Human ABCA1 BAC transgenic mice show increased high density lipoprotein cholesterol and ApoAI-dependent efflux stimulated by an internal promoter containing liver X receptor response elements in intron 1

Hepatic nonvesicular cholesterol transport is critical for systemic lipid homeostasis

In cell models, changes in the 'accessible' pool of plasma membrane (PM) cholesterol are linked with the regulation of endoplasmic reticulum sterol synthesis and metabolism by the Aster family of nonvesicular transporters; however, the relevance of such nonvesicular transport mechanisms for lipid homeostasis in vivo has not been defined. Here we reveal two physiological contexts that generate accessible PM cholesterol and engage the Aster pathway in the liver: fasting and reverse cholesterol transport. During fasting, adipose-tissue-derived fatty acids activate hepatocyte sphingomyelinase to liberate sequestered PM cholesterol. Aster-dependent cholesterol transport during fasting facilitates cholesteryl ester formation, cholesterol movement into bile and very low-density lipoprotein production. During reverse cholesterol transport, high-density lipoprotein delivers excess cholesterol to the hepatocyte PM through scavenger receptor class B member 1. Loss of hepatic Asters impairs cholesterol movement into feces, raises plasma cholesterol levels and causes cholesterol accumulation in peripheral tissues. These results reveal fundamental mechanisms by which Aster cholesterol flux contributes to hepatic and systemic lipid homeostasis.

Current models of apolipoprotein A-I lipidation by adenosine triphosphate binding cassette transporter A1

PURPOSE OF REVIEW

The primary cardioprotective function of high-density lipoprotein (HDL) is to remove excess cellular free cholesterol (FC) from peripheral tissues and deliver it to the liver. Here, we summarize recent research that examines apolipoprotein A-I (apoA-I) lipidation models by adenosine triphosphate binding cassette transporter A1 (ABCA1) and discuss its relevance in atherosclerotic cardiovascular disease (ASCVD).

RECENT FINDINGS

The first step in HDL formation involves the interaction between apoA-I and ABCA1, where ABCA1 mediates the removal of FC and phospholipids from lipid-laden macrophages to form discoidal nascent HDL (nHDL). However, there are currently no clear-cut systematic models that characterize HDL formation. A number of recent studies have investigated the importance of apoA-I C- and N-terminal domains required for optimal cholesterol efflux and nHDL production. Furthermore, functional ABCA1 is required for direct or indirect binding to apoA-I where ABCA1 dimer-monomer interconversion facilitates apoA-I lipidation from plasma membrane microdomains. Microparticles are also another lipid source for apoA-I solubilization into nHDL.

SUMMARY

ApoA-I and ABCA1 are key factors in macrophage-mediated cholesterol efflux and nHDL production. Understanding of the key steps in HDL formation may unlock the therapeutic potential of HDL and improve clinical management of ASCVD.

Rosmarinic Acid Exhibits a Lipid-Lowering Effect by Modulating the Expression of Reverse Cholesterol Transporters and Lipid Metabolism in High-Fat Diet-Fed Mice

Hyperlipidemia is a potent risk factor for the development of cardiovascular diseases. The reverse cholesterol transport (RCT) process has been shown to alleviate hyperlipidemia and protect against cardiovascular diseases. Recently, rosmarinic acid was reported to exhibit lipid-lowering effects. However, the underlying mechanism is still unclear. This study aims to investigate whether rosmarinic acid lowers lipids by modulating the RCT process in high-fat diet (HFD)-induced hyperlipidemic C57BL/6J mice. Our results indicated that rosmarinic acid treatment significantly decreased body weight, blood glucose, and plasma total cholesterol and triglyceride levels in HFD-fed mice. Rosmarinic acid increased the expression levels of cholesterol uptake-associated receptors in liver tissues, including scavenger receptor B type 1 (SR-B1) and low-density lipoprotein receptor (LDL-R). Furthermore, rosmarinic acid treatment notably increased the expression of cholesterol excretion molecules, ATP-binding cassette G5 (ABCG5) and G8 (ABCG8) transporters, and cholesterol 7 alpha-hydroxylase A1 (CYP7A1) as well as markedly reduced cholesterol and triglyceride levels in liver tissues. In addition, rosmarinic acid facilitated fatty acid oxidation through AMP-activated protein kinase (AMPK)-mediated carnitine palmitoyltransferase 1A (CPT1A) induction. In conclusion, rosmarinic acid exhibited a lipid-lowering effect by modulating the expression of RCT-related proteins and lipid metabolism-associated molecules, confirming its potential for the prevention or treatment of hyperlipidemia-derived diseases.

Reciprocal association of serum Mac-2 binding protein and HDL-cholesterol concentrations

BACKGROUND

Mac-2 binding protein (Mac-2BP) is used as a serum biomarker of nonalcoholic steatohepatitis, considered to be a liver phenotype of metabolic syndrome (MetS). In this study, we investigated the serum Mac-2BP concentrations-correlated MetS-related clinical parameters in vivo, and the underlying mechanism in vitro.

MATERIALS & METHODS

We enrolled 54 healthy Japanese men who underwent health examination at Osaka University Health Care Center in this study. Physical and serum biochemical parameters were obtained from all the subjects. In the cultured HepG2 cells, the effects of interferon (IFN)-γ on the expression of Mac-2BP, apolipoprotein (apo) A-I, and ATP binding cassette transporter A1 (ABCA1) were studied.

RESULTS

Serum Mac-2BP concentrations correlated negatively with HDL-C, and positively with body mass index and systolic blood pressure in univariate analysis. These results suggested the association between Mac-2BP and MetS, although none of these 3 parameters had significant correlation with serum Mac-2BP concentrations in multivariate analysis. In HepG2 cells, IFN-γ stimulation resulted in the increased Mac-2BP and the decreased ABCA1 and apo A-I mRNA concentrations, while Mac-2BP had no effects on ABCA1 and apo A-I concentrations.

CONCLUSIONS

The serum Mac-2BP concentrations are negatively correlated with HDL-C concentrations in healthy subjects, as a result of chronic inflammation.

Rapid degradation of ABCA1 protein following cAMP withdrawal and treatment with PKA inhibitor suggests ABCA1 is a short-lived protein primarily regulated at the transcriptional level

Objectives

ATP-binding cassette transporter A1 (ABCA1) is a key player in the reverse cholesterol transport (RCT) and HDL biogenesis. Since RCT is compromised as a result of ABCA1 dysfunction in diabetic state, the objective of this study was to investigate the regulation of ABCA1 in a stably transfected 293 cells expressing ABCA1 under the control of cAMP response element.

Methods

To delineate transcriptional and posttranscriptional regulation of ABCA1, 293 cells were stably transfected with the full length ABCA1 cDNA under the control of CMV promoter harboring cAMP response element. cAMP-mediated regulation of ABCA1 and cholesterol efflux were studied in the presence of 8-Br-cAMP and after withdrawal of 8-Br-cAMP. The mechanism of cAMP-mediated transcriptional induction of the ABCA1 gene was studied in protein kinase A (PKA) inhibitors-treated cells.

Results

The transfected 293 cells expressed high levels of ABCA1, while non-transfected wild-type 293 cells showed very low levels of ABCA1. Treatments of transfected cells with 8-Br-cAMP increased ABCA1 protein by 10-fold and mRNA by 20-fold. Cholesterol efflux also increased in parallel. Withdrawal of 8-Br-cAMP caused time-dependent rapid diminution of ABCA1 protein and mRNA, suggesting ABCA1 regulation at the transcriptional level. Treatment with PKA inhibitors abolished the cAMP-mediated induction of the ABCA1 mRNA and protein, resulting dampening of ABCA1-dependent cholesterol efflux.

Conclusions

These results demonstrate that transfected cell line mimics cAMP response similar to normal cells with natural ABCA1 promoter and suggest that ABCA1 is a short-lived protein primarily regulated at the transcriptional level to maintain cellular cholesterol homeostasis.

The interaction between ABCA1 polymorphism and physical activity on the HDL-cholesterol levels in a Japanese population

Few studies have investigated the interactions between HDL-C-related SNPs identified by genome-wide association (GWA) study and physical activity (PA) on HDL-C. First, we conducted a sex-stratified GWA study in a discovery sample (2,231 men and 2,431 women) and replication sample (2,599 men and 3,109 women) to identify SNPs influencing log-transformed HDL-C in Japanese participants in the baseline survey of the Japan Multi-Institutional Collaborative Cohort Study. We also replicated previously reported HDL-C-related SNPs in a combined (discovery plus replication) sample (4,830 men and 5,540 women). We then analyzed the interactions of the HDL-C-related SNPs with PA on HDL-C. The sex-stratified GWA analyses identified 11 and 10 HDL-C-related SNPs in men and women as targets for an interaction analysis. Among these, only one interaction of ABCA1 rs1883025 with PA was statistically significant in men, after Bonferroni correction [P-interaction = 0.001 (α = 0.05/21 = 0.002)]. The per-major-allele (C allele) increase in log-transformed HDL-C was lost in men with low PA (β = 0.008) compared with those with medium (β = 0.032) or high PA (β = 0.034). These findings suggest that the benefit of carrying a C allele of ABCA1 rs1883025 on enhancing HDL-C may be attenuated in inactive men.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

Single cell transcriptome in aneuploidies reveals mechanisms of gene dosage imbalance

Aneuploidy is a major source of gene dosage imbalance due to copy number alterations (CNA), and viable human trisomies are model disorders of altered gene expression. We study gene and allele-specific expression (ASE) of 9668 single-cell fibroblasts from trisomy 21 (T21) discordant twins and from mosaic T21, T18, T13 and T8. We examine 928 single cells with deep scRNAseq. Expected and observed overexpression of trisomic genes in trisomic vs. diploid bulk RNAseq is not detectable in trisomic vs. diploid single cells. Instead, for trisomic genes with low-to-average expression, their altered gene dosage is mainly due to the higher fraction of trisomic cells simultaneously expressing these genes, in agreement with a stochastic 2-state burst-like model of transcription. These results, confirmed in a further analysis of 8740 single fibroblasts with shallow scRNAseq, suggest that the specific transcriptional profile of each gene contributes to the phenotypic variability of trisomies. We propose an improved model to understand the effects of CNA and, generally, of gene regulation on gene dosage imbalance.

Characterization of cholesterol metabolism in Sertoli cells and spermatogenesis (Review)

The Sertoli cell, which is the supporting cell of spermatogenesis, has an important role in the endocrine and paracrine control of spermatogenesis. Functionally, it provides the cells of the seminiferous epithelium with nutrition, conveys mature spermatids to the lumen of seminiferous tubules, secretes androgen‑binding protein and interacts with endocrine Leydig cells. In addition, the levels of cholesterol, as well as its intermediates, vary greatly between nongonadal tissues and the male reproductive system. Throughout spermatogenesis, a dynamic and constant alteration in the membrane lipid composition of Sertoli cells occurs. In several mammalian species, testis meiosis‑activating sterol and desmosterol, as well as other cholesterol precursors, accumulate in the testes and spermatozoa. In addition, certain cholesterogenic genes exhibit stage‑specific expression patterns during spermatogenesis, including the cytochrome P450 enzyme lanosterol 14α‑demethylase. Inconsistency in the patterns of gene expression during spermatogenesis indicates a cell‑type specific and complex temporary modulation of lipids and cholesterol, which also implicates the dynamic interactions between Sertoli cells and germ cells. Furthermore, in the female reproductive tract and during epididymal transit, which is a prerequisite for valid fertilization, the modulation of cholesterol occurring in spermatozoal membranes further indicates the functional importance of sterol compounds in spermatogenesis. However, the exact role of cholesterol metabolism in Sertoli cells in sperm production is unknown. The present review article describes the progress made in the research regarding the characteristics of the Sertoli cell, particularly the regulation of its cholesterol metabolism during spermatogenesis.

Chinese Olive (Canarium album L.) Fruit Extract Attenuates Metabolic Dysfunction in Diabetic Rats

Hyperglycemia and dysregulation of lipid metabolism play a crucial role in metabolic dysfunction. The aims of present study were to evaluate the ameliorative effect of the ethyl acetate fraction of Chinese olive fruit extract (CO-EtOAc) on high-fat diet (HFD) and streptozotocin (STZ)-induced diabetic rats. CO-EtOAc, rich in gallic acid and ellagic acid, could markedly decreased the body weight and epididymal adipose mass. In addition, CO-EtOAc increased serum HDL-C levels, hepatic GSH levels, and antioxidant enzyme activities; lowered blood glucose, serum levels of total cholesterol (TC), triglycerides (TG), bile acid, and tumor necrosis factor alpha (TNFα); and reduced TC and TG in liver. We further demonstrated that CO-EtOAc mildly suppressed hepatic levels of phosphorylated IRS-1, TNF-α, and IL-6, but enhanced Akt phosphorylation. The possible mechanisms of cholesterol metabolism were assessed by determining the expression of genes involved in cholesterol transportation, biosynthesis, and degradation. It was found that CO-EtOAc not only inhibited mRNA levels of SREBP-2, HMG-CoAR, SR-B1, and CYP7A1 but also increased the expression of genes, such as ABCA1 and LDLR that governed cholesterol efflux and cholesterol uptake. Moreover, the protein expressions of ABCA1 and LDLR were also significantly increased in the liver of rats supplemented with CO-EtOAc. We suggest that Chinese olive fruit may ameliorate metabolic dysfunction in diabetic rats under HFD challenge.

Farnesoid X Receptor and Liver X Receptor Ligands Initiate Formation of Coated Platelets

OBJECTIVES

The liver X receptors (LXRs) and farnesoid X receptor (FXR) have been identified in human platelets. Ligands of these receptors have been shown to have nongenomic inhibitory effects on platelet activation by platelet agonists. This, however, seems contradictory with the platelet hyper-reactivity that is associated with several pathological conditions that are associated with increased circulating levels of molecules that are LXR and FXR ligands, such as hyperlipidemia, type 2 diabetes mellitus, and obesity.

APPROACH AND RESULTS

We, therefore, investigated whether ligands for the LXR and FXR receptors were capable of priming platelets to the activated state without stimulation by platelet agonists. Treatment of platelets with ligands for LXR and FXR converted platelets to the procoagulant state, with increases in phosphatidylserine exposure, platelet swelling, reduced membrane integrity, depolarization of the mitochondrial membrane, and microparticle release observed. Additionally, platelets also displayed features associated with coated platelets such as P-selectin exposure, fibrinogen binding, fibrin generation that is supported by increased serine protease activity, and inhibition of integrin αIIbβ3. LXR and FXR ligand-induced formation of coated platelets was found to be dependent on both reactive oxygen species and intracellular calcium mobilization, and for FXR ligands, this process was found to be dependent on cyclophilin D.

CONCLUSIONS

We conclude that treatment with LXR and FXR ligands initiates coated platelet formation, which is thought to support coagulation but results in desensitization to platelet stimuli through inhibition of αIIbβ3 consistent with their ability to inhibit platelet function and stable thrombus formation in vivo.

ABCA1/ApoE/HDL Pathway Mediates GW3965-Induced Neurorestoration After Stroke

BACKGROUND AND PURPOSE

ATP-binding cassette transporter A1 (ABCA1) is a major reverse cholesterol transporter and plays critical role in the formation of brain high-density lipoprotein (HDL) cholesterol. Apolipoprotein E (ApoE) is the most abundant apolipoprotein and transports cholesterol into cells in brain. ABCA1 and ApoE are upregulated by liver-X receptors. Activation of liver-X receptors has neurorestorative benefit for stroke. The current study investigates whether ABCA1/ApoE/HDL pathway mediates GW3965, a synthetic dual liver-X receptor agonist, induced neurorestoration after stroke.

METHODS

Middle-aged male specific brain ABCA1-deficient (ABCA1^-B/-B) and floxed-control (ABCA1^fl/fl) mice were subjected to distal middle-cerebral artery occlusion (dMCAo) and gavaged with saline or GW3965 (10 mg/kg) or intracerebral infusion of artificial cerebrospinal fluid or human plasma HDL3 in ABCA1^-B/-B stroke mice, starting 24 hours after dMCAo and daily until euthanization 14 days after dMCAo.

RESULTS

No differences in the blood level of total cholesterol and triglyceride and lesion volume were found among the groups. Compared with ABCA1^fl/fl ischemic mice, ABCA1^-B/-B ischemic mice exhibited impairment functional outcome and decreased ABCA1/ApoE expression and decreased gray/white matter densities in the ischemic boundary zone 14 days after dMCAo. GW3965 treatment of ABCA1^fl/fl ischemic mice led to increased brain ABCA1/ApoE expression, concomitantly to increased blood HDL, gray/white matter densities and oligodendrocyte progenitor cell numbers in the ischemic boundary zone, as well as improved functional outcome 14 days after dMCAo. GW3965 treatment had negligible beneficial effects in ABCA1^-B/-B ischemic mice. However, intracerebral infusion of human plasma HDL3 significantly attenuated ABCA1^-B/-B-induced deficits. In vitro, GW3965 treatment (5 μM) increased ABCA1/synaptophysin level and neurite/axonal outgrowth in primary cortical neurons derived from ABCA1^fl/fl embryos, but not in neurons derived from ABCA1^-B/-B embryos. HDL treatment (80 μg/mL) attenuated the reduction of neurite/axonal outgrowth in neurons derived from ABCA1^-B/-B embryos.

CONCLUSIONS

ABCA1/ApoE/HDL pathway, at least partially, contributes to GW3965-induced neurorestoration after stroke.

Cellular Cholesterol Accumulation Facilitates Ubiquitination and Lysosomal Degradation of Cell Surface–Resident ABCA1

Objective—

By excreting cellular cholesterol to apolipoprotein A-I, ATP-binding cassette transporter A1 (ABCA1) mediates the biogenesis of high-density lipoprotein in hepatocytes and prevents foam cell formation from macrophages. We recently showed that cell surface–resident ABCA1 (csABCA1) undergoes ubiquitination and later lysosomal degradation through the endosomal sorting complex required for transport system. Herein, we investigated the relevance of this degradation pathway to the turnover of csABCA1 in hypercholesterolemia.

Approach and Results—

Immunoprecipitation and cell surface-biotinylation studies with HepG2 cells and mouse peritoneal macrophages showed that the ubiquitination level and degradation of csABCA1 were facilitated by treatment with a liver X receptor (LXR) agonist and acetylated low-density lipoprotein. The effects of an LXR agonist and acetylated low-density lipoprotein on the degradation of csABCA1 were repressed completely by treatment with bafilomycin, an inhibitor of lysosomal degradation, and by depletion of tumor susceptibility gene 101, a major component of endosomal sorting complex required for transport-I. RNAi analysis indicated that LXRβ inhibited the accelerated lysosomal degradation of csABCA1 by the LXR agonist, regardless of its transcriptional activity. Cell surface coimmunoprecipitation with COS1 cells expressing extracellularly hemagglutinin-tagged ABCA1 showed that LXRβ interacted with csABCA1 and inhibited the ubiquitination of csABCA1. Immunoprecipitates with anti-ABCA1 antibodies from the liver plasma membranes showed less LXRβ and a higher ubiquitination level of ABCA1 in high-fat diet–fed mice than in normal chow-fed mice.

Conclusions—

Under conditions of high cellular cholesterol content, csABCA1 became susceptible to ubiquitination by dissociation of LXRβ from csABCA1, which facilitated the lysosomal degradation of csABCA1 through the endosomal sorting complex required for transport system.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

High-density lipoprotein and atherosclerosis: Roles of lipid transporters

Various previous studies have found a negative correlation between the risk of cardiovascular events and serum high-density lipoprotein (HDL) cholesterol levels. The reverse cholesterol transport, a pathway of cholesterol from peripheral tissue to liver which has several potent antiatherogenic properties. For instance, the particles of HDL mediate to transport cholesterol from cells in arterial tissues, particularly from atherosclerotic plaques, to the liver. Both ATP-binding cassette transporters (ABC) A1 and ABCG1 are membrane cholesterol transporters and have been implicated in mediating cholesterol effluxes from cells in the presence of HDL and apolipoprotein A-I, a major protein constituent of HDL. Previous studies demonstrated that ABCA1 and ABCG1 or the interaction between ABCA1 and ABCG1 exerted antiatherosclerotic effects. As a therapeutic approach for increasing HDL cholesterol levels, much focus has been placed on increasing HDL cholesterol levels as well as enhancing HDL biochemical functions. HDL therapies that use injections of reconstituted HDL, apoA-I mimetics, or full-length apoA-I have shown dramatic effectiveness. In particular, a novel apoA-I mimetic peptide, Fukuoka University ApoA-I Mimetic Peptide, effectively removes cholesterol via specific ABCA1 and other transporters, such as ABCG1, and has an antiatherosclerotic effect by enhancing the biological functions of HDL without changing circulating HDL cholesterol levels. Thus, HDL-targeting therapy has significant atheroprotective potential, as it uses lipid transporter-targeting agents, and may prove to be a therapeutic tool for atherosclerotic cardiovascular diseases.

Peripheral Blood Mononuclear Cell ABCA1 Transcripts and Protein Expression in Acute Myocardial Infarction

BACKGROUND

ATP binding cassette transporter-A1 (ABCA1) facilitates the formation of high density lipoprotein (HDL). HDL due to its anti-atherosclerotic, anti-inflammatory and anti-thrombotic activities provides protection against atherothrombosis or myocardial infarction (MI). The aim was to investigate the role of peripheral blood mononuclear cell (PBMNC) ABCA1 expression in MI.

METHODS

The participants comprised 29 males with acute MI (AMI) and 20 healthy controls. AMI patients were normotensive, not on statins, with triglycerides < 200mg/dl and categorized into AMI with type 2 diabetes (T2DM) (N = 12) and without T2DM (N = 17). The PBMNC ABCA1 mRNA transcripts were analysed by quantitative real-time polymerase chain reaction (qRTPCR) and protein by enzyme linked immunosorbent assay (ELISA).

RESULTS

PBMNC ABCA1 mRNA transcript and protein levels were not significantly different in AMI patients or when sub-grouped into with/without T2DM, as compared to controls. ABCA1 protein correlated positively with HDL-cholesterol (r = 0.655, p = 0.021) in AMI patients with T2DM and negatively with age (r = - 0.525, p = 0.031) in AMI patients without T2DM and it was more strongly associated in latter group with smoking and alcohol habit.

CONCLUSION

In the present study, the effects of metabolites of diabetes and ischemia were observed on PBMNC ABCA1 protein and thus on HDL-C in AMI patients. Further influence of risk factors such as smoking and alcohol consumption observed in the present study can be evaluated in larger sample size. The control of these cardiovascular associated risk factors may increase stability of PBMNC ABCA1 protein and thus HDL-C levels.

The high-fat high-fructose hamster as an animal model for niacin's biological activities in humans

OBJECTIVE

Niacin has been used for more than 50 years to treat dyslipidemia, yet the mechanisms underlying its lipid-modifying effects remain unknown, a situation stemming at least in part from a lack of validated animal models. The objective of this study was to determine if the dyslipidemic hamster could serve as such a model.

MATERIALS/METHODS

Dyslipidemia was induced in Golden Syrian hamsters by feeding them a high-fat, high-cholesterol, and high-fructose (HF/HF) diet. The effect of high-dose niacin treatment for 18 days and 28 days on plasma lipid levels and gene expression was measured.

RESULTS

Niacin treatment produced significant decreases in plasma total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and free fatty acids (FFA), but had no measureable effect on high-density lipoprotein cholesterol (HDL-C) in the dyslipidemic hamster. Niacin treatment also produced significant increases in hepatic adenosine ATP-Binding Cassette A1 (ABCA1) mRNA, ABCA1 protein, apolipoprotein A-I (Apo A-I) mRNA, and adipose adiponectin mRNA in these animals.

CONCLUSIONS

With the exception of HDL-C, the lipid effects of niacin treatment in the dyslipidemic hamster closely parallel those observed in humans. Moreover, the effects of niacin treatment on gene expression of hepatic proteins related to HDL metabolism are similar to those observed in human cells in culture. The HF/HF-fed hamster could therefore serve as an animal model for niacin's lowering of proatherogenic lipids and mechanisms of action relative to lipid metabolism.

Effects of compound X, a novel potent inhibitor of acyl-coenzyme A:cholesterol O-acyltransferase, on the adrenal gland of rats

To investigate the adrenal toxicity of a novel inhibitor of acyl-coenzyme A:cholesterol O-acyltransferase, compound X (CX), histopathological examinations, fat staining, adrenal cholesterol measurement, blood biochemistry, plasma corticosterone and ACTH measurement, ACTH-stimulation assay, and adrenal gene-expression analyses were done in rats in repeated-dose studies (experiment 1: 0, 3, 10, 30 and 150mg/kg for 4, 8, 15 and 28 days; experiment 2: 0, 3, 10,30 and 150mg/kg for 28 days; experiment 3: 0, 10, 30, 100 and 300mg/kg for 28 days). CX induced morphologic changes such as vacuolation and hypertrophy in the zona fasciculata (ZF) at ≥10mg/kg, and eosinophilic changes in the ZF at 150mg/kg. Vacuolation decreased in a dose-dependent manner and was replaced by eosinophilic changes. Inflammatory and fibrous changes were observed at ≥30mg/kg. These changes were expressed at early stages of dosing and were not exacerbated by extension of the administration period. Oil-red-O/Filipin staining showed depletion of cholesterol ester in dose-dependent manner and enabled adrenal cholesterol measurement. Filipin staining also revealed vacuoles to be composed of cholesterol esters. No significant changes were observed during the dosing period of CX for plasma corticosterone and ACTH levels. Gene-expression analyses showed up-regulation of Star and Abca1 mRNA levels at 300mg/kg. In conclusion, CX induced adrenal toxicity, but CX did not influence adrenocortical functions, and exacerbation of adrenal toxicities by extension of the administration period was not observed. Up-regulation of genes related to the transport of FC, such as Star and Abca1, were observed in CX groups, and these genes may be involved in the maintenance of adrenal structure and function in rats given CX.

Reduced biliary sterol output with no change in total faecal excretion in mice expressing a human apolipoprotein A-I variant

BACKGROUND/AIMS

Apolipoprotein (apo)A-I(M) (ilano), is a molecular variant of apoA-I(wild-type), associated with dramatically low HDL-cholesterol levels, but no increased risk for cardiovascular disease. In view of the present uncertainties on the role of apoA-I in liver cholesterol removal by way of bile acids and neutral sterols, and of the greater capacity of apoA-I(M) (ilano) to remove arterial cholesterol, biliary sterol metabolism was evaluated in transgenic mice expressing apoA-I(M) (ilano).

METHODS

ApoA-I(M) (ilano) mice were fed a high-cholesterol/high-fat diet, and compared with human apoA-I(wild-type) mice. Plasma lipid levels, hepatic bile flow and composition, hepatic and intestinal cholesterol and bile acid content, and faecal sterol content were measured. Moreover, the expression of hepatic ABCA1, SR-B1 and that of hepatic and intestinal genes involved in bile acid metabolism were evaluated.

RESULTS

The dietary treatment led to a strong elevation in HDL-cholesterol levels in A-I(M) (ilano) mice, associated with an increased expression of hepatic ABCA1. ApoA-I(M) (ilano) mice showed lower cholesterol output from the liver compared with apoA-I(wild-type) mice, in the absence of liver sterol accumulation. Faecal excretion of neutral sterols and bile acids was similar in the two mouse lines.

CONCLUSIONS

In spite of a different response to the dietary challenge, with an increased ABCA1 expression and a lower hepatic cholesterol output in apoA-I(M) (ilano) mice, the net sterol excretion is comparable in the two transgenic lines.

Hepatic ABC transporters and triglyceride metabolism

PURPOSE OF REVIEW

Elevated plasma triglyceride and reduced HDL concentrations are prominent features of metabolic syndrome and type 2 diabetes. Individuals with Tangier disease also have elevated plasma triglyceride concentrations and very low HDL, resulting from mutations in ATP-binding cassette transporter A1 (ABCA1), an integral membrane protein that facilitates nascent HDL particle assembly. Past studies attributed the inverse relationship between plasma HDL and triglyceride to intravascular lipid exchange and catabolic events. However, recent studies also suggest that hepatic signaling and lipid mobilization and secretion may explain how HDL affects plasma triglyceride concentrations.

RECENT FINDINGS

Hepatocyte-specific ABCA1 knockout mice have markedly reduced plasma HDL and a two-fold increase in triglyceride due to failure to assemble nascent HDL particles by hepatocytes, causing increased catabolism of HDL apolipoprotein A-I and increased hepatic production of triglyceride-enriched VLDL. In-vitro studies suggest that nascent HDL particles may induce signaling to decrease triglyceride secretion. Inhibition of microRNA 33 expression in nonhuman primates augments hepatic ABCA1, genes involved in fatty acid oxidation, and decreases expression of lipogenic genes, causing increased plasma HDL and decreased triglyceride levels.

SUMMARY

New evidence suggests potential mechanisms by which hepatic ABCA1-mediated nascent HDL formation regulates VLDL-triglyceride production and contributes to the inverse relationship between plasma HDL and triglyceride.

Novel evidence of the involvement of calreticulin in major psychiatric disorders

Calreticulin (CALR) is a multi-functional protein that is strictly conserved across species. Two mRNA transcripts have been recognized for the CALR gene in humans, which use a common promoter sequence. We have recently reported mutations in the CALR promoter that co-occur with psychosis. One of those mutations at -220A increases gene expression in human BE(2)-C and HEK-293 cell lines. This mutation is the first instance of a functional cognition-deficit mutation reversing a human gene promoter to the primitive type. In the current study, we analyzed the effect of the most widely-used mood-stabilizing drug, valproic acid (VPA), on nucleotide -220 in two neuronal cell lines, LAN-5 and N2A. Remarkably, VPA increased gene expression in the cells with the wild-type -220C construct, whereas a dramatic decrease in gene expression was observed in the cell lines with the mutant construct (p<0.000004 and p<0.016, respectively). We also sequenced the 600-bp CALR promoter, and the highly conserved intron 1 sequence in an independent sample of patients afflicted with major psychiatric disorders and controls. A new case of major depressive disorder with psychotic features with the -220A mutation was identified. A novel 1-bp insertion was also detected in intron 1 at IVSI-310, in a case of amphetamine-induced psychosis. As for the psychosis-linked CALR promoter mutations identified to-date, the IVSI mutation was not detected in the control pool. This mutation creates a RREB-1 transcription factor binding site within the first intron. Our present findings identify the site of action of VPA in the CALR promoter, and introduce a novel mutation in a case of substance-induced psychosis in the first intron of CALR.

HNF4α Increases Liver-Specific Human ATP-Binding Cassette Transporter A1 Expression and Cholesterol Efflux to Apolipoprotein A-I in Response to Cholesterol Depletion

Objective—

Hepatic ATP-binding cassette transporter A1 (ABCA1) plays the major role in maintaining plasma high-density lipoprotein levels by producing cholesterol-accepting nascent high-density lipoprotein, whereas peripheral ABCA1 is responsible for releasing cellular cholesterol. We previously reported that in rodents, cholesterol depletion reduces ABCA1 expression in peripheral but not hepatic cells by increasing a liver-specific ABCA1 transcript via the sterol regulatory element-binding protein-2 system. However, the regulatory element is not conserved in humans. Here we investigated the mechanism of sterol-regulated human hepatic ABCA1 gene expression.

Methods and Results—

ABCA1 mRNA variant type L3 is a novel and human-liver-specific transcript accounting for ≈25% of total ABCA1 mRNA in the liver and is induced by cellular cholesterol depletion. Specific knockdown or forced expression revealed that type L3 produces functional ABCA1 protein in cholesterol efflux. We identified a regulatory enhancer element for L3 expression lying within intron 3 of the human ABCA1 gene, to which hepatocyte nuclear factor (HNF) 4α binds in response to cholesterol depletion. HNF4α knockdown abolished induction of liver-specific L3 and L2b transcripts (and consequently the liver-type response of ABCA1 expression to cellular cholesterol status) and diminished cholesterol efflux activity.

Conclusion—

These findings indicate that HNF4α regulates human hepatic ABCA1 expression in response to cholesterol depletion.

Niacin increases HDL biogenesis by enhancing DR4-dependent transcription of ABCA1 and lipidation of apolipoprotein A-I in HepG2 cells

The lipidation of apoA-I in liver greatly influences HDL biogenesis and plasma HDL levels by stabilizing the secreted apoA-I. Niacin is the most effective lipid-regulating agent clinically available to raise HDL. This study was undertaken to identify regulatory mechanisms of niacin action in hepatic lipidation of apoA-I, a critical event involved in HDL biogenesis. In cultured human hepatocytes (HepG2), niacin increased: association of apoA-I with phospholipids and cholesterol by 46% and 23% respectively, formation of lipid-poor single apoA-I molecule-containing particles up to ~2.4-fold, and pre β 1 and α migrating HDL particles. Niacin dose-dependently stimulated the cell efflux of phospholipid and cholesterol and increased transcription of ABCA1 gene and ABCA1 protein. Mutated DR4, a binding site for nuclear factor liver X receptor alpha (LXR α ) in the ABCA1 promoter, abolished niacin stimulatory effect. Further, knocking down LXR α or ABCA1 by RNA interference eliminated niacin-stimulated apoA-I lipidation. Niacin treatment did not change apoA-I gene expression. The present data indicate that niacin increases apoA-I lipidation by enhancing lipid efflux through a DR4-dependent transcription of ABCA1 gene in HepG2 cells. A stimulatory role of niacin in early hepatic formation of HDL particles suggests a new mechanism that contributes to niacin action to increase the stability of newly synthesized circulating HDL.

Contribution of monocyte-derived macrophages and smooth muscle cells to arterial foam cell formation

Smooth muscle cells (SMCs) are the main cell type in intimal thickenings and some stages of human atherosclerosis. Like monocyte-derived macrophages, SMCs accumulate excess lipids and contribute to the total intimal foam cell population. In contrast, apolipoprotein (Apo)E-deficient and LDL receptor-deficient mice develop atherosclerotic lesions that are macrophage- as opposed to SMC-rich. The lesser contribution of SMCs to lesion development in these mouse models has distracted attention away from the importance of SMC cholesterol homeostasis in the artery wall. Intimal SMCs accumulate excess amounts of cholesteryl esters when compared with medial layer SMCs, possibly explained by reduced ATP-binding cassette transporter A1 expression and ApoA-I binding to intimal-type SMCs. The aim of this review is to compare the relative contribution of monocyte-derived macrophages and SMCs to human vs. mouse atherosclerosis, and describe what is known about lipid uptake and removal mechanisms contributing to arterial macrophage and SMC foam cell formation. An increased understanding of the contribution of these cell types to lesion development will help to delineate their relative importance in atherogenesis and as potential therapeutic targets.

Endothelial expression of human ABCA1 in mice increases plasma HDL cholesterol and reduces diet-induced atherosclerosis

The role of endothelial ABCA1 expression in reverse cholesterol transport (RCT) was examined in transgenic mice, using the endothelial-specific Tie2 promoter. Human ABCA1 (hABCA1) was significantly expressed in endothelial cells (EC) of most tissues except the liver. Increased expression of ABCA1 was not observed in resident peritoneal macrophages. ApoA-I-mediated cholesterol efflux from aortic EC was 2.6-fold higher (P < 0.0001) for cells from transgenic versus control mice. On normal chow diet, Tie2 hABCA1 transgenic mice had a 25% (P < 0.0001) increase in HDL-cholesterol (HDL-C) and more than a 2-fold increase of eNOS mRNA in the aorta (P < 0.04). After 6 months on a high-fat, high-cholesterol (HFHC) diet, transgenic mice compared with controls had a 40% increase in plasma HDL-C (P < 0.003) and close to 40% decrease in aortic lesions (P < 0.02). Aortas from HFHC-fed transgenic mice also showed gene expression changes consistent with decreased inflammation and apoptosis. Beneficial effects of the ABCA1 transgene on HDL-C levels or on atherosclerosis were absent when the transgene was transferred onto ApoE or Abca1 knockout mice. In summary, expression of hABCA1 in EC appears to play a role in decreasing diet-induced atherosclerosis in mice and is associated with increased plasma HDL-C levels and beneficial gene expression changes in EC.

Regulation of ABCA1 functions by signaling pathways

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

LXR as a novel antithrombotic target

Liver X receptors (LXRs) are transcription factors involved in the regulation of cholesterol homeostasis. LXR ligands have athero-protective properties independent of their effects on cholesterol metabolism. Platelets are involved in the initiation of atherosclerosis and despite being anucleate express nuclear receptors. We hypothesized that the athero-protective effects of LXR ligands could be in part mediated through platelets and therefore explored the potential role of LXR in platelets. Our results show that LXR-β is present in human platelets and the LXR ligands, GW3965 and T0901317, modulated nongenomically platelet aggregation stimulated by a range of agonists. GW3965 caused LXR to associate with signaling components proximal to the collagen receptor, GPVI, suggesting a potential mechanism of LXR action in platelets that leads to diminished platelet responses. Activation of platelets at sites of atherosclerotic lesions results in thrombosis preceding myocardial infarction and stroke. Using an in vivo model of thrombosis in mice, we show that GW3965 has antithrombotic effects, reducing the size and the stability of thrombi. The athero-protective effects of GW3965, together with its novel antiplatelet/thrombotic effects, indicate LXR as a potential target for prevention of athero-thrombotic disease.

Physiological effects of Type 2 diabetes on mRNA processing and gene expression

Characteristics of Type 2 diabetes include both high blood glucose (hyperglycemia) and raised cholesterol and triglycerides (hyperlipidemia). Several studies have now shown that both hyperglycemia and hyperlipidemia can alter gene expression by disrupting physiological mechanisms of gene regulation, including alternative mRNA splicing, epigenetic gene regulation and miRNA-mediated regulation of gene expression. These processes may also be influenced by intracellular oxidative stress, which is increased in diabetes and in response to hyperglycemia and hyperlipidemia. Many pathways relevant to diabetes are affected by altered gene expression, including lipid and glucose metabolism and oxidative phosphorylation. This article considers how hyperglycemia and hyperlipidemia can alter gene expression in diabetes, which could potentially contribute to the worsening of the diabetic phenotype and diabetic complications.

Expression of miR-33 from an SREBP2 intron inhibits cholesterol export and fatty acid oxidation

The regulation of synthesis, degradation, and distribution of lipids is crucial for homeostasis of organisms and cells. The sterol regulatory element-binding protein (SREBP) transcription factor family is post-translationally activated in situations of reduced lipid abundance and activates numerous genes involved in cholesterol, fatty acid, and phospholipid synthesis. In this study, we provide evidence that the primary transcript of SREBP2 contains an intronic miRNA (miR-33) that reduces cellular cholesterol export via inhibition of translation of the cholesterol export pump ABCA1. Notably, miR-33 also inhibits translation of several transcripts encoding proteins involved in fatty acid β-oxidation including CPT1A, HADHB, and CROT, thereby reducing fatty acid degradation. The genetic locus encoding SREBP2 and miR-33 therefore contains a protein that increases lipid synthesis and a miRNA that prevents export and degradation of newly synthesized lipids. These results add an additional layer of complexity to our understanding of lipid homeostasis and might open possibilities for future therapeutic intervention.

Panax notoginseng saponins decrease cholesterol ester via up-regulating ATP-binding cassette transporter A1 in foam cells

AIM

Accumulating evidence has indicated that Panax notoginseng saponins (PNS), the major ingredients in Panax notoginseng (Burk.) F.H. Chen which could be found widely in Asia, can attenuate atherogenesis in vivo. This study was designed to examine the relationship of PNS with cholesterol ester in foam cells sourced from macrophages and the effect of PNS on the expression of ATP-binding cassette transporter A1 (ABCA1).

MATERIALS AND METHODS

Foam cells sourced from macrophages were cultured with PNS. The content of cholesterol ester in foam cells was analyzed and expressions of ABCA1 and liver X receptor α (LXRα) in foam cells were measured by real-time PCR and western blotting methods.

RESULTS

The results showed that PNS could significantly decrease the level of cholesterol ester in foam cells at middle and high dosages. The real-time PCR and western blotting assays indicated that the expression of ABCA1 was up-regulated by PNS in a dose-dependent manner. Analysis based on these results showed that the cholesterol ester level was negatively correlated with ABCA1 expression.

CONCLUSIONS

As a result, we conclude that by up-regulating the expression of ABCA1, PNS could lower the cholesterol ester level, which resulted in the attenuation of the foam cell formation. This bioactivity might be associated with the special chemical structures of PNS that are similar to the natural agonist of LXRα.

The effects of ABCG5/G8 polymorphisms on HDL-cholesterol concentrations depend on ABCA1 genetic variants in the Boston Puerto Rican Health Study

BACKGROUND AND AIMS

ATP-binding cassette transporters G5/G8 (ABCG5/G8) are associated with HDL-C concentrations. To assess whether the effect of ABCG5/G8 genetic variants on HDL-C concentrations is dependent on ATP-binding cassette transporters A1 (ABCA1), we studied potential interactions between single nucleotide polymorphisms (SNPs) at ABCG5/G8 (i7892T > C, 5U145A > C, T54CA > G, T400KC > A) and ABCA1 (i27943G > A, i48168G > A, K219RG > A, i125970G > C, 3U8995A > G) genes with HDL-C concentrations.

METHODS AND RESULTS

ABCG5/G8 and ABCA1 SNPs were genotyped in 788 subjects (228 men and 560 women) who participated in the Boston Puerto Rican Health Study. Biochemical measurements were determined by standard procedures. Genotyping was performed using TaqMan assays according to routine laboratory protocols. Significant gene-gene interactions for HDL-C were found between ABCG8 (5U145A > C, T54CA > G, T400KC > A) SNPs and ABCA1_i48168G > A genetic variant (P = 0.009, P = 0.042 and P = 0.036, respectively), in which carriers of the 5U145C and 54C alleles, and homozygotes for the T400 allele at ABCG8 genetic variants displayed lower HDL-C concentrations than homozygotes for the 5U145A and T54 alleles, and heterozygotes for the 400K allele at ABCG8 SNPs, only if they were also homozygous for the minor allele (A) at the aforementioned ABCA1 SNP.

CONCLUSIONS

The gene-gene interactions reported in the present study support the hypothesis that the effect of ABCG5/G8 genetic variants on HDL-C concentrations is dependent on ABCA1 expression. Replication of these analyses to further populations, particularly with low HDL-C, is clearly warranted.

miR-33 links SREBP-2 induction to repression of sterol transporters

The sterol regulatory element binding protein 2 (SREBP-2) and the liver X receptor (LXR) control antagonistic transcriptional programs that stimulate cellular cholesterol uptake and synthesis, and cholesterol efflux, respectively. The clinical importance of SREBP-2 is revealed in patients with hypercholesterolemia treated with statins, which reduce low-density lipoprotein (LDL) cholesterol levels by increasing hepatic expression of SREBP-2 and its target, the LDL receptor. Here we show that miR-33 is encoded within SREBP-2 and that both mRNAs are coexpressed. We also identify sequences in the 3' UTR of ABCA1 and ABCG1, sterol transporter genes both previously shown to be regulated by LXR, as targets for miR-33-mediated silencing. Our data show that LXR-dependent cholesterol efflux to both ApoAI and serum is ameliorated by miR-33 overexpression and, conversely, stimulated by miR-33 silencing. Finally, we show that ABCA1 mRNA and protein and plasma HDL levels decline after hepatic overexpression of miR-33, whereas they increase after hepatic miR-33 silencing. These results suggest novel ways to manage hypercholesterolemic patients.

Inhibition of ERK1/2 and activation of liver X receptor synergistically induce macrophage ABCA1 expression and cholesterol efflux

ATP-binding cassette transporter A1 (ABCA1), a molecule mediating free cholesterol efflux from peripheral tissues to apoAI and high density lipoprotein (HDL), inhibits the formation of lipid-laden macrophage/foam cells and the development of atherosclerosis. ERK1/2 are important signaling molecules regulating cellular growth and differentiation. The ERK1/2 signaling pathway is implicated in cardiac development and hypertrophy. However, the role of ERK1/2 in the development of atherosclerosis, particularly in macrophage cholesterol homeostasis, is unknown. In this study, we investigated the effects of ERK1/2 activity on macrophage ABCA1 expression and cholesterol efflux. Compared with a minor effect by inhibition of other kinases, inhibition of ERK1/2 significantly increased macrophage cholesterol efflux to apoAI and HDL. In contrast, activation of ERK1/2 reduced macrophage cholesterol efflux and ABCA1 expression. The increased cholesterol efflux by ERK1/2 inhibitors was associated with the increased ABCA1 levels and the binding of apoAI to cells. The increased ABCA1 by ERK1/2 inhibitors was due to increased ABCA1 mRNA and protein stability. The induction of ABCA1 expression and cholesterol efflux by ERK1/2 inhibitors was concentration-dependent. The mechanism study indicated that activation of liver X receptor (LXR) had little effect on ERK1/2 expression and activation. ERK1/2 inhibitors had no effect on macrophage LXRalpha/beta expression, whereas they did not influence the activation or the inhibition of the ABCA1 promoter by LXR or sterol regulatory element-binding protein (SREBP). However, inhibition of ERK1/2 and activation of LXR synergistically induced macrophage cholesterol efflux and ABCA1 expression. Our data suggest that ERK1/2 activity can play an important role in macrophage cholesterol trafficking.

LRP1 controls cPLA2 phosphorylation, ABCA1 expression and cellular cholesterol export

BACKGROUND

ATP-binding cassette transporter A1 mediates apolipoprotein AI-dependent efflux of cholesterol and thereby removes cholesterol from peripheral tissues. ABCA1 expression is tightly regulated and deficiency of this cholesterol transporter results in cholesterol accumulation within cells. Low-density lipoprotein receptor-related protein 1 (LRP1) participates in lipid metabolism and energy homeostasis by endocytosis of apolipoprotein E-containing lipoproteins and modulation of cellular proliferation signals.

METHODS AND PRINCIPAL FINDINGS

In the present study, we demonstrate a new role for LRP1 in reverse cholesterol transport. Absence of LRP1 expression results in increased PDGFRbeta signaling and sequential activation of the mitogen-activated protein kinase signaling pathway, which increases phosphorylation of cytosolic phospholipase A(2) (cPLA(2)). Phosphorylated and activated cPLA(2) releases arachidonic acid from the phospholipid pool. Overproduction of arachidonic acid suppresses the activation of LXR/RXR heterodimers bound to the promoter of LXR regulated genes such as ABCA1, resulting in greatly reduced ABCA1 expression.

CONCLUSIONS AND SIGNIFICANCE

LRP1 regulates LXR-mediated gene transcription and participates in reverse cholesterol transport by controlling cPLA(2) activation and ABCA1 expression. LRP1 thus functions as a physiological integrator of cellular lipid homeostasis with signals that regulate cellular proliferation and vascular wall integrity.

Omega-3 fatty acids regulate gene expression levels differently in subjects carrying the PPARalpha L162V polymorphism

Omega-3 fatty acids (FAs) are natural ligands of the peroxisome proliferator-activated receptor-alpha (PPARalpha), a nuclear receptor that modulates expression levels of genes involved in lipid metabolism. The L162V polymorphism of the PPARalpha gene is associated with a deteriorated metabolic profile. We postulate that subjects carrying the PPARalpha-V162 allele exhibit differences in the expression of PPARalpha and its target genes after incubation with omega-3 FAs compared with L162 homozygotes. Peripheral blood monocytes from six men carrying the PPARalpha-V162 allele paired for age and for body mass index with six L162 homozygotes were differentiated into macrophages and activated with eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), or mixtures of EPA:DHA. Data demonstrates that gene expression levels of PPARalpha and apolipoprotein AI (APOA1) were significantly lower for carriers of the PPARalpha-V162 allele compared to L162 homozygotes after the addition of DHA and a mixture of EPA:DHA. Additionally, lipoprotein lipase (LPL) gene expression displayed a tendency to be lower in the PPARalpha L162V polymorphism subgroup after the addition of a mixture of EPA:DHA. Consequently, individuals carrying the PPARalpha-V162 allele may demonstrate inferior improvements in their lipid profile due to alterations in gene expression rates in response to omega-3 FA supplementation.

ABCA1 expression and statins: inhibitory effect in peripheral blood mononuclear cells

The ATP-binding cassette transporter A1 (ABCA1) has an essential role in the formation of nascent high-density lipoprotein particles and also participates in the cholesterol efflux from macrophages in the artery wall. Several substances, such as statins, or even gene variants are able to modulate ABCA1 expression. There is strong evidence that statin treatment downregulates the ABCA1 expression in nonloaded macrophages. Interestingly, in cholesterol-loaded macrophages, which are more relevant to atherogenesis, this effect is lost. We observed an inhibitory effect of atorvastatin in peripheral blood mononuclear cells of hypercholesterolemic individuals. Moreover, in these individuals, the ABCA1 -14C>T polymorphism was associated with high baseline gene-expression levels. Other studies are needed to evaluate how relevant these findings are to the formation of arterial foam cells in vivo.

Liver X Receptors as Therapeutic Targets for Managing Cholesterol: Implications for Atherosclerosis and Other Inflammatory Conditions

Atherosclerosis is a disease characterized by excess cholesterol and inflammation in the blood vessels. The liver X receptors (alpha and beta) are members of the nuclear hormone receptor family that are activated by endogenous cholesterol metabolites. These receptors are widely expressed with a tissue distribution that includes the liver, intestine and macrophage. Upon activation, these receptors have been shown to increase reverse cholesterol transport from the macrophage back to the liver to aid in the removal of excess cholesterol. More recently, they have also been shown to inhibit the inflammatory response in macrophages. These functions are accomplished through direct regulation of gene transcription. Herein, we will describe the key benefits and potential risks of targeting the LXRs for the treatment of atherosclerosis.

Overexpression of Human ABCG1 Does Not Affect Atherosclerosis in Fat-Fed ApoE-Deficient Mice

Objective— The purpose of this study was to evaluate the effects of whole body overexpression of human ABCG1 on atherosclerosis in apoE −/− mice.

Methods and Results— We generated BAC transgenic mice in which human ABCG1 is expressed from endogenous regulatory signals, leading to a 3- to 7-fold increase in ABCG1 protein across various tissues. Although the ABCG1 BAC transgene rescued lung lipid accumulation in ABCG1 −/− mice, it did not affect plasma lipid levels, macrophage cholesterol efflux to HDL, atherosclerotic lesion area in apoE −/− mice, or levels of tissue cholesterol, cholesterol ester, phospholipids, or triglycerides. Subtle changes in sterol biosynthetic intermediate levels were observed in liver, with chow-fed ABCG1 BAC Tg mice showing a nonsignificant trend toward decreased levels of lathosterol, lanosterol, and desmosterol, and fat-fed mice exhibiting significantly elevated levels of each intermediate. These changes were insufficient to alter ABCA1 expression in liver.

Conclusions— Transgenic human ABCG1 does not influence atherosclerosis in apoE −/− mice but may participate in the regulation of tissue cholesterol biosynthesis.

The RXR agonists PA024 and HX630 have different abilities to activate LXR/RXR and to induce ABCA1 expression in macrophage cell lines

Release of cellular cholesterol by ATP-binding cassette transporter (ABC)A1 and apolipoproteins is a major source of plasma high-density lipoprotein (HDL). Expression of ABC transporter A1 (ABCA1) is directly stimulated by liver X receptor (LXR)/retinoid X receptor (RXR) activation. We evaluated the abilities of two RXR agonists, PA024 and HX630, to increase ABCA1 expression. In differentiated THP-1 cells, the two agonists efficiently enhanced ABCA1 mRNA expression and apoA-I-dependent cellular cholesterol release. However, in RAW264 cells and undifferentiated THP-1 cells, PA024 was highly effective while HX630 was inactive in increasing ABCA1 mRNA. In parallel, the two agonists had different abilities to activate ABCA1 promoter in an LXR-responsive-element (LXRE)-dependent manner and to directly stimulate LXRalpha/RXR transactivation. The ability of HX630 to enhance ABCA1 expression was correlated closely with the cellular PPARgamma mRNA level. Moreover, HX630 was able to activate PPARgamma/RXR. Transfection of PPARgamma in RAW264 cells induced HX630-mediated activation of LXRE-dependent transcription and ABCA1 promoter, suggesting the ability of HX630 to activate PPARgamma-LXR-ABCA1 pathway. We conclude that RXR agonist PA024 and HX630 have different abilities to activate LXR/RXR, and that the cell-type-dependent effect of HX630 on ABCA1 expression and HDL generation is closely associated with this defect.

A novel enzyme immunoassay specific for ABCA1 protein quantification in human tissues and cells

ATP-binding cassette transporter A1 (ABCA1) mediates the transport of cholesterol and phospholipids from cells to lipid-poor HDL and maintains cellular lipid homeostasis. Impaired ABCA1 function plays a role in lipid disorders, cardiovascular disease, atherosclerosis, and metabolic disorders. Despite the clinical importance of ABCA1, no method is available for quantifying ABCA1 protein. We developed a sensitive indirect competitive ELISA for measuring ABCA1 protein in human tissues using a commercial ABCA1 peptide and a polyclonal anti-ABCA1 antibody. The ELISA has a detection limit of 8 ng/well (0.08 mg/l) with a working range of 9-1000 ng/well (0.09-10 mg/l). Intra- and interassay coefficient of variations (CVs) were 6.4% and 9.6%, respectively. Good linearity (r = 0.97-0.99) was recorded in serial dilutions of human arterial and placental crude membrane preparations, and fibroblast lysates. The ELISA measurements for ABCA1 quantification in reference arterial tissues corresponded well with immunoblot analysis. The assay performance and clinical utility was evaluated with arterial tissues obtained from 15 controls and 44 patients with atherosclerotic plaques. ABCA1 protein concentrations in tissue lysates were significantly lower in patients (n = 24) as compared with controls (n = 5; 9.37 +/- 0.82 vs. 17.03 +/- 4.25 microg/g tissue; P < 0.01). The novel ELISA enables the quantification of ABCA1 protein in human tissues and confirms previous semiquantitative immunoblot results.

Curcumin inhibits cellular cholesterol accumulation by regulating SREBP-1/caveolin-1 signaling pathway in vascular smooth muscle cells

AIM

To investigate the protective effect and the possible mechanism of curcumin on anti-atherosclerosis.

METHODS

Morphological changes of atherosclerotic lesions taken from apoE knockout (apoE-/-) mice were determined by hematoxylin- eosin staining. Intracellular lipid droplets and lipid levels were assayed by oil red O staining and HPLC. The protein expression of caveolin-1 was quantified by Western blotting. Translocation and the expression of sterol response element-binding protein-1 (SREBP-1) were indirectly detected by an immunofluorescence analysis.

RESULTS

The administration of 20 mg. kg(-1 ). d(-1 )curcumin to apoE-/- mice for 4 months induced a 50% reduction of atherosclerotic lesions and yielded a 5- fold increase in the caveolin-1 expression level as compared to the model group. Rat vascular smooth muscle cells (VSMC) pretreated with 50 mg/L ox-lipid density lipoprotein(ox-LDL) for 48 h increased cellular lipid contents, and stimulated SREBP-1 translocation, but decreased the caveolin-1 expression level. Lipid-loaded cells exposed to curcumin at various concentrations (12.5, 25, and 50 micromol/L) for different durations (0, 6, 12, 24, and 48 h) significantly diminished the number and area of cellular lipid droplets, total cholesterol, cholesterol ester, and free cholesterol accompanying the elevation of the caveolin-1 expression level (approximately 3-fold); the translocation of SREBP-1 from the cytoplasm to the nucleus was inhibited compared with the models. Lipid-loaded VSMC exposed to N-acetyl- Leu-Leu-norleucinal, a SREBP-1 protease inhibitor, showed increased nuclear translocation of SREBP-1, reduced caveolin-1 expression level, and upregulated cellular lipid levels.

CONCLUSION

Curcumin inhibits ox-LDL-induced cholesterol accumulation in cultured VSMC through increasing the caveolin-1 expression via the inhibition of nuclear translocation of SREBP-1.

ABCA2 as a therapeutic target in cancer and nervous system disorders

BACKGROUND

Overexpression of ATP-binding cassette (ABC) transporters is a major adaptive advantage used by tumor cells to evade the accumulation of cytotoxic agents. ABCA2, a transporter highly expressed in the cells of the nervous and haematopoetic systems, is associated with lipid transport and drug resistance in cancer cells, including tumor stem cells. Recently, a single nucleotide polymorphism (SNP) in Abca2 was linked to early onset Alzheimer's disease (AD). The characterization of two independent knockout mouse models has shed light on putative in vivo functions of this transporter in the development and maintenance of myelin membrane lipids in the CNS.

OBJECTIVE

The objective of this review is to guide the reader through the existing scope of literature on the ABCA2 transporter, focusing on its potential as a future target in human pathologies, specifically cancer and neurological disease.

METHODS

An NCBI PubMed literature search was conducted to address the growing body of ABCA2 literature that, at the time of publication, included 39 reports. From these, we focused on papers that provided insight into the functional importance of this transporter in tumor stem cells, cancer, drug resistance, Alzheimer's disease and myelination.

RESULTS/CONCLUSION

These studies have implicated ABCA2 as a therapeutic target in modulating the drug resistance phenotype prevalent in human cancers and in the treatment of neuropathies, including Alzheimer's disease and myelin-related disorders.

Overexpression of ABCA1 reduces amyloid deposition in the PDAPP mouse model of Alzheimer disease

APOE genotype is a major genetic risk factor for late-onset Alzheimer disease (AD). ABCA1, a member of the ATP-binding cassette family of active transporters, lipidates apoE in the CNS. Abca1(-/-) mice have decreased lipid associated with apoE and increased amyloid deposition in several AD mouse models. We hypothesized that mice overexpressing ABCA1 in the brain would have increased lipidation of apoE-containing lipoproteins and decreased amyloid deposition. To address these hypotheses, we created PrP-mAbca1 Tg mice that overexpress mouse Abca1 throughout the brain under the control of the mouse prion promoter. We bred the PrP-mAbca1 mice to the PDAPP AD mouse model, a transgenic line overexpressing a mutant human amyloid precursor protein. PDAPP/Abca1 Tg mice developed a phenotype remarkably similar to that seen in PDAPP/Apoe(-/-) mice: there was significantly less amyloid beta-peptide (Abeta) deposition, a redistribution of Abeta to the hilus of the dentate gyrus in the hippocampus, and an almost complete absence of thioflavine S-positive amyloid plaques. Analyses of CSF from PrP-mAbca1 Tg mice and media conditioned by PrP-mAbca1 Tg primary astrocytes demonstrated increased lipidation of apoE-containing particles. These data support the conclusions that increased ABCA1-mediated lipidation of apoE in the CNS can reduce amyloid burden and that increasing ABCA1 function may have a therapeutic effect on AD.

PPAR and LXR Activators Regulate ABCA12 Expression in Human Keratinocytes

ATP-binding cassette (ABC) transporter, family 12 (ABCA12), a member of the ABC superfamily, facilitates the delivery of lipids to lamellar bodies (LB) in keratinocytes, which is critical for permeability barrier function. Recently, gene mutations of ABCA12 were found to underlie Harlequin ichthyosis and lamellar ichthyosis, two devastating skin disorders. Previously we and others have demonstrated that peroxisome proliferators-activated receptors (PPARs) and liver X receptor (LXR) activation improved epidermal permeability barrier homeostasis by stimulating keratinocyte differentiation, lipid synthesis, and increasing LB formation/secretion. Here we report that both PPAR-gamma and -beta/delta activators markedly stimulate ABCA12 mRNA expression in cultured human keratinocyte (CHK) in a dose- and time-dependent manner. Increased ABCA12 mRNA levels are accompanied by an increase in ABCA12 protein, suggesting biological importance of this upregulation. LXR activators also increase ABCA12 mRNA levels in CHK, but to a lesser extent. In contrast, activators of PPAR-alpha, RAR, RXR, or vitamin D receptor did not alter ABCA12 expression. Two major ABCA12 alternative transcripts and their corresponding proteins are also expressed and upregulated by PPAR or LXR activator in both undifferentiated and differentiated CHK. Together, our data demonstrate that PPAR and LXR activators increase ABCA12 expression, providing an additional mechanism by which PPAR and LXR activators promote epidermal permeability barrier homeostasis.