Liver X receptors as therapeutic targets in metabolism and atherosclerosis

Therapeutic potential of Mediator complex subunits in metabolic diseases

The multisubunit Mediator is an evolutionary conserved transcriptional coregulatory complex in eukaryotes. It is needed for the transcriptional regulation of gene expression in general as well as in a gene specific manner. Mediator complex subunits interact with different transcription factors as well as components of RNA Pol II transcription initiation complex and in doing so act as a bridge between gene specific transcription factors and general Pol II transcription machinery. Specific interaction of various Mediator subunits with nuclear receptors (NRs) and other transcription factors involved in metabolism has been reported in different studies. Evidences indicate that ligand-activated NRs recruit Mediator complex for RNA Pol II-dependent gene transcription. These NRs have been explored as therapeutic targets in different metabolic diseases; however, they show side-effects as targets due to their overlapping involvement in different signaling pathways. Here we discuss the interaction of various Mediator subunits with transcription factors involved in metabolism and whether specific interaction of these transcription factors with Mediator subunits could be potentially utilized as therapeutic strategy in a variety of metabolic diseases.

Impaired adipose expansion caused by liver X receptor activation is associated with insulin resistance in mice fed a high-fat diet

Liver X receptors (LXR) are deemed as potential drug targets for atherosclerosis, whereas a role in adipose tissue expansion and its relation to insulin sensitivity remains unclear. To assess the metabolic effects of LXR activation by the dual LXRα/β agonist T0901317, C57BL/6 mice fed a high-fat diet (HFD) were treated with T0901317 (30 mg/kg once daily by intraperitoneal injection) for 3 weeks. Differentiated 3T3-L1 adipocytes were used for analysing the effect of T0901317 on glucose uptake. The following results were obtained from this study. T0901317 reduced fat mass, accompanied by a massive fatty liver and lower serum adipokine levels in HFD mice. Increased adipocyte apoptosis was found in epididymal fat of T0901317-treated HFD mice. In addition, T0901317 treatment promoted basal lipolysis, but blunted the anti-lipolytic action of insulin. Furthermore, LXR activation antagonised PPARγ target genes in epididymal fat and PPARγ-PPRE-binding activity in 3T3-L1 adipocytes. Although the glucose tolerance was comparable to that in HFD mice, the insulin response during IPGTT was significantly higher and the insulin tolerance was significantly impaired in T0901317-treated HFD mice, indicating decreased insulin sensitivity by T0901317 administration, and which was further supported by impaired insulin signalling found in epididymal fat and decreased insulin-induced glucose uptake in 3T3-L1 adipocytes by T0901317 administration. In conclusion, these findings reveal that LXR activation impairs adipose expansion by increasing adipocyte apoptosis, lipolysis and antagonising PPARγ-mediated transcriptional activity, which contributes to decreased insulin sensitivity in whole body. The potential of LXR activation being a therapeutic target for atherosclerosis might be limited by the possibility of exacerbating insulin resistance.

Hataedock Treatment Has Preventive Therapeutic Effects in Atopic Dermatitis-Induced NC/Nga Mice under High-Fat Diet Conditions

This study investigated the preventive therapeutic effects of Hataedock (HTD) treatment on inflammatory regulation and skin protection in AD-induced NC/Nga mice under high-fat diet conditions. Before inducing AD, the extract of Coptidis Rhizoma and Glycyrrhiza uralensis was administered orally to the 3-week-old mice. After that, AD-like skin lesions were induced by applying DNFB. All groups except the control group were fed a high-fat diet freely. We identified the effects of HTD on morphological changes, cytokine release and the induction of apoptosis through histochemistry, immunohistochemistry, and TUNEL assay. HTD downregulated the levels of IL-4 and PKC but increased the levels of LXR. HTD also suppressed the mast cell degranulation and release of MMP-9, Substance P. The levels of TNF-α, p-IκB, iNOS, and COX-2 were also decreased. The upregulation of inflammatory cell's apoptosis is confirmed by our results as increase of apoptotic body and cleaved caspase-3 and decrease of Bcl-2. HTD also reduced edema, angiogenesis, and skin lesion inflammation. Our results indicate HTD suppresses various inflammatory response on AD-induced mice with obesity through the regulation of Th2 differentiation and the protection of lipid barrier. Therefore, HTD could be used as an alternative and preventive therapeutic approach in the management of AD.

The effect of the SIRT1 2827 A>G polymorphism, resveratrol, exercise, age and occupation in Turkish population with cardiovascular disease

Objective:

Cardiovascular disease (CVD) is the leading cause of death in Europe. One of the candidate molecule affecting epigenetic mechanisms of CVD is the SIRT1, a subclass of sirtuins, is located on the long arm of chromosome 10 (10q21.3). Particularly, the relation between 2827 A>G polymorphism of the SIRT1 positioned on exon 2, leading to conversion of histidine to arginine, and the formation of CVD is not known yet. One of the activator of SIRT1, resveratrol, is also known as a cardioprotective molecule. On the other hand, the parameters including exercise, occupation and age affect CVD. The aim of the present study was to investigate the effect of the rs144124002 (2827 A>G) single nucleotide polymorphisms (SNP) of SIRT1 and exercise-occupation-age parameters on CVD.

Methods:

SNP of SIRT1 were analyzed using DNA isolation, the polymerase chain reaction (PCR) and restriction fragment length polymorphism. To do so, large cohorts of CVD patients (n=293) and healthy controls (n=117) who directed Cardiology Department of Bezmialem Vakıf University, Bezmialem Vakıf University Hospital were used.

Results:

In this study, when we assessed CVD and control groups about 2827 A>G polymorphism, all individuals were determined as homozygous genotype. We found a positive effect between the modifications of resveratrol, exercise, age and occupation and CVD (OR=0.17; CI 95%, 0.1-0.2; p≤0.001).

Conclusion:

This is the first study demonstrating the correlation between the SIRT1 rs144124002 polymorphism and CVD in Turkish population.

The role of SIRT1 in ocular aging

The sirtuins are a highly conserved family of nicotinamide adenine dinucleotide (NAD+)-dependent histone deacetylases that helps regulate the lifespan of diverse organisms. The human genome encodes seven different sirtuins (SIRT1-7), which share a common catalytic core domain but possess distinct N- and C-terminal extensions. Dysfunction of some sirtuins have been associated with age-related diseases, such as cancer, type II diabetes, obesity-associated metabolic diseases, neurodegeneration, and cardiac aging, as well as the response to environmental stress. SIRT1 is one of the targets of resveratrol, a polyphenolic SIRT1 activator that has been shown to increase the lifespan and to protect various organs against aging. A number of animal studies have been conducted to examine the role of sirtuins in ocular aging. Here we review current knowledge about SIRT1 and ocular aging. The available data indicate that SIRT1 is localized in the nucleus and cytoplasm of cells forming all normal ocular structures, including the cornea, lens, iris, ciliary body, and retina. Upregulation of SIRT1 has been shown to have an important protective effect against various ocular diseases, such as cataract, retinal degeneration, optic neuritis, and uveitis, in animal models. These results suggest that SIRT1 may provide protection against diseases related to oxidative stress-induced ocular damage, including cataract, age-related macular degeneration, and optic nerve degeneration in glaucoma patients.

SIRT1 prevents atherosclerosis via liver‑X‑receptor and NF‑κB signaling in a U937 cell model

Atherosclerosis is a chronic immunoinflammatory disease associated with blood lipid disorders. Previous studies in mice have demonstrated that liver X receptor (LXR)‑ATP‑binding cassette (ABC) A1/ABCG1/C‑C chemokine receptor type 7 (CCR7) and nuclear factor κB (NF‑κB) signaling pathways are important for atherosclerotic plaque formation. In addition, Sirtuin 1 (SIRT1) has been reported as a key regulator in the protection from risk of atherosclerosis. However, the exact mechanism by which SIRT1 prevents atherosclerosis remains largely unknown. To explore the possible mechanisms, the expression of SIRT1 and the association between SIRT1, LXR and NF‑κB in the process of foam cell formation was investigated in an in vitro human mononuclear U937 cell line. Monocyte‑derived foam cells were induced by palmitate and Ox‑LDL treatment. Oil Red O staining revealed an accumulation of a large number of lipid droplets in foam cells. Results of reverse transcription polymerase chain reaction (RT-PCR) revealed that SIRT1 expression was downregulated during foam cell formation. In addition, the expression of LXRα and its targets, ABCA1, ABCG1 and CCR7, were downregulated. However, NF‑κB and its targets, tumor necrosis factor α (TNFα) and interleukin (IL)‑1β, were upregulated in foam cells. Following activation of SIRT1 by SRT1720, the expression of LXRα and its targets increased, whereas expression of NF‑κB and its targets decreased. Furthermore, the formation of foam cells was blocked. The SIRT1 inhibitor, nicotinamide, was found to eliminate the effects of SRT1720. Results of the present study indicate that SIRT1 may prevent the formation and progression of atherosclerosis by enhancing the LXR‑ABCA1/ABCG1/CCR7 and inhibiting the NF‑κB pathways.

Nuclear receptors and their selective pharmacologic modulators

Nuclear receptors are ligand-activated transcription factors and include the receptors for steroid hormones, lipophilic vitamins, sterols, and bile acids. These receptors serve as targets for development of myriad drugs that target a range of disorders. Classically defined ligands that bind to the ligand-binding domain of nuclear receptors, whether they are endogenous or synthetic, either activate receptor activity (agonists) or block activation (antagonists) and due to the ability to alter activity of the receptors are often termed receptor "modulators." The complex pharmacology of nuclear receptors has provided a class of ligands distinct from these simple modulators where ligands display agonist/partial agonist/antagonist function in a tissue or gene selective manner. This class of ligands is defined as selective modulators. Here, we review the development and pharmacology of a range of selective nuclear receptor modulators.

Cholesterol metabolite, 5-cholesten-3β-25-diol-3-sulfate, promotes hepatic proliferation in mice

Oxysterols are well known as physiological ligands of liver X receptors (LXRs). Oxysterols, 25-hydroxycholesterol (25HC) and 27-hydroxycholesterol as endogenous ligands of LXRs, suppress cell proliferation via LXRs signaling pathway. Recent reports have shown that sulfated oxysterol, 5-cholesten-3β-25-diol-3-sulfate (25HC3S) as LXRs antagonist, plays an opposite direction to oxysterols in lipid biosynthesis. The present report was to explore the effect and mechanism of 25HC3S on hepatic proliferation in vivo. Following administration, 25HC3S had a 48 h half life in the circulation and widely distributed in mouse tissues. Profiler™ PCR array and RTqPCR analysis showed that either exogenous or endogenous 25HC3S generated by overexpression of oxysterol sulfotransferase (SULT2B1b) plus administration of 25HC significantly up-regulated the proliferation gene expression of Wt1, Pcna, cMyc, cyclin A, FoxM1b, and CDC25b in a dose-dependent manner in liver while substantially down-regulating the expression of cell cycle arrest gene Chek2 and apoptotic gene Apaf1. Either exogenous or endogenous administration of 25HC3S significantly induced hepatic DNA replication as measured by immunostaining of the PCNA labeling index and was associated with reduction in expression of LXR response genes, such as ABCA1 and SREBP-1c. Synthetic LXR agonist T0901317 effectively blocked 25HC3S-induced hepatic proliferation.

CONCLUSIONS

25HC3S may be a potent regulator of hepatocyte proliferation and oxysterol sulfation may represent a novel regulatory pathway in liver proliferation via inactivating LXR signaling.

Cytosolic sulfotransferase 2B1b promotes hepatocyte proliferation gene expression in vivo and in vitro

Cytosolic sulfotransferase 2B1b (SULT2B1b) catalyzes the sulfation of 3β-hydroxysteroids and functions as a selective cholesterol and oxysterol sulfotransferase. Activation of liver X receptors (LXRs) by oxysterols has been known to be an antiproliferative factor. Overexpression of SULT2B1b impairs LXR's response to oxysterols, by which it regulates lipid metabolism. The aim of this study was to investigate in vivo and in vitro effects of SULT2B1b on liver proliferation and the underlying mechanisms. Primary rat hepatocytes and C57BL/6 mice were infected with adenovirus encoding SULT2B1b. Liver proliferation was determined by measuring the proliferating cell nuclear antigen (PCNA) immunostaining labeling index. The correlation between SULT2B1b and PCNA expression in mouse liver tissues was determined by double immunofluorescence. Gene expressions were evaluated by quantitative real-time PCR and Western blot analysis. SULT2B1b overexpression in mouse liver tissues increased PCNA-positive cells in a dose- and time-dependent manner. The increased expression of PCNA in mouse liver tissues was only observed in the SULT2B1b transgenic cells. Small interference RNA SULT2B1b significantly inhibited cell cycle regulatory gene expressions in primary rat hepatocytes. LXR activation by T0901317 effectively suppressed SULT2B1b-induced gene expression in vivo and in vitro. SULT2B1b may promote hepatocyte proliferation by inactivating oxysterol/LXR signaling.

Control of metabolism by nutrient-regulated nuclear receptors acting in the brain

Today, we are witnessing a rising incidence of obesity worldwide. This increase is due to a sedentary life style, an increased caloric intake and a decrease in physical activity. Obesity contributes to the appearance of type 2 diabetes, dyslipidemia and cardiovascular complications due to atherosclerosis, and nephropathy. Therefore, the development of new therapeutic strategies may become a necessity. Given the metabolism controlling properties of nuclear receptors in peripheral organs (such as liver, adipose tissues, pancreas) and their implication in various processes underlying metabolic diseases, they constitute interesting therapeutic targets for obesity, dyslipidemia, cardiovascular disease and type 2 diabetes. The recent identification of the central nervous system as a player in the control of peripheral metabolism opens new avenues to our understanding of the pathophysiology of obesity and type 2 diabetes and potential novel ways to treat these diseases. While the metabolic functions of nuclear receptors in peripheral organs have been extensively investigated, little is known about their functions in the brain, in particular with respect to brain control of energy homeostasis. This review provides an overview of the relationships between nuclear receptors in the brain, mainly at the hypothalamic level, and the central regulation of energy homeostasis. In this context, we will particularly focus on the role of PPARα, PPARγ, LXR and Rev-erbα.

SIRTUIN 1 gene polymorphisms are associated with cholesterol metabolism and coronary artery calcification in Japanese hemodialysis patients

OBJECTIVES

Sirtuin 1 (SIRT1), a longevity gene, protects cells against oxidative and genotoxic stress. This study aimed to investigate the association of SIRT 1 gene single-nucleotide polymorphisms, namely, rs7895833, rs7069102, and rs2273773 with lipid profiles and coronary artery calcification score in 219 Japanese hemodialysis (HD) patients.

METHODS

Genotyping of these polymorphisms was performed using polymerase chain reaction with confronting two-pair primers assay.

RESULTS

The A allele frequency of rs7895833 and G allele frequency of rs7069102 were significantly lower in HD patients (0.228 and 0.131, respectively) than those in 803 control subjects (general population) (0.289 and 0.181, respectively) (P = .010 and P = .012, respectively). However, the allele frequency of rs2273773 was not significantly different from that in the control subjects. Multivariate analysis adjusted for age and duration on HD demonstrated that the serum levels of total and low-density lipoprotein cholesterol were significantly high in G allele carriers of rs7069102 compared with CC genotype in male HD patients. Coronary artery calcification score was significantly high in C allele carriers of rs2273773 in all and male HD patients.

CONCLUSIONS

SIRT 1 polymorphisms, rs7069102 and rs2273773, are associated with abnormal cholesterol metabolism and coronary artery calcification, respectively, in Japanese HD patients, especially in males.

Piperine, a component of black pepper, inhibits adipogenesis by antagonizing PPARγ activity in 3T3-L1 cells

This study investigated the antiadipogenic activity of black pepper extract and its constituent piperine in 3T3-L1 preadipocytes as well as the underlying molecular mechanisms. Both black pepper extract and piperine, without affecting cytotoxicity, strongly inhibited the adipocyte differentiation of 3T3-L1 cells. The mRNA expression of the master adipogenic transcription factors, PPARγ, SREBP-1c, and C/EBPβ, was markedly decreased. Intriguingly, mRNA levels of PPARγ target genes were also down-regulated. Moreover, a luciferase reporter assay indicated that pipierine significantly represses the rosiglitazone-induced PPARγ transcriptional activity. Finally, GST-pull down assays demonstrated that piperine disrupts the rosiglitazone-dependent interaction between PPARγ and coactivator CBP. Genome-wide analysis using microarray further supports the role of piperine in regulating genes associated with lipid metabolism. Overall, these results suggest that piperine, a major component of black pepper, attenuates fat cell differentiation by down-regulating PPARγ activity as well as suppressing PPARγ expression, thus leading to potential treatment for obesity-related diseases.

Plasma sterol evidence for decreased absorption and increased synthesis of cholesterol in insulin resistance and obesity

BACKGROUND

The rise in LDL with egg feeding in lean insulin-sensitive (LIS) participants is 2- and 3-fold greater than in lean insulin-resistant (LIR) and obese insulin-resistant (OIR) participants, respectively.

OBJECTIVE

We determined whether differences in cholesterol absorption, synthesis, or both could be responsible for these differences by measuring plasma sterols as indexes of cholesterol absorption and endogenous synthesis.

DESIGN

Plasma sterols were measured by gas chromatography-mass spectrometry in a random subset of 34 LIS, 37 LIR, and 37 OIR participants defined by the insulin sensitivity index (S(I)) and by BMI criteria selected from a parent group of 197 participants. Cholestanol and plant sterols provide a measure of cholesterol absorption, and lathosterol provides a measure of cholesterol synthesis.

RESULTS

The mean (±SD) ratio of plasma total absorption biomarker sterols to cholesterol was 4.48 ± 1.74 in LIS, 3.25 ± 1.06 in LIR, and 2.82 ± 1.08 in OIR participants. After adjustment for age and sex, the relations of the absorption sterol-cholesterol ratios were as follows: LIS > OIR (P < 0.001), LIS > LIR (P < 0.001), and LIR > OIR (P = 0.11). Lathosterol-cholesterol ratios were 0.71 ± 0.32 in the LIS participants, 0.95 ± 0.47 in the LIR participants, and 1.29 ± 0.55 in the OIR participants. After adjustment for age and sex, the relations of lathosterol-cholesterol ratios were as follows: LIS < OIR (P < 0.001), LIS < LIR (P = 0.03), and LIR < OIR (P = 0.002). Total sterol concentrations were positively associated with S(I) and negatively associated with obesity, whereas lathosterol correlations were the opposite.

CONCLUSIONS

Cholesterol absorption was highest in the LIS participants, whereas cholesterol synthesis was highest in the LIR and OIR participants. Therapeutic diets for hyperlipidemia should emphasize low-cholesterol diets in LIS persons and weight loss to improve S(I) and to decrease cholesterol overproduction in LIR and OIR persons.

Mammalian Sirt1: insights on its biological functions

Sirt1 (member of the sirtuin family) is a nicotinamide adenosine dinucleotide (NAD)-dependent deacetylase that removes acetyl groups from various proteins. Sirt1 performs a wide variety of functions in biological systems. The current review focuses on the biological functions of Sirt1 in obesity-associated metabolic diseases, cancer, adipose tissue, aging, cellular senescence, cardiac aging and stress, prion-mediated neurodegeneration, inflammatory signaling in response to environmental stress, development and placental cell survival.

Liver X receptor α and farnesoid X receptor are major transcriptional regulators of OATP1B1

Organic anion transporting polypeptide 1B1 (OATP1B1) is a liver-enriched transporter involved in the hepatocellular uptake of many endogenous molecules and several structurally divergent drugs in clinical use. Although OATP1B1 coding region polymorphisms are known to make an impact on substrate drug disposition in humans, little is known regarding the mechanisms underlying the transcriptional regulation of this transporter. In this study, we note that messenger RNA (mRNA) expression of OATP1B1 in a large human liver bank exhibited marked interindividual variability that was not associated with coding region polymorphisms. Accordingly, we hypothesized that such variability in expression is reflective of nuclear receptor-mediated transcriptional regulation of this transporter. We tested prototypical ligands for the nuclear receptors pregnane X receptor (PXR), constitutive androstane receptor (CAR), liver X receptor (LXR) α, and farnesoid X receptor (FXR) in a human hepatoma-derived cell line and noted induction of OATP1B1 mRNA when the cells were treated with LXRα or FXR ligands. To confirm a direct role for LXRα and FXR to OATP1B1 expression, we performed detailed promoter analysis and cell-based reporter gene assays resulting in the identification of two functional FXR response elements and one LXRα response element. The direct interaction between nuclear receptors with the identified response elements was assessed using chromatin immunoprecipitation assays. Using isolated primary human hepatocytes, we show that LXRα or FXR agonists, but not PXR or CAR agonists, are capable of OATP1B1 induction.

CONCLUSION

We note that OATP1B1 transcriptional regulation is under dual nuclear receptor control through the oxysterol sensing LXRα and the bile acid sensor FXR. Accordingly, the interplay between OATP1B1 and nuclear receptors may play an important and heretofore unrecognized role during cholestasis, drug-induced liver injury, and OATP1B1 induction-related drug interactions.

Gut hormones and the brain

The dialogue between gut hormone, bile acids, and the brain plays an important role in energy homeostasis and the onset of Type 2 diabetes mellitus (T2DM). The present review focuses on: (i) bile acid metabolism and the role of bile acids in the regulation of both glucose homeostasis and the control of hypercholesterolemia; (ii) the role of gut hormones in energy homeostasis; and (iii) translation of the pathophysiology of bile acids and gut hormones into clinical practice. Although definitive mechanisms of action of gut hormones and bile acids have not been elucidated completely, these concepts allow us to understand several pharmacological interventions in the treatment of T2DM. Results from further clinical studies with related therapies will help us determine the role of these treatments in the management of energy homeostasis.

LXR{beta} is the dominant LXR subtype in skeletal muscle regulating lipogenesis and cholesterol efflux

Liver X receptors (LXRs) are important regulators of cholesterol, lipid, and glucose metabolism and have been extensively studied in liver, macrophages, and adipose tissue. However, their role in skeletal muscle is poorly studied and the functional role of each of the LXRalpha and LXRbeta subtypes in skeletal muscle is at present unknown. To study the importance of each of the receptor subtypes, myotube cultures derived from wild-type (WT) and LXRalpha and LXRbeta knockout (KO) mice were established. The present study showed that treatment with the LXR agonist T0901317 increased lipogenesis and apoA1-dependent cholesterol efflux in LXRalpha KO and WT myotubes but not in LXRbeta KO cells. The functional studies were confirmed by T0901317-induced increase in mRNA levels of LXR target genes involved in lipid and cholesterol metabolism in myotubes established from WT and LXRalpha KO mice, whereas only minor changes were observed for these genes in myotubes from LXRbeta KO mice. Gene expression analysis using microarrays showed that very few genes other than the classical, well-known LXR target genes were regulated by LXR in skeletal muscle. The present study also showed that basal glucose uptake was increased in LXRbeta KO myotubes compared with WT myotubes, suggesting a role for LXRbeta in glucose metabolism in skeletal muscle. In conclusion, LXRbeta seems to be the main LXR subtype regulating lipogenesis and cholesterol efflux in skeletal muscle.

The probioticLactobacillus acidophilusreduces cholesterol absorption through the down-regulation of Niemann-Pick C1-like 1 in Caco-2 cells

Elevated blood cholesterol is an important risk factor associated with atherosclerosis and CHD. The search for mediators that fine tune cholesterol homeostasis has recognised probiotics as being potentially beneficial. Here, we present data describing bacterial regulation of Niemann-Pick C1-like 1 (NPC1L1), which, when weakly expressed, results in a marked reduction in intestinal absorption of cholesterol. The probioticLactobacillus acidophilusATCC 4356 reduced NPC1L1 gene expression and inhibited the cellular uptake of micellar cholesterol in Caco-2 cells. Soluble effector molecules secreted by ATCC 4356 were shown to be responsible for the decrease in NPC1L1. Furthermore, ATCC 4356 mediated this effect partly through the liver X receptors (LXR). The role of NPC1L1 and the LXR in cholesterol metabolism underscores the basis for the use of probiotics, such as ATCC 4356, in managing hypercholesterolaemia.

Reverse cholesterol transport in type 2 diabetes mellitus

High-density lipoprotein (HDL) plays an important protective role against atherosclerosis, and the anti-atherogenic properties of HDL include the promotion of cellular cholesterol efflux and reverse cholesterol transport (RCT), as well as antioxidant, anti-inflammatory and anticoagulant effects. RCT is a complex pathway, which transports cholesterol from peripheral cells and tissues to the liver for its metabolism and biliary excretion. The major steps in the RCT pathway include the efflux of free cholesterol mediated by cholesterol transporters from cells to the main extracellular acceptor HDL, the conversion of free cholesterol to cholesteryl esters and the subsequent removal of cholesteryl ester in HDL by the liver. The efficiency of RCT is influenced by the mobilization of cellular lipids for efflux and the intravascular remodelling and kinetics of HDL metabolism. Despite the increased cardiovascular risk in people with type 2 diabetes, current knowledge on RCT in diabetes is limited. In this article, abnormalities in RCT in type 2 diabetes mellitus and therapeutic strategies targeting HDL and RCT will be reviewed.

Liver X receptor is a regulator of orphan nuclear receptor NOR-1 gene transcription in adipocytes

OBJECTIVE

The liver X receptors (LXRs) are ligand-activated nuclear transcription factors that have been shown to play major roles in lipid, glucose and cholesterol metabolism. Recently, members of the NR4A orphan nuclear receptor family have also been shown to regulate the expression of important genes in metabolically active tissues such as liver, adipose and skeletal muscle. Here, we investigated the role of LXRs to regulate the expression of the nuclear receptor NOR-1 (neuron-derived orphan receptor-1) in adipocytes.

APPROACH

White and brown adipose tissues from wild-type, LXRalpha-/-- and LXRalpha:beta-deficient mice were collected from animals at room temperature or following cold exposure to measure NOR-1 mRNA. The expression of NOR-1 and its promoter activity in response to LXR ligands were determined in cultured primary brown adipocytes or mouse embryo fibroblasts derived from wild-type or LXRalpha-/- mice differentiated into adipocytes.

RESULTS

In LXRalpha-/-- and LXRalpha:beta-deficient adipocytes, basal levels of NOR-1 were significantly reduced while retaining an equivalent proportional induction by beta-adrenergic agonists. This reduced basal expression of NOR-1 in adipose tissue from LXR-deficient mice is a cell-autonomous event as it was also preserved in adipocytes differentiated from mouse embryo fibroblasts derived from these mice. In cultured primary brown adipocytes or cell lines, the expression of NOR-1 increased in response to an LXR agonist. A DNA sequence element (DR-4) capable of binding LXRs was found at -997 bp of the NOR-1 promoter, which was shown to be functional by promoter reporter gene activity, gel shift and chromatin immunoprecipitation assays.

CONCLUSION

These data describe a new role for LXR to regulate NOR-1 gene expression in adipocytes and demonstrate that these two nuclear receptors have an interdependent regulatory relationship, in addition to each being involved in the control of metabolic fuel usage.