ROS production in brown adipose tissue mitochondria: the question of UCP1-dependence

Age-Dependent Changes in the Production of Mitochondrial Reactive Oxygen Species in Human Skeletal Muscle

A decrease in muscle mass and its functionality (strength, endurance, and insulin sensitivity) is one of the integral signs of aging. One of the triggers of aging is an increase in the production of mitochondrial reactive oxygen species. Our study was the first to examine age-dependent changes in the production of mitochondrial reactive oxygen species related to a decrease in the proportion of mitochondria-associated hexokinase-2 in human skeletal muscle. For this purpose, a biopsy was taken from m. vastus lateralis in 10 young healthy volunteers and 70 patients (26-85 years old) with long-term primary arthrosis of the knee/hip joint. It turned out that aging (comparing different groups of patients), in contrast to inactivity/chronic inflammation (comparing young healthy people and young patients), causes a pronounced increase in peroxide production by isolated mitochondria. This correlated with the age-dependent distribution of hexokinase-2 between mitochondrial and cytosolic fractions, a decrease in the rate of coupled respiration of isolated mitochondria and respiration when stimulated with glucose (a hexokinase substrate). It is discussed that these changes may be caused by an age-dependent decrease in the content of cardiolipin, a potential regulator of the mitochondrial microcompartment containing hexokinase. The results obtained contribute to a deeper understanding of age-related pathogenetic processes in skeletal muscles and open prospects for the search for pharmacological/physiological approaches to the correction of these pathologies.

Enhanced ROS Production in Mitochondria from Prematurely Aging mtDNA Mutator Mice

An increase in mitochondrial DNA (mtDNA) mutations and an ensuing increase in mitochondrial reactive oxygen species (ROS) production have been suggested to be a cause of the aging process ("the mitochondrial hypothesis of aging"). In agreement with this, mtDNA-mutator mice accumulate a large amount of mtDNA mutations, giving rise to defective mitochondria and an accelerated aging phenotype. However, incongruously, the rates of ROS production in mtDNA mutator mitochondria have generally earlier been reported to be lower - not higher - than in wildtype, thus apparently invalidating the "mitochondrial hypothesis of aging". We have here re-examined ROS production rates in mtDNA-mutator mice mitochondria. Using traditional conditions for measuring ROS (succinate in the absence of rotenone), we indeed found lower ROS in the mtDNA-mutator mitochondria compared to wildtype. This ROS mainly results from reverse electron flow driven by the membrane potential, but the membrane potential reached in the isolated mtDNA-mutator mitochondria was 33 mV lower than that in wildtype mitochondria, due to the feedback inhibition of succinate oxidation by oxaloacetate, and to a lower oxidative capacity in the mtDNA-mutator mice, explaining the lower ROS production. In contrast, in normal forward electron flow systems (pyruvate (or glutamate) + malate or palmitoyl-CoA + carnitine), mitochondrial ROS production was higher in the mtDNA-mutator mitochondria. Particularly, even during active oxidative phosphorylation (as would be ongoing physiologically), higher ROS rates were seen in the mtDNA-mutator mitochondria than in wildtype. Thus, when examined under physiological conditions, mitochondrial ROS production rates are indeed increased in mtDNA-mutator mitochondria. While this does not prove the validity of the mitochondrial hypothesis of aging, it may no longer be said to be negated in this respect. This paper is dedicated to the memory of Professor Vladimir P. Skulachev.

Impairment of adrenergically-regulated thermogenesis in brown fat of obesity-resistant mice is compensated by non-shivering thermogenesis in skeletal muscle

OBJECTIVE

Non-shivering thermogenesis (NST) mediated by uncoupling protein 1 (UCP1) in brown adipose tissue (BAT) can be activated via the adrenergic system in response to cold or diet, contributing to both thermal and energy homeostasis. Other mechanisms, including metabolism of skeletal muscle, may also be involved in NST. However, relative contribution of these energy dissipating pathways and their adaptability remain a matter of long-standing controversy.

METHODS

We used warm-acclimated (30 °C) mice to characterize the effect of an up to 7-day cold acclimation (6 °C; CA) on thermoregulatory thermogenesis, comparing inbred mice with a genetic background conferring resistance (A/J) or susceptibility (C57BL/6 J) to obesity.

RESULTS

Both warm-acclimated C57BL/6 J and A/J mice exhibited similar cold endurance, assessed as a capability to maintain core body temperature during acute exposure to cold, which improved in response to CA, resulting in comparable cold endurance and similar induction of UCP1 protein in BAT of mice of both genotypes. Despite this, adrenergic NST in BAT was induced only in C57BL/6 J, not in A/J mice subjected to CA. Cold tolerance phenotype of A/J mice subjected to CA was not based on increased shivering, improved insulation, or changes in physical activity. On the contrary, lipidomic, proteomic and gene expression analyses along with palmitoyl carnitine oxidation and cytochrome c oxidase activity revealed induction of lipid oxidation exclusively in skeletal muscle of A/J mice subjected to CA. These changes appear to be related to skeletal muscle NST, mediated by sarcolipin-induced uncoupling of sarco(endo)plasmic reticulum calcium ATPase pump activity and accentuated by changes in mitochondrial respiratory chain supercomplexes assembly.

CONCLUSIONS

Our results suggest that NST in skeletal muscle could be adaptively augmented in the face of insufficient adrenergic NST in BAT, depending on the genetic background of the mice. It may provide both protection from cold and resistance to obesity, more effectively than BAT.

Thermogenic Adipose Redox Mechanisms: Potential Targets for Metabolic Disease Therapies

Metabolic diseases, such as diabetes and non-alcoholic fatty liver disease (NAFLD), have several negative health outcomes on affected humans. Dysregulated energy metabolism is a key component underlying the pathophysiology of these conditions. Adipose tissue is a fundamental regulator of energy homeostasis that utilizes several redox reactions to carry out the metabolism. Brown and beige adipose tissues, in particular, perform highly oxidative reactions during non-shivering thermogenesis to dissipate energy as heat. The appropriate regulation of energy metabolism then requires coordinated antioxidant mechanisms to counterbalance the oxidation reactions. Indeed, non-shivering thermogenesis activation can cause striking changes in concentrations of both oxidants and antioxidants in order to adapt to various oxidative environments. Current therapeutic options for metabolic diseases either translate poorly from rodent models to humans (in part due to the challenges of creating a physiologically relevant rodent model) or tend to have numerous side effects, necessitating novel therapies. As increased brown adipose tissue activity results in enhanced energy expenditure and is associated with beneficial effects on metabolic health, such as decreased obesity, it has gathered great interest as a modulator of metabolic disease. One potential reason for the beneficial health effects may be that although non-shivering thermogenesis is enormously oxidative, it is also associated with decreased oxidant formation after its activation. However, targeting its redox mechanisms specifically to alter metabolic disease remains an underexplored area. Therefore, this review will discuss the role of adipose tissue in energy homeostasis, non-shivering thermogenesis in adults, and redox mechanisms that may serve as novel therapeutic targets of metabolic disease.

Architecture of the outbred brown fat proteome defines regulators of metabolic physiology

Brown adipose tissue (BAT) regulates metabolic physiology. However, nearly all mechanistic studies of BAT protein function occur in a single inbred mouse strain, which has limited the understanding of generalizable mechanisms of BAT regulation over physiology. Here, we perform deep quantitative proteomics of BAT across a cohort of 163 genetically defined diversity outbred mice, a model that parallels the genetic and phenotypic variation found in humans. We leverage this diversity to define the functional architecture of the outbred BAT proteome, comprising 10,479 proteins. We assign co-operative functions to 2,578 proteins, enabling systematic discovery of regulators of BAT. We also identify 638 proteins that correlate with protection from, or sensitivity to, at least one parameter of metabolic disease. We use these findings to uncover SFXN5, LETMD1, and ATP1A2 as modulators of BAT thermogenesis or adiposity, and provide OPABAT as a resource for understanding the conserved mechanisms of BAT regulation over metabolic physiology.

Conditions Conducive to the Glutathionylation of Complex I Subunit NDUFS1 Augment ROS Production following the Oxidation of Ubiquinone Linked Substrates, Glycerol-3-Phosphate and Proline

Mitochondrial complex I can produce large quantities of reactive oxygen species (ROS) by reverse electron transfer (RET) from the ubiquinone (UQ) pool. Glutathionylation of complex I does induce increased mitochondrial superoxide/hydrogen peroxide (O2●-/H2O2) production, but the source of this ROS has not been identified. Here, we interrogated the glutathionylation of complex I subunit NDUFS1 and examined if its modification can result in increased ROS production during RET from the UQ pool. We also assessed glycerol-3-phosphate dehydrogenase (GPD) and proline dehydrogenase (PRODH) glutathionylation since both flavoproteins have measurable rates for ROS production as well. Induction of glutathionylation with disulfiram induced a significant increase in O2●-/H2O2 production during glycerol-3-phosphate (G3P) and proline (Pro) oxidation. Treatment of mitochondria with inhibitors for complex I (rotenone and S1QEL), complex III (myxothiazol and S3QEL), glycerol-3-phosphate dehydrogenase (iGP), and proline dehydrogenase (TFA) confirmed that the sites for this increase were complexes I and III, respectively. Treatment of liver mitochondria with disulfiram (50-1000 nM) did not induce GPD or PRODH glutathionylation, nor did it affect their activities, even though disulfiram dose-dependently increased the total number of protein glutathione mixed disulfides (PSSG). Immunocapture of complex I showed disulfiram incubations resulted in the modification of NDUFS1 subunit in complex I. Glutathionylation could be reversed by reducing agents, restoring the deglutathionylated state of NDUFS1 and the activity of the complex. Reduction of glutathionyl moieties in complex I also significantly decreased ROS production by RET from GPD and PRODH. Overall, these findings demonstrate that the modification of NDUFS1 can result in increased ROS production during RET from the UQ pool, which has implications for understanding the relationship between mitochondrial glutathionylation reactions and induction of oxidative distress in several pathologies.

The Uncoupling Proteins: A Systematic Review on the Mechanism Used in the Prevention of Oxidative Stress

Mitochondrial uncoupling proteins (UCP) 1-3 fulfill many physiological functions, ranging from non-shivering thermogenesis (UCP1) to glucose-stimulated insulin release (GSIS) and satiety signaling (UCP2) and muscle fuel metabolism (UCP3). Several studies have suggested that UCPs mediate these functions by facilitating proton return to the matrix. This would decrease protonic backpressure on the respiratory chain, lowering the production of hydrogen peroxide (H₂O₂), a second messenger. However, controlling mitochondrial H₂O₂ production to prevent oxidative stress by activating these leaks through these proteins is still enthusiastically debated. This is due to compelling evidence that UCP2/3 fulfill other function(s) and the inability to reproduce findings that UCP1-3 use inducible leaks to control reactive oxygen species (ROS) production. Further, other studies have found that UCP2/3 may serve as Ca²⁺. Therefore, we performed a systematic review aiming to summarize the results collected on the topic. A literature search using a list of curated keywords in Pubmed, BIOSIS Citation Index and Scopus was conducted. Potentially relevant references were screened, duplicate references eliminated, and then literature titles and abstracts were evaluated using Rayyan software. A total of 1101 eligible studies were identified for the review. From this total, 416 studies were evaluated based on our inclusion criteria. In general, most studies identified a role for UCPs in preventing oxidative stress, and in some cases, this may be related to the induction of leaks and lowering protonic backpressure on the respiratory chain. However, some studies also generated evidence that UCP2/3 may mitigate oxidative stress by transporting Ca²⁺ into the matrix, exporting lipid hydroperoxides, or by transporting C-4 metabolites. Additionally, some showed that activating UCP1 or 3 can increase mitochondrial ROS production, even though there is still augmented protection from oxidative stress. Conclusion: Overall, most available studies demonstrate that UCPs, particularly UCP2/3, prevent oxidative stress. However, the mechanism utilized to do so remains elusive and raises the question that UCP2/3 should be renamed, since they may still not be true “uncoupling proteins”.

Engineering Functional Vascularized Beige Adipose Tissue from Microvascular Fragments of Models of Healthy and Type II Diabetes Conditions

Engineered beige adipose tissues could be used for screening therapeutic strategies or as a direct treatment for obesity and metabolic disease. Microvascular fragments are vessel structures that can be directly isolated from adipose tissue and may contain cells capable of differentiation into thermogenic, or beige, adipocytes. In this study, culture conditions were investigated to engineer three-dimensional, vascularized functional beige adipose tissue using microvascular fragments isolated from both healthy animals and a model of type II diabetes (T2D). Vascularized beige adipose tissues were engineered and exhibited increased expression of beige adipose markers, enhanced function, and improved cellular respiration. While microvascular fragments isolated from both lean and diabetic models were able to generate functional tissues, differences were observed in regard to vessel assembly and tissue function. This study introduces an approach that could be employed to engineer vascularized beige adipose tissues from a single, potentially autologous source of cells.

Catalase deficiency facilitates the shuttling of free fatty acid to brown adipose tissue through lipolysis mediated by ROS during sustained fasting

Background

Fatty acids (FA) derived from adipose tissue and liver serve as the main fuel in thermogenesis of brown adipose tissue (BAT). Catalase, a peroxisomal enzyme, plays an important role in maintaining intracellular redox homeostasis by decomposing hydrogen peroxide to either water or oxygen that oxidize and provide fuel for cellular metabolism. Although the antioxidant enzymatic activity of catalase is well known, its role in the metabolism and maintenance of energy homeostasis has not yet been revealed. The present study investigated the role of catalase in lipid metabolism and thermogenesis during nutrient deprivation in catalase-knockout (KO) mice.

Results

We found that hepatic triglyceride accumulation in KO mice decreased during sustained fasting due to lipolysis through reactive oxygen species (ROS) generation in adipocytes. Furthermore, the free FA released from lipolysis were shuttled to BAT through the activation of CD36 and catabolized by lipoprotein lipase in KO mice during sustained fasting. Although the exact mechanism for the activation of the FA receptor enzyme, CD36 in BAT is still unclear, we found that ROS generation in adipocytes mediated the shuttling of FA to BAT.

Conclusions

Taken together, our findings uncover the novel role of catalase in lipid metabolism and thermogenesis in BAT, which may be useful in understanding metabolic dysfunction.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00710-5.

Spatial Regulation of Reactive Oxygen Species via G6PD in Brown Adipocytes Supports Thermogenic Function

Reactive oxygen species (ROS) are associated with various roles of brown adipocytes. Glucose-6-phosphate dehydrogenase (G6PD) controls cellular redox potentials by producing NADPH. Although G6PD upregulates cellular ROS levels in white adipocytes, the roles of G6PD in brown adipocytes remain elusive. Here, we found that G6PD defect in brown adipocytes impaired thermogenic function through excessive cytosolic ROS accumulation. Upon cold exposure, G6PD-deficient mutant (G6PD^mut) mice exhibited cold intolerance and downregulated thermogenic gene expression in brown adipose tissue (BAT). In addition, G6PD-deficient brown adipocytes had increased cytosolic ROS levels, leading to extracellular signal-regulated kinase (ERK) activation. In BAT of G6PD^mut mice, administration of antioxidant restored the thermogenic activity by potentiating thermogenic gene expression and relieving ERK activation. Consistently, body temperature and thermogenic execution were rescued by ERK inhibition in cold-exposed G6PD^mut mice. Taken together, these data suggest that G6PD in brown adipocytes would protect against cytosolic oxidative stress, leading to cold-induced thermogenesis.

Mitochondrial Uncoupling Proteins (UCPs) as Key Modulators of ROS Homeostasis: A Crosstalk between Diabesity and Male Infertility?

Uncoupling proteins (UCPs) are transmembrane proteins members of the mitochondrial anion transporter family present in the mitochondrial inner membrane. Currently, six homologs have been identified (UCP1-6) in mammals, with ubiquitous tissue distribution and multiple physiological functions. UCPs are regulators of key events for cellular bioenergetic metabolism, such as membrane potential, metabolic efficiency, and energy dissipation also functioning as pivotal modulators of ROS production and general cellular redox state. UCPs can act as proton channels, leading to proton re-entry the mitochondrial matrix from the intermembrane space and thus collapsing the proton gradient and decreasing the membrane potential. Each homolog exhibits its specific functions, from thermogenesis to regulation of ROS production. The expression and function of UCPs are intimately linked to diabesity, with their dysregulation/dysfunction not only associated to diabesity onset, but also by exacerbating oxidative stress-related damage. Male infertility is one of the most overlooked diabesity-related comorbidities, where high oxidative stress takes a major role. In this review, we discuss in detail the expression and function of the different UCP homologs. In addition, the role of UCPs as key regulators of ROS production and redox homeostasis, as well as their influence on the pathophysiology of diabesity and potential role on diabesity-induced male infertility is debated.

Management of Oxidative Stress: Crosstalk Between Brown/Beige Adipose Tissues and Skeletal Muscles

Exercise plays an important role in the physiology, often depending on its intensity, duration, and frequency. It increases the production of reactive oxygen species (ROS). Meanwhile, it also increases antioxidant enzymes involved in the oxidative damage defense. Prolonged, acute, or strenuous exercise often leads to an increased radical production and a subsequent oxidative stress in the skeletal muscles, while chronic regular or moderate exercise results in a decrease in oxidative stress. Notably, under pathological state, such as obesity, aging, etc., ROS levels could be elevated in humans, which could be attenuated by proper exercise. Significantly, exercise stimulates the development of beige adipose tissue and potentially influence the function of brown adipose tissue (BAT), which is known to be conducive to a metabolic balance through non-shivering thermogenesis (NST) and may protect from oxidative stress. Exercise-related balance of the ROS levels is associated with a healthy metabolism in humans. In this review, we summarize the integrated effects of exercise on oxidative metabolism, and especially focus on the role of brown and beige adipose tissues in this process, providing more evidence and knowledge for a better management of exercise-induced oxidative stress.

Mitochondrial proton leaks and uncoupling proteins

Non-shivering thermogenesis in brown adipose tissue is mediated by uncoupling protein 1 (UCP1), which provides a carefully regulated proton re-entry pathway across the mitochondrial inner membrane operating in parallel to the ATP synthase and allowing respiration, and hence thermogenesis, to be released from the constraints of respiratory control. In the 40 years since UCP1 was first described, an extensive, and frequently contradictory, literature has accumulated, focused on the acute physiological regulation of the protein by fatty acids, purine nucleotides and possible additional factors. The purpose of this review is to examine, in detail, the experimental evidence underlying these proposed mechanisms. Emphasis will be placed on the methodologies employed and their relation to the physiological constraints under which the protein functions in the intact cell. The nature of the endogenous, UCP1-independent, proton leak will also be discussed. Finally, the troubled history of the putative novel uncoupling proteins, UCP2 and UCP3, will be evaluated.

Predictive and reactive changes in antioxidant defence system in a heterothermic rodent

Living in a seasonal environment requires periodic changes in animal physiology, morphology and behaviour. Winter phenotype of small mammals living in Temperate and Boreal Zones may differ considerably from summer one in multiple traits that enhance energy conservation or diminish energy loss. However, there is a considerable variation in the development of winter phenotype among individuals in a population and some, representing the non-responding phenotype (non-responders), are insensitive to shortening days and maintain summer phenotype throughout a year. Differences in energy management associated with the development of different winter phenotypes should be accompanied by changes in antioxidant defence capacity, leading to effective protection against oxidative stress resulting from increased heat production in winter. To test it, we analysed correlation of winter phenotypes of Siberian hamsters (Phodopus sungorus) with facultative non-shivering thermogenesis capacity (NST) and oxidative status. We found that in both phenotypes acclimation to winter-like conditions increased NST capacity and improved antioxidant defence resulting in lower oxidative stress (OS) than in summer, and females had always lower OS than males. Although NST capacity did not correlate with the intensity of OS, shortly after NST induction responders had lower OS than non-responders suggesting more effective mechanisms protecting from detrimental effects of reactive oxygen metabolites generated during rewarming from torpor. We suggest that seasonal increase in antioxidant defence is programmed endogenously to predictively prevent oxidative stress in winter. At the same time reactive upregulation of antioxidant defence protects against reactive oxygen species generated during NST itself. It suggests that evolution of winter phenotype with potentially harmful characteristics was counterbalanced by the development of protective mechanisms allowing for the maintenance of phenotypic adjustments to seasonally changing environment.

Deletion of the Glutaredoxin-2 Gene Protects Mice from Diet-Induced Weight Gain, Which Correlates with Increased Mitochondrial Respiration and Proton Leaks in Skeletal Muscle

Aims: The aim of this study was to determine whether deleting the gene encoding glutaredoxin-2 (GRX2) could protect mice from diet-induced weight gain. Results: Subjecting wild-type littermates to a high fat diet (HFD) induced a significant increase in overall body mass, white adipose tissue hypertrophy, lipid droplet accumulation in hepatocytes, and higher circulating insulin and triglyceride levels. In contrast, GRX2 heterozygotes (GRX2^+/-) fed an HFD had a body mass, white adipose tissue weight, and hepatic and circulating lipid and insulin levels similar to littermates fed a control diet. Examination of the bioenergetics of muscle mitochondria revealed that this protective effect was associated with an increase in respiration and proton leaks. Muscle mitochondria from GRX2^+/- mice had a ∼2- to 3-fold increase in state 3 (phosphorylating) respiration when pyruvate/malate or succinate served as substrates and a ∼4-fold increase when palmitoyl-carnitine was being oxidized. Proton leaks were ∼2- to 3-fold higher in GRX2^+/- muscle mitochondria. Treatment of mitochondria with either guanosine diphosphate, genipin, or octanoyl-carnitine revealed that the higher rate of O₂ consumption under state 4 conditions was associated with increased leaks through uncoupling protein-3 and adenine nucleotide translocase. GRX2^+/- mitochondria also had better protection from oxidative distress. Innovation: For the first time, we demonstrate that deleting the Grx2 gene can protect from diet-induced weight gain and the development of obesity-related disorders. Conclusions: Deleting the Grx2 gene protects mice from diet-induced weight gain. This effect was related to an increase in muscle fuel combustion, mitochondrial respiration, proton leaks, and reactive oxygen species handling. Antioxid. Redox Signal. 31, 1272-1288.

Redox state of adipose tissue for patients with gastric cancer and its connection with the body mass index and distance from the tumor

Excess body weight has been causally linked to an increased risk of different cancer types, including gastric cancer but the mechanisms underlying this relationship are not well understood. Superoxide generation rate, activity of complex I in electron transport chain of mitochondria, activity of matrix metalloproteinase (MMP-2 and 9) of adipose tissues (AT) of patients with gastric cancer in AT located adjacent to tumor (ATAT) and at a distance of 3 cm (ATD) are measured to follow the connection of the redox state with some of the microenvironment indicators (HIF-1α, CD68, Plin5), body mass index (BMI) and cancer metastasis. Superoxide generation rate in ATAT positively correlates with BMI (r = 0.59, p < 0.05) being 4 times higher than in control (p < 0.05). MMP-2, 9 activities in ATAT positively correlate with BMI (r = 0.67, p < 0.05) being 3.3-4.0 higher than in control (p < 0.05). In ATD a statistically significant increase of MMP-2 activity is found. In ATAT for the group of patients with distant metastasis (M1) the superoxide generation rate, MMP-2, 9 activities are about 2 times higher (p < 0.05) than in the subgroup without distant metastases (M0). M1 is also characterized by the increased values of HIF-1α+ (factor 1.25), CD68+ (factor 1.4) and Plin5+ (factor 2.1) compared to M0 category in tumor tissues (p < 0.05). The results can be used for better understanding the mechanism(s) of symbiosis of tumor and adipose tissues as well as serve as a basis for new therapeutic approaches.

Mitochondrial Uncoupling: A Key Controller of Biological Processes in Physiology and Diseases

Mitochondrial uncoupling can be defined as a dissociation between mitochondrial membrane potential generation and its use for mitochondria-dependent ATP synthesis. Although this process was originally considered a mitochondrial dysfunction, the identification of UCP-1 as an endogenous physiological uncoupling protein suggests that the process could be involved in many other biological processes. In this review, we first compare the mitochondrial uncoupling agents available in term of mechanistic and non-specific effects. Proteins regulating mitochondrial uncoupling, as well as chemical compounds with uncoupling properties are discussed. Second, we summarize the most recent findings linking mitochondrial uncoupling and other cellular or biological processes, such as bulk and specific autophagy, reactive oxygen species production, protein secretion, cell death, physical exercise, metabolic adaptations in adipose tissue, and cell signaling. Finally, we show how mitochondrial uncoupling could be used to treat several human diseases, such as obesity, cardiovascular diseases, or neurological disorders.

Mitochondrial electron transport chain, ROS generation and uncoupling (Review)

The mammalian mitochondrial electron transport chain (ETC) includes complexes I-IV, as well as the electron transporters ubiquinone and cytochrome c. There are two electron transport pathways in the ETC: Complex I/III/IV, with NADH as the substrate and complex II/III/IV, with succinic acid as the substrate. The electron flow is coupled with the generation of a proton gradient across the inner membrane and the energy accumulated in the proton gradient is used by complex V (ATP synthase) to produce ATP. The first part of this review briefly introduces the structure and function of complexes I-IV and ATP synthase, including the specific electron transfer process in each complex. Some electrons are directly transferred to O₂ to generate reactive oxygen species (ROS) in the ETC. The second part of this review discusses the sites of ROS generation in each ETC complex, including sites I_F and I_Q in complex I, site II_F in complex II and site III_Qo in complex III, and the physiological and pathological regulation of ROS. As signaling molecules, ROS play an important role in cell proliferation, hypoxia adaptation and cell fate determination, but excessive ROS can cause irreversible cell damage and even cell death. The occurrence and development of a number of diseases are closely related to ROS overproduction. Finally, proton leak and uncoupling proteins (UCP_S) are discussed. Proton leak consists of basal proton leak and induced proton leak. Induced proton leak is precisely regulated and induced by UCPs. A total of five UCPs (UCP1-5) have been identified in mammalian cells. UCP1 mainly plays a role in the maintenance of body temperature in a cold environment through non-shivering thermogenesis. The core role of UCP2-5 is to reduce oxidative stress under certain conditions, therefore exerting cytoprotective effects. All diseases involving oxidative stress are associated with UCPs.

Redox control of non-shivering thermogenesis

Background

Thermogenic adipocytes reorganize their metabolism during cold exposure. Metabolic reprogramming requires readily available bioenergetics substrates, such as glucose and fatty acids, to increase mitochondrial respiration and produce heat via the uncoupling protein 1 (UCP1). This condition generates a finely-tuned production of mitochondrial reactive oxygen species (ROS) that support non-shivering thermogenesis.

Scope of review

Herein, the findings underlining the mechanisms that regulate ROS production and control of the adaptive responses tuning thermogenesis in adipocytes are described. Furthermore, this review describes the metabolic responses to substrate availability and the consequence of mitochondrial failure to switch fuel oxidation in response to changes in nutrient availability. A framework to control mitochondrial ROS threshold to maximize non-shivering thermogenesis in adipocytes is provided.

Major conclusions

Thermogenesis synchronizes fuel oxidation with an acute and transient increase of mitochondrial ROS that promotes the activation of redox-sensitive thermogenic signaling cascade and UCP1. However, an overload of substrate flux to mitochondria causes a massive and damaging mitochondrial ROS production that affects mitochondrial flexibility. Finding novel thermogenic redox targets and manipulating ROS concentration in adipocytes appears to be a promising avenue of research for improving thermogenesis and counteracting metabolic diseases.

•

Mitochondrial ROS support non-shivering thermogenesis.

•

Thermogenic ROS are tightly related to mitochondrial metabolic reorganization.

•

Uncontrolled mitochondrial ROS production is causative of metabolic inflexibility.

Selenium-Rich Diet Induces Myocardial Structural and Functional Abnormalities by Activating Caspase-9 and Caspase-3 in Gpx-1P198L-Overexpression Transgenic Mice

Background

Selenium (Se) deficiency and supplementation result in multiple effects. GPx-1 (Pro198Leu) polymorphism is associated with Se deficiency. This study aimed to observe associations between Se-deficiency/supplement and GPx-1-198Leu overexpression in myocardial injuries.

Material/Methods

GPx-1P198L transgenic (Tg) mice and non-transgenic wild-type (WT) littermates were divided into Control (CON, 0.1–0.2 mg/kg), Se-deficiency (SD, <0.02 mg/kg), and Se-supplement (SS, 0.4 mg/kg) groups. Cardiac functions were observed with animal M-mode echocardiography. Se level was measured using 2,3-diamino Kenai fluorospectrophotometry. Total cardiac GPx activity was also measured. Myocardial histopathology was determined with HE and Masson’s trichrome staining. Caspase-9 and caspase-3 were measured with Western blot analysis.

Results

In WT Se-deficient mice, cardiac GPx activity was significantly decreased, and was not elevated by overexpression of GPx-1-198Leu gene. Increased GPx activity was observed in WT Se-supplemented mice and Tg Se-supplemented mice (much more). Se deficiency as well as supplementation resulted in cardiac systolic dysfunction, which was not affected by GPx-1-198Leu gene. Se deficiency led to myocardial fibrosis and pathological changes accompanied by increased activation of caspase-9 and caspase-3. Se supplementation significantly reduced pathological changes, as well as caspase-9 and caspase-3 levels in the presence of increased myocardial fibrosis. In Se-deficient mice, GPx-1-198Leu overexpression did not significantly decrease myocardial pathological injuries and fibrosis. In Se-supplemented Tg mice, myocardial fibrosis and caspase-9 level were increased, although pathological injuries and caspase-3 were similar to that in Se-supplemented WT mice.

Conclusions

Se deficiency as well as supplementation induced myocardial structural and functional abnormalities through activation of caspase-9 and caspase-3 in GPx-1P198L overexpression transgenic mice.

Cysteine Switches and the Regulation of Mitochondrial Bioenergetics and ROS Production

Mitochondria are dynamic organelles that perform a number of interconnected tasks that are elegantly intertwined with the regulation of cell functions. This includes the provision of ATP, reactive oxygen species (ROS), and building blocks for the biosynthesis of macromolecules while also serving as signaling platforms for the cell. Although the functions executed by mitochondria are complex, at its core these roles are, to a certain degree, fulfilled by electron transfer reactions and the establishment of a protonmotive force (PMF). Indeed, mitochondria are energy conserving organelles that extract electrons from nutrients to establish a PMF, which is then used to drive ATP and NADPH production, solute import, and many other functions including the propagation of cell signals. These same electrons extracted from nutrients are also used to produce ROS, pro-oxidants that can have potentially damaging effects at high levels, but also serve as secondary messengers at low amounts. Mitochondria are also enriched with antioxidant defenses, which are required to buffer cellular ROS. These same redox buffering networks also fulfill another important role; regulation of proteins through the reversible oxidation of cysteine switches. The modification of cysteine switches with the antioxidant glutathione, a process called protein S-glutathionylation, has been found to play an integral role in controlling various mitochondrial functions. In addition, recent findings have demonstrated that disrupting mitochondrial protein S-glutathionylation reactions can have some dire pathological consequences. Accordingly, this chapter focuses on the role of mitochondrial cysteine switches in the modulation of different physiological functions and how defects in these pathways contribute to the development of disease.

Comparative Secretome Analyses of Primary Murine White and Brown Adipocytes Reveal Novel Adipokines

The adipose organ, including white and brown adipose tissues, is an important player in systemic energy homeostasis, storing excess energy in form of lipids while releasing energy upon various energy demands. Recent studies have demonstrated that white and brown adipocytes also function as endocrine cells and regulate systemic metabolism by secreting factors that act locally and systemically. However, a comparative proteomic analysis of secreted factors from white and brown adipocytes and their responsiveness to adrenergic stimulation has not been reported yet. Therefore, we studied and compared the secretome of white and brown adipocytes, with and without norepinephrine (NE) stimulation. Our results reveal that carbohydrate-metabolism-regulating proteins are preferably secreted from white adipocytes, while brown adipocytes predominantly secrete a large variety of proteins. Upon NE stimulation, an increased secretion of known adipokines is favored by white adipocytes while brown adipocytes secreted higher amounts of novel adipokines. Furthermore, the secretory response between NE-stimulated and basal state was multifaceted addressing lipid and glucose metabolism, adipogenesis, and antioxidative reactions. Intriguingly, NE stimulation drastically changed the secretome in brown adipocytes. In conclusion, our study provides a comprehensive catalogue of novel adipokine candidates secreted from white and brown adipocytes with many of them responsive to NE. Given the beneficial effects of brown adipose tissue activation on its endocrine function and systemic metabolism, this study provides an archive of novel batokine candidates and biomarkers for activated brown adipose tissue.

Colorectal Cancer and Mitochondrial Dysfunctions of the Adjunct Adipose Tissues: A Case Study

Excess body weight has been causally linked to an increased risk of different cancer types, including colorectal cancer (CRC) but the mechanisms underlying this association are practically unknown. We investigate redox state-superoxide (SO) generation rate, activity of complex I in electron transport chain (ETC) of mitochondria and of dinitrosyl iron complexes by electron paramagnetic resonance; activity of matrix metalloproteinase (gelatinase) MMP-2 and MMP-9 by gel zymography of adipose tissues (AT) from 46 patients (64.0 ± 1.6 y.o.) with CRC (II-III stages, pT2-3N0-2M0) in the AT adjacent to tumor (ATAT) and at a distance of 3 cm from the tumor (ATD) to follow the connection of the AT redox state with some of the tumor microenvironment indicators. We have incubated the AT species with the tumor necrosis factor α (TNF-α) to follow its influence on the measured values. As a control, normal AT (NAT) obtained during the liposuction is used. Tumor-induced changes in mitochondrial ETC of ATAT, particularly for Complex I, lead to the enhanced SO generation and consequent oxidative modifications of DNA in ATAT (up to 6.1 times higher than that in NAT and 3.7 times higher than that in ATD, p < 0.05). Gelatinase activity in ATAT is significantly higher than in ATD. A considerable effect of TNF-α on ATAT and ATD (but not on NAT, i.e., only on the tissues where the reprogramming of metabolism has already occurred under the influence of tumor) manifested in increase of cellular hypoxia, gelatinase activity, and SO generation rate is observed. The results can be used for better understanding the mechanism(s) of metabolic symbiosis of tumor and AT as well as serving as a basis for new therapeutic approaches.

Uncoupling protein-1 deficiency promotes brown adipose tissue inflammation and ER stress

Inflammation and endoplasmic reticulum (ER) stress are hallmarks of metabolic syndrome. While these metabolic derangements have been well-investigated in white adipose tissue, their existence and etiology in brown adipose tissue (BAT) are poorly understood. Here, we aimed to investigate ER homeostasis and the inflammatory status and of BAT lacking uncoupling protein-1 (UCP1), a protein required for BAT thermogenesis. H&E staining illustrated lipid accumulation and crown-like structures surrounding adipocytes in BAT of UCP1-/- mice housed at room temperature compared to control mice. Further, immunohistological evaluation of F4/80 and gene expression studies demonstrated BAT macrophage infiltration and robust elevation of pro-inflammatory markers in UCP1-/- BAT. ER stress was also present in BAT of UCP1-/- mice, as evidenced by elevated gene expression and post-translational modifications of unfolded protein response components. After four weeks of thermoneutral housing, UCP1-/- mice did not exhibit elevated BAT inflammation and ER stress gene expression compared to WT mice, but depot expansion persisted. Collectively, we demonstrate that the effects of UCP1 deficiency in BAT are not restricted to mitochondrial uncoupling. We conclude that brown adipose tissue of UCP1-/- mice exhibits pro-inflammatory immune cell infiltration and perturbations in ER homeostasis and that this phenotype is driven by cold exposure rather than lipid accumulation.

Dihydronortanshinone, a natural product, alleviates LPS-induced inflammatory response through NF-κB, mitochondrial ROS, and MAPK pathways

Inflammation is considered to be the common pathophysiological basis for a series of diseases. Documented data showed the anti-inflammatory effects of Salvia miltiorrhiza Bunge (Danshen), a traditional herb. The pharmacological activities of dihydronortanshinone (DNT), a tanshinone isolated from Danshen, remain unknown. In this study, the anti-inflammatory effects and underlying mechanisms of DNT were investigated with a lipopolysaccharide (LPS)-induced RAW264.7 macrophage model. DNT significantly suppressed LPS-induced inflammatory mediators such as nitrite oxide (NO), tumor necrosis factor α (TNF-α), interleukin 6 (IL-6), inducible nitric oxide synthase (iNOS). LPS-induced reactive oxygen species (ROS) generation was inhibited by DNT, rotenone (Rot), thenoyltrifluoroacetone (TTFA), and antimycin A (AA). Furthermore, DNT inhibited LPS-induced NF-κBp65 phosphorylation, nuclear translocation, as well as JNK1/2 and p38MAPK phosphorylation. In addition, DNT interrupted Toll-like receptor 4 (TLR4) dimerization and molecular docking results suggested that it was buried in the pocket of TLR4-MD2 complex. In conclusion, DNT inhibited LPS-induced inflammation mainly through NF-κB, mitochondrial ROS, and MAPK pathways possibly mediated by interfering LPS-TLR4-MD2 complex.

Mitophagy is required for brown adipose tissue mitochondrial homeostasis during cold challenge

Brown adipose tissue (BAT) is a specialized thermogenic organ in mammals. The ability of BAT mitochondria to generate heat in response to cold-challenge to maintain core body temperature is essential for organismal survival. While cold activated BAT mitochondrial biogenesis is recognized as critical for thermogenic adaptation, the contribution of mitochondrial quality control to this process remains unclear. Here, we show mitophagy is required for brown adipocyte mitochondrial homeostasis during thermogenic adaptation. Mitophagy is significantly increased in BAT from cold-challenged mice (4 °C) and in β-agonist treated brown adipocytes. Blockade of mitophagy compromises brown adipocytes mitochondrial oxidative phosphorylation (OX-PHOS) capacity, as well as BAT mitochondrial integrity. Mechanistically, cold-challenge induction of BAT mitophagy is UCP1-dependent. Furthermore, our results indicate that mitophagy coordinates with mitochondrial biogenesis, maintaining activated BAT mitochondrial homeostasis. Collectively, our in vivo and in vitro findings identify mitophagy as critical for brown adipocyte mitochondrial homeostasis during cold adaptation.

Effects of exercise on brown and beige adipocytes

Physical exercise leads to beneficial effects in numerous tissues and organ systems and offers protection against obesity and type 2 diabetes. Recent studies have investigated the role of exercise on brown adipose tissue (BAT) and white adipose tissue (WAT), and have indicated marked adaptations to each tissue with exercise. Studies investigating the effects of exercise on BAT have produced conflicting results, with some showing an increase in the thermogenic activity of BAT and some demonstrating a decrease in the thermogenic activity of BAT. Human studies have observed a down-regulation of BAT activity (measured by a reduction in glucose uptake) in response to exercise. In WAT, exercise decreases adipocyte size, alters gene expression, and increases mitochondrial activity. Transplantation of exercise-trained subcutaneous WAT (scWAT) improves whole-body metabolic health. In rodents, exercise also results in a beiging of scWAT. Thus, exercise-induced changes to adipose tissue may be part of the mechanism by which exercise improves metabolic health.

Increase of reactive oxygen species in different tissues during lipopolysaccharide-induced fever and antipyresis: an electron paramagnetic resonance study

Fever is a regulated increase in body temperature and a component of the acute-phase response, triggered mainly after the invasion of pathogens in the body. Reactive oxygen species (ROS) are generated during the physiological and pathological processes, and can act as both signalling molecules as well as promoters of oxidative stress. Male Wistar rats, pretreated with oral doses of acetaminophen, celecoxib, dipyrone, or ibuprofen 30 min before an intravenous lipopolysaccharide (LPS) or sterile saline injection, showed a reduced febrile response in all animals tested. The formation of ROS in the fresh blood, liver, brown adipose tissue (BAT), and hypothalamus of febrile and antipyretic-treated animals was assessed by electron paramagnetic resonance using the spin probe 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (CMH). While the CM^• concentrations remained unaltered in the blood samples examined 5 h after the induction of fever, we found increased CM^• levels in the liver (in µM, saline: 290 ± 42; LPS: 512 ± 34), BAT (in µM, saline: 509 ± 79, LPS: 855 ± 79), and hypothalamus (in µM, saline: 292 ± 35; LPS: 467 ± 8) at the same time point. Importantly, none of the antipyretics were seen to alter the CM^• accumulation profile. Data from this study suggest that there is an increased formation of ROS in the different tissues during fever, which may cause oxidative stress, and that the antipyretics tested do not interfere with ROS production.

Subcutaneous inguinal white adipose tissue is responsive to, but dispensable for, the metabolic health benefits of exercise

Exercise training has robust effects on subcutaneous inguinal white adipose tissue (iWAT), characterized by a shift to a brown adipose tissue (BAT)-like phenotype. Consistent with this, transplantation of exercise-trained iWAT into sedentary rodents activates thermogenesis and improves glucose homeostasis, suggesting that iWAT metabolism may contribute to the beneficial effects of exercise. However, it is yet to be determined if adaptations in iWAT are necessary for the beneficial systemic effects of exercise. To test this, male C57BL/6 mice were provided access to voluntary wheel running (VWR) or remained as a cage control (SED) for 11 nights after iWAT removal via lipectomy (LIPX) or SHAM surgery. We found that SHAM and LIPX mice with access to VWR ran similar distances and had comparable reductions in body mass, increased food intake, and increased respiratory exchange ratio (RER). Further, VWR improved indexes of glucose homeostasis and insulin tolerance in both SHAM and LIPX mice. The lack of effect of LIPX in the response to VWR was not explained by compensatory increases in markers of mitochondrial biogenesis and thermogenesis in skeletal muscle, epididymal white adipose tissue, or interscapular brown adipose tissue. Together, these data demonstrate that mice with and without iWAT have comparable adaptations to VWR, suggesting that iWAT may be dispensable for the metabolic health benefits of exercise.

Improved health-span and lifespan in mtDNA mutator mice treated with the mitochondrially targeted antioxidant SkQ1

MtDNA mutator mice exhibit marked features of premature aging. We find that these mice treated from age of ≈100 days with the mitochondria-targeted antioxidant SkQ1 showed a delayed appearance of traits of aging such as kyphosis, alopecia, lowering of body temperature, body weight loss, as well as ameliorated heart, kidney and liver pathologies. These effects of SkQ1 are suggested to be related to an alleviation of the effects of an enhanced reactive oxygen species (ROS) level in mtDNA mutator mice: the increased mitochondrial ROS released due to mitochondrial mutations probably interact with polyunsaturated fatty acids in cardiolipin, releasing malondialdehyde and 4-hydroxynonenal that form protein adducts and thus diminishes mitochondrial functions. SkQ1 counteracts this as it scavenges mitochondrial ROS. As the results, the normal mitochondrial ultrastructure is preserved in liver and heart; the phosphorylation capacity of skeletal muscle mitochondria as well as the thermogenic capacity of brown adipose tissue is also improved. The SkQ1-treated mice live significantly longer (335 versus 290 days). These data may be relevant in relation to treatment of mitochondrial diseases particularly and the process of aging in general.

Progress in understanding the molecular oxygen paradox - function of mitochondrial reactive oxygen species in cell signaling

The molecular oxygen (O2) paradox was coined to describe its essential nature and toxicity. The latter characteristic of O2 is associated with the formation of reactive oxygen species (ROS), which can damage structures vital for cellular function. Mammals are equipped with antioxidant systems to fend off the potentially damaging effects of ROS. However, under certain circumstances antioxidant systems can become overwhelmed leading to oxidative stress and damage. Over the past few decades, it has become evident that ROS, specifically H2O2, are integral signaling molecules complicating the previous logos that oxyradicals were unfortunate by-products of oxygen metabolism that indiscriminately damage cell structures. To avoid its potential toxicity whilst taking advantage of its signaling properties, it is vital for mitochondria to control ROS production and degradation. H2O2 elimination pathways are well characterized in mitochondria. However, less is known about how H2O2 production is controlled. The present review examines the importance of mitochondrial H2O2 in controlling various cellular programs and emerging evidence for how production is regulated. Recently published studies showing how mitochondrial H2O2 can be used as a secondary messenger will be discussed in detail. This will be followed with a description of how mitochondria use S-glutathionylation to control H2O2 production.

Pleiotropic Effects of Biguanides on Mitochondrial Reactive Oxygen Species Production

Metformin is widely prescribed as a first-choice antihyperglycemic drug for treatment of type 2 diabetes mellitus, and recent epidemiological studies showed its utility also in cancer therapy. Although it is in use since the 1970s, its molecular target, either for antihyperglycemic or antineoplastic action, remains elusive. However, the body of the research on metformin effect oscillates around mitochondrial metabolism, including the function of oxidative phosphorylation (OXPHOS) apparatus. In this study, we focused on direct inhibitory mechanism of biguanides (metformin and phenformin) on OXPHOS complexes and its functional impact, using the model of isolated brown adipose tissue mitochondria. We demonstrate that biguanides nonspecifically target the activities of all respiratory chain dehydrogenases (mitochondrial NADH, succinate, and glycerophosphate dehydrogenases), but only at very high concentrations (10^-2-10^-1 M) that highly exceed cellular concentrations observed during the treatment. In addition, these concentrations of biguanides also trigger burst of reactive oxygen species production which, in combination with pleiotropic OXPHOS inhibition, can be toxic for the organism. We conclude that the beneficial effect of biguanides should probably be associated with subtler mechanism, different from the generalized inhibition of the respiratory chain.

Regulation of glycolysis in brown adipocytes by HIF-1α

Brown adipose tissue takes up large amounts of glucose during cold exposure in mice and humans. Here we report an induction of glucose transporter 1 expression and increased expression of several glycolytic enzymes in brown adipose tissue from cold-exposed mice. Accordingly, these genes were also induced after β-adrenergic activation of cultured brown adipocytes, concomitant with accumulation of hypoxia inducible factor-1α (HIF-1α) protein levels. HIF-1α accumulation was dependent on uncoupling protein 1 and generation of mitochondrial reactive oxygen species. Expression of key glycolytic enzymes was reduced after knockdown of HIF-1α in mature brown adipocytes. Glucose consumption, lactate export and glycolytic capacity were reduced in brown adipocytes depleted of Hif-1α. Finally, we observed a decreased β-adrenergically induced oxygen consumption in Hif-1α knockdown adipocytes cultured in medium with glucose as the only exogenously added fuel. These data suggest that HIF-1α-dependent regulation of glycolysis is necessary for maximum glucose metabolism in brown adipocytes.

UCP1 deficiency causes brown fat respiratory chain depletion and sensitizes mitochondria to calcium overload-induced dysfunction

Brown adipose tissue (BAT) mitochondria exhibit high oxidative capacity and abundant expression of both electron transport chain components and uncoupling protein 1 (UCP1). UCP1 dissipates the mitochondrial proton motive force (Δp) generated by the respiratory chain and increases thermogenesis. Here we find that in mice genetically lacking UCP1, cold-induced activation of metabolism triggers innate immune signaling and markers of cell death in BAT. Moreover, global proteomic analysis reveals that this cascade induced by UCP1 deletion is associated with a dramatic reduction in electron transport chain abundance. UCP1-deficient BAT mitochondria exhibit reduced mitochondrial calcium buffering capacity and are highly sensitive to mitochondrial permeability transition induced by reactive oxygen species (ROS) and calcium overload. This dysfunction depends on ROS production by reverse electron transport through mitochondrial complex I, and can be rescued by inhibition of electron transfer through complex I or pharmacologic depletion of ROS levels. Our findings indicate that the interscapular BAT of Ucp1 knockout mice exhibits mitochondrial disruptions that extend well beyond the deletion of UCP1 itself. This finding should be carefully considered when using this mouse model to examine the role of UCP1 in physiology.

Sodium fluoride induces nephrotoxicity via oxidative stress-regulated mitochondrial SIRT3 signaling pathway

Accumulation of mitochondrial reactive oxygen species (mROS) has been implicated in the pathogenesis of fluorosis. As the main mitochondrial deacetylase, SIRT3 is closely associated with oxidative stress. To investigate the role of SIRT3 in response to sodium fluoride (NaF)-induced nephrotoxicity. Our results showed that NaF treatment impaired mitochondrial ultrastructure, decreased cell viability and increased apoptosis in TCMK-1 cells. Oxidative stress, detected by mROS and 8-Hydroxy-2’-deoxyguanosine (8-OHdG) were higher in NaF-treated cells, accompanied by decreased level of reduced glutathione (GSH). NaF reduces manganese superoxide dismutase (SOD2) expression through SIRT3-mediated DNA-binding activity of FoxO3a and decrease SOD2 activity by inhibiting SIRT3-mediated deacetylation. These effects were ameliorated by overexpression of SIRT3. Peroxisome proliferator-activated receptor-coactivator 1a (PGC-1α) interacted with nuclear factor erythroid 2 (NF-E2)-related factor 2 (NRF2) that bound to SIRT3 promoter to regulate SIRT3 expression. The study provides new insights into a critical NRF2/PGC-1α-SIRT3 pathway in response to NaF-induced nephritic oxidative injury. In vivo treatment of SIRT3-expressing adenovirus protects against NaF-induced nephritic injury in mice. Moreover, mechanistic study revealed that ERK1/2 activation was associated with increased apoptosis induced by NaF. In conclusion, these data shedding light on new approaches for treatment of NaF-induced nephrotoxicity.

Reversible temperature-dependent differences in brown adipose tissue respiration during torpor in a mammalian hibernator

Although seasonal modifications of brown adipose tissue (BAT) in hibernators are well documented, we know little about functional regulation of BAT in different phases of hibernation. In the 13-lined ground squirrel, liver mitochondrial respiration is suppressed by up to 70% during torpor. This suppression is reversed during arousal and interbout euthermia (IBE), and corresponds with patterns of maximal activities of electron transport system (ETS) enzymes. Uncoupling of BAT mitochondria is controlled by free fatty acid release stimulated by sympathetic activation of adipocytes, so we hypothesized that further regulation at the level of the ETS would be of little advantage. As predicted, maximal ETS enzyme activities of isolated BAT mitochondria did not differ between torpor and IBE. In contrast to this pattern, respiration rates of mitochondria isolated from torpid individuals were suppressed by ~60% compared with rates from IBE individuals when measured at 37°C. At 10°C, however, mitochondrial respiration rates tended to be greater in torpor than IBE. As a result, the temperature sensitivity (Q₁₀) of mitochondrial respiration was significantly lower in torpor (~1.4) than IBE (~2.4), perhaps facilitating energy savings during entrance into torpor and thermogenesis at low body temperatures. Despite the observed differences in isolated mitochondria, norepinephrine-stimulated respiration rates of isolated BAT adipocytes did not differ between torpor and IBE, perhaps because the adipocyte isolation requires lengthy incubation at 37°C, potentially reversing any changes that occur in torpor. Such changes may include remodeling of BAT mitochondrial membrane phospholipids, which could change in situ enzyme activities and temperature sensitivities.

Chronic hyperinsulinemia reduces insulin sensitivity and metabolic functions of brown adipocyte

The growing pandemics of diabetes have become a real threat to world economy. Hyperinsulinemia and insulin resistance are closely associated with the pathophysiology of type 2 diabetes. In pretext of brown adipocytes being considered as the therapeutic strategy for the treatment of obesity and insulin resistance, we have tried to understand the effect of hyperinsulinemia on brown adipocyte function. We here with for the first time report that hyperinsulinemia-induced insulin resistance in brown adipocyte is also accompanied with reduced insulin sensitivity and brown adipocyte characteristics. CI treatment decreased expression of brown adipocyte-specific markers (such as PRDM16, PGC1α, and UCP1) and mitochondrial content as well as activity. CI-treated brown adipocytes showed drastic decrease in oxygen consumption rate (OCR) and spare respiratory capacity. Morphological study indicates increased accumulation of lipid droplets in CI-treated brown adipocytes. We have further validated these findings in vivo in C57BL/6 mice implanted with mini-osmotic insulin pump for 8weeks. CI treatment in mice leads to increased body weight gain, fat mass and impaired glucose intolerance with reduced energy expenditure and insulin sensitivity. CI-treated mice showed decreased BAT characteristics and function. We also observed increased inflammation and ER stress markers in BAT of CI-treated animals. The above results conclude that hyperinsulinemia has deleterious effect on brown adipocyte function, making it susceptible to insulin resistance. Thus, the above findings have greater implication in designing approaches for the treatment of insulin resistance and diabetes via recruitment of brown adipocytes.

Mitochondria-targeted dodecyltriphenylphosphonium (C12TPP) combats high-fat-diet-induced obesity in mice

Background:

A membrane-penetrating cation, dodecyltriphenylphosphonium (C₁₂TPP), facilitates the recycling of fatty acids in the artificial lipid membrane and mitochondria. C₁₂TPP can dissipate mitochondrial membrane potential and may affect total energy expenditure and body weight in animals and humans.

Methods:

We investigated the metabolic effects of C₁₂TPP in isolated brown-fat mitochondria, brown adipocyte cultures and mice in vivo. Experimental approaches included the measurement of oxygen consumption, carbon dioxide production, western blotting, magnetic resonance imaging and bomb calorimetry.

Results:

In mice, C₁₂TPP (50 μmol per (day•kg body weight)) in the drinking water significantly reduced body weight (12%, P<0.001) and body fat mass (24%, P<0.001) during the first 7 days of treatment. C₁₂TPP did not affect water palatability and intake or the energy and lipid content in feces. The addition of C₁₂TPP to isolated brown-fat mitochondria resulted in increased oxygen consumption. Three hours of pretreatment with C₁₂TPP also increased oligomycin-insensitive oxygen consumption in brown adipocyte cultures (P<0.01). The effects of C₁₂TPP on mitochondria, cells and mice were independent of uncoupling protein 1 (UCP1). However, C₁₂TPP treatment increased the mitochondrial protein levels in the brown adipose tissue of both wild-type and UCP1-knockout mice. Pair-feeding revealed that one-third of the body weight loss in C₁₂TPP-treated mice was due to reduced food intake. C₁₂TPP treatment elevated the resting metabolic rate (RMR) by up to 18% (P<0.05) compared with pair-fed animals. C₁₂TPP reduced the respiratory exchange ratio, indicating enhanced fatty acid oxidation in mice.

Conclusions:

C₁₂TPP combats diet-induced obesity by reducing food intake, increasing the RMR and enhancing fatty acid oxidation.

Metabolically inert perfluorinated fatty acids directly activate uncoupling protein 1 in brown-fat mitochondria

The metabolically inert perfluorinated fatty acids perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) can display fatty acid-like activity in biological systems. The uncoupling protein 1 (UCP1) in brown adipose tissue is physiologically (re)activated by fatty acids, including octanoate. This leads to bioenergetically uncoupled energy dissipation (heat production, thermogenesis). We have examined here the possibility that PFOA/PFOS can directly (re)activate UCP1 in isolated mouse brown-fat mitochondria. In wild-type brown-fat mitochondria, PFOS and PFOA overcame GDP-inhibited thermogenesis, leading to increased oxygen consumption and dissipated membrane potential. The absence of this effect in brown-fat mitochondria from UCP1-ablated mice indicated that it occurred through activation of UCP1. A competitive type of inhibition by increased GDP concentrations indicated interaction with the same mechanistic site as that utilized by fatty acids. No effect was observed in heart mitochondria, i.e., in mitochondria without UCP1. The stimulatory effect of PFOA/PFOS was not secondary to non-specific mitochondrial membrane permeabilization or to ROS production. Thus, metabolic effects of perfluorinated fatty acids could include direct brown adipose tissue (UCP1) activation. The possibility that this may lead to unwarranted extra heat production and thus extra utilization of food resources, leading to decreased fitness in mammalian wildlife, is discussed, as well as possible negative effects in humans. However, a possibility to utilize PFOA-/PFOS-like substances for activating UCP1 therapeutically in obesity-prone humans may also be envisaged.

Peroxisomes compensate hepatic lipid overflow in mice with fatty liver

Major causes of lipid accumulation in liver are increased import or synthesis or decreased catabolism of fatty acids. The latter is caused by dysfunction of cellular organelles controlling energy homeostasis, i.e., mitochondria. Peroxisomes also appear to be an important organelle in lipid metabolism of hepatocytes, but little is known about their role in the development of non-alcoholic fatty liver disease (NAFLD). To investigate the role of peroxisomes alongside mitochondria in excessive hepatic lipid accumulation, we used leptin-resistant db/db mice on C57BLKS background, a mouse model that develops hyperphagia-induced diabetes with obesity and NAFLD. Proteome and gene expression analyses along with lipid analyses in the liver revealed differential expression of genes related to lipid metabolism and β-oxidation, whereas genes for peroxisomal proteins were predominantly regulated.

CONCLUSION

Our investigations show that in fatty liver disease in combination with obesity and diabetes, the hepatocyte-protecting organelle peroxisome is altered. Hence, peroxisomes might indicate a stage of pre-NAFLD, play a role in the early development of NAFLD and appear to be a potential target for treatment and prevention of NAFLD.

Teaching the fundamentals of electron transfer reactions in mitochondria and the production and detection of reactive oxygen species

Mitochondria fulfill a number of biological functions which inherently depend on ATP and O2(-•)/H2O2 production. Both ATP and O2(-•)/H2O2 are generated by electron transfer reactions. ATP is the product of oxidative phosphorylation whereas O2(-•) is generated by singlet electron reduction of di-oxygen (O2). O2(-•) is then rapidly dismutated by superoxide dismutase (SOD) producing H2O2. O2(-•)/H2O2 were once viewed as unfortunately by-products of aerobic respiration. This characterization is fitting considering over production of O2(-•)/H2O2 by mitochondria is associated with range of pathological conditions and aging. However, O2(-•)/H2O2 are only dangerous in large quantities. If produced in a controlled fashion and maintained at a low concentration, cells can benefit greatly from the redox properties of O2(-•)/H2O2. Indeed, low rates of O2(-•)/H2O2 production are required for intrinsic mitochondrial signaling (e.g. modulation of mitochondrial processes) and communication with the rest of the cell. O2(-•)/H2O2 levels are kept in check by anti-oxidant defense systems that sequester O2(-•)/H2O2 with extreme efficiency. Given the importance of O2(-•)/H2O2 in cellular function, it is imperative to consider how mitochondria produce O2(-•)/H2O2 and how O2(-•)/H2O2 genesis is regulated in conjunction with fluctuations in nutritional and redox states. Here, I discuss the fundamentals of electron transfer reactions in mitochondria and emerging knowledge on the 11 potential sources of mitochondrial O2(-•)/H2O2 in tandem with their significance in contributing to overall O2(-•)/H2O2 emission in health and disease. The potential for classifying these different sites in isopotential groups, which is essentially defined by the redox properties of electron donator involved in O2(-•)/H2O2 production, as originally suggested by Brand and colleagues is also surveyed in detail. In addition, redox signaling mechanisms that control O2(-•)/H2O2 genesis from these sites are discussed. Finally, the current methodologies utilized for measuring O2(-•)/H2O2 in isolated mitochondria, cell culture and in vivo are reviewed.

Sestrin2, a Regulator of Thermogenesis and Mitohormesis in Brown Adipose Tissue

Sestrin2 is a stress-inducible protein that functions as an antioxidant and inhibitor of mTOR complex 1. In a recent study, we found that Sestrin2 overexpression in brown adipocytes interfered with normal metabolism by reducing mitochondrial respiration through the suppression of uncoupling protein 1 (UCP1) expression. The metabolic effects of Sestrin2 in brown adipocytes were dependent on its antioxidant activity, and chemical antioxidants produced similar effects in inhibiting UCP1-dependent thermogenesis. These observations suggest that low levels of reactive oxygen species (ROS) in brown adipocytes can actually be beneficial and necessary for proper metabolic homeostasis. In addition, considering that Sestrins are ROS inducible and perform ROS detoxifying as well as other metabolism-controlling functions, they are potential regulators of mitohormesis. This is a concept in which overall beneficial effects result from low-level oxidative stress stimuli, such as the ones induced by caloric restriction or physical exercise. In this perspective, we incorporate our recent insight obtained from the Sestrin2 study toward a better understanding of the relationship between ROS, Sestrin2, and mitochondrial metabolism in the context of brown adipocyte physiology.

Unraveling biochemical pathways affected by mitochondrial dysfunctions using metabolomic approaches

Mitochondrial dysfunction(s) (MDs) can be defined as alterations in the mitochondria, including mitochondrial uncoupling, mitochondrial depolarization, inhibition of the mitochondrial respiratory chain, mitochondrial network fragmentation, mitochondrial or nuclear DNA mutations and the mitochondrial accumulation of protein aggregates. All these MDs are known to alter the capacity of ATP production and are observed in several pathological states/diseases, including cancer, obesity, muscle and neurological disorders. The induction of MDs can also alter the secretion of several metabolites, reactive oxygen species production and modify several cell-signalling pathways to resolve the mitochondrial dysfunction or ultimately trigger cell death. Many metabolites, such as fatty acids and derived compounds, could be secreted into the blood stream by cells suffering from mitochondrial alterations. In this review, we summarize how a mitochondrial uncoupling can modify metabolites, the signalling pathways and transcription factors involved in this process. We describe how to identify the causes or consequences of mitochondrial dysfunction using metabolomics (liquid and gas chromatography associated with mass spectrometry analysis, NMR spectroscopy) in the obesity and insulin resistance thematic.

Mitochondrial uncoupling as a regulator of life-history trajectories in birds: an experimental study in the zebra finch

Mitochondria have a fundamental role in the transduction of energy from food into ATP. The coupling between food oxidation and ATP production is never perfect, but may nevertheless be of evolutionary significance. The 'uncoupling to survive' hypothesis suggests that 'mild' mitochondrial uncoupling evolved as a protective mechanism against the excessive production of damaging reactive oxygen species (ROS). Because resource allocation and ROS production are thought to shape animal life histories, alternative life-history trajectories might be driven by individual variation in the degree of mitochondrial uncoupling. We tested this hypothesis in a small bird species, the zebra finch (Taeniopygia guttata), by treating adults with the artificial mitochondrial uncoupler 2,4-dinitrophenol (DNP) over a 32-month period. In agreement with our expectations, the uncoupling treatment increased metabolic rate. However, we found no evidence that treated birds enjoyed lower oxidative stress levels or greater survival rates, in contrast to previous results in other taxa. In vitro experiments revealed lower sensitivity of ROS production to DNP in mitochondria isolated from skeletal muscles of zebra finch than mouse. In addition, we found significant reductions in the number of eggs laid and in the inflammatory immune response in treated birds. Altogether, our data suggest that the 'uncoupling to survive' hypothesis may not be applicable for zebra finches, presumably because of lower effects of mitochondrial uncoupling on mitochondrial ROS production in birds than in mammals. Nevertheless, mitochondrial uncoupling appeared to be a potential life-history regulator of traits such as fecundity and immunity at adulthood, even with food supplied ad libitum.

Putative roles of Ca(2+) -independent phospholipase A2 in respiratory chain-associated ROS production in brain mitochondria: influence of docosahexaenoic acid and bromoenol lactone

Ca(2+) -independent phospholipase A2 (iPLA2 ) is hypothesized to control mitochondrial reactive oxygen species (ROS) generation. Here, we modulated the influence of iPLA2 -induced liberation of non-esterified free fatty acids on ROS generation associated with the electron transport chain. We demonstrate enzymatic activity of membrane-associated iPLA2 in native, energized rat brain mitochondria (RBM). Theoretically, enhanced liberation of free fatty acids by iPLA2 modulates mitochondrial ROS generation, either attenuating the reversed electron transport (RET) or deregulating the forward electron transport of electron transport chain. For mimicking such conditions, we probed the effect of docosahexaenoic acid (DHA), a major iPLA2 product on ROS generation. We demonstrate that the adenine nucleotide translocase partly mediates DHA-induced uncoupling, and that low micromolar DHA concentrations diminish RET-dependent ROS generation. Uncoupling proteins have no effect, but the adenine nucleotide translocase inhibitor carboxyatractyloside attenuates DHA-linked uncoupling effect on RET-dependent ROS generation. Under physiological conditions of forward electron transport, low micromolar DHA stimulates ROS generation. Finally, exposure of RBM to the iPLA2 inhibitor bromoenol lactone (BEL) enhanced ROS generation. BEL diminished RBM glutathione content. BEL-treated RBM exhibits reduced Ca(2+) retention capacity and partial depolarization. Thus, we rebut the view that iPLA2 attenuates oxidative stress in brain mitochondria. However, the iPLA2 inhibitor BEL has detrimental activities on energy-dependent mitochondrial functions. The Ca(2+) -independent phospholipase A2 (iPLA2 ), a FFA (free fatty acids)-generating membrane-attached mitochondrial phospholipase, is potential to regulate ROS (reactive oxygen species) generation by mitochondria. FFA can either decrease reversed electron transport (RET)-linked or enhance forward electron transport (FET)-linked ROS generation. In the physiological mode of FET, iPLA2 activity increases ROS generation. The iPLA2 inhibitor BEL exerts detrimental effects on energy-dependent mitochondrial functions.