Cidea promotes hepatic steatosis by sensing dietary fatty acids

Apigenin Ameliorates Dyslipidemia, Hepatic Steatosis and Insulin Resistance by Modulating Metabolic and Transcriptional Profiles in the Liver of High-Fat Diet-Induced Obese Mice

Several in vitro and in vivo studies have reported the anti-inflammatory, anti-diabetic and anti-obesity effects of the flavonoid apigenin. However, the long-term supplementary effects of low-dose apigenin on obesity are unclear. Therefore, we investigated the protective effects of apigenin against obesity and related metabolic disturbances by exploring the metabolic and transcriptional responses in high-fat diet (HFD)-induced obese mice. C57BL/6J mice were fed an HFD or apigenin (0.005%, w/w)-supplemented HFD for 16 weeks. In HFD-fed mice, apigenin lowered plasma levels of free fatty acid, total cholesterol, apolipoprotein B and hepatic dysfunction markers and ameliorated hepatic steatosis and hepatomegaly, without altering food intake and adiposity. These effects were partly attributed to upregulated expression of genes regulating fatty acid oxidation, tricarboxylic acid cycle, oxidative phosphorylation, electron transport chain and cholesterol homeostasis, downregulated expression of lipolytic and lipogenic genes and decreased activities of enzymes responsible for triglyceride and cholesterol ester synthesis in the liver. Moreover, apigenin lowered plasma levels of pro-inflammatory mediators and fasting blood glucose. The anti-hyperglycemic effect of apigenin appeared to be related to decreased insulin resistance, hyperinsulinemia and hepatic gluconeogenic enzymes activities. Thus, apigenin can ameliorate HFD-induced comorbidities via metabolic and transcriptional modulations in the liver.

Ces3/TGH Deficiency Attenuates Steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease in developed countries. NAFLD describes a wide range of liver pathologies from simple steatosis to nonalcoholic steatohepatitis (NASH) and cirrhosis. NASH is distinguished from simple steatosis by inflammation, cell death and fibrosis. In this study we found that mice lacking triacylglycerol hydrolase (TGH, also known as carboxylesterase 3 or carboxylesterase 1d) are protected from high-fat diet (HFD) - induced hepatic steatosis via decreased lipogenesis, increased fatty acid oxidation and improved hepatic insulin sensitivity. To examine the effect of the loss of TGH function on the more severe NAFLD form NASH, we ablated Tgh expression in two independent NASH mouse models, Pemt(-/-) mice fed HFD and Ldlr(-/-) mice fed high-fat, high-cholesterol Western-type diet (WTD). TGH deficiency reduced liver inflammation, oxidative stress and fibrosis in Pemt(-/-) mice. TGH deficiency also decreased NASH in Ldlr(-/-) mice. Collectively, these findings indicate that TGH deficiency attenuated both simple hepatic steatosis and irreversible NASH.

Differential Roles of Cell Death-inducing DNA Fragmentation Factor-α-like Effector (CIDE) Proteins in Promoting Lipid Droplet Fusion and Growth in Subpopulations of Hepatocytes

Lipid droplets (LDs) are dynamic subcellular organelles whose growth is closely linked to obesity and hepatic steatosis. Cell death-inducing DNA fragmentation factor-α-like effector (CIDE) proteins, including Cidea, Cideb, and Cidec (also called Fsp27), play important roles in lipid metabolism. Cidea and Cidec are LD-associated proteins that promote atypical LD fusion in adipocytes. Here, we find that CIDE proteins are all localized to LD-LD contact sites (LDCSs) and promote lipid transfer, LD fusion, and growth in hepatocytes. We have identified two types of hepatocytes, one with small LDs (small LD-containing hepatocytes, SLHs) and one with large LDs (large LD-containing hepatocytes, LLHs) in the liver. Cideb is localized to LDCSs and promotes lipid exchange and LD fusion in both SLHs and LLHs, whereas Cidea and Cidec are specifically localized to the LDCSs and promote lipid exchange and LD fusion in LLHs. Cideb-deficient SLHs have reduced LD sizes and lower lipid exchange activities. Fasting dramatically induces the expression of Cidea/Cidec and increases the percentage of LLHs in the liver. The majority of the hepatocytes from the liver of obese mice are Cidea/Cidec-positive LLHs. Knocking down Cidea or Cidec significantly reduced lipid storage in the livers of obese animals. Our data reveal that CIDE proteins play differential roles in promoting LD fusion and lipid storage; Cideb promotes lipid storage under normal diet conditions, whereas Cidea and Cidec are responsible for liver steatosis under fasting and obese conditions.

The brown adipocyte protein CIDEA promotes lipid droplet fusion via a phosphatidic acid-binding amphipathic helix

Maintenance of energy homeostasis depends on the highly regulated storage and release of triacylglycerol primarily in adipose tissue, and excessive storage is a feature of common metabolic disorders. CIDEA is a lipid droplet (LD)-protein enriched in brown adipocytes promoting the enlargement of LDs, which are dynamic, ubiquitous organelles specialized for storing neutral lipids. We demonstrate an essential role in this process for an amphipathic helix in CIDEA, which facilitates embedding in the LD phospholipid monolayer and binds phosphatidic acid (PA). LD pairs are docked by CIDEA trans-complexes through contributions of the N-terminal domain and a C-terminal dimerization region. These complexes, enriched at the LD–LD contact site, interact with the cone-shaped phospholipid PA and likely increase phospholipid barrier permeability, promoting LD fusion by transference of lipids. This physiological process is essential in adipocyte differentiation as well as serving to facilitate the tight coupling of lipolysis and lipogenesis in activated brown fat.

DOI:http://dx.doi.org/10.7554/eLife.07485.001

If other energy sources become unavailable, cells fall back on stores of fatty molecules called lipids. These are held in membrane-enclosed compartments in the cell called lipid droplets, which in mammals are particularly abundant in fat cells called adipocytes. There are two main types of adipocytes: white adipocytes have a single giant lipid droplet, whereas brown adipocytes contain many smaller droplets.

Proteins embedded in the membrane that surrounds a lipid droplet help to control the droplet’s growth and when it releases lipids. For example, a protein called CIDEA, which is only found in brown adipocytes, helps lipid droplets to grow by enabling one droplet to transfer its contents to another droplet. However, little is known about how this occurs.

By combining cell biology, biophysical and computer modelling approaches, Barneda et al. investigated how normal and mutant forms of CIDEA affect the growth of lipid droplets. These experiments identified a helix in the structure of CIDEA that embeds it in the membrane, from where it can then interact with CIDEA proteins on other lipid droplets to hold the droplets together. In addition, the helix interacts with a molecule in the lipid droplet membrane called phosphatidic acid. Barneda et al. suggest that this interaction helps to transfer the contents of one droplet to another by making it easier for lipids to move through the droplets’ membranes.

The next challenge is to characterize the mechanisms that control CIDEA activity to influence the formation of the multiple lipid droplets that distinguish brown and BRITE (brown-in-white) adipocytes from white adipocytes. The lipid droplets in brown adipocytes are an important target for research to combat obesity, due to the 'burning' rather than storing of lipids that occurs in these cells.

DOI:http://dx.doi.org/10.7554/eLife.07485.002

IL-25 or IL-17E Protects against High-Fat Diet-Induced Hepatic Steatosis in Mice Dependent upon IL-13 Activation of STAT6

IL-25 or IL-17E is a member of IL-17 cytokine family and has immune-modulating activities. The role of IL-25 in maintaining lipid metabolic homeostasis remains unknown. We investigated the effects of exogenous IL-25 or deficiency of IL-25 on hepatic lipid accumulation. IL-25 expression was examined in paraffin-embedded tissue sections of liver from patients or in the livers from mice. Mouse model of steatosis was induced by feeding a high-fat diet (HFD). Extent of steatosis as well as expression of cytokines, key enzymes for lipid metabolic pathways, markers for Kupffer cells/macrophages, and lipid droplet (LD) proteins, were analyzed. Our results show that hepatic steatosis in mice was accompanied by increased LD proteins, but decreased IL-25 in the liver. Decreased hepatic IL-25 was also observed in patients with fatty liver. Administration of IL-25 to HFD-fed wild-type mice led to a significant improvement in hepatic steatosis. This effect was associated with increased expression of IL-13, development of alternatively activated Kupffer cells/macrophages, and decreased expression of LD proteins in the liver. In contrast, administration of IL-25 to HFD-fed mice deficient in STAT6 or IL-13 had no effects. In addition, stimulation of primary hepatocytes with IL-13, but not IL-25, resulted in downregulation of LD proteins. Finally, mice deficient in IL-25 had exacerbated hepatic lipid accumulation when fed the HFD. These data demonstrate that dysregulated IL-25 expression contributes to lipid accumulation, whereas exogenous IL-25 protects against hepatic steatosis through IL-13 activation of STAT6. IL-25 and IL-13 are potential therapeutic agents for hepatic steatosis and associated pathologies.

Berry intake changes hepatic gene expression and DNA methylation patterns associated with high-fat diet

The liver is a critical organ for regulation of energy homeostasis and fatty liver disease is closely associated with obesity and insulin resistance. We have previously found that lingonberries, blackcurrants and bilberries prevent, whereas açai berries exacerbate, the development of hepatic steatosis and obesity in the high-fat (HF)-fed C57BL/6J mouse model. In this follow-up study, we investigated the mechanisms behind these effects. Genome-wide hepatic gene expression profiling indicates that the protective effects of lingonberries and bilberries are accounted for by several-fold downregulation of genes involved in acute-phase and inflammatory pathways (e.g. Saa1, Cxcl1, Lcn2). In contrast, açai-fed mice exhibit marked upregulation of genes associated with steatosis (e.g. Cfd, Cidea, Crat) and lipid and cholesterol biosynthesis, which is in line with the exacerbation of HF-induced hepatic steatosis in these mice. In silico transcription factor analysis together with immunoblot analysis identified NF-κB, STAT3 and mTOR as upstream regulators involved in mediating the observed transcriptional effects. To gain further insight into mechanisms involved in the gene expression changes, the HELP-tagging assay was used to identify differentially methylated CpG sites. Compared to the HF control group, lingonberries induced genome-wide hypermethylation and specific hypermethylation of Ncor2, encoding the corepressor NCoR/SMRT implicated in the regulation of pathways of metabolic homeostasis and inflammation. We conclude that the beneficial metabolic effects of lingonberries and bilberries are associated with downregulation of inflammatory pathways, whereas for blackcurrants, exerting similar metabolic effects, different mechanisms of action appear to dominate. NF-κB, STAT3 and mTOR are potential targets of the health-promoting effects of berries.

Soybean Oil Is More Obesogenic and Diabetogenic than Coconut Oil and Fructose in Mouse: Potential Role for the Liver

The obesity epidemic in the U.S. has led to extensive research into potential contributing dietary factors, especially fat and fructose. Recently, increased consumption of soybean oil, which is rich in polyunsaturated fatty acids (PUFAs), has been proposed to play a causal role in the epidemic. Here, we designed a series of four isocaloric diets (HFD, SO-HFD, F-HFD, F-SO-HFD) to investigate the effects of saturated versus unsaturated fat, as well as fructose, on obesity and diabetes. C57/BL6 male mice fed a diet moderately high in fat from coconut oil and soybean oil (SO-HFD, 40% kcal total fat) showed statistically significant increases in weight gain, adiposity, diabetes, glucose intolerance and insulin resistance compared to mice on a diet consisting primarily of coconut oil (HFD). They also had fatty livers with hepatocyte ballooning and very large lipid droplets as well as shorter colonic crypt length. While the high fructose diet (F-HFD) did not cause as much obesity or diabetes as SO-HFD, it did cause rectal prolapse and a very fatty liver, but no balloon injury. The coconut oil diet (with or without fructose) increased spleen weight while fructose in the presence of soybean oil increased kidney weight. Metabolomics analysis of the liver showed an increased accumulation of PUFAs and their metabolites as well as γ-tocopherol, but a decrease in cholesterol in SO-HFD. Liver transcriptomics analysis revealed a global dysregulation of cytochrome P450 (Cyp) genes in SO-HFD versus HFD livers, most notably in the Cyp3a and Cyp2c families. Other genes involved in obesity (e.g., Cidec, Cd36), diabetes (Igfbp1), inflammation (Cd63), mitochondrial function (Pdk4) and cancer (H19) were also upregulated by the soybean oil diet. Taken together, our results indicate that in mice a diet high in soybean oil is more detrimental to metabolic health than a diet high in fructose or coconut oil.

SREBP-1c overactivates ROS-mediated hepatic NF-κB inflammatory pathway in dairy cows with fatty liver

Dairy cows with fatty liver are characterized by hepatic lipid accumulation and a severe inflammatory response. Sterol receptor element binding protein-1c (SREBP-1c) and nuclear factor κB (NF-κB) are components of the main pathways for controlling triglyceride (TG) accumulation and inflammatory levels, respectively. A previous study demonstrated that hepatic inflammatory levels are positively correlated with hepatic TG content. We therefore speculated that SREBP-1c might play an important role in the overactivation of the hepatic NF-κB inflammatory pathway in cows with fatty liver. Compared with healthy cows, cows with fatty liver exhibited severe hepatic injury and high blood concentrations of the inflammatory cytokines TNF-α, IL-6 and IL-1β. Hepatic SREBP-1c-mediated lipid synthesis and the NF-κB inflammatory pathway were both overinduced in cows with fatty liver. In vitro, treatment with non-esterified fatty acids (NEFA) further increased SREBP-1c expression and NF-κB pathway activation, which then promoted TG and inflammatory cytokine synthesis. SREBP-1c overexpression overactivated the NF-κB inflammatory pathway in hepatocytes by increasing ROS content and not through TLR4. Furthermore, SREBP-1c silencing decreased ROS content and further attenuated the activation of the NEFA-induced NF-κB pathway, thereby decreasing TNF-α, IL-6 and IL-1β synthesis. SREBP-1c-overexpressing mice exhibited hepatic steatosis and an overinduced hepatic NF-κB pathway. Taken together, these results indicate that SREBP-1c enhances the NEFA-induced overactivation of the NF-κB inflammatory pathway by increasing ROS in cow hepatocytes, thereby further increasing hepatic inflammatory injury in cows with fatty liver.

Effects of a High Fat Diet and Voluntary Wheel Running Exercise on Cidea and Cidec Expression in Liver and Adipose Tissue of Mice

Cidea and Cidec play an important role in regulating triglyceride storage in liver and adipose tissue. It is not known if the Cidea and Cidec genes respond to a high fat diet (HFD) or exercise training, two interventions that alter lipid storage. The purpose of the present study was to determine the effect of a HFD and voluntary wheel running (WR) on Cidea and Cidec mRNA and protein expression in adipose tissue and liver of mice. A HFD promoted a significant increase in Cidea and Cidec mRNA levels in adipose tissue and liver. The increase in Cidea and Cidec mRNAs in adipose tissue and liver in response to a HFD was prevented by WR. Similar to the changes in Cidea mRNA, Cidea protein levels in adipose tissue significantly increased in response to a HFD, a process that was, again, prevented by WR. However, in adipose tissue the changes in Cidec mRNA did not correspond to the changes in Cidec protein levels, as a HFD decreased Cidec protein abundance. Interestingly, in adipose tissue Cidea protein expression was significantly related to body weight (R=.725), epididymal adipose tissue (EWAT) mass (R=.475) and insulin resistance (R=.706), whereas Cidec protein expression was inversely related to body weight (R=-.787), EWAT mass (R=-.706), and insulin resistance (R=-.679). Similar to adipose tissue, Cidea protein expression in liver was significantly related to body weight (R=.660), EWAT mass (R=.468), and insulin resistance (R=.599); however, unlike adipose tissue, Cidec protein levels in liver were not related to body weight or EWAT mass and only moderately associated with insulin resistance (R=-.422, P=0.051). Overall, our findings indicate that Cidea is highly associated with adiposity and insulin resistance, whereas Cidec is related to insulin sensitivity. The present study suggests that Cide proteins might play an important functional role in the development of obesity, hepatic steatosis, as well as the pathogenesis of type 2 diabetes.

Coordinated regulation of scopoletin at adipose tissue-liver axis improved alcohol-induced lipid dysmetabolism and inflammation in rats

There is increasing evidence that alcohol-induced white adipose tissue (WAT) dysfunction contributes to disturbance of hepatic lipid metabolism. This study investigated the effects of scopoletin on lipid homeostasis and inflammation at the WAT and liver in chronic alcohol-fed rats. Rats were fed a liquid diet containing 5% alcohol with or without two doses of scopoletin (0.001% and 0.005%) for 8 weeks. Scopoletin decreased serum triglyceride and cytokines (TNFα and IL-6) levels and hepatic and WAT lipid levels, whereas it increased WAT adiponectin mRNA and serum adiponectin levels, up-regulated hepatic gene and protein expression of AdipoR2 and activated AMPK. Additionally, scopoletin inhibited the expression of lipogenic genes (SREBP-1c and Fasn) and increased the expression of fatty acid oxidative genes (PPARα, Acsl1, CPT, Acox, and Acaa1a) in both WAT and liver. Alcohol led to significant up-regulation of WAT lipolysis and hepatic Cidea gene expression, whereas it decreased the WAT Cidea gene level; however, scopoletin reversed these changes. Scopoletin significantly down-regulated TLR4 signaling genes such as MyD88, TRIF, NFκB, TNFα and IL-6 in WAT and liver. These results indicated that coordinated regulation of scopoletin at the WAT-liver axis may play an important role in improvement of alcohol-induced lipid dysregulation and inflammation.

Luteolin attenuates hepatic steatosis and insulin resistance through the interplay between the liver and adipose tissue in mice with diet-induced obesity

The flavonoid luteolin has various pharmacological activities. However, few studies exist on the in vivo mechanism underlying the actions of luteolin in hepatic steatosis and obesity. The aim of the current study was to elucidate the action of luteolin on obesity and its comorbidity by analyzing its transcriptional and metabolic responses, in particular the luteolin-mediated cross-talk between liver and adipose tissue in diet-induced obese mice. C57BL/6J mice were fed a normal, high-fat, and high-fat + 0.005% (weight for weight) luteolin diet for 16 weeks. In high fat-fed mice, luteolin improved hepatic steatosis by suppressing hepatic lipogenesis and lipid absorption. In adipose tissue, luteolin increased PPARγ protein expression to attenuate hepatic lipotoxicity, which may be linked to the improvement in circulating fatty acid (FA) levels by enhancing FA uptake genes and lipogenic genes and proteins in adipose tissue. Interestingly, luteolin also upregulated the expression of genes controlling lipolysis and the tricarboxylic acid (TCA) cycle prior to lipid droplet formation, thereby reducing adiposity. Moreover, luteolin improved hepatic insulin sensitivity by suppressing SREBP1 expression that modulates Irs2 expression through its negative feedback and gluconeogenesis. Luteolin ameliorates the deleterious effects of diet-induced obesity and its comorbidity via the interplay between liver and adipose tissue.

The Lipid Droplet Protein Hypoxia-inducible Gene 2 Promotes Hepatic Triglyceride Deposition by Inhibiting Lipolysis

The liver is a major site of glucose, fatty acid, and triglyceride (TG) synthesis and serves as a major regulator of whole body nutrient homeostasis. Chronic exposure of humans or rodents to high-calorie diets promotes non-alcoholic fatty liver disease, characterized by neutral lipid accumulation in lipid droplets (LD) of hepatocytes. Here we show that the LD protein hypoxia-inducible gene 2 (Hig2/Hilpda) functions to enhance lipid accumulation in hepatocytes by attenuating TG hydrolysis. Hig2 expression increased in livers of mice on a high-fat diet and during fasting, two states associated with enhanced hepatic TG content. Hig2 expressed in primary mouse hepatocytes localized to LDs and promoted LD TG deposition in the presence of oleate. Conversely, tamoxifen-inducible Hig2 deletion reduced both TG content and LD size in primary hepatocytes from mice harboring floxed alleles of Hig2 and a cre/ERT2 transgene controlled by the ubiquitin C promoter. Hepatic TG was also decreased by liver-specific deletion of Hig2 in mice with floxed Hig2 expressing cre controlled by the albumin promoter. Importantly, we demonstrate that Hig2-deficient hepatocytes exhibit increased TG lipolysis, TG turnover, and fatty acid oxidation as compared with controls. Interestingly, mice with liver-specific Hig2 deletion also display improved glucose tolerance. Taken together, these data indicate that Hig2 plays a major role in promoting lipid sequestration within LDs in mouse hepatocytes through a mechanism that impairs TG degradation.

CIDEC/FSP27 is regulated by peroxisome proliferator-activated receptor alpha and plays a critical role in fasting- and diet-induced hepatosteatosis

The cell death-inducing DNA fragmentation factor alpha-like effector c (CIDEC; also known in rodents as FSP27 or fat-specific protein 27) is a lipid droplet-associated protein that promotes intracellular triglyceride (TAG) storage. CIDEC/Fsp27 is highly expressed in adipose tissue, but undetectable in normal liver. However, its hepatic expression rises during fasting or under genetic or diet-induced hepatosteatosis in both mice and patients. Herein, we demonstrate that CIDEC/Fsp27 is a direct transcriptional target of the nuclear receptor PPARα (peroxisome proliferator-activated receptor alpha) in both mouse and human hepatocytes, and that preventing Fsp27 induction accelerates PPARα-stimulated fatty acid oxidation. We show that adenoviral-mediated silencing of hepatic Fsp27 abolishes fasting-induced liver steatosis in the absence of changes in plasma lipids. Finally, we report that anti-Fsp27 short hairpin RNA and PPARα agonists synergize to ameliorate hepatosteatosis in mice fed a high fat diet.

CONCLUSIONS

Together, our data highlight the physiological importance of CIDEC/Fsp27 in TAG homeostasis under both physiological and pathological liver steatosis. Our results also suggest that patients taking fibrates likely have elevated levels of hepatic CIDEC, which may limit the efficient mobilization and catabolism of hepatic TAGs.

A weekly alternating diet between caloric restriction and medium fat protects the liver from fatty liver development in middle-aged C57BL/6J mice

Scope

We investigated whether a novel dietary intervention consisting of an every-other-week calorie-restricted diet could prevent nonalcoholic fatty liver disease (NAFLD) development induced by a medium-fat (MF) diet.

Methods and results

Nine-week-old male C57BL/6J mice received either a (i) control (C), (ii) 30E% calorie restricted (CR), (iii) MF (25E% fat), or (iv) intermittent (INT) diet, a diet alternating weekly between 40E% CR and an ad libitum MF diet until sacrifice at the age of 12 months. The metabolic, morphological, and molecular features of NAFLD were examined. The INT diet resulted in healthy metabolic and morphological features as displayed by the continuous CR diet: glucose tolerant, low hepatic triglyceride content, low plasma alanine aminotransferase. In contrast, the C- and MF-exposed mice with high body weight developed signs of NAFLD. However, the gene expression profiles of INT-exposed mice differed to those of CR-exposed mice and showed to be more similar with those of C- and MF-exposed mice with a comparable body weight.

Conclusions

Our study reveals that the INT diet maintains metabolic health and reverses the adverse effects of the MF diet, thus effectively prevents the development of NAFLD in 12-month-old male C57BL/6J mice.

Rorα deficiency and decreased adiposity are associated with induction of thermogenic gene expression in subcutaneous white adipose and brown adipose tissue

The Rar-related orphan receptor-α (Rorα) is a nuclear receptor that regulates adiposity and is a potential regulator of energy homeostasis. We have demonstrated that the Rorα-deficient staggerer (sg/sg) mice display a lean and obesity-resistant phenotype. Adaptive Ucp1-dependent thermogenesis in beige/brite and brown adipose tissue serves as a mechanism to increase energy expenditure and resist obesity. DEXA and MRI analysis demonstrated significantly decreased total fat mass and fat/lean mass tissue ratio in male chow-fed sg/sg mice relative to wt mice. In addition, we observed increased Ucp1 expression in brown adipose and subcutaneous white adipose tissue but not in visceral adipose tissue from Rorα-deficient mice. Moreover, this was associated with significant increases in the expression of the mRNAs encoding the thermogenic genes (i.e., markers of brown and beige adipose) Pparα, Errα, Dio2, Acot11/Bfit, Cpt1β, and Cidea in the subcutaneous adipose in the sg/sg relative to WT mice. These changes in thermogenic gene expression involved the significantly increased expression of the (cell-fate controlling) histone-lysine N-methyltransferase 1 (Ehmt1), which stabilizes the Prdm16 transcriptional complex. Moreover, primary brown adipocytes from sg/sg mice displayed a higher metabolic rate, and further analysis was consistent with increased uncoupling. Finally, core body temperature analysis and infrared thermography demonstrated that the sg/sg mice maintained greater thermal control and cold tolerance relative to the WT littermates. We suggest that enhanced Ucp1 and thermogenic gene expression/activity may be an important contributor to the lean, obesity-resistant phenotype in Rorα-deficient mice.

Insulin resistance and white adipose tissue inflammation are uncoupled in energetically challenged Fsp27-deficient mice

Fsp27 is a lipid droplet-associated protein almost exclusively expressed in adipocytes where it facilitates unilocular lipid droplet formation. In mice, Fsp27 deficiency is associated with increased basal lipolysis, ‘browning’ of white fat and a healthy metabolic profile, whereas a patient with congenital CIDEC deficiency manifested an adverse lipodystrophic phenotype. Here we reconcile these data by showing that exposing Fsp27-null mice to a substantial energetic stress by crossing them with ob/ob mice or BATless mice, or feeding them a high-fat diet, results in hepatic steatosis and insulin resistance. We also observe a striking reduction in adipose inflammation and increase in adiponectin levels in all three models. This appears to reflect reduced activation of the inflammasome and less adipocyte death. These findings highlight the importance of Fsp27 in facilitating optimal energy storage in adipocytes and represent a rare example where adipose inflammation and hepatic insulin resistance are disassociated.

Fsp27 mediates ‘fusion’ of lipid droplets in mouse adipose tissue. Here, the authors investigate the physiological consequences of loss of Fsp27 in three different mouse models of ‘energetic overload’, and observe hepatic steatosis and insulin resistance but reduced adipose tissue inflammation.

Adipocyte-specific disruption of fat-specific protein 27 causes hepatosteatosis and insulin resistance in high-fat diet-fed mice

White adipose tissue (WAT) functions as an energy reservoir where excess circulating fatty acids are transported to WAT, converted to triglycerides, and stored as unilocular lipid droplets. Fat-specific protein 27 (FSP27, CIDEC in humans) is a lipid-coating protein highly expressed in mature white adipocytes that contributes to unilocular lipid droplet formation. However, the influence of FSP27 in adipose tissue on whole-body energy homeostasis remains unclear. Mice with adipocyte-specific disruption of the Fsp27 gene (Fsp27(ΔAd)) were generated using an aP2-Cre transgene with the Cre/LoxP system. Upon high-fat diet feeding, Fsp27(ΔAd) mice were resistant to weight gain. In the small WAT of these mice, small adipocytes containing multilocular lipid droplets were dispersed. The expression levels of the genes associated with mitochondrial abundance and brown adipocyte identity were increased, and basal lipolytic activities were significantly augmented in adipocytes isolated from Fsp27(ΔAd) mice compared with the Fsp27(F/F) counterparts. The impaired fat-storing function in Fsp27(ΔAd) adipocytes and the resultant lipid overflow from WAT led to marked hepatosteatosis, dyslipidemia, and systemic insulin resistance in high-fat diet-treated Fsp27(ΔAd) mice. These results demonstrate a critical role for FSP27 in the storage of excess fat in WAT with minimizing ectopic fat accumulation that causes insulin-resistant diabetes and non-alcoholic fatty liver disease. This mouse model may be useful for understanding the significance of fat-storing properties of white adipocytes and the role of local FSP27 in whole-body metabolism and estimating the pathogenesis of human partial lipodystrophy caused by CIDEC mutations.

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.

Dietary umbelliferone attenuates alcohol-induced fatty liver via regulation of PPARα and SREBP-1c in rats

This study investigated the effects of umbelliferone (UF) on alcoholic fatty liver and its underlying mechanism. Rats were fed a Lieber-DeCarli liquid diet with 36% of calories as alcohol with or without UF (0.05 g/L) for 8 weeks. Pair-fed rats received an isocaloric carbohydrate liquid diet. UF significantly reduced the severity of alcohol-induced body weight loss, hepatic lipid accumulation and droplet formation, and dyslipidemia. UF decreased plasma AST, ALT, and γGTP activity. UF significantly reduced hepatic cytochrome P450 2E1 activities and increased alcohol dehydrogenase and aldehyde dehydrogenase 2 activities compared to the alcohol control group, which resulted in a lower plasma acetaldehyde level in the rats that received UF. Chronic alcohol exposure inhibited hepatic AMPK activation compared to the pair-fed rats, which was reversed by UF supplementation. UF also significantly suppressed the lipogenic gene expression (SREBP-1c, SREBP-2, FAS, CIDEA, and PPARγ) and elevated the fatty acid oxidation gene expression (PPARα, Acsl1, CPT, Acox, and Acaa1a) compared to the alcohol control group, which could lead to inhibition of FAS activity and stimulation of CPT and fatty acid β-oxidation activities in the liver of chronic alcohol-fed rats. These results indicated that UF attenuated alcoholic steatosis through down-regulation of SREBP-1c-mediated lipogenesis and up-regulation of PPARα-mediated fatty acid oxidation. Therefore, UF may provide a promising natural therapeutic strategy against alcoholic fatty liver.

MicroRNA-33 regulates sterol regulatory element-binding protein 1 expression in mice

MicroRNAs (miRs) are small non-protein-coding RNAs that bind to specific mRNAs and inhibit translation or promote mRNA degradation. Recent reports have indicated that miR-33, which is located within the intron of sterol regulatory element-binding protein (SREBP) 2, controls cholesterol homoeostasis and may be a potential therapeutic target for the treatment of atherosclerosis. Here we show that deletion of miR-33 results in marked worsening of high-fat diet-induced obesity and liver steatosis. Using miR-33(-/-)Srebf1(+/-) mice, we demonstrate that SREBP-1 is a target of miR-33 and that the mechanisms leading to obesity and liver steatosis in miR-33(-/-) mice involve enhanced expression of SREBP-1. These results elucidate a novel interaction between SREBP-1 and SREBP-2 mediated by miR-33 in vivo.

Inhibiting monoacylglycerol acyltransferase 1 ameliorates hepatic metabolic abnormalities but not inflammation and injury in mice

Abnormalities in hepatic lipid metabolism and insulin action are believed to play a critical role in the etiology of nonalcoholic steatohepatitis. Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol, which is the penultimate step in one pathway for triacylglycerol synthesis. Hepatic expression of Mogat1, which encodes an MGAT enzyme, is increased in the livers of mice with hepatic steatosis, and knocking down Mogat1 improves glucose metabolism and hepatic insulin signaling, but whether increased MGAT activity plays a role in the etiology of nonalcoholic steatohepatitis is unclear. To examine this issue, mice were placed on a diet containing high levels of trans fatty acids, fructose, and cholesterol (HTF-C diet) or a low fat control diet for 4 weeks. Mice were injected with antisense oligonucleotides (ASOs) to knockdown Mogat1 or a scrambled ASO control for 12 weeks while remaining on diet. The HTF-C diet caused glucose intolerance, hepatic steatosis, and induced hepatic gene expression markers of inflammation, macrophage infiltration, and stellate cell activation. Mogat1 ASO treatment, which suppressed Mogat1 expression in liver and adipose tissue, attenuated weight gain, improved glucose tolerance, improved hepatic insulin signaling, and decreased hepatic triacylglycerol content compared with control ASO-treated mice on HTF-C chow. However, Mogat1 ASO treatment did not reduce hepatic diacylglycerol, cholesterol, or free fatty acid content; improve histologic measures of liver injury; or reduce expression of markers of stellate cell activation, liver inflammation, and injury. In conclusion, inhibition of hepatic Mogat1 in HTF-C diet-fed mice improves hepatic metabolic abnormalities without attenuating liver inflammation and injury.

SREBP-1c overexpression induces triglycerides accumulation through increasing lipid synthesis and decreasing lipid oxidation and VLDL assembly in bovine hepatocytes

The natural incidence of fatty liver in ruminants is significantly higher than in monogastric animals. Fatty liver is associated with sterol regulatory element-binding protein 1c (SREBP-1c). The aim of this study was to investigate the regulatory network effects of SREBP-1c on the lipid metabolic genes involved in fatty acid uptake, activation, oxidation, synthesis, and very low-density lipoprotein (VLDL) assembly in bovine hepatocytes. In vitro, bovine hepatocytes were transfected with an adenovirus-mediated SREBP-1c overexpression vector. SREBP-1c overexpression significantly up-regulated the expression and activity of the fatty acid uptake, activation, and synthesis enzymes: liver fatty acid binding protein, fatty acid translocase, acyl-CoA synthetase long-chain 1, acetyl-CoA carboxylase 1, and fatty acid synthase, increasing triglyceride (TG) synthesis and accumulation. SREBP-1c overexpression down-regulated the expression and activity of the lipid oxidation enzymes: carnitine palmitoyltransferase 1 and carnitine palmitoyltransferase 2. Furthermore, the apolipoprotein B100 expression and microsomal triglyceride transfer protein activity were significantly decreased. SREBP-1c overexpression reduced lipid oxidation and VLDL synthesis, thereby decreasing TG disposal and export. Therefore, large amounts of TG accumulated in the bovine hepatocytes. Taken together, these results indicate that SREBP-1c overexpression increases lipid synthesis and decreases lipid oxidation and VLDL export, thereby inducing TG accumulation in bovine hepatocytes.

Thrombin inhibition with dabigatran protects against high-fat diet-induced fatty liver disease in mice

Nonalcoholic fatty liver disease (NAFLD) is the hepatic manifestation of obesity and metabolic syndrome. Robust coagulation cascade activation is common in obese patients with NAFLD. We identified a critical temporal relationship between thrombin generation and the manifestation of hepatic steatosis, inflammation, and injury in C57BL/6J mice fed a high-fat diet (HFD) for 1, 2, and 3 months. Mice fed a HFD exhibited dramatic increases in hepatocellular injury and inflammation over time. Hepatic fibrin deposition preceded an increase in serum alanine aminotransferase, and the most dramatic changes in liver histopathology occurred in conjunction with a detectable increase in plasma thrombin-antithrombin levels at 3 months. To directly determine whether thrombin activity promotes NAFLD pathogenesis, mice were fed a HFD and simultaneously treated with the direct thrombin inhibitor dabigatran etexilate for 3 months. Notably, dabigatran treatment significantly reduced hepatic fibrin deposition, hepatic inflammation, hepatocellular injury, and steatosis in mice fed a HFD. Of interest, dabigatran treatment also significantly attenuated HFD-induced body weight gain. Gene expression analysis suggested that thrombin potentially drives NAFLD pathogenesis by altering the expression of genes associated with lipid metabolism and bile acid synthesis. Collectively, the results suggest that thrombin activity is central to HFD-induced body weight gain, liver injury, and inflammation and provide the proof-of-principle evidence that pharmacological thrombin inhibition could be effective in limiting NAFLD and associated pathologies.

Investigating the antiviral role of cell death-inducing DFF45-like effector B in HCV replication

Cell-death-inducing DFF45-like effector B (CIDEB) is an apoptotic host factor, which was recently found to also regulate hepatic lipid homeostasis. Herein we delineate the relevance of these dual roles of CIDEB in apoptosis and lipid metabolism in the context of hepatitis C virus (HCV) replication. We demonstrate that HCV upregulates CIDEB expression in human serum differentiated hepatoma cells. CIDEB overexpression inhibits HCV replication in HCV replicon expressing Huh7.5 cells, while small interfering RNA knockdown of CIDEB expression in human serum differentiated hepatoma cells promotes HCV replication and secretion of viral proteins. Furthermore, we characterize a CIDEB mutant, KRRA, which is deficient in lipid droplet clustering and fusion and demonstrate that CIDEB-mediated inhibition of HCV is independent of the protein's lipid droplet fusogenic role. Our results suggest that higher levels of CIDEB expression, which favour an apoptotic role for the host factor, inhibit HCV. Collectively, our data demonstrate that CIDEB can act as an anti-HCV host factor and contribute to altered triglyceride homeostasis.

Dissecting adipose tissue lipolysis: molecular regulation and implications for metabolic disease

Lipolysis is the process by which triglycerides (TGs) are hydrolyzed to free fatty acids (FFAs) and glycerol. In adipocytes, this is achieved by sequential action of adipose TG lipase (ATGL), hormone-sensitive lipase (HSL), and monoglyceride lipase. The activity in the lipolytic pathway is tightly regulated by hormonal and nutritional factors. Under conditions of negative energy balance such as fasting and exercise, stimulation of lipolysis results in a profound increase in FFA release from adipose tissue (AT). This response is crucial in order to provide the organism with a sufficient supply of substrate for oxidative metabolism. However, failure to efficiently suppress lipolysis when FFA demands are low can have serious metabolic consequences and is believed to be a key mechanism in the development of type 2 diabetes in obesity. As the discovery of ATGL in 2004, substantial progress has been made in the delineation of the remarkable complexity of the regulatory network controlling adipocyte lipolysis. Notably, regulatory mechanisms have been identified on multiple levels of the lipolytic pathway, including gene transcription and translation, post-translational modifications, intracellular localization, protein-protein interactions, and protein stability/degradation. Here, we provide an overview of the recent advances in the field of AT lipolysis with particular focus on the molecular regulation of the two main lipases, ATGL and HSL, and the intracellular and extracellular signals affecting their activity.

Targeting Hepatic Glycerolipid Synthesis and Turnover to Treat Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) encompasses a spectrum of metabolic abnormalities ranging from simple hepatic steatosis (accumulation of neutral lipid) to development of steatotic lesions, steatohepatitis, and cirrhosis. NAFLD is extremely prevalent in obese individuals and with the epidemic of obesity; nonalcoholic steatohepatitis (NASH) has become the most common cause of liver disease in the developed world. NASH is rapidly emerging as a prominent cause of liver failure and transplantation. Moreover, hepatic steatosis is tightly linked to risk of developing insulin resistance, diabetes, and cardiovascular disease. Abnormalities in hepatic lipid metabolism are part and parcel of the development of NAFLD and human genetic studies and work conducted in experimentally tractable systems have identified a number of enzymes involved in fat synthesis and degradation that are linked to NAFLD susceptibility as well as progression to NASH. The goal of this review is to summarize the current state of our knowledge on these pathways and focus on how they contribute to etiology of NAFLD and related metabolic diseases.

Maternal diet amplifies the hepatic aging trajectory of Cidea in male mice and leads to the development of fatty liver

The importance of the early environment on long-term heath and life span is well documented. However, the molecular mechanisms mediating these effects remain poorly understood. Male offspring from a maternal protein restriction model, in which animals are exposed to a low-protein diet while in utero and then are cross-fostered to normally fed dams, demonstrate low birth weight, catch-up growth, and reduced life span (recuperated offspring). In the current study, we used microarray analysis to identify hepatic genes that changed with age. Cell death-inducing DNA fragmentation factor, α subunit-like effector A (Cidea), a transcriptional coactivator that has been implicated in lipid accumulation demonstrated one of the largest age-associated increases in expression (200-fold, P<0.001). This increase was exaggerated ∼3-fold in recuperated offspring. These demonstrated increased hepatic lipid accumulation, higher levels of transcription factors important in lipid regulation, and greater oxidative stress. In vitro analysis revealed that Cidea expression was regulated by oxidative stress and DNA methylation. These findings suggest that maternal diet modulates the age-associated changes in Cidea expression through several mechanisms. This expression affects hepatic lipid metabolism in these animals and thus provides a mechanism by which maternal diet can contribute to the metabolic health and ultimately the life span of the offspring.

Regulation of steatohepatitis and PPARγ signaling by distinct AP-1 dimers

Nonalcoholic fatty liver disease (NAFLD) affects up to 30% of the adult population in Western societies, yet the underlying molecular pathways remain poorly understood. Here, we identify the dimeric Activator Protein 1 as a regulator of NAFLD. Fos-related antigen 1 (Fra-1) and Fos-related antigen 2 (Fra-2) prevent dietary NAFLD by inhibiting prosteatotic PPARγ signaling. Moreover, established NAFLD and the associated liver damage can be efficiently reversed by hepatocyte-specific Fra-1 expression. In contrast, c-Fos promotes PPARγ expression, while c-Jun exerts opposing, dimer-dependent functions. Interestingly, JunD was found to be essential for PPARγ signaling and NAFLD development. This unique antagonistic regulation of PPARγ by distinct AP-1 dimers occurs at the transcriptional level and establishes AP-1 as a link between obesity, hepatic lipid metabolism, and NAFLD.

Maternal western diet primes non-alcoholic fatty liver disease in adult mouse offspring

AIM

Metabolic programming via components of the maternal diet during gestation may play a role in the development of different aspects of the metabolic syndrome. Using a mouse model, we aimed to characterize the role of maternal western-type diet in the development of non-alcoholic fatty liver disease (NAFLD) in the offspring.

METHODS

Female mice were fed either a western (W) or low-fat control (L) semisynthetic diet before and during gestation and lactation. At weaning, male offspring were assigned either the W or the L diet, generating four experimental groups: WW, WL, LW and LL offspring. Biochemical, histological and epigenetic indicators were investigated at 29 weeks of age.

RESULTS

Male offspring exposed to prenatal and post-weaning western-style diet (WW) showed hepatomegaly combined with accumulation of hepatic cholesterol and triglycerides. This accumulation was associated with up-regulation of de novo lipid synthesis, inflammation and dysregulation of lipid storage. Elevated hepatic transaminases and increased expression of Tnfa, Cd11, Mcp1 and Tgfb underpin the severity of liver injury. Histopathological analysis revealed the presence of advanced steatohepatitis in WW offspring. In addition, alterations in DNA methylation in key metabolic genes (Ppara, Insig, and Fasn) were detected.

CONCLUSION

Maternal dietary fat intake during early development programmes susceptibility to liver disease in male offspring, mediated by disturbances in lipid metabolism and inflammatory response. Long-lasting epigenetic changes may underlie this dysregulation.

Cidea controls lipid droplet fusion and lipid storage in brown and white adipose tissue

Excess lipid storage in adipose tissue results in the development of obesity and other metabolic disorders including diabetes, fatty liver and cardiovascular diseases. The lipid droplet (LD) is an important subcellular organelle responsible for lipid storage. We previously observed that Fsp27, a member of the CIDE family proteins, is localized to LD-contact sites and promotes atypical LD fusion and growth. Cidea, a close homolog of Fsp27, is expressed at high levels in brown adipose tissue. However, the exact role of Cidea in promoting LD fusion and lipid storage in adipose tissue remains unknown. Here, we expressed Cidea in Fsp27-knockdown adipocytes and observed that Cidea has similar activity to Fsp27 in promoting lipid storage and LD fusion and growth. Next, we generated Cidea and Fsp27 double-deficient mice and observed that these animals had drastically reduced adipose tissue mass and a strong lean phenotype. In addition, Cidea/Fsp27 double-deficient mice had improved insulin sensitivity and were intolerant to cold. Furthermore, we observed that the brown and white adipose tissues of Cidea/Fsp27 double-deficient mice had significantly reduced lipid storage and contained smaller LDs compared to those of Cidea or Fsp27 single deficient mice. Overall, these data reveal an important role of Cidea in controlling lipid droplet fusion, lipid storage in brown and white adipose tissue, and the development of obesity.

Human serum activates CIDEB-mediated lipid droplet enlargement in hepatoma cells

Human hepatocytes constitutively express the lipid droplet (LD) associated protein cell death-inducing DFFA-like effector B (CIDEB). CIDEB mediates LD fusion, as well as very-low-density lipoprotein (VLDL) maturation. However, there are limited cell culture models readily available to study CIDEB's role in these biological processes, as hepatoma cell lines express negligible levels of CIDEB. Recent work has highlighted the ability of human serum to differentiate hepatoma cells. Herein, we demonstrate that culturing Huh7.5 cells in media supplemented with human serum activates CIDEB expression. This activation occurs through the induced expression of PGC-1α, a positive transcriptional regulator of CIDEB. Coherent anti-Stokes Raman scattering (CARS) microscopy revealed a correlation between CIDEB levels and LD size in human serum treated Huh7.5 cells. Human serum treatment also resulted in a rapid decrease in the levels of adipose differentiation-related protein (ADRP). Furthermore, individual overexpression of CIDEB was sufficient to down-regulate ADRP protein levels. siRNA knockdown of CIDEB revealed that the human serum mediated increase in LD size was CIDEB-dependent. Overall, our work highlights CIDEB's role in LD fusion, and presents a new model system to study the PGC-1α/CIDEB pathway's role in LD dynamics and the VLDL pathway.

Highly expressed MT-ND3 positively associated with histological severity of hepatic steatosis

Hepatic steatosis is the accumulation of an excess amount of triglycerides and other fats inside liver cells resulting from abnormal hepatic lipid metabolism. Mitochondrial structural and molecular defects are involved in the progression of hepatic steatosis pathogenesis. Hepatic methylation and transcriptional activity of the mitochondrial-encoded NADH dehydrogenase (MT-ND) play a critical role in the progression of non-alcoholic fatty liver disease (NAFLD). However, the expression of MT-ND3 in hepatic steatosis has not been extensively studied. In this study, liver specimens were collected from different patients, and were subjected to immunohistochemistry. Primary hepatocytes were treated with oxidative stress, hypoxia, and lipotoxicity to investigate the respective roles of these factors on MT-ND3 expression and cell apoptosis by western blotting and flow cytometry, respectively. We found that increased MT-ND3 expression in human hepatic steatosis was positively associated with histological severity of hepatic steatosis. Hypoxia, H2O2 , and saturated fatty acid treatment induced cell apoptosis mediated by mitochondria. These three factors all had effects on MT-ND3 expression in cultured hepatocytes. Taken together, MT-ND3 may play important roles in hepatic steatosis progress. Hypoxia, oxidative stress, and lipotoxicity could all influence expression of MT-ND3 and thus may play a role in the progression of hepatic steatosis.

Expression of CIDE proteins in clear cell renal cell carcinoma and their prognostic significance

Clear cell renal cell carcinoma (ccRCC) is the major and aggressive subtype of renal cell carcinoma. It is known to derive its histologic appearance from accumulation of abundant lipids and glycogens. The cell death-inducing DFF45-like effector (CIDE) family has been characterized as the lipid droplet proteins involved in the metabolism of lipid storage droplets. The purpose of this study was to evaluate the expression of CIDE proteins in ccRCC cells and to investigate their prognostic significance. We examined consecutive patients with sporadic ccRCC, who underwent nephrectomy, to measure their mRNA and protein expression of CIDE proteins. We found that Cidec and ADRP expression were significantly up-regulated in ccRCC, compared with normal kidney tissues. Cideb was down-regulated. We also found that Cideb was expressed more in low-grade ccRCC than in high-grade tumors. To further clarify the relationship between Cideb expression and patient prognosis, we evaluated 57 ccRCC patients followed up for 120 months. Reduced ccRCC Cideb expression was associated with a higher Fuhrman nuclear grade. Patients with high Cideb expression had better overall survival rate than those with low expression (p < 0.05). Cideb expression was an independent predictor of survival (p = 0.001). Although the biologic function of Cideb in ccRCC remains unknown, the expression level of Cideb might be a novel predictor of prognosis in ccRCC.

CD44 plays a critical role in regulating diet-induced adipose inflammation, hepatic steatosis, and insulin resistance

CD44 is a multifunctional membrane receptor implicated in the regulation of several biological processes, including inflammation. CD44 expression is elevated in liver and white adipose tissue (WAT) during obesity suggesting a possible regulatory role for CD44 in metabolic syndrome. To study this hypothesis, we examined the effect of the loss of CD44 expression on the development of various features of metabolic syndrome using CD44 null mice. Our study demonstrates that CD44-deficient mice (CD44KO) exhibit a significantly reduced susceptibility to the development of high fat-diet (HFD)-induced hepatic steatosis, WAT-associated inflammation, and insulin resistance. The decreased expression of genes involved in fatty acid synthesis and transport (Fasn and Cd36), de novo triglyceride synthesis (Mogat1), and triglyceride accumulation (Cidea, Cidec) appears in part responsible for the reduced hepatic lipid accumulation in CD44KO(HFD) mice. In addition, the expression of various inflammatory and cell matrix genes, including several chemokines and its receptors, osteopontin, and several matrix metalloproteinases and collagen genes was greatly diminished in CD44KO(HFD) liver consistent with reduced inflammation and fibrogenesis. In contrast, lipid accumulation was significantly increased in CD44KO(HFD) WAT, whereas inflammation as indicated by the reduced infiltration of macrophages and expression of macrophage marker genes, was significantly diminished in WAT of CD44KO(HFD) mice compared to WT(HFD) mice. CD44KO(HFD) mice remained considerably more insulin sensitive and glucose tolerant than WT(HFD) mice and exhibited lower blood insulin levels. Our study indicates that CD44 plays a critical role in regulating several aspects of metabolic syndrome and may provide a new therapeutic target in the management of insulin resistance.

Cellular steatosis in ethanol oxidizing-HepG2 cells is partially controlled by the transcription factor, early growth response-1

Recent studies have shown that the transcription factor early growth response-1 (Egr-1) regulates ethanol-induced fatty liver. However, the mechanism(s) through which ethanol oxidation controls Egr-1 is unknown. Here, using recombinant hepatoma (HepG2; VL-17A) cells that metabolize ethanol, we show that alcohol dehydrogenase catalysis of ethanol oxidation and subsequent acetaldehyde production controls Egr-1 expression. Further, the induction of Egr-1 enhances expression of other steatosis-related genes, resulting in triglyceride accumulation. Ethanol exposure increased Egr-1 promoter activity, messenger RNA and Egr-1 protein levels in VL-17A cells. Elevated Egr-1 protein was sustained by an ethanol-induced decrease in proteasome activity, thereby stabilizing the Egr-1 protein. Egr-1 induction depended on ethanol oxidation, as it was prevented when ethanol oxidation was blocked. Ethanol exposure induced Egr-1 and triglyceride accumulation only in alcohol dehydrogenase-expressing cells that produced acetaldehyde. Such induction did not occur in parental, non-metabolizing HepG2 cells or in cells that express only cytochrome P450 2E1. However, direct exposure of HepG2 cells to acetaldehyde induced both Egr-1 protein and triglycerides. Egr-1 over-expression elevated triglyceride levels, which were augmented by ethanol exposure. However, these triglyceride levels did not exceed those in ethanol-exposed cells that had normal Egr-1 expression. Conversely, Egr-1 knockdown by siRNA only partially blocked ethanol-induced triglyceride accumulation and was associated not only with lower Egr-1 expression but also attenuation of SREBP1c and TNF-α mRNAs. Double knockdown of both Egr-1 and SREBP-1c abolished ethanol-elicited steatosis. Collectively, our findings provide important new insights into the temporal regulation by ethanol oxidation of Egr-1 and cellular steatosis.

CIDE proteins and lipid metabolism

Lipid homeostasis is maintained through the coordination of lipid metabolism in various tissues, including adipose tissue and the liver. The disruption of lipid homeostasis often results in the development of metabolic disorders such as obesity, diabetes mellitus, liver steatosis, and cardiovascular diseases. Cell death-inducing DNA fragmentation factor 45-like effector family proteins, including Cidea, Cideb, and Fsp27 (Cidec), are emerging as important regulators of various lipid metabolic pathways and play pivotal roles in the development of metabolic disorders. This review summarizes the latest cell death-inducing DNA fragmentation factor 45-like effector protein discoveries related to the control of lipid metabolism, with emphasis on the role of these proteins in lipid droplet growth in adipocytes and in the regulation of very low-density lipoprotein lipidation and maturation in hepatocytes.

Opposing roles of cell death-inducing DFF45-like effector B and perilipin 2 in controlling hepatic VLDL lipidation

Regulation of hepatic very low density lipoprotein (VLDL) assembly and maturation is crucial in controlling lipid homeostasis and in the development of metabolic disorders, including obesity, hepatic steatosis, and insulin resistance. Cideb, a member of cell death-inducing DFF45-like effector (CIDE) protein family, has been previously shown to promote VLDL lipidation and maturation. However, the precise subcellular location of Cideb-mediated VLDL lipidation and the factors modulating its activity remain elusive. In addition to its localization to endoplasmic reticulum (ER) and lipid droplets (LD), we observed that Cideb was also localized to the Golgi apparatus. Mature and lipid-rich VLDL particles did not accumulate in the Golgi apparatus in Cideb(-/-) livers. Interestingly, we observed that hepatic perilipin 2/adipose differentiation-related protein (ADRP) levels were markedly increased in Cideb(-/-) mice. Liver-specific knockdown of perilipin 2 in Cideb(-/-) mice resulted in the reduced accumulation of hepatic triglycerides (TAG), increased VLDL-TAG secretion, and the accumulation of mature TAG-rich VLDL in the Golgi apparatus. These data reveal that Cideb and perilipin 2 play opposing roles in controlling VLDL lipidation and hepatic lipid homeostasis.