HDL: structure, function and metabolism.

The major advances in our knowledge of the structure, function and metabolism of the plasma lipoproteins have occurred as a result of the rapid increase in our knowledge of the structure and function of the apolipoproteins, lipoproteins, and the heterogeneity of the individual classes of lipoproteins. Over the last decade, there has been a tremendous increase in our knowledge of the structure and molecular properties of ApoA-I and ApoA-II which has permitted an analysis of the functions of these apolipoproteins in lipid and lipoprotein metabolism and the initiation of kinetic studies of HDL metabolism. The elucidation of the structures of the ApoA-I and ApoA-II genes has permitted the determination of genetic defects resulting in decreased levels of HDL and premature cardiovascular disease, as well as the identification of new diseases (e.g. hereditary systemic amyloidosis). The future focus of research on HDL will be the analysis of the individual lipoprotein particles within HDL which have different physiological functions and important roles in reverse cholesterol transport. An improved understanding of the role of HDL in the transport of cellular cholesterol to the liver and the exchange of cholesterol between plasma lipoproteins will provide critical information on cholesterol metabolism in normal subjects and permit the elucidation of the molecular defects of new genetic diseases which may be associated with the development of premature cardiovascular disease.     

Lipid profiling of lipoproteins by electrospray ionization tandem mass spectrometry

Lipoproteins are of fundamental importance for the lipid transport and cardiovascular disease. The function and metabolism of lipoproteins is intimately linked to the biophysical properties of their surface lipids. Although a number of disease associations were found for lipid species in plasma, only a few studies reported lipid profiles of lipoproteins. Here, we provide an overview of techniques for lipoprotein separation, methods for lipid species analysis based on electrospray ionization tandem mass spectrometry (ESI-MS/MS) as well as data from recent lipidomic studies on lipoprotein fractions. We also discuss the different analytical strategies and how lipid profiling can expand our understanding of the biology and structures of lipoproteins.     

Clinical significance of apolipoprotein A5

PURPOSE OF REVIEW: We have examined the evidence from recent human studies examining the role of apolipoprotein A-V in triglyceride-rich lipoprotein metabolism and cardiovascular disease risk. Special emphasis was placed on the evidence emerging from the association between genetic variability at the apolipoprotein A5 locus, lipid phenotypes and disease outcomes. Moreover, we address recent reports evaluating apolipoprotein A5 gene-environment interactions in relation to cardiovascular disease and its common risk factors. RECENT FINDINGS: Several genetic association studies have continued to strengthen the position of APOA5 as a major gene that is involved in triglyceride metabolism and modulated by dietary factors and pharmacological therapies. Moreover, genetic variants at this locus have been significantly associated with both coronary disease and stroke risks. SUMMARY: Apolipoprotein A-V has an important role in lipid metabolism, specifically for triglyceride-rich lipoproteins. However, its mechanism of action is still poorly understood. Clinical significance at present comes largely from genetic studies showing a consistent association with plasma triglyceride concentrations. Moreover, the effects of common genetic variants on triglyceride concentrations and disease risk are further modulated by other factors such as diet, pharmacological interventions and BMI. Therefore, these genetic variants could be potentially used to predict cardiovascular disease risk and individualize therapeutic options to decrease cardiovascular disease risk.     

Mammalian alpha beta hydrolase domain (ABHD) proteins: Lipid metabolizing enzymes at the interface of cell signaling and energy metabolism

Dysregulation of lipid metabolism underlies many chronic diseases such as obesity, diabetes, cardiovascular disease, and cancer. Therefore, understanding enzymatic mechanisms controlling lipid synthesis and degradation is imperative for successful drug discovery for these human diseases. Genes encoding α/β hydrolase fold domain (ABHD) proteins are present in virtually all reported genomes, and conserved structural motifs shared by these proteins predict common roles in lipid synthesis and degradation. However, the physiological substrates and products for these lipid metabolizing enzymes and their broader role in metabolic pathways remain largely uncharacterized. Recently, mutations in several members of the ABHD protein family have been implicated in inherited inborn errors of lipid metabolism. Furthermore, studies in cell and animal models have revealed important roles for ABHD proteins in lipid metabolism, lipid signal transduction, and metabolic disease. The purpose of this review is to provide a comprehensive summary surrounding the current state of knowledge regarding mammalian ABHD protein family members. In particular, we will discuss how ABHD proteins are ideally suited to act at the interface of lipid metabolism and signal transduction. Although, the current state of knowledge regarding mammalian ABHD proteins is still in its infancy, this review highlights the potential for the ABHD enzymes as being attractive targets for novel therapies targeting metabolic disease.     

Single-molecule 3D imaging of human plasma intermediate-density lipoproteins reveals a polyhedral structure

Intermediate-density lipoproteins (IDLs), the remnants of very-low-density lipoproteins via lipolysis, are rich in cholesteryl ester and are associated with cardiovascular disease. Despite pharmacological interest in IDLs, their three-dimensional (3D) structure is still undetermined due to their variation in size, composition, and dynamic structure. To explore the 3D structure of IDLs, we reconstructed 3D density maps from individual IDL particles using cryo-electron microscopy (cryo-EM) and individual-particle electron tomography (IPET, without averaging from different molecules). 3D reconstructions of IDLs revealed an unexpected polyhedral structure that deviates from the generally assumed spherical shape model (Frias et al., 2007; Olson, 1998; Shen et al., 1977). The polyhedral-shaped IDL contains a high-density shell formed by flat surfaces that are similar to those of very-low-density lipoproteins but have sharper dihedral angles between nearby surfaces. These flat surfaces would be less hydrophobic than the curved surface of mature spherical high-density lipoprotein (HDL), leading to a lower binding affinity of IDL to hydrophobic proteins (such as cholesteryl ester transfer protein) than HDL. This is the first visualization of the IDL 3D structure, which could provide fundamental clues for delineating the role of IDL in lipid metabolism and cardiovascular disease.     

Recent advances in cytoplasmic lipid droplet metabolism in intestinal enterocyte

Processing of dietary fats in the intestine is a highly regulated process that influences whole-body energy homeostasis and multiple physiological functions. Dysregulated lipid handling in the intestine leads to dyslipidemia and atherosclerotic cardiovascular disease. In intestinal enterocytes, lipids are incorporated into lipoproteins and cytoplasmic lipid droplets (CLDs). Lipoprotein synthesis and CLD metabolism are inter-connected pathways with multiple points of regulation. This review aims to highlight recent advances in the regulatory mechanisms of lipid processing in the enterocyte, with particular focus on CLDs. In-depth understanding of the regulation of lipid metabolism in the enterocyte may help identify therapeutic targets for the treatment and prevention of metabolic disorders.     

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.     

Genes influencing HDL metabolism: new perspectives and implications for atherosclerosis prevention

Atherosclerotic cardiovascular disease (ASCVD) is the most common cause of morbidity and mortality in Western societies. Current therapies, such as reduction of plasma cholesterol, significantly reduce, but do not come close to eliminating, the complications of ASCVD. Therefore, novel therapeutic approaches to the prevention of acute coronary events and progression of atherosclerosis are still needed. The complex metabolism of high density lipoproteins represents an attractive potential target for therapeutic intervention. Here, we will discuss those components of the high density lipoprotein metabolism and lipid transport pathways that are potential preventative or therapeutic targets for ASCVD.     

Molecular mechanisms,prevalence, and molecular methods for familial combined hyperlipidemia disease: A review

Familial combined hyperlipidemia (FCHL) is the most common genetic dyslipidemia disorder which is accompanied by increasing of triglyceride and cholesterol. This disorder is a complex genetic disease although it also has monogenic forms. The familial form has several criteria for diagnosis that can be distinguished of nonfamilial position. It has been shown that a variety of internal and external risk factors are involved in the pathogenesis of FCHL. Environmental factors and the genetic background also play an important role in the FCHL pathogenesis. Many mechanisms and pathways are involved in lipid metabolism (ie, dysfunctional adipose tissue, hepatic fat and very low-density lipoprotein overproduction, triglyceride-rich lipoproteins, and clearance of low-density lipoprotein particles) that could lead to FCHL. Individuals with a positive family history like those who have a positive family history of cardiovascular diseases are more predispositions for this disorder. To date several methods have been used to identify the genetic background of the FCHL. In the current review, we summarized the prevalence and the molecular mechanisms involved in the FCHL disease. Moreover, we highlighted the used molecular methods for determining the genes involved in the FCHL.     

Determination of the surface charge of lipoproteins and its changes during lipid peroxidation

Surface potential of human plasma lipoproteins was studied by the use of positively charged spin probe. The calculated values of surface potential of high and low density lipoproteins appeared to be -29 +/- 1 and -16 +/- 1 respectively. It was shown that lipid peroxidation process induces an increase of surface potential of both high and low density lipoproteins. Probably, it is connected with the increase of the negative charge density on their surface. This fact can play an important role in pathogenesis of diseases with lipid metabolism and lipid peroxidation level disorders in plasma (atherosclerosis, ischemic heart disease etc.).     

脂蛋白质组学研究进展

脂蛋白质组学(lipoproteomics)是一门应用蛋白质组学技术对脂蛋白(lipoprotein)进行全面、系统地分析和鉴定,进而了解脂蛋白的组成和功能以及与相关疾病发生、发展之间关系的新兴学科.近些年来,脂蛋白质组学研究促进了脂蛋白中蛋白质组分的急性期响应、补体激活、免疫响应、炎症响应、蛋白酶抑制等新功能的发现,显示了广阔的应用前景.本文对脂蛋白的功能和分类,以及目前应用于脂蛋白质组学研究的脂蛋白分离方法和蛋白质鉴定方法进行了简述,并综述了高密度脂蛋白、低密度脂蛋白和极低密度脂蛋白的脂蛋白质组学研究,及其在冠状动脉疾病、糖尿病、类风湿性关节炎等疾病研究的最新进展.     

Tumor necrosis factor, interleukin, and interferon induced changes in lipid metabolism as part of host defense.

As the immune response is activated during infection, multiple changes in lipid metabolism, especially increased production of VLDL, occur. Many of the cytokines that mediate the immune response are able to produce such changes in lipid metabolism in vivo. The induction of hypertriglyceridemia or other changes in lipid metabolism during infection do not directly cause the wasting syndrome. It appears that such changes in lipid metabolism may be beneficial to the host, as lipoproteins inactivate a variety of infectious agents. Cytokine-driven hepatic VLDL production during infection most likely represents a part of the acute phase response. The body is thus able to increase serum lipids during infection, or at least maintain triglyceride-rich lipoproteins despite the anorexia of infection. In this manner, the anti-infective, protective effects of lipoproteins are maintained.     

Thematic Review Series: High Density Lipoprotein Structure,Function, and Metabolism: Proteomic diversity of high density lipoproteins: our emerging understanding of its importance in lipid transport and beyond

Recent applications of mass spectrometry technology have dramatically increased our understanding of the proteomic diversity of high density lipoproteins (HDL). Depending on the method of HDL isolation, upwards of 85 proteins have been identified, and the list continues to grow. In addition to proteins consistent with traditionally accepted roles in lipid transport, HDL carries surprising constituents, such as members of the complement pathway, protease inhibitors involved in hemostasis, acute-phase response proteins, immune function mediators, and even metal-binding proteins. This compositional diversity fits well with hundreds of studies demonstrating a wide functional pleiotrophy, including roles in lipid transport, oxidation, inflammation, hemostasis, and immunity. This review summarizes the progression of our understanding of HDL proteomic complexity and points out key experimental observations that reinforce the functional diversity of HDL. The possibility of specific HDL subspecies with distinct functions, the evidence supporting this concept, and some of the best examples of experimentally defined HDL subspecies are also discussed. Finally, key challenges facing the field are highlighted, particularly the need to identify and define the function of HDL subspecies to better inform attempts to pharmacologically manipulate HDL for the benefit of cardiovascular disease and possibly other maladies.     

Composition and lipid spatial distribution of HDL particles in subjects with low and high HDL-cholesterol

A low level of high density lipoprotein cholesterol (HDL-C) is a powerful risk factor for cardiovascular disease. However, despite the reported key role of apolipo-proteins, specifically, apoA-I, in HDL metabolism, lipid molecular composition of HDL particles in subjects with high and low HDL-C levels is currently unknown. Here lipidomics was used to study HDL derived from well-characterized high and low HDL-C subjects. Low HDL-C subjects had elevated triacylglycerols and diminished lysophosphatidylcholines and sphingomyelins. Using information about the lipid composition of HDL particles in these two groups, we reconstituted HDL particles in silico by performing large-scale molecular dynamics simulations. In addition to confirming the measured change in particle size, we found that the changes in lipid composition also induced specific spatial distributions of lipids within the HDL particles, including a higher amount of triacylglycerols at the surface of HDL particles in low HDL-C subjects. Our findings have important implications for understanding HDL metabolism and function. For the first time we demonstrate the power of combining molecular profiling of lipoproteins with dynamic modeling of lipoprotein structure.     

Cellular mechanisms of vitamin E uptake: relevance in alpha-tocopherol metabolism and potential implications for disease

alpha-Tocopherol is an essential micronutrient involved in various oxidative stress-related processes. Because of its hydrophobic nature, alpha-tocopherol is transported in plasma lipoproteins, and the pathways involved in its cellular uptake are closely related to the lipoprotein metabolism. alpha-Tocopherol transfer from plasma to cells can occur by different mechanisms such as uptake facilitated by lipid transfer proteins and lipases, receptor-mediated lipoprotein endocytosis, and selective lipid uptake. Here we discuss recent progress in understanding the physiological and pathophysiological relevance of these different pathways for cellular uptake of vitamin E in vivo. This review is mainly focused on the role of the scavenger receptor class B type I (SR-BI) on alpha-tocopherol metabolism and its potential implications for disease conditions.     

Modification by acrolein, a component of tobacco smoke and age-related oxidative stress, mediates functional impairment of human apolipoprotein E

Oxidative damage to proteins such as apolipoprotein B-100 increases the atherogenicity of low-density lipoproteins (LDL). However, little is known about the potential oxidative damage to apolipoprotein E (apoE), an exchangeable antiatherogenic apolipoprotein. ApoE plays an integral role in lipoprotein metabolism by regulating the plasma cholesterol and triglyceride levels. Hepatic uptake of lipoproteins is facilitated by apoE's ability to bind with cell surface heparan sulfate proteoglycans and to lipoprotein receptors via basic residues in its 22 kDa N-terminal domain (NT). We investigated the effect of acrolein, an aldehydic product of endogenous lipid peroxidation and a tobacco smoke component, on the conformation and function of recombinant human apoE3-NT. Acrolein caused oxidative modification of apoE3-NT as detected by Western blot with acrolein-lysine-specific antibodies, and tertiary conformational alterations. Acrolein modification impairs the ability of apoE3-NT to interact with heparin and the LDL receptor. Furthermore, acrolein-modified apoE3-NT displayed a 5-fold decrease in its ability to interact with lipid surfaces. Our data indicate that acrolein disrupts the functional integrity of apoE3, which likely interferes with its role in regulating plasma cholesterol homeostasis. These observations have implications regarding the role of apoE in the pathogenesis of smoking- and oxidative stress-mediated cardiovascular and cerebrovascular diseases.     

Macrophages heterogeneity in atherosclerosis – implications for therapy

Atherosclerosis is a chronic inflammatory disease occurring within the artery wall and is an underlying cause of cardiovascular complications, including myocardial infarction, stroke and peripheral vascular disease. Its pathogenesis involves many immune cell types with a well accepted role for monocyte/macrophages. Cholesterol-loaded macrophages are a characteristic feature of plaques and are major players in all stages of plaque development. As well as modulating lipid metabolism, macrophages secrete inflammatory cytokines, chemokines and reactive oxygen and nitrogen species that drive pathogenesis. They also produce proteases and tissue factor that contribute to plaque rupture and thrombosis. Macrophages are however heterogeneous cells and when appropriately activated, they phagocytose cytotoxic lipoproteins, clear apoptotic bodies, secrete anti-inflammatory cytokines and synthesize matrix repair proteins that stabilize vulnerable plaques. Pharmacological modulation of macrophage activity therefore represents a potential therapeutic strategy for atherosclerosis. The aim of this review is to provide an overview of the current understanding of the different macrophage subsets and their monocyte precursors, and, the implications of these subsets for atherosclerosis. This will present a foundation for highlighting novel opportunities to exploit the heterogeneity of macrophages as important diagnostic and therapeutic targets for atherosclerosis and its associated diseases.     

A monomeric, biologically active, full-length human apolipoprotein E

Apolipoprotein E (apoE) is an exchangeable apolipoprotein that plays an important role in lipid/lipoprotein metabolism and cardiovascular diseases. Recent evidence indicates that apoE is also critical in several other important biological processes, including Alzheimer's disease, cognitive function, immunoregulation, cell signaling, and infectious diseases. Although the X-ray crystal structure of the apoE N-terminal domain was solved in 1991, the structural study of full-length apoE is hindered by apoE's oligomerization property. Using protein-engineering techniques, we generated a monomeric, biologically active, full-length apoE. Cross-linking experiments indicate that this mutant is nearly 95-100% monomeric even at 20 mg/mL. CD spectroscopy and guanidine hydrochloride denaturation demonstrate that the structure and stability of the monomeric mutant are identical to wild-type apoE. Monomeric and wild-type apoE display similar lipid-binding activities in dimyristoylphosphatidylcholine clearance assays and formation of reconstituted high-density lipoproteins. Furthermore, the monomeric and wild-type apoE proteins display an identical LDL receptor binding activity. Availability of this monomeric, biologically active, full-length apoE allows us to collect high quality NMR data for structural determination. Our initial NMR data of lipid-free apoE demonstrates that the N-terminal domain in the full-length apoE adopts a nearly identical structure as the isolated N-terminal domain, whereas the C-terminal domain appears to become more structured than the isolated C-terminal domain fragment, suggesting a weak domain-domain interaction. This interaction is confirmed by NMR examination of a segmental labeled apoE, in which the N-terminal domain is deuterated and the C-terminal domain is double-labeled. NMR titration experiments further suggest that the hinge region (residues 192-215) that connects apoE's N- and C-terminal domains may play an important role in mediating this domain-domain interaction.     

综述: HDL蛋白质组分临床研究新进展

高密度脂蛋白胆固醇(HDL-C)水平与冠心病风险呈负相关,低HDL-C水平增加心血管疾病风险,是心血管疾病的独立危险因素．然而升高HDL-C水平的药物治疗并没有明显的临床获益,没有起到降低心血管疾病风险的预期效果,因此高密度脂蛋白(HDL)功能比HDL-C水平更好地预测心血管事件的发生．HDL是蛋白质含量最高的脂蛋白,由于蛋白质组学技术的进步,越来越多的HDL蛋白质成分被发现,除了传统的载脂蛋白、酶类,还包括脂质转移蛋白、急性期反应蛋白、补体成分、蛋白酶抑制剂,HDL的功能也从脂质转运扩展到感染免疫、急性期反应、补体激活、离子结合等,不仅参与动脉粥样硬化的发生发展,在终末期肾病、糖尿病等高心血管风险疾病中也发挥重要作用．本文就HDL蛋白质成分、功能及在冠心病和高心血管风险疾病中的作用做一综述．     

The role of CD36 in the regulation of myocardial lipid metabolism

Since the heart has one of the highest energy requirements of all organs in the body, it requires a constant and plentiful supply of fuel to function properly. Mitochondrial oxidation of lipids provides a major source of ATP for the heart, and the cellular processes that regulate lipid uptake and utilization are important contributors to maintaining proper myocardial energetic status. Although numerous proteins are coordinately regulated in order to ensure proper fatty acid utilization in the cardiomyocyte, a key first step in this process is the entry of fatty acids into the cell. An important protein involved in the transport of fatty acids into the cardiomyocyte is the plasma membrane-associated protein known as fatty acid translocase (FAT; also known as CD36). While multiple proteins are involved in facilitating fatty acid uptake in the heart, CD36 accounts for approximately 50–70% of the total fatty acid taken up in cardiomyocytes. As such, myocardial metabolism of fatty acids may depend upon proper CD36 function. Consistent with this, changes in CD36 levels/function have been implicated in the alteration of myocardial metabolism in the pathophysiology of certain cardiovascular diseases. As such, a better understanding of the role and function of CD36 in the heart may provide important insights for the development of new treatments for specific cardiovascular diseases. Herein, we review the role of CD36 in myocardial lipid metabolism in the healthy heart and describe how CD36-mediated alterations in lipid metabolism may contribute to cardiovascular disease. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.