Dieter M. Kramsch

The Protein and Lipid Composition of Arterial Elastin and Its Relationship to Lipid Accumulation in the Atherosclerotic Plaque

Elastin preparations from intimal layers and the media of normal and atherosclerotic human aortae were analyzed for protein and lipid content. In atherosclerotic aortae, elastin from plaques was compared with elastin from adjacent normal appearing areas of the same aorta. Arterial elastin purified by alkaline extraction appeared to be a protein-lipid complex containing free and ester cholesterol, phospholipids, and triglycerides. The lipid component of normal arterial elastin was small (1-2%). With increasing severity of atherosclerosis, there was a progressive accumulation of lipid in intimal elastin from plaques, reaching a mean lipid content of 37% in severe plaques. The increase in the lipid content of plaque elastic preparations was mainly due to large increases in cholesterol, over 80% of which was cholesteryl ester. This deposition of cholesterol in plaque elastin accounted for 20-34% of the total cholesterol content of the plaque. The increased lipid deposition in plaque elastin was associated with alterations in the amino acid composition of plaque elastin. In elastin from plaque intima, the following polar amino acids were increased significantly: aspartic acid, threonine, serine, glutamic acid, lysine, histidine, and arginine; whereas, cross-linking amino acids: desmosine, isodesmosine, and lysinonorleucine were decreased significantly. The amino acid and lipid composition of elastin from normal appearing aortic areas was comparable to that of normal arterial elastin except for intimal elastin directly adjacent to and medial elastin directly below the most severe plaques.The data indicate that the focal lipid deposition in early atherosclerotic plaques is due to a large extent to lipid accumulations in altered elastin protein of localized intimal areas. Continued lipid deposition in altered elastin appears to contribute substantially to the progressive lipid accumulation in the plaque. The study suggests that elastin of intimal elastic membranes may play an important role in the pathogenesis and progression of atherosclerosis.

The Interaction of Serum and Arterial Lipoproteins with Elastin of the Arterial Intima and Its Role in the Lipid Accumulation in Atherosclerotic Plaques

Arterial elastin appears to be a proteinlipid complex with the lipid component being bound to elastin peptide groups. In atherosclerotic lesions the lipid content of elastin increases progressively with increasing severity of atherosclerosis. The increases in the lipid content of plaque elastin are mainly due to large increases in cholesterol with about 80% of the cholesterol being cholesterol ester. This deposition of cholesterol in elastin accounts for a substantial part of the total cholesterol accumulation in atherosclerotic lesions of all stages. The present in vitro study suggests that the mechanism involved in the deposition of lipids in arterial elastin may be an interaction of the elastin protein with serum or arterial low density or very low density lipoproteins (LDL and VLDL) resulting in a transfer of lipids, but not of lipoprotein protein to the elastin. No significant lipid transfer occurred from the high density lipoproteins or chylomicrons. The amount of lipid taken up by plaque elastin was strikingly higher than by normal elastin and consisted mainly of cholesterol with over 80% of the cholesterol being cholesterol ester. The precondition for the lipid accumulation in plaque elastin appeared to be an altered amino acid composition of the elastin protein consisting of an increase in polar amino acids and a reduction in cross-linking amino acids. Subsequent treatment of lipoprotein-incubated arterial elastin with hot alkali and apolipoproteins did not reverse the binding of lipoprotein lipid to diseased elastin.

Occlusive atherosclerotic disease of the coronary arteries in monkey (Macaca irus) induced by diet

Abstract The M. irus is a monkey species that is highly susceptible to experimental atheroslerosis induced by feeding a diet moderately high in cholesterol content. The dietary-induced atherosclerosis is associated with moderate serum cholesterol elevations without concomitant elevations of the serum triglycerides. All monkeys on the atherogenic diet that had serum cholesterol levels that rose above 350 mg/100 ml, developed severe atherosclerosis of the proximal segment of the coronary arteries with occlusions of the coronary arterial lumen of more than 50%. Two monkeys, which died suddenly on exertion after 12 months on the atherogenic diet, had coronary artery occlusions of more than 75%. One of these animals showed histological evidence suggestive of myocardial ischemia. After 12 months on the atherogenic diet, only the coronary arteries revealed gross atherosclerosis; after 18 months the aorta and other arteries also showed gross atherosclerotic lesions, but always to a lesser extent than the coronary arteries. The predict able early involvement of the coronary arteries with severe occlusive atherosclerotic disease, and the consistent predominance of coronary atherosclerotic lesions over lesions in other arteries are unusual features of this monkey species. The M. irus monkey lends itself well as an experimental model for the study of coronary atherosclerosis and its sequelae.

Components of the Protein-Lipid Complex of Arterial Elastin: Their Role in the Retention of Lipid in Atherosclerotic Lesions

Atherosclerosis is associated with accumulations of lipids, especially ester cholesterol, in focal areas of the arterial intima and frequently in the subintimal media below plaques. The lipid accumulations commonly occur intracellularly as well as extracellularly. As many morphological studies have shown, the increased amounts of intracellular lipids are deposited mainly in proliferated and modified smooth muscle cells (Constantinides, 1965; Scott et al., 1967; Geer et al., 1968; Daoud et al., 1968). The extracellular lipids are located at the necrotic core of lesions or are associated with connective tissue which frequently has proliferated. A common finding is reduplication (or splitting) and fragmentation of the arterial elastic lamellae of the intima and subintima of lesions. These altered elastic lamellae appear to play an important role in the retention of stainable lipids in atherosclerotic lesions of all stages (Zugibe and Brown, 1960; McGill et al., 1960; Parker, 1960; Adams and Tuqan, 1961; Friedman, 1963; Lindsay and Chaikoff, 1965; Smith et al., 1967; Kramsch and Hollander, 1968).

Calcium Antagonists and Atherosclerosis

The clinically important lesion of human atherosclerosis is the fibrous atheromatous plaque which often is calcified.1 The connective tissue changes of fibrous lesions begin to develop in the second decade of life but it usually takes decades of slow growth before they produce clinical symptoms. However, it is these connective tissue changes which threaten health and life by seriously impairing arterial function through: luminal stenoses by the raised fibrous intima, through loss of elasticity by elastica destruction and collagen accumulation, as well as through increased rigidity and brittleness by calcium mineralization. These impairments of arterial function in turn lead to insufficiency of blood flow, thrombosis, hypertension, aneurysms and vessel rupture.

Prevention of Arterial Calcium Deposition with Diphosphonates and Calcium Entry Blockers

The arterial disease most commonly associated with an increase in arterial calcium is atherosclerosis. In the past, the deposition of calcium in atherosclerotic lesions has been considered to be an end-stage of advanced atheromata formation only [1]. While such end-stage calcification of plaques certainly is one of the prominent features of late atherosclerosis, it has long been recognized that discrete calcium mineral deposition may occur, especially on intimo-medial elastica, in otherwise normal-appearing arteries [2] and, therefore, may be considered an early event in atherogenesis. In fact, in recent years there has been mounting evidence that localized increases in arterial ionic calcium may play a key role in early atherogenesis, stimulating cellular functions of arterial smooth muscle cells and macrophages such as increased cell migration, mitosis, endocytosis of lipoproteins, as well as excessive synthesis and/or secretion of connective tissue macromolecules [3,4]. Focal increases in arterial calcium content also have been implicated in the early pathobiochemistry of the intercellular matrix molecules. Calcium ions are required for the complexing of low- density (LDL) and very-low-density lipoproteins (VLDL) to sulfated glycosaminoglycans. Calcium-dependent mechanisms may be responsible for the excessive degradation of arterial connective tissue such as the activation of elastolysis by macrophage elastase [5] or the release of collagenolytic, elastolytic, and mucolytic enzymes from platelets [6].

Selected Papers on Pathogenesis of Atherosclerosis, Including Thrombosis

A number of epidemiological and clinical studies have recently shown a considerable increase in incidence of occlusive arterial diseases in smokers, most conspicuously among young individuals. A central role of tobacco smoking in the development of arteriosclerotic cardiovascular disorders has long been suspected, but the basic pathogenetic mechanisms responsible for such an atherogenic effect of tobacco smoke remains to be elucidated. For several years attention was focused on the action of nicotine and other alkaloids on the cardiovascular system; but it is important to stress that it has never been possible through the action of such substances to produce lesions in the vasculature which can be confidently accepted as atherosclerotic in nature.

Metabolism and Distribution of Intravenously Administered d-Aldosterone-l,2-H3 in the Arteries, Kidneys, and Heart of Dog

The initial disappearance of intravenously administered H3-d-aldosterone from the plasma was associated with a rapid uptake of the labeled aldosterone in the tissues. The subsequent, slower fall off of plasma aldosterone radioactivity appeared to parallel the biological decay of H3-aldosterone in the tissues. H3-d-aldosterone attained a maximal level in the kidney, liver, heart, and aorta of dog in five to 15 minutes after the aldosterone injection. During and after the time of peak tissue radioactivity, H3-aldosterone radioactivity was higher in the kidney, liver, heart, and thoracic aorta than in the plasma. No consistent relationship was observed between the H3-aldosterone uptake and the salt, water, and acid mucopolysaccharide content of different portions of the aorta. Subcellular fractionation of the tissues indicated that most of the H3-aldosterone taken up by the tissues was present in the supernatant fraction and not bound to subcellular particles. Radioautography revealed a high concentration of H3-aldosterone along the cell walls of the convoluted tubular cells and along the walls of the glomerular capillaries, arterioles, and Bowmans capsule. In the heart H3-aldosterone localized along the cell walls of the heart muscle cells, whereas in the aorta the hormone appeared to accumulate over the cytoplasm of the smooth muscle cells. The uptake and distribution of H3-aldosterone in the tissues were not altered by pretreatment with the aldosterone antagonist, spironolactone.

Some General Considerations on Vascular Reactivity In Vitro Effects of Vasopressor Agents on the Metabolism of the Vascular Wall

Norepinephrine and epinephrine in pharmacological doses significantly inhibited the synthesis of acid mucopolysaccharides and lipids in incubated dog arterial tissue, whereas angiotensin had no detectable effect on these metabolic functions. Although these findings suggested different metabolic effects of the catecholamines and angiotensin, differences in rates of inactivation of these agents could not be excluded as a cause for their different actions. The inactivation of angiotensin by incubated dog arterial tissue appeared to be over a hundred times more rapid than that of the catecholamines. The differences in the in vitro and the in vivo inactivationof the catecholamines are most likely related to the inability of the denervated tissue to bind, and hence inactivate, the amines. Tissue binding, rather than enzyme degradation of catecholamines, appears to be the major mechanism responsible for the rapid inactivation of the pharmacological activity of the catecholamines. It is suggested that the changes in the reactivity of the blood pressure to cold, norepinephrine, chlorothiazide, and reserpine are related to the binding of catecholamines in the tissues. It is also postulated that one of the basic disturbances in essential hypertension might be an accumulation of pharmacologically active norepinephrine in the arterioles due to insufficient binding of the amine in the tissues.