Cholesterol efflux mediated by apolipoproteins is an active cellular process distinct from efflux mediated by passive diffusion

Effect of a PCSK9 inhibitor and a statin on cholesterol efflux capacity: A limitation of current cholesterol-lowering treatments?

BACKGROUND

Cellular cholesterol efflux is a key step in reverse cholesterol transport that may impact on atherosclerotic cardiovascular risk. The process may be reliant on the availability of apolipoprotein (apo) B-100-containing lipoproteins to accept cholesterol from high-density lipoprotein. Evolocumab and atorvastatin are known to lower plasma apoB-100-containing lipoproteins that could impact on cholesterol efflux capacity (CEC).

METHODS

We conducted a 2-by-2 factorial trial of the effects of subcutaneous evolocumab (420 mg every 2 weeks) and atorvastatin (80 mg daily) for 8 weeks on CEC in 81 healthy, normolipidaemic men. The capacity of whole plasma and apoB-depleted plasma, including ATP-binding cassette transporter A1 (ABCA1)-mediated and passive diffusion, to efflux cholesterol, was measured.

RESULTS

Evolocumab and atorvastatin independently decreased whole plasma CEC (main effect p < .01 for both). However, there were no significant effects of evolocumab and atorvastatin on apoB-depleted plasma, ABCA1-mediated and passive diffusion-mediated CEC (p > .05 in all). In the three intervention groups combined, the reduction in whole plasma CEC was significantly correlated with the corresponding reduction in plasma apoB-100 concentration (r = .339, p < .01). In the evolocumab monotherapy group, the reduction in whole plasma CEC was also significantly correlated with the corresponding reduction in plasma lipoprotein(a) concentration (r = .487, p < .05).

CONCLUSIONS

In normolipidaemic men, evolocumab and atorvastatin decrease the capacity of whole plasma to efflux cellular cholesterol. These effects may be chiefly owing to a fall in the availability of apoB-100-containing lipoproteins. Reduction in circulating lipoprotein(a) may also contribute to the decrease in whole plasma cholesterol efflux with evolocumab monotherapy.

Sterolight as imaging tool to study sterol uptake, trafficking and efflux in living cells

Information about cholesterol subcellular localization and transport pathways inside cells is essential for understanding and treatment of cholesterol-related diseases. However, there is a lack of reliable tools to monitor it. This work follows the fate of Sterolight, a BODIPY-labelled sterol, within the cell and demonstrates it as a suitable probe for visualization of sterol/lipid trafficking. Sterolight enters cells through an energy-independent process and knockdown experiments suggest caveolin-1 as its potential cellular carrier. Intracellular transport of Sterolight is a rapid process, and transfer from ER and mitochondria to lysosomes and later to lipid droplets requires the participation of active microtubules, as it can be inhibited by the microtubule disruptor nocodazole. Excess of the probe is actively exported from cells, in addition to being stored in lipid droplets, to re-establish the sterol balance. Efflux occurs through a mechanism requiring energy and may be selectively poisoned with verapamil or blocked in cells with mutated cholesterol transporter NPC1. Sterolight is efficiently transferred within and between different cell populations, making it suitable for monitoring numerous aspects of sterol biology, including the live tracking and visualization of intracellular and intercellular transport.

Cellular cholesterol prediction of mammalian ATP-binding cassette (ABC) proteins based on fuzzy c-means with support vector machine algorithms

Over the years protein interaction and prediction of membrane protein have been a pivotal research area for all researchers. For both prokaryotes and eukaryotes Adenosine Triphosphate-(ATP) binding cassette (ABC) genes plays a significant role. In our analysis, we concentrate on human part of ABC genes. In case of living organisms transport of precise molecules across lipid membranes has been treated as vital part and for that reason a bigger transporter is required to carry out the molecules. Here ABC transporter families are evolved to transport the specific molecules such as sugars, amino acid, peptides, proteins, ions etc. within the plasma membrane. As we know another important component of human being is cholesterol, which is a major component in cell membrane and its main functions are to maintain integrity and mechanical stability. Each and every time, membrane cholesterolsareinteracted with membrane protein in both N-C terminuses and target valid sequence(s) which has relevance in human diseases. In this manuscript we have applied Fuzzy C-Means (FCM) with Support Vector Machine (SVM) algorithm for prediction of cellular cholesterol with ABC genes. Our experiments have been performed well using ABCdata set.

Intracellular and Plasma Membrane Events in Cholesterol Transport and Homeostasis

Cholesterol transport between intracellular compartments proceeds by both energy- and non-energy-dependent processes. Energy-dependent vesicular traffic partly contributes to cholesterol flux between endoplasmic reticulum, plasma membrane, and endocytic vesicles. Membrane contact sites and lipid transfer proteins are involved in nonvesicular lipid traffic. Only “active" cholesterol molecules outside of cholesterol-rich regions and partially exposed in water phase are able to fast transfer. The dissociation of partially exposed cholesterol molecules in water determines the rate of passive aqueous diffusion of cholesterol out of plasma membrane. ATP hydrolysis with concomitant conformational transition is required to cholesterol efflux by ABCA1 and ABCG1 transporters. Besides, scavenger receptor SR-B1 is involved also in cholesterol efflux by facilitated diffusion via hydrophobic tunnel within the molecule. Direct interaction of ABCA1 with apolipoprotein A-I (apoA-I) or apoA-I binding to high capacity binding sites in plasma membrane is important in cholesterol escape to free apoA-I. ABCG1-mediated efflux to fully lipidated apoA-I within high density lipoprotein particle proceeds more likely through the increase of “active” cholesterol level. Putative cholesterol-binding linear motifs within the structure of all three proteins ABCA1, ABCG1, and SR-B1 are suggested to contribute to the binding and transfer of cholesterol molecules from cytoplasmic to outer leaflets of lipid bilayer. Together, plasma membrane events and intracellular cholesterol metabolism and traffic determine the capacity of the cell for cholesterol efflux.

Direct Measurement of Free and Esterified Cholesterol Mass in Differentiated Human Podocytes: A TLC and Enzymatic Assay-Based Method

Esterified cholesterol content is often lower than free cholesterol content in biological systems and thus the determination of the esterified cholesterol content of cells is often challenging. Traditional methods use enzymatic assays in which an indirect measurement of the esterified cholesterol content is obtained by subtracting the measurements of the free from the total cholesterol content. However, this approach fails in the case where the total cholesterol content of cells is unchanged while the ratio of free to esterified cholesterol changes such that total and free cholesterol content are very similar and thus the difference may fall within the background noise of the enzymatic assay. To overcome this challenge, we here describe a method that utilizes a TLC-based technique to isolate esterified cholesterol. Isolated esterified cholesterol can then be measured using traditional enzymatic methods. Therefore, this method provides a practical and more sensitive assay to measure esterified cholesterol content in cellular extracts.

Local TNF causes NFATc1-dependent cholesterol-mediated podocyte injury

High levels of circulating TNF and its receptors, TNFR1 and TNFR2, predict the progression of diabetic kidney disease (DKD), but their contribution to organ damage in DKD remains largely unknown. Here, we investigated the function of local and systemic TNF in podocyte injury. We cultured human podocytes with sera collected from DKD patients, who displayed elevated TNF levels, and focal segmental glomerulosclerosis (FSGS) patients, whose TNF levels resembled those of healthy patients. Exogenous TNF administration or local TNF expression was equally sufficient to cause free cholesterol-dependent apoptosis in podocytes by acting through a dual mechanism that required a reduction in ATP-binding cassette transporter A1-mediated (ABCA1-mediated) cholesterol efflux and reduced cholesterol esterification by sterol-O-acyltransferase 1 (SOAT1). TNF-induced albuminuria was aggravated in mice with podocyte-specific ABCA1 deficiency and was partially prevented by cholesterol depletion with cyclodextrin. TNF-stimulated free cholesterol-dependent apoptosis in podocytes was mediated by nuclear factor of activated T cells 1 (NFATc1). ABCA1 overexpression or cholesterol depletion was sufficient to reduce albuminuria in mice with podocyte-specific NFATc1 activation. Our data implicate an NFATc1/ABCA1-dependent mechanism in which local TNF is sufficient to cause free cholesterol-dependent podocyte injury irrespective of TNF, TNFR1, or TNFR2 serum levels.

Cholesterol Transporters ABCA1 and ABCG1 Gene Expression in Peripheral Blood Mononuclear Cells in Patients with Metabolic Syndrome

ABCA1 and ABCG1 genes encode the cholesterol transporter proteins that play a key role in cholesterol and phospholipids homeostasis. This study was aimed at evaluating and comparing ABCA1 and ABCG1 genes expression in metabolic syndrome patients and healthy individuals. This case-control study was performed on 36 patients with metabolic syndrome and the same number of healthy individuals in Hamadan (west of Iran) during 2013-2014. Total RNA was extracted from mononuclear cells and purified using RNeasy Mini Kit column. The expression of ABCA1 and ABCG1 genes was performed by qRT-PCR. Lipid profile and fasting blood glucose were measured using colorimetric procedures. ABCG1 expression in metabolic syndrome patients was significantly lower (about 75%) compared to that of control group, while for ABCA1 expression, there was no significant difference between the two studied groups. Comparison of other parameters such as HDL-C, FBS, BMI, waist circumference, and systolic and diastolic blood pressure between metabolic syndrome patients and healthy individuals showed significant differences (P < 0.05). Decrease in ABCG1 expression in metabolic syndrome patients compared to healthy individuals suggests that hyperglycemia, related metabolites, and hyperlipidemia over the transporter capacity resulted in decreased expression of ABCG1. Absence of a significant change in ABCA1 gene expression between two groups can indicate a different regulation mechanism for ABCA1 expression.

Cyclodextrin protects podocytes in diabetic kidney disease

Diabetic kidney disease (DKD) remains the most common cause of end-stage kidney disease despite multifactorial intervention. We demonstrated that increased cholesterol in association with downregulation of ATP-binding cassette transporter ABCA1 occurs in normal human podocytes exposed to the sera of patients with type 1 diabetes and albuminuria (DKD(+)) when compared with diabetic patients with normoalbuminuria (DKD(-)) and similar duration of diabetes and lipid profile. Glomerular downregulation of ABCA1 was confirmed in biopsies from patients with early DKD (n = 70) when compared with normal living donors (n = 32). Induction of cholesterol efflux with cyclodextrin (CD) but not inhibition of cholesterol synthesis with simvastatin prevented podocyte injury observed in vitro after exposure to patient sera. Subcutaneous administration of CD to diabetic BTBR (black and tan, brachiuric) ob/ob mice was safe and reduced albuminuria, mesangial expansion, kidney weight, and cortical cholesterol content. This was followed by an improvement of fasting insulin, blood glucose, body weight, and glucose tolerance in vivo and improved glucose-stimulated insulin release in human islets in vitro. Our data suggest that impaired reverse cholesterol transport characterizes clinical and experimental DKD and negatively influences podocyte function. Treatment with CD is safe and effective in preserving podocyte function in vitro and in vivo and may improve the metabolic control of diabetes.

Probing nanoscale self-assembly of nonfluorescent small molecules inside live mammalian cells

Like cellular proteins that form fibrillar nanostructures, small hydrogelator molecules self-assemble in water to generate molecular nanofibers. In contrast to the well-defined (dys)functions of endogenous protein filaments, the fate of intracellular assembly of small molecules remains largely unknown. Here we demonstrate the imaging of enzyme-triggered self-assembly of nonfluorescent small molecules by doping the molecular assemblies with a fluorescent hydrogelator. The cell fractionation experiments, fluorescent imaging, and electron microscopy indicate that the hydrogelators self-assemble and localize to the endoplasmic reticulum (ER) and are likely processed via the cellular secretory pathway (i.e., ER-Golgi-lysosomes/secretion). This work, as the first example of the use of correlative light and electron microscopy for probing the self-assembly of nonfluorescent small molecules inside live mammalian cells, not only establishes a general strategy to provide the spatiotemporal profile of the assemblies of small molecules inside cells but may lead to a new paradigm for regulating cellular functions based on the interactions between the assemblies of small molecules (e.g., molecular nanofibers) and subcellular organelles.

Tailoring of biomimetic high-density lipoprotein nanostructures changes cholesterol binding and efflux

Gold nanoparticles (Au NPs) were employed as templates to synthesize spherical, high-density lipoprotein (HDL) biomimics (HDL Au NPs) of different sizes and surface chemistries. The effect of size and surface chemistry on the cholesterol binding properties and the ability of the HDL Au NPs to efflux cholesterol from macrophage cells were measured. Results demonstrate that Au NPs may be utilized as templates to generate nanostructures with different physical characteristics that mimic natural HDL. Furthermore, the properties of the HDL Au NPs may be tailored to modulate the ability to bind cholesterol in solution and efflux cholesterol from macrophages. From the conjugates tested, the optimum size and surface chemistry for preparing functional Au NP-templated HDL biomimics were identified.

The emerging role of HDL in glucose metabolism

A low plasma level of HDL cholesterol is an atherosclerotic risk factor; however, emerging evidence suggests that low HDL levels might also contribute to the pathophysiology of type 2 diabetes mellitus (T2DM) through direct effects on plasma glucose. In the past decade, animal and clinical studies have uncovered a previously undescribed spectrum of HDL actions, indicating that HDL may control glucose homeostasis through mechanisms including insulin secretion, direct glucose uptake by muscle via the AMP-activated protein kinase, and possibly enhanced insulin sensitivity. These effects are mediated by multiple cell types via mechanisms including preservation of cell function through cellular lipid removal and also via direct signaling events. We suggest a paradigm shift from HDL being a bystander to being an active player in diabetic pathophysiology, which raises the possibility that HDL elevation could be a novel therapeutic avenue for T2DM. The entry of HDL-raising agents of the cholesteryl ester transfer protein (CETP) inhibitor class into late-phase clinical trials creates potential for rapid clinical translation. This Review will discuss the emerging evidence for a role of HDL-mediated glucose regulation in the pathophysiology of T2DM, and will also outline the therapeutic potential for HDL elevation for the prevention and management of T2DM.

Laboratory investigation of dysfunctional HDL

High-density lipoprotein (HDL) particles are anti-atherosclerotic, by virtue of their functions in reverse cholesterol transportation, anti-inflammation and anti-oxidation. However, recent studies have cast doubt on the cardio-protective role of HDL. Structural modification and composition alteration of HDL due to chronic inflammation and acute phase responses may result in loss of normal biological function and even convert HDL into a pro-inflammatory and pro-oxidative agent. Therefore, the assessment of dysfunctional HDL has become a novel target to investigate the association between HDL and coronary artery disease risk. This review article summarizes the laboratory assessment of dysfunctional HDL.

Mechanism underlying apolipoprotein E (ApoE) isoform-dependent lipid efflux from neural cells in culture

We determined the molecular mechanisms underlying apolipoprotein E (ApoE)-isoform-dependent lipid efflux from neurons and ApoE-deficient astrocytes in culture. The ability of ApoE3 to induce lipid efflux was 2.5- to 3.9-fold greater than ApoE4. To explore the contributions of the amino- and carboxyl-terminal tertiary structure domains of ApoE to cellular lipid efflux, each domain was studied separately. The amino-terminal fragment of ApoE3 (22-kDa-ApoE3) induced lipid efflux greater than 22-kDa-ApoE4, whereas the common carboxyl-terminal fragment of ApoE induced very low levels of lipid efflux. Addition of segments of the carboxyl-terminal domain to 22-kDa-ApoE3 additively induced lipid efflux in a length-dependent manner; in contrast, this effect did not occur with ApoE4. This observation, coupled with the fact that introduction of the E255A mutation (which disrupts domain-domain interaction) into ApoE4 increases lipid efflux, indicates that interaction between the amino- and carboxyl-terminal domains in ApoE4 reduces the ability of this isoform to mediate lipid efflux from neural cells. Dimeric 22-kDa or intact ApoE3 induced higher lipid efflux than monomeric 22-kDa or intact ApoE3, respectively, indicating that dimerization of ApoE3 enhances the ability to release lipids. The adenosine triphosphate-binding cassette protein A1 (ABCA1) is involved in ApoE-induced lipid efflux. In conclusion, there are two major factors, intramolecular domain interaction and intermolecular dimerization, that cause ApoE-isoform-dependent lipid efflux from neural cells in culture.

Interferon gamma: a master regulator of atherosclerosis

Atherosclerosis is a chronic inflammatory disease that is characterized by the development of fibrotic plaques in the arterial wall. The disease exhibits a complex aetiology and its progression is influenced by a number of environmental and genetic risk factors. The cytokine interferon-gamma (IFN-gamma), a key regulator of immune function, is highly expressed in atherosclerotic lesions and has emerged as a significant factor in atherogenesis. Evidence from both mouse models of atherosclerosis and in vitro cell culture has suggested that the role of IFN-gamma is complex since both pro- and anti-atherogenic actions have been affiliated to it. This review will focus on evaluating the contribution of IFN-gamma to atherosclerosis and, in particular, how it regulates immune responses to the disease.

Biochemical and structural characterization of apolipoprotein A-I binding protein, a novel phosphoprotein with a potential role in sperm capacitation

The physiological changes that sperm undergo in the female reproductive tract rendering them fertilization-competent constitute the phenomenon of capacitation. Cholesterol efflux from the sperm surface and protein kinase A (PKA)-dependent phosphorylation play major regulatory roles in capacitation, but the link between these two phenomena is unknown. We report that apolipoprotein A-I binding protein (AI-BP) is phosphorylated downstream to PKA activation, localizes to both sperm head and tail domains, and is released from the sperm into the media during in vitro capacitation. AI-BP interacts with apolipoprotein A-I, the component of high-density lipoprotein involved in cholesterol transport. The crystal structure demonstrates that the subunit of the AI-BP homodimer has a Rossmann-like fold. The protein surface has a large two compartment cavity lined with conserved residues. This cavity is likely to constitute an active site, suggesting that AI-BP functions as an enzyme. The presence of AI-BP in sperm, its phosphorylation by PKA, and its release during capacitation suggest that AI-BP plays an important role in capacitation possibly providing a link between protein phosphorylation and cholesterol efflux.

Laboratory assessment of HDL heterogeneity and function

BACKGROUND

Plasma concentrations of HDL cholesterol (HDL-C) and its major protein component apolipoprotein (apo) A-I are strongly inversely associated with cardiovascular risk, leading to the concept that therapy to increase HDL-C and apoA-I concentrations would be antiatherosclerotic and protective against cardiovascular events. The recent failure of the drug torcetrapib, a cholesteryl ester transfer protein inhibitor that substantially increased HDL-C concentrations, has brought focus on the issues of HDL heterogeneity and function as distinct from HDL-C concentrations.

CONTENT

This review addresses the current state of knowledge regarding assays of HDL heterogeneity and function and their relationship to cardiovascular disease. HDL is highly heterogeneous, with subfractions that can be identified on the basis of density, size, charge, and protein composition, and the concept that certain subfractions of HDL may be better predictors of cardiovascular risk is attractive. In addition, HDL has been shown to have a variety of functions that may contribute to its cardiovascular protective effects, including promotion of macrophage cholesterol efflux and reverse cholesterol transport and antiinflammatory and nitric oxide-promoting effects.

SUMMARY

Robust laboratory assays of HDL subfractions and functions and validation of the usefulness of these assays for predicting cardiovascular risk and assessing response to therapeutic interventions are critically important and of great interest to cardiovascular clinicians and investigators and clinical chemists.

An analysis of the role of a retroendocytosis pathway in ABCA1-mediated cholesterol efflux from macrophages

The ATP binding cassette transporter A-1 (ABCA1) is critical for apolipoprotein-mediated cholesterol efflux, an important mechanism employed by macrophages to avoid becoming lipid-laden foam cells, the hallmark of early atherosclerotic lesions. It has been proposed that lipid-free apolipoprotein A-I (apoA-I) enters the cell and is resecreted as a lipidated particle via a retroendocytosis pathway during ABCA1-mediated cholesterol efflux from macrophages. To determine the functional importance of such a pathway, confocal microscopy was used to characterize the internalization of a fully functional apoA-I cysteine mutant containing a thiol-reactive fluorescent probe in cultured macrophages. ApoA-I was also endogenously labeled with (35)S-methionine to quantify cellular uptake and to determine the metabolic fate of the internalized protein. It was found that apoA-I was specifically taken inside macrophages and that a small amount of intact apoA-I was resecreted from the cells. However, a majority of the label that reappeared in the media was degraded. We estimate that the mass of apoA-I retroendocytosed is not sufficient to account for the HDL produced by the cholesterol efflux reaction. Furthermore, we have demonstrated that lipid-free apoA-I-mediated cholesterol efflux from macrophages can be pharmacologically uncoupled from apoA-I internalization into cells. On the basis these findings, we present a model in which the ABCA1-mediated lipid transfer process occurs primarily at the membrane surface in macrophages, but still accounts for the observed specific internalization of apoA-I.

Total body ABCG1 expression protects against early atherosclerotic lesion development in mice

OBJECTIVE

ABCG1 has recently been identified as a facilitator of cholesterol and phospholipid efflux from macrophages to HDL. In bone marrow transplantation studies, we and others have now shown that the absence of macrophage ABCG1 may differentially influence atherosclerotic lesions dependent on the experimental setting and/or the stage of atherosclerotic lesion development. To further define the role of ABCG1 in atherogenesis, we investigated in the current study the effect of total body deficiency of ABCG1 on atherosclerotic lesion development.

METHODS AND RESULTS

ABCG1-/- mice and wild-type littermates were fed an atherogenic diet for 12 weeks to induce atherosclerotic lesion formation. Both before and after the start of the atherogenic diet, serum lipid levels and lipoprotein profiles did not differ significantly between the two groups. In addition no significant difference in serum apoE levels was found after diet feeding. In wild-type mice the atherogenic diet induced the formation of macrophage-rich early lesions (size: 24+/-7x10(3) microm2 [n=6]). Feeding ABCG1-/- mice the atherogenic diet led to a significant 1.9-fold stimulation of atherosclerotic lesion size (46+/-6x10(3) microm2 [n=7]; Student t test P=0.034 and Mann-Whitney test P=0.050) compared with controls, suggesting a clear antiatherogenic role for ABCG1. At the same time, excessive lipid accumulation was observed in macrophage-rich areas of the lungs and spleens of ABCG1-/- mice as compared with wild-type mice.

CONCLUSIONS

Total body ABCG1 expression protects against early atherosclerotic lesion development.

Isolation and characterization of human apolipoprotein M-containing lipoproteins

Apolipoprotein M (apoM) is a novel apolipoprotein with unknown function. In this study, we established a method for isolating apoM-containing lipoproteins and studied their composition and the effect of apoM on HDL function. ApoM-containing lipoproteins were isolated from human plasma with immunoaffinity chromatography and compared with lipoproteins lacking apoM. The apoM-containing lipoproteins were predominantly of HDL size; approximately 5% of the total HDL population contained apoM. Mass spectrometry showed that the apoM-containing lipoproteins also contained apoJ, apoA-I, apoA-II, apoC-I, apoC-II, apoC-III, paraoxonase 1, and apoB. ApoM-containing HDL (HDL(apoM+)) contained significantly more free cholesterol than HDL lacking apoM (HDL(apoM-)) (5.9 +/- 0.7% vs. 3.2 +/- 0.5%; P < 0.005) and was heterogeneous in size with both small and large particles. HDL(apoM+) inhibited Cu(2+)-induced oxidation of LDL and stimulated cholesterol efflux from THP-1 foam cells more efficiently than HDL(apoM-). In conclusion, our results suggest that apoM is associated with a small heterogeneous subpopulation of HDL particles. Nevertheless, apoM designates a subpopulation of HDL that protects LDL against oxidation and stimulates cholesterol efflux more efficiently than HDL lacking apoM.

Ability of serum to decrease cellular acylCoA:cholesterol acyl transferase activity predicts cardiovascular outcomes

BACKGROUND

We evaluated whether cholesterol efflux activity of serum is associated with the presence of angiographic coronary artery disease (CAD) and the risk of major adverse cardiovascular events (MACE) and death.

METHODS AND RESULTS

We studied 168 men undergoing coronary angiography. Cholesterol efflux activity was measured in vitro by incubation of patient serum with human skin fibroblasts and defined as the ability of serum to decrease the pool of cholesterol available for esterification by the acylCoA:cholesterol acyl transferase (ACAT) reaction. We evaluated whether this activity was associated with the presence of CAD and the risk of MACE and death during a 4.5-year follow-up. Serum-induced changes in ACAT activity did not correlate with HDL levels or the presence of CAD. Patients in the highest tertile of change in ACAT activity had a significantly higher risk for MACE (HR, 2.15; 95% CI, 1.36 to 3.39; P=0.001) and death (HR, 2.23; 95% CI, 1.17 to 4.26; P=0.01). These correlations were independent of other risk markers including LDL, HDL, and C-reactive protein levels.

CONCLUSIONS

Serum-induced depletion of cellular cholesterol available for esterification by ACAT was a strong, independent predictor of MACE and death. We speculate that the ability of serum to decrease ACAT activity depends on ATP binding cassette transporter A1 (ABCA1)-mediated efflux. Furthermore, serum samples that induce larger changes in ACAT activity contain increased levels of HDL particles that preferentially interact with ABCA1 and that these particles accumulate in the serum of patients because of low activity of ABCA1 in vivo preventing or limiting the extent of apoA-I lipidation.

Lysophosphatidylcholine promotes cholesterol efflux from mouse macrophage foam cells via PPARγ-LXRα-ABCA1-dependent pathway associated with apoE

Formation of macrophage-derived foam cells is a hallmark in earlier stages of atherosclerosis (AS). Increased cholesterol efflux from macrophage foam cells promote atherosclerotic regression. In the present study, lysophosphatidylcholine (LPC) promoting cholesterol efflux from macrophage foam cells was observed, and the mechanism underlying the action was investigated. Macrophage foam cells from mice were incubated with different concentrations of LPC (10, 20, 40, 80 microM), and the free cholesterol in medium increased but total intracellular cholesterol decreased. At the same time, the expression of PPARgamma, LXRalpha, ABCA1 was enhanced in a dose-dependent manner. The treatment of macrophage foam cells with 40 microM LPC for 12, 24 and 48 h promoted cellular cholesterol efflux in a time-dependent manner, meanwhile expression of PPARgamma, LXRalpha, ABCA1 was also raised respectively. Addition of different specific inhibitors of PPARgamma (GW9662), LXRalpha (GGPP), ABCA1 (DIDS) to the foam cells significantly suppressed LPC-induced cholesterol efflux. Also treatment with specific inhibitors of PPARgamma or LXRalpha decreased ABCA1 mRNA and protein expressions. LPC (40 microM)-induced cholesterol efflux was significantly lower in macrophage foam cells from apoE deficient mice than from normal C57BL/6J mice. In contrast, 10 microg apoAI-induced cholesterol efflux from foam cells remained in apoE deficient mice. The present results indicate that LPC promotes cholesterol efflux from macrophage foam cells via a PPARgamma-LXRalpha-ABCA1-dependent pathway. Furthermore, apoE may be involved in this process.

Distinct spatio-temporal expression of ABCA and ABCG transporters in the developing and adult mouse brain

Using in situ hybridization for the mouse brain, we analyzed developmental changes in gene expression for the ATP-binding cassette (ABC) transporter subfamilies ABCA1-4 and 7, and ABCG1, 2, 4, 5 and 8. In the embryonic brains, ABCA1 and A7 were highly expressed in the ventricular (or germinal) zone, whereas ABCA2, A3 and G4 were enriched in the mantle (or differentiating) zone. At the postnatal stages, ABCA1 was detected in both the gray and white matter and in the choroid plexus. On the other hand, ABCA2, A3 and A7 were distributed in the gray matter. In addition, marked up-regulation of ABCA2 occurred in the white matter at 14 days-of-age when various myelin protein genes are known to be up-regulated. In marked contrast, ABCA4 was selective to the choroid plexus throughout development. ABCG1 was expressed in both the gray and white matters, whereas ABCG4 was confined to the gray matter. ABCG2 was diffusely and weakly detected throughout the brain at all stages examined. Immunohistochemistry of ABCG2 showed its preferential expression on the luminal membrane of brain capillaries. Expression signals for ABCG5 and G8 were barely detected at any stages. The distinct spatio-temporal expressions of individual ABCA and G transporters may reflect their distinct cellular expressions in the developing and adult brains, presumably, to regulate and maintain lipid homeostasis in the brain.

Dyslipidemia and inflammation: an evolutionary conserved mechanism

Inflammation leads to changes in lipid metabolism aimed at decreasing the toxicity of a variety of harmful agents and tissue repair by redistributing nutrients to cells involved in host defence. Acute phase response, mediated by cytokines, preserves the host from acute injury. When this inflammation becomes chronic, it might lead to chronic disorders as atherosclerosis and the metabolic syndrome. The activation of the inflammatory cascade will induce a decrease in HDL-cholesterol (HDL-C), with impairment in reverse cholesterol transport, and parallel changes in apolipoproteins, enzymes, anti-oxidant capacity and ATP binding cassette A1-dependent efflux. This decrease in HDL-C and phospholipids could stimulate compensatory changes, as synthesis and accumulation of phospholipid-rich VLDL which binds bacterial products and other toxic substances, resulting in hypertriglyceridemia. The final consequence is an increased accumulation of cholesterol in cells. When the compensatory response (inflammation) is not able to repair injury, it turns into a harmful reaction, and the lipid changes will become chronic, either by repeated or overwhelming stimulus, enhancing the formation of atherosclerotic lesions. Thus, the classical lipid changes associated with the metabolic syndrome (increased triglycerides and decreased HDL-C) may be envisioned as a highly conserved evolutionary response aimed at tissue repair. Under this assumption, the problem is not the response but the persistence of the stimulus.

Macrophage apolipoprotein A-I expression protects against atherosclerosis in ApoE-deficient mice and up-regulates ABC transporters

The antiatherogenic effect of high-density lipoprotein (HDL) and its major protein component apolipoprotein A-I (apoA-I) has been largely attributed to their key roles in reverse cholesterol transport (RCT) and cellular cholesterol efflux. Substantial evidence shows that overexpression of human apoA-I reduces atherosclerosis in animal models. However, it is uncertain whether this protection is due to an increase in plasma HDL level or to a local effect in the artery wall. To test the hypothesis that expression of human apoA-I in macrophages can promote RCT in the artery wall, we used a retroviral construct expressing human apoA-I cDNA (MFG-HAI) to transduce ApoE(-/-) bone marrow cells and then transplanted these cells into ApoE(-/-) mice with preexisting atherosclerosis. ApoE(-/-) mice reconstituted with MFG-HAI marrow had a significant reduction (30%) in atherosclerotic lesions in the proximal aorta compared to control mice that received marrow expressing MFG parental virus. Peritoneal macrophages isolated from MFG-HAI mice showed a four- to fivefold increase in mRNA expression levels of both ATP-binding cassette (ABC) A1 and ABCG1 compared to controls. Our data demonstrate that gene transfer-mediated expression of human apoA-I in macrophages can compensate in part for apoE deficiency and delay the progression of atherosclerotic lesions by stimulating ABC-dependent cholesterol efflux and RCT.

ABCA1-dependent lipid efflux to apolipoprotein A-I mediates HDL particle formation and decreases VLDL secretion from murine hepatocytes

High levels of expression of the ATP binding cassette transporter A1 (ABCA1) in the liver and the need to over- or underexpress hepatic ABCA1 to impact plasma HDL levels in mice suggest a major role of the liver in HDL formation and in determining circulating HDL levels. Cultured murine hepatocytes were used to examine the role of hepatic ABCA1 in mediating the lipidation of apolipoprotein A-I (apoA-I) for HDL particle formation. Exogenous apoA-I stimulated cholesterol efflux to the medium from wild-type hepatocytes, but not from ABCA1-deficient (abca1(-/-)) hepatocytes. ApoA-I induced the formation of new HDL particles and enhanced the lipidation of endogenously secreted murine apoA-I in ABCA1-expressing but not abca1(-/-) hepatocytes. ABCA1-dependent cholesterol mobilization to apoA-I increased new cholesterol synthesis, indicating depletion of the regulatory pool of hepatocyte cholesterol during HDL formation. Secretion of triacylglycerol and apoB was decreased following apoA-I incubation with ABCA1-expressing but not abca1(-/-) hepatocytes. These results support a major role for hepatocyte ABCA1 in generating a critical pool of HDL precursor particles that enhance further HDL generation and passive cholesterol mobilization in the periphery. The results also suggest that diversion of hepatocyte cholesterol into the "reverse" cholesterol transport pathway diminishes cholesterol availability for apoB-containing lipoprotein secretion by the liver.

The ABCA1 transporter modulates late endocytic trafficking: insights from the correction of the genetic defect in Tangier disease

We have previously established that the ABCA1 transporter, which plays a critical role in the lipidation of extracellular apolipoprotein acceptors, traffics between late endocytic vesicles and the cell surface (Neufeld, E. B., Remaley, A. T., Demosky, S. J., Jr., Stonik, J. A., Cooney, A. M., Comly, M., Dwyer, N. K., Zhang, M., Blanchette-Mackie, J., Santamarina-Fojo, S., and Brewer, H. B., Jr. (2001) J. Biol. Chem. 276, 27584-27590). The present study provides evidence that ABCA1 in late endocytic vesicles plays a role in cellular lipid efflux. Late endocytic trafficking was defective in Tangier disease fibroblasts that lack functional ABCA1. Consistent with a late endocytic protein trafficking defect, the hydrophobic amine U18666A retained NPC1 in abnormally tubulated, cholesterol-poor, Tangier disease late endosomes, rather than cholesterol-laden lysosomes, as in wild type fibroblasts. Consistent with a lipid trafficking defect, Tangier disease late endocytic vesicles accumulated both cholesterol and sphingomyelin and were immobilized in a perinuclear localization. The excess cholesterol in Tangier disease late endocytic vesicles retained massive amounts of NPC1, which traffics lysosomal cholesterol to other cellular sites. Exogenous apoA-I abrogated the cholesterol-induced retention of NPC1 in wild type but not in Tangier disease late endosomes. Adenovirally mediated ABCA1-GFP expression in Tangier disease fibroblasts corrected the late endocytic trafficking defects and restored apoA-I-mediated cholesterol efflux. ABCA1-GFP expression in wild type fibroblasts also reduced late endosome-associated NPC1, induced a marked uptake of fluorescent apoA-I into ABCA1-GFP-containing endosomes (that shuttled between late endosomes and the cell surface), and enhanced apoA-I-mediated cholesterol efflux. The combined results of this study suggest that ABCA1 converts pools of late endocytic lipids that retain NPC1 to pools that can associate with endocytosed apoA-I, and be released from the cell as nascent high density lipoprotein.

Apolipoprotein A-1 interaction with plasma membrane lipid rafts controls cholesterol export from macrophages

Cholesterol efflux to apolipoprotein A-1 (apoA-1) from cholesterol-loaded macrophages is an important anti-atherosclerotic mechanism in reverse cholesterol transport. We recently provided kinetic evidence for two distinct pathways for cholesterol efflux to apoA-1 [Gaus et al. (2001) Biochemistry 40, 9363]. Cholesterol efflux from two membrane pools occurs sequentially with different kinetics; a small pool rapidly effluxed over the first hour, followed by progressive release from a major, slow efflux pool over several hours. In the present study, we propose that the rapid and slow cholesterol efflux pools represent cholesterol derived from lipid raft and nonraft domains of the plasma membrane, respectively. We provide direct evidence that apoA-1 binds to both lipid raft and nonraft domains of the macrophage plasma membrane. Conditions that selectively deplete plasma membrane lipid raft cholesterol, such as incorporation of 7-ketocholesterol or rapid exposure to cyclodextrins, block apoA-1 binding to these domains but also inhibit cholesterol efflux from the major, slow pool. We propose that cholesterol exported to apoA-1 from this major slow efflux pool derives from nonraft regions of the plasma membrane but that the interaction of apoA-1 with lipid rafts is necessary to stimulate this efflux.

Periodontitis decreases the antiatherogenic potency of high density lipoprotein

Periodontitis, a consequence of persistent bacterial infection and chronic inflammation, has been suggested to predict coronary heart disease (CHD). The aim of this study was to investigate the impact of periodontitis on HDL structure and antiatherogenic function in cholesterol efflux in vitro. HDL was isolated from 30 patients (age 43.6 +/- 6.1 years, mean +/- SD) with periodontitis before and after (3.2 +/- 1.4 months) periodontal treatment. The capacity of HDL for cholesterol efflux from macrophages (RAW 264.7), HDL composition, and key proteins of HDL metabolism were determined. After periodontal treatment, phospholipid transfer protein (PLTP) activity was 6.2% (P<0.05) lower, and serum HDL cholesterol concentration, PLTP mass, and cholesteryl ester transfer protein activity were 10.7% (P<0.001), 7.1% (P=0.078), and 19.4% (P<0.001) higher, respectively. The mean HDL2/HDL3 ratio increased from 2.16 +/- 0.87 to 3.56 +/- 0.48 (P<0.05). HDL total phospholipid mass and sphingomyelin-phosphatidylcholine ratio were 7.4% (P<0.05) and 36.8% (P<0.001) higher, respectively. The HDL-mediated cholesterol efflux tended to be higher after periodontal treatment; interestingly, this increase was significant (P<0.05) among patients whose C-reactive protein decreased (53.7% reduction, P=0.015) and who were positive by PCR for Actinobacillus actinomycetemcomitans. These results suggest that periodontitis causes similar, but milder, changes in HDL metabolism than those that occur during the acute-phase response and that periodontitis may diminish the antiatherogenic potency of HDL, thus increasing the risk for CHD.

The ins and outs of reverse cholesterol transport

It is generally assumed that HDL is the obligate transport vehicle for 'reverse cholesterol transport', the pathway for removal of excess cholesterol from peripheral tissues via the liver into bile and subsequent excretion via the feces. During the last few years, intensive research has generated exciting new data on the separate processes involved in reverse cholesterol transport. Many 'new' proteins, particularly members of the ABC transporter and nuclear receptor subfamilies, that mediate or influence cholesterol fluxes have been identified and characterized. An important role of the intestine in regulation of cholesterol homeostasis is emerging. In this paper, new insights into mechanisms of reverse cholesterol are reviewed.

Impaired ABCA1-dependent lipid efflux and hypoalphalipoproteinemia in human Niemann-Pick type C disease

The cholesterol trafficking defect in Niemann-Pick type C (NPC) disease leads to impaired regulation of cholesterol esterification, cholesterol synthesis, and low density lipoprotein receptor activity. The ATP-binding cassette transporter A1 (ABCA1), which mediates the rate-limiting step in high density lipoprotein (HDL) particle formation, is also regulated by cell cholesterol content. To determine whether the Niemann-Pick C1 protein alters the expression and activity of ABCA1, we determined the ability of apolipoprotein A-I (apoA-I) to deplete pools of cellular cholesterol and phospholipids in human fibroblasts derived from NPC1+/+, NPC1+/-, and NPC1-/- subjects. Efflux of low density lipoprotein-derived, non-lipoprotein, plasma membrane, and newly synthesized pools of cell cholesterol by apoA-I was diminished in NPC1-/- cells, as was efflux of phosphatidylcholine and sphingomyelin. NPC1+/- cells showed intermediate levels of lipid efflux compared with NPC1+/+ and NPC1-/- cells. Binding of apoA-I to cholesterol-loaded and non-cholesterol-loaded cells was highest for NPC1+/- cells, with NPC1+/+ and NPC1-/- cells showing similar levels of binding. ABCA1 mRNA and protein levels increased in response to cholesterol loading in NPC1+/+ and NPC1+/- cells but showed low levels at base line and in response to cholesterol loading in NPC1-/- cells. Consistent with impaired ABCA1-dependent lipid mobilization to apoA-I for HDL particle formation, we demonstrate for the first time decreased plasma HDL-cholesterol levels in 17 of 21 (81%) NPC1-/- subjects studied. These results indicate that the cholesterol trafficking defect in NPC disease results in reduced activity of ABCA1, which we suggest is responsible for the low HDL-cholesterol in the majority of NPC subjects and partially responsible for the overaccumulation of cellular lipids in this disorder.

Cholesterol distribution in the Golgi complex of DITNC1 astrocytes is differentially altered by fresh and aged amyloid beta-peptide-(1-42)

The Golgi complex plays an important role in cholesterol trafficking in cells, and amyloid beta-peptides (Abetas) alter cholesterol trafficking. The hypothesis was tested that fresh and aged Abeta-(1-42) would differentially modify Golgi cholesterol content in DINTC1 astrocytes and that the effects of Abeta-(1-42) would be associated with the region of the Golgi complex. Two different methods were used to determine the effects of Abeta-(1-42) on Golgi complex cholesterol. Confocal microscopy showed that fresh Abeta-(1-42) significantly increased cholesterol and that aged Abeta-(1-42) significantly reduced cholesterol content in the Golgi complex. Isolation of the Golgi complex into two fractions using density gradient centrifugation showed effects of aged Abeta-(1-42) similar to those observed with confocal microscopy but revealed the novel finding that fresh Abeta-(1-42) had opposite effects on the two Golgi fractions suggesting a specificity of Abeta-(1-42) perturbation of the Golgi complex. Phosphatidylcholine-phospholipase D activity, cell membrane cholesterol, and apolipoprotein E levels were associated with effects of fresh Abeta-(1-42) on cholesterol distribution but not with effects of aged Abeta-(1-42), arguing against a common mechanism. Extracellular Abeta-(1-42) targets the Golgi complex and disrupts cell cholesterol homeostasis, and this action of Abeta-(1-42) could alter cell functions requiring optimal levels of cholesterol.

Synthetic amphipathic helical peptides promote lipid efflux from cells by an ABCA1-dependent and an ABCA1-independent pathway

In order to examine the necessary structural features for a protein to promote lipid efflux by the ABCA1 transporter, synthetic peptides were tested on ABCA1-transfected cells (ABCA1 cells) and on control cells. L-37pA, an l amino acid peptide that contains two class-A amphipathic helices linked by proline, showed a 4-fold increase in cholesterol and phospholipid efflux from ABCA1 cells compared to control cells. The same peptide synthesized with a mixture of l and d amino acids was less effective than L-37pA in solubilizing dimyristoyl phosphatidyl choline vesicles and in effluxing lipids. In contrast, the 37pA peptide synthesized with all d amino acids (D-37pA) was as effective as L-37pA. Unlike apoA-I, L-37pA and D-37pA were also capable, although at a reduced rate, of causing lipid efflux independent of ABCA1 from control cells, Tangier disease cells, and paraformaldehyde fixed ABCA1 cells. The ability of peptides to bind to cells correlated with their lipid affinity. In summary, the amphipathic helix was found to be a key structural motif for peptide-mediated lipid efflux from ABCA1, but there was no stereoselective requirement. In addition, unlike apoA-I, synthetic peptides can also efflux lipid by a passive, energy-independent pathway that does not involve ABCA1 but does depend upon their lipid affinity.

Contribution of HDL-apolipoproteins to the inhibition of low density lipoprotein oxidation and lipid accumulation in macrophages

High-density lipoprotein (HDL) is known as a protective factor against atherosclerosis. However, whether HDL-apolipoproteins (apo-HDL) contribute to the protection in arterial cells remains unclear. The localization patterns of human apolipoproteins in atherosclerotic arteries were determined using immunohistochemical examination. The results indicate that several apolipoproteins are retained in component cells of the coronary artery walls. To elucidate the possible roles of apo-HDL in the protection of atherosclerotic lesion formation, we investigated the effects of apo-HDL on the formation of conjugated diene (CD) in a cell-free system and thiobarbituric acid-reactive substances (TBARS) in the medium of a macrophage-mediated LDL oxidation system. The results showed that apo-HDL significantly exerted an inhibitory effect on LDL lipid oxidation in vitro. In addition, apo-HDL decreased cholesterol influx but enhanced cholesterol efflux from J774 macrophages in a dose-dependent manner. These results are consistent with the notion that there is reduced intracellular lipid accumulation in apo-HDL treated macrophages. These data provide a direct evidence for apo-HDL in protecting LDL from oxidative modification and in reducing the accumulation of cholesterol and lipid droplets by J774 macrophages.

The ABCA1 transporter functions on the basolateral surface of hepatocytes

ABCA1 on the cell surface and in endosomes plays an essential role in the cell-mediated lipidation of apoA-I to form nascent HDL. Our previous studies of transgenic mice overexpressing ABCA1 suggested that ABCA1 in the liver plays a major role in regulating plasma HDL levels. The site of function of ABCA1 in the polarized hepatocyte was currently assessed by expression of an adenoviral construct encoding a human ABCA1-GFP fusion protein in the polarized hepatocyte-like WIF-B cell line. Consistent with localization of ABCA1 at the basolateral (vascular) cell surface, expression of ABCA1-GFP stimulated apoA-I mediated efflux of WIF-B cell cholesterol into the culture medium. Confocal fluorescence microscopy revealed that ABCA1-GFP was expressed solely on the basolateral surface and associated endocytic vesicles. These findings suggest an important role for hepatocyte basolateral membrane ABCA1 in the regulation of the levels of intracellular hepatic cholesterol, as well as plasma HDL.

Naturally occurring mutations in the largest extracellular loops of ABCA1 can disrupt its direct interaction with apolipoprotein A-I

The ABCA1 transporter contains two large domains into which many of the genetic mutations in individuals with Tangier disease fall. To investigate the structural requirements for the cellular cholesterol efflux mediated by ABCA1, we have determined the topology of these two domains and generated transporters harboring five naturally occurring missense mutations in them. These mutants, unlike wild type ABCA1, produced little or no apoA-I-stimulated cholesterol efflux when transfected into 293 cells, establishing their causality in Tangier disease. Because all five mutant proteins were well expressed and detectable on the plasma membrane, their interaction with the ABCA1 ligand, apolipoprotein (apo) A-I, was measured using bifunctional cross-linking agents. Four of five mutants had a marked decline in cross-linking to apoA-I, whereas one (W590S) retained full cross-linking activity. Cross-linking of apoA-I was temperature-dependent, rapid in onset, and detectable with both lipid- and water-soluble cross-linking agents. These results suggest that apoA-I-stimulated cholesterol efflux cannot occur without a direct interaction between the apoprotein and critical residues in two extracellular loops of ABCA1. The behavior of the W590S mutant indicates that although binding of apoA-I by ABCA1 may be necessary, it is not sufficient for stimulation of cholesterol efflux.

Genetic disorders associated with ATP binding cassette cholesterol transporters

Coronary artery disease is the most prevalent form of mortality and morbidity in Western countries. Studies in the last several decades have identified high LDL cholesterol and low HDL cholesterol as major risk factors leading to the disease. Human genetic studies have provided significant insight into the regulation of lipoprotein metabolism. In the last several years, the genes associated with several rare genetic diseases of lipid metabolism have been revealed. These landmark discoveries that identified mutant ABC cholesterol transporters as the underlying causes of these genetic disorders have paved the way for better understanding of the cellular cholesterol transport process and HDL biogenesis. This summary provides an overview and discussion of the most recent progress that includes molecular mechanism and regulation of cholesterol transport mediated by these ABC transporters.

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.

Apolipoprotein E (ApoE) isoform-dependent lipid release from astrocytes prepared from human ApoE3 and ApoE4 knock-in mice

We have reported previously (Michikawa, M., Fan, Q.-W., Isobe, I., and Yanagisawa, K. (2000) J. Neurochem. 74, 1008-1016) that exogenously added recombinant human apolipoprotein E (apoE) promotes cholesterol release in an isoform-dependent manner. However, the molecular mechanism underlying this isoform-dependent promotion of cholesterol release remains undetermined. In this study, we demonstrate that the cholesterol release is mediated by endogenously synthesized and secreted apoE isoforms and clarify the mechanism underlying this apoE isoform-dependent cholesterol release using cultured astrocytes prepared from human apoE3 and apoE4 knock-in mice. Cholesterol and phospholipids were released into the culture media, resulting in the generation of two types of high density lipoprotein (HDL)-like particles; one was associated with apoE and the other with apoJ. The amount of cholesterol released into the culture media from the apoE3-expressing astrocytes was approximately 2.5-fold greater than that from apoE4-expressing astrocytes. In contrast, the amount of apoE3 released in association with the HDL-like particles was similar to that of apoE4, and the sizes of the HDL-like particles released from apoE3- and apoE4-expressing astrocytes were similar. The molar ratios of cholesterol to apoE in the HDL fraction of the culture media of apoE3- and apoE4-expressing astrocytes were 250 +/- 6.0 and 119 +/- 5.1, respectively. These data indicate that apoE3 has an ability to generate similarly sized lipid particles with less number of apoE molecules than apoE4, suggesting that apoE3-expressing astrocytes can supply more cholesterol to neurons than apoE4-expressing astrocytes. These findings provide a new insight into the issue concerning the putative alteration of apoE-related cholesterol metabolism in Alzheimer's disease.

Lipopolysaccharide down regulates both scavenger receptor B1 and ATP binding cassette transporter A1 in RAW cells

Lipopolysaccharide (LPS) has recently been shown to facilitate macrophage foam cell formation and has been suggested to be a proatherogenic factor. The mechanism of LPS induced cholesterol accumulation, however, is unclear. In this report, using the macrophage-like RAW 264.7 cell line, we provide experimental evidence that LPS's proatherogenic effects may at least in part reflect altered cholesterol metabolism. Data presented demonstrate that in a dose-dependent manner, LPS is able to down regulate the mRNA expression of the two primary high-density lipoprotein (HDL) receptors, scavenger receptor B1 (SR-B1) and ATP binding cassette A1 (ABCA1), with a 50% inhibitory concentration of less than 0.2 ng/ml, as well as to decrease SR-B1 protein expression by 80%. We also found that LPS treatment resulted in a significant decrease (to 20% of the control level) of the specific (125)I-HDL binding as well as in 50% inhibition of the HDL-mediated cholesterol efflux compared to untreated cells. In addition, we compared the potencies of various modified LPS preparations and demonstrated that the phosphorylated lipid A portion of LPS, which is highly conserved among gram-negative microorganisms, including Chlamydia, is primarily responsible for the effects of LPS on SR-B1 and ABCA1 expression. Inhibitors of NF-kappaB activation were observed to efficiently block the suppressive effect of LPS on SR-B1 and ABCA1, suggesting a mechanism involving NF-kappaB. These data indicate that the LPS effects on cholesterol metabolism may contribute to the proatherogenic properties of LPS.

Effects of enrichment of fibroblasts with unesterified cholesterol on the efflux of cellular lipids to apolipoprotein A-I

This study elucidates the factors underlying the enhancement in efflux of human fibroblast unesterified cholesterol and phospholipid (PL) by lipid-free apolipoprotein (apo) A-I that is induced by cholesterol enrichment of the cells. Doubling the unesterified cholesterol content of the plasma membrane by incubation for 24 h with low density lipoprotein and lipid/cholesterol dispersions increases the pools of PL and cholesterol available for removal by apoA-I from about 0.8-5%; the initial rates of mass release of cholesterol and PL are both increased about 6-fold. Expression of the ATP binding cassette transporter A1 (ABCA1) is critical for this increased efflux of lipids, and cholesterol loading of the fibroblasts over 24 h increases ABCA1 mRNA about 12-fold. The presence of more ABCA1 and cholesterol in the plasma membrane results in a 2-fold increase in the level of specific binding of apoA-I to the cells with no change in binding affinity. Characterization of the species released from either control or cholesterol-enriched cells indicates that the plasma membrane domains from which lipids are removed are cholesterol-enriched with respect to the average plasma membrane composition. Cholesterol enrichment of fibroblasts also affects PL synthesis, and this leads to enhanced release of phosphatidylcholine (PC) relative to sphingomyelin (SM); the ratios of PC to SM solubilized from control and cholesterol-enriched fibroblasts are approximately 2/1 and 5/1, respectively. Biosynthesis of PC is critical for this preferential release of PC and the enhanced cholesterol efflux because inhibition of PC synthesis by choline depletion reduces cholesterol efflux from cholesterol-enriched cells. Overall, it is clear that enrichment of fibroblasts with unesterified cholesterol enhances efflux of cholesterol and PL to apoA-I because of three effects, 1) increased PC biosynthesis, 2) increased PC transport via ABCA1, and 3) increased cholesterol in the plasma membrane.

Insights of high-density lipoprotein apolipoprotein-mediated lipid efflux from cells

High-density lipoprotein (HDL) protects against cardiovascular diseases by removal of excess lipids from cells. HDL apolipoprotein-mediated lipid efflux involves multiple cellular proteins to remove both cholesterol and phospholipids that are otherwise stored in the cells. This article reviews recent progress in the understanding of receptors, signal mediators, Golgi and vesicle transport related to the pathway and proposes a model of HDL apolipoprotein receptor-mediated exocytosis of cellular cholesterol. Such an exocytotic pathway could provide the most effective mechanism to remove excess cellular lipids and prevent atherogenesis.

Inhibition of cholesterol efflux by 7-ketocholesterol: comparison between cells, plasma membrane vesicles, and liposomes as cholesterol donors

Cholesterol removal from lipid-loaded macrophages is an important, potentially antiatherogenic process, and we have previously shown that an oxysterol, 7-ketocholesterol (7K), can impair efflux to lipid-free apoprotein A-1 (apoA-1). This publication investigates whether incorporation of 7K into membranes could account for this impairment of cholesterol efflux. Cholesterol efflux was studied from lipoprotein-loaded THP-1 cells, from plasma membrane vesicles obtained from these cells, and from artificial, protein-free liposomes. Impairment of cholesterol efflux by 7K was observed for all cholesterol donor systems whether measured as decline in cholesterol removal rates or as the percentage mass of total cellular cholesterol exported. 7-Ketocholesterol itself was not removed by apoA-1 from any of the cholesterol donor systems. Increasing membrane cholesterol content increased the rate of cholesterol removal by apoA-1 (as seen with plasma membrane vesicles), the quantity of cholesterol removed at equilibrium (liposomes), or both (whole cells). Although the minimum inhibitory 7K concentrations varied between the cholesterol donor systems, 7K inhibited cholesterol efflux in all systems. It was concluded that 7K induces alteration in membranes which decreased the efficiency of cholesterol efflux and the quantity of removed cholesterol induced by apoA-1. As cell membrane proteins are not essential for cholesterol efflux in these systems, the impairment of such by 7K suggests that its effect on membrane lipid composition and its structure are key regulatory elements in this efflux process.

Ethanol and lipid metabolic signaling

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were Shivendra D. Shukla and Grace Y. Sun. The presentations were (1) Metabolic turnover of ethanol into cellular lipids and platelet activating factor, by Shivendra D. Shukla; (2) Ethanol action on the phospholipase A2 signaling pathways in astrocytes, by Grace Y. Sun; (3) Mechanisms of ethanol-induced perturbation of lipoprotein cholesterol transport, by W. Gibson Wood; (4) Transfer of an abnormal ethanol-induced phospholipid, phosphatidylethanol, between lipoproteins, by Markku J. Savolainen; (5) Phospholipase-d-mediated formation of phosphatidylethanol, by Christer Alling; and (6) Changes in phosphoinositide signaling after chronic ethanol treatment, by Jan B. Hoek.

Regulation and activity of the human ABCA1 gene in transgenic mice

The ABCA1 transporter is one of the limiting steps in cellular cholesterol efflux. To study the expression and activity of the human ABCA1 gene in vivo we have examined mice containing two human BAC transgenes with different 5' ends. Mice containing a 255-kilobase (kb) BAC transgene, including 70 kb upstream of the previously defined exon 1, demonstrated a pattern of tissue-specific expression mimicking that of the endogenous mouse gene. Compared with macrophages from control mice, macrophages from these transgenics had increases in apoA-I cholesterol efflux heightened in response to increases in cell cholesterol content. The observed increase in macrophage apoA-I-mediated cholesterol efflux was not accompanied by alterations in plasma high density lipoprotein in the transgenics. Although mice containing a smaller 171-kb human BAC transgene, lacking the previously described exon 1 and ABCA1 promoter, did not express human ABCA1 in macrophages, they did express the human transgene in liver at levels comparable with those of the orthologous mouse gene. Analysis by 5' rapid amplification of cDNA ends of liver mRNA from these animals revealed a new ABCA1 exon 1 (exon 1A) and a previously unrecognized promoter. Analysis of human tissue revealed that exon 1A containing transcripts accounted for a high proportion of the ABCA1 mRNAs present in human liver. This analysis of ABCA1 transgenics showed that the expression of human ABCA1 transgenes can result in increased cholesterol efflux from macrophages, unaccompanied by changes in plasma high density lipoprotein, and identified a new ABCA1 promoter in humans.