Liver LXRα expression is crucial for whole body cholesterol homeostasis and reverse cholesterol transport in mice

Feedback modulation of cholesterol metabolism by the lipid-responsive non-coding RNA LeXis

Liver X receptors (LXRs) are transcriptional regulators of cellular and systemic cholesterol homeostasis. Under conditions of excess cholesterol, LXR activation induces the expression of several genes involved in cholesterol efflux, facilitates cholesterol esterification by promoting fatty acid synthesis, and inhibits cholesterol uptake by the low-density lipoprotein receptor. The fact that sterol content is maintained in a narrow range in most cell types and in the organism as a whole suggests that extensive crosstalk between regulatory pathways must exist. However, the molecular mechanisms that integrate LXRs with other lipid metabolic pathways are incompletely understood. Here we show that ligand activation of LXRs in mouse liver not only promotes cholesterol efflux, but also simultaneously inhibits cholesterol biosynthesis. We further identify the long non-coding RNA LeXis as a mediator of this effect. Hepatic LeXis expression is robustly induced in response to a Western diet (high in fat and cholesterol) or to pharmacological LXR activation. Raising or lowering LeXis levels in the liver affects the expression of genes involved in cholesterol biosynthesis and alters the cholesterol levels in the liver and plasma. LeXis interacts with and affects the DNA interactions of RALY, a heterogeneous ribonucleoprotein that acts as a transcriptional cofactor for cholesterol biosynthetic genes in the mouse liver. These findings outline a regulatory role for a non-coding RNA in lipid metabolism and advance our understanding of the mechanisms that coordinate sterol homeostasis.

Overexpression of Peroxiredoxin 4 Affects Intestinal Function in a Dietary Mouse Model of Nonalcoholic Fatty Liver Disease

Background

Accumulating evidence has shown that methionine- and choline-deficient high fat (MCD+HF) diet induces the development of nonalcoholic fatty liver disease (NAFLD), in which elevated reactive oxygen species play a crucial role. We have reported that peroxiredoxin 4 (PRDX4), a unique secretory member of the PRDX antioxidant family, protects against NAFLD progression. However, the detailed mechanism and potential effects on the intestinal function still remain unclear.

Methods & Results

Two weeks after feeding mice a MCD+HF diet, the livers of human PRDX4 transgenic (Tg) mice exhibited significant suppression in the development of NAFLD compared with wild-type (WT) mice. The serum thiobarbituric acid reactive substances levels were significantly lower in Tg mice. In contrast, the Tg small intestine with PRDX4 overexpression showed more suppressed shortening of total length and villi height, and more accumulation of lipid in the jejunum, along with lower levels of dihydroethidium binding. The enterocytes exhibited fewer apoptotic but more proliferating cells, and inflammation was reduced in the mucosa. Furthermore, the small intestine of Tg mice had significantly higher expression of cholesterol absorption-regulatory factors, including liver X receptor-α, but lower expression of microsomal triglyceride-transfer protein.

Conclusion

Our present data provide the first evidence of the beneficial effects of PRDX4 on intestinal function in the reduction of the severity of NAFLD, by ameliorating oxidative stress-induced local and systemic injury. We can suggest that both liver and intestine are spared, to some degree, by the antioxidant properties of PRDX4.

An Essential Role for Liver ERα in Coupling Hepatic Metabolism to the Reproductive Cycle

Lipoprotein synthesis is controlled by estrogens, but the exact mechanisms underpinning this regulation and the role of the hepatic estrogen receptor α (ERα) in cholesterol physiology are unclear. Utilizing a mouse model involving selective ablation of ERα in the liver, we demonstrate that hepatic ERα couples lipid metabolism to the reproductive cycle. We show that this receptor regulates the synthesis of cholesterol transport proteins, enzymes for lipoprotein remodeling, and receptors for cholesterol uptake. Additionally, ERα is indispensable during proestrus for the generation of high-density lipoproteins efficient in eliciting cholesterol efflux from macrophages. We propose that a specific interaction with liver X receptor α (LXRα) mediates the broad effects of ERα on the hepatic lipid metabolism.

Cucumis melo ssp. Agrestis var. Agrestis Ameliorates High Fat Diet Induced Dyslipidemia in Syrian Golden Hamsters and Inhibits Adipogenesis in 3T3-L1 Adipocytes

Background:

Cucumis melo ssp. agrestis var. agrestis (CMA) is a wild variety of C. melo. This study aimed to explore anti-dyslipidemic and anti-adipogenic potential of CMA.

Materials and Methods:

For initial anti-dyslipidemic and antihyperglycemic potential of CMA fruit extract (CMFE), male Syrian golden hamsters were fed a chow or high-fat diet with or without CMFE (100 mg/kg). Further, we did fractionation of this CMFE into two fractions namely; CMA water fraction (CMWF) and CMA hexane fraction (CMHF). Phytochemical screening was done with liquid chromatography-mass spectrometry LC- (MS)/MS and direct analysis in real time-MS to detect active compounds in the fractions. Further, high-fat diet fed dyslipidemic hamsters were treated with CMWF and CMHF at 50 mg/kg for 7 days.

Results:

Oral administration of CMFE and both fractions (CMWF and CMHF) reduced the total cholesterol, triglycerides, low‐density lipoprotein cholesterol, and very low‐density lipoprotein-cholesterol levels in high fat diet-fed dyslipidemic hamsters. CMHF also modulated expression of genes involved in lipogenesis, lipid metabolism, and reverse cholesterol transport. Standard biochemical diagnostic tests suggested that neither of fractions causes any toxicity to hamster liver or kidneys. CMFE and CMHF also decreased oil-red-O accumulation in 3T3-L1 adipocytes.

Conclusion:

Based on these results, it is concluded that CMA possesses anti-dyslipidemic and anti-hyperglycemic activity along with the anti-adipogenic activity.

SUMMARY

The oral administration of Cucumis melo agrestis fruit extract (CMFE) and its fractions (CMWF and CMHF) improved serum lipid profile in HFD fed dyslipidemic hamsters.

CMFE, CMWF and CMHF significantly attenuated body weight gain and eWAT hypertrophy.

The CMHF decreased lipogenesis in both liver and adipose tissue.

CMFE and CMHF also inhibited adipogenesis in 3T3-L1 adipocytes.

Abbreviation used: CMA: Cucumis melo ssp. agrestis var. agrestis, CMFE: CMA fruit extract, CMWF: CMA water fraction, CMHF: CMA hexane fraction, FAS: Fatty acid synthase, SREBP1c: Sterol regulatory element binding protein 1c, ACC: Acetyl CoA carboxylase, LXR α: Liver X receptor α.

Nuclear receptors and nonalcoholic fatty liver disease

Nuclear receptors are transcription factors which sense changing environmental or hormonal signals and effect transcriptional changes to regulate core life functions including growth, development, and reproduction. To support this function, following ligand-activation by xenobiotics, members of subfamily 1 nuclear receptors (NR1s) may heterodimerize with the retinoid X receptor (RXR) to regulate transcription of genes involved in energy and xenobiotic metabolism and inflammation. Several of these receptors including the peroxisome proliferator-activated receptors (PPARs), the pregnane and xenobiotic receptor (PXR), the constitutive androstane receptor (CAR), the liver X receptor (LXR) and the farnesoid X receptor (FXR) are key regulators of the gut:liver:adipose axis and serve to coordinate metabolic responses across organ systems between the fed and fasting states. Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease and may progress to cirrhosis and even hepatocellular carcinoma. NAFLD is associated with inappropriate nuclear receptor function and perturbations along the gut:liver:adipose axis including obesity, increased intestinal permeability with systemic inflammation, abnormal hepatic lipid metabolism, and insulin resistance. Environmental chemicals may compound the problem by directly interacting with nuclear receptors leading to metabolic confusion and the inability to differentiate fed from fasting conditions. This review focuses on the impact of nuclear receptors in the pathogenesis and treatment of NAFLD. Clinical trials including PIVENS and FLINT demonstrate that nuclear receptor targeted therapies may lead to the paradoxical dissociation of steatosis, inflammation, fibrosis, insulin resistance, dyslipidemia and obesity. Novel strategies currently under development (including tissue-specific ligands and dual receptor agonists) may be required to separate the beneficial effects of nuclear receptor activation from unwanted metabolic side effects. The impact of nuclear receptor crosstalk in NAFLD is likely to be profound, but requires further elucidation. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Hepatocyte-specific Keap1 deletion reduces liver steatosis but not inflammation during non-alcoholic steatohepatitis development

Generation of reactive oxygen species (ROS) in response to fatty acids accumulation has been classically proposed as a possible "second hit" triggering progression from simple steatosis to non-alcoholic steatohepatitis (NASH). In this study we challenged hepatocyte-specific Keap1 knockout mice (Keap1(Δhepa)) and littermate Cre- controls (Keap1(fx/fx)) with two different diet models of NASH in order to evaluate the effects of the anti-oxidant transcription factor Nrf2 over-activation on hepatic metabolism and disease progression. After 4 weeks of MCD diet the liver/body weight ratio of Keap1(Δhepa) mice was significantly higher compared to littermate controls with no differences in total body weight. Strikingly, liver histology revealed a dramatic reduction of lipid droplets confirmed by a decreased content of intra-hepatic triglycerides in Keap1(Δhepa) compared to controls. In parallel to reduced expression of genes involved in lipid droplet formation, protein expression of Liver X Receptor (LXRα/β) and Peroxisome proliferator-activated receptor α (PPARα) was significantly decreased. In contrast, genes involved in mitochondrial lipid catabolism were markedly up-regulated in Keap1(Δhepa) livers. A similar phenotype characterized by inhibition of lipogenesis in favor of increased mitochondrial catabolic activity was also observed after 13 weeks of western diet administration. MCD-induced apoptosis was significantly dampened in Keap1(Δhepa) compared to Keap1(fx/fx) as detected by TUNEL, cleaved caspase-3 and Bcl-2 protein expression analyses. However, no differences in inflammatory F4/80- and CD11b-positive cells and pro-fibrogenic genes were detected between the two groups. Although hepatic lack of Keap1 did not ameliorate inflammation, the resulting constitutive Nrf2 over-activation in hepatocytes strongly reduced hepatic steatosis via enhanced lipid catabolism and repressed de novo lipogenesis during murine NASH development.

Liver X receptors as regulators of metabolism

The liver X receptors (LXR) are crucial regulators of metabolism. After ligand binding, they regulate gene transcription and thereby mediate changes in metabolic pathways. Modulation of LXR and their downstream targets has appeared to be a promising treatment for metabolic diseases especially atherosclerosis and cholesterol metabolism. However, the complexity of LXR action in various metabolic tissues and the liver side effect of LXR activation have slowed down the interest for LXR drugs. In this review, we summarized the role of LXR in the main metabolically active tissues with a special focus on obesity and associated diseases in mammals. We will also discuss the dual interplay between the two LXR isoforms suggesting that they may collaborate to establish a fine and efficient system for the maintenance of metabolism homeostasis.

Emerging role of liver X receptors in cardiac pathophysiology and heart failure

Liver X receptors (LXRs) are master regulators of metabolism and have been studied for their pharmacological potential in vascular and metabolic disease. Besides their established role in metabolic homeostasis and disease, there is mounting evidence to suggest that LXRs may exert direct beneficial effects in the heart. Here, we aim to provide a conceptual framework to explain the broad mode of action of LXRs and how LXR signaling may be an important local and systemic target for the treatment of heart failure. We discuss the potential role of LXRs in systemic conditions associated with heart failure, such as hypertension, diabetes, and renal and vascular disease. Further, we expound on recent data that implicate a direct role for LXR activation in the heart, for its impact on cardiomyocyte damage and loss due to ischemia, and effects on cardiac hypertrophy, fibrosis, and myocardial metabolism. Taken together, the accumulating evidence supports the notion that LXRs may represent a novel therapeutic target for the treatment of heart failure.

Loss of the liver X receptor LXRα/β in peripheral sensory neurons modifies energy expenditure

Peripheral neural sensory mechanisms play a crucial role in metabolic regulation but less is known about the mechanisms underlying vagal sensing itself. Recently, we identified an enrichment of liver X receptor alpha and beta (LXRα/β) in the nodose ganglia of the vagus nerve. In this study, we show mice lacking LXRα/β in peripheral sensory neurons have increased energy expenditure and weight loss when fed a Western diet (WD). Our findings suggest that the ability to metabolize and sense cholesterol and/or fatty acids in peripheral neurons is an important requirement for physiological adaptations to WDs.

DOI:http://dx.doi.org/10.7554/eLife.06667.001

The vagus nerves run from the brainstem to the heart and the digestive system and help to control several processes including digestion and heart rate. Because of their role in regulating food intake, these nerves are attractive targets for scientists hoping to develop treatments for obesity.

There are two types of fat tissue found in mammals: white fat, which is used as an energy store and makes up most of the extra fat seen in obese individuals; and brown fat, which can generate body heat. The vagus nerves monitor fat and cholesterol levels in the body via receptor proteins that respond to messages sent from the fat tissues and the liver. Previous research unexpectedly found that mice genetically engineered to lack these receptor proteins—called LXRα and LXRβ—do not become obese even when fed a high-fat, high-cholesterol diet that would make normal mice gain excessive weight.

Mansuy-Aubert et al. have now investigated in more detail why mice without these receptor proteins are resistant to obesity. When fed a high-fat, high-cholesterol diet, mice that lacked the LXRα and LXRβ receptors in sensory neurons had higher cholesterol levels in their nerve cells than normal mice on the same diet. Mice lacking these receptors also burned more energy and gained less weight than normal mice.

Next, Mansuy-Aubert et al. examined fat tissue from both types of mice. This revealed that the heat-generating brown fat was more active in mice lacking the LXRα and LXRβ receptors. Some of the white fat in these mice had also become more like brown fat, allowing the mice to burn more energy and so gain less weight.

In many Western countries, many people also eat a diet that is high in fat and cholesterol. This raises the possibility that drugs that block the LXRα and LXRβ receptors in sensory neurons in humans could help to treat or prevent obesity, although further work will be needed to investigate this.

DOI:http://dx.doi.org/10.7554/eLife.06667.002

Inhibition of ABCA1 protein degradation promotes HDL cholesterol efflux capacity and RCT and reduces atherosclerosis in mice

ABCA1 plays a key role in the initial lipidation of apoA-I, which generates circulating HDL cholesterol. Whereas it is known that the transcriptional upregulation of ABCA1 promotes HDL formation and reverse cholesterol transport (RCT), it is not known how the inhibition of ABCA1 protein degradation impacts HDL function. Employing the small molecule triacetyl-3-hydroxyphenyladenosine (IMM-H007), we determined how the attenuation of ABCA1 protein degradation affects HDL cholesterol efflux capacity, RCT, and atherosclerotic lesion formation. Pulse-chase analysis revealed that IMM-H007 inhibits ABCA1 degradation and facilitates its cell-surface localization in macrophages, and additional studies in macrophages showed that IMM-H007 thereby promotes cholesterol efflux. IMM-H007 treatment of Paigen diet-fed mice caused an increase in circulating HDL level, it increased the cholesterol efflux capacity of HDL, and it enhanced in vivo RCT from macrophages to the plasma, liver, and feces. Furthermore, ABCA1 degradation suppression by IMM-H007 reduced atherosclerotic plaque formation in apoE(-/-) mice. Thus, via effects on both ABCA1-expressing cells and circulating HDL function, the inhibition of ABCA1 protein degradation by IMM-H007 promotes HDL cholesterol efflux capacity and RCT and attenuates atherogenesis. IMM-H007 potentially represents a lead compound for the development of agents to augment HDL function.

Liver X receptor activation promotes polyunsaturated fatty acid synthesis in macrophages: relevance in the context of atherosclerosis

OBJECTIVE

Liver X receptors (LXRs) modulate cholesterol and fatty acid homeostasis as well as inflammation. This study aims to decipher the role of LXRs in the regulation of polyunsaturated fatty acid (PUFA) synthesis in macrophages in the context of atherosclerosis.

APPROACH AND RESULTS

Transcriptomic analysis in human monocytes and macrophages was used to identify putative LXR target genes among enzymes involved in PUFA biosynthesis. In parallel, the consequences of LXR activation or LXR invalidation on PUFA synthesis and distribution were determined. Finally, we investigated the impact of LXR activation on PUFA metabolism in vivo in apolipoprotein E-deficient mice. mRNA levels of acyl-CoA synthase long-chain family member 3, fatty acid desaturases 1 and 2, and fatty acid elongase 5 were significantly increased in human macrophages after LXR agonist treatment, involving both direct and sterol responsive element binding protein-1-dependent mechanisms. Subsequently, pharmacological LXR agonist increased long chain PUFA synthesis and enhanced arachidonic acid content in the phospholipids of human macrophages. Increased fatty acid desaturases 1 and 2 and acyl-CoA synthase long-chain family member 3 mRNA levels as well as increased arachidonic acid to linoleic acid and docosahexaenoic acid to eicosapentaenoic acid ratios were also found in atheroma plaque and peritoneal foam cells from LXR agonist-treated mice. By contrast, murine LXR-deficient macrophages displayed reduced expression of fatty acid elongase 5, acyl-CoA synthase long-chain family member 3 and fatty acid desaturases 1, as well as decreased cellular levels of docosahexaenoic acid and arachidonic acid.

CONCLUSIONS

Our results indicate that LXR activation triggers PUFA synthesis in macrophages, which results in significant alterations in the macrophage lipid composition. Moreover, we demonstrate here that LXR agonist treatment modulates PUFA metabolism in atherosclerotic arteries.

Inhibition of ERK1/2 and activation of LXR synergistically reduce atherosclerotic lesions in ApoE-deficient mice

OBJECTIVE

Activation of liver X receptor (LXR) inhibits atherosclerosis but induces hypertriglyceridemia. In vitro, it has been shown that mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor synergizes LXR ligand-induced macrophage ABCA1 expression and cholesterol efflux. In this study, we determined whether MEK1/2 (U0126) and LXR ligand (T0901317) can have a synergistic effect on the reduction of atherosclerosis while eliminating LXR ligand-induced fatty livers and hypertriglyceridemia. We also set out to identify the cellular mechanisms of the actions.

APPROACH AND RESULTS

Wild-type mice were used to determine the effect of U0126 on a high-fat diet or high-fat diet plus T0901317-induced transient dyslipidemia and liver injury. ApoE deficient (apoE(-/-)) mice or mice with advanced lesions were used to determine the effect of the combination of T0901317 and U0126 on atherosclerosis and hypertriglyceridemia. We found that U0126 protected animals against T0901317-induced transient or long-term hepatic lipid accumulation, liver injury, and hypertriglyceridemia. Meanwhile, the combination of T0901317 and U0126 inhibited the development of atherosclerosis in a synergistic manner and reduced advanced lesions. Mechanistically, in addition to synergistic induction of macrophage ABCA1 expression, the combination of U0126 and T0901317 maintained arterial wall integrity, inhibited macrophage accumulation in aortas and formation of macrophages/foam cells, and activated reverse cholesterol transport. The inhibition of T0901317-induced lipid accumulation by the combined U0126 might be attributed to inactivation of lipogenesis and activation of lipolysis/fatty acid oxidation pathways.

CONCLUSIONS

Our study suggests that the combination of mitogen-activated protein kinase kinase 1/2 inhibitor and LXR ligand can function as a novel therapy to synergistically reduce atherosclerosis while eliminating LXR-induced deleterious effects.

Effect of nine diets on xenobiotic transporters in livers of mice

1. Lifestyle diseases are often caused by inappropriate nutrition habits and attempted to be treated by polypharmacotherapy. Therefore, it is important to determine whether differences in diet affect the disposition of drugs. Xenobiotic transporters in the liver are essential in drug disposition. 2. In the current study, mice were fed one of nine diets for 3 weeks. The mRNAs of 23 known xenobiotic transporters in livers of mice were quantified by microarray analysis, and validated by branched DNA assay. The mRNAs of 15 transporters were altered by at least one diet. Diet-restriction (10) and the atherogenic diet (10) altered the expression of the most number of transporters, followed by western diet (8), high-fat diet (4), lab chow (2), high-fructose diet (2) and EFA-deficient diet (2), whereas the low n-3 FA diet had no effect on these transporters. Seven of the 11 xenobiotic transporters in the Slc family, three of four in the Abcb family, two of four in the Abcc family and all three in the Abcg family were changed significantly. 3. This first comprehensive study indicates that xenobiotic transporters are altered by diet, and suggests there are likely diet-drug interactions due to changes in the expression of drug transporters.

PRMT3 regulates hepatic lipogenesis through direct interaction with LXRα

Arginine methylation is responsible for diverse biological functions and is mediated by protein arginine methyltransferases (PRMTs). Nonalcoholic fatty liver disease (NAFLD) is accompanied by excessive hepatic lipogenesis via liver X receptor α (LXRα). Thus we examined the pathophysiological role of PRMTs in NAFLD and their relationship with LXRα. In this study, palmitic acid (PA) treatment increased PRMT3, which is correlated with the elevation of hepatic lipogenic proteins. The expression of lipogenic proteins was increased by PRMT3 overexpression, but decreased by PRMT3 silencing and use of the PRMT3 knockout (KO) mouse embryonic fibroblast cell line. PRMT3 also increased the transcriptional activity of LXRα by directly binding with LXRα in a methylation-independent manner. In addition, PA treatment translocated PRMT3 to the nucleus. In animal models, a high-fat diet increased the LXRα and PRMT3 expressions and binding, which was not observed in LXRα KO mice. Furthermore, increased PRMT3 expression and its binding with LXRα were observed in NAFLD patients. Taken together, LXRα and PRMT3 expression was increased in cellular and mouse models of NAFLD and human patients, and PRMT3 translocated into the nucleus bound with LXRα as a transcriptional cofactor, which induced lipogenesis. In conclusion, PRMT3 translocation by PA is coupled to the binding of LXRα, which is responsible for the onset of fatty liver.

Hepatic TRAP80 selectively regulates lipogenic activity of liver X receptor

Inflammation in response to excess low-density lipoproteins in the blood is an important driver of atherosclerosis development. Due to its ability to enhance ATP-binding cassette A1-dependent (ABCA1-dependent) reverse cholesterol transport (RCT), liver X receptor (LXR) is an attractive target for the treatment of atherosclerosis. However, LXR also upregulates the expression of sterol regulatory element-binding protein 1c (SREBP-1c), leading to increased hepatic triglyceride synthesis, an independent risk factor for atherosclerosis. Here, we developed a strategy to separate the favorable and unfavorable effects of LXR by exploiting the specificity of the coactivator thyroid hormone receptor-associated protein 80 (TRAP80). Using human hepatic cell lines, we determined that TRAP80 selectively promotes the transcription of SREBP-1c but not ABCA1. Adenovirus-mediated expression of shTRAP80 inhibited LXR-dependent SREBP-1c expression and RNA polymerase II recruitment to the LXR responsive element (LXRE) of SREBP-1c, but not to the LXRE of ABCA1. In murine models, liver-specific knockdown of TRAP80 ameliorated liver steatosis and hypertriglyceridemia induced by LXR activation and maintained RCT stimulation by the LXR ligand. Together, these data indicate that TRAP80 is a selective regulator of hepatic lipogenesis and is required for LXR-dependent SREBP-1c activation. Moreover, targeting the interaction between TRAP80 and LXR should facilitate the development of potential LXR agonists that effectively prevent atherosclerosis.

FGF21 acts centrally to induce sympathetic nerve activity, energy expenditure, and weight loss

The mechanism by which pharmacologic administration of the hormone FGF21 increases energy expenditure to cause weight loss in obese animals is unknown. Here we report that FGF21 acts centrally to exert its effects on energy expenditure and body weight in obese mice. Using tissue-specific knockout mice, we show that βKlotho, the obligate coreceptor for FGF21, is required in the nervous system for these effects. FGF21 stimulates sympathetic nerve activity to brown adipose tissue through a mechanism that depends on the neuropeptide corticotropin-releasing factor. Our findings provide an unexpected mechanistic explanation for the strong pharmacologic effects of FGF21 on energy expenditure and weight loss in obese animals.

28-Homobrassinolide: a novel oxysterol transactivating LXR gene expression

Cholesterol is the template for steroid hormone biosynthesis. Cholesterol homeostasis is regulated by Cyt-P450 oxygenated cholesterols acting as ligands on LXR-α and LXR-β transcription factors that are now emerging as drug targets. Heterodimerization of LXRs with retinoic acid receptor is considered a prerequisite for target gene activation. Dietary plant oxysterol 28-homobrassinolide (28-HB) is a proven antihyperglycemic and a pro-steroidogenic agent in the rat. Whether 28-HB has a role in LXR gene expression was therefore investigated using oral gavage (15 days) of 28-HB (333 µg/kg b w) to normal and diabetic rat. PCR amplified LXR-α and β mRNA transcripts from treated rat liver and testis exhibited quantitative differences in their expression. Conformational differences in 28-HB docking to LXR-α and β binding domains were also noted through in silico studies, LXR-β adopting lesser specificity. We report that 28-HB transactivates LXR genes in the rat tissues.

Lxr-driven enterocyte lipid droplet formation delays transport of ingested lipids

Liver X receptors (Lxrs) are master regulators of cholesterol catabolism, driving the elimination of cholesterol from the periphery to the lumen of the intestine. Development of pharmacological agents to activate Lxrs has been hindered by synthetic Lxr agonists' induction of hepatic lipogenesis and hypertriglyceridemia. Elucidating the function of Lxrs in regulating enterocyte lipid handling might identify novel aspects of lipid metabolism that are pharmacologically amenable. We took a genetic approach centered on the single Lxr gene nr1h3 in zebrafish to study the role of Lxr in enterocyte lipid metabolism. Loss of nr1h3 function causes anticipated gene regulatory changes and cholesterol intolerance, collectively reflecting high evolutionary conservation of zebrafish Lxra function. Intestinal nr1h3 activation delays transport of absorbed neutral lipids, with accumulation of neutral lipids in enterocyte cytoplasmic droplets. This delay in transport of ingested neutral lipids protects animals from hypercholesterolemia and hepatic steatosis induced by a high-fat diet. On a gene regulatory level, Lxra induces expression of acsl3a, which encodes acyl-CoA synthetase long-chain family member 3a, a lipid droplet-anchored protein that directs fatty acyl chains into lipids. Forced overexpression of acls3a in enterocytes delays, in part, the appearance of neutral lipids in the vasculature of zebrafish larvae. Activation of Lxr in the intestine cell-autonomously regulates the rate of delivery of absorbed lipids by inducting a temporary lipid intestinal droplet storage depot.

Macrophage-independent regulation of reverse cholesterol transport by liver X receptors

OBJECTIVE

The ability of high-density lipoprotein (HDL) particles to accept cholesterol from peripheral cells, such as lipid-laden macrophages, and to transport cholesterol to the liver for catabolism and excretion in a process termed reverse cholesterol transport (RCT) is thought to underlie the beneficial cardiovascular effects of elevated HDL. The liver X receptors (LXRs; LXRα and LXRβ) regulate RCT by controlling the efflux of cholesterol from macrophages to HDL and the excretion, catabolism, and absorption of cholesterol in the liver and intestine. Importantly, treatment with LXR agonists increases RCT and decreases atherosclerosis in animal models. Nevertheless, LXRs are expressed in multiple tissues involved in RCT, and their tissue-specific contributions to RCT are still not well defined.

APPROACH AND RESULTS

Using tissue-specific LXR deletions together with in vitro and in vivo assays of cholesterol efflux and fecal cholesterol excretion, we demonstrate that macrophage LXR activity is neither necessary nor sufficient for LXR agonist-stimulated RCT. In contrast, the ability of LXR agonists primarily acting in the intestine to increase HDL mass and HDL function seems to underlie the ability of LXR agonists to stimulate RCT in vivo.

CONCLUSIONS

We demonstrate that activation of LXR in macrophages makes little or no contribution to LXR agonist-stimulated RCT. Unexpectedly, our studies suggest that the ability of macrophages to efflux cholesterol to HDL in vivo is not regulated by macrophage activity but is primarily determined by the quantity and functional activity of HDL.

Cyanidin-3-O-β-glucoside ameliorates lipopolysaccharide-induced acute lung injury by reducing TLR4 recruitment into lipid rafts

Cyanidin-3-O-β-glucoside (C3G), a typical anthocyanin pigment that exists in the human diet, has been reported to have anti-inflammatory properties. The aim of this study was to detect the effect of C3G on LPS-induced acute lung injury and to investigate the molecular mechanisms. Acute lung injury was induced by intratracheal administration of LPS in mice. Alveolar macrophages from mice were stimulated with LPS and were treated with C3G. Our results showed that C3G attenuated lung histopathologic changes, myeloperoxidase (MPO) activity, TNF-α, IL-1β and IL-6 production in LPS-induced acute lung injury model. In vitro, C3G dose-dependently inhibited TNF-α, IL-1β, IL-6, IL-10 and IFN-β production, as well as NF-κB and IRF3 activation in LPS-stimulated alveolar macrophages. Furthermore, C3G disrupted the formation of lipid rafts by depleting cholesterol and inhibited TLR4 translocation into lipid rafts. Moreover, C3G activated LXRα-ABCG1-dependent cholesterol efflux. Knockout of LXRα abrogated the anti-inflammatory effects of C3G. In conclusion, C3G has a protective effect on LPS-induced acute lung injury. The promising anti-inflammatory mechanisms of C3G is associated with up-regulation of the LXRα-ABCG1 pathway which result in disrupting lipid rafts by depleting cholesterol and reducing translocation of TLR4 to lipid rafts, thereby suppressing TLR4 mediated inflammatory response.

Hepatic nuclear corepressor 1 regulates cholesterol absorption through a TRβ1-governed pathway

Transcriptional coregulators are important components of nuclear receptor (NR) signaling machinery and provide additional mechanisms for modulation of NR activity. Expression of a mutated nuclear corepressor 1 (NCoR1) that lacks 2 NR interacting domains (NCoRΔID) in the liver leads to elevated expression of genes regulated by thyroid hormone receptor (TR) and liver X receptor (LXR), both of which control hepatic cholesterol metabolism. Here, we demonstrate that expression of NCoRΔID in mouse liver improves dietary cholesterol tolerance in an LXRα-independent manner. NCoRΔID-associated cholesterol tolerance was primarily due to diminished intestinal cholesterol absorption as the result of changes in the composition and hydrophobicity of the bile salt pool. Alterations of the bile salt pool were mediated by increased expression of genes encoding the bile acid metabolism enzymes CYP27A1 and CYP3A11 as well as canalicular bile salt pump ABCB11. We have determined that these genes are regulated by thyroid hormone and that TRβ1 is recruited to their regulatory regions. Together, these data indicate that interactions between NCoR1 and TR control a specific pathway involved in regulation of cholesterol metabolism and clearance.

Vitamin D receptor activation down-regulates the small heterodimer partner and increases CYP7A1 to lower cholesterol

BACKGROUND & AIMS

Little is known about the effects of the vitamin D receptor (VDR) on hepatic activity of human cholesterol 7α-hydroxylase (CYP7A1) and cholesterol metabolism. We studied these processes in mice in vivo and mouse and human hepatocytes.

METHODS

Farnesoid X receptor (Fxr)(-/-), small heterodimer partner (Shp)(-/-), and C57BL/6 (wild-type control) mice were fed normal or Western diets for 3 weeks and were then given intraperitoneal injections of vehicle (corn oil) or 1α,25-dihydroxyvitamin D3 (1,25[OH]2D3; 4 doses, 2.5 μg/kg, every other day). Plasma and tissue samples were collected and levels of Vdr, Shp, Cyp7a1, Cyp24a1, and rodent fibroblast growth factor (Fgf) 15 expression, as well as levels of cholesterol, were measured. We studied the regulation of Shp by Vdr using reporter and mobility shift assays in transfected human embryonic kidney 293 cells, quantitative polymerase chain reaction with mouse tissues and mouse and human hepatocytes, and chromatin immunoprecipitation assays with mouse liver.

RESULTS

We first confirmed the presence of Vdr mRNA and protein expression in livers of mice. In mice fed normal diets and given injections of 1,25(OH)2D3, liver and plasma concentrations of 1,25(OH)2D3 increased and decreased in unison. Changes in hepatic Cyp7a1 messenger RNA (mRNA) correlated with those of Cyp24a1 (a Vdr target gene) and inversely with Shp mRNA, but not ileal Fgf15 mRNA. Similarly, incubation with 1,25(OH)2D3 increased levels of Cyp24a1/CYP24A1 and Cyp7a1/CYP7A1 mRNA in mouse and human hepatocytes, and reduced levels of Shp mRNA in mouse hepatocytes. In Fxr(-/-) and wild-type mice with hypercholesterolemia, injection of 1,25(OH)2D3 consistently reduced levels of plasma and liver cholesterol and Shp mRNA, and increased hepatic Cyp7a1 mRNA and protein; these changes were not observed in Shp(-/-) mice given 1,25(OH)2D3 and fed Western diets. Truncation of the human small heterodimer partner (SHP) promoter and deletion analyses revealed VDR-dependent inhibition of SHP, and mobility shift assays showed direct binding of VDR to enhancer regions of SHP. In addition, chromatin immunoprecipitation analysis of livers from mice showed that injection of 1,25(OH)2D3 increased recruitment of Vdr and rodent retinoid X receptor to the Shp promoter.

CONCLUSIONS

Activation of the VDR represses hepatic SHP to increase levels of mouse and human CYP7A1 and reduce cholesterol.

Genetic inhibition of hepatic acetyl-CoA carboxylase activity increases liver fat and alters global protein acetylation

Lipid deposition in the liver is associated with metabolic disorders including fatty liver disease, type II diabetes, and hepatocellular cancer. The enzymes acetyl-CoA carboxylase 1 (ACC1) and ACC2 are powerful regulators of hepatic fat storage; therefore, their inhibition is expected to prevent the development of fatty liver. In this study we generated liver-specific ACC1 and ACC2 double knockout (LDKO) mice to determine how the loss of ACC activity affects liver fat metabolism and whole-body physiology. Characterization of LDKO mice revealed unexpected phenotypes of increased hepatic triglyceride and decreased fat oxidation. We also observed that chronic ACC inhibition led to hyper-acetylation of proteins in the extra-mitochondrial space. In sum, these data reveal the existence of a compensatory pathway that protects hepatic fat stores when ACC enzymes are inhibited. Furthermore, we identified an important role for ACC enzymes in the regulation of protein acetylation in the extra-mitochondrial space.

Activation of liver X receptors inhibits experimental fibrosis by interfering with interleukin-6 release from macrophages

OBJECTIVES

To investigate the role of liver X receptors (LXRs) in experimental skin fibrosis and evaluate their potential as novel antifibrotic targets.

METHODS

We studied the role of LXRs in bleomycin-induced skin fibrosis, in the model of sclerodermatous graft-versus-host disease (sclGvHD) and in tight skin-1 (Tsk-1) mice, reflecting different subtypes of fibrotic disease. We examined both LXR isoforms using LXRα-, LXRβ- and LXR-α/β-double-knockout mice. Finally, we investigated the effects of LXRs on fibroblasts and macrophages to establish the antifibrotic mode of action of LXRs.

RESULTS

LXR activation by the agonist T0901317 had antifibrotic effects in bleomycin-induced skin fibrosis, in the sclGvHD model and in Tsk-1 mice. The antifibrotic activity of LXRs was particularly prominent in the inflammation-driven bleomycin and sclGvHD models. LXRα-, LXRβ- and LXRα/β-double-knockout mice showed a similar response to bleomycin as wildtype animals. Low levels of the LXR target gene ABCA-1 in the skin of bleomycin-challenged and control mice suggested a low baseline activation of the antifibrotic LXR signalling, which, however, could be specifically activated by T0901317. Fibroblasts were not the direct target cells of LXRs agonists, but LXR activation inhibited fibrosis by interfering with infiltration of macrophages and their release of the pro-fibrotic interleukin-6.

CONCLUSIONS

We identified LXRs as novel targets for antifibrotic therapies, a yet unknown aspect of these nuclear receptors. Our data suggest that LXR activation might be particularly effective in patients with inflammatory disease subtypes. Activation of LXRs interfered with the release of interleukin-6 from macrophages and, thus, inhibited fibroblast activation and collagen release.

NDRG1 deficiency attenuates fetal growth and the intrauterine response to hypoxic injury

Intrauterine mammalian development depends on the preservation of placental function. The expression of the protein N-myc downstream-regulated gene 1 (NDRG1) is increased in placentas of human pregnancies affected by fetal growth restriction and in hypoxic primary human trophoblasts, where NDRG1 attenuates cell injury. We sought to assess the function of placental NDRG1 in vivo and tested the hypothesis that NDRG1 deficiency in the mouse embryo impairs placental function and consequently intrauterine growth. We found that Ndrg1 knock-out embryos were growth restricted in comparison to wild-type or heterozygous counterparts. Furthermore, hypoxia reduced the survival of female, but not male, knock-out embryos. Ndrg1 deletion caused significant alterations in placental gene expression, with a marked reduction in transcription of several lipoproteins in the placental labyrinth. These transcriptional changes were associated with reduced fetal:maternal serum cholesterol ratio exclusively in hypoxic female embryos. Collectively, our findings indicate that NDRG1 promotes fetal growth and regulates the metabolic response to intrauterine hypoxic injury in a sexually dichotomous manner.

LXRα fuels fatty acid-stimulated oxygen consumption in white adipocytes

Liver X receptors (LXRs) are transcription factors known for their role in hepatic cholesterol and lipid metabolism. Though highly expressed in fat, the role of LXR in this tissue is not well characterized. We generated adipose tissue LXRα knockout (ATaKO) mice and showed that these mice gain more weight and fat mass on a high-fat diet compared with wild-type controls. White adipose tissue (WAT) accretion in ATaKO mice results from both a decrease in WAT lipolytic and oxidative capacities. This was demonstrated by decreased expression of the β2- and β3-adrenergic receptors, reduced level of phosphorylated hormone-sensitive lipase, and lower oxygen consumption rates (OCRs) in WAT of ATaKO mice. Furthermore, LXR activation in vivo and in vitro led to decreased adipocyte size in WAT and increased glycerol release from primary adipocytes, respectively, with a concomitant increase in OCR in both models. Our findings show that absence of LXRα in adipose tissue results in elevated adiposity through a decrease in WAT oxidation, secondary to attenuated FA availability.

Effect of garlic on lipid profile and expression of LXR alpha in intestine and liver of hypercholesterolemic mice

BACKGROUND

Garlic is one of the medicinal plants which has showed beneficial effects on atherosclerosis risk factors. The liver X receptor α (LXRα) is an important regulator of cholesterol, triglyceride and glucose homeostasis that belongs to the nuclear receptor superfamily. In this study we investigated the effect of garlic on lipid profile, glucose as well as LXRα expression in intestine and liver of mice.

METHODS

Forty male N-Mary mice were randomly divided into 3 groups (n = 8): group1 received chow + 2% cholesterol + 0.5% cholic acid, group 2: chow + 4% (w/w) garlic extract + 2% cholesterol + 0.5% cholic acid, and group 3: chow only. After one month of treatment, mice were anesthetized, blood was collected from their heart, and the first 10 cm of the small intestine and liver were removed. Glucose was measured by a glucometer; other biochemical factors were measured by enzymatic methods. LXR expression was checked by RT-PCR and western blotting.

RESULTS

Compared with hypercholesterolemic mice, treatment with garlic extract significantly decreased total cholesterol, low-density lipoprotein cholesterol (LDL-C), triglycerides, very low density lipoprotein-cholesterol (VLDL-C), atherogenic index, alanine aminotranferease (ALT) and aspartate aminotransferase (AST) (all of them P < 0.05). Change in HDL-C levels was not significant in garlic-extract treated animals compared with hypercholesterolemic group. LXR protein and mRNA in the intestine were increased in garlic-extract treated group compared with chow group (P < 0.05), while in the liver, only mRNA of LXR was increased in hypercholesterolemic control mice (P < 0.05).

CONCLUSIONS

The present study demonstrated that garlic extract reduced LXRα expression in the liver and increased its expression in the intestine. These effects probably have an important role in reducing serum triglyceride and cholesterol.

Metabolic consequences of microRNA-122 inhibition in rainbow trout, Oncorhynchus mykiss

Background

MicroRNAs (miRNAs) are small regulatory molecules which post-transcriptionally regulate mRNA stability and translation. Several microRNAs have received attention due to their role as key metabolic regulators. In spite of the high evolutionary conservation of several miRNAs, the role of miRNAs in lower taxa of vertebrates has not been studied with regard to metabolism. The liver-specific and highly abundant miRNA-122 is one of the most widely studied miRNA in mammals, where it has been implicated in the control of hepatic lipid metabolism. Following our identification of acute postprandial, nutritional and endocrine regulation of hepatic miRNA-122 isomiRNA expression in rainbow trout, we used complementary in silico and in vivo approaches to study the role of miRNA-122 in rainbow trout metabolism. We hypothesized that the role of miRNA-122 in regulating lipid metabolism in rainbow trout is conserved to that in mammals and that modulation of miRNA-122 function would result in altered lipid homeostasis and secondarily altered glucose homeostasis, since lipogenesis has been suggested to act as glucose sink in trout.

Results

Our results show that miRNA-122 was functionally inhibited in vivo in the liver. Postprandial glucose concentrations increased significantly in rainbow trout injected with a miRNA-122 inhibitor, and this effect correlated with decreases in hepatic FAS protein abundance, indicative of altered lipogenic potential. Additionally, miRNA-122 inhibition resulted in a 20% decrease in plasma cholesterol concentration, an effect associated with increased expression of genes involved in cholesterol degradation and excretion.

Conclusions

Overall evidence suggests that miRNA-122 may have evolved in early vertebrates to support liver-specific metabolic functions. Nevertheless, our data also indicate that metabolic consequences of miRNA-122 inhibition may differ quantitatively between vertebrate species and that distinct direct molecular targets of miRNA-122 may mediate metabolic effects between vertebrate species, indicating that miRNA-122 - mRNA target relationships may have undergone species-specific evolutionary changes.

Genetics of HDL-C: a causal link to atherosclerosis?

Prospective epidemiological studies have consistently reported an inverse association between HDL cholesterol (HDL-C) levels and the risk of cardiovascular disease (CVD). However, large intervention trials on HDL-C-increasing drugs and recent Mendelian randomization studies have questioned a causal relationship between HDL-C and atherosclerosis. HDL-C levels have been shown to be highly heritable, and the combination of HDL-C-associated SNPs in recent large-scale genome-wide association studies (GWAS) only explains a small proportion of this heritability. As a large part of our current understanding of HDL metabolism comes from genetic studies, further insights in this research field may aid us in elucidating HDL functionality in relation to CVD risk. In this review we focus on the question of whether genetically defined HDL-C levels are associated with risk of atherosclerosis. We also discuss the latest insights for HDL-C-associated genes and recent GWAS data.

The orphan nuclear receptors at their 25-year reunion

The nuclear receptor superfamily includes many receptors, identified based on their similarity to steroid hormone receptors but without a known ligand. The study of how these receptors are diversely regulated to interact with genomic regions to control a plethora of biological processes has provided critical insight into development, physiology, and the molecular pathology of disease. Here we provide a compendium of these so-called orphan receptors and focus on what has been learned about their modes of action, physiological functions, and therapeutic promise.

Bile acid metabolism and signaling

Bile acids are important physiological agents for intestinal nutrient absorption and biliary secretion of lipids, toxic metabolites, and xenobiotics. Bile acids also are signaling molecules and metabolic regulators that activate nuclear receptors and G protein-coupled receptor (GPCR) signaling to regulate hepatic lipid, glucose, and energy homeostasis and maintain metabolic homeostasis. Conversion of cholesterol to bile acids is critical for maintaining cholesterol homeostasis and preventing accumulation of cholesterol, triglycerides, and toxic metabolites, and injury in the liver and other organs. Enterohepatic circulation of bile acids from the liver to intestine and back to the liver plays a central role in nutrient absorption and distribution, and metabolic regulation and homeostasis. This physiological process is regulated by a complex membrane transport system in the liver and intestine regulated by nuclear receptors. Toxic bile acids may cause inflammation, apoptosis, and cell death. On the other hand, bile acid-activated nuclear and GPCR signaling protects against inflammation in liver, intestine, and macrophages. Disorders in bile acid metabolism cause cholestatic liver diseases, dyslipidemia, fatty liver diseases, cardiovascular diseases, and diabetes. Bile acids, bile acid derivatives, and bile acid sequestrants are therapeutic agents for treating chronic liver diseases, obesity, and diabetes in humans.

27-Hydroxycholesterol promotes cell-autonomous, ER-positive breast cancer growth

To date, estrogen is the only known endogenous estrogen receptor (ER) ligand that promotes ER+ breast tumor growth. We report that the cholesterol metabolite 27-hydroxycholesterol (27HC) stimulates MCF-7 cell xenograft growth in mice. More importantly, in ER+ breast cancer patients, 27HC content in normal breast tissue is increased compared to that in cancer-free controls, and tumor 27HC content is further elevated. Increased tumor 27HC is correlated with diminished expression of CYP7B1, the 27HC metabolizing enzyme, and reduced expression of CYP7B1 in tumors is associated with poorer patient survival. Moreover, 27HC is produced by MCF-7 cells, and it stimulates cell-autonomous, ER-dependent, and GDNF-RET-dependent cell proliferation. Thus, 27HC is a locally modulated, nonaromatized ER ligand that promotes ER+ breast tumor growth.

Transcriptional control of hepatic lipid metabolism by SREBP and ChREBP

The liver is a central organ that controls systemic energy homeostasis and nutrient metabolism. Dietary carbohydrates and lipids, and fatty acids derived from adipose tissue are delivered to the liver, and utilized for gluconeogenesis, lipogenesis, and ketogenesis, which are tightly regulated by hormonal and neural signals. Hepatic lipogenesis is activated primarily by insulin that is secreted from the pancreas after a high-carbohydrate meal. Sterol regulatory element binding protein-1c (SREBP-1c) and carbohydrate-responsive element-binding protein (ChREBP) are major transcriptional regulators that induce key lipogenic enzymes to promote lipogenesis in the liver. Sterol regulatory element binding protein-1c is activated by insulin through complex signaling cascades that control SREBP-1c at both transcriptional and posttranslational levels. Carbohydrate-responsive element-binding protein is activated by glucose independently of insulin. Here, the authors attempt to summarize the current understanding of the molecular mechanism for the transcriptional regulation of hepatic lipogenesis, focusing on recent studies that explore the signaling pathways controlling SREBPs and ChREBP.

Endoplasmic reticulum stress impairs cholesterol efflux and synthesis in hepatic cells

Metabolic disorders such as type 2 diabetes cause hepatic endoplasmic reticulum (ER) stress, which affects neutral lipid metabolism. However, the role of ER stress in cholesterol metabolism is incompletely understood. Here, we show that induction of acute ER stress in human hepatic HepG2 cells reduced ABCA1 expression and caused ABCA1 redistribution to tubular perinuclear compartments. Consequently, cholesterol efflux to apoA-I, a key step in nascent HDL formation, was diminished by 80%. Besides ABCA1, endogenous apoA-I expression was reduced upon ER stress induction, which contributed to reduced cholesterol efflux. Liver X receptor, a key regulator of ABCA1 in peripheral cells, was not involved in this process. Despite reduced cholesterol efflux, cellular cholesterol levels remained unchanged during ER stress. This was due to impaired de novo cholesterol synthesis by reduction of HMG-CoA reductase activity by 70%, although sterol response element-binding protein-2 activity was induced. In mice, ER stress induction led to a marked reduction of hepatic ABCA1 expression. However, HDL cholesterol levels were unaltered, presumably because of scavenger receptor class B, type I downregulation under ER stress. Taken together, our data suggest that ER stress in metabolic disorders reduces HDL biogenesis due to impaired hepatic ABCA1 function.

Fatty acids regulation of inflammatory and metabolic genes

PURPOSE OF REVIEW

Fatty acids influence human health and diseases in various ways. In recent years, much work has been carried out to elucidate the mechanisms by which dietary fatty acids control short-term and long-term cellular functions. We have reviewed herein the most recent studies on modulation of gene expression by fatty acids. A number of genes respond to transcription factors and present a transcription factor response element in their promoter regions. Fatty acids may exert their effects on metabolism by regulating gene transcription via transcription factors. Understanding how fatty acids control expression of metabolic genes is a promising strategy to be investigated by aiming to treat metabolic diseases such as insulin resistance, obesity, and type 2 diabetes mellitus.

RECENT FINDINGS

Fatty acids exert many of their biological effects through the modulation of the activity of transcription factors, such as sterol regulatory element-binding proteins, peroxisome proliferator-activated receptors, and liver X receptors.

SUMMARY

Fatty acid action through transcription factors controls the expression of several inflammatory and metabolic genes.

Targeting liver X receptors in inflammation

INTRODUCTION

The two oxysterol receptors, 'liver X receptors (LXRs)' LXRα and LXRβ, are amongst the emerging newer drug targets within the nuclear receptor family and targeting LXRs represents novel strategies needed for prevention and treatment of diseases where current therapeutics is inadequate.

AREAS COVERED

This review discusses the current understanding of LXR biology with an emphasis on the molecular aspects of LXR signalling establishing their potential as drug targets. Recent advances of their transcriptional mechanisms in inflammatory pathways and their physiological roles in inflammation and immunity are described.

EXPERT OPINION

The new discoveries of LXR-regulated inflammatory pathways have ignited new promises for LXRs as drug targets. The broad physiological roles of LXRs involve a high risk of unwanted side effects. Recent insights into LXR biology of the brain indicate a highly important role in neuronal development and a clinical trial testing an LXR agonist reported adverse neurological side effects. This suggests that drug development must focus on limiting the range of LXR signalling - possibly achieved through subtype, tissue specific, promoter specific or pathway specific activation of LXRs where a successful candidate drug must be carefully studied for its effect in the central nervous system.

Differential regulation of gene expression by LXRs in response to macrophage cholesterol loading

The ability of cells to precisely control gene expression in response to intracellular and extracellular signals plays an important role in both normal physiology and in pathological settings. For instance, the accumulation of excess cholesterol by macrophages initiates a genetic response mediated by the liver X receptors (LXRs)-α (NR1H3) and LXRβ (NR1H2), which facilitates the transport of cholesterol out of cells to high-density lipoprotein particles. Studies using synthetic LXR agonists have also demonstrated that macrophage LXR activation simultaneously induces a second network of genes that promotes fatty acid and triglyceride synthesis that may support the detoxification of excess free cholesterol by storage in the ester form. We now show that treatment of human THP-1 macrophages with endogenous or synthetic LXR ligands stimulates both transcriptional and posttranscriptional pathways that result in the selective recruitment of the LXRα subtype to LXR-regulated promoters. Interestingly, when human or mouse macrophages are loaded with cholesterol under conditions that mimic the development of atherogenic macrophage foam cells, a selective LXR response is generated that induces genes mediating cholesterol transport but does not coordinately regulate genes involved in fatty acid synthesis. The gene-selective response to cholesterol loading occurs, even in the presence of LXRα binding to the promoter of the gene encoding the sterol regulatory element-binding protein-1c, the master transcriptional regulator of fatty acid synthesis. The ability of promoter bound LXRα to recruit RNA polymerase to the sterol regulatory element-binding protein-1c promoter, however, appears to be ligand selective.

Targeting high density lipoproteins in the prevention of cardiovascular disease?

Recent studies involving HDL-raising therapeutics have greatly changed our understanding of this field. Despite effectively raising HDL-C levels, niacin remains of uncertain clinical benefit. Synthetic niacin receptor agonists are unlikely to raise HDL-C or have other beneficial effects on plasma lipids. Despite the failure in phase 3 of 2 CETP inhibitors, 2 potent CETP inhibitors that raise HDL-C levels by >100 % (and reduce LDL-C substantially) are in late stage clinical development. Infusions of recombinant HDL containing 'wild-type' apoA-I or apoA-I Milano, as well as autologous delipidated HDL, all demonstrated promising early results, and remain in clinical development. A small molecule that causes upregulation of endogenous apoA-I production is also in clinical development. Finally, upregulation of macrophage cholesterol efflux pathways through agonism of liver X receptors or antagonism of miR-33 remains of substantial interest. The field of HDL therapeutics is poised to transition from the 'HDL-cholesterol hypothesis' to the 'HDL flux hypothesis' in which the impact on flux from macrophage to feces is deemed to be of greater therapeutic benefit than the increase in steady-state concentrations of HDL cholesterol.

Phenotypic polarization of macrophages in atherosclerosis

Macrophages orchestrate the inflammatory response in inflamed tissues, and recent work indicates that these cells can alter their phenotypes and functions accordingly in response to changes in the microenvironment. Initial work in models of cardiovascular disease used immunologic markers to characterize macrophage phenotypes present in atherosclerotic plaque, and these studies have lately been extended through the use of markers that are more specific for atherosclerosis and metabolic disease. Together, these studies have led to a novel view of the function of macrophages in the development of atherosclerosis that suggests dynamic plasticity. Understanding this plasticity and the ensuing macrophage heterogeneity could lead to novel strategies of pharmacological intervention to combat chronic inflammation in metabolic diseases. Most importantly, revealing the functional characteristics of individual macrophage phenotypes will lead to a better understanding of their contribution to lesion development and plaque stability.

Essential fatty acids deficiency promotes lipogenic gene expression and hepatic steatosis through the liver X receptor

BACKGROUND & AIMS

Nutrients influence non-alcoholic fatty liver disease. Essential fatty acids deficiency promotes various syndromes, including hepatic steatosis, through increased de novo lipogenesis. The mechanisms underlying such increased lipogenic response remain unidentified.

METHODS

We used wild type mice and mice lacking Liver X Receptors to perform a nutrigenomic study that aimed at examining the role of these transcription factors.

RESULTS

We showed that, in the absence of Liver X Receptors, essential fatty acids deficiency does not promote steatosis. Consistent with this, Liver X Receptors are required for the elevated expression of genes involved in lipogenesis in response to essential fatty acids deficiency.

CONCLUSIONS

This work identifies, for the first time, the central role of Liver X Receptors in steatosis induced by essential fatty acids deficiency.

Cholesterol and male fertility: what about orphans and adopted?

The link between cholesterol homeostasis and male fertility has been clearly suggested in patients who suffer from hyperlipidemia and metabolic syndrome. This has been confirmed by the generation of several transgenic mouse models or in animals fed with high cholesterol diet. Next to the alteration of the endocrine signaling pathways through steroid receptors (androgen and estrogen receptors); "orphan" and "adopted" nuclear receptors, such as the Liver X Receptors (LXRs), the Proliferating Peroxisomal Activated Receptors (PPARs) or the Liver Receptor Homolog-1 (LRH-1), have been involved in this cross-talk. These transcription factors show distinct expression patterns in the male genital tract, explaining the large panel of phenotypes observed in transgenic male mice and highlighting the importance of lipid homesostasis and the complexity of the molecular pathways involved. Increasing our knowledge of the roles of these nuclear receptors in male germ cell differentiation could help in proposing new approaches to either treat infertile men or define new strategies for contraception.

A liver-selective LXR inverse agonist that suppresses hepatic steatosis

Fatty liver, which often accompanies obesity and type 2 diabetes, frequently leads to a much more debilitating hepatic disease including non-alcoholic steatohepatitis, cirrhosis, and hepatocellular carcinoma. Current pharmacological therapies lack conclusive efficacy and thus treatment options are limited. Novel therapeutics that suppress either hepatic lipogenesis and/or hepatic inflammation may be useful. Here, we describe the development of the first selective synthetic LXR inverse agonist (SR9238) and demonstrate that this compound effectively suppresses hepatic lipogenesis, inflammation, and hepatic lipid accumulation in a mouse model of non-alcoholic hepatosteatosis. SR9238 displays high potency for both LXRα and LXRβ (40-200 nM IC50) and was designed to display liver specificity so as to avoid potential side effects due to suppression of LXR in the periphery. Unexpectedly, treatment of diet-induced obese mice with SR9238 suppressed plasma cholesterol levels. These data indicate that liver-selective LXR inverse agonists may hold utility in the treatment of liver disease.