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Uncoupling Proteins: Functional Characteristics and Role in the Pathogenesis of Obesity and Type II Diabetes

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Uncoupling Proteins: Functional Characteristics and Role in the Pathogenesis of Obesity and Type II Diabetes Diabetologia 2001) 44: 946±965 Ó Springer-Verlag 2001 Uncoupling proteins: functional characteristics and role in the pathogenesis of obesity and Type II diabetes L. T.Dalgaard, O.Pedersen Steno Diabetes Center, Gentofte, Denmark Abstract obesity. The three uncoupling protein homologue genes UCP1, UCP2, and UCP3 have been investigat- Uncoupling proteins are mitochondrial carrier pro- ed for polymorphisms and mutations and their impact teins which are able to dissipate the proton gradient on Type II diabetes mellitus, obesity, and body weight of the inner mitochondrial membrane. This uncou- gain or BMI. The main conclusion is that variation in pling process reduces the amount of ATP generated the UCP1, UCP2 or UCP3 genes is not associated through an oxidation of fuels. The hypothesis that un- with major alterations of body weight gain. The con- coupling proteins UCPs) are candidate genes for hu- tribution of UCP genes towards polygenic obesity man obesity or Type II non-insulin-dependent) dia- and Type II diabetes is evaluated and discussed. [Dia- betes mellitus is based on the finding that a chemical betologia 2001) 44: 946±965] uncoupling of the mitochondrial membrane reduces body adiposity, and that lower metabolic rates predict Keywords Uncoupling proteins, Type II diabetes weight gain. It is straightforward to hypothesize that mellitus, obesity, genetics, body weight regulation, common polymorphisms of UCP1, UCP2 and UCP3 energy expenditure, metabolic rate, brown adipose genes lower metabolic rate by a more efficient energy tissue, white adipose tissue, reactive oxygen species, coupling in the mitochondria. Furthermore, geneti- polymorphism, mutation, transgenics, gene knock- cally engineered mice over expressing different UCP out. homologues are lean and resistant to diet-induced An enormous amount of data has been collected on the equivalent of two new papers a week on the sub- the uncoupling proteins 2 and ±3 UCP2 and ±3) since ject during the last four years, and that is excluding their discovery in 1997and well over 400 publications those papers on UCP1 alone. Despite the intense fo- with either UCP2 or UCP3 or both as a keyword cur- cus on these proteins their function and role in me- rently exist in public bibliographic databases. This is tabolism is not clear. There have been numerous clues to the possible functions of these proteins, and many theories have been offered but it is still not Received: 28 February 2001 and in revised form: 24 April 2001 known what these uncoupling protein homologues Corresponding author: Oluf Pedersen, MD, DMSc, Steno Dia- actually do [1±6]. betes Center, Niels Steensens Vej 2, DK-2820 Gentofte, Den- mark, E-mail: [email protected] Abbreviations: UCP, Uncoupling protein; RMR, resting meta- Mitochondrial bioenergetics in brief bolic rate; BMR, basal metabolic rate; BAT, brown adipose tis- sue: ROS, reactive oxygen species; BMCP1, brain mitochon- The mitochondrial oxidation of substrates results in drial carrier protein 1; WAT, white adipose tissue; skm, skeletal reduced NADH and FADH , which deliver their muscle; LPS, lipopolysaccharide; PPAR, peroxisome prolifera- 2 tor activated receptor; 24h-EE, 24 hours energy expenditure; electrons to the electron transport chain in the inner b3-AR, beta 3 adrenergic receptor; UTR, untranslated region; mitochondrial membrane. The chemiosmotic hypoth- SMR, sleeping metabolic rate esis proposes that oxidation is coupled by the elec- L. T.Dalgaard and O. Pedersen: Uncoupling proteins in the pathogenesis of obesity and Type II diabetes 947 tron transport chain to the pumping of protons into over years [23]. Increasing the energy expenditure the mitochondrial intramembrane space [7]. This cre- by increasing the proton conductance of mitochon- ates a pH-gradient the proton motive force), and dria has long been recognised as an effective way to proton re-entry into the mitochondrial matrix is cou- obtain weight loss. This has been achieved by intake pled to phosphorylation of ADP to ATP.Protons can- of the chemical uncoupler dinitrophenol, which was not enter the matrix through ATP synthase when used in the 1930's for a review see [24]). ADP is not available. The steeper proton gradient in the absence of ADP inhibits the electron transport chain and the reduction of NAD+ and FADH. As a The original uncoupling protein of brown fat: UCP1 result, substrate oxidation is closely coupled to ener- gy needs. Brown adipose tissue BAT) is a thermogenic organ, However, mitochondria uses oxygen when ADP is present in almost all mammals where it is a major not available state 4 respiration), an indication that site of cold-induced non-shivering thermogenesis as the coupling of substrate oxidation to ATP synthesis well as contributing to diet-induced thermogenesis is imperfect. State 4 respiration is mostly due to un- [25]. BAT is a metabolically active tissue, which con- coupling [8]. Uncoupling could, in theory, be due to sists of adipocytes rich in mitochondria and small lip- proton leak for example catalysed by an uncoupling id droplets Fig.1). This is in contrast to white adi- protein) or to the slippage of the electron transport pose tissue WAT), which contains large fat droplets chain, so that it does not, for example, exclude a pro- and few mitochondria Fig.1). In large mammals, ton each time it transfers an electron. such as humans, BAT is mainly active in infancy after which it largely disappears. Thermogenesis in BAT is due to uncoupling pro- Proton leaks and variations in energy expenditure tein 1 UCP1). The special function of BAT has made the underlying molecular mechanisms an issue Proton leaks constitute a considerable part of the of great interest, and UCP1 previously just UCP or resting metabolic rate RMR). An estimated thermogenin) was purified as early as 1978 [26]. 20±50% of total energy expenditure is due to proton The amino acid sequence was determined and its leaks with skeletal muscle as the main contributor [9, coding DNA sequence cloned in 1985 [27]. UCP1 is 10]. Energy expenditure in humans can be divided a dimeric protein present in the inner mitochondrial into the basal energy expenditure measured under membrane Fig.2A), and it dissipates the pH-gradi- resting conditions, the energy expenditure caused by ent generated by oxidative phosphorylation, releas- physical activity and the adaptive energy expenditure ing chemical energy as heat. UCP1 is exclusively ex- including thermogenesis) observed in the metabo- pressed in brown adipocytes, where the gene expres- lism of substrates. Variations in the resting metabolic sion is increased by cold, adrenergic stimulation, b3- rate are due to several determinants, including body agonists, retinoids and thyroid hormone [28] Ta- composition fat vs fat free mass), concentrations of ble 1). thyroid and steroid hormones, genetics as well as the UCP1 is activated by non-esterified fatty acids activity of the sympathetic nervous system [11]. Vari- and inhibited by purine nucleotides [10]. Although ations in energy expenditure could be a source of coenzyme Q ubiquinone) has been identified as a body weight variation in human beings [12, 13]. co-factor for the proton transport of UCP1 the What evidence is there that energy expenditure is an mechanism by which UCP1 transports protons inherited trait? Several studies have addressed this across the inner mitochondrial membrane is still to question by computing the intrafamilial correlations be determined [29, 30]. However, two models ac- for basal metabolic rate or 24-h energy expenditure count for important facts concerning the kinetics giving heritabilities of 0.26 to 0.70 [13±16]. It is clear and the regulation of UCP1 activity: the fatty acid that variations in energy expenditure between sub- protonophore or flip-flop) model Fig.2B) [31] and jects have genetic determinants. the channel model Fig.2C) [32]. Both models pre- Low energy expenditure could predict future dict a net transport of protons. The arguments for weight gain. In infants, children and adults, lower en- and against each mechanism have been reviewed ergy expenditure rather than increased energy intake elsewhere [10]. has been reported to predict later weight gain [17±19]. However, other reports have thrown doubt on the predictive value of resting metabolic rate UCP2 and UCP3: homologues to UCP1 RMR) for weight gain [20, 21]. In one study, children of obese subjects had a lower RMR than the children Uncoupling of the proton gradient of the mitochon- of lean parents [22]. Only a slight imbalance between dria is thought to be important for energy dissipation intake and expenditure an estimated surplus as heat, maintenance of respiration, activation of sub- of ~ 0.3%) is necessary for a weight gain if it persists strate oxidation and prevention of reactive oxygen 948 L. T.Dalgaard and O.Pedersen: Uncoupling proteins in the pathogenesis of obesity and Type II diabetes A B Fig.1. A Brown adipose tissue. The lipid droplets are small Tissue distribution of UCP2 and UCP3 and plentiful and the nuclei are large and placed centrally haematoxylin-eosin, magnification 580). B White adipose UCP2 is expressed widely and in humans, high ex- tissue. Cells contain one large lipid droplet and the nucleus is placed peripherally haematoxylin-eosin, magnification pression is seen in white adipose tissue. Other tissues 295). From Histology, Munksgaard, with permission skeletal muscle, heart, placenta, liver, kidney, pan- creas, brain, and cells of the immune system) express considerable amounts of UCP2 [35,36]. In mice, the tissue distribution of UCP2 mRNA is even wider, species ROS ± oxygen radicals) [33,34]. In newborn with high expression also found in BAT, kidney, children and rodents, UCP1 is a major contributor to spleen and macrophages [35,36,46]. the energy expenditure, but since human adults con- The tissue distribution of UCP3 is much narrower tain very little BAT, the contribution of UCP1 to than UCP2. It is expressed, albeit abundantly, but whole body energy expenditure is probably less.
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