Hyperlipemia: a role in regulating UCP3 gene expression in skeletal muscle during cancer cachexia?

Cancer-Mediated Muscle Cachexia: Etiology and Clinical Management

Muscle cachexia has a major detrimental impact on cancer patients, being responsible for 30% of all cancer deaths. It is characterized by a debilitating loss in muscle mass and function, which ultimately deteriorates patients' quality of life and dampens therapeutic treatment efficacy. Muscle cachexia stems from widespread alterations in whole-body metabolism as well as immunity and neuroendocrine functions and these global defects often culminate in aberrant signaling within skeletal muscle, causing muscle protein breakdown and attendant muscle atrophy. This review summarizes recent landmark discoveries that significantly enhance our understanding of the molecular etiology of cancer-driven muscle cachexia and further discuss emerging therapeutic approaches seeking to simultaneously target those newly discovered mechanisms to efficiently curb this lethal syndrome.

Nutraceutical targeting of TLR4 signaling has potential for prevention of cancer cachexia

The mechanisms underlying cancer cachexia - the proximate cause of at least 20% of cancer-related deaths - have until recently remained rather obscure. New research, however, clarifies that cancers evoking cachexia release microvesicles rich in heat shock proteins 70 and 90, and that these extracellular heat shock proteins induce cachexia by serving as agonists for toll-like receptor 4 (TLR4) in skeletal muscle, macrophages, and adipocytes. Hence, safe nutraceutical measures which can down-regulate TLR4 signaling can be expected to aid prevention and control of cancer cachexia. There is reason to suspect that phycocyanobilin, ferulic acid, glycine, long-chain omega-3s, green tea catechins, β-hydroxy-β-methylbutyrate, carnitine, and high-dose biotin may have some utility in this regard.

An automatic diagnostic system based on deep learning, to diagnose hyperlipidemia

Background: Using artificial intelligence to assist in diagnosing diseases has become a contemporary research hotspot. Conventional automatic diagnostic method uses a conventional machine learning algorithm to distinguish features from which a professional doctor manually extracts features in diagnostic reports. But it can be difficult to collect large amounts of necessary medical data. Therefore, these methods face challenges with efficiency and accuracy. Method: Here, we proposed an automatic diagnostic system based on a deep learning algorithm to diagnose hyperlipidemia by using human physiological parameters. This model is a neural network which uses technologies of data extension and data correction. Firstly, we corrected and supplemented the original data by the method mentioned previously to solve the problem of lacking data. Secondly, the processed data were used to train a deep learning model. Deep learning model can automatically extract all the available information instead of artificially reducing the raw data. Therefore, it can reduce labor costs. The classifiers classify the data by using features previously mentioned. Finally, the system was evaluated with data from a test dataset. Result: It achieved 91.49% accuracy, 87.50% sensitivity, 93.33% specificity, and 87.50% precision with data from the test dataset. Conclusion: The proposed diagnostic method has a highly robust and accurate performance, and can be used for tentative diagnosis. It can automatically diagnose diseases by using human physiological parameters, thereby reducing labor cost, which results in effective improvement of clinical diagnostic efficiency.

Expression and putative role of mitochondrial transport proteins in cancer

Cancer cells undergo major changes in energy and biosynthetic metabolism. One of them is the Warburg effect, in which pyruvate is used for fermentation rather for oxidative phosphorylation. Another major one is their increased reliance on glutamine, which helps to replenish the pool of Krebs cycle metabolites used for other purposes, such as amino acid or lipid biosynthesis. Mitochondria are central to these alterations, as the biochemical pathways linking these processes run through these organelles. Two membranes, an outer and inner membrane, surround mitochondria, the latter being impermeable to most organic compounds. Therefore, a large number of transport proteins are needed to link the biochemical pathways of the cytosol and mitochondrial matrix. Since the transport steps are relatively slow, it is expected that many of these transport steps are altered when cells become cancerous. In this review, changes in expression and regulation of these transport proteins are discussed as well as the role of the transported substrates. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.

The emerging role of skeletal muscle oxidative metabolism as a biological target and cellular regulator of cancer-induced muscle wasting

While skeletal muscle mass is an established primary outcome related to understanding cancer cachexia mechanisms, considerable gaps exist in our understanding of muscle biochemical and functional properties that have recognized roles in systemic health. Skeletal muscle quality is a classification beyond mass, and is aligned with muscle's metabolic capacity and substrate utilization flexibility. This supplies an additional role for the mitochondria in cancer-induced muscle wasting. While the historical assessment of mitochondria content and function during cancer-induced muscle loss was closely aligned with energy flux and wasting susceptibility, this understanding has expanded to link mitochondria dysfunction to cellular processes regulating myofiber wasting. The primary objective of this article is to highlight muscle mitochondria and oxidative metabolism as a biological target of cancer cachexia and also as a cellular regulator of cancer-induced muscle wasting. Initially, we examine the role of muscle metabolic phenotype and mitochondria content in cancer-induced wasting susceptibility. We then assess the evidence for cancer-induced regulation of skeletal muscle mitochondrial biogenesis, dynamics, mitophagy, and oxidative stress. In addition, we discuss environments associated with cancer cachexia that can impact the regulation of skeletal muscle oxidative metabolism. The article also examines the role of cytokine-mediated regulation of mitochondria function, followed by the potential role of cancer-induced hypogonadism. Lastly, a role for decreased muscle use in cancer-induced mitochondrial dysfunction is reviewed.

Cachexia: a problem of energetic inefficiency

An alteration of energy balance is the immediate cause of the so-called cachexia. Although alterations of energy intake are often associated with cachexia, it has lately became clear that an increased energy expenditure is the main cause of wasting associated with different types of pathological conditions, such as cancer, infections or chronic heart failure among others. Different types of molecular mechanisms contribute to energy expenditure and, therefore, involuntary body weight loss; among them, adenosine triphosphate (ATP) consumption by sarcoplasmic reticulum Ca(2+) pumps could represent a key mechanism. In other cases, an increase in energy inefficiency will further contribute to energy imbalance.

Cancer cachexia is associated with a decrease in skeletal muscle mitochondrial oxidative capacities without alteration of ATP production efficiency

BACKGROUND

Cancer cachexia is a complex syndrome related to a negative energy balance resulting in muscle wasting. Implication of muscle mitochondrial bioenergetics alterations during cancer cachexia was suggested. Therefore, the aim of this study was to explore the efficiency of oxidative phosphorylation in skeletal muscle mitochondria in a preclinical model of cancer cachexia.

METHODS

Berlin-Druckrey IX rats with peritoneal carcinosis (PC) were used as a model of cancer cachexia with healthy pair-fed rats (PF) as control. Hindlimb muscle morphology and fibre type composition were analysed in parallel with ubiquitin ligases and UCP gene expression. Oxidative phosphorylation was investigated in isolated muscle mitochondria by measuring oxygen consumption and ATP synthesis rate.

RESULTS

PC rats underwent significant muscle wasting affecting fast glycolytic muscles due to a reduction in fibre cross-sectional area. MuRF1 and MAFbx gene expression were significantly increased (9- and 3.5-fold, respectively) in the muscle of PC compared to PF rats. Oxygen consumption in non-phosphorylating state and the ATP/O were similar in both groups. Muscle UCP2 gene was overexpressed in PC rats. State III and the uncoupled state were significantly lower in muscle mitochondria from PC rats with a parallel reduction in complex IV activity (-30 %).

CONCLUSION

This study demonstrated that there was neither alteration in ATP synthesis efficiency nor mitochondrial uncoupling in skeletal muscle of cachectic rats despite UCP2 gene overexpression. Muscle mitochondrial oxidative capacities were reduced due to a decrease in complex IV activity. This mitochondrial bioenergetics alteration could participate to insulin resistance, lipid droplet accumulation and lactate production.

Cancer cachexia

Cancer cachexia is a wasting syndrome characterized by weight loss, anorexia, asthenia and anemia. The pathogenicity of this syndrome is multifactorial, due to a complex interaction of tumor and host factors. The signs and symptoms of cachexia are considered as the prognostic parameters in cancer patients. This review gives an emphasis on the various mechanisms involved in cachexia and an insight into head and neck cancer cachexia.

L-Carnitine: an adequate supplement for a multi-targeted anti-wasting therapy in cancer

BACKGROUND & AIMS

Tumour growth is associated with weight loss resulting from both adipose and muscle wasting.

METHODS

Administration of L-carnitine (1 g/kg body weight) to rats bearing the AH-130 Yoshida ascites hepatoma, a highly cachectic rat tumour.

RESULTS

The treatment results in a significant improvement of food intake and in muscle weight (gastrocnemius, EDL and soleus). These beneficial effects are directly related to improved physical performance (total physical activity, mean movement velocity and total travelled distance). Administration of L-carnitine decreases proteasome activity and the expression of genes related with this activity, such as ubiquitin, C8 proteasome subunit and MuRF-1. Interestingly, L-carnitine treatment also decreases caspase-3 mRNA content therefore suggesting a modulation of apoptosis. Moreover, addition of 50 μM of L-carnitine to isolated EDL muscles results in a significant decrease in the proteolytic rate suggesting a direct effect.

CONCLUSIONS

It can be concluded that L-carnitine supplementation may be a good approach for a multi-targeted therapy for the treatment of cancer-related cachexia.

New insights into adipose tissue atrophy in cancer cachexia

Profound loss of adipose and other tissues is a hallmark of cancer cachexia, a debilitating condition associated with increased morbidity and mortality. Fat loss cannot be attributable to reduced appetite alone as it precedes the onset of anorexia and is much more severe in experimental models of cachexia than in food restriction. Morphological examination has shown marked remodelling of adipose tissue in cancer cachexia. It is characterised by the tissue containing shrunken adipocytes with a major reduction in cell size and increased fibrosis in the tissue matrix. The ultrastructure of 'slimmed' adipocytes has revealed severe delipidation and modifications in cell membrane conformation. Although the molecular mechanisms remain to be established, evidence suggests that altered adipocyte metabolism may lead to adipose atrophy in cancer cachexia. Increased lipolysis appears to be a key factor underlying fat loss, while inhibition of adipocyte development and lipid deposition may also contribute. Both tumour and host-derived factors are implicated in adipose atrophy. Zinc-alpha2-glycoprotein (ZAG), which is overexpressed by certain malignant tumours, has been identified as a novel adipokine. ZAG transcripts and protein expression in adipose tissue are up regulated in cancer cachexia but reduced with adipose tissue expansion in obesity. Studies in vitro demonstrate that recombinant ZAG stimulates lipolysis. ZAG may therefore act locally, as well as systemically, to promote lipid mobilisation in cancer cachexia. Further elucidation of ZAG function in adipose tissue may lead to novel targets for preventing adipose atrophy in malignancy.

Mechanisms of cancer cachexia

Up to 50% of cancer patients suffer from a progressive atrophy of adipose tissue and skeletal muscle, called cachexia, resulting in weight loss, a reduced quality of life, and a shortened survival time. Anorexia often accompanies cachexia, but appears not to be responsible for the tissue loss, particularly lean body mass. An increased resting energy expenditure is seen, possibly arising from an increased thermogenesis in skeletal muscle due to an increased expression of uncoupling protein, and increased operation of the Cori cycle. Loss of adipose tissue is due to an increased lipolysis by tumor or host products. Loss of skeletal muscle in cachexia results from a depression in protein synthesis combined with an increase in protein degradation. The increase in protein degradation may include both increased activity of the ubiquitin-proteasome pathway and lysosomes. The decrease in protein synthesis is due to a reduced level of the initiation factor 4F, decreased elongation, and decreased binding of methionyl-tRNA to the 40S ribosomal subunit through increased phosphorylation of eIF2 on the alpha-subunit by activation of the dsRNA-dependent protein kinase, which also increases expression of the ubiquitin-proteasome pathway through activation of NFkappaB. Tumor factors such as proteolysis-inducing factor and host factors such as tumor necrosis factor-alpha, angiotensin II, and glucocorticoids can all induce muscle atrophy. Knowledge of the mechanisms of tissue destruction in cachexia should improve methods of treatment.

Glycerolipid metabolism and signaling in health and disease

Maintenance of body temperature is achieved partly by modulating lipolysis by a network of complex regulatory mechanisms. Lipolysis is an integral part of the glycerolipid/free fatty acid (GL/FFA) cycle, which is the focus of this review, and we discuss the significance of this pathway in the regulation of many physiological processes besides thermogenesis. GL/FFA cycle is referred to as a "futile" cycle because it involves continuous formation and hydrolysis of GL with the release of heat, at the expense of ATP. However, we present evidence underscoring the "vital" cellular signaling roles of the GL/FFA cycle for many biological processes. Probably because of its importance in many cellular functions, GL/FFA cycling is under stringent control and is organized as several composite short substrate/product cycles where forward and backward reactions are catalyzed by separate enzymes. We believe that the renaissance of the GL/FFA cycle is timely, considering the emerging view that many of the neutral lipids are in fact key signaling molecules whose production is closely linked to GL/FFA cycling processes. The evidence supporting the view that alterations in GL/FFA cycling are involved in the pathogenesis of "fatal" conditions such as obesity, type 2 diabetes, and cancer is discussed. We also review the different enzymatic and transport steps that encompass the GL/FFA cycle leading to the generation of several metabolic signals possibly implicated in the regulation of biological processes ranging from energy homeostasis, insulin secretion and appetite control to aging and longevity. Finally, we present a perspective of the possible therapeutic implications of targeting this cycling.

Muscle unloading-induced metabolic remodeling is associated with acute alterations in PPARδ and UCP-3 expression

A number of physiological changes follow prolonged skeletal muscle unloading as occurs in spaceflight, bed rest, and hindlimb suspension (HLS) and also in aging. These include muscle atrophy, fiber type switching, and loss of the ability to switch between lipid and glucose usage, or metabolic inflexibility. The signaling and genomic events that precede these physiological manifestations have not been investigated in detail, particularly in regard to loss of metabolic flexibility. Here we used gene arrays to determine the effects of 24-h HLS on metabolic remodeling in mouse muscle. Acute unloading resulted in differential expression of a number of transcripts in soleus and gastrocnemius muscle, including many involved in lipid and glucose metabolism. These include the peroxisome proliferator-activated receptors (PPARs). In contrast to Ppar-α and Ppar-γ, which were downregulated by acute HLS, Ppar-δ was upregulated concomitant with increased expression of its downstream target, uncoupling protein-3 ( Ucp-3). However, differential expression of Ppar-δ was both acute and transient in nature, suggesting that regulation of PPARδ may represent an adaptive, compensatory response aimed at regulating fuel utilization and maintaining metabolic flexibility.

Cachexia in chronic kidney disease: role of inflammation and neuropeptide signaling

PURPOSE OF REVIEW

This review will update clinicians and basic scientists who are interested in the clinical relevance and molecular mechanism of uremic cachexia. Recent studies that examine the role of cytokines and hypothalamic neuropeptides are emphasized.

RECENT FINDINGS

A current hypothesis of the cause of cachexia in chronic illness is that proinflammatory cytokines, such as tumor necrosis factor-alpha, interleukin-6, and leptin, act on the central nervous system to alter the release and function of several key neurotransmitters, thereby altering both appetite and metabolic rate. Proinflammatory cytokines also activate the transcription factor nuclear factor-kappaB, resulting in decreased protein synthesis, and activate the ubiquitin-mediated proteolytic system, which is the major system involved in increased protein degradation.

SUMMARY

This review highlights the importance of melanocortin signaling in the pathogenesis of uremia-associated cachexia and the potential of peripheral administration of melanocortin-4 receptor antagonists as a novel therapeutic approach.

Energy homeostasis and cachexia in chronic kidney disease

Loss of protein stores, presenting as clinical wasting, is reported to have a prevalence of 30-60% and is an important risk factor for mortality in chronic kidney disease (CKD) patients. There is debate as to whether the clinical wasting in CKD patients represents malnutrition or cachexia. Malnutrition results from inadequate intake of nutrients, despite a good appetite, and manifests as weight loss associated with adaptive metabolic responses such as decreased basic metabolic rate and preservation of lean body mass at the expense of fat mass. Furthermore, the abnormalities in malnutrition can usually be overcome simply by supplying more food or altering the composition of the diet. In contrast, cachexia is characterized by maladaptive responses such as anorexia, elevated basic metabolic rate, wasting of lean body tissue, and underutilization of fat tissue for energy. Diet supplementation and intradialytic parenteral nutrition have not been successful in reversing cachexia in CKD. The etiology of cachexia in CKD is complex and multifactorial. Two major factors causing muscle wasting in uremia are acidosis and decreased insulin responses. Inflammation secondary to cytokines may also play a significant role. The hypoalbuminemia of CKD patients is principally associated with inflammation and not changes in food intake. There is also recent evidence that hypothalamic neuropeptides may be important in the downstream signaling of cytokines in the pathogenesis of cachexia in CKD. Elevated circulating levels of cytokines, such as leptin, may be an important cause of uremia-associated cachexia via signaling through the central melanocortin system. Further research into the molecular pathways leading to cachexia may lead to novel therapeutic therapy for this devastating and potentially fatal complication of CKD.

UCP3 in muscle wasting, a role in modulating lipotoxicity?

UCP3 has been postulated to function in the defense against lipid-induced oxidative muscle damage (lipotoxicity). We explored this hypothesis during cachexia in rats (zymosan-induced sepsis), a condition characterized by increased oxidative stress and supply of fatty acids to the muscle. Muscle UCP3 protein content was increased 2, 6 and 11 days after zymosan injection. Plasma FFA levels were increased at day 2, but dropped below control levels on days 6 and 11. Muscular levels of the lipid peroxidation byproduct 4-hydroxy-2-nonenal (4-HNE) were increased at days 6 and 11 in zymosan-treated rats, supporting a role for UCP3 in modulating lipotoxicity during cachexia.

Molecular pathways leading to cancer cachexia

Loss of body weight in cancer patients strongly influences morbidity and mortality. Recent studies have suggested that both tumor and host factors play a major role in tissue catabolism in cachexia, leading to upregulation of degradative pathways in both skeletal muscle and adipose tissue.

Cancer cachexia modifies the zonal distribution of lipid metabolism-related proteins in rat liver

Cancer cachexia is a syndrome that causes profound metabolic disruption. Lipid metabolism in the liver is markedly affected. We investigated the effect of cachexia upon liver-acinus lipid-metabolism zonation in Walker 245 carcinosarcoma-bearing rats (TB). The expression of protein (by Western blotting) and mRNA (by semi-quantitative polymerase chain reaction) of the enzymes of the carnitine palmitoyltransferase system (CPT I and CPT II) and of liver fatty-acid-binding protein (L-FABP) was studied. Although no changes were found for these parameters, the maximal activities (by radioassay) of CPT I and II were reduced (P<0.05) in TB compared with controls. CPT II activity in the perivenous (PV) region was higher in TB compared with controls. The distribution of CPT II and L-FABP (by immunohistochemistry) within the acinus was modified by cachexia: whereas CPT II positivity was restricted to the PV zone, L-FABP labelling shifted from periportal (control) to perivenous (TB) zone. These changes in metabolic zonation, together with decreased CPT II activity, may contribute to the aggravation of cachexia.

Activation of UCPs gene expression in skeletal muscle can be independent on both circulating fatty acids and food intake. Involvement of ROS in a model of mouse cancer cachexia

Implantation of a fast growing tumour to mice (Lewis lung carcinoma) resulted in a clear cachectic state characterized by a profound muscle wasting. This was accompanied by a significant increase in both UCP2 and UCP3 gene expression in skeletal muscle and heart. Interestingly, this increase in gene expression was not linked to a rise in circulating fatty acids or in a decrease in food intake, as previously reported in other pathophysiological states. These results question the concept that hyperlipaemia is the only factor controlling UCP gene expression in different pathophysiological conditions. In addition, the present work suggests that UCPs might participate in a counter-regulatory mechanism to lower the production of ROS.

Long–term dietary high protein intake up–regulates tissue specific gene expression of uncoupling proteins 1 and 2 in rats

BACKGROUND

The consequences of chronic high protein (HP) diets are discussed controversially and are not well understood. Rats adapted to HP exposure show an increased amino acid and fat oxidation and lower feed energy efficiency. We hypothesized that the dietary protein level can affect gene expression of uncoupling protein (UCP) homologues which is suggested to be involved in thermogenesis, substrate oxidation, and energy expenditure.

AIM OF THE STUDY

To assess the mRNA expression of UCP homologues in various tissues of rats fed HP diets and to relate UCP gene expression to various parameters of substrate and energy metabolism. To obtain further indications for the possible involvement of UCP in reducing feed energy efficiency under conditions of HP exposure.

METHODS

Adult rats were adapted to casein based diets containing either 13.8% (adequate, AP), 25.7% (medium, MP), or 51.3 % (high, HP) crude protein. Rats were fed for 8 wk and killed in the postabsorptive state. Energy expenditure and mRNA expression were measured using indirect calorimetry and Northern blot analysis, respectively. Pearson correlation coefficients were calculated to determine relationships between UCP mRNA expression and metabolic parameters.

RESULTS

Hepatic UCP2 mRNA expression was increased by MP and HP diets compared to AP diet. In skeletal muscle UCP2 mRNA expression was lowest under MP conditions. UCP1 mRNA expression in brown adipose tissue (BAT) was significantly increased by HP exposure. The values were inversely associated with feed energy efficiency and positively with energy expenditure and oxygen consumption in the dark period. Skeletal muscle UCP2 and -3 mRNA expression strongly correlated with the plasma free fatty acid concentration, whereas BAT UCP1 and hepatic UCP2 gene expression did not.

CONCLUSIONS

Our results indicate that hepatic UCP2 and BAT UCP1 mRNA expression is related to the level of dietary protein intake. This suggests a role of UCPs in substrate oxidation and in thermogenesis under conditions of HP exposure.

Peripheral mechanisms involved with catabolism

PURPOSE OF REVIEW

This review summarizes recent developments concerning the mechanisms of skeletal muscle and adipose tissue breakdown, which are a hallmark of cachexia during many diseases. Current knowledge on the hypermetabolism which often contributes to cachexia is also considered.

RECENT FINDINGS

Recent studies have identified interactions between Ca2+, proinflammatory cytokines (in particular tumor necrosis factor-alpha) and the activation of transcription factors (e.g. nuclear factor-kappaB) in the stimulation of major proteolytic pathways in cachexia. Progress has also been made in explaining the inhibiting effects of several drugs on protein breakdown. Advances in our understanding of the mechanisms of adipose tissue catabolism in cachexia include demonstrations that (1) tumor necrosis factor-alpha, in addition to its direct lipolytic effect, promotes adipose tissue breakdown by inhibiting adipocyte differentiation and increasing adipocyte apoptosis, (2) interleukin-6 has a lipolytic effect, and (3) chemokines are expressed by adipocytes and interact with tumor necrosis factor-alpha to cause lipolysis. Concerning the hypermetabolism in cachexia, new evidence supports previous theories that uncoupling protein-2 and 3 are primarily involved in the generation of reactive oxygen species and in the control of fatty acid flux across the mitochondrial membrane, respectively. Furthermore, the cytokine-induced transcriptional coactivator-1 for the peroxysome proliferator-activated receptor-gamma was recently identified as a contributor to hypermetabolism.

SUMMARY

These new insights into major catabolic pathways during cachexia provide a focus for future studies in this area and may help to develop promising therapeutic approaches.

The role of uncoupling proteins in pathophysiological states

Until very recently, the uncoupling protein-1 (UCP1), present only in brown adipose tissue (BAT), was considered to be the only mitochondrial carrier protein that stimulated heat production by dissipating the proton gradient generated during respiration across the inner mitochondrial membrane and therefore uncoupling respiration from ATP synthesis. Recently, new uncoupling proteins, UCP2, UCP3, and UCP4, and brain mitochondrial carrier protein-1 (BMCP-1) have been described in mammalian tissues. The present review deals with the possible role of these proteins in different pathological conditions involving alterations in energy balance such as obesity or cachexia. In conclusion, the emergence of the UCP family has altered the approaches to bioenergetics and stressed the importance of uncoupling respiration in different pathophysiological conditions. An extensive qualitative and quantitative characterization of the new members of the UCP family in mammalian tissues will allow a better understanding of the molecular and regulatory mechanisms of thermogenesis and energy metabolism. At this point, we hope that the knowledge presented in the present review will not only stimulate a debate about the role of the UCP family in disease but also lead to applications beneficial for human health.

Skeletal muscle uncoupling protein 3 (UCP3): mitochondrial uncoupling protein in search of a function

The uncoupling protein 1 homologue, uncoupling protein 3, is able to uncouple adenosine triphosphate production from mitochondrial respiration, thereby dissipating energy as heat and affecting the efficiency of energy metabolism. Uncoupling protein 3 is expressed predominantly in skeletal muscle, and has been associated with whole-body energy metabolism. However, on the basis of present evidence it has been concluded that the primary function of uncoupling protein 3 is not in the regulation of energy expenditure. For example, fasting, an energy expenditure attenuating condition, upregulates uncoupling protein 3 expression, and uncoupling protein 3 knockout mice have a normal metabolic rate. The exact function of uncoupling protein 3 remains to be elucidated, but at present putative roles for uncoupling protein 3 include involvement in the regulation of the production of reactive oxygen species, mitochondrial fatty acid transport and the regulation of glucose metabolism in skeletal muscle. Because all these putative functions assume that uncoupling protein 3 affects mitochondrial coupling, a secondary effect of the function of uncoupling protein 3 might still be that it influences (but not regulates) energy metabolism, consistent with observations in linkage and association studies. Therefore, uncoupling protein 3 remains an interesting target for pharmacological upregulation in the treatment of obesity and diabetes.