Nikolai N. Anitschkow and the lipid hypothesis of atherosclerosis

Evolving concepts of low-density lipoprotein: From structure to function

BACKGROUND

Low-density lipoprotein (LDL) is a central player in atherogenesis and has long been referred to as 'bad cholesterol.' However, emerging evidence indicates that LDL functions in multifaceted ways beyond cholesterol transport that include roles in inflammation, immunity, and cellular signaling. Understanding LDL's structure, metabolism and function is essential for advancing cardiovascular disease research and therapeutic strategies.

METHODS

This narrative review examines the history, structural properties, metabolism and functions of LDL in cardiovascular health and disease. We analyze key milestones in LDL research, from its early identification to recent advancements in molecular biology and omics-based investigations. Structural and functional insights are explored through imaging, proteomic analyses and lipidomic profiling, providing a deeper understanding of LDL heterogeneity.

RESULTS

Low-density lipoprotein metabolism, from biosynthesis to receptor-mediated clearance, plays a crucial role in lipid homeostasis and atherogenesis. Beyond cholesterol transport, LDL contributes to plaque inflammation, modulates adaptive immunity and regulates cellular signaling pathways. Structural studies reveal its heterogeneous composition, which influences its pathogenic potential. Evolving perspectives on LDL redefine its clinical significance, affecting cardiovascular risk assessment and therapeutic interventions.

CONCLUSIONS

A holistic understanding of LDL biology challenges traditional perspectives and underscores its complexity in cardiovascular health. Future research should focus on further elucidating LDL's structural and functional diversity to refine risk prediction models and therapeutic strategies, ultimately improving cardiovascular outcomes.

Insights from autopsy-initiated pathological studies of the pathogenesis and clinical manifestations of atherosclerosis and ischemic heart disease: Part I. Atherosclerosis

CONTEXT

Ischemic heart disease (IHD) due to coronary atherosclerosis constitutes the leading cause of morbidity and mortality worldwide. This review was undertaken to document the historical basis for our contemporary understanding of atherosclerosis-based disease and to provide a rationale for continued support for autopsy-based research to make further progress in reducing the morbidity and mortality from atherosclerosis-related disease.

OBJECTIVES

To analyze the contributions of the autopsy-initiated pathological studies to complement and validate other lines of investigation in determining the pathology and pathogenesis of the leading worldwide cause of morbidity and mortality, namely, atherosclerosis and its major complications of coronary atherosclerosis, ischemic heart disease, coronary thrombosis, acute myocardial infarction, and sudden cardiac death.

DATA SOURCES

Systematic search on PubMed to gather relevant studies concerning autopsy studies and reviews of the pathology and pathogenesis of atherosclerosis, ischemic heart disease, coronary atherosclerosis, coronary thrombosis, myocardial infarction, and sudden cardiac death CONCLUSIONS: Extensive published reports have confirmed the continuing importance of the autopsy as a powerful tool to understand the pathogenesis, clinical features, and therapeutic options for major diseases. This specifically has been shown by the analysis of atherosclerosis and its major manifestation of ischemic heart disease, as presented in this (Part I) and its companion (Part II) review. Autopsy-initiated pathological studies have documented the prevalence and natural history of atherosclerosis in different human populations in relationship to the prevalence of risk factors and established that the clinically silent phase of the disease begins in the first decades of life. Insights from these studies have been essential in developing and evaluating strategies for continued progress in preventing and controlling the disability and death associated with atherosclerotic heart disease.

Homocysteine Metabolites, Endothelial Dysfunction, and Cardiovascular Disease

Atherosclerosis is accompanied by inflammation that underlies cardiovascular disease (CVD) and its vascular manifestations, including acute stroke, myocardial infarction, and peripheral artery disease, the leading causes of morbidity/mortality worldwide. The monolayer of endothelial cells formed on the luminal surface of arteries and veins regulates vascular tone and permeability, which supports vascular homeostasis. Endothelial dysfunction, the first step in the development of atherosclerosis, is caused by mechanical and biochemical factors that disrupt vascular homeostasis and induce inflammation. Together with increased plasma levels of low-density lipoprotein (LDL), diabetes, hypertension, cigarette smoking, infectious microorganisms, and genetic factors, epidemiological studies established that dysregulated metabolism of homocysteine (Hcy) causing hyperhomocysteinemia (HHcy) is associated with CVD. Patients with severe HHcy exhibit severe CVD and die prematurely due to vascular complications. Biochemically, HHcy is characterized by elevated levels of Hcy and related metabolites such as Hcy-thiolactone and N-Hcy-protein, seen in genetic and nutritional deficiencies in Hcy metabolism in humans and animals. The only known source of Hcy in humans is methionine released in the gut from dietary protein. Hcy is generated from S-adenosylhomocysteine (AdoHcy) and metabolized to cystathionine by cystathionine β-synthase (CBS) and to Hcy-thiolactone by methionyl-tRNA synthetase. Hcy-thiolactone, a chemically reactive thioester, modifies protein lysine residues, generating N-homocysteinylated (N-Hcy)-protein. N-Hcy-proteins lose their normal native function and become cytotoxic, autoimmunogenic, proinflammatory, prothrombotic, and proatherogenic. Accumulating evidence, discussed in this review, shows that these Hcy metabolites can promote endothelial dysfunction, CVD, and stroke in humans by inducing pro-atherogenic changes in gene expression, upregulating mTOR signaling, and inhibiting autophagy through epigenetic mechanisms involving specific microRNAs, histone demethylase PHF8, and methylated histone H4K20me1. Clinical studies, also discussed in this review, show that cystathionine and Hcy-thiolactone are associated with myocardial infarction and ischemic stroke by influencing blood clotting. These findings contribute to our understanding of the complex mechanisms underlying endothelial dysfunction, atherosclerosis, CVD, and stroke and identify potential targets for therapeutic intervention.

Identification of Important Genes Associated with the Development of Atherosclerosis

Atherosclerosis is one of the most important medical problems due to its prevalence and significant contribution to the structure of temporary and permanent disability and mortality. Atherosclerosis is a complex chain of events occurring in the vascular wall over many years. Disorders of lipid metabolism, inflammation, and impaired hemodynamics are important mechanisms of atherogenesis. A growing body of evidence strengthens the understanding of the role of genetic and epigenetic factors in individual predisposition and development of atherosclerosis and its clinical outcomes. In addition, hemodynamic changes, lipid metabolism abnormalities, and inflammation are closely related and have many overlapping links in regulation. A better study of these mechanisms may improve the quality of diagnosis and management of such patients.

Novel wine in an old bottle: Preventive and therapeutic potentials of andrographolide in atherosclerotic cardiovascular diseases

Atherosclerotic cardiovascular disease (ASCVD) frequently results in sudden death and poses a serious threat to public health worldwide. The drugs approved for the prevention and treatment of ASCVD are usually used in combination but are inefficient owing to their side effects and single therapeutic targets. Therefore, the use of natural products in developing drugs for the prevention and treatment of ASCVD has received great scholarly attention. Andrographolide (AG) is a diterpenoid lactone compound extracted from Andrographis paniculata. In addition to its use in conditions such as sore throat, AG can be used to prevent and treat ASCVD. It is different from drugs that are commonly used in the prevention and treatment of ASCVD and can not only treat obesity, diabetes, hyperlipidaemia and ASCVD but also inhibit the pathological process of atherosclerosis (AS) including lipid accumulation, inflammation, oxidative stress and cellular abnormalities by regulating various targets and pathways. However, the pharmacological mechanisms of AG underlying the prevention and treatment of ASCVD have not been corroborated, which may hinder its clinical development and application. Therefore, this review summarizes the physiological and pathological mechanisms underlying the development of ASCVD and the in vivo and in vitro pharmacological effects of AG on the relative risk factors of AS and ASCVD. The findings support the use of the old pharmacological compound ('old bottle') as a novel drug ('novel wine') for the prevention and treatment of ASCVD. Additionally, this review summarizes studies on the availability as well as pharmaceutical and pharmacokinetic properties of AG, aiming to provide more information regarding the clinical application and further research and development of AG.

Association between Intestinal Microecological Changes and Atherothrombosis

Atherosclerosis (AS) is a chronic inflammatory disease of large- and medium-sized arteries that causes ischemic heart disease, strokes, and peripheral vascular disease, collectively called cardiovascular disease (CVD), and is the leading cause of CVD resulting in a high rate of mortality in the population. AS is pathological by plaque development, which is caused by lipid infiltration in the vessel wall, endothelial dysfunction, and chronic low-grade inflammation. Recently, more and more scholars have paid attention to the importance of intestinal microecological disorders in the occurrence and development of AS. Intestinal G-bacterial cell wall lipopolysaccharide (LPS) and bacterial metabolites, such as oxidized trimethylamine (TMAO) and short-chain fatty acids (SCFAs), are involved in the development of AS by affecting the inflammatory response, lipid metabolism, and blood pressure regulation of the body. Additionally, intestinal microecology promotes the progression of AS by interfering with the normal bile acid metabolism of the body. In this review, we summarize the research on the correlation between maintaining a dynamic balance of intestinal microecology and AS, which may be potentially helpful for the treatment of AS.

The Multifaceted Biology of PCSK9

This article reviews the discovery of PCSK9, its structure-function characteristics, and its presently known and proposed novel biological functions. The major critical function of PCSK9 deduced from human and mouse studies, as well as cellular and structural analyses, is its role in increasing the levels of circulating low-density lipoprotein (LDL)-cholesterol (LDLc), via its ability to enhance the sorting and escort of the cell surface LDL receptor (LDLR) to lysosomes. This implicates the binding of the catalytic domain of PCSK9 to the EGF-A domain of the LDLR. This also requires the presence of the C-terminal Cys/His-rich domain, its binding to the secreted cytosolic cyclase associated protein 1, and possibly another membrane-bound "protein X". Curiously, in PCSK9-deficient mice, an alternative to the downregulation of the surface levels of the LDLR by PCSK9 is taking place in the liver of female mice in a 17β-estradiol-dependent manner by still an unknown mechanism. Recent studies have extended our understanding of the biological functions of PCSK9, namely its implication in septic shock, vascular inflammation, viral infections (Dengue; SARS-CoV-2) or immune checkpoint modulation in cancer via the regulation of the cell surface levels of the T-cell receptor and MHC-I, which govern the antitumoral activity of CD8+ T cells. Because PCSK9 inhibition may be advantageous in these processes, the availability of injectable safe PCSK9 inhibitors that reduces by 50% to 60% LDLc above the effect of statins is highly valuable. Indeed, injectable PCSK9 monoclonal antibody or small interfering RNA could be added to current immunotherapies in cancer/metastasis.

Molecular Pharmacology of Inflammation Resolution in Atherosclerosis

Atherosclerosis is one of the most important problems of modern medicine as it is the leading cause of hospitalizations, disability, and mortality. The key role in the development and progression of atherosclerosis is the imbalance between the activation of inflammation in the vascular wall and the mechanisms of its control. The resolution of inflammation is the most important physiological mechanism that is impaired in atherosclerosis. The resolution of inflammation has complex, not fully known mechanisms, in which lipid mediators derived from polyunsaturated fatty acids (PUFAs) play an important role. Specialized pro-resolving mediators (SPMs) represent a group of substances that carry out inflammation resolution and may play an important role in the pathogenesis of atherosclerosis. SPMs include lipoxins, resolvins, maresins, and protectins, which are formed from PUFAs and regulate many processes related to the active resolution of inflammation. Given the physiological importance of these substances, studies examining the possibility of pharmacological effects on inflammation resolution are of interest.

Pathological Investigations of Intracranial Atherosclerosis Using Multiple Hypercholesterolemic Rabbit Models

BACKGROUND

Intracranial atherosclerosis (ICAS) is one of the most common causes of ischemic stroke, but there are few animal models that can recapitulate its pathological features. In this study, we examined ICAS pathological features and anatomic distributions using three types of hyperlipidemic rabbit models. We also investigated the effect of different lipoprotein profiles and hypertension on ICAS.

MATERIALS AND METHODS

We examined Watanabe heritable hyperlipidemic (WHHL) rabbits, apoE knockout (KO) rabbits and wild-type rabbits (WT) fed a cholesterol diet, in addition to WT rabbits fed a standard diet as a control. The whole brain was dissected and embedded in paraffin. Serial sections were stained with either hematoxylin/eosin or elastica van Gieson, or immunohistochemically stained with monoclonal antibodies against macrophages and smooth muscle cells. We investigated (1) the presence of cerebral atherosclerosis; (2) the lesion locations in the cerebral arteries; (3) the degree of lumen stenosis; (4) pathological features and cellular components of the lesions in these rabbits; and (5) whether hypertension affects ICAS.

RESULTS

ICAS was detected in apoE and WHHL rabbits, but not in WT rabbits. Compared with apoE KO rabbits, WHHL rabbits had greater ICAS. The lesions of cerebral atherosclerosis were mainly distributed at the bifurcations of the posterior cerebral artery, basilar artery and vertebral artery, and they were basically characterized by smooth muscle cells and extracellular matrix with few macrophages. The extent of the ICAS in WHHL rabbits was significantly increased by hypertension.

CONCLUSIONS

ICAS was detected in WHHL and apoE KO rabbits, and occurred in specific locations in the cerebral arteries. Hypertension promotes the development of ICAS in the setting of hypercholesterolemia.

Association between Toxoplasma gondii infection and coronary atherosclerosis

BACKGROUND

Toxoplasma gondii is a worldwide protozoon that can infect all nucleated vertebrate cells. Little information is available about the association between T. gondii infection and coronary atherosclerosis.

METHODS

A total of 320 cases were enrolled (160 patients with coronary atherosclerosis and 160 non-atherosclerotic individuals). Blood samples were collected to measure anti-T. gondii immunoglobulin G (IgG) antibodies using enzyme-linked immunosorbent assay (ELISA) and serum lipid profile. Coronary angiogram was also performed.

RESULTS

The seroprevalence of anti-Toxoplasma antibodies in atherosclerotic and non-atherosclerotic individuals was 63.1% and 46.2%, respectively, with higher levels of anti-T. gondii IgG in atherosclerotic patients. Consumption of contaminated water, unwashed fruits and vegetables and raw meat and contact with soil were significant risk factors for Toxoplasma infection. Significant differences were detected in serum levels of low-density lipoproteins, triglycerides and cholesterol between both groups. Positive correlations were detected between ELISA titres and serum levels of low-density lipoproteins, triglycerides and cholesterol, disease severity and the number of affected vessels. Male gender and contact with soil had a significant association with positive T. gondii serology in atherosclerotic patients.

CONCLUSIONS

Patients with coronary atherosclerosis have a high prevalence of T. gondii infection. More studies are crucial to elucidate the mechanisms underlying the effects of chronic toxoplasmosis on coronary atherosclerosis.

Mast Cells as Potential Accelerators of Human Atherosclerosis-From Early to Late Lesions

Mast cells are present in atherosclerotic lesions throughout their development. The process of atherogenesis itself is characterized by infiltration and retention of cholesterol-containing blood-derived low-density lipoprotein (LDL) particles in the intimal layer of the arterial wall, where the particles become modified and ingested by macrophages, resulting in the formation of cholesterol-filled foam cells. Provided the blood-derived high-density lipoproteins (HDL) particles are able to efficiently carry cholesterol from the foam cells back to the circulation, the early lesions may stay stable or even disappear. However, the modified LDL particles also trigger a permanent local inflammatory reaction characterized by the presence of activated macrophages, T cells, and mast cells, which drive lesion progression. Then, the HDL particles become modified and unable to remove cholesterol from the foam cells. Ultimately, the aging foam cells die and form a necrotic lipid core. In such advanced lesions, the lipid core is separated from the circulating blood by a collagenous cap, which may become thin and fragile and susceptible to rupture, so causing an acute atherothrombotic event. Regarding the potential contribution of mast cells in the initiation and progression of atherosclerotic lesions, immunohistochemical studies in autopsied human subjects and studies in cell culture systems and in atherosclerotic mouse models have collectively provided evidence that the compounds released by activated mast cells may promote atherogenesis at various steps along the path of lesion development. This review focuses on the presence of activated mast cells in human atherosclerotic lesions. Moreover, some of the molecular mechanisms potentially governing activation and effector functions of mast cells in such lesions are presented and discussed.

A cynomolgus monkey model of carotid atherosclerosis induced by puncturing and scratching of the carotid artery combined with a high-fat diet

Cardio-cerebrovascular disease is one of the three major causes of mortality in humans and constitutes a major socioeconomic burden. Carotid atherosclerosis (CAS) is a very common lesion of the arterial walls, which leads to narrowing of the arteries, in some cases occluding them entirely, increasing the risk of cardiovascular events. The aim of the present study was to evaluate a cynomolgus monkey model of carotid atherosclerosis (CAS) induced by puncturing and scratching combined with a high-fat diet. A total of 12 cynomolgus monkeys were randomly divided into four groups: A, puncturing and scratching carotid artery intimas + high-fat diet (n=3); B, puncturing and scratching carotid artery intimas + regular diet (n=3); C, high-fat diet only (n=3); and D, regular diet only (n=3). Blood was harvested at weeks 4, 6 and 8 and plasma lipid levels were assessed. At week 8, monkeys were sacrificed and carotid arteries were harvested for hematoxylin and eosin (H&E) staining to observe pathological changes. The results revealed that a high-fat diet led to increased plasma lipid levels and accelerated plaque formation. Carotid color Doppler ultrasonography was performed and, along with H&E staining, revealed plaque formation in group A. In summary, the results of the present study suggest that a cynomolgus monkey model of CAS model may be successfully constructed by puncturing and scratching of the carotid artery intimas in combination with a high-fat diet.

PCSK9 inhibitor valuation: A science-based review of the two recent models

Low‐density lipoprotein cholesterol (LDL‐C) has been extensively evaluated. Prospective cohort studies, randomized controlled trials, biology, pathophysiology, genetics, and Mendelian randomization studies, have clearly taught us that LDL‐C causes atherosclerotic cardiovascular disease. The newest class of drugs to lower LDL‐C, the proprotein convertase subtilisin/kexin type 9 (PCSK9) monoclonal antibodies, have been found to safely reduce LDL‐C approximately 60% when added to high‐intensity statin therapy. Because their cost is much greater than that of the currently available agents, their value has been questioned. In late August, 2017, two groups assessed the value of this class of drugs looking at cost‐effectiveness; however, the Institute for Clinical and Economic Review and Fonarow and colleagues found disparate results when assessing PCSK9 valuation. Herein, we review the evolution of LDL‐C from hypothesis to fact, and then attempt to adjudicate the 2 models, shedding light on the complex modeling process. We find that models of cost‐effectiveness are helpful adjuncts to decision making, but that their conclusions depend on many assumptions. Ultimately, clinician judgment regarding their clinical benefit, balanced by some estimation of cost, may be more productive to target the right patients for whom the benefits can be well‐justified.

The Evidence for Saturated Fat and for Sugar Related to Coronary Heart Disease

Dietary guidelines continue to recommend restricting intake of saturated fats. This recommendation follows largely from the observation that saturated fats can raise levels of total serum cholesterol (TC), thereby putatively increasing the risk of atherosclerotic coronary heart disease (CHD). However, TC is only modestly associated with CHD, and more important than the total level of cholesterol in the blood may be the number and size of low-density lipoprotein (LDL) particles that contain it. As for saturated fats, these fats are a diverse class of compounds; different fats may have different effects on LDL and on broader CHD risk based on the specific saturated fatty acids (SFAs) they contain. Importantly, though, people eat foods, not isolated fatty acids. Some food sources of SFAs may pose no risk for CHD or possibly even be protective. Advice to reduce saturated fat in the diet without regard to nuances about LDL, SFAs, or dietary sources could actually increase people's risk of CHD. When saturated fats are replaced with refined carbohydrates, and specifically with added sugars (like sucrose or high fructose corn syrup), the end result is not favorable for heart health. Such replacement leads to changes in LDL, high-density lipoprotein (HDL), and triglycerides that may increase the risk of CHD. Additionally, diets high in sugar may induce many other abnormalities associated with elevated CHD risk, including elevated levels of glucose, insulin, and uric acid, impaired glucose tolerance, insulin and leptin resistance, non-alcoholic fatty liver disease, and altered platelet function. A diet high in added sugars has been found to cause a 3-fold increased risk of death due to cardiovascular disease, but sugars, like saturated fats, are a diverse class of compounds. The monosaccharide, fructose, and fructose-containing sweeteners (e.g., sucrose) produce greater degrees of metabolic abnormalities than does glucose (either isolated as a monomer, or in chains as starch) and may present greater risk of CHD. This paper reviews the evidence linking saturated fats and sugars to CHD, and concludes that the latter is more of a problem than the former. Dietary guidelines should shift focus away from reducing saturated fat, and from replacing saturated fat with carbohydrates, specifically when these carbohydrates are refined. To reduce the burden of CHD, guidelines should focus particularly on reducing intake of concentrated sugars, specifically the fructose-containing sugars like sucrose and high-fructose corn syrup in the form of ultra-processed foods and beverages.

Arteriosclerosis: facts and fancy

Arterial vascular diseases comprise the leading cause of death in the industrialized world. Every physician learns about the pathology of these diseases in medical school. All pathologists evaluate arterial disease in surgical pathology and/or autopsy specimens. All clinicians encounter patients with clinical manifestations of these diseases. With such a common and clinically-important group of entities one would think there would be a general understanding of the "known" information that exists. That is, physicians and scientists should be able to separate what is fact and what is fancy. This review article is intended to generate thought in this regard.

Consensus statement on surgical pathology of the aorta from the Society for Cardiovascular Pathology and the Association for European Cardiovascular Pathology: I. Inflammatory diseases

Inflammatory diseases of the aorta include routine atherosclerosis, aortitis, periaortitis, and atherosclerosis with excessive inflammatory responses, such as inflammatory atherosclerotic aneurysms. The nomenclature and histologic features of these disorders are reviewed and discussed. In addition, diagnostic criteria are provided to distinguish between these disorders in surgical pathology specimens. An initial classification scheme is provided for aortitis and periaortitis based on the pattern of the inflammatory infiltrate: granulomatous/giant cell pattern, lymphoplasmacytic pattern, mixed inflammatory pattern, and the suppurative pattern. These inflammatory patterns are discussed in relation to specific systemic diseases including giant cell arteritis, Takayasu arteritis, granulomatosis with polyangiitis (Wegener's), rheumatoid arthritis, sarcoidosis, ankylosing spondylitis, Cogan syndrome, Behçet's disease, relapsing polychondritis, syphilitic aortitis, and bacterial and fungal infections.

Oxidized lipids and lysophosphatidylcholine induce the chemotaxis, up-regulate the expression of CCR9 and CXCR4 and abrogate the release of IL-6 in human monocytes

Lipids through regulation of chronic inflammation play key roles in the development of various diseases. Here, we report that a mixed population of human primary monocytes migrated towards LPC, as well as oxidized linoleic acid isoforms 9-S-HODE, 9-R-HODE and 13-R-HODE. Incubation with 9-R-HODE, 13-R-HODE and LPC resulted in increased expression of CXCR4, the receptor for SDF-1α/CXCL12, correlated with increased monocyte migration towards SDF-1α/CXCL12. Further, we report increased expression of CCR9, the receptor for TECK/CCL25, after stimulation with these lipids. Upon examining the migratory response towards TECK/CCL25, it was observed that an increase in CCR9 expression upon pre-treatment with 9-S-HODE, 9-R-HODE, 13-R-HODE and LPC resulted in increased migration of monocytes expressing CCR9. Only LPC but not any other lipid examined increased the influx of intracellular Ca²⁺ in monocytes. Finally, 9-S-HODE, 9-R-HODE, 13-R-HODE, or LPC inhibited the release of IL-6 from monocytes suggesting that these lipids may play important role in controlling inflammatory responses.