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

Bone marrow transplantation reveals the in vivo expression of the mitochondrial uncoupling protein 2 in immune and nonimmune cells during inflammation

The mitochondrial uncoupling protein 2 (UCP2) is expressed in spleen, lung, intestine, white adipose tissue, and immune cells. Bone marrow transplantation in mice was used to assess the contribution of immune cells to the expression of UCP2 in basal condition and during inflammation. Immune cells accounted for the total amount of UCP2 expression in the spleen, one-third of its expression in the lung, and did not participate in its expression in the intestine. LPS injection stimulated UCP2 expression in lung, spleen, and intestine in both immune and non-immune cells. Successive injections of LPS and dexamethasone or N-acetyl-cysteine prevented the induction of UCP2 in all three tissues, suggesting that oxygen free radical generation plays a role in UCP2 regulation. Finally, both previous studies and our data show that there is down-regulation of UCP2 in immune cells during their activation in the early stages of the LPS response followed by an up-regulation in UCP2 during the later stages to protect all cells against oxidative stress.

Overexpression of c-myc in the liver prevents obesity and insulin resistance

Alterations in hepatic glucose metabolism play a key role in the development of the hyperglycemia observed in type 2 diabetes. Because the transcription factor c-Myc induces hepatic glucose uptake and utilization and blocks gluconeogenesis, we examined whether hepatic overexpression of c-myc counteracts the insulin resistance induced by a high-fat diet. After 3 months on this diet, control mice became obese, hyperglycemic, and hyperinsulinemic, indicating that they had developed insulin resistance. In contrast, transgenic mice remained lean and showed improved glucose disposal and normal levels of blood glucose and insulin, indicating that they had developed neither obesity nor insulin resistance. These findings were concomitant with normalization of hepatic glucokinase and pyruvate kinase gene expression and enzyme activity, which led to normalization of intrahepatic glucose-6-phosphate and glycogen content. In the liver of control mice fed a high-fat diet, the expression of genes encoding proteins that control energy metabolism, such as sterol receptor element binding protein 1-c, peroxisome proliferator activated receptor alpha, and uncoupling protein-2, was altered. In contrast, in the liver of transgenic mice fed a high-fat diet, the expression of these genes was normal. These results suggest that c-myc overexpression counteracted the obesity and insulin resistance induced by a high-fat diet by modulating the expression of genes that regulate hepatic metabolism.

Decrease of intracellular ATP content downregulated UCP2 expression in mouse hepatocytes

Mitochondrial uncoupling protein 2 (UCP2) plays an important role in regulating energy metabolism. We previously reported that UCP2 expression in steatotic livers is increased which leads to diminished hepatic ATP stores and renders steatotic hepatocytes vulnerable to ischemic damage. In this study, reagents that inhibit the production of ATP were used to mimic an ischemic state in the liver in order to investigate the effects of decreased intracellular ATP levels on UCP2 expression in a murine hepatocyte cell line (HEP6-16). Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP), an oxidative phosphorylation uncoupler, was found to decrease intracellular ATP levels in a dose- and time-dependent manner. Relatively high concentrations of FCCP from 8 to 80 microM were required to reduce the intracellular concentration of ATP. The inhibitory effect of FCCP on intracellular ATP was significantly potentiated by 2-deoxy-D-glucose, an inhibitor of glycolysis that when administered alone had no negative effect on cellular ATP levels in mouse hepatocytes. Decreased intracellular ATP levels were accompanied by lower UCP2 mRNA expression. Upon removal of FCCP and/or 2-deoxy-D-glucose and reculture with normal medium, ATP and UCP2 mRNA levels returned to normal within a few hours. Mitochondrial membrane potential in HEP6-16 cells was dissipated by 80 microM FCCP but not 8 microM FCCP, suggesting that the downregulation of UCP2 expression by FCCP was not related to mitochondrial potential changes. Consequently, the in vitro manipulation of ATP stores is consistent with the in vivo observations associated with ischemia/reperfusion injury.

Current best treatment for non-alcoholic fatty liver disease

Treatment of patients with non-alcoholic fatty liver disease (NAFLD) has typically been focused on the management of associated conditions such as obesity, diabetes mellitus and hyperlipidaemia. NAFLD associated with obesity may be resolved by weight reduction, although the benefits of weight loss have been inconsistent. Improving insulin sensitivity with lifestyle modifications or medications usually improves glucose and lipid levels in patients with diabetes and hyperlipidaemia. Improving insulin sensitivity is expected to improve the liver disease but in many diabetic/hyperlipidaemic patients with NAFLD, the appropriate control of glucose and lipid levels is not always accompanied by improvement of the liver condition. Results of pilot studies evaluating ursodeoxycholic acid, gemfibrozil, betaine, N-acetylcysteine, alphatocopherol, metformin and thiazolidinedione derivatives suggest that these medications may be of potential benefit. This article reviews the treatment modalities currently available for patients with NAFLD, including emerging data from clinical trials evaluating promising medications as well as possibilities for the future.

Review article: Non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease is a clinicopathological condition that comprises a wide spectrum of liver damage, ranging from simple steatosis to steatohepatitis, advanced fibrosis and cirrhosis. Non-alcoholic steatohepatitis represents only a stage within the spectrum of non-alcoholic fatty liver disease and is defined pathologically by the presence of steatosis together with necro-inflammatory activity. The true prevalence of non-alcoholic fatty liver disease is unknown, but it is estimated that it affects 10-24% of the general population in different countries. The diagnosis of non-alcoholic fatty liver disease is based upon convincing evidence of absent or minimal alcohol consumption, compatible histological changes in liver biopsy and the exclusion of other liver diseases. The natural history of non-alcoholic fatty liver disease remains to be defined. Patients with pure steatosis on liver biopsy follow a relatively benign course, whereas patients with histological necro-inflammatory changes and/or fibrosis may progress to end-stage liver disease. An initial step in the treatment of non-alcoholic fatty liver disease is the management of associated conditions, such as obesity, diabetes mellitus and hyperlipidaemia. Non-alcoholic fatty liver disease patients with steatohepatitis and/or fibrosis on liver biopsy may benefit from investigational pharmacological therapy. Patients with decompensated cirrhosis from non-alcoholic fatty liver disease may be candidates for liver transplantation.

Probiotics and antibodies to TNF inhibit inflammatory activity and improve nonalcoholic fatty liver disease

Ob/ob mice, a model for nonalcoholic fatty liver disease (NAFLD), develop intestinal bacterial overgrowth and overexpress tumor necrosis factor alpha (TNF-alpha). In animal models for alcoholic fatty liver disease (AFLD), decontaminating the intestine or inhibiting TNF-alpha improves AFLD. Because AFLD and NAFLD may have a similar pathogenesis, treatment with a probiotic (to modify the intestinal flora) or anti-TNF antibodies (to inhibit TNF-alpha activity) may improve NAFLD in ob/ob mice. To evaluate this hypothesis, 48 ob/ob mice were given either a high-fat diet alone (ob/ob controls) or the same diet + VSL#3 probiotic or anti-TNF antibodies for 4 weeks. Twelve lean littermates fed a high-fat diet served as controls. Treatment with VSL#3 or anti-TNF antibodies improved liver histology, reduced hepatic total fatty acid content, and decreased serum alanine aminotransferase (ALT) levels. These benefits were associated with decreased hepatic expression of TNF-alpha messenger RNA (mRNA) in mice treated with anti-TNF antibodies but not in mice treated with VSL#3. Nevertheless, both treatments reduced activity of Jun N-terminal kinase (JNK), a TNF-regulated kinase that promotes insulin resistance, and decreased the DNA binding activity of nuclear factor kappaB (NF-kappaB), the target of IKKbeta, another TNF-regulated enzyme that causes insulin resistance. Consistent with treatment-related improvements in hepatic insulin resistance, fatty acid beta-oxidation and uncoupling protein (UCP)-2 expression decreased after treatment with VSL#3 or anti-TNF antibodies. In conclusion, these results support the concept that intestinal bacteria induce endogenous signals that play a pathogenic role in hepatic insulin resistance and NAFLD and suggest novel therapies for these common conditions.

Hepatic ATP reserve and efficiency of replenishing: comparison between obese and nonobese normal individuals

OBJECTIVE

Although obesity-associated fatty liver disease is emerging as one of the most common diseases in hepatology practice, it is unclear why liver disease prevalence increases with obesity. Because impaired energy homeostasis enhances the susceptibility of hepatocytes to injury, the aim of this study was to determine whether increased body mass index (BMI) is associated with decreased basal hepatic adenosine triphosphate (ATP) stores or impaired recovery from fructose-induced hepatic ATP depletion.

METHODS

Hepatic ATP stores were assessed by nuclear magnetic resonance spectroscopy in 19 healthy subjects with varying BMI. After obtaining the baseline spectra, 0.5 ml/kg of 50% fructose solution was administered to all subjects to deplete the ATP reserve, and follow-up nuclear magnetic resonance spectra was obtained at 5-min intervals for the ensuing hour. AST and ALT were determined at 24 h to assess whether ATP depletion caused any appreciable hepatocyte injury.

RESULTS

Among the 19 subjects who participated in the study, five had BMI of < or =25, seven had BMI between 25-30, and seven had BMI of >30. The baseline ATP content was inversely related to BMI (correlation coefficient -0.63, p = 0.02), decreasing steadily with increasing BMI. Fructose injection decreased hepatic ATP stores in all subjects and did not increase transaminases in anyone. Neither the postfructose ATP nadir values nor the extent of ATP recovery correlated with BMI.

CONCLUSIONS

Reduced hepatic ATP stores are more prevalent in overweight and obese subjects than in lean subjects. However, a cause-effect relationship between these abnormalities was not demonstrated by our study.

Advances in the Understanding and Treatment of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a well recognised form of chronic liver disease that has recently gained greater recognition. Originally described in the late 1950s, NAFLD is currently considered the leading cause of abnormal liver enzyme levels in the US, closely paralleling the increase in obesity and diabetes mellitus. NAFLD has a worldwide distribution, affecting both adults and children, and typically is seen in association with obesity, diabetes, hypertension and hypertriglyceridaemia. Most patients are asymptomatic and usually present with mild elevations in aminotransferases. The natural history of NAFLD is not clearly defined but progression to cirrhosis and end-stage liver disease is well recognised in some patients. The accumulation of hepatic steatosis is thought to occur initially, primarily through hepatic and peripheral insulin resistance, which leads to altered glucose and free fatty acid metabolism. The progression from simple fatty liver to more severe forms of NAFLD (nonalcoholic steatohepatitis and cirrhosis) is much less clear but evidence suggests that oxidative stress may preferentially enhance proinflammatory cytokines, which leads to cellular adaptations and dysfunction followed by development of inflammation, necrosis and fibrosis. Therapeutic modalities remain limited and are largely focused on correcting the underlying insulin resistance or reducing oxidative stress. However, at the present time, there are several limitations to the current potential therapies, mainly because of the lack of large-scale, prospective, randomised studies, as well as clearly defined histological endpoints. Ultimately, the future for potential therapeutic modalities to treat this disease are quite promising, but further research is needed to clearly demonstrate which therapy or therapies will be effective at eliminating fatty liver disease and its potential complications.

The liver in obesity and type 2 diabetes mellitus

Obesity and type 2 diabetes are associated strongly with NAFLD. It is not clear if one of these conditions causes the others, or if all are consequences of another process. Although NAFLD is known to occur in overly lean individuals, which indicates that excessive adiposity is not required for the development of NAFLD, the severities of insulin resistance and NAFLD tend to parallel each other, and the greatest prevalence of type 2 diabetes occurs in patients with NAFLD and cirrhosis. This observation suggests that insulin resistance and NAFLD may be related pathogenically. Experiments in mice demonstrate that insulin resistance and NAFLD result from a chronic inflammatory state that is characterized by increased levels of TNF alpha. The mechanisms that drive this chronic inflammation are unknown but might involve the oxidative stress that develops during fatty acid metabolism or when products from intestinal bacteria escape into the mesenteric blood to trigger a sustained hepatic inflammatory cytokine response in genetically susceptible individuals, promoting a positive feedback loop that reinforces insulin resistance and inflammation. This hypothesis is supported by some animal and human studies; however, more research is needed to evaluate this theory. Additional studies also are required to determine the benefits of treatments that interrupt this pathogenic cascade at various points. Preliminary work in animal and human studies suggests that diverse strategies that inhibit production of TNF alpha and improve insulin resistance also ameliorate NAFLD.

Nonalcoholic steatohepatitis: what we know in the new millennium

Nonalcoholic steatohepatitis (NASH) is a liver disease characterized by diffuse fatty infiltration and inflammation. The exact prevalence of NASH is unclear, but it is becoming more evident that the disease is much more common than previously thought. Although generally a benign, indolent process, it can progress to advanced liver disease in approximately 15-20% of patients. Clinical characteristics associated with NASH include obesity, hyperlipidemia, diabetes mellitus, and hypertension, all of which have been associated with underlying insulin resistance. Typically, this disease becomes evident in the fourth or fifth decade of life with an equal sex predilection. NASH is thought to be caused, in part, by impaired insulin signaling, leading to elevated circulating insulin levels and subsequent altered lipid homeostasis. This process is likely multifactorial and includes both genetic and environmental factors. Treatment options to date are limited and are based on very small clinical trials. Current investigations are focusing on improving the underlying insulin resistance that has been associated with NASH as well as other therapies that decrease oxidative stress or improve hepatocyte survival.

Treatment of non-alcoholic steatohepatitis

Treatment of patients with non-alcoholic steatohepatitis (NASH) has typically been focused on the management of associated conditions such as obesity, diabetes mellitus and hyperlipidaemia. NASH associated with obesity may resolve with weight reduction, although the benefits of weight loss have been inconsistent. Appropriate control of glucose and lipid levels is always recommended, but is not always effective in reversing the liver condition. Results of pilot studies evaluating ursodeoxycholic acid, gemfibrozil, betaine, N-acetylcysteine, alpha-tocopherol, metformin and thiazolidinedione derivatives suggest that these medications may be of potential benefit for patients with NASH. These medications, however, need first to be tested in well-controlled trials with clinically relevant end-points and extended follow-up. A better understanding of the pathogenesis and natural history of NASH will help to identify the subset of patients at risk of progressing to advanced liver disease and, hence, those patients who should derive the most benefit from medical therapy.

Interaction of non-alcoholic fatty liver disease with other liver diseases

Obesity-related steatosis is an increasingly common histological finding in liver biopsies and may co-exist with other chronic liver diseases. Although non-alcoholic fatty liver disease (NAFLD) without true steatohepatitis is generally a benign condition, when another liver disease is present, steatosis may exacerbate the liver damage. In this review, we discuss the interaction of obesity-related steatosis with chronic hepatitis C, alcoholic liver disease, disorders of hepatic iron storage and drug-induced liver disease. The role of weight reduction in minimizing liver injury in patients with chronic hepatitis C is discussed. Finally, we discuss the problems associated with orthotopic liver transplantation for patients with NAFLD.

Possible physiological roles of mitochondrial uncoupling proteins--UCPn

Five mitochondrial uncoupling proteins exist in the human gemone: UCP2, expressed ubiquitously; UCP1, exclusively in brown adipose tissue (BAT); UCP3, predominantly in muscle; UCP4 and BMCP (UCP5), in brain. UCP4 is the ancestral prototype from which the other UCPn diverged. Findings on the level of organism and reconstituted recombinant proteins demonstrated that UCPn exhibit a protonophoric function, documented by overexpression in mice, L6 myotubes, INS1 cells, muscle, and yeast. In a few cases (yeast), this protonophoric function was correlated with elevated fatty acid (FA) levels. Reconstituted UCPn exhibited nucleotide-sensitive FA induced H(+) uniport. Two mechanisms, local buffering or FA cycling were suggested as an explanation. A basic UCPn role with mild uncoupling is to accelerate metabolism and reduce reactive oxygen species. UCP2 (UCP3) roles were inferred from transcriptional up-regulation mediated by FAs via peroxisome proliferator-activated receptors, cytokines, leptin signalling via hypothalamic pathway, and by thyroide and beta2 adrenergic stimulation. The latter indicated a role in catecholamine-induced thermogenesis in skeletal muscle. UCP2 (UCP3) may contribute to body weight regulation, although obesity was not induced in knockout (KO) mice. An obesity reduction in middle-aged humans was associated with the less common allele of -866 G/A polymorphism in the ucp2 gene promoter enhancing the exon 8 insertion: deletion transcript ratio. Up-regulated UCP2 transcription by pyrogenic cytokines (tumour necrosis factor alpha (TNFalpha)) suggested a role in fever. UCP2 could induce type 2 diabetes as developed from obesity due to up-regulated UCP2 transcription by FAs in pancreatic beta-cells. UCPn might be pro-apoptotic as well as anti-apoptotic, depending on transcriptional and biochemical regulation.

Animal models of steatohepatitis

Animal models of hepatic steatosis and steatohepatitis have improved our understanding of the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Three models, genetically obese ob/ob mice, lipoatrophic mice and normal rats fed choline-deficient, methionine-restricted diets, have been particularly informative. All support the multiple 'hit' hypothesis for NAFLD pathogenesis that suggests that fatty livers are unusually vulnerable to oxidants and develop steatohepatitis when secondary insults generate sufficient oxidants to cause liver cell death and inflammation. Steatohepatitis, in turn, increases sensitivity to other insults that induce hepatic fibrosis, promoting the evolution of cirrhosis. Early during NAFLD pathogenesis, inhibitor kappa kinase beta (IKKbeta), an enzyme that induces tumour necrosis factor alpha (TNFalpha) and other proinflammatory cytokines, is activated and this causes insulin resistance. Inhibition of IKKbeta or TNFalpha improves insulin sensitivity, steatosis and steatohepatitis in animals, suggesting novel strategies to prevent and treat early NAFLD in humans.

Pathogenesis of steatohepatitis

Understanding the pathogenesis of non-alcoholic steatohepatitis has recently assumed great importance with the recognition that it has the potential to progress to fibrosis and cirrhosis. The 'two-hit' model of pathogenesis was proposed in 1998, with the first 'hit' - steatosis - increasing the sensitivity of the liver to the second 'hits' mediating liver injury. The main aim of this chapter is to review this model in the light of studies that have been published over the subsequent 4 years. Particular attention will be focused on the role of insulin resistance and recent advances in our understanding of the basic cellular mechanisms linking obesity and insulin resistance. Based on this information I will propose a modification of the two-hit model that places more emphasis on the role of free fatty acids. This model will provide the basis for further research and enable the rational design of treatment strategies.

Spontaneous oxidative stress and liver tumors in mice lacking methionine adenosyltransferase 1A

In mammals, methionine metabolism occurs mainly in the liver via methionine adenosyltransferase-catalyzed conversion to S-adenosylmethionine. Of the two genes that encode methionine adenosyltransferase(MAT1Aand MAT2A), MAT1A is mainly expressed in adult liver whereas MAT2A is expressed in all extrahepatic tissues. Mice lacking MAT1A have reduced hepatic S-adenosylmethionine content and hyperplasia and spontaneously develop nonalcoholic steatohepatitis. In this study, we examined whether chronic hepatic S-adenosylmethionine deficiency generates oxidative stress and predisposes to injury and malignant transformation. Differential gene expression in MAT1A knockout mice was analyzed following the criteria of the Gene Ontology Consortium. Susceptibility of MAT1A knockout mice to CCl4-induced hepatotoxicity and malignant transformation was determined in 3- and 18-month-old mice, respectively. Analysis of gene expression profiles revealed an abnormal expression of genes involved in the metabolism of lipids and carbohydrates in MAT1A knockout mice, a situation that is reminiscent of that found in diabetes, obesity, and other conditions associated with nonalcoholic steatohepatitis. This aberrant expression of metabolic genes in the knockout mice was associated with hyperglycemia, increased hepatic CYP2E1 and UCP2 expression and triglyceride levels, and reduced hepatic glutathione content. The knockout animals have increased lipid peroxidation and enhanced sensitivity to CCl4-induced liver damage, which was largely due to increased CYP2E1 expression because diallyl sulfide, an inhibitor of CYP2E1, prevented CCl4-induced liver injury. Hepatocellular carcinoma developed in more than half of the knockout mice by 18 months of age. Taken together, our findings define a critical role for S-adenosylmethionine in maintaining normal hepatic function and tumorigenesis of the liver.

Subacute liver failure in obese women

OBJECTIVES

Steatosis or steatohepatitis, common conditions associated with obesity, are usually considered to be stable or only slowly progressive. We have encountered a small number of patients with a history of obesity and a subacute course of liver failure over a period of 4-16 wk from the onset of symptoms. The patients had findings suggestive of an acute exacerbation of previously unrecognized nonalcoholic steatohepatitis (NASH).

METHODS

The patients were ascertained from our liver disease registry, which, at the time of the study, contained 2380 patients: 167 had NASH and 215 had cryptogenic cirrhosis. Five of these patients were identified with a subacute course of their illness.

RESULTS

The patients were female, aged 41-65 yr, and obese (BMI >30, mean 41 +/- 9, range 32-52). One patient had type 2 diabetes treated by diet alone and one had a history of glucose intolerance. None had known prior liver disease and two had no prior medical problems. All five presented with fatigue and lethargy. Over 4-16 wk, the patients developed frank encephalopathy, ascites, jaundice, and multiorgan failure. An extensive evaluation revealed no clear etiology of their disease, although initial imaging studies consistently showed evidence of previously unrecognized cirrhosis. Four patients died from complications of liver failure and the fifth patient underwent OLT. Histology revealed cirrhosis with variable numbers of balloon cells in all five patients, frank steatohepatitis in three, necrosis in two, and microvesicular (with macrovesicular) steatosis in one.

CONCLUSIONS

These patients, all obese and middle-aged women with no history of liver disease, had previously unrecognized cirrhosis and sudden deterioration of uncertain cause. We speculate, based on the clinical and histological findings, that these patients had undiagnosed NASH with silent progression to cirrhosis followed by subacute liver failure. We propose that obesity-related liver disease may infrequently present as severe, subacute illness.

Regulation of cellular oncosis by uncoupling protein 2

Cell death can proceed through at least two distinct pathways. Apoptosis is an energy-dependent process characterized morphologically by cell shrinkage, whereas oncosis is a form of cell death induced by energy depletion and initially characterized by cell swelling. We demonstrate in HeLa cells but not in normal diploid fibroblasts that modest increases in the expression level of uncoupling protein 2 (UCP-2) leads to a rapid and dramatic fall in mitochondrial membrane potential and to a reduction of mitochondrial NADH and intracellular ATP. In HeLa cells, increased UCP-2 expression leads to a form of cell death that is not inhibited by the anti-apoptotic gene product Bcl-2 and that morphologically resembles cellular oncosis. We further describe the creation of a dominant interfering mutant of UCP-2 whose expression increases resting mitochondrial membrane potential and selectively increases the resistance to cell death following oncotic but not apoptotic stimuli. These results suggest that distinct genetic programs may regulate the cellular response to either apoptotic or oncotic stimuli.

Altered gene expressions involved in energy expenditure in 5-HT(2C) receptor mutant mice

Mice with a targeted null mutation of the serotonin 5-HT(2C) receptor gene exhibit hyperphagia that leads to a late-onset obesity. Here we show that oxygen consumption was decreased in fed and fasted obese mutants. No phenotypic differences were observed in uncoupling protein-1 (UCP-1) mRNA levels in brown adipose tissues and UCP-3 mRNA in skeletal muscle. UCP-2 mRNA levels were significantly increased in white adipose tissue (4-fold) and skeletal muscle (47%) in older obese mutant mice, whereas UCP-2 mRNA in liver are significantly increased in both young lean (54% increase) and older obese (52% increase) mutant mice. In contrast, 5-HT(2C) receptor mutants displayed age-dependent decreases in beta 3-adrenergic receptor (beta 3-AR) mRNA levels in white adipose tissue, however, no such changes were observed in brown adipose tissue. These results indicate that a mutation of 5-HT(2C) receptor gene leads to a secondary decrease in beta 3-AR gene expression that is related to enhanced adiposity.

Hepatic steatosis and type 2 diabetes mellitus

Type 2 diabetes is strongly associated with nonalcoholic fatty liver disease (NAFLD), a spectrum of liver damage that ranges from relatively benign hepatic steatosis to potentially fatal cirrhosis. The severities of insulin resistance and liver damage parallel each other, with the greatest prevalence of cirrhosis occurring in cirrhotics. However, it is unknown whether one of these conditions causes the other, or if both are consequences of another process. Experimental evidence suggests that both insulin resistance and NAFLD result from a chronic inflammatory state. The mechanisms driving this chronic inflammation are unknown but might include the egress of products from intestinal bacteria into the portal blood, liver, and systemic circulation to trigger a sustained inflammatory cytokine response in genetically susceptible individuals. More research is needed to evaluate this hypothesis and to determine the benefits of treatments that interrupt this pathogenic cascade.

The role of uncoupling proteins in pathophysiological states

Until very recently, the uncoupling protein-1 (UCP1), present only in brown adipose tissue (BAT), was considered to be the only mitochondrial carrier protein that stimulated heat production by dissipating the proton gradient generated during respiration across the inner mitochondrial membrane and therefore uncoupling respiration from ATP synthesis. Recently, new uncoupling proteins, UCP2, UCP3, and UCP4, and brain mitochondrial carrier protein-1 (BMCP-1) have been described in mammalian tissues. The present review deals with the possible role of these proteins in different pathological conditions involving alterations in energy balance such as obesity or cachexia. In conclusion, the emergence of the UCP family has altered the approaches to bioenergetics and stressed the importance of uncoupling respiration in different pathophysiological conditions. An extensive qualitative and quantitative characterization of the new members of the UCP family in mammalian tissues will allow a better understanding of the molecular and regulatory mechanisms of thermogenesis and energy metabolism. At this point, we hope that the knowledge presented in the present review will not only stimulate a debate about the role of the UCP family in disease but also lead to applications beneficial for human health.

Alcoholic and nonalcoholic steatohepatitis

The constellation of histopathologic lesions that characterize alcoholic and nonalcoholic steatohepatitis has been well described and has served as the basis for clinical diagnosis, natural history studies, and experimental models for analyses of etiopathogenesis. The lesions common to both entities include, to varying degrees, steatosis, liver cell ballooning, lobular inflammation with a notable component of polymorphonuclear leukocytes, and a characteristic form of fibrosis that is initially located in the perisinusoidal regions of acinar zone 3. Cirrhosis with or without steatosis or steatohepatitis may occur in both entities. Mallory's hyaline is common but not necessary; megamitochondria and varying amounts of iron may be observed in either process. Hepatocellular carcinoma is a recognized complication of both processes, albeit with greater frequency in the former. Alcoholic hepatitis may present with more severe clinical and histologic manifestations than the nonalcoholic counterpart, including significant morbidity and mortality. The perivenular lesions collectively referred to as sclerosing hyaline necrosis are markers of severity, and are not common in nonalcoholics. In many instances, however, the microscopic lesions of these two processes are similar, likely as a reflection of common pathogenetic pathways, and the distinction between the two is ultimately clinically derived.

Uncoupling proteins and thermoregulation

Energy balance in animals is a metabolic state that exists when total body energy expenditure equals dietary energy intake. Energy expenditure, or thermogenesis, can be subcategorized into groups of obligatory and facultative metabolic processes. Brown adipose tissue (BAT), through the activity of uncoupling protein 1 (UCP1), is responsible for nonshivering thermogenesis, a major component of facultative thermogenesis in newborn humans and in small mammals. UCP1, found in the mitochondrial inner membrane in BAT, uncouples energy substrate oxidation from mitochondrial ATP production and hence results in the loss of potential energy as heat. Mice that do not express UCP1 (UCP1 knockouts) are markedly cold sensitive. The recent identification of four new homologs to UCP1 expressed in BAT, muscle, white adipose tissue, brain, and other tissues has been met by tremendous scientific interest. The hypothesis that the novel UCPs may regulate thermogenesis and/or fatty acid metabolism guides investigations worldwide. Despite several hundred publications on the new UCPs, there are a number of significant controversies, and only a limited understanding of their physiological and biochemical properties has emerged. The discovery of UCP orthologs in fish, birds, insects, and even plants suggests the widespread importance of their metabolic functions. Answers to fundamental questions regarding the metabolic functions of the new UCPs are thus pending and more research is needed to elucidate their physiological functions. In this review, we discuss recent findings from mammalian studies in an effort to identify potential patterns of function for the UCPs.

Obesity-related fatty liver is unchanged in mice deficient for mitochondrial uncoupling protein 2

Nonalcoholic fatty liver disease (NAFLD), a prevalent condition associated with obesity, has the potential of evolving into end-stage liver disease. The biochemical mechanisms that define the progression of NAFLD are not well known, but reactive oxygen species (ROS) have been implicated in this process. Uncoupling protein (UCP) 2 is a mitochondrial inner-membrane protein that mediates proton leak, uncouples adenosine triphosphate (ATP) synthesis, and negatively regulates ROS production. UCP2 expression is increased in various animal models of NAFLD. Up-regulation of UCP2 may compromise cellular ATP levels and worsen liver damage, or it may be protective by ROS reduction in NAFLD. This study aimed to obtain a definitive answer as to whether increased UCP2 expression contributes to NAFLD. UCP2-/- mice were exposed to obesity by crossbreeding with ob/ob mice and by long-term high-fat feeding to study the effect of UCP2 deficiency on the outcome of NAFLD. Steatohepatitis score of crossbred mice (ob/ob/ko) was similar to that of ob/ob mice at 25 weeks. No compensatory increase was observed in the expression of UCP5 in ob/ob/ko livers. To unmask the effects of absent leptin and its potential proinflammatory actions, steatosis was also induced in UCP2-/- mice by a high-fat diet continued for 6 months. Serum alanine aminotransferase (ALT) levels remained normal, and the steatohepatitis score in UCP2-/- mice was the same as in wild-type controls. We conclude that increased expression of UCP2 in the livers of mice with genetically or diet-induced obesity exerts neither protective nor deleterious effects on the severity of fatty liver disease.

Mechanism for peroxisome proliferator-activated receptor-alpha activator-induced up-regulation of UCP2 mRNA in rodent hepatocytes

Peroxisome proliferator-activated receptor-alpha (PPARalpha)activators, fish oil feeding, or fibrate administration up-regulated mitochondrial uncoupling protein (UCP2) mRNA expression in mouse liver by 5-9-fold, whereas tumor necrosis factor-alpha (TNFalpha) also up-regulated UCP2 in liver. In this study, the mechanisms for PPARalpha activators-induced up-regulation of UCP2 mRNA, related to TNFalpha and reactive oxygen species (ROS), were investigated. PPARalpha activators-induced UCP2 up-regulation in mouse/rat liver tissues was due to their increases in hepatocytes but not in non-parenchymal cells. Addition of PPARalpha activators, WY14,643 or fenofibrate, to cultured hepatocytes up-regulated UCP2 mRNA by 5-10-fold. PPARalpha activators-induced up-regulation of UCP2 mRNA was not due to increased mRNA stability and required cycloheximide-sensitive short term turnover protein(s). However, expression of PPARalpha/retinoid X receptor-alpha and PGC-1 was not rate-limiting for WY14,643-induced UCP2 up-regulation. In primary hepatocytes, an exogenous oxidant, tert-butyl-hydroperoxide (TBHP), which increased ROS production, up-regulated UCP2 mRNA, whereas WY14,643 treatment did not produce detectable ROS under the condition that fibrate markedly up-regulated UCP2. In in vivo studies, PPARalpha activators moderately up-regulated TNFalpha mRNA expression in mouse liver. An anti-oxidant pyrrolidine dithiocarbamate ammonium salt injection completely prevented their TNFalpha mRNA increases but did not prevent most of their UCP2 mRNA increases. These data indicate that PPARalpha activators up-regulate UCP2 expression in hepatocytes through unknown proteins by increased transcription, and neither ROS nor TNFalpha production are the major causes for PPARalpha activators-induced UCP2 up-regulation.

Update on nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is now recognized as the most common liver disease in the United States, with a prevalence of approximately 5% in the general population and up to 25% to 75% in patients with obesity and type II diabetes mellitus. Nonalcoholic fatty liver disease is a clinicopathologic syndrome with a wide spectrum of histologic abnormalities and clinical outcomes. Hepatic steatosis has a benign clinical course. In contrast, nonalcoholic steatohepatitis (NASH) may progress to cirrhosis and liver-related death in 25% and 10% of patients, respectively. Cases occur most commonly in obese, middle-aged women with diabetes. However, NASH may also occur in children and normal-weight men with normal glucose and lipid metabolism. The pathophysiology involves two steps. The first is insulin resistance, which causes steatosis. The second is oxidative stress, which produces lipid peroxidation and activates inflammatory cytokines resulting in NASH. Liver biopsy provides prognostic information and identifies NASH patients who may benefit from therapy. Treatment consists of managing the comorbidities: obesity, diabetes, and hyperlipidemia. Although antioxidant therapy with vitamin E is often used, ursodeoxycholic acid is the only drug that has shown benefit and is the most promising of the drugs currently being investigated. Future therapies will depend on a greater understanding of the pathophysiology and should focus on diminishing fibrosis.

Non-alcoholic steatohepatitis: clinical significance and pathogenesis

Non-alcoholic steatohepatitis (NASH) is a form of liver disease resembling alcoholic liver disease in a patient who does not consume significant amounts of alcohol. Since its first description in 1980 it has been recognized with increasing frequency. The natural course is relatively benign, but liver cirrhosis. together with all its sequelae, may develop; sometimes liver transplantation is indicated. NASH should probably be regarded as a two-stage acquired metabolic disorder consisting of the development of the insulin resistance syndrome in a patient with pre-existing metabolic abnormalities. The insulin resistance syndrome may well be the most important metabolic abnormality giving rise to hepatic steatosis. The preexisting metabolic abnormalities can be diverse, and may well be multifactorial and/or polymorphogenetic. A steatotic liver may be more susceptible to the deleterious effects of the pre-existing metabolic abnormalities. Pre-existing metabolic abnormalities of particular interest are increased hepatic iron storage and derangements of lipoprotein metabolism. While awaiting the complete resolution of the pathogenesis, current treatment is largely conservative. Every patient should be encouraged to lose weight and to avoid alcohol and other hepatotoxins. In addition, diabetes, lipid abnormalities and increased iron stores should be looked for.

UCP2-dependent proton leak in isolated mammalian mitochondria

A role for uncoupling protein (UCP) homologues in mediating the proton leak in mammalian mitochondria is controversial. We subjected insulinoma (INS-1) cells to adenoviral expression of UCP2 or UCP1 and assessed the proton leak as the kinetic relationship between oxygen use and the inner mitochondrial membrane potential. Cells were infected with different amounts of rat UCP2, and, in other experiments, with either UCP2 or UCP1. The relative molar expression of these subtypes was quantified through comparison with histidine-tagged UCP1 or UCP2 proteins engineered by expression in Escherichia coli. Adenoviral infection with UCP2, compared with beta-galactosidase, resulted in a dose-dependent shift in kinetics indicating increased H(+) flux at any given membrane potential. UCP1 also enhanced H(+) flux, but, on a relative molar basis, the overexpression of the endogenous protein, UCP2, was more potent than UCP1. These results were not due to nonspecific overexpression of mitochondrial protein since UCP1 activity was inhibited by GDP and because overexpression of another membrane carrier protein, the oxoglutarate malate carrier had no effect. UCP2-mediated H(+) conduction was not GDP sensitive. These data suggest that the UCP homologue, UCP2, mediates the proton leak in mitochondria of a mammalian cell wherein UCP2 is the native subtype.

Nonalcoholic steatosis and steatohepatitis. V. Mitochondrial dysfunction in steatohepatitis

Rich diet and lack of exercise are causing a surge in the prevalence of obesity and hepatic steatosis, which causes "primary" steatohepatitis in some patients. Ultrastructural mitochondrial lesions, decreased activity of respiratory chain complexes, and impaired ability to synthesize ATP are observed in these patients. Reactive oxygen species (ROS) may increase tumor necrosis factor-alpha (TNF-alpha) production and also oxidize fat deposits. TNF-alpha and lipid peroxidation products impair the flow of electrons along the respiratory chain, causing overreduction of respiratory chain components and enhanced mitochondrial ROS formation. Steatohepatitis can also be due to alcohol, drugs, or other causes that either directly increase ROS formation or first impair respiration, which secondarily increases ROS formation. Higher ROS formation in secondary steatohepatitis could cause more lipid peroxidation, cytokine induction, and fibrogenesis than in primary steatohepatitis.

Nonalcoholic steatosis and steatohepatitis IV. Nonalcoholic fatty liver disease abnormalities in macrophage function and cytokines

Macrophage products, such as cytokines, prostanoids, nitric oxide, and reactive oxygen intermediates, influence the function and viability of macrophages and neighboring cells. Given that the liver has one of the largest resident macrophage populations in the body, it is not surprising that hepatic macrophages [i.e., Kupffer cells (KC)] are involved in the pathogenesis of many kinds of liver disease. This review summarizes the abnormalities that have been demonstrated in bone marrow, peritoneal and hepatic macrophage of leptin-resistant (fa/fa) rats and leptin-deficient (ob/ob) mice, two animal models for nonalcoholic fatty liver disease (NAFLD). Evidence supports the concept that altered KC function influences the viability of other cells, such as lymphocytes and hepatocytes, in fatty livers, thereby contributing to the pathogenesis of NAFLD in animals with reduced leptin activity. Further work is needed to determine whether KC dysfunction is a component of more generalized mechanisms that lead to NAFLD.

LDL receptor but not apolipoprotein E deficiency increases diet-induced obesity and diabetes in mice

The aim of this study was to determine whether phenotypes associated with type 2 diabetes are altered in dyslipidemic obese mice. C57BL/6 wild-type, low-density lipoprotein (LDL) receptor-deficient (LDLR-/-), and apolipoprotein E-deficient (apoE-/-) mice were fed a high-fat, high-carbohydrate diet (diabetogenic diet), and the development of obesity, diabetes, and hypertriglyceridemia was examined. Wild-type mice became obese and developed hyperglycemia, but not hypertriglyceridemia, in response to this diet. LDLR-/- mice fed the diabetogenic diet became more obese than wild-type mice and developed severe hypertriglyceridemia and hyperleptinemia. Surprisingly, glucose levels were only modestly higher and insulin levels and insulin-to-glucose ratios were not strikingly different from those of wild-type mice. In contrast, diabetogenic diet-fed apoE-/- mice were resistant to changes in glucose and lipid homeostasis despite becoming obese. These data suggest that modifications in lipoprotein profiles associated with loss of the LDL receptor or apoE function have profound and unique consequences on susceptibility to diet-induced obesity and type 2 diabetic phenotypes.

Transcriptional profiling reveals global defects in energy metabolism, lipoprotein, and bile acid synthesis and transport with reversal by leptin treatment in ob/ob mouse liver

Leptin, a hormone secreted by adipose tissue, has been shown to have a major influence on hepatic lipid and lipoprotein metabolism. To characterize changes in lipid and lipoprotein gene expression in mouse liver, suppression subtractive hybridization and cDNA microarray analysis were used to identify mRNAs differentially expressed after leptin treatment of ob/ob mice. Ob/ob mice showed a profound decrease in mRNAs encoding genes controlling bile acid synthesis and transport as well as a variety of apolipoprotein genes and hepatic lipase with reversal upon leptin administration, suggesting that leptin coordinately regulates high density lipoprotein and bile salt metabolism. Leptin administration also resulted in decreased expression of genes involved in fatty acid and cholesterol synthesis, glycolysis, gluconeogenesis, and urea synthesis, and increased expression of genes mediating fatty acid oxidation, ATP synthesis, and oxidant defenses. The changes in mRNA expression are consistent with a switch in energy metabolism from glucose utilization and fatty acid synthesis to fatty acid oxidation and increased respiration. The latter changes may produce oxidant stress, explaining the unexpected finding that leptin induces a battery of genes involved in antioxidant defenses. Expression cluster analysis revealed responses of several sets of genes that were kinetically linked. Thus, the mRNA levels of genes involved in fatty acid and cholesterol synthesis are rapidly (<1 h) repressed by leptin administration, in association with an acute decrease in plasma insulin levels and decreased sterol regulator element-binding protein-1 expression. In contrast, genes participating in fatty acid oxidation and ketogenesis were induced more slowly (24 h), following an increase in expression of their common regulatory factor, peroxisome proliferator-activated receptor alpha. However, the regulation of genes involved in high density lipoprotein and bile salt metabolism shows complex kinetics and is likely to be mediated by novel transcription factors.

Polyunsaturated fatty acids stimulate hepatic UCP-2 expression via a PPARalpha-mediated pathway

The discovery of homologs of the brown fat uncoupling protein(s) (UCP) UCP-2 and UCP-3 revived the hypothesis of uncoupling protein involvement in the regulation of energy metabolism. Thus we hypothesized that UCP-2 would be regulated in the hepatocyte by fatty acids, which are known to control other energy-related metabolic processes. Treatment with 250 microM palmitic acid was without effect on UCP-2 expression, whereas 250 microM oleic acid exhibited a modest eightfold increase. Eicosapentaenoic acid (EPA), a polyunsaturated fatty acid, exerted a 50-fold upregulation of UCP-2 that was concentration dependent. This effect was seen within 12 h and was maximal by 36 h. Aspirin blocked the induction of UCP-2 by EPA, indicating involvement of the prostaglandin pathway. Hepatocytes treated with arachidonic acid, the immediate precursor to the prostaglandins, also exhibited an aspirin-inhibitable increase in UCP-2 levels, further supporting the involvement of prostaglandins in regulating hepatic UCP-2. The peroxisome proliferator-activated receptor-alpha (PPARalpha) agonist Wy-14643 stimulated UCP-2 mRNA levels as effectively as EPA. These data indicate that UCP-2 is upregulated by polyunsaturated fatty acids, potentially through a prostaglandin/PPARalpha-mediated pathway.

Nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a condition characterized by hepatomegaly, elevated serum aminotransferase levels, and a histologic picture similar to alcoholic hepatitis in the absence of alcohol abuse. Most patients with NASH are obese women, and many have diabetes mellitus, hypercholesterolemia, or hypertriglyceridemia. NASH has also been associated with a number of metabolic conditions, surgical procedures, and drug treatments. Most patients are asymptomatic. The most common sign of NASH is hepatomegaly. Stigmata of chronic liver disease are rare. Laboratory abnormalities include a 2-4-fold elevation of serum aminotransferase levels; other liver function test results are usually normal. Histologically, there is moderate to severe macrovesicular steatosis and lobular hepatitis with necrosis or ballooning degeneration and/or fibrosis. The pathogenesis of NASH is poorly understood, but lipid peroxidation and oxidative stress are the leading culprits. The natural history of NASH is unknown, but NASH seems to be a stable disease in most patients. Treatment of NASH is unproven, but weight reduction is recommended in obese patients. Small pilot studies of several drugs have shown promise, but large randomized clinical trials are awaited. Orthotopic liver transplantation is the treatment of choice for end-stage liver disease secondary to NASH.

Fatty liver vulnerability to endotoxin-induced damage despite NF-kappaB induction and inhibited caspase 3 activation

Fatty livers are sensitive to lipopolysaccharide (LPS) damage. This study tests the hypothesis that this vulnerability occurs because protective, antiapoptotic mechanisms are not upregulated appropriately. Genetically obese, leptin-deficient ob/ob mice, a model for nonalcoholic fatty liver disease, and their lean litter mates were treated with a small dose of LPS. General measures of liver injury, early (i.e., cytochrome c release) and late (i.e., activation of caspase 3) events that occur during hepatocyte apoptosis, and various aspects of the signal transduction pathways that induce nuclear factor-kappaB (NF-kappaB) and several of its antiapoptotic transcriptional targets (e.g., inducible nitric oxide synthase, bfl-1, and bcl-xL) were compared. Within 0.5-6 h after LPS exposure, cytochrome c begins to accumulate in the cytosol of normal livers, and procaspase 3 cleavage increases. Coincident with these events, kinases (e.g., AKT and Erk-1 and -2) that result in the degradation of inhibitor kappa-B are activated; NF-kappaB activity is induced, and NF-kappaB-regulated gene products accumulate. Throughout this period, there is negligible histological evidence of liver damage, and serum alanine aminotransferase values barely increase over baseline values. Although ob/ob livers have significant histological liver injury and 11-fold greater serum alanine aminotransferase values than those of lean mice by 6 h post-LPS, they exhibit greater activation of AKT and Erk, more profound reductions in inhibitor kappa-B, enhanced activation of NF-kappaB, and greater induction of NF-kappaB-regulated genes. Consistent with this heightened antiapoptotic response, increases in cytochrome c and procaspase 3 cleavage products are inhibited. Together with evidence that ob/ob hepatocytes have a reduced ATP content and undergo increased lysis after in vitro exposure to tumor necrosis factor-alpha, these findings suggest that fatty livers are sensitive to LPS damage because of vulnerability to necrosis, rather than because of apoptosis.

Metabolic changes induced by cold stress in rat liver mitochondria

The mechanisms involved in the metabolic changes induced by cold stress in isolated rat liver mitochondria were studied. Respiration, ATP synthesis, and membrane potential as well as the contents of several metabolites were determined in liver mitochondria from cold-exposed rats. At different times of cold exposure, the force-flux relationships showed net variation in flux (enhanced respiration, diminished ATP synthesis) with no associated variation in force (H+ gradient); this suggested that decoupling rather than classical uncoupling was involved in the effects of cold stress. The flux control coefficient of the H+ leak on basal respiration was slightly increased by 380 h of cold exposure. Cold stress also induced a diminution in total membrane fatty acids, Zn2+, Fe3+, ATP, and ADP/O ratios; the content of cytochromes c + c1 and b oscillated. The contents of Ca2+, Na+, Pi, and cytochromes a + a3 were not affected, whereas matrix ADP, AMP, K+, and Mg2+ were markedly increased. Basal and oleic acid-stimulated respiration of mitochondria from cold-stressed rats was inhibited by GDP, carboxyatractyloside, or albumin. These agents did not affect basal respiration in control mitochondria. Western blot analysis showed enhanced expression of a protein of about 35 kDa, presumably the uncoupling protein 2, induced by long-term cold exposure. The overall data suggest that cold stress promoted decoupling of oxidative phosphorylation, and hence, changes in several matrix metabolites, by increasing free fatty acids and the UCP2 content.

Nuclear magnetic resonance spectroscopy and imaging in animal research

Nuclear magnetic resonance (NMR) spectroscopy and imaging can be used to investigate, noninvasively, a wide range of biological processes in systems as diverse as protein solutions, single cells, isolated perfused organs, and tissues in vivo. It is also possible to combine different NMR techniques enabling metabolic, anatomical, and physiological information to be obtained in the same experiment. This review provides a simple overview of the basic principles of NMR and outlines both the advantages and disadvantages of NMR spectroscopy and imaging. A few examples of potential applications of NMR spectroscopy and imaging are presented, which demonstrate the range of questions that can be asked using these techniques. The potential impact of using NMR techniques in a biomedical research program on the total number of animals required for specific investigations, as well as the number of animals used in research, are discussed. The article concludes with a personal perspective on the impact of continuing improvements in NMR technology for future applications in animal research.

Current concepts in the pathogenesis of alcoholic liver injury

Alcoholic liver disease (ALD) develops as a consequence of priming and sensitizing mechanisms rendered by cross-interactions of primary mechanistic factors and secondary risk factors. This concept, albeit not novel, is becoming widely accepted by the field, and more research is directed toward identifying and characterizing the interfaces of the cross-interactions to help understand individual predisposition to the disease. Another pivotal development is the beginning of cell type-specific research to elucidate specific contributions not only of hepatocytes, but also of hepatic macrophages, liver-associated lymphocytes, sinusoidal endothelial cells, and hepatic stellate cells to sensitizing and priming mechanisms. In particular, the critical role of hepatic macrophages has been highlighted and the priming mechanisms concerning this paracrine effect have been proposed. Glutathione depletion in hepatocyte mitochondria is considered the most important sensitizing mechanism. One of the contributing factors is decreased methionine metabolism. Remaining key questions include how altered methionine metabolism contribute to the pathogenesis of ALD; how cross-talk among nonparenchymal liver cells or between nonparenchymal cells and hepatocytes leads to ALD; how dysfunctional mitochondria determine the type of cell death in ALD; and what secondary factors are critical for the development of advanced ALD such as alcoholic hepatitis and cirrhosis.

Role of hepatocytes and bile duct cells in preservation-reperfusion injury of liver grafts

In liver transplantation, it is currently hypothesized that nonparenchymal cell damage and/or activation is the major cause of preservation-related graft injury. Because parenchymal cells (hepatocytes) appear morphologically well preserved even after extended cold preservation, their injury after warm reperfusion is ascribed to the consequences of nonparenchymal cell damage and/or activation. However, accumulating evidence over the past decade indicated that the current hypothesis cannot fully explain preservation-related liver graft injury. We review data obtained in animal and human liver transplantation and isolated perfused animal livers, as well as isolated cell models to highlight growing evidence of the importance of hepatocyte disturbances in the pathogenesis of normal and fatty graft injury. Particular attention is given to preservation time-dependent decreases in high-energy adenine nucleotide levels in liver cells, a circumstance that (1) sensitizes hepatocytes to various stimuli and insults, (2) correlates well with graft function after liver transplantation, and (3) may also underlie the preservation time-dependent increase in endothelial cell damage. We also review damage to bile duct cells, which is increasingly being recognized as important in the long-lasting phase of reperfusion injury. The role of hydrophobic bile salts in that context is particularly assessed. Finally, a number of avenues aimed at preserving hepatocyte and bile duct cell integrity are discussed in the context of liver transplantation therapy as a complement to reducing nonparenchymal cell damage and/or activation.

Uncoupling protein 2: a possible link between fatty acid excess and impaired glucose-induced insulin secretion?

The mechanism by which long-term exposure of the beta-cell to elevated concentrations of fatty acid alters glucose-induced insulin secretion has been examined. Exposure of INS-1 beta-cells to 0.4 mmol/l oleate for 72 h increased basal insulin secretion and decreased insulin release in response to high glucose, but not in response to agents acting at the level of the K(ATP) channel (tolbutamide) or beyond (elevated KCl). This also suppressed the glucose-induced increase in the cellular ATP-to-ADP ratio. The depolarization of the plasma membrane promoted by glucose was decreased after oleate exposure, whereas the response to KCl was unchanged. Cells exposed to free fatty acids displayed a lower mitochondrial membrane potential and a decreased glucose-induced hyperpolarization. The possible implication of uncoupling protein (UCP)-2 in the altered secretory response was examined by measuring UCP2 gene expression after chronic exposure of the cells to fatty acids. UCP2 mRNA and protein were increased twofold by oleate. Palmitate and the nonoxidizable fatty acid bromopalmitate had similar effects on UCP2 mRNA, suggesting that UCP2 gene induction by fatty acids does not require their metabolism. The data are compatible with a role of UCP2 and partial mitochondrial uncoupling in the decreased secretory response to glucose observed after chronic exposure of the beta-cell to elevated fatty acids, and suggest that the expression and/or activity of the protein may modulate insulin secretion in response to glucose.

Uncoupling protein 2, in vivo distribution, induction upon oxidative stress, and evidence for translational regulation

Uncoupling protein 2 (UCP2) belongs to the mitochondrial anion carrier family and partially uncouples respiration from ATP synthesis when expressed in recombinant yeast mitochondria. We generated a highly sensitive polyclonal antibody against human UCP2. Its reactivity toward mitochondrial proteins was compared between wild type and ucp2(-/-) mice, leading to non-ambiguous identification of UCP2. We detected UCP2 in spleen, lung, stomach, and white adipose tissue. No UCP2 was detected in heart, skeletal muscle, liver, and brown adipose tissue. The level of UCP2 in spleen mitochondria is less than 1% of the level of UCP1 in brown adipose tissue mitochondria. Starvation and LPS treatments increase UCP2 level up to 12 times in lung and stomach, which supports the hypothesis that UCP2 responds to oxidative stress situations. Stimulation of the UCP2 expression occurs without any change in UCP2 mRNA levels. This is explained by translational regulation of the UCP2 mRNA. We have shown that an upstream open reading frame located in exon two of the ucp2 gene strongly inhibits the expression of the protein. This further level of regulation of the ucp2 gene provides a mechanism by which expression can be strongly and rapidly induced under stress conditions.

Mitochondrial proton leak: a role for uncoupling proteins 2 and 3?

In mitochondria ATP synthesis is not perfectly coupled to oxygen consumption due to proton leak across the mitochondrial inner membrane. Quantitative studies have shown that proton leak contributes to approximately 25% of the resting oxygen consumption of mammals. Proton leak plays a role in accounting for differences in basal metabolic rate. Thyroid studies, body mass studies, phylogenic studies and obesity studies have all shown that increased mass-specific metabolic rate is linked to increased mitochondrial proton leak. The mechanism of the proton leak is unclear. Evidence suggests that proton leak occurs by a non-specific diffusion process across the mitochondrial inner membrane. However, the high degree of sequence homology of the recently cloned uncoupling proteins UCP 2 and UCP 3 to brown adipose tissue UCP 1, and their extensive tissue distribution, suggest that these novel uncoupling proteins play a role in proton leak. Early indications from reconstitution experiments and several in vitro expression studies suggest that the novel uncoupling proteins uncouple mitochondria. Furthermore, mice overexpressing UCP 3 certainly show a phenotype consistent with increased metabolism. The evidence for a role for these novel UCPs in mitochondrial proton leak is reviewed.

UCP1: the only protein able to mediate adaptive non-shivering thermogenesis and metabolic inefficiency

The uniqueness of UCP1 (as compared to UCP2/UCP3) is evident from expression analysis and ablation studies. UCP1 expression is positively correlated with metabolic inefficiency, being increased by cold acclimation (in adults or perinatally) and overfeeding, and reduced in fasting and genetic obesity. Such a simple relationship is not observable for UCP2/UCP3. Studies with UCP1-ablated animals substantiate the unique role of UCP1: the phenomenon of adaptive adrenergic non-shivering thermogenesis in the intact animal is fully dependent on the presence of UCP1, and so is any kind of cold acclimation-recruited non-shivering thermogenesis; thus UCP2/UCP3 (or any other proteins or metabolic processes) cannot substitute for UCP1 physiologically, irrespective of their demonstrated ability to show uncoupling in reconstituted systems or when ectopically expressed. Norepinephrine-induced thermogenesis in brown-fat cells is absolutely dependent on UCP1, as is the uncoupled state and the recoupling by purine nucleotides in isolated brown-fat mitochondria. Although very high UCP2/UCP3 mRNA levels are observed in brown adipose tissue of UCP1-ablated mice, there is no indication that the isolated brown-fat mitochondria are uncoupled; thus, high expression of UCP2/UCP3 does not necessarily confer to the mitochondria of a tissue a propensity for being innately uncoupled. Whereas the thermogenic effect of fatty acids in brown-fat cells is fully UCP1-dependent, this is not the case in brown-fat mitochondria; this adds complexity to the issues concerning the mechanisms of UCP1 function and the pathway from beta(3)-adrenoceptor stimulation to UCP1 activation and thermogenesis. In addition to amino acid sequences conserved in all UCPs as part of the tripartite structure, all UCPs contain certain residues associated with nucleotide binding. However, conserved amongst all UCP1s so far sequenced, and without parallel in all UCP2/UCP3, are two sequences: 144SHLHGIKP and the C-terminal sequence RQTVDC(A/T)T; these sequences may therefore be essential for the unique thermogenic function of UCP1. The level of UCP1 in the organism is basically regulated at the transcriptional level (physiologically probably mainly through the beta(3)-adrenoceptor/CREB pathway), with influences from UCP1 mRNA stability and from the delay caused by translation. It is concluded that UCP1 is unique amongst the uncoupling proteins and is the only protein able to mediate adaptive non-shivering thermogenesis and the ensuing metabolic inefficiency.

Fast decline of hematopoiesis and uncoupling protein 2 content in human liver after birth: location of the protein in Kupffer cells

Hepatic hematopoiesis is prominent during fetal life and ceases around birth. In rodent liver, the decline of the hepatic hematopoiesis starts abruptly at birth being accompanied by a decrease of mitochondrial uncoupling protein 2 (UCP2) expression in monocytes/macrophages, whereas hepatocytes may express UCP2 only under pathologic situations. The goals of this study were to characterize hepatic hematopoiesis in humans around birth, and to identify cells expressing UCP2. Hematopoiesis was evaluated histologically in the liver of 22 newborns (mostly very premature neonates), who died between 45 min and 140 d after birth, and one fetus. UCP2 expression was characterized by Northern blots, immunoblotting, immunohistochemistry, and by in situ hybridization. The number of hematopoietic cells started to decrease rapidly at birth, irrespectively of the gestational age (23-40 wk) of neonates. A similar decline was observed for UCP2 expression, which was relatively high in fetal liver. UCP2 was detected only in myeloid cells (mainly in Kupffer cells), but not in hepatocytes, although sepsis or other pathologies occurred in the critically ill newborns. Kupffer cells represent the major site of mitochondrial UCP2 expression in the human newborn. UCP2 may be essential for the differentiation and function of macrophages and serve as a marker for these cells in human liver during the perinatal period.

UCP2 muscle gene transfer modifies mitochondrial membrane potential

OBJECTIVE

The aim of this work was to evaluate the effect of uncoupling protein 2 (UCP2) muscle gene transfer on mitochondrial activity.

DESIGN

Five week-old male Wistar rats received an intramuscular injection of plasmid pXU1 containing UCP2 cDNA in the right tibialis anterior muscles. Left tibialis anterior muscles were injected with vehicle as control. Ten days after DNA injection, tibialis anterior muscles were dissected and muscle mitochondria isolated and analyzed.

RESULTS

There were two mitochondrial populations in the muscle after UCP2 gene transfer, one of low fluorescence and complexity and the other, showing high fluorescence and complexity. UCP2 gene transfer resulted in a 3.6 fold increase in muscle UCP2 protein levels compared to control muscles assessed by Western blotting. Furthermore, a significant reduction in mitochondria membrane potential assessed by spectrofluorometry and flow cytometry was observed. The mitochondria membrane potential reduction might account for a decrease in fluorescence of the low fluorescence mitochondrial subpopulation.

CONCLUSION

It has been demonstrated that UCP2 muscle gene transfer in vivo is associated with a lower mitochondria membrane potential. Our results suggest the potential involvement of UCP2 in uncoupling respiration. International Journal of Obesity (2001) 25, 68-74