GUJHS. 2006 March; Vol. 3, No. 1
Karah Salaets, Jennifer Schliessman, Robin Speiser,
Anh-Minh Tran, Emily Wang, Dr. Alan Angerio
Abstract: Endothelin-1 (ET-1) is a 21 amino acid complex that is released from vascular endothelial cells, smooth muscle cells in the blood vessels, and macrophages. ET-1 is normally inactive, but can become active if stimulated by angiotensin II (Ang II), thrombin, and transforming growth factor beta. Through conducting a literature review, we have determined that ET-1 plays a significant role in contributing to the pathogenicity of an atherosclerotic plaque by acting as a growth factor. More specifically, research indicates that ET-1 acts as a growth factor through up regulating macrophages, fibroblasts, and smooth muscle cells. An understanding of ET-1’s role in atherosclerosis may help widen the range of possible remedies beyond the traditional treatments, bypass surgery and drug administration for resulting diseases, such as coronary artery disease.
Key words: Endothelin, Endothelin-1, Coronary Artery Disease, Atherosclerosis, Angiotensin II, Vascular Remodeling, Macrophage, Smooth Muscle Cells, Fibroblasts, Low-Density Lipoprotein Cholesterol.
Endothelins, a group of polypeptides, assist in vasomotor activity, including transitory vasodilation and sustained vasoconstriction, proliferation of smooth muscle cells, and hormone production. There are three endothelins: endothelin-1 (ET-1), endothelin-2 (ET-2), and endothelin-3 (ET-3). Each bind with three types of endothelin receptors: endothelin A (ETA), endothelin B (ETB), and endothelin C (ETC). ETAreceptors are found in vascular smooth muscle tissue and cause vasoconstriction. ETB are located in the lining of the blood vessel wall and are associated with the release of vasodilators nitric oxide and prostacyclin. The role of ETC is still being researched (Luscher and Wenzel, 1995).
Endothelin-1 (ET-1) contains 21 amino acids and is released from vascular endothelial cells, smooth muscle cells in the blood vessels, and macrophages. Under normal conditions, ET-1 is not activated. However, the production of ET-1 can be stimulated by angiotensin II (Ang II), thrombin, and transforming growth factor beta. Ang II is a protein that causes blood vessels to constrict, which increases resistance and the arterial pressure. Its formation is regulated by the circulation of angiotensinogen and renin in the bloodstream. Angiotensinogen is produced by the liver, and renin is produced by the kidney’s juxtaglomerular cells when the body is under stress or in a state of low blood pressure. Renin cleaves off a portion of angiotensinogen, transforming it into angiotensin I. Next, angiotensin converting enzyme (ACE), which is found in many parts of the body, especially the lungs, converts the circulating angiotensin I to angiotensin II. Ang II is a potent vasoconstrictor that also has renal effects. Thrombin is a major component in the extrinsic and intrinsic clotting cascades (D’Uscio, Barton, Shaw, and Luscher, 2000). The production of ET-1 is also marked by an increase in intracellular Ca2+ (Luscher and Wenzel, 1995).
ET-1 is an important peptide because of its ability to initiate vasoconstriction in the blood vessels. Although endothelins have exhibited vasodilatory effects, the most notable effects of ET-1 are its long term vasoconstriction of blood vessels. Other capabilities of ET-1 include increasing expression of endothelial adhesion molecules, acting as chemotactic agent for monocytes, and stimulating migration and proliferation of vascular smooth muscle cells (Fan, Unoki, Iwasa, and Watanabe, 2000). These various abilities of ET-1 have initiated research on the role of ET-1 in atherosclerosis, specifically coronary artery disease.
Atherosclerosis begins with an accumulation of macrophages and lipids on the endothelial surface of an arterial wall. This fatty streak often develops without noticeable symptoms or warnings. After this long asymptomatic phase, the buildup evolves into a mature fibro-fatty plaque (Davies, Rudd, and Weissberg, 2004). Low-density-lipoproteins (LDL cholesterol) attach to the surface of the endothelial lining, which then increases its permeability to lipoproteins. Mediators of this increased permeability include nitric oxide, platelet derived growth factor, angiotensin II, and ET-1. LDL buildup along the surface of the endothelium causes damage to the lining. Platelets, specialized proteins that circulate in plasma to assist in healing damaged tissue, attach themselves to the blood vessel lining in an attempt to seal the injury (Ross, 1999). Atherosclerotic plaques have a characteristic central lipid core and an endothelial fibrous cap covering. The cap is comprised of vascular smooth muscle cells, macrophages, and connective tissue, especially collagen (Davies, Rudd, and Weissberg, 2004). Cap thickness depends on the main contents of the plaque. Unstable plaques have a higher macrophage-to-smooth muscle cell ratio, have a lipid-filled necrotic core, and have a thinner cap. Unstable plaques are more vulnerable than stable plaques, easily rupturing and eroding (Ross, 1999). The following figure displays the formation of a fibrous plaque.
Over time the plaque grows in size, causing the vessel to compensate by stretch so that blood flow can continue unobstructed. This expansion is called positive remodeling. Unfortunately, positive remodeling goes unnoticed until the blood vessel can no longer expand, compromising blood flow. The plaque buildup is recognized only when tissues are completely deprived of blood flow. The absence of symptoms and warning signs makes atherosclerosis an especially hazardous health risk. Buildup of atherosclerotic plaques can also be noticed if the plaque erodes or ruptures. Underlying collagen and lipids become exposed, triggering the activation of platelets in the blood. This activation, through clotting factors VII and XI, causes the production of thrombin, fibrinogen, and fibrin, and eventually a thrombus of platelets and fibrin is formed. This additional clotting escalates the already dangerous situation and can cause immediate health effects, such as coronary artery disease. Fatty accumulation in coronary arteries can form plaques, leading to the development of coronary artery disease (Ross, 1999). Increased levels of ET-1 have been found in patients with risk factors for plaque development, as well as in atherosclerotic lesions themselves. These correlations indicate a link between ET-1, atherosclerosis, and coronary artery disease (Luscher and Wenzel, 1995).
The presence of ET-1 varies depending on the type of hypertension involved and vascular area studied. Although several studies have indicated a reduced response of ET-1 in hypertension, others have discovered contractions due to ET-1 in the renal circulation of spontaneously hypertensive rats. Experiments demonstrate that when levels of Ang II in these rats are increased, an increase in the production of ET-1 follows. In Angiotensin II-induced hypertension, higher levels of ET-1 are seen in conduit, small resistance arteries and kidneys. They have not been found, however, in ventricular myocardium. Because the plasma levels of ET-1 remain unchanged, impaired clearance of ET-1 does not affect local ET-1 concentrations. This suggests that the effects of ET-1 are carried out through paracrine action. In salt-induced hypertension, endothelial nitric oxide synthase activity is reduced. This may cause an increased production of ET-1, thus possibly contributing to hypertrophic remodeling. This production of ET-1 locally may therefore alter the vessels (Luscher and Wenzel, 1995).
Hypercholesterolemia presents with endothelial dysfunction, an imbalance between vasodilators and vasoconstrictors. It also affects ET-1, increasing the adhesion of platelets and monocytes, as well as the migration and creation of vascular smooth muscle cells. Oxidized LDL cholesterol adds to this development by decreasing the vasoconstrictors present and by increasing the production of preproendothelin-1 mRNA. This increased transcription triggers the release of ET-1 from endothelial cells. The ET-1 present may then cause migration of monocytes to the area through chemotaxis, cell proliferation, and movement. Both of these characteristics of ET-1 thus advance the lesion, which then attracts macrophages. These macrophages then take lipids and transform them into the foam cells that compose the fatty streak. The foam cells become the source of ET-1. This cycle causes the lesion to worsen, leading to atherosclerosis (Fan, Unoki, Iwasa, and Watanabe, 2000).
As mentioned above, atherosclerotic plaques are made up of many different cell types, such as macrophages, smooth muscle cells, and fibroblasts. Macrophages are phagocytic cells important to tissue homeostasis. Developing from circulating monocytes which originate in bone marrow, macrophages migrate to a site after receiving constitutive or inflammatory signals. Though macrophage functions are diverse, one very important role involves tissue remodeling during development and healing. They attract other cells important in wound repair, such as fibroblasts and smooth muscle cells, and secrete cytokines, growth factors, and proteases necessary for cell matrix remodeling (Lucas and Greaves, 2001).
Macrophage concentration at sites of atherosclerosis is high because of their role in wound-healing and immunity. Stimulated endothelial cells secrete macrophage colony-stimulating factor (MCSF) which increases the incidence of macrophages. Various factors, including ET-1, attract macrophages to a plaque site. Here, Angiotensin II, acting through the type 1 angiotensin receptor, causes macrophages and circulating platelets to adhere to the intima of the arteries. This adhesion, seen in figure 3, contributes to the size of the plaque (Wenzel, Siffert, Bruck, Philipp, and Schagers, 2002).
Another constituent of atherosclerotic lesions are fibroblasts. Fibroblasts are connective tissue cells, which are the main producers of cellular matrix. Similar to macrophages, they are involved in the body’s inflammatory response, including wound repair and tissue remodeling. In atherosclerotic damage, fibroblasts are called to the site by macrophages and other chemical signals (Solini, Santini, Ferrannini, 2005). It has been discovered that ET-1, which has mitogenic properties, stimulates fibroblast proliferation with the aid of intracellular reactive oxygen species and mitogen-activated protein kinases (Cheng, Liu, Shih, et.al., 2003).
ET-1 also causes the proliferation of smooth muscle cells and various proteins along blood vessels. These cause plaque build up which could lead to atherosclerosis (Touyz and Schiffrin, 2003). ET-1 stimulates the ETA receptors on the blood vessel’s smooth muscle cells, causing vasoconstriction. (Luscher and Wenzel, 1995; Goto and Miyauchi, 2003). This causes the smooth muscle to contract, constricting the blood vessel.
It is seen that ET-1 has mitogenic properties and acts as a growth factor (Sihvola, Pulkkinin, Koskinen, and Lemstrom, 2002). As mentioned above, LDLs are able to attach to the surface of the endothelial lining, and the endothelial lining increases its permeability to lipoproteins. LDL migration occurs in the deeper areas of the blood vessel, until the lipoproteins come into contact with the smooth muscle lining of the blood vessel (Ross, 1999).
As blood viscosity increases with the large accumulation of fats, cholesterols, and low-density lipoproteins, the blood pressure must increase in order to move blood more forcefully through the narrower passageway. The endothelium then releases ET-1, causing smooth muscle contraction via ETA receptor interaction. The ET-1 can also be released by smooth muscles themselves. The constant contraction and stimulation of the smooth muscle and ET-1 secretion allows the fat molecules and the smooth muscle cells to migrate together to form foam-cells. Platelets, whose purpose was to heal the injured tissue, become part of the foam-cells as well. Also, fibroblasts, macrophages, and other white blood cells that have made an attempt to eliminate the lipoproteins from the area become part of the foam-cell accumulation (Timm, Kaski, and Dashwood, 1995). Figure 4 illustrates the process of accumulation of foam cells.
The foam-cells may rupture as a result of macrophages releasing metalloproteinase and other enzymes into the area. The rupture causes plaque to chip off and be released into the blood. Plaque fragments may block blood vessels. Such a blockage occurring in a coronary artery can result in a heart attack.
Treatment of atherosclerosis is imperative. If left untreated, the atherosclerotic process can lead to very serious health conditions, such as coronary artery disease, a heart attack, or stroke. Various treatments have been developed to control atherosclerosis. Medication, surgery, angioplasty, exercise, diet, vitamin therapy, and refraining from smoking are all possible treatments for this deadly disease.
Medications are often taken to control atherosclerotic symptoms. Pain relievers, blood thinners, and medications are administered to enlarge or dilate the affected arteries. The most common of these medications are nitrates, which act through the effects of nitric oxide. Such medications include nitroglycerine, isosorbide, and comparable agents. There are also long-acting oral agents or skin patches that release small amounts of medication through the skin and into the bloodstream continuously over the course of a day. These drugs function through dilating the coronary arteries, which in turn decreases the venous return to the heart. Nitrates are the foundation of medical treatment for angina. They are used for both the management of symptoms and the treatment of the illness when the patient presents warning signs (Angleworth, 2004). There are many unpleasant side effects associated with these drugs, such as headaches, dizziness, tachycardia and postural hypotension.
Beta blockers are another effective medicine type. They block the activity of the beta-receptors in the nervous system, causing blood pressure elevation, tachycardia, and powerful heart contractions. Blocking these effects decreases the metabolic load on the heart, so angina or even the degree of a heart attack may be reduced. The side effects associated with these drugs are most commonly gastro-intestinal disturbances and heart failure. Heart failure results because, although resting overactivation of beta receptors can be detrimental, the beta receptors do have the ability to inotropically (in terms of heart contraction and stroke volume) and chronotropically (in terms of heart rate) maintain heart function under stress (Johnson, 2005).
It is widely accepted that high blood cholesterol increases the danger of developing atherosclerosis. Although some may believe otherwise, the cholesterol is not the caustic mechanism. Determining the comparative relationship between HDL cholesterol and LDL cholesterol is essential to correctly evaluating atherosclerotic risk. Although the correlation is not completely clear, it reflects the body’s ability to reduce free radical damage to the internal walls of the artery (Johnson, 2005). Recent research shows that free radical damage to the artery walls sets off a natural repair progression that results in the patching and increasing of calcium and cholesterol deposits. Therefore, although not highly discussed as of yet, controlling free radicals is a very fascinating and progressive idea for preventing cardiovascular disease. Cholesterol-lowering drugs are often prescribed when cholesterol levels remain high. The most commonly prescribed drugs to lower LDL are known as the statin drugs: lovastatin (Mevacor), pravastatin (Pravacho) simvastatin (Zocor), and atorvastatin (Lipitor). Bile acid drug sequestrants are another class of drugs prescribed for improper LDL levels (Singer, 2000).
Many believe that plaque growth can be stopped and even reversed by a routine of vitamin B intake, such as B6, B12 and folate. This vitamin B therapy notably lessens the amount of a toxic amino acid, homocysteine, circulating in the blood. Homocysteine, a derivative of digesting protein, has been shown to harm the inside of blood vessels, causing the arteries and plaques to harden which can lead to heart disease. This study concluded that B6, B12, and folate lowered the level of homocysteine to normal levels, regressed arterial plaque, and restored the ability of blood vessels to dilate. It is important to note that homocysteine is just as dangerous in causing heart disease as cholesterol. Recent clinical trials using Policosanol, derived from sugar cane, demonstrated similar clinical improvements in measuring cholesterol compared with the statin drugs Mevacor and Zocor. It was found that Policosanol is as effective as 100 mg of aspirin per day in opposing platelet aggregation. It was also found that it lowered blood pressure, providing an added advantage for high risk coronary patients. Aspirin functions as a blood thinner, thereby lessening pressure on the heart (Singer, 2000).
Vasodilators are medications that expand the blood vessels, decreasing resistance, and, therefore, lowering blood pressure. Vasodilators are prescribed to treat hypertension, as well as to relieve symptoms related to congestive heart failure. Doctors recommend healthy lifestyle changes for people watching their blood pressure, such as eating a nutritious diet, reducing stress, and getting regular exercise. In addition, patients are expected to avoid excessive alcohol and tobacco consumption, which constrict the arteries, decrease the ability of blood to carry oxygen, and increase the risk of forming clots (Singer, 2000).
Surgery is another option, though only performed on severe cases. One type of surgery is grafting, which involves repairing or replacing a vessel. Undergoing an endarterectomy, where the lining of an artery is removed, is also an alternative. The most common surgery, however, is bypass surgery. Heart bypass surgery forms an alternative route around the blocked segment of the coronary artery in order to re-establish blood supply to the heart tissue (Angleworth, 2004). Coronary bypass surgery has recently been performed with the assistance of a robot, which permits the surgeon to perform the operation without being in direct contact with the patient. Bypass grafts made of synthetic material are inserted in place of the blocked segments or the area may be surgically dilated. If the blockage is in several smaller vessels then surgery is not appropriate. A procedure called “balloon dilation” is sometimes suggested if there is one severe. This is when a tube is placed into the artery under the direction of an x-ray and is inflated to alleviate the obstruction (Johnson, 2005).
Sometimes, blood-thinning medications are prescribed to stop the formation of a blood clot. In most cases a stent is also placed at the spot of the narrowing or blockage to keep the artery open. A stent can be used during a coronary artery bypass graft surgery to keep the grafted vessel open, and after balloon angioplasty to prevent the reclosure of the blood vessel (Johnson, 2005).
There are also a group of drugs called calcium channel blockers. Calcium channels pertain to the region of heart membranes and other cells where calcium streams in and out. The calcium reacts with other chemicals to adjust the rate and force of contractions. In the heart, these drugs can lessen the force and rate of contractions and electrical excitability, ultimately having a calming effect on the heart (Johnson, 2005).
Studies of ET-1 antagonists have been used to examine the effects of ET-1 in hypertension and atherosclerosis. ETA antagonist increased the amount of nitric oxide present, lowered the arterial pressure rise, and prevented hypertrophic remodeling and endothelial dysfunction. The blockade of these receptors also reduced the formation of the fatty streak, inhibited the formation vascular lesions, and normalized nitric oxide-mediated endothelium-dependent relaxations. This information contributes to the connection between ET-1 and the occurrence of atherosclerosis (Luscher and Wenzel, 1995). It is clear, therefore, that through the up-regulation of macrophages, smooth muscle cells, and fibroblasts, ET-1 contributes to the pathogenicity of an atherosclerotic plaque. Such research about the role of ET-1 in atherosclerosis, as well as the studied positive effect of ET-1 antagonists in hypertension and atherosclerosis, may lead to new remedies and prevention methods for coronary artery disease.
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[Fig. 1] ET-1’s role in atherosclerosis. This flow chart describes the general abilities of ET-1 and its result, inflammation, which is thought to lead to atherosclerosis.
[Fig. 2] Formation of fibrous plaques. Atherosclerotic plaques have a characteristic central lipid core and an endothelial fibrous cap covering. This illustration shows several factors that cause the build up of a plaque. National Institute on Aging. The BBID-Biological Biochemical Image Database page. Available at: http://bbid.grc.nia.nih.gov/geneimages/134.Atherosclerosis-Fibrous_plaques.jpeg. Accessed March 22, 2005.
[Fig. 3] Platelet adhesion in an atherosclerotic vessel. Platelets are one of the many components found in plaques. They adhere to the intima of arteries and cause a blocking of the vessel. The Stroke Center at Barnes-Jewish Hospital and Washington University School of Medicine. The Internet Stroke Center at Washington University page. Available at:http://www.strokecenter.org/education/ais_pathogenesis/images/platelet_adhesion.jpg. Accessed March 22, 2005.
[Fig. 4] Growth of a plaque. Monocytes, macrophages, and smooth muscle cells all become foam cells which then results in a fatty streak, as illustrated. The aggregation of foam cells narrows the blood vessel and can eventually lead to atherosclerosis. University of Washington. Professor Karen E. Petersen’s Home page. Available at:http://faculty.washington.edu/kepeter/118/photos/atherosclerosis_damage.jpg. Accessed March 22, 2005.