Regulation of plasminogen binding to neutrophils

Crosstalk between the plasminogen/plasmin system and inflammation resolution

The plasminogen/plasmin (Plg/Pla) system, best known for its classical role in thrombolysis, has been recently highlighted as a regulator of other biological processes in mammals, including key steps involved in the resolution of inflammation. Inflammation resolution is a complex process coordinated by different cellular effectors, notably leukocytes, and active mediators, and is initiated shortly after the inflammatory response begins. Once the inflammatory insult is eliminated, an effective and timely engagement of proresolution programs prevents persistent inflammation, thereby avoiding excessive tissue damage, fibrosis, and the development of autoimmunity. Interestingly, recent studies demonstrate that Plg/Pla and their receptor, plasminogen receptor KT (Plg-RKT), regulate key steps in inflammation resolution. The number of studies investigating the involvement of the Plg/Pla system in these and other aspects of inflammation, including degradation of extracellular matrices, immune cell migration, wound healing, and skeletal growth and maintenance, highlights key roles of the Plg/Pla system during physiological and pathologic conditions. Here, we discuss robust evidence in the literature for the emerging roles of the Plg/Pla system in key steps of inflammation resolution. These findings suggest that dysregulation in Plg production and its activation plays a role in the pathogenesis of inflammatory diseases. Elucidating central mechanisms underlying the role of Plg/Pla in key steps of inflammation resolution either in preclinical models of inflammation or in human inflammatory conditions, can provide a rationale for the development of new pharmacologic interventions to promote resolution of inflammation, and open new pathways for the treatment of thromboinflammatory conditions.

Fibrinolysis in COVID-19: Impact on Clot Lysis and Modulation of Inflammation

COVID-19 is a multisystem disease caused by SARS-CoV-2 and is associated with an imbalance between the coagulation and fibrinolytic systems. Overall, hypercoagulation, hypofibrinolysis and fibrin-clot resistance to fibrinolysis predispose patients to thrombotic and thromboembolic events. In the lungs, the virus triggers alveolar and interstitial fibrin deposition, endothelial dysfunction, and pulmonary intravascular coagulation, all events intrinsically associated with the activation of inflammation and organ injury. Adding to the pathogenesis of COVID-19, there is a positive feedback loop by which local fibrin deposition in the lungs can fuel inflammation and consequently dysregulates coagulation, a process known as immunothrombosis. Therefore, fibrinolysis plays a central role in maintaining hemostasis and tissue homeostasis during COVID-19 by cleaning fibrin clots and controlling feed-forward products of coagulation. In addition, components of the fibrinolytic system have important immunomodulatory roles, as evidenced by studies showing the contribution of Plasminogen/Plasmin (Plg/Pla) to the resolution of inflammation. Herein, we review clinical evidence for the dysregulation of the fibrinolytic system and discuss its contribution to thrombosis risk and exacerbated inflammation in severe COVID-19. We also discuss the current concept of an interplay between fibrinolysis and inflammation resolution, mirroring the well-known crosstalk between inflammation and coagulation. Finally, we consider the central role of the Plg/Pla system in resolving thromboinflammation, drawing attention to the overlooked consequences of COVID-19-associated fibrinolytic abnormalities to local and systemic inflammation.

Histone 2B Facilitates Plasminogen-Enhanced Endothelial Migration through Protease-Activated Receptor 1 (PAR1) and Protease-Activated Receptor 2 (PAR2)

Plasminogen and its multiple receptors have been implicated in the responses of many different cell types. Among these receptors, histone 2B (H2B) has been shown to play a prominent role in macrophage responses. The contribution of H2B to plasminogen-induced endothelial migration, an event relevant to wound healing and angiogenesis, is unknown. Plasminogen enhanced the migration of endothelial cells, which was inhibited by both Protease-Activated Receptor-1 (PAR1) and 2 (PAR2) antagonists. H2B was detected on viable endothelial cells of venous and arterial origin, and an antibody to H2B that blocks plasminogen binding also inhibited the plasminogen-dependent migration by these cells. The antibody blockade was as effective as PAR1 or PAR2 antagonists in inhibiting endothelial cell migration. In pull-down experiments, H2B formed a complex with both PAR1 and PAR2 but not β3 integrin, another receptor implicated in endothelial migration in the presence of plasminogen. H2B was found to be associated with clathrin adapator protein, AP2µ (clathrin AP2µ) and β-arrestin2, which are central to the internationalization/signaling machinery of the PARs. These associations with PAR1-clathrin adaptor AP2µ- and PAR2-β-arrestin2-dependent internalization/signaling pathways provide a mechanism to link plasminogen to responses such as wound healing and angiogenesis.

Functions of the plasminogen receptor Plg-RKT

Plg-RKT is a structurally unique transmembrane plasminogen receptor with both N- and C-terminal domains exposed on the extracellular face of the cell. Its C-terminal lysine functions to tether plasminogen to cell surfaces. Overexpression of Plg-RKT increases cell surface plasminogen binding capacity while genetic deletion of Plg-RKT decreases plasminogen binding. Plasminogen binding to Plg-RKT results in promotion of plasminogen activation to the broad spectrum serine protease plasmin. This function is promoted by the physical association of Plg-RKT with the urokinase receptor (uPAR). Plg-RKT is broadly expressed in cells and tissues throughout the organism and its sequence is remarkably conserved phylogenetically. Plg-RKT also is required for lactation and, thus, is necessary for survival of the species. This review provides an overview of established and emerging functions of Plg-RKT and highlights major roles for Plg-RKT in both the initiation and resolution of inflammation. While the roles for Plg-RKT in the inflammatory response are predominantly plasmin(ogen)-dependent, its role in lactation requires both plasminogen-dependent and plasminogen-independent mechanisms. Furthermore, the functions of Plg-RKT are dependent on sex. In view of the broad tissue distribution of Plg-RKT , its role in a broad array of physiological and pathological processes should provide a fruitful area for future investigation.

Human Plasminogen Exacerbates Clostridioides difficile Enteric Disease and Alters the Spore Surface

BACKGROUND & AIMS

The protease plasmin is an important wound healing factor, but it is not clear how it affects gastrointestinal infection-mediated damage, such as that resulting from Clostridioides difficile. We investigated the role of plasmin in C difficile-associated disease. This bacterium produces a spore form that is required for infection, so we also investigated the effects of plasmin on spores.

METHODS

C57BL/6J mice expressing the precursor to plasmin, the zymogen human plasminogen (hPLG), or infused with hPLG were infected with C difficile, and disease progression was monitored. Gut tissues were collected, and cytokine production and tissue damage were analyzed by using proteomic and cytokine arrays. Antibodies that inhibit either hPLG activation or plasmin activity were developed and structurally characterized, and their effects were tested in mice. Spores were isolated from infected patients or mice and visualized using super-resolution microscopy; the functional consequences of hPLG binding to spores were determined.

RESULTS

hPLG localized to the toxin-damaged gut, resulting in immune dysregulation with an increased abundance of cytokines (such as interleukin [IL] 1A, IL1B, IL3, IL10, IL12B, MCP1, MP1A, MP1B, GCSF, GMCSF, KC, TIMP-1), tissue degradation, and reduced survival. Administration of antibodies that inhibit plasminogen activation reduced disease severity in mice. C difficile spores bound specifically to hPLG and active plasmin degraded their surface, facilitating rapid germination.

CONCLUSIONS

We found that hPLG is recruited to the damaged gut, exacerbating C difficile disease in mice. hPLG binds to C difficile spores, and, upon activation to plasmin, remodels the spore surface, facilitating rapid spore germination. Inhibitors of plasminogen activation might be developed for treatment of C difficile or other infection-mediated gastrointestinal diseases.

Thrombin and Plasmin Alter the Proteome of Neutrophil Extracellular Traps

Neutrophil extracellular traps (NETs) consist of a decondensed DNA scaffold decorated with neutrophil-derived proteins. The proteome of NETs, or "NETome," has been largely elucidated in vitro. However, components such as plasma and extracellular matrix proteins may affect the NETome under physiological conditions. Here, using a reductionistic approach, we explored the effects of two proteases active during injury and wounding, human thrombin and plasmin, on the NETome. Using high-resolution mass spectrometry, we identified a total of 164 proteins, including those previously not described in NETs. The serine proteases, particularly thrombin, were also found to interact with DNA and bound to NETs in vitro. Among the most abundant proteins were those identified previously, including histones, neutrophil elastase, and antimicrobial proteins. We observed reduced histone (H2B, H3, and H4) and neutrophil elastase levels upon the addition of the two proteases. Analyses of NET-derived tryptic peptides identified subtle changes upon protease treatments. Our results provide evidence that exogenous proteases, present during wounding and inflammation, influence the NETome. Taken together, regulation of NETs and their proteins under different physiological conditions may affect their roles in infection, inflammation, and the host response.

A Neutrophil Proteomic Signature in Surgical Trauma Wounds

Non-healing wounds continue to be a clinical challenge for patients and medical staff. These wounds have a heterogeneous etiology, including diabetes and surgical trauma wounds. It is therefore important to decipher molecular signatures that reflect the macroscopic process of wound healing. To this end, we collected wound sponge dressings routinely used in vacuum assisted therapy after surgical trauma to generate wound-derived protein profiles via global mass spectrometry. We confidently identified 311 proteins in exudates. Among them were expected targets belonging to the immunoglobulin superfamily, complement, and skin-derived proteins, such as keratins. Next to several S100 proteins, chaperones, heat shock proteins, and immune modulators, the exudates presented a number of redox proteins as well as a discrete neutrophil proteomic signature, including for example cathepsin G, elastase, myeloperoxidase, CD66c, and lipocalin 2. We mapped over 200 post-translational modifications (PTMs; cysteine/methionine oxidation, tyrosine nitration, cysteine trioxidation) to the proteomic profile, for example, in peroxiredoxin 1. Investigating manually collected exudates, we confirmed presence of neutrophils and their products, such as microparticles and fragments containing myeloperoxidase and DNA. These data confirmed known and identified less known wound proteins and their PTMs, which may serve as resource for future studies on human wound healing.

Genes Critical for Developing Periodontitis: Lessons from Mouse Models

Since the etiology of periodontitis in humans is not fully understood, genetic mouse models may pinpoint indispensable genes for optimal immunological protection of the periodontium against tissue destruction. This review describes the current knowledge of genes that are involved for a proper maintenance of a healthy periodontium in mice. Null mutations of genes required for leukocyte cell–cell recognition and extravasation (e.g., Icam-1, P-selectin, Beta2-integrin/Cd18), for pathogen recognition and killing (e.g., Tlr2, Tlr4, Lamp-2), immune modulatory molecules (e.g., Cxcr2, Ccr4, IL-10, Opg, IL1RA, Tnf-α receptor, IL-17 receptor, Socs3, Foxo1), and proteolytic enzymes (e.g., Mmp8, Plasmin) cause periodontitis, most likely due to an inefficient clearance of bacteria and bacterial products. Several mechanisms resulting in periodontitis can be recognized: (1) inefficient bacterial control by the polymorphonuclear neutrophils (defective migration, killing), (2) inadequate antigen presentation by dendritic cells, or (3) exaggerated production of pro-inflammatory cytokines. In all these cases, the local immune reaction is skewed toward a Th1/Th17 (and insufficient activation of the Th2/Treg) with subsequent osteoclast activation. Finally, genotypes are described that protect the mice from periodontitis: the SCID mouse, and mice lacking Tlr2/Tlr4, the Ccr1/Ccr5, the Tnf-α receptor p55, and Cathepsin K by attenuating the inflammatory reaction and the osteoclastogenic response.

Idiopathic linear IgA bullous dermatosis: prognostic factors based on a case series of 72 adults

BACKGROUND

Linear IgA bullous dermatosis (LABD) is a clinically and immunologically heterogeneous, subepidermal, autoimmune bullous disease (AIBD), for which the long-term evolution is poorly described.

OBJECTIVES

To investigate the clinical and immunological characteristics, follow-up and prognostic factors of adult idiopathic LABD.

METHODS

This retrospective study, conducted in our AIBD referral centre, included adults, diagnosed between 1995 and 2012, with idiopathic LABD, defined as pure or predominant IgA deposits by direct immunofluorescence. Clinical, histological and immunological findings were collected from charts. Standard histology was systematically reviewed, and indirect immunofluorescence (IIF) on salt-split skin (SSS) and immunoblots (IBs) on amniotic membrane extracts using anti-IgA secondary antibodies were performed, when biopsies and sera obtained at diagnosis were available. Prognostic factors for complete remission (CR) were identified using univariate and multivariate analyses.

RESULTS

Of the 72 patients included (median age 54 years), 60% had mucous membrane (MM) involvement. IgA IIF on SSS was positive for 21 of 35 patients tested; 15 had epidermal and dermal labellings. Immunoelectron microscopy performed on the biopsies of 31 patients labelled lamina lucida (LL) (26%), lamina densa (23%), anchoring-fibril zone (AFz) (19%) and LL+AFz (23%). Of the 34 IgA IBs, 22 were positive, mostly for LAD-1/LABD97 (44%) and full-length BP180 (33%). The median follow-up was 39 months. Overall, 24 patients (36%) achieved sustained CR, 19 (29%) relapsed and 35% had chronic disease. CR was significantly associated with age > 70 years or no MM involvement. No prognostic immunological factor was identified.

CONCLUSIONS

Patients with LABD who are < 70 years old and have MM involvement are at risk for chronic evolution.

Plasmin induces in vivo monocyte recruitment through protease-activated receptor-1-, MEK/ERK-, and CCR2-mediated signaling

The plasminogen (Plg)/plasmin (Pla) system is associated with a variety of biological activities beyond the classical dissolution of fibrin clots, including cell migration, tissue repair, and inflammation. Although the capacity of Plg/Pla to induce cell migration is well defined, the mechanism underlying this process in vivo is elusive. In this study, we show that Pla induces in vitro migration of murine fibroblasts and macrophages (RAW 264.7) dependent on the MEK/ERK pathway and by requiring its proteolytic activity and lysine binding sites. Plasmin injection into the pleural cavity of BALB/c mice induced a time-dependent influx of mononuclear cells that was associated with augmented ERK1/2 and IκB-α phosphorylation and increased levels of CCL2 and IL-6 in pleural exudates. The inhibition of protease activity by using a serine protease inhibitor leupeptin or two structurally different protease-activated receptor-1 antagonists (SCH79797 and RWJ56110) abolished Pla-induced mononuclear recruitment and ERK1/2 and IκB-α phosphorylation. Interestingly, inhibition of the MEK/ERK pathway abolished Pla-induced CCL2 upregulation and mononuclear cell influx. In agreement with a requirement for the CCL2/CCR2 axis to Pla-induced cell migration, the use of a CCR2 antagonist (RS504393) prevented the Plg/Pla-induced recruitment of mononuclear cells to the pleural cavity and migration of macrophages at transwell plates. Therefore, Pla-induced mononuclear cell recruitment in vivo was dependent on protease-activated receptor-1 activation of the MEK/ERK/NF-κB pathway, which led to the release of CCL2 and activation of CCR2.

Serum-derived plasminogen is activated by apoptotic cells and promotes their phagocytic clearance

The elimination of apoptotic cells, called efferocytosis, is fundamentally important for tissue homeostasis and prevents the onset of inflammation and autoimmunity. Serum proteins are known to assist in this complex process. In the current study, we performed a multistep chromatographic fractionation of human serum and identified plasminogen, a protein involved in fibrinolysis, wound healing, and tissue remodeling, as a novel serum-derived factor promoting apoptotic cell removal. Even at levels significantly lower than its serum concentration, purified plasminogen strongly enhanced apoptotic prey cell internalization by macrophages. Plasminogen acted mainly on prey cells, whereas on macrophages no enhancement of the engulfment process was observed. We further demonstrate that the efferocytosis-promoting activity essentially required the proteolytic activation of plasminogen and was completely abrogated by the urokinase plasminogen activator inhibitor-1 and serine protease inhibitor aprotinin. Thus, our study assigns a new function to plasminogen and plasmin in apoptotic cell clearance.

Characterization of plasminogen binding to NB4 promyelocytic cells using monoclonal antibodies against receptor-induced binding sites in cell-bound plasminogen

The NB4 promyelocytic cell line exhibits many of the characteristics of acute promyelocytic leukemia blast cells, including the translocation (15 : 17) that fuses the PML gene on chromosome 15 to the RARα gene on chromosome 17. These cells have a very high fibrinolytic capacity. In addition to a high secretion of urokinase, NB4 cells exhibit a 10-fold higher plasminogen binding capacity compared with other leukemic cell lines. When tissue-type plasminogen activator was added to acid-treated cells, plasmin generation was 20–26-fold higher than that generated by U937 cells or peripheral blood neutrophils, respectively. We found that plasminogen bound to these cells can be detected by fluorescence-activated cell sorting using an antiplasminogen monoclonal antibody that specifically reacts with this antigen when it is bound to cell surfaces. All-trans retinoid acid treatment of NB4 cells markedly decreased the binding of this monoclonal antibody. This cell line constitutes a unique model to explore plasminogen binding and activation on cell surfaces that can be modulated by all-trans retinoid acid treatment.

Plasminogen is a key proinflammatory regulator that accelerates the healing of acute and diabetic wounds

Despite decades of research on wound healing, effective biologic agents for the treatment of chronic wounds, especially diabetic wounds, are still lacking. In the present study, we report that the inert plasma protein plasminogen (plg) acts as a key regulatory molecule that potentiates wound healing in mice. Early in the healing process, plg bound to inflammatory cells is transported to the wound area, where the level of plg is increased locally, leading to the induction of cytokines and intracellular signaling events and to a potentiation of the early inflammatory response. Systemic administration of additional plg not only accelerates the healing of acute burn wounds in wild-type mice, but also improves the healing of chronic diabetic wounds in a mouse model of diabetes. Our results suggest that the administration of plg may be a novel therapeutic strategy to treat many different types of wounds, especially chronic wounds such as those caused by diabetes.

Naloxone inhibits immune cell function by suppressing superoxide production through a direct interaction with gp91phox subunit of NADPH oxidase

BACKGROUND

Both (-) and (+)-naloxone attenuate inflammation-mediated neurodegeneration by inhibition of microglial activation through superoxide reduction in an opioid receptor-independent manner. Multiple lines of evidence have documented a pivotal role of overactivated NADPH oxidase (NOX2) in inflammation-mediated neurodegeneration. We hypothesized that NOX2 might be a novel action site of naloxone to mediate its anti-inflammatory actions.

METHODS

Inhibition of NOX-2-derived superoxide by (-) and (+)-naloxone was measured in lipopolysaccharide (LPS)-treated midbrain neuron-glia cultures and phorbol myristate acetate (PMA)-stimulated neutrophil membranes by measuring the superoxide dismutase (SOD)-inhibitable reduction of tetrazolium salt (WST-1) or ferricytochrome c. Further, various ligand (3H-naloxone) binding assays were performed in wild type and gp91phox-/- neutrophils and transfected COS-7 and HEK293 cells. The translocation of cytosolic subunit p47phox to plasma membrane was assessed by western blot.

RESULTS

Both (-) and (+)-naloxone equally inhibited LPS- and PMA-induced superoxide production with an IC50 of 1.96 and 2.52 μM, respectively. Competitive binding of 3H-naloxone with cold (-) and (+)-naloxone in microglia showed equal potency with an IC50 of 2.73 and 1.57 μM, respectively. 3H-Naloxone binding was elevated in COS-7 and HEK293 cells transfected with gp91phox; in contrast, reduced 3H-naloxone binding was found in neutrophils deficient in gp91phox or in the presence of a NOX2 inhibitor. The specificity and an increase in binding capacity of 3H-naloxone were further demonstrated by 1) an immunoprecipitation study using gp91phox antibody, and 2) activation of NOX2 by PMA. Finally, western blot studies showed that naloxone suppressed translocation of the cytosolic subunit p47phox to the membrane, leading to NOX2 inactivation.

CONCLUSIONS

Strong evidence is provided indicating that NOX2 is a non-opioid novel binding site for naloxone, which is critical in mediating its inhibitory effect on microglia overactivation and superoxide production.

Plasmin is essential in preventing periodontitis in mice

Periodontitis involves bacterial infection, inflammation of the periodontium, degradation of gum tissue, and alveolar bone resorption, which eventually leads to loss of teeth. To study the role of the broad-spectrum protease plasmin in periodontitis, we examined the oral health of plasminogen (Plg)-deficient mice. In wild-type mice, the periodontium was unaffected at all time points studied; in Plg-deficient mice, periodontitis progressed rapidly, within 20 weeks. Morphological study results of Plg-deficient mice revealed detachment of gingival tissue, resorption of the cementum layer, formation of necrotic tissue, and severe alveolar bone degradation. IHC staining showed massive infiltration of neutrophils in the periodontal tissues. Interestingly, doubly deficient mice, lacking both tissue- and urokinase-type plasminogen activators, developed periodontal disease similar to that in Plg-deficient mice; however, mice lacking only tissue- or urokinase-type plasminogen activator remained healthy. Supplementation by injection of Plg-deficient mice with human plasminogen for 10 days led to necrotic tissue absorption, inflammation subsidence, and full regeneration of gum tissues. Notably, there was also partial regrowth of degraded alveolar bone. Taken together, our results show that plasminogen is essential for the maintenance of a healthy periodontium and plays an important role in combating the spontaneous development of chronic periodontitis. Moreover, reversal to healthy status after supplementation of Plg-deficient mice with plasminogen suggests the possibility of using plasminogen for therapy of periodontal diseases.

Plasminogen and its receptors as regulators of cardiovascular inflammatory responses

In addition to its role in fibrinolysis, plasminogen (Plg) influences inflammatory cell migration and thereby plays a prominent role in cardiovascular pathology. The contribution of Plg to inflammatory cell recruitment depends on its tethering to the surface of responding cells. Plasminogen receptors (Plg-Rs) are heterogeneous and can be classified as tailless, lacking cytoplasmic tails, or tailed (having cytoplasmic tails). In vivo observations implicate several tailless Plg-Rs in inflammatory responses. Tailed Plg-Rs on leukocytes include several integrins, which have also been implicated in Plg-dependent responses. Surface expression of both tailless and tailed Plg-Rs can be modulated in number and/or function. A common mechanism involving intracellular calcium mobilization and calcium channels regulates expression of both classes of Plg-Rs. Data are emerging to indicate that targeting Plg and Plg-Rs may limit inflammation and cardiovascular pathology.

Plasmin plays an essential role in amplification of psoriasiform skin inflammation in mice

Background

Although increased levels of plasminogen activators have been found in psoriatic lesions, the role of plasmin converted from plasminogen by plasminogen activators in pathogenesis of psoriasis has not been investigated.

Methodology/Principal Findings

Here we examined the contribution of plasmin to amplification of inflammation in patients with psoriasis. We found that plasminogen was diminished, but that the amount and activity of its converted product plasmin were markedly increased in psoriasis. Moreover, annexin II, a receptor for plasmin was dramatically increased in both dermis and epidermis in psoriasis. Plasmin at sites of inflammation was pro-inflammatory, eliciting production of inflammatory factors, including CC chemokine ligand 20 (CCL20) and interleukin-23 (IL-23), that was mediated by the nuclear factor-kappaB (NF-κB) signaling pathway and that had an essential role in the recruitment and activation of pathogenic C-C chemokine receptor type 6 (CCR6)⁺ T cells. Moreover, intradermal injection of plasmin or plasmin together with recombinant monocyte/macrophage chemotactic protein-1 (MCP-1) resulted in induction of psoriasiform skin inflammation around the injection sites with several aspects of human psoriasis in mice.

Conclusions/Significance

Plasmin converted from plasminogen by plasminogen activators plays an essential role in amplification of psoriasiform skin inflammation in mice, and targeting plasmin receptor - annexin II - may harbor therapeutic potential for the treatment of human psoriasis.

Phosphatidylserine as an anchor for plasminogen and its plasminogen receptor, histone H2B, to the macrophage surface

BACKGROUND

Plasminogen (Plg) binding to cell surface Plg receptors (Plg-Rs) on the surface of macrophages facilitates Plg activation and migration of these cells. Histone H2B (H2B) acts as a Plg-R and its cell surface expression is up-regulated when monocytes are differentiated to macrophages via a pathway dependent on L-type Ca(2+) channels and intracellular Ca(2+).

OBJECTIVES

We sought to investigate the mechanism by which H2B, a protein without a transmembrane domain, is retained on the macrophage surface.

METHODS

THP-1 monocytoid cells were induced to differentiate with interferon gamma + Vitamin D3 or to undergo apoptosis by treatment with camptothecin. Flow cytometry and cell surface biotinylation followed by Western blotting were used to measure the interrelationship between Plg binding, cell surface expression of H2B and outer membrane exposure of phosphatidylserine (PS).

RESULTS

H2B interacted directly with PS via an electrostatic interaction. Anti-PS or PS binding proteins, annexin V and protein S, diminished H2B interaction with PS on the surface of differentiated or apoptotic cells and these same reagents inhibited Plg binding to these cells. L-type Ca(2+) channels played a significant role in PS exposure, H2B surface expression and Plg binding induced either by differentiation or apoptosis.

CONCLUSIONS

These data suggest that H2B tethers to the surface of cells by interacting with PS on differentiated or apoptotic monocytoid cells. L-type Ca(2+) channels regulate PS exposure on the surface of these cells. The exposed PS interacts directly with H2B and hence provides sites for Plg to bind to.

Histone H2B as a functionally important plasminogen receptor on macrophages

Plasminogen (Plg) facilitates inflammatory cell recruitment, a function that depends upon its binding to Plg receptors (Plg-Rs). However, the Plg-Rs that are critical for cell migration are not well defined. Three previously characterized Plg-Rs (alpha-enolase, annexin 2, and p11) and a recently identified Plg-R (histone H2B [H2B]) were assessed for their contribution to Plg binding and function on macrophages. Two murine macrophage cell lines (RAW 264.7 and J774A.1) and mouse peritoneal macrophages induced by thioglycollate were analyzed. All 4 Plg-Rs were present on the surface of these cells and showed enhanced expression on the thioglycollate-induced macrophages compared with peripheral blood monocytes. Using blocking Fab fragments to each Plg-R, H2B supported approximately 50% of the Plg binding capacity, whereas the other Plg-Rs contributed less than 25%. Anti-H2B Fab also demonstrated a major role of this Plg-R in plasmin generation and matrix invasion. When mice were treated intravenously with anti-H2B Fab, peritoneal macrophage recruitment in response to thioglycollate was reduced by approximately 45% at 24, 48, and 72 hours, with no effect on blood monocyte levels. Taken together, these data suggest that multiple Plg-Rs do contribute to Plg binding to macrophages, and among these, H2B plays a very prominent and functionally important role.

Plasminogen binding and activation at the breast cancer cell surface: the integral role of urokinase activity

Introduction

The regulation of extracellular proteolytic activity via the plasminogen activation system is complex, involving numerous activators, inhibitors, and receptors. Previous studies on monocytic and colon cell lines suggest that plasmin pre-treatment can increase plasminogen binding, allowing the active enzyme to generate binding sites for its precursor. Other studies have shown the importance of pre-formed receptors such as annexin II heterotetramer. However, few studies have used techniques that exclusively characterise cell-surface events and these mechanisms have not been investigated at the breast cancer cell surface.

Methods

We have studied plasminogen binding to MCF-7 in which urokinase plasminogen activator receptor (uPAR) levels were upregulated by PMA (12-O-tetradecanoylphorbol-13-acetate) stimulation, allowing flexible and transient modulation of cell-surface uPA. Similar experiments were also performed using MDA-MB-231 cells, which overexpress uPAR/uPA endogenously. Using techniques that preserve cell integrity, we characterise the role of uPA as both a plasminogen receptor and activator and quantify the relative contribution of pre-formed and cryptic plasminogen receptors to plasminogen binding.

Results

Cell-surface plasminogen binding was significantly enhanced in the presence of elevated levels of uPA in an activity-dependent manner and was greatly attenuated in the presence of the plasmin inhibitor aprotinin. Pre-formed receptors were also found to contribute to increased plasminogen binding after PMA stimulation and to co-localise with uPA/uPAR and plasminogen. Nevertheless, a relatively modest increase in plasminogen-binding capacity coupled with an increase in uPA led to a dramatic increase in the proteolytic capacity of these cells.

Conclusion

We show that the majority of lysine-dependent plasminogen binding to breast cancer cells is ultimately regulated by plasmin activity and is dependent on the presence of significant levels of active uPA. The existence of a proteolytic positive feedback loop in plasminogen activation has profound implications for the ability of breast cancer cells expressing high amounts of uPA to accumulate a large proteolytic capacity at the cell surface, thereby conferring invasive potential.

Identification of pH-regulated antigen 1 released from Candida albicans as the major ligand for leukocyte integrin alphaMbeta2

Candida albicans is a common opportunistic fungal pathogen and is the leading cause of invasive fungal disease in immunocompromised individuals. The induction of cell-mediated immunity to C. albicans is of critical importance in host defense and the prime task of cells of the innate immune system. We previously demonstrated that the integrin alpha(M)beta(2) (CD11b/CD18) is the major leukocyte receptor involved in C. albicans recognition, mediating both adhesive and migratory responses to the fungus. In the present study, we demonstrate that various C. albicans strains release a protease-sensitive activity into their conditioned medium that supports alpha(M)beta(2)-mediated cell adhesion and migration. The isolation and characterization of this protein was undertaken by two independent approaches: 1) immunoaffinity purification on a mAb raised to conditioned medium which blocked alpha(M)beta(2)-dependent adhesion and migration; and 2) affinity chromatography on purified alpha(M)beta(2). Each approach led to the isolation of the same protein, which was unequivocally identified as pH-regulated Ag 1 (Pra1p), based on mass spectrometry and amino acid sequence analyses. C. albicans mutant strains lacking Pra1p were unable to support leukocyte adhesion or migration. In a neutrophil-mediated fungal killing assay, such mutant strains were resistant to killing and/or phagocytosis. Addition of purified Pra1p or reagents that block alpha(M)beta(2) function prevented killing of Pra1p-expressing but not Pra1p-deficient strains of C. albicans. Together, these data indicate that Pra1p is a ligand of alpha(M)beta(2) on C. albicans and that the soluble form of Pra1p may assist the fungus in escaping host surveillance.

Endogenous tissue-type plasminogen activator is protective during Escherichia coli-induced abdominal sepsis in mice

Sepsis is associated with enhanced production of tissue-type plasminogen activator (tPA). We investigated the function of endogenous tPA in the immune responses to Escherichia coli-induced abdominal sepsis using tPA gene-deficient (tPA(-/-)) and normal wild-type (WT) mice. tPA(-/-) mice demonstrated an impaired defense against E. coli peritonitis as indicated by higher bacterial loads at the primary site of the infection, enhanced dissemination, and reduced survival. The protective function of tPA was independent of plasmin since plasminogen gene-deficient (Plg(-/-)) mice were indistinguishable from WT mice. Relative to WT mice, tPA(-/-) mice demonstrated similar neutrophil counts in the peritoneal cavity despite much higher bacterial loads and higher local concentrations of neutrophil attracting chemokines, suggesting a reduced migratory response. In line, tPA(-/-) mice demonstrated a reduced thioglycolate-induced neutrophil influx into the peritoneal cavity and i.p. injection of WT mice with a replication-defective adenoviral vector expressing tPA caused an enhanced cell migration to the peritoneal cavity during E. coli peritonitis. These findings identify a novel protective function of tPA in abdominal sepsis caused by E. coli that seems independent of its role in the generation of plasmin.

Plasminogen/plasmin regulates α-enolase expression through the MEK/ERK pathway

Previously we have demonstrated that both plasminogen (Plg) and plasmin (Pla) regulate the expression of the transcription factors c-FOS and EGR-1 [L.P. De Sousa, B.S. Brasil, B.M. Silva, M.H. Freitas, S.V. Nogueira, P.C. Ferreira, E.G. Kroon, C.A. Bonjardim, Plasminogen/plasmin regulates c-fos and egr-1 expression via the MEK/ERK pathway, Biochem. Biophys. Res. Commun. 329 (2005) 237-245]. Here we show that Plg activates the mitogen-activated protein kinases MEK and ERK which leads to alpha-enolase (alpha-ENO) gene expression not only in fibroblasts, but also in peripheral blood mononuclear cells. The alpha-ENO mRNA accumulation was apparent three hours post-Plg treatment and remained elevated out to 28h, a process that seems to require both de novo protein synthesis and active gene transcription. Pla mimics Plg-stimulated alpha-ENO expression through its serine protease activity, suggesting that conversion of Plg to active Pla is required. Pharmacological and genetic blockade of MEK caused inhibition of alpha-ENO mRNA accumulation, implicating MEK/ERK as the transduction pathway that leads to alpha-ENO expression upon Plg stimulation. Furthermore, Plg stimulated DNA binding activity of the transcription factors activator-protein 1 and early growth response gene-1 to their cognate regulatory sequences at alpha-ENO promoter. Altogether, our data show that Plg/Pla regulates alpha-ENO expression through the MEK/ERK pathway.

The functions of plasminogen in cardiovascular disease

Plasminogen (Plg) and its derivative serine protease, plasmin, together with the activators, inhibitors, modulators, and substrates of the Plg network, are postulated to regulate a wide variety of biologic responses that could influence cardiovascular disease. The development of Plg-deficient mice has provided an incisive approach to test these proposed functions in vivo. Several different models of atherosclerosis, restenosis, aneurysm, and thrombosis have been analyzed in these mice and have demonstrated profound effects of Plg on these events as well as on the inflammatory response, which contributes to these cardiovascular diseases. Plasmin (ogen) may influence the progression of cardiovascular diseases through its degradation of matrix proteins, including fibrin; its activation of matrix metalloproteinases; its regulation of growth factor and chemokine pathways; or its influence on directed cell migration. Dissection of these mechanisms represents a future challenge toward understanding the roles of Plg in vivo.

A central role for plasminogen in the inflammatory response to biomaterials

The inflammatory response to implanted biomaterials severely limits their deployment in patients. Plasminogen has been shown to play a central role in cell migration, and therefore could regulate this inflammatory response. We sought to determine if plasminogen influences recruitment of inflammatory cells to a biomaterial implanted into plasminogen-deficient (Plg(-/-)) mice. Small disks of polyethylene terephthalate, a material used in vascular grafts, were surgically implanted into the peritoneum of wild-type and Plg(-/-) mice. Recruitment of neutrophils and monocytes/macrophages into the peritoneum and onto the disks was measured, primarily at 18 h. Monocyte/macrophage recruitment was markedly blunted in Plg(-/-) mice compared with wild-type mice. Unexpectedly, neutrophil recruitment was also markedly decreased in the Plg(-/-) mice. While recruitment of leukocytes into the peritoneum was plasminogen-dependent, the adhesion of the emigrating cells to the implants was not. In contrast, adhesion but not recruitment was reduced in fibrinogen-deficient mice. Reconstitution of Plg(-/-) mice with intravenous or intraperitoneal plasminogen differentially restored monocyte/macrophage and neutrophil recruitment. Tranexamic acid, an inhibitor of the lysine binding sites of plasminogen, suppressed leukocyte recruitment in wild-type mice, but aprotinin, a plasmin inhibitor, did not. Plasminogen exerts a marked influence on both neutrophil and monocyte/macrophage recruitment to implanted biomaterials. This role is distinct from that of fibrinogen, and the two inflammatory cell types use plasminogen in different ways. Plasminogen represents a therapeutic target for controlling the inflammatory response to implanted materials.

Characterization of plasminogen as an adhesive ligand for integrins alphaMbeta2 (Mac-1) and alpha5beta1 (VLA-5)

Plasminogen (Pg) has been implicated in many biologic processes involving extracellular proteolysis. We investigated whether Pg, by virtue of its capacity to be deposited within the extracellular matrix, can serve as a ligand for cell surface integrins. We report here that Pg supports cell adhesion by engaging integrins alphaMbeta2 and alpha5beta1. The immobilized Glu-Pg, but not its derivatives with the N-terminal peptide lacking, plasmin and Lys-Pg, supported efficient adhesion that was abolished by anti-alphaMbeta2 and anti-alpha5beta1 integrin-specific monoclonal antibodies (mAbs). In addition, lysine binding sites of Glu-Pg contributed to cell adhesion inasmuch as tranexamic acid and epsilon-aminocaproic acid inhibited cell adhesion. The involvement of alphaMbeta2 and alpha5)beta1 in adhesion to Glu-Pg was demonstrable with blood neutrophils, U937 monocytoid cells, and genetically engineered alphaMbeta2-transfected human embryonic kidney (HEK) 293 cells. In alphaMbeta2, the alphaMI-domain is the binding site for Glu-Pg because the "I-less" form of alphaMbeta2 did not support cell adhesion and the recombinant alphaMI-domain bound Glu-Pg directly. In comparison with cell adhesion, the binding of soluble Glu-Pg to cells and the concomitant generation of plasmin activity was inhibited by anti-alpha5beta1 but not by anti-alphaMbeta2. These findings identify Glu-Pg as an adhesive ligand for integrins alphaMbeta2 and alpha5beta1 and suggest that alpha5beta1 may participate in the binding of soluble Glu-Pg and assist in its activation.

Urokinase-type plasminogen activator potentiates lipopolysaccharide-induced neutrophil activation

Urokinase plasminogen activator (uPA) is a serine protease that catalyzes the conversion of plasminogen to plasmin. Although increased circulating levels of uPA are present in endotoxemia and sepsis, conditions in which activated neutrophils contribute to the development of acute organ dysfunction, the ability of uPA to participate directly in LPS-induced neutrophil activation has not been examined. In the present experiments, we show that uPA can enhance activation of neutrophils exposed to submaximal stimulatory doses of LPS. In particular, uPA increased LPS-induced activation of intracellular signaling pathways, including Akt and c-Jun N-terminal kinase, nuclear translocation of the transcriptional regulatory factor NF-kappa B, and expression of proinflammatory cytokines, including IL-1 beta, macrophage-inflammatory protein-2, and TNF-alpha. There was no effect of uPA on LPS-induced activation of p38 mitogen-activated protein kinase in neutrophils. Transgenic mice unable to produce uPA (uPA(-/-)) were protected from endotoxemia-induced lung injury, as determined by development of lung edema, pulmonary neutrophil accumulation, lung IL-1 beta, macrophage-inflammatory protein-2, and TNF-alpha cytokine levels. These results demonstrate that uPA can potentiate LPS-induced neutrophil responses and also suggest that such effects are sufficiently important in vivo to play a major contributory role in neutrophil-mediated inflammatory responses, such as the development of acute lung injury.

In vivo regulation of plasminogen function by plasma carboxypeptidase B

The major functions of plasminogen (Plg) in fibrinolysis and cell migration depend on its binding to carboxy-terminal lysyl residues. The ability of plasma carboxypeptidase B (pCPB) to remove these residues suggests that it may act as a suppressor of these Plg functions. To evaluate this role of pCPB in vivo, homozygote pCPB-deficient mice were generated by homologous recombination, and the resulting pCPB(-/-) mice, which were viable and healthy, were mated to Plg(-/-) mice. Plg(+/-) mice show intermediate levels of fibrinolysis and cell migration compared with Plg wild-type and deficient mice, reflecting the intermediate levels of the Plg antigen in their plasma. Differences in Plg-dependent functions between pCPB(+/+), pCPB(+/-), and pCPB(-/-) mice were then analyzed in a Plg(+/-) background. In a pulmonary clot lysis model, fibrinolysis was significantly increased in mice with partial (pCPB(+/-)) or total absence (pCPB(-/-)) of pCPB compared with their wild-type counterparts (pCPB(+/+)). In a thioglycollate model of peritoneal inflammation, leukocyte migration at 72 hours increased significantly in Plg(+/-)/pCPB(+/-) and Plg(+/-)/pCPB(-/-) compared with their wild-type counterparts. These studies demonstrate a definitive role of pCPB as a modulator of the pivotal functions of Plg in fibrinolysis and cell migration in vivo.

Bivalency of plasminogen monoclonal antibodies is required for plasminogen bridging to fibrin and enhanced plasmin formation

Binding of plasminogen to fibrin and cell surfaces is essential for fibrinolysis and pericellular proteolysis. We used surface plasmon resonance and enzyme kinetic analyses to study the effect of two mAbs (A10.2, CPL15) on plasminogen binding and activation at fibrin surfaces. A10.2 is directed against the lysine-binding site (LBS) of kringle 4, whereas CPL15 recognises a region in kringle 1 outside the LBS. In the presence of CPL15 and A10.2 mAbs, binding of plasminogen (K(d)=1.16+/-0.22 micromol/l) to fibrin was characterised by a mAb concentration-dependent bell-shaped isotherm. A progressive increase in the concentration of mAbs at the surface was also detected, and reached a plateau corresponding to the maximum of plasminogen bound. These data indicated that at low mAb concentration, bivalent plasminogen-mAb-plasminogen ternary complexes are formed, whereas at high mAb concentration, a progressive shift to monovalent plasminogen-mAb binary complexes is observed. Plasmin formation in the presence of mAbs followed a similar bell-shaped profile. Monovalent Fab fragments of mAb A10.2 showed no effect on the binding of plasminogen, confirming the notion that a bivalent mAb interaction is essential to increase plasminogen binding and activation at the surface of fibrin.

Neutrophils as a key cellular target for angiostatin: implications for regulation of angiogenesis and inflammation

Angiostatin effectively blocks tumor angiogenesis through still poorly understood mechanisms. Given the close association between immune and vascular regulation, we investigated the effects of angiostatin on angiogenesis-associated leukocytes. Angiostatin inhibited the migration of monocytes and, even more markedly, neutrophils. Angiostatin blocked chemotaxis of neutrophils to CXCR2 chemokine receptor agonists (IL-8, MIP-2, and GROalpha), formyl-Met-Leu-Phe (fMLP), and 12-O-tetradecanoylphorbol 13-acetate, and repressed fMLP-induced mitochondrial activity. Two different angiostatin forms (kringles 1-4 and 1-3) were effective, whereas whole plasminogen had no effect. IL-8, MIP-2, and GROalpha induced intense angiogenic reactions in vivo, but no angiogenic response to these factors was observed in neutropenic mice, demonstrating an essential role for neutrophils. Angiostatin potently inhibited chemokine-induced angiogenesis in vivo, and consistent with in vitro observations, both angiostatin forms were active and whole plasminogen had little effect. Angiostatin inhibition of angiogenesis in vivo was accompanied by a striking reduction in the number of recruited leukocytes. In vivo, the inflammatory agent lipopolysaccharide also induced extensive leukocyte infiltration and angiogenesis that were blocked by angiostatin. Neutrophils expressed mRNAs for ATP synthase and angiomotin, two known angiostatin receptors. These data show that angiostatin directly inhibits neutrophil migration and neutrophil-mediated angiogenesis and indicate that angiostatin might inhibit inflammation.

Neutrophil enhancement of fibrin deposition under flow through platelet-dependent and -independent mechanisms

We examined the effect of adherent neutrophils on fibrin deposition under laminar flow conditions. Perfusion of recalcified citrated platelet-free plasma (PFP) over neutrophils adherent to fibrinogen-coated glass at a venous wall shear rate of 62.5 s(-1) for 15 minutes resulted in dense deposition of fibrin around each neutrophil, whereas fibrin deposition on glass alone was sparse. Fibrin deposition on neutrophils was markedly reduced by anti-CD18 or anti-CD11b or a higher shear rate (250 s(-1)). Significantly less fibrin was deposited around adherent fibrinogen-coated beads, indicating that nonspecific "cross-sectional capture" effects were not responsible for the massive fibrin deposition on neutrophils. Direct visualization of fibrin capture by neutrophils and elimination of fibrin deposition at 15 minutes by a factor XIIa inhibitor (50 microg/mL corn trypsin inhibitor [CTI]) or elastase/cathepsin G inhibitors (Methoxysuccinyl-Ala-Ala-Pro-Ala-Chloromethyl-Ketone/Z-Gly-Leu-Phe-CMK, 100 micromol/L) indicated that neutrophils can capture short fibrin strands flowing in recalcified PFP lacking CTI and can also promote thrombin generation through pathways attenuated by inhibitors of factor XIIa, elastase, and cathepsin G. When neutrophils were allowed to interact with platelets on a fibrinogen surface before perfusion of recalcified CTI-treated PFP, the fibrin deposition was observed to be dramatic compared with that over surfaces coated with platelets alone or neutrophils alone and compared with that formed on platelets adherent to collagen. This neutrophil promotion of platelet-mediated fibrin formation was attenuated by inhibitors of elastase or cathepsin G but not anti-tissue factor antibody. Neutrophils can interact with platelets via released proteases to increase platelet procoagulant activity and fibrin formation in CTI-treated plasma under the low-flow conditions expected in venous thrombosis or inflammation.