Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

Vitronectin

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101907

Synonyms

Historical Background

VN was first described as a “serum spreading factor” to promote the attachment and spreading of cells to various surfaces, and it is this property of plasma/serum that enables mammalian cells to adhere to culture dishes and propagate (Leavesley et al. 2013). Unrelated investigations in cellular movement and epiboly as well as in innate immunity lead to the term “epibolin” or the discovery of VN as the complement inhibitor “S-protein,” respectively, whereby the identity of both factors was uncovered 30 years ago by the author.

VN Gene and Expression of the VN Protein

Gene Structure

The human VN gene (about 3.5 kb) is localized on chromosome 17 (centromeric region 17q11); the murine VN gene is found on chromosome 10. Both VN genes consist of eight exons and seven introns, resulting in 1.7 kb transcripts without indication for alternative splicing (Jenne and Stanley 1987). The first exon of VN encodes for the 19 residues of the leader peptide and the first 2 residues of the 459 amino acid mature protein. The second exon encodes residues 3–42 of the N-terminal somatomedin B domain (residues 1–44) and is flanked on each side by a phase I intron. The subsequent exons encode the hemopexin-type repeats in VN which may have been derived from a primordial hemopexin precursor.

Gene Expression and Regulation

The liver is the predominant source of VN, and respective cDNAs have been isolated from rodent and human liver cell libraries (Seiffert et al. 1991). Although VN mRNA is also detectable in other somatic tissues, the biosynthesis of VN in extrahepatic cell types is minimal as compared to the liver. The constitutive expression of VN in hepatocytes is increased by stimulation with endotoxin or cytokines, indicating that VN is an acute phase protein.

Protein Processing and Fate

As a major liver-derived protein, VN is present in normal plasma at 200–500 μg/ml (Preissner et al. 1985) and thus constitutes 0.2–0.5% of total plasma proteins. A second circulatory pool of VN in the blood that accounts for approximately 0.8% of total VN is contained within platelets: At a very early stage of their 10–12-day life span, platelets endocytose VN from plasma and incorporate the adhesive glycoprotein into their α-granules, to be rapidly released upon platelet activation and aggregation in situations of inflammation and tissue repair. While plasma VN mainly circulates in monomeric or dimeric form, a major portion of platelet VN is present in a high-molecular-weight, multimeric form that has undergone conformational alterations. VN has been found also in other tissue fluids including seminal plasma, urine, amniotic fluid, cerebrospinal fluid, and bronchoalveolar lavage fluid, here, particularly in patients with interstitial lung disease.

Specific mechanisms, which include receptor-mediated endo- and transcytosis, appear to mediate the uptake of VN into tissues. These processes are likely to regulate turnover of high-molecular-weight forms of VN that are part of the provisional wound healing matrix in areas of tissue injury and necrosis. Unlike fibronectin, VN is not a typical structural protein of the extracellular matrix, whereby VN deposits can be viewed as markers of tissue remodeling that define a role for VN as protective factor against tissue damage.

Structural Characteristics

Sequence and Size

The primary structures of human, mouse, rat, rabbit, and porcine VN share more than 80% sequence homology, whereby porcine VN has a truncated compact functional form that lacks the C-terminal 10–12 kDa fragment. All VN species contain N-linked and O-linked carbohydrates which differ in their composition among the various counterparts. A 54 kDa protein in chick egg yolk plasma, designated nectinepsin, exhibits high structural similarity to VN but is encoded by a different gene.

The glycosylated form of human VN (459 amino acid residues) has a molecular mass of 75–78 kDa and circulates as single-chain and/or disulfide-bridged two-chain (65 kDa and 10–12 kDa) forms, the latter being genetically determined by the amino acid residue at position 381 and derived from proteolytic cleavage in the carboxy-terminal region. A Met381Thr polymorphism increases the risk by 18-fold of hemangioblastoma in patients with von Hippel-Lindau gene defect, possibly related to enhanced tumor angiogenesis. Other posttranslational modifications include tyrosyl sulfation and phosphorylation, which can alter the binding function of VN for different ligands. Physicochemical parameters of VN are summarized in Table 1.
Vitronectin, Table 1

Physicochemical parameters of human vitronectin

459 amino acids encoded by a 1.7 kb transcript

  

Sedimentation coefficient, s 20,w

(Monomeric)

4.2–4.6 S

 

(Oligomeric)

6.5 S

Diffusion coefficient, D 20,w

 

5.7 × 10−7 cm2/s

Mr (from cDNA)

 

52,400

Mr (gel electrophoretic analysis, gel filtration)

 

75,000–78,000

Partial specific volume, v

 

0.711 cm3/g

Stokes radius, rH

(Monomeric)

3.7–3.9 nm

 

(Oligomeric)

5.6 nm

Frictional ratio, f/f0

 

1.37

Extinction coefficient, A (1%, 1 cm, 280 nm)

 

13.8

Isoelectric point (8 M urea), pI

 

4.75–5.25

Carbohydrate composition (mol/mol glycoprotein)

 

GlcN15, Man11, Gal9, NeuAc5

Posttranslational modifications

 

Phosphorylation (Ser378)

  

Sulfation (Tyr56, Tyr59)

Two to four free Cys in the native form

  

Endogenous proteolytic cleavage site

 

Arg379/Ala 380

Protein Conformation

VN is a conformationally labile molecule, and structural alterations are intimately linked to changes in ligand or cell-binding properties. In plasma, only 2% of VN is in the heparin-binding (multimeric) conformation, whereas in serum this portion of VN increases to 7%. Likewise, degranulated platelet VN predominantly consists of the multimeric heparin-binding form. In addition, complex formation of VN with other proteins such as plasminogen activator inhibitor-1 (PAI-1), thrombin-antithrombin complex, or complement C5b-C9 induces similar conformational changes to produce the multimeric conformer (Tomasini and Mosher 1991). Thus, circulating plasma VN is considered to be a conformationally “leached” proform of the reactive protein that requires induction through binding interactions (also with surfaces) to expose formerly cryptic epitopes. Such conformational changes in VN can be mimicked by denaturation with chaotropic agents or detergents, heat treatment, and acidification, resulting in the spontaneous formation of disulfide-linked VN multimers. Experimental evidence indicates that conformational changes expose the C-terminal heparin-binding domain and the N-terminal acidic portion. Within these regions, particular ligand-binding sites are cryptic in native plasma VN but become exposed upon denaturation/multimerization of the protein. Although both ionic and hydrophobic interactions may contribute to subunit interfaces, multimerization is likely to include the complementary charged C- and N-terminal regions of the molecule (Fig. 1).
Vitronectin, Fig. 1

Schematic representation of ligand-binding domains in vitronectin. The domain organization of human vitronectin within the protein indicates (from left to right) the somatomedin B domain (SMB) as major binding site for PAI-1; the major cell attachment site (RGD), recognized by integrins; the adjacent phosphorylation site (P) and acidic region (−−) as binding site for anti-adhesive high MW kininogen; the binding site for thrombin-serine protease inhibitor (SERPIN) complexes; the hemopexin-like repeats (green), entailing the collagen-binding region; the inhibitory binding region for terminal complements complexes (C5b-7); and the glycosaminoglycan-binding site (+++) with the sites for plasminogen and bacteria binding as well as an endogenous protease cleavage site (arrow), which lie adjacent to the protein kinase A–dependent phosphorylation site (P). The respective macromolecular ligands and receptors for vitronectin are indicated. Note that many binding domains are clustered around the acidic amino terminus as well as the basic carboxy-terminus, both of which become exposed upon “activation” of plasma vitronectin and concomitant conformational transition into multimeric complexes. HSPG heparan sulfate proteoglycan, MW molecular weight, PAI plasminogen activator inhibitor

Domain Structure

Different domains along the VN sequence are involved in various binding interactions or in self-association. The N-terminal somatomedin B domain contains 8 of the 14 cysteine residues which are predicted to form four intra-chain disulfide links. This domain is followed by a connecting region which contains the RGD sequence (residues 45–47), known as the versatile integrin recognition motif, and a highly acidic region (residues 53–64) containing two tyrosyl sulfation sites. In hemofiltrate of diabetic patients, functionally active RGD-containing somatomedin B peptides were identified, indicative for a protease-sensitive region in the N-terminus of VN. Such peptides were erroneously attributed to contain mitogenic activity.

This part of the molecule is followed C-terminally by seven hemopexin-type repeats, which were first described in hemopexin, the heme-binding protein in plasma that has a symmetrical two-domain structure connected by a flexible hinge region, whereby both domains are made up of four homologous repeats. The second hemopexin-like domain region of VN contains the basic heparin-binding site which is represented by a 40 amino acid stretch rich in Arg and Lys residues, representing two heparin-binding consensus sequences. Adjacent to the basic domain is the site for cAMP-dependent phosphorylation by protein kinase A, which may change the functional properties of this region. Although these protein domains determine the spacial arrangement of ligand interactions with the VN molecule, they are not superimposable with respective ligand-binding sites of the protein, particularly at different conformational states.

Homologies with Other Proteins

Based on identical spacing of cysteine residues, the amino-terminal somatomedin B typical domain is found in several other proteins. Likewise, many extracellular (matrix) proteins such as fibronectin, laminin, or fibrinogen share the integrin-binding RGD motif with VN, yet, the characteristics of ligand recognition by various integrins may differ and mostly depend on the RGD-flanking regions in these proteins. The proximal acidic peptide stretch downstream of the RGD site is found with appreciable sequence homology in some proteins relevant for hemostasis. VN may also be considered as a distant member of the hemopexin protein family, including hemopexin itself and matrix metalloproteinases. The C-terminal heparin-binding consensus sequences share this property with other glycosaminoglycan-binding proteins. Other sites, such as the internal plasminogen-binding motif or the protein kinase A recognition site, exhibit high homology with respective consensus sequences found in other proteins (Table 2).
Vitronectin, Table 2

Structure-function homologies of vitronectin domains in other factors

Somatomedin B region (cysteine pattern)

 Megakaryocyte-stimulating factor, plasma cell-expressed protein 1, placental protein 11

RGD cell adhesion motif

 Several adhesive glycoproteins, including bone sialoglycoprotein, osteopontin, von-Willebrand factor, fibronectin, denatured collagen, prothrombin, HIV-Tat protein

Acidic region, containing sulfated tyrosines

 Microfibril-associated protein, hirudin, platelet glycoprotein Ib, protease-activated receptor-1 (PAR-1, thrombin receptor), selectin ligand PSGL-1

Hemopexin-type repeats

 Hemopexin, several matrix metalloproteinases (MMP, e.g., interstitial collagenase, type IV collagenase), pea seed albumin 2, nectinepsin

Glycosaminoglycan-binding consensus site

 Several heparin-binding proteins, including apolipoprotein E, osteopontin, lipoprotein lipase, tissue factor pathway inhibitor

Binding Sites for Macromolecular Ligands

Various macromolecular protein ligands can (simultaneously) bind to soluble or matrix-associated VN, which contains respective domains that are mainly concentrated within the amino and carboxy termini of the protein (Tomasini and Mosher 1991; Preissner 1991). The conformational states of VN greatly determine the affinity and the extent of associations with diverse ligands (Fig. 1).

Somatomedin B Domain

At the amino terminus, the somatomedin B domain comprises the major PAI-1 binding site (Seiffert and Loskutoff 1991), whereby the positions of all eight Cys residues are highly conserved also in VN from other species; destroying the Cys-knot structure will abrogate PAI-1 binding activity. Due to appreciable overlap between binding and interaction sites for urokinase/urokinase receptor (uPAR) and PAI-1 within this VN domain, both proteins may compete with each other and influence cell migration or proteolysis at VN-rich matrix sites. Likewise, Zn2+-dependent tight binding of activated high-molecular-weight kininogen (HKa) to the amino-terminal region of VN overlaps with PAI-1 and uPAR binding, resulting in ligand displacement and anti-adhesive properties of HKa in uPAR-mediated cell adhesion to VN.

RGD Adhesive Site and Acidic Stretch

The adjacent RGD motif is crucial for VN’s adhesive functions due to binding of immobilized or aggregated VN to several cellular integrins, including the αIIb/ßIIIa integrin on platelets as well as different αv integrins, each containing the ß1, ß3, ß5, ß6, or ß8 subunit. In close proximity to the RGD motif, a highly acidic region (amino acid residues 53–64) follows, encompassing two tyrosyl sulfation and potential phosphorylation sites. This short peptide portion represents considerable sequence homology with regions in other proteins, relevant for hemostasis, such as platelet glycoprotein I, protease-activated receptor-1, or hirudin and has been implicated in binding interactions with the thrombin-antithrombin complex, or HKa. This region is followed by a cross-linking site (Gln-93) that is required for intermolecular linkage of VN mediated by coagulation factor XIIIa or tissue transglutaminase.

Hemopexin Repeats and Heparin-Binding Site

The carboxy-terminal half of VN is characterized by a long connecting segment encompassing a collagen-binding domain followed by two substructured hemopexin-like domains. Repetitive hydrophobic sequences within the first one serve as specific binding sites for infectious group A streptococci, whereby the middle portion of the protein molecule (residues 51–310) provides the functional part for VN being an inhibitor of the terminal complement membrane attack complex (Podack et al. 1984). The second hemopexin-like domain contains the basic heparin-binding site that is composed of a 40 amino acid residue stretch rich in Arg and Lys residues, constituting two typical heparin-binding consensus sequences. This basic cluster serves as a primary, conformation-dependent binding site of VN for heparin and other glycosaminoglycans. Likewise, cell surface-associated heparan sulfate proteoglycans provide cellular recognition sites for VN-containing protein complexes. This site is particularly exposed in VN multimers and engaged in specific binding interactions with fibrillar collagen, osteonectin, PAI-1 (secondary binding site), and plasminogen/angiostatin. The substrate site for protein kinase A, which may change the functional properties of this region following cyclic adenosine monophosphate-dependent phosphorylation, is located in close proximity to the C-terminus.

VN Ligands and Receptors

In addition to the mentioned macromolecular ligands and receptors, several growth factors, cytokines, proteolytic enzymes, as well as protein complexes directly interact with VN in vitro and in vivo, supporting the functional repertoire of VN as matricellular organizing molecule to regulate tissue remodeling and wound repair (Leavesley et al. 2013). The major macromolecular ligands are depicted in Fig. 1, and major cellular receptors for VN are listed in Table 3.
Vitronectin, Table 3

Distribution and functions of different vitronectin receptors

Receptor

Distribution

(Patho-)physiological function (ligand)

αvß3-integrin

Vascular cells

Cell migration, angiogenesis, restenosis (vitronectin, fibrin and other RGD-proteins) cell-cell contacts (PECAM-1, CD31) adenovirus binding and uptake (penton-base)

Tumor cells

Migration, chemotaxis (vitronectin-PAI-1, osteopontin, matrix-metalloproteinase-2)

αvß5-integrin

Adhesive cells

Endocytosis, phagocytosis and bacterial invasion

Heparansulfate-proteoglycans

Endothelial cells

Endocytosis, transcytosis (vitronectin-complexes)

Urokinase-receptor

Vascular, blood cells, tumor cells

Modulation of adhesion, cell invasion, pericellular proteolysis (vitronectin-rich extracellular matrix) receptor turnover (vitronectin-PAI-1)

Globular C1q-receptor (gC1qR)

Vascular, blood cells

Phagocytosis, clearance (vitronectin-complexes, foreign particles)

Biological Characteristics of VN

As a multifunctional matricellular protein, VN not only promotes cellular adhesion and migration of a variety of cell types but also provides cofactor activity for the regulation of pericellular proteolysis relevant in vascular remodeling and wound healing and serves as scavenging factor for protease inhibitor complexes of the hemostasis and complement systems (Tomasini and Mosher 1991). Cellular activities of VN are mediated in great part by unrelated members of different cell surface receptors, expressed on vascular and other cell types that are of importance at VN-rich sites in the body, and differences in the conformation of VN also determine the affinity and avidity for recognition/interaction of these ligands or cellular receptors by VN (Preissner and Reuning 2011).

Cell Adhesion, Migration, and Integrin Interactions of VN

Defense mechanisms of the organism, including wound healing, tissue repair, the humoral and cellular immune systems, and hemostasis, require different types of adhesive and mobile cells (platelets, leukocytic) which all bear variable subtypes of adhesion receptors. In particular, VN-binding integrins are associated with these events and participate in signal transduction and gene regulation depending on the adhesion receptor involved. The integrins αvß3, αvß5, αvß1, and αIIbß3 are established receptors for VN but are promiscuous in recognizing the RGD motif in other adhesive proteins, including fibrinogen, von Willebrand factor, fibronectin, osteopontin, or denatured collagen. The most selective VN-binding αvß5 also recognizes basic regions in HIV-Tat protein as well as the heparin-binding domain of VN, indicating that a variability of integrin-recognizing alternative recognition motifs exists. With regard to mediating cell contact and motility, both αvß3 and αvß5 serve complementary functions on vascular and tumor cells: While αvß3 is localized to focal adhesion sites and promotes cell spreading and migration on various RGD substrata, αvß5 supports cell attachment and is clustered in focal adhesion sites only in some cell types. Moreover, coordination with the urokinase/uPAR complex appears to direct cellular migration on VN-rich substrata (Chapman 1997): Due to the proximity of binding sites for PAI-1 and integrins within the N-terminal portion of VN, active PAI-1 but not the latent or cleaved form acts as a potent inhibitor for VN-mediated cell adhesion via the integrins αvß3, αvß5, and αIIbß3 (Andreasen et al. 1997). This indicates that excess PAI-1 favors inhibition of cellular motility at VN-rich matrix sites in a proteolysis-independent manner.

While such cellular interactions relate to the activity of immobilized VN, the binding of soluble multimeric VN to the major αMß2 integrin on neutrophils is RGD independent but is competed for by fibrinogen and exhibits a specific integrin affinity-regulated interaction. VN thereby significantly enhances leukocyte adhesion and extravasation, e.g., at sites of bacteria-infiltrated wounds.

VN was also co-localized with αvß3 and αIIbß3 in regions of pro-platelet generation in maturating bone marrow-derived megakaryocytes, suggesting an important intracellular role for this integrin-ligand system during platelet formation or for translocation of VN and fibrinogen between intra- and extracellular space. Binding of VN to the luminal phase of endothelial cells is only mediated by αvß3 if highly clustered soluble forms of the adhesion protein or if VN-coated microspheres were employed.

Proteolysis, Hemostasis, and the Urokinase Receptor System

Among the functional interactions of VN with proteolytic systems, binding to the serine protease inhibitor PAI-1 and to thrombin-inhibitor complexes are of major importance for regulation of hemostatic events and pericellular proteolysis. As a functional consequence, complex formation with VN does not only increase the half-life of PAI-1 in the circulation by two- to fourfold (Declerck et al. 1988) but enables the active inhibitor to become stabilized at sites of vascular injury and initial platelet plug formation. PAI-1 has also been identified as “heparin cofactor” and kinetic data indicate that both, VN and heparin, drastically change the specificity of PAI-1 toward thrombin, such that this procoagulant enzyme becomes recognized and subsequently neutralized by PAI-1.

During the cause of blood clotting, a unique property of VN is the formation of stable, disulfide-bridged ternary products with thrombin-inhibitor complexes, accompanied by concomitant multimerization of VN (Preissner and Reuning 2011). VN thereby serves as clearance protein for the spent thrombin-antithrombin complex that subsequently becomes recognized by endothelial heparan sulfate proteoglycans and is removed by endocytosis from the blood circulation. These ternary complexes exhibit binding to PAI-1 as well and promote cell adhesion identical to isolated multimeric VN. As potent heparin scavengers in the circulation, VN multimers as well as ternary complexes may neutralize exogenously administered heparin and thereby substantially lower its catalytic anticoagulant potential in serine protease inhibitor-dependent control of coagulation enzymes such as thrombin or factor Xa. The constant supply of the vessel wall-associated matrix with VN may explain the (age-related) accumulation of the protein along large vessels or elastic fibers in the skin or during changes in vascular permeability associated with angiogenesis.

In addition to heparan sulfate proteoglycans, specific and saturable interactions of heparin-binding soluble VN multimers with cells engage uPAR as a high-affinity receptor, whereby VN serves a regulatory function in uPAR-related invasiveness or differentiation of cells. The glycolipid-anchored uPAR is expressed on a variety of cell types, strongly correlating with their migratory and invasive potential. uPAR coordinates cell–cell and cell–matrix interactions by integrating pericellular proteolysis, cell adhesion, and signal transduction events (Chapman 1997; Andreasen et al. 1997), the outcome of which depends on the respective cell type, the availability of urokinase, PAI-1, VN, and other cofactors/receptors. As a consequence, immobilized VN can mediate cell adhesion via uPAR on cells that exhibit high expression of the GPI-anchored protein. The binding affinity (and adhesive capacity) of multimeric VN for uPAR is increased almost tenfold in the presence of urokinase, whereas active PAI-1 totally abrogates the VN–uPAR interaction due to overlapping binding sites within the somatomedin B domain. Thus, in a VN-dependent but proteolysis-independent fashion, urokinase acts as pro-adhesive factor, whereas PAI serves as potent anti-adhesive protein. Another ligand of uPAR is HK, and due to its competitive interaction with VN for uPAR, HK serves an anti-adhesive function in this type of cell adhesion.

The ternary complex between uPAR, VN, and urokinase is detectable on the surface of normal and tumor cells (e.g., breast cancer tissue), and increased expression of uPAR at the leading edge of migrating (tumor) cells is essential for their locomotion or invasiveness, independent of urokinase activity (Andreasen et al. 1997), whereby VN-complexed PAI-1 regulates cell adhesion by promoting urokinase/uPAR turnover. Likewise, removal of the thrombin-PAI-1 complex by lipoprotein receptor-related proteins 1 and 2 is facilitated by VN. The urokinase/uPAR and VN-PAI-1/integrin systems are thus essential in the regulation of plasmin-dependent and plasmin-independent adhesive cellular activities, particularly relevant for the dynamic remodeling of vascular and tumor tissue.

VN and Growth Factors

Based on in vitro studies, several growth factors and cytokines directly or indirectly interact with immobilized matrix-bound VN, resulting in coordination of adhesive and cell signaling events. In particular, insulin-like growth factor (IGF)-II directly binds to VN, whereas IGF-I interacts only indirectly with VN via IGF-binding proteins (IGF-BP)-2, -3, -4 and -5, respectively. The formed dimeric or trimeric complexes, respectively, significantly enhance migration and proliferation of, e.g., smooth muscle or epithelial cells of different origin (Upton et al. 2008). To induce cellular responses, the activation of the respective IGF receptor together with VN binding to αv integrins is required. Other growth factors, such as transforming growth factor-ß, interact with VN, may thereby become concentrated in the extracellular matrix and be oriented for induction of cellular signaling. VN-directed binding of growth factors can protect them against proteolytic degradation and thus facilitates their recognition by cognate receptors also in proximity to integrins. In addition, IGF-binding proteins in concert with IGF-I and VN are capable of stimulating cell migration in the context of wound healing.

As a consequence, delivery of growth factors coupled to VN may hold great promise for the development of more effective therapeutic strategies in wound repair (Upton et al. 2008). These aspects may also point to an integrated view on VN’s functions in the different phases of wound repair that include (a) promotion of platelet aggregation in a fibrin-dependent manner with the concomitant release of growth factors and chemoattractants from platelet α-granules, (b) fixation of PAI-1 to the fibrin clot to achieve clot stabilization and to limit premature fibrinolysis, (c) extravasation of VN into the wound site and fixation of growth factors, and (d) promotion of cell migration and proliferation through the coordinated action between integrins, uPAR, pericellular proteolysis, and growth factor signaling. Finally, integrin ligation and clustering on VN substratum are linked to different intracellular signals that prevent endothelial cells from undergoing apoptosis, whereas soluble VN was unable to reverse this event.

VN and Innate Immunity

VN and Bacterial Invasion

As a ubiquitously distributed host factor, VN mediates bacterial binding and colonization for a variety of gram-positive and gram-negative bacterial strains, reminiscent of the function of fibronectin and other matrix proteins (Preissner and Chhatwal 2005), that may likely occur in damaged or infected tissues. Various strains of staphylococci as well as groups A, C, and G streptococci express typical surface-associated, VN-binding adhesins, some of which are inhibited by heparin, while hemopexin-type repeats in VN have been identified as primary binding sites for group A streptococci as well. As a consequence, microbial binding of VN mediates the adherence of gram-positive bacteria to host cells. The respective adhesin of S. aureus has striking similarity to a heparan sulfate-binding protein from the same staphylococcal strain, suggesting that VN undergoes multiple interactions with different bacterial surface recognition sites. The impact of a bridging function of VN on bacterial invasion has been demonstrated for Neisseria gonorrhoeae and S. pneumonia, and once bacteria are attached to the VN-integrin complex, host cell signaling events are activated to promote internalization of microbes. The latter occurs in a VN-mediated, αvß3 integrin-dependent manner in pneumococcal adherence to and invasion into human epithelial and endothelial cells, involving the host cell signaling molecules integrin-linked kinase, PI3-kinase, and Akt (Bergmann et al. 2009).

VN and Complement Inhibition

In the host, complement-mediated opsonization and killing of microbes as well as protection of host bystander cells against complement attack are achieved by several regulatory proteins at different levels of the complement cascade (Schmidt et al. 2016). These include C4b-binding protein, factor H, clusterin, and VN. VN exerts its complement regulatory function by blocking the metastable membrane-binding site of the nascent C5b-7 complex and by inhibiting C9 polymerization and thereby the assembly of the membranolytic attack complex (Podack et al. 1984).

Another cell surface receptor for multimeric VN, gC1q receptor, that independently recognizes the globular heads of C1q (Lim et al. 1996), may contribute to the mechanism that serves as scavenger or opsonizing complex in phagocytosis. In contrast, VN was found to inhibit ingestion of apoptotic cells by macrophages (“efferocytosis”) due to interactions with both cells types. The functional properties of VN as an indirect opsonin, a participating component in apoptosis, and a major complement inhibitor are likely to be expressed in concert. The distribution of VN and gC1q receptor on blood and vascular cells may very well indicate a functional linkage in processes of wound healing and immune defense.

The pathogenicity of many microbes relies on their capacity to resist innate immunity (including complement attack), and bacteria have developed highly efficient complement evasion strategies to survive and persist in an immune-competent human host. Different human bacterial pathogens as well as Candida albicans associate with VN via its C-terminal heparin-binding domain, leaving the complement regulatory region of VN intact and thereby allowing inhibition of C5b-7 membrane insertion and C9 polymerization. VN complexed with various microbes not only enhanced bacterial adhesion to host cells but protected microbes (including Moraxella catarrhalis, Haemophilus influenzae, and Neisseria gonorrhoeae) from complement-mediated killing (Hallström et al. 2016).

Vitronectin and Pathologies

VN Deposition in Diseased Tissues

Reduced levels of circulating VN in patients are associated with severe liver failure, whereas increased plasma levels of VN are observed in patients undergoing elective orthopedic surgery but also in rodents upon systemic stimulation with endotoxin or inflammatory cytokines, indicative for VN being an acute phase protein. Prominent deposition of VN was documented in areas of sclerosis and necrosis in necrotic foci in hepatitis, degenerative skin disease, or in degenerative central nervous system disorders (Seiffert 1997). Increased deposits of VN in association with atherosclerotic plaques or acute myocardial infarction are indicative for the protein as a marker of injury/repair. Hyperglycemia-mediated irreversible modification of VN related to nonenzymatic glycation in diabetes resulted in the loss of functional properties of VN.

The indicated specific uptake mechanisms for VN seem to be operative for translocation of circulating VN into tissues, supported by the fact that VN is detectable by immunohistochemical staining in various tissues, but no association with VN mRNA is observed at these sites. The constant supply of the vessel wall-associated matrix with VN by these mechanisms may also explain the (accelerated) accumulation of the protein along large vessels or elastic fibers in the skin.

VN and Thrombus Formation

Information from different animal models of stenosis/thrombosis, including VN-deficient mice, points to the fact that VN serves thrombus-stabilizing functions by different mechanisms. PAI-1 and VN were found to be essential for stabilization of arterial thrombi (Eitzman et al. 2000) due to the indicated antifibrinolytic and pro-aggregatory action of the complex. The destabilization of thrombi observed in VN-deficient mice was reminiscent of the function of an antibody against the platelet aIIbß3 integrin that prevented aggregate formation and led to thrombus embolization. Furthermore, in contrast to platelet-derived/platelet-bound VN, plasma VN was shown to act in an anti-aggregatory manner by competing with fibrinogen for binding to aIIbß3 integrin. Thus, the balance between pro- and anti-aggregatory activities of different forms of VN as well as the contribution of fibrin and vascular cells is a major determinant for the outcome of thrombus formation.

VN and Atherosclerosis

During atherogenesis, the composition of the vascular wall extracellular matrix changes considerably, and like many other adhesive proteins, VN is found in association with atherosclerotic plaques originating from plasma leakage and activated platelets as well as from local expression by neointimal cells. Moreover, colocalization of VN, active PAI-1, and thrombin was found in the atherosclerotic vessel wall. Migration of neointimal smooth muscle cells depends on αv integrins, and blockade by an anti-VN antibody in a balloon injury model in rats prevents neointimal formation. Although VN and PAI-1 contribute to a significant decrease in fibrinolysis during the neointimal formation following vessel injury/ligation in mice, respective therapeutic regimen in delaying atherogenesis in humans has not been studied.

VN and Cancer

VN is found in association with a variety of (human) tumors, may induce cancer stem cell differentiation by downregulation of stem cell genes through αvß3 integrin-mediated mechanisms, and seems to influence cell adhesion and motility as well as formation of cancer cell aggregates. Based on expression studies, biosynthesis of VN was documented in cancer cell lines derived from the cervix, lung, pancreas, and ovary as well as in stromal cells, disclosing its distribution along the cell surface and within thin deposits at the cell periphery, whereby the VN–αvß3- integrin system was made responsible for adhesion and proliferation of ovarian cancer cells. The initial steps of ovarian cancer cell metastasis are mediated by cleavage of VN and fibronectin by the matrix metalloproteinase MMP-2, facilitating α5ß1- and αvß3-mediated cancer cell motility (Kenny et al. 2008).

Antagonists against the VN-binding αvß3 integrin, which is highly expressed on angiogenic blood vessels and on malignant tumors, were effective in suppressing physiological and tumor angiogenesis in different in vivo models. Conversely, VN and fibronectin are needed for in vivo activity of endogenous angiostatic molecules such as anastellin (derived from fibronectin) or modified antithrombin because these polypeptides require complex formation with the respective adhesion protein to confer activity (Yi et al. 2003).

Also, proteolysis-dependent and proteolysis-independent activities of urokinase and PAI-1 apply for tumor situations as well, and elevation of both factors (e.g., in breast cancer) is associated with a poor prognosis for the patient (Harbeck et al. 2004). Likewise, this proteolytic system regulates tumor angiogenesis, as demonstrated in various mouse models, whereby PAI-1 deficiency in mice prevents cancer invasion and neovascularization. Here, proteases rather than VN control PAI-1 activity in this in vivo tumor vascularization model.

Summary

Vitronectin (VN) belongs to the group of Arg-Gly-Asp (RGD)-type multifunctional adhesive glycoproteins that are recognized by different cellular adhesion receptors and thereby play key roles in the attachment of cells to their surrounding matrix. Moreover, VN exerts several regulatory functions in immunity (e.g., recognition of microbes, complement) and wound repair (hemostasis, pericellular proteolysis), such that this “matricellular” protein is particularly involved in defense processes and tissue remodeling (Preissner and Reuning 2011).

The predominant biosynthesis of VN in the liver and its minimal production in other somatic cell types may be altered in disease states, and as a circulating and diffusible plasma protein, VN is found immobilized in various tissues under (patho-) physiological conditions or in association with tumors. As a structurally labile molecule, many of VN’s biological functions are dependent on and/or regulated by its conformational state(s), whereby multimeric forms of the protein are predominantly recognized by cell surface receptors, including integrins, the urokinase receptor, and proteoglycans. Although deletion of the VN gene in mice is compatible with life (Zheng et al. 1995), related functional deficiencies in vivo may become apparent after challenge of VN knockout mice with different pathologies or in certain diseases in humans. Thus, VN provides a versatile molecular link in different tissues and the vasculature, particularly relevant for connecting biological defense systems with cell mobility and survival, cell damage/wound repair, or infection.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Department of BiochemistryMedical School, Justus-Liebig-UniversityGiessenGermany