Proteolysis of thrombospondin during cathepsin-G-induced platelet aggregation: functional role of the 165-kDa carboxy-terminal fragment

Thrombospondin-1 Signaling Through the Calreticulin/LDL Receptor Related Protein 1 Axis: Functions and Possible Roles in Glaucoma

Thrombospondin-1 (TSP-1) is a matricellular extracellular matrix protein. Matricellular proteins are components of the extracellular matrix (ECM) that regulate key cellular functions and impact ECM organization, but which lack direct primary structural roles in the ECM. TSP-1 expression is upregulated in response to injury, hypoxia, growth factor stimulation, inflammation, glucose, and by reactive oxygen species. Relevant to glaucoma, TSP-1 is also a mechanosensitive molecule upregulated by mechanical stretch. TSP-1 expression is increased in ocular remodeling in glaucoma in both the trabecular meshwork and in the optic nerve head. The exact roles of TSP-1 in glaucoma remain to be defined, however. It plays important roles in cell behavior and in ECM remodeling during wound healing, fibrosis, angiogenesis, and in tumorigenesis and metastasis. At the cellular level, TSP-1 can modulate cell adhesion and migration, protease activity, growth factor activity, anoikis resistance, apoptosis, and collagen secretion and matrix assembly and cross-linking. These multiple functions and macromolecular and receptor interactions have been ascribed to specific domains of the TSP-1 molecule. In this review, we will focus on the cell regulatory activities of the TSP-1 N-terminal domain (NTD) sequence that binds to cell surface calreticulin (Calr) and which regulates cell functions via signaling through Calr complexed with LDL receptor related protein 1 (LRP1). We will describe TSP-1 actions mediated through the Calr/LRP1 complex in regulating focal adhesion disassembly and cytoskeletal reorganization, cell motility, anoikis resistance, and induction of collagen secretion and matrix deposition. Finally, we will consider the relevance of these TSP-1 functions to the pathologic remodeling of the ECM in glaucoma.

Hindrance of the Proteolytic Activity of Neutrophil-Derived Serine Proteases by Serine Protease Inhibitors as a Management of Cardiovascular Diseases and Chronic Inflammation

During inflammation neutrophils become activated and segregate neutrophil serine proteases (NSPs) to the surrounding environment in order to support a natural immune defense. However, an excess of proteolytic activity of NSPs can cause many complications, such as cardiovascular diseases and chronic inflammatory disorders, which will be elucidated on a biochemical and immunological level. The application of selective serine protease inhibitors is the logical consequence in the management of the indicated comorbidities and will be summarized in this briefing.

Therapeutic effects of platelet derived growth factor overexpressed-mesenchymal stromal cells and sheets in canine skin wound healing model

Adipose-derived mesenchymal stromal cells (Ad-MSCs) have excellent potential for skin wound repair. Moreover, platelet-derived growth factor (PDGF) has strong wound healing properties. The purpose of the present study was to compare the healing effects of PDGF-overexpressing canine allogeneic Ad-MSCs (PDGF-MSCs) and their cell sheets (PDGF-CSs) as compared to unexpressed Ad-MSCs (U-MSCs) and their cell sheets (UCSs) in a cutaneous wound healing model induced upon dogs. In in vitro study, the expression of immunomodulatory and growth factors was assessed by qRT-PCR. In in vivo study, cells and sheets were transplanted into a square-shaped full-thickness (1.5×1.5 cm) skin defect model created in 12 dogs. After 5 and 10 days, wounds were harvested and evaluated macroscopically and histopathologically. The qRT-PCR results showed that the PDGF-B gene was significantly upregulated (p<0.05) in PDGF-CS and PDGF-MSCs groups. Upon gross analysis of the wound, all stromal cells and their sheet groups showed accelerated (p<0.05) cutaneous wound healing compared to the negative control groups. As compared to U-MSCs and UCSs, the PDGF-MSCs showed significant epithelization on days 5 and 10 of healing, whereas PDGF-CSs showed improved epithelization only on day 10. In the granulation tissue analysis, PDGF-CSs and UCSs promoted more formation (p<0.05) of upper granulation tissue, collagen, and activated fibroblasts than PDGF-MSCs, and U-MSCs. Especially, the PDGF-CSs presented the highest formation and maturation of granulation tissue among all groups. All considered, PDGF overexpressed stromal cells or cells sheets can improve cutaneous wound healing in a canine model.

Thrombospondin 1 and Its Diverse Roles as a Regulator of Extracellular Matrix in Fibrotic Disease

Thrombospondin 1 (TSP1) is a matricellular extracellular matrix protein that has diverse roles in regulating cellular processes important for the pathogenesis of fibrotic diseases. We will present evidence for the importance of TSP1 control of latent transforming growth factor beta activation in renal fibrosis with an emphasis on diabetic nephropathy. Other functions of TSP1 that affect renal fibrosis, including regulation of inflammation and capillary density, will be addressed. Emerging roles for TSP1 N-terminal domain regulation of collagen matrix assembly, direct effects of TSP1-collagen binding, and intracellular functions of TSP1 in mediating endoplasmic reticulum stress responses in extracellular matrix remodeling and fibrosis, which could potentially affect renal fibrogenesis, will also be discussed. Finally, we will address possible strategies for targeting TSP1 functions to treat fibrotic renal disease.

Advanced drug delivery systems and artificial skin grafts for skin wound healing

Cutaneous injuries, especially chronic wounds, burns, and skin wound infection, require painstakingly long-term treatment with an immense financial burden to healthcare systems worldwide. However, clinical management of chronic wounds remains unsatisfactory in many cases. Various strategies including growth factor and gene delivery as well as cell therapy have been used to enhance the healing of non-healing wounds. Drug delivery systems across the nano, micro, and macroscales can extend half-life, improve bioavailability, optimize pharmacokinetics, and decrease dosing frequency of drugs and genes. Replacement of the damaged skin tissue with substitutes comprising cell-laden scaffold can also restore the barrier and regulatory functions of skin at the wound site. This review covers comprehensively the advanced treatment strategies to improve the quality of wound healing.

Problems and Solutions for Platelet-Rich Plasma in Facial Rejuvenation: A Systematic Review

BACKGROUND

In recent years, platelet-rich plasma (PRP) has been widely applied in orthopedics, maxillofacial surgery, burns, and plastic surgery, especially in facial rejuvenation. Research is ongoing into new indications and mechanisms of PRP to promote its wider, safer, and more effective use in the clinic. This article reviews the possible mechanisms of PRP in facial rejuvenation and related research. It is expected that the application of PRP in this field will increase.

METHODS

The use of PRP in facial rejuvenation was screened using inclusion and exclusion criteria. The relevant articles were searched through Pubmed digest database, SCI full-text database, ScienceDirect full-text database, and the CNKI full-text database. The different effects and limitations of PRP were extracted.

RESULTS

A total of 108 articles were obtained, including 18 articles researching PRP in cells, 10 articles on animal research using PRP, 16 articles on the clinical study of PRP, 24 articles involving signs of skin aging, and four articles on the limitations of PRP. The remaining articles were related to the preparation of PRP, the introduction of PRP, and other aspects.

CONCLUSION

Based on in vitro and in vivo research, PRP may play a role in promoting tissue regeneration, oxidative stress and revascularization, which form the theoretical basis for the use of PRP in the clinical treatment of facial rejuvenation.

LEVEL OF EVIDENCE III

This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Integration of Two In-depth Quantitative Proteomics Approaches Determines the Kallikrein-related Peptidase 7 (KLK7) Degradome in Ovarian Cancer Cell Secretome

Kallikrein-related peptidase 7 (KLK7) is a serine peptidase that is over expressed in ovarian cancer. In vitro functional analyses have suggested KLK7 to play a cancer progressive role, although monitoring of KLK7 expression has suggested a contradictory protective role for KLK7 in ovarian cancer patients. In order to help delineate its mechanism of action and thereby the functional roles, information on its substrate repertoire is crucial. Therefore, in this study a quantitative proteomics approach-PROtein TOpography and Migration Analysis Platform (PROTOMAP)-coupled with SILAC was used for in-depth analysis of putative KLK7 substrates from a representative ovarian cancer cell line, SKOV-3, secreted proteins. The Terminal Amine Isotopic Labeling of Substrates (TAILS) approach was used to determine the exact cleavage sites and to validate qPROTOMAP-identified putative substrates. By employing these two technically divergent approaches, exact cleavage sites on 16 novel putative substrates and two established substrates, matrix metalloprotease (MMP) 2 and insulin growth factor binding protein 3 (IGFBP3), were identified in the SKOV-3 secretome. Eight of these substrates were also identified on TAILS analysis of another ovarian cancer cell (OVMZ-6) secretome, with a further seven OVMZ-6 substrates common to the SKOV-3 qPROTOMAP profile. Identified substrates were significantly associated with the common processes of cell adhesion, extracellular matrix remodeling and cell migration according to the gene ontology (GO) biological process analysis. Biochemical validation supports a role for KLK7 in directly activating pro-MMP10, hydrolysis of IGFBP6 and cleavage of thrombospondin 1 with generation of a potentially bioactive N-terminal fragment. Overall, this study constitutes the most comprehensive analysis of the putative KLK7 degradome in any cancer to date, thereby opening new avenues for KLK7 research.

Neutrophil-Mediated Proteolysis of Thrombospondin-1 Promotes Platelet Adhesion and String Formation

Thrombospondin-1 (TSP-1) is primarily expressed by platelets and endothelial cells (ECs) and rapidly released upon their activation. It functions in haemostasis as a bridging molecule in platelet aggregation, by promoting platelet adhesion to collagen and by protecting von Willebrand factor strings from degradation. In blood of patients undergoing surgery and in co-cultures of neutrophils with platelets or ECs, we observed proteolysis of the 185 kDa full-length TSP-1 to a 160-kDa isoform. We hypothesized that TSP-1 processing may alter its haemostatic properties. Selective enzyme inhibitors in co-cultures revealed that neutrophil proteases elastase and cathepsin G mediate TSP-1 processing. The cut site of cathepsin G was mapped to TSP-1 amino acids R237/T238 by Edman sequencing. Formation of neutrophil extracellular traps protected TSP-1 from complete degradation and promoted controlled processing to the 160-kDa isoform. Haemostatic properties were tested by platelet aggregation, adhesion, coagulation and string formation under flow. Platelets from TSP-1 deficient mice did not differ from wild-type in platelet aggregation but showed severe impairment of platelet adhesion to collagen and string formation under flow. Reconstitution experiments revealed that the 160-kDa TSP-1 isoform was markedly more potent than the 185-kDa full-length molecule in restoring function. Thus, TSP-1 processing by neutrophil proteases yields a 160-kDa isoform which shows enhanced potency to promote platelet adhesion and string formation. This finding reveals a novel mechanism of neutrophil-mediated thrombus formation and provides first evidence for the impact of TSP-1 proteolysis on its haemostatic properties.

N-Terminomics identifies HtrA1 cleavage of thrombospondin-1 with generation of a proangiogenic fragment in the polarized retinal pigment epithelial cell model of age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in the elderly population. Variants in the HTRA1-ARMS2 locus have been linked to increased AMD risk. In the present study we investigated the impact of elevated HtrA1 levels on the retina pigment epithelial (RPE) secretome using a polarized culture system. Upregulation of HtrA1 alters the abundance of key proteins involved in angiogenesis and extracellular matrix remodeling. Thrombospondin-1, an angiogenesis modulator, was identified as a substrate for HtrA1 using terminal amine isotope labeling of substrates in conjunction with HtrA1 specificity profiling. HtrA1 cleavage of thrombospondin-1 was further corroborated by in vitro cleavage assays and targeted proteomics together with small molecule inhibition of HtrA1. While thrombospondin-1 is anti-angiogenic, the proteolytically released N-terminal fragment promotes the formation of tube-like structure by endothelial cells. Taken together, our findings suggest a mechanism by which increased levels of HtrA1 may contribute to AMD pathogenesis. The proteomic data has been deposited to the ProteomeXchange Consortium via the PRIDE partner repository with the data set identifier. For quantitative secretome analysis, project accession: PXD007691, username: reviewer45093@ebi.ac.uk, password: 1FUpS6Yq. For TAILS analysis, project accession: PXD007139, username: reviewer76731@ebi.ac.uk, password: sNbMp7xK.

Clinical application of growth factors and cytokines in wound healing

Wound healing is a complex and dynamic biological process that involves the coordinated efforts of multiple cell types and is executed and regulated by numerous growth factors and cytokines. There has been a drive in the past two decades to study the therapeutic effects of various growth factors in the clinical management of nonhealing wounds (e.g., pressure ulcers, chronic venous ulcers, diabetic foot ulcers). For this review, we conducted an online search of Medline/PubMed and critically analyzed the literature regarding the role of growth factors and cytokines in the management of these wounds. We focused on currently approved therapies, emerging therapies, and future research possibilities. In this review, we discuss four growth factors and cytokines currently being used on and off label for the healing of wounds. These include granulocyte-macrophage colony-stimulating factor, platelet-derived growth factor, vascular endothelial growth factor, and basic fibroblast growth factor. While the clinical results of using growth factors and cytokines are encouraging, many studies involved a small sample size and are disparate in measured endpoints. Therefore, further research is required to provide definitive evidence of efficacy.

A motif within the N-terminal domain of TSP-1 specifically promotes the proangiogenic activity of endothelial colony-forming cells

Thrombospondin-1 (TSP-1) gives rise to fragments that have both pro- and anti-angiogenic effects in vitro and in vivo. The TSP-HepI peptide (2.3 kDa), located in the N-terminal domain of TSP-1, has proangiogenic effects on endothelial cells. We have previously shown that TSP-1 itself exhibits a dual effect on endothelial colony-forming cells (ECFC) by enhancing their adhesion through its TSP-HepI fragment while reducing their proliferation and differentiation into vascular tubes (tubulogenesis) in vitro. This effect is likely mediated through CD47 binding to the TSP-1 C-terminal domain. Here we investigated the effect of TSP-HepI peptide on the angiogenic properties of ECFC in vitro and in vivo. TSP-HepI peptide potentiated FGF-2-induced neovascularisation by enhancing ECFC chemotaxis and tubulogenesis in a Matrigel plug assay. ECFC exposure to 20 μg/mL of TSP-HepI peptide for 18 h enhanced cell migration (p < 0.001 versus VEGF exposure), upregulated alpha 6-integrin expression, and enhanced their cell adhesion to activated endothelium under physiological shear stress conditions at levels comparable to those of SDF-1α. The adhesion enhancement appeared to be mediated by the heparan sulfate proteoglycan (HSPG) syndecan-4, as ECFC adhesion was significantly reduced by a syndecan-4-neutralising antibody. ECFC migration and tubulogenesis were stimulated neither by a TSP-HepI peptide with a modified heparin-binding site (S/TSP-HepI) nor when the glycosaminoglycans (GAGs) moieties were removed from the ECFC surface by enzymatic treatment. Ex vivo TSP-HepI priming could potentially serve to enhance the effectiveness of therapeutic neovascularisation with ECFC.

Thrombospondin-1: a unique marker to identify in vitro platelet activation when monitoring in vivo processes

BACKGROUND

Measuring platelet activation in patients has become a potent method to investigate pathophysiological processes. However, the commonly applied markers are sensitive to detrimental influences by in vitro platelet activation during blood analysis.

OBJECTIVES

Protein isoforms of platelet-derived thrombospondin-1 (TSP-1) were investigated for their potential to identify in vitro platelet activation when monitoring in vivo processes.

METHODS

TSP-1 was determined in plasma, serum or supernatant of purified platelets by ELISA and immunoblotting and was compared with standard markers of platelet activation. A collective of 20 healthy individuals and 30 cancer patients was analyzed.

RESULTS

 While in vitro platelet degranulation led to a selective increase in the 200-kDa full-length molecule, an in vivo process involving platelet activation such as wound healing resulted in the predominant rise of the 140-kDa TSP-1 protein. The physiological ratio of circulating TSP-1 variants was determined and a cut-off level at 1.0 was defined to identify plasma samples with artificial in vitro platelet activation exceeding the cut-off level. In contrast, cancer patients known to frequently exhibit increased in vivo activation of platelets presented with a significantly decreased ratio of TSP-1 variants as compared with healthy volunteers.

CONCLUSIONS

In comparison to standard platelet markers, TSP-1 constitutes a sensitive and stable parameter suited to monitor in vitro platelet activation. The analysis of TSP-1 protein isoforms further offers a valuable tool to reliably discriminate between in vitro and in vivo effects, to exclude variability introduced during blood processing and improve clinical monitoring.

Bone and wound healing augmentation with platelet-rich plasma

Over the past two decades, autologous platelets that have been sequestered, concentrated, and mixed with thrombin to generate growth factor-concentrated platelet-rich plasma for application to bone and wounds to aide healing have been a subject of great interest. This article reviews the literature related to the use of autologous platelet-rich plasma in bone and wound healing, and reviews the processes necessary to secure a high concentration of viable platelets. Although not yet definitive, autologous platelet-rich plasma has been shown to be safe, reproducible, and effective in mimicking the natural process of bone and wound healing.

Regulation of thrombospondin1 by extracellular proteases

The contribution of proteases to developmental, physiological and pathological processes has been well accepted. Cleavage of matrix proteins is a key requirement for cell migration and remodeling of the extracellular environment. The constant process of matrix turnover is dependent on the delicate balance between degradation and synthesis. In addition, regulated proteolysis also allows for the release and activation of growth factors and cytokines. Similarly to other extracellular matrix proteins, thrombospondins are also targets of proteolysis. While in some cases enzymatic activity is associated with degradation of the protein; in other situations, targeted and selective cleavage offers the means to release polypeptides with either alternative or enhanced function. Here, we provide a summary of the published information related to thrombospondin proteolysis within the context of how proteolysis of extracellular matrix proteins impacts diversification of protein function. We also discuss its biological relevance and potential therapeutic value of thrombospondin proteolysis with particular emphasis on angiogenesis.

LDL receptor-related protein 1: unique tissue-specific functions revealed by selective gene knockout studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis

Anoikis, apoptotic cell death due to loss of cell adhesion, is critical for regulation of tissue homeostasis in tissue remodeling. Fibrogenesis is associated with reduced fibroblast apoptosis. The matricellular protein thrombospondin 1 (TSP1) regulates cell adhesion and motility during tissue remodeling and in fibrogenesis. The N-terminal domain of TSP1 binds to the calreticulin-LRP1 receptor co-complex to signal down-regulation of cell adhesion and increased cell motility through focal adhesion disassembly. TSP1 signaling through calreticulin-LRP1 activates cell survival signals such as PI3-kinase. Therefore, we tested the hypothesis that TSP1 supports cell survival under adhesion-independent conditions to facilitate tissue remodeling. Here, we show that platelet TSP1, its N-terminal domain (NoC1) as a recombinant protein, or a peptide comprising the calreticulin-LRP1 binding site [amino acids 17-35 (hep I)] in the N-terminal domain promotes fibroblast survival under anchorage-independent conditions. TSP1 activates Akt and decreases apoptotic signaling through caspase 3 and PARP1 in suspended fibroblasts. Inhibition of PI3K/Akt activity blocks TSP1-mediated anchorage-independent survival. Fibroblasts lacking LRP1 or expressing calreticulin lacking the TSP1 binding site do not respond to TSP1 with anchorage-independent survival. These data define a novel role for TSP1 signaling through the calreticulin/LRP1 co-complex in tissue remodeling and fibrotic responses through stimulation of anoikis resistance.-Pallero, M. A., Elzie, C. A., Chen, J., Mosher, D. F., Murphy-Ullrich, J. E. Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis.

High glucose reduces cathepsin L activity and impairs invasion of circulating progenitor cells

Endothelial progenitor cells (EPC) significantly contribute to neovascularization and endothelial regeneration. Risk factors for coronary artery disease, particularly diabetes mellitus, reduce the number and functional activity of EPC. As we have recently shown, expression and activity of the matrix degrading cysteine protease cathepsin L in EPC is required for tissue invasion and EPC-mediated improvement of neovascularization. Therefore, we investigated the effect of high glucose and diabetes mellitus on EPC invasion and cathepsin L activity. Incubation of EPC with high levels of glucose (10-30 mM) dose-dependently decreased cathepsin L activity (glucose 20 mM: 67+/-4% compared to control; p<0.05) and protein expression (48+/-5% of control, p<0.05). In contrast, other proteases of the cathepsin family such as cathepsins D and O, and the matrix metalloproteinases MMP-2 and MMP-9 were not altered with high glucose. Cathepsin L mRNA was not affected suggesting that a posttranscriptional mechanism is responsible for cathepsin L down-regulation. As a functional consequence, high glucose significantly reduced the gelatinolytic activity and invasion of EPC (50+/-5% of control). Importantly, EPC of patients with type 2 diabetes revealed profoundly decreased cathepsin L expression and activity as compared to EPC derived from healthy controls. Taken together, high glucose significantly reduces the protein expression and activity of cathepsin L, which is involved in matrix degradation and required for invasion of EPC into the ischemic tissue, and, thereby, may limit the functional capacity of EPC to improve neovascularization in diabetics.

Functions of the conserved thrombospondin carboxy-terminal cassette in cell-extracellular matrix interactions and signaling

Thrombospondins (TSPs) are extracellular, multidomain, calcium-binding glycoproteins that function at cell surfaces, in extracellular matrix (ECM) and as bridging molecules in cell-cell interactions. TSPs are multifunctional and modulate cell behavior during development, wound-healing, immune response, tumor growth and in the homeostasis of adult tissues. TSPs are assembled as oligomers that are composed of homologous polypeptides. In all the TSP polypeptides, the most highly-conserved region is the carboxyl-region, which contains a characteristic set of domains comprising EGF domains, TSP type 3 repeats and a globular carboxy-terminal domain. This large region is termed here the thrombospondin carboxy-terminal cassette (TSP-CTC). The strong conservation of the TSP-CTC suggests that it may mediate ancestral functions that are shared by all TSPs. This review summarizes the current knowledge of the TSP-CTC and areas of future interest.

The N-terminus of thrombospondin: the domain stands apart

Thrombospondin 1 (TSP1) was first recognized as a thrombin-sensitive protein associated with platelet membranes. It is secreted by numerous cell types and its expression is predominant in areas of active tissue remodeling. Thrombospondins 1 and 2 are large, trimeric, matricellular proteins, composed of multiple structural motifs which interact with a diverse array of receptors and molecules. Thrombospondin's capacity to bind multiple receptors renders it multifunctional. The functions of its isolated domains can be overlapping or contradictory. In this review, we focus on the N-terminus of the molecule, first recognized for its strong heparin binding properties and characterized by its susceptibility to proteolytic cleavage from the stalk region of thrombospondin. The N-terminus, called the heparin binding domain (HBD), interacts with a variety of macromolecules including heparan sulfate proteoglycans at the membrane and in the matrix, LDL receptor-related protein (LRP), sulfated glycolipids, calreticulin, and integrins. The HBD mediates endocytosis of thrombospondin. It functions both as a soluble and an insoluble modulator of cell adhesion and motility. In contrast to thrombospondin, the HBD has pro-angiogenic activity. We propose that the HBD of thrombospondins 1 and 2 are found primarily in the cellular microenvironment in conditions of cellular injury, stress and tissue remodeling and that the HBD conveys multiple signals involved in cellular adaptation to injury.

Matrix metalloproteinases and matrikines in angiogenesis

Neoangiogenesis, the formation of new blood capillaries from pre-existing vessels, plays an important role in a number of physiological and pathological processes, particularly in tumor growth and metastasis. Extracellular proteolysis by matrix metalloproteinases or other neutral proteinases is an absolute requirement for initiating tumor invasion and angiogenesis. Cryptic segments or pre-existing domains within larger proteins, most of them belonging to the extracellular matrix, can be exposed by conformational changes and/or generated by partial enzymatic hydrolysis. They can positively or negatively regulate important functions of endothelial cells including adhesion, migration, proliferation, cell survival and cell-cell interactions. Such regulations by cryptic segments and proteolytic fragments led to the concept of matricryptins and matrikines, respectively. Matrix metalloproteinases and matrikines in conjunction with other pro- or anti-angiogenic factors might act in concert at any step of the angiogenesis process. A number of matrikines have been identified as potent anti-angiogenic factors, which could provide a new alternative to anti-proteolytic strategies for the development of anti-angiogenic therapeutic molecules aimed at inhibiting tumor growth and metastasis. Some of them are currently being investigated in clinical trials.

Opposite effects of thrombospondin-1 via CD36 and CD47 on homotypic aggregation of monocytic cells

Thrombospondin-1 (TSP-1), an extracellular matrix protein, has a multimodular structure and each domain specifies a distinct biological function through interaction with a specific ligand. In this study we found that exogenously added TSP-1 inhibits phorbol myristate acetate (PMA)/LPS-induced homotypic aggregation of human monocytic U937 cells, whereas the 70-kDa fragment of TSP-1 generated by the proteolytic cleavage of the intact molecule promotes the homotypic aggregation. The aggregation was also inhibited by anti-CD47 mAb or the 4N1K peptide, of which sequence is derived from the CD47-binding site of TSP-1 and absent in the 70-kDa fragment. In contrast, the augmented cell aggregation by the 70-kDa fragment was hampered by anti-CD36 mAb or antibody against the CD36-binding site of TSP-1. The cell aggregation of U937 cells was completely blocked, even in the presence of the 70-kDa fragment, by mAb against leukocyte function associated antigen-1 (LFA-1) or intercellular adhesion molecule-1 (ICAM-1). We therefore propose that TSP-1 may regulate LFA-1/ICAM-1-mediated cell adhesion of monocytes/macrophages by either the inhibitory effect through CD47 or the promoting effect through CD36 depending on which domain/fragment is functional in a given biological setting.

Trimeric assembly of the C-terminal region of Thrombospondin-1 or Thrombospondin-2 is necessary for cell spreading and fascin spike organisation

Thrombospondin-1 (TSP-1) and the highly related protein thrombospondin-2(TSP-2) are trimeric extracellular molecules that have complex roles in wound healing, angiogenesis and matrix organisation. At the cellular level, TSP-1 supports cell adhesion and migration by the organisation of fascin spike cytoskeletal structures. To define the molecular requirements for assembly of fascin spikes by thrombospondins, we developed a panel of recombinant protein units of TSP-1 and TSP-2; these were designed according to the domain boundaries and included matched monomeric and trimeric units. These proteins were tested for their effects on cell attachment and fascin spike organisation using C2C12 skeletal myoblasts and vascular smooth muscle cells. In monomeric units, cell attachment activity was localised to the type 1 repeats or type 3 repeats/C-terminal globule, and both regions need to be present in the same molecule for maximal activity. On a molar basis, cell-attachment activities with monomeric units were low compared with intact TSP-1, and no monomeric unit induced cell spreading. Trimeric versions of the type 1 repeats were more adhesive but did not induce cell spreading. Strikingly, trimers that contained the type 3 repeats/C-terminal globule of either TSP-1 or TSP-2 supported cell spreading and fascin spike organisation, producing a similar activity to intact TSP-1. We conclude that trimeric assembly of the highly conserved TSP C-terminal region is necessary for organisation of the fascin-based cytoskeletal structures that are needed for thrombospondin-induced cell motility.

Characterization of polymorphic structure of cathepsin G gene: role in cardiovascular and cerebrovascular diseases

Cathepsin G (CTSG), a serine protease released from activated neutrophils, may cause platelet activation, leading to intravascular thrombosis, thus contributing to cardiovascular and cerebrovascular disease. Applying the candidate gene approach, we screened the 5'-flanking region and the entire coding region of the CTSG gene for genetic variation by using polymerase chain reaction/single-strand conformation polymorphism analysis from 96 patients at high risk for myocardial infarction (MI). We identified 4 polymorphisms in the 5'-flanking region (G-618C, G-315A, C-179T, and C-160T) and 1 polymorphism in the coding region (Asn125Ser) of the gene and genotyped the participants in the Etude Cas-Temoins sur l'Infarctus du Myocarde (ECTIM Study), a case-control study for MI, and in the Etude du Profil Génétique de l'Infarctus Cérébral (GENIC Study), a case-control study for brain infarction (BI), for all identified genetic variants. The potential in vitro functionality of the 4 variants in the 5'-flanking region was investigated with transient transfection analyses in U937 cells with different allelic promoter constructs by using a luciferase assay. Our in vitro analyses did not reveal any differences for the investigated allelic constructs with respect to promoter activity, and none of the polymorphisms in the 5'-flanking region was associated with the available phenotypes in either study. Allele and genotype distributions of all identified polymorphisms did not globally differ between cases and controls in the ECTIM Study. However, in patients from the ECTIM Study, the Ser125 allele was significantly associated with elevated plasma fibrinogen levels (P=0.006), but this effect was not seen in controls (case-control heterogeneity, P=0.04). There was a significant interaction between CTSG Asn125Ser and the beta-fibrinogen gene polymorphism G-455A on plasma fibrinogen levels (P=0.04). In the GENIC Study, the odds ratio for BI associated with CTSG Ser125 carrying was 1.82 (95% CI 1.16 to 2.84, P=0.008) in patients without a history of cardiovascular or cerebrovascular diseases. Our results indicate that the CTSG Ser125 allele is associated with plasma fibrinogen levels in MI patients from the ECTIM Study and with BI in the GENIC Study. Further studies should be carried out to define the underlying mechanisms.

A model of platelet aggregation involving multiple interactions of thrombospondin-1, fibrinogen, and GPIIbIIIa receptor

Thrombospondin-1 (TSP) may, after secretion from platelet alpha granules, participate in platelet aggregation, but its mode of action is poorly understood. We evaluated the capacity of TSP to form inter-platelet cross-bridges through its interaction with fibrinogen (Fg), using either Fg-coated beads or Fg bound to the activated GPIIbIIIa integrin (GPIIbIIIa*) immobilized on beads or on activated fixed platelets (AFP), i.e. in a system free of platelet signaling and secretion mechanisms. Aggregation at physiological shear rates (100-2000 s(-1)) was studied in a microcouette device and monitored by flow cytometry. Soluble TSP bound to and induced aggregation of Fg-coated beads dose-dependently, which could be blocked by the amino-terminal heparin-binding domain of TSP, TSP18. Soluble TSP did not bind to GPIIbIIIa*-coated beads or AFP, unless they were preincubated with Fg. The interaction of soluble TSP with Fg-GPIIbIIIa*-coated beads or Fg-AFP resulted in the formation of aggregates via Fg-TSP-Fg cross-bridges, as demonstrated in a system where direct cross-bridges mediated by GPIIbIIIa*-Fg on one particle and free GPIIbIIIa* on a second particle were blocked by the RGD mimetic Ro 44-9883. Soluble TSP increased the efficiency of Fg-mediated aggregation of AFP by 30-110% over all shear rates and GPIIbIIIa* occupancies evaluated. Surprisingly, TSP binding to Fg already bound to its GPIIbIIIa* receptor appears to block the ability of this occupied Fg to recognize another GPIIbIIIa* receptor, but this TSP can indeed cross-bridge to another Fg molecule on a second platelet. Finally, TSP-coated beads could directly coaggregate at shear rates from 100 to 2000 s(-1). Our studies provide a model for the contribution of secreted TSP in reinforcing inter-platelet interactions in flowing blood, through direct Fg-TSP-Fg and TSP-TSP cross-bridges.

Proteolysis of subendothelial adhesive glycoproteins (fibronectin, thrombospondin, and von Willebrand factor) by plasmin, leukocyte cathepsin G, and elastase

The degradation of extracellular matrix (ECM) adhesive glycoproteins, fibronectin (FN), thrombospondin (TSP) and von Willebrand factor (vWF), by human leukocyte cathepsin G and elastase, and by plasmin or thrombin, was analysed by immunoblotting after incubation of physiologic doses of the proteases with confluent human umbilical vein endothelial cells. Elastase induced an almost complete disappearance of intact FN, TSP, and vWF from the ECM at 0.02 units/ml within 5 minutes of incubation at 37 degrees C. Plasmin (0.2 units/ml) was also active on all three substrates, whereas cathepsin G (0.2 units/ml) had a preferential effect on TSP. Most remarkably, these degradations occurred with no apparent change in endothelial cell morphology, as shown by phase-contrast microscopy. In contrast, thrombin (0.2 units/ml) had no apparent proteolytic action on ECM glycoproteins, where it induced cell retraction and rounding. The release of adhesive glycoproteins from the ECM was accompanied by the detection of proteolytic fragments in the conditioned medium. Kinetic studies indicated that proteolysis started within minutes and proceeded for at least 1 hour. TSP was extremely sensitive to degradation by all enzymes except thrombin, whereas vWF released from the ECM was more resistant to proteolysis than constitutively secreted vWF, and FN was poorly degraded by plasmin. Our results indicate that serine proteinases, locally produced during inflammation and/or thrombolysis, can release extracellular matrix components and generate proteolytic fragments with potential biological activities.

Activation of rat alveolar macrophage-derived latent transforming growth factor beta-1 by plasmin requires interaction with thrombospondin-1 and its cell surface receptor, CD36

Transforming growth factor-beta-1 (TGF-beta1) is secreted by cells in a latent form (L-TGF-beta1) noncovalently bound to a latency-associated peptide. Activated alveolar macrophages obtained from rat lungs after bleomycin-induced pulmonary injury released increased amounts of active TGF-beta1 as well as plasmin, a protease, and thrombospondin-1 (TSP-1), a trimeric glycoprotein. Previously we had demonstrated that plasmin was critical to the activation of L-TGF- beta1. In the present study we demonstrated that TSP-1 is also important for the activation of L-TGF- beta1 because the activation can be inhibited by anti-TSP-1 monoclonal antibody. Proteins obtained from alveolar macrophage cell lysates immunoprecipitated with antibodies specific for TSP-1 were identified on immunoblots as LAP and TGF-beta1, indicating that TSP-1/L-TGF-beta1 complexes are present on alveolar macrophages. However, in the presence of plasmin both latency-associated peptide and TGF-beta1 were decreased in the same cell lysates, indicating that L-TGF-beta1 associated with TSP-1 is released by plasmin. Using immunofluorescence and antibodies to TGF-beta1 and CD36, a receptor for TSP-1, there was colocalization of TGF-beta1 with CD36. Because TSP-1 but not TGF-beta1 is a natural ligand for CD36, these findings suggest that the L-TGF-beta1 in a complex with TSP-1 localizes to the macrophage cell surface when TSP-1 interacts with its receptor, CD36. Furthermore, the association of TSP-1/L-TGF-beta1 complex with CD36 is necessary to the activation of L-TGF-beta1 because antibodies to CD36 prevent the colocalization of TGF-beta1 with CD36 as observed by immunofluorescence and inhibit activation of the L-TGF-beta1 by explanted alveolar macrophages. These findings suggest that activation of L-TGF-beta1 by plasmin occurs at the cell surface of activated alveolar macrophages and requires a TSP-1/CD36 interaction.