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(Arteriosclerosis, Thrombosis, and Vascular Biology. 1997;17:1859-1867.)
© 1997 American Heart Association, Inc.

Articles

The Unstable Atheroma

Richard T. Lee; ; Peter Libby

From the Vascular Medicine and Atherosclerosis Unit, Cardiovascular Division, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Mass.

Correspondence to Richard T. Lee, MD, Cardiovascular Division, Brigham and Women's Hospital, 75 Francis St, Boston, MA 02115. E-mail rtlee{at}bics.bwh.harvard.edu

Key Words: • unstable lesions • fibrous cap • plaque rupture

	Introduction

Top Introduction The Unstable Lesion and... Clinical Correlations With the... Major Gaps in Our... Steps to Prevent Plaque... References

 
For several decades, our understanding of the pathogenesis of unstable atherosclerotic lesions has continued to grow. A consistent picture of unstable plaque structure, inflammation, matrix degradation, and prothrombotic activity is emerging. Meanwhile, the mortality rates for coronary artery disease in the United States have steadily declined.¹ Changes in the American life style likely account for much of this decline,² and we can expect further improvements through the beneficial use of aspirin and cholesterol-lowering therapy in subsets of the population.^{3 4 5} Many of these improvements were based on careful epidemiological studies, with fundamental vascular biology serving a largely supportive role.

In light of this major clinical progress, it is reasonable to consider whether further basic understanding of the unstable atherosclerotic lesion will be translated into a clinical benefit beyond these recent advances.⁶ We propose that the basic biology of the unstable lesion will now assume even greater importance for two fundamental reasons. First, even with aspirin and cholesterol-lowering therapy, acute vascular events will continue to be a major cause of morbidity and mortality in developed countries. Performing clinical trials and epidemiological studies in the United States and other developed countries will become more difficult and expensive as the number of patients needed for each study increases. Thus, developing new successful approaches to the prevention of acute vascular syndromes will require a sound understanding of the pathogenesis of lesion destabilization, and proceeding to clinical breakthroughs will require strong justification from vascular biology before clinical trials are undertaken. Second, even as the age-adjusted incidence of ischemic heart disease falls in the United States, the worldwide importance of ischemic heart disease is increasing. In fact, a recent comprehensive survey of the global patterns of disease has projected that by the year 2020, ischemic heart disease (currently the fifth leading cause of global disease burden) will surpass communicable diseases to become the leading cause of disability in the world.⁷

This review will discuss our current understanding of the unstable lesion as well as the important gaps in that understanding. The difficult process of filling in these gaps will be critical to translating vascular biology into the prevention of clinical plaque rupture. However, the stakes are high, because the benefits of successful lesion stabilization may be enormous. These benefits potentially include the prevention not only of acute myocardial infarction but also of many or most cases of unstable angina, peripheral vascular disease,⁸ stroke, and aortic aneurysm formation. Indeed, prevention of myocardial ischemic injury by plaque stabilization is an excellent way to reduce the incidence of congestive heart failure.

	The Unstable Lesion and the Biology of Fibrous Cap Strength

Top Introduction The Unstable Lesion and... Clinical Correlations With the... Major Gaps in Our... Steps to Prevent Plaque... References

 
Two major mechanisms of plaque disruption may lead to coronary thrombosis: frank rupture of a plaque's fibrous cap and superficial erosion of the endothelium. As early as 1934, plaque rupture was proposed as a mechanism for acute arterial thrombosis.⁹ In the past 15 years, careful pathological examinations of human lesions have refocused attention on plaque rupture as the cause of the majority of acute coronary events. These studies, which have recently been reviewed in depth,^{10 11} have consistently identified several morphological features that characterize the unstable atheroma, including a thin, eccentric fibrous cap and a large necrotic core of lipid and cellular debris. Analyses of human lesions by modern computer methods and biomechanical testing have established the probable link between this characteristic morphology and the actual rupture event. This plaque configuration is particularly unstable because large mechanical stresses develop in the thinnest portions of the fibrous cap.^{12 13} The soft lipid core is unable to bear these mechanical forces, and excess stresses are thus "concentrated" in the fibrous cap, particularly at the junction with the normal vessel (the "shoulder" region). Like the Achilles' tendon of an athlete, under some circumstances the collagenous extracellular matrix of the fibrous cap cannot withstand these stresses, and the fibrous cap ruptures.

However, numerous recent observations tell us that other critical factors contribute to the ultimate fracture of a fibrous cap. First, many plaques with the typical geometric features of an unstable atheroma do not rupture and are only incidentally found at autopsy.¹⁴ Second, the mechanical strength of human fibrous caps varies considerably.^{15 16} This variability may explain why some atheromas with similar geometric features rupture and others do not. Third, a body of consistent evidence supports the view that the extracellular matrix of the fibrous cap is a dynamic, biologically active environment. Together, these three observations suggest that there is potential for modulating the biological activity of the fibrous cap to increase its strength, possibly by converting the unstable plaque that is highly prone to rupture to a more stable lesion. In this approach, we can consider the intact fibrous cap a "healed wound" and plaque rupture a "wound failure." Because most of the mass of the fibrous cap is extracellular matrix, particularly fibrillar collagen, understanding how cells of the atheroma secrete, organize, and degrade the collagenous matrix is a central theme of plaque stabilization.

Molecular Regulation of Matrix Synthesis
Extracellular matrix accumulation is characteristic of atherosclerotic as well as hypertensive arteries. Vascular smooth muscle cells are the primary source of this matrix, which primarily comprises collagens, elastin, proteoglycans, and microfibrillar proteins. In the normal artery, both the synthesis and degradation of extracellular matrix proteins are remarkably slow; for example, collagen turnover in the normal artery has been estimated to be in years.¹⁷ Atherosclerosis and injury to the artery lead to increased synthesis of many matrix components, including elastin, collagen types I and III, and several proteoglycans. In the later stages of atherosclerosis, nonfibrillar collagen types IV (usually a basement membrane collagen) and V can be found in the fibrous cap.¹⁸ Cytokines and growth factors in the atheroma regulate the synthesis of matrix components; for example, transforming growth factor-ß (TGF-ß) potently stimulates collagen synthesis whereas interferon- suppresses expression of collagen.¹⁹ For atherosclerotic lesions causing chronic stable ischemia, excess matrix accumulation is the primary mechanism of occlusion of the atherosclerotic artery lumen. In contrast, unstable lesions may fail to synthesize a sufficient mass of matrix to provide strength to the fibrous cap.

Matrix Organization
Once extracellular matrix constituents are secreted, they must organize into a functional three-dimensional structure. The smooth muscle cell probably participates in this process, particularly with respect to collagen organization. Smooth muscle cells (as well as fibroblasts and other cells) not only secrete collagen but also have ß1 integrins that serve as receptors for collagen. Cultured vascular smooth muscle cells use ß1 integrins to organize type I collagen in a three-dimensional culture as shown by experiments using monoclonal antibodies specific to integrin subunits.²⁰ In addition, we have found that promoting excess adhesion with anti-integrin monoclonal antibodies also inhibits collagen organization, suggesting that collagen organization is a dynamic process that requires both adhesion and release of adhesion to collagen. This process may be analogous to cellular migration, which may be inhibited by either insufficient or excess adhesion to matrix components.

Many extracellular matrix proteins interact with other matrix molecules, growth factors, and cell surface receptors in complex ways. For example, decorin, a proteoglycan abundant in the artery wall, binds to collagen as well as TGF-ß, a growth factor that regulates both collagen synthesis and degradation.²¹ Another well described example is the binding of basic fibroblast growth factor (FGF-2) to heparan sulfate proteoglycans in the extracellular space or on the cell surface, thereby regulating the presentation of FGF-2 to the high-affinity FGF receptor.²² The extent and physiological relevance of these extracellular matrix molecule interactions are incompletely understood, but these interactions likely influence the ultimate strength of fibrous atherosclerotic tissue.

Matrix Degradation
Let us suppose that the cells of the atheroma construct a structurally stable, healed wound so that any increased mechanical stress in the fibrous cap is adequately balanced by the strength of the fibrous cap collagenous matrix. There are two possible ways this structure could become unstable. First, the stresses in the fibrous cap could increase, perhaps by further lipid accumulation or a surge in blood pressure.²³ Second, the matrix could be weakened somehow, such as through enzymatic degradation. In the past decade, knowledge of the mechanism of matrix degradation has rapidly expanded, and it is now clear that some degree of ongoing matrix degradation is a highly controlled and essential component of normal tissue homeostasis. It is also clear that increased matrix-degrading activity is a common finding in unstable atherosclerotic lesions, thus presenting a potential therapeutic target. There are three major pathways of extracellular matrix degradation: the serine proteases, which include urokinase-type plasminogen activator and plasmin; the cysteine proteases; and the matrix metalloproteinases (MMPs; see the Table). Membrane-associated serine proteases participate in cellular migration and metastasis and can digest proteoglycans, fibronectin, and laminin.²⁴ Noda-Heiny et al²⁵ reported augmented urokinase receptor expression in human atherosclerotic lesions; excess serine protease activity has not been demonstrated in unstable lesions. The cysteine proteases, such as cathepsins S, B, and L, usually localize in the cell within lysosomes and function under acidic conditions. However, cathepsin S retains substantial proteolytic activity at the neutral pH of the extracellular space and can digest elastin, an important constituent of arterial matrix. Sukhova et al²⁶ found that both macrophages and nonmacrophage cells express cathepsin S in advanced fibrous lesions near the shoulder region of the lesion, suggesting that this elastolytic enzyme may participate in fibrous cap degradation

View this table: [in this window] [in a new window]   Table 1. Extracellular Matrix Degradation Mechanisms

The MMPs are members of a family of related enzymes that function at neutral pH in the extracellular space.²⁷ To date, 12 members of the MMP family have been identified and numbered from 1 to 15 (numbers 4, 5, and 6 were initially thought to be members of the MMP family but were later removed). Each of the MMPs depends on zinc and calcium ions for enzymatic activity. Most MMPs are secreted as proenzymes into the extracellular space. In 1994 Sato et al²⁸ described a new membrane-type metalloproteinase (MT-MMP, or MMP-14) that had a transmembrane domain and was cell-surface associated. Specific MMPs have preferential substrates, but often more than one MMP can digest a given substrate. For example, collagen type I, a prominent component of the atheroma's fibrous cap, can be digested by either MMP-1 or MMP-8.

Given the ability of MMPs to digest all of the major extracellular matrix components, it is not surprising that the activity of these enzymes is controlled at multiple levels. First, MMPs are under transcriptional regulation. For example, both MMP-1 and MMP-3 have consensus sequences for phorbol ester–responsive elements and activator protein-1 sites, and in a broad variety of cells, cytokines induce synthesis of these enzymes.²⁹ Second, after synthesis and secretion into the extracellular space, the enzymes must be activated.³⁰ This activation step is generally associated with cleavage of 10 kDa of the N-terminus from the proenzyme, allowing the enzyme to obtain full activity. In vitro, the activation step can be achieved by a variety of agents, such as aminophenylmercuric acetate, trypsin, plasmin, or SDS.³¹ In vivo, the mechanism of activation is less clear but may involve plasmin,³² urokinase-type plasminogen activator, membrane-type MMPs, or even autoactivation. Even after MMPs are activated, they may be inhibited. This third level of MMP regulation involves a family of specific endogenous inhibitors called tissue inhibitors of metalloproteinases (TIMPs).^{27 33} TIMPs have an amino-terminal domain that binds to available MMP active sites on a 1:1 stoichiometric basis, thereby inhibiting these enzymes.

The expression of MMPs increases in the human atheroma. Henney et al³⁴ found evidence for increased stromelysin-1 expression in the human atheroma by in situ hybridization and increased protein expression of MMP-3 (stromelysin-1), MMP-1 (interstitial collagenase) and gelatinases were identified by immunohistochemistry in human lesions. Nikkari et al³⁵ found intense MMP-1 expression at the borders of lipid cores as well as subsets of smooth muscle cells and endothelial cells of human carotid atherosclerotic lesions.

Brown et al³⁶ found increased levels of MMP-9 in human coronary atherectomy specimens. However, increased gene expression or the presence of immunoreactive enzymes does not necessarily indicate excess matrix degradation, because to acquire activity, these enzymes must undergo a conformational change (usually by cleavage) and even when activated, endogenous TIMPs can inhibit the MMPs. By the technique of in situ zymography we have demonstrated that excess matrix-degrading activity is indeed present in the human atheroma, particularly in the shoulder region of the atheroma, where mechanical stresses are highest.³⁷ We also studied the localization of MMP-1 by systematically comparing the distribution of stress in human coronary lesions with the expression of MMP-1. We found that expression of MMP-1 increased severalfold in regions of increased mechanical stress.³⁸ Because this region is where pathologists have frequently noted plaque rupture to occur, it is attractive to speculate that the combination of excess matrix degradation and excess mechanical stress at these locations leads to failure of the fibrous cap.

Which cells secrete MMPs at these high-stress locations, and what stimulates them to turn on the matrix degradation system? Turning to cell culture experiments helps to define factors that may regulate MMP expression by cell type, such as the macrophage and the vascular smooth muscle cell. Vascular smooth muscle cells in culture constitutively express MMP-2, a gelatinase that degrades nonfibrillar collagen and participates in cellular migration.^{39 40} However, this MMP-2 appears to be in its inactive precursor form and bound to its selective inhibitor TIMP-2. Resting vascular smooth muscle cells in vitro express little MMP-1, an enzyme required to cleave fibrillar collagens I and III, or MMP-3, a metalloproteinase that may activate MMP-1 and also degrade vascular proteoglycans. However, in the presence of cytokines such as tumor necrosis factor- (TNF-) or interleukin (IL)-1ß, smooth muscle cells markedly increase synthesis of MMP-1 and MMP-3. One potential sequence of events is that infiltrating macrophages secrete cytokines that stimulate smooth muscle cells to produce MMPs. In support of this potential mechanism, we have found that coculture of human monocytes with vascular smooth muscle cells provokes a dramatic induction of MMP-1 and MMP-3.⁴¹ The source of these MMPs is the vascular smooth muscle cell, and this induction can be abolished by inhibiting IL-1ß.

However, the human monocyte/macrophage can also be an abundant source of MMPs.⁴² The differentiated macrophage of the atheroma differs in important ways from the circulating monocyte. The macrophage accumulates lipid through several surface molecules that bind modified lipoproteins and may also be differentiated by locally secreted factors, including macrophage colony stimulating factor and monocyte chemotactic protein-1, which are found at increased levels in the atheroma. Galis et al⁴³ studied macrophage foam cells from arterial lesions in cholesterol-fed rabbits and compared them with macrophages isolated by bronchoalveolar lavage. Alveolar macrophages harvested from hypercholesterolemic rabbits did not express MMP-1 MMP-3, but could produce these enzymes when exposed to phorbol esters. In contrast, foam cells from the atheromatous aortas of these animals expressed these MMPs without additional stimulation. This suggests that the environment of the atheroma provides sufficient signals to promote MMP synthesis by macrophages. Lendon et al¹⁵ found that the mechanical strength of human aortic plaques was reduced when macrophage density was increased, and Shah et al⁴⁴ have demonstrated that macrophages in culture may promote collagen breakdown of human fibrous caps. Because macrophage infiltration of the fibrous cap is a ubiquitous finding in unstable lesions,^{45 46} these studies provide a direct link between the macrophage and fibrous cap weakening, even in the absence of smooth muscle cell–derived MMPs.

The Necrotic Lipid Core
The typical unstable atheroma has a substantial volume of necrotic, lipid-laden debris beneath the fibrous cap. This relatively avascular and hypocellular pool contains cholesterol monohydrate, cholesterol esters, and phospholipids. The lipid core is several orders of magnitude softer than the typical fibrous cap,⁴⁷ with a semifluid consistency at body temperature. This mechanical property is likely to be a critical factor in determining plaque stability; the softer the lipid core, the more stress the overlying fibrous cap must bear. One potentially important effect of cholesterol-lowering therapy might be to change the proportions of the constituents of the lipid core. Early in experimental cholesterol-lowering studies, the proportion of cholesterol esters decreased owing to hydrolysis, leading to an increase in the proportion of insoluble cholesterol monohydrate. The net effect is a stiffening of the lipid core region; in vitro experiments suggest that the lipid core may become as much as fivefold stiffer during regression of the atheroma.⁴⁷ If such changes occur in humans, decreased stress in the fibrous cap and improved lesion stability should result.

In addition to prominently contributing to the biomechanical instability of the atheroma, the lipid core is a source of prothrombotic material. Fernandez-Ortiz et al⁴⁸ reported that compared with matrix derived from foam cell–rich lesions, sclerotic plaques, or fibrolipid plaques, the atheromatous core was markedly more thrombogenic in vitro. Tissue factor (TF; thromboplastin) binds the serine protease coagulation factor VIIa, leading to activation of factors IX and X.⁴⁹ TF protein and mRNA colocalize with foam cells in human atheromatous plaques, and TF can be found in both the necrotic core and cellular areas of the plaque. Interestingly, TF expression is diminished in restenotic lesions (which rarely cause acute thrombosis) but is associated with plaque thrombus in human atherectomy specimens.⁵⁰ Thus, when the fibrous cap fractures, contact of the blood with the lipid core may initiate thrombosis.

Cellular Death
A fascinating pathological observation is that fibrous caps that have ruptured have not only twice as many macrophages as unruptured fibrous caps but also half as many smooth muscle cells.¹⁰ Therefore, at the same time that inflammation and matrix degradation decrease plaque strength, inadequate numbers of smooth muscle cells may be present to repair the degradation. The relative decrease in smooth muscle cell number in these regions might result from growth inhibition due to the lymphokine interferon-, which is secreted by T cells that localize in this very region.⁵¹ In addition, smooth muscle cells may die in this zone of the plaque. Cellular death in the vasculature may sometimes result from lytic injury and necrosis, but recent findings suggest that smooth muscle cells of the atheroma undergo programmed cell death, or apoptosis.^{51 52} The apoptotic pathway of cell death requires de novo gene induction and protein synthesis. A mammalian cysteine protease, IL-1ß–converting enzyme, catalyzes the cleavage of the precursor of IL-1ß to its active form. This enzyme and/or other closely related proteinases participate in the induction of apoptosis. In human atheromas, regions of cells with DNA damage characteristic of apoptosis, as disclosed by labeling with the TUNEL technique (terminal deoxynucleotidyl transferase–mediated dUTP nick-end labeling), colocalize with IL-1ß–converting enzyme.⁵³

The precise signals that regulate apoptosis in the atheroma are unknown, as multiple stimuli can provoke this process. Some proteins, including IL-1ß–converting enzyme, TNF-, interferon-, or the tumor suppressor p53 sometimes function as "apoptosis on" signals, while genes such as bcl-2 can function as "apoptosis off" genes.⁵⁴ Bennett et al⁵⁴ found that stable expression of bcl-2 can protect vascular smooth muscle cells from apoptosis and that specific cytokines and growth factors, such as insulin-like growth factor-1 and platelet-derived growth factor, are potent survival factors. It is possible that withdrawal of these factors "steers" smooth muscle cells in the atheroma toward apoptosis. Lack of sufficient smooth muscle cells to secrete and organize the matrix in response to mechanical stress could render the fibrous cap even more vulnerable to weakening by extracellular matrix degradation.

	Clinical Correlations With the Biology of the Unstable Lesion

Top Introduction The Unstable Lesion and... Clinical Correlations With the... Major Gaps in Our... Steps to Prevent Plaque... References

 
Our knowledge of the importance of plaque rupture has primarily grown from careful pathological studies, since we do not have routine tools for diagnosing and treating unstable lesions prior to a vascular catastrophe. It is therefore not surprising that our proven clinical successes have largely focused on decreasing the impact of plaques that have already ruptured, such as thrombolytic therapy, angiotensin-converting enzyme therapy, and aspirin. However, a number of clinical studies provide perspectives on the potential impact of a therapy that could prevent plaque rupture.

Lessons From Angiography
Although considered almost heretical only a decade ago, it is now widely accepted that stenosis severity assessed by coronary angiography poorly predicts the propensity of that lesion to rupture. Ambrose et al⁵⁵ showed that the average degree of stenosis of lesions that progressed to acute myocardial infarction was only 48% and that fewer than half of myocardial infarctions are caused by the most severe coronary stenosis. Clinical-angiographic studies of this relation have been consistent.⁵⁶ With our current understanding of plaque rupture, it is not surprising that angiography cannot reliably predict lesion stability, particularly since the calculated mechanical stress in the lesion actually decreases in more severe stenoses.¹³

Angiography can sometimes detect the irregular or ulcerated lesions of ruptured plaques, particularly after thrombolytic therapy, but cannot determine the intimal structure or biological features characteristic of the unstable lesion. Intravascular ultrasound and angioscopy^{57 58 59} have promise for visualizing some of these features, but systematic prospective evaluation of these techniques for predicting subsequent plaque rupture has not been performed.

Lessons From Cholesterol Lowering
The marked clinical success of cholesterol-lowering therapy has reinforced the inadequacy of coronary arteriography in assessing lesion vulnerability and has raised new questions regarding the mechanism of this success.⁶⁰ These studies have constantly shown a pattern of a decrease in acute cardiovascular events in the absence of impressive changes in coronary artery stenosis severity by angiography. The answer to this apparent paradox seems to lie on the surface or beneath the lumen that is visualized by angiography. The two dominant hypotheses for the benefit of cholesterol-reduction therapy are plaque stabilization and improved endothelial function.⁶¹ Restoration of endothelial vasodilator properties could surely benefit "demand"-induced ischemia. The recovery of endothelial dysfunction and improved vasomotor function following lipid-lowering therapy could also limit vasospasm that accompanies plaque rupture, decrease recruitment of inflammatory cells, and promote fibrinolysis. The improvement in endothelium-dependent vasomotor function occurs rapidly after lipid lowering, in only a matter of weeks following effective pharmacotherapy.⁶²

The second hypothesis is that lipid lowering alters intimal plaque stability in an endothelium-independent manner. Not only do lipids in the atheroma create mechanical instability but also biologically active lipids participate in promoting the oxidative stress and inflammatory responses such as monocyte migration. Lipid lowering may therefore influence the matrix degradation cascade that appears most active in macrophage-rich areas of the atheroma. These nonendothelial mechanisms of event reduction probably require many months or even years. It is important to consider that the hypotheses of endothelial recovery and plaque stabilization are not mutually exclusive explanations for the benefits of lipid lowering. In fact, the dramatic benefits (such as the 34% reduction in coronary deaths in the 4S Study⁴ ) may be the synergistic result of improved endothelial function at the blood-atheroma interface and improved stability of the extracellular matrix in the fibrous cap.

Complex Aortic Lesions and Stroke
Strong evidence has emerged that complex atheromatous lesions of the aortic arch are a cause of stroke, particularly among patients without severe carotid artery stenoses. This association first emerged from pathological observations⁶³ but has been confirmed repeatedly through new methods of imaging the thoracic aorta, particularly with transesophageal echocardiography.^{64 65} Aortic lesions frequently develop inflammation and surface fractures, and macrophage infiltration of these aortic lesions decreases their strength. Although a prospective study demonstrating that unruptured aortic atheromas evolve into complex lesions that cause stroke has not been performed, current data suggest that strategies to decrease plaque rupture would also affect this mechanism of stroke.

Systemic Inflammation in Acute Coronary Syndromes: Cause or Consequence?
Up to this point we have focused attention on the characteristics that make an individual lesion unstable. The foci of inflammation and matrix degradation that weaken the fibrous cap are so small that systemic signs of inflammation might not be expected. However, acute-phase reactants are increased in unstable coronary syndromes, even without myocardial infarction. Berk et al⁶⁶ reported increases in C-reactive protein in patients with unstable angina, and Liuzzo et al⁶⁷ found that elevations in C-reactive protein and serum amyloid A protein in patients with unstable angina predicted poor clinical outcomes. Jude et al⁶⁸ reported that the circulating peripheral monocytes in patients with unstable angina expressed higher levels of TF than those in patients with acute myocardial infarction or stable angina. TF expression suggests activation of monocytes and could promote coagulation. These studies raise the intriguing possibility that a systemic state of inflammation promotes the plaque rupture process. However, it is also possible that C-reactive protein increases in response to a lesion that has ruptured or to many lesions in the arterial system that may have become inflamed. Thus, it is unclear whether acute-phase reactants are risk factors that will predict future unstable coronary lesions, are markers of generally inflamed lesions in the arterial system, or are markers of a single, highly active lesion that may become symptomatic. Prospective clinical studies that follow C-reactive protein or other markers of inflammation in asymptomatic patients would provide insight into this distinction. At this point, the relationship of such systemic markers of inflammation to the individual unstable coronary lesion remains undefined.

	Major Gaps in Our Understanding of Plaque Rupture

Top Introduction The Unstable Lesion and... Clinical Correlations With the... Major Gaps in Our... Steps to Prevent Plaque... References

 
Time: A Critical Dimension of Plaque Rupture
As much as pathological studies have revealed the mechanisms of plaque rupture, these studies represent "snapshots" in the life of the atheroma. We do not know the time course of inflammation in the fibrous cap or of changes in the lipid pool, two key determinants of plaque stability. Is it possible that inhibition of matrix degradation or inflammatory infiltration could be limited to specific time periods, or would such therapy be required long-term? Insight into the time dimension of plaque instability could be provided by high-resolution imaging techniques that define intimal structure, such as optical coherence tomography or magnetic resonance imaging. In addition, a truly representative animal model would also allow us to serially examine the sequence of events that lead to plaque rupture.

Plaque Rupture and Asymptomatic Lesion Growth
Davies and colleagues¹⁴ have reported that plaque ruptures can be found in 9% of subjects who died of noncardiac causes and in as many as 22% of patients with diabetes or hypertension. Fractured fibrous caps with intense inflammation are also a common finding in the abdominal aorta at necropsy, and asymptomatic carotid plaque rupture may be found in almost one fifth of elderly persons at autopsy.⁶⁹ Thus, not all plaque ruptures cause catastrophic arterial occlusion, and in fact, most plaque ruptures are probably asymptomatic. Lack of symptoms may result from limited, nonocclusive mural or intramural thrombus or from maintained perfusion by collateral vessels. Angiographic studies have suggested that progression of the atheroma may occur in a stepwise fashion. Chester et al⁷⁰ reported that angiographically complex lesions in patients awaiting balloon angioplasty were more likely to progress in severity, even in the absence of acute coronary ischemic symptoms. Thus, it is likely that many lesions grow through plaque rupture, asymptomatic healing of the mural thrombus, and wound repair of the breached fibrous cap. Prevention of rupture of nonobstructive plaques could therefore be an important strategy for limiting their evolution to more stenotic lesions that might provoke chronic ischemia as well as acute ischemic syndromes.

Sudden Death
While the evidence that plaque rupture causes most acute myocardial infarctions is overwhelming, the role of plaque rupture in sudden death is less clear. Sudden death is usually precipitated by ventricular arrhythmia in the setting of extensive coronary disease. Friedman et al⁷¹ found 19 ruptured plaques with thrombi in 37 patients (51%). Farb et al⁷² studied 50 consecutive cases of sudden cardiac death and found that typical plaque rupture with fibrous cap tearing was present in 28 of 50 cases (65%). However, the remaining 22 cases had thrombi without rupture, and the thrombi were associated with superficial erosions of a proteoglycan-rich plaque. Erosions were less often associated with calcification and infiltration by macrophages and T cells and more often associated with clusters of smooth muscle cells adjacent to the thrombi. Thus, it appears that acute plaque rupture plays a role in at least half of the cases of sudden death but that the pathophysiological sequence that leads to thrombosis in the remaining cases may be different from fibrous cap fracture.

Lack of a Representative Animal Model
While numerous models of intimal thickening, restenosis, and intravascular thrombosis have contributed to vascular biology, an animal model of plaque rupture remains elusive. Abela et al⁷³ reported characterization of a rabbit model of intra-aortic thrombosis that was initially developed by Constantinides and Chakravarti in 1961. In this model, cholesterol-fed rabbits undergo balloon injury and are later injected with Russell's viper venom (which contains potent procoagulant and endothelial toxin activity) and histamine. These animals frequently develop acute thrombi of the aorta, but the thrombi appear to be associated with endothelial toxicity rather than the more typical collagenous, fibrous cap fracture found in human lesions. In this regard, this model may be more representative of the inflammatory erosions that accompany sudden death.

Mouse models of atherosclerosis develop aortic lesions with many features of those in humans, including foam cells and collagenous fibrous caps. Spontaneous plaque rupture has not been a feature of the apoE- or LDL receptor–deficient mouse, however. One potential explanation is simply that the mouse aorta has a diameter on the order of a millimeter. Since circumferential stress, the mechanical component most associated with plaque rupture, is directly proportional to the radius of the artery, it follows that the tensile forces on the mouse atheroma are much smaller than those in the human fibrous cap.

Skinner et al⁷⁵ reported a study of a double balloon injury–hypercholesterolemic rabbit model in which they found one rabbit aortic thrombus 14 months after beginning the protocol. This thrombus developed over a lesion very similar to the typical fibrous cap fracture of human myocardial infarction. An exciting component of this report was the use of magnetic resonance imaging with a phased-array receiving coil, which allowed the investigators to visualize the features of plaque rupture while the animal was still alive. This study demonstrated not only the potential of noninvasive imaging for revealing the natural history of the atheroma, but also the likely necessity of long observation periods to develop spontaneous plaque rupture in animal models.

	Steps to Prevent Plaque Rupture

Top Introduction The Unstable Lesion and... Clinical Correlations With the... Major Gaps in Our... Steps to Prevent Plaque... References

 
The current information available on the mechanism of plaque rupture suggests that several potential therapeutic options could provide additional clinical benefit beyond our current therapies. First, we could identify unstable lesions with improved imaging modalities and intervene on the lesion before it ruptures. Second, we could modulate the strength of the extracellular matrix of the fibrous cap by directly delivering growth factors, drugs, or genes that can regulate extracellular matrix, steps that are analogous to the intense efforts to improve dermal wound healing. Third, we can attempt to inhibit a step in the inflammation sequence between monocyte recruitment, migration, differentiation, and secretion of cytokines and proteases. Fourth, we can directly inhibit matrix degradation and weakening in the extracellular space. MMP inhibition is attractive for this purpose because the enzymes are extracellular and orally active inhibitors are already in clinical trials for noncardiovascular indications. MMP inhibition could have the added benefit of inhibiting smooth muscle cell migration into the intima. However, not all of the matrix-degrading enzymes in the plaque are MMPs, and the contribution of these non-MMPs to plaque destabilization requires consideration as well. Currently, we can only speculate about the potential risks and benefits of these strategies.

While coronary death rates have fallen in the United States, our appreciation of the broad implications of the unstable atheroma have just as dramatically increased. Attacking this problem could benefit the prevention not only of acute myocardial infarction but also acute peripheral vascular ischemia, stroke, and even the general progression of atherosclerosis. However, the pursuit of plaque rupture as a primary end point in clinical trials using any of the above strategies will likely require convincing vascular biology evidence that a particular strategy holds promise.

Establishing a robust, representative animal model of plaque rupture is imperative. Anecdotal evidence indicates that this is possible, and the cardiovascular community should recognize that establishing this model will require more patience and perseverance than animal models of intimal thickening or restenosis have required. In addition, we need improved imaging modalities, both for studying animal models and for following individual human atherosclerotic lesions, preferably noninvasively. The combination of a suitable animal model and imaging would likely turn the snapshots of postmortem examination of plaque rupture into a moving picture of vascular biology that should improve our understanding of when and how to intervene to prevent plaque rupture.

View larger version (52K): [in this window] [in a new window]   Figure 1. The unstable atheroma. Plaque rupture occurs when the mechanical stresses in the fibrous cap exceed a critical level that the tissue can withstand. Several factors increase stress in the atheroma. A thin, fibrous cap overlying a large lipid pool causes very high stresses in the fibrous cap. In addition, moderately stenotic lesions may have higher stresses than very severe lesions. The ratio of cholesterol esters to monohydrate cholesterol may determine how "soft" the lipid pool is, so that the nature of the lipid may be important. While these factors may be acting to increase stresses, a number of biological factors may be weakening the fibrous cap. These include inappropriately low collagen synthesis, an increase in collagen degradation activity, and infiltration of the fibrous cap with cells that promote these activities (macrophages, T cells). At the same time, there may be an inappropriately low number of smooth muscle cells at critical locations, since these cells are primarily responsible for maintaining the mechanical integrity of the fibrous cap.

	Acknowledgments

 
This study was supported in part by a Grant-in-Aid from the American Heart Association and grants from the National Heart, Lung, and Blood Institute (HL-47840, HL-48743, HL-54759).

	Footnotes

 
Arterioscler Thromb Vasc Biol. 1997;17:1859-1867.

Received May 13, 1997; accepted July 27, 1997.

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Top Introduction The Unstable Lesion and... Clinical Correlations With the... Major Gaps in Our... Steps to Prevent Plaque... References

 

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