Cellular cholesterol efflux is modulated by phospholipid-derived signaling molecules in familial HDL deficiency/Tangier disease fibroblasts

Role and therapeutic potential of gelsolin in atherosclerosis

Atherosclerosis is the major pathophysiological basis of a variety of cardiovascular diseases and has been recognized as a lipid-driven chronic inflammatory disease. Gelsolin (GSN) is a member of the GSN family. The main function of GSN is to cut and seal actin filaments to regulate the cytoskeleton and participate in a variety of biological functions, such as cell movement, morphological changes, metabolism, apoptosis and phagocytosis. Recently, more and more evidences have demonstrated that GSN is Closely related to atherosclerosis, involving lipid metabolism, inflammation, cell proliferation, migration and thrombosis. This article reviews the role of GSN in atherosclerosis from inflammation, apoptosis, angiogenesis and thrombosis.

ABCA1-Mediated Cholesterol Efflux Capacity to Cerebrospinal Fluid Is Reduced in Patients With Mild Cognitive Impairment and Alzheimer's Disease

Background

Animal and human studies indicate that ABCA1‐mediated cholesterol transport is important in Alzheimer's disease (AD). We hypothesized that the efficiency of cerebrospinal fluid (CSF) to facilitate ABCA1‐mediated cholesterol efflux would be reduced in participants with mild cognitive impairment (MCI) or AD compared with cognitively healthy participants.

Methods and Results

CSF was collected from a cross‐sectional study of cognitively healthy participants (n=47) and participants with MCI (n=35) or probable AD (n=26).The capacity of CSF to mediate cholesterol transport was assessed using a BHK cell line that can be induced to express the ABCA1 transporter. ABCA1‐mediated cholesterol efflux capacity was 30% less in participants with MCI or AD compared with cognitively healthy participants (P<0.001 for both). Cholesterol efflux capacity correlated with CSF cholesterol content (r=0.37, P<0.001). CSF phosphatidylcholine decreased in participants with MCI and AD compared with cognitively healthy participants (9% less in MCI and 27% less in AD compared with cognitively healthy participants, P=0.01) and correlated with CSF efflux capacity (r=0.3, P=0.001). CSF sphingomyelin also correlated with the efflux capacity (r=0.24, P=0.02). Concentrations of CSF apoA‐I and apoE did not significantly correlate with measures of efflux capacity.

Conclusions

In people with MCI and AD, the capacity of CSF to facilitate ABCA1‐mediated cholesterol efflux is impaired. This lesser cholesterol efflux in MCI supports a pathophysiological role for ABCA1‐mediated cholesterol transport in early neurodegeneration.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

Defective functionality of small, dense HDL3 subpopulations in ST segment elevation myocardial infarction: Relevance of enrichment in lysophosphatidylcholine, phosphatidic acid and serum amyloid A

BACKGROUND

Low plasma levels of high-density lipoprotein-cholesterol (HDL-C) are typical of acute myocardial infarction (MI) and predict risk of recurrent cardiovascular events. The potential relationships between modifications in the molecular composition and the functionality of HDL subpopulations in acute MI however remain indeterminate.

METHODS AND RESULTS

ST segment elevation MI (STEMI) patients were recruited within 24h after diagnosis (n=16) and featured low HDL-C (-31%, p<0.05) and acute-phase inflammation (determined as marked elevations in C-reactive protein, serum amyloid A (SAA) and interleukin-6) as compared to age- and sex-matched controls (n=10). STEMI plasma HDL and its subpopulations (HDL2b, 2a, 3a, 3b, 3c) displayed attenuated cholesterol efflux capacity from THP-1 cells (up to -32%, p<0.01, on a unit phospholipid mass basis) vs.

CONTROLS

Plasma HDL and small, dense HDL3b and 3c subpopulations from STEMI patients exhibited reduced anti-oxidative activity (up to -68%, p<0.05, on a unit HDL mass basis). HDL subpopulations in STEMI were enriched in two proinflammatory bioactive lipids, lysophosphatidylcholine (up to 3.0-fold, p<0.05) and phosphatidic acid (up to 8.4-fold, p<0.05), depleted in apolipoprotein A-I (up to -23%, p<0.05) and enriched in SAA (up to +10.2-fold, p<0.05); such changes were most marked in the HDL3b subfraction. In vitro HDL enrichment in both lysophosphatidylcholine and phosphatidic acid exerted deleterious effects on HDL functionality.

CONCLUSIONS

In the early phase of STEMI, HDL particle subpopulations display marked, concomitant alterations in both lipidome and proteome which are implicated in impaired HDL functionality. Such modifications may act synergistically to confer novel deleterious biological activities to STEMI HDL.

SIGNIFICANCE

Our present data highlight complex changes in the molecular composition and functionality of HDL particle subpopulations in the acute phase of STEMI, and for the first time, reveal that concomitant modifications in both the lipidome and proteome contribute to functional deficiencies in cholesterol efflux and antioxidative activities of HDL particles. These findings may provide new biomarkers and new insights in therapeutic strategy to reduce cardiovascular risk in this clinical setting where such net deficiency in HDL function, multiplied by low circulating HDL concentrations, can be expected to contribute to accelerated atherogenesis.

Signal transduction by HDL: agonists, receptors, and signaling cascades

Numerous epidemiologic studies revealed that high-density lipoprotein (HDL) is an important risk factor for coronary heart disease. There are several well-documented HDL functions such as reversed cholesterol transport, inhibition of inflammation, or inhibition of platelet activation that may account for the atheroprotective effects of this lipoprotein. Mechanistically, these functions are carried out by a direct interaction of HDL particle or its components with receptors localized on the cell surface followed by generation of intracellular signals. Several HDL-associated receptor ligands such as apolipoprotein A-I (apoA-I) or sphingosine-1-phosphate (S1P) have been identified in addition to HDL holoparticles, which interact with surface receptors such as ATP-binding cassette transporter A1 (ABCA1); S1P receptor types 1, 2, and 3 (S1P1, S1P2, and S1P3); or scavenger receptor type I (SR-BI) and activate intracellular signaling cascades encompassing kinases, phospholipases, trimeric and small G-proteins, and cytoskeletal proteins such as actin or junctional protein such as connexin43. In addition, depletion of plasma cell cholesterol mediated by ABCA1, ATP-binding cassette transporter G1 (ABCG1), or SR-BI was demonstrated to indirectly inhibit signaling over proinflammatory or proliferation-stimulating receptors such as Toll-like or growth factor receptors. The present review summarizes the current knowledge regarding the HDL-induced signal transduction and its relevance to athero- and cardioprotective effects as well as other physiological effects exerted by HDL.

Small, dense high-density lipoprotein-3 particles are enriched in negatively charged phospholipids: relevance to cellular cholesterol efflux, antioxidative, antithrombotic, anti-inflammatory, and antiapoptotic functionalities

OBJECTIVE

High-density lipoprotein (HDL) displays multiple atheroprotective activities and is highly heterogeneous in structure, composition, and function; the molecular determinants of atheroprotective functions of HDL are incompletely understood. Because phospholipids represent a major bioactive lipid component of HDL, we characterized the phosphosphingolipidome of major normolipidemic HDL subpopulations and related it to HDL functionality.

APPROACH AND RESULTS

Using an original liquid chromatography-mass spectrometry/mass spectrometry methodology for phospholipid and sphingolipid profiling, 162 individual molecular lipid species were quantified across the 9 lipid subclasses, in the order of decreasing abundance, phosphatidylcholine>sphingomyelin>lysophosphatidylcholine>phosphatidylethanolamine>phosphatidylinositol>ceramide>phosphatidylserine>phosphatidylglycerol>phosphatidic acid. When data were expressed relative to total lipid, the contents of lysophosphatidylcholine and of negatively charged phosphatidylserine and phosphatidic acid increased progressively with increase in hydrated density of HDL, whereas the proportions of sphingomyelin and ceramide decreased. Key biological activities of HDL subpopulations, notably cholesterol efflux capacity from human THP-1 macrophages, antioxidative activity toward low-density lipoprotein oxidation, antithrombotic activity in human platelets, cell-free anti-inflammatory activity, and antiapoptotic activity in endothelial cells, were predominantly associated with small, dense, protein-rich HDL3. The biological activities of HDL particles were strongly intercorrelated, exhibiting significant correlations with multiple components of the HDL phosphosphingolipidome. Specifically, the content of phosphatidylserine revealed positive correlations with all metrics of HDL functionality, reflecting enrichment of phosphatidylserine in small, dense HDL3.

CONCLUSIONS

Our structure-function analysis thereby reveals that the HDL lipidome may strongly affect atheroprotective functionality.

Sphingomyelin in high-density lipoproteins: structural role and biological function

High-density lipoprotein (HDL) levels are an inverse risk factor for cardiovascular diseases, and sphingomyelin (SM) is the second most abundant phospholipid component and the major sphingolipid in HDL. Considering the marked presence of SM, the present review has focused on the current knowledge about this phospholipid by addressing its variable distribution among HDL lipoparticles, how they acquire this phospholipid, and the important role that SM plays in regulating their fluidity and cholesterol efflux from different cells. In addition, plasma enzymes involved in HDL metabolism such as lecithin-cholesterol acyltransferase or phospholipid transfer protein are inhibited by HDL SM content. Likewise, HDL SM levels are influenced by dietary maneuvers (source of protein or fat), drugs (statins or diuretics) and modified in diseases such as diabetes, renal failure or Niemann-Pick disease. Furthermore, increased levels of HDL SM have been shown to be an inverse risk factor for coronary heart disease. The complexity of SM species, described using new lipidomic methodologies, and their distribution in different HDL particles under many experimental conditions are promising avenues for further research in the future.

Effect of apoA-I on cholesterol release and apoE secretion in human mature adipocytes

Background

The risk of cardiovascular disease is inversely correlated to level of plasma HDL-c. Moreover, reverse cholesterol transport (RCT) from peripheral tissues to the liver is the most widely accepted mechanism linked to the anti-atherosclerotic activity of HDL. The apolipoprotein A-I (apoA-I) and the ABC transporters play a key role in this process.

Adipose tissue constitutes the body's largest pool of free cholesterol. The adipose cell could therefore be regarded as a key factor in cholesterol homeostasis. The present study investigates the capacity of primary cultures of mature human adipocytes to release cholesterol and explores the relationships between apoA-I, ABCA1, and apoE as well as the signaling pathways that could be potentially involved.

Results

We demonstrate that apoA-I induces a strong increase in cholesterol release and apoE secretion from adipocytes, whereas it has no transcriptional effect on ABCA1 or apoE genes. Furthermore, brefeldin A (BFA), an intracellular trafficking inhibitor, reduces basal cholesterol and apoE secretion, but does not modify induction by apoA-I. The use of statins also demonstrates that apoA-I stimulated cholesterol release is independent of HMG-CoA reductase activation.

Conclusion

Our work highlights the fact that adipose tissue, and particularly adipocytes, may largely contribute to RCT via a mechanism specifically regulated within these cells. This further supports the argument that adipose tissue must be regarded as a major factor in the development of cardiovascular diseases, in particular atherosclerosis.

Replacement of dietary saturated FAs by PUFAs in diet and reverse cholesterol transport

Dietary intervention is the first and usually successful approach in the treatment of high LDL cholesterol (LDL-C) concentration, but it is frequently accompanied by a decrease in HDL concentration. We studied 14 male volunteers on two different diets, high saturated fatty acid (SFA) and high PUFA, in a crossover design to test whether a decrease in HDL can affect reverse cholesterol transport from relabeled macrophages. A significant decrease of LDL-C (in mmol/l) after a PUFA diet compared with an SFA diet from 3.15 +/- 0.65 to 2.80 +/- 0.56 (P < 0.01) was accompanied by a significant decrease of HDL cholesterol (HDL-C) (in mmol/l) from 1.21 +/- 0.30 to 1.10 +/- 0.32 (P < 0.05). These changes did not affect cholesterol efflux (CHE) from macrophages (9.74 +/- 1.46% vs. 9.53 +/- 1.41%). There was no correlation between individual changes of HDL-C and changes of CHE. It is concluded that the decrease of HDL-C after successful dietary intervention of LDL-C is not accompanied by a decrease of CHE.

New insights into the biogenesis of human high-density lipoproteins

PURPOSE OF REVIEW

The interest for the human HDL system was recently revived by the identification of the ABCA1 as a critical component in the formation and maintenance of plasma HDL levels. The present review focuses on recent progress in our understanding of the basic mechanisms underlying HDL biogenesis pathways.

RECENT FINDINGS

Several novel mechanisms governing ABCA1/apoA-I interactions have recently been identified: apolipoprotein A-I activates ABCA1 phosphorylation through the cAMP/protein kinase A-dependent pathway; the majority of ABCA1 exists as a tetramer in human living cell, supporting the concept that the homotetrameric ABCA1 complex constitutes the minimum functional unit for the formation of nascent HDL particles; apolipoprotein A-I has been shown to have a recycling retroendocytic pathway with uptake and resecretion of the lipidated nascent HDL particles by the cell, most likely through the ABCA1 transporter pathway; there is evidence that the speciation of nascent HDL into pre-beta and alpha-HDL is linked to specific cell lines, and occurs by both ABCA1-dependent and independent pathways.

SUMMARY

The fundamental mechanisms underlying the biogenesis, speciation and maturation of HDL remain complex and not well understood. Understanding the mechanisms governing HDL genesis at the cellular level could provide novel insights into the human atheroprotective system in health and disease.

Lipid efflux by the ATP-binding cassette transporters ABCA1 and ABCG1

Plasma levels of high-density lipoproteins (HDL) and apolipoprotein A-I (apoA-I) are inversely correlated with the risk of cardiovascular disease. One major atheroprotective mechanism of HDL and apoA-I is their role in reverse cholesterol transport, i.e., the transport of excess cholesterol from foam cells to the liver for secretion. The ATP-binding cassette transporters ABCA1 and ABCG1 play a pivotal role in this process by effluxing lipids from foam cells to apoA-I and HDL, respectively. In the liver, ABCA1 activity is one rate-limiting step in the formation of HDL. In macrophages, ABCA1 and ABCG1 prevent the excessive accumulation of lipids and thereby protect the arteries from developing atherosclerotic lesions. However, the mechanisms by which ABCA1 and ABCG1 mediate lipid removal are still unclear. Particularly, three questions remain controversial and are discussed in this review: (1) Do apoA-I and HDL directly interact with ABCA1 and ABCG1, respectively? (2) Does cholesterol efflux involve retroendocytosis of apoA-I or HDL? (3) Which lipids are directly transported by ABCA1 and ABCG1?

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.

Lipid efflux in human and mouse macrophagic cells: evidence for differential regulation of phospholipid and cholesterol efflux

ABCA1 is a critical regulator of lipid efflux from cells, which is highly regulated at the transcriptional and posttranslational levels. However, cells from different species and different tissues, and primary versus immortalized cells, show different modes of regulation. We have carried out a comparative analysis of basic signaling pathways of lipid efflux in mouse J774 cells, mouse peritoneal macrophages (MPMs), human THP-1 cells, and human monocyte-derived macrophages. Cyclic AMP (cAMP) was a potent stimulator of lipid efflux in mouse macrophages, but not in human macrophages. Moreover, this cAMP-inducible component of efflux from MPMs was inhibitable by H89 [a protein kinase A (PKA) inhibitor], but H89 did not affect basal efflux. On the other hand, cAMP failed to show any stimulatory effect in human macrophages, but basal efflux was inhibitable by H89. In MPMs and THP-1 cells, protein kinase C (PKC) inhibitors blocked cholesterol efflux but had no effect on phospholipid efflux, demonstrating the separation of the regulation of phospholipid efflux and cholesterol efflux in macrophages. We conclude that: 1) cAMP regulates lipid efflux predominantly in a PKA-dependent fashion; 2) cholesterol efflux is modulated by a PKC-dependent mechanism; and 3) mouse and human macrophages exhibit different modes of regulation of lipid efflux.

Mechanism of platelet-derived growth factor-dependent caveolin-1 phosphorylation: relationship to sterol binding and the role of serine-80

In human vascular smooth muscle cells, inhibitors of protein kinase C activity reduced serine phosphorylation of caveolin-1 and increased sterol binding by this protein. This was measured after immunoprecipitation of caveolin-1 from cells labeled with tritiated cholesterol or the photoactivable cholesterol analogue FCBP [Fielding et al. (2002) Biochemistry 41, 4929-4937]. At the same time cellular sterol efflux was inhibited. Mutagenesis within a caveolin-1 central domain (residues 80-104) suggested a major role for serine-80 in mediating both of these effects. To perturb sterol binding, platelet-derived growth factor was added to the cells, leading to a transient loss of caveolin-1-associated sterol. Under these conditions, sterol efflux was stimulated, and caveolin-1 phosphorylation at tyrosine(14), assayed with a selective antibody, was substantially increased above baseline levels. These changes were also blocked by inhibitors of protein kinase C activity. Selective inhibitors of the platelet-derived growth factor receptor and downstream kinases were used to show that loss of sterol from caveolin-1 preceded tyrosine phosphorylation, but relipidation was dependent on phosphotyrosine hydrolysis.

Regulation of ATP-binding cassette transporter A1 transcription by thyroid hormone receptor

Transcriptional regulation of the ATP-binding cassette transporter (ABCA1) gene is complex. It involves multiple transcription start sites and the binding of several different transcription factors to the ABCA1 promoter region. Cholesterol- and oxysterol-mediated up-regulation of ABCA1 transcription includes the binding of the liver X receptor and retinoid X receptor (LXR/RXR) heterodimer to the DR-4 element of the ABCA1 promoter. In this study we show that another nuclear hormone receptor, thyroid hormone receptor (TR), can suppress ABCA1 transcription. Electrophoretic mobility shift assays using both purified proteins and isolated nuclear extracts from primary human fibroblasts and 293T cells demonstrate that the TR/RXR heterodimer is able to bind to the DR-4 element of the ABCA1 promoter. This binding is also demonstrated in vivo by chromatin immunoprecipitation studies. Luciferase assays from 293T cells transfected with TRbeta or LXRalpha expression plasmids show that TR, together with its ligand T3, suppresses ABCA1 transcriptional activity, even in the presence of LXR-activating oxysterols. Finally, competition between TR/RXR and LXR/RXR heterodimers to suppress or activate ABCA1 transcription is shown to be dynamic and dependent on the amount of nuclear receptor present in the cells. These data identify a novel regulatory mechanism for ABCA1 and suggest new strategies to modify its expression.

Triglyceride-rich lipoproteins inhibit cholesterol efflux to apolipoprotein (apo) A1 from human macrophage foam cells

High circulating levels of triglyceride-rich lipoproteins (TGRL) represent an independent risk factor for coronary artery disease. Here, we show that TGRL inhibit the efflux of cholesterol from 'foam cell' macrophages to lipid-poor apolipoprotein (apo) A1, and may thereby inhibit arterial reverse cholesterol transport and promote the formation of atherosclerotic lesions. Human (THP-1) monocyte-derived macrophages were pre-incubated (48 h) with acetylated low-density lipoprotein (AcLDL) to provide a foam cell model of cholesterol efflux to apoA1. Pre-incubation of macrophage 'foam cells' with TGRL (0-200 microg/ml, 0-24 h) inhibited the efflux of exogenously radiolabelled ([3H]), endogenously synthesised ([14C]) and cellular cholesterol mass to lipid-poor apoA1, but not control medium, during a (subsequent) efflux period. This inhibition is dependent upon the length of prior exposure to, and concentration of, TGRL employed, but is independent of changes in intracellular triglyceride accumulation or turnover of the cholesteryl ester pool. Despite the negative impact of TGRL on cholesterol efflux, major proteins involved in this process--namely apoE, ABCA1, SR-B1 and caveolin-1--were unaffected by TGRL pre-incubation, suggesting that exposure to these lipoproteins inhibits an alternate, and possibly novel, anti-atherogenic pathway.

Apolipoprotein A-I activates cellular cAMP signaling through the ABCA1 transporter

It has been suggested that the signal transduction pathway initiated by apoA-I activates key proteins involved in cellular lipid efflux. We investigated apoA-I-mediated cAMP signaling in cultured human fibroblasts induced with (22R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Treatment of stimulated fibroblasts with apoA-I for short periods of time (<or=45 min) increased ATP binding cassette A1 (ABCA1) phosphorylation in a concentration-dependent manner. Concomitantly, apoA-I increased the intracellular level of cAMP in a concentration- and time-dependent manner. The maximal cAMP level was reached within 10 min at 10 microg/ml apoA-I representing a 1-fold increase. The ability of apoA-I to mediate cAMP production was only observed in stimulated fibroblasts. Furthermore, overexpression of ABCA1 in Chinese hamster ovary cells resulted in a 1.5-fold increase in apoA-I-mediated cAMP accumulation as compared with untransfected cells. In contrast, forskolin increased cAMP production significantly in unstimulated fibroblasts as well as in untransfected Chinese hamster ovary cells. Pharmacological inhibition of protein kinase A (H89) completely blocked apoA-I-mediated ABCA1 phosphorylation. Naturally occurring mutations of ABCA1 associated with Tangier disease (C1477R, 2203X, and 2145X) severely reduced apoA-I-mediated cAMP production, ABCA1 phosphorylation, (125)I-apoA-I binding, and lipid efflux, without affecting forskolin-mediated cAMP elevation. In contrast, the protein kinase A catalytic subunit was able to phosphorylate ABCA1 similarly from mutant and normal cell lines in vitro. Together, our results indicate that apoA-I activates ABCA1 phosphorylation through the cAMP/protein kinase A-dependent pathway, apoA-I-mediated cAMP production required high level expression of functional ABCA1, and Tangier disease mutants have defective apoA-I-mediated cAMP signaling. These findings suggest that apoA-I may activate cAMP signaling through G protein-coupled ABCA1 transporter.

Apolipoprotein A-I activates protein kinase C alpha signaling to phosphorylate and stabilize ATP binding cassette transporter A1 for the high density lipoprotein assembly

ATP-binding cassette transporter A1 (ABCA1) plays an essential role in the helical apolipoprotein-mediated assembly of high density lipoprotein, and the apolipoporteins stabilize ABCA1 against calpain-mediated degradation during the reaction ((2002) J. Biol. Chem. 277, 22426-22429). Protein kinase C (PKC) inhibitors suppressed both ABCA1 stabilization and cellular lipid release mediated by apolipoprotein A-I (apoA-I) but not ABCA1 increase by calpain inhibitors. The increase of ABCA1 and the cellular lipid release by apoA-I were both suppressed by a phosphatidylcholine phospholipase C (PC-PLC) inhibitor but not by the inhibitors of phosphatidylinositol-PLC and phosphatidylinositol 3-kinase. A protein phosphatase inhibitor further enhanced the ABCA1 increase by apoA-I. Biochemical and microscopic evidence indicated that apoA-I activated PKC alpha, and phosphorylation of ABCA1 was directly demonstrated by apoA-I via PKC. Finally, digestion of sphingomyelin increased ABCA1, and a PC-PLC inhibitor suppressed it. We conclude that apoA-I activates PKC alpha by PC-PLC-mediated generation of diacylglycerol initiated by the removal of cellular sphingomyelin ((2002) J. Biol. Chem. 277, 44709-44714), and subsequently phosphorylates and stabilizes ABCA1.

Ceramide enhances cholesterol efflux to apolipoprotein A-I by increasing the cell surface presence of ATP-binding cassette transporter A1

It is widely accepted that functional ATP-binding cassette transporter A1 (ABCA1) is critical for the formation of nascent high density lipoprotein particles. However, the cholesterol pool(s) and the cellular signaling processes utilized by the ABCA1-mediated pathway remain unclear. Sphingomyelin maintains a preferential interaction with cholesterol in membranes, and its catabolites, especially ceramide, are potent signaling molecules that could play a role in ABCA1 regulation or function. To study the potential role of ceramide in this process, we treated a variety of cell lines with 20 microM C2-ceramide and examined apolipoprotein-mediated cholesterol efflux to lipid-free apoA-I. We found that cell lines expressing ABCA1 displayed 2-3-fold increases in cholesterol efflux to apoA-I. Cell lines not expressing ABCA1 were unaffected by ceramide. We further characterized the cholesterol efflux effect in Chinese hamster ovary cells. Ceramide treatment did not cause significant cytotoxicity or apoptosis and did not affect cholesterol efflux to non-apolipoprotein acceptors. Raising endogenous ceramide levels increased cholesterol efflux to apoA-I. Using a cell surface biotinylation method, we found that the total cellular ABCA1 and that at the plasma membrane were increased with ceramide treatment. Also ceramide enhanced the binding of fluorescently labeled apoA-I to Chinese hamster ovary cells. These data suggest that ceramide may increase the plasma membrane content of ABCA1, leading to increased apoA-I binding and cholesterol efflux.

Cellular phospholipid and cholesterol efflux in high-density lipoprotein deficiency

BACKGROUND

Prospective studies have examined the relationship between coronary artery disease and low plasma levels of high-density lipoprotein cholesterol (HDL-C).

METHODS AND RESULTS

We investigated the causes of hypoalphalipoproteinemia (HypoA; HDL-C <5th percentile) in 64 subjects (12 women and 52 men). Apolipoprotein AI-mediated cellular cholesterol and phospholipid efflux were measured in fibroblasts from HypoA subjects, 9 controls, 2 patients with Tangier disease, and 5 patients with hyperalphalipoproteinemia. A phospholipid efflux defect was defined as <70% of controls. Mean HDL-C was 0.49+/-0.21 mmol/L. Cholesterol and phospholipid efflux correlated strongly (r=0.72, P<0.001). Phospholipid efflux and HDL-C (r=0.64, P<0.001) correlated in HypoA subjects. However, phospholipid or cholesterol efflux was no longer a determinant of HDL-C levels at higher levels (> approximately 1.0 mmol/L) of HDL-C. In HypoA subjects, 4 cases of Tangier disease and 6 of familial HDL deficiency (heterozygous Tangier disease) were identified (10 of 64; 16%). In the remaining 54 subjects, mean lipid efflux was not significantly different from controls and subjects with hyperalphalipoproteinemia. A phospholipid efflux defect was identified in 7 additional HypoA subjects, and a cholesterol efflux defect was detected in 11 subjects. In 2 of these subjects, the ABCA1 gene was ruled out as the cause of the efflux defect, while in 3, the low HDL-C trait segregated with the ABCA1 gene locus.

CONCLUSIONS

Lipidation of lipid-poor apolipoprotein AI may not be a major determinant of cholesterol accumulation within more mature HDL particles and increasing cholesterol or phospholipid efflux beyond normal levels may not lead to increase in plasma HDL-C levels. ABCA1 is essential in the initial steps of HDL formation but other plasma events are major modulators of HDL-C levels.

cAMP induces ABCA1 phosphorylation activity and promotes cholesterol efflux from fibroblasts

ATP-binding cassette transporter A1 (ABCA1) plays a crucial role in apoA-I lipidation, a key step in reverse cholesterol transport. cAMP induces apoA-I binding activity and promotes cellular cholesterol efflux. We investigated the role of the cAMP/protein kinase A (PKA) dependent pathway in the regulation of cellular cholesterol efflux. Treatment of normal fibroblasts with 8-bromo-cAMP (8-Br-cAMP) increased significantly apoA-I-mediated cholesterol efflux, with specificity for apoA-I, but not for cyclodextrin. Concomitantly, 8-Br-cAMP increased ABCA1 phosphorylation in a time-dependent manner. Maximum phosphorylation was reached in <10 min, representing a 260% increase compared to basal ABCA1 phosphorylation level. Forskolin, a known cAMP regulator, increased both cellular cholesterol efflux and ABCA1 phosphorylation. In contrast, H-89 PKA inhibitor reduced cellular cholesterol efflux by 70% in a dose-dependent manner and inhibited almost completely ABCA1 phosphorylation. To determine whether naturally occurring mutants of ABCA1 may affect its phosphorylation activity, fibroblasts from subjects with familial HDL deficiency (FHD, heterozygous ABCA1 defect) and Tangier disease (TD, homozygous/compound heterozygous ABCA1 defect) were treated with 8-Br-cAMP or forskolin. Cellular cholesterol efflux and ABCA1 phosphorylation were increased in FHD but not in TD cells. Taken together, these findings provide evidence for a link between the cAMP/PKA-dependent pathway, ABCA1 phosphorylation, and apoA-I mediated cellular cholesterol efflux.

A model for PKC involvement in the pathogenesis of inborn errors of metabolism

A model for the possible involvement of Protein Kinase C (PKC) in the pathogenesis of inborn errors of metabolism has been proposed. According to this model, perturbation of PKC activity by the accumulation of naturally occurring compounds serves as a unifying functional link between genotype and phenotype. Recent reports regarding an increasing number of modulating metabolites, specific PKC-subtypes activities, their effect on transcription factors and gene expression in various diseases and additional PKC-substrates expand the model. A re-examination of the proposed model in view of these reports and, vice versa, a review of these reports in the context of the proposed model reveal some common phenotypic outcomes in inborn errors of fatty acid-, cholesterol- and homocystine-metabolism as well as lysosomal and peroxisomal diseases.

Protein kinase A site-specific phosphorylation regulates ATP-binding cassette A1 (ABCA1)-mediated phospholipid efflux

ATP-binding cassette A1 (ABCA1) is a key mediator of cholesterol and phospholipid efflux to apolipoprotein particles. We show that ABCA1 is a constitutively phosphorylated protein in both RAW macrophages and in a human embryonic kidney cell line expressing ABCA1. Furthermore, we demonstrate that phosphorylation of ABCA1 is mediated by protein kinase A (PKA) or a PKA-like kinase in vivo. Through site-directed mutagenesis studies of consensus PKA phosphorylation sites and in vitro PKA kinase assays, we show that Ser-1042 and Ser-2054, located in the nucleotide binding domains of ABCA1, are major phosphorylation sites for PKA. ApoA-I-dependent phospholipid efflux was decreased significantly by mutation of Ser-2054 alone and Ser-1042/Ser-2054 but was not significantly impaired with Ser-1042 alone. The mechanism by which ABCA1 phosphorylation affected ApoA-I-dependent phospholipid efflux did not involve either alterations in ApoA-I binding or changes in ABCA1 protein stability. These studies demonstrate a novel serine (Ser-2054) on the ABCA1 protein crucial for PKA phosphorylation and for regulation of ABCA1 transporter activity.

ATP-binding cassette transporter A1 and cholesterol trafficking

PURPOSE OF REVIEW

Two hallmarks of cardiovascular disease are the presence of sterol-laden macrophages in the artery wall and reduced plasma HDL levels. A cell membrane protein named ATP-binding cassette transporter A1 (ABCA1) mediates secretion of excess cholesterol from cells into the HDL metabolic pathway. The discovery of ABCA1 in 1999 triggered a deluge of studies conducted to characterize the properties of this important transporter. The present review summarizes the more recent of those studies and evaluates their implications for the role of ABCA1 in cholesterol transport, HDL metabolism, and atherogenesis.

RECENT FINDINGS

Cell culture experiments have shown that ABCA1 transports cholesterol, phospholipids, and other lipophilic molecules across the plasma membrane, where they are picked up by apolipoproteins containing little or no lipids, but the mechanisms involved are still unclear. It is now apparent that factors in addition to sterols modulate ABCA1 expression by diverse transcriptional and post-transcriptional processes. Studies in humans and mice with ABCA1 mutations revealed that the relative activity of ABCA1 determines plasma HDL levels and influences susceptibility to cardiovascular disease. Mouse models are beginning to provide insights into the function of ABCA1 in vivo but are also raising new questions regarding the contribution of ABCA1 to total cholesterol flux.

SUMMARY

Recent studies underscore the critical role of ABCA1 in clearing excess cholesterol from macrophages and generating HDL particles, implicating ABCA1 as an attractive new therapeutic target for treating cardiovascular disease.

ABCA1: regulation, trafficking and association with heteromeric proteins

HDL metabolism is crucial in maintaining cellular cholesterol and phospholipid homeostasis and prevention of atherosclerosis progression. Recent work identified the ATP-binding cassette transporter A1 (ABCA1) as the major regulator of plasma high density lipoprotein (HDL) cholesterol responsible for the removal of excess cholesterol from peripheral cells and tissues. Here we discuss some novel aspects of the ABCA1 network: 1) the cellular pathways involved in cholesterol and phospholipid efflux, 2) regulation of ABCA1, 3) sulfonylurea receptor 1 (SUR1)- or cystic fibrosis transmembrane conductance regulator (CFTR)-like function of ABCA1, 4) interaction of the ABCA1 C-terminus with beta2-syntrophin, 5) ABCA1 modulation of the Rho GTPase Cdc42, 6) localization of ABCA1 in plasma membrane microdomains and intracellular sites, 7) differential effects of prebeta-HDL precursors on ABCA1 mediated alpha-HDL particle formation and 8) ABCA1 in platelets and its relation to phosphatidylserine-flippase activity. A complex regulatory network and additional antiatherogenic features that may depend on the composition of prebeta-HDL precursor particles are believed to coordinate ABCA1 function in reverse cholesterol and phospholipid transport. Distinct prebeta-HDL ligand-specific receptor-clusters are involved that may modulate specific signaling pathways with varying outcomes related to prebeta-HDL particle composition, the cell-type and the cellular response status.

Endocytosis is enhanced in Tangier fibroblasts: possible role of ATP-binding cassette protein A1 in endosomal vesicular transport

A human genetic disorder, Tangier disease, has been linked recently to mutations in ATP-binding cassette protein A1 (ABCA1). In addition to its function in apoprotein A-I-mediated lipid removal, ABCA1 was also shown to be a phosphatidylserine (PS) translocase that facilitates PS exofacial flipping. This PS translocation is crucial for the plasma membrane to produce protrusions enabling the engulfment of apoptotic cells. In this report, we show that ABCA1 also plays a role in endocytosis. Receptor-mediated endocytosis, probed by both transferrin and low density lipoprotein, is up-regulated by more than 50% in homozygous Tangier fibroblasts in comparison with controls. Fluid-phase uptake is increased similarly. We also demonstrate that bulk membrane flow, including lipid endocytosis and exocytosis, is accelerated greatly in Tangier cells. Moreover, endocytosis is similarly enhanced in normal fibroblasts when ABCA1 function is inhibited by glyburide, whereas glyburide has no effect on endocytosis in Tangier cells. In addition, we demonstrate a decreased annexin V binding in Tangier fibroblasts as compared with controls, supporting the notion that PS transmembrane distribution is indeed defective in the presence of ABCA1 mutations. Furthermore, adding a PS analog to the exofacial leaflet of the plasma membrane normalizes endocytosis in Tangier cells. Taken together, these data demonstrate that ABCA1 plays an important role in endocytosis. We speculate that this is related to the PS translocase function of ABCA1. A loss of functional ABCA1, as in the case of Tangier cells, enhances membrane inward bending and facilitates endocytosis.