Plaque rupture in humans and mice

Human paraoxonase gene cluster transgenic overexpression represses atherogenesis and promotes atherosclerotic plaque stability in ApoE-null mice

The paraoxonase (PON) gene cluster consists of the PON1, PON2, and PON3 genes, each of which can individually inhibit atherogenesis. To analyze the functions of the PON gene cluster (PC) in atherogenesis and plaque stability, human PC transgenic (Tg) mice were generated using bacterial artificial chromosome. The high-density lipoprotein from Tg mice exhibited increased paraoxonase activity. When crossed to the ApoE-null background and challenged by high-fat diet, PC Tg/ApoE-null mice formed significantly fewer atherosclerotic lesions. However overexpression of the PC transgene had no additive effect on atherosclerosis compared to the overexpression of the single PON1 or PON3 transgene. Plaques from PC Tg/ApoE-null mice exhibited increased levels of collagen and smooth muscle cells, and reduced levels of macrophages and lipid, compared with those from ApoE-null mice, indicating lesions of PC Tg/ApoE-null mice had characteristics of more stable plaques than those of ApoE-null mice. PC transgene enhanced high-density lipoprotein ability to protect low-density lipoprotein against oxidation in vitro. Serum intercellular adhesion molecule-1 and monocyte chemoattractant protein-1 were also repressed by PC transgene. Proatherogenic reactions of Tg mouse peritoneal macrophages induced by oxidized low-density lipoprotein were inhibited by PC transgene, as indicated by reduced reactive oxygen species generation, inflammation, matrix metalloproteinase-9 expression, and foam cell formation. Our results demonstrate that the PC transgene not only represses atherogenesis but also promotes atherosclerotic plaque stability in vivo. PC may therefore be a useful target for atherosclerosis treatment.

Vulnerable plaque in a Swine model of carotid atherosclerosis

BACKGROUND AND PURPOSE

Distal embolism and acute thrombosis due to rupture of a vulnerable atherosclerotic plaque are the common mechanisms of stroke in patients with carotid disease. The purpose of this study was to develop the first animal model of vulnerable carotid atherosclerotic plaque.

MATERIALS AND METHODS

Carotid atherosclerotic models were created in 12 Yucatan minipigs by using a combination of partial ligation and high cholesterol diet. Retia mirabilia from these animals were examined histopathologically to identify distal embolism. The association of distal embolism with advanced atherosclerosis and a thin fibrous cap was analyzed by using the Fisher exact test.

RESULTS

Typical features of vulnerable plaques, including a thin fibrous cap, necrotic core, and intraplaque hemorrhage, were observed in this swine model of carotid atherosclerosis. Distal embolism was detected in retia mirabilia supplied by 7 of 10 carotid arteries with advanced atherosclerotic plaques, compared with 3 of 14 carotid arteries without advanced plaque (P < .05).

CONCLUSIONS

This swine model of carotid atherosclerosis contains the salient features of vulnerable plaques, including plaque rupture and distal embolism.

Monocytes in acute coronary syndromes

The aim of this overview is to summarize the available data on the involvement of monocytes in the pathological processes related to the development of acute coronary syndromes and the recovery of damaged areas, the prevention of excessive inflammatory and procoagulant response, and the restoration of microcirculation (angiogenesis).

Genetic regulation of atherosclerotic plaque size and morphology in the innominate artery of hyperlipidemic mice

OBJECTIVE

We sought to determine the genetic factors contributing to atherosclerotic plaque size and cellular composition in the innominate artery, a murine model of advanced atherosclerosis.

METHODS AND RESULTS

We examined genetic contributions to innominate atherosclerotic plaque size and cellular composition in an intercross between C57BL/6J.Apoe(-/-), a strain susceptible to aortic lesions, and C3H/HeJ.Apoe(-/-), a strain resistant to aortic lesions. Surprisingly, total innominate lesion size was similar in the two strains. Genetic analyses identified one novel locus on Chromosome 2 for innominate artery lesion size, a significant locus for fibrous cap thickness on Chromosome 15, and several suggestive loci for cellular composition, all distinct from loci influencing aortic lesions. The Chromosome 2 locus contains a candidate, CD44. We show that CD44 is expressed in the innominate artery and differs strikingly in expression between the parental strains.

CONCLUSION

Multiple aspects of innominate lesion composition are genetically determined, but in a manner largely independent of the genetic contributions to aortic lesions.

Modes of defining atherosclerosis in mouse models: relative merits and evolving standards

Mouse models have become the most common model for defining mechanisms of atherosclerotic disease. Many genetic manipulations have enabled the development of atherosclerosis in mice due to either endogenous or diet-induced hypercholesterolemia. This availability of lesion-susceptible mice has facilitated many studies using pharmacological and genetic approaches. Unfortunately, this expansive literature on mouse atherosclerosis has generated many contradictions on the role of specific pathways. A contributor to these inconsistencies may be the multiple modes in which atherosclerosis is evaluated. Also, for each specific technique, there are no consistent standards applied to the measurements. This chapter will discuss the imaging, biochemical, and compositional modes of evaluating atherosclerosis with suggestions for standard execution of these techniques.

Regulatory T-cells, FoxP3 and atherosclerosis

Innate immune responses follow accumulation of modified lipids within the arterial wall thereby influencing atherosclerotic plaque progression. One of the mechanisms evolved in maintaining immunologic self-tolerance involves upregulation of regulatory T-cells, among which the CD4+CD25+ FoxP3+ regulatory T-cells (Treg) are best characterized. The putative important role of Treg in the initiation of atherosclerotic lesions as well as in the progression towards unstable plaques leading to ischemic events, supported by human studies and, indirectly, by murine models. Herein, we summarize the experimental approaches taken in order to study the possible mechanisms ofTreg involvement in atherosclerosis as well as the beneficial clinical potential of Treg in stabilizing atherosclerotic plaques.

Complementary roles of platelets and coagulation in thrombus formation on plaques acutely ruptured by targeted ultrasound treatment: a novel intravital model

BACKGROUND

Atherothrombosis is a major cause of cardiovascular events. However, animal models to study this process are scarce.

OBJECTIVES

We describe the first murine model of acute thrombus formation upon plaque rupture to study atherothrombosis by intravital fluorescence microscopy.

METHODS

Localized rupture of an atherosclerotic plaque in a carotid artery from Apoe(-/-) mice was induced in vivo using ultrasound. Rupture of the plaque and formation of localized thrombi were verified by two-photon laser scanning microscopy (TPLSM) in isolated arteries, and by immunohistochemistry. The thrombotic reaction was quantified by intravital fluorescence microscopy.

RESULTS

Inspection of the ultrasound-treated plaques by histochemistry and TPLSM demonstrated local damage, collagen exposure, luminal thrombus formation as well as intra-plaque intrusion of erythrocytes and fibrin. Ultrasound treatment of healthy carotid arteries resulted in endothelial damage and limited platelet adhesion. Real-time intravital fluorescence microscopy demonstrated rapid platelet deposition on plaques and formation of a single thrombus that remained subocclusive. The thrombotic process was antagonized by thrombin inhibition, or by blocking of collagen or adenosine diphosphate receptor pathways. Multiple thrombi were formed in 70% of mice lacking CD40L.

CONCLUSIONS

Targeted rupture of murine plaques results in collagen exposure and non-occlusive thrombus formation. The thrombotic process relies on platelet activation as well as on thrombin generation and coagulation, and is sensitive to established and novel antithrombotic medication. This model provides new possibilities to study atherothrombosis in vivo.

Molecular pathology in vulnerable carotid plaques: correlation with [18]-fluorodeoxyglucose positron emission tomography (FDG-PET)

OBJECTIVES

Atherosclerosis is recognised as an inflammatory disease, and new diagnostic tools are warranted to evaluate plaque inflammatory activity and risk of cardiovascular events. We investigated [18]-fluorodeoxyglucose (FDG) uptake in vulnerable carotid plaques visualised by positron emission tomography (PET). Uptake was correlated to quantitative gene expression of known markers of inflammation and plaque vulnerability.

METHODS

Ten patients with recent transient ischaemic attack and carotid artery stenosis (>50%) underwent combined FDG-PET and computed tomography angiography (CTA) the day before carotid endarterectomy. Plaque mRNA expression of the inflammatory cytokine interleukin 18 (IL-18), the macrophage-specific marker CD68 and the two proteinases, Cathepsin K and matrix metalloproteinase 9 (MMP-9), were quantified using real-time quantitative polymerase chain reaction.

RESULTS

Consistent up-regulation of CD68 (3.8-fold+/-0.9; mean+/-standard error), Cathepsin K (2.1-fold+/-0.5), MMP-9 (122-fold+/-65) and IL-18 (3.4-fold+/-0.7) were found in the plaques, compared to reference-artery specimens. The FDG uptake by plaques was strongly correlated with CD68 gene expression (r=0.71, P=0.02). Any correlations with Cathepsin K, MMP-9 or IL-18 gene expression were weaker.

CONCLUSIONS

FDG-PET uptake in carotid plaques is correlated to gene expression of CD68 and other molecular markers of inflammation and vulnerability.

Effects and underlying mechanisms of curcumin on the proliferation of vascular smooth muscle cells induced by Chol:MbetaCD

Proliferation of vascular smooth muscle cells (VSMCs) contributes to the development of various cardiovascular diseases. Curcumin, extracted from Curcumae longae, has been shown a variety of beneficial effects on human health, including anti-atherosclerosis by mechanisms poorly understood. In the present study, we attempted to investigate whether curcumin has any effect on VSMCs proliferation and the potential mechanisms involved. Our data showed curcumin concentration-dependently abrogated the proliferation of primary rat VSMCs induced by Chol:MbetaCD. To explore the underlying cellular and molecular mechanisms, we found that curcumin was capable of restoring caveolin-1 expression which was reduced by Chol:MbetaCD treatment. Moreover, curcumin abrogated the increment of phospho-ERK1/2 and nuclear accumulation of ERK1/2 in primary rat VSMCs induced by Chol:MbetaCD, which led to a suppression of AP-1 promoter activity stimulated by Chol:MbetaCD. In addition, curcumin was able to reverse cell cycle progression induced by Chol:MbetaCD, which was further supported by its down-regulation of cyclinD1 and E2F promoter activities in the presence of Chol:MbetaCD. Taking together, our data suggest curcumin inhibits Chol:MbetaCD-induced VSMCs proliferation via restoring caveolin-1 expression that leads to the suppression of over-activated ERK signaling and causes cell cycle arrest at G1/S phase. These novel findings support the beneficial potential of curcumin in cardiovascular disease.

Unstable coronary plaques and cardiac events in myocardial infarction-prone Watanabe heritable hyperlipidemic rabbits: questions and quandaries

PURPOSE OF REVIEW

Use of suitable animal models is essential for investigation of the mechanisms underlying cardiac events and development of the therapeutic strategies; however, ideal animal models that can recapitulate human coronary atherosclerosis and subsequent acute myocardial infarction are still lacking. In this article, we review the insights learned from myocardial infarction-prone Watanabe heritable hyperlipidemic (designated as WHHLMI) rabbits and discuss the possibility of using this model for the study of human acute coronary syndromes.

RECENT FINDINGS

The vulnerable plaques of human coronary arteries are histologically characterized by a large lipid core and a thin fibrous cap with inflammatory cells. Recent studies have revealed that inflammatory cells and inflammatory mediators (such as cytokines and matrix metalloproteinases) play an important role in the plaque rupture.

SUMMARY

We developed the WHHLMI rabbit that shows spontaneous myocardial infarction caused by coronary atherosclerosis. The coronary lesions of WHHLMI rabbits have features of fatty streaks, fibrous plaques, and fibroatheromatous plaques. Some plaques contain a lipid core and a thin fibrous cap similar to human vulnerable plaques. In spite of this, the plaque rupture is not observed in WHHLMI rabbits, suggesting that other additional factors such as mechanical stress are required to trigger the rupture. WHHLMI rabbits may become an important means for elucidating the possible mechanisms of plaque rupture by exposing the plaques to additional risk factors beyond hyperlipidemia.

Metabolic syndrome: from epidemiology to systems biology

Metabolic syndrome (MetSyn) is a group of metabolic conditions that occur together and promote the development of cardiovascular disease (CVD) and diabetes. Recent genome-wide association studies have identified several novel susceptibility genes for MetSyn traits, and studies in rodent models have provided important molecular insights. However, as yet, only a small fraction of the genetic component is known. Systems-based approaches that integrate genomic, molecular and physiological data are complementing traditional genetic and biochemical approaches to more fully address the complexity of MetSyn.

Inflammation and plaque vulnerability

Development of a thrombus at the site of an atherosclerotic plaque initiates abrupt arterial occlusion and is the proximate event responsible for the vast majority of acute ischemic syndromes. In nearly 75% of cases thrombus overlies a disrupted or ruptured plaque whereas the remainder of the thrombi overly an intact plaque with superficial endothelial erosion. Over the past several years, it has been recognized that plaque composition rather than plaque size or stenosis severity is important for plaque rupture and subsequent thrombosis. Ruptured plaques, and by inference, plaques prone to rupture, tend to be large in size with associated expansive arterial remodeling, thin fibrous cap with a thick or large necrotic lipid core with immuno-inflammatory cell infiltration in fibrous cap and adventitia and increased plaque neovascularity and intraplaque hemorrhage. The size of the necrotic lipid core and extent and location of plaque inflammation appear to be key factors in determining plaque instability. Inflammation and immune cell activation appears to play a key role in the loss of collagen in the fibrous cap, a prelude to fibrous cap rupture, through release of collagen degrading enzymes. Furthermore, inflammation may also play a key role in the death of collagen synthesizing smooth muscle cells which further contributes to loss of fibrous cap integrity. Inflammation also is likely a key player in the ensuing thrombosis that follows plaque disruption through the elaboration of the pro-coagulant protein, tissue factor. An improved understanding of the pathophysiology of plaque vulnerability and subsequent athero-thrombosis should provide novel insights into improved prevention of athero-thrombotic cardiovascular events.

Loss of SR-A and CD36 activity reduces atherosclerotic lesion complexity without abrogating foam cell formation in hyperlipidemic mice

OBJECTIVE

The scavenger receptors SR-A and CD36 have been implicated in macrophage foam cell formation during atherogenesis and in the regulation of inflammatory signaling pathways, including those leading to lesional macrophage apoptosis and plaque necrosis. To test the impact of deleting these receptors, we generated Apoe(-/-) mice lacking both SR-A and CD36 and fed them a Western diet for 12 weeks.

METHODS AND RESULTS

We analyzed atheroma in mice, assessing lesion size, foam cell formation, inflammatory gene expression, apoptosis, and necrotic core formation. Aortic root atherosclerosis in Apoe(-/-)Cd36(-/-)Msr1(-/-) mice, as assessed by morphometry, electron microscopy, and immunohistochemistry, showed no decrease in lesion area or in vivo foam cell formation when compared to Apoe(-/-) mice. However, Apoe(-/-)Cd36(-/-)Msr1(-/-) lesions showed reduced expression of inflammatory genes and morphological analysis revealed a approximately 30% decrease in macrophage apoptosis and a striking approximately 50% decrease in plaque necrosis in aortic root lesions of these mice.

CONCLUSIONS

Although targeted deletion of SR-A and CD36 does not abrogate macrophage foam cell formation or substantially reduce atherosclerotic lesion area in Apoe(-/-) mice, loss of these pathways does reduce progression to more advanced necrotic lesions. These data suggest that targeted inhibition of these pathways in vivo may reduce lesional inflammation and promote plaque stability.

Vascular endothelium in atherosclerosis

Their strategic location between blood and tissue and their constitutive properties allow endothelial cells (EC) to monitor the transport of plasma molecules, by employing bidirectional receptor-mediated and receptor-independent transcytosis and endocytosis, and to regulate vascular tone, cellular cholesterol and lipid homeostasis. These cells are also involved in signal transduction, immunity, inflammation and haemostasis. Cardiovascular risk factors, such as hyperlipaemia/dyslipidaemia trigger the molecular machinery of EC to respond to insults by modulation of their constitutive functions followed by dysfunction and ultimately by injury and apoptosis. The gradual activation of EC consists initially in the modulation of two constitutive functions: (1) permeability, i.e. increased transcytosis of lipoproteins, and (2) biosynthetic activity, i.e. enhanced synthesis of the basement membrane and extracellular matrix. The increased transcytosis and the reduced efflux of beta-lipoproteins (betaLp) lead to their retention within the endothelial hyperplasic basal lamina as modified lipoproteins (MLp) and to their subsequent alteration (oxidation, glycation, enzymatic modifications). MLp generate chemoattractant and inflammatory molecules, triggering EC dysfunction (appearance of new adhesion molecules, secretion of chemokines, cytokines), characterised by monocyte recruitment, adhesion, diapedesis and residence within the subendothelium. In time, EC in the athero-prone areas alter their net negative surface charge, losing their non-thrombogenic ability, become loaded with lipid droplets and turn into foam cells. Prolonged and/or repeated exposure to cardiovascular risk factors can ultimately exhaust the protective effect of the endogenous anti-inflammatory system within EC. As a consequence, EC may progress to senescence, lose their integrity and detach into the circulation.

Genetic and pharmacological inhibition of the 5-lipoxygenase/leukotriene pathway in atherosclerotic lesion development in ApoE deficient mice

The 5-lipoxygenase (5-LO) catalyzed formation of leukotriene (LT) lipid mediators is a pathway contributing to inflammatory events in asthma and more recently has been associated with cardiovascular disease. However, the relative impact of this pathway in atherogenesis has been controversial and a variety of mixed results reported. The goal of these studies was to assess the importance of the 5-LO/LT pathway in mice with either genetic (5-LO(-/-)) or pharmacological (L-739,010) inhibition of the 5-LO pathway on an apolipoprotein E deficient (apoE(-/-)) background when subjected to either an 8-week (Paigen) or 6 months (Western) atherosclerotic diet regimen. Atherosclerotic lesion analysis at the aortic root, brachiocephalic artery and throughout the whole aorta by en face Sudan IV staining was determined, as well as blood lipid levels. Ex vivo calcium ionophore-stimulation of whole blood demonstrated a significant reduction in the capacity to form LTB(4) in 5-LO(-/-) and drug-treated 5-LO(+/+) mice. Quantitative analysis of atherosclerotic lesions did not differ between groups at all three sites. Moreover, the composition of advanced lesions in the brachiocephalic arteries did not indicate altered plaque disruption as a result of 5-LO gene inactivation. These results do not support a role for the 5-LO/LT pathway in intermediate to advanced atherosclerotic lesion development in mice.

Mechanisms of disease: the evolving role of regulatory T cells in atherosclerosis

Atherosclerosis and related complications still represent the major cause of morbidity and mortality in the western world. The mechanisms that govern the progression and destabilization of atheromatous lesions are multiple and complex. Despite their widespread use, lipid-lowering agents do not provide sufficient protection from future clinical cardiovascular-associated events. Interest in the role of immunity in atherosclerosis and support for this relationship has grown significantly over recent years. This paradigm, in which inflammation is an instrumental process in plaque development and rupture, is further supported by studies showing that immune subsets are operative in atherosclerosis. Regulatory T-cell subpopulations consist of lymphocytes--with several phenotypic markers--that share the ability to suppress, by various mechanisms, inflammatory responses. These regulatory T cells consist of subsets such as interleukin-10 secreting type I regulatory cells, type 3 effector T-helper cells that produce transforming growth factor-beta, as well as adaptive and natural CD4(+)CD25(+) regulatory T cells. In this Review, I focus on the direct and indirect evidence for the involvement of regulatory T cells in atherogenesis in experimental models and in humans. The growing knowledge of the role of regulatory T cells could result in the future development of novel therapeutic modalities to attenuate atherosclerosis and stabilize vulnerable plaques.

The pathology of scleroderma vascular disease

Systemic sclerosis is characterized by three distinct pathologic processes: fibrosis, cellular/humoral autoimmunity, and specific vascular changes. Although a mild vasculitis may sometimes be present, the vascular pathology of scleroderma is not necessarily inflammatory and is best characterized as a vasculopathy. In this article, the authors propose that SSc vasculopathy is the result of an early event involving vascular injury that eventuates in a vicious cycle mediated in part by the immune process. The subsequent vascular malformation and rarefaction may be a function of systemic angiogenic dysregulation, with over expression of vascular endothelial growth factor but a lack of proper interactions with smooth muscle cells needed to stabilize and organize blood vessels.

From vulnerable plaque to atherothrombosis

Plaque rupture precipitates approximately 75% of all fatal coronary thrombi. Therefore, the plaque prone to rupture is the primary focus of this review. The lipid-rich core and fibrous cap are pivotal in the understanding of plaque rupture. Plaque rupture is a localized process within the plaque caused by degradation of a tiny fibrous cap rather than by diffuse inflammation of the plaque. Atherosclerosis is a multifocal disease, but plaques prone to rupture seem to be oligofocal at most.

Translating molecular discoveries into new therapies for atherosclerosis

Atherosclerosis is characterized by the thickening of the arterial wall and is the primary cause of coronary artery disease and cerebrovascular disease, two of the most common causes of illness and death worldwide. Clinical trials have confirmed that certain lipoproteins and the renin-angiotensin-aldosterone system are important in the pathogenesis of atherosclerotic cardiovascular disease, and that interventions targeted towards these are beneficial. Furthermore, efforts to understand how risk factors such as high blood pressure, dysregulated blood lipids and diabetes contribute to atherosclerotic disease, as well as to understand the molecular pathogenesis of atherosclerotic plaques, are leading to new targets for therapy.

The potential link between atherosclerosis and the 5-lipoxygenase pathway: investigational agents with new implications for the cardiovascular field

The 5-lipoxygenase pathway is responsible for the production of leukotrienes--inflammatory lipid mediators that have a role in innate immunity, but that can also have pathological effects in inflammatory diseases. Recently, a potential link between leukotriene production and atherosclerosis has been proposed. The expression of leukotriene biosynthetic enzymes and leukotriene receptors has been identified in coronary and carotid atherosclerotic plaques, and the levels of biosynthetic enzymes have been correlated with the clinical symptoms of unstable plaques. Genetic variants in 5-lipoxygenase pathway genes have also been associated with a relative risk of developing myocardial infarction and stroke. On the basis of these discoveries, antileukotriene compounds are now being evaluated for the treatment of cardiovascular disease. Several tool compounds have been shown to limit the progression of lesion development in preclinical models of atherosclerosis, and three compounds, including two drugs previously developed for asthma, are undergoing clinical trials in patients with acute coronary syndromes.

Mast cells and degradation of pericellular and extracellular matrices: potential contributions to erosion, rupture and intraplaque haemorrhage of atherosclerotic plaques

Mast cells are present in advanced human atherosclerotic plaques, where they are thought to exert multiple effects on their neighbouring cells and on the extracellular matrix of the plaque. Extensive efforts at delineating their role(s) in atherosclerotic plaques have unravelled mechanisms by which plaque mast cells may render advanced atherosclerotic plaques susceptible to erosion, rupture or intraplaque haemorrhage and so modulate their stability. In these mechanisms, the key effector molecules are mast-cell-derived neutral proteases and pro-inflammatory cytokines. These effector molecules are synthesized and stored in the cytoplasmic secretory granules of mast cells and, once the mast cells are activated to degranulate, are released into the microenvironment surrounding the activated mast cells. In the plaques, the key target cells are endothelial cells and smooth muscle cells and their pericellular matrices. In addition, the various components of the extracellular matrix of the plaques, notably collagen, are degraded when the released mast cell proteases activate matrix metalloproteinases in the plaques. By rendering the plaque susceptible to erosion, to rupture or to intraplaque haemorrhage, the mast cells may contribute to the onset of acute atherothrombotic complications of coronary atherosclerosis, such as myocardial infarction.

Type 1 diabetes promotes disruption of advanced atherosclerotic lesions in LDL receptor-deficient mice

Cardiovascular disease, largely because of disruption of atherosclerotic lesions, accounts for the majority of deaths in people with type 1 diabetes. Recent mouse models have provided insights into the accelerated atherosclerotic lesion initiation in diabetes, but it is unknown whether diabetes directly worsens more clinically relevant advanced lesions. We therefore used an LDL receptor-deficient mouse model, in which type 1 diabetes can be induced at will, to investigate the effects of diabetes on preexisting lesions. Advanced lesions were induced by feeding mice a high-fat diet for 16 weeks before induction of diabetes. Diabetes, independently of lesion size, increased intraplaque hemorrhage and plaque disruption in the brachiocephalic artery of mice fed low-fat or high-fat diets for an additional 14 weeks. Hyperglycemia was not sufficient to induce plaque disruption. Furthermore, diabetes resulted in increased accumulation of monocytic cells positive for S100A9, a proinflammatory biomarker for cardiovascular events, and for a macrophage marker protein, without increasing lesion macrophage content. S100A9 immunoreactivity correlated with intraplaque hemorrhage. Aggressive lowering primarily of triglyceride-rich lipoproteins prevented both plaque disruption and the increased S100A9 in diabetic atherosclerotic lesions. Conversely, oleate promoted macrophage differentiation into an S100A9-positive population in vitro, thereby mimicking the effects of diabetes. Thus, diabetes increases plaque disruption, independently of effects on plaque initiation, through a mechanism that requires triglyceride-rich lipoproteins and is associated with an increased accumulation of S100A9-positive monocytic cells. These findings indicate an important link between diabetes, plaque disruption, and the innate immune system.

CD36 is significantly correlated with adipophilin in human carotid lesions and inversely correlated with plasma ApoAI

OxLDL uptake and cholesterol efflux inhibition in macrophages play a key role in atherosclerotic plaque formation, rupture, and thrombotic ischemia. This study investigates genes implicated in OxLDL uptake (CD36, SRA), cholesterol efflux inhibition (adipophilin, ADFP), and inflammatory recruitments of leukocytes (IL-8) in plaque lesion areas (PLAs) compared to nonplaque lesion areas (NPLAs) in human carotid endarterectomy specimens. Gene and protein expressions were assayed using quantitative PCR and quantitative immunohistochemistry. Pearson tests were used to investigate potential correlation between (a) different gene expressions and (b) gene expression and patient's plasma constituents. CD36, SRA, ADFP, and IL-8 were shown to be significantly more expressed in PLA compared to NPLA. In PLA, a significant correlation was observed between CD36, SRA, ADFP, and IL-8 mRNA levels. Moreover, CD36 expression level was significantly inversely correlated to plasma marker ApoAI. The above investigated genes/proteins may play a key role in the maturation of atherosclerotic lesions.

Expression of the Lyst(beige) mutation is atheroprotective in chow-fed apolipoprotein E-deficient mice

Lyst(beige) mice crossed with hyperlipidemic low density lipoprotein receptor-deficient mice (BgLDLr(-/-)) display increased lesion area and a more stable lesion morphology. To verify that the beige phenotype is not unique to LDLr(-/-) mice, we examined atherosclerosis in beige, apolipoprotein E-deficient mutant mice (BgApoE(-/-)). Severe diet-induced hyperlipidemia in BgApoE(-/-) mice resulted in increased aortic sinus lesion areas compared with controls. Minimal aortic lesions were observed in both genotypes on a chow diet. Nevertheless, BgApoE(-/-) mice displayed drastically reduced aortic sinus lesion growth. Reconstitution with bone marrow (BM) from green fluorescent protein mice created chimeric animals that allowed for the identification of donor-derived cells within lesions. Expressing the beige mutation exclusively in BM-derived cells had no impact on plaque development, yet the beige mutation in all cells except the BM-derived cells led to significantly larger aortic sinus lesion areas. Both mRNA and secreted protein levels of monocyte chemoattractant protein 1 were altered in quiescent and phorbol ester-stimulated cultured macrophages, vascular smooth muscle cells, and aortic endothelial cells isolated from BgApoE(-/-) mice. Thus, expression of the beige mutation in all cell types involved in lesion development contributed to atheroprotection in chow-fed ApoE(-/-) mice.

Prediction of atherosclerotic plaque ruptures with high-frequency ultrasound imaging and serum inflammatory markers

Atherosclerotic plaque rupture and thrombosis are the main causes of acute coronary syndrome. In the present study, we investigated whether ultrasound imaging and inflammatory parameters are predictive of plaque rupture in a newly established animal model. We developed a rabbit model for plaque rupture by locally delivering recombinant p53 adenovirus to plaques in rabbits fed a high-cholesterol diet for 10 wk, and plaque rupture was triggered using Chinese Russell's viper venom and histamine. We found that 81.1% of rabbits transfected with p53 ( n = 37) had the ruptured plaques, which was significantly higher than results in rabbits transfected with the control vector (26.3%, n = 38; P < 0.001). Among measured biomarkers, high-sensitive C-reactive protein, soluble intercellular adhesion molecule-1, and soluble vascular cell adhesion molecule-1 were significantly different between rabbits with and without ruptured plaques. Using high-frequency duplex and intravascular ultrasound imaging techniques, we obtained a list of parameters. With the multivariate logistic regression model, we identified that plaque eccentric index, plaque area, high-sensitive C-reactive protein, and corrected integrated backscatter intensity were significant predictors of plaque rupture, with odds ratios of 7.056 [95% confidence interval (CI): 1.958, ∼25.430], 1.942 (95% CI: 1.058, ∼3.564), 1.025 (95% CI: 1.007, ∼1.043), and 0.856 (95% CI: 0.775, ∼0.946), respectively. Localized p53 overexpression technique induces plaque rupture, and the combined measurement of ultrasound and biochemical markers is a valuable tool in predicting plaque rupture.

Molecular mechanisms of plaque instability

PURPOSE OF REVIEW

Coronary artery thrombosis superimposed on a disrupted atherosclerotic plaque initiates abrupt arterial occlusion and is the proximate event responsible for 60-80% cases of acute coronary syndromes. This article provides a concise update on the evolving concepts in the pathophysiology of plaque rupture and thrombosis.

RECENT FINDINGS

Over the past several years, the critical role of plaque composition rather than plaque size or stenosis severity, in plaque rupture and thrombosis have been recognized. The necrotic lipid core and plaque inflammation appear to be key factors. Extracellular matrix loss in the fibrous cap, a prelude to rupture, is attributed to matrix degrading enzymes as well as to death of matrix synthesizing smooth muscle cells; inflammation appears to play a critical role in both these processes. Inflammatory cell derived tissue factor is a key contributor to plaque thrombogenicity. Inflammation has also been implicated in plaque neovascularity, intraplaque hemorrhage and plaque expansion. Recent observations have also highlighted the important modulatory role of immune system in atherosclerosis and plaque composition.

SUMMARY

Improved understanding of mechanisms causing plaque instability should provide novel insights into prevention of athero-thrombotic cardiovascular events.

Antiangiogenic therapy for normalization of atherosclerotic plaque vasculature: a potential strategy for plaque stabilization

Angiogenesis within human atherosclerotic plaques has an important role in plaque progression as immature blood vessels leak red blood cells and inflammatory mediators into the plaque center. Accumulation of free cholesterol from red blood cell membranes potentially increases the size of the necrotic core and triggers a chain of events that promote plaque destabilization. Antiangiogenic agents have been shown to prune some tumor vessels and 'normalize' the structure and function of the remaining vasculature, thereby improving the access of chemotherapeutic agents to tumors. We propose that antiangiogenic therapy can similarly stabilize vulnerable 'rupture-prone' plaques by pruning and normalizing immature intraplaque vessels, preventing further intraplaque hemorrhage. This normalization would limit necrotic core enlargement, further luminal narrowing and the degree of inflammation. Such normalization has been realized using vascular endothelial growth factor antagonists for the treatment of cancer and age-related macular degeneration. The development of this novel approach to prevent plaque progression might add to the armamentarium of preventive measures for acute myocardial infarction, stroke and sudden cardiac death.

Biomarkers of atherosclerosis and the potential of MRI for the diagnosis of vulnerable plaque

Atherosclerosis is a chronic inflammatory vascular disease. As it is an inflammation process, many cellular and molecular events are involved at each step of the progression of atherosclerosis from an early fatty streak lesion to a highly dangerous rupture-prone plaque. Magnetic resonance imaging (MRI) is a well-established diagnostic tool for many kinds of chronic inflammation in various systems and organs, and recent improvements in spatial resolution and contrast strategies make it a promising technique for the characterization of inflammatory vessel walls. The first part of this review will briefly introduce the main cellular and molecular processes involved in atherosclerotic lesions; the second part will focus on the use of high-resolution MRI and present-generation contrast agents for plaque characterization; and the third part will present some recent and ongoing cellular and molecular MRI studies of atherosclerosis.