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

Ablation of uncoupling protein 3 affects interrelated factors leading to lipolysis and insulin resistance in visceral white adipose tissue

The physiological role played by uncoupling protein 3 (UCP3) in white adipose tissue (WAT) has not been elucidated so far. In the present study, we evaluated the impact of the absence of the whole body UCP3 on WAT physiology in terms of ability to store triglycerides, oxidative capacity, response to insulin, inflammation, and adipokine production. Wild type (WT) and UCP3 Knockout (KO) mice housed at thermoneutrality (30°C) have been used as the animal model. Visceral gonadic WAT (gWAT) from KO mice showed an impaired capacity to store triglycerides (TG) as indicated by its lowered weight, reduced adipocyte diameter, and higher glycerol release (index of lipolysis). The absence of UCP3 reduces the maximal oxidative capacity of gWAT, increases mitochondrial free radicals, and activates ER stress. These processes are associated with increased levels of monocyte chemoattractant protein-1 and TNF-α. The response of gWAT to in vivo insulin administration, revealed by (ser473)-AKT phosphorylation, was blunted in KO mice, with a putative role played by eif2a, JNK, and inflammation. Variations in adipokine levels in the absence of UCP3 were observed, including reduced adiponectin levels both in gWAT and serum. As a whole, these data indicate an important role of UCP3 in regulating the metabolic functionality of gWAT, with its absence leading to metabolic derangement. The obtained results help to clarify some aspects of the association between metabolic disorders and low UCP3 levels.

Ethnicity Differences in the Association of UCP1-3826A/G, UCP2-866G/A and Ala55Val, and UCP3-55C/T Polymorphisms with Type 2 Diabetes Mellitus Susceptibility: An Updated Meta-Analysis

BACKGROUND

The relationship between uncoupling protein (UCP) 1-3 polymorphisms and susceptibility to type 2 diabetes mellitus (T2DM) has been extensively studied, while conclusions remain contradictory. Thus, we performed this meta-analysis to elucidate whether the UCP1-3826A/G, UCP2-866G/A, Ala55Val, and UCP3-55C/T polymorphisms are associated with T2DM.

METHODS

Eligible studies were searched from PubMed, Cochrane Library, and Web of Science database before 12 July 2020. Pooled odds ratios (ORs) with corresponding 95% confidence intervals (CIs) were calculated to evaluate the strength of the association. Heterogeneity analysis, subgroup analysis, sensitivity analysis, and publication bias were also performed.

RESULTS

A total of 38 case-control studies were included in this meta-analysis. The overall results revealed significant association between T2DM and the UCP2 Ala55Val polymorphism (recessive model: OR = 1.25, 95% CI 1.12-1.40, P < 0.01; homozygous model: OR = 1.33, 95% CI 1.03-1.72, P = 0.029, respectively). In subgroup analysis stratified by ethnicity, T2DM risk was increased with the UCP2 Ala55Val polymorphism (allele model: OR = 1.17, 95% CI 1.02-1.34, P = 0.023; recessive model: OR = 1.28, 95% CI 1.13-1.45, P < 0.01; homozygous model: OR = 1.39, 95% CI 1.05-1.86, P = 0.023, respectively), while decreased with the UCP2-866G/A polymorphism in Asians (dominant model: OR = 0.86, 95% CI 0.74-1.00, P = 0.045).

CONCLUSIONS

Our results demonstrate that the UCP2-866G/A polymorphism is protective against T2DM, while the UCP2 Ala55Val polymorphism is susceptible to T2DM in Asians.

Absence of uncoupling protein 3 at thermoneutrality influences brown adipose tissue mitochondrial functionality in mice

The physiological role played by uncoupling protein 3 (UCP3) in brown adipose tissue (BAT) has not been fully elucidated so far. In the present study, we evaluated the impact of the absence of UCP3 on BAT mitochondrial functionality and morphology. To this purpose, wild type (WT) and UCP3 Knockout (KO) female mice were housed at thermoneutrality (30°C), a condition in which BAT contributes to energy homeostasis independently of its cold-induced thermogenic function. BAT mitochondria from UCP3 KO mice presented a lower ability to oxidize the fatty acids and glycerol-3-phosphate, and an enhanced oxidative stress as revealed by enhanced mitochondrial electron leak, lipid hydroperoxide levels, and induction of antioxidant mitochondrial enzymatic capacity. The absence of UCP3 also influenced the mitochondrial super-molecular protein aggregation, an important feature for fatty acid oxidation rate as well as for adequate cristae organization and mitochondrial shape. Indeed, electron microscopy revealed alterations in mitochondrial morphology in brown adipocytes from KO mice. In the whole, data here reported show that the absence of UCP3 results in a significant alteration of BAT mitochondrial physiology and morphology. These observations could also help to clarify some aspects of the association between metabolic disorders associated with low UCP3 levels, as previously reported in human studies.

Sepsis Impairs Thyroid Hormone Signaling and Mitochondrial Function in the Mouse Diaphragm

Background: Sepsis can cause the nonthyroidal illness syndrome (NTIS), resulting in perturbed thyroid hormone (TH) signaling and reduced thyroxine (T4) levels. TH is a major regulator of muscle function, via its influence on mitochondria. This study aimed at evaluating the relationship between TH signaling, mitochondrial function, and the antioxidant defense system in the diaphragms of septic mice. Methods: Male C57Bl/6 mice were divided into two groups: cecal ligation and puncture (CLP) and sham. Twenty-four hours after surgery, plasma, diaphragms, and livers were collected. TH metabolism and responses were analyzed by measuring messenger RNA (mRNA) expression of Dio1 in the liver, and Thra, Thrb, Dio2, Slc16a10, and Slc16a2 (encodes MCT 10 and 8), in the diaphragm. T4 plasma levels were measured by radioimmunoassay. Damage to diaphragm mitochondria was assessed by electron microscopy and real-time polymerase chain reaction (qPCR), and function with oxygraphy. The diaphragm antioxidative defense system was examined by qPCR, analyzing superoxide dismutase (SOD) 1 (Sod1), mitochondrial superoxide dismutase (SOD 2; Sod2), extracellular superoxide dismutase (SOD 3; Sod3), glutathione peroxidase 1 (Gpx1), and catalase (Cat) expression. The effect of TH replacement was tested by treating the mice with T4 and triiodothyronine (T3) (CLP+TH) after surgery. Results: CLP mice presented reduced total plasma T4 concentrations, downregulated Dio1, and upregulated Il1b mRNA expression in the liver. CLP mice also displayed downregulated Thra, Thrb, Slc16a10, and Slc16a2 expression in the diaphragm, suggesting that TH signaling was compromised. The expression of Ppargc1a (encoding PGC1a) was downregulated, which correlated with the decrease in the number of total mitochondria, increase in the percentage of injured mitochondria, downregulation of respiratory chain complex 2 and 3 mRNA expression, and reduced maximal respiration. In addition, septic animals presented a three-fold increase in Ucp3 and G6pdh expression; downregulated Sod3, Gpx1, and Cat expression; and upregulated Sod2 expression, potentially due to elevated reactive oxygen species levels. The mitochondrial number and the percentage of injured mitochondrial were similar between sham and CLP+TH mice. Conclusions: Sepsis induced responses consistent with NTIS, resulted in mitochondrial damage and functional impairment, and modulated the expression of key antioxidant enzymes in the diaphragm. Thus, impaired diaphragm function during sepsis seems to involve altered local TH signaling, mitochondrial dysfunction, and oxidative stress defense.

Mitochondria in Health and Diseases

Mitochondria are subcellular organelles evolved by endosymbiosis of bacteria with eukaryotic cells characteristics. They are the main source of ATP in the cell and play a pivotal role in cell life and cell death. Mitochondria are engaged in the pathogenesis of human diseases and aging directly or indirectly through a broad range of signaling pathways. However, despite an increased interest in mitochondria over the past decades, the mechanisms of mitochondria-mediated cell/organ dysfunction in response to pathological stimuli remain unknown. The Special Issue, “Mitochondria in Health and Diseases,” organized by Cells includes 24 review and original articles that highlight the latest achievements in elucidating the role of mitochondria under physiological (healthy) conditions and, in various cell/animal models of human diseases and, in patients. Altogether, the Special Issue summarizes and discusses different aspects of mitochondrial metabolism and function that open new avenues in understanding mitochondrial biology.

Exercise with food withdrawal at thermoneutrality impacts fuel use, the microbiome, AMPK phosphorylation, muscle fibers, and thyroid hormone levels in rats

Exercise under fasting conditions induces a switch to lipid metabolism, eliciting beneficial metabolic effects. Knowledge of signaling responses underlying metabolic adjustments in such conditions may help to identify therapeutic strategies. Therefore, we studied the effect of mild exercise on rats submitted to food withdrawal at thermoneutrality (28°C) for 3 days. Animals were housed at thermoneutrality rather than the standard housing temperature (22°C) to avoid beta-adrenergic signaling responses that themselves affect metabolism and well-being. Quantitative analysis of multi-organ mRNA levels, myofibers, and serum metabolites shows that this protocol (a) boosts fat oxidation in muscle and liver, (b) reduces lipogenesis and increases gluconeogenesis in liver, (c) increases serum acylcarnitines (especially C4 OH) and ketone bodies and the use of the latter as fuel in muscle, (d) increases Type I myofibers, and (e) is associated with an increased thyroid hormone uptake and metabolism in muscle. In addition, stool microbiome DNA analysis revealed that food withdrawal dramatically alters the presence of bacterial genera associated with ketone metabolism. Taken together, this protocol induces a drastic switch toward increased lipid and ketone metabolism compared to exercise or food withdrawal alone, which may prove beneficial and may involve local thyroid hormones, which may be regarded as exercise mimetics.