Effects Of High-density Lipoprotein Research Articles

Effects of high-density lipoprotein on endothelium-dependent vasorelaxation

Studies using cultured endothelial cells have shown that high-density lipoprotein (HDL) positively modulates endothelial nitric oxide synthase (eNOS). The purpose of this study was to test the hypotheses that positive modulation of eNOS by HDL occurs in whole vessels and that it augments endothelium-dependent vasorelaxation. To test these hypotheses, brachial arteries were obtained from swine. Endothelium-dependent and endothelium-independent vasorelaxation were determined in vitro to assess the effects of acute administration of HDL (50 microg.mL-1; n = 8) and chronic exposure to relatively high HDL concentration on vascular function (low HDL, 0.89 +/- 0.02 mmol.L-1, n = 4; high HDL, 1.16 +/- 0.05 mmol.L-1, n = 4; p < 0.005). Acute administration of HDL did not augment maximal endothelium-dependent vasorelaxation to bradykinin (BK) (no HDL, 82.6% +/- 2.2%; HDL, 76.7% +/- 3.5%; not significant (ns)). Similarly, maximal relaxation to BK was not enhanced by chronic exposure to high HDL concentrations. NO synthase (NOS) activity was also similar between groups (low HDL, 129.0 +/- 19.2 counts.h-1.microg-1 protein; high HDL, 113.9 +/- 47.1 counts.h-1.microg-1; ns). Consistent with NOS activity, the extent of eNOS phosphorylation at several sites was similar between low HDL and high HDL. Both apolipoprotein A-I (ApoA-I) and scavenger receptor class B type I (SR-BI) were associated with eNOS. Similar to cultured cell studies, this study demonstrates that both ApoA-I and SR-BI associate with eNOS in the vascular wall. Binding of ApoA-I and SR-BI to eNOS does not, however, result in modulation of either NO formation or endothelial function.

Effects of High-Density Lipoproteins on Pancreatic β-Cell Insulin Secretion

Type 2 diabetes is characterized by impaired beta-cell secretory function, insulin resistance, reduced high-density lipoprotein (HDL) levels, and increased cardiovascular risk. Given the current interest in therapeutic interventions that raise HDLs levels, this study investigates the effects of HDLs on insulin secretion from beta-cells. Incubation of Min6 cells and primary islets under basal or high-glucose conditions with either apolipoprotein (apo) A-I or apoA-II in the lipid-free form, as a constituent of discoidal reconstituted HDLs (rHDLs), or with HDLs isolated from human plasma increased insulin secretion up to 5-fold in a calcium-dependent manner. The increase was time and concentration dependent. It was also K(ATP) channel and glucose metabolism dependent under high-glucose, but not low-glucose, conditions. The lipid-free apolipoprotein-mediated increase in insulin secretion was ATP binding cassette (ABC) transporter A1 and scavenger receptor-B1 dependent. The rHDL-mediated increase in insulin secretion was ABCG1 dependent. Exposure of beta-cells to lipid-free apolipoproteins also increased insulin mRNA expression and insulin secretion without significantly depleting intracellular insulin or cholesterol levels. These results establish that lipid-free and lipid-associated apoA-I and apoA-II increase beta-cell insulin secretion and indicate that interventions that raise HDLs levels may be beneficial in type 2 diabetes.

Relevance of Sphingolipids in the Pleiotropic Protective Effects of High-Density Lipoproteins

Atherosclerosis is the major cause of cardiovascular morbidity and mortality. A multitude of pro-atherogenic mediators are known liable for the initiation and progression of atherogenic vascular lesions. Only few endogenous molecules are known so far with cardiovascular protective properties, whereas high-density lipoprotein (HDL) is one of the most important. Beside cholesterol efflux, many pleiotropic cell-mediated functions of HDL are known so far. HDL is a spherical particle that contains different proteins and lipids. Especially sphingolipids, like sphingosine-1-phosphate (S1P), has gained great attention. The HDL associated S1P seems to be responsible for many of the pleiotropic effects of HDL by activating special S1P receptors. This review focuses on HDL associated sphingolipids as mediators of known protective pleiotropic effects of HDL and their possible therapeutic relevance.

In Vitro Intestinal Absorption of Carotenoids Delivered as Molecular Inclusion Complexes with β-Cyclodextrin Is Not Inhibited by High-Density Lipoproteins

This study analyzed the assimilation efficiency of carotenoids when they are delivered as inclusion complexes with beta-cyclodextrin (CyDIC) in water. The in vitro assimilation model used was the brush border membrane vesicles (BBMV) system in which the BBMVs were incubated with CyDIC. Carotenoid suspensions in Tween were used as a reference. Regardless of the form in which the carotenoids were delivered to the BBMV preparation, a higher assimilation efficiency was observed for carotenes than for the xanthophyll lutein. At the highest donor solution concentration, supplying carotenoids in CyDIC produced a significant increase in carotenoid assimilation compared to the corresponding carotenoid suspensions in Tween. The assimilation process using CyDIC takes place by means of a dissociation process in which the carotenoids are released from the beta-cyclodextrin to later be assimilated. At the highest concentration of CyDIC in the donor solution, the dissociation equilibrium will be shifted toward the free forms of the complex, thus increasing the amount of carotenoids available for assimilation. In another set of experiments, the effect of high-density lipoproteins as activity inhibitors for the receptors involved in carotenoid assimilation was analyzed. In carotenoid suspensions in Tween, with an inhibitor, a significant decrease in the assimilated quantity compared was observed with values reached without the inhibitor. Lutein presented the most significant decrease (70%). The fact that complete inhibition was not reached suggests that both simple and facilitated diffusion contributes to the assimilation process. When the donor solution composed of CyDIC and inhibitor was added, significant increases were observed in beta-carotene and lycopene assimilation for all concentrations and in lutein for the highest concentration. This effect is due to the exchange between lipoprotein lipid components and CyDIC, which promotes the dissociation and liberation processes of the carotenoid, which then becomes available for assimilation.

Apolipoprotein A-I inhibits chemotaxis, adhesion, activation of THP-1 cells and improves the plasma HDL inflammatory index

The anti-inflammatory effects of high density lipoprotein (HDL) are well described, however, such effects of Apolipoprotein A-I (ApoA-I) are less studied. Building on our previous study, we further explored the mechanism of anti-inflammatory effects of ApoA-I, and focused especially on the interaction between monocyte and endothelial cells and plasma HDL inflammatory index in LPS-challenged rabbits. Our results show that ApoA-I significantly decreased LPS-induced MCP-1 release from THP-1 cells and ox-LDL-induced THP-1 migration ratio ( P < 0.01, respectively). ApoA-I significantly decreased sL-selectin, sICAM-1 and sVCAM-1 release ( P < 0.01, P < 0.01, P < 0.05, respectively) from LPS-stimulated THP-1 cells. Furthermore, ApoA-I significantly inhibited LPS-induced CD11b and VCAM-1 expression on THP-1 cells ( P < 0.01, P < 0.05, respectively). ApoA-I diminished LPS-induced mCD14 expression ( P < 0.01) and NFκB nuclear translocation in THP-1 cells. After single dose treatment of ApoA-I, the value of plasma HDL inflammatory index in LPS-challenged rabbits was improved significantly ( P < 0.05). These results suggest that ApoA-I can inhibit chemotaxis, adhesion and activation of human monocytes and improve plasma HDL inflammatory index with presenting beneficial anti-inflammatory effects.

Vasculoprotective Effects of Apolipoprotein Mimetic Peptides: An Evolving Paradigm in HDL Therapy

Anti-atherogenic effects of high density lipoprotein (HDL) and its major protein component apolipoprotein A-I (apoA-I) are principally thought to be due to their ability to mediate reverse cholesterol transport. These agents also possess anti-oxidant properties that prevent the oxidative modification of low density lipoprotein (LDL) and anti-inflammatory properties that include inhibition of endothelial cell adhesion molecule expression. Results of the Framingham study revealed that a reduction in HDL levels is an independent risk factor for coronary artery disease (CAD). Accordingly, there has been considerable interest in developing new therapies that specifically elevate HDL cholesterol. However, recent evidence suggests that increasing circulating HDL cholesterol levels alone is not sufficient as a mode of HDL therapy. Rather, therapeutic approaches that increase the functional properties of HDL may be superior to simply raising the levels of HDL per se. Our laboratory has pioneered the development of synthetic, apolipoprotein mimetic peptides which are structurally and functionally similar to apoA-I but possess unique structural homology to the lipid-associating domains of apoA-I. The apoA-I mimetic peptide 4F inhibits atherogenic lesion formation in murine models of atherosclerosis. This effect is related to the ability of 4F to induce the formation of pre-β HDL particles that are enriched in apoA-I and paraoxonase. 4F also possesses anti-inflammatory and anti-oxidant properties that are independent of its effect on HDL quality per se. Recent studies suggest that 4F stimulates the expression of the antioxidant enzymes heme oxygenase and superoxide dismutase and inhibits superoxide anion formation in blood vessels of diabetic, hypercholesterolemic and sickle cell disease mice. The goal of this review is to discuss HDL-dependent and -independent mechanisms by which apoA-I mimetic peptides reduce vascular injury in experimental animal models.

The Anti Inflammatory Effects of High Density Lipoproteins

It is well recognised that increased levels of high density lipoprotein (HDL) protect against atherosclerosis and correlate with improved prognosis for vascular disease associated events. While many of the atheroprotective effects of HDL are ascribed to the ability to remove cholesterol from the vasculature through the reverse cholesterol transport system, recent work has shown that HDL may be atheroprotective through its other functions, such as regulation of endothelial adhesion molecule expression, stimulation of endothelial nitric oxide synthase and inhibition of the damaging effects of oxidised low density lipoproteins. Recently, HDL has also been described to interact with circulating cells inhibiting both leukocyte and platelet activation, therefore having further systemic anti-inflammatory functions. This review summarises the studies and models used to examine the anti-inflammatory effects of HDL and details data describing the ability to inhibit leukocyte activation, contributing to the hypothesis that raised HDL is beneficial in the context of inflammation in atherosclerosis. Further, HDL modification in disease and current therapeutic strategies such as reconstituted HDL particles and apoA I mimetic peptides is discussed to provide insights to the potential applicability of raising HDL to regress cardiovascular disease.

Critical role of scavenger receptor-BI–expressing bone marrow–derived endothelial progenitor cells in the attenuation of allograft vasculopathy after human apo A-I transfer

AbstractAllograft vasculopathy is the leading cause of death in patients with heart transplantation. Accelerated endothelial regeneration mediated by enhanced endothelial progenitor cell (EPC) incorporation may attenuate the development of allograft vasculopathy. We investigated the hypothesis that modulation of EPC biology and attenuation of allograft vasculopathy by increased high-density lipoprotein cholesterol after human apo A-I (AdA-I) transfer requires scavenger receptor (SR)–BI expression in bone marrow–derived EPCs. After AdA-I transfer, the number of circulating EPCs increased 2.0-fold (P < .001) at different time points in C57BL/6 mice transplanted with SR-BI+/+ bone marrow but remained unaltered in mice with SR-BI−/− bone marrow. The effect of high-density lipoprotein on EPC migration in vitro requires signaling via SR-BI and extracellular signal-regulated kinases and is dependent on increased nitric oxide (NO) production in EPCs. Human apo A-I transfer 2 weeks before paratopic artery transplantation reduced intimal area at day 21 3.7-fold (P < .001) in mice with SR-BI+/+ bone marrow but had no effect in mice with SR-BI−/− bone marrow. AdA-I transfer potently stimulated EPC incorporation and accelerated endothelial regeneration in chimeric SR-BI+/+ mice but not in chimeric SR-BI−/− mice. In conclusion, human apo A-I transfer accelerates endothelial regeneration mediated via SR-BI expressing bone marrow–derived EPCs, thereby preventing allograft vasculopathy.

Increased methionine sulfoxide content of apoA-I in type 1 diabetes

Cardiovascular disease is a major cause of morbidity and premature mortality in diabetes. HDL plays an important role in limiting vascular damage by removing cholesterol and cholesteryl ester hydroperoxides from oxidized low density lipoprotein and foam cells. Methionine (Met) residues in apolipoprotein A-I (apoA-I), the major apolipoprotein of HDL, reduce peroxides in HDL lipids, forming methionine sulfoxide [Met(O)]. We examined the extent and sites of Met(O) formation in apoA-I of HDL isolated from plasma of healthy control and type 1 diabetic subjects to assess apoA-I exposure to lipid peroxides and the status of oxidative stress in the vascular compartment in diabetes. Three tryptic peptides of apoA-I contain Met residues: Q(84)-M(86)-K(88), W(108)-M(112)-R(116), and L(144)-M(148)-R(149). These peptides and their Met(O) analogs were identified and quantified by mass spectrometry. Relative to controls, Met(O) formation was significantly increased at all three locations (Met(86), Met(112), and Met(148)) in diabetic patients. The increase in Met(O) in the diabetic group did not correlate with other biomarkers of oxidative stress, such as N(epsilon)-malondialdehyde-lysine or N(epsilon)-(carboxymethyl)lysine, in plasma or lipoproteins. The higher Met(O) content in apoA-I from diabetic patients is consistent with increased levels of lipid peroxidation products in plasma in diabetes. Using the methods developed here, future studies can address the relationship between Met(O) in apoA-I and the risk, development, or progression of the vascular complications of diabetes.

Cholesterol efflux pathways and other potential mechanisms involved in the athero‐protective effect of high density lipoproteins

Plasma high density lipoprotein (HDL) levels bear a strong independent inverse relationship with atherosclerotic cardiovascular disease. Although HDL has anti-oxidant, anti-inflammatory, vasodilating and anti-thrombotic properties, the central anti-atherogenic activity of HDL is likely to be its ability to remove cholesterol and oxysterols from macrophage foam cells, smooth muscle cells and endothelial cells in the arterial wall. To some extent, the pleotropic athero-protective properties of HDL may be related to its ability to promote sterol and oxysterol efflux from arterial wall cells, as well as to detoxify oxidized phospholipids. In cholesterol-loaded macrophages, activation of liver X receptors (LXRs) leads to increased expression of adenosine triphosphate (ATP) binding cassetter transporter (ABCA1), ATP binding cassetter transporter gene (ABCG1) and apoE and promotes cholesterol efflux. ABCA1 stimulates cholesterol efflux to lipid-poor apolipoproteins, whilst ABCG1 promotes efflux of cholesterol and oxysterols to HDL. Despite some recent setbacks in the clinical arena, there is still intense interest in therapeutically targeting HDL and macrophage cholesterol efflux pathways, via treatments with niacin, cholesterol ester transfer protein inhibitors, LXR activators and infusions of apoA-1, phospholipids and peptides.

Abstract 978: Novel Endothelial-protective Effect of High Density Lipoprotein: Down-regulation of Toll-like Receptor 4

Introduction: Endothelium-derived Toll-like receptor (TLR) 4 is known to be the key molecule in lipopolysaccharide (LPS)-induced neutrophil sequestration into lungs. The aim of our study was to investigate whether the endothelial-protective effects of HDL also include regulation of Toll-like receptor (TLR) 4. Therefore, we analyzed the effect of HDL supplementation on TLR4 expression in human aortic endothelial cells (HAEC) and the effect of increased HDL following adenoviral apo AI ( Ad.AI) gene transfer (GT) on lung TLR4 expression in an experimental model of LPS-induced endotoxic shock. Methods: HDL (50 μg/ml) was supplemented to HAEC in the presence/absence of LPS (100 ng/ml). Surface-TLR4 expression on HAEC was analyzed by FACS. C57BL/6 mice were i.v. injected with 5 x 10 10 total particles of Ad.AI or with Ad.Null . Fourteen days hereafter, mice were i.p. injected with LPS (80 mg/kg) or with saline and 20 hours afterwards sacrificed. For survival studies, endotoxic shock was induced in 25 mice per group. Lung myeloperoxidase activity (MPO), as a marker of neutrophil infiltration was analysed. Lung TLR4-mRNA expression was determined by real-time PCR, plasma interferon-gamma (INFγ) levels by ELISA. Results: HDL supplementation significantly reduced TLR4 expression on HAEC. Ad.AI resulted in human apo AI expression levels of 80 ± 10 mg/dl at day 14. By 24 hours after LPS injection, 81% of LPS-treated Ad.Null and saline mice died, in contrast to 50% of Ad.AI LPS-treated mice. This was associated with a 1.6- and 1.7-fold (p&lt;0.05) lower lung/body weight ratio in Ad.AI compared to Ad.Null and saline LPS-injected mice. LPS-induced lung TLR4 mRNA expression was 2.0-fold (p&lt;0.05) reduced after human apo AI-GT compared to LPS controls. Moreover, human apo AI-GT significantly reduced neutrophil infiltration in LPS-treated mice, as indicated by 1.8-fold (p&lt;0.05) and 1.9-fold (p&lt;0.05) reduced MPO activity compared to controls, respectively. Plasma INFγ-levels were 2.8-fold and 2.2-fold (p&lt;0.05) lower versus LPS-treated Ad.Null and saline mice, respectively. Conclusion: The HDL-mediated reduction in lung TLR4 expression contributes for the reduced neutrophil infiltration and subsequently may account for improved mortality rate.

Emerging HDL-based therapies for atherothrombotic vascular disease

Statin therapy has been a significant advance in the management of dyslipidemia and atherothrombotic cardiovascular disease with a resultant 30% to 40% reduction in cardiovascular events; however, a significant number of events continue to occur in statin-treated patients, including in patients treated with high-dose statins targeted to achieve mean low-density lipoprotein cholesterol levels in the range of 60 to 80 mg/dL. Therefore, development and testing of new therapies that exploit the vascular protective effects of high-density lipoprotein (HDL) constitutes a rational and complementary approach. A number of HDL-based therapies are in various stages of development and testing. It is hoped that one or more of these new HDL-based therapies, if proven effective and safe, will become a part of our armamentarium against vaso-occlusive cardiovascular disease. A paradigm could emerge in which patients recovering from acute coronary syndromes and at high risk of recurrent events could be treated with rapid-acting HDL-based therapy, such as infusions of recombinant HDL or even HDL delipidation, followed by more sustained long-term HDL-based therapies, such as oral agents and perhaps even HDL-based gene therapy.

Modifying the anti-inflammatory effects of high-density lipoprotein

The anti-inflammatory effects of high-density lipoproteins (HDL) are well documented and include inhibition of low-density lipoprotein (LDL) oxidation, reduction of inflammatory cytokines and vascular leukocyte adhesion molecules, and participation in innate immunity. However, certain conditions, including coronary disease, diabetes mellitus, systemic inflammation, and a diet high in saturated fat, are associated with modification of HDL such that it paradoxically enhances LDL oxidation and/or vascular inflammation. Treatment with statins and/or apolipoprotein A1 mimetic peptides improves HDL's anti-inflammatory functions, and these as well as other medications may represent a novel pathway through which to target atherosclerosis.

Molecular Mechanisms Responsible for the Anti-Inflammatory and Protective Effect of High-Density Lipoprotein on the Endothelium

In addition to their role in reverse cholesterol transport, high-density lipoproteins (HDLs) exert several beneficial effects, including the prevention and correction of endothelial dysfunction. HDLs promote endothelium proliferation and diminish endothelial apoptosis; they play a key role in vasorelaxation by increasing the release of nitric oxide and prostacyclin through the induction of the expression and the activity of endothelial nitric oxide synthase and the coupling of cyclo-oxygenase 2 and prostacyclin synthase. In addition, HDLs affect coagulation, fibrinolysis, platelet adhesion, adhesion molecules and protease expression, and exert antioxidant activity. These effects are achieved at the gene expression level and are dependent on the activation of several intracellular signalling pathways, including PI3K/Akt, extracellular signal-regulated kinase (ERK) 1/2, protein kinase C, and p38MAPK (mitogen-activated protein kinase). The complexity of the signalling pathways modulated by HDL reflects the different effects of the components of this class of lipoproteins such as apolipoproteins or lipids on endothelial cell gene expression and the subsequent observed modulation of endothelial function. The in vivo relevance of these findings to endothelial recovery during physiological or pathological conditions remain to be addressed; nevertheless, the results of clinical studies with synthetic HDL, apolipoprotein A-I mimetics and drugs selectively affecting HDL plasma levels and biological functions that are becoming available support the importance of the correction of endothelial function by HDL.

High density lipoprotein 3 inhibits oxidized low density lipoprotein-induced apoptosis via promoting cholesterol efflux in RAW264.7 cells1

To investigate the protective effect of high density lipoprotein 3 (HDL3) on oxidized low density lipoprotein (ox-LDL)-induced apoptosis in RAW264.7 cells. RAW264.7 cells were exposed to 50 mg/L ox-LDL for various durations up to 48 h, and apoptosis was detected using Hoechst 33258 staining and flow cytometric analysis. Total cholesterol levels were detected by high performance liquid chromatography, cholesterol efflux was determined by Tritium labeling, and the cellular lipid droplets were assayed by oil red O staining. Treatment with 50 mg/L ox-LDL for 12, 24, and 48 h increased the apoptotic rate of RAW264.7 cells in a time-dependent manner. The peak apoptotic rate (47.7%) was observed after 48 h incubation. HDL3 at various concentrations (50 mg/L, 100 mg/L, and 200 mg/L) inhibited the ox-LDL (50 mg/L for 48 h)-mediated apoptosis that was accompanied by an increased rate of intracellular cholesterol efflux, and decreased total cholesterol levels in cells in a concentration-dependent manner. Blockage of cholesterol efflux by brefeldin decreased the protective effect of HDL3 on ox-LDL-induced apoptosis. Increase of the cholesterol efflux effected by another cholesterol acceptor,beta-cyclodextrin, led to a dramatic decrease in the apoptotic rate of cells. HDL3 antagonizes ox-LDL-induced apoptosis in RAW264.7 cells, through reducing the accumulation of toxic cholesterol.

Agents for the Inhibition of Cholesteryl Ester Transfer Protein (CETP) and Prospects for the Future Treatment of Atherosclerosis

The atheroprotective effects of high density lipoprotein (HDL) have been established over the past several decades and this in turn has led to efforts to develop more effective and well-tolerated means for elevating this lipoprotein. The high levels of HDL associated with human deficiency of cholesteryl ester transfer protein (CETP) motivated the pharmaceutical industry to initiate programs throughout the 1990s aimed towards the inhibition of this transfer protein. Because the CETP mediated transfer of lipid from donor to acceptor lipoproteins appears to occur by a carrier mechanism there exists several means for achieving inhibition. These include interference with lipid binding to CETP and causing insufficient or excessive binding of CETP to its lipoprotein substrates. Many CETP inhibitors have now been identified and several mechanisms of inhibition are represented. Two of these synthetic inhibitors, JTT-705 and torcetrapib, have progressed into clinical trials. The results from these initial trials are consistent with prior reports regarding partial CETP deficiency in humans. For both inhibitors, whether administered as a monotherapy or in combination with a statin, a reduction in plasma CETP activity has resulted in an elevation of HDL, as well as a lowering of low density lipoprotein (LDL). Whether this class of agents will prove beneficial in the treatment and prevention of atherosclerosis awaits the results of longer trials. However, emerging data, including the role of the ABCG1/ABCG4 transporters in the efflux of cholesterol to mature HDL and the importance to reverse cholesterol transport of free cholesterol transport via HDL, reinforce the case for optimism.

Effect of High Density Lipoproteins on Protein Expression in Myoblast Cell Lines

We have studied the effects of high-density lipoprotein (HDL) on rat heart (H9c2) and skeletal (L6) myoblasts using two-dimensional gel electrophoresis and tandem mass spectrometry. Gels generated from control cells and cells treated with 250 microg/mL HDL showed significant differences in the 7-10 pI region and the 30-50 kDa mass region. In particular, the membrane binding protein, annexin II, the voltage-dependent anion channel, glyceraldehyde-3-phosphate dehydrogenase and other glycolytic proteins are differentially expressed.

High density lipoprotein subfractions and the risk of coronary heart disease: 9-years follow-up in the Caerphilly Study

Whether the protective effect of high density lipoprotein (HDL) on incident coronary heart disease (CHD) can be attributed to one or both HDL subfractions remains controversial. The associations of HDL 2 and HDL 3 cholesterol with the incidence of CHD in the 9-year follow-up of the Caerphilly study are described. A total of 2398 middle-aged British men were recruited from the general population between 1984 and 1988 and were followed, on average, for 9 years. Total and HDL 3 cholesterol were measured by a two-step precipitation technique on fresh, fasting samples from 2225 men. HDL 2 cholesterol was calculated by subtracting HDL 3 from total HDL cholesterol. Relative odds and 95% confidence intervals (CI) for incident CHD were obtained by use of a logistic regression model. During follow-up, 282 (12%) men developed a major new CHD event. Total HDL and HDL 3 cholesterol were significantly and inversely associated with the risk of incident CHD. When divided into fifths of the distributions of total HDL and HDL 3 cholesterol, multivariate-adjusted relative odds were 1.00, 0.95, 0.72, 0.85, 0.38 and 1.00, 1.05, 0.92, 0.67, 0.39, respectively graded from the least to the most quintile, with the lowest quintile group as referent. Tests for trend were significant ( P for trend 0.003 and 0.001, respectively). In a multivariate model, the contribution of HDL 3 was significant (standardized relative odds, 0.76; 95% CI, 0.64–0.91), whereas HDL 2 was not significant. No linear combination of the two subfractions was a better predictor of CHD than total HDL cholesterol alone. HDL 3 cholesterol was an independent predictor of incident CHD and may be more closely related to the development of CHD than HDL 2 cholesterol. The prediction of the risk of CHD from total HDL cholesterol alone could not be improved upon by measurement of the two HDL subfractions. In our view, the only way to improve our understanding of this situation is to measure both subfractions independently of each other and not to calculate one by subtraction.

Ectopic beta-chain of ATP synthase is an apolipoprotein A-I receptor in hepatic HDL endocytosis.

The effect of high-density lipoprotein (HDL) in protecting against atherosclerosis is usually attributed to its role in 'reverse cholesterol transport'. In this process, HDL particles mediate the efflux and the transport of cholesterol from peripheral cells to the liver for further metabolism and bile excretion. Thus, cell-surface receptors for HDL on hepatocytes are chief partners in the regulation of cholesterol homeostasis. A high-affinity HDL receptor for apolipoprotein A-I (apoA-I) was previously identified on the surface of hepatocytes. Here we show that this receptor is identical to the beta-chain of ATP synthase, a principal protein complex of the mitochondrial inner membrane. Different experimental approaches confirm this ectopic localization of components of the ATP synthase complex and the presence of ATP hydrolase activity at the hepatocyte cell surface. Receptor stimulation by apoA-I triggers the endocytosis of holo-HDL particles (protein plus lipid) by a mechanism that depends strictly on the generation of ADP. We confirm this effect on endocytosis in perfused rat liver ex vivo by using a specific inhibitor of ATP synthase. Thus, membrane-bound ATP synthase has a previously unsuspected role in modulating the concentrations of extracellular ADP and is regulated by a principal plasma apolipoprotein.

The effect of high-density lipoproteins on the formation of lipid/protein conjugates during in vitro oxidation of low-density lipoprotein

High-density lipoprotein (HDL) incubated with low-density lipoprotein (LDL) under oxidising conditions has previously been reported to decrease the accumulation of lipid peroxides on LDL and to diminish the biological effects of LDL, which would have been present had it been oxidatively modified in the absence of HDL. Thus far direct evidence that oxidative modification of LDL is diminished by HDL has, however, been lacking. We used electrospray ionisation mass spectrometry (ESI-MS) to detect 4-hydroxy-2-nonenal (HNE)-modified histidine residues of tryptic fragments of LDL which had been subject to Cu 2+ induced oxidation both in the presence and absence of human or avian HDL. HNE-modified angiotensin II was introduced into the incubation mixture as an internal standard and to check that HDL did not interfere in the detection of HNE-modified peptides non-specifically. Our results confirmed earlier reports that HNE modification of histidine occurs during the oxidation of LDL and for the first time revealed a marked attenuation of the process in the presence of human HDL with no effect on the detection of HNE-modified angiotensin II by ESI-MS. Avian HDL, which lacks the anti-oxidative enzyme paraoxonase, did not affect the formation of apo B adducts. Our findings therefore suggest that covalent linkage of lipid peroxidation products to LDL protein as well as the accumulation of lipid peroxides on LDL is diminished in the presence of HDL containing paraoxonase.