Uncoupling protein 3 content is decreased in skeletal muscle of patients with type 2 diabetes

Muscle-UCP3 in the regulation of energy metabolism

Uncoupling protein-3 (UCP3) is a mitochondria-regulatory protein with potential energy- homeostatic functions. This study explores the role of UCP3 in the regulation of muscle- and energy metabolism. UCP3 is critical for tuning substrate utilization, favoring lipid oxidation, particularly in conditions of high-fat availability. While UCP3 is non-essential for lipid oxidation during energy excess, it proves vital during fasting, indicating an energy-homeostatic trait. Preliminary evidence indicates UCP3' promotion of glucose uptake and oxidation, at least in conditions of high glucose/low fat availability. However, the dynamics of how fats and glucose differentially influence UCP3 remain undefined. UCP3 exhibits inducible proton transport and uncoupling activity, operating in a dual manner: a resting state with no/low activity and an activated state in the presence of activators. Uncoupling may enhance thermogenesis in specific conditions and in the presence of activators such as fatty acids, thyroid hormones, and catecholamines. This energy-dissipative activity adapts to varying energy availability, balancing energy dissipation with fatty acid oxidation to optimize whole-body energy homeostasis: fasting triggers UCP3 upregulation, enhancing lipid utilization while suppressing uncoupling. Additionally, UCP3 upregulation induces glucose and lipid disposal from the bloodstream and decreases tri-/diglyceride storage in muscle. This process improves mitochondrial functionality and insulin signaling, leading to enhanced systemicgluco-metabolic balance and protection from metabolic conditions. Reviewed evidence suggests that UCP3 plays a crucial role in adapting the system to changing energy conditions. However, the precise role of UCP3 in regulating metabolism requires further elucidation.

Overexpression of UCP3 decreases mitochondrial efficiency in mouse skeletal muscle in vivo

Uncoupling protein-3 (UCP3) is a mitochondrial transmembrane protein highly expressed in the muscle that has been implicated in regulating the efficiency of mitochondrial oxidative phosphorylation. Increasing UCP3 expression in skeletal muscle enhances proton leak across the inner mitochondrial membrane and increases oxygen consumption in isolated mitochondria, but its precise function in vivo has yet to be fully elucidated. To examine whether muscle-specific overexpression of UCP3 modulates muscle mitochondrial oxidation in vivo, rates of ATP synthesis were assessed by ³¹ P magnetic resonance spectroscopy (MRS), and rates of mitochondrial oxidative metabolism were measured by assessing the rate of [2-¹³ C]acetate incorporation into muscle [4-¹³ C]-, [3-¹³ C]-glutamate, and [4-¹³ C]-glutamine by high-resolution ¹³ C/¹ H MRS. Using this approach, we found that the overexpression of UCP3 in skeletal muscle was accompanied by increased muscle mitochondrial inefficiency in vivo as reflected by a 42% reduction in the ratio of ATP synthesis to mitochondrial oxidation.

ANT1 overexpression models: Some similarities with facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant disorder characterized by progressive muscle weakness. Adenine nucleotide translocator 1 (ANT1), the only 4q35 gene involved in mitochondrial function, is strongly expressed in FSHD skeletal muscle biopsies. However, its role in FSHD is unclear. In this study, we evaluated ANT1 overexpression effects in primary myoblasts from healthy controls and during Xenopus laevis organogenesis. We also compared ANT1 overexpression effects with the phenotype of FSHD muscle cells and biopsies. Here, we report that the ANT1 overexpression-induced phenotype presents some similarities with FSHD muscle cells and biopsies. ANT1-overexpressing muscle cells showed disorganized morphology, altered cytoskeletal arrangement, enhanced mitochondrial respiration/glycolysis, ROS production, oxidative stress, mitochondrial fragmentation and ultrastructure alteration, as observed in FSHD muscle cells. ANT1 overexpression in Xenopus laevis embryos affected skeletal muscle development, impaired skeletal muscle, altered mitochondrial ultrastructure and led to oxidative stress as observed in FSHD muscle biopsies. Moreover, ANT1 overexpression in X. laevis embryos affected heart structure and mitochondrial ultrastructure leading to cardiac arrhythmia, as described in some patients with FSHD. Overall our data suggest that ANT1 could contribute to mitochondria dysfunction and oxidative stress in FSHD muscle cells by modifying their bioenergetic profile associated with ROS production. Such interplay between energy metabolism and ROS production in FSHD will be of significant interest for future prospects.

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.

Effects of statins on mitochondrial pathways

Statins are a family of drugs that are used for treating hyperlipidaemia with a recognized capacity to prevent cardiovascular disease events. They inhibit β-hydroxy β-methylglutaryl-coenzyme A reductase, i.e. the rate-limiting enzyme in mevalonate pathway, reduce endogenous cholesterol synthesis, and increase low-density lipoprotein clearance by promoting low-density lipoprotein receptor expression mainly in the hepatocytes. Statins have pleiotropic effects including stabilization of atherosclerotic plaques, immunomodulation, anti-inflammatory properties, improvement of endothelial function, antioxidant, and anti-thrombotic action. Despite all beneficial effects, statins may elicit adverse reactions such as myopathy. Studies have shown that mitochondria play an important role in statin-induced myopathies. In this review, we aim to report the mechanisms of action of statins on mitochondrial function. Results have shown that statins have several effects on mitochondria including reduction of coenzyme Q10 level, inhibition of respiratory chain complexes, induction of mitochondrial apoptosis, dysregulation of Ca2+ metabolism, and carnitine palmitoyltransferase-2 expression. The use of statins has been associated with the onset of additional pathological conditions like diabetes and dementia as a result of interference with mitochondrial pathways by various mechanisms, such as reduction in mitochondrial oxidative phosphorylation, increase in oxidative stress, decrease in uncoupling protein 3 concentration, and interference in amyloid-β metabolism. Overall, data reported in this review suggest that statins may have major effects on mitochondrial function, and some of their adverse effects might be mediated through mitochondrial pathways.

Effect of -55C/T Polymorphism of Uncoupling Protein 3 Gene on Risk for New-Onset Diabetes in Chinese Peritoneal Dialysis Patients: A Prospective Cohort Study

BACKGROUND

A high-glucose load in therapy can cause new-onset diabetes (NOD) in peritoneal dialysis (PD) patients. Genetic variability may result in risk modulation.

OBJECTIVES

This study aims to investigate the association between -55C/T polymorphism of uncoupling protein 3 (UCP3) gene and the risk of NOD in PD patients.

METHODS

Nondiabetic incident PD patients between May 2005 and January 2017 were recruited (n = 154). -55C/T polymorphism of the UCP3 was genotyped in all participants at baseline. The cohort of wild group (-55CC) and mutant group (-55CT or -55TT) was built based on the genotypic difference. Insulin resistance was evaluated by the homeostasis model assessment method (HOMA-IR) during the follow-up. Binary logistic regression was performed to explore the association between HOMA-IR and genotypes. Competitive risk analysis was used to analyze the impact of -55C/T polymorphism of UCP3 on risk for NOD.

RESULTS

The cohort was followed for up to 164.6 months (median: 58.3 months; interquartile range: 30.7 months). During the follow-up, 14 NODs occurred in the mutant group, while only 3 occurred in the wild group. Patients in the mutant group had higher HOMA-IR (Odd ratio: 2.210; 95% CI: 1.043-4.680; p = 0.038). Genotype with the variant T allele turned out to be an independent predictor for NOD morbidity (HR: 7.639; 95% CI: 1.798-32.451; p = 0.006).

CONCLUSIONS

The variant of T allele of UCP3 -55C/T polymorphism was an independent predictor for NOD in PD patients. Early identification of the genotype may provide scientific basis for patients' clinic management.

EGCG Upregulates UCP3 Levels to Protect MIN6 Pancreatic Islet Cells from Interleukin-1β-Induced Apoptosis

Objective

The protective effects of epigallocatechin gallate (EGCG) on interleukin-1β (IL-1β)-induced apoptosis were investigated in murine MIN₆ pancreatic β-cells. The role of uncoupling protein-3 (UCP₃) signaling in this process was also explored.

Methods

After treatment with IL-1β and EGCG, cells were collected and analyzed. Cell viability was measured using the CCK₈ assay and the function of β-cells was evaluated by analyzing insulin secretion. Detection of mitochondrial function in cells was performed by measuring mitochondrial membrane potential, the concentration of ATP and activity of ROS. Apoptosis was analyzed by Hochest33258 staining and flow cytometry. Expression levels of UCP₃ were interrogated using immunohistochemistry, RT-PCR and Western blotting.

Results

Compared with the control group, IL-1β treatment (20nM) for 24 h significantly decreased cell viability and insulin secretion, damaged mitochondrial function and increased ROS activity. Results also showed increased apoptosis and a decrease in UCP₃ expression levels (p<0.01). However, treatment with low (1mM) or high (5mM) concentrations of EGCG significantly decreased IL-1β-induced apoptosis (p<0.01), restored mitochondrial function and subsequently increased UCP₃ levels in IL-1β-induced β-cells (p<0.01).

Conclusion

These results suggest that EGCG protects against IL-1β-induced mitochondrial injury and apoptosis in β-cells through the up-regulation of UCP₃.

Modelling of energy-driven switch for glucagon and insulin secretion

We present a mathematical model of the energy-driven metabolic switch for glucagon and insulin secretion from pancreatic alpha and beta cells, respectively. The energy status related to hormone secretion is studied for various glucose concentrations. Additionally, the physiological response is studied with regards to the presence of other metabolites, particularly the free-fatty acids. At low glucose, the ATP production in alpha cells is high due to free-fatty acids oxidation in mitochondria, which enables glucagon secretion. When the glucose concentration is elevated above the threshold value, the glucagon secretion is switched off due to the contribution of glycolytic ATP production, representing an "anaerobic switch". On the other hand, during hypoglycemia, the ATP production in beta cells is low, reflecting a "waiting state" for glucose as the main metabolite. When glucose is elevated above the threshold value, the oxidative fate of glucose in mitochondria is the main source of energy required for effective insulin secretion, i.e. the "aerobic switch". Our results show the importance of well-regulated and fine-tuned energetic processes in pancreatic alpha and beta cells required for efficient hormone secretion and hence effective blood glucose regulation. These energetic processes have to be appropriately switched on and off based on the sensing of different metabolites by alpha and beta cells. Our computational results indicate that disturbances in cell energetics (e.g. mitochondrial dysfunction), and dysfunctional metabolite sensing and distribution throughout the cell might be related to pathologies such as metabolic syndrome and diabetes.

Absence of Uncoupling Protein-3 at Thermoneutrality Impacts Lipid Handling and Energy Homeostasis in Mice

The role of uncoupling protein-3 (UCP3) in energy and lipid metabolism was investigated. Male wild-type (WT) and UCP3-null (KO) mice that were housed at thermoneutrality (30 °C) were used as the animal model. In KO mice, the ability of skeletal muscle mitochondria to oxidize fatty acids (but not pyruvate or succinate) was reduced. At whole animal level, adult KO mice presented blunted resting metabolic rates, energy expenditure, food intake, and the use of lipids as metabolic substrates. When WT and KO mice were fed with a standard/low-fat diet for 80 days, since weaning, they showed similar weight gain and body composition. Interestingly, KO mice showed lower fat accumulation in visceral adipose tissue and higher ectopic fat accumulation in liver and skeletal muscle. When fed with a high-fat diet for 80 days, since weaning, KO mice showed enhanced energy efficiency and an increased lipid gain (thus leading to a change in body composition between the two genotypes). We conclude that UCP3 plays a role in energy and lipid homeostasis and in preserving lean tissues by lipotoxicity, in mice that were housed at thermoneutrality.

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

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

The role of gut in type 2 diabetes mellitus during whole body gamma irradiation in high-fat diet Wistar rats

PURPOSE

The effects of a low rate (100 mGy/min) fractionated whole body gamma irradiation (FWBGI) at different doses were assessed using a real-time PCR technique on the expression of some target genes implicated in the development of type 2 diabetes mellitus in high-fat diet (HFD) Wistar rats.

METHOD

HFD Wistar rats were exposed to different doses (12, 24 and 48 Gy) divided into 24 fractions (three times a week for two months), thus, the daily doses were 0.5, 1, 2 Gy, respectively. Total RNA was extracted and the expression of target genes was measured in the four intestinal segments (duodenum, jejunum, ileum and colon).

RESULTS

The pre-diabetic state already induced by HFD was found to be improved by irradiation exposure. This irradiation effect occurs mainly via altered anti-diabetic gene expressions (mRNA and protein levels) of the incretin glucagon-like peptide-1 (GLP-1) overall bowel segments except the colon which has its own specific response to irradiation exposure by the induction of the insulin receptor substrate 4 (IRS-4) and the uncoupling protein 3 (UCP3).

CONCLUSIONS

Results could be of great importance suggesting for the first time, a protective role for FWBGI on HFD animal models by increasing GLP-1 and UCP3 levels.

Insulin restores UCP3 activity and decreases energy surfeit to alleviate lipotoxicity in skeletal muscle

An early insulin regimen ameliorates glucotoxicity but also lipotoxicity in type 2 diabetes; however, the underlying mechanism remains elusive. In the present study, we investigated the role of mitochondria in lipid regulation following early insulin administration in insulin-resistant skeletal muscle cells. Male C57BL/6 mice, fed a high-fat diet (HFD) for 8 weeks, were treated with insulin for 3 weeks, and L6 myotubes cultured with palmitate (PA) for 24 h were incubated with insulin for another 12 h. The results showed that insulin facilitated systemic glucose disposal and attenuated muscular triglyceride accumulation in vivo. Recovery of AMP-activated protein kinase (AMPK) phosphorylation, inhibition of sterol-regulated element binding protein-1c (SREBP-1c) and increased carnitine palmitoyltransferase‑1B (CPT1B) expression were observed after insulin administration. Moreover, increased ATP concentration and cellular energy charge elicited by over-nutrition were suppressed by insulin. Despite maintaining respiratory complex activities, insulin restored muscular uncoupling protein 3 (UCP3) protein expression in vitro and in vivo. By contrast, knockdown of UCP3 abrogated insulin-induced restoration of AMPK phosphorylation in vitro. Importantly, the PA-induced decrease in UCP3 was blocked by the proteasome inhibitor MG132, and insulin reduced UCP3 ubiquitination, thereby prohibiting its degradation. Our findings, focusing on energy balance, provide a mechanistic understanding of the promising effect of early insulin initiation on lipotoxicity. Insulin, by recovering UCP3 activity, alleviated energy surfeit and potentiated AMPK-mediated lipid homeostasis in skeletal muscle cells following exposure to PA and in gastrocnemius of mice fed HFD.

Single nucleotide polymorphisms linked to mitochondrial uncoupling protein genes UCP2 and UCP3 affect mitochondrial metabolism and healthy aging in female nonagenarians

Energy expenditure decreases with age, but in the oldest-old, energy demand for maintenance of body functions increases with declining health. Uncoupling proteins have profound impact on mitochondrial metabolic processes; therefore, we focused attention on mitochondrial uncoupling protein genes. Alongside resting metabolic rate (RMR), two SNPs in the promoter region of UCP2 were associated with healthy aging. These SNPs mark potential binding sites for several transcription factors; thus, they may affect expression of the gene. A third SNP in the 3'-UTR of UCP3 interacted with RMR. This UCP3 SNP is known to impact UCP3 expression in tissue culture cells, and it has been associated with body weight and mitochondrial energy metabolism. The significant main effects of the UCP2 SNPs and the interaction effect of the UCP3 SNP were also observed after controlling for fat-free mass (FFM) and physical-activity related energy consumption. The association of UCP2/3 with healthy aging was not found in males. Thus, our study provides evidence that the genetic risk factors for healthy aging differ in males and females, as expected from the differences in the phenotypes associated with healthy aging between the two sexes. It also has implications for how mitochondrial function changes during aging.

Chronic Hyperinsulinemia Causes Selective Insulin Resistance and Down-regulates Uncoupling Protein 3 (UCP3) through the Activation of Sterol Regulatory Element-binding Protein (SREBP)-1 Transcription Factor in the Mouse Heart

The risk for heart failure and death after myocardial infarction is abnormally high in diabetic subjects. We and others have shown previously that mitochondrial uncoupling protein 3 (UCP3) improves functional recovery of the rodent heart during reperfusion. Here, we demonstrate that pharmacological induction of hyperinsulinemia in mice down-regulates myocardial UCP3. Decreased UCP3 expression was linked to the development of selective insulin resistance in the heart, characterized by decreased basal activity of Akt but preserved activity of the p44/42 mitogen-activated protein kinase, and overactivation of the sterol regulatory element-binding protein (SREBP)-1-mediated lipogenic program. In cultured myocytes, insulin treatment and SREBP-1 overexpression decreased, whereas SREBP-1 interference increased, peroxisome proliferator-activated receptor-stimulated expression of UCP3. Promoter deletion and site-directed mutagenesis identified three functional sterol regulatory elements in the vicinity of a known complex intronic enhancer. Increased binding of SREBP-1 to this DNA region was confirmed in the heart of hyperinsulinemic mice. In conclusion, we describe a hitherto unknown regulatory mechanism by which insulin inhibits cardiac UCP3 expression through activation of the lipogenic factor SREBP-1. Sustained down-regulation of cardiac UCP3 by hyperinsulinemia may partly explain the poor prognosis of type 2 diabetic patients after myocardial infarction.

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.

Lack of UCP3 does not affect skeletal muscle mitochondrial function under lipid-challenged conditions, but leads to sudden cardiac death

UCP3's exact physiological function in lipid handling in skeletal and cardiac muscle remains unknown. Interestingly, etomoxir, a fat oxidation inhibitor and strong inducer of UCP3, is proposed for treating both diabetes and heart failure. We hypothesize that the upregulation of UCP3 upon etomoxir serves to protect mitochondria against lipotoxicity. To evaluate UCP3's role in skeletal muscle (skm) and heart under lipid-challenged conditions, the effect of UCP3 ablation was examined in a state of dysbalance between fat availability and oxidative capacity. Wild type (WT) and UCP3(-/-) mice were subjected to high-fat feeding for 14 days. From day 6 onwards, they were given either saline or etomoxir. Etomoxir treatment induced an increase in markers of lipotoxicity in skm compared to saline. This increase upon etomoxir was similar for both, WT and UCP3(-/-) mice, suggesting that UCP3 does not play a role in protection against lipotoxicity. Interestingly, we observed 25 % mortality in UCP3(-/-)s upon etomoxir administration vs. 11 % in WTs. This increased mortality in UCP3(-/-) compared to WT mice could not be explained by differences in cardiac lipotoxicity, apoptosis, fibrosis (histology, immunohistochemistry), oxidative capacity (respirometry) or function (echocardiography). Electrophysiology demonstrated, however, prolonged QRS and QTc intervals and greater susceptibility to ventricular tachycardia upon programmed electrical stimulation in etomoxir-treated UCP3(-/-)s versus WTs. Isoproterenol administration after pacing resulted in 75 % mortality in UCP3(-/-)s vs. 14 % in WTs. Our results argue against a protective role for UCP3 on skm metabolism under lipid overload, but suggest UCP3 to be crucial in prevention of arrhythmias upon lipid-challenged conditions.

Lipid Replacement Therapy: a natural medicine approach to replacing damaged lipids in cellular membranes and organelles and restoring function

Lipid Replacement Therapy, the use of functional oral supplements containing cell membrane phospholipids and antioxidants, has been used to replace damaged, usually oxidized, membrane glycerophospholipids that accumulate during aging and in various clinical conditions in order to restore cellular function. This approach differs from other dietary and intravenous phospholipid interventions in the composition of phospholipids and their defense against oxidation during storage, ingestion, digestion and uptake as well as the use of protective molecules that noncovalently complex with phospholipid micelles and prevent their enzymatic and bile disruption. Once the phospholipids have been taken in by transport processes, they are protected by several natural mechanisms involving lipid receptors, transport and carrier molecules and circulating cells and lipoproteins until their delivery to tissues and cells where they can again be transferred to intracellular membranes by specific and nonspecific transport systems. Once delivered to membrane sites, they naturally replace and stimulate removal of damaged membrane lipids. Various chronic clinical conditions are characterized by membrane damage, mainly oxidative but also enzymatic, resulting in loss of cellular function. This is readily apparent in mitochondrial inner membranes where oxidative damage to phospholipids like cardiolipin and other molecules results in loss of trans-membrane potential, electron transport function and generation of high-energy molecules. Recent clinical trials have shown the benefits of Lipid Replacement Therapy in restoring mitochondrial function and reducing fatigue in aged subjects and patients with a variety of clinical diagnoses that are characterized by loss of mitochondrial function and include fatigue as a major symptom. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Decreased long-chain fatty acid oxidation impairs postischemic recovery of the insulin-resistant rat heart

Diabetic patients with acute myocardial infarction are more likely to die than nondiabetic patients. In the present study we examined the effect of insulin resistance on myocardial ischemia tolerance. Hearts of rats, rendered insulin resistant by high-sucrose feeding, were subjected to ischemia/reperfusion ex vivo. Cardiac power of control hearts from chow-fed rats recovered to 93%, while insulin-resistant hearts recovered only to 80% (P<0.001 vs. control). Unexpectedly, impaired contractile recovery did not result from an impairment of glucose oxidation (576±36 vs. 593±42 nmol/min/g dry weight; not significant), but from a failure to increase and to sustain oxidation of the long-chain fatty acid oleate on reperfusion (1878±56 vs. 2070±67 nmol/min/g dry weight; P<0.05). This phenomenon was due to a reduced ability to transport oleate into mitochondria and associated with a 38-58% decrease in the mitochondrial uncoupling protein 3 (UCP3) levels. Contractile function was rescued by replacing oleate with a medium-chain fatty acid or by restoring UCP3 levels with 24 h of food withdrawal. Lastly, the knockdown of UCP3 in rat L6 myocytes also decreased oleate oxidation by 13-18% following ischemia. Together the results expose UCP3 as a critical regulator of long-chain fatty acid oxidation in the stressed heart postischemia and identify octanoate as an intervention by which myocardial metabolism can be manipulated to improve function of the insulin-resistant heart.

Simvastatin effects on skeletal muscle: relation to decreased mitochondrial function and glucose intolerance

OBJECTIVES

Glucose tolerance and skeletal muscle coenzyme Q(10) (Q(10)) content, mitochondrial density, and mitochondrial oxidative phosphorylation (OXPHOS) capacity were measured in simvastatin-treated patients (n = 10) and in well-matched control subjects (n = 9).

BACKGROUND

A prevalent side effect of statin therapy is muscle pain, and yet the basic mechanism behind it remains unknown. We hypothesize that a statin-induced reduction in muscle Q(10) may attenuate mitochondrial OXPHOS capacity, which may be an underlying mechanism.

METHODS

Plasma glucose and insulin concentrations were measured during an oral glucose tolerance test. Mitochondrial OXPHOS capacity was measured in permeabilized muscle fibers by high-resolution respirometry in a cross-sectional design. Mitochondrial content (estimated by citrate synthase [CS] activity, cardiolipin content, and voltage-dependent anion channel [VDAC] content) as well as Q(10) content was determined.

RESULTS

Simvastatin-treated patients had an impaired glucose tolerance and displayed a decreased insulin sensitivity index. Regarding mitochondrial studies, Q(10) content was reduced (p = 0.05), whereas mitochondrial content was similar between the groups. OXPHOS capacity was comparable between groups when complex I- and complex II-linked substrates were used alone, but when complex I + II-linked substrates were used (eliciting convergent electron input into the Q intersection [maximal ex vivo OXPHOS capacity]), a decreased (p < 0.01) capacity was observed in the patients compared with the control subjects.

CONCLUSIONS

These simvastatin-treated patients were glucose intolerant. A decreased Q(10) content was accompanied by a decreased maximal OXPHOS capacity in the simvastatin-treated patients. It is plausible that this finding partly explains the muscle pain and exercise intolerance that many patients experience with their statin treatment.

Insulin resistance improves metabolic and contractile efficiency in stressed rat heart

Insulin resistance is a prominent feature in heart failure, while hyperglycemia impairs cardiac contraction. We propose that decreased insulin-mediated glucose uptake by the heart preserves cardiac function in response to metabolic and hemodynamic stress. To test this hypothesis, we fed rats a high-sucrose diet (HSD). Energy substrate metabolism and cardiac work were determined ex vivo in a sequential protocol simulating metabolic and hemodynamic stress. Compared to chow-fed, control rats, HSD impaired myocardial insulin responsiveness and induced profound metabolic changes in the heart, characterized by reduced rates of glucose uptake (7.91 ± 0.30 vs. 10.73 ± 0.67 μmol/min/g dry weight; P<0.001) but increased rates of glucose oxidation (2.38 ± 0.17 vs. 1.50 ± 0.15 μmol/min/g dry weight; P<0.001) and oleate oxidation (2.29 ± 0.11 vs. 1.96 ± 0.12 μmol/min/g dry weight; P<0.05). Tight coupling of glucose uptake and oxidation and improved cardiac efficiency were associated with a reduction in glucose 6-phosphate and oleoyl-CoA levels, as well as a reduction in the content of uncoupling protein 3. Our results suggest that insulin resistance lessens fuel toxicity in the stressed heart. This calls for a new exploration of the mechanisms regulating substrate uptake and oxidation in the insulin-resistant heart.

Pathological aspects of lipid peroxidation

Lipid peroxidation (LPO) product accumulation in human tissues is a major cause of tissular and cellular dysfunction that plays a major role in ageing and most age-related and oxidative stress-related diseases. The current evidence for the implication of LPO in pathological processes is discussed in this review. New data and literature review are provided evaluating the role of LPO in the pathophysiology of ageing and classically oxidative stress-linked diseases, such as neurodegenerative diseases, diabetes and atherosclerosis (the main cause of cardiovascular complications). Striking evidences implicating LPO in foetal vascular dysfunction occurring in pre-eclampsia, in renal and liver diseases, as well as their role as cause and consequence to cancer development are addressed.

Adaptations in mitochondrial function parallel, but fail to rescue, the transition to severe hyperglycemia and hyperinsulinemia: a study in Zucker diabetic fatty rats

Cross-sectional human studies have associated mitochondrial dysfunction to type 2 diabetes. We chose Zucker diabetic fatty (ZDF) rats as a model of progressive insulin resistance to examine whether intrinsic mitochondrial defects are required for development of type 2 diabetes. Muscle mitochondrial function was examined in 6-, 12-, and 19-week-old ZDF (fa/fa) and fa/+ control rats (n = 8-10 per group) using respirometry with pyruvate, glutamate, and palmitoyl-CoA as substrates. Six-week-old normoglycemic-hyperinsulinemic fa/fa rats had reduced mitochondrial fat oxidative capacity. Adenosine diphosphate (ADP)-driven state 3 and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP)-stimulated state uncoupled (state u) respiration on palmitoyl-CoA were lower compared to controls (62.3 +/- 9.5 vs. 119.1 +/- 13.8 and 87.8 +/- 13.3 vs. 141.9 +/- 14.3 nmol O(2)/mg/min.). Pyruvate oxidation in 6-week-old fa/fa rats was similar to controls. Remarkably, reduced fat oxidative capacity in 6-week-old fa/fa rats was compensated for by an adaptive increase in intrinsic mitochondrial function at week 12, which could not be maintained toward week 19 (140.9 +/- 11.2 and 57.7 +/- 9.8 nmol O(2)/mg/min, weeks 12 and 19, respectively), whereas hyperglycemia had developed (13.5 +/- 0.6 and 16.1 +/- 0.3 mmol/l, weeks 12 and 19, respectively). This mitochondrial adaptation failed to rescue the progressive development of insulin resistance in fa/fa rats. The transition of prediabetes state toward advanced hyperglycemia and hyperinsulinemia was accompanied by a blunted increase in uncoupling protein-3 (UCP3). Thus, in ZDF rats insulin resistance develops progressively in the absence of mitochondrial dysfunction. In fact, improved mitochondrial capacity in hyperinsulinemic hyperglycemic rats does not rescue the progression toward advanced stages of insulin resistance.

Short report: Tissue-specific expression profiles of the uncoupling protein family in normal control mice and genetically ob/ob mice

Uncoupling proteins (UCPs) located in the inner mitochondrial membrane are involved in the regulation of energy balance. Thus far, 5 UCP isoforms have been identified, but controversies exist in the research focused on the function of the UCPs (except UCP1) in the pathogenesis of obesity. Because of the known cross-reactivity of the antibodies presently available for the detection of UCP proteins, this study systematically analyzed the differential tissue expression profiles of the 5 UCP isoforms in lean control mice and ob/ob mice by using real-time polymerase chain reaction (PCR) analysis. The results show that the tissue-specific expression patterns of individual isoforms in normal and ob/ob mice are considerably different; this will provide new insights into the functions of UCPs in the pathogenesis of genetic obesity.

Mitochondrial dysfunction in diabetes: from molecular mechanisms to functional significance and therapeutic opportunities

Given their essential function in aerobic metabolism, mitochondria are intuitively of interest in regard to the pathophysiology of diabetes. Qualitative, quantitative, and functional perturbations in mitochondria have been identified and affect the cause and complications of diabetes. Moreover, as a consequence of fuel oxidation, mitochondria generate considerable reactive oxygen species (ROS). Evidence is accumulating that these radicals per se are important in the pathophysiology of diabetes and its complications. In this review, we first present basic concepts underlying mitochondrial physiology. We then address mitochondrial function and ROS as related to diabetes. We consider different forms of diabetes and address both insulin secretion and insulin sensitivity. We also address the role of mitochondrial uncoupling and coenzyme Q. Finally, we address the potential for targeting mitochondria in the therapy of diabetes.

Restoration of muscle mitochondrial function and metabolic flexibility in type 2 diabetes by exercise training is paralleled by increased myocellular fat storage and improved insulin sensitivity

OBJECTIVE

Mitochondrial dysfunction and fat accumulation in skeletal muscle (increased intramyocellular lipid [IMCL]) have been linked to development of type 2 diabetes. We examined whether exercise training could restore mitochondrial function and insulin sensitivity in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

Eighteen male type 2 diabetic and 20 healthy male control subjects of comparable body weight, BMI, age, and VO2max participated in a 12-week combined progressive training program (three times per week and 45 min per session). In vivo mitochondrial function (assessed via magnetic resonance spectroscopy), insulin sensitivity (clamp), metabolic flexibility (indirect calorimetry), and IMCL content (histochemically) were measured before and after training.

RESULTS

Mitochondrial function was lower in type 2 diabetic compared with control subjects (P = 0.03), improved by training in control subjects (28% increase; P = 0.02), and restored to control values in type 2 diabetic subjects (48% increase; P < 0.01). Insulin sensitivity tended to improve in control subjects (delta Rd 8% increase; P = 0.08) and improved significantly in type 2 diabetic subjects (delta Rd 63% increase; P < 0.01). Suppression of insulin-stimulated endogenous glucose production improved in both groups (-64%; P < 0.01 in control subjects and -52% in diabetic subjects; P < 0.01). After training, metabolic flexibility in type 2 diabetic subjects was restored (delta respiratory exchange ratio 63% increase; P = 0.01) but was unchanged in control subjects (delta respiratory exchange ratio 7% increase; P = 0.22). Starting with comparable pretraining IMCL levels, training tended to increase IMCL content in type 2 diabetic subjects (27% increase; P = 0.10), especially in type 2 muscle fibers.

CONCLUSIONS

Exercise training restored in vivo mitochondrial function in type 2 diabetic subjects. Insulin-mediated glucose disposal and metabolic flexibility improved in type 2 diabetic subjects in the face of near-significantly increased IMCL content. This indicates that increased capacity to store IMCL and restoration of improved mitochondrial function contribute to improved muscle insulin sensitivity.

Compromised respiratory adaptation and thermoregulation in aging and age-related diseases

Mitochondrial dysfunction and reactive oxygen species (ROS) production are at the heart of the aging process and are thought to underpin age-related diseases. Mitochondria are not only the primary energy-generating system but also the dominant cellular source of metabolically derived ROS. Recent studies unravel the existence of mechanisms that serve to modulate the balance between energy metabolism and ROS production. Among these is the regulation of proton conductance across the inner mitochondrial membrane that affects the efficiency of respiration and heat production. The field of mitochondrial respiration research has provided important insight into the role of altered energy balance in obesity and diabetes. The notion that respiration and oxidative capacity are mechanistically linked is making significant headway into the field of aging and age-related diseases. Here we review the regulation of cellular energy and ROS balance in biological systems and survey some of the recent relevant studies that suggest that respiratory adaptation and thermodynamics are important in aging and age-related diseases.

Role of lipid-derived mediators in skeletal muscle insulin resistance

Imbalance between nutritional intake and energy expenditure has been described to culminate in obesity, which predisposes to insulin resistance and type 2 diabetes mellitus. In such states of energy oversupply, excess amounts of lipids are available in tissues and circulation. Over the past years, an increasingly important role in development of skeletal muscle (SkM) insulin resistance has been attributed to lipids and impaired fatty acid metabolism. In this review, we reflect the current state of knowledge about the effects of various lipid-derived mediators on SkM insulin sensitivity. Furthermore, potential mechanisms underlying the biogenesis of intramyocellular ectopic lipid stores are discussed. Previously, a pivotal role was attributed to mitochondrial dysfunction. However, results of recent studies have suggested an important role for exercise deficiency, accompanied by decreased expression levels of peroxisome proliferator-activated receptor-γ coactivator-1α and subsequent, incomplete β-oxidation. Additionally, we summarize the implications of increased levels of lipid-derived endocannabinoids (ECs) for metabolic control in peripheral tissue and highlight the benefits of targeting the EC system.

Increased susceptibility to oxidative damage in post-diabetic human myotubes

AIMS/HYPOTHESIS

Obesity is an important risk factor for the development of type 2 diabetes, but not all obese individuals develop this complication. The clinical signs of type 2 diabetes can often be reversed with weight loss; however, it is unknown whether the skeletal muscle oxidative stress associated with type 2 diabetes remains after weight loss. We hypothesised that chronic exposure to high glucose and insulin would re-elicit impaired metabolism in primary myotubes from patients with a history of type 2 diabetes.

METHODS

Obese participants with or without type 2 diabetes completed a standardised weight loss protocol, following which all participants were euglycaemic and had similar indices of insulin sensitivity. Satellite cells were isolated from muscle biopsies and differentiated under low or high glucose and insulin conditions (HGI).

RESULTS

Cells from participants with no history of type 2 diabetes showed robust increases in mitochondrial content, citrate synthase and cytochrome c oxidase activities when exposed to HGI. This increase in oxidative capacity was absent in cells from patients with a history of type 2 diabetes. High glucose and insulin caused increased oxidative damage in cells from the latter, despite higher superoxide dismutase expression. Cells from patients with a history of type 2 diabetes were unable to decrease mitochondrial membrane potential in response to HGI, potentially due to lower levels of uncoupling protein-3.

CONCLUSIONS/INTERPRETATION

This is the first report to note that primary myotubes from patients with a history of type 2 diabetes are unable to adapt to a hyperglycaemic-hyperinsulinaemic challenge. We have demonstrated that impaired mitochondrial biogenesis and an inability to manage oxidative stress define a muscle phenotype at risk of obesity-associated type 2 diabetes.

Mitochondrial dysfunction and lipotoxicity

Mitochondrial dysfunction in skeletal muscle has been suggested to underlie the development of insulin resistance and type 2 diabetes mellitus. Reduced mitochondrial capacity will contribute to the accumulation of lipid intermediates, desensitizing insulin signaling and leading to insulin resistance. Why mitochondrial function is reduced in the (pre-)diabetic state is, however, so far unknown. Although it is tempting to suggest that skeletal muscle insulin resistance may result from an inherited or acquired reduction in mitochondrial function in the pre-diabetic state, it cannot be excluded that mitochondrial dysfunction may in fact be the consequence of the insulin-resistant/diabetic state. Lipotoxicity, the deleterious effects of accumulating fatty acids in skeletal muscle cells, may lie at the basis of mitochondrial dysfunction: next to producing energy, mitochondria are also the major source of reactive oxygen species (ROS). Fatty acids accumulating in the vicinity of mitochondria are vulnerable to ROS-induced lipid peroxidation. Subsequently, these lipid peroxides could have lipotoxic effects on mtDNA, RNA and proteins of the mitochondrial machinery, leading to mitochondrial dysfunction. Indeed, increased lipid peroxidation has been reported in insulin resistant skeletal muscle and the mitochondrial uncoupling protein-3, which has been suggested to prevent lipid-induced mitochondrial damage, is reduced in subjects with an impaired glucose tolerance and in type 2 diabetic patients. These findings support the hypothesis that fat accumulation in skeletal muscle may precede the reduction in mitochondrial function that is observed in type 2 diabetes mellitus.

Transcription of the human uncoupling protein 3 gene is governed by a complex interplay between the promoter and intronic sequences

AIMS/HYPOTHESIS

Uncoupling protein (UCP) 3 is an inner mitochondrial membrane transporter mainly produced in skeletal muscle in humans. UCP3 plays a role in fatty acid metabolism and energy homeostasis and modulates insulin sensitivity. In humans, UCP3 content is higher in fast-twitch glycolytic muscle than in slow-twitch oxidative muscle and is dysregulated in type 2 diabetes. Here, we studied the molecular mechanisms determining human UCP3 levels in skeletal muscle and their regulation by fasting in transgenic mice.

METHODS

We produced a series of transgenic lines with constructs bearing different putative regulatory regions of the human UCP3 gene, including promoter and intron sequences. UCP3 mRNA and reporter gene expression and activity were measured in different skeletal muscles and tissues.

RESULTS

The profile of expression and the response to fasting and thyroid hormone of human UCP3 mRNA in transgenic mice with 16 kb of the human UCP3 gene were similar to that of the endogenous human gene. Various parts of the UCP3 promoter did not confer expression in transgenic lines. Inclusion of intron 1 resulted in an expression profile in skeletal muscle that was identical to that of human UCP3 mRNA. Further dissection of intron 1 revealed that distinct regions were involved in skeletal muscle expression, distribution among fibre types and response to fasting.

CONCLUSIONS/INTERPRETATION

The control of human UCP3 transcription in skeletal muscle is not solely conferred by the promoter, but depends on several cis-acting elements in intron 1, suggesting a complex interplay between the promoter and intronic sequences.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

The effect of UCP3 overexpression on mitochondrial ROS production in skeletal muscle of young versus aged mice

Uncoupling protein 3 (UCP3) is suggested to protect mitochondria against aging and lipid-induced damage, possibly via modulation of reactive oxygen species (ROS) production. Here we show that mice overexpressing UCP3 (UCP3Tg) have a blunted age-induced increase in ROS production, assessed by electron spin resonance spectroscopy, but only after addition of 4-hydroxynonenal (4-HNE). Mitochondrial function, assessed by respirometry, on glycolytic substrate was lower in UCP3Tg mice compared to wild types, whereas this tended to be higher on fatty acids. State 4o respiration was higher in UCP3Tg animals. To conclude, UCP3 overexpression leads to increased state 4o respiration and, in presence of 4-HNE, blunts the age-induced increase in ROS production.

Long-term high-fat feeding induces greater fat storage in mice lacking UCP3

Uncoupling protein-3 (UCP3) is a mitochondrial inner-membrane protein highly expressed in skeletal muscle. While UCP3's function is still unknown, it has been hypothesized to act as a fatty acid (FA) anion exporter, protecting mitochondria against lipid peroxidation and/or facilitating FA oxidation. The aim of this study was to determine the effects of long-term feeding of a 45% fat diet on whole body indicators of muscle metabolism in congenic C57BL/6 mice that were either lacking UCP3 (Ucp3(-/-)) or had a transgenically induced approximately twofold increase in UCP3 levels (UCP3tg). Mice were fed the high-fat (HF) diet for a period of either 4 or 8 mo immediately following weaning. After long-term HF feeding, UCP3tg mice weighed an average of 15% less than wild-type mice (P < 0.05) and were 20% less metabolically efficient than both wild-type and Ucp3(-/-) mice (P < 0.01). Additionally, wild-type mice had 21% lower, whereas UCP3tg mice had 36% lower, levels of adiposity compared with Ucp3(-/-) mice (P < 0.05 and P < 0.001, respectively), indicating a protective effect of UCP3 against fat gain. No differences in whole body oxygen consumption were detected following long-term HF feeding. Glucose and insulin tolerance tests revealed that both the UCP3tg and Ucp3(-/-) mice were more glucose tolerant and insulin sensitive compared with wild-type mice after short-term HF feeding, but this protection was not maintained in the long term. Findings indicate that UCP3 is involved in protection from fat gain induced by long-term HF feeding, but not in protection from insulin resistance.

Genetic variants in the UCP2-UCP3 gene cluster and risk of diabetes in the Women's Health Initiative Observational Study

OBJECTIVE

Mitochondrial uncoupling proteins (UCPs) are involved in body weight regulation and glucose homeostasis. Genetic variants in the UCP2-UCP3 gene cluster, located on chromosome 11q13, may play a significant role in the development of type 2 diabetes.

RESEARCH DESIGN AND METHODS

We conducted a comprehensive assessment of common single nucleotide polymorphisms (SNPs) at the 70-kb UCP2-UCP3 gene cluster in relation to type 2 diabetes risk in a prospective, case-control study nested in the Women's Health Initiative Observational Study, an ethnically diverse cohort of postmenopausal women including Caucasian, African, Hispanic, and Asian American subjects. We genotyped 14 tag SNPs in 1,584 incident type 2 diabetes case and 2,198 control subjects matched by age, ethnicity, clinical center, time of blood draw, and length of follow-up.

RESULTS

We identified a haplotype set (rs591758-rs668514- rs647126-rs1800006, spanning the UCP2-UCP3 intergenic and UCP3 regions) as significantly associated with greater type 2 diabetes risk (nominal P = 0.0011, permutation P = 0.046) in Caucasian women, especially among overweight Caucasians (BMI >25 kg/m(2)) (nominal P = 0.0006, permutation P = 0.032). Compared with the most common haplotype (h1010 as the referent), haplotype h0001 (19.5% in control subjects) had odds ratios of 2.0 (95% CI 1.13-3.37) in Caucasians and 3.8 (1.44-9.93) in Caucasian overweight women. Similar haplotype-type 2 diabetes association was also observed among Hispanic women who were overweight.

CONCLUSIONS

These findings suggest a role of UCP2-UCP3 gene cluster haplotypes in diabetes; in particular, the effects of the high-risk haplotypes were more apparent in overweight Caucasian women. These data warrant further confirmation in future prospective and experimental studies.

Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake

Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions. Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1-2 h, 75% of peak Vo(2)) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein]. Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g.kg body wt(-1) .h(-1)). Peak Vo(2) (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups. After training, pre-exercise glycogen content was higher in CHO (545 +/- 19 mmol/kg dry wt; P = 0.02), but not in F (434 +/- 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training. Thus short-term training elicits similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

UCP-2 and UCP-3 proteins are differentially regulated in pancreatic beta-cells

Background

Increased uncoupling protein-2 (UCP-2) expression has been associated with impaired insulin secretion, whereas UCP-3 protein levels are decreased in the skeleton muscle of type-2 diabetic subjects. In the present studies we hypothesize an opposing effect of glucose on the regulation of UCP-2 and UCP-3 in pancreatic islets.

Methodology

Dominant negative UCP-2 and wild type UCP-3 adenoviruses were generated, and insulin release by transduced human islets was measured. UCP-2 and UCP-3 mRNA levels were determined using quantitative PCR. UCP-2 and UCP-3 protein expression was investigated in human islets cultured in the presence of different glucose concentrations. Human pancreatic sections were analyzed for subcellular localization of UCP-3 using immunohistochemistry.

Principal Findings

Dominant negative UCP-2 expression in human islets increased insulin secretion compared to control islets (p<0.05). UCP-3 mRNA is expressed in human islets, but the relative abundance of UCP-2 mRNA was 8.1-fold higher (p<0.05). Immunohistochemical analysis confirmed co-localization of UCP-3 protein with mitochondria in human beta-cells. UCP-2 protein expression in human islets was increased ∼2-fold after high glucose exposure, whereas UCP-3 protein expression was decreased by ∼40% (p<0.05). UCP-3 overexpression improved glucose-stimulated insulin secretion.

Conclusions

UCP-2 and UCP-3 may have distinct roles in regulating beta-cell function. Increased expression of UCP-2 and decreased expression of UCP-3 in humans with chronic hyperglycemia may contribute to impaired glucose-stimulated insulin secretion. These data imply that mechanisms that suppress UCP-2 or mechanisms that increase UCP-3 expression and/or function are potential therapeutic targets to offset defects of insulin secretion in humans with type-2 diabetes.

Metabolic syndrome and mitochondrial function: molecular replacement and antioxidant supplements to prevent membrane peroxidation and restore mitochondrial function

Metabolic syndrome consists of a cluster of metabolic conditions, such as hypertriglyceridemia, hyper-low-density lipoproteins, hypo-high-density lipoproteins, insulin resistance, abnormal glucose tolerance and hypertension, that-in combination with genetic susceptibility and abdominal obesity-are risk factors for type 2 diabetes, vascular inflammation, atherosclerosis, and renal, liver and heart disease. One of the defects in metabolic syndrome and its associated diseases is excess cellular oxidative stress (mediated by reactive oxygen and nitrogen species, ROS/RNS) and oxidative damage to mitochondrial components, resulting in reduced efficiency of the electron transport chain. Recent evidence indicates that reduced mitochondrial function caused by ROS/RNS membrane oxidation is related to fatigue, a common complaint of MS patients. Lipid replacement therapy (LRT) administered as a nutritional supplement with antioxidants can prevent excess oxidative membrane damage, restore mitochondrial and other cellular membrane functions and reduce fatigue. Recent clinical trials have shown the benefit of LRT plus antioxidants in restoring mitochondrial electron transport function and reducing moderate to severe chronic fatigue. Thus LRT plus antioxidant supplements should be considered for metabolic syndrome patients who suffer to various degrees from fatigue.

Oxidative stress biology and cell injury during type 1 and type 2 diabetes mellitus

Diabetes mellitus (DM) affects approximately 170 million individuals worldwide and is expected to alter the lives of at least 366 million individuals within a future span of 25 years. Of even greater concern is the premise that these projections are underestimated since they assume obesity levels will remain constant. Type 1 insulin-dependent DM accounts for only 5-10 percent of all diabetics but represents a highly significant health concern, since this disorder begins early in life and leads to long-term complications. In contrast, Type 2 DM is recognized as the etiology of over 80 percent of all diabetics and is dramatically increasing in incidence as a result of changes in human behavior and increased body mass index. Yet, the pathological consequences of these disorders that involve the both the neuronal and vascular systems are intimately linked through the pathways that mediate oxidative stress. Here we highlight some of the relevant oxidative pathways that determine insulin resistance through reactive oxygen species, mitochondrial dysfunction, uncoupling proteins, and endoplasmic reticulum stress. These pathways are ultimately linked to protein kinase B (Akt) and the insulin signaling pathways that determine the initial onset of glucose intolerance and the subsequent course to apoptotic cell injury. Through the elucidation of these targets, improvement in current strategies as well as the development of future clinical applications can move forward for both the prevention and treatment of Type 1 and Type 2 DM.

The role of uncoupling protein 3 in fatty acid metabolism: protection against lipotoxicity?

The physiological function of the mitochondrial uncoupling protein (UCP), UCP3, is still under debate. There is, however, ample evidence to indicate that, in contrast to UCP1, the primary function of UCP3 is not the dissipation of energy. Rather, several lines of evidence suggest that UCP3 is associated with cellular fatty acid metabolism. The highest levels of expression of UCP3 have been found in type 2 glycolytic muscle fibres, and fasting and high-fat diets up regulate UCP3. This up-regulation is most pronounced in muscle with a low fat oxidative capacity. Acute exercise also up regulates UCP3, and this effect has been shown to be a result of the exercise-induced increase in plasma fatty acid levels. In contrast, regular physical activity, which increases fat oxidative capacity, reduces UCP3 content. Based on these data it has been postulated that UCP3 functions to export those fatty acids that cannot be oxidized from the mitochondrial matrix, in order to prevent fatty acid accumulation inside the matrix. Several experiments have been conducted to test this hypothesis. Blocking carnitine palmitoyltransferase 1, thereby reducing fat oxidative capacity, rapidly induces UCP3. High-fat diets, which increase the mitochondrial supply of fatty acids, also up regulate UCP. However, feeding a similar amount of medium-chain fatty acids, which can be oxidized inside the mitochondrial matrix and therefore does not need to be exported from the matrix, does not affect UCP3 protein levels. In addition, UCP3 is increased in patients with defective β-oxidation and is reduced after restoring oxidative capacity. In conclusion, it is suggested that UCP3 has an important physiological function in facilitating outward transport from the mitochondrial matrix of fatty acid anions that cannot be oxidized, thereby protecting against lipid-induced mitochondrial damage.

Mechanistic insights into diabetes mellitus and oxidative stress

Diabetes mellitus (DM) is a significant healthcare concern worldwide that affects more than 165 million individuals leading to cardiovascular disease, nephropathy, retinopathy, and widespread disease of both the peripheral and central nervous systems. The incidence of undiagnosed diabetes, impaired glucose tolerance, and impaired fasting glucose levels raises future concerns in regards to the financial and patient care resources that will be necessary to care for patients with DM. Interestingly, disease of the nervous system can become one of the most debilitating complications and affect sensitive cognitive regions of the brain, such as the hippocampus that modulates memory function, resulting in significant functional impairment and dementia. Oxidative stress forms the foundation for the induction of multiple cellular pathways that can ultimately lead to both the onset and subsequent complications of DM. In particular, novel pathways that involve metabotropic receptor signaling, protein-tyrosine phosphatases, Wnt proteins, Akt, GSK-3beta, and forkhead transcription factors may be responsible for the onset and progression of complications form DM. Further knowledge acquired in understanding the complexity of DM and its ability to impair cellular systems throughout the body will foster new strategies for the treatment of DM and its complications.

Expression of uncoupling protein 3 and GLUT4 gene in skeletal muscle of preterm newborns: possible control by AMP-activated protein kinase

We seek to understand the mechanism for the delayed postnatal switch between glycolytic and oxidative metabolism in preterm newborns. Our previous study [Brauner et al. (Pediatr Res 53: 691-697, 2003)] suggested impaired postnatal recruitment of the gene for mitochondrial uncoupling protein 3 (UCP3) by nutritional lipids in skeletal muscle of neonates delivered before approximately 26 wk of gestation. UCP3 is linked to lipid oxidation and may be involved in the defective development of energy metabolism in skeletal muscles of very preterm newborns. In extension of our previous study, autopsy samples of musculus quadriceps femoris from 40 mostly preterm neonates and 5 fetuses were used for quantification of transcripts for UCP3, GLUT4, and their transcriptional regulator, AMP-activated protein kinase (AMPK). The new analysis confirmed the defect in the recruitment of the UCP3 gene expression by lipids in very preterm neonates. It also suggested involvement of AMPK in the control of expression of both metabolic genes, UCP3 and GLUT4, in the skeletal muscle of the newborns. Experiments on adult C57BL/6J mice confirmed the relationships between the transcripts and supported the involvement of AMPK in the control of UCP3 gene expression.

Does coffee consumption reduce the risk of type 2 diabetes in individuals with impaired glucose?

OBJECTIVE

The purpose of this study was to investigate the association between coffee intake and incident diabetes based on an oral glucose tolerance test (OGTT) and examine coffee habits in those with impaired glucose separately from those with normal glucose at baseline.

RESEARCH DESIGN AND METHODS

In this prospective study, 910 adults aged >/=50 years without diabetes at baseline in 1984-1987 were followed to 1992-1996, an average of 8 years after assessment of coffee intake. Logistic regression models were adjusted for sex, age, physical activity, BMI, smoking, alcohol, hypertension, and baseline fasting plasma glucose.

RESULTS

Past and current coffee drinkers had a reduced risk of incident diabetes (odds ratio 0.38 [95% CI 0.17-0.87] and 0.36 [0.19-0.68], respectively) compared with those who never drank coffee. The 317 participants with baseline impaired glucose who were past or current coffee drinkers were also at reduced risk for incident diabetes (0.31 [0.11-0.87] and 0.36 [0.16-0.83], respectively).

CONCLUSIONS

This study confirms a striking protective effect of caffeinated coffee against incident diabetes and extends these findings to incident diabetes based on OGTT independent of multiple plausible confounders.

Low-intensity exercise increases skeletal muscle protein expression of PPARdelta and UCP3 in type 2 diabetic patients

BACKGROUND

Physical exercise provides health benefits for people with type 2 diabetes mellitus, partly by enhancing skeletal muscle insulin action. We tested the hypothesis that changes in expression of key genes in skeletal muscles relate to exercise-induced improvements in type 2 diabetic patients.

METHODS

We determined mRNA expression of 20 selected genes following a self-supervised program of walking (> 150 min per week) over a 4-month period.

RESULTS

This level of physical activity improved clinical parameters in approximately half the participants, as determined by reduced hypertension and enhanced insulin sensitivity (defined by reduced plasma-insulin levels and improved homeostasis model assessment (HOMA)). Skeletal muscle mRNA expression of Cbl-associated protein (CAP), diacylglycerol kinase (DGK)delta, uncoupling protein (UCP) 3, nuclear respiratory factor (NRF)-1, and peroxisome proliferator-activated receptor (PPAR)delta tended to increase in type 2 diabetic patients with an improved clinical profile. Skeletal muscle protein expression of PPARdelta and UCP3 was increased significantly after physical exercise in patients with an improved clinical profile, but were unchanged in patients who did not show exercise-mediated improvements in clinical parameters.

CONCLUSIONS

This study provides clinical evidence that improvements in insulin sensitivity can be achieved in type 2 diabetic patients after individually executed low-intensity exercise training. Moreover, the positive clinical response to exercise is correlated with changes in skeletal muscle proteins involved in the regulation of mitochondrial biogenesis and metabolism. These changes in skeletal muscle gene expression offer a possible molecular explanation for the improvements in clinical outcomes.

Peroxisome proliferator-activated receptor-gamma coactivator-1 and insulin resistance: acute effect of fatty acids

AIMS/HYPOTHESIS

Peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1 (PPARGC1), a coactivator regulating the transcription of genes involved in oxidative metabolism, is downregulated in patients with type 2 diabetes and in their first-degree relatives. Whether this downregulation is a cause or effect of early aberrations in the development of insulin resistance, such as disturbances in fat metabolism, is unknown. We examined whether lipid-induced insulin resistance was associated with downregulation of expression of skeletal muscle genes involved in oxidative metabolism and mitochondrial biogenesis in humans.

MATERIALS AND METHODS

Nine healthy lean male subjects underwent a 6-h hyperinsulinaemic-euglycaemic clamp with simultaneous infusion of either a lipid emulsion or glycerol as a control. Blood was sampled at regular time points and muscle biopsies were taken before and after every test. Intramuscular triacylglycerol (IMTG) content was determined by Oil Red O staining and gene expression was measured by quantitative PCR.

RESULTS

Lipid infusion resulted in a approximately 2.7-fold increase in plasma NEFA levels and a 31+/-6% decrease in insulin sensitivity (p=0.001). The infusion of lipids resulted in a approximately 1.6-fold increase in IMTG (p=0.02), whereas during the clamp with glycerol infusion IMTG tended to decrease to approximately 53% of preinfusion levels (p=0.065). Lipid infusion decreased PPARGC1A, PPARGC1B and PPARA expression to approximately 61, 77 and approximately 52% of basal values respectively, whereas expression of uncoupling protein 3 was upregulated 1.8-fold (all p<0.05).

CONCLUSIONS/INTERPRETATION

Acute elevation of plasma NEFA levels, leading to muscular fat accumulation and insulin resistance, downregulates PPARGC1A, PPARGC1B and PPARA expression, suggesting that the decrease in PPARGC1 expression observed in the (pre)diabetic state may be the result, rather than the cause of lipid-induced insulin resistance.

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

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