Role of inflammation and metalloproteinases in plaque disruption and thrombosis

Quantitative ?magnetic resonance immunohistochemistry? with ligand-targeted19F nanoparticles

Unstable atherosclerotic plaques exhibit microdeposits of fibrin that may indicate the potential for a future rupture. However, current methods for evaluating the stage of an atherosclerotic lesion only involve characterizing the level of vessel stenosis, without delineating which lesions are beginning to rupture. Previous work has shown that fibrin-targeted, liquid perfluorocarbon nanoparticles, which carry a high payload of gadolinium, have a high sensitivity and specificity for detecting fibrin with clinical (1)H MRI. In this work, the perfluorocarbon content of the targeted nanoparticles is exploited for the purposes of (19)F imaging and spectroscopy to demonstrate a method for quantifiable molecular imaging of fibrin in vitro at 4.7 T. Additionally, the quantity of bound nanoparticles formulated with different perfluorocarbon species was calculated using spectroscopy. Results indicate that the high degree of nanoparticle binding to fibrin clots and the lack of background (19)F signal allow accurate quantification using spectroscopy at 4.7 T, as corroborated with proton relaxation rate measurements at 1.5 T and trace element (gadolinium) analysis. Finally, the extension of these techniques to a clinically relevant application, the evaluation of the fibrin burden within an ex vivo human carotid endarterectomy sample, demonstrates the potential use of these particles for uniquely identifying unstable atherosclerotic lesions in vivo.

Supplementary taurine may stabilize atheromatous plaque by antagonizing the activation of metalloproteinases by hypochlorous acid

The rupture of atherosclerotic plaque, responsible for triggering the majority of myocardial infarctions, presumably requires proteolysis of collagen fibers and other protein components of the intercellular matrix. This is achieved by activated matrix metalloproteases (MMPs) secreted by intimal macrophages and foam cells. MMPs are synthesized as inactive pro-enzymes in which coordinate binding of the thiol group of a key cysteine residue to the active-site zinc atom blocks proteolytic activity. Physiological activation of MMPs is mediated, in large measure, by phagocyte-derived hypochlorous acid (HOCL), which can oxidize the zinc-bound thiol to sulfinic acid, thus freeing the active-site zinc. HOCL also encourages proteolysis of ground substance by inactivating proteins such as TIMP-1 that are physiological inhibitors of MMPs. In vivo, the unrestrained oxidant activity of HOCL is opposed by taurine, which reacts spontaneously with HOCL to generate taurine chloramine, much more stable than HOCL. Taurine chloramine has less impact than HOCL on MMP activation, and does not impair the activity of TIMP-1. Since tissue levels of taurine can be boosted via supplementation, taurine may thus have potential for stabilizing plaque and thereby warding off infarction--an effect that should be reinforced by taurine's platelet-stabilizing activity. In light of recent epidemiological evidence that increased expression of myeloperoxidase - the enzyme which generates HOCL--is an important risk factor for coronary disease, supplemental taurine may indeed have broader utility for suppressing both the genesis and the rupture of atherosclerotic plaque.

Hypoxia-induced gene expression in human macrophages: implications for ischemic tissues and hypoxia-regulated gene therapy

Macrophages accumulate in ischemic areas of such pathological tissues as solid tumors, atherosclerotic plaques and arthritic joints. Studies have suggested that hypoxia alters the phenotype of macrophages in a way that promotes these lesions. However, the genes up-regulated by macrophages in such hypoxic tissues are poorly characterized. Here, we have used cDNA array hybridization to investigate the effects of hypoxia on the mRNAs of 1185 genes in primary human monocyte-derived macrophages. As shown previously in other cell types, mRNA levels for vascular endothelial growth factor (VEGF) and glucose transporter 1 (GLUT-1) were up-regulated by hypoxia. However, the mRNAs of other genes were also up-regulated including matrix metalloproteinase-7 (MMP-7), neuromedin B receptor, and the DNA-binding protein inhibitor, Id2. The promoters of GLUT-1 and MMP-7 confer hypoxic inducibility on a reporter gene in RAW 264.7 macrophages, indicating that the hypoxic up-regulation of these mRNAs may occur, at least in part, at the transcriptional level. GLUT-1 and MMP-7 mRNA were also shown to be up-regulated in hypoxic macrophages in vitro by real-time RT-PCR, and these proteins were elevated in hypoxic macrophages in vitro and in hypoxic areas of human breast tumors. The hypoxia up-regulated genes identified could be important for the survival and functioning of macrophages in hypoxic diseased tissues, and their promoters could prove useful in macrophage-delivered gene therapy.

Different expression of MMPs/TIMP-1 in human atherosclerotic lesions. Relation to plaque features and vascular bed

BACKGROUND

Proteolytic imbalance might determine arterial remodeling and plaque destabilization in atherosclerotic vessels. The aim of this study was to examine differences in the patterns of metalloproteinases (MMPs) and MMP inhibitor (TIMP-1) expression in advanced human atheromas, both in relation to the plaque features and the vascular bed involved.

METHODS AND RESULTS

Immunohistochemistry for MMP-1, -3, -9 and TIMP-1 as well as the collagen content were measured in vascular sections from patients undergoing peripheral revascularization (carotid n=11, femoral n=23) and aorto-coronary bypass surgery (mammary arteries n=20, as controls). Increased expression of all MMPs was detected in atherosclerotic as compared with control sections (P<0.01). Aneurysmal plaques showed a significant increase of MMP-1 and-3 and a reduction in total collagen (P<0.05) in relation to occlusive lesions. Calcification areas in atherosclerotic plaques were consistently associated with increased TIMP-1 expression (P<0.01). Finally, MMP-9 expression was higher in occlusive lesions from carotid than femoral arteries (P<0.01).

CONCLUSIONS

Aneurysm lesions expressed higher MMP-1 and-3 expression than occlusive plaques, and MMP-9 was mainly detected in carotid as compared with femoral arteries. TIMP-1 was associated with arterial calcification. These differences in the MMPs/TIMP-1 expression might determine the evolution of advanced atherosclerotic plaques and contribute to its vulnerability.

Pathophysiology of plaque rupture and the concept of plaque stabilization

Atherosclerotic coronary artery disease is the major cause of death, in men and women, in the United States and in much of the Western world. Atherosclerosis is responsible for coronary heart disease, limb ischemia, and most strokes. Although luminal narrowing by an atherosclerotic plaque and exaggerated or anomalous vasoconstriction contribute to some of the clinical manifestations of atherosclerotic arterial disease, it is the superim-position of a thrombus over an underlying ruptured or eroded plaque that results in the acute coronary syndromes (unstable angina, acute myocardial infarction, and sudden death) that are the most serious clinical manifestations of this disease.

Thermography basket catheter: in vivo measurement of the temperature of atherosclerotic plaques for detection of vulnerable plaques

We have developed an intravascular thermography basket catheter to measure the temperature of the vessel wall to locate foci of inflammation. Our 3 Fr thermography basket catheter is a thermocouple-based catheter made of a nitinol expandable and externally controllable basket system loaded with nine small and flexible built-in thermosensors. It is equipped with real-time data acquisition software with a thermal resolution of 0.0001 degrees C and a sampling rate of 20 readings per second. In 10 inbred cholesterol-fed dogs with femoral (but not carotid) atherosclerosis, we found foci of warmth on the surface of atherosclerotic but not disease-free regions (P < 0.05). Marked temperature heterogeneity was also observed in the aortas of atherosclerotic Watanabe rabbits but not in normal rabbits. The catheters showed satisfactory accuracy, reproducibility, and safety. If confirmed in further studies, it has the potential to be utilized in detection of vulnerable plaques.

Mechanisms of plaque vulnerability and rupture

Rupture of atherosclerotic plaque has been identified as the proximate event in the majority of cases of acute ischemic syndromes. Plaque rupture exposes thrombogenic components of the plaque, activating the clotting cascade and promoting thrombus formation. Future culprit lesions are difficult to identify, however, and angiographic assessment of stenosis severity is prone to underestimation. Compared with plaques that cause severe luminal stenosis, vulnerable plaques may cause relatively minor stenosis, although they account for more cases of rupture and thrombosis. Such unstable, vulnerable plaques may be associated with outward remodeling of the vessel. Because severely stenotic plaques are more likely to stimulate collateral circulation to the post-stenotic segment, plaque rupture and thrombosis at such sites may be clinically silent. Characteristic histomorphologic features of vulnerable plaques include a high lipid content, increased numbers of inflammatory cells, and extensive adventitial and intimal neovascularity. The fibrous cap of an atherosclerotic plaque may become thin and rupture as a result of the depletion of matrix components through the activation of enzymes, such as matrix-degrading proteinases and cystine and aspartate proteases, and through the reduction in the number of smooth muscle cells. Activated T cells may also inhibit matrix synthesis through the production of interferon-gamma. A number of triggers of plaque rupture have been identified. Also, some thrombi may occur without rupture of the fibrous cap. Reducing the lipid component and inflammation in atherosclerotic plaques may help reduce the risk of plaque rupture. This may account for the clinical benefit of risk-factor reduction gained from changes in lifestyle and from drug therapy.

Influenza infection exerts prominent inflammatory and thrombotic effects on the atherosclerotic plaques of apolipoprotein E-deficient mice

BACKGROUND

The role of infection in the development and complications of atherosclerosis has been the focus of much attention. We reported previously that influenza vaccination was associated with reduced risk of recurrent myocardial infarction. Here, we report the effect of influenza A virus on the apolipoprotein E-deficient (apoE(-/-)) mouse, an animal model of atherosclerosis.

METHODS AND RESULTS

Twenty-four apoE(-/-) mice >24 months old were injected with 1 LD(50) (lethal dose 50) of influenza A virus. Ten wild-type C57BL/6 infected mice and 11 noninfected age-matched apoE(-/-) mice served as controls. Multiple aortic sections were studied histologically 3, 5, and 10 days later. The infected mice showed markedly increased intimal cellularity compared with the noninfected apoE(-/-) mice. No aortic abnormalities were seen in infected wild-type mice. Ten infected apoE(-/-) mice had a significant subendothelial infiltrate composed of a heterogeneous group of cells that stained positively for smooth muscle cell actin, F4/80 (macrophages), and CD3 (T lymphocytes). One case of subocclusive platelet and fibrin-rich thrombus was seen.

CONCLUSIONS

This study shows that influenza infection promotes inflammation, smooth muscle cell proliferation, and fibrin deposition in atherosclerotic plaques.

Left ventricular muscle mass and elevated heart rate are associated with coronary plaque disruption

BACKGROUND

Plaque disruption is the central pathophysiological mechanism underlying acute coronary syndromes and the progression of coronary atherosclerosis. There exists only scant information about the factors that are associated with its development. The aim of the current study was to analyze the contribution of hemodynamic forces in the pathogenesis of plaque disruption. Plaque disruption was diagnosed by coronary angiography of stenosed but not completely occluded coronary arteries.

METHODS AND RESULTS

This study retrospectively analyzed 106 patients who underwent 2 coronary angiography procedures within 6 months. We investigated 53 patients with initially smooth stenoses who developed plaque disruption by the time of the second coronary angiogram and compared these patients with 53 age- and sex-matched individuals with smooth stenoses without angiographic signs of plaque disruption. The 2 groups were compared by analyzing central hemodynamics, echocardiographic measurements, and cardiovascular medication use. Logistic regression analysis identified positive associations between plaque disruption, left ventricular muscle mass >270 g, and a mean heart rate >80 bpm and a negative association with the use of beta-blockers.

CONCLUSIONS

The associations documented by our investigation indicate that hemodynamic forces may play a crucial role in the pathogenesis of plaque disruption. These findings may help to identify patients who are at an increased risk of plaque disruption and who might gain benefit from pharmacological interventions aimed at reducing heart rate, for example, by the use of beta-blockers, or a reduction of left ventricular hypertrophy.

Expression of MMP2, MMP9, MT1-MMP, TIMP1, and TIMP2 mRNA in valvular lesions of the heart

Matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) play an important role in several diseases. This study was undertaken to investigate the mRNA synthesis of MMP2, MMP9, membrane-type 1 (MT1)-MMP, and matrix metalloproteinase inhibitors TIMP1 and TIMP2 by in situ hybridization in a set of heart mitral and aortic valves operatively removed due to degenerative or inflammatory valvular diseases. The material consisted of 21 valves, eight with endocarditis and 13 with a degenerative valvular disease. The samples were studied by in situ hybridization with specific probes for MMP2, MMP9, MT1-MMP, TIMP1, and TIMP2. Synthesis of MMP2 mRNA was found in seven valves, five with endocarditis and two with degenerative valvular disease. Signals for MMP9 mRNA were found in two cases with endocarditis and five cases with degenerative valvular disease. No signal for MT1-MMP mRNA was found in the lesions. TIMP1 mRNA, on the other hand, was found in 17 cases, both endocarditis and degenerative valvular disease. TIMP2 mRNA was found in three cases of endocarditis. The signals for MMP2, MMP9, TIMP1, and TIMP2 mRNA were localized in endothelial cells and in fibroblast-like cells expressing alpha-smooth muscle actin, thus showing myofibroblast-type differentiation. The results show that matrix metalloproteinases MMP2 and MMP9, and matrix metalloproteinase inhibitors TIMP1 and TIMP2 mRNAs are synthesized in diseased valves and suggest that they may contribute to matrix remodelling in valvular disease.

Pravastatin treatment increases collagen content and decreases lipid content, inflammation, metalloproteinases, and cell death in human carotid plaques: implications for plaque stabilization

BACKGROUND

The clinical benefits of lipid lowering with statins are attributed to changes in plaque composition leading to lesion stability, but supporting clinical data from human studies are lacking. Therefore, we investigated the effect of 3 months of pravastatin treatment on composition of human carotid plaques removed during carotid endarterectomy.

METHODS AND RESULTS

Consecutive patients with symptomatic carotid artery stenosis received 40 mg/d pravastatin (n=11) or no lipid-lowering therapy (n=13; control subjects) for 3 months before scheduled carotid endarterectomy. Carotid plaque composition was assessed with special stains and immunocytochemistry with quantitative image analysis. Plaques from the pravastatin group had less lipid by oil red O staining (8.2+/-8.4% versus 23.9+/-21.1% of the plaque area, P<0.05), less oxidized LDL immunoreactivity (13.3+/-3.6% versus 22.0+/-6.5%, P<0.001), fewer macrophages (15.0+/-10.2% versus 25.3+/-12.5%, P<0.05), fewer T cells (11.2+/-9.3% versus 24.3+/-13.4%, P<0.05), less matrix metalloproteinase 2 (MMP-2) immunoreactivity (3.6+/-3.9% versus 8.4+/-5.3%, P<0.05), greater tissue inhibitor of metalloproteinase 1 (TIMP-1) immunoreactivity (9.0+/-6.2% versus 3.1+/-3.9%, P<0.05), and a higher collagen content by Sirius red staining (12.4+/-3.1% versus 7.5+/-3.5%, P<0.005). Cell death by TUNEL staining was reduced in the pravastatin group (17.7+/-7.8% versus 32.0+/-12.6%, P<0.05).

CONCLUSIONS

-Pravastatin decreased lipids, lipid oxidation, inflammation, MMP-2, and cell death and increased TIMP-1 and collagen content in human carotid plaques, confirming its plaque-stabilizing effect in humans.

Overall cardiovascular profile of sildenafil citrate

Sildenafil, a selective inhibitor of phosphodiesterase type 5 (PDE5), is the first in a new class of orally effective treatments for erectile dysfunction. During sexual stimulation, the cavernous nerves release nitric oxide (NO), which induces cyclic guanosine monophosphate (cGMP) formation and smooth muscle relaxation in the corpus cavernosum. Sildenafil facilitates the erectile process during sexual stimulation by inhibiting PDE5 and thus blocking the breakdown of cGMP. Sildenafil alone can cause mean peak reductions in systolic/diastolic blood pressure of 10/7 mm Hg that are not dose related, whereas the heart rate is unchanged. Sildenafil and nitrates both increase cGMP levels in the systemic circulation but at different points along the NO-cGMP pathway. The combination is contraindicated because they synergistically potentiate vasodilation and may cause excessive reductions in blood pressure. Erectile dysfunction is a significant medical condition that shares numerous risk factors with ischemic heart disease, and hence a substantial overlap exists between these patient groups. From extensive clinical trials, the most commonly reported cardiovascular adverse events in patients treated with sildenafil were headache (16%), flushing (10%), and dizziness (2%). The incidences of hypotension, orthostatic hypotension, and syncope and the rate of discontinuation of treatment due to adverse events were <2% and were the same in patients taking sildenafil and those taking placebo. Retrospective analysis of the concomitant use of antihypertensive medications (beta blockers, alpha blockers, diuretics, angiotensin-converting enzyme inhibitors, and calcium antagonists) in patients taking sildenafil did not indicate an increase in the reports of adverse events or significant episodes of hypotension compared with patients treated with sildenafil alone. In clinical trials, the incidence of serious cardiovascular adverse events, including stroke and myocardial infarction, was the same for patients treated with sildenafil or placebo. Concurrent disease states, such as renal or hepatic impairment, or concomitant use of inhibitors of the cytochrome P450 isozyme CYP3A4 could increase systemic exposure to sildenafil. Since the US market launch in April 1998, monitoring of spontaneous adverse event reports in association with sildenafil has demonstrated a pattern that is generally consistent with the experience observed during clinical development, with the exception of infrequent reports of priapism. In conclusion, extensive clinical testing has shown that overall treatment with sildenafil for up to 1 year is well tolerated and is associated with a low incidence of adverse events that result in discontinuation of treatment in <3% of patients.