ReviewMitochondrial uncoupling protein 3 and its role in cardiac- and skeletal muscle metabolism
Introduction
The process of photosynthesis, which occurs in plants, is undoubtedly one of the most elementary chemical reactions in life, converting carbon dioxide and water to glucose and oxygen, fuelled by the energy of light. Just as essential to human and animal life is the chemistry of combustion, the reversed reaction that breaks down what photosynthesis creates. This process provides energy for numerous cellular processes and finally leads to the release of heat. Approximately half a century ago, mitochondria, cellular organelles bounded by a highly folded inner and fairly smooth outer membrane, were recognized as the cellular fireplaces, harboring the energy needed for these processes. The mechanism, that underlies the energy-generating capacity of mitochondria was described by Mitchell in 1961 [1] and awarded with the 1978 Nobel Prize in chemistry. Mitchell's chemiosmotic theory describes how the oxidation of nutritional substrates is coupled to the synthesis of ATP, the compound in which cellular energy is conserved.
In mitochondria, NADH and FADH2, reducing equivalents derived from the degradation of nutritional substrates, undergo a series of oxidation–reduction reactions by a system of enzymes present in the inner mitochondrial membrane, collectively known as the electron transport chain. These reactions give rise to a flow of electrons from the substrates to oxygen, the final electron acceptor, and the energy released during this process is used to pump protons from the mitochondrial matrix to the intermembrane space. As a result, an electrochemical gradient (a difference in proton concentration, or pH, and a difference in electric transmembrane potential) is created across the inner mitochondrial membrane that Mitchell refers to as “protonmotive force”. The exported protons flow back into the mitochondrial matrix through ATP synthase (also known as the F0–F1 ATPase), a protein also present in the inner mitochondrial membrane that uses the energy derived from the proton flow to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi). In this way, the oxidation of substrates and the use of oxygen (respiration) maintain the electron flow, and are coupled to the synthesis of ATP. The efficiency of ATP synthesis driven by substrate oxidation is however not 100% since leakage of electrons and/or protons from the electron transport chain or mitochondrial inner membrane respectively, can diminish the proton gradient thereby releasing energy that is not coupled to the synthesis of ATP. This “uncoupling” is illustrated by the fact that incubation of isolated mitochondria, in the presence of substrate but with depleted ADP levels, still consume a certain amount of oxygen (state 4 respiration). In other words, the consumed oxygen (substrate oxidation) under these circumstances is used solely to compensate for the loss of proton gradient due to proton/electron leak, since ATP synthesis is excluded. Although the exact causes for this phenomenon are not fully understood, uncoupling proteins are likely candidates in mediating mitochondrial proton leak. These proteins are a family of mitochondrial transport proteins that might be involved in adaptive thermogenesis by regulating uncoupling of substrate oxidation from the production of ATP. In 1978 the first member of this family, uncoupling protein 1 (UCP1), was discovered in brown adipose tissue [2] that is present in rodents and some other mammals. It is now well established that UCP1, present in the inner mitochondrial membrane, is responsible for non-shivering thermogenesis in response to cold exposure, by catalyzing a back-flux of protons not related to ATP synthesis, thereby regulating heat production. Twenty years after the discovery of UCP1, several other UCP1 homologues were reported and referred to as UCP2 to UCP5 respectively, although differences in nomenclature exist [3], [4].
This review focuses on UCP3 which is primarily located in skeletal muscle and, in analogy to UCP1, was hypothesized to be able to modulate energy expenditure by regulating mitochondrial uncoupling. Since humans in general lack brown adipose tissue (the site of UCP1 expression), the discovery of a potential uncoupling protein in an organ highly contributing to energy expenditure was regarded as a potential anti-obesity target. However, the bulk of research conducted after the discovery of UCP3 in 1997 suggested that this protein is more likely to be involved in skeletal muscle fatty acid metabolism and mitochondrial reactive oxygen species (ROS) production. Although the exact physiological function of UCP3 is thus so far unknown, the first aim of this review is to give a short overview on the available literature that collectively seems to propose a role for UCP3 in the protection of mitochondria against lipid-induced oxidative damage.
Interestingly, the UCP3 protein is also present in the heart although UCP3 protein levels have been shown to be low in this highly fat-oxidative tissue in comparison with skeletal muscle [5]. The physiological role of cardiac UCP3 is relatively unexplored despite the potential important impact of an uncoupling protein in this tissue. Energy deficiency in the heart is related to heart failure and high amounts of UCP3 might contribute to this energy deficit via the uncoupling mechanism. On the other hand, high levels of cardiac UCP3 might also serve a protective role against lipid-related oxidative stress, similar to the suggested function of UCP3 in skeletal muscle. Therefore, the second aim of this review is to give insight in the physiological function of UCP3 in the heart and to address the question whether cardiac UCP3 should be regarded as protective or harmful.
Section snippets
Skeletal muscle UCP3 and energy metabolism
Since the discovery of UCP3 many studies, using cell systems and transgenic animals, were conducted to establish the true uncoupling capacities of the protein. Cell studies over expressing UCP3 have shown that the protein indeed possesses uncoupling activity, indicated by changes in proton flux, increased thermogenesis, increased state 4 respiration and decreases in mitochondrial membrane potential [6], [7], [8]. However, serious questions were raised concerning the physiological relevance of
UCP3 and fat metabolism
The finding that plasma FFAs, most likely through activation of PPAR responsive elements in the UCP3 promotor [40], [41], [42], are responsible for the induced expression of UCP3 during fasting (and possibly during β-adrenergic stimulation and thyroid hormone treatment), initiated numerous physiological studies interfering with fat metabolism. To elucidate its physiological role, UCP3 expression was measured during several conditions of lipid oversupply and in muscles with different
Suggested physiological functions of uncoupling protein 3
Throughout the years, several functions have been ascribed to skeletal muscle UCP3, based on the available data discussed in this review so far (see Fig. 1). We proposed that UCP3 protects mitochondria against the potentially detrimental effects of intra-mitochondrial fat accumulation [56]. When cytosolic levels of fatty acids increase inside muscle cells due to a mismatch between fat supply and oxidation capacity, the load of fatty acids on the mitochondria would drive neutral non-esterified
Heart failure and cardiac energy metabolism
Besides skeletal muscle, UCP3 is also expressed in cardiac tissue and the presence of an uncoupling protein in heart could potentially have a major impact on cardiac ATP synthesis. Interestingly, disturbances in cardiac ATP production have been implicated in the pathogenesis of heart failure. Thus, ATP levels were shown to be reduced in myocardium of individuals with dilated cardiomyopathy [63], and significant linear correlations between ATP levels and indices of systolic and diastolic
Cardiac UCP3 and energy metabolism
As stated earlier, the support for UCP3 as a true uncoupling protein in skeletal muscle is equivocal, although numerous studies were conducted to investigate this issue. Specific data on the uncoupling activity of cardiac UCP3 is however limited. Nonetheless, several (pathological) conditions exist that may link cardiac energy metabolism to UCP3.
First of all, thyroid hormones are known to affect basal metabolic rate and UCP3 has been suggested to mediate this increase in energy expenditure by
UCP3 and cardiac fatty acid metabolism
A clear relation between high fatty acid availability and UCP3 expression (both mRNA and protein) has already been shown in skeletal muscle [37], [43], [44], and although data is scarce, there is support for a similar relation in the heart. Reduction in plasma FFA levels via a high-carbohydrate/low-fat diet decreased cardiac UCP3 expression [79] whereas increasing plasma FFAs by 24 h of lipid infusion resulted in a 1.8-fold induction of cardiac UCP3 gene expression compared to saline-infused
Conclusion
The primary role for skeletal muscle UCP3 does not seem to be the regulation of energy expenditure, as was initially predicted based on its homology to the known regulator of adaptive thermogenesis in brown adipose tissue. However, UCP3 has been shown to possess uncoupling activity and correlates with energy metabolism in certain situations. It was suggested that the protein could mediate a mechanism of mild uncoupling, thereby diminishing the production of ROS at the cost of a slight increase
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