Human prothrombin fragment 1 and 2 inhibit bFGF-induced BCE cell growth

Pathophysiological Role of Microglial Activation Induced by Blood-Borne Proteins in Alzheimer's Disease

The blood-brain barrier (BBB) restricts entry of neurotoxic plasma components, blood cells, and pathogens into the brain, leading to proper neuronal functioning. BBB impairment leads to blood-borne protein infiltration such as prothrombin, thrombin, prothrombin kringle-2, fibrinogen, fibrin, and other harmful substances. Thus, microglial activation and release of pro-inflammatory mediators commence, resulting in neuronal damage and leading to impaired cognition via neuroinflammatory responses, which are important features observed in the brain of Alzheimer's disease (AD) patients. Moreover, these blood-borne proteins cluster with the amyloid beta plaque in the brain, exacerbating microglial activation, neuroinflammation, tau phosphorylation, and oxidative stress. These mechanisms work in concert and reinforce each other, contributing to the typical pathological changes in AD in the brain. Therefore, the identification of blood-borne proteins and the mechanisms involved in microglial activation and neuroinflammatory damage can be a promising therapeutic strategy for AD prevention. In this article, we review the current knowledge regarding the mechanisms of microglial activation-mediated neuroinflammation caused by the influx of blood-borne proteins into the brain via BBB disruption. Subsequently, the mechanisms of drugs that inhibit blood-borne proteins, as a potential therapeutic approach for AD, along with the limitations and potential challenges of these approaches, are also summarized.

Features of angiogenesis in eye diseases

Based on the analysis of published data, the review provides information on the role and mechanisms of angiogenesis in the development of eye diseases. It has been shown that the developing inflammatory process associated with infections or damage to the organ of vision almost always leads to the appearance of newly formed vessels in the avascular cornea. The progression, in particular, of age-related macular degeneration is associated with the immune-mediated development of angiogenesis processes. A key inducer of angiogenesis is vascular endothelial growth factor (VEGF), whose activity can be enhanced by a number of pro-inflammatory cytokines (tumor necrosis factor alpha, TNF-), growth (fibroblast growth factor, FGF) and transforming factors (transforming growth factor beta, TGF- ). In addition, VEGF overproduction is mediated by an imbalance of pro-angiogenic (angiogenin) and anti-angiogenic (angiostatin, vasostatin, endostatin; tissue inhibitors of matrix metalloproteinases) factors. Antiangiogenic activity based on inhibition of vascular endothelial growth factor (VEGF) has been successfully used in the treatment of a number of eye diseases, such as exudative age-related macular degeneration and diabetic macular edema, the pathogenesis of which is based on the growth of newly formed vessels. The review presents information on the main anti-angiogenic drugs for intravitreal administration, used in ophthalmology.

Angiogenesis: Basic Mechanisms and Clinical Applications

The development and maintenance of an adequate vascular supply is critical for the viability of normal and neoplastic tissues. Angiogenesis, the development of new blood vessels from preexisting capillary networks, plays an important role in a number of physiologic and pathologic processes, including reproduction, wound repair, inflammatory diseases, and tumor growth. Angiogenesis involves sequential steps that are triggered in response to angiogenic growth factors released by inflammatory, mesenchymal, or tumor cells that act as ligands for endothelial cell receptor tyrosine kinases. Stimulated endothelial cells detach from neighboring cells and migrate, proliferate, and form tubes. The immature tubes are subsequently invested and stabilized by pericytes or smooth muscle cells. Angiogenesis depends upon complex interactions among various classes of molecules, including adhesion molecules, proteases, structural proteins, cell surface receptors, and growth factors. The therapeutic manipulation of angiogenesis targeted against ischemic and neoplastic diseases has been investigated in preclinical animal models and in clinical trials. Proangiogenic trials that have stimulated vessel growth in ischemic coronary or peripheral tissues through expression, delivery, or stimulated release of growth factors have shown efficacy in animal models and mixed results in human clinical trials. Antiangiogenic trials have used strategies to block the function of molecules critical for new vessel growth or maturation in the treatment of a variety of malignancies, mostly with results less encouraging than those seen in preclinical models. Pro-and antiangiogenic clinical trials demonstrate that strategies for optimal drug delivery, dosing schedules, patient selection, and endpoint measurements need further investigation and refinement before the therapeutic manipulation of angiogenesis will realize its full clinical potential.

Prothrombin Kringle-2: A Potential Inflammatory Pathogen in the Parkinsonian Dopaminergic System

Although accumulating evidence suggests that microglia-mediated neuroinflammation may be crucial for the initiation and progression of Parkinson's disease (PD), and that the control of neuroinflammation may be a useful strategy for preventing the degeneration of nigrostriatal dopaminergic (DA) projections in the adult brain, it is still unclear what kinds of endogenous biomolecules initiate microglial activation, consequently resulting in neurodegeneration. Recently, we reported that the increase in the levels of prothrombin kringle-2 (pKr-2), which is a domain of prothrombin that is generated by active thrombin, can lead to disruption of the nigrostriatal DA projection. This disruption is mediated by neurotoxic inflammatory events via the induction of microglial Toll-like receptor 4 (TLR4) in vivo , thereby resulting in less neurotoxicity in TLR4-deficient mice. Moreover, inhibition of microglial activation following minocycline treatment, which has anti-inflammatory activity, protects DA neurons from pKr-2-induced neurotoxicity in the substantia nigra (SN) in vivo. We also found that the levels of pKr-2 and microglial TLR4 were significantly increased in the SN of PD patients compared to those of age-matched controls. These observations suggest that there may be a correlation between pKr-2 and microglial TLR4 in the initiation and progression of PD, and that inhibition of pKr-2-induced microglial activation may be protective against the degeneration of the nigrostriatal DA system in vivo. To describe the significance of pKr-2 overexpression, which may have a role in the pathogenesis of PD, we have reviewed the mechanisms of pKr-2-induced microglial activation, which results in neurodegeneration in the SN of the adult brain.

Fibrinogen-derived fibrinostatin inhibits tumor growth through anti-angiogenesis

Angiogenesis is a prerequisite of tumor growth and metastasis and, thus, anti‐angiogenesis treatment has become an important part of cancer therapy. A 15‐amino acid peptide of the fibrinogen α chain, fibrinostatin, was previously found in serum samples of gastric cancer patients. Herein we demonstrated that fibrinostatin has anti‐angiogenesis activity in several angiogenesis models and it reduces tumor growth in mouse xenografts and allografts. Increased tumor necrosis and reduced microvessel density in tumors were observed in mouse xenograft models. Fibrinostatin inhibited proliferation and induced apoptosis in HUVEC, but not in cancer cells. In addition, fibrinostatin specifically entered HUVEC. Fibrinostatin also prevented migration, adhesion and tubule formation of HUVEC in vitro. A single‐dose acute toxicity testing and a repeated‐dose chronic toxicity study in the mouse, rat and monkey indicated that fibrinostatin had a wide margin of safety. Taken together, fibrinostatin shows promise as a potential anti‐angiogenesis therapeutic agent.

Proteolytically Derived Endogenous Angioinhibitors Originating from the Extracellular Matrix

Angiogenesis, a neovascularization process induced from the existing parent blood vessels, is a prerequisite for many physiological and pathological conditions. Under physiological conditions it is regulated by a balance between endogenous angioinhibitors and angioactivators, and an imbalance between them would lead to pathological conditions such as cancer, age-related macular degeneration (AMD), diabetic retinopathy, cardiovascular diseases, etc. Several proteolytically generated endogenous molecules have been identified which exhibit angioinhibition and/or antitumor activities. These angioinhibitors interact with endothelial and tumor cells by binding to distinct integrins and initiate many of their intracellular signaling mechanisms regulating the cell survival and or apoptotic pathways. The present review will focus on the extracellular matrix derived angioinhibitors, and their mechanisms of actions that point to the clinical significance and therapeutic implications.

Prothrombin kringle-2 induces death of mesencephalic dopaminergic neurons in vivo and in vitro via microglial activation

We have shown that prothrombin kringle-2 (pKr-2), a domain of human prothrombin distinct from thrombin could activate cultured rat brain microglia in vitro. However, little is known whether pKr-2-induced microglial activation could cause neurotoxicity on dopaminergic (DA) neurons in vivo. To address this question, pKr-2 was injected into the rat substantia nigra (SN). Tyrosine hydroxylase (TH) immunohistochemistry experiments demonstrate significant loss of DA neurons seven days after injection of pKr-2. In parallel, pKr-2-activated microglia were detected in the SN with OX-42 and OX-6 immunohistochemistry. Reverse transcription PCR and double-label immunohistochemistry revealed that activated microglia in vivo exhibit early and transient expression of inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2) and several proinflammatory cytokines. The pKr-2-induced loss of SN DA neurons was partially inhibited by the NOS inhibitor N(G)-nitro-L-arginine methyl ester hydrochloride, and the COX-2 inhibitor DuP-697. Extracellular signal-regulated kinase 1/2, c-Jun N-terminal kinase and p38 mitogen-activated protein kinase were activated in the SN as early as 1 hr after pKr-2 injection, and localized within microglia. Inhibition of these kinases led to attenuation of mRNA expression of iNOS, COX-2 and several proinflammatory cytokines, and rescue of DA neurons in the SN. Intriguingly, following treatment with pKr-2 in vitro, neurotoxicity was detected exclusively in co-cultures of mesencephalic neurons and microglia, but not microglia-free neuron-enriched mesencephalic cultures, indicating that microglia are required for pKr-2 neurotoxicity. Our results strongly suggest that microglia activated by endogenous compound(s), such as pKr-2, are implicated in the DA neuronal cell death in the SN.

The Chick Embryo Chorioallantoic Membrane as an In Vivo Assay to Study Antiangiogenesis

Antiangiogenesis, e.g., inhibition of blood vessel growth, is being investigated as a way to prevent the growth of tumors and other angiogenesis-dependent diseases. Pharmacological inhibition interferes with the angiogenic cascade or the immature neovasculature with synthetic or semi-synthetic substances, endogenous inhibitors or biological antagonists.The chick embryo chorioallantoic membrane (CAM) is an extraembryonic membrane, which serves as a gas exchange surface and its function is supported by a dense capillary network. Because its extensive vascularization and easy accessibility, CAM has been used to study morphofunctional aspects of the angiogenesis process in vivo and to study the efficacy and mechanism of action of pro- and anti-angiogenic molecules. The fields of application of CAM in the study of antiangiogenesis, including our personal experience, are illustrated in this review article.

Plasmin-cleaved beta-2-glycoprotein 1 is an inhibitor of angiogenesis

beta-2-Glycoprotein 1, an abundant plasma glycoprotein, binds anionic cell surfaces and functions as a regulator of thrombosis. Here, we show that cleavage of the kringle domain at Lys317/Thr318 switches its function to a regulator of angiogenesis. In vitro, the cleaved protein specifically inhibited the proliferation and migration of endothelial cells. The protein was without effect on preformed endothelial cell tubes. In vivo, the cleaved protein inhibited neovascularization into subcutaneously implanted Matrigel and Gelfoam sponge implants and the growth of orthotopically injected tumors. Collectively, these data indicate that plasmin-cleaved beta-2-glycoprotein 1 is a potent antiangiogenic and antitumor molecule of potential therapeutic significance.

Growth inhibitory actions of prothrombin on normal hepatocytes: influence of matrix

Most hepatomas have a defect in prothrombin carboxylation, and can secrete under-carboxylated prothrombin or des-gamma-carboxy-prothrombin (DCP), the function of which is unknown. We considered that the prothrombin-DCP axis might also be involved in growth control. Hepatocytes and hepatoma cells were treated with prothrombin and DNA synthesis and cytoskeletal changes were studied. Prothrombin inhibited DNA synthesis in hepatocytes on fibronectin, but not collagen matrix. Hepatoma cell lines were not inhibited. We found that hepatoma cell matrix conferred resistance to hepatocytes. Prothrombin decreased fibronectin but not collagen amounts, but only in the presence of hepatocytes and not hepatoma cells, indicating that it has a differential action on matrix proteins. It also caused changes in cell shape and actin depolymerization. In vivo, there was a decrease in plasma prothrombin activity after a partial hepatectomy (PH), concomitant with the peak of DNA synthesis in the hepatocytes at 24h after PH. Injection of warfarin at the time of PH, further inhibited PT activity and enhanced this 24h peak of DNA synthesis. Furthermore, repeated injection of prothrombin lowered the peak DNA synthesis after PH. The data support the hypothesis that prothrombin can act as a hepatocyte growth inhibitor, likely at the level of fibronectin loss and result in cytoskeletal changes. Hepatomas resist this action, possibly due to their different matrix proteins. This represents a novel mechanism for growth regulation and provides a possible biological significance for the tumor marker DCP.

Endostatin: Current concepts about its biological role and mechanisms of action

Endogenous inhibitors of angiogenesis are proved to be a major factor preventing the emergence of clinically manifested stages of human cancer. The protein endostatin, a 20-kD proteolytic fragment of type XVIII collagen, is one of the most active natural inhibitors of angiogenesis. Endostatin specifically inhibits the in vitro and in vivo proliferation of endothelial cells, inducing their apoptosis through inhibition of cyclin D1. On the surface of endothelial cells, endostatin binds with the integrin alpha(5)beta(1) that activates the Src-kinase pathway. The binding of endostatin with integrins also down-regulates the activity of RhoA GTPase and inhibits signaling pathways mediated by small kinases of the Ras and Raf families. All these events promote disassembly of the actin cytoskeleton, disorders in cell-matrix interactions, and decrease in endotheliocyte mobility, i.e., promote the suppression of angiogenesis. Endostatin displays a high antitumor activity in vivo: it inhibits the progression of more than 60 types of tumors. This review summarizes results of numerous studies concerning the biological activity and action mechanism of endostatin.

Effect of FXIII on Monocyte and Fibroblast Function

Factor XIII is a plasma transglutaminase that participates in the final stage of the coagulation cascade. Thrombin-activated FXIII (FXIIIa) catalyzes the formation of covalent crosslinks between gamma-glutamyl and epsilon-lysyl residues on fibrin molecules to yield the mature clot. In addition to its role in hemostasis, FXIIIa was previously shown by us to stimulate endothelial cells to exhibit pro-angiogenic activity. In this work, we studied the effect of FXIIIa on other cells that participate in angiogenesis and tissue repair, such as monocytes and fibroblasts. FXIIIa significantly enhanced migration and proliferation, and inhibited apoptosis of monocytes and fibroblasts. Similar to our previous observations with endothelial cells, the stimulating effect of FXIIIa on monocytes and fibroblasts was elicited via its binding to alpha (v)beta (3) integrin leading to cJun upregulation and TSP-1 downregulation. Since monocytes and fibroblasts are essential components of the tissue repair process, the results of this study, together with the proangiogenic activity of FXIIIa, further substantiate a significant role of FXIII in tissue repair.

Factor XIII (FXIII) and angiogenesis

Factor XIII is a plasma transglutaminase that participates in the final stage of the coagulation cascade. Thrombin-activated FXIII (FXIIIa) catalyzes the formation of covalent cross-links between gamma-glutamyl and epsilon-lysyl residues on adjacent fibrin chains in polymerized fibrin to yield the mature clot. In addition to its role in hemostasis, FXIII is known to participate in wound healing and embryo implantation, which are processes involving angiogenesis. In this review, we discuss the role of FXIII in angiogenesis and the molecular mechanisms underlying its proangiogenic effects. The FXIII role in tissue repair and remodeling may at least in part be attributed to its pro-angiogenic activity.

Angiogenesis inhibitors found within the haemostasis pathway

Angiogenesis, the development of new blood vessels from the existing vasculature, and haemostasis, the coagulation cascade leading to formation of a clot, are among the most consistent host responses associated with cancer. Importantly, these two pathways interrelate, with blood coagulation and fibrinolysis influencing tumor angiogenesis directly, thereby contributing to tumor growth. Moreover, many endogenous inhibitors of angiogenesis are found within platelets or harboured as cryptic fragments of haemostatic proteins. In this review we outline ways in which angiogenesis is coordinated and regulated by haemostasis in human cancer. Then we detail the experimental and pre-clinical evidence for the ability of many of these endogenous proteins to inhibit tumor angiogenesis and thus their potential to be anti-cancer agents, with particular reference to any clinical trials.

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.

Antiangiogenic property of human thrombin

Using protein chromatography, we purified and identified human prothrombin from human plasma as antiangiogenic. Prothrombin significantly inhibited endothelial cell tube formation in vitro at 10 microg/ml. Importantly, it also inhibited bFGF-induced angiogenesis in Matrigel-plug assays performed in mice. The proteolytic activity of thrombin appeared to be critical for the antiangiogenic activity of prothrombin. For example, thrombin exhibited inhibitory effects on endothelial cell tube formation in vitro at 10 U/ml. Addition of lepirudin, a specific inhibitor of thrombin, completely blocked prothrombin's and thrombin's antiangiogenic effects in vitro. We also assessed the importance of thrombin receptors in angiogenesis. Using small peptides that activate different protease-activated receptors (PARs), we showed that activation of PAR-1 led to inhibition of endothelial cell tube formation in vitro and bFGF-induced angiogenesis in vivo. Collectively, our data suggest that thrombin's proteolytic activity can be antiangiogenic.

Mechanisms and future directions for angiogenesis-based cancer therapies

Targeting angiogenesis represents a new strategy for the development of anticancer therapies. New targets derived from proliferating endothelial cells may be useful in developing anticancer drugs that prolong or stabilize the progression of tumors with minimal systemic toxicities. These drugs may also be used as novel imaging and radiommunotherapeutic agents in cancer therapy. In this review, the mechanisms and control of angiogenesis are discussed. Genetic and proteomic approaches to defining new potential targets on tumor vasculature are then summarized, followed by discussion of possible antiangiogenic treatments that may be derived from these targets and current clinical trials. Such strategies involve the use of endogenous antiangiogenic agents, chemotherapy, gene therapy, antiangiogenic radioligands, immunotherapy, and endothelial cell-based therapies. The potential biologic end points, toxicities, and resistance mechanisms to antiangiogenic agents must be considered as these therapies enter clinical trials.

Prothrombin kringle-2 activates cultured rat brain microglia

Microglia, the major immune effector cells in the CNS, become activated when the brain suffers injury. In this study, we observed that prothrombin, a zymogen of thrombin, induced NO release and mRNA expression of inducible NO synthase, IL-1beta, and TNF-alpha in rat brain microglia. The effect of prothrombin was independent of the protease activity of thrombin since hirudin, a specific inhibitor of thrombin, did not inhibit prothrombin-induced NO release. Furthermore, factor Xa enhanced the effect of prothrombin on microglial NO release. Kringle-2, a domain of prothrombin distinct from thrombin, mimicked the effect of prothrombin in inducing NO release and mRNA expression of inducible NO synthase, IL-1beta, and TNF-alpha. Prothrombin and kringle-2 both triggered the same intracellular signaling pathways. They both activated mitogen-activated protein kinases and NF-kappaB in a similar pattern. NO release stimulated by either was similarly reduced by inhibitors of the extracellular signal-regulated kinase pathway (PD98059), p38 (SB203580), NF-kappaB (N-acetylcysteine), protein kinase C (Go6976, bisindolylmaleimide, and Ro31-8220), and phospholipase C (D609 and U73122). These results suggest that prothrombin can activate microglia, and that, in addition to thrombin, kringle-2 is a domain of prothrombin independently capable of activating microglia.

A peptide derived from human prothrombin fragment 2 inhibits prothrombinase and angiogenesis

We constructed the synthetic peptide library representing human prothrombin fragment 2 (F2) sequence and explored the inhibitory sequence for prothrombinase, which was reconstituted in vitro by adding factor Xa, factor Va, and calcium into phospholipids. The nonapeptide NSAVLQVEN (NSA9) suppressed prothrombinase reconstituted not only on phospholipid vesicles but also on the bovine capillary endothelial (BCE) cell surface. Kinetic analyses demonstrated that NSA9 is a mixed-type inhibitor of Xa. Furthermore, the nonapeptide inhibited the proliferation of BCE cells and also suppressed angiogenesis in chicken embryos. The inhibitory activities of NSA9 were abrogated by pre-incubation with anti-F2 monoclonal antibody, 4E7. These data demonstrate that anti-angiogenic activity of F2 may be related to its ability to inhibit prothrombinase.

The role of the coagulation system in tumour angiogenesis

The coagulation system, which is activated in most cancer patients, has an important role in tumour biology. It may make a substantial contribution to tumour angiogenesis, which represents an imbalance in the normal mechanisms that allow organised healing after injury. The recently recognised, but steadily growing, knowledge of the relationship between the coagulation and angiogenesis pathways has research and clinical implications. Manipulation of these systems may minimise both the neoangiogenesis essential for tumour growth and associated thromboembolic complications. However, since surgery is the primary treatment for most cancers, the angiogenesis of wound healing and haemostatic competence must be maintained. In this article, we summarise the complex interactions between the coagulation system and the angiogenic process that occur in cancer growth. We focus upon the contributions of the vascular endothelium, platelets, and coagulation factors to the angiogenic process and explore the coagulation system as a therapeutic target.

The hemostatic system and angiogenesis in malignancy

Coagulopathy and angiogenesis are among the most consistent host responses associated with cancer. These two respective processes, hitherto viewed as distinct, may in fact be functionally inseparable as blood coagulation and fibrinolysis, in their own right, influence tumor angiogenesis and thereby contribute to malignant growth. In addition, tumor angiogenesis appears to be controlled through both standard and non-standard functions of such elements of the hemostatic system as tissue factor, thrombin, fibrin, plasminogen activators, plasminogen, and platelets. "Cryptic" domains can be released from hemostatic proteins through proteolytic cleavage, and act systemically as angiogenesis inhibitors (e.g., angiostatin, antiangiogenic antithrombin III aaATIII). Various components of the hemostatic system either promote or inhibit angiogenesis and likely act by changing the net angiogenic balance. However, their complex influences are far from being fully understood. Targeted pharmacological and/or genetic inhibition of pro-angiogenic activities of the hemostatic system and exploitation of endogenous angiogenesis inhibitors of the angiostatin and aaATIII variety are under study as prospective anti-cancer treatments.

No grip, no growth: the conceptual basis of excessive proteolysis in the treatment of cancer

The formation of new bloodvessels, called angiogenesis, is critical for a tumour to grow beyond a few mm(3) in size. A provisional matrix promotes endothelial cell adhesion, migration, proliferation and survival. Synthesis and degradation of this matrix closely resemble processes that occur during coagulation and fibrinolysis. Degradation of the matrix and fibrinolysis are tightly controlled and balanced by stimulators and inhibitors of the plasminogen activation system. Here we give an overview of these processes during tumour progression. We postulate a novel way to inhibit angiogenesis by removal of the matrix through specific and localised overstimulation of the plasminogen activation system.

The chemotactic action of urokinase on smooth muscle cells is dependent on its kringle domain. Characterization of interactions and contribution to chemotaxis

Urokinase plasminogen activator (uPA) is thought to exert its effects on cell growth, adhesion, and migration by mechanisms involving proteolysis and interaction with its cell surface receptor (uPAR). The functional properties of uPA and the significance of its various domains for chemotactic activity were analyzed using human airway smooth muscle cells (hAWSMC). The wild-type uPA (r-uPAwt), inactive urokinase with single mutation (His(204) to Gln) (r-uPA(H/Q)), urokinase with mutation of His(204) to Gln together with a deletion of growth factor-like domain (r-uPA(H/Q)-GFD), the catalytic domain of urokinase (r-uPA(LMW)), and its kringle domain (r-KD) were expressed in Escherichia coli. We demonstrate that glycosylated uPA, r-uPAwt, r-uPA(H/Q), and r-uPA(H/Q)-GFD elicited similar chemotactic effects. Half-maximal chemotaxis (EC(50)) were apparent at approximately 2 nm with all the uPA variants. The kringle domain induced cell migration with an EC(50) of about 6 nm, whereas the denaturated r-KD and r-uPA(LMW) were without effect. R-uPAwt-induced chemotaxis was dependent on an association with uPAR and a uPA-kringle domain-binding site, determined using a monoclonal uPAR antibody to prevent the uPA-uPAR interaction, and a monoclonal antibody to the uPA-kringle domain. The binding of iodinated r-uPAwt with hAWSMC was due to interaction with a high affinity binding site on the uPAR, and a lower affinity binding site on an unidentified cell surface target, which was mediated exclusively through the kringle domain of urokinase. Specific binding of r-uPA(H/Q)-GFD to hAWSMC involved an interaction with a single site whose characteristics were similar to those of the low affinity site of r-uPAwt binding to hAWSMC. uPAR-deficient HEK 293 cells specifically bound r-uPAwt and r-uPA(H/Q)-GFD via a single, similar type of binding site. These cells migrated when stimulated by r-uPA(H/Q)-GFD and uPAwt, but not r-uPA(LMW). HEK 293 cells transfected with the uPAR cDNA expressed two classes of sites that bound r-uPAwt; however, only a single site was responsible for the binding of r-uPA(H/Q)-GFD. Together, these findings indicate that uPA-induced chemotaxis is dependent on the binding of the uPA-kringle to the membrane surface of cells and the association of uPA with uPAR.

Disruption of interkringle disulfide bond of plasminogen kringle 1-3 changes the lysine binding capability of kringle 2, but not its antiangiogenic activity

Kringle 1-3 of human plasminogen is a potent inhibitor of endothelial cell proliferation. To understand a possible role for the unique cystine bridge between kringle 2 and kringle 3, we disrupted the interkringle disulfide bond by mutating Cys(169) and Cys(297) to serine residues. The yield of the mutant during the refolding process was decreased significantly. Anti-endothelial cell proliferative activity of the mutant was similar to that of the wild type. There was no significant difference in in vivo antiangiogenic activity between the wild type and the mutant in chorioallantoic membrane assay. However, in the mutant, the weak lysine binding capability of kringle 2 was not detected and its mobility in nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis is different from that of the wild type. These results support the notion that the overall antiangiogenic function of angiostatin is mediated by individual kringles, and suggest that the lysine binding capability of kringle 2 is likely not important for the antiangiogenic activity of kringle 1-3.