ATVB In Focus
Role of ABCA1 in Cellular Cholesterol Efflux and Reverse Cholesterol Transport
Series Editor:
Alan R. Tall
Previous Brief Reviews in this Series:
• Yancy PG, Bortnick AE, Kellner-Weibel G, de la Llera-Moya M, Phillips MC, Rothblat GH. Importance of different
pathways of cellular cholesterol efflux. 2003;23:712–719.
• Oram JF. HDL Apolipoproteins and ABCA1: partner in the removal of excess cellular cholesterol. 2003;23:720 –727.
• Joyce C, Freeman L, Brewer HB Jr, Sanatamarina-Fojo S. Study of ABCA1 function in transgenic mice.
2003;23:965–971.
• Aiello RJ, Brees D, Francone OL. ABCA1-deficient mice: insights into the role of monocyte lipid efflux in HDL
formation and inflammation. 2003;23:972–980.
• Lund EG, Menke JG, Sparrow CP. Liver X receptor agonists as potential therapeutic agents for dyslipidemia and
atherosclerosis. 2003;23:1169 –1177.
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Regulation and Mechanisms of ATP-Binding Cassette
Transporter A1-Mediated Cellular Cholesterol Efflux
Nan Wang, Alan R. Tall
Abstract—ATP-binding cassette transporter A1 (ABCA1) plays a major role in cholesterol homeostasis and HDL
metabolism. ABCA1 mediates cellular cholesterol and phospholipid efflux to lipid-poor apolipoproteins, and upregulation of
ABCA1 activity is antiatherogenic. ApoA-I, the major apolipoprotein component of HDL, promotes ABCA1-mediated
cholesterol and phospholipid efflux, probably by directly binding to ABCA1. ABCA1 gene expression is markedly increased
in cholesterol-loaded cells as a result of activation of LXR/RXR. ABCA1 protein turnover is rapid. ABCA1 contains a
PEST—proline (P), glutamate (E), serine (S), and threonine (T)—sequence in the intracellular segment that mediates ABCA1
degradation by a thiol protease, calpain. ApoA-I and apoE stabilize ABCA1 in a novel mode of regulation by decreasing
PEST sequence-mediated calpain proteolysis. ABCA1-mediated cholesterol and phospholipid efflux are distinctly regulated
and affected by the activity of other gene products. Stearyol CoA desaturase decreases ABCA1-mediated cholesterol efflux
but not phospholipid efflux, likely by decreasing the cholesterol pool available to ABCA1. This and other evidence suggest
that ABCA1 promotes cholesterol and phospholipid efflux, probably by directly transporting both lipids as substrates.
(Arterioscler Thromb Vasc Biol. 2003;23:1178-1184.)
Key Words: lipoprotein metabolism 䡲 risk factors 䡲 cell biology 䡲 gene regulation
A
therosclerosis, the major cause of death in industrialized
societies, is initiated by the retention in arteries of
cholesterol-enriched, apoB-containing lipoproteins and their
subsequent uptake by arterial wall macrophages, giving rise
to macrophage foam cells.1 HDL and HDL apolipoproteins
protect against the development of atherosclerosis, probably
primarily by stimulating the efflux of cholesterol from macrophage foam cells.2,3 A breakthrough in this area of research
See cover
has been the elucidation of mutations in the ATP-binding
cassette transporter A1 (ABCA1)4 – 6 as the genetic defect in
Tangier disease, a disorder characterized by HDL deficiency,
defective apolipoprotein-mediated phospholipid and cholesterol efflux from cells, and the accumulation of macrophage
foam cells in various tissues, including arteries. The pheno-
Received March 4, 2003; revision accepted April 24, 2003.
From the Division of Molecular Medicine, Department of Medicine, Columbia University, New York, NY.
Correspondence to Nan Wang, Division of Molecular Medicine, Department of Medicine, Columbia University, 630 West 168 Street, New York, NY
10032. E-mail nw30@columbia.edu
© 2003 American Heart Association, Inc.
Arterioscler Thromb Vasc Biol. is available at http://www.atvbaha.org
1178
DOI: 10.1161/01.ATV.0000075912.83860.26
Wang and Tall
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Figure 1. ApoA-I–mediated stabilization of ABCA1 by inhibiting
PEST-dependent calpain proteolysis. ApoA-I binds to ABCA1
and causes inhibition of calpain-catalyzed proteolysis of ABCA1
by promoting cellular phospholipid efflux or by inducing a conformational change of ABCA1 that inhibits access of calpain to
the PEST sequence. Calpeptin is a specific inhibitor of calpain
and increases ABCA1 protein by reducing calpain proteolysis.
typic defects of Tangier disease are consistent with the
proposed function of ABCA1 as a cell surface transporter that
promotes the efflux of cellular phospholipids and cholesterol
to lipid-poor apolipoproteins, a process that constitutes the
initial step in HDL formation.7,8
The finding that ABCA1 mutations cause Tangier disease
has stimulated studies of ABCA1 functions in vivo and in
vitro. As reviewed elsewhere in this series, these studies
confirm the pivotal role of ABCA1 in HDL formation and
indicate an antiatherogenic role of ABCA1, especially
ABCA1 in the macrophage.9 –11 Studies of the regulation of
ABCA1 gene expression have revealed that ABCA1 is
induced in cholesterol-loaded cells as a result of activation of
LXR/RXR heterodimer and induction of ABCA1 expression
is probably central to the antiatherogenic effects of LXR
ligands in vivo.12–14 This review will focus on the regulation
and function of ABCA1, emphasizing recent work on the
apolipoprotein-mediated post-transcriptional regulation of
ABCA1 turnover and function.
Role of Apolipoprotein Binding to ABCA1 in
the Regulation of Lipid Efflux
ABCA1 is a large membrane protein with 2 transmembrane
domains and 2 nucleotide-binding folds linked by an intracellular peptide segment (Figure 1). After translation,
ABCA1 is further processed by glycosylation and presented
at the cell surface.15 The primary function of ABCA1 is to
promote cellular cholesterol and phospholipid efflux, which
requires the presence of extracellular lipid-poor apolipoproteins.16,17 However, the detailed molecular events linking
apolipoproteins to ABCA1-facilitated cellular lipid efflux are
still not clear. Before the discovery of ABCA1, a number of
different laboratories had demonstrated apolipoproteinmediated phospholipids and cholesterol efflux from cells,18 –20
and Francis et al21 discovered that this process was defective
in Tangier disease fibroblasts. Subsequent to the discovery
that ABCA1 mutations cause Tangier disease, we and others
ABCA1-Mediated Cellular Cholesterol Efflux
1179
found that transfection of the full-length cDNA of ABCA1 in
cells resulted in increased apoA-I binding to the cell surface.17,22 These studies also showed that apoA-I could be
chemically cross-linked to ABCA1, suggesting close proximity of apoA-I to ABCA1 (ie, within about 11 Å).17 These
findings led us to suggest that apoA-I directly binds ABCA1
to form a complex that is required for ABCA1-facilitated
cellular cholesterol and phospholipid efflux.17 In contrast to
this idea, Chambenoit et al23 proposed that apoA-I binding
might not involve a direct molecular interaction between
ABCA1 and apoA-I but rather a modified distribution of
plasma membrane lipids induced by ABCA1 expression,
leading to “docking” of apoA-I molecules at the cell surface,
perhaps in close proximity to ABCA1. The authors provided
3 lines of evidence to support this hypothesis. First, apoA-I
binding requires functional ABCA1 because an ABCA1
mutant with defective ATPase activity fails to bind apoA-I.
Second, ABCA1 expression at the cell surface is not linearly
correlated with apoA-I binding, as determined by fluorescence quantification of apoA-I binding and cell surface
ABCA1 protein levels. Third, the lateral mobility of ABCA1
green fluorescent protein in membranes appears to be different from that of membrane-bound apoA-I.23 The authors
suggested that apoA-I is likely bound to phosphatidylserine
(PS) that is presented to the exofacial leaflet by the “floppase”
activity of ABCA1 because PS levels on the exofacial leaflet
are increased by ABCA1 expression.23 However, Smith et
al24 reported that although ABCA1 expression increased PS
levels on the exofacial leaflet, the increased PS was insufficient to mediate cellular apoA-I binding and lipid efflux
because annexin V, a PS-binding protein, did not compete
with ABCA1-induced apoA-I binding, nor did it affect
ABCA1-mediated lipid efflux to apoA-I.24,25 Using a fluorescence photobleaching technique, this study also suggested
that apoA-I bound to an integral membrane protein in
ABCA1-expressing cells.24
The controversy about apoA-I/ABCA1 interaction has also
been looked into from a different angle using apoA-I mutants.26 The C-terminal lipid-binding helix 10 of apoA-I was
identified to be critical for apoA-I–mediated cholesterol
efflux from ABCA-expressing cells,26 and a positive correlation between cholesterol efflux and the lipid-binding characteristics of apoA-I was observed.26 These findings led to a
composite model to explain the interaction between apoA-I
and ABCA1, that is, helix 10 of apoA-I tethers the lipid-free
apolipoprotein to the ABCA1-generated lipid domain that
then diffuses within the plane of the membrane until it comes
in contact with ABCA1, where a protein–protein interaction
could lead to the lipidation of apoA-I.26 However, the data do
not distinguish whether apoA-I/lipid binding precedes the
interaction of apoA-I with ABCA1 or the other way around.
The ABCA1 molecule has 2 large extracellular loops,
probably linked by a disulfide bond, located on each of the 2
transmembrane domains (Figure 1 and27,28). Importantly,
many ABCA1 missense mutations causing Tangier disease
have been identified in these loops.29 A functional test of
these mutations in transfected cells revealed defects in apoA-I
binding and cellular lipid efflux.27,28 Interestingly, one of the
mutants, W590S, showed lipid efflux deficiency but moder-
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ately increased apoA-I binding.27 These findings have been
confirmed independently30,31 and strongly suggested that
apoA-I directly binds ABCA1 at the cell surface without
requiring the ability of ABCA1 to mediate lipid efflux.
Together, these results also suggest that apoA-I binding
requires an ABCA1 molecule assuming an optimal structural
conformation that is maintained by a functional ATPase
because cell-surface ABCA1 mutants with defective ATPase
fail to bind apoA-I.23,32 The moderate increase in apoA-I
binding to W590S may imply that dissociation of apoA-I
from ABCA1 could be facilitated by lipid efflux to apolipoproteins bound to ABCA1 and is consistent with the earlier
finding that ABCA1 binds apoA-I but not HDL3.7
Extrahepatic ABCA1 functions to promote cholesterol
efflux from peripheral tissues to lipid-poor apolipoproteins
that in turn deliver the lipid load back to liver for disposal, a
process likely involving scavenger receptor BI, which has
high affinity for lipid-rich HDLs but low affinity for lipidpoor apolipoproteins.33 Hepatic ABCA1 is likely involved in
pre-␤ HDL formation in liver, demonstrated by increased
pre-␤ HDL as well as more mature HDL levels in mice with
adenovirus-mediated expression of ABCA1.34 Like scavenger receptor BI, ABCA1 binds not only apoA-I but also other
apolipoproteins, including apoE.35 Thus, the antiatherogenic
effect of apoE in vivo could be partially explained by lipid
efflux from macrophage foam cells coordinated with
ABCA1.
Apolipoprotein Binding in the Regulation of
ABCA1 Turnover
Cellular expression of ABCA1 is highly regulated. As a
consequence of increased ABCA1 expression,
apolipoprotein-mediated cellular cholesterol efflux is also
enhanced,17,22 leading to a decreased cellular cholesterol
accumulation. Highly regulated ABCA1 expression may
reflect the necessity to tightly regulate ABCA1 protein levels
by the cell to maintain cellular cholesterol and phospholipid
homeostasis. During cholesterol loading of macrophages,
cellular cholesterol and phospholipid content are increased,
the latter is caused by increased phospholipid synthesis36 and
is a protective response against cytotoxicity caused by cholesterol accumulation.37 Therefore, induction of ABCA1
expression could help the cell shed excess cholesterol and
phospholipids and reach a new steady state of cellular lipid
metabolism. However, overexpressed ABCA1 in the absence
of cholesterol loading causes an altered membrane structure,7
which could be detrimental to the cell. Oram et al22 showed
that the turnover of ABCA1 protein was rapid with a short
half life less than 1 hour in murine macrophage-like cells.
Thus, the induction of ABCA1 expression by cholesterol
loading and rapid turnover of ABCA1 protein suggest the
existence of cellular mechanisms tightly regulating the protein levels of ABCA1 to maintain lipid homeostasis.
The cellular processes regulating the turnover of ABCA1
protein have only recently received attention. ABCA1 is
present at the cell surface and proposed to function primarily
at plasma membranes.32 However, intracellular localizations
of ABCA1 have been observed,38,39 and ABCA1-mediated
lipid efflux may involve intracellular lipid trafficking38,39 or
apoA-I endocytotic recycling.40 Using ABCA1 green fluorescent protein fusion protein and confocal and time-lapse
fluorescence microscopy, Neufeld et al39 found that ABCA1
is on the cell surface and in intracellular vesicles, including
early endosomes, late endosomes, and lysosomes. They have
suggested that delivery of ABCA1 to lysosomes could be a
mechanism to regulate ABCA1 protein turnover and to
modulate its cell surface expression and function.39 A key
role of late endosomes/lysosomes in providing cholesterol for
ABCA1-mediated efflux is indicated by the severe defect in
apolipoprotein-mediated cholesterol efflux in Niemann-Pick
C macrophages.41
Regulation of protein turnover of membrane receptors by
their ligands has been reported.42 Because apoA-I directly
binds ABCA1, we speculated that apoA-I might regulate
ABCA1 protein turnover. Indeed, we found that apoA-I
binding increased ABCA1 protein levels in mouse primary
hepatocytes, peritoneal macrophages, and transfected cells
without affecting ABCA1 mRNA levels.31 We also showed
that apoE had a similar positive effect on ABCA1 protein
levels.31 Arakawa and Yokoyama43 independently reported
that apoA-I increased ABCA1 protein in human THP-1 cells.
Furthermore, they demonstrated that the positive effect of
apoA-I on ABCA1 was likely mediated by inhibiting ABCA1
degradation by an unknown thiol protease.43 We obtained
similar results showing that nonspecific thiol protease inhibitors increased ABCA1 protein levels in transfected 293
cells.31
Many short-lived proteins have specific peptide motifs that
target proteins for rapid degradation. One of the motifs is
PEST, which is defined as a peptide sequence enriched in
proline (P), glutamate (E), serine (S), and threonine (T), and
usually flanked by lysine (K), arginine (R), or histidine (H)
residues.44 Using the program PESTfind, we identified a
conserved potential PEST sequence in ABCA1 (Figure 1).
This PEST sequence appeared to be important in regulation
of ABCA1 function because PEST deletion resulted in a 4- to
5-fold increase in cell surface ABCA1 protein and significantly increased ABCA1-mediated lipid efflux and apoA-I
binding. 31 PEST sequences often increase protein turnover
by enhancing protein ubiquitination and proteasomal degradation.44 In contrast with the lack of effect of lactacystin, a
specific inhibitor of proteasome, on ABCA1 protein levels in
THP-1 cells as reported by Arakawa and Yokoyama,43
ABCA1 was ubiquitinated in 293 cells, and lactacystin
moderately increased ABCA1 protein levels in both 293 cells
and mouse peritoneal macrophages,31 suggesting that the
proteasomal degradation pathway is involved in ABCA1
turnover. Additional evidence supporting the involvement of
the proteasomal pathway in ABCA1 turnover came from
another study showing that free cholesterol accumulation in
mouse peritoneal macrophages resulted in reduced ABCA1
protein levels that could be reversed by lactacystin treatment.45 However, we found that the PEST deletion ABCA1
mutant was still ubiquitinated and the protein level of the
mutant was also increased by lactacystin treatment,31 suggesting that ubiquitination and proteasomal degradation of
ABCA1 is not controlled by the PEST sequence. In a few
instances, PEST sequences have been implicated in proteol-
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ysis of the target proteins by calpains, a subfamily of thiol
proteases.46 Indeed, a specific calpain protease inhibitor,
calpeptin, substantially increased total and cell surface wildtype ABCA1 levels but had no effect on the protein level of
the PEST deletion mutant,31 suggesting that ABCA1 is a
target of calpain proteolysis and that the PEST sequence
helps to target calpain to cell-surface ABCA1. Further,
purified ␮-calpain protease added to permeabilized cells
efficiently degraded wild-type ABCA1 but not the PEST
deletion mutant, providing direct evidence that PESTdependent degradation of ABCA1 is mediated by calpain
protease.31 Calpeptin treatment also increased ABCA1 protein levels in primary mouse hepatocytes and mouse peritoneal macrophages,31 suggesting a physiological role of calpain protease in regulation of ABCA1 turnover.
Because apoA-I binding increased ABCA1 protein levels,
we then tested whether apoA-I-mediated ABCA1 stabilization was mediated by inhibiting calpain proteolysis in a PEST
sequence-dependent fashion. ApoA-I increased levels of
wild-type ABCA1 but not of the PEST deletion mutant, and
apoA-I pretreatment blocked the degradation of the wild-type
ABCA1 by purified calpain protease added to the permeabilized cells.31 Together, these results have provided convincing evidence for a novel mode of regulation of ABCA1:
apoA-I stabilizes ABCA1 by inhibiting PEST sequencemediated calpain proteolysis. To evaluate the effect of apoA-I
on ABCA1 as a potential mechanism for its anti-atherogenic
role in vivo, we injected a bolus of apoA-I into mice and
determined ABCA1 protein levels. Intravenous apoA-I injection resulted in an induction of ABCA1 protein in liver and
peritoneal macrophages.31 However, hepatic and macrophage
ABCA1 protein levels showed no change in apoA-I knockout
or transgenic mice,31 suggesting that some form of chronic
adaptation may occur as a result of sustained alterations in
apoA-I levels. Therefore, these studies may provide a potential explanation for previously observed antiatherogenic effects of apoA-I infusion, that occurred even without sustained
elevation of HDL levels, and suggest a rationale for apoA-I
infusion trials in humans.47 Our findings also suggest that
intermittent dosing with apoA-I could be more effective than
sustained overexpression in upregulating ABCA1 and cellular cholesterol efflux.
To further explore the mechanism for apoA-I–mediated
ABCA1 stabilization, we used several ABCA1 mutants with
defective transporter functions. ApoA-I failed to increase
protein levels of an ATP-binding motif mutant that had been
shown to be defective in both apoA-I binding and apoA-I
mediated lipid efflux,32 indicating that apoA-I binding to
ABCA1 is likely necessary for apoA-I–mediated ABCA1
stabilization. Binding of apoA-I to ABCA1 leads to cellular
cholesterol and phospholipid efflux. It may also cause conformational changes of ABCA1, as reported for ligandinduced conformational changes of other ABC transporters.48
Cholesterol efflux appears to have no effect on ABCA1
protein levels because HDL and cyclodextrin treatment promoted cholesterol efflux to an extent similar to or greater than
that by apoA-I–mediated cholesterol efflux but failed to alter
ABCA1 protein levels.31 Another mutation of ABCA1 that
causes Tangier disease (W590S) has been shown to cause a
ABCA1-Mediated Cellular Cholesterol Efflux
1181
moderate increase in apoA-I binding but defective lipid
efflux.27 ApoA-I failed to increase ABCA1-W590S levels
whereas calpeptin significantly increased ABCA1-W590S
proteins, suggesting that apoA-I binding is not sufficient for
ABCA1 stabilization.31 These results favor the hypothesis
that apoA-I–mediated ABCA1 stabilization may result from a
local change in membrane phospholipids that decreases the
binding of a hydrophobic, glycine-rich sequence of the small
calpain subunit.49 However, an equally valid interpretation is
that apoA-I fails to bind the W590S mutant in the correct
orientation and therefore the appropriate conformational
change of ABCA1 required to decrease calpain proteolysis
does not occur. Calpain proteolysis is also regulated by other
cellular processes, such as protein phosphorylation.46 Phosphorylation of the target protein often increases its degradation by calpain protease.50 Martinez et al have obtained
preliminary results showing that ABCA1 is phosphorylated in
the PEST sequence and that apoA-I decreases ABCA1
phosphorylation (Martinez LO, Wang N, Tall AR. Unpublished observation). Therefore, it is likely that apoA-I binds
ABCA1, promotes lipid efflux, and causes a conformational
change of ABCA1. These changes are accompanied by
decreased ABCA1 PEST sequence phosphorylation and decreased ABCA1 degradation by calpain.
The finding that ABCA1 is ubiquitinated31 and degraded
by the proteasome pathway31,45 likely reflects an independent
mechanism in the regulation of ABCA1 turnover. The cellular compartment in which ABCA1 ubiquitination occurs is
unknown and endoplasmic reticulum or plasma membrane
could both be potential sites. Although marked free cholesterol accumulation in macrophages leads to ABCA1 degradation, which is reversed by inhibition of proteasome pathway,45 the physiological significance of this pathway in
regulation of ABCA1 turnover in vivo is still not clear.
Interestingly, the cystic fibrosis transporter regulator (CFTR),
another ABC transporter, has also been shown to be ubiquitinated and degraded by the proteasome pathway.51 Conformational maturation of wild-type CFTR in the endoplasmic
reticulum is an inefficient process, where approximately 75%
of newly synthesized CFTR molecules are degraded by
cytoplasmic proteasomes shortly after synthesis.52 Defective
protein folding of some CFTR mutants has been proposed as
the cause for cystic fibrosis, and great efforts have been made
to explore different mechanisms to help the folding of the
mutant CFTR.52 Similarly, a recent study suggests that some
ABCA1 missense mutations leading to Tangier disease also
show defective ABCA1 presentation at the cell surface,28
perhaps reflecting defective protein folding.
Regulation of ABCA1 Protein Turnover as a
Result of Altered Lipid Metabolism
In addition to apolipoproteins, ABCA1 turnover is also
modulated by changes of cellular lipid metabolism. Wang and
Oram53 reported that unsaturated fatty acid accelerate the
degradation of ABCA1 without altering ABCA1 mRNA
levels and thus decreases apoA-I–mediated lipid efflux,
although the specific proteolytic pathway is not known.
Because type 2 diabetes and insulin resistance are characterized by elevated fatty acids, low plasma HDL levels, and
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July 2003
increased risk of cardiovascular diseases, impaired ABCA1mediated cholesterol efflux from macrophages may contribute to the enhanced atherosclerosis associated with these
metabolic disorders.53 Indeed, Uehara et al54 reported that
hepatic and macrophage ABCA1 expression were markedly
decreased in diabetic mice and this was reversed by insulin
treatment. Furthermore, they also showed that ABCA1
mRNA and protein levels were reduced by unsaturated, but
not saturated, fatty acids in hepatoma and macrophage-like
cell lines.54
Mechanisms of ABCA1-Mediated Lipid Efflux
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The molecular mechanism for ABCA1-mediated lipid efflux
is still poorly understood, and several models have been
proposed. Some studies have suggested that ABCA1 is
localized in and promotes lipid efflux from a membrane lipid
domain distinct from cholesterol- and sphingomyelin-rich
rafts.55 Several other studies have suggested the opposite
interpretation56 or suggested that both raft and nonraft domains contribute to ABCA1-facilitated lipid efflux to apoA-I
depending on the cell types.57 Phospholipid and cholesterol
efflux promoted by ABCA1 can be dissociated,25,58 and
different cell lines have shown nonparallel apoA-I–mediated
cholesterol and phospholipid efflux,59 suggesting distinctly
regulated ABCA1-dependent cholesterol and phospholipid
efflux pathways. Studies from our group suggest that this
lipid efflux pathway is in part regulated by stearyol CoA
desaturase (SCD), a rate-limiting enzyme in the cellular
synthesis of monounsaturated fatty acids from saturated fatty
acids.58 Coexpression of ABCA1 and SCD reduced ABCA1mediated cholesterol efflux but not phospholipid efflux to
apoA-I, whereas SCD expression increased cholesterol efflux
to HDL2,58 a process independent of ABCA1.25 SCD expression did not alter ABCA1 mRNA or protein levels,58 excluding the potential effect of increased unsaturated fatty acid
levels in SCD expressing cells as a cause of reduced ABCA1
protein levels.53,54 Confocal microscopy showed that SCD
overexpression led to a decrease in Triton X-100 –resistant
membrane liquid-ordered domains and an increase in membrane liquid domains.58 Because cellular free cholesterol,
cholesterol ester, and phospholipid content did not change in
SCD-overexpressing cells, a likely explanation for SCDinduced reduction of ABCA1-mediated cholesterol efflux
would be that less cholesterol is available for ABCA1facilitated efflux in the expanded Triton X-100 –soluble
domains containing ABCA1. These data are consistent with
coordinated cholesterol and phospholipid efflux by ABCA1
and with the results of kinetic studies of apoA-I–mediated
lipid efflux from macrophages, showing that cholesterol and
phospholipid efflux are parallel in cells with upregulated
ABCA1 expression,60,61 as reviewed elsewhere in this series.
However, they are in contrast with the 2-step model proposed
for ABCA1-mediated lipid efflux, which suggests an initial
formation of apoA-I/phospholipid complexes promoted by
ABCA1 followed by cholesterol efflux from a membrane
domain in an ABCA1-independent fashion.25,62 In addition to
ABCA1, several other ABC transporters have been shown to
primarily facilitate lipid exo-transport. ABCB4 (human
MDR3 and mouse MDR2) is localized on the canalicular
Figure 2. ABCA1-mediated phospholipid and cholesterol efflux
to apoA-I and its regulation. ABCA1 functions primarily at cell
surface directly promoting phospholipid and cholesterol efflux to
apoA-I (A). The ratio of cholesterol/phospholipids efflux may be
modulated by local membrane changes in the ratio of these lipids, which determine the availability of the lipids to the transporter. For example, SCD expression increases membrane
mono-unsaturated fatty acids at the expense of saturated fatty
acid and decreases the cholesterol pool available for ABCA1mediated efflux but has no effect on the phospholipid pool (B).
membranes of hepatocytes and plays an essential role in
phosphatidylcholine secretion into bile.63 Recently, ABCG5
and ABCG8, mutated genes in sitosterolemia,64 have been
shown to form functional heterodimers promoting plant sterol
and cholesterol secretion into bile and possibly intestinal
lumen.65,66 ABCA4 (ABCR), a close relative of ABCA1 and
the mutant gene in Stargardt’s disease,67 is specifically
expressed in retina and proposed to transport N-retinylidenephosphatidylethanolamine, a product generated during the
visual cycle.68 ABCA1 is distinct from other lipid ABC
transporters in that it promotes both phospholipid and cholesterol efflux. Given the high specificity of lipid substrates
for other ABC transporters and the above-described evidence
supporting the coordinated simultaneous lipid release by
ABCA1, we suggest that ABCA1-mediated cholesterol and
phospholipid efflux may reflect entirely a substrate specificity, that is, ABCA1 specifically transports both phospholipid
and cholesterol as substrates across membranes; the availability of different lipids in the vicinity of ABCA1 may result in
a modification of the ratio of cholesterol/phospholipid undergoing efflux (Figure 2).
Conclusion
ApoA-I binds specifically to ABCA1, likely involving a
direct molecular interaction. Binding of apoA-I to ABCA1
promotes cellular cholesterol and phospholipid efflux, probably as a result of a coordinated specific transmembrane
transport of both lipids by ABCA1. ABCA1 expression is
highly regulated, both on transcriptional and posttranscriptional levels. The interaction of apoA-I with ABCA1
modulates both forms of regulation. Thus, cholesterol efflux
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promoted by ABCA1 leads to decreased activation of LXR/
RXR by oxysterols and ultimately decreases ABCA1 transcription and protein levels. In contrast, apoA-I and apoE
have a positive effect on ABCA1 protein expression, and this
is likely to be important in hepatocytes and macrophage foam
cells. Intense interest has recently centered on the possibility
that increasing macrophage cholesterol efflux could represent
a novel approach to treatment of atherosclerosis. LXR/RXR
target a battery of genes mediating cholesterol efflux, transport, and excretion, and LXR activators are antiatherogenic.14
However, LXR/RXR also increases transcription of
SREBP1c and its target genes, causing fatty liver and hypertriglyceridemia.69,70 Therefore, calpain protease inhibitors,71
or small molecules that modulate the local interaction of
ABCA1 with calpain protease at the plasma membrane, might
be an alternative way to upregulate ABCA1 protein and
function. Low-affinity small peptides that mimic the binding
of lipid-free apolipoproteins to ABCA1 could stabilize the
transporter while allowing lipid efflux to apoA-I or apoE.
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Regulation and Mechanisms of ATP-Binding Cassette Transporter A1-Mediated Cellular
Cholesterol Efflux
Nan Wang and Alan R. Tall
Arterioscler Thromb Vasc Biol. 2003;23:1178-1184; originally published online May 8, 2003;
doi: 10.1161/01.ATV.0000075912.83860.26
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