Energy Metabolism in Uncoupling Protein 3 Gene Knockout Mice

Abstract

Uncoupling protein 3 (UCP3) is a member of the mitochondrial anion carrier superfamily. Based upon its high homology with UCP1 and its restricted tissue distribution to skeletal muscle and brown adipose tissue, UCP3 has been suggested to play important roles in regulating energy expenditure, body weight, and thermoregulation. Other postulated roles for UCP3 include regulation of fatty acid metabolism, adaptive responses to acute exercise and starvation, and prevention of reactive oxygen species (ROS) formation. To address these questions, we have generated mice lacking UCP3 (UCP3 knockout (KO) mice). Here, we provide evidence that skeletal muscle mitochondria lacking UCP3 are more coupled (i.e. increased state 3/state 4 ratio), indicating that UCP3 has uncoupling activity. In addition, production of ROS is increased in mitochondria lacking UCP3. This study demonstrates that UCP3 has uncoupling activity and that its absence may lead to increased production of ROS. Despite these effects on mitochondrial function, UCP3 does not seem to be required for body weight regulation, exercise tolerance, fatty acid oxidation, or cold-induced thermogenesis. The absence of such phenotypes in UCP3 KO mice could not be attributed to up-regulation of other UCP mRNAs. However, alternative compensatory mechanisms cannot be excluded. The consequence of increased mitochondrial coupling in UCP3 KO mice on metabolism and the possible role of yet unidentified compensatory mechanisms, remains to be determined. Uncoupling protein 3 (UCP3)1 (1-3) is a member of the mitochondrial anion carrier superfamily with high homology (57%) to UCP1, a well characterized uncoupling protein (4, 5). UCP3 together with UCP1, UCP2 (6, 7), and possibly BMCP1 (brain mitochondrial carrier protein) (8) and UCP4 (9), form a family of uncoupling proteins located in the inner mitochondrial membrane. The evidence supporting the uncoupling activity of these proteins comes from studies where UCPs have been heterologously expressed in yeast or reconstituted into proteoliposomes. The expression of UCP2 and -3 decreases the mitochondrial membrane potential, as assessed by uptake of fluorescent membrane potential-sensitive dyes in whole yeast. They also increase state 4 respiration in isolated mitochondria, which serves as an indicator of inner membrane proton leak (3, 6, 10). More recently, reconstitution of UCPs into liposomes has shown that UCP2 and UCP3, like UCP1, mediate proton transport across bilipid layers (11). It is well established that UCP1 is exclusively expressed in brown fat, where it plays a key role in facultative thermogenesis in rodents. Although there is controversy about the molecular mechanisms involved (12-16), it is clear that activated UCP1 catalyzes a proton leak across the mitochondrial inner membrane leading to thermogenesis. The activity of UCP1 is highly regulated, facilitated by fatty acids and inhibited by purine ribose di-and trinucleotides (ATP, ADP, GTP, GDP) (17). UCP1 is also highly regulated at the transcriptional level (18) by cat...

Capsiate administration results in an uncoupling protein-3 downregulation, an enhanced muscle oxidative capacity and a decreased abdominal fat content in vivo

“…Experiments conducted in UCP3 knockout mice have actually not illustrated a major involvement of UCP3 in the regulation of whole-body energy metabolism. [14][15][16] These genetically modified mice did not show changes in body mass and their metabolic rates were similar to those measured in the wild-type animals. Alternatively, on the basis of the present results, we hypothesize that UCP3 is involved in the control of ATP production in skeletal muscle at rest and during muscle activity.…”

Adaptive thermogenesis and uncoupling proteins: a reappraisal of their roles in fat metabolism and energy balance

“…(iv) State 4 respiration controversies: It can always be argued that changes (or lack of it) in the gene or protein expression of UCP3 do not reflect changes in the activity of the protein. Indeed, the most direct evidence so far that UCP3 has physiologically relevant uncoupling properties rests à priori upon the report that state 4 respiration-which is often postulated to reflect basal proton leak respiration [62]-is lower in skeletal muscle from UCP3-knockout mice [63]. Both UCP3 gene expression and state 4 mitochondrial respiration were also found to be lowest in skeletal muscle from patients with the least weight losses than in those with the greatest weight losses, following a hypocaloric slimming therapy [64].…”

Energetics and longevity in birds

“…Mild uncoupling produces the same effect, reducing the protonmotive force, increasing the oxidation of electron carriers in the transport chain and reducing the rate at which ROS-particularly superoxide-is generated. In addition, UCP2 and UCP3 knockout mice have been shown to exhibit increased superoxide production compared to that of wild-type mice (Negre-Salvayre et al 1997;Vidal-Puig et al 2000), but overexpression of UCP3 did not decrease ROS generation. This suggests that uncoupling over and above that which occurs naturally is unnecessary (Brand et al 2002).…”