Peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC1alpha) is a metabolic regulator of intestinal epithelial cell fate.

Peroxisome proliferator-activated receptor-γ coactivator 1-α (PGC1α) is a transcriptional coactivator able to up-regulate mitochondrial biogenesis, respiratory capacity, oxidative phosphorylation, and fatty acid ß-oxidation with the final aim of providing a more efficient pathway for aerobic energy production. In the continuously renewed intestinal epithelium, proliferative cells in the crypts migrate along the villus axis and differentiate into mature enterocytes, increasing their respiratory capacity and finally undergoing apoptosis. Here we show that in the intestinal epithelial surface, PGC1α drives mitochondrial biogenesis and respiration in the presence of reduced antioxidant enzyme activities, thus determining the accumulation of reactive oxygen species and fostering the fate of enterocytes toward apoptosis. Combining gain- and loss-of-function genetic approaches in human cells and mouse models of intestinal cancer, we present an intriguing scenario whereby PGC1α regulates enterocyte cell fate and protects against tumorigenesis.

Targeting fat to prevent diabetes.

An emerging view is that obesity causes metabolic problems when adipose tissue fails to meet the increased demands for fat storage. A study in this issue of Cell Metabolism (Waki et al., 2007) has identified harmine as a proadipogenic small molecule that promotes energy expenditure in white adipose tissue and delays the onset of obesity-associated diabetes.

Dysregulation of glucose homeostasis in nicotinamide nucleotide transhydrogenase knockout mice is independent of uncoupling protein 2.

Glucose intolerance in C57Bl/6 mice has been associated with mutations in the nicotinamide nucleotide transhydrogenase (Nnt) gene. It has been proposed that the absence of NNT from mitochondria leads to increased mitochondrial reactive oxygen species production and subsequent activation of uncoupling protein 2 (UCP2). Activation of UCP2 has been suggested to uncouple electron transport from ATP synthesis in pancreatic beta cell mitochondria thereby decreasing glucose tolerance due to decreased insulin secretion through lower ATP/ADP ratios. The hypothesis tested in this paper is that UCP2 function is required for the dysregulation of glucose homeostasis observed in NNT ablated mice. Single and double Nnt and Ucp2 knockout mouse lines were used to measure glucose tolerance, whole animal energy balance and biochemical characteristics of mitochondrial uncoupling. As expected, glucose tolerance was diminished in mice lacking NNT. This was independent of UCP2 as it was observed either in the presence or absence of UCP2. The range of metabolic parameters examined in the mice and the proton conductance of isolated mitochondria remained unaltered in this double NNT and UCP2 knockout model. Ablation of UCP2 did not itself affect glucose tolerance and therefore previous observations of increased glucose tolerance of mice lacking UCP2 were not confirmed. We conclude that the decreased glucose tolerance in Nnt knockout mice observed in our experiments does not require UCP2.

Uncoupling protein-1 (UCP1) contributes to the basal proton conductance of brown adipose tissue mitochondria.

Proton leak pathways uncouple substrate oxidation from ATP synthesis in mitochondria. These pathways are classified as basal (not regulated) or inducible (activated and inhibited). Previously it was found that over half of the basal proton conductance of muscle mitochondria was catalyzed by the adenine nucleotide translocase (ANT), an abundant mitochondrial anion carrier protein. To determine whether ANT is the unique protein catalyst, or one of many proteins that catalyze basal proton conductance, we measured proton leak kinetics in mitochondria isolated from brown adipose tissue (BAT). BAT can express another mitochondrial anion carrier, UCP1, at concentrations similar to ANT. Basal proton conductance was measured under conditions where UCP1 and ANT were catalytically inactive and was found to be lower in mitochondria from UCP1 knockout mice compared to wild-type. Ablation of another abundant inner membrane protein, nicotinamide nucleotide transhydrogenase, had no effect on proton leak kinetics in mitochondria from liver, kidney or muscle, showing that basal proton conductance is not catalyzed by all membrane proteins. We identify UCP1 as a second protein propagating basal proton leak, lending support to the hypothesis that basal leak pathways are perpetrated by members of the mitochondrial anion carrier family but not by other mitochondrial inner membrane proteins.

Wnt signalling at the crossroads of nutritional regulation.

The ability to sense and respond to nutritional cues is among the most fundamental processes that support life in living organisms. At the cellular level, a number of biochemical mechanisms have been proposed to mediate cellular glucose sensing. These include ATP-sensitive potassium channels, AMP-activated protein kinase, activation of PKC (protein kinase C), and flux through the hexosamine pathway. Less well known is how cellularly heterogenous organs couple nutrient availability to prioritization of cell autonomous functions and appropriate growth of the entire organ. Yet what is clear is that when such mechanisms fail or become inappropriately active they can lead to dire consequences such as diabetes, metabolic syndromes, cardiovascular diseases and cancer. In this issue of the Biochemical Journal, Anagnostou and Shepherd report the identification of an important link between cellular glucose sensing and the Wnt/beta-catenin signalling pathway in macrophages. Their data strongly indicate that the Wnt/beta-catenin pathway of Wnt signalling is responsive to physiological concentrations of nutrients but also suggests that that this system could be inappropriately activated in the diabetic (hyperglycaemic) or other metabolically compromised pathological states. This opens the exciting possibility that organ-selective modulation of Wnt signalling may become an attractive therapeutic target to treat these diseases.

Feeding and metabolism in mice lacking pituitary adenylate cyclase-activating polypeptide.

Disruption of the pituitary adenylate cyclase-activating polypeptide (PACAP) gene in mice has demonstrated a role for this highly conserved neuropeptide in the regulation of metabolism and temperature control. Localization of PACAP neurons within hypothalamic nuclei that regulate appetite suggest PACAP may affect feeding and thus energy balance. We used PACAP-null mice to address this question, examining both food intake and energy expenditure. PACAP-null mice were leaner than wild-type littermates due to decreased adiposity and displayed increased insulin sensitivity. The lean phenotype in the PACAP-null mice was completely eliminated if animals were fed a high-fat diet or housed near thermoneutrality (28 C). Further metabolic analyses of PACAP-null mice housed at 21 C indicated that the reduced body weight could not be explained by decreased food intake, increased metabolic rate, or increased locomotor activity. The thyroid hormone axis of PACAP-null mice was affected, because mRNA levels of hypothalamic TRH and brown adipose tissue type 2 deiodinase were reduced in PACAP-null mice housed at room temperature, and brain deiodinase activity was lower in PACAP-null mice after an acute cold challenge compared with wild-type controls. These results demonstrate that PACAP is not required for the regulation of food intake yet is necessary to maintain normal energy homeostasis, likely playing a role in central cold-sensing mechanisms.

Fed-EXosome: extracellular vesicles and cell-cell communication in metabolic regulation.

Extracellular vesicles (EVs) have emerged as a novel messaging system of the organism, mediating cell-cell and interorgan communication. Through their content of proteins and nucleic acids, as well as membrane proteins and lipid species, EVs can interact with and modulate the function of their target cells. The regulation of whole-body metabolism requires cross-talk between key metabolic tissues including adipose tissue (AT), the liver and skeletal muscle. Furthermore, the regulation of nutrient/energy allocation during pregnancy requires co-ordinated communication between the foetus and metabolic organs of the mother. A growing body of evidence is suggesting that EVs play a role in communication between and within key metabolic organs, both physiologically during metabolic homoeostasis but also contributing to pathophysiology during metabolic dysregulation observed in metabolic diseases such as obesity and diabetes. As obesity and its associated metabolic complications are reaching epidemic proportions, characterization of EV-mediated communication between key metabolic tissues may offer important insights into the regulation of metabolic functions during disease and offer global therapeutic opportunities. Here, we focus on the role of EVs in metabolic regulation and, in particular, EV-mediated cross-talk between cells of the AT.

Tamoxifen-induced anorexia is associated with fatty acid synthase inhibition in the ventromedial nucleus of the hypothalamus and accumulation of malonyl-CoA.

Fatty acid metabolism in the hypothalamus has recently been shown to regulate feeding. The selective estrogen receptor modulator tamoxifen (TMX) exerts a potent anorectic effect. Here, we show that the anorectic effect of TMX is associated with the accumulation of malonyl-CoA in the hypothalamus and inhibition of fatty acid synthase (FAS) expression specifically in the ventromedial nucleus of the hypothalamus (VMN). Furthermore, we demonstrate that FAS mRNA expression is physiologically regulated by fasting and refeeding in the VMN but not in other hypothalamic nuclei. Thus, the VMN appears to be the hypothalamic site where regulation of FAS and feeding converge. Supporting the potential clinical relevance of these observations, reanalysis of a primary breast cancer prevention study showed that obese women treated with TMX gained significantly less body weight over a 6-year period than obese women given placebo. The finding that TMX can modulate appetite through alterations in FAS expression and malonyl-CoA levels suggests a link between hypothalamic sex steroid receptors, fatty acid metabolism, and feeding behavior.

Adipose tissue expandability in the maintenance of metabolic homeostasis.

Adipose tissue expands to accommodate increased lipid through hypertrophy of existing adipocytes and by initiating differentiation of preadipocytes. The capacity of adipose tissue to expand is critical for accommodating changes in energy availability, but this capacity is not an unlimited process and likely varies between individuals. We suggest that it is not the absolute amount of adipose tissue but rather the capacity of adipose tissue to expand that affects metabolic homeostasis. Here we highlight examples of disease states and transgenic animal models with altered adipose tissue function that support this hypothesis and discuss possible mechanisms by which altered adipose tissue expandability impairs metabolic homeostasis.

Lipotoxicity, overnutrition and energy metabolism in aging.

The safest place to store lipids is the white adipose tissue, but its storage capacity may become saturated resulting in excess of fat "overspilled" to non-adipose tissues. This overspill of fat occurs in apparently opposite pathological states such as lipodistrophy or obesity. When the excess of energy is redirected towards peripheral organs, their initial response is to facilitate the storage of the surplus in the form of triacylglycerol, but the limited triacylglycerol buffer capacity becomes saturated soon. Under these conditions excess of lipids enter alternative non-oxidative pathways that result in production of toxic reactive lipid species that induce organ-specific toxic responses leading to apoptosis. Reactive lipids can accumulate in non-adipose tissues of metabolically relevant organs such as pancreatic beta-cells, liver, heart and skeletal muscle leading to lipotoxicity, a process that contributes substantially to the pathophysiology of insulin resistance, type 2 diabetes, steatotic liver disease and heart failure. The effects of this lipotoxic insult can be minimised by several strategies: (a) decreased incorporation of energy, (b) a less orthodox approach such as increased adipose tissue expandability and/or (c) increased oxidation of fat in peripheral organs. Aging should be considered as physiological degenerative process potentially accelerated by concomitant lipotoxic insults. Conversely, the process of aging can sensitise cells to effects of lipid toxicity.

Human obesity and type 2 diabetes are associated with alterations in SREBP1 isoform expression that are reproduced ex vivo by tumor necrosis factor-alpha.

Sterol regulatory element binding protein (SREBP)-1 is a transcription factor with important roles in the control of fatty acid metabolism and adipogenesis. Little information is available regarding the expression of this molecule in human health or disease. Exposure of isolated human adipocytes to insulin enhanced SREBP1 gene expression and promoted its proteolytic cleavage to the active form. Furthermore, 3 h of in vivo hyperinsulinemia also significantly increased SREBP1 gene expression in human skeletal muscle. Transcript levels of SREBP1c, the most abundant isoform in adipose tissue, were significantly decreased in the subcutaneous adipose tissue of obese normoglycemic and type 2 diabetic subjects compared with that of nonobese normoglycemic control subjects. In skeletal muscle, SREBP1 expression was significantly reduced in type 2 diabetic subjects but not in obese subjects. Within the diabetic group, the extent of SREBP1 suppression was inversely related to metabolic control and was normalized by 3 h of in vivo hyperinsulinemia. Exposure of isolated human adipocytes to tumor necrosis factor-alpha (TNF-alpha) produced a marked and specific decrease in the mRNA encoding the SREBP1c isoform and completely blocked the insulin-induced cleavage of SREBP1 protein. Thus, both the expression and proteolytic maturation of human SREBP1 are positively modulated by insulin. The specific reduction in the SREBP1c isoform seen in the adipose tissue of obese and type 2 diabetic subjects can be recapitulated ex vivo by TNF-alpha, suggesting a possible mechanism for this association.

Isomer-dependent metabolic effects of conjugated linoleic acid: insights from molecular markers sterol regulatory element-binding protein-1c and LXRalpha.

Conjugated linoleic acid (CLA) is a heterogeneous group of positional and geometric isomers of linoleic acid. This study demonstrates the divergent effects of the cis-9 trans-11 (c9,t11-CLA) and trans-10 cis-12 (t10,c12-CLA) isomers of CLA on lipid metabolism and nutrient regulation of gene expression in ob/ob mice. The c9, t11-CLA diet decreased serum triacylglycerol (P = 0.01) and nonesterified fatty acid (NEFA) (P = 0.05) concentrations, and this was associated with reduced hepatic sterol regulatory element-binding protein-1c (SREBP-1c; P = 0.0045) mRNA expression, coupled with reduced levels of both the membrane-bound precursor and the nuclear forms of the SREBP-1 protein. C9,t11-CLA significantly reduced hepatic LXRalpha (P = 0.019) mRNA expression, a novel regulator of SREBP-1c. In contrast, c9,t11-CLA increased adipose tissue SREBP-1c mRNA expression (P = 0.0162) proportionally to the degree of reduction of tumor necrosis factor alpha (TNF-alpha) mRNA (P = 0.012). Recombinant TNF-alpha almost completely abolished adipose tissue SREBP-1c mRNA expression in vivo. The t10,c12-CLA diet promoted insulin resistance and increased serum glucose (P = 0.025) and insulin (P = 0.01) concentrations. T10, c12-CLA induced profound weight loss (P = 0.0001) and increased brown and white adipose tissue UCP-2 (P = 0.001) and skeletal muscle UCP-3 (P = 0.008) mRNA expression. This study highlights the contrasting molecular and metabolic effect of two isomers of the same fatty acids. The ameliorative effect of c9,t11-CLA on lipid metabolism may be ascribed to reduced synthesis and cleavage of hepatic SREBP-1, which in turn may be regulated by hepatic LXRalpha expression.

Not all mitochondrial carrier proteins support permeability transition pore formation: no involvement of uncoupling protein 1.

The mPTP (mitochondrial permeability transition pore) is a non-specific channel that is formed in the mitochondrial inner membrane in response to several stimuli, including elevated levels of matrix calcium. The pore is proposed to be composed of the ANT (adenine nucleotide translocase), voltage-dependent anion channel and cyclophilin D. Knockout studies, however, have demonstrated that ANT is not essential for permeability transition, which has led to the proposal that other members of the mitochondrial carrier protein family may be able to play a similar function to ANT in pore formation. To investigate this possibility, we have studied the permeability transition properties of BAT (brown adipose tissue) mitochondria in which levels of the mitochondrial carrier protein, UCP1 (uncoupling protein 1), can exceed those of ANT. Using an improved spectroscopic assay, we have quantified mPTP formation in de-energized mitochondria from wild-type and Ucp1KO (Ucp1-knockout) mice and assessed the dependence of pore formation on UCP1. When correctly normalized for differences in mitochondrial morphology, we find that calcium-induced mPTP activity is the same in both types of mitochondria, with similar sensitivity to GDP (approximately 50% inhibited), although the portion sensitive to cyclosporin A is higher in mitochondria lacking UCP1 (approximately 80% inhibited, compared with approximately 60% in mitochondria containing UCP1). We conclude that UCP1 is not a component of the cyclosporin A-sensitive mPTP in BAT and that playing a role in mPTP formation is not a general characteristic of the mitochondrial carrier protein family but is, more likely, restricted to specific members including ANT.

Dact1, a nutritionally regulated preadipocyte gene, controls adipogenesis by coordinating the Wnt/beta-catenin signaling network.

OBJECTIVE: Wnt signaling inhibits adipogenesis, but its regulation, physiological relevance, and molecular effectors are poorly understood. Here, we identify the Wnt modulator Dapper1/Frodo1 (Dact1) as a new preadipocyte gene involved in the regulation of murine and human adipogenesis. RESEARCH DESIGN AND METHODS: Changes in Dact1 expression were investigated in three in vitro models of adipogenesis. In vitro gain- and loss-of-function studies were used to investigate the mechanism of Dact1 action during adipogenesis. The in vivo regulation of Dact1 and Wnt/beta-catenin signaling were investigated in murine models of altered nutritional status, of pharmacological stimulation of in vivo adipogenesis, and during the development of dietary and genetic obesity. RESULTS: Dact1 is a preadipocyte gene that decreases during adipogenesis. However, Dact1 knockdown impairs adipogenesis through activation of the Wnt/beta-catenin signaling pathway, and this is reversed by treatment with the secreted Wnt antagonist, secreted Frizzled-related protein 1 (Sfrp1). In contrast, constitutive Dact1 overexpression promotes adipogenesis and confers resistance to Wnt ligand-induced antiadipogenesis through increased expression of endogenous Sfrps and reduced expression of Wnts. In vivo, in white adipose tissue, Dact1 and Wnt/beta-catenin signaling also exhibit coordinated expression profiles in response to altered nutritional status, in response to pharmacological stimulation of in vivo adipogenesis, and during the development of dietary and genetic obesity. CONCLUSIONS: Dact1 regulates adipogenesis through coordinated effects on gene expression that selectively alter intracellular and paracrine/autocrine components of the Wnt/beta-catenin signaling pathway. These novel insights into the molecular mechanisms controlling adipose tissue plasticity provide a functional network with therapeutic potential against diseases, such as obesity and associated metabolic disorders.

The metabolic profile of early Huntington's disease--a combined human and transgenic mouse study.

Huntington's disease (HD) is a debilitating autosomal dominant, neurodegenerative disease with a fatal prognosis. Classical symptoms include motor disturbances, subcortical dementia and psychiatric symptoms but are not restricted to this triad. Patients often experience other problems such as weight loss, although why and when this occurs in the disease course is not known. We studied metabolism using whole body indirect calorimetry in both early stage HD patients and in the R6/2 transgenic mouse model of HD, at times before and after they displayed signs of disease. Using this combined approach we found that patients with early HD tended to be in negative energy balance for reasons not related to their movement disorder, which was paralleled in the transgenic R6/2 mice. These mice had significantly elevated total energy expenditure as they developed overt disease with weight loss due primarily to a loss of muscle bulk. This study has shown for the first time that in HD there is the development of early negative energy balance, which in turn may cause weight loss with loss of muscle bulk in particular. The reason for this is not known but may reflect a catabolic state secondary to hypothalamic pathology, as abnormalities have been reported in the hypothalamus early in the disease course.

Thematic review series: adipocyte biology. Adipose tissue function and plasticity orchestrate nutritional adaptation.

This review focuses on adipose tissue biology and introduces the concept of adipose tissue plasticity and expandability as key determinants of obesity-associated metabolic dysregulation. This concept is fundamental to our understanding of adipose tissue as a dynamic organ at the center of nutritional adaptation. Here, we summarize the current knowledge of the mechanisms by which adipose tissue can affect peripheral energy homeostasis, particularly in the context of overnutrition. Two mechanisms emerge that provide a molecular understanding for obesity-associated insulin resistance. These are a) the dysregulation of adipose tissue expandability and b) the abnormal production of adipokines. This knowledge has the potential to pave the way for novel therapeutic concepts and strategies for managing and/or correcting complications associated with obesity and the metabolic syndrome.

An update on visfatin/pre-B cell colony-enhancing factor, an ubiquitously expressed, illusive cytokine that is regulated in obesity.

PURPOSE OF REVIEW: The aim of this article is to summarize all of the recent studies on pre-B cell colony-enhancing factor visfatin, a ubiquitously expressed secreted protein that has been implicated in obesity and insulin resistance. Although pre-B-cell colony-enhancing factor was discovered over 10 years ago, there are many remaining questions about the regulation and function of this protein. RECENT FINDINGS: Studies in the last decade have revealed the endocrine properties of fat cells. One of the most recent proteins shown to be highly expressed in adipose tissue is visfatin, originally identified as pre-B-cell colony-enhancing factor. Visfatin/pre-B-cell colony-enhancing factor appears to be preferentially produced by the visceral adipose tissue and has insulin mimetic actions. Studies by many groups indicate that obesity-related diabetes and accompanying metabolic disorders in humans have been specifically linked to increased visceral adipose tissue mass. The different roles of various adipocyte depots, however, are still poorly understood. It has been hypothesized that understanding the differences in the biology of visceral and subcutaneous human adipose tissue may hold the key to therapeutic strategies aimed at reducing obesity-induced insulin resistance and alleviating symptoms of the metabolic syndrome. Interestingly, some observed actions of visfatin indicate that this secreted protein may be an interesting therapeutic target. Several recent studies, however, indicate that our understanding of visfatin is still speculative. SUMMARY: This review summarizes all of the papers in the last year on the expression and function of visfatin/pre-B-cell colony-enhancing factor and highlights inconsistent observations from various investigators studying this protein. It also highlights previous observations on the role of pre-B-cell colony-enhancing factor. We suggest that the pathophysiologic role of visfatin/pre-B-cell colony-enhancing factor in humans remains largely unknown.

The peroxisome proliferator-activated receptor-gamma regulates murine pyruvate carboxylase gene expression in vivo and in vitro.

Pyruvate carboxylase (PC) plays a crucial role in various metabolic pathways, including gluconeogenesis, lipogenesis, and glucose-induced insulin secretion. Here we showed for the first time that the PC gene is transcriptionally regulated by peroxisome proliferator-activated receptor-gamma (PPARgamma) in vitro and in vivo in white and brown adipose tissue. PC mRNA and protein are markedly increased during differentiation of 3T3-L1 cells and HIB-1B, in parallel with the expression of the adipogenic transcription factors, CCAAT-enhancer binding protein alpha, PPARgamma1, and PPARgamma2. Tumor necrosis factor-alpha, a cytokine that blocks differentiation of 3T3-L1 cells, suppressed PC expression. Co-transfection studies in 3T3-L1 preadipocytes or HEK293T cells with a 2.3-kb promoter fragment of mouse PC gene linked to a luciferase reporter construct and with plasmids overexpressing retinoid X receptor alpha/PPARgamma1 or retinoid X receptor alpha/PPARgamma2 showed a 6-8-fold increase above the basal promoter activity. Furthermore, treatment of these transfected cells with the PPARgamma agonist doubled the promoter activity. Mutation of the putative PPAR-response element-(-386/-374) of this 2.3-kb PC promoter fragment abolished the PPARgamma response. Gel shift and chromatin immunoprecipitation assays demonstrated that endogenous PPARgamma binds to this functional PPAR-response element of the PC promoter. Mice with targeted disruption of the PPARgamma2 gene displayed approximately 50-60% reduction of PC mRNA and protein in white adipose tissue. Similarly, in brown adipose tissue of PPARgamma2-deficient mice subjected to cold exposure, PC mRNA was 40% lower than that of wild type mice. Impaired in vitro differentiation of white adipocytes of PPARgamma2 knock-out mice was also associated with a marked reduction of PC mRNA. Our findings identified PC as a PPARgamma-regulated gene and suggested a role for PPARgamma regulating intermediary metabolism.

Characterization of the human, mouse and rat PGC1 beta (peroxisome-proliferator-activated receptor-gamma co-activator 1 beta) gene in vitro and in vivo.

PGC1 alpha is a co-activator involved in adaptive thermogenesis, fatty-acid oxidation and gluconeogenesis. We describe the identification of several isoforms of a new human PGC1 alpha homologue, cloned independently and named PGC1 beta. The human PGC1 beta gene is localized to chromosome 5, has 13 exons and spans more than 78 kb. Two different 5' and 3' ends due to differential splicing were identified by rapid amplification of cDNA ends PCR and screening of human cDNA libraries. We show that PGC1 beta variants in humans, mice and rats are expressed predominantly in heart, brown adipose tissue, brain and skeletal muscle. PGC1 beta expression, unlike PGC1 alpha, is not up-regulated in brown adipose tissue in response to cold or obesity. Fasting experiments showed that PGC1 alpha, but not PGC1 beta, is induced in liver and this suggests that only PGC1 alpha is involved in the hepatic gluconeogenesis. No changes in PGC1 beta gene expression were observed associated with exercise. Human PGC1 beta-1a and -2a isoforms localized to the cell nucleus and, specifically, the isoform PGC1 beta-1a co-activated peroxisome-proliferator-activated receptor-gamma, -alpha and the thyroid hormone receptor beta1. Finally, we show that ectopic expression PGC1 beta leads to increased mitochondrial number and basal oxygen consumption. These results suggest that PGC1 beta may play a role in constitutive adrenergic-independent mitochondrial biogenesis.

A signalling role for 4-hydroxy-2-nonenal in regulation of mitochondrial uncoupling.

Oxidative stress and mitochondrial dysfunction are associated with disease and aging. Oxidative stress results from overproduction of reactive oxygen species (ROS), often leading to peroxidation of membrane phospholipids and production of reactive aldehydes, particularly 4-hydroxy-2-nonenal. Mild uncoupling of oxidative phosphorylation protects by decreasing mitochondrial ROS production. We find that hydroxynonenal and structurally related compounds (such as trans-retinoic acid, trans-retinal and other 2-alkenals) specifically induce uncoupling of mitochondria through the uncoupling proteins UCP1, UCP2 and UCP3 and the adenine nucleotide translocase (ANT). Hydroxynonenal-induced uncoupling was inhibited by potent inhibitors of ANT (carboxyatractylate and bongkrekate) and UCP (GDP). The GDP-sensitive proton conductance induced by hydroxynonenal correlated with tissue expression of UCPs, appeared in yeast mitochondria expressing UCP1 and was absent in skeletal muscle mitochondria from UCP3 knockout mice. The carboxyatractylate-sensitive hydroxynonenal stimulation correlated with ANT content in mitochondria from Drosophila melanogaster expressing different amounts of ANT. Our findings indicate that hydroxynonenal is not merely toxic, but may be a biological signal to induce uncoupling through UCPs and ANT and thus decrease mitochondrial ROS production.