Porcine vitronectin, the most compact form of single-chain vitronectin: the smallest molecular mass among vitronectins was ascribed to deletion and substitution of base pairs, and proteolytic trimming of the peptide

Regulatory properties of vitronectin and its glycosylation in collagen fibril formation and collagen-degrading enzyme cathepsin K activity

Vitronectin (VN) is a glycoprotein found in extracellular matrix and blood. Collagen, a major extracellular matrix component in mammals, is degraded by cathepsin K (CatK), which is essential for bone resorption under acidic conditions. The relationship between VN and cathepsins has been unclear. We discovered that VN promoted collagen fibril formation and inhibited CatK activity, and observed its activation in vitro. VN accelerated collagen fibril formation at neutral pH. Collagen fibers formed with VN were in close contact with each other and appeared as scattered flat masses in scanning electron microscopy images. VN formed collagen fibers with high acid solubility and significantly inhibited CatK; the IC₅₀ was 8.1–16.6 nM and competitive, almost the same as those of human and porcine VNs. VN inhibited the autoprocessing of inactive pro-CatK from active CatK. DeN-glycosylation of VN attenuated the inhibitory effects of CatK and its autoprocessing by VN, but had little effect on acid solubilization of collagen and VN degradation via CatK. CatK inhibition is an attractive treatment approach for osteoporosis and osteoarthritis. These findings suggest that glycosylated VN is a potential biological candidate for CatK inhibition and may help to understand the molecular mechanisms of tissue re-modeling.

Sialylation of vitronectin regulates stress fiber formation and cell spreading of dermal fibroblasts via a heparin-binding site

Vitronectin (VN) plays an important role in tissue regeneration. We previously reported that VN from partial hepatectomized (PH) rats results in a decrease of sialylation of VN and de-sialylation of VN decreases the cell spreading of hepatic stellate cells. In this study, we analyzed the mechanism how sialylation of VN regulates the properties of mouse primary cultured dermal fibroblasts (MDF) and a dermal fibroblast cell line, Swiss 3T3 cells. At first, we confirmed that VN from PH rats or de-sialylated VN also decreased cell spreading in MDF and Swiss 3T3 cells. The de-sialylation suppressed stress fiber formation in Swiss 3T3 cells. Next, we analyzed the effect of the de-sialylation of VN on stress fiber formation in Swiss 3T3 cells. RGD peptide, an inhibitor for a cell binding site of VN, did not affect the cell attachment of Swiss 3T3 cells on untreated VN but significantly decreased it on de-sialylated VN, suggesting that the de-sialylation attenuates the binding activity of an RGD-independent binding site in VN. To analyze a candidate RGD-independent binding site, an inhibition experiment of stress fiber formation for a heparin binding site was performed. The addition of heparin and treatment of cells with heparinase decreased stress fiber formation in Swiss 3T3 cells. Furthermore, de-sialylation increased the binding activity of VN to heparin, as detected by surface plasmon resonance (SPR). These results demonstrate that sialylation of VN glycans regulates stress fiber formation and cell spreading of dermal fibroblast cells via a heparin binding site.

Survival signals of hepatic stellate cells in liver regeneration are regulated by glycosylation changes in rat vitronectin, especially decreased sialylation

The extracellular matrix (ECM) molecules play important roles in many biological and pathological processes. During tissue remodeling, the ECM molecules that are glycosylated are different from those of normal tissue owing to changes in the expression of many proteins that are responsible for glycan synthesis. Vitronectin (VN) is a major ECM molecule that recognizes integrin on hepatic stellate cells (HSCs). The present study attempted to elucidate how changes in VN glycans modulate the survival of HSCs, which play a critical role in liver regeneration. Plasma VN was purified from partially hepatectomized (PH) and sham-operated (SH) rats at 24 h after operation and non-operated (NO) rats. Adhesion of rat HSCs (rHSCs), together with phosphorylation of focal adhesion kinase, in PH-VN was decreased to one-half of that in NO- or SH-VN. Spreading of rHSCs on desialylated NO-VN was decreased to one-half of that of control VN, indicating the importance of sialylation of VN for activation of HSCs. Liquid chromatography/multiple-stage mass spectrometry analysis of Glu-C glycopeptides of each VN determined the site-specific glycosylation. In addition to the major biantennary complex-type N-glycans, hybrid-type N-glycans were site-specifically present at Asn(167). Highly sialylated O-glycans were found to be present in the Thr(110)-Thr(124) region. In PH-VN, the disialyl O-glycans and complex-type N-glycans were decreased while core-fucosylated N-glycans were increased. In addition, immunodetection after two-dimensional PAGE indicated the presence of hyper- and hyposialylated molecules in each VN and showed that hypersialylation was markedly attenuated in PH-VN. This study proposes that the alteration of VN glycosylation modulates the substrate adhesion to rat HSCs, which is responsible for matrix restructuring.

Modulation of integrin antagonist signaling by ligand binding of the heparin-binding domain of vitronectin to the alphaVbeta3 integrin

The interaction between the arginine glycine and aspartic acid motif (RGD) of integrin ligands such as vitronectin and the integrin receptor alphaVbeta3 in mediating cell attachment has been well described. Similarly, the ability of disintegrins, small RGD containing peptides, to inhibit cell attachment and other cellular processes has also been studied extensively. Recently, we characterized a second site of interaction between vitronectin and its integrin partner. We determined that amino acids within the heparin-binding domain of vitronectin bind to a cysteine loop (C-loop) region of beta3 and that this interaction is required for the positive effects of alphaVbeta3 ligand occupancy on IGF-I signaling in smooth muscle cells. In this study we examine the signaling events activated following ligand binding of disintegrins to the alphaVbeta3 and the ability of these signals to be regulated by binding of the heparin-binding domain of vitronectin. We demonstrate that disintegrin ligand binding activates a series of events including the sequential activation of the tyrosine kinases c-Src and Syk. This leads to the activation of calpain and the cleavage of the beta3 cytoplasmic tail. Addition of vitronectin or a peptide homologous to the heparin-binding domain inhibited activation of this pathway. Our results suggest that the signaling events that occur following ligand binding to the alphaVbeta3 integrin reflects a balance between the effects mediated through the RGD binding site interaction and the effects mediated by the heparin binding site interaction and that for intact vitronectin the effect of the heparin-binding domain predominates.

The vitronectin gene in rainbow trout: cloning, expression and phylogenetic analysis

Vitronectin is a major cell adhesion glycoprotein that is found in plasma and the extracellular matrix. Vitronectin consists of an N-terminal somatomedin B domain and two hemopexin-like domains and controls functions including cell adhesion, migration, haemostasis and immune defence. In order to study the molecular evolution of the complement lytic pathway regulation, we have cloned and characterized the vitronectin gene from rainbow trout (Oncorhynchus mykiss). The deduced amino acid sequence of trout vitronectin exhibits 45%, 46%, 47% and 63% identity with human, chicken, Xenopus and zebrafish orthologs, respectively. The domain architecture of the trout vitronectin, consisting of a somatomedin B domain and two hemopexin-like domains, resembles that of mammalian vitronectins. Analysis of partial genomic clones shows that trout vitronectin gene exhibits the same exon-intron organization profile as the human ortholog gene. The trout vitronectin gene is probably present as a single copy in the trout genome, showing a differential expression pattern among tissues investigated.

Vitronectin in atherosclerotic disease

Atherosclerosis is characterized by the development of an intimal thickening that contains monocytes, T lymphocytes, and smooth muscle cells within an accumulation of lipid and extracellular matrix proteins. Vitronectin is a plasma glycoprotein implicated as a regulator of diverse physiological process, including blood coagulation, fibrinolysis, pericellular proteolysis, complement dependent immune responses, and cell attachment and spreading. Because of its ability to bind platelet glycoproteins and mediate platelet adhesion and aggregation at sites of vascular injury, vitronectin has become an important mediator in the pathogenesis of coronary atherosclerosis.

New insights into heparin binding to vitronectin: studies with monoclonal antibodies

Vitronectin is a plasma glycoprotein that binds to a variety of ligands. There is considerable debate regarding the dependency of these binding interactions upon the conformational status of vitronectin, the role of multimerization and how the binding of different ligands can change vitronectin's conformational state. We have developed a method of capturing vitronectin directly from fresh plasma using solid-phase monoclonal antibodies. Various biotin-labelled secondary monoclonal antibodies were used to quantify the bound vitronectin and to measure its degree of denaturation. Using these tools we demonstrated that one monoclonal antibody partially denatured vitronectin without direct multimerization. Treatment of vitronectin in plasma with soluble heparin produced a similar degree of denaturation. These results led to a proposed adaptation of the unfolding/refolding pathways for chemically denatured vitronectin originally presented by Zhuang and co-workers in 1996 [Zhuang, Blackburn and Peterson (1996) J. Biol. Chem. 271, 14323-14332 and Zhuang, Li, Williams, Wagner, Seiffert and Peterson (1996) J. Biol. Chem. 271, 14333-14343]. The adapted version allows for the production of a more stable partially unfolded intermediate, resulting from the binding of particular ligands. We also demonstrated that the avidity of heparin binding to vitronectin is governed by both the conformational state of the monomer and multimerization of the molecule.

The changes in glycosylation after partial hepatectomy enhance collagen binding of vitronectin in plasma

Vitronectin is a multifunctional glycoprotein present in the extracellular matrix and plasma. Changes in rat vitronectin were studied during liver regeneration after partial hepatectomy. Carbohydrate concentrations of vitronectin decreased to 2/3 of sham-operated rats at 24 h after partial hepatectomy. Carbohydrate composition and lectin reactivity indicated that N-glycosylation and sialylation of vitronectin changed markedly after partial hepatectomy, while amino acid composition did not change significantly. We previously showed that deN-glycosylation of vitronectin in vitro affects collagen binding among various ligands (Yoneda et al., Biochemistry (1998) 37, 6351-6360). Vitronectins from partially hepatectomized rats at 24 h were found to exhibit markedly enhanced binding to type I collagen. The effect of sialylation on collagen binding was further examined using enzymatically deglycosylated vitronectin of nonoperated rats. Collagen binding increased by 1.2 times after deN-glycosylation of vitronectin, while it increased more than 2.9 times after desialylation. Various glycosyltransferases in liver are known to change after partial hepatectomy, including the attenuation of N-oligosaccharide transferase. The findings therefore suggest that the collagen binding of vitronectin is modulated by the alteration of peptide glycosylation caused by postoperative physiological changes of glycosyltransferases and that the change may contribute to tissue remodeling processes.

Vitronectin deficiency is associated with increased wound fibrinolysis and decreased microvascular angiogenesis in mice

BACKGROUND

Vitronectin has several putative functions including regulating hemostasis, cell adhesion, and cell migration. However, the targeted deletion of vitronectin in mice results in normal development and normal coagulation parameters. To determine whether vitronectin may be necessary for nondevelopmental processes, we examined the response to tissue injury in vitronectin-null mice.

METHODS

We examined wound healing in control and vitronectin-null mice by healing rate, zymography, reverse zymography, and Western blots.

RESULTS

We found that dermal wound healing was slightly delayed in mice lacking vitronectin. More importantly, we found extensive areas of delayed hemorrhage near the sprouting tips of microvessels between days 7 and 14, which temporally coincided with increased urokinase-type plasminogen activator and tissue-type plasminogen activator activity by zymography. Though Western blots confirmed the presence of plasminogen activator inhibitor-1 protein throughout wound repair and reverse zymograms showed decreased plasminogen activator inhibitor-1 activity between days 7 and 14.

CONCLUSIONS

Loss of vitronectin in mice was associated with changes in the fibrinolytic balance, and this may have led to focal sites of delayed hemorrhage. The mechanism that resulted in decreased angiogenesis and the formation of larger blood vessels in response to tissue injury remains unknown. This study suggests that vitronectin may have several distinct functions that are not required for normal development but are manifested in response to tissue injury.

Interaction of a lectin from Psathyrella velutina mushroom with N-acetylneuraminic acid

A lectin from the fruiting body of Psathyrella velutina has been used as a specific probe for non-reducing terminal N-acetylglucosamine residues. We reveal in this report that P. velutina lectin recognizes a non-reducing terminal N-acetylneuraminic acid residue in glycoproteins and oligosaccharides. Binding of biotinyl P. velutina lectin to N-acetylneuraminic acid residues was prevented by desialylation of glycoconjugates and was distinguished from the binding to N-acetylglucosamine. Sialooligosaccharides were retarded or bound and eluted with N-acetylglucosamine on a P. velutina lectin column, being differentiated from each other and also from the oligosaccharides with non-reducing terminal N-acetylglucosamine which bound more strongly to the column.

Characterization of the ligand binding activities of vitronectin: interaction of vitronectin with lipids and identification of the binding domains for various ligands using recombinant domains

Vitronectin is a multifunctional plasma glycoprotein which may regulate the systems related to protease cascades such as the coagulation, fibrinolysis, and complement systems as well as cell adhesion. Solid-phase assays and affinity chromatography on immobilized glycolipids indicated that vitronectin purified under denaturing conditions bound to sulfatide (Gal(3-SO4)beta1-1ceramide), cholesterol 3-sulfate, and various phospholipids, but not gangliosides. Only the unfolded or multimeric form of vitronectin bound to sulfatide, suggesting a conformational dependency of the binding activity, while vitronectin bound to cholesterol 3-sulfate regardless of its conformational state. The recombinant domains of human vitronectin and mutants with certain domains deleted were separately expressed in E. coli as fusion proteins. Using the recombinants, sulfatide-, phosphatidylserine-, cholesterol 3-sulfate-, Type I collagen-, heparin-, and beta-endorphin-binding activities were found to be attributable to hemopexin domain 2 and hemopexin domain 1. The possibility was suggested that the presence of a somatomedin domain and/or connecting region flanking hemopexin domain 1 inactivated its heparin binding. De-N-glycosylation of plasma vitronectin significantly affected the cholesterol sulfate- and collagen-binding activities, although its effects were opposite. These findings suggest that diverse ligand-binding activities could be attributed to pexin family motifs but that the interdomain interactions and glycosylations modulate the ligand binding activities of vitronectin.