Effect of obesity on high-density lipoprotein metabolism

Reduced levels of high-density lipoproteins (HDL) in non-obese and obese states are associated with increased risk for the development of coronary artery disease. Therefore, it is imperative to determine the mechanisms responsible for reduced HDL in obese states and, conversely, to examine therapies aimed at increasing HDL levels in these individuals. This paper examines the multiple causes for reduced HDL in obese states and the effect of exercise and diet--two non-pharmacologic therapies--on HDL metabolism in humans. In general, the concentration of HDL-cholesterol is adversely altered in obesity, with HDL-cholesterol levels associated with both the degree and distribution of obesity. More specifically, intra-abdominal visceral fat deposition is an important negative correlate of HDL-cholesterol. The specific subfractions of HDL that are altered in obese states include the HDL2, apolipoprotein A-I, and pre-beta1 subfractions. Decreased HDL levels in obesity have been attributed to both an enhancement in the uptake of HDL2 by adipocytes and an increase in the catabolism of apolipoprotein A-I on HDL particles. In addition, there is a decrease in the conversion of the pre-beta1 subfraction, the initial acceptor of cholesterol from peripheral cells, to pre-beta2 particles. Conversely, as a means of reversing the decrease in HDL levels in obesity, sustained weight loss is an effective method. More specifically, weight loss achieved through exercise is more effective at raising HDL levels than dieting. Exercise mediates positive effects on HDL levels at least partly through changes in enzymes of HDL metabolism. Increased lipid transfer to HDL by lipoprotein lipase and reduced HDL clearance by hepatic triglyceride lipase as a result of endurance training are two important mechanisms for increases in HDL observed from exercise.     

Apolipoprotein M--a novel player in high-density lipoprotein metabolism and atherosclerosis

PURPOSE OF REVIEW: Apolipoprotein M is a recently described apolipoprotein predominantly associated with high-density lipoprotein, but also found in chylomicrons, very low-density lipoproteins, and low-density lipoprotein. The purpose is to review recent information on the unusual structural properties of apolipoprotein M and its possible role in formation of pre-beta high-density lipoprotein and reverse cholesterol metabolism. RECENT FINDINGS: Apolipoprotein M is a lipocalin having a coffee filter-like structure with a hydrophobic ligand-binding pocket. Mature apolipoprotein M retains its signal peptide, which serves as a hydrophobic anchor. In mice, silencing of expression in the liver with siRNA led to disappearance of pre-beta high-density lipoprotein and appearance of unusually large high-density lipoproteins. This suggests that apolipoprotein M is important for the formation of pre-beta high-density lipoprotein and reverse cholesterol transport. In accordance with this idea, hepatic overexpression of apolipoprotein M with an adenovirus in low-density lipoprotein-receptor deficient mice led to an approximately 70% reduction of atherosclerosis. In addition to the liver, apolipoprotein M is also expressed in the kidney. Kidney-derived apolipoprotein M binds to megalin, a member of the low-density lipoprotein-receptor family, which interacts with many lipocalins in renal tubuli. Apolipoprotein M is excreted in the urine of mice with a kidney-specific megalin deficiency but not in the urine of normal mice, suggesting megalin-mediated uptake of apolipoprotein M in the tubular epithelium of normal mice. SUMMARY: Apolipoprotein M is a novel apolipoprotein with unusual structural features that appears to play important roles in high-density lipoprotein metabolism and prevention of atherosclerosis.     

The role of the high-density lipoprotein receptor SR-BI in cholesterol metabolism

The HDL receptor scavenger receptor class B type I (SR-BI), which mediates selective HDL cholesterol uptake, plays a role in murine HDL metabolism, reverse cholesterol transport and whole-body cholesterol homeostasis. SR-BI is found in the liver, where its expression is regulated by estrogen, dietary cholesterol and fat, and controls murine plasma HDL cholesterol levels and bile cholesterol secretion. SR-BI is also highly expressed in rodent steroidogenic cells, where it facilitates cholesterol uptake for storage or steroid hormone synthesis and where its expression is regulated by trophic hormones. The detailed mechanism(s) underlying SR-BI-mediated selective cholesterol uptake have not yet been elucidated. Further analysis of the molecular and cellular bases of SR-BI regulation and function should provide new insights into the physiology and pathophysiology of cholesterol metabolism.     

Liver disease alters high-density lipoprotein composition,metabolism and function

High-density lipoproteins (HDL) are important endogenous inhibitors of inflammatory responses. Functional impairment of HDL might contribute to the excess mortality experienced by patients with liver disease, but the effect of cirrhosis on HDL metabolism and function remain elusive. To get an integrated measure of HDL quantity and quality, we assessed several metrics of HDL function using apolipoprotein (apo) B-depleted sera from patients with compensated cirrhosis, patients with acutely decompensated cirrhosis and healthy controls. We observed that sera of cirrhotic patients showed reduced levels of HDL-cholesterol and profoundly suppressed activities of several enzymes involved in HDL maturation and metabolism. Native gel electrophoresis analyses revealed that cirrhotic serum HDL shifts towards the larger HDL₂ subclass. Proteomic assessment of isolated HDL identified several proteins, including apoA-I, apoC-III, apoE, paraoxonase 1 and acute phase serum amyloid A to be significantly altered in cirrhotic patients. With regard to function, these alterations in levels, composition and structure of HDL were strongly associated with metrics of function of apoB-depleted sera, including cholesterol efflux capability, paraoxonase activity, the ability to inhibit monocyte production of cytokines and endothelial regenerative activities. Of particular interest, cholesterol efflux capacity appeared to be strongly associated with liver disease mortality. Our findings may be clinically relevant and improve our ability to monitor cirrhotic patients at high risk.     

Acute changes in high-density lipoprotein cholesterol with exercise of different intensities

The purpose of this study was to investigate the acute effects of exercise on plasma high-density lipoprotein cholesterol (HDL-C) and to determine whether the magnitude of this response would be affected by the intensity of the exercise. Twelve men (19-41 yr) ran an equivalent distance (9-12 km) on a treadmill on two separate occasions. On one occasion the exercise was performed at a speed that elicited 60% of the subject's maximal O2 uptake (VO2max), and on the other occasion exercise was performed at a speed that elicited 90% of VO2max. Changes in total cholesterol, triglycerides (TG), HDL-C, HDL apoprotein A (HDL-A), HDL saturation, lactate (LA), and free fatty acids (FFA) were measured during the course of each run, and all values were corrected for changes in plasma volume as indicated by hematocrit. There were significant increases (P less than 0.01) in HDL-C, HDL-A, and HDL saturation with exercise at both intensities, but greater increases in HDL-C (25 vs. 14%) and HDL-A (18 vs. 8%) were observed with the higher intensity exercise. Plasma FFA and TG did not differ between conditions, but LA concentrations rose significantly during the high-intensity exercise. These results indicate that increases in HDL components can occur with a relatively moderate exercise session and that the magnitude of these increases are directly related to the exercise intensity.     

Phosphatidylcholine metabolism after transfer from lipid emulsions injected intravenously in rats. Implications for high-density lipoprotein metabolism

When injected intravenously in rats, emulsion models of triacylglycerol-rich lipoproteins were metabolized like natural lipoproteins and during the hydrolysis of emulsion triacylglycerols, a large fraction of the emulsion phosphatidylcholine was transferred to the plasma high-density lipoproteins. The removal from plasma of emulsion phosphatidylcholine was followed for 2 h in unanaesthetized rats. The half-lives for removal of phospholipid after injection of emulsions stabilized with dioleoylphosphatidylcholine or 1-palmitoyl-2-oleolyphosphatidylcholine were 58-63 min when traced with isologous label. In comparison, the published half-lives of HDL mixed phospholipids in rats are approx. 40 min, indicating that much of the clearance of the emulsion phospholipid could be accounted for by HDL catabolism. Measured LCAT activity was sufficient to account for not more than 2% of the catabolism of the HDL phospholipids labelled by this physiological procedure. Removal from plasma of label was more rapid when the same emulsions were labelled with tracer amounts of the heterologous dipalmitoylphosphatidylcholine, showing that individual phosphatidylcholine species were handled distinctly even when present only in tracer amounts in a bulk of another phosphatidylcholine differing in acyl chains.     

Detection of oxidized high-density lipoprotein

This paper reviews working procedures for the separation and detection of oxidized high-density lipoproteins (ox-HDL) and their constituents. It begins with an introductory overview of structural alterations of the HDL particle and its constituents generated during oxidation. The main body of the review delineates various procedures for the isolation and detection of ox-HDL as well as the purification and separation of phosphatidylcholine metabolites and denatured apolipoproteins in the particle. The useful methods published more recently are picked up and the utility of the separation techniques is described. The last section covers a clinical evaluation of changes in these factors in ox-HDL as well as future directions of ox-HDL research.     

Proteomic analysis of high-density lipoprotein

Plasma lipoproteins, such as high-density lipoprotein (HDL), can serve as carriers for a wide range of proteins that are involved in processes such as lipid metabolism, thrombosis, inflammation and atherosclerosis. The identification of HDL-associated proteins is essential with regards to understanding these processes at the molecular level. In this study, a combination of proteomic approaches including 1-DE and 2-DE MALDI-TOF, isotope-coded affinity tag and Western blot analysis were employed to identify proteins associated with human HDL. To minimize potential losses of HDL-associated proteins during isolation, a one-step ultracentrifugation technique was applied and the quality of purified HDL was confirmed by nephelometry, high-performance gel chromatography, and Western blot analysis. MS analysis revealed the presence of 56 HDL-associated proteins including all known apolipoproteins and lipid transport proteins. Furthermore, proteins involved in hemostasis and thrombosis, the immune and complement system were found. In addition, growth factors, receptors, hormone-associated proteins and many other proteins were found to be associated with HDL. Our approach thus resulted in the identification of a large number of proteins associated with HDL. The combination of proteomic technologies proved to be a powerful and comprehensive tool for the identification of proteins on HDL.     

Serum amyloid A protein generates pre beta 1 high-density lipoprotein from alpha-migrating high-density lipoprotein

Serum amyloid A protein (SAA), an acute-phase reactant in reactive amyloidosis, has high affinity for high-density lipoprotein (HDL). When SAA is added to HDL, SAA displaces apolipoprotein A-I (apoA-I) and phospholipid from the HDL particles. These dissociated components may form prebeta1-HDL because free apoA-I can associate with phospholipid to become a lipoprotein having prebeta mobility. To determine whether SAA generates prebeta1-HDL from alpha-migrating HDL, we investigated the effects of recombinant SAA on HDL subfraction concentration using nondenaturing two-dimensional gradient gel electrophoresis. When we added SAA (0.5 mg/mL) to plasma, the prebeta1-HDL concentration increased by 164% (from 4.7% +/- 1.3% to 12.4% +/- 3.2% of apoA-I, p < 0.005). The increase in prebeta1-HDL was proportional to the dose of SAA. When we added SAA to a column of Sepharose beads coupled to the isolated HDL (alpha-migrating HDL), prebeta1-HDL was dissociated from the column together with the SAA-associated HDL. In summary, we demonstrate that SAA generates prebeta1-HDL from alpha-migrating HDL. We speculate that SAA-mediated HDL remodeling may take place in inflammation.     

Membrane fusogenic high-density lipoprotein nanoparticles

Membrane fusion under mildly acidic pH occurs naturally during viral infection in cells and has been exploited in the field of nanoparticle-mediated drug delivery to circumvent endosomal entrapment of the cargo. Herein, we aimed to confer virus-like fusogenic activity to HDL in the form of a ca. 10-nm disc comprising a discoidal lipid bilayer and two copies of a lipid-binding protein at the edge. A series of HDL mutants were prepared with a mixture of three lipids and a cell-penetrating peptide (TAT, penetratin, or Arg8) fused to the protein. In a lipid-mixing assay with anionic liposomes at pH 5.5, one HDL mutant showed the fusogenic activity higher than known fusogenic liposomes. In live mammalian cells, this HDL mutant showed high plasma membrane-binding activity in the presence of serum independent of pH. In the absence of serum, a mildly acidic pH dependency for binding to the plasma membrane and the subsequent lipid mixing between them was observed for this mutant. We propose a novel strategy to develop HDL-based drug carriers by taking advantage of the HDL lipid/protein composite structure.     

Reconstituted high-density lipoprotein infusion modulates fatty acid metabolism in patients with type 2 diabetes mellitus

We recently demonstrated that reconstituted high-density lipoprotein (rHDL) modulates glucose metabolism in humans via both AMP-activated protein kinase (AMPK) in muscle and by increasing plasma insulin. Given the key roles of both AMPK and insulin in fatty acid metabolism, the current study investigated the effect of rHDL infusion on fatty acid oxidation and lipolysis. Thirteen patients with type 2 diabetes received separate infusions of rHDL and placebo in a randomized, cross-over study. Fatty acid metabolism was assessed using steady-state tracer methodology, and plasma lipids were measured by mass spectrometry (lipidomics). In vitro studies were undertaken in 3T3-L1 adipocytes. rHDL infusion inhibited fasting-induced lipolysis (P = 0.03), fatty acid oxidation (P < 0.01), and circulating glycerol (P = 0.04). In vitro, HDL inhibited adipocyte lipolysis in part via activation of AMPK, providing a possible mechanistic link for the apparent reductions in lipolysis observed in vivo. In contrast, circulating NEFA increased after rHDL infusion (P < 0.01). Lipidomic analyses implicated phospholipase hydrolysis of rHDL-associated phosphatidylcholine as the cause, rather than lipolysis of endogenous fat stores. rHDL infusion inhibits fasting-induced lipolysis and oxidation in patients with type 2 diabetes, potentially through both AMPK activation in adipose tissue and elevation of plasma insulin. The phospholipid component of rHDL also has the potentially undesirable effect of increasing circulating NEFA.     

Altered lipoprotein metabolism in chronic inflammatory states: proinflammatory high-density lipoprotein and accelerated atherosclerosis in systemic lupus erythematosus and rheumatoid arthritis

In this review, the authors discuss the formation and structure of high-density lipoproteins (HDLs) and how those particles are altered in inflammatory or stress states to lose their capacity for reverse cholesterol transport and for antioxidant activity. In addition, abnormal HDLs can become proinflammatory (piHDLs) and actually contribute to oxidative damage. The assay by which piHDLs are identified involves studying the ability of test HDLs to prevent oxidation of low-density lipoproteins. Finally, the authors discuss the potential role of piHDLs (found in some 45% of patients with systemic lupus erythematosus and 20% of patients with rheumatoid arthritis) in the accelerated atherosclerosis associated with some chronic rheumatic diseases.     

Low- and high-density lipoprotein metabolism in HepG2 cells expressing various levels of apolipoprotein E

To determine the importance of hepatic apolipoprotein (apo) E in lipoprotein metabolism, HepG2 cells were transfected with a constitutive expression vector (pRc/CMV) containing either the complete or the first 474 base pairs of the human apoE cDNA inserted in an antisense orientation, for apoE gene inactivation, or the full-length human apoE cDNA inserted in a sense orientation for overexpression of apoE. Stable transformants were obtained that expressed 15, 24, 226, and 287% the apoE level of control HepG2 cells. The metabolism of low-density lipoprotein (LDL) and high-density lipoprotein-3 (HDL(3)), two lipoprotein classes following both holoparticle and cholesteryl esters (CE)-selective uptake pathways, was compared between all these cells. LDL-protein degradation, an indicator of the holoparticle uptake, was greater in low apoE expressing cells than in control or high expressing cells, while HDL(3)-protein degradation paralleled the apoE levels of the cells (r(2) = 0.989). LDL- and HDL(3)-protein association was higher in low apoE expressing cells compared to control cells. In opposition, LDL- and HDL(3)-CE association was not different from control cells in low apoE expressing cells but rose in high apoE expressing cells. In consequence, the CE-selective uptake (CE/protein association ratio) was positively correlated with the level of apoE expression in all cells for both LDL (r(2) = 0.977) and HDL(3) (r(2) = 0.998). We also show that, although in normal and low apoE expressor cells, 92% of LDL- and 80% HDL(3)-CE hydrolysis is sensitive to chloroquine suggesting a pathway linked to lysosomes for both lipoproteins, cells overexpressing apoE lost 60% of chloroquine-sensitive HDL(3)-CE hydrolysis without affecting that of LDL-CE. Thus, the level of apoE expression in HepG2 cells determines the fate of LDL and HDL(3).     

Cholesteryl ester transfer protein inhibition, high-density lipoprotein metabolism and heart disease risk reduction

PURPOSE OF REVIEW: Cholesteryl ester transfer protein (CETP) inhibitors (JTT-705 and torcetrapib) are currently in clinical testing, and significantly raise high-density lipoprotein (HDL) cholesterol levels. Low HDL cholesterol is a significant independent predictor of coronary heart disease (CHD) and HDL raising has been associated with coronary heart disease risk reduction, but there is debate about whether CETP inhibition will reduce coronary heart disease risk. RECENT FINDINGS: It has been documented in transgenic mouse models that apolipoprotein (apo) C-I inhibits CETP, and that high mono-unsaturated fat diets prevent the normal stimulation of CETP activity by dietary cholesterol. In rabbits, torcetrapib markedly decreases clearance of HDL cholesteryl ester via an indirect pathway, but has no effect on total plasma cholesteryl ester clearance. In humans, torcetrapib raises HDL apoA-I by modestly decreasing its fractional catabolic rate, while having a very profound effect on raising HDL cholesterol and large alpha-1 migrating HDL particles by more than 50%, with no effect on fecal cholesterol excretion. When JTT-705 at 600 mg/day was given to hypercholesterolemic patients already on pravastatin 40 mg/day, the combination was well tolerated and increases in HDL cholesterol of 28% were noted. SUMMARY: In our view, CETP inhibitors in combination with statins will be profoundly beneficial in reducing human atherosclerosis, primarily because they normalize HDL particles and prevent the transfer of cholesteryl ester from HDL to atherogenic lipoproteins.     

Cubilin, a high-density lipoprotein receptor

The metabolism of HDL particles is a complex biological process involving various regulating factors in plasma and different cellular receptors. In addition to the well-established scavenger receptor BI-mediated selective HDL-cholesteryl ester uptake in liver and steroidogenic tissues, evidence has been provided that HDL also undergoes holoparticle endocytosis in different tissues. Recently, a novel receptor expressed in various absorptive epithelia was disclosed as a high affinity receptor for endocytosis of HDL and lipid-poor apolipoprotein AI. This receptor, designated cubilin, may play an important role in the renal clearance of filterable apolipoprotein AI/HDL and in the maternal-fetal transport of cholesterol.     

New targets of high-density lipoprotein therapy

PURPOSE OF REVIEW: Increasing attention has focused on the development of therapeutic strategies to promote the biologic activity of HDL particles, which possess a number of functional properties that contribute to their role in cardioprotection. Currently available therapies raise levels of HDL-cholesterol by relatively modest amounts. This review describes experimental strategies that promote HDL activity. RECENT FINDINGS: The functional quality of HDL may be more important than the absolute level of HDL-cholesterol found in the systemic circulation. This is supported by the observation that small rises in HDL-cholesterol with current therapies is associated with clinical benefit. This has major implications for the development of new therapies. A number of therapeutic strategies have been developed that promote reverse cholesterol transport, inhibit inflammatory events in the vessel wall, and modify remodeling of HDL particles within the systemic circulation. SUMMARY: A number of emerging therapies appear to promote the biologic activity of HDL. These agents can be administered as acute infusions in the setting of acute ischemic syndromes or as oral therapy for chronic prevention of cardiovascular disease.     

Isoferulic acid action against glycation-induced changes in structural and functional attributes of human high-density lipoprotein

Glycation-induced high-density lipoprotein (HDL) modification by aldehydes can result in loss of its antiinflammatory/antioxidative properties, contributing to diabetes-associated cardiovascular diseases. Isoferulic acid, a major active ingredient of Cimicifuga heracleifolia, shows antiinflammatory, antiviral, antioxidant, and antidiabetic properties. Thus, this study investigated the antiglycation effect of isoferulic acid against compositional modifications of HDL and loss of biological activity of HDL-paraoxonase induced on incubation with different aldehydes. Protective effect of isoferulic acid was assessed by subjecting purified HDL from human plasma to glycation with methylglyoxal, glyoxal, or glycolaldehyde and varying concentrations of isoferulic acid. The effect of isoferulic acid was analyzed by determining amino group number, tryptophan and advanced glycation end-product fluorescence, thermal denaturation studies, carboxymethyl lysine content, and activity of HDL-paraoxonase. Concentration-dependent inhibitory action of isoferulic acid was observed against extensive structural perturbations, decrease in amino group number, increase in carboxymethyl lysine content, and decrease in the activity of HDL-paraoxonase caused by aldehyde-associated glycation in the HDL molecule. Isoferulic acid, when taken in concentration equal to that of aldehydes, was most protective, as 82-88% of paraoxonase activity was retained for all studied aldehydes. Isoferulic acid shows antiglycation action against aldehyde-associated glycation in HDL, which indicates its therapeutic potential for diabetic patients, especially those with micro-/macrovascular complications.     

The paradox of dysfunctional high-density lipoprotein

PURPOSE OF REVIEW: This review addresses how, in atherosclerosis or systemic inflammation, HDL can lose its usual atheroprotective characteristics and even paradoxically assume proinflammatory properties. RECENT FINDINGS: Specific chemical and structural changes within HDL particles can impede reverse cholesterol transport, enhance oxidation of LDL, and increase vascular inflammation. HDL may be viewed as a shuttle that can be either anti-inflammatory or proinflammatory, depending on its cargo of proteins, enzymes, and lipids. Some therapeutic approaches that reduce coronary risk, such as statins and therapeutic lifestyle changes, can favorably moderate the characteristics of proinflammatory HDL. In addition, apolipoprotein A-I mimetic peptides and other compounds that target functional aspects of HDL may offer novel approaches to reduction in cardiovascular risk. SUMMARY: Current data suggest that under some conditions HDL can become dysfunctional and even proinflammatory, but this characterization can change with resolution of systemic inflammation or use of certain treatments.     

Diet-induced weight loss in overweight or obese women and changes in high-density lipoprotein levels and function

Diet-induced weight loss in women may be associated with decreases not only in plasma levels of low-density lipoprotein cholesterol (LDL-C), but also in high-density lipoprotein cholesterol (HDL-C). Whether a decrease in HDL-C is associated with altered HDL function is unknown. One hundred overweight or obese women (age 46 ± 11 years, 60 black; 12 diabetic) were enrolled in the 6-month program of reduced fat and total energy diet and low-intensity exercise. Serum cholesterol efflux capacity was measured in (3)H-cholesterol-labeled BHK cells expressing ABCA1, ABCG1, or SR-B1 transporters and incubated with 1% apolipoprotein B (apoB)-depleted serum. Antioxidant properties of HDL were estimated by paraoxonase-1 (PON1) activity and oxygen radical absorbance capacity (ORAC). Endothelial nitric oxide synthase (eNOS) activation was measured by conversion of L-arginine to L-citrulline in endothelial cells incubated with HDL from 49 subjects. Participants achieved an average weight loss of 2.2 ± 3.9 kg (P < 0.001), associated with reductions in both LDL-C (-6 ± 21 mg/dl, P = 0.004) and HDL-C (-3 ± 9 mg/dl, P = 0.016). Cholesterol efflux capacity by the ABCA1 transporter decreased by 10% (P = 0.006); efflux capacities by the ABCG1 and SR-B1 transporters were not significantly altered. ORAC decreased by 15% (P = 0.018); neither PON1 activity nor eNOS activation was significantly altered by reduction in HDL-C. Findings were similar for diabetic and nondiabetic subjects. Diet-induced weight loss in overweight or obese women is associated with a decrease in HDL-C levels, but overall HDL function is relatively spared, suggesting that decrease in HDL-C in this setting is not deleterious to cardiovascular risk.