ATP-binding cassette transporter G1 and lipid homeostasis

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.

Lipid and Lipoprotein Metabolism

The exogenous lipoprotein pathway starts with the incorporation of dietary lipids into chylomicrons in the intestine. Chylomicron triglycerides are metabolized in muscle and adipose tissue and chylomicron remnants are formed, which are removed by the liver. The endogenous lipoprotein pathway begins in the liver with the formation of very low-density lipoprotein particles (VLDL). VLDL triglycerides are metabolized in muscle and adipose tissue forming intermediate-density lipoprotein (IDL), which may be taken up by the liver or further metabolized to low-density lipoprotein (LDL). Reverse cholesterol transport begins with the formation of nascent high-density lipoprotein (HDL) by the liver and intestine that acquire cholesterol from cells resulting in mature HDL. The HDL then transports the cholesterol to the liver either directly or indirectly by transferring the cholesterol to VLDL or LDL.

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.

Studies in a Murine Granuloma Model of Instilled Carbon Nanotubes: Relevance to Sarcoidosis

Poorly soluble environmental antigens, including carbon pollutants, are thought to play a role in the incidence of human sarcoidosis, a chronic inflammatory granulomatous disease of unknown causation. Currently, engineered carbon products such as multiwall carbon nanotubes (MWCNT) are manufactured commercially and have been shown to elicit acute and chronic inflammatory responses in experimental animals, including the production of granulomas or fibrosis. Several years ago, we hypothesized that constructing an experimental model of chronic granulomatosis resembling that associated with sarcoidosis might be achieved by oropharyngeal instillation of MWCNT into mice. This review summarizes the results of our efforts to define mechanisms of granuloma formation and identify potential therapeutic targets for sarcoidosis. Evidence is presented linking findings from the murine MWCNT granuloma model to sarcoidosis pathophysiology. As our goal was to determine what pulmonary inflammatory pathways might be involved, we utilized mice of knock-out (KO) backgrounds which corresponded to deficiencies noted in sarcoidosis patients. A primary example of this approach was to study mice with a myeloid-specific knock-out of the lipid-regulated transcription factor, peroxisome proliferator-activated receptor gamma (PPARγ) which is strikingly depressed in sarcoidosis. Among the major findings associated with PPARγ KO mice compared to wild-type were: (1) exacerbation of granulomatous and fibrotic histopathology in response to MWCNT; (2) elevation of inflammatory mediators; and (3) pulmonary retention of a potentially antigenic ESAT-6 peptide co-instilled with MWCNT. In line with these data, we also observed that activation of PPARγ in wild-type mice by the PPARγ-specific ligand, rosiglitazone, significantly reduced both pulmonary granuloma and inflammatory mediator production. Similarly, recognition of a deficiency of ATP-binding cassette (ABC) lipid transporter ABCG1 in sarcoidosis led us to study MWCNT instillation in myeloid-specific ABCG1 KO mice. As anticipated, ABCG1 deficiency was associated with larger granulomas and increased levels of inflammatory mediators. Finally, a transcriptional survey of alveolar macrophages from MWCNT-instilled wild-type mice and human sarcoidosis patients revealed several common themes. One of the most prominent mediators identified in both human and mouse transcriptomic analyses was MMP12. Studies with MMP12 KO mice revealed similar acute reactions to those in wild-type but at chronic time points where wild-type maintained granulomatous disease, resolution occurred with MMP12 KO mice suggesting MMP12 is necessary for granuloma progression. In conclusion, these studies suggest that the MWCNT granuloma model has relevance to human sarcoidosis study, particularly with respect to immune-specific pathways.

Levels of Prebeta-1 High-Density Lipoprotein Are a Strong Independent Positive Risk Factor for Coronary Heart Disease and Myocardial Infarction: A Meta-Analysis

Background We previously showed that levels of prebeta-1 high-density lipoprotein (HDL), the principal acceptor of cholesterol effluxed from cells, including artery wall macrophages, are positively associated with coronary heart disease (CHD) and myocardial infarction (MI) risk. Methods and Results In a multiethnic follow-up cohort of 1249 individuals from University of California-San Francisco clinics, we determined the degree to which prebeta-1 HDL levels, both absolute and percentage of apolipoprotein AI, are associated with CHD and history of MI. Independent, strong, positive associations were found. Meta-analysis revealed for the absolute prebeta-1 HDL for the top tertile versus the lowest, unadjusted odds ratios of 1.90 (95% CI, 1.40-2.58) for CHD and 1.79 (95% CI, 1.35-2.36) for MI. For CHD, adjusting for established risk factors, the top versus bottom tertiles, quintiles, and deciles yielded sizable odds ratios of 2.37 (95% CI, 1.74-3.25, P<0.001), 3.20 (95% CI, 2.07-4.94, P<0.001), and 4.00 (95% CI, 2.11-7.58, P<0.001), respectively. Men and women were analyzed separately in a combined data set of 2507 individuals. The odds ratios for CHD and MI risk were similar. Higher levels of prebeta-1 HDL were associated with all 5 metabolic syndrome features. Addition of prebeta-1 HDL to these 5 features resulted in significant improvements in risk-prediction models. Conclusions Analysis of 2507 subjects showed conclusively that levels of prebeta-1 HDL are strongly associated with a history of CHD or MI, independently of traditional risk factors. Addition of prebeta-1 HDL can significantly improve clinical assessment of risk of CHD and MI.

Alveolar Macrophage ABCG1 Deficiency Promotes Pulmonary Granulomatous Inflammation

Pulmonary granuloma formation is a complex and poorly understood response to inhaled pathogens and particulate matter. To explore the mechanisms of pulmonary granuloma formation and maintenance, our laboratory has developed a multiwall carbon nanotube (MWCNT)-induced murine model of chronic granulomatous inflammation. We have demonstrated that the MWCNT model closely mimics pulmonary sarcoidosis pathophysiology, including the deficiency of alveolar macrophage ATP-binding cassette (ABC) lipid transporters ABCA1 and ABCG1. We hypothesized that deficiency of alveolar macrophage ABCA1 and ABCG1 would promote pulmonary granuloma formation and inflammation. To test this hypothesis, the effects of MWCNT instillation were evaluated in ABCA1, ABCG1, and ABCA1/ABCG1 myeloid-specific knockout (KO) mice. Histological examination revealed significantly larger pulmonary granulomas in ABCG1-KO and ABCA1/ABCG1 double-KO animals when compared with wild-type animals. Evaluation of BAL cells indicated increased expression of CCL2 and osteopontin, genes shown to be involved in the formation and maintenance of pulmonary granulomas. Single deficiency of alveolar macrophage ABCA1 did not affect MWCNT-induced granuloma formation or proinflammatory gene expression. These observations indicate that the deficiency of alveolar macrophage ABCG1 promotes pulmonary granulomatous inflammation and that this is augmented by additional deletion of ABCA1.

ABCG1 regulates pulmonary surfactant metabolism in mice and men

Idiopathic pulmonary alveolar proteinosis (PAP) is a rare lung disease characterized by accumulation of surfactant. Surfactant synthesis and secretion are restricted to epithelial type 2 (T2) pneumocytes (also called T2 cells). Clearance of surfactant is dependent upon T2 cells and macrophages. ABCG1 is highly expressed in both T2 cells and macrophages. ABCG1-deficient mice accumulate surfactant, lamellar body-loaded T2 cells, lipid-loaded macrophages, B-1 lymphocytes, and immunoglobulins, clearly demonstrating that ABCG1 has a critical role in pulmonary homeostasis. We identify a variant in the ABCG1 promoter in patients with PAP that results in impaired activation of ABCG1 by the liver X receptor α, suggesting that ABCG1 basal expression and/or induction in response to sterol/lipid loading is essential for normal lung function. We generated mice lacking ABCG1 specifically in either T2 cells or macrophages to determine the relative contribution of these cell types on surfactant lipid homeostasis. These results establish a critical role for T2 cell ABCG1 in controlling surfactant and overall lipid homeostasis in the lung and in the pathogenesis of human lung disease.

Autoantibody-Mediated Pulmonary Alveolar Proteinosis in Rasgrp1-Deficient Mice

Pulmonary alveolar proteinosis (PAP) is a rare lung syndrome caused by the accumulation of surfactants in the alveoli. The most prevalent clinical form of PAP is autoimmune PAP (aPAP) whereby IgG autoantibodies neutralize GM-CSF. GM-CSF is a pleiotropic cytokine that promotes the differentiation, survival, and activation of alveolar macrophages, the cells responsible for surfactant degradation. IgG-mediated neutralization of GM-CSF thereby inhibits alveolar macrophage homeostasis and function, leading to surfactant accumulation and innate immunodeficiency. Importantly, there are no rodent models for this disease; therefore, underlying immune mechanisms regulating GM-CSF-specific IgG in aPAP are not well understood. In this article, we identify that autoimmune-prone Rasgrp1-deficient mice develop aPAP: 1) Rasgrp1-deficient mice exhibit reduced pulmonary compliance and lung histopathology characteristic of PAP; 2) alveolar macrophages from Rasgrp1-deficient mice are enlarged and exhibit reduced surfactant degradation; 3) the concentration of GM-CSF-specific IgG is elevated in both serum and bronchoalveolar lavage fluid from Rasgrp1-deficient mice; 4) GM-CSF-specific IgG is capable of neutralizing GM-CSF bioactivity; and 5) Rasgrp1-deficient mice also lacking CD275/ICOSL, a molecule necessary for conventional T cell-dependent Ab production, have reduced GM-CSF-specific autoantibody and do not develop PAP. Collectively, these studies reveal that Rasgrp1-deficient mice, to our knowledge, represent the first rodent model for aPAP.

Functional Cooperativity between ABCG4 and ABCG1 Isoforms

ABCG4 belongs to the ABCG subfamily, the members of which are half transporters composed of a single transmembrane and a single nucleotide-binding domain. ABCG proteins have a reverse domain topology as compared to other mammalian ABC transporters, and have to form functional dimers, since the catalytic sites for ATP binding and hydrolysis, as well as the transmembrane domains are composed of distinct parts of the monomers. Here we demonstrate that ABCG4 can form homodimers, but also heterodimers with its closest relative, ABCG1. Both the full-length and the short isoforms of ABCG1 can dimerize with ABCG4, whereas the ABCG2 multidrug transporter is unable to form a heterodimer with ABCG4. We also show that contrary to that reported in some previous studies, ABCG4 is predominantly localized to the plasma membrane. While both ABCG1 and ABCG4 have been suggested to be involved in lipid transport or regulation, in accordance with our previous results regarding the long version of ABCG1, here we document that the expression of both the short isoform of ABCG1 as well as ABCG4 induce apoptosis in various cell types. This apoptotic effect, as a functional read-out, allowed us to demonstrate that the dimerization between these half transporters is not only a physical interaction but functional cooperativity. Given that ABCG4 is predominantly expressed in microglial-like cells and endothelial cells in the brain, our finding of ABCG4-induced apoptosis may implicate a new role for this protein in the clearance mechanisms within the central nervous system.

LXR regulate cholesterol homeostasis in the proximal mouse epididymis

Oxysterol nuclear receptors liver x receptors (LXRalpha and LXRbeta) regulate lipid homeostasis when cells have to face high amounts of cholesterol and/or fatty acids. Male mice invalidated for both lxr (LXR-/-) are infertile by 5 months of age, and become sterile by the age of 9 months. The epididymis was previously shown to be affected by the gene invalidation, a phenotype specifically located in the two proximal segments of this organ. We demonstrate here that cholesteryl esters are accumulated in a specific cell type of the epididymal epithelium, the apical cells, in these two first segments, in LXR-/- male mice. These accumulations are correlated to a decrease in the amount of a specific membrane cholesterol transporter, ATP-binding cassette A1 (ABCA1) in the caput epididymidis of LXR-/- mice. This decrease is due to a transcriptional down-regulation, and we further demonstrate that ABCA1, in the two first segments of the caput epididymidis, is located in the apical cells, and that its accumulation is lost in these cells for LXR-/- male mice as soon as 4 months of age. These data bring new elements in the cholesterol trafficking pathways in the epididymis, and will help a better understanding of the molecular mechanisms occurring in this organ in relation to the sperm cells maturation process.

C/EBPβ in bone marrow is essential for diet induced inflammation, cholesterol balance, and atherosclerosis

BACKGROUND AND OBJECTIVE

Atherosclerosis is both a chronic inflammatory disease and a lipid metabolism disorder. C/EBPβ is well documented for its role in the development of hematopoietic cells and integration of lipid metabolism. However, C/EBPβ's role in atherosclerotic progression has not been examined. We assessed the impact of hematopoietic CEBPβ deletion in ApoE(-/-) mice on hyperlipidemia, inflammatory responses and lesion formation in the aorta.

METHODS AND RESULTS

ApoE(-/-) mice were reconstituted with bone marrow cells derived from either WT or C/EBPβ(-/-) mice and placed on low fat or high fat/high cholesterol diet for 11 weeks. Hematopoietic C/EBPβ deletion in ApoE(-/-) mice reduced blood and hepatic lipids and gene expression of hepatic stearoyl CoA desaturase 1 and fatty acid synthase while expression of ATP binding cassette transporter G1, cholesterol 7-alpha-hydroxylase, and liver X receptor alpha genes were significantly increased. ApoE(-/-) mice reconstituted with C/EBPβ(-/-) bone marrow cells also significantly reduced blood cytokine levels and reduced lesion area in aortic sinuses compared with ApoE(-/-) mice reconstituted with WT bone marrow cells. Silencing of C/EBPβ in RAW264.7 macrophage cells prevented oxLDL-mediated foam cell formation and inflammatory cytokine secretion in conditioned medium.

CONCLUSION

C/EBPβ in hematopoietic cells is crucial to regulate diet-induced inflammation, hyperlipidemia and atherosclerosis development.

Circadian Regulation of Macronutrient Absorption

Various intestinal functions exhibit circadian rhythmicity. Disruptions in these rhythms as in shift workers and transcontinental travelers are associated with intestinal discomfort. Circadian rhythms are controlled at the molecular level by core clock and clock-controlled genes. These clock genes are expressed in intestinal cells, suggesting that they might participate in the circadian regulation of intestinal functions. A major function of the intestine is nutrient absorption. Here, we will review absorption of proteins, carbohydrates, and lipids and circadian regulation of various transporters involved in their absorption. A better understanding of circadian regulation of intestinal absorption might help control several metabolic disorders and attenuate intestinal discomfort associated with disruptions in sleep-wake cycles.

Genome-wide identification of whole ATP-binding cassette (ABC) transporters in the intertidal copepod Tigriopus japonicus

Backgrounds

The ATP-binding cassette (ABC) transporter superfamily is one of the largest transporter gene families and is observed in all animal taxa. Although a large set of transcriptomic data was recently assembled for several species of crustaceans, identification and annotation of the large ABC transporter gene family have been very challenging.

Results

In the intertidal copepod Tigriopus japonicus, 46 putative ABC transporters were identified using in silico analysis, and their full-length cDNA sequences were characterized. Phylogenetic analysis revealed that the 46 T. japonicus ABC transporters are classified into eight subfamilies (A-H) that include all the members of all ABC subfamilies, consisting of five ABCA, five ABCB, 17 ABCC, three ABCD, one ABCE, three ABCF, seven ABCG, and five ABCH subfamilies. Of them, unique isotypic expansion of two clades of ABCC1 proteins was observed. Real-time RT-PCR-based heatmap analysis revealed that most T. japonicus ABC genes showed temporal transcriptional expression during copepod development. The overall transcriptional profile demonstrated that half of all T. japonicus ABC genes were strongly associated with at least one developmental stage. Of them, transcripts TJ-ABCH_88708 and TJ-ABCE1 were highly expressed during all developmental stages.

Conclusions

The whole set of T. japonicus ABC genes and their phylogenetic relationships will provide a better understanding of the comparative evolution of essential gene family resources in arthropods, including the crustacean copepods.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-651) contains supplementary material, which is available to authorized users.

Identification of biomarkers of human skin ageing in both genders. Wnt signalling - a label of skin ageing?

The goal of our work has been to investigate the mechanisms of gender-independent human skin ageing and examine the hypothesis of skin being an adequate model of global ageing. For this purpose, whole genome gene profiling was employed in sun-protected skin obtained from European Caucasian young and elderly females (mean age 26.7±4 years [n1 = 7] and 70.75±3.3 years [n2 = 4], respectively) and males (mean age 25.8±5.2 years [n3 = 6] and 76±3.8 years [n4 = 7], respectively) using the Illumina array platform. Confirmation of gene regulation was performed by real-time RT-PCR and immunohistochemistry. 523 genes were significantly regulated in female skin and 401 genes in male skin for the chosen criteria. Of these, 183 genes exhibited increased and 340 decreased expression in females whereas 210 genes showed increased and 191 decreased expression in males with age. In total, 39 genes were common in the target lists of significant regulated genes in males and females. 35 of these genes showed increased (16) or decreased (19) expression independent of gender. Only 4 overlapping genes (OR52N2, F6FR1OP2, TUBAL3 and STK40) showed differential regulation with age. Interestingly, Wnt signalling pathway showed to be significantly downregulated in aged skin with decreased gene and protein expression for males and females, accordingly. In addition, several genes involved in central nervous system (CNS) ageing (f.i. APP, TAU) showed to be expressed in human skin and were significanlty regulated with age. In conclusion, our study provides biomarkers of endogenous human skin ageing in both genders and highlight the role of Wnt signalling in this process. Furthermore, our data give evidence that skin could be used as a good alternative to understand ageing of different tissues such as CNS.

Specific Kv1.3 blockade modulates key cholesterol-metabolism-associated molecules in human macrophages exposed to ox-LDL

Cholesterol-metabolism-associated molecules, including scavenger receptor class A (SR-A), lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1), CD36, ACAT1, ABCA1, ABCG1, and scavenger receptor class B type I, can modulate cholesterol metabolism in the transformation from macrophages to foam cells. Voltage-gated potassium channel Kv1.3 has increasingly been demonstrated to play an important role in the modulation of macrophage function. Here, we investigate the role of Kv1.3 in modulating cholesterol-metabolism-associated molecules in human acute monocytic leukemia cell-derived macrophages (THP-1 macrophages) and human monocyte-derived macrophages exposed to oxidized LDL (ox-LDL). Human Kv1.3 and Kv1.5 channels (hKv1.3 and hKv1.5) are expressed in macrophages and form a heteromultimeric channel. The hKv1.3-E314 antibody that we had generated as a specific hKv1.3 blocker inhibited outward delayed rectifier potassium currents, whereas the hKv1.5-E313 antibody that we had generated as a specific hKv1.5 blocker failed. Accordingly, the hKv1.3-E314 antibody reduced percentage of cholesterol ester and enhanced apoA-I-mediated cholesterol efflux in THP-1 macrophages and human monocyte-derived macrophages exposed to ox-LDL. The hKv1.3-E314 antibody downregulated SR-A, LOX-1, and ACAT1 expression and upregulated ABCA1 expression in THP-1 macrophages and human monocyte-derived macrophages. Our results reveal that specific Kv1.3 blockade represents a novel strategy modulating cholesterol metabolism in macrophages, which benefits the treatment of atherosclerotic lesions.

Effect of reconstituted discoidal high-density lipoproteins on lipid mobilization in RAW 264.7 and CHOK1 cells

Reconstituted discoidal high-density lipoproteins (rHDL) resemble nascent HDL, which are formed at the early reverse cholesterol transport steps, and constitute the initial cholesterol (Chol) acceptors from cell membranes. We have used different sized rHDL containing or not Chol, to test their abilities to promote cholesterol and phospholipid efflux from two different cell lines: Raw 264.7 macrophages and CHOK1 cells. All rHDL and lipid-free apolipoprotein A-I (apoA-I) were found to be bound to CHO and RAW cells. In RAW cells, a positive correlation between cellular binding and Chol removal was found for 78 and 96 Å rHDL. Chol-free rHDL were more effective than Chol-containing ones in binding to RAW cells and promoting Chol removal. These results were more evident in the 96 Å rHDL. On the other hand, rHDL binding to CHO cells was relatively independent of disc size and Chol content. In spite of the fact that apoA-I and rHDL promoted Chol efflux from both cellular lines, only in CHOK1 cells this result was also associated to decrease Chol esterification. Among choline-containing phospholipids, only phosphatidylcholine (PC) (but not sphingomyelin) was detected to be effuxed from both cellular lines. With the only exception of Chol-free 96 Å discs, the other rHDL as well as apoA-I promoted PC efflux from RAW cells. Chol-containing rHDL were more active than Chol-free ones of comparable size to promote PC efflux from RAW macrophages. Regarding CHO cells, only apoA-I and Chol-free 78 Å rHDL were active enough to remove PC.

High-density lipoprotein subclass and particle size in coronary heart disease patients with or without diabetes

BACKGROUND

A higher prevalence of coronary heart disease (CHD) in people with diabetes. We investigated the high-density lipoprotein (HDL) subclass profiles and alterations of particle size in CHD patients with diabetes or without diabetes.

METHODS

Plasma HDL subclasses were quantified in CHD by 1-dimensional gel electrophoresis coupled with immunodetection.

RESULTS

Although the particle size of HDL tend to small, the mean levels of low density lipoprotein cholesterol(LDL-C) and total cholesterol (TC) have achieved normal or desirable for CHD patients with or without diabetes who administered statins therapy. Fasting plasma glucose (FPG), triglyceride (TG), TC, LDL-C concentrations, and HDL₃ (HDL(3b) and (3a)) contents along with Gensini Score were significantly higher; but those of HDL-C, HDL(2b+preβ2), and HDL(2a) were significantly lower in CHD patients with diabetes versus CHD patients without diabetes; The preβ₁-HDL contents did not differ significantly between these groups. Multivariate regression analysis revealed that Gensini Score was significantly and independently predicted by HDL(2a), and HDL(2b+preβ2).

CONCLUSIONS

The abnormality of HDL subpopulations distribution and particle size may contribute to CHD risk in diabetes patients. The HDL subclasses distribution may help in severity of coronary artery and risk stratification, especially in CHD patients with therapeutic LDL, TG and HDL levels.

Long term induction by pterostilbene results in autophagy and cellular differentiation in MCF-7 cells via ROS dependent pathway

This study shows the effect of pterostilbene on intracellular neutral lipid accumulation in MCF-7 breast cancer cells leading to growth arrest and autophagy. On exposing the breast cancer cells with 30 μM pterostilbene for 72 h there was almost 2-folds increase in neutral lipids and triglycerides. Also the phytochemical caused a 4-folds increase in the expression of adipogenic differentiation marker c/EBPα. Further, pterostilbene inhibited 3β-hydroxylsterol-Δ(7)-reductase, the enzyme which catalyzes the last step conversion of 7-dehydrocholesterol to cholesterol, and thereby causes the intracellular accumulation of the former sterol. These results were associated with over-expression of oxysterol binding protein homologue and liver X receptor (LXR) by ~7-folds. Pterostilbene also caused a simultaneous increase in the expression autophagic marker proteins Beclin 1 and LC3 II (microtubule-associated protein 1 light chain 3) by approximately 6-folds, which leads to an alternative pathway of autophagy. These effects were observed in association with the loss of mitotic and metastatic potential of MCF-7 cells which was abolished in the presence of catalase (ROS scavenger) or 3MA (autophagic inhibitor). Thus the present data shows that the long term exposure to pterostilbene causes growth arrest in MCF-7 cells which may be due to differentiation of the mammary carcinoma cells into normal epithelial cell like morphology and activation of autophagy.

Sterol lipid metabolism in down syndrome revisited: down syndrome is associated with a selective reduction in serum brassicasterol levels

Over the past 15 years, insights into sterol metabolism have improved our understanding of the relationship between lipids and common conditions such as atherosclerosis and Alzheimer's Disease (AD). A better understanding of sterol lipid metabolism in individuals with Down Syndrome (DS) may help elucidate how this population's unique metabolic characteristics influence their risks for atherosclerosis and AD. To revisit the question of whether sterol lipid parameters may be altered in DS subjects, we performed a pilot study to assess traditional serum sterol lipids and lipoproteins, as well as markers of sterol biosynthesis, metabolites, and plant sterols in 20 subjects with DS compared to age-matched controls. Here we report that the levels of nearly all lipids and lipoproteins examined are similar to control subjects, suggesting that trisomy 21 does not lead to pronounced general alterations in sterol lipid metabolism. However, the levels of serum brassicasterol were markedly reduced in DS subjects.

Obesity and weight loss result in increased adipose tissue ABCG1 expression in db/db mice

The prevalence of obesity has reached epidemic proportions and is associated with several co-morbid conditions including diabetes, dyslipidemia, cancer, atherosclerosis and gallstones. Obesity is associated with low systemic inflammation and an accumulation of adipose tissue macrophages (ATMs) that are thought to modulate insulin resistance. ATMs may also modulate adipocyte metabolism and take up lipids released during adipocyte lipolysis and cell death. We suggest that high levels of free cholesterol residing in adipocytes are released during these processes and contribute to ATM activation and accumulation during obesity and caloric restriction. Db/db mice were studied for extent of adipose tissue inflammation under feeding conditions of ad libitum (AL) and caloric restriction (CR). The major finding was a marked elevation in epididymal adipose ABCG1 mRNA levels with obesity and CR (6-fold and 16-fold, respectively) over that seen for lean wild-type mice. ABCG1 protein was also elevated for CR as compared to AL adipose tissue. ABCG1 is likely produced by cholesterol loaded ATMs since this gene is not highly expressed in adipocytes and ABCG1 expression is sterol mediated. Our data supports the concept that metabolic changes in adipocytes due to demand lipolysis and cell death lead to cholesterol loading of ATMs. Based on finding cholesterol-loaded peritoneal leukocytes with elevated levels of ABCG1 in CR as compared to AL mice, we suggest that pathways for cholesterol trafficking out of adipose tissue involve ATM egress as well as ABCG1 mediated cholesterol efflux. 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).

Update on HDL receptors and cellular cholesterol transport

Efflux is central to maintenance of tissue and whole body cholesterol homeostasis. The discovery of cell surface receptors that bind high-density lipoprotein (HDL) with high specificity and affinity to promote cholesterol release has significantly advanced our understanding of cholesterol efflux. We now know that 1) cells have several mechanisms to promote cholesterol release, including a passive mechanism that depends on the physico-chemical properties of cholesterol molecules and their interactions with phospholipids; 2) a variety of HDL particles can interact with receptors to promote cholesterol transport from tissues to the liver for excretion; and 3) interactions between HDL and receptors show functional synergy. Therefore, efflux efficiency depends both on the arrays of receptors on tissue cells and HDL particles in serum.

High density lipoprotein (HDL) promotes glucose uptake in adipocytes and glycogen synthesis in muscle cells

BACKGROUND

High density lipoprotein (HDL) was reported to decrease plasma glucose and promote insulin secretion in type 2 diabetes patients. This investigation was designed to determine the effects and mechanisms of HDL on glucose uptake in adipocytes and glycogen synthesis in muscle cells.

METHODS AND RESULTS

Actions of HDL on glucose uptake and GLUT4 translocation were assessed with 1-[(3)H]-2-deoxyglucose and plasma membrane lawn, respectively, in 3T3-L1 adipocytes. Glycogen analysis was performed with amyloglucosidase and glucose oxidase-peroxidase methods in normal and palmitate-treated L6 cells. Small interfering RNA was used to observe role of scavenger receptor type I (SR-BI) in glucose uptake of HDL. Corresponding signaling molecules were detected by immunoblotting. HDL stimulated glucose uptake in a time- and concentration-dependent manner in 3T3-L1 adipocytes. GLUT4 translocation was significantly increased by HDL. Glycogen deposition got enhanced in L6 muscle cells paralleling with elevated glycogen synthase kinase3 (GSK3) phosphorylation. Meanwhile, increased phosphorylations of Akt-Ser473 and AMP activated protein kinase (AMPK) α were detected in 3T3-L1 adipocytes. Glucose uptake and Akt-Ser473 activation but not AMPK-α were diminished in SR-BI knock-down 3T3-L1 cells.

CONCLUSIONS

HDL stimulates glucose uptake in 3T3-L1 adipocytes through enhancing GLUT4 translocation by mechanisms involving PI3K/Akt via SR-BI and AMPK signaling pathways, and increases glycogen deposition in L6 muscle cells through promoting GSK3 phosphorylation.

Endothelin-1 exacerbates lipid accumulation by increasing the protein degradation of the ATP-binding cassette transporter G1 in macrophages

Endothelin-1 (ET-1), a potent proatherogenic vasoconstrictive peptide, is known to promote macrophage foam cell formation via mechanisms that are not fully understood. Excessive lipid accumulation in macrophages is a major hallmark during the early stages of atherosclerotic lesions. Cholesterol homeostasis is tightly regulated by scavenger receptors (SRs) and ATP-binding cassette (ABC) transporters during the transformation of macrophage foam cells. The aim of this study was to investigate the possible mechanisms by which ET-1 affects lipid accumulation in macrophages. Our results demonstrate that oxidized low-density lipoprotein (oxLDL) treatment increases lipid accumulation in rat bone marrow-derived macrophages. Combined treatment with ET-1 and oxLDL significantly exacerbated lipid accumulation in macrophages as compared to treatment with oxLDL alone. The results of Western blotting show that ET-1 markedly decreased the ABCG1 levels via ET type A and B receptors and activation of the phosphatidylinositol 3-kinase pathway; however, ET-1 had no effect on the protein expression of CD36, SR-BI, SR-A, or ABCA1. In addition, real-time PCR analysis showed that ET-1 treatment did not affect ABCG1 mRNA expression. We also found that ET-1 decreases ABCG1 possibly due to the enhancement of the proteosome/calpain pathway-dependent degradation of ABCG1. Moreover, ET-1 significantly reduced the efficiency of the cholesterol efflux in macrophages. Taken together, these findings suggest that ET-1 may impair cholesterol efflux and further exacerbate lipid accumulation during the transformation of macrophage foam cells.

Effects of lipid-lowering drugs on reverse cholesterol transport gene expressions in peripheral blood mononuclear and HepG2 cells

AIMS

The ATP-binding cassette transporters, ABCA1 and ABCG1, are LXR-target genes that play an important role in reverse cholesterol transport. We examined the effects of inhibitors of the cholesterol absorption (ezetimibe) and synthesis (statins) on expression of these transporters in HepG2 cells and peripheral blood mononuclear cells (PBMCs) of individuals with primary (and nonfamilial) hypercholesterolemia (HC).

MATERIALS & METHODS

A total of 48 HC individuals were treated with atorvastatin (10 mg/day/4 weeks) and 23 were treated with ezetimibe (10 mg/day/4 weeks), followed by simvastatin (10 mg/day/8 weeks) and simvastatin plus ezetimibe (10 mg of each/day/4 weeks). Gene expression was examined in statin- or ezetimibe-treated and control HepG2 cells as well as PBMCs using real-time PCR.

RESULTS

In PBMCs, statins and ezetimibe downregulated ABCA1 and ABCG1 mRNA expression but did not modulate NR1H2 (LXR-β) and NR1H3 (LXR-α) levels. Positive correlations of ABCA1 with ABCG1 and of NR1H2 with NR1H3 expressions were found in all phases of the treatments. In HepG2 cells, ABCA1 mRNA levels remained unaltered while ABCG1 expression was increased by statin (1.0-10.0 µM) or ezetimibe (5.0 µM) treatments. Atorvastatin upregulated NR1H2 and NR1H3 only at 10.0 µM, meanwhile ezetimibe (1.0-5.0 µM) downregulated NR1H2 but did not change NR1H3 expression.

CONCLUSION

Our findings reveal that lipid-lowering drugs downregulate ABCA1 and ABCG1 mRNA expression in PBMCs of HC individuals and exhibit differential effects on HepG2 cells. Moreover, they indicate that the ABCA1 and ABCG1 transcript levels were not correlated directly to LXR mRNA expression in both cell models treated with lipid-lowering drugs.

Epididymis cholesterol homeostasis and sperm fertilizing ability

Cholesterol, being the starting point of steroid hormone synthesis, is a long known modulator of both female and male reproductive physiology especially at the level of the gonads and the impact cholesterol has on gametogenesis. Less is known about the effects cholesterol homeostasis may have on postgonadic reproductive functions. Lately, several data have been reported showing how imbalanced cholesterol levels may particularly affect the post-testicular events of sperm maturation that lead to fully fertile male gametes. This review will focus on that aspect and essentially centers on how cholesterol is important for the physiology of the mammalian epididymis and spermatozoa.

Regulation of ABCG1 expression in human keratinocytes and murine epidermis

ABCG1, a member of the ATP binding cassette superfamily, facilitates the efflux of cholesterol from cells to HDL. In this study, we demonstrate that ABCG1 is expressed in cultured human keratinocytes and murine epidermis, and induced during keratinocyte differentiation, with increased levels in the outer epidermis. ABCG1 is regulated by liver X receptor (LXR) and peroxisome proliferator-activated receptor-δ (PPAR-δ) activators, cellular sterol levels, and acute barrier disruption. Both LXR and PPAR-δ activators markedly stimulate ABCG1 expression in a dose- and time-dependent fashion. PPAR-γ activators also increase ABCG1 expression, but to a lesser degree. In contrast, activators of PPAR-α, retinoic acid receptor, retinoid X receptor, and vitamin D receptor do not alter ABCG1 expression. In response to increased intracellular sterol levels, ABCG1 expression increases, whereas inhibition of cholesterol biosynthesis decreases ABCG1 expression. In vivo, ABCG1 is stimulated 3-6 h after acute barrier disruption by either tape stripping or acetone treatment, an increase that can be inhibited by occlusion, suggesting a potential role of ABCG1 in permeability barrier homeostasis. Although Abcg1-null mice display normal epidermal permeability barrier function and gross morphology, abnormal lamellar body (LB) contents and secretion leading to impaired lamellar bilayer formation could be demonstrated by electron microscopy, indicating a potential role of ABCG1 in normal LB formation and secretion.

Regulation of cholesterol homeostasis by liver X receptors

Cellular cholesterol levels reflect a balance between uptake, efflux, and endogenous synthesis. The sterol-responsive transcription factors liver X receptors (LXRalpha and LXRbeta) help maintain cholesterol homeostasis, not only through promotion of cholesterol efflux from peripheral tissues but also through suppression of de novo synthesis and exogenous cholesterol uptake. In this review, we summarize the important role of LXRs in regulating expression of key members that keep cholesterol levels in balance.

Caveolin, sterol carrier protein-2, membrane cholesterol-rich microdomains and intracellular cholesterol trafficking

While the existence of membrane lateral microdomains has been known for over 30 years, interest in these structures accelerated in the past decade due to the discovery that cholesterol-rich microdomains serve important biological functions. It is increasingly appreciated that cholesterol-rich microdomains in the plasma membranes of eukaryotic cells represent an organizing nexus for multiple cellular proteins involved in transmembrane nutrient uptake (cholesterol, fatty acid, glucose, etc.), cell-signaling, immune recognition, pathogen entry, and many other roles. Despite these advances, however, relatively little is known regarding the organization of cholesterol itself in these plasma membrane microdomains. Although a variety of non-sterol markers indicate the presence of microdomains in the plasma membranes of living cells, none of these studies have demonstrated that cholesterol is enriched in these microdomains in living cells. Further, the role of cholesterol-rich membrane microdomains as targets for intracellular cholesterol trafficking proteins such as sterol carrier protein-2 (SCP-2) that facilitate cholesterol uptake and transcellular transport for targeting storage (cholesterol esters) or efflux is only beginning to be understood. Herein, we summarize the background as well as recent progress in this field that has advanced our understanding of these issues.

Effects of cholesterol on thermal stability of discoidal high density lipoproteins

Reverse cholesterol transport in plasma involves variations in HDL cholesterol concentration. To understand physicochemical and functional implications of such variations, we analyzed stability of reconstituted HDL containing human apolipoproteins (apoA-I, apoA-II, or apoC-I), phosphatidylcholines varying in chain length (12-18 carbons) and unsaturation (0 or 1), and 0-35 mol% cholesterol. Lipoprotein heat denaturation was monitored by circular dichroism for protein unfolding/dissociation and by light scattering for particle fusion. We found that cholesterol stabilizes relatively unstable complexes; for example, incorporation of 10-30 mol% cholesterol in apoC-I:dimyristoyl phosphatidylcholine complexes increased their kinetic stability by deltaDeltaG* congruent with 1 kcal/mol. In more stable complexes containing larger proteins and/or longer-chain lipids, incorporation of 10% cholesterol did not significantly alter the disk stability; however, 15% or more cholesterol destabilized the apoA-I-containing complexes and led to vesicle formation. Thus, cholesterol tends to stabilize less stable lipoproteins, apparently by enhancing favorable packing interactions, but in more stable lipoproteins, where such interactions are already highly optimized, the stabilizing effect of cholesterol decreases and, eventually, becomes destabilizing. These results help uncouple the functional roles of particle stability and chain fluidity and suggest that structural disorder in HDL surface, rather than chain fluidity, is an important physicochemical determinant of HDL function.

Emerging new paradigms for ABCG transporters

Every cell is separated from its external environment by a lipid membrane. Survival depends on the regulated and selective transport of nutrients, waste products and regulatory molecules across these membranes, a process that is often mediated by integral membrane proteins. The largest and most diverse of these membrane transport systems is the ATP binding cassette (ABC) family of membrane transport proteins. The ABC family is a large evolutionary conserved family of transmembrane proteins (>250 members) present in all phyla, from bacteria to Homo sapiens, which require energy in the form of ATP hydrolysis to transport substrates against concentration gradients. In prokaryotes the majority of ABC transporters are involved in the transport of nutrients and other macromolecules into the cell. In eukaryotes, with the exception of the cystic fibrosis transmembrane conductance regulator (CFTR/ABCC7), ABC transporters mobilize substrates from the cytoplasm out of the cell or into specific intracellular organelles. This review focuses on the members of the ABCG subfamily of transporters, which are conserved through evolution in multiple taxa. As discussed below, these proteins participate in multiple cellular homeostatic processes, and functional mutations in some of them have clinical relevance in humans.

Microsomal antiestrogen-binding site ligands induce growth control and differentiation of human breast cancer cells through the modulation of cholesterol metabolism

The microsomal antiestrogen-binding site (AEBS) is a high-affinity membranous binding site for the antitumor drug tamoxifen that selectively binds diphenylmethane derivatives of tamoxifen such as PBPE and mediates their antiproliferative properties. The AEBS is a hetero-oligomeric complex consisting of 3beta-hydroxysterol-Delta8-Delta7-isomerase and 3beta-hydroxysterol-Delta7-reductase. High-affinity AEBS ligands inhibit these enzymes leading to the massive intracellular accumulation of zymostenol or 7-dehydrocholesterol (DHC), thus linking AEBS binding to the modulation of cholesterol metabolism and growth control. The aim of the present study was to gain more insight into the control of breast cancer cell growth by AEBS ligands. We report that PBPE and tamoxifen treatment induced differentiation in human breast adenocarcinoma cells MCF-7 as indicated by the arrest of cells in the G0-G1 phase of the cell cycle, the increase in the cell volume, the accumulation and secretion of lipids, and a milk fat globule protein found in milk. These effects were observed with other AEBS ligands and with zymostenol and DHC. Vitamin E abrogates the induction of differentiation and reverses the control of cell growth produced by AEBS ligands, zymostenol, and DHC, showing the importance of the oxidative processes in this effect. AEBS ligands induced differentiation in estrogen receptor-negative mammary tumor cell lines SKBr-3 and MDA-MB-468 but with a lower efficiency than observed with MCF-7. Together, these data show that AEBS ligands exert an antiproliferative effect on mammary cancer cells by inducing cell differentiation and growth arrest and highlight the importance of cholesterol metabolism in these effects.

The ABCs of sterol transport

Mammalian cells have developed various responses to minimize accumulation of unesterified cholesterol, as the latter can result in cell toxicity and death [reviewed in this edition by Björkhem (Björkhem, I. 2009. Are side-chain oxidized oxysterols regulators also in vivo? J. Lipid Res. In press)]. These responses include esterification to sequester excess sterol in intracellular lipid droplets, repression of both cholesterol synthesis and LDL receptor expression (thus reducing endocytosis of LDL), and induction of a panoply of genes that promote sterol efflux and affect lipid metabolism. The nuclear receptor liver-X-receptor (LXR) functions as a cellular "sterol sensor" and plays a critical role in these latter transcriptional changes [reviewed in this edition by Glass (Shibata, N., and Glass C, K. 2009. Regulation of macrophage function in inflammation and atherosclerosis. J. Lipid Res. In press)]. Activation of LXR by either endogenous oxysterols or synthetic agonists induces the expression of many genes, including those encoding ATP-binding cassette (ABC) transporters ABCA1, ABCG1, ABCG5, and ABCG8. As discussed below, these four proteins function to promote sterol efflux from cells.

Correlation of structural stability with functional remodeling of high-density lipoproteins: the importance of being disordered

High-density lipoproteins (HDLs) are protein-lipid assemblies that remove excess cell cholesterol and prevent atherosclerosis. HDLs are stabilized by kinetic barriers that decelerate protein dissociation and lipoprotein fusion. We propose that similar barriers modulate metabolic remodeling of plasma HDLs; hence, changes in particle composition that destabilize HDLs and accelerate their denaturation may accelerate their metabolic remodeling. To test this notion, we correlate existing reports on HDL-mediated cell cholesterol efflux and esterification, which are obligatory early steps in cholesterol removal, with our kinetic studies of HDL stability. The results support our hypothesis and show that factors accelerating cholesterol efflux and esterification in model discoidal lipoproteins (including reduced protein size, reduced fatty acyl chain length, and/or increased level of cis unsaturation) destabilize lipoproteins and accelerate their fusion and apolipoprotein dissociation. Oxidation studies of plasma spherical HDLs show a similar trend: mild oxidation by Cu(2+) or OCl(-) accelerates cell cholesterol efflux, protein dissociation, and HDL fusion, while extensive oxidation inhibits these reactions. Consequently, moderate destabilization may be beneficial for HDL functions by facilitating insertion of cholesterol and lipophilic enzymes, promoting dissociation of lipid-poor apolipoproteins, which are primary acceptors of cell cholesterol, and thereby accelerating HDL metabolism. Therefore, HDL stability must be delicately balanced to maintain the structural integrity of the lipoprotein assembly and ensure structural specificity necessary for interactions of HDL with its metabolic partners, while facilitating rapid HDL remodeling and turnover at key junctures of cholesterol transport. The inverse correlation between HDL stability and remodeling illustrates the functional importance of structural disorder in macromolecular assemblies stabilized by kinetic barriers.

Effects of acceptor composition and mechanism of ABCG1-mediated cellular free cholesterol efflux

Among the known mechanisms of reverse cholesterol transport (RCT), ATP binding cassette transporter G1 (ABCG1)-mediated free cholesterol (FC) transport is the most recent and least studied. Here, we have characterized the efficiencies of different acceptors using baby hamster kidney (BHK) cells transfected with human ABCG1 cDNA, which is inducible upon treatment with mifepristone. When normalized on particle number and particle surface area, the acceptor efficiency for FC efflux was as follows: small unilamellar vesicles (SUV)>LDL>reconstituted HDL>HDL(2) = HDL(3). Based on phospholipid content, the order was reversed. ABCG1 also mediated phospholipid efflux to human serum and HDL(3). ABCG1-mediated FC efflux correlated significantly with a number of HDL subfractions and components in serum collected from 25 normolipidemic individuals: apolipoprotein A-II (apoA-II) (r(2) = 0.7), apolipoprotein A-I (apoA-I) (r(2) = 0.5), HDL-C (r(2) = 0.4), HDL-PL (r(2) = 0.4), alpha-2 HDL (r(2) = 0.4), and prebeta HDL (r(2) = 0.2). ABCG1 did not enhance influx of FC or cholesteryl oleyl ether (COE) when cells were incubated with radiolabeled HDL(3). ABCG1 expression did not increase the association of HDL(3) with cells. Compared with control cells, ABCG1 expression significantly increased the FC pool available for efflux and the rate constant for efflux. In conclusion, composition and particle size determine the acceptor efficiency for ABCG1-mediated efflux. ABCG1 increases cell membrane FC pools and changes its rate of desorption into the aqueous phase without enhancing the association with the acceptor.

Loss of ABCG1 results in chronic pulmonary inflammation

ABCG1, a member of the ATP-binding cassette transporter superfamily, is highly expressed in multiple cells of the lung. Loss of ABCG1 results in severe pulmonary lipidosis in mice, with massive deposition of cholesterol in both alveolar macrophages and type 2 cells and the accumulation of excessive surfactant phospholipids. These observations are consistent with ABCG1 controlling cellular sterol metabolism. Herein, we report on the progressive and chronic inflammatory process that accompanies the lipidosis in the lungs of Abcg1-/- mice. Compared with wild-type animals, the lungs of aged chow-fed mice deficient in ABCG1 show distinctive signs of inflammation that include macrophage accumulation, lymphocytic infiltration, hemorrhage, eosinophilic crystals, and elevated levels of numerous cytokines and cytokine receptors. Analysis of bronchoalveolar lavages obtained from Abcg1-/- mice revealed elevated numbers of foamy macrophages and leukocytes and the presence of multiple markers of inflammation including crystals of chitinase-3-like proteins. These data suggest that cholesterol and/or cholesterol metabolites that accumulate in Abcg1-/- lungs can trigger inflammatory signaling pathways. Consistent with this hypothesis, the expression of a number of cytokines was found to be significantly increased following increased cholesterol delivery to either primary peritoneal macrophages or Raw264.7 cells. Finally, cholesterol loading of primary mouse macrophages induced cytokine mRNAs to higher levels in Abcg1-/-, as compared with wild-type cells. These results demonstrate that ABCG1 plays critical roles in pulmonary homeostasis, balancing both lipid/cholesterol metabolism and inflammatory responses.

Functional ABCG1 expression induces apoptosis in macrophages and other cell types

The expression of the ATP-binding cassette transporter ABCG1 is greatly increased in macrophages by cholesterol loading via the activation of the nuclear receptor LXR. Several recent studies demonstrated that ABCG1 expression is associated with increased cholesterol efflux from macrophages to high-density lipoprotein, suggesting an atheroprotective role for this protein. Our present study uncovers an as yet not described cellular function of ABCG1. Here we demonstrate that elevated expression of human ABCG1 is associated with apoptotic cell death in macrophages and also in other cell types. We found that overexpression of the wild type protein results in phosphatidyl serine (PS) translocation, caspase 3 activation, and subsequent cell death, whereas neither the inactive mutant variant of ABCG1 (ABCG1K124M) nor the ABCG2 multidrug transporter had such effect. Induction of ABCG1 expression by LXR activation in Thp1 cells and in human monocyte-derived macrophages was accompanied by a significant increase in the number of apoptotic cells. Thyroxin and benzamil, previously identified inhibitors of ABCG1 function, selectively prevented ABCG1-promoted apoptosis in transfected cells as well as in LXR-induced macrophages. Collectively, our results suggest a causative relationship between ABCG1 function and apoptotic cell death, and may offer new insights into the role of ABCG1 in atherogenesis.

Enhanced placental cholesterol efflux by fetal HDL in Smith-Lemli-Opitz syndrome

Previous studies from this laboratory have shown that maternal-derived cholesterol can be effluxed from trophoblasts to fetal HDL and plasma. We had the opportunity to study for the first time the ability of HDL and plasma from a fetus with the Smith-Lemli-Opitz syndrome (SLOS) to efflux cholesterol from trophoblasts. It was unclear whether cholesterol could be effluxed to fetuses with SLOS since lipoprotein levels are often very low. To answer this question, cord blood was collected from the placentas of an SLOS fetus and unaffected fetuses just after delivery. Plasma cholesterol concentrations were very low in the affected fetus; cholesterol, 7-dehydrocholesterol, and 8-dehydocholesterol concentrations were 14.1, 4.5, and 5.2 mg/dl, respectively. The HDL from the fetal SLOS effluxed approximately 50% more cholesterol from a trophoblast cell line, were smaller in size, and had a lower cholesterol to phospholipid ratio as compared to HDL from unaffected fetuses or adults. Plasma from the SLOS fetus effluxed cholesterol to a similar percentage as unaffected fetal plasma or adult plasma, possibly due to fewer HDL particles as demonstrated in previous SLOS patients. These novel data demonstrate that the cholesterol-deficient SLOS fetus is able to obtain cholesterol from trophoblasts at a time when cholesterol is playing a critical role in development, and has implications for design of treatments for cholesterol deficiency syndromes as well as understanding of prenatal cholesterol transport in humans.

ATP-binding cassette transporters G1 and G4 mediate cholesterol and desmosterol efflux to HDL and regulate sterol accumulation in the brain

Transporters in the ABCG family appear to be involved in the cellular excretion of cholesterol and other sterols in a cell- and tissue-specific fashion. Overexpression of ATP-binding cassette transporters G1 (Abcg1) and G4 (Abcg4) can promote cellular cholesterol efflux to high-density lipoprotein (HDL), but the in vivo functions of Abcg4 are poorly understood. We used mice with knockouts of Abcg1 or Abcg4 singly or together to further elucidate the function of these transporters. Abcg1 and Abcg4 are highly expressed in the brain and are found in both astrocytes and neurons. Whereas Abcg1(-/-) or Abcg4(-/-) mice showed essentially normal levels of brain sterols, in Abcg1(-/-)/Abcg4(-/-) mice, levels of several sterol intermediates in the cholesterol biosynthetic pathway, namely desmosterol, lathosterol, and lanosterol, as well as 27-OH cholesterol, were increased 2- to 3-fold. Overexpression of Abcg1 or Abcg4 promoted efflux of desmosterol and cholesterol from cells to HDL, and combined deficiency of these transporters led to defective efflux and accumulation of these sterols in primary astrocytes. Consistent with defective efflux and sterol accumulation, cholesterol biosynthesis was reduced in Abcg1(-/-)/Abcg4(-/-) astrocytes. The accumulation of desmosterol, a known liver-X receptor (LXR) activator, was associated with increased expression of LXR target genes, including ATP-binding cassette transporter A1, and increased apolipoprotein E secretion in Abcg1(-/-)/Abcg4(-/-) astrocytes. Our findings provide the first in vivo demonstration of a role for Abcg4 in sterol efflux in the brain and show that Abcg1 and Abcg4 have overlapping functions in astrocytes, promoting efflux of cholesterol, desmosterol, and possibly other sterol biosynthetic intermediates to HDL.

ABCG1 and ABCG4 are coexpressed in neurons and astrocytes of the CNS and regulate cholesterol homeostasis through SREBP-2

Here, we describe the initial characterization of Abcg4(-/-) mice and identify overlapping functions of ABCG4 and ABCG1 in the brain. Histological examination of tissues from Abcg4(+/-)/nlsLacZ and Abcg1(+/-)/nlsLacZ mice demonstrates that coexpression of Abcg4 and Abcg1 is restricted to neurons and astrocytes of the central nervous system (CNS). Interestingly, Abcg4 mRNA is undetectable outside the CNS, in contrast with the broad tissue and cellular expression of Abcg1. We also used primary astrocytes, microglia, neurons, and macrophages to demonstrate that the expression of Abcg1, but not Abcg4, is induced after the activation of liver X receptor. Cellular localization studies demonstrated that both proteins reside in RhoB-positive endocytic vesicle membranes. Furthermore, overexpression of either ABCG1 or ABCG4 increased the processing of sterol-regulatory element binding protein 2 (SREBP-2) to the transcriptionally active protein, thus accounting for the observed increase in the expression of SREBP-2 target genes and cholesterol synthesis. Consistent with these latter results, we show that the expression levels of the same SREBP-2 target genes are repressed in the brains of Abcg1(-/-) and, to a lesser extent, Abcg4(-/-) mice. Based on the results of the current study, we propose that ABCG1 and ABCG4 mediate the intracellular vesicular transport of cholesterol/sterols within both neurons and astrocytes to regulate cholesterol transport in the brain.

The paradox of dysfunctional high-density lipoprotein

PURPOSE OF REVIEW

This review addresses how, in atherosclerosis or systemic inflammation, HDL can lose its usual atheroprotective characteristics and even paradoxically assume proinflammatory properties.

RECENT FINDINGS

Specific chemical and structural changes within HDL particles can impede reverse cholesterol transport, enhance oxidation of LDL, and increase vascular inflammation. HDL may be viewed as a shuttle that can be either anti-inflammatory or proinflammatory, depending on its cargo of proteins, enzymes, and lipids. Some therapeutic approaches that reduce coronary risk, such as statins and therapeutic lifestyle changes, can favorably moderate the characteristics of proinflammatory HDL. In addition, apolipoprotein A-I mimetic peptides and other compounds that target functional aspects of HDL may offer novel approaches to reduction in cardiovascular risk.

SUMMARY

Current data suggest that under some conditions HDL can become dysfunctional and even proinflammatory, but this characterization can change with resolution of systemic inflammation or use of certain treatments.

Lipid rafts: dream or reality for cholesterol transporters?

As a key constituent of the cell membranes, cholesterol is an endogenous component of mammalian cells of primary importance, and is thus subjected to highly regulated homeostasis at the cellular level as well as at the level of the whole body. This regulation requires adapted mechanisms favoring the handling of cholesterol in aqueous compartments, as well as its transfer into or out of membranes, involving membrane proteins. A membrane exhibits functional properties largely depending on its lipid composition and on its structural organization, which very often involves cholesterol-rich microdomains. Then there is the appealing possibility that cholesterol may regulate its own transmembrane transport at a purely functional level, independently of any transcriptional regulation based on cholesterol-sensitive nuclear factors controling the expression level of lipid transport proteins. Indeed, the main cholesterol "transporters" presently believed to mediate for instance the intestinal absorption of cholesterol, that are SR-BI, NPC1L1, ABCA1, ABCG1, ABCG5/G8 and even P-glycoprotein, all present privileged functional relationships with membrane cholesterol-containing microdomains. In particular, they all more or less clearly induce membrane disorganization, supposed to facilitate cholesterol exchanges with the close aqueous medium. The actual lipid substrates handled by these transporters are not yet unambiguously determined, but they likely concern the components of membrane microdomains. Conversely, raft alterations may provide specific modulations of the transporter activities, as well as they can induce indirect effects via local perturbations of the membrane. Finally, these cholesterol transporters undergo regulated intracellular trafficking, with presumably some relationships to rafts which remain to be clarified.

ATP-Binding cassette cholesterol transporters and cardiovascular disease

A hallmark of atherosclerotic cardiovascular disease (CVD) is the accumulation of cholesterol in arterial macrophages. Factors that modulate circulating and tissue cholesterol levels have major impacts on initiation, progression, and regression of CVD. Four members of the ATP-binding cassette (ABC) transporter family play important roles in this modulation. ABCA1 and ABCG1 export excess cellular cholesterol into the HDL pathway and reduce cholesterol accumulation in macrophages. ABCG5 and ABCG8 form heterodimers that limit absorption of dietary sterols in the intestine and promote cholesterol elimination from the body through hepatobiliary secretion. All 4 transporters are induced by the same sterol-sensing nuclear receptor system. ABCA1 expression and activity are also highly regulated posttranscriptionally by diverse processes. ABCA1 mutations can cause a severe HDL-deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. ABCG5 or ABCG8 mutations can cause sitosterolemia, in which patients accumulate cholesterol and plant sterols in the circulation and develop premature CVD. Disrupting Abca1 or Abcg1 in mice promotes accumulation of excess cholesterol in macrophages, and manipulating mouse macrophage ABCA1 expression affects atherogenesis. Overexpressing ABCG5 and ABCG8 in mice attenuates diet-induced atherosclerosis in association with reduced circulating and liver cholesterol. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and inhibit transcription of all 4 transporters. Thus, impaired ABC cholesterol transporters might contribute to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. Their beneficial effects on cholesterol homeostasis have made these transporters important new therapeutic targets for preventing and reversing CVD.