Overall, stages of atherosclerosis
Arterial injury causes endothelial dysfunction promoting modification of apoB containing lipoproteins and infiltration of monocytes into the subendothelial space. Internalization of the apoB containing lipoproteins by macrophages promotes foam cell formation, which is the hallmark of the fatty streak phase of atherosclerosis. Macrophage inflammation results in enhanced oxidative stress and cytokine/chemokine secretion, causing more LDL/remnant oxidation, endothelial cell activation, monocyte recruitment, and foam cell formation. HDL, apoA-I, and endogenous apoE prevent inflammation and oxidative stress and promote cholesterol efflux to reduce lesion formation. Macrophage inflammatory chemoattractants stimulate infiltration and proliferation of smooth muscle cells. Smooth muscle cells produce the extracellular matrix providing a stable fibrous barrier between plaque prothrombotic factors and platelets. Unresolved inflammation results in formation of vulnerable plaques characterized by enhanced macrophage apoptosis and defective efferocytosis of apoptotic cells resulting in necrotic cell death leading to increased smooth muscle cell death, decreased extracellular matrix production, and collagen degradation by macrophage proteases. Rupture of the thinning fibrous cap promotes thrombus formation resulting in clinical ischemic ASCVE. Surprisingly, native LDL is not taken up by macrophages in vitro but has to be modified to promote foam cell formation. Oxidative modification converts LDL into atherogenic particles that initiate inflammatory responses. Uptake and accumulation of oxidatively modified LDL (oxLDL) by macrophages initiates a wide range of bioactivities that may drive development of atherosclerotic lesions. Lowering LDL-cholesterol with statins reduces risk for cardiovascular events, providing ultimate proof of the cholesterol hypothesis. All of the apoB containing lipoproteins are atherogenic, and both triglyceride rich remnant lipoproteins and Lp(a) promote atherothrombosis. Non-HDL cholesterol levels capture all of the apoB containing lipoproteins in one number and are useful in assessing risk in the setting of hypertriglyceridemia. Measures of apoB and LDL-P are superior at predicting risk for ASCVE, when levels of LDL-C and LDL-P are discordant. Here, we also describe the current landscape of HDL metabolism. Epidemiological studies have consistently shown that HDL-C levels are inversely related to ASCVE. By contrast, high density lipoproteins possess the capacity to remove this excess cholesterol from the artery wall – explaining the epidemiologic links between high circulating concentrations of HDL and protection against CHD.
Highlight recent clinical trials aimed at raising HDL-C that failed to reduce CVE and the shifting clinical targets of HDL-C, HDL particle numbers, and HDL function (e.g. cholesterol efflux capacity). Furthermore, describe many beneficial properties of HDL that antagonize atherosclerosis and how HDL dysfunction may promote cardiometabolic disease
Atherosclerosis: A progressive process
Aetiology of atherosclerosis
The atherosclerotic process is initiated by inflammation of the endothelium lining the artery wall. Upregulation of chemo-attractants and adhesion molecules mediates the recruitment of monocytes and lymphocytes from the bloodstream to the artery wall.
The presence of macrophage foam cells denotes the ‘fatty streak’ – the earliest type of lesion shown in 4) – which can develop within the first or second decades of life. As the lesion develops, smooth muscle cells become involved, producing extracellular matrix and contributing to the ‘fibro-fatty’ lesion. The lesion may undergo several cycles of inflammation, monocyte recruitment and lipid accumulation, and fibrosis, with calcification occurring in the later stages, before plaque rupture and the MI or stroke is precipitated.
Structure of a typical large artery A normal healthy artery is lined with a single layer of endothelial cells, forming the endothelium which provides a non-thrombogenic, non-adhesive surface through which the blood can flow. It produces factors regulating tissue homeostasis, including nitric oxide to maintain the vessel in an appropriate state of relaxation. However, the endothelium is also a highly responsive organ and provides the key to initiation of the intimal atherosclerotic lesion. Beneath the endothelium lies the internal elastic lamina, formed of elastin fibres and providing structural support, and beneath that, the media which is formed of smooth muscle cells. Medial SMC are arranged concentrically so that they maintain the vessel structure and contract or relax as required. The adventitia is formed of fibroblasts, collagen and elastin fibres, and also provides structure and support to the vessel.
Endothelial activation
The first step in initiating the formation of an atherosclerotic lesion is thought to be the activation of the endothelial layer. A number of species can achieve this, including many of the risk factors discussed earlier. Bacterial or viral infection (chylamydiae pneumoniae, herpes simplex virus, cytomegalovirus and various peridonal pathogens) have been proposed as activating factors: however, a major trial, in which individuals who had suffered a coronary event were dosed with antibiotics during the next five years showed no effect in terms of reducing mortality. Elevated LDL cholesterol, and triglyceride-rich lipoproteins, can induce endothelial activation, as can hypoxia and turbulent blood flow (‘shear stress’). Inflammatory cytokines such as tumour necrosis factor and interleukins-1 and -6 can trigger endothelial dysfunction, as can advanced glycation end (AGE) products seen in diabetic individuals, and the products derived from cigarette smoking. Thus, the endothelium is subject to activation by a number of stimuli, and responds by producing growth factors and chemoattractant cytokines (chemokines), by increasing its procoagulant activity (tissue factor), by increasing the permeability of the endothelial layer, and by increasing the adhesion of blood leucocytes to its surface. Thus, the first stage in atherogenesis is essentially a classic inflammatory response. However, instead of resolving over a day or two, atherosclerotic lesions are characterised by a chronic low grade inflammation where the response is sustained over long periods.
Monocyte recruitment Activation of the endothelium results in the enhanced transcription of adhesion molecules and chemokines, resulting in the recruitment of monocytes from the bloodstream to the artery wall. Normally, blood leucocytes roll gently and continuously along the endothelium. However, when the latter is activated, this process is slowed and then halted by adhesion molecules, like E- and P-selectin, VCAM-1 and ICAM-1. The increased expression of chemokines, like Monocyte Chemotactic Protein-1 (MCP-1), attracts leucocytes to the site of inflammation; they also induce changes in integrin receptors on blood leucocytes so that tight binding to adhesion molecules occurs. The chemotactic gradient facilitates passage of monocytes between the endothelial cells and into the artery wall. Monocytes differentiate into tissue macrophages under the influence of colony stimulating factors such as Macrophage Colony Stimulating Factor (M-CSF) that promote differentiation, survival and retention of these cells within the inflammatory tissue.
Inflammatory responses in atherosclerosis –
role of nuclear factor kB family of transcription factors
Nuclear factor kB transcription factors The factors triggering endothelial activation converge on the NF-kB family of transcription factors. Normally, this transcription factor is held in the cytoplasm in an inactive form. However, pro-inflammatory stimuli trigger the movement of this molecule into the nucleus, where (together with a complex of other transcription factors) it binds to the promoter region and initiates the transcription of adhesion molecules, chemokines, CSFs and cytokines. This explains the recruitment of monocytes by the endothelium and their subsequent differentiation into tissue macrophages. If NF-kB activation is blocked, by using dominant negative constructs or pharmacological inhibitors, such as antioxidants or nitric oxide donors, then the process of arterial inflammation is reduced and lesion formation decreased.
Monocyte adhesion Blood monocytes are recruited from the bloodstream to adhere (a) and then migrate (b) across the endothelium into the intima of the artery wall.
Arterial lipid accumulation Macrophages play a very important part in lesion progression and particularly in mediating lipid accumulation within the artery wall. Macrophages release free radical species which can oxidize LDL to either mildly oxidized or highly oxidized forms. Mildly oxidized LDL can activate the endothelium, ensuring further monocyte recruitment. Highly oxidized LDL is recognized by macrophage ‘scavenger receptors’ as foreign particles and, as part of the non-adaptive innate immune response, is engulfed by the macrophages. This occurs to a marked degree, leading to the intracellular accumulation of lipid, particularly cholesteryl ester, forming macrophage ‘foam cells’. At later stages of lesion development, these cells can undergo apoptosis and necrosis, releasing this lipid and forming the extracellular lipid core characteristic of more advanced atherosclerotic lesions.
Recruitment of white blood cells to sites of inflammation – which most commonly arise at regions of the artery where blood flow is disturbed, rather than in regions of linear flow. Monocytes, for example, then undergo differentiation into tissue macrophages, and become resident in the artery wall, sustaining and exacerbating the chronic low-grade inflammation characteristic of atherosclerotic lesions.
Macrophages are also a source of oxidizing free radical species, which can modify the LDL particles, to oxidized LDL – which, unlike ‘normal’ LDL, are recognized as ‘foreign’ particles by scavenger receptors on the macrophages . So – just as macrophages ‘recognise’ foreign particles or organisms (e.g. bacteria) and clear them from the bloodstream – macrophages within the artery wall recognise oxidized LDL as ‘foreign’ to the body, and take up these particles in an unregulated manner, accumulating large deposits of cholesterol as ‘foamy’ lipid droplets within the cytosol of these cells.
By contrast, high density lipoproteins possess the capacity to remove this excess cholesterol from the artery wall – explaining the epidemiologic links between high circulating concentrations of HDL and protection against CHD.
Smooth muscle cell migration and proliferation Macrophage foam cells are not, however, merely ‘bags of lipid’ within the artery wall. They release a number of factors that influence lesion development, such as inflammatory cytokines (IL-6, IL-1b), thus chronically prolonging and exacerbating the inflammatory response. They release free radicals (oxidizing species, such as superoxide, peroxynitrite etc,) that can increase oxidative stress within the artery wall, and further enhance LDL oxidation and macrophage lipid accumulation.
Macrophages also release matrix-degrading enzymes, and growth factors. The combination of these factors leads to the migration of smooth muscle cells (SMC) from the underlying media into the intima of the vessel. SMC in the media express cell surface integrins which detect the change in extracellular matrix induced by matrix metalloproteases. This, in combination with the expression of mitogenic growth factors induces the smooth muscle cells to change in phenotype from the contractile quiescent state to enter a proliferative state. This phenotypic switch is also involved in a change in function – the SMC lose their contractile function and switch to a synthetic state, synthesizing large amounts of extracellular matrix, like proteoglycans and collagen, and forming a fibrous cap over the lesion.
Enhanced matrix formation, particularly proteoglycan formation, appears to be another factor enhancing the retention of lipoproteins, like LDL, within the lesion. The presence of SMC and fibrotic extracellular material also increases the stability of the lesion. By contrast, lesions which contain a large amount of extracellular lipid, plus macrophages at the edges of the lesions secreting matrix degrading enzymes, are much more liable to rupture. So, the involvement of SMC exerts dual influences on lesion development – they add to lesion size, but also to lesion stability.
Formation of complex lesions
The final stages of the atherosclerotic process involve the formation of complex lesions. There is now a substantial extracellular lipid pool, made up of macrophage foam cells, necrotic material, cholesterol crystals and this atherosclerotic ‘gruel’ is also prone to calcification. The macrophages present at the plaque edges tend to make the plaque prone to rupture, and here, at this point, we have a small mural thrombus forming. It is also common to see a number of small thrombi incorporated within the body of the plaque due to fissures within the plaque – these do not precipitate the final MI but can contribute to the bulk of the lesion.
MACROPHAGE SCAVENGER RECEPTOR (SR-AI)
In 1979, Brown and Goldstein demonstrated that macrophages had specific binding sites for acetylated LDL (AcLDL) that allowed uptake of this modified LDL even in the presence of high cellular cholesterol levels (298). This was in contrast to LDL uptake by the LDLR, which is markedly downregulated when cellular cholesterol levels rise (Figure 2). Cholesterol synthesis is also downregulated by LDL uptake by LDLR (337). The lack of feedback inhibition during uptake of modified LDL by this unidentified receptor suggested a plausible mechanism for the massive accumulation of cholesterol in macrophages that generates foam cells. The putative receptor mediating this binding was named the macrophage scavenger receptor (MSR). Later, oxLDL (338) and MDA-LDL (248) were shown to compete with AcLDL for binding and uptake by macrophages, suggesting they were native ligands for MSR. In 1990, Kodama et al. purified and sequenced this scavenger receptor, allowing identification of the MSR gene (339). Through alternative gene splicing, this gene gives rise to Scavenger Receptor A–I (SR-AI), SRA-II, and SRA-III. Deletion of the MSR gene in C57BL6 mice fed butterfat diet substantially reduced atherosclerotic lesions and deletion of MSR in Ldlr-/- mice also reduced lesion formation (340).
https://www.ncbi.nlm.nih.gov/books/NBK343489/