UCP2, not a physiologically relevant uncoupler but a glucose sparing switch impacting ROS production and glucose sensing

In mammals the two proteins UCP2 and UCP3 are highly similar to the mitochondrial uncoupling protein found in the brown adipose tissue (UCP1). Accordingly, it was proposed that UCP2 and UCP3 are also uncoupling proteins i.e. protonophores with impact on mitochondrial ROS production and glucose signaling. However, it appears now impossible to explain the physiological relevance of the new UCPs uniquely by their uncoupling activity as observed in vitro. Therefore, we propose a metabolic hypothesis in which UCP2 acts through a transport distinct of the proton transport. A consequence of this transport activity would be a decrease of the mitochondrial oxidation of the pyruvate originating from glucose. This would put UCP2 and UCP3 in a crucial position to influence cellular metabolism. The tight control exerted on UCP2 expression appears consistent with it. In this hypothesis, UCP2/3 would allow a cell to remain glycolytic within an aerobic organism. This tallies with the high expression level of UCP2 or UCP3 in glycolytic cells. The metabolic hypothesis would explain the spectacular modifications associated with UCP2 manipulation as well as the uncoupling activity usually called for and which in fact remains elusive in vivo.     

Energy metabolism in uncoupling protein 3 gene knockout mice

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.     

Loss of UCP2 Attenuates Mitochondrial Dysfunction without Altering ROS Production and Uncoupling Activity

Although mitochondrial dysfunction is often accompanied by excessive reactive oxygen species (ROS) production, we previously showed that an increase in random somatic mtDNA mutations does not result in increased oxidative stress. Normal levels of ROS and oxidative stress could also be a result of an active compensatory mechanism such as a mild increase in proton leak. Uncoupling protein 2 (UCP2) was proposed to play such a role in many physiological situations. However, we show that upregulation of UCP2 in mtDNA mutator mice is not associated with altered proton leak kinetics or ROS production, challenging the current view on the role of UCP2 in energy metabolism. Instead, our results argue that high UCP2 levels allow better utilization of fatty acid oxidation resulting in a beneficial effect on mitochondrial function in heart, postponing systemic lactic acidosis and resulting in longer lifespan in these mice. This study proposes a novel mechanism for an adaptive response to mitochondrial cardiomyopathy that links changes in metabolism to amelioration of respiratory chain deficiency and longer lifespan.     

Cancer cells metabolically “fertilize” the tumor microenvironment with hydrogen peroxide,driving the Warburg effect: Implications for PET imaging of human tumors

Previously, we proposed that cancer cells behave as metabolic parasites, as they use targeted oxidative stress as a “weapon” to extract recycled nutrients from adjacent stromal cells. Oxidative stress in cancer-associated fibroblasts triggers autophagy and mitophagy, resulting in compartmentalized cellular catabolism, loss of mitochondrial function, and the onset of aerobic glycolysis, in the tumor stroma. As such, cancer-associated fibroblasts produce high-energy nutrients (such as lactate and ketones) that fuel mitochondrial biogenesis and oxidative metabolism in cancer cells. We have termed this new energy-transfer mechanism the “reverse Warburg effect.” To further test the validity of this hypothesis, here we used an in vitro MCF7-fibroblast co-culture system and quantitatively measured a variety of metabolic parameters by FACS analysis (analogous to laser-capture micro-dissection). Mitochondrial activity, glucose uptake and ROS production were measured with highly-sensitive fluorescent probes (MitoTracker, NBD-2-deoxy-glucose and DCF-DA). Interestingly, using this approach, we directly show that cancer cells initially secrete hydrogen peroxide that then triggers oxidative stress in neighboring fibroblasts. Thus, oxidative stress is contagious (spreads like a virus) and is propagated laterally and vectorially from cancer cells to adjacent fibroblasts. Experimentally, we show that oxidative stress in cancer-associated fibroblasts quantitatively reduces mitochondrial activity and increases glucose uptake, as the fibroblasts become more dependent on aerobic glycolysis. Conversely, co-cultured cancer cells show significant increases in mitochondrial activity and corresponding reductions in both glucose uptake and GLUT1 expression. Pre-treatment of co-cultures with extracellular catalase (an anti-oxidant enzyme that detoxifies hydrogen peroxide) blocks the onset of oxidative stress and potently induces the death of cancer cells, likely via starvation. Given that cancer-associated fibroblasts show the largest increases in glucose uptake, we suggest that PET imaging of human tumors, with Fluoro-2-deoxy-D-glucose (F-2-DG), may be specifically detecting the tumor stroma, rather than epithelial cancer cells.Key words: tumor stroma, microenvironment, hydrogen peroxide, aerobic glycolysis, mitochondrial oxidative phosphorylation, glucose uptake, oxidative stress, reactive oxygen species (ROS), cancer associated fibroblasts, PET imaging, the field effect, caveolin-1     

Glycolytic Switch in Response to Betulinic Acid in Non-Cancer Cells

The naturally occurring triterpenoid betulinic acid (BA) shows pronounced polypharmacology ranging from anti-inflammatory to anti-lipogenic activities. Recent evidence suggests that rather diverse cellular signaling events may be attributed to the same common upstream switch in cellular metabolism. In this study we therefore examined the metabolic changes induced by BA (10 µM) administration, with focus on cellular glucose metabolism. We demonstrate that BA elevates the rates of cellular glucose uptake and aerobic glycolysis in mouse embryonic fibroblasts with concomitant reduction of glucose oxidation. Without eliciting signs of obvious cell death BA leads to compromised mitochondrial function, increased expression of mitochondrial uncoupling proteins (UCP) 1 and 2, and liver kinase B1 (LKB1)-dependent activation AMP-activated protein kinase. AMPK activation accounts for the increased glucose uptake and glycolysis which in turn are indispensable for cell viability upon BA treatment. Overall, we show for the first time a significant impact of BA on cellular bioenergetics which may be a central mediator of the pleiotropic actions of BA.     

Overexpression of UCP3 in cultured human muscle lowers mitochondrial membrane potential, raises ATP/ADP ratio, and favors fatty acid vs. glucose oxidation.

The skeletal muscle mitochondrial uncoupling protein-3 (UCP3) promotes substrate oxidation, but direct evidence for its metabolic role is lacking. Here, we show that UCP3 overexpression in cultured human muscle cells decreased mitochondrial membrane potential (DYm). Despite this, the ATP content was not significantly decreased compared with control cells, whereas ADP content was reduced and thus the ATP/ADP ratio raised. This finding was contrasts with the effect caused by the chemical protonophoric uncoupler, CCCP, which lowered DYm, ATP, and the ATP/ADP ratio. UCP3-overexpression enhanced oxidation of oleate, regardless of the presence of glucose, whereas etomoxir, which blocks fatty acid entry to mitochondria, suppressed the UCP3 effect. Glucose oxidation was stimulated in UCP3-overexpressing cells, but this effect was inhibited by oleate. UCP3 caused weak increase of both 2-Deoxyglucose uptake and glycolytic rate, which differed from the marked stimulation by CCCP. We concluded that UCP3 promoted nutrient oxidation by lowering DYm and enhanced fatty acid-dependent inhibition of glucose oxidation. Unlike the uncoupler CCCP, however, UCP3 raised the ATP/ADP ratio and modestly increased glucose uptake and glycolysis. We propose that this differential effect provides a biological significance to UCP3, which is up-regulated in metabolic stress situations where it could be involved in nutrient partitioning.     

UCP2 regulates energy metabolism and differentiation potential of human pluripotent stem cells

It has been assumed, based largely on morphologic evidence, that human pluripotent stem cells (hPSCs) contain underdeveloped, bioenergetically inactive mitochondria. In contrast, differentiated cells harbour a branched mitochondrial network with oxidative phosphorylation as the main energy source. A role for mitochondria in hPSC bioenergetics and in cell differentiation therefore remains uncertain. Here, we show that hPSCs have functional respiratory complexes that are able to consume O(2) at maximal capacity. Despite this, ATP generation in hPSCs is mainly by glycolysis and ATP is consumed by the F(1)F(0) ATP synthase to partially maintain hPSC mitochondrial membrane potential and cell viability. Uncoupling protein 2 (UCP2) plays a regulating role in hPSC energy metabolism by preventing mitochondrial glucose oxidation and facilitating glycolysis via a substrate shunting mechanism. With early differentiation, hPSC proliferation slows, energy metabolism decreases, and UCP2 is repressed, resulting in decreased glycolysis and maintained or increased mitochondrial glucose oxidation. Ectopic UCP2 expression perturbs this metabolic transition and impairs hPSC differentiation. Overall, hPSCs contain active mitochondria and require UCP2 repression for full differentiation potential.     

Effects of the presence, absence, and overexpression of uncoupling protein-3 on adiposity and fuel metabolism in congenic mice

Uncoupling protein-3 (UCP3) is a poorly understood mitochondrial inner membrane protein expressed predominantly in skeletal muscle. The aim of this study was to examine the effects of the absence or constitutive physiological overexpression of UCP3 on whole body energy metabolism, glucose tolerance, and muscle triglyceride content. Congenic male UCP3 knockout mice (Ucp3-/-), wild-type, and transgenic UCP3 overexpressing (UCP3Tg) mice were fed a 10% fat diet for 4 or 8 mo after they were weaned. UCP3Tg mice had lower body weights and were less metabolically efficient than wild-type or Ucp3-/- mice, but they were not hyperphagic. UCP3Tg mice had smaller epididymal white adipose tissue and brown adipose tissue (BAT) depots; however, there were no differences in muscle weights. Glucose and insulin tolerance tests revealed that both UCP3Tg and Ucp3-/- mice were protected from development of impaired glucose tolerance and were more sensitive to insulin. 2-Deoxy-D-[1-3H]glucose tracer studies showed increased uptake of glucose into BAT and increased storage of liver glycogen in Ucp3-/- mice. Assessments of intramuscular triglyceride (IMTG) revealed decreases in quadriceps of UCP3Tg mice compared with wild-type and Ucp3-/- mice. When challenged with a 45% fat diet, Ucp3-/- mice showed increased accumulation of IMTG compared with wild-type mice, which in turn had greater IMTG than UCP3Tg mice. Results are consistent with a role for UCP3 in preventing accumulation of triglyceride in both adipose tissue and muscle.     

Shc depletion stimulates brown fat activity in vivo and in vitro

Adipose tissue is an important metabolic organ that integrates a wide array of homeostatic processes and is crucial for whole‐body insulin sensitivity and energy metabolism. Brown adipose tissue (BAT) is a key thermogenic tissue with a well‐established role in energy expenditure. BAT dissipates energy and protects against both hypothermia and obesity. Thus, BAT stimulation therapy is a rational strategy for the looming pandemic of obesity, whose consequences and comorbidities have a huge impact on the aged. Shc‐deficient mice (ShcKO) were previously shown to be lean, insulin sensitive, and resistant to high‐fat diet and obesity. We investigated the contribution of BAT to this phenotype. Insulin‐dependent BAT glucose uptake was higher in ShcKO mice. Primary ShcKO BAT cells exhibited increased mitochondrial respiration; increased expression of several mitochondrial and lipid‐oxidative enzymes was observed in ShcKO BAT. Levels of brown fat‐specific markers of differentiation, UCP1, PRDM16, ELOVL3, and Cox8b, were higher in ShcKO BAT. In vitro, Shc knockdown in BAT cell line increased insulin sensitivity and metabolic activity. In vivo, pharmacological stimulation of ShcKO BAT resulted in higher energy expenditure. Conversely, pharmacological inhibition of BAT abolished the improved metabolic parameters, that is the increased insulin sensitivity and glucose tolerance of ShcKO mice. Similarly, in vitro Shc knockdown in BAT cell lines increased their expression of UCP1 and metabolic activity. These data suggest increased BAT activity significantly contributes to the improved metabolic phenotype of ShcKO mice.     

Effects of fasting on muscle mitochondrial energetics and fatty acid metabolism in Ucp3(-/-) and wild-type mice

Uncoupling protein-3 (UCP3) is a mitochondrial carrier protein of as yet undefined physiological function. To elucidate characteristics of its function, we studied the effects of fasting on resting metabolic rate, respiratory quotient, muscle Ucp3 expression, and mitochondrial proton leak in wild-type and Ucp3(-/-) mice. Also analyzed were the fatty acid compositions of skeletal muscle mitochondria in fed and fasted Ucp3(-/-) and wild-type mice. In wild-type mice, fasting caused significant increases in Ucp3 (4-fold) and Ucp2 (2-fold) mRNA but did not significantly affect mitochondrial proton leak. State 4 oxygen consumption was not affected by fasting in either of the two groups. However, protonmotive force was consistently higher in mitochondria of Ucp3(-/-) animals (P = 0.03), and fasting further augmented protonmotive force in Ucp3(-/-) mice; there was no effect in wild-type mitochondria. Resting metabolic rates decreased with fasting in both groups. Ucp3(-/-) mice had higher respiratory quotients than wild-type mice in fed resting states, indicating impaired fatty acid oxidation. Altogether, results show that the fasting-induced increases in Ucp2 and Ucp3 do not correlate with increased mitochondrial proton leak but support a role for UCP3 in fatty acid metabolism.     

UCP2 modulates cell proliferation through the MAPK/ERK pathway during erythropoiesis and has no effect on heme biosynthesis

UCP2, an inner membrane mitochondrial protein, has been implicated in bioenergetics and reactive oxygen species (ROS) modulation. High levels of UCP2 mRNA were recently found in erythroid cells where UCP2 is hypothesized to function as a facilitator of heme synthesis and iron metabolism by reducing ROS production. We examined UCP2 protein expression and role in mice erythropoiesis in vivo. UCP2 was mainly expressed at early stages of erythroid maturation when cells are not fully committed in heme synthesis. Iron incorporation into heme was unaltered in reticulocytes from UCP2-deficient mice. Although heme synthesis was not influenced by UCP2 deficiency, mice lacking UCP2 had a delayed recovery from chemically induced hemolytic anemia. Analysis of progenitor cells from bone marrow and fetal liver both in vitro and in vivo revealed that UCP2 deficiency results in a significant decrease in cell proliferation at the erythropoietin-dependent phase of erythropoiesis. This was accompanied by reduction in the phosphorylated form of ERK, a ROS-dependent cytosolic regulator of cell proliferation. Analysis of ROS in UCP2 null erythroid cells revealed altered distribution of ROS, resulting in decreased cytosolic and increased mitochondrial ROS. Restoration of the cytosol oxidative state of erythroid progenitor cells by the pro-oxidant Paraquat reversed the effect of UCP2 deficiency on cell proliferation in in vitro differentiation assays. Together, these results indicate that UCP2 is a regulator of erythropoiesis and suggests that inhibition of UCP2 function may contribute to the development of anemia.     

Remodeling of Oxidative Energy Metabolism by Galactose Improves Glucose Handling and Metabolic Switching in Human Skeletal Muscle Cells

Cultured human myotubes have a low mitochondrial oxidative potential. This study aims to remodel energy metabolism in myotubes by replacing glucose with galactose during growth and differentiation to ultimately examine the consequences for fatty acid and glucose metabolism. Exposure to galactose showed an increased [¹⁴C]oleic acid oxidation, whereas cellular uptake of oleic acid uptake was unchanged. On the other hand, both cellular uptake and oxidation of [¹⁴C]glucose increased in myotubes exposed to galactose. In the presence of the mitochondrial uncoupler carbonylcyanide p-trifluormethoxy-phenylhydrazone (FCCP) the reserve capacity for glucose oxidation was increased in cells grown with galactose. Staining and live imaging of the cells showed that myotubes exposed to galactose had a significant increase in mitochondrial and neutral lipid content. Suppressibility of fatty acid oxidation by acute addition of glucose was increased compared to cells grown in presence of glucose. In summary, we show that cells grown in galactose were more oxidative, had increased oxidative capacity and higher mitochondrial content, and showed an increased glucose handling. Interestingly, cells exposed to galactose showed an increased suppressibility of fatty acid metabolism. Thus, galactose improved glucose metabolism and metabolic switching of myotubes, representing a cell model that may be valuable for metabolic studies related to insulin resistance and disorders involving mitochondrial impairments.     

Uncoupling proteins and the control of mitochondrial reactive oxygen species production

Reactive oxygen species (ROS), natural by-products of aerobic respiration, are important cell signaling molecules, which left unchecked can severely impair cellular functions and induce cell death. Hence, cells have developed a series of systems to keep ROS in the nontoxic range. Uncoupling proteins (UCPs) 1-3 are mitochondrial anion carrier proteins that are purported to play important roles in minimizing ROS emission from the electron transport chain. The function of UCP1 in this regard is highly contentious. However, UCPs 2 and 3 are generally thought to be activated by ROS or ROS by-products to induce proton leak, thus providing a negative feedback loop for mitochondrial ROS production. In our laboratory, we have not only confirmed that ROS activate UCP2 and UCP3, but also demonstrated that UCP2 and UCP3 are controlled by covalent modification by glutathione. Furthermore, the reversible glutathionylation is required to activate/inhibit UCP2 and UCP3, but not UCP1. Hence, our findings are consistent with the notion that UCPs 2 and 3 are acutely activated by ROS, which then directly modulate the glutathionylation status of the UCP to decrease ROS emission and participate in cell signaling mechanisms.     

Cancer cells metabolically "fertilize" the tumor microenvironment with hydrogen peroxide,driving the Warburg effect

Previously, we proposed that cancer cells behave as metabolic parasites, as they use targeted oxidative stress as a “weapon” to extract recycled nutrients from adjacent stromal cells. Oxidative stress in cancer-associated fibroblasts triggers autophagy and mitophagy, resulting in compartmentalized cellular catabolism, loss of mitochondrial function, and the onset of aerobic glycolysis, in the tumor stroma. As such, cancer-associated fibroblasts produce high-energy nutrients (such as lactate and ketones) that fuel mitochondrial biogenesis, and oxidative metabolism in cancer cells. We have termed this new energy-transfer mechanism the “reverse Warburg effect.” To further test the validity of this hypothesis, here we used an in vitro MCF7-fibroblast co-culture system, and quantitatively measured a variety of metabolic parameters by FACS analysis (analogous to laser-capture micro-dissection). Mitochondrial activity, glucose uptake, and ROS production were measured with highly-sensitive fluorescent probes (MitoTracker, NBD-2-deoxy-glucose, and DCF-DA). Interestingly, using this approach, we directly show that cancer cells initially secrete hydrogen peroxide that then triggers oxidative stress in neighboring fibroblasts. Thus, oxidative stress is contagious (spreads like a virus) and is propagated laterally and vectorially from cancer cells to adjacent fibroblasts. Experimentally, we show that oxidative stress in cancer-associated fibroblasts quantitatively reduces mitochondrial activity, and increases glucose uptake, as the fibroblasts become more dependent on aerobic glycolysis. Conversely, co-cultured cancer cells show significant increases in mitochondrial activity, and corresponding reductions in both glucose uptake and GLUT1 expression. Pre-treatment of co-cultures with extracellular catalase (an anti-oxidant enzyme that detoxifies hydrogen peroxide) blocks the onset of oxidative stress, and potently induces the death of cancer cells, likely via starvation. Given that cancer-associated fibroblasts show the largest increases in glucose uptake, we suggest that PET imaging of human tumors, with Fluoro-2-deoxy-D-glucose (F-2-DG), may be specifically detecting the tumor stroma, rather than epithelial cancer cells.     

The contraction induced increase in gene expression of peroxisome proliferator-activated receptor (PPAR)-gamma coactivator 1alpha (PGC-1alpha), mitochondrial uncoupling protein 3 (UCP3) and hexokinase II (HKII) in primary rat skeletal muscle cells is dependent on reactive oxygen species

We evaluated the role of reactive oxygen species (ROS) for the contraction induced increase in expression of PGC-1alpha, HKII and UCP3 mRNA. Rat skeletal muscle cells were subjected to acute or repeated electrostimulation in the presence and absence of antioxidants. Contraction of muscle cells lead to an increased H2O2 formation, as measured by oxidation of H2HFF. Acute contraction of the muscle cells lead to a transient increase in PGC-1alpha and UCP3 mRNA by 172 and 65%, respectively (p<0.05), whereas this increase was absent in the presence of antioxidants. Repeated contraction sessions induced a sustained elevation in PGC-1alpha and UCP3 mRNA and a transient increase in HKII (p<0.05) and this effect was not present with treatment of cells with either an antioxidant cocktail or with GPX+GSH. Incubation of cells for 10 days with ROS produced by xanthine oxidase/xanthine increased the level of PGC-1alpha, HKII and UCP3 mRNA by 175, 58 and 115%, respectively (p<0.05). A 10-day incubation of cells with antioxidants was found to have no effect on the basal mRNA content (p>0.05). The present data demonstrate that contraction of skeletal muscle cells leads to an enhanced formation of ROS and an elevation in PGC-1alpha, UCP3 and HKII mRNA content which is abolished in the presence of antioxidants, suggesting that ROS are of importance for the contraction induced increase in expression of these genes in skeletal muscle.     

Stimulation of mitochondrial oxidative capacity in white fat independent of UCP1: A key to lean phenotype

We are facing a revival of the strategy to counteract obesity and associated metabolic disorders by inducing thermogenesis mediated by mitochondrial uncoupling protein-1 (UCP1). Thus, the main focus is on the adaptive non-shivering thermogenesis occurring both in the typical depots of brown adipose tissue (BAT) and in UCP1-containing cells that could be induced in white adipose tissue (WAT). Because contribution of WAT to resting metabolic rate is relatively small, the possibility to reduce adiposity by enhancing energy expenditure in classical white adipocytes is largely neglected. However, several pieces of evidence support a notion that induction of energy expenditure based on oxidation of fatty acids (FA) in WAT may be beneficial for health, namely: (i) studies in both humans and rodents document negative association between oxidative capacity of mitochondria in WAT and obesity; (ii) pharmacological activation of AMPK in rats as well as cold-acclimation of UCP1-ablated mice results in obesity resistance associated with increased oxidative capacity in WAT; and (iii) combined intervention using long-chain n-3 polyunsaturated FA (omega 3) and mild calorie restriction exerted synergism in the prevention of obesity in mice fed a high-fat diet; this was associated with strong hypolipidemic and insulin-sensitizing effects, as well as prevention of inflammation, and synergistic induction of mitochondrial oxidative phosphorylation (OXPHOS) and FA oxidation, specifically in epididymal WAT. Importantly, these changes occurred without induction of UCP1 and suggested the involvement of: (i) futile substrate cycle in white adipocytes, which is based on lipolysis of intracellular triacylglycerols and re-esterification of FA, in association with the induction of mitochondrial OXPHOS capacity, β-oxidation, and energy expenditure; (ii) endogenous lipid mediators (namely endocannabinoids, eicosanoids, prostanoids, resolvins, and protectins) and their cognate receptors; and (iii) AMP-activated protein kinase in WAT. Quantitatively, the strong induction of FA oxidation in WAT in response to the combined intervention is similar to that observed in the transgenic mice rendered resistant to obesity by ectopic expression of UCP1 in WAT. The induction of UCP1-independent FA oxidation and energy expenditure in WAT in response to the above physiological stimuli could underlie the amelioration of obesity and low-grade WAT inflammation, and it could reduce the release of FA from adipose tissue and counteract harmful consequences of lipid accumulation in other tissues. In this respect, new combination treatments may be designed using naturally occurring micronutrients (e.g. omega 3), reduced calorie intake or pharmaceuticals, exerting synergism in the induction of the mitochondrial OXPHOS capacity and stimulation of lipid catabolism in white adipocytes, and improving metabolic flexibility of WAT. The role of mutual interactions between adipocytes and immune cells contained in WAT in tissue metabolism should be better characterised. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.     

解偶联蛋白2与衰老

解偶联蛋白（uncoupling protein,UCP）属于内膜上的一类载体蛋白,其生理作用是消除线粒体膜电位,使氧化磷酸化解偶联,从而抑制酸腺苷（adenosine triphosphate,ATP）合成,能量以热能形式散发。研究发现UCP2具有一种质子漏功能,表现对线粒体活性氧（reactive oxygen species,ROS）产生的调控和降低ROS的功能：在不同组织器官,不同代谢状态下UCP2的生理功能对细胞的影响不完全相同。特别是近年来的研究发现,UCP2参与了能量代谢、ROS的产生、子宫内膜退化、衰老等过程,并且与非酒精性脂肪肝、抗肥胖、动脉粥样硬化、局部缺血以及缺血再灌注损伤和2型糖尿病等有一定的相关性,倍受人们的关注。     

Exercise training reverses impaired skeletal muscle metabolism induced by artificial selection for low aerobic capacity

We have used a novel model of genetically imparted endurance exercise capacity and metabolic health to study the genetic and environmental contributions to skeletal muscle glucose and lipid metabolism. We hypothesized that metabolic abnormalities associated with low intrinsic running capacity would be ameliorated by exercise training. Selective breeding for 22 generations resulted in rat models with a fivefold difference in intrinsic aerobic capacity. Low (LCR)- and high (HCR)-capacity runners remained sedentary (SED) or underwent 6 wk of exercise training (EXT). Insulin-stimulated glucose transport, insulin signal transduction, and rates of palmitate oxidation were lower in LCR SED vs. HCR SED (P < 0.05). Decreases in glucose and lipid metabolism were associated with decreased β?-adrenergic receptor (β?-AR), and reduced expression of Nur77 target proteins that are critical regulators of muscle glucose and lipid metabolism [uncoupling protein-3 (UCP3), fatty acid transporter (FAT)/CD36; P < 0.01 and P < 0.05, respectively]. EXT reversed the impairments to glucose and lipid metabolism observed in the skeletal muscle of LCR, while increasing the expression of β?-AR, Nur77, GLUT4, UCP3, and FAT/CD36 (P < 0.05) in this tissue. However, no metabolic improvements were observed following exercise training in HCR. Our results demonstrate that metabolic impairments resulting from genetic factors (low intrinsic aerobic capacity) can be overcome by an environmental intervention (exercise training). Furthermore, we identify Nur77 as a potential mechanism for improved skeletal muscle metabolism in response to EXT.     

Role of mitochondria-associated hexokinase II in cancer cell death induced by 3-bromopyruvate

It has long been observed that cancer cells rely more on glycolysis to generate ATP and actively use certain glycolytic metabolic intermediates for biosynthesis. Hexokinase II (HKII) is a key glycolytic enzyme that plays a role in the regulation of the mitochondria-initiated apoptotic cell death. As a potent inhibitor of hexokinase, 3-bromopyruvate (3-BrPA) is known to inhibit cancer cell energy metabolism and trigger cell death, supposedly through depletion of cellular ATP. The current study showed that 3-BrPA caused a covalent modification of HKII protein and directly triggered its dissociation from mitochondria, leading to a specific release of apoptosis-inducing factor (AIF) from the mitochondria to cytosol and eventual cell death. Co-immunoprecipitation revealed a physical interaction between HKII and AIF. Using a competitive peptide of HKII, we showed that the dissociation of hexokinase II from mitochondria alone could cause apoptotic cell death, especially in the mitochondria-deficient ρ⁰ cells that highly express HKII. Interestingly, the dissociation of HKII itself did not directly affect the mitochondrial membrane potential, ROS generation, and oxidative phosphorylation. Our study suggests that the physical association between HKII and AIF is important for the normal localization of AIF in the mitochondria, and disruption of this protein complex by 3-BrPA leads to their release from the mitochondria and eventual cell death.