Pathophysiology and Medical Treatment of Carotid Artery Stenosis

Abstract

Stroke is the third leading cause of mortality. Approximately 80 to 85% strokes are ischemic due to carotid artery stenosis (CAS). The prevalence of significant CAS is 7% in women and 9% in men. Severe asymptomatic CAS varies from 0 to 3.1%. Prevalence of symptomatic CAS is high in patients with peripheral arterial disease. CAS is due to atherosclerosis, the major risk factors for which include dyslipidemia, hypertension, diabetes, obesity, cigarette smoking, advanced glycation end products (AGEs) and its receptors (RAGE, soluble RAGE [sRAGE]), lack of exercise and C-reactive protein (CRP). This article discusses the basic mechanism of atherosclerosis and the mechanisms by which these risk factors induce atherosclerosis. The role of AGEs and its receptors in the development and progression of CAS has been discussed in detail. Lifestyle changes and medical treatment of CAS such as lifestyle changes, lipid-lowering agents, antihypertensive agents, antidiabetic drugs, anti-AGE therapy, measures to elevate soluble receptors of AGE (sRAGE, esRAGE). CRP-lowering agents have been discussed in detail. The drugs especially lipid-lowering agents, and antihypertensive and antidiabetic drugs suppress, regress, and slow the progression of CAS. The possible role of lowering the levels of AGEs and raising the levels of sRAGE in the treatment of CAS has been proposed. Lifestyle changes besides medical treatment have been stressed. Lifestyle changes and medical treatment not only would slow the progression of CAS but would also regress the CAS.

Stroke is the third cause of disability in the world and third leading cause of mortality.¹ Approximately 80 to 85% of strokes are ischemic due to stenosis, clot, and embolism. Approximately 20 to 30% of all strokes are caused by extracranial carotid artery stenosis (CAS),² while intracranial CAS accounts for 5 to 10% of strokes.³ CAS is due to atherosclerosis. As the atherosclerosis progresses the atherosclerotic plaques rupture resulting in the formation of thrombus and arterial occlusion or dislodged materials from the plaques blocking the smaller branches of the carotid artery. Transient ischemic attacks (TIA) are a brief period of symptoms similar to stroke due to temporary blockage of blood supply to a section of the brain and often lasts less than 24 hours. Carotid artery disease is responsible for nearly 50% of all TIAs.⁴ Risk of developing stroke after TIAs is as high as 20% within the 1st month.² If untreated, TIAs result in development of stroke within 2 years. Risk of stroke events remains high for 10 to 15 years after TIAs.⁵

CAS manifests into clinical syndromes, that is, asymptomatic, TIAs, and ischemic stroke. CAS is considered symptomatic when ipsilateral retinal or cerebral ischemia occurs and asymptomatic when these symptoms are absent. Around 5 to 10% of the general population over 65 years of age has an asymptomatic CAS of 50% or greater.^{6 7} Prevalence of asymptomatic CAS of 50% or greater is highest in patients with peripheral arterial disease (15%) and abdominal aortic aneurysm (12%).⁸ Risk of stroke increases with increasing CAS.⁹ There is less than 1% stroke per year for a CAS less than 80% but increases to 4.8% per year for a CAS greater than 90%.⁸ A stenosis of the carotid artery greater than 50% is considered significant carotid artery disease. Differentiation between asymptomatic and symptomatic CAS is important for treatment of CAS. In general, medical treatment is provided for asymptomatic patients while invasive treatment is considered for symptomatic patients with CAS greater than 50%¹⁰ and for asymptomatic patients with CAS greater than 60%.¹¹ For invasive or medical treatments, the risk factors for CAS have to be considered.

This article discusses the epidemiology and risk factor for CAS, and medical treatment of carotid stenosis.

Epidemiology

Carotid artery atherosclerosis is mostly present at the carotid bifurcation into external and internal carotid artery. The ostium of the internal carotid artery is mostly affected. Intracranial internal carotid artery and its branches are less affected with atherosclerosis. The prevalence of CAS varies with the study population, use of equipment, and the criteria employed. The prevalence of significant CAS was 7% in women and 9% men when carotid ultrasound was used for measurement of stenosis and when stenosis was less than 50%.¹² Prevalence is high in individuals with high risk of atherosclerosis (11%), cardiac disease (18%), and acute stroke (60%).¹³ The prevalence of moderate asymptomatic CAS in general population is between 0.2% in men aged less than 50 years and 7.5% in men aged greater than or equal to 80 years.⁹ For women, this prevalence ranged from 0 to 5%. de Weerd et al⁹ also reported that prevalence of severe asymptomatic CAS varies from 0.1% in men aged less than 50 years to 3.1% in men aged greater than or equal to 80 years. For women, this prevalence increased from 0 to 0.9%. They concluded that the prevalence of severe asymptomatic CAS varies from 0 to 3.1%. Around 15% of the patients with extracranial CAS had intracranial CAS.¹⁴ Mild intracranial CAS was observed in 33% of the patients with extracranial CAS.¹⁵ O'Leary et al¹⁶ reported that 25% of the 4,476 elderly patients without clinical evidence of cardiovascular disease in the 5th quartile intimal–medical thickness had myocardial infarction or stroke at 6.2 year follow-up compared with less than 5% in the 1st quantile.

It has been reported that the prevalence of asymptomatic CAS is high in patients with peripheral arterial disease.^{17 18 19} Bavil et al²⁰ reported that the prevalence of significant internal CAS in Iranian patients with peripheral arterial disease was low (4.2%). Tanimoto et al²¹ reported the prevalence of CAS (less than 50%) was 14% with one-vessel disease, 21% with two-vessel disease, and 36% with three-vessel disease in Japanese patients with coronary artery disease (CAD). Kallikazaros et al²² showed that the prevalence of CAS of less than 50% was present in 5% with one-vessel disease, 13% with two-vessel disease, 25% with three-vessel disease, and 40% with left main disease. In a recent large study, Steinvil et al²³ reported the prevalence of CAS (greater than 50%) as 5.9% with normal or nonobstructive CAD, 6.6% with one-vessel disease, 13% with two-vessel disease, 17.8% with three-vessel disease, and 31.3% with left main disease. Prevalence of severe CAS (greater than 70%) was 2.1% with normal or nonobstructive CAD, 3.1% with one-vessel disease, 3.6% with two-vessel disease, 7% with three-vessel disease, and 10.8% with left main disease in the same study. The prevalence of significant CAS has been reported to be 30% in patients with CAD.²⁴ Mathiesen et al²⁵ reported that prevalence of CAS was higher in men than women (3.8 vs. 2.7%). CAS was associated with history of cerebrovascular disease, CAD, and peripheral arterial disease. They also reported that with each 10% increase in the extent of CAS, the risk of cerebrovascular event increased by 26%.These data suggest that prevalence of CAS is high and is associated with peripheral vascular disease. Association of prevalence with CAD is dependent upon the number of affected coronary artery.

Pathogenesis of Carotid Artery Stenosis

CAS is a progressive narrowing of the carotid artery due to development of atherosclerosis, characterized by local thickening of the interior arterial wall. Pathogenesis of atherosclerosis has been reviewed early by Prasad.²⁶ Classical types of lesion are fatty streaks, fibrous cap, and complicated lesion, based on the progression of atherosclerosis. Carotid plaques consist of lipid core with infiltration of inflammatory cells covered with a fibrous cap. A typical fibrous cap consists of the following: (1) fibrous cap composed of smooth muscle cells, few leukocytes, dense connective tissue that contains elastin, collagen fibrils, proteoglycans, and a basement membrane; (2) a cellular area beneath that consists of a mixture of macrophages, smooth muscle cells, and T lymphocytes; and (3) a deeper necrotic core that contains cellular debris, lipids, cholesterol crystals, and calcium deposit. With time the plaque can become large and produce narrowing in the carotid artery. A plaque can be stable and asymptomatic or it may be a source of embolization. Rupture-prone carotid plaques are called vulnerable or unstable plaques which are characterized by active inflammation, extensive accumulation of macrophages, thin cap with a large lipid core, endothelial denudation with superficial platelet aggregation, fissures, and severe stenosis.²⁷ Vulnerable plaques are more susceptible to rupture when the fibrous cap becomes thin with remodeling of extracellular matrix by matrix metalloproteinase secreted by leukocytes within the intima.²⁸ The vulnerable plaques are more likely to rupture resulting in thromboembolic events.²⁹ Inflammation of the fibrous cap occurs most likely in noncalcified plaque as compared with calcified plaque suggesting that calcification of plaque is a marker of stability of plaque.³⁰ Asymptomatic plaques are more calcified and less inflamed than symptomatic plaques.³¹

Atherosclerotic plaques generally develop at branch ostia and bifurcation of the common carotid artery into the external and internal carotid artery. The ostium of the internal carotid artery is mostly affected, involving posterior wall of carotid sinus. It also extends into the distal common carotid artery. Intracranial internal carotid artery and its branches are affected by atherosclerosis. Besides the general risk factor for atherosclerosis, the fluid dynamics and vessel geometry also play a role in the development of atherosclerosis.^{32 33} Hemodynamic forces at the carotid bifurcation have a role in localization of intimal thickening. Both in vivo and in vitro studies it has been reported that disturbed flow and low-shear conditions produces endothelial dysfunction.^{32 33 34}

Mauriello et al³⁵ reported that the thrombotic plaques are more frequently detected in patients with stroke (66.9%) than with TIA (36.1%) and asymptomatic patients (26.8%). They also reported that out of 45 carotid plaques removed during carotid endarterectomy 39.6% were thrombotic plaques, 15.8% were vulnerable plaques, and 44.6% were stable plaques.

Mechanism of Atherosclerosis

The mechanism of atherosclerosis in carotid artery is similar to the atherosclerosis in other arterial locations. Reactive oxygen species (ROS) have been implicated in the initiation and progression of atherosclerosis.^{36 37 38 39 40} This hypothesis is universally accepted and it has been described in detail elsewhere by Prasad.²⁶ ROS increases the expression of cell adhesion molecules including intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and endothelial leukocyte adhesion molecule on endothelial cells (ELAMs).^{41 42 43 44} Monocyte adherence to endothelial cells is mediated through adhesion molecules. Low-density lipoprotein cholesterol (LDL-C) is mildly oxidized to minimally modified LDL (MM-LDL) which stimulates endothelial and smooth muscle cells to produce chemoattractant proten-1 (MCP-1).⁴⁵ MM-LDL is further oxidized to fully oxidized LDL (OX-LDL).⁴⁶ MCP-1 and OX-LDL accelerate migration of monocytes to subendothelial area.^{47 48 49} Monocyte macrophages express the LDL receptor, but the rate of uptake of native LDL is insufficient to produce foam cells.⁵⁰ OX-LDL is a ligand for the scavenger receptor expressed in monocyte differentiated into tissue macrophage.^{51 52} This monocyte/macrophage differentiation is facilitated by release of monocyte colony stimulating factor (MCSF) from endothelial cells under the influence of MM-LDL.⁴⁵ The differentiated macrophage develops receptor for OX-LDL which is taken up by receptors to produce foam cells. The production of foam cells is the early stage of atherosclerosis. Foam cells also generate ROS.⁵³

Macrophages generate a host on growth-regulating molecules (platelet-derived growth factor [PDGF], basic fibroblast growth factor [bFGF],⁵⁴ heparin-binding epidermal growth factor [HB-EGF], and transforming growth factors [TGF-α and TGF-β]), and cytokines (IL-1, TNF-α) that affect neighboring cells. Gene expression and transcription in smooth muscle cells could result in the formation of collagen, elastic fiber proteins, and growth-regulating molecules (bFGF, insulin-like growth factor I [IGF-I], HB-EGF, TGF-β) and cytokines (IL-1, TNF-α). Endothelial cells produce growth-promoting molecules (PDGF, bFGF, TGF, IGF-I) and cytokines. T lymphocytes produce TGF-β and cytokines. Smooth muscle cells also produce colony-stimulating factor (macrophage colony-stimulating factor [M-CSF] and granulocyte-monocyte colony-stimulating factor [GM-CSF]). PDGF, bFGF, and IGF-I are critical to the proliferation of smooth muscle cells and possibly of endothelium. Colony-stimulating factor plays a role in macrophage stability and replication. TGF-β is a potent stimulator of synthesis of connective tissue and matrix including collagens, proteoglycans, and elastic fiber proteins. It inhibits the replication of many cells including smooth muscle cells. Cytokines promote smooth muscle cell proliferation. Both PDGF and IGF-I are chemoattractants for smooth muscle cells. Fibroblast growth factors and M-CSF are chemoattractants for endothelium and for monocyte-derived macrophages, respectively.

Smooth muscle cell proliferation and migration, synthesis of connective tissue and matrix, migration of monocytes and formation of foam cells results in the development and progression of atherosclerosis.

Risk Factors

Risk factors for development of CAS are similar to those for CAD and other peripheral vascular disease. These risk factors include: dyslipidemia, hypertension, diabetes, advanced glycation end products (AGEs) and its receptors, obesity, cigarette smoking, lack of exercise, age, and C-reactive protein. The following section will provide the mechanism of development of atherosclerosis/CAS related to risk factors. Mathiesen et al²⁵ have reported that total cholesterol (TC), LDL-C, systolic blood pressure, and current smoking were independently associated with CAS both in men and women. In multivariate models adjusted for age and sex. Raitakari et al⁵⁵ have shown that carotid artery intima-media thickness (IMT) in adulthood was significantly associated with levels of LDL-C, systolic blood pressure, and smoking in childhood, and with systolic blood pressure and smoking in adulthood.

Dyslipidemia

Total Cholesterol

High levels of serum TC and LDL-C are risk factors for development of atherosclerosis. Hypercholesterolemia increases the generation of ROS through various mechanisms.^{26 56} Hypercholesterolemia also increases the activity of nicotinamide adenine dinucleotide phosphate-oxidase (NADPH-oxidase), xanthine oxidase,⁵⁷ and myeloperoxidase,⁵⁸ resulting in the generation of ROS. Cholesterol also increases the secretion of proinflammatory cytokines.⁵⁹ It has been reported that TNF-α, IL-β, and IFN-y increase the mitochondrial and NAPDH-oxidase-generated ROS.⁶⁰ The facts that hypercholesterolemia produces atherosclerosis in animal model,^{36 37 38} and antioxidants suppress hypercholesterolemic atherosclerosis,^{36 37 38 61 62} suggest that ROS plays a role in the development of atherosclerosis.

Low-Density Lipoprotein Cholesterol

LDL-C takes a major part in the development of atherosclerosis. LDL⁶³ and OX-LDL⁶⁴ activate endothelial NADPH-oxidase resulting in ROS generation. The other effects of MM-LDL and OX-LDL have been described in the section on mechanism of atherosclerosis. Native LDL increases the expression of ICAM-1,⁶⁵ VCAM-1, and p-selectin.⁶⁶ OX-LDL also increases the expression of ICAM-1 and E-selectin.⁶⁷

High-Density Lipoprotein Cholesterol

High-density lipoprotein cholesterol (HDL-C) is protective against atherosclerosis. Classical function of HDL-C is reverse cholesterol transport, that is, removing cholesterol from peripheral tissue and delivering in the liver for metabolism. This way it reduces the serum levels of TC and its consequences. Besides its function as reverse cholesterol transport, it has numerous other functions which may reduce the development of atherosclerosis. Protective effect of HDL-C could be due to its effect on generation of ROS and vascular cell adhesion molecules. Reconstituted HDL-C inhibits leukocyte NADPH-oxidase.⁶⁸ This effect of HDL-C may protect vascular injury and development of atherosclerosis. HDL-C suppresses the expression of cell adhesion molecules on the endothelial cells activated by cytokines and inhibits adhesion of monocyte to endothelium.^{69 70} Paraoxonase-containing HDLs significantly protects LDL-C oxidation and inhibits expression of MCP-1.⁷¹ LDL-C in a coculture of endothelial and smooth muscle cells in human serum increased the MCP-1 expression and sevenfold increase in monocyte migration into subendothelial space while purified HDL in this culture reduced the monocyte migration by approximately 90%.⁴⁸ HDL-C is positively correlated with plasminogen inhibitor-1 in humans.⁷² This would reduce the formation of blood clot and reduce the stroke. Tolle et al⁷³ have reported that HDL-associated lysosphingolipids inhibit NAPDH-oxidase-dependent ROS generation and MCP-1 generation.

Triglycerides

Triglyceride (TG) is a critical and a well-documented risk factor for atherosclerosis.⁷⁴ TGs in the presence of high concentration of very low-density lipoprotein generates small dense LDL,⁷⁵ which exhibits atherogenic property. High TG also reduces the levels of HDL. Triglyceride-rich lipoprotein (TGRL) increases the generation of ROS.⁷⁶ TGRL increases secretion of TNF-α, expression of cell adhesion molecules, and increases oxidative stress.⁷⁷ It increases the expression of VCAM-1.⁷⁸ This effect of TG on VCAM-1 would produce atherosclerosis. High TG levels produce a procoagulant state by increasing factor VII, and activated factor VII phospholipid complexes,^{79 80} factor X,⁸¹ tissue plasminogen activator inhibitor,⁸² and thrombin generation.⁸³

These data, on serum lipids, suggest that high levels of TC, LDL-C, and TG increase the risk factors which are responsible for the genesis and progression of atherosclerosis. HDL-C protects the development of atherosclerosis by affecting the factors responsible for development of atherosclerosis adversely.

Hypertension

Hypertension may induce atherosclerosis by increasing oxidative stress, levels of cytokines, cell adhesion molecules, and chemokines.

There is evidence that oxidative stress is involved in the development and progression of atherosclerosis. In spontaneously hypertensive rats (SHR), the increased levels of ROS precedes the development of hypertension.⁸⁴ Superoxide (O₂ ^-) generation is increased in the vasculature of SHR⁸⁵ and SHR and deoxycorticosterone acetate (DOCA) salt-sensitive hypertension.⁸⁶ There are increased levels of thiobarbituric acid reactive substances (TBARs) and F2α-isoprostane the markers of oxidative stress, tissue concentrations of hydrogen peroxide (H₂O₂) and O⁻ ₂, and decreased levels of antioxidant enzymes in experimental hypertension.^{87 88 89} Manning et al⁹⁰ reported an increased O₂ ^- production in aortic and mesenteric microvascular of DOCA-salt sensitive rats. Hypertension is attenuated and prevented with antioxidant.⁹¹ H₂O₂ levels are elevated in hypertensive patients also.⁹² Polymorphonuclear leukocytes of hypertensive patients generate increased amounts of ROS.^{93 94} Patients with essential, salt-sensitive, and renovascular hypertension have higher levels of TBARs and 8-isoprostane in the plasma.^{95 96 97} There is an increased levels of ROS and decreased levels of antioxidants or both in experimental models of hypertension.^{98 99 100} These data suggest that ROS levels are elevated in hypertension.

The studies have shown that the plasma levels of proinflammatory cytokines (IL-1β, IL-6, and TNF-α)^{101 102 103} are higher in hypertensive patients compared with normotensive patients. Inflammation contributes to atherosclerosis.¹⁰⁴ Vascular cell adhesion molecules are affected by hypertension. Shalia et al¹⁰⁵ have reported that circulating levels of soluble (s) ICAM-1, sE-selectin, and sP-selectin were significantly elevated in hypertensive patients and the levels of ICAM-1, VCAM-1, and E-selectin are elevated in hypertensive patients with increased IMT and left-ventricular hypertrophy.¹⁰⁶ MCP-1 levels in plasma are elevated in idiopathic pulmonary hypertension,¹⁰⁷ and systematic hypertension.^{108 109} MCSF levels in plasma are elevated in hypertensive patients.¹¹⁰

These data suggest that the levels of ROS, inflammatory cytokines, cell adhesion molecules, MCP-1, and MCSF which are relevant to development of atherosclerosis are elevated in hypertensive patients.

Cigarette Smoking

Cigarette smoking contributes to CAS. Tell et al¹¹¹ have reported that increasing cigarette smoke increases the thickening of internal and common carotid artery, and internal CAS. The prevalence of clinically significant internal CAS increased from 4.4% in never-smokers to 7.3% in former smokers and to 9.5% in current smokers. It has been reported that smoking was independently associated with severe CAS.¹¹² They also found that there was a significant association with smoking 20 pack-years, however no significant effect was observed with lower amounts of cigarette smoke. This association was significant with white smokers, less strong for black smokers, and no association for Hispanics. The development of CAS in cigarette smokers could be due to increased levels of ROS, increased expression of vascular cell adhesion molecules, cytokines, MCP-1 and MCSF.

Cigarette smokers have elevated levels of ROS in the serum.¹¹³ Cigarette smoking could generate ROS through gas/tar, activation of macrophage, and polymorphonuclear leukocytes (PMNLs), xanthine oxidase, and AGEs.^{113 114 115 116 117} It has been reported that tobacco smoke is a source of toxic reactive substances that are involved in the generation of ROS. Kalra and Prasad¹¹³ have shown that cigarette smoke increases the generation of ROS by PMNLs and also increase the serum levels of malondialdehyde, a measure of levels of ROS. Cigarette smoke can generate ROS through activation of xanthine oxidase.¹¹⁵ The levels of serum xanthine oxidase are elevated in cigarette smokers.¹¹⁸ Nicotine in cigarette smoke enhances PMNL responsiveness to complement C5a,¹¹⁹ and hence increases the generation of ROS. Cigarette smoke contains peroxyl radicals and can damage the endothelium. Cigarette smoke decreases the levels of vitamin C and E which are antioxidants and hence increases the levels of ROS. Cigarette smoke decreases the serum levels of HDL-C¹²⁰ and increases the levels of non-HDL-C.¹²¹ HDL is known to inhibit leukocyte NADPH-oxidase,⁶⁸ so reduction in HDL-C levels would increase the activity of NADPH-oxidase resulting in generation of ROS. Increases in non-HDL cholesterol would increase generation of ROS through mechanisms already discussed in dyslipidemia section of risk factors.

Serum levels of AGEs are elevated in cigarette smokers.^{116 122 123} Interaction of AGEs with its full length receptor (RAGE) increases the generation of ROS, activates NF-kB and increases the expression of vascular cell adhesion molecules and cytokines.^{124 125 126} Cytokines are also known to stimulate granulocytes to generate ROS.^{127 128}

Other factors involved in the genesis of atherosclerosis are also affected in cigarette smokers. The serum levels of MCP-1¹²⁹ and MCSF¹³⁰ are elevated in cigarette smokers. The serum levels of cytokines (IL-1 β, IL-8, IL-17) are elevated in smokers.¹³¹

The serum levels of cell adhesion molecules are also elevated in cigarette smokers.^{132 133} The levels of sICAM-1¹²⁵ and VCAM-1¹²⁶ are elevated in cigarette smokers.

These data suggest that cigarette smoke increases all the factors involved in the mechanism of atherosclerosis.

Diabetes

Diabetes, both type-1 and type-2, are strong and independent risk factors for CAD, stroke, and peripheral vascular disease.¹³⁴ Chronic hyperglycemia is considered as a primary causal factor in diabetic complications.^{135 136} Glucose can induce atherosclerosis through various mechanisms including oxidative stress, AGEs, and protein kinase C (PKC).

Oxidative Stress

The sources of oxidative stress include: mitochondria, NADPH-oxidase, insulin, autoxidation of glucose, and uric acid.

Increase in the glucose metabolism in the mitochondria-generate reduced NADPH and reduced flavin adenine dinucleotide (FADH), resulting in the generation of O₂ ^- which is converted into H₂O₂ and •OH¹³⁷ NADPH-oxidase is a membrane-associated enzyme complex that lies dormant and is present in the vascular endothelium and smooth muscle cells, cardiomyocytes, macrophages, and neutrophils. High glucose levels activate NADPH-oxidase.^{138 139 140} Activated NADPH-oxidase catalyzes the reduction of O₂ to O₂ ^-.

The other mechanism of hyperglycemia-induced ROS involves transition metal catalyzed autoxidation of free glucose in which glucose initiates autoxidation reaction and production of O₂ ^- and H₂O₂.¹⁴¹ Glucose intake increases the secretion of insulin,¹⁴² which activates plasma membrane enzyme system with the properties of NADPH-oxidase resulting in the generation of H₂O₂.¹⁴³ Insulin-induced generation of H₂O₂ is through activation of NOX₄, a homologous family of NADPH-oxidase.¹⁴⁴ Fructose in sugar increases uric acid in humans.¹⁴⁵ Uric acid is pro-oxidant in lipid membrane through interacting with peroxynitrite¹⁴⁶ and oxidized lipids.¹⁴⁷

Advanced Glycation End Products

AGEs are heterogeneous group of molecules formed from nonenzymatic reaction of reducing sugars with amino group of proteins, lipids, and nucleic acids.^{124 148} Initially, glycation occurs through binding of aldehyde or ketone groups of reducing sugars to free amino groups of proteins, resulting in formation of Schiff base which undergoes rearrangements that form more stable Amadori products. Schiff base and Amadori products (hemoglobin AI_C) react at equilibrium state in hours and weeks, respectively. These initial and intermediate glycation products undergo a complex series of further chemical rearrangements to yield a stable and irreversible AGEs.¹⁴⁹ AGEs comprise of chemical structures such as N-€-carboxy-methyl-lysine (CML), N-€-carboxy-ethyl-lysine, pyrraline, pentosidine, and argpyramidine.¹⁵⁰ CML modifications of proteins are predominant AGEs.¹⁵¹

AGEs can induce atherosclerosis through a nonreceptor dependent and receptor-mediated mechanism.

Nonreceptor-Mediated Mechanism

AGEs affect functional properties of extracellular matrix molecules. It enhances the synthesis of extracellular matrix components,¹⁴⁹ traps subendothelial LDL,¹⁵² and crossbinds with collagen.¹⁵³

AGEs affects the lipids. Glycation process affect apoprotein B and phospholipid component of LDL, resulting in functional alteration in LDL clearance, and an increased susceptibility to oxidative modification.^{154 155} These nonreceptor-mediated mechanisms would induce the initiation and progression of atherosclerosis.

Receptor-Mediated Mechanism

The cellular interactions of AGEs are mediated through specific receptors on the cell membrane. There are three receptors of AGEs (RAGE): Full length RAGE, N-truncated RAGE, and C-truncated RAGE. Full length RAGE is a member of immunoglobulin superfamily of receptors.¹⁵⁶ It has a single transmembrane domain and a highly charged cytosolic tail, which is vital for RAGE ligands. N-truncated RAGE is present in the plasma membrane and its functions are not clearly understood. C-truncated RAGE lacks cytosolic tail and transmembrane domain, circulates in the blood, and binds with AGEs but does not activate intracellular signaling.¹⁵⁷ There are two isoforms of C-truncated RAGE; cleaved RAGE (cRAGE) and endogenous secretory RAGE (esRAGE). cRAGE is proteolytically cleavage of full length RAGE from cell surface¹⁵⁸ and esRAGE formed from alternative splicing of full length RAGE mRNA.¹⁵⁹ Both cRAGE and sRAGE are extracellular soluble receptors. Measurement of total sRAGE includes both cRAGE and esRAGE (measured by sRAGE enzyme-linked immunosorbent assay [ELISA] kits) while esRAGE is measured by esRAGE ELISA kits. Serum sRAGE levels are five times higher than esRAGE in healthy humans.¹⁶⁰ AGEs interact with sRAGE before they interact with full length RAGE.^{161 162} sRAGE acts as a decoy for RAGE ligands by sequestering/competing with full length RAGE for ligand binding and hence has a cytoprotective effects.

RAGE has been shown to be expressed in variety of tissues including cells associated with atherosclerotic process such as vascular endothelial and smooth muscle cells, and monocyte-derived macrophages.^{163 164} The expression of RAGE is upregulated in various diseases including atherosclerosis and diabetes.^{126 161} The interaction of AGEs with RAGE results in generation of ROS and activation of NF-kB. Interaction of AGE with vascular endothelial surface RAGE in vasculature generates ROS through activation of NADPH-oxidase¹⁶⁵ The generated ROS then activates NF-kB. NF-kB leads to transcriptional activation of many genes including TNF-α, IL-1, IL-6, IL-8, interferon-γ and VCAM-1, and ICAM-1.^{166 167 168} Also, interaction of AGE with its receptor leads to reduced endothelial barrier function, increasing the permeability of endothelial cell layer.^{169 170} Increased permeability would increase the transmigration of lipids into the subendothelial space.

Interaction of AGEs with RAGE of monocyte induces chemotaxis, which accelerates the monocyte infiltration into subendothelial space.^{171 172} This interaction of AGEs with monocyte-macrophage results in the expression and production of IL-β, TNF-α, platelet-derived growth factor, and insulin-like growth factor-1,^{173 174 175} and increases uptake of AGE–LDL by macrophage.¹⁷⁶ Interaction of AGEs with RAGE in vascular smooth muscle cells increases cell proliferation and production of fibronectin.^{177 178}

The following data support the involvement of AGE–RAGE axis in the development of atherosclerosis and vascular hyperplasia. Zhou et al¹⁷⁹ have reported that the levels of AGEs and RAGE in carotid arterial wall are elevated in Zucker diabetic rats as compared with euglycemic control rats. They also showed that the balloon injury in the carotid artery of these rats further increases the levels of AGEs and RAGE and produced neointimal hyperplasia. Treatment with sRAGE before and for up to 21 days after balloon injury reduced the neointimal growth significantly. sRAGE also reduced the vascular smooth muscle cell growth in vitro and vascular smooth cell proliferation in vivo. Sakaguchi et al¹⁷⁸ reported that arterial de-endothelialization in wild type mice upregulated RAGE in the injured vessels, particularly smooth muscle, and increased AGE deposition in expanding neointima. sRAGE administration decreased the neointimal expansion, and decreased smooth muscle proliferation, migration, and expression of extracellular matrix proteins. In another study, Wendt el al¹⁸⁰ reported that strepotozotocin-induced diabetes accelerated atherosclerosis in apoE-deficient mice and this was associated with an increased expression of VCAM-1 in aorta, compared with nondiabetic mice. Treatment of diabetic mice with sRAGE significantly decreased VCAM-1 and reduced atherosclerotic lesion in a glycemia—and lipid-independent manner. Treatment of diabetic apoE-deficient mice with sRAGE completely suppressed atherosclerosis in a glycemia- and lipid- independent manner.¹⁶² It has been reported that aorta of diabetic apoE-deficient mice showed an increased expression of RAGE, and VCAM-1 compared with nondiabetic aorta.¹⁸¹ Treatment with sRAGE suppressed the levels of VCAM-1 and RAGE in aorta of diabetic rats in this study.

Author's laboratory has reported that serum levels of sRAGE were low in patients with non-ST elevation myocardial infarction.¹⁸² Also, it has been reported that low levels of serum sRAGE is predictor for restenosis following percutaneous coronary intervention.¹⁸³

These data suggest that AGE–RAGE axis is involved in the pathogenesis of atherosclerosis.

Hyperglycemia and Protein Kinase C

Hyperglycemia in diabetes increases intracellular glucose which increases the concentration of intracellular diacylglycerol (DAG). DAG activates PKC in the vascular system.¹⁸⁴ PKC activation in vascular smooth muscle cells modulates vascular growth and DNA synthesis.¹⁸⁵ It increases the platelet-derived growth factor-β receptor expression in a smooth muscle cells.¹⁸⁶ PKC activation increases the expression of transforming growth factor-β which is involved in the regulation of extracellular matrix and collagen synthesis, and decreases the synthesis of matrix metalloproteinase.¹⁸⁷ These data suggest that hyperglycemia accelerates the development of atherosclerosis by activating PKC.

Obesity

Obesity increases the risk of diabetes,¹⁸⁸ hypertension,¹⁸⁹ and insulin resistance¹⁹⁰ which are involved in the development of atherosclerosis/CAS.

Age

Age is a risk factor for CAS. The prevalence of CAS (50% stenosis) below the age of 60 years is 0.5% and increases to 10% above 80 years of age. In general men under age 75 have a greater chance of developing CAS than women, but women have greater chance of developing CAS than men after age 75 years.

C-Reactive Protein

Elevated levels of serum C-reactive proteins (CRP) predict the risk of future ischemic stroke and TIAs irrespective of other risk factors.¹⁹¹ CRP is moderate but statistically significant marker for CAS.¹⁹² Xiao-Jun et al¹⁹³ have reported that high sensitivity CRP (hs-CRP) levels in serum of hypertensive patients with carotid atherosclerosis were higher than those without carotid artery atherosclerosis, suggesting that hs-CRP plays a role in CAS. Preprocedural CRP predicts the stroke and death in patients undergoing carotid stenting.¹⁹⁴ CRP increases the generation of ROS through activation of neutrophils.¹⁹⁵ CRP induces the development of atherosclerosis through various mechanisms including release of ROS, increased expression of vascular cell adhesion molecule, and foam cell formation.¹⁹⁶

Medical Treatments of Carotid Artery Stenosis

The treatment of CAS is directed toward the risk factors for CAS. Some risk factors for CAS such as smoking, diabetes, obesity, hypertension, CRP, and dyslipidemia can be controlled while others such as age and heredity cannot be controlled. Certainly aging process can be delayed.

The treatment of asymptomatic patients with CAS includes lifestyle changes and use of pharmacological agents.

Lifestyle Changes

Lifestyle changes that could slow the progression of CAS include the following:

1. Cessation of smoking and use of tobacco products.

2. Use of foods low in saturated fats, cholesterol, and sodium.

3. Control of body weight.

4. Daily physical exercise.

5. Reduction of dietary calories intake.

6. Limitation of alcohol use.

Pharmacological Agents

Asymptomatic patients with low-grade CAS (less than 50%) should receive intensive medical treatment.¹⁹⁷ Medical treatment is targeted at risk factors of CAS and includes: lipid lowering agents, antihypertensive agents, AGE-lowering agents, agents that increase the levels of sRAGE, and CRP-lowering agents. Only the guidelines will be described in this section. Details of the medical treatment are not feasible for this article.

Lipid-Lowering Agents

The objectives of this treatment is to reduce the serum levels of LDL-C to < 100 mg/dL but should be reduced to < 70 mg/dL in patients with diabetes and CAD. Statins are used to lower the serum lipids. Statins are known to slow the progression of carotid atherosclerosis.¹⁹⁸ Statins have pleotropic effects and this has been discussed in details by Prasad.¹⁹⁹ They affect all the risk parameters involved in the development of atherosclerosis. Pleotropic effects include, anti-inflammatory, inhibition of expression and secretion of matrix-metalloproteinase, antioxidant, CRP-lowering effects, antithrombotic, anticell proliferation, and antimitotic.²⁰⁰

Antihypertensive Agents

There are numerous articles which show that antihypertensive agents slow the progression²⁰¹ and regression²⁰² of CAS.

The blood pressure should be reduced to below 140/90 mm Hg,²⁰³ but 130/80 mm Hg in patients with diabetes and renal disease. The general guidelines recommended for management of hypertension by the Seventh Joint National Committee on Prevention, Detection, Evaluation and Treatment of High blood pressure (JNC-7),²⁰³ the American Diabetic Association, the National Kidney Foundation and Canadian guideline²⁰⁴ should be followed for the treatment of hypertension.

The initial therapy for stage I hypertension are monotherapy with thiazide diuretics, angiotensin converting enzyme inhibitor (ACEI), β receptor blockers (BBs), angiotensin receptor blockers (ARBs), calcium channel blockers (CCBs) or combination. For stage II without compelling indications the choices are combinations of thiazide diuretics and ACEI, or ARBs, or BBs or CCBs.

For additional hypotensive effects in dual therapy a drug combination from thiazide or ACEI, and CCBs, or ACEIs should be considered. Initial therapy with more than one drug increases the chances of achieving the blood pressure goal. Combination therapy produces greater reduction in blood pressure at lower doses, resulting in lower side effects.

Hypertension with compelling indications require following regimens:

1. Hypertension with heart failure: diuretics, ACEI, ARBs, CCBs, BBs.

2. Hypertension with CAD risk: diuretics, ACEI, CCBs, BBs.

3. Hypertension with diabetes: diuretics, ACEI, ARBs, CCBs, BBs.

4. Hypertension with chronic renal disease: ACEI, ARBs.

5. Hypertension and recurrent stroke prevention: diuretics, ACEI.

Antidiabetic Agents

Antidiabetic drugs are used for the treatment of diabetes. The main objective is to keep the blood sugar levels closer to normal and prevent complications of diabetes. Other objective is to prevent, slows the progression, and regress the CAS. Hypoglycemic agents prevent,²⁰⁵ slows the progression,^{206 207} and regress^{208 209} CAS. Depending upon the stages and type of diabetes the following antidiabetic agents should be used: Metformin, glipizide, prandin, piaglitazone, DPP-4 inhibitors (Januvia, Merck), GLP-1 receptor antagonist (sulfonylurea), SGLT₂ inhibitors and insulin.

Agents that Reduce the Levels of AGEs and Increase the Levels of sRAGE: Anti-AGEs Therapy

Diet and Cooking

Food should be cooked at low temperature. Cooking at high heat increases the formation of AGEs.²¹⁰ Cooking in oil produces more AGEs than cooking in dry heat. Avoid eating diet with high content of AGEs. It has been reported that bread, cookies, meat, and fish are main contributors of AGEs in the standard diet.²¹¹ Avoid fat consumption. Certain diet has high AGEs content than others, for example, fat and oil.²¹²

Prevention of AGE Formation

The drugs that prevent the formation of AGEs include: aminoguanidine, pyrido-xanthine (natural vitamin 6), benfotiamine (a lipid-soluble derivative of thiamine), aspirin, metformin, candesartan, and orlistat.

AGE Crosslink Breaker

The drugs that break AGE crosslink are capable of breaking α-carbonyl compounds by cleaving the carbon–carbon bond between carbonyls. This drug include alagebrium^{157 212} and reduces the levels of AGE.

Exercise

Regular moderate exercise reduces the serum levels of AGEs than irregular severe one.²¹³

Agents that Increase the Levels of sRAGE/esRAGE

Exercise

Aerobic exercise increases the serum levels of sRAGE in patients with type 2 diabetes.²¹⁴

Pharmacological Agents

1. Antidiabetic agents: Insulin²¹⁵ and rosiglitazone²¹⁶ increase the serum levels of sRAGE and esRAGE.

2. Vitamins: Vitamin D increases the serum levels of sRAGE in women with polycystic ovary syndrome.²¹⁶

3. Statins: Statins increase the serum levels of sRAGE.^{217 218}

4. Angiotensin converting enzyme inhibitors (ACE-I): ACE-I (ramipril) increases the plasma levels of sRAGE and decreases the levels AGEs in diabetic rats.²¹⁹

Anti-C-Reactive Protein

CRP-lowering agents have been described in details by Prasad.^{196 220} The following agents reduce the serum levels of CRP: Celecoxib, clopidogrel. Statins, rosiglitazone, carvedilol, antioxidants (α-tocopherol, vitamin C), ramipril, quinapril, valsartan, candesartan, calcium channel blockers (amlodipine), and combination of hydrochlorothiazide and amlodipine.

Antiplatelet Therapy

Antiplatelet therapy with aspirin, clopidogrel, or ticlopidine should be instituted in the patients with CAS.

Antioxidant Therapy

The antioxidant therapy in patients with CAS has been tried to a limited extent and with variable results. Azen et al²²¹ in a controlled clinical trial have shown that supplementation of vitamin E (≥ 100 IU/d) appears to be effective in reducing the progression of common carotid artery wall IMT in subjects not treated with lipid-lowering drugs. Vitamin C had no effect on the IMT of patients within the drug or placebo group. Kritchevsky et al²²² in the Atherosclerosis Risk in Community Study, measured carotid artery wall thickness in 6,318 female and 4,989 male participants and made a correlation study between carotid artery IMT and intake of dietary and supplemental vitamin C, α-tocopherol, and provitamin A carotenoid. They concluded that there is a limited support for hypothesis that antioxidants protect carotid artery disease, especially in individuals greater than 55 years of age. Devaraj et al²²³ in a randomized controlled double-blind trial, showed that high dose of RRR-α-tocopherol (1,200 IU/d, for 2 years) had no significant effect on the carotid IMT. However, it significantly reduced the biomarkers of oxidative stress. McQuillan et al²²⁴ reported that there was an inverse association between carotid artery IMT and plasma lycopene (an antioxidant) in women, but not in men. They also showed that other antioxidant vitamins A, C, and E, and α- and β- carotene were not associated with carotid artery IMT or focal carotid artery plaques.

However, there are reports that suggest a beneficial effect of antioxidants in carotid artery disease. Karppi et al²²⁵ reported that there was an inverse association between carotid artery IMT and lycopene or α-carotene and β-carotene. This was a population-based study in Finland. Gale et al²²⁶ in a study comprising of 468 men and women aged between 66 and 75 years reported that antioxidants (vitamin E, C, and β-carotene) prevented the progression of CAS. In another study, bilateral duplex ultrasonography revealed carotid artery atherosclerotic regression in 7 and progression in 2 of the 25 tocotrienol (antioxidant) group of patients while none of the control group of patients showed regression, and 10 of 25 patients showed progression.²²⁷ Aviram et al²²⁸ reported that consumption of pomegranate juice resulted in a significant reduction in carotid artery IMT. These data suggest that the antioxidants have variable and limited effects on the slowing of progression and regression of CAS. It is to note that statins that have antioxidant activity, also regress and slow the progression of CAS.^{198 199}

The variable data could be due to small doses used in these studies. Also, the ineffectiveness of vitamin E in slowing the progression of CAS could be due to conversion of α-tocopherol to α-tocopheroxyl radical, which is prooxidant.^{229 230} Vitamin C rapidly reduces α-tocopheroxyl radical to α-tocopherol.²³¹ Considering this vitamin E should be used in combination with vitamin C to have maximum effect and avoid the adverse effects of vitamin E.

Medical Treatment for Asymptomatic Patients with Severe CAS

Carotid endarterectomy (CEA) is generally recommended in asymptomatic patients with CAS of 50 to 60%.²²⁰ Abbott²³² reported that ipsilateral stroke was 1.5% per year in patients treated with CEA in patients with asymptomatic severe CAS and 2.3% for patients with medical treatment. He also reported that medical treatment is three to eight times more cost-effective than surgical treatment. Current data suggest that only 50% of patients with asymptomatic CAS benefit from CEA in this period of advanced medical treatment.²³³

Perspectives

In the section of medical treatment there are not enough clinical trial based data for lowering of AGEs and CRP and elevation of sRAGE. However the drugs, which are used in hypertension, diabetes, and dyslipidemia, lower the serum levels of AGEs and CRP, and raises the levels of sRAGE. Little attention has been focused on the effects of exercise and diet in the management of CAS. As mention earlier exercise raises levels of sRAGE and HDL-C, and lowers the levels of CRP and TC. Exercise regimen should be included in the treatment of CAS. Also, diet with low AGEs and cooking at low temperature should be considered along with the drug treatment for CAS.