Formation and Metabolism of Prebeta-Migrating, Lipid-Poor Apolipoprotein A-I

The preferred extracellular acceptor of cell phospholipids and unesterified cholesterol in the process mediated by the ATP-binding cassette A1 (ABCA1) transporter is a monomolecular, prebeta-migrating, lipid-poor or lipid-free form of apolipoprotein (apo) A-I. This monomolecular form of apoA-I is quite distinct from the prebeta-migrating, discoidal high-density lipoprotein (HDL) that contains two or three molecules of apoA-I per particle and which are present as minor components of the HDL fraction in human plasma. The mechanism of the ABCA1-mediated efflux of phospholipid and cholesterol from cells has been studied extensively. In contrast, much less attention has been given to the origin and subsequent metabolism of the acceptor lipid-free/lipid-poor apoA-I. There is a substantial body of evidence from studies conducted in vitro that a monomolecular, lipid-free/lipid-poor form of apoA-I dissociates from HDL during the remodeling of HDLs by plasma factors such as cholesteryl ester transfer protein, hepatic lipase, and phospholipid transfer protein. The rate at which apoA-I dissociates from HDL is influenced by the phospholipid composition of the particles and by the presence of apoA-II. This review describes current knowledge regarding the formation, metabolism, and regulation of monomolecular, lipid-free/lipid-poor apoA-I in plasma.

The structure and refinement of apocrustacyanin C2 to 1.3 A resolution and the search for differences between this protein and the homologous apoproteins A1 and C1

The blue carotenoprotein alpha-crustacyanin of Homarus gammarus lobster carapace is comprised chemically of five 20 kDa subunits. Only two genes for the proteins have been isolated (J. B. C. Findlay, personal communication) and the five apoproteins fall into two sets of homologous proteins based on their chemical properties (CRTC, consisting of apoproteins C(1), C(2) and A(1), and CRTA, consisting of apoproteins A(2) and A(3)). The diffraction quality of apo C(2) has been improved from 2.2 to 1.3 A and its structure solved. The structure is compared with the A(1) and C(1) proteins determined at 1.4 A [Cianci et al. (2001), Acta Cryst. D57, 1219-1229] and 1.15 A, respectively [Gordon et al. (2001), Acta Cryst. D57, 1230-1237] and found to be very similar. Normalized B-factor difference plots per residue of different types were used to try to find chemically modified residues; none were found at these resolutions. It remains possible that the differences between the CRTC proteins result from differences in amidation. By comparison of a crystal grown with glycerol (studied at 1.6 A) and one grown without glycerol (studied at 1.3 A) it was seen that glycerol bound at the astaxanthin site.

Dietary linoleic acid-induced hypercholesterolemia and accumulation of very light HDL in steers

This experiment was designed to study the effects in fattening steers of n-6 PUFA supplementation on the plasma distribution and chemical composition of major lipoproteins (TG-rich lipoproteins: d < 1.006 g/mL; intermediate density lipoproteins + LDL: 1.019 < d < 1.060 g/mL; light HDL: 1.060 < d < 1.091 g/mL; and heavy HDL: 1.091 < d < 1.180 g/mL). For a period of 70 d, animals [454 +/- 20 d; 528 +/- 36 kg (mean +/- SD)] were given a control diet (diet C, n = 6) consisting of hay and concentrate mixture (54 and 46% of diet dry matter, respectively) or the same diet supplemented with sunflower oil (4% of dry matter), given either as crushed seeds (diet S, n = 6) or as free oil continuously infused into the duodenum through a chronic canula to avoid ruminal PUFA hydrogenation (diet O, n = 6). Plasma lipids increased in steers given diet S (x1.4, P < 0.05) and diet O (x2.3, P < 0.05), leading to hyperphospholipemia and hypercholesterolemia. With diet S, hypercholesterolemia was associated with higher levels of light (x1.4, P < 0.05) and heavy HDL (x1.3, NS). With diet O, it was linked to higher levels of light HDL (x1.8, P < 0.005) and to very light HDL accumulation within density limits of 1.019 to 1.060 g/mL, as demonstrated by the apolipoprotein A-I profile. Diet O favored incorporation of 18:2n-6 into polar (x2.2, P < 0.05) and neutral lipids (x1.5 to x8, P < 0.05) at the expense of SFA, MUFA, and n-3 PUFA. Thus, protection of dietary PUFA against ruminal hydrogenation allowed them to accumulate in plasma lipoproteins, but the effects of hypercholesterolemia on animal health linked to very light HDL accumulation remain to be elucidated.

Lysine Residues Direct the Chlorination of Tyrosines in YXXK Motifs of Apolipoprotein A-I When Hypochlorous Acid Oxidizes High Density Lipoprotein

Oxidized lipoproteins may play an important role in the pathogenesis of atherosclerosis. Elevated levels of 3-chlorotyrosine, a specific end product of the reaction between hypochlorous acid (HOCl) and tyrosine residues of proteins, have been detected in atherosclerotic tissue. Thus, HOCl generated by the phagocyte enzyme myeloperoxidase represents one pathway for protein oxidation in humans. One important target of the myeloperoxidase pathway may be high density lipoprotein (HDL), which mobilizes cholesterol from artery wall cells. To determine whether activated phagocytes preferentially chlorinate specific sites in HDL, we used tandem mass spectrometry (MS/MS) to analyze apolipoprotein A-I that had been oxidized by HOCl. The major site of chlorination was a single tyrosine residue located in one of the protein's YXXK motifs (where X represents a nonreactive amino acid). To investigate the mechanism of chlorination, we exposed synthetic peptides to HOCl. The peptides encompassed the amino acid sequences YKXXY, YXXKY, or YXXXY. MS/MS analysis demonstrated that chlorination of tyrosine in the peptides that contained lysine was regioselective and occurred in high yield if the substrate was KXXY or YXXK. NMR and MS analyses revealed that the N(epsilon) amino group of lysine was initially chlorinated, which suggests that chloramine formation is the first step in tyrosine chlorination. Molecular modeling of the YXXK motif in apolipoprotein A-I demonstrated that these tyrosine and lysine residues are adjacent on the same face of an amphipathic alpha-helix. Our observations suggest that HOCl selectively targets tyrosine residues that are suitably juxtaposed to primary amino groups in proteins. This mechanism might enable phagocytes to efficiently damage proteins when they destroy microbial proteins during infection or damage host tissue during inflammation.

Cross-Linking and Lipid Efflux Properties of ApoA-I Mutants Suggest Direct Association between ApoA-I Helices and ABCA1

To explore the functional interactions between apoA-I and ABCA1, we correlated the cross-linking properties of several apoA-I mutants with their ability to promote cholesterol efflux. In a competitive cross-linking assay, amino-terminal deletion and double amino- and carboxy-terminal deletion mutants of apoA-I competed effectively the cross-linking of WT (125)I-apoA-I to ABCA1, while the carboxy-terminal deletion mutant apoA-I[Delta(220-243)] competed poorly. Direct cross-linking of WT apoA-I, amino-terminal, and double deletion mutants of apoA-I to ABCA1 showed similar apparent K(d) values (49-74 nM), whereas the apparent K(d) values of the carboxy-terminal deletion mutants apoA-I[Delta(185-243)] and apoA-I[Delta(220-243)] were increased 3-fold. Analysis of several internal deletions and point mutants of apoA-I showed that apoA-I[Delta(61-78)], apoA-I[Delta(89-99)], apoA-I[Delta(136-143)], apoA-I[Delta(144-165)], apoA-I[D102A/D103A], apoA-I[E125K/E128K/K133E/E139K], apoA-I[L141R], apoA-I[R160V/H162A], and WT apoA-I had similar ABCA1-mediated lipid efflux, and all competed efficiently the cross-linking of WT (125)I-apoA-I to ABCA1. WT apoA-I and ABCA1 could be cross-linked with a 3 A cross-linker. The WT apoA-I, amino, carboxy and double deletion mutants of apoA-I showed differences in the cross-linking to WT ABCA1 and the mutant ABCA1[W590S]. The findings are consistent with a direct association of different combinations of apoA-I helices with a complementary ABCA1 domain. Mutations that alter ABCA1/apoA-I association affect cholesterol efflux and inhibit biogenesis of HDL.

Phosphorylation and stabilization of ATP binding cassette transporter A1 by synthetic amphiphilic helical peptides

To investigate structural requirement of helical apolipoprotein to phosphorylate and stabilize ATP-binding cassette transporter A1 (ABCA1), synthetic peptides (Remaley, A. T., Thomas, F., Stonik, J. A., Demosky, S. J., Bark, S. E., Neufeld, E. B., Bocharov, A. V., Vishnyakova, T. G., Patterson, A. P., Eggerman, T. L., Santamarina-Fojo, S., and Brewer, H. B. (2003) J. Lipid Res. 44, 828-836) were examined for these activities. L37pA, an L amino acid peptide that contains two class-A amphiphilic helices, and D37pA, the same peptide with all D amino acids, both removed cholesterol and phospholipid from differentiated THP-1 cells more than apolipoproteins (apos) A-I, A-II, and E. Both peptides also mediated lipid release from human fibroblasts WI-38 similar to apoA-I. L2D37pA, an L-peptide whose valine and tyrosine were replaced with D amino acids also promoted lipid release from WI-38 but less so with THP-1, whereas L3D37pA, in which alanine, lysine, and asparatic acid were replaced with D amino acids was ineffective in lipid release for both cell lines. ABCA1 protein in THP-1 and WT-38 was stabilized against proteolytic degradation by apoA-I, apoA-II, and apoE and by all the peptides tested except for L3D37pA, and ABCA1 phosphorylation closely correlated with its stabilization. The analysis of the relationship among these parameters indicated that removal of phospholipid triggers signals for phosphorylation and stabilization of ABCA1. We thus concluded that an amphiphilic helical motif is the minimum structural requirement for a protein to stabilize ABCA1 against proteolytic degradation.

Interaction of apolipoprotein A-I with lecithin-cholesterol vesicles in the presence of phospholipase C

Here we study the anti-nucleating mechanism of apolipoprotein A-I (apo A-I) on model biliary vesicles in the presence of phospholipase C (PLC) utilizing dynamic light scattering (DLS), steady-state fluorescence spectroscopy, cryogenic transmission electron microscopy (cryo-TEM), and UV/Vis spectroscopy. PLC induces aggregation of cholesterol-free lecithin vesicles from an initial, average size of 100 nm to a maximal size of 600 nm. The presence of apo A-I likely inhibits vesicle aggregation by shielding the PLC-generated hydrophobic moieties, which results in vesicles of an average size of 200 nm. A similar phenomenon is observed in cholesterol-enriched lecithin vesicles. Whereas PLC alone produces aggregates of 300 nm, no aggregation is observed when apo A-I is present along with PLC. However, the ability of apo A-I to inhibit aggregation is temporary, and after 8 h, a broad particle size distribution with sizes as high as 800 nm is observed. Apo A-I possibly induces the formation of small apo A-I/lecithin/cholesterol complexes of about 5-20 nm similar to the discoidal pre-HDL complexes found in blood when it can no longer effectively shield all the DAG molecules. Concomitant with formation of complexes, DAG molecules coalesce into large oil droplets, which account for the large particles observed by light scattering. Thus, apo A-I acts as an anti-nucleating agent by two mechanisms, anti-aggregation and microstructural transition. The mode of protection is dependent on the cholesterol content and the relative amounts of DAG and apo A-I present. This study supports the possibility of apo A-I solubilizing lipids in bile in a similar fashion as it does in blood and also delineates the mechanism of formation of the complexes.

Tetrahydrocortisol-apolipoprotein A-I complex specifically interacts with eukaryotic DNA and GCC elements of genes

Tetrahydrocortisol stimulates DNA and protein biosynthesis in hepatocytes only when it enters the complex with apolipoprotein A-I. Tetrahydrocortisol-apolipoprotein A-I (THC-apoA-I) complex specifically interacts with eukaryotic DNA isolated from rat liver. In the process of interaction, rupture of hydrogen bonds between the pairs of nitrous bases occurs with the formation of single-stranded DNA structures. In such state DNA forms complexes with DNA-dependent RNA-polymerase. The most probable site of binding the tetrahydrocortisol-apolipoprotein A-I complex with DNA is the sequence of CC(GCC)(n) type entering the structure of many genes, among them the structure of human apolipoprotein A-I gene. Oligonucleotide of this type has been synthesized. Association constant (K(ass)) of it with tetrahydrocortisol-apolipoprotein A-I complex was shown to be 1.66 x 10(6)M(-1). Substitution of tetrahydrocortisol for cortisol in the complex results in a considerable decrease of K(ass). It was assumed that in the GC-pairs of the given sequence tetrahydrocortisol itself participates in the formation of hydrogen bonds with cytosine, favoring their rupture with complementary base-guanine.

The concept of apolipoprotein-defined lipoprotein families and its clinical significance

Classification of plasma lipoproteins on the basis of apolipoprotein (apo) composition recognizes two lipoprotein (Lp) classes, one of which is characterized by apoA-I and the other by apoB as major protein constituents. The former lipoprotein class consists of three major subclasses referred to (according to their apolipoprotein constituents) as Lp-A-I, Lp-A-I:A-II, and Lp-A-II, and the latter one of five subclasses called Lp-B, Lp-B:E, Lp-B:C, Lp-B:C:E, and Lp-A-II:B:C:D:E. As polydisperse systems of particles, the apoA-I-containing lipoproteins overlap in high-density segments and apoB- containing lipoproteins in low-density segments of the density gradient. Each subclass is characterized by a specific chemical composition and metabolic property. Normolipidemia and dyslipoproteinemias are characterized by quantitative rather than qualitative differences in the levels of apoA- and apoB-containing subclasses. Furthermore, apoA-containing subclasses seem to differ with respect to their relative antiatherogenic capacities, and apoB-containing subclasses regarding their relative atherogenic potentials. Whereas Lp-A-I may have a greater antiatherogenic capacity than other apoA-containing subclasses, the cholesterol-enriched Lp-B:C appears to be the most atherogenic subclass among apoB-containing lipoprotein families. The use of pharmacologic and/or dietary interventions to treat dyslipoproteinemias has already shown that these therapeutic modalities may affect selectively individual apolipoprotein-defined lipoproteins, and thus allow the selection of individualized treatments targeted at decreasing harmful and/or increasing beneficial lipoprotein subclasses.

Involvement of Cdc42 signaling in apoA-I-induced cholesterol efflux

Cholesterol efflux, an important mechanism by which high density lipoproteins (HDL) protect against atherosclerosis, is initiated by docking of apolipoprotein A-I (apoA-I), a major HDL protein, to specific binding sites followed by activation of ATP-binding cassette transporter A1 (ABCA1) and translocation of cholesterol from intracellular compartments to the exofacial monolayer of the plasma membrane where it is accessible to HDL. In this report, we investigated potential signal transduction pathways that may link apoA-I binding to cholesterol translocation to the plasma membrane and cholesterol efflux. By using pull-down assays we found that apoA-I substantially increased the amount of activated Cdc42, Rac1, and Rho in human fibroblasts. Moreover, apoA-I induced actin polymerization, which is known to be controlled by Rho family G proteins. Inhibition of Cdc42 and Rac1 with Clostridium difficile toxin B inhibited apoA-I-induced cholesterol efflux, whereas inhibition of Rho with Clostridium botulinum C3-exoenzyme exerted opposite effects. Adenoviral expression of a Cdc42(T17N) dominant negative mutant substantially reduced apoA-I-induced cholesterol efflux, whereas dominant negative Rac1(T17N) had no effect. We further found that two downstream effectors of Cdc42/Rac1 signaling, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), are activated by apoA-I. Pharmacological inhibition of JNK but not p38 MAPK decreased apoA-I-induced cholesterol efflux, whereas anisomycin and hydrogen peroxide, two direct JNK activators, could partially substitute for apoA-I in its ability to induce cholesterol efflux. These results for the first time demonstrate activation of Rho family G proteins and stress kinases by apoA-I and implicate the involvement of Cdc42 and JNK in the apoA-I-induced cholesterol efflux.

Interfacial properties of an amphipathic alpha-helix consensus peptide of exchangeable apolipoproteins at air/water and oil/water interfaces

Amphipathic alpha-helices are the main structure and the major lipid binding motif of exchangeable apolipoproteins. To understand how these apolipoproteins behave at an hydrophobic lipoprotein interface, the interfacial properties of a consensus sequence peptide (CSP) derived from three exchangeable apolipoproteins (A-I, A-IV, and E) were studied using an oil drop tensiometer at air/water (A/W) and dodecane/water (DD/W) interfaces. CSP ((PLAEELRARLRAQLEELRERLG)2-NH2) contains two 22-amino acid tandem repeat sequences that form amphipathic alpha-helices. CSP, when added into the aqueous phase, lowered the interfacial tension (gamma) of A/W and DD/W in a concentration-dependent fashion. The gammaA/W was lowered approximately 24 mn/m, and gammaDD/W approximately 31 mn/m, indicating a greater affinity of CSP for DD/W. Using the Gibbs equation for surface, the surface area per CSP molecule was estimated at approximately 702 A2 ( approximately 16 A2/amino acid) on A/W and approximately 622 A2 on DD/W ( approximately 14 A2/amino acid) suggesting that adsorbed CSP lies flat with alpha-helices in the plane of both interfaces. At equilibrium gamma, CSP desorbed from the interface when compressed and re-adsorbed when expanded. The adsorption rate was concentration-dependent, but the desorption rate was not. Less CSP desorbed from DD/W than A/W indicating that CSP has higher affinity for DD/W. Dynamic analysis of elasticity shows that the faster the oscillation period (4, 8 s) and the lower the oscillation amplitude the more elastic the surfaces. CSP can be compressed 6-12% while remaining on the surface, but large increases in pressure eject it from the surface. We suggest that surface pressure-mediated desorption and readsorption of amphipathic alpha-helices provide lipoprotein stability during remodeling reactions in plasma.

The spatial organization of apolipoprotein A-I on the edge of discoidal high density lipoprotein particles: a mass specrometry study

The three-dimensional structure of human apoA-I on nascent, discoidal HDL particles has been debated extensively over the past 25 years. Recent evidence has demonstrated that the alpha-helical domains of apoA-I are arranged in a belt-like orientation with the long axis of the helices perpendicular to the phospholipid acyl chains on the disc edge. However, experimental information on the spatial relationships between apoA-I molecules on the disc is lacking. To address this issue, we have taken advantage of recent advances in mass spectrometry technology combined with cleavable cross-linking chemistry to derive a set of distance constraints suitable for testing apoA-I structural models. We generated highly homogeneous, reconstituted HDL particles containing two molecules of apoA-I. These were treated with a thiol-cleavable cross-linking agent, which covalently joined Lys residues in close proximity within or between molecules of apoA-I in the disc. The cross-linked discs were then exhaustively trypsinized to generate a discrete population of peptides. The resulting peptides were analyzed by liquid chromatography/mass spectrometry before and after cleavage of the cross-links, and resulting peaks were identified based on the theoretical tryptic cleavage of apoA-I. We identified at least 8 intramolecular and 7 intermolecular cross-links in the particle. The distance constraints are used to analyze three current models of apoA-I structure. The results strongly support the presence of the salt-bridge interactions that were predicted to occur in the "double belt" model of apoA-I, but a helical hairpin model containing the same salt-bridge docking interface is also consistent with the data.

[Advances in apolipoprotein A- I and it's anti-atherosclerosis properties]

Human apolipoprotein A- I, the major protein component of high density lipoproteins and the main activator of the enzyme lecithin: cholesterol acyltransferase, defines the structure and stability and functions of HDL. It is clearly demonstrated that high concentration of the apoA- I not only inhibits the initiation and progression of atherosclerosis, but also makes the preexisting atherosclerotic lesions regress. This review gives an overview of the apoA- I structure, production, relation between apoA- I and HDL, and several mechanisms of the apoA-I anti-atherosclerosis. These mechanisms include directing excess celluar cholesterol from the peripheral tissues to the liver in reverse cholesterol transport, inhibiting oxidative modification of LDL, and modulating inflammatory responses to favour vasoprotection.

ABCA1 redistributes membrane cholesterol independent of apolipoprotein interactions

ATP binding cassette transporter A1 (ABCA1) mediates the transport of phospholipids and cholesterol from cells to lipid-poor HDL apolipoproteins. Cholesterol loading of cells induces ABCA1, implicating cholesterol as its major physiologic substrate. It is believed, however, that ABCA1 is primarily a phospholipid transporter and that cholesterol efflux occurs by diffusion to ABCA1-generated phospholipid-rich apolipoproteins. Here we show that overexpression of ABCA1 in baby hamster kidney cells in the absence of apolipoproteins redistributed membrane cholesterol to cell-surface domains accessible to treatment with the enzyme cholesterol oxidase. The cholesterol removed by apolipoprotein A-I (apoA-I), but not by HDL phospholipids, was derived exclusively from these domains. ABCA1 overexpression also increased cholesterol esterification, which was prevented by addition of apoA-I, suggesting that some of the cell-surface cholesterol not removed by apolipoproteins is transported to the intracellular esterifying enzyme acyl-CoA:cholesterol acyltransferase. ABCA1 expression was essential for cholesterol efflux even when apolipoproteins had already acquired phospholipids during prior exposure to ABCA1-expressing cells. These studies show that ABCA1 redistributes cholesterol to cell-surface domains, where it becomes accessible for removal by apolipoproteins, consistent with a direct role of ABCA1 in cholesterol transport.

Domain structure and lipid interaction in human apolipoproteins A-I and E, a general model

Detailed structural information on human exchangeable apolipoproteins (apo) is required to understand their functions in lipid transport. Using a series of deletion mutants that progressively lacked different regions along the molecule, we probed the structural organization of lipid-free human apoA-I and the role of different domains in lipid binding, making comparisons to apoE, which is a member of the same gene family and known to have two structural domains. Measurements of alpha-helix content by CD in conjunction with tryptophan and 8-anilino-1-naphthalenesulfonic acid fluorescence data demonstrated that deletion of the amino-terminal or central regions disrupts the tertiary organization, whereas deletion of the carboxyl terminus has no effect on stability and induces a more cooperative structure. These data are consistent with the lipid-free apoA-I molecule being organized into two structural domains similar to apoE; the amino-terminal and central parts form a helix bundle, whereas the carboxyl-terminal alpha-helices form a separate, less organized structure. The binding of the apoA-I variants to lipid emulsions is modulated by reorganization of the helix bundle structure, because the rate of release of heat on binding is inversely correlated with the stability of the helix bundle. Based on these observations, we propose that there is a two-step mechanism for lipid binding of apoA-I: apoA-I initially binds to a lipid surface through amphipathic alpha-helices in the carboxyl-terminal domain, followed by opening of the helix bundle in the amino-terminal domain. Because apoE behaves similarly, this mechanism is probably a general feature for lipid interaction of other exchangeable apolipoproteins, such as apoA-IV.

Apolipoprotein A-II inhibits high density lipoprotein remodeling and lipid-poor apolipoprotein A-I formation

The high density lipoproteins (HDL) in human plasma are classified on the basis of apolipoprotein composition into those containing apolipoprotein (apo) A-I but not apoA-II, (A-I)HDL, and those containing both apoA-I and apoA-II, (A-I/A-II)HDL. Cholesteryl ester transfer protein (CETP) transfers core lipids between HDL and other lipoproteins. It also remodels (A-I)HDL into large and small particles in a process that generates lipid-poor, pre-beta-migrating apoA-I. Lipid-poor apoA-I is the initial acceptor of cellular cholesterol and phospholipids in reverse cholesterol transport. The aim of this study is to determine whether lipid-poor apoA-I is also formed when (A-I/A-II)rHDL are remodeled by CETP. Spherical reconstituted HDL that were identical in size had comparable lipid/apolipoprotein ratios and either contained apoA-I only, (A-I)rHDL, or (A-I/A-II)rHDL were incubated for 0-24 h with CETP and Intralipid(R). At 6 h, the apoA-I content of the (A-I)rHDL had decreased by 25% and there was a concomitant formation of lipid-poor apoA-I. By 24 h, all of the (A-I)rHDL were remodeled into large and small particles. CETP remodeled approximately 32% (A-I/A-II)rHDL into small but not large particles. Lipid-poor apoA-I did not dissociate from the (A-I/A-II)rHDL. The reasons for these differences were investigated. The binding of monoclonal antibodies to three epitopes in the C-terminal domain of apoA-I was decreased in (A-I/A-II)rHDL compared with (A-I)rHDL. When the (A-I/A-II)rHDL were incubated with Gdn-HCl at pH 8.0, the apoA-I unfolded by 15% compared with 100% for the apoA-I in (A-I)rHDL. When these incubations were repeated at pH 4.0 and 2.0, the apoA-I in the (A-I)rHDL and the (A-I/A-II)rHDL unfolded completely. These results are consistent with salt bridges between apoA-II and the C-terminal domain of apoA-I, enhancing the stability of apoA-I in (A-I/A-II)rHDL and possibly contributing to the reduced remodeling and absence of lipid poor apoA-I in the (A-I/A-II)rHDL incubations.

High-density lipoprotein increases the abundance of eNOS protein in human vascular endothelial cells by increasing its half-life

OBJECTIVES

Given the importance of endothelial nitric oxide synthase (eNOS) in regulating endothelium-dependent vasorelaxation, we investigated the effects of high-density lipoprotein in (HDL) on eNOS protein abundance in cultured human vascular endothelial cells.

BACKGROUND

Endothelial dysfunction, characterized by decreased nitric oxide production, is one of the early features in the development of atherosclerosis. We have recently shown in vivo that niacin therapy increases plasma HDL concentration and improves endothelium-dependent vasorelaxation in patients with coronary artery disease.

METHODS

Human vascular endothelial cells were cultured in the presence or absence of HDL or apolipoprotein (apo)A-I. The eNOS protein abundance was assessed by immunoblotting, and protein half-life was assessed by pulse-chase techniques. The eNOS messenger ribonucleic acid (mRNA) abundance was measured using real-time quantitative polymerase chain reaction.

RESULTS

High density lipoprotein, or apoA-I alone, increased eNOS protein abundance by 3.5 +/- 0.7 and 2.7 +/- 0.5-fold, respectively (p < 0.05 for both). However, neither HDL nor apoA-I increased eNOS mRNA abundance. It was shown that HDL increased eNOS protein half-life up to 3.3 +/- 0.2-fold (p = 0.001). Both HDL and apoA-I activated mitogen-activated protein-kinase and phosphatidylinositol 3-kinase (PI3K) Akt-pathways in human arterial endothelial cells, and inhibition of either of these pathways by specific pharmacologic inhibitors abolished the effect of HDL on eNOS.

CONCLUSIONS

We demonstrate that HDL activates both extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt, resulting in enhanced eNOS protein stability and subsequent accumulation of eNOS protein. This posttranslational regulation represents a previously unrecognized mechanism for regulating eNOS.

Structural studies of N- and C-terminally truncated human apolipoprotein A-I

Apolipoprotein A-I (apoA-I) plays an important structural and functional role in lipid transport and metabolism. This work is focused on the central region of apoA-I (residues 60-183) that is predicted to contain exclusively amphipathic alpha-helices. Six N- and/or C-terminally truncated mutants, delta(1-41), delta(1-59), delta(198-243), delta(209-243), delta(1-41,185-243), and delta(1-59,185-243), were analyzed in their lipid-free state in solution at pH 4.7-7.8 by far- and near-UV CD spectroscopy. At pH 7.8, all mutants show well-defined secondary structures consisting of 40-52% alpha-helix. Comparison of the alpha-helix content in the wild type and mutants suggests that deletion of either the N- or C-terminal region induces helical unfolding elsewhere in the structure, indicating that the terminal regions are important for the integrity of the solution conformation of apoA-I. Near-UV CD spectra indicate significant tertiary and/or quaternary structural changes resulting from deletion of the N-terminal 41 residues. Reduction in pH from 7.8 to 4.7 leads to an increase in the mutant helical content by 5-20% and to a large increase in thermal unfolding cooperativity. Van't Hoff analysis of the mutants at pH 4.7 indicates melting temperatures T(m) ranging from 51 to 59 degrees C and effective enthalpies deltaH(v)(T(m)) = 35 +/- 5 kcal/mol, similar to the values for plasma apoA-I at pH 7.8 (T(m) = 57 degrees C, deltaH(v) = 32 kcal/mol). Our results provide the first report of the pH effects on the secondary, tertiary, and/or quaternary structure of apoA-I variants and indicate the importance of the electrostatic interactions for the solution conformation of apoA-I.

The C-terminal domain of apolipoprotein A-I contains a lipid-sensitive conformational trigger

Exchangeable apolipoproteins can convert between lipid-free and lipid-associated states. The C-terminal domain of human apolipoprotein A-I (apoA-I) plays a role in both lipid binding and self-association. Site-directed spin-label electron paramagnetic resonance spectroscopy was used to examine the structure of the apoA-I C terminus in lipid-free and lipid-associated states. Nitroxide spin-labels positioned at defined locations throughout the C terminus were used to define discrete secondary structural elements. Magnetic interactions between probes localized at positions 163, 217 and 226 in singly and doubly labeled apoA-I gave inter- and intramolecular distance information, providing a basis for mapping apoA-I tertiary and quaternary structure. Spectra of apoA-I in reconstituted HDL revealed a lipid-induced transition of defined random coils and beta-strands into alpha-helices. This conformational switch is analogous to triggered events in viral fusion proteins and may serve as a means to overcome the energy barriers of lipid sequestration, a critical step in cholesterol efflux and HDL assembly.

High-density lipoprotein concentrations relate to the clinical course of HIV viral load in patients undergoing antiretroviral therapy

OBJECTIVE

To determine whether levels of HDL are associated with viral load response in HIV-treated patients, and to seek an explanation based on amino acid sequence similarity between the key apolipoprotein A1 and HIV proteins concerned in viral replication.

DESIGN

The major HDL lipoprotein is apolipoprotein A1, which is able to inhibit HIV-induced syncytium formation. This retrospective clinical study assessed the relationship between the response to antiretroviral treatment (time of undetectable viral load/duration of viral suppression below the limit of detection) and HDL-cholesterol levels on commencing antiretroviral treatment.

PATIENTS AND METHODS

HIV-treated patients with undetectable HIV viral loads were followed every 3 months for 36 months. We measured total cholesterol, HDL-cholesterol, triglycerides, previous responses to antiretroviral treatment, opportunistic infections, sex and age. These variables were assessed in relation to the time of undetectable viral load until viral rebound. Amino acid sequence alignment was performed with HIV proteins and apolipoprotein A1 to detect shared similarity.

RESULTS

The Cox proportional hazards model showed a significant association between HDL-cholesterol and the time of undetectable viral load. The other variables studied were not associated. There was 30% sequence similarity in an area of 50 amino acids shared between apolipoprotein A1 and p17 Gag-HIV protein.

CONCLUSION

High levels of HDL-cholesterol are associated with a better viral response in treated HIV patients. This association could be related to the sequence similarity and structure homology between apolipoprotein A1 and p17 Gag-HIV protein, which raises the intriguing clinical possibility that inducing an increase in HDL could assist HIV therapy.

L-4F, an apolipoprotein A-1 mimetic, dramatically improves vasodilation in hypercholesterolemia and sickle cell disease

BACKGROUND

Hypercholesterolemia and sickle cell disease (SCD) impair endothelium-dependent vasodilation by dissimilar mechanisms. Hypercholesterolemia impairs vasodilation by a low-density lipoprotein (LDL)-dependent mechanism. SCD has been characterized as a chronic state of inflammation in which xanthine oxidase (XO) from ischemic tissues increases vascular superoxide anion (O2*-) generation. Recent reports indicate that apolipoprotein (apo) A-1 mimetics inhibit atherosclerosis in LDL receptor-null (Ldlr-/-) mice fed Western diets. Here we hypothesize that L-4F, an apoA-1 mimetic, preserves vasodilation in hypercholesterolemia and SCD by decreasing mechanisms that increase O2*- generation.

METHODS AND RESULTS

Arterioles were isolated from hypercholesterolemic Ldlr-/- mice and from SCD mice that were treated with either saline or L-4F (1 mg/kg per day). Vasodilation in response to acetylcholine was determined by videomicroscopy. Effects of L-4F on LDL-induced increases in endothelium-dependent O2*- generation were determined on arterial segments via the hydroethidine assay and on stimulated endothelial cell cultures via superoxide dismutase-inhibitable ferricytochrome c reduction. Effects of L-4F on XO bound to pulmonary arterioles and content in livers of SCD mice were determined by immunofluorescence. Hypercholesterolemia impaired vasodilation in Ldlr-/- mice, which L-4F dramatically improved. L-4F inhibited LDL-induced increases in O2*- in arterial segments and in stimulated cultures. SCD impaired vasodilation, increased XO bound to pulmonary endothelium, and decreased liver XO content. L-4F dramatically improved vasodilation, decreased XO bound to pulmonary endothelium, and increased liver XO content compared with levels in untreated SCD mice.

CONCLUSIONS

These data show that L-4F protects endothelium-dependent vasodilation in hypercholesterolemia and SCD. Our findings suggest that L-4F restores vascular endothelial function in diverse models of disease and may be applicable to treating a variety of vascular diseases.

Structure-function relationships of apolipoprotein A-I: a flexible protein with dynamic lipid associations

PURPOSE OF REVIEW

Apolipoprotein A-I is the major structural protein of HDL. Its physicochemical properties maintain a delicate balance between maintenance of stable lipoproteins and the ability to associate with and dissociate from the lipid transported. Here we review the progress made in the last 2-3 years on the structure-function relationships of apolipoprotein A-I, including elements related to the ATP binding cassette transporter A1.

RECENT FINDINGS

Current evidence now supports the so-called 'belt' or 'hairpin' models for apolipoprotein A-I conformation when bound to discoidal lipoproteins. In-vivo expression of apolipoprotein A-I mutant proteins has shown that both the N- and C-terminal domains are important for lipid association as well as for the esterification reaction, particularly binding of cholesteryl esters and formation of mature alpha-migrating lipoproteins. This property is apparently quite distinct from the activation of the enzyme lecithin cholesterol acyl transferase, which requires interaction with the central helix 6. The interaction of apolipoprotein A-I with the ATP binding cassette transporter A1 has been shown to require the C-terminal domain, which is proposed to mediate the opening of the helix bundle formed by lipid-free or lipid-poor apolipoprotein A-I and allow its association with hydrophobic binding sites.

SUMMARY

Significant progress has been made in the understanding of the molecular mechanisms controlling the folding of apolipoprotein A-I and its interaction with lipids and various other protein factors involved in HDL metabolism.

Novel mass spectrometric immunoassays for the rapid structural characterization of plasma apolipoproteins

Novel mass spectrometric immunoassays (MSIAs) for the isolation and structural characterization of plasma apolipoprotein A-I (apoA-I), apoA-II, and apoE have been developed. The assays combine selective isolation of apolipoprotein species via affinity capture with mass-specific detection using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. In application, plasma (from 50 microl of whole blood drawn from individuals, using finger lancet) was addressed with affinity pipette tips derivatized with antibodies toward the specific apolipoprotein. The time required for each assay was approximately 15 min, less if assays on multiple individuals were performed in parallel. In a brief study of five individuals, several recently reported apoA-II variants were identified and observed consistently in all individuals. Additionally, the apoE phenotype of E3/E3 was observed in three of the individuals, and E2/E3 and E3/E4 observed in the remaining two individuals, the latter of whom suffers from Alzheimer's disease. Overall, the MSIA approach offers a rapid, sensitive, and highly accurate means of profiling apolipoproteins from small volumes of plasma.

The central helices of ApoA-I can promote ATP-binding cassette transporter A1 (ABCA1)-mediated lipid efflux. Amino acid residues 220-231 of the wild-type ApoA-I are required for lipid efflux in vitro and high density lipoprotein formation in vivo

We have mapped the domains of lipid-free apoA-I that promote cAMP-dependent and cAMP-independent cholesterol and phospholipid efflux. The cAMP-dependent lipid efflux in J774 mouse macrophages was decreased by approximately 80-92% by apoA-I[delta(185-243)], only by 15% by apoA-I[delta(1-41)] or apoA-I[delta(1-59)], and was restored to 75-80% of the wild-type apoA-I control value by double deletion mutants apoA-I[delta(1-41)delta(185-243)] and apoA-I[delta(1-59)delta(185-243)]. Similar results were obtained in HEK293 cells transfected with an ATP-binding cassette transporter A1 (ABCA1) expression plasmid. The double deletion mutant of apoA-I had reduced thermal and chemical stability compared with wild-type apoA-I. Sequential carboxyl-terminal deletions showed that cAMP-dependent cholesterol efflux was diminished in all the mutants tested, except the apoA-I[delta(232-243)] which had normal cholesterol efflux. In cAMP-untreated or in mock-transfected cells, cholesterol efflux was not affected by the amino-terminal deletions, but decreased by 30-40% and 50-65% by the carboxyl-terminal and double deletions, respectively. After adenovirus-mediated gene transfer in apoA-I-deficient mice, wild-type apoA-I and apoA-I[delta(1-41)] formed spherical high density lipoprotein (HDL) particles, whereas apoA-I[delta(1-41)delta(185-243)] formed discoidal HDL. The findings suggest that although the central helices of apoA-I alone can promote ABCA1-mediated lipid efflux, residues 220-231 are necessary to allow functional interactions between the full-length apoA-I and ABCA1 that are required for lipid efflux and HDL biogenesis.