This review article discusses the historical origin of our continuously evolving model of the etiology of atherosclerotic cardiovascular disease. The basic molecular biologic concepts underlying the development of coronary artery disease and the dynamic connection between the immune system and arterial integrity are explored. Emphasis is placed on the role of inflammation as a driving force in the process of atherosclerosis and vascular endothelium as a modulating factor in the pathogenesis of coronary artery disease.
Atherosclerosis is a complex, chronic, and multifactorial pathologic change in the arterial wall. It is a disease of large arteries characterized by cholesterol deposition, lipid accumulation, and fibrosis. An inflammatory response of the arterial intima to injury or denudation initiates this process.1A crucial early step in the genesis of atherosclerosis is the recruitment of circulating monocytes to the damaged endothelial lining. Vascular endothelium may contribute to the development of atherosclerosis by expressing adhesive molecules that mediate recruitment of monocytes to the blood vessel wall, where they bind, transmigrate into the subintima, and then differentiate into macrophages.2These macrophages ingest oxidized low-density lipoprotein (LDL) cholesterol particles, which are proinflammatory, and become engorged with cholesterol, ultimately transforming into foam cells. Elevated levels of LDL cholesterol thus play a role in the causation of atherosclerosis and are therefore considered a major risk factor for cardiovascular disease.3Fatty streaks are primarily composed of lipid-laden foam cells beneath the endothelium.4Overproliferating smooth muscle cells (SMCs) are another major component of the fatty streak. SMCs secrete a connective tissue matrix rich in collagen. Activated T cells also participate.5Eventually, fatty streak lesions develop into fibrous plaques by the continual deposition of lipid and the accumulation of SMCs and connective tissue. These fibrous plaques have a lipid-rich core formed by living and dead foam cells. As the fibrous plaques expand, they ultimately cause the onset of acute clinical syndromes, such as heart attack or stroke, by the action of a variety of changes, such as hemorrhage, ulceration, thrombosis, or calcification.
BLOOD CHOLESTEROL AND CORONARY HEART DISEASE
The link between cholesterol and cardiovascular disease was first acknowledged in the 1960s and 1970s.6At that time, total cholesterol was the only measurement available. In 1982, the Nutrition Committee of the American Heart Association recommended that no more than 10% of calories be derived from polyunsaturated fat, but they acknowledged that epidemiologic data did not support a correlation among dietary fat, serum cholesterol, and the risk of coronary heart disease (CHD).7Nonetheless, in 1985, with the goal of reducing the prevalence of hypercholesterolemia in the United States, the National Heart, Lung, and Blood Institute initiated the National Cholesterol Education Program.8Cholesterol was identified as the villain in CHD, and the idea that foods rich in saturated fat and cholesterol cause cholesterol deposition in arteries in the form of plaque dominated medical thinking. The American Medical Association launched a high-profile “war on cholesterol.”
Elevated blood cholesterol is one of the major modifiable risk factors for CHD.9However, it is now recognized that high LDL cholesterol and low high-density lipoprotein (HDL) cholesterol are more specifically linked to cardiovascular disease than is total cholesterol.10LDL transports most of the cholesterol in the blood and is the main source of atherogenic cholesterol.11,12There is also a strong, independent, and inverse association between HDL levels and CHD risk.13,14The primary function of HDL is to retrieve cholesterol from cells and tissues for delivery via the bloodstream back to the liver, which leads to its elimination from the body.15In many studies, measures of HDL or the ratio of total cholesterol to HDL are better predictors of CHD risk than is serum cholesterol alone.16
LIVER METABOLISM OF CHOLESTEROL
Cholesterol is essential for many biologic functions of mammalian cells and is a key component of the eukaryotic plasma membrane. It is a specific precursor for the synthesis of the steroid hormones, D vitamins, and bile acids. Since cholesterol is insoluble in the blood, both dietary cholesterol and that synthesized de novo are transported through the circulation in lipoprotein particles. The same is true of cholesteryl esters, the form in which cholesterol is stored in cells. The synthesis and use of cholesterol must be tightly regulated to prevent overaccumulation and abnormal deposition within the body. There is homeostatic regulation at multiple steps to prevent excess production or intake of cholesterol.17The liver is the organ responsible for both synthesis and catabolism of cholesterol. The average person makes about 75% of blood cholesterol in his or her liver.18Hepatocyte lipoprotein uptake mediates the clearance of circulating plasma cholesterol. The end products of cholesterol use are the water-soluble bile acids, synthesized exclusively in the hepatocytes in liver. Synthesis of bile acids is the only significant mechanism for the elimination of excess cholesterol in mammals.19The adult human liver synthesizes approximately 500 mg of bile acids per day from cholesterol.20
3-Hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors (statin drugs) lower serum cholesterol by targeting the rate-limiting step in the metabolic cascade leading to cholesterol biosynthesis in hepatocytes, thus blocking formation of cholesterol in the liver. These drugs may prevent or slow atherosclerosis. Statins reduce LDL concentration by up to 60%. This drug class is the first-line treatment for hypercholesterolemia.21A number of properties other than lipid lowering may help explain the impact of statins on atherosclerotic cardiovascular disease (ASCVD). The clinical action of many cholesterol-lowering drugs is the result of multimodal pleiotropic effects rather than simply a reduction in cholesterol. The documented beneficial effects are antiatherosclerotic, anti-inflammatory, and antithrombotic. Statins have been shown to reduce the levels of proinflammatory cytokines and markers of acute-phase response, including C-reactive protein and serum amyloid A.22They have antioxidant properties and can both inhibit the generation of reactive oxygen species and blunt their destructive effects.23
INFLAMMATORY PROCESSES, INTERFERON-γ, AND ATHEROSCLEROSIS
Cholesterol does not directly clog arteries in the way that grease clogs pipes. Half of all heart attacks occur in people with normal cholesterol levels.24Inflammation of the vessel wall is a characteristic feature of atherosclerosis, and evidence continues to accumulate implicating a state of chronic, low-grade inflammation in the development of cardiovascular disease.25Inflammation activates monocytes and macrophages, which are recruited to the artery, adhere to the vessel wall, release cytokines, and take up oxidized LDL via nonautoregulated scavenger receptors to form foam cells.26Persons suffering from chronic immunologically mediated diseases, such as systemic lupus erythematosus and rheumatoid arthritis, are at increased risk of developing premature ASCVD.27,28
Endothelial cells and leukocytes within atherosclerotic lesions produce a variety of immune and inflammatory mediators, such as interleukin (IL)-1, IL-6, monocyte chemoattractant peptide 1, and platelet-derived growth factor.29,30Macrophages and T cells are present in the atherosclerotic tissue of humans and animal models.31T-cell clones from human atherosclerotic plaques respond to oxidized LDL exposure by proliferating and secreting cytokines, specifically interferon (IFN)-γ. These data suggest that a T cell-dependent autoimmune response to oxidized LDL occurs within the plaque.32Infiltration of T cells and monocyte-derived macrophages into the intimal wall is believed to result in progression of the atherosclerotic lesion from the early fatty streak, which is an accumulation of lipid-laden macrophages (foam cells), to an advanced fibroproliferative atherosclerotic lesion. Macrophages play a key role in the development of these early lesions by the uptake and metabolism of modified LDL particles, as well as in lesion progression by secreting chemokines, cytokines, proteases, and coagulation factors. Zhou and colleagues evaluated the role of immunity in atherosclerosis by crossing atherosclerosis-prone apolipoprotein E (apoE) knockout (KO) mice with severe combined immunodeficiency disorder (scid/scid) mice.33When compared with immunocompetent apoE-deficient mice, the doubly deficient offspring showed a 73% reduction in aortic fatty streak lesions. When CD4+ T cells from the immunocompetent apoE KO mice were transferred to immunodeficient apoE-deficient scid/scid mice, lesions increased by 164% and the transferred T cells infiltrated into the lesions, increased circulating IFN-γ levels, and increased major histocompatibility complex (MHC) class II antigen I-A expression in lesions. Thus, CD4+ T cells carry disease-promoting immunity in atherosclerosis.
Activated CD4+ T cells present in the atherosclerotic lesion can secrete IFN-γ, as has been demonstrated in human atherosclerotic plaques by immunofluorescence and polymerase chain reaction (PCR).34,35The proatherogenic effects of IFN-γ include induction of vascular cellular adhesion molecule 1 on endothelial cells, MHC-II on macrophages and SMCs, lipoprotein receptors on SMCs, and decreased secretion of apoE and expression of lipoprotein receptors on macrophages.36-38Consistent with these in vitro observations is the report that apoE KO mice crossed with IFN-γ receptor KO mice display reduced lesion size, lipid accumulation, and cellularity.39Further, when apoE KO mice were given intraperitoneal recombinant IFN-γ daily for 30 days, atherosclerotic lesion size in the ascending aorta increased twofold compared with phosphate buffered saline-treated controls.40
ARTERIAL WALL: SITE OF ATHEROSCLEROSIS
It is now widely recognized that the artery wall is itself a metabolically active organ system that maintains vascular homeostasis. The arterial endothelium, smooth muscle, and macrophages are active participants in the atherosclerotic process. Endothelial dysfunction may contribute to the development and clinical expression of atherosclerosis. However, it was previously believed that all significant cholesterol metabolism occurred in the liver. In 1989, Andersson and colleagues cloned and sequenced cholesterol 27-hydroxylase, a cholesterol-metabolizing enzyme that produces the oxysterol product 27-hydroxycholesterol.41Then, in 1994, two seminal findings indicated that extrahepatic metabolism of cholesterol could affect atherogenesis locally within the vessel wall: (1) our laboratory discovered that aortic endothelial cells express 27-hydroxylase and thus have the ability to convert cholesterol to 27-hydroxycholesterol,42and (2) Bjorkhem and colleagues demonstrated that human macrophages have a very high 27-hydroxylase activity.43
Uptake of modified LDL into macrophages via scavenger receptors with consequent foam cell formation is an early and crucial step in the atherosclerotic process. 27-Hydroxylase in macrophages counteracts the lipid overload of foam cells by ridding the cells of excess cholesterol. Support for this function of the 27-hydroxylase is derived from experiments that demonstrate that macrophages treated with the 27-hydroxylase inhibitor cyclosporin A accumulate intracellular cholesterol.44
CHOLESTEROL 27-HYDROXYLASE, ADENOSINE TRIPHOSPHATE BINDING CASSETTE TRANSPORTER 1, AND REVERSE CHOLESTEROL TRANSPORT
The mitochondrial cytochrome P-450 enzyme cholesterol 27-hydroxylase (CYP27A1) catalyzes the first step in the oxidation of the side chain of sterol intermediates in the bile acid synthesis pathway. It catalyzes the 27-hydroxylation of cholesterol and several other bile acid intermediates.41,45Mammalian cells lack the capacity to directly degrade the four-member ring structure of cholesterol. Therefore, the conversion of cholesterol into bile acids represents the major catabolic pathway for removal of cholesterol from the organism.
The 27-hydroxylase enzyme is widely distributed in most organs and tissues but is expressed at particularly high levels in macrophages46and in arterial endothelium.42High levels of the enzyme are found in atherosclerotic arteries.43Mounting evidence suggests that this enzyme participates in physiologic events that provide a defense mechanism against atherosclerosis by removing cholesterol from the arterial wall, by inhibiting peripheral cholesterol synthesis, and through the regulatory effect of its metabolites on key steps in the pathogenesis of atherosclerosis.47,48
Cholesterol 27-hydroxylase specifically hydroxylates carbon 27 on the cholesterol molecule and converts it into 27-hydroxycholesterol (Figure 1). The CYP27A1 enzyme has the ability to hydroxylate the same methyl group three times to give a carboxylic acid: 3β-hydroxy-5-cholestenoic acid.49These oxygenated derivatives of cholesterol are more polar in nature and thus are conveyed from peripheral tissues back to the liver more easily (a process known as reverse cholesterol transport), facilitating their degradation and excretion. The conversion of cholesterol into 27-hydroxycholesterol and 3β-hydroxy-5-cholestenoic acid represents a general defense mechanism for macrophages and other peripheral cells exposed to cholesterol.4327-Hydroxycholesterol is a component of the major circulating lipoproteins and is the most abundant hydroxycholesterol in human circulation.50It inhibits the rate-limiting enzyme in cholesterol synthesis, HMG CoA reductase, suppresses smooth muscle cell proliferation, and diminishes foam cell formation by macrophages.51-53
The most convincing argument for the antiatherogenic effect of cholesterol 27-hydroxylase is the accelerated atherosclerosis that occurs in patients with cerebrotendinous xanthomatosis (CTX), a deficiency of the enzyme caused at the molecular level by mutations in the CYP27 locus. This autosomal recessive heritable disorder results in the absence of 27-hydroxylase enzyme activity with reduced synthesis of bile acids from cholesterol in the liver. Also, the lack of 27-hydroxylase activity in vascular endothelium and macrophages leads to compromised reverse cholesterol transport from peripheral arterial tissue to liver, with consequent vascular lipid accumulation and premature atherosclerosis despite a normal serum lipid profile.54-56
The clinical features in patients with CTX are tendon and brain xanthomas, premature atherosclerosis, juvenile bilateral cataracts, and nervous system dysfunction, including mental retardation, dementia, cerebellar ataxia, epileptic seizures, and peripheral neuropathy. 27-Hydroxycholesterol production is less than one-tenth that of normal individuals,57and atherosclerosis proceeds despite normal to low levels of circulating LDL and normal HDL levels.56Defects in the gene lead to reduced bile acid synthesis, with accumulation of 7α-hydroxylated intermediates, one of which is a precursor to cholestanol. Treatment with chenodeoxycholic acid may slow disease progression by reducing up-regulation of cholesterol 7α-hydroxylase and therefore formation of cholestanol. HMG-CoA reductase inhibitors are combined with chenodeoxycholic acid for a synergistic effect on lipoprotein metabolism.58The clinical presentation and severity for CTX are variable. In CTX, the same mutation may result in different phenotypes, or mutations at different sites of the CYP27 gene may result in the same or different phenotypes, with little correlation between genotype and phenotype. Thus, patients may be diagnosed at an early age or well into adulthood, and the symptoms or signs may range from being asymptomatic, to the presence of tendon xanthomas only, or to early neurologic dysfunction.59
Rosen and colleagues disrupted the 27-hydroxylase gene in the C57BL/6J mouse for the purposes of studying bile acid synthesis and observed no CTX-related pathologic abnormalities when the mice were fed a normal diet.60However, in some preliminary experiments when these KO mice were fed an atherogenic diet, an increased death rate was observed. Dubrac and colleagues found that mice heterozygous for the CYP27 gene KO exhibited mild hypercholesterolemia with elevated LDL and increased body weight.61The 27-hydroxylase KO mouse needs to be crossed into a genetic atherosclerosis model, such as the apoE-deficient mouse, for more information.
In a recent study, Babiker and colleagues found that although increased circulating levels of 27-hydroxycholesterol and its derivatives might be associated with atherosclerosis, the data did not support clinical utility or predictive value for this measure.45In light of the multiple potential sources of these oxysterols, as well as the very localized impact of concentration variations within the vessel, this result is not surprising.
27-Hydroxylase is not the only factor that influences cholesterol metabolism and efflux. Rather, it is one mechanism that is part of a complex regulatory system. Adenosine triphosphate binding cassette transporter 1 (ABCA1) is a second reverse cholesterol transport protein that plays a crucial role in facilitating efflux of cellular cholesterol to extracellular apoA-I or HDL (Figure 2). ABCA1, a 220 kD protein, mediates cholesterol secretion from cells and functions as a rate-controlling protein in the apoA-I-dependent active transport of cholesterol and phospholipids.62Understanding of the role of ABCA1 in cellular cholesterol efflux came with the discovery that mutations in the ABCA1 gene result in Tangier disease (TD).63TD is a rare genetic disorder characterized by extremely low plasma HDL and apoA-I levels, cholesteryl ester deposition in tissues, macrophage cholesterol ester accumulation, and a high prevalence of ASCVD. The discovery that fibroblasts from TD patients have a marked defect in efflux of cholesterol and phospholipids to apoA-I suggests that ABCA1 mediates or regulates the efflux of cellular cholesterol and phospholipids to apoA-I.64Lipidation of apoA-I by the ABCA1 pathway is required for generating HDL particles and clearing sterol from macrophages.
Expression of ABCA1 is induced by oxysterols, including 27-hydroxycholesterol, via the nuclear receptor liver X receptor (LXR).65LXR regulates the metabolism of several important lipids, including cholesterol and bile acids. LXR functions as a sterol sensor by responding to increases in oxysterol concentration with up-regulated transcription of gene products that control cholesterol catabolism and efflux.66LXR forms an obligate heterodimeric complex with the retinoid X receptor nuclear hormone receptor, which binds retinoic acid, and together these molecules induce transcription of genes containing LXR response elements. Activation of the LXR by oxysterol ligands induces transcription of ABCA1 in macrophages and fibroblasts. Retroviral expression and activation of LXR promote cellular cholesterol outflow to apoA-I.65,6727-Hydroxycholesterol functionally activates LXR, and 27-hydroxylation of cholesterol may be an important pathway for LXR activation in response to cholesterol overload in human monocyte-derived macrophages.68ABCA1 induction by cholesterol loading is ablated in cholesterol 27-hydroxylase-deficient fibroblasts.
IMMUNOREGULATION OF REVERSE CHOLESTEROL TRANSPORT: LINK BETWEEN IMMUNITY AND ATHEROGENESIS
Human arterial endothelial cells and monocytes and macrophages have high levels of 27-hydroxylase expression and enzymatic activity. Hence, these cells, which are intimately involved in the atherosclerotic process, possess the ability to defend against lipid overload through metabolism of cholesterol to 27-hydroxycholesterol within the arterial wall.46,69
Immune reactants have been implicated in the pathogenesis of atherosclerosis, although their effect on cholesterol metabolism has not been thoroughly understood. Accumulating evidence strongly suggests a connection between suppression of 27-hydroxylase expression by immunologic reactants, cellular cholesterol overload, and, by extension, development of atherosclerosis. We have demonstrated that specific immunologic reactants, immune complexes that have fixed C1q and IFN-γ, suppress cholesterol 27-hydroxylase message and protein in human arterial endothelium and monocytes and macrophages.67We have observed that these same agents also down-regulate ABCA1 message (Figure 3) and promote foam cell formation in response to acetylated LDL, an early step in the development of atherosclerosis.70Increased foam cell formation results, at least in part, from diminished export of cholesterol owing, most likely, to diminished expression of transporter proteins.
Autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus accelerate the progression of atherosclerosis.71Methotrexate (MTX) is the most frequent choice of disease-modifying antirheumatic therapy for rheumatoid arthritis. MTX may provide a substantial survival benefit in rheumatoid arthritis, largely by reducing cardiovascular mortality.72Although the mechanism of action of the anti-inflammatory effects of MTX is a subject of debate, MTX increases the extracellular concentration of the endogenous autocoid adenosine, a potent anti-inflammatory agent.73Adenosine is also a powerful coronary vasodilator that plays an important role in metabolic regulation of coronary blood flow.74Adenosine mediates many of its effects through cell surface G protein-coupled receptors.75Receptors are subdivided into four classes, A1, A2A, A2B, and A3, based on their differential selectivity of adenosine analogues and molecular structure.76A1 and A3 receptors inhibit stimulated adenylate cyclase activity, wherease A2A and A2B receptors are stimulatory. The vasodilatory effect of adenosine is mediated via extracellular A2 receptors located on the SMCs and endothelium of the coronary artery.74Adenosine is known to act as an anti-inflammatory agent, suppressing expression of inflammatory cytokines.77Anti-inflammatory effects of adenosine on leukocytes and endothelial cells are mediated through its A2A receptor. The antiatherogenic effects of MTX may be mediated through adenosine as well. Since inflammatory processes are intricately involved in the pathogenesis of atheroma,78we investigated whether adenosine, acting at one of its four known receptors, modulates expression of reverse cholesterol transport proteins and thereby influences atherogenesis. We were able to demonstrate that activation of the adenosine A2A receptor substantially up-regulates 27-hydroxylase and ABCA1 expression.70This was the first evidence that any physiologic regulator could increase expression of the antiatherogenic 27-hydroxylase. A2A receptor activation inhibited foam cell transformation, measured after Oil Red O staining, under cholesterol loading conditions in both human and murine macrophages.70Further, a selective A2A receptor antagonist, ZM241385, completely blocked agonist-enhanced expression of both 27-hydroxylase and ABCA1, as well as abrogated inhibition of foam cell formation. Our results suggest that adenosine contributes to the capacity of MTX to reduce death rates owing to ASCVD via a novel physiologic mechanism that underscores a need to explore the role of adenosine in atherosclerosis.
The cytokine transforming growth factor β1 (TGF-β1) is a multifunctional polypeptide and potent growth inhibitor. It modulates a variety of cellular behaviors, including cell proliferation, differentiation, and apoptosis. TGF-β1 increases 27-hydroxylase expression and activity in cultured human monocyte-derived macrophages.79Low levels of circulating TGF-β have been linked to severe atherosclerosis.80
HUMAN TOLL-LIKE RECEPTORS AND ATHEROSCLEROSIS
Human Toll-like receptors (TLRs), a family of phylogenetically conserved receptors that recognize self versus nonself molecular patterns, are required to activate innate immune defense reactions, such as release of inflammatory cytokines, and play a role in the development of adaptive immune responses.81,82TLRs have a leucine-rich extracellular domain and a cytoplasmic domain with sequence homology to the IL-1 receptor. Bacterial factors, such as lipopolysaccharide (LPS; endotoxin), activate innate immunity, stimulate the antigen-specific immune response, and trigger the inflammatory response. TLR family members convey signals stimulated by these factors, activating signal transduction pathways that result in transcriptional regulation and stimulate immune function. Engagement of TLR proteins leads to up-regulation of costimulatory molecules and proinflammatory cytokines, as well as reactive nitrogen and oxygen products. Engagement of TLR proteins leads to sequential activation of the adapter protein MyD88, the IL-1 receptor-associated kinases, tumor necrosis factor (TNF) receptor-associated factor 6, and, ultimately, the IκB kinase complex. Most TLR-dependent responses in innate immune cells (eg, proinflammatory cytokine production) are MyD88 dependent. However, there are also MyD88-independent signaling pathways, as demonstrated by the finding that macrophages from MyD88-deficient mice can be stimulated with LPS to secrete IFN-β.83,84
TLR2 is activated by bacterial lipoproteins. TLR4, the key LPS-signaling receptor, resides within the Golgi network and plasma membrane rafts. TLRs are expressed in cell types involved in the first line of defense against atherosclerosis, including cells of the arterial wall: macrophages and endothelial cells. LPS, the major cell wall constituent of gram-negative bacteria, activates multiple macrophage effector functions that coordinate the host immune and inflammatory responses, mostly through induction of the secretion of inflammatory cytokines, such as TNF-α, IL-6, and IL-1. Quantitative real-time PCR studies show that human monocytes and macrophages express TLR1, TLR2, and TLR4 most abundantly.85Human vascular endothelial cells in culture express TLR4 under baseline conditions, and their expression of TLR2 and TLR4 is induced to high levels on stimulation with proinflammatory cytokines.86
Evidence is accumulating that TLRs are important in cardiovascular pathologies and points to a role of TLR4 in neointima formation and atherosclerosis. In humans, a TLR4 polymorphism (Asp299Gly TLR4 allele) has been associated with a higher risk of developing severe gram-negative sepsis and a lower risk of atherosclerosis. Individuals with this allele also exhibit lower levels of certain proinflammatory cytokines, acute-phase reactants, and soluble adhesion molecules.87TLR4 is preferentially expressed by macrophages in murine and human lipid-rich atherosclerotic lesions.88In human atherosclerotic plaques, expression of TLR1, TLR2, and TLR4 is markedly up-regulated, particularly in the endothelium and macrophages.89TLR2 and TLR4 expression colocalizes with the redox-sensitive transcription factor nuclear factor κB (NF-κB). This suggests that expression of TLR2 and TLR4 may be a direct result of NF-κB activation, is itself triggering NF-κB activation, or both. This observation is particularly relevant because NF-κB activation plays a major role in the regulation of proinflammatory and prothrombotic responses in human atherosclerosis, including expression of adhesion molecules, tissue factor, and scavenger receptor lectin-like oxidized low-density lipoprotein receptor-1.90
Finally, TLR3 or TLR4 inhibits LXR function, thereby decreasing ABCA1 transporter expression. ABCA1 mediates a pathway for cholesterol efflux from macrophages. The effect on ABCA1 is independent of MyD88 and NF-κB but depends on another transcription factor, IRF3, a specific effector of TLR3/4 that inhibits the transcriptional activity of LXR on its target promoters.91
Atherosclerosis is a complex disorder involving much more than the clogging of a pipe with fatty material. Inflammatory factors contribute to all stages of atherosclerosis, including the disruption of cholesterol metabolism in macrophages and endothelial cells in the artery wall. The connection between inflammation and atherosclerosis underscores the importance of defining key points of interaction between inflammatory stimuli and the arterial wall that ultimately lead to plaque formation. Circulating immune reactants may contribute to the development of atherosclerosis by mediating down-regulation of enzymes involved in cholesterol efflux, resulting in accumulation of cholesterol in the cell. Cellular cholesterol homeostasis is maintained by feedback repression of genes involved in cholesterol synthesis and supply. Adenosine A2A receptor occupancy increases expression of the reverse cholesterol transport proteins cholesterol 27-hydroxylase and ABCA1 and decreases foam cell transformation in cultured macrophages. Up-regulation of 27-hydroxylase and ABCA1 via A2A receptors restores a critical defense mechanism against atherosclerosis. The antiatherogenic effect of adenosine may explain reduced cardiovascular mortality and survival benefit in rheumatoid arthritis patients treated with MTX, a drug whose anti-inflammatory properties are mediated through adenosine. Increased knowledge of the molecular mechanisms that lead to derangements in cholesterol homeostasis with consequent lipid overload and development of ASCVD is likely to provide an effective path to improved efficacy of clinical treatment with decreased morbidity and mortality.
Thanks to Dr. Michael Vagell for his assistance with preparation of the figures.