The Pathogenic Etiology of Atherosclerosis: A Review.
Special Topics in Pathophysiology
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To understand the basis of this paper, the pathophysiology of atherosclerosis, it is vital to appreciate the basic physiology of the heart, circulatory system, and most importantly, the coronary arteries. This fundamental comprehension will lay the foundation to better understand the devastation caused to the coronary arteries by the pathogenesis of atherosclerosis. This may also provide insight into prevention and treatment strategies to counteract the destructive mechanism of this disease.
The heart is a very small, vitally important organ composed of four muscular chambers: the right and left atria, and the right and left ventricles. The atria have relatively thin muscular walls, allowing them to be highly distensible ; whereas the ventricles are of greater muscular thickness, which is vital for pumping the blood to the pulmonary and systemic circuits. A normal healthy heart has two main functions: to pump blood to the pulmonary circuit where the blood becomes oxygenated and to pump the oxygen-rich blood to the systemic circuit. The heart is essentially a small, muscular pump that is responsible for propelling deoxygenated blood to the lungs, while correspondingly pumping nutrient rich, oxygenated blood to the body. Once the blood leaves the left ventricle, it enters the aorta and corresponding network of arteries that constitute the circulatory system.
Blood vessels are divided into four categories: arteries (take oxygenated blood away from the heart to the body), arterioles (branch out from the arteries leading into the capillaries), capillaries (smallest of blood vessels where gas and nutrient exchange occurs), and veins (carry deoxygenated blood from the body to the heart). Arteries and veins have different functions; however, they both are composed of three distinct layers: tunica intima, tunica media, and the tunica adventita . The tunica intima is the innermost layer of any given blood vessel; it includes the endothelial lining and a layer of connective tissue containing variable amounts of elastic fibers . The tunica media is the middle layer which contains concentric sheets of smooth muscle composed of elastin and collagen fibers . It is this smooth muscle that when stimulated by the sympathetic nervous system either constricts, decreasing the diameter of the lumen (vasoconstriction), or it relaxes, increasing the diameter of the vessel lumen (vasodilation) ; the role of these vasoactivators will be discussed later in this paper. Lastly, the tunica adventitia is the outer most layer, which is composed of collagen and elastin fibers. Often, this outer layer is blended into adjacent tissues allowing the anchoring and stabilization of some vessels .
As the heart is an organ continuously doing work, the cardiac muscle cells are in need of a constant supply of oxygen and nutrients. It is the coronary circulation that is responsible for the blood supply to the cardiac tissues, via an extensive network of coronary arteries. Both the left and right coronary arteries originate from the base of the ascending aorta within the aortic sinus [1,3]. The autonomic nervous system (ANS) plays an important role as neurogenic stimuli have the ability to restrain the extent of coronary vasodilation. This neuromodulation governs the rate of release of vasoconstrictive norepinephrine (NE), which is increased by the adrenergic activation and angiotension II (AII) . Other vasoconstrictors include α1 and α2 adrenergic activity, AII, and endothelin. Vasoconstrictive stimuli are also responsible for an increase in free cytosolic calcium in the vascular smooth muscle, resulting in the homeostasis of myocardial contraction .
Importantly, these vasoconstrictive adrenergic influences are opposed by vasodilatory influences such as β-adrenergic vascular receptors and metabolic mechanisms such as nitric oxide (NO), adenosine (ATP) and the activation of vascular ATP dependent potassium channels (KATP) . With this, there are three essential regulators of coronary tone: i) the metabolic vasodilatory system; ii) the neurogenic control system (more vasoconstrictive than vasodilatory); and iii) the vascular epithelium, which can be either vasodilatory by releasing NO or vasoconstrictive by releasing endothelin-1 [1, 4]. Thus, we must keep in mind that endothelin-1 is one of the more powerful vasoconstrictors, especially when endothelial damage is extensive [1, 4]. These vasoactive substances are activated by their respective and very different, signaling pathways; thus contributing to the complexities of atherosclerosis, making it a true multifactorial disease.
As with other vessels within the body, when there is an increased demand for oxygen, vasodilation of the coronary arteries occurs. This vasodilation is usually mediated by the release of NO from healthy endothelium; in contrast, when the endothelium is damaged, it releases vasoconstrictive endothelin . It is because of their vital importance that the coronary arteries have gained popular attention when they are partially or completely occluded by atherosclerotic plaques. These atherosclerotic plaques cause inadequate oxygen supply to the cardiac tissue resulting in tissue death (myocardial infarction), and various other forms of heart diseases . Therefore without an adequate supply of oxygen and nutrients to the myocardial muscle, the heart will cease to function properly.
This basic foundation will give us a better idea on how a healthy cardiovascular system functions. Therefore allowing us to understand the drastic effects a disease such as atherosclerosis can have on this system. The main focus of this paper will be on atherosclerosis; however other forms of heart disease will be discussed to solidify the idea of how destructive atherosclerosis can be. Thus, the remainder of this paper will focus on the cellular mechanisms behind atherosclerosis, along with old and new thoughts in regards to the etiology and treatment options for this type of heart disease.
Cardiovascular disease (CVD) has emerged as the dominant chronic disease in many parts of the world, and early in the 21st century it is predicted to become the main cause of disability and death worldwide . CVD represents a very broad category of conditions that affect the heart and circulatory system. Common risk factors include: blood pressure (hypertension), total cholesterol (LDL and HDL), diabetes, obesity, left ventricular hypertrophy, and genetic predisposition . The most prominent and worrisome of these diseases are those that contribute to coronary heart disease. The coronary heart diseases of interest include: ischemic heart disease, angina pectoris, myocardial infarction, and most importantly, atherosclerosis. As a result of these coronary heart diseases, cardiac output is often depressed and often increases the oxygen demand needed by the cardiac tissues. Therefore the effects of coronary heart disease cannot be taken lightly, as the effects can be highly variable, ranging from diffuse damage, to localized narrowing or stenosis of the coronary arteries . Importantly, these coronary diseases have direct vasodilatory effects of the coronary circulation, acting by the formation of adenosine and NO, and the opening of the KATP channels; also the vascular endothelium is damaged, causing the vasodilatory stimuli to be overcome by the vasoconstrictors such as endothelin and AII . By discussing these other forms of coronary heart disease, the reader will better understand the relationship between these diseases and atherosclerosis; allowing a better understanding of the importance for prevention and treatment strategies of coronary heart disease.
Traditionally, it has been thought that the major cause of myocardial ischemia is the result of fixed vessel narrowing and abnormal vascular tone, caused by atherosclerosis-induced endothelial cell dysfunction . This narrowing of the coronary arteries reduces the blood and oxygen flow to the myocardial tissues. It is the cessation of the myocardial blood flow due to atherosclerotic occlusions that results in the immediate physiological and metabolic changes. Unfortunately, the heart cannot increase oxygen extraction on demand, therefore any additional oxygen requirements are met by increasing the blood flow and autoregulation of the coronary vasculature . This oxygen imbalance may also be an underlying cause for not only myocardial ischemia, but contractile cardiac dysfunction, arrhythmias, infarction, and sometimes death . However, important to note is the heart’s unique ability to adapt to these sudden changes in coronary blood flow by correspondingly decreasing the rate of cardiac contraction [1,5]. Thus, the decreased work during ischemia proportionately decreases the oxygen demand and helps conserve the underperfused myocardium ; this protective mechanism prevents further damage and cell death due to decreased oxygen levels.
Besides physiological factors, there are also metabolic changes that occur immediately after the initial onset of ischemia. The myocardial energy metabolism shifts from aerobic (mitochondrial) metabolism to anaerobic glycolysis within a few seconds ; simultaneously, the energy depletion causes the myocardial contraction to diminish, eventually ceasing altogether. Consequently, due to the inhibited mitochondrial metabolism, there is an increase in adenosine concentrations; which causes the adenosine to bind to the smooth muscle receptors, decreasing calcium entry into the cells, thus causing relaxation due to vasodilation [7,8]. Overall, the inability to meet the myocardial oxygen demand often results in severe, vice-like chest pain, or more commonly known as angina pectoris.
Angina pectoris often is an associated symptom of myocardial ischemia and is the common medical term used to describe chest pain or discomfort due to coronary heart disease without myocardial necrosis. Interestingly, angina can also occur in people with valvular disease, hypertrophic cardiomyopathy, and uncontrolled high blood pressure (hypertension). Currently there are three major variations of angina pectoris. The first is known as stable angina, or more commonly, chronic stable angina. This form of angina is characterized by a fixed, obstructive atheromous plaque in one or more coronary arteries [1,7,9]. Patients who suffer from chronic stable angina usually have episodes of discomfort that are usually predictable. The discomfort is experienced shortly after over exertion and/or mental or emotional stress; these symptoms are usually relieved by rest, nitroglycerin, or a combination of both. Again, the major contributing factor in stable angina is due to the coronary vasoconstriction caused by atherosclerotic endothelial dysfunction .
A second form of angina is known as unstable angina. Unstable angina is characterized by unexpected chest pain which usually occurs at rest without any type of physical exertion. This chest pain is due to coronary artery stenosis caused by atherosclerotic plaque or the narrowing of the vessels obstructed by blood clots. Also other key factors in unstable angina include inflammation and infection [7,9]. The last form of angina is the variant angina, or more commonly known as Prinzmetal’s Angina . This form of angina is manifested by episodes of focal coronary artery spasm in the absence of atherosclerotic lesions [7,9]. The coronary vasospasm alone reduces coronary oxygen supply and is thought to be caused in response to abnormal endothelial dependent vasodilators (Acetylcholine – ACh, and serotonin) [1,7]. These coronary spasms are often manifested by the coronary atheroma which damages the vascular endothelium, causing a decreased production of vasodilators (NO and prostaglandin – PGI2) and an increase in vasoconstrictive factors such as endothelin and AII . Often when someone is diagnosed with either form of angina, they are usually monitored closely, as they are at an increased risk of a heart attack (myocardial infarction), cardiac arrest, or sudden cardiac death.
A myocardial infarction (heart attack) is the resultant complication when the blood supply to part of the heart is interrupted. This ischemic oxygen shortage causes damage and sometimes death to the heart tissues. Important associated risk factors include: atherosclerosis, previous heart attack or stroke, smoking, high LDL and low HDL cholesterol levels, diabetes, obesity, and high blood pressure . Often referred to as an acute myocardial infarction, it is part of the acute coronary syndromes which includes ST segment elevation myocardial infarction (STEMI), non-ST segment elevation myocardial infarction (NSTEMI) and unstable angina [1,7,10].
As with angina, the pain experienced may result from the release of mediators such as adenosine and lactate from the ischemic myocardial cells onto the local nerve endings . This ischemic persistence triggers a process called the ischemic cascade , which usually results in tissue death due to necrosis. Certain factors such as psychological stressors and physical exertion have been identified as major triggering factors involved with acute myocardial infarctions. Often these acute myocardial infarctions are brought on by the rupturing of atherosclerotic plaques, which then promote thrombus (blood clot) formation causing further occlusion of the arteries. This atherosclerotic blockage thus initiates myocardial necrosis, which in turn activates systemic responses to inflammation causing the release of cytokines interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFα) [7,10]. Damaged caused by myocardial necrosis includes: i) loss of critical amount of ATP, ii) membrane damage induced metabolically or mechanically, iii) formation of free radicals, iv) calcium overload, and v) sodium pump inhibition .
Apart from damaging the myocardial tissue, an acute myocardial infarction can cause varying pathophysiological changes in other organ systems. Some of these changes include: decreased pulmonary function – gas exchange, ventilation, and distribution of perfusion, decreased vital capacity; reduction in hemoglobin’s affinity for oxygen, causes hyperglycemia and impaired glucose function, increases the plasma and urinary catecholamine levels (thus enhancing platelet aggregation), and also has been found to increase blood viscosity . From the above evidence, we can see that coronary heart disease should not be looked at light heartedly. It is due to their similarity that the different coronary heart diseases can be diagnosed using a given set of molecular markers and other diagnostic tools.
Serum cardiac markers have become widely used when it comes to diagnosing the extent and type of coronary heart disease a patient is symptomatic of. Also, these tests have allowed physicians to diagnose an additional one third of patients that do not exhibit all criteria of a given disease , thus preventing more premature deaths. The most common of these cardiac markers are myocardial bound creatine kinase (CK-MB), and cardiac troponin l and t (cTnl and cTnT). These markers are often found within a blood sample as levels start to rise between 3-8 hours and 3-4 hours respectively . More recently, new ‘risk factor’ biomarkers such as C-reactive protein (CRP), myeloperoxidase (MPO) [11, 12], and lipoprotein-associated phospholipase A2  are being studied more in depth as alternative cardiac markers. Although cardiac biomarkers are heavily used, the role of noninvasive technologies also plays a major role in diagnosing coronary heart disease. These noninvasive methods include electrocardiography, exercise stress testing, echocardiography, cardiovascular MRI, and CT imaging of the heart . Some invasive, intravascular techniques include ultrasound, thermography, near infrared spectroscopy, cardiac catheterization, and cardiac angiography .
As coronary heart disease is the leading cause of hospitalization and death among today’s population, primary and secondary prevention strategies need to be considered with the utmost importance. Primary prevention generally means the effort set forth to modify risk factors and prevent their development delaying or preventing new onset coronary heart disease . As for secondary prevention, this often refers to the therapy involved to reduce recurrent coronary heart disease events; thus secondary preventions are essentially treatment strategies. The most common and less intensive of these treatment strategies are that of the pharmaceutical therapies. Often, these drug regimes range from the daily aspirin intake to angiotension-converting enzyme inhibitors (ACEi), to β-blockers and nitrates . These drug therapies often lower the risk of recurrent cardiovascular events. Unfortunately daily drug regimes do not work for everyone. Some people have their coronary heart disease surgically corrected either by angioplasty (insertion of stent to keep the blocked vessel open) or by means of a more complex surgery consisting of a single to multiple coronary artery bypass. With everything considered, drug therapies and surgical correction are only a means of correcting the problem; patients are also encouraged to increase physical activity and change their daily dietary habits in becoming more successful in reducing risk of development or progression of coronary artery disease.
These different forms of coronary heart disease are very closely related to one another, more importantly, closely related to atherosclerosis. As discussed previously, coronary heart diseases are characterized by the narrowing or stenosis of the coronary vessels, usually caused by the atherosclerotic plaque formation due to endothelial cell dysfunction. As a result, atherosclerosis is the underlying mechanism for ischemic heart disease, angina pectoris (stable, unstable, and variant), myocardial infarction and sudden cardiac death . Therefore it is important to understand the cellular pathogenesis of atherosclerosis, which will lead to a better understanding resulting in better prevention and treatment strategies for all forms of atheroma induced coronary heart disease.
Atherosclerosis, the primary etiology of cardiovascular disease, is characterized by intimal plaque that forms as a time-dependent response to arterial injury . Atherosclerosis is a disease affecting the arterial blood vessels, which is commonly known as “hardening of the arteries.” This form of coronary heart disease is the principle source of both cerebral and myocardial infarction, gangrene of the extremities, and loss of function of both organs and tissues ; this disease is ultimately responsible for a majority of deaths in North America, Europe, and Japan . The method of atherogenesis is not fully understood, however there are a number of current models that suggest that stressors corrupt the vascular integrity allowing the abnormal accumulation of lipids, cells and extracellular matrix within the arterial wall . Due to its very slow progression, it is not surprising that atherosclerosis goes undetected and remains asymptomatic until the atheroma obstructs the blood flow within the artery [14,16]; hence atherosclerosis is often referred to as the “silent killer”.
Often, the atherosclerotic plaque can be divided into three distinct components. The first being the atheroma, which is the nodular accumulation of the soft, flaky, and yellow material of the plaques, usually composed of macrophages closest to the lumen of the artery. The second component is the underlying areas of cholesterol crystals, and the third is the calcification at the outer base of the older/more advanced lesions . Collectively, these components constitute the basis of the atherosclerotic plaques. These atherosclerotic plaques are responsible for the arterial narrowing (stenosis) or they may rupture and provoke thrombosis [7, 14, 15]; either way the atherosclerotic plaque causes an insufficient blood supply to the heart and other organs. As discussed previously, the atherosclerotic plaques lead to other major complications such as ischemia, angina pectoris, myocardial infarction, stroke, and causes impaired blood flow to the kidneys and lower extremities. Interestingly, arteries without many branches (internal mammary or radial arteries) tend not to develop atherosclerosis .
One of the most evidence-based hypotheses regarding atherogenesis is that of the response-to-injury hypothesis. This hypothesis suggests that the atherosclerotic lesions represent a specialized form of a protective, inflammatory, fibroproliferative response to various forms of insult to the arterial wall . This seems to be a reoccurring theme, as now atherosclerosis is considered to be a form of chronic inflammation between modified lipoproteins, monocyte derived macrophages, T cells, and normal cellular elements of the arterial wall [16, 18]. As with other diseases, there are a number of physiological factors that increases one’s risk for developing atherosclerosis. These factors include: age, sex, diabetes or impaired glucose tolerance, hypertension, tobacco smoking, estrogen status, physical inactivity, metabolic syndrome, and dyslipidemia [7, 19].
The remainder of this paper will shift its focus to the pathogenesis of atherosclerosis including the ideas of endothelial dysfunction, lipoprotein entry and modification, recruitment of leukocytes, recruitment of smooth muscle; as well as other contributing factors such as dyslipidemia, hypertension, and diabetes. Also, the cellular complications of atherosclerosis will be discussed.
Healthy arteries are often responsive to various stimuli, including the shear stress of blood flow and various neurogenic signals. These endothelial cells secrete substances that modulate contraction and dilation of the smooth muscle cells of the underlying medial layer . These healthy endothelial cells are also responsible for the inhibition of migration of smooth muscle cells to the intimal layer  and they also play an important role in immune responses. Normal functional characteristics of healthy endothelium includes: i) ability to act as a permeable barrier between the intravascular and tissue space, ii) ability to modify and transport lipoproteins into the vessel wall, iii) acts as a non-thrombogenic and non-leukocyte adherent surface, iv) acting as a source of vasoactive molecules, v) act as a source of growth regulatory molecules, and vi) a source of connective tissue matrix molecules [14, 15]. Overall, in a normal, healthy state, the endothelial layer provides a protective, non-thrombogenic surface with homeostatic vasodilatory and anti-inflammatory properties .
It is widely known that the endothelium is responsible for the synthesis and release of several vasodilators such as: NO, endothelium derived hyperpolarizing factors (EDHFs), endothelial derived relaxing factors (EDRFs), and prostacyclin (PGI2) [7, 20]. These vasodilators utilize a G-coupled signaling pathway, where NO diffuses from the endothelium to the vascular smooth muscle where it activates guanylyl cyclase (G-cyclase) . The G-cyclase in turn forms cyclic guanosine monophosphate (cGMP) from cGTP; an increase in cGMP results in smooth muscle relaxation which subsequently involves a reduction of cytosolic Ca2+. Aside from these anti-thrombic substances, the endothelium also produces prothrombic molecules including endothelin-1 and other endothelium derived contracting factors (EDFCs) . Importantly, the endothelium derived NO not only modulates the tone of the underlying vascular smooth muscle, but is also responsible for the inhibition of several proatherogenic processes. These processes include smooth muscle proliferation and recruitment, platelet aggregation, oxidation of low density lipoproteins (LDLs), monocyte and leukocyte recruitment, platelet adhesion, and the synthesis of inflammatory cytokines . Therefore, relating back to the response-to-injury hypothesis, loss of these endothelial functions promotes endothelial dysfunction, thus acting as the primary event in atherogenesis.
Endothelial dysfunction is considered to be an initiating event which leads to the pathogenesis of atherosclerosis. For this reason endothelial dysfunction has been shown to be of prognostic significance in predicting such vascular events as heart attacks or strokes . It has been established that endothelial cell dysfunction is characterized by alterations in vascular permeability and inadequate production of NO [4, 22, 23]; thus predisposing the endothelium to the development of atheromas. Interestingly, in response to initial atheroma formation, the arteries often dilate, causing outward remodeling of the vessel for this accommodation ; however if this remodeling is insufficient, the blood flow is impaired, thus causing ischemia . Several physical and chemical factors are responsible for affecting normal endothelial function. Some common factors discussed previously include diabetes, hypertension, hypercholesterolemia, smoking, age, diet, and physical inactivity. However, more importantly are the physiological factors: i) impairment of the permeable barrier, ii) release of inflammatory cytokines, iii) increase transcription of cell-surface adhesion molecules, iv) altered release of vasoactive substances (PGI2 and NO), and v) interference with normal anti-thrombotic properties .
Commonly, endothelial dysfunction is characterized by the reduction of vasodilators NO and PGI2, and the increase of various endothelial derived contracting factors [23, 24]. This impairment may also predispose the vessels to vasospasm . This decrease in NO bioavailability is thought to cause a decreased level of expression of endothelial cell NO synthetase (eNOS) , thus reducing the likelihood of vasodilation from occurring. Apart from its vasodilatory role, NO is also responsible for resisting inflammatory activation of endothelial functions such as expression of the adhesion molecule VCAM-1 . NO has also appeared to exert anti-inflammatory action at the level of gene expression by interfering with nuclear factor kappa B (NFκB), which is important in regulating numerous genes involved in inflammatory responses ; these inflammatory responses will be discussed later on. The other common vasodilator, PGI2 is also reduced during endothelial dysfunction. PGI2 is a major product of vascular cyclooxygenase (COX) and is considered a potent inhibitor of platelet aggregation . Like NO, PGI2 is an endothelial derived product which is often produced in response to shear stress (commonly caused by blood flow) and hypoxia . By understanding the other roles NO and PGI2 play within the endothelium, we can see that a decrease in one or the other ultimately leads to dysfunction and disruption of the endothelium. As a result of vasodilator reduction, the endothelium often synthesizes and releases EDCFs causing endothelial constriction. The major constrictors include superoxide anions (which act by scavenging NO – thus further reducing NO levels), thromboxane A2, endothelin-1, AII, and α-adrenergic factors . Unlike the vasodilators, the vasoconstrictors utilize two signaling pathways. The α 1-adrenergic receptor signaling pathways utilize the same G-coupled pathway as the vasodilators (discussed previously) however instead of cGMP; it utilizes cyclic adenosine monophosphate (cAMP) . The other constrictors including thromboxane A2, endothelin-1 and AII utilize the cAMP-dependent protein kinase pathway; where the activated kinase acts as a trigger for various physiological effects, including increased contractile activity on the arterioles .
Apart from the imbalance of vasoactivators, endothelial dysfunction is responsible for initiating two other separate pathways that also participate in the progression of plaque formation and growth. Lipoprotein entry is the next initial stage in atherogenesis. This is then followed by the modification and entry of lipoproteins, the recruitment of leukocytes, and the migration and proliferation of smooth muscle cells. Overall this “evolutionary” process best represents the formation of atherosclerotic plaques within the vessels.
Lipid accumulation is another major manifestation of the vascular response to injury, and is accelerated by the entry and modification of lipoproteins. Lipoproteins are composed of both lipids and proteins, and help transport water-insoluble fats throughout the bloodstream [7, 25]. The lipid core is surrounded by hydrophilic phospholipids, free cholesterol and apoliporoteins; where the protein portion has a charged group, aimed outwards to attack water molecules, thus making the lipoproteins soluble in the plasma of the blood [26, 27]. In total, there are five major classes of lipoproteins: the chylomicrons, very low density lipoproteins (VLDLs), intermediate low density lipoproteins (ILDLs), low density lipoproteins (LDLs), and the high density lipoproteins (HDLs). The chylomicrons provide the primary means of transport of dietary lipids, while the VLDLs, ILDLs, LDLs, and HDLs function to transport endogenous lipids [16, 25]. Of the lipoproteins, the LDLs are of most interest. Interestingly high LDL levels often correlate closely with atherosclerosis development, whereas high HDL levels protect against atherosclerosis; the HDL protection is thought to be related to its ability to transport lipids away from the peripheral tissues back to the liver for disposal .
A key component to the accumulation of lipids is due to the endothelial dysfunction, which causes a loss of selective permeability and barrier function. This ineffective permeability allows for the entry of LDLs into the intima lining of the vessels [7, 16]. The highly elevated circulating levels of LDLs are colloquially referred to as having hyperlipidemia, hypercholesterolemia, or dyslipidemia [7, 25-27]. In either case, once the LDL has entered the intima of the vessel, the LDL starts accumulating in the subendothelial space by binding to components of the extracellular matrix, the proteoglycans; lipolytic and lysosomal enzymes also play a role in lipid accumulation . Importantly, statins lower circulating cholesterol levels by indirectly inhibiting HMG CoA-reductase (rate limiting enzyme required for endogenous cholesterol biosynthesis . This results in the decrease of intracellular cholesterol levels, which leads to the activation of SREBP, upregulation of LDL receptors, and the clearance from plasma degradation of LDL; thus reducing circulating LDL levels .
When the lipid accumulation increases the residence time that the LDL occupies within the vessel wall, it allows more time for lipoprotein modification ; which appears to play a key role in the continued progression of the atherosclerotic plaque. Often, endothelial cell dysfunction leads to the altered expression of lipoprotein receptors used to internalize and modify various lipoproteins . These changes usually occur via oxidative modifications. The oxidative modification hypothesis (figure 2) focuses on the concept that LDLs in their native state are often not atherogenic . It is believed, however, that LDLs are modified chemically by the endothelial cells  and are readily internalized by macrophages (formation of the foam cell) via the ‘scavenger-receptor’ pathway . Essentially the “trapped” LDL within the subendothelial space is oxidized by the resident vascular smooth muscle cells, endothelial cells, and macrophages. As a result this stage in atherogenesis, several major consequences follow including: i) modified LDL (mLDL) acts a chemoattractant that recruits circulating monocytes to the vessel wall; ii) mLDL increases the endothelial expression of genes encoding mediators of inflammation (monocyte colony stimulating factor [M-CSF], monocyte chemoattractant protein [MCP-1], and certain leukocyte adhesion molecules); and iii) mLDLs can be ingested by macrophages and other cells in large quantities as they are not regulated by negative feedback inhibition .
Galle et al (1991) had demonstrated that increased levels of LDLs appeared to halt NO production. These authors also hypothesized that the NO may actually be sequestered and inactivated within the hydrophobic core of the lipoprotein. It is also believed that the superoxide anions, discussed previously, may be responsible for initiating and accelerating the oxidative modifications of the LDLs [25, 27]. The mLDLs may play an important role for the further recruitment of leukocytes and macrophages during atherogenesis.
Although endothelial dysfunction is the primary hallmark by initiating the atherosclerotic cascade, leukocyte and macrophage recruitment also play a key role in the progressive plaque formation. The most common inflammatory factors that aid in the progressive development include: monocytes, macrophages, T lymphocytes, leukocyte adhesion molecules, and chemoattractant cytokines. Monocytes are produced in the bone marrow, enter the bloodstream, and migrate to the site of injury; this is often the primary stage in the inflammatory response. The monocytes can later develop into macrophages which are phagocytic and digest invading organisms and foreign bodies, as well as damaged senescent cells. These newly formed macrophages are also responsible for the ingestion of the oxidized (oxLDL) and mLDLs, forming initial foam cells. The T cells belong to a group of white blood cells, the lymphocytes, and play a central role in cell mediated immunity. These cells develop within the thymus and differentiate into cytotoxic T cells, regulatory T cells, memory T cells, natural killer T cells, үδ T cells, and most importantly the helper T cells . It has been established that the TH1 subset of the CD4+ helper T cells are the predominate lymphocytes found in the atherosclerotic lesions [29-32]. Importantly, these TH1 cells secrete IFN-γ, a potent proinflammatory cytokine which induces the expression of major histocompatibility complex (MHC) class II and activation of macrophages . The secreted IFN-γ has been implicated in the down regulation of ABC1, a protein that regulates cholesterol efflux in macrophages (foam cell formation), and antagonizes the production of collagen which is believed to stabilize plaque structure .
The leukocyte adhesion molecules and chemoattractant cytokines also play an important role in atherogenesis. Vascular cell adhesion molecule-1 (VCAM-1) is a type 1 membrane protein that mediates leukocyte endothelial cell adhesion and signal transduction through the upregulated response to tumor necrosis factor alpha (TNF-α) and interleukin-1 (IL-1) [28, 32]. VCAM-1 also acts as an endothelial ligand for the integrins, very late antigen-4 (VLA-4) and α4β1. A similar adhesion molecule, intracellular adhesion molecule-1 (ICAM-1), is present in low concentrations in the membranes of leukocytes and endothelial cells, and like VCAM-1 is induced by TNF-α and IL-1. The smaller adhesion molecules include the selectins, which are cell-surface carbohydrate binding proteins that mediate a variety of Ca2+-dependent, cell-cell adhesion interactions . Endothelial (E)-selectin, preferentially recruits polymorphonuclear leukocytes and is responsible for mediating the slow rolling and transition to firm adhesion of leukocytes during the inflammatory response (figure 3) . On the other hand, platelet (P)-selectin tends to promote tighter adhesion interactions and immobilization of leukocytes .
The chemoattractant cytokines are of equal importance. One of the more important molecules, monocyte chemoattractant protein-1 (MCP-1) is produced in the endothelium in response to oxLDLs and other stimuli . MCP-1 selectively promotes the directed migration (chemotaxis) of monocytes; therefore it is not surprising that studies have shown that by lacking MCP-1 or its receptor (CCR2), causes a delay and attenuation in atheroma formation . Other important cytokines include and are not limited to, IL-1, IL-1β, TNF-α, TNF-β, IL-6, MCP-1, M-CSF, IL-8, and CD-40L. The involvement of the inflammatory response has undoubtedly complicated the cellular pathogenesis of atherosclerosis.
It has become increasingly apparent that leukocytes, adhesion molecules, and cytokines play an important role in the further development of the atherosclerotic plaque. It has been previously established that the healthy endothelial cells tend to resist adhesion interactions with leukocytes; in inflamed tissues, the recruitment and trafficking of leukocytes occurs in the post capillary venules and not in the arteries [30-35]. In atherogenesis, leukocyte recruitment and adhesion usually begins shortly after initiation of the hypercholesterolemia  as seen in figure 1 parts 3-5. Many recent studies have shown that the expression of VCAM-1 on the endothelial surface is an early and necessary step for the adherence of monocytes [29-32]. The increased cellular adhesion, along with the initial endothelial dysfunction allows for the further recruitment of inflammatory cells, release of cytokines and further recruitment of lipids into the atherosclerotic plaque [29, 31], which is mediated by the inflammatory cascade. These leukocyte adhesion molecules are mainly regulated transcriptionally through the involvement of NFβ, which is a transcription factor that is transactivated when certain proinflammatory cytokines ligate their receptors in the endothelial surface . E-selectin, along with VCAM-1 and ICAM-1 can be induced this way.
The initiation of expression of these endothelial leukocyte adhesion molecules is often simply stimulated by varying components of oxLDLs. Therefore it is the lipid accumulation with in the intima of the vessel that promotes the inflammatory cascade. In the study by Crowther et al (2005) it was demonstrated that VCAM-1 expression increases the recruitment of monocytes and T lymphocytes to the site of injury. This promoted the subsequent release of MCP-1 via leukocytes, which caused magnification of the inflammatory response, thus activating leukocytes within the media of the vessel; subsequently signaling for the recruitment and proliferation of smooth muscle cells [29, 30]. Soon after monocytes adhere to the endothelium, they migrate through the luminal surface of the intima into the subendothelial space . Once localized in the subendothelial space, the monocytes differentiate into macrophages with phagocytic capabilities  allowing for the ingestion of mLDLs and oxLDLs. Further recruitment of macrophages is enhanced by the local release of M-CSF which causes further monocytic proliferation and activation of monocytes which leads to cytokine mediated progression and further oxidation of LDLs . These macrophages subsequently produce growth factors such as AII and cytokines such as TNF-α. In addition, the AII produced promotes the formation of free radicals and stimulates fibroblasts to form fibrous tissue ; thus increasing vasoconstrictive properties. It is important to note that VCAM-1 is usually induced by oxLDLs, whereas ICAM-1 is induced via a cytokinedependent pathway or through increased hemodynamic stress . Either way, they both mediate the firm adhesion of leukocytes  which is important in their migration. The leukocyte recruitment and atherogenic progression involving all components discussed.
Leukocyte adhesion is a multistep process that starts with the leukocyte rolling on the endothelial surface [31-35]. This initial adhesion is often referred to as the primary and transient stage. During this step endothelial P and E-selectins bind with the carbohydrate ligands on leukocytes and the leukocyte integrin VLA-4, which interacts with expressed VCAM-1 and ICAM-1 [31, 34]. This leukocyte rolling is hypothesized to commence upon leukocyte capture from free flow .
This initial step allows the flowing cells to tether and subsequently roll along the vessel wall which is followed by the rapid activation of certain cytokines (released from the localized endothelium). This is primarily mediated by VCAM-1 along with both P and E-selectins. The second stage in migration involves the arrest and firm adhesion of the rolling monocytes and T lymphocytes. This stage is moderately mediated by VCAM-1 in conjunction with α4β1 ; however it is ICAM-1 that strongly mediates the arrest and firm adhesion of T lymphocytes monocytes and neutrophils . The combination of leukocytes, adhesion molecules, and cytokines plays a critical role in the initial development of the atherosclerotic lesion, the fatty streak, as seen in figure 6.
Fatty streaks represent the earliest visible lesion of atherosclerosis. These fatty streaks are often observed as areas of yellow discoloration on the artery’s inner surface . The fatty streaks do not protrude into the arterial lumen and do not disturb normal blood flow ; thus these initial stages of atherosclerosis remain undetected as this stage of progression is largely asymptomatic. Surprisingly, fatty streaks exist in the aorta and coronary arteries of most individuals by the age of 20 [7, 16]. Often the fatty streaks within the coronary arteries develop into more ominous fibrous plaques  through the recruitment of smooth muscle cells.
Smooth muscle is the muscle of a variety of internal organs including the gut, bladder, and importantly, the blood vessels. The smooth muscle cells (SMCs) of interest for this paper are the vascular smooth muscle cells. The vascular smooth muscle cells (VSMCs) have both contractile and synthetic capabilities with the main function of maintaining vascular tone and regulating peripheral vascular resistance [1, 7]. The VSMCs differ from the myocardial cells in various ways: the VSMCs are approximately fusiform in shape (allowing them to form the muscular tube of the arteries), and they do not need to respond rapidly to a wave of depolarization, as contraction and relaxation are much slower . The contractile properties are due largely in part from the stimulation induced by various vasoactive substances discussed previously (AII, ACh, and NO, etc.). These vasoactive molecules bind to the receptors of the SMCs exciting myocyte contraction and relaxation which alters the diameter of the vessel’s lumen .
The VSMCs of healthy vessels also have synthetic properties including the production of collagen, elastin, and proteoglycans (form the vascular extracellular matrix); while SMCs produce IL-6 and TNF-α which promotes the proliferation of leukocytes and increased expression of leukocyte adhesion molecules  discussed previously. Interestingly in the initial atheroma formation, the VSMCs grow larger by hypertrophy in response to inarterial pressure due to hypertension. As discussed previously, the VSMCs can manufacture components of the vascular extracellular matrix including collagen and elastin fibers, all of which contribute to the vascular damage in atherosclerosis .
The majority of SMCs likely arrive in the arterial intima in early life, while others that accumulate in the advancing atheroma likely arise from cells that have migrated from the underlying media into the intima . The accepted concept for atherosclerosis involves migration of medial SMCs through fenestrations in the internal elastic lamina to the intima in response to injury; subsequently, proliferation and extracellular matrix production by these SMCs contributes to the excessive plaque volume [36-38]. The very potent chemoattractant secreted by the foam cells responsible for additional recruitment of SMCs is the platelet-derived growth factor (PDGF) [5, 7]. The secreted PDGF stimulates the migration of SMCs into the intimal subendothelial space where they subsequently replicate. In addition, the foam cells release TNF-α, IL-1, fibroblast growth factor (FGF), and transforming growth factor β (TGF-β), all of which modulate further SMC proliferation and synthesis of extracellular matrix proteins, as well as stimulating leukocyte activation . Figure 7 shows a simplified flowchart of all key components involved in the transition from fatty streak to fibrous plaque.
Lesions of atherosclerosis are found in the intima and surprisingly the SMCs involved in these lesions are clonal . Murry et al (1997) demonstrated that the atherosclerotic clone is located in the SMC of the fibrous cap itself, however smaller clones were present in the normal intima. It was then hypothesized that clonal origins of the intimal cells could reflect trapping of rare SMCs during the formation of the internal elastic lamina [37-39]. As progression continues, the medial SMCs undergo mitogenic stimulation and change phenotype causing them to lose contractile elements, thus allowing the ability to replicate and migrate within the artery wall . In addition to SMC replication, death of these cells also participates in the further complication of atherosclerosis (seen in figure 1 part 8). Apoptosis can occur in response to inflammatory cytokines known to be present in the evolving atheroma ; often in conjunction with these solute cytokines that trigger apoptosis, the T cells in the atheroma can participate by eliminating some of the SMCs. Some of these T cell populations are known to accumulate in the atherosclerotic plaques and have been found to express the fas ligand on their surface [5, 39]. It is the interaction of fas in conjunction with the cytokines that further promotes the death and accumulation of SMCs .
Overall the increase in SMC mass growing from the medial layer to the diseased intima, the accumulation of leukocytes and foam cells, and the development of a surrounding “fibrous cap” of the extracellular matrix tissue with embedded SMC, in what characterized the fibrous plaque of advanced atherosclerotic disease . This fibrous plaque often localizes itself in the same site as the previous fatty streak. The major difference is that the fibrous plaques have a pale gray appearance and are often firm and elevated. These lesions often project into the arterial lumen and if large enough, significantly reduce blood flow [7, 38]. The fibrous plaques regularly contain a necrotic core of cell debris which may also contain degenerating foam cells that were derived from the SMCs. Overall the core of the fibrous plaque us excessively thrombogenic and is usually covered by a fibrous cap and endothelium . These fibrous plaques frequently lead to further complications such as calcification, rupture, hemorrhage, and embolization.
One major complication as a result of atherosclerotic plaque formation is the narrowing and calcification of the vessel. This usually increases the rigidity of the vessel along with its fragility [7, 37]. The calcification is usually the result of the progressive development if the fibrous plaque due to the organization of microthrombi with the lesion itself . This form of complication is usually an initiating step for the development of myocardial ischemia and limb claudication (peripheral vascular disease) [1, 7].
A second complication is the formation of a thrombus which occludes the arterial lumen. This formation is largely due to the rupture or ulceration of the fibrous plaque which exposes thrombogenic materials in the core of the circulating blood. Such thrombosis can either occlude the vessel resulting in myocardial infarction, unstable angina, or thrombic stroke (cerebral infarction); alternatively, the thrombus material can incorporate into the lesion adding to the bulk of the plaque . Susceptibility of rupture is largely due to the thickness of the fibrous cap which separates from the foam cells . These “vulnerable plaques” usually have a very rich lipid core and a high concentration of inflammatory cells (macrophages and T lymphocytes) [7, 16]. Closely related to this is the complication of peripheral emboli, or more commonly, embolism. This is characterized by the fragmentation and passage of atheromous material from large proximal vessels to the more distal vascular sites . Embolism has also been known to cause embolic stroke and atheroembolic renal failure [14, 16]. One of the last, more common complications associated with atherosclerosis is the weakening of the vessel wall. This weakening is dues to increased pressure caused by the fibrous plaque, thus provoking atrophy and loss of elastic tissue, which ultimately leads to the formation of an aneurysm [7, 16].
To reiterate, these atherosclerotic plaques grow gradually, restrict blood flow, and alter the integrity of the arteries. These atherosclerotic lesions in the coronary arteries impair perfusion to the myocardium which produces intermittent chest discomfort. Thus, the coronary arteries experience alterations in the associated metabolic substances, but most importantly, alterations with the vasoactive substances.
The diseased coronary arteries experience a direct hemodynamic effect which decreases coronary perfusion . Another indirect effect causes contractile failure which compresses the subendothelial tissue, further reducing perfusion. The atherosclerotic plaques have direct vasodilatory effects on the coronary circulation due to the formation of adenosine, NO and the opening of the vascular KATP channels [1, 16]. It has been demonstrated that vasodilation in atherosclerosis is initiated by the breakdown of ATP to adenosine. This decreasing level of ATP activates the ATP-sensitive KATP channels; this early channel opening may cause early potassium loss from the myocardium.
Often as a result of atherosclerosis, the coronary arteries experience coronary spasm in response to increased amounts of vasoconstrictors. Possible explanations for this coronary spasm includes and are not limited to: plaque formations liberate serotonin and thromboxane A2 which are vasoconstrictors usually found shortly after initial endothelial dysfunction; and the coronary atheroma damages the vascular endothelium causing a decreased production of the vasodilators NO and PGI2 and an increased production of vasoconstrictors, AII and endothelin-1 .
Lack of adequate oxygen supply to the myocardium, due to vessel occlusion, often leads to important metabolic and mechanical changes. As oxygen levels decrease, there is a subsequent decrease in the level of available ATP; however, metabolic cycles are also responsible for this depletion. These various metabolic cycles utilize ATP for the internal cycling of excess Ca2+, and for the breakdown of triglycerides and glycogen . In addition to the decreased oxygen levels, the metabolism shifts to anaerobic glycolysis. During anaerobic glycolysis, glycolytic ATP is produced in small amounts (not enough to meet the demand of the myocardium); however this small amount of ATP is believed to play a crucial role in maintaining the ion gradients within the myocardium .
Changes in fatty acid metabolism are also observed in the damaged arteries. Initially fatty acid oxidation leads to the accumulation of lipid metabolites including intracellular free fatty acid acetyl-CoA and acylcarnitine . Meanwhile, the membrane phospholipids are broken down by phopholipases which are activated by the accumulation of excess Ca2+ ; thus aiding in the lipid accumulation within the subendothelial space. Other metabolic changes include: increased cytosolic calcium and magnesium, decreased production of the sodium and sodium/proton exchange, and lactate accumulation [1, 5].
It has been established that atherosclerosis is an underlying initiator for other forms of coronary artery diseases. These diseases ultimately change the metabolic and biological mechanisms necessary form normal myocardial function, which lead to severe physiological changes and sometimes death. Therefore it is important to understand the cellular pathogenesis of atherosclerosis which allows for better comprehension of the causes and common risk factors associated with this disease, hence better prevention and treatment strategies.
Atherosclerosis is a multifactorial disease which has been previously thought to solely arise through a number of common risk factors. It has been widely accepted that increases levels of LDLs in the circulating blood as a major risk facto for the development of atherosclerosis. This hyperlipidemia or dyslipidemia is often characterized when the excess of LDL begins accumulating within the subendothelial space, where it undergoes oxidative modification. High levels of LDL are generally caused by high fat diets or defects with the LDL-receptor clearance mechanisms [41-43]. The mutations in the LDL receptors cause insufficient removal of LDLs; this condition is known as familial hypercholesterolemia and the high levels of LDL often develop into premature atherosclerosis [5, 42, 44]. There is also increasing evidence that triglyceride-rich lipoproteins such as VLDLs and IDLS may have a direct role in further plaque development .
In conjunction with low HDL levels, high LDL levels, poorly controlled type II diabetes and hypertension is the clustered risk factor known as metabolic syndrome (MetS) [45-47]. Nishida et al (2007) found that intima-media thickness (narrowing) and pulse wave velocity (blood pressure) significantly increased in men and women with MetS. Adult onset (type II) diabetes is often part of the MetS and central to this syndrome is the presence of insulin resistance [5, 45]. Interestingly, the presence of insulin resistance appears to promote atherosclerosis long before patients are found to be overly diabetic . Hyperglycemia has been found to induce a large number of alterations in the vasculature accelerating atherosclerosis. These pathological alterations include: i) glycosylated proteins interact with specific receptors present on cells such as monocyte-derived macrophages, endothelial cells and SMCs; ii) glycosylated protein interactions result in the induction of oxidative stress and proinflammatory responses; and iii) protein kinase C (PKC) is activated with subsequent alterations in growth factor expressions . In addition, diabetic individuals frequently have impaired endothelial function with reduced NO expression and increased leukocyte adhesion.
Another common risk factor associated with atherogenesis is the combination of tobacco smoking and hypertension. Cigarette consumption remains the single most important modifiable risk factor for coronary artery disease and the leading preventable cause of death in North America. Beyond the acute unfavorable effects on blood pressure, sympathetic tone, and reduction in myocardial oxygen supply, smoking affects atherothrombosis by several other mechanisms . Long term smoking may enhance oxidation of LDLs and impairs endothelium-dependent coronary artery vasodilation which has been linked directly to dysfunctional endothelial NO biosynthesis from chronic and acute consumption . In addition to enhancing oxLDLs, smoking has been found to decrease circulatory HDL levels, increase platelet adhesiveness, increase in soluble leukocyte adhesion molecules (VCAM-1 and ICAM-1), and the displacement of oxygen by carbon monoxide from hemoglobin . Not surprisingly, continued smoking not only aids in atherosclerotic progression but increases the risk of recurrent myocardial infarctions .
In contrast to cigarette smoking, hypertension if often a silent cardiovascular risk factor with its prevalence steadily increasing. The average normal blood pressure is found to be 120/80 mmHg (+10) in healthy individuals. It has been shown that individuals with blood pressures greater than 140/90 mmHg are at increased risk of coronary artery diseases. Hypertension has been thought to accelerate atherosclerosis in various ways. These include: elevated blood pressure injures the endothelium which increases permeability of the vessel wall to lipoproteins; increased hemodynamic stress increases the number of scavenger receptors on macrophages, enhancing the development of foam cells; and may cause augmentation of the production by SMCs of proteoglycans that bind and retain LDLs (facilitating oxidation) [7, 15]. Lastly, hypertension promotes the increased release of AII, which not only acts as a vasoconstrictor but also acts as a proinflammatory cytokines . Therefore it is thought that hypertension promotes atherosclerosis by contributing to inflammation.
Exercise may mitigate atherogenesis in several ways. It has been demonstrated that exercise has beneficial effects on the lipid profile and blood pressure; it also enhances insulin sensitivity and the endothelial production of NO [5, 7]. Long term studies have also shown that even moderate activities such as brisk walking or various activities of 30 minute intervals have been proven to lower the risk of cardiovascular disease. In addition, various social determinants of health such as income, age, sex, education, occupation, living conditions, and lifestyles have an impact on cardiovascular disease . Sedentary Death Syndrome (SeDS) is an associated symptom with an inactive lifestyle. SeDS is a major public health burden due to its causing multiple chronic diseases and millions of premature deaths each year . This form of physical inactivity is responsible for a severe decline in one’s health and is also associated with other health conditions such as: arthritis pain, arrhythmias, breast cancer, colon cancer, congestive heart failure, depression, gallstone disease, heart attack, hypertension, obesity, osteoporosis, peripheral vascular disease, respiratory problems, Type 2 diabetes, sleep apnea and stroke. Thus physical inactivity adds to the impact of atherosclerosis on the healthcare system.
As with most diseases there are a number of treatment options available for treating and preventing atherosclerosis. Strategies that improve abnormal lipid levels can limit atherosclerosis and its clinical complications. Often diet and exercise are the most important components of the lipid lowering arsenal [7, 15]. However, when these lifestyle modifications fail, there are a variety of pharmaceutical options that can treat abnormal lipid levels. The major group of lipid altering agents include: HMG CoA reductase inhibitors (statins), bile-acid binding agents, niacin, and fibric acid derivatives [5-7, 15]. These statins have been exposed to lower LDL and enhance HDL levels which are thought to reduce the lipid content of atherosclerotic plaques. Other pharmaceutical therapies have also increased NO synthesis, inhibited SMC proliferation and monocyte recruitment, and reduced macrophage production . Various studies have shown that statins also reduce inflammation by inhibiting macrophage cytokines TNF-α, IL-1 and IL-6, thereby reducing endothelial expression of leukocyte adhesion molecules [7, 15].
As with treating the dyslipidemias, treatments for hypertension start with lifestyle modifications, but usually require a pharmaceutical regime. Studies have indicated that diets high in fruits and vegetables with dairy products low in fat and reduced sodium have had dramatic effects on reducing blood pressure levels . The cessation of smoking has also dramatically reduced the risk of cardiovascular disease as well as various forms of cancers [6, 11, 15]. Medications often used to lower blood pressure include diuretics, β-adrenergic receptor antagonists, AII inhibitors, calcium channel blockers, and α-adrenergic inhibitors . Overall it is the combination of lifestyle changes and drug therapies that significantly lower ones chances of the development or further progression of coronary artery diseases.
In addition to the risk factors and treatments discussed, a number of novel atherosclerotic markers, including new emerging risk factors, are being further investigated. As previously discussed, inflammatory responses play a critical role in atherosclerotic progression. There is considerable evidence that suggests that impaired endogenous atheroprotective mechanisms occur at branch points in arteries, where the endothelial cells experience disturbed flow [52-54]. For example, absence of normal laminar shear stress may reduce the endothelial-derived NO , which is found to have anti-inflammatory properties limiting expression of VCAM-1. These natural protective mechanisms such as disturbed blood flow have also been found to augment the production of ICAM-1 . As the inflammatory process continues, the activated leukocytes and intrinsic arterial cells can release fibrogenic mediators including a variety of peptide growth factors that can promote replication of SMCs and contributes to the elaboration of these cells [52-56].
In addition macrophages express scavenger receptors for mLDLs permitting them to ingest lipids, thus forming foam cells. These macrophages can express MHC class II molecules and have been found to process and display antigenic peptides for recognition by CD4+ T cells. From these findings, it has been established that CD4+ and CD8+ T lymphocytes are present within the fatty streaks [57-61]. The concurrence of macrophages and CD4+ T cells in atherosclerotic lesions suggests that the macrophages may present an antigen to the T lymphocyte within the vessel wall . Recent studies have revealed that T cell cytokines IL-2 and IFN-γ are expressed in the plaques, which further indicates that local T cell activation has occurred; furthermore, expression of MHC class II molecules are induced on the endothelium and SMCs within the atheroma are thought to be a consequence of the local production of INF- γ by the plaque T cells [57-59]. Studies have also indicated that analysis of plaque T cell clones suggests that it is the TH1 phenotype that is dominant in atherosclerotic plaques [59, 61]. Also, the macrophage foam cells are starting to be investigated as possible therapeutic targets due to their role in atherosclerosis [62-64].
It has also been demonstrated that chronic inflammation of large blood vessels associated with atherosclerosis are associated with proliferation of vaso vasorum . These vaso vasorum capillaries may facilitate leukocyte entry into lesions or cause intraplaque hemorrhage [53-55]. Therefore it has been hypothesized that plaque neovascularzation may also function as a conduit for leukocytes and plasma entry into the arterial wall [52, 53]. Xu et al. (2007) were able to demonstrate that expression of VCAM-1, ICAM-1, and E-selectin were 2-to-3-fold higher in the neovessels; thus confirming that these neovessles serve as a pathway of leukocyte infiltration. In addition to these inflammatory components, the fas ligand is also under further investigation. There is further research on the involvement of the fas ligand, as it is believed that FasL may serve an atheroprotective function of the endothelium through its ability to induce apoptosis in mononuclear cells that attempt to invade the vessel wall in the absence of normal inflammatory stimuli . All inflammatory evidence considered, it is no wonder that the role of inflammation is aiding in the development of pharmaceuticals as new therapeutic methods to help reduce atherosclerotic progression.
In addition to inflammation, infectious agents are starting to be investigated. Several studies have addressed the possible role of infectious agents, both bacterial and viral, in the pathogenesis of coronary artery diseases such as atherosclerosis [64-66]. Serological tests have suggested that many agents affect the inflammatory process in atherosclerosis; such pathogens include: cytomegalovirus (CMV), herpes simplex virus 1 and 2 (HSV-1 and HSV-2), Chlamydia pneumoniae, Helicobacter pylori, hepatitis A, and enterovirsuses [64-66]. C. pneumoniae has gained special attention for its ability to produce a heat shock protein 60 (HSP60) which activates macrophages and stimulates the production of enzymes that impair the stability of the fibrous cap [5, 7]. These infectious agents have also been linked to inducing foam cell formation, lipoprotein oxidation, and increasing procoagulant activity . A schematic diagram representing how C. pneumoniae can lead to atherosclerosis, coronary heart disease and plaque rupture.
As discussed previously, the pathogenesis of atherosclerosis involves inflammation at every stage; thus markers for inflammation are being further investigated as predictors of cardiovascular disease . The acute phase reactant, C-reactive protein (CRP), is a simple marker of inflammation which has emerged as a major cardiovascular risk factor . CRP may influence vascular vulnerability through several mechanisms including the enhanced expression of ICAM-1 and VCAM-1, increased expression of endothelial PAI-1, reduced endothelial NO bioactivity, and altered LDL uptake by macrophages . Therefore CRP within the serum proves to be an excellent marker and useful screening tool for signs of cardiovascular diseases.
Another emerging risk factor is the increasingly significant relationship between circulating levels of the amino acid, homocysteine. The mechanism by which homocysteine might increase atherosclerosis is not completely understood, however there is current evidence that suggests that abnormally high levels of homocysteine may promote thrombosis . Other proposed mechanisms include: aiding in endothelial dysfunction, accelerating oxidation of LDLs, impairing flow-mediated EDRFs (reducing vasodilation), increasing expression of MCP-1 and IL-8 (increasing proinflammatory response), and increasing oxidative stress . Often this risk factor is the common result of a genetic defect in methionine metabolism or the result of insufficient dietary intake of folic acid. Unfortunately, even though these deficiencies are reversed by folic acid and vitamin B supplements, there has been no current proof that these therapies help reduce atherosclerotic disease [5, 7]. One of the last, more common, emerging risk factors to consider is lipoprotein (a) – Lp(a). Lp(a) consists of an LDL particle with its apo B-100 component linked by a disulfide bridge to apo(a) . The apo(a) portion structurally resembles plasminogen, a plasma protein that is important for the lysis of fibrin clots. Therefore Lp(a) may have a detrimental affect as it believed to compete with plasminogen binding on the endothelium thus inhibiting fibrinolysis [5, 7], which further links lipoproteins and thrombosis.
In regards to new advances in treatments for atherosclerosis, it has become evident that the best preventative medicine is lifestyle and behavioral modifications. As already discussed, studies have proven that a minimum of 30 minutes of activity a day dramatically lowers ones risk of developing cardiovascular diseases. Additionally, diets low in trans saturated fats, high in fiber, fruits and vegetables, and low in sodium have also lowered the risk of development or further progression of cardiovascular diseases. Currently there a number of pharmaceutical therapies that have been proven to lower certain risk factors, these include blood pressure reducing agents such as diuretics, β-adrenergic receptor antagonists, AII inhibitors, calcium channel blockers, α-adrenergic inhibitors; as well as lipid altering agents: HMG CoA reductase inhibitors (statins), bile-acid binding agents, niacin, and fibric acid derivatives. Sometimes when lifestyle modifications and drug therapies fail to reduce coronary artery diseases, surgical correction is often considered. Depending on the extent of the coronary artery disease, sometimes balloon angioplasty or the insertion of medicated or non-medicated stents help relieve any plaque occlusion. However when these noninvasive methods cannot be utilized, often patients are required to undergo bypass surgery, where vessels grafted from the limbs of the individual are used to reroute blood flow of the coronary arteries; this is commonly known as CABG surgery.
To understand the pathophysiology of atherosclerosis, a brief overview of the cardiovascular system was presented. This allowed the reader to gain a basic comprehension to better understand the devastation caused to the coronary arteries by the pathogenesis of atherosclerosis. This hopefully provided insight into various prevention and treatment strategies that could be utilized to counteract the destructive mechanism of this disease. Overall by understanding the importance of the cardiovascular system, we are able to fully appreciate the destructive nature of the atherosclerotic plaques.
Atherosclerosis is also known as the hardening of the arteries; this is mainly due to the increased plaque accumulation within the arterial wall. This accumulation is due to a number of key events such as endothelial dysfunction, lipid accumulation and modification, leukocyte and macrophage recruitment, as well as SMC proliferation and migration. Recall that endothelial dysfunction is the initiating event that impairs the release of NO into the endothelium, as well as making the endothelium more permeable to lipoproteins and leukocytes. These accumulated lipids undergo oxidative modifications which have been demonstrated to induce cytokine production. The cytokines elicit an increased expression of adhesion molecules (VCAM-1, ICAM-1, and E and P-selectins) which allow for leukocyte adhesion; thus releasing more chemoattractant cytokines such as MCP-1 and M-CSF. If the monocytes encounter M-CSF their expression of scavenger receptors becomes augmented; this allows for further uptake of lipoprotein particles forming the foam cells. In addition, SMCs replicate and migrate into the intima where they replicate and produce extracellular matrix; thus the first initial signs of the fatty streak. This fatty streak can develop into a fibrous plaque due to the additional accumulation of SMCs and leukocytes.
By occluding the arterial lumen, the atherosclerotic plaques also alters the release of vasoactive substances into the endothelial space. As a result vasoconstricting molecules such as α and β-adrenergic vascular receptors, angiotension II (AII), and endothelin. These vasoconstrictors are also responsible for an increase in free cytosolic calcium resulting in the homeostasis of myocardial contraction. On the other hand, atherosclerosis decreases the production and bioactivity of the vasodilators: NO, adenosine, and PGI2; thus limiting the amount of myocardial perfusion. These atherosclerotic plaques also cause inadequate oxygen supply to the cardiac tissue resulting in tissue death and various other forms of heart disease. Therefore without an adequate supply of oxygen or nutrients to the myocardial muscle, the heart will cease to function properly. Recall that atherosclerosis has also been demonstrated to be an initiator for other coronary heart diseases as: ischemia, angina pectoris, and myocardial infarction. Also, the fibrous plaques of atherosclerosis have been shown to cause other complications including calcification, rupture, hemorrhage, and embolization.
There has been new indication that suggests inflammation plays a much greater role in the development of atherosclerosis. Along side this, there is emerging evidence suggesting that infectious agents such as C. pneumoniae, HSV-1 and HSV-2, as well as CMV may have an underlying role in atherosclerotic progression. Also in addition to the common risk factors, there has been substantial evidence that supports the role of homocysteine, CRP, and Lp(a) as plausible markers for detection of atherosclerosis. The combination of these components has solidified that atherosclerosis is very much a multifactorial disease, which is provoked by a number of common risk factors. These risk factors include high LDL levels, hypertension, cigarette smoking, diabetes, physical activity levels, as well as socioeconomic factors. Treatments for these risk factors include of a range of lipid altering agents (HMG CoA reductase inhibitors (statins), bile-acid binding agents, niacin, and fibric acid derivatives), a range of hypertensive reducing agents (lifestyle and behavior modifications), as well as increased physical activity levels. Lastly, these pharmaceuticals treatments, along with changes in diet and exercise have been proven to successfully lower ones risk of development or further progression of atherosclerosis.
The combination of all factors involved in atherosclerosis, makes it one of the more prevalent diseases throughout North America, and Europe. It has been estimated that atherosclerosis in conjunction with other cardiovascular diseases are responsible for more than 50% of deaths in developed countries and approximately 25% of deaths in underdeveloped countries; and it is estimated that by 2020, cardiovascular related diseases will account for roughly 25 million deaths annually. It has already been discussed that atherosclerosis is responsible for other forms of cardiovascular disease, congestive heart failure and death; however there is increasing evidence that supports the idea that they may also contribute to diabetes, renal failure and sedentary death syndrome. Therefore by contributing to other potentially deadly disorders, cardiovascular diseases can have a huge impact on the healthcare system. Unfortunately, sometimes this can lead to decreased funding in other areas, thus causing a downward domino effect.
As there are many different processes involved in the pathogenesis of atherosclerosis, there are a number of avenues future research could follow. As it has been established, the endothelial dysfunction increases the permeability, it would be interesting to find out whether or not, and to what extent the integrity of the cellular junctions involved in the endothelium are compromised. It would also be interesting to further test the role of infectious diseases, and CRPs to truly find out whether or not they do contribute to cardiovascular diseases. Lastly, as the inflammatory response is gaining greater attention for its role in atherogenesis, we shouldn’t be surprised if these inflammatory molecules start being utilized in more therapeutic studies, thus allowing alternative treatments and better prevention strategies for this disease.
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