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Cardiovascular Research 2001 52(3):372-386; doi:10.1016/S0008-6363(01)00399-6
© 2001 by European Society of Cardiology
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Copyright © 2001, European Society of Cardiology

Adhesion receptors of vascular smooth muscle cells and their functions

Elena P. Moiseeva*

Division of Cardiology, Department of Medicine, University of Leicester, Clinical Sciences Wing, Glenfield General Hospital, Leicester LE3 9QP, UK

* Tel.: +44-116-256-3048; fax: +44-116-287-5792 em9{at}le.ac.uk

Received 9 January 2001; accepted 20 June 2001


   Abstract
Top
 Abstract
1 Introduction
 2 SMC integrins
 3 Dystrophin-glycoprotein...
 4 Syndecans
 5 Cell adhesion molecules
 6 Cadherins
 7 Closing remarks
 References
 
Vascular smooth muscle cells (SMCs) are present in several phenotypic states in blood vessels. They show a high degree of plasticity, undergoing rapid and reversible phenotypic changes in response to environmental stresses and vascular injury. Thereby, SMCs play an important role in development of atherosclerosis and restenosis after angioplasty and coronary bypass grafting. Many functions of SMCs, such as adhesion, migration, proliferation, contraction, differentiation and apoptosis are determined by surface adhesion receptors involved in cell–cell binding and interactions between cells and extracellular matrix (ECM) proteins. Some cell adhesion receptors are involved in intracellular signalling and participate in cellular response to different stimuli. The adhesion receptors of vascular SMCs discussed here include the ECM adhesion receptors integrins, {alpha}-dystroglycan and syndecans, as well as the cell–cell adhesion receptors cadherins and cell adhesion molecules. This review is intended to provide a generalised overview of the receptors expressed in vascular SMCs in relation to their functions and implications for vascular pathology.

KEYWORDS DGPC, dystrophin–glycoprotein complex; ECM, extracellular matrix; ECs, endothelial cells; IAP, integrin-associated protein; ICAM-1, intercellular cell adhesion molecule-1; PIP2, phosphatidylinositol (4,5)-biphosphate; PKC, protein kinase C; RGD, arginine–glycine–aspartic acid; SM, smooth muscle; SMCs, SM cells; uPAR, urokinase-type plasminogen activator receptor, VCAM-1, vascular cell adhesion molecule-1


   1 Introduction
Top
 Abstract
 1 Introduction
2 SMC integrins
 3 Dystrophin-glycoprotein...
 4 Syndecans
 5 Cell adhesion molecules
 6 Cadherins
 7 Closing remarks
 References
 
There is a considerable level of heterogeneity of vascular smooth muscle cells (SMCs) in blood vessels. Vascular SMCs exhibit distinct morphological and functional properties within the same blood vessel [1,2], as well as in different types of blood vessels, e.g. arteries and veins [3]. The principle function of vascular SMCs is to maintain vascular tone and resistance. The differentiated SMCs exhibit a contractile phenotype, and express smooth muscle (SM) specific contractile and cytoskeletal proteins [4]. Adhesion of contractile SMCs to the extracellular matrix (ECM) is fortified in order to withstand considerable tensions due to SMC contraction and haemodynamic forces. In addition to integrins and syndecans, found in non-muscle cell types, contractile SMCs contain a muscle specific dystrophin–glycoprotein complex (DGPC), linking actin filaments to ECM. The other important function of SMCs is related to the SMC response to environmental stresses and the repair of the blood vessel wall after vascular injury [1,4]. These responses require modulation of the SM phenotype, synthetic and migratory activities of SMCs, which in turn need disengagement of adhesion receptors anchoring cells to ECM during cell contraction. The dedifferentiated SMCs are motile and express an altered set of adhesion receptors. Therefore, modulation of SM phenotype is crucial in the development of many vascular diseases including atherosclerosis and restenosis after angioplasty and coronary artery bypass grafting.

To date, the factors and the mechanisms involved in regulation of SMC phenotype remain obscure. However, the ECM is likely to play a pivotal role in regulation of SMC phenotype [5,6]. Vascular contractile SMCs are embedded in the ECM and surrounded by an incomplete basement membrane in vivo [7,8]. A basement membrane and one of its essential components laminin, surrounding contractile SMCs in the blood vessels [7,9], play an important role in the differentiation of SMCs. The temporal phenotypic modulation of SMCs during vascular injury leads to the destruction of the basement membrane, which reappears when SMCs revert to the contractile phenotype [7]. Laminin is also implicated in maintaining the SMC contractile phenotype in vitro [10–14].

ECM adhesion receptors provide a link between the cytoskeleton and the ECM in cell–ECM contacts. ECM adhesion receptors play a major role in SMC adhesion, although cell–cell adhesion receptors are also found in SMCs. A vast number of studies has revealed a central role for integrins in SMC adhesion and related SMC functions. There is accumulating evidence on the role of cell adhesion molecules for SMCs. Other adhesion receptors, syndecans, cadherins and {alpha}-dystroglycan, have been studied to a much lesser extent, although {alpha}-dystroglycan is likely to be relevant to SM contraction. This review focuses on adhesion receptors including integrins, {alpha}-dystroglycan, syndecans, cell adhesion molecules and cadherins with an intention to relate expression of the receptors to their functions in a specific SM phenotype and/or a particular pathophysiological process. Fig. 1 schematically shows some of the information presented below.


Figure 1
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  		Fig. 1 Schematic representation of phenotypic modulation of smooth muscle (SM) cells, resulting in the shift in the expression pattern of adhesion receptors. The adhesion receptors, specifically expressed in a particular SM phenotype, are shown. Other receptors are shown in Table 2. Left side: differentiated contractile SM cells express integrins {alpha}1β1 and {alpha}7β1 and the dystrophin–glycoprotein complex (DGPC) with {alpha}-dystroglycan. Integrins localise in the adherens junctions on longitudinal ribs of cells (shown in black) (after [128]). The DGPC is found in caveolar domain of cells (shown in grey) between the ribs. Vascular injury and atherogenesis cause SMC dedifferentiation. Integrins {alpha}vβ1 and {alpha}vβ3 are involved in phenotypic modulation of SM cells. Right side: dedifferentiated SM cells have rearranged the cytoskeleton and adhesion apparatus. They express integrin {alpha}4β1 and cell adhesion molecules VCAM-1 and ICAM-1, responsible for interaction between SMCs and other cell types. Integrin {alpha}4β1 and VCAM-1 may be involved in SM differentiation.

 

   2 SMC integrins
Top
 Abstract
 1 Introduction
 2 SMC integrins
3 Dystrophin-glycoprotein...
 4 Syndecans
 5 Cell adhesion molecules
 6 Cadherins
 7 Closing remarks
 References
 
Integrin receptors are the principle receptors for the ECM and serve as a transmembrane link between the ECM and the actin cytoskeleton (Fig. 2). Integrins are bound to an intracellular protein complex, which includes the cytoskeletal proteins talin, vinculin, {alpha}-actinin and filamin [15], in adherens junctions of SMCs. Integrins are composed of {alpha} and β subunits. Each {alpha}β combination has its own ligand-binding specificity [16,17] and signalling properties (reviewed in [18,19]).


Figure 2
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  		Fig. 2 Schematic drawings of cell–ECM adhesion complexes of smooth muscle cells. ECM proteins collagen (COL), fibronectin (FN), laminin (LN), tenascin (TC), thrombospondin (TSP) and fibrinogen (FG) are shown. Black lines extending from adhesion receptors indicate polysaccharide chains in proteoglycans. (A) The extracellular domains of integrins and syndecans interact with ECM proteins. The intracellular domains of integrins and syndecans bind to cytoskeletal proteins linking adhesion receptors to the actin filaments. (B) {alpha}-Dystroglycan ({alpha}DG) interacts with the C-terminal part of {alpha} chain of LN, {alpha}1β1 and {alpha}2β1 integrins interact with the N-terminal part of {alpha} chain of LN. In the dystrophin–glycoprotein complex, {alpha}-dystroglycan binds to a complex of integral membrane proteins β-dystroglycan (β-DG), β-, {delta}- and {epsilon}- sarcoglycans and sarcospan (SP); they bind to dystrophin (D). Dystrophin has several actin-binding sites and provides the connection to the actin filaments. (C) ICAM-1 interacts with FG. The intracellular domain of ICAM-1 binds to cytoskeletal proteins linking it to the actin filaments.

 
The main integrin ligands are ECM proteins, although some integrins also interact with other transmembrane counter-receptors and mediate cell–cell interactions. Integrin–ligand binding is tightly regulated via conformational changes of integrins by cell signalling. Resting, inactive integrins have low affinity for their ligands. Integrins in an active state bind to their ligands with high affinity. Integrin activation increases integrin affinity and, thereby, regulates SMC functions, such as adhesion, migration and proliferation (see below).

2.1 Integrin expression in vascular SM related to the SMC phenotype and interactions of SMCs with other cell types via integrins
Integrins expressed in vascular SM are shown in Table 1. Integrins β1 are predominant in vascular SM in vivo and in cultured SMCs [26,32]. They are present in the activated high affinity form in arteries [65]. Vascular injury reduces expression of total and activated β1 integrin in neointima [65]. Deactivation of β1 integrins correlates with increased proliferative and migratory activities of SMCs [65,66]. Hence, expression of activated β1 integrins may contribute to maintaining the contractile SM phenotype.


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  		Table 1 Integrin expression in vascular smooth muscle

 
The major {alpha} integrin subunits present in vascular SM in vivo are {alpha}1, {alpha}3 and {alpha}5 [26,29]. Expression of {alpha}1β1 and {alpha}7β1 integrins correlates with the differentiated SM phenotype [20,47]. Integrin {alpha}7β1 is known to inhibit surface expression of other β1 integrins in non-muscle cell types [45,67], therefore, it may influence expression of other integrins in contractile SMCs. Noteworthy, both {alpha}1β1 and {alpha}7β1 bind to laminin, which is involved in maintaining the contractile SM phenotype. Interestingly, the promoter of the {alpha}1 integrin subunit gene is more active in differentiated SMCs [68], which may account for huge variations in reported levels of {alpha}1β1 integrin.

In contrast to {alpha}1β1, {alpha}4β1 integrin is not expressed in differentiated SM, but is expressed in developing blood vessels, where it co-localises with its ligand cellular fibronectin and a counter-receptor vascular cell adhesion molecule-1 (VCAM-1) [37,38]. This integrin binds to an additional site of fibronectin, encoded by an alternatively spliced exon. Expression of {alpha}4β1 integrin and VCAM-1 is a prerequisite for expression of SM phenotype markers in human cultured SMCs [38]. This integrin is also found in atherosclerotic intimal thickening together with VCAM-1 (see below) and may be involved in cell–cell interactions between SMCs and VCAM-1-expressing endothelial cells (ECs) [38]. Since another integrin, {alpha}9β1, also binds to VCAM-1 [54] and is expressed in vascular SM [55], it may mediate interactions between SMCs and VCAM-1-expressing ECs.

Integrins {alpha}v are expressed in the blood vessel wall, although they are not the major integrins. Expression of {alpha}vβ3 integrin varies dramatically in SMCs of different origin [57,58]. Microscopic studies have shown co-localisation of integrins {alpha}vβ3 and {alpha}vβ5 with their ligand vitronectin in the atherosclerotic intima [60]. Some ECM ligands for {alpha}vβ3, such as fibrin, fibrinogen, thrombospondin, vitronectin and osteopontin, are not detectable in normal blood vessels and found only in diseased blood vessels [60,69–72]. Integrins {alpha}vβ3 and {alpha}vβ5 are upregulated early after vascular injury and are likely to be involved in postangioplasty events and neointima formation [62].

2.2 Adhesion, migration, ECM assembly and ECM contraction
Integrins β1 are considered to play a major role in adhesion [22], ECM assembly, repair and remodelling. Several SMC integrins bind to laminin, but only {alpha}7β1 integrin has been shown to affect laminin adhesion in SMCs [47]. Integrin {alpha}7β1 also plays an important role in laminin polymerisation [73]. Among integrins {alpha}1β1, {alpha}2β1 and {alpha}3β1, involved in collagen adhesion, only {alpha}1β1 and {alpha}2β1 are responsible for collagen contraction [23,30,31,74]. Since only {alpha}1β1 is expressed in vivo, it is likely to be involved in collagen remodelling after vascular injury.

Adhesion to fibronectin is mediated by integrins {alpha}3β1 and {alpha}5β1 [23,74]. Integrin {alpha}5β1, highly abundant in cultured SMCs, is also involved in fibronectin polymerisation [41]. This has important implications for SMC proliferation, since inhibition of the fibronectin matrix assembly inhibits proliferation of human primary SMCs [75]. After vascular injury, {alpha}5β1 is induced at the lumenal surface of neointima and may be involved in blood vessel wall repair and incorporation of plasma fibronectin into ECM [41]. Recently this integrin has been implicated in fibrin adhesion and fibrin clot contraction [33]. Since fibrin is present in atherosclerotic plaques [69] and fibrinogen is abundant in the intima of atherosclerotic aorta [70], {alpha}5β1 integrin is thought to cause vessel narrowing during the progression of atherosclerosis [33]. The other fibrin-binding integrin {alpha}vβ3 is implicated only in fibrin adhesion, but not in fibrin clot contraction [33,76]. Recent data on {alpha}5β1 and {alpha}vβ3 integrins indicate that migration of SMCs into fibrin clot may require co-operation of both integrins [77].

Integrins β1 are implicated in migration of cultured SMCs [22,66,78,79]. Among β1 integrins, only {alpha}2β1, {alpha}5β1 and {alpha}7β1 are known to participate in SMC migration and motility [26,47,77,79,80]. In contrast to β1 integrins, {alpha}vβ3 is implicated in SMC migration both in vitro [32,57,58,77–79,81–85] and in vivo [61,83,86,87]. A blockade of {alpha}vβ3 integrin with cyclic arginine–glycine–aspartic acid (RGD) peptide or RGD mimetic in animal vascular injury models reduces vessel wall thickening [61,83,86,87]. Recent investigations have demonstrated that this effect may be also related to apoptosis (see below).

While all {alpha}v-containing integrins interact with vitronectin and osteopontin and are involved in the adhesion to these ECM proteins, only {alpha}vβ3 is implicated in SMC migration [57,58,84]. SMCs, which express {alpha}vβ5 integrin and do not express {alpha}vβ3, do not migrate to osteopontin [57,58].

2.3 Integrin signalling in cell adhesion, proliferation, apoptosis and other SMC functions
Integrins participate in several signalling pathways. Integrin ligation and clustering in response to the ECM initiate an assembly of integrin-cytoskeleton complexes to support cell adhesion. Several integrin-associated proteins, such as urokinase-type plasminogen activator receptor (uPAR), integrin-associated protein (IAP) and tetraspanin CD9, modulate integrin-mediated adhesion in SMCs. The uPAR associates with and stabilises β1 integrin–caveolin complexes in human SMCs; this association is required for β1 integrin-mediated SMC migration [88–90]. SMC migration to collagen is potentiated by thrombospondin via thrombospondin-binding IAP, which is associated with {alpha}2β1 integrin in SMCs [91,92]. In human vascular SMCs, CD9 is mainly associated with {alpha}2β1, {alpha}3β1 and {alpha}5β1 integrins and affects collagen gel contraction [93].

Some integrins participate in another category of signalling leading to cell proliferation, differentiation and apoptosis (reviewed in [18,19]). Integrins also contribute to the cell response to growth factors. Integrin-mediated signalling, which is related to these SMC functions, is summarised below.

Integrins {alpha}v affect many functions of SMCs in contrast to other integrins. Integrin {alpha}vβ3 together with {alpha}5β1 affects Ca2+ influx in arteriolar SMCs, indicating the link through intracellular signalling pathways to the Ca2+ channels [94,95]. Blocking {alpha}vβ3 integrin causes a decrease in Ca2+ influx in parallel to the arteriolar lumen diameter increase [94]. Therefore, the effect of {alpha}vβ3 integrin on arteriolar vasodilation in response to tissue injury, observed earlier [96], may be mediated by {alpha}vβ3 integrin signalling involving Ca2+. Another intriguing example of integrin-mediated signalling involves SMC dedifferentiation induced by serum or vitronectin, abundant in serum, but the underlying mechanisms remain obscure. SMC adhesion to ligands from whole serum or vitronectin via integrin {alpha}vβ1 inhibits SMC contractility. Blocking vitronectin with specific antibodies or RGD-containing peptides diminishes the loss of SMC contractility in cell culture [97].

Integrin {alpha}vβ3 has been implicated in SMC proliferation in response to several stimuli. Integrins {alpha}vβ3 and {alpha}vβ5 augment the mitogenic response to cyclic stretch, mediated by PDGF [64]. Integrin {alpha}vβ3 enhances proliferative and other responses of SMCs to growth factors [82,85,98,99] and modulates proliferation induced by thrombospondin [63]. The exact mechanisms of these co-operative effects are not clear, but the physical association between growth factor receptors and {alpha}vβ3 in other cell types has been confirmed, linking the growth factor and {alpha}vβ3 integrin signalling pathways [100,101].

Recently a new wave of research has demonstrated the importance of cell adhesion in apoptosis. The role for {alpha}vβ3 integrin in SMC apoptosis has been studied to provide insight into the mechanisms of restenosis. The blockade of {alpha}vβ3 integrin in an animal model of vascular injury reduces intimal thickening, which correlates with abundant apoptosis in injured vessels [102,103]. The {alpha}vβ3 integrin-mediated signalling in apoptosis may be also relevant to plaque stability, since blocking of {alpha}vβ3 integrin with the peptide products of ECM degradation may lead to SMC apoptosis and promote plaque rupture.


   3 Dystrophin–glycoprotein complex
Top
 Abstract
 1 Introduction
 2 SMC integrins
 3 Dystrophin-glycoprotein...
4 Syndecans
 5 Cell adhesion molecules
 6 Cadherins
 7 Closing remarks
 References
 
The dystrophin–glycoprotein complex (DGPC) is expressed in all types of muscle and is critical for muscle function. Mutations in genes encoding DGPC components lead to different forms of muscular dystrophy and cardiomyopathy [104–107]. The function of the DGPC is to provide mechanical stability of the cell membrane during contraction and protect cells from overstretching [108–111]. The importance of this complex for vascular SM has just started to be unravelled.

The DGPC serves as a link between the basement membrane around muscle cells and the actin filaments (Fig. 2B). The cell surface receptor for the DGPC is a proteoglycan, called {alpha}-dystroglycan. It connects other components of the DGPC and laminin and perlecan [112,113], which are essential ECM components produced by vascular SMCs [9,25,114,115]. {alpha}-Dystroglycan is also involved in the laminin assembly on cell surfaces followed by incorporation of other ECM proteins, such as perlecan and collagen IV, to form a basement membrane [73,116]. An antibody blocking study has revealed that laminin polymerisation by myoblasts is supported much more by {alpha}-dystroglycan than by β1 or {alpha}7 integrins [73]. Studies of {alpha}-dystroglycan null mice and {alpha}-dystroglycan-deficient ES cells also indicate that {alpha}-dystroglycan is indispensable in basement membrane assembly compared with {alpha}7β1 integrin [116–118].

{alpha}-Dystroglycan is an extracellular protein in contrast to other integral membrane adhesion receptors. It binds to a transmembrane protein β-dystroglycan in complex with several other transmembrane proteins, sarcoglycans and sarcospan (reviewed in [104,119]). Sarcoglycans contribute to stability of the whole DGPC [111,120] and cell membrane [110,111]. Thus, several proteins in the DGPC provide a durable link between the ECM and intracellular proteins, whereas this is the function of an adhesion receptor in other ECM adhesion complexes. For this reason, it seems feasible to consider the functioning of the DGPC as a whole unit, rather than to consider the function of {alpha}-dystroglycan alone.

The SM DGPC consists of {alpha}- and β-dystroglycans, β-, {delta}- and {epsilon}-sarcoglycans, sarcospan and dystrophin [121,122]. The SM DGPC differs from that in striated muscle, which contains a different set of sarcoglycans: {alpha}, β, {gamma} and {delta} [121,122]. There are two differently glycosylated {alpha}-dystroglycan isoforms, 100- and 156-kDa, in SM [121]. The 156-kDa isoform is a component of the DGPC [121]. Dystrophin expression strongly correlates with the contractile performance of vascular SM both in vivo and in vitro [123–125]. Dystrophin is not expressed in cultured SMCs in the presence of serum [123], which causes SMC dedifferentiation. In vivo, dystrophin is found in the caveolar domain of SMCs outside integrin-containing adherens junctions [126–128].

In parallel to the DGPC in SMCs, there is another complex with utrophin [122], which has been studied to a lesser extent than the DGPC. Utrophin is ubiquitously expressed in cells, including vascular SMCs [125,129]; therefore, in contrast to dystrophin, it is not related to muscle contraction. The utrophin glycoprotein complex in SM is likely to include 100-kDa {alpha}-dystroglycan and β-dystroglycan [121,122]. In cultured aortic SMCs, utrophin co-localises with {alpha}-dystroglycan in the focal adhesions, containing β1 integrin and the cytoskeletal protein vinculin [130]. Recently a new dystrophin/utrophin-binding protein aciculin has been characterised in SM [131,132]. Aciculin (also called PGMRP) is expressed predominantly in vascular and visceral SM and to a lesser extent in cardiac muscle in humans [132]. Aciculin expression strongly correlates with the differentiated contractile SM phenotype [132,133]. Aciculin is likely to be involved in cell adhesion, as it co-localises with dystrophin, utrophin and {alpha}-dystroglycan in adhesion structures of muscle cells [130,131], but whether PGMRP has structural, signalling or other function is unknown.

The role of {alpha}-dystroglycan in SMC adhesion has not been studied, although the effect of other proteins of the DGPC on SM is known. In mdx mice, with the inactivated dystrophin gene, the mechanical activities in portal vein were severely disrupted [134]. Duchenne muscular dystrophy patients (with the mutated dystrophin gene) undergoing surgery tend to bleed more during surgery compared with other patients [135]. Mutations in the sarcoglycan genes in limb-girdle muscular dystrophy patients [107] lead to vascular SM dysfunction and an increased baseline of myocardial blood flow [136]. In both groups of patients vascular abnormalities are likely to be caused by weakness of vascular vessel walls and a poor vascular SM vasoconstrictive response due to mutations in the dystrophin and sarcoglycan genes.

Recently the importance of DGPC for vascular smooth muscle has been demonstrated directly in animal models with mutated DGPC components [120,137–139]. Mouse and BIO14.6 hamster animal models deficient in {delta}-sarcoglycan develop cardiac abnormalities caused by primary vascular dysfunction [137]. The {delta}-sarcoglycan-null mutation leads to irregularities of the coronary arteries, which trigger the onset of myocardial necrosis in mice [137,140]. The β-sarcoglycan-deficient mice develop vascular constrictions in the heart, diaphragm and kidneys, which result in muscular dystrophy and cardiomyopathy with focal areas of necrosis, associated with the disrupted DGPC in all muscle types [120]. Thus, mutations in β- and {delta}-sarcoglycans in mice perturb vascular function, which in turn initiates cardiomyopathy.


   4 Syndecans
Top
 Abstract
 1 Introduction
 2 SMC integrins
 3 Dystrophin-glycoprotein...
 4 Syndecans
5 Cell adhesion molecules
 6 Cadherins
 7 Closing remarks
 References
 
Syndecans are members of a proteoglycan family of adhesion transmembrane receptors with the glycosaminoglycan chains covalently attached to the extracellular part of the receptor (reviewed in [141,142]). There are four known mammalian syndecans. Syndecans bind to a variety of ECM proteins (Fig. 2A), cell adhesion molecules, growth factors, lipoproteins, lipoprotein lipases and components of the blood-coagulation cascade. To date, all known syndecans interact with their ligands via polysaccharide chains, although syndecan-4 may also support cell adhesion by a protein core [143]. These receptors play important roles in cell–ECM and cell–cell adhesion and migration (reviewed in [141,142,144]). Syndecans are thought to be co-receptors providing additional sites for ECM ligands in order to increase the strength of cell adhesion (Fig. 2A).

All syndecans bind to PDZ-containing proteins (the PDZ domain is called after proteins PSD-95/DlgA/ZO-1, where it was originally found) linking syndecans to the cytoskeleton [145]. Syndecan-4 is consistently found in focal adhesions of several cell types, including SMCs [146]. Recent data have implicated syndecans, and syndecan-4 in particular, in the assembly of focal adhesions and actin cytoskeleton [147–152]. Similar to integrins, syndecans are involved both in cytoskeleton and ECM assembly. Syndecan-2 affects the assembly of laminin and fibronectin fibrils and can override the function of activated integrins in ECM assembly [153]. Growing evidence suggests that syndecans are involved in cell signalling. Syndecan-4-mediated signalling involves protein kinase PKC{alpha} and phosphatidylinositol-4,5-biphosphate (PIP2) via direct binding of syndecan-4 to PKC{alpha} and PIP2 (reviewed in [145,154]).

An intriguing feature of syndecans is their involvement in other cellular functions, such as proliferation, growth factor signalling and lipid metabolism (reviewed in [141,142,144]). Syndecans bind to the heparin-binding growth factors, such as basic FGF, VEFG and EGF, and provide secondary binding sites for them on the cell surface. Therefore, syndecans may be involved in mediating growth factor-induced responses in vascular SM. Syndecans, in particular syndecan-1, may also participate in lipid metabolism, as syndecan-1 may mediate LDL receptor-independent clearance of lipoproteins via binding to lipoprotein lipase, an enzyme involved in triglyceride catabolism [155].

All four known syndecans are expressed in normal arterial SMCs [156]. Syndecan-1 is enriched in the whole artery, syndecans-2 and -4 are enriched in the medial layer. The arterial medial layer contains most of syndecan-1 and syndecan-4 mRNA. In contrast to syndecan-2 and -3, syndecan-1 and -4 are increased after arterial injury in animal models [156,157]. Since syndecans participate in the ECM assembly, they may be involved in the ECM repair after vascular injury. Syndecan-1, -2 and –4 are expressed in cultured vascular SMCs [156,158,159]. Expression of syndecan-1 and -4 varies in cultured SMCs in response to serum and growth factors, while syndecan-2 is expressed constitutively. The significance of syndecans in SMC adhesion and migration has not been studied.

Since syndecans bind to growth factors, lipoprotein lipases and components of the blood-coagulation cascade, they may have pleiotropic effects on SMC functioning. Thrombin treatment of vascular SMCs augments expression of syndecan-1, exhibiting high affinity antithrombin III-binding activity [160]. Therefore, SMCs could play an important role in controlling local thrombus formation subsequent to vascular injury, via a feedback mechanism that involves thrombin-induced stimulation of SMC surface syndecan-1 expression, which generates anticoagulant activity via binding of antithrombin III, an inhibitor of thrombin activity [160].


   5 Cell adhesion molecules
Top
 Abstract
 1 Introduction
 2 SMC integrins
 3 Dystrophin-glycoprotein...
 4 Syndecans
 5 Cell adhesion molecules
6 Cadherins
 7 Closing remarks
 References
 
Cell adhesion molecules belong to the immunoglobulin family of cell–cell adhesion receptors with a transmembrane domain. There is not much known about the link between these receptors and the actin cytoskeleton. Intercellular cell adhesion molecule (ICAM-1) is known to interact with the cytoskeletal proteins {alpha}-actinin and ezrin [161,162]. Two adhesion molecules VCAM-1 and ICAM-1 have been detected in vascular SM. VCAM-1 is a counter-receptor for the {alpha}4β1, {alpha}9β1, {alpha}4β7 and {alpha}Dβ2 integrins, present on lymphocytes and monocytes [54,163,164]. ICAM-1 interacts with β2 integrins, present on neutrophils, monocytes and lymphocytes [163,165]. ICAM-1 also binds to fibrinogen and, thereby, can participate in ECM–cell interactions [166–168] (Fig. 2C). The expression of VCAM-1 and ICAM-1 in vasculature and their potential roles in the development of atherosclerosis were thoroughly reviewed recently [169].

VCAM-1 is found in human foetal aortic SMCs expressing {alpha}4β1 integrin (see above), but not in the normal adult aortic media [38]. VCAM-1 expression in SMCs in atherosclerotic lesions is well-documented [170–172]. VCAM-1 is also upregulated in SMCs after vascular injury and co-expressed with monocyte chemotactic protein (MCP)-1 in SMCs within 4 h of injury [173]. Macrophage infiltration occurs in parallel with the expression of VCAM-1 and MCP-1; these inflammatory cells are present on the lumenal surface of injured vessels during intimal lesion formation [173]. Similar to VCAM-1, ICAM-1 is expressed in atherosclerotic SMCs, but not in normal contractile SMCs [172]. ICAM-1 is induced in SMCs of saphenous vein after culturing with serum or TNF-{alpha} ex vivo [174]. ICAM-1 expression in vitro is phenotype-dependent and can only be induced in SMCs with decreased or modulated myofilament volume [175]. In cultured SMCs expression of VCAM-1 and ICAM-1 can be induced by cytokines [170,176–178]. Increased surface expression of VCAM-1 and ICAM-1 on SMCs leads to augmented adhesion of {alpha}4 integrin-positive lymphocytes and β2 integrin-containing monocytes [178,179]. Since the expression of VCAM-1 and ICAM-1 can be induced by cytokines in SMCs, these adhesion molecules are likely to play important roles in the pathophysiology of inflammatory and immune processes in atherosclerosis by enabling leukocyte trafficking within atherosclerotic vascular tissue.

ICAM-1 expression of SMCs affects not only adhesion of monocytes, but also expression of tissue factor and procoagulant activity of monocytes [180]. Thereby, the interactions between monocytes and ICAM-1-expressing SMCs within atherosclerotic plaques would increase the thrombogenic properties of the plaques; in the case of plaque rupture it would lead to thrombus formation and SMC adhesion to fibrinogen via integrins and ICAM-1. Since ICAM-1 participates in cell signalling in other cell types, SMC adhesion to fibrinogen via ICAM-1 may lead to other events. ICAM-1 ligation induces VCAM-1 expression in ECs [181]. Interaction between ICAM-1 and fibrinogen results in proliferation and increased cell survival of non-SMCs [182,183]. Ligation of fibrinogen to ICAM-1 on the cell surface of ECs stimulates the tyrosine phosphorylation of several proteins involved in signalling [183,184]. As mentioned above, fibrinogen is abundant in the intima of atherosclerotic aorta, where SMCs express ICAM-1. The effect of fibrinogen on SMCs expressing ICAM-1 remains to be investigated.


   6 Cadherins
Top
 Abstract
 1 Introduction
 2 SMC integrins
 3 Dystrophin-glycoprotein...
 4 Syndecans
 5 Cell adhesion molecules
 6 Cadherins
7 Closing remarks
 References
 
All cadherins, except T-cadherin, belong to a family of transmembrane Ca2+-dependent homophilic cell–cell adhesion receptors. T-Cadherin is anchored to the cell membrane via glycosylphosphatidylinositol. N- and R-cadherins can also support heterophilic adhesion [185]. Cadherin homodimers form a ‘zipper’ structure at the intercellular contact zone on the cell surface [186]. These receptors bind to the cytoplasmic plaque proteins catenins, which provide a link with vinculin and the actin cytoskeleton. Cadherins are involved in signalling mediated by β-catenin (reviewed in [186,187]). Expression of N-, R- and T-cadherins has been reported in normal vascular SMCs [188–192]. Cadherin 6B, expressed in chicken aorta, is characterised as a specific marker for differentiated SM phenotype [193].

N-Cadherin is expressed at a higher level in human venous SM compared to arterial SM [194]. In rat aorta N-cadherin is found in a population of SMCs adjacent to ECs [188]. An anti-N-cadherin antibody inhibits adhesion between SMCs and ECs in a Ca2+-dependent manner, indicating that N-cadherin may mediate EC–SMC interactions in vivo [188]. However, as ECs are separated from the SM layer by the basement membrane, they can only be in contact with SMCs after vascular injury and in ulcerated atherosclerotic plaques.

T-Cadherin was originally identified as a novel lipoprotein-binding protein in a membrane fraction of aortic SMCs [190]. T-Cadherin amounts are significantly higher in fatty streaks and aortic fibrous plaque tissue compared to normal aorta [195]. The expression of T-cadherin is high in vascular SMCs, but reduced by 75% in culture [194,195]. T-Cadherin expression is cell density dependent with the highest level in confluent cells and is downregulated by serum and growth factors [196,197].

In contrast to other cadherins, E-cadherin is not found in normal vascular SMCs, but the majority of atherosclerotic lesions contain cells positively stained for E-cadherin [198]. In atherosclerotic intima, E-cadherin is expressed by intimal cells showing varying degrees of transformation into foam cells [198]. After balloon injury an increased level of E-cadherin coincides with the initiation of SMC proliferation, whereas the levels of R- and T-cadherins temporarily decrease [192].

Since normal contractile SMCs are surrounded by ECM, SMC adhesion to ECM is predominant in vascular SM, although cell–cell contacts may increase during tissue remodelling or in vascular injury, when the basement membranes, underlying ECs and around SMCs, are degraded or damaged. Cadherin expression in adult vascular SMCs seems to be to some extent redundant, however, in other cell types E- and N-cadherins are implicated in contact inhibition of growth [199,200]. Furthermore, E-cadherin is also implicated in a physiological pathway required for cell survival [201]. Therefore, the significance of cadherin interactions in normal and especially pathological conditions, such as vascular injury with induced ECM rearrangement, SMC migration and proliferation, requires further investigation.


   7 Closing remarks
Top
 Abstract
 1 Introduction
 2 SMC integrins
 3 Dystrophin-glycoprotein...
 4 Syndecans
 5 Cell adhesion molecules
 6 Cadherins
 7 Closing remarks
References
 
The data, summarised in this review, demonstrate the fundamental importance of cell adhesion receptors in the physiology of vascular SM. SMCs use a range of adhesion receptors involved in diverse biological functions of SMCs under normal and pathological conditions, such as vascular injury and atherosclerosis (summarised in Table 2).


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  		Table 2 Involvement of adhesion receptors in smooth muscle functions in atherosclerosis and vascular injury

 
A growing body of evidence indicates that cell adhesion, spreading, migration and ECM assembly involve several types of adhesion receptors. Some types of adhesion receptors can regulate other types and/or may require co-operation with other types of adhesion receptors. Some integrins can regulate expression and activity of other integrins [45,67] and N-cadherin [202] or require co-operation with other integrins [77] and syndecans [150,152,153]. Syndecans are known to affect both integrins and cadherins [203]. ICAM-1 ligation induces expression of the other adhesion receptor VCAM-1 [181]. Such interactions between adhesion receptors in SM have not been studied; nevertheless they may be significant for the co-ordinated cell–cell and cell–ECM adhesion during vascular injury and/or in development of vascular diseases.

In conclusion, the following points should be emphasised

1. Laminin, found in the basement membrane around SMCs, plays the pivotal role in SMC differentiation. Adhesion receptors to laminin, integrins {alpha}1β1 and {alpha}7β1 and {alpha}-dystroglycan in the DGPC, are expressed only in contractile SMCs. While the adhesion receptor responsible for maintaining the SM contractile phenotype has not been identified, {alpha}-dystroglycan is indispensable for the laminin assembly.
2. Integrins {alpha}1β1 and {alpha}5β1 are likely to participate in collagen remodelling and fibrin clot contraction, respectively, after vascular injury. ECM remodelling activities of these integrins within the blood vessel wall may cause blood vessel narrowing.
3. Although integrin {alpha}vβ3 is expressed both in normal and diseased blood vessels, some ECM ligands of {alpha}vβ3 (thrombospondin, vitronectin, osteopontin, fibrin and fibrinogen) are found only in injured or diseased blood vessels. Furthermore, this integrin is also involved in signalling. For these reasons, SMC adhesion via {alpha}vβ3 may have a profound effect on many other SMC functions, such as migration, proliferation and apoptosis in vascular injury. Blocking of the integrin {alpha}vβ3 with products of degradation of ECM proteins may contribute to apoptotic process within the atherosclerotic tissue and affect plaque stability. The adhesion and signalling properties of {alpha}vβ3 integrin ensure its unique role in pathophysiological conditions.
4. Adhesion receptors ICAM-1, VCAM-1 and integrin {alpha}4β1 are expressed in adult vascular SM only in pathological conditions. These receptors may participate in SMC interactions with ECs and leukocytes. ICAM-1 and VCAM-1 expressed on SMC surface may contribute to inflammatory and immune cell trafficking within atherosclerotic tissue. ICAM-1 expression in SMCs may also increase thrombogenic properties of atherosclerotic plaques due to the increased production of tissue factor in monocytes.
5. Some adhesion receptors, such as cadherins and syndecans, may have important additional roles, e.g. syndecan-1 and T-cadherin in lipid metabolism, syndecan-1 in anticoagulant activity.

It is becoming clear that cell adhesion is a complex process with many players. The research on the role of integrins in SMC adhesion has dominated the field. There is an urgent need to accelerate research on other SM adhesion receptors, especially ECM receptors syndecans and {alpha}-dystroglycan, and the interactions between various types of receptors in order to understand in depth the functioning of vascular SM in norm and disease.

Time for primary review 28 days.


   Acknowledgements
 
The author would like to warmly thank Professor A.C. Newby for helpful discussion and critical comments on the manuscript. The author would like to thank E. Coates and departmental colleagues for proof-reading of the manuscript. E.P. Moiseeva’s work is funded by the grant PG/99138 from British Heart Foundation.


   References
Top
 Abstract
 1 Introduction
 2 SMC integrins
 3 Dystrophin-glycoprotein...
 4 Syndecans
 5 Cell adhesion molecules
 6 Cadherins
 7 Closing remarks
 References
 

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