Preservation and structural adaptation of endothelium over experimental foam cell lesions. Quantitative ultrastructural study

LDL Transcytosis by the Arterial Endothelium-Atherosclerosis by a Thousand Cuts?

PURPOSE OF REVIEW

The accumulation of LDL in the arterial intima is an initiating event in atherosclerosis. After decades of controversy, it is now clear that transcytosis of LDL across an intact endothelial monolayer contributes to its intimal deposition. We review recent observations in this field and address the question of whether LDL transcytosis can be manipulated therapeutically.

RECENT FINDINGS

The development of a live-cell imaging method for studying transcytosis using total internal reflection fluorescence (TIRF) microscopy has catalyzed recent discoveries. LDL transcytosis is mediated by SR-BI and ALK1. Estrogen down-regulates SR-BI and inhibits LDL transcytosis, while the nuclear structural protein HMGB1 promotes LDL transcytosis. LDL transcytosis by ALK1 is independent of the receptor's kinase activity and is antagonized by BMP9, ALK1's canonical ligand. Inflammation stimulates LDL transcytosis. Identifying the function and mechanisms of LDL transcytosis may ultimately permit its therapeutic manipulation.

Inhibition of endoplasmic reticulum stress ameliorates cardiovascular injury in a rat model of metabolic syndrome

Metabolic (Met) syndrome is characterized by hypertension, insulin resistance and dyslipidaemia with high risk of cardiovascular disease. Endoplasmic reticulum (ER) stress is a key contributor in the pathogenesis of Met syndrome. The current study investigates the effect of Tauroursodeoxycholate (TUDCA), an ER stress inhibitor, on Met syndrome-induced cardiovascular complications and the possible underlying signalling mechanisms. Met syndrome was induced in rats, which were then treated with TUDCA. Body weight, blood pressure, glucose tolerance and insulin tolerance tests were performed. ER stress, survival and oxidative stress markers were measured in heart and aorta tissue. The results showed that TUDCA improved metabolic parameters in rats with Met syndrome. Treatment mitigated the Met syndrome-induced cardiovascular complications through upregulating survival markers and downregulating ER and oxidative stress markers. These results highlight the protective effect of ER stress inhibition as a potential target in the management of cardiovascular complications associated with Met syndrome.

The Endothelial Cytoskeleton: Organization in Normal and Regenerating Endothelium*

The components of the endothelial cell cytoskeleton that have been shown to be important in maintaining endothelial structural integrity and in regulating endothelial repair include F-actin microfilament bundles, including stress fibers, and microtubules, and centrosomes. Endothelial cells contain peripheral and central actin microfilaments. The dense peripheral band (DPB) consists of peripheral actin microfilament bundles which are associated with vinculin adhesion plaques and are most prominent in low or no hemodynamic shear stress conditions. The central microfilaments are very prominent in areas of elevated hemodynamic shear stress. There is a redistribution of actin microfilaments characterized by a decrease of peripheral actin and an increase in central microfilaments under a variety of conditions, including exposure to thrombin, phorbol-esters, and hemodynamic shear stress. During reendothelialization, there is a sequential series of cytoskeletal changes. The DPB remains intact during the rapid lamellipodia mediated repair of very small wounds except at the base of the lamellipodia where it is splayed. The DPB is reduced or absent when cell locomotion occurs to repair a wound. In addition, when cell locomotion is required, the centrosome, in the presence of intact microtubules, redistributes to the front of the cell to establish cell polarity and acts as a modulator of the directionality of migration. This occurs prior to the loss of the DPB but does not occur in very small wounds that close without migration. Thus, the cytoskeleton is a dynamic intracellular system which regulates endothelial integrity and repair and is modulated by external stimuli that are present at the vessel wall-blood interface.

Current concepts in atherosclerosis

Atherosclerosis is a complex disease process. It is increasingly recognised that both lipoprotein retention and inflammatory cellular components are intricately related in the initiation and development of atherosclerotic plaque. LDL-c (cholesterol) has been long established as a cause for atherosclerosis; additionally, inflammatory cells such as monocytes and subsequently foam cells have also been directly linked to the progression of atherosclerotic disease. Emerging data suggest that structures outside vascular intima and media are also closely related to atherosclerosis. Perivascular adipose tissue (PVAT) may be a determinant of the inflammatory status of the atherosclerotic plaque. All these features are becoming extremely relevant as therapies against atherosclerosis are targeting both lipid retention and inflammation. Recently, there has been some success in these novel therapies, such as the proprotein convertase subtilisin-kexin type 9 (PCSK-9) inhibitor evolocumab and the interleukin-1ß neutralising antibody, canakinumab, in reducing cardiovascular events when added to standard therapy such as statin. This review will discuss the pathogenesis of atherosclerosis, including some novel features, and its management using new anti-atherosclerotic drugs.

Ephrin-A1/EphA4-mediated adhesion of monocytes to endothelial cells

The Eph receptors represent the largest family of receptor tyrosine kinases. Both Eph receptors and their ephrin ligands are cell-surface proteins, and they typically mediate cell-to-cell communication by interacting at sites of intercellular contact. The major aim of the present study was to investigate the involvement of EphA4-ephrin-A1 interaction in monocyte adhesion to endothelial cells, as this process is a crucial step during the initiation and progression of the atherosclerotic plaque. Immunohistochemical analysis of human atherosclerotic plaques revealed expression of EphA4 receptor and ephrin-A1 ligand in major cell types within the plaque. Short-time stimulation of endothelial cells with the soluble ligand ephrin-A1 leads to a fourfold increase in adhesion of human monocytes to endothelial cells. In addition, ephrin-A1 further increases monocyte adhesion to already inflamed endothelial cells. EphrinA1 mediates its effect on monocyte adhesion via the activated receptor EphA4. This ephrinA1/EphA4 induced process involves the activation of the Rho signaling pathway and does not require active transcription. Rho activation downstream of EphA4 leads to increased polymerization of actin filaments in endothelial cells. This process was shown to be crucial for the proadhesive effect of ephrin-A1. The results of the present study show that ephrin-A1-induced EphA4 forward signaling promotes monocyte adhesion to endothelial cells via activation of RhoA and subsequent stress-fiber formation by a non-transcriptional mechanism.

Hemodynamics of Carotid Artery Atherosclerotic Occlusive Disease

Hemodynamic mechanisms for the initiation and progression of carotid bifurcation atherosclerotic occlusive disease have been extensively researched during the past few decades. Attention has focused on the carotid bulb, or sinus, where most atherosclerotic plaques are found. Herein, the authors review the seminal works that have led to an understanding of not only complex local hemodynamics but also the elicited specific biologic response. In addition, new analysis of the age-dependent morphologic maturation of the human carotid bifurcation is unveiled. Understanding the role of hemodynamics in atherogenesis may lead to the improvement of minimally invasive endovascular therapy and noninvasive strategies.

Pathobiology of plaque modeling and complication

The artery wall adapts to changes in wall tension and wall shear stress by means of enlargement and changes in both thickness and composition. The intima may participate in these changes, and these compensatory adaptive-reactive modifications continue in the presence of atherogenesis. Further understanding of the interaction of the evolving plaque with the artery wall and the associated effects of the physical forces associated with the circulation should provide new insights into the nature of plaque instability and into the outcome of direct interventions.

Endothelial cell heterogeneity in experimentally-induced rabbit atherosclerosis. Demonstration of multinucleated giant endothelial cells by scanning electron microscopy and cell culture

We investigated the aortic endothelial cells of cholesterol-fed rabbits, using scanning electron microscopy and a cell culture technique. Rabbits were given a 1% cholesterol diet intermittently for up to 40 weeks. In these animals, the area of endothelial cells was increased and the cells showed polymorphism in relation to the progression of atherosclerosis. In animals fed the cholesterol diet for 12, 28 and 40 weeks, the average area of the endothelial cells was 436 +/- 15, 762 +/- 153, and 836 +/- 165 microns2, respectively. In the cholesterol-fed 40-week group, in particular, giant endothelial cells, measuring more than 1200 microns2, accounted for 14% of the population. In animals fed a standard diet there was no significant difference in endothelial cell morphology between control 0-week and control 40-week groups; in both, the luminal surface of the thoracic aorta formed a homogeneous sheet covered by small rhomboidal endothelial cells, the area of most being less than 400 microns2. Primary cultured endothelial cells harvested from those control groups were mononuclear typical small cells with a centrally located nucleus; the proportion of binucleated cells was less than 2% and no multinucleated giant cells with three or more nuclei were detected. Endothelial cells from the cholesterol-fed groups, however, contained larger numbers of binucleated cells, with the number increasing in proportion to the duration of cholesterol feeding. The major distinguishing feature of the endothelial cells in the cholesterol-fed groups was the presence of multinucleated giant cells with three or more nuclei; these accounted for 2.3% and 3.3% of the total cell population in the cholesterol-fed 28- and 40-week groups, respectively. No bromodeoxyuridine uptake was found in the nuclei of the cultured multinucleated giant cells. Heterogeneity of endothelial cells, with the concomitant appearance of multinucleated giant cells, emerges with the progression of diet-induced atherosclerosis. The morphological alterations of endothelial cells observed in the present study intimately reflect changes in their function associated with the progression of atherosclerotic lesions.

A definition of initial, fatty streak, and intermediate lesions of atherosclerosis. A report from the Committee on Vascular Lesions of the Council on Arteriosclerosis, American Heart Association

The compositions of lesion types that precede and that may initiate the development of advanced atherosclerotic lesions are described and the possible mechanisms of their development are reviewed. While advanced lesions involve disorganization of the intima and deformity of the artery, such changes are absent or minimal in their precursors. Advanced lesions are either overtly clinical or they predispose to the complications that cause ischemic episodes; precursors are silent and do not lead directly to complications. The precursors are arranged in a temporal sequence of three characteristic lesion types. Types I and II are generally the only lesion types found in children, although they may also occur in adults. Type I lesions represent the very initial changes and are recognized as an increase in the number of intimal macrophages and the appearance of macrophages filled with lipid droplets (foam cells). Type II lesions include the fatty streak lesion, the first grossly visible lesion, and are characterized by layers of macrophage foam cells and lipid droplets within intimal smooth muscle cells and minimal coarse-grained particles and heterogeneous droplets of extracellular lipid. Type III (intermediate) lesions are the morphological and chemical bridge between type II and advanced lesions. Type III lesions appear in some adaptive intimal thickenings (progression-prone locations) in young adults and are characterized by pools of extracellular lipid in addition to all the components of type II lesions.

A definition of initial, fatty streak, and intermediate lesions of atherosclerosis. A report from the Committee on Vascular Lesions of the Council on Arteriosclerosis, American Heart Association

The compositions of lesion types that precede and that may initiate the development of advanced atherosclerotic lesions are described and the possible mechanisms of their development are reviewed. While advanced lesions involve disorganization of the intima and deformity of the artery, such changes are absent or minimal in their precursors. Advanced lesions are either overtly clinical or they predispose to the complications that cause ischemic episodes; precursors are silent and do not lead directly to complications. The precursors are arranged in a temporal sequence of three characteristic lesion types. Types I and II are generally the only lesion types found in children, although they may also occur in adults. Type I lesions represent the very initial changes and are recognized as an increase in the number of intimal macrophages and the appearance of macrophages filled with lipid droplets (foam cells). Type II lesions include the fatty streak lesion, the first grossly visible lesion, and are characterized by layers of macrophage foam cells and lipid droplets within intimal smooth muscle cells and minimal coarse-grained particles and heterogeneous droplets of extracellular lipid. Type III (intermediate) lesions are the morphological and chemical bridge between type II and advanced lesions. Type III lesions appear in some adaptive intimal thickenings (progression-prone locations) in young adults and are characterized by pools of extracellular lipid in addition to all the components of type II lesions.

Effect of coarctation on matrix content of experimental aortic atherosclerosis: relation to location, plaque size and blood pressure

Cynomolgus monkeys were fed an atherogenic diet for 6 months following surgically produced high-grade (n = 10) or mild (n = 16) mid-thoracic aortic coarctation. A diet-control (DC) group (n = 13) was fed the diet without coarctation. High-grade coarctation (HGC) resulted in 74.1% +/- 8.3% stenosis by aortography prior to sacrifice and was associated with systolic brachial blood pressures of 143.3 +/- 26.0 mmHg and gradients across the stenoses of 36.8 +/- 23.6 mmHg. Mild coarctation (MC) resulted in stenoses of 50.9% +/- 12.9%, brachial systolic pressures of 119.4 +/- 25.7 and gradients of 12.5 +/- 15.2 mm Hg (P < 0.01, P = 0.03 and P < 0.005, respectively, compared with HGC). When total plaque cross-sectional area exceeded 0.8 mm2, the entire arterial circumference was usually involved. HGC resulted in complete sparing or minimal plaque formation in sections distal to the stenoses compared with proximal sections (P < 0.001). There were no significant differences between MC and DC animals in plaque location or size. Matrix content increased with plaque area regardless of degree of stenosis or sampling level (P < 0.01), but lesions with more than 75% matrix content were more numerous in distal than in proximal sections despite their smaller size. The number of plaques with greater than 75% matrix content was increased proximal to HGC (P < 0.04). Thus, distal location and plaque size were independent determinants of plaque matrix content and matrix content was increased proximal to HGC regardless of plaque size. Attempts to evaluate effects of various regimens and interventions on plaque composition need to take location and plaque size, as well as blood pressure differences, into account.

Experimental atherosclerosis at the carotid bifurcation of the cynomolgus monkey. Localization, compensatory enlargement, and the sparing effect of lowered heart rate

We have characterized plaque localization, the extent of compensatory artery enlargement, and the effect of heart rate in experimental atherosclerosis at the carotid bifurcation of the cynomolgus monkey. We altered heart rate by sino-atrial node ablation (SNA) and then fed the animals an atherogenic diet for 6 months. Heart rate was measured at four time points by 24-hour telemetry. Of nine animals with SNA, heart rate was reduced significantly in six (from 148 +/- 11 to 103 +/- 20 beats/min, p < 0.001) and was unchanged in three. Sham-operated monkeys had no significant change in heart rate. On the basis of comparison with the preoperative mean for all 17 animals (136 +/- 22 beats/min), animals were separated into a low-heart-rate (LHR) group (111 +/- 16 beats/min, n = 12) and a high-heart-rate (HHR) group (150 +/- 16 beats/min, n = 5). Blood pressure, serum cholesterol level, and body weight did not differ for the two groups. As in the human, plaques formed predominantly in the proximal portion of the internal carotid artery at the lateral wall opposite the flow divider. Plaque cross-sectional area increased progressively from the relatively uninvolved, adjacent common carotid artery to the mid-sinus region of the internal carotid artery and decreased from the mid-sinus region to the internal carotid artery beyond the sinus. Plaque distribution was the same for the LHR and HHR groups, but lesion area and percent stenosis were greater for the HHR group than for the LHR animals (2.01 +/- 1.19 compared with 0.76 +/- 0.42 mm2 for lesion area [p < 0.02] and 30.7 +/- 4.4% compared with 15.2 +/- 7.3% for stenosis [p < 0.002]).(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure appearance of atherosclerosis in human and experimentally-induced animal models

We describe here the basic structure of the aorta, the changes with aging and ultrastructural appearance of atherosclerosis of human and animal models. The architecture of the aortic wall is highly organized, for adaptation to changes of blood pressure. The main cells composing the vessel are endothelial cells and smooth muscle cells. They maintain the integrity and homeostasis of the aorta along with the extracellular matrix of collagen fibrils, elastic fibers and glycosaminoglycans. The structural changes with aging and atherogenesis are a compensative or degenerative phenomenon caused by many factors. Three major cells are the endothelial cell, smooth muscle cell and monocyte-derived macrophages (as well as platelets) all of which are involved in atherogenesis. Foam cells in atheromatous lesions are derived from macrophages and smooth muscle cells. Recently, the molecular biological nature and function of these cells and their derived-factors have been thoroughly investigated in cell culture and in experimental animal models caused by a mechanical injury of the endothelium or by a dietary induced hypercholesterolemia. However, the mechanism of the endothelial injury in vivo as well as formation of atheromatous cores of human atherosclerosis is not exactly understood. Some structural and functional changes inherent to the arterial wall during aging may play an important role in initiation or progression of human atherosclerosis.

Vitronectin-induced haptotaxis of vascular smooth muscle cells in vitro

Vitronectin, a multifunctional glycoprotein present in the plasma and interstitial tissues, has recently been found to be localized in atherosclerotic lesions. In this study we examined the effects of vitronectin on the migration of cultured bovine aortic smooth muscle cells using a modified Boyden chamber assay. The cells migrated to fluid-phase vitronectin in a concentration-dependent fashion. The cells also migrated to membrane filter surfaces precoated with vitronectin for a few minutes in the absence of additional vitronectin in the fluid phase, suggesting that this substance binds easily to the filters and stimulates cell migration by haptotaxis under the conditions described. These observations suggest that vitronectin deposited in the intima may be involved in the pathogenesis of atherosclerosis by recruiting smooth muscle cells from the media into the intima.

Intimal alterations in rabbit aortas during the first 6 months of alloxan-induced diabetes

Diabetes mellitus is a major risk factor for atherosclerosis. Since endothelial alteration is probably associated with the development of atherosclerosis, we questioned whether morphological evidence of endothelial injury could be observed during the first 6 months of diabetes induced by a single intravenous injection of alloxan in normally fed rabbits compared with age-matched controls. Diabetes (plasma glucose greater than 16 mM) was established by 5 days after alloxan injection. Endothelial alterations consistent with injury, including adhesion of white blood cells, platelets, and fibrin-like material to the endothelial surface, were seen in diabetic rabbit aortas by 2 weeks. These alterations became more severe during the next 6 months. Increased endothelial replication in diabetic vessels was shown by the uptake of tritium-labeled thymidine at 2 weeks and at 3 and 6 months. Hyperplasia of intimal smooth muscle cells progressed during 3 months after treatment. About one third of the diabetic rabbits also showed an elevated plasma cholesterol level, which correlated with increased intimal proliferation but not with endothelial injury or replication. The onset of alloxan-induced diabetes in rabbits is associated with nondenuding endothelial injury and subsequent intimal hypertrophy, changes that are consistent with atherogenesis.

Changes in plasma factor VIII complex and serum lipid profile during atherogenesis in cynomolgus monkeys

If endothelial injury plays a prominent role in early atherogenesis, the plasma levels of von Willebrand factor (VWF), which is made within and normally released from endothelial cells, might be expected to rise as a marker of the cellular damage. To evaluate this hypothesis, we measured plasma VWF (as VIIIR:Ag), factor VIII:C, and serum lipids serially up to 37 weeks in 29 adult male cynomolgus monkeys on an atherogenic diet. Factor VIII:C peaked at 113% above baseline by week 10 (p less than 0.0001), then fell and remained 53% below baseline (p less than 0.04) during weeks 20 to 37. However, the overall rise in VWF was not significant. In contrast, serum cholesterol continued to rise after week 21. Serum phospholipids (PL), triglycerides (TG), and free fatty acids (FFA) showed a temporal pattern similar to VIII:C. Significant positive correlations with VIII:C were noted for PL (r = 0.59, p = 0.0001) and TG (r = 0.36, p = 0.0096). At autopsy, small to moderately advanced atherosclerotic lesions were distributed throughout the aortas of the majority of the animals. We conclude that changes in plasma VIIIR:Ag do not correlate with atherogenesis in this model. However, the similar course of VIII:C, TG, and PL suggests that these substances may be involved and perhaps interrelated early in atherogenesis.