Limited proteolysis of high density lipoprotein abolishes its interaction with cell-surface binding sites that promote cholesterol efflux

ExoProK: A Practical Method for the Isolation of Small Extracellular Vesicles from Pleural Effusions

Extracellular vesicles (EVs) are cell-secreted, lipid membrane-enclosed nanoparticles without functional nucleus. EV is a general term that includes various subtypes of particles named microvesicles, microparticles, ectosomes or exosomes. EVs transfer RNA, DNA and protein cargo between proximal and distant cells and tissues, thus constituting an organism-wide signal transduction network. Pathological tissues secrete EVs that differ in their cargo composition compared to their healthy counterparts. The detection of biomarkers in EVs from biological fluids may aid the diagnosis of disease and/or monitor its progression in a minimally invasive manner. Among biological fluids, pleural effusions (PEs) are integrated to clinical practice, as they accompany a wide variety of lung disorders. Due to the proximity with the pleura and the lungs, PEs are expected to be especially enriched in EVs that originate from diseased tissues. However, PEs are among the least studied biofluids regarding EV-specialized isolation methods and related biomarkers. Herein, we describe a practical EV isolation method from PEs for the screening of EV RNA biomarkers in clinical routine. It is based on a Proteinase K treatment step to digest contaminants prior to standard polyethylene-glycol precipitation. The efficiency of the method was confirmed by transmission electron microscopy, nanoparticle tracking analysis and Western blot. The reliability and sensitivity of the method towards the detection of EV-enriched RNA biomarkers from multiple PEs was also demonstrated.

Dysfunctional HDL in diabetes mellitus and its role in the pathogenesis of cardiovascular disease

Coronary artery disease, the leading cause of death in the developed and developing countries, is prevalent in diabetes mellitus with 68% cardiovascular disease (CVD)-related mortality. Epidemiological studies suggested inverse correlation between HDL and CVD occurrence. Therefore, low HDL concentration observed in diabetic patients compared to non-diabetic individuals was thought to be one of the primary causes of increased risks of CVD. Efforts to raise HDL level via CETP inhibitors, Torcetrapib and Dalcetrapib, turned out to be disappointing in outcome studies despite substantial increases in HDL-C, suggesting that factors beyond HDL concentration may be responsible for the increased risks of CVD. Therefore, recent studies have focused more on HDL function than on HDL levels. The metabolic environment in diabetes mellitus condition such as hyperglycemia-induced advanced glycation end products, oxidative stress, and inflammation promote HDL dysfunction leading to greater risks of CVD. This review discusses dysfunctional HDL as one of the mechanisms of increased CVD risks in diabetes mellitus through adversely affecting components that support HDL function in cholesterol efflux and LDL oxidation. The dampening of reverse cholesterol transport, a key process that removes cholesterol from lipid-laden macrophages in the arterial wall, leads to increased risks of CVD in diabetic patients. Therapeutic approaches to keep diabetes under control may benefit patients from developing CVD.

The "beta-clasp" model of apolipoprotein A-I--a lipid-free solution structure determined by electron paramagnetic resonance spectroscopy

Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoproteins (HDL) and plays a central role in cholesterol metabolism. The lipid-free/lipid-poor form of apoA-I is the preferred substrate for the ATP-binding cassette transporter A1 (ABCA1). The interaction of apoA-I with ABCA1 leads to the formation of cholesterol laden high density lipoprotein (HDL) particles, a key step in reverse cholesterol transport and the maintenance of cholesterol homeostasis. Knowledge of the structure of lipid-free apoA-I is essential to understanding its critical interaction with ABCA1 and the molecular mechanisms underlying HDL biogenesis. We therefore examined the structure of lipid-free apoA-I by electron paramagnetic resonance spectroscopy (EPR). Through site directed spin label EPR, we mapped the secondary structure of apoA-I and identified sites of spin coupling as residues 26, 44, 64, 167, 217 and 226. We capitalize on the fact that lipid-free apoA-I self-associates in an anti-parallel manner in solution. We employed these sites of spin coupling to define the central plane in the dimeric apoA-I complex. Applying both the constraints of dipolar coupling with the EPR-derived pattern of solvent accessibility, we assembled the secondary structure into a tertiary context, providing a solution structure for lipid-free apoA-I. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Small discoidal pre-beta1 HDL particles are efficient acceptors of cell cholesterol via ABCA1 and ABCG1

The aim of this study was to correlate the lipid content and size of discoidal reconstituted HDL particles with their ability to promote cellular cholesterol efflux. Homogeneous discoidal rHDL particles containing apoA-I and POPC, with diameters of 7.8, 9.6, 10.8, 12.5, and 17.0 nm, were prepared by the cholate dialysis technique. Cholesterol efflux to rHDL was evaluated in pathway-specific cell models for ABCA1-, ABCG1-, and SR-BI-mediated efflux. ABCA1-mediated efflux was efficiently promoted by the 7.8 nm rHDL containing 82 POPC molecules per particle. This rHDL also promoted ABCG1, but not SR-BI, cholesterol efflux. All large and lipid-rich rHDLs, with a diameter of >or=9.6 nm and a phospholipid content of >/=202 molecules per particle, promoted both SR-BI- and ABCG1-mediated efflux. Our results indicated that the ABCA1-mediated cell cholesterol efflux can be efficiently driven not only by monomolecular lipid free/poor apoA-I but also by a small discoidal phospholipid-containing particle resembling plasma pre-beta1 HDL. This same particle also promotes ABCG1- but not SR-BI-mediated efflux. These results help to clarify the role of plasma pre-beta1 HDL in reverse cholesterol transport.

Surface plasmon resonance analysis of the mechanism of binding of apoA-I to high density lipoprotein particles

The partitioning of apolipoprotein A-I (apoA-I) molecules in plasma between HDL-bound and -unbound states is an integral part of HDL metabolism. We used the surface plasmon resonance (SPR) technique to monitor in real time the reversible binding of apoA-I to HDL. Biotinylated human HDL(2) and HDL(3) were immobilized on a streptavidin-coated SPR sensor chip, and apoA-I solutions at different concentrations were flowed across the surface. The wild-type (WT) human and mouse apoA-I/HDL interaction involves a two-step process; apoA-I initially binds to HDL with fast association and dissociation rates, followed by a step exhibiting slower kinetics. The isolated N-terminal helix bundle domains of human and mouse apoA-I also exhibit a two-step binding process, consistent with the second slower step involving opening of the helix bundle domain. The results of fluorescence experiments with pyrene-labeled apoA-I are consistent with the N-terminal helix bundle domain interacting with proteins resident on the HDL particle surface. Dissociation constants (K(d)) measured for WT human apoA-I interactions with HDL(2) and HDL(3) are about 10 microM, indicating that the binding is low affinity. This K(d) value does not apply to all of the apoA-I molecules on the HDL particle but only to a relatively small, labile pool.

An amphipathic helical region of the N-terminal barrel of phospholipid transfer protein is critical for ABCA1-dependent cholesterol efflux

Phospholipid lipid transfer protein (PLTP) mimics high-density lipoprotein apolipoproteins in removing cholesterol and phospholipids from cells through the ATP-binding cassette transporter A1 (ABCA1). Because amphipathic alpha-helices are the structural determinants for ABCA1 interactions, we examined the ability of synthetic peptides corresponding to helices in PLTP to remove cellular cholesterol by the ABCA1 pathway. Of the seven helices tested, only one containing PLTP residues 144-163 (p144), located at the tip of the N-terminal barrel, promoted ABCA1-dependent cholesterol efflux and stabilized ABCA1 protein. Mutating methionine 159 (Met-159) in this helix in PLTP to aspartate (M159D) or glutamate (M159E) nearly abolished the ability of PLTP to remove cellular cholesterol and dramatically reduced PLTP binding to phospholipid vesicles and its phospholipid transfer activity. These mutations impaired PLTP binding to ABCA1-generated lipid domains and PLTP-mediated stabilization of ABCA1 but increased PLTP binding to ABCA1. PLTP interactions with ABCA1 also mimicked apolipoproteins in activating Janus kinase 2; however, the M159D/E mutants were also able to activate this kinase. Structural analyses showed that the M159D/E mutations had only minor effects on PLTP conformation. These findings indicate that PLTP helix 144-163 is critical for removing lipid domains formed by ABCA1, stabilizing ABCA1 protein, interacting with phospholipids, and promoting phospholipid transfer. Direct interactions with ABCA1 and activation of signaling pathways likely involve other structural determinants of PLTP.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become an important new therapeutic target for treating cardiovascular disease.

Janus kinase 2 modulates the lipid-removing but not protein-stabilizing interactions of amphipathic helices with ABCA1

ABCA1 mediates the transport of cellular cholesterol and phospholipids to HDL apolipoproteins. Apolipoprotein A-I (apoA-I) interactions with ABCA1-expressing cells elicit several responses, including removing cellular lipids, stabilizing ABCA1 protein, and activating Janus kinase 2 (JAK2). Here, we used synthetic apolipoprotein-mimetic peptides to characterize the relationship between these responses. Peptides containing one amphipathic helix of L- or D-amino acids (2F, D-2F, or 4F) and a peptide containing two helices (37pA) all promoted ABCA1-dependent cholesterol efflux, competed for apoA-I binding to ABCA1-expressing cells, blocked covalent cross-linking of apoA-I to ABCA1, and inhibited ABCA1 degradation. 37pA was cross-linked to ABCA1, confirming the direct binding of amphipathic helices to ABCA1. 2F, 4F, 37pA, and D-37pA all stimulated JAK2 autophosphorylation. Inhibition of JAK2 greatly reduced peptide-mediated cholesterol efflux, peptide binding to ABCA1-expressing cells, and peptide cross-linking to ABCA1, indicating that these processes require an active JAK2. In contrast, apoA-I and peptides stabilized ABCA1 protein even in the absence of an active JAK2, implying that this process is independent of JAK2 and lipid efflux-promoting binding of amphipathic helices to ABCA1. These findings show that amphipathic helices coordinate the activity of ABCA1 by several distinct mechanisms that are likely to involve different cell surface binding sites.

Apolipoprotein E isoforms and apolipoprotein AI protect from amyloid precursor protein carboxy terminal fragment-associated cytotoxicity

Inheritance of the apolipoprotein (APO) E gene epsilon4 or epsilon2 allele alters the risk of developing Alzheimer disease (AD), while increased alpha-tocopherol (AT) intake appears to lower the risk of AD. As APOE is a major apolipoprotein in the CNS and AT in vivo is transported in lipoproteins, we tested the hypothesis that CNS lipoproteins, as modeled by relevant concentrations of high density lipoprotein (HDL), and AT would interact to suppress neurotoxicity in a cell culture model of amyloid beta (Abeta)- related toxicity. These cells conditionally express C99-derived peptides, proposed to be a key step in AD pathogenesis; this expression is closely associated with subsequent cell death. We found that physiologic concentrations of lipoproteins present in the CNS protected from C99-associated toxicity and provided evidence for two mechanisms of protection. The first was AT-independent, APOE isoform-dependent, and most potent for the APOE2 isoform. The second was a synergistic protection afforded by a combination of APOAI, or less so APOE, and AT. These data provide a novel explanation for the apparent AD-protective effect of inheriting an epsilon2 APOE allele, and suggest that optimizing AT enrichment of CNS lipoproteins or devising APOAI mimetics may augment AT efficacy in treating AD.

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

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

Depletion of pre-beta-high density lipoprotein by human chymase impairs ATP-binding cassette transporter A1- but not scavenger receptor class B type I-mediated lipid efflux to high density lipoprotein

The ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of cellular unesterified cholesterol and phospholipid to lipid-poor apolipoprotein A-I. Chymase, a protease secreted by mast cells, selectively cleaves pre-beta-migrating particles from high density lipoprotein (HDL)(3) and reduces the efflux of cholesterol from macrophages. To evaluate whether this effect is the result of reduction of ABCA1-dependent or -independent pathways of cholesterol efflux, in this study we examined the efflux of cholesterol to preparations of chymase-treated HDL(3) in two types of cell: 1) in J774 murine macrophages endogenously expressing low levels of scavenger receptor class B, type I (SR-BI), and high levels of ABCA1 upon treatment with cAMP; and 2) in Fu5AH rat hepatoma cells endogenously expressing high levels of the SR-BI and low levels of ABCA1. Treatment of HDL(3) with the human chymase resulted in rapid depletion of pre-beta-HDL and a concomitant decrease in the efflux of cholesterol and phospholipid (2-fold and 3-fold, respectively) from the ABCA1-expressing J774 cells. In contrast, efflux of free cholesterol from Fu5AH to chymase-treated and to untreated HDL(3) was similar. Incubation of HDL(3) with phospholipid transfer protein led to an increase in pre-beta-HDL contents as well as in ABCA1-mediated cholesterol efflux. A decreased cholesterol efflux to untreated HDL(3) but not to chymase-treated HDL(3) was observed in ABCA1-expressing J774 with probucol, an inhibitor of cholesterol efflux to lipid-poor apoA-I. Similar results were obtained using brefeldin and gliburide, two inhibitors of ABCA1-mediated efflux. These results indicate that chymase treatment of HDL(3) specifically impairs the ABCA1-dependent pathway without influencing either aqueous or SR-BI-facilitated diffusion and that this effect is caused by depletion of lipid-poor pre-beta-migrating particles in HDL(3). Our results are compatible with the view that HDL(3) promotes ABCA1-mediated lipid efflux entirely through its lipid-poor fraction with pre-beta mobility.

Phospholipid transfer protein interacts with and stabilizes ATP-binding cassette transporter A1 and enhances cholesterol efflux from cells

Phospholipid lipid transfer protein (PLTP) is ubiquitously expressed in animal tissues and plays multiple roles in lipoprotein metabolism, but the function of peripheral PLTP is still poorly understood. Here we show that one of its possible functions is to transport cholesterol and phospholipids from cells to lipoprotein particles by a process involving PLTP interactions with cellular ATP-binding cassette transporter A1 (ABCA1). When ABCA1 was induced in murine macrophages or ABCA1-transfected baby hamster kidney cells, PLTP gained the ability to promote cholesterol and phospholipid efflux from cells. Although PLTP alone had lipid efflux activity, its maximum activity was observed in the presence of high density lipoprotein particles. Pulsechase studies showed that the interaction of PLTP with ABCA1-expressing cells played a role in promoting lipid efflux. Overexpression of ABCA1 dramatically increased binding of both PLTP and apoA-I to common sites on the cell surface. Both PLTP and apoA-I were covalently cross-linked to ABCA1, each protein blocked cross-linking of the other, and both PLTP and apoA-I stabilized ABCA1 protein. These results are consistent with PLTP and apoA-I binding to ABCA1 at the same or closely related sites. Thus, PLTP mimics apolipoproteins in removing cellular lipids by the ABCA1 pathway, except that PLTP acts more as an intermediary in the transfer of cellular lipids to lipoprotein particles.

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.

HDL apolipoproteins and ABCA1: partners in the removal of excess cellular cholesterol

It is widely believed that HDL protects against atherosclerosis by removing excess cholesterol from arterial cells. Lipid-poor HDL apolipoproteins promote efflux of cholesterol, phospholipids, and other lipophilic molecules from cells by an active process mediated by a cell-membrane transporter called the ATP binding cassette transporter A-1 (ABCA1). ABCA1 either directly or indirectly translocates phospholipids and cholesterol to the cell surface, where they appear to form lipid domains that interact with amphipathic alpha-helixes in apolipoproteins. This interaction solubilizes these lipids and generates nascent HDL particles that dissociate from the cell. Binding of apolipoproteins to ABCA1 may also enhance the activity of this lipid-transport pathway. Thus, the apolipoprotein/ABCA1 pathway efficiently clears cells of excess cholesterol that would otherwise accumulate as intracellular lipid droplets. ABCA1 expression is highly induced by cholesterol loading of cells and is also modulated by sterol-independent mechanisms at both the transcriptional and posttranslational level. Studies of human disease and animal models have shown that both an increased availability of apolipoproteins and an enhanced macrophage ABCA1 activity are atheroprotective. These findings implicate the apolipoprotein/ABCA1 pathway as an important therapeutic target for treating cardiovascular disease.

Promoting export of macrophage cholesterol: the physiological role of a major acute-phase protein, serum amyloid A 2.1

We show that murine macrophages that have ingested cell membranes as a source of cholesterol exhibit a marked increase in acyl-CoA:cholesterol acyl transferase (ACAT) activity. Exposure of these macrophages to acute-phase high-density lipoprotein (HDL) results in a marked reduction of ACAT and enhancement of cholesteryl ester hydrolase (CEH) activities, phenomena not seen with native HDL. These complementary but opposite effects of acute-phase HDL on the two enzyme systems that regulate the balance between esterified (storage) cholesterol and unesterified (transportable) cholesterol are shown to reside with serum amyloid A (SAA) 2.1, an acute-phase apolipoprotein of HDL whose plasma concentration increases 500- to 1,000-fold within 24 h of acute tissue injury. Mild trypsin treatment of acute-phase HDL almost completely abolishes the apolipoprotein-mediated effects on the cholesteryl ester cycle in cholesterol-laden macrophages. The physiological effect of SAA2.1 on macrophage cholesterol is to shift it into a transportable state enhancing its rate of export, which we confirm in tissue culture and in vivo. The export process is shown to be coupled to the ATP binding cassette transport system. Our findings integrate previous isolated observations about SAA into the sphere of cholesterol transport, establish a function for a major acute-phase protein, and offer a novel approach to mobilizing macrophage cholesterol at sites of atherogenesis.

ATP binding cassette transporter A1--key roles in cellular lipid transport and atherosclerosis

ATP-binding cassette transporter A1 (ABCA1) was recently recognized as the mutant molecule responsible for Tangier disease with low HDL levels, accumulation of cholesteryl esters in tissues, and increased risk of cardiovascular disease. Extensive studies for the past 2 years have recognized the critical role of ABCA1 in cholesterol and phospholipid trafficking. Since the removal of cholesterol from tissues is a key step in the prevention of atherosclerosis, significant attention has been focused on this molecule. Natural ABCA1 mutations in Tangier disease (TD) patients and WHAM chickens together with induced mutation in ABCA1 knock-out mice unequivocally established the important role of ABCA1 in maintaining circulating HDL levels and promoting cholesterol efflux from the arterial wall. Mice lacking ABCA1 showed similar phenotypes observed in Tangier disease patients with low levels of HDL. Further understanding of the roles of ABCA1 in lipid transport and atherosclerosis became clear from studies with ABCA1 transgenic mice. These mice showed enhanced cholesterol efflux from macrophages and reduced atherosclerotic lesion formation. The promoter of the ABCA1 gene has been mapped to a large extent, with the exception of cAMP response element. The present review summarizes recent developments on the role of ABCA1 in cholesterol efflux and prevention of atherosclerosis. Given the antiatherogenic properties of ABCA1, this molecule can serve as an appropriate target for developing drugs to treat individuals with low levels of HDL.

Oxidation of methionine residues affects the structure and stability of apolipoprotein A-I in reconstituted high density lipoprotein particles

To determine the effect of oxidative damage to lipid-bound apolipoprotein A-I (apo A-I) on its structure and stability that might be related to previously observed functional disorders of oxidized apo A-I in high density lipoproteins (HDL), we prepared homogeneous reconstituted HDL (rHDL) particles containing unoxidized apo A-I and its commonly occurring oxidized form (Met-112, 148 bis-sulfoxide). The size of the obtained discoidal rHDL particles ranged from 9.0 to 10.0 nm and did not depend upon the content of the oxidized protein. Using circular dichroism methods, no change in the secondary structure of lipid-bound oxidized apo A-I was found. Isothermal and thermal denaturation experiments showed a significant destabilization of the oxidized protein to denaturation by guanidine hydrochloride or heat. This effect was observed with and without co-reconstituted apolipoprotein A-II. Limited tryptic digestion indicated that the central region of oxidatively damaged apo A-I becomes exposed to proteolysis in the rHDL particles. Implications of these data for apolipoprotein function are discussed.

ATP-binding cassette transporter A1 mediates cellular secretion of alpha-tocopherol

Alpha-tocopherol (alpha-TOH) is associated with plasma lipoproteins and accumulates in cell membranes throughout the body, suggesting that lipoproteins play a role in transporting alpha-TOH between tissues. Here we show that secretion of alpha-TOH from cultured cells is mediated in part by ABCA1, an ATP-binding cassette protein that transports cellular cholesterol and phospholipids to lipid-poor high density lipoprotein (HDL) apolipoproteins such as apoA-I. Treatment of human fibroblasts and murine RAW264 macrophages with cholesterol and/or 8-bromo-cyclic AMP, which induces ABCA1 expression, enhanced apoA-I-mediated alpha-TOH efflux. ApoA-I lacked the ability to remove alpha-TOH from Tangier disease fibroblasts that have a nonfunctional ABCA1. BHK cells that lack an active ABCA1 pathway markedly increased secretion of alpha-TOH to apoA-I when forced to express ABCA1. ABCA1 also mediated a fraction of the alpha-TOH efflux promoted by lipid-containing HDL particles, indicating that HDL promotes alpha-TOH efflux by both ABCA1-dependent and -independent processes. Exposing apoA-I to ABCA1-expressing cells did not enhance its ability to remove alpha-TOH from cells lacking ABCA1, consistent with this transporter participating directly in the translocation of alpha-TOH to apolipoproteins. These studies provide evidence that ABCA1 mediates secretion of cellular alpha-TOH into the HDL metabolic pathway, a process that may facilitate vitamin transport between tissues and influence lipid oxidation.

Proteolytic degradation and impaired secretion of an apolipoprotein A-I mutant associated with dominantly inherited hypoalphalipoproteinemia

We have devised a combined in vivo, ex vivo, and in vitro approach to elucidate the mechanism(s) responsible for the hypoalphalipoproteinemia in heterozygous carriers of a naturally occurring apolipoprotein A-I (apoA-I) variant (Leu(159) to Arg) known as apoA-I Finland (apoA-I(FIN)). Adenovirus-mediated expression of apoA-I(FIN) decreased apoA-I and high density lipoprotein cholesterol concentrations in both wild-type C57BL/6J mice and in apoA-I-deficient mice expressing native human apoA-I (hapoA-I). Interestingly, apoA-I(FIN) was degraded in the plasma, and the extent of proteolysis correlated with the most significant reductions in murine apoA-I concentrations. ApoA-I(FIN) had impaired activation of lecithin:cholesterol acyltransferase in vitro compared with hapoA-I, but in a mixed lipoprotein preparation consisting of both hapoA-I and apoA-I(FIN) there was only a moderate reduction in the activation of this enzyme. Importantly, secretion of apoA-I was also decreased from primary apoA-I-deficient hepatocytes when hapoA-I was co-expressed with apoA-I(FIN) following infection with recombinant adenoviruses, a condition that mimics secretion in heterozygotes. Thus, this is the first demonstration of an apoA-I point mutation that decreases LCAT activation, impairs hepatocyte secretion of apoA-I, and makes apoA-I susceptible to proteolysis leading to dominantly inherited hypoalphalipoproteinemia.

Release of cellular cholesterol: molecular mechanism for cholesterol homeostasis in cells and in the body

Most mammalian somatic cells are unable to catabolize cholesterol and therefore need to export it in order to maintain sterol homeostasis. This mechanism may also function to reduce excessively accumulated cholesterol, which would thereby contribute to prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) has been believed to play a main role in this reaction based on epidemiological evidence and in vitro experimental data. At least two independent mechanisms are identified for this reaction. One is non-specific diffusion-mediated cholesterol 'efflux' from cell surface. Cholesterol molecules desorbed from cells can be trapped by various extracellular acceptors including various lipoproteins and albumin, and extracellular cholesterol esterification mainly on HDL may provide a driving force for the net removal of cell cholesterol by maintaining a cholesterol gradient between lipoprotein surface and cell membrane. The other is apolipoprotein-mediated process to generate new HDL by removing cellular phospholipid and cholesterol. The reaction is initiated by the interaction of lipid-free or lipid-poor helical apolipoproteins with cellular surface resulting in assembly of HDL particles with cellular phospholipid and incorporation of cellular cholesterol into the HDL being formed. Thus, HDL has dual functions as an active cholesterol acceptor in the diffusion-mediated pathway and as an apolipoprotein carrier for the HDL assembly reaction. The impairment of the apolipoprotein-mediated reaction was found in Tangier disease and other familial HDL deficiencies to strongly suggest that this is a main mechanism to produce plasma HDL. The causative mutations for this defect was identified in ATP binding cassette transporter protein A1, as a significant step for further understanding of the reaction and cholesterol homeostasis.

The correlation of ATP-binding cassette 1 mRNA levels with cholesterol efflux from various cell lines

Studies show that lipid-free apoA-I stimulates release of cholesterol and phospholipid from fibroblasts and macrophages. ATP-binding cassette 1 (ABC1) is implicated in this release and has been identified as the genetic defect in Tangier disease, evidence that ABC1 is critical to the biogenesis of high density lipoprotein. We quantified levels of ABC1 mRNA, protein, and cholesterol efflux from J774 mouse macrophages +/- exposure to a cAMP analog. Up-regulating ABC1 mRNA correlated to increased cholesterol efflux in a dose- and time-dependent manner. mRNA levels rose after 15 min of exposure while protein levels rose after 1 h, with increased efflux 2-4 h post-treatment. In contrast to cells from wild-type mice, peritoneal macrophages from the Abc1 -/- mouse showed a lower level of basal efflux and no increase with cAMP treatment. The stimulation of efflux exhibits specificity for apoA-I, high density lipoprotein, and other apolipoproteins as cholesterol acceptors, but not for small unilamellar vesicles, bile acid micelles, or cyclodextrin. We have studied a number of cell types and found that while other cell lines express ABC1 constitutively, only J774 and elicited mouse macrophages show a substantial increase of mRNA and efflux with cAMP treatment. ApoA-I-stimulated efflux was detected from the majority of cell lines examined, independent of treatment.

Apolipoprotein binding to protruding membrane domains during removal of excess cellular cholesterol

High-density lipoproteins (HDL) are believed to protect against cardiovascular disease by removing excess cholesterol from cells. Lipid-free HDL apolipoproteins remove cellular cholesterol and phospholipids by an active, Golgi-dependent process that is still poorly understood. Here we characterized the morphology of apolipoprotein binding sites on cultured cells by immunogold electron microscopy. After 6 h incubations with lipid-free apoA-I or apoE, immunogold-labeled apolipoproteins were distributed sparsely along the planar surface of human fibroblasts and THP-1 macrophages. Overloading these cells with cholesterol led to a several-fold increase in the concentration of immunogold-labeled apoA-I and apoE on the cell surface, and over 80% of these gold particles were associated with novel electron-opaque structures protruding from the plasma membrane. Protrusions binding apoE were larger (100-200 nm) than those binding apoA-I (10-60 nm), and similar apoA-I-binding structures appeared when cells were incubated with either purified apoA-I or HDL particles. These structures were formed and enlarged by a time-dependent process inhibited by the Golgi disruptor brefledin A, the energy poison NaF, and low temperature. Moreover, formation of these structures was nearly absent in fibroblasts from a subject with Tangier disease, cells that lack a functioning apolipoprotein-mediated lipid removal pathway. Thus, formation of novel apolipoprotein binding structures protruding from the cell surface is an intermediate step in the cellular pathway by which apolipoproteins remove excess cholesterol.

Apolipoprotein E exhibits isoform-specific promotion of lipid efflux from astrocytes and neurons in culture

Many studies have shown that apolipoprotein E (apoE) plays important roles in maintaining intracellular lipid homeostasis in nonneuronal cells. However, little is known about the extracellular transport of lipids in the CNS. In this study, we determined whether and to what degree lipid efflux from astrocytes and neurons depended on apoE. Our results showed that exogenously added apoE promoted the efflux of cholesterol and phosphatidylcholine from both astrocytes and neurons in culture, resulting in the generation of high-density lipoprotein-like particles. The order of potency of the apoE isoforms as lipid acceptors was apoE2 > apoE3 = apoE4 in astrocytes and apoE2 > apoE3 > apoE4 in neurons. Treatment with brefeldin A, monensin, and a protein kinase C inhibitor, H7, abolished the ability of apoE to promote cholesterol efflux from cultured astrocytes, without altering apoE-mediated phosphatidylcholine efflux. In contrast, the efflux of both cholesterol and phosphatidylcholine promoted by apoE was abolished following treatment with heparinase or lactoferrin, which block the interaction of apoE with heparan sulfate proteoglycans (HSPGs) or low-density lipoprotein receptor-related protein (LRP), respectively. This study suggests that apoE promotes lipid efflux from astrocytes and neurons in an isoform-specific manner and that cell surface HSPGs and/or HSPG-LRP pathway may mediate this apoE-promoted lipid efflux.

Matrix metalloproteinases-3, -7, and -12, but not -9, reduce high density lipoprotein-induced cholesterol efflux from human macrophage foam cells by truncation of the carboxyl terminus of apolipoprotein A-I. Parallel losses of pre-beta particles and the high affinity component of efflux

Matrix metalloproteinases (MMPs) have been suggested to function in remodeling of the arterial wall, but no information is available on their possible role in early atherogenesis, when cholesterol accumulates in the cells of the arterial intima, forming foam cells. Here, we incubated the major component responsible for efflux of cholesterol from foam cells, high density lipoprotein 3 (HDL(3)), with MMP-1, -3, -7, -9, or -12 at 37 degrees C before adding it to cholesterol-loaded human monocyte-derived macrophages. After incubation with MMP-3, -7, or -12, the ability of HDL(3) to induce the high affinity component of cholesterol efflux from the macrophage foam cells was strongly reduced, whereas preincubation with MMP-1 reduced cholesterol efflux only slightly and preincubation with MMP-9 had no effect. These differential effects of the various MMPs were reflected in their differential abilities to degrade the small pre-beta migrating particles present in the HDL(3) fraction. NH(2)-terminal sequence and mass spectrometric analyses of the apolipoprotein (apo) A-I fragments generated by MMPs revealed that those MMPs that strongly reduced cholesterol efflux (MMPs-3, -7, and -12) cleaved the COOH-terminal region of apoA-I and produced a major fragment of about 22 kDa, whereas MMPs-1 and -9, which had little and no effect on cholesterol efflux, degraded apoA-I only slightly and not at all, respectively. These results show, for the first time, that some members of the MMP family can degrade the apoA-I of HDL(3), so blocking cholesterol efflux from macrophage foam cells. This expansion of the substrate repertoire of MMPs to include apoA suggests that these proteinases are directly involved in the accumulation of cholesterol in atherosclerotic lesions.

Targeting HDL-mediated cellular cholesterol efflux for the treatment and prevention of atherosclerosis

The hallmark of the atherosclerotic lesion is the overaccumulation of cholesterol in arterial wall cells. As no pathway exists for the degradation of cholesterol in peripheral cells, a mechanism is necessary to prevent its accumulation to toxic levels in these cells and to allow its delivery to the liver for excretion in bile. Promoting this reverse cholesterol transport pathway is believed to be the main cardioprotective action of high density lipoprotein (HDL). The rate-limiting step in this pathway is likely the initial removal of cholesterol from peripheral cells by HDL. The pathway HDL utilizes for inducing efflux of excess cellular cholesterol represents an important and as-yet untapped mechanism to employ for the treatment and prevention of atherosclerotic vascular disease. This review summarizes the potential cardioprotective actions of HDL, the mechanisms of HDL-mediated cellular cholesterol efflux, and evidence that the specific pathway of cholesterol removal by HDL may be enhanced and used as a novel target in the therapy of atherosclerosis.

Phospholipid transfer protein enhances removal of cellular cholesterol and phospholipids by high-density lipoprotein apolipoproteins

High-density lipoprotein (HDL) apolipoproteins remove excess cholesterol from cells by an active transport pathway that may protect against atherosclerosis. Here we show that treatment of cholesterol-loaded human skin fibroblasts with phospholipid transfer protein (PLTP) increased HDL binding to cells and enhanced cholesterol and phospholipid efflux by this pathway. PLTP did not stimulate lipid efflux in the presence of albumin, purified apolipoprotein A-I, and phospholipid vesicles, suggesting specificity for HDL particles. PLTP restored the lipid efflux activity of mildly trypsinized HDL, presumably by regenerating active apolipoproteins. PLTP-stimulated lipid efflux was absent in Tangier disease fibroblasts, induced by cholesterol loading, and inhibited by brefeldin A treatment, indicating selectivity for the apolipoprotein-mediated lipid removal pathway. The lipid efflux-stimulating effect of PLTP was not attributable to generation of prebeta HDL particles in solution but instead required cellular interactions. These interactions increased cholesterol efflux to minor HDL particles with electrophoretic mobility between alpha and prebeta. These findings suggest that PLTP promotes cell-surface binding and remodeling of HDL so as to improve its ability to remove cholesterol and phospholipids by the apolipoprotein-mediated pathway, a process that may play an important role in enhancing flux of excess cholesterol from tissues and retarding atherogenesis.

Depletion of pre beta 1LpA1 and LpA4 particles by mast cell chymase reduces cholesterol efflux from macrophage foam cells induced by plasma

Exposure of the LpA1-containing particles present in HDL3 and plasma to a minimal degree of proteolysis by the neutral protease chymase from exocytosed rat mast cell granules (granule remnants) leads to a reduction in the high-affinity component of cholesterol efflux from macrophage foam cells. In this study, we demonstrate for the first time, a role for mast cell chymase in the depletion of the lipid-poor minor components of HDL that are specifically involved in reverse cholesterol transport as initial acceptors of cellular cholesterol. Thus, addition of proteolytically active granule remnants or human skin chymase to cholesterol-loaded macrophages of mouse or human origin incubated with human apoA1, ie, a system in which prebeta1LpA1 is generated, resulted in a sharp reduction in the high-affinity cholesterol efflux promoted by apoA1. As determined by nondenaturing 2-dimensional polyacrylamide gradient gel electrophoresis, the granule remnants effectively depleted the prebeta1LpA1, but not the alphaLpA1, in HDL3 and in plasma during incubation at 37 degrees C for <1 hour. Incubation of plasma with granule remnants for 1 hour also led to near disappearance of the LpA4-1 and LpA4-2 particles, but did not affect the distribution of the apoA2-containing lipoproteins present in the plasma. We conclude that the reduced ability of granule remnant-treated HDL3 and granule remnant-treated plasma to induce cholesterol efflux from macrophage foam cells is caused by selective depletion by mast cell chymase of quantitatively minor A1- and A4-containing subpopulations of HDL. Because these particles, ie, prebeta1LpA1 and LpA4, are efficient acceptors of cholesterol from cell surfaces, their depletion by mast cells may block the initiation of reverse cholesterol transport in vivo and thereby favor foam cell formation in the arterial intima, the site of atherogenesis.