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Query: UMLS:C0028754 (obesity)
 124,988 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

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.
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PMID:Uncoupling protein-2 (UCP2): molecular and genetic studies. 1045 20

Genetic studies in humans provide a method to test hypotheses about the biological roles of specific genes. So far, ten published papers have chosen to examine the hypothesis that uncoupling protein-2 (UCP2) and/or UCP3 influence energy expenditure and/or body fat accumulation. These genes were chosen because they are candidate energy expenditure genes, based on their homology to UCP1. Studies of UCP2 and UCP3 are intrinsically intertwined because the two genes are separated by only 6000 base pairs on human chromosome 11. Linkage studies in families have suggested that UCP2 and/or UCP3, or a closely linked gene, may influence resting metabolic rate (RMR) Some association studies using a 3' untranslated region insertion/deletion variant of UCP2 have produced statistically positive evidence for association with body mass index (BMI) and RMR. In contrast, association studies of UCP2 using an Ala to Val variant at amino acid 55 have produced negative results. Positive results have also been reported for association of a UCP3 splice variant with respiratory quotient in African Americans. In addition, no studies have reported linkage or association of UCP2 or UCP3 with diabetes. Overall, the results suggest that some variants of UCP2 and UCP3 may be associated with obesity traits in some populations. The UCPs, to date, show positive results in associations with obesity traits but not with diabetes traits. Further work will be needed to settle the role of UCP2 and UCP3 alleles in human body weight regulation.
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PMID:Genetics of uncoupling proteins in humans. 1045 22

Uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) are mitochondrial proteins that may play a role in the control of energy expenditure by uncoupling respiration from ATP synthesis. The present review focuses on data obtained in humans. UCP2 is widely expressed in the body, whereas UCP3 expression is restricted to skeletal muscle. Positive correlations have been reported between UCP2 mRNA concentrations in adipose tissue, UCP3 mRNA concentrations in skeletal muscle, and components of the metabolic rate. Fasting induces an up-regulation of UCP2 and UCP3 mRNA expression. In vivo and in vitro studies suggest that fatty acids could modulate uncoupling protein gene expression. The putative relationship between obesity, energy expenditure and uncoupling protein expression, and the unexpected rise in UCP2 and UCP3 mRNA concentrations during short-term fasting, are discussed in view of the recent data obtained in rodents and cell lines.
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PMID:Uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) expression in adipose tissue and skeletal muscle in humans. 1045 28

The newly discovered uncoupling protein-2 (UCP2) has been proposed to play a critical role in thermoregulatory and substrate oxidation processes. Skeletal muscle mRNA expression and, more recently, the protein content of UCP2 were investigated in humans. These studies have shown that the content of this protein varies quite substantially and that several factors could be responsible for its variation in human skeletal muscle. The aim of this review is to determine whether obesity and low-intensity increased contractile activity contribute to variation in muscle UCP2 content. A recent study from our laboratories revealed that, in obesity, UCP2 content in skeletal muscle is over-expressed by about 1.5-fold compared to lean. Body weight loss in obese subjects did not cause any change in skeletal muscle UCP2 content. On the other hand, when increased muscular contractile activity of knee extensor muscles is induced by several weeks of low-frequency electrical stimulation, UCP2 content increased by about 15%. Obesity and increased contractile activity do not appear sufficient, however, to explain the magnitude of the human skeletal muscle variation in UCP2 content. Since intensive efforts are being devoted to this area of research, it is expected that our understanding of the causes contributing to its variation in humans will soon be substantially improved.
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PMID:Obesity and increased contractile activity influence the protein content of UCP2 in human skeletal muscle. 1045 29

Uncoupling protein 1 (UCP1) dissipates energy and generates heat by catalyzing back-flux of protons into the mitochondrial matrix, probably by a fatty acid cycling mechanism. If the newly discovered UCP2 and UCP3 function similarly, they will enhance peripheral energy expenditure and are potential molecular targets for the treatment of obesity. We expressed UCP2 and UCP3 in Escherichia coli and reconstituted the detergent-extracted proteins into liposomes. Ion flux studies show that purified UCP2 and UCP3 behave identically to UCP1. They catalyze electrophoretic flux of protons and alkylsulfonates, and proton flux exhibits an obligatory requirement for fatty acids. Proton flux is inhibited by purine nucleotides but with much lower affinity than observed with UCP1. These findings are consistent with the hypothesis that UCP2 and UCP3 behave as uncoupling proteins in the cell.
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PMID:Transport function and regulation of mitochondrial uncoupling proteins 2 and 3. 1047 45

In eukaryotic cells ATP is generated by oxidative phosphorylation, an energetic coupling at the mitochondrial level. The oxidative reactions occurring in the respiratory chain generate an electrochemical proton gradient on both sides of the inner membrane. This gradient is used by the ATPsynthase to phosphorylate ADP into ATP. The coupling between respiration and ADP phosphorylation is only partial in brown adipose tissue (BAT) mitochondria, where the uncoupling protein UCP1 causes a reentry of protons into the matrix and abolishes the electrochemical proton gradient. The liberated energy is then dissipated as heat and ATP synthesis is reduced. This property was for a long time considered as an exception and specific to the non-shivering thermogenesis found in BAT. The recent cloning of new UCPs expressed in other tissues revealed the importance of this kind of regulation of respiratory control in metabolism and energy expenditure. The newly characterised UCPs are potential targets for obesity treatment drugs which could favour energy expenditure and diminish the metabolic efficiency. In 1997, we cloned UCP2 and proposed a role for this new uncoupling protein in diet-induced thermogenesis, obesity, hyperinsulinemia, fever and resting metabolic rate. Currently, an abundant literature deals with UCP2, but its biochemical and physiological functions and regulation remain unclear. The present review reports the status of our knowledge of this mitochondrial carrier in terms of sequence, activity, tissue distribution and regulation of expression. The putative physiological roles of UCP2 will be introduced and discussed.
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PMID:The mitochondrial uncoupling protein-2: current status. 1060 19

The availability of a UCP1-ablated mouse has enabled critical studies of the function of UCP1, UCP2, and UCP3. Concerning UCP1, its presence in brown-fat mitochondria is associated with innate uncoupling, high GDP-binding capacity, and GDP-inhibitable Cl- permeability and uncoupling--but the high fatty acid sensitivity found in these mitochondria is observed even in the absence of UCP1. The absence of UCP1 leads to low cold tolerance but not to obesity. UCP1 ablation also leads to an augmented expression of UCP2 and UCP3 in brown adipose tissue, making this tissue probably the one that boasts the highest expression of these UCPs. However, these very high expression levels are not associated with any inherent uncoupling, or with a specific GDP-binding capacity, or with a GDP-sensitive Cl- permeability, or with any effect of GDP on mitochondrial membrane potential, or with an increased basal metabolism of cells, or with the presence of norepinephrine- or fatty acid-induced thermogenesis in cells, and not with a cold-acclimation recruited, norepinephrine-induced thermogenic response in the intact animal. Therefore, it can be discussed whether any uncoupling effect is associated with UCP2 or UCP3 when they are endogenously expressed and, consequently, whether (loss of) uncoupling (thermogenic) effects of UCP2 or UCP3 can be invoked to explain metabolic phenomena, such as obesity.
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PMID:UCP1: the original uncoupling protein--and perhaps the only one? New perspectives on UCP1, UCP2, and UCP3 in the light of the bioenergetics of the UCP1-ablated mice. 1065 76

The recently cloned uncoupling protein homolog UCP3 is expressed primarily in muscle and therefore may play a significant role in the regulation of energy expenditure and body weight. However, investigation into the regulation of uncoupling protein has been hampered by the inability to assess its activity in vivo. In this report, we demonstrate the use of a noninvasive NMR technique to assess mitochondrial energy uncoupling in skeletal muscle of awake rats by combining (13)C NMR to measure rates of mitochondrial substrate oxidation with (31)P NMR to assess unidirectional ATP synthesis flux. These combined (31)P/(13)C NMR measurements were performed in control, 10-day triiodo-l-thyronine (T(3))-treated (model of increased UCP3 expression), and acute 2,4-dinitrophenol (DNP)-treated (protonophore and mitochondrial uncoupler) rats. UCP3 mRNA and protein levels increased 8.1-fold (+/- 1.1) and 2.8-fold (+/- 0.8), respectively, in the T(3)-treated vs. control rat gastrocnemius muscle. (13)C NMR measurements of tricarboxylic acid cycle flux as an index of mitochondrial substrate oxidation were 61 +/- 21, 148 +/- 25, and 310 +/- 48 nmol/g per min in the control, T(3), and DNP groups, respectively. (31)P NMR saturation transfer measurements of unidirectional ATP synthesis flux were 83 +/- 14, 84 +/- 14, and 73 +/- 7 nmol/g per s in the control, T(3), and DNP groups, respectively. Together, these flux measurements, when normalized to the control group, suggest that acute administration of DNP (mitochondrial uncoupler) and chronic administration of T(3) decrease energy coupling by approximately 80% and approximately 60%, respectively, and that the latter treatment correlates with an increase in UCP3 mRNA and protein expression. This NMR approach could prove useful for exploring the regulation of uncoupling protein activity in vivo and elucidating its role in energy metabolism and obesity.
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PMID:Assessment of mitochondrial energy coupling in vivo by 13C/31P NMR. 1082 16

The physiological significance of changes in uncoupling protein-2 (UCP-2) gene expression is controversial. In this study we investigated the biochemical and functional correlates of UCP-2 gene expression in sc abdominal adipose tissue in humans in vivo. UCP-2 messenger ribonucleic acid expression was quantified by nuclease protection in adipose tissue from lean and obese humans in both the fasting and postprandial states. Plasma fatty acids, insulin, and leptin were all determined in paired samples from the superficial epigastric vein and radial artery, and local production rates were calculated from 133Xe washout. In the fasting state UCP-2 expression correlated inversely with body mass index (r = -0.45; P = 0.026), percent body fat (r = -0.41; P = 0.05), plasma insulin (r = -0.47; P = 0.02), epigastric venous fatty acids (r = -0.45; P = 0.04), and leptin (r = -0.50; P = 0.018). UCP-2 expression remained inversely related with plasma leptin after controlling for percent body (r = -0.45; P = 0.038). At 2 or 4 h postprandially, there were no significant relationships between UCP-2 expression and biochemical parameters. In conclusion, 1) UCP-2 messenger ribonucleic acid expression in sc adipose tissue is inversely related to adiposity and independently linked to local plasma leptin levels; and 2) UCP-2 expression is not acutely regulated by food intake, insulin, or fatty acids. Reduced UCP-2 expression may be a maladaptive response to sustained energy surplus and could contribute to the pathogenesis and maintenance of obesity.
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PMID:Physiological relationships of uncoupling protein-2 gene expression in human adipose tissue in vivo. 1085 69

We investigated the relative importance of overeating, thermogenesis, and uncoupling protein (UCP) expression in determining the severity of obesity in male Wistar rats fed a highly palatable diet. After 2 wk of feeding, body weight did not differ significantly from controls (248 +/- 4 vs. 229 +/- 3 g; P > 0.3), but rectal temperature, brown adipose tissue (BAT) mass, UCP3 expression in gastrocnemius muscle, and UCP2 expression in white adipose tissue (WAT) were all elevated in diet-fed animals. In a further study, rats fed a palatable diet for 8 wk exhibited higher energy intake and rectal temperature than controls. Dietary-obese rats were divided into high (427-490 g; n = 8) and low (313-410 g; n = 10) weight gainers. The high gainers ate significantly more than the low gainers, and energy intake was positively correlated with weight gain (r(2) = 0.72, P < 0.01). UCP2 and UCP3 mRNA levels in gastrocnemius muscle were significantly increased above lean controls in all diet-fed animals, whereas UCPs in WAT and BAT did not differ significantly from controls. Whereas rats fed palatable food exhibited a thermogenic response, there was no significant difference in core temperature between high and low gain groups (37. 5 +/- 0.1 vs. 37.6 +/- 0.1 degrees C; P > 0.5). We conclude that a higher energy intake is the critical factor determining susceptibility to dietary obesity in unselected Wistar rats.
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PMID:Individual severity of dietary obesity in unselected Wistar rats: relationship with hyperphagia. 1091 34


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