1 and thrombospondin 2 (TSP1 and TSP2), which comprise the subgroup A thrombospondins, are matricellular proteins. As matricellular proteins, they modulate interactions between cells and the cellular environment, regulate cell adhesion and typically are expressed during tissue formative processes. In general, TSP1 and TSP2 counter angiogenesis (including tumour angiogenesis) and play important but contrasting roles during cutaneous repair. The two proteins are involved in development, including that of the eye, although evidence suggests that they have their greatest impact during tissue production in the adult. In the normal adult eye, they tend to be found at sites of ongoing matrix synthesis or cell-matrix interactions. At these sites, the two proteins possibly influence cellular differentiation and/or basement membrane deposition. TSP1 is also present in the intraocular fluids and drainage pathway, where it may function in maintaining the anti-angiogenic environment and in intraocular pressure control, respectively. TSP1 could also be involved in ocular immune privilege. Unlike in skin wounds, where TSP1 is derived from the blood and is present only in the early phases of repair, ocular tissue damage appears to lead to protacted TSP1 synthesis by local cells. This response might help suppress angiogenesis in the transparent tissues of the eye and so lessen visual axis opacification following injury. However, TSP2, which is also produced by damaged ophthalmic tissue and may be especially important in matrix organisation, seems to augment contraction in anomalous intraocular fibrosis. Elucidating the roles of TSP1 and TSP2 in ocular physiology and pathobiology may lead to improved therapies for neovascular, neoplastic, reparative and other ophthalmic diseases.

Do antiangiogenic protein fragments have amyloid properties?

Tumor growth requires proteolytic activity. As a consequence, protein breakdown products are present in the circulation of patients with cancer. Within the past decade a large number of proteolytic fragments have been identified that inhibit angiogenesis and tumor growth. The mechanism of action of these inhibitors is still poorly understood. We recently found that the effects of the angiogenesis inhibitor endostatin on endothelial cells is critically dependent on the presence of cross-beta structure, a structure also present in amyloidogenic polypeptides in plaques of patients with amyloidosis, such as Alzheimer disease. We also showed that cross-beta structure containing endostatin is a ligand for tissue-type plasminogen activator (tPA). We noted that many angiogenesis inhibitors stimulate tPA-mediated plasminogen activation. Because the presence of cross-beta structure is the common denominator in tPA-binding ligands, we hypothesize that these endogenous antiangiogenic proteolytic fragments share features with amyloidogenic polypeptides. We postulate that the cross-beta structural fold is present in these antiangiogenic polypeptide fragments and that this structure mediates the inhibitory effects. The hypothesis provides new insights in the potential mechanisms of these angiogenesis inhibitors and offers opportunities to improve their use.

Thrombospondin-4 and its variants: expression and differential effects on endothelial cells

BACKGROUND

In a recent large-scale genetic association study, a single nucleotide polymorphism in the thrombospondin-4 (TSP-4) gene, resulting in a proline-for-alanine substitution at position 387, was associated with a significantly increased risk for premature atherosclerosis. TSP-4 had not previously been implicated in vascular pathology, and very little information is available on its expression and functions.

METHODS AND RESULTS

The goal of this study was to assess TSP-4 expression in vessel wall and to identify differences in functions of TSP-4 variants that could account for the proatherogenic effects of the (P387)TSP-4 variant. TSP-4 expression was demonstrated in human endothelial cells (ECs) and vascular smooth muscle cells from brain blood vessels and coronary arteries. (P387)TSP-4 and its fragment (residues 326 to 722), but not the A(387) forms, suppressed EC adhesion and proliferation. The (P387)TSP-4 was more active in inducing the phosphorylation of focal adhesion kinase, consistent with inhibition of proliferation. Both variant fragments increased the proliferation of human aortic smooth muscle cells.

CONCLUSIONS

TSP-4 is expressed by vascular cells and influences the vessel wall by modulating the proliferation of ECs and smooth muscle cells. The A387P substitution is a "gain-of-function" mutation, favoring a form of TSP-4 that interferes with EC adhesion and proliferation and may thereby be proatherogenic.

Use of recombinant tissue-plasminogen activator in a dog with chylothorax secondary to catheter-associated thrombosis of the cranial vena cava

A 4-year-old, castrated male Maltese developed cranial vena caval thrombosis and chylothorax following central venous catheterization for treatment of postoperative sepsis. Vena caval thrombolysis was attempted using recombinant human tissue-plasminogen activator (t-PA). Thrombolytic therapy led to an acute reduction in the size of the caval thrombus and was followed by prompt resolution of the chylothorax. Hemorrhage at the entry sites of a jugular catheter and esophagostomy tube placed at the time of treatment was a dose-limiting complication of t-PA therapy in this dog.

Plasmin-mediated release of the guidance molecule F-spondin from the extracellular matrix

Serine proteases are implicated in a variety of processes during neurogenesis, including cell migration, axon outgrowth, and synapse elimination. Tissue-type plasminogen activator and urokinase-type activator are expressed in the floor plate during embryonic development. F-spondin, a gene also expressed in the floor plate, encodes a secreted, extracellular matrix-attached protein that promotes outgrowth of commissural axons and inhibits outgrowth of motor axons. F-spondin is processed in vivo to yield an amino half protein that contains regions of homology to reelin and mindin, and a carboxyl half protein that contains either six or four thrombospondin type I repeats (TSRs). We have tested F-spondin to see whether it is subjected to processing by plasmin and to determine whether the processing modulates its biological activity. Plasmin cleaves F-spondin at its carboxyl terminus. By using nested deletion proteins and mutating potential plasmin cleavage sites, we have identified two cleavage sites, the first between the fifth and sixth TSRs, and the second at the fifth TSR. Analysis of the extracellular matrix (ECM) attachment properties of the TSRs revealed that the fifth and sixth TSRs bind to the ECM, but repeats 1-4 do not. Structural functional experiments revealed that two basic motives are required to elicit binding of TSR module to the ECM. We demonstrate further that plasmin releases the ECM-bound F-spondin protein.

Impaired activation of the fibrinolytic system in children with Henoch-Schönlein purpura: beneficial effect of hydrocortisone plus Sigma-aminocaproic acid therapy on disappearance rate of cutaneous vasculitis and fibrinolysis

Systemic vasculitis is a predominant clinical symptom in Henoch-Schönlein purpura (HSP), and some studies suggested that decreased blood fibrinolytic activity, as well as blood platelets, is of importance in the development of cutaneous vasculitis. Although patients with HSP have normal blood coagulation, little is known about the fibrinolytic system. On the other hand, it is known that the focus of Sigma-aminocaproic acid (EACA) activity in vivo is probably the blood platelet-vessel wall interaction or a vascular component alone. The aim of this study was, therefore, to investigate blood coagulation and fibrinolytic system as well as the effect of hydrocortisone (H) plus EACA therapy (Group I) on plasma antithrombin-III (AT-III), alpha1-proteinase inhibitor (alpha1-PI), alpha2-antiplasmin (alpha2-A), alpha2-macroglobulin (alpha2-M) activity, fibrinogen and plasminogen concentrations in plasma, euglobulin clot lysis time (ELT), and disappearance rate of cutaneous vasculitis in 14 children with HSP aged 7.6 +/- 3.1 (SD) years. Ten patients (8.6 +/- 2.5 years old) were treated with H alone (Group II), and 8 healthy, age-matched children served as controls. Plasma proteinase inhibitor activity was estimated with the kinetic method using Boehringer chromozyme tests before administration of H (9.2 +/- 3.3 mg/kg/d, i.v.) plus EACA (140 +/- 52 mg/kg/d, p.o.) for 5.93 +/- 2.05 days, and 24 hours after the last dose of EACA, as well as before and after treatment with H alone (8.25 +/- 1.74 mg/kg/24 h, i.v.) for 7.1 +/- 1.2 days. It was found that patients with HSP had the initial fibrinogen and plasminogen plasma concentrations significantly increased compared with the controls (Group I: 3.93 +/- 1.3 g/L and 124 +/- 38%; Group II: 4.24 +/- 0.89 g/L and 134 +/- 42% vs. 2.96 +/- 0.34 g/L, and 90 +/- 14%, respectively). Also, there was a marked decrease of the initial plasma alpha2-A activity in Group II compared with the controls (0.69 +/- 0.29 vs. 0.94 +/- 0.11 IU/mL, respectively, t = 2.33, P <.045). Both treatment regimens significantly improved fibrinolysis, which manifested as a shortening of ELT, but the mean values of this parameter remained within normal range. After treatment with H plus EACA, the skin lesions started to disappear significantly faster compared with the H alone regimen (2.28 +/- 0.45 days vs. 4.12 +/- 1.05, t = 4.41, P <.0023). In four of six patients receiving H plus EACA therapy, an approximately 20% decrease of systolic and diastolic arterial blood pressure lasting for 5 to 7 hours after administration of EACA was observed. These results may suggest that children with HSP have impaired plasma fibrinolytic activity and that an increased release of plasminogen activator inhibitor-1 (PAI-1) might be, at least in part, responsible for this phenomenon. Concomitant use of H (approximately 10 mg/kg/d) plus EACA (approximately 100 mg/kg/d) for a few days opens new therapeutic possibilities in some children with HSP.

Cartilage oligomeric matrix protein (thrombospondin-5) is expressed by human vascular smooth muscle cells

Cartilage oligomeric matrix protein (COMP/thrombospondin [TSP]-5) belongs to the thrombospondin gene family and is an extracellular glycoprotein found predominantly in cartilage and tendon. To date, there is limited evidence of COMP/TSP-5 expression outside of the skeletal system. The aim of the present study was to investigate the expression of COMP/TSP-5 in cultured human vascular smooth muscle cells and human arteries. COMP/TSP-5 mRNA and protein expression was detected in cultured human vascular smooth muscle cells with both Northern blotting and immunoprecipitation. Serum, as well as transforming growth factor (TGF)beta1 and TGF-beta3, stimulated COMP/TSP-5 mRNA expression. COMP/TSP-5 was detected in normal as well as atherosclerotic and restenotic human arteries with immunohistochemistry. The majority of COMP/TSP-5 was expressed in close proximity to vascular smooth muscle cells. In vitro attachment assays demonstrated strong adhesion of smooth muscle cells to COMP/TSP-5-coated surfaces, with the majority of cells spreading and forming stress fibers. In addition, COMP/TSP-5 supported the migration of smooth muscle cells in vitro. The present study shows that COMP/TSP-5 is present in human arteries and may play a role in the adhesion and migration of vascular smooth muscle cells during vasculogenesis and in vascular disease settings such as atherosclerosis.

Tissue factor pathway inhibitor binds to platelet thrombospondin-1

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type serine proteinase inhibitor that down-regulates tissue factor-initiated blood coagulation. The most biologically active pool of TFPI is associated with the vascular endothelium, however, the biochemical mechanisms responsible for its cellular binding are not entirely defined. Proposed cellular binding sites for TFPI include nonspecific association with cell surface glycosaminoglycans and binding to glycosyl phosphatidylinositol-anchored proteins. Here, we report that TFPI binds specifically and saturably to thrombospondin-1 (TSP-1) purified from platelet alpha-granules with an apparent K(D) of approximately 7.5 nm. Binding is inhibited by polyclonal antibodies against TFPI and partially inhibited by the B-7 monoclonal anti-TSP-1 antibody. TFPI bound to immobilized TSP-1 remains an active proteinase inhibitor. Additionally, in solution phase assays measuring TFPI inhibition of factor VIIa/tissue factor catalytic activity, the rate of factor Xa generation was decreased 55% in the presence of TSP-1 compared with TFPI alone. Binding experiments done in the presence of heparin and with altered forms of TFPI suggest that the basic C-terminal region of TFPI is required for TSP-1 binding. The data provide a mechanism for the recruitment and localization of TFPI to extravascular surfaces within a bleeding wound, where it can efficiently down-regulate the procoagulant activity of tissue factor and allow subsequent aspects of platelet-mediated healing to proceed.

Thrombospondin-1 binds to polyhistidine with high affinity and specificity

Thrombospondin-1 (TSP1) is a secreted trimeric glycoprotein of 450 kDa with demonstrated effects on cell growth, adhesion and migration. Its complex biological activity is attributed to its ability to bind to cell-surface receptors, growth factors and extracellular-matrix proteins. In this study, we used a (125)I solid-phase binding assay to demonstrate that TSP1 binds specifically to proteins containing polyhistidine stretches. Based on studies with three different six-histidine-containing recombinant proteins, we derived an average dissociation constant of 5 nM. The binding of (125)I-labelled TSP1 to these proteins was inhibited by peptides containing histidine residues, with the degree of competition being a function of the number of histidines within the peptide. Binding was not inhibited by excess histidine or imidazole, indicating that the imidazole ring is not sufficient for recognition by TSP1. Heparin was a potent inhibitor of binding with a K(i) of 50 nM, suggesting that the heparin-binding domain of TSP1 may be involved in this interaction. This was confirmed by the ability of a recombinant heparin-binding domain of TSP1 to directly compete for TSP1 binding to polyhistidine-containing proteins. Affinity chromatography with a polyhistidine-containing peptide immobilized on agarose revealed that TSP1 in platelet releasates is the major polypeptide retained on the six-histidine-peptide column. We conclude that TSP1 contains a high-affinity binding site for polyhistidine and this is likely to be the molecular basis for the observed binding of TSP1 to histidine-rich glycoprotein. The possibility that other polyhistidine-containing proteins also interact with TSP1 warrants further study.

Solution structure and dynamics of the plasminogen kringle 2-AMCHA complex: 3(1)-helix in homologous domains

The kringle 2 (K2) module of human plasminogen (Pgn) binds L-lysine and analogous zwitterionic compounds, such as the antifibronolytic agent trans-(aminomethyl)cyclohexanecarboxylic acid (AMCHA). Far-UV CD and NMR spectra reveal little conformational change in K2 upon ligand binding. However, retarded (1)H-(2)H isotope exchange kinetics induced by AMCHA indicate stabilization of the K2 conformation by the ligand. Assessment of secondary structure content from CD spectra yields approximately 26% beta-STRAND, approximately 13% beta-TURN, approximately 15% 3(1)-HELIX, and approximately 6% 3(10)-HELIX. The NMR solution conformation of the K2 domain complexed to AMCHA has been determined [heavy atom rmsd = 0.49 +/- 0.09A (BACKBONE) AND 1.02+/- 0.08 (ALL)]. The K2 molecule has overall dimensions of approximately 34.5A times approximately 33.4A times approximately 22.7A . Analogous with the polypeptide outline of homologous domains, K2 contains three short antiparallel beta-sheets (paired strands 15-16/20-21, 24-25/48-49, and 62-64/72-74) and four defined beta-turns (residues 6-9, 16-19, 53-56, AND 67-70). Consistent with the CD analysis, albeit novel in the context of kringle folding, the NMR structure reveals an unpaired beta-strand structured by residues 30-32, a turn of 3(10)-helix compromising residues 38-41, and a 3(1)-helix for residues 21-24 and 74-79. We also identify alignable 3(1)-helices in previously reported homologous kringle structures. Rather high order parameter S(2) values (<S(2)>= approximately 0.85 +/- 0.04) characterize the K2 backbone dynamics. The lowest flexibility is observed for the two inner loop segments of residues 51-63 AND 63-75 (<S(2)>= approximately 0.86-0.87 +/- 0.03). Overhauser connectivities reveal close hydrophobic contacts of the ligand ring with side chains of Tyr(36), Trp(62), Phe(64), Trp(72), AND Leu(74). In most K2 structures, the N atom of AMCHA places itself approximately 3.9 and 4.4A from the anionic groups of Glu(57) and Asp(55), respectively, while its carboxylate group, H-bonded to the Tyr(36) side chain OH(eta), ion-pairs the Arg(71) guanidinium group. Consistent with the preference of K2 for binding 5-aminopentanoic acid over 6-aminohexanoic acid, the positions of the ionic centers within the K2 binding site approach each other approximately 1A closer relative to what is observed in lysine binding sites of homologous Pgn modules.

Sertolin is a novel gene marker of cell-cell interactions in the rat testis

A novel testicular protein designated sertolin was cloned. The full-length sertolin cDNA consists of 853 base pairs with an open reading frame of 381 base pairs coding for a 127-amino acid polypeptide that shares limited identities with antaxin/josephin and thrombospondin proteins. Sertolin (calculated molecular mass, 13,759 daltons) has two mRNA transcripts of 2.3 and 1 kilobase. A 22-amino acid peptide based on the deduced amino acid sequence of sertolin (NH(2)-KKEHFNLFKAASVSHLVQVVPQ) was synthesized and used for polyclonal antibody production. Immunoblot analysis detected a 17-kDa immunoreactive band in the Sertoli cell cytosol. Using Sertoli-germ cell cocultures, sertolin expression was found to be reduced by as much as 5-fold at the time when germ cells attach onto Sertoli cells but preceding the establishment of specialized inter-Sertoli-germ cell junctions. Neither FSH nor 17beta-hydroxy-5alpha-androstan-3-one was able to affect sertolin expression, whereas estradiol-17beta and progesterone induced a significant increase in Sertoli cell sertolin expression in vitro. In addition, interleukin-1alpha, a germ cell-derived cytokine, was also able to elicit a transient but significant increase in Sertoli cell sertolin expression. Sertolin expression was also shown to increase with testicular development and is likely to be associated with the onset of spermatogenesis. In addition, sertolin expression increased in the testis when generalized inflammation was induced in adult rats by injection of fermented yeast. These results show that sertolin will be useful in characterizing cell-cell interactions in the testis.

Functional changes in the conformation of thrombospondin-1 during complexation with fibronectin or heparin

Thrombospondin-1 (TSP-1) interacts specifically with heparin and fibronectin in vitro and colocalizes with fibronectin and heparan sulfate in the extracellular matrix (ECM). Its conformation is strongly dependent on Ca2+ concentration. We have previously shown that both heparin and fibronectin have two binding sites on the TSP-1 subunit which may require conformational change for their occupancy (R. Dardik and J. Lahav, 1987, Eur. J. Biochem. 168, 347; ibid 1989, 185, 581). To investigate the effect of TSP-1 binding to fibronectin and heparin on its functional conformation, TSP-1 was subjected to proteolysis in the presence and absence of ligands and of Ca2+. We found that while trypsin cleavage of free TSP-1 resulted in the inactivation of ligand binding, TSP-1 bound to either fibronectin or heparin remained stably associated with these ligands. Cleavage by thrombin or tissue plasminogen activator (tPA) showed that Ca2+-depleted TSP-1, when bound to fibronectin or to heparin, yielded proteolytic cleavage patterns typical of the Ca2+-containing form. Cleavage by chymotrypsin was not affected by binding to fibronectin or heparin; hence loss of proteolytic susceptibility was not due to steric hindrance by the ligands. Taken together, these results indicate that: (A) binding of TSP-1 to fibronectin or heparin is a two-step mechanism where binding to one site leads to conformational changes that enable binding to the second site; (B) TSP-1 in complex with fibronectin or heparin adopts the Ca2+-containing conformation in the absence of Ca2+; and (C) such complexes are highly resistant to cleavage by tPA and, if cleaved by other enzymes, the TSP-1 fragments remain bound to other ECM components. These characteristics have profound significance for platelet adhesion and cell migration into wounds where Ca2+ concentrations are reduced.

Thrombospondin expression in nerve regeneration II. Comparison of optic nerve crush in the mouse and goldfish

Expression of the extracellular matrix molecule thrombospondin (TSP) was examined following retrobulbar crush injury of the goldfish and mouse optic nerve. TSP was present within the glia limitans and surrounding axon fascicles of the control normal goldfish optic nerve, but was absent from the normal mouse optic nerve. Following crush injury of the goldfish optic nerve, TSP expression increased dramatically along the path of regenerating axons and returned to near normal levels following axonal outgrowth. In contrast, during the unsuccessful attempt at regeneration following crush injury of the mouse optic nerve, TSP expression was present only in glial fibrillary acidic protein (GFAP)-negative, macrophage-rich regions distal to ganglion cell axons. These results indicate that TSP expression is increased in a temporal pattern along the path of regenerating goldfish optic nerve axons and therefore may be involved in successful central nervous system regeneration. The absence of TSP in the environment encountered by damaged mouse optic nerve axons may correlate with the lack of regeneration observed in the mouse optic nerve.

Interaction of recombinant procollagen and properdin modules of thrombospondin-1 with heparin and fibrinogen/fibrin

Many properties have been assigned to the procollagen and properdin (Type I) modules of thrombospondin-1 (TSP1) based on activities of large proteolytic fragments of TSP1 or peptides containing TSP1-derived sequences. To examine the activities of the modules more exactly, we expressed the first properdin module (P1); the third properdin module (P3); the first and second properdin modules (P12); the first, second, and third properdin modules (P123); and the procollagen module with the first, second, and third properdin modules (CP123) in the GELEX expression vector (GE1) using the baculovirus system. GE1 encodes the pre-pro sequence, the transglutaminase cross-linking site(s), the protease-sensitive site, and the gelatin binding domain from the amino terminus of rat fibronectin. All five recombinant proteins were expressed by insect cells, secreted into the culture medium, and purified by gelatin-agarose affinity chromatography. P123 shared with TSP1 a resistance to trypsin unless reduced and alkylated. P12/GE1, P123/GE1, and CP123/GE1 bound poorly to heparin-agarose except in the absence of sodium chloride, whereas peptides based on P2 are known to bind to heparin in up to 150 mM sodium chloride. In cross-linking experiments employing activated recombinant factor XIII and the transglutaminase cross-linking site in the fibronectin-derived sequence, P12/GE1, P123/GE1, CP123/GE1, and P3/GE1 but not P1/GE1 became incorporated into a fibrin clot more than GE1 alone. Analysis of the complex indicated that cross-linking was to the portion of the fibrin alpha-chain remaining in the D-dimer of plasmin digests. P123 also cross-linked to the Aalpha-chain of unclotted fibrinogen. P123 competed for 125I-TSP1 incorporation into the fibrin clot. P123 did not cross-link to plasminogen, histidine-rich glycoprotein, fibronectin, or plasma globulins other than fibrinogen/fibrin. These results indicate that the properdin modules of TSP1 specifically interact with fibrinogen/fibrin but not with heparin under physiologic conditions.

Anti-CD36 autoantibodies in thrombotic thrombocytopenic purpura and other thrombotic disorders: identification of an 85 kD form of CD36 as a target antigen

The presence of anti-CD36 antibodies in plasma of patients with thrombotic thrombocytopenic purpura (TTP), idiopathic thrombocytopenic purpura (ITP), and heparin-induced thrombocytopenia without/with thrombosis (HIT/HITT) has been examined by immunoblots, and a monoclonal antibody capture assay, the platelet-associated IgG characterization assay (PAICA). Results with PAICA showed that 73% (8/11) of patients with TTP were positive, and 71% (10/14) by immunoblots. With ITP, 20% (6/30) were positive by PAICA and 19% (3/16) by immunoblots; HIT, 30% (3/10) were positive by PAICA and 60% (6/10) by immunoblot; HITT, 50% (2/4) by PAICA and 100% (4/4) by immunoblot. Purification of CD36 by fast protein liquid chromatography (FPLC) from Triton X-100 extracts of normal platelet membranes resulted in the isolation of two different forms: the classic 88 kD form, and a second, lighter 85 kD form. Our data indicated that the patients' plasma autoantibodies reacted strongly with the 85 kD form. Conventional monoclonal and polyclonal antisera produced to the 88 kD form reacted strongly with the 88 kD form but weakly with the 85 kD form. These results confirm the possible importance of anti-CD36 antibodies in the pathophysiology of TTP and other thrombocytopenias and demonstrate the presence of a previously unrecognized target antigen for these antibodies.

Thrombospondin-1 expression in human myometrium before and during pregnancy, before and during labor, and in human myometrial cells in culture

The activation of latent transforming growth factor beta (L-TGFbeta) is essential for the action of TGFbeta, which, in turn, is involved in the regulation of expression of some progesterone-responsive genes. One mechanism by which TGFbeta is activated involves thrombospondin (TSP), a protein that binds extracellular proteins. Immunoreactive TSP (irTSP) protein and TSP-1 mRNA in myometrial tissues of ovulatory and pregnant women were localized by immunohistochemistry and in situ hybridization. IrTSP and TSP-1 mRNA were randomly distributed in myometrial smooth muscle cells of some, but not all, tissues of pregnant women at term before labor; but in some areas of most of these tissues, irTSP was intense and commonly localized extracellularly. Intense irTSP and TSP-1 mRNA in myocytes were more common in myometrium during labor. In myometrium from ovulatory women (n = 26), irTSP was localized primarily in vascular smooth muscle cells and was detected occasionally in scattered myocytes. Little TSP-1 mRNA was demonstrable by in situ hybridization in vessels or myocytes of myometrial tissue from ovulatory women (n = 7). By Northern analysis of total RNA, TSP-1 mRNA was detected in myometrial tissue of pregnant women and in human myometrial smooth muscle cells in culture. The levels of TSP-1 mRNA in myometrial tissues of pregnant women during labor (n = 18) were greater than those in myometrium at > 37 wk gestation before labor began (n = 25, p < 0.001). The ratios of TSP-1 to glyceraldehyde 3-phosphate dehydrogenase mRNAs in 3 myometrial tissues during oxytocin-induced labor were not statistically different from those in myometrium during spontaneous labor but were greater than those in myometrium before labor (p < 0.05). The level of TSP-1 mRNA in confluent human myometrial cells in culture was relatively high, was increased by treatment with fetal bovine serum, and was decreased by treatment with platelet-derived growth factor or activators of adenylyl cyclase or protein kinase C. Myometrial cells in culture constitute a useful model for studying the regulation of TSP-1 gene expression in human myometrium.

Lysis of Plasma Clots by Urokinase-Soluble Urokinase Receptor Complexes

Single-chain urokinase plasminogen activator (scuPA), the unique form secreted by cells, expresses little intrinsic plasminogen activator activity. scuPA can be activated by proteolytic cleavage to form a two-chain enzyme (tcuPA), which is susceptible to inhibition by plasminogen activator inhibitor type I (PAI-1). scuPA is also activated when it binds to its cellular receptor (uPAR), in which case the protein remains as a single chain molecule with less susceptibility to PAIs. Fibrin clots are invested with PAI-1 derived from plasma and from activated platelets. Therefore, we compared the fibrinolytic activity of complexes between scuPA and recombinant soluble uPAR (suPAR) to that of scuPA, tcuPA, and tcuPA/suPAR complexes. scuPA/suPAR complexes mediated the lysis of plasma-derived fibrin clots 14-fold more extensively than did equimolar concentrations of scuPA and threefold more extensively than did tcuPA or tcuPA/suPAR, respectively. The enhanced catalytic activity of scuPA/suPAR required that all three domains of the receptor be present, correlated with its PAI-1 resistance, was not dependent on fibrin alone, and required a plasma cofactor that was identified as IgG. Human IgG bound specifically to suPAR and scuPA/suPAR as determined by using affinity chromatography and immunoprecipitation. Plasma depleted of IgG lost most of its capacity to promote the fibrinolytic activity of scuPA/suPAR, and the activity of the complex was restored by adding plasma concentrations of purified IgG. These studies indicate that scuPA/suPAR can function as a plasminogen activator in a physiological milieu.

© 1998 by The American Society of Hematology.

Lysis of Plasma Clots by Urokinase-Soluble Urokinase Receptor Complexes

Single-chain urokinase plasminogen activator (scuPA), the unique form secreted by cells, expresses little intrinsic plasminogen activator activity. scuPA can be activated by proteolytic cleavage to form a two-chain enzyme (tcuPA), which is susceptible to inhibition by plasminogen activator inhibitor type I (PAI-1). scuPA is also activated when it binds to its cellular receptor (uPAR), in which case the protein remains as a single chain molecule with less susceptibility to PAIs. Fibrin clots are invested with PAI-1 derived from plasma and from activated platelets. Therefore, we compared the fibrinolytic activity of complexes between scuPA and recombinant soluble uPAR (suPAR) to that of scuPA, tcuPA, and tcuPA/suPAR complexes. scuPA/suPAR complexes mediated the lysis of plasma-derived fibrin clots 14-fold more extensively than did equimolar concentrations of scuPA and threefold more extensively than did tcuPA or tcuPA/suPAR, respectively. The enhanced catalytic activity of scuPA/suPAR required that all three domains of the receptor be present, correlated with its PAI-1 resistance, was not dependent on fibrin alone, and required a plasma cofactor that was identified as IgG. Human IgG bound specifically to suPAR and scuPA/suPAR as determined by using affinity chromatography and immunoprecipitation. Plasma depleted of IgG lost most of its capacity to promote the fibrinolytic activity of scuPA/suPAR, and the activity of the complex was restored by adding plasma concentrations of purified IgG. These studies indicate that scuPA/suPAR can function as a plasminogen activator in a physiological milieu.

© 1998 by The American Society of Hematology.

Lysosomal integral membrane protein II binds thrombospondin-1. Structure-function homology with the cell adhesion molecule CD36 defines a conserved recognition motif

LIMPII (lysosomal integral membrane protein II) is one of a family of proteins structurally related to the cell surface glycoprotein CD36. We recently defined a single structural domain on CD36 that mediates binding to adhesive glycoprotein thrombospondin-1 (TSP1). The CD36-TSP1 interaction is known to play a role in platelet-tumor and platelet-monocyte adhesion, angiogenesis, and in monocyte uptake of apoptotic cells. To test whether LIMPII also binds TSP1, a LIMPII peptide corresponding to the TSP1 binding domain of CD36 was expressed as a recombinant glutathione S-transferase (GST) fusion protein. In solid phase binding assays, purified 125I-TSP1 bound to immobilized GST/LIMPII in a time-dependent and saturable manner. Inhibition by excess unlabeled TSP1 or EDTA demonstrated specificity. LIMPII.TSP1 complex formation was specifically blocked by soluble LIMPII fusion protein, by monospecific rabbit IgG directed against the LIMPII peptide and by CD36 fusion proteins containing the TSP1 binding domain. Transfection of Bowes melanoma cells with a chimeric LIMPII cDNA that targets expression to the plasma membrane conferred the ability to bind 125I-TSP1 and to adhere to TSP1-coated surfaces. This study defines a TSP1 binding site conserved between LIMPII and CD36 and suggests that cell surface LIMPII may function in some circumstances as an adhesion receptor for TSP1. Computer-assisted homology searches suggest that the TSP1 recognition motif identified from study of CD36 family members may be widely expressed in nature.

High-molecular-mass and low-molecular-mass kininogens block plasmin-induced platelet aggregation by forming a complex with kringle 5 of plasminogen/plasmin

We have previously demonstrated a low-affinity (0.8 microM, non-covalent complex formation between high-molecular-mass kininogen (HK) and plasminogen (Plg) which prevented Plg interaction with glioma and endothelial cells. We have now extended our previous observations by exploring the potential complex formation between Plg and low-molecular-mass kininogen (LK) and between LK and HK with Plg cleaved with human neutrophil elastase (HNE). Plg cleavage by HNE (PlgHNE) yielded kringles 1-3, kringle 4 and mini-plasminogen. PlgHNE was subjected to SDS/PAGE under non-reducing conditions, followed by western blotting, and incubated with either 125I-HK or 125I-LK. Autoradiograms revealed that 125I-HK bound to miniplasminogen and to kringles 1-3 but not to kringle 4 and the presence of 10 mM 6-aminohexanoic acid (Ahx) disrupted only the interaction with kringles 1-3. In contrast, 125I-LK bound to miniplasminogen but not to kringles 1-3 or 4 and Ahx had no effect at all. The complex formation of either HK (0.67 microM) or LK (3 microM) with Plg (1.5 microM) did not affect its conversion to plasmin by tissue plasminogen activator (t-PA) (10 U/ml) in the presence of a tissue plasminogen stimulator (0.14 microM). However, the rate of conversion of plasminogen to plasmin by t-PA was affected when platelets were added to the reaction mixture. Since HK (0.83 microM) has been shown to inhibit plasmin-induced platelet aggregation, we investigated whether this inhibitory property is found within the heavy chain shared by HK and LK. We found that LK inhibited plasmin-induced platelet aggregation, but a 4-fold molar excess was required when compared to HK. Compared to plasmin, 3-5-fold molar excess of miniplasmin is required to induce platelet aggregation, indicating the important role of kringles 1-3 for plasmin interactions with these cells. These results indicate that HK and LK-mediated inhibition of plasmin-induced platelet aggregation is likely due to complex formation with kringle 5 without interfering with plasmin's active site. We found an additional interaction between HK and kringles 1-3 enhancing the inhibitory effect, presumably by interfering with plasmin's interaction with platelets. This HK and LK-associated modulation of plasmin-induced platelet aggregation may serve as a template to develop synthetic peptides as novel therapeutic agents to prevent some of the plasmin-associated thrombocytopenia seen during thrombolytic therapy.

The urokinase-type plasminogen activator system in cancer metastasis: a review

The urokinase-type plasminogen activator (u-PA) system consists of the serine proteinases plasmin and u-PA; the serpin inhibitors alpha2-anti-plasmin, PAI-1 and PAI-2; and the u-PA receptor (u-PAR). Two lines of evidence have strongly suggested an important and apparently causal role for the u-PA system in cancer metastasis: results from experimental model systems with animal tumor metastasis and the finding that high levels of u-PA, PAI-1 and u-PAR in many tumor types predict poor patient prognosis. We discuss here recent observations related to the molecular and cellular mechanisms underlying this role of the u-PA system. Many findings suggest that the system does not support tumor metastasis by the unrestricted enzyme activity of u-PA and plasmin. Rather, pericellular molecular and functional interactions between u-PA, u-PAR, PAI-1, extracellular matrix proteins, integrins, endocytosis receptors and growth factors appear to allow temporal and spatial re-organizations of the system during cell migration and a selective degradation of extracellular matrix proteins during invasion. Differential expression of components of the system by cancer and non-cancer cells, regulated by paracrine mechanisms, appear to determine the involvement of the system in cancer cell-directed tissue remodeling. A detailed knowledge of these processes is necessary for utilization of the therapeutic potential of interfering with the action of the system in cancers.

Factor H co-purifies with thrombospondin isolated from platelet secretate

Thrombospondin is a trimeric glycoprotein that has several known functions, including roles in platelet aggregation, phagocytosis and an inhibitor of angiogenesis. Typically the molecule is isolated from platelet secretate by heparin affinity followed by sizing chromatography. In this study, purity is analysed by 7.5% SDS-PAGE under reducing conditions when thrombospondin monomers run as a band at around 180 kDa. Under nonreducing conditions of 7.5% SDS-PAGE, thrombospondin does not penetrate beyond the stacking gel; however, under these conditions a major contaminating band can be seen which, upon reduction, merges into the thrombospondin band. Further purification of this contaminating protein was achieved by DEAE chromatography and it was identified as Factor H by peptide sequencing and immunoblotting. Factor H function was demonstrated by the ability of the protein to function as a cofactor in the Factor-I-mediated cleavage of C3b. Since Factor H has several known functions, such contamination could confound functional studies of thrombospondin thus purified and a pre-elution step of the heparin affinity column is recommended.

Electron spin resonance and fluorescence studies of the conformational environment of the thiol groups of thrombospondin: interactions with thrombin

The free thiols of platelet thrombospondin (TSP) were modified with thiol-specific spin labels and fluorescence probes. The conformational effects of thrombin complexation with TSP were monitored by thiol-specific spin labels covalently attached to TSP and active site specific spin labels on thrombin. The results provide evidence supporting speculations that the thiols of the three polypeptide chains in TSP are not conformationally identical. Studies on the effects of Ca2+ and temperature confirm that TSP exists in multiple conformations which are under dynamic equilibrium. The ESR spectra of spin-labeled TSP are sensitive to the proteolytic effects of thrombin in the presence and absence of calcium. Phenylsulfonyl fluoride spin labels specific for the active site of thrombin are excellent indicators of thrombin: TSP complex formation in the absence of calcium. The anticoagulant thrombin inhibitor hirudin competes with TSP for the same binding locus on thrombin (which includes the requirement of an intact anion exosite). The results suggest that the species observed here is the noncovalent complex formed during the first step of the TSP--thrombin interaction, showing also that thrombin activity is not essential for complex formation. ESR and fluorescence studies of thiol-labeled TSP indicate that the sulfhydryls are not affected in the noncovalent thrombin: TSP complex, although they must be playing a major role in the second step, i.e., formation of the covalent complex, through intermolecular thiol exchange.

Location of the major epsilon-(gamma-glutamyl)lysyl cross-linking site in transglutaminase-modified human plasminogen

Tissue and plasma transglutaminases cross-link human plasminogen into high molecular weight complexes (Bendixen, E., Borth, W., and Harpel, P. C. (1993) J. Biol. Chem. 268, 21962-21967). A major cross-linking site in plasminogen involved in the tissue transglutaminase-mediated polymerization process has been identified. The epsilon-(gamma-glutamyl)lysyl bridges of the polymer are formed between Lys-298 and Gln-322. Both the acyl donor Gln residue and the acyl acceptor Lys residue are located in the kringle 3 domain of plasminogen, i.e. cross-linking of plasminogen by tissue transglutaminase involves neither the catalytic domain nor the lysine-dependent binding sites of plasminogen. This study documents that kringle 3 contains a novel functional site with the potential to participate in transglutaminase-mediated cross-linking interactions with plasma, cell-surface, and extracellular proteins.

Recombinant GST/CD36 fusion proteins define a thrombospondin binding domain. Evidence for a single calcium-dependent binding site on CD36

CD36 is a multifunctional cell surface glycoprotein that acts as a surface receptor for thrombospondin (TSP), and thereby may mediate adhesive interactions between cells and substrata, platelets and other cells, and macrophages and apoptotic neutrophils. The identity of the TSP binding site on CD36 is controversial and may involve more than one structural domain. We have constructed a series of recombinant bacterial GST/CD36 fusion proteins that span nearly all of the CD36 molecule and have demonstrated that fusion proteins containing the region extending from amino acid 93 to 120 formed specific, saturable, and reversible complexes with TSP. As with intact CD36, binding was calcium-dependent, was independent of which ligand was immobilized, and was blocked by monoclonal antibodies to both CD36 and TSP. Stoichiometry and affinity of the fusion proteins for TSP were consistent with that of the intact protein. We also demonstrated that these fusion proteins competitively inhibited binding of TSP to purified platelet CD36 and to cell surface CD36 on peripheral blood monocytes and CD36 cDNA-transfected melanoma cells. These data demonstrate that the region between amino acids 93 and 120 has all of the characteristics required of the TSP binding domain.

Binding and activation of plasminogen on the surface of osteosarcoma cells

Plasmin (Pm) is a broad action serine protease implicated in numerous physiological functions. In bone, Pm may play a role in growth, resorption, metastasis, and the activation of growth factors. The various components of the Pm system are known to bind and function on the cell surface of various cell types, but no pertinent data are available describing membrane-bound Pm or its zymogen, plasminogen (Pg), in either normal or neoplastic bone cells. We report here that Pg binds to the surface of the human osteosarcoma cell line MG-63 and is activated to Pm by endogenous urokinase plasminogen activator (uPA). These conclusions are based on experiments utilizing radiolabeled compounds and a cell surface proteolytic assay measuring amidolytic activity of Pm. 125I-Pg binding to cells was time dependent, saturable, reversible, and specific. Binding was characterized by a relatively low affinity (Kd approximately 0.9 microM) and a high capacity (approximately 7.5 x 10(6) sites/cell). The binding of 125I-Pg was associated with lysine binding sites of the plasminogen molecule. Activation of 125I-Pg to 125I-Pm occurred on the cell surface and was dependent upon cell bound uPA, as determined by inhibitory antibodies. Binding of Pg to MG-63 monolayers represented approximately 80% bound specifically to the cell surface and the remainder to the surrounding extra-cellular matrix. Either co-incubation with uPA or pre-incubation with Pm resulted in increased 125I-Pg binding to osteosarcoma cells. Cell surface Pm proteolytic activity was confirmed by an amidolytic chromogenic assay. Both Pm and Pg bound to cells with Pg being activated by endogenous uPA. Plasmin activated on the cell surface was partially protected from inhibition by alpha 2-antiPm (requiring Pm lysine binding site interaction) but inhibited by aprotinin, (interacting directly with the Pm catalytic site). Resistance of cell bound Pm to alpha 2-antiPm inhibition suggests that cell surface proteolysis can occur in the presence of a soluble Pm inhibitor known to exist in the extracellular space. Based on these results, we speculate that the various bone physiological processes implicating Pm may occur at or near the bone cell surface.

Expression of thrombospondin in the adult nervous system

Thrombospondin (TSP) is an extracellular matrix molecule that has been previously associated with neural development and neurite outgrowth in vitro. Little is known, however, about the expression of TSP in the adult nervous system. In this study, TSP localization was examined in nervous tissue from adult mouse, goldfish, newt, and adult and juvenile Xenopus. TSP was associated with neurons in the brains of all species examined. TSP was present in central nerve tracts capable of regeneration, such as the goldfish, Xenopus, and newt optic nerves, but was absent from tracts not capable of regeneration, such as the mouse optic nerve. TSP was also present in the neuropil of goldfish and newt spinal cord, but was restricted to motor neurons in mice and adult Xenopus. In addition, TSP was observed in sciatic nerves of mice, Xenopus, and newt. These results indicate a correlation between the presence of TSP and the potential for successful nerve regeneration across a wide range of animal classes.

Thrombospondin as an agglutinin of platelets

To study a hypothesis that thrombospondin (TSP) might function as an agglutinin in platelet aggregation, we designed two experiments. First, we prepared fibrinogen-coated agarose beads (fbg-beads) as a model of platelets, and subjected them to aggregometry using TSP as an inducer. TSP induced agglutination of fbg-beads in a dose-dependent manner. Calcium (Ca) and Magnesium (Mg) were necessary for the agglutination, and the aggregability was dependent on the concentration of Ca. These results confirmed the function of TSP as an agglutinin, suggesting some characteristics of the fbg-TSP interaction as well. Secondly, a variety of platelets were subjected to TSP-induced aggregation assay. Both gel-filtrated and washed-platelets were aggregated by TSP in a dose dependent manner and dissociated with EDTA. The same aggregation was observed in formalin-fixed platelets. Both Ca and Mg were required for the aggregation, and the maximum aggregation rate was dependent on the Ca concentration. Ca seemed to regulate the capacity as well as the affinity of the binding sites for TSP on platelets. Fibrinogen and some aminosugars inhibited the aggregation. These data suggest TSP may function as an agglutinin of platelets, and Ca may regulate the interaction between platelets and TSP. As one of the candidates for the receptor for TSP on platelet, fbg-GPIIb/IIIa was suggested because of the similarity between fbg-beads and platelets aggregation induced by TSP, and the Ca-dependency in both the GPIIb/IIIa induction and the TSP-induced platelet aggregation.