Nucleic acids potentiate Factor VII-activating protease (FSAP)-mediated cleavage of platelet-derived growth factor-BB and inhibition of vascular smooth muscle cell proliferation

Factor VII Activating Protease (FSAP) and Its Importance in Hemostasis—Part I: FSAP Structure, Synthesis and Activity Regulation: A Narrative Review

Factor VII activating protease (FSAP) was first isolated from human plasma less than 30 years ago. Since then, many research groups have described the biological properties of this protease and its role in hemostasis and other processes in humans and other animals. With the progress of knowledge about the structure of FSAP, several of its relationships with other proteins or chemical compounds that may modulate its activity have been explained. These mutual axes are described in the present narrative review. The first part of our series of manuscripts on FSAP describes the structure of this protein and the processes leading to the enhancement and inhibition of its activities. The following parts, II and III, concern the role of FSAP in hemostasis and in the pathophysiology of human diseases, with particular emphasis on cardiovascular diseases.

Danger signals in traumatic hemorrhagic shock and new lines for clinical applications

Hemorrhage is the leading cause of death in severe trauma injuries. When organs or tissues are subjected to prolonged hypoxia, danger signals-known as damage-associated molecular patterns (DAMPs)-are released into the intercellular environment. The endothelium is both the target and a major provider of damage-associated molecular patterns, which are directly involved in immuno-inflammatory dysregulation and the associated tissue suffering. Although damage-associated molecular patterns release begins very early after trauma, this release and its consequences continue beyond the initial treatment. Here we review a few examples of damage-associated molecular patterns to illustrate their pathophysiological roles, with emphasis on emerging therapeutic interventions in the context of severe trauma. Therapeutic intervention administered at precise points during damage-associated molecular patterns release may have beneficial effects by calming the inflammatory storm triggered by traumatic hemorrhagic shock.

Factor VII activating protease

Factor VII activating protease (FSAP) is a circulating serine protease with high homology to fibrinolytic enzymes. A role in the regulation of coagulation and fibrinolysis is suspected based on in vitro studies demonstrating activation of FVII or pro-urokinase plasminogen activator (uPA). However, considering the paucity of any studies in animal models or any correlative studies in humans the role of FSAP in haemostasis remains unclear. In relation to vascular remodeling processes or inflammation it has been convincingly shown that FSAP interacts with growth factors as well as protease activated receptors (PAR). Against this sparse background there are a plethora of studies which have investigated the linkage of single nucleotide polymorphisms (SNP) in the FSAP gene (HABP2) to various diseases. The G534E SNP of FSAP is associated with a low proteolytic activity due to an amino acid exchange in the protease domain. This and other SNPs have been linked to carotid stenosis, stroke as well as thrombosis in the elderly and plaque calcification. These SNP analyses indicate an important role for FSAP in the regulation of the haemostasis system as well as fibroproliferative inflammatory processes.

FSAP, a new player in inflammation?

Factor VII-activating protease (FSAP) is a serine protease in plasma that has a role in coagulation and fibrinolysis. FVII could be activated by purified FSAP in a tissue factor independent manner and pro-urokinase has been demonstrated to be a substrate for purified FSAP in-vitro. However, the physiological role of FSAP in haemostasis remains unclear. More recently FSAP is suggested to be involved in inflammation. It modulates vascular permeability directly and indirectly by the generation of bradykinin. Furthermore, FSAP is activated by dead cells induced by the inflammatory response and subsequently removes nucleosomes from apoptotic cells. FSAP activation can be detected in sepsis patients as well. However, whether FSAP activation upon inflammation is beneficial or detrimental remains an open question.

In this review the structure, activation mechanisms and the possible role of FSAP in inflammation are discussed.

Analysis of the substrate specificity of Factor VII activating protease (FSAP) and design of specific and sensitive peptide substrates

Factor VII (FVII) activating protease (FSAP) is a circulating serine protease that is likely to be involved in a number of disease conditions such as stroke, atherosclerosis, liver fibrosis, thrombosis and cancer. To date, no systematic information is available about the substrate specificity of FSAP. Applying phage display and positional scanning substrate combinatorial library (PS-SCL) approaches we have characterised the specificity of FSAP towards small peptides. Results were evaluated in the context of known protein substrates as well as molecular modelling of the peptides in the active site of FSAP. The representative FSAP-cleaved sequence obtained from the phage display method was Val-Leu-Lys-Arg-Ser (P4-P1'). The sequence X-Lys/Arg-Nle-Lys/Arg (P4-P1) was derived from the PS-SCL method. These results show a predilection for cleavage at a cluster of basic amino acids on the nonprime side. Quenched fluorescent substrate (Ala-Lys-Nle-Arg-AMC) (amino methyl coumarin) and (Ala-Leu-Lys-Arg-AMC) had a higher selectivity for FSAP compared to other proteases from the hemostasis system. These substrates could be used to measure FSAP activity in a complex biological system such as plasma. In histone-treated plasma there was a specific activation of pro-FSAP as validated by the use of an FSAP inhibitory antibody, corn trypsin inhibitor to inhibit Factor XIIa and hirudin to inhibit thrombin, which may account for some of the haemostasis-related effects of histones. These results will aid the development of further selective FSAP activity probes as well as specific inhibitors that will help to increase the understanding of the functions of FSAP in vivo.

Factor VII-activating protease deficiency promotes neointima formation by enhancing leukocyte accumulation

Essentials Factor VII-activating protease (FSAP) is a plasma protease involved in vascular processes. Neointima formation was investigated after vascular injury in FSAP-/- mice. The neointimal lesion size and the accumulation of macrophages were increased in FSAP-/- mice. This was due to an increased activity of the chemokine (C-C motif) ligand 2 (CCL2).

SUMMARY

Background Factor VII-activating protease (FSAP) is a multifunctional circulating plasma serine protease involved in thrombosis and vascular remodeling processes. The Marburg I single-nucleotide polymorphism (MI-SNP) in the FSAP-coding gene is characterized by low proteolytic activity, and is associated with increased rates of stroke and carotid stenosis in humans. Objectives To determine whether neointima formation after vascular injury is increased in FSAP-/- mice. Methods and Results The neointimal lesion size and the proliferation of vascular smooth muscle cells (VSMCs) were significantly enhanced in FSAP-/- mice as compared with C57BL/6 control mice after wire-induced injury of the femoral artery. Accumulation of leukocytes and macrophages was increased within the lesions of FSAP-/- mice at day 3 and day 14. Quantitative zymography demonstrated enhanced activity of gelatinases/matrix metalloproteinase (MMP)-2 and MMP-9 within the neointimal lesions of FSAP-/- mice, and immunohistochemistry showed particular costaining of MMP-9 with accumulating leukocytes. Using intravital microscopy, we observed that FSAP deficiency promoted the intravascular adherence and the subsequent transmigration of leukocytes in vivo in response to chemokine ligand 2 (CCL2). CCL2 expression was increased in FSAP-/- monocytes but not in the vessel wall. There was no difference in the expression of platelet-derived growth factor (PDGF-BB). Conclusions FSAP deficiency causes an increase in CCL2 expression and CCL2-mediated infiltration of leukocytes into the injured vessel, thereby promoting SMC proliferation and migration by the activation of leukocyte-derived gelatinases. These results provide a possible explanation for the observed association of the loss-of-function MI-SNP with vascular proliferative diseases.

The PDGF system and its antagonists in liver fibrosis

Platelet derived growth factor (PDGF) signaling plays an important role in activated hepatic stellate cells and portal fibroblast proliferation, chemotaxis, migration and cell survival. PDGF receptors and ligands are upregulated in experimental liver fibrotic models as well as in human liver fibrotic diseases. Blocking of PDGF signaling ameliorates experimental liver fibrogenesis. The plurality of molecular and cellular activities of PDGF and its involvement in initiation, progression and resolution of hepatic fibrogenesis offers an infinite number of therapeutic possibilities. These include the application of therapeutic antibodies (e.g. AbyD3263, MOR8457) which specifically sequester individual PDGF isoforms or the inhibition of PDGF isoforms by synthetic aptamers. In particular, the isolation of innovative slow off-rate modified aptamers (e.g., SOMAmer SL1 and SL5) that carry functional groups absent in natural nucleic acids by the Systematic Evolution of Ligands by EXponential (SELEX) enrichment technique offers the possibility to design high affinity aptamers that target PDGF isoforms for clinical purposes. Dominant-negative soluble PDGF receptors are also effective in attenuation of hepatic stellate cell proliferation and hepatic fibrogenesis. Moreover, some multikinase inhibitors targeting PDGF signaling have been intensively tested during the last decade and are on the way into advanced preclinical studies and clinical trials. This narrative review aims to gauge the recent progression of research into PDGF systems and liver fibrosis.

Increased Nucleosomes and Neutrophil Activation Link to Disease Progression in Patients with Scrub Typhus but Not Murine Typhus in Laos

Cell-mediated immunity is essential in protection against rickettsial illnesses, but the role of neutrophils in these intracellular vasculotropic infections remains unclear. This study analyzed the plasma levels of nucleosomes, FSAP-activation (nucleosome-releasing factor), and neutrophil activation, as evidenced by neutrophil-elastase (ELA) complexes, in sympatric Lao patients with scrub typhus and murine typhus. In acute scrub typhus elevated nucleosome levels correlated with lower GCS scores, raised respiratory rate, jaundice and impaired liver function, whereas neutrophil activation correlated with fibrinolysis and high IL-8 plasma levels, a recently identified predictor of severe disease and mortality. Nucleosome and ELA complex levels were associated with a 4.8-fold and 4-fold increased risk of developing severe scrub typhus, beyond cut off values of 1,040 U/ml for nucleosomes and 275 U/ml for ELA complexes respectively. In murine typhus, nucleosome levels associated with pro-inflammatory cytokines and the duration of illness, while ELA complexes correlated strongly with inflammation markers, jaundice and increased respiratory rates. This study found strong correlations between circulating nucleosomes and neutrophil activation in patients with scrub typhus, but not murine typhus, providing indirect evidence that nucleosomes could originate from neutrophil extracellular trap (NET) degradation. High circulating plasma nucleosomes and ELA complexes represent independent risk factors for developing severe complications in scrub typhus. As nucleosomes and histones exposed on NETs are highly cytotoxic to endothelial cells and are strongly pro-coagulant, neutrophil-derived nucleosomes could contribute to vascular damage, the pro-coagulant state and exacerbation of disease in scrub typhus, thus indicating a detrimental role of neutrophil activation. The data suggest that increased neutrophil activation relates to disease progression and severe complications, and increased plasma levels of nucleosomes and ELA complexes represent independent risk factors for developing severe scrub typhus.

Tropical rickettsial illnesses, especially scrub typhus and murine typhus, are increasingly recognized as a leading cause of treatable undifferentiated febrile illness in Asia, but remain severely neglected and under appreciated diseases in many areas. In this study we investigated the relationship of markers of neutrophil activation and cell death with disease severity in patients with acute scrub typhus and murine typhus in Laos. These easily measurable circulating markers were associated with a 4 to 5-fold increased risk of developing severe clinical disease manifestations in scrub typhus and represent independent predictors of severe disease, and possibly death. We also found strong correlations between circulating markers of cell death and neutrophil activation in patients with scrub typhus, but not murine typhus, providing indirect evidence that neutrophil extracellular traps could contribute to the vascular damage and pro-coagulant state leading to exacerbation of disease in scrub typhus, thus indicating a detrimental role of neutrophil activation. The data suggest that increased neutrophil activation relates to disease progression and severe complications, and increased plasma levels of nucleosomes and ELA complexes represent independent risk factors for developing severe scrub typhus.

Extracellular nucleic acids as novel alarm signals in the vascular system. Mediators of defence and disease

Upon vascular injury or tissue damage, the exposed intracellular material such as nucleic acids, histones and other macromolecules may come into contact with vessel wall cells and circulating blood cells and may thus, have an enduring influence on wound healing and body defence processes. This short review summarizes recent work related to extracellular DNA and RNA and their role as prominent alarm signals and inducers of different defence reactions related to innate immunity and thrombus formation. Of particular importance are DNA-histone complexes (nucleosome material) that, having been expelled during stimulation of the neutrophils, not only trap and eliminate bacteria but also promote thrombus formation in the arterial and venous system. Consequently therefore, the administration of DNase exhibits strong antithrombotic functions. Similarly, extracellular RNA provokes activation of the contact phase system of blood coagulation and, by interacting with specific proteins and cytokines, it promotes vascular permeability and oedema formation. The development of RNA-mediated thrombosis, vasogenic oedema or proinflammatory responses are counteracted by the administration of RNase1 in several pathogenetic animal models. Thus, extracellular nucleic acids appear not only to function as host alarm signals that serve to amplify the defence response, but they also provide important links to thrombus formation as part of the innate immune system.

Plasma factor VII-activating protease antigen levels and activity are increased in ischemic stroke

BACKGROUND

Factor VII-activating protease (FSAP) is a recently discovered plasma protease with a role in the regulation of hemostasis and vascular remodeling processes. Higher levels and activity of FSAP have been reported in patients with deep vein thrombosis, but there are no data on plasma FSAP in ischemic stroke (IS).

OBJECTIVE

To investigate whether FSAP antigen levels and activity are associated with IS and/or etiologic subtypes of IS.

PATIENTS AND METHODS

To assess the potential association between FSAP and IS, plasma FSAP antigen levels and activity were measured in 600 consecutive IS patients and 600 population-based controls from the case-control study the Sahlgrenska Academy Study on Ischemic Stroke (SAHLSIS). Blood sampling was performed in the acute phase and 3 months after the index stroke. FSAP was also investigated at the genetic level by genotyping of 33 single-nucleotide polymorphisms.

RESULTS

Increased FSAP antigen level and activity, at both time-points, were independently associated with IS. Subtype analysis revealed similar associations for both FSAP measures, at both time-points, in all main IS subtypes. FSAP genotypes showed association with both FSAP plasma measurements, but not with IS.

CONCLUSIONS

Increased plasma FSAP antigen levels and activity were associated with IS and all main etiologic subtypes, suggesting a possible role for FSAP in the pathophysiology of IS, irrespective of the underlying etiology.

The platelet-derived growth factor system in renal disease: an emerging role of endogenous inhibitors

The platelet-derived growth factor (PDGF) family consists of four isoforms which are secreted as homodimers (PDGF-AA, PDGF-BB, PDGF-CC and PDGF-DD) or heterodimers (PDGF-AB), and two receptor chains (PDGFR-α and -β). All members of the PDGF system are constitutively or inducibly expressed in renal cells and are involved in the regulation of cell proliferation and migration, the accumulation of extracellular matrix proteins and the secretion of pro- and anti-inflammatory mediators. Particular roles have been identified in mediating mesangioproliferative changes, renal interstitial fibrosis and glomerular angiogenesis. Different endogenous inhibitors of PDGF-induced biological responses exist which affect the activation/deactivation of PDGF isoforms, the activity of the PDGFRs, or which block downstream signaling pathways of the autophosphorylated PDGFRs. The novel endogenous inhibitor nephroblastoma overexpressed gene (NOV, CCN3) reduces PDGF-induced cell proliferation and is downregulated by PDGF isoforms itself. Among all identified inhibitors only few "true" PDGF antagonists have been identified. A better understanding of these inhibitors may aid in the design of novel therapeutic approaches to PDGF-mediated diseases.

Activation of factor VII-activating protease in human inflammation: a sensor for cell death

INTRODUCTION

Cell death is a central event in the pathogenesis of sepsis and is reflected by circulating nucleosomes. Circulating nucleosomes were suggested to play an important role in inflammation and were demonstrated to correlate with severity and outcome in sepsis patients. We recently showed that plasma can release nucleosomes from late apoptotic cells. Factor VII-activating protease (FSAP) was identified to be the plasma serine protease responsible for nucleosome release. The aim of this study was to investigate FSAP activation in patients suffering from various inflammatory diseases of increasing severity.

METHODS

We developed ELISAs to measure FSAP-C1-inhibitor and FSAP-α2-antiplasmin complexes in plasma. FSAP-inhibitor complexes were measured in the plasma of 20 adult patients undergoing transhiatal esophagectomy, 32 adult patients suffering from severe sepsis and 8 from septic shock and 38 children suffering from meningococcal sepsis.

RESULTS

We demonstrate plasma FSAP to be activated upon contact with apoptotic and necrotic cells by an assay detecting complexes between FSAP and its target serpins α2-antiplasmin and C1-inhibitor, respectively. By means of that assay we demonstrate FSAP activation in post-surgery patients, patients suffering from severe sepsis, septic shock and meningococcal sepsis. Levels of FSAP-inhibitor complexes correlate with nucleosome levels and correlate with severity and mortality in these patients.

CONCLUSIONS

These results suggest FSAP activation to be a sensor for cell death in the circulation and that FSAP activation in sepsis might be involved in nucleosome release, thereby contributing to lethality.

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.

Altered factor VII activating protease expression in murine hepatic fibrosis and its influence on hepatic stellate cells

BACKGROUND

Platelet-derived growth factor-BB (PDGF-BB) is a profibrotic factor in liver fibrosis through its ability to stimulate hepatic stellate cells (HSC). The liver-derived serine protease factor VII activating protease (FSAP) regulates the activities of PDGF-BB in a cell-specific manner.

AIMS

Our aim was to determine the influence of FSAP on the activation of HSC and to analyse the regulation of FSAP in hepatic fibrogenesis.

METHODS

The effect of FSAP on PDGF-stimulated p42/p44 mitogen-activated protein kinase (MAPK) activation in primary rat HSC was determined by Western blotting. Migration and proliferation of HSC was evaluated in Boyden chamber experiments and (3)H-thymidine incorporation assays respectively. Expression of FSAP was analysed in a CCl(4) mouse model of liver fibrosis by Western blot, quantitative real-time polymerase chain reaction and immunohistochemistry.

RESULTS

FSAP inhibited PDGF-BB-stimulated p42/p44 MAPK phosphorylation, proliferation and migration of HSC. FSAP mRNA expression level was increased 3 h after CCl(4) application and decreased after 18 h and, in established fibrosis, after chronic CCl(4) administration. In parallel, there was a decrease in the circulating FSAP protein in chronic fibrosis. Concurrently, the homogenous hepatic expression pattern of FSAP was disturbed. Immunohistochemistry revealed a decrease of FSAP in hepatocytes in inflammatory and fibrotic lesions.

CONCLUSIONS

Our results demonstrate an inhibitory effect of FSAP on PDGF-mediated activation of HSC. In addition, FSAP expression is transiently increased in acute-phase reaction but decreased during chronic fibrogenesis, which in turn may influence PDGF-BB availability and myofibroblast activity.

Factor VII-activating protease in patients with acute deep venous thrombosis

Factor VII-activating protease (FSAP) is involved in haemostasis and inflammation. FSAP cleaves single chain urokinase-type plasminogen activator (scu-PA). The 1601GA genotype of the 1601G/A polymorphism in the FSAP gene leads to the expression of a FSAP variant with reduced ability to activate scu-PA, without affecting the ability to activate coagulation Factor VII (FVII). Previous studies have investigated the association of the 1601GA genotype with incidence and progression of carotid stenosis and deep venous thrombosis (DVT). The present study is the first to evaluate the potential association between the FSAP phenotype and DVT. We studied the association between the 1601G/A polymorphism, FSAP activity, FSAP antigen, Factor VIIa (FVIIa), prothrombin fragment 1+2 (F1+2), and C-reactive protein (CRP) in plasmas of 170 patients suspected for DVT. FSAP genotypes were equally distributed in patients with (n=64) and without DVT (n=106), (P=0.94). The 1601GA genotype was associated with significant reduction of FSAP activity (P<0.001) and FSAP antigen levels (P=0.04). Patients with DVT showed significantly higher FSAP activity (P=0.008), FSAP antigen (P=0.003), and F1+2 levels (P<0.001) than patients without DVT. The association between the FSAP measures and DVT disappeared when adjusted for CRP levels. F1+2 correlated positively to FSAP antigen (P=0.01), while FVIIa-levels were comparable in patients with and without DVT. We conclude that even though FSAP measures are significantly increased in patients with acute DVT, alterations in the scu-PA activating properties of FSAP are presumably not markedly involved in the development of acute DVT, and that the association between FSAP and DVT disappears after adjustment for CRP.