A human in vivo study of the extent to which 31-phosphorus neurospectroscopy phosphomonoesters index cerebral cell membrane phospholipid anabolism

The phosphomonoester narrow resonance of human in vivo 31-phosphorus neurospectroscopy studies is believed to index the anabolism of cell membrane phospholipids and has therefore been used to study phospholipid anabolism in the brain non-invasively. However, it is an indirect measure of phospholipid metabolism and although it does contain major contributions from phosphocholine, phosphoethanolamine and L-phosphoserine, which are important precursors of membrane phospholipids, many other metabolites, including sugar phosphates, can contribute to this region of the spectrum, and separation of these different peaks is not achieved with the present in vivo methodology. Recently, it has become possible to analyze signal directly from the cell membrane motion-restricted phospholipids by analysis of a broad resonance signal. We therefore hypothesized that there should be a positive correlation between the phosphomonoester narrow resonance and the broad resonance signal if the former does indeed index cell membrane phospholipid anabolism. Cerebral 31-phosphorus magnetic resonance spectroscopy was carried out in 54 human subjects, including normal volunteers and patients with schizophrenia in order to widen the range of phosphomonoester and broad resonance values. Spectra were obtained from 70×70×70 mm³ voxels using an image-selected in vivo spectroscopy pulse sequence. There was a highly significant positive correlation between the phosphomonoester resonances and the broad resonance signals (r=0.404, P<0.005). These results are consistent with the hypothesis that the phosphomonoester narrow resonance does indeed index cell membrane phospholipid anabolism in brain studies.     

In vivo assessment of hepatic triglycerides in murine non-alcoholic fatty liver disease using magnetic resonance spectroscopy

In vivo¹H magnetic resonance spectroscopy (MRS) was used to examine the progression of fatty liver in two murine models of progressive hepatic steatosis: leptin-deficient obese (ob/ob) mice and mice maintained on a diet deficient in methionine and choline (MCDD). Ob/ob mice displayed high levels of intracellular hepatic triglycerides as early as 9 weeks after birth, as observed with MRS and histopathology. Single voxel spectra of ob/ob liver displayed strong resonances arising from saturated (1.3 ppm) and unsaturated (2.8 and 5.3 ppm) fatty acyl chains that could be resolved in the absence of water suppression. Hepatic inflammation, induced by lipopolysaccharide administration, led to a significant increase in unsaturated and polyunsaturated fatty acyl chain resonances (P < 0.05), indicating a change in the composition of hepatic triglycerides in lipid droplets. Mice maintained on the MCDD displayed histological evidence of hepatic steatosis as early as two weeks, progressing to macrovesicular steatohepatitis at 10 weeks. The histological changes were accompanied by significant increases in saturated and unsaturated fatty acyl chain resonances and a significant decrease in the lipid/(water + lipid) ratio (P < 0.05). These results indicate that in vivo¹H MRS may be a suitable method to monitor the progression of steatohepatitis.     

Association between 12α-hydroxylated bile acids and hepatic steatosis in rats fed a high-fat diet

High-fat (HF) diet induces hepatic steatosis that is a risk factor for noncommunicable diseases such as obesity, type 2 diabetes and cardiovascular disease. Previously, we found that HF feeding in rats increases the excretion of fecal bile acids (BAs), specifically 12α-hydroxylated (12αOH) BAs. Although the liver is the metabolic center in our body, the association between hepatic steatosis and 12αOH BAs in HF-fed rats is unclear. Thus, we investigated extensively BA composition in HF-fed rats and evaluated the association between hepatic steatosis and 12αOH BAs. Acclimated male inbred WKAH/HkmSlc rats were divided into two groups and fed either control or HF diet for 8 weeks. Feeding HF diet increased hepatic triglyceride and total cholesterol concentrations, which correlated positively with 12αOH BAs concentrations but not with non-12αOH BAs in the feces, portal plasma and liver. Accompanied by the increase in 12αOH BAs, the rats fed HF diet showed increased fat absorption and higher mRNA expression of liver Cidea. The enhancement of 12αOH BA secretion may contribute to hepatic steatosis by the promotion of dietary fat absorption and hepatic Cidea mRNA expression. The increase in 12αOH BAs was associated with enhanced liver cholesterol 7α-hydroxylase (Cyp7a1) and sterol 12α-hydroxylase (Cyp8b1) mRNA expression. There was a significant increase in 7α-hydroxycholesterol, a precursor of BAs, in the liver of HF-fed rats. Altogether, these data suggest that the HF diet increases preferentially 12αOH BAs synthesis by utilizing the accumulated hepatic cholesterol and enhancing mRNA expression of Cyp7a1 and Cyp8b1 in the liver.     

In vivo hepatic lipid quantification using MRS at 7 Tesla in a mouse model of glycogen storage disease type 1a

The assessment of liver lipid content and composition is needed in preclinical research to investigate steatosis and steatosis-related disorders. The purpose of this study was to quantify in vivo hepatic fatty acid content and composition using a method based on short echo time proton magnetic resonance spectroscopy (MRS) at 7 Tesla. A mouse model of glycogen storage disease type 1a with inducible liver-specific deletion of the glucose-6-phosphatase gene (L-G6pc^−/−) mice and control mice were fed a standard diet or a high-fat/high-sucrose (HF/HS) diet for 9 months. In control mice, hepatic lipid content was found significantly higher with the HF/HS diet than with the standard diet. As expected, hepatic lipid content was already elevated in L-G6pc^−/− mice fed a standard diet compared with control mice. L-G6pc^−/− mice rapidly developed steatosis which was not modified by the HF/HS diet. On the standard diet, estimated amplitudes from olefinic protons were found significantly higher in L-G6pc^−/− mice compared with that in control mice. L-G6pc^−/− mice showed no noticeable polyunsaturation from diallylic protons. Total unsaturated fatty acid indexes measured by gas chromatography were in agreement with MRS measurements. These results showed the great potential of high magnetic field MRS to follow the diet impact and lipid alterations in mouse liver.     

The DNA damage checkpoint protein ATM promotes hepatocellular apoptosis and fibrosis in a mouse model of non-alcoholic fatty liver disease

Steatoapoptosis is a hallmark of non-alcoholic fatty liver disease (NAFLD) and is an important factor in liver disease progression. We hypothesized that increased reactive oxygen species resulting from excess dietary fat contribute to liver disease by causing DNA damage and apoptotic cell death, and tested this by investigating the effects of feeding mice high fat or standard diets for 8 weeks. High fat diet feeding resulted in increased hepatic H₂O₂, superoxide production, and expression of oxidative stress response genes, confirming that the high fat diet induced hepatic oxidative stress. High fat diet feeding also increased hepatic steatosis, hepatitis and DNA damage as exemplified by an increase in the percentage of 8-hydroxyguanosine (8-OHG) positive hepatocytes in high fat diet fed mice. Consistent with reports that the DNA damage checkpoint kinase Ataxia Telangiectasia Mutated (ATM) is activated by oxidative stress, ATM phosphorylation was induced in the livers of wild type mice following high fat diet feeding. We therefore examined the effects of high fat diet feeding in Atm-deficient mice. The prevalence of apoptosis and expression of the pro-apoptotic factor PUMA were significantly reduced in Atm-deficient mice fed the high fat diet when compared with wild type controls. Furthermore, high fat diet fed Atm^−/− mice had significantly less hepatic fibrosis than Atm^+/+ or Atm^+/− mice fed the same diet. Together, these data demonstrate a prominent role for the ATM pathway in the response to hepatic fat accumulation and link ATM activation to fatty liver-induced steatoapoptosis and fibrosis, key features of NAFLD progression.     

Antioxidant properties of tea blunt ROS-dependent lipogenesis: beneficial effect on hepatic steatosis in a high fat-high sucrose diet NAFLD obese rat model

Oxidative stress could trigger lipid accumulation in liver and thus hepatic steatosis. Tea is able to prevent liver disorders, but a direct link between antioxidant capacities and prevention of steatosis has not been reported yet. We aimed to investigate such relationship in a rat model of high fat-high sucrose diet (HFS)-induced obesity and to explore more deeply the mechanisms in isolated hepatocytes. Wistar rats were divided into a control group (standard diet), an HFS group (high fat-sucrose diet) and an HFS + tea group (HFS diet with ad-libitum access to tea drink). Body weight, fat mass, glycemic parameters in blood, lipid and oxidative stress parameters in blood and liver were measured in each group after 14 weeks. Isolated hepatocytes were treated with the reactive oxygen species (ROS) inducer t-BHP in the presence or not of antioxidants (tempol or tea), and superoxide anion production and lipid accumulation were measured using specific fluorescent probes. We reported that the HFS diet highly increased hepatic lipids content, while tea consumption attenuated steatosis and improved the oxidative status (decrease in hepatic oxidative stress, increase in plasma total antioxidant capacity). The role of antioxidant properties of tea in such phenomenon was confirmed in primary cultured rat hepatocytes. Indeed, the increase of mitochondrial ROS production with t-BHP resulted in lipid accumulation in hepatocytes (positive linear regression), and antioxidants (tempol or tea) normalized both. We reported that the antioxidant properties of tea protect rats from an obesogenic HFS diet-induced hepatic steatosis by counteracting the ROS-dependent lipogenesis.     

GCN2 deficiency protects against high fat diet induced hepatic steatosis and insulin resistance in mice

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic lipid deposition and oxidative stress. It has been demonstrated that general control nonderepressible 2 (GCN2) is required to maintain hepatic fatty acid homeostasis under conditions of amino acid deprivation. However, the impact of GCN2 on the development of NAFLD has not been investigated. In this study, we used Gcn2^?/? mice to investigate the effect of GCN2 on high fat diet (HFD)-induced hepatic steatosis. After HFD feeding for 12?weeks, Gcn2^?/? mice were less obese than wild-type (WT) mice, and Gcn2^?/? significantly attenuated HFD-induced liver dysfunction, hepatic steatosis and insulin resistance. In the livers of the HFD-fed mice, GCN2 deficiency resulted in higher levels of lipolysis genes, lower expression of genes related to FA synthesis, transport and lipogenesis, and less induction of oxidative stress. Furthermore, we found that knockdown of GCN2 attenuated, whereas overexpression of GCN2 exacerbated, palmitic acid-induced steatosis, oxidative & ER stress, and changes of peroxisome proliferator-activated receptor gamma (PPARγ), fatty acid synthase (FAS) and metallothionein (MT) expression in HepG2 cells. Collectively, our data provide evidences that GCN2 deficiency protects against HFD-induced hepatic steatosis by inhibiting lipogenesis and reducing oxidative stress. Our findings suggest that strategies to inhibit GCN2 activity in the liver may provide a novel approach to attenuate NAFLD development.     

Hepatic steatosis and plasma dyslipidemia induced by a high-sucrose diet are corrected by an acute leptin infusion.

High sucrose (HS) feeding in rats induces hepatic steatosis and plasma dyslipidemia. In previous reports (Huang W, Dedousis N, Bhatt BA, O'Doherty RM. J Biol Chem 279: 21695-21700, 2004; and Huang W, Dedousis N, Bandi A, Lopaschuk GD, O'Doherty RM. Endocrinology 147: 1480-1487, 2006), our laboratory demonstrated a rapid ( approximately 100 min) leptin-induced decrease in liver and plasma VLDL triglycerides (TG) in lean rats, effects that were abolished in obese rats fed a high-fat diet, a model that also presents with hepatic steatosis and plasma dyslipidemia. To further examine the capacity of acute leptin treatment to improve metabolic abnormalities induced by nutrient excess, hepatic leptin action was studied in rats after 5 wk of HS feeding. HS feeding induced hepatic steatosis (TG+80+/-8%; P=0.001), plasma hyperlipidemia (VLDL-TG+102+/-14%; P=0.001), hyperinsulinemia (plasma insulin +67+/-12%; P=0.04), and insulin resistance as measured by homeostasis model assessment (+125+/-20%; P=0.02), without increases in adiposity or plasma leptin concentration compared with standard chow-fed controls. A 120-min infusion of leptin (plasma leptin 13.6+/-0.7 ng/ml) corrected hepatic steatosis (liver TG-29+/-3%; P=0.003) and plasma hyperlipidemia in HS (VLDL-TG-42+/-4%; P=0.001) and increased plasma ketones (+45+/-3%; P=0.006), without altering plasma glucose, insulin, or homeostasis model assessment compared with saline-infused HS controls. In addition, leptin activated liver phosphatidylinositol 3-kinase (+70+/-18%; P=0.01) and protein kinase B (Akt; +90+/-29%; P=0.02), and inhibited acetyl-CoA carboxylase (40+/-7%; P=0.04) in HS, further demonstrating that hepatic leptin action was intact in these animals. We conclude that 1) leptin action on hepatic lipid metabolism remains intact in HS-fed rats, 2) leptin rapidly reverses hepatic steatosis and plasma dyslipidemia induced by sucrose, and 3) the preservation of hepatic leptin action after a HS diet is associated with the maintenance of low adiposity and plasma leptin concentrations.     

Bile acids override steatosis in farnesoid X receptor deficient mice in a model of non-alcoholic steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) is one of the most common liver diseases, and the pathogenesis is still not well known. The farnesoid X receptor (FXR) is a member of the nuclear hormone receptor superfamily and plays an essential role in maintaining bile acid and lipid homeostasis. In this study, we study the role of FXR in the pathogenesis of NFALD. We found that FXR deficient (FXR^−/−) mice fed methionine- and choline-deficient (MCD) diet had higher serum ALT and AST activities and lower hepatic triglyceride levels than wild-type (WT) mice fed MCD diet. Expression of genes involved in inflammation (VCAM-1) and fibrosis (α-SMA) was increased in FXR^−/− mice fed MCD diet (FXR^−/−/MCD) compared to WT mice fed MCD diet (WT/MCD). Although MCD diet significantly induced hepatic fibrosis in terms of liver histology, FXR^−/−/MCD mice showed less degree of hepatic steatosis than WT/MCD mice. Moreover, FXR deficiency synergistically potentiated the elevation effects of MCD diet on serum and hepatic bile acids levels. The super-physiological concentrations of hepatic bile acids in FXR^−/−/MCD mice inhibited the expression of genes involved in fatty acid uptake and triglyceride accumulation, which may be an explanation for less steatosis in FXR^−/−/MCD mice in contrast to WT/MCD mice. These results suggest that hepatic bile acids accumulation could override simple steatosis in hepatic injury during the progression of NAFLD and further emphasize the role of FXR in maintaining hepatic bile acid homeostasis in liver disorders and in hepatic protection.     

12α-Hydroxylated bile acid induces hepatic steatosis with dysbiosis in rats

There is an increasing need to explore the mechanism of the progression of non-alcoholic fatty liver disease. Steroid metabolism is closely linked to hepatic steatosis and steroids are excreted as bile acids (BAs). Here, we demonstrated that feeding WKAH/HkmSlc inbred rats a diet supplemented with cholic acid (CA) at 0.5 g/kg for 13 weeks induced simple steatosis without obesity. Liver triglyceride and cholesterol levels were increased accompanied by mild elevation of aminotransferase activities. There were no signs of inflammation, insulin resistance, oxidative stress, or fibrosis. CA supplementation increased levels of CA and taurocholic acid (TCA) in enterohepatic circulation and deoxycholic acid (DCA) levels in cecum with an increased ratio of 12α-hydroxylated BAs to non-12α-hydroxylated BAs. Analyses of hepatic gene expression revealed no apparent feedback control of BA and cholesterol biosynthesis. CA feeding induced dysbiosis in cecal microbiota with enrichment of DCA producers, which underlines the increased cecal DCA levels. The mechanism of steatosis was increased expression of Srebp1 (positive regulator of liver lipogenesis) through activation of the liver X receptor by increased oxysterols in the CA-fed rats, especially 4β-hydroxycholesterol (4βOH) formed by upregulated expression of hepatic Cyp3a2, responsible for 4βOH formation. Multiple regression analyses identified portal TCA and cecal DCA as positive predictors for liver 4βOH levels. The possible mechanisms linking these predictors and upregulated expression of Cyp3a2 are discussed. Overall, our observations highlight the role of 12α-hydroxylated BAs in triggering liver lipogenesis and allow us to explore the mechanisms of hepatic steatosis onset, focusing on cholesterol and BA metabolism.     

Effects of soluble corn bran hemicellulose on lipid metabolism

• 1.1. Since soluble corn bran hemicellulose (CBH) was found to reduce serum cholesterol level in the rat fed with a high cholesterol diet, rats were fed with diets containing orotic acid (OA) to investigate the effect of CBH on lipid metabolism.
• 2.2. Hepatic lipid accumulation induced by OA was reduced by feeding with CBH in rats. The reduction was not due to inhibition of intestinal absorption of OA by CBH.
• 3.3. Administration of acetate or propionate, colonie fermentation products of CBH, tended to alleviate the hepatic lipid accumulation by OA in rats.
• 4.4. OA feeding decreased activities of some hepatic enzymes involved in fatty acid synthesis except for acetyl CoA carboxylase. The decreases were reversed by the concurrent feeding of CBH.

     

The role of mitochondrial glycerol-3-phosphate acyltransferase-1 in regulating lipid and glucose homeostasis in high-fat diet fed mice

Glycerol-3-phosphate acyltransferase (GPAT) is involved in triacylglycerol (TAG) and phospholipid synthesis, catalyzing the first committed step. In order to further investigate the in vivo importance of the dominating mitochondrial variant, GPAT1, a novel GPAT1^−/− mouse model was generated and studied. Female GPAT1^−/− mice had reduced body weight-gain and adiposity when fed chow diet compared with littermate wild-type controls. Furthermore, GPAT1^−/− females on chow diet showed decreased liver TAG content, plasma cholesterol and TAG levels and increased ex vivo liver fatty acid oxidation and plasma ketone bodies. However, these beneficial effects were abolished and the glucose tolerance tended to be impaired when GPAT1^−/− females were fed a long-term high-fat diet (HFD). GPAT1-deficiency was not associated with altered whole body energy expenditure or respiratory exchange ratio. In addition, there were no changes in male GPAT1^−/− mice fed either diet except for increased plasma ketone bodies on chow diet, indicating a gender-specific phenotype. Thus, GPAT1-deficiency does not protect against HFD-induced obesity, hepatic steatosis or whole body glucose intolerance.     

Absence of Perilipin 2 Prevents Hepatic Steatosis,Glucose Intolerance and Ceramide Accumulation in Alcohol-Fed Mice

Background

Perilipin 2 (Plin2) is a lipid droplet protein that has roles in both lipid and glucose homeostasis. An increase in Plin2 in liver is associated with the development of steatosis, glucose intolerance, and ceramide accumulation in alcoholic liver disease. We investigated the role of Plin2 on energy balance and glucose and lipid homeostasis in wildtype and Plin2 knockout (Plin2KO) mice chronically fed a Lieber-DeCarli liquid ethanol or control diet for six weeks.

Methods

We performed in vivo measurements of energy intake and expenditure; body composition; and glucose tolerance. After sacrifice, liver was dissected for histology and lipid analysis.

Results

We found that neither genotype nor diet had a significant effect on final weight, body composition, or energy intake between WT and Plin2KO mice fed alcohol or control diets. Additionally, alcohol feeding did not affect oxygen consumption or carbon dioxide production in Plin2KO mice. We performed glucose tolerance testing and observed that alcohol feeding failed to impair glucose tolerance in Plin2KO mice. Most notably, absence of Plin2 prevented hepatic steatosis and ceramide accumulation in alcohol-fed mice. These changes were related to downregulation of genes involved in lipogenesis and triglyceride synthesis.

Conclusions

Plin2KO mice chronically fed alcohol are protected from hepatic steatosis, glucose intolerance, and hepatic ceramide accumulation, suggesting a critical pathogenic role of Plin2 in experimental alcoholic liver disease.     

Feeding a corn oil/sucrose-enriched diet enhances steatohepatitis in sedentary rats

The current study investigated the combined effects of feeding a high-fat/high-sucrose (HF/HS) diet to rodents rendered sedentary via hindlimb unloading (HU). For 3 wk before HU, male Wistar rats were fed chow or a diet in which 32% of calories were derived from corn oil fat and 48% of calories from sucrose. Feeding continued during an additional 3-wk period of HU. Subsequently, blood samples were collected for determination of circulating leukocyte counts, insulin levels, and portal vein endotoxin. Inflammation, necrosis, and steatosis were assessed in formalin-fixed liver sections. No biochemical or histological evidence of injury was observed in control rats fed chow or HF/HS. HU increased circulating neutrophils and resulted in hyperinsulinemia. Mild hepatic fat accumulation and minimal focal necroinflammation were observed in this group. Feeding HF/HS during HU exacerbated hyperinsulinemia, hepatic steatosis, Kupffer cell content, and cytokine expression. Significant portal endotoxemia was noted in HU rats but was not influenced by HF/HS diet. On the other hand, feeding HF/HS significantly enhanced lipid peroxidation end products in liver of HU rats by approximately threefold compared with chow-fed rats. In summary, these findings demonstrate that feeding a high-calorie diet potentiates steatosis and injury in sedentary HU rats. Mechanisms underlying enhanced injury most likely involved lipid peroxidation. Importantly, these findings suggest that dietary manipulation combined with physical inactivity can be used to model steatohepatitis.     

CD36-deficient mice are resistant to alcohol- and high-carbohydrate-induced hepatic steatosis

CD36 is a scavenger receptor with multiple ligands and cellular functions, including facilitating cellular uptake of free fatty acids (FFAs). Chronic alcohol consumption increases hepatic CD36 expression, leading to the hypothesis that this promotes uptake of circulating FFAs, which then serve as a substrate for triglyceride (TG) synthesis and the development of alcoholic steatosis. We investigated this hypothesis in alcohol-fed wild-type and Cd36-deficient (Cd36^−/−) mice using low-fat/high-carbohydrate Lieber-DeCarli liquid diets, positing that Cd36^−/− mice would be resistant to alcoholic steatosis. Our data show that the livers of Cd36^−/− mice are resistant to the lipogenic effect of consuming high-carbohydrate liquid diets. These mice also do not further develop alcoholic steatosis when chronically fed alcohol. Surprisingly, we did not detect an effect of alcohol or CD36 deficiency on hepatic FFA uptake; however, the lower baseline levels of hepatic TG in Cd36^−/− mice fed a liquid diet were associated with decreased expression of genes in the de novo lipogenesis pathway and a lower rate of hepatic de novo lipogenesis. In conclusion, Cd36^−/− mice are resistant to hepatic steatosis when fed a high-carbohydrate liquid diet, and they are also resistant to alcoholic steatosis. These studies highlight an important role for CD36 in hepatic lipid homeostasis that is not associated with hepatic fatty acid uptake.     

The phosphorylation status of T522 modulates tissue‐specific functions of SIRT1 in energy metabolism in mice

SIRT1, the most conserved mammalian NAD⁺‐dependent protein deacetylase, is an important metabolic regulator. However, the mechanisms by which SIRT1 is regulated in vivo remain unclear. Here, we report that phosphorylation modification of T522 on SIRT1 is crucial for tissue‐specific regulation of SIRT1 activity in mice. Dephosphorylation of T522 is critical for repression of its activity during adipogenesis. The phospho‐T522 level is reduced during adipogenesis. Knocking‐in a constitutive T522 phosphorylation mimic activates the β‐catenin/GATA3 pathway, repressing PPARγ signaling, impairing differentiation of white adipocytes, and ameliorating high‐fat diet‐induced dyslipidemia in mice. In contrast, phosphorylation of T522 is crucial for activation of hepatic SIRT1 in response to over‐nutrition. Hepatic SIRT1 is hyperphosphorylated at T522 upon high‐fat diet feeding. Knocking‐in a SIRT1 mutant defective in T522 phosphorylation disrupts hepatic fatty acid oxidation, resulting in hepatic steatosis after high‐fat diet feeding. In addition, the T522 dephosphorylation mimic impairs systemic energy metabolism. Our findings unveil an important link between environmental cues, SIRT1 phosphorylation, and energy homeostasis and demonstrate that the phosphorylation of T522 is a critical element in tissue‐specific regulation of SIRT1 activity in vivo.     

Lipoic acid administration prevents nonalcoholic steatosis linked to long-term high-fat feeding by modulating mitochondrial function

Nonalcoholic steatosis is an important hepatic complication of obesity linked to mitochondrial dysfunction and insulin resistance. Furthermore, lipoic acid has been reported to have beneficial effects on mitochondrial function. In this study, we analyzed the potential protective effect of lipoic acid supplementation against the development of nonalcoholic steatosis associated with a long-term high-fat diet feeding and the potential mechanism of this effect. Wistar rats were fed on a standard diet (n=10), a high-fat diet (n=10) and a high-fat diet supplemented with lipoic acid (n=10). A group pair-fed to the latter group (n=6) was also included. Lipoic acid prevented hepatic triglyceride accumulation and liver damage in rats fed a high-fat diet (?68%±11.3% vs. obese group) through the modulation of genes involved in lipogenesis and mitochondrial β-oxidation and by improving insulin sensitivity. Moreover, this molecule showed an inhibitory action on electron transport chain complexes activities (P<.01–P<.001) and adenosine triphosphate synthesis (P<.05), and reduced significantly energy efficiency. By contrast, lipoic acid induced an increase in mitochondrial copy number and in Ucp2 gene expression (P<.001 vs. obese). In summary, this investigation demonstrated the ability of lipoic acid to prevent nonalcoholic steatosis induced by a high-fat intake. Finally, the novelty and importance of this study are the finding of how lipoic acid modulates some of the mitochondrial processes involved in energy homeostasis. The reduction in mitochondrial energy efficiency could also explain, at least in part, the beneficial effects of lipoic acid not only in fatty liver but also in preventing excessive body weight gain.     

The DNA damage checkpoint protein ATM promotes hepatocellular apoptosis and fibrosis in a mouse model of non-alcoholic fatty liver disease

Steatoapoptosis is a hallmark of non-alcoholic fatty liver disease (NAFLD) and is an important factor in liver disease progression. We hypothesized that increased reactive oxygen species resulting from excess dietary fat contribute to liver disease by causing DNA damage and apoptotic cell death, and tested this by investigating the effects of feeding mice high fat or standard diets for 8 weeks. High fat diet feeding resulted in increased hepatic H₂O₂, superoxide production, and expression of oxidative stress response genes, confirming that the high fat diet induced hepatic oxidative stress. High fat diet feeding also increased hepatic steatosis, hepatitis and DNA damage as exemplified by an increase in the percentage of 8-hydroxyguanosine (8-OHG) positive hepatocytes in high fat diet fed mice. Consistent with reports that the DNA damage checkpoint kinase Ataxia Telangiectasia Mutated (ATM) is activated by oxidative stress, ATM phosphorylation was induced in the livers of wild type mice following high fat diet feeding. We therefore examined the effects of high fat diet feeding in Atm-deficient mice. The prevalence of apoptosis and expression of the pro-apoptotic factor PUMA were significantly reduced in Atm-deficient mice fed the high fat diet when compared with wild type controls. Furthermore, high fat diet fed Atm^?/? mice had significantly less hepatic fibrosis than Atm^+/+ or Atm^+/? mice fed the same diet. Together, these data demonstrate a prominent role for the ATM pathway in the response to hepatic fat accumulation and link ATM activation to fatty liver-induced steatoapoptosis and fibrosis, key features of NAFLD progression.     

High Fat Feeding Induces Hepatic Fatty Acid Elongation in Mice

Background

High-fat diets promote hepatic lipid accumulation. Paradoxically, these diets also induce lipogenic gene expression in rodent liver. Whether high expression of these genes actually results in an increased flux through the de novo lipogenic pathway in vivo has not been demonstrated.

Methodology/Principal Findings

To interrogate this apparent paradox, we have quantified de novo lipogenesis in C57Bl/6J mice fed either chow, a high-fat or a n-3 polyunsaturated fatty acid (PUFA)-enriched high-fat diet. A novel approach based on mass isotopomer distribution analysis (MIDA) following 1-¹³C acetate infusion was applied to simultaneously determine de novo lipogenesis, fatty acid elongation as well as cholesterol synthesis. Furthermore, we measured very low density lipoprotein-triglyceride (VLDL-TG) production rates. High-fat feeding promoted hepatic lipid accumulation and induced the expression of lipogenic and cholesterogenic genes compared to chow-fed mice: induction of gene expression was found to translate into increased oleate synthesis. Interestingly, this higher lipogenic flux (+74 µg/g/h for oleic acid) in mice fed the high-fat diet was mainly due to an increased hepatic elongation of unlabeled palmitate (+66 µg/g/h) rather than to elongation of de novo synthesized palmitate. In addition, fractional cholesterol synthesis was increased, i.e. 5.8±0.4% vs. 8.1±0.6% for control and high fat-fed animals, respectively. Hepatic VLDL-TG production was not affected by high-fat feeding. Partial replacement of saturated fat by fish oil completely reversed the lipogenic effects of high-fat feeding: hepatic lipogenic and cholesterogenic gene expression levels as well as fatty acid and cholesterol synthesis rates were normalized.

Conclusions/Significance

High-fat feeding induces hepatic fatty acid synthesis in mice, by chain elongation and subsequent desaturation rather than de novo synthesis, while VLDL-TG output remains unaffected. Suppression of lipogenic fluxes by fish oil prevents from high fat diet-induced hepatic steatosis in mice.