Uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) are mitochondrial proteins that may play a role in the control of energy expenditure by ...
International Journal of Obesity (1999) 23, Suppl 6, S64±S67 ß 1999 Stockton Press All rights reserved 0307±0565/99 $12.00 http://www.stockton-press.co.uk/ijo
Uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) expression in adipose tissue and skeletal muscle in humans D Langin1*, D Larrouy1, P Barbe1, L Millet1, N Viguerie-Bascands1, F Andreelli2, M Laville2 and H Vidal2 1
UniteÂ INSERM 317, Institut Louis Bugnard, UniversiteÂ Paul Sabatier, HoÃpital Rangueil, Toulouse, France; and 2UniteÂ INSERM 449 et Centre de Recherche en Nutrition Humaine de Lyon, FaculteÂ de MeÂdecine LaeÈnnec, Lyon, France
Uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) are mitochondrial proteins that may play a role in the control of energy expenditure by uncoupling respiration from ATP synthesis. The present review focuses on data obtained in humans. UCP2 is widely expressed in the body, whereas UCP3 expression is restricted to skeletal muscle. Positive correlations have been reported between UCP2 mRNA concentrations in adipose tissue, UCP3 mRNA concentrations in skeletal muscle, and components of the metabolic rate. Fasting induces an up-regulation of UCP2 and UCP3 mRNA expression. In vivo and in vitro studies suggest that fatty acids could modulate uncoupling protein gene expression. The putative relationship between obesity, energy expenditure and uncoupling protein expression, and the unexpected rise in UCP2 and UCP3 mRNA concentrations during short-term fasting, are discussed in view of the recent data obtained in rodents and cell lines. Keywords: energy expenditure; fatty acid; adipose tissue; skeletal muscle; obesity;
Uncoupling protein-2 (UCP2) and uncoupling protein-3 (UCP3) expression and isoforms in humans The two novel uncoupling proteins known as UCP2 and UCP3 may be the long-awaited `missing link' in the understanding of the molecular basis of energy expenditure in humans. The resting metabolic rate (RMR), that is, the obligatory energy expenditure required to maintain physiological tissue function in the resting state, is the largest component of daily energy expenditure. A substantial part of the RMR results from a leaking of protons across the mitochondrial inner membrane, which results in energy dissipation because of uncoupling of oxygen consumption to ATP synthesis.1,2 UCP2 and UCP3 are candidates to explain the proton leak in many tissues.3 ± 6 UCP2 and UCP3 expression in yeast indeed causes a decrease in mitochondrial membrane potential. The two proteins show sequence identity with UCP1, an uncoupling protein expressed in brown adipose tissue (BAT). In rodents, but probably not in adult humans, BAT is an important site of adaptive thermogenesis.7 The tissue distribution of UCP2 and
Correspondence: Dr Dominique Langin, INSERM U317, Institut Louis Bugnard, BaÃtiment L3, CHU Rangueil, 31403 Toulouse Cedex 4, France.
UCP3 mRNAs are markedly different. UCP2 mRNA is widely expressed in the body whereas the main site of UCP3 mRNA expression is skeletal muscle in adult humans. UCP3 mRNA exists as long and short form transcripts.3 The two transcripts are generated from a single gene, through alternative splicing and use of polyadenylation signals.8 The short form transcript encodes a putative protein, designated UCP3S, that does not contain the last 37 amino acids present in the long form UCP3 (UCP3L), UCP1 and UCP2. This region contains the putative sixth transmembrane domain and motifs likely to be critical for uncoupling activity. UCP3S could show an increased uncoupling activity because of the lack of a conserved motif that mediates inhibition of UCP1 uncoupling activity by purine nucleotides. It is also possible that UCP3S may not be stable or may not be functional in the mitochondrial inner membrane. The effect of mutations recently found in the human UCP3 gene9 is in accordance with a defective function of UCP3S. A mutation at the exon 6-splice donor site, detected in African-Americans, results in the premature termination of the protein product which is identical to UCP3S. Heterozygotes for the mutation showed a 50% reduction in fat oxidation and an elevation of the nonprotein respiratory quotient (RQ), compared with wild-type subjects. The frequency of the mutation was twice as high in obese, compared with lean, individuals. Variation in the ratio between UCP3L and UCP3S could therefore modulate skeletal muscle uncoupling activity.
Tissue UCP2 and UCP3 D Langin et al
Uncoupling protein mRNA concentrations, energy expenditure and obesity Because a low rate of energy expenditure is a predisposing factor for weight gain and the potential link between uncoupling protein expression and RMR, several laboratories, including ours, have sought to determine the factors modulating UCP2 and UCP3 gene expression in human skeletal muscle and adipose tissue. We have developed a reverse transcriptioncompetitive polymerase chain reaction (RT-cPCR) assay to determine UCP2 and UCP3 mRNA concentrations.11 Because of the important differences between UCP3L and UCP3S, the two mRNAs were quanti®ed separately.12 In the vastus lateralis muscle, UCP3 mRNA is more abundant than UCP2 mRNA, as also found in rodents.3,6 UCP3L and UCP3S are expressed in equal amounts. No large difference in the level of the three transcripts was observed between Caucasian obese and lean individuals.11,12 Using in situ hybridisation, a 30% lower UCP2 mRNA expression was reported by Norfors et al13 in skeletal muscle of obese subjects. In agreement with our data,11 UCP3 mRNA concentrations were similar in the two groups. In contrast, Schrauwen et al14 showed a negative correlation between UCP3L mRNA concentrations and body mass index (BMI) in Pima Indians that was not found with UCP2 mRNA levels. The lower susceptibility to obesity of Caucasians compared with Pima Indians might explain such a discrepancy. Studies on a large number of individuals with a wide range of BMI are clearly needed to clarify this point. Nevertheless, in line with a putative role of skeletal muscle UCP3 in the control of energy expenditure, a positive correlation was found in Pima Indians between sleeping metabolic rate (SMR) adjusted for fat-free mass (FFM) and fat mass (FM) and UCP3L mRNA concentrations.14 In subcutaneous adipose tissue, we found a positive correlation between BMI and UCP2 mRNA concentrations.11 This data is consistent with the higher UCP2 mRNA concentration found in white adipose tissue of ob=ob and db=db obese mice, compared with lean littermates,5 suggesting a possible relationship between the concentration of UCP2 mRNA and fat cell hypertrophy. A potential link between UCP2 and energy expenditure was studied in obese premenopausal women.15 After standardisation of food intake by a four week very-low-calorie diet (VLCD), a positive correlation was found between subcutaneous adipose tissue UCP2 mRNA concentration and RMR adjusted for FFM, but the correlation failed to reach signi®cance before the diet. Factors other than RMR (for example, the diet composition) are therefore likely to be associated with UCP2 mRNA concentrations in adipose tissue. A strong correlation was found between subcutaneous and visceral adipose tissue UCP2 mRNA concentrations. The two fat depots
represent most of the adipose tissue mass in humans. Therefore, the data suggest that the concentration of UCP2 mRNA in subcutaneous adipose tissue re¯ects the overall level of UCP2 gene expression in total body fat. Hence, the positive correlation found between RMR and subcutaneous adipose tissue UCP2 mRNA concentration might be extended to UCP2 mRNA concentration in whole body fat. Oberko¯er et al16 have reported a reduced UCP2 mRNA expression in visceral adipose tissue of a large number of morbidly obese subjects. As the authors indicate, the possible role of the lower UCP2 expression in the pathophysiology of obesity remains to be proven. In rodents, a potential role for white adipose tissue (WAT) UCP2 in the regulation of body weight and energy expenditure is suggested by several lines of evidence. A high fat diet increases WAT UCP2 gene expression in the obesity-resistant A=J and C57BL= KsJ strains, but not in the obesity-prone C57BL=6J mice. Interestingly, the diet does not affect UCP2 and UCP3 mRNA expression in skeletal muscle.4,17 UCP1-de®cient mice do not become obese and it was proposed that the lack of UCP1 may be compensated by UCP2.18 Moreover, the ectopic expression of UCP1 in WAT results, in transgenic mice, in a decrease of adiposity attributed to an increase of energy dissipation in this tissue.19, 20 A similar role for adipose tissue UCP2 in energy dissipation remains to be demonstrated.
Variations in uncoupling protein gene expression during fasting Possible modulation of uncoupling protein gene expression was studied during a ®ve-day severe calorie restriction, a situation associated with a decrease in energy expenditure.11,12 Unexpectedly, the fasting condition induced a 2 ± 3 fold increase of UCP2, UCP3L and UCP3S mRNA concentrations. Similar ®ndings have been reported in rodents.21,22 The induction was similar in lean and obese subjects suggesting that, during fasting, there is no major alteration of uncoupling protein gene regulation in obesity. The signi®cance of this observation is not clear at the present time. Study of UCP2 and UCP3 gene expression during fasting and refeeding in rodents has led to the proposal that UCP2 and UCP3 would act as regulators of lipids as a fuel substrate rather than as a mediator of regulatory thermogenesis.22 In agreement with this hypothesis, mutations in the human UCP3 gene have been associated with a decreased capacity of fat oxidation.9 The hypothesis needs, however, to be substantiated by studying the relationship between mitochondrial uncoupling proteins and intracellular lipid metabolism. It is important to stress that the increase in uncoupling protein mRNA concentrations might be transient and modulated by
Tissue UCP2 and UCP3 D Langin et al
calorie intake. During a four week VLCD providing more energy per day than the ®ve day severe calorie restricted diet, no changes in subcutaneous adipose tissue UCP2 mRNA concentrations were observed.15 This situation is reminiscent of the effect of food intake on the UCP3 mRNA concentration in rodent skeletal muscle.23 A 48 h fast markedly increased the UCP3 mRNA concentration, whereas one week of 50% food restriction resulted in a decrease of UCP3 mRNA expression. A biphasic regulation is also found during exercise and endurance training in rodents. A single bout of exercise results in a transient increase of UCP3 mRNA concentrations.24 On the other hand, skeletal muscle UCP2 and UCP3 mRNA concentrations were reduced after an eight week endurance training.25 In both calorie restriction and exercise, the short-term up-regulation of uncoupling protein mRNA concentrations seems to be transient, maybe as discussed below, in response to acute metabolic and hormonal changes. The long-term down-regulation is consistent with the increased feeding ef®ciency which contributes to the increased weight gain observed after cessation of endurance training or calorie restriction. Fasting provokes a complex physiological adaptation with numerous hormonal and metabolic changes that could explain the increase in UCP2 and UCP3 gene expression. Among the hormones that have been shown to upregulate UCP2 and UCP3 gene expression, leptin and triiodothyronine are not likely to play a role because their plasma concentrations decrease during fasting.26 ± 30 Moreover, we have shown that insulin does not acutely regulate UCP2 and UCP3 mRNA concentrations in humans.11 Calorie restriction induces an increase in adipose tissue lipolysis,31 resulting in an important fatty acid release from body fat stores. Fatty acids and other ligands of the peroxisome proliferator-activated receptors stimulate UCP2 gene expression in adipocyte and skeletal muscle cell lines,32,33 and UCP2 and UCP3 mRNA concentrations are increased by high-fat feeding in mice. 17,34 In rats, elevation of plasma free fatty acids (FFA) concentrations by Intralipid plus heparin infusion induces UCP3 mRNA expression in skeletal muscle.35 A positive correlation was found between UCP3 mRNA concentration and plasma FFA concentration in 10 h fasted obese subjects.36 Further investigation is required to determine whether the increase in plasma FFA concentrations occurring during fasting contributes to the up-regulation of uncoupling protein mRNAs in humans. Acknowledgements
The laboratories are parts of the FATLINK Concerted Action supported by the European Commission (FAIR programme). References
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