Cooperation between engulfment receptors: the case of ABCA1 and MEGF10

The engulfment of dying cells is a specialized form of phagocytosis that is extremely conserved across evolution. In the worm, it is genetically controlled by two parallel pathways, which are only partially reconstituted in mammals. We focused on the recapitulation of the CED-1 defined pathway in mammalian systems. We first explored and validated MEGF10, a novel receptor bearing striking structural similarities to CED-1, as a bona fide functional ortholog in mammals and hence progressed toward the analysis of molecular interactions along the corresponding pathway. We ascertained that, in a system of forced expression by transfection, MEGF10 function can be modulated by the ATP binding cassette transporter ABCA1, ortholog to CED-7. Indeed, the coexpression of either a functional or a mutant ABCA1 exerted a transdominant positive or negative modulation on the MEGF10-dependent engulfment. The combined use of biochemical and biophysical approaches indicated that this functional cooperation relies on the alternate association of these receptors with a common partner, endogenously expressed in our cell system. We provide the first working model structuring in mammals the CED-1 dependent pathway.

Phosphatidylserine exposure in B lymphocytes: a role for lipid packing

Plasma membrane lipids are usually distributed asymmetrically, with phosphatidylserine (PS) confined to the inner leaflet. PS exposure at the outer leaflet occurs early in apoptosis, but it is also constitutive on some nonapoptotic cell populations where it plays a role in cell signaling. How PS is transported (“flopped”) to the cell surface is unknown. Contrary to previous reports that normal murine B lymphocytes lack lipid asymmetry, we show that PS is normally restricted to the inner leaflet of these cells. PS exposure on normal B cells did, however, occur spontaneously ex vivo. Consistent with the hypothesis that loss of PS asymmetry is regulated by CD45, PS is constitutively exposed on viable, CD45-deficient B cells. We show that calcium-stimulated PS exposure in B cells is strain variable, ABCA1 independent, and both preceded by and dependent on a decrease in lipid packing. This decrease in lipid packing is concomitant with cell shrinkage and consequent membrane distortion, both of which are potently inhibited by blockers of volume-regulatory K+ and Cl- ion channels. Thus, changes in plasma membrane organization precede PS translocation. The data suggest a model in which PS redistribution may occur by a translocase-independent mechanism at energetically favorable sites of membrane perturbation where lipid packing is decreased.

Specific Mutations in ABCA1 Have Discrete Effects on ABCA1 Function and Lipid Phenotypes Both In Vivo and In Vitro

Mutations in ATP-binding cassette transporter A1 (ABCA1) cause Tangier disease and familial hypoalphalipoproteinemia, resulting in low to absent plasma high-density lipoprotein cholesterol levels. However, wide variations in clinical lipid phenotypes are observed in patients with mutations in ABCA1. We hypothesized that the various lipid phenotypes would be the direct result of discrete and differing effects of the mutations on ABCA1 function. To determine whether there is a correlation between the mutations and the resulting phenotypes, we generated in vitro 15 missense mutations that have been described in patients with Tangier disease and familial hypoalphalipoproteinemia. Using localization of ABCA1, its ability to induce cell surface binding of apolipoprotein A-I, and its ability to elicit efflux of cholesterol and phospholipids to apolipoprotein A-I we determined that the phenotypes of patients correlate with the severity and nature of defects in ABCA1 function.

ABCA1, from pathology to membrane function

The ABCA1 transporter is the prototype of the A class of mammalian adenosine triphosphate binding cassette transporters and one of the largest members of this family. ABCA1 has been originally identified as an engulfment receptor on macrophages and, more recently, it has been shown to play an essential role in the handling of cellular lipids. Indeed by promoting the effluxes of membrane phospholipids and cholesterol to lipid-poor apoprotein acceptors, ABCA1 controls the formation of high-density lipoproteins and thus the whole process of reverse cholesterol transport. A number of additional phenotypes have been found in the mouse model of invalidation of the ABCA1 gene. In spite of their clinical diversity, they all are extremely sensitive to variations in the physicochemical properties of the cell membrane, which ABCA1 controls as a lipid translocator.

Cell vesiculation and immunopathology: implications in cerebral malaria

Microparticles are plasma membrane fragments that are generated and released under physiological conditions. They are also released when tissue and/or systemic homeostasis is disrupted. These microparticles display different physiological features of the cells from which they originate. They are detected in some pathological conditions, but rarely suspected of participating in the disease's pathogenesis. In the present review, we summarise data about the production of the microparticles, their biological significance and potential role during microorganism-driven processes, especially in cerebral malaria.

Transition from dimers to higher oligomeric forms occurs during the ATPase cycle of the ABCA1 transporter

Fluorescence resonance energy transfer and native PAGE analytical techniques were employed to assess the quaternary structure of ABCA1, an ATP binding cassette transporter playing a crucial role in cellular lipid handling. These experimental approaches support the conclusion that ABCA1 is associated in dimeric structures that undergo transition into higher order structures, i.e. tetramers, during the ATP catalytic cycle. Our data hence underline molecular assembly as a crucial parameter in ABCA1 function and the advantage of native PAGE as analytical tool for intractable membrane proteins.

ABCA1 mediates high-affinity uptake of 25-hydroxycholesterol by membrane vesicles and rapid efflux of oxysterol by intact cells

ATP Binding Cassette (ABC) transporter, ABCA1, plays a pivotal role in reverse cholesterol transport by mediating the cellular efflux of phospholipid and cholesterol. Studies using intact cells strongly suggest that ABCA1 acts as a phospholipid floppase, but there has been no direct demonstration that the protein is a primary active sterol transporter. Using membrane vesicles from insect Sf21 cells, we found that ABCA1 mediated ATP-dependent uptake of [(3)H]25-hydroxycholesterol with an apparent K(m) of 0.7 muM. Consistent with this high apparent affinity, expression of ABCA1 in human embryonic kidney cells both increased rapid efflux of 25-hydroxcholesterol and prevented oxysterol-mediated repression of low-density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase mRNAs. Comparison of wild-type and ABCA1(-/-) murine fibroblasts indicates that 25-hydroxycholesterol is effluxed approximately 5-fold more rapidly by wild-type cells. In addition, the rate of efflux from the wild-type but not the ABCA1(-/-) fibroblasts is increased a further twofold by inducers of ABCA1 expression. Thus under the experimental conditions employed, endogenous ABCA1 is a major contributor to 25-hydroxycholesterol efflux from wild-type fibroblasts. Evidence from in vitro studies indicates that oxysterols are potent inducers of genes involved in cellular cholesterol efflux and metabolism, including the ABCA1 gene, and repressors of genes involved in cholesterol synthesis or uptake. Our observations raise the possibility that efflux of oxysterols by ABCA1 could contribute to a homeostatic mechanism, which both attenuates oxysterol-induced expression of its cognate gene and alleviates repression of genes encoding proteins, such as HMG-CoA reductase and LDL receptor.

Immunopathological consequences of the loss of engulfment genes: the case of ABCA1

Programmed cell death plays a crucial role in the maintenance of cell homeostasis. An initial, effector phase leads to the generation of apoptotic corpses and is closely followed by a swift clearance by professional or amateur phagocytes. Several aspects distinguish this latter process of engulfment of dying cells from the classical forms of phagocytosis. They concern all aspects of the process from the recognition of the prey to the final outcome, i.e. immunological silence. The engulfment of dead cells is a process highly conserved through evolution and it has been studied in parallel in two systems, mammalian cells and the nematode C. elegans. ABCA1 and its ortholog CED-7 in the nematode are key players of engulfment. Their mode of action is somehow original in the panorama of engulfment receptors since they act as lipid transporters. While in the worm the loss of CED-7 has phenotypic consequences exclusively on engulfment, in the mouse the deletion of ABCA1 by homologous recombination has highlighted broad consequences on macrophage biology. Among those we will discuss here the aberrant responses of ABCA1-/- mice to Plasmodium berghei ANKA infection, concerning in particular the development of cerebral malaria (CM), a cytokine-induced immunopathology. This syndrome involves a central role of monocytes and, as shown recently, high levels of circulating microparticles. It was found that ABCA1 loss completely protects against CM and its associated mortality. This observation, together with the demonstration of quantitative and functional modifications of microparticles, suggests that microparticles may be involved in CM pathogenesis. The ABCA1 transporter thus appears to control susceptibility to CM, thereby providing new insights in its pathophysiological mechanisms and potential new therapeutic avenues.

ABCA2 is a marker of neural progenitors and neuronal subsets in the adult rodent brain

The notion that the ATP-binding cassette transporter-A2 (ABCA2) may be involved in brain sterol homeostasis and is associated with early onset Alzheimer's disease led us to explore its neural expression. Our data support and extend the previous reports on ABCA2 expression by oligodendrocytes. They evidence that ABCA2 (i) is located in intracellular vesicles, identified in transfected cells as lysosome-related organelles only partially overlapping with classical endolysosomes; (ii) is a marker of neural progenitors as it is expressed in the subventricular zone of the lateral ventricle and the dentate gyrus of the hippocampal formation, sites of continual neurogenesis in the adult brain, and in nestin(+) cells differentiated in vitro from embryonic stem cells; (iii) persists, in the adult rodent brain, in a subset of GABAergic and glutamatergic neurons. Considering that the latter are targets of Alzheimer's lesions, these data provide a new rationale to explore the neuropathological consequences of ABCA2 functional dysregulations.

Headgroup-specific exposure of phospholipids in ABCA1-expressing cells

ABCA1 has been established to be required for the efflux of cholesterol and phospholipids to apolipoproteins such as apoA-I. At present, it is unclear whether ABCA1-mediated lipid exposure is specific with regard to lipid headgroups and whether it requires calcium activation and the presence of a lipid acceptor. In the present work, we found exofacial exposure of endogenous phosphatidylserine in the absence of apoA-I to be enhanced in ABCA1-GFP expressing MDCKII and HeLa cells compared with control cells. By using C6-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) (NBD)-labeled phospholipid analogues, we observed elevated redistribution of phosphatidylserine and phosphatidylethanolamine but not of phosphatidylcholine analogues from the cytoplasmic to the exoplasmic leaflet of the plasma membrane of ABCA1-GFP expressing cells. Whereas glyburide affected neither the level of exofacial endogenous PS nor the outward movement of the amino phospholipid analogues, the latter was sensitive to intracellular Ca2+ in ABCA1-GFP expressing cells, further enhancing outward analogue redistribution with respect to control cells. Both receptor-mediated endocytosis and fluidphase endocytosis were reduced in MDCKII cells expressing ABCA1-GFP. Glyburide raised the level of receptor-mediated endocytosis in the ABCA1-GFP expressing cell to the level of control cells in the absence of glyburide. In control cells, however, fluid-phase endocytosis but not receptor-mediated endocytosis was significantly reduced upon glyburide treatment.

ABCA1 gene deletion protects against cerebral malaria: potential pathogenic role of microparticles in neuropathology

The ATP-binding cassette transporter A1 (ABCA1) modulates the transbilayer distribution of phosphatidylserine at the outer leaflet of the plasma membrane. This external exposure of phosphatidylserine is a hallmark of microparticle production and is impaired in ABCA1(-/-) mice. In this study, we report about the complete resistance to cerebral malaria of these mice. On analysis of histological and systemic parameters we evidenced an impairment of cellular responses to Plasmodium berghei ANKA infection in ABCA1(-/-) mice, as shown by lower plasma tumor necrosis factor levels, a weaker up-regulation of endothelial adhesion molecules in brain microvessels, a reduced leukocyte sequestration, as well as an ablated platelet accumulation. Besides, the number and the procoagulant activity of microparticles were dramatically reduced in the plasma of ABCA1(-/-) compared to ABCA1(+/+) mice. Moreover, microparticles derived from Plasmodium berghei ANKA-infected ABCA1(+/+) mice induced a significant increase of tumor necrosis factor release by noninfected macrophages. In ABCA1(-/-) mice platelet and macrophage responses to vesiculation agonists were ablated and reduced, respectively. Altogether, by pointing out the ABCA1 transporter as a major element controlling cerebral malaria susceptibility, these data provide a novel insight into its pathophysiological mechanisms and are consistent with a pathogenic role of microparticles in this neurological syndrome.

Reduced cholesterol absorption upon PPARdelta activation coincides with decreased intestinal expression of NPC1L1

Peroxisome proliferator-activated receptors (PPARs) control the transcription of genes involved in lipid metabolism. Activation of PPARdelta may have antiatherogenic effects through the increase of plasma HDL, theoretically promoting reverse cholesterol transport from peripheral tissues toward the liver for removal via bile and feces. Effects of PPARdelta activation by GW610742 were evaluated in wild-type and Abca1-deficient (Abca1(-/-)) mice that lack HDL. Treatment with GW610742 resulted in an approximately 50% increase of plasma HDL-cholesterol in wild-type mice, whereas plasma cholesterol levels remained extremely low in Abca1(-/-) mice. Yet, biliary cholesterol secretion rates were similar in untreated wild-type and Abca1(-/-) mice and unaltered upon treatment. Unexpectedly, PPARdelta activation led to enhanced fecal neutral sterol loss in both groups without any changes in intestinal Abca1, Abcg5, Abcg8, and 3-hydroxy-3-methylglutaryl-coenzyme A reductase expression. Moreover, GW610742 treatment resulted in a 43% reduction of fractional cholesterol absorption in wild-type mice, coinciding with a significantly reduced expression of the cholesterol absorption protein Niemann-Pick C1-like 1 (Npc1l1) in the intestine. PPARdelta activation is associated with increased plasma HDL and reduced intestinal cholesterol absorption efficiency that may be related to decreased intestinal Npc1l1 expression. Thus, PPARdelta is a promising target for drugs aimed to treat or prevent atherosclerosis.

Phosphorylation by Protein Kinase CK2 Modulates the Activity of the ATP Binding Cassette A1 Transporter

In a previous characterization of the ABCA subfamily of the ATP-binding cassette (ABC) transporters, we identified potential protein kinase 2 (CK2) phosphorylation sites, which are conserved in eukaryotic and prokaryotic members of the ABCA transporters. These phosphorylation residues are located in the conserved cytoplamic R1 and R2 domains, downstream of the nucleotide binding domains NBD1 and NBD2. To study the possible regulation of the ABCA1 transporter by CK2, we expressed the recombinant cytoplasmic domains of ABCA1, NBD1+R1 and NBD2+R2. We demonstrated that in vitro ABCA1 NBD1+R1, and not NBD2+R2, is phosphorylated by CK2, and we identified Thr-1242, Thr-1243, and Ser-1255 as the phosphorylated residues in the R1 domain by mass spectrometry. We further investigated the functional significance of the threonine and serine phosphorylation sites in NBD1 by site-directed mutagenesis of the entire ABCA1 followed by transfection into Hek-293 Tet-Off cells. The ABCA1 flippase activity, apolipoprotein AI and AII binding, and cellular phospholipid and cholesterol efflux were enhanced by mutations preventing CK2 phosphorylation of the threonine and serine residues. This was confirmed by the effect of specific protein kinase CK2 inhibitors upon the activity of wild type and mutant ABCA1 in transfected Hek-293 Tet-Off cells. The activities of the mutants mimicking threonine phosphorylation were close to that of wild type ABCA1. Our data, therefore, suggest that besides protein kinase A and C, protein kinase CK2 might play an important role in vivo in regulating the function and transport activity of ABCA1 and possibly of other members of the ABCA subfamily.

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

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

The lipidation by hepatocytes of human apolipoprotein A-I occurs by both ABCA1-dependent and -independent pathways

The pathways of hepatic intra- and peri-cellular lipidation of apolipoprotein A-I (apoA-I) were studied by infecting primary mouse hepatocytes from either apoA-I-deficient or ABCA1-deficient mice with a recombinant adenovirus expressing the human apoA-I (hapoA-I) cDNA (endo apoA-I) or incubating the hepatocytes with exogenously added hapoA-I (exo apoA-I) and examining the hapoA-I-containing lipoproteins formed. The cells, maintained in serum-free medium, were labeled with [(3)H]choline, and the cell medium was separated by fast protein liquid chromatography or immunoprecipitated to quantify labeled choline phospholipids specifically associated with hapoA-I. With the apoA-I-deficient hepatocytes, the high density lipoprotein fraction formed with endo apoA-I contained proportionally more phospholipids than that formed with exo apoA-I. However, the lipoprotein size and electrophoretic mobility and phospholipid profiles were similar for exo apoA-I and endo apoA-I. Taken together, these data demonstrate that a significant proportion of hapoA-I is secreted from hepatocytes in a phospholipidated state but that hapoA-I is also phospholipidated peri-cellularly. With primary hepatocytes from ABCA1-deficient mice, the expression and net secretion of adenoviral-generated endogenous apoA-I was unchanged compared with control mice, but (3)H-phospholipids associated with endo apoA-I and exo apoA-I decreased by 63 and 25%, respectively. The lipoprotein size and electrophoretic migration and their phospholipid profiles remained unchanged. In conclusion, we demonstrated that intracellular and peri-cellular lipidation of apoA-I represent distinct and additive pathways that may be regulated independently. Hepatocyte expression of ABCA1 is central to the lipidation of newly synthesized apoA-I but also contributes to the lipidation of exogenous apoA-I. However, a significant basal level of phospholipidation occurs in the absence of ABCA1.

ABCA1 and the engulfment of apoptotic cells

Programmed cell death is one of the major devices controlling cellular homeostasis. However, the generation of cell debris that follows the execution phase of apoptosis has to be backed up by their efficient removal by phagocyte. This highly dynamic process requires the concerted action of a number of surface molecules able to recognize early signals of membrane modifications on the apoptotic prey. Among those, the loss of phospholipid asymmetry and exposure of phosphatidylserine on the prey to be is determinant to engage phagocyte receptors and trigger the removal of corpses. A loss of membrane lipid asymmetry occurs also on the phagocyte determining its efficiency as an undertaker. Here we will discuss how, in our mind, the ATP binding cassette transporter, ABCA1, by its action on the arrangement of lipids at the phagocyte membrane, may actively promote their competence to engulf.

Distinct sites on ABCA1 control distinct steps required for cellular release of phospholipids

The loss of ABCA1 function leads to Tangier dyslipidemia in humans and to a Tangier-like phenotype in mice, by impairing the transformation of nascent apolipoproteins into mature HDL particles. Mechanistically this ensues from the inability of cells to release membrane lipids and cholesterol. Whereas the ability of ABCA1 to promote phospholipid effluxes, surface binding of apolipoproteins and outward flip of membrane lipids has been documented, the relationship between this series of ABCA1-dependent events is still elusive. Here we provide evidence that i) lipid effluxes require both flip of membrane lipids and binding of apolipoproteins to the cell surface, ii) apolipoprotein A-I binding depends on structural determinants on ABCA1, and iii) phospholipid effluxes can be modulated by engineered mutations on the structural determinants identified on ABCA1.

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

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

Identification and functional analysis of a naturally occurring E89K mutation in the ABCA1 gene of the WHAM chicken

The Wisconsin hypoalpha mutant (WHAM) chicken has a >90% reduction in plasma HDL due to hypercatabolism by the kidney of lipid-poor apoA-I. The WHAM chickens have a recessive white skin phenotype caused by a single-gene mutation that maps to the chicken Z-chromosome. This corresponds to human 9q31.1, a chromosomal segment that contains the ATP-binding cassette protein-1 (ABCA1) gene, which is mutated in Tangier Disease and familial hypoalphalipoproteinemia. Complete sequencing of the WHAM ABCA1 cDNA identified a missense mutation near the N-terminus of the protein (E89K). The substitution of this evolutionary conserved glutamate residue for lysine in the mouse ABCA1 transporter leads to complete loss of function, resulting principally from defective intracellular trafficking and very little ABCA1 reaching the plasma membrane. The WHAM chicken is a naturally occurring animal model for Tangier Disease.

The ABCA1 transporter and ApoA-I: obligate or facultative partners?

Premature cardiovascular disease, the leading cause of death in the Western world, is frequently associated with disorders of lipid metabolism and, in particular, with low levels of circulating high-density lipoprotein (HDL) cholesterol. However, the relationship among HDL, centripetal cholesterol transport, and early atherogenesis has remained elusive until the characterization of the molecular defect leading to Tangier disease. In this disorder, the loss of function of the adenosine triphosphate-binding cassette transporter, ABCA1, leads to an impaired formation of nascent HDL particles by preventing the release of cellular lipids and cholesterol to the acceptor apolipoprotein (apo)A-I. Lipids bound to circulating apoA-I are derived from cell membranes via active effluxes experimentally elicited by the interaction of nascent lipid-free apoA-I with the membrane itself. The nature of this key interaction is still enigmatic, however, and a large number of controversial results (discussed in this article) have been reported. Indeed, although the active mechanism that assists the extraction of cellular lipids entails as a simplest option the existence of a dedicated receptor at the membrane, the unambiguous identification of this molecule has not been achieved. This lack of precise evidence makes it necessary to consider alternative models, taking into account the dynamic and functional constraints that regulate the interaction between apoA-I and the plasma membrane.

Increased hepatobiliary and fecal cholesterol excretion upon activation of the liver X receptor is independent of ABCA1

The ATP-binding cassette transporter ABCA1 is essential for high density lipoprotein (HDL) formation and considered rate-controlling for reverse cholesterol transport. Expression of the Abca1 gene is under control of the liver X receptor (LXR). We have evaluated effects of LXR activation by the synthetic agonist T0901317 on hepatic and intestinal cholesterol metabolism in C57BL/6J and DBA/1 wild-type mice and in ABCA1-deficient DBA/1 mice. In wild-type mice, T0901317 increased expression of Abca1 in liver and intestine, which was associated with an approximately 60% rise in HDL. Biliary cholesterol excretion rose 2.7-fold upon treatment, and fecal neutral sterol output was increased by 150-300%. Plasma cholesterol levels also increased in treated Abca1(-/-) mice (+120%), but exclusively in very low density lipoprotein-sized fractions. Despite the absence of HDL, hepatobiliary cholesterol output was stimulated upon LXR activation in Abca1(-/-) mice, leading to a 250% increase in the biliary cholesterol/phospholipid ratio. Most importantly, fecal neutral sterol loss was induced to a similar extent (+300%) by the LXR agonist in DBA/1 wild-type and Abca1(-/-) mice. Expression of Abcg5 and Abcg8, recently implicated in biliary excretion of cholesterol and its intestinal absorption, was induced in T0901317-treated mice. Thus, activation of LXR in mice leads to enhanced hepatobiliary cholesterol secretion and fecal neutral sterol loss independent of (ABCA1-mediated) elevation of HDL and the presence of ABCA1 in liver and intestine.