**13. Mechanisms of cellular lipidosis**

Bearing in mind that the "splash" of proliferative activity and collagen production occur in lipid-rich lesions, it can be suggested that there is a relationship between the main athero‐ sclerotic manifestations: lipid accumulation (lipidosis), proliferation, and secretion of the extracellular matrix (fibrosis). Since the proliferative activity and the collagen-synthetic activity are the highest in the zones of lipidosis, it was important to find out how the accu‐ mulation of lipids in cells (cellular lipidosis) is related to other manifestations of atheroscle‐ rosis, namely, proliferation and fibrosis. To investigate this relationship, lipid accumulation was induced in cultured cells with maximum adequacy to cellular lipidosis occurring in the vascular wall [77, 78].

Disturbances in cell metabolism of lipids may be one of the causes of intracellular lipid accumulation. In the cells cultured from atherosclerotic lesions, the rate of the synthesis of main classes of lipids is higher than that in cells cultured from uninvolved human aortic intima [79]. The rate of lipid synthesis is directly correlated with the intracellular lipid content: the higher intracellular excess of fat, the higher the intensity of lipid metabolism in atherosclerotic cells [79]. However, these findings provide no answer to the following question: what is the cause of lipid metabolism dysfunction in the vascular wall that led to excessive lipid accumu‐ lation in vascular cells?

Low-density lipoprotein (LDL), the major lipid carrier in the blood, is the main candidate for the source of lipids that overload cells of atherosclerotic lesions. Considerable efforts have been concentrated on induction of lipid accumulation in cultured cells by adding LDL to the culture medium. However, most attempts failed: even high concentrations of LDL in culture medium induced no lipid accumulation [80]. This is due to reverse regulation of cholesterol metabolism in the cell, when the number of specific LDL receptors on the cell surface decreases in response to increased intracellular cholesterol content, which prevents LDL internalization via the receptor pathway [81].

It was demonstrated in several laboratories that intracellular lipid accumulation can be induced by LDL-containing insoluble complexes, such as LDL associated with glycosamino‐ glycans, proteoglycans, fibronectin, collagen, elastin, and other components of the arterial wall connective tissue matrix [77, 80, 82-92]. This pathway for intracellular lipid accumulation is quite possible, since all conditions are present in the vascular wall.

In addition, LDL association (formation of complexes including several LDL particles) is a necessary and sufficient condition for the accumulation of intracellular lipids [86, 87]. Presumably, LDL associates, like LDL-containing insoluble associates, which are essential‐ ly large particles, are internalized into the cell not via the receptor pathway but by nonspecific phagocytosis. This may result in unregulated lipid deposition and lipid overload. Native LDL generally does not form associates, while chemically modified lipoprotein readily associates, forming rather large particles. It has been shown that all known chemical modifications of the lipoprotein, including naturally occurring multiplemodified LDL, stimulate LDL association [86-88].

Hypersecretion of extracellular matrix may be a cause or a consequence of disintegration of the intimal cellular system in atherosclerotic lesions. In initial lesions, where the cellular network is preserved, collagen-producing cells are not numerous and are integrated into a network [53]. The network is destroyed in fatty streaks and atherosclerotic plaques. The proportion of collagen-producing cells in these lesions is higher than in initial lesions, and the cells are located at the sites of the network disintegration [53]. This suggests that synthetic activity of cells increases considerably after they have lost contact with neighboring cells.

Bearing in mind that the "splash" of proliferative activity and collagen production occur in lipid-rich lesions, it can be suggested that there is a relationship between the main athero‐ sclerotic manifestations: lipid accumulation (lipidosis), proliferation, and secretion of the extracellular matrix (fibrosis). Since the proliferative activity and the collagen-synthetic activity are the highest in the zones of lipidosis, it was important to find out how the accu‐ mulation of lipids in cells (cellular lipidosis) is related to other manifestations of atheroscle‐ rosis, namely, proliferation and fibrosis. To investigate this relationship, lipid accumulation was induced in cultured cells with maximum adequacy to cellular lipidosis occurring in the

Disturbances in cell metabolism of lipids may be one of the causes of intracellular lipid accumulation. In the cells cultured from atherosclerotic lesions, the rate of the synthesis of main classes of lipids is higher than that in cells cultured from uninvolved human aortic intima [79]. The rate of lipid synthesis is directly correlated with the intracellular lipid content: the higher intracellular excess of fat, the higher the intensity of lipid metabolism in atherosclerotic cells [79]. However, these findings provide no answer to the following question: what is the cause of lipid metabolism dysfunction in the vascular wall that led to excessive lipid accumu‐

Low-density lipoprotein (LDL), the major lipid carrier in the blood, is the main candidate for the source of lipids that overload cells of atherosclerotic lesions. Considerable efforts have been concentrated on induction of lipid accumulation in cultured cells by adding LDL to the culture medium. However, most attempts failed: even high concentrations of LDL in culture medium induced no lipid accumulation [80]. This is due to reverse regulation of cholesterol metabolism in the cell, when the number of specific LDL receptors on the cell surface decreases in response to increased intracellular cholesterol content, which prevents LDL internalization via the

It was demonstrated in several laboratories that intracellular lipid accumulation can be induced by LDL-containing insoluble complexes, such as LDL associated with glycosamino‐ glycans, proteoglycans, fibronectin, collagen, elastin, and other components of the arterial wall connective tissue matrix [77, 80, 82-92]. This pathway for intracellular lipid accumulation is

quite possible, since all conditions are present in the vascular wall.

**13. Mechanisms of cellular lipidosis**

vascular wall [77, 78].

218 Muscle Cell and Tissue

lation in vascular cells?

receptor pathway [81].

Atherogenic modified LDL circulating in the blood, but not native LDL, induces lipid accu‐ mulation in cultured cells [88]. Modifications occur in protein, carbohydrate, and lipid moieties of an LDL particle; as a result, its size, charge, density, immunogenicity, and many other properties change [89]. As mentioned above, lipid accumulation caused by multiple-modified LDL is observed only when modified LDL forms associates. Similar to native LDL, unassoci‐ ated modified LDL does not induce intracellular lipid accumulation [77]. A direct and strong correlation was established between the ability to induce lipid accumulation and the size of LDL associate [88].

Multiple-modified LDL has been isolated from the blood of patients with assessed atheroscle‐ rosis and from healthy subjects. However, the content of modified LDL in the blood of patients with atherosclerosis of any localization is considerably higher than in healthy subjects [90]. The positive correlation was revealed between the blood level of modified LDL and athero‐ genic potential of blood serum, i.e., its ability to induce intracellular lipid accumulation [90].
