MiR-26 controls LXR-dependent cholesterol efflux by targeting ABCA1 and ARL7

MicroRNAs in lipid metabolism and atherosclerosis

Numerous studies have examined the role of microRNAs (miRNAs) in cell homeostasis and cardiovascular disease and have markedly improved our understanding of RNA biology in general and the potential role of miRNAs in atherosclerosis. In atherosclerosis, several miRNAs, such as miR-33a,b, miR-92a, miR-126 and others, have been identified that are relevant mediators of pathological processes, including regulation of cholesterol and lipid biosynthesis, lipoprotein metabolism and cholesterol efflux, but also immune responses, endothelial cell biology and vascular function. Further understanding of the specific roles of miRNAs in the distinct cell types involved in atherosclerosis initiation, progression and resolution may reveal new intervention strategies for the prevention and treatment of atherosclerotic cardiovascular disease.

MicroRNA-33 Regulates the Innate Immune Response via ATP Binding Cassette Transporter-mediated Remodeling of Membrane Microdomains

MicroRNAs (miRNAs) are short non-coding RNAs that regulate gene expression by promoting degradation and/or repressing translation of specific target mRNAs. Several miRNAs have been identified that regulate the amplitude of the innate immune response by directly targeting Toll-like receptor (TLR) pathway members and/or cytokines. miR-33a and miR-33b (the latter present in primates but absent in rodents and lower species) are located in introns of the sterol regulatory element-binding protein (SREBP)-encoding genes and control cholesterol/lipid homeostasis in concert with their host gene products. These miRNAs regulate macrophage cholesterol by targeting the lipid efflux transporters ATP binding cassette (ABC)A1 and ABCG1. We and others have previously reported that Abca1(-/-) and Abcg1(-/-) macrophages have increased TLR proinflammatory responses due to augmented lipid raft cholesterol. Given this, we hypothesized that miR-33 would augment TLR signaling in macrophages via a raft cholesterol-dependent mechanism. Herein, we report that multiple TLR ligands down-regulate miR-33 in murine macrophages. In the case of lipopolysaccharide, this is a delayed, Toll/interleukin-1 receptor (TIR) domain-containing adapter-inducing interferon-β-dependent response that also down-regulates Srebf-2, the host gene for miR-33. miR-33 augments macrophage lipid rafts and enhances proinflammatory cytokine induction and NF-κB activation by LPS. This occurs through an ABCA1- and ABCG1-dependent mechanism and is reversible by interventions upon raft cholesterol and by ABC transporter-inducing liver X receptor agonists. Taken together, these findings extend the purview of miR-33, identifying it as an indirect regulator of innate immunity that mediates bidirectional cross-talk between lipid homeostasis and inflammation.

[Epigenetic factors in atherogenesis: microRNA]

MicroRNAs (miRNAs) are small (~22 nucleotides in length) noncoding RNA sequences regulating gene expression at posttranscriptional level. MicroRNAs bind complementarily to certain mRNA and cause gene silencing. The involvement of miRNAs in the regulation of lipid metabolism, inflammatory response, cell cycle progression and proliferation, oxidative stress, platelet activation, endothelial and vascular smooth muscle cells (VSMC) function, angiogenesis and plaque formation and rapture indicates important roles in the initiation and progression of atherosclerosis. The key role of microRNAs in pathophysiology of cardiovascular diseases (CVDs), including atherosclerosis, was demonstrated in recent studies. Creating antisense oligonucleotides is a novel technique for selective changes in gene expression both in vitro and in vivo. In this review, we draw attention to the role of miRNAs in atherosclerosis progression, using miRNA as the potential biomarkers and targets in the CVDs, as well as possible application of antisense oligonucleotides.

Identification of novel microRNA genes in freshwater and marine ecotypes of the three-spined stickleback (Gasterosteus aculeatus)

The three-spined stickleback (Gasterosteus aculeatus L.) is an important model organism for studying the molecular mechanisms of speciation and adaptation to salinity. Despite increased interest to microRNA discovery and recent publication on microRNA prediction in the three-spined stickleback using bioinformatics approaches, there is still a lack of experimental support for these data. In this paper, high-throughput sequencing technology was applied to identify microRNA genes in gills of the three-spined stickleback. In total, 595 miRNA genes were discovered; half of them were predicted in previous computational studies and were confirmed here as microRNAs expressed in gill tissue. Moreover, 298 novel microRNA genes were identified. The presence of miRNA genes in selected 'divergence islands' was analysed and 10 miRNA genes were identified as not randomly located in 'divergence islands'. Regulatory regions of miRNA genes were found enriched with selective SNPs that may play a role in freshwater adaptation.

Circulating microRNAs predict future fatal myocardial infarction in healthy individuals - The HUNT study

Coronary heart disease is the most common cause of death, and the number of individuals at risk is increasing. To better manage this pandemic, improved tool for risk prediction, including more accurate biomarkers are needed. The objective of this study was to assess the utility of circulating microRNAs (miRs) to predict future fatal acute myocardial infarction (AMI) in healthy participants. We performed a prospective nested case-control study with 10-year observation period and fatal AMI as endpoint. In total, 179 miRs were quantified by real-time polymerase chain reaction in serum of 112 healthy participants (40-70years) that either (1) suffered from fatal AMI within 10years [n=56], or (2) remained healthy [n=56, risk factor-matched controls]. Candidate miRs were validated in a separate cohort of healthy individuals (n=100). Twelve miRs were differently expressed in cases and controls in the derivation cohort (p<0.05). Among these, 10 miRs differed significantly between cases and controls in the validation cohort (p<0.05). We identified gender dimorphisms, as miR-424-5p and miR-26a-5p were associated exclusively with risk in men and women, respectively. The best model for predicting future AMI consisted of miR-106a-5p, miR-424-5p, let-7g-5p, miR-144-3p and miR-660-5p, providing 77.6% correct classification for both genders, and 74.1% and 81.8% for men and women, respectively. Adding these 5 miRs to the Framingham Risk Score, increased the AUC from 0.72 to 0.91 (p<0.001). In conclusion, we identified several miRs associated with future AMI, revealed gender-specific associations, and proposed a panel of 5 miRs to enhance AMI risk prediction in healthy individuals.

Micro-RNAs and High-Density Lipoprotein Metabolism

Improved prevention and treatment of cardiovascular diseases is one of the challenges in Western societies, where ischemic heart disease and stroke are the leading cause of death. Early epidemiological studies have shown an inverse correlation between circulating high-density lipoprotein-cholesterol (HDL-C) and cardiovascular diseases. The cardioprotective effect of HDL is because of its ability to remove cholesterol from plaques in the artery wall to the liver for excretion by a process known as reverse cholesterol transport. Numerous studies have reported the role that micro-RNAs (miRNA) play in the regulation of the different steps in reverse cholesterol transport, including HDL biogenesis, cholesterol efflux, and cholesterol uptake in the liver and bile acid synthesis and secretion. Because of their ability to control different aspects of HDL metabolism and function, miRNAs have emerged as potential therapeutic targets to combat cardiovascular diseases. In this review, we summarize the recent advances in the miRNA-mediated control of HDL metabolism. We also discuss how HDL particles serve as carriers of miRNAs and the potential use of HDL-containing miRNAs as cardiovascular diseases biomarkers.

MicroRNA-26a/b and their host genes synergistically regulate triacylglycerol synthesis by targeting the INSIG1 gene

The microRNA-26 (miR-26) family is known to control adipogenesis in non-ruminants. The genomic loci of miR-26a and miR-26b have been localized in the introns of genes encoding for the proteins of the C-terminal domain RNA polymerase II polypeptide A small phosphatase (CTDSP) family. Insulin-induced gene 1 (INSIG1) encodes a protein with a key role in the regulation of lipogenesis in rodent liver. In the present study, we investigated the synergistic function of the miR-26 family and their host genes in goat mammary epithelial cells (GMEC). Downregulation of miR-26a/b and their host genes in GMEC decreased the expression of genes relate to fatty acid synthesis (PPARG, LXRA, SREBF1, FASN, ACACA, GPAM, LPIN1, DGAT1 and SCD1), triacylglycerol accumulation and unsaturated fatty acid synthesis. Luciferase reporter assays confirmed INSIG1 as a direct target of miR-26a/b. Furthermore, inhibition of the CTDSP family also downregulated the expression of INSIG1. Taken together, our findings highlight a functional association of miR-26a/b, their host genes and INSIG1, and provide new insights into the regulatory network controlling milk fat synthesis in GMEC. The data indicate that targeting this network via nutrition might be important for regulating milk fat synthesis in ruminants.

MicroRNA Regulation of Atherosclerosis

Atherosclerosis and its attendant clinical complications, such as myocardial infarction, stroke, and peripheral artery disease, are the leading cause of morbidity and mortality in Western societies. In response to biochemical and biomechanical stimuli, atherosclerotic lesion formation occurs from the participation of a range of cell types, inflammatory mediators, and shear stress. Over the past decade, microRNAs (miRNAs) have emerged as evolutionarily conserved, noncoding small RNAs that serve as important regulators and fine-tuners of a range of pathophysiological cellular effects and molecular signaling pathways involved in atherosclerosis. Accumulating studies reveal the importance of miRNAs in regulating key signaling and lipid homeostasis pathways that alter the balance of atherosclerotic plaque progression and regression. In this review, we highlight current paradigms of miRNA-mediated effects in atherosclerosis progression and regression. We provide an update on the potential use of miRNAs diagnostically for detecting increasing severity of coronary disease and clinical events. Finally, we provide a perspective on therapeutic opportunities and challenges for miRNA delivery in the field.

Macrophage miRNAs in atherosclerosis

The discovery of endogenous microRNAs (miRNAs) in the early 1990s has been followed by the identification of hundreds of miRNAs and their roles in regulating various biological processes, including proliferation, apoptosis, lipid metabolism, glucose homeostasis and viral infection Esteller (2011), Ameres and Zamore (2013) [1,2]. miRNAs are small (~22 nucleotides) non-coding RNAs that function as "rheostats" to simultaneously tweak the expression of multiple genes within a genetic network, resulting in dramatic functional modulation of biological processes. Although the last decade has brought the identification of miRNAs, their targets and function(s) in health and disease, there remains much to be deciphered from the human genome and its complexities in mechanistic regulation of entire genetic networks. These discoveries have opened the door to new and exciting avenues for therapeutic interventions to treat various pathological diseases, including cardiometabolic diseases such as atherosclerosis, diabetes and obesity. In a complex multi-factorial disease like atherosclerosis, many miRNAs have been shown to contribute to disease progression and may offer novel targets for future therapy. This article is part of a Special Issue entitled: MicroRNAs and lipid/energy metabolism and related diseases edited by Carlos Fernández-Hernando and Yajaira Suárez.

miRNAs and High-Density Lipoprotein metabolism

Altered lipoprotein metabolism plays a key role during atherogenesis. For over 50years, epidemiological data have fueled the proposal that HDL-cholesterol (HDL-c) in circulation is inversely correlated to cardiovascular risk. However, the atheroprotective role of HDL is currently the focus of much debate and remains an active field of research. The emerging picture from research in the past decade suggests that HDL function, rather than HDL-c content, is important in disease. Recent developments demonstrate that miRNAs play an important role in fine-tuning the expression of key genes involved in HDL biogenesis, lipidation, and clearance, as well as in determining the amounts of HDL-c in circulation. Thus, it has been proposed that miRNAs that affect HDL metabolism might be exploited therapeutically in patients. Whether HDL-based therapies, alone or in combination with LDL-based treatments (e.g. statins), provide superior outcomes in patients has been recently questioned by human genetics studies and clinical trials. The switch in focus from "HDL-cholesterol" to "HDL function" opens a new paradigm to understand the physiology and therapeutic potential of HDL, and to find novel modulators of cardiovascular risk. In this review we summarize the current knowledge on the regulation of HDL metabolism and function by miRNAs. This article is part of a Special Issue entitled: MicroRNAs and lipid/energy metabolism and related diseases edited by Carlos Fernández-Hernando and Yajaira Suárez.

microRNAs in lipoprotein metabolism and cardiometabolic disorders

Circulating levels of low-density lipoprotein cholesterol (LDL), and high-density lipoprotein cholesterol (HDL) are two of the most important risk factors for the development of cardiovascular disease (CVD), the leading cause of death worldwide. Recently, miRNAs have emerged as critical regulators of cholesterol metabolism and promising therapeutic targets for the treatment of CVD. A great deal of work has established numerous miRNAs as important regulators of HDL metabolism. This includes miRNAs that target ABCA1, a critical factor for HDL biogenesis and reverse cholesterol transport (RCT), the process through which cells, including arterial macrophages, efflux cellular cholesterol for transport to and removal by the liver. The most well studied of these miRNAs, miR-33, has been demonstrated to target ABCA1, as well as numerous other genes involved in metabolic function and RCT, and therapeutic inhibition of miR-33 was found to increase HDL levels in mice and non-human primates. Moreover, numerous studies have demonstrated the beneficial effects of miR-33 inhibition or knockout on reducing atherosclerotic plaque burden. Even more recent work has identified miRNAs that regulate LDL cholesterol levels, including direct modulation of LDL uptake in the liver through targeting of the LDL receptor. Among these, inhibition of miR-128-1, miR-148a, or miR-185 was found to reduce plasma LDL levels, and inhibition of miR-185 was further demonstrated to reduce atherosclerotic plaque size in ApoE(-/-) mice. Due to their ability to target many different genes, miRNAs have the ability to mediate complex physiologic changes through simultaneous regulation of multiple interrelated pathways. Of particular importance for CVD, inhibition of miR-148a may prove an important therapeutic approach for combating dyslipidemia, as this has been demonstrated to both raise plasma HDL levels and lower LDL levels in mice by targeting both ABCA1 and LDLR, respectively. In this review we highlight recent advances in our understanding of how miRNAs regulate cholesterol metabolism and the development of atherosclerotic plaques and discuss the potential of anti-miRNA therapies for the treatment and prevention of CVD.

miRNA and cholesterol homeostasis

MicroRNAs (miRNAs) have recently emerged as a novel class of epigenetic regulators of gene expression. They are systemically involved in the control of lipid metabolism through a complex interactive mechanism that involves gene regulatory networks. Hence, they can contribute to defective lipid metabolism and metabolic diseases. Here, we review recent advances in the roles of lipid-sensing transcription factors in regulating miRNA gene networks, as well as miRNA expression and function in the regulation of cholesterol metabolism. This article is part of a Special Issue entitled: MicroRNAs and lipid/energy metabolism and related diseases edited by Carlos Fernández-Hernando and Yajaira Suárez.

Macrophage-mediated cholesterol handling in atherosclerosis

Formation of foam cells is a hallmark at the initial stages of atherosclerosis. Monocytes attracted by pro-inflammatory stimuli attach to the inflamed vascular endothelium and penetrate to the arterial intima where they differentiate to macrophages. Intimal macrophages phagocytize oxidized low-density lipoproteins (oxLDL). Several scavenger receptors (SR), including CD36, SR-A1 and lectin-like oxLDL receptor-1 (LOX-1), mediate oxLDL uptake. In late endosomes/lysosomes of macrophages, oxLDL are catabolysed. Lysosomal acid lipase (LAL) hydrolyses cholesterol esters that are enriched in LDL to free cholesterol and free fatty acids. In the endoplasmic reticulum (ER), acyl coenzyme A: cholesterol acyltransferase-1 (ACAT1) in turn catalyses esterification of cholesterol to store cholesterol esters as lipid droplets in the ER of macrophages. Neutral cholesteryl ester hydrolases nCEH and NCEH1 are involved in a secondary hydrolysis of cholesterol esters to liberate free cholesterol that could be then out-flowed from macrophages by cholesterol ATP-binding cassette (ABC) transporters ABCA1 and ABCG1 and SR-BI. In atherosclerosis, disruption of lipid homoeostasis in macrophages leads to cholesterol accumulation and formation of foam cells.

MicroRNAs in obesity-associated disorders

The emergence of a worldwide obesity epidemic has dramatically increased the prevalence of insulin resistance and metabolic syndrome, predisposing individuals to a greater risk for the development of non-alcoholic fatty liver disease, type II diabetes and atherosclerotic cardiovascular diseases. Current available pharmacological interventions combined with diet and exercise-based managements are still poorly effective for weight management, likely in part due to an incomplete understanding of regulatory mechanisms and pathways contributing to the systemic metabolic abnormalities under disturbed energy homeostasis. MicroRNAs, small non-coding RNAs that regulate posttranscriptional gene expression, have been increasingly described to influence shifts in metabolic pathways under various obesity-related disease settings. Here we review recent discoveries of the mechanistic role that microRNAs play in regulating metabolic functions in liver and adipose tissues involved in obesity associated disorders, and briefly discusses the potential candidates that are being pursued as viable therapeutic targets.

Mammalian hibernation and regulation of lipid metabolism: a focus on non-coding RNAs

Numerous species will confront severe environmental conditions by undergoing significant metabolic rate reduction. Mammalian hibernation is one such natural model of hypometabolism. Hibernators experience considerable physiological, metabolic, and molecular changes to survive the harsh challenges associated with winter. Whether as fuel source or as key signaling molecules, lipids are of primary importance for a successful bout of hibernation and their careful regulation throughout this process is essential. In recent years, a plethora of non-coding RNAs has emerged as potential regulators of targets implicated in lipid metabolism in diverse models. In this review, we introduce the general characteristics associated with mammalian hibernation, present the importance of lipid metabolism prior to and during hibernation, as well as discuss the potential relevance of non-coding RNAs such as miRNAs and lncRNAs during this process.

MicroRNAs and Noncoding RNAs in Hepatic Lipid and Lipoprotein Metabolism: Potential Therapeutic Targets of Metabolic Disorders

Noncoding RNAs and microRNAs (miRNAs) represent an important class of regulatory molecules that modulate gene expression. The role of miRNAs in diverse cellular processes such as cancer, apoptosis, cell differentiation, cardiac remodeling, and inflammation has been intensively explored. Recent studies further demonstrated the important roles of miRNAs and noncoding RNAs in modulating a broad spectrum of genes involved in lipid synthesis and metabolic pathways. This overview focuses on the role of miRNAs in hepatic lipid and lipoprotein metabolism and their potential as therapeutic targets for metabolic syndrome. This includes recent advances made in the understanding of their target pathways and the clinical development of miRNAs in lipid metabolic disorders.

The Regulation of Reverse Cholesterol Transport and Cellular Cholesterol Homeostasis by MicroRNAs

MicroRNAs (miRNAs) are small, non-coding RNAs that have the ability to post-transcriptionally regulate gene expression. Hundreds of miRNAs have been identified in humans and they are involved in the regulation of almost every process, including cholesterol transport, metabolism, and maintenance of cholesterol homeostasis. Because of their small size and their ability to very specifically regulate gene expression, miRNAs are attractive targets for the regulation of dyslipidemias and other lipid-related disorders. However, the complex interactions between miRNAs, transcription factors, and gene expression raise great potential for side effects as a result of miRNA overexpression or inhibition. Many dietary components can also target specific miRNAs, altering the expression of downstream genes. Therefore, much more research is necessary to fully understand the role(s) of each miRNA in the body and how they may be impacted by diet and health. The present review aims to summarize the known roles of miRNAs in the regulation of reverse cholesterol transport and the maintenance of cholesterol homeostasis, as well as the potential clinical consequences of their manipulation.

MicroRNA Levels as Prognostic Markers for the Differentiation Potential of Human Mesenchymal Stromal Cell Donors

The ability of human mesenchymal stromal/stem cells (hMSCs) to differentiate into various mesenchymal cell lineages makes them a promising cell source for the use in tissue repair strategies. Since the differentiation potential of hMSCs differs between donors, it is necessary to establish biomarkers for the identification of donors with high differentiation potential. In this study, we show that microRNA (miRNA) expression levels are effective for distinguishing donors with high differentiation potential from low differentiation potential. Twenty hMSC donors were initially tested for marker expression and differentiation potential. In particular, the chondrogenic differentiation potential was evaluated on the basis of histological matrix formation, mRNA expression levels of chondrogenic marker genes, and quantitative glycosaminoglycan deposition. Three donors out of twenty were identified as donors with high chondrogenic potential, whereas nine showed moderate and eight showed low chondrogenic potential. Expression profiles of miRNAs involved in chondrogenesis and cartilage homeostasis were used for the distinction between high-performance hMSCs and low-performance hMSCs. Global mRNA expression profiles of the donors before the onset of chondrogenic differentiation revealed minor differences in gene expression between low and high chondrogenic performers. However, analysis of miRNA expression during a 7-day differentiation period identified miR-210 and miR-630 as positive regulators of chondrogenesis. In contrast, miR-181 and miR-34a, which are negative regulators of chondrogenesis, were upregulated during differentiation in low-performing donors. In conclusion, profiling of hMSC donors for a specific panel of miRNAs may have a prognostic value for selecting donors with high differentiation potential to improve hMSC-based strategies for tissue regeneration.

snoRNA U17 regulates cellular cholesterol trafficking

Cholesterol is required for the growth and viability of mammalian cells and is an obligate precursor for steroid hormone synthesis. Using a loss-of-function screen for mutants with defects in intracellular cholesterol trafficking, a Chinese hamster ovary cell mutant with haploinsufficiency of the U17 snoRNA was isolated. U17 is an H/ACA orphan snoRNA, for which a function other than ribosomal processing has not previously been identified. Through expression profiling, we identified hypoxia-upregulated mitochondrial movement regulator (HUMMR) mRNA as a target that is negatively regulated by U17 snoRNA. Upregulation of HUMMR in U17 snoRNA-deficient cells promoted the formation of ER-mitochondrial contacts, decreasing esterification of cholesterol and facilitating cholesterol trafficking to mitochondria. U17 snoRNA and HUMMR regulate mitochondrial synthesis of steroids in vivo and are developmentally regulated in steroidogenic tissues, suggesting that the U17 snoRNA-HUMMR pathway may serve a previously unrecognized, physiological role in gonadal tissue maturation.

MicroRNA-101 overexpression by IL-6 and TNF-α inhibits cholesterol efflux by suppressing ATP-binding cassette transporter A1 expression

BACKGROUND

MicroRNAs play key roles in regulating cholesterol homeostasis. Here, we investigated the role of microRNA-101 (miR-101) in regulating ATP-binding cassette transporter A1 (ABCA1) expression and cholesterol efflux under non-inflammatory and inflammatory conditions in human THP-1-derived macrophages and HepG2 hepatoblastoma cells.

METHODS

The cell lines were transfected with one of four lentiviral vectors: miR-101, miR-101 control, anti-miR-101, or anti-miR-101 control. A luciferase reporter assay was used to examine miR-101 binding to the 3' untranslated region (UTR) of ABCA1. Western blotting was conducted to assess ABCA1 protein expression. Cells were loaded with BODIPY-cholesterol and stained with oil red O to assess cholesterol efflux.

RESULTS

The luciferase activity assay revealed that wild-type miR-101 binding at site 2 significantly repressed ABCA1 3' UTR activity, suggesting that miR-101 directly targets the ABCA1 mRNA at site 2. In both cell lines, Western blotting revealed that miR-101 expression negatively regulates ABCA1 protein expression and significantly suppresses cholesterol efflux to ApoA1 under both low-density lipoprotein (LDL) and non-LDL conditions, which was confirmed by pronounced lipid inclusions visible by oil red O staining. In HepG2 cells, both IL-6 and TNF-α treatments produced significant miR-101 overexpression; however, in THP-1-derived macrophages, only IL-6 treatment produced significant miR-101 overexpression. Anti-mir-101 transfection under both IL-6 and TNF-α treatment conditions led to ABCA1 upregulation, indicating that miR-101 expression represses ABCA1 expression under inflammatory conditions.

CONCLUSIONS

miR-101 promotes intracellular cholesterol retention under inflammatory conditions through suppressing ABCA1 expression and suggests that the miR-101-ABCA1 axis may play an intermediary role in the development of NAFLD and vascular atherosclerosis.

Cholesterol efflux and reverse cholesterol transport

Both alterations of lipid/lipoprotein metabolism and inflammatory events contribute to the formation of the atherosclerotic plaque, characterized by the accumulation of abnormal amounts of cholesterol and macrophages in the artery wall. Reverse cholesterol transport (RCT) may counteract the pathogenic events leading to the formation and development of atheroma, by promoting the high-density lipoprotein (HDL)-mediated removal of cholesterol from the artery wall. Recent in vivo studies established the inverse relationship between RCT efficiency and atherosclerotic cardiovascular diseases (CVD), thus suggesting that the promotion of this process may represent a novel strategy to reduce atherosclerotic plaque burden and subsequent cardiovascular events. HDL plays a primary role in all stages of RCT: (1) cholesterol efflux, where these lipoproteins remove excess cholesterol from cells; (2) lipoprotein remodeling, where HDL undergo structural modifications with possible impact on their function; and (3) hepatic lipid uptake, where HDL releases cholesterol to the liver, for the final excretion into bile and feces. Although the inverse association between HDL plasma levels and CVD risk has been postulated for years, recently this concept has been challenged by studies reporting that HDL antiatherogenic functions may be independent of their plasma levels. Therefore, assessment of HDL function, evaluated as the capacity to promote cell cholesterol efflux may offer a better prediction of CVD than HDL levels alone. Consistent with this idea, it has been recently demonstrated that the evaluation of serum cholesterol efflux capacity (CEC) is a predictor of atherosclerosis extent in humans.

Antisense oligonucleotides, microRNAs, and antibodies

The specificity of Watson-Crick base pairing and the development of several chemical modifications to oligonucleotides have enabled the development of novel drug classes for the treatment of different human diseases. This review focuses on promising results of recent preclinical or clinical studies on targeting HDL metabolism and function by antisense oligonucleotides and miRNA-based therapies. Although many hurdles regarding basic mechanism of action, delivery, specificity, and toxicity need to be overcome, promising results from recent clinical trials and recent approval of these types of therapy to treat dyslipidemia suggest that the treatment of HDL dysfunction will benefit from these unique clinical opportunities. Moreover, an overview of monoclonal antibodies (mAbs) developed for the treatment of dyslipidemia and cardiovascular disease and currently being tested in clinical studies is provided. Initial studies have shown that these compounds are generally safe and well tolerated, but ongoing large clinical studies will assess their long-term safety and efficacy.

miRNAs in atherosclerotic plaque initiation, progression, and rupture

Atherosclerosis is a chronic immune-inflammatory disorder that integrates multiple cell types and a diverse set of inflammatory mediators. miRNAs are emerging as important post-transcriptional regulators of gene expression in most, if not all, vertebrate cells, and constitute central players in many physiological and pathological processes. Rapidly accumulating experimental studies reveal their key role in cellular and molecular processes related to the development of atherosclerosis. We review current evidence for the involvement of miRNAs in early atherosclerotic lesion formation and in plaque rupture and erosion. We conclude with a perspective on the clinical relevance, therapeutic opportunities, and future challenges of miRNA biology in understanding the pathogenesis of this complex disease.

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.

Mechanistic Role of MicroRNAs in Coupling Lipid Metabolism and Atherosclerosis

MicroRNAs (miRNAs, miRs) represent a group of powerful and versatile posttranscriptional regulators of gene expression being involved in the fine control of a plethora of physiological and pathological processes. Besides their well-established crucial roles in the regulation of cell cycle, embryogenesis or tumorigenesis, these tiny molecules have also been shown to participate in the regulation of lipid metabolism. In particular, miRs orchestrate cholesterol and fatty acids synthesis, transport, and degradation and low-density and high-density lipoprotein (LDL and HDL) formation. It is thus not surprising that they have also been reported to affect the development and progression of several lipid metabolism-related disorders including liver steatosis and atherosclerosis. Mounting evidence suggests that miRs might represent important "posttranscriptional hubs" of lipid metabolism, which means that one miR usually targets 3'-untranslated regions of various mRNAs that are involved in different steps of one precise metabolic/signaling pathway, e.g., one miR targets mRNAs of enzymes important for cholesterol synthesis, degradation, and transport. Therefore, changes in the levels of one key miR affect various steps of one pathway, which is thereby promoted or inhibited. This makes miRs potent future diagnostic and even therapeutic tools for personalized medicine. Within this chapter, the most prominent microRNAs involved in lipid metabolism, e.g., miR-27a/b, miR-33/33*, miR-122, miR-144, or miR-223, and their intracellular and extracellular functions will be extensively discussed, in particular focusing on their mechanistic role in the pathophysiology of atherosclerosis. Special emphasis will be given on miR-122, the first microRNA currently in clinical trials for the treatment of hepatitis C and on miR-223, the most abundant miR in lipoprotein particles.

Association of plasma MiR-17-92 with dyslipidemia in patients with coronary artery disease

Circulating microRNAs (miRNAs) have already been proposed as sensitive and informative biomarkers for the diagnosis of multiple diseases. We investigated the miRNA expression patterns in plasma samples of patients with coronary artery disease (CAD) and explored the potential functions of certain miRNAs.Deep sequencing analysis was performed to determine the miRNA expression profiles using RNA samples isolated from 20 healthy subjects and 20 patients with CAD. Quantitative reverse transcription polymerase chain reaction was applied to confirm the differential expression of the miR-17-92 cluster in 81 patients and 50 healthy volunteers. The association between the miR-17-92 cluster and clinical characteristics of patients with CAD were analyzed using SPSS16.0, SPSS Inc, Chicago, IL.Hundreds of miRNAs were detected and most members from the miR-17-92 cluster and its paralogs, including miR-18a, miR-92a, miR-106b, and miR-17, exhibited differential expression in the plasma of patients with CAD compared with controls. Moreover, these miRNAs were found widely related to the blood lipids in the patients with CAD, as miR-17 was positively correlated with total cholesterol, low-density lipoprotein cholesterol, and apolipoprotein B, while miR-92a was found positively related to high-density lipoprotein cholesterol (HDL-C) but negatively related to lipoprotein-a. Additionally, miR-106b was positively related to HDL-C and apolipoprotein A-I.Taken together with existing evidence from mechanistic studies, the current results of our study support a relationship between the miR-17-92 family and lipid metabolism, which merits further study.

Non-coding RNAs and lipid metabolism

A high percentage of the mammalian genome consists of non-coding RNAs (ncRNAs). Among ncRNAs two main subgroups have been identified: long ncRNAs (lncRNAs) and micro RNAs (miRNAs). ncRNAs have been demonstrated to play a role in a vast variety of diseases, since they regulate gene transcription and are involved in post-transcriptional regulation. They have the potential to function as molecular signals or as guides for transcription factors and to regulate epigenetic modifiers. In this literature review we have summarized data on miRNAs and lncRNAs and their involvement in dyslipidaemia, atherosclerosis, insulin resistance and adipogenesis. Outlining certain ncRNAs as disease biomarkers and/or therapeutic targets, and testing them in vivo, will be the next steps in future research.

microRNA regulation of lipoprotein metabolism

PURPOSE OF REVIEW

The objective of this review article is to summarize the recent findings about the importance of microRNAs (miRNAs) in regulating lipoprotein metabolism. We highlight the recent findings that uncover the importance of miRNAs in controlling plasma LDL-cholesterol (LDL-C) levels.

RECENT FINDINGS

In 2013, several studies reported a number of miRNAs that regulate plasma LDL-C levels, including miR-30c. In this review article, we discuss those miRNAs that modulate LDL-C levels and lipoprotein secretion. We also discuss the numerous studies that demonstrate the critical role of miRNAs in governing the many facets of HDL metabolism, such as the ATP transporters, ABCA1, and ABCG1, and the scavenger receptor, SRB1.

SUMMARY

The understanding of how these miRNAs modulate lipoprotein metabolism promises to reveal new therapeutic targets to treat dyslipidemias and related cardiovascular disorders.

Relevance of microRNA in metabolic diseases

Metabolic syndrome is a complex metabolic condition caused by abnormal adipose deposition and function, dyslipidemia and hyperglycemia, which affects >47 million American adults and ∼1 million children. Individuals with the metabolic syndrome have essentially twice the risk for developing cardiovascular disease (CVD) and Type 2 diabetes mellitus (T2D), compared to those without the syndrome. In the search for improved and novel therapeutic strategies, microRNAs (miRNA) have been shown to be interesting targets due to their regulatory role on gene networks controlling different crucial aspects of metabolism, including lipid and glucose homeostasis. More recently, the discovery of circulating miRNAs suggest that miRNAs may be involved in facilitating metabolic crosstalk between organs as well as serving as novel biomarkers of diseases, including T2D and atherosclerosis. These findings highlight the importance of miRNAs for regulating pathways that underlie metabolic diseases, and their potential as therapeutic targets for the development of novel treatments.

The emerging role of epigenetics in cardiovascular disease

There is a worldwide epidemic of cardiovascular diseases causing not only a public health issue but also accounting for trillions of dollars of healthcare expenditure. Studies pertaining to epidemiology, pathophysiology, molecular biology, gene identification and genetic linkage maps have been able to lay a strong foundation for both the diagnosis and treatment of cardiovascular medicine. Although the concept of 'epigenetics' is not recent, the term in current usage is extended from the initial concept of 'controlling developmental gene expression and signaling pathways in undifferentiated zygotes' to include heritable changes to gene expression that are not from differences in the genetic code. The impact of epigenetics in cardiovascular disease is now emerging as an important regulatory key player at different levels from pathophysiology to therapeutics. This review focuses on the emerging role of epigenetics in major cardiovascular medicine specialties such as coronary artery disease, heart failure, cardiac hypertrophy and diabetes.

microRNAs and HDL life cycle

miRNAs have emerged as important regulators of lipoprotein metabolism. Work over the past few years has demonstrated that miRNAs control the expression of most of the genes associated with high-density lipoprotein (HDL) metabolism, including the ATP transporters, ABCA1 and ABCG1, and the scavenger receptor SRB1. These findings strongly suggest that miRNAs regulate HDL biogenesis, cellular cholesterol efflux, and HDL cholesterol (HDL-C) uptake in the liver, thereby controlling all of the steps of reverse cholesterol transport. Recent work in animal models has demonstrated that manipulating miRNA levels including miR-33 can increase circulating HDL-C. Importantly, antagonizing miR-33 in vivo enhances the regression and reduces the progression of atherosclerosis. These findings support the idea of developing miRNA inhibitors for the treatment of dyslipidaemia and related cardiovascular disorders such as atherosclerosis. This review article focuses on how HDL metabolism is regulated by miRNAs and how antagonizing miRNA expression could be a potential therapy for treating cardiometabolic diseases.

MicroRNAs involved in the lipid metabolism and their possible implications for atherosclerosis development and treatment

Hyperlipidemia is a well-accepted risk factor in the development of atherosclerosis. MicroRNAs (miRNAs), a novel class of posttranscriptional regulators of gene expression, are involved in a variety of biological and pathological processes, including the regulation of the lipid metabolism and atherosclerosis. As our knowledge of miRNAs expands, a new class of "circulating miRNAs" has recently been described. It includes miRNAs which may be found in various bodily fluids packaged in microvesicles/exosomes, or bound to specific transporting proteins. High-density lipoprotein (HDL) particles have been identified as one such carrier. As this class of miRNAs likely plays a role in intercellular communication, it may also contribute to the atherosclerosis development and progression. This review aims to provide a comprehensive explanation of the roles of distinct miRNAs involved in the regulation of the lipid metabolism. These microRNAs seem to be promising therapeutic agents, as documented in rodents and African green monkeys. The second part of the review focuses on circulating miRNAs and their involvement in the atherosclerosis, especially as their levels have been described as altered in patients with dyslipidemia/hyperlipidemia. Special emphasis is placed on miRNAs transported in a complex with HDL particles and on those which may be considered potential atherosclerosis biomarkers.

MicroRNAs in Lipid Metabolism and Atherosclerosis

BACKGROUND: MicroRNAs (miRNA) are mediators of post-transcriptional gene expression that likely regulate most biological pathways and networks. The study of miRNAs is a rapidly emerging field; recent findings have revealed a significant role for miRNAs in atherosclerosis and lipoprotein metabolism.CONTENT: Results from recent studies demonstrated a role for miRNAs in endothelial integrity, macrophage inflammatory response to oxidized low-density lipoprotein, vascular smooth muscle cell proliferation and cholesterol synthesis. These mechanisms are all vital to the initiation and proliferation of atherosclerosis and cardiovascular disease. The importance of miRNAs has recently been recognized in cardiovascular sciences and miRNAs will likely become an integral part of our fundamental comprehension of atherosclerosis and lipoprotein metabolism. The extensive impact of miRNA mediated gene regulation and the relative ease of in vivo applicable modifications highlight the enormous potential of miRNA-based therapeutics in cardiovascular diseases.SUMMARY: miRNA studies in the field of lipid metabolism and atherosclerosis are in their infancy, and thus there is tremendous opportunity for discovery in this understudied area. The ability to target miRNAs in vivo through delivery of miRNA-mimics to enhance miRNA function, or antimiRNAs which inhibit miRNAs, has opened new avenues for the development of therapeutics for dyslipidemias and atherosclerosis, offers a unique approach to treating disease by modulating entire biological pathways. These exciting findings support the development of miRNA antagonists as potential therapeutics for the treatment of dyslipidaemia, atherosclerosis and related metabolic diseases.KEYWORDS: atherosclerosis, lipoprotein, HDL, miRNA

miR-206 controls LXRα expression and promotes LXR-mediated cholesterol efflux in macrophages

Liver X receptors (LXRα and LXRβ) are key transcription factors in cholesterol metabolism that regulate cholesterol biosynthesis/efflux and bile acid metabolism/excretion in the liver and numerous organs. In macrophages, LXR signaling modulates cholesterol handling and the inflammatory response, pathways involved in atherosclerosis. Since regulatory pathways of LXR transcription control are well understood, in the present study we aimed at identifying post-transcriptional regulators of LXR activity. MicroRNAs (miRs) are such post-transcriptional regulators of genes that in the canonical pathway mediate mRNA inactivation. In silico analysis identified miR-206 as a putative regulator of LXRα but not LXRβ. Indeed, as recently shown, we found that miR-206 represses LXRα activity and expression of LXRα and its target genes in hepatic cells. Interestingly, miR-206 regulates LXRα differently in macrophages. Stably overexpressing miR-206 in THP-1 human macrophages revealed an up-regulation and miR-206 knockdown led to a down-regulation of LXRα and its target genes. In support of these results, bone marrow-derived macrophages (BMDMs) from miR-206 KO mice also exhibited lower expression of LXRα target genes. The physiological relevance of these findings was proven by gain- and loss-of-function of miR-206; overexpression of miR-206 enhanced cholesterol efflux in human macrophages and knocking out miR-206 decreased cholesterol efflux from MPMs. Moreover, we show that miR-206 expression in macrophages is repressed by LXRα activation, while oxidized LDL and inflammatory stimuli profoundly induced miR-206 expression. We therefore propose a feed-back loop between miR-206 and LXRα that might be part of an LXR auto-regulatory mechanism to fine tune LXR activity.

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Functional differences of miR-206 in the liver and macrophages

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In the liver, miR-206 suppresses LXRα expression and signaling.

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In macrophages, miR-206 increases LXRα abundance and promotes cholesterol efflux.

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In macrophages, LXRα activation represses miR-206 expression.

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In macrophages, pro-inflammatory stimuli increase miR-206 expression.

Regulation of high-density lipoprotein metabolism

There is compelling evidence from human population studies that plasma levels of high-density lipoprotein (HDL) cholesterol correlate inversely with cardiovascular risk. Identification of this relationship has stimulated research designed to understand how HDL metabolism is regulated. The ultimate goal of these studies has been to develop HDL-raising therapies that have the potential to decrease the morbidity and mortality associated with atherosclerotic cardiovascular disease. However, the situation has turned out to be much more complex than originally envisaged. This is partly because the HDL fraction consists of multiple subpopulations of particles that vary in terms of shape, size, composition, and surface charge, as well as in their potential cardioprotective properties. This heterogeneity is a consequence of the continual remodeling and interconversion of HDL subpopulations by multiple plasma factors. Evidence that the remodeling of HDLs may impact on their cardioprotective properties is beginning to emerge. This serves to highlight the importance of understanding not only how the remodeling and interconversion of HDL subpopulations is regulated but also how these processes are affected by agents that increase HDL levels. This review provides an overview of what is currently understood about HDL metabolism and how the subpopulation distribution of these lipoproteins is regulated.

MicroRNA-27a/b regulates cellular cholesterol efflux, influx and esterification/hydrolysis in THP-1 macrophages

RATIONALE

Macrophage cholesterol homeostasis maintenance is the result of a balance between influx, endogenous synthesis, esterification/hydrolysis and efflux. Excessive accumulation of cholesterol leads to foam cell formation, which is the major pathology of atherosclerosis. Previous studies have shown that miR-27 (miR-27a and miR-27b) may play a key role in the progression of atherosclerosis.

OBJECTIVE

We set out to investigate the molecular mechanisms of miR-27a/b in intracellular cholesterol homeostasis.

METHODS AND RESULTS

In the present study, our results have shown that the miR-27 family is highly conserved during evolution, present in mammals and directly targets the 3' UTR of ABCA1, LPL, and ACAT1. apoA1, ABCG1 and SR-B1 lacking miR-27 bind sites should not be influenced by miR-27 directly. miR-27a and miR-27b directly regulated the expression of endogenous ABCA1 in different cells. Treatment with miR-27a and miR-27b mimics reduced apoA1-mediated cholesterol efflux by 33.08% and 44.61% in THP-1 cells, respectively. miR-27a/b also regulated HDL-mediated cholesterol efflux in THP-1 macrophages and affected the expression of apoA1 in HepG2 cells. However, miR-27a/b had no effect on total cellular cholesterol accumulation, but regulated the levels of cellular free cholesterol and cholesterol ester. We further found that miR-27a/b regulated the expression of LPL and CD36, and then affected the ability of THP-1 macrophages to uptake Dil-oxLDL. Finally, we identified that miR-27a/b regulated cholesterol ester formation by targeting ACAT1 in THP-1 macrophages.

CONCLUSION

These findings indicate that miR-27a/b affects the efflux, influx, esterification and hydrolysis of cellular cholesterol by regulating the expression of ABCA1, apoA1, LPL, CD36 and ACAT1.

Small RNA overcomes the challenges of therapeutic targeting of microsomal triglyceride transfer protein

The plasma level of apolipoprotein B (apoB) is among the strongest risk factors for coronary artery disease. Microsomal triglyceride transfer protein (MTP) plays a key role in the lipidation of nascent apoB and the secretion of apoB-containing lipoproteins enriched with triglycerides and is thus a promising target for the treatment of hyperlipidemia. Yet, the development of MTP inhibitors to lower plasma lipid concentrations has been hindered by adverse effects on hepatic steatosis. A study recently published in Nature Medicine identifies microRNA-30c (miR-30c) as a potent repressor of MTP that controls plasma apoB-containing lipoprotein levels, in addition to decreasing hepatic lipid synthesis through direct targeting of lysophosphatidylglycerol acyltransferase 1 (LPGAT1). These findings identify miR-30c as a novel therapeutic target that coordinately reduces lipid biosynthesis and lipoprotein secretion to suppress circulating apoB lipoproteins, while sparing the liver from steatosis.

MicroRNA control of high-density lipoprotein metabolism and function

Recent discoveries of microRNAs (miRNAs) that control high-density lipoprotein abundance and function have expanded our knowledge of the mechanisms regulating this important lipoprotein subclass. miRNAs have been shown to regulate gene networks that control high-density lipoprotein biogenesis and uptake, as well as discrete steps in the reverse cholesterol transport pathway. Furthermore, high-density lipoprotein itself has been shown to transport miRNAs selectively in health and disease, offering new possibilities of how this lipoprotein may alter gene expression in distal target cells and tissues. Collectively, these discoveries offer new insights into the mechanisms governing high-density lipoprotein metabolism and function and open new avenues for the development of therapeutics for the treatment of cardiovascular disease.

ARF-Like (ARL) Proteins

The ARF-like (ARL) proteins, within the ARF family, are a collection of functionally diverse GTPases that share extensive (>40 %) identity with the ARFs and each other and are assumed to share basic mechanisms of regulation and a very incompletely documented degree of overlapping regulators. At least four ARLs were already present in the last eukaryotic common ancestor, along with one ARF, and these have been expanded to >20 members in mammals. We know little about the majority of these proteins so our review will focus on those about which the most is known, including ARL1, ARL2, ARL3, ARL4s, ARL6, ARL13s, and ARFRP1. From this fragmentary information we extract some generalizations and conclusions regarding the sources and extent of specificity and functions of the ARLs.

MicroRNAs in the onset and development of cardiovascular disease

Physiological and pathological roles for small non-encoding miRNAs (microRNAs) in the cardiovascular system have recently emerged and are now widely studied. The discovery of widespread functions of miRNAs has increased the complexity of gene-regulatory processes and networks in both the cardiovascular system and cardiovascular diseases. Indeed, it has recently been shown that miRNAs are implicated in the regulation of many of the steps leading to the development of cardiovascular disease. These findings represent novel aspects in miRNA biology and, therefore, our understanding of the role of these miRNAs during the pathogenesis of cardiovascular disease is critical for the development of novel therapies and diagnostic interventions. The present review will focus on understanding how miRNAs are involved in the onset and development of cardiovascular diseases.

MicroRNAs and Lipoprotein Metabolism

MicroRNAs(miRNAs; miRs) are small, non-protein-coding RNAs that negatively regulate the gene expression. They bind to specific mRNAs and inhibit translation or promote mRNA degradation. Recently, some miRNAs have been shown to be involved in lipid homoeostasis. In particular, miR122 and miR-33 have a significant impact on lipid homeostasis and are potential therapeutic targets for treating lipid disorders and/or atherosclerosis. In this review, we describe the current understanding of the function of miRNAs in lipid homeostasis, with a focus on lipoprotein metabolism.

The role of microRNA-26b in human adipocyte differentiation and proliferation

Recent findings indicate that microRNAs (miRNAs) are involved in the regulatory network of adipogenesis and obesity. Thus far, only a few human miRNAs are known to function as adipogenic regulators, fanning interest in studies on the functional role of miRNAs during adipogenesis in humans. In a previous study, we used a microarray to assess miRNA expression during human preadipocyte differentiation. We found that expression of the miR-26b was increased in mature adipocytes. MiR-26b is an intronic miRNA located in the intron of CTDSP1 (carboxy terminal domain, RNA polymerase II, polypeptide A, small phosphatase 1). Target prediction and Renilla luciferase analyses revealed the phosphatase and tensin homolog gene (PTEN) as a putative target gene. In this study, we found that miR-26b was gradually upregulated during adipocyte differentiation. To understand the roles of miR-26b in adipogenesis, we adopted a loss-of-function approach to silence miR-26b stably in human preadipocytes. We found that miR-26b inhibition effectively suppressed adipocyte differentiation, as evidenced by decreased lipid droplets and the ability of miR-26b to decrease mRNA levels of adipocyte-specific molecular markers and triglyceride accumulation. Furthermore, the cell growth assay revealed that miR-26b inhibition promoted proliferation. Nevertheless, it had no effect on apoptosis. Taken together, these data indicate that miR-26b may be involved in adipogenesis and could be targeted for therapeutic intervention in obesity.

MicroRNAs and atherosclerosis

MicroRNAs (miRNAs) are small, ~22 nucleotide (nt) sequences of RNA that regulate gene expression at posttranscriptional level. These endogenous gene expression inhibitors were primarily described in cancer but recent exciting findings have also demonstrated a key role in cardiovascular diseases (CVDs), including atherosclerosis. MiRNAs control endothelial cell (EC), vascular smooth muscle cell (VSMC), and macrophage functions, and thereby regulate the progression of atherosclerosis. MiRNA expression is modulated by different stimuli involved in every stage of atherosclerosis, and conversely miRNAs modulates several pathways implicated in plaque development such as cholesterol metabolism. In the present review, we focus on the importance of miRNAs in atherosclerosis, and we further discuss their potential use as biomarkers and therapeutic targets in CVDs.

LXR is a negative regulator of glucose uptake in human adipocytes

AIMS/HYPOTHESIS

Obesity increases the risk of developing type 2 diabetes mellitus, characterised by impaired insulin-mediated glucose uptake in peripheral tissues. Liver X receptor (LXR) is a positive regulator of adipocyte glucose transport in murine models and a possible target for diabetes treatment. However, the levels of LXRα are increased in obese adipose tissue in humans. We aimed to investigate the transcriptome of LXR and the role of LXR in the regulation of glucose uptake in primary human adipocytes.

METHODS

The insulin responsiveness of human adipocytes differentiated in vitro was characterised, adipocytes were treated with the LXR agonist GW3965 and global transcriptome profiling was determined by microarray, followed by quantitative RT-PCR (qRT-PCR), western blot and ELISA. Basal and insulin-stimulated glucose uptake was measured and the effect on plasma membrane translocation of glucose transporter 4 (GLUT4) was assayed.

RESULTS

LXR activation resulted in transcriptional suppression of several insulin signalling genes, such as AKT2, SORBS1 and CAV1, but caused only minor changes (<15%) in microRNA expression. Activation of LXR impaired the plasma membrane translocation of GLUT4, but not the expression of its gene, SLC2A4. LXR activation also diminished insulin-stimulated glucose transport and lipogenesis in adipocytes obtained from overweight individuals. Furthermore, AKT2 expression was reduced in obese adipose tissue, and AKT2 and SORBS1 expression was inversely correlated with BMI and HOMA index.

CONCLUSIONS/INTERPRETATION

In contrast to murine models, LXR downregulates insulin-stimulated glucose uptake in human adipocytes from overweight individuals. This could be due to suppression of Akt2, c-Cbl-associated protein and caveolin-1. These findings challenge the idea of LXR as a drug target in the treatment of diabetes.

From evolution to revolution: miRNAs as pharmacological targets for modulating cholesterol efflux and reverse cholesterol transport

There has been strong evolutionary pressure to ensure that an animal cell maintains levels of cholesterol within tight limits for normal function. Imbalances in cellular cholesterol levels are a major player in the development of different pathologies associated to dietary excess. Although epidemiological studies indicate that elevated levels of high-density lipoprotein (HDL)-cholesterol reduce the risk of cardiovascular disease, recent genetic evidence and pharmacological therapies to raise HDL levels do not support their beneficial effects. Cholesterol efflux as the first and probably the most important step in reverse cholesterol transport is an important biological process relevant to HDL function. Small non-coding RNAs (microRNAs), post-transcriptional control different aspects of cellular cholesterol homeostasis including cholesterol efflux. miRNA families miR-33, miR-758, miR-10b, miR-26 and miR-106b directly modulates cholesterol efflux by targeting the ATP-binding cassette transporter A1 (ABCA1). Pre-clinical studies with anti-miR therapies to inhibit some of these miRNAs have increased cellular cholesterol efflux, reverse cholesterol transport and reduce pathologies associated to dyslipidemia. Although miRNAs as therapy have benefits from existing antisense technology, different obstacles need to be solved before we incorporate such research into clinical care. Here we focus on the clinical potential of miRNAs as therapeutic target to increase cholesterol efflux and reverse cholesterol transport as a new alternative to ameliorate cholesterol-related pathologies.

Role of microRNAs in the regulation of drug metabolism and disposition genes in diabetes and liver disease

INTRODUCTION

The pathogenesis of diabetes mellitus and nonalcoholic fatty liver disease (NAFLD) is complex, and the underlying molecular mechanisms are only partially understood.

AREAS COVERED

This review summarizes current knowledge of the role of microRNAs (miRNAs) in the regulation of drug absorption, distribution, metabolism, and excretion genes in the pathogenesis of diabetes and NAFLD. The literature search was performed using the PubMed database (up to February 2013).

EXPERT OPINION

miRNAs play a fundamental role in diabetes and NAFLD. This review focuses on the dysregulation of miRNAs involved in the regulation of drug metabolism and disposition in the pathogenesis of these metabolic syndromes. The evidence presented indicates that better understanding of the underlying molecular mechanisms associated with dysregulation of miRNAs controlling the cellular drug metabolizing system is of great importance not only from a scientific, but also from a clinical perspective. More importantly, an association between these metabolic disorders and miRNA dysregulation suggests that correcting miRNA expression by either their up-regulation or inhibition holds a promise for treating these metabolic syndrome and alleviating disease progression.