Atherogenesis and atherothrombosis – focus on diabetes mellitus

Best Practice & Research Clinical Endocrinology & Metabolism 23 (2009) 291–303

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Atherogenesis and atherothrombosis – focus on diabetes mellitus Bernd Stratmann, PhD, Research Director, Diethelm Tschoepe, MD, Dr h.c., Medical Director * Heart and Diabetes Center NRW, Ruhr University of Bochum, Georgstrasse 11, D-32545 Bad Oeynhausen, Germany

Keywords: atherosclerosis atherothrombosis platelet activation endothelial dysfunction diabetes mellitus

Diabetes mellitus represents a major cause of cardiovascular morbidity and mortality in developed countries, and atherothrombosis accounts for most deaths among patients with diabetes mellitus. Atherothrombosis is defined as atherosclerotic lesion disruption with superimposed thrombus formation. As a long-term, progressive disease process, atherosclerosis often results in an acute atherothrombotic event through plaque rupture and formation of a platelet-rich thrombus. The principal clinical manifestations of atherothrombosis are sudden cardiac death, myocardial infarction, ischaemic stroke, and peripheral arterial ischaemia comprising both intermittent claudication and critical limb ischaemia. Atherosclerosis is the leading cause of morbidity and mortality in the industrialized world, and diabetes mellitus magnifies the risk of cardiovascular events. In addition to the well-known microvascular complications of diabetes mellitus – such as nephropathy, retinopathy and neuropathy – the risk of macrovascular complications affecting the large conduit arteries markedly increases in patients with diabetes mellitus. Ó 2008 Published by Elsevier Ltd.

Defined as atherosclerotic plaque disruption with superimposed thrombus formation, atherothrombosis is the leading cause of mortality in the Western world. Diabetes mellitus has been recognized as an independent cardiovascular risk factor, and atherothrombosis accounts for nearly 80% of deaths among patients with diabetes mellitus.1,2 Atherothrombosis is the result of atherosclerosis progression, and the principal clinical manifestations are sudden cardiac death, myocardial infarction

* Corresponding author. Tel.: þ49 57 31 97 2292; Fax: þ49 57 31 97 2122. E-mail address: [email protected] (D. Tschoepe). 1521-690X/$ – see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.beem.2008.12.004

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(MI), ischaemic stroke, and peripheral arterial ischaemia (both intermittent claudication and critical limb ischaemia). These events are mostly secondary to atherosclerotic plaques disruption and subsequent thrombus formation.3 According to an international registry on outpatients with atherothrombosis, the 1-year incidence of death, MI, stroke, or hospitalization for an ischaemic event is 14% for patients with established atherosclerotic disease, and 5% for patients with multiple (more than two) risk factors.4 Atherosclerosis is a diffuse process that starts early in childhood and progresses – mostly in the absence of symptoms – throughout adult life.5 This systemic arterial disease preferentially affects the intima of large- and medium-sized systemic vessels, including the carotid, aorta, coronary, and peripheral arteries. The main components of atherothrombotic plaques are6–10:  connective tissue extracellular matrix such as collagen, proteoglycans, and fibronectin elastic fibres;  crystalline cholesterol, cholesteryl esters, and phospholipids;  cells such as monocyte-derived macrophages, T-lymphocytes, and smooth-muscle cells;  thrombotic material with platelets and fibrin deposition. Furthermore, atherothrombotic events result from a complex inflammatory response to a multifaceted vascular pathology. Key inflammatory factors in atherothrombosis include activated endothelial cells; inflammatory leukocytes (which are a source of thrombogenic stimuli); smooth muscle cells (which act as a source of procoagulants and an amplifier of inflammatory responses during thrombosis); and platelets (which act as an important source of further inflammatory mediators). Diabetes mellitus is associated with a hypercoaguable state which is pronounced in the postprandial period. Hyperactivated platelets at injured endothelial surfaces act together with an increased availability of thrombotic precursors, reduced coagulation inhibitors and dimished fibrinolysis.11 The United Kingdom Prospective Diabetes Study (UKPDS) clearly showed that macrovascular events among people with diabetes mellitus account for more than 50% of total mortality, and that vascular events represent the most relevant clinical endpoint in type-2 diabetes mellitus.12 In conclusion, in diabetes mellitus atherosclerosis develops more aggressively and faster, and leads more frequently to thrombotic events through the interaction between the vessel wall and the hypercoagulable blood.13,14

Atherothrombosis – the role of the endothelium A single layer of endothelial cells covers the inner surface of all blood vessels and provides a metabolically active interface between blood and tissue. The endothelium modulates blood flow, nutrient flux, coagulation and thrombosis, and leukocyte diapedesis.15 Important bioactive substances such as nitric oxide, prostaglandins, endothelin, and angiotensin II, regulating blood vessel function and structure, are synthesized by endothelial cells. Endothelial dysfunction is a systemic, reversible disorder considered as the earliest pathological process of atherosclerosis.16,17 It is defined as the partial or complete loss of balance between vasoconstrictors (endothelin, angiotensin II) and vasodilators (nitric oxide, prostacycline), growth-promoting and -inhibiting factors, proatherogenic and antiatherogenic factors, and procoagulant and anticoagulant factors.17 It is involved in the recruitment of inflammatory cells into the vessel wall and in the initiation of atherosclerosis. Endothelial cells produce cytokines, express adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and selectins, and assist leukocytes and other blood-derived cells in ‘homing’ and atheroma infiltration.7 Thus, the endothelium is a dynamic autocrine and paracrine organ that relies on regulation by nitric oxide (NO). Endothelial NO synthase (eNOS) synthesizes NO in the endothelial cells from L-arginine in presence of tetrahydrobiopterin. NO is released mainly in response to blood-flow-induced shear stress and pharmacological stimulants such as acetylcholine. Besides the function as a vasodilator, NO inhibits adhesion and migration of leukocytes into the arterial wall, proliferation of smooth muscle cell, and adhesion and aggregation of platelets. Secondary changes may occur in the underlying media and adventitia, particularly in advanced stages of the disease. A dysfunctional endothelium facilitates

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vessel wall entry and oxidation of circulating lipoproteins, monocyte entry and internalization or inflammation, smooth-cell proliferation and deposition of extracellular matrix, vasoconstriction, as well as a prothrombotic state within the vessel lumen. Endothelial dysfunction often manifests at points where there is blood flow and flow reversal or oscillating shear stress near bifurcations. Activation of the arterial endothelium may arise from a number of cardiometabolic derangements, including oxidative stress (e.g. from smoking or formation of advanced glycation end products in the setting of hyperglycaemia), mechanical stress due to hypertension, or reduced endothelial function secondary to insulin resistance (Fig. 1). The activated endothelium expresses cell adhesion molecules (CAMs), such as ICAM and VCAM, or proteins from the selectin family such as E- or P-selectin, which bind monocytes loosely to the endothelial surface. The chemotactic factor monocyte chemoattractant protein-1 (MCP-1) mediates

Diabetes mellitus

Hyperglycaemia

Excess free fatty acids

Insulin resistance

Oxidative stress PKC activation RAGE activation

ENDOTHELIUM Nitric oxide Endothelin-1 Angiotensin II

Nitric oxide Activation of NF-KB Angiotensin II Activation of activator protein-1

Nitric oxide Tissue factor Plasminogen activator inhibitor-1 Prostacycline

Vasoconstriction

Inflammation

Thrombosis

Hypertension Vascular smooth Muscle cell growth

Release of chemokines Release of cytokines Expression of cellular adhesion molecules

Hypercoagulation Platelet activation Decreased fibrinolysis

ATHEROGENESIS Fig. 1. Endothelial dysfunction in diabetes mellitus. PKC, protein kinase C; RAGE, receptor for advanced glycation end products; NF-kB, nucelar factor kB. Adapted from Beckman et al (2002, JAMA 287: 2570–2581) with permission.

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tight binding of monocytes and infiltration into the vessel wall where they readily take up lipids. This process seems to relate to an ionic interaction of apolipoproteine B with matrix proteins such as proteoglycans, collagen, and fibronectin. The interaction of low-density lipoprotein (LDL) with proteoglycans is of major concern in early atherosclerosis; intravascular accumulation of LDL leads to chemical modification (oxidation) and induction of inflammation. Monocytes then differentiate into macrophages under the influence of macrophage colony-stimulating factor and other chemokines. These macrophages, in turn, become foam cells, and then break down to form fatty streaks, thus providing the beginnings of the lipid core of the mature atherosclerotic plaque. Typically, this accumulation of lipids drives the growth of the atherosclerotic plaque. However, the inflammatory cells that invade the arterial wall during atherogenesis also secrete a range of inflammatory substances and growth factors which profoundly influence the properties of the arterial wall. Thus, in the later steps of atherosclerosis development, proliferation of smooth muscle cells and deposition of collagen contribute increasingly to the overall formation of the plaque. The contents of the plaque are contained within a collagen-rich fibrous cap which stabilizes the plaque and prevents access of its thrombogenic core to the bloodstream. This cap itself is continually remodelled, with simultaneous removal and replacement of collagen. Clearly, any reduction in the strength of the fibrous cap during this process may increase the likelihood of plaque rupture, which is believed to be the most common precipitating event for coronary thrombosis and MI, and an important cause of unstable angina pectoris where coronary occlusion is incomplete. The normally protective endothelium is capable of modulating interaction between platelet and vessel wall by activating vascular smooth muscle cells in a thromboxane-A2- and serotonin-dependent way. Plaque composition, rather than luminal stenosis, is recognized as the major determinant of this disease. Since tissue factor is found within atheroma, and also in the bloodstream of atherosclerotic patients, it probably plays a key role in determining both plaque and blood thrombogenicity. Tissue factor (TF) is expressed in subendothelial cells such as vascular smooth muscle cells and fibroblasts, resulting in an immediate initiation of blood coagulation once vessel damage occurs. Endothelial cells do not express tissue factor under physiological conditions, but may be stimulated by various cytokines, such as tumour necrosis factor a (TNFa), interleukin 1b (IL-1b), and CD40 ligand (CD40L). Other mediators such as thrombin, oxidized LDL (oxLDL), vascular endothelial growth factor (VEGF) and serotonin or histamine are also able to initiate TF expression of endothelial cells. TF expression is not only regulated by in vivo expression as a latent form but also through the inhibition by tissue factor pathway inhibitor (TFPI). The inflammatory environment with increased levels of expression of cytokines further induces TF expression in monocytes, foam cells, endothelial cells, and vascular smooth muscle cells, and therefore drives the process of coagulation.18,19 One key event besides inflammation in atherosclerotic progression is the production of reactive oxygen species (ROS) which finally lead to oxidative stress. Hypercholesterinaemia, hypertension, diabetes mellitus and smoking promote vascular oxidative stress. Superoxide anions act directly as vasoconstrictors, and may scavenge NO to form peroxynitrate, which is highly reactive. Oxidative stress further triggers the formation of oxLDL, promoting the vicious circle of atherogenesis. Endotheliumdependent vasodilation is a prognostic marker in atherosclerosis. The measurement of flow-mediated dilatation is an accepted tool, as the arterial diameter response to reactive hyperaemia correlates well with the progression of endothelial dysfunction.20 Besides its ability to accept cholesterol from lipidloaded macrophages and transport it out of the artery wall for catabolism and excretion (reverse cholesterol transport)21–23, high-density lipoprotein (HDL) particles contain antioxidant enzymes such as paraoxonase-1, an enzyme that can inactivate potentially proinflammatory phospholipids.24 The HDL particles can mitigate expression of cytokine-induced leukocyte adhesion molecules by endothelial cells. Thus, low levels of circulating HDL deprive the atheroma’s microenvironment of an endogenous antioxidant and anti-inflammatory particle. However, it is well recognized that a cluster of other cardiovascular risk factors related to thrombosis are also associated with the metabolic syndrome, including impaired endogenous fibrinolysis secondary to increased levels of plasminogen activator inhibitor-1 (PAI-1), and increases in circulating fibrinogen, and liquid-phase coagulation factors such as factor VII, von Willebrand factor, etc.25,26 The atherothrombotic disturbances associated with the metabolic syndrome are likely to markedly increase the risk of intravascular occlusion at the site of a vulnerable plaque and may account for

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a substantial proportion of the excess cardiovascular risk associated with the metabolic syndrome. Establishing adequate control of these risk factors therefore remains a principal goal of therapy to prevent or delay the onset of established cardiovascular disease.27 The mechanisms involved in the progression of atherosclerosis are summarized in Fig. 2. The atherosclerotic process in patients with diabetes mellitus in not really distinct from that in nondiabetic people. However, it starts earlier, progresses faster, and is localized more peripherally.28 Patients with diabetes mellitus experience cardiovascular events earlier than unaffected individuals, and may have more diffuse and severe underlying pathologies. Simple lipid accumulation in the arterial walls is no longer considered sufficient to be the sole driver of atherothrombotic events, and an endothelial microenvironment favouring plaque rupture is required. The prognosis for the outcome is worsened in the preceding phase of glucose intolerance, which is in accordance with the ‘common soil’ hypothesis for both diabetes mellitus and vascular disease implementing the metabolic disease in a wider network of classical risk factors such as hypertension, dyslipoproteinaemia or obesity in a situation of a genetically programmed susceptibility.29,30 Metabolic syndrome has to be regarded as metabolic vascular syndrome. Atherothrombosis – the role of platelets Platelet activation Platelets are anuclear cellular structures, originating from megakaryocytes in the bone marrow, that are activated upon stimulation with arachidonic acid, adenosine diphosphate (ADP), thrombin,

HYPERGLYCAEMIA

INFLAMMATION ↑IL-1β ↑IL-6 INFECTION ↑CRP ↑SAA ↓ defence ↑ pathogen burden

↑AGE ↑ROS

INSULIN RESISTANCE

DYSLIPIDAEMIA hypertension, endothelial dysfunction ↑ LDL

↑ TG THROMBOSIS ↓ HDL ↑ PAI-I ↑ TF ↓ tPA

subclinical atherosclerosis

DISEASE PROGRESSION

atherosclerotic clinical events (acute and chronic coronay syndromes, stroke, peripheral arterial disease)

Fig. 2. Pathogenic mechanisms involved in the initiation and progression of atherosclerosis in patients with diabetes mellitus, AGE, advanced glycation end products, CRP, C-reactive protein; HDL, high-density lipoprotein; IL-1b, interleukin-1b; IL-6, interleukin-6; LDL, low-density lipoprotein; PAI-I, plasminogen activator inhibitor-I; ROS, reactive oxygen species; SAA, serum amyloid A protein; TF, tissue factor; TG, triglycerides; tPA, tissue-type plasminogen activator. Adapted from Biondi-Zoccai et al (2003, Journal of the American College of Cardiology 41: 1071–1077) with permission.

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advanced glycation end products, or when in contact with surface molecules of the subendothelial layer. Although activation is caused by a variety of substances, platelets respond with the same series of distinguishable actions: (a) change of shape from a discoid to a pseudopodial structure; (b) aggregation with platelets and other blood-derived cells; (c) release of substances from the three secretory granules (a-granules, dense bodies, and lysosomes); and (d) liberation of arachidonic acid, which is rapidly converted to prostaglandins and lipoxygenase products.31 The a-granules are the most abundant granules and contain a variety of high-molecular-weight proteins like P-selectin, von Willebrand factor, fibrinogen, GPIIb/IIIa, Factor V, Factor X, PAI, growth factors, cytokine-like proteins such as b-thrombomodulin, platelet factor 4, CD40L and IL-1. The dense granules contain low-molecular-weight compounds which promote platelet activation (e.g. ADP, serotonin, and Ca2þ). Lysosomes contain hydrolytic enzymes such as elastase, which may affect the vessel wall and thus promote atherosclerosis. The platelet plasma membrane contains a large number of receptors that specifically bind agonists which stimulate the physiological platelet response, e.g. ADP, epinephrine, collagen, thrombin, serotonin, and platelet-activating factor (PAF). The interaction between a platelet-activating agonist and its receptor causes rapid mobilization of signalling molecules within the platelet – mainly calcium, diacylglycerol (DAG), and inositol 1,4,5-trisphosphate (IP3) – which are sufficient to initiate and complete shape change and aggregation responses.31 Platelet activation is a hallmark reaction of atherothrombosis. Platelets provide the haemostatic interface between the vascular wall and the liquid phase of coagulation and fibrinolysis, resulting in physiological balance at sites of vascular injury.32 Under physiological conditions, the aggregation response is instigated when circulating platelets meet a ruptured atherosclerotic plaque and are exposed to agonists. Loose adhesion to vessel-wall matrix ligands, such as von Willebrand factor and collagen, triggers via platelet membrane glycoprotein receptors a signal promoting ion flux, protein kinase activation, cytoskeletal polymerization, and arachidonic acid metabolism (‘outside-in’ signalling). Such intraplatelet events lead to conformational changes in the glycoprotein surface receptor IIb/IIIa (GPIIb/IIIa), exposing the high-affinity binding site for fibrinogen, which cross-links the activated platelets (‘inside-out’ signalling).33 The adhesion or aggregation of platelets is regulated by pro-aggregants and anti-aggregants within the circulation. Prostacycline and NO are anti-aggregants, supporting a vasodilatory, anticoagulant phenotype to prevent the formation of thrombi.34 Insulin antagonizes the platelet activation/aggregation of several antagonists such as ADP, PAF and collagen via cell-surface receptors.35,36 However, in vivo studies show that platelets of insulin-resistant subjects are resistant to the stimulation by insulin, NO or prostacycline, suggesting that in cases of insulin resistance platelet aggregation is upregulated.36–38 Furthermore, in patients with type-2 diabetes mellitus, platelets adhere to the vascular endothelium more readily than in healthy subjects.39 Platelet activation in diabetes mellitus In diabetes mellitus, activation of the intrinsic coagulation pathway occurs more easily and fibrinolysis is effectively decreased.40 Increased platelet reactivity involves intensified adhesion and aggregation in patients with diabetes mellitus or those at high risk for the disease. Subjects at various stages of diabetes proved to have increased numbers of CD62P- and CD63-positive platelets (‘activated platelets’) compared to healthy controls. Whereas such a finding was anticipated, in line with the socalled ‘response-to-injury’ theory in subjects with clinically overt angiopathy, newly diagnosed insulin-dependent diabetes patients already show clearly increased levels of circulating activated platelets with exposed adhesion molecules. Moreover, this activation was not related to the improvement of glycaemic control with intensified insulin therapy. Most strikingly, CD62P-positive platelets can also be detected in metabolically healthy first-degree relatives of clinically manifest type1 diabetes patients. Basal thromboxane B(2) is significantly increased in resting platelets from both type-1 and type-2 diabetes patients, even in the absence of vascular complications and in cases of wellcontrolled diabetes. Flow cytometry has revealed that a subpopulation of large, hyperactive platelets circulates in patients with diabetes mellitus, at a level similar to that in patients who have experienced an MI. This suggests that the increased potential for aggregation of such platelets lowers their threshold for activation, thus contributing to the increased incidence of acute cardiovascular events in diabetes mellitus.

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Contribution of platelets to atherothrombosis Platelets are involved in the very early phase of atherothrombosis. Blood-flow disturbances lead to a loose adhesion of platelets to the arterial wall, even in the absence of endothelial dysfunction, resulting in the formation of lesions.41,42 Whereas the intact endothelium prevents platelet adhesion, inflammatory conditions lead to an activation of the endothelial monolayer, to which platelets adhere independently from shear stress.43,44 Platelet adherence occurs through several steps involving platelet tethering, rolling, and finally firm adhesion to the vessel wall.45,46 These processes involve selectins, integrins, and immunoglobin-like receptors, which induce receptor-specific activation signals in both platelets and endothelial cells. Selectins mediate the rolling of platelets on the endothelial surface and are present on the platelet surface as well as on the endothelial cell.47,48 Platelets interact with the inflamed endothelium with an endothelial ligand via P-selectin.49 The GPIIb/IIIa receptor plays a key role in platelet accumulation on activated endothelium, as it is the major integrin on platelets. In the presence of fibrinogen, the GPIIb/IIIa receptor mediates the adhesion to avb3-expressing cells including endothelial cells.44,50,51 The vitronectin-receptor (avb3) appears to play a crucial role in platelets adhesion. It is up-regulated upon endothelial cell activation via IL-1b or thrombin.44 Activated platelets show an increased adhesiveness and aggregation in response to collagen, thrombin, and platelet-activating factor. Thus, abnormalities in platelet function may exacerbate the progression of atherosclerosis and the consequences of plaque rupture. Platelet activation may lead to increased microembolism in the capillaries and local progression of pre-existing vascular lesions.52,53 Patients with diabetes mellitus frequently have hypercoagulable blood, as evidenced by increased plasmatic coagulators, depressed fibrinolysis, reduced endothelial thromboresistance, and platelet hyperreactivity.11,54 These alterations lead to a downshift of the coagulation threshold in the arterial circulation where occlusive thrombi induce hypoxic damage to parenchymal organs. The immediate aggregation response of platelets after exposure to physiological agonists is identical to the reaction as platelets bypass a ruptured atherosclerotic plaque in vivo. Clinically, this situation is comparable to the acute coronary syndrome which is a completely platelet-driven process and frequently spins off towards a mixed occlusive thrombus containing leukocytes, erythrocytes, and fibrin strands. This corresponds to the infarction of the parenchymatous organ.55,56 Occlusion of the capillaries by platelet or mixed platelet–leukocyte emboli may cause sustained occlusion of the functional microcirculation even without any acute clinical symptoms. Basement membrane thickening, microaneurysms and capillary occlusions have been reported, and would serve as pathological substrates for the diabetic microangiopathy.57 Platelets, together with neutrophils, have been shown to modulate vasoconstrictor responses under these conditions as they are strongly involved in the network of reperfusion injury mechanisms. Both of these processes are clearly worsened in diabetes mellitus.58 Summarizing, in atherothrombosis platelets contribute to the amplification of atherosclerotic lesions, to thrombogenesis and to distal embolization into the microvasculature (Fig. 3). Atherothrombosis – the phases of progression The progression of atherothrombosis can be divided into five phases8,59: In the early phase (phase 1), plaques are small and are seen predominantly in young people. Type-I lesions consist of macrophage-derived lipid droplets containing foam cells. Type-II lesions contain both macrophages and smooth muscle cells with mild lipid deposits. Type-III lesions only contain smooth-muscle cells that are surrounded by extracellular connective tissue, fibrils and lipid deposits. In the advanced phase (phase 2), lesions may rupture because of their lipid content, their increased inflammation, and the thin fibrous cap. Type-IV lesions consist of a confluent cellular lesion and extracellular lipid in a mixture with normal intima or type-Va lesions which show an extracellular lipid core covered by an acquired fibrous cap. In phase 3, acute complicated type-VI lesions appear, originating from ruptured or eroded lesions (type-IV or -Va) and leading to mural, non-obstructive thrombosis.

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Atherogenic blood interaction

Plaque formation

Plaque stressors: Plaque rupture

Thrombogenic blood interaction

torsion pressure pressure oscillations stretching shear postprandial state

endogenous thrombolysis

Sudden vessel closure Fig. 3. Clinical ischaemia event cascade in patients with diabetes mellitus.

Acute complicated type-VI lesions with fixed or repetitive occlusive thrombosis characterize phase 4. Even in the coronary vasculature, most of the time this process is clinically silent.60,61 Just in one third of acute coronary syndrome (ACS) cases, thrombus formation occurs on the surface of a stenotic plaque, while in the remainder the process develops on a non-stenotic plaque. Geometrical changes in ruptured plaques and organization of the occlusive or mural thrombus by connective tissue may lead to the occlusive or significantly stenotic and fibrotic plaques. Type Vb (calcific) or Vc (fibrotic) lesions that may cause angina are observed in phase 5. If preceded by stenosis or occlusion with associated ischaemia, the myocardium may be protected by collateral circulation, and such lesions may be clinically inapparent.62,63 The development and maturation of a plaque in different phases is excellently described by Libby and Ridker.64

Atherothrombosis – the rupture of plaques Stable angina is associated with smooth, fibrous coronary artery plaques, whereas unstable angina, acute MI, and sudden cardiac death are invariably associated with irregular or ruptured plaques.65 In the coronary arteries, small plaques tend to be lipid-rich and prone to disruption. Markedly stenotic plaques tend to be fibrotic and stable.66 Features of vulnerability are: a large lipid core, a thin fibrous cap, and an inflammatory filtrate rich in monocytes and macrophages.67 Physical forces may disrupt the thin foam-cell-infiltrated cap.68 Vulnerability of the plaque depends on several factors, including circumferential wall stress or cap fatigue, blood-flow characteristics, location, size, and consistency of

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the atheromatous core.68–70 The second mechanism that leads to plaque rupture involves macrophages71 and mast cells72 which degrade extracellular matrix compounds by phagocytosis or secretion of proteolytic enzymes such as plasminogen activator and matrix metalloproteases (MMPs).73,74 Studies of ruptured plaques indicate that large necrotic cores, a fibrous cap of <65 mm in thickness, intraplaque haemorrhage, and infiltrating macrophages within the cap are indicators of instability.75–77 The most common substrate for coronary thrombosis is plaque rupture, with 30–40% of thrombotic embolisms occurring at sites without obvious plaque rupture.78 Besides plaque rupture, plaque erosion occurs as a process which describes plaques as less stenotic, showing lower macrophage infiltration and lower incidence of calcification. A terminology for high-risk and vulnerable coronary artery plaques was recently published (Table 1).

Atherothrombosis – therapeutic interventions A significant decrease in major cardiovascular events by aggressive lipid-lowering therapy with the use of intensive statin therapy in patients with either unstable or stable coronary artery disease was achieved, but all-cause mortality was only reduced in the unstable disease group.79 The rise in HDL cholesterol levels in combination with a decrease in LDL is effective in the regression of atheroma volume.80 Furthermore, a 1.1% reduction in the 5-year risk of major cardiovascular events was achieved per 1-mg/dL increment in HDL concentration.81 Until recently, no antidiabetic drug has demonstrated the ability to reduce the progression of coronary atherosclerosis. The PERISCOPE (Pioglitazone Effect on Regression of Intravascular Sonographic Coronary Obstruction Prospective Evaluation) trial enrolled 547 patients to analyse the effect of an insulin sensitizer, pioglitazone, with an insulin secretagogue, glimepiride, on the progression of coronary atherosclerosis in patients with type-2 diabetes mellitus. The change in atheroma volume from baseline to study completion was defined as the main outcome measure. Treatment with pioglitazone in patients with type-2 diabetes mellitus and coronary artery disease resulted in a significantly lower rate of atherosclerosis progression compared to the treatment with glimepiride. In addition, the group treated with pioglitazone showed significantly improved HDL cholesterol values and significantly improved HbA1c values compared to the glimepiride group and baseline values.82

Table 1 Main characteristics of plaque and correct terminology commonly used in atherothrombosis and acute coronary syndromes.86 Culprit lesion

Eroded plaque

High-risk, vulnerable, and thrombosis-prone plaque Inflamed thin-cap fibroatheroma Plaque with a calcified nodule

Ruptured plaque

Thrombosed plaque Vulnerable patient

A lesion in a coronary artery considered on the basis of angiographic, autopsy, or other findings to be responsible for the clinical event. In unstable angina, myocardial infarction, and sudden coronary death, the culprit lesion is often a plaque complicated by thrombosis extending into the lumen A plaque with loss and/or dysfunction of the luminal endothelial cells leading to thrombosis. There is usually no additional defect or gap in the plaque, which is often rich in smooth muscle cells and proteoglycans These terms can be used as synonyms to describe a plaque that is at increased risk of thrombosis and rapid stenosis progression An inflamed plaque with a thin cap covering a lipid-rich, necrotic core. An inflamed thin-cap fibroatheroma is suspected to be a high-risk/vulnerable plaque A heavily calcified plaque with the loss and/or dysfunction of endothelial cells over a calcified nodule, resulting in loss of fibrous cap, that makes the plaque high-risk/vulnerable. This is the least common of the three types of suspected high-risk/vulnerable plaques A plaque with deep injury with a real defect or gap in the fibrous cap that had separated its lipid-rich atheromatous core from the flowing blood, thereby exposing the thrombogenic core of the plaque. This is the most common cause of thrombosis A plaque with an overlying thrombus extending into the lumen of the vessel. The thrombus may be occlusive or non-occlusive A patient at high risk (vulnerable, prone) for experiencing a cardiovascular ischaemic event due to a high atherosclerotic burden, high-risk vulnerable plaques, and/or thrombogenic blood

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The post-hoc analysis of the CHARISMA trial (Clopidogrel for High Atherothrombotic Risk and Ischemic Stabilization Management and Avoidance) answers clearly the question of whether to treat or not to treat patients at risk for atherothrombosis with a combination of acetylsalicylic acid (ASA) and clopidogrel. Patients with risk factors do not benefit from this combination therapy if compared with ASA alone, but those with clinically evident atherothrombosis do.83 Furthermore, the TRITON-TIMI-38 trial (Trial to assess Improvement in Therapeutic Outcomes by optimizing platelet inhibition with prasugrel – Thrombolysis in Myocardial Infarction)-38 evaluated the clinical benefit of the thienopyridine prasugrel in comparison to clopidogrel in patients with acute coronary syndrome undergoing percutaneous coronary intervention, and showed a superiority of prasugrel regarding the primary end point cardiovascular death, non-fatal MI, or non-fatal stroke.84 Patients at risk for atherosclerosis should be treated regarding their risk profile and according to the atherothrombotic phase. As the easiest way, lifestyle modification can yield regression of endothelial dysfunction which is regarded as a first step in atherothrombis.85 Conclusion Atherothrombosis is a complex disease characterized by cholesterol deposition, inflammation, and thrombus formation. High-risk vulnerable plaques are responsible for acute coronary thrombosis, and plaque rupture is the most common trigger for thrombosis. The disease is best characterized by a description of five phases. Therapeutic interventions are multifunctional; besides blood pressure regulation to reduce shear stress and endothelial dysfunction, lipid-lowering and antiplatelet therapies are beneficial. The abnormalities in haemostasis in patients with insulin resistance and diabetes mellitus generate a prothrombotic phenotype. The more pronounced disease progression in subjects with diabetes mellitus and the increased risk of thrombotic vascular occlusion among these patients emphasizes the need for a distinctive and multimodal disease management. The cornerstones of pharmacological treatment, in both primary and secondary prevention as well as in the setting of acute coronary syndromes, are statins and lipid-lowering agents.

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