Unsaturated fatty acids phosphorylate and destabilize ABCA1 through a phospholipase D2 pathway

Impact of various dietary lipids on amelioration of biomarkers linked to metabolic syndrome in both healthy and diabetic Wistar rats

BACKGROUND

The present study was designed to investigate the influence of different dietary lipids (sheep's fat, olive oil, coconut oil, and corn oil) on specific biomarkers associated with metabolic syndrome in both healthy and diabetic rats.

METHODS

The study designed for 45 days, utilized a male diabetic wistar rat (body weight, 180-220 g) model induced by streptozotocin (45 mg/kg bw). The rats were divided into two sections: five non-diabetic and five diabetic groups, each containing six rats. The first group in each section serving as the control, received a standard diet. Both non-diabetic or diabetic groups, were provided with a standard diet enriched with 15% sheep fat, 15% coconut oil, 15% olive oil, and 15% corn oil, respectively for a duration of 45 days.

RESULTS

Post-supplementation, both healthy and diabetic control rats exhibited a higher food intake compared to rats supplemented with lipid diet; notably food intake was higher in diabetic control than healthy control. However, rats fed with coconut oil, olive oil and sheep fat showed weight gain at the end of the experiment, in both healthy and diabetic groups. Coconut oil supplementation significantly (p ≤ 0.05) increased HDL-C and total cholesterol level in diabetic groups compared to healthy group, it was confirmed by an increased PPAR-α and ABCA-1 protein level. Olive oil significantly decreased triglyceride, total cholesterol, and LDL-C levels in diabetic rats when compared to sheep fat or coconut oil. Corn oil significantly decreased fasting glucose, total cholesterol and LDL-C levels compared to all other groups. Corn and olive oil supplemented normal groups, found with significant increase in hepatic glucose-lipid oxidative metabolism associated protein, like FGF-21, MSH, ABCA-1, PPAR-γ and decreased lipogenesis proteins like, SREBP and PPAR-α levels. In contrast, sheep grease and coconut oil increased SREBP and PPAR-α expression in both normal and diabetic groups. Most notably, normal and diabetic groups pretreated with sheep grease resulted in increased inflammatory (MCP-1, IL-1β, TLR-4, TNF-α), and oxidative stress markers (LPO, GSH, GPx, SOD and CAT) linked with metabolic complications.

CONCLUSION

The combination or alternative use of olive oil and corn oil in daily diet may play a significant role in preventing proinflammatory condition associated with insulin resistance and cardiovascular diseases.

In Situ Remodeling of Efferocytosis via Lesion-Localized Microspheres to Reverse Cartilage Senescence

Efferocytosis, an intrinsic regulatory mechanism to eliminate apoptotic cells, will be suppressed due to the delayed apoptosis process in aging-related diseases, such as osteoarthritis (OA). In this study, cartilage lesion-localized hydrogel microspheres are developed to remodel the in situ efferocytosis to reverse cartilage senescence and recruit endogenous stem cells to accelerate cartilage repair. Specifically, aldehyde- and methacrylic anhydride (MA)-modified hyaluronic acid hydrogel microspheres (AHM), loaded with pro-apoptotic liposomes (liposomes encapsulating ABT263, A-Lipo) and PDGF-BB, namely A-Lipo/PAHM, are prepared by microfluidic and photo-cross-linking techniques. By a degraded porcine cartilage explant OA model, the in situ cartilage lesion location experiment illustrated that aldehyde-functionalized microspheres promote affinity for degraded cartilage. In vitro data showed that A-Lipo induced apoptosis of senescent chondrocytes (Sn-chondrocytes), which can then be phagocytosed by the efferocytosis of macrophages, and remodeling efferocytosis facilitated the protection of normal chondrocytes and maintained the chondrogenic differentiation capacity of MSCs. In vivo experiments confirmed that hydrogel microspheres localized to cartilage lesion reversed cartilage senescence and promoted cartilage repair in OA. It is believed this in situ efferocytosis remodeling strategy can be of great significance for tissue regeneration in aging-related diseases.

Mononuclear phagocyte system-related multi-omics features yield head and neck squamous cell carcinoma subtypes with distinct overall survival, drug, and immunotherapy responses

BACKGROUND

Recent research reported that mononuclear phagocyte system (MPS) can contribute to immune defense but the classification of head and neck squamous cell carcinoma (HNSCC) patients based on MPS-related multi-omics features using machine learning lacked.

METHODS

In this study, we obtain marker genes for MPS through differential analysis at the single-cell level and utilize "similarity network fusion" and "MoCluster" algorithms to cluster patients' multi-omics features. Subsequently, based on the corresponding clinical information, we investigate the prognosis, drugs, immunotherapy, and biological differences between the subtypes. A total of 848 patients have been included in this study, and the results obtained from the training set can be verified by two independent validation sets using "the nearest template prediction".

RESULTS

We identified two subtypes of HNSCC based on MPS-related multi-omics features, with CS2 exhibiting better predictive prognosis and drug response. CS2 represented better xenobiotic metabolism and higher levels of T and B cell infiltration, while the biological functions of CS1 were mainly enriched in coagulation function, extracellular matrix, and the JAK-STAT signaling pathway. Furthermore, we established a novel and stable classifier called "getMPsub" to classify HNSCC patients, demonstrating good consistency in the same training set. External validation sets classified by "getMPsub" also illustrated similar differences between the two subtypes.

CONCLUSIONS

Our study identified two HNSCC subtypes by machine learning and explored their biological difference. Notably, we constructed a robust classifier that presented an excellent classifying prediction, providing new insight into the precision medicine of HNSCC.

The role and regulation of phospholipase D in metabolic disorders

Phospholipase D (PLD) is an enzyme that catalyzes the hydrolysis of phosphatidylcholine into phosphatidic acid and free choline. In mammals, PLD exists in two well-characterized isoforms, PLD1 and PLD2, and it plays pivotal roles as signaling mediators in various cellular functions, such as cell survival, differentiation, and migration. These isoforms are predominantly expressed in diverse cell types, including many immune cells, such as monocytes and macrophages, as well as non-immune cells, such as epithelial and endothelial cells. Several previous studies have revealed that the stimulation of these cells leads to an increase in PLD expression and its enzymatic products, potentially influencing the pathological responses in a wide spectrum of diseases. Metabolic diseases, exemplified by conditions, such as diabetes, obesity, hypertension, and atherosclerosis, pose significant global health challenges. Abnormal activation or dysfunction of PLD emerges as a potential contributing factor to the pathogenesis and progression of these metabolic disorders. Therefore, it is crucial to thoroughly investigate and understand the intricate relationship between PLD and metabolic diseases. In this review, we provide an in-depth overview of the functional roles and molecular mechanisms of PLD involved in metabolic diseases. By delving into the intricate interplay between PLD and metabolic disorders, this review aims to offer insights into the potential therapeutic interventions.

Drugging the efferocytosis process: concepts and opportunities

The daily removal of billions of apoptotic cells in the human body via the process of efferocytosis is essential for homeostasis. To allow for this continuous efferocytosis, rapid phenotypic changes occur in the phagocytes enabling them to engulf and digest the apoptotic cargo. In addition, efferocytosis is actively anti-inflammatory and promotes resolution. Owing to its ubiquitous nature and the sheer volume of cell turnover, efferocytosis is a point of vulnerability. Aberrations in efferocytosis are associated with numerous inflammatory pathologies, including atherosclerosis, cancer and infections. The recent exciting discoveries defining the molecular machinery involved in efferocytosis have opened many avenues for therapeutic intervention, with several agents now in clinical trials.

Role of ABCA1 in Cardiovascular Disease

Cholesterol homeostasis plays a significant role in cardiovascular disease. Previous studies have indicated that ATP-binding cassette transporter A1 (ABCA1) is one of the most important proteins that maintains cholesterol homeostasis. ABCA1 mediates nascent high-density lipoprotein biogenesis. Upon binding with apolipoprotein A-I, ABCA1 facilitates the efflux of excess intracellular cholesterol and phospholipids and controls the rate-limiting step of reverse cholesterol transport. In addition, ABCA1 interacts with the apolipoprotein receptor and suppresses inflammation through a series of signaling pathways. Thus, ABCA1 may prevent cardiovascular disease by inhibiting inflammation and maintaining lipid homeostasis. Several studies have indicated that post-transcriptional modifications play a critical role in the regulation of ABCA1 transportation and plasma membrane localization, which affects its biological function. Meanwhile, carriers of the loss-of-function ABCA1 gene are often accompanied by decreased expression of ABCA1 and an increased risk of cardiovascular diseases. We summarized the ABCA1 transcription regulation mechanism, mutations, post-translational modifications, and their roles in the development of dyslipidemia, atherosclerosis, ischemia/reperfusion, myocardial infarction, and coronary heart disease.

Metabolite patterns link diet, obesity, and type 2 diabetes in a Hispanic population

INTRODUCTION

Obesity is a precursor of type 2 diabetes (T2D).

OBJECTIVES

Our aim was to identify metabolic signatures of T2D and dietary factors unique to obesity.

METHODS

We examined a subsample of the Boston Puerto Rican Health Study (BPRHS) population with a high prevalence of obesity and T2D at baseline (n = 806) and participants (without T2D at baseline) at 5-year follow-up (n = 412). We determined differences in metabolite profiles between T2D and non-T2D participants of the whole sample and according to abdominal obesity status. Enrichment analysis was performed to identify metabolic pathways that were over-represented by metabolites that differed between T2D and non-T2D participants. T2D-associated metabolites unique to obesity were examined for correlation with dietary food groups to understand metabolic links between dietary intake and T2D risk. False Discovery Rate method was used to correct for multiple testing.

RESULTS

Of 526 targeted metabolites, 179 differed between T2D and non-T2D in the whole sample, 64 in non-obese participants and 120 unique to participants with abdominal obesity. Twenty-four of 120 metabolites were replicated and were associated with T2D incidence at 5-year follow-up. Enrichment analysis pointed to three metabolic pathways that were overrepresented in obesity-associated T2D: phosphatidylethanolamine (PE), long-chain fatty acids, and glutamate metabolism. Elevated intakes of three food groups, energy-dense takeout food, dairy intake and sugar-sweetened beverages, associated with 13 metabolites represented by the three pathways.

CONCLUSION

Metabolic signatures of lipid and glutamate metabolism link obesity to T2D, in parallel with increased intake of dairy and sugar-sweetened beverages, thereby providing insight into the relationship between dietary habits and T2D risk.

Participation of ABCA1 Transporter in Pathogenesis of Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) is the important medical and social problem. According to modern concepts, COPD is a chronic inflammatory disease, macrophages play a key role in its pathogenesis. Macrophages are heterogeneous in their functions, which is largely determined by their immunometabolic profile, as well as the features of lipid homeostasis, in which the ATP binding cassette transporter A1 (ABCA1) plays an essential role. The objective of this work is the analysis of the ABCA1 protein participation and the function of reverse cholesterol transport in the pathogenesis of COPD. The expression of the ABCA1 gene in lung tissues takes the second place after the liver, which indicates the important role of the carrier in lung function. The participation of the transporter in the development of COPD consists in provision of lipid metabolism, regulation of inflammation, phagocytosis, and apoptosis. Violation of the processes in which ABCA1 is involved may be a part of the pathophysiological mechanisms, leading to the formation of a heterogeneous clinical course of the disease.

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.

Impact of natural products on the cholesterol transporter ABCA1

ETHNOPHARMACOLOGICAL RELEVANCE

In different countries and areas of the world, traditional medicine has been and is still used for the treatment of various disorders, including chest pain or liver complaints, of which we now know that they can be linked with altered lipid and cholesterol homeostasis. As ATP-binding cassette transporter A1 (ABCA1) plays an essential role in cholesterol metabolism, its modulation may be one of the molecular mechanisms responsible for the experienced benefit of traditional recipes. Intense research activity has been dedicated to the identification of natural products from traditional medicine that regulate ABCA1 expression.

AIMS OF THE REVIEW

This review surveys natural products, originating from ethnopharmacologically used plants, fungi or marine sources, which influence ABCA1 expression, providing a reference for future study.

MATERIALS AND METHODS

Information on regulation of ABCA1 expression by natural compounds from traditional medicine was extracted from ancient and modern books, materia medica, and electronic databases (PubMed, Google Scholar, Science Direct, and ResearchGate).

RESULTS

More than 60 natural compounds from traditional medicine, especially traditional Chinese medicine (TCM), are reported to regulate ABCA1 expression in different in vitro and in vivo models (such as cholesterol efflux and atherosclerotic animal models). These active compounds belong to the classes of polyketides, terpenoids, phenylpropanoids, tannins, alkaloids, steroids, amino acids and others. Several compounds appear very promising in vivo, which need to be further investigated in animal models of diseases related to ABCA1 or in clinical studies.

CONCLUSION

Natural products from traditional medicine constitute a large promising pool for compounds that regulate ABCA1 expression, and thus may prevent/treat diseases related to cholesterol metabolism, like atherosclerosis or Alzheimer's disease. In many cases, the molecular mechanisms of these natural products remain to be investigated.

Post-translational modifications of transporters

Drug transporter proteins are critical to the distribution of a wide range of endogenous compounds and xenobiotics such as hormones, bile acids, peptides, lipids, sugars, and drugs. There are two classes of drug transporters- the solute carrier (SLC) transporters and ATP-binding cassette (ABC) transporters -which predominantly differ in the energy source utilized to transport substrates across a membrane barrier. Despite their hydrophobic nature and residence in the membrane bilayer, drug transporters have dynamic structures and adopt many conformations during the translocation process. Whereas there is significant literature evidence for the substrate specificity and structure-function relationship for clinically relevant drug transporters proteins, there is less of an understanding in the regulatory mechanisms that contribute to the functional expression of these proteins. Post-translational modifications have been shown to modulate drug transporter functional expression via a wide range of molecular mechanisms. These modifications commonly occur through the addition of a functional group (e.g. phosphorylation), a small protein (e.g. ubiquitination), sugar chains (e.g. glycosylation), or lipids (e.g. palmitoylation) on solvent accessible amino acid residues. These covalent additions often occur as a result of a signaling cascade and may be reversible depending on the type of modification and the intended fate of the signaling event. Here, we review the significant role in which post-translational modifications contribute to the dynamic regulation and functional consequences of SLC and ABC drug transporters and highlight recent progress in understanding their roles in transporter structure, function, and regulation.

Is ABCA1 a lipid transfer protein?

ABCA1 functions as a lipid transporter because it mediates the transfer of cellular phospholipid (PL) and free (unesterified) cholesterol (FC) to apoA-I and related proteins present in the extracellular medium. ABCA1 is a membrane PL translocase and its enzymatic activity leads to transfer of PL molecules from the cytoplasmic leaflet to the exofacial leaflet of a cell plasma membrane (PM). The presence of active ABCA1 in the PM promotes binding of apoA-I to the cell surface. About 10% of this bound apoA-I interacts directly with ABCA1 and stabilizes the transporter. Most of the pool of cell surface-associated apoA-I is bound to lipid domains in the PM that are created by the activity of ABCA1. The amphipathic α-helices in apoA-I confer detergent-like properties on the protein enabling it to solubilize PL and FC in these membrane domains to create a heterogeneous population of discoidal nascent HDL particles. This review focuses on current understanding of the structure-function relationships of human ABCA1 and the molecular mechanisms underlying HDL particle production.

Phospholipase D2 loss results in increased blood pressure via inhibition of the endothelial nitric oxide synthase pathway

The Phospholipase D (PLD) superfamily is linked to neurological disease, cancer, and fertility, and a recent report correlated a potential loss-of-function PLD2 polymorphism with hypotension. Surprisingly, PLD2 -/- mice exhibit elevated blood pressure accompanied by associated changes in cardiac performance and molecular markers, but do not have findings consistent with the metabolic syndrome. Instead, expression of endothelial nitric oxide synthase (eNOS), which generates the potent vasodilator nitric oxide (NO), is decreased. An eNOS inhibitor phenocopied PLD2 loss and had no further effect on PLD2 -/- mice, confirming the functional relationship. Using a human endothelial cell line, PLD2 loss of function was shown to lower intracellular free cholesterol, causing upregulation of HMG Co-A reductase, the rate-limiting enzyme in cholesterol synthesis. HMG Co-A reductase negatively regulates eNOS, and the PLD2-deficiency phenotype of decreased eNOS expression and activity could be rescued by cholesterol supplementation and HMG Co-A reductase inhibition. Together, these findings identify a novel pathway through which the lipid signaling enzyme PLD2 regulates blood pressure, creating implications for on-going therapeutic development of PLD small molecule inhibitors. Finally, we show that the human PLD2 polymorphism does not trigger eNOS loss, but rather creates another effect, suggesting altered functioning for the allele.

Dysfunctional HDL in diabetes mellitus and its role in the pathogenesis of cardiovascular disease

Coronary artery disease, the leading cause of death in the developed and developing countries, is prevalent in diabetes mellitus with 68% cardiovascular disease (CVD)-related mortality. Epidemiological studies suggested inverse correlation between HDL and CVD occurrence. Therefore, low HDL concentration observed in diabetic patients compared to non-diabetic individuals was thought to be one of the primary causes of increased risks of CVD. Efforts to raise HDL level via CETP inhibitors, Torcetrapib and Dalcetrapib, turned out to be disappointing in outcome studies despite substantial increases in HDL-C, suggesting that factors beyond HDL concentration may be responsible for the increased risks of CVD. Therefore, recent studies have focused more on HDL function than on HDL levels. The metabolic environment in diabetes mellitus condition such as hyperglycemia-induced advanced glycation end products, oxidative stress, and inflammation promote HDL dysfunction leading to greater risks of CVD. This review discusses dysfunctional HDL as one of the mechanisms of increased CVD risks in diabetes mellitus through adversely affecting components that support HDL function in cholesterol efflux and LDL oxidation. The dampening of reverse cholesterol transport, a key process that removes cholesterol from lipid-laden macrophages in the arterial wall, leads to increased risks of CVD in diabetic patients. Therapeutic approaches to keep diabetes under control may benefit patients from developing CVD.

Identification of glaikit in a genome-wide expression profiling for axonal bifurcation of the mushroom body in Drosophila

Axonal branching is a fundamental requirement for sending electrical signals to multiple targets. However, despite the importance of axonal branching in neural development and function, the molecular mechanisms that control branch formation are poorly understood. Previous studies have hardly addressed the intracellular signaling cascade of axonal bifurcation characterized by growth cone splitting. Recently we reported that DISCO interacting protein 2 (DIP2) regulates bifurcation of mushroom body axons in Drosophila melanogaster. DIP2 mutant displays ectopic bifurcations in α/β neurons. Taking advantage of this phenomenon, we tried to identify genes involved in branching formation by comparing the transcriptome of wild type with that of DIP2 RNAi flies. After the microarray analysis, Glaikit (Gkt), a member of the phospholipase D superfamily, was identified as a downstream target of DIP2 by RNAi against gkt and qRT-PCR experiment. Single cell MARCM analysis of gkt mutant phenocopied the ectopic axonal branches observed in DIP2 mutant. Furthermore, a genetic analysis between gkt and DIP2 revealed that gkt potentially acts in parallel with DIP2. In conclusion, we identified a novel gene underlying the axonal bifurcation process.

Cell lipid metabolism modulators 2-bromopalmitate, D609, monensin, U18666A and probucol shift discoidal HDL formation to the smaller-sized particles: implications for the mechanism of HDL assembly

ATP-binding cassette transporter A1 (ABCA1) mediates formation of disc-shaped high-density lipoprotein (HDL) from cell lipid and lipid-free apolipoprotein A-I (apo A-I). Discoidal HDL particles are heterogeneous in physicochemical characteristics for reasons that are understood incompletely. Discoidal lipoprotein particles similar in characteristics and heterogeneity to cell-formed discoidal HDL can be reconstituted from purified lipids and apo A-I by cell-free, physicochemical methods. The heterogeneity of reconstituted HDL (rHDL) is sensitive to the lipid composition of the starting lipid/apo A-I mixture. To determine whether the heterogeneity of cell-formed HDL is similarly sensitive to changes in cell lipids, we investigated four compounds that have well-established effects on cell lipid metabolism and ABCA1-mediated cell cholesterol efflux. 2-Bromopalmitate, D609, monensin and U18666A decreased formation of the larger-sized, but dramatically increased formation of the smaller-sized HDL. 2-Bromopalmitate did not appear to affect ABCA1 activity, subcellular localization or oligomerization, but induced dissolution of the cholesterol-phospholipid complexes in the plasma membrane. Arachidonic and linoleic acids shifted HDL formation to the smaller-sized species. Tangier disease mutations and inhibitors of ABCA1 activity wheat germ agglutinin and AG 490 reduced formation of both larger-sized and smaller-sized HDL. The effect of probucol was similar to the effect of 2-bromopalmitate. Taking rHDL formation as a paradigm, we propose that ABCA1 mutations and activity inhibitors reduce the amount of cell lipid available for HDL formation, and the compounds in the 2-bromopalmitate group and the polyunsaturated fatty acids change cell lipid composition from one that favors formation of the larger-sized HDL particles to one that favors formation of the smaller-sized species.

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.

HDLs, diabetes, and metabolic syndrome

The prevalence of type 2 diabetes mellitus and of the metabolic syndrome is rising worldwide and reaching epidemic proportions. These pathologies are associated with significant morbidity and mortality, in particular with an excess of cardiovascular deaths. Type 2 diabetes mellitus and the cluster of pathologies including insulin resistance, central obesity, high blood pressure, and hypertriglyceridemia that constitute the metabolic syndrome are associated with low levels of HDL cholesterol and the presence of dysfunctional HDLs. We here review the epidemiological evidence and the potential underlying mechanisms of this association. We first discuss the well-established association of type 2 diabetes mellitus and insulin resistance with alterations of lipid metabolism and how these alterations may lead to low levels of HDL cholesterol and the occurrence of dysfunctional HDLs. We then present and discuss the evidence showing that HDL modulates insulin sensitivity, insulin-independent glucose uptake, insulin secretion, and beta cell survival. A dysfunction in these actions could play a direct role in the pathogenesis of type 2 diabetes mellitus.

Activation of soluble guanylyl cyclase prevents foam cell formation and atherosclerosis

AIMS

Soluble guanylyl cyclase (sGC) is a key modulator in the regulation of vascular tone. However, its role and involving mechanism in cholesterol metabolism of macrophages and atherosclerosis remain unclear.

METHODS

Oil red O staining, Dil-oxidized low-density lipoprotein (oxLDL)-binding assay and cholesterol efflux assay were performed in biology of foam cells. Levels of cytokines or intracellular lipid were evaluated by ELISA or colorimetric kits. Expression of gene or protein was determined by quantitative real-time PCR or Western blotting. Histopathology was examined by haematoxylin and eosin staining.

RESULTS

Soluble guanylyl cyclase was expressed in macrophages of mouse atherosclerotic lesions. Treatment with 1H-[1, 2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, sGC inhibitor) exacerbated oxLDL-induced cholesterol accumulation in macrophages. In contrast, 3-(5'-hydroxymethyl-2'furyl)-1-benzyl indazole (YC-1, sGC activator) attenuated the oxLDL-induced cholesterol accumulation because of increased cholesterol efflux. Additionally, YC-1 dose dependently increased the protein expression of ATP-binding cassette transporter A1 (ABCA1) but did not alter that of scavenger receptor class A (SR-A), CD36, SR-BI or ABCG1. Moreover, YC-1-upregulated ABCA1 level depended on liver X receptor α (LXRα). Inhibition of the LXRα-ABCA1 pathway by LXRα small interfering RNA (siRNA), ABCA1 neutralizing antibody or ABCA1 siRNA abolished the effect of YC-1 on cholesterol accumulation and cholesterol efflux. In vivo, YC-1 retarded the development of atherosclerosis, accompanied by reduced serum levels of cholesterol and pro-inflammatory cytokines, in apolipoprotein E-deficient mice.

CONCLUSION

Activation of sGC by YC-1 leads to LXRα-dependent upregulation of ABCA1 in macrophages and may confer protection against atherosclerosis.

Linoleic acid suppresses cholesterol efflux and ATP-binding cassette transporters in murine bone marrow-derived macrophages

Individuals with type 2 diabetes mellitus (T2DM) are at increased risk of developing cardiovascular disease (CVD), possibly associated with elevated plasma free fatty acid concentrations. Paradoxically, evidence suggests that unsaturated, compared to saturated fatty acids, suppress macrophage cholesterol efflux, favoring cholesterol accumulation in the artery wall. Murine bone marrow-derived macrophages (BMDM) were used to further explore the relationship between saturated and unsaturated fatty acids, and cholesterol efflux mediated by ATP-binding cassette transporters (ABCA1 and ABCG1) through transcription factors liver-x-receptor-alpha (LXR-α) and sterol receptor element binding protein (SREBP)-1. BMDM isolated from C57BL/6 mice were exposed to 100 μM linoleic acid (18:2) or palmitic acid (16:0) for 16 h, and 25 μg/mL oxidized low density lipoprotein for an additional 24 h. ABCA1 and ABCG1 mRNA expression was suppressed to a greater extent by 18:2 (60 % and 54 %, respectively) than 16:0 (30 % and 29 %, respectively) relative to the control (all p < 0.01). 18:2 decreased ABCA1 protein levels by 94 % and high density lipoprotein (HDL) mediated cholesterol efflux by 53 % (both p < 0.05), and had no significant effect on ABCG1, LXR-α or SREBP-1 protein levels. 16:0 had no effect on ABCA1, ABCG1, LXR-α or SREBP-1 protein expression or HDL-mediated cholesterol efflux. These results suggest that 18:2, relative to 16:0, attenuated macrophage HDL-mediated cholesterol efflux through down regulation of ABCA1 mRNA and protein levels but not through changes in LXR-α or SREBP-1 expression. The effect of 18:2 relative to 16:0 on macrophages cholesterol homeostasis may exacerbate the predisposition of individuals with T2DM to increased CVD risk.

The role of signalling in cellular cholesterol homeostasis

Cholesterol is a vital lipid and performs diverse functions on a whole body and cellular level. However, excess cellular cholesterol is toxic, and thus, elegant mechanisms have evolved to tightly regulate this important lipid. The regulation of cholesterol homeostasis is an area of intense research, and the role that signalling plays is gradually becoming more widely recognised. Cholesterol homeostasis is achieved through intricate mechanisms involving synthesis, uptake, and efflux. Although there is a large body of work elucidating these cholesterol-related pathways, less is known about the role of signalling in these processes. Here, we discuss the variety of ways that signalling impacts on these modes and levels of cholesterol homeostasis, including transcriptional regulation. Most work thus far has investigated the role of kinases in cholesterol efflux (especially on ATP-binding cassette transporter A1, ABCA1), and therefore constitutes a major focus of this review. We also indicate further avenues to explore in the area of signalling in cellular cholesterol homeostasis.

The influence of polyunsaturated fatty acids on the phospholipase D isoforms trafficking and activity in mast cells

The impact of polyunsaturated fatty acid (PUFA) supplementation on phospholipase D (PLD) trafficking and activity in mast cells was investigated. The enrichment of mast cells with different PUFA including α-linolenic acid (LNA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), linoleic acid (LA) or arachidonic acid (AA) revealed a PUFA-mediated modulation of the mastoparan-stimulated PLD trafficking and activity. All PUFA examined, except AA, prevented the migration of the PLD1 to the plasma membrane. For PLD2 no PUFA effects on trafficking could be observed. Moreover, PUFA supplementation resulted in an increase of mastoparan-stimulated total PLD activity, which correlated with the number of double bonds of the supplemented fatty acids. To investigate, which PLD isoform was affected by PUFA, stimulated mast cells were supplemented with DHA or AA in the presence of specific PLD-isoform inhibitors. It was found that both DHA and AA diminished the inhibition of PLD activity in the presence of a PLD1 inhibitor. By contrast, only AA diminished the inhibition of PLD activity in the presence of a PLD2 inhibitor. Thus, PUFA modulate the trafficking and activity of PLD isoforms in mast cells differently. This may, in part, account for the immunomodulatory effect of unsaturated fatty acids and contributes to our understanding of the modulation of mast cell activity by PUFA.

Cholesterol efflux is differentially regulated in neurons and astrocytes: implications for brain cholesterol homeostasis

Disruption of cholesterol homeostasis in the central nervous system (CNS) has been associated with neurological, neurodegenerative, and neurodevelopmental disorders. The CNS is a closed system with regard to cholesterol homeostasis, as cholesterol-delivering lipoproteins from the periphery cannot pass the blood-brain-barrier and enter the brain. Different cell types in the brain have different functions in the regulation of cholesterol homeostasis, with astrocytes producing and releasing apolipoprotein E and lipoproteins, and neurons metabolizing cholesterol to 24(S)-hydroxycholesterol. We present evidence that astrocytes and neurons adopt different mechanisms also in regulating cholesterol efflux. We found that in astrocytes cholesterol efflux is induced by both lipid-free apolipoproteins and lipoproteins, while cholesterol removal from neurons is triggered only by lipoproteins. The main pathway by which apolipoproteins induce cholesterol efflux is through ABCA1. By upregulating ABCA1 levels and by inhibiting its activity and silencing its expression, we show that ABCA1 is involved in cholesterol efflux from astrocytes but not from neurons. Furthermore, our results suggest that ABCG1 is involved in cholesterol efflux to apolipoproteins and lipoproteins from astrocytes but not from neurons, while ABCG4, whose expression is much higher in neurons than astrocytes, is involved in cholesterol efflux from neurons but not astrocytes. These results indicate that different mechanisms regulate cholesterol efflux from neurons and astrocytes, reflecting the different roles that these cell types play in brain cholesterol homeostasis. These results are important in understanding cellular targets of therapeutic drugs under development for the treatments of conditions associated with altered cholesterol homeostasis in the CNS.

Quantitative genomics of 30 complex phenotypes in Wagyu x Angus F₁ progeny

In the present study, a total of 91 genes involved in various pathways were investigated for their associations with six carcass traits and twenty-four fatty acid composition phenotypes in a Wagyu×Angus reference population, including 43 Wagyu bulls and their potential 791 F₁ progeny. Of the 182 SNPs evaluated, 102 SNPs that were in Hardy-Weinberg equilibrium with minor allele frequencies (MAF>0.15) were selected for parentage assignment and association studies with these quantitative traits. The parentage assignment revealed that 40 of 43 Wagyu sires produced over 96.71% of the calves in the population. Linkage disequilibrium analysis identified 75 of 102 SNPs derived from 54 genes as tagged SNPs. After Bonferroni correction, single-marker analysis revealed a total of 113 significant associations between 44 genes and 29 phenotypes (adjusted P<0.05). Multiple-marker analysis confirmed single-gene associations for 10 traits, but revealed two-gene networks for 9 traits and three-gene networks for 8 traits. Particularly, we observed that TNF (tumor necrosis factor) gene is significantly associated with both beef marbling score (P=0.0016) and palmitic acid (C16:0) (P=0.0043), RCAN1 (regulator of calcineurin 1) with rib-eye area (P=0.0103), ASB3 (ankyrin repeat and SOCS box-containing 3) with backfat (P=0.0392), ABCA1 (ATP-binding cassette A1) with both palmitic acid (C16:0) (P=0.0025) and oleic acid (C18:1n9) (P=0.0114), SLC27A1(solute carrier family 27 A1) with oleic acid (C18:1n9) (P=0.0155), CRH (corticotropin releasing hormone) with both linolenic acid (OMEGA-3) (P=0.0200) and OMEGA 6:3 RATIO (P=0.0054), SLC27A2 (solute carrier family 27 A2) with both linoleic acid (OMEGA-6) (P=0.0121) and FAT (P=0.0333), GNG3 (guanine nucleotide binding protein gamma 3 with desaturase 9 (P=0.0115), and EFEMP1 (EGF containing fibulin-like extracellular matrix protein 1), PLTP (phospholipid transfer protein) and DSEL (dermatan sulfate epimerase-like) with conjugated linoleic acid (P=0.0042-0.0044), respectively, in the Wagyu x Angus F₁ population. In addition, we observed an interesting phenomenon that crossbreeding of different breeds might change gene actions to dominant and overdominant modes, thus explaining the origin of heterosis. The present study confirmed that these important families or pathway-based genes are useful targets for improving meat quality traits and healthful beef products in cattle.

Molecular targets of omega 3 and conjugated linoleic Fatty acids - "micromanaging" cellular response

Essential fatty acids cannot be synthesized de novo by mammals and need to be ingested either with the diet or through the use of supplements/functional foods to ameliorate cardiovascular prognosis. This review focus on the molecular targets of omega 3 fatty acids and conjugated linoleic acid, as paradigmatic molecules that can be exploited both as nutrients and as pharmacological agents, especially as related to cardioprotection. In addition, we indicate novel molecular targets, namely microRNAs that might contribute to the observed biological activities of such essential fatty acids.

Genome-wide association study identifies novel loci associated with circulating phospho- and sphingolipid concentrations

Phospho- and sphingolipids are crucial cellular and intracellular compounds. These lipids are required for active transport, a number of enzymatic processes, membrane formation, and cell signalling. Disruption of their metabolism leads to several diseases, with diverse neurological, psychiatric, and metabolic consequences. A large number of phospholipid and sphingolipid species can be detected and measured in human plasma. We conducted a meta-analysis of five European family-based genome-wide association studies (N = 4034) on plasma levels of 24 sphingomyelins (SPM), 9 ceramides (CER), 57 phosphatidylcholines (PC), 20 lysophosphatidylcholines (LPC), 27 phosphatidylethanolamines (PE), and 16 PE-based plasmalogens (PLPE), as well as their proportions in each major class. This effort yielded 25 genome-wide significant loci for phospholipids (smallest P-value = 9.88×10⁻²⁰⁴) and 10 loci for sphingolipids (smallest P-value = 3.10×10⁻⁵⁷). After a correction for multiple comparisons (P-value<2.2×10⁻⁹), we observed four novel loci significantly associated with phospholipids (PAQR9, AGPAT1, PKD2L1, PDXDC1) and two with sphingolipids (PLD2 and APOE) explaining up to 3.1% of the variance. Further analysis of the top findings with respect to within class molar proportions uncovered three additional loci for phospholipids (PNLIPRP2, PCDH20, and ABDH3) suggesting their involvement in either fatty acid elongation/saturation processes or fatty acid specific turnover mechanisms. Among those, 14 loci (KCNH7, AGPAT1, PNLIPRP2, SYT9, FADS1-2-3, DLG2, APOA1, ELOVL2, CDK17, LIPC, PDXDC1, PLD2, LASS4, and APOE) mapped into the glycerophospholipid and 12 loci (ILKAP, ITGA9, AGPAT1, FADS1-2-3, APOA1, PCDH20, LIPC, PDXDC1, SGPP1, APOE, LASS4, and PLD2) to the sphingolipid pathways. In large meta-analyses, associations between FADS1-2-3 and carotid intima media thickness, AGPAT1 and type 2 diabetes, and APOA1 and coronary artery disease were observed. In conclusion, our study identified nine novel phospho- and sphingolipid loci, substantially increasing our knowledge of the genetic basis for these traits.

Phospho- and sphingolipids are integral to membrane formation and are involved in crucial cellular functions such as signalling, membrane fluidity, membrane protein trafficking, neurotransmission, and receptor trafficking. In addition to severe monogenic diseases resulting from defective phospho- and sphingolipid function and metabolism, the evidence suggests that variations in these lipid levels at the population level are involved in the determination of cardiovascular and neurologic traits and subsequent disease. We took advantage of modern laboratory methods, including microarray-based genotyping and electrospray ionization tandem mass spectrometry, to hunt for genetic variation influencing the levels of more than 350 phospho- and sphingolipid phenotypes. We identified nine novel loci, in addition to confirming a number of previously described loci. Several other genetic regions provided substantial evidence of their involvement in these traits. All of these loci are strong candidates for further research in the field of lipid biology and are likely to yield considerable insights into the complex metabolic pathways underlying circulating phospho- and sphingolipid levels. Understanding these mechanisms might help to illuminate factors leading to the development of common cardiovascular and neurological diseases and might provide molecular targets for the development of new therapies.

Regulation of the expression of key genes involved in HDL metabolism by unsaturated fatty acids

The cardioprotective effects of HDL have been largely attributed to their role in the reverse cholesterol transport pathway, whose efficiency is affected by many proteins involved in the formation and remodelling of HDL. The aim of the present study was to determine the effects, and possible mechanisms of action, of unsaturated fatty acids on the expression of genes involved in HDL metabolism in HepG2 cells. The mRNA concentration of target genes was assessed by real-time PCR. Protein concentrations were determined by Western blot or immunoassays. PPAR and liver X receptor (LXR) activities were assessed in transfection experiments. Compared with the SFA palmitic acid (PA), the PUFA arachidonic acid (AA), EPA and DHA significantly decreased apoA-I, ATP-binding cassette A1 (ABCA1), lecithin-cholesterol acyltransferase (LCAT) and phospholipid transfer protein mRNA levels. EPA and DHA significantly lowered the protein concentration of apoA-I and LCAT in the media, as well as the cellular ABCA1 protein content. In addition, DHA repressed the apoA-I promoter activity. AA lowered only the protein concentration of LCAT in the media. The activity of PPAR was increased by DHA, while the activity of LXR was lowered by both DHA and AA, relative to PA. The regulation of these transcription factors by PUFA may explain some of the PUFA effects on gene expression. The observed n-3 PUFA-mediated changes in gene expression are predicted to reduce the rate of HDL particle formation and maturation.

Acyl-CoA synthetase 1 is required for oleate and linoleate mediated inhibition of cholesterol efflux through ATP-binding cassette transporter A1 in macrophages

Diabetes and insulin resistance increase the risk of cardiovascular disease caused by atherosclerosis through mechanisms that are poorly understood. Lipid-loaded macrophages are key contributors to all stages of atherosclerosis. We have recently shown that diabetes associated with increased plasma lipids reduces cholesterol efflux and levels of the reverse cholesterol transporter ABCA1 (ATP-binding cassette transporter A1) in mouse macrophages, which likely contributes to macrophage lipid accumulation in diabetes. Furthermore, we and others have shown that unsaturated fatty acids reduce ABCA1-mediated cholesterol efflux, and that this effect is mediated by the acyl-CoA derivatives of the fatty acids. We therefore investigated whether acyl-CoA synthetase 1 (ACSL1), a key enzyme mediating acyl-CoA synthesis in macrophages, could directly influence ABCA1 levels and cholesterol efflux in these cells. Mouse macrophages deficient in ACSL1 exhibited reduced sensitivity to oleate- and linoleate-mediated ABCA1 degradation, which resulted in increased ABCA1 levels and increased apolipoprotein A-I-dependent cholesterol efflux in the presence of these fatty acids, as compared with wildtype mouse macrophages. Conversely, overexpression of ACSL1 resulted in reduced ABCA1 levels and reduced cholesterol efflux in the presence of unsaturated fatty acids. Thus, the reduced ABCA1 and cholesterol efflux in macrophages subjected to conditions of diabetes and elevated fatty load may, at least in part, be mediated by ACSL1. These observations raise the possibility that ABCA1 levels could be increased by inhibition of acyl-CoA synthetase activity in vivo. 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).

Deleterious impact of elaidic fatty acid on ABCA1-mediated cholesterol efflux from mouse and human macrophages

Consumption of trans fatty acids (TFA) increase cardiovascular risk more than do saturated FA, but the mechanisms explaining their atherogenicity are still unclear. We investigated the impact of membrane incorporation of TFA on cholesterol efflux by exposing J774 mouse macrophages or human monocyte-derived macrophages (HMDM) to media enriched or not (standard medium) with industrially produced elaidic (trans-9 18:1) acid, naturally produced vaccenic (trans-11 18:1) acid (34 h, 70 μM) or palmitic acid. In J774 macrophages, elaidic and palmitic acid, but not vaccenic acid, reduced ABCA1-mediated efflux by ~23% without affecting aqueous diffusion, SR-BI or ABCG1-mediated pathways, and this effect was maintained in cholesterol-loaded cells. The impact of elaidic acid on the ABCA1 pathway was weaker in cholesterol-normal HMDM, but elaidic acid induced a strong reduction of ABCA1-mediated efflux in cholesterol-loaded cells (-36%). In J774 cells, the FA supplies had no impact on cellular free cholesterol or cholesteryl ester masses, the abundance of ABCA1 mRNA or the total and plasma membrane ABCA1 protein content. Conversely, TFA or palmitic acid incorporation induced strong modifications of the membrane FA composition with a decrease in the ratio of (cis-monounsaturated FA+polyunsaturated FA):(saturated FA+TFA), with elaidic and vaccenic acids representing each 20% and 13% of the total FA composition, respectively. Moreover, we demonstrated that cellular ATP was required for the effect of elaidic acid, suggesting that it contributes to atherogenesis by impairing ABCA1-mediated cholesterol efflux in macrophages, likely by decreasing the membrane fluidity, which could thereby reduce ATPase activity and the function of the transporter.

Phosphatidylcholine biosynthesis and lipoprotein metabolism

Phosphatidylcholine (PC) is the major phospholipid component of all plasma lipoprotein classes. PC is the only phospholipid which is currently known to be required for lipoprotein assembly and secretion. Impaired hepatic PC biosynthesis significantly reduces the levels of circulating very low density lipoproteins (VLDLs) and high density lipoproteins (HDLs). The reduction in plasma VLDLs is due in part to impaired hepatic secretion of VLDLs. Less PC within the hepatic secretory pathway results in nascent VLDL particles with reduced levels of PC. These particles are recognized as being defective and are degraded within the secretory system by an incompletely defined process that occurs in a post-endoplasmic reticulum compartment, consistent with degradation directed by the low-density lipoprotein receptor and/or autophagy. Moreover, VLDL particles are taken up more readily from the circulation when the PC content of the VLDLs is reduced, likely due to a preference of cell surface receptors and/or enzymes for lipoproteins that contain less PC. Impaired PC biosynthesis also reduces plasma HDLs by inhibiting hepatic HDL formation and by increasing HDL uptake from the circulation. These effects are mediated by elevated expression of ATP-binding cassette transporter A1 and hepatic scavenger receptor class B type 1, respectively. Hepatic PC availability has recently been linked to the progression of liver and heart disease. These findings demonstrate that hepatic PC biosynthesis can regulate the amount of circulating lipoproteins and suggest that hepatic PC biosynthesis may represent an important pharmaceutical target. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

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).

Ubiquitination is associated with lysosomal degradation of cell surface-resident ATP-binding cassette transporter A1 (ABCA1) through the endosomal sorting complex required for transport (ESCRT) pathway

ATP-binding cassette transporter A1 (ABCA1) plays an essential role in the biogenesis of high-density lipoprotein in liver and in the prevention of foam cell formation in macrophages by mediating the efflux of cellular cholesterol and phospholipids to apolipoprotein A-I (apoA-I). Our current study investigated the mechanism of degradation of cell surface-resident ABCA1, focusing on ubiquitination. A coimmunoprecipitation study indicated the presence of ubiquitinated ABCA1 in the plasma membrane of the human hepatoma cell line, HuH-7, of cells from mouse liver, and of macrophages differentiated from the human acute monocytic leukemia cell line, THP-1 (THP-1 macrophages). In HuH-7 cells, degradation of cell surface-resident ABCA1 was inhibited by the overexpression of a dominant-negative form of ubiquitin. Moreover, the disruption of the endosomal sorting complex required for transport (ESCRT) pathway, a dominant mechanism for ubiquitination-mediated lysosomal degradation, by the knockdown of hepatocyte growth factor-regulated tyrosine kinase substrate (HRS), significantly delayed the degradation of cell surface-resident ABCA1. This was accompanied by an increase in ABCA1 expression as well as in apoA-I-mediated [3H]-cholesterol efflux function. The effect of HRS knockdown was also observed after calpain inhibitor treatment, which is reported to retard ABCA1 degradation. The induction of ABCA1 by HRS knockdown was confirmed in THP-1 macrophages.

CONCLUSION

Together with the fact that lysosomal inhibitor treatments increased ABCA1 expression in HuH-7 and THP-1 macrophages, these results suggest that ubiquitination mediates the lysosomal degradation of cell surface-resident ABCA1 through the ESCRT pathway, and thereby controls the expression and cholesterol efflux function of ABCA1. This mechanism seems to mediate ABCA1 degradation independently of the calpain-involving pathway. The modulation of ABCA1 ubiquitination could thus be a potential new therapeutic target for antiatherogenic drugs.

Calpain-mediated ABCA1 degradation: post-translational regulation of ABCA1 for HDL biogenesis

Helical apolipoproteins remove cellular phospholipid and cholesterol to generate nascent HDL and this reaction is the major source of plasma HDL. ABCA1 is mandatory and rate-limiting for this reaction. Besides regulation of the gene expression by transcriptional factors including LXR, AP2 and SREBP, the ABCA1 activity is regulated post-translationally by calpain-mediated proteolytic degradation of ABCA1 protein that occurs in the early endosome after its endocytosis. When the HDL biogenesis reaction is ongoing as helical apolipoproteins interact with ABCA1, ABCA1 becomes resistant to calpain and is recycled to cell surface after endocytosis. Biogenesis of HDL is most likely to take place on cell surface. Clearance rate of ABCA1 by this mechanism is also retarded by various factors that interact with ABCA1, such as α1-syntrophin, LXRβ and calmodulin. Physiological relevance of the retardation by these factors is not entirely clear. Pharmacological inhibition of the calpain-mediated ABCA1 degradation results in the increase of the ABCA1 activity and HDL biogenesis in vitro and in vivo, and potentially suppresses atherogenesis. 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).

ATP-binding cassette transporter A1 and HDL metabolism: effects of fatty acids

Ample evidence indicates that dietary fatty acids alter the plasma levels of high-density lipoprotein cholesterol (HDL-C). However, the mechanisms underlying the effects of fatty acids still remain elusive. Recent advances in our understanding of ATP-binding cassette transporter A1 (ABCA1) function and regulation have provided a valuable insight into the mechanisms by which fatty acids may affect plasma HDL-C levels. ABCA1 mediates the assembly of phospholipids and free cholesterol with apolipoprotein A-I, which is a critical step for HDL biogenesis. Studies have shown that unsaturated fatty acids, but not saturated fatty acids, repress the expression of ABCA1 in vitro. Although information on mechanisms for the fatty-acid-mediated regulation of ABCA1 expression is still limited and controversial, recent evidence suggests that unsaturated fatty acids inhibit the expression of ABCA1 at the transcriptional and posttranscriptional levels. The transcriptional repression of ABCA1 expression by unsaturated fatty acids is likely liver X receptor dependent. Evidence also suggests that histone deacetylation may play a role in the repression. Posttranscriptionally, unsaturated fatty acids may facilitate ABCA1 protein degradation, which may involve phosphorylation of ABCA1 by protein kinases. Further studies are warranted to better understand the role of dietary fatty acids in HDL metabolism and their effects on cardiovascular health.

High-density lipoprotein metabolism in human apolipoprotein B(100) transgenic/brown adipose tissue deficient mice: a model of obesity-induced hyperinsulinemia

Obese and diabetic humans display decreased plasma high-density lipoprotein cholesterol (HDL-C) concentrations and an increased risk for coronary heart disease. However, investigation on HDL metabolism in obesity with a particular emphasis on hepatic ATP-binding cassette transporter A1 (ABCA1), the primary factor for HDL formation, has not been well studied. Human apolipoprotein B(100) transgenic (hApoB(tg)) and brown adipose tissue deficient (BATless) mice were crossed to generate hApoB(tg)/BATless mice. Male and female hApoB(tg) and hApoB(tg)/BATless mice were maintained on either a regular rodent chow diet or a diet high in fat and cholesterol until 24 weeks of age. The hApoB(tg)/BATless mice that were fed a HF/HC diet became obese, developed hepatic steatosis, and had significantly elevated plasma insulin levels compared with their hApoB(tg) counterparts, but plasma concentrations of total cholesterol, HDL-C, triglycerides, and free fatty acids and lipoprotein distribution between genotypes were not significantly different. Hepatic expression of genes encoding HDL-modifying factors (e.g., scavenger receptor, class B, type I, hepatic lipase, lecithin:cholesterol acyltransferase, and phospholipid transfer protein) was either altered significantly or showed a trend of difference between 2 genotypes of mice. Importantly, hepatic protein levels of ABCA1 were significantly lowered by ∼35% in male obese hApoB(tg)/BATless mice with no difference in mRNA levels compared with hApoB(tg) counterparts. Despite reduced hepatic ABCA1 protein levels, plasma HDL-C concentrations were not altered in male obese hApoB(tg)/BATless mice. The result suggests that hepatic ABCA1 may not be a primary contributing factor for perturbations in HDL metabolism in obesity-induced hyperinsulinemia.

Relative roles of various efflux pathways in net cholesterol efflux from macrophage foam cells in atherosclerotic lesions

PURPOSE OF REVIEW

Cholesterol efflux mechanisms are essential for macrophage cholesterol homeostasis. HDL, an important cholesterol efflux acceptor, comprises a class of heterogeneous particles that induce cholesterol efflux via distinct pathways. This review focuses on the understanding of the different cholesterol efflux pathways and physiological acceptors involved, and their regulation in atherosclerotic lesions.

RECENT FINDINGS

The synergistic interactions of ATP-binding cassette transporters A1 and G1 as well as ATP-binding cassette transporter A1 and scavenger receptor class B type I are essential for cellular cholesterol efflux and the prevention of macrophage foam cell formation. However, the importance of aqueous diffusion should also not be underestimated. Significant progress has been made in understanding the mechanisms underlying ATP-binding cassette A1-mediated cholesterol efflux and regulation of its expression and trafficking. Conditions locally in the atherosclerotic lesion, for example, lipids, cytokines, oxidative stress, and hypoxia, as well as systemic factors, including inflammation and diabetes, critically influence the expression of cholesterol transporters on macrophage foam cells. Furthermore, HDL modification and remodeling in atherosclerosis, inflammation, and diabetes impairs its function as an acceptor for cellular cholesterol.

SUMMARY

Recent advances in the understanding of the regulation of cholesterol transporters and their acceptors in atherosclerotic lesions indicate that HDL-based therapies should aim to enhance the activity of cholesterol transporters and improve both the quantity and quality of HDL.

Procoagulant and prothrombotic activation of human erythrocytes by phosphatidic acid

Increased phosphatidic acid (PA) and phospholipase D (PLD) activity are frequently observed in various disease states including cancers, diabetes, sepsis, and thrombosis. Previously, PA has been regarded as just a precursor for lysophosphatidic acid (LPA) and diacylglycerol (DAG). However, increasing evidence has suggested independent biological activities of PA itself. In the present study, we demonstrated that PA can enhance thrombogenic activities in human erythrocytes through phosphatidylserine (PS) exposure in a Ca2+-dependent manner. In freshly isolated human erythrocytes, treatment of PA or PLD induced PS exposure. PA-induced PS exposure was not attenuated by inhibitors of phospholipase A2or phosphatidate phosphatase, which converts PA to LPA or DAG. An intracellular Ca2+increase and the resultant activation of Ca2+-dependent PKC-α appeared to underlie the PA-induced PS exposure through the activation of scramblase. A marginal decrease in flippase activity was also noted, contributing further to the maintenance of exposed PS on the outer membrane. PA-treated erythrocytes showed strong thrombogenic activities, as demonstrated by increased thrombin generation, endothelial cell adhesion, and erythrocyte aggregation. Importantly, these procoagulant activations by PA were confirmed in a rat in vivo venous thrombosis model, where PA significantly enhanced thrombus formation. In conclusion, these results suggest that PA can induce thrombogenic activities in erythrocytes through PS exposure, which can increase thrombus formation and ultimately contribute to the development of cardiovascular diseases.

EGb761 ameliorates the formation of foam cells by regulating the expression of SR-A and ABCA1: role of haem oxygenase-1

AIMS

Accumulation of foam cells in the intima is a hallmark of early-stage atherosclerotic lesions. Ginkgo biloba extract (EGb761) has been reported to exert anti-oxidative and anti-inflammatory properties in atherosclerosis, yet the significance and the molecular mechanisms of action of EGb761 in the formation of macrophage foam cells are not fully understood.

METHODS AND RESULTS

Treatment with EGb761 resulted in a dose-dependent decrease in oxidized low-density lipoprotein (oxLDL)-mediated cholesterol accumulation in macrophages, a consequence that was due to a decrease in cholesterol uptake and an increase in cholesterol efflux. Additionally, EGb761 significantly down-regulated the mRNA and protein expression of class A scavenger receptor (SR-A) by decreasing expression of activator protein 1 (AP-1); however, EGb761 increased the protein stability of ATP-binding cassette transporter A1 (ABCA1) by reducing calpain activity without affecting ABCA1 mRNA expression. Small interfering RNA (siRNA) targeting haem oxygenase-1 (HO-1) abolished the EGb761-induced protective effects on the expression of AP-1, SR-A, ABCA1, and calpain activity. Accordingly, EGb761-mediated suppression of lipid accumulation in foam cells was also abrogated by HO-1 siRNA. Moreover, the lesion size of atherosclerosis was smaller in EGb761-treated, apolipoprotein E-deficient mice compared with the vehicle-treated mice, and the expression of HO-1, SR-A, and ABCA1 in aortas was modulated similar to that observed in macrophages.

CONCLUSION

These findings suggest that EGb761 confers a protection from the formation of foam cells by a novel HO-1-dependent regulation of cholesterol homeostasis in macrophages.

Suppression of ABCA1 by unsaturated fatty acids leads to lipid accumulation in HepG2 cells

Abnormal lipid metabolism may contribute to the pathogenesis of non-alcoholic steatohepatitis (NASH). ATP-binding cassette transporter A1 (ABCA1) mediates the transport of cholesterol and phospholipids from cells to HDL apolipoproteins. We previously reported that unsaturated fatty acids destabilise ABCA1 in murine macrophages and ABCA1-transfected baby hamster kidney cells by increasing its protein degradation. Here, we examined the correlation between ABCA1 and hepatic lipids. In HepG2 cells, unsaturated but not saturated fatty acids suppressed ABCA1 protein levels by promoting its protein degradation. Over-expression of ABCA1 resulted in a decrease of cellular fatty acids and triglycerides, while repression by ABCA1 siRNA increased both cellular fatty acids and triglycerides. Rats with NASH also showed lower ABCA1 protein levels in liver cells, compared with that of the normal rats. These data indicate that steatosis is associated with a decrease in ABCA1 protein expression leading to an increase in lipid storage in hepatocytes. And it further suggests that this effect could be due to an excess of unsaturated fatty acids.