Calorie restriction in mice overexpressing UCP3: evidence that prior mitochondrial uncoupling alters response

2,4-Dinitrophenol is toxic on a low caloric diet but extends lifespan of Drosophila melanogaster on nutrient-rich diets without an impact on metabolism

Uncouplers of mitochondrial electron transport chain, such as 2,4-dinitrophehol (DNP), can mimic calorie restriction by decreasing efficiency of adenosine triphosphate (ATP) synthesis. However, DNP is also a toxic substance, whose overdosage can be lethal. In the fruit fly, Drosophila melanogaster model, we have found that DNP in concentrations of 0.05-0.2 g/L, led to a drastic decrease in fruit fly survival on a low caloric diet (1% sucrose and 1% yeast; 1S-1Y). On the 5S-5Y diet, DNP decreased lifespan of flies reared only in concentration 0.2 g/L, whilst on the diet 15S-15Y DNP either did not significantly shortened fruit fly lifespan or extended it. The lifespan extension on the high caloric 15S-15Y diet with DNP was accompanied by lower activity of lactate dehydrogenase and a decrease in activities of mitochondrial respiratory chain complexes I, II, and V, determined by blue native electrophoresis followed by in-gel activity assays. The exposure to DNP also did not affect key glycolytic enzymes, antioxidant and related enzymes, and markers of oxidative stress, such as aconitase activity and amount protein carbonyls. Consumption of DNP-supplemented diet did not affect flies' resistance to heat stress, though made male flies slightly more resistant to starvation compared with males reared on the control food. We also did not observe substantial changes in the contents of metabolic stores, triacylglycerols and glycogen, in the DNP-treated flies. All this suggest that a nutrient-rich diets provide effective protection against DNP, providing a mild uncoupling of the respiratory chain that allows lifespan extension without considerable changes in metabolism.

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.

Altered mitochondrial metabolism in the insulin-resistant heart

Obesity-induced insulin resistance and type 2 diabetes mellitus can ultimately result in various complications, including diabetic cardiomyopathy. In this case, cardiac dysfunction is characterized by metabolic disturbances such as impaired glucose oxidation and an increased reliance on fatty acid (FA) oxidation. Mitochondrial dysfunction has often been associated with the altered metabolic function in the diabetic heart, and may result from FA-induced lipotoxicity and uncoupling of oxidative phosphorylation. In this review, we address the metabolic changes in the diabetic heart, focusing on the loss of metabolic flexibility and cardiac mitochondrial function. We consider the alterations observed in mitochondrial substrate utilization, bioenergetics and dynamics, and highlight new areas of research which may improve our understanding of the cause and effect of cardiac mitochondrial dysfunction in diabetes. Finally, we explore how lifestyle (nutrition and exercise) and pharmacological interventions can prevent and treat metabolic and mitochondrial dysfunction in diabetes.

Important Trends in UCP3 Investigation

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

Skeletal muscle mitoflashes, pH, and the role of uncoupling protein-3

Mitochondrial reactive oxygen species (ROS) are important cellular signaling molecules, but can cause oxidative damage if not kept within tolerable limits. An important proximal form of ROS in mitochondria is superoxide. Its production is thought to occur in regulated stochastic bursts, but current methods using mitochondrial targeted cpYFP to assess superoxide flashes are confounded by changes in pH. Accordingly, these flashes are generally referred to as 'mitoflashes'. Here we provide regulatory insights into mitoflashes and pH fluctuations in skeletal muscle, and the role of uncoupling protein-3 (UCP3). Using quantitative confocal microscopy of mitoflashes in intact muscle fibers, we show that the mitoflash magnitude significantly correlates with the degree of mitochondrial inner membrane depolarization and ablation of UCP3 did not affect this correlation. We assessed the effects of the absence of UCP3 on mitoflash activity in intact skeletal muscle fibers, and found no effects on mitoflash frequency, amplitude or duration, with a slight reduction in the average size of mitoflashes. We further investigated the regulation of pH flashes (pHlashes, presumably a component of mitoflash) by UCP3 using mitochondrial targeted SypHer (mt-SypHer) in skeletal muscle fibers. The frequency of pHlashes was significantly reduced in the absence of UCP3, without changes in other flash properties. ROS scavenger, tiron, did not alter pHlash frequency in either WT or UCP3KO mice. High resolution respirometry revealed that in the absence of UCP3 there is impaired proton leak and Complex I-driven respiration and maximal coupled respiration. Total cellular production of hydrogen peroxide (H₂O₂) as detected by Amplex-UltraRed was unaffected. Altogether, we demonstrate a correlation between mitochondrial membrane potential and mitoflash magnitude in skeletal muscle fibers that is independent of UCP3, and a role for UCP3 in the control of pHlash frequency and of proton leak- and Complex I coupled-respiration in skeletal muscle fibers. The differential regulation of mitoflashes and pHlashes by UCP3 and tiron also indicate that the two events, though may be related, are not identical events.

Combined effects of resistance training and calorie restriction on mitochondrial fusion and fission proteins in rat skeletal muscle

Recent studies have demonstrated that resistance exercise leads not only to muscle hypertrophy, but it also improves mitochondrial function. Because calorie restriction (CR) has been suggested as a way to induce mitochondrial biogenesis, we examined the effects of resistance training with or without CR on muscle weight and key mitochondrial parameters in rat skeletal muscle. Four weeks of resistance training (thrice/wk) resulted in increased gastrocnemius muscle weight by 14% in rats fed ad libitum (AL). The degree of muscle-weight increase via resistance training was lower in rats with CR (7.4%). CR showed no effect on phosphorylation of mammalian target of rapamycin (mTOR) signaling proteins rpS6 and ULK1. Our results revealed that CR resulted in elevated levels of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) protein, a known master regulator of mitochondrial biogenesis. Resistance training alone also resulted in increased PGC-1α levels in skeletal muscle. The magnitude of the increase in PGC-1α was similar in rats in both the CR and AL groups. Moreover, we found that resistance training with CR resulted in elevated levels of proteins involved in mitochondrial fusion (Opa1 and Mfn1), and oxidative phosphorylation, whereas there was no effect of CR on the fission-regulatory proteins Fis1 and Drp1. These results indicate that CR attenuates resistance training-induced muscle hypertrophy, and that it may enhance mitochondrial adaptations in skeletal muscle.

Low birth weight is associated with adiposity, impaired skeletal muscle energetics and weight loss resistance in mice

BACKGROUND

In utero undernutrition is associated with obesity and insulin resistance, although its effects on skeletal muscle remain poorly defined. Therefore, in the current study we explored the effects of in utero food restriction on muscle energy metabolism in mice.

METHODS

We used an experimental mouse model system of maternal undernutrition during late pregnancy to examine offspring from undernourished dams (U) and control offspring from ad libitum-fed dams (C). Weight loss of 10-week-old offspring on a 4-week 40% calorie-restricted diet was also followed. Experimental approaches included bioenergetic analyses in isolated mitochondria, intact (permeabilized) muscle and at the whole body level.

RESULTS

U have increased adiposity and decreased glucose tolerance compared to C. Strikingly, when U are put on a 40% calorie-restricted diet they lose half as much weight as calorie-restricted controls. Mitochondria from muscle overall from U had decreased coupled (state 3) and uncoupled (state 4) respiration and increased maximal respiration compared to C. Mitochondrial yield was lower in U than C. In permeabilized fiber preparations from mixed fiber-type muscle, U had decreased mitochondrial content and decreased adenylate-free leak respiration, fatty acid oxidative capacity and state 3 respiratory capacity through complex I. Fiber maximal oxidative phosphorylation capacity did not differ between U and C but was decreased with calorie restriction.

CONCLUSIONS

Our results reveal that in utero undernutrition alters metabolic physiology through a profound effect on skeletal muscle energetics and blunts response to a hypocaloric diet in adulthood. We propose that mitochondrial dysfunction links undernutrition in utero with metabolic disease in adulthood.

Mitochondrial uncoupling in skeletal muscle by UCP1 augments energy expenditure and glutathione content while mitigating ROS production

Enhancement of proton leaks in muscle tissue represents a potential target for obesity treatment. In this study, we examined the bioenergetic and physiological implications of increased proton leak in skeletal muscle. To induce muscle-specific increases in proton leak, we used mice that selectively express uncoupling protein-1 (UCP1) in skeletal muscle tissue. UCP1 expression in muscle mitochondria was ∼13% of levels in brown adipose tissue (BAT) mitochondria and caused increased GDP-sensitive proton leak. This was associated with an increase in whole body energy expenditure and a decrease in white adipose tissue content. Muscle UCP1 activity had divergent effects on mitochondrial ROS emission and glutathione levels compared with BAT. UCP1 in muscle increased total mitochondrial glutathione levels ∼7.6 fold. Intriguingly, unlike in BAT mitochondria, leak through UCP1 in muscle controlled mitochondrial ROS emission. Inhibition of UCP1 with GDP in muscle mitochondria increased ROS emission ∼2.8-fold relative to WT muscle mitochondria. GDP had no impact on ROS emission from BAT mitochondria from either genotype. Collectively, these findings indicate that selective induction of UCP1-mediated proton leak in muscle can increase whole body energy expenditure and decrease adiposity. Moreover, ectopic UCP1 expression in skeletal muscle can control mitochondrial ROS emission, while it apparently plays no such role in its endogenous tissue, brown fat.

Metabolic adaptations to short-term every-other-day feeding in long-living Ames dwarf mice

Restrictive dietary interventions exert significant beneficial physiological effects in terms of aging and age-related disease in many species. Every other day feeding (EOD) has been utilized in aging research and shown to mimic many of the positive outcomes consequent with dietary restriction. This study employed long living Ames dwarf mice subjected to EOD feeding to examine the adaptations of the oxidative phosphorylation and antioxidative defense systems to this feeding regimen. Every other day feeding lowered liver glutathione (GSH) concentrations in dwarf and wild type (WT) mice but altered GSH biosynthesis and degradation in WT mice only. The activities of liver OXPHOS enzymes and corresponding proteins declined in WT mice fed EOD while in dwarf animals, the levels were maintained or increased with this feeding regimen. Antioxidative enzymes were differentially affected depending on the tissue, whether proliferative or post-mitotic. Gene expression of components of liver methionine metabolism remained elevated in dwarf mice when compared to WT mice as previously reported however, enzymes responsible for recycling homocysteine to methionine were elevated in both genotypes in response to EOD feeding. The data suggest that the differences in anabolic hormone levels likely affect the sensitivity of long living and control mice to this dietary regimen, with dwarf mice exhibiting fewer responses in comparison to WT mice. These results provide further evidence that dwarf mice may be better protected against metabolic and environmental perturbations which may in turn, contribute to their extended longevity.

Muscle uncoupling protein 3 overexpression mimics endurance training and reduces circulating biomarkers of incomplete β-oxidation

Exercise substantially improves metabolic health, making the elicited mechanisms important targets for novel therapeutic strategies. Uncoupling protein 3 (UCP3) is a mitochondrial inner membrane protein highly selectively expressed in skeletal muscle. Here we report that moderate UCP3 overexpression (roughly 3-fold) in muscles of UCP3 transgenic (UCP3 Tg) mice acts as an exercise mimetic in many ways. UCP3 overexpression increased spontaneous activity (∼40%) and energy expenditure (∼5-10%) and decreased oxidative stress (∼15-20%), similar to exercise training in wild-type (WT) mice. The increase in complete fatty acid oxidation (FAO; ∼30% for WT and ∼70% for UCP3 Tg) and energy expenditure (∼8% for WT and 15% for UCP3 Tg) in response to endurance training was higher in UCP3 Tg than in WT mice, showing an additive effect of UCP3 and endurance training on these two parameters. Moreover, increases in circulating short-chain acylcarnitines in response to acute exercise in untrained WT mice were absent with training or in UCP3 Tg mice. UCP3 overexpression had the same effect as training in decreasing long-chain acylcarnitines. Outcomes coincided with a reduction in muscle carnitine acetyltransferase activity that catalyzes the formation of acylcarnitines. Overall, results are consistent with the conclusions that circulating acylcarnitines could be used as a marker of incomplete muscle FAO and that UCP3 is a potential target for the treatment of prevalent metabolic diseases in which muscle FAO is affected.

Reduced mitochondrial ROS, enhanced antioxidant defense, and distinct age-related changes in oxidative damage in muscles of long-lived Peromyscus leucopus

Comparing biological processes in closely related species with divergent life spans is a powerful approach to study mechanisms of aging. The oxidative stress hypothesis of aging predicts that longer-lived species would have lower reactive oxygen species (ROS) generation and/or an increased antioxidant capacity, resulting in reduced oxidative damage with age than in shorter-lived species. In this study, we measured ROS generation in the young adult animals of the long-lived white-footed mouse, Peromyscus leucopus (maximal life span potential, MLSP = 8 yr) and the common laboratory mouse, Mus musculus (C57BL/6J strain; MLSP = 3.5 yr). Consistent with the hypothesis, our results show that skeletal muscle mitochondria from adult P. leucopus produce less ROS (superoxide and hydrogen peroxide) compared with M. musculus. Additionally, P. leucopus has an increase in the activity of antioxidant enzymes superoxide dismutase 1, catalase, and glutathione peroxidase 1 at young age. P. leucopus compared with M. musculus display low levels of lipid peroxidation (isoprostanes) throughout life; however, P. leucopus although having elevated protein carbonyls at a young age, the accrual of protein oxidation with age is minimal in contrast to the linear increase in M. musculus. Altogether, the results from young animals are in agreement with the predictions of the oxidative stress hypothesis of aging with the exception of protein carbonyls. Nonetheless, the age-dependent increase in protein carbonyls is more pronounced in short-lived M. musculus, which supports enhanced protein homeostasis in long-lived P. leucopus.

Fatty acid metabolism and thyroid hormones

The importance of thyroid hormone signaling in the acute regulation of metabolic activity has been recognized for decades. Slowly, the underlying mechanisms responsible for this activity are being elucidated. A prominent characteristic of thyroid signaling is rapid increases in oxygen consumption and ATP production. This discovery implicated a non-genomic regulation of mitochondrial metabolism by thyroid hormones. Another important clue came from the discovery that thyroid hormones stimulated fatty acid oxidation (FAO) in a variety of tissues in a receptor-dependent, but transcriptional-independent manner. Recently, key linkages between thyroid hormone signaling and specific mitochondrial-targeted pathways have been discovered. This review focuses on the molecular mechanisms by which mitochondrial FAO can be increased through thyroid hormone signaling. The roles of both the full-length and shortened mitochondrial isoforms of thyroid hormone receptor will be discussed. Additionally, the impact of thyroid hormone signaling on dyslipidemias such as obesity, type II diabetes, and fatty liver disease will be considered.