Society is obsessed with its weight. Seventy percent of us are overweight and 70 percent of us would kill for a drug that would cure us. Obesity is becoming a worldwide epidemic, one in three people in the United States are now obese and there is an increasing prevalence of obesity in developing countries. The discovery of the hormone leptin in 1994 was critical to our understanding of some of the mechanisms of obesity, and an explosion of research into the molecules involved in the normal control of appetite and energy expenditure has resulted (Zang, 1994). Once these are better defined, potential genetic and pharmacological approaches to treating obesity will be opened up. Of particular interest are leptin itself (and how this circulating hormone may regulate appetite via receptors in the hypothalamus) and uncoupling proteins, which are molecules outside the brain that may influence energy expenditure.

The rate at which food is broken down to release energy could provide a physiological route to manipulating body weight. In theory, people with high metabolic rates "burn off" more food as heat energy, while those with low metabolic rates lay down more fat. According to the chemiosmotic theory, cellular energy production takes place across the inner mitochondrial membrane. Adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP) using a gradient of protons produced by the respiratory chain. If protons leak back, abolishing the proton gradient, heat is produced instead of useful energy. This disruption of the connection between food breakdown and energy production is known as "uncoupling."

Uncoupling protein (UCP) 2 and UCP3 are newly discovered proteins that can uncouple ATP production from mitochondrial respiration, thereby dissipating energy as heat and affecting energy metabolism efficiency. The proteins are similar to another energy protein, UCP1, found in brown fat adipose tissue, which is found in small quantities in adult humans. In contrast to UCP1, UCP2 has a wide tissue distribution and responds to fat intake, whereas UCP3 is expressed predominantly in skeletal muscle (Schrauwen, 1999). Several types of cells use the respiratory chain for thermogenesis, the generation of heat, rather than the formation of ATP. Newborn mammals and the adults of some mammals that are adapted to live in cold climates do this in the specialized brown fat adipose tissue. The brown color is due largely to the cytochromes in the tissue’s abundant mitochondria. Heat produced by brown-fat mitochondria is important for maintaining body temperature in the newborn and for the arousal of hibernating animals (Zubay, 1998).

The maintenance of a proton-motive force requires that the inner mitochondrial membrane remain highly impermeable to protons. If not, the gradient established by electrontransport is immediately dissipated by the diffusion of protons back into the matrix, leading to the release of energy as heat. This expenditure of energy in the form of heat as opposed to converting it into biomass is exactly how the UCPs work. However, in the 1920’s a drug called DNP (2,4-dinitrophenol) was prescribed as a diet pill that could uncouple oxidation and phosphorylation because it combines with protons. Due to it’s lipid solubility, it is able to carry the protons across the inner mitochondrial membrane down the electrochemical gradient (Karp, 1999). The prescription of this drug ceased after several deaths occurred. The UPCs do basically the same thing by acting as membrane transporters which are involved in dissipating the proton electrochemical gradient thereby releasing stored energy as heat, but in a way as a natural uncoupler.

This paper will focus on uncoupling proteins by reviewing papers that deal with the topics of the discovery of uncoupling proteins and how they work, what compounds have shown to induced the activity of uncoupling proteins, and if mutations and/or polymorphisms in the uncoupling protein genes can lead to the manifestation of obesity.

In 1997 proteins were identified in human cells which may catalyze the uncoupling process in humans, and these have also become known as uncoupling proteins: UCP2 in brain, muscle, and fat and UCP3 in skeletal muscle (Fleury, 1997). Of the three forms of energy expenditure- physical activity, resting metabolic rate, and thermogenesis or heat production-UCPs affect the last of these by burning calories as heat rather than storing them as fat. These proteins have already been picked out as potential targets for antiobesity agents as more evidence for their uncoupling function emerges. The evidence of the uncoupling action of these proteins as it stands comes from several sources.

Firstly, the genes that encode for these human proteins are 59% identical with the genes that encode proteins performing the same function in hibernating animals (UCP1) (Fleury, 1997). In yeast experiments, the UCP2 gene was expressed in mitochondria and the electrical potential across the mitochondria decreased. This drop in electrical potential suggests that a proton leak-and hence uncoupling-is occurring. In addition, when the modified mitochondria were purified, they were less coupled, producing more heat and less useful energy, compared with wild type yeast cells.

Similar studies done on the transport function of both UCP2 and UCP3 are consistent with the hypothesis that UCP2 and UCP3 behave as uncoupling proteins in the cell (Jaburek, 1999). Initially the transport and physiological functions of UCP2 and UCP3 had been deduced primarily from their striking sequence identities with UCP1. To address this assumption of functioning similarly to UCP1, UCP2 and UCP3 were expressed in Escherichia coli and the detergent-extracted proteins were reconstituted into liposomes to measure H⁺ and K⁺ fluxes. The ion flux studies showed that purified UCP2 and UCP3 behave identically to UCP1 (Jaburek, 1999). They both catalyzed electrophoretic flux of protons and alkylsulfonates, and proton flux exhibited an obligatory requirement for fatty acids. The findings that fatty acids are obligatory for proton flux mediated by UCP2 and UCP3, just as they are for UCP1, has important implications for the biophysical transport mechanisms of UCPs.

The importance of the research showing how these proteins work as uncouplers show that the three mammalian UCPs are qualitatively identical in mediating fatty acid dependent proton transport. Studies on whole cells and isolated mitochondria containing UCPs are urgently needed. It is hoped that the biophysical approach described in these two previous studies will prove to be useful as a guide to investigations on the human system.

In addition to understanding how uncoupling proteins function, it is also important to know what chemicals induce or inhibit the activity of UCPs. The effects of hyperthyroid state induced by chronic treatment with triiodthyroine (T3) on UCP2 mRNA expression in male rats was examined to assess the functional modulation of UCP2 gene expression in relation to body weight control (Masaki, 1997). The ubiquitous expression of UCP2 suggests that the protein may be important for determining basal metabolic rate, and possibly for regulation of body weight in mammals including humans. Thyroid hormone is known to increase the basal metabolic rate in humans. Taken together, these findings led to a hypothesis based on possible mechanisms that increased basal metabolic rate during hyperthyroidism may depend on increases not only in UCP1 but also in UCP2 mRNA expression (Masaki, 1997).

To test this hypothesis, hyperthyroid rats treated with chronic administration of T3 were examined for UCP2 mRNA expression in brown adipose tissue, white adipose tissue and skeletal muscle. Daily subcutaneous injection of T3 for 7 days increased UCP2 mRNA expression in all three tissues. The study provides evidence that thyroid hormone is a potent activator of UCP2. This is important because it identifies a molecule that may directly increase UCP2 expression. Knowing what increases UCP2 expression can lead to a breakthrough that is likely to have important implications for the treatment of human obesity.

In addition to understanding the function of UCPs and what compounds induce their activity, it is also important to examine what the relationship is between variants in the uncoupling protein genes and pathogenesis of obesity. UCP2 maps to regions of human chromosome 11 and mouse chromosome 7 that have been linked to hyperinsulinemia and obesity. The coding region of the human UCP2 gene is comprised of six coding exons, covering 5kb of chromosome 11q13. A study screening for mutations in the entire coding region of UCP2 by polymerase chain reaction (PCR) and single-strand conformation polymorphism analysis on 25 Japanese patients with obesity and noninsulin-dependent diabetes mellitus (NIDDM) and 25 subjects with simple obesity (Kubota, 1998). Two nucleotide polymorphisms resulting in Ala55à Val and Ala232à Thr were detected. Then, with the use of PCR and restriction fragment length polymorphism analysis, the allele frequencies for each of these polymorphisms were determined in 210 Japanese patient with NIDDM, 42 obese individuals, and 218 normal control subjects. The frequency of the Val55 allele did not differ significantly among the NIDDM group (46.0%), the obesity group (48.8%), and the normal control group (48.4%). The Thr232 allele was detected in only three subjects, who were in the NIDDM group (allele frequency, 0.7%). However, expression in yeast of the human wild-type UCP2 protein and UCP2 containing Thr232 revealed no difference in functional activity. The results indicate that the Ala55à Val and Ala232à Thr variants of UCP2 do not play an important role in the pathogenesis of NIDDM or obesity (Kubota, 1998). Although the two UCP2 variants do not appear to represent disease-causing mutations, it is possible that other polymorphisms that affect UCP2 function remain undetected.

Energy expenditure (how many calories we burn) may prove to be a more important variable in obesity than energy consumption, or how many calories we eat. There is surprisingly little evidence to support the general assumption that fat children or adults overeat. Instead, it appears that they burn too little energy. Obesity is a complex problem, and effective treatment will probably call for many different approaches. But the discovery of the UCP2 gene makes it theoretically possible to increase expression of the gene just in targeted tissues to reduce the size of fat deposits in certain areas of the body. Some might actually call this "genetic body shaping."