**2. Cellular Mechanisms of Atherosclerosis**

Recent studies of the cellular mechanisms of atherosclerosis carried out on cultured human aortic cells have revealed the outlined below regularities.

Modified LDL circulates in the bloodstream. We have discovered modified (desialylated) LDL in blood plasma of patients with coronary atherosclerosis [4-7]. This LDL induces accumula‐ tion of cholesterol in arterial cells [4-7]. Naturally occurring modified LDL has lesser sialic acid, triglyceride and cholesterol contents, lesser particle size, greater density and negative charge, higher aggregative activity and some other specific features [8]. We have discovered an enzyme, trans-sialydase, responsible for desialylation of LDL particle in blood [9].

In addition to desialylated LDL, more electronegative LDL and small dense LDL were de‐ tected in human blood [10,11]. We have performed a comparative study of in vivo modified LDLs. This study showed that more electronegative LDL isolated by ion-exchange chroma‐ tography is desialylated LDL [12]. Desialylated LDL isolated from patient blood [4-7] is more electronegative LDL. These facts suggest that both desialylated LDL and electronega‐ tive LDL are similar if not identical.

We have found that a particle of desialylated LDL is smaller and denser than that of native LDL, i.e., this LDL is small dense lipoprotein. On the other hand, La Belle and Krauss showed that small dense LDL has a low content of sialic acid, i.e., is desialylated [13]. These findings point out to a similarity between the two types of modified LDL.

Glycosylation is another type of in vivo LDL modification. Glycosylated LDL was found in the blood of patients with diabetes mellitus [14]. This LDL is also atherogenic, i.e. induces intracellular lipid accumulation [15]. Oxidation is probably also one type of an atherogenic modification of LDL in vivo. There are indirect evidences of the presence of oxidized LDL in vivo [16].

In epidemiological studies of hypercholesterolemia, a high level of plasma cholesterol and the plasma concentration of LDL are significantly associated with the development of pre‐ mature atherosclerosis [1]. Cholesterol accumulation in the arterial wall is the main sign of atherosclerosis. It was suggested that LDL is the major source of cholesterol deposited in the

Accumulation of cholesterol and other lipids is the most prominent manifestation of athero‐ sclerosis at the arterial cell level. In addition to lipid accumulation, elevated proliferative ac‐ tivity of vascular cells and enhanced synthesis of the extracellular matrix are characteristics of cellular atherogenesis. Collagen and glycoproteins are the main components of the extrac‐

Intracellular lipid accumulation can be induced by LDL; however native lipoprotein does not increase the cholesterol content of the cell [2]. On the other hand, incubation of cell cul‐ ture with chemically modified LDLs results in a massive accumulation of cholesterol in the cells [2]. The in vitro studies revealed a great number of atherogenic modifications of LDL, i.e. modifications which lead to cellular lipidosis [2]. This findings suggest that modified, but not native LDLs are the source of lipids accumulated in arterial cells. Arterial intimal cells populating atherosclerotic lesion are overloaded with lipids, their cytoplasm is almost

Recent studies of the cellular mechanisms of atherosclerosis carried out on cultured human

Modified LDL circulates in the bloodstream. We have discovered modified (desialylated) LDL in blood plasma of patients with coronary atherosclerosis [4-7]. This LDL induces accumula‐ tion of cholesterol in arterial cells [4-7]. Naturally occurring modified LDL has lesser sialic acid, triglyceride and cholesterol contents, lesser particle size, greater density and negative charge, higher aggregative activity and some other specific features [8]. We have discovered

In addition to desialylated LDL, more electronegative LDL and small dense LDL were de‐ tected in human blood [10,11]. We have performed a comparative study of in vivo modified LDLs. This study showed that more electronegative LDL isolated by ion-exchange chroma‐ tography is desialylated LDL [12]. Desialylated LDL isolated from patient blood [4-7] is more electronegative LDL. These facts suggest that both desialylated LDL and electronega‐

We have found that a particle of desialylated LDL is smaller and denser than that of native LDL, i.e., this LDL is small dense lipoprotein. On the other hand, La Belle and Krauss showed that small dense LDL has a low content of sialic acid, i.e., is desialylated [13]. These

findings point out to a similarity between the two types of modified LDL.

an enzyme, trans-sialydase, responsible for desialylation of LDL particle in blood [9].

completely filled with lipid inclusions [3]. These cells are referred to as foam cells.

vessel wall.

188 Current Trends in Atherogenesis

ellular matrix which forms a fibrous plaque.

**2. Cellular Mechanisms of Atherosclerosis**

aortic cells have revealed the outlined below regularities.

tive LDL are similar if not identical.

Autoantibodies are produced in response to the appearance of modified LDL (either desialy‐ lated, glycosylated or oxidized) in the bloodstream [16-18]. Autoantibodies to desialylated LDL react with both modified and, though with a lesser affinity, native lipoproteins [17,19,20]. The interaction between anti-LDL autoantibodies and the lipoprotein results in the formation of LDL-containing immune complexes [12]. Desialylated LDL which enter the cells as a component of immune complexes possess a higher atherogenic potential compared with free lipoprotein, i.e. induce a more intense cholesterol accumulation in the cell [21,22]. The interaction with anti-LDL converts native non-atherogenic LDL into atherogenic, i.e. en‐ ables it to induce intracellular cholesterol accumulation which accompanied by enhanced cell proliferation and the extracellular matrix production [17,20]. We have found circulating immune complexes consisting of LDL and anti-LDL autoantibodies in the blood of most atherosclerotic patients [21,22]. A positive correlation between level of LDL-containing im‐ mune complexes and the severity of atherosclerosis has been demonstrated [23-25].

We and others have demonstrated that LDL is able to form complexes with cellular debris, collagen, elastin, and proteoglycans of human aortic intima [26-28]. Addition of these com‐ plexes to cultured cells stimulated intracellular accumulation of lipids. Experiments with io‐ dinated LDL showed an increased uptake and decreased intracellular degradation of lipoproteins in complexes.

In 1989 we showed that in vivo and in vitro modified LDLs are spontaneously self-associat‐ ed under cell culture conditions, while native LDLs do not forms self-associates [29]. A posi‐ tive correlation between atherogenic activity of modified LDLs and the degree of LDL selfassociation has been established [30,31]. Lipoprotein associates isolated by gel filtration induced a dramatic increase in the lipid accumulation by cultured human aortic intimal cells. Removal of LDL associates from the incubation medium by filtration through filter with pore diameter 0.1 µm completely prevented intracellular lipid accumulation. Thus, self-association increases atherogenic potential of LDL.

Thus, we can conclude that formation of large complexes (self-associates, immune com‐ plexes, and complexes with connective tissue matrix) by modified LDL leads to intracellular lipid accumulation through enhanced cellular uptake and slow intracellular degradation of lipoprotein particles.
