Lack of ABCA1 considerably decreases brain ApoE level and increases amyloid deposition in APP23 mice

ABCA1 (ATP-binding cassette transporter A1) is a major regulator of cholesterol efflux and high density lipoprotein (HDL) metabolism. Mutations in human ABCA1 cause severe HDL deficiencies characterized by the virtual absence of apoA-I and HDL and prevalent atherosclerosis. Recently, it has been reported that the lack of ABCA1 causes a significant reduction of apoE protein level in the brain of ABCA1 knock-out (ABCA1-/-) mice. ApoE isoforms strongly affect Alzheimer disease (AD) pathology and risk. To determine further the effect of ABCA1 on amyloid deposition, we used APP23 transgenic mice in which the human familial Swedish AD mutant is expressed only in neurons. We demonstrated that the targeted disruption of ABCA1 increases amyloid deposition in APP23 mice, and the effect is manifested by an increased level of Abeta immunoreactivity, as well as thioflavine S-positive plaques in brain parenchyma. We found that the lack of ABCA1 also considerably increased the level of cerebral amyloid angiopathy and exacerbated cerebral amyloid angiopathy-related microhemorrhage in APP23/ABCA1-/- mice. Remarkably, the elevation in parenchymal and vascular amyloid in APP23/ABCA1-/- mice was accompanied by a dramatic decrease in the level of soluble brain apoE, although insoluble apoE was not changed. The elevation of insoluble Abeta fraction in old APP23/ABCA1-/- mice, accompanied by a lack of changes in APP processing and soluble beta-amyloid in young APP23/ABCA1-/- animals, supports the conclusion that the ABCA1 deficiency increases amyloid deposition. These results suggest that ABCA1 plays a role in the pathogenesis of parenchymal and cerebrovascular amyloid pathology and thus may be considered a therapeutic target in AD.     

Activation of liver X receptor decreases BACE1 expression and activity by reducing membrane cholesterol levels

The synthetic Liver X receptor (LXR) activator T0901317 has been reported to exert neuroprotective effect in Alzheimer’s disease, but the relationship between LXR activation and beta-site amyloid precursor protein cleaving enzyme 1 (BACE-1) remains uncertain. This study investigated the effect of T0901317 on membrane cholesterol levels, BACE1 expression and activity. We found that T0901317 decreased membrane cholesterol levels, reduced BACE1 expression and activity as well as β-secretase cleaved C-terminal fragment (β-CTF) levels in vivo and in vitro. Meanwhile, the expression of ATP-binding membrane cassette transport protein A1 (ABCA1) enhanced. Additionally, inhibition of ABCA1 abrogated the effects of T0901317 on membrane cholesterol levels and β-secretase activity. Moreover, addition of LXR antagonist reversed the effect of T0901317 on ABCA1 mRNA expression, membrane cholesterol levels and β-secretase activity. Our results suggest that activation of LXR may decrease BACE1 expression and activity through a pathway associated with ABCA1-mediated reduction in membrane cholesterol levels.     

LXR/RXR activation enhances basolateral efflux of cholesterol in CaCo-2 cells

Regulation of gene expression of ATP-binding cassette transporter (ABC)A1 and ABCG1 by liver X receptor/retinoid X receptor (LXR/RXR) ligands was investigated in the human intestinal cell line CaCo-2. Neither the RXR ligand, 9-cis retinoic acid, nor the natural LXR ligand 22-hydroxycholesterol alone altered ABCA1 mRNA levels. When added together, ABCA1 and ABCG1 mRNA levels were increased 3- and 7-fold, respectively. T0901317, a synthetic non-sterol LXR agonist, increased ABCA1 and ABCG1 gene expression 11- and 6-fold, respectively. ABCA1 mass was increased by LXR/RXR activation. T0901317 or 9-cis retinoic acid and 22-hydroxycholesterol increased cholesterol efflux from basolateral but not apical membranes. Cholesterol efflux was increased by the LXR/RXR ligands to apolipoprotein (apo)A-I or HDL but not to taurocholate/phosphatidylcholine micelles. Actinomycin D prevented the increase in ABCA1 and ABCG1 mRNA levels and the increase in cholesterol efflux induced by the ligands. Glyburide, an inhibitor of ABCA1 activity, attenuated the increase in basolateral cholesterol efflux induced by T0901317. LXR/RXR activation decreased the esterification and secretion of cholesterol esters derived from plasma membranes. Thus, in CaCo-2 cells, LXR/RXR activation increases gene expression of ABCA1 and ABCG1 and the basolateral efflux of cholesterol, suggesting that ABCA1 plays an important role in intestinal HDL production and cholesterol absorption.     

The absence of ABCA1 decreases soluble ApoE levels but does not diminish amyloid deposition in two murine models of Alzheimer disease

ABCA1, a cholesterol transporter expressed in the brain, has been shown recently to be required to maintain normal apoE levels and lipidation in the central nervous system. In addition, ABCA1 has been reported to modulate beta-amyloid (Abeta) production in vitro. These observations raise the possibility that ABCA1 may play a role in the pathogenesis of Alzheimer disease. Here we report that the deficiency of ABCA1 does not affect soluble or guanidine-extractable Abeta levels in Tg-SwDI/B or amyloid precursor protein/presenilin 1 (APP/PS1) mice, but rather is associated with a dramatic reduction in soluble apoE levels in brain. Although this reduction in apoE was expected to reduce the amyloid burden in vivo, we observed that the parenchymal and vascular amyloid load was increased in Tg-SwDI/B animals and was not diminished in APP/PS1 mice. Furthermore, we observed an increase in the proportion of apoE retained in the insoluble fraction, particularly in the APP/PS1 model. These data suggested that ABCA1-mediated effects on apoE levels and lipidation influenced amyloidogenesis in vivo.     

Expression of liver X receptor target genes decreases cellular amyloid beta peptide secretion

A hallmark of Alzheimer's disease is the deposition of plaques of amyloid beta peptide (Abeta) in the brain. Abeta is thought to be formed from the amyloid precursor protein (APP) in cholesterol-enriched membrane rafts, and cellular cholesterol depletion decreases Abeta formation. The liver X receptors (LXR) play a key role in regulating genes that control cellular cholesterol efflux and membrane composition and are widely expressed in cells of the central nervous system. We show that treatment of APP-expressing cells with LXR activators reduces the formation of Abeta. LXR activation resulted in increased levels of the ATP-binding cassette transporter A1 (ABCA1) and stearoyl CoA desaturase, and expression of these genes individually decreased formation of Abeta. Expression of ABCA1 led to both decreased beta-cleavage product of APPSw (i.e. C99 peptide) and reduced gamma-secretase-cleavage of C99 peptide. Remarkably, these effects of ABCA1 on APP processing were independent of cellular lipid efflux. LXR and ABCA1-induced changes in membrane lipid organization had favorable effects on processing of APP, suggesting a new approach to the treatment of Alzheimer's disease.     

The LXR ligand T0901317 induces severe lipogenesis in the db/db diabetic mouse

Liver X receptor (LXR) ligands are currently being evaluated as potential therapeutic agents for the treatment of low HDL. The LXR ligand T0901317 elevates ATP binding cassette transporter A1 (ABCA1) and HDL levels in animal models and induces moderate lipogenesis through upregulation of sterol regulatory element binding protein 1c (SREBP1c). Because insulin may also regulate lipogenesis through SREBP1c and fatty acid synthase (FAS), we investigated the effect of an LXR ligand in hyperinsulinemic mice. Administration of T0901317 to male db/db mice for 12 days resulted in a more severe hypertriacylglycerolemia and hepatic triacylglycerol accumulation than observed in nondiabetic mice. The LXR target genes ABCA1, SREBP1c, FAS, and stearoyl-CoA desaturase 1 were upregulated by T0901317 treatment in both diabetic db/db and nondiabetic C57BLKS mice. Changes in lipogenic gene expression were independent of mouse strain, indicating that the severe lipogenesis observed in LXR ligand-treated db/db mice was not due to additive effects of insulin on lipogenic gene expression. Phosphoenolpyruvate carboxykinase expression was suppressed, suggesting that a shift from gluconeogenesis toward lipogenesis could partially explain our observations in db/db mice. Our data suggest that LXR ligands that have effects on both fatty acid and carbohydrate metabolism should be carefully evaluated in obesity, insulin, and leptin resistance.     

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.     

Physiologically regulated transgenic ABCA1 does not reduce amyloid burden or amyloid-beta peptide levels in vivo

ABCA1-deficient mice have low levels of poorly lipidated apolipoprotein E (apoE) and exhibit increased amyloid load. To test whether excess ABCA1 protects from amyloid deposition, we crossed APP/PS1 mice to ABCA1 bacterial artificial chromosome (BAC) transgenic mice. Compared with wild-type animals, the ABCA1 BAC led to a 50% increase in cortical ABCA1 protein and a 15% increase in apoE abundance, demonstrating that this BAC supports modest ABCA1 overexpression in brain. However, this was observed only in animals that do not deposit amyloid. Comparison of ABCA1/APP/PS1 mice with APP/PS1 controls revealed no differences in levels of brain ABCA1 protein, amyloid, Abeta, or apoE, despite clear retention of ABCA1 overexpression in the livers of these animals. To further investigate ABCA1 expression in the amyloid-containing brain, we then compared ABCA1 mRNA and protein levels in young and aged cortex and cerebellum of APP/PS1 and ABCA1/APP/PS1 animals. Compared with APP/PS1 controls, aged ABCA1/APP/PS1 mice exhibited increased ABCA1 mRNA, but not protein, selectively in cortex. Additionally, ABCA1 mRNA levels were not increased before amyloid deposition but were induced only in the presence of extensive Abeta and amyloid levels. These data suggest that an induction of ABCA1 expression may be associated with late-stage Alzheimer's neuropathology.     

The effects of ABCA1 on cholesterol efflux and Abeta levels in vitro and in vivo

ABCA1 promotes cholesterol efflux from cells and is required for maintaining plasma cholesterol levels. Cholesterol homeostasis is important in the production of beta-amyloid (Abeta), a peptide that is overproduced in Alzheimer's disease (AD). Overexpression of ABCA1 can be achieved by stimulating Liver X Receptors (LXR), and changes in Abeta have been reported after LXR stimulation in vitro. To determine whether ABCA1 could alter endogenous Abeta levels, we used two different in vivo systems. We first examined the effects of an LXR agonist (TO-901317) on wild-type mice and found an increase in brain ABCA1 and apoE levels, which caused an increase in plasma cholesterol. This was accompanied by a decrease in brain Abeta levels. We then examined endogenous Abeta levels in ABCA1 knockout mice and found that, despite having no ABCA1, lowered brain apoE levels, and lowered plasma cholesterol, there was no change in Abeta levels. To assess these in vivo models in an in vitro system, we designed a model in which cholesterol transport via ABCA1 (or related transporters) was prevented. Switching off cholesterol efflux, even in the presence of TO-901317, caused no change in Abeta levels. However, when efflux capability was restored, TO-901317 reduced Abeta levels. These data show that promoting cholesterol efflux is a viable target for Abeta reducing strategies; however, knockout of cholesterol transporters is not sufficient to alter Abeta in vitro or in vivo.     

Protein kinase C and amyloid precursor protein processing in skin fibroblasts from sporadic and familial Alzheimer's disease cases

Non-amyloidogenic alpha-secretase processing of amyloid precursor protein (APP) is stimulated by protein kinase C (PKC). Levels and activity of PKC are decreased in sporadic Alzheimer's disease skin fibroblasts. We investigated whether alterations in PKC and PKC-mediated APP processing occur also in fibroblasts established from individuals with familial Alzheimer's disease APP KM670/671NL, PS1 M146V and H163Y mutations. These pathogenic mutations are known to alter APP metabolism to increase Abeta. PKC activities, but not levels, were decreased by 50% in soluble fractions from sporadic Alzheimer's disease cases. In contrast, familial Alzheimer's disease fibroblasts showed no significant changes in PKC enzyme activity. Fibroblasts bearing the APP KM670/671NL mutation showed no significant differences in either PKC levels or PKC-mediated soluble APP (APPs) secretion, compared to controls. Fibroblasts bearing PS1 M146V and H163Y mutations showed a 30% increase in soluble PKC levels and a 40% decrease in PKC-mediated APPs secretion. These results indicate that PKC deficits are unlikely to contribute to increased Abeta seen with APP and PS1 mutations, and also that PS1 mutations decrease alpha-secretase derived APPs production independently of altered PKC activity.     

Enzymatic reduction of oxysterols impairs LXR signaling in cultured cells and the livers of mice

Liver X receptors (LXRs) are nuclear receptors that play crucial roles in lipid metabolism in vivo and are activated by oxysterol ligands in vitro. The identity of the ligand that activates LXRs in vivo is uncertain. Here we provide two lines of evidence that oxysterols are LXR ligands in vitro and in vivo. First, overexpression of an oxysterol catabolic enzyme, cholesterol sulfotransferase, inactivates LXR signaling in several cultured mammalian cell lines but does not alter receptor response to the nonsterol agonist T0901317. Adenovirus-mediated expression of the enzyme in mice prevents dietary induction of hepatic LXR target genes by cholesterol but not by T0901317. Second, triple-knockout mice deficient in the biosynthesis of three oxysterol ligands of LXRs, 24S-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol, respond to dietary T0901317 by inducing LXR target genes in liver but show impaired responses to dietary cholesterol. We conclude that oxysterols are in vivo ligands for LXR.     

A liver X receptor and retinoid X receptor heterodimer mediates apolipoprotein E expression, secretion and cholesterol homeostasis in astrocytes

Apolipoprotein E (apoE) is an important protein involved in lipoprotein clearance and cholesterol redistribution. ApoE is abundantly expressed in astrocytes in the brain and is closely linked to the pathogenesis of Alzheimer's disease (AD). We report here that small molecule ligands that activate either liver X receptors (LXR) or retinoid X receptor (RXR) lead to a dramatic increase in apoE mRNA and protein expression as well as secretion of apoE in a human astrocytoma cell line (CCF-STTG1 cells). Examination of primary mouse astrocytes also revealed significant induction of apoE mRNA, and protein expression and secretion following incubation with LXR/RXR agonists. Moreover, treatment of mice with a specific synthetic LXR agonist T0901317 resulted in up-regulation of apoE mRNA and protein in both hippocampus and cerebral cortex, indicating that apoE expression in brain can be up-regulated by LXR agonists in vivo. Along with a dramatic induction of ABCA1 cholesterol transporter expression, these ligands effectively mediate cholesterol efflux in both CCF-STTG1 cells and mouse astrocytes in the presence or absence of apolipoprotein AI (apoAI). Our studies provide strong evidence that small molecule LXR/RXR agonists can effectively mediate apoE synthesis and secretion as well as cholesterol homeostasis in astrocytes. LXR/RXR agonists may have significant impact on the pathogenesis of multiple neurological diseases, including AD.     

Reduced insulin-mediated inhibition of VLDL secretion upon pharmacological activation of the liver X receptor in mice

The nuclear liver X receptor (LXR) regulates multiple aspects of cholesterol, triacylglycerol (TG), and carbohydrate metabolism. Activation of LXR induces the expression of genes encoding enzymes involved in de novo lipogenesis (DNL) resulting in hepatic steatosis in mice. Pharmacological LXR activation has also been reported to improve insulin sensitivity and glucose homeostasis in diabetic rodents. The effects of pharmacological LXR ligands on insulin''s action on hepatic lipid metabolism are not known. We evaluated secretion of VLDL during a hyperinsulinemic euglycemic clamp in mice treated with the LXR-ligand T0901317. In untreated mice, hyperinsulinemia reduced the availability of plasma NEFA for VLDL-TG synthesis, increased the contribution of DNL to VLDL-TG, reduced VLDL particle size, and suppressed overall VLDL-TG production rate by approximately 50%. Upon T0901317 treatment, hyperinsulinemia failed to reduce VLDL particle size or suppress VLDL-TG production rate, but the contribution of DNL to VLDL-TG was increased. In conclusion, the effects of LXR activation by T0901317 on lipid metabolism can override the normal control of insulin to suppress VLDL particle secretion.     

Deletion of Abca1 increases Abeta deposition in the PDAPP transgenic mouse model of Alzheimer disease

Apolipoprotein E (apoE) genotype has a major influence on the risk for Alzheimer disease (AD). Different apoE isoforms may alter AD pathogenesis via their interactions with the amyloid beta-peptide (Abeta). Mice lacking the lipid transporter ABCA1 were found to have markedly decreased levels and lipidation of apoE in the central nervous system. We hypothesized that if Abca1-/- mice were bred to the PDAPP mouse model of AD, PDAPP Abca1-/ mice would have a phenotype similar to that of PDAPP Apoe+/- and PDAPP Apoe-/- mice, which develop less amyloid deposition than PDAPP Apoe+/+ mice. In contrast to this prediction, 12-month-old PDAPP Abca -/- mice had significantly higher levels of hippocampal Abeta, and cerebral amyloid angiopathy was significantly more common compared with PDAPP Abca1+/+ mice. Amyloid precursor protein (APP) C-terminal fragments were not different between Abca1 genotypes prior to plaque deposition in 3-month-old PDAPP mice, suggesting that deletion of Abca1 did not affect APP processing or Abeta production. As expected, 3-month-old PDAPP Abca1-/- mice had decreased apoE levels, but they also had a higher percentage of carbonate-insoluble apoE, suggesting that poorly lipidated apoE is less soluble in vivo. We also found that 12-month-old PDAPP Abca1-/- mice had a higher percentage of carbonate-insoluble apoE and that apoE deposits co-localize with amyloid plaques, demonstrating that poorly lipidated apoE co-deposits with insoluble Abeta. Together, these data suggest that despite substantially lower apoE levels, poorly lipidated apoE produced in the absence of ABCA1 is strongly amyloidogenic in vivo.     

Intracellular copper deficiency increases amyloid-beta secretion by diverse mechanisms

In Alzheimer's disease there is abnormal brain copper distribution, with accumulation of copper in amyloid plaques and a deficiency of copper in neighbouring cells. Excess copper inhibits Abeta (amyloid beta-peptide) production, but the effects of deficiency have not yet been determined. We therefore studied the effects of modulating intracellular copper levels on the processing of APP (amyloid precursor protein) and the production of Abeta. Human fibroblasts genetically disposed to copper accumulation secreted higher levels of sAPP (soluble APP ectodomain)alpha into their medium, whereas fibroblasts genetically manipulated to be profoundly copper deficient secreted predominantly sAPPbeta and produced more amyloidogenic beta-cleaved APP C-termini (C99). The level of Abeta secreted from copper-deficient fibroblasts was however regulated and limited by alpha-secretase cleavage. APP can be processed by both alpha- and beta-secretase, as copper-deficient fibroblasts secreted sAPPbeta exclusively, but produced primarily alpha-cleaved APP C-terminal fragments (C83). Copper deficiency also markedly reduced the steady-state level of APP mRNA whereas the APP protein level remained constant, indicating that copper deficiency may accelerate APP translation. Copper deficiency in human neuroblastoma cells significantly increased the level of Abeta secretion, but did not affect the cleavage of APP. Therefore copper deficiency markedly alters APP metabolism and can elevate Abeta secretion by either influencing APP cleavage or by inhibiting its degradation, with the mechanism dependent on cell type. Overall our results suggest that correcting brain copper imbalance represents a relevant therapeutic target for Alzheimer's disease.     

Expression profiling in APP23 mouse brain: inhibition of Aβ amyloidosis and inflammation in response to LXR agonist treatment

Background

Recent studies demonstrate that in addition to its modulatory effect on APP processing, in vivo application of Liver X Receptor agonist T0901317 (T0) to APP transgenic and non-transgenic mice decreases the level of Aβ_42. Moreover, in young Tg2576 mice T0 completely reversed contextual memory deficits. Compared to other tissues, the regulatory functions of LXRs in brain remain largely unexplored and our knowledge so far is limited to the cholesterol transporters and apoE. In this study we applied T0 to APP23 mice for various times and examined gene and protein expression. We also performed a series of experiments with primary brain cells derived from wild type and LXR knockout mice subjected to various LXR agonist treatments and inflammatory stimuli.

Results

We demonstrate an upregulation of genes related to lipid metabolism/transport, metabolism of xenobiotics and detoxification. Downregulated genes are involved in immune response and inflammation, cell death and apoptosis. Additional treatment experiments demonstrated an increase of soluble apolipoproteins E and A-I and a decrease of insoluble Aβ. In primary LXR^wt but not in LXRα^-/-β^-/- microglia and astrocytes LXR agonists suppressed the inflammatory response induced by LPS or fibrillar Aβ.

Conclusion

The results show that LXR agonists could alleviate AD pathology by acting on amyloid deposition and brain inflammation. An increased understanding of the LXR controlled regulation of Aβ aggregation and clearance systems will lead to the development of more specific and powerful agonists targeting LXR for the treatment of AD.     

Role of ABCG1 and ABCA1 in regulation of neuronal cholesterol efflux to apolipoprotein E discs and suppression of amyloid-beta peptide generation

Maintenance of an adequate supply of cholesterol is important for neuronal function, whereas excess cholesterol promotes amyloid precursor protein (APP) cleavage generating toxic amyloid-beta (Abeta) peptides. To gain insights into the pathways that regulate neuronal cholesterol level, we investigated the potential for reconstituted apolipoprotein E (apoE) discs, resembling nascent lipoprotein complexes in the central nervous system, to stimulate neuronal [3H]cholesterol efflux. ApoE discs potently accelerated cholesterol efflux from primary human neurons and cell lines. The process was saturable (17.5 microg of apoE/ml) and was not influenced by APOE genotype. High performance liquid chromatography analysis of cholesterol and cholesterol metabolites effluxed from neurons indicated that <25% of the released cholesterol was modified to polar products (e.g. 24-hydroxycholesterol) that diffuse from neuronal membranes. Thus, most cholesterol (approximately 75%) appeared to be effluxed from neurons in a native state via a transporter pathway. ATP-binding cassette transporters ABCA1, ABCA2, and ABCG1 were detected in neurons and neuroblastoma cell lines and expression of these cDNAs revealed that ABCA1 and ABCG1 stimulated cholesterol efflux to apoE discs. In addition, ABCA1 and ABCG1 expression in Chinese hamster ovary cells that stably express human APP significantly reduced Abeta generation, whereas ABCA2 did not modulate either cholesterol efflux or Abeta generation. These data indicate that ABCA1 and ABCG1 play a significant role in the regulation of neuronal cholesterol efflux to apoE discs and in suppression of APP processing to generate Abeta peptides.     

Apolipoprotein A-I activates Cdc42 signaling through the ABCA1 transporter

It has been suggested that the signal transduction initiated by apolipoprotein A-I (apoA-I) activates key proteins involved in cholesterol efflux. ABCA1 serves as a binding partner for apoA-I, but its participation in apoA-I-induced signaling remains uncertain. We show that the exposure of human fibroblasts to ABCA1 ligands (apolipoproteins and amphipathic helical peptides) results in the generation of intracellular signals, including activation of the small G-protein Cdc42, protein kinases (PAK-1 and p54JNK), and actin polymerization. ApoA-I-induced signaling was abrogated by glyburide, an inhibitor of the ABC transporter family, and in fibroblasts from patients with Tangier disease, which do not express ABCA1. Conversely, induction of ABCA1 expression with the liver X receptor agonist, T0901317, and the retinoid X receptor agonist, R0264456, potentiated apoA-I-induced signaling. Similar effects were observed in HEK293 cells overexpressing ABCA1-green fluorescent protein (GFP) fusion protein, but not ABCA1-GFP (K939M), which fails to hydrolyze ATP, or a nonfunctional ABCA1-GFP with a truncated C terminus. We further found that Cdc42 coimmunoprecipitates with ABCA1 in ABCA1-GFP-expressing HEK293 cells exposed to apoA-I but not in cells expressing ABCA1 mutants. We conclude that ABCA1 transduces signals from apoA-I by complexing and activating Cdc42 and downstream kinases and, therefore, acts as a full apoA-I receptor.