Sex and race contribute to variation in mitochondrial function and insulin sensitivity.

OBJECTIVE: Insulin sensitivity is lower in African American (AA) versus Caucasian American (CA). We tested the hypothesis that lower insulin sensitivity in AA could be explained by mitochondrial respiratory rates, coupling efficiency, myofiber composition, or H2 O2 emission. A secondary aim was to determine whether sex affected the results. METHODS: AA and CA men and women, 19-45 years, BMI 17-43 kg m2 , were assessed for insulin sensitivity (SIClamp ) using a euglycemic clamp at 120 mU/m2 /min, muscle mitochondrial function using high-resolution respirometry, H2 O2 emission using amplex red, and % myofiber composition. RESULTS: SIClamp was greater in CA (p < 0.01) and women (p < 0.01). Proportion of type I myofibers was lower in AA (p < 0.01). Mitochondrial respiratory rates, coupling efficiency, and H2 O2 production did not differ with race. Mitochondrial function was positively associated with insulin sensitivity in women but not men. Statistical adjustment for mitochondrial function, H2 O2 production, or fiber composition did not eliminate the race difference in SIClamp . CONCLUSION: Neither mitochondrial respiratory rates, coupling efficiency, myofiber composition, nor mitochondrial reactive oxygen species production explained lower SIClamp in AA compared to CA. The source of lower insulin sensitivity in AA may be due to other aspects of skeletal muscle that have yet to be identified.

Exercise Effects on Mitochondrial Function and Lipid Metabolism during Energy Balance.

INTRODUCTION/PURPOSE: Aerobic exercise training (AET) has been shown to improve mitochondrial bioenergetics and upregulate proteins related to lipid metabolism. However, it remains to be determined if these alterations associated with AET persist when measured in energy balance (EB) in the days after the last bout of training. The purpose of the study was to test the hypothesis that improvements in skeletal muscle mitochondrial function induced by AET observed in previous literature would persist when measured after restoring EB conditions 72 h removed from the last exercise bout. METHODS: Participants were 14 premenopausal women (age = 31.2 ± 6.7 yr, BMI = 26.6 ± 5.1 kg·m). The AET program required three monitored training sessions per week for 8-16 wk. Skeletal muscle biopsies were obtained at baseline and after 8-16 wk of AET (≥72 h after the last exercise bout). All food was provided for 72 h before biopsies, and EB was managed 24 h before testing within ±100 kcal of measured energy requirements using a whole-room calorimeter. Mitochondrial oxidative capacity was quantified in permeabilized muscle fibers from the vastus lateralis. RESULTS: We found that AET increased coupled respiration (154%) and uncoupled respiration (90%) rates using a fatty acid substrate (palmitoyl carnitine) (P < 0.05). However, when rates were normalized to complex IV activity (a marker of mitochondrial content), no significant differences were observed. In addition, there were no changes in proteins known to mediate mitochondrial biogenesis or lipid transport and metabolism after AET. CONCLUSION: Eight to 16 wk of AET improved mitochondrial capacity under fatty acid substrate when assessed in EB, which appears to be due to mitochondrial biogenesis.

Regulation of Adaptive Immune Cells by Sirtuins.

It is now well-established that the pathways that control lymphocyte metabolism and function are intimately linked, and changes in lymphocyte metabolism can influence and direct cellular function. Interestingly, a number of recent advances indicate that lymphocyte identity and metabolism is partially controlled via epigenetic regulation. Epigenetic mechanisms, such as changes in DNA methylation or histone acetylation, have been found to alter immune function and play a role in numerous chronic disease states. There are several enzymes that can mediate epigenetic changes; of particular interest are sirtuins, protein deacetylases that mediate adaptive responses to a variety of stresses (including calorie restriction and metabolic stress) and are now understood to play a significant role in immunity. This review will focus on recent advances in the understanding of how sirtuins affect the adaptive immune system. These pathways are of significant interest as therapeutic targets for the treatment of autoimmunity, cancer, and transplant tolerance.

Dietary R, S-1,3-butanediol diacetoacetate reduces body weight and adiposity in obese mice fed a high-fat diet.

The dietary R-3-hydroxybutyrate- R-1,3-butanediol monoester increases resting energy expenditure (REE) and markers of brown and white adipose thermogenesis in lean mice. The purpose of this investigation was to determine whether the ketone ester, R, S-1,3-butanediol diacetoacetate (BD-AcAc2), increases energy expenditure and markers of adipose tissue thermogenesis in the context of high-fat diet (HFD)-induced obesity. Thirty-five-week-old male C57BL/6J mice were placed on an ad libitum HFD (45% kcal) for 10 wk. The mice were then randomized to 1 of 3 groups ( n = 10 per group) for an additional 12 wk: 1) control (Con), continuous HFD, 2) pair-fed (PF) to ketone ester (KE); and 3) KE: HFD+30% energy from BD-AcAc2. Mean energy intake throughout the study was ∼26% lower in the KE compared to the Con group (8.2 ± 0.5 vs. 11.2 ± 0.7 kcal/d; P < 0.05). Final body weight (26.8 ± 3.6 vs. 34.9 ± 4.8 g; P < 0.001) and fat mass (5.2 ± 1.2 vs. 11.3 ± 4.5 g; P < 0.001) of the KE group was significantly lower than PF, despite being matched for energy provisions. Differences in body weight and adiposity were accompanied by higher REE and total energy expenditure in the KE group compared to PF after adjustment for lean body mass and fat-mass ( P = 0.001 and 0.007, respectively). Coupled or uncoupled mitochondrial respiratory rates in skeletal muscle were not different among groups, but markers of mitochondrial uncoupling and thermogenesis (uncoupling protein-1, deiodinase-2, and peroxisome proliferator-activated receptor γ coactivator-1α) were higher in interscapular brown adipose tissue (BAT) of mice receiving the KE diet. The absence of mitochondrial uncoupling in skeletal muscle and increased markers of mitochondrial uncoupling in BAT suggest that BD-AcAc2 initiates a transcriptional signature consistent with BAT thermogenesis in the context of HFD-induced obesity.-Davis, R. A. H., Deemer, S. E., Bergeron, J. M., Little, J. T., Warren, J. L., Fisher, G., Smith, D. L., Jr., Fontaine, K. R., Dickinson, S. L., Allison, D. B., Plaisance, E. P. Dietary R, S-1,3-butanediol diacetoacetate reduces body weight and adiposity in obese mice fed a high-fat diet.

Associations of Mitochondrial Fatty Acid Oxidation with Body Fat in Premenopausal Women.

Higher in vivo fatty acid (FA) oxidation rates have been reported in obese individuals compared to lean counterparts; however whether this reflects a shift in substrate-specific oxidative capacity at the level of the skeletal muscle mitochondria has not been examined. The purpose of this study was to test the hypothesis that in situ measures of skeletal muscle mitochondria FA oxidation would be positively associated with total body fat. Participants were 38 premenopausal women (BMI = 26.5 ± 4.3 kg/m2). Total and regional fat were assessed by dual-energy X-ray absorptiometry (DXA). Mitochondrial FA oxidation was assessed in permeabilized myofibers using high-resolution respirometry and a palmitoyl carnitine substrate. We found positive associations of total fat mass with State 3 (ADP-stimulated respiration) (r = 0.379, p < 0.05) and the respiratory control ratio (RCR, measure of mitochondrial coupling) (r = 0.348, p < 0.05). When participants were dichotomized by high or low body fat percent, participants with high total body fat displayed a higher RCR compared to those with low body fat (p < 0.05). There were no associations between any measure of regional fat and mitochondrial FA oxidation independent of total fat mass. In conclusion, greater FA oxidation in obesity may reflect molecular processes that enhance FA oxidation capacity at the mitochondrial level.

Effects of acute hyperinsulinemia on skeletal muscle mitochondrial function, reactive oxygen species production, and metabolism in premenopausal women.

BACKGROUND: Acute metabolic demands that promote excessive and/or prolonged reactive oxygen species production may stimulate changes in mitochondrial oxidative capacity. PURPOSE: To assess changes in skeletal muscle H2O2 production, mitochondrial function, and expression of genes at the mRNA and protein levels regulating energy metabolism and mitochondrial dynamics following a hyperinsulinemic-euglycemic clamp in a cohort of 11 healthy premenopausal women. METHODS: Skeletal muscle biopsies of the vastus lateralis were taken at baseline and immediately following the conclusion of a hyperinsulinemic-euglycemic clamp. Mitochondrial production of H2O2 was quantified fluorometrically and mitochondrial oxidation supported by pyruvate, malate, and succinate (PMS) or palmitoyl carnitine and malate (PCM) was measured by high-resolution respirometry in permeabilized muscle fiber bundles. mRNA and protein levels were assessed by real time PCR and Western blotting. RESULTS: H2O2 emission increased following the clamp (P<0.05). Coupled respiration (State 3) supported by PMS and the respiratory control ratio (index of mitochondrial coupling) for both PMS and PCM were lower following the clamp (P<0.05). IRS1 mRNA decreased, whereas PGC1α and GLUT4 mRNA increased following the clamp (P≤0.05). PGC1α, IRS1, and phosphorylated AKT protein levels were higher after the clamp compared to baseline (P<0.05). CONCLUSIONS: This study demonstrated that acute hyperinsulinemia induced H2O2 production and a concurrent decrease in coupling of mitochondrial respiration with ATP production in a cohort of healthy premenopausal women. Future studies should determine if this uncoupling ameliorates peripheral oxidative damage, and if this mechanism is impaired in diseases associated with chronic oxidative stress.

Reduction of Syndecan Transcript Levels in the Insulin-Producing Cells Affects Glucose Homeostasis in Adult Drosophila melanogaster.

Signaling by direct cell-matrix interactions has been shown to impact the transcription, secretion, and storage of insulin in mammalian ß cells. However, more research is still needed in this area. Syndecans are transmembrane heparan sulfate proteoglycans that function independently and in synergy with integrin-mediated signaling to mediate cell adhesion to the extracellular matrix. In this study, we used the model organism Drosophila melanogaster to determine whether knockdown of the Syndecan (Sdc) gene expression specifically in the insulin-producing cells (IPCs) might affect insulin-like peptide (ILP) production and secretion. IPCs of adult flies produce three ILPs (ILP2, ILP3, and ILP5), which have significant homology to mammalian insulin. We report that flies with reduced Sdc expression in the IPCs did not show any difference in the expression of ilp genes compared to controls. However, they had significantly reduced levels of the circulating ILP2 protein, higher circulating carbohydrates, and were less glucose tolerant than control flies. Finally, we found that IPCs-specific Sdc knockdown led to reduced levels of head Glucose transporter1 gene expression, extracellular signal-regulated kinase phosphorylation, and reactive oxygen species. Taken together, our findings suggest a cell autonomous role for Sdc in insulin release in D. melanogaster.

Associations of human skeletal muscle fiber type and insulin sensitivity, blood lipids, and vascular hemodynamics in a cohort of premenopausal women.

PURPOSE: Cardiometabolic disease remains a leading cause of morbidity and mortality in developed nations. Consequently, identifying and understanding factors associated with underlying pathophysiological processes leading to chronic cardio metabolic conditions is critical. Metabolic health, arterial elasticity, and insulin sensitivity (SI) may impact disease risk, and may be determined in part by myofiber type. Therefore, the purpose of this study was to test the hypothesis that type I myofiber composition would be associated with high SI, greater arterial elasticity, lower blood pressure, and blood lipids; whereas, type IIx myofibers would be associated with lower SI, lower arterial elasticity, higher blood pressure, blood lipids. METHODS: Muscle biopsies were performed on the vastus lateralis in 16 subjects (BMI = 27.62 ± 4.71 kg/m2, age = 32.24 ± 6.37 years, 43% African American). The distribution of type I, IIa, and IIx myofibers was determined via immunohistochemistry performed on frozen cross-sections. Pearson correlation analyses were performed to assess associations between myofiber composition, SI, arterial elasticity, blood pressure, and blood lipid concentrations. RESULTS: The percentage of type I myofibers positively correlated with SI and negatively correlated with systolic blood pressure SBP, diastolic blood pressure, and mean arterial pressure (MAP); whereas, the percentage of type IIx myofibers were negatively correlated with SI and large artery elasticity, and positively correlated with LDL cholesterol, SBP, and MAP. CONCLUSIONS: These data demonstrate a potential link between myofiber composition and cardiometabolic health outcomes in a cohort of premenopausal women. Future research is needed to determine the precise mechanisms in which myofiber composition impacts the pathophysiology of impaired glucose and lipid metabolism, as well as vascular dysfunction.