High-density lipoprotein: our elusive friend

Relationships between HDL subpopulation proteome and HDL function in overweight/obese people with and without coronary heart disease

BACKGROUND AND AIMS

The structure-function relationships of high-density lipoprotein (HDL) subpopulations are not well understood. Our aim was to examine the interrelationships between HDL particle proteome and HDL functionality in subjects with and without coronary heart disease (CHD).

METHODS

We isolated 5 different HDL subpopulations based on charge, size, and apolipoprotein A1 (APOA1) content from the plasma of 33 overweight/obese CHD patients and 33 age-and body mass index (BMI)-matched CHD-free subjects. We measured the relative molar concentration of HDL-associated proteins by liquid chromatography tandem mass spectrometry (LC-MS/MS) and assessed particle functionality.

RESULTS

We quantified 110 proteins associated with the 5 APOA1-containing HDL subpopulations. The relative molar concentration of these proteins spanned five orders of magnitude. Only 10 proteins were present in >1% while 73 were present in <0.1% concentration. Only 6 of the 10 most abundant proteins were apolipoproteins. Interestingly, the largest (α-1) and the smallest (preβ-1) HDL particles contained the most diverse proteomes. The protein composition of each HDL subpopulation was altered in CHD cases as compared to controls with the most prominent differences in preβ-1 and α-1 particles. APOA2 concentration was positively correlated with preβ-1 particle functionality (ABCA1-CEC/mg APOA1 in preβ-1) (R2 = 0.42, p = 0.005), while APOE concentration was inversely correlated with large-HDL particle functionality (SRBI-CEC/mg APOA1 in α-1+α-2) (R2 = 0.18, p = 0.01).

CONCLUSIONS

The protein composition of the different HDL subpopulations was altered differentially in CHD patients. The functionality of the small and large HDL particles correlated with the protein content of APOA2 and APOE, respectively. Our data indicate that distinct particle subspecies and specific particle associated proteins provide new information about the role of HDL in CHD.

Lipid and Lipoprotein Metabolism

The exogenous lipoprotein pathway starts with the incorporation of dietary lipids into chylomicrons in the intestine. Chylomicron triglycerides are metabolized in muscle and adipose tissue and chylomicron remnants are formed, which are removed by the liver. The endogenous lipoprotein pathway begins in the liver with the formation of very low-density lipoprotein particles (VLDL). VLDL triglycerides are metabolized in muscle and adipose tissue forming intermediate-density lipoprotein (IDL), which may be taken up by the liver or further metabolized to low-density lipoprotein (LDL). Reverse cholesterol transport begins with the formation of nascent high-density lipoprotein (HDL) by the liver and intestine that acquire cholesterol from cells resulting in mature HDL. The HDL then transports the cholesterol to the liver either directly or indirectly by transferring the cholesterol to VLDL or LDL.

A Modern Approach to Dyslipidemia

Lipid disorders involving derangements in serum cholesterol, triglycerides, or both are commonly encountered in clinical practice and often have implications for cardiovascular risk and overall health. Recent advances in knowledge, recommendations, and treatment options have necessitated an updated approach to these disorders. Older classification schemes have outlived their usefulness, yielding to an approach based on the primary lipid disturbance identified on a routine lipid panel as a practical starting point. Although monogenic dyslipidemias exist and are important to identify, most individuals with lipid disorders have polygenic predisposition, often in the context of secondary factors such as obesity and type 2 diabetes. With regard to cardiovascular disease, elevated low-density lipoprotein cholesterol is essentially causal, and clinical practice guidelines worldwide have recommended treatment thresholds and targets for this variable. Furthermore, recent studies have established elevated triglycerides as a cardiovascular risk factor, whereas depressed high-density lipoprotein cholesterol now appears less contributory than was previously believed. An updated approach to diagnosis and risk assessment may include measurement of secondary lipid variables such as apolipoprotein B and lipoprotein(a), together with selective use of genetic testing to diagnose rare monogenic dyslipidemias such as familial hypercholesterolemia or familial chylomicronemia syndrome. The ongoing development of new agents-especially antisense RNA and monoclonal antibodies-targeting dyslipidemias will provide additional management options, which in turn motivates discussion on how best to incorporate them into current treatment algorithms.

Purification and Analysis of Circulating Lipid Particles

Lipid particles found in circulating extracellular fluids such as blood or lymph are essential for cellular homeostasis, metabolism and survival. Such particles provide essential lipids and fats which enable cells to synthesize new membranes and regulate different biochemical pathways. Imbalance in lipid particle metabolism can cause pathological states such as atherosclerosis. Here, elevated low-density lipoprotein (LDL) accumulation leads to fat-filled lesions or plaques in arterial walls. In this chapter, we provide a detailed set of protocols for the rapid and safe purification of lipid particles from human blood using high-speed ultracentrifugation. We provide a detailed set of assays for further analysis of the biochemical and cellular properties of these lipid particles. By combining these assays, we can better understand the complex roles of different lipid particles in normal physiology and disease pathology.

Lipidomic risk scores are independent of polygenic risk scores and can predict incidence of diabetes and cardiovascular disease in a large population cohort

Type 2 diabetes (T2D) and cardiovascular disease (CVD) represent significant disease burdens for most societies and susceptibility to these diseases is strongly influenced by diet and lifestyle. Physiological changes associated with T2D or CVD, such has high blood pressure and cholesterol and glucose levels in the blood, are often apparent prior to disease incidence. Here we integrated genetics, lipidomics, and standard clinical diagnostics to assess future T2D and CVD risk for 4,067 participants from a large prospective population-based cohort, the Malmö Diet and Cancer-Cardiovascular Cohort. By training Ridge regression-based machine learning models on the measurements obtained at baseline when the individuals were healthy, we computed several risk scores for T2D and CVD incidence during up to 23 years of follow-up. We used these scores to stratify the participants into risk groups and found that a lipidomics risk score based on the quantification of 184 plasma lipid concentrations resulted in a 168% and 84% increase of the incidence rate in the highest risk group and a 77% and 53% decrease of the incidence rate in lowest risk group for T2D and CVD, respectively, compared to the average case rates of 13.8% and 22.0%. Notably, lipidomic risk correlated only marginally with polygenic risk, indicating that the lipidome and genetic variants may constitute largely independent risk factors for T2D and CVD. Risk stratification was further improved by adding standard clinical variables to the model, resulting in a case rate of 51.0% and 53.3% in the highest risk group for T2D and CVD, respectively. The participants in the highest risk group showed significantly altered lipidome compositions affecting 167 and 157 lipid species for T2D and CVD, respectively. Our results demonstrated that a subset of individuals at high risk for developing T2D or CVD can be identified years before disease incidence. The lipidomic risk, which is derived from only one single mass spectrometric measurement that is cheap and fast, is informative and could extend traditional risk assessment based on clinical assays.

Apolipoprotein A1, the neglected relative of Apolipoprotein E and its potential role in Alzheimer's disease

Lipoproteins are multi-molecule assemblies with the primary function of transportation and processing of lipophilic substances within aqueous bodily fluids (blood, cerebrospinal fluid). Nevertheless, they also exert other physiological functions such as immune regulation. In particular, neurons are both sensitive to uncontrolled responses of the immune system and highly dependent on a controlled and sufficient supply of lipids. For this reason, the role of certain lipoproteins and their protein-component (apolipoproteins, Apo's) in neurological diseases is perceivable. ApoE, for example, is well-accepted as one of the major risk factors for sporadic Alzheimer's disease with a protective allele variant (ε2) and a risk-causing allele variant (ε4). ApoA1, the major protein component of high-density lipoproteins, is responsible for transportation of excess cholesterol from peripheral tissues to the liver. The protein is synthesized in the liver and intestine but also can enter the brain via the choroid plexus and thereby might have an impact on brain lipid homeostasis. This review focuses on the role of ApoA1 in Alzheimer's disease and discusses whether its role within this neurodegenerative disorder is specific or represents a general neuroprotective mechanism.

Early Signs of Atherogenic Features in the HDL Lipidomes of Normolipidemic Patients Newly Diagnosed with Type 2 Diabetes

Cardiovascular disease (CVD) is the major cause of death in patients with type-2 diabetes mellitus (T2DM), although the factors that accelerate atherosclerosis in these patients are poorly understood. The identification of the altered quantity and quality of lipoproteins, closely related to atherogenesis, is limited in routine to a pattern of high triglycerides and low HDL-cholesterol (HDL-C) and in research as dysfunctional HDLs. We used the emerging NMR-based lipidomic technology to investigate compositional features of the HDLs of healthy individuals with normal coronary arteries, drug-naïve; recently diagnosed T2DM patients with normal coronary arteries; and patients with recent acute coronary syndrome. Patients with T2DM and normal serum lipid profiles even at diagnosis presented significant lipid alterations in HDL, characterized by higher triglycerides, lysophosphatidylcholine and saturated fatty acids; and lower cholesterol, phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, plasmalogens and polyunsaturated fatty acids, an atherogenic pattern that may be involved in the pathogenesis of atherosclerosis. These changes are qualitatively similar to those found, more profoundly, in normolipidemic patients with established Coronary Heart Disease (CHD). We also conclude that NMR-based lipidomics offer a novel holistic exploratory approach for identifying and quantifying lipid species in biological matrixes in physiological processes and disease states or in disease biomarker discovery.

OX-HDL: A Starring Role in Cardiorenal Syndrome and the Effects of Heme Oxygenase-1 Intervention

In this review, we will evaluate how high-density lipoprotein (HDL) and the reverse cholesterol transport (RCT) pathway are critical for proper cardiovascular-renal physiology. We will begin by reviewing the basic concepts of HDL cholesterol synthesis and pathway regulation, followed by cardiorenal syndrome (CRS) pathophysiology. After explaining how the HDL and RCT pathways become dysfunctional through oxidative processes, we will elaborate on the potential role of HDL dysfunction in CRS. We will then present findings on how HDL function and the inducible antioxidant gene heme oxygenase-1 (HO-1) are interconnected and how induction of HO-1 is protective against HDL dysfunction and important for the proper functioning of the cardiovascular-renal system. This will substantiate the proposal of HO-1 as a novel therapeutic target to prevent HDL dysfunction and, consequently, cardiovascular disease, renal dysfunction, and the onset of CRS.

Deepening our understanding of HDL proteome

Introduction: High-density lipoprotein (HDL) particles are heterogeneous and their proteome is complex and distinct from HDL cholesterol. However, it is largely unknown whether HDL proteins are associated with cardiovascular protection. Areas covered: HDL isolation techniques and proteomic analyses are reviewed. A list of HDL proteins reported in 37 different studies was compiled and the effects of different isolation techniques on proteins attributed to HDL are discussed. Mass spectrometric techniques used for HDL analysis and the need for precise and robust methods for quantification of HDL proteins are discussed. Expert opinion: Proteins associated with HDL have the potential to be used as biomarkers and/or help to understand HDL functionality. To achieve this, large cohorts must be studied using precise quantification methods. Key factors in HDL proteome quantification are the isolation methodology and the mass spectrometry technique employed. Isolation methodology affects what proteins are identified in HDL and the specificity of association with HDL particles needs to be addressed. Shotgun proteomics yields imprecise quantification, but the majority of HDL studies relied on this approach. Few recent studies used targeted tandem mass spectrometry to quantify HDL proteins, and it is imperative that future studies focus on the application of these precise techniques.