Stably transfected ABCA1 antisense cell line has decreased ABCA1 mRNA and cAMP-induced cholesterol efflux to apolipoprotein AI and HDL

A critical appraisal of the measurement of serum 'cholesterol efflux capacity' and its use as surrogate marker of risk of cardiovascular disease

The 'cholesterol efflux capacity (CEC)' assay is a simple in vitro measure of the capacities of individual sera to promote the first step of the reverse cholesterol transport pathway, the delivery of cellular cholesterol to plasma HDL. This review describes the cell biology of this model and critically assesses its application as a marker of cardiovascular risk. We describe the pathways for cell cholesterol export, current cell models used in the CEC assay with their limitations and consider the contribution that measurement of serum CEC provides to our understanding of HDL function in vivo.

Cell lipid metabolism modulators 2-bromopalmitate, D609, monensin, U18666A and probucol shift discoidal HDL formation to the smaller-sized particles: implications for the mechanism of HDL assembly

ATP-binding cassette transporter A1 (ABCA1) mediates formation of disc-shaped high-density lipoprotein (HDL) from cell lipid and lipid-free apolipoprotein A-I (apo A-I). Discoidal HDL particles are heterogeneous in physicochemical characteristics for reasons that are understood incompletely. Discoidal lipoprotein particles similar in characteristics and heterogeneity to cell-formed discoidal HDL can be reconstituted from purified lipids and apo A-I by cell-free, physicochemical methods. The heterogeneity of reconstituted HDL (rHDL) is sensitive to the lipid composition of the starting lipid/apo A-I mixture. To determine whether the heterogeneity of cell-formed HDL is similarly sensitive to changes in cell lipids, we investigated four compounds that have well-established effects on cell lipid metabolism and ABCA1-mediated cell cholesterol efflux. 2-Bromopalmitate, D609, monensin and U18666A decreased formation of the larger-sized, but dramatically increased formation of the smaller-sized HDL. 2-Bromopalmitate did not appear to affect ABCA1 activity, subcellular localization or oligomerization, but induced dissolution of the cholesterol-phospholipid complexes in the plasma membrane. Arachidonic and linoleic acids shifted HDL formation to the smaller-sized species. Tangier disease mutations and inhibitors of ABCA1 activity wheat germ agglutinin and AG 490 reduced formation of both larger-sized and smaller-sized HDL. The effect of probucol was similar to the effect of 2-bromopalmitate. Taking rHDL formation as a paradigm, we propose that ABCA1 mutations and activity inhibitors reduce the amount of cell lipid available for HDL formation, and the compounds in the 2-bromopalmitate group and the polyunsaturated fatty acids change cell lipid composition from one that favors formation of the larger-sized HDL particles to one that favors formation of the smaller-sized species.

HDL particle size is a critical determinant of ABCA1-mediated macrophage cellular cholesterol export

RATIONALE

High-density lipoprotein (HDL) is a heterogeneous population of particles. Differences in the capacities of HDL subfractions to remove cellular cholesterol may explain variable correlations between HDL-cholesterol and cardiovascular risk and inform future targets for HDL-related therapies. The ATP binding cassette transporter A1 (ABCA1) facilitates cholesterol efflux to lipid-free apolipoprotein A-I, but the majority of apolipoprotein A-I in the circulation is transported in a lipidated state and ABCA1-dependent efflux to individual HDL subfractions has not been systematically studied.

OBJECTIVE

Our aims were to determine which HDL particle subfractions are most efficient in mediating cellular cholesterol efflux from foam cell macrophages and to identify the cellular cholesterol transporters involved in this process.

METHODS AND RESULTS

We used reconstituted HDL particles of defined size and composition, isolated subfractions of human plasma HDL, cell lines stably expressing ABCA1 or ABCG1, and both mouse and human macrophages in which ABCA1 or ABCG1 expression was deleted. We show that ABCA1 is the major mediator of macrophage cholesterol efflux to HDL, demonstrating most marked efficiency with small, dense HDL subfractions (HDL3b and HDL3c). ABCG1 has a lesser role in cholesterol efflux and a negligible role in efflux to HDL3b and HDL3c subfractions.

CONCLUSIONS

Small, dense HDL subfractions are the most efficient mediators of cholesterol efflux, and ABCA1 mediates cholesterol efflux to small dense HDL and to lipid-free apolipoprotein A-I. HDL-directed therapies should target increasing the concentrations or the cholesterol efflux capacity of small, dense HDL species in vivo.

Factors controlling nascent high-density lipoprotein particle heterogeneity: ATP-binding cassette transporter A1 activity and cell lipid and apolipoprotein AI availability

Nascent high-density lipoprotein (HDL) particles arise in different sizes. We have sought to uncover factors that control this size heterogeneity. Gel filtration, native PAGE, and protein cross-linking were used to analyze the size heterogeneity of nascent HDL produced by BHK-ABCA1, RAW 264.7, J774, and HepG2 cells under different levels of two factors considered as a ratio, the availability of apolipoprotein AI (apoAI) -accessible cell lipid, and concentration of extracellular lipid-free apoAI. Increases in the available cell lipid:apoAI ratio due to either elevated ATP-binding cassette transporter A1 (ABCA1) expression and activity or raised cell density (i.e., increasing numerator) shifted the production of nascent HDL from smaller particles with fewer apoAI molecules per particle and fewer molecules of choline-phospholipid and cholesterol per apoAI molecule to larger particles that contained more apoAI and more lipid per molecule of apoAI. A further shift to larger particles was observed in BHK-ABCA1 cells when the available cell lipid:apoAI ratio was raised still higher by decreasing the apoAI concentration (i.e., the denominator). These changes in nascent HDL biogenesis were reminiscent of the transition that occurs in the size composition of reconstituted HDL in response to an increasing initial lipid:apoAI molar ratio. Thus, the ratio of available cell lipid:apoAI is a fundamental cause of nascent HDL size heterogeneity, and rHDL formation is a good model of nascent HDL biogenesis.—Lyssenko, N. N., Nickel, M., Tang, C., Phillips, M. C. Factors controlling nascent high-density lipoprotein particle heterogeneity: ATP-binding cassette transporter A1 activity and cell lipid and apolipoprotein AI availability.

Apolipoprotein A-I lysine modification: effects on helical content, lipid binding and cholesterol acceptor activity

We examined the role of the positively charged lysine residues in apoAI by chemical modification. Lysine modification by reductive methylation did not alter apoAI's net charge, secondary or tertiary structure as observed by circular dichroism and trytophan fluorescence, respectively, or have much impact on lipid binding or ABCA1-dependent cholesterol acceptor activity. Acetylation of lysine residues lowered the isoelectric point of apoAI, altered its secondary and tertiary structure, and led to a 40% decrease in cholesterol acceptor activity, while maintaining 93% of its lipid binding activity. Exhaustive lysine acetoacetylation lowered apoAI's isoelectric point, profoundly disrupted its secondary and tertiary structure, and led to 90% and 82% reductions in cholesterol acceptor and lipid binding activities, respectively. The dose-dependent acetoacetylation of an increasing proportion of apoAI lysine residues demonstrated that cholesterol acceptor activity was more sensitive to this modification than lipid binding activity, suggesting that apoAI lysine positive charges play an important role in ABCA1 mediated lipid efflux beyond the role needed to maintain alpha-helical content and lipid binding activity.

Identification of the cAMP-responsive enhancer of the murine ABCA1 gene: requirement for CREB1 and STAT3/4 elements

OBJECTIVE

To determine the mechanism by which expression of the murine ABCA1 gene is highly induced by cAMP analogues.

METHODS AND RESULTS

ABCA1 mRNA turnover cannot account for its induction by cAMP. Thus cAMP induction of ABCA1 mRNA occurs at a transcriptional level. Shotgun cloning DNA fragments from the murine ABCA1 locus identified a strong cAMP responsive enhancer located in the first intron, which led to 25- to 100-fold cAMP-mediated induction of reporter gene activity. Deletions and mutations of this enhancer led to the identification a cAMP-responsive element (CRE) that was essential for the cAMP induction. Furthermore, the capacity of this CRE site to mediate the cAMP induction required the presence of a STAT3/4 element located 81 bp away. A dominant-negative CREB expression vector inhibited the cAMP induction of ABCA1, demonstrating that CREB was required for cAMP induction of ABCA1 expression in RAW264.7 cells.

CONCLUSIONS

Phospho-CREB1 controls the cAMP-mediated induction of murine ABCA1 gene expression through a CRE site acting in cooperation with a nearby STAT element. This CRE site is not conserved in the human ABCA1 gene, explaining why human ABCA1 is not strongly stimulated by cAMP analogs.

Tyrosine modification is not required for myeloperoxidase-induced loss of apolipoprotein A-I functional activities

Apolipoprotein A-I (apoAI), the major protein of high density lipoprotein, plays an important role in reverse cholesterol transport via its activity as an ABCA1-dependent acceptor of cellular cholesterol. We reported recently that myeloperoxidase (MPO) modification of apoAI inhibits its ABCA1-dependent cholesterol acceptor activity (Zheng, L., Nukuna, B., Brennan, M. L., Sun, M., Goormastic, M., Settle, M., Schmitt, D., Fu, X., Thomson, L., Fox, P. L., Ischiropoulos, H., Smith, J. D., Kinter, M., and Hazen, S. L. (2004) J. Clin. Invest. 114, 529-541). We also reported that MPO-mediated chlorination preferentially modifies two of the seven tyrosines in apoAI, and loss of parent peptides containing these residues dose-dependently correlates with loss in ABCA1-mediated cholesterol acceptor activity (Zheng, L., Settle, M., Brubaker, G., Schmitt, D., Hazen, S. L., Smith, J. D., and Kinter, M. (2005) J. Biol. Chem. 280, 38-47). To determine whether oxidative modification of apoA-I tyrosine residues was responsible for the MPO-mediated inactivation of cholesterol acceptor activity, we made recombinant apoAI with site-specific substitutions of all seven tyrosine residues to phenylalanine. ApoAI and the tyrosine-free apoAI were equally susceptible to dose-dependent MPO-mediated loss of ABCA1-dependent cholesterol acceptor activity, as well as lipid binding activity. MPO modification altered the migration of apoAI on SDS gels and decreased its alpha-helix content. MPO-induced modification also targeted apoAI tryptophan and lysine residues. Specifically, we detected apoAI tryptophan oxidation to mono- and dihydroxytryptophan and apoAI lysine modification to chlorolysine and 2-aminoadipic acid. Thus, tyrosine modification of apoAI is not required for its MPO-mediated inhibition of cholesterol acceptor activity.

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.

Cyclosporin A traps ABCA1 at the plasma membrane and inhibits ABCA1-mediated lipid efflux to apolipoprotein A-I

OBJECTIVE

ABCA1 mediates cellular cholesterol and phospholipid efflux to apolipoprotein A-I and other apolipoprotein acceptors. In this study, we analyzed the effect of the immunosuppressant cyclosporin A on the ABCA1-mediated lipid effluxes reactions.

METHODS AND RESULTS

Cyclosporin A acted as a potent inhibitor of ABCA1 activity in several cell lines. Using the RAW264.7 mouse macrophage cell line, in which ABCA1 and its associated cholesterol efflux activity are inducible by cAMP analogues, cyclosporin A inhibition of cholesterol efflux to apolipoprotein A-I was rapidly reversible after its removal from the culture media, implying that ABCA1 levels were not drastically reduced by cyclosporin A. In fact, cyclosporin A treatment decreased ABCA1 turnover and yielded a 2-fold increase in cell-surface ABCA1. Despite the increase in cell-surface ABCA1, cyclosporin A decreased apolipoprotein A-I uptake, resecretion, and degradation in RAW cells. Finally, consistent with the inhibition of ABCA1 in vitro, cyclosporin A treatment induced a 33% reduction of high-density lipoprotein (HDL) levels in mice.

CONCLUSIONS

ABCA1 inhibition by cyclosporin A supports a role for ABCA1 endocytic trafficking in ABCA1-mediated lipid efflux and could explain in part the low HDL levels observed in some patients with transplants.

Adenosine A2A receptor occupancy stimulates expression of proteins involved in reverse cholesterol transport and inhibits foam cell formation in macrophages

Transport of cholesterol out of macrophages is critical for prevention of foam cell formation, the first step in the pathogenesis of atherosclerosis. Proteins involved in this process include cholesterol 27-hydroxylase and adenosine 5'-triphosphate-binding cassette transporter A1 (ABCA1). Proinflammatory cytokines and immune complexes (IC) down-regulate cholesterol 27-hydroxylase and impede cholesterol efflux from macrophages, leading to foam cell formation. Prior studies have suggested occupancy of the anti-inflammatory adenosine A2A receptor (A2AR) minimizes early atherosclerotic changes in arteries following injury. We therefore asked whether A2AR occupancy affects macrophage foam cell formation in response to IC and the cytokine interferon-gamma. We found that the selective A2AR agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adenosine (CGS-21680) inhibited foam cell formation in stimulated THP-1 human macrophages, and the effects of CGS-21680 were reversed by the selective A2AR antagonist 4-(2-[7-amino-2-(2-furyl) [1, 2, 4]triazolo[2,3-a] [1, 3, 5]triazin-5-ylamino]ethyl)phenol. In confirmation of the role of A2AR in prevention of foam cell formation, CGS-21680 also inhibited foam cell formation in cultured murine peritoneal macrophages but did not affect foam cell formation in A2AR-deficient mice. Agents that increase foam cell formation also down-regulate cholesterol 27-hydroxylase and ABCA1 expression. Therefore, we determined the effect of A2AR occupancy on expression of these reverse cholesterol transport (RCT) proteins and found that A2AR occupancy stimulates expression of message for both proteins. These results indicate that one mechanism for the antiatherogenic effects of adenosine is stimulation of the expression of proteins involved in RCT. These findings suggest a novel approach to the development of agents that prevent progression of atherosclerosis.

ABCA1 mediates concurrent cholesterol and phospholipid efflux to apolipoprotein A-I

Prior studies provide data supporting the notion that ATP binding cassette transporter A1 (ABCA1) promotes lipid efflux to extracellular acceptors in a two-step process: first, ABCA1 mediates phospholipid efflux to an apolipoprotein, and second, this apolipoprotein-phospholipid complex accepts free cholesterol in an ABCA1-independent manner. In the current study using RAW264.7 cells, ABCA1-mediated free cholesterol and phospholipid efflux to apolipoprotein A-I (apoA-I) were tightly coupled to each other both temporally and after treatment with ABCA1 inhibitors. The time course and temperature dependence of ABCA1-mediated lipid efflux to apoA-I support a role for endocytosis in this process. Cyclodextrin treatment of RAW264.7 cells partially inhibited 8Br-cAMP-induced efflux of free cholesterol and phospholipid to apoA-I. ABCA1-expressing cells are more sensitive to cell damage by high-dose cyclodextrin and vanadate, leading to increased lactate dehydrogenase leakage and phospholipid release even in the absence of the acceptor apoA-I. Finally, we could not reproduce a two-step effect on lipid efflux using conditioned medium from ABCA1-expressing cells pretreated with cyclodextrin.

Transdifferentiation of mouse aortic smooth muscle cells to a macrophage-like state after cholesterol loading

Mouse aortic smooth muscle cells (SMCs) were loaded for 72 h with cholesterol by using cholesterol:methyl-beta-cyclodextrin complexes, leading to approximately 2-fold and approximately 10-fold increases in the contents of total cholesterol and cholesteryl ester, respectively. Foam-cell formation was demonstrated by accumulation of intracellular, Oil Red O-stained lipid droplets. Immunostaining showed decreased protein levels of smooth muscle alpha-actin and alpha-tropomyosin and increased levels of macrophage markers CD68 and Mac-2 antigen. Quantitative real-time RT-PCR revealed that after cholesterol loading, the expression of SMC-related genes alpha-actin, alpha-tropomyosin, myosin heavy chain, and calponin H1 decreased (to 11.5 +/- 0.5%, 29.3 +/- 1.4%, 23.8 +/- 1.4%, and 3.8 +/- 0.5% of unloaded cells, respectively; P < 0.05 for all), whereas expression of macrophage-related genes CD68, Mac-2, and ABCA1 mRNA increased (to 709 +/- 84%, 330 +/- 11%, and 207 +/- 13% of unloaded cells, respectively; P < 0.05 for all), thereby demonstrating that the protein changes were regulated at the mRNA level. Furthermore, these changes were accompanied by a gain in macrophage-like function as assessed by phagocytotic activity. Expression of vascular cell adhesion molecule 1 and monocyte chemoattractant protein 1, known responders to inflammation, were not changed. In conclusion, cholesterol loading of SMC causes phenotypic changes regulated at the mRNA level that result in a transdifferentiation to a macrophage-like state. This finding suggests that not all foam cells in lesions may have a macrophage origin, despite what is indicated by immunostaining for macrophage-related markers. Furthermore, inflammatory changes in foam cells observed in vivo may not be simple consequences of cholesterol accumulation.

ABCG5 and ABCG8 are expressed in gallbladder epithelial cells

Gallbladder epithelial cells (GBEC) are exposed to high biliary cholesterol concentrations on their apical (AP) surface. The mechanisms of cholesterol absorption and efflux by these cells are not known. We hypothesized that ABCG5 and ABCG8 are expressed in GBEC and mediate AP cholesterol efflux. Human gallbladder cDNA expressed message for ABCG5 and ABCG8. Cultured murine GBEC also expressed abcg5 and abcg8 mRNA and protein, as did cultured canine GBEC. Interestingly, treatment with model bile containing supersaturating concentrations of cholesterol, or treatment with LXRalpha/RXR ligands, did not lead to differences in expression of ABCG5 or ABCG8 in the murine or the canine cells. The subcellular localization of ABCG5 and ABCG8 did show alterations, with predominantly intracellular localization at baseline and predominantly AP localization following treatment with model bile or LXRalpha ligand. GBEC therefore express ABCG5 and ABCG8; these sterol transporters may play a role in mediating AP cholesterol efflux in the gallbladder epithelium.

Human plasma platelet-activating factor acetylhydrolase binds to all the murine lipoproteins, conferring protection against oxidative stress

OBJECTIVE

Plasma platelet-activating factor (PAF) acetylhydrolase (AH) is an enzyme bound with lipoproteins that degrades not only PAF but also PAF-like oxidized phospholipids that are proposed to promote atherosclerosis. In this study, we investigated the distribution of PAF-AH protein among lipoprotein classes by using adenovirus-mediated gene transfer in mice, and we examined its effects on lipoprotein oxidation and foam cell formation of macrophages.

METHODS AND RESULTS

Adenovirus-mediated overexpression of PAF-AH in mice resulted in a 76- to 140-fold increase in plasma PAF-AH activity. Contrary to the previous report, overexpressed human PAF-AH protein was bound to very low density lipoprotein, intermediate density lipoprotein, low density lipoprotein, and high density lipoprotein (HDL). All the lipoproteins with overexpressed human PAF-AH revealed more resistance against oxidative stress, which was associated with lower levels in autoantibody against oxidized low density lipoprotein in the plasma. In addition, HDL with human PAF-AH inhibited foam cell formation and facilitated cholesterol efflux in macrophages.

CONCLUSIONS

These results suggest that human plasma PAF-AH exerts an antiatherogenic effect by binding to all the lipoproteins and thereby protecting them from oxidation, producing less proatherogenic lipoproteins and preserving HDL functions.

Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization

After receptor-mediated endocytosis of triglyceride-rich lipoproteins (TRL) into the liver, TRL particles are immediately disintegrated in peripheral endosomal compartments. Whereas core lipids and apoprotein B are delivered for degradation into lysosomes, TRL-derived apoE is efficiently recycled back to the plasma membrane. This is followed by apoE re-secretion and association of apoE with high density lipoproteins (HDL). Because HDL and apoE can independently promote cholesterol efflux, we investigated whether recycling of TRL-derived apoE in human hepatoma cells and fibroblasts could be linked to intracellular cholesterol transport. In this study we demonstrate that HDL(3) does not only act as an extracellular acceptor for recycled apoE but also stimulates the recycling of internalized TRL-derived apoE. Furthermore, radioactive pulse-chase experiments indicate that apoE recycling is accompanied by cholesterol efflux. Confocal imaging reveals co-localization of apoE and cholesterol in early endosome antigen 1-positive endosomes. During apoE re-secretion, HDL(3)-derived apoA-I is found in these early endosome antigen 1, cholesterol-containing endosomes. As shown by time-lapse fluorescence microscopy, apoE recycling involves the intracellular trafficking of apoA-I to pre-existing and TRL-derived apoE/cholesterol-containing endosomes in the periphery. Thus, these studies provide evidence for a new intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism.

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

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

ABCA1-mediated cholesterol efflux is defective in free cholesterol-loaded macrophages. Mechanism involves enhanced ABCA1 degradation in a process requiring full NPC1 activity

In advanced atherosclerosis, macrophage foam cells progressively accumulate large amounts of unesterified or "free" cholesterol (FC), a process that is thought to contribute to foam cell death and lesional necrosis. The cellular consequences of early FC accumulation, including those that lead to further FC accumulation, are poorly understood. In this context, we show that cholesterol and phospholipid efflux mediated by ABCA1, which is initially induced in the cholesterol-loaded macrophage, was inhibited by approximately 80% in pre-toxic FC-loaded macrophages. Cholesterol efflux to HDL(2), which is mediated by a non-ABCA1 pathway, was inhibited by only approximately 20% in FC-loaded macrophages. FC loading led to decreased levels of ABCA1 protein via increased degradation of ABCA1, and not by decreased transcription or translation of AbcA1 mRNA. The decrease in ABCA1 protein occurred relatively early and was not prevented by caspase inhibitors, indicating that it was not a consequence of FC-induced apoptosis. However, inhibition of proteasomal function by lactacystin largely prevented the degradation of ABCA1. Importantly, the FC-induced decrease in ABCA1 function and protein was almost entirely prevented in macrophages that had partial deficiency of npc1 or were exposed to nanomolar concentrations of U18666A, both of which lead to defective cholesterol trafficking to the endoplasmic reticulum, but leave trafficking to the plasma membrane largely intact. Thus, a relatively early event during FC loading of macrophages is increased degradation of ABCA1, which appears to require trafficking of cholesterol to a peripheral cellular site, but not bulk trafficking of excess cholesterol to the plasma membrane. These findings provide new insight into the post-translational regulation of ABCA1 and the pathobiology of the FC-loaded macrophage.

Evaluation of the role of phosphatidylserine translocase activity in ABCA1-mediated lipid efflux

The following two theories for the mechanism of ABCA1 in lipid efflux to apolipoprotein acceptors have been proposed: 1) that ABCA1 directly binds the apolipoprotein ligand and then facilitates lipid efflux and 2) that ABCA1 acts as a phosphatidylserine (PS) translocase, increasing PS levels in the plasma membrane exofacial leaflet, and that this is sufficient to facilitate apolipoprotein binding and lipid assembly. Upon induction of ABCA1 in RAW264.7 cells by cAMP analogues there was a moderate increase in cell surface PS as detected by annexin V binding, whereas apoAI binding was increased more robustly. Apoptosis induced large increases in annexin V and apoAI binding; however, apoptotic cells did not efflux lipids to apoAI. Annexin V did not act as a cholesterol acceptor, and it did not compete for the cholesterol acceptor or cell binding activity of apoAI. ApoAI binds to ABCA1-expressing cells, and with incubation at 37 degrees C apoAI is co-localized within the cells in ABCA1-containing endosomes. Fluorescent recovery after photobleaching demonstrated that apoAI bound to ABCA1-expressing cells was relatively immobile, suggesting that it was bound either directly or indirectly to an integral membrane protein. Although ABCA1 induction was associated with a small increase in cell surface PS, these results argue against the notion that this cell surface PS is sufficient to mediate cellular apoAI binding and lipid efflux.