Binding of phospholipid transfer protein (PLTP) to apolipoproteins A-I and A-II: location of a PLTP binding domain in the amino terminal region of apoA-I

Human HDL subclasses modulate energy metabolism in skeletal muscle cells

In addition to its antiatherogenic role, HDL reportedly modulates energy metabolism at the whole-body level. HDL functionality is associated with its structure and composition, and functional activities can differ between HDL subclasses. Therefore, we studied if HDL2 and HDL3, the two major HDL subclasses, are able to modulate energy metabolism of skeletal muscle cells. Differentiated mouse and primary human skeletal muscle myotubes were used to investigate the influences of human HDL2 and HDL3 on glucose and fatty uptake and oxidation. HDL-induced changes in lipid distribution and mRNA expression of genes related to energy substrate metabolism, mitochondrial function, and HDL receptors were studied with human myotubes. Additionally, we examined the effects of apoA-I and discoidal, reconstituted HDL particles on substrate metabolism. In mouse myotubes, HDL subclasses strongly enhanced glycolysis upon high and low glucose concentrations. HDL3 caused a minor increase in ATP-linked respiration upon glucose conditioning but HDL2 improved complex I-mediated mitochondrial respiration upon fatty acid treatment. In human myotubes, glucose metabolism was attenuated but fatty acid uptake and oxidation were markedly increased by both HDL subclasses, which also increased mRNA expression of genes related to fatty acid metabolism and HDL receptors. Finally, both HDL subclasses induced incorporation of oleic acid into different lipid classes. These results, demonstrating that HDL subclasses enhance fatty acid oxidation in human myotubes but improve anaerobic metabolism in mouse myotubes, support the role of HDL as a circulating modulator of energy metabolism. Exact mechanisms and components of HDL causing the change, require further investigation.

A Current Update on the Role of HDL-Based Nanomedicine in Targeting Macrophages in Cardiovascular Disease

High-density lipoproteins (HDL) are complex endogenous nanoparticles involved in important functions such as reverse cholesterol transport and immunomodulatory activities, ensuring metabolic homeostasis and vascular health. The ability of HDL to interact with a plethora of immune cells and structural cells places it in the center of numerous disease pathophysiologies. However, inflammatory dysregulation can lead to pathogenic remodeling and post-translational modification of HDL, rendering HDL dysfunctional or even pro-inflammatory. Monocytes and macrophages play a critical role in mediating vascular inflammation, such as in coronary artery disease (CAD). The fact that HDL nanoparticles have potent anti-inflammatory effects on mononuclear phagocytes has opened new avenues for the development of nanotherapeutics to restore vascular integrity. HDL infusion therapies are being developed to improve the physiological functions of HDL and to quantitatively restore or increase the native HDL pool. The components and design of HDL-based nanoparticles have evolved significantly since their initial introduction with highly anticipated results in an ongoing phase III clinical trial in subjects with acute coronary syndrome. The understanding of mechanisms involved in HDL-based synthetic nanotherapeutics is critical to their design, therapeutic potential and effectiveness. In this review, we provide a current update on HDL-ApoA-I mimetic nanotherapeutics, highlighting the scope of treating vascular diseases by targeting monocytes and macrophages.

Alterations of HDL's to piHDL's Proteome in Patients with Chronic Inflammatory Diseases, and HDL-Targeted Therapies

Chronic inflammatory diseases, such as rheumatoid arthritis, steatohepatitis, periodontitis, chronic kidney disease, and others are associated with an increased risk of atherosclerotic cardiovascular disease, which persists even after accounting for traditional cardiac risk factors. The common factor linking these diseases to accelerated atherosclerosis is chronic systemic low-grade inflammation triggering changes in lipoprotein structure and metabolism. HDL, an independent marker of cardiovascular risk, is a lipoprotein particle with numerous important anti-atherogenic properties. Besides the essential role in reverse cholesterol transport, HDL possesses antioxidative, anti-inflammatory, antiapoptotic, and antithrombotic properties. Inflammation and inflammation-associated pathologies can cause modifications in HDL's proteome and lipidome, transforming HDL from atheroprotective into a pro-atherosclerotic lipoprotein. Therefore, a simple increase in HDL concentration in patients with inflammatory diseases has not led to the desired anti-atherogenic outcome. In this review, the functions of individual protein components of HDL, rendering them either anti-inflammatory or pro-inflammatory are described in detail. Alterations of HDL proteome (such as replacing atheroprotective proteins by pro-inflammatory proteins, or posttranslational modifications) in patients with chronic inflammatory diseases and their impact on cardiovascular health are discussed. Finally, molecular, and clinical aspects of HDL-targeted therapies, including those used in therapeutical practice, drugs in clinical trials, and experimental drugs are comprehensively summarised.

Glycosylation of HDL-Associated Proteins and Its Implications in Cardiovascular Disease Diagnosis, Metabolism and Function

High-density lipoprotein (HDL) particles, long known for their critical role in the prevention of cardiovascular disease (CVD), were recently identified to carry a wide array of glycosylated proteins, and the importance of this glycosylation in the structure, function and metabolism of HDL are starting to emerge. Early studies have demonstrated differential glycosylation of HDL-associated proteins in various pathological states, which may be key to understanding their etiological role in these diseases and may be important for diagnostic development. Given the vast array and specificity of glycosylation pathways, the study of HDL-associated glycosylation has the potential to uncover novel mechanisms and biomarkers of CVD. To date, no large studies examining the relationships between HDL glycosylation profiles and cardiovascular outcomes have been performed. However, small pilot studies provide promising preliminary evidence that such a relationship may exist. In this review article we discuss the current state of the evidence on the glycosylation of HDL-associated proteins, the potential for HDL glycosylation profiling in CVD diagnostics, how glycosylation affects HDL function, and the potential for modifying the glycosylation of HDL-associated proteins to confer therapeutic value.

AGTR1, PLTP, and SCG2 associated with immune genes and immune cell infiltration in calcific aortic valve stenosis: analysis from integrated bioinformatics and machine learning

Background: Calcific aortic valve stenosis (CAVS) is a crucial cardiovascular disease facing aging societies. Our research attempts to identify immune-related genes through bioinformatics and machine learning analysis. Two machine learning strategies include Least Absolute Shrinkage Selection Operator (LASSO) and Support Vector Machine Recursive Feature Elimination (SVM-RFE). In addition, we deeply explore the role of immune cell infiltration in CAVS, aiming to study the potential therapeutic targets of CAVS and explore possible drugs. Methods: Download three data sets related to CAVS from the Gene Expression Omnibus. Gene set variation analysis (GSVA) looks for potential mechanisms, determines differentially expressed immune-related genes (DEIRGs) by combining the ImmPort database with CAVS differential genes, and explores the functions and pathways of enrichment. Two machine learning methods, LASSO and SVM-RFE, screen key immune signals and validate them in external data sets. Single-sample GSEA (ssGSEA) and CIBERSORT analyze the subtypes of immune infiltrating cells and integrate the analysis with DEIRGs and key immune signals. Finally, the possible targeted drugs are analyzed through the Connectivity Map (CMap). Results: GSVA analysis of the gene set suggests that it is highly correlated with multiple immune pathways. 266 differential genes (DEGs) integrate with immune genes to obtain 71 DEIRGs. Enrichment analysis found that DEIRGs are related to oxidative stress, synaptic membrane components, receptor activity, and a variety of cardiovascular diseases and immune pathways. Angiotensin II Receptor Type 1(AGTR1), Phospholipid Transfer Protein (PLTP), Secretogranin II (SCG2) are identified as key immune signals of CAVS by machine learning. Immune infiltration found that B cells naï ve and Macrophages M2 are less in CAVS, while Macrophages M0 is more in CAVS. Simultaneously, AGTR1, PLTP, SCG2 are highly correlated with a variety of immune cell subtypes. CMap analysis found that isoliquiritigenin, parthenolide, and pyrrolidine-dithiocarbamate are the top three targeted drugs related to CAVS immunity. Conclusion: The key immune signals, immune infiltration and potential drugs obtained from the research play a vital role in the pathophysiological progress of CAVS.

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

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

Modified lipoproteins in periodontitis: a link to cardiovascular disease?

There is a strong association between periodontal disease and atherosclerotic cardiovascular disorders. A key event in the development of atherosclerosis is accumulation of modified lipoproteins within the arterial wall. We hypothesise that patients with periodontitis have an altered lipoprotein profile towards an atherogenic form. Therefore, the present study aims at identifying modifications of plasma lipoproteins in periodontitis. Lipoproteins from ten female patients with periodontitis and gender- and age-matched healthy controls were isolated by density-gradient ultracentrifugation. Proteins were separated by 2D gel-electrophoresis and identified by map-matching or by nano-LC followed by MS. Apolipoprotein (Apo) A-I (ApoA-I) methionine oxidation, Oxyblot, total antioxidant capacity and a multiplex of 71 inflammation-related plasma proteins were assessed. Reduced levels of apoJ, phospholipid transfer protein, apoF, complement C3, paraoxonase 3 and increased levels of α-1-antichymotrypsin, apoA-II, apoC-III were found in high-density lipoprotein (HDL) from the patients. In low-density lipoprotein (LDL)/very LDL (VLDL), the levels of apoL-1 and platelet-activating factor acetylhydrolase (PAF-AH) as well as apo-B fragments were increased. Methionine oxidation of apoA-I was increased in HDL and showed a relationship with periodontal parameters. α-1 antitrypsin and α-2-HS glycoprotein were oxidised in LDL/VLDL and antioxidant capacity was increased in the patient group. A total of 17 inflammation-related proteins were important for group separation with the highest discriminating proteins identified as IL-21, Fractalkine, IL-17F, IL-7, IL-1RA and IL-2. Patients with periodontitis have an altered plasma lipoprotein profile, defined by altered protein levels as well as post-translational and other structural modifications towards an atherogenic form, which supports a role of modified plasma lipoproteins as central in the link between periodontal and cardiovascular disease (CVD).

Vitamin E Metabolic Effects and Genetic Variants: A Challenge for Precision Nutrition in Obesity and Associated Disturbances

Vitamin E (VE) has a recognized leading role as a contributor to the protection of cell constituents from oxidative damage. However, evidence suggests that the health benefits of VE go far beyond that of an antioxidant acting in lipophilic environments. In humans, VE is channeled toward pathways dealing with lipoproteins and cholesterol, underlining its relevance in lipid handling and metabolism. In this context, both VE intake and status may be relevant in physiopathological conditions associated with disturbances in lipid metabolism or concomitant with oxidative stress, such as obesity. However, dietary reference values for VE in obese populations have not yet been defined, and VE supplementation trials show contradictory results. Therefore, a better understanding of the role of genetic variants in genes involved in VE metabolism may be crucial to exert dietary recommendations with a higher degree of precision. In particular, genetic variability should be taken into account in targets concerning VE bioavailability per se or concomitant with impaired lipoprotein transport. Genetic variants associated with impaired VE liver balance, and the handling/resolution of oxidative stress might also be relevant, but the core information that exists at present is insufficient to deliver precise recommendations.

Structural basis of the lipid transfer mechanism of phospholipid transfer protein (PLTP)

Human phospholipid transfer protein (PLTP) mediates the transfer of phospholipids among atheroprotective high-density lipoproteins (HDL) and atherogenic low-density lipoproteins (LDL) by an unknown mechanism. Delineating this mechanism would represent the first step towards understanding PLTP-mediated lipid transfers, which may be important for treating lipoprotein abnormalities and cardiovascular disease. Here, using various electron microscopy techniques, PLTP is revealed to have a banana-shaped structure similar to cholesteryl ester transfer protein (CETP). We provide evidence that PLTP penetrates into the HDL and LDL surfaces, respectively, and then forms a ternary complex with HDL and LDL. Insights into the interaction of PLTP with lipoproteins at the molecular level provide a basis to understand the PLTP-dependent lipid transfer mechanisms for dyslipidemia treatment.

Influence of Isoforms and Carboxyl-Terminal Truncations on the Capacity of Apolipoprotein E To Associate with and Activate Phospholipid Transfer Protein

Phospholipid transfer protein (PLTP), a main protein in lipid and lipoprotein metabolism, exists in high-activity (HA-PLTP) and low-activity (LA-PLTP) forms in human plasma. Proper phospholipid transfer activity of PLTP is modulated by interactions with various apolipoproteins (apo) including apoE. The domains of apoE involved in interactions with PLTP are not known. Here we analyzed the capacity of recombinant apoE isoforms and apoE4 mutants with progressive carboxyl-terminal deletions to bind to and activate HA-PLTP and LA-PLTP. Our analyses demonstrated that lipid-free apoE isoforms bind to both HA-PLTP and LA-PLTP, resulting in phospholipid transfer activation, with apoE3 inducing the highest PLTP activation. The isoform-specific differences in apoE/PLTP binding and PLTP activation were abolished following apoE lipidation. Lipid-free apoE4[Δ(260-299)], apoE4[Δ(230-299)], apoE4[Δ(203-299)], and apoE4[Δ(186-299)] activated HA-PLTP by 120-160% compared to full-length apoE4. Lipid-free apoE4[Δ(186-299)] also activated LA-PLTP by 85% compared to full-length apoE4. All lipidated truncated apoE4 forms displayed a similar effect on HA-PLTP and LA-PLTP activity as full-length apoE4. Strikingly, lipid-free or lipidated full-length apoE4 and apoE4[Δ(186-299)] demonstrated similar binding capacity to LA-PLTP and HA-PLTP. Biophysical studies showed that the carboxyl-terminal truncations of apoE4 resulted in small changes of the structural or thermodynamic properties of lipidated apoE4, that were much less pronounced compared to changes observed previously for lipid-free apoE4. Overall, our findings show an isoform-dependent binding to and activation of PLTP by lipid-free apoE. Furthermore, the domain of apoE4 required for PLTP activation resides within its amino-terminal 1-185 region. The apoE/PLTP interactions can be modulated by the conformation and lipidation state of apoE.

Native protein mapping and visualization of protein interactions in the area of human plasma high-density lipoprotein by combining nondenaturing micro 2DE and quantitative LC-MS/MS

A human plasma sample was subjected to nondenaturing micro 2DE and a gel area (5 mm × 18 mm) that includes high-density lipoprotein (HDL) was cut into 1 mm × 1 mm squares, then the proteins in the 90 gel pieces were analyzed by quantitative LC-MS/MS. Grid-cutting of the gel was employed to; (i) ensure the total analysis of the proteins in the area, (ii) standardize the conditions of analysis by LC-MS/MS, (iii) reconstruct the protein distribution patterns from the quantity data. Totally 154 proteins were assigned in the 90 gel pieces and the quantity distribution of each was reconstructed as a color density pattern (a native protein map). The map of apolipoprotein (Apo) A-I showed a wide apparent mass distribution characteristic to HDL and was compared with the maps of the other 153 proteins. Eleven proteins showed maps of wide distribution that overlapped with the map of Apo A-I, and all have been reported to be the components of HDL. Further, seven minor proteins associated with HDL were detected at the gel positions of high Apo A-I quantity. These results for the first time visualized the localization of HDL apolipoproteins on a nondenaturing 2DE gel and strongly suggested their interactions.

Spontaneous remodeling of HDL particles at acidic pH enhances their capacity to induce cholesterol efflux from human macrophage foam cells

HDL particles may enter atherosclerotic lesions having an acidic intimal fluid. Therefore, we investigated whether acidic pH would affect their structural and functional properties. For this purpose, HDL(2) and HDL(3) subfractions were incubated for various periods of time at different pH values ranging from 5.5 to 7.5, after which their protein and lipid compositions, size, structure, and cholesterol efflux capacity were analyzed. Incubation of either subfraction at acidic pH induced unfolding of apolipoproteins, which was followed by release of lipid-poor apoA-I and ensuing fusion of the HDL particles. The acidic pH-modified HDL particles exhibited an enhanced ability to promote cholesterol efflux from cholesterol-laden primary human macrophages. Importantly, treatment of the acidic pH-modified HDL with the mast cell-derived protease chymase completely depleted the newly generated lipid-poor apoA-I, and prevented the acidic pH-dependent increase in cholesterol efflux. The above-found pH-dependent structural and functional changes were stronger in HDL(3) than in HDL(2). Spontaneous acidic pH-induced remodeling of mature spherical HDL particles increases HDL-induced cholesterol efflux from macrophage foam cells, and therefore may have atheroprotective effects.

Role of apolipoprotein A-II in the structure and remodeling of human high-density lipoprotein (HDL): protein conformational ensemble on HDL

High-density lipoproteins (HDL, or "good cholesterol") are heterogeneous nanoparticles that remove excess cell cholesterol and protect against atherosclerosis. The cardioprotective action of HDL and its major protein, apolipoprotein A-I (apoA-I), is well-established, yet the function of the second major protein, apolipoprotein A-II (apoA-II), is less clear. In this review, we postulate an ensemble of apolipoprotein conformations on various HDL. This ensemble is based on the crystal structure of Δ(185-243)apoA-I determined by Mei and Atkinson combined with the "double-hairpin" conformation of apoA-II(dimer) proposed in the cross-linking studies by Silva's team, and is supported by the wide array of low-resolution structural, biophysical, and biochemical data obtained by many teams over decades. The proposed conformational ensemble helps integrate and improve several existing HDL models, including the "buckle-belt" conformation of apoA-I on the midsize disks and the "trefoil/tetrafoil" arrangement on spherical HDL. This ensemble prompts us to hypothesize that endogenous apoA-II (i) helps confer lipid surface curvature during conversion of nascent discoidal HDL(A-I) and HDL(A-II) containing either apoA-I or apoA-II to mature spherical HDL(A-I/A-II) containing both proteins, and (ii) hinders remodeling of HDL(A-I/A-II) by hindering the expansion of the apoA-I conformation. Also, we report that, although endogenous apoA-II circulates mainly on the midsize spherical HDL(A-I/A-II), exogenous apoA-II can bind to HDL of any size, thereby slightly increasing this size and stabilizing the HDL assembly. This suggests distinctly different effects of the endogenous and exogenous apoA-II on HDL. Taken together, the existing results and models prompt us to postulate a new structural and functional role of apoA-II on human HDL.

Apolipoprotein A-I, A-II, and H mRNA and protein accumulation sites in the developing lung in late gestation

BACKGROUND

Expression of apolipoprotein A-I (apoA-I), A-II, and H was previously observed at 16 to 50-fold higher levels in the fetal than the adult mouse lung. Here, sites of apoA-I, A-II, and H mRNA and protein accumulation were determined in mouse fetal lungs by in situ hybridization and immunohistochemistry in late gestation.

RESULTS

Expression sites vary for the three genes and change for the distal epithelium before the end of the canalicular stage, thus where and when the surge of surfactant synthesis occurs. Messenger of apoH, but not those of apoA-I and A-II, was also observed in the proximal epithelium and smooth muscles surrounding arteries. In contrast to apoC-II protein, none of the three studied apolipoproteins accumulated within secretory granule-like structures. Immunohistochemistry revealed that apoA-I and apoH accumulated mainly in capillaries. Three different positive signals with the anti-apoA-II antibody were found: one transient signal in the nucleus of a portion of mesenchymal cells, a second at lower levels throughout the mesenchyme, and another in capillaries with a specific increase from gestation day 17.5/18.5.

CONCLUSION

Temporal and geographic co-expression of apoAI, AII, and H genes with surfactant production site suggests that the three apolipoproteins are secreted to play roles supporting the lung-specific surfactant lipid-related metabolism.

"Sticky" and "promiscuous", the yin and yang of apolipoprotein A-I termini in discoidal high-density lipoproteins: a combined computational-experimental approach

Apolipoprotein (apo) A-I-containing lipoproteins in the form of high-density lipoproteins (HDL) are inversely correlated with atherosclerosis. Because HDL is a soft form of condensed matter easily deformable by thermal fluctuations, the molecular mechanisms for HDL remodeling are not well understood. A promising approach to understanding HDL structure and dynamics is molecular dynamics (MD). In the present study, two computational strategies, MD simulated annealing (MDSA) and MD temperature jump, were combined with experimental particle reconstitution to explore molecular mechanisms for phospholipid- (PL-) rich HDL particle remodeling. The N-terminal domains of full-length apoA-I were shown to be "sticky", acting as a molecular latch largely driven by salt bridges, until, at a critical threshold of particle size, the associated domains released to expose extensive hydrocarbon regions of the PL to solvent. The "sticky" N-termini also associate with other apoA-I domains, perhaps being involved in N-terminal loops suggested by other laboratories. Alternatively, the overlapping helix 10 C-terminal domains of apoA-I were observed to be extremely mobile or "promiscuous", transiently exposing limited hydrocarbon regions of PL. Based upon these models and reconstitution studies, we propose that separation of the N-terminal domains, as particles exceed a critical size, triggers fusion between particles or between particles and membranes, while the C-terminal domains of apoA-I drive the exchange of polar lipids down concentration gradients between particles. This hypothesis has significant biological relevance since lipid exchange and particle remodeling are critically important processes during metabolism of HDL particles at every step in the antiatherogenic process of reverse cholesterol transport.

Global proteomic profiling reveals altered proteomic signature in schizophrenia serum

Schizophrenia is one of the most severe psychiatric disorders affecting 1% of the world population. There is yet no empirical method to validate the diagnosis of the disease. The identification of an underlying molecular alteration could lead to an improved disease understanding and may yield an objective panel of biomarkers to aid in the diagnosis of this devastating disease. Presented is the largest reported liquid chromatography-mass spectrometry-based proteomic profiling study investigating serum samples taken from first-onset drug-naive patients compared with samples collected from healthy volunteers. The results of this large-scale study are presented along with enzyme-linked immunosorbent assay-based validation data.

The role of phospholipid transfer protein in lipoprotein-mediated neutralization of the procoagulant effect of anionic liposomes

BACKGROUND

Serum has the ability to neutralize the procoagulant properties of anionic liposomes, with transfer of phospholipids (PLs) to both high-density lipoprotein (HDL) and low-density lipoprotein (LDL) particles. Phospholipid transfer protein (PLTP) mediates transfer of PLs between HDL and other lipoproteins and conversion of HDL into larger and smaller particles.

OBJECTIVES

To examine the role of PLTP in the neutralization of procoagulant liposomes.

METHODS

Procoagulant liposomes were incubated with different lipoproteins in the presence or absence of PLTP, and then tested for their ability to stimulate thrombin formation.

RESULTS AND CONCLUSIONS

In the absence of added PLTP, the lipoprotein-enriched fraction, total HDL, HDL(3) and very high-density lipoprotein (VHDL) were all able to neutralize the procoagulant properties of the liposomes. In these samples, endogenous PLTP was present, as judged by Western blotting. In contrast, no PLTP was present in LDL, HDL(2) and lipoprotein-deficient serum, all of which displayed no ability to neutralize the procoagulant liposomes. The phospholipid (PL) transfer activity was dependent on both enzyme (PLTP) and PL acceptor (lipoproteins). After treatment of the VHDL fraction with antiserum against PLTP, the neutralization of procoagulant activity was reduced, but could be regained by the addition of active PLTP. The neutralizing activity was dependent on a catalytically active form of PLTP, and addition of a low activity form of PLTP had no effect. In conclusion, PLTP was found to mediate transfer of anionic PLs to HDL and LDL, thereby neutralizing the effect of procoagulant liposomes, resulting in a reduction of procoagulant activity.

Thermal stability of apolipoprotein A-I in high-density lipoproteins by molecular dynamics

Apolipoprotein (apo) A-I is an unusually flexible protein whose lipid-associated structure is poorly understood. Thermal denaturation, which is used to measure the global helix stability of high-density lipoprotein (HDL)-associated apoA-I, provides no information about local helix stability. Here we report the use of temperature jump molecular dynamics (MD) simulations to scan the per-residue helix stability of apoA-I in phospholipid-rich HDL. When three 20 ns MD simulations were performed at 500 K on each of two particles created by MD simulations at 310 K, bilayers remained intact but expanded by 40%, and total apoA-I helicity decreased from 95% to 72%. Of significance, the conformations of the overlapping N- and C-terminal domains of apoA-I in the particles were unusually mobile, exposing hydrocarbon regions of the phospholipid to solvent; a lack of buried interhelical salt bridges in the terminal domains correlated with increased mobility. Nondenaturing gradient gels show that 40% expansion of the phospholipid surface of 100:2 particles by addition of palmitoyloleoylphosphatidylcholine exceeds the threshold of particle stability. As a unifying hypothesis, we propose that the terminal domains of apoA-I are phospholipid concentration-sensitive molecular triggers for fusion/remodeling of HDL particles. Since HDL remodeling is necessary for cholesterol transport, our model for remodeling has substantial biomedical implications.

Discrete roles of apoA-I and apoE in the biogenesis of HDL species: lessons learned from gene transfer studies in different mouse models

Using adenovirus-mediated gene transfer in apolipoprotein A-I (apoA-I)-deficient mice, we have established that apoA-I mutations inhibit discrete steps in a pathway that leads to the biogenesis and remodeling of high-density lipoprotein (HDL). To this point, five discrete categories of apoA-I mutants have been characterized that may affect the interactions of apoA-I with ATP-binding cassette superfamily A, member 1 (ABCA1) or lecithin:cholesterol acyl transferase (LCAT) or may influence the plasma phospholipid transfer protein activity or may cause various forms of dyslipidemia. Biogenesis of HDL is not a unique property of apoA-I. Using adenovirus-mediated gene transfer of apoE in apoA-I- or ABCA1-deficient mice, we have established that apolipoprotein E (apoE) also participates in a novel pathway of biogenesis of apoE-containing HDL particles. This process requires the functions of the ABCA1 lipid transporter and LCAT, and it is promoted by substitution of hydrophobic residues in the 261 to 269 region of apoE by Ala. The apoE-containing HDL particles formed in the circulation may have atheroprotective properties. ApoE-containing HDL may also have important biological functions in the brain that confer protection from Alzheimer's disease.

Atherogenic, enlarged, and dysfunctional HDL in human PLTP/apoA-I double transgenic mice

In low density lipoprotein receptor (LDLR)-deficient mice, overexpression of human plasma phospholipid transfer protein (PLTP) results in increased atherosclerosis. PLTP strongly decreases HDL levels and might alter the antiatherogenic properties of HDL particles. To study the potential interaction between human PLTP and apolipoprotein A-I (apoA-I), double transgenic animals (hPLTPtg/hApoAItg) were compared with hApoAItg mice. PLTP activity was increased 4.5-fold. Plasma total cholesterol and phospholipid were decreased. Average HDL size (analyzed by gel filtration) increased strongly, hPLTPtg/hApoAItg mice having very large, LDL-sized, HDL particles. Also, after density gradient ultracentrifugation, a substantial part of the apoA-I-containing lipoproteins in hPLTPtg/hApoAItg mice was found in the LDL density range. In cholesterol efflux studies from macrophages, HDL isolated from hPLTPtg/hApoAItg mice was less efficient than HDL isolated from hApoAItg mice. Furthermore, it was found that the largest subfraction of the HDL particles present in hPLTPtg/hApoAItg mice was markedly inferior as a cholesterol acceptor, as no labeled cholesterol was transferred to this fraction. In an LDLR-deficient background, the human PLTP-expressing mouse line showed a 2.2-fold increased atherosclerotic lesion area. These data demonstrate that the action of human PLTP in the presence of human apoA-I results in the formation of a dysfunctional HDL subfraction, which is less efficient in the uptake of cholesterol from cholesterol-laden macrophages.

Buffering mechanisms in aging: a systems approach toward uncovering the genetic component of aging

An unrealized potential to understand the genetic basis of aging in humans, is to consider the immense survival advantage of the rare individuals who live 100 years or more. The Longevity Gene Study was initiated in 1998 at the Albert Einstein College of Medicine to investigate longevity genes in a selected population: the “oldest old” Ashkenazi Jews, 95 years of age and older, and their children. The study proved the principle that some of these subjects are endowed with longevity-promoting genotypes. Here we reason that some of the favorable genotypes act as mechanisms that buffer the deleterious effect of age-related disease genes. As a result, the frequency of deleterious genotypes may increase among individuals with extreme lifespan because their protective genotype allows disease-related genes to accumulate. Thus, studies of genotypic frequencies among different age groups can elucidate the genetic determinants and pathways responsible for longevity. Borrowing from evolutionary theory, we present arguments regarding the differential survival via buffering mechanisms and their target age-related disease genes in searching for aging and longevity genes. Using more than 1,200 subjects between the sixth and eleventh decades of life (at least 140 subjects in each group), we corroborate our hypotheses experimentally. We study 66 common allelic site polymorphism in 36 candidate genes on the basis of their phenotype. Among them we have identified a candidate-buffering mechanism and its candidate age-related disease gene target. Previously, the beneficial effect of an advantageous cholesteryl ester transfer protein (CETP-VV) genotype on lipoprotein particle size in association with decreased metabolic and cardiovascular diseases, as well as with better cognitive function, have been demonstrated. We report an additional advantageous effect of the CETP-VV (favorable) genotype in neutralizing the deleterious effects of the lipoprotein(a) (LPA) gene. Finally, using literature-based interaction discovery methods, we use the set of longevity genes, buffering genes, and their age-related target disease genes to construct the underlying subnetwork of interacting genes that is expected to be responsible for longevity. Genome wide, high-throughput hypothesis-free analyses are currently being utilized to elucidate unknown genetic pathways in many model organisms, linking observed phenotypes to their underlying genetic mechanisms. The longevity phenotype and its genetic mechanisms, such as our buffering hypothesis, are similar; thus, the experimental corroboration of our hypothesis provides a proof of concept for the utility of high-throughput methods for elucidating such mechanisms. It also provides a framework for developing strategies to prevent some age-related diseases by intervention at the appropriate level.

Previous research showed that the frequency of deleterious genotype of some age-related disease decreases its prevalence as the population ages, as expected, since subjects with deleterious genotype are weeded out due to mortality. There exists, however, a set of age-related genes whose deleterious genotype indeed decreases up to ages 80–85, but subsequently increases monotonically, until by age 100 its prevalence is similar to that at age ∼60. Why is a known harmful genotype so prevalent among centenarians? Most likely because this genotype is protected by longevity genes. We corroborated this hypothesis by studying gene–gene interactions between age-related disease genotypes and longevity genotypes. Our findings suggest that individuals with the favorable longevity genotype can have just as many deleterious aging genotypes as the rest of the population because their longevity genotype protects them from the harmful effects of the other. We identify genes contributing to extreme lifespan as well as their counterpart, age-related disease genes. Our findings provide a proof of concept for the utility of high-throughput methods, and for elucidating mechanisms by which longevity genes buffer the effects of disease genes. Our approach gives hope for developing new medications that will protect against several age-related diseases.

Plasma apolipoprotein E concentration is an important determinant of phospholipid transfer protein activity in type 2 diabetes mellitus

BACKGROUND

Phospholipid transfer protein (PLTP) transfers phospholipids between lipoproteins and plays an important role in HDL metabolism. PLTP exists as a high-activity and a low-activity form in the circulation. In vitro studies have shown that apolipoprotein (apo) E is involved in maintaining PLTP in the active form, while the low-activity form is associated with apo AI. We have therefore investigated whether plasma apo AI, B and E concentrations are important determinants of plasma PLTP activity in type 2 diabetes, a condition associated with increased plasma PLTP activity.

METHODS

Plasma PLTP activity was assayed by measuring the transfer of radiolabelled phosphatidylcholine from liposomes to HDL; apo AI and B by rate nephelometry and apo E by a 2-point turbidimetric assay.

RESULTS

Type 2 diabetic patients (n = 230) had higher PLTP activity than controls (n = 97) (2374 +/- 628 nmol/mL/h versus 1862 +/- 585 respectively, p < 0.01). They also had increased fasting triglyceride and low HDL. Plasma apo B (p < 0.01) and apo E (p < 0.05) were increased, whereas apo AI was reduced (p < 0.01). Univariate analysis showed that plasma PLTP activity correlated mainly with apolipoproteins AI and E. Stepwise regression analysis showed that apo E was the main determinant of plasma PLTP activity, accounting for 23% of its variability in the diabetic subjects and 8% in the controls respectively.

CONCLUSIONS

The associations between plasma apo AI and E concentrations and PLTP activity suggest that these apolipoproteins are important regulators of PLTP activity in vivo. The increase in PLTP activity in type 2 diabetes is partly related to the changes in these apolipoproteins.

Role of apoA-I, ABCA1, LCAT, and SR-BI in the biogenesis of HDL

The concentration, composition, shape, and size of plasma high-density lipoprotein (HDL) are determined by numerous proteins that influence its biogenesis, remodeling, and catabolism. The discoveries of the HDL receptor (scavenger receptor class B type I, SR-BI) and the ABCA1 (ATP-binding cassette transporter A1) lipid transporter provided two missing links that were necessary to understand the biogenesis and some of the functions of HDL. Existing data indicate that functional interactions between apoA-I and ABCA1 are necessary for the initial lipidation of apoA-I. Through a series of intermediate steps, lipidated apoA-I proceeds to form discoidal HDL particles that can be converted to spherical particles by the action of lecithin:cholesterol acyltransferase (LCAT). Discoidal and spherical HDL can interact functionally with SR-BI and these interactions lead to selective lipid uptake and net efflux of cholesterol and thus remodel HDL. Defective apoA-I/ABCA1 interactions prevent lipidation of apoA-I that is necessary for the formation of HDL particles. In the same way, specific mutations in apoA-I or LCAT prevent the conversion of discoidal to spherical HDL particles. The interactions of lipid-bound apoA-I with SR-BI are affected in vitro by specific mutations in apoA-I or SR-BI. Furthermore, deficiency of SR-BI affects the lipid and apolipoprotein composition of HDL and is associated with increased susceptibility to atherosclerosis. Here we review the current status of the pathway of HDL biogenesis and mutations in apoA-I, ABCA1, and SR-BI that disrupt different steps of the pathway and may lead to dyslipidemia and atherosclerosis in mouse models. The phenotypes generated in experimental mouse models for apoA-I, ABCA1, LCAT, SR-BI, and other proteins of the HDL pathway may facilitate early diagnosis of similar phenotypes in the human population and provide guidance for proper treatment.

Deletions of helices 2 and 3 of human apoA-I are associated with severe dyslipidemia following adenovirus-mediated gene transfer in apoA-I-deficient mice

The objective of this study was to determine the effect of two amino-terminal apolipoprotein A-I (apoA-I) deletions on high-density lipoprotein (HDL) biosynthesis and lipid homeostasis. Adenovirus-mediated gene transfer showed that the apoA-I[Delta(89-99)] deletion mutant caused hypercholesterolemia, characterized by increased plasma cholesterol and phospholipids, that were distributed in the very low density/intermediate density/low-density lipoprotein (VLDL/IDL/LDL) region, and normal triglycerides. The capacity of the mutant protein to promote ATP-binding cassette transporter A1- (ABCA1-) mediated cholesterol efflux and to activate lecithin:cholesterol acyltranserase (LCAT) was approximately 70-80% of the wild-type (WT) control. The phospholipid transfer protein (PLTP) activity of plasma containing the apoA-I[Delta(89-99)] mutant was decreased to 32% of the WT control. Similar analysis showed that the apoA-I[Delta(62-78)] deletion mutant in apoA-I-deficient mice caused combined hyperlipidemia characterized by increased triglycerides, cholesterol, and phospholipids in the VLDL/IDL region. There was enrichment of the VLDL/IDL with mutant apoA-I that resulted in reduction of in vitro lipolysis. The capacity of this mutant to promote ABCA1-mediated cholesterol efflux was normal, and the capacity to activate LCAT in vitro was reduced by 53%. The WT apoA-I and the apoA-I[Delta(62-78)] mutant formed spherical HDL particles, whereas the apoA-I[Delta(89-99)] mutant formed discoidal HDL particles. We conclude that alterations in apoA-I not only may have adverse effects on HDL biosynthesis but also may promote dyslipidemia due to interference of the apoA-I mutants on the overall cholesterol and triglycerides homeostasis.

Apolipoprotein E activates the low-activity form of human phospholipid transfer protein

Phospholipid transfer protein (PLTP) exists in a high-activity (HA-PLTP) and a low-activity form (LA-PLTP) in the circulation. LA-PLTP is associated with apoA-I while the HA-PLTP complex is enriched with apoE. To study the interaction of PLTP with apolipoproteins, we carried out surface plasmon resonance analyses. These demonstrated a concentration-dependent binding of recombinant human PLTP, which represents an active PLTP form, and LA-PLTP to apoE, apoA-I, and apoA-IV within a nanomolar K(D) range. To study whether LA-PLTP can be transformed into an active form, we incubated it in the presence of proteoliposomes containing apoE, apoA-I or apoA-IV. The apoE proteoliposomes induced a concentration-dependent activation of LA-PLTP. ApoA-IV proteoliposomes also activated LA-PLTP in a concentration-dependent manner, whereas apoA-I proteoliposomes had no such effect. These observations suggest that PLTP is capable of interacting with apoE, apoA-I, and apoA-IV, and that these interactions regulate PLTP-activity levels in plasma.

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.

Plasma phospholipid transfer protein-mediated reactions are impaired by hypochlorite-modification of high density lipoprotein

The two main functions of phospholipid transfer protein (PLTP) are the transfer of phospholipids between plasma lipoproteins and the conversion of high density lipoprotein (HDL), where prebeta-HDL particles are generated. HDL is considered an anti-atherogenic lipoprotein due to its function in the reverse cholesterol transport, where prebeta-HDL accepts cellular membrane cholesterol from peripheral tissues. However, the anti-atherogenic properties of native HDL may be abolished by oxidation/modification. Hypochlorous acid/hypochlorite (HOCl/OCl-)-a potent oxidant generated in vivo only by the myeloperoxidase-H2O2-chloride system of activated phagocytes-alters the physiological properties of HDL by generating a pro-atherogenic lipoprotein particle. Therefore, we have studied the effect of HOCl on the function of HDL subclass 3 (HDL3) and triglyceride-enriched HDL3 (TG-HDL3) in PLTP-mediated processes in vitro. Modification of HDL3 and TG-HDL3 with increasing HOCl concentrations (oxidant:lipoprotein molar ratio between 25:1 and 200:1) decreased the capacity of the corresponding lipoprotein particles to accept phospholipids. Although binding of PLTP to unmodified and HOCl-modified lipoprotein particles was similar, the degree of PLTP-mediated HDL conversion was decreased upon HOCl oxidation. PLTP released apolipoprotein A-I (apoA-I) from HOCl-modified HDL3, but the particles formed displayed no prebeta-mobility. Based on these findings, we conclude that the substrate properties of HOCl-modified HDL3 and TG-HDL3 in PLTP-mediated processes are impaired, which indicates that the anti-atherogenic properties of HDL are impaired.

Isolation and partial characterization of the inactive and active forms of human plasma phospholipid transfer protein (PLTP)

Plasma phospholipid transfer protein (PLTP) plays an important role in lipoprotein metabolism. Two forms of PLTP exist in human plasma, one catalytically active (high activity form, HA-PLTP) and the other inactive (low activity form, LA-PLTP) (Oka, T., Kujiraoka, T., Ito, M., Egashira, T., Takahashi, S., Nanjee, N. M., Miller, N. E., Metso, J., Olkkonen, V. M., Ehnholm, C., Jauhiainen, M., and Hattori, H. (2000) J. Lipid Res. 41, 1651-1657). The two forms are associated with macromolecular complexes of different size. The apparent size of LA-PLTP is 520 kDa and that of HA-PLTP is 160 kDa. Of the circulating PLTP mass only a minor portion is in the HA-PLTP form in normolipidemic subjects. In the present study we have isolated and partially characterized the LA and HA forms of PLTP. Both LA- and HA-PLTP bind to heparin-Sepharose and can be separated by elution with 0-0.5 m NaCl gradient, with HA-PLTP displaying higher affinity for the matrix. LA-PLTP was further purified using hydrophobic butyl-Sepharose and anti-PLTP immunoaffinity chromatography steps. HA-PLTP was subjected to a second heparin-Sepharose step and hydroxylapatite chromatography. Analysis of the two forms of PLTP by SDS-PAGE, Western blotting, immunoprecipitation, and gel filtration demonstrates that LA-PLTP is complexed with apoA-I whereas HA-PLTP is not. Instead, HA-PLTP copurified with apoE. Based on these findings we suggest a model in which nascent PLTP enters the circulation as a high specific activity form not associated with apoA-I. During or after the transfer of lipolytic surface remnants to HDL, PLTP is transferred to apoA-I-containing HDL particles and thereby becomes part of the low activity complex.

Distribution and kinetics of lipoprotein-bound endotoxin

Lipopolysaccharide (LPS), the major glycolipid component of gram-negative bacterial outer membranes, is a potent endotoxin responsible for pathophysiological symptoms characteristic of infection. The observation that the majority of LPS is found in association with plasma lipoproteins has prompted the suggestion that sequestering of LPS by lipid particles may form an integral part of a humoral detoxification mechanism. Previous studies on the biological properties of isolated lipoproteins used differential ultracentrifugation to separate the major subclasses. To preserve the integrity of the lipoproteins, we have analyzed the LPS distribution, specificity, binding capacity, and kinetics of binding to lipoproteins in human whole blood or plasma by using high-performance gel permeation chromatography and fluorescent LPS of three different chemotypes. The average distribution of O111:B4, J5, or Re595 LPS in whole blood from 10 human volunteers was 60% (+/-8%) high-density lipoprotein (HDL), 25% (+/-7%) low-density lipoprotein, and 12% (+/-5%) very low density lipoprotein. The saturation capacity of lipoproteins for all three LPS chemotypes was in excess of 200 microg/ml. Kinetic analysis however, revealed a strict chemotype dependence. The binding of Re595 or J5 LPS was essentially complete within 10 min, and subsequent redistribution among the lipoprotein subclasses occurred to attain similar distributions as O111:B4 LPS at 40 min. We conclude that under simulated physiological conditions, the binding of LPS to lipoproteins is highly specific, HDL has the highest binding capacity for LPS, the saturation capacity of lipoproteins for endotoxin far exceeds the LPS concentrations measured in clinical situations, and the kinetics of LPS association with lipoproteins display chemotype-dependent differences.

The impact of phospholipid transfer protein (PLTP) on HDL metabolism

High-density lipoproteins (HDL) play a major protective role against the development of coronary artery disease. Phospholipid transfer protein (PLTP) is a main factor regulating the size and composition of HDL in the circulation and plays an important role in controlling plasma HDL levels. This is achieved via both the phospholipid transfer activity of PLTP and its capability to cause HDL conversion. The present review focuses on the impact of PLTP on HDL metabolism. The basic characteristics and structure of the PLTP protein are described. The two main functions of PLTP, PLTP-mediated phospholipid transfer and HDL conversion are reviewed, and the mechanisms and control, as well as the physiological significance of these processes are discussed. The relationship between PLTP and the related cholesteryl ester transfer protein (CETP) is reviewed. Thereafter other functions of PLTP are recapitulated: the ability of PLTP to transfer cholesterol, alpha-tocopherol and lipopolysaccharide (LPS), and the suggested involvement of PLTP in cellular cholesterol traffic. The discussion on PLTP activity and mass in (patho)physiological settings includes new data on the presence of two forms of PLTP in the circulation, one catalytically active and the other inactive. Finally, future directions for PLTP research are outlined.

Acute-phase HDL in phospholipid transfer protein (PLTP)-mediated HDL conversion

In reverse cholesterol transport, plasma phospholipid transfer protein (PLTP) converts high density lipoprotein(3) (HDL(3)) into two new subpopulations, HDL(2)-like particles and prebeta-HDL. During the acute-phase reaction (APR), serum amyloid A (SAA) becomes the predominant apolipoprotein on HDL. Displacement of apo A-I by SAA and subsequent remodeling of HDL during the APR impairs cholesterol efflux from peripheral tissues, and might thereby change substrate properties of HDL for lipid transfer proteins. Therefore, the aim of this work was to study the properties of SAA-containing HDL in PLTP-mediated conversion. Enrichment of HDL by SAA was performed in vitro and in vivo and the SAA content in HDL varied between 32 and 58 mass%. These HDLs were incubated with PLTP, and the conversion products were analyzed for their size, composition, mobility in agarose gels, and apo A-I degradation. Despite decreased apo A-I concentrations, PLTP facilitated the conversion of acute-phase HDL (AP-HDL) more effectively than the conversion of native HDL(3), and large fusion particles with diameters of 10.5, 12.0, and 13.8 nm were generated. The ability of PLTP to release prebeta from AP-HDL was more profound than from native HDL(3). Prebeta-HDL formed contained fragmented apo A-I with a molecular mass of about 23 kDa. The present findings suggest that PLTP-mediated conversion of AP-HDL is not impaired, indicating that the production of prebeta-HDL is functional during the ARP. However, PLTP-mediated in vitro degradation of apo A-I in AP-HDL was more effective than that of native HDL, which may be associated with a faster catabolism of inflammatory HDL.

A hydrophobic cluster at the surface of the human plasma phospholipid transfer protein is critical for activity on high density lipoproteins

The plasma phospholipid transfer protein (PLTP) belongs to the lipid transfer/lipopolysaccharide binding protein (LT/LBP) family, together with the cholesteryl ester transfer protein, the lipopolysaccharide binding protein (LBP) and the bactericidal permeability increasing protein (BPI). In the present study, we used the crystallographic data available for BPI to build a three-dimensional model for PLTP. Multiple sequence alignment suggested that, in PLTP, a cluster of hydrophobic residues substitutes for a cluster of positively charged residues found on the surface of LBP and BPI, which is critical for interaction with lipopolysaccharides. According to the PLTP model, these hydrophobic residues are situated on an exposed hydrophobic patch at the N-terminal tip of the molecule. To assess the role of this hydrophobic cluster for the functional activity of PLTP, single point alanine mutants were engineered. Phospholipid transfer from liposomes to high density lipoprotein (HDL) by the W91A, F92A, and F93A PLTP mutants was drastically reduced, whereas their transfer activity toward very low density lipoprotein and low density lipoprotein did not change. The HDL size conversion activity of the mutants was reduced to the same extent as the PLTP transfer activity toward HDL. Based on these results, we propose that a functional solvent-exposed hydrophobic cluster in the PLTP molecule specifically contributes to the PLTP transfer activity on HDL substrates.

Characterization of apolipoprotein A-I structure using a cysteine-specific fluorescence probe

Two new Cys mutants of proapolipoprotein A-I, D9C and A232C, were created and expressed in Escherichia coli systems. Specific labeling with the thiol-reactive fluorescence probe, 6-acryloyl-2-dimethylaminonaphthalene (acrylodan), was used to study the structural organization and dynamic properties of the extreme regions of human apolipoprotein A-I (apoA-I) in lipid-free and lipid-bound states. Spectroscopic approaches, including circular dichroism and various fluorescence methods, were used to examine the properties of the mutant proteins and of their covalent adducts with the fluorescence probe. The mutations themselves had no effect on the structure and stability of apoA-I in the lipid-free state and in reconstituted HDL (rHDL) complexes. Furthermore, covalent modification with acrylodan did not alter the properties of the apoA-I variants in the lipid-bound state nor in the lipid-free A232C mutant, but it affected the structure and local stability of the lipid-free protein in the D9C mutant. Fluorescence results using the acrylodan probe confirmed a well-organized structure in the N-terminal region of apoA-I. Also, they suggested a three-dimensional structure in the C-terminal region, stabilized by protein-protein contacts. When Trp residues and acrylodan were used as donor-acceptor pairs for fluorescence resonance energy transfer (FRET), average distances could be measured. Both intensity and lifetime changes of the Trp emission indicated a protein folding in solution that brings the C-terminus of the protein near the Trp residues in the N-terminal half of the sequence. Also, the N- and C-terminal domains of apoA-I appeared to be near each other in rHDL having two apoA-I per particle.

Monoclonal antibodies to human apolipoproteins: application to the study of high density lipoprotein subpopulations

We produced, selected and cloned hybridomas that secrete monoclonal antibodies against human apolipoprotein (apo) A-I. All of the antibodies corresponded to the IgG(1) subclass and were named 1C11, 2B4, 2C10, 7C5, 8A4 and 8A5. The antibodies were characterized by their reactivity with whole lipoproteins, apolipoproteins, synthetic peptides and fragments generated by cleavage of the apo A-I. Three of the monoclonal antibodies studied (2B4, 2C10 and 7C5) were similarly inhibited by an amino-terminal peptide (amino acid sequence 1-20) of apo A-I, whereas antibodies 1C11, 8A4 and 8A5 had no reaction. Other results show that monoclonal antibody 1C11 recognizes an epitope located between amino acids 135-148. We evaluated the monoclonal antibody 8A4 against different HDL subpopulations by competitive displacement analysis and it showed a similar reactivity with the HDL particles: LpA-I and LpA-I:A-II. This antibody was used to standardize a sandwich ELISA to quantitate LpA-I in plasma. We conclude that these monoclonal antibodies are relevant for the study of apo A-I epitope expression and for quantitating apo A-I containing lipoparticles.

Measurement of Human Plasma Phospholipid Transfer Protein by Sandwich ELISA

Background: Plasma phospholipid transfer protein (PLTP) plays a central role in the remodeling of HDLs. Reliable and accurate methods for assaying PLTP concentration are required.

Methods: A sandwich ELISA for PLTP has been developed, using two monoclonal antibodies against recombinant human PLTP (rhPLTP) expressed in Chinese hamster ovary cells. The ELISA allows for the quantification of PLTP in the range 0.625–15.0 ng/assay (1.2–30.0 mg/L). Intra- and interassay CVs were &lt;3.0% and &lt;4.2% respectively. The assay was used to quantify plasma PLTP concentrations in 132 Japanese subjects (75 males and 57 females).

Results: PLTP concentrations were 12.0 ± 3.0 mg/L (mean ± SD; range, 4.9–20.5 mg/L). No sex difference was observed. Plasma PLTP concentration was positively correlated with HDL-cholesterol (r = 0.72; P &lt;0.001), apolipoprotein (apo) A-I (r = 0.62; P &lt;0.001) and HDL2-cholesterol (r = 0.72; P &lt;0.001), and was negatively correlated with triacylglycerol (r = −0.45; P &lt;0.001). There was no correlation with plasma apo A-II. These results agree with other evidence that plasma PLTP is associated with large apo A-I-containing lipoproteins. There was no correlation (r = −0.01) between plasma PLTP and plasma phosphatidylcholine transfer activity (range, 3.5–10.5 μmol · mL−1 · h−1), suggesting that PLTP may exist in active and inactive forms.

Conclusion: This new ELISA will be of value for further studies of PLTP in health and disease.

Identification of a sequence of apolipoprotein A-I associated with the activation of Lecithin:Cholesterol acyltransferase

We aimed to distinguish between the effects of mutations in apoA-I on the requirements for the secondary structure and a specific amino acid sequence for lecithin:cholesterol acyltransferase (LCAT) activation. Several mutants were constructed targeting region 140-150: (i) two mutations affecting alpha-helical structure, deletion of amino acids 140-150 and substitution of Ala(143) for proline; (ii) two mutations not affecting alpha-helical structure, substitution of Val(149) for arginine and substitution of amino acids 63-73 for sequence 140-150; and (iii) a mutation in a similar region away from the target area, deletion of amino acids 63-73. All mutations affecting region 140-150 resulted in a 4-42-fold reduction in LCAT activation. Three mutations, apoA-I(Delta140-150), apoA-I(P143A), and apoA-I(140-150 --> 63-73), affected both the apparent V(max) and K(m), whereas the mutation apoA-I(R149V) affected only the V(max). The mutation apoA-I(Delta63-73) caused only a 5-fold increase in the K(m). All mutants, except apoA-I(P143A) and apoA-I(Delta63-73), were active in phospholipid binding assay. All mutants, except apoA-I(P143A), formed normal discoidal complexes with phospholipid. The mutation apoA-I(Delta63-73) caused a significant reduction in the stability of apoA-I.phospholipid complexes in denaturation experiments. Combined, our results strongly suggest that although the correct conformation and orientation of apoA-I in the complex with lipids are crucial for activation of LCAT, when these conditions are fulfilled, activation also strongly depends on the sequence that includes amino acids 140-150.

Structure and function of apolipoprotein A-I and high-density lipoprotein

Structural biology and molecular modeling have provided intriguing insights into the atomic details of the lipid-associated structure of the major protein component of HDL, apo A-I. For the first time, an atomic resolution map is available for future studies of the molecular interactions of HDL in such biological processes as ABC1-regulated HDL assembly, LCAT activation, receptor binding, reverse lipid transport and HDL heterogeneity. Within the context of this paradigm, the current review summarizes the state of HDL research.

High density associated enzymes: their role in vascular biology

Enzymes associated with circulating HDL include lecithin: cholesterol acyl transferase, phospholipid transfer protein, cholesterol ester transfer protein, paraoxonase 1 and platelet activating factor acetylhydrolase. Together with lipoprotein lipase and hepatic lipase these enzymes produce important lipoprotein remodeling and modulate their structure and function and therefore their role in artery wall metabolism.