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

Molecular mechanisms for ABCA1-mediated cholesterol efflux

The maintenance of cellular cholesterol homeostasis is essential for normal cell function and viability. Excessive cholesterol accumulation is detrimental to cells and serves as the molecular basis of many diseases, such as atherosclerosis, Alzheimer's disease, and diabetes mellitus. The peripheral cells do not have the ability to degrade cholesterol. Cholesterol efflux is therefore the only pathway to eliminate excessive cholesterol from these cells. This process is predominantly mediated by ATP-binding cassette transporter A1 (ABCA1), an integral membrane protein. ABCA1 is known to transfer intracellular free cholesterol and phospholipids to apolipoprotein A-I (apoA-I) for generating nascent high-density lipoprotein (nHDL) particles. nHDL can accept more free cholesterol from peripheral cells. Free cholesterol is then converted to cholesteryl ester by lecithin:cholesterol acyltransferase to form mature HDL. HDL-bound cholesterol enters the liver for biliary secretion and fecal excretion. Although how cholesterol is transported by ABCA1 to apoA-I remains incompletely understood, nine models have been proposed to explain this effect. In this review, we focus on the current view of the mechanisms underlying ABCA1-mediated cholesterol efflux to provide an important framework for future investigation and lipid-lowering therapy.

Brothers in Arms: ABCA1- and ABCG1-Mediated Cholesterol Efflux as Promising Targets in Cardiovascular Disease Treatment

Atherosclerosis is a leading cause of cardiovascular disease worldwide, and hypercholesterolemia is a major risk factor. Preventive treatments mainly focus on the effective reduction of low-density lipoprotein cholesterol, but their therapeutic value is limited by the inability to completely normalize atherosclerotic risk, probably due to the disease complexity and multifactorial pathogenesis. Consequently, high-density lipoprotein cholesterol gained much interest, as it appeared to be cardioprotective due to its major role in reverse cholesterol transport (RCT). RCT facilitates removal of cholesterol from peripheral tissues, including atherosclerotic plaques, and its subsequent hepatic clearance into bile. Therefore, RCT is expected to limit plaque formation and progression. Cellular cholesterol efflux is initiated and propagated by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1. Their expression and function are expected to be rate-limiting for cholesterol efflux, which makes them interesting targets to stimulate RCT and lower atherosclerotic risk. This systematic review discusses the molecular mechanisms relevant for RCT and ABCA1 and ABCG1 function, followed by a critical overview of potential pharmacological strategies with small molecules to enhance cellular cholesterol efflux and RCT. These strategies include regulation of ABCA1 and ABCG1 expression, degradation, and mRNA stability. Various small molecules have been demonstrated to increase RCT, but the underlying mechanisms are often not completely understood and are rather unspecific, potentially causing adverse effects. Better understanding of these mechanisms could enable the development of safer drugs to increase RCT and provide more insight into its relation with atherosclerotic risk. SIGNIFICANCE STATEMENT: Hypercholesterolemia is an important risk factor of atherosclerosis, which is a leading pathological mechanism underlying cardiovascular disease. Cholesterol is removed from atherosclerotic plaques and subsequently cleared by the liver into bile. This transport is mediated by high-density lipoprotein particles, to which cholesterol is transferred via ATP-binding cassette transporters ABCA1 and ABCG1. Small-molecule pharmacological strategies stimulating these transporters may provide promising options for cardiovascular disease treatment.

Is ABCA1 a lipid transfer protein?

ABCA1 functions as a lipid transporter because it mediates the transfer of cellular phospholipid (PL) and free (unesterified) cholesterol (FC) to apoA-I and related proteins present in the extracellular medium. ABCA1 is a membrane PL translocase and its enzymatic activity leads to transfer of PL molecules from the cytoplasmic leaflet to the exofacial leaflet of a cell plasma membrane (PM). The presence of active ABCA1 in the PM promotes binding of apoA-I to the cell surface. About 10% of this bound apoA-I interacts directly with ABCA1 and stabilizes the transporter. Most of the pool of cell surface-associated apoA-I is bound to lipid domains in the PM that are created by the activity of ABCA1. The amphipathic α-helices in apoA-I confer detergent-like properties on the protein enabling it to solubilize PL and FC in these membrane domains to create a heterogeneous population of discoidal nascent HDL particles. This review focuses on current understanding of the structure-function relationships of human ABCA1 and the molecular mechanisms underlying HDL particle production.

Exposure to High Glucose Concentration Decreases Cell Surface ABCA1 and HDL Biogenesis in Hepatocytes

Aim: To study atherosclerosis risk in diabetes, we investigated ATP-binding cassette transporter A1 (ABCA1) expression and high-density lipoprotein (HDL) biogenesis in the liver and hepatocytes under hyperglycemic conditions.

Methods and Results: In streptozotocin-induced diabetic mice, plasma HDL decreased while ABCA1 protein increased without changing its mRNA in the liver, only in the animals that responded to the treatment to show hypoinsulinemia and fasting hyperglycemia but not in the poor responders not showing those. To study the mechanism for this finding, hepatocytes were isolated from the control and diabetic mice, and they showed no difference in expression of ABCA1 protein, its mRNA, and HDL biogenesis in 1 g/l d-glucose but showed decreased HDL biogenesis in 4.5 g/l d-glucose although ABCA1 protein increased without change in its mRNA. Similar findings were confirmed in HepG2 cells with d-glucose but not with l-glucose. Thus, these cell models reproduced the in vivo findings in hyperglycemia. Labeling of cell surface protein revealed that surface ABCA1 decreased in high concentration of d-glucose in HepG2 cells despite the increase of cellular ABCA1 while not with l-glucose. Immunostaining of ABCA1 in HepG2 cells demonstrated the decrease of surface ABCA1 but increase of intracellular ABCA1 with high d-glucose. Clearance of ABCA1 was retarded both in primary hepatocytes and HepG2 cells exposed to high d-glucose but not to l-glucose, being consistent with the decrease of surface ABCA1.

Conclusions: It is suggested that localization of ABCA1 to the cell surface is decreased in hepatocytes in hyperglycemic condition to cause decrease of HDL biogenesis.

Lipid-free apoA-I structure - Origins of model diversity

Apolipoprotein A-I (apoA-I) is a prominent member of the exchangeable apolipoprotein class of proteins, capable of transitioning between lipid-bound and lipid-free states. It is the primary structural and functional protein of high density lipoprotein (HDL). Lipid-free apoA-I is critical to de novo HDL formation as it is the preferred substrate of the lipid transporter, ATP Binding Cassette Transporter A1 (ABCA1) Remaley et al. (2001) [1]. Lipid-free apoA-I is an important element in reverse cholesterol transport and comprehension of its structure is a core issue in our understanding of cholesterol metabolism. However, lipid-free apoA-I is highly conformationally dynamic making it a challenging subject for structural analysis. Over the past 20years there have been significant advances in overcoming the dynamic nature of lipid-free apoA-I, which have resulted in a multitude of proposed conformational models.

ABCA1 (ATP-Binding Cassette Transporter A1) Mediates ApoA-I (Apolipoprotein A-I) and ApoA-I Mimetic Peptide Mobilization of Extracellular Cholesterol Microdomains Deposited by Macrophages

OBJECTIVE

We examined the function of ABCA1 (ATP-binding cassette transporter A1) in ApoA-I (apolipoprotein A-I) mobilization of cholesterol microdomains deposited into the extracellular matrix by cholesterol-enriched macrophages. We have also determined whether an ApoA-I mimetic peptide without and with complexing to sphingomyelin can mobilize macrophage-deposited cholesterol microdomains.

APPROACH AND RESULTS

Extracellular cholesterol microdomains deposited by cholesterol-enriched macrophages were detected with a monoclonal antibody, 58B1. ApoA-I and an ApoA-I mimetic peptide 5A mobilized cholesterol microdomains deposited by ABCA1^+/+ macrophages but not by ABCA1^-/- macrophages. In contrast, ApoA-I mimetic peptide 5A complexed with sphingomyelin could mobilize cholesterol microdomains deposited by ABCA1^-/- macrophages.

CONCLUSIONS

Our findings show that a unique pool of extracellular cholesterol microdomains deposited by macrophages can be mobilized by both ApoA-I and an ApoA-I mimetic peptide but that mobilization depends on macrophage ABCA1. It is known that ABCA1 complexes ApoA-I and ApoA-I mimetic peptide with phospholipid, a cholesterol-solubilizing agent, explaining the requirement for ABCA1 in extracellular cholesterol microdomain mobilization. Importantly, ApoA-I mimetic peptide already complexed with phospholipid can mobilize macrophage-deposited extracellular cholesterol microdomains even in the absence of ABCA1.

Small GTPase ARF6 Regulates Endocytic Pathway Leading to Degradation of ATP-Binding Cassette Transporter A1

OBJECTIVE

ABCA1 (ATP-binding cassette transporter A1) is the principal protein responsible for cellular cholesterol efflux. Abundance and functionality of ABCA1 is regulated both transcriptionally and post-translationally, with endocytosis of ABCA1 being an important element of post-translational regulation. Functional ABCA1 resides on the plasma membrane but can be internalized and either degraded or recycled back to the plasma membrane. The interaction between the degradative and recycling pathways determines the abundance of ABCA1 and may contribute to the efflux of intracellular cholesterol.

APPROACH AND RESULTS

Here, we show that the principal pathway responsible for the internalization of ABCA1 leading to its degradation in macrophages is ARF6-dependent endocytic pathway. This pathway was predominant in the regulation of ABCA1 abundance and efflux of plasma membrane cholesterol. Conversely, the efflux of intracellular cholesterol was predominantly controlled by ARF6-independent pathways, and inhibition of ARF6 shifted ABCA1 into recycling endosomes enhancing efflux of intracellular cholesterol.

CONCLUSIONS

We conclude that ARF6-dependent pathway is the predominant route responsible for the ABCA1 internalization and degradation, whereas ARF6-independent endocytic pathways may contribute to ABCA1 recycling and efflux of intracellular cholesterol.

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.

The unsolved mystery of apoA-I recycling in adipocyte

As the major storage site for triglycerides and free cholesterol, adipose tissue plays a central role in energy metabolism. ApoA-I is the main constituent of HDL and plays an important role in removal of excess cholesterol from peripheral tissues. Recently, multiple studies have shown beneficial effects of apoA-I on adipose metabolism and function. ApoA-I was reported to improve insulin sensitivity and exert anti-inflammatory, anti-obesity effect in animal studies. Interestingly, Uptake and resecretion of apoA-I by adipocytes has been detected. However, the significance of apoA-I recycling by adipocytes is still not clear. This article reviewed methods used to study cellular recycling of apoA-I and summarized the current knowledge on the mechanisms involved in apoA-I uptake by adipocytes. Since the main function of apoA-I is to mediate reverse cholesterol transport from peripheral tissues, the role of apoA-I internalization and re-secretion by adipocytes in intracellular cholesterol transport under physiological and pathological conditions were discussed. In addition, findings on the correlation between apoA-I recycling and obesity were discussed. Finally, it was proposed that during intracellular transport, apoA-I-protein complex may acquire cargoes other than lipids and deliver regulatory information when they were resecreted into the plasma. Although apoA-I recycling by adipocytes is still an unsolved mystery, it's likely that it is more than a redundant pathway especially under pathological conditions.

ABCA1-dependent sterol release: sterol molecule specificity and potential membrane domain for HDL biogenesis

Mammalian cells synthesize various sterol molecules, including the C30 sterol, lanosterol, as cholesterol precursors in the endoplasmic reticulum. The build-up of precursor sterols, including lanosterol, displays cellular toxicity. Precursor sterols are found in plasma HDL. How these structurally different sterols are released from cells is poorly understood. Here, we show that newly synthesized precursor sterols arriving at the plasma membrane (PM) are removed by extracellular apoA-I in a manner dependent on ABCA1, a key macromolecule for HDL biogenesis. Analysis of sterol molecules by GC-MS and tracing the fate of radiolabeled acetate-derived sterols in normal and mutant Niemann-Pick type C cells reveal that ABCA1 prefers newly synthesized sterols, especially lanosterol, as the substrates before they are internalized from the PM. We also show that ABCA1 resides in a cholesterol-rich membrane domain resistant to the mild detergent, Brij 98. Blocking ACAT activity increases the cholesterol contents of this domain. Newly synthesized C29/C30 sterols are transiently enriched within this domain, but rapidly disappear from this domain with a half-life of less than 1 h. Our work shows that substantial amounts of precursor sterols are transported to a certain PM domain and are removed by the ABCA1-dependent pathway.

ABCA1 and nascent HDL biogenesis

ABCA1 mediates the secretion of cellular free cholesterol and phospholipids to an extracellular acceptor, apolipoprotein AI, to form nascent high-density lipoprotein (HDL). Thus, ABCA1 is a key molecule in cholesterol homeostasis. Functional studies of certain Tangier disease mutations demonstrate that ABCA1 has multiple activities, including plasma membrane remodeling and apoAI binding to cell surface, which participate in nascent HDL biogenesis. Recent advances in our understanding of ABCA1 have demonstrated that ABCA1also mediates unfolding the N terminus of apoAI on the cell surface, followed by lipidation of apoAI and release of nascent HDL. Although ABCA1-mediated cholesterol efflux to apoAI can occur on the plasma membrane, the role of apoAI retroendocytosis during cholesterol efflux may play a role in macrophage foam cells that store cholesterol esters in cytoplasmic lipid droplets.

Molecular mechanisms of cellular cholesterol efflux

Most types of cells in the body do not express the capability of catabolizing cholesterol, so cholesterol efflux is essential for homeostasis. For instance, macrophages possess four pathways for exporting free (unesterified) cholesterol to extracellular high density lipoprotein (HDL). The passive processes include simple diffusion via the aqueous phase and facilitated diffusion mediated by scavenger receptor class B, type 1 (SR-BI). Active pathways are mediated by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1, which are membrane lipid translocases. The efflux of cellular phospholipid and free cholesterol to apolipoprotein A-I promoted by ABCA1 is essential for HDL biogenesis. Current understanding of the molecular mechanisms involved in these four efflux pathways is presented in this minireview.

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.

Induction of apolipoprotein A-I gene expression by glucagon-like peptide-1 and exendin-4 in hepatocytes but not intestinal cells

OBJECTIVE

Diabetic dyslipidemia is an important risk factor for the development of macrovascular complications. Recent clinical trials suggest that diabetics treated with glucagon-like peptide-1 (GLP-1) have normalized lipid levels, including an increase in plasma high-density lipoprotein cholesterol (HDLc) levels.

METHODS

To determine if GLP-1 (7-36 amide) and the GLP-1-like insulinotropic peptide exendin-4 regulate expression of apolipoprotein A-I (apo A-I), the primary anti-atherogenic component of high-density lipoprotein (HDL), HepG2 hepatocytes and Caco-2 intestinal cells, representative of tissues that express the majority of apo A-I, were treated with increasing amounts of each peptide and apo A-I gene expression was measured in the conditioned medium.

RESULTS

Apo A-I secretion increased in both GLP-1 and exendin-4-treated HepG2, but not Caco-2 cells, and this was accompanied by similar changes in apo A-I mRNA levels and apo A-I promoter activity. Induction of apo A-I promoter activity by GLP-1 and exendin-4 required an SP1-responsive element. Hepatic ATP binding cassette protein A1 (ABCA1) expression, but not scavenger receptor class B type1 receptor expression was also induced by GLP-1 and exendin-4.

CONCLUSIONS

These results suggest that GLP-1- and exendin-4-mediated changes in HDLc are likely due to changes in hepatic expression of apo A-I and ABCA1.

Cyclosporine A and PSC833 inhibit ABCA1 function via direct binding

ATP-binding cassette protein A1 (ABCA1) plays a key role in generating high-density lipoprotein (HDL). However, the detailed mechanism of HDL formation remains unclear; in order to reveal it, chemicals that specifically block each step of HDL formation would be useful. Cyclosporine A inhibits ABCA1-mediated cholesterol efflux, but it is not clear whether this is mediated via inhibition of calcineurin. We analyzed the effects of cyclosporine A and related compounds on ABCA1 function in BHK/ABCA1 cells. Cyclosporine A, FK506, and pimecrolimus inhibited ABCA1-mediated cholesterol efflux in a concentration-dependent manner, with IC(50) of 7.6, 13.6, and 7.0μM, respectively. An mTOR inhibitor, rapamycin also inhibited ABCA1, with IC(50) of 18.8μM. The primary targets for these drugs were inhibited at much lower concentrations in BHK/ABCA1 cells, suggesting that they were not involved. Binding of [(3)H] cyclosporine A to purified ABCA1 could be clearly detected. Furthermore, a non-immunosuppressive cyclosporine, PSC833, inhibited ABCA1-mediated cholesterol efflux with IC(50) of 1.9μM, and efficiently competed with [(3)H] cyclosporine A binding to ABCA1. These results indicate that cyclosporine A and PSC833 inhibit ABCA1 via direct binding, and that the ABCA1 inhibitor PSC833 is an excellent candidate for further investigations of the detailed mechanisms underlying formation of HDL.

The interaction of ApoA-I and ABCA1 triggers signal transduction pathways to mediate efflux of cellular lipids

Reverse cholesterol transport (RCT) has been characterized as a crucial step for antiatherosclerosis, which is initiated by ATP-binding cassette A1 (ABCA1) to mediate the efflux of cellular phospholipids and cholesterol to lipid-free apolipoprotein A-I (apoA-I). However, the mechanisms underlying apoA-I/ABCA1 interaction to lead to the lipidation of apoA-I are poorly understood. There are several models proposed for the interaction of apoA-I with ABCA1 as well as the lipidation of apoA-I mediated by ABCA1. ApoA-I increases the levels of ABCA1 protein markedly. In turn, ABCA1 can stabilize apoA-I. The interaction of apoA-I with ABCA1 could activate signaling molecules that modulate posttranslational ABCA1 activity or lipid transport activity. The key signaling molecules in these processes include protein kinase A (PKA), protein kinase C (PKC), Janus kinase 2 (JAK2), Rho GTPases and Ca²⁺, and many factors also could influence the interaction of apoA-I with ABCA1. This review will summarize these mechanisms for the apoA-I interaction with ABCA1 as well as the signal transduction pathways involved in these processes.

A novel compound inhibits reconstituted high-density lipoprotein assembly and blocks nascent high-density lipoprotein biogenesis downstream of apolipoprotein AI binding to ATP-binding cassette transporter A1-expressing cells

OBJECTIVE

Nascent high-density lipoprotein (HDL) particles form from cellular lipids and extracellular lipid-free apolipoprotein AI (apoAI) in a process mediated by ATP-binding cassette transporter A1 (ABCA1). We have sought out compounds that inhibit nascent HDL biogenesis without affecting ABCA1 activity.

METHODS AND RESULTS

Reconstituted HDL (rHDL) formation and cellular cholesterol efflux assays were used to show that 2 compounds that bond via hydrogen with phospholipids inhibit rHDL and nascent HDL production. In rHDL formation assays, the inhibitory effect of compound 1 (methyl 3α-acetoxy-7α,12α-di[(phenylaminocarbonyl)amino]-5β-cholan-24-oate), the more active of the 2, depended on its ability to associate with phospholipids. In cell assays, compound 1 suppressed ABCA1-mediated cholesterol efflux to apoAI, the 18A peptide, and taurocholate with high specificity, without affecting ABCA1-independent cellular cholesterol efflux to HDL and endocytosis of acetylated low-density lipoprotein and transferrin. Furthermore, compound 1 did not affect ABCA1 activity adversely, as ABCA1-mediated shedding of microparticles proceeded unabated and apoAI binding to ABCA1-expressing cells increased in its presence.

CONCLUSION

The inhibitory effects of compound 1 support a 3-step model of nascent HDL biogenesis: plasma membrane remodeling by ABCA1, apoAI binding to ABCA1, and lipoprotein particle assembly. The compound inhibits the final step, causing accumulation of apoAI in ABCA1-expressing cells.

Cytoskeleton disruption in J774 macrophages: consequences for lipid droplet formation and cholesterol flux

Macrophages store excess unesterified cholesterol (free, FC) in the form of cholesteryl ester (CE) in cytoplasmic lipid droplets. The hydrolysis of droplet-CE in peripheral foam cells is critical to HDL-promoted reverse cholesterol transport because it represents the first step in cellular cholesterol clearance, as only FC is effluxed from cells to HDL. Cytoplasmic lipid droplets move within the cell utilizing the cytoskeletal network, but, little is known about the influence of the cytoskeleton on lipid droplet formation. To understand this role we employed cytochalasin D (cyt.D) to promote actin depolymerization in J774 macrophages. Incubating J774 with acetylated LDL creates foam cells having a 4-fold increase in cellular cholesterol content (30-40% cholesterol present as cholesteryl ester (CE)) in cytoplasmic droplets. Lipid droplets formed in the presence of cyt.D are smaller in diameter. CE-deposition and -hydrolysis are decreased when cells are cholesterol-enriched in the presence of cyt.D or latrunculin A, another cytoskeleton disrupting agent. However, when lipid droplets formed in the presence of cyt.D are isolated and incubated with an exogenous CE hydrolase, the CE is more rapidly metabolized compared to droplets from control cells. This is apparently due to the smaller size and altered lipid composition of the droplets formed in the presence of cyt.D. Cytoskeletal proteins found on CE droplets influence droplet lipid composition and maturation in model foam cells. In J774 macrophages, cytoskeletal proteins are apparently involved in facilitating the interaction of lipid droplets and a cytosolic neutral CE hydrolase and may play a role in foam cell formation. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Update on HDL receptors and cellular cholesterol transport

Efflux is central to maintenance of tissue and whole body cholesterol homeostasis. The discovery of cell surface receptors that bind high-density lipoprotein (HDL) with high specificity and affinity to promote cholesterol release has significantly advanced our understanding of cholesterol efflux. We now know that 1) cells have several mechanisms to promote cholesterol release, including a passive mechanism that depends on the physico-chemical properties of cholesterol molecules and their interactions with phospholipids; 2) a variety of HDL particles can interact with receptors to promote cholesterol transport from tissues to the liver for excretion; and 3) interactions between HDL and receptors show functional synergy. Therefore, efflux efficiency depends both on the arrays of receptors on tissue cells and HDL particles in serum.

Regulation of ABCA1 functions by signaling pathways

ATP-binding cassette transporter A1 (ABCA1) is an integral cell membrane protein that protects cardiovascular disease by at least two mechanisms: by export of excess cholesterol from cells and by suppression of inflammation. ABCA1 exports cholesterol and phospholipids from cells by multiple steps that involve forming cell surface lipid domains, binding of apolipoproteins to ABCA1, activating signaling pathways, and solubilizing these lipids by apolipoproteins. ABCA1 executes its anti-inflammatory effect by modifying cell membrane lipid rafts and directly activating signaling pathways. The interaction of apolipoproteins with ABCA1 activates multiple signaling pathways, including Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3), protein kinase A, Rho family G protein CDC42 and protein kinase C. Activating protein kinase A and Rho family G protein CDC42 regulates ABCA1-mediated lipid efflux, activating PKC stabilizes ABCA1 protein, and activating JAK2/STAT3 regulates both ABCA1-mediated lipid efflux and anti-inflammation. Thus, ABCA1 behaves both as a lipid exporter and a signaling receptor. Targeting ABCA1 receptor-like property using agonists for ABCA1 protein could become a promising new therapeutic target for increasing ABCA1 function and treating cardiovascular disease. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Caveolin-1-mediated apolipoprotein A-I membrane binding sites are not required for cholesterol efflux

Caveolin-1 (Cav1), a structural protein required for the formation of invaginated membrane domains known as caveolae, has been implicated in cholesterol trafficking and homeostasis. Here we investigated the contribution of Cav1 to apolipoprotein A-I (apoA-I) cell surface binding and intracellular processing using mouse embryonic fibroblasts (MEFs) derived from wild type (WT) or Cav1-deficient (Cav1(-/-)) animals. We found that cells expressing Cav1 have 2.6-fold more apoA-I binding sites than Cav1(-/-) cells although these additional binding sites are not associated with detergent-free lipid rafts. Further, Cav1-mediated binding targets apoA-I for internalization and degradation and these processes are not correlated to cholesterol efflux. Despite lower apoA-I binding, cholesterol efflux from Cav1(-/-) MEFs is 1.7-fold higher than from WT MEFs. Stimulation of ABCA1 expression with an LXR agonist enhances cholesterol efflux from both WT and Cav1(-/-) cells without increasing apoA-I surface binding or affecting apoA-I processing. Our results indicate that there are at least two independent lipid binding sites for apoA-I; Cav1-mediated apoA-I surface binding and uptake is not linked to cholesterol efflux, indicating that membrane domains other than caveolae regulate ABCA1-mediated cholesterol efflux.

Calpain-mediated ABCA1 degradation: post-translational regulation of ABCA1 for HDL biogenesis

Helical apolipoproteins remove cellular phospholipid and cholesterol to generate nascent HDL and this reaction is the major source of plasma HDL. ABCA1 is mandatory and rate-limiting for this reaction. Besides regulation of the gene expression by transcriptional factors including LXR, AP2 and SREBP, the ABCA1 activity is regulated post-translationally by calpain-mediated proteolytic degradation of ABCA1 protein that occurs in the early endosome after its endocytosis. When the HDL biogenesis reaction is ongoing as helical apolipoproteins interact with ABCA1, ABCA1 becomes resistant to calpain and is recycled to cell surface after endocytosis. Biogenesis of HDL is most likely to take place on cell surface. Clearance rate of ABCA1 by this mechanism is also retarded by various factors that interact with ABCA1, such as α1-syntrophin, LXRβ and calmodulin. Physiological relevance of the retardation by these factors is not entirely clear. Pharmacological inhibition of the calpain-mediated ABCA1 degradation results in the increase of the ABCA1 activity and HDL biogenesis in vitro and in vivo, and potentially suppresses atherogenesis. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

ABC proteins protect the human body and maintain optimal health

Human MDR1, a multi-drug transporter gene, was isolated as the first of the eukaryote ATP Binding Cassette (ABC) proteins from a multidrug-resistant carcinoma cell line in 1986. To date, over 25 years, many ABC proteins have been found to play important physiological roles by transporting hydrophobic compounds. Defects in their functions cause various diseases, indicating that endogenous hydrophobic compounds, as well as water-soluble compounds, are properly transported by transmembrane proteins. MDR1 transports a large number of structurally unrelated drugs and is involved in their pharmacokinetics, and thus is a key factor in drug interaction. ABCA1, an ABC protein, eliminates excess cholesterol in peripheral cells by generating HDL. Because ABCA1 is a key molecule in cholesterol homeostasis, its function and expression are highly regulated. Eukaryote ABC proteins function on the body surface facing the outside and in organ pathways to adapt to the extracellular environment and protect the body to maintain optimal health.

Reverse cholesterol transport: From classical view to new insights

Cholesterol is of vital importance for the human body. It is a constituent for most biological membranes, it is needed for the formation of bile salts, and it is the precursor for steroid hormones and vitamin D. However, the presence of excess cholesterol in cells, and in particular in macrophages in the arterial vessel wall, might be harmful. The accumulation of cholesterol in arteries can lead to atherosclerosis, and in turn, to other cardiovascular diseases. The route that is primarily thought to be responsible for the disposal of cholesterol is called reverse cholesterol transport (RCT). Therefore, RCT is seen as an interesting target for the development of drugs aimed at the prevention of atherosclerosis. Research on RCT has taken off in recent years. In this review, the classical concepts about RCT are discussed, together with new insights about this topic.

The β-subunit of ATP synthase is involved in cellular uptake and resecretion of apoA-I but does not control apoA-I-induced lipid efflux in adipocytes

Cellular uptake and resecretion of apoA-I (apoA-I recycling) could be an important factor in determining the circulating plasma levels of apoA-I and/or HDL. Using a novel method to study protein recycling, we have recently demonstrated recycling of apoA-I by adipocytes and suggested that this is a receptor mediated process independent of ABCA1 function. In the present study, it is shown that apoA-I recycling by adipocytes can be blocked by a monoclonal antibody against the β-subunit of ATP synthase, a protein that had been previously identified as an apoA-I receptor. Investigation of the cellular recycling of two other proteins, an apolipoprotein and a small globular protein, showed that recycling of apoA-I is a selective process. The present study also shows that blocking apoA-I recycling has no effect on the rate of apoA-I-induced cholesterol or phospholipid efflux. It is concluded that cellular recycling of apoA-I is a selective process that involves the ectopically expressed β-subunit of ATP synthase. The physiological function of apoA-I recycling remains to be elucidated. However, this study shows that the process of apoA-I uptake and resecretion is not required for apoA-I lipidation.

Relative roles of various efflux pathways in net cholesterol efflux from macrophage foam cells in atherosclerotic lesions

PURPOSE OF REVIEW

Cholesterol efflux mechanisms are essential for macrophage cholesterol homeostasis. HDL, an important cholesterol efflux acceptor, comprises a class of heterogeneous particles that induce cholesterol efflux via distinct pathways. This review focuses on the understanding of the different cholesterol efflux pathways and physiological acceptors involved, and their regulation in atherosclerotic lesions.

RECENT FINDINGS

The synergistic interactions of ATP-binding cassette transporters A1 and G1 as well as ATP-binding cassette transporter A1 and scavenger receptor class B type I are essential for cellular cholesterol efflux and the prevention of macrophage foam cell formation. However, the importance of aqueous diffusion should also not be underestimated. Significant progress has been made in understanding the mechanisms underlying ATP-binding cassette A1-mediated cholesterol efflux and regulation of its expression and trafficking. Conditions locally in the atherosclerotic lesion, for example, lipids, cytokines, oxidative stress, and hypoxia, as well as systemic factors, including inflammation and diabetes, critically influence the expression of cholesterol transporters on macrophage foam cells. Furthermore, HDL modification and remodeling in atherosclerosis, inflammation, and diabetes impairs its function as an acceptor for cellular cholesterol.

SUMMARY

Recent advances in the understanding of the regulation of cholesterol transporters and their acceptors in atherosclerotic lesions indicate that HDL-based therapies should aim to enhance the activity of cholesterol transporters and improve both the quantity and quality of HDL.

Influence of apolipoprotein (Apo) A-I structure on nascent high density lipoprotein (HDL) particle size distribution

The principal protein of high density lipoprotein (HDL), apolipoprotein (apo) A-I, in the lipid-free state contains two tertiary structure domains comprising an N-terminal helix bundle and a less organized C-terminal domain. It is not known how the properties of these domains modulate the formation and size distribution of apoA-I-containing nascent HDL particles created by ATP-binding cassette transporter A1 (ABCA1)-mediated efflux of cellular phospholipid and cholesterol. To address this issue, proteins corresponding to the two domains of human apoA-I (residues 1-189 and 190-243) and mouse apoA-I (residues 1-186 and 187-240) together with some human/mouse domain hybrids were examined for their abilities to form HDL particles when incubated with either ABCA1-expressing cells or phospholipid multilamellar vesicles. Incubation of human apoA-I with cells gave rise to two sizes of HDL particles (hydrodynamic diameter, 8 and 10 nm), and removal or disruption of the C-terminal domain eliminated the formation of the smaller particle. Variations in apoA-I domain structure and physical properties exerted similar effects on the rates of formation and sizes of HDL particles created by either spontaneous solubilization of phospholipid multilamellar vesicles or the ABCA1-mediated efflux of cellular lipids. It follows that the sizes of nascent HDL particles are determined at the point at which cellular phospholipid and cholesterol are solubilized by apoA-I; apparently, this is the rate-determining step in the overall ABCA1-mediated cellular lipid efflux process. The stability of the apoA-I N-terminal helix bundle domain and the hydrophobicity of the C-terminal domain are important determinants of both nascent HDL particle size and their rate of formation.

ABC transporters, atherosclerosis and inflammation

Atherosclerosis, driven by inflamed lipid-laden lesions, can occlude the coronary arteries and lead to myocardial infarction. This chronic disease is a major and expensive health burden. However, the body is able to mobilize and excrete cholesterol and other lipids, thus preventing atherosclerosis by a process termed reverse cholesterol transport (RCT). Insight into the mechanism of RCT has been gained by the study of two rare syndromes caused by the mutation of ABC transporter loci. In Tangier disease, loss of ABCA1 prevents cells from exporting cholesterol and phospholipid, thus resulting in the build-up of cholesterol in the peripheral tissues and a loss of circulating HDL. Consistent with HDL being an athero-protective particle, Tangier patients are more prone to develop atherosclerosis. Likewise, sitosterolemia is another inherited syndrome associated with premature atherosclerosis. Here mutations in either the ABCG5 or G8 loci, prevents hepatocytes and enterocytes from excreting cholesterol and plant sterols, including sitosterol, into the bile and intestinal lumen. Thus, ABCG5 and G8, which from a heterodimer, constitute a transporter that excretes cholesterol and dietary sterols back into the gut, while ABCA1 functions to export excess cell cholesterol and phospholipid during the biogenesis of HDL. Interestingly, a third protein, ABCG1, that has been shown to have anti-atherosclerotic activity in mice, may also act to transfer cholesterol to mature HDL particles. Here we review the relationship between the lipid transport activities of these proteins and their anti-atherosclerotic effect, particularly how they may reduce inflammatory signaling pathways. Of particular interest are recent reports that indicate both ABCA1 and ABCG1 modulate cell surface cholesterol levels and inhibit its partitioning into lipid rafts. Given lipid rafts may provide platforms for innate immune receptors to respond to inflammatory signals, it follows that loss of ABCA1 and ABCG1 by increasing raft content will increase signaling through these receptors, as has been experimentally demonstrated. Moreover, additional reports indicate ABCA1, and possibly SR-BI, another HDL receptor, may directly act as anti-inflammatory receptors independent of their lipid transport activities. Finally, we give an update on the progress and pitfalls of therapeutic approaches that seek to stimulate the flux of lipids through the RCT pathway.

High density lipoprotein structure-function and role in reverse cholesterol transport

High density lipoprotein (HDL) possesses important anti-atherogenic properties and this review addresses the molecular mechanisms underlying these functions. The structures and cholesterol transport abilities of HDL particles are determined by the properties of their exchangeable apolipoprotein (apo) components. ApoA-I and apoE, which are the best characterized in structural terms, contain a series of amphipathic alpha-helical repeats. The helices located in the amino-terminal two-thirds of the molecule adopt a helix bundle structure while the carboxy-terminal segment forms a separately folded, relatively disorganized, domain. The latter domain initiates lipid binding and this interaction induces changes in conformation; the alpha-helix content increases and the amino-terminal helix bundle can open subsequently. These conformational changes alter the abilities of apoA-I and apoE to function as ligands for their receptors. The apoA-I and apoE molecules possess detergent-like properties and they can solubilize vesicular phospholipid to create discoidal HDL particles with hydrodynamic diameters of ~10 nm. In the case of apoA-I, such a particle is stabilized by two protein molecules arranged in an anti-parallel, double-belt, conformation around the edge of the disc. The abilities of apoA-I and apoE to solubilize phospholipid and stabilize HDL particles enable these proteins to be partners with ABCA1 in mediating efflux of cellular phospholipid and cholesterol, and the biogenesis of HDL particles. ApoA-I-containing nascent HDL particles play a critical role in cholesterol transport in the circulation whereas apoE-containing HDL particles mediate cholesterol transport in the brain. The mechanisms by which HDL particles are remodeled by lipases and lipid transfer proteins, and interact with SR-BI to deliver cholesterol to cells, are reviewed.

Role of HDL, ABCA1, and ABCG1 transporters in cholesterol efflux and immune responses

Atherosclerosis has been characterized as a chronic inflammatory response to cholesterol deposition in arteries, but the mechanisms linking cholesterol accumulation in macrophage foam cells to inflammation are poorly understood. Macrophage cholesterol efflux occurs at all stages of atherosclerosis and protects cells from free cholesterol and oxysterol-induced toxicity. The ATP-binding cassette transporters ABCA1 and ABCG1 are responsible for the major part of macrophage cholesterol efflux to serum or HDL in macrophage foam cells, but other less efficient pathways such as passive efflux are also involved. Recent studies have shown that the sterol efflux activities of ABCA1 and ABCG1 modulate macrophage expression of inflammatory cytokines and chemokines as well as lymphocyte proliferative responses. In macrophages, transporter deficiency causes increased signaling via various Toll-like receptors including TLR4. These studies have shown that the traditional roles of HDL and ABC transporters in cholesterol efflux and reverse cholesterol transport are mechanistically linked to antiinflammatory and immunosuppressive functions of HDL. The underlying mechanisms may involve modulation of sterol levels and lipid organization in cell membranes.

Apolipoprotein M expression increases the size of nascent pre beta HDL formed by ATP binding cassette transporter A1

Apolipoprotein M (apoM) is a novel apolipoprotein that is reportedly necessary for pre beta HDL formation; however, its detailed function remains unknown. We investigated the biogenesis and properties of apoM and its effects on the initial steps of nascent pre beta HDL assembly by ABCA1 in HEK293 cells. Transiently transfected apoM was localized primarily in the endomembrane compartment. Pulse-chase analyses demonstrated that apoM is inefficiently secreted, relative to human serum albumin, and that approximately 50% remains membrane-associated after extraction with sodium carbonate, pH 11.5. To investigate the role of apoM in nascent pre beta HDL formation, ABCA1-expressing or control cells, transfected with empty vector, apoM, or C-terminal epitope-tagged apoM (apoM-C-FLAG), were incubated with (125)I-apoA-I for 24 h. Conditioned media were harvested and fractionated by fast-protein liquid chromatography (FPLC) to monitor HDL particle size. Pre beta HDL particles were formed effectively in the absence of apoM expression; however, increased apoM expression stimulated the formation of larger-sized nascent pre beta HDLs. Immunoprecipitation with anti-apoA-I antibody followed by apoM Western blot analysis revealed that little secreted apoM was physically associated with pre beta HDL. Our results suggest that apoM is an atypical secretory protein that is not necessary for ABCA1-dependent pre beta HDL formation but does stimulate the formation of larger-sized pre beta HDL. We propose that apoM may function catalytically at an intracellular site to transfer lipid onto pre beta HDL during or after their formation by ABCA1.

An induction in hepatic HDL secretion associated with reduced ATPase expression

Linoleic acid-phospholipids stimulate high-density lipoprotein (HDL) net secretion from liver cells by blocking the endocytic recycling of apoA-I. Experiments were undertaken to determine whether apoA-I accumulation in the cell media is associated with membrane ATPase expression. Treatment of HepG2 cells with dilinoeoylphosphatidylcholine (DLPC) increased apoA-I secretion fourfold. DLPC also significantly reduced cell surface F1-ATPase expression and reduced cellular ATP binding cassette (ABC)A1 and ABCG1 protein levels by approximately 50%. In addition, treatment of HepG2 cells with the ABC transporter inhibitor, glyburide, stimulated the apoA-I secretory effects of both DLPC and clofibrate. Pretreatment of HepG2 cells with compounds that increased ABC transport protein levels (TO901317, N-Acetyl-L-leucyl-L-leucyl-L-norleucinal, and resveratrol) blocked the DLPC-induced stimulation in apoA-I net secretion. Furthermore, whereas HepG2 cells normally secrete nascent prebeta-HDL, DLPC treatment promoted secretion of alpha-migrating HDL particles. These data show that an linoleic acid-phospholipid induced stimulation in hepatic HDL secretion is related to the expression and function of membrane ATP metabolizing proteins.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

The cell cholesterol exporter ABCA1 as a protector from cardiovascular disease and diabetes

ATP-binding cassette transporter A1 (ABCA1) is an integral cell membrane protein that exports cholesterol from cells and suppresses macrophage inflammation. ABCA1 exports cholesterol by a multistep pathway that involves forming cell-surface lipid domains, solubilizing these lipids by apolipoproteins, binding of apolipoproteins to ABCA1, and activating signaling processes. Thus, ABCA1 behaves both as a lipid exporter and a signaling receptor. ABCA1 transcription is highly induced by sterols, and its expression and activity are regulated post-transcriptionally by diverse processes. ABCA1 mutations can reduce plasma HDL levels, accelerate cardiovascular disease, and increase the risk for type 2 diabetes. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels, inflammation, atherogenesis, and pancreatic beta cell function. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages, raising 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 a promising new therapeutic target for treating cardiovascular disease and diabetes.

The ABCs of sterol transport

Mammalian cells have developed various responses to minimize accumulation of unesterified cholesterol, as the latter can result in cell toxicity and death [reviewed in this edition by Björkhem (Björkhem, I. 2009. Are side-chain oxidized oxysterols regulators also in vivo? J. Lipid Res. In press)]. These responses include esterification to sequester excess sterol in intracellular lipid droplets, repression of both cholesterol synthesis and LDL receptor expression (thus reducing endocytosis of LDL), and induction of a panoply of genes that promote sterol efflux and affect lipid metabolism. The nuclear receptor liver-X-receptor (LXR) functions as a cellular "sterol sensor" and plays a critical role in these latter transcriptional changes [reviewed in this edition by Glass (Shibata, N., and Glass C, K. 2009. Regulation of macrophage function in inflammation and atherosclerosis. J. Lipid Res. In press)]. Activation of LXR by either endogenous oxysterols or synthetic agonists induces the expression of many genes, including those encoding ATP-binding cassette (ABC) transporters ABCA1, ABCG1, ABCG5, and ABCG8. As discussed below, these four proteins function to promote sterol efflux from cells.

ApoA-I facilitates ABCA1 recycle/accumulation to cell surface by inhibiting its intracellular degradation and increases HDL generation

OBJECTIVE

Calpain-mediated proteolysis is one of the major regulatory factors for activity of ATP-binding cassette transporter (ABC) A1. Helical apolipoproteins protect ABCA1 against this degradation and increase generation of HDL. We investigated the mechanism for this reaction focusing on roles of endocytotic internalization of ABCA1.

METHODS AND RESULTS

Surface ABCA1 was labeled with biotin and traced for its internalization and degradation. ABCA1 in the cell surface was internalized within 10 minutes regardless of the presence of apoA-I. ABCA1 was intracellularly degraded and was protected against this only when exposed to extracellular apoA-I before its endocytosis. Consequently, recycle of ABCA1 to the surface was enhanced, and surface ABCA1 was increased by apoA-I. Direct inhibition of ABCA1 endocytosis led to decrease of its degradation and increase of surface ABCA1. Generation of HDL increased in parallel with surface ABCA1.

CONCLUSIONS

Surface ABCA1 is internalized and degraded, and apoA-I interferes with only the latter step to recycle ABCA1 to the surface. Increase of surface ABCA1 results in the increase of generation of HDL.

Regulation of cholesterol efflux from macrophages

PURPOSE OF REVIEW

The lipid efflux pathway is important for both HDL formation and the reverse cholesterol transport pathway. This review is focused on recent findings on the mechanism of lipid efflux and its regulation, particularly in macrophages.

RECENT FINDINGS

Significant progress has been made on understanding the sequence of events that accompany the interaction of apolipoproteins A-I with cell surface ATP-binding cassette transporter A1 and its subsequent lipidation. Continued research on the regulation of ATP-binding cassette transporter A1 and ATP-binding cassette transporter G1 expression and traffic has also generated new paradigms for the control of lipid efflux from macrophages and its contribution to reverse cholesterol transport. In addition, the mobilization of cholesteryl esters from lipid droplets represents a new step in the control of cholesterol efflux.

SUMMARY

The synergy between lipid transporters is a work in progress, but its importance in reverse cholesterol transport is clear. The regulation of efflux implies both the regulation of relevant transporters and the cellular trafficking of cholesterol.

Brefeldin A inhibits cholesterol efflux without affecting the rate of cellular uptake and re-secretion of apolipoprotein A-I in adipocytes

A possible role of cellular uptake and re-secretion of apoA-I in the mechanism of cholesterol efflux induced by apoA-I was investigated using a novel experimental approach. Incubation of adipocytes with a recombinant human apoA-I containing a consensus PKA phosphorylation site, pka-ApoA-I, leads to the appearance of phosphorylated protein in the cell culture medium unambiguously proving cellular uptake and re-secretion of pka-ApoA-I. Phosphorylation of apoA-I is abolished by PKA inhibitors and enhanced by PKA activators demonstrating the specific involvement of PKA. Studies on the concentration dependence of pka-apoA-I phosphorylation and competition experiments with human apoA-I suggest that apolipoprotein uptake is a receptor mediated process. A possible role of apoA-I recycling in the mechanism of cholesterol efflux was investigated by determining the rates of apoA-I induced cholesterol efflux and apoA-I recycling in the presence and in the absence of Brefeldin A (BFA). The studies showed that BFA strongly inhibits cholesterol efflux without affecting the rate of apoA-I recycling. Since BFA affects vesicular trafficking of ABCA1, this study suggests that the interaction of apoA-I with ABCA1 does not mediate apolipoprotein uptake and re-secretion. This result suggests that lipidation of apoA-I and apolipoprotein uptake/re-secretion are independent processes.

Initial interaction of apoA-I with ABCA1 impacts in vivo metabolic fate of nascent HDL

We investigated the in vivo metabolic fate of pre-beta HDL particles in human apolipoprotein A-I transgenic (hA-I (Tg)) mice. Pre-beta HDL tracers were assembled by incubation of [(125)I]tyramine cellobiose-labeled apolipoprotein A-I (apoA-I) with HEK293 cells expressing ABCA1. Radiolabeled pre-beta HDLs of increasing size (pre-beta1, -2, -3, and -4 HDLs) were isolated by fast-protein liquid chromatography and injected into hA-I (Tg)-recipient mice, after which plasma decay, in vivo remodeling, and tissue uptake were monitored. Pre-beta2, -3, and -4 had similar plasma die-away rates, whereas pre-beta1 HDL was removed 7-fold more rapidly. Radiolabel recovered in liver and kidney 24 h after tracer injection suggested increased (P < 0.001) liver and decreased kidney catabolism as pre-beta HDL size increased. In plasma, pre-beta1 and -2 were rapidly (<5 min) remodeled into larger HDLs, whereas pre-beta3 and -4 were remodeled into smaller HDLs. Pre-beta HDLs were similarly remodeled in vitro with control or LCAT-immunodepleted plasma, but not when incubated with phospholipid transfer protein knockout plasma. Our results suggest that initial interaction of apoA-I with ABCA1 imparts a unique conformation that partially determines the in vivo metabolic fate of apoA-I, resulting in increased liver and decreased kidney catabolism as pre-beta HDL particle size increases.

SPTLC1 binds ABCA1 to negatively regulate trafficking and cholesterol efflux activity of the transporter

ABCA1 transport of cholesterol and phospholipids to nascent HDL particles plays a central role in lipoprotein metabolism and macrophage cholesterol homeostasis. ABCA1 activity is regulated both at the transcriptional level and at the post-translational level. To explore mechanisms involved in the post-translational regulation of the transporter, we have used affinity purification and mass spectrometry to identify proteins that bind ABCA1 and influence its activity. Previously, we demonstrated that an interaction between beta1-syntrophin stimulated ABCA1 activity, at least in part, be slowing the degradation of the transporter. This work demonstrates that one subunit of the serine palmitoyltransferase enzyme, SPTLC1, but not subunit 2 (SPTLC2), is copurified with ABCA1 and negatively regulates its function. In human THP-I macrophages and in mouse liver, the ABCA1-SPTLC1 complex was detected by co-immunoprecipitation, demonstrating that the interaction occurs in cellular settings where ABCA1 activity is critical for HDL genesis. Pharmacologic inhibition of SPTLC1 with myriocin, which resulted in the disruption of the SPTLC1-ABCA1 complex, and siRNA knockdown of SPTLC1 expression both stimulated ABCA1 efflux by nearly 60% ( p < 0.05). In contrast, dominant-negative mutants of SPTLC1 inhibited ABCA1 efflux, indicating that a reduced level of sphingomyelin synthesis could not explain the effect of myriocin on ABCA1 activity. In 293 cells, the SPTLC1 inhibition of ABCA1 activity led to the blockade of the exit of ABCA1 from the endoplasmic reticulum. In contrast, myriocin treatment of macrophages increased the level of cell surface ABCA1. In composite, these results indicate that the physical interaction of ABCA1 and SPTLC1 results in reduction of ABCA1 activity and that inhibition of this interaction produces enhanced cholesterol efflux.

Retroendocytosis pathway of ABCA1/apoA-I contributes to HDL formation

ATP-binding cassette protein A1 (ABCA1) mediates transfer of cellular free cholesterol and phospholipids to apolipoprotein A-I (apoA-I), an extracellular acceptor in plasma, to form high-density lipoprotein (HDL). It is currently unknown to what extent ABCA1 endocytosis and recycling contribute to the HDL formation. To address this issue, we expressed human ABCA1 constructs with either an extracellular HA tag or an intracellular GFP tag in cells, and used this system to characterize endocytosis and recycling of ABCA1 and apoA-I. Under basal conditions, ABCA1 and apoA-I are endocytosed via a clathrin- and Rab5-mediated pathway and recycled rapidly back to the cell surface, at least in part via a Rab4-mediated route; approximately 30% of the endocytosed ABCA1 is recycled back to the cell surface. When receptor-mediated endocytosis is inhibited, the level of ABCA1 at the cell surface increases and apoA-I internalization is blocked. Under these conditions, apoA-I mediated cholesterol efflux from cells that have accumulated lipoprotein-derived cholesterol is decreased, whereas efflux from cells without excess cholesterol is increased. These results suggest that the retroendocytosis pathway of ABCA1/apoA-I contributes to HDL formation when excess lipoprotein-derived cholesterol has accumulated in cells.