Reduced antioxidant capacity and diet-induced atherosclerosis in uncoupling protein-2-deficient mice

Mitochondria and endothelial function

In contrast to their role in cell types with higher energy demands, mitochondria in endothelial cells primarily function in signaling cellular responses to environmental cues. This article provides an overview of key aspects of mitochondrial biology in endothelial cells, including subcellular location, biogenesis, dynamics, autophagy, reactive oxygen species production and signaling, calcium homeostasis, regulated cell death, and heme biosynthesis. In each section, we introduce key concepts and then review studies showing the importance of that mechanism to endothelial control of vasomotor tone, angiogenesis, and/or inflammatory activation. We particularly highlight the small number of clinical and translational studies that have investigated each mechanism in human subjects. Finally, we review interventions that target different aspects of mitochondrial function and their effects on endothelial function. The ultimate goal of such research is the identification of new approaches for therapy. The reviewed studies make it clear that mitochondria are important in endothelial physiology and pathophysiology. A great deal of work will be needed, however, before mitochondria-directed therapies are available for the prevention and treatment of cardiovascular disease.

TRPV1-mediated UCP2 upregulation ameliorates hyperglycemia-induced endothelial dysfunction

Background

Diabetic cardiovascular complications are characterised by oxidative stress-induced endothelial dysfunction. Uncoupling protein 2 (UCP2) is a regulator of mitochondrial reactive oxygen species (ROS) generation and can antagonise oxidative stress, but approaches that enhance the activity of UCP2 to inhibit ROS are scarce. Our previous studies show that activation of transient receptor potential vanilloid 1 (TRPV1) by capsaicin can prevent cardiometabolic disorders. In this study, we conducted experiments in vitro and in vivo to investigate the effect of capsaicin treatment on endothelial UCP2 and oxidative stress. We hypothesised that TRPV1 activation by capsaicin attenuates hyperglycemia-induced endothelial dysfunction through a UCP2-mediated antioxidant effect.

Methods

TRPV1^-/-, UCP2 ^-/- and db/db mice, as well as matched wild type (WT) control mice, were included in this study. Some mice were subjected to dietary capsaicin for 14 weeks. Arteries isolated from mice and endothelial cells were cultured. Endothelial function was examined, and immunohistological and molecular analyses were performed.

Results

Under high-glucose conditions, TRPV1 expression and protein kinase A (PKA) phosphorylation were found to be decreased in the cultured endothelial cells, and the effects of high-glucose on these molecules were reversed by the administration of capsaicin. Furthermore, high-glucose exposure increased ROS production and reduced nitric oxide (NO) levels both in endothelial cells and in arteries that were evaluated respectively by dihydroethidium (DHE) and DAF-2 DA fluorescence. Capsaicin administration decreased the production of ROS, restored high-glucose-induced endothelial dysfunction through the activation of TRPV1 and acted in a UCP2-dependent manner in vivo. Administration of dietary capsaicin for 14 weeks increased the levels of PKA phosphorylation and UCP2 expression, ameliorated the vascular oxidative stress and increased NO levels observed in diabetic mice. Prolonged dietary administration of capsaicin promoted endothelium-dependent relaxation in diabetic mice. However, the beneficial effect of capsaicin on vasorelaxation was absent in the aortas of UCP2 ^-/- mice exposed to high-glucose levels.

Conclusion

TRPV1 activation by capsaicin might protect against hyperglycemia-induced endothelial dysfunction through a mechanism involving the PKA/UCP2 pathway.

Functionalization of gadolinium metallofullerenes for detecting atherosclerotic plaque lesions by cardiovascular magnetic resonance

BACKGROUND

The hallmark of atherosclerosis is the accumulation of plaque in vessel walls. This process is initiated when monocytic cells differentiate into macrophage foam cells under conditions with high levels of atherogenic lipoproteins. Vulnerable plaque can dislodge, enter the blood stream, and result in acute myocardial infarction and stroke. Imaging techniques such as cardiovascular magnetic resonance (CMR) provides one strategy to identify patients with plaque accumulation.

METHODS

We synthesized an atherosclerotic-targeting contrast agent (ATCA) in which gadolinium (Gd)-containing endohedrals were functionalized and formulated into liposomes with CD36 ligands intercalated into the lipid bilayer. In vitro assays were used to assess the specificity of the ATCA for foam cells. The ability of ATCA to detect atherosclerotic plaque lesions in vivo was assessed using CMR.

RESULTS

The ATCA was able to detect scavenger receptor (CD36)-expressing foam cells in vitro and were specifically internalized via the CD36 receptor as determined by focused ion beam/scanning electron microscopy (FIB-SEM) and Western blotting analysis of CD36 receptor-specific signaling pathways. The ATCA exhibited time-dependent accumulation in atherosclerotic plaque lesions of ApoE -/- mice as determined using CMR. No ATCA accumulation was observed in vessels of wild type (C57/b6) controls. Non-targeted control compounds, without the plaque-targeting moieties, were not taken up by foam cells in vitro and did not bind plaque in vivo. Importantly, the ATCA injection was well tolerated, did not demonstrate toxicity in vitro or in vivo, and no accumulation was observed in the major organs.

CONCLUSIONS

The ATCA is specifically internalized by CD36 receptors on atherosclerotic plaque providing enhanced visualization of lesions under physiological conditions. These ATCA may provide new tools for physicians to non-invasively detect atherosclerotic disease.

Uncoupling and reactive oxygen species (ROS)--a double-edged sword for β-cell function? "Moderation in all things"

The ability of the mitochondrion to (a) manage fuel import to oxidize for adenosine tri-phosphate (ATP) generation while (b) protecting itself and the cellular environment from electron leak, which can generate highly reactive oxygen species (ROS) is a delicate balancing act. ATP is the currency of the cell and as such serves a signaling function as a substrate partner to many kinases and ion channels. While various ROS species have been viewed as a dangerous and toxic group of molecules, it also has a role as a signal derived from mitochondria, as well as other enzymatic sources: a double-edged sword. Current efforts to understand the biochemical mechanisms affected by ROS as a signal--usually noted to be hydrogen peroxide (H(2)O(2))--are exciting, but this duality of ROS effects also pose challenges in managing its levels to protect cells. The mitochondrial uncoupling protein-2 (UCP2), UCP3, and the permeability transition pore have been integral to efforts to try to understand what role mitochondrial-derived ROS have in cells. In this piece we reflect on mitochondrial ROS and uncoupling proteins as signaling regulators. It seems that when it comes to ROS and uncoupling the proverb "Moderation in all things" is apt.

Involvement of Kv1.5 protein in oxidative vascular endothelial cell injury

Endothelial injury related to oxidative stress is a key event in cardiovascular diseases, such as hypertension and atherosclerosis. The activation of the redox-sensitive Kv1.5 potassium channel mediates mitochondrial reactive oxygen species (ROS)-induced apoptosis in vascular smooth muscle cells and some cancer cells. Kv1.5 channel is therefore taken as a new potential therapeutic target for pulmonary hypertension and cancers. Although Kv1.5 is abundantly expressed in vascular endothelium, there is little knowledge of its role in endothelial injury related to oxidative stress. We found that DPO-1, a specific inhibitor of Kv1.5, attenuated H₂O₂-evoked endothelial cell apoptosis in an in vivo rat carotid arterial model. In human umbilical vein endothelial cells (HUVECs) and human pulmonary arterial endothelial cells (HPAECs), angiotensin II and oxLDL time- or concentration-dependently enhanced Kv1.5 protein expression in parallel with the production of intracellular ROS and endothelial cell injury. Moreover, siRNA-mediated knockdown of Kv1.5 attenuated, whereas adenovirus-mediated Kv1.5 cDNA overexpression enhanced oxLDL–induced cellular damage, NADPH oxidase and mitochondria-derived ROS production and restored the decrease in protein expression of mitochondria uncoupling protein 2 (UCP2). Collectively, these data suggest that Kv1.5 may play an important role in oxidative vascular endothelial injury.

Mitochondria in vascular health and disease

The eukaryote's mitochondrial network is perhaps the cell's most sophisticated and dynamic responsive sensing system. Integrating metabolic, oxygen, or danger signals with inputs from other organelles, as well as local and systemic signals, mitochondria have a profound impact on vascular function in both health and disease. This review highlights recently discovered aspects of mitochondrial function (oxygen sensing, inflammation, autophagy, and apoptosis) and discusses their role in diseases of both systemic and pulmonary vessels. We also emphasize the role of mitochondria as therapeutic targets for vascular disease. We highlight the intriguing similarities of mitochondria-driven molecular mechanisms in terms of both pathogenesis and therapies in very diverse diseases, such as atherosclerosis, pulmonary hypertension, and cancer, to support the foundation of a new field in medicine: mitochondrial medicine.

Inhibition of uncoupling protein 2 with genipin exacerbates palmitate-induced hepatic steatosis

Background

Uncoupling protein 2 (UCP2) was reported to be involved in lipid metabolism through regulating the production of superoxide anion. However, the role of UCP2 in hepatocytes steatosis has not been determined. We hypothesized that UCP2 might regulate hepatic steatosis via suppressing oxidative stress.

Results

We tested this hypothesis in an in vitro model of hepatocytic steatosis in HepG2 cell lines induced by palmitic acid (PA). We found that treatment with PA induced an obvious lipid accumulation in HepG2 cells and a significant increase in intracellular triglyceride content. Moreover, the specific inhibition of UCP2 by genipin remarkably exacerbated PA-induced hepatocytes steatosis. Interestingly, the PA-induced superoxide overproduction can also be enhanced by incubation with genipin. In addition, administration with the antioxidant tempol abolished genipin-induced increase in intracellular lipid deposition. We further found that genipin significantly increased the protein expression of fatty acid translocase (FAT)/CD36.

Conclusions

These findings suggest that UCP2 plays a protective role in PA-induced hepatocytic steatosis through ameliorating oxidative stress.

Role of lipotoxicity in endothelial dysfunction

Vascular endothelial dysfunction is determined by both genetic and environmental factors that cause decreased bioavailability of the vasodilator nitric oxide. This is a hallmark of atherosclerosis, hypertension, and coronary heart disease, which are major complications of metabolic disorders, including diabetes and obesity. Several therapeutic interventions, including changes in lifestyle as well as pharmacologic treatments, are useful for improving endothelial dysfunction in the face of lipotoxicity. This review discusses the current understanding of molecular and physiologic mechanisms underlying lipotoxicity-mediated endothelial dysfunction as well as relevant therapeutic approaches to ameliorate dyslipidemia and consequent endothelial dysfunction that have the potential to improve cardiovascular and metabolic outcomes.

Endothelial mitochondria--less respiration, more integration

Lining the inner surface of the circulatory system, the vascular endothelium accomplishes a vast variety of specialized functions. Even slight alterations of these functions are implicated in the development of certain cardiovascular diseases that represent major causes of morbidity and mortality in developed countries. Endothelial mitochondria are essential to the functional integrity of the endothelial cell as they integrate a wide range of cellular processes including Ca²⁺ handling, redox signaling and apoptosis, all of which are closely interrelated. Growing evidence supports the notion that impairment of mitochondrial signaling in the endothelium is an early event and a causative factor in the development of diseases such as atherosclerosis or diabetic complications. In this review, we want to outline the significance of mitochondria in both physiology and pathology of the vascular endothelium.

Uncoupling protein-2 protects endothelial function in diet-induced obese mice

RATIONALE

Previous studies indicate uncoupling protein-2 (UCP2) as an antioxidant defense against endothelial dysfunction in hypertension. UCP2 also regulates insulin secretion and action. However, the role of UCP2 in endothelial dysfunction associated with diabetes and obesity is unclear.

OBJECTIVE

UCP2 protects against endothelial dysfunction induced by high-fat diet through inhibition of reactive oxygen species (ROS) production, and subsequent increase of nitric oxide bioavailability.

METHODS AND RESULTS

Endothelium-dependent relaxation (EDR) in aortae and mesenteric arteries in response to acetylcholine was measured in wire myograph. Flow-mediated vasodilatation in 2(nd)-order mesenteric arteries was measured in pressure myograph. ROS production is measured by CM-H(2)DCFDA and DHE fluorescence. High-glucose exposure reduced EDR in mouse aortae, which was exaggerated in UCP2 knockout (KO) mice, whereas UCP2 overexpression by adenoviral infection (AdUCP2) restored the impaired EDR. Impairment of EDR and flow-mediated vasodilatation in aortae and mesenteric arteries from high-fat diet-induced obese mice (DIO) was exaggerated in UCP2KO DIO mice compared with wild-type DIO littermates, whereas AdUCP2 i.v. injection restored both EDR and flow-mediated vasodilatation in DIO mice. Improved EDR in mesenteric arteries was inhibited by nitric oxide synthase inhibitor. UCP2 overexpression also inhibited intracellular ROS production in the en face endothelium of aorta and mesenteric artery of DIO mice, whereas UCP2 deficiency enhanced ROS production.

CONCLUSIONS

UCP2 preserves endothelial function through increasing nitric oxide bioavailability secondary to the inhibition of ROS production in the endothelium of obese diabetic mice.

Uncoupling proteins: molecular, functional, regulatory, physiological and pathological aspects

Uncoupling proteins are a subfamily of the mitochondrial anion carrier family. They are widespread in the whole eukaryotic world with a few exceptions and present tissue specific isoforms in higher organisms. They mediate purine nucleotide-sensitive free fatty acid-activated proton inward flux through the inner mitochondrial membrane. This proton flux occurs at the expense of the proton motive force build up by the respiration and weakens the coupling between respiration and ATP synthesis. In this chapter we describe current and reliable knowledge of uncoupling proteins. A new methodology allowing study of their activity and regulation during phosphorylating respiration is described. It has entitled us to assert that all uncoupling proteins share common mechanisms of activation and regulation. This is of the utmost importance in order to understand the physiological roles of UCPs as well as their participation in pathological processes since every role of the UCPs in every cell is an integral part of their function and regulation. The central role of reduction level of ubiquinone in the control of their regulation is well-argued. Their potential and reliable roles in thermogenesis, reactive oxygen species prevention and energy flow are discussed as well as their role in some pathological disorders.

The regulation and physiology of mitochondrial proton leak

Mitochondria couple respiration to ATP synthesis through an electrochemical proton gradient. Proton leak across the inner membrane allows adjustment of the coupling efficiency. The aim of this review is threefold: 1) introduce the unfamiliar reader to proton leak and its physiological significance, 2) review the role and regulation of uncoupling proteins, and 3) outline the prospects of proton leak as an avenue to treat obesity, diabetes, and age-related disease.

Activation of AMP-activated protein kinase is required for berberine-induced reduction of atherosclerosis in mice: the role of uncoupling protein 2

AIMS

Berberine, a botanical alkaloid purified from Coptidis rhizoma, is reported to activate the AMP-activated protein kinase (AMPK). Whether AMPK is required for the protective effects of berberine in cardiovascular diseases remains unknown. This study was designed to determine whether AMPK is required for berberine-induced reduction of oxidative stress and atherosclerosis in vivo.

METHODS

ApoE (ApoE⁻/⁻) mice and ApoE⁻/⁻/AMPK alpha 2⁻/⁻ mice that were fed Western diets were treated with berberine for 8 weeks. Atherosclerotic aortic lesions, expression of uncoupling protein 2 (UCP2), and markers of oxidative stress were evaluated in isolated aortas.

RESULTS

In ApoE⁻/⁻ mice, chronic administration of berberine significantly reduced aortic lesions, markedly reduced oxidative stress and expression of adhesion molecules in aorta, and significantly increased UCP2 levels. In contrast, in ApoE⁻/⁻/AMPK alpha 2⁻/⁻ mice, berberine had little effect on those endpoints. In cultured human umbilical vein endothelial cells (HUVECs), berberine significantly increased UCP2 mRNA and protein expression in an AMPK-dependent manner. Transfection of HUVECs with nuclear respiratory factor 1 (NRF1)-specific siRNA attenuated berberine-induced expression of UCP2, whereas transfection with control siRNA did not. Finally, berberine promoted mitochondrial biogenesis that contributed to up-regulation of UCP2 expression.

CONCLUSION

We conclude that berberine reduces oxidative stress and vascular inflammation, and suppresses atherogenesis via a mechanism that includes stimulation of AMPK-dependent UCP2 expression.

Inhibition of glucose-induced vascular endothelial growth factor expression by Salvia miltiorrhiza hydrophilic extract in human microvascular endothelial cells: evidence for mitochondrial oxidative stress

AIM OF STUDY

Diabetes mellitus is frequently combined with vascular diseases, which are associated with the expression of vascular endothelial growth factor (VEGF). An approach that can reverse the induction of VEGF by hyperglycemia may potentially benefit the outcome of diabetic patients. Therefore, in the present study, we investigated the effect of Salvia miltiorrhiza (S. miltiorrhiza) hydrophilic extract on the expression of VEGF induced by high concentration of glucose.

MATERIALS AND METHODS

Vector of VEGF promoter luc was transiently transfected into HMEC-1 cells, and luciferase activity was measured to determine the promoter activity. In order to investigate the mechanism of Salvia miltiorrhiza hydrophilic extract, mitochondrial uncoupling protein 2(UCP2) was knockdown by using UCP2 siRNA. The expression of VEGF was obtained by using quantitative RT-PCR and dot blot. The level of reactive oxygen species (ROS) was expressed by the level of 2',7'-dichlorfluorescein.

RESULTS

Exposure of HMEC-1 cells to 30 mM glucose resulted in a significant increase in the expression of VEGF mRNA (5.7 fold at 3mM glucose, P<0.005), and an increase of ROS formation (2.4 fold at 3mM glucose, P<0.005). These effects were completely antagonized by an inhibitor of electron transport chain complex II, thenoyltrifluoroacetone (TTFA) and an uncoupler of oxidative phosphorylation, carbonylcyanide-m-chlorophenylhydrazone (CCCP). Addition of Salvia miltiorrhiza hydrophilic extract (10 μg/ml) led to a significant decrease of VEGF mRNA and ROS formation in 30 mM glucose condition. Interestingly, knockdown of mitochondrial UCP-2 by UCP-2 siRNA abolished the reduction of VEGF expression and ROS formation by Salvia miltiorrhiza hydrophilic extract.

CONCLUSIONS

These findings indicated that Salvia miltiorrhiza hydrophilic extract effectively reversed induction of VEGF expression by high glucose via ameliorating mitochondrial oxidative stress. Salvia miltiorrhiza hydrophilic extract can potentially be an effective antioxidant therapy for the treatment of diabetic chronic vascular complication.

Continued clearance of apoptotic cells critically depends on the phagocyte Ucp2 protein

Rapid and efficient removal of apoptotic cells by phagocytes is important during development, tissue homeostasis and in immune responses. Efficient clearance depends on the capacity of a single phagocyte to ingest multiple apoptotic cells successively, and to process the corpse-derived cellular material. However, the factors that influence continued clearance by phagocytes are not known. Here we show that the mitochondrial membrane potential of the phagocyte critically controls engulfment capacity, with lower potential enhancing engulfment and vice versa. The mitochondrial membrane protein Ucp2, which acts to lower the mitochondrial membrane potential, was upregulated in phagocytes engulfing apoptotic cells. Loss of Ucp2 reduced phagocytic capacity, whereas Ucp2 overexpression enhanced engulfment. Mutational and pharmacological studies indicated a direct role for Ucp2-mediated mitochondrial function in phagocytosis. Macrophages from Ucp2-deficient mice were impaired in phagocytosis in vitro, and Ucp2-deficient mice showed profound in vivo defects in clearing dying cells in the thymus and testes. Collectively, these data indicate that mitochondrial membrane potential and Ucp2 are key molecular determinants of apoptotic cell clearance. As Ucp2 is linked to metabolic diseases and atherosclerosis, this newly discovered role for Ucp2 in apoptotic cell clearance has implications for the complex aetiology and pathogenesis of these diseases.

Role of obesity-associated dysfunctional adipose tissue in cancer: a molecular nutrition approach

Obesity is a complex disease caused by the interaction of a myriad of genetic, dietary, lifestyle and environmental factors, which favors a chronic positive energy balance, leading to increased body fat mass. There is emerging evidence of a strong association between obesity and an increased risk of cancer. However, the mechanisms linking both diseases are not fully understood. Here, we analyze the current knowledge about the potential contribution that expanding adipose tissue in obesity could make to the development of cancer via dysregulated secretion of pro-inflammatory cytokines, chemokines and adipokines such as TNF-α, IL-6, leptin, adiponectin, visfatin and PAI-1. Dietary factors play an important role in the risk of suffering obesity and cancer. The identification of bioactive dietary factors or substances that affect some of the components of energy balance to prevent/reduce weight gain as well as cancer is a promising avenue of research. This article reviews the beneficial effects of some bioactive food molecules (n-3 PUFA, CLA, resveratrol and lipoic acid) in energy metabolism and cancer, focusing on the molecular mechanisms involved, which may provide new therapeutic targets in obesity and cancer.

Current perspectives of oxidative stress and its measurement in chronic obstructive pulmonary disease

Cigarette smoking, the principal aetiology of chronic obstructive pulmonary disease (COPD) in the developed countries, delivers and generates oxidative stress within the lungs. This imbalance of oxidant burden and antioxidant capacity has been implicated as an important contributing factor in the pathogenesis of COPD. Oxidative processes and free radical generation orchestrate the inflammation, mucous gland hyperplasia, and apoptosis of the airway lining epithelium which characterises COPD. Pivotal oxidative stress/pro-inflammatory molecules include reactive oxygen species such as the superoxides and hydroxyl radicals, pro-inflammatory cytokines including leukotrienes, interleukins, tumour necrosis factor alpha, and activated transcriptional factors such as nuclear factor kappa-B and activator protein 1. The lung has a large reserve of antioxidant agents such as glutathione and superoxide dismutase to counter oxidants. However, smoking also causes the depletion of antioxidants, which further contributes to oxidative tissue damage. The downregulation of antioxidant pathways has also been associated with acute exacerbations of COPD. The delivery of redox-protective antioxidants may have preventative and therapeutic potential of COPD. Although these observations have yet to translate into common clinical practice, preliminary clinical trials and studies of animal models have shown that interventions to counter this oxidative imbalance may have potential to better manage COPD. There is, thus, a need for the ability to monitor such interventions and exhaled breath condensate is rapidly emerging as a novel and noninvasive approach in the sampling of airway epithelial lining fluid which could be used for repeated analysis of oxidative stress and inflammation in the lungs.

Reactive oxygen species and uncoupling protein 2 in pancreatic β-cell function

Growing evidence indicates that reactive oxygen species (ROS) are not just deleterious by-products of respiratory metabolism in mitochondria, but can be essential elements for many biological responses, including in pancreatic β-cells. ROS can be a 'second-messenger signal' in response to hormone/receptor activation that serves as part of the 'code' to trigger the ultimate biological response, or it can be a 'protective signal' to increase the levels of antioxidant enzymes and small molecules to scavenge ROS, thus restoring cellular redox homeostasis. In pancreatic β-cells evidence is emerging that acute and transient glucose-dependent ROS contributes to normal glucose-stimulated insulin secretion (GSIS). However, chronic and persistent elevation of ROS, resulting from inflammation or excessive metabolic fuels such as glucose and fatty acids, may elevate antioxidant enzymes such that they blunt ROS and redox signalling, thus impairing β-cell function. An interesting mitochondrial protein whose main function appears to be the control of ROS is uncoupling protein 2 (UCP2). Despite continuing investigation of the exact mechanism by which UCP2 is 'activated', it is clear that UCP2 levels and/or activity impact the efficacy of GSIS in pancreatic islets. This review will focus on the paradoxical roles of ROS in pancreatic β-cell function and the regulatory role of UCP2 in ROS signalling and GSIS.

Elevated systemic TGF-beta impairs aortic vasomotor function through activation of NADPH oxidase-driven superoxide production and leads to hypertension, myocardial remodeling, and increased plaque formation in apoE(-/-) mice

The role of circulating, systemic TGF-beta levels in endothelial function is not clear. TGF-beta(1) may cause endothelial dysfunction in apolipoprotein E-deficient (apoE(-/-)) mice via stimulation of reactive oxygen species (ROS) production by the NADPH oxidase (NOX) system and aggravate aortic and heart remodeling and hypertension. Thoracic aorta (TA) were isolated from 4-mo-old control (C57Bl/6), apoE(-/-), TGF-beta(1)-overexpressing (TGFbeta(1)), and crossbred apoE(-/-) x TGFbeta(1) mice. Endothelium-dependent relaxation was measured before and after incubation with apocynin (NOX inhibitor) or superoxide dismutase (SOD; ROS scavenger). Superoxide production within the vessel wall was determined by dihydroethidine staining under confocal microscope. In 8-mo-old mice, aortic and myocardial morphometric changes, plaque formation by en face fat staining, and blood pressure were determined. Serum TGF-beta(1) levels (ELISA) were elevated in TGFbeta(1) mice without downregulation of TGF-beta-I receptor (immunohistochemistry). In the aortic wall, superoxide production was enhanced and NO-dependent relaxation diminished in apoE(-/-) x TGFbeta(1) mice but improved significantly after apocynin or SOD. Myocardial capillary density was reduced, fibrocyte density increased, aortic wall was thicker, combined lesion area was greater, and blood pressure was higher in the apoE(-/-) x TGFbeta vs. C57Bl/6 mice. Our results demonstrate that elevated circulating TGF-beta(1) causes endothelial dysfunction through NOX activation-induced oxidative stress, accelerating atherosclerosis and hypertension in apoE(-/-) mice. These findings may provide a mechanism explaining accelerated atherosclerosis in patients with elevated plasma TGFbeta(1).

The regulation and turnover of mitochondrial uncoupling proteins

Uncoupling proteins (UCP1, UCP2 and UCP3) are important in regulating cellular fuel metabolism and as attenuators of reactive oxygen species production through strong or mild uncoupling. The generic function and broad tissue distribution of the uncoupling protein family means that they are increasingly implicated in a range of pathophysiological processes including obesity, insulin resistance and diabetes mellitus, neurodegeneration, cardiovascular disease, immunity and cancer. The significant recent progress describing the turnover of novel uncoupling proteins, as well as current views on the physiological roles and regulation of UCPs, is outlined.

Uncoupling protein-2 and cancer

Cancer cells respond to unfavorable microenvironments such as nutrient limitation, hypoxia, oxidative stress, and host defense by comprehensive metabolic reprogramming. Mitochondria are linked to this complex adaptive response and emerging evidence indicates that uncoupling protein-2 (UCP2), a mitochondrial inner membrane anion carrier, may contribute to this process. Effects of UCP2 on mitochondrial bioenergetics, redox homeostasis, and oxidant production in cancer cells may modulate molecular pathways of macromolecular biosynthesis, antioxidant defense, apoptosis, cell growth and proliferation, enhancing robustness and promoting chemoresistance. Elucidation of these interactions may identify novel anti-cancer strategies.

UCP2, a metabolic sensor coupling glucose oxidation to mitochondrial metabolism?

Mitochondrial uncoupling of oxidative phosphorylation may serve a variety of purposes such as the regulation of substrate oxidation, free radical production (a major by-product of mitochondrial respiration) and ATP production and turnover. As regulators of energy expenditure and antioxidant defenses, uncoupling proteins would seem to offer an attractive mechanism by which to explain the control of body weight, resting metabolic rate and aging. As a result, the discovery of UCP1 homologues has led to an impressive number of publications. However, 10 years after their identification, no consensus has been found concerning the function of UCP homologues, and there are controversies as to whether or not they even have physiologically significant uncoupling activity. Here, we discuss a potential new function for UCP2, as a carrier involved in the coupling between glucose oxidation and mitochondrial metabolism.

Persistent oxidative stress due to absence of uncoupling protein 2 associated with impaired pancreatic beta-cell function

Uncoupling protein (UCP) 2 is a widely expressed mitochondrial protein whose precise function is still unclear but has been linked to mitochondria-derived reactive oxygen species production. Thus, the chronic absence of UCP2 has the potential to promote persistent reactive oxygen species accumulation and an oxidative stress response. Here, we show that Ucp2-/- mice on three highly congenic (N >10) strain backgrounds (C57BL/6J, A/J, 129/SvImJ), including two independently generated sources of Ucp2-null animals, all exhibit increased oxidative stress. Ucp2-null animals exhibit a decreased ratio of reduced glutathione to its oxidized form in blood and tissues that normally express UCP2, including pancreatic islets. Islets from Ucp2-/- mice exhibit elevated levels of numerous antioxidant enzymes, increased nitrotyrosine and F4/80 staining, but no change in insulin content. Contrary to results in Ucp2-/- mice of mixed 129/B6 strain background, glucose-stimulated insulin secretion in Ucp2-/- islets of each congenic strain was significantly decreased. These data show that the chronic absence of UCP2 causes oxidative stress, including in islets, and is accompanied by impaired glucose-stimulated insulin secretion.

Dysregulation of glucose homeostasis in nicotinamide nucleotide transhydrogenase knockout mice is independent of uncoupling protein 2

Glucose intolerance in C57Bl/6 mice has been associated with mutations in the nicotinamide nucleotide transhydrogenase (Nnt) gene. It has been proposed that the absence of NNT from mitochondria leads to increased mitochondrial reactive oxygen species production and subsequent activation of uncoupling protein 2 (UCP2). Activation of UCP2 has been suggested to uncouple electron transport from ATP synthesis in pancreatic beta cell mitochondria thereby decreasing glucose tolerance due to decreased insulin secretion through lower ATP/ADP ratios. The hypothesis tested in this paper is that UCP2 function is required for the dysregulation of glucose homeostasis observed in NNT ablated mice. Single and double Nnt and Ucp2 knockout mouse lines were used to measure glucose tolerance, whole animal energy balance and biochemical characteristics of mitochondrial uncoupling. As expected, glucose tolerance was diminished in mice lacking NNT. This was independent of UCP2 as it was observed either in the presence or absence of UCP2. The range of metabolic parameters examined in the mice and the proton conductance of isolated mitochondria remained unaltered in this double NNT and UCP2 knockout model. Ablation of UCP2 did not itself affect glucose tolerance and therefore previous observations of increased glucose tolerance of mice lacking UCP2 were not confirmed. We conclude that the decreased glucose tolerance in Nnt knockout mice observed in our experiments does not require UCP2.