The nuclear receptor LXR is a glucose sensor

Carbohydrate response element binding protein gene expression is positively regulated by thyroid hormone

The molecular mechanism of thyroid hormone (TH) effects to fatty acid metabolism in liver is yet to be clear. The carbohydrate response element-binding protein (ChREBP) as well as sterol response element-binding protein (SREBP)-1c plays a pivotal role in hepatic lipogenesis. Both SREBP-1c and ChREBP are target genes of liver X receptors (LXRs). Because LXRs and TH receptors (TRs) cross talk mutually in many aspects of transcription, we examined whether TRs regulate the mouse ChREBP gene expression. In the current study, we demonstrated that TH up-regulated mouse ChREBP mRNA and protein expression in liver. Run-on and luciferase assays showed that TH and TR-beta1 positively regulated the ChREBP gene transcription. The mouse ChREBP gene promoter contains two direct repeat-4 sites (LXRE1 and LXRE2) and EMSAs demonstrated that LXR-alpha and TR-beta1 prefer to bind LXRE1 and LXRE2, respectively. The direct repeat-4 deletion and LXRE2 mutants of the promoter deteriorate the positive regulation by TR-beta1, indicating that LXRE2 is functionally important for the regulation. We also showed that human ChREBP gene expression and promoter activities were up-regulated by TH. These data suggest that ChREBP mRNA expression is positively regulated by TR-beta1 and TH at the transcriptional level in mammals. This novel observation indicates that TH fine-tunes hepatic lipogenesis via regulating SREBP-1c and ChREBP gene expression reciprocally.

Therapeutic potential of dithiolethiones for hepatic diseases

Comprehensive studies support the notion that oltipraz [4-methyl-5-(2-pyrazynyl)-1,2-dithiole-3-thione] and its congeners exert cancer chemopreventive effects by the prevention, inhibition or reversal of carcinogenic processes. Recently, it was found that dithiolethione compounds had the activities to prevent or treat fibrosis, insulin resistance, and mitochondrial protective effects in the liver by a mechanism involving AMP-activated protein kinase (AMPK) and/or 70-kDa ribosomal protein S6 kinase 1 (S6K1). Moreover, chemical regulation of the AMPK-S6K1 pathway was found to affect Liver X receptor (LXR) activity and lipogenesis, leading to the identification of AMPK and S6K1 as targets for treating hepatic steatosis. These biological activities of dithiolethiones may offer a novel approach to pharmaceutical intervention. This review focuses on the interaction between oltipraz and the AMPK-mTOR-S6K1 pathway, which regulates genes that confer hepatocyte protection from intoxication, disrupted energy metabolism, and inflammation. In terms of therapeutic potential, the findings reviewed here demonstrate a new therapeutic potential for dithiolethiones, which function in a unique manner, and offer the possibility of new treatments for hepatic diseases.

Role of adenosine monophosphate-activated protein kinase-p70 ribosomal S6 kinase-1 pathway in repression of liver X receptor-alpha-dependent lipogenic gene induction and hepatic steatosis by a novel class of dithiolethiones

Dithiolethiones, a novel class of adenosine monophosphate-activated protein kinase (AMPK) activators, prevent insulin resistance through AMPK-dependent p70 ribosomal S6 kinase-1 (S6K1) inhibition. There is no known effect of S6K1 for liver X receptor-alpha (LXRalpha)-mediated lipogenic gene expression and steatosis, a cause of chronic liver disease. This study investigated the role of S6K1 in LXRalpha activation and the effects of oltipraz (prototype) and other dithiolethiones on LXRalpha-dependent lipogenesis in hepatocytes and high-fat diet animal model. Oltipraz prevented the ability of LXRalpha agonist (T0901317) to activate sterol regulatory element binding protein-1c (SREBP-1c), inhibiting its own mRNA and protein induction. Impaired SREBP-1c activity by oltipraz caused inhibition of LXRalpha-induced transcription of the fatty acid synthase, LXRalpha, acetyl-CoA carboxylase, stearoyl-CoA desaturase-1, and adenosine triphosphate-binding cassette transporter A1 genes. S6K1 activation antagonized the inhibitory effect of oltipraz on SREBP-1c activation, whereas dominant negative (DN) mutant S6K1 and rapamycin inhibited the T0901317-induced SREBP-1c expression. Oltipraz impaired LXRalpha DNA binding activity and LXR agonist-induced CYP7A1-LXRE-luciferase (CYP7A1) transactivation. Moreover, in vitro S6K1 directly phosphorylated LXRalpha at serine residues for gene transactivation, which was antagonized by its DN mutant. S6K1 inhibition antagonized CYP7A1 induction promoted by AMPK inhibition, whereas AMPK activation abrogated S6K1-dependent CYP7A1 induction, supporting the opposing role of S6K1 and AMPK in LXR activity. Finally, oltipraz was found to inhibit hepatic triglyceride accumulation and lipogenic gene induction in mice fed a high-fat diet. Other dithiolethiones also inhibited SREBP-1c induction by T0901317.

CONCLUSION

Our findings showing the role of AMPK-S6K1 pathway in LXR activity and S6K1-dependent inhibition of LXRalpha-induced lipogenic gene transactivation by a novel class of dithiolethiones led to the identification of S6K1 as a particularly attractive target for intervention in hepatic steatosis.

Elevated insulin secretion from liver X receptor-activated pancreatic beta-cells involves increased de novo lipid synthesis and triacylglyceride turnover

Increased basal and loss of glucose-stimulated insulin secretion (GSIS) are hallmarks of beta-cell dysfunction associated with type 2 diabetes. It has been proposed that elevated glucose promotes insulin secretory defects by activating sterol regulatory element binding protein (SREBP)-1c, lipogenic gene expression, and neutral lipid storage. Activation of liver X receptors (LXRs) also activates SREBP-1c and increases lipogenic gene expression and neutral lipid storage but increases basal and GSIS. This study was designed to characterize the changes in de novo fatty acid and triacylglyceride (TAG) synthesis in LXR-activated beta-cells and determine how these changes contribute to elevated basal and GSIS. Treatment of INS-1 beta-cells with LXR agonist T0901317 and elevated glucose led to markedly increased nuclear localization of SREBP-1, lipogenic gene expression, de novo synthesis of monounsaturated fatty acids and TAG, and basal and GSIS. LXR-activated cells had increased fatty acid oxidation and expression of genes involved in mitochondrial beta-oxidation, particularly carnitine palmitoyltransferase-1. Increased basal insulin release from LXR-activated cells coincided with rapid turnover of newly synthesized TAG and required acyl-coenzyme A synthesis and mitochondrial beta-oxidation. GSIS from LXR-activated INS-1 cells required influx of extracellular calcium and lipolysis, suggesting production of lipid-signaling molecules from TAG. Inhibition of diacylglyceride (DAG)-binding proteins, but not classic isoforms of protein kinase C, attenuated GSIS from LXR-activated INS-1 cells. In conclusion, LXR activation in beta-cells exposed to elevated glucose concentrations increases de novo TAG synthesis; subsequent lipolysis produces free fatty acids and DAG, which are oxidized to increase basal insulin release and activate DAG-binding proteins to enhance GSIS, respectively.

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.

Glycolytic flux signals to mTOR through glyceraldehyde-3-phosphate dehydrogenase-mediated regulation of Rheb

The mammalian target of rapamycin (mTOR) interacts with raptor to form the protein complex mTORC1 (mTOR complex 1), which plays a central role in the regulation of cell growth in response to environmental cues. Given that glucose is a primary fuel source and a biosynthetic precursor, how mTORC1 signaling is coordinated with glucose metabolism has been an important question. Here, we found that the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) binds Rheb and inhibits mTORC1 signaling. Under low-glucose conditions, GAPDH prevents Rheb from binding to mTOR and thereby inhibits mTORC1 signaling. High glycolytic flux suppresses the interaction between GAPDH and Rheb and thus allows Rheb to activate mTORC1. Silencing of GAPDH or blocking of the Rheb-GAPDH interaction desensitizes mTORC1 signaling to changes in the level of glucose. The GAPDH-dependent regulation of mTORC1 in response to glucose availability occurred even in TSC1-deficient cells and AMPK-silenced cells, supporting the idea that the GAPDH-Rheb pathway functions independently of the AMPK axis. Furthermore, we show that glyceraldehyde-3-phosphate, a glycolytic intermediate that binds GAPDH, destabilizes the Rheb-GAPDH interaction even under low-glucose conditions, explaining how high-glucose flux suppresses the interaction and activates mTORC1 signaling. Taken together, our results suggest that the glycolytic flux regulates mTOR's access to Rheb by regulating the Rheb-GAPDH interaction, thereby allowing mTORC1 to coordinate cell growth with glucose availability.

Colesevelam hydrochloride for the treatment of type 2 diabetes mellitus

BACKGROUND

Colesevelam hydrochloride is a bile acid sequestrant approved in January 2008 by the US Food and Drug Administration (FDA) for the treatment of adult patients with type 2 diabetes mellitus (DM) in combination with a sulfonylurea, metformin, and/or insulin therapy.

OBJECTIVE

The purpose of this article was to review the pharmacology, pharmacokinetics, efficacy, adverse effects and tolerability, drug-drug interactions, contraindications/precautions, dosage and administration, pharmacoeconomics, and the overall role of colesevelam in the management of adult patients with type 2 DM.

METHODS

A literature search using MEDLINE (1966-October 27, 2008), PubMed (1950-October 27, 2008), Science Direct (1994-October 27, 2008), Web of Science (1980-October 27, 2008), American Diabetes Association Scientific Abstracts (2004-2008), and International Pharmaceutical Abstracts (1970-October 27, 2008) was performed using the term colesevelam. English-language, original research and review articles were examined, and citations from these articles were assessed. Manufacturer prescribing information and the FDA review of the new drug application for colesevelam were also examined.

RESULTS

Colesevelam is a hydrophilic, water-insoluble polymer, with negligible absorption and systemic distribution, that is excreted primarily in the feces. Through a mechanism still under investigation, colesevelam effectively lowers glycosylated hemoglobin (HbA(1c)) when used in combination with a sulfonylurea, metformin, and/or insulin therapy. Three completed, published Phase III clinical trials investigating colesevelam for the treatment of type 2 DM were evaluated for information, data, and conclusions. At dosing of 1.875 g BID or 3.75 g once daily in combination with one of the aforementioned agents versus placebo, reductions in HbA(1c) in all 3 Phase III clinical trials of colesevelam ranged from 0.5% to 0.7% (P < 0.02). In clinical trials, colesevelam was well tolerated, with hypoglycemia occurring in approximately 3% of studied patients.

CONCLUSIONS

When used in combination with a sulfonylurea, metformin, and/or insulin therapy, colesevelam has been reported to significantly reduce HbA(1c) in adult patients with type 2 DM. Colesevelam's role in the management of type 2 DM remains undefined, however; further investigation into its mechanism of action and long-term efficacy and safety should be performed.

Liver X receptors contribute to the protective immune response against Mycobacterium tuberculosis in mice

Liver X receptors (LXRs) are key regulators of macrophage function, controlling transcriptional programs involved in lipid homeostasis and inflammation. However, exactly how LXRs modulate inflammation during infection remains unknown. To explore this, we used a mouse model of Mycobacterium tuberculosis infection. Upon intratracheal infection with M. tuberculosis, LXRs and LXR target genes were induced in CD11c+ lung and alveolar cells. Furthermore, mice deficient in both LXR isoforms, LXRalpha and LXRbeta (Lxra-/-Lxrb-/- mice), were more susceptible to infection, developing higher bacterial burdens and an increase in the size and number of granulomatous lesions. Interestingly, mice solely deficient in LXRalpha, but not those lacking only LXRbeta, mirrored the susceptibility of the Lxra-/-Lxrb-/- animals. Lxra-/-Lxrb-/- mice failed to mount an effective early neutrophilic airway response to infection and showed dysregulation of both pro- and antiinflammatory factors in CD11c+ lung cells. T cell responses were strongly affected in Lxra-/-Lxrb-/- mice, showing near-complete abrogation of the infection-induced Th1 function - and even more so Th17 function - in the lungs. Treatment of WT mice with the LXR agonists TO901317 and GW3965 resulted in a 10-fold decrease of the pulmonary bacterial burden and a comparable increase of Th1/Th17 function in the lungs. The dependence of LXR signaling on the neutrophil IL-17 axis represents what we believe to be a novel function for these nuclear receptors in resistance to M. tuberculosis infection and may provide a new target for therapeutics.

New functions of cholesterol binding proteins

Cholesterol is an essential component of eukaryotic cell membranes with an important role in signal transduction. Although cholesterol can operate to auto-regulate its own disposal via gene transcriptional mechanisms, glucose also binds to the same cholesterol-binding transcription factors to regulate gene expression. Different sterol binding proteins bind different lipids to regulate both lipid homeostasis and antigen presentation. This mini-review examines the recently reported new functions of cholesterol binding proteins in cholesterol homeostasis, function and trafficking, and explores the molecular mechanisms whereby sterol sensors respond to glucose and other ligands to regulate diverse cellular functions in metabolism. Several new models are proposed from studies on a range of sterol binding proteins including Insig, SCAP, LXR, HMG-CoAR, NPC1 and NPC2.

Adiponectin suppresses hepatic SREBP1c expression in an AdipoR1/LKB1/AMPK dependent pathway

Adiponectin, one of the insulin-sensitizing adipokines, has been shown to activate fatty acid oxidation in liver and skeletal muscle, thus maintaining insulin sensitivity. However, the precise roles of adiponectin in fatty acid synthesis are poorly understood. Here we show that adiponectin administration acutely suppresses expression of sterol regulatory element-binding protein (SREBP) 1c, the master regulator which controls and upregulates the enzymes involved in fatty acid synthesis, in the liver of +Lepr(db)/+Lepr(db) (db/db) mouse as well as in cultured hepatocytes. We also show that adiponectin suppresses SREBP1c by AdipoR1, one of the functional receptors for adiponetin, and furthermore that suppressing either AMP-activated protein kinase (AMPK) via its upstream kinase LKB1 deletion cancels the negative effect of adiponectin on SREBP1c expression. These data show that adiponectin suppresses SREBP1c through the AdipoR1/LKB1/AMPK pathway, and suggest a possible role for adiponectin in the regulation of hepatic fatty acid synthesis.

Molecular cloning, tissue expression and regulation of liver X receptor (LXR) transcription factors of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss)

Fish are important sources of high quality protein, essential minerals such as iodine and selenium, vitamins including A, D and E, and omega-3 fatty acids in the human diet. With declining fisheries worldwide, farmed fish constitute an ever-increasing proportion of fish in the food basket. Sustainable development of aquaculture dictates that diets will have to contain increasing levels of plant products that are devoid of cholesterol, but contain phytosterols that are known to have physiological effects in mammals. Liver X receptors (LXR) are transcription factors whose activity is modulated by sterols, with activation inducing cholesterol catabolism and de novo fatty acid biosynthesis in liver. Transcriptomic analysis has shown that substitution of fish meal and oil with plant products induces genes of cholesterol and fatty acid metabolism in salmonids. Here we report the cloning of LXR cDNAs from two species of salmonid fish that are important in aquaculture. The full-length cDNA (mRNA) of LXR obtained from salmon was shown to be 3766 bp, which included a 5'-untranslated region (UTR) of 412 bp and a 3'-UTR of 1960 bp and an open reading frame (ORF) of 1394 bp, which specified a protein of 462 amino acids. The trout LXR full-length cDNA was 2056 bp, including 5'- and 3'-UTRs of 219 and 547 bp, respectively, and an ORF of 1290 bp, which specified a protein of 427 amino acids. The protein sequences included characteristic features of mammalian LXRs, including the DNA binding (DBD), containing P-box, ligand binding (LBD) and activation function-2 (AF-2) domains, D-box, D (hinge) region, and eight cysteines that belong to the two zinc fingers. Phylogenetic analysis clustered the salmonid LXRs together, more closely with zebrafish and more distantly from medaka and stickleback. A pair-wise comparison among vertebrate LXR sequences showed the amino acid sequence predicted by the salmon LXR ORF showed greatest identity to that of trout 97%, and 97%, 87% and 81% identity to LXRs of zebrafish, frog and human (LXRalpha). The trout LXR ORF showed 96%, 92% and 82% identity to LXRs of zebrafish, frog and human (LXRalpha). Surprisingly, the expression of LXR was lowest in liver of all tissues examined and in salmon the greatest expression was observed in pyloric caeca with liver showing intermediate expression. It is likely that tissue expression was affected by the physiological status of the sampled animals. Certainly, nutritional, environmental and/or developmental regulation was evident in salmon, where the expression of LXR in liver was higher in fish in seawater than in freshwater, and higher in fish fed fish oil compared to fish fed vegetable oil in adult salmon.

The structural basis of gas-responsive transcription by the human nuclear hormone receptor REV-ERBbeta

Heme is a ligand for the human nuclear receptors (NR) REV-ERBalpha and REV-ERBbeta, which are transcriptional repressors that play important roles in circadian rhythm, lipid and glucose metabolism, and diseases such as diabetes, atherosclerosis, inflammation, and cancer. Here we show that transcription repression mediated by heme-bound REV-ERBs is reversed by the addition of nitric oxide (NO), and that the heme and NO effects are mediated by the C-terminal ligand-binding domain (LBD). A 1.9 A crystal structure of the REV-ERBbeta LBD, in complex with the oxidized Fe(III) form of heme, shows that heme binds in a prototypical NR ligand-binding pocket, where the heme iron is coordinately bound by histidine 568 and cysteine 384. Under reducing conditions, spectroscopic studies of the heme-REV-ERBbeta complex reveal that the Fe(II) form of the LBD transitions between penta-coordinated and hexa-coordinated structural states, neither of which possess the Cys384 bond observed in the oxidized state. In addition, the Fe(II) LBD is also able to bind either NO or CO, revealing a total of at least six structural states of the protein. The binding of known co-repressors is shown to be highly dependent upon these various liganded states. REV-ERBs are thus highly dynamic receptors that are responsive not only to heme, but also to redox and gas. Taken together, these findings suggest new mechanisms for the systemic coordination of molecular clocks and metabolism. They also raise the possibility for gas-based therapies for the many disorders associated with REV-ERB biological functions.

Activation of liver X receptors inhibits pancreatic islet beta cell proliferation through cell cycle arrest

AIMS/HYPOTHESIS

Liver X receptors (LXRs) are important transcriptional regulators of lipid homeostasis and proliferation in several cell types. However, the roles of LXRs in pancreatic beta cells have not been fully established. The aim of this study was to investigate the effects of LXRs on pancreatic beta cell proliferation.

METHODS

Gene expression was analysed using real-time RT-PCR. Transient transfection and reporter gene assays were used to determine the transcriptional activity of LXRs in pancreatic beta cells. Cell viability and proliferation were analysed using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), DNA fluorometric, BrdU labelling and [(3)H]thymidine incorporation assays. Cell cycle distribution was investigated by flow cytometry analysis. Adenovirus-based RNA interference was used to knockdown LXRalpha, LXRbeta and p27 in MIN6 cells and mouse islets.

RESULTS

We found that both Lxralpha (also known as Nr1h3) and Lxrbeta (also known as Nr1h2) were expressed and transactivated the LXR response element in HIT-T15 and MIN6 cells. Activation of LXRs dose-dependently inhibited pancreatic beta cell viability and proliferation. This was accompanied by beta cell cycle arrest at the G1 phase. Furthermore, LXR activation increased levels of the p27 protein by inhibiting its degradation. Knockdown of p27 reversed these effects of LXR activation on growth inhibition and cell cycle arrest.

CONCLUSIONS/INTERPRETATION

Our observations indicate that LXR activation inhibits pancreatic beta cell proliferation through cell cycle arrest. A well-known regulator of pancreatic beta cell cycle progression, p27, is upregulated and mediates the effects of LXRs on growth inhibition in beta cells. These observations suggest the involvement of aberrant activation of LXR in beta cell mass inadequacy, which is an important step in the development of type 2 diabetes.

Transcriptional regulation of hepatic fatty acid metabolism

The liver is a major site of fatty acid synthesis and degradation. Transcriptional regulation is one of several mechanisms controlling hepatic metabolism of fatty acids. Two transcription factors, namely SREBP1-c and PPARalpha, appear to be the main players controlling synthesis and degradation of fatty acids respectively. This chapter briefly presents fatty acid metabolism. The first part focuses on SREBP1-c contribution to the control of gene expression relevant to fatty acid synthesis and the main mechanisms of activation for this transcriptional program. The second part reviews the evidence for the involvement of PPARalpha in the control of fatty acid degradation and the key features of this nuclear receptor. Finally, the third part aims at summarizing recent advances in our current understanding of how these two transcription factors fit in the regulatory networks that sense hormones or nutrients, including cellular fatty acids, and govern the transcription of genes implicated in hepatic fatty acid metabolism.

Minireview: liver X receptor beta: emerging roles in physiology and diseases

Liver X receptors, LXRalpha and LXRbeta, are nuclear receptors belonging to the large family of transcription factors. After activation by oxysterols, LXRs play a central role in the control of lipid and carbohydrate metabolism as well as inflammation. The role of LXRalpha has been extensively studied, particularly in the liver and macrophages. In the liver it prevents cholesterol accumulation by increasing bile acid synthesis and secretion into the bile through ATP-binding cassette G5/G8 transporters, whereas in macrophages it increases cholesterol reverse transport. The function of LXRbeta is still under investigation with most of the current knowledge coming from the study of phenotypes of LXRbeta-/- mice. With these mice new emerging roles for LXRbeta have been demonstrated in the pathogenesis of diseases such as amyotrophic lateral sclerosis and chronic pancreatitis. The present review will focus on the abnormalities described so far in LXRbeta-/- mice and the insight gained into the possible roles of LXRbeta in human diseases.

Identification of adropin as a secreted factor linking dietary macronutrient intake with energy homeostasis and lipid metabolism

Obesity and nutrient homeostasis are linked by mechanisms that are not fully elucidated. Here we describe a secreted protein, adropin, encoded by a gene, Energy Homeostasis Associated (Enho), expressed in liver and brain. Liver Enho expression is regulated by nutrition: lean C57BL/6J mice fed high-fat diet (HFD) exhibited a rapid increase, while fasting reduced expression compared to controls. However, liver Enho expression declines with diet-induced obesity (DIO) associated with 3 months of HFD or with genetically induced obesity, suggesting an association with metabolic disorders in the obese state. In DIO mice, transgenic overexpression or systemic adropin treatment attenuated hepatosteatosis and insulin resistance independently of effects on adiposity or food intake. Adropin regulated expression of hepatic lipogenic genes and adipose tissue peroxisome proliferator-activated receptor gamma, a major regulator of lipogenesis. Adropin may therefore be a factor governing glucose and lipid homeostasis, which protects against hepatosteatosis and hyperinsulinemia associated with obesity.

Role of nuclear receptors in the modulation of insulin secretion in lipid-induced insulin resistance

In healthy individuals, a hyperbolic relationship exists between whole-body insulin-sensitivity and insulin secretion. Thus, for any difference in insulin-sensitivity, a reciprocal proportionate change occurs in insulin secretion. Such a feedback loop is evident in healthy individuals ingesting diets high in saturated fat and in late pregnancy where, despite lipid-induced insulin resistance, glucose tolerance is maintained through augmented GSIS (glucose-stimulated insulin secretion). NRs (nuclear receptors) are members of a superfamily of ligand-regulated and orphan transcription factors. On activation by a cognate ligand, many ligand-activated NRs recruit the RXR (retinoid X receptor) for heterodimer formation. Such NRs include the PPARs (peroxisome-proliferator-activated receptors), which are involved in lipid sensing and liporegulation. PPARs exert important lipid-lowering effects in vivo, thereby opposing the development of lipid-induced insulin resistance by relieving the inhibition of insulin-stimulated glucose disposal by muscle and lowering the necessity for augmented GSIS to counter lipid-induced insulin resistance. Long-chain fatty acids are proposed as natural PPAR ligands and some specific endogenous pathways of lipid metabolism are believed to generate PPAR agonists. Other NRs, e.g. the LXR (liver X receptor), which senses expansion of the metabolically active pool of cholesterol, and the FXR (farnesoid X receptor; NR1H4), which, like the LXR, is involved in sterol metabolism, also modulate systemic lipid levels and insulin-sensitivity. In this review, we discuss how these NRs impact insulin secretion via effects on the insulin-sensitivity-insulin secretion feedback loop and, in some cases, via direct effects on the islet itself. In addition, we discuss interactions between these nutrient/metabolite-responsive NRs and NRs that are central to the action of metabolically important hormones, including (i) the glucocorticoid receptor, critical for maintaining glucose homoeostasis in stress, inflammation and during fasting, and (ii) the thyroid hormone receptors, vital for maintenance of oxidative functions. We present data indicating that the RXR occupies a key role in directly modulating islet function and that its heterodimerization with at least two of its partners modulates GSIS.

Differential effects of 24-hydroxycholesterol and 27-hydroxycholesterol on tyrosine hydroxylase and alpha-synuclein in human neuroblastoma SH-SY5Y cells

Evidence suggests that environmental and dietary factors may contribute to the pathogenesis of Parkinson's disease (PD). High dietary intake of cholesterol is such a factor that has been shown to increase or decrease the risk of PD. However, because circulating cholesterol does not cross the blood-brain barrier, the mechanisms linking dietary cholesterol to the pathogenesis of PD remain to be understood. In contrast to cholesterol, the oxidized cholesterol metabolites (oxysterols), 24S-hydroxycholesterol (24-OHC) and 27-hydroxycholesterol (27-OHC), can cross the blood-brain barrier and may place the brain at risk of degeneration. In this study, we incubated the human neuroblastoma SH-SY5Y cells for 24 h with 24-OHC, 27-OHC, or a mixture of 24-OHC plus 27-OHC, and have determined effects on tyrosine hydroxylase (the rate-limiting enzyme in dopamine synthesis) levels, alpha-synuclein levels, and apoptosis. We demonstrate that while 24-OHC increases the levels of tyrosine hydroxylase, 27-OHC increases levels of alpha-synuclein, and induces apoptosis. Our findings show for the first time that oxysterols trigger changes in levels of proteins that are associated with the pathogenesis of PD. As steady state levels of 24-OHC and 27-OHC are tightly regulated in the brain, disturbances in these levels may contribute to the pathogenesis of PD.

Modulation of intestinal cholesterol absorption by high glucose levels: impact on cholesterol transporters, regulatory enzymes, and transcription factors

Growing evidence suggests that the small intestine may contribute to excessive postprandial lipemia, which is highly prevalent in insulin-resistant/Type 2 diabetic individuals and substantially increases the risk of cardiovascular disease. The aim of the present study was to determine the role of high glucose levels on intestinal cholesterol absorption, cholesterol transporter expression, enzymes controlling cholesterol homeostasis, and the status of transcription factors. To this end, we employed highly differentiated and polarized cells (20 days of culture), plated on permeable polycarbonate filters. In the presence of [(14)C]cholesterol, glucose at 25 mM stimulated cholesterol uptake compared with Caco-2/15 cells supplemented with 5 mM glucose (P < 0.04). Because combination of 5 mM glucose with 20 mM of the structurally related mannitol or sorbitol did not change cholesterol uptake, we conclude that extracellular glucose concentration is uniquely involved in the regulation of intestinal cholesterol transport. The high concentration of glucose enhanced the protein expression of the critical cholesterol transporter NPC1L1 and that of CD36 (P < 0.02) and concomitantly decreased SR-BI protein mass (P < 0.02). No significant changes were observed in the protein expression of ABCA1 and ABCG8, which act as efflux pumps favoring cholesterol export out of absorptive cells. At the same time, 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity was decreased (P < 0.007), whereas ACAT activity remained unchanged. Finally, increases were noted in the transcription factors LXR-alpha, LXR-beta, PPAR-beta, and PPAR-gamma along with a drop in the protein expression of SREBP-2. Collectively, our data indicate that glucose at high concentrations may regulate intestinal cholesterol transport and metabolism in Caco-2/15 cells, thus suggesting a potential influence on the cholesterol absorption process in Type 2 diabetes.

Identification of cis-acting promoter sequences required for expression of the glycerol-3-phosphate acyltransferase 1 gene in mice

Glycerol-3-phosphate acyltransferase 1 (GPAT1) is a rate limiting enzyme in de novo glycerophospholipid synthesis. The murine GPAT1 promoter sequence (the "classical" sequence) was reported previously. However, the organization of this DNA sequence does not fully match the mouse genome sequences on NCBI/GenBank. Here we have identified net cis-acting promoter sequences for the mouse GPAT1 gene: promoter 1a which includes part of the classical sequence and the downstream promoter 1b. Promoter 1a facilitates transcription of two alternative GPAT1 transcript variants, GPAT1-V1 and V2, while promoter 1b produces a third transcript variant, GPAT1-V3. Upstream stimulating factor-1 (USF-1) controlled both promoters whereas sterol regulatory element-binding protein-1 (SREBP-1) exclusively regulated promoter 1a activity in vitro. Feeding increased GPAT1-V1 and V2, but not V3 mRNA levels in mouse liver. The obese condition of db/db mice did not alter the hepatic expression levels of any of the three GPAT1 variants. Feeding enhanced hepatic mRNA levels, intranuclear protein levels and promoter 1a-binding levels of SREBP-1, but not of USF-1. Thus, promoter 1a was exclusively activated by routine feeding in vivo. Our results indicate differential roles of the two promoters in the regulation of hepatic GPAT1 gene expression in mice.

Transcriptional control of energy homeostasis by the estrogen-related receptors

Transcriptional control of cellular energy metabolic pathways is achieved by the coordinated action of numerous transcription factors and associated coregulators. Several members of the nuclear receptor superfamily have been shown to play important roles in this process because they can translate hormonal, nutrient, and metabolite signals into specific gene expression networks to satisfy energy demands in response to distinct physiological cues. Estrogen-related receptor (ERR) alpha, ERRbeta, and ERRgamma are nuclear receptors that have yet to be associated with a natural ligand and are thus considered as orphan receptors. However, the transcriptional activity of the ERRs is exquisitely sensitive to the presence of coregulatory proteins known to be essential for the control of energy homeostasis, and for all intents and purposes, these coregulators function as protein ligands for the ERRs. In particular, functional genomics and biochemical studies have shown that ERRalpha and ERRgamma operate as the primary conduits for the activity of members of the family of PGC-1 coactivators. As transcription factors, the ERRs control vast gene networks involved in all aspects of energy homeostasis, including fat and glucose metabolism as well as mitochondrial biogenesis and function. Phenotypic analyses of knockout mouse models have shown that all three ERRs are indispensable for proper development and/or survival of the organism when subjected to a variety of physiological challenges. The focus of this review is on the recent and rapid advances in understanding the functions of the ERRs in regulating bioenergetic pathways, with an emphasis on their roles in the specification of energetic properties required for cell- and tissue-specific functions.

Pancreatic exocrine insufficiency in LXRbeta-/- mice is associated with a reduction in aquaporin-1 expression

Liver X receptors (LXRs) alpha and beta are nuclear oxysterol receptors with a key role in cholesterol, triglyceride, and glucose metabolism. In LXRbeta(-/-) mice on a normal diet, there is a reduction in size of perigonadal fat pad and, on high-fat diet there is resistance to obesity. In the present study, we investigated the reason for the resistance of LXRbeta(-/-) mice to weight gain. In LXRbeta(-/-) mice we found pancreatic exocrine insufficiency with reduced serum levels of amylase and lipase, reduced proteolytic activity in feces, chronic inflammatory infiltration, and, in the ductal epithelium, an increased apoptosis without compensatory proliferation. Electron microscopy revealed ductal dilatation with intraductal laminar structures characteristic of cystic fibrosis. To investigate the relationship between LXRbeta and pancreatic secretion, we studied the expression of LXRbeta and the water channel, aquaporin-1 (AQP1), in the ductal epithelium of the pancreas. In WT mice, ductal epithelial cells expressed LXRbeta in the nuclei and AQP1 on the plasma membrane. In LXRbeta(-/-) mice neither LXRbeta nor AQP1 was detectable. Moreover, in WT mice the LXR agonist (T2320) increased AQP1 gene expression. These data demonstrate that in LXRbeta(-/-) mice dietary resistance to weight gain is caused by pancreatic insufficiency and that LXRbeta regulates pancreatic exocrine secretion through the control of AQP1 expression. Pancreatic exocrine insufficiency is the main cause of malabsorption syndrome responsible for weight loss in adults and growth failure in children. Several genes are known to be involved in the pathogenesis and susceptibility to pancreatic insufficiency. LXRbeta should be included in that list.

Effects of liver X receptor agonist treatment on pulmonary inflammation and host defense

Liver X receptor (LXR) alpha and beta are members of the nuclear receptor superfamily of ligand-activated transcription factors. Best known for triggering "reverse cholesterol transport" gene programs upon their activation by endogenous oxysterols, LXRs have recently also been implicated in regulation of innate immunity. In this study, we define a role for LXRs in regulation of pulmonary inflammation and host defense and identify the lung and neutrophil as novel in vivo targets for pharmacologic LXR activation. LXR is expressed in murine alveolar macrophages, alveolar epithelial type II cells, and neutrophils. Treatment of mice with TO-901317, a synthetic LXR agonist, reduces influx of neutrophils to the lung triggered by inhaled LPS, intratracheal KC chemokine, and intratracheal Klebsiella pneumoniae and impairs pulmonary host defense against this bacterium. Pharmacologic LXR activation selectively modulates airspace cytokine expression induced by both LPS and K. pneumoniae. Moreover, we report for the first time that LXR activation impairs neutrophil motility and identify inhibition of chemokine-induced RhoA activation as a putative underlying mechanism. Taken together, these data define a novel role for LXR in lung pathophysiology and neutrophil biology and identify pharmacologic activation of LXR as a potential tool for modulation of innate immunity in the lung.

Liver X receptor in cooperation with SREBP-1c is a major lipid synthesis regulator in nonalcoholic fatty liver disease

AIM

Nonalcoholic fatty liver disease (NAFLD) is one of the most frequent causes of liver dysfunction and its incidence has increased markedly. However, the mechanisms involved in the pathogenesis of NAFLD in humans have not been thoroughly investigated. Sterol regulatory element binding protein (SREBP)-1c and carbohydrate responsive element binding protein (ChREBP) are transcriptional factors that regulate the expression of lipogenic genes, including acetyl-CoA carboxylases (ACCs) and fatty acid synthase (FAS). SREBP-1c and ChREBP are transactivated by liver X receptor (LXR), a nuclear receptor that regulates the metabolism of cholesterol and fatty acids. To understand the mechanisms involved in the pathogenesis of NAFLD, we investigated the transcriptional factors and lipogenic genes activated in the liver with NAFLD.

METHODS

Real-time PCR was carried out on liver biopsy samples from 20 NAFLD patients. The target genes studied were: ACC1, FAS, SREBP-1c, ChREBP, AMP-activated protein kinase (AMPK), and LXRalpha.

RESULTS

LXRalpha, SREBP-1c, ACC1, and FAS were upregulated in NAFLD patients. Expression levels of LXR were four times greater than those of the controls and correlated significantly with SREBP-1c, but not with ChREBP, levels.

CONCLUSIONS

These findings suggest that LXR acts as one of the main regulators of lipid metabolism by regulating SREBP-1c expression in NAFLD.

The early steps of glucose signalling in yeast

In the presence of glucose, yeast undergoes an important remodelling of its metabolism. There are changes in the concentration of intracellular metabolites and in the stability of proteins and mRNAs; modifications occur in the activity of enzymes as well as in the rate of transcription of a large number of genes, some of the genes being induced while others are repressed. Diverse combinations of input signals are required for glucose regulation of gene expression and of other cellular processes. This review focuses on the early elements in glucose signalling and discusses their relevance for the regulation of specific processes. Glucose sensing involves the plasma membrane proteins Snf3, Rgt2 and Gpr1 and the glucose-phosphorylating enzyme Hxk2, as well as other regulatory elements whose functions are still incompletely understood. The similarities and differences in the way in which yeasts and mammalian cells respond to glucose are also examined. It is shown that in Saccharomyces cerevisiae, sensing systems for other nutrients share some of the characteristics of the glucose-sensing pathways.

Pck1 gene silencing in the liver improves glycemia control, insulin sensitivity, and dyslipidemia in db/db mice

OBJECTIVE

Cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C; encoded by Pck1) catalyzes the first committed step in gluconeogenesis. Extensive evidence demonstrates a direct correlation between PEPCK-C activity and glycemia control. Therefore, we aimed to evaluate the metabolic impact and their underlying mechanisms of knocking down hepatic PEPCK-C in a type 2 diabetic model.

RESEARCH DESIGN AND METHODS

PEPCK-C gene targeting was achieved using adenovirus-transduced RNAi. The study assessed several clinical symptoms of diabetes and insulin signaling in peripheral tissues, in addition to changes in gene expression, protein, and metabolites in the liver. Liver bioenergetics was also evaluated.

RESULTS

Treatment resulted in reduced PEPCK-C mRNA and protein. After treatment, improved glycemia and insulinemia, lower triglyceride, and higher total and HDL cholesterol were measured. Unsterified fatty acid accumulation was observed in the liver, in the absence of de novo lipogenesis. Despite hepatic lipidosis, treatment resulted in improved insulin signaling in the liver, muscle, and adipose tissue. O(2) consumption measurements in isolated hepatocytes demonstrated unaltered mitochondrial function and a consequent increased cellular energy charge. Key regulatory factors (FOXO1, hepatocyte nuclear factor-4alpha, and peroxisome proliferator-activated receptor-gamma coactivator [PGC]-1alpha) and enzymes (G6Pase) implicated in gluconeogenesis were downregulated after treatment. Finally, the levels of Sirt1, a redox-state sensor that modulates gluconeogenesis through PGC-1alpha, were diminished.

CONCLUSIONS

Our observations indicate that silencing PEPCK-C has direct impact on glycemia control and energy metabolism and provides new insights into the potential significance of the enzyme as a therapeutic target for the treatment of diabetes.

ChREBP: a glucose-activated transcription factor involved in the development of metabolic syndrome

Excess carbohydrate intake leads to fat accumulation and insulin resistance. Glucose and insulin coordinately regulate de novo lipogenesis from glucose in the liver, and insulin activates several transcription factors including SREBP1c and LXR, while those activated by glucose remain unknown. Recently, a carbohydrate response element binding protein (ChREBP), which binds to the carbohydrate response element (ChoRE) in the promoter of rat liver type pyruvate kinase (LPK), has been identified. The target genes of ChREBP are involved in glycolysis, lipogenesis, and gluconeogenesis. Although the regulation of ChREBP remains unknown in detail, the transactivity of ChREBP is partly regulated by a phosphorylation/dephosphorylation mechanism. During fasting, protein kinase A and AMP-activated protein kinase phosphorylate ChREBP and inactivate its transactivity. During feeding, xylulose-5-phosphate in the hexose monophosphate pathway activates protein phosphatase 2A, which dephosphorylates ChREBP and activates its transactivity. ChREBP controls 50% of hepatic lipogenesis by regulating glycolytic and lipogenic gene expression. In ChREBP (-/-) mice, liver triglyceride content is decreased and liver glycogen content is increased compared to wild-type mice. These results indicate that ChREBP can regulate metabolic gene expression to convert excess carbohydrate into triglyceride rather than glycogen. Furthermore, complete inhibition of ChREBP in ob/ob mice reduces the effects of the metabolic syndrome such as obesity, fatty liver, and glucose intolerance. Thus, further clarification of the physiological role of ChREBP may be useful in developing treatments for the metabolic syndrome.

Determinants of leukocyte adenosine triphosphate-binding cassette transporter G1 gene expression in type 2 diabetes mellitus

Cellular cholesterol efflux is regulated by cholesterol transporters including adenosine triphosphate-binding cassette transporter A1 (ABCA1), ABCG1, and scavenger receptor class B type I (SR-BI). We have investigated whether the expression of these transporters is affected by type 2 diabetes mellitus and the association with glycemic indexes and oxidized low-density lipoprotein (oxLDL). Messenger RNA of ABCA1, ABCG1, and SR-BI in peripheral monocytes was measured in 30 diabetic patients and 30 matched controls. Plasma oxLDL and advanced glycation end products (AGEs) were assayed by enzyme-linked immunosorbent assay. Cellular cholesterol efflux from monocytes to serum was determined in a subgroup chosen randomly. The expression of ABCG1 was decreased in diabetic patients (P < .05), whereas the levels of ABCA1 and SR-BI were comparable between the 2 groups. Fasting glucose, hemoglobin A(1c), AGEs, and oxLDL were all significantly increased in diabetic patients. There was an inverse correlation between serum AGEs and ABCG1 (r = -0.44, P < .05) that remained significant after adjusting for potential confounding factors. No associations between fasting glucose, hemoglobin A(1c), plasma lipids, or oxLDL and the expression of ABCG1, ABCA1, or SR-BI were found. Cholesterol efflux from monocytes to standard serum or autologous serum was significantly impaired in diabetic patients, and the reduction in efflux to autologous serum correlated with the expression of ABCG1 (r = 0.60, P < .05). The expression of ABCG1 in monocytes is reduced in type 2 diabetes mellitus and is partly related to serum AGEs concentration. The reduction in ABCG1 is associated with impairment in cholesterol efflux and may contribute to accelerated foam cell formation in diabetic patients.

Research resource: nuclear hormone receptor expression in the endocrine pancreas

The endocrine pancreas comprises the islets of Langerhans, tiny clusters of cells that contribute only about 2% to the total pancreas mass. However, this little endocrine organ plays a critical role in maintaining glucose homeostasis by the regulated secretion of insulin (by beta-cells) and glucagon (by alpha-cells). The rapid increase in the incidence of diabetes worldwide has spurred renewed interest in islet cell biology. Some of the most widely prescribed oral drugs for treating type 2 diabetes include agents that bind and activate the nuclear hormone receptor, peroxisome proliferator-activated receptor-gamma. As a first step in addressing potential roles of peroxisome proliferator-activated receptor-gamma and other nuclear hormone receptors (NHRs) in the biology of the endocrine pancreas, we have used quantitative real-time PCR to profile the expression of all 49 members of the mouse NHR superfamily in primary islets, and cell lines that represent alpha-cells (alphaTC1) and beta-cells (betaTC6 and MIN6). In summary, 19 NHR members were highly expressed in both alpha- and beta-cell lines, 13 receptors showed predominant expression (at least an 8-fold difference) in alpha- vs. beta-cell lines, and 10 NHRs were not expressed in the endocrine pancreas. In addition we evaluated the relative expression of these transcription factors during hyperglycemia and found that 16 NHRs showed significantly altered mRNA levels in mouse islets. A similar survey was conducted in primary human islets to reveal several significant differences in NHR expression between mouse and man. These data identify potential therapeutic targets in the endocrine pancreas for the treatment of diabetes mellitus.

Comparative transcriptomics of human multipotent stem cells during adipogenesis and osteoblastogenesis

Background

A reciprocal relationship between bone and fat development in osteoporosis is clinically well established. Some of the key molecular regulators involved in this tissue replacement process have been identified. The detailed mechanisms governing the differentiation of mesenchymal stem cells (MSC) – the key cells involved – are however only now beginning to emerge. In an attempt to address the regulation of the adipocyte/osteoblast balance at the level of gene transcription in a comprehensive and unbiased manner, we performed a large-scale gene expression profiling study using a unique cellular model, human multipotent adipose tissue-derived stem cells (hMADS).

Results

The analysis of 1606 genes that were found to be differentially expressed between adipogenesis and osteoblastogenesis revealed gene repression to be most prevalent prior to commitment in both lineages. Computational analyses suggested that this gene repression is mediated by miRNAs. The transcriptional activation of lineage-specific molecular processes in both cases occurred predominantly after commitment. Analyses of the gene expression data and promoter sequences produced a set of 65 genes that are candidates for genes involved in the process of adipocyte/osteoblast commitment. Four of these genes were studied in more detail: LXRα and phospholipid transfer protein (PLTP) for adipogenesis, the nuclear receptor COUP-TF1 and one uncharacterized gene, TMEM135 for osteoblastogenesis. PLTP was secreted during both early and late time points of hMADS adipocyte differentiation. LXRα, COUP-TF1, and the transmembrane protein TMEM135 were studied in primary cultures of differentiating bone marrow stromal cells from healthy donors and were found to be transcriptionally activated in the corresponding lineages.

Conclusion

Our results reveal gene repression as a predominant early mechanism before final cell commitment. We were moreover able to identify 65 genes as candidates for genes controlling the adipocyte/osteoblast balance and to further evaluate four of these. Additional studies will explore the precise role of these candidate genes in regulating the adipogenesis/osteoblastogenesis switch.

Lxralpha deficiency hampers the hepatic adaptive response to fasting in mice

Besides its well established role in control of cellular cholesterol homeostasis, the liver X receptor (LXR) has been implicated in the regulation of hepatic gluconeogenesis. We investigated the role of the major hepatic LXR isoform in hepatic glucose metabolism during the feeding-to-fasting transition in vivo. In addition, we explored hepatic glucose sensing by LXR during carbohydrate refeeding. Lxralpha(-/-) mice and their wild-type littermates were subjected to a fasting-refeeding protocol and hepatic carbohydrate fluxes as well as whole body insulin sensitivity were determined in vivo by stable isotope procedures. Lxralpha(-/-) mice showed an impaired response to fasting in terms of hepatic glycogen depletion and triglyceride accumulation. Hepatic glucose 6-phosphate turnover was reduced in 9-h fasted Lxralpha(-/-) mice as compared with controls. Although hepatic gluconeogenic gene expression was increased in 9-h fasted Lxralpha(-/-) mice compared with wild-type controls, the actual gluconeogenic flux was not affected by Lxralpha deficiency. Hepatic and peripheral insulin sensitivity were similar in Lxralpha(-/-) and wild-type mice. Compared with wild-type controls, the induction of hepatic lipogenic gene expression was blunted in carbohydrate-refed Lxralpha(-/-) mice, which was associated with lower plasma triglyceride concentrations. Yet, expression of "classic" LXR target genes Abca1, Abcg5, and Abcg8 was not affected by Lxralpha deficiency in carbohydrate-refed mice. In summary, these studies identify LXRalpha as a physiologically relevant mediator of the hepatic response to fasting. However, the data do not support a role for LXR in hepatic glucose sensing.

Carbohydrate responsive element binding protein and lipid homeostasis

PURPOSE OF REVIEW

The liver is responsible for the conversion of excess dietary carbohydrates into fatty acids, through de-novo lipogenesis. A clear understanding of the control of lipogenesis is crucial since excess fatty acids leads to hepatic steatosis and associated metabolic diseases. The transcription factor sterol regulatory element binding protein 1c and the nuclear receptor liver X receptor are implicated in the insulin-mediated induction of lipogenic genes. Recently, the transcription factor carbohydrate responsive element binding protein has emerged as the hepatic glucose sensor required for the induction of lipogenic genes in response to glucose.

RECENT FINDINGS

We have recently demonstrated that the liver-specific inhibition of carbohydrate responsive element binding protein decreases the rate of lipogenesis and improves hepatic steatosis and insulin resistance in obese ob/ob mice. These results suggest that carbohydrate responsive element binding protein is a potential therapeutic target, and an accurate knowledge of the mechanisms involved in regulating its expression or activation is needed for the development of pharmacological approaches for the treatment of metabolic diseases. Recent studies report that carbohydrate responsive element binding protein is regulated at the transcriptional level by glucose and by liver X receptor but that posttranslational modifications are needed for carbohydrate responsive element binding protein to become active.

SUMMARY

Here we review some of the studies that provided a better understanding of the role and regulation of the newly identified transcription factor carbohydrate responsive element binding protein in lipid homeostasis.

Role of stearoyl-coenzyme A desaturase in regulating lipid metabolism

PURPOSE OF REVIEW

Stearoyl-coenzyme A desaturase 1 is a delta-9 fatty acid desaturase that catalyzes the synthesis of monounsaturated fatty acids and has emerged as a key regulator of metabolism. This review evaluates the latest advances in our understanding of the pivotal role of stearoyl-coenzyme A desaturase 1 in health and disease.

RECENT FINDINGS

Scd1-deficient mice have reduced lipid synthesis and enhanced lipid oxidation, thermogenesis and insulin sensitivity in various tissues including liver, muscle and adipose tissue due to transcriptional and posttranscriptional effects. These metabolic changes protect Scd1-deficient mice from a variety of dietary, pharmacological and genetic conditions that promote obesity, insulin resistance and hepatic steatosis. Stearoyl-coenzyme A desaturase 1 is required to guard against dietary unsaturated fat deficiency, leptin deficiency-induced diabetes, and palmitate-induced lipotoxic insults in muscle and pancreatic beta-cells. Paradoxical observations of increased muscle stearoyl-coenzyme A desaturase 1 during obesity, starvation and exercise raise questions as to the role of stearoyl-coenzyme A desaturase 1 in this tissue. Mice with a liver-specific loss of stearoyl-coenzyme A desaturase 1, and inhibition of stearoyl-coenzyme A desaturase 1 via antisense or RNA interference, recapitulate only a subset of the phenotypes observed in global Scd1 deficiency, indicating the involvement of multiple tissues.

SUMMARY

Recent studies in humans and animal models have highlighted that modulation of stearoyl-coenzyme A desaturase 1 activity by dietary intervention or genetic manipulation strongly influences several facets of energy metabolism to affect susceptibility to obesity, insulin resistance, diabetes and hyperlipidemia.

N-3 polyunsaturated fatty acid regulation of hepatic gene transcription

PURPOSE OF REVIEW

The liver plays a central role in whole body lipid metabolism and adapts rapidly to changes in dietary fat composition. This adaption involves changes in the expression of genes involved in glycolysis, de-novo lipogenesis, fatty acid elongation, desaturation and oxidation. This review brings together metabolic and molecular studies that help explain n-3 (omega-3) polyunsaturated fatty acid regulation of hepatic gene transcription.

RECENT FINDINGS

Dietary n-3 polyunsaturated fatty acid regulates hepatic gene expression by targeting three major transcriptional regulatory networks: peroxisome proliferator-activated receptor alpha, sterol regulatory element binding protein-1 and the carbohydrate regulatory element binding protein/Max-like factor X heterodimer. 22:6,n-3, the most prominent n-3 polyunsaturated fatty acid in tissues, is a weak activator of peroxisome proliferator-activated receptor alpha. Hepatic metabolism of 22:6,n-3, however, generates 20:5,n-3, a strong peroxisome proliferator-activated receptor alpha activator. In contrast to peroxisome proliferator-activated receptor alpha, 22:6,n-3 is the most potent fatty acid regulator of hepatic sterol regulatory element binding protein-1. 22:6,n-3 suppresses sterol regulatory element binding protein-1 gene expression while enhancing degradation of nuclear sterol regulatory element binding protein-1 through 26S proteasome and Erk1/2-dependent mechanisms. Both n-3 and n-6 polyunsaturated fatty acid suppress carbohydrate regulatory element binding protein and Max-like factor X nuclear abundance and interfere with glucose-regulated hepatic metabolism.

SUMMARY

These studies have revealed unique mechanisms by which specific polyunsaturated fatty acids control peroxisome proliferator activated receptor alpha, sterol regulatory element binding protein-1 and carbohydrate regulatory element binding protein/Max-like factor X function. As such, specific metabolic and signal transduction pathways contribute significantly to the fatty acid regulation of these transcription factors and their corresponding regulatory networks.

Liver X Receptor: Crosstalk Node for the Signaling of Lipid Metabolism, Carbohydrate Metabolism, and Innate Immunity

Liver X Receptor-α (LXRα, also known as NR1H3) and LXRβ (NR1H2) are members of the nuclear receptor superfamily of ligand-activated transcription factors, a superfamily which includes the more widely known glucocorticoid receptor, estrogen receptor, thyroid receptor, and peroxisome proliferator-activated receptors. The LXRs are activated by physiologic sterol ligands (e.g., oxysterols) and by synthetic agonists. In recent years, our understanding of the importance of LXRs has expanded across several fields of (patho-)physiology. Perhaps best known from a sizeable literature as homeostatic 'cholesterol sensors' that drive transcriptional programs promoting cellular cholesterol efflux, 'reverse cholesterol transport,' and bile acid synthesis, more recent roles for LXRs in glucose homeostasis, atherosclerosis, and innate immunity have also been identified. These discoveries complement an emerging literature that continues to draw surprisingly intimate connections between host metabolism and host defense. The present review will discuss the roles of LXR in the signaling of metabolism and innate immunity, and the potential for synthetic LXR agonists as novel therapeutics in dyslipidemia, atherosclerosis, disordered glucose metabolism, and inflammation.

A cholesterol-free, high-fat diet suppresses gene expression of cholesterol transporters in murine small intestine

Transporters present in the epithelium of the small intestine determine the efficiency by which dietary and biliary cholesterol are taken up into the body and thus control whole-body cholesterol balance. Niemann-Pick C1 Like Protein 1 (Npc1l1) transports cholesterol into the enterocyte, whereas ATP-binding cassette transporters Abca1 and Abcg5/Abcg8 are presumed to be involved in cholesterol efflux from the enterocyte toward plasma HDL and back into the intestinal lumen, respectively. Abca1, Abcg5, and Abcg8 are well-established liver X receptor (LXR) target genes. We examined the effects of a high-fat diet on expression and function of cholesterol transporters in the small intestine in mice. Npc1l1, Abca1, Abcg5, and Abcg8 were all downregulated after 2, 4, and 8 wk on a cholesterol-free, high-fat diet. The high-fat diet did not affect biliary cholesterol secretion but diminished fractional cholesterol absorption from 61 to 42% (P < 0.05). In an acute experiment in which triacylglycerols of unsaturated fatty acids were given by gavage, we found that this downregulation occurs within a 6-h time frame. Studies in LXRalpha-null mice, confirmed by in vitro data, showed that fatty acid-induced downregulation of cholesterol transporters is LXRalpha independent and associated with a posttranslational increase in 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity that reflects induction of cholesterol biosynthesis as well as with a doubling of neutral fecal sterol loss. This study highlights the induction of adaptive changes in small intestinal cholesterol metabolism during exposure to dietary fat.