Lesional Overexpression of Matrix Metalloproteinase-9 Promotes Intraplaque Hemorrhage in Advanced Lesions But Not at Earlier Stages of Atherogenesis

Role of endothelial shear stress in the natural history of coronary atherosclerosis and vascular remodeling: molecular, cellular, and vascular behavior

Although the entire coronary tree is exposed to the atherogenic effect of the systemic risk factors, atherosclerotic lesions form at specific arterial regions, where low and oscillatory endothelial shear stress (ESS) occur. Low ESS modulates endothelial gene expression through complex mechanoreception and mechanotransduction processes, inducing an atherogenic endothelial phenotype and formation of an early atherosclerotic plaque. Each early plaque exhibits an individual natural history of progression, regression, or stabilization, which is dependent not only on the formation and progression of atherosclerosis but also on the vascular remodeling response. Although the pathophysiologic mechanisms involved in the remodeling of the atherosclerotic wall are incompletely understood, the dynamic interplay between local hemodynamic milieu, low ESS in particular, and the biology of the wall is likely to be important. In this review, we explore the molecular, cellular, and vascular processes supporting the role of low ESS in the natural history of coronary atherosclerosis and vascular remodeling and indicate likely mechanisms concerning the different natural history trajectories of individual coronary lesions. Atherosclerotic plaques associated with excessive expansive remodeling evolve to high-risk plaques, because low ESS conditions persist, thereby promoting continued local lipid accumulation, inflammation, oxidative stress, matrix breakdown, and eventually further plaque progression and excessive expansive remodeling. An enhanced understanding of the pathobiologic processes responsible for atherosclerosis and vascular remodeling might allow for early identification of a high-risk coronary plaque and thereby provide a rationale for innovative diagnostic and/or therapeutic strategies for the management of coronary patients and prevention of acute coronary syndromes.

Pathological mechanisms and dose dependency of erythrocyte-induced vulnerability of atherosclerotic plaques

To test our hypothesis that erythrocytes may induce plaque vulnerability and investigate the mechanism involved, we established a novel model of intraplaque hemorrhage in 56 New Zealand white rabbits with established plaques. Three distinct abdominal aortic plaques with similar thickness were identified in each rabbit with use of intravascular ultrasound (IVUS) imaging. Rabbits were equally divided into 4 groups depending on dosage of treatment; with the guidance of IVUS, one of the three plaques from each rabbit was injected from adventitia with autologous erythrocytes (RBC) or cholesterol (CH) for the following groups: RBC, 50 microL or 100 microL, and CH, 50 microL or 100 microL. One of the other two plaques in each rabbit received an equal volume of normal saline (NS) and one received no injection. Plaques in the 100 microL RBC group had a higher plaque rupture rate than its respective NS or blank controls plaques (57.1% vs. 14.3% or 14.3%, P<0.05). Plaques from the RBC or cholesterol groups showed, dose-dependently, more macrophage infiltration, more superoxide and lipid content, thinner plaque fibrous cap, higher mRNA level of MCP-1, IL-1 or IFN-gamma and higher vulnerability index than controls, especially in the RBC group. Thus, erythrocyte treatment can dose-dependently induce the vulnerability of plaques. Accumulation of lipid content and augmentation of oxidative stress and inflammation in the plaques are the probable pathological mechanisms involved.

Quantitative X-ray imaging of intraplaque hemorrhage in aortas of apoE(-/-)/LDL(-/-) double knockout mice

OBJECTIVES

To determine if hemorrhage into an arterial wall can be detected in CT images by virtue of the iron content.

MATERIALS AND METHODS

Aortas from male apoE(-/-)/LDL(-/-) mice (n = 31) were infused in situ with contrast agent, for micro-CT scanning and histology. Roentgen-opacities within the aortic walls were identified by histology and micro-x-ray fluorescence to be iron or calcium. Dual-energy scanning was performed at 2 energy levels using synchrotron-based micro-CT [(2 microm)(3) voxels, 16 and 20 keV] and 64-slice CT (0.4 x 0.4 x 0.6 mm voxels, 80 and 120 kVp).

RESULTS

Opacities were identified as hemorrhage-related clusters of multiple punctate deposits, containing both Fe (0.48 x 10(-12) g/voxel) and Ca (3.18 x 10(-2) g/voxel), or as isolated confluent accumulations of exclusively calcium. Subtraction of the dual-energy CT scans discriminated iron from calcium deposits.

CONCLUSION

Detection and quantification of iron deposits in hemorrhaged atherosclerotic lesions is feasible by dual-energy CT imaging.

Perivascular carotid collar placement induces neointima formation and outward arterial remodeling in mice independent of apolipoprotein E deficiency or Western-type diet feeding

We investigated the influence of apolipoprotein E deficiency and Western-type diet feeding on the development and composition of neointimal lesions induced by periadventitial carotid placement of a non-occlusive collar in mice. ApoE-/- and wild-type mice were fed a Western-type diet or chow diet for 4 weeks before collar surgery. Diets were continued after collar placement for 6 or 12 weeks. Compared to sham-operated arteries, collared carotids showed significant neointima formation, lumen loss, and outward remodeling in both apoE-/- and wild-type mice. These changes were not affected by either the genotype or the diet. Conversely, significant differences in neointima composition were detected between the two genotypes, with apoE-/- mice showing greater lipid deposition and lower SMC accumulation compared to wild-type mice, independent of the dietetic regimen. Altogether, the results of the present study indicate that although lesion composition may be influenced by genotype, neointima formation and arterial remodeling in the murine perivascular carotid collar model occur independent of the exposure to atherogenic diet or the presence of a sensitized genotype such as apoE-/-. The murine perivascular carotid collar model would thus be suitable for investigating neointima formation, arterial remodeling, and their potential pharmacological modulation in the setting of different genetic and dietary conditions.

Do glucose and lipids exert independent effects on atherosclerotic lesion initiation or progression to advanced plaques?

It is becoming increasingly clear that suboptimal blood glucose control results in adverse effects on large blood vessels, thereby accelerating atherosclerosis and cardiovascular disease, manifested as myocardial infarction, stroke, and peripheral vascular disease. Cardiovascular disease is accelerated by both type 1 and type 2 diabetes. In type 1 diabetes, hyperglycemia generally occurs in the absence of elevated blood lipid levels, whereas type 2 diabetes is frequently associated with dyslipidemia. In this review article, we discuss hyperglycemia versus hyperlipidemia as culprits in diabetes-accelerated atherosclerosis and cardiovascular disease, with emphasis on studies in mouse models and isolated vascular cells. Recent studies on LDL receptor-deficient mice that are hyperglycemic, but exhibit no marked dyslipidemia compared with nondiabetic controls, show that diabetes in the absence of diabetes-induced hyperlipidemia is associated with an accelerated formation of atherosclerotic lesions, similar to what is seen in fat-fed nondiabetic mice. These effects of diabetes are masked in severely dyslipidemic mice, suggesting that the effects of glucose and lipids on lesion initiation might be mediated by similar mechanisms. Recent evidence from isolated endothelial cells demonstrates that glucose and lipids can induce endothelial dysfunction through similar intracellular mechanisms. Analogous effects of glucose and lipids are also seen in macrophages. Furthermore, glucose exerts many of its cellular effects through lipid mediators. We propose that diabetes without associated dyslipidemia accelerates atherosclerosis by mechanisms that can also be activated by hyperlipidemia.

Matrix metalloproteinases: influence on smooth muscle cells and atherosclerotic plaque stability

Atherosclerotic plaque rupture, with subsequent occlusive thrombosis, is the underlying cause of most cases of sudden cardiac death. Matrix metalloproteinases (MMPs) are thought to mediate the progression of stable atherosclerotic lesions to an unstable phenotype that is prone to rupture through the destruction of strength-giving extracellular matrix (ECM) proteins. Smooth muscle cells secrete and deposit ECM proteins and are, therefore, considered protective against atherosclerotic plaque destabilization. However, similar to inflammatory cells (e.g., macrophages), smooth muscle cells release numerous MMPs that are capable of digesting ECM proteins. Thus, the interaction of smooth muscle cells and MMPs in atherosclerotic plaques is complex and not fully understood. Recently, research into the roles of MMPs and their endogenous inhibitors (tissue inhibitors of metalloproteinases), and their effects on smooth muscle behavior during plaque destabilization has been aided by the development of reproducible animal models of plaque instability. A plethora of studies has demonstrated that MMPs directly modulate smooth muscle behavior with both beneficial and deleterious effects on atherosclerotic plaque stability, in addition to their canonical effects on ECM remodeling. Consequently, broad-spectrum MMP inhibition may inhibit plaque-stabilizing mechanisms, such as smooth muscle cell growth, while conversely retarding ECM destruction and subsequent rupture. Hence the development of selective MMP inhibitors, that spare inhibitory effects on smooth muscle cell function, may be useful therapies to prevent plaque rupture and in this regard MMP-12 appears to be a particularly attractive target.

Mouse models for studies of cardiovascular complications of type 1 diabetes

Mouse models represent a powerful tool for investigating the underlying mechanisms of disease. Type 1 diabetes results in a markedly increased risk of cardiovascular disease. The cardiovascular complications are manifested primarily as ischemic heart disease caused by accelerated atherosclerosis, but also as cardiomyopathy, defined as ventricular dysfunction in the absence of clear ischemic heart disease. Several mouse models are now available to study atherosclerosis and cardiomyopathy associated with type 1 diabetes. For studies of diabetes-accelerated atherosclerosis, these models include low-density lipoprotein (LDL) receptor-deficient and apolipoprotein E-deficient mice in which diabetes is induced by streptozotocin or viral infection. In these mouse models, type 1 diabetes can be induced without marked changes in plasma lipid levels, thereby mimicking the accelerated atherosclerosis seen in patients with type 1 diabetes. However, mouse models that exhibit thrombotic events and myocardial infarctions as a result of diabetes still need to be developed. Conversely, cardiomyopathy associated with diabetes has now been extensively evaluated in streptozotocin-treated C57BL/6 mice, and in transgenic mice expressing calmodulin under a beta-cell-specific promoter. These mouse models have given significant insight into the molecular mechanisms causing cardiomyopathy, and indicate that increased oxidative stress contributes to diabetes-associated cardiomyopathy. In this review, we will discuss the available mouse models for studies of cardiovascular complications of type 1 diabetes, the potential mechanisms underlying these complications, and the need for new and improved mouse models.

Plaque rupture in humans and mice

Despite the many studies of murine atherosclerosis, we do not yet know the relevance of the natural history of this model to the final events precipitated by plaque disruption of human atherosclerotic lesions. The literature has become particularly confused because of the common use of terms such as "instability", "vulnerable", "rupture", or even "thrombosis" for features of plaques in murine model systems not yet shown to rupture spontaneously and in an animal surprisingly resistant to formation of thrombi at sites of atherosclerosis. We suggest that use of conclusory terms like "vulnerable" and "stable" should be discouraged. Similarly, terms such as "buried fibrous caps" that imply preceding events that are unproven tend to create confusion. We will argue that such terminology may mislead readers by implying knowledge that does not yet exist. We suggest, instead, a focus on specific processes that various forms of data have implicated in plaque progression. For example, formation of the fibrous cap, protease activation, and cell death in the necrotic core can be well described and have all been modeled in well-defined experiments. The relevance of such well-defined, objective, descriptive observations in the mouse can be tested for relevance against data from human pathology.

Assessment of unstable atherosclerosis in mice

There is an urgent need for representative animal models where prospective examination of the events leading up to plaque rupture and the rupture process itself can be performed. Recently, reports have begun to emerge that apolipoprotein E and low density lipoprotein receptor knockout mice may spontaneously develop unstable atherosclerosis, with plaques in certain parts of the arterial tree showing features suggestive of plaque rupture. Here we discuss the problems inherent in applying definitions of plaque rupture as seen in human arteries to mice; the anatomic locations in mice where unstable plaques do and do not occur; methods of inducing plaque instability in mice; and how to assess plaque stability in mice. These considerations lead us to a number of general recommendations.

Magnetic resonance imaging of vulnerable atherosclerotic plaques: Current imaging strategies and molecular imaging probes

The vulnerability or destabilization of atherosclerotic plaques has been directly linked to plaque composition. Imaging modalities, such as magnetic resonance (MR) imaging, that allow for evaluation of plaque composition at a cellular and molecular level, could further improve the detection of vulnerable plaque and may allow for monitoring the efficacy of antiatherosclerotic therapies. In this review we focus on MR imaging strategies for the detection and evaluation of atherosclerotic plaques and their composition. We highlight recent advancements in the development of MR pulse sequences, computer image analysis, and the use of commercially available MR contrast agents, such as gadopentic acid (Gd-DTPA), for plaque characterization. We also discuss molecular imaging strategies that are currently being used to design specific imaging probes targeted to biochemical and cellular markers of atherosclerotic plaque vulnerability.

Adventitial lymphatic vessels – An important role in atherosclerosis

Arterial inflammation is a significant component of atherosclerotic disease-specific immune responses directed against autoantigens or pathogen-derived antigens in the vascular wall could initiate and/or maintain atherosclerotic processes. Atherosclerosis is now regarded as a chronic inflammatory disease. Developing in response to injury in the vessel wall, it is characterized by the infiltration of mononuclear lymphocytes into the intima, local expansion of vascular smooth muscle cells, and accumulation of extracellular matrix. A number of potential mechanisms have been implicated in the development of inflammatory reactions in the vascular system. Adventitia provides cells and molecules with the ability to influence neointimal formation and vascular remodeling implemented in part by vasa vasorum. We hypothesize that lymphatic vessels, existing in adventitia in the atherosclerotic artery, could drain local inflammatory cells and cytokines to the lymphatic nodes and lymphoid tissues where inflammatory cells can be sensitized and activated. Or, blood vessels may deliver sensitized inflammatory cells and cytokines to the inflammatory site of the vascular wall. Therefore, both lymphatic and blood vessels constitute a complete circle of immune response, whereby the inflammatory cells and cytokines are effectively delivered to tissues and their effects magnified. Under certain circumstances, this situation may lead to a vicious circle of inflammation such as in atherosclerosis, resulting in perpetuating intimal hyperplasia and vascular remodeling. Inhibition of lymphangiogenesis may interrupt this self-perpetuating vicious circle of inflammation in atherosclerosis and provide a new approach to the prevention and treatment of the disease.

Do metalloproteinases destabilize vulnerable atherosclerotic plaques?

PURPOSE OF REVIEW

Atherosclerotic plaque rupture and thrombosis underlie most myocardial infarctions. Matrix metalloproteinases are a family of enzymes that remodel the extracellular matrix. Metalloproteinases could stabilize rupture-prone plaques by promoting smooth muscle cell migration and proliferation. Alternatively, metalloproteinases could destabilize vulnerable plaques by promoting matrix destruction, angiogenesis, leucocyte infiltration, and apoptosis. Evidence is reviewed from genetically modified mice and human biomarker and genetic studies that sheds light on this dual role of metalloproteinases.

RECENT FINDINGS

Inhibition of metalloproteinases in mice using tissue inhibitors of metalloproteinases increases plaque stability; however, double knockouts of apolipoprotein E with matrix metalloproteinase 2, 3, 7, 9, 12, and 13 have more or less stable plaques, consistent with harmful or protective effects of individual metalloproteinases. Overexpression studies in mice or rabbits show that high activities of matrix metalloproteinase 9 and 12 decrease stability. Biomarker and human genetic studies demonstrate that increased metalloproteinase activity is associated with vascular repair or myocardial infarction.

SUMMARY

Recent studies reinforce evidence for a dual role of matrix metalloproteinases in plaque stabilization and rupture, which probably depends on the stage, site, and severity of disease. Dysregulated metalloproteinase activity in end-stage coronary artery disease appears a valid target for therapy.

Vascular endothelial growth factor-A induces plaque expansion in ApoE knock-out mice by promoting de novo leukocyte recruitment

Vascular endothelial growth factor-A is widely used in clinical trials for the treatment of cardiac ischemia. VEGF-A was recently suggested to act in a proinflammatory manner, which could aggravate adjacent atherogenesis in VEGF-A–based therapy. To assess potential bystander effects, VEGF-A was focally overexpressed in advanced atherosclerotic plaques in ApoE−/− mice. Sheer-induced carotid artery plaques were transluminally incubated with Ad.hVEGF-A leading to neointimal overexpression of VEGF-A. Ad.hVEGF-A treatment of pre-existing lesions was seen to promote plaque expansion, with a concomitant increase in macrophage and lipid content, whereas it lowered collagen content. In general, Ad.hVEGF-A–treated plaques displayed a more vulnerable phenotype. VEGF-A overexpression was not accompanied by increased microvessel development in the neointima, suggesting that VEGF-A destabilizes atherosclerotic plaques through an angiogenesis-independent mechanism. Intravital microscopy confirmed that treatment with Ad.hVEGF-A led to an increased monocyte adhesion, which was mediated by a VCAM-1/PECAM-1–dependent pathway. VEGF-A indeed induced a differential expression of VCAM-1 and PECAM-1 in endothelial cells. Our data underline the importance of regular monitoring of stenotic vessels adjacent to the site of VEGF-A application. We propose that VCAM-1/PECAM-1–directed cotherapy may be an efficient strategy to prevent bystander effects of focal VEGF-A therapy in patients suffering from cardiovascular disease.

Cyclic strain-mediated matrix metalloproteinase regulation within the vascular endothelium: a force to be reckoned with

The vascular endothelium is a dynamic cellular interface between the vessel wall and the bloodstream, where it regulates the physiological effects of humoral and biomechanical stimuli on vessel tone and remodeling. With respect to the latter hemodynamic stimulus, the endothelium is chronically exposed to mechanical forces in the form of cyclic circumferential strain, resulting from the pulsatile nature of blood flow, and shear stress. Both forces can profoundly modulate endothelial cell (EC) metabolism and function and, under normal physiological conditions, impart an atheroprotective effect that disfavors pathological remodeling of the vessel wall. Moreover, disruption of normal hemodynamic loading can be either causative of or contributory to vascular diseases such as atherosclerosis. EC-matrix interactions are a critical determinant of how the vascular endothelium responds to these forces and unquestionably utilizes matrix metalloproteinases (MMPs), enzymes capable of degrading basement membrane and interstitial matrix molecules, to facilitate force-mediated changes in vascular cell fate. In view of the growing importance of blood flow patterns and mechanotransduction to vascular health and pathophysiology, and considering the potential value of MMPs as therapeutic targets, a timely review of our collective understanding of MMP mechanoregulation and its impact on the vascular endothelium is warranted. More specifically, this review primarily summarizes our current knowledge of how cyclic strain regulates MMP expression and activation within the vascular endothelium and subsequently endeavors to address the direct and indirect consequences of this on vascular EC fate. Possible relevance of these phenomena to vascular endothelial dysfunction and pathological remodeling are also addressed.