Obesity induces expression of uncoupling protein-2 in hepatocytes and promotes liver ATP depletion

Angiotensin receptor blockade recovers hepatic UCP2 expression and aconitase and SDH activities and ameliorates hepatic oxidative damage in insulin resistant rats

Metabolic syndrome (MetS) is commonly associated with elevated renin-angiotensin system, oxidative stress, and steatohepatitis with down-regulation of uncoupling proteins (UCPs). However, the mechanisms linking renin-angiotensin system, steatosis, and UCP2 to hepatic oxidative damage during insulin resistance are not described. To test the hypothesis that angiotensin receptor activation contributes to decreased hepatic UCP2 expression and aconitase activity and to increased oxidative damage after increased glucose intake in a model of MetS, lean and obese Long Evans rats (n = 10/group) were randomly assigned to the following groups: 1) untreated Long Evans Tokushima Otsuka (lean, strain control), 2) untreated Otsuka Long Evans Tokushima Fatty (OLETF) (MetS model), 3) OLETF + angiotensin receptor blocker (ARB) (10 mg olmesartan/kg·d × 6 wk), 4) OLETF + high glucose (HG) (5% in drinking water × 6 wk), and 5) OLETF + ARB + HG (ARB/HG × 6 wk). HG increased body mass (37%), plasma triglycerides (TGs) (35%), plasma glycerol (87%), plasma free fatty acids (28%), and hepatic nitrotyrosine (74%). ARB treatment in HG decreased body mass (12%), plasma TG (15%), plasma glycerol (23%), plasma free fatty acids (14%), and hepatic TG content (42%), suggesting that angiotensin receptor type 1 (AT1) activation and increased adiposity contribute to the development of obesity-related dyslipidemia. ARB in HG also decreased hepatic nitrotyrosine and increased hepatic UCP2 expression (59%) and aconitase activity (40%), as well as antioxidant enzyme activities (50-120%), suggesting that AT1 activation also contributes to protein oxidation, impaired lipid metabolism, and antioxidant metabolism in the liver. Thus, in addition to promoting obesity-related hypertension, AT1 activation may also impair lipid metabolism and antioxidant capacity, resulting in steatosis via decreased UCP2 and tricarboxylic acid cycle activity.

Inhibition of uncoupling protein 2 with genipin exacerbates palmitate-induced hepatic steatosis

Background

Uncoupling protein 2 (UCP2) was reported to be involved in lipid metabolism through regulating the production of superoxide anion. However, the role of UCP2 in hepatocytes steatosis has not been determined. We hypothesized that UCP2 might regulate hepatic steatosis via suppressing oxidative stress.

Results

We tested this hypothesis in an in vitro model of hepatocytic steatosis in HepG2 cell lines induced by palmitic acid (PA). We found that treatment with PA induced an obvious lipid accumulation in HepG2 cells and a significant increase in intracellular triglyceride content. Moreover, the specific inhibition of UCP2 by genipin remarkably exacerbated PA-induced hepatocytes steatosis. Interestingly, the PA-induced superoxide overproduction can also be enhanced by incubation with genipin. In addition, administration with the antioxidant tempol abolished genipin-induced increase in intracellular lipid deposition. We further found that genipin significantly increased the protein expression of fatty acid translocase (FAT)/CD36.

Conclusions

These findings suggest that UCP2 plays a protective role in PA-induced hepatocytic steatosis through ameliorating oxidative stress.

Protective effect of grape seed proanthocyanidins against liver ischemic reperfusion injury: particularly in diet-induced obese mice

Background: Hepatic ischemia and reperfusion injury (IRI) is a major complication in liver surgery, and hepatic steatosis is a primary factor aggravating cellular injury during IRI. Both pro-inflammatory cytokines and reactive oxygen species (ROS) are key mediators of hepatic IRI. Ischemic preconditioning (IpreC), remote ischemia preconditioning (RIPC) and ischemic postconditioning (IpostC) have offered protections on hepatic IRI, but all these methods have their own shortcomings. Grape seed proanthocyanidins (GSP) has a broad spectrum of pharmacological properties against oxidative stress. Thus, GSP has potential protective effects against hepatic IRI.

Methods: C57BL/6 mice suffering 30mins hepatic ischemia process were sacrificed after 1h reperfusion to build murine warm hepatic IRI model. The mice were injected GSP intraperitoneally 10, 20, 40mg/kg/day for 3 weeks as pharmacological preconditioning. Obese mice fed with high-fat diet for 24 weeks before used. Three pathways related to IRI, including ROS elimination, pro-inflammatory cytokines release and hypoxia responses were examined.

Results: Our data show that GSP could significantly reduce hepatic IRI by protecting hepatocyte function and increasing the activity of ROS scavengers, as well as decreasing cytokines levels. At the same time, GSP also enhance the hypoxia tolerance response. Combined GSP and postconditioning can provided synergistic protection. In the obese mice suffering hepatic IRI group, GSP was more effective than postconditioning on protecting liver against IRI, and the combined strategy was obviously superior to the solo treatment.

Conclusion: GSP could protect liver against IRI: particularly in high-fat diet induced obese mice. GSP used as pharmacological preconditioning and combined with other protocols have huge potential to be used in clinical.

Hyperinsulinemia-induced vascular smooth muscle cell (VSMC) migration and proliferation is mediated by converging mechanisms of mitochondrial dysfunction and oxidative stress

Atherosclerosis is one of the major complications of diabetes and involves endothelial dysfunction, matrix alteration, and most importantly migration and proliferation of vascular smooth muscle cells (VSMCs). Although hyperglycemia and hyperinsulinemia are known to contribute to atherosclerosis, little is known about the specific cellular signaling pathways that mediate the detrimental hyperinsulinemic effects in VSMCs. Therefore, we investigated the cellular mechanisms of hyperinsulinemia-induced migration and proliferation of VSMCs. VSMCs were treated with insulin (100 nM) for 6 days and subjected to various physiological and molecular investigations. VSMCs subjected to hyperinsulinemia exhibited increased migration and proliferation, and this is paralleled by oxidative stress [increased NADPH oxidase activity, NADPH oxidase 1 mRNA expression, and reactive oxygen species (ROS) generation], alterations in mitochondrial physiology (membrane depolarization, decreased mitochondrial mass, and increased mitochondrial ROS), changes in mitochondrial biogenesis-related genes (mitofusin 1, mitofusin 2, dynamin-related protein 1, peroxisome proliferator-activated receptor gamma coactivator 1-alpha, peroxisome proliferator-activated receptor gamma coactivator 1-beta, nuclear respiratory factor 1, and uncoupling protein 2), and increased Akt phosphorylation. Diphenyleneiodonium, a known NADPH oxidase inhibitor significantly inhibited migration and proliferation of VSMCs and normalized all the above physiological and molecular perturbations. This study suggests a plausible crosstalk between mitochondrial dysfunction and oxidative stress under hyperinsulinemia and emphasizes counteracting mitochondrial dysfunction and oxidative stress as a novel therapeutic strategy for atherosclerosis.

Resveratrol up-regulates hepatic uncoupling protein 2 and prevents development of nonalcoholic fatty liver disease in rats fed a high-fat diet

Obesity is associated with a markedly increased risk of nonalcoholic fatty liver disease. The anti-inflammatory polyphenol resveratrol possess promising properties in preventing this metabolic condition by dampening the pathological inflammatory reaction in the hepatic tissue. However, in the current study, we hypothesize that the beneficial effect of resveratrol is not solely attributable to its anti-inflammatory potential. Eight-week-old male Wistar rats were randomly distributed into 3 groups of 12 animals each: control diet (C), high-fat diet (HF), and HF supplemented with 100 mg resveratrol daily (HFR). After 8 weeks of dietary treatment, the rats were euthanized and relevant tissues were prepared for subsequent analysis. Resveratrol prevented the high fat-induced steatosis assessed by semiquantitative grading, which furthermore corresponded with a complete normalization of the hepatic triglyceride content (P < .001), despite no change in total body fat. In HFR, the hepatic uncoupling protein 2 expression was significantly increased by 76% and 298% as compared with HF and C, respectively. Moreover, the hepatic mitochondria content in HFR was significantly higher as compared with both C and HF (P < .001 and P = .004, respectively). We found no signs of hepatic inflammation, hereby demonstrating that resveratrol protects against fatty liver disease independently of its proposed anti-inflammatory potential. Our data might indicate that an increased number of mitochondria and, particularly, an increase in hepatic uncoupling protein 2 expression are involved in normalizing the hepatic fat content due to resveratrol supplementation in rodents fed a high-fat diet.

Higher dietary fructose is associated with impaired hepatic adenosine triphosphate homeostasis in obese individuals with type 2 diabetes

Fructose consumption predicts increased hepatic fibrosis in those with nonalcoholic fatty liver disease (NAFLD). Because of its ability to lower hepatic adenosine triphosphate (ATP) levels, habitual fructose consumption could result in more hepatic ATP depletion and impaired ATP recovery. The degree of ATP depletion after an intravenous (IV) fructose challenge test in low- versus high-fructose consumers was assessed. We evaluated diabetic adults enrolled in the Action for Health in Diabetes Fatty Liver Ancillary Study (n = 244) for whom dietary fructose consumption estimated by a 130-item food frequency questionnaire and hepatic ATP measured by phosphorus magnetic resonance spectroscopy and uric acid (UA) levels were performed (n = 105). In a subset of participants (n = 25), an IV fructose challenge was utilized to assess change in hepatic ATP content. The relationships between dietary fructose, UA, and hepatic ATP depletion at baseline and after IV fructose challenge were evaluated in low- (<15 g/day) versus high-fructose (≥ 15 g/day) consumers. High dietary fructose consumers had slightly lower baseline hepatic ATP levels and a greater absolute change in hepatic α-ATP/ inorganic phosphate (Pi) ratio (0.08 versus 0.03; P = 0.05) and γ-ATP /Pi ratio after an IV fructose challenge (0.03 versus 0.06; P = 0.06). Patients with high UA (≥ 5.5 mg/dL) showed a lower minimum liver ATP/Pi ratio postfructose challenge (4.5 versus 7.0; P = 0.04).

CONCLUSIONS

High-fructose consumption depletes hepatic ATP and impairs recovery from ATP depletion after an IV fructose challenge. Subjects with high UA show a greater nadir in hepatic ATP in response to fructose. Both high dietary fructose intake and elevated UA level may predict more severe hepatic ATP depletion in response to fructose and hence may be risk factors for the development and progression of NAFLD.

Progression of type 2 diabetes in GK rats affects muscle and liver mitochondria differently: pronounced reduction of complex II flux is observed in liver only

Impaired mitochondrial function is implicated in the development of type 2 diabetes mellitus (T2DM). This was investigated in mitochondria from skeletal muscle and liver of the Goto-Kakizaki (GK) rat, which spontaneously develops T2DM with age. The early and the manifest stage of T2DM was studied in 6- and 16-wk-old GK rats, respectively. In GK16 compared with GK6 animals, a decrease in state 3 respiration with palmitoyl carnitine (PC) as substrate was observed in muscle. Yet an increase was seen in liver. To test the complex II contribution to the state 3 respiration, succinate was added together with PC. In liver mitochondria, this resulted in an ∼50% smaller respiratory increase in the GK6 group compared with control and no respiratory increase at all in the GK16 animals. Yet no difference between groups was seen in muscle mitochondria. RCR and P/O ratio was increased (P < 0.05) in liver but unchanged in muscle in both GK groups. We observed increased lipid peroxidation and decreased Akt phosphorylation in liver with the progression of T2DM but no change in muscle. We conclude that, during the progression of T2DM in GK rats, liver mitochondria are affected earlier and/or more severely than muscle mitochondria. Succinate dehydrogenase flux in the presence of fatty acids was reduced severely in liver but not in muscle mitochondria during manifest T2DM. The observations support the notion that T2DM pathogenesis is initiated in the liver and that only later are muscle mitochondria affected.

Role of inducible nitric oxide synthase in mitochondrial depolarization and graft injury after transplantation of fatty livers

This study investigated the role of inducible nitric oxide synthase (iNOS) in failure of ethanol-induced fatty liver grafts. Rat livers were explanted 20 h after gavaging with ethanol (5 g/kg) and storing in UW solution for 24h before implantation. Hepatic oil red O staining-positive areas increased from ∼2 to ∼33% after ethanol treatment, indicating steatosis. iNOS expression increased ∼8-fold after transplantation of lean grafts (LG) and 25-fold in fatty grafts (FG). Alanine aminotransferase release, total bilirubin, hepatic necrosis, TUNEL-positive cells, and cleaved caspase-3 were higher in FG than LG. A specific iNOS inhibitor 1400W (5 μM in the cold-storage solution) blunted these alterations by >42% and increased survival of fatty grafts from 25 to 88%. Serum nitrite/nitrate and hepatic nitrotyrosine adducts increased to a greater extent after transplantation of FG than LG, indicating reactive nitrogen species (RNS) overproduction. Phospho-c-Jun and phospho-c-Jun N-terminal kinase-1/2 (JNK1/2) were higher in FG than in LG, indicating more JNK activation in fatty grafts. RNS formation and JNK activation were blunted by 1400W. Mitochondrial polarization and cell death were visualized by intravital multiphoton microscopy of rhodamine 123 and propidium iodide, respectively. After implantation, viable cells with depolarized mitochondria were 3-fold higher in FG than in LG and 1400W decreased mitochondrial depolarization in FG to the levels of LG. Taken together, iNOS is upregulated after transplantation of FG, leading to excessive RNS formation, JNK activation, mitochondrial dysfunction, and severe graft injury. The iNOS inhibitor 1400W could be an effective therapy for primary nonfunction of fatty liver grafts.

Mitochondrial uncouplers inhibit hepatic stellate cell activation

Background

Mitochondrial dysfunction participates in the progression of several pathologies. Although there is increasing evidence for a mitochondrial role in liver disease, little is known about its contribution to hepatic stellate cell (HSC) activation. In this study we investigated the role of mitochondrial activity through mild uncoupling during in vitro activation of HSCs.

Methods

Cultured primary human and mouse HSCs were treated with the chemical uncouplers FCCP and Valinomycin. ATP levels were measured by luciferase assay and production of reactive oxygen species was determined using the fluorescent probe DCFH-DA. Possible cytotoxicity by uncoupler treatment was evaluated by caspase 3/7 activity and cytoplasmic protease leakage. Activation of HSCs and their response to the pro-fibrogenic cytokine TGF-β was evaluated by gene expression of activation markers and signal mediators using RT-qPCR. Proliferation was measured by incorporation of EdU and protein expression of α-smooth muscle actin was analyzed by immunocytochemistry and western blot.

Results

FCCP and Valinomycin treatment mildly decreased ATP and reactive oxygen species levels. Both uncouplers increased the expression of mitochondrial genes such as Tfam and COXIV while inducing morphological features of quiescent mouse HSCs and abrogating TGF-β signal transduction. Mild uncoupling reduced HSC proliferation and expression of pro-fibrogenic markers of mouse and human HSCs.

Conclusions

Mild mitochondrial uncoupling inhibits culture-induced HSC activation and their response to pro-fibrogenic cytokines like TGF-β. These results therefore suggest mitochondrial uncoupling of HSCs as a strategy to reduce progression of liver fibrosis.

The current state of knowledge of hepatic ischemia-reperfusion injury based on its study in experimental models

The present review focuses on the numerous experimental models used to study the complexity of hepatic ischemia/reperfusion (I/R) injury. Although experimental models of hepatic I/R injury represent a compromise between the clinical reality and experimental simplification, the clinical transfer of experimental results is problematic because of anatomical and physiological differences and the inevitable simplification of experimental work. In this review, the strengths and limitations of the various models of hepatic I/R are discussed. Several strategies to protect the liver from I/R injury have been developed in animal models and, some of these, might find their way into clinical practice. We also attempt to highlight the fact that the mechanisms responsible for hepatic I/R injury depend on the experimental model used, and therefore the therapeutic strategies also differ according to the model used. Thus, the choice of model must therefore be adapted to the clinical question being answered.

Hepatic ratio of phosphatidylcholine to phosphatidylethanolamine predicts survival after partial hepatectomy in mice

A major predictor of failed liver resection and transplantation is nonalcoholic fatty liver disease (NAFLD). NAFLD is linked to a wide spectrum of diseases including obesity and diabetes that are increasingly prevalent in Western populations. Thus, it is important to develop therapies aimed at improving posthepatectomy outcomes in patients with NAFLD, as well as to improve the evaluation of patients slated for hepatic surgery. Decreased hepatic phosphatidylcholine (PC) content and decreased ratio of hepatic PC to phosphatidylethanolamine (PE) have previously been linked to NAFLD. To determine if decreased hepatic PC/PE could predict survival after hepatectomy, we used mouse models lacking key enzymes in PC biosynthesis, namely, phosphatidylethanolamine N-methyltransferase and hepatic-specific CTP:phosphocholine cytidylyltransferase α. These mice were fed a high-fat diet to induce NAFLD. We then performed a 70% partial hepatectomy and monitored postoperative survival. We identified hepatic PC/PE to be inversely correlated with the development of steatosis and inflammation in the progression of NAFLD. Decreased hepatic PC/PE before surgery was also strongly associated with decreased rates of survival after partial hepatectomy. Choline supplementation to the diet increased hepatic PC/PE in Pemt(-/-) mice with NAFLD, decreased inflammation, and increased the survival rate after partial hepatectomy.

CONCLUSION

Decreased hepatic PC/PE is a predictor of NAFLD and survival following partial hepatectomy. Choline supplementation may serve as a potential therapy to prevent the progression of NAFLD and to improve postoperative outcome after liver surgery.

Genetically modified mouse models for the study of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is associated with obesity, insulin resistance, and type 2 diabetes. NAFLD represents a large spectrum of diseases ranging from (1) fatty liver (hepatic steatosis); (2) steatosis with inflammation and necrosis; to (3) cirrhosis. The animal models to study NAFLD/nonalcoholic steatohepatitis (NASH) are extremely useful, as there are still many events to be elucidated in the pathology of NASH. The study of the established animal models has provided many clues in the pathogenesis of steatosis and steatohepatitis, but these remain incompletely understood. The different mouse models can be classified in two large groups. The first one includes genetically modified (transgenic or knockout) mice that spontaneously develop liver disease, and the second one includes mice that acquire the disease after dietary or pharmacological manipulation. Although the molecular mechanism leading to the development of hepatic steatosis in the pathogenesis of NAFLD is complex, genetically modified animal models may be a key for the treatment of NAFLD. Ideal animal models for NASH should closely resemble the pathological characteristics observed in humans. To date, no single animal model has encompassed the full spectrum of human disease progression, but they can imitate particular characteristics of human disease. Therefore, it is important that the researchers choose the appropriate animal model. This review discusses various genetically modified animal models developed and used in research on NAFLD.

[Effects of the mixture of Swertia pseudochinensis Hara and Silybum marianum Gaertn extracts on CCl(4)-induced liver injury in rats with non-alcoholic fatty liver disease]

OBJECTIVE

To study the mechanism of liver injury induced by carbon tetrachloride (CCl(4)) in rats with non-alcoholic fatty liver disease (NAFLD), and the therapeutic effects of the extract mixture of Dangyao (Swertia pseudochinensis Hara) and Shuifeiji (Silybum marianum Gaertn) on NAFLD rats with liver injury.

METHODS

Male Wistar rats were randomized into normal control group, CCl(4) group, high-fat diet group, high-fat diet plus CCl(4) injection group (model group), diammonium glycyrrhizinate group and extract mixture group. Except the normal control and CCl(4) groups, rats were fed with high-fat diet (88% normal chow, 10% lard and 2% cholesterol) to induce NAFLD. Diammonium glycyrrhizinate and extracts were given by gavage. After eight weeks, a nonlethal dose of CCl(4) was injected intraperitoneally to all rats except the normal and high-fat diet groups. And 48 h later, all rats were sacrificed, and serum and liver tissues were collected for further study. Paraffin-processed liver tissue was stained with hematoxylin-eosin (HE) to observe the pathological changes. Serum alamine aminotransferase (ALT) and aspartate aminotransferase (AST) were measured. The levels of triacylglycerol (TAG), malondialdehyde (MDA) and glutathione (GSH) in liver tissues were also examined. Expression of uncoupling protein 2 (UCP2) was determined by reverse transcription-polymerase chain reaction and Western blotting.

RESULTS

Liver sections stained with HE showed that the histopathological changes in the normal control group and the CCl(4) group were mild; massive hepatosteatosis diffusing in lobules was shown in the high-fat diet groups; steatosis, hepatocellular ballooning degeneration and inflammatory infiltration were severe around the central vein in sections of the model group. Compared with the model group, hepatosteatosis and ballooning were significantly attenuated in the treatment groups. Levels of serum ALT and AST, contents of TAG and MDA and the UCP2 expression in liver tissues of the model group increased obviously, while the level of liver GSH decreased. Compared with rats in the model group, the above biomarkers in the treatment groups were improved significantly.

CONCLUSION

The mixture of Dangyao and Shuifeiji extracts can decrease the susceptibility and degree of liver injury induced by hepatotoxin in rats with NAFLD. Regulation of the balance of pro- and anti-oxidative stress factors is involved in the mechanism.

Mitochondrial uncoupling protein-2 deficiency protects steatotic mouse hepatocytes from hypoxia/reoxygenation

Steatotic livers are sensitive to ischemic events and associated ATP depletion. Hepatocellular necrosis following these events may result from mitochondrial uncoupling protein-2 (UCP2) expression. To test this hypothesis, we developed a model of in vitro steatosis using primary hepatocytes from wild-type (WT) and UCP2 knockout (KO) mice and subjected them to hypoxia/reoxygenation (H/R). Using cultured hepatocytes treated with emulsified fatty acids for 24 h, generating a steatotic phenotype (i.e., microvesicular and broad-spectrum fatty acid accumulation), we found that the phenotype of the WT and UCP2 KO were the same; however, cellular viability was increased in the steatotic KO hepatocytes following 4 h of hypoxia and 24 h of reoxygenation; Hepatocellular ATP levels decreased during hypoxia and recovered after reoxygenation in the control and UCP2 KO steatotic hepatocytes but not in the WT steatotic hepatocytes; mitochondrial membrane potential in WT and UCP2 KO steatotic groups was less than control groups but higher than UCP2 KO hepatocytes. Following reoxygenation, lipid peroxidation, as measured by thiobarbituric acid reactive substances, increased in all groups but to a greater extent in the steatotic hepatocytes, regardless of UCP2 expression. These results demonstrate that UCP2 sensitizes steatotic hepatocytes to H/R through mitochondrial depolarization and ATP depletion but not lipid peroxidation.

Animal models of cutaneous and hepatic fibrosis

Fibrosis occurs as a part of normal wound healing. However, excessive or dysregulated fibrosis can lead to severe organ dysfunction and is a feature of a variety of diseases. Due to its insidious onset, fibrosis tends to go undetected in its early stages. This is in part why these diseases remain so poorly understood. Animal models have provided a means to examine these early stages and to isolate and understand the effect of perturbations in signaling pathways, chemokines, and cytokines. Here, we summarize recent progress in the understanding of the molecular pathogenesis of fibrosis, both its initiation and its maintenance phases, from animal models of fibrosis in the skin and liver. Due to these organs' properties, modeling fibrosis in them poses unique challenges. Elegant solutions have therefore been developed for modeling fibrosis in each, and now, great potential for animal models to contribute to our understanding appears scientifically imminent.

Uncoupling proteins: molecular, functional, regulatory, physiological and pathological aspects

Uncoupling proteins are a subfamily of the mitochondrial anion carrier family. They are widespread in the whole eukaryotic world with a few exceptions and present tissue specific isoforms in higher organisms. They mediate purine nucleotide-sensitive free fatty acid-activated proton inward flux through the inner mitochondrial membrane. This proton flux occurs at the expense of the proton motive force build up by the respiration and weakens the coupling between respiration and ATP synthesis. In this chapter we describe current and reliable knowledge of uncoupling proteins. A new methodology allowing study of their activity and regulation during phosphorylating respiration is described. It has entitled us to assert that all uncoupling proteins share common mechanisms of activation and regulation. This is of the utmost importance in order to understand the physiological roles of UCPs as well as their participation in pathological processes since every role of the UCPs in every cell is an integral part of their function and regulation. The central role of reduction level of ubiquinone in the control of their regulation is well-argued. Their potential and reliable roles in thermogenesis, reactive oxygen species prevention and energy flow are discussed as well as their role in some pathological disorders.

Role of oxidative stress in the pathogenesis of nonalcoholic steatohepatitis

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

Hog millet (Panicum miliaceum L.)-supplemented diet ameliorates hyperlipidemia and hepatic lipid accumulation in C57BL/6J-ob/ob mice

Dietary intake of whole grains reduces the incidence of chronic diseases such as obesity, diabetes, cardiovascular disease, and cancer. In an earlier study, we showed that Panicum miliaceum L. extract (PME) exhibited the highest anti-lipogenic activity in 3T3-L1 cells among extracts of nine different cereal grains tested. In this study, we hypothesized that PME in the diet would lead to weight loss and augmentation of hyperlipidemia by regulating fatty acid metabolism. PME was fed to ob/ob mice at 0%, 0.5%, or 1% (w/w) for 4 weeks. After the experimental period, body weight changes, blood serum and lipid profiles, hepatic fatty acid metabolism-related gene expression, and white adipose tissue (WAT) fatty acid composition were determined. We found that the 1% PME diet, but not the 0.5%, effectively decreased body weight, liver weight, and blood triglyceride and total cholesterol levels (P < 0.05) compared to obese ob/ob mice on a normal diet. Hepatic lipogenic-related gene (PPARα, L-FABP, FAS, and SCD1) expression decreased, whereas lipolysis-related gene (CPT1) expression increased in animals fed the 1% PME diet (P < 0.05). Long chain fatty acid content and the ratio of C18:1/C18:0 fatty acids decreased significantly in adipose tissue of animals fed the 1% PME diet (P < 0.05). Serum inflammatory mediators also decreased significantly in animals fed the 1% PME diet compared to those of the ob/ob control group (P < 0.05). These results suggest that PME is useful in the chemoprevention or treatment of obesity and obesity-related disorders.

UCP2 mRNA expression is dependent on glucose metabolism in pancreatic islets

Uncoupling Protein 2 (UCP2) is expressed in the pancreatic β-cell, where it partially uncouples the mitochondrial proton gradient, decreasing both ATP-production and glucose-stimulated insulin secretion (GSIS). Increased glucose levels up-regulate UCP2 mRNA and protein levels, but the mechanism for UCP2 up-regulation in response to increased glucose is unknown. The aim was to examine the effects of glucokinase (GK) deficiency on UCP2 mRNA levels and to characterize the interaction between UCP2 and GK with regard to glucose-stimulated insulin secretion in pancreatic islets. UCP2 mRNA expression was reduced in GK+/- islets and GK heterozygosity prevented glucose-induced up-regulation of islet UCP2 mRNA. In contrast to UCP2 protein function UCP2 mRNA regulation was not dependent on superoxide generation, but rather on products of glucose metabolism, because MnTBAP, a superoxide dismutase mimetic, did not prevent the glucose-induced up-regulation of UCP2. Glucose-stimulated insulin secretion was increased in UCP2-/- and GK+/- islets compared with GK+/- islets and UCP2 deficiency improved glucose tolerance of GK+/- mice. Accordingly, UCP2 deficiency increased ATP-levels of GK+/- mice. Thus, the compensatory down-regulation of UCP2 is involved in preserving the insulin secretory capacity of GK mutant mice and might also be implicated in limiting disease progression in MODY2 patients.

High-sucrose diet increases ROS generation, FFA accumulation, UCP2 level, and proton leak in liver mitochondria

Obesity, a risk factor for insulin resistance, contributes to the development of type 2 diabetes and cardiovascular diseases. The relationship between increased levels of free fatty acids in the liver mitochondria, mitochondrial function, and ROS generation in rat model of obesity induced by a high-sucrose diet was not sufficiently established. We determined how the bioenergetic functions and ROS generation of the mitochondria respond to a hyperlipidemic environment. Mitochondria from sucrose-fed rats generated H(2)O(2) at a higher rate than the control mitochondria. Adding fatty acid-free bovine serum albumin to mitochondria from sucrose-fed rats significantly reduced the rate of H(2)O(2) generation. In contrast, adding exogenous oleic or linoleic acid exacerbated the rate of H(2)O(2) generation in both sucrose-fed and control mitochondria, and the mitochondria from sucrose-fed rats were more sensitive than the control mitochondria. The increased rate of H(2)O(2) generation in sucrose-fed mitochondria corresponded to decreased levels of reduced GSH and vitamin E and increased levels of Cu/Zn-SOD in the intermembrane space. There was no difference between the levels of lipid peroxidation and protein carbonylation in the two types of mitochondria. In addition to the normal activity of Mn-SOD, GPX and catalase detected an increased activity of complex II, and upregulation of UCP2 was observed in mitochondria from sucrose-fed rats, all of which may accelerate respiration rates and reduce generation of ROS. In turn, these effects may protect the mitochondria of sucrose-fed rats from oxidative stress and preserve their function and integrity. However, in whole liver these adaptive mechanisms of the mitochondria were inefficient at counteracting redox imbalances and inhibiting oxidative stress outside of the mitochondria.

Role of UCP2 expression after hepatic warm ischemia-reperfusion in the rat

Background/Aims

The role of uncoupling protein-2 (UCP2) in the liver is currently unclear. Emerging evidence suggests a relationship between UCP2 and oxidative stress. In the present study, we tested the hypothesis that UCP2 expression in the liver might change during warm ischemia-reperfusion (I/R) according to oxidative stress.

Methods

Wistar rats were subjected to 40 (short ischemia) or 90 (long ischemia) minutes of partial lobar ischemia followed by 4 hours of reperfusion. UCP2 expression in the ischemic and nonischemic lobes was assessed using reverse transcription-polymerase chain reaction and immunohistochemistry. Malondialdehyde concentrations in the liver tissue were also compared.

Results

Malondialdehyde concentrations in the ischemic lobes were significantly higher in the long ischemia group. In the ischemic lobes of the short ischemia group, UCP2 protein expression was induced in hepatocytes, which did not express the protein prior to treatment, and the expression levels were higher than in the long ischemia group. The intralobular distribution of UCP2 seemed to correlate inversely with that of the necrotic area. UCP2 expression was observed, even in nonischemic lobes with similar intralobular heterogeneity.

Conclusions

UCP2 was induced in hepatocytes after warm I/R. Although the primitive role of UCP2 expression may be cytoprotective in nature, its actual protective effect in hepatic I/R may be minimal

Sustained activation of PPARα by endogenous ligands increases hepatic fatty acid oxidation and prevents obesity in ob/ob mice

Obesity, a major health concern, results from an imbalance between energy intake and expenditure. Leptin-deficient ob/ob mice are paradigmatic of obesity, resulting from excess energy intake and storage. Mice lacking acyl-CoA oxidase 1 (Acox1), the first enzyme of the peroxisomal fatty acid β-oxidation system, are characterized by increased energy expenditure and a lean body phenotype caused by sustained activation of peroxisome proliferator-activated receptor α (PPARα) by endogenous ligands in liver that remain unmetabolized in the absence of Acox1. We generated ob/ob mice deficient in Acox1 (Acox1(-/-)) to determine how the activation of PPARα by endogenous ligands might affect the obesity of ob/ob mice. In contrast to Acox1(-/-) (14.3±1.2 g at 6 mo) and the Acox1-deficient (ob/ob) double-mutant mice (23.8±4.6 g at 6 mo), the ob/ob mice are severely obese (54.3±3.2 g at 6 mo) and had significantly more (P<0.01) epididymal fat content. The resistance of Acox1(-/-)/ob/ob mice to obesity is due to increased PPARα-mediated up-regulation of genes involved in fatty acid oxidation in liver. Activation of PPARα in Acox1-deficient ob/ob mice also reduces serum glucose and insulin (P<0.05) and improves glucose tolerance and insulin sensitivity. Further, PPARα activation reduces hepatic steatosis and increases hepatocellular regenerative response in Acox1(-/-)/ob/ob mice at a more accelerated pace than in mice lacking only Acox1. However, Acox1(-/-)/ob/ob mice manifest hepatic endoplasmic reticulum (ER) stress and also develop hepatocellular carcinomas (8 of 8 mice) similar to those observed in Acox1(-/-) mice (10 of 10 mice), but unlike in ob/ob (0 of 14 mice) and OB/OB (0 of 6 mice) mice, suggesting that superimposed ER stress and PPARα activation contribute to carcinogenesis in a fatty liver. Finally, absence of Acox1 in ob/ob mice can impart resistance to high-fat diet (60% fat)-induced obesity, and their liver had significantly (P<0.01) more cell proliferation. These studies with Acox1(-/-)/ob/ob mice indicate that sustained activation of lipid-sensing nuclear receptor PPARα attenuates obesity and restores glucose homeostasis by ameliorating insulin resistance but increases the risk for liver cancer development, in part related to excess energy combustion.

Oxidative phosphorylation flexibility in the liver of mice resistant to high-fat diet-induced hepatic steatosis

OBJECTIVE

To identify metabolic pathways that may underlie susceptibility or resistance to high-fat diet-induced hepatic steatosis.

RESEARCH DESIGN AND METHODS

We performed comparative transcriptomic analysis of the livers of A/J and C57Bl/6 mice, which are, respectively, resistant and susceptible to high-fat diet-induced hepatosteatosis and obesity. Mice from both strains were fed a normal chow or a high-fat diet for 2, 10, and 30 days, and transcriptomic data were analyzed by time-dependent gene set enrichment analysis. Biochemical analysis of mitochondrial respiration was performed to confirm the transcriptomic analysis.

RESULTS

Time-dependent gene set enrichment analysis revealed a rapid, transient, and coordinate upregulation of 13 oxidative phosphorylation genes after initiation of high-fat diet feeding in the A/J, but not in the C57Bl/6, mouse livers. Biochemical analysis using liver mitochondria from both strains of mice confirmed a rapid increase by high-fat diet feeding of the respiration rate in A/J but not C57Bl/6 mice. Importantly, ATP production was the same in both types of mitochondria, indicating increased uncoupling of the A/J mitochondria.

CONCLUSIONS

Together with previous data showing increased expression of mitochondrial β-oxidation genes in C57Bl/6 but not A/J mouse livers, our present study suggests that an important aspect of the adaptation of livers to high-fat diet feeding is to increase the activity of the oxidative phosphorylation chain and its uncoupling to dissipate the excess of incoming metabolic energy and to reduce the production of reactive oxygen species. The flexibility in oxidative phosphorylation activity may thus participate in the protection of A/J mouse livers against the initial damages induced by high-fat diet feeding that may lead to hepatosteatosis.

Redox balance in the pathogenesis of nonalcoholic fatty liver disease: mechanisms and therapeutic opportunities

Nonalcoholic fatty liver disease (NAFLD) is currently the most common liver disease in the world. It encompasses a histological spectrum, ranging from simple, nonprogressive steatosis to nonalcoholic steatohepatitis (NASH), which may progress to cirrhosis and hepatocellular carcinoma. While liver-related complications are confined to NASH, emerging evidence suggests both simple steatosis and NASH predispose to type 2 diabetes and cardiovascular disease. The pathogenesis of NAFLD is currently unknown, but accumulating data suggest that oxidative stress and altered redox balance play a crucial role in the pathogenesis of steatosis, steatohepatitis, and fibrosis. We will examine intracellular mechanisms, including mitochondrial dysfunction and impaired oxidative free fatty acid metabolism, leading to reactive oxygen species generation; additionally, the potential pathogenetic role of extracellular sources of reactive oxygen species in NAFLD, including increased myeloperoxidase activity and oxidized low density lipoprotein accumulation, will be reviewed. We will discuss how these mechanisms converge to determine the whole pathophysiological spectrum of NAFLD, including hepatocyte triglyceride accumulation, hepatocyte apoptosis, hepatic inflammation, hepatic stellate cell activation, and fibrogenesis. Finally, available animal and human data on treatment opportunities with older and newer antioxidant will be presented.

Multidrug donor preconditioning protects steatotic liver grafts against ischemia-reperfusion injury

BACKGROUND

Graft dysfunction of steatotic livers (SL) still remains a major challenge in liver transplantation. Different mechanisms are thought to be involved in the impaired tolerance of SL to ischemia-reperfusion injury. Thus, different pharmacologic strategies may need to be combined to effectively protect SL and to reduce graft dysfunction after transplantation. Therefore, we analyzed the effectiveness of a multidrug donor preconditioning (MDDP) procedure to protect SL from cold ischemia-reperfusion injury.

METHODS

Liver steatosis was induced by a high-carbohydrate, fat-free diet. A total of 24 Sprague-Dawley rats were divided into 3 groups (n = 8 each), including a control group with nonsteatotic livers (Con), a vehicle-treated SL group (SL-Con), and a SL group undergoing MDDP (SL-MDDP), including pentoxyphylline, glycine, deferoxamine, N-acetylcysteine, erythropoietin, melatonin, and simvastatin. MDDP was applied before liver perfusion with 4°C histidine-tryptophan-ketoglutarate (HTK) solution and organ harvest. After 24 hours of cold storage in HTK, postischemic reperfusion was performed in an isolated liver reperfusion model using 37°C Krebs-Henseleit bicarbonate buffer.

RESULTS

After 60 minutes of reperfusion, SL showed a significant reduction of bile flow as well as a marked increase of liver enzyme levels and apoptotic cell death compared with Con. This was associated with an increased malondialdehyde formation, interleukin-1 production, and leukocytic tissue infiltration. MDDP completely abolished the inflammatory response and was capable of significantly reducing parenchymal dysfunction and injury.

CONCLUSIONS

MDDP decreases SL injury after cold storage and reperfusion. The concept of MDDP as a simple and safe preoperative regime, thus may be of interest in clinical use, expanding the donor pool from marginal donors.

Mitochondria and redox signaling in steatohepatitis

Alcoholic and nonalcoholic fatty liver diseases are potentially pathological conditions that can progress to steatohepatitis, fibrosis, and cirrhosis. These conditions affect millions of people throughout the world in part through poor lifestyle choices of excess alcohol consumption, overnutrition, and lack of regular physical activity. Abnormal mitochondrial and cellular redox homeostasis has been documented in steatohepatitis and results in alterations of multiple redox-sensitive signaling cascades. Ultimately, these changes in signaling lead to altered enzyme function and transcriptional activities of proteins critical to mitochondrial and cellular function. In this article, we review the current hypotheses linking mitochondrial redox state to the overall pathophysiology of alcoholic and nonalcoholic steatohepatitis and briefly discuss the current therapeutic options under investigation.

Chronic exposure to a high-fat diet induces hepatic steatosis, impairs nitric oxide bioavailability, and modifies the mitochondrial proteome in mice

Obesity-related pathologies, such as nonalcoholic fatty liver disease, are linked to mitochondrial dysfunction and nitric oxide (NO) deficiency. Herein, we tested the hypothesis that a high-fat diet (HFD) modifies the liver mitochondrial proteome and alters proteins involved in NO metabolism, namely arginase 1 and endothelial NO synthase. Male C57BL/6 mice were fed a control or HFD and liver mitochondria were isolated for proteomics and reactive oxygen species measurements. Steatosis and hepatocyte ballooning were present in livers of HFD mice, with no pathology observed in the controls. HFD mice had increased serum glucose and decreased adiponectin. Mitochondrial reactive oxygen species was increased after 8 weeks in the HFD mice, but decreased at 16 weeks compared with the control, which was accompanied by increased uncoupling protein 2. Using proteomics, 22 proteins were altered as a consequence of the HFD. This cohort consists of oxidative phosphorylation, lipid metabolism, sulfur amino acid metabolism, and chaperone proteins. We observed a HFD-dependent increase in arginase 1 and decrease in activated endothelial NO synthase. Serum and liver nitrate + nitrite were decreased by HFD. In summary, these data demonstrate that a HFD causes steatosis, alters NO metabolism, and modifies the liver mitochondrial proteome; thus, NO may play an important role in the processes responsible for nonalcoholic fatty liver disease.

The Anti-Obesity Effect of the Palatinose-Based Formula Inslow is Likely due to an Increase in the Hepatic PPAR-alpha and Adipocyte PPAR-gamma Gene Expressions

Abdominal obesity is a principal risk factor in the development of metabolic syndrome. Previously, we showed that a palatinose-based liquid formula, Inslow/MHN-01, suppressed postprandial plasma glucose level and reduced visceral fat accumulation better than the standard formula (SF). To elucidate the mechanism of Inslow-mediated anti-obesity effect, expression levels of genes involved in the glucose and lipid metabolism were compared in Inslow- and SF-fed rats. Both fasting plasma insulin level and average islet sizes were reduced in the Inslow group. We also found less abdominal fat accumulation and reduced hepatic triacylglycerol content in the Inslow group. Expression of the β-oxidation enzymes and uncoupling potein-2 (UCP-2) mRNAs in the liver of the Inslow group were higher than the SF group, which was due to a concomitant higher expression of the peroxisome proliferator-activated receptor (PPAR)-α mRNA in the former. Furthermore, expression of the UCP-2 and adiponectin mRNAs in the epididymal fat were higher in the Inslow group than the SF group, and were stimulated by a concomitant increase of the PPAR-γ gene expression in the former. These results strongly suggested that the anti-obesity effect of Inslow was due to an increase in the hepatic PPAR-α and adipocyte PPAR-γ gene expressions.

Mitochondrial recoupling: a novel therapeutic strategy for cancer?

Recent findings link metabolic transformation of cancer cells to aberrant functions of mitochondrial uncoupling proteins (UCPs). By inducing proton leak, UCPs interfere with mitochondrial synthesis of adenosine 5'-triphosphate, which is also a key determinant of glycolytic pathways. In addition, UCP suppress the generation of superoxide, a byproduct of mitochondrial electron transport and a major source of oxidative stress. The near ubiquitous UCP2 becomes highly abundant in some cancers and may advance metabolic reprogramming, further disrupt tumour suppression, and promote chemoresistance. Here we review current evidence to suggest that inhibition of mitochondrial uncoupling may eliminate these responses and reveal novel anti-cancer strategies.

Contribution of endogenous inhibitor of nitric oxide synthase to hepatic mitochondrial dysfunction in streptozotocin-induced diabetic rats

AIMS

Mitochondrial dysfunction plays important roles in the development of diabetes. Elevated nitric oxide (NO) synthase inhibitor asymmetric dimethylarginine (ADMA) has been shown to be closely related to diabetes. But the relationship between them in diabetes has not been determined. This study was to explore the role of ADMA in hepatic mitochondrial dysfunction and its potential mechanisms in diabetic rats and hepatocytes.

METHODS

Respiratory enzymes activities, mitochondrial transmembrane potential and ATP content were measured to evaluate mitochondrial function. The copy number ratio of mitochondrial gene to nuclear gene was used to represent mitochondrial biogenesis. The activity of superoxide dismutase and malondialdehyde content were detected to reflect oxidative stress. Furthermore, changes in ADMA and NO contents, uncoupling protein 2 (UCP2) and peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α) transcriptions were determined.

RESULTS

Elevated ADMA levels in serum of diabetic rats were found to be associated with hepatic mitochondrial dysfunction reflected by reductions of respiratory enzyme activities, mitochondrial membrane potential and ATP contents. Similar mitochondrial dysfunction also occurred in ADMA-treated hepatocytes. The mitochondrial dysfunction observed in diabetic rats or hepatocytes was accompanied with suppressions of mitochondrial biogenesis, PGC-1α transcription and NO synthesis as well as enhances of UCP 2 transcription and oxidative stress. These effects of ADMA could be attenuated by treatments with antioxidant or NO donor.

CONCLUSIONS

These results indicate that elevated endogenous ADMA contributes to hepatic mitochondrial dysfunction in diabetic rats, and underlying mechanisms may be related to the suppression of mitochondrial biogenesis and mitochondrial uncoupling via inhibiting NO synthesis and enhancing oxidative stress.

Mitochondrial dysfunction in nonalcoholic steatohepatitis

The pathogenesis of nonalcoholic steatohepatitis (NASH) is poorly understood and the mechanisms are still being elucidated. Mitochondrial dysfunction participates at different levels in NASH pathogenesis since it impairs fatty liver homeostasis and induces overproduction of free radicals that in turn trigger lipid peroxidation and cell death. In this article, we review the role of mitochondria in fat metabolism, energy homeostasis and reactive oxygen species production, with a focus on the role of mitochondrial impairment and uncoupling proteins in the pathophysiology of NASH progression. The potential effects of some molecules targeted to mitochondria are also discussed.

Cell death and nonalcoholic steatohepatitis: where is ballooning relevant?

Nonalcoholic fatty liver disease (NAFLD) is the most common form of liver disease in the Western world. Progression to more aggressive forms of liver injury, such as nonalcoholic steatohepatitis (NASH) and cirrhosis, occurs in less than a third of affected subjects. Human data and both in vivo and in vitro models demonstrate that cell death, particularly apoptosis, is increased in NAFLD and NASH patients, suggesting that it is crucial in disease progression. Indeed, fatty acids - more specifically, saturated fatty acids - strongly induce hepatocyte apoptosis. In addition, hepatic steatosis renders hepatocytes more susceptible to apoptotic injury. Ballooned hepatocytes and Mallory-Denk bodies are important hallmarks of NASH and correlate with disease progression. There are complex correlations between ballooning, Mallory-Denk bodies and apoptosis through keratin metabolism and depletion, as well as through the endoplasmic reticulum stress response. Whether apoptosis may promote hepatocellular ballooning, or vice versa, will be discussed in this article.

Organelle dysfunction in hepatitis C virus-associated steatosis: anything to learn from nonalcoholic steatohepatitis?

Nonalcoholic fatty liver disease (NAFLD) spans a pathological spectrum from nonalcoholic steatosis to steatohepatitis. The pathophysiology of this disorder is complex, but includes insulin resistance and disrupted lipid and carbohydrate homeostasis, which at a subcellular level results in oxidative stress, free fatty acid-mediated lipotoxicity, defects in mitochondrial function, endoplasmic reticulum stress and cytokine-mediated toxicity. In chronic hepatitis C (CHC), systemic metabolic derangements similar to NAFLD may be operative, but in addition, virus-specific factors contribute to steatosis. The mechanisms for steatosis in CHC appear to share common pathways with those observed in NAFLD. This article outlines our current understanding of the subcellular mechanisms of steatosis in NAFLD and CHC, including their similarities and differences.

Effect of berberine on expressions of uncoupling protein-2 mRNA and protein in hepatic tissue of non-alcoholic fatty liver disease in rats

OBJECTIVE

To observe the effect of berberine on uncoupling protein-2 (UCP2) mRNA and protein expressions in the hepatic tissue of non-alcoholic fatty liver disease (NAFLD) in rats, and to explore the molecular mechanism.

METHODS

To establish the NAFLD rat model; the rats were fed by high fat forage and were randomly divided into four groups: normal group, model group, berberine high-dose group (324 mg/kg), and berberine low-dose group (162 mg/kg). After treatment for 12 weeks, the expression of UCP2 mRNA in the liver tissue was analyzed by semiquantitative reverse transcription polymerase chain reaction (RT-RTPCR). The expression level of UCP2 protein in the liver tissue was examined by immunohistochemistry. Total PCR). cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) contents in blood serum, and TG and TC contents in the liver were detected by an automatic biochemical analyzer. The other is to observe the axungia degree of the liver.

RESULTS

The expression of UCP2 mRNA and positive cell numbers in the liver tissue were dramatically increased in the model group (P<0.01). Lipid in the serum and hepatic tissues increased significantly, and the liver was fatty. But in the treatment groups, the expression levels of mRNA and UCP2 proteins were significantly down-regulated (P<0.01). Liver steatosis was improved.

CONCLUSIONS

Berberine can down-regulate the expression levels of UCP2 mRNA and UCP2 proteins of hepatic tissue in NAFLD rats. It can promote the recovery of hepatocyte steatosis and improve lipid metabolism disorder in NAFLD rats. Berberine shows a potential therapeutic effect on NAFLD.

Effects of a high-fat diet on energy metabolism and ROS production in rat liver

BACKGROUND & AIMS

A high-fat diet affects liver metabolism, leading to steatosis, a complex disorder related to insulin resistance and mitochondrial alterations. Steatosis is still poorly understood since diverse effects have been reported, depending on the different experimental models used.

METHODS

We hereby report the effects of an 8 week high-fat diet on liver energy metabolism in a rat model, investigated in both isolated mitochondria and hepatocytes.

RESULTS

Liver mass was unchanged but lipid content and composition were markedly affected. State-3 mitochondrial oxidative phosphorylation was inhibited, contrasting with unaffected cytochrome content. Oxidative phosphorylation stoichiometry was unaffected, as were ATPase and adenine nucleotide translocator proteins and mRNAs. Mitochondrial acylcarnitine-related H(2)O(2) production was substantially higher and the mitochondrial quinone pool was smaller and more reduced. Cellular consequences of these mitochondrial alterations were investigated in perifused, freshly isolated hepatocytes. Ketogenesis and fatty acid-dependent respiration were lower, indicating a lower β-oxidation rate contrasting with higher RNA contents of CD36, FABP, CPT-1, and AcylCoA dehydrogenases. Concomitantly, the cellular redox state was more reduced in the mitochondrial matrix but more oxidized in the cytosol: these opposing changes are in agreement with a significantly higher in situ mitochondrial proton motive force.

CONCLUSIONS

A high-fat diet results in both a decrease in mitochondrial quinone pool and a profound modification in mitochondrial lipid composition. These changes appear to play a key role in the resulting inhibition of fatty acid oxidation and of mitochondrial oxidative-phosphorylation associated with an increased mitochondrial ROS production. Mitochondrial quinone pool could have prospects as a crucial event, potentially leading to interesting therapeutic perspectives.

Characterization of the insulin sensitivity of ghrelin receptor KO mice using glycemic clamps

Background

We and others have demonstrated previously that ghrelin receptor (GhrR) knock out (KO) mice fed a high fat diet (HFD) have increased insulin sensitivity and metabolic flexibility relative to WT littermates. A striking feature of the HFD-fed GhrR KO mouse is the dramatic decrease in hepatic steatosis. To characterize further the underlying mechanisms of glucose homeostasis in GhrR KO mice, we conducted both hyperglycemic (HG) and hyperinsulinemic-euglycemic (HI-E) clamps. Additionally, we investigated tissue glucose uptake and specifically examined liver insulin sensitivity.

Results

Consistent with glucose tolerance-test data, in HG clamp experiments, GhrR KO mice showed a reduction in glucose-stimulated insulin release relative to WT littermates. Nevertheless, a robust 1^st phase insulin secretion was still achieved, indicating that a healthy β-cell response is maintained. Additionally, GhrR KO mice demonstrated both a significantly increased glucose infusion rate and significantly reduced insulin requirement for maintenance of the HG clamp, consistent with their relative insulin sensitivity. In HI-E clamps, both LFD-fed and HFD-fed GhrR KO mice showed higher peripheral insulin sensitivity relative to WT littermates as indicated by a significant increase in insulin-stimulated glucose disposal (Rd), and decreased hepatic glucose production (HGP). HFD-fed GhrR KO mice showed a marked increase in peripheral tissue glucose uptake in a variety of tissues, including skeletal muscle, brown adipose tissue and white adipose tissue. GhrR KO mice fed a HFD also showed a modest, but significant decrease in conversion of pyruvate to glucose, as would be anticipated if these mice displayed increased liver insulin sensitivity. Additionally, the levels of UCP2 and UCP1 were reduced in the liver and BAT, respectively, in GhrR KO mice relative to WT mice.

Conclusions

These results indicate that improved glucose homeostasis of GhrR KO mice is characterized by robust improvements of glucose disposal in both normal and metabolically challenged states, relative to WT controls. GhrR KO mice have an intact 1^st phase insulin response but require significantly less insulin for glucose disposal. Our experiments reveal that the insulin sensitivity of GhrR KO mice is due to both BW independent and dependent factors. We also provide several lines of evidence that a key feature of the GhrR KO mouse is maintenance of hepatic insulin sensitivity during metabolic challenge.

Suboptimal maternal nutrition, during early fetal liver development, promotes lipid accumulation in the liver of obese offspring

Maternal nutrition during the period of early organ development can modulate the offspring's ability to metabolise excess fat as young adults when exposed to an obesogenic environment. This study examined the hypothesis that exposing offspring to nutrient restriction coincident with early hepatogenesis would result in endocrine and metabolic adaptations that subsequently lead to increased ectopic lipid accumulation within the liver. Pregnant sheep were fed either 50 or 100% of total metabolisable energy requirements from 30 to 80 days gestation and 100% thereafter. At weaning, offspring were made obese, and at ∼1 year of age livers were sampled. Lipid infiltration and molecular indices of gluconeogenesis, lipid metabolism and mitochondrial function were measured. Although hepatic triglyceride accumulation was not affected by obesity per se, it was nearly doubled in obese offspring born to nutrient-restricted mothers. This adaptation was accompanied by elevated gene expression for peroxisome proliferator-activated receptor γ (PPARG) and its co-activator PGC1α, which may be indicative of changes in the rate of hepatic fatty acid oxidation. In contrast, maternal diet had no influence on the stimulatory effect of obesity on gene expression for a range of proteins involved in glucose metabolism and energy balance including glucokinase, glucocorticoid receptors and uncoupling protein 2. Similarly, although gene expressions for the insulin and IGF1 receptors were suppressed by obesity they were not influenced by the prenatal nutritional environment. In conclusion, excess hepatic lipid accumulation with juvenile obesity is promoted by suboptimal nutrition coincident with early development of the fetal liver.

Increased fructose consumption is associated with fibrosis severity in patients with nonalcoholic fatty liver disease

The rising incidence of obesity and diabetes coincides with a marked increase in fructose consumption. Fructose consumption is higher in individuals with nonalcoholic fatty liver disease (NAFLD) than in age-matched and body mass index (BMI)-matched controls. Because fructose elicits metabolic perturbations that may be hepatotoxic, we investigated the relationship between fructose consumption and disease severity in NAFLD. We studied 427 adults enrolled in the NASH Clinical Research Network for whom Block food questionnaire data were collected within 3 months of a liver biopsy. Fructose consumption was estimated based on reporting (frequency x amount) of Kool-aid, fruit juices, and nondietary soda intake, expressed as servings per week, and classified into none, minimum to moderate (<7 servings/week), and daily (> or =7 servings/week). The association of fructose intake with metabolic and histological features of NAFLD was analyzed using multiple linear and ordinal logistic regression analyses with and without controlling for other confounding factors. Increased fructose consumption was univariately associated with decreased age (P < 0.0001), male sex (P < 0.0001), hypertriglyceridemia (P < 0.04), low high-density lipoprotein (HDL) cholesterol (<0.0001), decreased serum glucose (P < 0.001), increased calorie intake (P < 0.0001), and hyperuricemia (P < 0.0001). After controlling for age, sex, BMI, and total calorie intake, daily fructose consumption was associated with lower steatosis grade and higher fibrosis stage (P < 0.05 for each). In older adults (age > or = 48 years), daily fructose consumption was associated with increased hepatic inflammation (P < 0.05) and hepatocyte ballooning (P = 0.05).

CONCLUSION

In patients with NAFLD, daily fructose ingestion is associated with reduced hepatic steatosis but increased fibrosis. These results identify a readily modifiable environmental risk factor that may ameliorate disease progression in patients with NAFLD.

Uncoupling protein-2 deficient mice are not protected against warm ischemia/reperfusion injury of the liver

BACKGROUND

Uncoupling protein-2 (UCP2) might play an important role in mediating ischemia/reperfusion (I/R) injury due to its function in uncoupling of oxidative phosphorylation and in the proton leak-associated increase of reactive oxygen species (ROS) production. The aim of this study was to elucidate the role of UCP2 in hepatic I/R injury.

MATERIALS AND METHODS

UCP2 wild type and UCP2 deficient mice were subjected to I/R of the left liver lobe. Sham-operated animals without I/R served as controls. Intravital fluorescence microscopy was used for assessing postischemic microcirculatory dysfunction. Indicators of hepatic inflammatory response, oxidative stress, and bioenergetic status as well as histomorphology were investigated.

RESULTS

Under sham conditions UCP2-/-mice presented slightly but not significantly higher levels of hepatic ATP and energy charge than wild type mice. In addition, they exhibited higher systemic IL-6 levels and intrahepatic leukocyte adherence. After exposure to I/R, the extent of reperfusion injury did not differ between UCP2+/+ and UCP2-/-mice, as indicated by a comparable loss of sinusoidal perfusion, hepatic ATP, and energy charge levels, as well as rise of transaminases and disintegration of liver structures. Intrahepatic leukocyte adherence and plasma IL-6 levels of postischemic UCP2-/-mice still exceeded those of UCP2+/+mice.

CONCLUSIONS

UCP2 appears to be of minor relevance for the manifestation and extent of postischemic reperfusion injury in nondiseased livers with the increased ATP availability being counteracted by the higher pro-inflammatory IL-6 levels in UCP2 deficient mice.

Mice with null mutation of Ceacam I develop nonalcoholic steatohepatitis

Transgenic liver-specific inactivation of the carcinoembryonic antigen-related cell adhesion molecule (CEACAM1) impairs hepatic insulin clearance and causes hyperinsulinemia, insulin resistance, elevation in hepatic and serum triglyceride levels, and visceral obesity. It also predisposes to nonalchoholic steatohepatitis (NASH) in response to a high-fat diet. To discern whether this phenotype reflects a physiological function of CEACAM1 rather than the effect of the dominant-negative transgene, we investigated whether Ceacam1 (gene encoding CEACAM1 protein) null mice with impaired insulin clearance also develop a NASH-like phenotype on a prolonged high-fat diet. Three-month-old male null and wild-type mice were fed a high-fat diet for 3 months and their NASH phenotype was examined. While high-fat feeding elevated hepatic triglyceride content in both strains of mice, it exacerbated macrosteatosis and caused NASH-characteristic fibrogenic changes and inflammatory responses more intensely in the null mouse. This demonstrates that CEACAM1-dependent insulin clearance pathways are linked with NASH pathogenesis.

Vitamin E succinate enhances steatotic liver energy status and prevents oxidative damage following ischemia/reperfusion

We have previously shown that treatment of steatotic livers with vitamin E succinate decreases liver injury and increases survival after ischemia/reperfusion (I/R). It is now understood that compromised energy status is associated with increased injury following liver ischemia in the setting of hepatic steatosis at least partially as a result of increased reactive oxygen species (ROS) and induction of mitochondrial uncoupling protein-2 (UCP2). Given the association between ROS, mitochondrial function, and UCP2, it was our goal to determine whether the protective effects of vitamin E succinate were associated with decreased ROS injury, down-regulation of UCP2, or improvement of ATP levels following I/R. To test this, leptin deficient (ob/ob) mice with steatotic livers that had received other 50 IU of vitamin E succinate supplement per day or control chow for 7 days were subjected to total hepatic ischemia (15 minutes) followed by reperfusion. We measured liver expressions of ATP, glutathione (GSH), and UCP2 as well as mitochondrial DNA damage. Vitamin E treatment decreased hepatic UCP2 expression and increased ATP and GSH levels prior to I/R. These levels were maintained at 1 hour after I/R. At 24 hours, while hepatic UCP2 expression, ATP, and GSH levels were similar to those of mice not receiving vitamin E, mitochondrial DNA damage was blocked. These results revealed that vitamin E succinate decreased hepatic UCP2 expression, reduced oxidative stress, and improved mitochondrial function in mice with steatotic livers before and after I/R, identifying mechanisms of protection in this setting.

Nonalcoholic steatohepatitis, animal models, and biomarkers: what is new?

Nonalcoholic fatty liver disease (NAFLD) is a clinicopathological term that encompasses a spectrum of abnormalities ranging from simple triglyceride accumulation in the hepatocytes (hepatic steatosis) to hepatic steatosis with inflammation (steatohepatitis, also known as nonalcoholic steatohepatitis or NASH). NASH can also progress to cirrhosis and hepatocellular carcinoma (HCC). Steatohepatitis has been estimated to affect around 5% of the total population and 20% of those who are overweight. The mechanisms leading to NASH and its progression to cirrhosis and HCC remain unclear, but it is a condition typically associated with obesity, insulin resistance, diabetes, and hypertriglyceridemia. This point corroborates the need for animal models and molecular markers that allow us to understand the mechanisms underlying this disease. Nowadays, there are numerous mice models to study abnormal liver function such as steatosis, NASH, and hepatocellular carcinoma. The study of the established animal models has provided many clues in the pathogenesis of steatosis and steatohepatitis, although these remain incompletely understood and no mice model completely fulfills the clinical features observed in humans. In addition, there is a lack of accurate sensitive diagnostic tests that do not involve invasive procedures. Current laboratory tests include some biochemical analysis, but their utility for diagnosing NASH is still poor. For that reason, a great effort is being made toward the identification and validation of novel biomarkers to assess NASH using high-throughput analysis based on genomics, proteomics, and metabolomics. The most recent discoveries and their validation will be discussed.