News Physiol Sci 15: 313-318, 2000;
1548-9213/00 $5.00
News in Physiological Sciences, Vol. 15, No. 6, 313-318,
December 2000
© 2000 Int. Union Physiol. Sci./Am. Physiol. Soc.
Uncoupling Proteins: Do They Have a Role in Body Weight Regulation?
Abdul.G. Dulloo and
Sonia Samec
A. G. Dulloo and S. Samec are at the Institute of Physiology, University of Fribourg, CH-1700 Fribourg, Switzerland.
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Abstract
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Several members of the mitochondrial carrier protein family are classified as uncoupling proteins. In contrast to the uncoupling protein specific to brown adipose tissue (UCP1), the physiological role of skeletal muscle uncoupling proteins (UCP2 and UCP3) in weight regulation seems more closely associated with the regulation of lipids as fuel substrate than as mediators of adaptive thermogenesis.
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Introduction
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Since Lavoisier's realization in 1780 that "life is combustion," there have been numerous debates about the extent to which humans are able to adapt to changes in food availability by turning down the rate of heat production during undernutrition so as to conserve energy or by turning it up during overnutrition to burn excess calories. As we enter a new millennium, these debates will certainly continue, but they will be marked by several reevaluations of human studies of starvation and overfeeding, which suggest that the control of heat production plays a more important role in the regulation of body weight and body composition than previously recognized. First, in an elegant reanalysis of human overfeeding studies published between 1967 and 1999, Stock (14) has produced strong arguments for the notion that variations in the capacity to activate heat production in response to diet [i.e., diet-induced thermogenesis (DIT)] contributes significantly to the ability of certain individuals to resist obesity (the so-called fast burners) while others become readily obese (the slow burners). Second, the application of more robust analysis to new and older data on changes in basal metabolic rate and body composition in response to starvation and therapeutic slimming has confirmed the long-held view that the rate of heat production falls to an extent well beyond that explained by the loss of body weight and lean tissues (3, 7). Third, the reanalysis of data from the classic Minnesota Experiment of semistarvation and refeeding has revealed that the extent to which this phenomenon of suppressed thermogenesis occurs during weight loss is partly determined by the degree of depletion of the fat stores and that it persists during subsequent weight recovery, with the energy thus conserved being directed at accelerating the replenishment of the fat stores (3). These reevaluations therefore reinforce the notion that adaptive changes in thermogenesis can contribute importantly to human variability in susceptibility to obesity and to the ease with which obesity relapse occurs after slimming. An understanding of the physiological mechanisms that underlie such adaptive changes in thermogenesis contributing to the defense of body weight may lead to the development of more effective thermogenic antiobesity therapy. In light of the recent cloning of novel members of the mitochondrial carrier protein family that apparently possess uncoupling (hence thermogenic) properties (9), this paper first traces the physiological rationale that led to these discoveries and then addresses the ongoing debate about whether the novel uncoupling proteins (UCPs) in the skeletal muscle and brown adipose tissue (BAT) have physiological importance in the regulation of body weight and body composition.
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Role of UCP specific to brown fat in DIT
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Since the classic human overfeeding studies of the 1960s implicating a role for DIT in susceptibility to leanness and fatness, several neurohormonal systems and numerous biochemical pathways have been proposed to account for adaptive changes in thermogenesis in body weight regulation (5). These include the thyroid hormones, insulin, the autonomic nervous system, and their potential control over biochemical mechanisms whose activation leads either to an increased use of ATP (e.g., sodium pumping and substrate cycling) or to a high rate of mitochondrial oxidation with poor coupling of ATP synthesis. This search for mechanisms underlying DIT in weight regulation has been intimately linked to that for nonshivering thermogenesis in body temperature regulation. By the late 1970s, this search culminated in the proposal (11) that these two forms of thermogenesis (in response to diet and to cold) have a common origin in BAT, whose thermogenic activity is mediated by the uncoupling of mitochondrial respiration.
A schematic diagram indicating the principle of uncoupling in BAT is depicted in Fig. 1
. According to the chemiosmotic model of oxidative phosphorylation, the driving force in capturing useful energy via synthesis of ATP along the mitochondrial respiratory chain is linked to the transport of protons across the inner mitochondrial membrane. Brown adipocytes, however, are unique in expressing a UCP, which, under control of noradrenaline released from the sympathetic nervous system (SNS), allows protons to leak back across the inner mitochondrial membrane. The resulting dissipation of the proton electrochemical gradient (a phenomenon referred to as "proton leak") allows substrate oxidation to occur without concomitant capture of some of the useful energy via ATP synthesis. The net effect during activation of UCP by cold or diet is that substrate oxidation is effectively uncoupled from phosphorylation, with a resultant increase in heat production.
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FIGURE 1. Schematic diagram illustrating how brown adipose tissue (BAT) generates heat. When activated by cold or diet, the uncoupling protein (UCP) in brown fat cells allows protons (H+) to pass through the inner mitochondrial membrane, thereby abolishing the proton gradient needed to drive ATP synthesis. FFA, free fatty acid. Reproduced with permission from Obesity Matters 2: 5–8, 1999 (Mediscript, London, UK).
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During much of the 1980s, a considerable body of evidence converged in support of an important role for the SNS-BAT-UCP axis in the control of thermogenesis in laboratory animals (6, 9). The results are embodied in the schematic diagram presented in Fig. 2. In rats and mice kept at laboratory temperature (21–23°C), the SNS-BAT-UCP axis is activated by overfeeding and suppressed during starvation, and these states are hence believed to play a contributory role in mediating these adaptive increases and decreases in thermogenesis, respectively. In addition to such dietary energy regulation, the SNS-BAT-UCP axis is modulated by diet composition and is markedly increased in response to dietary deficiencies, such as in protein, essential fatty acids, or certain minerals (e.g., Fe), all of which result in rapid and marked activation of both BAT and whole-body thermogenesis.
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FIGURE 2. Schematic diagram showing the response of the sympathetic nervous system (SNS)-BAT-UCP axis participating in the control of thermogenesis to changes in diet and environmental temperature. Note that human experiments are generally conducted under thermoneutral conditions and that, in rat/mouse experiments conducted at their respective thermoneutral environments, the dietary regulation of the SNS-BAT-UCP axis is considerably blunted, although it is still evident for extremes of dietary manipulation such as in response to protein deficiency or to fasting.
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Also depicted in Fig. 2
is the fact that if the above-mentioned experiments are conducted in the cold (usually <10°C), this dietary regulation or modulation of the SNS-BAT-UCP axis is virtually abolished, thereby underlining the overriding effect of the cold stimulus in activating this axis for thermoregulatory needs independently of diet. Conversely, if the experiments are conducted in rats and mice housed within their thermoneutral zone, i.e., when the energy expended for thermal regulation is minimal, the effects of diet on the SNS-BAT-UCP axis are also considerably blunted, although not entirely abolished, by extremes of dietary manipulation. Indeed, even under such conditions of thermoneutrality, this axis is also activated in response to low-protein diets and suppressed during total starvation (fasting). The subsequent demonstrations (5) that several animal models of genetic obesity show reduced capacity for DIT in parallel to defects in the SNS-BAT-UCP axis and that bilateral sympathetic denervation of interscapular BAT in lean mice leads to greater fat deposition due to an elevated efficiency of energy use (hence decreased thermogenesis) have reinforced the notion that the SNS-BAT-UCP axis plays a role in the control of DIT and energy balance regulation.
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Search for skeletal muscle UCP
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Although in humans several lines of evidence are consistent with an important role for SNS in the regulation of thermogenesis (5, 6), the importance of BAT as a site of adaptive thermogenesis in adult humans and its role in the etiology of human obesity have remained elusive. This situation has served to shift greater attention to the skeletal muscle, which by its sheer size (40% of body weight) has long been advocated as the major site for adaptive thermogenesis in large mammals. Besides, even in the small laboratory rat, skeletal muscle makes a substantial contribution to the increase and decrease in nonshivering thermogenesis in response to cold or to starvation (8), respectively. Unlike BAT, however, the mechanisms underlying skeletal muscle thermogenesis are poorly understood. But reports in the mid 1990s that the phenomenon of mitochondrial proton leak also exists in tissues other than BAT and could contribute as much as 50% of skeletal muscle heat production at rest (10) prompted the search for UCPs in the skeletal muscle. This led to the discovery of several new members of the UCP family (UCP2, UCP3, UCP4, and BMCP1) on the basis of their high sequence homology to the UCP in BAT (renamed UCP1). Unlike UCP1, which is expressed only in BAT, UCP2 is expressed in all tissues so far examined and UCP3 is highly expressed in skeletal muscles and BAT, whereas UCP4 and BMCP1 are expressed solely in the brain. Considered breakthrough discoveries, the cloning of UCP2 and UCP3 has generated considerable enthusiasm for the hypothesis that, by analogy to UCP1 in BAT, these UCP homologues may be mediators of thermogenesis in other tissues and thus became the long-awaited candidate genes controlling skeletal muscle heat production for adaptive thermogenesis. This thermogenesis hypothesis received a further boost following demonstrations that transfection or overexpression of these genes in yeast or mammalian cells yield data that are consistent with mitochondrial uncoupling properties (9).
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Starvation paradox for novel UCPs
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However, as research progressed from genetic manipulations of cell systems into integrative physiology, difficulties arose in the interpretation of skeletal muscle UCP expression vis-a-vis adaptive thermogenesis. A major challenge for the thermogenesis hypothesis is how to reconcile the uncoupling properties of UCP2 and UCP3 found in cell cultures with the fact that their gene expression in the skeletal muscle is upregulated during starvation, a directional change in gene expression that is opposite to that expected for any putative mediator of adaptive thermogenesis in a well established condition of energy conservation. Proponents of this hypothesis have argued that the upregulation of muscle UCP during starvation may be attributed to an increased thermoregulatory need consequential to starvation-induced body wasting, loss of fat insulation, and energy conservation in other tissues. This explanation is, however, difficult to accept in the light of evidence that, during fasting conducted at thermoneutrality, to minimize thermoregulatory needs, the gene expression of muscle UCP homologues remained markedly upregulated (12). Besides, direct measurements of regional metabolic rate have clearly demonstrated that the skeletal muscle is a quantitatively important site of energy conservation during starvation (8). Furthermore, the more recent demonstration that, during fasting, UCP2 and UCP3 gene expression (as well as the protein product of UCP3) in the skeletal muscle is upregulated but that the kinetics of the proton conductance pathway are unchanged further dissociates skeletal muscle UCP2 and UCP3 regulation from changes in mitochondrial proton leak (2). What then could be the physiological significance of UCP2 and UCP3 gene upregulation in the skeletal muscle during starvation?
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Novel UCPs: uncoupling proteins or substrate carriers?
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A reevaluation of our data on these UCP homologues and the current state of knowledge about skeletal muscle and BAT metabolism during starvation reveals that the only common association in both tissues is in the parallel changes in the expression and use of lipids as fuel substrate (Fig. 3). In BAT, the decreases in UCP homologue expression are consistent with the decrease in the use of lipids as metabolic fuel due to a general downregulation of metabolic activity in this tissue consequential to the suppressive effect of fasting on the SNS-BAT-UCP1 axis. In the skeletal muscle, the increased UCP2 and UCP3 expression is in line with the well known fasting-induced shift in substrate use in favor of lipids as the predominant metabolic fuel, hence allowing the sparing of glucose for organs/tissues with an obligatory requirement for glucose (e.g., the brain). Could it then be that the structural homology of UCP2 and UCP3 to UCP1 has been made with the wrong function of UCP1, since the primary function of this UCP as a mediator of adaptive thermogenesis via mitochondrial proton leak has often been linked to a putative secondary function as an anion/substrate transporter across the mitochondrial membranes? After all, the gene sequences of these novel UCPs, like UCP1, all possess mitochondrial carrier domains, which have served to classify them as members of the mitochondrial carrier protein family. Like the novel UCPs, several of these substrate carriers, e.g., adenine nucleotide and glutamate/aspartate transporters, are also known to be able to catalyze a fatty acid-dependent proton leak and hence possess uncoupling properties, although the physiological importance of these pathways is also unclear. On the basis of the results shown in Fig. 3, the hypothesis therefore arises that the primary function of UCP2 and UCP3 in the skeletal muscle and BAT may somehow be involved, either actively or passively, with the regulation of lipids as fuel substrate rather than in the mediation of adaptive thermogenesis (12).
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FIGURE 3. Changes in BAT and skeletal muscle in response to starvation. 0, no change; downward arrow, decrease; upward arrow, increase; double upward arrow, more pronounced increase. All of the results and/or specific references for the various results presented are provided in Ref. 12.
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Lipid handling hypothesis for novel UCPs
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Several other lines of evidence are consistent with this alternative hypothesis for a physiological role of these UCP homologues in lipid handling. First, on transition from starvation to refeeding, the gene expressions of these muscle UCP homologues were found to be altered from a state of upregulation to one of downregulation below control levels (12). These findings could be interpreted as being consistent with a role for these UCP homologues in the switching of muscle substrate metabolism from a state of enhanced lipid use during starvation when glucose is limited to one of reduced fat utilization during refeeding on a low-fat diet when lipids need to be "spared" for deposition during a phase of increased metabolic efficiency and accelerated replenishment of the fat stores. Second, support for the latter contention linking muscle UCP downregulation and lipid sparing can be derived from the finding (13) that the downregulation of muscle expression of UCP3 and UCP2 during refeeding on a low-fat diet is prevented by high-fat refeeding, i.e., when the availability of dietary lipids for rapid fat replenishment is no longer a limiting factor and the need for de novo lipogenesis is obviated. Third, the observation that the changes in UCP expression during starvation and refeeding are more pronounced in the predominantly fast glycolytic "white" muscles than in the predominantly slow oxidative "red" muscles (12) is consistent with the greater dependency of slow oxidative muscles on lipids as fuel substrate and the greater capacity of fast glycolytic muscles to shift between glucose and lipids as fuel substrate. Finally, the recent association in humans between polymorphism in UCP3 and marked reduction in basal lipid oxidation (1) would also be consistent with the proposal of a physiological role for these UCP homologues in the regulation of lipids as fuel substrate.
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Interpreting the outcome of UCP transgenic models
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To date, the use of gene knockout (KO) technology to elucidate the role of UCPs in energy balance and substrate metabolism have been disappointing, since mice lacking UCP1, UCP2, UCP3, or both UCP1 and UCP3 do not appear to show impairments in resting metabolic rate, DIT, or substrate use. There are, however, many pitfalls in interpreting results from transgenic experiments. As recently emphasized by Williams and Wagner (15), unexpected consequences of genomic modifications are frequent, and the phenotype or lack of phenotype observed in any transgenic experiment is a function both of the planned genetic modification and of secondary responses of the organism to that perturbation. A dramatic example of this principle was recently demonstrated in the case of myoglobin KO mice, which either die in utero or survive by adaptive responses that compensate for the absence of myoglobin by steepening the PO2 gradient and reducing the diffusion path length for O2 between capillaries and the mitochondria (15). Thus, since the knocking out of genes for reasonably well established functions often fail to reveal the expected impairment in these functions because of compensatory mechanisms (known or unknown), the failure of UCP KO mice to reveal impairment in weight regulation via thermogenesis or substrate oxidation is not sufficient to reject the hypothesis that UCPs play a role in thermogenesis or in lipid handling. An even stronger argument can be made in the case of UCP1, since the failure to show changes in body fat stores in UCP1 KO mice can be weighed against ample pharmacological and surgical evidence that implicates UCP1 in DIT and in weight regulation (9). Similarly, preliminary reports that transgenic mice overexpressing human UCP3 show markedly elevated metabolic rates and lipid oxidation neither establish the normal physiological role of that UCP homologue nor do they tell us which is the primary effect of UCP3 overexpression: is it an increase in thermogenesis that could have resulted from an exaggerated increase in UCP3-mediated proton leak, which then drives lipid use, or is it primarily due to a massive increase in UCP3-mediated free fatty acid (FFA) flux across the mitochondria, with resulting "uncoupling effects" of FFAs and/or increased proton leak that is secondary to carrier-mediated FFA transport?
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Perspectives
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The arguments put forward against the thermogenesis hypothesis or in favor of the lipid handling hypothesis as the primary functional role of UCP2 and UCP3 are based almost entirely on their gene expression and, in a few instances, on their protein expression; they are limited by the current lack of suitable assays for the activities of their protein products. Nonetheless, the available evidence casts serious doubts on the role of skeletal muscle UCP2 and UCP3 in the mediation of adaptive thermogenesis and at the same time opens new perspectives for these UCP homologues in metabolic regulation: that of candidate genes for the regulation of substrate metabolism, particularly in the skeletal muscle. Given evidence from the past decade linking the regulation of substrate balance (particularly between the intake and oxidation of fats and carbohydrates) to energy balance regulation via changes in appetite, these UCP homologues, via their postulated role in regulating lipids as metabolic fuel, may still play a role in body weight regulation via the control of food intake. Paradoxically, the current difficulties in attributing a physiological role to UCP2 and UCP3 in adaptive thermogenesis is occurring at a time when the biological significance of DIT is being reassessed. Humans seem to possess a much larger capacity for DIT than is generally recognized, but like in the laboratory rats/mice at thermoneutrality (Fig. 2), that capacity is poorly recruited on well balanced diets but much more pronounced on diets poor in essential nutrients. As Stock (14) has recently argued, the necessity to increase DIT in the face of nutrient-deficient diets probably had evolutionary survival advantage since it enables overeating (on an energy basis) of nutrient-deficient diets in an attempt to achieve an adequate intake of the specific nutrient without an excessive weight gain, which would be a hindrance to optimal locomotion, hunting capabilities, and the ability to fight or flee. He went on to propose that DIT may have evolved as a mechanism for regulating the metabolic supply of essential nutrients (protein, minerals, vitamins) with only a secondary role in regulating energy balance and body weight. Indeed, using different approaches, both Stock's (14) and our own (4) reanalysis of human overfeeding studies reveal that relatively small individual differences in DIT on balanced normal-protein diets are amplified with protein-deficient diets (Fig. 4). Consequently, short-term overfeeding on low-protein diets could provide a very sensitive method for discriminating between those who are metabolically predisposed to leanness or fatness. Given the potent effect of protein deficiency on thermogenesis via the SNS-BAT-UCP1 axis in the laboratory rats/ mice, it remains to be seen whether the use of such low-protein diets as a tool to unmask some of the genetic and metabolic basis underlying human susceptibility to obesity will pinpoint BAT and its UCP (UCP1) as a source of this human variability in thermogenesis.
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FIGURE 4. Unmasking of interindividual variability in diet-induced thermogenesis (DIT) by low-protein overfeeding in humans. The data represents the energy cost of weight gain (excess energy in megajoules consumed per kilogram of weight gained) during 3–4 weeks of overfeeding in 5 volunteer subjects (# 27, 28, 30, 33, and 34) who participated in both the normal-protein and low-protein overfeeding in the gluttony experiments of Miller and co-workers (4). The two horizontal broken lines (enclosing the shaded area) correspond to predicted energy cost of weight gain on the assumption that weight gain is either 100% fat (45 MJ/kg) or 60% fat (30 MJ/kg), the latter value including cost of fat-free mass gain. The greater the deviation from the predicted values, the greater the likelihood that the excess calories were dissipated via enhanced DIT. Reproduced with permission from Ref. 4 (Stockton, Basingstoke, UK).
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Acknowledgments
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We thank Dr. Bruce Van Vliet for useful suggestions.
This work is supported by the Swiss National Science Foundation (grant no. 32000-61684).
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References
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Introduction
Role of UCP specific...
