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aim to review the literature currently available on UCP3 in relation to its regulation and function and the implications for obesity. UCP3 may possess uncoupling activity in vivo. Indeed, it was shown that expression of recombinant human UCP3 in yeast lowered the mitochondrial membrane potential (8), increased basal oxygen consumption by 31%, induced a 20% increase in state 4 respiration in isolated mitochondria, and retarded growth (9). It was also shown that overexpress- ing UCP3 in yeast induced a decline in cellular respiration coupled to oxidative phosphorylation from 57% to 11%; this decline was concerted with a 33% increase in cellular heat production (10). These findings seem to confirm un- coupling activity of UCP3 in vitro. However, it was also noted that, in a heterologous yeast expression system, the uncoupling activity of UCP3 was hardly affected by guanosine diphosphate (GDP) or ATP (11), which was expected based on the similarity of UCP3 with UCP1. studies have also been criticized (12). The main criticism relates to computation of the respiratory control values (the ratio of fully uncoupled respiration over state 4 respiration, see Figure 1). Adding a chemical uncoupler to the incuba- tion medium and measuring maximal respiration rates is referred to as fully uncoupled respiration. Thus, if UCP3 is considered a true uncoupler, it should increase state 4 res- piration but leave the fully uncoupled state unaffected. However, the fully uncoupled respiration in in vitro UCP3 overexpression conditions was lower compared with control values. These results can be interpreted as a malfunctioning of the electron transfer chain (12). Interestingly, it was shown that yeast overexpressing UCP1 at modest concen- trations possessed the expected uncoupling behavior (i.e., GDP inhibitable uncoupling and unaffected rates of the fully uncoupled state), whereas yeast overexpressing UCP1 at higher concentrations were growth retarded, partly GDP insensitive, and had decreased chemically uncoupled respi- ration (13). These observations are similar to UCP3-ex- pressing yeast and may indicate that expression of UCP3 in yeast induces artifactual uncoupling, possibly induced by improper incorporation of UCP3 in the inner mitochondrial membrane. With respect to this, it is important to note that it was shown that after sonication and differential centrifu- gation, UCP3 and its degradation products were primarily present in extramitochondrial fractions (14) rather than folded properly into the mitochondria. respiration, and mildly increases state 4 respiration, and that the observed uncoupling cannot be inhibited by GDP or ATP, as in UCP1. Therefore, care should be taken when extrapolating these findings to in vivo (human) conditions. mice overexpressing UCP3 (UCP3-tg) were lean despite the fact that they are hyperphagic compared with their wild- type littermates. Additional phenotypical changes, next to the 66-fold up-regulation of UCP3 mRNA in skeletal mus- cle, include 25% increase in resting oxygen consumption, decreased fasting blood glucose and insulin levels, im- proved glucose tolerance, decreased total cholesterol, and a 44% to 57% reduction in adipose tissue over total animal volume. Shortly after this work, Li et al. overexpressed the BAT-specific UCP1 in skeletal muscle (by coupling UCP1 cDNA to a myosin light chain promoter) (16) and showed a similar phenotype (reduced body mass, decreased fasting blood glucose and triglycerides, and prevention of diet- induced obesity and insulin resistance). The straightforward interpretation of these data is that UCP3 overexpression results in increased energy expenditure and the accompany- ing phenotype. The results would suggest that UCP3 would be a good candidate for pharmaceutical up-regulation in the prevention and/or treatment of obesity. However, along the same lines as the overexpression of UCP3 in yeast was challenged (the lack of inducible uncoupling and the lack of inhibition by GDP), the increased proton leak and related phenotype in the UCP3-tg mice were considered artificial (17). Quantifying the absolute concentration of UCP3 in mitochondria isolated from UCP3-tg and wild-type mice (15) revealed that UCP3-tg mice had mitochondrial protein). According to Cadenas et al. (17), the uncoupling observed at this supraphysiological level of expression does not represent native uncoupling, because it cannot be induced by fatty acids or be inhibited by purine nucleotides as was shown for UCP3 expressed at physio- logical levels (18). The inability of overexpressed UCP3 to catalyze proton conductance was attributed to poor insertion or folding of UCP3 in the mitochondria, whereas the authors leave open the option that an unidentified endogenous co- factor is required to provoke UCP3 to affect proton conduc- tance. However, no attempts have been made to examine if the protein was indeed poorly inserted or misfolded into the mitochondria in mice expressing supraphysiological levels of UCP3. |
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