Identification of the specific molecular and functional signatures of pre-beta-HDL: relevance to cardiovascular disease

While low concentrations of high-density lipoprotein-cholesterol (HDL-C) are widely accepted as an independent cardiovascular risk factor, HDL-C-rising therapies largely failed, suggesting the importance of both HDL functions and individual subspecies. Indeed HDL particles are highly heterogeneous, with small, dense pre-beta-HDLs being considered highly biologically active but remaining poorly studied, largely reflecting difficulties for their purification. We developed an original experimental approach allowing the isolation of sufficient amounts of human pre-beta-HDLs and revealing the specificity of their proteomic and lipidomic profiles and biological activities. Pre-beta-HDLs were enriched in highly poly-unsaturated species of phosphatidic acid and phosphatidylserine, and in an unexpectedly high number of proteins implicated in the inflammatory response, including serum paraoxonase/arylesterase-1, vitronectin and clusterin, as well as in complement regulation and immunity, including haptoglobin-related protein, complement proteins and those of the immunoglobulin class. Interestingly, amongst proteins associated with lipid metabolism, phospholipid transfer protein, cholesteryl ester transfer protein and lecithin:cholesterol acyltransferase were strongly enriched in, or restricted to, pre-beta-HDL. Furthermore, pre-beta-HDL potently mediated cellular cholesterol efflux and displayed strong anti-inflammatory activities. A correlational network analysis between lipidome, proteome and biological activities highlighted 15 individual lipid and protein components of pre-beta-HDL relevant to cardiovascular disease, which may constitute novel diagnostic targets in a pathological context of altered lipoprotein metabolism.

Associations of lipoprotein particle profile and objectively measured physical activity and sedentary time in schoolchildren: a prospective cohort study

Background

Our understanding of the mechanisms through which physical activity might benefit lipoprotein metabolism is inadequate. Here we characterise the continuous associations between physical activity of different intensities, sedentary time, and a comprehensive lipoprotein particle profile.

Methods

Our cohort included 762 fifth grade (mean [SD] age = 10.0 [0.3] y) Norwegian schoolchildren (49.6% girls) measured on two separate occasions across one school year. We used targeted proton nuclear magnetic resonance (1H NMR) spectroscopy to produce 57 lipoprotein measures from fasted blood serum samples. The children wore accelerometers for seven consecutive days to record time spent in light-, moderate-, and vigorous-intensity physical activity, and sedentary time. We used separate multivariable linear regression models to analyse associations between the device-measured activity variables—modelled both prospectively (baseline value) and as change scores (follow-up minus baseline value)—and each lipoprotein measure at follow-up.

Results

Higher baseline levels of moderate-intensity and vigorous-intensity physical activity were associated with a favourable lipoprotein particle profile at follow-up. The strongest associations were with the larger subclasses of triglyceride-rich lipoproteins. Sedentary time was associated with an unfavourable lipoprotein particle profile, the pattern of associations being the inverse of those in the moderate-intensity and vigorous-intensity physical activity analyses. The associations with light-intensity physical activity were more modest; those of the change models were weak.

Conclusion

We provide evidence of a prospective association between time spent active or sedentary and lipoprotein metabolism in schoolchildren. Change in activity levels across the school year is of limited influence in our young, healthy cohort.

Trial registration

ClinicalTrials.gov, #NCT02132494. Registered 7th April 2014

Analysis of pyrene-labelled apolipoprotein A-I oligomerization in solution: Spectra deconvolution and changes in P-value and excimer formation

ApoA-I is the main protein of HDL which has anti-atherogenic properties attributed to reverse cholesterol transport. It shares with other exchangeable apolipoproteins a high level of structural plasticity. In the lipid-free state, the apolipoprotein amphipathic α-helices interact intra- and inter-molecularly, providing structural stabilization by a complex self-association mechanism. In this study, we employed a multi-parametric fluorescent probe to study the self-association of apoA-I. We constructed six single cysteine mutants spanning positions along three helices: F104C, K107C (H4), K133C, L137C (H5), F225C and K226C (H10); and labelled them with N-Maleimide Pyrene. Taking advantage of its spectral properties, namely formation of an excited dimer (excimer) and polarity-dependent changes in its fluorescence fine structure (P-value), we monitored the apoA-I self-association in its lipid-free form as a function of its concentration. Interactions in helices H5 (K133C) and H10 (F225C and K226C) were highlighted by excimer emission; while polarity changes were reported in helix H4 (K107C), as well as in helices H5 and H10. Mathematical models were developed to enrich data analysis and estimate association constants (K_A) and oligomeric species distribution. Furthermore, we briefly discuss the usefulness of the multi-parametric fluorescent probe to monitor different equilibria, even at a single labelling position. Results suggest that apoA-I self-association must be considered to fully understand its physiological roles. Particularly, some contacts that stabilize discoidal HDL particles seem to be already present in the lipid-free apoA-I oligomers.

Denaturation of human plasma high-density lipoproteins by urea studied by apolipoprotein A-I dissociation

We studied the mechanism of HDL denaturation with concomitant apoA-I dissociation with HDL preparations from 48 patients with a wide range of plasma HDL-C and evaluated the contribution of lipid-free apoA-I into cholesterol efflux from macrophage, in particular, mediated by cholesterol transporter ABCA1. We prepared HDL by precipitation of apoB-containing lipoproteins by polyethylene glycol and used the chaotropic agent urea to denature HDL preparations. Apo-I dissociation from urea-treated HDL was assessed by the increase of preβ-band fraction with agarose gel electrophoresis followed by electro transfer and immunodetection and by the increase of ABCA1-mediated efflux of fluorescent analogue BODIPY-Cholesterol from RAW 264.7 macrophages. The HDL denaturation is governed by a single transition to fully dissociated apoA-I and the transition cooperativity decreases with increasing HDL-C. The apoA-I release depends on phospholipid concentration of HDL preparation and HDL compositional and structural heterogeneity and is well described by apolipoprotein partition between aqueous and lipid phases. Dissociated apoA-I determines the increase of ABCA1-mediated efflux of BODIPY-Cholesterol from RAW 264.7 macrophages to patient HDL. The increase in apoA-I dissociation is associated with the increase of ABCA1 gene transcript in peripheral blood mononuclear cells from patients. The low level of plasma HDL particles may be compensated by their increased potency for apoA-I release, thus suggesting apoA-I dissociation as a new HDL functional property.

Humoral Immunity Against HDL Particle: A New Perspective in Cardiovascular Diseases?

BACKGROUND

Autoimmune diseases are closely associated with cardiovascular diseases (CVD). Over the last decades, the comprehension of atherosclerosis, the principal initiator of CVD, evolved from a lipidcentered disease to a predominant inflammatory and immune response-driven disease displaying features of autoimmunity against a broad range of auto-antigens, including lipoproteins. Among them, high density lipoproteins (HDL) are important actors of cholesterol transport and bear several anti-atherogenic properties, raising a growing interest as therapeutic targets to decrease atherosclerosis and CVD burden, with nevertheless rather disappointing results so far. Reflecting HDL composition complexity, autoimmune responses and autoantibodies against various HDL components have been reported.

RESULTS

In this review, we addressed the important complexity of humoral autoimmunity towards HDL and particularly how this autoimmune response could help improving our understanding of HDL biological implication in atherosclerosis and CVD. We also discussed several issues related to specific HDL autoantibody subclasses characteristics, including etiology, prognosis and pathological mechanisms according to Rose criteria.

CONCLUSION

Finally, we addressed the possible clinical value of using these antibodies not only as potential biomarkers of atherogenesis and CVD, but also as a factor potentially mitigating the benefit of HDL-raising therapies.

Nascent HDL (High-Density Lipoprotein) Discs Carry Cholesterol to HDL Spheres: Effects of HDL Particle Remodeling on Cholesterol Efflux

OBJECTIVE

To characterize the fate of protein and lipid in nascent HDL (high-density lipoprotein) in plasma and explore the role of interaction between nascent HDL and mature HDL in promoting ABCA1 (ATP-binding cassette transporter 1)-dependent cholesterol efflux. Approach and Results: Two discoidal species, nascent HDL produced by RAW264.7 cells expressing ABCA1 (LpA-I [apo AI containing particles formed by incubating ABCA1-expressing cells with apo AI]), and CSL112, human apo AI (apolipoprotein AI) reconstituted with phospholipids, were used for in vitro incubations with human plasma or purified spherical plasma HDL. Fluorescent labeling and biotinylation of HDL were employed to follow the redistribution of cholesterol and apo AI, cholesterol efflux was measured using cholesterol-loaded cells. We show that both nascent LpA-I and CSL112 can rapidly fuse with spherical HDL. Redistribution of the apo AI molecules and cholesterol after particle fusion leads to the formation of (1) enlarged, remodeled, lipid-rich HDL particles carrying lipid and apo AI from LpA-I and (2) lipid-poor apo AI particles carrying apo AI from both discs and spheres. The interaction of discs and spheres led to a greater than additive elevation of ABCA1-dependent cholesterol efflux.

CONCLUSIONS

These data demonstrate that although newly formed discs are relatively poor substrates for ABCA1, they can interact with spheres to produce lipid-poor apo AI, a much better substrate for ABCA1. Because the lipid-poor apo AI generated in this interaction can itself become discoid by the action of ABCA1, cycles of cholesterol efflux and disc-sphere fusion may result in net ABCA1-dependent transfer of cholesterol from cells to HDL spheres. This process may be of particular importance in atherosclerotic plaque where cholesterol acceptors may be limiting.

Pharmacological inhibition of the F1 -ATPase/P2Y1 pathway suppresses the effect of apolipoprotein A1 on endothelial nitric oxide synthesis and vasorelaxation

AIM

The contribution of apolipoprotein A1 (APOA1), the major apolipoprotein of high-density lipoprotein (HDL), to endothelium-dependent vasodilatation is unclear, and there is little information regarding endothelial receptors involved in this effect. Ecto-F₁ -ATPase is a receptor for APOA1, and its activity in endothelial cells is coupled to adenosine diphosphate (ADP)-sensitive P2Y receptors (P2Y ADP receptors). Ecto-F₁ -ATPase is involved in APOA1-mediated cell proliferation and HDL transcytosis. Here, we investigated the effect of lipid-free APOA1 and the involvement of ecto-F₁ -ATPase and P2Y ADP receptors on nitric oxide (NO) synthesis and the regulation of vascular tone.

METHOD

Nitric oxide synthesis was assessed in human endothelial cells from umbilical veins (HUVECs) and isolated mouse aortas. Changes in vascular tone were evaluated by isometric force measurements in isolated human umbilical and placental veins and by assessing femoral artery blood flow in conscious mice.

RESULTS

Physiological concentrations of lipid-free APOA1 enhanced endothelial NO synthesis, which was abolished by inhibitors of endothelial nitric oxide synthase (eNOS) and of the ecto-F₁ -ATPase/P2Y₁ axis. Accordingly, APOA1 inhibited vasoconstriction induced by thromboxane A2 receptor agonist and increased femoral artery blood flow in mice. These effects were blunted by inhibitors of eNOS, ecto-F₁ -ATPase and P2Y₁ receptor.

CONCLUSIONS

Using a pharmacological approach, we thus found that APOA1 promotes endothelial NO production and thereby controls vascular tone in a process that requires activation of the ecto-F₁ -ATPase/P2Y₁ pathway by APOA1. Pharmacological targeting of this pathway with respect to vascular diseases should be explored.

Methionine oxidized apolipoprotein A-I at the crossroads of HDL biogenesis and amyloid formation

Apolipoprotein A-I (apoA-I) shares with other exchangeable apolipoproteins a high level of structural plasticity. In the lipid-free state, the apolipoprotein amphipathic α-helices interact intra- and intermolecularly, providing structural stabilization by self-association. We have reported that lipid-free apoA-I becomes amyloidogenic upon physiologically relevant (myeloperoxidase-mediated) Met oxidation. In this study, we established that Met oxidation promotes amyloidogenesis by reducing the stability of apoA-I monomers and irreversibly disrupting self-association. The oxidized apoA-I monomers also exhibited increased cellular cholesterol release capacity and stronger association with macrophages, compared to nonoxidized apoA-I. Of physiologic relevance, preformed oxidized apoA-I amyloid fibrils induced amyloid formation in nonoxidized apoA-I. This process was enhanced when self-association of nonoxidized apoA-I was disrupted by thermal treatment. Solid state NMR analysis revealed that aggregates formed by seeded nonoxidized apoA-I were structurally similar to those formed by the oxidized protein, featuring a β-structure-rich amyloid fold alongside α-helices retained from the native state. In atherosclerotic lesions, the conditions that promote apoA-I amyloid formation are readily available: myeloperoxidase, active oxygen species, low pH, and high concentration of lipid-free apoA-I. Our results suggest that even partial Met oxidation of apoA-I can nucleate amyloidogenesis, thus sequestering and inactivating otherwise antiatherogenic and HDL-forming apoA-I.-Witkowski, A., Chan, G. K. L., Boatz, J. C., Li, N. J., Inoue, A. P., Wong, J. C., van der Wel, P. C. A., Cavigiolio, G. Methionine oxidized apolipoprotein A-I at the crossroads of HDL biogenesis and amyloid formation.

Technical note: Characterization of ceramide in bovine lipoproteins

The hepatic synthesis and export of ceramide is enhanced in diabetic monogastrics. Moreover, ceramide in lipoproteins can mediate the development of insulin resistance. We have previously demonstrated that circulating ceramide increases during the progression of insulin resistance in postpartum dairy cows. Considering that the origins of circulating ceramide required investigation, our objective was to develop a method to characterize the ceramide profile of lipoprotein fractions collected from dairy cows. Serum was collected from 4 nonpregnant and nonlactating Holstein dairy cows. Serum lipoproteins were isolated using size exclusion chromatography by fast protein liquid chromatography (SEC-FPLC). Measurement of triacylglycerol (TAG), phospholipid, total cholesterol, and protein was performed using standard colorimetry practices. Following lipid extraction, fractions were analyzed using electrospray ionization tandem mass spectrometry. Data were analyzed as repeated measures using a mixed model. Lipoprotein isolation using SEC-FPLC and subsequent colorimetric analyses confirmed the presence of 4 distinct fractions: TAG-rich, low density (LDL), and large (buoyant) and small (dense) high density lipoprotein (HDL) subclasses. As expected, the fraction representing mixed very low density lipoproteins and chylomicrons primarily contained TAG. Low density lipoprotein fractions were equally enriched with cholesterol and phospholipid. Buoyant HDL contained elevated levels of cholesterol, phospholipid, and protein. In contrast, the fraction containing dense HDL primarily contained protein. Our method revealed that LDL are enriched with ceramides. Ceramides were also compartmentalized to a lesser extent within both HDL subclasses and TAG-rich lipoproteins. Comparable to whole serum, C16:0-ceramide was the predominant ceramide quantified in all lipoprotein subclasses. Interestingly, the proportion of C24:0-ceramide to total ceramide was elevated in TAG-rich lipoproteins, relative to all other lipoprotein subclasses. We conclude that SEC-FPLC coupled with mass spectrometry is a means to quantify ceramides in lipoprotein fractions. Moreover, ceramides are enriched within bovine LDL, and lipoprotein ceramide profiles reflect levels observed in whole serum. Future investigation will need to determine the biological importance of lipoprotein ceramides with distinct C-chains at amide residues.

Analysis of "On/Off" Kinetics of a CETP Inhibitor Using a Mechanistic Model of Lipoprotein Metabolism and Kinetics

RG7232 is a potent inhibitor of cholesteryl-ester transfer protein (CETP). Daily oral administration of RG7232 produces a dose- and time-dependent increase in high-density lipoprotein-cholesterol (HDL-C) and apolipoproteinA-I (ApoA-I) levels and a corresponding decrease in low-density lipoprotein-cholesterol (LDL-C) and apolipoproteinB (ApoB) levels. Due to its short plasma half-life (∼3 hours), RG7232 transiently inhibits CETP activity during each dosing interval ("on/off" kinetics), as reflected by the temporal effects on HDL-C and LDL-C. The influence of RG7232 on lipid-poor ApoA-I (i.e., pre-β 1) levels and reverse cholesterol transport rates is unclear. To investigate this, a published model of lipoprotein metabolism and kinetics was combined with a pharmacokinetic model of RG7232. After calibration and validation of the combined model, the effect of RG7232 on pre-β 1 levels was simulated. A dose-dependent oscillation of pre-β 1, driven by the "on/off" kinetics of RG7232 was observed. The possible implications of these findings are discussed.

An in-silico model of lipoprotein metabolism and kinetics for the evaluation of targets and biomarkers in the reverse cholesterol transport pathway

High-density lipoprotein (HDL) is believed to play an important role in lowering cardiovascular disease (CVD) risk by mediating the process of reverse cholesterol transport (RCT). Via RCT, excess cholesterol from peripheral tissues is carried back to the liver and hence should lead to the reduction of atherosclerotic plaques. The recent failures of HDL-cholesterol (HDL-C) raising therapies have initiated a re-examination of the link between CVD risk and the rate of RCT, and have brought into question whether all target modulations that raise HDL-C would be atheroprotective. To help address these issues, a novel in-silico model has been built to incorporate modern concepts of HDL biology, including: the geometric structure of HDL linking the core radius with the number of ApoA-I molecules on it, and the regeneration of lipid-poor ApoA-I from spherical HDL due to remodeling processes. The ODE model has been calibrated using data from the literature and validated by simulating additional experiments not used in the calibration. Using a virtual population, we show that the model provides possible explanations for a number of well-known relationships in cholesterol metabolism, including the epidemiological relationship between HDL-C and CVD risk and the correlations between some HDL-related lipoprotein markers. In particular, the model has been used to explore two HDL-C raising target modulations, Cholesteryl Ester Transfer Protein (CETP) inhibition and ATP-binding cassette transporter member 1 (ABCA1) up-regulation. It predicts that while CETP inhibition would not result in an increased RCT rate, ABCA1 up-regulation should increase both HDL-C and RCT rate. Furthermore, the model predicts the two target modulations result in distinct changes in the lipoprotein measures. Finally, the model also allows for an evaluation of two candidate biomarkers for in-vivo whole-body ABCA1 activity: the absolute concentration and the % lipid-poor ApoA-I. These findings illustrate the potential utility of the model in drug development.

Evidence for the presence of lipid-free monomolecular apolipoprotein A-1 in plasma

The first step in reverse cholesterol transport is a process by which lipid-free or lipid-poor apoA-1 removes cholesterol from cells through the action of ATP binding cassette transporter A1 at the plasma membrane. However the structure and composition of lipid-free or -poor apoA-1 in plasma remains obscure. We previously obtained a monoclonal antibody (MAb) that specifically recognizes apoA-1 in preβ1-HDL, the smallest apoA-1-containing particle in plasma, which we used to establish a preβ1-HDL ELISA. Here, we purified preβ1-HDL from fresh normal plasma using said antibody, and analyzed the composition and structure. ApoA-1 was detected, but neither phospholipid nor cholesterol were detected in the purified preβ1-HDL. Only globular, not discoidal, particles were observed by electron microscopy. In nondenaturing PAGE, no difference in the mobility was observed between the purified preβ1-HDL and original plasma preβ1-HDL, or between the preβ1-HDL and lipid-free apoA-1 prepared by delipidating HDL. In sandwich ELISA using two anti-preβ1-HDL MAbs, reactivity with intact plasma preβ1-HDL was observed in ELISA using two MAbs with distinct epitopes but no reactivity was observed in ELISA using a single MAb, and the same phenomenon was observed with monomolecular lipid-free apoA-1. These results suggest that plasma preβ1-HDL is lipid-free monomolecular apoA-1.

FAMP, a novel apoA-I mimetic peptide, suppresses aortic plaque formation through promotion of biological HDL function in ApoE-deficient mice

Background

Apolipoprotein (apo) A‐I is a major high‐density lipoprotein (HDL) protein that causes cholesterol efflux from peripheral cells through the ATP‐binding cassette transporter A1 (ABCA1), thus generating HDL and reversing the macrophage foam cell phenotype. Pre‐β₁ HDL is the smallest subfraction of HDL, which is believed to represent newly formed HDL, and it is the most active acceptor of free cholesterol. Furthermore it has a possible protective function against cardiovascular disease (CVD). We developed a novel apoA‐I mimetic peptide without phospholipids (Fukuoka University ApoA‐I Mimetic Peptide, FAMP).

Methods and Results

FAMP type 5 (FAMP5) had a high capacity for cholesterol efflux from A172 cells and mouse and human macrophages in vitro, and the efflux was mainly dependent on ABCA1 transporter. Incubation of FAMP5 with human HDL or whole plasma generated small HDL particles, and charged apoA‐I‐rich particles migrated as pre‐β HDL on agarose gel electrophoresis. Sixteen weeks of treatment with FAMP5 significantly suppressed aortic plaque formation (scrambled FAMP, 31.3±8.9% versus high‐dose FAMP5, 16.2±5.0%; P<0.01) and plasma C‐reactive protein and monocyte chemoattractant protein‐1 in apoE‐deficient mice fed a high‐fat diet. In addition, it significantly enhanced HDL‐mediated cholesterol efflux capacity from the mice.

Conclusions

A newly developed apoA‐I mimetic peptide, FAMP, has an antiatherosclerotic effect through the enhancement of the biological function of HDL. FAMP may have significant atheroprotective potential and prove to be a new therapeutic tool for CVD.

Lipoprotein remodeling generates lipid-poor apolipoprotein A-I particles in human interstitial fluid

Although much is known about the remodeling of high density lipoproteins (HDLs) in blood, there is no information on that in interstitial fluid, where it might have a major impact on the transport of cholesterol from cells. We incubated plasma and afferent (prenodal) peripheral lymph from 10 healthy men at 37°C in vitro and followed the changes in HDL subclasses by nondenaturing two-dimensional crossed immunoelectrophoresis and size-exclusion chromatography. In plasma, there was always initially a net conversion of small pre-β-HDLs to cholesteryl ester (CE)-rich α-HDLs. By contrast, in lymph, there was only net production of pre-β-HDLs from α-HDLs. Endogenous cholesterol esterification rate, cholesteryl ester transfer protein (CETP) concentration, CE transfer activity, phospholipid transfer protein (PLTP) concentration, and phospholipid transfer activity in lymph averaged 5.0, 10.4, 8.2, 25.0, and 82.0% of those in plasma, respectively (all P < 0.02). Lymph PLTP concentration, but not phospholipid transfer activity, was positively correlated with that in plasma (r = +0.63, P = 0.05). Mean PLTP-specific activity was 3.5-fold greater in lymph, reflecting a greater proportion of the high-activity form of PLTP. These findings suggest that cholesterol esterification rate and PLTP specific activity are differentially regulated in the two matrices in accordance with the requirements of reverse cholesterol transport, generating lipid-poor pre-β-HDLs in the extracellular matrix for cholesterol uptake from neighboring cells and converting pre-β-HDLs to α-HDLs in plasma for the delivery of cell-derived CEs to the liver.

Folded functional lipid-poor apolipoprotein A-I obtained by heating of high-density lipoproteins: relevance to high-density lipoprotein biogenesis

HDL (high-density lipoproteins) remove cell cholesterol and protect from atherosclerosis. The major HDL protein is apoA-I (apolipoprotein A-I). Most plasma apoA-I circulates in lipoproteins, yet ~5% forms monomeric lipid-poor/free species. This metabolically active species is a primary cholesterol acceptor and is central to HDL biogenesis. Structural properties of lipid-poor apoA-I are unclear due to difficulties in isolating this transient species. We used thermal denaturation of human HDL to produce lipid-poor apoA-I. Analysis of the isolated lipid-poor fraction showed a protein/lipid weight ratio of 3:1, with apoA-I, PC (phosphatidylcholine) and CE (cholesterol ester) at approximate molar ratios of 1:8:1. Compared with lipid-free apoA-I, lipid-poor apoA-I showed slightly altered secondary structure and aromatic packing, reduced thermodynamic stability, lower self-associating propensity, increased adsorption to phospholipid surface and comparable ability to remodel phospholipids and form reconstituted HDL. Lipid-poor apoA-I can be formed by heating of either plasma or reconstituted HDL. We propose the first structural model of lipid-poor apoA-I which corroborates its distinct biophysical properties and postulates the lipid-induced ordering of the labile C-terminal region. In summary, HDL heating produces folded functional monomolecular lipid-poor apoA-I that is distinct from lipid-free apoA-I. Increased adsorption to phospholipid surface and reduced C-terminal disorder may help direct lipid-poor apoA-I towards HDL biogenesis.

Impact of self-association on function of apolipoprotein A-I

Self-association is an inherent property of the lipid-free forms of several exchangeable apolipoproteins, including apolipoprotein A-I (apoA-I), the main protein component of high density lipoproteins (HDL) and an established antiatherogenic factor. Monomeric lipid-free apoA-I is believed to be the biologically active species, but abnormal conditions, such as specific natural mutations or oxidation, produce an altered state of self-association that may contribute to apoA-I dysfunction. Replacement of the tryptophans of apoA-I with phenylalanines (ΔW-apoA-I) leads to unusually large and stable self-associated species. We took advantage of this unique solution property of ΔW-apoA-I to analyze the role of self-association in determining the structure and lipid-binding properties of apoA-I as well as ATP-binding cassette A1 (ABCA1)-mediated cellular lipid release, a relevant pathway in atherosclerosis. Monomeric ΔW-apoA-I and wild-type apoA-I activated ABCA1-mediated cellular lipid release with similar efficiencies, whereas the efficiency of high order self-associated species was reduced to less than 50%. Analysis of specific self-associated subclasses revealed that different factors influence the rate of HDL formation in vitro and ABCA1-mediated lipid release efficiency. The α-helix-forming ability of apoA-I is the main determinant of in vitro lipid solubilization rates, whereas loss of cellular lipid release efficiency is mainly caused by reduced structural flexibility by formation of stable quaternary interactions. Thus, stabilization of self-associated species impairs apoA-I biological activity through an ABCA1-mediated mechanism. These results afford mechanistic insights into the ABCA1 reaction and suggest self-association as a functional feature of apoA-I. Physiologic mechanisms may alter the native self-association state and contribute to apoA-I dysfunction.

Relation of increased prebeta-1 high-density lipoprotein levels to risk of coronary heart disease

Preβ-1 high-density lipoprotein (HDL) plays a key role in reverse cholesterol transport by promoting cholesterol efflux. Our aims were (1) to test previous associations between preβ-1 HDL and coronary heart disease (CHD) and (2) to investigate whether preβ-1 HDL levels also are associated with risk of myocardial infarction (MI). Plasma preβ-1 HDL was measured by an ultrafiltration-isotope dilution technique in 1,255 subjects recruited from the University of California-San Francisco Lipid and Cardiovascular Clinics and collaborating cardiologists. Preβ-1 HDL was significantly and positively associated with CHD and MI even after adjustment for established risk factors. Inclusion of preβ-1 HDL in a multivariable model for CHD led to a modest improvement in reclassification of subjects (net reclassification index 0.15, p = 0.01; integrated discrimination improvement 0.003, p = 0.2). In contrast, incorporation of preβ-1 HDL into a risk model of MI alone significantly improved reclassification of subjects (net reclassification index 0.21, p = 0.008; integrated discrimination improvement 0.01, p = 0.02), suggesting that preβ-1 HDL has more discriminatory power for MI than for CHD in our study population. In conclusion, these results confirm previous associations between preβ-1 HDL and CHD in a large well-characterized clinical cohort. Also, this is the first study in which preβ-1 HDL was identified as a novel and independent predictor of MI above and beyond traditional CHD risk factors.

High pre-beta1 HDL concentrations and low lecithin: cholesterol acyltransferase activities are strong positive risk markers for ischemic heart disease and independent of HDL-cholesterol

BACKGROUND

We hypothesized that patients with high HDL-cholesterol (HDL-C) and ischemic heart disease (IHD) may have dysfunctional HDL or unrecognized nonconventional risk factors.

METHODS

Individuals with IHD (Copenhagen University Hospital) and either high HDL-C (n = 53; women >or=735 mg/L; men >or=619 mg/L) or low HDL-C (n = 42; women <or=387 mg/L; men <or=341 mg/L) were compared with individuals without IHD (Copenhagen City Heart Study) matched by age, sex, and HDL-C concentrations (n = 110). All participants had concentrations within reference intervals for LDL-C (<1600 mg/L) and triglyceride (<1500 mg/L), and none were treated with lipid-lowering medications. Pre-beta(1) HDL and phospholipid transfer protein concentrations were measured by using commercial kits and lecithin:cholesterol acyltransferase (LCAT) activity by using a proteoliposome cholesterol esterification assay.

RESULTS

Pre-beta(1) HDL concentrations were 2-fold higher in individuals with IHD vs no IHD in both the high [63 (5.7) vs 35 (2.3) mg/L; P < 0.0001] and low HDL-C [49 (5.0) vs 27 (1.5) mg/L; P = 0.001] groups. Low LCAT activity was also associated with IHD in the high [95.2 (6.7) vs 123.0 (5.3) micromol x L(-1) x h(-1); P = 0.002] and low [93.4 (8.3) vs 113.5 (4.9) micromol x L(-1) . h(-1); P = 0.03] HDL-C groups. ROC curves for pre-beta(1) HDL in the high-HDL-C groups yielded an area under the curve of 0.71 (95% CI: 0.61-0.81) for predicting IHD, which increased to 0.92 (0.87-0.97) when LCAT was included. Similar results were obtained for low HDL-C groups. An inverse correlation between LCAT activity and pre-beta(1) HDL was observed (r(2) = 0.30; P < 0.0001) in IHD participants, which was stronger in the low HDL-C group (r(2) = 0.56; P < 0.0001).

CONCLUSIONS

IHD was associated with high pre-beta(1) HDL concentrations and low LCAT levels, yielding correct classification in more than 90% of the IHD cases for which both were measured, thus making pre-beta(1) HDL concentration and LCAT activity level potentially useful diagnostic markers for cardiovascular disease.

Integrated approach for the comprehensive characterization of lipoproteins from human plasma using FPLC and nano-HPLC-tandem mass spectrometry

The implication of the various lipoprotein classes in the development of atherosclerotic cardiovascular disease has served to focus a great deal of attention on these particles over the past half-century. Using knowledge gained by the sequencing of the human genome, recent research efforts have been directed toward the elucidation of the proteomes of several lipoprotein subclasses. One of the challenges of such proteomic experimentation is the ability to initially isolate plasma lipoproteins subsequent to their analysis by mass spectrometry. Although several methods for the isolation of plasma lipoproteins are available, the most commonly utilized techniques require large sample volumes and may cause destruction and dissociation of lipoprotein particle-associated proteins. Fast protein liquid chromatography (FPLC) is a nondenaturing technique that has been validated for the isolation of plasma lipoproteins from relatively small sample volumes. In this study, we present the use of FPLC in conjunction with nano-HPLC-ESI-tandem mass spectrometry as a new integrated methodology suitable for the proteomic analysis of human lipoprotein fractions. Results from our analysis show that only 200 microl of human plasma suffices for the isolation of whole high density lipoprotein (HDL) and the identification of the majority of all known HDL-associated proteins using mass spectrometry of the resulting fractions.

Formation of prebeta1-HDL during lipolysis of triglyceride-rich lipoprotein

Prebeta1-HDL, a putative discoid-shaped high-density lipoprotein (HDL) is known to participate in the retrieval of cholesterol from peripheral tissues. In this study, to clarify potential sources of this lipoprotein, we conducted heparin injection on four Japanese volunteer men and found that serum triglyceride (TG) level decreased in parallel with the increase in serum nonesterified fatty acids and plasma lipoprotein lipase (LPL) protein mass after heparin injection. Plasma prebeta1-HDL showed considerable increases at 15 min after the heparin injection in all of the subjects. In contrast, serum HDL-C levels did not change. Gel filtration with fast protein liquid chromatography system (FPLC) study on lipoprotein profile revealed that in post-heparin plasma, low-density lipoprotein and alphaHDL fractions did not change, whereas there was a considerable decrease in very low-density lipoprotein (VLDL) fraction and an increase in prebeta1-HDL fraction when compared with those in pre-heparin plasma. We also conducted in vitro analysis on whether prebeta1-HDL was produced during VLDL lipolysis by LPL. One hundred microliters of VLDL extracted from pooled serum by ultracentrifugation was incubated with purified bovine milk LPL at 37 degrees C for 0-120 min. Prebeta1-HDL concentration increased in a dose dependent manner with increased concentration of added LPL in the reaction mixture and with increased incubation time, indicating that prebeta1-HDL was produced during lipolysis of VLDL by LPL. Taken these in vivo and in vitro analysis together, we suggest that lipolysis of VLDL particle by LPL is an important source for formation of prebeta1-HDL.

Functionally defective high-density lipoprotein: a new therapeutic target at the crossroads of dyslipidemia, inflammation, and atherosclerosis

High-density lipoproteins (HDL) possess key atheroprotective biological properties, including cellular cholesterol efflux capacity, and anti-oxidative and anti-inflammatory activities. Plasma HDL particles are highly heterogeneous in physicochemical properties, metabolism, and biological activity. Within the circulating HDL particle population, small, dense HDL particles display elevated cellular cholesterol efflux capacity, afford potent protection of atherogenic low-density lipoprotein against oxidative stress and attenuate inflammation. The antiatherogenic properties of HDL can, however be compromised in metabolic diseases associated with accelerated atherosclerosis. Indeed, metabolic syndrome and type 2 diabetes are characterized not only by elevated cardiovascular risk and by low HDL-cholesterol (HDL-C) levels but also by defective HDL function. Functional HDL deficiency is intimately associated with alterations in intravascular HDL metabolism and structure. Indeed, formation of HDL particles with attenuated antiatherogenic activity is mechanistically related to core lipid enrichment in triglycerides and cholesteryl ester depletion, altered apolipoprotein A-I (apoA-I) conformation, replacement of apoA-I by serum amyloid A, and covalent modification of HDL protein components by oxidation and glycation. Deficient HDL function and subnormal HDL-C levels may act synergistically to accelerate atherosclerosis in metabolic disease. Therapeutic normalization of attenuated antiatherogenic HDL function in terms of both particle number and quality of HDL particles is the target of innovative pharmacological approaches to HDL raising, including inhibition of cholesteryl ester transfer protein, enhanced lipidation of apoA-I with nicotinic acid and infusion of reconstituted HDL or apoA-I mimetics. A preferential increase in circulating concentrations of HDL particles possessing normalized antiatherogenic activity is therefore a promising therapeutic strategy for the treatment of common metabolic diseases featuring dyslipidemia, inflammation, and premature atherosclerosis.

Natriuretic peptide Val7Met substitution and risk of coronary artery disease in Greek patients with familial hypercholesterolemia

Atrial natriuretic peptide (ANP or NPPA) is the precursor protein of the form of amyloidosis called isolated atrial amyloid (IAA), which is related to the increased incidence of cardiac pathological conditions with age. Familial hypercholesterolemia (FH) patients are characterized by high concentrations of low-density lipoprotein cholesterol (LDL-C), which frequently gives rise to premature coronary artery disease (CAD). However, not all FH patients have the same clinical phenotype. The aim of the present study was to assess the relationship between ANP polymorphisms and apolipoprotein (Apo) A1 levels and CAD risk in FH patients. Transition T2238C, which leads to ANP with two additional arginines, and G664A (Val7Met) were investigated with lipid values and clinical phenotype in 83 FH patients. ApoA1 and HDL cholesterol levels were lower in GA patients compared to GG homozygotes for the G664A polymorphism. No association was found between the G664A polymorphism and CAD in our population. Moreover, ApoA1 and high-density lipoprotein cholesterol (HDL-C) levels did not differ among the different genotypes of the T2238C polymorphism, even after adjusting for age and sex. The 664A allele of the ANP polymorphism is associated with lower levels of ApoA1 and HDL-C in FH patients, but not with CAD risk. Concerning the T2238C polymorphism, no effect was found on lipid parameters or CAD incidence.

LCAT-dependent conversion rate is a determinant of plasma prebeta1-HDL concentration in healthy Japanese

BACKGROUND

Prebeta1-HDL acts as a primary acceptor of cellular cholesterol. Prebeta1-HDL is converted into alpha-migrating high-density lipoprotein (HDL) by lecithin/cholesterol acyltransferase (LCAT). We examined whether the LCAT-dependent conversion rate of prebeta1-HDL is a determinant of the plasma prebeta1-HDL concentration in healthy Japanese.

METHODS

We measured the conversion half time (CHT(prebeta1)), the time required for 50% of baseline prebeta1-HDL to be changed into alpha-migrating HDL by LCAT, in 100 healthy Japanese (47 men, 53 women, 22-88 years).

RESULTS

Prebeta1-HDL concentration, as determined by immunoassay, was significantly lower in younger women (<50 years, n=24) than in older women (>or=50 years, n=29) (16.8+/-3.3 vs. 21.7+/-8.0 mg/l apolipoprotein AI (apoAI), p<0.01). There was no significant difference in prebeta1-HDL concentration between younger (n=24) and older (n=23) men (21.2+/-6.8 vs. 22.5+/-6.6 mg/l apoAI). The mean CHT(prebeta1) for all subjects was 47.4+/-13.0 min, and was not influenced by gender or age. Prebeta1-HDL concentration was positively correlated with CHT(prebeta1) in both men and women, suggesting that high prebeta1-HDL levels may reflect delayed conversion of prebeta1-HDL.

CONCLUSION

LCAT-dependent conversion rate is a determinant of plasma prebeta1-HDL concentration in healthy Japanese. We speculate that prebeta1-HDL concentration may be used as a metabolic marker for HDL maturation.

Increased plasma lipid-poor apolipoprotein A-I in patients with coronary artery disease

BACKGROUND

Pre-beta 1-HDL participates in a cyclic process involved in the retrieval of cholesterol from peripheral tissues. Although pre-beta 1-HDL can be measured by two-dimensional electrophoresis or crossed immunoelectrophoresis, these methods are time-consuming and require technical expertise. In this study, we separated plasma lipid-poor apolipoprotein A-I (apo A-I) by high-performance size-exclusion chromatography.

METHODS

We measured plasma lipid-poor apo A-I in 20 male patients with coronary artery disease [CAD; mean (SD) age, 64.0 (18) years] and 15 male controls [54.7 (17) years] and in 7 female CAD patients [70.3 (7.7) years] and 9 female controls [65.1 (4.7) years].

RESULTS

Lipid-poor apo A-I was most stable when stored at -80 degrees C in the presence of aprotinin (final concentration, 50 kIU/L). The lipid-poor apo A-I concentration decreased during incubation at 37 degrees C, and this was not prevented by the addition of 2 mmol/L of the lecithin:cholesterol acyltransferase (LCAT) inhibitor 5,5'-dithiobis(2-nitrobenzoic acid). Lipid-poor apo A-I was significantly higher in CAD patients than in controls [38.3 (7.9) mg/L for male CAD patients vs 29.3 (7.3) mg/L for male controls; 43.3 (11) mg/L for female CAD patients vs 27.1 (7.4) mg/L for female controls (P <0.01 for both)]. There were no significant differences in LCAT activity or cholesteryl ester transfer protein (CETP) concentration between patients and controls. Moreover, the plasma lipid-poor apo A-I concentration was not significantly correlated with LCAT or CETP activities.

CONCLUSIONS

Although the production of lipid-poor apo A-I in plasma is not fully understood, our results indicate that lipid-poor apo A-I could be used as a marker for arteriosclerosis and demonstrate that it is not identical to the pre-beta1-HDL measured by other methods.

SR-BI- and ABCA1-mediated cholesterol efflux to serum from patients with Alagille syndrome

Alagille syndrome is associated with bile duct paucity resulting in liver disease. Patients can be divided into mildly and severely icteric groups, with both groups having altered lipoproteins. The incidence of ischemic heart disease is rare in severely cholestatic children despite increased total cholesterol and decreased high density lipoprotein cholesterol (HDL-C). The present studies examine the impact of altered lipid and lipoproteins on scavenger receptor class B type I (SR-BI)- and ABCA1-mediated efflux to serum from both groups. Efflux was compared with serum from 29 patients (15 with normal plasma cholesteryl ester, 14 with low cholesteryl ester). Efflux via SR-BI and ABCA1 was studied using cell systems having either low or high expression levels of these receptors. SR-BI efflux was lower (P = 0.04) with serum from severely icteric patients (3.9 +/- 1.4%) compared with serum from mildly icteric patients (5.1 +/- 1.4%) and was positively correlated with HDL-C and its apolipoproteins. SR-BI-mediated efflux was not correlated with any particular mature HDL but was negatively correlated with small lipid-poor prebeta-1 HDL. Consistent with severely icteric patients having high prebeta-1 HDL levels, the ABCA1 efflux was significantly higher with their serum (4.8 +/- 2.2%) compared with serum from mildly icteric patients (2.0 +/- 0.6%) and was positively correlated with prebeta-1 HDL. These studies demonstrated that prebeta-1 HDL is the preferred acceptor for ABCA1 efflux, whereas many particles mediate SR-BI efflux.

Small HDL form via apo A-I a complex with atrial natriuretic peptide

The goal of this study was to test the ability of small high density lipoproteins (small HDL) to bind human alpha-atrial natriuretic peptide (alpha-hANP), an amyloidogenic peptide whose involvement in cardiac pathologies is gaining increasing clinical evidence. After incubation of HDL with labeled ANP, the peptide associated to lipoprotein was detectable only in small HDL containing preparations. HDL-associated alpha-[(125)I]hANP was subjected to chromatography, electrophoresis, and autoradiography. The autoradiograph showed two radioactive bands, whose molecular weight was consistent with the chromatographic pattern. Immunoblotting showed the presence of apo A-I in both autoradiographic bands. The proteins of the main band were electroeluted, incubated with labeled ANP, and subjected to two-dimensional electrophoresis followed by autoradiography. The mass spectrometry and molecular weight analyses of the radioactive spot demonstrated the presence of an apo A-I dimer. This finding provided a novel solid evidence that small HDL via apo A-I dimer are involved in the ANP sequestration and thus may play a role in preventing amyloid fibril formation.

Association of Coronary Heart Disease with Pre-β-HDL Concentrations in Japanese Men

Background: In individuals heterozygous for ABCA1 transporter mutations, defective reverse cholesterol transport (RCT) causes low HDL-cholesterol and premature coronary heart disease (CHD). However, the extent to which impaired RCT underlies premature CHD in others with low HDL-cholesterol is not known. The primary acceptors of cell cholesterol are a minor subclass of lipid-poor pre-β-HDLs. These are generated during remodeling of α-HDLs, which account for almost all HDL-cholesterol. We studied the strength of the association of CHD with pre-β-HDL concentrations in Japanese men.

Methods: Blood was collected from 42 men with clinical CHD and 44 healthy controls 40–70 years of age. Pre-β-HDL was assayed by crossed immunoelectrophoresis.

Results: Cases had lower HDL-cholesterol (−23%), total apolipoprotein A-I (−26%), and pre-β-HDL (−55%; all P &lt;0.001) concentrations; lower pre-β-HDL:α-HDL ratios (−45%; P &lt;0.001); and higher plasma triglycerides (20%; P &lt;0.03) than the controls. On stepwise logistic regression, CHD was associated most strongly with pre-β-HDL concentrations. On ROC analysis, pre-β-HDL concentration discriminated between cases and controls better than any other lipoprotein measurement. When plasma was incubated for 16 h at 37 °C, mean (SD) pre-β-HDL increased by 47 (36)% in controls, but was unchanged in cases (group difference, P &lt;0.001).

Conclusions: Our results suggest that inefficient RCT, secondary to a low pre-β-HDL concentration and production rate in plasma, contributes to premature CHD in Japanese men with low HDL-cholesterol.

Assessing low levels of high-density lipoprotein cholesterol as a risk factor in coronary heart disease

Clinical data show that a 1% increase in serum concentrations of high-density lipoprotein cholesterol (HDL-C) can decrease cardiovascular risk by 2% to 3%. Therefore, mechanisms affecting the level and functionality of high-density lipoprotein (HDL) and its constituents are being investigated as targets for the rational development of drugs to prevent or treat cardiovascular disease. High-density lipoprotein-related research may also increase our understanding of the link between atherosclerosis and metabolic disorders. This report and update of the HDL Working Group discusses HDL metabolism and reverse cholesterol transport, impaired HDL as a marker and a cause of proatherogenic states, and experimental and current approaches to HDL-related therapy.

Potential involvement of dissociated apoA-I in the ABCA1-dependent cellular lipid release by HDL

Helical apolipoproteins of high density lipoprotein (HDL) remove phospholipid and cholesterol from cells and generate HDL particles being mediated by ATP binding cassette transporter A1 (ABCA1). In murine macrophage cell line RAW264 cells, cAMP induced expression of ABCA1, release of cellular phospholipid and cholesterol by apolipoprotein A-I (apoA-I), and reversible binding of apoA-I to the cell. The apoA-I-dependent lipid release was directly proportional to the cAMP-induced binding of apoA-I, and was inhibited 70% by a monoclonal antibody selective to lipid-free apoA-I, 725-1E2. In contrast, apparent cellular cholesterol release to HDL was substantial even without ABCA1 induction, and it was increased only by 27% after the cAMP treatment. The antibody inhibited this increment by 70%. Lipid-free apoA-II liberated apoA-I from HDL by displacement and thereby markedly expanded the cAMP-induced part of the cholesterol release to the HDL-containing medium, and the antibody inhibited this part also by 70%. Binding experiments of the double-labeled reconstituted HDL showed that cAMP induced reversible binding of apoA-I but not the association of cholesteryl ester with the cells. The effect of the antibody on the cellular cholesterol release to the reconstituted HDL was similar to that of the HDL-mediated release. The data implicated that the ABCA1-dependent cholesterol release to HDL is mediated by apoA-I dissociated from HDL.

Prebeta high density lipoprotein has two metabolic fates in human apolipoprotein A-I transgenic mice

We compared the in vivo metabolism of prebeta HDL particles isolated by anti-human apolipoprotein A-I (apoA-I) immunoaffinity chromatography (LpA-I) in human apoA-I transgenic (hA-I Tg) mice with that of lipid-free apoA-I (LFA-I) and small LpA-I. After injection, prebeta LpA-I were removed from plasma more rapidly than were LFA-I and small LpA-I. Prebeta LpA-I and LFA-I were preferentially degraded by kidney compared with liver; small LpA-I were preferentially degraded by the liver. Five minutes after tracer injection, 99% of LFA-I in plasma was found to be associated with medium-sized (8.6 nm) HDL, whereas only 37% of prebeta tracer remodeled to medium-sized HDL. Injection of prebeta LpA-I doses into C57Bl/6 recipients resulted in a slower plasma decay compared with hA-I Tg recipients and a greater proportion (>60%) of the prebeta radiolabel that was associated with medium-sized HDL. Prebeta LpA-I contained one to four molecules of phosphatidylcholine per molecule of apoA-I, whereas LFA-I contained less than one. We conclude that prebeta LpA-I has two metabolic fates in vivo, rapid removal from plasma and catabolism by kidney or remodeling to medium-sized HDL, which we hypothesize is determined by the amount of lipid associated with the prebeta particle and the particle's ability to bind to medium-sized HDL.

PLTP secreted by HepG2 cells resembles the high-activity PLTP form in human plasma

Plasma phospholipid transfer protein (PLTP) is an important regulator of plasma HDL levels and HDL particle distribution. PLTP is present in plasma in two forms, one with high and the other with low phospholipid transfer activity. We have used the human hepatoma cell line, HepG2, as a model to study PLTP secreted from hepatic cells. PLTP activity was secreted by the cells into serum-free culture medium as a function of time. However, modification of a previously established ELISA assay to include a denaturing sample pretreatment with the anionic detergent sodium dodecyl sulphate was required for the detection of the secreted PLTP protein. The HepG2 PLTP could be enriched by Heparin-Sepharose affinity chromatography and eluted in size-exclusion chromatography at a position corresponding to the size of 160 kDa. PLTP coeluted with apolipoprotein E (apoE) but not with apoB-100 or apoA-I. A portion of PLTP was retained by an anti-apoE immunoaffinity column together with apoE, suggesting an interaction between these two proteins. Furthermore, antibodies against apoE but not those against apoB-100 or apoA-I were capable of inhibiting PLTP activity. These results show that the HepG2-derived PLTP resembles in several aspects the high-activity form of PLTP found in human plasma.

Analytical performance of a sandwich enzyme immunoassay for pre beta 1-HDL in stabilized plasma

We have established an immunoassay for pre beta 1-HDL (the initial acceptor of cellular cholesterol) using a monoclonal antibody, MAb55201. Because pre beta 1-HDL is unstable during storage, fresh plasma must be used for pre beta 1-HDL measurements. In this study, we describe a method of stabilizing pre beta 1-HDL, and evaluate the analytical performance of the immunoassay for pre beta 1-HDL. Fresh plasma was stored under various conditions with or without a pretreatment consisting of a 21-fold dilution into 50% (v/v) sucrose. Pre beta 1-HDL concentration was measured by immunoassay. In nonpretreated samples, pre beta 1-HDL decreased significantly from the baseline after 6 h at room temperature. Although pre beta 1-HDL was more stable at 0 degrees C than at room temperature, it increased from 30.2 +/- 8.5 (SE) to 56.5 +/- 5.5 mg/l apolipoprotein A-I (apoA-I) (P < 0.001) in hyperlipidemics, and from 18.4 +/- 1.2 to 37.9 +/- 3.3 mg/l apoA-I (P < 0.001) in normolipidemics after 5-day storage. After 30-day storage at -80 degrees C, pre beta 1-HDL increased from 29.0 +/- 4.0 to 38.0 +/- 5.7 mg/l apoA-I (P < 0.001) in hyperlipidemics, whereas it did not change in normolipidemics. In pretreated samples, pre beta 1-HDL concentration did not change significantly under any of the above conditions. Moreover, pre beta 1-HDL concentrations determined by immunoassay correlated with those determined by native two-dimensional gel electrophoresis (n = 24, r = 0.833, P < 0.05). An immunoassay using MAb55201 with pretreated plasma is useful for clinical measurement of pre beta 1-HDL.

High-density lipoprotein cholesterol and triglycerides as therapeutic targets for preventing and treating coronary artery disease

Epidemiologic and clinical trials show that elevated triglycerides and low levels of high-density lipoprotein cholesterol (HDL-C) are independent risk factors for coronary heart disease (CHD). However, adjustment for covariates frequently weakens or abolishes the predictive significance of triglycerides, whereas the evidence for HDL-C is more consistently strong. Data indicate that there is a 2% to 3% decrease in coronary risk for each 1 mg/dL increase in HDL-C, whereas the benefit of triglyceride lowering appears to occur largely in patients with the highest baseline levels. The 2001 National Cholesterol Education Program Adult Treatment Panel III (ATP III) guidelines for detecting and treating high blood cholesterol reflect our improved understanding of triglycerides and HDL as CHD risk factors. However, the guidelines place more emphasis on lowering triglycerides than on raising HDL-C by identifying non-HDL-C (ie, low-density lipoprotein cholesterol [LDL-C] + very-low-density lipoprotein cholesterol [VLDL-C]) as a secondary target of therapy. In clinical practice, VLDL-C is the most readily available measure of atherogenic triglyceride-rich remnant lipoproteins. On the basis of the available epidemiologic and clinical evidence, refinement of the NCEP guidelines to include more emphasis on raising HDL-C levels should be considered. Novel drugs are being developed that have the potential to increase HDL-C concentrations and/or improve the functionality of HDL.

The differential apoA-I enrichment of prebeta1 and alphaHDL is detectable by gel filtration separation

The aim of the study was to assess the isolation of HDL by fast protein liquid chromatography (FPLC) to perform kinetics studies of apolipoprotein (apo)A-I-HDL labelled with a stable isotope. Comparison between FPLC and ultracentrifugation has been made. ApoA-I-HDL kinetics were studied by infusion of [5.5.5-(2)H(3)]leucine for 14 h in five subjects. Using FPLC, prebeta(1) HDL and alphaHDL (HDL(2) and HDL(3)) were separated from 200 microl of plasma samples. Total HDL was isolated by sequential ultracentrifugation (HDL-UC). The tracer-to-tracee ratio was higher in prebeta(1) HDL than in total HDL-UC. The higher leucine enrichment found in total HDL-UC compared to alphaHDL suggested the existence of a mixture of apoA-I-HDL sub-classes. From this difference in enrichments, the turnover rate of total HDL-UC, usually assumed to be alphaHDL, was probably overestimated in previous studies. To our knowledge, this study is the first report which provides a convenient tool to distinguish enrichments of apoA-I in prebeta(1) HDL and alphaHDL from total HDL previously used for kinetic measurements. This original and new method should help to understand the kinetics of HDL in humans and the reverse cholesterol transport dynamics.

Plasma factors controlling atrial natriuretic peptide (ANP) aggregation: role of lipoproteins

We have previously shown that human plasma atrial alpha-natriuretic peptide (alpha-hANP) sequestering is a protective phenomenon against amyloid aggregation. In the present work, the possible role of lipoproteins as alpha-hANP binding factors has been investigated in vitro using an experimental model, developed in our laboratory, that allows to work at physiological concentrations. This approach consists of gel filtration on Sephacryl S-300 HR of big alpha-[(125)I]hANP generated in phosphate buffered saline or in human normal plasma supplemented or not with lipoproteins. The results of these experiments indicate that high density lipoproteins (HDL) are responsible for the ANP binding phenomenon observed in vitro, while low density lipoproteins and very low density lipoproteins do not directly interact with ANP. Moreover, the HDL remodeling process occurring in vitro has been analyzed during plasma incubation by monitoring the redistribution of lipids and apolipoproteins among the HDL subclasses. The changes in HDL size and composition observed in incubated plasma were compared with the redistribution of endogenous and labeled big ANP. The obtained results revealed that both tend to follow the molecular rearrangement in plasma of apolipoprotein A-I containing particles and suggested that, among HDL species, the small particles are mainly involved in the ANP binding phenomenon. This hypothesis was further demonstrated by ligand blotting experiments that confirmed the existence of differences in the ability of HDL particles to bind alpha-[(125)I]hANP.

Concentrations of electrophoretic and size subclasses of apolipoprotein A-I-containing particles in human peripheral lymph

When cultured cells are exposed to plasma, the initial acceptors of unesterified cholesterol are small lipid-poor apolipoprotein A-I (apoA-I)-containing high density lipoproteins (HDLs) with pre-beta electrophoretic mobility. These are converted by lecithin:cholesterol acyltransferase into larger spheroidal cholesteryl ester-rich HDLs with alpha mobility. To study the determinants of the concentration of small pre-beta HDLs in tissue fluids, we collected prenodal peripheral lymph from 34 fasted normal men. By crossed immunoelectrophoresis, the concentration of pre-beta HDLs in lymph averaged 20% of that in plasma. On multiple regression analysis, pre-beta apoA-I concentration in lymph was directly related to pre-beta apoA-I concentration in plasma and independently to alpha apoA-I concentration in lymph. Similar results were obtained when the same apoA-I-containing particles were quantified by size exclusion chromatography. Lymph pre-beta apoA-I concentration was low in a subject with familial lecithin:cholesterol acyltransferase deficiency, despite a normal plasma pre-beta apoA-I concentration, but was normal in a subject with familial lipoprotein lipase deficiency. These results suggest that the concentration of small pre-beta HDLs in human tissue fluids is determined only in part by the transfer of pre-beta HDLs across capillary endothelium from plasma. Local production, by remodeling of spheroidal alpha HDLs in tissue fluids, may be equally important. Lipolysis of triglyceride-rich lipoproteins by lipoprotein lipase appears to have little effect.