Mast cells in vulnerable coronary plaques: potential mechanisms linking mast cell activation to plaque erosion and rupture

PURPOSE OF REVIEW: A novel link between inflammation and acute coronary syndromes is emerging, in that infiltrating inflammatory cells may convert a clinically silent coronary plaque into a dangerous and potentially lethal plaque. The majority of acute atherothrombotic events now relate to erosion or rupture of such unstable plaques. Here we summarize the molecular mechanisms by which activated mast cells may contribute to plaque erosion or rupture. RECENT FINDINGS: In-vitro experiments have revealed a multitude of paracrine effects exerted by activated mast cells. By secreting heparin proteoglycans and chymase, activated mast cells efficiently inhibit the proliferation of smooth muscle cells in vitro, and reduce their ability to produce collagen by a transforming growth factor beta-dependent and -independent mechanism. Mast cell chymase and tryptase are capable of activating matrix metalloproteinases types 1 and 3, causing degradation of the extracellular matrix component, collagen, necessary for the stability of the plaque. Activated mast cells also secrete matrix metalloproteinases types 1 and 9 themselves. Furthermore, chymase induces SMC apoptosis by degrading fibronectin, a pericellular matrix component necessary for SMC adhesion and survival, with the subsequent disruption of focal adhesions and loss of outside-in survival signaling. By secreting chymase and tumour necrosis factor alpha, activated mast cells also induce endothelial cell apoptosis. SUMMARY: Locally activated mast cells may participate in the weakening of atherosclerotic plaques by secreting heparin proteoglycans, chymase, and cytokines, which affect the growth, function and death of arterial endothelial cells and smooth muscle cells, thereby predisposing to plaque erosion or rupture.     

Molecular mechanisms of plaque instability

PURPOSE OF REVIEW: Coronary artery thrombosis superimposed on a disrupted atherosclerotic plaque initiates abrupt arterial occlusion and is the proximate event responsible for 60-80% cases of acute coronary syndromes. This article provides a concise update on the evolving concepts in the pathophysiology of plaque rupture and thrombosis. RECENT FINDINGS: Over the past several years, the critical role of plaque composition rather than plaque size or stenosis severity, in plaque rupture and thrombosis have been recognized. The necrotic lipid core and plaque inflammation appear to be key factors. Extracellular matrix loss in the fibrous cap, a prelude to rupture, is attributed to matrix degrading enzymes as well as to death of matrix synthesizing smooth muscle cells; inflammation appears to play a critical role in both these processes. Inflammatory cell derived tissue factor is a key contributor to plaque thrombogenicity. Inflammation has also been implicated in plaque neovascularity, intraplaque hemorrhage and plaque expansion. Recent observations have also highlighted the important modulatory role of immune system in atherosclerosis and plaque composition. SUMMARY: Improved understanding of mechanisms causing plaque instability should provide novel insights into prevention of athero-thrombotic cardiovascular events.     

Thrombosis, physical activity, and acute coronary syndromes

Acute coronary syndromes (ACS) are common, life-threatening cardiac disorders that typically are triggered by rupture or erosion of an atherosclerotic plaque. Platelet deposition and activation of the blood coagulation cascade in response to plaque disruption lead to the formation of a platelet-fibrin thrombus, which can grow rapidly, obstruct coronary blood flow, and cause myocardial ischemia and/or infarction. Several clinical studies have examined the relationship between physical activity and ACS, and numerous preclinical and clinical studies have examined specific effects of sustained physical training and acute physical activity on atherosclerotic plaque rupture, platelet function, and formation and clearance of intravascular fibrin. This article reviews the available literature regarding the role of physical activity in determining the incidence of atherosclerotic plaque rupture and the pace and extent of thrombus formation after plaque rupture.     

Platelet interactions in thrombosis

Patho/physiological platelet aggregate (thrombus) formation is initiated by engagement of platelet surface receptors, glycoprotein (GP)Ib-IX-V and GPVI that bind von Willebrand factor or collagen. Although beneficial in response to vascular injury by preventing blood loss (haemostasis), platelet aggregation in a sclerotic coronary artery or other diseased blood vessel (thrombosis) can cause thrombotic diseases like heart attack and stroke. At the molecular level, ligand interactions with GPIb-IX-V or GPVI trigger signalling responses, including elevation of cytosolic Ca2+, dissociation of calmodulin from their cytoplasmic domains, cytoskeletal actin-filament rearrangements, activation of src-family kinases or PI 3-kinase, and 'inside-out' activation of the integrin, alphaIIbbeta3 (GPIIb-llla), that binds von Willebrand factor or fibrinogen and mediates platelet aggregation. Furthermore, emerging evidence supports a topographical co-association of these receptors of the leucine-rich repeat family (GPIb-IX-V) and immunoglobulin superfamily (GPVI) in an adhesive cluster or 'adhesosome'. This arrangement may underlie common mechanisms of initiating thrombus formation in haemostasis or thrombotic disease.     

The emerging role of thrombus aspiration in contemporary percutaneous coronary intervention practice

Coronary atherosclerosis may lead towards thrombogenesis, usually triggered by rupture or erosion of a vulnerable epicardial coronary artery plaque. Most acute coronary syndromes are caused by ruptured atherosclerotic plaques with superimposed thrombus.     

Biomechanics and Inflammation in Atherosclerotic Plaque Erosion and Plaque Rupture: Implications for Cardiovascular Events in Women

Objective

Although plaque erosion causes approximately 40% of all coronary thrombi and disproportionally affects women more than men, its mechanism is not well understood. The role of tissue mechanics in plaque rupture and regulation of mechanosensitive inflammatory proteins is well established, but their role in plaque erosion is unknown. Given obvious differences in morphology between plaque erosion and rupture, we hypothesized that inflammation in general as well as the association between local mechanical strain and inflammation known to exist in plaque rupture may not occur in plaque erosion. Therefore, our objective was to determine if similar mechanisms underlie plaque rupture and plaque erosion.

Methods and Results

We studied a total of 74 human coronary plaque specimens obtained at autopsy. Using lesion-specific computer modeling of solid mechanics, we calculated the stress and strain distribution for each plaque and determined if there were any relationships with markers of inflammation. Consistent with previous studies, inflammatory markers were positively associated with increasing strain in specimens with rupture and thin-cap fibroatheromas. Conversely, overall staining for inflammatory markers and apoptosis were significantly lower in erosion, and there was no relationship with mechanical strain. Samples with plaque erosion most closely resembled those with the stable phenotype of thick-cap fibroatheromas.

Conclusions

In contrast to classic plaque rupture, plaque erosion was not associated with markers of inflammation and mechanical strain. These data suggest that plaque erosion is a distinct pathophysiological process with a different etiology and therefore raises the possibility that a different therapeutic approach may be required to prevent plaque erosion.     

Apoptosis and acute coronary syndromes

Atherosclerosis is an inflammatory disease of the arterial wall. Ischemic manifestations of atherosclerosis are mainly due to thrombus formation upon a superficially eroded (denudation of luminal endothelium, 40% of cases) or deeply ruptured (fibrous cap rupture, 60% of cases) plaques. Recent studies have unraveled potentially critical roles for both inflammatory and apoptotic processes in plaque destabilization leading to thrombus formation. Pro-inflammatory mediators have been particularly implicated in the loss of smooth muscle cell and the promotion of collagen degradation that are responsible for fibrous cap rupture, whereas apoptosis has been identified as one of the major determinants of plaque thrombogenicity.     

24-Hour Assessment of Performance on a Palmtop Computer: Validating a Self-Constructed Test Battery

Serious adverse cardiovascular events, including myocardial infarction, sudden cardiac death, and stroke, frequently result from rupture of atherosclerotic plaques with superimposed thrombosis and exhibit a pronounced circadian rhythmicity, peaking in the morning hours. Two potentially synergistic mechanisms play a pathogenic role in the circadian variation of arterial thrombotic events. A morning surge in sympathetic activity alters hemodynamic forces and predisposes vulnerable coronary atherosclerotic plaques to rupture. Day–night variations of hemostatic and fibrinolytic factors result in morning hypercoagulability and hypofibrinolysis, promoting intraluminal thrombus formation at the same time when the risk for plaque rupture is highest. Diabetic patients have a very high cardiac event rate but fail to show normal circadian fluctuations in the occurrence of myocardial infarction. Alterations in the circadian variation autonomic tone, blood pressure, and the thrombotic–thrombolytic equilibrium have been documented in diabetic patients. These include reduced or absent 24-h periodicity in autonomic tone, fibrinolytic activity, and thrombotic tendency, and a blunted decline in nocturnal blood pressure. Disruption of these circadian rhythms explains the lack of significant circadian distribution of cardiac events in diabetic patients. Moreover, the loss of these normal biorhythms results in a continuous susceptibility to thrombotic events throughout the day and may contribute to the excess cardiovascular mortality and morbidity in these patients. (Chronobiology International, 18(1), 109–121, 2001)     

Impaired modulation of circadian rhythms in patients with diabetes mellitus: a risk factor for cardiac thrombotic events?

Serious adverse cardiovascular events, including myocardial infarction, sudden cardiac death, and stroke, frequently result from rupture of atherosclerotic plaques with superimposed thrombosis and exhibit a pronounced circadian rhythmicity, peaking in the morning hours. Two potentially synergistic mechanisms play a pathogenic role in the circadian variation of arterial thrombotic events. A morning surge in sympathetic activity alters hemodynamic forces and predisposes vulnerable coronary atherosclerotic plaques to rupture. Day-night variations of hemostatic and fibrinolytic factors result in morning hypercoagulability and hypofibrinolysis, promoting intraluminal thrombus formation at the same time when the risk for plaque rupture is highest. Diabetic patients have a very high cardiac event rate but fail to show normal circadian fluctuations in the occurrence of myocardial infarction. Alterations in the circadian variation autonomic tone, blood pressure, and the thrombotic-thrombolytic equilibrium have been documented in diabetic patients. These include reduced or absent 24-h periodicity in autonomic tone, fibrinolytic activity, and thrombotic tendency, and a blunted decline in nocturnal blood pressure. Disruption of these circadian rhythms explains the lack of significant circadian distribution of cardiac events in diabetic patients. Moreover, the loss of these normal biorhythms results in a continuous susceptibility to thrombotic events throughout the day and may contribute to the excess cardiovascular mortality and morbidity in these patients. (Chronobiology International, 18(1), 109-121, 2001)     

Detection of the vulnerable coronary atheromatous plaque. Where are we now?

Atherosclerosis is a progressive process with potentially devastating consequences and has been identified as the leading cause of morbidity and mortality, especially in the industrial countries. The underlying mechanisms include endothelial dysfunction, lipid accumulation and enhanced inflammatory involvement resulting in plaque disruption or plaque erosion and subsequent thrombosis. However, it has been made evident, that the majority of rupture prone plaques that produce acute coronary syndromes are not severely stenotic. Conversely, lipid-rich plaques with thin fibrous cap, heavily infiltrated by inflammatory cells have been shown to predispose to rupture and thrombosis, independently of the degree of stenosis. Therefore, given the importance of plaque composition, a continuously growing interest in the development and improvement of diagnostic modalities will promptly and most importantly, accurately detect and characterize the high-risk atheromatous plaque. Use of these techniques may help risk stratification and allow the selection of the most appropriate therapeutic approach.     

Pre-activated blood platelets and a pro-thrombotic phenotype in APP23 mice modeling Alzheimer's disease

Platelet activation and thrombus formation play a critical role in primary hemostasis but also represent a pathophysiological mechanism leading to acute thrombotic vascular occlusions. Besides, platelets modulate cellular processes including inflammation, angiogenesis and neurodegeneration. On the other hand, platelet activation and thrombus formation are altered in different diseases leading to either bleeding complications or pathological thrombus formation. For many years platelets have been considered to play a role in neuroinflammatory diseases such as Alzheimer's disease (AD). AD is characterized by deposits of amyloid-β (Aβ) and strongly related to vascular diseases with platelets playing a critical role in the progression of AD because exposure of platelets to Aβ induces platelet activation, platelet Aβ release, and enhanced platelet adhesion to collagen in vitro and at the injured carotid artery in vivo. However, the molecular mechanisms and the relation between vascular pathology and amyloid-β plaque formation in the pathogenesis of AD are not fully understood. Compelling evidence is suggestive for altered platelet activity in AD patients. Thus we analyzed platelet activation and thrombus formation in aged AD transgenic mice (APP23) known to develop amyloid-β deposits in the brain parenchyma and cerebral vessels. As a result, platelets are in a pre-activated state in blood of APP23 mice and showed strongly enhanced integrin activation, degranulation and spreading kinetics on fibrinogen surfaces upon stimulation. This enhanced platelet signaling translated into almost unlimited thrombus formation on collagen under flow conditions in vitro and accelerated vessel occlusion in vivo suggesting that these mice are at high risk of arterial thrombosis leading to cerebrovascular and unexpectedly to cardiovascular complications that might be also relevant in AD patients.     

The thrombin mutant W215A/E217A shows safe and potent anticoagulant and antithrombotic effects in vivo

Administration of the thrombin mutant W215A/E217A (WE), rationally designed for selective activation of the anticoagulant protein C, elicits safe and potent anticoagulant and antithrombotic effects in a baboon model of platelet-dependent thrombosis. The lowest dose of WE tested (0.011 mg/kg bolus) reduced platelet thrombus accumulation by 80% and was at least as effective as the direct administration of 40-fold more (0.45 mg/kg bolus) activated protein C. WE-treated animals showed no detectable hemorrhage or organ failure. No procoagulant activity could be detected for up to 1 week in baboon plasma obtained following WE administration. These results show that engineered thrombin derivatives that selectively activate protein C may represent useful therapeutic agents for the treatment of thrombotic disorders.     

A new role in hemostasis for the adhesion receptor P-selectin

The adhesion receptor P-selectin has long been known to support leukocyte rolling and emigration at sites of inflammation. Recently, P-selectin was also revealed to be a key molecule in hemostasis and thrombosis, mediating platelet rolling, generating procoagulant microparticles containing active tissue factor and enhancing fibrin deposition. Elevated levels of plasma P-selectin are indicative of thrombotic disorders and predictive of future cardiovascular events. Because the interaction between P-selectin and its receptor P-selectin glycoprotein ligand-1 (PSGL-1) represents an important mechanism by which P-selectin induces the formation of procoagulant microparticles and recruits the microparticles to thrombi, anti-thrombotic strategies are currently aimed at inhibiting this interaction. Recent developments also suggest that the procoagulant potential of P-selectin could be used to treat coagulation disorders such as hemophilia A.     

Vascular Smooth Muscle Cells Stimulate Platelets and Facilitate Thrombus Formation through Platelet CLEC-2: Implications in Atherothrombosis

The platelet receptor CLEC-2 is involved in thrombosis/hemostasis, but its ligand, podoplanin, is expressed only in advanced atherosclerotic lesions. We investigated CLEC-2 ligands in vessel walls. Recombinant CLEC-2 bound to early atherosclerotic lesions and normal arterial walls, co-localizing with vascular smooth muscle cells (VSMCs). Flow cytometry and immunocytochemistry showed that recombinant CLEC-2, but not an anti-podoplanin antibody, bound to VSMCs, suggesting that CLEC-2 ligands other than podoplanin are present in VSMCs. VSMCs stimulated platelet granule release and supported thrombus formation under flow, dependent on CLEC-2. The time to occlusion in a FeCl₃-induced animal thrombosis model was significantly prolonged in the absence of CLEC-2. Because the internal elastic lamina was lacerated in our FeCl₃-induced model, we assume that the interaction between CLEC-2 and its ligands in VSMCs induces thrombus formation. Protein arrays and Biacore analysis were used to identify S100A13 as a CLEC-2 ligand in VSMCs. However, S100A13 is not responsible for the above-described VSMC-induced platelet activation, because S100A13 is not expressed on the surface of normal VSMCs. S100A13 was released upon oxidative stress and expressed in the luminal area of atherosclerotic lesions. Suspended S100A13 did not activate platelets, but immobilized S100A13 significantly increased thrombus formation on collagen-coated surfaces. Taken together, we proposed that VSMCs stimulate platelets through CLEC-2, possibly leading to thrombus formation after plaque erosion and stent implantation, where VSMCs are exposed to blood flow. Furthermore, we identified S100A13 as one of the ligands on VSMCs.     

Antithrombotic Activities of Luteolin In Vitro and In Vivo

The objectives of present study were to investigate whether luteolin affects procoagulant proteinase activity and fibrin clot formation and influences thrombosis and coagulation in Sprague–Dawle rats. Luteolin significantly inhibited the enzymatic activity of thrombin and FXa activity by 29.1% and 16.2%. Luteolin also inhibited fibrin polymer formation in turbidity and microscopic analysis using fluorescent conjugate. Coagulation assay of luteolin was found to prolong activated partial thromboplastin time and prothrombin time. Moreover, luteolin protected the development of oxidative stress induced thrombosis in the FeCl₃‐induced carotid arterial thrombus model. This study demonstrated that luteolin may be useful by reducing or preventing thrombotic challenge and can help us better understand the antithrombotic action of luteolin.     

Force-mediated dissociation of proteoglycan aggregate in articular cartilage

Proteoglycan aggregate is the primary component in articular cartilage responsible for resisting compressive loading. It consists of a core molecule of hyaluronan and a number of side chains of aggrecan bound to hyaluronan non-covalently. The loss of aggrecan from articular cartilage is considered to be a major factor in the development of osteoarthritis. Though enzymatic digestion of aggrecan is believed to be responsible for the release of aggrecan from osteoarthritic cartilage, other mechanisms, such as direct force-mediated detachment of aggrecan from hyaluronan may also be involved. In this study, the rupture force of the single bond between hyaluronan and aggrecan in articular cartilage was directly quantified using experimental measurement and Monte Carlo simulation. Low rupture force of this bond, as determined in this study suggested a possible direct force-mediated detachment of aggrecan from proteoglycan aggregate in osteoarthritic cartilage.     

大鼠实验性血栓模型的建立及其评价

根据Ｖｉｒｃｈｏｗ血栓形成原理,采用机械性损伤内膜,同时降低血流速度方法制备大鼠实验性血栓模型。结果显示,利用此方法制备血栓模型成功率为７８．４％,血栓平均长度８．５３±１．４２ｍｍ,血栓形成后,腹主动脉的收缩功能明显下降。     

Atherothrombosis: role of tissue factor; link between diabetes, obesity and inflammation

Atherothrombotic vascular disease is a complex disorder in which inflammation and coagulation play a pivotal role. Rupture of high-risk, vulnerable plaques with the subsequent tissue factor (TF) exposure is responsible for coronary thrombosis, the main cause of unstable angina, acute myocardial infarction, and sudden cardiac death. Tissue factor (TF), the key initiator of coagulation is an important modulator of inflammation. TF is widely expressed in atherosclerotic plaques and found in macrophages, smooth muscle cells, extracellular matrix and acellular lipid-rich core. TF expression can be induced by various stimulants such as C-reactive protein, oxLDL, hyperglycemia and adipocytokines. The blood-born TF encrypted on the circulating microparticles derived from vascular cells is a marker of vascular injury and a source of procoagulant activity. Another form of TF, called alternatively spliced has been recently identified in human and murine. It is soluble, circulates in plasma and initiates coagulation and thrombus propagation. Evidence indicates that elevated levels of blood-borne or circulating TF has been associated with metabolic syndrome, type 2 diabetes and cardiovascular risk factors and is a candidate biomarker for future cardiovascular events. Therapeutic strategies have been developed to specifically interfere with TF activity in the treatment of cardiovascular disease.     

Endotoxin enhances microvascular thrombosis in mouse cremaster venules via a TLR4-dependent, neutrophil-independent mechanism

Endotoxemia promotes adhesive interactions between platelets and microvascular endothelium in vivo. We sought to determine whether endotoxin (lipopolysaccharide, LPS) modified platelet thrombus formation in mouse cremaster venules and whether Toll-like receptor 4 (TLR4) and neutrophils were involved in the response. Intravital videomicroscopy was performed in the cremaster microcirculation of pentobarbital-anesthetized mice; venular platelet thrombi were induced with a light/dye endothelial injury model. C57BL/6 mice treated with Escherichia coli endotoxin had enhanced rates of venular platelet thrombus formation: the time to microvessel occlusion was reduced by approximately 50% (P < 0.005) compared with saline-treated animals. Enhanced microvascular thrombosis was evident as early as 2 h after LPS administration. LPS had no effect on thrombosis in either of two mouse strains with altered TLR4 signaling (C57BL/10ScNJ or C3H/HeJ), whereas it enhanced thrombosis in the control strains (C57BL/10J and C3H/HeN). LPS also enhanced platelet adhesion to endothelium in the absence of light/dye injury. Platelet adhesion, but not enhanced thrombosis, was inhibited by depletion of circulating neutrophils. LPS failed to enhance platelet aggregation ex vivo and did not influence platelet P-selectin expression, a marker of platelet activation. These findings support the notion that endotoxemia promotes platelet thrombus formation independent of neutrophils and without enhancement of platelet aggregation, via a TLR4-dependent mechanism.     

Role of plaque inflammation in acute and recurrent coronary syndromes

Inflammation plays an important role in the initiation, development, progression and complications of atherosclerotic vascular disease. Our present knowledge of the elementary role of inflammation for the onset of plaque rupture in atherosclerotic coronary lesions primarily stems from autopsy studies. However, the introduction of directional coronary atherectomy catheters has provided a unique opportunity to directly investigate the role of inflammation in coronary syndromes. In this report we describe the role of coronary plaque inflammation, as determined by immunohistochemistry, on the presentation of coronary syndromes and on the clinical outcome following percutaneous interventions.