The adipose organ of obesity-prone C57BL/6J mice is composed of mixed white and brown adipocytes

White and brown adipocytes are believed to occupy different sites in the body. We studied the anatomical features and quantitative histology of the fat depots in obesity and type 2 diabetes-prone C57BL/6J mice acclimated to warm or cold temperatures. Most of the fat tissue was contained in depots with discrete anatomical features, and most depots contained both white and brown adipocytes. Quantitative analysis showed that cold acclimation induced an increase in brown adipocytes and an almost equal reduction in white adipocytes; however, there were no significant differences in total adipocyte count or any signs of apoptosis or mitosis, in line with the hypothesis of the direct transformation of white into brown adipocytes. The brown adipocyte increase was accompanied by enhanced density of noradrenergic parenchymal nerve fibers, with a significant correlation between the density of these fibers and the number of brown adipocytes. Comparison with data from obesity-resistant Sv129 mice disclosed a significantly different brown adipocyte content in C57BL/6J mice, suggesting that this feature could underpin the propensity of the latter strain to develop obesity. However, the greater C57BL/6J browning capacity can hopefully be harnessed to curb obesity and type 2 diabetes in patients with constitutively low amounts of brown adipose tissue.

Kinetic analyses guide the therapeutic decision in a novel form of moderate aromatic Acid decarboxylase deficiency

BACKGROUND

Aromatic amino acid decarboxylase (AADC) deficiency is a rare autosomal recessive disorder resulting in a combined dopamine and serotonin deficiency. About 50% of the cases set in the neonatal period. Here, we report an atypical clinical presentation with moderate symptoms.

PATIENT

At 10months old, the patient presented paroxysmal eye movements without seizures, and feeding difficulties which were attributed to gastroesophageal reflux. She was investigated at the age of 7years, because of orofacial dyspraxia, hypomimie, axial hypotonia and focal segmental dystonia, bilateral ptosis, without evidence for cognitive impairment.

RESULTS

HVA [110nM; (reference value (rv): 202-596)] and HIAA (12nM; rv: 87-366) decreased, OMD (520nM; rv: 5-60) and 5-HTP (56nM; rv: 2-16) increased in CSF. We confirmed the diagnosis of AADC deficiency because the activity in plasma was low: 4pmol/min/ml; rv: 16-137. The kinetic analysis revealed a sixfold increase in the apparent affinity for L-dopa (4.26mM; control=0.71), but the V (max) was unchanged (37.5pmol dopamine/min/ml; control=39.1), suggesting a modification in the substrate binding-site. Molecular analysis revealed two heterozygous mutations in the DDC gene: c1040G > A; pR347Q already described, and a novel mutation c478C > T, pR160W.

CONCLUSION

(1) CSF neurotransmitters metabolites suggested a moderate AADC deficiency; (2) The initial velocity saturation curve for L-dopa displayed a cooperative ligand binding behavior, in keeping with the modifications of the three-dimensional structure, induced by the amino acid substitutions (3) The treatment combination of L-dopa with pyridoxine dramatically improved the quality of life, the fatigability, and the paroxysmal eye movements.

Measurement of cystine in granulocytes using liquid chromatography-tandem mass spectrometry

BACKGROUND

Cystinosis is a rare autosomal recessive disorder characterized by an accumulation of intralysosomal cystine due to a defect in cystine transport across the lysosomal membrane. This disorder can be treated specifically using high doses of cysteamine. Accurate measurement of intracellular cystine content is necessary for the diagnosis and monitoring of treatment with cysteamine. Here we describe a new method to measure intracellular cystine. It relies on a liquid chromatography-tandem mass spectrometry assay. We compare this novel method with the cystine-binding protein assay.

METHOD

Cells were isolated and lysed in the presence of N-ethylmaleimide to avoid interference from cysteine. After deproteinization, addition of stable isotope d6 cystine and butylation, cystine was measured using an API 3000 MSMS.

RESULTS

The cystine assay was linear to at least 50 micromol/L. Within-run and between-run coefficients of variation were 2.9% and 5.7% respectively.

CONCLUSION

It is possible to measure very low concentrations of intracellular cystine with liquid chromatography-tandem mass spectrometry. The results obtained with this novel method correlate very well with those obtained using the cystine-binding protein assay.

Respiration under control of uncoupling proteins: Clinical perspective

The term 'uncoupling protein' was originally used for the mitochondrial membrane protein UCP1, which is uniquely present in mitochondria of brown adipocytes, thermogenic cells that regulate body temperature in small rodents, hibernators and mammalian newborns. In these cells, UCP1 acts as a proton carrier activated by free fatty acids and creates a shunt between complexes of the respiratory chain and ATP-synthase resulting in a futile proton cycling and dissipation of oxidation energy as heat. Recent identification of new homologues to UCP1 expressed in brown and white adipose tissue, muscle, brain and other tissues together with the hypothesis that these novel uncoupling proteins (UCPs) may regulate thermogenesis and/or fatty acid metabolism and furthermore may protect against free radical oxygen species production have generated considerable optimism for rapid advances in the identification of new targets for pharmacological management of complex pathological syndromes such as obesity, type 2 diabetes or chronic inflammatory diseases. However, since the physiological and biochemical roles of the novel UCPs are not yet clear, the main challenge today consists first of all in providing mechanistic explanation for their functions in cellular physiology. This lively awaited information may be the basis for potential pharmacological targeting of the UCPs in future.

Protective role of uncoupling protein 2 in atherosclerosis

BACKGROUND

Uncoupling protein 2 (UCP2) regulates the production of reactive oxygen species in macrophages. However, its role in atherosclerosis is unknown.

METHODS AND RESULTS

Irradiated low-density lipoprotein receptor deficient mice (LDLR-/-) were transplanted with bone marrow from either UCP2 deficient mice (Ucp2-/-) or wild type mice (Ucp2+/+). Mice were fed an atherogenic diet for 7 weeks. Engraftment of bone marrow cells was confirmed by the presence of UCP2 protein expression in spleen cell mitochondria of Ucp2+/+ transplanted mice and its absence in Ucp2-/- transplanted mice. Leukocyte counts and plasma cholesterol levels were comparable in both groups. We found a marked increase in atherosclerotic lesion size in the thoracic aorta of Ucp2-/- transplanted mice compared with control Ucp2+/+ transplanted mice (8.3+/-0.9% versus 4.3+/-0.4%, respectively; P<0.005), as well as in the aortic sinus (150 066+/-12 388 microm2 versus 105 689+/-9 727 microm2, respectively; P<0.05). This was associated with increased nitrotyrosine staining, which suggests enhanced oxidative stress. Analysis of plaque composition revealed a significant increase in macrophage accumulation (P<0.05) and apoptosis (P<0.05), along with a decrease in collagen content (P<0.05), suggesting a potentially more vulnerable phenotype.

CONCLUSION

These results suggest a protective role for UCP2 against atherosclerosis.

[To burn or to store]

Energy exists as organic molecules and heat in living organisms. In adult mammals, body weight and fat content remain unchanged if energy intake is strictly equivalent to energy expenditure. In other words, regulation of body weight requires energy of foods to be entirely dissipated as heat. Imbalance between ingested energy and thermogenesis induces obesity or thinness. Alterations of food intake or energy expenditure represent the two causes of body weight disturbance. It is accepted that individuals differ in food efficiency i.e. ability to metabolize foods and store fat or totally burn nutrients. Mechanisms of food efficiency and futile cycles are presented. I started my research work analysing thermogenic mechanism in brown adipose tissue. Actually, in addition to white adipose tissue which is the major type of adipose tissue, mammals own another type of adipose tissue referred to as brown adipose tissue. This later tissue is an activatable thermogenic organ which oxidizes fatty acids and releases heat in blood stream. Brown fat is activated during exposure to the cold (in rodents), at birth, and during arousal in hibernators. My initial work helped to characterize a mitochondrial protein named uncoupling protein or UCP which is responsible for activation of fatty acid oxidation and heat production in brown adipocytes. Actually, in most cells, fifty per cent of oxidation energy is recovered as ATP in mitochondria through the process of coupling of respiration to ADP phosphorylation. In contrast to mitochondria of most tissues, brown adipocyte mitochondria can escape the obligatorily coupling of respiration and waste almost ninety per cent of respiration energy as thermogenesis. UCP characterization and its molecular cloning as well as antibodies obtention were used to better understand cellular thermogenesis. Brown adipocytes were identified in babies and adult patients with pheochromocytoma. More recently, research on the brown fat UCP helped us to identify UCP2, a UCP homolog present in most human and animal tissues. A family of UCPs exist in animals and plants. These UCPs may function as mitochondrial uncouplers. However, the ancient function of the UCPs may be rather associated to adaptation to oxygen and control of free radicals than to thermogenesis. Further studies of UCPs will improve the knowledge of mitochondrial metabolism and substrate oxidation. In other respects, analysis of molecular mechanisms controlling respiration uncoupling may contribute to new strategies of treatment of metabolic disorders such as obesity.

Changes in FAT/CD36, UCP2, UCP3 and GLUT4 gene expression during lipid infusion in rat skeletal and heart muscle

OBJECTIVE

It has been reported that an increased availability of free fatty acids (NEFA) not only interferes with glucose utilization in insulin-dependent tissues, but may also result in an uncoupling effect of heart metabolism. We aimed therefore to investigate the effect of an increased availability of NEFA on gene expression of proteins involved in transmembrane fatty acid (FAT/CD36) and glucose (GLUT4) transport and of the uncoupling proteins UCP2 and 3 at the heart and skeletal muscle level.

STUDY DESIGN

Euglycemic hyperinsulinemic clamp was performed after 24 h Intralipid(R) plus heparin or saline infusion in lean Zucker rats. Skeletal and heart muscle glucose utilization was calculated by 2-deoxy-[1-(3)H]-D-glucose technique. Quantification of FAT/CD36, GLUT4, UCP2 and UCP3 mRNAs was obtained by Northern blot analysis or RT-PCR.

RESULTS

In Intralipid(R) plus heparin infused animals a significant decrease in insulin-mediated glucose uptake was observed both in the heart (22.62+/-2.04 vs 10.37+/-2.33 ng/mg/min; P<0.01) and in soleus muscle (13.46+/-1.53 vs 6.84+/-2.58 ng/mg/min; P<0.05). FAT/CD36 mRNA was significantly increased in skeletal muscle tissue (+117.4+/-16.3%, P<0.05), while no differences were found at the heart level in respect to saline infused rats. A clear decrease of GLUT4 mRNA was observed in both tissues. The 24 h infusion of fat emulsion resulted in a clear enhancement of UCP2 and UCP3 mRNA levels in the heart (99.5+/-15.3 and 80+/-4%) and in the skeletal muscle (291.5+/-24.7 and 146.9+/-12.7%).

CONCLUSIONS

As a result of the increased availability of NEFA, FAT/CD36 gene expression increases in skeletal muscle, but not at the heart level. The augmented lipid fuel supply is responsible for the depression of insulin-mediated glucose transport and for the increase of UCP2 and 3 gene expression in both skeletal and heart muscle.

A new polymorphic site located in the human UCP1 gene controls the in vitro binding of CREB-like factor

Uncoupling protein 1 (UCP1) is uniquely expressed in brown adipose tissue (BAT) and generates heat by uncoupling respiration from ATP synthesis. A defect in BAT thermogenesis has been described in different models of rodent obesity. In humans, the implication of BAT in energy expenditure is still under discussion. A BclI polymorphism associated with fat gain over time has been described in the upstream region of the human UCP1 (hUCP1) gene. In this study, a new polymorphic site linked to the BclI site is described which results in a C to A point mutation, 89 bp downstream of the BclI site, ie at position -3737 bp. This site is located in the recently analysed regulatory region of the hUCP1 gene. The mutation disrupts a consensus site for the binding of ATF/CREB transcription factor family and inhibits the factor binding in vitro. However, transient transfection of a rodent brown adipocyte cell line shows that the isoproterenol (ISO) stimulation of the hUCP1 gene transcription is not significantly affected by this mutation. However, we postulate that the C/A substitution, in human, may contribute to a minor defect in the regulation of hUCP1 transcription and that would explain fat gain over time.

Uncoupling protein 2 in the brain: distribution and function

Uncoupling protein 2 (UCP2) mRNA is expressed in a panoply of tissues, including the brain, where it is widely distributed. In the mouse brain, it is expressed in the hypothalamus (suprachiasmatic, paraventricular, dorsomedial, ventromedial and arcuate nuclei), the thalamus (submedius nucleus) and the brain-stem (dorsal motor nucleus of the vagus nerve). In the rat brain, it is also expressed in the hippocampus. The presence of UCP2 mRNA in neurons expressing corticotropin-releasing factor and arginine-vasopressin suggests a role for UCP2 in the control of neuroendocrine and behavioural functions. We have recently demonstrated that UCP2-deficient mice can resist the lethal effect of toxoplasmosis through an enhanced production of reactive oxygen species (ROS) from the macrophages. This finding provides evidence that UCP2 can be part of a mechanism preventing ROS production. UCP2 could therefore be involved in protecting the brain against oxidative stress. The involvement of UCP2 in neuroprotection is also consistent with the recent observation that kainic acid, which promotes Ca(2+) uptake in the glutamate-activated neurons in the hippocampal CA1 field, can induce the UCP2 gene in the activated CA1 cells. The role of UCP2 in neuroprotection warrants further investigation.

Rapid increase of mitochondrial uncoupling protein and its mRNA in stimulated brown adipose tissue

The increase in mitochondrial uncoupling protein in brown adipose tissue during acute stimulation by exposure of animals to cold was examined. Uncoupling protein level increased during the first hours of tissue stimulation. Use of a cDNA probe shows that synthesis of uncoupling protein mRNA was quickly stimulated. Animals treated with propranolol exhibited neither increase in uncoupling protein mRNA nor increase in the protein itself.

Genetic and physiological analysis of the role of uncoupling proteins in human energy homeostasis

The metabolic utilization of substrates results in ATP synthesis and energy loss as heat. In tissues and cells the mitochondria reoxidize reduced coenzymes and phosphorylate ADP. A significant proportion of the energy is released through thermogenesis by mitochondria. This is due to a less than perfect coupling of cellular respiration to ATP synthesis. Previous studies of brown adipocytes, which are cells specialized in regulatory thermogenesis, have shown that heat production is due to the regulated activity and synthesis of a particular proton transporter in the inner membrane of brown adipocyte mitochondria--uncoupling protein (UCP) 1. UCP homologues have recently been identified. UCP2 is widely expressed in human tissues, whereas UCP3 is expressed predominantly in human skeletal muscles. These novel UCPs represent genes which are potentially important for regulation of metabolic pathways and energy expenditure in humans. Biochemical and genetic studies support a role for these novel UCPs in metabolic regulations in humans. However, several physiological studies question such a role. Importantly, UCP2 and UCP3 seem to be able to control the activity of mitochondria in response to oxidants.

Transcriptional regulation of the mouse uncoupling protein-2 gene. Double E-box motif is required for peroxisome proliferator-activated receptor-gamma-dependent activation

Uncoupling protein-2 (UCP2) is present in many tissues with relevance to fuel metabolism, and its expression is increased in fat and muscle in response to elevated circulating free fatty acids resulting from fasting and high fat feeding. We proposed a role for peroxisome proliferator-activated receptor-gamma (PPARgamma) as a mediator of these physiological changes in UCP2, because thiazolidinediones also increase expression of UCP2 in these cell types (). To determine the molecular basis for this regulation, we isolated the 7.3-kilobase promoter region of the mouse UCP2 gene. The -7.3-kilobase/+12-base pair fragment activates transcription of a reporter gene by 50-100-fold. Deletion and point mutation analysis, coupled with gel shift assays, indicate the presence of a 43-base pair enhancer (-86/-44) that is responsible for the majority of both basal and PPARgamma-dependent transcriptional activity. The distal (-86/-76) part of the enhancer specifically binds Sp1, Sp2, and Sp3 and is indistinguishable from a consensus Sp1 element in competition experiments. Point mutation in this sequence reduces basal activity by 75%. A second region (-74/-66) is identical to the sterol response element consensus and specifically binds ADD1/SREBP1. However, deletion of this sequence does not affect basal transcriptional activity or the response to PPARgamma. The proximal portion of the enhancer contains a direct repeat of two E-Box motifs, which contributes most strongly to basal and PPARgamma-dependent transcription of the UCP2 promoter. Deletion of this region results in a 10-20-fold reduction of transcriptional activity and complete loss of PPARgamma responsiveness. Point mutations in either E-Box, but not in the spacer region between them, eliminate the stimulatory response to PPARgamma. However, gel shift assays show that PPARgamma does not bind to this region. Taken together, these data indicate that PPARgamma activates the UCP2 gene indirectly by altering the activity or expression of other transcription factors that bind to the UCP2 promoter.

Mitochondrial respiratory chain deficiency revealed by hypothermia

We observed a 17-month-old girl with profound and initially isolated episodes of hypothermia. Thereafter, she developed growth delay, repetitive corneal and bone lesions. Persistent hyperlactataemia in plasma and in CSF prompted us to investigate respiratory chain enzymes. A deficit in respiratory chain complexes III and IV was demonstrated in isolated skeletal muscle mitochondria, circulating lymphocytes and fibroblasts by spectrophotometric and polarographic studies. Moreover, UCP3 mRNA expression in muscle was decreased.

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.

An uncoupling protein homologue putatively involved in facultative muscle thermogenesis in birds

The cDNA of an uncoupling protein (UCP) homologue was obtained by screening a chicken skeletal-muscle library. The predicted 307-amino-acid sequence of avian UCP (avUCP) is 55, 70, 70 and 46% identical with mammalian UCP1, UCP2 and UCP3 and plant UCP respectively. avUCP mRNA expression is restricted to skeletal muscle and its abundance was increased 1.3-fold in a chicken line showing diet-induced thermogenesis, and 3.6- and 2.6-fold in cold-acclimated and glucagon-treated ducklings developing muscle non-shivering thermogenesis respectively. The present data support the implication of avUCP in avian energy expenditure.

Disruption of the uncoupling protein-2 gene in mice reveals a role in immunity and reactive oxygen species production

The gene Ucp2 is a member of a family of genes found in animals and plants, encoding a protein homologous to the brown fat uncoupling protein Ucp1 (refs 1-3). As Ucp2 is widely expressed in mammalian tissues, uncouples respiration and resides within a region of genetic linkage to obesity, a role in energy dissipation has been proposed. We demonstrate here, however, that mice lacking Ucp2 following targeted gene disruption are not obese and have a normal response to cold exposure or high-fat diet. Expression of Ucp2 is robust in spleen, lung and isolated macrophages, suggesting a role for Ucp2 in immunity or inflammatory responsiveness. We investigated the response to infection with Toxoplasma gondii in Ucp2-/- mice, and found that they are completely resistant to infection, in contrast with the lethality observed in wild-type littermates. Parasitic cysts and inflammation sites in brain were significantly reduced in Ucp2-/- mice (63% decrease, P<0.04). Macrophages from Ucp2-/- mice generated more reactive oxygen species than wild-type mice (80% increase, P<0.001) in response to T. gondii, and had a fivefold greater toxoplasmacidal activity in vitro compared with wild-type mice (P<0.001 ), which was absent in the presence of a quencher of reactive oxygen species (ROS). Our results indicate a role for Ucp2 in the limitation of ROS and macrophage-mediated immunity.

Mitochondrial uncoupling proteins: from mitochondria to the regulation of energy balance

The coupling of oxygen consumption to ADP phosphorylation is incomplete, as is particularly evident in brown adipocyte mitochondria which use a regulated uncoupling mechanism to dissipate heat produced by substrate oxidation. In brown adipose tissue, uncoupling is effected by a specific protein in the inner mitochondrial membrane referred to as uncoupling protein-1 (UCP1). UCP1 gene disruption in mice has confirmed UCP1's role in cold-induced thermogenesis. Genetic analysis of human cohorts has suggested that UCP1 plays a minor role in the control of fat content and body weight. The recent cloning of UCP2 and UCP3, two homologues of UCP1, has boosted research on the importance of respiration control in metabolic processes, metabolic diseases and energy balance. UCP2 is widely expressed in different organs whereas UCP3 is mainly present in skeletal muscle. The chromosomal localization of UCP2 as well as UCP2 mRNA induction by a lipid-rich diet in obesity-resistant mice suggested that UCP2 is involved in diet-induced thermogenesis. A strong linkage between markers in the vicinity of human UCP2 and UCP3 (which are adjacent genes) and resting metabolic rate was calculated. UCPs are known or supposed to participate in basal and regulatory thermogenesis, but their exact biochemical and physiological functions have yet to be elucidated. UCPs may constitute novel targets in the development of drugs designed to modulate substrate oxidation. However, very recent data suggest an important role for the UCPs in the control of production of free radicals by mitochondria, and in response to oxidants.

Transcriptional activation of the human ucp1 gene in a rodent cell line. Synergism of retinoids, isoproterenol, and thiazolidinedione is mediated by a multipartite response element

Uncoupling protein 1 (UCP1) is uniquely expressed in brown adipocytes and generates heat production by uncoupling respiration from ATP synthesis. The activatory effects of norepinephrine and retinoic acid (RA) on rodent ucp1 gene transcription have been well characterized. These effects are mediated by a 211-base pair (bp) enhancer which is also sufficient to restrict expression to brown adipose tissue. The molecular mechanisms controlling the transcription of the human ucp1 gene are unknown. In order to study the transcriptional regulation of the human gene, we set up chloramphenicol acetyltransferase constructs containing the entire or deleted 5' regions upstream of the transcriptional start site of the gene. These constructs were transiently transfected in a mouse cell line. A 350-bp hormone response region showing a significant homology with the rat ucp1 enhancer and located between the BclI polymorphic site and an AatII site (bp -3820/-3470) was detected. This region was sufficient to mediate the stimulation by RA and by combined treatments (RA + isoproterenol (ISO), RA + thiazolidinedione (TZD), or RA + ISO + TZD). The highest stimulation, a 26-fold increase in basal activity, was obtained by RA + ISO + TZD treatment. In contrast to the rodent gene, under our conditions, the effect of ISO and/or TZD is dependent on RA stimulation. Analysis of 105 bp inside the 350-bp element by site-directed mutagenesis and gel retardation experiments demonstrated that a multipartite response element mediates the drug stimulation. This region binds RARs and RXRs nuclear factors, CREB/ATF factors, and also PPARgamma despite the absence of a consensus peroxisome-proliferator response element. The activation of the human ucp1 gene transcription by certain hormones or drugs, and the identification of the cis-elements involved, will help to identify new compounds activating fat oxidation and energy expenditure in humans.

The human uncoupling protein-1 gene (UCP1): present status and perspectives in obesity research

Energy expenditure through brown adipose tissue thermogenesis contributes either to maintenance of body temperature in a cold environment or to wasted food energy, i.e. cold-induced or diet-induced thermogenesis. Both mechanisms are due to a specific and unique protein: the uncoupling protein-1. Uncoupling protein-1 is exclusively expressed in mitochondria of brown adipocytes where it uncouples respiration from ATP synthesis, dissipating the proton gradient as heat. In humans, although uncoupling protein-1 can be detected, the inability to quantify brown adipose tissue makes it difficult to argue for a role for uncoupling protein-1 in thermogenesis and energy expenditure. This review summarizes data supporting the existence of brown adipocytes and the role of UCP1 in energy dissipation in adult humans. Understanding the mechanisms which regulate transcription and expression of the human UCP1 gene will facilitate the identification of molecules able to increase the levels of this protein in order to modulate energy expenditure in adult humans.

Effect of high-fat diet, surrounding temperature, and enterostatin on uncoupling protein gene expression

Nonshivering thermogenesis induced in brown adipose tissue (BAT) during high-fat feeding is mediated through uncoupling protein 1 (UCP1). UCP2 is a recently identified homologue found in many tissues. To determine the role of UCP1 and UCP2 in thermoregulation and energy balance, we investigated the long-term effect of high-fat feeding on mRNA levels in mice at two different ambient temperatures. We also treated mice with the anorectic peptide enterostatin and compared mRNA levels in BAT, white adipose tissue (WAT), stomach, and duodenum. Here, we report that high-fat feeding at 23 degrees C increased UCP1 and UCP2 levels in BAT four- and threefold, respectively, and increased UCP2 levels fourfold in WAT. However, at 29 degrees C, UCP1 decreased, whereas UCP2 remained unchanged in BAT and increased twofold in WAT. Enterostatin increased UCP1 and decreased UCP2 mRNA in BAT. In stomach and duodenum, high-fat feeding decreased UCP2 mRNA, whereas enterostatin increased it. Our results suggest that the regulation of uncoupling protein mRNA levels by high-fat feeding is dependent on ambient temperature and that enterostatin is able to modulate it.

Endocrine regulation of uncoupling proteins and energy expenditure

Regulatory thermogenesis occurs upon exposure to the cold or during food intake. Among a variety of mechanisms leading to heat production, uncoupling of respiration in brown adipocyte mitochondria appears to be a major contributor to resistance to the cold in rodents. This uncoupling mechanism is due to the activity of uncoupling protein-1 (UCP-1), a specific carrier present in the inner membrane of mitochondria. The recent identification of UCP-2 and UCP-3, two homologues of the brown fat UCP, suggested that respiration uncoupling could contribute to thermogenesis in most tissues. Activity and expression of the three UCP's are stimulated by several neuromediators and hormones such as noradrenaline, tri-iodothyronine and leptin.

The uncoupling protein homologues: UCP1, UCP2, UCP3, StUCP and AtUCP

Animal and plant uncoupling protein (UCP) homologues form a subfamily of mitochondrial carriers that are evolutionarily related and possibly derived from a proton/anion transporter ancestor. The brown adipose tissue (BAT) UCP1 has a marked and strongly regulated uncoupling activity, essential to the maintenance of body temperature in small mammals. UCP homologues identified in plants are induced in a cold environment and may be involved in resistance to chilling. The biochemical activities and biological functions of the recently identified mammalian UCP2 and UCP3 are not well known. However, recent data support a role for these UCPs in State 4 respiration, respiration uncoupling and proton leaks in mitochondria. Moreover, genetic studies suggest that UCP2 and UCP3 play a part in energy expenditure in humans. The UCPs may also be involved in adaptation of cellular metabolism to an excessive supply of substrates in order to regulate the ATP level, the NAD(+)/NADH ratio and various metabolic pathways, and to contain superoxide production. A major goal will be the analysis of mice that either lack the UCP2 or UCP3 gene or overexpress these genes. Other aims will be to investigate the possible roles of UCP2 and UCP3 in response to oxidative stress, lipid peroxidation, inflammatory processes, fever and regulation of temperature in certain specific parts of the body.

[Genetic and molecular aspects of obesity: recent data]

Obesity is a disease responsible for many serious complications. The sharp rise in the prevalence of obesity in many countries is supplying a powerful drive to basic and clinical research. Several genes responsible for monogenic murine obesity have recently been identified. One of these genes encodes the OB protein, or leptine, which is secreted by fat tissue and inhibits appetite by means of an effect on the hypothalamus. In humans, obese subjects carrying a mutation of this gene or of the leptine receptor have been identified. Several other genes implicated in human obesity have been mapped to chromosomes 1, 11, 18, and 20. Several transcription factors that control fat cell differentiation have been identified, such as C/ERB alpha, beta, and delta; ADD1/SREBP1, and PPAR gamma 2. It has been established that fat tissue can secrete many factors, including TNF alpha, CETP, IGF beta, TGF beta, PGE2, and LPA. Mitochondrial uncoupling proteins (UCPs) are recently characterized proteins capable of uncoupling respiration and contributing to energy expenditures. The hypothalamic neuropeptides and their receptors are a focus of active research. About ten of these neuropeptides have been identified.

Retinoids activate proton transport by the uncoupling proteins UCP1 and UCP2

In mammalian brown adipose tissue, thermogenesis is explained by uncoupling mitochondrial respiration from ATP synthesis. Uncoupling protein-1 (UCP1) is responsible for this uncoupled state, because it allows proton re-entry into the matrix and thus dissipates the proton gradient generated by the respiratory chain. Proton transport by UCP1 is regulated negatively by nucleotides and positively by fatty acids. Adrenergic stimulation of brown adipocytes stimulates lipolysis and therefore enhances uncoupling and thermogenesis. Adrenergic stimulation also boosts ucp1 gene transcription. Since retinoic acid also promotes ucp1 gene transcription and its structure makes it a possible activator of UCP1, we hypothesized that retinoic acid, like noradrenaline, could have a dual action and trigger the activity of the protein UCP1 itself. Here we show that retinoic acid strongly increases proton transport by UCP1 in brown adipose tissue mitochondria and that it is much more potent than fatty acids. These data are corroborated with yeast mitochondria where UCP1 was introduced by genetic manipulation. The yeast expression system allows the comparison of the UCP1 with the newly described homologues UCP2 and UCP3. The search for regulators of UCP2 has demonstrated that it is positively regulated by retinoids in a pH-dependent manner.

Contribution to the identification and analysis of the mitochondrial uncoupling proteins

This review is primarily focused on the contribution of our laboratory to study of the mitochondrial uncoupling UCPs. The initial stage was the description of a 32-kDa membranous protein specifically induced in brown adipose tissue mitochondria of cold-adapted rats. This protein was then shown by others to be responsible for brown fat thermogenesis and was referred to as the uncoupling protein-UCP (recently renamed UCP1). cDNA and genomic clones of UCP1 were isolated and used to investigate the topology and functional organization of the protein in the membrane and the mechanisms of control of UCP1 gene transcription. Orientation of the transmembrane fragments was proposed and specific amino acid residues involved in the inhibition of UCP1 by purine nucleotides were identified in recombinant yeast. A potent enhancer mediating the response of the UCP1 gene to retinoids and controlling the specific transcription in brown adipocytes was identified using transgenic mice. More recently, we identified UCP2, an UCP homolog widely expressed in human and rodent tissues we also collaborated to characterize the plant UCP. Although the biochemical activities and physiological roles of the novel UCPs are not well understood, these recent data stimulate research on mitochondrial carriers, mitochondrial bioenergetics, and energy expenditure.

Mitochondrial uncoupling proteins

The mechanisms explaining the imperfect coupling of respiration to ADP phosphorylation in mitochondria are not well understood. In the case of a thermogenic organ such as brown adipose tissue, heat production results from a regulated uncoupling of respiration due to a specific uncoupler present in the inner mitochondrial uncoupling protein, referred to as uncoupling protein (UCP)-1. UCP1 functions as a proton translocator regulated by fatty acids. Two UCP homologues were identified very recently; UCP2 is expressed in most organs, whereas UCP3 expression is restricted to skeletal muscle and brown adipose tissue. Experimental data support the respiration uncoupling activity of UCP2 and UCP3. Physiological and genetic data are in agreement with thermogenic activity of the two proteins, although other physiological data favor a role for UCP2 and UCP3 in lipid handling rather than in energy expenditure. UCP2 and UCP3 may also be involved in inflammation, immune response and fever. Many hormones and certain pharmacological regulators affect expression level of UCP2 and/or UCP3 gene.

Uncoupling protein-2 (UCP2): molecular and genetic studies

Thermogenesis is associated to oxygen consumption and cellular respiration. This process is coupled to adenosine-diphosphate (ADP) phosphorylation through the existence of a proton gradient across the inner mitochondrial membrane. It was postulated that proton leaks through this membrane would uncouple respiration from adenosine-triphosphate (ATP) synthesis and induce energy dissipation as heat. Such a mechanism was identified in thermogenic brown adipose tissue mitochondria which contain a unique proton carrier referred to as uncoupling protein (UCP). This UCP is activated by fatty acids and its synthesis is positively controlled by retinoids, thyroid hormones, catecholamines and rexinoids. In fact, in most types of cells, respiring mitochondria release heat and the coupling of substrate oxidation to ADP phosphorylation is under 100%. It suggested that the partial coupling of respiration to ADP phosphorylation was due to proton leaks possibly related to the brown fat UCP. This approach led to the identification of UCP2 and UCP3, two homologues of the brown fat UCP (renamed UCP1). UCP2 gene is widely expressed in tissues and cell types, whereas the UCP3 gene is dominantly expressed in skeletal muscles (and brown fat in mice). Recent genetic, biochemical and physiological studies suggest that these novel UCP2 contribute to resting metabolic rate, fat oxidation and may represent new targets for anti-obesity compounds.

Contributions of studies on uncoupling proteins to research on metabolic diseases

The coupling of O2 consumption to ADP phosphorylation in mitochondria is partial. This is particularly obvious in brown adipocyte mitochondria which use a regulated uncoupling mechanism generating heat production from substrate oxidation, and catalysing thermogenesis in rodents or infants in response to cold, and arousing hibernators. In the case of brown adipose tissue, the uncoupling mechanism is related to a specific protein in the inner mitochondrial membrane referred to as UCP1. Although the biological importance of UCP1 in human adults is not demonstrated, genetic analysis of various human cohorts suggested a participation of UCP1 to control of fat content and body weight. Very recently, the cloning of UCP2 and UCP3, two homologues of UCP1, has renewed the field of research on the importance of respiration control in metabolic processes and metabolic diseases. UCP2 is widely expressed in organs, whereas UCP3 is mainly present in muscles. These proteins may explain why the coupling of respiration to ADP phosphorylation is less than perfect. Their biological importance should be studied. They also represent new putative targets for drugs against metabolic diseases such as obesity.

An uncoupling protein 2 gene variant is associated with a raised body mass index but not Type II diabetes

AIMS/HYPOTHESIS

Linkage between markers close to the uncoupling protein 2 and 3 genes (11q13) and resting metabolic rate and a pre-diabetic phenotype have been found. The syntenic region in mouse has been found to be linked to quantitative traits associated with obesity and diabetes. UCP2 and UCP3 could therefore have an important role in body weight regulation and susceptibility to diabetes. We investigated a recently identified variant of the UCP2 gene in exon 8 as a marker for glucose and weight homeostasis.

METHODS

Length variation of the UCP2 exon 8 variant was studied by the polymerase chain reaction and agarose gel electrophoresis. Sequence variants of the UCP3 gene were sought by semi-automated DNA sequencing.

RESULTS

In 453 South Indian subjects, we found an association in women between the UCP2 exon variant and body mass index (p = 0.018). These findings were replicated in a separate group of South Indian subjects (n = 143, p < 0.001) irrespective of sex. Although no association was found between the UCP2 exon 8 variant and overt obesity in British subjects, the UCP2 genotype of obese women (n = 83) correlated with fasting serum leptin concentration (p = 0.006) in the presence of extreme obesity. These observations could not be explained by tight linkage disequilibrium with a coding region variant in the region of the UCP3 gene of biological significance. Lastly, no association was found between UCP2 and Type II (non-insulin-dependent) diabetes using either a family based design (85 families) or case control study (normal glucose tolerance n = 335, impaired glucose tolerance n = 42, Type II diabetes n = 76).

CONCLUSION/INTERPRETATION

We have described a UCP2 gene exon 8 variant that may affect susceptibility to weight gain by influencing regulation of leptin.

Brain distribution of UCP2 mRNA: in situ hybridization histochemistry studies

Uncoupling protein-2 (UCP2) is expressed in large amounts in several tissues. In the mouse brain, in situ hybridization studies have revealed an abundant expression of UCP2 mRNA in the ventral septal region, the hypothalamus, the hindbrain (medulla), the ventricular regions and the cerebellum. In the hypothalamus, a very highly intense hybridization signal is apparent in the suprachiasmatic nucleus, in the medial parvicellular and magnocellular lateral parts of the paraventricular hypothalamic nucleus, and in the arcuate nucleus. In the brainstem, UCP2 is found to be strongly expressed in the dorsal motor nucleus of the vagus nerve. The expression of UCP2 mRNA is also clearly noticeable in the choroid plexuses and in the cerebellum. The expression of UCP2 mRNA in specific regions of the brain as well as its presence in neurons with a known chemical identity suggest that UCP2 mRNA is expressed in neurons. It is as yet premature to conclude about a specific function of UCP2 in the brain. The brain distribution pattern of its transcript suggests that this mitochondrial protein could be part of neuronal circuitries involved in the control of neuroendocrine functions and autonomic responses. Assuming that the UCP2 mRNA encodes a functional uncoupling protein, it can be argued that UCP2 contributes to the metabolic rate and thermoregulation of the neuronal structures to which it is associated. In addition, by elevating oxygen consumption in the brain, UCP2 could in specific regions control the production of reactive oxygen species and thereby influence the process of neural degeneration.

Acute effects of exercise on circulating leptin in lean and genetically obese fa/fa rats

Mechanisms of regulation of plasma leptin in lean and genetically obese animals are not completely understood. In particular a relation has been proposed between energy metabolism and leptin. However, it is not clear how energy expenditure and leptin are related under exercise in lean and obese animals. To clarify these aspects we investigated lean and genetically obese (fa/fa) Zucker rats undergoing a single bout (30 min) of swimming and measured several biochemical and hormonal parameters of energy metabolism and leptin changes throughout the study. Moreover ob-gene expression in adipose tissue was also measured. Our results showed that plasma leptin is decreased by 30% at the end of exercise in lean animals while resulting unaffected in obese animals. Leptin changes in lean rats are concomitant with the peak of NEFA and glycerol release from adipose tissue rather than with the reduction of plasma insulin. Ob-gene expression in adipose tissue was markedly increased in fa/fa compared to lean rats, but was not modified by exercise both in lean and obese animals. In conclusion our data show that leptin changes during exercise are related to lipolytic events in adipose tissue and support a link between leptin and energy expenditure.

Functional organization of the human uncoupling protein-2 gene, and juxtaposition to the uncoupling protein-3 gene

Human and mouse UCP2 genes were cloned and sequenced. Transcriptional start sites were identified using primer extension analysis. The transcription unit of UCP2 gene is made of 2 untranslated exons followed by 6 exons encoding UCP2. In vitro translation analysis demonstrated that an open-reading-frame for a putative peptide of 36 residues present in exon 2 did not prevent UCP2 translation and confirmed that the initiation site of translation was in exon 3 as predicted from sequencing data. Short (bp -125 to +93) and long (bp -1383 and +93) CAT-constructs containing DNA upstream of the transcriptional start site of the human gene were made and transfected in adipocytes or HeLa cells allowing characterization of a potent promoter. Analysis of several genomic clones encompassing UCP2 and/or UCP3 genes demonstrated that the 2 genes are adjacent, the human UCP2 gene being located 7 kb downstream of the UCP3 gene.

Induction of obesity and hyperleptinemia by central glucocorticoid infusion in the rat

It has been claimed that factors favoring the development or maintenance of animal or human obesity may include increases in glucocorticoid production or hyperresponsiveness of the hypothalamic-pituitary-adrenal axis. In normal rats, glucocorticoids have been shown to be necessary for chronic intracerebroventricular infusion of neuropeptide Y to produce obesity and related abnormalities. Conversely, glucocorticoids inhibited the body weight-lowering effect of leptin. Such dual action of glucocorticoids may occur within the central nervous system, since both neuropeptide Y and leptin act within the hypothalamus. The aim of this study was to determine the effects of glucocorticoids (dexamethasone) given intracerebroventricularly to normal rats on body weight homeostasis and hypothalamic levels of neuropeptide Y and corticotropin-releasing hormone. Continuous central glucocorticoid infusion for 3 days resulted in marked sustained increases in food intake and body weight relative to saline-infused controls. The infusion abolished endogenous corticosterone output and produced hyperinsulinemia, hypertriglyceridemia, and hyperleptinemia, three salient abnormalities of obesity syndromes. Central glucocorticoid infusion also produced a marked decrease in the expression of uncoupling protein (UCP)-1 and UCP-3 in brown adipose tissue and UCP-3 in muscle. Finally, chronic central glucocorticoid administration increased the hypothalamic levels of neuropeptide Y and decreased those of corticotropin-releasing hormone. When the same dose of glucocorticoids was administered peripherally, it resulted in decreases in food intake and body weight, in keeping with the decrease in hypothalamic neuropeptide Y levels. These results suggest that glucocorticoids induce an obesity syndrome in rodents by acting centrally and not peripherally.

BMCP1, a novel mitochondrial carrier with high expression in the central nervous system of humans and rodents, and respiration uncoupling activity in recombinant yeast

We report here the cloning and functional analysis of a novel homologue of the mitochondrial carriers predominantly expressed in the central nervous system and referred to as BMCP1 (brain mitochondrial carrier protein-1). The predicted amino acid sequence of this novel mitochondrial carrier indicates a level of identity of 39, 31, or 30%, toward the mitochondrial oxoglutarate carrier, phosphate carrier, or adenine nucleotide translocator, respectively, and a level of identity of 34, 38, or 39% with the mitochondrial uncoupling proteins UCP1, UCP2, or UCP3, respectively. Northern analysis of mouse, rat, or human tissues demonstrated that mRNA of this novel gene is mainly expressed in brain, although it is 10-30-fold less expressed in other tissues. In situ hybridization analysis of brain showed it is particularly abundant in cortex, hippocampus, thalamus, amygdala, and hypothalamus. Chromosomal mapping indicates that BMCP1 is located on chromosome X of mice and at Xq24 in man. Expression of the protein in yeast strongly impaired growth rate. Analysis of respiration of total recombinant yeast or yeast spheroplasts and in particular of the relationship between respiratory rate and membrane potential of yeast spheroplasts revealed a marked uncoupling activity of respiration, suggesting that although BMCP1 sequence is more distant from the uncoupling proteins (UCPs), this protein could be a fourth member of the UCP family.

Increased uncoupling protein-2 and -3 gene expressions in skeletal muscle of STZ-induced diabetic rats

Streptozotocin (STZ)-induced diabetic animals are vulnerable to cold stress. Uncoupling proteins (UCPs) play an important role in regulating thermogenesis. We investigated the gene expressions of UCPs in brown adipose tissue (BAT), white adipose tissue (WAT), liver and gastrocnemius muscle of STZ-diabetic rats using Northern blot. UCP-1, -2 and -3 mRNA expressions in BAT were all remarkably lower in STZ-diabetic rats than those in control rats. Both UCP-2 and -3 gene expressions in gastrocnemius muscle were substantially elevated in STZ-diabetic rats and insulin treatment restored UCP gene expressions to normal levels. These results suggest that in STZ-diabetic rats, the overexpression of UCP-2 and UCP-3 in skeletal muscle provides a defense against hypothermogenesis caused by decreased UCPs in BAT.

In the rat, tumor necrosis factor alpha administration results in an increase in both UCP2 and UCP3 mRNAs in skeletal muscle: a possible mechanism for cytokine-induced thermogenesis?

Since the discovery of the new members of the UCP (uncoupling protein) family, UCP2 and UCP3, very few studies have dealt with the regulation of their expression. Bearing this in mind, administration of a single intravenous injection of TNF-alpha (100 microg/kg body weight) to rats resulted in a significant increase in UCP2 (242%) and UCP3 (113%) gene expression in skeletal muscle. The results suggest a possible role for UCP2 and UCP3 in the increase of energy expenditure associated with cytokine treatment.

TH-, NPY-, SP-, and CGRP-immunoreactive nerves in interscapular brown adipose tissue of adult rats acclimated at different temperatures: an immunohistochemical study

Interscapular brown adipose tissue (IBAT), a site of nonshivering thermogenesis in mammals, is neurally controlled. The co-existence of sympathetic and peptidergic innervation has been demonstrated in different brown adipose depots. We studied the morphological profile of IBAT innervation and tested by immunohistochemical methods whether cold and warm stimulation are accompanied by modifications in the density of parenchymal noradrenergic nerve fibers. We also studied the immunoreactivity of afferent fibers--which contain calcitonin gene-related peptide (CGRP) and substance P (SP)--in different functional conditions. IBAT was obtained from adult rats (6 weeks old) acclimated at different temperatures (4 degrees, 20 degrees, and 28 degrees C). Tissue activity was evaluated by studying the immunolocalization of uncoupling protein (UCP-1), a specific marker of brown adipose tissue. Noradrenergic and peptidergic innervation were seen to arise from morphologically different nerves. Fibers staining for tyrosine hydroxylase (TH) were thin, unmyelinated hilar nerves, and CGRP- and SP-positive fibers were in thick nerves containing both myelinated and unmyelinated fibers. Under cold stimulation, noradrenergic neurons produce greater amounts of TH, and their axons branch, resulting in increased parenchymal nerve fibers density. Neuropeptide Y (NPY) probably co-localizes with TH in noradrenergic neurons, but only in the perivascular nerve fiber network. The parenchymal distribution of NPY to interlobular arterioles and capillaries suggests that this peptide must have other functions besides that of innervating arteriovenous anastomoses, as hypothesized by other researchers. The different distribution of CGRP and SP suggests the existence of different sensory neuronal populations. The detection of CGRP at the parenchymal level is in line with the hypothesis of a trophic action of this peptide.

Leptin and UCP1 genes are reciprocally regulated in brown adipose tissue

In a previous work we showed that only unilocular brown adipocytes express leptin. In order to investigate the relationship between leptin gene expression, brown adipocyte activity (UCP1) and morphology, we studied brown adipose tissues of mice (C57BL, female, 7 weeks old) acclimated at different temperatures (19 degrees C and 28 degrees C). Northern blot analysis revealed higher leptin and lower UCP1 mRNA levels in mice exposed to 28 degrees C than in the group acclimated at 19 degrees C. Also protein expression (immunohistochemistry) differed in the two groups: at 28 degrees C brown adipocytes were positive for leptin and only weakly positive for UCP1, while at 19 degrees C they were leptin-negative and UCP1-positive. In the former group the morphology was mainly unilocular. Our data suggest that in brown adipocytes of warm-acclimated mice leptin expression is closely related to their hypoactive functional stage, as evidenced by their low level of UCP1 synthesis and the morphological rearrangement of the lipid content (unilocularity).