Adipose tissue mitochondrial dysfunction triggers a lipodystrophic syndrome with insulin resistance, hepatosteatosis, and cardiovascular complications

AMP-activated protein kinase, fatty acid metabolism, and insulin sensitivity

PURPOSE OF REVIEW

Insulin resistance is an important risk factor for metabolic diseases such as type 2 diabetes, cardiovascular disease and certain cancers. A common characteristic of strategies that improve insulin sensitivity involves the activation of the energy sensing enzyme of the cell, AMP-activated protein kinase (AMPK). The purpose of this review is to explore the mechanisms associated with AMPK activation to improve insulin sensitivity with a focus on fatty acid metabolism. We will also discuss the literature surrounding direct AMPK activators to improve insulin resistance and important considerations for the design of direct AMPK activators.

RECENT FINDINGS

AMPK activation can decrease de novo lipogenesis, increase fatty acid oxidation and promote mitochondrial integrity to improve insulin sensitivity. Drugs targeted to directly activate AMPK show therapeutic promise, yet in vivo data is lacking.

SUMMARY

Designing a drug to directly activate AMPK may improve insulin resistance by reducing liver de novo lipogenesis and increasing brown and white adipose tissue mitochondrial function. However, in vivo experimental procedures to support this notion are not extensive and more research is required.

A fish oil diet induces mitochondrial uncoupling and mitochondrial unfolded protein response in epididymal white adipose tissue of mice

White adipose tissue (WAT) mitochondrial dysfunction is linked to the pathogenesis of obesity driven insulin resistance. Dietary conditions that alter fat mass are known to affect white adipocyte mitochondrial function, however, the impact of high calorie diets on white adipocyte mitochondria is not fully understood. The aim of this study is to assess the effect of a diet rich in saturated or polyunsaturated fat on mitochondrial unfolded protein response (UPR^mt), a retrograde signaling response that maintains mitochondrial homeostasis, in epididymal WAT (eWAT). Mice were fed a low fat diet (LFD), saturated fat diet (SFD) or fish oil (unsaturated fat diet, UFD) and assessed changes in eWAT mitochondria. Compared to mice fed a LFD, SFD-fed mice have reduced mitochondrial biogenesis markers, mitochondrial fatty acid oxidation enzymes and TCA cycle enzymes, suggesting an impaired mitochondrial function that could contribute to increased fat mass. In contrast, isocaloric UFD-fed mice have increased expression of mitochondrial uncoupling protein 1 (UCP1) and peroxisomal fatty acid oxidation enzymes suggesting that elevated mitochondrial uncoupling and peroxisomal fatty acid oxidation could contribute to the reduction in fat mass. Interestingly, expression of UPR^mt-associated proteins caseinolytic peptidase (ClpP) and heat shock protein 60 (Hsp60) are induced by UFD, whereas SFD reduced the expression of ClpP. Based on our data, we propose that induction of UPR^mt helps to preserve a functional mitochondria and efficient utilization of fat by UFD whereas a dampened UPR^mt response might impair mitochondrial function and promote fat accumulation by SFD. Thus, our findings suggest a potential role of UPR^mt in mediating the beneficial effects of fish oil.

Supramolecular structure of dietary fat in early life modulates expression of markers for mitochondrial content and capacity in adipose tissue of adult mice

Background

Previous studies have shown that early life nutrition can modulate the development of white adipose tissue and thereby affect the risk on obesity and metabolic disease later in life. For instance, postnatal feeding with a concept infant milk formula with large, phospholipid coated lipid droplets (Concept, Nuturis®), resulted in reduced adiposity in adult mice. The present study investigated whether differences in cell energy metabolism, using markers of mitochondrial content and capacity, may contribute to the observed effects.

Methods

C57Bl/6j male mice were exposed to a rodent diet containing the Concept (Concept) or standard (CTRL) infant milk formula from postnatal day 16 until postnatal day 42, followed by a western style diet challenge until postnatal day 98. Markers for mitochondrial content and capacity were analyzed in retroperitoneal white adipose tissue and gene expression of metabolic markers was measured in both retroperitoneal white adipose tissue and muscle tibialis (M. tibialis) at postnatal day 98.

Results

In retroperitoneal white adipose tissue, the Concept group showed higher citrate synthase activity and mitochondrial DNA expression compared to the CTRL group (p < 0.05). In addition, protein expression of mitochondrial cytochrome c oxidase subunit I of the oxidative phosphorylation pathway/cascade was increased in the Concept group compared to CTRL (p < 0.05). In the M. tibialis, gene expression of uncoupling protein 3 was higher in the Concept compared to the CTRL group. Other gene and protein expression markers for mitochondrial oxidative capacity were not different between groups.

Conclusion

Postnatal feeding with large, phospholipid coated lipid droplets generating a different supramolecular structure of dietary lipids enhances adult gene and protein expression of specific mitochondrial oxidative capacity markers, indicative of increased substrate oxidation in white adipose tissue and skeletal muscle. Although functional mitochondrial capacity was not measured, these results may suggest that adaptations in mitochondrial function via early feeding with a more physiological structure of dietary lipids, could underlie the observed beneficial effects on later life adiposity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-017-0191-5) contains supplementary material, which is available to authorized users.

Mitochondria and ageing: role in heart, skeletal muscle and adipose tissue

Age is the most important risk factor for most diseases. Mitochondria play a central role in bioenergetics and metabolism. In addition, several lines of evidence indicate the impact of mitochondria in lifespan determination and ageing. The best-known hypothesis to explain ageing is the free radical theory, which proposes that cells, organs, and organisms age because they accumulate reactive oxygen species (ROS) damage over time. Mitochondria play a central role as the principle source of intracellular ROS, which are mainly formed at the level of complex I and III of the respiratory chain. Dysfunctional mitochondria generating less ATP have been observed in various aged organs. Mitochondrial dysfunction comprises different features including reduced mitochondrial content, altered mitochondrial morphology, reduced activity of the complexes of the electron transport chain, opening of the mitochondrial permeability transition pore, and increased ROS formation. Furthermore, abnormalities in mitochondrial quality control or defects in mitochondrial dynamics have also been linked to senescence. Among the tissues affected by mitochondrial dysfunction are those with a high-energy demand and thus high mitochondrial content. Therefore, the present review focuses on the impact of mitochondria in the ageing process of heart and skeletal muscle. In this article, we review different aspects of mitochondrial dysfunction and discuss potential therapeutic strategies to improve mitochondrial function. Finally, novel aspects of adipose tissue biology and their involvement in the ageing process are discussed.

Mitochondrial retrograde signaling connects respiratory capacity to thermogenic gene expression

Mitochondrial respiration plays a crucial role in determining the metabolic state of brown adipose tissue (BAT), due to its direct roles in thermogenesis, as well as through additional mechanisms. Here, we show that respiration-dependent retrograde signaling from mitochondria to nucleus contributes to genetic and metabolic reprogramming of BAT. In mouse BAT, ablation of LRPPRC (LRP130), a potent regulator of mitochondrial transcription and respiratory capacity, triggers down-regulation of thermogenic genes, promoting a storage phenotype in BAT. This retrograde regulation functions by inhibiting the recruitment of PPARγ to the regulatory elements of thermogenic genes. Reducing cytosolic Ca²⁺ reverses the attenuation of thermogenic genes in brown adipocytes with impaired respiratory capacity, while induction of cytosolic Ca²⁺ is sufficient to attenuate thermogenic gene expression, indicating that cytosolic Ca²⁺ mediates mitochondria-nucleus crosstalk. Our findings suggest respiratory capacity governs thermogenic gene expression and BAT function via mitochondria-nucleus communication, which in turn leads to either a thermogenic or storage mode.

Mfn2 is critical for brown adipose tissue thermogenic function

Mitochondrial fusion and fission events, collectively known as mitochondrial dynamics, act as quality control mechanisms to ensure mitochondrial function and fine‐tune cellular bioenergetics. Defective mitofusin 2 (Mfn2) expression and enhanced mitochondrial fission in skeletal muscle are hallmarks of insulin‐resistant states. Interestingly, Mfn2 is highly expressed in brown adipose tissue (BAT), yet its role remains unexplored. Using adipose‐specific Mfn2 knockout (Mfn2‐adKO) mice, we demonstrate that Mfn2, but not Mfn1, deficiency in BAT leads to a profound BAT dysfunction, associated with impaired respiratory capacity and a blunted response to adrenergic stimuli. Importantly, Mfn2 directly interacts with perilipin 1, facilitating the interaction between the mitochondria and the lipid droplet in response to adrenergic stimulation. Surprisingly, Mfn2‐adKO mice were protected from high‐fat diet‐induced insulin resistance and hepatic steatosis. Altogether, these results demonstrate that Mfn2 is a mediator of mitochondria to lipid droplet interactions, influencing lipolytic processes and whole‐body energy homeostasis.

Adipose tissue NAD+-homeostasis, sirtuins and poly(ADP-ribose) polymerases -important players in mitochondrial metabolism and metabolic health

Obesity, a chronic state of energy overload, is characterized by adipose tissue dysfunction that is considered to be the major driver for obesity associated metabolic complications. The reasons for adipose tissue dysfunction are incompletely understood, but one potential contributing factor is adipose tissue mitochondrial dysfunction. Derangements of adipose tissue mitochondrial biogenesis and pathways associate with obesity and metabolic diseases. Mitochondria are central organelles in energy metabolism through their role in energy derivation through catabolic oxidative reactions. The mitochondrial processes are dependent on the proper NAD⁺/NADH redox balance and NAD⁺ is essential for reactions catalyzed by the key regulators of mitochondrial metabolism, sirtuins (SIRTs) and poly(ADP-ribose) polymerases (PARPs). Notably, obesity is associated with disturbed adipose tissue NAD⁺ homeostasis and the balance of SIRT and PARP activities. In this review we aim to summarize existing literature on the maintenance of intracellular NAD⁺ pools and the function of SIRTs and PARPs in adipose tissue during normal and obese conditions, with the purpose of comprehending their potential role in mitochondrial derangements and obesity associated metabolic complications. Understanding the molecular mechanisms that are the root cause of the adipose tissue mitochondrial derangements is crucial for developing new effective strategies to reverse obesity associated metabolic complications.

Obesity and cancer: the role of adipose tissue and adipo-cytokines-induced chronic inflammation

Adipose tissue in addition to its ability to keep lipids is now recognized as a real organ with both metabolic and endocrine functions. Recent studies demonstrated that in obese animals is established a status of adipocyte hypoxia and in this hypoxic state interaction between adipocytes and stromal vascular cells contribute to tumor development and progression. In several tumors such as breast, colon, liver and prostate, obesity represents a poor predictor of clinical outcomes. Dysfunctional adipose tissue in obesity releases a disturbed profile of adipokines with elevated levels of pro-inflammatory factors and a consequent alteration of key signaling mediators which may be an active local player in establishing the peritumoral environment promoting tumor growth and progression. Therefore, adipose tissue hypoxia might contribute to cancer risk in the obese population. To date the precise mechanisms behind this obesity-cancer link is not yet fully understood. In the light of information provided in this review that aims to identify the key mechanisms underlying the link between obesity and cancer we support that inflammatory state specific of obesity may be important in obesity-cancer link.

Adipose-Specific Deficiency of Fumarate Hydratase in Mice Protects Against Obesity, Hepatic Steatosis, and Insulin Resistance

Obesity and type 2 diabetes are associated with impaired mitochondrial function in adipose tissue. To study the effects of primary deficiency of mitochondrial energy metabolism in fat, we generated mice with adipose-specific deficiency of fumarate hydratase (FH), an integral Krebs cycle enzyme (AFHKO mice). AFHKO mice have severe ultrastructural abnormalities of mitochondria, ATP depletion in white adipose tissue (WAT) and brown adipose tissue, low WAT mass with small adipocytes, and impaired thermogenesis with large unilocular brown adipocytes. AFHKO mice are strongly protected against obesity, insulin resistance, and fatty liver despite aging and high-fat feeding. AFHKO white adipocytes showed normal lipolysis but low triglyceride synthesis. ATP depletion in normal white adipocytes by mitochondrial toxins also decreased triglyceride synthesis, proportionally to ATP depletion, suggesting that reduced triglyceride synthesis may result nonspecifically from adipocyte energy deficiency. At thermoneutrality, protection from insulin resistance and hepatic steatosis was diminished. Taken together, the results show that under the cold stress of regular animal room conditions, adipocyte-specific FH deficiency in mice causes mitochondrial energy depletion in adipose tissues and protects from obesity, hepatic steatosis, and insulin resistance, suggesting that in cold-stressed animals, mitochondrial function in adipose tissue is a determinant of fat mass and insulin sensitivity.

Severe Brown Fat Lipoatrophy Aggravates Atherosclerotic Process in Male Mice

Obesity is one of the major risk factors for the development of cardiovascular diseases and is characterized by abnormal accumulation of adipose tissue, including perivascular adipose tissue (PVAT). However, brown adipose tissue (BAT) activation reduces visceral adiposity. To demonstrate that severe brown fat lipoatrophy might accelerate atherosclerotic process, we generated a new mouse model without insulin receptor (IR) in BAT and without apolipoprotein (Apo)E (BAT-specific IR knockout [BATIRKO];ApoE(-/-) mice) and assessed vascular and metabolic alterations associated to obesity. In addition, we analyzed the contribution of the adipose organ to vascular inflammation. Brown fat lipoatrophy induces visceral adiposity, mainly in gonadal depot (gonadal white adipose tissue [gWAT]), severe glucose intolerance, high postprandial glucose levels, and a severe defect in acute insulin secretion. BATIRKO;ApoE(-/-) mice showed greater hypertriglyceridemia than the obtained in ApoE(-/-) and hypercholesterolemia similar to ApoE(-/-) mice. BATIRKO;ApoE(-/-) mice, in addition to primary insulin resistance in BAT, also showed a significant decrease in insulin signaling in liver, gWAT, heart, aorta artery, and thoracic PVAT. More importantly, our results suggest that severe brown fat lipoatrophy aggravates the atherosclerotic process, characterized by a significant increase of lipid depots, atherosclerotic coverage, lesion size and complexity, increased macrophage infiltration, and proinflammatory markers expression. Finally, an increase of TNF-α and leptin as well as a decrease of adiponectin by BAT, gWAT, and thoracic PVAT might also be responsible of vascular damage. Our results suggest that severe brown lipoatrophy aggravates atherosclerotic process. Thus, BAT activation might protect against obesity and its associated metabolic alterations.

Type 2 diabetes is associated with decreased PGC1α expression in epicardial adipose tissue of patients with coronary artery disease

Background

Although recent studies indicate that epicardial adipose tissue expresses brown fat-like genes, such as PGC1α, UCP1 and PRDM16, the association of these genes with type 2 diabetes mellitus (DM2) in coronary artery disease (CAD) remains unknown.

Methods

PGC1α, UCP1, and PRDM16 mRNAs expression levels were measured by real-time PCR in epicardial and thoracic subcutaneous adipose tissue from 44 CAD patients (22 with DM2 [CAD-DM2] and 22 without DM2 [CAD-NDM2]) and 23 non-CAD patients (NCAD).

Results

The CAD-DM2 patients had significantly lower PGC1α and UCP1 expression in epicardial adipose tissue than the CAD-NDM2 and NCAD patients. However, PGC1α and UCP1 mRNA trended upward in subcutaneous adipose tissue from CAD-DM2 patients. At multiple regression analysis, age, body mass index, left ventricular ejection fraction, UCP1 expression of epicardial adipose tissue and diabetes came out to be independent predictors of PGC1α levels. Epicardial adipose tissue PGC1α expression was dependent on the number of injured coronary arteries and logistic regression analysis showed that PGC1α expression in epicardial adipose tissue could exert a protective effect against coronary lesions.

Conclusions

DM2 is associated with decreased expression of PGC1α and UCP1 mRNA in epicardial adipose tissue of patients with CAD, likely reflecting a loss of brown-like fat features. Decreased expression of PGC1α in human epicardial adipose tissue is associated with higher prevalence of coronary lesions.

Differential Roles of Insulin and IGF-1 Receptors in Adipose Tissue Development and Function

To determine the roles of insulin and insulin-like growth factor 1 (IGF-1) action in adipose tissue, we created mice lacking the insulin receptor (IR), IGF-1 receptor (IGF1R), or both using Cre-recombinase driven by the adiponectin promoter. Mice lacking IGF1R only (F-IGFRKO) had a ∼25% reduction in white adipose tissue (WAT) and brown adipose tissue (BAT), whereas mice lacking both IR and IGF1R (F-IR/IGFRKO) showed an almost complete absence of WAT and BAT. Interestingly, mice lacking only the IR (F-IRKO) had a 95% reduction in WAT, but a paradoxical 50% increase in BAT with accumulation of large unilocular lipid droplets. Both F-IRKO and F-IR/IGFRKO mice were unable to maintain body temperature in the cold and developed severe diabetes, ectopic lipid accumulation in liver and muscle, and pancreatic islet hyperplasia. Leptin treatment normalized blood glucose levels in both groups. Glucose levels also improved spontaneously by 1 year of age, despite sustained lipodystrophy and insulin resistance. Thus, loss of IR is sufficient to disrupt white fat formation, but not brown fat formation and/or maintenance, although it is required for normal BAT function and temperature homeostasis. IGF1R has only a modest contribution to both WAT and BAT formation and function.

Alterations of Mitochondrial Function and Insulin Sensitivity in Human Obesity and Diabetes Mellitus

Mitochondrial function refers to a broad spectrum of features such as resting mitochondrial activity, (sub)maximal oxidative phosphorylation capacity (OXPHOS), and mitochondrial dynamics, turnover, and plasticity. The interaction between mitochondria and insulin sensitivity is bidirectional and varies depending on tissue, experimental model, methodological approach, and features of mitochondrial function tested. In human skeletal muscle, mitochondrial abnormalities may be inherited (e.g., lower mitochondrial content) or acquired (e.g., impaired OXPHOS capacity and plasticity). Abnormalities ultimately lead to lower mitochondrial functionality due to or resulting in insulin resistance and type 2 diabetes mellitus. Similar mechanisms can also operate in adipose tissue and heart muscle. In contrast, mitochondrial oxidative capacity is transiently upregulated in the liver of obese insulin-resistant humans with or without fatty liver, giving rise to oxidative stress and declines in advanced fatty liver disease. These data suggest a highly tissue-specific interaction between insulin sensitivity and oxidative metabolism during the course of metabolic diseases in humans.

Tissue-Specific and Genetic Regulation of Insulin Sensitivity-Associated Transcripts in African Americans

Integrative multiomics analyses of adipose and muscle tissue transcripts, S, and genotypes revealed novel genetic regulatory mechanisms of insulin resistance in African Americans.

Mitochondria in White, Brown, and Beige Adipocytes

Mitochondria play a key role in energy metabolism in many tissues, including cardiac and skeletal muscle, brain, liver, and adipose tissue. Three types of adipose depots can be identified in mammals, commonly classified according to their colour appearance: the white (WAT), the brown (BAT), and the beige/brite/brown-like (bAT) adipose tissues. WAT is mainly involved in the storage and mobilization of energy and BAT is predominantly responsible for nonshivering thermogenesis. Recent data suggest that adipocyte mitochondria might play an important role in the development of obesity through defects in mitochondrial lipogenesis and lipolysis, regulation of adipocyte differentiation, apoptosis, production of oxygen radicals, efficiency of oxidative phosphorylation, and regulation of conversion of white adipocytes into brown-like adipocytes. This review summarizes the main characteristics of each adipose tissue subtype and describes morphological and functional modifications focusing on mitochondria and their activity in healthy and unhealthy adipocytes.

Adipose tissue and metabolic syndrome: too much, too little or neither

Obesity is strongly associated with metabolic syndrome. Recent research suggests that excess adipose tissue plays an important role in development of the syndrome. On the other hand, persons with a deficiency of adipose tissue (e.g. lipodystrophy) also manifest the metabolic syndrome. In some animal models, expansion of adipose tissue pools mitigates adverse metabolic components (e.g. insulin resistance, hyperglycaemia and dyslipidemia). Hence, there are conflicting data as to whether adipose tissue worsens the metabolic syndrome or protects against it. This conflict may relate partly to locations of adipose tissue pools. For instance, lower body adipose tissue may be protective whereas upper body adipose tissue may promote the syndrome. One view holds that in either case, the accumulation of ectopic fat in muscle and liver is the driving factor underlying the syndrome. If so, there may be some link between adipose tissue fat and ectopic fat. But the mechanisms underlying this connection are not clear. A stronger association appears to exist between excessive caloric intake and ectopic fat accumulation. Adipose tissue may act as a buffer to reduce the impact of excess energy consumption by fat storage; but once a constant weight has been achieved, it is unclear whether adipose tissue influences levels of ectopic fat. Another mechanism whereby adipose tissue could worsen the metabolic syndrome is through release of adipokines. This is an intriguing mechanism, but the impact of adipokines on metabolic syndrome risk factors is uncertain. Thus, many potential connections between adipose tissue and metabolic syndrome remain to unravelled.

The role of perivascular adipose tissue in vasoconstriction, arterial stiffness, and aneurysm

Since the "rediscovery" of brown adipose tissue in adult humans, significant scientific efforts are being pursued to identify the molecular mechanisms to promote a phenotypic change of white adipocytes into brown-like cells, a process called "browning". It is well documented that white adipose tissue (WAT) mass and factors released from WAT influence the vascular function and positively correlate with cardiac arrest, stroke, and other cardiovascular complications. Similar to other fat depots, perivascular adipose tissue (PVAT) is an active endocrine organ and anatomically surrounds vessels. Both brown-like and white-like PVAT secrete various adipokines, cytokines, and growth factors that either prevent or promote the development of cardiovascular diseases (CVDs) depending on the relative abundance of each type and their bioactivity in the neighboring vasculature. Notably, pathophysiological conditions, such as obesity, hypertension, or diabetes, induce the imbalance of PVAT-derived vasoactive products that promote the infiltration of inflammatory cells. This then triggers derangements in vascular smooth muscle cells and endothelial cell dysfunction, resulting in the development of vascular diseases. In this review, we discuss the recent advances on the contribution of PVAT in CVDs. Specifically, we summarize the current proposed roles of PVAT in relationship with vascular contractility, endothelial dysfunction, neointimal formation, arterial stiffness, and aneurysm.

Metabolic imaging in obesity: underlying mechanisms and consequences in the whole body

Obesity is a phenotype resulting from a series of causative factors with a variable risk of complications. Etiologic diversity requires personalized prevention and treatment. Imaging procedures offer the potential to investigate the interplay between organs and pathways underlying energy intake and consumption in an integrated manner, and may open the perspective to classify and treat obesity according to causative mechanisms. This review illustrates the contribution provided by imaging studies to the understanding of human obesity, starting with the regulation of food intake and intestinal metabolism, followed by the role of adipose tissue in storing, releasing, and utilizing substrates, including the interconversion of white and brown fat, and concluding with the examination of imaging risk indicators related to complications, including type 2 diabetes, liver pathologies, cardiac and kidney diseases, and sleep disorders. The imaging modalities include (1) positron emission tomography to quantify organ-specific perfusion and substrate metabolism; (2) computed tomography to assess tissue density as an indicator of fat content and browning/ whitening; (3) ultrasounds to examine liver steatosis, stiffness, and inflammation; and (4) magnetic resonance techniques to assess blood oxygenation levels in the brain, liver stiffness, and metabolite contents (triglycerides, fatty acids, glucose, phosphocreatine, ATP, and acetylcarnitine) in a variety of organs.

Diabetes mellitus: The epidemic of the century

The epidemic nature of diabetes mellitus in different regions is reviewed. The Middle East and North Africa region has the highest prevalence of diabetes in adults (10.9%) whereas, the Western Pacific region has the highest number of adults diagnosed with diabetes and has countries with the highest prevalence of diabetes (37.5%). Different classes of diabetes mellitus, type 1, type 2, gestational diabetes and other types of diabetes mellitus are compared in terms of diagnostic criteria, etiology and genetics. The molecular genetics of diabetes received extensive attention in recent years by many prominent investigators and research groups in the biomedical field. A large array of mutations and single nucleotide polymorphisms in genes that play a role in the various steps and pathways involved in glucose metabolism and the development, control and function of pancreatic cells at various levels are reviewed. The major advances in the molecular understanding of diabetes in relation to the different types of diabetes in comparison to the previous understanding in this field are briefly reviewed here. Despite the accumulation of extensive data at the molecular and cellular levels, the mechanism of diabetes development and complications are still not fully understood. Definitely, more extensive research is needed in this field that will eventually reflect on the ultimate objective to improve diagnoses, therapy and minimize the chance of chronic complications development.

The Whitening of Brown Fat and Its Implications for Weight Management in Obesity

Systemic inflammation resulting from dysfunction of white adipose tissue (WAT) accelerates the pathologies of diabetes and cardiovascular diseases. In contrast to WAT, brown adipose tissue (BAT) is abundant in mitochondria that produce heat by uncoupling respiratory chain process of ATP synthesis. Besides BAT's role in thermogenesis, accumulating evidence has shown that it is involved in regulating systemic metabolism. Studies have analyzed the "browning" processes of WAT as a means to combat obesity, whereas few studies have focused on the impact and molecular mechanisms that contribute to obesity-linked BAT dysfunction--a process that is associated with the "whitening" of this tissue. Compared to WAT, a dense vascular network is required to support the high energy consumption of BAT. Recently, vascular rarefaction was shown to be a significant causal factor in the whitening of BAT in mouse models. Vascular insufficiency leads to mitochondrial dysfunction and loss in BAT and contributes to systemic insulin resistance. These data suggest that BAT "whitening," resulting from vascular dysfunction, can impact obesity and obesity-linked diseases. Conversely, agents that promote BAT function could have utility in the treatment of these conditions.

Nitric oxide and mitochondria in metabolic syndrome

Metabolic syndrome (MS) is a cluster of metabolic disorders that collectively increase the risk of cardiovascular disease. Nitric oxide (NO) plays a crucial role in the pathogeneses of MS components and is involved in different mitochondrial signaling pathways that control respiration and apoptosis. The present review summarizes the recent information regarding the interrelations of mitochondria and NO in MS. Changes in the activities of different NO synthase isoforms lead to the formation of metabolic disorders and therefore are highlighted here. Reduced endothelial NOS activity and NO bioavailability, as the main factors underlying the endothelial dysfunction that occurs in MS, are discussed in this review in relation to mitochondrial dysfunction. We also focus on potential therapeutic strategies involving NO signaling pathways that can be used to treat patients with metabolic disorders associated with mitochondrial dysfunction. The article may help researchers develop new approaches for the diagnosis, prevention and treatment of MS.

Role of Energy Metabolism in the Brown Fat Gene Program

In murine and human brown adipose tissue (BAT), mitochondria are powerful generators of heat that safely metabolize fat, a feature that has great promise in the fight against obesity and diabetes. Recent studies suggest that the actions of mitochondria extend beyond their conventional role as generators of heat. There is mounting evidence that impaired mitochondrial respiratory capacity is accompanied by attenuated expression of Ucp1 and other BAT-selective genes, implying that mitochondria exert transcriptional control over the brown fat gene program. In this review, we discuss the current understanding of brown fat mitochondria, their potential role in transcriptional control of the brown fat gene program, and potential strategies to treat obesity in humans by leveraging thermogenesis in brown adipocytes.