Carbohydrate responsive element binding protein (ChREBP) and sterol regulatory element binding protein-1c (SREBP-1c): two key regulators of glucose metabolism and lipid synthesis in liver

Role of carbohydrate response element-binding protein (ChREBP) in generating an aerobic metabolic phenotype and in breast cancer progression

BACKGROUND

The lipogenic transcription factor carbohydrate response element-binding protein (ChREBP) may play a key role in malignant progression of breast cancer by allowing metabolic adaptations to take place in response to changes in oxygenation.

METHODS

Immunohistochemical analysis of ChREBP was carried out in human breast tumour tissue microarrays representative of malignant progression from normal breast through to metastatic cancer. The ChREBP protein and mRNA expressions were then analysed in a series of breast cancers for correlative analysis with common and breast-specific hypoxia signatures, and survival.

RESULTS

In invasive ductal carcinoma, ChREBP correlated significantly with mean 'downregulated' hypoxia scores (r=0.3, P<0.015, n=67) and in two distinct breast progression arrays, ChREBP protein also increased with malignant progression (P<0.001). However, bioinformatic analysis of a large data set (2136 cases) revealed an apparent reversal in the relationship between ChREBP mRNA level and clinical outcome - not only being significantly correlated with increased survival (log rank P<0.001), but also downregulated in malignant tissue compared with adjacent normal tissue.

CONCLUSION

The ChREBP expression may be reflective of an aerobic metabolic phenotype that may conflict with hypoxia-induced signalling but provide a mechanism for growth at the oxygenated edge of the tumours.

Non-alcoholic steatohepatitis and hepatocellular carcinoma: implications for lycopene intervention

Increased prevalence of non-alcoholic fatty liver disease (NAFLD) is one of the consequences of the current obesity epidemic. NAFLD is a major form of chronic liver disease that is highly prevalent in obese and overweight adults and children. Nonalcoholic steatohepatitis (NASH) is the severe form of NAFLD, and uncontrolled inflammation as displayed in NASH has been identified as one of the key events in enhancing hepatic carcinogenesis. Lycopene is a non-provitamin A carotenoid and the pigment principally responsible for the characteristic deep-red color of ripe tomato and tomato products, as well as some fruits and vegetables. Lycopene's innate antioxidant and anti-inflammatory properties have generated research interests on its capacity to protect against human diseases that are associated with oxidative stress and inflammation. In addition, differential mechanisms of lycopene metabolism including endogenous cleavage by carotenoid cleavage oxygenases (BCOs), generate lycopene metabolites that may also have significant impact on human disease development. However, it remains to be elucidated as to whether lycopene or its metabolites apolycopenoids have protective effects against obesity-related complications including inflammation and tumorigenesis. This article summarizes the in vivo experiments that elucidated molecular mechanisms associated with obesity-related hepatic inflammation and carcinogenesis. This review also provides an overview of lycopene metabolism, and the molecular pathways involved in the potential beneficial properties of lycopene and apolycopenoids. More research is clearly needed to fully unravel the importance of BCOs in tomato carotenoid metabolism and the consequence on human health and diseases.

Long-term effects of subacute ruminal acidosis (SARA) on milk quality and hepatic gene expression in lactating goats fed a high-concentrate diet

PURPOSE

The mechanism underlying the decline in milk quality during periods of feeding high-concentrate diets to dairy ruminants is not well documented. The aim of this study was to investigate the metabolic changes in the liver that contribute to the input of substrate precursors to the mammary gland after feeding a high-concentrate diet to lactating goats for a long period.

EXPERIMENTAL DESIGN

Eight mid-lactating goats with rumen cannulas were randomly assigned to two groups. For 9 weeks, the treatment group was fed a high-concentrate diet (60% concentrate of dry matter, HC) and the control group was fed a low-concentrate diet (40% concentrate of dry matter, LC). Ruminal fluid, plasma, and liver tissues were sampled, microarray techniques and real-time polymerase chain reaction were used to evaluate metabolic parameters and gene expression in liver.

RESULTS

Feeding a 60%-concentrate diet for 9 weeks resulted in a significant decrease in rumen pH. Changes in fat and protein content also occurred, which negatively affected milk quality. Plasma levels of leptin (p = 0.058), non-esterified fatty acid (p = 0.071), and glucose (p = 0.014) increased markedly in HC group. Plasma cortisol concentration was significantly elevated in the treatment group (p<0.05). Expression of the glucocorticoid receptor protein gene was significantly down-regulated (p<0.05) in the liver. The expression of genes for interleukin 1β, serum amyloid A, C-reactive protein, and haptoglobin mRNA was significantly increased (p<0.05) in the HC group. GeneRelNet analysis showed that gene expression involved in inflammatory responses and the metabolism of lipids, protein, and carbohydrate were significantly altered by feeding a high-concentrate diet for 9 weeks.

CONCLUSIONS

Activation of the acute phase response and the inflammatory response may contribute to nutrient partitioning and re-distribution of energy in the liver, and ultimately lead to a decline in milk quality.

The inflammatory profile and liver damage of a sucrose-rich diet in mice

It is still unclear if an isoenergetic, sucrose-rich diet leads to health consequences.

AIMS

To investigate the effects of excessive sucrose within an isoenergetic diet on metabolic parameters in male C57BL/6 mice.

METHODS

Animals were fed a control diet (10% fat, 8% sucrose - SC group), a high-sucrose diet (10% fat, 32% sucrose - HSu group), a high-fat diet (42% fat, 8% sucrose - HF group) or a high-fat/high-sucrose diet (42% fat, 32% sucrose - HF/HSu group) for 8 weeks.

RESULTS

Mice fed HF and HF/HSu diets gained more body mass (BM) and more body adiposity than SC- or Hsu-fed mice. Despite the unchanged BM and adiposity indices, HSu mice presented adipocyte hypertrophy, which was also observed in the HF and HF/HSu groups (P<.0001). The HF, HSu and HF/HSu mice were glucose intolerant and had elevated serum insulin levels (P<.05). The levels of leptin, resistin and monocyte chemotactic protein-1 increased, while the serum adiponectin decreased in the HF, HSu and HF/HSu groups (P<.05). In the adipose tissue, the HF, HSu and HF/HSu groups showed higher levels of leptin expression and lower levels of adiponectin expression in comparison with the SC group (P<.05). Liver steatosis was higher in the HF, HSu and HF/HSu groups than in the SC group (P<.0001). Hepatic cholesterol was higher in the HF and HF/HSu groups, while hepatic TG was higher in the HSu and HF/HSu groups (P<.05). In hepatic tissue, the sterol receptor element-binding protein-1c expression was increased in the HF, HSu and HF/HSu groups, unlike the peroxisome proliferator-activated receptor-alpha expression that decreased in the HF, HSu and HF/HSu groups in comparison with the SC group (P<.05).

CONCLUSION

A sucrose-rich diet does not lead to a state of obesity but has the potential to cause changes in the adipocytes (hypertrophy) as well as glucose intolerance, hyperinsulinemia, hyperlipidemia, hepatic steatosis and increases in the number of inflammatory cytokines. The deleterious effects of a sucrose-rich diet in an animal model, even when the sucrose replaces starch isocalorically in the feed, can have far-reaching consequences for health.

Steatosis in the liver

Accumulation of triacylglycerols within the cytoplasm of hepatocytes to the degree that lipid droplets are visible microscopically is called liver steatosis. Most commonly, it occurs when there is an imbalance between the delivery or synthesis of fatty acids in the liver and their disposal through oxidative pathways or secretion into the blood as a component of triacylglycerols in very low density lipoprotein. This disorder is called nonalcoholic fatty liver disease (NAFLD) in the absence of alcoholic abuse and viral hepatitis, and it is often associated with insulin resistance, obesity and type 2 diabetes. Also, liver steatosis can be induced by many other causes including excessive alcohol consumption, infection with genotype 3 hepatitis C virus and certain medications. Whereas hepatic triacylglycerol accumulation was once considered the ultimate effector of hepatic lipotoxicity, triacylglycerols per se are quite inert and do not induce insulin resistance or cellular injury. Rather, lipotoxic injury in the liver appears to be mediated by the global ongoing fatty acid enrichment in the liver, paralleling the development of insulin resistance. A considerable number of fatty acid metabolites may be responsible for hepatic lipotoxicity and liver injury. Additional key contributors include hepatic cytosolic lipases and the "lipophagy" of lipid droplets, as sources of hepatic fatty acids. The specific origin of the lipids, mainly triacylglycerols, accumulating in liver has been unraveled by recent kinetic studies, and identifying the origin of the accumulated triacylglycerols in the liver of patients with NAFLD may direct the prevention and treatment of this condition.

Current role of fenofibrate in the prevention and management of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is a common health problem with a high mortality burden due to its liver- and vascular-specific complications. It is associated with obesity, high-fat diet as well as with type 2 diabetes mellitus (T2DM) and metabolic syndrome (MetS). Impaired hepatic fatty acid (FA) turnover together with insulin resistance are key players in NAFLD pathogenesis. Peroxisome proliferator-activated receptors (PPARs) are involved in lipid and glucose metabolic pathways. The novel concept is that the activation of the PPARα subunit may protect from liver steatosis. Fenofibrate, by activating PPARα, effectively improves the atherogenic lipid profile associated with T2DM and MetS. Experimental evidence suggested various protective effects of the drug against liver steatosis. Namely, fenofibrate-related PPARα activation may enhance the expression of genes promoting hepatic FA β-oxidation. Furthermore, fenofibrate reduces hepatic insulin resistance. It also inhibits the expression of inflammatory mediators involved in non-alcoholic steatohepatitis pathogenesis. These include tumor necrosis factor-α, intercellular cell adhesion molecule-1, vascular cell adhesion molecule-1 and monocyte chemoattractant protein-1. Consequently, fenofibrate can limit hepatic macrophage infiltration. Other liver-protective effects include decreased oxidative stress and improved liver microvasculature function. Experimental studies showed that fenofibrate can limit liver steatosis associated with high-fat diet, T2DM and obesity-related insulin resistance. Few studies showed that these benefits are also relevant even in the clinical setting. However, these have certain limitations. Namely, these were uncontrolled, their sample size was small, fenofibrate was used as a part of multifactorial approach, while histological data were absent. In this context, there is a need for large prospective studies, including proper control groups and full assessment of liver histology.

Ezetimibe prevents hepatic steatosis induced by a high-fat but not a high-fructose diet

Nonalcoholic fatty liver disease is the most frequent liver disease. Ezetimibe, an inhibitor of intestinal cholesterol absorption, has been reported to ameliorate hepatic steatosis in human and animal models. To explore how ezetimibe reduces hepatic steatosis, we investigated the effects of ezetimibe on the expression of lipogenic enzymes and intestinal lipid metabolism in mice fed a high-fat or a high-fructose diet. CBA/JN mice were fed a high-fat diet or a high-fructose diet for 8 wk with or without ezetimibe. High-fat diet induced hepatic steatosis accompanied by hyperinsulinemia. Treatment with ezetimibe reduced hepatic steatosis, insulin levels, and glucose production from pyruvate in mice fed the high-fat diet, suggesting a reduction of insulin resistance in the liver. In the intestinal analysis, ezetimibe reduced the expression of fatty acid transfer protein-4 and apoB-48 in mice fed the high-fat diet. However, treatment with ezetimibe did not prevent hepatic steatosis, hyperinsulinemia, and intestinal apoB-48 expression in mice fed the high-fructose diet. Ezetimibe decreased liver X receptor-α binding to the sterol regulatory element-binding protein-1c promoter but not expression of carbohydrate response element-binding protein and fatty acid synthase in mice fed the high-fructose diet, suggesting that ezetimibe did not reduce hepatic lipogenesis induced by the high-fructose diet. Elevation of hepatic and intestinal lipogenesis in mice fed a high-fructose diet may partly explain the differences in the effect of ezetimibe.

Medium chain triglycerides dose-dependently prevent liver pathology in a rat model of non-alcoholic fatty liver disease

Metabolic syndrome is often accompanied by development of hepatic steatosis and less frequently by non-alcoholic fatty liver disease (NAFLD) leading to non-alcoholic steatohepatitis (NASH). Replacement of corn oil with medium chain triacylglycerols (MCT) in the diets of alcohol-fed rats has been shown to protect against steatosis and alcoholic liver injury. The current study was designed to determine if a similar beneficial effect of MCT occurs in a rat model of NAFLD. Groups of male rats were isocalorically overfed diets containing 10%, 35% or 70% total energy as corn oil or a 70% fat diet in which corn oil was replaced with increasing concentrations of saturated fat (18:82, beef tallow:MCT oil) from 20% to 65% for 21 days using total enteral nutrition (TEN). As dietary content of corn oil increased, hepatic steatosis and serum alanine amino transferases were elevated (P < 0.05). This was accompanied by greater expression of cytochrome P450 enzyme CYP2E1 (P < 0.05) and higher concentrations of polyunsaturated 18:2 and 20:4 fatty acids (FA) in the hepatic lipid fractions (P < 0.05). Keeping the total dietary fat at 70%, but increasing the proportion of MCT-enriched saturated fat resulted in a dose-dependent reduction in steatosis and necrosis without affecting CYP2E1 induction. There was no incorporation of C8-C10 FAs into liver lipids, but increasing the ratio of MCT to corn oil: reduced liver lipid 18:2 and 20:4 concentrations; reduced membrane susceptibility to radical attack; stimulated FA β- and ω-oxidation as a result of activation of peroxisomal proliferator activated receptor (PPAR)α, and appeared to increase mitochondrial respiration through complex III. These data suggest that replacing unsaturated fats like corn oil with MCT oil in the diet could be utilized as a potential treatment for NAFLD.

Molecular and metabolic changes in human liver clear cell foci resemble the alterations occurring in rat hepatocarcinogenesis

BACKGROUND & AIMS

Activation of the AKT/mTOR and Ras/MAPK pathways and the lipogenic phenotype occurs in both a rat model of insulin-induced hepatocarcinogenesis and in human hepatocellular carcinoma (HCC). In the rat model, activation of these pathways is evident within the earliest morphologic detectable alterations, i.e., clear cell foci (CCF) of altered hepatocytes. CCF have also been described in the human liver, but molecular and metabolic alterations within these foci remain to be determined.

METHODS

A collection of human liver specimens was examined using electron microscopy, histology, enzyme- and immunohistochemistry, and molecular analysis. Human data were compared to rat preneoplastic CCF and HCC induced by N-nitrosomorpholine administration.

RESULTS

CCF occurred in ∼33% of extrafocal tissues of human non-cirrhotic livers. Electron microscopy showed massive glycogen storage within CCF, largely due to the reduced activity of the glycogenolytic enzyme glucose-6-phosphatase. Hepatocytes in CCF overexpressed the insulin receptor and glucose transporter proteins. AKT/mTOR and Ras/MAPK pathways as well as enzymes of glycolysis, de novo lipogenesis, beta-oxidation, and cholesterol synthesis were upregulated, both in human CCF, and in CCF and HCC of N-nitrosomorpholine-treated rats. The Ki-67 proliferation index was 2-fold higher in human CCF than in extrafocal tissue.

CONCLUSIONS

The high degree of similarity between human CCF and pre-neoplastic lesions from experimental models of hepatocarcinogenesis in terms of morphologic, molecular and metabolic features suggests a low-grade dysplastic nature of these lesions in human non-cirrhotic livers.

Postprandial regulation of growth- and metabolism-related factors in zebrafish

Zebrafish (Danio rerio) have been proposed as a possible model organism for nutritional physiology. However, this potential has not yet been realized and studies on the field remain scarce. In this work, we investigated in this species the effect of a single meal as well as that of an increase in the ratio of dietary carbohydrates/proteins on the postprandial expression of several hepatic and muscle metabolism-related genes and proteins. Fish were fed once either a commercial diet (experiment 1) or one of two experimental diets (experiment 2) containing different protein and carbohydrate levels after 72 h of starvation. Refeeding induced the postprandial expression of genes of glycolysis (GK, HK1) and lipogenesis (FAS, G6PDH, ACCa) and inhibited those of gluconeogenesis (cPEPCK) and beta-oxidation (CPT1b) in the viscera. In the muscle, refeeding increased transcript levels of myogenesis (Myf5, Myogenin), inhibited those of Ub-proteasomal proteolytic system (Atrogin1, Murf1a, Murf1b), and induced the activation of key signaling factors of protein synthesis (Akt, 4EBP1, S6K1, S6). However, diet composition had a low impact on the studied factors. Together, these results highlight some specificity of the zebrafish metabolism and demonstrate the interest and the limits of this species as a model organism for nutritional physiology studies.

PGC-1β and ChREBP partner to cooperatively regulate hepatic lipogenesis in a glucose concentration-dependent manner

Peroxisome proliferator-activated receptorγ coactivators (PGC-1α and PGC-1β) play important roles in the transcriptional regulation of intermediary metabolism. To evaluate the effects of overexpressing PGC-1α or PGC-1β at physiologic levels in liver, we generated transgenic mice with inducible overexpression of PGC-1α or PGC-1β. Gene expression array profiling revealed that whereas both PGC-1 family proteins induced mitochondrial oxidative enzymes, the expression of several genes involved in converting glucose to fatty acid was induced by PGC-1β, but not PGC-1α. The increased expression of enzymes involved in carbohydrate utilization and de novo lipogenesis by PGC-1β required carbohydrate response element binding protein (ChREBP). The interaction between PGC-1β and ChREBP, as well as PGC-1β occupancy of the liver-type pyruvate kinase promoter, was influenced by glucose concentration and liver-specific PGC-1β(-/-) hepatocytes were refractory to the lipogenic response to high glucose conditions. These data suggest that PGC-1β-mediated coactivation of ChREBP is involved in the lipogenic response to hyperglycemia.

Fructose during pregnancy affects maternal and fetal leptin signaling

Fructose intake from added sugars correlates with the epidemic rise in obesity, metabolic syndrome and cardiovascular diseases. Fructose intake also causes features of metabolic syndrome in laboratory animals. Therefore, we have investigated whether fructose modifies lipidemia in pregnant rats and produces changes in their fetuses. Thus, fructose administration (10% wt/vol.) in the drinking water of rats throughout gestation leads to maternal hypertriglyceridemia. This change was not observed in glucose-fed rats, although both carbohydrates produced similar changes in liver triglycerides and in the expression of transcription factors and enzymes involved in lipogenesis. After fasting overnight, mothers fed with carbohydrates were found to be hyperleptinemic. However, after a bolus of glucose, leptinemia in fructose-fed mothers showed no response, whereas it increased in parallel in glucose-fed and control mothers. Fetuses from fructose-fed mothers showed hypotriglyceridemia and a higher hepatic triglyceride content than fetuses from control or glucose-fed mothers. A higher expression of genes related to lipogenesis and a lower expression of fatty acid catabolism genes were also found in fetuses from fructose-fed mothers. Moreover, although hyperleptinemic, these fetuses exhibited increased tyrosine phosphorylation of the signal transducer and activator of transcription-3 (STAT-3) protein, without a parallel increase in the serine phosphorylation of STAT-3 nor in the suppressor of cytokine signaling-3 protein levels whose expression is regulated by leptin through STAT-3 activation. Thus, fructose intake during gestation provoked a diminished maternal leptin response to fasting and refeeding and an impairment in the transduction of the leptin signal in the fetuses, which could be responsible for their hepatic steatosis.

Oleanolic Acid diminishes liquid fructose-induced Fatty liver in rats: role of modulation of hepatic sterol regulatory element-binding protein-1c-mediated expression of genes responsible for de novo Fatty Acid synthesis

Oleanolic acid (OA), contained in more than 1620 plants and as an aglycone precursor for naturally occurred and synthesized triterpenoid saponins, is used in China for liver disorders in humans. However, the underlying liver-protecting mechanisms remain largely unknown. Here, we found that treatment of rats with OA (25 mg/kg/day, gavage, once daily) over 10 weeks diminished liquid fructose-induced excess hepatic triglyceride accumulation without effect on total energy intake. Attenuation of the increased vacuolization and Oil Red O staining area was evident on histological examination of liver in OA-treated rats. Hepatic gene expression profile demonstrated that OA suppressed fructose-stimulated overexpression of sterol regulatory element-binding protein-(SREBP-) 1/1c mRNA and nuclear protein. In accord, overexpression of SREBP-1c-responsive genes responsible for fatty acid synthesis was also downregulated. In contrast, overexpressed nuclear protein of carbohydrate response element-binding protein and its target genes liver pyruvate kinase and microsomal triglyceride transfer protein were not altered. Additionally, OA did not affect expression of peroxisome proliferator-activated receptor-gamma- and -alpha and their target genes. It is concluded that modulation of hepatic SREBP-1c-mediated expression of the genes responsible for de novo fatty acid synthesis plays a pivotal role in OA-elicited diminishment of fructose-induced fatty liver in rats.

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.

Role of patatin-like phospholipase domain-containing 3 on lipid-induced hepatic steatosis and insulin resistance in rats

Genome-wide array studies have associated the patatin-like phospholipase domain-containing 3 (PNPLA3) gene polymorphisms with hepatic steatosis. However, it is unclear whether PNPLA3 functions as a lipase or a lipogenic enzyme and whether PNPLA3 is involved in the pathogenesis of hepatic insulin resistance. To address these questions we treated high-fat-fed rats with specific antisense oligonucleotides to decrease hepatic and adipose pnpla3 expression. Reducing pnpla3 expression prevented hepatic steatosis, which could be attributed to decreased fatty acid esterification measured by the incorporation of [U-(13) C]-palmitate into hepatic triglyceride. While the precursors for phosphatidic acid (PA) (long-chain fatty acyl-CoAs and lysophosphatidic acid [LPA]) were not decreased, we did observe an ∼20% reduction in the hepatic PA content, ∼35% reduction in the PA/LPA ratio, and ∼60%-70% reduction in transacylation activity at the level of acyl-CoA:1-acylglycerol-sn-3-phosphate acyltransferase. These changes were associated with an ∼50% reduction in hepatic diacylglycerol (DAG) content, an ∼80% reduction in hepatic protein kinase Cε activation, and increased hepatic insulin sensitivity, as reflected by a 2-fold greater suppression of endogenous glucose production during the hyperinsulinemic-euglycemic clamp. Finally, in humans, hepatic PNPLA3 messenger RNA (mRNA) expression was strongly correlated with hepatic triglyceride and DAG content, supporting a potential lipogenic role of PNPLA3 in humans.

CONCLUSION

PNPLA3 may function primarily in a lipogenic capacity and inhibition of PNPLA3 may be a novel therapeutic approach for treatment of nonalcoholic fatty liver disease-associated hepatic insulin resistance.

Resveratrol suppresses T0901317-induced hepatic fat accumulation in mice

Liver X receptor (LXR) has been identified as a potential target for treatment of atherosclerosis and diabetes. Activation of LXR, however, is associated with increased lipogenesis and fat accumulation in the liver. The objective of the current study was to examine the effect of resveratrol on LXR activator-induced fat accumulation in liver using mice as an animal model. Three groups of C57BL/6 mice were studied. Animals in group 1 were treated with T0901317, a potent activator of LXR in mice. Animals in group 2 served as the control and were treated with carrier solution and those in group 3 were treated with T0901317/resveratrol combination. Using histochemical and biochemical methods, we demonstrate that resveratrol treatment significantly suppressed fat accumulation in the liver induced by T0901317. In addition, resveratrol completely blocked elevation of blood levels of triglyceride and cholesterol and reduced blood glucose level. Quantitative PCR analysis revealed that resveratrol treatment did not change the mRNA levels of abca1, abcg1, cyp7a1, srebp-1c, chrebp, and acc genes compared to that of animals treated with T0901317 alone but reduced pepck and g6p gene expressions. Immunohistochemistry and Western blot analyses show resveratrol treatment activated AMP-activated protein kinase (AMPK) and increased phosphorylation of acetyl-CoA carboxylase. Treatment with T0901317 on hepatocytes increased intracellular fat accumulation and this increase was suppressed by resveratrol; the suppressive effect of resveratrol was greatly repressed by Compound C which is an inhibitor of AMPK. Collectively, these data suggest that resveratrol blocks T0901317-induced lipid accumulation in the liver and can be considered for inclusion into the treatment of diseases involving activation of liver X receptor.

A pathway approach to investigate the function and regulation of SREBPs

The essential function of sterol regulatory element-binding proteins (SREBPs) in cellular lipid metabolism and homeostasis has been recognized for a long time, and the basic biological pathway involving SREBPs has been well described; however, a rapidly growing number of studies reveal the complex regulation of these SREBP transcription factors at multiple levels. This regulation allows the integration of signals of diverse pathways involving nutrients, contributing to cellular lipid and energy homeostasis. This review attempts to integrate this knowledge. The description of the SREBP pathway is Web-linked as it refers to the online version of the pathway on wikipathways.org , which is interactively linked to genomics databases and literature. This allows a more extensive study of the pathway through reviewing these links.

Reduced expression of FASN through SREBP-1 down-regulation is responsible for hypoxic cell death in HepG2 cells

Cells under hypoxic stress either activate an adaptive response or undergo cell death. Although some mechanisms have been reported, the exact mechanism behind hypoxic cell death remains unclear. Recently, increased expression of fatty acid synthase (FASN) has been observed in various human cancers. In highly proliferating cells, tumor-associated FASN is considered necessary for both membrane lipids production and post-translational protein modification, but the exact mechanisms are not fully understood. Further, FASN overexpression is associated with aggressive and malignant cancer diseases and FASN inhibition induces apoptosis in cancer cells. For this reason, FASN is emerging as a key target for the potential diagnosis and treatment of various cancers. Here, we observed decreased FASN expression under hypoxic cell death conditions in HepG2 cells. Thus, we examined the effect of decreased FASN expression on hypoxia-induced cell death in HepG2 cells and also investigated the mechanism responsible for reduction of FASN expression under hypoxic cell death conditions. As a result, reduction of FASN expression resulted in hypoxic cell death via malonyl-CoA accumulation. In addition, SREBP-1 restored FASN reduction and hypoxia-induced apoptosis. Taken together, we suggest that hypoxic cell death is promoted by the reduced expression of FASN through SREBP-1 down-regulation.

Anti-atherogenic effect of chromium picolinate in streptozotocin-induced experimental diabetes

BACKGROUND

Several studies have implicated changes in the levels of trace elements in diabetes. Chromium is one such element that seems to potentiate insulin action, thereby regulating carbohydrate and lipid metabolism. The aim of the present study was to evaluate the effect of chromium supplementation as chromium picolinate on the lipid profile of streptozotocin (STZ)-induced diabetic rats.

METHODS

Rats were rendered diabetic by a single injection of STZ (50 mg/kg, i.p.). Chromium picolinate (1 mg/kg per day, p.o.) was administered to rats for a period of 4 weeks. At the end of the treatment period, plasma total lipids, triglycerides, total cholesterol and lipoprotein levels were determined, as was hepatic glucose-6-phosphate dehydrogenase activity.

RESULTS

Total plasma lipids increased significantly in diabetic rats and this increase was ameliorated by chromium treatment for 4 weeks. Elevated total lipids in diabetic rats were due to increased plasma triglyceride and cholesterol levels. Chromium supplementation lowered plasma triglyceride and cholesterol levels to near normal. Chromium treatment also normalized low-density lipoprotein-cholesterol (LDL-C) and very low-density lipoprotein-cholesterol levels and improved the total cholesterol:high-density lipoprotein-cholesterol (HDL-C) and HDL-C:LDL-C ratios, suggesting an anti-atherogenic effect. In addition to improving the plasma lipid profile, chromium supplementation normalized liver glucose-6-phosphate dehydrogenase activity in diabetic rats.

CONCLUSIONS

These results provide evidence that chromium picolinate effectively attenuates the dyslipidemia associated with diabetes and thus can be used as an adjuvant therapy in the treatment of diabetes and its associated complications.

G proteins, p60TRP, and neurodegenerative diseases

Alzheimer's disease (AD) is a complex brain disorder of the limbic system and association cortices. The disease is characterized by the production and deposition of the amyloid β-peptide (Aβ) in the brain, and the neuropathological mechanisms involved must be deciphered to gain further insights into the fundamental aspects of the protein biology responsible for the development and progression of this disease. Aβ is generated by the intramembranous cleavage of the β-amyloid precursor protein, which is mediated by the proteases β- and γ-secretase. Accumulating evidence suggests the importance of the coupling of this cleavage mechanism to G protein signaling. Heterotrimeric G proteins play pivotal roles as molecular switches in signal transduction pathways mediated by G protein-coupled receptors (GPCRs). Extracellular stimuli activate these receptors, which in turn catalyze guanosine triphosphate-guanosine diphosphate exchange on the G protein α-subunit. The activation-deactivation cycles of G proteins underlie their crucial functions as molecular switches for a vast array of biological responses. The novel transcription regulator protein p60 transcription regulator protein and its related GPCR signaling pathways have recently been described as potential targets for the development of alternative strategies for inhibiting the early signaling mechanisms involved in neurodegenerative diseases such as AD.

Impacts of birth weight on plasma, liver and skeletal muscle neutral amino acid profiles and intestinal amino acid transporters in suckling Huanjiang mini-piglets

Genetic selection strategies towards increased prolificacy have resulted in more and more increased littler size and incidences of impaired fetal development. Low birth weight (LBW) piglets, with long-term alterations in structure, physiology and metabolism, have lower survival rates and poor growth performance. The aim of the study was to compare the plasma, liver and skeletal muscle contents of neutral amino acids (NAA) and the intestinal expression of NAA transporters between LBW and high birth weight (HBW) suckling Huanjiang mini-piglets. Forty piglets with either LBW or HBW (20 piglets per group) were sampled on day 0, 7, 14 and 21 of age to give 5 observations per day per group. The contents of NAA in plasma, liver and skeletal muscle were measured, and jejunal expression of transporters for NAA, including Slc6a19 (B⁰AT1) and Slc1a5 (ASCT2), were determined by real-time RT-PCR and Western Blot, respectively. Results showed that the suckling piglets with LBW had higher contents of Thr, Ser, Gly, Ala, Val, Met, Ile, Leu, Tyr, Phe and Pro in liver, and Gly in skeletal muscle, whereas lower contents of Met, Ser and Ala in plasma when compared with the HBW littermates. Consistent with the content differences in plasma NAA, the jejunal expression profiles of both Slc6a19 (B⁰AT1) and Slc1a5 (ASCT2) in the LBW piglets were lower in compared with the HBW littermates during the early suckling period. These findings suggested that intestinal dysfunction in the LBW piglets may be one of the reasons in altered physiology and metabolism states of other organs, which result in lower survival and growth rate.

Lipoic acid prevents liver metabolic changes induced by administration of a fructose-rich diet

BACKGROUND

To evaluate whether co-administration of R/S-α-lipoic acid can prevent the development of oxidative stress and metabolic changes induced by a fructose-rich diet (F).

METHODS

We assessed glycemia in the fasting state and during an oral glucose tolerance test, triglyceridemia and insulinemia in rats fed with standard diet (control) and fructose without or with R/S-α-lipoic acid. Insulin resistance and hepatic insulin sensitivity were also calculated. In liver, we measured reduced glutathione, protein carbonyl groups, antioxidant capacity by ABTS assay, antioxidant enzymes (catalase and superoxide dismutase 1 and 2), uncoupling protein 2, PPARδ and PPARγ protein expressions, SREBP-1c, fatty acid synthase and glycerol-3-phosphate acyltransferase-1 gene expression, and glucokinase activity.

RESULTS

R/S-α-lipoic acid co-administration to F-fed rats a) prevented hyperinsulinemia, hypertriglyceridemia and insulin resistance, b) improved hepatic insulin sensitivity and glucose tolerance, c) decreased liver oxidative stress and increased antioxidant capacity and antioxidant enzymes expression, d) decreased uncoupling protein 2 and PPARδ protein expression and increased PPARγ levels, e) restored the basal gene expression of PPARδ, SREBP-1c and the lipogenic genes fatty acid synthase and glycerol-3-phosphate acyltransferase, and f) decreased the fructose-mediated enhancement of glucokinase activity.

CONCLUSIONS

Our results suggest that fructose-induced oxidative stress is an early phenomenon associated with compensatory hepatic metabolic mechanisms, and that treatment with an antioxidant prevented the development of such changes.

GENERAL SIGNIFICANCE

This knowledge would help to better understand the mechanisms involved in liver adaptation to fructose-induced oxidative stress and to develop effective strategies to prevent and treat, at early stages, obesity and type 2 diabetes mellitus.

Nutrition-/diet-induced changes in gene expression in pancreatic β-cells

β-Cell dysfunction is a critical component in the development of type 2 diabetes. Whilst both genetic and environmental factors contribute to the development of the disease, relatively little is known about the molecular network that is responsible for diet-induced functional changes in pancreatic β-cells. Recent genome-wide association studies for diabetes-related traits have generated a large number of candidate genes that constitute possible links between dietary factors and the genetic susceptibility for β-cell failure. Here, we summarize recent approaches for identifying nutritionally regulated transcripts in islets on a genome-wide scale. Polygenic mouse models for type 2 diabetes have been instrumental for investigating the mechanism of diet-induced β-cell dysfunction. Enhanced oxidative metabolism, triggered by a combination of dietary carbohydrates and fat, appears to play a critical role in the pathophysiology of diet-induced impairment of islets. More systematic studies of gene-diet interactions in β-cells of rodent models in combination with genetic profiling might reveal the regulatory circuits fundamental for the understanding of diet-induced impairments of β-cell function in humans.

Dietary fat source alters hepatic gene expression profile and determines the type of liver pathology in rats overfed via total enteral nutrition

To determine if dietary fat composition affects the progression of nonalcoholic fatty liver disease (NAFLD), we overfed male Sprague-Dawley rats low (5%) or high (70%) fat diets with different fat sources: olive oil (OO), corn oil (CO), or echium oil (EO), with total enteral nutrition (TEN) for 21 days. Overfeeding of the 5% CO or 5% EO diets resulted in less steatosis than 5% OO (P < 0.05). Affymetrix array analysis revealed significant differences in hepatic gene expression signatures associated with greater fatty acid synthesis, ChREBP, and SREBP-1c signaling and increased fatty acid transport (P < 0.05) in the 5% OO compared with 5% CO group. The OO groups had macrosteatosis, but no evidence of oxidative stress or necrosis. The 70% CO and 70% EO groups had a mixture of micro- and macrosteatosis or only microsteatosis, respectively; increased oxidative stress; and increased necrotic injury relative to their respective 5% groups (P < 0.05). Oxidative stress and necrosis correlated with increasing peroxidizability of the accumulated triglycerides. Affymetrix array analysis comparing the 70% OO and 70% CO groups revealed increased antioxidant pathways and lower expression of genes linked to inflammation and fibrosis in the 70% OO group. A second study in which 70% OO diet was overfed for 50 days produced no evidence of progression of injury beyond simple steatosis. These data suggest that dietary fat type strongly influences the progression of NAFLD and that a Mediterranean diet high in olive oil may reduce the risk of NAFLD progressing to nonalcoholic steatohepatitis.

Vitamin A status affects obesity development and hepatic expression of key genes for fuel metabolism in Zucker fatty rats

We hypothesized that vitamin A (VA) status may affect obesity development. Male Zucker lean (ZL) and fatty (ZF) rats after weaning were fed a synthetic VA deficient (VAD) or VA sufficient (VAS) diet for 8 weeks before their plasma parameters and hepatic genes' expression were analyzed. The body mass (BM) of ZL or ZF rats fed the VAD diet was lower than that of their corresponding controls fed the VAS diet at 5 or 2 weeks, respectively. The VAD ZL and ZF rats had less food intake than the VAS rats after 5 weeks. The VAD ZL and ZF rats had lower plasma glucose, triglyceride, insulin, and leptin levels, as well as lower liver glycogen content, net mass of epididymal fat, and liver/BM and epididymal fat/BM ratios (ZL only) than their respective VAS controls. VAD rats had lower hepatic Cyp26a1, Srebp-1c, Fas, Scd1, Me1, Gck, and Pklr (ZL and ZF); and higher Igfbp1 (ZL and ZF), Pck1(ZF only), and G6pc (ZF only) mRNA levels than their respective VAS controls. We conclude that ZL and ZF rats responded differently to dietary VA deficiency. VA status affected obesity development and altered the expression of hepatic genes for fuel metabolism in ZF rats. The mechanisms will help us to combat metabolic diseases.

Target metabolomics revealed complementary roles of hexose- and pentose-phosphates in the regulation of carbohydrate-dependent gene expression

Carbohydrate response element-binding protein (ChREBP) is a transcription factor that mediates glucose signaling in mammalian liver, leading to the expression of different glycolytic and lipogenic genes, such as pyruvate kinase (L-PK) and fatty acid synthase (FAS). The current model for ChREBP activation in response to sugar phosphates holds that glucose metabolization to xylulose 5-phosphate (X-5-P) triggers the activation of protein phosphatase 2A, which dephosphorylates ChREBP and leads to its nuclear translocation and activation. However, evidence indicates that glucose 6-phosphate (G-6-P) is the most likely signal metabolite for the glucose-induced transcription of these genes. The glucose derivative that is responsible for carbohydrate-dependent gene expression remains to be identified. The difficulties in measuring G-6-P and X-5-P concentrations simultaneously and in changing them independently have hindered such identification. To discriminate between these possibilities, we adapted a liquid chromatography mass spectrometry method to identify and quantify sugar phosphates in human hepatocarcinoma cells (Hep G2) and rat hepatocytes in response to different carbon sources and in the presence/absence of a glucose-6-phosphate dehydrogenase inhibitor. We also used this method to demonstrate that these cells could not metabolize 2-deoxyglucose beyond 2-deoxyglucose-6-phosphate. The simultaneous quantification of sugar phosphates and FAS and L-PK expression levels demonstrated that both X-5-P and G-6-P play a role in the modulation of gene expression. In conclusion, this report presents for the first time a single mechanism that incorporates the effects of X-5-P and G-6-P on the enhancement of the expression of carbohydrate-responsive genes.

Diet high in fat and sucrose induces rapid onset of obesity-related metabolic syndrome partly through rapid response of genes involved in lipogenesis, insulin signalling and inflammation in mice

BACKGROUND

Frequent consumption of a diet high in fat and sucrose contributes to lifestyle-related diseases. However, limited information is available regarding the short-term effects of such a diet on the onset of obesity-associated metabolic abnormalities.

METHODS

Male C57BL/6 J mice were divided into two groups and fed a standard chow diet (control group) or a high fat-high sucrose diet containing 21% fat and 34% sucrose (HF-HS diet group) for 2 or 4 weeks.

RESULTS

The HF-HS diet significantly induced body weight gain beginning at week 1 and similarly increased mesenteric white adipose tissue weight and plasma insulin levels at weeks 2 and 4. Plasma resistin levels were notably elevated after feeding with the HF-HS diet for 4 weeks. Measurement of hepatic triglycerides and Oil Red O staining clearly indicated increased hepatic lipid accumulation in response to the HF-HS diet as early as 2 weeks. Quantitative PCR analysis of liver and white adipose tissue indicated that, starting at week 2, the HF-HS diet upregulated mRNA expression from genes involved in lipid metabolism and inflammation and downregulated genes involved in insulin signalling. Although plasma cholesterol levels were also rapidly increased by the HF-HS diet, no differences were found between the control and HF-HS diet-fed animals in the expression of key genes involved in cholesterol biosynthesis.

CONCLUSIONS

Our study demonstrates that the rapid onset of hepatosteatosis, adipose tissue hypertrophy and hyperinsulinemia by ingestion of a diet high in fat and sucrose may possibly be due to the rapid response of lipogenic, insulin signalling and inflammatory genes.

Hepatic mTORC2 activates glycolysis and lipogenesis through Akt, glucokinase, and SREBP1c

Mammalian target of rapamycin complex 2 (mTORC2) phosphorylates and activates AGC kinase family members, including Akt, SGK1, and PKC, in response to insulin/IGF1. The liver is a key organ in insulin-mediated regulation of metabolism. To assess the role of hepatic mTORC2, we generated liver-specific rictor knockout (LiRiKO) mice. Fed LiRiKO mice displayed loss of Akt Ser473 phosphorylation and reduced glucokinase and SREBP1c activity in the liver, leading to constitutive gluconeogenesis, and impaired glycolysis and lipogenesis, suggesting that the mTORC2-deficient liver is unable to sense satiety. These liver-specific defects resulted in systemic hyperglycemia, hyperinsulinemia, and hypolipidemia. Expression of constitutively active Akt2 in mTORC2-deficient hepatocytes restored both glucose flux and lipogenesis, whereas glucokinase overexpression rescued glucose flux but not lipogenesis. Thus, mTORC2 regulates hepatic glucose and lipid metabolism via insulin-induced Akt signaling to control whole-body metabolic homeostasis. These findings have implications for emerging drug therapies that target mTORC2.

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.

Adenosine A₁ receptors do not play a major role in the regulation of lipogenic gene expression in hepatocytes

Activation of adenosine A₁ receptors was reported to promote fatty acid synthesis in AML-12 cells, by increasing the expression of SREBP-(1c) (sterol regulatory binding protein 1c) and FAS (fatty acid synthase). Since these findings have important therapeutic implications for the discovery of adenosine A₁ receptor agonists, further studies were undertaken to determine the expression and functional relevance of adenosine A₁ receptor in the liver. To that end, we used two classes of distinct adenosine A₁ receptor agonists: CPA (N⁶-cyclopentyl-adenosine), a full agonist and GS-9667 (2-{6-[((1R,2R)-2-hydroxycyclopentyl)-amino]purin-9-yl}(4S,5S,2R,3R)-5-[(2-fluorophenylthio)methyl]-oxolane-3,4-diol), a partial agonist. Treatment of AML-12 cells, HepG2 cells and primary human hepatocytes with either CPA or GS-9667 did not increase the gene expression of SREBP-(1c) or FAS. Furthermore, in AML-12 and HepG2 cells, CPA did not antagonize forskolin-stimulated cAMP production, a characteristic of adenosine A₁ receptor activation, indicating that these cells lack adenosine A₁ receptor function. Consistent with this finding, adenosine A₁ receptor gene expression was found to be very low and adenosine A₁ receptor protein levels were hardly detectable by radioligand binding assays in hepatic cell lines such as AML-12 and HepG2 as well as in both mouse and human liver tissues. Finally, acute treatment with adenosine A₁ receptor agonist GS-9667 had no significant effect on gene expression of both SREBP-(1c) and FAS in livers of Sprague Dawley rats. Taken together, our data suggest that the expression of adenosine A₁ receptor is too low to play a major role in the regulation of lipogenic gene expression in hepatocytes.

Glucose sensing by ChREBP/MondoA-Mlx transcription factors

The paralogous transcription factors ChREBP and MondoA, together with their common binding partner Mlx, have emerged as key mediators of intracellular glucose sensing. By regulating target genes involved in glycolysis and lipogenesis, they mediate metabolic adaptation to changing glucose levels. As disturbed glucose homeostasis plays a central role in human metabolic diseases and as cancer cells often display altered glucose metabolism, better understanding of cellular glucose sensing will likely uncover new therapeutic opportunities. Here we review the regulation, function and evolutionary conservation of the ChREBP/MondoA-Mlx glucose sensing system and discuss possible directions for future research.

Differing endoplasmic reticulum stress response to excess lipogenesis versus lipid oversupply in relation to hepatic steatosis and insulin resistance

Mitochondrial dysfunction and endoplasmic reticulum (ER) stress have been implicated in hepatic steatosis and insulin resistance. The present study investigated their roles in the development of hepatic steatosis and insulin resistance during de novo lipogenesis (DNL) compared to extrahepatic lipid oversupply. Male C57BL/6J mice were fed either a high fructose (HFru) or high fat (HFat) diet to induce DNL or lipid oversupply in/to the liver. Both HFru and HFat feeding increased hepatic triglyceride within 3 days (by 3.5 and 2.4 fold) and the steatosis remained persistent from 1 week onwards (p<0.01 vs Con). Glucose intolerance (iAUC increased by ∼60%) and blunted insulin-stimulated hepatic Akt and GSK3β phosphorylation (∼40–60%) were found in both feeding conditions (p<0.01 vs Con, assessed after 1 week). No impairment of mitochondrial function was found (oxidation capacity, expression of PGC1α, CPT1, respiratory complexes, enzymatic activity of citrate synthase & β-HAD). As expected, DNL was increased (∼60%) in HFru-fed mice and decreased (32%) in HFat-fed mice (all p<0.05). Interestingly, associated with the upregulated lipogenic enzymes (ACC, FAS and SCD1), two (PERK/eIF2α and IRE1/XBP1) of three ER stress pathways were significantly activated in HFru-fed mice. However, no significant ER stress was observed in HFat-fed mice during the development of hepatic steatosis. Our findings indicate that HFru and HFat diets can result in hepatic steatosis and insulin resistance without obvious mitochondrial defects via different lipid metabolic pathways. The fact that ER stress is apparent only with HFru feeding suggests that ER stress is involved in DNL per se rather than resulting from hepatic steatosis or insulin resistance.

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.

Fructose containing sugars modulate mRNA of lipogenic genes ACC and FAS and protein levels of transcription factors ChREBP and SREBP1c with no effect on body weight or liver fat

The aim of this study was to determine the effects of high-glucose, high-fructose and high-sucrose diets on weight gain, liver lipid metabolism and gene expression of proteins involved with hepatic fat metabolism. Rats were fed a diet containing either 60% glucose, 60% fructose, 60% sucrose, or a standard chow for 28 days. Results indicated that high-fructose and high-sucrose diets were associated with higher mRNA levels of gene transcripts involved with fat synthesis; ACC, FAS and ChREBP, with no change in SREBP-1C mRNA. The protein level of ChREBP and SREBP1c was similar in liver homogenates from all groups, but were higher in nuclear fractions from the liver of high-fructose and high-sucrose fed rats. The mRNA level of gene transcripts involved with fat oxidation was the same in all three diets, whilst a high-fructose diet was associated with greater amount of mRNA of the fat transporter CD36. Despite the changes in mRNA of lipogenic proteins, the body weight of animals from each group was the same and the livers from rats fed high-fructose and high-sucrose diets did not contain more fat than control diet livers. In conclusion, changing the composition of the principal monosaccharide in the diet to a fructose containing sugar elicits changes in the level of hepatic mRNA of lipogenic and fat transport proteins and protein levels of their transcriptional regulators; however this is not associated with any changes in body weight or liver fat content.

Role of transcription factor modifications in the pathogenesis of insulin resistance

Non-alcoholic fatty liver disease (NAFLD) is characterized by fat accumulation in the liver not due to alcohol abuse. NAFLD is accompanied by variety of symptoms related to metabolic syndrome. Although the metabolic link between NAFLD and insulin resistance is not fully understood, it is clear that NAFLD is one of the main cause of insulin resistance. NAFLD is shown to affect the functions of other organs, including pancreas, adipose tissue, muscle and inflammatory systems. Currently efforts are being made to understand molecular mechanism of interrelationship between NAFLD and insulin resistance at the transcriptional level with specific focus on post-translational modification (PTM) of transcription factors. PTM of transcription factors plays a key role in controlling numerous biological events, including cellular energy metabolism, cell-cycle progression, and organ development. Cell type- and tissue-specific reversible modifications include lysine acetylation, methylation, ubiquitination, and SUMOylation. Moreover, phosphorylation and O-GlcNAcylation on serine and threonine residues have been shown to affect protein stability, subcellular distribution, DNA-binding affinity, and transcriptional activity. PTMs of transcription factors involved in insulin-sensitive tissues confer specific adaptive mechanisms in response to internal or external stimuli. Our understanding of the interplay between these modifications and their effects on transcriptional regulation is growing. Here, we summarize the diverse roles of PTMs in insulin-sensitive tissues and their involvement in the pathogenesis of insulin resistance.

Roles of PPARs in NAFLD: potential therapeutic targets

Non-alcoholic fatty liver disease (NAFLD) is a liver pathology with increasing prevalence due to the obesity epidemic. Hence, NAFLD represents a rising threat to public health. Currently, no effective treatments are available to treat NAFLD and its complications such as cirrhosis and liver cancer. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear receptors which regulate lipid and glucose metabolism as well as inflammation. Here we review recent findings on the pathophysiological role of PPARs in the different stages of NAFLD, from steatosis development to steatohepatitis and fibrosis, as well as the preclinical and clinical evidence for potential therapeutical use of PPAR agonists in the treatment of NAFLD. PPARs play a role in modulating hepatic triglyceride accumulation, a hallmark of the development of NAFLD. Moreover, PPARs may also influence the evolution of reversible steatosis toward irreversible, more advanced lesions. Presently, large controlled trials of long duration are needed to assess the long-term clinical benefits of PPAR agonists in humans. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Effects of the total replacement of fish-based diet with plant-based diet on the hepatic transcriptome of two European sea bass (Dicentrarchus labrax) half-sibfamilies showing different growth rates with the plant-based diet

Background

Efforts towards utilisation of diets without fish meal (FM) or fish oil (FO) in finfish aquaculture have been being made for more than two decades. Metabolic responses to substitution of fishery products have been shown to impact growth performance and immune system of fish as well as their subsequent nutritional value, particularly in marine fish species, which exhibit low capacity for biosynthesis of long-chain poly-unsaturated fatty acids (LC-PUFA). The main objective of the present study was to analyse the effects of a plant-based diet on the hepatic transcriptome of European sea bass (Dicentrarchus labrax).

Results

We report the first results obtained using a transcriptomic approach on the liver of two half-sibfamilies of the European sea bass that exhibit similar growth rates when fed a fish-based diet (FD), but significantly different growth rates when fed an all-plant diet (VD). Overall gene expression was analysed using oligo DNA microarrays (GPL9663). Statistical analysis identified 582 unique annotated genes differentially expressed between groups of fish fed the two diets, 199 genes regulated by genetic factors, and 72 genes that exhibited diet-family interactions. The expression of several genes involved in the LC-PUFA and cholesterol biosynthetic pathways was found to be up-regulated in fish fed VD, suggesting a stimulation of the lipogenic pathways. No significant diet-family interaction for the regulation of LC-PUFA biosynthesis pathways could be detected by microarray analysis. This result was in agreement with LC-PUFA profiles, which were found to be similar in the flesh of the two half-sibfamilies. In addition, the combination of our transcriptomic data with an analysis of plasmatic immune parameters revealed a stimulation of complement activity associated with an immunodeficiency in the fish fed VD, and different inflammatory status between the two half-sibfamilies. Biological processes related to protein catabolism, amino acid transaminations, RNA splicing and blood coagulation were also found to be regulated by diet, while the expression of genes involved in protein and ATP synthesis differed between the half-sibfamilies.

Conclusions

Overall, the combined gene expression, compositional and biochemical studies demonstrated a large panel of metabolic and physiological effects induced by total substitution of both FM and FO in the diets of European sea bass and revealed physiological characteristics associated with the two half-sibfamilies.

The AMPK signalling pathway coordinates cell growth, autophagy and metabolism

One of the central regulators of cellular and organismal metabolism in eukaryotes is AMP-activated protein kinase (AMPK), which is activated when intracellular ATP production decreases. AMPK has critical roles in regulating growth and reprogramming metabolism, and has recently been connected to cellular processes such as autophagy and cell polarity. Here we review a number of recent breakthroughs in the mechanistic understanding of AMPK function, focusing on a number of newly identified downstream effectors of AMPK.

Fasted zebrafish mimic genetic and physiological responses in mammals: a model for obesity and diabetes?

With worldwide rates of obesity and type-II diabetes increasing, it is essential to identify and understand the mechanisms involved during nutrient absorption and fuel allocation. Recent studies demonstrate that nutrients (e.g., lipids and carbohydrates) play a major regulatory role in gene transcription of glycolytic and lipogenic enzymes in addition to hormones, including insulin and glucagon. These nutrients generally exert their effects through key cellular nutrient/energy receptors. Fasting was used to identify these nutrient/energy receptors known from mammalian studies to ascertain if zebrafish (Danio rerio) are a suitable model for the study of metabolic disorders. Zebrafish were subjected to a fasting/re-feeding regime for 3 weeks, and gene expression of sterol responsive binding protein 1 and 2 (SREBP), the mammalian target of rapamycin (mTOR), cAMP response element binding protein 3-like 3 (CREB3l3), and AMP-activated protein kinase alpha (AMPKα) was assessed. Fasted zebrafish lost ∼10% of their body mass over the 3-week experiment, with an associated depression in oxygen consumption. Increases in liver AMPKα and CREB3l3 mRNA transcript level were noted, concurrent with increases in the activities of the β-oxidation and gluconeogenic markers β-hydroxyacyl CoA dehydrogenase and phosphoenolpyruvate carboxykinase, respectively. Conversely, a depression in liver mTOR and SREBP1 and 2 expression was noted, with a decrease in pyruvate kinase and alanine aminotransferase activities and decreases in liver lipid and glycogen contents. Twenty-four hours after re-feeding, zebrafish rapidly recover, and the majority of parameters return to control values. Taken together, these data suggest adult zebrafish are an appropriate model for the further study of human metabolic disorders.

The link between fibroblast growth factor 21 and sterol regulatory element binding protein 1c during lipogenesis in hepatocytes

Recently fibroblast growth factor 21 (FGF21) has been identified as a potent regulator in glucose and lipid homeostasis. Here, we firstly investigated the metabolic role of FGF21 in human liver-derived HepG2 cells, and suggested that overexpression of FGF21 suppressed triglyceride accumulation by inhibiting the transcription of the gene necessary for de novo lipogenesis. The potential mechanism of FGF21 regulating lipogenesis was also explored, which revealed that FGF21 repressed the transcription of sterol regulatory element binding protein 1c (SREBP1c), an essential transcription factor promoting expression of lipogenesis-related genes. Overexpression of FGF21 ameliorated the up-regulation of SREBP1c and fatty acid synthase (FAS) in HepG2 cells elicited by FFAs treatment. Moreover, FGF21 could inhibit the transcriptional levels of the key genes involved in processing and nuclear translocation of SREBP1c, and decrease the protein amount of mature SREBP1c. Unexpectedly, overexpression of SREBP1c in HepG2 cells could also inhibit the endogenous FGF21 transcription. Further experiments demonstrated that SREBP1c could significantly attenuate the promoter activity of FGF21. In conclusion, our data identifies a clear link between FGF21 and SREBP1c during lipogenesis in hepatocyte in culture.

The Role of Hepatic Liver X Receptor α-and Sterol Regulatory Element Binding Protein-1c-Mediated Lipid Disorder in the Pathogenesis of Non-Alcoholic Steatohepatitis in Rats

Liver X receptor α (LXRα) and sterol regulatory element binding protein-1c (SREBP-1c) were studied in rats with non-alcoholic steatohepatitis (NASH) induced by a high-fat diet. Forty 5-week-old rats were fed either a high-fat diet ( n = 30) or a normal diet ( n = 10) for 9, 13 or 17 weeks. The mRNA and protein levels for LXRα and SREBP-1c were measured at each time point, as was fatty acid synthase (FAS) activity and the serum levels of free fatty acid (FFA) and triglyceride (TG). The mRNA and protein levels for LXRα and SREBP-1c, FAS activity and serum levels of FFA and TG all significantly increased from week 9 in the high-fat diet rats versus controls. In conclusion, a high-fat diet upregulates LXRα which, in turn, upregulates SREBP-1c, increasing the activity of FAS and FFA and accumulation of TG in hepatocytes. Thus, LXRα and SREBP-1c contribute to the development of NASH.

Differential effects of krill oil and fish oil on the hepatic transcriptome in mice

Dietary supplementation with ω-3 polyunsaturated fatty acids (ω-3 PUFAs), specifically the fatty acids docosahexaenoic acid (DHA; 22:6 ω-3) and eicosapentaenoic acid (EPA; 20:5 ω-3), is known to have beneficial health effects including improvements in glucose and lipid homeostasis and modulation of inflammation. To evaluate the efficacy of two different sources of ω-3 PUFAs, we performed gene expression profiling in the liver of mice fed diets supplemented with either fish oil (FO) or krill oil (KO). We found that ω-3 PUFA supplements derived from a phospholipid krill fraction (KO) downregulated the activity of pathways involved in hepatic glucose production as well as lipid and cholesterol synthesis. The data also suggested that KO-supplementation increases the activity of the mitochondrial respiratory chain. Surprisingly, an equimolar dose of EPA and DHA derived from FO modulated fewer pathways than a KO-supplemented diet and did not modulate key metabolic pathways regulated by KO, including glucose metabolism, lipid metabolism and the mitochondrial respiratory chain. Moreover, FO upregulated the cholesterol synthesis pathway, which was the opposite effect of krill-supplementation. Neither diet elicited changes in plasma levels of lipids, glucose, or insulin, probably because the mice used in this study were young and were fed a low-fat diet. Further studies of KO-supplementation using animal models of metabolic disorders and/or diets with a higher level of fat may be required to observe these effects.

High-fat diet causes increased serum insulin and glucose which synergistically lead to renal tubular lipid deposition and extracellular matrix accumulation

Renal tubular lipid accumulation is associated with renal injury in the metabolic syndrome, but its mechanisms are not fully elucidated. The purpose of the present study was to investigate the exact mechanism of renal tubular lipid accumulation in the diet-induced metabolic syndrome. The in vivo experiments showed that a high-fat diet induced hyperglycaemia, hyperinsulinaemia and hypertriacylglycerolaemia, subsequent increases in sterol regulatory element binding protein-1 (SREBP-1) and transforming growth factor-β1 (TGF-β1), lipid droplet deposit in renal tubular cells and interstitial extracellular matrix accumulation in Wistar rats. A human renal proximal tubular epithelial cell line (HKC) was used to determine the direct role of insulin, and the results revealed that insulin induced SREBP-1, fatty acid synthase (FASN), TGF-β1 expressions, lipid droplet and extracellular matrix deposits. Knockdown of SREBP-1 by RNA interference technology significantly inhibited FASN, TGF-β1 up-regulation, lipid and extracellular matrix accumulation caused by insulin. In addition, we found that insulin and high glucose could synergistically increase SREBP-1, FASN, TGF-β1 and fibronectin expressions in HKC cells. These results indicate that high-fat diet-induced increased serum insulin and glucose synergistically cause renal tubular lipid deposit and extracellular matrix accumulation via the SREBP-1 pathway.

Effects of xylitol on metabolic parameters and visceral fat accumulation

Xylitol is widely used as a sweetener in foods and medications. Xylitol ingestion causes a small blood glucose rise, and it is commonly used as an alternative to high-energy supplements in diabetics. In previous studies, a xylitol metabolite, xylulose-5-phosphate, was shown to activate carbohydrate response element binding protein, and to promote lipogenic enzyme gene transcription in vitro; however, the effects of xylitol in vivo are not understood. Here we investigated the effects of dietary xylitol on lipid metabolism and visceral fat accumulation in rats fed a high-fat diet. Sprague-Dawley rats were fed a high-fat diet containing 0 g (control), 1.0 g/100 kcal (X1) or 2.0 g/100 kcal (X2) of xylitol. After the 8-week feeding period, visceral fat mass and plasma insulin and lipid concentrations were significantly lower in xylitol-fed rats than those in high-fat diet rats. Gene expression levels of ChREBP and lipogenic enzymes were higher, whereas the expression of sterol regulatory-element binding protein 1c was lower and fatty acid oxidation-related genes were significantly higher in the liver of xylitol-fed rats as compared with high-fat diet rats. In conclusion, intake of xylitol may be beneficial in preventing the development of obesity and metabolic abnormalities in rats with diet-induced obesity.

O-GlcNAcylation increases ChREBP protein content and transcriptional activity in the liver

OBJECTIVE

Carbohydrate-responsive element-binding protein (ChREBP) is a key transcription factor that mediates the effects of glucose on glycolytic and lipogenic genes in the liver. We have previously reported that liver-specific inhibition of ChREBP prevents hepatic steatosis in ob/ob mice by specifically decreasing lipogenic rates in vivo. To better understand the regulation of ChREBP activity in the liver, we investigated the implication of O-linked β-N-acetylglucosamine (O-GlcNAc or O-GlcNAcylation), an important glucose-dependent posttranslational modification playing multiple roles in transcription, protein stabilization, nuclear localization, and signal transduction.

RESEARCH DESIGN AND METHODS

O-GlcNAcylation is highly dynamic through the action of two enzymes: the O-GlcNAc transferase (OGT), which transfers the monosaccharide to serine/threonine residues on a target protein, and the O-GlcNAcase (OGA), which hydrolyses the sugar. To modulate ChREBP(OG) in vitro and in vivo, the OGT and OGA enzymes were overexpressed or inhibited via adenoviral approaches in mouse hepatocytes and in the liver of C57BL/6J or obese db/db mice.

RESULTS

Our study shows that ChREBP interacts with OGT and is subjected to O-GlcNAcylation in liver cells. O-GlcNAcylation stabilizes the ChREBP protein and increases its transcriptional activity toward its target glycolytic (L-PK) and lipogenic genes (ACC, FAS, and SCD1) when combined with an active glucose flux in vivo. Indeed, OGT overexpression significantly increased ChREBP(OG) in liver nuclear extracts from fed C57BL/6J mice, leading in turn to enhanced lipogenic gene expression and to excessive hepatic triglyceride deposition. In the livers of hyperglycemic obese db/db mice, ChREBP(OG) levels were elevated compared with controls. Interestingly, reducing ChREBP(OG) levels via OGA overexpression decreased lipogenic protein content (ACC, FAS), prevented hepatic steatosis, and improved the lipidic profile of OGA-treated db/db mice.

CONCLUSIONS

Taken together, our results reveal that O-GlcNAcylation represents an important novel regulation of ChREBP activity in the liver under both physiological and pathophysiological conditions.

Screening and identification of differentially expressed genes in goose hepatocytes exposed to free fatty acid

The overaccumulation of triglycerides in hepatocytes induces hepatic steatosis; however, little is known about the mechanism of goose hepatic steatosis. The aim of this study was to define an experimental model of hepatocellular steatosis with TG overaccumulation and minimal cytotoxicity, using a mixture of various proportions of oleate and palmitate free fatty acids (FFAs) to induce fat-overloading, then using suppressive subtractive hybridization and a quantitative PCR approach to identify genes with higher or lower expression levels after the treatment of cells with FFA mixtures. Overall, 502 differentially expressed clones, representing 21 novel genes and 87 known genes, were detected by SSH. Based on functional clustering, up- and down-regulated genes were mostly related to carbohydrate and lipid metabolism, enzyme activity and signal transduction. The expression of 20 selected clones involved with carbohydrate and lipid metabolism pathways was further studied by quantitative PCR. The data indicated that six clones similar to the genes ChREBP, FoxO1, apoB, IHPK2, KIF1B, and FSP27, which participate in de novo synthesis of fatty acid and secretion of very low density lipoproteins, had significantly lower expression levels in the hepatocytes treated with FFA mixtures. Meanwhile, 13 clones similar to the genes DGAT-1, ACSL1, DHRS7, PPARα, L-FABP, DGAT-2, PCK, ACSL3, CPT-1, A-FABP, PPARβ, MAT, and ALDOB had significantly higher expression levels in the hepatocytes treated with FFA mixtures. These results suggest that several metabolic pathways are altered in goose hepatocytes, which may be useful for further research into the molecular mechanism of goose hepatic steatosis.