Modified Crossed Immunoelectrophoresis to Study with Whole Plasma the Reversible Complex Formation of Histidine-Rich Glycoprotein with Plasminogen

Histidine-rich Glycoprotein Specifically Binds to Necrotic Cells via Its Amino-terminal Domain and Facilitates Necrotic Cell Phagocytosis

Cells that become necrotic or apoptotic through tissue damage or during normal cellular turnover are usually rapidly cleared from the circulation and tissues by phagocytic cells. A number of soluble proteins have been identified that facilitate the phagocytosis of apoptotic cells, but few proteins have been defined that selectively opsonize necrotic cells. Previous studies have shown that histidine-rich glycoprotein (HRG), an abundant (approximately 100 microg/ml) 75-kDa plasma glycoprotein, binds to cell surface heparan sulfate on viable cells and cross-links other ligands, such as plasminogen, to the cell surface. In this study we have demonstrated that HRG also binds very strongly, in a heparan sulfate-independent manner, to cytoplasmic ligand(s) exposed in necrotic cells. This interaction is mediated by the amino-terminal domain of HRG and results in enhanced phagocytosis of the necrotic cells by a monocytic cell line. In contrast, it was found that HRG binds poorly to and does not opsonize early stage apoptotic cells. Thus, HRG has the unique property of selectively recognizing necrotic cells and may play an important physiological role in vivo by facilitating the uptake and clearance of necrotic, but not apoptotic, cells by phagocytes.

Histidine‐rich glycoprotein: A novel adaptor protein in plasma that modulates the immune, vascular and coagulation systems

Histidine-rich glycoprotein (HRG) is an abundant plasma glycoprotein that has a multidomain structure, interacts with many ligands, and has been shown to regulate a number of important biological processes. HRG ligands include Zn(2+) and haem, tropomyosin, heparin and heparan sulphate, plasminogen, plasmin, fibrinogen, thrombospondin, IgG, FcgammaR and complement. In many cases, the histidine-rich region of the molecule enhances ligand binding following interaction with Zn(2+) or exposure to low pH, conditions associated with sites of tissue injury or tumour growth. The multidomain nature of HRG indicates that it can act as an extracellular adaptor protein, bringing together disparate ligands, particularly on cell surfaces. HRG binds to most cells primarily via heparan sulphate proteoglycans, binding which is also potentiated by elevated free Zn(2+) levels and low pH. Recent reports have shown that HRG can modulate angiogenesis and additional studies have shown that it may regulate other physiological processes such as cell adhesion and migration, fibrinolysis and coagulation, complement activation, immune complex clearance and phagocytosis of apoptotic cells. This review outlines the molecular, structural, biological and clinical properties of HRG as well as describing the role of HRG in various physiological processes.

Effects of histidine-proline-rich glycoprotein on plasminogen activation in solution and on surfaces

Histidine-proline-rich glycoprotein (HPRG) has long been known to associate with plasminogen (Plg) in solution, but the consequences of this interaction have not been defined. Here we show that HPRG adsorbed to a glycosaminoglycan (GAG) surface also binds Plg with a Kd value of 0.7 micromol/l. Moreover, we present evidence that HPRG acts as a modulator of the activation of Plg by tissue-type Plg activator. Specifically, Plg complexed with HPRG on a GAG surface is more readily activated by tissue-type Plg activator than free Plg, with a 10-fold difference in apparent catalytic efficiency (kcat/Km). HPRG also augments the increase in Plg activation caused by fibrinogen fragments either in solution or on GAG surfaces. In contrast, HPRG abrogates the stimulatory effects of fibrinogen on Plg activation in solution. These observations demonstrate that HPRG can act as either a positive or negative effector of Plg activation in vitro and may serve as a modulator of fibrinolysis in vivo.

Plasminogen and tissue plasminogen activator interact with antithrombin III

Human antithrombin III was demonstrated to bind plasminogen specifically in a time and concentration-dependent manner. The above binding was also confirmed using ligand western blot assays. The interaction of plasminogen was significantly (>90%) inhibited by lysine, indicating the involvement of kringles in binding antithrombin III. Plasminogen also bound to heparin-antithrombin III complex. In converse experiments, antithrombin III also interacted with immobilized plasminogen. Using carboxypeptidase B digestion, the plasminogen-binding site of antithrombin III was localized to the carboxy-terminus lysine of the anticoagulant protein. Tissue plasminogen activator also interacted with antithrombin III in a time- and concentration-dependent manner and its binding was also significantly (>90%) inhibited by lysine. Moreover, the interaction of plasminogen and tissue plasminogen activator with antithrombin III was competitive. These results provide the first evidence for the interaction of antithrombin III with fibrinolytic factors and suggest that antithrombin III may serve to localize these factors at the site of clot formation.

Comparison of the effect of histidine-rich glycoprotein and 6-aminohexanoic acid on plasmin production and fibrinolysis in vitro

Because histidine-rich glycoprotein binds to the kringle 1-3 domain of plasminogen, it may affect fibrinolysis by reducing fibrin-dependent plasmin production, and in this way it could be mechanistically analogous to 6-aminohexanoic acid. We tested this hypothesis by comparing the effects of histidine-rich glycoprotein and 6-aminohexanoic acid in an in vitro assay of fibrin-dependent plasmin production mediated by tissue plasminogen activator. Whereas 1 mM of 6-aminohexanoic acid increased the K(m) of the reaction from approximately 0.22 microM to approximately 1.7 microM, 2 microM of histidine-rich glycoprotein had no discernible effect. Similar results were obtained in an assay based upon fibrin clot lysis. Therefore, we could not document an effect of histidine-rich glycoprotein on the rate of fibrin-dependent plasmin production.

A specific allele of the histidine-rich glycoprotein (HRG) locus is linked with elevated plasma levels of HRG in a Dutch family with thrombosis

Recent studies describe families with both elevated plasma HRG levels and thrombosis. In order to study the possibility that allelic variants of the HRG locus are associated with differences in HRG level, we studied linkage between HRG levels and a dinucleotide repeat polymorphism in a Dutch family which was selected on the presence of both thrombosis and elevated plasma HRG levels. No other known risk factors from thrombosis were found in this family. Linkage was calculated between the dinucleotide repeat and the HRG level considering the HRG level as a quantitative phenotype assuming a population prevalence of elevated HRG of 5%. Two classes of HRG levels were defined by a mean and a variance: one class with normal HRG levels and a second class with high HRG levels. Using a mean HRG level of 99% for individuals with a normal HRG level and 145% for individuals with high HRG, a maximum lod score of 4.17 (odds in favour of linkage of 22,000:1) was found at a recombination fraction of 0, indicating linkage. Considering the pedigree, an association was found between the presence of a specific allele (no. 6) of the dinucleotide repeat polymorphism and plasma HRG levels. Family members carrying allele 6 were found to have higher HRG plasma levels compared with family members lacking allele 6 (149% v 109% respectively). We conclude that in this family, linkage is found between the HRG locus and the HRG level, and that a HRG gene coupled to allele 6 of the dinucleotide polymorphism is associated with elevated plasma HRG levels. No evidence was found for a causal relationship between elevated plasma HRG levels and thrombosis in this family.

Functional analogy between lipoprotein(a) and plasminogen in the binding to the kringle 4 binding protein, tetranectin

Apolipoprotein(a), apo(a), contains 37 repeats structurally homologous to kringle 4 structures of the fibrinolysis zymogen plasminogen. The aim of the study was to explore the functional analogy between apolipoprotein(a) and plasminogen in the binding to the kringle-4-binding plasma protein, tetranectin. With a modified crossed immunoelectrophoresis technique, reversible binding between lipoprotein(a) and tetranectin could be demonstrated with an apparent Kd of 0.013 muMol/l. Lys- and Glu-plasminogen showed an apparent Kd of 0.5 muMol/l. Binding of lipoprotein(a) to fibrin and to fibrin-bound tetranectin was found to be negligible. The absence of fibrin binding of lipoprotein(a) excludes a potential mechanism of coexistence of fibrin and lipid deposits in arterial diseases and does not provide for a link between lipoprotein and the clotting system. Plasminogen and lipoprotein(a) show functional analogy in their binding to tetranectin, but tetranectin primarily targets at lipoprotein(a).