Liver-specific mRNA for Insig-2 down-regulated by insulin: implications for fatty acid synthesis

Association between the insulin-induced gene 2 (INSIG2) and weight gain in a German sample of antipsychotic-treated schizophrenic patients: perturbation of SREBP-controlled lipogenesis in drug-related metabolic adverse effects?

Atypical antipsychotics are nowadays the most widely used drugs to treat schizophrenia and other psychosis. Unfortunately, some of them can cause major metabolic adverse effects, such as weight gain, dyslipidemia and type 2 diabetes. The underlying lipogenic mechanisms of the antipsychotic drugs are not known, but several studies have focused on a central effect in the hypothalamic control of appetite regulation and energy expenditure. In a functional convergent genomic approach we recently used a cellular model and demonstrated that orexigenic antipsychotics that induce weight gain activate the expression of lipid biosynthesis genes controlled by the sterol regulatory element-binding protein (SREBP) transcription factors. We therefore hypothesized that the major genes involved in the SREBP activation of fatty acids and cholesterol production (SREBF1, SREBF2, SCAP, INSIG1 and INSIG2) would be strong candidate genes for interindividual variation in drug-induced weight gain. We genotyped a total of 44 HapMap-selected tagging single nucleotide polymorphisms in a sample of 160 German patients with schizophrenia that had been monitored with respect to changes in body mass index during antipsychotic drug treatment. We found a strong association (P=0.0003-0.00007) between three markers localized within or near the INSIG2 gene (rs17587100, rs10490624 and rs17047764) and antipsychotic-related weight gain. Our finding is supported by the recent involvement of the INSIG2 gene in obesity in the general population and implicates SREBP-controlled lipogenesis in drug-induced metabolic adverse effects.

Down-regulation of hepatic HNF4alpha gene expression during hyperinsulinemia via SREBPs

Mutations in the coding region of hepatocyte nuclear factor 4alpha (HNF4alpha), and its upstream promoter (P2) that drives expression in the pancreas, are known to lead to maturity-onset diabetes of the young 1 (MODY1). HNF4alpha also controls gluconeogenesis and lipid metabolism in the liver, where the proximal promoter (P1) predominates. However, very little is known about the role of hepatic HNF4alpha in diabetes. Here, we examine the expression of hepatic HNF4alpha in two diabetic mouse models, db/db mice (type 2, insulin resistant) and streptozotocin-treated mice (type 1, insulin deficient). We found that the level of HNF4alpha protein and mRNA was decreased in the liver of db/db mice but increased in streptozotocin-treated mice. Because insulin increases the activity of sterol regulatory element-binding proteins (SREBP)-1c and -2, we also examined the effect of SREBPs on hepatic HNF4alpha gene expression and found that, like insulin, ectopic expression of SREBPs decreases the level of hepatic HNF4alpha protein and mRNA both in vitro in primary hepatocytes and in vivo in the liver of C57BL/6 mice. Finally, we use gel shift, chromatin immunoprecipitation, small interfering RNA, and reporter gene analysis to show that SREBP2 binds the human HNF4alpha P1 promoter and negatively regulates its expression. These data indicate that hyperinsulinemia down-regulates HNF4alpha in the liver through the up-regulation of SREBPs, thereby establishing a link between these two critical transcription factor pathways that regulate lipid and glucose metabolism in the liver. These findings also provide new insights into diabetes-associated complications such as fatty liver disease.

Insulin enhances post-translational processing of nascent SREBP-1c by promoting its phosphorylation and association with COPII vesicles

The regulation of lipid homeostasis by insulin is mediated in part by the enhanced transcription of the gene encoding SREBP-1c (sterol regulatory element-binding protein-1c). Nascent SREBP-1c is synthesized and embedded in the endoplasmic reticulum (ER) and must be transported to the Golgi in coatomer protein II (COPII) vesicles where two sequential cleavages generate the transcriptionally active NH(2)-terminal fragment, nSREBP-1c. There is limited indirect evidence to suggest that insulin may also regulate the posttranslational processing of the nascent SREBP-1c protein. Therefore, we designed experiments to directly assess the action of insulin on the post-translational processing of epitope-tagged full-length SREBP-1c and SREBP-2 proteins expressed in cultured hepatocytes. We demonstrate that insulin treatment led to enhanced post-translational processing of SREBP-1c, which was associated with phosphorylation of ER-bound nascent SREBP-1c protein that increased affinity of the SREBP-1c cleavage-activating protein (SCAP)-SREBP-1c complex for the Sec23/24 proteins of the COPII vesicles. Furthermore, chemical and molecular inhibitors of the phosphoinositide 3-kinase pathway and its downstream kinase protein kinase B (PKB)/Akt prevented both insulin-mediated phosphorylation of nascent SREBP-1c protein and its posttranslational processing. Insulin had no effect on the proteolysis of nascent SREBP-2 under identical conditions. We also show that in vitro incubation of an active PKB/Akt enzyme with recombinant full-length SREBP-1c led to its phosphorylation. Thus, insulin selectively stimulates the processing of SREBP-1c in rat hepatocytes by enhancing the association between the SCAP-SREBP-1c complex and COPII proteins and subsequent ER to Golgi transport and proteolytic cleavage. This effect of insulin is tightly linked to phosphoinositide 3-kinase and PKB/Akt-dependent serine phosphorylation of the precursor SREBP-1c protein.

PPAR-delta in Vascular Pathophysiology

Peroxisome proliferator-activated receptors belong to the superfamily of ligand-dependent nuclear receptor transcription factors, which include three subtypes: PPAR-α, β/δ, and γ. PPAR-δ, play important roles in the regulation of cell growth and differentiation as well as tissue wound and repair. Emerging evidence has also demonstrated that PPAR-δ is implicated in lipids and glucose metabolism. Most recently, the direct effects of PPAR-δ on cardiovascular processes such as endothelial function and angiogenesis have also been investigated. Therefore, it is suggested that PPAR-δ may have critical roles in cardiovascular pathophysiology and is a potential target for therapeutic intervention of cardiovascular disorders such as atherosclerosis.

Suppression of 2,3-oxidosqualene cyclase by high fat diet contributes to liver X receptor-alpha-mediated improvement of hepatic lipid profile

The liver X receptors (LXRs) sense oxysterols and regulate genes involved in cholesterol metabolism. Synthetic agonists of LXRs are potent stimulators of fatty acid synthesis, which is mediated largely by sterol regulatory element-binding protein-1c (SREBP-1c). Paradoxically, an improved hepatic lipid profile by LXR was observed in mice fed a Western high fat (HF) diet. To explore the underlying mechanism, we administered mice normal chow or an HF diet and overexpressed LXRalpha in the liver. The HF diet with tail-vein injection of adenovirus of LXRalpha increased the expression of LXR-targeted genes involved in cholesterol reverse transport but not those involved in fatty acid synthesis. A similar effect was also observed with the use of 22R-hydroxycholesterol, an LXR ligand, in cultured hepatocytes. Consequently, SREBP-1c maturation was inhibited by the HF diet, which resulted from the induction of Insig-2a. Importantly, increased cholesterol level suppressed the expression of 2,3-oxidosqualene cyclase (OSC), which led to an increase in endogenous LXR ligand(s). Furthermore, siRNA-mediated knockdown of OSC expression enhanced LXR activity and selectively up-regulated LXR-targeted genes involved in cholesterol reverse transport. Thus, down-regulation of OSC may account for a novel mechanism underlying the LXR-mediated lipid metabolism in the liver of mice fed an HF diet.

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.

Peroxisome deficiency causes a complex phenotype because of hepatic SREBP/Insig dysregulation associated with endoplasmic reticulum stress

Regulation of hepatic cholesterol biosynthesis, lipogenesis, and insulin signaling intersect at the transcriptional level by control of SREBP and Insig genes. We previously demonstrated that peroxisome-deficient PEX2-/- mice activate SREBP-2 pathways but are unable to maintain normal cholesterol homeostasis. In this study, we demonstrate that oral bile acid treatment normalized hepatic and plasma cholesterol levels and hepatic cholesterol synthesis in early postnatal PEX2 mutants, but SREBP-2 and its target gene expressions remained increased. SREBP-2 pathway induction was also observed in neonatal and longer surviving PEX2 mutants, where hepatic cholesterol levels were normal. Abnormal expression patterns for SREBP-1c and Insig-2a, and novel regulation of Insig-2b, further demonstrate that peroxisome deficiency widely affects the regulation of related metabolic pathways. We have provided the first demonstration that peroxisome deficiency activates hepatic endoplasmic reticulum (ER) stress pathways, especially the integrated stress response mediated by PERK and ATF4 signaling. Our studies suggest a mechanism whereby ER stress leads to dysregulation of the endogenous sterol response mechanism and concordantly activates oxidative stress pathways. Several metabolic derangements in peroxisome-deficient PEX2-/- liver are likely to trigger ER stress, including perturbed flux of mevalonate metabolites, altered bile acid homeostasis, changes in fatty acid levels and composition, and oxidative stress.

Switch-like control of SREBP-2 transport triggered by small changes in ER cholesterol: a delicate balance

Animal cells control their membrane lipid composition within narrow limits, but the sensing mechanisms underlying this control are largely unknown. Recent studies disclosed a protein network that controls the level of one lipid-cholesterol. This network resides in the endoplasmic reticulum (ER). A key component is Scap, a tetrameric ER membrane protein that binds cholesterol. Cholesterol binding prevents Scap from transporting SREBPs to the Golgi for activation. Using a new method to purify ER membranes from cultured cells, we show that Scap responds cooperatively to ER cholesterol levels. When ER cholesterol exceeds 5% of total ER lipids (molar basis), SREBP-2 transport is abruptly blocked. Transport resumes when ER cholesterol falls below the 5% threshold. The 5% threshold is lowered to 3% when cells overexpress Insig-1, a Scap-binding protein. Cooperative interactions between cholesterol, Scap, and Insig create a sensitive switch that controls the cholesterol composition of cell membranes with remarkable precision.

A cholesterol toggle switch

Cholesterol levels in mammalian cells are controlled by an intricate mechanism in which the transcription factor SREBP plays a key role. Work in this issue (Radhakrishnan et al., 2008) employing direct measurement of endoplasmic reticulum cholesterol levels offers insights into the "switch" that controls this system with surprising precision.

Deletion of ELOVL5 leads to fatty liver through activation of SREBP-1c in mice

Elongation of very long chain fatty acids (ELOVL)5 is one of seven mammalian fatty acid condensing enzymes involved in microsomal fatty acid elongation. To determine the in vivo substrates and function of ELOVL5, we generated Elovl5(-/-) mice. Studies using liver microsomal protein from wild-type and knockout mice demonstrated that the elongation of gamma-linolenic (C18:3, n-6) to dihomo-gamma-linolenic (C20:3, n-6) and stearidonic (C18:4, n-3) to omega3-arachidonic acid (C20:4, n-3) required ELOVL5 activity. Tissues of Elovl5(-/-) mice accumulated the C18 substrates of ELOVL5 and the levels of the downstream products, arachidonic acid (C20:4, n-6) and docosahexaenoic acid (DHA, C22:6, n-3), were decreased. A consequence of decreased cellular arachidonic acid and DHA concentrations was the activation of sterol regulatory element-binding protein (SREBP)-1c and its target genes involved in fatty acid and triglyceride synthesis, which culminated in the development of hepatic steatosis in Elovl5(-/-) mice. The molecular and metabolic changes in fatty acid metabolism in Elovl5(-/-) mice were reversed by dietary supplementation with arachidonic acid and DHA. These studies demonstrate that reduced ELOVL5 activity leads to hepatic steatosis, and endogenously synthesized PUFAs are key regulators of SREBP-1c activation and fatty acid synthesis in livers of mice.

Altered expression of transcription factors and genes regulating lipogenesis in liver and adipose tissue of mice with high fat diet-induced obesity and nonalcoholic fatty liver disease

OBJECTIVE

To determine whether expression of transcription factors and lipogenic enzymes is altered in liver and adipose tissue of mice with obesity, insulin resistance, and nonalcoholic fatty liver disease.

METHODS

Mice were fed chow containing 9% of calories from standard fat (SF) or 20% of calories from high fat (HF) and killed after 9 months in the fasted or fed state.

MEASUREMENTS

Liver injury was evaluated by histology and serum aminotransferase levels. Transcription factor expression was measured by real-time PCR. Lipogenic enzymes were measured by real-time PCR and Western blots.

RESULTS

HF mice weighed more, had insulin resistance, hepatic steatosis, and focal pericellular hepatic fibrosis. Hepatic expression of sterol regulatory element-binding protein-1c, carbohydrate response element-binding protein, liver X receptor-alpha, acetyl-CoA carboxylase (ACC), and fatty acid synthase (FAS) decreased during fasting in SF and HF mice; however, FAS expression and protein content were higher in the liver of fasted HF mice than of fasted SF mice. In adipose tissue, expression of sterol response element-binding protein-1c, carbohydrate response element-binding protein, liver X receptor-alpha, peroxisome proliferator-activated receptor-gamma, ACC, and FAS decreased with fasting in mice fed SF, but not in HF mice. ACC and FAS expression and protein content remained higher during fasting in HF than in SF mice.

CONCLUSION

Feeding a nutritionally complete diet containing a moderate increase in fat produces obesity and steatohepatitis. During fasting, hepatic FAS expression and protein content are increased in HF mice. Transcription factor expression, and lipogenic enzyme expression and protein concentration do not decline during fasting in adipose tissue from HF mice. De-novo lipogenesis may persist in liver and adipose tissue during fasting in obesity/nonalcoholic fatty liver disease.

Hedgehog signaling regulates sensory cell formation and auditory function in mice and humans

Auditory perception is mediated through a finite number of mechanosensory hair cells located in a specialized sensory epithelium within the inner ear. The formation of the appropriate number of hair cells and the location of those cells is crucial for normal auditory function. However, the factors that regulate the formation of this epithelium remain poorly understood. Truncating mutations in the transcription factor GLI3, a downstream effector of the Hedgehog (HH) pathway, lead to a partial loss of HH signaling and cause Pallister-Hall syndrome (PHS). Here, we report that cochleae from a mouse model of PHS (Gli3(Delta699)), which produces only the truncated, repressor form of GLI3, have a variably penetrant phenotype that includes an increase in the size of the sensory epithelium and the development of large ectopic sensory patches in Kölliker's organ (KO). Consistent with the mouse model, some PHS individuals exhibit hearing loss across a broad range of frequencies. Moreover, inhibition of HH signaling in vitro results in an increase in the size of the prosensory domain, a precursor population that gives rise to the sensory epithelium, whereas treatment with Sonic hedgehog (SHH) inhibits prosensory formation. Finally, we demonstrate that HH signaling within the cochlea regulates expression of prosensory markers and that the effects of HH in KO are dependent on activation of Notch, an inducer of prosensory fate. These results suggest that HH signaling plays a key role in the specification, size, and location of the prosensory domain, and therefore of hair cells, within the cochlea.

Nutrigenomics, beta-cell function and type 2 diabetes

INTRODUCTION

The present investigation was designed to investigate the accuracy and precision of lactate measurement obtained with contemporary biosensors (Chiron Diagnostics, Nova Biomedical) and standard enzymatic photometric procedures (Sigma Diagnostics, Abbott Laboratories, Analyticon).

MATERIALS AND METHODS

Measurements were performed in vitro before and after the stepwise addition of 1 molar sodium lactate solution to samples of fresh frozen plasma to systematically achieve lactate concentrations of up to 20 mmol/l.

RESULTS

Precision of the methods investigated varied between 1% and 7%, accuracy ranged between 2% and -33% with the variability being lowest in the Sigma photometric procedure (6%) and more than 13% in both biosensor methods.

CONCLUSION

Biosensors for lactate measurement provide adequate accuracy in mean with the limitation of highly variable results. A true lactate value of 6 mmol/l was found to be presented between 4.4 and 7.6 mmol/l or even with higher difference. Biosensors and standard enzymatic photometric procedures are only limited comparable because the differences between paired determinations presented to be several mmol. The advantage of biosensors is the complete lack of preanalytical sample preparation which appeared to be the major limitation of standard photometry methods.

INSIG2 gene rs7566605 polymorphism is associated with severe obesity in Japanese

The single nucleotide polymorphism (SNP) rs7566605 in the upstream region of the insulin-induced gene 2 (INSIG2) is associated with the obesity phenotype in many Caucasian populations. In Japanese, this association with the obesity phenotype is not clear. To investigate the relationship between rs7566605 and obesity in Japanese, we genotyped rs7566605 from severely obese subjects [n = 908, body mass index (BMI) ≥ 30 kg/m²] and normal-weight control subjects (n = 1495, BMI < 25 kg/m²). A case–control association analysis revealed that rs7566605 was significantly associated with obesity in Japanese. The P value in the minor allele recessive mode was 0.00020, and the odds ratio (OR) adjusted for gender and age was 1.61 [95% confidential interval (CI) = 1.24–2.09]. Obesity-associated phenotypes, which included the level of BMI, plasma glucose, hemoglobin A1c, total cholesterol, triglycerides, high-density lipoprotein (HDL) cholesterol, and blood pressure, were not associated with the rs7566605 genotype. Thus, rs7566605 in the upstream region of the INSIG2 gene was found to be associated with obesity, i.e., severe obesity, in Japanese.

The single nucleotide polymorphism upstream of insulin-induced gene 2 (INSIG2) is associated with the prevalence of hypercholesterolaemia, but not with obesity, in Japanese American women

Insulin-induced gene 2 (insig-2) protein is known to play important roles in cholesterol and TAG metabolism bothin vivoandin vitro. One particularly interesting single nucleotide polymorphism (SNP), rs7566605, located 10 kb upstream ofINSIG2was reported to have the strongest association with obesity among 86 604 SNP, while the relationship with dyslipidaemia is uncertain. Eight hundred and eighty-five Japanese Americans (347 men and 538 women) and 378 Japanese (182 men and 196 women) were enrolled, and the rs7566605 SNP, which is consistent with either G or C, was determined. We investigated the association between the rs7566605 SNP and the prevalence of hypercholesterolaemia or hypertriacylglycerolaemia, or obesity parameters, as assessed by BMI, waist girth and percentage body fat. There were no significant differences in BMI, waist girth and percentage body fat according to the genotype in each of the four groups, which was divided by population and sex. The prevalence of hypercholesterolaemia was significantly different between the genotypes in Japanese American female subjects (GG, 62·2 %; GC, 57·1 %; CC, 42·1 %;P = 0·021), but not in the other subjects. In Japanese American women, the subjects with the CC genotype had a 0·43-fold decreased risk (95 % CI 0·24, 0·80) for hypercholesterolaemia compared with the GG genotype after adjustment for age, percentage body fat, smoking status and hormone replacement therapy. The CC genotype of the rs7566605 SNP is suggested to be a protective genetic factor against the progression of hypercholesterolaemia on a high-fat diet, especially in Japanese female subjects.

Dual effect of the adapter growth factor receptor-bound protein 14 (grb14) on insulin action in primary hepatocytes

Tight control of insulin action in liver is a crucial determinant for the regulation of energy homeostasis. Growth factor receptor-bound protein 14 (Grb14) is a molecular adapter, highly expressed in liver, which binds to the activated insulin receptor and inhibits its tyrosine kinase activity. The physiological role of Grb14 in liver metabolism was unexplored. In this study we used RNA interference to investigate the consequences of Grb14 decrease on insulin-regulated intracellular signaling, and on glucose and lipid metabolism in mouse primary cultured hepatocytes. In Grb14-depleted hepatocytes, insulin-induced phosphorylation of Akt, and of its substrates glycogen synthase kinase 3 and fork-head box protein 1, was increased. These effects on insulin signaling are in agreement with the selective inhibitory effect of Grb14 on the receptor kinase. However, the metabolic and genic effects of insulin were differentially regulated after Grb14 down-regulation. Indeed, the insulin-mediated inhibition of hepatic glucose production and gluconeogenic gene expression was slightly increased. Surprisingly, despite the improved Akt pathway, the induction by insulin of sterol regulatory element binding protein-1c maturation was totally blunted. As a result, in the absence of Grb14, glycogen synthesis as well as glycolytic and lipogenic gene expression were not responsive to the stimulatory effect of insulin. This study provides evidence that Grb14 exerts a dual role on the regulation by insulin of hepatic metabolism. It inhibits insulin receptor catalytic activity, and acts also at a more distal step, i.e. sterol regulatory element binding protein-1c maturation, which effect is predominant under short-term inhibition of Grb14 expression.

Regulation of sterol synthesis in eukaryotes

Cholesterol is an essential component of mammalian cell membranes and is required for proper membrane permeability, fluidity, organelle identity, and protein function. Cells maintain sterol homeostasis by multiple feedback controls that act through transcriptional and posttranscriptional mechanisms. The membrane-bound transcription factor sterol regulatory element binding protein (SREBP) is the principal regulator of both sterol synthesis and uptake. In mammalian cells, the ER membrane protein Insig has emerged as a key component of homeostatic regulation by controlling both the activity of SREBP and the sterol-dependent degradation of the biosynthetic enzyme HMG-CoA reductase. In this review, we focus on recent advances in our understanding of the molecular mechanisms of the regulation of sterol synthesis. A comparative analysis of SREBP and HMG-CoA reductase regulation in mammals, yeast, and flies points toward an equilibrium model for how lipid signals regulate the activity of sterol-sensing proteins and their downstream effectors.

Insulin-induced gene 2 involvement in human adipocyte metabolism and body weight regulation

BACKGROUND

Insulin-induced genes (INSIGs) encode proteins that block proteolytic activation of sterol regulatory element-binding proteins, transcription factors that regulate lipogenic enzymes, and adipocyte differentiation.

OBJECTIVE

Here, we analyzed the relative significance of INSIG1 and INSIG2 in human liver and adipocyte metabolism, and defined a novel, functional polymorphism in the promoter of INSIG2 associated with body mass index.

RESEARCH METHODS

Variations in gene expression of different human tissues, of hepatoma cells exposed to INSIG1 and INSIG2 gene silencing probes, and of differentiating 3T3-L1 adipocytes were determined by real-time quantitative PCR. The functional significance of a novel polymorphism in the promoter of INSIG2 was analyzed using in vitro methods and gene expression analysis of human adipose tissue, whereas the phenotype associated with this polymorphism was studied in two cohorts of middle-aged men.

RESULTS

Gene expression analysis of 17 human tissues demonstrated that INSIG1 is highly expressed in the liver, whereas INSIG2 is ubiquitously expressed. Gene silencing experiments confirmed that INSIG1, but not INSIG2, regulates the expression of sterol regulatory element-binding proteins target genes in human hepatoma cells. In contrast, adipocyte differentiation of 3T3-L1 cells was associated with a 13-fold increase in expression of INSIG2. Significant relationships between the INSIG2-102G/A polymorphism and body mass index were observed in two cohorts of middle-aged men (ANOVA P = 0.017 and 0.044, respectively). In vitro studies and analysis of allele-specific expression in human adipose tissue substantiated the functional significance of the INSIG2-102G/A polymorphism.

CONCLUSION

INSIG2 is involved in adipocyte metabolism and body weight regulation.

INSIG2 polymorphism is neither associated with BMI nor with phenotypes of lipoprotein metabolism

OBJECTIVE

A previous epidemiological study showed an association of the insulin-induced gene 2 (INSIG2) gene with BMI. Additionally, experimental investigations in animals and cell culture provided evidence that this gene might be involved in lipoprotein and free fatty acid (FFA) metabolism. Therefore, the aim of this study was to examine the association between the rs7566605 variant near the INSIG2 gene and BMI and to extend it to other quantitative measures of obesity, as well as parameters of lipoprotein and FFA metabolism.

METHODS AND PROCEDURES

We genotyped rs7566605 in a group of severely obese white patients (n = 1,026) with an average BMI of 46.0 kg/m(2) and a control group (n = 818) from Utah, as well as in the Salzburg Atherosclerosis Prevention Program in Subjects at High Individual Risk (SAPHIR) study from Austria, which is based on a healthy working population (n = 1,696).

RESULTS

We observed no difference in the genotype frequency of rs7566605 of INSIG2 between obese subjects and population-based controls from Utah. Furthermore, we did not find evidence of an association with measures of body composition (BMI, waist, waist-to-hip ratio, percentage body fat, amount of visceral and subcutaneous abdominal adipose fat) or lipoprotein metabolism (total cholesterol, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol, triglycerides, and FFAs) in the Utah study population or in the independent SAPHIR study.

DISCUSSION

Our results do not support an association of the INSIG2 gene with the regulation of body weight or parameters related to lipoprotein metabolism.

SREBPs: the crossroads of physiological and pathological lipid homeostasis

The uptake, biosynthesis and metabolism of cholesterol and other lipids are exquisitely regulated by feedback and feed-forward pathways in organisms ranging from Caenorhabditis elegans to humans. As endoplasmic reticulum (ER) membrane-embedded transcription factors that are activated in the Golgi apparatus, sterol regulatory element-binding proteins (SREBPs) are central to the intracellular surveillance of lipid catabolism and de novo biogenesis. The biosynthesis of SREBP proteins, their migration from the ER to the Golgi compartment, intra-membrane proteolysis, nuclear translocation and trans-activation potential are tightly controlled in vivo. Here we summarize recent studies elucidating the transcriptional and post-transcriptional regulation of SREBP-1c through nutrition and the action of hormones, particularly insulin, and the resulting implications for dyslipidemia of obesity, metabolic syndrome and type 2 diabetes.

Docosahexaenoic acid (DHA) and hepatic gene transcription

The type and quantity of dietary fat ingested contributes to the onset and progression of chronic diseases, like diabetes and atherosclerosis. The liver plays a central role in whole body lipid metabolism and responds rapidly to changes in dietary fat composition. Polyunsaturated fatty acids (PUFA) play a key role in membrane composition and function, metabolism and the control of gene expression. Certain PUFA, like the n-3 PUFA, enhance hepatic fatty acid oxidation and inhibit fatty acid synthesis and VLDL secretion, in part, by regulating gene expression. Our studies have established that key transcription factors, like PPARalpha, SREBP-1, ChREBP and MLX, are regulated by n-3 PUFA, which in turn control levels of proteins involved in lipid and carbohydrate metabolism. Of the n-3 PUFA, 22:6,n-3 has recently been established as a key controller of hepatic lipid synthesis. 22:6,n-3 controls the 26S proteasomal degradation of the nuclear form of SREBP-1. SREBP-1 is a major transcription factor that controls the expression of multiple genes involved fatty acid synthesis and desaturation. 22:6,n-3 suppresses nuclear SREBP-1, which in turn suppresses lipogenesis. This mechanism is achieved, in part, through control of the phosphorylation status of protein kinases. This review will examine both the general features of PUFA-regulated hepatic gene transcription and highlight the unique mechanisms by which 22:6,n-3 impacts gene expression. The outcome of this analysis will reveal that changes in hepatic 22:6,n-3 content has a major impact on hepatic lipid and carbohydrate metabolism. Moreover, the mechanisms involve 22:6,n-3 control of several well-known signaling pathways, such as Akt, Erk1/2, Gsk3beta and PKC (novel or atypical). 22:6,n-3 control of these same signaling pathways in non-hepatic tissues may help to explain the diverse actions of n-3 PUFA on such complex physiological processes as visual acuity and learning.

Effects of fish oil and conjugated linoleic acids on expression of target genes of PPAR alpha and sterol regulatory element-binding proteins in the liver of laying hens

In mammals, (n-3) PUFA and conjugated linoleic acids (CLA) act as activators of PPAR alpha and alter nuclear concentrations of sterol regulatory element-binding proteins (SREBP) in the liver, and thereby influence hepatic lipid catabolism and synthesis. In this study, we investigated the hypothesis that (n-3) PUFA and CLA exert similar effects in the liver of laying hens. Thirty hens (64 weeks old) were fed diets containing 30 g/kg of sunflower oil (control), fish oil (salmon oil) or CLA in TAG form (containing predominantly cis-9, trans-11 CLA and trans-10, cis-12 CLA) for 5 weeks. Hens fed fish oil had a higher expression of some PPAR alpha target genes and a lower nuclear concentration of SREBP-2 in the liver and lower concentrations of cholesterol and TAG in plasma than control hens. Nuclear concentration of SREBP-1 and its target genes involved in lipogenesis were not altered in hens fed fish oil. Hens fed CLA had increased concentrations of TAG and cholesterol in the liver. However, their mRNA levels of PPAR alpha target genes and nuclear concentrations of SREBP-1 and SREBP-2 as well as mRNA levels of their target genes in the liver were largely unchanged compared to control hens. The results of this study suggest that (n-3) PUFA cause a moderate activation of PPAR alpha and lower cholesterol synthesis but do not impair fatty acid synthesis in the liver of laying hens. CLA lead to an accumulation of TAG and cholesterol in the liver of hens by mechanisms to be elucidated in further studies.

Suppression of phosphoenolpyruvate carboxykinase gene expression by reduced endogenous glutathione level

Glutathione is a small tripeptide to maintain overall reducing environment in vivo. Reduced endogenous glutathione level has been associated with aging, obesity and diabetes. In this study, the direct impact of low endogenous glutathione level on energy homeostasis is investigated at molecular level. Depletion of endogenous glutathione in rat primary hepatocytes by BSO, an inhibitor of gamma-glutamylcysteine synthase, leads to reduced mRNA levels of several key enzymes in energy homeostasis, including phosphoenolpyruvate carboxylkinase (PEPCK), the rate-limiting enzyme in gluconeogenesis. Supplementation of various reducing reagents, including N-acetylcysteine, DTT and glutathione, reverses the inhibitory effect of BSO on PEPCK mRNA level. The suppressive effect of BSO on PEPCK mRNA level is also reversed through co-treatment with either SB210290, a specific p38 kinase inhibitor, or wortmannin and LY294002, the well-established PI-3 kinase inhibitors, suggesting the involvement of these kinases in this process. These observations correlate well with the observations that reduced endogenous glutathione level and reduced gluconeogenesis coincide with aging process, implying a causal relationship between these changes in aged population. More importantly, this study suggests that endogenous glutathione level tightly associates with energy homeostasis at molecular level, identifying reduced endogenous glutathione level as a potential contributing factor to dysregulated metabolic processes in aging, obese and diabetic populations. In addition, the different responses of PEPCK expression to the alteration of endogenous glutathione level in rat hepatoma cells from primary hepatocytes raises caution against using established cell lines in examining the dysregulated metabolic process related to altered endogenous glutathione level.

Chenodeoxycholic acid suppresses the activation of acetyl-coenzyme A carboxylase-alpha gene transcription by the liver X receptor agonist T0-901317

The therapeutic utility of liver X receptor (LXR) agonists in treating atherosclerosis is limited by an undesired accumulation of triglycerides in the blood and liver. This effect is caused by an increase in the transcription of genes involved in fatty acid synthesis. Here, we show that the primary bile acid, chenodeoxycholic acid (CDCA), antagonizes the stimulatory effect of the synthetic LXR agonist, T0-901317, on the expression of acetyl-coenzyme A carboxylase-alpha (ACCalpha) and other lipogenic enzymes in chick embryo hepatocyte cultures. CDCA inhibits T0-901317-induced ACCalpha transcription by suppressing the enhancer activity of a LXR response unit (-101 to -71 bp) that binds LXR and sterol-regulatory element binding protein-1 (SREBP-1). We also demonstrate that CDCA decreases the expression of SREBP-1 in the nucleus and the acetylation of histone H3 and H4 at the ACCalpha LXR response unit. The CDCA-mediated reduction in ACCalpha expression is associated with a decrease in the expression of peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) and small heterodimer partner and an increase in the expression of fibroblast growth factor-19 (FGF-19). Ectopic expression of FGF-19 decreases T0-901317-induced ACCalpha expression. Inhibition of p38 mitogen-activated protein kinase (MAPK) and/or extracellular signal-regulated kinase (ERK) suppresses the effects of CDCA on the expression of ACCalpha, SREBP-1, PGC-1alpha, and FGF-19. These results demonstrate that CDCA inhibits T0-901317-induced ACCalpha transcription by suppressing the activity of LXR and SREBP-1. We postulate that p38 MAPK, ERK, PGC-1alpha, and FGF-19 are components of the signaling pathway(s) mediating the regulation of ACCalpha gene transcription by CDCA.

Thermally oxidized oil increases the expression of insulin-induced genes and inhibits activation of sterol regulatory element-binding protein-2 in rat liver

Administration of oxidized oils to rats or pigs causes a reduction of their cholesterol concentrations in liver and plasma. The reason for this effect is unknown. We tested the hypothesis that oxidized oils lower cholesterol concentrations by inhibiting the proteolytic activation of sterol regulatory element-binding protein (SREBP)-2 in the liver and transcription of its target genes involved in cholesterol synthesis and uptake through an upregulation of gene expression of insulin-induced genes (Insig). For 6 d, 18 rats were orally administered either sunflower oil (control group) or an oxidized oil prepared by heating sunflower oil. Rats administered the oxidized oil had higher messenger RNA (mRNA) concentrations of acyl-CoA oxidase and cytochrome P450 4A1 in the liver than control rats (P < 0.05), indicative of activation of PPARalpha. Furthermore, rats administered the oxidized oil had higher mRNA concentrations of Insig-1 and Insig-2a, a lower concentration of the mature SREBP-2 in the nucleus, lower mRNA concentrations of the SREBP-2 target genes 3-hydroxy-3-methylglutaryl CoA reductase and LDL receptor in their livers, and a lower concentration of cholesterol in liver, plasma, VLDL, and HDL than control rats (P < 0.05). In conclusion, this study shows that reduced cholesterol concentrations in liver and plasma of rats administered an oxidized oil were due to an inhibition of the activation of SREBP-2 by an upregulation of Insig, which in turn inhibited transcription of proteins involved in hepatic cholesterol synthesis and uptake.

Characterization of insulin resistance syndrome in children and young adults. When to screen for prediabetes?

CONTEXT

Insulin resistance syndrome (IRS) is associated with the development of type 2 diabetes mellitus (DM2). However, it is unclear which individuals with insulin resistance will develop DM2.

AIM

To study the prevalence of IRS in childhood and to identify the group with the highest risk of further progression to DM2.

METHODS

In a cross-sectional study, 86 obese individuals underwent an intravenous glucose tolerance test (IVGTT). Insulin resistance index (Si(IVGTT)), acute insulin response (AIR) and disposition index (DI) were calculated from IVGTT.

RESULTS

For analysis the participants were divided into insulin-sensitive (IS) (n = 25, 13.3 +/- 5.9 yr) and insulin-resistant (IR) groups on the basis of having an Si(IVGTT) greater or lesser than 4.5 x 10(-4) mU/ml/min, respectively. The IR group was then subdivided according to DI, with the standard cut-off value of 0.13 min(-1), into compensated IR (CIR) (n = 37, 13.0 +/- 3.5 yr) and decompensated IR (DIR) (n = 24, 21.9 +/- 12.6 yr) groups. The frequency of IRS was 43% in children, 78% in adolescents and 83.6% in adults. Decompensated insulin response first appeared during adolescence. The frequency of decompensation increased from 22% in adolescence to 67% in adulthood. The DIR group had increased triglycerides (TG) and urinary free cortisol levels.

CONCLUSIONS

The frequency and severity of IR increases with age. Decompensation first presents in adolescence with low AIR and elevated TG. Decompensated adolescents are the group at highest risk for further progression to DM2.

Hypoxia Stimulates Degradation of 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase through Accumulation of Lanosterol and Hypoxia-Inducible Factor-mediated Induction of Insigs

Endoplasmic reticulum-associated degradation of the enzyme 3-hydroxy-3-methylglutaryl-CoA reductase represents one mechanism by which cholesterol synthesis is controlled in mammalian cells. The key reaction in this degradation is binding of reductase to Insig proteins in the endoplasmic reticulum, which is stimulated by the cholesterol precursor lanosterol. Conversion of lanosterol to cholesterol requires removal of three methyl groups, which consumes nine molecules of dioxygen. Here, we report that oxygen deprivation (hypoxia) slows demethylation of lanosterol and its metabolite 24,25-dihydrolanosterol, causing both sterols to accumulate in cells. In addition, hypoxia increases the amount of Insig-1 and Insig-2 in a response mediated by hypoxia-inducible factor (HIF)-1alpha. Accumulation of lanosterol together with increased Insigs accelerates degradation of reductase, which ultimately slows a rate-determining step in cholesterol synthesis. These results define a novel oxygen-sensing mechanism mediated by the combined actions of methylated intermediates in cholesterol synthesis and the hypoxia-activated transcription factor HIF-1alpha.

TNF-alpha interferes with lipid homeostasis and activates acute and proatherogenic processes

The interaction between disrupted lipid homeostasis and immune response is implicated in the pathogenesis of several diseases, but the molecular bridges between the major players are still a matter of controversy. Our systemic study of the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) in the livers of mice exposed to 20-h cytokine/fasting for the first time shows that TNF-alpha interferes with adaptation to fasting and activates harmful proatherogenic pathways, partially through interaction with the insulin-Insig-sterol regulatory element binding protein (Srebp) signaling pathway. In addition to the increased expression of acute-phase inflammatory genes, the most prominent alterations represent modified lipid homeostasis observed on the gene expression and metabolite levels. These include reduction of HDL-cholesterol, increase of LDL-cholesterol, and elevated expression of cholesterogenic genes, accompanied by increase of potentially harmful precholesterol metabolites and suppression of cholesterol elimination through bile acids, likely by farnesoid X receptor-independent mechanisms. On the transcriptional level, a shift from fatty oxidation toward fatty acid synthesis is observed. The concept of the influence of TNF-alpha on the Srebp regulatory network, followed by downstream effects on sterol metabolism, is novel. Observed acute alterations in lipid metabolism are in agreement with chronic disturbances found in patients.

Oxysterol Binding Protein Induces Upregulation of SREBP-1c and Enhances Hepatic Lipogenesis

BACKGROUND

Oxysterol binding protein (OSBP) has previously been implicated as a sterol sensor that regulates sphingomyelin synthesis and the activity of extracellular signal-regulated kinases (ERK).

METHODS AND RESULTS

We determined the effects of adenovirus-mediated hepatic overexpression of OSBP and its homologues ORP1L and ORP3 on mouse serum lipids. Whereas ORP1L and ORP3 had no effect on serum lipids, OSBP induced a marked increase of VLDL triglycerides (TG). Also, the liver tissue TG were elevated in the AdOSBP-injected mice, and their TG secretion rate was increased by 70%. The messenger RNAs for enzymes of fatty acid synthesis and their transcriptional regulator, SREBP-1c, as well as the Insig-1 mRNA, were upregulated two-fold in the OSBP-expressing livers. No change occurred in the messages of liver X receptor target genes ABCA1, ABCG5, and CYP7A1, and the Insig-2a mRNA was reduced. The phosphorylation of ERK was decreased in AdOSBP-infected liver and cultured hepatocytes. Importantly, silencing of OSBP in hepatocytes suppressed the induction of SREBP1-c by insulin and resulted in a reduction of TG synthesis.

CONCLUSION

Our results demonstrate that OSBP regulates hepatic TG metabolism and suggest the involvement of OSBP in the insulin signaling pathways that control hepatic lipogenesis.

Generation of a conditional knockout allele for mammalian Spen protein Mint/SHARP

The Spen protein family is found in worms, flies, and mammals, and is implicated in diverse biological processes from embryogenesis to aging. Spen proteins have three N-terminal RNA recognition motifs and a C-terminal SPOC domain. The mammalian Spen proteins Mint and its human ortholog SHARP interact with the Notch-signaling mediator RBP-J as well as Msx2 and several unliganded nuclear hormone receptors, and impart transcription-repressing activity to these molecules by recruiting corepressors through the SPOC domain. Despite these in vitro findings, Mint/SHARP's physiological role is largely unknown, because Mint germline knockouts are embryonic lethal. To analyze Mint/SHARP function in postnatal mice, we created Mint-floxed mice that allow the Cre/loxP-mediated conditional knockout of Mint. We analyzed Mint and RBP-J epistasis during Notch-dependent splenic B-lymphocyte development, and found that Mint suppresses Notch signaling through RBP-J. In addition, Mint deficiency caused severe hypoplasia in postnatal brain, suggesting it may regulate neuronal cell survival.

Transcriptional regulation by insulin: from the receptor to the gene

Insulin, after binding to its receptor, regulates many cellular processes and the expression of several genes. For a subset of genes, insulin exerts a negative effect on transcription; for others, the effect is positive. Insulin controls gene transcription by modifying the binding of transcription factors on insulin-response elements or by regulating their transcriptional activities. Different insulin-signaling cascades have been characterized as mediating the insulin effect on gene transcription. In this review, we analyze recent data on the molecular mechanisms, mostly in the liver, through which insulin exerts its effect. We first focus on the key transcription factors (viz. Foxo, sterol-response-element-binding protein family (SREBP), and Sp1) involved in the regulation of gene transcription by insulin. We then present current information on the way insulin downregulates and upregulates gene transcription, using as examples of downregulation phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor binding protein 1 (IGFBP-1) genes and of upregulation the fatty acid synthase and malic enzyme genes. The last part of the paper focuses on the signaling cascades activated by insulin in the liver, leading to the modulation of gene transcription.

Conditional animal models for the study of lipid metabolism and lipid disorders

The advent of technologies that allow conditional mutagenesis has revolutionized our ability to explore gene functions and to establish animal models of human diseases. Both aspects have proven to be of particular importance in the study of lipid-related disorders. Classical approaches to gene inactivation by conventional gene targeting strategies have been successfully applied to generate animal models like the LDL receptor- and the apolipoprotein E-knockout mice, which are still widely used to study diverse aspects of atherosclerosis, lipid transport, and neurodegenerative disease. In many cases, however, simply inactivating the gene of interest has resulted in early lethal or complex phenotypes which are difficult to interpret. In recent years, additional tools have therefore been developed that allow the spatiotemporally controlled manipulation of the genome, as described in detail in Part I of this volume. Our aim is to provide an exemplary survey of the application of different conditional mutagenesis techniques in lipid research in order to illustrate their potential to unravel physiological functions of a broad range of genes involved in lipid homeostasis.

Activation of PPARα lowers synthesis and concentration of cholesterol by reduction of nuclear SREBP-2

To elucidate the mechanisms underlying the cholesterol lowering effects of PPARalpha agonists we investigated key regulators of cholesterol synthesis and uptake in rats and in the rat hepatoma cell line Fao after treatment with the PPARalpha agonists clofibrate and WY 14,643, respectively. In rat liver as well as in Fao cells, PPARalpha activation led to a decrease of transcriptionally active nuclear SREBP-2. mRNA concentrations of the key regulators of SREBP processing, Insig-1 in rat liver and Insig-1 and Insig-2a in Fao cells, were increased upon PPARalpha activation. Thus we suggest, that the observed reduction of the amount of nuclear SREBP-2 was due to an inhibition of the processing of the precursor protein. Both, in rat liver and in Fao cells, mRNA concentrations of the SREBP-2 target genes HMG-CoA reductase (EC1.1.1.34) and LDL receptor were reduced after treatment with the PPARalpha agonists. Furthermore, treatment of Fao cells with WY 14,643 reduced cholesterol synthesis. As a result, the amount of total cholesterol in liver, plasma and lipoproteins of clofibrate treated rats and in WY 14,643 treated Fao cells was decreased compared to control animals and cells, respectively. In conclusion, we could show a novel link between PPARalpha and cholesterol metabolism by demonstrating that PPARalpha activation lowers cholesterol concentration by reducing the abundance of nuclear SREBP-2.

Comment on "A common genetic variant is associated with adult and childhood obesity"

Contrary to the findings of Herbert et al. (Reports, 14 April 2006, p. 279), homozygous carriers of the C allele of the rs7566605 variant near the INSIG2 gene did not exhibit a significantly increased risk for obesity in a large population-based cross-sectional German study. A subgroup analysis, however, revealed that this allele significantly increased the risk for obesity in already overweight individuals.

Activation-induced cytidine deaminase (AID) promotes B cell lymphomagenesis in Emu-cmyc transgenic mice

Activation-induced cytidine deaminase (AID), which is essential to both class switch recombination and somatic hypermutation of the Ig gene, is expressed in many types of human B cell lymphoma/leukemia. AID is a potent mutator because it is involved in DNA breakage not only of Ig but also of other genes, including proto-oncogenes. Recent studies suggest that AID is required for chromosomal translocation involving cmyc and Ig loci. However, it is unclear whether AID plays other roles in tumorigenesis. We examined the effect of AID deficiency on the generation of surface Ig-positive B cell lymphomas in Emu-cmyc transgenic mice. Almost all lymphomas that developed in AID-deficient transgenic mice were pre-B cell lymphomas, whereas control transgenic mice had predominantly B cell lymphomas, indicating that AID is required for development of B but not pre-B cell lymphomas from cmyc overexpressing tumor progenitors. Thus, AID may play multiple roles in B cell lymphomagenesis.

Msx2-interacting nuclear target protein (Mint) deficiency reveals negative regulation of early thymocyte differentiation by Notch/RBP-J signaling

Notch/RBP-J signaling is required for generation of early T progenitors (ETP) and promotion of double-negative (DN) 4 cells from DN3 cells in thymocyte differentiation. However, whether Notch affects other steps during thymocyte differentiation remains unknown. Msx2-interacting nuclear target protein (Mint) is an endogenous inhibitor of Notch regulation. Concordantly, by ex vivo analyses of embryonic thymi and in vitro differentiation studies of fetal liver progenitors, we find that Mint deficiency enhances generation of ETP and DN4 cells. Unexpectedly, however, Mint deficiency impairs differentiation of ETP into DN2 cells, suggesting that Notch/RBP-J signaling negatively regulates DN1-DN2 transition.

Insulin activates human sterol-regulatory-element-binding protein-1c (SREBP-1c) promoter through SRE motifs

In the present study, we aimed to decipher the mechanisms involved in the transcriptional effect of insulin on the SREBP-1c specific promoter of the human srebf-1 gene. Using luciferase reporter gene constructs in HEK-293 cells (human embryonic kidney cells), we demonstrated that the full effect of insulin requires the presence of SREs (sterol response elements) in the proximal region of the promoter. Furthermore, insulin increases the binding of SREBP-1 (sterol-regulatory-element-binding protein-1) to this promoter region in chromatin immunoprecipitation assay. We also found that the nuclear receptors LXRs (liver X receptors) strongly activate SREBP-1c gene expression and identified the LXRE (LXR-response element) involved in this effect. However, our results suggested that these LXREs do not play a major role in the response to insulin. Finally, using expression vectors and adenoviruses allowing ectopic overexpressions of the human mature forms of SREBP-1a or SREBP-1c, we demonstrated the direct role of SREBP-1 in the control of SREBP-1c gene expression in human skeletal-muscle cells. Altogether, these results strongly suggest that the SREBP-1 transcription factors are the main mediators of insulin action on SREBP-1c expression in human tissues.

Chromatin remodeling complex interacts with ADD1/SREBP1c to mediate insulin-dependent regulation of gene expression

Insulin plays a critical role in whole-body energy homeostasis by regulating lipid and glucose metabolism. In fat and liver tissues, ADD1/SREBP1c is a key transcription factor to mediate insulin-dependent regulation of gene expression. Although transcriptional and proteolytic activation of ADD1/SREBP1c has been studied intensively, the mechanism by which insulin regulates expression of its target genes with ADD1/SREBP1c at the chromatin level is unclear. Here, we reveal that SWI/SNF chromatin remodeling factors interact with the ADD1/SREBP1c and actively regulate insulin-dependent gene expression. Insulin enhanced recruitment of SWI/SNF chromatin remodeling factors to its target gene promoters with concomitant changes in the chromatin structures as well as gene expression. Furthermore, in vivo overexpression of BAF155/SRG3, a component of the SWI/SNF complex, substantially promoted insulin target gene expression and insulin sensitivity. Taken together, our results suggest that the SWI/SNF chromatin remodeling complexes confer not only insulin-dependent gene expression but also insulin sensitivity in vivo via interaction with ADD1/SREBP1c.

Regulation of GLUT4 gene expression by SREBP-1c in adipocytes

Expression of the GLUT4 (glucose transporter type 4 isoform) gene in adipocytes is subject to hormonal or metabolic control. In the present study, we have characterized an adipose tissue transcription factor that is influenced by fasting/refeeding regimens and insulin. Northern blotting showed that refeeding increased GLUT4 mRNA levels for 24 h in adipose tissue. Consistent with an increased GLUT4 gene expression, the mRNA levels of SREBP (sterol-regulatory-element-binding protein)-1c in adipose tissue were also increased by refeeding. In streptozotocin-induced diabetic rats, insulin treatment increased the mRNA levels of GLUT4 in adipose tissue. Serial deletion, luciferase reporter assays and electrophoretic mobility-shift assay studies indicated that the putative sterol response element is located in the region between bases -109 and -100 of the human GLUT4 promoter. Transduction of the SREBP-1c dominant negative form to differentiated 3T3-L1 adipocytes caused a reduction in the mRNA levels of GLUT4, suggesting that SREBP-1c mediates the transcription of GLUT4. In vivo chromatin immunoprecipitation revealed that refeeding increased the binding of SREBP-1 to the putative sterol-response element in the GLUT4. Furthermore, treating streptozotocin-induced diabetic rats with insulin restored SREBP-1 binding. In addition, we have identified an Sp1 binding site adjacent to the functional sterol-response element in the GLUT4 promoter. The Sp1 site appears to play an additive role in SREBP-1c mediated GLUT4 gene upregulation. These results suggest that upregulation of GLUT4 gene transcription might be directly mediated by SREBP-1c in adipose tissue.

Modified HMG-CoA reductase and LDLr regulation is deeply involved in age-related hypercholesterolemia

During the ageing process in rats hypercholesterolemia occurs in concert with full activation, lowered degradation rate and an unchanged level of the rate limiting cholesterol biosynthesis enzyme, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG-CoAR). The molecular bases of the HMG-CoAR unchanged level and lowered degradation rate in aged rats is not clear. In fact no data are available during ageing, on transcription and degradation of HMG-CoAR, so well defined in adult animal. So, aim of this work was to measure mRNA levels of the enzyme and the level of the proteins of the regulatory complex responsible of the cholesterol metabolism. To complete the picture, the level of sterol regulatory element binding proteins (SREBPs), SREBP cleavage activating protein, and insulin-induced gene has been measured. The levels of other related proteins, whose transcription is SREBP dependent, that is low density lipoprotein receptor (LDLr) and Caveolin 1, have been also measured. The age-related reduced Insigs levels, joined to a reduced insulin sensitivity, could explain the decreased degradation rate of the HMG-CoAR and the increased active SREBP-2. The SREBP-2 in particular seems to be committed in multiple way to gene transcription. The obtained data represent a good contribution to explain the age-related hypercholesterolemia.

Insulin and glucose play a role in foam cell formation and function

Background

Foam cell formation in diabetic patients often occurs in the presence of high insulin and glucose levels. To test whether hyperinsulinemic hyperglycemic conditions affect foam cell differentiation, we examined gene expression, cytokine production, and Akt phosphorylation in human monocyte-derived macrophages incubated with two types of oxidized low density lipoprotein (LDL), minimally modified LDL (mmLDL) and extensively oxidized LDL (OxLDL).

Methods and results

Using Affymetrix GeneChip^® arrays, we found that several genes directly related to insulin signaling were changed. The insulin receptor and glucose-6-phosphate dehydrogenase were upregulated by mmLDL and OxLDL, whereas insulin-induced gene 1 was significantly down-regulated. In hyperinsulinemic hyperglycemic conditions, modified LDL upregulated Akt phosphorylation and expression of the insulin-regulated aminopeptidase. The level of proinflammatory cytokines, IL-lβ, IL-12, and IL-6, and of a 5-lipoxygenase eicosanoid, 5-hydroxyeicosatetraenoic acid (5-HETE), was also increased.

Conclusion

These results suggest that the exposure of macrophages to modified low density lipoproteins in hyperglycemic hyperinsulinemic conditions affects insulin signaling and promotes the release of proinflammatory stimuli, such as cytokines and eicosanoids. These in turn may contribute to the development of insulin resistance.

From mice to men: insights into the insulin resistance syndromes

The insulin resistance syndrome refers to a constellation of findings, including glucose intolerance, obesity, dyslipidemia, and hypertension, that promote the development of type 2 diabetes, cardiovascular disease, cancer, and other disorders. Defining the pathophysiological links between insulin resistance, the insulin resistance syndrome, and its sequelae is critical to understanding and treating these disorders. Over the past decade, two approaches have provided important insights into how changes in insulin signaling produce the spectrum of phenotypes associated with insulin resistance. First, studies using tissue-specific knockouts or tissue-specific reconstitution of the insulin receptor in vivo in mice have enabled us to deconstruct the insulin resistance syndromes by dissecting the contributions of different tissues to the insulin-resistant state. Second, in vivo and in vitro studies of the complex network of insulin signaling have provided insight into how insulin resistance can develop in some pathways whereas insulin sensitivity is maintained in others. These data, taken together, give us a framework for understanding the relationship between insulin resistance and the insulin resistance syndromes.

Transcriptional regulation of metabolism

Our understanding of metabolism is undergoing a dramatic shift. Indeed, the efforts made towards elucidating the mechanisms controlling the major regulatory pathways are now being rewarded. At the molecular level, the crucial role of transcription factors is particularly well-illustrated by the link between alterations of their functions and the occurrence of major metabolic diseases. In addition, the possibility of manipulating the ligand-dependent activity of some of these transcription factors makes them attractive as therapeutic targets. The aim of this review is to summarize recent knowledge on the transcriptional control of metabolic homeostasis. We first review data on the transcriptional regulation of the intermediary metabolism, i.e., glucose, amino acid, lipid, and cholesterol metabolism. Then, we analyze how transcription factors integrate signals from various pathways to ensure homeostasis. One example of this coordination is the daily adaptation to the circadian fasting and feeding rhythm. This section also discusses the dysregulations causing the metabolic syndrome, which reveals the intricate nature of glucose and lipid metabolism and the role of the transcription factor PPARgamma in orchestrating this association. Finally, we discuss the molecular mechanisms underlying metabolic regulations, which provide new opportunities for treating complex metabolic disorders.

Protein sensors for membrane sterols

Cholesterol is an essential component of animal cell membranes, and its concentration is tightly controlled by a feedback system that operates at transcriptional and posttranscriptional levels. Here, we discuss recent advances that explain how cells employ an ensemble of membrane-embedded proteins to monitor sterol concentrations and adjust sterol synthesis and uptake.

Androgen activation of the sterol regulatory element-binding protein pathway: Current insights

The cellular effects of androgens are mediated by a cognate receptor, the androgen receptor. Typically, the androgen receptor is viewed to exert its activity by binding to androgen response elements located in or near the promoter region of target genes, thereby directly affecting the expression of these genes. However, increasing evidence indicates that androgens may also indirectly influence the expression of genes that do not contain androgen response elements by modulating the activity of secondary transcription factors, mediating the expression of growth factors acting in a paracrine or autocrine fashion, or by inducing changes in the production of other hormones. These indirect effects of androgens can induce cascade-like actions and may play an important role in more complex processes involving coordinated responses of genes, cells, and organs. Previously, our laboratory has identified and characterized a novel indirect mechanism of androgen action involving proteolytical activation of the key lipogenic transcription factor sterol regulatory element-binding protein (SREBP), resulting in the coordinate up-regulation of entire cellular lipogenic pathways. Interestingly, activation of SREBPs by androgens occurs not only under normal physiological conditions but has also been observed in a growing number of pathologies, and more in particular in the setting of steroid-regulated cancers, where increased lipogenesis has been shown to have remarkable diagnostic and prognostic potential and is considered a prime target for novel therapeutic approaches. This review aims to analyze current insights into the molecular mechanism(s) underlying androgen activation of the SREBP pathway and to ascertain the extent to which this phenomenon can be generalized to androgen-responsive cell systems.

In vivo expression of carbohydrate responsive element binding protein in lean and obese rats

ChREBP (Carbohydrate response element binding protein) is considered to mediate the stimulatory effect of glucose on the expression of lipogenic genes. Its activity is stimulated by glucose. Less is known on the control of its expression. This expression could be controlled by nutritional (glucose, fatty acids) and hormonal (insulin) factors. We examined the in vivo nutritional control of ChREBP expression in liver and adipose tissue of Wistar rats. Compared respectively to the fed state and to a high carbohydrate diet, ChREBP mRNA concentrations were not modified by fasting or a high fat diet in rat liver and adipose tissue. FAS and ACC1 mRNA concentrations were on the contrary decreased as expected by fasting and high fat diets and these variations of FAS and ACC1 mRNA were positively related to those of SREBP-1c mRNA and protein, but not of ChREBP mRNA. Therefore i) ChREBP expression appears poorly responsive to modifications of nutritional condition, ii) modifications of the expression of ChREBP do not seem implicated in the physiological control of lipogenesis. To investigate the possible role of ChREBP in pathological situations we measured its mRNA concentrations in the liver and adipose tissue of obese Zucker rats. ChREBP expression was increased in the liver but not the adipose tissue of obese rats compared to their lean littermates. These results support a role of ChREBP in the development of hepatic steatosis and hypertriglyceridemia but not of obesity in this experimental model.

Sterol regulatory element-binding protein (SREBP)-1: gene regulatory target for insulin resistance?

The combined appearance of different cardiovascular risk factors seems to be more prevalent in individuals with decreased insulin sensitivity and increased visceral obesity, thereby being components of the so-called metabolic syndrome. Alterations in transcription factors result in complex dysregulation of gene expression, which might be the key to understanding insulin resistance-associated clinical clustering of coronary risk factors at the cellular or gene regulatory level. Recent examples are peroxisome proliferator-activated receptors and sterol regulatory element-binding proteins (SREBPs), which also appear to be novel drug targets. The authors have recently shown that SREBPs are substrates of mitogen-activated protein kinases, and propose that SREBP-1 might play a role in the development of cellular features belonging to lipotoxicity and, possibly, syndrome X.