Neutralization of Heparin-Like Saccharides by Complement S Protein/Vitronectin

The ligand binding functions of vitronectin (Vn) are regulated by its conformational state/degree of multimerization. In the native plasma form of Vn, the C-terminal glycosaminoglycan (GAG) binding domain is believed to be cryptic. Here, evidence is provided that the addition of fucoidan or dextran sulfate to unfractionated plasma results in the formation of covalently and non-covalently stabilized Vn multimers. These multimers express conformationally sensitive antibody epitopes and ligand binding sites located in the N terminus of the Vn molecule. While heparin forms complexes with monomeric plasma Vn and induces conformational changes, a reduction in ionic strength is required for induction of multimerization. In addition, heparin serves as a template for the assembly of type 1 plasminogen activator inhibitor-induced disulfide--linked Vn multimers. These results support a new model for the structure of native Vn. The C-terminal GAG binding domain is predicted to be exposed in the native conformation, whereas the N terminus is cryptic. Ligand binding to the GAG binding site unfolds the N terminus, thereby exposing cryptic ligand binding sites.

Section: Discussionmentioning

confidence: 85%

The Glycosaminoglycan Binding Site Governs Ligand Binding to the Somatomedin B Domain of Vitronectin

Seiffert

1997

“…Interestingly, no binding of the thrombin-antithrombin complex to gC1qR was observed, whereas thrombin as well as prothrombin presented appreciable binding. 3 Heparan sulfate proteoglycans of the vessel wall can interact with multimeric forms of vitronectin (19), and luminal heparinoids of endothelial cells have been proposed to mediate binding and uptake of the ternary complex (11). In light of the present findings, gC1qR expressed on vascular cells seems to share certain properties with these proteoglycans and may very well be another candidate for specific binding and clearance of heparin binding forms of vitronectin.…”

Section: Figmentioning

confidence: 81%

The Binding Protein for Globular Heads of Complement C1q, gC1qR

Lim

Reid

Ghebrehiwet

et al. 1996

126

A binding protein for the globular head domains of complement component C1q, designated gC1qR, recently described to be present on vascular and blood cells (Ghebrehiwet, B., Lim, B.-L., Peerschke, E. I. B., Willis, A. C., and Reid, K. B. M. (1994) J. Exp. Med. 179, 1809 -1821 was expressed in recombinant form in bacteria to investigate its functional and structural properties. The recombinant gC1qR was found to be functional because tetramerization of the 24.3-kDa polypeptide occurred as described for the native protein, and the binding of the ligand C1q by recombinant gC1qR was indistinguishable from binding shown by gC1qR isolated from Raji cells. Recombinant gC1qR immobilized to microspheres was used to search for additional binding proteins unrelated to C1q. Surprisingly, it was found that vitronectin or complexes containing vitronectin were retained from plasma or serum, and subsequent analysis revealed the specific binding of the ternary vitronectin-thrombin-antithrombin complex to gC1qR. Because the thrombin-antithrombin complex was unable to interact with gC1qR, direct binding with vitronectin was investigated in a purified system. The heparin binding multimeric form of vitronectin but not the plasma form of vitronectin was found to bind specifically to gC1qR isolated from Raji cell membrane as well as to recombinant gC1qR. This interaction was saturable (K D ϳ20 nM) and inhibitable by glycosaminoglycans such as heparin but not by chondroitin sulfate. C1q and vitronectin did not compete with each other for binding to gC1qR, and both ligands seem to interact with different parts of the gC1qR because a truncated version of recombinant gC1qR lacking the N-terminal 22-amino acid portion hardly interacted with vitronectin but bound C1q as well as the intact gC1qR. These findings establish gC1qR as a novel vitronectin-binding protein that may participate in the clearance of vitronectincontaining complexes or opsonized particles or cooperate with vitronectin in the inhibition of complementmediated cytolysis.

“…HRGP has been shown to interact with heparin and HS (35,36,39,40), a feature shared by many pro-and anti-angiogenic factors (for a review, see Ref. 9).…”

Section: Discussionmentioning

confidence: 99%

The Anti-angiogenic His/Pro-rich Fragment of Histidine-rich Glycoprotein Binds to Endothelial Cell Heparan Sulfate in a Zn2+-dependent Manner

Vanwildemeersch

Olsson

Gottfridsson

et al. 2006

The plasma protein histidine-rich glycoprotein (HRGP), which has been identified as an angiogenesis inhibitor, binds to heparan sulfate (HS) in a Zn 2؉ -dependent manner. We wished to test whether this interaction is mechanistically important in mediation of the anti-angiogenic effect of HRGP. Inhibition of angiogenesis by HRGP is exerted through its central His/Pro-rich domain, which is proteolytically released. A 35-amino-acid residue synthetic peptide, HRGP330, derived from the His/Pro-rich domain retains the inhibitory effect on blood vessel formation in vitro and in vivo, an effect dependent on the presence of Zn 2؉ . We now show that HRGP330 binds heparin/HS with the same capacity as full-length HRGP, and the binding is Angiogenesis, the formation of new capillary blood vessels, is essential during development and physiological conditions, such as wound healing and the menstrual cycle (1). However, prolonged and excessive angiogenesis has been implicated in a number of pathological processes, for instance rheumatoid arthritis, diabetic retinopathy, and tumor growth (2, 3). The adult vasculature is tightly regulated by naturally occurring pro-and anti-angiogenic factors. Fibroblast growth factor (FGF) 3 and vascular endothelial growth factor (VEGF) are well known stimulators of endothelial cells in vitro and in vivo. To date, a number of endogenous factors negatively regulating angiogenesis have also been identified (4). The inhibitors described so far mainly fall into three groups: plasma proteins, basement membrane proteins, and serine protease inhibitors (serpins). A common feature of many of these antiangiogenic molecules is inhibition of endothelial cell chemotaxis in vitro.We have identified histidine-rich glycoprotein (HRGP) as a potent inhibitor of angiogenesis in vivo (5). HRGP is a heparin-binding plasma protein highly conserved through vertebrate species (for review, see Ref. 6). The N-terminal part of the protein contains two cysteine protease inhibitor (cystatin)-like stretches, hence the classification of HRGP as a member of the cystatin superfamily (Fig. 1A). The central domain is rich in histidine and proline residues, and the human form contains 12 more or less conserved tandem repeats of the pentapeptide HHPHG. Multiple binding partners for HRGP have been reported, such as heparin/heparan sulfate (HS), plasminogen, fibrinogen, tropomyosin, and divalent cations (6). The heparin-binding affinity of HRGP can be modulated and is increased in the presence of Zn 2ϩ and at low pH (7), a common environment in hypoxic tumors. The anti-angiogenic effect of HRGP is mediated via its His/Pro-rich domain, which needs to be released from the full-length protein to exert its effects (5). HRGP attenuates endothelial cell migration and adhesion to vitronectin in vitro and tumor vascularization and growth in vivo, by interfering with endothelial cell focal adhesion function. We have narrowed down the minimal active domain of HRGP to a 35-amino-acid (aa) peptide, HRGP330, corresponding to a sequence in the H...