Hepatic FoxO1 Integrates Glucose Utilization and Lipid Synthesis through Regulation of Chrebp O-Glycosylation

In liver, glucose utilization and lipid synthesis are inextricably intertwined. When glucose availability exceeds its utilization, lipogenesis increases, leading to increased intrahepatic lipid content and lipoprotein secretion. Although the fate of three-carbon metabolites is largely determined by flux rate through the relevant enzymes, insulin plays a permissive role in this process. But the mechanism integrating insulin receptor signaling to glucose utilization with lipogenesis is unknown. Forkhead box O1 (FoxO1), a downstream effector of insulin signaling, plays a central role in hepatic glucose metabolism through the regulation of hepatic glucose production. In this study, we investigated the mechanism by which FoxO1 integrates hepatic glucose utilization with lipid synthesis. We show that FoxO1 overexpression in hepatocytes reduces activity of carbohydrate response element binding protein (Chrebp), a key regulator of lipogenesis, by suppressing O-linked glycosylation and reducing the protein stability. FoxO1 inhibits high glucose- or O-GlcNAc transferase (OGT)-induced liver-pyruvate kinase (L-PK) promoter activity by decreasing Chrebp recruitment to the L-PK promoter. Conversely, FoxO1 ablation in liver leads to the enhanced O-glycosylation and increased protein level of Chrebp owing to decreased its ubiquitination. We propose that FoxO1 regulation of Chrebp O-glycosylation is a mechanism linking hepatic glucose utilization with lipid synthesis.     

Hepatic Hdac3 promotes gluconeogenesis by repressing lipid synthesis and sequestration

Fatty liver disease is associated with obesity and type 2 diabetes, and hepatic lipid accumulation may contribute to insulin resistance. Histone deacetylase 3 (Hdac3) controls the circadian rhythm of hepatic lipogenesis. Here we show that, despite severe hepatosteatosis, mice with liver-specific depletion of Hdac3 have higher insulin sensitivity without any changes in insulin signaling or body weight compared to wild-type mice. Hdac3 depletion reroutes metabolic precursors towards lipid synthesis and storage within lipid droplets and away from hepatic glucose production. Perilipin 2, which coats lipid droplets, is markedly induced upon Hdac3 depletion and contributes to the development of both steatosis and improved tolerance to glucose. These findings suggest that the sequestration of hepatic lipids in perilipin 2–coated droplets ameliorates insulin resistance and establish Hdac3 as a pivotal epigenomic modifier that integrates signals from the circadian clock in the regulation of hepatic intermediary metabolism.     

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.     

Enhanced prereceptor glucocorticoid metabolism and lipogenesis impair insulin signaling in the liver of fructose-fed rats

Overconsumption of fructose, as a highly lipogenic sugar, may profoundly affect hepatic metabolism and has been associated with many components of the metabolic syndrome, particularly with insulin resistance and Type 2 diabetes. In this study, we proposed that high fructose diet may enhance lipogenesis and decrease insulin sensitivity in the liver through dysregulation of glucocorticoid signaling. Therefore, we examined the effects of long-term consumption of 10% fructose solution on triglyceridemia, liver histology and intracellular corticosterone level, as well as on 11β-hydroxysteroid dehydrogenase type 1 (11βHSD1) and hexose-6-phosphate dehydrogenase (H6PDH) mRNA and protein levels in the rat liver. Glucocorticoid action was assessed by glucocorticoid receptor (GR) expression and intracellular redistribution. We also analyzed the expression of enzymes involved in gluconeogenesis and lipogenesis, phosphoenolpyruvate carboxykinase (PEPCK) and lipin-1. The results have shown that fructose-rich diet led to increase in 11βHSD1 and H6PDH protein levels, while hepatic corticosterone concentration remained unchanged. Concomitantly, GR was increasingly accumulated in the cytoplasm, whereas its nuclear level was unchanged and accompanied by diminished PEPCK mRNA level. Elevation of lipin-1 in the liver microsomes suggested that fructose diet led to an increase in lipogenesis and consequently to hypertriglyceridemia. The observed increase of insulin receptor supstrate-1 phosphorylation on Ser³⁰⁷ represents a hallmark of impaired insulin signaling in the liver of fructose-fed rat and probably is a consequence of the alterations in 11βHSD1 and lipin-1 levels. Overall, our findings suggest that fructose-rich diet may perturb hepatic prereceptor glucocorticoid metabolism and lipogenesis, resulting in hypertriglyceridemia and attenuated hepatic insulin sensitivity.     

Irisin ameliorates hepatic glucose/lipid metabolism and enhances cell survival in insulin-resistant human HepG2 cells through adenosine monophosphate-activated protein kinase signaling

Irisin is a newly identified myokine that promotes the browning of white adipose tissue, enhances glucose uptake in skeletal muscle and modulates hepatic metabolism. However, the signaling pathways involved in the effects on hepatic glucose and lipid metabolism have not been resolved. This study aimed to examine the role of irisin in the regulation of hepatic glucose/lipid metabolism and cell survival, and whether adenosine monophosphate-activated protein kinase (AMPK), a master metabolic regulator in the liver, is involved in irisin’s actions. Human liver-derived HepG2 cells were cultured in normal glucose-normal insulin (NGNI) or high glucose-high insulin (HGHI/insulin-resistant) condition. Hepatic glucose and lipid metabolism was evaluated by glucose output and glycogen content or triglyceride accumulation assays, respectively. Our results showed that irisin stimulated phosphorylation of AMPK and acetyl-CoA-carboxylase (ACC) via liver kinase B1 (LKB1) rather than Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ) in HepG2 cells. Irisin ameliorated hepatic insulin resistance induced by HGHI condition. Irisin reduced hepatic triglyceride content and glucose output, but increased glycogen content, with those effects reversed by dorsomorphin, an AMPK inhibitor. Furthermore, irisin also stimulated extracellular signal-regulated kinase (ERK) 1/2 phosphorylation and promoted cell survival in an AMPK-dependent manner. In conclusion, our data indicate that irisin ameliorates dysregulation of hepatic glucose/lipid metabolism and cell death in insulin-resistant states via AMPK activation. These findings reveal a novel irisin-mediated protective mechanism in hepatic metabolism which provides a scientific basis for irisin as a potential therapeutic target for the treatment of insulin resistance and type 2 diabetes mellitus.     

Gut peptide and neuroendocrine regulation of hepatic lipid and lipoprotein metabolism in health and disease

Non-alcoholic fatty liver disease (NAFLD) is a continuum of disorders that can range from simple steatosis to non-alcoholic steatohepatitis (NASH). As a complex metabolic disorder, the pathophysiology of NAFLD is incompletely understood. Recently glucagon-like peptide (GLP)-1 and -2 signalling has been implicated in the pathogenesis of NAFLD. The role of these gut hormones in the hepatic abnormalities is complicated by lack of consensus on the presence of GLP-1 and GLP-2 receptors within the liver. Nevertheless, GLP-1 and GLP-2 receptor agonists have been associated with alterations in lipid metabolism and hepatic and systemic inflammation, pathological abnormalities characteristic of NAFLD. Treatment with GLP-1 analogues has been shown to reverse features of NAFLD including insulin resistance, and alterations in hepatic de novo lipogenesis and reactive oxygen species. In this review, we provide an overview of the role of GLP-1 and GLP-2 in lipid homeostasis and metabolic disease including NAFLD and NASH.     

Determination of Fatty Acid Oxidation and Lipogenesis in Mouse Primary Hepatocytes

Lipid metabolism in liver is complex. In addition to importing and exporting lipid via lipoproteins, hepatocytes can oxidize lipid via fatty acid oxidation, or alternatively, synthesize new lipid via de novo lipogenesis. The net sum of these pathways is dictated by a number of factors, which in certain disease states leads to fatty liver disease. Excess hepatic lipid accumulation is associated with whole body insulin resistance and coronary heart disease. Tools to study lipid metabolism in hepatocytes are useful to understand the role of hepatic lipid metabolism in certain metabolic disorders.In the liver, hepatocytes regulate the breakdown and synthesis of fatty acids via β-fatty oxidation and de novo lipogenesis, respectively. Quantifying metabolism in these pathways provides insight into hepatic lipid handling. Unlike in vitro quantification, using primary hepatocytes, making measurements in vivo is technically challenging and resource intensive. Hence, quantifying β-fatty acid oxidation and de novo lipogenesis in cultured mouse hepatocytes provides a straight forward method to assess hepatocyte lipid handling. Here we describe a method for the isolation of primary mouse hepatocytes, and we demonstrate quantification of β-fatty acid oxidation and de novo lipogenesis, using radiolabeled substrates.     

Liver Clock Protein BMAL1 Promotes de Novo Lipogenesis through Insulin-mTORC2-AKT Signaling

The clock protein BMAL1 (brain and muscle Arnt-like protein 1) participates in circadian regulation of lipid metabolism, but its contribution to insulin AKT-regulated hepatic lipid synthesis is unclear. Here we used both Bmal1^−/− and acute liver-specific Bmal1-depleted mice to study the role of BMAL1 in refeeding-induced de novo lipogenesis in the liver. Both global deficiency and acute hepatic depletion of Bmal1 reduced lipogenic gene expression in the liver upon refeeding. Conversely, Bmal1 overexpression in mouse liver by adenovirus was sufficient to elevate the levels of mRNA of lipogenic enzymes. Bmal1^−/− primary mouse hepatocytes displayed decreased levels of de novo lipogenesis and lipogenic enzymes, supporting the notion that BMAL1 regulates lipid synthesis in hepatocytes in a cell-autonomous manner. Both refed mouse liver and insulin-treated primary mouse hepatocytes showed impaired AKT activation in the case of either Bmal1 deficiency or Bmal1 depletion by adenoviral shRNA. Restoring AKT activity by a constitutively active mutant of AKT nearly normalized de novo lipogenesis in Bmal1^−/− hepatocytes. Finally, Bmal1 deficiency or knockdown decreased the protein abundance of RICTOR, the key component of the mTORC2 complex, without affecting the gene expression of key factors of insulin signaling. Thus, our study uncovered a novel metabolic function of hepatic BMAL1 that promotes de novo lipogenesis via the insulin-mTORC2-AKT signaling during refeeding.     

Long-term adjustment of hepatic lipid metabolism after chronic stress and the role of FGF21

Chronic stress leads to post-traumatic stress disorder (PTSD) and metabolic disorders including fatty liver. We hypothesized that stress-induced molecular mechanisms alter energy metabolism, thereby promoting hepatic lipid accumulation even after a stress-free recovery period. In this context, we investigated fibroblast growth factor-21 (FGF21) as protective for energy and glucose homeostasis. FGF21 knockout mice (B6.129S6(SJL)-Fgf21tm1.2Djm; FGF21KO) and control mice (C57BL6; WT) were subjected to chronic variable stress. Mice were examined directly after acute intervention (Cvs) and long-term after 3 months of recovery (3mCvs). In WT, Cvs reduced insulin sensitivity and hepatic lipid accumulation, whilst fatty acid uptake increased. FGF21KO mice responded to Cvs with improved glucose tolerance, insulin resistance but liver triglycerides and plasma lipids were unaltered. Hepatic gene expression was specifically altered by genotype and stress e.g. by PPARa and SREBP-1 regulated genes. The stress-induced alteration of hepatic metabolism persisted after stress recovery. In hepatocytes at 3mCvs, differential gene regulation and secreted proteins indicated a genotype specific progression of liver dysfunction. Overall, at 3mCvs FGF21 was involved in maintaining mitochondrial activity, attenuating de novo lipogenesis, increased fatty acid uptake and histone acetyltransferase activity. Glucocorticoid release and binding to the FGF21 promoter may contribute to prolonged FGF21 release and protection against hepatic lipid accumulation. In conclusion, we showed that stress favors fatty liver disease and FGF21 protected against hepatic lipid accumulation after previous chronic stress loading by i) restored physiological function, ii) modulated gene expression via DNA-modifying enzymes, and iii) maintained energy metabolism.     

Toll-like receptor 3 ablation prevented high-fat diet-induced obesity and metabolic disorder

Inflammation in insulin-sensitive tissues (e.g., liver, visceral adipose tissue [VAT]) plays a major role in obesity and insulin resistance. Recruitment of innate immune cells drives the dysregulation of glucose and lipid metabolism. We aimed to seek the role of Toll like receptor 3 (TLR3), a pattern recognition receptor involved in innate immunity, obesity and the metabolic disorder. TLR3 expression in liver and VAT from diet induced obese mice and in VAT from overweight women was examined. Body weight, glucose homeostasis and insulin sensitivity were evaluated in TLR3 wild-type and knockout (KO) mice on a chow diet (CD) or high-fat diet for 15 weeks. At euthanasia, blood was collected, and plasma biochemical parameters and adipokines were determined with commercial kits. Flow cytometry was used to measure macrophage infiltration and activation in VAT. Standard western blot, immunohistochemistry and quantative PCR were used to assess molecules in pathways about lipid and glucose metabolism, insulin and inflammation in tissues of liver and VAT. Utilizing human and animal samples, we found that expression of TLR3 was upregulated in the liver and VAT in obese mice as well as VAT in overweight women. TLR3-deficiency protected against high-fat diet induced obesity, glucose intolerance, insulin resistance and lipid accumulation. Lipolysis was enhanced in VAT and hepatic lipogenesis was inhibited in TLR3 KO animals. Macrophages infiltration into adipose tissue was attenuated in TLR3 KO mice, accompanied with inhibition of NF-κB-dependent AMPK/Akt signaling pathway. These findings demonstrated that TLR3 ablation prevented obesity and metabolic disorders, thereby providing new mechanistic links between inflammation and obesity and associated metabolic abnormalities in lipid/glucose metabolism.     

Azoxystrobin,a mitochondrial complex III Qo site inhibitor,exerts beneficial metabolic effects in vivo and in vitro

Background

Several anti-diabetes drugs exert beneficial effects against metabolic syndrome by inhibiting mitochondrial function. Although much progress has been made toward understanding the role of mitochondrial function inhibitors in treating metabolic diseases, the potential effects of these inhibitors on mitochondrial respiratory chain complex III remain unclear.

Methods

We investigated the metabolic effects of azoxystrobin (AZOX), a Q_o inhibitor of complex III, in a high-fat diet-fed mouse model with insulin resistance in order to elucidate the mechanism by which AZOX improves glucose and lipid metabolism at the metabolic cellular level.

Results

Acute administration of AZOX in mice increased the respiratory exchange ratio. Chronic treatment with AZOX reduced body weight and significantly improved glucose tolerance and insulin sensitivity in high-fat diet-fed mice. AZOX treatment resulted in decreased triacylglycerol accumulation and down-regulated the expression of genes involved in liver lipogenesis. AZOX increased glucose uptake in L6 myotubes and 3T3-L1 adipocytes and inhibited de novo lipogenesis in HepG2 cells. The findings indicate that AZOX-mediated alterations to lipid and glucose metabolism may depend on AMP-activated protein kinase (AMPK) signaling.

Conclusions

AZOX, a Q_o inhibitor of mitochondrial respiratory complex III, exerts whole-body beneficial effects on the regulation of glucose and lipid homeostasis in high-fat diet-fed mice.

General significance

These findings provide evidence that a Q_o inhibitor of mitochondrial respiratory complex III could represent a novel approach for the treatment of obesity.     

Programming of Obesity and Comorbidities in the Progeny: Lessons from a Model of Diet-Induced Obese Parents

Aim

To determine the impact of paternal obesity, maternal obesity or the combination of two obese parents on markers of adult offspring metabolism, with a focus on body mass (BM), lipid and carbohydrate, components of lipogenesis and beta-oxidation in the liver, sex dimorphism in the offspring that received a SC diet during the postnatal period.

Materials and Methods

Male and female C57BL/6 mice were fed a high-fat diet (HF; 49% lipids) or standard chow (SC; 17% lipids) for 8 weeks before mating until lactation. The offspring were labeled according to sex, maternal diet (first letters), paternal diet (second letters), and received a SCdiet until 12-weeks of age when they were sacrificed. BM, eating behavior, glucose tolerance, plasma analysis, gene and protein expression of the components of lipogenesis and beta-oxidation in the liver of offspring were evaluated.

Results

HF diet-fed mothers and fathers were overweight, hyperglycemic and glucose intolerant and had a deteriorating lipid profile. The adult male and female offspring of HF-mothers were overweight, with an increased adiposity index, hyperphagic, had an impaired glucose metabolism, increased total cholesterol and triacylglycerol levels, increased lipogenesis concomitant with decreased beta-oxidation resulting in liver steatosis. The male and female offspring of HF-father had impaired glucose metabolism, exacerbated lipogenesis without influencing beta-oxidation and enhanced hepatic steatosis. These findings are independent of BM. Male and female offspring of a mother and father that received a HF diet demonstrated these effects most prominently in adult life.

Conclusion

Paternal obesity leads to alterations in glucose metabolism, increase in components of lipogenesis and liver steatosis. In contrast, maternal obesity leads to overweight and changes in the metabolic profile and liver resulting from activation of hepatic lipogenesis with impaired beta-oxidation. When both parents are obese, the effects observed in the male and female offspring are exacerbated.     

Chromium attenuates high-fat diet-induced nonalcoholic fatty liver disease in KK/HlJ mice

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease associated with insulin resistance, oxidative stress, and inflammation. Evidence indicates that chromium has a role in the regulation of glucose and lipid metabolism and may improve insulin sensitivity. In this study, we report that chromium supplementation has a beneficial effect against NAFLD. We found that KK/HlJ mice developed obesity and progressed to NAFLD after feeding with high-fat diet for 8 weeks. High-fat-fed KK/HlJ mice showed hepatocyte injury and hepatic triglyceride accumulation, which was accompanied by insulin resistance, oxidative stress, and inflammation. Chromium supplementation prevented progression of NAFLD and the beneficial effects were accompanied by reduction of hepatic triglyceride accumulation, elevation of hepatic lipid catabolic enzyme, improvement of glucose and lipid metabolism, suppression of inflammation as well as resolution of oxidative stress, probably through enhancement of insulin signaling. Our findings suggest that chromium could serve as a hepatoprotective agent against NAFLD.     

Dietary citrate acutely induces insulin resistance and markers of liver inflammation in mice

Citrate is widely used as a food additive being part of virtually all processed foods. Although considered inert by most of the regulatory agencies in the world, plasma citrate has been proposed to play immunometabolic functions in multiple tissues through altering a plethora of cellular pathways. Here, we used a short-term alimentary intervention (24 hours) with standard chow supplemented with citrate in amount corresponding to that found in processed foods to evaluate its effects on glucose homeostasis and liver physiology in C57BL/6J mice. Animals supplemented with dietary citrate showed glucose intolerance and insulin resistance as revealed by glucose and insulin tolerance tests. Moreover, animals supplemented with citrate in their food displayed fed and fasted hyperinsulinemia and enhanced insulin secretion during an oral glucose tolerance test. Citrate treatment also amplified glucose-induced insulin secretion in vitro in INS1-E cells. Citrate supplemented animals had increased liver PKCα activity and altered phosphorylation at serine or threonine residues of components of insulin signaling including IRS-1, Akt, GSK-3 and FoxO1. Furthermore, citrate supplementation enhanced the hepatic expression of lipogenic genes suggesting increased de novo lipogenesis, a finding that was reproduced after citrate treatment of hepatic FAO cells. Finally, liver inflammation markers were higher in citrate supplemented animals. Overall, the results demonstrate that dietary citrate supplementation in mice causes hyperinsulinemia and insulin resistance both in vivo and in vitro, and therefore call for a note of caution on the use of citrate as a food additive given its potential role in metabolic dysregulation.     

Reduced biliverdin reductase-A levels are associated with early alterations of insulin signaling in obesity

Biliverdin reductase-A (BVR-A) is a serine/threonine/tyrosine kinase involved in the regulation of insulin signaling. In vitro studies have demonstrated that BVR-A is a substrate of the insulin receptor and regulates IRS1 by avoiding its aberrant activation, and in animal model of obesity the loss of hepatic BVR-A has been associated with glucose/insulin alterations and fatty liver disease. However, no studies exist in humans. Here, we evaluated BVR-A expression levels and activation in peripheral blood mononuclear cells (PBMC) from obese subjects and matched lean controls and we investigated the related molecular alterations of the insulin along with clinical correlates. We showed that BVR-A levels are significantly reduced in obese subjects and associated with a hyper-activation of the IR/IRS1/Akt/GSK-3β/AS160/GLUT4 pathway. Low BVR-A levels also associate with the presence of obesity, metabolic syndrome, NASH and visceral adipose tissue inflammation. These data suggest that the reduction of BVR-A may be responsible for early alterations of the insulin signaling pathway in obesity and in this context may represent a novel molecular target to be investigated for the comprehension of the process of insulin resistance development in obesity.     

Insulin resistance after a 72-h fast is associated with impaired AS160 phosphorylation and accumulation of lipid and glycogen in human skeletal muscle

During fasting, human skeletal muscle depends on lipid oxidation for its energy substrate metabolism. This is associated with the development of insulin resistance and a subsequent reduction of insulin-stimulated glucose uptake. The underlying mechanisms controlling insulin action on skeletal muscle under these conditions are unresolved. In a randomized design, we investigated eight healthy subjects after a 72-h fast compared with a 10-h overnight fast. Insulin action on skeletal muscle was assessed by a hyperinsulinemic euglycemic clamp and by determining insulin signaling to glucose transport. In addition, substrate oxidation, skeletal muscle lipid content, regulation of glycogen synthesis, and AMPK signaling were assessed. Skeletal muscle insulin sensitivity was reduced profoundly in response to a 72-h fast and substrate oxidation shifted to predominantly lipid oxidation. This was associated with accumulation of both lipid and glycogen in skeletal muscle. Intracellular insulin signaling to glucose transport was impaired by regulation of phosphorylation at specific sites on AS160 but not TBC1D1, both key regulators of glucose uptake. In contrast, fasting did not impact phosphorylation of AMPK or insulin regulation of Akt, both of which are established upstream kinases of AS160. These findings show that insulin resistance in muscles from healthy individuals is associated with suppression of site-specific phosphorylation of AS160, without Akt or AMPK being affected. This impairment of AS160 phosphorylation, in combination with glycogen accumulation and increased intramuscular lipid content, may provide the underlying mechanisms for resistance to insulin in skeletal muscle after a prolonged fast.     

Loss of insulin signaling in hepatocytes leads to severe insulin resistance and progressive hepatic dysfunction

The liver plays a central role in the control of glucose homeostasis and is subject to complex regulation by substrates, insulin, and other hormones. To investigate the effect of the loss of direct insulin action in liver, we have used the Cre-loxP system to inactivate the insulin receptor gene in hepatocytes. Liver-specific insulin receptor knockout (LIRKO) mice exhibit dramatic insulin resistance, severe glucose intolerance, and a failure of insulin to suppress hepatic glucose production and to regulate hepatic gene expression. These alterations are paralleled by marked hyperinsulinemia due to a combination of increased insulin secretion and decreased insulin clearance. With aging, the LIRKO liver exhibits morphological and functional changes, and the metabolic phenotype becomes less severe. Thus, insulin signaling in liver is critical in regulating glucose homeostasis and maintaining normal hepatic function.