Genetic and physiologic analysis of the role of uncoupling protein 3 in human energy homeostasis

Mitochondrial Phenotype as a Driver of the Racial Dichotomy in Obesity and Insulin Resistance

African Americans (AA) are disproportionately burdened by metabolic diseases. While largely unexplored between Caucasian (C) and AA, differences in mitochondrial bioenergetics may provide crucial insight to mechanisms for increased susceptibility to metabolic diseases. AA display lower total energy expenditure and resting metabolic rate compared to C, but paradoxically have a higher amount of skeletal muscle mass, suggestive of inherent energetic efficiency differences between these races. Such adaptations would increase the chances of overnutrition in AA; however, these disparities would not explain the racial difference in insulin resistance (IR) in healthy subjects. Hallmarks associated with insulin resistance (IR), such as reduced mitochondrial oxidative capacity and metabolic inflexibility are present even in healthy AA without a metabolic disease. These adaptations might be influential of mitochondrial “substrate preference” and could play a role in disproportionate IR rates among races. A higher glycolytic flux and provision of shuttles transferring electrons from cytosol to mitochondrial matrix could be a contributing factor in development of IR via heightened reactive oxygen species (ROS) production. This review highlights the above concepts and provides suggestions for future studies that could help delineate molecular premises behind potential impairments in insulin signaling and metabolic disease susceptibility in AA.

The role of uncoupling protein 2 and 3 genes polymorphism and energy expenditure in obese Indonesian children

AIM

Uncoupling protein (UCP) genes, which may contribute to energy metabolism in mitochondria, may be involved in the pathogenesis of obesity. We analyzed the differences in energy expenditure between single nucleotide polymorphisms (SNPs) UCP3-55C/T, UCP3 Y210Y, and UCP2 A55V among Indonesian children.

METHODS

The study included 76 schoolchildren (36 obese and 40 healthy; mean age, 12.8 years) in Semarang, Indonesia. Body composition was measured by bioelectrical impedance analysis; resting energy expenditure (REE) by indirect calorimetry; physical activity by uniaxial accelerometer; and total energy expenditure (TEE) by the equations extrapolated from REE and physical activity. UCP3-55C/T, UCP3 Y210Y, and UCP2 A55V were examined by restriction length fragment polymorphism analysis.

RESULTS

The TEE of the subjects with the T/T genotype at UCP3-55C/T after adjusting for fat-free mass (63.2±7.2 kcal/kg/day) and T/T at UCP2 A55V (62.8±5.6 kcal/kg/day) was lower than that of the subjects with the C/C and C/T genotypes (p<0.05). The REE of the subjects with these T/T genotypes tended to be lower than that of the subjects with C/C and C/T (p≥0.05). No significant differences in REE or TEE were found between the UCP3 Y210Y genotypes.

CONCLUSIONS

The subjects with the T/T genotypes of UCP3-55C/T or UCP2 A55V had lower TEE than those with other genotypes.

Human Uncoupling Protein-3 and Obesity: An Update

The cloning of the uncoupling protein (UCP)1 homologs UCP2 and UCP3 has raised considerable interest in the mechanism. The expression of UCP3 mainly in skeletal muscle mitochondria and the potency of the skeletal muscle as a thermogenic organ made UCP3 an attractive target for studies toward manipulation of energy expenditure to fight disorders such as obesity and type 2 diabetes. Overexpressing UCP3 in mice resulted in lean, hyperphagic mice. However, the lack of an apparent phenotype in mice lacking UCP3 triggered the search for alternative functions of UCP3. The observation that fatty acid levels significantly affect UCP3 expression has given UCP3 a position in fatty acid handling and/or oxidation. Emerging data indicate that the primary physiological role of UCP3 may be the mitochondrial handling of fatty acids rather than the regulation of energy expenditure through thermogenesis. It has been proposed that UCP3 functions to export fatty acid anions away from the mitochondrial matrix. In doing so, fatty acids are exchanged with protons, explaining the uncoupling activity of UCP3. The exported fatty acid anions may originate from hydrolysis of fatty acid esters by a mitochondrial thioesterase, or they may have entered the mitochondria as nonesterified fatty acids by incorporating into and flip-flopping across the mitochondrial inner membrane. Regardless of the origin of the fatty acid anions, this putative function of UCP3 might be of great importance in protecting mitochondria against fatty acid accumulation and may help to maintain muscular fat oxidative capacity.

Population genetic analysis of the uncoupling proteins supports a role for UCP3 in human cold resistance

Production of heat via nonshivering thermogenesis (NST) is critical for temperature homeostasis in mammals. Uncoupling protein UCP1 plays a central role in NST by uncoupling the proton gradients produced in the inner membranes of mitochondria to produce heat; however, the extent to which UCP1 homologues, UCP2 and UCP3, are involved in NST is the subject of an ongoing debate. We used an evolutionary approach to test the hypotheses that variants that are associated with increased expression of these genes (UCP1 -3826A, UCP2 -866A, and UCP3 -55T) show evidence of adaptation with winter climate. To that end, we calculated correlations between allele frequencies and winter climate variables for these single-nucleotide polymorphisms (SNPs), which we genotyped in a panel of 52 worldwide populations. We found significant correlations with winter climate for UCP1 -3826G/A and UCP3 -55C/T. Further, by analyzing previously published genotype data for these SNPs, we found that the peak of the correlation for the UCP1 region occurred at the disease-associated -3826A/G variant and that the UCP3 region has a striking signal overall, with several individual SNPs showing interesting patterns, including the -55C/T variant. Resequencing of the regions in a set of three diverse population samples helped to clarify the signals that we found with the genotype data. At UCP1, the resequencing data revealed modest evidence that the haplotype carrying the -3826A variant was driven to high frequency by selection. In the UCP3 region, combining results from the climate analysis and resequencing survey suggest a more complex model in which variants on multiple haplotypes may independently be correlated with temperature. This is further supported by an excess of intermediate frequency variants in the UCP3 region in the Han Chinese population. Taken together, our results suggest that adaptation to climate influenced the global distribution of allele frequencies in UCP1 and UCP3 and provide an independent source of evidence for a role in cold resistance for UCP3.

Mitochondrial dysfunction in diabetes: from molecular mechanisms to functional significance and therapeutic opportunities

Given their essential function in aerobic metabolism, mitochondria are intuitively of interest in regard to the pathophysiology of diabetes. Qualitative, quantitative, and functional perturbations in mitochondria have been identified and affect the cause and complications of diabetes. Moreover, as a consequence of fuel oxidation, mitochondria generate considerable reactive oxygen species (ROS). Evidence is accumulating that these radicals per se are important in the pathophysiology of diabetes and its complications. In this review, we first present basic concepts underlying mitochondrial physiology. We then address mitochondrial function and ROS as related to diabetes. We consider different forms of diabetes and address both insulin secretion and insulin sensitivity. We also address the role of mitochondrial uncoupling and coenzyme Q. Finally, we address the potential for targeting mitochondria in the therapy of diabetes.

The energetic implications of uncoupling protein-3 in skeletal muscle

Despite almost a decade of research since the identification of uncoupling protein-3 (UCP3), the molecular mechanisms and physiological functions of this mitochondrial anion carrier protein are not well understood. Because of its highly selective expression in skeletal muscle and the existence of mitochondrial proton leak in this tissue, early reports proposed that UCP3 caused a basal proton leak and increased thermogenesis. However, gene expression data and results from knockout and overexpression studies indicated that UCP3 does not cause basal proton leak or physiological thermogenesis. UCP3 expression is associated with increases in circulating fatty acids and in fatty acid oxidation (FAO) in muscle. Fatty acids are also well recognized as activators of the prototypic UCP1 in brown adipose tissue. This has led to hypotheses implicating UCP3 in mitochondrial fatty acid translocation. The corresponding hypothesized physiological roles include facilitated FAO and protection from the lipotoxic effects of fatty acids. Recent in vitro studies of physiological increases in UCP3 in muscle cells demonstrate increased FAO, and decreased reactive oxygen species (ROS) production. Detailed mechanistic studies indicate that ROS or lipid by-products of ROS can activate a UCP3-mediated proton leak, which in turn acts in a negative feedback loop to mitigate ROS production. Altogether, UCP3 appears to play roles in muscle FAO and mitigated ROS production. Future studies will need to elucidate the molecular mechanisms underlying increased FAO, as well as the physiological relevance of ROS-activated proton leak.

Uncoupling protein-3: clues in an ongoing mitochondrial mystery

Uncoupling protein (UCP) 3 (UCP3) is a mitochondrial anion carrier protein with highly selective expression in skeletal muscle. Despite a great deal of interest, to date neither its molecular mechanism nor its biochemical and physiological functions are well understood. Based on its high degree of homology to the original UCP (UCP1), early studies examined a role for UCP3 in thermogenesis. However, evidence for such a function is lacking. Recent studies have focused on two distinct, but not mutually exclusive, hypotheses: 1) UCP3 mitigates reactive oxygen species (ROS) production, and 2) UCP3 is somehow involved in fatty acid (FA) translocation. While supportive evidence exists for both hypotheses, the interpretation of the corresponding evidence has created some controversy. Mechanistic studies examining mitigated ROS production have been largely conducted in vitro, and the physiological significance of the findings is questioned. Conversely, while physiological evidence exists for FA translocation hypotheses, the evidence is largely correlative, leaving causal relationships unexplored. This review critically assesses evidence for the hypotheses and attempts to link the outcomes from mechanistic studies to physiological implications. Important directions for future studies, using current and novel approaches, are discussed.

Consequences of food energy excess and positive energy balance

This paper discusses possible consequences of energy excess throughout the life cycle. Firstly we consider the effects of foods on hunger, satiety and satiation. Also, the changes in food availability and consumption in relation to changes in social and economic determinants of energy excess. The relationship between physical activity and energy intake (EI) is also considered. Secondly we explore the definition of energy excess and the metabolic effects of macronutrients (mainly in relation to fuel partitioning oxidation/storage) on energy balance. The cellular and molecular regulation determined by specific genes involved in lipogenesis, fuel partitioning and/or in energy dissipation are explored. Thirdly, we examine the main consequences induced by energy excess and positive energy balance, starting with the alterations in glucose utilisation (insulin resistance) leading to type 2 diabetes and the linkage of energy excess with other non-communicable diseases (NCDs). Biological, social and psychological consequences during perinatal, childhood and adolescence periods are specifically analysed. Fourthly, the transition from energy deficit to excess, under the optic of a developing country is analysed with country examples drawn from Latin America. The possible role of supplementary food programmes in determining positive energy balance is discussed especially in relation to pre-school and school feeding programmes. Fifthly, we deal with the economic costs of energy excess and obesity related diseases. Finally, some areas where further research is needed are described; biological and genetic determinants of individual and population energy requirements, foods and food preparations as actually consumed, consumer education and research needs on social determinants of energy imbalances.

Topical ER36009, a RARgamma-selective retinoid, decreases abdominal white adipose tissue and elicits changes in expression of genes related to adiposity and thermogenesis

Chronic topical treatment of rats with a new RARgamma-selective retinoid, ER36009, resulted in a significant reduction of epididymal white adipose tissue and a significant increase of interscapular brown adipose tissue without affecting food intake. ER36009 markedly decreased PPARgamma, 11beta-HSD1, and Bcl-2 mRNA levels, and increased Bax mRNA in white adipose tissue, while it upregulated UCP1 and UCP3 mRNAs in brown adipose tissue and UCP3 mRNA in gastrocnemial muscle. These results suggest that ER36009 has multiple effects on adipose tissue biology and the energy balance. Topically applied ER36009 may have potential for the treatment of obesity.

Physiological increases in uncoupling protein 3 augment fatty acid oxidation and decrease reactive oxygen species production without uncoupling respiration in muscle cells

Decreased uncoupling protein (UCP)3 is associated with insulin resistance in muscle of pre-diabetic and diabetic individuals, but the function of UCP3 remains unclear. Our goal was to elucidate mechanisms underlying the negative correlation between UCP3 and insulin resistance in muscle. We determined effects of physiologic UCP3 overexpression on glucose and fatty acid oxidation and on mitochondrial uncoupling and reactive oxygen species (ROS) production in L6 muscle cells. An adenoviral construct caused a 2.2- to 2.5-fold increase in UCP3 protein. Palmitate oxidation was increased in muscle cells incubated under normoglycemic or hyperglycemic conditions, whereas adenoviral green fluorescent protein infection or chronic low doses of the uncoupler dinitrophenol had no effect. Increased UCP3 did not affect glucose oxidation, whereas dinitrophenol and insulin treatments caused increases. Basal oxygen consumption, assessed in situ using self-referencing microelectrodes, was not significantly affected, whereas dinitrophenol caused increases. Mitochondrial membrane potential was decreased by dinitrophenol but was not affected by increased UCP3 expression. Finally, mitochondrial ROS production decreased significantly with increased UCP3 expression. Results are consistent with UCP3 functioning to facilitate fatty acid oxidation and minimize ROS production. As impaired fatty acid metabolism and ROS handling are important precursors in muscular insulin resistance, UCP3 is an important therapeutic target in type 2 diabetes.

The reactions catalysed by the mitochondrial uncoupling proteins UCP2 and UCP3

The mitochondrial uncoupling proteins UCP2 and UCP3 may be important in attenuating mitochondrial production of reactive oxygen species, in insulin signalling (UCP2), and perhaps in thermogenesis and other processes. To understand their physiological roles, it is necessary to know what reactions they are able to catalyse. We critically examine the evidence for proton transport and anion transport by UCP2 and UCP3. There is good evidence that they increase mitochondrial proton conductance when activated by superoxide, reactive oxygen species derivatives such as hydroxynonenal, and other alkenals or their analogues. However, they do not catalyse proton leak in the absence of such acute activation. They can also catalyse export of fatty acid and other anions, although the relationship of anion transport to proton transport remains controversial.

The mitochondrial uncoupling-protein homologues

Uncoupling protein(UCP)1 is an integral membrane protein that is located in the mitochondrial inner membrane of brown adipocytes. Its physiological role is to mediate a regulated, thermogenic proton leak. UCP2 and UCP3 are recently identified UCP1 homologues. They also mediate regulated proton leak, and might function to control the production of superoxide and other downstream reactive oxygen species. However, their role in normal physiology remains unknown. Recent studies have shown that UCP2 has an important part in the pathogenesis of type-2 diabetes. The obscure roles of the UCP homologues in normal physiology, together with their emerging role in pathophysiology, provide exciting potential for further investigation.

Current limitations of SNP data from the public domain for studies of complex disorders: a test for ten candidate genes for obesity and osteoporosis

BACKGROUND

Public SNP databases are frequently used to choose SNPs for candidate genes in the association and linkage studies of complex disorders. However, their utility for such studies of diseases with ethnic-dependent background has never been evaluated.

RESULTS

To estimate the accuracy and completeness of SNP public databases, we analyzed the allele frequencies of 41 SNPs in 10 candidate genes for obesity and/or osteoporosis in a large American-Caucasian sample (1,873 individuals from 405 nuclear families) by PCR-invader assay. We compared our results with those from the databases and other published studies. Of the 41 SNPs, 8 were monomorphic in our sample. Twelve were reported for the first time for Caucasians and the other 29 SNPs in our sample essentially confirmed the respective allele frequencies for Caucasians in the databases and previous studies. The comparison of our data with other ethnic groups showed significant differentiation between the three major world ethnic groups at some SNPs (Caucasians and Africans differed at 3 of the 18 shared SNPs, and Caucasians and Asians differed at 13 of the 22 shared SNPs). This genetic differentiation may have an important implication for studying the well-known ethnic differences in the prevalence of obesity and osteoporosis, and complex disorders in general.

CONCLUSION

A comparative analysis of the SNP data of the candidate genes obtained in the present study, as well as those retrieved from the public domain, suggests that the databases may currently have serious limitations for studying complex disorders with an ethnic-dependent background due to the incomplete and uneven representation of the candidate SNPs in the databases for the major ethnic groups. This conclusion attests to the imperative necessity of large-scale and accurate characterization of these SNPs in different ethnic groups.

Increased uncoupling protein 3 content does not affect mitochondrial function in human skeletal muscle in vivo

Phosphocreatine (PCr) resynthesis rate following intense anoxic contraction can be used as a sensitive index of in vivo mitochondrial function. We examined the effect of a diet-induced increase in uncoupling protein 3 (UCP3) expression on postexercise PCr resynthesis in skeletal muscle. Nine healthy male volunteers undertook 20 one-legged maximal voluntary contractions with limb blood flow occluded to deplete muscle PCr stores. Exercise was performed following 7 days consumption of low-fat (LF) or high-fat (HF) diets. Immediately following exercise, blood flow was reinstated, and muscle was sampled after 20, 60, and 120 seconds of recovery. Mitochondrial coupling was assessed by determining the rate of PCr resynthesis during recovery. The HF diet increased UCP3 protein content by approximately 44% compared with the LF diet. However, this HF diet-induced increase in UCP3 expression was not associated with any changes in the rate of muscle PCr resynthesis during conditions of maximal flux through oxidative phosphorylation. Muscle acetylcarnitine, free-creatine, and lactate concentrations during recovery were unaffected by the HF diet. Taken together, our findings demonstrate that increasing muscle UCP3 expression does not diminish the rate of PCr resynthesis, allowing us to conclude that the primary role of UCP3 in humans is not uncoupling.

UCP2 and UCP3 in muscle controlling body metabolism

The uncoupling protein-1 (UCP1) homologues UCP2 and UCP3 are able to uncouple ATP production from mitochondrial respiration, thereby dissipating energy as heat and affecting energy metabolism efficiency. In contrast to UCP1, which plays an important role in adaptive thermogenesis, UCP2 and UCP3 do not have a primary role in the regulation of energy metabolism. UCP2, which is expressed in a wide variety of tissues, including white adipose tissue,skeletal muscle and tissues of the immune system, has been suggested to affect the production of reactive oxygen species. UCP2 has also been suggested to regulate the [ATP]/[ADP] ratio and was recently shown to influence insulin secretion in the β-cells of the pancreas. UCP3, in contrast, is expressed predominantly in skeletal muscle and has been associated with whole-body energy metabolism. However, the primary function of UCP3 is not the regulation of energy metabolism. For example, fasting, a condition attenuating energy expenditure, upregulates UCP3 expression. Moreover, UCP3-knockout mice have a normal metabolic rate. The exact function of UCP3 therefore remains to be elucidated, but putative roles for UCP3 include involvement in the regulation of ROS, in mitochondrial fatty acid transport and in the regulation of glucose metabolism in skeletal muscle. Whatever the primary function of these novel uncoupling proteins, a secondary effect via uncoupling might allow them to influence (but not to regulate) energy metabolism, which would be consistent with the observations from linkage and association studies. Therefore, UCP2 and UCP3 remain interesting targets for pharmacological upregulation in the treatment of obesity and diabetes.

Decreased mitochondrial proton leak and reduced expression of uncoupling protein 3 in skeletal muscle of obese diet-resistant women

Weight loss in response to caloric restriction is variable. Because skeletal muscle mitochondrial proton leak may account for a large proportion of resting metabolic rate, we compared proton leak in diet-resistant and diet-responsive overweight women and compared the expression and gene characteristics of uncoupling protein (UCP)2 and UCP3. Of 1,129 overweight women who completed the University of Ottawa Weight Management Clinic program, 353 met compliance criteria and were free of medical conditions that could affect weight loss. Subjects were ranked according to percent body weight loss during the first 6 weeks of a 900-kcal meal replacement protocol. The highest and lowest quintiles of weight loss were defined as diet responsive and diet resistant, respectively. After body weight had been stable for at least 10 weeks, 12 of 70 subjects from each group consented to muscle biopsy and blood sampling for determinations of proton leak, UCP mRNA expression, and genetic studies. Despite similar baseline weight and age, weight loss was 43% greater, mitochondrial proton leak-dependent (state 4) respiration was 51% higher (P = 0.0062), and expression of UCP3 mRNA abundance was 25% greater (P < 0.001) in diet-responsive than in diet-resistant subjects. There were no differences in UCP2 mRNA abundance. None of the known polymorphisms in UCP3 or its 5' flanking sequence were associated with weight loss or UCP3 mRNA abundance. Thus, proton leak and the expression of UCP3 correlate with weight loss success and may be candidates for pharmacological regulation of fat oxidation in obese diet-resistant subjects.

Uncoupling proteins and thermoregulation

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

Racial differences in the relation between uncoupling protein genes and resting energy expenditure

BACKGROUND

Lower resting energy expenditure (REE) in African American women may contribute to their obesity. The identification of uncoupling protein (UCP) genes has fueled a search for genes involved in energy metabolism in humans.

OBJECTIVE

We examined variation in REE in relation to variation in UCP1, UCP2, and UCP3 in 141 women aged 18-21 y.

DESIGN

Standard methods were used for REE measurements and genetic analysis. Body composition was determined with the use of dual-energy X-ray absorptiometry. Multivariate analysis was used to examine the effect of genotypes on REE and on fat mass in relation to other potentially confounding variables.

RESULTS

REE was 295 kJ/d lower in African American women than in white women. No significant variation in REE was seen for UCP1, UCP2, and UCP3 (p-55; exon 3a; and exon 3b) variants after adjustment for other variables including smoking status. For the UCP3 exon 5 variant, REE was significantly (P = 0.019) lower in African American women with the CC genotype than in those with the TT genotype. In African American women, there was a significant trend (P = 0.012) toward lower REE and a weak but nonsignificant trend (P = 0.1) toward greater fat mass across the 3 genotypes (TT, CT, and CC).

CONCLUSIONS

The significant and dose-dependent relation between lower REE and the C allele suggests that it may be a thrifty allele. The presence of this parsimonious energy metabolism in African American women, possibly linked to UCP3, may be implicated in their susceptibility to obesity. The absence of a UCP3 effect in white women is intriguing and needs to be explored to further understand possible interactions between UCP3 and other genes.

Uncoupling protein 3 gene is associated with body composition changes with training in HERITAGE study

The uncoupling protein 3 (UCP3) is a mitochondrial membrane transporter mainly expressed in skeletal muscle that we have shown to be associated with obesity. We have analyzed UCP3 polymorphisms, Val102Ile, Tyr210Tyr, and a new microsatellite GAIVS6 located in the sixth intron, among 276 black and 503 white subjects from the HERITAGE Family Study. Linkage and association studies were undertaken with body composition variables measured in a sedentary state (baseline) and after 20 wk of endurance training (changes). Allele and genotype frequencies were found to be significantly different between whites and blacks. Suggestive linkages (0.009 < or = P < or = 0.033) with Tyr210Tyr were found in blacks and whites for baseline body mass index, fat mass, or leptin level and with GAIVS6 in whites for changes in fat mass and percent body fat. Associations were also found in whites between GAIVS6 and changes in the sum of eight skinfold thicknesses (P = 0.0006), with a borderline result for body mass index (P = 0.06). We concluded that UCP3 could be involved in body composition changes after regular exercise.

Genetic variation at the uncoupling protein 1, 2 and 3 loci and the response to long-term overfeeding

OBJECTIVE

To evaluate the effects of uncoupling protein (UCP) 1, UCP2 and UCP3 gene variants on body composition and metabolic changes in response to chronic overfeeding and the recovery after the period of overfeeding.

SUBJECTS AND DESIGN

Twenty-four normal weight men (21+/-2 y), who constituted 12 pairs of identical twins, ate a 4.2 MJ/day energy surplus, 6 days a week, during a period of 100 days. The subjects were asked to return to the laboratory for testing at 4 months and for a final examination 5 y after completion of the overfeeding protocol.

METHODS

Resting metabolic rate (RMR) measurements were performed before and after overfeeding. A 4.2 MJ test meal was consumed, after which calorimetric measurements were continued for 240 min. Total body fat was assessed by hydrodensitometry and total subcutaneous fat by the sum of eight skinfolds. Polymorphisms were typed by PCR and PCR-RFLP-techniques. Thyroid stimulating hormone (TSH) concentrations after a thyrotropin releasing hormone (TRH) injection were measured by radioimmunoassay (RIA).

RESULTS

The changes in body weight and adiposity were not different between UCP1 Bcl I, UCP2 alanine to valine (A55V), UCP2 insertion/deletion (I/D) or UCP3 Rsa I genotypes. However, the recovery from overfeeding was worse among G-allele carriers of the UCP1 Bcl I, I allele non-carriers of the UCP2 I/D, AV heterozygote subjects of the UCP2 A55V and CC subjects of the UCP3 Rsa I polymorphisms. RMR was lower both before (P=0.01) and after (P=0.001) overfeeding in subjects with the CC genotype of the UCP3 Rsa I polymorphism. Moreover, after overfeeding, the UCP2 A55V heterozygote and UCP3 Rsa I CC homozygote subjects had significantly higher respiratory quotient (RQ) values at rest (P<0.01) and during the meal test (P from<0.01 to<0.05). Also mean plasma TSH concentrations 20, 30 and 45 min after the TRH injection increased more with overfeeding among UCP2 A55V (P<0.005) and UCP3 Rsa I CC (P=0.017) subjects.

CONCLUSIONS

These data suggest that UCP polymorphisms may play a role in the recovery from the overfeeding by regulating substrate oxidation in response to long-term caloric surplus. The association of the UCP2 A55V and UCP3 Rsa I CC genotypes with a greater increase in the TSH response to TRH load could reflect a compensatory mechanism counteracting the effects of overfeeding. A longer period of exposure to chronic positive energy balance conditions may be necessary before sequence variation in UCP2 and UCP3 makes an impact on thyroid metabolism to influence body mass and composition changes.

Effects of fasting on muscle mitochondrial energetics and fatty acid metabolism in Ucp3(-/-) and wild-type mice

Uncoupling protein-3 (UCP3) is a mitochondrial carrier protein of as yet undefined physiological function. To elucidate characteristics of its function, we studied the effects of fasting on resting metabolic rate, respiratory quotient, muscle Ucp3 expression, and mitochondrial proton leak in wild-type and Ucp3(-/-) mice. Also analyzed were the fatty acid compositions of skeletal muscle mitochondria in fed and fasted Ucp3(-/-) and wild-type mice. In wild-type mice, fasting caused significant increases in Ucp3 (4-fold) and Ucp2 (2-fold) mRNA but did not significantly affect mitochondrial proton leak. State 4 oxygen consumption was not affected by fasting in either of the two groups. However, protonmotive force was consistently higher in mitochondria of Ucp3(-/-) animals (P = 0.03), and fasting further augmented protonmotive force in Ucp3(-/-) mice; there was no effect in wild-type mitochondria. Resting metabolic rates decreased with fasting in both groups. Ucp3(-/-) mice had higher respiratory quotients than wild-type mice in fed resting states, indicating impaired fatty acid oxidation. Altogether, results show that the fasting-induced increases in Ucp2 and Ucp3 do not correlate with increased mitochondrial proton leak but support a role for UCP3 in fatty acid metabolism.

The dysmetabolic syndrome

The first unifying definition for the metabolic syndrome was proposed by WHO in 1998. In accordance to this, patients with type 2 diabetes mellitus or impaired glucose tolerance have the syndrome if they fulfil two of the criteria: hypertension, dyslipidaemia, obesity/abdominal obesity and microalbuminuria. Persons with normal glucose tolerance (NGT) should also be insulin resistant. About 40% of persons with impaired glucose tolerance (IGT) and 70% of patients with type 2 diabetes have features of the syndrome. Importantly, presence of the dysmetabolic syndrome is associated with reduced survival, particularly because of increased cardiovascular mortality. The dysmetabolic syndrome most likely results from interplay between several genes and an affluent environment. Compatible with the thrifty gene theory, common variants in genes regulating lipolysis, thermogenesis and glucose uptake in skeletal muscle account for a large part of such thrifty genes. However, hitherto unknown genes may still be identified by random gene approaches.

Uncoupling protein 3 genetic variants in human obesity: the c-55t promoter polymorphism is negatively correlated with body mass index in a UK Caucasian population

OBJECTIVE

To investigate whether genetic variation at the UCP3 locus contributes to human obesity.

SUBJECTS

Ninety-one obese children (BMI>4 standard deviations from age related mean) and 419 Caucasian adults from the Isle of Ely Study.

DESIGN

Single strand conformation polymorphism (SSCP) analysis was used to scan the coding region of the UCP3 gene in 91 severely obese children. A common polymorphism identified in this gene (c-55t) has been shown to associate with lower UCP3 mRNA expression. Polymerase chain reaction-based forced restriction digestion was used to detect this allele in Caucasian adults. Multiple regression analysis was used to determine associations between the c-55t genotype and anthropometric, energetic and biochemical indices relevant to obesity.

MEASUREMENTS

For the obese children, SSCP analysis and sequencing of variants were carried out. For the Isle of Ely Study, c-55t genotype and anthropometric (body mass index, waist-hip ratio, percentage body fat), energetic (dietary fat intake, physical activity index, adjusted metabolic rate, maximum oxygen consumption) and biochemical indices (pre- and post-glucose challenge plasma triglycerides, non-esterified fatty acids, insulin and glucose) were determined.

RESULTS

A previously reported missense mutation (V102I) was detected in a single obese Afro-Carribean child. Twenty-one percent of the genes examined in the Isle of Ely study carried the c-55t promoter variant. Age-adjusted body mass index (BMI) was significantly (P=0.0037) lower in carriers of this variant.

CONCLUSION

Mutations in the coding sequence of UCP3 are unlikely to be a common monogenic cause of severe human obesity. In a Caucasian population the UCP3 c-55t polymorphism is negatively associated with BMI.

13C/31P NMR assessment of mitochondrial energy coupling in skeletal muscle of awake fed and fasted rats. Relationship with uncoupling protein 3 expression

To examine the relationship between mitochondrial energy coupling in skeletal muscle and change in uncoupling protein 3 (UCP3) expression during the transition from the fed to fasted state, we used a novel noninvasive (31)P/(13)C NMR spectroscopic approach to measure the degree of mitochondrial energy coupling in the hind limb muscles of awake rats before and after a 48-h fast. Compared with fed levels, UCP3 mRNA and protein levels in the gastrocnemius increased 1.7- (p < 0.01) and 2.9-fold (p < 0.001), respectively, following a 48-h fast. Tricarboxylic acid cycle flux measured using (13)C NMR as an index of mitochondrial substrate oxidation was 212 +/- 23 and 173 +/- 25 nmol/g/min (p not significant) in the fed and 48-h fasted groups, respectively. Unidirectional ATP synthesis flux measured using (31)P NMR was 79 +/- 15 and 57 +/- 9 nmol/g/s (p not significant) in the fed and 48-h fasted groups, respectively. Mitochondrial energy coupling as expressed by the ratio of ATP synthesis to tricarboxylic acid cycle flux was not different between the fed and fasted states. To test the hypothesis that UCP3 may be involved in the translocation of long chain free fatty acids (FFA) into the mitochondrial matrix under conditions of elevated FFA availability, [U-(13)C]palmitate/albumin was administered in a separate group of rats with (+) or without (-) etomoxir (an inhibitor of carnitine palmitoyltransferase I). The ratio of glutamate enrichment ((+) etomoxir/(-) etomoxir) in the hind limb muscles was the same between groups, indicating that UCP3 does not appear to function as a translocator for long chain FFA in skeletal muscle following a 48-h fast. In summary, these data demonstrate that despite a 2-3-fold increase in UCP3 mRNA and protein expression in skeletal muscle during the transition from the fed to fasted state, mitochondrial energy coupling does not change. Furthermore, UCP3 does not appear to have a major role in FFA translocation into the mitochondria. The physiological role of UCP3 following a 48-h fast in skeletal muscle remains to be elucidated.

The uncoupling protein-3 gene is transcribed from tissue-specific promoters in humans but not in rodents

Uncoupling protein-3 (UCP3), a mitochondrial membrane transporter, is a candidate effector of thermogenesis. Even though mice with targeted disruption of the UCP3 gene are not obese, indirect evidence suggests that this protein contributes to the control of energy expenditure in humans. We therefore characterized the human UCP3 gene and compared it with its rodent homologues with respect to tissue-specific expression and regulatory regions. Like rodent UCP3, human UCP3 was expressed in skeletal muscle and brown adipose tissue (BAT). The short mRNA isoform, UCP3(S), which is absent in rodents, was relatively more abundant in human skeletal muscle in comparison to human BAT. Two tissue-specific transcription start sites for each skeletal muscle and BAT were delineated for human UCP3. Tissue-specific transcript initiation was maintained in both tissues and cultured cells over a wide range of expression levels. In contrast, rodent transcripts were initiated at the same site in BAT and muscle tissue. Comparison of human and rodent promoters indicated a rapid phylogenetic evolution suggesting functional diversification. The transcription from tissue-specific promoters in humans is a novel finding that may provide the basis for therapeutic interventions aimed at regulating energy expenditure in a tissue-specific fashion.

Skeletal muscle respiratory uncoupling prevents diet-induced obesity and insulin resistance in mice

To determine whether uncoupling respiration from oxidative phosphorylation in skeletal muscle is a suitable treatment for obesity and type 2 diabetes, we generated transgenic mice expressing the mitochondrial uncoupling protein (Ucp) in skeletal muscle. Skeletal muscle oxygen consumption was 98% higher in Ucp-L mice (with low expression) and 246% higher in Ucp-H mice (with high expression) than in wild-type mice. Ucp mice fed a chow diet had the same food intake as wild-type mice, but weighed less and had lower levels of glucose and triglycerides and better glucose tolerance than did control mice. Ucp-L mice were resistant to obesity induced by two different high-fat diets. Ucp-L mice fed a high-fat diet had less adiposity, lower levels of glucose, insulin and cholesterol, and an increased metabolic rate at rest and with exercise. They were also more responsive to insulin, and had enhanced glucose transport in skeletal muscle in the setting of increased muscle triglyceride content. These data suggest that manipulating respiratory uncoupling in muscle is a viable treatment for obesity and its metabolic sequelae.

Energy metabolism in uncoupling protein 3 gene knockout mice

Uncoupling protein 3 (UCP3) is a member of the mitochondrial anion carrier superfamily. Based upon its high homology with UCP1 and its restricted tissue distribution to skeletal muscle and brown adipose tissue, UCP3 has been suggested to play important roles in regulating energy expenditure, body weight, and thermoregulation. Other postulated roles for UCP3 include regulation of fatty acid metabolism, adaptive responses to acute exercise and starvation, and prevention of reactive oxygen species (ROS) formation. To address these questions, we have generated mice lacking UCP3 (UCP3 knockout (KO) mice). Here, we provide evidence that skeletal muscle mitochondria lacking UCP3 are more coupled (i.e. increased state 3/state 4 ratio), indicating that UCP3 has uncoupling activity. In addition, production of ROS is increased in mitochondria lacking UCP3. This study demonstrates that UCP3 has uncoupling activity and that its absence may lead to increased production of ROS. Despite these effects on mitochondrial function, UCP3 does not seem to be required for body weight regulation, exercise tolerance, fatty acid oxidation, or cold-induced thermogenesis. The absence of such phenotypes in UCP3 KO mice could not be attributed to up-regulation of other UCP mRNAs. However, alternative compensatory mechanisms cannot be excluded. The consequence of increased mitochondrial coupling in UCP3 KO mice on metabolism and the possible role of yet unidentified compensatory mechanisms, remains to be determined.

Lack of obesity and normal response to fasting and thyroid hormone in mice lacking uncoupling protein-3

Uncoupling protein-3 (UCP3) is a mitochondrial protein that can diminish the mitochondrial membrane potential. Levels of muscle Ucp3 mRNA are increased by thyroid hormone and fasting. Ucp3 has been proposed to influence metabolic efficiency and is a candidate obesity gene. We have produced a Ucp3 knockout mouse to test these hypotheses. The Ucp3 (-/-) mice had no detectable immunoreactive UCP3 by Western blotting. In mitochondria from the knockout mice, proton leak was greatly reduced in muscle, minimally reduced in brown fat, and not reduced at all in liver. These data suggest that UCP3 accounts for much of the proton leak in skeletal muscle. Despite the lack of UCP3, no consistent phenotypic abnormality was observed. The knockout mice were not obese and had normal serum insulin, triglyceride, and leptin levels, with a tendency toward reduced free fatty acids and glucose. Knockout mice showed a normal circadian rhythm in body temperature and motor activity and had normal body temperature responses to fasting, stress, thyroid hormone, and cold exposure. The base-line metabolic rate and respiratory exchange ratio were the same in knockout and control mice, as were the effects of fasting, a beta3-adrenergic agonist (CL316243), and thyroid hormone on these parameters. The phenotype of Ucp1/Ucp3 double knockout mice was indistinguishable from Ucp1 single knockout mice. These data suggest that Ucp3 is not a major determinant of metabolic rate but, rather, has other functions.

Genetics of the metabolic syndrome

The clustering of cardiovascular risk factors such as abdominal obesity, hypertension, dyslipidaemia and glucose intolerance in the same persons has been called the metabolic or insulin-resistance syndrome. In 1998 WHO proposed a unifying definition for the syndrome and chose to call it the metabolic syndrome rather than the insulin-resistance syndrome. Although insulin resistance has been considered as a common denominator for the different components of the syndrome, there is still debate as to whether it is pathogenically involved in all of the different components of the syndrome. Clustering of the syndrome in families suggests a genetic component. It is plausible that so-called thrifty genes, which have ensured optimal storage of energy during periods of fasting, could contribute to the phenotype of the metabolic syndrome. Common variants in a number of candidate genes influencing fat and glucose metabolism can probably, together with environmental triggers, increase susceptibility to the syndrome. Among these, the genes for beta 3-adrenergic receptor, hormone-sensitive lipase, lipoprotein lipase, IRS-1, PC-1, skeletal muscle glycogen synthase, etc. appear to increase the risk of the metabolic syndrome. In addition, novel genes may be identified by genome-wide searches.

The human obesity gene map: the 1999 update

This report constitutes the sixth update of the human obesity gene map incorporating published results up to the end of October 1999. Evidence from the rodent and human obesity cases caused by single gene mutations, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTL) uncovered in human genome-wide scans and in crossbreeding experiments with mouse, rat, pig and chicken models, association and linkage studies with candidate genes and other markers is reviewed. Twenty-five human cases of obesity can now be explained by variation in five genes. Twenty Mendelian disorders exhibiting obesity as one of their clinical manifestations have now been mapped. The number of different QTLs reported from animal models reaches now 98. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 89 reports of positive associations pertaining to 40 candidate genes. Finally, 44 loci have linked to obesity indicators in genomic scans and other linkage study designs. The obesity gene map depicted in Figure 1 reveals that putative loci affecting obesity-related phenotypes can be found on all autosomes, with chromosomes 14 and 21 showing each one locus only. The number of genes, markers, and chromosomal regions that have been associated or linked with human obesity phenotypes continues to increase and is now well above 200.

Endogenous mutations in human uncoupling protein 3 alter its functional properties

Human uncoupling protein (UCP3) is a mitochondrial transmembrane carrier that uncouples oxidative phosphorylation and is a candidate gene for obesity. Expression of native human UCP3 mutations in yeast showed complete loss (R70W), significant reduction (R143X), or no effect (V102I and IVS6+1G > A) on the uncoupling activity of UCP3. It is concluded that certain mutations in UCP3 alter its functional impact on membrane potential (deltaphi), possibly conferring susceptibility to develop metabolic diseases.