Insolubilization of low density lipoprotein induces cholesterol accumulation in cultured subendothelial cells of human aorta

Opinion: On the Way towards the New Paradigm of Atherosclerosis

Atherosclerosis is a multicausal disease characterized by the formation of cholesterol-containing plaque in the pronounced intima nearest to the heart's elastic-type arteries that have high levels of blood circulation. Plaques are formed due to arterial pressure-induced damage to the endothelium in areas of turbulent blood flow. It is found in the majority of the Western population, including young people. This denies the monogenic mechanism of atherogenesis. In 1988, Orekhov et al. and Kawai et al. discovered that the presence of atherogenic (modified, including oxidized ones) LDLs is necessary for atherogenesis. On the basis of our discovery, suggesting that the overloading of enterocytes with lipids could lead to the formation of modified LDLs, we proposed a new hypothesis explaining the main factors of atherogenesis. Indeed, when endothelial cells are damaged and then pass through the G2 phase of their cell cycle they secrete proteins into their basement membrane. This leads to thickening of the basement membrane and increases its affinity to LDL especially for modified ones. When the enterocyte transcytosis pathway is overloaded with fat, very large chylomicrons are formed, which have few sialic acids, circulate in the blood for a long time, undergo oxidation, and can induce the production of autoantibodies. It is the sialic acids that shield the short forks of the polysaccharide chains to which autoantibodies are produced. Here, these data are evaluated from the point of view of our new model.

Natural products: The role and mechanism in low-density lipoprotein oxidation and atherosclerosis

Atherosclerosis is a chronic inflammatory, metabolic, and epigenetic disease, which leads to the life-threatening coronary artery disease. Emerging studies from bench to bedside have demonstrated the pivotal role of low-density lipoprotein (LDL) oxidation in the initiation and progression of atherosclerosis. This article hereby reviews oxidation mechanism of LDL, and the pro-atherogenic and biomarker role of oxidized LDL in atherosclerosis. We also review the pharmacological effects of several representative natural products (vitamin E, resveratrol, quercetin, probucol, tanshinone IIA, epigallocatechin gallate, and Lycopene) in protecting against LDL oxidation and atherosclerosis. Clinical and basic research supports the beneficial effects of these natural products in inhibiting LDL oxidation and preventing atherosclerosis, but the data are still controversial. This may be related to factors such as the population and the dosage and time of taking natural products involved in different studies. Understanding the mechanism of LDL oxidation and effect of oxidized LDL help researchers to find novel therapies against atherosclerosis.

The Atherogenic Role of Circulating Modified Lipids in Atherosclerosis

Lipid accumulation in the arterial wall is a crucial event in the development of atherosclerotic lesions. Circulating low-density lipoprotein (LDL) is the major source of lipids that accumulate in the atherosclerotic plaques. It was discovered that not all LDL is atherogenic. In the blood plasma of atherosclerotic patients, LDL particles are the subject of multiple enzymatic and non-enzymatic modifications that determine their atherogenicity. Desialylation is the primary and the most important atherogenic LDL modification followed by a cascade of other modifications that also increase blood atherogenicity. The enzyme trans-sialidase is responsible for the desialylation of LDL, therefore, its activity plays an important role in atherosclerosis development. Moreover, circulating modified LDL is associated with immune complexes that also have a strong atherogenic potential. Moreover, it was shown that antibodies to modified LDL are also atherogenic. The properties of modified LDL were described, and the strong evidence indicating that it is capable of inducing intracellular accumulation of lipids was presented. The accumulated evidence indicated that the molecular properties of modified LDL, including LDL-containing immune complexes can serve as the prognostic/diagnostic biomarkers and molecular targets for the development of anti-atherosclerotic drugs.

Modified and Dysfunctional Lipoproteins in Atherosclerosis: Effectors or Biomarkers?

Atherosclerotic diseases are the leading cause of mortality in industrialized countries. Correspondingly, studying the pathogenesis of atherosclerosis and developing new methods for its diagnostic and treatment remain in the focus of current medicine and health care. This review aims to discuss the mechanistic role of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) in atherogenesis. In particular, the generally accepted hypothesis about the key role of oxidized LDL in atherogenesis is questioned, and an alternative concept of multiple modification of LDL is presented. The fundamental question discussed in this review is whether LDL and HDL are effectors or biomarkers, or both. This is important for understanding whether lipoproteins are a therapeutic target or just diagnostic indicators.

Anti-atherosclerotic effects of garlic preparation in freeze injury model of atherosclerosis in cholesterol-fed rabbits

BACKGROUND

Garlic (Allium sativum L.) is one of the most popular substances used to reduce various risks associated with cardiovascular disease. However, little is known on the direct effects of garlic on atherosclerosis.

PURPOSE

In the present study we have examined the effect of per oral administration of the time-released garlic herbal preparation on serum atherogenicity and formation of intimal thickening after freeze injury in cholesterol-fed rabbits.

METHODS

Group 1 rabbits maintained on the standard cholesterol-rich diet served as the control. Group 2 rabbits were fed the cholesterol-rich diet and treated with garlic preparation containing 300 mg garlic powder.

RESULTS

Local thickening of the aortic media (i.e., the neointima formation) in the freeze injury zone was observed in all the rabbits. Regular garlic preparation therapy prevented the neointima formation and the accumulation of free and esterified cholesterol, triglycerides, phospholipids and collagen in the neointima, the effects being statistically significant. Garlic preparation also decreased serum lipid content by 1.5-fold and lowered atherogenic activity of blood serum (ability to induce lipid accumulation in cultured cells) induced by cholesterol-rich diet.

CONCLUSION

The results obtained indicate that garlic preparation prevents the development of cholesterol-induced experimental atherosclerosis and possesses the direct anti-atherogenic activity.

Low density lipoprotein-containing circulating immune complexes: role in atherosclerosis and diagnostic value

It has been suggested that low density lipoprotein-containing circulating immune complexes (LDL-CIC) play a role in atherogenesis and are involved in the formation of early atherosclerotic lesion. These complexes, as well as anti-LDL autoantibodies, have been found in the blood and in the atherosclerotic lesions of patients with different cardiovascular diseases, as well as in the blood of animals with experimental atherosclerosis. It can be suggested that the presence of anti-LDL antibodies in the blood is a result of immune response induced by lipoprotein modification. LDL-CIC differs from native LDL in many aspects. It has much lower sialic acid content, smaller diameter, and higher density and is more electronegative than native LDL. Fraction of LDL-CICs is fundamental to the serum atherogenicity manifested at the cellular level. LDL-CIC, unlike native LDL, is able to induce intracellular accumulation of neutral lipids, especially esterified cholesterol, in cells cultured from uninvolved human aortic intima and in macrophage cultures. After removal of LDL-CIC, the CHD patient's sera lose their atherogenic properties. Titer of LDL-CIC in blood serum significantly correlates with progression of atherosclerosis in human in vivo and has the highest diagnostic value among other measured serum lipid parameters. Elevated CIC-cholesterol might well be a possible risk factor of coronary atherosclerosis.

Subendothelial smooth muscle cells of human aorta express macrophage antigen in situ and in vitro

Cells bearing a smooth muscle cell marker--alpha-actin and a macrophage marker--CD68 antigen were immunocytochemically identified on 'en face' preparations of human aortic intima. Cells, expressing smooth muscle alpha-actin, macrophage CD68 antigen and both markers, i.e. smooth muscle cells possessing the macrophage antigen, were identified both in grossly normal aortic areas and in atherosclerotic lesions (fatty streaks and atherosclerotic plaques). CD68-positive smooth muscle cells were most common in the lipid-rich areas: fatty streaks and atherosclerotic plaque shoulders. Cells expressing smooth muscle alpha-actin and CD68 were also revealed in primary cultures prepared from grossly normal and atherosclerotic intima. Cells expressing both antigens were found in all examined cultures. The proportion of these cells in cultures from grossly normal areas and atherosclerotic plaques was similar: 14.5 +/- 4.1 and 14.6 +/- 4.8%, respectively. Cultures from fatty streaks had a higher content of cells expressing both antigens: 25.1 +/- 7.0%. Modified low density lipoprotein-induced intracellular lipid accumulation in cells cultured from grossly normal intima led to a three-fold increase in the number of cells sharing alpha-actin and CD68 antigen. Accumulation of latex beads by phagocytosis had a similar effect. It was suggested that in atherosclerotic lesions intracellular lipid accumulation and other stimulators of phagocytosis may provoke the expression of macrophage-associated antigen CD68 in settled cells of the subendothelial intima of human aorta.

Mast cell granule remnants carry LDL into smooth muscle cells of the synthetic phenotype and induce their conversion into foam cells

We report the effect of mast cells on the uptake of LDL by smooth muscle cells (SMCs) and their conversion into foam cells in vitro. The mast cells were stimulated to exocytose their cytoplasmic secretory granules, and the granule remnants formed were recovered from the extracellular fluid and added to cultures of SMCs of either the synthetic or contractile phenotype in LDL-containing medium. In the presence but not in the absence of granule remnants, SMCs of the synthetic phenotype took up LDL with ensuing stimulation of intracellular cholesteryl ester synthesis and cytoplasmic accumulation of neutral lipid droplets. Using methylated LDL (mLDL), a modified species of LDL that binds to granule remnants but not to LDL receptors, we demonstrated that this uptake (leading to foam cell formation) occurred only when LDL was bound to granule remnants. After the addition of colloidal gold-LDL and granule remnants to the incubation system, electron microscopy revealed that within phagosomes of the SMCs there were granule remnants (diameter, 0.5 to 1 micron) coated with LDL, confirming that LDL had been carried into the cells with the remnants. SMCs of the contractile phenotype were less efficient than their synthetic counterparts at phagocytosing LDL-coated granule remnants and were not converted into foam cells. This difference in the rate of phagocytosis of granule remnants was present even in the absence of LDL, revealing that the more active phagocytosis by SMCs of the synthetic phenotype was not specifically related to uptake of lipids but rather reflected a general phenotype characteristic of these cells. These observations indicate a phagocytic mechanism by which SMCs of the synthetic phenotype are converted into cholesteryl ester-filled foam cells, and they also suggest that degranulation of mast cells plays a role in the development of fatty streak lesions.

Effect of lipoprotein(a) on lipid metabolism of cultured human intimal aortic cells

The effect of Lp(a) on lipid metabolism in cells cultured from unaffected human aortic intima has been investigated. Lp(a) isolated from the blood of healthy subjects failed to alter intracellular neutral lipid content. On the other hand, Lp(a) obtained from coronary atherosclerosis patients induced a 1.5- to 2-5-fold increase in intracellular levels of free and esterified cholesterol and triglycerides. The sialic acid content of patients' Lp(a) was 2.5-fold lower as compared with that of healthy subjects' Lp(a). Healthy donors' Lp(a) in vitro desialylated with neuraminidase were able to accumulate lipids within the cells. Using lectin-chromatography on Ricinus Communis agglutinin-agarose, Lp(a) was divided into subfractions differing by sialic acid content. Desialylated (sialic acid-poor) Lp(a), but not sialylated lipoproteins, were capable of increasing the intracellular content of total cholesterol. Desialylated but not sialylated Lp(a) formed aggregates during incubation under cell culture conditions. Isolated aggregates of desialylated Lp(a) induced lipid accumulation in cells. Elimination of Lp(a) aggregates from cultural medium prevented the increase of intracellular lipids. Complexes of Lp(a) with the matrix components and antibodies increased lipid level in cultured cells. We assume that formation of large particles of desialylated Lp(a) aggregates or Lp(a)-containing complexes is a necessary step for lipid accumulation in human intimal cells.

Three types of naturally occurring modified lipoproteins induce intracellular lipid accumulation due to lipoprotein aggregation

Low density lipoprotein (LDL) from patients with coronary atherosclerosis and diabetes mellitus as well as in vitro desialylated LDL, glycosylated LDL, and lipoprotein (a) caused a twofold to fourfold rise in cholesteryl ester in cultured human blood monocytes and intimal smooth muscle cells isolated from normal aorta. Native LDL from healthy subjects failed to induce intracellular lipid accumulation. We have demonstrated by laser correlative photometry and gel filtration chromatography that in vivo and in vitro modified lipoproteins form aggregates under cell culture conditions. The degree of modified lipoprotein aggregation directly correlated with the ability of these lipoproteins to elevate the cholesteryl ester content of cultured cells. Modified lipoprotein aggregates isolated by gel filtration induced a threefold to fivefold elevation in cellular cholesteryl ester content. Aggregates of 125I-modified LDL were taken up and degraded fivefold to sevenfold more effectively as compared with nonaggregated lipoproteins. The uptake and degradation of 125I-labeled aggregates were strongly inhibited by unlabeled aggregates, latex beads, and cytochalasin B but not by native or acetylated LDL. These data indicate that uptake of lipoprotein aggregates occurred by phagocytosis. Obtained results suggest that modified lipoprotein aggregation may be the key condition for lipid accumulation.

Beta-blockers: propranolol, metoprolol, atenolol, pindolol, alprenolol and timolol, manifest atherogenicity on in vitro, ex vivo and in vivo models. Elimination of propranolol atherogenic effects by papaverine

The addition of the beta-blockers propranolol, metoprolol, atenolol, pindolol, alprenolol and timolol to a culture of peritoneal macrophages or smooth muscle cells induced an increase in the intracellular cholesterol content. Blood serum obtained from a rabbit after a peroral administration of beta-blockers also induced cholesterol accumulation. This property of drug or blood serum obtained after peroral administration is conventionally referred to as atherogenic potential or atherogenicity. Regular administration of propranolol during a 21-day period evoked stable atherogenicity of rabbit blood serum. This was accompanied by stimulation of manifestations of atherosclerosis in the aorta deendothelialized with a balloon catheter. Propranolol increased neointimal thickening, lipid accumulation, an increase in cell number and in the collagen content. In vitro, the combination of propranolol with papaverine eliminated the atherogenic effect of propranolol which manifested itself as stimulation of cholesterol accumulation in cultured cells. Simultaneous peroral administration of propranolol and papaverine prevented the appearance of serum atherogenicity. Papaverine eliminated neointimal thickening, an increase in cell number and in the lipid and collagen contents evoked by propranolol. Papaverine itself had no effect on these parameters. Thus, the atherogenicity of propranolol as well as capacity of papaverine to eliminate beta-blocker atherogenicity revealed in cell culture was confirmed in vivo. We hope that these results may be useful in the development of new drugs and optimization of antiatherosclerotic drug therapy.

The atherogenic effect of lupus sera: systemic lupus erythematosus-derived immune complexes stimulate the accumulation of cholesterol in cultured smooth muscle cells from human aorta

The influence of systemic lupus erythematosus (SLE) patients' sera on lipid accumulation in the cultured smooth muscle cells (SMC) from unaffected human aortic intima was examined. It was demonstrated that the cholesterol uptake in the SMC cultured in the presence of SLE sera is 1.5- to 6-fold higher than in the cells cultured with normal human sera (NHS) obtained from healthy donors. Incubation of the SMC with circulating immune complexes (CIC) isolated from lupus sera by precipitation with 2.5% polyethylene glycol 6000 (PEG) caused a 3- to 4-fold rise in the intracellular cholesterol level. The atherogenic effect of lupus sera, as well as isolated CIC, strongly correlated (r = 0.98) with the low density lipoprotein (LDL) content in the PEG-precipitated CIC. The cholesterol level in cultured SMC also increased 2- to 3-fold when growth medium was supplemented with LDL, DNA, and anti-DNA autoantibodies (IgG) affinity isolated from lupus sera. Using immunofluorescent staining, it was shown that the addition of a DNA-anti-DNA IgG mixture to the growth medium, together with NHS, stimulated LDL incorporation in the SMC. The results of double-label staining suggest the formation of LDL-DNA-IgG complexes which seem to be entrapped in cells more actively than free LDL. The composition of PEG-precipitated CIC was studied by electrophoresis and immunoblotting. Significant amounts of apolipoprotein B, as well as low molecular weight DNA and immunoglobulins, were found in SLE-derived CIC. The data obtained suggest that the atherogenic effect of human lupus sera in vitro is generally due to the appearance of LDL-containing immune complexes. Different mechanisms possibly involved in the lupus atherogenesis are discussed.

Autoantibodies against modified low density lipoprotein. Nonlipid factor of blood plasma that stimulates foam cell formation

The blood serum of patients with coronary atherosclerosis possesses an ability to induce the accumulation of cellular lipids in primary cultures of human aortic intimal cells. Factors responsible for this property of the atherosclerotic patients' sera are represented by modified (desialylated) low density lipoprotein (LDL) and a nonlipid factor interacting with LDL. It was assumed that the nonlipid factor was antibodies against LDL. Total immunoglobulin G (IgG) fraction was isolated from the sera of atherosclerotic patients, and IgGs interacting with LDL (anti-LDL) were then purified by affinity chromatography on a sorbent with immobilized LDL. From the sera of patients, a 30-fold greater amount of anti-LDL has been isolated than from the sera of healthy donors. The affinity constant of anti-LDL to the lipoprotein obtained from the blood of healthy donors was 2 x 10(7) M-1. The affinity of anti-LDL to the lipoprotein from the blood of atherosclerotic patients, as well as to LDL desialylated in vitro with neuraminidase, was much higher. Anti-LDL increased the uptake of LDL by cultured aortic cells by approximately 2.5-fold and substantially increased intracellular lipid accumulation. The obtained data suggest that autoantibodies against LDL are an essential factor of blood plasma responsible for its atherogenic potential.