Differential expression and function of ABCG1 and ABCG4 during development and aging

Molecular detection of lacrimal apparatus and ocular surface - related ABC transporter genes

The ocular system is in constant interaction with the environment and with numerous pathogens. The ATP-binding cassette (ABC) transporters represent one of the largest groups among the transmembrane proteins. Their relevance has been demonstrated for their defense function against biotic and abiotic stress factors, for metabolic processes in tumors and for their importance in the development of resistance to drugs. The aim of this study was to analyze which ABC transporters are expressed at the ocular surface and in the human lacrimal apparatus. Using RT-PCR, all ABC transporters known to date in humans were examined in tissue samples from human cornea, conjunctiva, meibomian glands and lacrimal glands. The RT-PCR analyses revealed the presence of all ABC transporters in the samples examined, although the results for some of the 48 transporters known in human and analyzed were different in the various tissues. The present results provide information on the expression of ABC transporters at the mRNA level on the ocular surface and in the lacrimal system. Their detection forms the basis for follow-up studies at the protein level, which will provide more information about their physiological significance at the ocular surface and in the lacrimal system and which may explain pathological effects such as drug resistance.

Behavioral and Metabolic Effects of ABCG4 KO in the APPswe,Ind (J9) Mouse Model of Alzheimer's Disease

The pathogenesis of Alzheimer's disease (AD) is complex and involves an imbalance between production and clearance of amyloid-ß peptides (Aß), resulting in accumulation of Aß in senile plaques. Hypercholesterolemia is a major risk factor for developing AD, with cholesterol shown to accumulate in senile plaques and increase production of Aß. ABCG4 is a member of the ATP-binding cassette transporters predominantly expressed in the CNS and has been suggested to play a role in cholesterol and Aß efflux from the brain. In this study, we bred Abcg4 knockout (KO) with the APPSwe,Ind (J9) mouse model of AD to test the hypothesis that loss of Abcg4 would exacerbate the AD phenotype. Unexpectedly, no differences were observed in novel object recognition (NOR) and novel object placement (NOP) behavioral tests, or on histologic examinations of brain tissues for senile plaque numbers. Furthermore, clearance of radiolabeled Aß from the brains did not differ between Abcg4 KO and control mice. Metabolic testing by indirect calorimetry, glucose tolerance test (GTT), and insulin tolerance test (ITT) were also mostly similar between groups with only a few mild metabolic differences noted. Overall, these data suggest that the loss of ABCG4 did not exacerbate the AD phenotype.

Blood-brain barrier transporters: An overview of function, dysfunction in Alzheimer's disease and strategies for treatment

The blood-brain-barrier (BBB) has a major function in maintaining brain homeostasis by regulating the entry of molecules from the blood to the brain. Key players in BBB function are BBB transporters which are highly expressed in brain endothelial cells (BECs) and critical in mediating the exchange of nutrients and waste products. BBB transporters can also influence drug delivery into the brain by inhibiting or facilitating the entry of brain targeting therapeutics for the treatment of brain disorders, such as Alzheimer's disease (AD). Recent studies have shown that AD is associated with a disrupted BBB and transporter dysfunction, although their roles in the development in AD are not fully understand. Modulation of BBB transporter activity may pose a novel approach to enhance the delivery of drugs to the brain for enhanced treatment of AD. In this review, we will give an overview of key functions of BBB transporters and known changes in AD. In addition, we will discuss current strategies for transporter modulation for enhanced drug delivery into the brain.

Adrenal Abcg1 Controls Cholesterol Flux and Steroidogenesis

Cholesterol is the precursor of all steroids, but how cholesterol flux is controlled in steroidogenic tissues is poorly understood. The cholesterol exporter ABCG1 is an essential component of the reverse cholesterol pathway and its global inactivation results in neutral lipid redistribution to tissue macrophages. The function of ABCG1 in steroidogenic tissues, however, has not been explored. To model this, we inactivated Abcg1 in the mouse adrenal cortex, which led to an adrenal-specific increase in transcripts involved in cholesterol uptake and de novo synthesis. Abcg1 inactivation did not affect adrenal cholesterol content, zonation, or serum lipid profile. Instead, we observed a moderate increase in corticosterone production that was not recapitulated by the inactivation of the functionally similar cholesterol exporter Abca1. Altogether, our data imply that Abcg1 controls cholesterol uptake and biosynthesis and regulates glucocorticoid production in the adrenal cortex, introducing the possibility that ABCG1 variants may account for physiological or subclinical variation in stress response.

Behavioral and metabolic and effects of ABCG4 KO in the APPswe,Ind (J9) mouse model of Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is complex and involves an imbalance between production and clearance of amyloid-ß peptides (Aß), resulting in accumulation of Aß in senile plaques. Hypercholesterolemia is a major risk factor for developing AD, with cholesterol shown to accumulate in senile plaques and increase production of Aß. ABCG4 is a member of the ATP-binding cassette transporters predominantly expressed in the CNS, and has been suggested to play a role in cholesterol and Aß efflux from the brain. In this study, we bred Abcg4 knockout (KO) with the APPSwe,Ind (J9) mouse model of AD to test the hypothesis that loss of Abcg4 would exacerbate the AD phenotype. Unexpectedly, no differences were observed in Novel object recognition (NOR) and Novel object placement (NOP) behavioral tests, or on histologic examinations of brain tissues for senile plaque numbers. Furthermore, clearance of radiolabeled Aß from the brains did not differ between Abcg4 KO and control mice. Metabolic testing by indirect calorimetry, glucose tolerance test (GTT) and insulin tolerance test (ITT), were also mostly similar between groups with only a few mild metabolic differences noted. Overall these data suggest that the loss of ABCG4 did not exacerbate the AD phenotype.

ABC transporters: human disease and pharmacotherapeutic potential

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are a 48-member superfamily of membrane proteins that actively transport a variety of biological substrates across lipid membranes. Their functional diversity defines an expansive involvement in myriad aspects of human biology. At least 21 ABC transporters underlie rare monogenic disorders, with even more implicated in the predisposition to and symptomology of common and complex diseases. Such broad (patho)physiological relevance places this class of proteins at the intersection of disease causation and therapeutic potential, underlining them as promising targets for drug discovery, as exemplified by the transformative CFTR (ABCC7) modulator therapies for cystic fibrosis. This review will explore the growing relevance of ABC transporters to human disease and their potential as small-molecule drug targets.

The Reentry Helix Is Potentially Involved in Cholesterol Sensing of the ABCG1 Transporter Protein

ABCG1 has been proposed to play a role in HDL-dependent cellular sterol regulation; however, details of the interaction between the transporter and its potential sterol substrates have not been revealed. In the present work, we explored the effect of numerous sterol compounds on the two isoforms of ABCG1 and ABCG4 and made efforts to identify the molecular motifs in ABCG1 that are involved in the interaction with cholesterol. The functional readouts used include ABCG1-mediated ATPase activity and ABCG1-induced apoptosis. We found that both ABCG1 isoforms and ABCG4 interact with several sterol compounds; however, they have selective sensitivities to sterols. Mutational analysis of potential cholesterol-interacting motifs in ABCG1 revealed altered ABCG1 functions when F571, L626, or Y586 were mutated. L430A and Y660A substitutions had no functional consequence, whereas Y655A completely abolished the ABCG1-mediated functions. Detailed structural analysis of ABCG1 demonstrated that the mutations modulating ABCG1 functions are positioned either in the so-called reentry helix (G-loop/TM5b,c) (Y586) or in its close proximity (F571 and L626). Cholesterol molecules resolved in the structure of ABCG1 are also located close to Y586. Based on the experimental observations and structural considerations, we propose an essential role for the reentry helix in cholesterol sensing in ABCG1.

Structural analysis of cholesterol binding and sterol selectivity by ABCG5/G8

The ATP-binding cassette (ABC) sterol transporters are responsible for maintaining cholesterol homeostasis in mammals by participating in reverse cholesterol transport (RCT) or transintestinal cholesterol efflux (TICE). The heterodimeric ABCG5/G8 carries out selective sterol excretion, preventing the abnormal accumulation of plant sterols in human bodies, while homodimeric ABCG1 contributes to the biogenesis and metabolism of high-density lipoproteins. A sterol-binding site on ABCG5/G8 was proposed at the interface of the transmembrane domain and the core of lipid bilayers. In this study, we have determined the crystal structure of ABCG5/G8 in a cholesterol-bound state. The structure combined with amino acid sequence analysis shows that in the proximity of the sterol-binding site, a highly conserved phenylalanine array supports functional implications for ABCG cholesterol/sterol transporters. Lastly, in silico docking analysis of cholesterol and stigmasterol (a plant sterol) suggests sterol-binding selectivity on ABCG5/G8, but not ABCG1. Together, our results provide a structural basis for cholesterol binding on ABCG5/G8 and the sterol selectivity by ABCG transporters.

Ether lipid transfer across the blood-brain and placental barriers does not improve by inactivation of the most abundant ABC transporters

Phospholipid transport from the periphery to the brain is an understudied topic. When certain lipid species are deficient due to impaired synthesis, though, transfer across the blood-brain barrier is essential for replenishing lipids in the brain. For example, the deficiency in plasmalogens, the most abundant ether lipids in mammals, has detrimental effects on the brain, which is a major issue in inherited peroxisomal disorders but also contributes to more common disorders like Alzheimer's disease. Oral administration of alkylglycerols like batyl alcohol, which carry a pre-formed ether bond, enables replenishment of ether lipids in various peripheral tissues. However, plasmalogen deficiency in the brain cannot be overcome by this approach. Here, we tried to increase cerebral plasmalogen uptake by modulating the efflux transport across the blood-brain barrier. We hypothesized, based on previous literature, that at least some ether lipid species readily enter endothelial cells of the barrier through the transporter MFSD2A but are re-exported by ATP-binding cassette (ABC) transporters. By crossbreeding Mdr1a^-/-/Mdr1b^-/-/Bcrp^-/- and ether lipid-deficient Gnpat^-/- mice as well as pharmacological inhibition with MK-571 to inactivate the major ABC transporters at the blood-brain barrier, we evaluated the potential of combined ABC transporter inhibition and oral batyl alcohol administration for the treatment of plasmalogen deficiency. We found that even in the absence of the most abundant ABC transporters, batyl alcohol supplementation did not restore plasmalogen levels in the brain, despite the presence of a wide spectrum of ether lipid subspecies in the plasma as demonstrated by lipidomic analysis. Surprisingly, batyl alcohol treatment of pregnant Gnpat^+/- dams had beneficial effects on the plasmalogen levels of Gnpat^-/- offspring with defective ether lipid biosynthesis, independently of ABC transporter status at the placental barrier. Our results underline the autonomy of brain lipid homeostasis and indicate that peripheral supplementation of ether lipids is not sufficient to supply the brain with larger amounts of plasmalogens. Yet, the findings suggest that alkylglycerol treatment during pregnancy may pose a viable option to ameliorate some of the severe developmental defects of inborn ether lipid deficiency.

Mutants of the white ABCG Transporter in Drosophila melanogaster Have Deficient Olfactory Learning and Cholesterol Homeostasis

Drosophila's white gene encodes an ATP-binding cassette G-subfamily (ABCG) half-transporter. White is closely related to mammalian ABCG family members that function in cholesterol efflux. Mutants of white have several behavioral phenotypes that are independent of visual defects. This study characterizes a novel defect of white mutants in the acquisition of olfactory memory using the aversive olfactory conditioning paradigm. The w¹¹¹⁸ mutants learned slower than wildtype controls, yet with additional training, they reached wildtype levels of performance. The w¹¹¹⁸ learning phenotype is also found in the w^apricot and w^coral alleles, is dominant, and is rescued by genomic white and mini-white transgenes. Reducing dietary cholesterol strongly impaired olfactory learning for wildtype controls, while w¹¹¹⁸ mutants were resistant to this deficit. The w¹¹¹⁸ mutants displayed higher levels of cholesterol and cholesterol esters than wildtype under this low-cholesterol diet. Increasing levels of serotonin, dopamine, or both in the white mutants significantly improved w¹¹¹⁸ learning. However, serotonin levels were not lower in the heads of the w¹¹¹⁸ mutants than in wildtype controls. There were also no significant differences found in synapse numbers within the w¹¹¹⁸ brain. We propose that the w¹¹¹⁸ learning defect may be due to inefficient biogenic amine signaling brought about by altered cholesterol homeostasis.

ABCG1 and ABCG4 as key transporters in the development of pulmonary alveolar proteinosis by nanoparticles

Pulmonary alveolar proteinosis (PAP) has been reported in rodents treated with nanoparticles (NPs). However, little is known about the type of NPs producing PAP and their toxicity mechanisms. Here, we assembled seven PAP-inducing NPs and TiO₂ NPs as a negative control. At 1 and 6 months after a single intratracheal instillation in rats, pulmonary inflammation and the gene expression of ATP-binding cassette (ABC) transporters and related genes were evaluated in separated alveolar macrophages (AMs). One month after intratracheal instillation, seven NPs (Eu₂O₃, In₂O₃, Pr₆O₁₁, Sm₂O₃, Tb₄O₇, and NiO) caused PAP, but only In₂O₃ NPs caused persistent PAP at 6 months after treatment. The levels of phospholipids, indicators of PAP, showed good correlations with the gene expression profile of five transporters (ABCA1, ABCB4, ABCB8, ABCG1, and ABCG4), which effluxing phospholipids in AMs. Among them, ABCG1 and ABCG4 might be key transporters involved in PAP development because both showed a negative correlation with the magnitude of PAP, while others might be compensatory transporters for PAP recovery, as they showed a positive correlation. In conclusion, the identification of seven PAP-producing NPs implies that PAP may be an emerging occupational disease and that ABCG1 and ABCG4 may be therapeutic targets for PAP.

Structure of the Human Cholesterol Transporter ABCG1

ABCG1 is an ATP binding cassette (ABC) transporter that removes excess cholesterol from peripheral tissues. Despite its role in preventing lipid accumulation and the development of cardiovascular and metabolic disease, the mechanism underpinning ABCG1-mediated cholesterol transport is unknown. Here we report a cryo-EM structure of human ABCG1 at 4 Å resolution in an inward-open state, featuring sterol-like density in the binding cavity. Structural comparison with the multidrug transporter ABCG2 and the sterol transporter ABCG5/G8 reveals the basis of mechanistic differences and distinct substrate specificity. Benzamil and taurocholate inhibited the ATPase activity of liposome-reconstituted ABCG1, whereas the ABCG2 inhibitor Ko143 did not. Based on the structural insights into ABCG1, we propose a mechanism for ABCG1-mediated cholesterol transport.

The Role of ABC Transporters in Lipid Metabolism and the Comorbid Course of Chronic Obstructive Pulmonary Disease and Atherosclerosis

Chronic obstructive pulmonary disease (COPD) ranks among the leading causes of morbidity and mortality worldwide. COPD rarely occurs in isolation and is often combined with various diseases. It is considered that systemic inflammation underlies the comorbid course of COPD. The data obtained in recent years have shown the importance of violations of the cross-links of lipid metabolism and the immune response, which are links in the pathogenesis of both COPD and atherosclerosis. The role of lipid metabolism disorders in the pathogenesis of the comorbid course of COPD and atherosclerosis and the participation of ATP-binding cassette (ABC) transporters in these processes is discussed in this article. It is known that about 20 representatives of a large family of ABC transporters provide lipid homeostasis of cells by moving lipids inside the cell and in its plasma membrane, as well as removing lipids from the cell. It was shown that some representatives of the ABC-transporter family are involved in various links of the pathogenesis of COPD and atherosclerosis, which can determine their comorbid course.

Cholesterol homeostasis in the vertebrate retina: biology and pathobiology

Cholesterol is a quantitatively and biologically significant constituent of all mammalian cell membrane, including those that comprise the retina. Retinal cholesterol homeostasis entails the interplay between de novo synthesis, uptake, intraretinal sterol transport, metabolism, and efflux. Defects in these complex processes are associated with several congenital and age-related disorders of the visual system. Herein, we provide an overview of the following topics: (a) cholesterol synthesis in the neural retina; (b) lipoprotein uptake and intraretinal sterol transport in the neural retina and the retinal pigment epithelium (RPE); (c) cholesterol efflux from the neural retina and the RPE; and (d) biology and pathobiology of defects in sterol synthesis and sterol oxidation in the neural retina and the RPE. We focus, in particular, on studies involving animal models of monogenic disorders pertinent to the above topics, as well as in vitro models using biochemical, metabolic, and omic approaches. We also identify current knowledge gaps and opportunities in the field that beg further research in this topic area.

The ABCs of Sterol Transport

Cholesterol homeostasis and trafficking are critical to the maintenance of the asymmetric plasma membrane of eukaryotic cells. Disruption or dysfunction of cholesterol trafficking leads to numerous human diseases. ATP-binding cassette (ABC) transporters play several critical roles in this process, and mutations in these sterol transporters lead to disorders such as Tangier disease and sitosterolemia. Biochemical and structural information on ABC sterol transporters is beginning to emerge, with published structures of ABCA1 and ABCG5/G8; these two proteins function in the reverse cholesterol transport pathway and mediate the efflux of cholesterol and xenosterols to high-density lipoprotein and bile salt micelles, respectively. Although both of these transporters belong to the ABC family and mediate the efflux of a sterol substrate, they have many distinct differences. Here, we summarize the current understanding of sterol transport driven by ABC transporters, with an emphasis on these two extensively characterized transporters.

Regulation of ABCG4 transporter expression by sterols and LXR ligands

BACKGROUND

Oxysterols, which are derivatives of cholesterol produced by enzymic or non-enzymic pathways, are potent regulators of cellular lipid homeostasis. Sterol homeostasis in the brain is an important area of interest with regards to neurodegenerative conditions like Alzheimer's disease (AD). Brain cells including neurons and astrocytes express sterol transporters belonging to the ABC transporter family of proteins, including ABCA1, ABCG1 and ABCG4, and these transporters are considered of interest as therapeutic targets. Although regulation of ABCA1 and ABCG1 is well established, regulation of ABCG4 is still controversial, in particular whether the transporter is an Liver X receptor (LXR) target. ABCG4 is thought to transport cholesterol, oxysterols and cholesterol synthesis intermediates, and was recently found on the blood brain barrier (BBB), implicated in amyloid-beta export. In this study, we investigate the regulation of ABCG4 by oxysterols, cholesterol-synthesis intermediates and cholesterol itself.

METHODS

ABC transporter expression was measured in neuroblastoma and gliablastoma cell lines and cells overexpressing ABCG4 in response to synthetic LXR ligands, oxysterols and cholesterol-synthesis intermediates.

RESULTS

In contrast to previous reports, ABCG4 expression was induced by a synthetic LXR ligand in U87-MG astrocytes but not in neuroblastoma and BBB endothelial cell lines. In addition, ABCG4 protein was stabilized by cholesterol as was previously shown for ABCG1. ABCG4 protein was furthermore stabilized by cholesterol-synthesis intermediates, desmosterol, lathosterol and lanosterol.

CONCLUSIONS

These results identify new aspects of the post-translational control of ABCG4 that warrant further exploration into the role of this transporter in the maintenance of sterol homeostasis in the brain.

High Density Lipoproteins: Metabolism, Function, and Therapeutic Potential

High Density Lipoproteins (HDLs) have long been considered as “good cholesterol,” beneficial to the whole body and, in particular, to cardio-vascular health. However, HDLs are complex particles that undergoes dynamic remodeling through interactions with various enzymes and tissues throughout their life cycle, making the complete understanding of its functions and roles more complicated than initially expected. In this review, we explore the novel understanding of HDLs' behavior in health and disease as a multifaceted class of lipoprotein, with different size subclasses, molecular composition, receptor interactions, and functionality. Further, we report on emergent HDL-based therapeutics tested in small and larger scale clinical trials and their mixed successes.

ABC Transporters in Neurological Disorders: An Important Gateway for Botanical Compounds Mediated Neuro-Therapeutics

Neurodegeneration is a distinguishing feature of many age related disorders and other vector borne neuroinflammatory diseases. There are a number of factors that can modulate the pathology of these disorders. ATP-binding cassette (ABC) transporters are primarily involved in the maintenance of normal brain homeostasis by eliminating toxic peptides and compounds from the brain. Also, ABC transporters protect the brain from the unwanted effects of endogenous and exogenous toxins that can enter the brain parenchyma. Therefore, these transporters have the ability to determine the pathological outcomes of several neurological disorders. For instance, ABC transporters like P-glycoprotein (ABCB1), and BCRP (ABCG2) have been reported to facilitate the clearance of peptides such as amyloid-β (Aβ) that accumulate in the brain during Alzheimer's disease (AD) progression. Other members such as ABCA1, ABCA2, ABCC8, ABCC9, ABCG1 and ABCG4 also have been reported to be involved in the progression of various brain disorders such as HIV-associated dementia, Multiple sclerosis (MS), Ischemic stroke, Japanese encephalitis (JE) and Epilepsy. However, these defective transporters can be targeted by numerous botanical compounds such as Verapamil, Berberine and Fascalpsyn as a therapeutic target to treat these neurological outcomes. These compounds are already reported to modulate ABC transporter activity in the CNS. Nonetheless, the exact mechanisms involving the ABC transporters role in normal brain functioning, their role in neuronal dysfunction and how these botanical compounds ensure and facilitate their therapeutic action in association with defective transporters still remain elusive. This review therefore, summarizes the role of ABC transporters in neurological disorders, with a special emphasis on its role in AD brains. The prospect of using botanical/natural compounds as modulators of ABC transporters in neurological disorders is discussed in the latter half of the article.

Role of cholesterol homeostasis and its efflux pathways in cancer progression

Tumor cells show high avidity for cholesterol in order to support their inherent nature to divide and proliferate. This results in the rewiring of cholesterol homeostatic pathways by influencing not only de novo synthesis but also uptake or efflux pathways of cholesterol. Recent findings have pointed towards the importance of cholesterol efflux in tumor pathogenesis. Cholesterol efflux is the first and foremost step in reverse cholesterol transport and any perturbation in this pathway may lead to the accumulation of intracellular cholesterol, thereby altering the cellular equilibrium. This review addresses the different mechanisms of cholesterol efflux from the cell and highlights their role and regulation in context to tumor development. There are four different routes by which cholesterol can be effluxed from the cell namely, 1) passive diffusion of cholesterol to mature HDL particles, 2) SR-B1 mediated facilitated diffusion, 3) Active efflux to apo A1 via ABCA1 and 4) ABCG1 mediated efflux to mature HDL. These molecular players facilitating cholesterol efflux are engaged in a complex interplay with different signaling pathways. Thus, an understanding of the efflux pathways, their regulation and cross-talk with signaling molecules may provide novel prognostic markers and therapeutic targets to combat the onset of carcinogenesis.

[Metabolism and Function of High-Density Lipoproteins (HDL)]

Metabolism and Function of High-Density Lipoproteins (HDL) Abstract. HDL has long been considered as 'good cholesterol', beneficial to the whole body and in particular to cardio-vascular health. However, HDL is a complex particle that undergoes dynamic remodeling through interactions with various enzymes and tissue types throughout its life cycle. In this review, we explore the novel understanding of HDL as a multifaceted class of lipoprotein, with multiple subclasses of different size, molecular composition, receptor interactions, and functionality, in health and disease. Further, we report on emergent HDL based therapeutics tested in small and larger scale clinical trials and their mixed successes.

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

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

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

Determination of system level alterations in host transcriptome due to Zika virus (ZIKV) Infection in retinal pigment epithelium

Previously, we reported that Zika virus (ZIKV) causes ocular complications such as chorioretinal atrophy, by infecting cells lining the blood-retinal barrier, including the retinal pigment epithelium (RPE). To understand the molecular basis of ZIKV-induced retinal pathology, we performed a meta-analysis of transcriptome profiles of ZIKV-infected human primary RPE and other cell types infected with either ZIKV or other related flaviviruses (Japanese encephalitis, West Nile, and Dengue). This led to identification of a unique ZIKV infection signature comprising 43 genes (35 upregulated and 8 downregulated). The major biological processes perturbed include SH3/SH2 adaptor activity, lipid and ceramide metabolism, and embryonic organ development. Further, a comparative analysis of some differentially regulated genes (ABCG1, SH2B3, SIX4, and TNFSF13B) revealed that ZIKV induced their expression relatively more than dengue virus did in RPE. Importantly, the pharmacological inhibition of ABCG1, a membrane transporter of cholesterol, resulted in reduced ZIKV infectivity. Interestingly, the ZIKV infection signature revealed the downregulation of ALDH5A1 and CHML, genes implicated in neurological (cognitive impairment, expressive language deficit, and mild ataxia) and ophthalmic (choroideremia) disorders, respectively. Collectively, our study revealed that ZIKV induces differential gene expression in RPE cells, and the identified genes/pathways (e.g., ABCG1) could potentially contribute to ZIKV-associated ocular pathologies.

Expression and function of Abcg4 in the mouse blood-brain barrier: role in restricting the brain entry of amyloid-β peptide

ABCG4 is an ATP-binding cassette transmembrane protein which has been shown, in vitro, to participate in the cellular efflux of desmosterol and amyloid-β peptide (Aβ). ABCG4 is highly expressed in the brain, but its localization and function at the blood-brain barrier (BBB) level remain unknown. We demonstrate by qRT-PCR and confocal imaging that mouse Abcg4 is expressed in the brain capillary endothelial cells. Modelling studies of the Abcg4 dimer suggested that desmosterol showed thermodynamically favorable binding at the putative sterol-binding site, and this was greater than for cholesterol. Additionally, unbiased docking also showed Aβ binding at this site. Using a novel Abcg4-deficient mouse model, we show that Abcg4 was able to export Aβ and desmosterol at the BBB level and these processes could be inhibited by probucol and L-thyroxine. Our assay also showed that desmosterol antagonized the export of Aβ, presumably as both bind at the sterol-binding site on Abcg4. We show for the first time that Abcg4 may function in vivo to export Aβ at the BBB, in a process that can be antagonized by its putative natural ligand, desmosterol (and possibly cholesterol).

Regulated efflux of photoreceptor outer segment-derived cholesterol by human RPE cells

Genetic studies have linked age-related macular degeneration (AMD) to genes involved in high-density lipoprotein (HDL) metabolism, including ATP-binding cassette transporter A1 (ABCA1). The retinal pigment epithelium (RPE) handles large amounts of lipids, among others cholesterol, partially derived from internalized photoreceptor outer segments (OS) and lipids physiologically accumulate in the aging eye. To analyze the potential function of ABCA1 in the eye, we measured cholesterol efflux, the first step of HDL generation, in RPE cells. We show the expression of selected genes related to HDL metabolism in mouse and human eyecups as well as in ARPE-19 and human primary RPE cells. Immunofluorescence staining revealed localization of ABCA1 on both sides of polarized RPE cells. This was functionally confirmed by directional efflux to apolipoprotein AI (ApoA-I) of ³H-labeled cholesterol given to the cells via serum or via OS. ABCA1 expression and activity was modulated using a liver-X-receptor (LXR) agonist and an ABCA1 neutralizing antibody, demonstrating that the efflux was ABCA1-dependent. We concluded that the ABCA1-mediated lipid efflux pathway, and hence HDL biosynthesis, is functional in RPE cells towards both the basal (choroidal) and apical (subretinal) space. Impaired activity of the pathway might cause age-related perturbations of lipid homeostasis in the outer retina and thus may contribute to disease development and/or progression.

Intracellular Localization of Endogenous Mouse ABCG1 Is Mimicked by Both ABCG1-L550 and ABCG1-P550-Brief Report

OBJECTIVE

In a recent article in Arteriosclerosis, Thrombosis, and Vascular Biology, it was reported that ATP-binding cassette transporter G1 (ABCG1) containing leucine at position 550 (ABCG1-L550) was localized to the plasma membrane, whereas ABCG1-P550 (proline at position 550) was intracellular. Because the published data on the subcellular localization of ABCG1 are controversial, we performed additional experiments to determine the importance of leucine or proline at amino acid 550.

APPROACH AND RESULTS

We transfected multiple cell lines (CHO-K1, Cos-7, and HEK293 [human embryonic kidney]) with untagged or FLAG-tagged ABCG1 containing either leucine or proline at position 550. Immunofluorescence studies demonstrated that in all cases, ABCG1 localized to intracellular endosomal vesicles. We also show that both ABCG1-L550 and ABCG1-P550 are equally active in both promoting the efflux of cellular cholesterol to exogenous high-density lipoprotein and in inducing the activity of sterol regulatory element-binding protein-2, presumably as a result of redistributing intracellular sterols away from the endoplasmic reticulum. Importantly, we treated nontransfected primary peritoneal macrophages with a liver X receptor agonist and demonstrate, using immunofluorescence, that although endogenous ABCG1 localizes to intracellular endosomes, none was detectable at the cell surface/plasma membrane.

CONCLUSIONS

ABCG1, irrespective of either a leucine or proline at position 550, is an intracellular protein that localizes to vesicles of the endosomal pathway where it functions to mobilize sterols away from the endoplasmic reticulum and out of the cell.

Effects of a disrupted blood-brain barrier on cholesterol homeostasis in the brain

The presence of the blood-brain barrier (BBB) is critical for cholesterol metabolism in the brain, preventing uptake of lipoprotein-bound cholesterol from the circulation. The metabolic consequences of a leaking BBB for cholesterol metabolism have not been studied previously. Here we used a pericyte-deficient mouse model, Pdgfb^ret/ret, shown to have increased permeability of the BBB to a range of low-molecular mass and high-molecular mass tracers. There was a significant accumulation of plant sterols in the brains of the Pdgfb^ret/ret mice. By dietary treatment with 0.3% deuterium-labeled cholesterol, we could demonstrate a significant flux of cholesterol from the circulation into the brains of the mutant mice roughly corresponding to about half of the measured turnover of cholesterol in the brain. We expected the cholesterol flux into the brain to cause a down-regulation of cholesterol synthesis. Instead, cholesterol synthesis was increased by about 60%. The levels of 24(S)-hydroxycholesterol (24S-OHC) were significantly reduced in the brains of the pericyte-deficient mice but increased in the circulation. After treatment with 1% cholesterol in diet, the difference in cholesterol synthesis between mutants and controls disappeared. The findings are consistent with increased leakage of 24S-OHC from the brain into the circulation in the pericyte-deficient mice. This oxysterol is an efficient suppressor of cholesterol synthesis, and the results are consistent with a regulatory role of 24S-OHC in the brain. To our knowledge, this is the first demonstration that a defective BBB may lead to increased flux of a lipophilic compound out from the brain. The relevance of the findings for the human situation is discussed.

Docosahexaenoic acid modulates the enterocyte Caco-2 cell expression of microRNAs involved in lipid metabolism

Consumption of the long-chain ω-3 (n-3) polyunsaturated fatty acid docosahexaenoic acid (DHA) is associated with a reduced risk of cardiovascular disease and greater chemoprevention. However, the mechanisms underlying the biologic effects of DHA remain unknown. It is well known that microRNAs (miRNAs) are versatile regulators of gene expression. Therefore, we aimed to determine if the beneficial effects of DHA may be modulated in part through miRNAs. Loss of dicer 1 ribonuclease type III (DICER) in enterocyte Caco-2 cells supplemented with DHA suggested that several lipid metabolism genes are modulated by miRNAs. Analysis of miRNAs predicted to target these genes revealed several miRNA candidates that are differentially modulated by fatty acids. Among the miRNAs modulated by DHA were miR-192 and miR-30c. Overexpression of either miR-192 or miR-30c in enterocyte and hepatocyte cells suggested an effect on the expression of genes related to lipid metabolism, some of which were confirmed by endogenous inhibition of these miRNAs. Our results show in enterocytes that DHA exerts its biologic effect in part by regulating genes involved in lipid metabolism and cancer. Moreover, this response is mediated through miRNA activity. We validate novel targets of miR-30c and miR-192 related to lipid metabolism and cancer including nuclear receptor corepressor 2, isocitrate dehydrogenase 1, DICER, caveolin 1, ATP-binding cassette subfamily G (white) member 4, retinoic acid receptor β, and others. We also present evidence that in enterocytes DHA modulates the expression of regulatory factor X6 through these miRNAs. Alteration of miRNA levels by dietary components in support of their pharmacologic modulation might be valuable in adjunct therapy for dyslipidemia and other related diseases.

Cholesterol efflux in megakaryocyte progenitors suppresses platelet production and thrombocytosis

Platelets play a key role in atherogenesis and its complications. Both hypercholesterolemia and increased platelet production promote athero-thrombosis; however, a potential link between altered cholesterol homeostasis and platelet production has not been explored. Transplantation of bone marrow (BM) deficient in ABCG4, a transporter of unknown function, into Ldlr^−/− mice resulted in thrombocytosis, accelerated thrombosis and atherosclerosis. While not detected in lesions, Abcg4 was highly expressed in BM megakaryocyte progenitors (MkP). Abcg4^−/− MkPs displayed defective cholesterol efflux to HDL, increased cell surface levels of thrombopoietin (TPO) receptor (c-MPL) and enhanced proliferation. This appeared to reflect disruption of the negative feedback regulation of c-MPL levels and signaling by E3 ligase c-CBL and cholesterol-sensing LYN kinase. HDL infusions reduced platelet counts in Ldlr^−/− mice and in a mouse model of myeloproliferative neoplasm, in a completely ABCG4-dependent fashion. HDL infusions may offer a novel approach to reducing athero-thrombotic events associated with increased platelet production.

Expanding roles of ABCG1 and sterol transport

PURPOSE OF REVIEW

To offer a comprehensive review on the role of ABCG1 in cellular sterol homeostasis.

RECENT FINDINGS

Early studies with Abcg1 mice indicated that ABCG1 was crucial for tissue lipid homeostasis, especially in the lung. More recent studies have demonstrated that loss of ABCG1 has wide-ranging consequences and impacts lymphocyte and stem cell proliferation, endothelial cell function, macrophage foam cell formation, as well as insulin secretion from pancreatic β cells. Recent studies have also demonstrated that ABCG1 functions as an intracellular lipid transporter, localizes to intracellular vesicles/endosomes, and that the transmembrane domains are sufficient for localization and transport function.

SUMMARY

ABCG1 plays a crucial role in maintaining intracellular sterol and lipid homeostasis. Loss of this transporter has significant, cell-type-specific consequences ranging from effects on cellular proliferation, to surfactant production and/or insulin secretion. Elucidation of the mechanisms by which ABCG1 affects intracellular sterol flux/movement should provide important information that may link ABCG1 to diseases of dysregulated tissue lipid homeostasis.

Lipid metabolism and neuroinflammation in Alzheimer's disease: a role for liver X receptors

Liver X receptors (LXR) are nuclear receptors that have emerged as key regulators of lipid metabolism. In addition to their functions as cholesterol sensors, LXR have also been found to regulate inflammatory responses in macrophages. Alzheimer's disease (AD) is a neurodegenerative disease characterized by a progressive cognitive decline associated with inflammation. Evidence indicates that the initiation and progression of AD is linked to aberrant cholesterol metabolism and inflammation. Activation of LXR can regulate neuroinflammation and decrease amyloid-β peptide accumulation. Here, we highlight the role of LXR in orchestrating lipid homeostasis and neuroinflammation in the brain. In addition, diabetes mellitus is also briefly discussed as a significant risk factor for AD because of the appearing beneficial effects of LXR on glucose homeostasis. The ability of LXR to attenuate AD pathology makes them potential therapeutic targets for this neurodegenerative disease.

Liver X receptor agonist modulation of cholesterol efflux in mice with intestine-specific deletion of microsomal triglyceride transfer protein

OBJECTIVE

Previous work demonstrated that intestinal cholesterol absorption and regulated expression of intestinal Niemann-Pick C1-like 1 and ATP-binding cassette protein A1 are required for liver X receptor (LXR) agonist-mediated increases in high-density lipoprotein biogenesis. We re-examined those conclusions in mice with intestine-specific deletion of the microsomal triglyceride transfer protein (MTTP-IKO), where chylomicron formation is eliminated.

METHODS AND RESULTS

MTTP-IKO mice demonstrated sustained ≈90% reduction in cholesterol absorption and >80% reduction in Niemann-Pick C1-like 1 expression, yet LXR agonist treatment increased serum high-density lipoprotein and upregulated intestinal ATP-binding cassette protein A1 expression. Hepatic lipogenesis and triglyceride content increased with LXR agonist treatment in both genotypes. Biliary cholesterol secretion was increased in MTTP-IKO mice without further increase upon LXR agonist administration. LXR agonist treatment caused a paradoxical increase in cholesterol absorption in MTTP-IKO mice and decreased fecal neutral sterol excretion, but to levels that still exceeded fecal neutral sterol excretion in LXR agonist-treated control mice. Finally, MTTP-IKO mice demonstrated indistinguishable patterns of increased cholesterol turnover and efflux after intravenous radiolabeled cholesterol administration, with or without LXR agonist treatment.

CONCLUSIONS

Both intestinal and hepatic cholesterol efflux pathways are basally upregulated in MTTP-IKO mice. Moreover, LXR-dependent pathways modulate intestinal cholesterol absorption, transport, efflux, and high-density lipoprotein production independent of chylomicron assembly and secretion.

Role of liver X receptors in cholesterol efflux and inflammatory signaling (review)

Liver X receptors (LXRs) are nuclear receptors that play a central role in cholesterol metabolism. When activated, LXRs induce a series of genes that are involved in cholesterol efflux, absorption, transport and excretion. In recent studies, LXRs have also been shown to play an important role in inflammatory signaling. LXR agonists show promise as potential therapeutics, given their anti-atherogenic and anti-inflammatory properties. The function of LXRs in cholesterol efflux and inflammatory signaling make them attractive as therapies for cardiovascular and inflammatory diseases.

ATP binding cassette transporter G1 (ABCG1) is an intracellular sterol transporter

Four members of the mammalian ATP binding cassette (ABC) transporter G subfamily are thought to be involved in transmembrane (TM) transport of sterols. The residues responsible for this transport are unknown. The mechanism of action of ABCG1 is controversial and it has been proposed to act at the plasma membrane to facilitate the efflux of cellular sterols to exogenous high-density lipoprotein (HDL). Here we show that ABCG1 function is dependent on localization to intracellular endosomes. Importantly, localization to the endosome pathway distinguishes ABCG1 and/or ABCG4 from all other mammalian members of this superfamily, including other sterol transporters. We have identified critical residues within the TM domains of ABCG1 that are both essential for sterol transport and conserved in some other members of the ABCG subfamily and/or the insulin-induced gene 2 (INSIG-2). Our conclusions are based on studies in which (i) biotinylation of peritoneal macrophages showed that endogenous ABCG1 is intracellular and undetectable at the cell surface, (ii) a chimeric protein containing the TM of ABCG1 and the cytoplasmic domains of the nonsterol transporter ABCG2 is both targeted to endosomes and functional, and (iii) ABCG1 colocalizes with multiple proteins that mark late endosomes and recycling endosomes. Mutagenesis studies identify critical residues in the TM domains that are important for ABCG1 to alter sterol efflux, induce sterol regulatory element binding protein-2 (SREBP-2) processing, and selectively attenuate the oxysterol-mediated repression of SREBP-2 processing. Our data demonstrate that ABCG1 is an intracellular sterol transporter that localizes to endocytic vesicles to facilitate the redistribution of specific intracellular sterols away from the endoplasmic reticulum (ER).

ATP-binding cassette transporters and their roles in protecting the brain

The blood-brain barrier is a network of endothelial cells that are tightly attached with each other via specialized cell-cell contacts. This passive diffusion barrier is complemented by ATP-binding cassette (ABC) transporters, which are localized on the surface of the endothelial cells. ABC transporters play important roles in the maintenance of blood-brain barrier integrity, as they carry a wide range of organic molecules, cell metabolites, and nutrients both out of the brain and into the brain. Recent studies have unraveled important roles of ABC transporters in the preservation of tissue homeostasis, pointing out the fact that ABC transporters protect both brain parenchymal cells and microvascular cells from injury. As such, ABC transporters have been involved in the pathogenesis of age-related neurodegenerative diseases, such as Parkinson and Alzheimer diseases, recently. This has led to the idea that neurodegenerative processes might be targeted by restoration of transport processes across the blood-brain barrier.

Involvement of low-density lipoprotein receptor-related protein and ABCG1 in stimulation of axonal extension by apoE-containing lipoproteins

Apolipoprotein E (apoE)-containing lipoproteins (LpE) are produced by glial cells in the central nervous system (CNS). When LpE are supplied to distal axons, but not cell bodies, of CNS neurons (retinal ganglion cells) the rate of axonal extension is increased. In this study we have investigated the molecular requirements underlying the stimulatory effect of LpE on axonal extension. We show that enhancement of axonal growth by LpE requires the presence of the low-density lipoprotein receptor-related protein-1 (LRP1) in neurons since RNA silencing of LRP1 in neurons, or antibodies directed against LRP, suppressed the LpE-induced axonal extension. In contrast, an alternative LRP1 ligand, α2-macroglobulin, failed to stimulate axonal extension, suggesting that LpE do not exert their growth-stimulatory effect solely by activation of a LRP1-mediated signaling pathway. In addition, although apoE3-containing LpE enhanced axonal extension, apoE4-containing LpE did not. Over-expression of ABCG1 in rat cortical glial cells resulted in production of LpE that increased the rate of axonal extension to a greater extent than did expression of an inactive, mutant form of ABGC1. Furthermore, reconstituted lipoprotein particles containing apoE3, phosphatidylcholine and sphingomyelin, but not cholesterol, stimulated axonal extension, suggesting that sphingomyelin, but not cholesterol, is involved in the stimulatory effect of LpE. These observations demonstrate that LpE and LRP1 promote axonal extension, and suggest that lipids exported to LpE by ABCG1 are important for the enhancement of axonal extension mediated by LpE.