Transcriptional regulatory mechanisms of the human apolipoprotein genes in vitro and in vivo

Liver knockout of MCU leads to greater dysregulation of lipid metabolism in MAFLD

Metabolic-associated fatty liver disease (MAFLD) is a common chronic condition that poses a significant threat to human health. Mitochondrial dysfunction, particularly involving the mitochondrial Ca2+ uniporter (MCU), plays a key role in its pathogenesis. This study aimed to investigate the impact of the MCU gene on hepatic lipid metabolism in mice fed a high-fat diet. Using MCU knockout and wild-type mice, subjected to either a high-fat or normal diet for 14 weeks, we observed notable Steatosis and liver weight gain in MCU-deficient mice. Liver function markers, serum triglycerides, very low-density lipoprotein (VLDL) levels, and ApoB protein expression were all significantly elevated. Mechanistic studies revealed that MCU deletion led to mitochondrial dysfunction, increased oxidative stress. These findings highlight the critical role of the MCU gene in maintaining hepatic lipid balance and suggest its potential as a therapeutic target for managing nonalcoholic fatty liver disease.

Peroxisome Proliferator-Activated Receptor α in Lipoprotein Metabolism and Atherosclerotic Cardiovascular Disease

Peroxisome proliferator-activated receptors (PPARs) are a group of ligand-binding transcription factors with pivotal action in regulating pleiotropic signaling pathways of energetic metabolism, immune responses and cell proliferation and differentiation. A significant body of evidence indicates that the PPARα receptor is an important modulator of plasma lipid and lipoprotein metabolism, with pluripotent effects influencing the lipid and apolipoprotein cargo of both atherogenic and antiatherogenic lipoproteins and their functionality. Clinical evidence supports an important role of PPARα agonists (fibric acid derivatives) in the treatment of hypertriglyceridemia and/or low high-density lipoprotein (HDL) cholesterol levels, although the effects of clinical trials are contradictory and point to a reduction in the risk of nonfatal and fatal myocardial infarction events. In this manuscript, we provide an up-to-date critical review of the existing relevant literature.

In vivo PAR-CLIP (viP-CLIP) of liver TIAL1 unveils targets regulating cholesterol synthesis and secretion

System-wide cross-linking and immunoprecipitation (CLIP) approaches have unveiled regulatory mechanisms of RNA-binding proteins (RBPs) mainly in cultured cells due to limitations in the cross-linking efficiency of tissues. Here, we describe viP-CLIP (in vivo PAR-CLIP), a method capable of identifying RBP targets in mammalian tissues, thereby facilitating the functional analysis of RBP-regulatory networks in vivo. We applied viP-CLIP to mouse livers and identified Insig2 and ApoB as prominent TIAL1 target transcripts, indicating an important role of TIAL1 in cholesterol synthesis and secretion. The functional relevance of these targets was confirmed by showing that TIAL1 influences their translation in hepatocytes. Mutant Tial1 mice exhibit altered cholesterol synthesis, APOB secretion and plasma cholesterol levels. Our results demonstrate that viP-CLIP can identify physiologically relevant RBP targets by finding a factor implicated in the negative feedback regulation of cholesterol biosynthesis.

A genome-wide association study of childhood adiposity and blood lipids

Background: The rising prevalence of childhood obesity and dyslipidaemia is a major public health concern due to its association with morbidity and mortality in later life. Previous studies have found that genetic variants inherited at birth can begin to exert their effects on cardiometabolic traits during the early stages of the lifecourse. Methods: In this study, we have conducted genome-wide association studies (GWAS) for eight measures of adiposity and lipids in a cohort of young individuals (mean age 9.9 years, sample sizes=4,202 to 5,766) from the Avon Longitudinal Study of Parents and Children (ALSPAC). These measures were body mass index (BMI), systolic and diastolic blood pressure, high- density and low-density lipoprotein cholesterol, triglycerides, apolipoprotein A-I and apolipoprotein B. We next undertook functional enrichment, pathway analyses and linkage disequilibrium (LD) score regression to evaluate genetic correlations with later-life cardiometabolic diseases. Results: Using GWAS we identified 14 unique loci associated with at least one risk factor in this cohort of age 10 individuals (P<5x10 -8), with lipoprotein lipid-associated loci being enriched for liver tissue-derived gene expression and lipid synthesis pathways. LD score regression provided evidence of various genetic correlations, such as childhood systolic blood pressure being genetically correlated with later-life coronary artery disease (rG=0.26, 95% CI=0.07 to 0.46, P=0.009) and hypertension (rG=0.37, 95% CI=0.19 to 0.55, P=6.57x10 -5), as well as childhood BMI with type 2 diabetes (rG=0.35, 95% CI=0.18 to 0.51, P=3.28x10 -5). Conclusions: Our findings suggest that there are genetic variants inherited at birth which begin to exert their effects on cardiometabolic risk factors as early as age 10 in the life course. However, further research is required to assess whether the genetic correlations we have identified are due to direct or indirect effects of childhood adiposity and lipid traits.

Apolipoprotein C3 facilitates internalization of cationic lipid nanoparticles into bone marrow-derived mouse mast cells

Mast cells (MCs), are hematopoetically-derived secretory immune cells that release preformed as well as de novo synthesized inflammatory mediators in response to activation by several stimuli. Based on their role in inflammatory responses, particularly in the lung and skin, MCs provide an effective target for anti-inflammatory therapeutic strategies. Drug-delivery of lipophilic payloads to MCs can be challenging due to their functionally distinct intracellular structures. In the present study, pH-sensitive cationic lipid-based nanoparticles (LNPs) composed of DODMA, DODAP or DOTAP lipids that encapsulated a GFP or eGFP plasmid were constructed using non-turbulent microfluidic mixing. This approach achieved up to 75-92% encapsulation efficiency. Dynamic light scattering revealed a uniformly sized and homogeneous dispersion of LNPs. To promote cellular internalization, LNPs were complexed with apolipoproteins, amphipathic proteins capable of binding lipids and facilitating their transport into cells. Cryo-TEM analysis showed that LNP structure was differentially modified when associated with different types of apolipoproteins. LNP preparations made up of DODMA or DODMA, DODAP and DOTAP lipids were coated with seven apolipoproteins (Apo A1, B, C3, D, E2, E4 and H). Differentiated bone-marrow derived mouse mast cells (BMMCs) were exposed to apolipoprotein-LNP and internalization was measured using flow cytometry. Out of all the apolipoproteins tested, ApoC3 most efficiently facilitated cellular internalization of the LNP into BMMCs as determined by GFP fluorescence using flow cytometry. These effects were confirmed in a less differentiated but also interleukin-3-dependent model of mouse mast cells, MC/9. ApoC3-LNP enhanced internalization by BMMC in a concentration-dependent manner and this was significantly increased when BMMC were pre-treated with inhibitors of actin polymerization, suggesting a dependence on intracellular shuttling. Activation of peroxisome proliferator-activated receptor gamma (PPARγ) decreased ApoC3-LNP internalization and reduced the expression of apolipoprotein E receptor 2 (ApoER2), suggesting that ApoC3-LNP binding to ApoER2 may be responsible for its enhanced internalization. Furthermore, ApoC3 fails to facilitate internalization of LNPs in Lrp8^-/- KO BMMC that do not express ApoER2 on their cell surface. Altogether, our studies reveal an important role of ApoC3 in facilitating internalization of cationic LNPs into MCs.

A genome-wide association study of childhood adiposity and blood lipids

Background: The rising prevalence of childhood obesity and dyslipidaemia is a major public health concern due to its association with morbidity and mortality in later life. Methods: In this study, we have conducted genome-wide association studies (GWAS) for eight measures of adiposity and lipids in a cohort of young individuals (mean age 9.9) from the Avon Longitudinal Study of Parents and Children (ALSPAC). These measures were body mass index (BMI), systolic and diastolic blood pressure, high- density and low-density lipoprotein cholesterol, triglycerides, apolipoprotein A-I and apolipoprotein B. We next undertook functional enrichment, pathway analyses and linkage disequilibrium (LD) score regression to evaluate genetic correlations with later-life cardiometabolic diseases. Results: Using GWAS we identified 14 unique loci associated with at least one risk factor in this cohort of age 10 individuals (P<5x10-8), with lipoprotein lipid-associated loci being enriched for liver tissue-derived gene expression and lipid synthesis pathways. LD score regression provided evidence of various genetic correlations, such as childhood systolic blood pressure being genetically correlated with later-life coronary artery disease (rG=0.26, 95% CI=0.07 to 0.46, P=0.009) and hypertension (rG=0.37, 95% CI=0.19 to 0.55, P=6.57x10-5), as well as childhood BMI with type 2 diabetes (rG=0.35, 95% CI=0.18 to 0.51, P=3.28x10-5). Conclusions: Our findings suggest that there are genetic variants inherited at birth which begin to exert their effects on cardiometabolic risk factors as early as age 10 in the life course. However, further research is required to assess whether the genetic correlations we have identified are due to direct or indirect effects of childhood adiposity and lipid traits.

Augmenting and directing long-range CRISPR-mediated activation in human cells

Epigenetic editing is an emerging technology that uses artificial transcription factors (aTFs) to regulate expression of a target gene. Although human genes can be robustly upregulated by targeting aTFs to promoters, the activation induced by directing aTFs to distal transcriptional enhancers is substantially less robust and consistent. Here we show that long-range activation using CRISPR-based aTFs in human cells can be made more efficient and reliable by concurrently targeting an aTF to the target gene promoter. We used this strategy to direct target gene choice for enhancers capable of regulating more than one promoter and to achieve allele-selective activation of human genes by targeting aTFs to single-nucleotide polymorphisms embedded in distally located sequences. Our results broaden the potential applications of the epigenetic editing toolbox for research and therapeutics.

Hepatic HuR modulates lipid homeostasis in response to high-fat diet

Lipid transport and ATP synthesis are critical for the progression of non-alcoholic fatty liver disease (NAFLD), but the underlying mechanisms are largely unknown. Here, we report that the RNA-binding protein HuR (ELAVL1) forms complexes with NAFLD-relevant transcripts. It associates with intron 24 of Apob pre-mRNA, with the 3′UTR of Uqcrb, and with the 5′UTR of Ndufb6 mRNA, thereby regulating the splicing of Apob mRNA and the translation of UQCRB and NDUFB6. Hepatocyte-specific HuR knockout reduces the expression of APOB, UQCRB, and NDUFB6 in mice, reducing liver lipid transport and ATP synthesis, and aggravating high-fat diet (HFD)-induced NAFLD. Adenovirus-mediated re-expression of HuR in hepatocytes rescues the effect of HuR knockout in HFD-induced NAFLD. Our findings highlight a critical role of HuR in regulating lipid transport and ATP synthesis.

Human antigen R (HuR) is a RNA binding protein involved in the regulation of many cellular functions. Here the authors show that, hepatocyte specific deletion of HuR exacerbates high-fat diet-induced NAFLD in mice by regulating transcripts involved in lipid transport and ATP synthesis.

mTORC1-dependent protein synthesis and autophagy uncouple in the regulation of Apolipoprotein A-I expression

BACKGROUND

Apolipoprotein A-I (ApoA-I) is involved in reverse cholesterol transport as a major component of HDL, but also conveys anti-thrombotic, anti-oxidative, anti-inflammatory and immune-regulatory properties that are pertinent to its protective roles in cardiovascular, inflammatory and malignant pathologies. Despite the pleiotropy in ApoA-I functions, the regulation of intracellular ApoA-I levels remains poorly explored.

METHODS

HepG2 hepatoma cells and primary mouse hepatocytes were used as in vitro models to study the impact of genetic and chemical inhibitors of autophagy and the proteasome on ApoA-I by immunoblot, immunofluorescence and electron microscopy. Different growth conditions were implemented in conjunction with mTORC inhibitors to model the influence of nutrient scarcity versus sufficiency on ApoA-I regulation. Hepatic ApoA-I expression was also evaluated in high fat diet-fed mice displaying blockade in autophagy.

RESULTS

Under nutrient-rich conditions, basal ApoA-I levels in liver cells are sustained by the balancing act of autophagy and of mTORC1-dependent de novo protein synthesis. ApoA-I proteolysis occurs through a canonical autophagic pathway involving Beclin1 and ULK1 and the receptor protein p62/SQSTM1 that targets ApoA-I to autophagosomes. However, upon aminoacid insufficiency, suppression of ApoA-I synthesis prevails, rendering mTORC1 inactivation dispensable for autophagy-mediated ApoA-I proteolysis.

CONCLUSION

These data underscore the major contribution of post-transcriptional mechanisms to ApoA-I levels which differentially involve mTORC1-dependent signaling to protein synthesis and autophagy, depending on nutrient availability. Given the established role of ApoA-I in HDL-mediated reverse cholesterol transport, this mode of ApoA-I regulation may reflect a hepatocellular response to the organismal requirement for maintenance of cholesterol and lipid reserves under conditions of nutrient scarcity.

Why Is Apolipoprotein CIII Emerging as a Novel Therapeutic Target to Reduce the Burden of Cardiovascular Disease?

ApoC-III was discovered almost 50 years ago, but for many years, it did not attract much attention. However, as epidemiological and Mendelian randomization studies have associated apoC-III with low levels of triglycerides and decreased incidence of cardiovascular disease (CVD), it has emerged as a novel and potentially powerful therapeutic approach to managing dyslipidemia and CVD risk. The atherogenicity of apoC-III has been attributed to both direct lipoprotein lipase-mediated mechanisms and indirect mechanisms, such as promoting secretion of triglyceride-rich lipoproteins (TRLs), provoking proinflammatory responses in vascular cells and impairing LPL-independent hepatic clearance of TRL remnants. Encouraging results from clinical trials using antisense oligonucleotide, which selectively inhibits apoC-III, indicate that modulating apoC-III may be a potent therapeutic approach to managing dyslipidemia and cardiovascular disease risk.

High levels of homocysteine downregulate apolipoprotein E expression via nuclear factor kappa B

AIM: To investigate the effect of high homocysteine (Hcy) levels on apolipoprotein E (apoE) expression and the signaling pathways involved in this gene regulation.

METHODS: Reverse transcriptase polymerase chain reaction (RT-PCR) and Western blot were used to assess apoE expression in cells treated with various concentrations (50-500 μmol/L) of Hcy. Calcium phosphate-transient transfections were performed in HEK-293 and RAW 264.7 cells to evaluate the effect of Hcy on apoE regulatory elements [promoter and distal multienhancer 2 (ME2)]. To this aim, plasmids containing the proximal apoE promoter [(-500/+73)apoE construct] alone or in the presence of ME2 [ME2/(-500/+73)apoE construct] to drive the expression of the reporter luciferase gene were used. Co-transfection experiments were carried out to investigate the downstream effectors of Hcy-mediated regulation of apoE promoter by using specific inhibitors or a dominant negative form of IKβ. In other co-transfections, the luciferase reporter was under the control of synthetic promoters containing multiple specific binding sites for nuclear factor kappa B (NF-κB), activator protein-1 (AP-1) or nuclear factor of activated T cells (NFAT). Chromatin immunoprecipitation (ChIP) assay was accomplished to detect the binding of NF-κB p65 subunit to the apoE promoter in HEK-293 treated with 500 μmol/L Hcy. As control, cells were incubated with similar concentration of cysteine. NF-κB p65 proteins bound to DNA were immunoprecipitated with anti-p65 antibodies and DNA was identified by PCR using primers amplifying the region -100/+4 of the apoE gene.

RESULTS: RT-PCR revealed that high levels of Hcy (250-750 μmol/L) induced a 2-3 fold decrease in apoE mRNA levels in HEK-293 cells, while apoE gene expression was not significantly affected by treatment with lower concentrations of Hcy (100 μmol/L). Immunoblotting data provided additional evidence for the negative role of Hcy in apoE expression. Hcy decreased apoE promoter activity, in the presence or absence of ME2, in a dose dependent manner, in both RAW 264.7 and HEK-293 cells, as revealed by transient transfection experiments. The downstream effectors of the signaling pathways of Hcy were also investigated. The inhibitory effect of Hcy on the apoE promoter activity was counteracted by MAPK/ERK kinase 1/2 (MEK1/2) inhibitor U0126, suggesting that MEK1/2 is involved in the downregulation of apoE promoter activity by Hcy. Our data demonstrated that Hcy-induced inhibition of apoE took place through activation of NF-κB. Moreover, we demonstrated that Hcy activated a synthetic promoter containing three NF-κB binding sites, but did not affect promoters containing AP-1 or NFAT binding sites. ChIP experiments revealed that NF-κB p65 subunit is recruited to the apoE promoter following Hcy treatment of cells.

CONCLUSION: Hcy-induced stress negatively modulates apoE expression via MEK1/2 and NF-κB activation. The decreased apoE expression in peripheral tissues may aggravate atherosclerosis, neurodegenerative diseases and renal dysfunctions.

Metformin reduces the endotoxin-induced down-regulation of apolipoprotein E gene expression in macrophages

The atheroprotective role of macrophage-derived apolipoprotein E (apoE) is well known. Our previous reports demonstrated that inflammatory stress down-regulates apoE expression in macrophages, aggravating atherogenesis. Metformin, extensively used as an anti-diabetic drug, has also anti-inflammatory properties, and thus confers vascular protection. In this study, we questioned whether metformin could have an effect on apoE expression in macrophages in normal conditions or under lipopolysaccharide (LPS)-induced stress. The results showed that metformin slightly increases the apoE expression only at high doses (5-10 mM). Low doses of metformin (1-3 mM) significantly reduce the LPS down-regulatory effect on apoE expression in macrophages. Our experiments demonstrated that LPS-induced NF-κB binds to the macrophage-specific distal regulatory element of apoE gene, namely to the multienhancer 2 (ME.2) and its 5'-deletion fragments. The NF-κB binding on ME.2 and apoE promoter has a down-regulatory effect. In addition, data revealed that metformin impairs NF-κB nuclear translocation, and thus, improves the apoE levels in macrophages under inflammatory stress. The positive effect of metformin in the inflammatory states, its clinical safety and low cost, make this drug a potential adjuvant in the therapeutic strategies for atherosclerosis.

Quercetin represses apolipoprotein B expression by inhibiting the transcriptional activity of C/EBPβ

Quercetin is one of the most abundant polyphenolic flavonoids found in fruits and vegetables and has anti-oxidative and anti-obesity effects. Because the small intestine is a major absorptive organ of dietary nutrients, it is likely that highly concentrated food constituents, including polyphenols, are present in the small intestinal epithelial cells, suggesting that food factors may have a profound effect in this tissue. To identify novel targets of quercetin in the intestinal enterocytes, mRNA profiling using human intestinal epithelial Caco-2 cells was performed. We found that mRNA levels of some apolipoproteins, particularly apolipoprotein B (apoB), are downregulated in the presence of quercetin. On the exposure of Caco-2 cells to quercetin, both mRNA and protein levels of apoB were decreased. Promoter analysis of the human apoB revealed that quercetin response element is localized at the 5′-proximal promoter region, which contains a conserved CCAAT enhancer-binding protein (C/EBP)-response element. We found that quercetin reduces the promoter activity of apoB, driven by the enforced expression of C/EBPβ. Quercetin had no effect on either mRNA or protein levels of C/EBPβ. In contrast, we found that quercetin inhibits the transcriptional activity of C/EBPβ but not its recruitment to the apoB promoter. On the exposure of Caco-2 cells to quercetin 3-O-glucuronide, which is in a cell-impermeable form, no notable change in apoB mRNA was observed, suggesting an intracellular action of quercetin. In vitro interaction experiments using quercetin-conjugated beads revealed that quercetin binds to C/EBPβ. Our results describe a novel regulatory mechanism of transcription of apolipoprotein genes by quercetin in the intestinal enterocytes.

Zebrafish as a model for apolipoprotein biology: comprehensive expression analysis and a role for ApoA-IV in regulating food intake

Improved understanding of lipoproteins, particles that transport lipids throughout the circulation, is vital to developing new treatments for the dyslipidemias associated with metabolic syndrome. Apolipoproteins are a key component of lipoproteins. Apolipoproteins are proteins that structure lipoproteins and regulate lipid metabolism through control of cellular lipid exchange. Constraints of cell culture and mouse models mean that there is a need for a complementary model that can replicate the complex in vivo milieu that regulates apolipoprotein and lipoprotein biology. Here, we further establish the utility of the genetically tractable and optically clear larval zebrafish as a model of apolipoprotein biology. Gene ancestry analyses were implemented to determine the closest human orthologs of the zebrafish apolipoprotein A-I (apoA-I), apoB, apoE and apoA-IV genes and therefore ensure that they have been correctly named. Their expression patterns throughout development were also analyzed, by whole-mount mRNA in situ hybridization (ISH). The ISH results emphasized the importance of apolipoproteins in transporting yolk and dietary lipids: mRNA expression of all apolipoproteins was observed in the yolk syncytial layer, and intestinal and liver expression was observed from 4-6 days post-fertilization (dpf). Furthermore, real-time PCR confirmed that transcription of three of the four zebrafish apoA-IV genes was increased 4 hours after the onset of a 1-hour high-fat feed. Therefore, we tested the hypothesis that zebrafish ApoA-IV performs a conserved role to that in rat in the regulation of food intake by transiently overexpressing ApoA-IVb.1 in transgenic larvae and quantifying ingestion of co-fed fluorescently labeled fatty acid during a high-fat meal as an indicator of food intake. Indeed, ApoA-IVb.1 overexpression decreased food intake by approximately one-third. This study comprehensively describes the expression and function of eleven zebrafish apolipoproteins and serves as a springboard for future investigations to elucidate their roles in development and disease in the larval zebrafish model.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

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

KRAB-Zinc Finger Proteins: A Repressor Family Displaying Multiple Biological Functions

Zinc finger proteins containing the Kruppel associated box (KRAB-ZFPs) constitute the largest individual family of transcriptional repressors encoded by the genomes of higher organisms. KRAB domain, positioned at the NH2 terminus of the KRAB-ZFPs, interacts with a scaffold protein, KAP-1, which is able to recruit various transcriptional factors causing repression of genes to which KRAB ZFPs bind. The relevance of such repression is reflected in the large number of the KRAB zinc finger protein genes in the human genome. However, in spite of their numerical abundance little is currently known about the gene targets and the physiological functions of KRAB- ZFPs. However, emerging evidence links the transcriptional repression mediated by the KRAB-ZFPs to cell proliferation, differentiation, apoptosis and cancer. Moreover, the fact that KRAB containing proteins are vertebrate-specific suggests that they have evolved recently, and that their key roles lie in some aspects of vertebrate development. In this review, we will briefly discuss some regulatory functions of the KRAB-ZFPs in different physiological and pathological states, thus contributing to better understand their biological roles.

The involvement of the monocytes/macrophages in chronic inflammation associated with atherosclerosis

Atherosclerosis is a progressive chronic disease of large and medium arteries, characterized by the formation of atherosclerotic plaques. Monocytes and macrophages are key factors in lesion development, participating to the processes that mediate the progression of the atherosclerotic plaque (lipid accumulation, secretion of pro-inflammatory and cytotoxic factors, extracellular matrix remodeling). The recruitment of the monocytes in the vascular wall represents a hallmark in the pathology of the atherosclerotic lesion. Monocyte adhesion and transmigration are dependent on the complementary adhesion molecules expressed on the endothelial surface, whose expression is modulated by chemical mediators. The atherosclerotic plaque is characterized by a heterogeneous population of macrophages reflecting the complexity and diversity of the micro-environment to which cells are exposed after entering the arterial wall. Within the atherosclerotic lesions, macrophages differentiate, proliferate and undergo apoptosis. Taking into account that their behavior has a direct and critical influence on all lesional stages, the development of therapeutic approaches to target monocytes/macrophages in the atherosclerotic plaque became a focal interest point for researchers in the field.

Changes in the hepatic mitochondrial and membrane proteome in mice fed a non-alcoholic steatohepatitis inducing diet

Non-alcoholic steatohepatitis (NASH) accounts for a large proportion of cryptic cirrhosis in the Western societies. Nevertheless, we lack a deeper understanding of the underlying pathomolecular processes, particularly those preceding hepatic inflammation and fibrosis. In order to gain novel insights into early NASH-development from the first appearance of proteomic alterations to the onset of hepatic inflammation and fibrosis, we conducted a time-course analysis of proteomic changes in liver mitochondria and membrane-enriched fractions of female C57Bl/6N mice fed either a mere steatosis or NASH inducing diet. This data was complemented by quantitative measurements of hepatic glycerol-containing lipids, cholesterol and intermediates of the methionine cycle. Aside from energy metabolism and stress response proteins, enzymes of the urea cycle and methionine metabolism were found regulated. Alterations in the methionine cycle occur early in disease progression preceding molecular signs of inflammation. Proteins that hold particular promise in the early distinction between benign steatosis and NASH are methyl-transferase Mettl7b, the glycoprotein basigin and the microsomal glutathione-transferase.

ApoA-IV promotes the biogenesis of apoA-IV-containing HDL particles with the participation of ABCA1 and LCAT

The objective of this study was to establish the role of apoA-IV, ABCA1, and LCAT in the biogenesis of apoA-IV-containing HDL (HDL-A-IV) using different mouse models. Adenovirus-mediated gene transfer of apoA-IV in apoA-I(-/-) mice did not change plasma lipid levels. ApoA-IV floated in the HDL2/HDL3 region, promoted the formation of spherical HDL particles as determined by electron microscopy, and generated mostly α- and a few pre-β-like HDL subpopulations. Gene transfer of apoA-IV in apoA-I(-/-) × apoE(-/-) mice increased plasma cholesterol and triglyceride levels, and 80% of the protein was distributed in the VLDL/IDL/LDL region. This treatment likewise generated α- and pre-β-like HDL subpopulations. Spherical and α-migrating HDL particles were not detectable following gene transfer of apoA-IV in ABCA1(-/-) or LCAT(-/-) mice. Coexpression of apoA-IV and LCAT in apoA-I(-/-) mice restored the formation of HDL-A-IV. Lipid-free apoA-IV and reconstituted HDL-A-IV promoted ABCA1 and scavenger receptor BI (SR-BI)-mediated cholesterol efflux, respectively, as efficiently as apoA-I and apoE. Our findings are consistent with a novel function of apoA-IV in the biogenesis of discrete HDL-A-IV particles with the participation of ABCA1 and LCAT, and may explain previously reported anti-inflammatory and atheroprotective properties of apoA-IV.

LDL receptor knock-out mice are a physiological model particularly vulnerable to study the onset of inflammation in non-alcoholic fatty liver disease

Background & Aims

Non-alcoholic steatohepatitis (NASH) involves steatosis combined with inflammation, which can progress into fibrosis and cirrhosis. Exploring the molecular mechanisms of NASH is highly dependent on the availability of animal models. Currently, the most commonly used animal models for NASH imitate particularly late stages of human disease. Thus, there is a need for an animal model that can be used for investigating the factors that potentiate the inflammatory response within NASH. We have previously shown that 7-day high-fat-high-cholesterol (HFC) feeding induces steatosis and inflammation in both APOE2ki and Ldlr^−/− mice. However, it is not known whether the early inflammatory response observed in these mice will sustain over time and lead to liver damage. We hypothesized that the inflammatory response in both models is sufficient to induce liver damage over time.

Methods

APOE2ki and Ldlr^−/− mice were fed a chow or HFC diet for 3 months. C57Bl6/J mice were used as control.

Results

Surprisingly, hepatic inflammation was abolished in APOE2ki mice, while it was sustained in Ldlr^−/− mice. In addition, increased apoptosis and hepatic fibrosis was only demonstrated in Ldlr^−/− mice. Finally, bone-marrow-derived-macrophages of Ldlr^−/− mice showed an increased inflammatory response after oxidized LDL (oxLDL) loading compared to APOE2ki mice.

Conclusion

Ldlr^−/− mice, but not APOE2ki mice, developed sustained hepatic inflammation and liver damage upon long term HFC feeding due to increased sensitivity for oxLDL uptake. Therefore, the Ldlr^−/− mice are a promising physiological model particularly vulnerable for investigating the onset of hepatic inflammation in non-alcoholic steatohepatitis.

Analysis of genes regulated by the transcription factor LUMAN identifies ApoA4 as a target gene in dendritic cells

Dendritic cells (DCs) are professional antigen presenting cells of the immune system that play a crucial role in initiating immune responses and maintaining self tolerance. Better understanding of the molecular basis of DC immunobiology is required to improve DC-based immunotherapies. We previously described the interaction of transcription factor LUMAN (also known as CREB3 or LZIP) with the DC-specific transmembrane protein DC-STAMP in DCs. Target genes of LUMAN and its role in DCs are currently unknown. In this study we set out to identify genes regulated by LUMAN in DCs using microarray analysis. Expression of a constitutively active form of LUMAN in mouse DC cell line D2SC/1 identified Apolipoprotein A4 (ApoA4) as its target gene. Subsequent validation experiments, bioinformatics-based promoter analysis, and silencing studies confirmed that ApoA4 is a true target gene of LUMAN in bone marrow-derived DCs (BMDCs).

Characterization of a hypertriglyceridemic transgenic miniature pig model expressing human apolipoprotein CIII

Hypertriglyceridemia has recently been considered to be an independent risk factor for coronary heart disease, in which apolipoprotein (Apo)CIII is one of the major contributory factors, as it is strongly correlated with plasma triglyceride levels. Although ApoCIII transgenic mice have been generated as an animal model for the study of hypertriglyceridemia, the features of lipoprotein metabolism in mice differ greatly from those in humans. Because of the great similarity between pigs and humans with respect to lipid metabolism and cardiovascular physiology, we generated transgenic miniature pigs expressing human ApoCIII by the transfection of somatic cells combined with nuclear transfer. The expression of human ApoCIII was detected in the liver and intestine of the transgenic pigs. As compared with nontransgenic controls, transgenic pigs showed significantly increased plasma triglyceride levels (83 ± 36 versus 38 ± 4 mg·dL(-1), P < 0.01) when fed a chow diet. Plasma lipoprotein profiling by FPLC in transgenic animals showed a higher peak in large-particle fractions corresponding to very low-density lipoprotein/chylomicrons when triglyceride content in the fractions was assayed. There was not much difference in cholesterol content in FPLC fractions, although a large low-density lipoprotein peak was identified in both nontransgenic and transgenic animals, resembling that found in humans. Further analysis revealed markedly delayed clearance of plasma triglyceride, accompanied by significantly reduced lipoprotein lipase activity in post-heparin plasma, in transgenic pigs as compared with nontransgenic controls. In summary, we have successfully generated a novel hypertriglyceridemic ApoCIII transgenic miniature pig model that could be of great value for studies on hyperlipidemia in relation to atherosclerotic disorders.

Macrophage-specific up-regulation of apolipoprotein E gene expression by STAT1 is achieved via long range genomic interactions

In atherogenesis, macrophage-derived apolipoprotein E (apoE) has an athero-protective role by a mechanism that is not fully understood. We investigated the regulatory mechanisms involved in the modulation of apoE expression in macrophages. The experiments showed that the promoters of all genes of the apoE/apoCI/apoCIV/apoCII gene cluster are enhanced by multienhancer 2 (ME.2), a regulatory region that is located 15.9 kb downstream of the apoE gene. ME.2 interacts with the apoE promoter in a macrophage-specific manner. Transient transfections in RAW 264.7 macrophages showed that the activity of ME.2 was strongly decreased by deletion of either 87 bp from the 5' end or 131 bp from the 3' end. We determined that the minimal fragment of this promoter that can be activated by ME.2 is the proximal -100/+73 region. The analysis of the deletion mutants of ME.2 revealed the importance of the 5' end of ME.2 in apoE promoter transactivation. Chromatin conformational capture assays demonstrated that both ME.2 and ME.1 physically interacted with the apoE promoter in macrophages. Our data showed that phorbol 12-myristate 13-acetate-induced differentiation of macrophages is accompanied by a robust induction of apoE and STAT1 expression. In macrophages (but not in hepatocytes), STAT1 up-regulated apoE gene expression via ME.2. The STAT1 binding site was located in the 174-182 region of ME.2. In conclusion, the specificity of the interactions between the two multienhancers (ME.1 and ME.2) and the apoE promoter indicates that these distal regulatory elements play an important role in the modulation of apoE gene expression in a cell-specific manner.

Regulation of hepatic ApoC3 expression by PGC-1β mediates hypolipidemic effect of nicotinic acid

Peroxisome proliferator-activated receptor (PPAR) γ coactivator-1β (PGC-1β) is a transcriptional coactivator that induces hypertriglyceridemia in response to dietary fats through activating hepatic lipogenesis and lipoprotein secretion. The expression of PGC-1β is regulated by free fatty acids. Here we show that PGC-1β regulates plasma triglyceride metabolism through stimulating apolipoprotein C3 (APOC3) expression and elevating APOC3 levels in circulation. Remarkably, liver-specific knockdown of APOC3 significantly ameliorates PGC-1β-induced hypertriglyceridemia in mice. Hepatic expression of PGC-1β and APOC3 is reduced in response to acute and chronic treatments with nicotinic acid, a widely prescribed drug for lowering plasma triglycerides. Adenoviral-mediated knockdown of PGC-1β or APOC3 in the liver recapitulates the hypolipidemic effect of nicotinic acid. Proteomic analysis of hepatic PGC-1β transcriptional complex indicates that it stimulates APOC3 expression through coactivating orphan nuclear receptor ERRα and recruiting chromatin-remodeling cofactors. Together, these studies identify PGC-1β as an important regulator of the APOC3 gene cluster and reveal a mechanism through which nicotinic acid achieves its therapeutic effects.

Regulation of human apolipoprotein m gene expression by orphan and ligand-dependent nuclear receptors

Apolipoprotein M (apoM) plays an important role in the biogenesis and the metabolism of anti-atherogenic HDL particles in plasma and is expressed primarily in the liver and the kidney. We investigated the role of hormone nuclear receptors in apoM gene regulation in hepatic cells. Overexpression via adenovirus-mediated gene transfer and siRNA-mediated gene silencing established that hepatocyte nuclear factor 4 (HNF-4) is an important regulator of apoM gene transcription in hepatic cells. apoM promoter deletion analysis combined with DNA affinity precipitation and chromatin immunoprecipitation assays revealed that HNF-4 binds to a hormone-response element (HRE) in the proximal apoM promoter (nucleotides -33 to -21). Mutagenesis of this HRE decreased basal hepatic apoM promoter activity to 10% of control and abolished the HNF4-mediated transactivation of the apoM promoter. In addition to HNF-4, homodimers of retinoid X receptor and heterodimers of retinoid X receptor with receptors for retinoic acid, thyroid hormone, fibrates (peroxisome proliferator-activated receptor), and oxysterols (liver X receptor) were shown to bind with different affinities to the proximal HRE in vitro and in vivo. Ligands of these receptors strongly induced human apoM gene transcription and apoM promoter activity in HepG2 cells, whereas mutations in the proximal HRE abolished this induction. These findings provide novel insights into the role of apoM in the regulation of HDL by steroid hormones and into the development of novel HDL-based therapies for diseases such as diabetes, obesity, metabolic syndrome, and coronary artery disease that affect a large proportion of the population in Western countries.

Role of Esrrg in the fibrate-mediated regulation of lipid metabolism genes in human ApoA-I transgenic mice

We have used a new ApoA-I transgenic mouse model to identify by global gene expression profiling, candidate genes that affect lipid and lipoprotein metabolism in response to fenofibrate treatment. Multilevel bioinformatical analysis and stringent selection criteria (2-fold change, 0% false discovery rate) identified 267 significantly changed genes involved in several molecular pathways. The fenofibrate-treated group did not have significantly altered levels of hepatic human APOA-I mRNA and plasma ApoA-I compared with the control group. However, the treatment increased cholesterol levels to 1.95-fold mainly due to the increase in high-density lipoprotein (HDL) cholesterol. The observed changes in HDL are associated with the upregulation of genes involved in phospholipid biosynthesis and lipid hydrolysis, as well as phospholipid transfer protein. Significant upregulation was observed in genes involved in fatty acid transport and β-oxidation, but not in those of fatty acid and cholesterol biosynthesis, Krebs cycle and gluconeogenesis. Fenofibrate changed significantly the expression of seven transcription factors. The estrogen receptor-related gamma gene was upregulated 2.36-fold and had a significant positive correlation with genes of lipid and lipoprotein metabolism and mitochondrial functions, indicating an important role of this orphan receptor in mediating the fenofibrate-induced activation of a specific subset of its target genes.

Differentiated CaCo-2 cells as an in-vitro model to evaluate de-novo apolipoprotein A-I production in the small intestine

BACKGROUND

Increasing HDL cholesterol concentrations by stimulating de-novo apolipoprotein A-I (apoA-I) production in the liver and/or in the small intestine is a potential strategy to reduce coronary heart disease risk. Although there is quite some knowledge concerning regulatory effects in the liver, less is known concerning potential agents that could elevate de-novo apoA-I production in the small intestine.

METHODS

Therefore, we compared side-by-side effects of various peroxisome proliferator-activated receptor (PPAR)alpha, PPARgamma, retinoid-X-receptor alpha, and farnesoid-X-receptor agonists on de-novo apoA-I production in differentiated CaCo-2 and HepG2 cells.

RESULTS

For PPARa agonists, we showed that GW7647 elevated apoA-I concentrations in the medium of both cell models, whereas WY14643 elevated only de-novo apoA-I concentrations in differentiated CaCo-2 cells. Unexpectedly, fenofibric acid lowered apoA-I medium concentrations in both cell lines, which could not be explained by a lack of PPAR transactivation or a lack of retinoid-X-receptor a activation. For farnesoid-X-receptor agonists, chenodeoxycholic acid strongly reduced apoA-I concentrations both in differentiated CaCo-2 and HepG2 cells, whereas GW4064 and taurocholate only lowered apoA-I in CaCo-2 cells (GW4064) or in HepG2 cells (taurocholate). However, overall effects of all individual components on apoA-I production in differentiated CaCo-2 and HepG2 cells were highly correlated (r = 0.68; P = 0.037; N=9).

CONCLUSION

We conclude that differentiated CaCo-2 cells are suitable models to study de-novo small intestinal apoA-I production in vitro enabling the possibility to screen for potential bioactive dietary components. This cell model may also determine small-intestinal-specific effects, as some discrepancy was found between both cell models.

Novel genes in cell cycle control and lipid metabolism with dynamically regulated binding sites for sterol regulatory element-binding protein 1 and RNA polymerase II in HepG2 cells detected by chromatin immunoprecipitation with microarray detection

Sterol regulatory element-binding proteins 1 and 2 (SREBP-1 and SREBP-2) are important regulators of genes involved in cholesterol and fatty acid metabolism, but have also been implicated in the regulation of the cell cycle and have been associated with the pathogenesis of type 2 diabetes, atherosclerosis and obesity, among others. In this study, we aimed to characterize the binding sites of SREBP-1 and RNA polymerase II through chromatin immunoprecipitation and microarray analysis in 1% of the human genome, as defined by the Encyclopaedia of DNA Elements consortium, in a hepatocellular carcinoma cell line (HepG2). Our data identified novel binding sites for SREBP-1 in genes directly or indirectly involved in cholesterol metabolism, e.g. apolipoprotein C-III (APOC3). The most interesting biological findings were the binding sites for SREBP-1 in genes for host cell factor C1 (HCFC1), involved in cell cycle regulation, and for filamin A (FLNA). For RNA polymerase II, we found binding sites at classical promoters, but also in intergenic and intragenic regions. Furthermore, we found evidence of sterol-regulated binding of SREBP-1 and RNA polymerase II to HCFC1 and FLNA. From the results of this work, we infer that SREBP-1 may be involved in processes other than lipid metabolism.

ABHD5/CGI-58 facilitates the assembly and secretion of apolipoprotein B lipoproteins by McA RH7777 rat hepatoma cells

Lipolysis of stored triacylglycerols provides lipid precursors for the assembly of apolipoprotein B (apoB) lipoproteins in hepatocytes. Abhydrolase domain containing 5 (ABHD5) is expressed in liver and facilitates the lipolysis of triacylglycerols. To study the function of ABHD5 in lipoprotein secretion, we silenced the expression of ABHD5 in McA RH7777 cells using RNA interference and studied the metabolism of lipids and secretion of apoB lipoproteins. McA RH7777 cells deficient in ABHD5 secreted reduced amounts of apoB, triacylglycerols, and cholesterol esters. Detailed analysis of liquid chromatography-mass spectrometry data for the molecular species of secreted triacylglycerols revealed that deficiency of ABHD5 significantly reduced secretion of triacylglycerols containing oleate, even when oleate was supplied in the culture medium; the ABHD5-deficient cells partially compensated by secreting higher levels of triacylglycerols containing saturated fatty acids. In experiments tracking the metabolism of [(14)C]oleate, silencing of ABHD5 reduced lipolysis of cellular triacylglycerols and incorporation of intermediates derived from stored lipids into secreted triacylglycerols and cholesterol esters. In contrast, the incorporation of exogenous oleate into secreted triacylglycerols and cholesterol esters was unaffected by deficiency of ABHD5. These findings suggest that ABHD5 facilitates the use of lipid intermediates derived from lipolysis of stored triacylglycerols for the assembly of lipoproteins.

The apolipoprotein CIII enhancer regulates both extensive histone modification and intergenic transcription of human apolipoprotein AI/CIII/AIV genes but not apolipoprotein AV

The apolipoprotein (apo) AI/CIII/AIV/AV cluster genes are expressed at different levels in the liver and intestine. The apoCIII enhancer, a common regulatory element, regulates the tissue-specific expression of apoAI, apoCIII, and apoAIV but not apoAV. To study this regulation at the chromatin level, the histone modifications and intergenic transcription in the human apoAI/CIII/AIV/AV cluster were investigated in HepG2 and Caco-2 cells and in the livers of transgenic mice carrying the human gene cluster constructs with or without the apoCIII enhancer. We found that both the promoters and the intergenic regions of the apoAI/CIII/AIV genes were hyperacetylated and formed an open subdomain that did not include the apoAV gene. Hepatic and intestinal intergenic transcripts were identified to transcribe bidirectionally with strand preferences along the cluster. The deletion of the apoCIII enhancer influenced both histone modification and intergenic transcription in the apoAI/CIII/AIV gene region. These results demonstrate that the apoCIII enhancer contributes to the maintenance of an active chromatin subdomain of the apoAI/CIII/AIV genes, but not apoAV.

Dietary cholesterol, rather than liver steatosis, leads to hepatic inflammation in hyperlipidemic mouse models of nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) involves liver lipid accumulation (steatosis) combined with hepatic inflammation. The transition towards hepatic inflammation represents a key step in pathogenesis, because it will set the stage for further liver damage, culminating in hepatic fibrosis, cirrhosis, and liver cancer. The actual risk factors that drive hepatic inflammation during the progression to NASH remain largely unknown. The role of steatosis and dietary cholesterol in the etiology of diet-induced NASH was investigated using hyperlipidemic mouse models fed a Western diet. Livers of male and female hyperlipidemic (low-density lipoprotein receptor-deficient [ldlr(-/-)] and apolipoprotein E2 knock-in [APOE2ki]) mouse models were compared with livers of normolipidemic wild-type (WT) C57BL/6J mice after short-term feeding with a high-fat diet with cholesterol (HFC) and without cholesterol. Whereas WT mice displayed only steatosis after a short-term HFC diet, female ldlr(-/-) and APOE2ki mice showed steatosis with severe inflammation characterized by infiltration of macrophages and increased nuclear factor kappaB (NF-kappaB) signaling. Remarkably, male ldlr(-/-) and APOE2ki mice developed severe hepatic inflammation in the absence of steatosis after 7 days on an HFC diet compared with WT animals. An HFC diet induced bloated, "foamy" Kupffer cells in male and female ldlr(-/-) and APOE2ki mice. Hepatic inflammation was found to be linked to increased plasma very low-density lipoprotein (VLDL) cholesterol levels. Omitting cholesterol from the HFC diet lowered plasma VLDL cholesterol and prevented the development of inflammation and hepatic foam cells.

CONCLUSION

These findings indicate that dietary cholesterol, possibly in the form of modified plasma lipoproteins, is an important risk factor for the progression to hepatic inflammation in diet-induced NASH.

Triglyceride-rich lipoprotein-associated apolipoprotein C-III production is stimulated by plasma free fatty acids in humans

OBJECTIVE

Insulin resistant states are associated with increased fatty acid flux to liver and intestine, which stimulates the production of triglyceride-rich lipoproteins (TRL). ApoC-III production and plasma and TRL concentrations are increased in insulin resistance and may contribute to the hypertriglyceridemia of these conditions. The mechanism underlying that increase is not known, but because apoC-III and VLDL production are closely linked we hypothesized that FFAs may stimulate TRL apoC-III production.

METHODS AND RESULTS

We used Intralipid/heparin (IH) to raise plasma FFA in 12 healthy men in the fed state, and stable isotopes to examine apoC-III metabolism. TRL apoC-III concentration was significantly higher in the IH study, and this increase was associated with higher production (PR) and fractional catabolic rate (FCR). The increase in production was greater than in FCR (90% versus 30%, respectively), accounting for the elevated concentration. Glycerol infusion had no effect on apoC-III concentration, PR, or FCR compared to saline, indicating that the effect was not attributable to glycerol released from intralipid.

CONCLUSIONS

These findings confirm that TRL apoC-III production is stimulated by an acute elevation of plasma FFAs, suggesting a novel regulatory pathway that may play a role in the overproduction of TRL apoC-III in insulin resistant states.

Transcriptional regulation of the human CYP46A1 brain-specific expression by Sp transcription factors

Brain defective cholesterol homeostasis has been associated with neurologic diseases, such as Alzheimer's and Huntington's disease. The elimination of cholesterol from the brain involves its conversion into 24(S)-hydroxycholesterol by CYP46A1, and the efflux of this oxysterol across the blood-brain barrier. Herein, we identified the regulatory elements and factors involved the human CYP46A1 expression. Functional 5'deletion analysis mapped a region spanning from nucleotides -236/-64 that is indispensable for basal expression of this TATA-less gene. Treatment of SH-SY5Y cells with mithramycin A resulted in a significant reduction of promoter activity, suggesting a role of Sp family of transcription factors in CYP46A1 regulation. Combination of Sp1, Sp3, and Sp4 over-expression studies in Drosophila SL-2 cells, and systematic promoter mutagenesis identified Sp3 and Sp4 binding to four GC-boxes as required and sufficient for high levels of promoter activity. Moreover, Sp3 and Sp4 were demonstrated to be the major components of the protein-DNA complexes observed in primary rat cortical extracts. Our results suggest that the cell-type specific expression of Sp transcription factors - substitution of Sp1 by Sp4 in neurons - is responsible for the basal expression of the CYP46A1 gene. This study delineates for the first time the mechanisms underlying the human CYP46A1 transcription and thereby elucidates potential pathways underlying cholesterol homeostasis in the brain.

Increasing apoA-I production as a target for CHD risk reduction

Dyslipidemia leading to coronary heart diseases (CHD) enables venues to prevent or treat CHD by other strategies than only lowering serum LDL cholesterol (LDL-C) concentrations, which is currently the most frequently targeted change. Unlike LDL-C, elevated high-density lipoprotein cholesterol (HDL-C) concentrations may protect against the development of CHD as demonstrated in numerous large-scale epidemiological studies. In this review we describe that besides elevating serum HDL-C concentrations by increasing alpha-HDL particles, approaches to elevate HDL-C concentrations by increasing pre-beta HDL particle concentrations seems more attractive. Besides infusion of apoA-I(Milano), using apoA-I mimetics, or delipidation of alpha-HDL particles, elevating de novo apoA-I production may be a suitable target to functionally increase pre-beta HDL particle concentrations. Therefore, a detailed description of the molecular pathways underlying apoA-I synthesis and secretion, completed with an overview of known effects of pharmacological and nutritional compounds on apoA-I synthesis will be presented. This knowledge may ultimately be applied in developing dietary intervention strategies to elevate apoA-I production and serum HDL-C concentrations and consequently lower CHD risk.

Inflammatory signaling pathways regulating ApoE gene expression in macrophages

The atheroprotective role of apolipoprotein E (apoE) is well established. During inflammation, expression of apoE in macrophages is reduced leading to enhanced atheromatous plaque development. In the present study, we investigated the signaling pathways involved in the repression of apoE gene expression in response to lipopolysaccharide (LPS) treatment, a condition that mimics the inflammatory stress, in mouse macrophages RAW 264.7. We identified Tpl-2 and MEKK1 as the kinases that are primarily responsible for the down-regulation of apoE promoter activity by LPS. Using a dominant negative form of IkappaB, we established that Tpl-2 and MEKK1 signaling pathways converge to NF-kappaB acting on the apoE core promoter -55/+73. In addition to NF-kappaB activation, LPS also activated c-Jun via its phosphorylation by JNK. The activity of the apoE promoter was repressed by c-Jun, whereas small interference RNA-mediated inhibition of endogenous c-Jun expression reversed the inhibitory effect of Tpl-2 on the apoE promoter. Transfection experiments and DNA binding assays showed that the binding site for c-Jun is in the -55/+73 region of the apoE promoter. Finally, we showed that LPS inhibited apoE gene expression via activation of the Tpl-2/MEK/ERK pathway acting on a different apoE promoter region. In summary, LPS represses apoE gene expression in macrophages via signaling pathways that involve the upstream kinases Tpl-2 and MEKK1, the intermediate mitogen-activated protein kinases ERK and JNK, and the downstream transcription factors AP-1 and NF-kappaB that inhibit the apoE promoter activity via distinct regions.

Genetic association between endothelial nitric oxide synthase and Alzheimer disease

Evidence suggests that vascular and inflammatory factors may be important in the etiology of Alzheimer disease (AD). The Glu/Glu genotype at the Glu298Asp variant of the endothelial nitric oxide synthase (NOS3) gene has been tested for association with AD in several Caucasian and Asian populations, with conflicting results. We tested the Glu298Asp variant for association in African American and Caucasian AD patients, unaffected siblings, and unrelated controls from the MIRAGE Study. To explore whether the inconsistent results in previous studies might be due to linkage disequilibrium with a polymorphism or haplotype not previously tested, we genotyped 10 additional NOS3 single nucleotide polymorphisms (SNPs) spanning 25.3 kb. Finally, we compiled results of previous studies of Glu298Asp using meta-analysis, to determine whether the aggregate studies support an association between Glu298Asp and AD. We found that the Glu298 allele was associated with higher risk of AD in the MIRAGE African American (p = 0.002) but not Caucasian (p = 0.9) groups. None of the additional SNPs were associated with AD in the Caucasians, whereas two showed evidence for association in the African Americans. The meta-analysis showed a small effect of the Glu298Asp GG genotype on AD risk across all studies (summary odds ratio = 1.15, 95% confidence interval: 0.97-1.35) and significant heterogeneity of this association among studies (p = 0.02).

Transcriptional regulation of the human hepatic lipase (LIPC) gene promoter

Hepatic lipase (HL) plays a key role in the metabolism of plasma lipoproteins, and its level of activity requires tight regulation, given the association of both low and high levels with atherosclerosis and coronary artery disease. However, little is known about the factors responsible for HL expression. Here, we report that the human hepatic lipase gene (LIPC) promoter is regulated by hepatocyte nuclear factor 4alpha (HNF4alpha), peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha), apolipoprotein A-I regulatory protein-1 (ARP-1), and hepatocyte nuclear factor 1alpha (HNF1alpha). Reporter analysis showed that HNF4alpha directly regulates the LIPC promoter via two newly identified direct repeat elements, DR1 and DR4. PGC-1alpha is capable of stimulating the HNF4alpha-dependent transactivation of the LIPC promoter. ARP-1 displaces HNF4alpha from the DR1 site and blocks its ability to activate the LIPC promoter. Induction by HNF1alpha requires the HNF1 binding site and upon cotransfection with HNF4alpha leads to an additive effect. In addition, the in vivo relevance of HNF4alpha in LIPC expression is shown by the ability of the HNF4alpha antagonist Medica 16 to repress endogenous LIPC mRNA expression. Furthermore, disruption of Hnf4alpha in mice prevents the expression of HL mRNA in liver. The overall effect these transcription factors have on HL expression will ultimately depend on the interplay between these various factors and their relative intracellular concentrations.

Associations among race/ethnicity, ApoC-III genotypes, and lipids in HIV-1-infected individuals on antiretroviral therapy

BACKGROUND

Protease inhibitors (PIs) are associated with hypertriglyceridemia and atherogenic dyslipidemia. Identifying HIV-1-infected individuals who are at increased risk of PI-related dyslipidemia will facilitate therapeutic choices that maintain viral suppression while reducing risk of atherosclerotic diseases. Apolipoprotein C-III (apoC-III) gene variants, which vary by race/ethnicity, have been associated with a lipid profile that resembles PI-induced dyslipidemia. However, the association of race/ethnicity, or candidate gene effects across race/ethnicity, with plasma lipid levels in HIV-1-infected individuals, has not been reported.

METHODS AND FINDINGS

A cross-sectional analysis of race/ethnicity, apoC-III/apoA-I genotypes, and PI exposure on plasma lipids was performed in AIDS Clinical Trial Group studies (n = 626). Race/ethnicity was a highly significant predictor of plasma lipids in fully adjusted models. Furthermore, in stratified analyses, the effect of PI exposure appeared to differ across race/ethnicity. Black/non-Hispanic, compared with White/non-Hispanics and Hispanics, had lower plasma triglyceride (TG) levels overall, but the greatest increase in TG levels when exposed to PIs. In Hispanics, current PI antiretroviral therapy (ART) exposure was associated with a significantly smaller increase in TGs among patients with variant alleles at apoC-III-482, -455, and Intron 1, or at a composite apoC-III genotype, compared with patients with the wild-type genotypes.

CONCLUSIONS

In the first pharmacogenetic study of its kind in HIV-1 disease, we found race/ethnic-specific differences in plasma lipid levels on ART, as well as differences in the influence of the apoC-III gene on the development of PI-related hypertriglyceridemia. Given the multi-ethnic distribution of HIV-1 infection, our findings underscore the need for future studies of metabolic and cardiovascular complications of ART that specifically account for racial/ethnic heterogeneity, particularly when assessing candidate gene effects.

RAR-related orphan receptor A isoform 1 (RORa1) is disrupted by a balanced translocation t(4;15)(q22.3;q21.3) associated with severe obesity

We have identified a family comprising a mother and two children with idiopathic and profound obesity body mass index (BMI) 41-49 kg/m(2). The three family members carry a balanced reciprocal chromosome translocation t(4;15). We present here the clinical features of the affected individuals as well as the physical mapping and cloning of the chromosomal breakpoints. A detailed characterisation of the chromosomal breakpoints at chromosomes 4 and 15 revealed that the translocation is almost perfectly balanced with a very short insertion/deletion. The chromosome 15 breakpoint is positioned in intron 1 of the RAR-related orphan receptor A isoform 1 (RORa1) and the chromosome 4 breakpoint is positioned 133 kb telomeric to the transcriptional start of the unc-5 homolog B (UNC5C) and 154 kb centromeric of the transcriptional start of the pyruvate dehydrogenase (lipoamide) alpha 2 (PDHA2). The rearrangement creates a fusion gene, which includes the RORa1 exon 1 and UNC5C that is expressed in frame in adipocytes from the affected patients. We also show that this transcript is translated into a protein. From previous reports, it is shown that RORa1 is implicated in the regulation of adipogenesis and lipoprotein metabolism. We hypothesise that the obesity in this family is caused by (i) haploinsufficiency for RORa1 or, (ii) a gain of function mechanism mediated by the RORa1-UNC5C fusion gene.

Hepatocyte Nuclear Factor-4α Regulates the Human Apolipoprotein AV Gene: Identification of a Novel Response Element and Involvement in the Control by Peroxisome Proliferator-Activated Receptor-γ Coactivator-1α, AMP-Activated Protein Kinase, and Mitogen-Activated Protein Kinase Pathway

The recently discovered apolipoprotein AV (apoAV) gene has been reported to be a key player in modulating plasma triglyceride levels. Here we identify the hepatocyte nuclear factor-4alpha (HNF-4alpha) as a novel regulator of human apoAV gene. Inhibition of HNF-4alpha expression by small interfering RNA resulted in down-regulation of apoAV. Deletion, mutagenesis, and binding assays revealed that HNF-4alpha directly regulates human apoAV promoter through DR1 [a direct repeat separated by one nucleotide (nt)], and via a novel element for HNF-4alpha consisting of an inverted repeat separated by 8 nt (IR8). In addition, we show that the coactivator peroxisome proliferator-activated receptor-gamma coactivator-1alpha was capable of stimulating the HNF-4alpha-dependent transactivation of apoAV promoter. Furthermore, analyses in human hepatic cells demonstrated that AMP-activated protein kinase (AMPK) and the MAPK signaling pathway regulate human apoAV expression and suggested that this regulation may be mediated, at least in part, by changes in HNF-4alpha. Intriguingly, EMSAs and mice with a liver-specific disruption of the HNF-4alpha gene revealed a species-distinct regulation of apoAV by HNF-4alpha, which resembles that of a subset of HNF-4alpha target genes. Taken together, our data provide new insights into the binding properties and the modulation of HNF-4alpha and underscore the role of HNF-4alpha in regulating triglyceride metabolism.

Binding sites for metabolic disease related transcription factors inferred at base pair resolution by chromatin immunoprecipitation and genomic microarrays

We present a detailed in vivo characterization of hepatocyte transcriptional regulation in HepG2 cells, using chromatin immunoprecipitation and detection on PCR fragment-based genomic tiling path arrays covering the encyclopedia of DNA element (ENCODE) regions. Our data suggest that HNF-4alpha and HNF-3beta, which were commonly bound to distal regulatory elements, may cooperate in the regulation of a large fraction of the liver transcriptome and that both HNF-4alpha and USF1 may promote H3 acetylation to many of their targets. Importantly, bioinformatic analysis of the sequences bound by each transcription factor (TF) shows an over-representation of motifs highly similar to the in vitro established consensus sequences. On the basis of these data, we have inferred tentative binding sites at base pair resolution. Some of these sites have been previously found by in vitro analysis and some were verified in vitro in this study. Our data suggests that a similar approach could be used for the in vivo characterization of all predicted/uncharacterized TF and that the analysis could be scaled to the whole genome.

Octanoate reduces very low-density lipoprotein secretion by decreasing the synthesis of apolipoprotein B in primary cultures of chicken hepatocytes

Fatty acids of varying lengths and saturation differentially affect plasma apolipoprotein B (apoB) levels. To identify the mechanisms underlying the effect of octanoate on very low-density lipoprotein (VLDL) secretion, chicken primary hepatocytes were incubated with either fatty acid-bovine serum albumin (BSA) complexes or BSA alone. Addition of octanoate to culture medium significantly reduced VLDL-triacylglycerol (TG), VLDL-cholesterol and apoB secretion from hepatocytes compared to both control cultures with BSA only and palmitate treatments, but did not modulate intracellular TG accumulation. However, no differences in cellular microsomal triglyceride transfer protein levels were observed in the cultures with saturated fatty acid. In pulse-chase studies, octanoate treatment resulted in reduced apoB-100 synthesis, in agreement with its promotion of secretion. This characteristic effect of octanoate was confirmed by addition of a protease inhibitor, N-acetyl-leucyl-leucyl-norleucinal (ALLN), to hepatocyte cultures. Analysis showed that the level of apoB mRNA was lower in cultures supplemented with octanoate than in the control cultures, but no significant changes were observed in the levels of apolipoprotein A-I, fatty acid synthase and 3-hydroxy-3-methylglutaryl-CoA reductase mRNA as a result of octanoate treatment. Time-course studies indicate that a 50% reduction in apoB mRNA levels requires 12 h of incubation with octanoate. We conclude that octanoate reduced VLDL secretion by the specific down-regulation of apoB gene expression and impairment of subsequent synthesis of apoB, not by the modulation of intracellular apoB degradation, which is known to be a major regulatory target of VLDL secretion of other fatty acids.

Expression of the pregnane X receptor in mice antagonizes the cholic acid-mediated changes in plasma lipoprotein profile

OBJECTIVE

Modification of lipoprotein metabolism by bile acids has been mainly explained by activation of the farnesyl X receptor (FXR). The aim of the present study was to determine the relative contribution of the pregnane X receptor (PXR), another bile acid-activated nuclear receptor to changes in plasma lipoprotein profile.

METHODS AND RESULTS

Wild-type mice, Pxr-deficient mice, and Pxr-null mice expressing human PXR (Pxr-null SXR-Tg mice) were fed a cholic acid-containing diet, and consequences on plasma lipoprotein profiles and target gene expression were assessed. Cholic acid produced significant decreases in high-density lipoprotein (HDL) cholesterol, plasma apolipoprotein (apo)A-I and hepatic apoA-I mRNA in wild-type mice. Interestingly, the effect of cholic acid was significantly more pronounced in Pxr-deficient mice, indicating that PXR contributes to the weakening of the effect of bile acids on lipoprotein metabolism. Reciprocally, changes in HDL/apoA-I profiles were abolished in Pxr-null SXR-Tg mice in which PXR-responsive genes, particularly those involved in bile acid detoxification were readily activated after cholic acid treatment.

CONCLUSIONS

PXR expression in mice antagonizes the cholic acid-mediated downregulation of plasma HDL cholesterol and apoA-I, and magnification of PXR/SXR-mediated changes may constitute a new mean to counteract the effects of bile acids on plasma lipoproteins.