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Cholesterol metabolism and atherosclerosis

Institution: University of California, San Diego
Investigator(s): Oswald Quehenberger, Ph.D.
Award Cycle: 1997 (Cycle 6) Grant #: 6IT-0133 Award: $74,500
Subject Area: Cardiovascular Disease
Award Type: Inno Dev & Exp Awards (IDEAS)
Abstracts

Initial Award Abstract
Atherosclerosis-the thickening of the arterial wall-and its consequences such as coronary heart disease and stroke, are the leading cause of death in western societies. Epidemiological studies have identified high serum cholesterol levels and exposure to cigarette smoke as major risk factors of atherosclerosis. Although the exact mechanisms remain unclear, there is evidence that cigarette smoke damages low density lipoproteins, the carrier of cholesterol in the blood, and increases the adhesion of white blood cells to the endothelium, the innermost layer of blood vessels. This initial adhesion is followed by the penetration of the adhering blood cells into deeper layers of the vessel wall. Among the cells that accumulate there, the monocytes are particularly important, because they take up the damaged lipoproteins. During this process they are transformed into cholesterol-loaded macrophages. This contributes substantially to the thickening of the arterial wall, obstructing blood flow.

We propose to examine the pathways by which macrophages recognize, take up and metabolize damaged low density lipoproteins. We have previously demonstrated that an enzyme, acylcoenzyme A:cholesterol acyltransferase, which modifies cholesterol in such a way that it can be stored by the cells, might also be important for the initial recognition of damaged lipoproteins by macrophages. Further, we also found that this enzyme is mainly expressed at sites of the arterial wall that also show clear signs of atherosclerosis, suggesting that this enzyme is involved in this process. To test this, we will examine whether acylcoenzyme A:cholesterol acyltransferase is on the surface of macrophages, taking advantage of an antibody we have developed, that specifically binds to this protein. By suitable genetic manipulation we will then induce the synthesis of acylcoenzyme A:chole-sterol acyltransferase in cells that do not normally make it and subsequently test if these cells can more effectively bind damaged lipoproteins. Finally, we will determine the type of cells of the arterial wall that synthesize acylcoenzyme A:cholesterol acyltransferase. This will be done by immunological techniques on aortas isolated from mice made susceptible to atherosclerosis by genetic engineering.

Exposure to cigarette smoke accelerates the atherogenic process, substantially increasing the risk of premature coronary heart disease. A precise understanding of the underlying mechanisms is essential to develop novel and effective therapeutic strategies for intervention and prevention smoke-related cardiovascular disorders.

Final Report
Atherosclerosis and related pathological disorders such as coronary heart disease are still the major causes of death in westefn societies. The development of this disease is a multifactorial process and a substantial body of data indicates that smoking contributes to atherogenesis by a number of mechanisms. Histologically, atherosclerotic lesions are characterized by the proliferation of cells of the arterial wall, accumulation of white blood cells and deposition of lipids. Although the exact mechanisms remain unclear, there is evidence that cigarette smoke induces damage to low density lipoproteins, the transport vehicle of cholesterol, and increases the adhesion of white blood cells to the endothelium which represent the innermost layer of blood vessels. Following cellular signals, the adhering blood cells then penetrate into deeper layers of the vessel wall. Among the cells that accumulate there, the so-called Imonocytes are particularly important, because they take up the damaged lipoproteins. During this process they are transformed into cholesterol-loaded macrophages. This contributes substantially to the thickening of the arterial wall, obstructing the blood-flow. Therefore, a precise understanding of the pathways by which macrophages recognize, take up and metabolize damaged low density lipoproteins is essential for the development of effective therapeutic means.

Acyl-CoA:cholesterol acyltransferase, which modifies cholesterol in such a way that it can be stored in cells, plays a key role in the formation of cholesterol-loaded macrophages/foam cells. A major effort of this study is to demonstrate the expression of the enzyme in atherosclerotic lesions and to study potential mechanisms of enzyme activation. Using genetically engineered mice that develop atherosclerosis when put on high fat diet, we were able to demonstrate that the expression of acylCoA:cholesterol acyltransferase is several-fold higher in lesions compared to non-lesion areas of the aorta. Histochemical staining experiments of atherosclerotic lesions showed that macrophages were the principal cell type that expressed high levels of acyl-CoA:cholesterol acyltransferase protein.

To further explore the regulation of acyl-CoA:cholesterol acyltransferase expression in atherosclerosis, we used a cell culture system. Isolated macrophages were loaded with cholesterol by incubation with modified lipoprotein under conditions that resemble a lesion. This treatment increased acyl-CoA:cholesterol acyltransferase enzyme activity several-fold. In addition, cholesterol induced the enzyme to aggregate, which was apparently necessary for enzyme activation and we hypothesize that the active enzyme consists of 4 subunits.

Exposure to cigarette smoke substantially accelerates the atherogenic process by promoting modification of lipoproteins that then become the source for the cholesterol deposited in lesions. A precise understanding of the underlying mechanisms is essential to develop novel therapeutic options.
Publications

Chemokine receptor CCR2 expression and monocyte chemoattractant protein-1-mediated chemotaxis in human monocytes
Periodical: Arteriosclerosis, Thrombosis, and Vascular Biology Index Medicus:
Authors: Han KH, Tangirala RK, Green SR, Quehenberger O ART
Yr: 1998 Vol: 18 Nbr: Abs: Pg: 1983-1991