Regulation of the fatty acid synthase promoter by insulin

Physiological and pathophysiological actions of insulin in the liver

The liver plays an important role in the control of glucose homeostasis. When insulin levels are low, such as in the fasting state, gluconeogenesis and glycogenolysis are stimulated to maintain the blood glucose levels. Conversely, in the presence of increased insulin levels, such as after a meal, synthesis of glycogen and lipid occurs to maintain the blood glucose levels within normal range. Insulin receptor signaling regulates glycogenesis, gluconeogenesis and lipogenesis through downstream pathways such as the insulin receptor substrate (IRS)-phosphoinositide 3 (PI3) kinase-Akt pathway. IRS-1 and IRS-2 are abundantly expressed in the liver and are thought to be responsible for transmitting the insulin signal from the insulin receptor to the intracellular effectors involved in the regulation of glucose and lipid homeostasis. Impaired insulin receptor signaling can cause hepatic insulin resistance and lead to type 2 diabetes. In the present study, we focus on a concept called "selective insulin resistance," which has received increasing attention recently: the frequent coexistence of hyperglycemia and hepatic steatosis in people with type 2 diabetes and obesity suggests that it is possible for the insulin signaling regulating gluconeogenesis to be impaired even while that regulating lipogenesis is preserved, suggestive of selective insulin resistance. In this review, we review the progress in research on the insulin actions and insulin signaling in the liver.

Plasma PFOA and PFOS Levels, DNA Methylation, and Blood Lipid Levels: A Pilot Study

Exposure to perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) is associated with blood lipids in adults, but the underlying mechanisms remain unclear. This pilot study aimed to investigate the associations between PFOA or PFOS and epigenome-wide DNA methylation and assess the mediating effect of DNA methylation on the PFOA/PFOS-blood lipid association. We measured plasma PFOA/PFOS and leukocyte DNA methylation in 98 patients enrolled from the hospital between October 2018 and August 2019. The median plasma PFOA/PFOS levels were 0.85 and 2.29 ng/mL. Plasma PFOA and PFOS levels were significantly associated with elevated total cholesterol (TC) and low-density lipoprotein cholesterol (LDL) levels. There were 63/87 CpG positions and 8/11 differentially methylated regions (DMRs) associated with plasma PFOA/PFOS levels, respectively. In addition, 5 CpG positions (annotated to AFF3, CREB5, NRG2, USF2, and intergenic region) and one DMR annotated to IRF6 may mediate the association between plasma PFOA/PFOS and LDL levels (mediated proportion from 7.29 to 46.77%); two CpG positions may mediate the association between plasma PFOA/PFOS and TC levels (annotated to CREB5 and USF2, mediated proportion is around 30%). The data suggest that PFOA/PFOS exposure alters DNA methylation. More importantly, the association of PFOA/PFOS with lipid indicators was partly mediated by DNA methylation changes in lipid metabolism-related genes.

Placental nutrient transporters adapt during persistent maternal hypoglycaemia in rats

Maternal malnutrition is associated with decreased nutrient transfer to the foetus, which may lead to foetal growth restriction, predisposing children to a variety of diseases. However, regulation of placental nutrient transfer during decreased nutrient availability is not fully understood. In the present study, the aim was to investigate changes in levels of placental nutrient transporters accompanying maternal hypoglycaemia following different durations and stages of gestation in rats. Maternal hypoglycaemia was induced by insulin-infusion throughout gestation until gestation day (GD)20 or until end of organogenesis (GD17), with sacrifice on GD17 or GD20. Protein levels of placental glucose transporters GLUT1 (45/55 kDa isotypes) and GLUT3, amino acid transporters SNAT1 and SNAT2, and insulin receptor (InsR) were assessed. On GD17, GLUT1-45, GLUT3, and SNAT1 levels were increased and InsR levels decreased versus controls. On GD20, following hypoglycaemia throughout gestation, GLUT3 levels were increased, GLUT1-55 showed the same trend. After cessation of hypoglycaemia at end of organogenesis, GLUT1-55, GLUT3, and InsR levels were increased versus controls, whereas SNAT1 levels were decreased. The increases in levels of placental nutrient transporters seen during maternal hypoglycaemia and hyperinsulinemia likely reflect an adaptive response to optimise foetal nutrient supply and development during limited availability of glucose.

Metabolism and Health Effects of Rare Sugars in a CACO-2/HepG2 Coculture Model

Non-alcoholic fatty liver disease (NAFLD) has become the most prevalent liver disease worldwide and is impacted by an unhealthy diet with excessive calories, although the role of sugars in NAFLD etiology remains largely unexplored. Rare sugars are natural sugars with alternative monomers and glycosidic bonds, which have attracted attention as sugar replacers due to developments in enzyme engineering and hence an increased availability. We studied the impact of (rare) sugars on energy production, liver cell physiology and gene expression in human intestinal colorectal adenocarcinoma (Caco-2) cells, hepatoma G2 (HepG2) liver cells and a coculture model with these cells. Fat accumulation was investigated in the presence of an oleic/palmitic acid mixture. Glucose, fructose and galactose, but not mannose, l-arabinose, xylose and ribose enhanced hepatic fat accumulation in a HepG2 monoculture. In the coculture model, there was a non-significant trend (p = 0.08) towards higher (20–55% increased) median fat accumulation with maltose, kojibiose and nigerose. In this coculture model, cellular energy production was increased by glucose, maltose, kojibiose and nigerose, but not by trehalose. Furthermore, glucose, fructose and l-arabinose affected gene expression in a sugar-specific way in coculture HepG2 cells. These findings indicate that sugars provide structure-specific effects on cellular energy production, hepatic fat accumulation and gene expression, suggesting a health potential for trehalose and l-arabinose, as well as a differential impact of sugars beyond the distinction of conventional and rare sugars.

Dietary copper supplementation enhances lipolysis in Rex rabbits

BACKGROUND

Copper is an important regulator of lipid metabolism in mammals, as a cofactor of many enzymes and is involved in the lipolysis. Copper deficiency has been considered as a significant factor in human diseases related to abnormal lipid metabolism, while adding copper to the diet seems to be the simplest and most effective way to prevent copper deficiency.

AIMS

The aim of this study was to investigate the effects of dietary copper level on lipid metabolism in Rex Rabbits.

METHODS

A total of 120 90-d-old Rex Rabbits were randomly allotted into three treatments, with 40 replicates (20 males, 20 females) in each treatment (1 rabbit per replicate). The diets included 1) control (8.4 mg/kg), normal-copper diet (39.1 mg/kg), 3) high-copper diet (67.5 mg/kg). The trial including a one-week adaptation period and a five-week experimental period.

RESULT

The results showed that copper (39.1 mg/kg) diet increased average daily feed intake (ADFI) (P＜0.05, N = 34), and tended to increase the final body weight (FBW) (P = 0.0556, N = 34). Moreover, dietary copper addition (39.1 and 67.5 mg/kg) significantly increased the foreleg and hindleg weight (P＜0.05, N = 8), and decreased the weight of Perirenal fat and the concentration of triglycerides (TG) in the liver (P＜0.05, N = 8). The concentration of triglycerides (TG), epinephrine (EPI), and glucagon (GC) in serum were obviously higher than that in control group (P＜0.05, N = 8), and the concentration of insulin (INS), and very low-density lipoprotein (VLDL) in serum were significantly decreased (P＜0.05, N = 8). The copper group (39.1 mg/kg) showed up-regulated gene expression levels of carnitine palmitoyl transferases (CPT-1 and CPT-2) and peroxisome proliferator-activated receptor (PPAR-α) in liver (P < 0.05, N = 8) and down-regulated gene expression levels of fatty acid synthase (FAS) and Acetyl-CoA carboxylase (ACC) (P < 0.05, N = 8). In skeletal muscle, CPT-1, CPT-2, PPAR-α, fatty acid transport protein (FATP), fatty acid-binding protein (FABP) and lipoprotein lipase (LPL) levels were significantly up-regulated by copper treatment (P < 0.05, N = 8). Rex Rabbits receiving copper addition had higher CPT-1, CPT-2, PPAR-a and hormone-sensitive lipase (HSL) mRNA levels in adipose tissue (P < 0.05, N = 8).

CONCLUSION

Copper diets promoted skeletal muscle growth and reduced fat accumulation by enhancing fatty acid oxidation, at the same time, dietary copper inhibited De novo lipogenesis in the liver. PPAR-α signaling in liver, skeletal muscle and adipose tissues were involved in the regulation of lipid metabolism by copper.

Immunonutritional Bioactives from Chenopodium quinoa and Salvia hispanica L. Flour Positively Modulate Insulin Resistance and Preserve Alterations in Peripheral Myeloid Population

Innate immunity plays a determinant role in high fat diet (HFD)-induced insulin resistance. This study compares the effects of immunonutritional bioactives from Chenopodium quinoa (WQ) or Salvia hispanica L. (Ch) when used to partially replace wheat flour (WB) into bread formulations. These flours were chosen to condition starch and lipid content in the products as well as because their immunonutritional activity. To be administered with different bread formulations, HFD-fed C57BL/6J mice were distributed in different groups: (i) wild type, (ii) displaying inherited disturbances in glucose homeostasis, and (iii) displaying dietary iron-mediated impairment of the innate immune TLR4/TRAM/TRIF pathway. We analyze the effects of the products on glycaemia and insulin resistance (HOMA-IR), plasmatic triglycerides, intestinal and hepatic gene expression and variations of myeloid (MY), and lymphoid (LY) cells population in peripheral blood. Our results show that feeding animals with WQ and Ch formulations influenced the expression of lipogenic and coronary risk markers, thus attaining a better control of hepatic lipid accumulation. WQ and Ch products also improved glucose homeostasis compared to WB, normalizing the HOMA-IR in animals with an altered glucose and lipid metabolism. These positive effects were associated with positive variations in the peripheral myeloid cells population.

Prepartum dietary energy intake alters adipose tissue transcriptome profiles during the periparturient period in Holstein dairy cows

Background

The aim of the study was to investigate the effect of energy overfeeding during the dry period on adipose tissue transcriptome profiles during the periparturient period in dairy cows.

Methods

Fourteen primiparous Holstein cows from a larger cohort receiving a higher-energy diet (1.62 Mcal of net energy for lactation/kg of dry matter; 15% crude protein) for ad libitum intake to supply 150% (OVR) or 100% (CTR) of energy requirements from dry off until parturition were used. After calving, all cows received the same lactation diet. Subcutaneous adipose tissue (SAT) biopsies were collected at - 14, 1, and 14 d from parturition (d) and used for transcriptome profiling using a bovine oligonucleotide microarray. Data mining of differentially expressed genes (DEG) between treatments and due to sampling time was performed using the Dynamic Impact Approach (DIA) and Ingenuity Pathway Analysis (IPA).

Results

There was a strong effect of over-feeding energy on DEG with 2434 (False discovery rate-corrected P < 0.05) between OVR and CTR at - 14 d, and only 340 and 538 at 1 and 14 d. The most-impacted and activated pathways in the Kyoto Encyclopedia of Genes and Genomes (KEGG) database that were highlighted by DIA analysis at - 14 d in OVR vs. CTR included 9 associated with carbohydrate metabolism, with 'Pyruvate metabolism', 'Glycolysis/gluconeogenesis', and 'Pentose phosphate pathway' among the most-activated. Not surprisingly, OVR led to marked activation of lipid metabolism (e.g. 'Fatty acid biosynthesis' and 'Glycerolipid metabolism'). Unexpected metabolic pathways that were activated at - 14 d in OVR included several related to metabolism of amino acids (e.g. branched chain) and of cofactors and vitamins (thiamin). Among endocrine and immune system pathways, at - 14 d OVR led to marked activation of 'PPAR signalling' and 'Antigen processing and presentation'. Among key pathways affected over time in OVR, a number were related to translation (e.g. mTOR signaling), endocrine/immune signaling (CXCR4 and IGF1), and lipid metabolism (oxidative phosphorylation) with greater activation in OVR vs. CTR specifically at - 14 d. Although statistical differences for several pathways in OVR vs. CTR nearly disappeared at 1 and 14 vs. - 14 d, despite the well-known catabolic state of adipose depots after calving, the bioinformatics analyses suggested important roles for a number of signaling mechanisms at - 14 vs. 14 than 1 vs. -14 d. This was particularly evident in cows fed to meet predicted energy requirements during the dry period (CTR).

Conclusions

Data underscored a strong activation by overfeeding energy of anabolic processes in the SAT exclusively prepartum. The study confirmed that higher-energy diets prepartum drive a transcriptional cascade of events orchestrated in part by the activation of PPARγ that regulate preadipocyte differentiation and lipid storage in SAT. Novel aspects of SAT biology to energy overfeeding or change in physiologic state also were uncovered, including the role of amino acid metabolism, mTOR signaling, and the immune system.

Adipocyte Glucocorticoid Receptor Deficiency Promotes Adipose Tissue Expandability and Improves the Metabolic Profile Under Corticosterone Exposure

Widely used for their anti-inflammatory and immunosuppressive properties, glucocorticoids are nonetheless responsible for the development of diabetes and lipodystrophy. Despite an increasing number of studies focused on the adipocyte glucocorticoid receptor (GR), its precise role in the molecular mechanisms of these complications has not been elucidated. In keeping with this goal, we generated a conditional adipocyte-specific murine model of GR invalidation (AdipoGR knockout [KO] mice). Interestingly, when administered a corticosterone treatment to mimic hypercorticism conditions, AdipoGR-KO mice exhibited an improved glucose tolerance and insulin sensitivity. This was related to the adipose-specific activation of the insulin-signaling pathway, which contributed to fat mass expansion, as well as a shift toward an anti-inflammatory macrophage polarization in adipose tissue of AdipoGR-KO animals. Moreover, these mice were protected against ectopic lipid accumulation in the liver and displayed an improved lipid profile, contributing to their overall healthier phenotype. Altogether, our results indicate that adipocyte GR is a key factor of adipose tissue expansion and glucose and lipid metabolism control, which should be taken into account in the further design of adipocyte GR-selective modulators.

A maternal high-fat, high-sucrose diet alters insulin sensitivity and expression of insulin signalling and lipid metabolism genes and proteins in male rat offspring: effect of folic acid supplementation

A maternal high-fat, high-sucrose (HFS) diet alters offspring glucose and lipid homoeostasis through unknown mechanisms and may be modulated by folic acid. We investigated the effect of a maternal HFS diet on glucose homoeostasis, expression of genes and proteins associated with insulin signalling and lipid metabolism and the effect of prenatal folic acid supplementation (HFS/F) in male rat offspring. Pregnant Sprague-Dawley rats were randomly fed control (CON), HFS or HFS/F diets. Offspring were weaned on CON; at postnatal day 70, fasting plasma insulin and glucose and liver and skeletal muscle gene and protein expression were measured. Treatment effects were assessed by one-way ANOVA. Maternal HFS diet induced higher fasting glucose in offspring v. HFS/F (P=0·027) and down-regulation (P<0·05) of genes coding for v-Akt murine thymoma viral oncogene homolog 2, resistin and v-Raf-1 murine leukaemia viral oncogene homolog 1 (Raf1) in offspring skeletal muscle and acetyl-CoA carboxylase (Acaca), fatty acid synthase and phosphatidylinositol-4,5-biphosphate 3-kinase, catalytic subunit β in offspring liver. Skeletal muscle neuropeptide Y and hepatic Kruppel-like factor 10 were up-regulated in HFS v. CON offspring (P<0·05). Compared with CON, Acaca and Raf1 protein expression levels were significantly lower in HFS offspring. Maternal HFS induced higher homoeostasis model of assessment index of insulin resistance v. CON (P=0·030) and HFS/F was associated with higher insulin (P=0·016) and lower glucose (P=0·025). Maternal HFS diet alters offspring insulin sensitivity and de novo hepatic lipogenesis via altered gene and protein expression, which appears to be potentiated by folate supplementation.

Liver-adipose tissue crosstalk: A key player in the pathogenesis of glucolipid metabolic disease

Glucolipid metabolic disease (GLMD), a complex of interrelated disorders in glucose and lipid metabolism, has become one of the leading chronic diseases causing public and clinical problem worldwide. As the metabolism of lipid and glucose is a highly coordinated process under both physiological and diseased conditions, the impairment in the signals corresponding to the metabolism of either lipid or glucose represents the common mechanism underlying the pathogenesis of GLMD. The liver and adipose tissue are the major metabolic organs responsible for energy utilization and storage, respectively. This review article aims to summarize the current advances in the investigation of the functional roles and the underling mechanisms of the interplay between the liver and adipose tissue in the modulation of GLMD development. Fibroblast growth factor 21 (FGF21) and adiponectin represent the two major hormones secreted from the liver and adipose tissues, respectively. FGF21 exerts pleiotropic effects on regulating glucose and lipid homeostasis majorly through inducing the expression and secretion of adiponectin. Therefore, FGF21-adiponectin axis functions as the key mediator for the crosstalk between the liver and adipose tissue to exert the beneficial effects on the maintenance of the homeostasis of energy consumption. The liver- and adipose tissue-derived factors with pleiotropic effects on regulating of lipid and glucose metabolism function as the key mediator for the crosstalk between these two highly active metabolic organs, thereby coordinating the initiation and development of GLMD.

Comprehensive functional screening of miRNAs involved in fat cell insulin sensitivity among women

The key pathological link between obesity and type 2 diabetes is insulin resistance, but the molecular mechanisms are not entirely identified. micro-RNAs (miRNA) are dysregulated in obesity and may contribute to insulin resistance. Our objective was to detect and functionally investigate miRNAs linked to insulin sensitivity in human subcutaneous white adipose tissue (scWAT). Subjects were selected based on the insulin-stimulated lipogenesis response of subcutaneous adipocytes. Global miRNA profiling was performed in abdominal scWAT of 18 obese insulin-resistance (OIR), 21 obese insulin-sensitive (OIS), and 9 lean women. miRNAs demonstrating differential expression between OIR and OIS women were overexpressed in human in vitro-differentiated adipocytes followed by assessment of lipogenesis and identification of miRNA targets by measuring mRNA/protein expression and 3'-untranslated region analysis. Eleven miRNAs displayed differential expression between OIR and OIS states. Overexpression of miR-143-3p and miR-652-3p increased insulin-stimulated lipogenesis in human in vitro differentiated adipocytes and directly or indirectly affected several genes/proteins involved in insulin signaling at transcriptional or posttranscriptional levels. Adipose expression of miR-143-3p and miR-652-3p was positively associated with insulin-stimulated lipogenesis in scWAT independent of body mass index. In conclusion, miR-143-3p and miR-652-3p are linked to scWAT insulin resistance independent of obesity and influence insulin-stimulated lipogenesis by interacting at different steps with insulin-signaling pathways.

WITHDRAWN: Modulation of adipocyte function by the TGF-β family

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Dietary glycemic and insulin scores and colorectal cancer survival by tumor molecular biomarkers

Accumulating evidence suggests that post-diagnostic insulin levels may influence colorectal cancer (CRC) survival. Yet, no previous study has examined CRC survival in relation to a post-diagnostic diet rich in foods that increase post-prandial insulin levels. We hypothesized that glycemic and insulin scores (index or load; derived from food frequency questionnaire data) may be associated with survival from specific CRC subtypes sensitive to the insulin signaling pathway. We prospectively followed 1,160 CRC patients from the Nurses' Health Study (1980-2012) and Health Professionals Follow-Up Study (1986-2012), resulting in 266 CRC deaths in 10,235 person-years. CRC subtypes were defined by seven tumor biomarkers (KRAS, BRAF, PIK3CA mutations, and IRS1, IRS2, FASN and CTNNB1 expression) implicated in the insulin signaling pathway. For overall CRC and each subtype, hazard ratio (HR) and 95% confidence interval (95% CI) for an increase of one standard deviation in each of glycemic and insulin scores were estimated using time-dependent Cox proportional hazards model. We found that insulin scores, but not glycemic scores, were positively associated with CRC mortality (HR = 1.19, 95% CI = 1.02-1.38 for index; HR = 1.23, 95% CI = 1.04-1.47 for load). The significant positive associations appeared more pronounced among PIK3CA wild-type cases and FASN-negative cases, with HR ranging from 1.36 to 1.60 across insulin scores. However, we did not observe statistically significant interactions of insulin scores with PIK3CA, FASN, or any other tumor marker (p interaction > 0.12). While additional studies are needed for definitive evidence, a high-insulinogenic diet after CRC diagnosis may contribute to worse CRC survival.

Metabolic Pathway Inhibition in Liver Cancer

Liver cancer is fundamentally physiologically different from the surrounding liver tissue. Despite multiple efforts to target the altered signaling pathways created by oncogenic mutations, not many have focused on targeting the altered metabolism that allows liver cancer to develop and grow. Still to be resolved is the question of whether the altered metabolic pathways in this cancer differ enough from the surrounding noncancerous cells to allow for the development of potent and specific compounds. Clinical studies of metabolic modulators would provide some more information with regard to the feasibility of this approach. Furthermore, as it appears that oncogenic signaling is essential to this cancer's altered metabolism, it stands to reason that targeting this altered signaling may allow the exploitation of specific metabolic vulnerabilities in combination with other drugs for enhanced efficacy. The identification of biomarkers of metabolic sensitivity will also be essential to determine whether these drugs will have the desired effect.

Iron Content Affects Lipogenic Gene Expression in the Muscle of Nelore Beef Cattle

Iron (Fe) is an essential mineral for metabolism and plays a central role in a range of biochemical processes. Therefore, this study aimed to identify differentially expressed (DE) genes and metabolic pathways in Longissimus dorsi (LD) muscle from cattle with divergent iron content, as well as to investigate the likely role of these DE genes in biological processes underlying beef quality parameters. Samples for RNA extraction for sequencing and iron, copper, manganese, and zinc determination were collected from LD muscles at slaughter. Eight Nelore steers, with extreme genomic estimated breeding values for iron content (Fe-GEBV), were selected from a reference population of 373 animals. From the 49 annotated DE genes (FDR<0.05) found between the two groups, 18 were up-regulated and 31 down-regulated for the animals in the low Fe-GEBV group. The functional enrichment analyses identified several biological processes, such as lipid transport and metabolism, and cell growth. Lipid metabolism was the main pathway observed in the analysis of metabolic and canonical signaling pathways for the genes identified as DE, including the genes FASN, FABP4, and THRSP, which are functional candidates for beef quality, suggesting reduced lipogenic activities with lower iron content. Our results indicate metabolic pathways that are partially influenced by iron, contributing to a better understanding of its participation in skeletal muscle physiology.

The influence of maternal energy status during mid-gestation on beef offspring tenderness, muscle characteristics, and gene expression

The objective of this study was to determine if maternal energy status during mid-gestation influences the expression of genes regulating muscle and fat development, and muscle characteristics that may impact meat tenderness. Cows grazed dormant, native range (Positive Energy Status [PES]) or were fed at 80% of maintenance energy requirements (Negative Energy Status [NES]) during mid-gestation. Steer offspring were harvested after 21 d in the feedlot (weaning subsample) or after 208 d in the feedlot (final subsample). Greater 21-d tenderness was observed in NES steers, resulting from reduced collagen content in longissimus lumborum steaks. In the semitendinosus, NES steers had greater soluble collagen, and down-regulated expression of MHC-IIA and TIMP-3 at weaning, while MHC-IIA expression was up-regulated in NES steers in the final harvest. Data show mid-gestational maternal energy status may impact offspring tenderness and collagen, but differences were not detected in expression of genes important in myogenesis and adipogenesis in muscle samples obtained from steers at weaning or slaughter.

TRIB3 mediates glucose-induced insulin resistance via a mechanism that requires the hexosamine biosynthetic pathway

In the current study, we investigated the role of tribbles homolog 3 (TRIB3) in glucose-induced insulin resistance and whether the induction of TRIB3 by glucose is dependent on the nutrient-sensing hexosamine biosynthetic pathway (HBP) known to mediate glucose toxicity in diabetes. In diabetic rats, TRIB3 expression in skeletal muscle was increased after 10 days of hyperglycemia, and glycemia and muscle TRIB3 were both restored toward normal by insulin therapy. In L6 myocytes, the induction of TRIB3 by high glucose or glucosamine was reversible upon removal of these substrates. To assess the role of HBP in the induction of TRIB3, we demonstrated that the ability of high glucose to augment TRIB3 expression was prevented by azaserine, an inhibitor of glutamine: fructose-6-phosphate amidotransferase (GFAT), which is the rate-limiting enzyme in the HBP pathway. TRIB3 expression was also substantially stimulated by glucosamine, which bypasses GFAT, accompanied by a decrease in the insulin-stimulated glucose transport rate, and neither response was affected by azaserine. Further, knockdown of TRIB3 inhibited, and TRIB3 overexpression enhanced, the ability of both high glucose and glucosamine to induce insulin resistance. These data provide the mechanistic link between the HBP flux and insulin resistance and point to TRIB3 as a novel target for treatment of glucose-induced insulin resistance.

Cirsium brevicaule A. GRAY leaf inhibits adipogenesis in 3T3-L1 cells and C57BL/6 mice

Background

Various flavonoids obtained from the genus Cirsium have been reported to exhibit beneficial effects on health. The present study evaluated the antiobesity effects of Cirsium brevicaule A. GRAY leaf (CL) by using 3T3-L1 cells and C57BL/6 mice that were fed a high-fat diet (HFD).

Methods

Dried CL powder was serially extracted with solvents of various polarities, and these extracts were tested for antiadipogenic activity using 3T3-L1 adipocytes. Mice were fed experimental HFD supplemented with dried CL powder for 4 wk. Lipid levels and mRNA levels of genes related to lipid metabolism were determined in 3T3-L1 adipocytes and the white adipose tissue (WAT) and liver of mice fed on a HFD.

Results

Treatment of 3T3-L1 adipocytes with a hexane extract of CL significantly reduced cellular lipid accumulation and expression of the fatty acid synthase (FASN) gene. Dietary CL reduced the serum levels of non-esterified fatty acids in HFD-fed mice. Significant decreases in subcutaneous WAT weight and associated FASN gene expression were observed in the mice fed the experimental CL diet. Dietary CL also reduced the hepatic lipid and serum levels of a hepatopathic indicator in the HFD-fed mice. A significant reduction in mRNA levels of FASN and HMG-CoA reductase were observed in the livers of the CL-diet group. Dietary CL, on the other hand, increased in the hepatic mRNA levels of genes related to β-oxidation, namely peroxisome proliferator-activated receptor α, calnitine palmitoyltrasferase 1A, and uncoupling protein 2. Expression of the insulin receptor gene was also significantly increased in the livers of mice-fed the CL diet.

Conclusions

The present study therefore demonstrated that CL suppresses lipid accumulation in the WAT and liver partly through inhibiting mRNA levels of FASN gene and enhancing the lipolysis-related gene expression.

Cholesterol-lowering activity of soy-derived glyceollins in the golden Syrian hamster model

Hypercholesterolemia is one of the major factors contributing to the risk of cardiovascular disease (CVD), which is the leading cause of death in developed countries. Consumption of soy foods has been recognized to lower the risk of CVD, and phytochemicals in soy are believed to contribute to the health benefits. Glyceollin is one of the candidate phytochemicals synthesized in stressed soy that may account for many unique biological activities. In this study, the in vivo cholesterol-lowering effect of glyceollins was investigated. Male golden Syrian hamsters were fed diets including (1) 36 kcal% fat diet, (2) 36 kcal% fat diet containing 250 mg/kg diet glyceollins, or (3) chow for 28 days. Hepatic cholesterol esters and free cholesterol, hepatic total lipid content, plasma lipoproteins, fecal bile acid, fecal total cholesterol, and cholesterol metabolism related gene expressions were measured. Glyceollin supplementation led to significant reduction of plasma VLDL, hepatic cholesterol esters, and total lipid content. Consistent with changes in circulating cholesterol, glyceollin supplementation also altered expression of the genes related to cholesterol metabolism in the liver. In contrast, no change in plasma LDL and HDL, fecal bile acid, or cholesterol content was observed. The cholesterol-lowering effect of glyceollins appeared not to go through the increase of bile excretion. These results supported glyceollins' role as novel soy-derived cholesterol-lowering phytochemicals that may contribute to soy's health effects.

Insulin signalling mechanisms for triacylglycerol storage

Insulin signalling is uniquely required for storing energy as fat in humans. While de novo synthesis of fatty acids and triacylglycerol occurs mostly in liver, adipose tissue is the primary site for triacylglycerol storage. Insulin signalling mechanisms in adipose tissue that stimulate hydrolysis of circulating triacylglycerol, uptake of the released fatty acids and their conversion to triacylglycerol are poorly understood. New findings include (1) activation of DNA-dependent protein kinase to stimulate upstream stimulatory factor (USF)1/USF2 heterodimers, enhancing the lipogenic transcription factor sterol regulatory element binding protein 1c (SREBP1c); (2) stimulation of fatty acid synthase through AMP kinase modulation; (3) mobilisation of lipid droplet proteins to promote retention of triacylglycerol; and (4) upregulation of a novel carbohydrate response element binding protein β isoform that potently stimulates transcription of lipogenic enzymes. Additionally, insulin signalling through mammalian target of rapamycin to activate transcription and processing of SREBP1c described in liver may apply to adipose tissue. Paradoxically, insulin resistance in obesity and type 2 diabetes is associated with increased triacylglycerol synthesis in liver, while it is decreased in adipose tissue. This and other mysteries about insulin signalling and insulin resistance in adipose tissue make this topic especially fertile for future research.

Interaction between sleep and metabolism in Drosophila with altered octopamine signaling

Sleep length and metabolic dysfunction are correlated, but the causal relationship between these processes is unclear. Octopamine promotes wakefulness in the fly by acting through the insulin-producing cells (IPCs) in the fly brain. To determine if insulin signaling mediates the effects of octopamine on sleep:wake behavior, we assayed flies in which insulin signaling activity was genetically altered. We found that increasing insulin signaling does not promote wake, nor does insulin appear to mediate the wake-promoting effects of octopamine. Octopamine also affects metabolism in invertebrate species, including, as we show here, Drosophila melanogaster. Triglycerides are decreased in mutants with compromised octopamine signaling and elevated in flies with increased activity of octopaminergic neurons. Interestingly, this effect is mediated at least partially by insulin, suggesting that effects of octopamine on metabolism are independent of its effects on sleep. We further investigated the relative contribution of metabolic and sleep phenotypes to the starvation response of flies with altered octopamine signaling. Hyperactivity (indicative of foraging) induced by starvation was elevated in octopamine receptor mutants, despite their high propensity for sleep, indicating that their metabolic state dictates their behavioral response under these conditions. Moreover, flies with increased octopamine signaling do not suppress sleep in response to starvation, even though they are normally hyper-aroused, most likely because of their high triglyceride levels. Together, these data suggest that observed correlations between sleep and metabolic phenotypes can result from shared molecular pathways rather than causality, and environmental conditions can lead to the dominance of one phenotype over the other.

The interaction of hepatic lipid and glucose metabolism in liver diseases

It is widely known that the liver is a central organ in lipogenesis, gluconeogenesis and cholesterol metabolism. However, over the last decades, a variety of pathological conditions highlighted the importance of metabolic functions within the diseased liver. As observed in Western societies, an increase in the prevalence of obesity and the metabolic syndrome promotes pathophysiological changes that cause non-alcoholic fatty liver disease (NAFLD). NAFLD increases the susceptibility of the liver to acute liver injury and may lead to cirrhosis and hepatocellular cancer. Alterations in insulin response, β-oxidation, lipid storage and transport, autophagy and an imbalance in chemokines and nuclear receptor signaling are held accountable for these changes. Furthermore, recent studies revealed a role for lipid accumulation in inflammation and ER stress in the clinical context of liver regeneration and hepatic carcinogenesis. This review focuses on novel findings related to nuclear receptor signaling - including the vitamin D receptor and the liver receptor homolog 1 - in hepatic lipid and glucose uptake, storage and metabolism in the clinical context of NAFLD, liver regeneration, and cancer.

Role of oxidative stress in the pathogenesis of nonalcoholic steatohepatitis

The worldwide rising prevalence of obesity and insulin resistance is associated with a parallel increase in nonalcoholic fatty liver disease (NAFLD). NAFLD is characterized by excess accumulation of triglyceride in the hepatocyte due to increased inflow of free fatty acids and/or de novo lipogenesis caused by various drugs and multiple defects in energy metabolism. Accumulation of lipids in the hepatocyte impairs the oxidative capacity of the mitochondria, increasing the reduced state of the electron transport chain (ETC) complexes and stimulating peroxisomal and microsomal pathways of fat oxidation. The consequent increased generation of reactive oxygen species (ROS) and reactive aldehydic derivatives causes oxidative stress and cell death, via ATP, NAD, and glutathione depletion and DNA, lipid, and protein damage. Oxidative stress also triggers production of inflammatory cytokines, causing inflammation and a fibrogenic response. This ultimately results in the development of nonalcoholic steatohepatitis (NASH), which can result in end-stage liver disease. The current therapeutic strategies for NASH treatment are mostly directed toward correction of the risk factors. Stimulation of mitochondrial function may also prevent NASH development, protecting the cell against the increased flux of reduced substrates to the ETC and ROS generation.

The protein level of isoenergetic formulae does not modulate postprandial insulin secretion in piglets and has no consequences on later glucose tolerance

Early postnatal nutrition is involved in metabolic programming, an excess of protein being suspected to enhance early growth and the propensity to later develop insulin resistance and type 2 diabetes mellitus. The aim of the present study was to test the hypothesis that excessive protein intake during the suckling period would overstimulate the endocrine pancreas in the short term and alter durably its maturation, contributing to the later disruption of glucose homeostasis. Normal-birth-weight and low-birth-weight piglets were fed isoenergetic formulae providing an adequate-protein (AP, equivalent to sow milk) or a high-protein (HP, +48 %) supply between 7 and 28 d of age and were fed a standard diet until 70 d of age. During the formula-feeding period, the HP formula did not modify postprandial insulin secretion but transiently increased fasting insulin and the homeostasis model assessment-insulin resistance index (HOMA-IR, P < 0·05). Fasting insulin and HOMA-IR were restored to AP piglets' values 1 month after weaning. The structure of the endocrine pancreas was not affected by the protein content of the formula. The weight at birth had no major effect on the studied parameters. We concluded that a high-protein supply during the suckling period does not interfere with insulin secretion and endocrine pancreas maturation in the short term. It has no consequences either on glucose tolerance 1 month after weaning. The present study demonstrated that up-regulation of postprandial insulin secretion is not involved in higher growth observed in piglets fed a HP formula.

Comparative Approach of the de novo Fatty Acid Synthesis (Lipogenesis) between Ruminant and Non Ruminant Mammalian Species: From Biochemical Level to the Main Regulatory Lipogenic Genes

Over the second half of 20^th century much research on lipogenesis has been conducted, especially focused on increasing the production efficiency and improving the quality of animal derived products. However, many diferences are observed in the physiology of lipogenesis between species. Recently, many studies have also elucidated the involvement of numerous genes in this procedure, highlighting diferences not only at physiology but also at the molecular level. The main scope of this review is to point out the major differences between ruminant and non ruminant species, that are observed in key regulatory genes involved in lipogenesis. Human is used as a central reference and according to the findinggs, main differences are analysed. These findings could serve not only as basis for understanding the main physiology of lipogenesis and further basic research, but also as a basis for any animal scientist to develop new concepts and methods for use in improving animal production and modern genetic improvement.

Fatty acid synthase modulates homeostatic responses to myocardial stress

Fatty acid synthase (FAS) promotes energy storage through de novo lipogenesis and participates in signaling by the nuclear receptor PPARα in noncardiac tissues. To determine if de novo lipogenesis is relevant to cardiac physiology, we generated and characterized FAS knockout in the myocardium (FASKard) mice. FASKard mice develop normally, manifest normal resting heart function, and have normal cardiac PPARα signaling as well as fatty acid oxidation. However, they decompensate with stress. Most die within 1 h of transverse aortic constriction, probably due to arrhythmia. Voltage clamp measurements of FASKard cardiomyocytes show hyperactivation of L-type calcium channel current that could not be reversed with palmitate supplementation. Of the classic regulators of this current, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) but not protein kinase A signaling is activated in FASKard hearts, and knockdown of FAS in cultured cells activates CaMKII. In addition to being intolerant of the stress of acute pressure, FASKard hearts were also intolerant of the stress of aging, reflected as persistent CaMKII hyperactivation, progression to dilatation, and premature death by ∼1 year of age. CaMKII signaling appears to be pathogenic in FASKard hearts because inhibition of its signaling in vivo rescues mice from early mortality after transverse aortic constriction. FAS was also increased in two mechanistically distinct mouse models of heart failure and in the hearts of humans with end stage cardiomyopathy. These data implicate a novel relationship between FAS and calcium signaling in the heart and suggest that FAS induction in stressed myocardium represents a compensatory response to protect cardiomyocytes from pathological calcium flux.

Insulin signaling in fatty acid and fat synthesis: a transcriptional perspective

Transcription of enzymes involved in FA and TAG synthesis is coordinately induced in lipogenic tissues by feeding and insulin treatment. The three major transcription factors involved are USF, SREBP-1c, and LXRα. New insights into the insulin-signaling pathway(s) that control(s) lipogenic gene transcription via these factors have recently been revealed. Dephosphorylation/activation of DNA-PK by PP1 causes phosphorylation of USF that in turn recruits P/CAF to be acetylated for transcriptional activation. SREBP-1c can be induced by mTORC1, bifurcating lipogenesis from AKT-activated gluconeogenesis. LXRα may serve as a glucose sensor and, along with ChREBP, may activate lipogenic genes in the fed state. Dysregulation of FA and TAG metabolism often contributes to metabolic diseases such as obesity, diabetes, and cardiovascular diseases. Transcription factors and signaling molecules involved in transcriptional activation of FA and TAG synthesis represent attractive targets for the prevention and treatment of metabolic diseases.

Dietary carbohydrate-to-protein ratio affects TOR signaling and metabolism-related gene expression in the liver and muscle of rainbow trout after a single meal

Most teleost fish are known to require high levels of dietary proteins. Such high-protein intake could have significant effects, particularly on insulin-regulated gene expression. We therefore analyzed the effects of an increase in the ratio of dietary carbohydrates/proteins on the refeeding activation of the Akt-target of rapamycin (TOR) signaling pathways in rainbow trout and the effects on the expression of several genes related to hepatic and muscle metabolism and known to be regulated by insulin, amino acids, and/or glucose. Fish were fed once one of three experimental diets containing high (H), medium (M), or low (L) protein (P) or carbohydrate (C) levels after 48 h of feed deprivation. Activation of the Akt/TOR signaling pathway by refeeding was severely impaired by decreasing the proteins-to-carbohydrates ratio. Similarly, postprandial regulation of several genes related to glucose (Glut4, glucose-6-phosphatase isoform 1), lipid (fatty acid synthase, ATP-citrate lyase, sterol responsive element binding protein, carnitine palmitoyltransferase 1, and 3-hydroxyacyl-CoA dehydrogenase), and amino acid metabolism (serine dehydratase and branched-chain α-keto acid dehydrogenase E2 subunit) only occurred when fish were fed the high-protein diet. On the other hand, diet composition had a low impact on the expression of genes related to muscle protein degradation. Interestingly, glucokinase was the only gene of those monitored whose expression was significantly upregulated by increased carbohydrate intake. In conclusion, this study demonstrated that macro-nutrient composition of the diet strongly affected the insulin/amino acids signaling pathway and expression pattern of genes related to metabolism.

Rainbow trout genetically selected for greater muscle fat content display increased activation of liver TOR signaling and lipogenic gene expression

Genetic selection is commonly used in farm animals to manage body fat content. In rainbow trout, divergent selection for low or high muscle fat content leads to differences in utilization of dietary energy sources between the fat muscle line (FL) and the lean muscle line (LL). To establish whether genetic selection on muscle fat content affects the hepatic insulin/nutrient signaling pathway, we analyzed this pathway and the expression of several metabolism-related target genes in the livers of the two divergent lines under fasting and then refeeding conditions. Whereas glycemia returned to basal level 24 h after refeeding in FL trout, it remained elevated in the LL trout. Target of rapamycin (TOR) protein was more abundant in the livers of FL trout than in LL trout, and refeeding activation of the hepatic TOR signaling pathway (TOR, S6K1, and S6) was therefore enhanced. Genes related to glycolysis (glucokinase and pyruvate kinase) and gluconeogenesis (glucose-6-phosphatase and phosphoenolpyruvate carboxykinase) were only slightly affected by refeeding and genetic selection. Refeeding stimulated expression of lipogenic genes and the sterol-responsive element binding protein (SREBP1), and expression of fatty acid synthase, glucose-6-phosphate dehydrogenase, and serine dehydratase was predominant in the livers of FL fish compared with LL fish. In agreement with recent findings linking TOR to lipogenesis control, we concluded that genetic selection for muscle fat content resulted in overactivation of the TOR signaling pathway-associated lipogenesis and probably also improved utilization of glucose.

Hepatic protein kinase B (Akt)-target of rapamycin (TOR)-signalling pathways and intermediary metabolism in rainbow trout (Oncorhynchus mykiss) are not significantly affected by feeding plant-based diets

The aim of the present study was to analyse the effects of partial or total replacement of fish meal (FM) and fish oil (FO) by a mixture of plant protein (PP) and a mixture of vegetable oils (VO) on the hepatic insulin-nutrient-signalling pathway and intermediary metabolism-related gene expression in rainbow trout (Oncorhynchus mykiss). Triplicate groups of fish were fed four practical diets containing graded levels of replacement of FM and FO by PP and VO for 12 weeks: diet 0/0 (100 % FM, 100 % FO); diet 50/50 (50 % FM and 50 % PP, 50 % FO and 50 % VO); diet 50/100 (50 % FM and 50 % PP, 100 % VO); diet 100/100 (100 % PP, 100 % VO). Samplings were performed on trout starved for 5 d then refed with their allocated diet. In contrast to partial substitution (diet 50/50), total substitution of FM and FO (diet 100/100) led to significantly lower growth compared with diet 0/0. The insulin-nutrient-signalling pathway (protein kinase B (Akt), target of rapamycin (TOR), S6 protein kinase 1 (S6K1) and S6) was characterised in trout liver and found to be activated by refeeding. However, changes in diet compositions did not differentially affect the Akt-TOR-signalling pathway. Moreover, expression of genes encoding fructose-1,6-biphosphatase, mitochondrial phosphoenolpyruvate carboxykinase, glucokinase, pyruvate kinase and carnitine palmitoyl transferase 1 were not affected by refeeding or by dietary changes. Refeeding down- and up-regulated the expression of gluconeogenic glucose-6-phosphatase isoform 1 and lipogenic fatty acid synthase genes, respectively. Expression of both genes was also increased with partial replacement of FM and total replacement of FO (diet 50/100). These findings indicate that plant-based diets barely affect glucose and lipid metabolism in trout.

A role of DNA-PK for the metabolic gene regulation in response to insulin

Fatty acid synthase (FAS) is a central enzyme in lipogenesis and transcriptionally activated in response to feeding and insulin signaling. The transcription factor USF is required for the activation of FAS transcription, and we show here that USF phosphorylation by DNA-PK, which is dephosphorylated by PP1 in response to feeding, triggers a switch-like mechanism. Under fasting conditions, USF-1 is deacetylated by HDAC9, causing promoter inactivation. In contrast, feeding induces the recruitment of DNA-PK to USF-1 and its phosphorylation, which then allows recruitment of P/CAF, resulting in USF-1 acetylation and FAS promoter activation. DNA break/repair components associated with USF induce transient DNA breaks during FAS activation. In DNA-PK-deficient SCID mice, feeding-induced USF-1 phosphorylation/acetylation, DNA breaks, and FAS activation leading to lipogenesis are impaired, resulting in decreased triglyceride levels. Our study demonstrates that a kinase central to the DNA damage response mediates metabolic gene activation.

Insulin regulates the expression of several metabolism-related genes in the liver and primary hepatocytes of rainbow trout (Oncorhynchus mykiss)

Rainbow trout have a limited ability to use dietary carbohydrates efficiently and are considered to be glucose intolerant. Administration of carbohydrates results in persistent hyperglycemia and impairs post-prandial down regulation of gluconeogenesis despite normal insulin secretion. Since gluconeogenic genes are mainly under insulin control, we put forward the hypothesis that the transcriptional function of insulin as a whole may be impaired in the trout liver. In order to test this hypothesis, we performed intraperitoneal administration of bovine insulin to fasted rainbow trout and also subjected rainbow trout primary hepatocytes to insulin and/or glucose stimulation. We demonstrate that insulin was able to activate Akt, a key element in the insulin signaling pathway, and to regulate hepatic metabolism-related target genes both in vivo and in vitro. In the same way as in mammals, insulin decreased mRNA expression of gluconeogenic genes, including glucose 6-phosphatase (G6Pase),fructose 1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK). Insulin also limited the expression of carnitine palmitoyltransferase 1 (CPT1), a limiting enzyme of fatty acid β-oxidation. In vitro studies revealed that, as in mammals,glucose is an important regulator of some insulin target genes such as the glycolytic enzyme pyruvate kinase (PK) and the lipogenic enzyme fatty acid synthase (FAS). Interestingly, glucose also stimulates expression of glucokinase (GK), which has no equivalent in mammals. This study demonstrates that insulin possesses the intrinsic ability to regulate hepatic gene expression in rainbow trout, suggesting that other hormonal or metabolic factors may counteract some of the post-prandial actions of insulin.

Liver lipid metabolism

The liver plays a key role in lipid metabolism. Depending on species it is, more or less, the hub of fatty acid synthesis and lipid circulation through lipoprotein synthesis. Eventually the accumulation of lipid droplets into the hepatocytes results in hepatic steatosis, which may develop as a consequence of multiple dysfunctions such as alterations in beta-oxidation, very low density lipoprotein secretion, and pathways involved in the synthesis of fatty acids. In addition an increased circulating pool of non-esterified fatty acid may also to be a major determinant in the pathogenesis fatty liver disease. This review also focuses on transcription factors such as sterol-regulatory-element-binding protein-1c and peroxisome proliferator-activated receptor alpha, which promote either hepatic fatty acid synthesis or oxidation.

Energy restriction enhances therapeutic efficacy of the PPARgamma agonist, rosiglitazone, through regulation of visceral fat gene expression

AIM

Consumption of a palatable diet can induce hyperphagia, leading to weight gain (dietary obesity) and insulin resistance in rats. Thiazolidinediones (TZDs) can also induce hyperphagia in rats but conversely have an insulin-sensitizing effect. The aim of this study was to investigate whether preventing TZD-induced hyperphagia (i.e. energy restriction) in dietary obese (DIO) rats would enhance the insulin-sensitizing effects of treatment at a therapeutic dose; and, within this paradigm, to produce an original survey of candidate TZD-gene targets in the clinically relevant visceral white adipose tissue (WAT) depot.

METHODS

DIO rats that were either freely fed or energy restricted (i.e. pair-fed to the level of untreated controls) were treated with rosiglitazone maleate (RSG; 3 mg/kg/day) for 2 weeks, the restricted group controlling for treatment-induced hyperphagia and weight gain. The outcome measures were circulating concentrations of various biochemical markers of insulin resistance, and gene expression was measured in epididymal WAT.

RESULTS

In both freely fed and pair-fed groups, compared to untreated DIO controls, RSG reduced plasma levels of insulin (-29% and -43%; p < 0.05 and p < 0.001, respectively), free fatty acids (FFAs; -45% and -48%; p < 0.01 and p < 0.001, respectively) and triglycerides (TGs; -63% and -72%; both p < 0.001), reflected in improved insulin sensitivity, as measured by homeostasis model assessment (-29% and -43%; p < 0.01 and p < 0.0001). RSG also increased the expression of the fatty acid transport/synthesis genes, fatty acid transport protein (2.4-3.2-fold), epidermal fatty acid-binding protein (FABP; 1.7-2.0-fold), heart FABP (25-29-fold) and fatty acid synthase (2.3-2.9-fold; all p < 0.05) in both groups. Adipocyte FABP was also increased by RSG treatment, but only in combination with energy restriction (1.52-fold; p < 0.05) as was hexokinase II expression (p < 0.001). In contrast, the drug had no effect on expression of several genes associated with lipolysis. Although obesity-induced hyperleptinaemia was normalized only in the energy-restricted group, leptin messenger RNA (mRNA) expression was reduced in both treated groups (all p < 0.01). Resistin and tumour necrosis factor-alpha expression was also reduced, though in the latter case, only with energy restriction (p < 0.05). Other adipokines were unaffected by RSG treatment.

CONCLUSION

Our results clearly show that energy restriction enhances the therapeutic efficacy of TZDs and suggest that this occurs, at least in part, through a modulatory effect on gene expression in visceral WAT. These findings improve our understanding of the underlying mechanistic basis for the clinical usefulness of dietary restriction as an adjunct to TZD therapy in type 2 diabetes.

Activation of peroxisome proliferator-activated receptor alpha by dietary fish oil attenuates steatosis, but does not prevent experimental steatohepatitis because of hepatic lipoperoxide accumulation

BACKGROUND AND AIM

Non-alcoholic fatty liver disease is the result of an imbalance in hepatic lipid partitioning that favors fatty acid synthesis and storage over fatty acid oxidation and triglyceride secretion. The progressive, inflammatory disorder of steatohepatitis can be prevented or reversed by correcting this lipid imbalance by activating peroxisome proliferator-activated receptor (PPAR) alpha, a transcription factor which regulates fatty acid oxidation. n-3 polyunsaturated fatty acids (PUFA), such as those found in fish oil (FO), are naturally occurring PPARalpha ligands which also suppress lipid synthesis.

METHODS

We tested the role of dietary activation of PPARalpha by feeding mice a n-3 PUFA-enriched FO diet in the methionine and choline deficient (MCD) model of steatohepatitis. Results were compared with mice fed the corresponding diet supplemented with monounsaturated fatty acids as olive oil (OO).

RESULTS

As expected, FO feeding led to robust hepatic PPARalpha activation in control mice, and decreased expression of genes involved with fatty acid synthesis. Such lipolytic gene expression profile was also clearly evident in FO MCD-fed mice, and was associated with reduced hepatic lipid accumulation in comparison with mice fed OO MCD diet. FO feeding in control mice also caused marked hepatic accumulation of lipoperoxides compared with OO and chow-fed mice. This was further exacerbated in FO MCD-fed animals, which developed steatohepatitis characterized by mild steatosis and moderate inflammation in comparison with OO MCD-fed mice; such inflammatory recruitment was not related to NF-kappaB activation or enhanced cyclooxygenase-2 activity.

CONCLUSIONS

Feeding an n-3 PUFA-enriched diet activated PPARalpha and suppressed hepatic de novo lipogenesis, but failed to prevent development of steatohepatitis in the presence of methionine and choline deficiency. Instead, the very high levels of hepatic lipoperoxides may have abrogated the protection that would otherwise be conferred by PPARalpha activation, and could also be responsible for lipotoxic hepatocellular injury and inflammatory recruitment.

Cloning, characterization and computational analysis of the 5' regulatory region of ovine glucose 6-phosphate dehydrogenase gene

To better understand the structure and the function of ovine glucose 6-phosphate dehydrogenase (G6PD) promoter region, a genome-walking procedure was followed to isolate and sequence a 1628 bp fragment, containing the 5' regulatory region of the G6PD gene. In silico analysis of the sequence showed many conserved blocks and features with other known mammalian G6PD promoter regions. The analysis also revealed the presence of one TATA box, three GC boxes, two E-boxes and several binding sites for Stimulating Protein 1 (Sp1) and Activator Protein 2 (AP2). Moreover, elements involved in the regulation of lipogenesis like USF (Upstream stimulating factor), HSF (Heat Shock Factor), F2F (Prolactin receptor), RAR (Retinoid Acid Receptor), STRE (STress Response Element), RORa (Retinoid related Orphan Receptor alpha), GATA (GATA binding factor), RFX (Regulatory Factor X), SREBP (Sterol Regulatory Element Binding Protein), MEP (Metal Element Protein), CREB (insulin receptor), PRE (Progesterone receptor), and HNF4 (Hepatic Nuclear Factor 4) were detected. The most important regulatory motifs were found to be conserved as compared to those in human and mouse counterparts. However, some differences were noted, likely indicating differences in the transcription regulation of G6PD gene between ruminant and non-ruminant species.

Direct interaction between USF and SREBP-1c mediates synergistic activation of the fatty-acid synthase promoter

To understand the molecular mechanisms underlying transcriptional activation of fatty-acid synthase (FAS), we examined the relationship between upstream stimulatory factor (USF) and SREBP-1c, two transcription factors that we have shown previously to be critical for FAS induction by feeding/insulin. Here, by using a combination of tandem affinity purification and coimmunoprecipitation, we demonstrate, for the first time, that USF and SREBP-1 interact in vitro and in vivo. Glutathione S-transferase pulldown experiments with various USF and sterol regulatory element-binding protein (SREBP) deletion constructs indicate that the basic helix-loop-helix domain of USF interacts directly with the basic helix-loop-helix and an N-terminal region of SREBP-1c. Furthermore, cotransfection of USF and SREBP-1c with an FAS promoter-luciferase reporter construct in Drosophila SL2 cells results in highly synergistic activation of the FAS promoter. We also show similar cooperative activation of the mitochondrial glycerol-3-phosphate acyltransferase promoter by USF and SREBP-1c. Chromatin immunoprecipitation analysis of mouse liver demonstrates that USF binds constitutively to the mitochondrial glycerol 3-phosphate acyltransferase promoter during fasting/refeeding in vivo, whereas binding of SREBP-1 is observed only during refeeding, in a manner identical to that of the FAS promoter. In addition, we show that the synergy we have observed depends on the activation domains of both proteins and that mutated USF or SREBP lacking the N-terminal activation domain could inhibit the transactivation of the other. Closely positioned E-boxes and sterol regulatory elements found in the promoters of several lipogenic genes suggest a common mechanism of induction by feeding/insulin.

Phosphatidylinositol 3-kinase-dependent modulation of carnitine palmitoyltransferase 1A expression regulates lipid metabolism during hematopoietic cell growth

An abundant supply of extracellular nutrients is believed to be sufficient to suppress catabolism of cellular macromolecules. Here we show that, despite abundant extracellular nutrients, interleukin-3-deprived hematopoietic cells begin to catabolize intracellular lipids. Constitutive Akt activation blunts the increased beta-oxidation that accompanies growth factor withdrawal, and in growth factor-replete cells, phosphatidylinositol 3-kinase (PI3K) signaling is required to suppress lipid catabolism. Surprisingly, PI3K and Akt exert these effects by suppressing expression of the beta-oxidation enzyme carnitine palmitoyltransferase 1A (CPT1A). Cells expressing a short hairpin RNA against CPT1A fail to induce beta-oxidation in response to growth factor withdrawal and are unable to survive glucose deprivation. When CPT1A is constitutively expressed, growth factor stimulation fails to repress beta-oxidation. As a result, both net lipid synthesis and cell proliferation are diminished. Together, these results demonstrate that modulation of CPT1A expression by PI3K-dependent signaling is the major mechanism by which cells suppress beta-oxidation during anabolic growth.

Differential expression of selected mitochondrial genes in hibernating little brown bats,Myotis lucifugus

High rates of non-shivering thermogenesis by brown adipose tissue accompanied by additional shivering thermogenesis in skeletal muscle provide the powerful reheating of body organs that allows hibernating mammals to return from their state of cold torpor back to euthermic function. Previous studies have suggested that changes to brown adipose mitochondria occur during hibernation and are partially responsible for its capacity for non-shivering thermogenesis. The current study shows that selected mitochondrial enzyme activities are elevated and selected genes and proteins are induced during torpor in brown adipose tissue of the little brown bat, Myotis lucifugus. Cytochrome oxidase activity in brown adipose tissue was more than 3-fold higher during torpor than in euthermic animals. Transcript levels of mitochondria-encoded genes, coxII and nad4, were also 3-4-fold higher during torpor, as evidenced by northern blotting. By contrast, transcripts of these genes were unchanged in skeletal muscle during torpor. Protein levels of carnitine palmitoyl transferase-1beta, an enzyme embedded in the outer membrane of the mitochondria that is the rate-limiting step enzyme in beta-oxidation, were also elevated by 2-fold during torpor in brown adipose but were unchanged in skeletal muscle. Cloning and sequencing of a 624 bp segment of cpt-1beta revealed a number of amino acid substitutions in the bat protein as compared to CPT-1beta from other mammals; these may be beneficial for enzyme function at low body temperatures during torpor. This study provides further evidence for a key role of mitochondria in hibernation.

Overexpression of suppressor of cytokine signaling 3 in adipose tissue causes local but not systemic insulin resistance

In adipocytes, suppressor of cytokine signaling (SOCS)3 deficiency increases insulin-stimulated insulin receptor substrate (IRS)-1 and -2 phosphorylation, IRS-associated phosphatidylinositol 3 kinase activity, and insulin-stimulated glucose uptake. Moreover, SOCS3 is required for tumor necrosis factor-alpha full inhibition of insulin-stimulated IRS-1 and -2 phosphorylation, phosphatidylinositol 3 kinase activity, and glucose uptake. Whether SOCS3 also inhibits adipocyte insulin signaling in vivo and whether this action further affects systemic insulin sensitivity is not clear. We therefore generated a transgenic mouse (aP2-SOCS3 mouse) overexpressing SOCS3 in adipose tissue. Overexpression of SOCS3 in adipocytes decreases IRS1 protein levels and subsequent insulin-stimulated IRS-1 and -2 phosphorylation, decreases p85 binding to IRS-1, and leads to decreased insulin-stimulated glucose uptake in adipocytes. This impaired insulin signaling in adipose tissue of aP2-SOCS3 mice causes decreased lipogenesis and blocks insulin's antilipolytic action. However, because of decreased energy partitioning in adipose tissue, aP2-SOCS3 mice are resistant to diet-induced obesity and are protected against systemic insulin resistance caused by a high-fat diet. Therefore, overexpression of SOCS3 in adipocytes causes local adipocyte insulin resistance, but it is not sufficient to cause systemic insulin resistance.

PKB/Akt induces transcription of enzymes involved in cholesterol and fatty acid biosynthesis via activation of SREBP

Protein kinase B (PKB/Akt) has been shown to play a role in protection from apoptosis, cell proliferation and cell growth. It is also involved in mediating the effects of insulin, such as lipogenesis, glucose uptake and conversion of glucose into fatty acids and cholesterol. Sterol-regulatory element binding proteins (SREBPs) are the major transcription factors that regulate genes involved in fatty acid and cholesterol synthesis. It has been postulated that constitutive activation of the phosphatidylinositol 3 kinase/Akt pathway may be involved in fatty acid and cholesterol accumulation that has been described in several tumour types. In this study, we have analysed changes in gene expression in response to Akt activation using DNA microarrays. We identified several enzymes involved in fatty acid and cholesterol synthesis as targets for Akt-regulated transcription. Expression of these enzymes has previously been shown to be regulated by the SREBP family of transcription factors. Activation of Akt induces synthesis of full-length SREBP-1 and SREBP-2 proteins as well as expression of fatty acid synthase (FAS), the key regulatory enzyme in lipid biosynthesis. We also show that Akt leads to the accumulation of nuclear SREBP-1 but not SREBP-2, and that activation of SREBP is required for Akt-induced activation of the FAS promoter. Finally, activation of Akt induces an increase in the concentration of cellular fatty acids as well as phosphoglycerides, the components of cellular membranes. Our data indicate that activation of SREBP by Akt leads to the induction of key enzymes of the cholesterol and fatty acid biosynthesis pathways, and thus membrane lipid biosynthesis.

FAS expression inversely correlates with PTEN level in prostate cancer and a PI 3-kinase inhibitor synergizes with FAS siRNA to induce apoptosis

Fatty acid synthase (FAS), a key enzyme of the fatty acid biosynthetic pathway, has been shown to be overexpressed in various types of human cancer and is, therefore, considered to be an attractive target for anticancer therapy. However, the exact mechanism of overexpression of the FAS gene in tumor cells is not well understood. In this report, we demonstrate that the expression of the tumor suppressor gene PTEN has a significant inverse correlation with FAS expression in the case of prostate cancer in the clinical setting, and inhibition of the PTEN gene leads to the overexpression of FAS in vitro. We also found that the combination of the expression status of these two genes is a better prognostic marker than either gene alone. Furthermore, our results indicate that the specific inhibition of FAS gene by siRNA leads to apoptosis of prostate tumor cells, and inhibition of PI 3-kinase pathway synergizes with FAS siRNA to enhance tumor cell death. These results provide a strong rationale for exploring the therapeutic use of an inhibitor of the PTEN signaling pathway in conjunction with the FAS siRNA to inhibit prostate tumor growth.

Insulin-mediated down-regulation of apolipoprotein A5 gene expression through the phosphatidylinositol 3-kinase pathway: role of upstream stimulatory factor

The apolipoprotein A5 gene (APOA5) has been repeatedly implicated in lowering plasma triglyceride levels. Since several studies have demonstrated that hyperinsulinemia is associated with hypertriglyceridemia, we sought to determine whether APOA5 is regulated by insulin. Here, we show that cell lines and mice treated with insulin down-regulate APOA5 expression in a dose-dependent manner. Furthermore, we found that insulin decreases human APOA5 promoter activity, and subsequent deletion and mutation analyses uncovered a functional E box in the promoter. Electrophoretic mobility shift and chromatin immunoprecipitation assays demonstrated that this APOA5 E box binds upstream stimulatory factors (USFs). Moreover, in transfection studies, USF1 stimulates APOA5 promoter activity, and the treatment with insulin reduced the binding of USF1/USF2 to the APOA5 promoter. The inhibition of the phosphatidylinositol 3-kinase (PI3K) pathway abolished insulin's effect on APOA5 gene expression, while the inhibition of the P70 S6 kinase pathway with rapamycin reversed its effect and increased APOA5 gene expression. Using an oligonucleotide precipitation assay for USF from nuclear extracts, we demonstrate that phosphorylated USF1 fails to bind to the APOA5 promoter. Taken together, these data indicate that insulin-mediated APOA5 gene transrepression could involve a phosphorylation of USFs through the PI3K and P70 S6 kinase pathways that modulate their binding to the APOA5 E box and results in APOA5 down-regulation. The effect of exogenous hyperinsulinemia in men showed a decrease in the plasma ApoAV level. These results suggest a potential contribution of the APOA5 gene in hypertriglyceridemia associated with hyperinsulinemia.

Identification of genes contributing to the obese yellow Avy phenotype: caloric restriction, genotype, diet x genotype interactions

The incidence and severity of obesity and type 2 diabetes are increasing in Western societies. The progression of obesity to type 2 diabetes is gradual with overlapping symptoms of insulin resistance, hyperinsulinemia, hyperglycemia, dyslipidemias, ion imbalance, and inflammation; this complex syndrome has been called diabesity. We describe here comparisons of gene expression in livers of A/a (agouti) vs. A(vy)/A (obese yellow) segregants (i.e., littermates) from BALB/cStCrlfC3H/Nctr x VYWffC3Hf/Nctr-A(vy)/a matings in response to 70% and 100% of ad libitum caloric intakes of a reproducible diet. Twenty-eight (28) genes regulated by diet, genotype, or diet x genotype interactions mapped to diabesity quantitative trait loci. A subset of the identified genes is linked to abnormal physiological signs observed in obesity and diabetes.

Seasonal, tissue-specific regulation of Akt/protein kinase B and glycogen synthase in hibernators

Yellow-bellied marmots ( Marmota flaviventris) exhibit a circannual cycle of hyperphagia and nutrient storage in the summer followed by hibernation in the winter. This annual cycle of body mass gain and loss is primarily due to large-scale accumulation of lipid in the summer, which is then mobilized and oxidized for energy during winter. The rapid and predictable change in body mass makes these animals ideal for studies investigating the molecular basis for body weight regulation. In the study described herein, we monitored seasonal changes in the protein levels and activity of a central regulator of anabolic metabolism, the serine-threonine kinase Akt-protein kinase B (Akt/PKB), during the months accompanying maximal weight gain and entry into hibernation (June-November). Interestingly, under fasting conditions, Akt/PKB demonstrated a tissue-specific seasonal activation. Specifically, although Akt/PKB levels did not change, the activity of Akt/PKB (isoforms 1/α and 2/β) in white adipose tissue (WAT) increased significantly in July. Moreover, glycogen synthase, which lies downstream of Akt/PKB on a linear pathway linking the enzyme to the stimulation of glycogen synthesis, demonstrated a similar pattern of seasonal activation. By contrast, Akt/PKB activity in skeletal muscle peaked much later (i.e., September). These data suggest the existence of a novel, tissue-specific mechanism regulating Akt/PKB activation during periods of marked anabolism.

Octanoate inhibits triglyceride synthesis in 3T3-L1 and human adipocytes

To understand how medium-chain fatty acids (FA) influence lipid metabolism in adipocytes, we studied the effects of octanoate on the oxidation of glucose and endogenous palmitate, cellular O(2) consumption, mitochondrial membrane potential, lipid synthesis from long-chain FA, glucose and lactate. We found that octanoate significantly suppressed the esterification of oleate into triglycerides (TG) in both 3T3-L1 and human adipocytes. Octanoate also significantly suppressed de novo FA synthesis. These effects were associated with octanoate-mediated reductions in the activities of acyl CoA:1,2-diacylglycerol acyltransferase (DGAT) and acetyl CoA carboxylase (ACC). Cells pretreated with octanoate had reduced mRNA levels for a number of lipid metabolism genes, including of DGAT, ACC and stearoyl CoA desaturase-1. On the other hand, octanoate did not acutely perturb cellular O(2) consumption or mitochondrial membrane potential. Together, these results suggest that octanoate affected adipocyte function by reducing TG synthesis but not by enhancing oxidation.