A function for novel uncoupling proteins: antioxidant defense of mitochondrial matrix by translocating fatty acid peroxides from the inner to the outer membrane leaflet

Human mitochondrial uncoupling protein 3 functions as a metabolite transporter

Since its discovery, a major debate about mitochondrial uncoupling protein 3 (UCP3) has been whether its metabolic actions result primarily from mitochondrial inner membrane proton transport, a process that decreases respiratory efficiency and ATP synthesis. However, UCP3 expression and activity are induced by conditions that would seem at odds with inefficient 'uncoupled' respiration, including fasting and exercise. Here, we demonstrate that the bacterially expressed human UCP3, reconstituted into liposomes, catalyses a strict exchange of aspartate, malate, sulphate and phosphate. The R282Q mutation abolishes the transport activity of the protein. Although the substrate specificity and inhibitor sensitivity of UCP3 display similarity with that of its close homolog UCP2, the two proteins significantly differ in their transport mode and kinetic constants.

Mechanism of Obesity-Related Lipotoxicity and Clinical Perspective

The link between cellular exposure to fatty acid species and toxicity phenotypes remains poorly understood. However, structural characterization and functional profiling of human plasma free fatty acids (FFAs) analysis has revealed that FFAs are located either in the toxic cluster or in the cluster that is transcriptionally responsive to lipotoxic stress and creates genetic risk factors. Genome-wide short hairpin RNA screen has identified more than 350 genes modulating lipotoxicity. Hypertrophic adipocytes in obese adipose are both unable to expand further to store excess lipids in the diet and are resistant to the antilipolytic action of insulin. In addition to lipolysis, the inability of packaging the excess lipids into lipid droplets causes circulating fatty acids to reach toxic levels in non-adipose tissues. Deleterious effects of accumulated lipid in non-adipose tissues are known as lipotoxicity. Although triglycerides serve a storage function for long-chain non-esterified fatty acid and their products such as ceramide and diacylglycerols (DAGs), overloading of palmitic acid fraction of saturated fatty acids (SFAs) raises ceramide levels. The excess DAG and ceramide load create harmful effects on multiple organs and systems, inducing chronic inflammation in obesity. Thus, lipotoxic inflammation results in β cells death and pancreatic islets dysfunction. Endoplasmic reticulum stress stimuli induce lipolysis by activating cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) and extracellular signal-regulated kinase (Erk) 1/2 signaling in adipocytes. However, palmitic acid-induced endoplasmic reticulum stress-c-Jun N-terminal kinase (JNK)-autophagy axis in hypertrophic adipocytes is a pro-survival mechanism against endoplasmic reticulum stress and cell death induced by SFAs. Endoplasmic reticulum-localized acyl-coenzyme A (CoA): glycerol-3-phosphate acyltransferase (GPAT) enzymes are mediators of lipotoxicity, and inhibiting these enzymes has therapeutic potential for lipotoxicity. Lipotoxicity increases the number of autophagosomes, which engulf palmitic acid, and thus suppress the autophagic turnover. Fatty acid desaturation promotes palmitate detoxification and storages into triglycerides. As therapeutic targets of glucolipotoxicity, in addition to caloric restriction and exercise, there are four different pharmacological approaches, which consist of metformin, glucagon-like peptide 1 (GLP-1) receptor agonists, peroxisome proliferator-activated receptor-gamma (PPARγ) ligands thiazolidinediones, and chaperones are still used in clinical practice. Furthermore, induction of the brown fat-like phenotype with the mixture of eicosapentanoic acid and docosahexaenoic acid appears as a potential therapeutic application for treatment of lipotoxicity.

Disruption of mitochondrial bioenergetics and calcium homeostasis by phytanic acid in the heart: Potential relevance for the cardiomyopathy in Refsum disease

Refsum disease is an inherited peroxisomal disorder caused by severe deficiency of phytanoyl-CoA hydroxylase activity. Affected patients develop severe cardiomyopathy of poorly known pathogenesis that may lead to a fatal outcome. Since phytanic acid (Phyt) concentrations are highly increased in tissues of individuals with this disease, it is conceivable that this branched-chain fatty acid is cardiotoxic. The present study investigated whether Phyt (10-30 μM) could disturb important mitochondrial functions in rat heart mitochondria. We also determined the influence of Phyt (50-100 μM) on cell viability (MTT reduction) in cardiac cells (H9C2). Phyt markedly increased mitochondrial state 4 (resting) and decreased state 3 (ADP-stimulated) and uncoupled (CCCP-stimulated) respirations, besides reducing the respiratory control ratio, ATP synthesis and the activities of the respiratory chain complexes I-III, II, and II-III. This fatty acid also reduced mitochondrial membrane potential and induced swelling in mitochondria supplemented by exogenous Ca²⁺, which were prevented by cyclosporin A alone or combined with ADP, suggesting the involvement of the mitochondrial permeability transition (MPT) pore opening. Mitochondrial NAD(P)H content and Ca²⁺ retention capacity were also decreased by Phyt in the presence of Ca²⁺. Finally, Phyt significantly reduced cellular viability (MTT reduction) in cultured cardiomyocytes. The present data indicate that Phyt, at concentrations found in the plasma of patients with Refsum disease, disrupts by multiple mechanisms mitochondrial bioenergetics and Ca²⁺ homeostasis, which could presumably be involved in the cardiomyopathy of this disease.

Comprehensive Analysis and Validation of Solute Carrier Family 25 (SLC25) and Its Correlation with Immune Infiltration in Pan-Cancer

As the largest gene family functioning in protein transport among human solute carriers, the SLC25 family (mitochondrial carrier family) can participate in development of cancer. However, a comprehensive exploration for the exactly roles of SLC family remains lacking. In the present study, a total of 15 functional SLC25 family genes were retrieved from all current publications. And multidimensional analyses were systematically performed based on the transcriptome and genome data of SLC25 family from a variety of online databases for their expression, immune cell infiltration, and cancer prognosis. Validation by qPCR and immunohistochemistry were further conducted for the expression of partial SLC25 family members in some tumor tissue. We found that the SLC25 family had strong correlation with immune cells, such as macrophages M2, CD8+ T cell, CD4+ T cell memory activated, and memory resting. Among them, SLC25A6 was most correlated with Macrophage M1 in uveal melanoma (r = -0.68, P = 1.9e - 0.5). Expression of mRNA level showed that SLC25A4 was downregulated in stomach adenocarcinoma and colon adenocarcinoma. SLC25A7 was highly expressed in stomach adenocarcinoma and colon adenocarcinoma. SLC25A23 was decreased in colon adenocarcinoma. qPCR and immunohistochemistry validation results were consistent with our bioinformatics prediction. SLC25A8 was associated with the prognosis of cancer. All these findings suggested that the SLC25 family might affects the immune microenvironment of the cancer and then had the potential to be predictive biomarkers for early diagnosis and prognosis as well as novel targets for individualized treatment of cancer.

Oxygen Consumption and Basal Metabolic Rate as Markers of Susceptibility to Malignant Hyperthermia and Heat Stroke

Calsequestrin 1 (CASQ1) and Ryanodine receptor 1 (RYR1) are two of the main players in excitation-contraction (EC) coupling. CASQ1-knockout mice and mice carrying a mutation in RYR1 (Y522S) linked to human malignant hyperthermia susceptibility (MHS) both suffer lethal hypermetabolic episodes when exposed to halothane (MHS crises) and to environmental heat (heat stroke, HS). The phenotype of Y522S is more severe than that of CASQ1-null mice. As MHS and HS are hypermetabolic responses, we studied the metabolism of adult CASQ1-null and Y522S mice using wild-type (WT) mice as controls. We found that CASQ1-null and Y522S mice have increased food consumption and higher core temperature at rest. By indirect calorimetry, we then verified that CASQ1-null and Y522S mice show an increased oxygen consumption and a lower respiratory quotient (RQ). The accelerated metabolism of CASQ1-null and Y522S mice was also accompanied with a reduction in body fat. Moreover, both mouse models displayed increased oxygen consumption and a higher core temperature during heat stress. The results collected suggest that metabolic rate, oxygen consumption, and body temperature at rest, all more elevated in Y522S than in CASQ1-null mice, could possibly be used as predictors of the level of susceptibility to hyperthermic crises of mice (and possibly humans).

Stimulation of adenosine A1 receptor prevents oxidative injury in H9c2 cardiomyoblasts: Role of Gβγ-mediated Akt and ERK1/2 signaling

Oxidative stress causes cellular injury and damage in the heart primarily through apoptosis resulting in cardiac abnormalities such as heart failure and cardiomyopathy. During oxidative stress, stimulation of adenosine receptor (AR) has been shown to protect against oxidative damage due to their cytoprotective properties. However, the subtype specificity and signal transductions of adenosine A₁ receptor (A₁R) on cardiac protection during oxidative stress have remained elusive. In this study, we found that stimulation of A₁Rs with N⁶-cyclopentyladenosine (CPA), a specific A₁R agonist, attenuated the H₂O₂-induced intracellular and mitochondrial reactive oxygen species (ROS) production and apoptosis. In addition, A₁R stimulation upregulated the synthesis of antioxidant enzymes (catalase and GPx-1), antiapoptotic proteins (Bcl-2 and Bcl-xL), and mitochondria-related markers (UCP2 and UCP3). Blockades of Gβγ subunit of heterotrimeric Gα_i protein antagonized A₁R-mediated antioxidant and antiapoptotic effects, confirming the potential role of Gβγ subunit-mediated A₁R signaling. Additionally, cardioprotective effects of CPA mediated through PI3K/Akt- and ERK1/2-dependent signaling pathways. Thus, we propose that A₁R represents a promising therapeutic target for prevention of oxidative injury in the heart.

Muscle-Derived Beige Adipose Precursors Secrete Promyogenic Exosomes That Treat Rotator Cuff Muscle Degeneration in Mice and Are Identified in Humans by Single-Cell RNA Sequencing

BACKGROUND

Muscle atrophy, fibrosis, and fatty infiltration are common to a variety of sports-related and degenerative conditions and are thought to be irreversible. Fibroadipogenic progenitors (FAPs) are multipotent resident muscle stem cells with the capacity to differentiate into fibrogenic as well as white and beige adipose tissue (BAT). FAPs that have assumed a BAT differentiation state (FAP-BAT) have proven efficacious in treating muscle degeneration in numerous injury models.

PURPOSE

To characterize the subpopulation of murine FAPs with FAP-BAT activity, determine whether their promyogenic effect is mediated via exosomes, and analyze human FAPs for an analogous promyogenic exosome-rich subpopulation.

STUDY DESIGN

Controlled laboratory study.

METHODS

FAPs from UCP1 reporter mice were isolated via fluorescence-activated cell sorting and sorted according to the differential intensity of the UCP1 signal observed: negative for UCP1 (UCP1-), intermediate intensity (UCP1+), and high intensity (UCP1++). Bulk RNA sequencing was performed on UCP1-, UCP1+, and UCP1++ FAPs to evaluate distinct characteristics of each population. Exosomes were harvested from UCP1++ FAP-BAT exosomes (Exo-FB) as well as UCP1- non-FAP-BAT exosomes (Exo-nFB) cells using cushioned-density gradient ultracentrifugation and used to treat C2C12 cells and mouse embryonic fibroblasts in vitro, and the myotube fusion index was assessed. Exo-FB and Exo-nFB were then used to treat wild type C57B/L6J mice that had undergone a massive rotator cuff tear. At 6 weeks mice were sacrificed, and supraspinatus muscles were harvested and analyzed for muscle atrophy, fibrosis, fatty infiltration, and UCP1 expression. Single-cell RNA sequencing was then performed on FAPs isolated from human muscle that were treated with the beta-agonist formoterol or standard media to assess for the presence of a parallel promyogenic subpopulation of FAP-BAT cells in humans.

RESULTS

Flow cytometry analysis of sorted UCP1 reporter mouse FAPs revealed a trimodal distribution of UCP1 signal intensity, which correlated with 3 distinct transcriptomic profiles characterized with bulk RNA sequencing. UCP1++ cells were marked by high mitochondrial gene expression, BAT markers, and exosome surface makers; UCP1- cells were marked by fibrogenic markers; and UCP1+ cells were characterized differential enrichment of white adipose tissue markers. Exo-FB treatment of C2C12 cells resulted in robust myotube fusion, while treatment of mouse embryonic fibroblasts resulted in differentiation into myotubes. Treatment of cells with Exo-nFB resulted in poor myotube formation. Mice that were treated with Exo-FB at the time of rotator cuff injury demonstrated markedly reduced muscle atrophy and fatty infiltration as compared with treatment with Exo-nFB or phosphate-buffered saline. Single-cell RNA sequencing of human FAPs from the rotator cuff revealed 6 distinct subpopulations of human FAPs, with one subpopulation demonstrating the presence of UCP1+ beige adipocytes with a distinct profile of BAT, mitochondrial, and extracellular vesicle-associated markers.

CONCLUSION

FAP-BAT cells form a subpopulation of FAPs with upregulated beige gene expression and exosome production that mediate promyogenic effects in vitro and in vivo, and they are present as a transcriptomically similar subpopulation of FAPs in humans.

CLINICAL RELEVANCE

FAP-BAT cells and their exosomes represent a potential therapeutic avenue for treating rotator cuff muscle degeneration.

The relationship between free fatty acids and mitochondrial oxidative stress damage to trophoblast cell in preeclampsia

Aim

To investigate the effects of free fatty acids on mitochondrial oxidative stress and the pathogenesis of preeclampsia.

Methods

Human primary trophoblast cells at 6–8 weeks of gestation were retrieved and cultured to 70–80% confluence, then incubated in serum from women with a normal pregnancy (normal pregnancy group), women with preeclampsia (PE group), and a combination of serum from women with 24 h preeclampsia-like symptoms and free fatty acids (FFA group). Mitochondrial membrane potential was assessed by fluorescent dye concurrent with detection of membrane channel conversion pore activity by fluorescence microscope. Enzyme labeling instruments and RT-PCR were used to detect mitochondrial DNA (mtDNA) levels.

Results

The preeclampsia and free fatty acids groups both exhibited significantly higher levels of mitochondria oxidative stress damage when compared to the normal pregnancy group. However, no significant differences in mitochondrial oxidative stress damage were observed between the FFA and PE groups.

Conclusions

Serum free fatty acids might play an important role in the pathogenesis of preeclampsia by enhancing mitochondrial oxidative stress damage.

Bioenergetic Aspects of Mitochondrial Actions of Thyroid Hormones

Much is known, but there is also much more to discover, about the actions that thyroid hormones (TH) exert on metabolism. Indeed, despite the fact that thyroid hormones are recognized as one of the most important regulators of metabolic rate, much remains to be clarified on which mechanisms control/regulate these actions. Given their actions on energy metabolism and that mitochondria are the main cellular site where metabolic transformations take place, these organelles have been the subject of extensive investigations. In relatively recent times, new knowledge concerning both thyroid hormones (such as the mechanisms of action, the existence of metabolically active TH derivatives) and the mechanisms of energy transduction such as (among others) dynamics, respiratory chain organization in supercomplexes and cristes organization, have opened new pathways of investigation in the field of the control of energy metabolism and of the mechanisms of action of TH at cellular level. In this review, we highlight the knowledge and approaches about the complex relationship between TH, including some of their derivatives, and the mitochondrial respiratory chain.

Oxidative damage and mitochondrial functionality in hearts from KO UCP3 mice housed at thermoneutrality

The antioxidant role of mitochondrial uncoupling protein 3 (UCP3) is controversial. This work aimed to investigate the effects of UCP3 on the heart of mice housed at thermoneutral temperature, an experimental condition that avoids the effects of thermoregulation on mitochondrial activity and redox homeostasis, preventing the alterations related to these processes from confusing the results caused by the lack of UCP3. WT and KO UCP3 mice were acclimatized at 30 °C for 4 weeks and hearts were used to evaluate metabolic capacity and redox state. Tissue and mitochondrial respiration, the activities of the mitochondrial complexes, and the protein expression of mitochondrial complexes markers furnished information on mitochondrial functionality. The levels of lipid and protein oxidative damage markers, the activity of antioxidant enzymes, the reactive oxygen species levels, and the susceptibility to in vitro Fe-ascorbate-induced oxidative stress furnished information on redox state. UCP3 ablation reduced tissue and mitochondrial respiratory capacities, not affecting the mitochondrial content. In KO UCP3 mice, the mitochondrial complexes activities were lower than in WT without changes in their content. These effects were accompanied by an increase in the level of oxidative stress markers, ROS content, and in vitro susceptibility to oxidative stress, notwithstanding that the activities of antioxidant enzymes were not affected by UCP3 ablation. Such modifications are also associated with enhanced activation/phosphorylation of EIF2α, a marker of integrated stress response and endoplasmic reticulum stress (GRP778 BIP). The lack of UCP3 makes the heart more prone to oxidative insult by reducing oxygen consumption and increasing ROS. Our results demonstrate that UCP3 helps the cell to preserve mitochondrial function by mitigating oxidative stress.

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.

Exercise prevents fatty liver by modifying the compensatory response of mitochondrial metabolism to excess substrate availability

Objective

Liver mitochondria adapt to high-calorie intake. We investigated how exercise alters the early compensatory response of mitochondria, thus preventing fatty liver disease as a long-term consequence of overnutrition.

Methods

We compared the effects of a steatogenic high-energy diet (HED) for six weeks on mitochondrial metabolism of sedentary and treadmill-trained C57BL/6N mice. We applied multi-OMICs analyses to study the alterations in the proteome, transcriptome, and lipids in isolated mitochondria of liver and skeletal muscle as well as in whole tissue and examined the functional consequences by high-resolution respirometry.

Results

HED increased the respiratory capacity of isolated liver mitochondria, both in sedentary and in trained mice. However, proteomics analysis of the mitochondria and transcriptomics indicated that training modified the adaptation of the hepatic metabolism to HED on the level of respiratory complex I, glucose oxidation, pyruvate and acetyl-CoA metabolism, and lipogenesis. Training also counteracted the HED-induced glucose intolerance, the increase in fasting insulin, and in liver fat by lowering diacylglycerol species and c-Jun N-terminal kinase (JNK) phosphorylation in the livers of trained HED-fed mice, two mechanisms that can reverse hepatic insulin resistance. In skeletal muscle, the combination of HED and training improved the oxidative capacity to a greater extent than training alone by increasing respiration of isolated mitochondria and total mitochondrial protein content.

Conclusion

We provide a comprehensive insight into the early adaptations of mitochondria in the liver and skeletal muscle to HED and endurance training. Our results suggest that exercise disconnects the HED-induced increase in mitochondrial substrate oxidation from pyruvate and acetyl-CoA-driven lipid synthesis. This could contribute to the prevention of deleterious long-term effects of high fat and sugar intake on hepatic mitochondrial function and insulin sensitivity.

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High-energy diet promotes mitochondrial respiration in liver independent of training.

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High-energy diet combined with training disconnects substrate oxidation from lipid synthesis.

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High-energy diet combined with training reduces complex I formation in the liver.

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Trained skeletal muscle unburdens the liver from substrate overload.

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Comprehensive resource of mitochondrial adaptations to high-energy diet and training.

Molecular Insights into the Recruiting Between UCP2 and DDX5/UBAP2L in the Metabolic Plasticity of Non-Small-Cell Lung Cancer

Mitochondrial uncoupling protein 2 (UCP2) is distributed in tumor cells with a link to the support of systemic metabolic deregulation, and the downregulation of UCP2 has been unveiled as a biomarker of oncogenesis and chemoresistance in non-small-cell lung cancer (NSCLC) cells. However, the underlying mechanism of how UCP2 cooperates with other proteins in this metabolic reprogramming remains largely unsolved. We employed a combined computational and experimental strategy to explore into the recruiting of DDX5 with other proteins, and we unraveled the underlying structural mechanisms. We found that recruiting by ATP-dependent RNA helicase DDX5 (DDX5)/ubiquitin-associated protein 2-like (UBAP2L) might help UCP2 to play the pathological roles in NSCLC cells. According to the view of thermodynamics in physics, UCP2 tends to recruit DDX5 rather than UBAP2L, as shown by the ensemble-based docking, molecular dynamics simulations and molecular mechanics generalized Born surface area (MM/GBSA) approach. Cellular immunofluorescence assays further demonstrated that UCP2 associate with DDX5, and the recruiting of DDX5 with UCP2 at least partially contribute to the metabolic plasticity of NSCLCs via the AKT/mTOR pathway. Our study proposed an efficient way for detecting the protein-protein association via the experimentally validated molecular simulation. Our results shed light on the functional annotation of UCP and DDX family proteins in dysregulated metabolism, and the identification of candidate therapeutic targets for NSCLC.

Podocyte Lipotoxicity in CKD

CKD represents the ninth most common cause of death in the United States but, despite this large health burden, treatment options for affected patients remain limited. To remedy this, several relevant pathways have been identified that may lead to novel therapeutic options. Among them, altered renal lipid metabolism, first described in 1982, has been recognized as a common pathway in clinical and experimental CKD of both metabolic and nonmetabolic origin. This observation has led many researchers to investigate the cause of this renal parenchyma lipid accumulation and its downstream effect on renal structure and function. Among key cellular components of the kidney parenchyma, podocytes are terminally differentiated cells that cannot be easily replaced when lost. Clinical and experimental evidence supports a role of reduced podocyte number in the progression of CKD. Given the importance of the podocytes in the maintenance of the glomerular filtration barrier and the accumulation of TG and cholesterol-rich lipid droplets in the podocyte and glomerulus in kidney diseases that cause CKD, understanding the upstream cause and downstream consequences of lipid accumulation in podocytes may lead to novel therapeutic opportunities. In this review, we hope to consolidate our understanding of the causes and consequences of dysregulated renal lipid metabolism in CKD development and progression, with a major focus on podocytes.

Solute Carrier Transporters as Potential Targets for the Treatment of Metabolic Disease

The solute carrier (SLC) superfamily comprises more than 400 transport proteins mediating the influx and efflux of substances such as ions, nucleotides, and sugars across biological membranes. Over 80 SLC transporters have been linked to human diseases, including obesity and type 2 diabetes (T2D). This observation highlights the importance of SLCs for human (patho)physiology. Yet, only a small number of SLC proteins are validated drug targets. The most recent drug class approved for the treatment of T2D targets sodium-glucose cotransporter 2, product of the SLC5A2 gene. There is great interest in identifying other SLC transporters as potential targets for the treatment of metabolic diseases. Finding better treatments will prove essential in future years, given the enormous personal and socioeconomic burden posed by more than 500 million patients with T2D by 2040 worldwide. In this review, we summarize the evidence for SLC transporters as target structures in metabolic disease. To this end, we identified SLC13A5/sodium-coupled citrate transporter, and recent proof-of-concept studies confirm its therapeutic potential in T2D and nonalcoholic fatty liver disease. Further SLC transporters were linked in multiple genome-wide association studies to T2D or related metabolic disorders. In addition to presenting better-characterized potential therapeutic targets, we discuss the likely unnoticed link between other SLC transporters and metabolic disease. Recognition of their potential may promote research on these proteins for future medical management of human metabolic diseases such as obesity, fatty liver disease, and T2D. SIGNIFICANCE STATEMENT: Given the fact that the prevalence of human metabolic diseases such as obesity and type 2 diabetes has dramatically risen, pharmacological intervention will be a key future approach to managing their burden and reducing mortality. In this review, we present the evidence for solute carrier (SLC) genes associated with human metabolic diseases and discuss the potential of SLC transporters as therapeutic target structures.

Channels and transporters for inorganic ions in plant mitochondria: Prediction and facts

Mitochondria are crucial bioenergetic organelles for providing different metabolites, including ATP, to sustain cell growth both in animals and in plants. These organelles, delimited by two membranes (outer and inner mitochondrial membrane), maintain their function by an intensive communication with other organelles as well as with the cytosol. Transport of metabolites across the two membranes, but also that of inorganic ions, takes place through specific ion channels and transporters and plays a crucial role in ensuring an adequate ionic milieu within the mitochondria. In the present review we briefly summarize the current knowledge about plant mitochondrial ion channels and transporters in comparison to those of animal mitochondria and examine the possible molecular identity of the so far unidentified transport systems taking into account subcellular targeting predictions and data from literature.

P2X7 Receptor in the Management of Energy Homeostasis: Implications for Obesity, Dyslipidemia, and Insulin Resistance

Whole-body energy metabolism entails the highly regulated balance between food intake, nutrient breakdown, energy generation (ATP), and energy storage for the preservation of vital functions and body mass. Purinergic signaling has attracted increasing attention in the regulatory mechanisms not only for the reverse processes of white adipose tissue lipogenesis and lipolysis, but also for brown adipocyte-dependent thermogenesis and leptin production. This regulatory role has remarkable implications in the handling of body's energy expenditure and energy reservoir. Hence, selected purinergic receptors can play a relevant function in lipid metabolism, endocrine activity, glucose uptake, ATP-dependent increased expression of uncoupling protein 1, and browning of adipose tissue. Indeed, purinergic P2 receptors regulate adipogenesis and lipid metabolism and are involved in adipogenic differentiation. In particular, the ionotropic ATP-activated P2X7 subtype is involved in fat distribution, as well as in the modulation of inflammatory pathways in white adipose tissue. Within this context, very recent evidence has established a direct function of P2X7 in energy metabolism. Specifically, either genetic deletion (P2X7 knockout mice) or subchronic pharmacological inhibition of the receptor produces a decrease of whole-body energy expenditure and, concurrently, an increase of carbohydrate oxidation. As further evidence, lipid accumulation, increased fat mass distribution, and weight gain are reported in P2X7-depleted mice. Conversely, the stimulation of P2X7 enhances energy expenditure. Altogether, this knowledge supports the role of P2X7 signaling in the fight against obesity and insulin resistance, as well as in the promotion of adaptive thermogenesis.

EPA is Cardioprotective in Male Rats Subjected to Sepsis, but ALA Is Not Beneficial

It has been proven that dietary eicosapentaenoic acid (C20:5 n-3 or EPA) protects the heart against the deleterious effects of sepsis in female rats. We do not know if this is the case for male rodents. In this case, the efficiency of other n-3 polyunsaturated fatty acids (PUFAs) remains to be determined in both female and male rats. This study aimed at (i) determining whether dietary EPA is cardioprotective in septic male rats; (ii) evaluating the influence of dietary α-linolenic (C18:3 n-3 or ALA) on cardiac function during this pathology; and (iii) finding out the physiological and molecular mechanisms responsible for the observed effects. Sixty male rats were divided into three dietary groups. The animals were fed a diet deficient in n-3 PUFAs (DEF group), a diet enriched with ALA (ALA group) or a diet fortified with EPA (EPA group) for 6 weeks. Thereafter, each group was subdivided into 2 subgroups, one being subjected to cecal ligation and puncture (CLP) and the other undergoing a fictive surgery. Cardiac function was determined in vivo and ex vivo. Several parameters related to the inflammation process and oxidative stress were determined. Finally, the fatty acid compositions of circulating lipids and cardiac phospholipids were evaluated. The results of the ex vivo situation indicated that sepsis triggered cardiac damage in the DEF group. Conversely, the ex vivo data indicated that dietary ALA and EPA were cardioprotective by resolving the inflammation process and decreasing the oxidative stress. However, the measurements of the cardiac function in the in vivo situation modulated these conclusions. Indeed, in the in vivo situation, sepsis deteriorated cardiac mechanical activity in the ALA group. This was suspected to be due to a restricted coronary flow which was related to a lack of cyclooxygenase substrates in membrane phospholipids. Finally, only EPA proved to be beneficial in sepsis. Its action necessitates both resolution of inflammation and increased coronary perfusion. In that sense, dietary ALA, which does not allow the accumulation of vasodilator precursors in membrane lipids, cannot be protective during the pathology.

Glucolipotoxicity: A Proposed Etiology for Wooden Breast and Related Myopathies in Commercial Broiler Chickens

Wooden breast is one of several myopathies of fast-growing commercial broilers that has emerged as a consequence of intensive selection practices in the poultry breeding industry. Despite the substantial economic burden presented to broiler producers worldwide by wooden breast and related muscle disorders such as white striping, the genetic and etiological underpinnings of these diseases are still poorly understood. Here we propose a new hypothesis on the primary causes of wooden breast that implicates dysregulation of lipid and glucose metabolism. Our hypothesis addresses recent findings that have suggested etiologic similarities between wooden breast and type 2 diabetes despite their phenotypic disparities. Unlike in mammals, dysregulation of lipid and glucose metabolism is not accompanied by an increase in plasma glucose levels but generates a unique skeletal muscle phenotype, i.e., wooden breast, in chickens. We hypothesize that these phenotypic disparities result from a major difference in skeletal muscle glucose transport between birds and mammals, and that the wooden breast phenotype most closely resembles complications of diabetes in smooth and cardiac muscle of mammals. Additional basic research on wooden breast and related muscle disorders in commercial broiler chickens is necessary and can be informative for poultry breeding and production as well as for human health and disease. To inform future studies, this paper reviews the current biological knowledge of wooden breast, outlines the major steps in its proposed pathogenesis, and examines how selection for production traits may have contributed to its prevalence.

Important Trends in UCP3 Investigation

Membrane uncoupling protein 3 (UCP3), a member of the mitochondrial uncoupling protein family, was discovered in 1997. UCP3's properties, such as its high homology to other mitochondrial carriers, especially to UCP2, its short lifetime and low specificity of UCP3 antibodies, have hindered progress in understanding its biological function and transport mechanism over decades. The abundance of UCP3 is highest in murine brown adipose tissue (BAT, 15.0 pmol/mg protein), compared to heart (2.7 pmol/mg protein) and the gastrocnemius muscle (1.7 pmol/mg protein), but it is still 400-fold lower than the abundance of UCP1, a biomarker for BAT. Investigation of UCP3 reconstituted in planar bilayer membranes revealed that it transports protons only when activated by fatty acids (FA). Although purine nucleotides (PN) inhibit UCP3-mediated transport, the molecular mechanism differs from that of UCP1. It remains a conundrum that two homologous proton-transporting proteins exist within the same tissue. Recently, we proposed that UCP3 abundance directly correlates with the degree of FA β-oxidation in cell metabolism. Further development in this field implies that UCP3 may have dual function in transporting substrates, which have yet to be identified, alongside protons. Evaluation of the literature with respect to UCP3 is a complex task because (i) UCP3 features are often extrapolated from its "twin" UCP2 without additional proof, and (ii) the specificity of antibodies against UCP3 used in studies is rarely evaluated. In this review, we primarily focus on recent findings obtained for UCP3 in biological and biomimetic systems.

The Expression of Uncoupling Protein 3 Coincides With the Fatty Acid Oxidation Type of Metabolism in Adult Murine Heart

The involvement of mitochondrial uncoupling proteins 2 and 3 in the pathogenesis of cardiovascular diseases is widely acknowledged. However, contradictory reports show that the functions of UCP2/UCP3 are still disputed. We have previously described that UCP2 is highly abundant in cells that rely on glycolysis, such as stem, cancer and activated immune cells. In contrast, high amounts of UCP3 are present in brown adipose tissue, followed by heart and skeletal muscles - all known to metabolize fatty acids (FA) to a high extent. Using two different models - mouse embryonic stem cell (mESC) differentiation to cardiomyocytes (CM) and murine heart at different developmental stages - we now tested the concept that the expression ratio between UCP2 and UCP3 indicates the metabolism type in CM. Our results revealed the tight correlation between UCP3 abundance, expression of mitochondrial fatty acid oxidation (FAO) markers and presence of multiple connections between mitochondria and lipid droplets. We further demonstrated that the time course of UCP3 expression neither coincided with the onset of the electrical activity in CM, derived from mESC, nor with the expression of respiratory chain proteins, the observation which rendered protein participation in ROS regulation unlikely. The present data imply that UCP3 may facilitate FAO by transporting FAs into mitochondria. In contrast, UCP2 was highly abundant at early stages of heart development and in mESC. Understanding, that the expression patterns of UCP3 and UCP2 in heart during development reflect the type of the cell metabolism is key to the uncovering their different functions. Their expression ratio may be an important diagnostic criterion for the degree of CM differentiation and/or severity of a heart failure.

Metabolite accumulation in VLCAD deficiency markedly disrupts mitochondrial bioenergetics and Ca2+ homeostasis in the heart

We studied the effects of the major long-chain fatty acids accumulating in very long-chain acyl-CoA dehydrogenase (VLCAD) deficiency, namely cis-5-tetradecenoic acid (Cis-5) and myristic acid (Myr), on important mitochondrial functions in isolated mitochondria from cardiac fibers and cardiomyocytes of juvenile rats. Cis-5 and Myr at pathological concentrations markedly reduced mitochondrial membrane potential (ΔΨ_m ), matrix NAD(P)H pool, Ca²⁺ retention capacity, ADP- (state 3) and carbonyl cyanide 3-chlorophenyl hydrazine-stimulated (uncoupled) respiration, and ATP generation. By contrast, these fatty acids increased resting (state 4) respiration (uncoupling effect) with the involvement of the adenine nucleotide translocator because carboxyatractyloside significantly attenuated the increased state 4 respiration provoked by Cis-5 and Myr. Furthermore, the classical inhibitors of mitochondrial permeability transition (MPT) pore cyclosporin A plus ADP, as well as the Ca²⁺ uptake blocker ruthenium red, fully prevented the Cis-5- and Myr-induced decrease in ΔΨ_m in Ca²⁺ -loaded mitochondria, suggesting, respectively, the induction of MPT pore opening and the contribution of Ca²⁺ toward these effects. The findings of the present study indicate that the major long-chain fatty acids that accumulate in VLCAD deficiency disrupt mitochondrial bioenergetics and Ca²⁺ homeostasis, acting as uncouplers and metabolic inhibitors of oxidative phosphorylation, as well as inducers of MPT pore opening, in the heart at pathological relevant concentrations. It is therefore presumed that a disturbance of bioenergetics and Ca²⁺ homeostasis may contribute to the cardiac manifestations observed in VLCAD deficiency.

Green tea supplementation upregulates uncoupling protein 3 expression in severe obese women adipose tissue but does not promote weight loss

This study aims (i) to verify expression of the UCPs, PLIN1, PPARG2, and ADRB3 genes in the abdominal subcutaneous adipose tissue of obese women at baseline and after 8 weeks of supplementation with decaffeinated green tea extract, and (ii) to associate findings with clinical parameters. This is a longitudinal study during which 11 women with obesity grade III were submitted to supplementation with 450 mg of (-)-epigallocatechin gallate (EGCG) (intervention group); the control group consisted of 10 eutrophic women. Anthropometric parameters [weight, height, and body mass index (BMI)], resting metabolic rate (RMR, measured by indirect calorimetry), and gene expression (measured by real-time PCR, RT-qPCR) were determined before and after supplementation. After 8 weeks, clinical parameters and UCP1, PLIN1, PPARG2, and ADRB3 expression remained unaltered in the intervention group (p > .05). Genetic analysis also showed that the UCP3 gene was upregulated (p = .026), but its upregulation did not promote weight loss.

Lipid profiling in serum from apolipoprotein E-knock out mice fed with different diets and its application to the study of the regulatory effect on lipid metabolism

A lipidome profiling platform was established that could evaluate the cardiovascular health products from lipid changes, their metabolism regulation and intervention mechanism in chronic disease, such as atherosclerosis.

Chronic endurance exercise antagonizes the cardiac UCP2 and UCP3 protein up-regulation induced by nandrolone decanoate

BACKGROUND

Several lines of evidence revealed that chronic treatment of anabolic androgenic steroids (AASs) is accompanied with some cardiovascular side effects and in addition they also negatively mask the beneficial effects of exercise training on cardiac performance.

METHODS

The present study examined whether the nandrolone decanoate (ND)-induced cardiac effects were mediated by changing the cardiac uncoupling protein 2 (UCP2) and 3 (UCP3) expression. Five groups of male wistar-albino rats including sedentary control (SC), sedentary vehicle (SV), sedentary nandrolone decanoate (SND), exercise control (EC), and exercise nandrolone decanoate (END) were used. ND was injected (10 mg/kg/week, intramuscular) to the animals in the SND and END groups and endurance exercise training was performed on a treadmill five times per week.

RESULTS

The protein expressions of cardiac UCP2 and UCP3 have significantly increased in both the SND and EC groups compared to the SC ones. In contrast to UCP3, no significant differences were found between UCP2 protein expressions of the END and SC groups. Compared with the SND group, the exercise training significantly decreased the UCP2 and UCP3 protein expressions in the END group.

CONCLUSIONS

The study has indicated that endurance exercise in combination with ND can result in that the exercise effectively antagonizes the effects of ND treatment on UCP2 and UCP3 up-regulation.

The evolution of metabolic regulation in animals

Energy metabolism is determined by a suite of regulatory mechanism, and their increasing complexity over evolutionary time provides the key to understanding the emergence of different metabolic phenotypes. Energy metabolism is at the core of biological processes because all organisms must maintain energy balance against thermodynamic gradients. Energy metabolism is regulated by a bewildering array of interacting molecular mechanisms, and much of what is known about metabolic regulation comes from the medical literature. However, ecology and evolutionary research would gain considerably by incorporating regulatory mechanisms more explicitly in research on topics such as the evolution of endothermy, metabolic plasticity, and energy balance. The purpose of this brief review is to summarise the main regulatory pathways of energy metabolism in animals and their evolutionary origins to make these complex interactions more accessible to researchers from a broad range of backgrounds. Some of the principal regulators of energy balance, such as the AMP-stimulated protein kinase, have an ancient prokaryotic origin. Most regulatory pathways (e.g. thyroid hormone, insulin, adipokines), however, are eukaryotic in origin and diversified substantially in metazoans and vertebrates. Diversification in vertebrates is at least partly due to genome duplications early in this lineage. The interaction between regulatory mechanisms permitted an increasingly sophisticated fine-tuning of energy balance and metabolism. Hence, regulatory complexity increased over evolutionary time, and taxa differ in their potential range of metabolic phenotypes. Choice of model organism therefore becomes important, and bacteria or even invertebrates are not good models for more derived vertebrates. Different metabolic phenotypes and their evolution, such as endothermy and metabolic plasticity, should be interpreted against this regulatory background.

What Is Lipotoxicity?

Enlarged fat cells in obese adipose tissue diminish capacity to store fat and are resistant to the anti-lipolytic effect of insulin. Insulin resistance (IR)-associated S-nitrosylation of insulin-signaling proteins increases in obesity. In accordance with the inhibition of insulin-mediated anti-lipolytic action, plasma free fatty acid (FFA) levels increase. Additionally, endoplasmic reticulum stress stimuli induce lipolysis by activating cyclic adenosine monophosphate/Protein kinase A (cAMP/PKA) and extracellular signal-regulated kinase ½ (ERK1/2) signaling in adipocytes. Failure of packaging of excess lipid into lipid droplets causes chronic elevation of circulating fatty acids, which can reach to toxic levels within non-adipose tissues. Deleterious effects of lipid accumulation in non-adipose tissues are known as lipotoxicity. In fact, triglycerides may also serve a storage function for long-chain non-esterified fatty acids and their products such as ceramides and diacylglycerols (DAGs). Thus, excess DAG, ceramide and saturated fatty acids in obesity can induce chronic inflammation and have harmful effect on multiple organs and systems. In this context, chronic adipose tissue inflammation, mitochondrial dysfunction and IR have been discussed within the scope of lipotoxicity.

Pemafibrate, a novel selective peroxisome proliferator-activated receptor alpha modulator, improves the pathogenesis in a rodent model of nonalcoholic steatohepatitis

The efficacy of peroxisome proliferator-activated receptor α-agonists (e.g., fibrates) against nonalcoholic fatty liver disease (NAFLD)/nonalcoholic steatohepatitis (NASH) in humans is not known. Pemafibrate is a novel selective peroxisome proliferator-activated receptor α modulator that can maximize the beneficial effects and minimize the adverse effects of fibrates used currently. In a phase-2 study, pemafibrate was shown to improve liver dysfunction in patients with dyslipidaemia. In the present study, we first investigated the effect of pemafibrate on rodent models of NASH. Pemafibrate efficacy was assessed in a diet-induced rodent model of NASH compared with fenofibrate. Pemafibrate and fenofibrate improved obesity, dyslipidaemia, liver dysfunction, and the pathological condition of NASH. Pemafibrate improved insulin resistance and increased energy expenditure significantly. To investigate the effects of pemafibrate, we analysed the gene expressions and protein levels involved in lipid metabolism. We also analysed uncoupling protein 3 (UCP3) expression. Pemafibrate stimulated lipid turnover and upregulated UCP3 expression in the liver. Levels of acyl-CoA oxidase 1 and UCP3 protein were increased by pemafibrate significantly. Pemafibrate can improve the pathogenesis of NASH by modulation of lipid turnover and energy metabolism in the liver. Pemafibrate is a promising therapeutic agent for NAFLD/NASH.

A novel amino acid and metabolomics signature in mice overexpressing muscle uncoupling protein 3

Uncoupling protein 3 (UCP3) is highly selectively expressed in skeletal muscle and is known to lower mitochondrial reactive oxygen species and promote fatty acid oxidation; however, the global impact of UCP3 activity on skeletal muscle and whole-body metabolism have not been extensively studied. We utilized untargeted metabolomics to identify novel metabolites that distinguish mice overexpressing UCP3 in muscle, both at rest and after exercise regimens that challenged muscle metabolism, to potentially unmask subtle phenotypes. Male wild-type (WT) and muscle-specific UCP3-overexpressing transgenic (UCP3 Tg) C57BL/6J mice were compared with or without a 5 wk endurance training protocol at rest or after an acute exercise bout (EB). Skeletal muscle, liver, and plasma samples were analyzed by gas chromatography time-of-flight mass spectrometry. Discriminant metabolites were considered if within the top 99th percentile of variable importance measurements obtained from partial least-squares discriminant analysis models. A total of 80 metabolites accurately discriminated UCP3 Tg mice from WT when modeled within a specific exercise condition (i.e., untrained/rested, endurance trained/rested, untrained/EB, and endurance trained/EB). Results revealed that several amino acids and amino acid derivatives in skeletal muscle and plasma of UCP3 Tg mice (e.g., Asp, Glu, Lys, Tyr, Ser, Met) were significantly reduced after an EB; that metabolites associated with skeletal muscle glutathione/Met/Cys metabolism (2-hydroxybutanoic acid, oxoproline, Gly, and Glu) were altered in UCP3 Tg mice across all training and exercise conditions; and that muscle metabolite indices of dehydrogenase activity were increased in UCP3 Tg mice, suggestive of a shift in tissue NADH/NAD+ ratio. The results indicate that mitochondrial UCP3 activity affects metabolism well beyond fatty acid oxidation, regulating biochemical pathways associated with amino acid metabolism and redox status. That select metabolites were altered in liver of UCP3 Tg mice highlights that changes in muscle UCP3 activity can also affect other organ systems, presumably through changes in systemic metabolite trafficking.-Aguer, C., Piccolo, B. D., Fiehn, O., Adams, S. H., Harper, M.-E. A novel amino acid and metabolomics signature in mice overexpressing muscle uncoupling protein 3.

Effects of the duration of hyperlipidemia on cerebral lipids, vessels and neurons in rats

Background

The present study was designed to investigate the effects of hyperlipidemia on the cerebral lipids, vessels and neurons of rats, and to provide experimental evidence for subsequent intervention.

Method

One hundred adult SD rats, half of which were male and half of which were female, were randomly divided into five groups on the basis of serum total cholesterol (TC) levels. Four groups were fed a hypercholesterolemic diet (rat chow supplemented with 4% cholesterol, 1% cholic acid and 0.5% thiouracil – this is also called a CCT diet) for periods of 1 week, 2 weeks, 3 weeks and 4 weeks, respectively. A control group was included. The levels of serum lipids, cerebral lipids, free fatty acids (FFA), interleukin-6 (IL-6), interleukin-1 (IL-1), tumor necrosis factor alpha (TNF-α), vascular endothelial growth factor (VEGF), oxidized low density lipoprotein (ox-LDL), A-beta precursor proteins (APP), amyloid beta (Aβ), glial fibrillary acidic protein (GFAP) and tight junction protein Claudin-5 were measured after the experiment. The pathologic changes and apoptosis of the rat brains were evaluated.

Results

Compared with the control group, after 1 week of a CCT diet, the levels of serum total cholesterol (TC), triglycerides (TG), low density lipoprotein cholesterol (LDL-C) and brain triglycerides had increased by 2.40, 1.29 and 1.75 and 0.3 times, respectively. The serum high density lipoprotein cholesterol (HDL-C) had decreased by 0.74 times (P < 0.05) and the expression of IL-1, TNF-α and GFAP in the brains had increased (P < 0.05). In the second week, the expression of FFA and APP in the brains, and the amount of apoptotic neurons, had increased (P < 0.05). In the third week, the levels of VEGF, Ox-LDL and Aβ had increased, and the expression of Claudin-5 had decreased in the brains (P < 0.05). In the fourth week, the levels of TC, LDL-C and the amount of apoptotic neurons had increased (P < 0.05). The correlation analysis showed a positive correlation among FFA, TNF-α, VEGF, ox-LDL, Aβ, GFAP and neuronal apoptosis in the rat brains, and they all were negatively correlated with Claudin-5 (P < 0.05).

Conclusion

Hyperlipidemia may activate astrocytes by means of high levels of TG that will have direct toxic effects on the cerebral vessels and neurons by causing the secretion of TNF-α and IL-1 in the brains of rats. In the metabolic procession, brain tissue was shown to generate FFA that aggravated the biosynthesis of ox-LDL. With the extension of the duration of hyperlipidemia, high levels of cerebral TC and LDL-C were shown to aggravate the deposition of Aβ, induce the secretion of VEGF, reduce the expression of tight junction protein Claudin-5 and change the permeability of blood–brain barriers to factors that could damage cerebral vessels and neurons.

Mitochondrial Actions of Thyroid Hormone

The hypermetabolic effects of thyroid hormones (THs), the major endocrine regulators of metabolic rate, are widely recognized. Although, the cellular mechanisms underlying these effects have been extensively investigated, much has yet to be learned about how TH regulates diverse cellular functions. THs have a profound impact on mitochondria, the organelles responsible for the majority of cellular energy production, and several studies have been devoted to understand the respective importance of the nuclear and mitochondrial pathways for organelle activity. During the last decades, several new aspects of both THs (i.e., metabolism, transport, mechanisms of action, and the existence of metabolically active TH derivatives) and mitochondria (i.e., dynamics, respiratory chain organization in supercomplexes, and the discovery of uncoupling proteins other than uncoupling protein 1) have emerged, thus opening new perspectives to the investigation of the complex relationship between thyroid and the mitochondrial compartment. In this review, in the light of an historical background, we attempt to point out the present findings regarding thyroid physiology and the emerging recognition that mitochondrial dynamics as well as the arrangement of the electron transport chain in mitochondrial cristae contribute to the mitochondrial activity. We unravel the genomic and nongenomic mechanisms so far studied as well as the effects of THs on mitochondrial energetics and, principally, uncoupling of oxidative phosphorylation via various mechanisms involving uncoupling proteins. The emergence of new approaches to the question as to what extent and how the action of TH can affect mitochondria is highlighted. © 2016 American Physiological Society. Compr Physiol 6:1591-1607, 2016.

Exercise-induced hormesis and skeletal muscle health

Hormesis refers to the phenomenon that an exposure or repeated exposures of a toxin can elicit adaptive changes within the organism to resist to higher doses of toxin with reduced harm. Skeletal muscle shows considerable plasticity and adaptions in response to a single bout of acute exercise or chronic training, especially in antioxidant defense capacity and metabolic functions mainly due to remodeling of mitochondria. It has thus been hypothesized that contraction-induced production of reactive oxygen species (ROS) may stimulate the hormesis-like adaptations. Furthermore, there has been considerable evidence that select ROS such as hydrogen peroxide and nitric oxide, or even oxidatively degraded macromolecules, may serve as signaling molecules to stimulate such hermetic adaptations due to the activation of redox-sensitive signaling pathways. Recent research has highlighted the important role of nuclear factor (NF) κB, mitogen-activated protein kinase (MAPK), and peroxisome proliferator-activated receptor γ co-activator 1α (PGC-1α), along with other newly discovered signaling pathways, in some of the most vital biological functions such as mitochondrial biogenesis, antioxidant defense, inflammation, protein turnover, apoptosis, and autophagy. The inability of the cell to maintain proper redox signaling underlies mechanisms of biological aging, during which inflammatory and catabolic pathways prevail. Research evidence and mechanisms connecting exercise-induced hormesis and redox signaling are reviewed.

Striatal Mitochondrial Disruption following Severe Traumatic Brain Injury

Traumatic brain injury (TBI) results in oxidative stress and calcium dysregulation in mitochondria. However, little work has examined perturbations of mitochondrial homeostasis in peri-injury tissue. We examined mitochondrial homeostasis after a unilateral controlled cortical impact over the sensorimotor cortex in adult male rats. There was a significant reduction in peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) messenger RNA (mRNA) at post-injury days 3 and 6 and a transient reduction in mitochondrial DNA copy number at 3 days post-injury that recovered by 6 days in the ipsi-injury striatum. In ipsilateral cortex, PGC-1α mRNA was reduced only at 6 days post-injury. Additionally, expression of mitochondrial-encoded mRNAs, cytochrome c oxidase subunit 1 and NADH dehydrogenase subunit 1, was decreased at 3 and 6 days post-injury in ipsilesional striatum and at 6 days post-injury in ipsilesional cortex. There was no observable decrease in nuclear-encoded mRNAs mitochondrial transcription factor A or NADH dehydrogenase (ubiquinone) Fe-S protein 1. We detected an acute increase in superoxide dismutase 2 mRNA expression, as well as an induction of microRNA (miR)-21 and miR-155, which have been previously demonstrated to disrupt mitochondrial homeostasis. Behaviorally, rats with TBI exhibited marked error rates in contrainjury forelimb performance on the ladder test. These findings reveal that there may be differential susceptibilities of various peri-injury brain structures to mitochondrial dysfunction and associated behavioral deficits, and that molecular pathways demonstrated to interfere with mitochondrial homeostasis and function are activated subacutely post-TBI.

The effect of chronic exposure to high palmitic acid concentrations on the aerobic metabolism of human endothelial EA.hy926 cells

A chronic elevation of circulating free fatty acids (FFAs) is associated with diseases like obesity or diabetes and can lead to lipotoxicity. The goals of this study were to assess the influence of chronic exposure to high palmitic acid (PAL) levels on mitochondrial respiratory functions in endothelial cells and isolated mitochondria. Human umbilical vein endothelial cells (EA.hy926 line) were grown for 6 days in a medium containing either 100 or 150 μM PAL. Growth at high PAL concentrations induced a considerable increase in fatty acid-supplied respiration and a reduction of mitochondrial respiration during carbohydrate and glutamine oxidation. High PAL levels elevated intracellular and mitochondrial superoxide generation; increased inflammation marker, acyl-coenzyme A (CoA) dehydrogenase, uncoupling protein 2 (UCP2), and superoxide dismutase 2 expression; and decreased hexokinase I and pyruvate dehydrogenase expression. No change in aerobic respiration capacity was observed, while fermentation was decreased. In mitochondria isolated from high PAL-treated cells, an increase in the oxidation of palmitoylcarnitine, a decrease in the oxidation of pyruvate, and an increase in UCP2 activity were observed. Our results demonstrate that exposure to high PAL levels induces a shift in endothelial aerobic metabolism toward the oxidation of fatty acids. Increased levels of PAL caused impairment and uncoupling of the mitochondrial oxidative phosphorylation system. Our data indicate that FFAs significantly affect endothelial oxidative metabolism, reactive oxygen species (ROS) formation, and cell viability and, thus, might contribute to endothelial and vascular dysfunction.

Disturbance of mitochondrial functions provoked by the major long-chain 3-hydroxylated fatty acids accumulating in MTP and LCHAD deficiencies in skeletal muscle

The pathogenesis of the muscular symptoms and recurrent rhabdomyolysis that are commonly manifested in patients with mitochondrial trifunctional protein (MTP) and long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) deficiencies is still unknown. In this study we investigated the effects of the major long-chain monocarboxylic 3-hydroxylated fatty acids (LCHFA) accumulating in these disorders, namely 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, on important mitochondrial functions in rat skeletal muscle mitochondria. 3HTA and 3HPA markedly increased resting (state 4) and decreased ADP-stimulated (state 3) and CCCP-stimulated (uncoupled) respiration. 3HPA provoked similar effects in permeabilized skeletal muscle fibers, validating the results obtained in purified mitochondria. Furthermore, 3HTA and 3HPA markedly diminished mitochondrial membrane potential, NAD(P)H content and Ca(2+) retention capacity in Ca(2+)-loaded mitochondria. Mitochondrial permeability transition (mPT) induction probably underlie these effects since they were totally prevented by cyclosporin A and ADP. In contrast, the dicarboxylic analogue of 3HTA did not alter the tested parameters. Our data strongly indicate that 3HTA and 3HPA behave as metabolic inhibitors, uncouplers of oxidative phosphorylation and mPT inducers in skeletal muscle. It is proposed that these pathomechanisms disrupting mitochondrial homeostasis may be involved in the muscle alterations characteristic of MTP and LCHAD deficiencies.

The Role of Uncoupling Protein 2 During Myocardial Dysfunction in a Canine Model of Endotoxin Shock

To explore the role of uncoupling protein 2 (UCP2) during myocardial dysfunction in a canine model of endotoxin shock, 26 mongrel canines were randomly divided into the following four groups: A (control group; n = 6), B2 (shock after 2 h; n = 7), B4 (shock after 4 h; n = 7), and B6 (shock after 6 h; n = 6). Escherichia coli endotoxin was injected into the canines via the central vein, and hemodynamics were monitored. Energy metabolism, UCP2 mRNA and protein expression, and UCP2 localization were analyzed, and the correlation between energy metabolism changes, and UCP2 expression was determined. After the canine endotoxin shock model was successfully established, the expression of UCP2 mRNA and protein was found to increase, with later time points showing significant increases (P < 0.05). Immunofluorescence assays of UCP2 in heart tissue showed that UCP2 was localized in the cytoplasm, and its expression pattern was the same as that found in the mRNA and protein analyses. The energy metabolism results revealed that the ADP levels increased, but the ATP and phosphocreatine (PCr) levels and ATP/ADP and PCr/ATP ratios decreased in the model. In particular, the PCr/ATP ratio was significantly different from that of the control group 6 h after shock (P < 0.05). Furthermore, correlation analysis showed that the UCP2 protein and mRNA levels were negatively correlated with myocardial energy levels. In summary, decreased energy synthesis can occur in the myocardium during endotoxin shock, and UCP2 may play an important role in this process. The negative correlation between UCP2 expression and energy metabolism requires further study, as the results might contribute to the treatment of sepsis with heart failure.

UCPs, at the interface between bioenergetics and metabolism

The first member of the uncoupling protein (UCP) family, brown adipose tissue uncoupling protein 1 (UCP1), was identified in 1976. Twenty years later, two closely related proteins, UCP2 and UCP3, were described in mammals. Homologs of these proteins exist in other organisms, including plants. Uncoupling refers to a deterioration of energy conservation between substrate oxidation and ADP phosphorylation. Complete energy conservation loss would be fatal but fine-tuning can be beneficial for processes such as thermogenesis, redox control, and prevention of mitochondrial ROS release. The coupled/uncoupled state of mitochondria is related to the permeability of the inner membrane and the proton transport mediated by activated UCPs underlies the uncoupling activity of these proteins. Proton transport by UCP1 is activated by fatty acids and this ensures thermogenesis. In vivo in absence of this activation UCP1 remains inhibited with no transport activity. A similar situation now seems unlikely for UCP2 and UCP3 and while activation of their proton transport has been described its physiological relevance remains uncertain and their influence can be envisaged as a result of another transport pathway that takes place in the absence of activation. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

The expression of UCP3 directly correlates to UCP1 abundance in brown adipose tissue

UCP1 and UCP3 are members of the uncoupling protein (UCP) subfamily and are localized in the inner mitochondrial membrane. Whereas UCP1's central role in non-shivering thermogenesis is acknowledged, the function and even tissue expression pattern of UCP3 are still under dispute. Because UCP3 properties regarding transport of protons are qualitatively identical to those of UCP1, its expression in brown adipose tissue (BAT) alongside UCP1 requires justification. In this work, we tested whether any correlation exists between the expression of UCP1 and UCP3 in BAT by quantification of protein amounts in mouse tissues at physiological conditions, in cold-acclimated and UCP1 knockout mice. Quantification using recombinant UCP3 revealed that the UCP3 amount in BAT (0.51ng/(μg total tissue protein)) was nearly one order of magnitude higher than that in muscles and heart. Cold-acclimated mice showed an approximate three-fold increase in UCP3 abundance in BAT in comparison to mice in thermoneutral conditions. Surprisingly, we found a significant decrease of UCP3 in BAT of UCP1 knockout mice, whereas the protein amount in skeletal and heart muscles remained constant. UCP3 abundance decreased even more in cold-acclimated UCP1 knockout mice. Protein quantification in UCP3 knockout mice revealed no compensatory increase in UCP1 or UCP2 expression. Our results do not support the participation of UCP3 in thermogenesis in the absence of UCP1 in BAT, but clearly demonstrate the correlation in abundance between both proteins. The latter is important for understanding UCP3's function in BAT.

H₂O₂-Activated Mitochondrial Phospholipase iPLA₂γ Prevents Lipotoxic Oxidative Stress in Synergy with UCP2, Amplifies Signaling via G-Protein-Coupled Receptor GPR40, and Regulates Insulin Secretion in Pancreatic β-Cells

AIMS

Pancreatic β-cell chronic lipotoxicity evolves from acute free fatty acid (FA)-mediated oxidative stress, unprotected by antioxidant mechanisms. Since mitochondrial uncoupling protein-2 (UCP2) plays antioxidant and insulin-regulating roles in pancreatic β-cells, we tested our hypothesis, that UCP2-mediated uncoupling attenuating mitochondrial superoxide production is initiated by FA release due to a direct H2O2-induced activation of mitochondrial phospholipase iPLA2γ.

RESULTS

Pro-oxidant tert-butylhydroperoxide increased respiration, decreased membrane potential and mitochondrial matrix superoxide release rates of control but not UCP2- or iPLA2γ-silenced INS-1E cells. iPLA2γ/UCP2-mediated uncoupling was alternatively activated by an H2O2 burst, resulting from palmitic acid (PA) β-oxidation, and it was prevented by antioxidants or catalase overexpression. Exclusively, nascent FAs that cleaved off phospholipids by iPLA2γ were capable of activating UCP2, indicating that the previously reported direct redox UCP2 activation is actually indirect. Glucose-stimulated insulin release was not affected by UCP2 or iPLA2γ silencing, unless pro-oxidant activation had taken place. PA augmented insulin secretion via G-protein-coupled receptor 40 (GPR40), stimulated by iPLA2γ-cleaved FAs (absent after GPR40 silencing).

INNOVATION AND CONCLUSION

The iPLA2γ/UCP2 synergy provides a feedback antioxidant mechanism preventing oxidative stress by physiological FA intake in pancreatic β-cells, regulating glucose-, FA-, and redox-stimulated insulin secretion. iPLA2γ is regulated by exogenous FA via β-oxidation causing H2O2 signaling, while FAs are cleaved off phospholipids, subsequently acting as amplifying messengers for GPR40. Hence, iPLA2γ acts in eminent physiological redox signaling, the impairment of which results in the lack of antilipotoxic defense and contributes to chronic lipotoxicity.

UCP-3 uncoupling protein confers hypoxia resistance to renal epithelial cells and is upregulated in renal cell carcinoma

Tumor cells can adapt to a hostile environment with reduced oxygen supply. The present study aimed to identify mechanisms that confer hypoxia resistance. Partially hypoxia/reoxygenation (H/R)-resistant proximal tubular (PT) cells were selected by exposing PT cultures to repetitive cycles of H/R. Thereafter, H/R-induced changes in mRNA and protein expression, inner mitochondrial membrane potential (ΔΨ_m), formation of superoxide, and cell death were compared between H/R-adapted and control PT cultures. As a result, H/R-adapted PT cells exhibited lower H/R-induced hyperpolarization of ΔΨ_m and produced less superoxide than the control cultures. Consequently, H/R triggered ΔΨ_m break-down and DNA degradation in a lower percentage of H/R-adapted than control PT cells. Moreover, H/R induced upregulation of mitochondrial uncoupling protein-3 (UCP-3) in H/R-adapted PT but not in control cultures. In addition, ionizing radiation killed a lower percentage of H/R-adapted as compared to control cells suggestive of an H/R-radiation cross-resistance developed by the selection procedure. Knockdown of UCP-3 decreased H/R- and radioresitance of the H/R-adapted cells. Finally, UCP-3 protein abundance of PT-derived clear cell renal cell carcinoma and normal renal tissue was compared in human specimens indicating upregulation of UCP-3 during tumor development. Combined, our data suggest functional significance of UCP-3 for H/R resistance.

Regulation of skeletal muscle mitochondrial activity by thyroid hormones: focus on the "old" triiodothyronine and the "emerging" 3,5-diiodothyronine

3,5,3′-Triiodo-L-thyronine (T3) plays a crucial role in regulating metabolic rate and fuel oxidation; however, the mechanisms by which it affects whole-body energy metabolism are still not completely understood. Skeletal muscle (SKM) plays a relevant role in energy metabolism and responds to thyroid state by remodeling the metabolic characteristics and cytoarchitecture of myocytes. These processes are coordinated with changes in mitochondrial content, bioenergetics, substrate oxidation rate, and oxidative phosphorylation efficiency. Recent data indicate that “emerging” iodothyronines have biological activity. Among these, 3,5-diiodo-L-thyronine (T2) affects energy metabolism, SKM substrate utilization, and mitochondrial functionality. The effects it exerts on SKM mitochondria involve more aspects of mitochondrial bioenergetics; among these, respiratory chain activity, mitochondrial thermogenesis, and lipid-handling are stimulated rapidly. This mini review focuses on signaling and biochemical pathways activated by T3 and T2 in SKM that influence the above processes. These novel aspects of thyroid physiology could reveal new perspectives for understanding the involvement of SKM mitochondria in hypo- and hyper-thyroidism.

Thyroid hormone in the frontier of cell protection, survival and functional recovery

Thyroid hormone (TH) exerts important actions on cellular energy metabolism, accelerating O2consumption with consequent reactive oxygen species (ROS) generation and redox signalling affording cell protection, a response that is contributed by redox-independent mechanisms. These processes underlie genomic and non-genomic pathways, which are integrated and exhibit hierarchical organisation. ROS production led to the activation of the redox-sensitive transcription factors nuclear factor-κB, signal transducer and activator of transcription 3, activating protein 1 and nuclear factor erythroid 2-related factor 2, promoting cell protection and survival by TH. These features involve enhancement in the homeostatic potential including antioxidant, antiapoptotic, antiinflammatory and cell proliferation responses, besides higher detoxification capabilities and energy supply through AMP-activated protein kinase upregulation. The above aspects constitute the molecular basis for TH-induced preconditioning of the liver that exerts protection against ischemia-reperfusion injury, a strategy also observed in extrahepatic organs of experimental animals and with other types of injury, which awaits application in the clinical setting. Noteworthy, re-adjusting TH to normal levels results in several beneficial effects; for example, it lengthens the cold storage time of organs for transplantation from brain-dead donors; allows a superior neurological outcome in infants of <28 weeks of gestation; reduces the cognitive side-effects of lithium and improves electroconvulsive therapy in patients with bipolar disorders.

Skeletal muscle mitochondrial energetic efficiency and aging

Aging is associated with a progressive loss of maximal cell functionality, and mitochondria are considered a key factor in aging process, since they determine the ATP availability in the cells. Mitochondrial performance during aging in skeletal muscle is reported to be either decreased or unchanged. This heterogeneity of results could partly be due to the method used to assess mitochondrial performance. In addition, in skeletal muscle the mitochondrial population is heterogeneous, composed of subsarcolemmal and intermyofibrillar mitochondria. Therefore, the purpose of the present review is to summarize the results obtained on the functionality of the above mitochondrial populations during aging, taking into account that the mitochondrial performance depends on organelle number, organelle activity, and energetic efficiency of the mitochondrial machinery in synthesizing ATP from the oxidation of fuels.

Uncoupling, metabolic inhibition and induction of mitochondrial permeability transition in rat liver mitochondria caused by the major long-chain hydroxyl monocarboxylic fatty acids accumulating in LCHAD deficiency

Patients with long-chain 3-hydroxy-acyl-CoA dehydrogenase (LCHAD) deficiency commonly present liver dysfunction whose pathogenesis is unknown. We studied the effects of long-chain 3-hydroxylated fatty acids (LCHFA) that accumulate in LCHAD deficiency on liver bioenergetics using mitochondrial preparations from young rats. We provide strong evidence that 3-hydroxytetradecanoic (3HTA) and 3-hydroxypalmitic (3HPA) acids, the monocarboxylic acids that are found at the highest tissue concentrations in this disorder, act as metabolic inhibitors and uncouplers of oxidative phosphorylation. These conclusions are based on the findings that these fatty acids decreased ADP-stimulated (state 3) and uncoupled respiration, mitochondrial membrane potential and NAD(P)H content, and, in contrast, increased resting (state 4) respiration. We also verified that 3HTA and 3HPA markedly reduced Ca2+ retention capacity and induced swelling in Ca2+-loaded mitochondria. These effects were mediated by mitochondrial permeability transition (MPT) induction since they were totally prevented by the classical MPT inhibitors cyclosporin A and ADP, as well as by ruthenium red, a Ca2+ uptake blocker. Taken together, our data demonstrate that the major monocarboxylic LCHFA accumulating in LCHAD deficiency disrupt energy mitochondrial homeostasis in the liver. It is proposed that this pathomechanism may explain at least in part the hepatic alterations characteristic of the affected patients.

Molecular cloning and tissue expression of uncoupling protein 1, 2 and 3 genes in Chinese perch (Siniperca chuatsi)

Uncoupling proteins (UCPs) are mitochondrial anion carrier proteins, which play important roles in several physiological processes, including thermogenesis, reactive oxygen species generation, growth, lipid metabolism and insulin secretion. Although the roles of UCPs are well understood in mammals, little is known in fish. To investigate the thermogenesis roles in Chinese perch (Siniperca chuatsi), we cloned the UCP1, 2 and 3. The UCP1 consisted of six exons and five introns, and the UCP2 consisted of eight exons and seven introns. The UCP1 was primarily expressed in liver, UCP2 was ubiquitously expressed, and UCP3 was primarily expressed in muscle. The mRNA levels of UCP1 and UCP2 in liver, and UCP3 in muscle were significantly increased after prolonged cold exposure, but did not change after prolonged heat exposure, suggesting that Chinese perch might have a mechanism of response to cold environment, but not to hot environment. The intestinal UCP1 mRNA level was significantly up-regulated after prolonged heat exposure, while the UCP2 mRNA level was significantly up-regulated after prolonged cold exposure, suggesting that the two paralogs might play different roles in intestine of Chinese perch. In addition, the phylogenetic analysis could shed new light on the evolutionary diversification of UCP gene family.

Mitochondrial uncoupling proteins and energy metabolism

Understanding the metabolic factors that contribute to energy metabolism (EM) is critical for the development of new treatments for obesity and related diseases. Mitochondrial oxidative phosphorylation is not perfectly coupled to ATP synthesis, and the process of proton-leak plays a crucial role. Proton-leak accounts for a significant part of the resting metabolic rate (RMR) and therefore enhancement of this process represents a potential target for obesity treatment. Since their discovery, uncoupling proteins have stimulated great interest due to their involvement in mitochondrial-inducible proton-leak. Despite the widely accepted uncoupling/thermogenic effect of uncoupling protein one (UCP₁), which was the first in this family to be discovered, the reactions catalyzed by its homolog UCP₃ and the physiological role remain under debate. This review provides an overview of the role played by UCP₁ and UCP₃ in mitochondrial uncoupling/functionality as well as EM and suggests that they are a potential therapeutic target for treating obesity and its related diseases such as type II diabetes mellitus.

Modulation of cadmium-induced mitochondrial dysfunction and volume changes by temperature in rainbow trout (Oncorhynchus mykiss)

We investigated how temperature modulates cadmium (Cd)-induced mitochondrial bioenergetic disturbances, metal accumulation and volume changes in rainbow trout (Oncorhynchus mykiss). In the first set of experiments, rainbow trout liver mitochondrial function and Cd content were measured in the presence of complex I substrates, malate and glutamate, following exposure to Cd (0-100 μM) at three (5, 13 and 25 °C) temperatures. The second set of experiments assessed the effect of temperature on Cd-induced mitochondrial volume changes, including the underlying mechanisms, at 15 and 25 °C. Although temperature stimulated both state 3 and 4 rates of respiration, the coupling efficiency was reduced at temperature extremes due to greater inhibition of state 3 at low temperature and greater stimulation of state 4 at the high temperature. Cadmium exposure reduced the stimulatory effect of temperature on state 3 respiration but increased that on state 4, consequently exacerbating mitochondrial uncoupling. The interaction of Cd and temperature yielded different responses on thermal sensitivity of state 3 and 4 respiration; the Q10 values for state 3 respiration increased at low temperature (5-13 °C) while those for state 4 increased at high temperature (13-25 °C). Importantly, the mitochondria accumulated more Cd at high temperature suggesting that the observed greater impairment of oxidative phosphorylation with temperature was due, at least in part, to a higher metal burden. Cadmium-induced mitochondrial volume changes were characterized by an early phase of contraction followed by swelling, with temperature changing the kinetics and intensifying the effects. Lastly, using specific modulators of mitochondrial ion channels, we demonstrated that the mitochondrial volume changes were associated with Cd uptake via the mitochondrial calcium uniporter (MCU) without significant contribution of the permeability transition pore and/or potassium channels. Overall, it appears that high temperature exacerbates Cd-induced mitochondrial dysfunction and volume changes in part by increasing metal uptake through the MCU.

Perspectives on mitochondrial uncoupling proteins-mediated neuroprotection

The integrity of mitochondrial function is essential to cell life. It follows that disturbances of mitochondrial function will lead to disruption of cell function, expressed as disease or even death. Considering that neuronal uncoupling proteins (UCPs) decrease reactive oxygen species (ROS) production at the expense of energy production, it is important to understand the underlying mechanisms by which UCPs control the balance between the production of adenosine triphosphate (ATP) and ROS in the context of normal physiological activity and in pathological conditions. Here we review the current understanding of neuronal UCPs-mediated respiratory uncoupling process by performing a survey in their physiology and regulation. The latest findings regarding neuronal UCPs physiological roles and their involvement and interest as potential targets for therapeutic intervention in brain diseases will also be exploited.

Subsarcolemmal and intermyofibrillar mitochondrial responses to short-term high-fat feeding in rat skeletal muscle

OBJECTIVES

We assessed the alterations in mitochondrial function in skeletal muscle that were elicited by short-term high-fat feeding in sedentary rats.

METHODS

Two groups of rats were pair-fed for 1 wk and received a low-fat or high-fat diet. Body composition, energy balance, and glucose homeostasis were measured. Mitochondrial mass, oxidative capacity, and energetic efficiency as well as parameters of oxidative stress and antioxidant defense were evaluated in subsarcolemmal and intermyofibrillar mitochondria from the skeletal muscle.

RESULTS

Body energy, lipid content, and metabolic efficiency were significantly higher and energy expenditure was significantly decreased among rats that were fed a high-fat diet, as compared with controls. Skeletal muscle mitochondrial energetic efficiency, oxidative capacity for lipid substrates, and antioxidant defense were significantly increased in rats that were fed a high-fat diet as compared with controls.

CONCLUSIONS

Acute isocaloric high-fat feeding is able to induce increased phosphorylation efficiency in skeletal muscle subsarcolemmal and intermyofibrillar mitochondria. This modification implies a reduced oxidation of energy substrates that may contribute to the early onset of obesity.