Plasma dyslipidemia: pathogenesis and diagnostic value. Literature review

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Introduction
Atherosclerosis of various localization is still the main cause of the occurrence and development of CVD both in our country and around the world. The problem of its detection is directly related to the improvement of methods for the early diagnosis of this disease. The unified biochemical methods for studying lipid metabolism at the disposal of clinical diagnostic laboratories today are clearly not enough to fully identify the molecular mechanisms underlying atherosclerotic damage to the vascular wall. Today, more than ever, new approaches and new principles of laboratory diagnostics of atherosclerosis are needed, which would cover not only the determination of atherogenic fractions of blood plasma, but would also include the diagnosis of digestion and absorption of lipids in the gastrointestinal tract, the determination of the components of lipid transport through the blood system, and the control of intermediate metabolism. (processes of biosynthesis and decay) of simple and complex lipids in the tissues of the human body. Undoubtedly, it is necessary to determine the modified lipids in the blood that have arisen in the process of peroxidation, to expand the range of application of physicochemical, immunological and molecular genetic methods for studying lipid metabolism disorders. Today, the much-needed diagnostics of disorders of the neuro-humoral regulation of lipid metabolism is not fully carried out. But it is well known that even emotional stress or physical activity increase the release of adrenaline and noradrenaline into the blood, which, by increasing the processes of lipolysis in the body, contribute to the release of fatty acids from adipose tissue, increasing their content in blood plasma.
Of course, there is no doubt that the main reason for the formation of atherosclerosis in the body is a change in the lipid composition of blood plasma and a violation of the immune properties of the body. But such violations, perhaps, are only a small part of the overall picture of pathological changes during the formation of an atherosclerotic plaque. Existing modern theories of atherosclerosis (lipid, autoimmune, inflammatory, monoclonal, infectious, endothelial, neurometabolic, thrombogenic and others), considering this problem from different angles, try to find new specific approaches to the diagnosis, treatment and prevention of atherosclerosis. Since today CVD is one of the main causes of death and disability in modern society. In this regard, attempts are continuing to search for new, more effective diagnostic approaches for the early detection of atherogenesis and signs-markers of this disease.
And although today there are quite clear ideas about the diagnostic significance of blood plasma lipids, we still do not clearly understand the functional role of each of the components of lipid metabolism and the metabolic pathways leading to the formation of an atherosclerotic pathological inflammatory focus. The molecular-cellular, immuno-biochemical mechanisms of the onset and development of atherosclerosis and the true role of T- and B-lymphocytes in this process remain unclear. The role of the blood plasma protein apolipoprotein A-1 (apoA-1) in the atherosclerotic process should also be clarified.
And if today immuno-biochemical methods cannot fully diagnose the development of atherosclerosis in the early stages of its formation, then it is necessary to develop and put into practice new methods for the molecular diagnosis of atherosclerosis. Based on this, we have every reason to talk about the need to refine the system for diagnosing atherosclerosis based on new principles and new approaches in biomedical science.
The purpose of the work is to summarize, on the basis of literature data, information about the pathogenetic mechanisms of the onset and development of blood plasma dyslipidemia in patients with atherosclerosis and show their significance in the immuno-biochemical diagnosis of lipid metabolism disorders in humans.

Lipid profile of blood plasma in atherosclerosis
In the clinical and laboratory diagnosis of ischemic disorders associated with atherosclerosis, a biochemical study of the lipid composition of blood serum is usually carried out: the determination of cholesterol (CHS), triglycerides (TG), high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C) [ 12]. And this is justified to a certain extent, since an increase in the level of simple and complex lipids in the blood serum is an indicator of atherosclerotic damage to the vascular wall. It is now generally accepted that lipid profiles of blood plasma are the basic criteria for assessing the risk of developing CVD. For example, not so long ago [3], subgroups of people with different lipoprotein profiles and, accordingly, different risk of coronary heart disease were identified. It was determined that a subgroup of people suffering from coronary heart disease had the highest concentrations of lipoproteins in the blood plasma, and not suffering - the lowest. It is interesting to note that not all lipid molecules exhibit pronounced atherogenic properties in the human body. There is evidence that a significant part of the risk of ischemic disorders is associated mainly with particles of lipoproteins containing apolipoprotein B (apoB).
However, the cholesterol theory of atherosclerosis assigns a leading role in this process to cholesterol. But note that in the human body, cholesterol homeostasis is of great importance in life, since in the process of evolution of eukaryotic cells, a divergent set of pathways has developed to meet the body's needs for cholesterol. One of these pathways is the transfer of amphiphilic lipids to the cell membrane after contact of lipoprotein particles with the membrane formation. Thus, in the human body there is a constant exchange of amphiphilic lipids between lipoprotein particles and the cell membrane [4]. But how does cholesterol, which performs the most important functions in the body, begin to play a key role in atherogenesis? Apparently, endogenous cholesterol, which is synthesized in the liver and transported as part of LDL to the organs and tissues of the body, plays the main role in this process. Indeed, elevated levels of total cholesterol and LDL cholesterol in blood plasma are statistically associated with an increase in the incidence of CVD and vice versa, a decrease in the level of plasma lipoproteins leads to a positive effect of blood lipid composition on the vascular wall and reduces the risk of CVD [5].
Note that the circulating sterols in the blood are formed as a result of cholesterol biosynthesis, or are supplied as a result of intestinal absorption, and they are mainly esterified. Meanwhile, it has been established that sterols are intensively accumulated in atherosclerotic plaques, so the ratio of their levels is of great clinical and diagnostic importance. However, there is evidence that cholesterol molecules, like sterols, are less etherified in plaques than in blood plasma [6].
We note another important circumstance, which is that the atherogenic lipid profile of blood plasma in humans can vary significantly depending on the state of health and the physiological state of the body. For example, an increased risk of lipid disorders and atherogenic dyslipidemia was found in more than half of women after menopause [7]. However, here we see a weakening of the effect of estrogens on lipid metabolism, and increased deposition of fat in adipose tissue may serve as an indicator of a weakening of extragenic activity. However, in some cases, the combined lesion of the carotid and coronary arteries is accompanied by changes in the fractional spectrum of blood plasma lipoproteins, which is an additional marker of atherogenic lipid profile [8]. On the contrary, when atherosclerosis is localized in the vessels of the lower extremities, a high concentration of lipoprotein (a) LP(a) and C-reactive protein (CRP) in leukocyte supernatants is detected in the blood serum [9]. On the other hand, during ischemia of cerebral vessels in patients, an active process of inflammation of the vascular wall is accompanied by an increase in the level of monocytic chemoactive protein -1 and CRP [10].
The data available in the literature indicate the relationship between indicators of the oxidative-antioxidant status, lipid and carbohydrate metabolism in the formation of atherosclerosis in males. The laboratory diagnostics performed showed the presence in the blood of such patients of an increase in the concentration of LDL-C, TG, apoB, a change in the ratio of apoB/apoA, LP (a), CRP and a decrease in HDL-C, retinol, β-carotene levels and a decrease in LDL resistance to oxidation [11].Indeed, it has been proven that the antioxidant properties of HDL can be weakened in an oxygen-rich environment of arterial blood. Therefore, HDL particles from arterial blood plasma have less pronounced antioxidant properties compared to particles of venous origin, which is consistent with the development of atherosclerosis in the arterial wall [12].

There is also strong evidence that elevated plasma levels of Lp(a) increase the risk of CVD. Since, like LDL particles, LP molecules (a) contain cholesterol, which contributes to the development of atherosclerosis [13]. In addition, plasma lipid fractions independent of LDL-C contribute significantly to the risk of CVD, since circulating lipoproteins represent only a small fraction of the total cholesterol present in the human body. Therefore, the mobilization and release of cholesterol from the blood plasma and tissue pool may be an important risk factor for CVD, which is a kind of protective agent against the onset and development of atherosclerosis. Thus, the reverse pathway of cholesterol transport, including the intestines and blood plasma, as well as the tissue cholesterol pool, may be a determinant of the risk of CVD [14].
On the other hand, metabolically active brown adipose tissue plays an important role in the metabolism of lipoproteins in atherosclerosis. Thus, activated adipocytes intensively use their intracellular reserves of triglycerides for the formation and oxidation of fatty acids [15]. However, excessive accumulation of fatty inclusions by adipocytes in obesity contributes to a decrease in the protective functions of HDL and increases the risk of atherosclerotic burden on the body [16].
And I would also like to dwell on the issue of the relationship between the main indicators of calcium and lipid metabolism in people with atherosclerosis. According to [17], the levels of calcium, magnesium, calcitonin, alkaline phosphatase, and triglycerides in men with coronary atherosclerosis were found to be higher than normal in the biochemical analysis of blood, and the level of HDL cholesterol was lower than the reference values. Such relationships indicate a direct effect of calcium on the atherosclerotic focus of damage to arterial vessels. This is evidenced by the data available in the literature, indicating the active participation of osteogenic progenitor cells in the development of atherosclerotic calcification of the vascular wall [18].
Thus, the generalization of the above data shows that the essence of normal lipid metabolism in the body in atherosclerosis is that some lipids must accumulate and perform their physiological functions, while others must be removed from the body to achieve the ideal state of the vascular wall. But in the pathogenesis of atherosclerosis, it is still necessary to specify the atherogenic properties of lipids and the molecular mechanisms responsible for their accumulation and decrease in blood plasma [19].
The mechanisms that control lipid exchange between cells, cell organelles and blood plasma are directly involved in the development of dyslipidemia. And these mechanisms of lipid homeostasis can be easily disturbed due to the disorder of neurohumoral mechanisms of regulation of digestion, absorption and intermediate lipid metabolism. Excitation of the autonomic nervous system enhances the mobilization of fat from the depot to the blood, and then to the liver, where they are oxidized. In turn, insulin enhances lipogenesis and the conversion of carbohydrates into fats, inhibits the oxidation of fatty acids. Contrinsular hormones, on the contrary, activate reverse processes and stimulate lipolysis. In other words, the disorder of regulation processes, especially with age, leads to dyslipidemia and metabolic diseases of civilization: atherosclerosis, obesity, diabetes mellitus. In [20], we presented a model of vascular aging, which is based on maladaptive damage to all layers of the vascular wall (intima, media, and adventitia) during aging of the body, due to violations of neurohumoral regulation of functions under the influence of a number of external and internal environmental factors. Also earlier [21], a regularity was described, which consists in a decrease in cell metabolism when the body is exposed to unfavorable environmental factors. In this regard, it was concluded that under conditions of violation of regulatory influences on the organ, metabolism is set at the level of incomplete physiological and biochemical adaptation (the principle of limiting cellular metabolism). Thus, we believe that atherosclerosis is associated with a violation of the entire metabolism and the neurohumoral apparatus that regulates blood circulation and nutrition of the vascular wall.

Immuno-biochemical signs-markers of atherosclerosis
By now, it is well known that numerous immunocompetent cells (macrophages, dendritic cells, lymphocytes), as well as endothelial and smooth muscle cells that interact with blood plasma lipoproteins, are involved in the development of atherosclerosis. In this process, CD4+ T-lymphocytes circulating in the blood differentiate predominantly into Th1 cells that respond to specific antigens (oxidized LDL). However, to date, it is not entirely clear whether the development of atherosclerosis depends on the balanced participation of regulatory T cells (Treg) with suppressor activity, since there is evidence of a decrease in the content of Treg in patients with severe atherosclerosis [22]. It can be seen from the above that oxidized LDL and antibodies to them play a key role in the immune-inflammatory process in atherosclerosis. In other words, oxidized LDL are self-antigens that induce a local immune response in arterial blood vessels. It is oxidative modification that converts LDL into a form that is captured by macrophages ten times faster than native LDL, which contributes to the progression of the atherosclerotic process. In a simplified form, the oxidative modification of LDL and VLDL in the bloodstream and in the arterial wall can be represented as follows: under the influence of reactive oxygen species (O3, 1O2, , O2, HO,, H2O2), lipoproteins, like other lipids, undergo peroxidation with the formation hydroperoxides (ROOH), malondialdehyde (CH2(CHO)2 and other peroxidation products. Subsequently, peroxide-modified lipoproteins acquire autoimmune properties, to which antibodies are formed in the human body. Chemically modified lipoproteins and autoimmune lipoprotein-antibody complexes in the arterial wall are actively captured by macrophages.Macrophages accumulate a large amount of cholesterol in the cytoplasm and turn into the so-called foam cells, which are subsequently destroyed and cholesterol in large quantities enters the intima of arterial vessels.Subsequently, a fibrous-cholesterol plaque is formed in this area of the vessel, followed by calcification (Fig. . one).

Fig. 1. Scheme of immuno-biochemical processes underlying the formation of atherosclerotic plaques in blood vessels

In this regard, the level of oxidized LDL in the blood serum can be a prognostic sign of the risk of CVD, and antibodies to them can be considered markers of LDL oxidation and a predictor of the progression of the atherosclerotic process [23].
Recall that LDL actively transport cholesterol to the tissues and organs of our body, where it undergoes various transformations and is used for the needs of cells. But how cholesterol began to play a significant role in the immunological reactions in atherosclerosis occurring in vivo is still not entirely clear? Nevertheless, there are some data in this respect indicating that, for example, in patients with coronary heart disease, compared with healthy individuals, the level of cholesterol in circulating immune complexes (CSIC) is increased and the level of antibodies (IgM) to hypochlorite is reduced. -LDL [24]. We are well aware that cholesterol causes inflammation of the arterial wall and a multilevel cellular immune response in humans. Thus, in vitro experiments have shown that cholesterol accumulates in human hematopoietic stem and progenitor cells. The data obtained suggest the presence of an inflammatory component in the atherogenicity of cholesterol, which contributes to an increased risk of CVD [25]. And although the development of atherosclerosis is largely accompanied by the accumulation of lipids in the walls of blood vessels, it is also associated with apoptosis of macrophages, smooth muscle and endothelial cells of blood vessels [26]. And in this process, perhaps, an important role is played by the excessive accumulation of fatty inclusions in the cells of adipose tissue (adipocytes), which, apparently, also activates the degenerative-dystrophic process in all layers of the vascular wall. The proof of this is the fact that against the background of visceral obesity in coronary heart disease, there is a change in the adipokino-cytokine profile of adipocytes of epicardial adipose tissue. The results of the work showed the presence of “metabolic inflammation” associated with the involvement of adipocytes in the pathogenesis of coronary disease due to the formation of adipokinosis imbalance and activation of anti-inflammatory reactions in the human body [27].
Thus, there is no doubt that atherosclerosis is a chronic inflammatory disease resulting from the interaction of lipoproteins, macrophages, T-lymphocytes and other cellular elements in the vascular wall. Moreover, macrophage derivatives, foam cells, play a key role in this process, both at an early stage of the development of the disease, and at a late one [28]. However, we note that in response to atherosclerotic damage to the vascular wall, protective and adaptive immunological reactions are formed in the human body that prevent these processes. So, not so long ago it became known that B-lymphocyte-mediated immunity in the body plays a protective role in atherosclerosis. Blood B cells have become important modulators of anti-inflammatory effects in atherosclerosis [29].

Diagnosis, treatment and prevention of atherosclerosis
So, a large variety of immuno-biochemical processes involved in atherosclerotic damage to the vascular wall suggests a directed search for diagnostic markers for early localization of this pathological process. A comparative analysis of biochemical markers in combined atherosclerosis shows that a good laboratory diagnostic criterion for atherosclerosis is an elevated concentration of cholesterol and LDL, which are accompanied by a progressive increase in CRP and the atherogenic index. The available results indicate a significant effect of atherogenic lipids and acute-phase inflammatory proteins on the atherosclerotic inflammatory process [30]. On the other hand, now an information panel has been developed for diagnosing the risk of myocardial infarction in patients with coronary heart disease, which includes quite significant laboratory indicators. Such laboratory biochemical factors are the level of chaperone activity and the concentration of hemocysteine in blood serum, as well as an increase in CRP and activity of the superoxide dismutase enzyme [31].
In the literature on the diagnosis of atherosclerosis, there is evidence that people with a concentration of Lp (a) in blood plasma > 200 mg/l have an increased risk of CVD due to its involvement in atherogenesis, thrombogenesis and inflammation. In addition, it is known for certain that LP (a) circulating in the blood for a long time can interact with the components of the coagulation cascade and thus influence the blood coagulation process, enhancing the development of the atherosclerotic process. Therefore, for the diagnosis of atherosclerosis, it is now very important to determine the molecular pathways by which LP(a) affects the vascular wall, since LP(a) molecules are very resistant to lipid-lowering treatments [32].
So, there is no doubt that the violation of lipid metabolism is indeed a significant risk factor for the development of CVD. Therefore, in the diagnosis of coronary disease or other cardiovascular pathology, as always, it is traditional to determine the level of total cholesterol LDL cholesterol, HDL cholesterol, TG. However, in recent years, clinicians and biochemists have begun to pay close attention to the problem of the intersection of metabolic pathways for the biosynthesis of cholesterol and sphingolipids. This is due to the fact that an increase in the level of ceramide and sphingosine, which is observed during ischemia, and a decrease in the level of sphingosine-1-phosphate (S-1-P) in blood plasma can be an important clinical diagnostic factor in the development of atherosclerosis. Thus, for early diagnosis of coronary heart disease and arterial hypertension, it is proposed to determine the level of sphingolipids in blood plasma [33].

determine the level of sphingolipids in blood plasma [33].
It is also justified to use other biomarkers in the laboratory diagnosis of atherosclerosis and dyslipidemia. Note that studies are currently underway on the following markers of atherosclerosis: lipoprotein-associated phospholipase A2, CRP, LP(a), pregnancy-associated plasma protein A, asymmetric dimethylarginine [34, 35].
Today, multi-marker diagnostic panels for non-invasive detection of coronary atherosclerosis, called atheromarkers, have already been created: 1) the K coefficient, which reflects the ratio between atherogenic and physiologically active subfractions of lipoproteins; 2) duplex diagnostic complexes in the form of adiponectin/endothelin ratio; 3) an integrated bimarker for noninvasive diagnosis of coronary atherosclerosis (i-BIO) [36].
In recent years, molecular genetic studies have been intensively carried out, the results of which are projected onto the diagnostic and treatment process. The role of miRNA is discussed, which can be used as a biomarker of atherogenesis due to the fact that miRNA molecules have a fairly high stability and indirectly reflect the level of expression of genes involved in the development of atherosclerosis [37]. The process of DNA methylation, which is one of the most important epigenetic mechanism that changes gene expression, is being studied. Moreover, this mechanism plays an important role in the initiation and development of atherosclerosis in humans [38].
It has long been known that cardiovascular risk factors for the development of atherosclerotic vascular lesions in patients are also male gender, smoking, arterial hypertension, a positive family history of hyperlipidemia, and elevated levels of lipoprotein (a) [39]. There is also a relationship between increased body weight and the level of LP (a) in blood plasma and the risk of CVD [40]. New results of clinical and laboratory studies show that the level of LP (a) and especially its low molecular weight apoA phenotype is a risk factor for coronary atherosclerosis. And lipid apheresis is the main lipid-lowering therapy in patients suffering from dyslipidemia [41].
Thus, the violation of the physicochemical parameters of VLDL is an important pathogenetic factor in the development of hyperlipoproteinemia and atherosclerosis. According to the phylogenetic theory of general pathology V.N. Tit's aphysiological induction by the substrate is associated with a high content of palmitic fatty acid and palmitic forms of triglycerides in food, which occurs when eating a large amount of meat food and insufficient carbohydrate intake. And what are the ways to counteract the development of atherosclerosis? To normalize the biological function of endoecology, the following conditions must be met: firstly, to reduce the flow of ligand-free palmitic LDL into the intima of blood vessels and, secondly, to inhibit atherosclerosis by normalizing the biological function of nutrition (that is, it is necessary to reduce the amount of meat food, replacing it with fish) and increase consumption of plant foods [42]. In our time, dyslipidemias can intensively form if a herbivore in the phylogeny of Homo sapiens begins to abuse meat food. In this case, the biological, energy and kinetic perfection will be violated and such metabolic pandemics as atherosclerosis and atheromatosis, insulin resistance, obesity and fatty liver infiltration will begin to prevail in the human population [43].
And, indeed, today, all over the world, the normalization of lipid metabolism is part of the global prevention of CVD caused by atherosclerosis. But now it is necessary to search for new statins that can effectively change the level of lipoproteins in the blood plasma in the right direction. In this regard, new combinations of drugs are being used today that have a lipid-modifying effect (statin + ezetimibe) and reduce the level of total cholesterol LDL-C, apoB, TG and at the same time increase the level of HDL-C in patients with hypercholesterolemia. Innovations in the field of lipid-lowering therapy are also the use of monoclonal antibodies against proprotein convertase subtilisin-kexin type 9 (PCSK9) (PCSK9 inhibitors), which can quickly reduce blood cholesterol levels, as well as the use of specific oligonucleotide sequences that block the translation of a certain protein, the use of inhibitors of microsomal triglyceride carrier protein [44].

Conclusion
Thus, the main cause of dyslipidemia in atherosclerosis is the imbalance between the amount of decomposed and newly synthesized lipids. Therefore, the study of the qualitative features of individual lipids (triglycerides, lipoproteins) and the determination of the fatty acid composition of chylomicrons are of great importance in the diagnosis of atherosclerosis. Triglycerides rich in saturated fatty acids and LDL have an adverse effect on the development of atherosclerosis, the accumulation of which leads to severe hypercholesterolemia and atheromatosis. Therefore, in the early stages of the development of atherosclerosis, the determination of not only the content of cholesterol in the blood, but also a wide range of lipid fractions of the blood plasma is of no small diagnostic importance.
The role of macrophages, which play a major role in the pathogenesis of atherosclerosis, remains insufficiently studied. Today we clearly know that the structure of an atherosclerotic plaque contains many components that control the activity of macrophages. The complex mapping of a wide range of macrophages and their phenotypes located in the atherosclerotic arterial wall made it possible to consider them as specific markers of atherosclerosis. A deep study of the phenotype of immune cells in atherosclerotic plaque is the basis for creating new approaches to the diagnosis and treatment of atherosclerosis.
Thus, now it is necessary to expand the use of physicochemical research methods (electrophoresis, liquid chromatography, mass spectrometry, nuclear magnetic resonance spectroscopy) in the diagnosis of atherosclerosis, which will allow to detect lipid metabolism pathology with high efficiency and study dyslipidemia of various origins. Great hopes in the future are associated with the use of molecular biology methods (polymerase chain reaction) in the diagnosis of atherosclerosis in the early stages of its development. We hope that in the long run, all the advanced technologies of biology and medicine will make it possible to maintain the biochemical homeostasis of the body at a constant level and extend a person's life.

About the authors

Aleksey A. Artemenkov

Author for correspondence.
Email: aaartemenkov@chsu.ru
ORCID iD: 0000-0001-7919-3690

Candidate of Biological Sciences, Associate Professor, Head of the Department of Theoretical Basis of Physical Culture, Sport and Health of the Faculty of Biology and Human Health

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