Smooth Muscle Cell-Proteoglycan-Lipoprotein Interactions as Drivers of Atherosclerosis

In humans, smooth muscle cells (SMCs) are the main cell type in the artery medial layer, in pre-atherosclerotic diffuse thickening of the intima, and in all stages of atherosclerotic lesion development. SMCs secrete the proteoglycans responsible for the initial binding and retention of atherogenic lipoproteins in the artery intima, with this retention driving foam cell formation and subsequent stages of atherosclerosis. In this chapter we review current knowledge of the extracellular matrix generated by SMCs in medial and intimal arterial layers, their relationship to atherosclerotic lesion development and stabilization, how these findings correlate with mouse models of atherosclerosis, and potential therapies aimed at targeting the SMC matrix-lipoprotein interaction for atherosclerosis prevention.

1. Introduction

Smooth muscle is the involuntary nonskeletal form of muscle cells, present in hollow organs such as the stomach, intestine, bladder and uterus, the respiratory tract, and arteries and veins of the circulatory system. Smooth muscle cells (SMCs) in the artery muscular or “medial” layer contract or relax to redistribute blood throughout the body and to regulate local blood pressure and blood vessel volume. In addition, in humans SMCs are the predominant cell type in the pre-atherosclerotic intima and all stages of atherosclerotic lesion development (Allahverdian et al. 2018). In the intimal layer, SMCs take on alternate phenotypes and roles including increased proliferation, increased production of matrix proteoglycans, uptake of lipoproteins and foam cell formation, loss of SMC markers and expression of macrophage markers, and stabilization of arteries against plaque rupture. In this chapter we review extracellular matrix (ECM) production by SMCs and its critical role in mediating the lipoprotein retention that drives atherosclerotic lesion development. We correlate these findings in humans with mouse models of atherosclerosis and how modification of the SMC-derived proteoglycan-lipoprotein interaction might be targeted as a means of preventing or treating atherosclerosis.

2. Smooth Muscle Cell Phenotype Switch and Extracellular Matrix Production

Vascular smooth muscle cells (SMCs), unlike skeletal and heart muscle cells, are highly plastic and show a spectrum of phenotypes in the medial and intimal artery layers, with contractile and synthetic being the extremes (Owens et al. 2004). SMCs switch their phenotypes depending on genetic preprogramming and local environmental signals including cytokines and mechanical forces. The majority of SMCs constituting the medial layer of healthy arteries exhibit a contractile phenotype with high levels of cytoplasmic contractile myofilaments, a low proliferation rate, and low protein synthesis. On the other end of the spectrum, intimal SMCs can exhibit either a contractile phenotype or a dedifferentiated, synthetic phenotype with reduced expression of contractile proteins and other differentiated SMC markers and high rates of proliferation and protein synthesis. The majority of SMCs in the intima are of synthetic phenotype. SMCs forming the fibrous cap over atheromas, however, retain SMC markers including α-SMA and show a more differentiated, contractile phenotype (Feil et al. 2014; Perisic Matic et al. 2016; Albarran-Juarez et al. 2016; Chappell et al. 2016). Although transdifferentiation of SMCs with partial or total loss of SMC markers and gain of some macrophage markers has been reported in both human and mouse atherosclerotic lesions, detailed studies have confirmed SMCs maintain their characteristics and do not function as macrophages (Feil et al. 2014; Allahverdian et al. 2014; Shankman et al. 2015; Vengrenyuk et al. 2015).

As the major cell type of the pre-atherosclerotic and atherosclerotic intima, the phenotypic characteristics and products generated by synthetic phenotype SMCs are key determinants of atherosclerotic lesion development (Nakashima et al. 2008). Some of these SMC products are components of the ECM (Wight 2018). ECM is a component of all tissues and organs including blood vessels that provides physical strength to the tissue and acts as a framework in which cells and other molecules are embedded and bound. In arteries, the composition of ECM varies in the intima, media, and adventitia, between healthy and atherosclerotic arteries, and with different stages of atherosclerosis. In particular, the marked increase in proteoglycan content of ECM in the pre-atherosclerotic diffuse intimal thickening (DIT) stage of intima development creates the conditions necessary for the initial retention of serum lipoproteins and initiation of foam cell and atherosclerotic lesion formation (Nakashima et al. 2008; Wight 2018).

3. Extracellular Matrix (ECM)

Extracellular matrix (ECM) is a non-cellular, well-organized, and highly dynamic three-dimensional network in which cells reside and function. ECM is comprised of both fibers and a non-fibrillar amorphous gelatinous material called ground substance. Fibrous protein components of ECM include collagens, elastin, fibronectin (FN), and laminins. The non-fibrillar substance consists of proteoglycans (including versican, perlecan, biglycan, and decorin), along with glycoproteins and hyaluronan. Proteoglycans consist of long chains of repeating disaccharides attached to a core protein, while glycoproteins are composed of short, highly branched chains of various monosaccharides with no repeating unit attached to a core protein (Fig. 1). Hyaluronan is a glycosaminoglycan molecule that has no protein core but forms a non-covalent complex with proteoglycans in the ECM.

Fig. 1

Model structures of proteoglycans and glycoproteins. (a) Proteoglycans consist of a core protein (green line) with glycosaminoglycans (GAGs, blue beaded string), consisting of repeating disaccharide units of various lengths that are attached via O-glycosidic (more...)

ECM components interact with resident cells through their cell surface receptors and regulate their morphology as well as functions such as growth, migration, differentiation, and survival (Theocharis et al. 2016). The composition and structure of ECM varies from tissue to tissue, and alteration of this composition changes the overall structure of the tissue and the function and properties of the embedded cells. Moreover, growth factors, cytokines, and chemokines that bind to ECM molecules are released in response to different stimuli and modify local cellular function (Theocharis et al. 2016). All cell types synthesize and secrete ECM, with alteration of cellular properties due to physiological and pathologic conditions modulating the composition of ECM produced. In the following sections, we focus primarily on ECM produced by, and affecting the function of, arterial SMCs.

3.2. Proteoglycans: Non-fibrillar Components of the ECM

Proteoglycans are complex macromolecules consisting of a core protein (e.g., versican; the entire proteoglycan molecule is named according to its core protein) to which one or more glycosaminoglycan molecules (GAGs) are covalently attached (Wight 2018) (Fig. 1). GAGs are linear negatively charged polysaccharides of varying length made up of repeating disaccharide units. There are six types of GAGs: chondroitin sulfate (CS), dermatan sulfate (DS), heparan sulfate (HS), heparin, keratan sulfate (KS), and hyaluronic acid (HA; also called hyaluronan). CS, DS, HS, and KS are linked to the different core proteins through O-glycosidic linkage to make distinct PG families: CSPGs, DSPGs, HSPGs, and KSPGs. Binding of more than one type of GAG to a single core protein creates various PG subtypes. Hyaluronan is the only GAG that is synthesized at the cell membrane and not in the Golgi apparatus, and is only present in a protein-free form. Proteoglycans are abundant in the ECM and are also present on the cell surface and intracellularly. They interact with growth factors, cytokines, cell surface receptors, and other ECM molecules, through which they participate in multiple cellular functions including signaling, proliferation, migration, adhesion, differentiation, and apoptosis (Wight 2018; Theocharis et al. 2016; Yue 2014). Almost all cell types in the vessel wall can synthesize PGs, with SMCs being the major contributor. Proteoglycan content of the normal vessel wall is low, only 4% of total ECM, but it increases dramatically up to 50% in early atherosclerosis and 20% in late lesions (Wight 2018). There are more than 20 different PGs in blood vessels. As discussed below, the negative charge of PGs plays a critical role in the binding and retention of positively charged lipoproteins in the artery intima, a pivotal step in atherosclerotic lesion initiation. Key features of PGs involved in atherosclerotic lesion development are discussed here and summarized in Table 1.

Table 1

Summary of the various proteoglycans found in the arterial wall, their respective GAG chains, location, and their key role in atherogenesis and vascular homeostasis

3.2.1. Versican

Versican is a large aggregating CSPG (Wight 2018) that exists in at least four different isoforms, created by alternative splicing of mRNA from a single gene (V0–V3). These variants differ by the length of their core proteins and the number of GAGs attached. Versican forms complexes with hyaluronan, a long-chain GAG, to make high molecular weight aggregates. These aggregates provide a swelling pressure that resists tissue shrinkage and also a viscoelastic pericellular matrix that allows SMCs to change their shape to prepare them for proliferation and migration (Wight and Merrilees 2004). Immunostaining of the ECM of human coronary arteries showed the presence of versican in all layers of the artery wall but increased in areas of atherosclerotic arteries with high SMC content such as diffuse intimal thickening (DIT), fibrous cap, and the edge of the lipid core in advanced lesions (Gutierrez et al. 1997; Kolodgie et al. 2002). SMCs are the main source of versican, and its expression is upregulated by factors including platelet-derived growth factor (PDGF) and transforming growth factor-β1 (TGF-β1). Both PDGF and TGF-β1 increase the number of versican transcripts and elongation of CS chains in cultured SMCs (Schonherr et al. 1991).

3.2.2. Biglycan

The small leucine-rich proteoglycan (SLRP) family includes small core proteins (biglycan, decorin) containing a number of leucine-rich repeats (LRRs) that can have CS, DS, or KS side chains (Wight 2018). Biglycan, together with decorin, belongs to the same class of SLRPs due to their homologous genes and proteins and the presence of ten LRRs sandwiched between cysteine-rich regions (Gutierrez et al. 1997). The N-terminal domain of biglycan typically contains two CS/DS side chains (Iozzo 1999) or two DSPG chains (O’Brien et al. 1998; Gutierrez et al. 1997). Biglycan is the primary proteoglycan found in the intima of normal and atherosclerotic human arteries (Little et al. 2008) and is enriched in areas such as DIT (discussed further in Sect. 4), the plaque core, and fibrous cap (Gutierrez et al. 1997). The presence of growth factors such as PDGF and TGF-β1 transforms biglycan into “hyperelongated biglycan,” which has more sulfate incorporated, conveying higher negative charge and longer GAG chains (Schonherr et al. 1993), and therefore increased binding capacity to low density lipoprotein (LDL) and other apolipoprotein B (apoB)-containing lipoproteins (Little et al. 2002). Biglycan concentrated in the deepest part of DIT colocalizes strongly with apoB in early and advancing fatty streaks and plays a critical role in mediating lipoprotein retention in the artery wall (outlined further in Sect. 4 below).

3.2.3. Decorin

Decorin is another member of the SLRP family (Iozzo 1999). Decorin’s N-terminus has a single CS/DS chain (Wight 2018). Decorin is expressed in the adventitia of normal and atherosclerotic arteries (Gutierrez et al. 1997). Decorin is also found, although to a somewhat lesser extent than biglycan, in the deep (outer) intima of coronary arteries in early stages of atherosclerosis, suggesting it also has a role in lipoprotein retention. It is also often found in association with collagen fibers in atherosclerotic plaque and may have a role in plaque stability by increasing collagen fibrillogenesis (Wight 2018).

3.2.4. Perlecan

Perlecan, the most abundant HSPG in the mouse artery wall (Nakashima et al. 2008), is located in the basement membrane between the endothelial cells and intimal SMCs (Tran-Lundmark et al. 2008). Perlecan functions as a permeability barrier but also facilitates lipoprotein retention (Tran-Lundmark et al. 2008). Mice that were deficient in both HS-perlecan and apoE were shown to have higher flux but lower arterial retention of lipoproteins compared to apoE-single null mice (Tran-Lundmark et al. 2008). Human atherosclerotic lesions have low expression of perlecan, and the role of perlecan in development of human atherosclerosis is not clear.

4. Interactions Between SMCs and the ECM

In addition to SMCs being the primary source of proteoglycans and other ECM components in arteries, the composition and organization of ECM itself plays an important role in the regulation of SMC phenotype, i.e., the interaction between SMCs and ECM is bidirectional. Integrins, transmembrane receptors linking the intracellular SMC cytoskeleton to the ECM, play a critical role in this bidirectional signaling across the plasma membrane and regulation of SMC phenotype by ECM components (Finney et al. 2017; Adiguzel et al. 2009).

Fibrillar collagen I, a major component of ECM in the medial layer, induces a contractile and quiescent phenotype in human SMCs grown in culture, whereas monomeric (non-fibrillar) collagen type I, present in atherosclerotic plaque intima, enhances SMC proliferation, a characteristic feature of synthetic phenotype (Ichii et al. 2001; Koohestani et al. 2013; Yeh et al. 2012). This phenotypic shift was accompanied by alteration of gene expression in cultured human SMCs induced by polymerized compared to monomeric collagens (Ichii et al. 2001). Fibrillar collagen type I has also been shown to inhibit PDGF-induced proliferation and migration of SMCs (Raines et al. 2000). Collagen type IV and laminin, components of the basement membrane that surrounds arterial SMCs in the subendothelial space, induce a differentiated contractile phenotype in vascular SMCs grown in culture and diminish the synthetic phenotype induced by PDGF (Thyberg and Hultgardh-Nilsson 1994). In the same manner, integrins α1β1 and α7β1, which bind to collagen IV and laminin, respectively, are highly expressed in quiescent contractile SMCs and serve to maintain the spindle morphology and contractile phenotype of SMCs (Finney et al. 2017). Fibronectin, a major component of provisional matrix in the intima, induces both contractile and synthetic phenotype in vascular SMCs depending on the nature of integrins expressed by the local SMCs (Finney et al. 2017). Altogether various combinations of integrins expressed by SMCs and a variety of ECM components result in a complex SMC-ECM interaction that affects both SMC and ECM properties.

Smooth muscle cells also interact with non-fibrillar PG components of the ECM. Bingley et al. found that HSPGs extracted from rabbit aortae inhibit phenotypic change of the cultured SMCs and maintain them in a quiescent state (Bingley et al. 1998). Interestingly, they found that when applied in a periadventitial gel following injury of rabbit carotid artery, HSPGs inhibit neointima formation (Bingley et al. 1998). Different subtypes of versican have been shown to alter SMC phenotype differently (Wight et al. 2014). Hyaluronan, present in both early and advanced lesions (Krolikoski et al. 2019), enhances proliferation and migration of SMCs (Evanko et al. 1999) and when overexpressed in SMCs of apoE-deficient mice accelerates the progression of atherosclerosis (Chai et al. 2005).

5. SMC, ECM, and Lipoprotein Interactions in Diffuse Intimal Thickening (DIT)

Diffuse intimal thickening (DIT) is a layer of thickened intima composed of SMCs, elastin, and PGs. DIT is initiated in utero and is present in all humans in atherosclerosis-prone arteries such as the coronary arteries and abdominal aorta by the age of 2 years (Nakashima et al. 2008). DIT represents a pre-atherosclerotic stage that provides the “soil” for the initial deposition and retention of lipoproteins and subsequent stages of atherosclerosis (Nakashima et al. 2008; Dubland and Francis 2016). Versican and biglycan are the most abundant proteoglycans in this DIT layer of atherosclerosis-prone arteries, while the thin intimal layer of atherosclerosis-resistant arteries, such as the internal thoracic artery, is enriched in decorin (Merrilees et al. 2001).

Using immunohistochemical techniques, Nakashima et al. showed that biglycan is localized in the deeper region of DIT in human coronary arteries (Nakashima et al. 2007). Lipoproteins that diffuse in from the plasma are trapped and deposited in the deep intima in the early stages of atherosclerosis due to a charge-charge interaction between this proteoglycan and apolipoprotein B of the lipoproteins. In an in vitro model, overexpression of biglycan in rat vascular SMCs increased lipoprotein binding to matrix formed by the cultured cells, while expression of a GAG-deficient mutated biglycan blocked this binding and accumulation (O’Brien et al. 2004). This observation and colocalization of biglycan with apoB- and apoE-containing lipoproteins in both early and advanced human (Nakashima et al. 2007; O’Brien et al. 2004) and murine lesions (Kunjathoor et al. 2002) indicate the presence of biglycan is crucial for the initial and ongoing deposition of lipids in the artery wall.

Unlike biglycan, immunohistochemical studies show versican is predominantly localized in the superficial part of the thickened intima of coronary arteries (Merrilees et al. 2001). Although in vitro studies have shown versican is capable of binding to LDL and has a larger number of binding sites for LDL compared to biglycan (Camejo et al. 1988), it has not been detected in lipoprotein-rich areas of atherosclerotic lesions nor colocalized with apoE or apoB epitopes in either human or mouse atherosclerotic lesions (Wight and Merrilees 2004; Kunjathoor et al. 2002).

Decorin, like biglycan, is distributed in the deep intima although at lower levels and colocalizes with lipids with less intensity compared to biglycan (Nakashima et al. 2007; Otsuka et al. 2015). In vitro studies have shown decorin links LDL with collagen type I (Pentikainen et al. 1997), and in vivo studies have demonstrated colocalization of decorin with collagens and apoB in atherosclerotic lesions (Nakashima et al. 2007; Riessen et al. 1994). Moreover, overexpression of decorin in rat SMCs after balloon injury of the carotid arteries resulted in compact collagen-rich lesions with reduced intimal thickness (Fischer et al. 2000). These findings suggest that decorin has a primary role in collagen organization in atherogenesis and a secondary role in lipid retention.

Perlecan is a heparan sulfate proteoglycan and the most abundant PG in the mouse artery wall, present in basement membranes in the intima and media (Nakashima et al. 2008). Human atherosclerotic lesions express only low amounts of perlecan (Tran et al. 2007). Like biglycan in humans, perlecan is involved in lipid retention in mouse atherosclerosis (Kunjathoor et al. 2002). Vikramadithyan et al. have shown that heterozygous deficiency of perlecan in chow-fed apoE-deficient mice results in more than 70% reduction in early-stage atherosclerosis lesion size. However, these authors did not find any difference in lesion size in advanced atherosclerosis (Vikramadithyan et al. 2004). Tran-Lundmark et al. also showed more than 50% reduction in lesion size in HS-deficient perlecan apoE^−/− mice compared to control apoE^−/− mice (Tran-Lundmark et al. 2008).

The expression of key artery wall ECM components and their roles including lipoprotein binding and fibrous cap formation are indicated in Fig. 2.

Fig. 2

Distribution of major ECM components in the normal artery wall and different stages of atherosclerosis. Composition of ECM varies in different layers of the artery wall and at different stages of atherosclerosis. (a) Collagen I and decorin in the adventitia, (more...)

6. SMC-ECM Interaction in Lipid Retention

The ability of SMC-generated PGs in the human arterial intima to bind atherogenic apoB-containing lipoproteins mediates what is considered to be the critical and rate-limiting step in atherogenesis, from which subsequent steps of atheroma development ensue. This is the so-called “response-to-retention” hypothesis of atherosclerosis (Williams and Tabas 1995). The interaction between atherogenic lipoproteins and PGs involves an ionic or electrostatic interaction between positively charged amino acids of aopB on lipoproteins and negatively charged sulfate groups on the PGs generated by SMCs (Camejo et al. 1998; Boren et al. 1998). The role of deposition of lipoproteins in the intimal space and specific interactions of SMC-ECM molecules and apolipoproteins that mediate retention of lipoproteins are outlined below.

7. Role of ECM Proteoglycans in Early Stages of Atherosclerosis Development in Animal Models

A key question, raised by the apparent pivotal importance of SMCs in producing the lipoprotein-binding PGs that drive lipoprotein retention and atherogenesis in humans, is how important this is in mice, the primary models used to study the determinants of atherosclerosis in man. Although small animals do not develop a pre-atherosclerotic SMC DIT layer like humans (Nakashima et al. 2008), several studies provide evidence that the initial binding of apoB-containing lipoproteins to subendothelial PGs in the artery wall is also a key step in atherogenesis in animal models of atherosclerosis. Perlecan and biglycan, but not versican, a major PG detected in human plaques, appear early in developing lesions in apoE^−/− and LDLR^−/− mice underneath the endothelium and in the absence of intimal thickening (Kunjathoor et al. 2002). The presence of these PGs around areas of extracellular lipid deposition suggests that they also contribute to lipoprotein retention in murine models of atherosclerosis. This supports the theory of PG-dependent lipoprotein retention, although these PGs are likely secreted by endothelial cells or activated macrophages rather than SMCs in the first stage of mouse atherosclerosis (Kinsella et al. 1997; Nugent et al. 2000; Chang et al. 2012). In intermediate and advanced lesions, PGs and lipoproteins are localized in SMC-rich areas of the mouse intima (Kunjathoor et al. 2002). In another study, Thompson et al. found that overexpression of biglycan by SMCs in LDLR^−/− mice increases lipid retention and atherosclerosis development (Thompson et al. 2014). Moreover, apoE^−/− mice heterozygous for deletion of perlecan have less atherosclerotic lesion area at early stages compared to apoE^−/− mice with no deletion. This might be attributed partly to decreased lipoprotein retention in the perlecan^+/− mice (Vikramadithyan et al. 2004). In another study, Skalen et al. used transgenic mice expressing recombinant LDL with proteoglycan binding site-defective apoB and showed those mice develop significantly less atherosclerosis compared to mice expressing wild-type control LDL (Skalen et al. 2002). These studies demonstrate a key role for PGs for initial subendothelial lipid retention at early stages of mouse atherosclerosis and a likely role for SMC-derived PGs in later stages of murine models of this disease.

8. The Role of the ECM-SMC Interaction in Plaque Stability and Rupture

Stable plaque is defined by the presence of a thick fibrous cap lying between the necrotic core and luminal surface of the plaque. It consists of SMCs embedded in a collagen-proteoglycan matrix with varying degrees of macrophage and lymphocyte infiltration and has a critical role in maintaining integrity of the plaque. Unstable plaques are prone to erosion and rupture that leads to platelet adhesion and luminal thrombus formation, the most common cause of acute coronary syndromes and sudden cardiac death (Allahverdian et al. 2018). Plaque rupture involves breakage of a large lipid/necrotic core with large luminal thrombus, thin fibrous cap, and a dense infiltration of inflammatory cells (Kolodgie et al. 2004).

Extracellular matrix components of atherosclerotic lesions have a key role in maintaining stability of plaques and accumulate in topographically distinct patterns in different plaque types. Kolodgie et al. found that the fibrous caps of stable lesions show increased amounts of versican, biglycan, and collagen type I staining and significantly less decorin and hyaluronan (Kolodgie et al. 2002). With SMCs being the main producer of collagen and PGs in the fibrous matrix (Pietila and Nikkari 1983), it is not surprising that the number of SMCs in fibrous caps is directly correlated with plaque stability (Allahverdian et al. 2018). One study found collagen VIII deficiency in apoE^−/− mice resulted in thinning of the fibrous cap, which suggests that this collagen plays an important role in protecting the plaque from rupture (Lopes et al. 2013).

Eroded plaques show an intense staining for hyaluronan, versican, and collagen type III at the plaque/thrombus interface and weak staining for biglycan and decorin. Cell surface receptor for hyaluronan, CD44, has been shown to be expressed by a subset of SMCs at the plaque/thrombus interface in erosions (Kolodgie et al. 2002) and mediates adhesion of platelets to hyaluronan (Day 1999). Exposure of versican-hyaluronan to flowing blood at the de-endothelialized surface of the eroded plaque promotes platelet attachment and may play an important role in the development of thrombosis via a CD44-dependent mechanism (Koshiishi et al. 1994).

Fibrous caps owe their strength and resistance to rupture to their collagen fiber content and therefore the content and synthetic capacity of collagen-producing SMCs (Doran et al. 2008; Libby 2008). Thinned fibrous cap areas contain very few SMCs with little to absent staining for PGs or hyaluronan and scattered strands of collagen type I (Kolodgie et al. 2002). Factors that alter the abundance or function of SMCs thereby alter ECM production by SMCs and plaque stability.

9. Therapeutic Targets and Interventions

The critical role of SMC-dependent PG synthesis and lipoprotein retention in subsequent atherosclerosis development makes the PG-lipoprotein interaction a potentially profound and novel point of therapeutic intervention to prevent ischemic vascular disease. Vazquez and colleagues introduced the chP3R99 monoclonal antibody (mAb) against ECM GAGs as a potential therapeutic for atherosclerosis in 2012 (Soto et al. 2012). The mAb binds to sulfated GAGs such as heparin, HS, and DS and had the strongest affinity for CS in aorta (Soto et al. 2012). It prevented 70% of LDL-CS complex formation and abolished 80% of LDL oxidation in vitro (Soto et al. 2012). The mAb also demonstrated the capacity to (1) prevent foam cell formation by the inhibition of LDL retention and oxidation in the arteries of rats; (2) decrease the intima-media ratio, a measure of DIT formation; and (3) decrease macrophage infiltration in the aorta of New Zealand white rabbits (Soto et al. 2012). Thereafter, they delivered the mAb chP3R99 via subcutaneous injections to apoE^−/− mice fed a high-fat and high-cholesterol diet and determined that the vaccination stimulated an immune response against various sulfated GAGs via the induction of a secondary anti-idiotype response (Ab2) that mirrors GAG antigenic determinants (Brito et al. 2012). The Ab2 gave rise to synthesis of anti-idiotype antibodies (Ab3) that have a specificity homologous to that of Ab1 (Brito et al. 2012). Further research using chP3R99 in the same mouse model established the chP3R99-LALA variant restricted the progression of atherosclerosis in apoE^−/− mice by preventing initial CS-LDL complex formation (Delgado-Roche et al. 2015). Both the efficiency of the chP3R99-LALA mAb in initiating the anti-idiotype cascade and anti-atherogenic benefits were dose-dependent in apoE^−/− mice but were independent of the sex and age of the mice (Sarduy et al. 2017). The mAb was also shown to decrease inflammation and halt lesion development of advanced stages of atherosclerosis in male apoE^−/− mice (Brito et al. 2017). These findings strongly confirm the relevance of the “response-to-retention” hypothesis of atherogenesis and provide the exciting possibility of developing an immunization that could block the PG-lipoprotein interaction lifelong as a novel means of preventing atherosclerosis.

10. Conclusion

Smooth muscle cells (SMCs) are the most abundant cell type in the healthy human artery wall as well as the pre-atherosclerotic and atherosclerotic intimal layer. The ECM components secreted by SMCs are critical factors in maintaining normal arterial architecture and stability but also in predicting propensity to atherosclerosis. In particular, the secretion of PGs by intimal SMCs promotes the initial and ongoing binding and retention of atherogenic lipoproteins in the artery wall. This retention of lipoproteins is necessary for further development of atherosclerosis, apparently in mice as well as in humans, making the proteoglycan-lipoprotein interaction a novel and exciting potential point of intervention for the future treatment and prevention of atherosclerosis.

Acknowledgments

This work is supported by a Heart and Stroke Foundation Grant-In-Aid to GAF.

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