The nuclear receptor LXR is a glucose sensor

May diet and dietary supplements improve the wellness of multiple sclerosis patients? A molecular approach

Multiple sclerosis is a complex and multifactorial neurological disease, and nutrition is one of the environmental factors possibly involved in its pathogenesis. At present, the role of nutrition is unclear, and MS therapy is not associated to a particular diet. MS clinical trials based on specific diets or dietary supplements are very few and in some cases controversial. To understand how diet can influence the course of MS and improve the wellness of MS patients, it is necessary to identify the dietary molecules, their targets and the molecular mechanisms involved in the control of the disease. The aim of this paper is to provide a molecular basis for the nutritional intervention in MS by evaluating at molecular level the effect of dietary molecules on the inflammatory and autoimmune processes involved in the disease.

Gene regulation mediating fiber-type transformation in skeletal muscle cells is partly glucose- and ChREBP-dependent

Adaptations in the oxidative capacity of skeletal muscle cells can occur under several physiological or pathological conditions. We investigated the effect of increasing extracellular glucose concentration on the expression of markers of energy metabolism in primary skeletal muscle cells and the C2C12 muscle cell line. Growth of myotubes in 25mM glucose (high glucose, HG) compared with 5.55mM led to increases in the expression and activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a marker of glycolytic energy metabolism, while oxidative markers peroxisome proliferator-activated receptor γ coactivator 1α and citrate synthase decreased. HG induced metabolic adaptations as are seen during a slow-to-fast fiber transformation. Furthermore, HG increased fast myosin heavy chain (MHC) IId/x but did not change slow MHCI/β expression. Protein phosphatase 2A (PP2A) was shown to mediate the effects of HG on GAPDH and MHCIId/x. Carbohydrate response element-binding protein (ChREBP), a glucose-dependent transcription factor downstream of PP2A, partially mediated the effects of glucose on metabolic markers. The glucose-induced increase in PP2A activity was associated with an increase in p38 mitogen-activated protein kinase activity, which presumably mediates the increase in MHCIId/x promoter activity. Liver X receptor, another possible mediator of glucose effects, induced only an incomplete metabolic shift, mainly increasing the expression of the glycolytic marker. Taken together, HG induces a partial slow-to-fast transformation comprising metabolic enzymes together with an increased expression of MHCIId/x. This work demonstrates a functional role for ChREBP in determining the metabolic type of muscle fibers and highlights the importance of glucose as a signaling molecule in muscle.

Carbohydrate-response-element-binding protein (ChREBP) and not the liver X receptor α (LXRα) mediates elevated hepatic lipogenic gene expression in a mouse model of glycogen storage disease type 1

GSD-1 (glycogen storage disease type 1) is caused by an inherited defect in glucose-6-phosphatase activity, resulting in a massive accumulation of hepatic glycogen content and an induction of de novo lipogenesis. The chlorogenic acid derivative S4048 is a pharmacological inhibitor of the glucose 6-phosphate transporter, which is part of glucose-6-phosphatase, and allows for mechanistic studies concerning metabolic defects in GSD-1. Treatment of mice with S4048 resulted in an ~60% reduction in blood glucose, increased hepatic glycogen and triacylglycerol (triglyceride) content, and a markedly enhanced hepatic lipogenic gene expression. In mammals, hepatic expression of lipogenic genes is regulated by the co-ordinated action of the transcription factors SREBP (sterol-regulatory-element-binding protein)-1c, LXRα (liver X receptor α) and ChREBP (carbohydrate-response-element-binding protein). Treatment of Lxra-/- mice and Chrebp-/- mice with S4048 demonstrated that ChREBP, but not LXRα, mediates the induction of hepatic lipogenic gene expression in this murine model of GSD-1. Thus ChREBP is an attractive target to alleviate derangements in lipid metabolism observed in patients with GSD-1.

Liver X receptors as regulators of macrophage inflammatory and metabolic pathways

The liver X receptors (LXRα and LXRβ) are members of the nuclear receptor family of transcription factors that play essential roles in the transcriptional control of lipid metabolism. LXRs are endogenously activated by modified forms of cholesterol known as oxysterols and control the expression of genes important for cholesterol uptake, efflux, transport, and excretion in multiple tissues. In addition to their role as cholesterol sensors, a number of studies have implicated LXRs in the modulation of innate and adaptive immune responses. Both through activation and repression mechanisms, LXRs regulate diverse aspects of inflammatory gene expression in macrophages. The ability of LXRs to coordinate metabolic and immune responses constitutes an attractive therapeutic target for the treatment of chronic inflammatory disorders. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease.

Transcriptional activation of apolipoprotein CIII expression by glucose may contribute to diabetic dyslipidemia

OBJECTIVE

Hypertriglyceridemia and fatty liver are common in patients with type 2 diabetes, but the factors connecting alterations in glucose metabolism with plasma and liver lipid metabolism remain unclear. Apolipoprotein CIII (apoCIII), a regulator of hepatic and plasma triglyceride metabolism, is elevated in type 2 diabetes. In this study, we analyzed whether apoCIII is affected by altered glucose metabolism.

METHODS AND RESULTS

Liver-specific insulin receptor-deficient mice display lower hepatic apoCIII mRNA levels than controls, suggesting that factors other than insulin regulate apoCIII in vivo. Glucose induces apoCIII transcription in primary rat hepatocytes and immortalized human hepatocytes via a mechanism involving the transcription factors carbohydrate response element-binding protein and hepatocyte nuclear factor-4α. ApoCIII induction by glucose is blunted by treatment with agonists of farnesoid X receptor and peroxisome proliferator-activated receptor-α but not liver X receptor, ie, nuclear receptors controlling triglyceride metabolism. Moreover, in obese humans, plasma apoCIII protein correlates more closely with plasma fasting glucose and glucose excursion after oral glucose load than with insulin.

CONCLUSIONS

Glucose induces apoCIII transcription, which may represent a mechanism linking hyperglycemia, hypertriglyceridemia, and cardiovascular disease in type 2 diabetes.

Both liver-X receptor (LXR) isoforms control energy expenditure by regulating brown adipose tissue activity

Brown adipocytes are multilocular lipid storage cells that play a crucial role in nonshivering thermogenesis. Uncoupling protein 1 (UCP1) is a unique feature of brown fat cells that allows heat generation on sympathetic nervous system stimulation. As conventional transcriptional factors that are activated in various signaling pathways, liver-X receptors (LXRs) play important roles in many physiological processes. The role of LXRs in the regulation of energy homeostasis remains unclear, however. Female WT, LXRαβ(-/-), LXRα(-/-), and LXRβ(-/-) mice were fed with either a normal diet (ND) or a high-carbohydrate diet (HCD) supplemented with or without GW3965-LXR agonist. LXRαβ(-/-) mice exhibited higher energy expenditure (EE) as well as higher UCP1 expression in brown adipose tissue (BAT) compared with WT mice on the HCD. In addition, long-term treatment of WT mice with GW3965 showed lower EE at thermoneutrality (30 °C) and lower Ucp1 expression level in BAT. Furthermore, H&E staining of the BAT of LXRαβ(-/-) mice exhibited decreased lipid droplet size compared with WT mice on the HCD associated with a more intense UCP1-positive reaction. Quantification of triglyceride (TG) content in BAT showed lower TG accumulation in LXRβ(-/-) mice compared with WT mice. Surprisingly, GW3965 treatment increased TG content (twofold) in the BAT of WT and LXRα(-/-) mice but not in LXRβ(-/-) mice. Furthermore, glucose transporter (GLUT4) in the BAT of LXRα(-/-) and LXRβ(-/-) mice was sixfold and fourfold increased, respectively, compared with WT mice on the ND. These findings suggest that LXRα as well as LXRβ could play a crucial role in the regulation of energy homeostasis in female mice and may be a potential target for the treatment of obesity and energy regulation.

Modification at the Lipophilic Domain of RXR Agonists Differentially Influences Activation of RXR Heterodimers

RXR permissive heterodimers are reported to be activated differently depending upon the chemical structure of RXR agonists, but the relationship of agonist structure to differential heterodimer activation has not been explored in detail. In this study, we performed systematic conversion of the alkoxy side chain of 5a (6-[ethyl-(3-isopropoxy-4-isopropylphenyl)amino]nicotinic acid, NEt-3IP) and evaluated the RXR-, PPAR/RXR-, and LXR/RXR-agonist activities of the products. The cyclopropylmethoxy analogue (5c) showed similar RXR- and LXR/RXR-agonistic activities to the benzyloxy analogue (5i) and n-propoxy analogue (5k) but exhibited more potent PPAR/RXR-agonistic activity than 5i or 5k. Differential modulation of RXR heterodimer-activating ability by conversion of the alkoxy group located in the lipophilic domain of the RXR-agonist common structure is expected be a useful approach in the design of new RXR agonists for the treatment of hyperlipidemia or type 2 diabetes.

Systems analysis of small signaling modules relevant to eight human diseases

Using eight newly generated models relevant to addiction, Alzheimer’s disease, cancer, diabetes, HIV, heart disease, malaria, and tuberculosis, we show that systems analysis of small (4–25 species), bounded protein signaling modules rapidly generates new quantitative knowledge from published experimental research. For example, our models show that tumor sclerosis complex (TSC) inhibitors may be more effective than the rapamycin (mTOR) inhibitors currently used to treat cancer, that HIV infection could be more effectively blocked by increasing production of the human innate immune response protein APOBEC3G, rather than targeting HIV’s viral infectivity factor (Vif), and how peroxisome proliferator-activated receptor alpha (PPARα) agonists used to treat dyslipidemia would most effectively stimulate PPARα signaling if drug design were to increase agonist nucleoplasmic concentration, as opposed to increasing agonist binding affinity for PPARα. Comparative analysis of system-level properties for all eight modules showed that a significantly higher proportion of concentration parameters fall in the top 15th percentile sensitivity ranking than binding affinity parameters. In infectious disease modules, host networks were significantly more sensitive to virulence factor concentration parameters compared to all other concentration parameters. This work supports the future use of this approach for informing the next generation of experimental roadmaps for known diseases.

Changes in adipocyte cell size, gene expression of lipid metabolism markers, and lipolytic responses induced by dietary fish oil replacement in gilthead sea bream (Sparus aurata L.)

The effects of fish oil (FO) substitution by 66% vegetable oils in a diet with already 75% vegetable protein (66VO) on adipose tissue lipid metabolism of gilthead sea bream were analysed after a 14-month feeding trial. In the last 3 months of the experiment, a FO diet was administrated to a 66VO group (group 66VO/FO) as a finishing diet. Hormone-sensitive lipase (HSL) activity was measured in adipose tissue and adipocyte size, and HSL, lipoprotein lipase and liver X receptor gene expression in isolated adipocytes, on which lipolysis and glucose uptake experiments were also performed. Lipolysis was measured after incubation with tumour necrosis factor-α (TNFα), linoleic acid, and two conjugated linoleic acid isomers. Glucose uptake was analysed after TNFα or insulin administration. Our results show that FO replacement increased lipolytic activity and adipocyte cell size. The higher proportion of large cells observed in the 66VO group could be involved in their observed lower response to fatty acid treatments and lower insulin sensitivity. The 66VO/FO group showed a moderate return to the FO conditions. Therefore, FO replacement can affect the morphology and metabolism of gilthead sea bream adipocytes which could potentially affect other organs such as the liver.

Transcriptional regulation of human and rat hepatic lipid metabolism by the grapefruit flavonoid naringenin: role of PPARalpha, PPARgamma and LXRalpha

Disruption of lipid and carbohydrate homeostasis is an important factor in the development of prevalent metabolic diseases such as diabetes, obesity, and atherosclerosis. Therefore, small molecules that could reduce insulin dependence and regulate dyslipidemia could have a dramatic effect on public health. The grapefruit flavonoid naringenin has been shown to normalize lipids in diabetes and hypercholesterolemia, as well as inhibit the production of HCV. Here, we demonstrate that naringenin regulates the activity of nuclear receptors PPARα, PPARγ, and LXRα. We show it activates the ligand-binding domain of both PPARα and PPARγ, while inhibiting LXRα in GAL4-fusion reporters. Using TR-FRET, we show that naringenin is a partial agonist of LXRα, inhibiting its association with Trap220 co-activator in the presence of TO901317. In addition, naringenin induces the expression of PPARα co-activator, PGC1α. The flavonoid activates PPAR response element (PPRE) while suppressing LXRα response element (LXRE) in human hepatocytes, translating into the induction of PPAR-regulated fatty acid oxidation genes such as CYP4A11, ACOX, UCP1 and ApoAI, and inhibition of LXRα-regulated lipogenesis genes, such as FAS, ABCA1, ABCG1, and HMGR. This effect results in the induction of a fasted-like state in primary rat hepatocytes in which fatty acid oxidation increases, while cholesterol and bile acid production decreases. Our findings explain the myriad effects of naringenin and support its continued clinical development. Of note, this is the first description of a non-toxic, naturally occurring LXRα inhibitor.

The effect of colesevelam hydrochloride on insulin sensitivity and secretion in patients with type 2 diabetes: a pilot study

BACKGROUND

This study evaluated the effect of colesevelam hydrochloride on insulin sensitivity, potential binding to glucose, and chronic effect(s) on fasting and postprandial glucose and insulin in patients with type 2 diabetes mellitus.

METHODS

Patients meeting inclusion criteria were withdrawn from all antidiabetes agents for 2 weeks and randomized to colesevelam 3.75 grams/day (n = 17) or placebo (n = 18) for 8 weeks. Hyperinsulinemic-euglycemic clamp studies were performed at baseline (week -1) and weeks 2 and 8. A meal tolerance test was conducted at weeks -1, 0, 2, and 8. The meal tolerance test and study drug were coadministered at week 0 to assess the direct effect of colesevelam on glucose absorption.

RESULTS

Insulin sensitivity as measured by the insulin clamp method did not change, but the Matsuda Index, a measure of whole-body insulin sensitivity calculated from postmeal tolerance test glucose and insulin levels, increased significantly within the colesevelam group from baseline to week 8 with last observation carried forward (P = 0.020). The postprandial area under the curve for glucose decreased with colesevelam versus placebo at weeks 2 and 8 with last observation carried forward (P = 0.012 and P = 0.061, respectively); the area under the curve for insulin did not decrease in concert with the decrease in area under the curve for glucose at week 2 (P = 0.585). Colesevelam had no effect on postmeal tolerance test glucose levels at week 0.

CONCLUSIONS

These results suggest that colesevelam has no effect on peripheral insulin sensitivity or glucose absorption, but may improve glucose control by improving whole-body insulin sensitivity, although not by an acute effect on glucose absorption. CLINICAL TRIAL IDENTIFIER: NCT00361153.

Coronary heart disease: Significance of liver X receptor α genomics

Crosstalk between lipid peroxidation and inflammation is known to be a pathognomonic feature for the development of coronary heart disease (CHD). In this regard ligand activated liver X receptor (LXR)-α has emerged as a key molecular switch by its inherent ability to modulate an array of genes involved in these two fundamental cellular processes. In addition, LXR-α has also been found to play a role in hepatic lipogenesis and innate immunity. Although several lines of evidence in experimental model systems have established the atheroprotective nature of LXR-α, human subjects have been reported to possess a paradoxical situation in which increased blood cellular LXR-α gene expression is always accompanied by increased coronary occlusion. This apparent paradox was resolved recently by the finding that CHD patients possess a deregulated LXR-α transcriptome due to impaired ligand-receptor interaction. This blood cellular mutated LXR-α gene expression correlated specifically with the extent of coronary occlusion and hence need is felt to devise new synthetic ligands that could restore the function of this mutated LXR-α protein in order to modulate genes involved in reverse cholesterol transport and suppression of the inflammatory response leading to the effective treatment of CHD.

The Role of Colesevelam Hydrochloride in Hypercholesterolemia and Type 2 Diabetes Mellitus

Objective:

To evaluate the safety and efficacy of colesevelam hydrochloride for the treatment of hypercholesterolemia and type 2 diabetes mellitus.

Data Sources:

Literature retrieval was accessed through MEDLINE/PubMed (1950–March 2010), Web of Science (1980–March 2010), and International Pharmaceutical Abstracts (1977–March 2010) using the terms colesevelam, dyslipidemia, hypercholesterolemia, and type 2 diabetes mellitus. References from publications identified were reviewed for additional resources. In addition, abstracts presented at the most recent (2009) American Diabetes Association, American Association of Clinical Endocrinologists, and European Association for the Study of Diabetes annual meetings were searched for relevant original research.

Study Selection and Data Extraction:

All articles in English identified from the data sources were evaluated. All relevant studies evaluating the safety and efficacy of colesevelam in hypercholesterolemia and/or type 2 diabetes mellitus were included. Priority was placed on data obtained from human randomized controlled trials.

Data Synthesis:

Seventeen clinical trials were reviewed and evaluated. Of the clinical trials evaluating colesevelam in hypercholesterolemia, 3 evaluated monotherapy, 4 evaluated combination therapy with hydroxymethylglutaryl coenzyme A (HMG-CoA) reductase inhibitors, and 6 evaluated combination therapy with other lipid-lowering therapies. In the type 2 diabetes mellitus trials, colesevelam was evaluated in combination with metformin, sulfonylureas, insulin, and rosiglitazone and sitagliptin. A review of the clinical trials provided evidence that colesevelam monotherapy effectively reduces low-density lipoprotein cholesterol (LDL-C). Additionally, the use of colesevelam in combination with other lipid-lowering therapies further reduces LDL-C. Colesevelam also effectively reduces hemoglobin A1c in patients with type 2 diabetes mellitus. The safety and tolerability of colesevelam appear to be improved from that of older-generation bile acid sequestrants, with adverse effects similar to those with placebo in monotherapy and type 2 diabetes mellitus trials.

Conclusions:

Colesevelam is a safe and effective option for the treatment of hypercholesterolemia and type 2 diabetes mellitus. It can fulfill a useful role in combination with HMG-CoA reductase inhibitors for hypercholesterolemia and should be considered in patients with type 2 diabetes mellitus with concomitant hypercholesterolemia.

A review of the response to oral antidiabetes agents in patients with type 2 diabetes

In patients with type 2 diabetes, a progressive decline in glycemic control exacerbates the microvascular (retinopathy, neuropathy, and nephropathy) and macrovascular (cardiovascular disease, stroke, and peripheral arterial disease) complications associated with this disease. In a consensus statement, the American Diabetes Association and the European Association for the Study of Diabetes stated that the main goal of antidiabetes therapy is to achieve glycemic control, defined as a glycated hemoglobin of < 7%, although goals should be individualized to each patient, especially in patients at highest risk of cardiovascular disease events. However, because type 2 diabetes is a progressive disease, the response to oral antidiabetes (OAD) agents declines over time and the majority of patients are unable to maintain glycemic control, thus necessitating additional therapy. This requirement for additional antidiabetes therapies as the disease progresses suggests that more aggressive and earlier use of synergistic therapies may help achieve and prolong glycemic control. This review examines the efficacy of OAD therapies to address the unmet need for prompt and maintained glycemic control to the recommended goal in patients with type 2 diabetes, the factors that can affect the glycemic response to the various classes of OAD agents, and ways in which the management of hyperglycemia can be improved in patients with type 2 diabetes.

Interplay between cholesterol and drug metabolism

Cholesterol biosynthetic and metabolic pathways contain several branching points towards physiologically active molecules, such as coenzyme Q, vitamin D, glucocorticoid and steroid hormones, oxysterols, or bile acids. Sophisticated regulatory mechanisms are involved in maintenance of the homeostasis of not only cholesterol but also other cholesterogenic molecules. In addition to endogenous cues, cholesterol homeostasis needs to accommodate also to exogenous cues that are imported into the body, such as chemicals and medications. Steroid and nuclear receptors together with sterol regulatory element-binding protein (SREBP) mediate the fine tuning of biosynthetic and metabolic routes as well as transports of cholesterol and its derivatives. Similarly, drug/xenobiotic metabolism is the subject to the feedback regulation of cytochrome P450 enzymes and transporters. The regulatory mechanisms that maintain the homeostasis of cholesterogenic molecules and are involved in drug metabolism share similarities. Cholesterol and cholesterogenic compounds (bile acids, glucocorticoids, vitamin D, etc.) regulate the xenosensor signaling in drug-mediated induction of the major drug-metabolizing cytochrome P450 enzymes. The key cellular receptors, pregnane X receptor (PXR), constitutive androstane receptor (CAR), vitamin D receptor (VDR), and glucocorticoid receptor (GR) provide a functional cross-talk between the pathways maintaining cholesterol homeostasis and controlling the expression of drug-metabolizing enzymes. These receptors serve as metabolic sensors, resulting in a coordinate regulation of cholesterogenic compounds metabolism and of the defense against xenobiotic and endobiotic toxicity. Herein we present a comprehensive review of functional interactions between cholesterol homeostasis and drug metabolism involving the main nuclear and steroid receptors.

Bowman-Birk inhibitor affects pathways associated with energy metabolism in Drosophila melanogaster

Bowman-Birk inhibitor (BBI) is toxic when fed to certain insects, including the fruit fly, Drosophila melanogaster. Dietary BBI has been demonstrated to slow growth and increase insect mortality by inhibiting the digestive enzymes trypsin and chymotrypsin, resulting in a reduced supply of amino acids. In mammals, BBI influences cellular energy metabolism. Therefore, we tested the hypothesis that dietary BBI affects energy-associated pathways in the D. melanogaster midgut. Through microarray and metabolomic analyses, we show that dietary BBI affects energy utilization pathways in the midgut cells of D. melanogaster. In addition, ultrastructure studies indicate that microvilli are significantly shortened in BBI-fed larvae. These data provide further insights into the complex cellular response of insects to dietary protease inhibitors.

Endothelial dysfunction in diabetes: multiple targets for treatment

Robert Furchgott's discovery of the obligatory role that the endothelium plays in the regulation of vascular tone has proved to be a major advance in terms of our understanding of the cellular basis of diabetic vascular disease. Endothelial dysfunction, as defined by a reduction in the vasodilatation response to an endothelium-dependent vasodilator (such as acetylcholine) or to flow-mediated vasodilatation, is an early indicator for the development of the micro- and macroangipathy that is associated with diabetes. In diabetes, hyperglycaemia plays a key role in the initiation and development of endothelial dysfunction; however, the cellular mechanisms involved as well as the importance of dyslipidaemia and co-morbidities such as hypertension and obesity remain incompletely understood. In this review, we discuss the mechanisms whereby hyperglycaemia, oxidative stress and dyslipidaemia can alter endothelial function and highlight their effects on endothelial nitric oxide synthase (eNOS), the endothelium-dependent hyperpolarising factor (EDHF) pathway(s), as well as on the role of endothelium-derived contracting factors (EDCFs) and adipocyte-derived vasoactive factors such as adipose-derived relaxing factor (ADRF).

The liver X receptor: control of cellular lipid homeostasis and beyond Implications for drug design

Liver X receptor (LXR) α and β are nuclear receptors that control cellular metabolism. LXRs modulate the expression of genes involved in cholesterol and lipid metabolism in response to changes in cellular cholesterol status. Because of their involvement in cholesterol homeostasis, LXRs have emerged as promising drug targets for anti-atherosclerotic therapies. In rodents, synthetic LXR agonists promote cellular cholesterol efflux, transport and excretion. As a result, the progression of atherosclerosis is halted. However, pharmacological LXR activation also induces hepatic steatosis and promotes the secretion of atherogenic triacylglycerol-rich VLDL particles by the liver, complicating the clinical application of LXR agonists. The more recently emerged roles of LXRs in fat tissue, pituitary and brain may have implications for treatment of obesity and Alzheimer disease. In addition to the improvements in atherosclerosis, LXR activation exerts beneficial effects on glucose control in mouse models of type 2 diabetes. Future therapeutic strategies aiming to exert beneficial effects on cholesterol and glucose homeostasis, while circumventing the undesired effects on hepatic lipid metabolism, should target specific LXR-mediated processes. Therefore, tissue and/or isotype-specific effects of LXR action need to be established. The consequences of combinatorial drug approaches and the identification of the co-regulatory networks involved in the LXR-mediated control of particular genes may contribute to development of novel LXR agonists. Finally, pathway analyses of LXR actions provide tools to evaluate and optimize the effectiveness of novel therapeutic strategies to prevent and/or treat metabolic diseases.

Acute inhibition of fatty acid import inhibits GLUT4 transcription in adipose tissue, but not skeletal or cardiac muscle tissue, partly through liver X receptor (LXR) signaling

OBJECTIVE

Insulin-mediated glucose uptake is highly sensitive to the levels of the facilitative GLUT protein GLUT4. Transcription of the GLUT4 gene is repressed in states of insulin deficiency and insulin resistance and can be induced by states of enhanced energy output, such as exercise. The cellular signals that regulate GLUT4 transcription are not well understood. We hypothesized that changes in energy substrate flux regulate GLUT4 transcription.

RESEARCH DESIGN AND METHODS

To test this hypothesis, we used transgenic mice in which expression of the chloramphenicol acetyltransferase (CAT) gene is driven by a functional 895-bp fragment of the human GLUT4 promoter, thereby acting as a reporter for transcriptional activity. Mice were treated with a single dose of etomoxir, which inhibits the transport of long-chain fatty acids into mitochondria and increases basal, but not insulin-mediated, glucose flux. GLUT4 and transgenic CAT mRNA were measured.

RESULTS

Etomoxir treatment significantly reduced CAT and GLUT4 mRNA transcription in adipose tissue, but did not change transcription in heart and skeletal muscle. Downregulation of GLUT4 transcription was cell autonomous, since etomoxir treatment of 3T3-L1 adipocytes resulted in a similar downregulation of GLUT4 mRNA. GLUT4 transcriptional downregulation required the putative liver X receptor (LXR) binding site in the human GLUT4 gene promoter in adipose tissue and 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with the LXR agonist, TO901317, partially restored GLUT4 expression in etomoxir-treated cells.

CONCLUSIONS

Our data suggest that long-chain fatty acid import into mitochondria in adipose tissue may produce ligands that regulate expression of metabolic genes.

Akt2 modulates glucose availability and downstream apoptotic pathways during development

Glucose is the primary energy substrate for eukaryotic cells and the predominant substrate for the brain. Studies suggest that glucose serves an additional role in the regulation of cellular functions, including viability. Zebrafish is a tractable system for defining the cellular and molecular mechanisms perturbed by impaired glucose transport and metabolism. Previously, we demonstrated a critical role for the facilitative glucose transporter, Glut1, in the regulation of embryonic brain development. In this study, we aim to identify mediators in this Glut1-sensitive process by investigating the role of the antiapoptotic kinase, Akt2. Results show that abrogating expression of akt2 causes a phenotype strikingly similar to that observed when glut1 expression is inhibited. akt2-deficient embryos exhibit increased neuronal apoptosis, impaired glucose uptake, and death by 72 h postfertilization. Similar to what was observed in the glut1 morphants, inhibiting the expression of the proapoptotic protein, bad, in the context of impaired akt2 expression results in the inhibition of apoptosis and rescue of the morphant embryos. Intriguingly, overexpression of glut1 in the akt2 morphants was also able to rescue these embryos. Quantitative reverse transcription-PCR analysis revealed decreased glut1 transcript expression in akt2 morphant embryos. Taken together, these data suggest that Akt2 modulates glucose availability by regulating Glut1 expression at the transcript level. These data support a role for akt2 in an integrative pathway directly linking glucose, Glut1 expression, and activation of apoptosis and demonstrate the dependence of akt2 on glucose availability for the maintenance of cellular viability, particularly in the central nervous system.

Characterization of cardiovascular outcomes in a type 2 diabetes glucose supply and insulin demand model

BACKGROUND

The nonsignificant reduction in macrovascular outcomes observed in Action to Control Cardiovascular Risk in Diabetes; Action in Diabetes and Vascular Disease: Preterax and Diamicron Modified Release Controlled Evaluation; and the Veterans Affairs Diabetes Trial have collectively created uncertainty with respect toward the proper extent of blood glucose reduction and also the optimal therapeutic choice to attain the reduction. In the article entitled "Glucose Supply and Insulin Demand Dynamics of Antidiabetic Agents" in this issue of Journal of Diabetes Science and Technology, we presented data for a pharmacokinetic/pharmacodynamic model that characterizes the effect of conventional antidiabetic therapies on the glucose supply and insulin demand dynamic. Here, it is our objective to test the hypothesis that, in conjunction with hemoglobin A1c (HbA1c), patients managed on the glucose supply side of the model would have fewer cardiovascular events versus those managed on the insulin demand side.

METHODS

To test this hypothesis, the electronic medical records of a group model health maintenance organization were queried to compile a population of patients meeting the following inclusion criteria: (1) type 2 diabetes mellitus (T2DM), (2) known date of T2DM diagnosis; (3) ICD-9 or CPT code identification and chart review confirmation of a first major cardiovascular event (myocardial infarction, coronary artery bypass graft, or angioplasty),(4) five years of continuous eligibility, and (5) on antidiabetic therapy at the beginning of the 5-year observation period. These patients were subsequently matched (1:1) to T2DM patients meeting the same criteria who had not experienced an event and were analyzed for differences in glucose control (HbA1C), the glucose supply:insulin demand dynamic (SD ratio), and categorical combinations of both parameters.

RESULTS

Fifty cardiovascular event patients met inclusion criteria and were matched to controls. No difference was observed for the average HbA1c or SD ratio between patients experiencing an event and controls (7.5 +/- 1.0% versus 7.3 +/- 0.9%, p = .275, and 1.2 +/- 0.3 versus 1.3 +/- 0.3, p = .205, respectively). Likewise, for categorical representations, there were no differences in event rate at the pre-identified breakpoints (HbA1c >or=7% versus <7%; 72% versus 64%, p = .391, and SD ratio >or=1 versus <1; 68% versus 76%, p = .373, >or=1.25 versus <1.25; 42% versus 56%, p = .161, >or=1.5 versus <1.5; 22% versus 30%, p = .362, respectively). Analyzing the combined effect of glucose control and the SD dynamic, patients managed at higher glucose values and on the insulin demand side of the model (HbA1c >or=7% and SD ratio <1.25) tended to have greater cardiovascular risk than those managed at an HbA1c <7%, or HbA1c >or=7% with an SD ratio >or=1.25 (61% versus 39%; p = .096).

CONCLUSION

Independently, more aggressive HbA1c reduction and higher SD ratio values were not independently associated with a reduction in cardiovascular outcomes. Combining the parameters, it would appear that patients managed at higher glucose values and on the insulin demand side of the model may have increased cardiovascular risk. Based on these findings, it is pertinent to conduct subsequent works to refine SD ratio estimates and apply the model to larger, long-term T2DM cardiovascular outcome trials. J Diabetes Sci Technol 2010;4(2):382-390.

Thyroid hormone crosstalk with nuclear receptor signaling in metabolic regulation

Thyroid hormone influences diverse metabolic pathways important in lipid and glucose metabolism, lipolysis and regulation of body weight. Recently, it has been recognized that thyroid hormone receptor interacts with transcription factors that predominantly respond to nutrient signals including the peroxisome proliferator-activated receptors, liver X receptor and others. Crosstalk between thyroid hormone signaling and these nutrient responsive factors occurs through a variety of mechanisms: competition for retinoid X receptor heterodimer partners, DNA binding sites and transcriptional cofactors. This review focuses on the mechanisms of interaction of thyroid hormone signaling with other metabolic pathways and the importance of understanding these interactions to develop therapeutic agents for treatment of metabolic disorders, such as dyslipidemias, obesity and diabetes.

Nuclear receptors and hepatic lipidogenic enzyme response to a dyslipidemic sucrose-rich diet and its reversal by fish oil n-3 polyunsaturated fatty acids

A sucrose-rich diet (SRD), compared with a starch diet, induces time-dependent metabolic disorders and insulin resistance with hypertriglyceridemia, similar to type 2 diabetes. In this study, we examined the effect of SRD, after 8 mo, on nuclear receptors peroxisome proliferator-activated receptor-alpha (PPARalpha), and liver X receptor-alpha (LXRalpha), stearoyl-CoA desaturase-1 (SCD-1), and Delta6 and Delta5 desaturases mRNA and activity, hepatic enzymes involved in lipid metabolism, and fatty acid (FA) composition as well as the reversal produced by cod liver oil. SRD induced triglyceride increase in plasma and liver, increasing the anabolic FA synthase, malic enzyme, and glucose-6-phosphate dehydrogenase, but not the prooxidative enzymes FA oxidase and carnitine palmitoyltransferase I, and correspondingly decreased PPARalpha and increased LXRalpha expressions. Results suggest a contribution of both nuclear receptors' interaction on these enzymatic activities. SRD depressed SCD-1 without altering oleic acid proportion and increased Delta6 and Delta5 desaturases and the proportion of n-6 arachidonic acid. Therefore, the data do not support that SRD hypertriglyceridemia is produced by increased SCD-1-dependent oleic acid biosynthesis. The administration of 7% cod liver oil for 2 mo depressed LXRalpha, enhancing PPARalpha in control and SRD-fed rats, reversing the activity of the hepatic enzymes involved in lipid metabolism and therefore the hyperlipidemia produced by the SRD. Fish oil increased n-3 PUFA and depressed n-6 PUFA of liver lipids without altering the 18:1/18:0 ratio, suggesting that its effects were produced mainly by competition of dietary n-6 and n-3 FA and not through desaturase activity modification.

Effects of Liver x receptor agonist treatment on signal transduction pathways in acute lung inflammation

Background

Liver × receptor α (LXRα) and β (LXRβ) are members of the nuclear receptor super family of ligand-activated transcription factors, a super family which includes the perhaps better known glucocorticoid receptor, estrogen receptor, thyroid receptor, and peroxisome proliferator-activated receptors. There is limited evidence that LXL activation may reduces acute lung inflammation. The aim of this study was to investigate the effects of T0901317, a potent LXR receptor ligand, in a mouse model of carrageenan-induced pleurisy.

Methods

Injection of carrageenan into the pleural cavity of mice elicited an acute inflammatory response characterized by: accumulation of fluid containing a large number of neutrophils (PMNs) in the pleural cavity, infiltration of PMNs in lung tissues and subsequent lipid peroxidation, and increased production of nitrite/nitrate (NOx), tumor necrosis factor-α, (TNF-α) and interleukin-1β (IL-1β). Furthermore, carrageenan induced the expression of iNOS, nitrotyrosine and PARP, as well as induced apoptosis (TUNEL staining and Bax and Bcl-2 expression) in the lung tissues.

Results

Administration of T0901317, 30 min after the challenge with carrageenan, caused a significant reduction in a dose dependent manner of all the parameters of inflammation measured.

Conclusions

Thus, based on these findings we propose that LXR ligand such as T0901317, may be useful in the treatment of various inflammatory diseases.

Liver X receptor agonist GW3965 dose-dependently regulates lps-mediated liver injury and modulates posttranscriptional TNF-alpha production and p38 mitogen-activated protein kinase activation in liver macrophages

Modulation of the host inflammatory response to infection may be a key approach to improve the outcome of patients with sepsis and organ injury. We previously reported that pretreatment of rats with the liver X receptor (LXR) agonist GW3965 reduced the liver injury associated with endotoxemia and attenuated the production of TNF-alpha by rat Kupffer cells. Here, we examine the dose-dependent effect of GW3965 on liver injury and cytokine production in a rat model of endotoxemia and explore the mechanisms underlying TNF-alpha attenuation in Kupffer cells. Low doses of GW3965 (0.1 or 0.3 mg/kg) administered 30 min before infusion of LPS and peptidoglycan significantly attenuated the increase in plasma levels of the liver injury markers alanine aminotransferase and bilirubin (6 h) as well as the inflammatory mediators TNF-alpha (1 h) and prostaglandin E2 (6 h) associated with endotoxemia. In contrast, pretreatment with a higher dose of GW3965 (1.0 mg/kg) had no such effect. Studies in primary cultures of rat Kupffer cells demonstrated that LXR agonist treatment attenuated both the secreted and cell-associated levels of TNF-alpha, whereas TNF-alpha mRNA levels were not altered. Phosphorylated p38 mitogen-activated protein kinase, which plays a major role in production of TNF-alpha at the posttranscriptional level, was attenuated by GW3965 treatment in Kupffer cells. Experiments in murine LXR-deficient Kupffer cells demonstrated enhanced production of TNF-alpha in Kupffer cells from LXR-alpha(-/-) mice when challenged with LPS compared with LXR-beta(-/-) and wild-type Kupffer cells. Taken together, these results argue in favor of a novel mechanism for LXR-mediated attenuation of liver injury by interfering with posttranscriptional regulation of TNF-alpha in Kupffer cells.

LXRbeta deficient mice have reduced hepatic insulin clearance during hyperinsulinemic euglucemic clamp

The present study addresses the insulin sensitivity in mice deficient in LXRbeta (LXRbeta(-/-)) as well as in wild type (wt) mice assessed by hyperinsulinemic euglycemic clamp. Wt and LXRbeta(-/-) mice were fed either a normal chow diet or a high fat and high cholesterol diet (HFCD), and insulin sensitivity was assessed by hyperinsulinemic euglycemic clamps. We show that LXRbeta(-/-) mice have reduced insulin clearance during hyperinsulinemic clamps upon feeding both HFCD and a regular chow diet. Moreover we also observed reduced hepatic inflammation in LXRbeta(-/-) mice compared to wt mice upon feeding an HFCD, despite equal levels of hepatic steatosis. In summary, our results indicate that LXRbeta(-/-) mice have reduced insulin clearance during hyperinsulinemic euglycemic clamps and also reduced hepatic inflammation upon feeding an HFCD for 26weeks.

New genetic loci implicated in fasting glucose homeostasis and their impact on type 2 diabetes risk

Levels of circulating glucose are tightly regulated. To identify new loci influencing glycemic traits, we performed meta-analyses of 21 genome-wide association studies informative for fasting glucose, fasting insulin and indices of beta-cell function (HOMA-B) and insulin resistance (HOMA-IR) in up to 46,186 nondiabetic participants. Follow-up of 25 loci in up to 76,558 additional subjects identified 16 loci associated with fasting glucose and HOMA-B and two loci associated with fasting insulin and HOMA-IR. These include nine loci newly associated with fasting glucose (in or near ADCY5, MADD, ADRA2A, CRY2, FADS1, GLIS3, SLC2A2, PROX1 and C2CD4B) and one influencing fasting insulin and HOMA-IR (near IGF1). We also demonstrated association of ADCY5, PROX1, GCK, GCKR and DGKB-TMEM195 with type 2 diabetes. Within these loci, likely biological candidate genes influence signal transduction, cell proliferation, development, glucose-sensing and circadian regulation. Our results demonstrate that genetic studies of glycemic traits can identify type 2 diabetes risk loci, as well as loci containing gene variants that are associated with a modest elevation in glucose levels but are not associated with overt diabetes.

Separate and overlapping metabolic functions of LXRalpha and LXRbeta in C57Bl/6 female mice

The two liver X receptors (LXRs), LXRalpha and LXRbeta, are transcriptional regulators of cholesterol, lipid, and glucose metabolism and are both activated by oxysterols. Impaired metabolism is linked with obesity, insulin resistance, and type 2-diabetes (T2D). In the present study, we aimed to delineate the specific roles of LXRalpha and -beta in metabolic processes. C57Bl/6 female mice were fed a normal or a high-fat diet (HFD) and metabolic responses in wild-type, LXRalpha(-/-), LXRbeta(-/-), and LXRalphabeta(-/-) mice were analyzed. Whole body fat and intramyocellular lipid contents were measured by nuclear magnetic resonance. Energy expenditure was measured in individual metabolic cages. Glucose, insulin, and pyruvate tolerance tests were performed and gene expression profiles analyzed by qPCR. We found that both LXRbeta(-/-) and LXRalphabeta(-/-) mice are resistant to HFD-induced obesity independently of the presence of high cholesterol. Using tolerance tests, we found that, on an HFD, LXRbeta(-/-) mice enhanced their endogenous glucose production and became highly insulin resistant, whereas LXRalpha(-/-) and LXRalphabeta(-/-) mice remained glucose tolerant and insulin sensitive. Gene expression profiling confirmed that LXRbeta is the regulator of lipogenic genes in visceral white adipose tissue (WAT) and muscle tissue and, surprisingly, that Ucp1 and Dio2 are not responsible for the protection against diet-induced obesity observed in LXRbeta(-/-) and LXRalphabeta(-/-) mice. LXRalpha is required for the control of cholesterol metabolism in the liver, while LXRbeta appears to be a major regulator of glucose homeostasis and energy utilization and of fat storage in muscle and WAT. We conclude that selective LXRbeta agonists would be novel pharmaceuticals in the treatment of T2D.

The role of the autonomic nervous liver innervation in the control of energy metabolism

Despite a longstanding research interest ever since the early work by Claude Bernard, the functional significance of autonomic liver innervation, either sympathetic or parasympathetic, is still ill defined. This scarcity of information not only holds for the brain control of hepatic metabolism, but also for the metabolic sensing function of the liver and the way in which this metabolic information from the liver affects the brain. Clinical information from the bedside suggests that successful human liver transplantation (implying a complete autonomic liver denervation) causes no life threatening metabolic derangements, at least in the absence of severe metabolic challenges such as hypoglycemia. However, from the benchside, data are accumulating that interference with the neuronal brain-liver connection does cause pronounced changes in liver metabolism. This review provides an extensive overview on how metabolic information is sensed by the liver, and how this information is processed via neuronal pathways to the brain. With this information the brain controls liver metabolism and that of other organs and tissues. We will pay special attention to the hypothalamic pathways involved in these liver-brain-liver circuits. At this stage, we still do not know the final destination and processing of the metabolic information that is transferred from the liver to the brain. On the other hand, in recent years, there has been a considerable increase in the understanding which brain areas are involved in the control of liver metabolism via its autonomic innervation. However, in view of the ever rising prevalence of type 2 diabetes, this potentially highly relevant knowledge is still by far too limited. Thus the autonomic innervation of the liver and its role in the control of metabolism needs our continued and devoted attention.

Hormonal regulation of acetyl-CoA carboxylase isoenzyme gene transcription

Both glucocorticoid and insulin are known to have an anabolic effect on lipogenesis. Acetyl-CoA, an intermediate product of glycolysis, is supplied for fatty acid synthesis when carbohydrate intake is sufficient. Acetyl-CoA carboxylase (ACC), consisting of two isoenzymes ACC1 and ACC2, mediates the conversion from acetyl-CoA to malonyl-CoA, and thus plays a key role for the regulation of lipogenesis. In this study, we surveyed the effects of glucocorticoid and insulin on the transcriptional activity of the alternative promoters of ACCs (PI-PIII for ACC1, and PI and PII for ACC2) using the HepG2 human hepatocyte cell line in vitro. We also examined the roles of the insulin and/or glucose-regulated transcriptional factor(s) such as SREBP1c, LXRalpha/beta, and ChREBP on each promoter of the ACC genes. We found that both insulin and glucocorticoid had potent positive effects on all the promoters examined, and additive effects of both hormones were recognized in ACC1 PI and ACC2 PI. Furthermore, a representative insulin-responsive transcription factor SREBP1c showed significant stimulatory effects on all the promoters of ACC genes, among which those on ACC1 PIII and ACC2 PI were most prominent. On the other hand, the effect of LXRalpha was rather selective; it showed a marked stimulatory effect only on ACC1 PII. LXRbeta and ChREBP had minimal, if any, effects on some of the promoters. Altogether, our data suggest that insulin and glucocorticoid have positive effects on both ACC1 and ACC2 gene transcription. SREBP1c might be a master regulator of the expression of both genes regardless of the promoter utilized, whereas LXRalpha seems to play a promoter-specific role. Since ACC1 facilitates lipogenesis by stimulating fatty acid synthesis and ACC2 inhibits lipolysis, both insulin and glucocorticoid seem to play an important role in the pathogenesis of obesity and/or hepatic steatosis.

Sterol-protein interactions in cholesterol and bile acid synthesis

Cholesterol and other cholesterol related metabolites, oxysterols, and bile acids, establish specific interactions with enzymes and other proteins involved in cholesterol and bile acid homeostasis, triggering a variety of biological responses. The substrate-enzyme binding represents the best-characterized type of complementary interaction between proteins and small molecules. Key enzymes in the pathway that converts cholesterol to bile acids belong to the cytochrome P450 superfamily. In contrast to the majority of P450 enzymes, those acting on cholesterol and related metabolites exhibit higher stringency with respect to substrate molecules. This stringency, coupled with the specificity of the reactions, dictates the chemical features of intermediate metabolites (oxysterols) and end products (bile acids). Both oxysterols and bile acids have emerged in recent years as new signalling molecules due to their ability to interact and activate nuclear receptors, and consequently to regulate the transcription of genes involved in cholesterol and bile acid homeostasis and metabolism, but also in glucose and fatty acid metabolism. Interestingly, other proteins function as bile acid or sterol receptors. New findings indicate that bile acids also interact with a membrane G protein-coupled receptor, triggering a signalling cascade that ultimately promote energy expenditure. On the other end, cholesterol and side chain oxysterols establish specific interactions with different proteins residing in the endoplasmic reticulum that result in controlled protein degradation and/or trafficking to the Golgi and the nucleus. These regulatory pathways converge and contribute to adapt cholesterol uptake and synthesis to the cellular needs.

Liver X receptor β: maintenance of epidermal expression in intrinsic and extrinsic skin aging

Aging in human skin is the composite of time-dependent intrinsic aging plus photoaging induced by chronic exposure to ultraviolet radiation. Nuclear hormone receptors coordinate diverse processes including metabolic homeostasis. Liver X receptor β (LXRβ) is a close human homologue of daf-12, a regulator of nematode longevity. LXRβ is positively regulated by sirtuin-1 and resveratrol, while LXRβ-null mice show transcriptional profiles similar to those seen in aged human skin. In these studies, we examined LXRβ expression in aged and photoaged human skin. Volunteers were recruited to assess intrinsic aging and photoaging. Epidermal LXRβ mRNA was examined by in situ hybridization while protein was identified by immunofluorescence. No significant changes were observed in either LXRβ mRNA or protein expression between young and aged volunteers (mRNA p = 0.90; protein p = 0.26). Similarly, LXRβ protein expression was unaltered in photoaged skin (p = 0.75). Our data therefore suggest that, while not playing a major role in skin aging, robust cutaneous expression implies a fundamental role for LXRβ in epidermal biology.

Nuclear receptor liver X receptor is O-GlcNAc-modified in response to glucose

Post-translational modification of nucleocytoplasmic proteins by O-linked beta-N-acetylglucosamine (O-GlcNAc) has for the last 25 years emerged as an essential glucose-sensing mechanism. The liver X receptors (LXRs) function as nutritional sensors for cholesterol-regulating lipid metabolism, glucose homeostasis, and inflammation. LXRs are shown to be post-translationally modified by phosphorylation, acetylation, and sumoylation, affecting their target gene specificity, stability, and transactivating and transrepressional activity, respectively. In the present study, we show for the first time that LXRalpha and LXRbeta are targets for glucose-hexosamine-derived O-GlcNAc modification in human Huh7 cells. Furthermore, we observed increased hepatic LXRalpha O-GlcNAcylation in vivo in refed mice and in streptozotocin-induced refed diabetic mice. Importantly, induction of LXRalpha O-GlcNAcylation in both mouse models was concomitant with increased expression of the lipogenic gene SREBP-1c (sterol regulatory element-binding protein 1c). Furthermore, glucose increased LXR/retinoic acid receptor-dependent activation of luciferase reporter activity driven by the mouse SREBP-1c promoter via the hexosamine biosynthetic pathway in Huh7 cells. Altogether, our results suggest that O-GlcNAcylation of LXR is a novel mechanism by which LXR acts as a glucose sensor affecting LXR-dependent gene expression, substantiating the crucial role of LXR as a nutritional sensor in lipid and glucose metabolism.

Gene expression modulation of liver energy metabolism by oleoyl-oestrone in overweight rats

We intended to determine how the liver copes with the massive handling of lipids induced by OE (oleoyl-oestrone), as well as to characterize and differentiate the actual OE effects from those that may be only the consequence of decreased food intake. Thus we used male rats treated with oral OE (10 nmol/g per day) compared with a vehicle only PF (pair-fed) group and controls fed ad libitum (vehicle only). Plasma parameters, and total liver lipids, glycogen, DNA and total mRNA were measured. RNA was extracted and used for real-time PCR analysis of the gene expression of enzymes and regulatory factors of liver energy metabolism. Most hepatic proteins showed similar gene expressions in OE and controls, but the differences widened between OE and PF rats, showing that OE effects could not be merely attributed to a lower energy intake. The liver of OE-treated rats largely maintained its ability to mobilize glucose for the synthesis of fats; this was achieved in part by a peculiar combination of regulative modifications that facilitate both fatty acid disposal and restrained glucose utilization under conditions of limited food supply but ample availability of internal energy stores. In conclusion, the results presented suggest that the effect of OE on liver metabolism may be (at least in part) mediated through an insulin-sensitivity-dependent modulation of the expression of SREBP-1c (sterol-regulatory-element-binding protein-1c), resulting in the unique combined effect of mildly increased (or maintained) glucose disposal but also limited enhancement of lipogenesis.

From sugar to fat: How the transcription factor XBP1 regulates hepatic lipogenesis

Lipogenesis occurs primarily in the liver, where dietary carbohydrates control the expression of key enzymes in glycolytic and lipogenic pathways. We have recently discovered that the transcription factor XBP1, best known as a key regulator of the unfolded protein response (UPR), is required for de novo fatty acid synthesis in the liver, a function unrelated to its role in the UPR.(1) XBP1 protein expression is induced in the liver by a high carbohydrate diet and directly controls the induction of critical genes involved in fatty acid synthesis. Specific deletion of XBP1 in adult liver using an inducible approach results in profound hypocholesterolemia and hypotriglyceridemia, which could be attributed to diminished production of lipids in the liver. Notably, this phenotype is not associated with fatty liver (hepatic steatosis) or significant compromise in protein secretion. XBP1 joins an already rich field of transcriptional regulatory proteins in the control of hepatic lipogenesis. Its function in lipogenesis appears to be highly significant as evidenced by the phenotype of the genetic mutant strain. A more complete understanding of the mechanisms by which XBP1 accelerates de novo fatty acid synthesis in the liver while preserving normal hepatic lipid composition is highly relevant to the treatment of diseases such as atherosclerosis and metabolic syndrome that are associated with dyslipidemia. Since excess fat accumulation in the liver could result from increased hepatic fatty acid synthesis, compounds that inhibit XBP1 activation may also be useful therapeutics for the treatment of human alcoholic liver disease (ALD) and nonalcoholic fatty liver disease (NAFLD), increasingly common causes of morbidity and mortality in the United States.

Regulation of metabolism by nuclear hormone receptors

The worldwide epidemic of metabolic disease indicates that a better understanding of the pathways contributing to the pathogenesis of this constellation of diseases need to be determined. Nuclear hormone receptors comprise a superfamily of ligand-activated transcription factors that control development, differentiation, and metabolism. Over the last 15 years a growing number of nuclear receptors have been identified that coordinate genetic networks regulating lipid metabolism and energy utilization. Several of these receptors directly sample the levels of metabolic intermediates and use this information to regulate the synthesis, transport, and breakdown of the metabolite of interest. In contrast, other family members sense metabolic activity via the presence or absence of interacting proteins. The ability of these nuclear receptors to impact metabolism and inflammation will be discussed and the potential of each receptor subfamily to serve as drug targets for metabolic disease will be highlighted.

Non-alcoholic fatty liver disease pathogenesis: the present and the future

Non-alcoholic fatty liver disease is the clinical hepatic expression of metabolic syndrome. The prevalence of non-alcoholic fatty liver disease is around 20-30%, and with a rapid increase in the metabolic risk factors in the general population, non-alcoholic fatty liver disease has become the most common cause of liver disease worldwide. A fraction (20-30%) of non-alcoholic fatty liver disease patients develop a potentially progressive hepatic disorder, namely non-alcoholic steatohepatitis, leading to end-stage liver disease. The pathogenesis of non-alcoholic fatty liver disease is not entirely understood, and even if insulin resistance is a major pathogenetic key, many other factors are implicated in both liver fat accumulation and disease progression to non-alcoholic steatohepatitis. In this review we aim to examine the literature, principally concerning human non-alcoholic fatty liver disease pathogenesis, and to identify the newest, most promising clinical and basic research data.

Glucolipotoxicity of the pancreatic beta cell

The concept of glucolipotoxicity refers to the combined, deleterious effects of elevated glucose and fatty acid levels on pancreatic beta-cell function and survival. Significant progress has been made in recent years towards a better understanding of the cellular and molecular basis of glucolipotoxicity in the beta cell. The permissive effect of elevated glucose on the detrimental actions of fatty acids stems from the influence of glucose on intracellular fatty acid metabolism, promoting the synthesis of cellular lipids. The combination of excessive levels of fatty acids and glucose therefore leads to decreased insulin secretion, impaired insulin gene expression, and beta-cell death by apoptosis, all of which probably have distinct underlying mechanisms. Recent studies from our laboratory have identified several pathways implicated in fatty acid inhibition of insulin gene expression, including the extracellular-regulated kinase (ERK1/2) pathway, the metabolic sensor Per-Arnt-Sim kinase (PASK), and the ATF6 branch of the unfolded protein response. We have also confirmed in vivo in rats that the decrease in insulin gene expression is an early defect which precedes any detectable abnormality in insulin secretion. While the role of glucolipotoxicity in humans is still debated, the inhibitory effects of chronically elevated fatty acid levels has been clearly demonstrated in several studies, at least in individuals genetically predisposed to developing type 2 diabetes. It is therefore likely that glucolipotoxicity contributes to beta-cell failure in type 2 diabetes as well as to the decline in beta-cell function observed after the onset of the disease.

Effect of 27-hydroxycholesterol on survival and death of human macrophages and vascular smooth muscle cells

The objective was to compare the effect of a LXR synthetic ligand (T0901317) on cell viability and lysosomal membrane destabilization in human U937 macrophage and aortic smooth muscle cell (HASMC) incubated in the presence of cholesterol or 27-OH and to verify whether the Akt signalling pathway is involved. In U937 macrophages, cholesterol triggered cell survival while 27-OH triggered either survival (low concentration) or a lysosomal independent apoptosis (high concentration). Despite a strong effect of T0901317 on macrophage survival, its effect on cell viability is hampered in cells incubated in the presence of cholesterol or 27-OH. In these cells, cholesterol triggers the phosphorylation of Akt on the Thr308 residue. In HASMC, cholesterol induced apoptosis but no additionnal effect of T0901317 prevented apoptosis. All together, cell survival triggered by LXRs is impaired in the presence of cholesterol or high concentrations of 27-OH in human U937 macrophages and is not effective in HASMC.

Retinoid X receptor heterodimer variants and cardiovascular risk factors

Nuclear receptors are transcription factors that can be activated by specific ligands. Recent progress has shown that retinoid X receptor (RXR) and its heterodimerization partners, including peroxisome proliferator-activated receptors, regulate many important genes involved in energy homeostasis and atherosclerosis, and should be promising therapeutic targets of metabolic syndrome. RXR heterodimers regulate a number of complex cellular processes, and genetic studies of RXR heterodimers have provided important clinical information in addition to knowledge gained from basic research. Genetic variants of RXR heterodimers were screened and investigated, and some variants were shown to have a considerable impact on metabolic disorders, including phenotypic components of familial combined hyperlipidemia. The combined efforts of basic and clinical science regarding nuclear receptors have achieved significant progress in unraveling the inextricably linked control system of energy expenditure, lipid and glucose homeostasis, inflammation, and atherosclerosis.This review summarizes the current understanding regarding RXR heterodimers based on their human genetic variants, which will provide new clues to uncover the background of multifactorial disease, such as metabolic syndrome or familial combined hyperlipidemia.

Glucose metabolism is required for oxidized LDL-induced macrophage survival: role of PI3K and Bcl-2 family proteins

OBJECTIVE

Oxidized low-density lipoprotein (oxLDL) induces survival of colony stimulating factor-1 (CSF-1)-dependent macrophages in vitro. Because atherosclerotic lesion-associated macrophages take up large amounts of glucose, we investigated whether, and how, oxLDL promotes glucose uptake and how glucose metabolism regulates oxLDL-induced macrophage survival.

METHODS AND RESULTS

OxLDL-induced macrophage survival required glucose metabolism. OxLDL stimulated 2 phases of glucose uptake, namely acute and chronic, which required PI3K but not MEK1/2 activity. PI3K appeared to regulate glucose transport via glucose transporter affinity and/or mobilization. OxLDL also maintained levels of the prosurvival proteins, Bcl-2 and Bcl-x(L), after CSF-1 had been removed through a combination of mechanisms including transcription, translation, and protein stabilization. Significantly, inhibition of glucose metabolism reduced Bcl-2 and Bcl-x(L) protein levels. MEK1/2 and PI3K activities were also required for oxLDL-induced Bcl-2 and Bcl-x(L) mRNA upregulation.

CONCLUSIONS

These results suggest that oxLDL enhances macrophage survival in the absence of CSF-1 by inducing PI3K-dependent glucose uptake, which is metabolized to maintain Bcl-2 and Bcl-x(L) protein levels.