ABCA1 in adipocytes regulates adipose tissue lipid content,glucose tolerance,and insulin sensitivity

Adipose tissue contains one of the largest reservoirs of cholesterol in the body. Adipocyte dysfunction in obesity is associated with intracellular cholesterol accumulation, and alterations in cholesterol homeostasis have been shown to alter glucose metabolism in cultured adipocytes. ABCA1 plays a major role in cholesterol efflux, suggesting a role for ABCA1 in maintaining cholesterol homeostasis in the adipocyte. However, the impact of adipocyte ABCA1 on adipose tissue function and glucose metabolism is unknown. Our aim was to determine the impact of adipocyte ABCA1 on adipocyte lipid metabolism, body weight, and glucose metabolism in vivo. To address this, we used mice lacking ABCA1 specifically in adipocytes (ABCA1^−ad/−ad). When fed a high-fat, high-cholesterol diet, ABCA1^−ad/−ad mice showed increased cholesterol and triglyceride stores in adipose tissue, developed enlarged fat pads, and had increased body weight. Associated with these phenotypic changes, we observed significant changes in the expression of genes involved in cholesterol and glucose homeostasis, including ldlr, abcg1, glut-4, adiponectin, and leptin. ABCA1^−ad/−ad mice also demonstrated impaired glucose tolerance, lower insulin sensitivity, and decreased insulin secretion. We conclude that ABCA1 in adipocytes influences adipocyte lipid metabolism, body weight, and whole-body glucose homeostasis.     

Sodium taurocholate-dependent lipid efflux by ABCA1: effects of W590S mutation on lipid translocation and apolipoprotein A-I dissociation

ABCA1 plays a major role in HDL metabolism. Cholesterol secretion by ABCA1 is dependent on the presence of extracellular acceptors, such as lipid-free apolipoprotein A-I (apoA-I). However, the importance of the direct interaction between apoA-I and ABCA1 in HDL formation remains unclear. In contrast, ABCB4 mediates the secretion of phospholipids and cholesterol in the presence of sodium taurocholate (NaTC) but not in the presence of apoA-I. In this study, we analyzed apoA-I binding and NaTC-dependent lipid efflux by ABCA1. ABCA1 mediated the efflux of cholesterol and phospholipids in the presence of NaTC as well as in the presence of apoA-I in an ATP-dependent manner. The Tangier disease mutation W590S, which resides in the extracellular domain and impairs apoA-I-dependent lipid efflux, greatly decreased NaTC-dependent cholesterol and phospholipid efflux. However, the W590S mutation did not impair apoA-I binding and, conversely, retarded the dissociation of apoA-I from ABCA1. These results suggest that the W590S mutation impairs ATP-dependent lipid translocation and that lipid translocation or possibly lipid loading, facilitates apoA-I dissociation from ABCA1. NaTC is a good tool for analyzing ABCA1-mediated lipid efflux and allows dissection of the steps of HDL formation by ABCA1.     

Existence of lipid microdomains in bilayer of dipalmitoyl phosphatidylcholine (DPPC) and 1-stearoyl-2-docosahexenoyl phosphatidylserine (SDPS) and their perturbation by chlorpromazine: a 13C and 31P solid-state NMR study

The polyunsaturated fatty acid docosahexaenoic acid (DHA, 22 : 6, n-3) is found at a level of about 50% in the phospholipids of neuronal tissue membranes and appears to be crucial to human health. Dipalmitoyl phosphatidylcholine (DPPC, 16 : 0/16 : 0 PC) and the DHA containing 1-stearoyl-2-docosahexenoyl phosphatidylserine (SDPS) were used to make DPPC (60%)/SDPS (40%) bilayers with and without 10 mol% chlorpromazine (CPZ), a cationic, amphiphilic phenothiazine.

Resonances that are present in ¹³C NMR spectrum of the DPPC (60%)/SDPS (40%) sample and that disappear in presence of 10% CPZ most probably are due to the special interface environment, e.g. the hydrophobic mismatch, at the interface of DPPC and SDPS microdomains in the DPPC/SDPS bilayer. In itself the appearance of resonances at novel chemical shift values is a clear demonstration of a unique chemical environment in the DPPC (60%)/SDPS (40%) bilayer. The findings of the study presented here suggest CPZ bound to the phosphate of SDPS will slow down and partially inhibit such a DHA acyl chain movement in the DPPC/SDPS bilayer. This would affect the area occupied by a SDPS molecule (in the bilayer) and probably the thickness of the bilayer where SDPS molecules reside as well. It is quite likely that such CPZ caused changes can affect the function of proteins embedded in the bilayer.     

Targeting the lipid metabolic axis ACSL/SCD in colorectal cancer progression by therapeutic miRNAs: miR-19b-1 role

An abnormal acyl-CoA synthetase/stearoyl-CoA desaturase (ACSL/SCD) lipid network fuels colon cancer progression, endowing cells with invasive and migratory properties. Therapies against this metabolic network may be useful to improve clinical outcomes. Because micro-RNAs (miRNAs/miRs) are important epigenetic regulators, we investigated novel miRNAs targeting this pro-tumorigenic axis; hence to be used as therapeutic or prognostic miRNAs. Thirty-one putative common miRNAs were predicted to simultaneously target the three enzymes comprising the ACSL/SCD network. Target validation by quantitative RT-PCR, Western blotting, and luciferase assays showed miR-544a, miR-142, and miR-19b-1 as major regulators of the metabolic axis, ACSL/SCD. Importantly, lower miR-19b-1 expression was associated with a decreased survival rate in colorectal cancer (CRC) patients, accordingly with ACSL/SCD involvement in patient relapse. Finally, miR-19b-1 regulated the pro-tumorigenic axis, ACSL/SCD, being able to inhibit invasion in colon cancer cells. Because its expression correlated with an increased survival rate in CRC patients, we propose miR-19b-1 as a potential noninvasive biomarker of disease-free survival and a promising therapeutic miRNA in CRC.     

Increased lipid rafts and accelerated lipopolysaccharide-induced tumor necrosis factor-alpha secretion in Abca1-deficient macrophages

Lipid rafts on the cell surface are believed to be very important as platforms for various cellular functions. The aim of this study was to know whether defective lipid efflux may influence lipid rafts on the cell surface and their related cellular functions. We investigated macrophages with defective lipid efflux from ATP binding cassette transporter A1-deficient (Abca1-KO) mice. Lipid rafts were evaluated by the following two novel probes: a biotinylated and protease (subtilisin Carlsberg)-nicked derivative of theta-toxin and a fluorescein ester of polyethylene glycol-derived cholesterol. Lipid rafts in Abca1-KO macrophages were increased, as demonstrated by both probes. Moreover, activities of nuclear factor kappaB, mRNA and intracellular distribution, and secretion of tumor necrosis factor-alpha (TNF-alpha) were examined after stimulation by lipopolysaccharides (LPSs). LPS-induced responses of the activation of nuclear factor kappaB and TNF-alpha were more prompt and accelerated in the Abca1-KO macrophages compared with wild-type macrophages. Modification of lipid rafts by cyclodextrin and nystatin corrected the abnormal response, suggesting an association between the increased lipid rafts and abnormal TNF-alpha secretion. We report here that Abca1-KO macrophages with defective lipid efflux exhibited increased lipid rafts on the cell surface and accelerated TNF-alpha secretion.     

Macronutrient-specific effect of the MTNR1B genotype on lipid levels in response to 2 year weight-loss diets

Compelling evidence indicates that lipid metabolism is in partial control of the circadian system. In this context, it has been reported that the melatonin receptor 1B (MTNR1B) genetic variant influences the dynamics of melatonin secretion, which is involved in the circadian system as a chronobiotic. The objective was to analyze whether the MTNR1B rs10830963 genetic variant was related to changes in lipid levels in response to dietary interventions with different macronutrient distribution in 722 overweight/obese subjects from the POUNDS Lost trial. We did not find a significant association between the MTNR1B genotype and changes in lipid metabolism. However, dietary fat intake significantly modified genetic effects on 2 year changes in total and LDL cholesterol (P interaction = 0.006 and 0.001, respectively). In the low-fat diet group, carriers of the sleep disruption G allele (minor allele) showed a greater reduction of total cholesterol (β ± SE = −5.78 ± 2.88 mg/dl, P = 0.04) and LDL cholesterol (β ± SE = −7.19 ± 2.37 mg/dl, P = 0.003). Conversely, in the high-fat diet group, subjects carrying the G allele evidenced a smaller decrease in total cholesterol (β ± SE = 5.81 ± 2.65 mg/dl, P = 0.03) and LDL cholesterol (β ± SE = 5.23 ± 2.21 mg/dl, P = 0.002). Subjects carrying the G allele of the circadian rhythm-related MTNR1B variant may present a bigger impact on total and LDL cholesterol when undertaking an energy-restricted low-fat diet.     

The impact on high‐grade serous ovarian cancer of obesity and lipid metabolism‐related gene expression patterns: the underestimated driving force affecting prognosis

To investigate whether specific obesity/metabolism‐related gene expression patterns affect the survival of patients with ovarian cancer. Clinical and genomic data of 590 samples from the high‐grade ovarian serous carcinoma (HGOSC) study of The Cancer Genome Atlas (TCGA) and 91 samples from the Australian Ovarian Cancer Study were downloaded from the International Cancer Genome Consortium (ICGC) portal. Clustering of mRNA microarray and reverse‐phase protein array (RPPA) data was performed with 83 consensus driver genes and 144 obesity and lipid metabolism‐related genes. Association between different clusters and survival was analyzed with the Kaplan–Meier method and a Cox regression. Mutually exclusive, co‐occurrence and network analyses were also carried out. Using RNA and RPPA data, it was possible to identify two subsets of HGOSCs with similar clinical characteristics and cancer driver mutation profiles (e.g. TP53), but with different outcome. These differences depend more on up‐regulation of specific obesity and lipid metabolism‐related genes than on the number of gene mutations or copy number alterations. It was also found that CD36 and TGF‐ß are highly up‐regulated at the protein levels in the cluster with the poorer outcome. In contrast, BSCL2 is highly up‐regulated in the cluster with better progression‐free and overall survival. Different obesity/metabolism‐related gene expression patterns constitute a risk factor for prognosis independent of the therapy results in the Cox regression. Prognoses were conditioned by the differential expression of obesity and lipid metabolism‐related genes in HGOSCs with similar cancer driver mutation profiles, independent of the initial therapeutic response.     

Interaction of membrane/lipid rafts with the cytoskeleton: Impact on signaling and function : Membrane/lipid rafts,mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.     

Characterization of palmitoylation of ATP binding cassette transporter G1: Effect on protein trafficking and function

ATP-binding cassette transporter G1 (ABCG1) mediates cholesterol efflux onto lipidated apolipoprotein A-I and HDL and plays a role in various important physiological functions. However, the mechanism by which ABCG1 mediates cholesterol translocation is unclear. Protein palmitoylation regulates many functions of proteins such as ABCA1. Here we investigated if ABCG1 is palmitoylated and the subsequent effects on ABCG1-mediated cholesterol efflux. We demonstrated that ABCG1 is palmitoylated in both human embryonic kidney 293 cells and in mouse macrophage, J774. Five cysteine residues located at positions 26, 150, 311, 390 and 402 in the NH₂-terminal cytoplasmic region of ABCG1 were palmitoylated. Removal of palmitoylation at Cys311 by mutating the residue to Ala (C311A) or Ser significantly decreased ABCG1-mediated cholesterol efflux. On the other hand, removal of palmitoylation at sites 26, 150, 390 and 402 had no significant effect. We further demonstrated that mutations of Cys311 affected ABCG1 trafficking from the endoplasmic reticulum. Therefore, our data suggest that palmitoylation plays a critical role in ABCG1-mediated cholesterol efflux through the regulation of trafficking.     

Nobiletin protects against insulin resistance and disorders of lipid metabolism by reprogramming of circadian clock in hepatocytes

Scope

Circadian clock plays a principal role in orchestrating our daily physiology and metabolism, and their perturbation can evoke metabolic diseases such as fatty liver and insulin resistance. Nobiletin (NOB) has been demonstrated to possess antitumor and neuroprotective activities. The objective of the current study is to determine potential effects of NOB on modulating the core clock gene Bmal1 regarding ameliorating glucolipid metabolic disorders.

Angiotensin-converting enzyme insertion/deletion (rs106180) and angiotensin type 1 receptor A1166C (rs106165) genotypes and psoriasis: Correlation with cellular immunity,lipid profile,and oxidative stress markers

Psoriasis is a chronic inflammatory skin condition and angiotensin-converting enzyme (ACE) is a key circulating enzyme converting angiotensin (Ang) I to the vasoactive peptide Ang II. The exact role of ACE insertion (I)/deletion (D) polymorphism (rs106180) in psoriasis is not clear. We aimed to examine whether the ACE I/D and Ang II type 1 receptor (AT1R) A₁₁₆₆C-polymorphisms (rs106165), lipid profile, and stress oxidative are associated with susceptibility to psoriasis. One hundred patients with psoriasis and 100 sex- and age-matched unrelated healthy controls were recruited for this case-control study. ACE I/D and AT1R A₁₁₆₆C polymorphisms were identified by the polymerase chain reaction (PCR) and PCR-restriction fragment length polymorphism, respectively, malondialdehyde (MDA) was detected by the high-performance liquid chromatography, serum arylesterase (ARE) activity of paraoxonase and catalase activities were detected by the spectrophotometry, superoxide dismutase (SOD) activity and vascular adhesion protein (VAP)-1 were measured by ELISA. The presence of C allele of AT1R A₁₁₆₆C and I allele of ACE considerably increased the risk of psoriasis by 6.42-fold (P < 0.001). The distribution of II-genotype of ACE was significantly higher in psoriasis patients than in control group and increased the risk of disease by 3.11-times (P = 0.023). The higher levels of MDA in patients and the higher activity of SOD, ARE, and CAT was observed in healthy controls with I/D+I/I-genotype of ACE I/D. This study for the first time demonstrated that the ACE I/D and AT1R A ₁₁₆₆C genes polymorphisms robustly increases the risk of developing psoriasis in population from west of Iran. In addition, these individuals had significantly higher VAP-1 and MDA concentration and lower enzymatic and nonenzymatic antioxidant-status, suggesting that psoriatic patients carrying C allele of AT1R₁₁₆₆ polymorphism may be more susceptible to cardiovascular disease and myocardial infarction compared with A allele.     

Cytokines, macrophage lipid metabolism and foam cells: implications for cardiovascular disease therapy

Cardiovascular disease is the biggest killer globally and the principal contributing factor to the pathology is atherosclerosis; a chronic, inflammatory disorder characterized by lipid and cholesterol accumulation and the development of fibrotic plaques within the walls of large and medium arteries. Macrophages are fundamental to the immune response directed to the site of inflammation and their normal, protective function is harnessed, detrimentally, in atherosclerosis. Macrophages contribute to plaque development by internalizing native and modified lipoproteins to convert them into cholesterol-rich foam cells. Foam cells not only help to bridge the innate and adaptive immune response to atherosclerosis but also accumulate to create fatty streaks, which help shape the architecture of advanced plaques. Foam cell formation involves the disruption of normal macrophage cholesterol metabolism, which is governed by a homeostatic mechanism that controls the uptake, intracellular metabolism, and efflux of cholesterol. It has emerged over the last 20 years that an array of cytokines, including interferon-γ, transforming growth factor-β1, interleukin-1β, and interleukin-10, are able to manipulate these processes. Foam cell targeting, anti-inflammatory therapies, such as agonists of nuclear receptors and statins, are known to regulate the actions of pro- and anti-atherogenic cytokines indirectly of their primary pharmacological function. A clear understanding of macrophage foam cell biology will hopefully enable novel foam cell targeting therapies to be developed for use in the clinical intervention of atherosclerosis.     

Cholesterol trafficking and raft-like membrane domain composition mediate scavenger receptor class B type 1-dependent lipid sensing in intestinal epithelial cells

Scavenger receptor Class B type 1 (SR-B1) is a lipid transporter and sensor. In intestinal epithelial cells, SR-B1-dependent lipid sensing is associated with SR-B1 recruitment in raft-like/ detergent-resistant membrane domains and interaction of its C-terminal transmembrane domain with plasma membrane cholesterol. To clarify the initiating events occurring during lipid sensing by SR-B1, we analyzed cholesterol trafficking and raft-like domain composition in intestinal epithelial cells expressing wild-type SR-B1 or the mutated form SR-B1-Q445A, defective in membrane cholesterol binding and signal initiation. These features of SR-B1 were found to influence both apical cholesterol efflux and intracellular cholesterol trafficking from plasma membrane to lipid droplets, and the lipid composition of raft-like domains. Lipidomic analysis revealed likely participation of d18:0/16:0 sphingomyelin and 16:0/0:0 lysophosphatidylethanolamine in lipid sensing by SR-B1. Proteomic analysis identified proteins, whose abundance changed in raft-like domains during lipid sensing, and these included molecules linked to lipid raft dynamics and signal transduction. These findings provide new insights into the role of SR-B1 in cellular cholesterol homeostasis and suggest molecular links between SR-B1-dependent lipid sensing and cell cholesterol and lipid droplet dynamics.