Effects of mutations in the human uncoupling protein 3 gene on the respiratory quotient and fat oxidation in severe obesity and type 2 diabetes

Identification and Characteristics of Novel Mutations in Nonsyndromic Monogenic Obesity

Nonsyndromic monogenic obesity (NSMO) is a class of individual obesity that is independent of the environment and caused by a single gene mutation. It is mostly caused by mutations in LEP, LEPR, PCSK1, as well as some rare mutations in UCP3, NR0B2, and PPARG. Among 30 obesity patients, five patients are identified with positive gene detection. For the first time, the c.624C>T mutation associated with PCSK1, and the c.50G>A and c.293_301delinsAC mutations associated with NR0B2, as well as the obesity phenotype mutation (c.284A>G) associated with PPARG is confirmed. Following this, the genotype-clinical phenotype, mutation hotspots, and mutation distributions of each gene are summarized, and the genetic characteristics of NSMO are analyzed. The locations of mutation c.50G>A, and c.284A>G are highly conserved according to the sequencing alignment. According to the findings, the c.624C>T mutation in PCSK1 is a newly discovered synonymous mutation, but it can result in significant early-onset obesity. Additionally, the mutation of c.284A>G(PPARG) can lead to a variety of clinical phenotypes and the mutation of UCP3 and NR0B2 may increase the risk of type 2 diabetes mellitus. This study enriches the human NSMO gene mutation database and provides a scientific basis for clinically accurate diagnosis and treatment.

Next-generation sequencing of 12 obesity genes in a Portuguese cohort of patients with overweight and obesity

We examined 12 monogenic obesity genes in 72 Portuguese individuals with overweight and obesity (class 1 and class 2), some of which with suspected genetic obesity, to identify known or unknown potential obesity variants. Genomic DNA was analyzed for variants in genes LEP, LEPR, MC4R, POMC, PCSK1, BDNF, NTRK2, SIM1, SH2B1, UCP3, GCG and ADCY3 through next generation sequencing (NGS). The impact of the rare variants was investigated in the ClinVar database and using in silico tools for prediction of pathogenicity. Four potential pathogenic missense variants were detected at the heterozygous state in five individuals: two in the ADCY3 gene, NM_004036.5:c.1153G > A (p.Val385Ile) (rs756783003) and NM_004036.5:c.1222G > A (p.Gly408Arg) (rs201606553), one in gene SH2B1, NM_001145795.1:c.127C > A (p.Arg43Ser) (rs547678855), and the fourth in gene POMC NM_000939.4:c.706C > G (p.Arg236Gly) (rs28932472), which was found in two individuals. Moreover, six rare variants near splicing sites were also identified, as well as eight rare synonymous variants. In summary, some potential pathogenic rare missense variants were identified, two of them in ADCY3 gene, the most recently identified gene as having a role in monogenic obesity. Further analysis should be performed to confirm the clinical relevance of these variants.

Targeting skeletal muscle mitochondrial health in obesity

Metabolic demands of skeletal muscle are substantial and are characterized normally as highly flexible and with a large dynamic range. Skeletal muscle composition (e.g., fiber type and mitochondrial content) and metabolism (e.g., capacity to switch between fatty acid and glucose substrates) are altered in obesity, with some changes proceeding and some following the development of the disease. Nonetheless, there are marked interindividual differences in skeletal muscle composition and metabolism in obesity, some of which have been associated with obesity risk and weight loss capacity. In this review, we discuss related molecular mechanisms and how current and novel treatment strategies may enhance weight loss capacity, particularly in diet-resistant obesity.

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.

Is Obesity/Adiposity-Based Chronic Disease Curable: The Set Point Theory, the Environment, and Second Generation Medications

Adiposity-Based Chronic Disease (ABCD) is a chronic disease and requires life-long treatment and follow-up. Obesity protects obesity through altered regulation of caloric intake and set point mechanisms that maintains a high equilibrium body weight. Lifestyle interventions and obesity medications do not permanently alter the set point which often makes weight loss achieved by lifestyle short-lived and operates to drive weight regain once medications are discontinued. Bariatric surgery procedures can alter appetite and lower the "set point" for equilibrium body weight via unknown mechanisms. However, few patients attain ideal body weight following surgery, many regain weight, and all require long-term follow-up for the disease. The excess adiposity of ABCD gives rise to complications that impair health and confer morbidity and mortality; however, the genetic risks and potential interactions between genes and environment that give rise to complications also cannot be eliminated. The equilibrium body weight around which set point mechanisms operate can be modified by environment, which underscores the importance of a less obesogenic environment for prevention and treatment of ABCD on a population basis. If ABCD will eventually be curable, this will depend on a clear understanding of the molecular mechanisms that determine the set point regulation of body weight, and an ability to permanently modulate the set point to oscillate around and a lean body mass. The conceptualization of ABCD as a chronic disease, however, does present us with opportunities for primary, secondary, and tertiary prevention to avert disease progression. For tertiary care, the advent of new, more effective, second-generation obesity medications will allow clinicians to treat-to-target via active management of body weight into a target range that will ameliorate specific complications.

Computational studies of the mitochondrial carrier family SLC25. Present status and future perspectives

The members of the mitochondrial carrier family, also known as solute carrier family 25 (SLC25), are transmembrane proteins involved in the translocation of a plethora of small molecules between the mitochondrial intermembrane space and the matrix. These transporters are characterized by three homologous domains structure and a transport mechanism that involves the transition between different conformations. Mutations in regions critical for these transporters’ function often cause several diseases, given the crucial role of these proteins in the mitochondrial homeostasis. Experimental studies can be problematic in the case of membrane proteins, in particular concerning the characterization of the structure–function relationships. For this reason, computational methods are often applied in order to develop new hypotheses or to support/explain experimental evidence. Here the computational analyses carried out on the SLC25 members are reviewed, describing the main techniques used and the outcome in terms of improved knowledge of the transport mechanism. Potential future applications on this protein family of more recent and advanced in silico methods are also suggested.

Identification of Genetic Variants in 65 Obesity Related Genes in a Cohort of Portuguese Obese Individuals

Obesity is a major public health problem, which has a strong genetic component that interplays with environmental factors. Several genes are known to be implicated in the regulation of body weight. The identification of alleles that can be associated with obesity is a key element to control this pandemic. On the basis of a Portuguese population, 65 obesity-related genes are sequenced using Next-Generation Sequencing (NGS) in 72 individuals with obesity, in order to identify variants associated with monogenic obesity and potential risk factors. A total of 429 variants are identified, 129 of which had already been associated with the phenotype. Comparing our results with the European and Global frequencies, from 1000 Genomes project, 23 potential risk variants are identified. Six new variants are discovered in heterozygous carriers: four missense (genes ALMS1-NM_015120.4:c.5552C>T; SORCS1-NM_001013031.2:c.1072A>G and NM_001013031.2: c.2491A>C; TMEM67-NM_153704.5:c.158A>G) and two synonymous (genes BBS1-NM_024649.4:c.1437C>T; TMEM67-NM_153704.5:c.2583T>C). Functional studies should be performed to validate these new findings and evaluate their penetrance and pathogenicity. Regardless of no cases of monogenic obesity being identified, this kind of investigational study is important when we are still trying to understand the aetiology and pathophysiology of obesity. This will allow the identification of rare variants associated with obesity and the study of their prevalence in specific populational groups.

Regulatory roles and mechanisms of alternative RNA splicing in adipogenesis and human metabolic health

Alternative splicing (AS) regulates gene expression patterns at the post-transcriptional level and generates a striking expansion of coding capacities of genomes and cellular protein diversity. RNA splicing could undergo modulation and close interaction with genetic and epigenetic machinery. Notably, during the adipogenesis processes of white, brown and beige adipocytes, AS tightly interplays with the differentiation gene program networks. Here, we integrate the available findings on specific splicing events and distinct functions of different splicing regulators as examples to highlight the directive biological contribution of AS mechanism in adipogenesis and adipocyte biology. Furthermore, accumulating evidence has suggested that mutations and/or altered expression in splicing regulators and aberrant splicing alterations in the obesity-associated genes are often linked to humans’ diet-induced obesity and metabolic dysregulation phenotypes. Therefore, significant attempts have been finally made to overview novel detailed discussion on the prospects of splicing machinery with obesity and metabolic disorders to supply featured potential management mechanisms in clinical applicability for obesity treatment strategies.

Streptozotocin-induced type II diabetic rat administered with nonobesogenic high-fat diet is highly susceptible to myocardial ischemia-reperfusion injury: An insight into the function of mitochondria

RATIONALE

Our recent study suggested that ischemia-reperfusion (I/R) induced oxidative stress was minimal in the rat heart during initial stage of diabetes and the one that progressed to diabetic cardiomyopathy (DCM), despite having higher infarct and low cardiac performance. Mitochondrial dysfunction is an important mediator for adverse outcome in rat heart affected with diabetes, which is also a potential contributor for the cardiac reperfusion injury.

OBJECTIVE

The current study aims to evaluate the susceptibility of diabetes heart with or without myopathy to I/R injury and its influence on cardiac mitochondrial function.

METHODS AND RESULTS

Male Wistar rats (3 weeks old) were fed with high-fat diet for 8 weeks followed by diabetes mellitus (DM) induction via streptozotocin (35 mg/kg body weight) and maintained for further 4 weeks. The animal displayed cardiomyopathy characteristics like hypertrophy, fibrosis, and insulin resistance-termed diabetic cardiomyopathy (DCM). To study the specific effect of DCM on I/R, we included diabetic rats without cardiomyopathy. Induction of I/R in different groups suggested higher vulnerability to injury in DCM rat hearts than DM and normal (measured via hemodynamics, triphenyltetrazolium chloride stain, and apoptotic markers). Mitochondrial function at the subpopulation level was evaluated with respect to adenosine triphosphate (ATP) concentration, membrane potential, swelling behavior, and oxidative stress, wherein the results confirmed I/R-induced mitochondrial dysfunction. Unlike normal heart, DM, and DCM heart challenged to I/R exhibited altered ATP producing capacity among subsarcolemmal and interfibrillar mitochondria.

CONCLUSION

The above results suggest that mitochondrial changes associated with diabetes and cardiomyopathy significantly contribute to the adverse outcome of I/R injury.

RETRACTED: Association between peroxisome proliferator-activated receptor, UCP3 and lipoprotein lipase gene polymorphisms and obesity in Chinese adolescents

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). Please note that this retraction notice has been updated in September 2021, as follows: This article has been retracted at the request of the Editors in Chief due to concerns regarding the authorship. Certain individuals were erroneously indicated as co-authors of the article when it was originally published. These individuals have informed the journal that they did not contribute to the article and that they had no knowledge of its submission for publication. We confirm, following investigation, that those individuals previously identified by the submitting author as co-authors did not author, approve or submit this article for publication, and the previous attribution of the article to them was in error. We have not had a response from Dr Mao and Dr Cai regarding authorship. As a result of the correspondence with Associate Professor Z. Zou and Dr L. Wang, we believe that the paper needs to be retracted and have elected to proceed with retraction.

Genetic association studies of obesity in Africa: a systematic review

Obesity is increasing in Africa, but the underlying genetic background largely remains unknown. We assessed existing evidence on genetic determinants of obesity among populations within Africa. MEDLINE and EMBASE were searched and the bibliographies of retrieved articles were examined. Included studies had to report on the association of a genetic marker with obesity indices and the presence/occurrence of obesity/obesity trait. Data were extracted on study design and characteristics, genetic determinants and effect estimates of associations with obesity indices. According to this data, over 300 polymorphisms in 42 genes have been studied in various population groups within Africa mostly through the candidate gene approach. Polymorphisms in genes such as ACE, ADIPOQ, ADRB2, AGRP, AR, CAPN10, CD36, C7orf31, DRD4, FTO, MC3R, MC4R, SGIP1 and LEP were found to be associated with various measures of obesity. Of the 36 polymorphisms previously validated by genome-wide association studies (GWAS) elsewhere, only FTO and MC4R polymorphisms showed significant associations with obesity in black South Africans, Nigerians and Ghanaians. However, these data are insufficient to establish the true nature of genetic susceptibility to obesity in populations within Africa. There has been recent progress in describing the genetic architecture of obesity among populations within Africa. This effort needs to be sustained via GWAS studies.

Mitochondrial uncoupling proteins and energy metabolism

Understanding the metabolic factors that contribute to energy metabolism (EM) is critical for the development of new treatments for obesity and related diseases. Mitochondrial oxidative phosphorylation is not perfectly coupled to ATP synthesis, and the process of proton-leak plays a crucial role. Proton-leak accounts for a significant part of the resting metabolic rate (RMR) and therefore enhancement of this process represents a potential target for obesity treatment. Since their discovery, uncoupling proteins have stimulated great interest due to their involvement in mitochondrial-inducible proton-leak. Despite the widely accepted uncoupling/thermogenic effect of uncoupling protein one (UCP₁), which was the first in this family to be discovered, the reactions catalyzed by its homolog UCP₃ and the physiological role remain under debate. This review provides an overview of the role played by UCP₁ and UCP₃ in mitochondrial uncoupling/functionality as well as EM and suggests that they are a potential therapeutic target for treating obesity and its related diseases such as type II diabetes mellitus.

Genetic deletion in uncoupling protein 3 augments 18F-fluorodeoxyglucose cardiac uptake in the ischemic heart

Background

We investigated the effects of uncoupling protein 3 (UCP3) genetic deletion on ¹⁸F-fluorodeoxyglucose (FDG) cardiac uptake by positron emission tomography (PET)/computed tomography (CT) dedicated animal system after permanent coronary artery ligation.

Methods

Cardiac ¹⁸F-FDG PET/CT was performed in UCP3 knockout (UCP3^−/−) and wild-type (WT) mice one week after induction of myocardial infarction or sham procedure.

Results

In sham-operated mice no difference in left ventricular (LV) volume was detectable between WT and UCP3^−/−. After myocardial infarction, LV volume was higher in both WT and UCP3^−/− compared to sham animals, with a significant interaction (p < 0.05) between genotype and myocardial infarction. In sham-operated animals no difference in FDG standardized uptake value (SUV) was detectable between WT (1.8 ± 0.6) and UCP3^−/− (1.8 ± 0.6). After myocardial infarction SUV was significantly higher in remote areas than in infarcted territories in both UCP3−/− and WT mice (both p < 0.01). Moreover, in remote areas, SUV was significantly higher (p < 0.001) in UCP3−/− as compared to WT, while in the infarcted territory SUV was comparable (p = 0.29). A significant relationship (r = 0.68, p < 0.001) between LV volume and SUV was found.

Conclusions

In a mice model of permanent coronary occlusion, UCP3 deficiency results in a metabolic shift that favored glycolytic metabolism and increased FDG uptake in remote areas.

Obesity pharmacotherapy: current perspectives and future directions

The rising tide of obesity and its related disorders is one of the most pressing health concerns worldwide, yet existing medicines to combat the problem are disappointingly limited in number and effectiveness. Recent advances in mechanistic insights into the neuroendocrine regulation of body weight have revealed an expanding list of molecular targets for novel, rationally designed antiobesity pharmaceutical agents. Antiobesity drugs act via any of four mechanisms: 1) decreasing energy intake, 2) increasing energy expenditure or modulating lipid metabolism, 3) modulating fat stores or adipocyte differentiation, and 4) mimicking caloric restriction. Various novel drug candidates and targets directed against obesity are currently being explored. A few of them are also in the later phases of clinical trials. This review discusses the development of novel antiobesity drugs based on current understanding of energy homeostasis

Muscle uncoupling protein 3 overexpression mimics endurance training and reduces circulating biomarkers of incomplete β-oxidation

Exercise substantially improves metabolic health, making the elicited mechanisms important targets for novel therapeutic strategies. Uncoupling protein 3 (UCP3) is a mitochondrial inner membrane protein highly selectively expressed in skeletal muscle. Here we report that moderate UCP3 overexpression (roughly 3-fold) in muscles of UCP3 transgenic (UCP3 Tg) mice acts as an exercise mimetic in many ways. UCP3 overexpression increased spontaneous activity (∼40%) and energy expenditure (∼5-10%) and decreased oxidative stress (∼15-20%), similar to exercise training in wild-type (WT) mice. The increase in complete fatty acid oxidation (FAO; ∼30% for WT and ∼70% for UCP3 Tg) and energy expenditure (∼8% for WT and 15% for UCP3 Tg) in response to endurance training was higher in UCP3 Tg than in WT mice, showing an additive effect of UCP3 and endurance training on these two parameters. Moreover, increases in circulating short-chain acylcarnitines in response to acute exercise in untrained WT mice were absent with training or in UCP3 Tg mice. UCP3 overexpression had the same effect as training in decreasing long-chain acylcarnitines. Outcomes coincided with a reduction in muscle carnitine acetyltransferase activity that catalyzes the formation of acylcarnitines. Overall, results are consistent with the conclusions that circulating acylcarnitines could be used as a marker of incomplete muscle FAO and that UCP3 is a potential target for the treatment of prevalent metabolic diseases in which muscle FAO is affected.

Genipin stimulates glucose transport in C2C12 myotubes via an IRS-1 and calcium-dependent mechanism

Genipin, a compound derived from Gardenia jasminoides Ellis fruits, has been used over the years in traditional Chinese medicine to treat symptoms of type 2 diabetes. However, the molecular basis for its antidiabetic effect has not been fully revealed. In this study, we investigated the effects of genipin on glucose uptake and signaling pathways in C(2)C(12) myotubes. Our study demonstrates that genipin stimulated glucose uptake in a time- and dose-dependent manner. The maximal effect was achieved at 2 h with a concentration of 10 μM. In myotubes, genipin promoted glucose transporter 4 (GLUT4) translocation to the cell surface, which was observed by analyzing their distribution in subcellular membrane fraction, and increased the phosphorylation of insulin receptor substrate-1 (IRS-1), AKT, and GSK3β. Meanwhile, genipin increased ATP levels, closed K(ATP) channels, and then increased the concentration of calcium in the cytoplasm in C(2)C(12) myotubes. Genipin-stimulated glucose uptake could be blocked by both the PI3-K inhibitor wortmannin and calcium chelator EGTA. Moreover, genipin increases the level of reactive oxygen species and ATP in C(2)C(12) myotubes. These results suggest that genipin activates IRS-1, PI3-K, and downstream signaling pathway and increases concentrations of calcium, resulting in GLUT4 translocation and glucose uptake increase in C(2)C(12) myotubes.

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.

The Human Obesity Gene Map: The 2003 Update

This is the tenth update of the human obesity gene map, incorporating published results up to the end of October 2003 and continuing the previous format. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) from human genome-wide scans and animal crossbreeding experiments, and association and linkage studies with candidate genes and other markers is reviewed. Transgenic and knockout murine models relevant to obesity are also incorporated (N = 55). As of October 2003, 41 Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. QTLs reported from animal models currently number 183. There are 208 human QTLs for obesity phenotypes from genome-wide scans and candidate regions in targeted studies. A total of 35 genomic regions harbor QTLs replicated among two to five studies. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 272 studies reporting positive associations with 90 candidate genes. Fifteen such candidate genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, more than 430 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful sites can be found at http://obesitygene.pbrc.edu.

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 166 genes which, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

Mutational Analysis of theUCP2Core Promoter and Relationships of Variants with Obesity

OBJECTIVE

To identify polymorphisms in the human uncoupling protein 2 gene (UCP2) promoter and to investigate whether these were associated with obesity or weight gain.

RESEARCH METHODS AND PROCEDURES

The human UCP2 promoter was characterized by reporter gene analysis in cell lines derived from skeletal muscle, white adipose tissue, and embryonic tissue. We analyzed the core promoter for polymorphisms in 60 obese subjects. A prevalent polymorphism, the -866 G/A variant, was investigated for association with obesity in 749 men obese as young adults and 816 men of the same age representing the background population. Genotype-phenotype interaction studies were performed in two other population-based samples: one group of middle-aged-to-elderly Danish subjects (mean age, 53 years; range, 30 to 88 years) and one group of 60-year-old Danish subjects.

RESULTS

The region up to -1202 bp relative to the UCP2 transcription initiation site gave rise to the highest promoter activity. Eight mutations in this region were identified comprising -866 G/A, -850 G/A, -337 G/C, -41 G/T, -28 insertion T, -5 insertion (cactgcgaagccc), +45 C/T, and +53 G/C, but none of these was associated with consistent alterations in BMI, body fat content, weight gain, or fasting levels of plasma glucose and serum insulin.

DISCUSSION

Variation of the UCP2 promoter including the single common variant (-866 A/G) is not associated with obesity or obesity-related intermediary phenotypes in Danish subjects.

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.

Ucp3 Expression during Weight Gain and Loss, Cold Exposure, and Fasting in the Collared Lemming

OBJECTIVE

To determine the gene sequence and tissue distribution of uncoupling protein 3 (Ucp3) in the collared lemming, we quantified mRNA expression of Ucp3 under known states of altered energy expenditure (photoperiod-induced weight gain and loss, cold exposure, and fasting) and measured mitochondrial oxygen consumption to assess possible functional changes in energy expenditure.

RESEARCH METHODS AND PROCEDURES

The Ucp3 gene sequence information was obtained using the reverse transcription-polymerase chain reaction and rapid amplification of cDNA ends methods. Northern blots were used to determine mRNA expression levels. Respirometry was used to measure oxygen consumption rates in isolated mitochondria.

RESULTS

The lemming Ucp3 gene has a 97% sequence similarity with other published Ucp3 sequences at the amino acid level. Ucp3 mRNA is expressed in muscle, heart, and brown adipose tissue of collared lemmings. Long-photoperiod lemmings have a higher expression of Ucp3 mRNA than short-photoperiod lemmings (p < 0.001) in both muscle and brown adipose tissue. Transferring lemmings from long to short photoperiods (inducing weight gain) significantly decreased Ucp3 mRNA expression (p < 0.01), whereas transferring lemmings from short to long photoperiods (inducing weight loss) significantly increased Ucp3 expression (p < 0.001). Muscle Ucp3 mRNA expression was significantly decreased by 10 days of mild (10 degrees C) cold exposure (p < 0.001). Muscle Ucp3 mRNA expression was significantly increased by fasting (p < 0.01) and was correlated to plasma free fatty acid levels (r = 0.7). Photoperiod transfer did not alter mitochondrial coupling.

DISCUSSION

These data suggest that UCP3 may not be involved in energy expenditure in the collared lemming.

Urinary F2-isoprostanes, obesity, and weight gain in the IRAS cohort

Obesity has been associated with increased F(2)-isoprostane (F(2)-IsoP) levels cross-sectionally. However, the prospective association may be inverse, based on our earlier finding that elevated urinary F(2)-IsoP levels predict lower risk of diabetes. This earlier finding led us to hypothesize that urinary F(2)-IsoPs reflect the intensity of oxidative metabolism and as such predict lower risk of both diabetes and weight gain. We examined cross-sectional relationships with obesity and prospective relationships with weight gain using the data from 299 participants of the Insulin Resistance Atherosclerosis Study (IRAS), all of whom were free of diabetes at baseline. Four urinary F(2)-IsoPs were assayed in stored baseline urine samples using liquid chromatography with tandem mass spectrometry: iPF(2α)-III, 2,3-dinor-iPF(2α)-III, iPF(2α)-VI, and 8,12-iso-iPF(2α)-VI (F(2)-IsoP 1-4, respectively). Baseline F(2)-IsoPs were positively associated with baseline measures of obesity; the strongest associations were found with two F(2)-IsoPs: odds ratios (95% confidence intervals) for overall and abdominal obesity were 1.74 (1.26-2.40) and 1.63 (1.18-2.24) for F(2)-IsoP2 and 1.47 (1.12-1.94) and 1.64 (1.22-2.20) for F(2)-IsoP4. F(2)-IsoP2 showed the strongest and significant inverse association with weight gain during the 5-year follow-up period: increase in F(2)-IsoP2 equal to 1 s.d. was associated with 0.90 kg lower weight gain (P = 0.02) and the odds ratios for relative (≥5%) and absolute (≥5 kg) weight gain were 0.67 (0.47-0.96) and 0.57 (0.37-0.87), respectively. The other three F(2)-IsoPs were consistently inversely associated with weight gain, although not significantly, suggesting that different F(2)-IsoPs vary in their ability to detect the association with weight gain.

Four novel UCP3 gene variants associated with childhood obesity: effect on fatty acid oxidation and on prevention of triglyceride storage

OBJECTIVE

The objective of the study was to look for uncoupling protein 3 (UCP3) gene variants in early-onset severe childhood obesity and to determine their effect on long-chain fatty acid oxidation and triglyceride storage.

METHODS AND RESULTS

We identified four novel mutations in the UCP3 gene (V56M, A111V, V192I and Q252X) in 200 children with severe, early-onset obesity (body mass index-standard deviation score >2.5; onset: <4 years) living in Southern Italy. We evaluated the role of wild-type (wt) and mutant UCP3 proteins in palmitate oxidation and in triglyceride storage in human embryonic kidney cells (HEK293). Palmitate oxidation was ∼60% lower (P<0.05; P<0.01) and triglyceride storage was higher in HEK293 cells expressing the four UCP3 mutants than in cells expressing wt UCP3. Moreover, mutants V56M and Q252X exerted a dominant-negative effect on wt protein activity (P<0.01 and P<0.05, respectively). Telmisartan, an angiotensin II receptor antagonist used in the management of hypertension, significantly (P<0.05) increased palmitate oxidation in HEK293 cells expressing wt and mutant proteins (P<0.05; P<0.01), including the dominant-negative mutants.

CONCLUSIONS

These data indicate that protein UCP3 affects long-chain fatty acid metabolism and can prevent cytosolic triglyceride storage. Our results also suggest that telmisartan, which increases fatty acid oxidation in rat skeletal muscle, also improves UCP3 wt and mutant protein activity, including the dominant-negative UCP3 mutants.

(-)-Epigallocatechin-3-gallate increases the expression of genes related to fat oxidation in the skeletal muscle of high fat-fed mice

(-)-Epigallocatechin-3-gallate (EGCG), the major polyphenol in green tea, has been shown to prevent the development of obesity in rodent models. Here, we examined the effect of EGCG on markers of fat oxidation in high fat-fed C57bl/6J mice. High fat-fed mice treated with 0.32% dietary EGCG for 16 weeks had reduced body weight gain and final body weight (19.2% and 9.4%, respectively) compared to high fat-fed controls. EGCG-treatment decreased fasting blood glucose, plasma insulin, and insulin resistance by 18.5%, 25.3%, and 33.9%, respectively. EGCG treatment also reduced markers of obesity-related fatty liver disease in high fat-fed mice. Gene expression analysis of skeletal muscle showed that EGCG increased mRNA levels of nuclear respiratory factor (nrf)1, medium chain acyl coA decarboxylase (mcad), uncoupling protein (ucp)3, and peroxisome proliferator responsive element (ppar)α by 1.4-1.9-fold compared to high fat-fed controls. These genes are all related to mitochondrial fatty acid oxidation. In addition, EGCG increased fecal excretion of lipids in high fat-fed mice. In summary, it appears that EGCG modulates body weight gain in high fat-fed mice both by increasing the expression of genes related fat oxidation in the skeletal muscle and by modulating fat absorption from the diet.

Plasma metabolomic profiles reflective of glucose homeostasis in non-diabetic and type 2 diabetic obese African-American women

Insulin resistance progressing to type 2 diabetes mellitus (T2DM) is marked by a broad perturbation of macronutrient intermediary metabolism. Understanding the biochemical networks that underlie metabolic homeostasis and how they associate with insulin action will help unravel diabetes etiology and should foster discovery of new biomarkers of disease risk and severity. We examined differences in plasma concentrations of >350 metabolites in fasted obese T2DM vs. obese non-diabetic African-American women, and utilized principal components analysis to identify 158 metabolite components that strongly correlated with fasting HbA1c over a broad range of the latter (r = −0.631; p<0.0001). In addition to many unidentified small molecules, specific metabolites that were increased significantly in T2DM subjects included certain amino acids and their derivatives (i.e., leucine, 2-ketoisocaproate, valine, cystine, histidine), 2-hydroxybutanoate, long-chain fatty acids, and carbohydrate derivatives. Leucine and valine concentrations rose with increasing HbA1c, and significantly correlated with plasma acetylcarnitine concentrations. It is hypothesized that this reflects a close link between abnormalities in glucose homeostasis, amino acid catabolism, and efficiency of fuel combustion in the tricarboxylic acid (TCA) cycle. It is speculated that a mechanism for potential TCA cycle inefficiency concurrent with insulin resistance is “anaplerotic stress” emanating from reduced amino acid-derived carbon flux to TCA cycle intermediates, which if coupled to perturbation in cataplerosis would lead to net reduction in TCA cycle capacity relative to fuel delivery.

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.

Plasma acylcarnitine profiles suggest incomplete long-chain fatty acid beta-oxidation and altered tricarboxylic acid cycle activity in type 2 diabetic African-American women

Inefficient muscle long-chain fatty acid (LCFA) combustion is associated with insulin resistance, but molecular links between mitochondrial fat catabolism and insulin action remain controversial. We hypothesized that plasma acylcarnitine profiling would identify distinct metabolite patterns reflective of muscle fat catabolism when comparing individuals bearing a missense G304A uncoupling protein 3 (UCP3 g/a) polymorphism to controls, because UCP3 is predominantly expressed in skeletal muscle and g/a individuals have reduced whole-body fat oxidation. MS analyses of 42 carnitine moieties in plasma samples from fasting type 2 diabetics (n = 44) and nondiabetics (n = 12) with or without the UCP3 g/a polymorphism (n = 28/genotype: 22 diabetic, 6 nondiabetic/genotype) were conducted. Contrary to our hypothesis, genotype had a negligible impact on plasma metabolite patterns. However, a comparison of nondiabetics vs. type 2 diabetics revealed a striking increase in the concentrations of fatty acylcarnitines reflective of incomplete LCFA beta-oxidation in the latter (i.e. summed C10- to C14-carnitine concentrations were approximately 300% of controls; P = 0.004). Across all volunteers (n = 56), acetylcarnitine rose and propionylcarnitine decreased with increasing hemoglobin A1c (r = 0.544, P < 0.0001; and r = -0.308, P < 0.05, respectively) and with increasing total plasma acylcarnitine concentration. In proof-of-concept studies, we made the novel observation that C12-C14 acylcarnitines significantly stimulated nuclear factor kappa-B activity (up to 200% of controls) in RAW264.7 cells. These results are consistent with the working hypothesis that inefficient tissue LCFA beta-oxidation, due in part to a relatively low tricarboxylic acid cycle capacity, increases tissue accumulation of acetyl-CoA and generates chain-shortened acylcarnitine molecules that activate proinflammatory pathways implicated in insulin resistance.

African genetic diversity: implications for human demographic history, modern human origins, and complex disease mapping

Comparative studies of ethnically diverse human populations, particularly in Africa, are important for reconstructing human evolutionary history and for understanding the genetic basis of phenotypic adaptation and complex disease. African populations are characterized by greater levels of genetic diversity, extensive population substructure, and less linkage disequilibrium (LD) among loci compared to non-African populations. Africans also possess a number of genetic adaptations that have evolved in response to diverse climates and diets, as well as exposure to infectious disease. This review summarizes patterns and the evolutionary origins of genetic diversity present in African populations, as well as their implications for the mapping of complex traits, including disease susceptibility.

Genome-wide linkage screen for stature and body mass index in 3.032 families: evidence for sex- and population-specific genetic effects

Stature (adult body height) and body mass index (BMI) have a strong genetic component explaining observed variation in human populations; however, identifying those genetic components has been extremely challenging. It seems obvious that sample size is a critical determinant for successful identification of quantitative trait loci (QTL) that underlie the genetic architecture of these polygenic traits. The inherent shared environment and known genetic relationships in family studies provide clear advantages for gene mapping over studies utilizing unrelated individuals. To these ends, we combined the genotype and phenotype data from four previously performed family-based genome-wide screens resulting in a sample of 9.371 individuals from 3.032 African-American and European-American families and performed variance-components linkage analyses for stature and BMI. To our knowledge, this study represents the single largest family-based genome-wide linkage scan published for stature and BMI to date. This large study sample allowed us to pursue population- and sex-specific analyses as well. For stature, we found evidence for linkage in previously reported loci on 11q23, 12q12, 15q25 and 18q23, as well as 15q26 and 19q13, which have not been linked to stature previously. For BMI, we found evidence for two loci: one on 7q35 and another on 11q22, both of which have been previously linked to BMI in multiple populations. Our results show both the benefit of (1) combining data to maximize the sample size and (2) minimizing heterogeneity by analyzing subgroups where within-group variation can be reduced and suggest that the latter may be a more successful approach in genetic mapping.

Human uncoupling protein 2 and 3 genes are associated with obesity in Japanese

Human uncoupling proteins (UCPs) are mitochondrial proteins that are involved in the control of energy metabolism and the pathophysiology of obesity. Although there have been several reports on the association between the UCP2/UCP3 locus and the obesity, there have been no haplotype-based case-control studies with gender-specific analysis. The aim of this study was to examine whether there is an association between the UCP2/UCP3 locus and the obesity in the Japanese population when using a single nucleotide polymorphism (SNP)-based and haplotype-based case-control study with gender-specific analysis. We examined a group consisting of 551 subjects, of which 369 were non-obese and 182 were overweight and/or obese. We selected one nonsynonymous SNP (rs660339: Ala55Val) as a genetic marker. Genotyping for all subjects was performed by the TaqMan polymerase chain reaction (PCR) method. Although the overall distributions of genotype and allele were not significantly different between the non-obese and the obese groups, the overall distributions of the genotype were significantly different in men (P = 0.030). In the obese group, male subjects with the Val allele were significantly more frequent in both association studies. There was a significant difference in the overall distribution of the haplotype (UCP3 rs180049, UCP3 rs2075577, UCP2 rs660339) between the weight groups (P = 0.010), and in women, there was a significant difference (P = 0.042) in the overall distribution of the haplotype (UCP3 rs2075577, UCP2 rs660339). Nonsynonymous rs660339 in the human UCP2 gene in men, and the haplotype (UCP3 rs2075577-UCP2 rs660339) in women might be good obesity markers.

Ethnogenetic layering (EL): an alternative to the traditional race model in human variation and health disparity studies

BACKGROUND

Traditionally, studies in human biodiversity, disease risk, and health disparities have defined populations in the context of typological racial models. However, such racial models are often imprecise generalizations that fail to capture important local patterns of human biodiversity.

AIM

More explicit, detailed, and integrated information on relevant geographic, environmental, cultural, genetic, historical, and demographic variables are needed to understand local group expressions of disease inequities. This paper details the methods used in ethnogenetic layering (EL), a non-typological alternative to the current reliance of the biological racial paradigm in public health, epidemiology, and biomedicine.

SUBJECTS AND METHODS

EL is focused on geographically identified microethnic groups or MEGs, a more nuanced and sensitive level of analysis than race. Using the MEG level of analysis, EL reveals clinical variations, details the causes of health disparities, and provides a foundation for bioculturally effective intervention strategies. EL relies on computational approaches by using GIS-facilitated maps to produce horizontally stratified geographical regional profiles which are then stacked and evaluated vertically. Each horizontal digital map details local geographic variation in the attributes of a particular database; usually this includes data on local historical demography, genetic diversity, cultural patterns, and specific chronic disease risks (e.g. dietary and toxicological exposures). Horizontal visual display of these layered maps permits vertical analysis at various geographic hot spots.

RESULTS AND CONCLUSIONS

From these analyses, geographical areas and their associated MEGs with highly correlated chronic disease risk factors can be identified and targeted for further study.

Essential role for uncoupling protein-3 in mitochondrial adaptation to fasting but not in fatty acid oxidation or fatty acid anion export

Uncoupling protein-3 (UCP3) is a mitochondrial inner membrane protein expressed most abundantly in skeletal muscle and to a lesser extent in heart and brown adipose tissue. Evidence supports a role for UCP3 in fatty acid oxidation (FAO); however, the underlying mechanism has not been explored. In 2001 we proposed a role for UCP3 in fatty acid export, leading to higher FAO rates (Himms-Hagen, J., and Harper, M. E. (2001) Exp. Biol. Med. (Maywood) 226, 78-84). Specifically, this widely held hypothesis states that during elevated FAO rates, UCP3 exports fatty acid anions, thereby maintaining mitochondrial co-enzyme A availability; reactivation of exported fatty acid anions would ultimately enable increased FAO. Here we tested mechanistic aspects of this hypothesis as well as its functional implications, namely increased FAO rates. Using complementary mechanistic approaches in mitochondria from wild-type and Ucp3(-/-) mice, we find that UCP3 is not required for FAO regardless of substrate type or supply rate covering a 20-fold range. Fatty acid anion export and reoxidation during elevated FAO, although present in skeletal muscle mitochondria, are independent of UCP3 abundance. Interestingly, UCP3 was found to be necessary for the fasting-induced enhancement of FAO rate and capacity, possibly via mitigated mitochondrial oxidative stress. Thus, although our observations indicate that UCP3 can impact FAO rates, the mechanistic basis is not via an integral function for UCP3 in the FAO machinery. Overall our data indicate a function for UCP3 in mitochondrial adaptation to perturbed cellular energy balance and integrate previous observations that have linked UCP3 to reduced oxidative stress and FAO.

Genetic variants in the UCP2-UCP3 gene cluster and risk of diabetes in the Women's Health Initiative Observational Study

OBJECTIVE

Mitochondrial uncoupling proteins (UCPs) are involved in body weight regulation and glucose homeostasis. Genetic variants in the UCP2-UCP3 gene cluster, located on chromosome 11q13, may play a significant role in the development of type 2 diabetes.

RESEARCH DESIGN AND METHODS

We conducted a comprehensive assessment of common single nucleotide polymorphisms (SNPs) at the 70-kb UCP2-UCP3 gene cluster in relation to type 2 diabetes risk in a prospective, case-control study nested in the Women's Health Initiative Observational Study, an ethnically diverse cohort of postmenopausal women including Caucasian, African, Hispanic, and Asian American subjects. We genotyped 14 tag SNPs in 1,584 incident type 2 diabetes case and 2,198 control subjects matched by age, ethnicity, clinical center, time of blood draw, and length of follow-up.

RESULTS

We identified a haplotype set (rs591758-rs668514- rs647126-rs1800006, spanning the UCP2-UCP3 intergenic and UCP3 regions) as significantly associated with greater type 2 diabetes risk (nominal P = 0.0011, permutation P = 0.046) in Caucasian women, especially among overweight Caucasians (BMI >25 kg/m(2)) (nominal P = 0.0006, permutation P = 0.032). Compared with the most common haplotype (h1010 as the referent), haplotype h0001 (19.5% in control subjects) had odds ratios of 2.0 (95% CI 1.13-3.37) in Caucasians and 3.8 (1.44-9.93) in Caucasian overweight women. Similar haplotype-type 2 diabetes association was also observed among Hispanic women who were overweight.

CONCLUSIONS

These findings suggest a role of UCP2-UCP3 gene cluster haplotypes in diabetes; in particular, the effects of the high-risk haplotypes were more apparent in overweight Caucasian women. These data warrant further confirmation in future prospective and experimental studies.

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.

Obesity and polymorphisms in genes regulating human adipose tissue

Obesity is the result of an imbalance between food intake and energy expenditure resulting in the storing of energy as fat. Adipose tissue contains the largest store of energy in the body and plays important roles in regulating energy partitioning. Developments in genomics, in particular microarray-based expression profiling, have provided scientists with a number of new candidate genes whose expression in adipose tissue is regulated by obesity. Integrating expression profiles with genome-wide linkage and/or association analyses is a promising strategy to identify new genes underlying susceptibility to obesity. This article provides a comprehensive review of adipose-tissue-expressed genes implicated in predisposition to human obesity. The authors consider the following genes of particular interest: peroxisome proliferator-activated receptor gamma and, potentially, INSIG2 acting in adipogenesis; the adrenoreceptors beta 2 and 3, as well as hormone-sensitive lipase acting on lipolysis; uncoupling protein 2 acting in mitochondria energy expenditure; and among secreted molecules the cytokine tumor necrosis factor alpha and the hormone leptin. With the rapid development in genome research, we predict that additional alleles in genes regulating adipose tissue function will be established as risk factors for common obesity in the coming years. This has important implications for the prevention of obesity and may also offer new therapeutic targets.

Medical sequencing at the extremes of human body mass

Body weight is a quantitative trait with significant heritability in humans. To identify potential genetic contributors to this phenotype, we resequenced the coding exons and splice junctions of 58 genes in 379 obese and 378 lean individuals. Our 96-Mb survey included 21 genes associated with monogenic forms of obesity in humans or mice, as well as 37 genes that function in body weight-related pathways. We found that the monogenic obesity-associated gene group was enriched for rare nonsynonymous variants unique to the obese population compared with the lean population. In addition, computational analysis predicted a greater fraction of deleterious variants within the obese cohort. Together, these data suggest that multiple rare alleles contribute to obesity in the population and provide a medical sequencing-based approach to detect them.

The role of uncoupling protein 3 in fatty acid metabolism: protection against lipotoxicity?

The physiological function of the mitochondrial uncoupling protein (UCP), UCP3, is still under debate. There is, however, ample evidence to indicate that, in contrast to UCP1, the primary function of UCP3 is not the dissipation of energy. Rather, several lines of evidence suggest that UCP3 is associated with cellular fatty acid metabolism. The highest levels of expression of UCP3 have been found in type 2 glycolytic muscle fibres, and fasting and high-fat diets up regulate UCP3. This up-regulation is most pronounced in muscle with a low fat oxidative capacity. Acute exercise also up regulates UCP3, and this effect has been shown to be a result of the exercise-induced increase in plasma fatty acid levels. In contrast, regular physical activity, which increases fat oxidative capacity, reduces UCP3 content. Based on these data it has been postulated that UCP3 functions to export those fatty acids that cannot be oxidized from the mitochondrial matrix, in order to prevent fatty acid accumulation inside the matrix. Several experiments have been conducted to test this hypothesis. Blocking carnitine palmitoyltransferase 1, thereby reducing fat oxidative capacity, rapidly induces UCP3. High-fat diets, which increase the mitochondrial supply of fatty acids, also up regulate UCP. However, feeding a similar amount of medium-chain fatty acids, which can be oxidized inside the mitochondrial matrix and therefore does not need to be exported from the matrix, does not affect UCP3 protein levels. In addition, UCP3 is increased in patients with defective β-oxidation and is reduced after restoring oxidative capacity. In conclusion, it is suggested that UCP3 has an important physiological function in facilitating outward transport from the mitochondrial matrix of fatty acid anions that cannot be oxidized, thereby protecting against lipid-induced mitochondrial damage.

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.

Uncoupling proteins: role in insulin resistance and insulin insufficiency

Uncoupling proteins (UCPs) are modulators of mitochondrial metabolism that have been implicated in the development of both insulin resistance and insulin insufficiency, the two major pathophysiological events associated with type 2 diabetes. UCP2 mRNA is expressed in a wide range of tissues; however UCP2 protein expression is restricted to fewer tissues, including the endocrine pancreas, spleen, stomach, brain and the lung. To date, its role in the pathophysiology of diabetes has been most strongly associated with impaired glucose-stimulated insulin secretion from the beta-cell, particularly after its induction by free fatty acids. The physiological role of UCP2 remains controversial, but it may act as a downstream signal transducer of superoxide. UCP3 mRNA and protein are expressed in relatively few tissues, predominantly skeletal muscle, brown adipose tissue and heart. Increased expression of UCP3 in skeletal muscle is associated with protection from diet-induced insulin resistance in mice. In patients with type 2 diabetes UCP3 protein in muscle is reduced by 50% compared to healthy controls. The primary physiological role of the novel UCPs does not appear to be protection against positive energy balance and obesity; this is based largely on findings from studies of UCP2 and UCP3 knockout mice and from observed increases in UCP3 expression with fasting. The mechanism(s) of action of UCP2 and UCP3 are poorly understood. However, findings support roles for UCP2 and UCP3 as modifiers of fatty acid metabolism and in mitigating damage from reactive oxygen species.

The human mitochondrial transport/carrier protein family. Nonsynonymous single nucleotide polymorphisms (nsSNPs) and mutations that lead to human diseases

There are 67 proteins in the human mitochondrial transport protein family. They have been identified from among the proteins of the RefSeq database on the basis of sequence similarity to proteins that have been functionally identified as mitochondrial transport proteins. They have also been identified by matching their predicted structure to the high resolution structure of the bovine ADP/ATP T1 transporter subunit/carboxyatractyloside complex. 74 nonsynonymous single nucleotide polymorphisms (nsSNP) have been identified in their gene sequences. These nsSNPs are present in genes of 30 of the proteins. No nsSNP has been found in 24 of the protein genes and no search has as yet been carried out on the rest (13) of them. The largest number of nsSNPs are in the ADP/ATP T3 transporter, the uncoupling protein 3 L, and the phosphate transporter genes with 7, 6, and 6, respectively. nsSNPs are located in groups along the protein sequence suggesting that certain protein domains are too critical for transport function to tolerate mutations. This interpretation has been validated with mutation and function studies of the phosphate transporter. Human diseases have been identified with replacement mutations in seven of these proteins. Their genes are not abnormally susceptible to mutations since they have the smallest number of nsSNPs. Disease causing mutations have also been observed as: substitution, silent (may affect stability of messages), frameshift (protein truncation or elongation), splicing (exon skipping), residue deletion. Disease causing mutations have only been identified in few transporter genes because others do not yield dramatic symptoms or are essential and thus lethal. Mutations in other transporter genes may also only have a major impact through their combination with other genes and their nsSNPs.

Variation in the UCP2-UCP3 gene cluster predicts the development of type 2 diabetes in healthy middle-aged men

The impact of the UCP2 -866G>A and UCP3 -55C>T variants on prospective risk of type 2 diabetes was examined over 15 years in 2,936 healthy middle-aged men (mean age 56 years). Conversion to diabetes (n = 169) was associated with higher BMI, blood pressure, cholesterol, triglycerides and C-reactive protein. The hazard ratio (HR) for diabetes of a BMI >30 kg/m(2) was 3.96 (95% CI 2.87-5.47). Homozygosity for the UCP2A or UCP3T alleles accelerated the onset of diabetes, with significant differences in risk of diabetes at 10 years (HR [95% CI] UCP2AA vs. GA+GG 1.94 [1.18-3.19], P = 0.009; UCP3TT vs. CC+ CT 2.06 [1.06-3.99], P = 0.03) but less so at 15 years (UCP2AA 1.42 [0.92-2.19], P = 0.1; UCP3TT 1.57 [0.87-2.04], P = 0.13). Men who were homozygous for both UCP2AA and UCP3TT (1.5% of men) had a risk for diabetes at 10 years of 4.20 (1.70-10.37), P = 0.002. These genotype effects were additive with obesity, and men with a BMI >30 kg/m(2) and this genotype combination had a 10-year risk of diabetes of 19.23 [5.63-63.69], P < 0.0001. Functional promoter variants UCP2 and UCP3 increase the prospective risk of diabetes. Although the mechanism of the UCP2 effect is likely to be caused by increased expression in the pancreas and subsequent reduced insulin secretion, the mechanism of the UCP3 effect is currently unknown. Both effects are exacerbated by obesity.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.

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.

Gene Polymorphism Association Studies in Dialysis: The Nutrition-Inflammation Axis

Although traditional risk factors for cardiovascular disease are common in dialysis patients, they alone cannot explain the unacceptably high prevalence of vascular disease in this patient group. Much recent interest has therefore focused on the role of various nontraditional cardiovascular risk factors, such as inflammation, wasting, obesity, vascular calcification, and oxidative stress. In addition, genetic factors such as single nucleotide polymorphisms (SNPs) may significantly influence the immune response, the levels of inflammatory markers and body composition, as well as the prevalence of vascular calcification in this patient group. While genetic variations in the tumor necrosis factor (TNF)-alpha-308 and interleukin (IL)-10 -1082 SNPs seem to be consistently associated with adverse clinical outcome in end-stage renal disease (ESRD) patients, the results regarding genetic variations in the IL-6 gene have been conflicting. To elucidate the respective role of DNA polymorphisms in the IL-6 and C-reactive protein (CRP) genes, as well as genes that encode vascular calcification inhibitors (such as fetuin-A, matrix Gla protein, and osteoprotegerin), sufficiently powered studies are needed in which both the protein product and the specific phenotype are determined. In addition, polymorphisms in genes related to body composition may be excellent candidates for analysis in the ESRD population, since nutritional parameters are strongly associated with adverse events in these patients. It seems conceivable that in the future, prognostic or predictive multigene DNA assays (which allow a simultaneous and rapid assessment of multiple genetic variants) will provide nephrologists with a more precise approach for the identification of "high-risk" ESRD patients and the development of accurate individualized treatment strategies.

Influence of size at birth on the endocrine profiles and expression of uncoupling proteins in subcutaneous adipose tissue, lung, and muscle of neonatal pigs

Epidemiological studies suggest that infants of low birth weight show poor neonatal growth and increased susceptibility to adult diseases such as diabetes and lung disease. Uncoupling protein 2 and 3 (UCP2 and UCP3) have been implicated in the development of such diseases; pigs provide an ideal model to examine the influence of birth weight due to the natural variance in piglet weight within a litter. This study examined whether birth weight influences the expression of UCP2 and UCP3 in adipose tissue, skeletal muscle, and lung. Piglets from 11 litters were ranked according to birth weight and three from each litter assigned to small (SFD), normal (NFD), or large for dates (LFD) groups. Blood samples and morphometric measurements were taken over the first 14 days of life, and tissue samples were taken on day 7 or 14. Plasma hormone and metabolite concentrations and the expression of UCP2 and UCP3 mRNA in adipose tissue, skeletal muscle, and lung were measured. UCP2 and UCP3 expression in adipose tissue was lower in the SFD compared with the LFD group on day 7. UCP3 expression in skeletal muscle was higher than that of adipose tissue. Lung UCP2 and skeletal muscle UCP3 mRNA expression were unaffected by size at birth. Regression analysis indicated that UCP3 expression was differentially associated with IGF-1, leptin, and insulin. In conclusion, low birth weight is associated with tissue-specific effects on UCP expression. It remains to be established whether these subsequently contribute to pathological conditions such as diabetes.