Dong Ho Han

Skeletal muscle respiratory uncoupling prevents diet-induced obesityand insulin resistance in mice

To determine whether uncoupling respiration from oxidative phosphorylation in skeletal muscle is a suitable treatment for obesity and type 2 diabetes, we generated transgenic mice expressing the mitochondrial uncoupling protein (Ucp) in skeletal muscle. Skeletal muscle oxygen consumption was 98% higher in Ucp-L mice (with low expression) and 246% higher in Ucp-H mice (with high expression) than in wild-type mice. Ucp mice fed a chow diet had the same food intake as wild-type mice, but weighed less and had lower levels of glucose and triglycerides and better glucose tolerance than did control mice. Ucp-L mice were resistant to obesity induced by two different high-fat diets. Ucp-L mice fed a high-fat diet had less adiposity, lower levels of glucose, insulin and cholesterol, and an increased metabolic rate at rest and with exercise. They were also more responsive to insulin, and had enhanced glucose transport in skeletal muscle in the setting of increased muscle triglyceride content. These data suggest that manipulating respiratory uncoupling in muscle is a viable treatment for obesity and its metabolic sequelae.

A Role for the Transcriptional Coactivator PGC-1α in Muscle Refueling

The transcriptional coactivator peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) has been identified as an inducible regulator of mitochondrial function. Skeletal muscle PGC-1α expression is induced post-exercise. Therefore, we sought to determine its role in the regulation of muscle fuel metabolism. Studies were performed using conditional, muscle-specific, PGC-1α gain-of-function and constitutive, generalized, loss-of-function mice. Forced expression of PGC-1α increased muscle glucose uptake concomitant with augmentation of glycogen stores, a metabolic response similar to post-exercise recovery. Induction of muscle PGC-1α expression prevented muscle glycogen depletion during exercise. Conversely, PGC-1α-deficient animals exhibited reduced rates of muscle glycogen repletion post-exercise. PGC-1α was shown to increase muscle glycogen stores via several mechanisms including stimulation of glucose import, suppression of glycolytic flux, and by down-regulation of the expression of glycogen phosphorylase and its activating kinase, phosphorylase kinase α. These findings identify PGC-1α as a critical regulator of skeletal muscle fuel stores.

A High Fat Diet Impairs Stimulation of Glucose Transport in Muscle FUNCTIONAL EVALUATION OF POTENTIAL MECHANISMS

A high fat diet causes resistance of skeletal muscle glucose transport to insulin and contractions. We tested the hypothesis that fat feeding causes a change in plasma membrane composition that interferes with functioning of glucose transporters and/or insulin receptors. Epitrochlearis muscles of rats fed a high (50% of calories) fat diet for 8 weeks showed ∼50% decreases in insulin- and contraction-stimulated 3-O-methylglucose transport. Similar decreases in stimulated glucose transport activity occurred in muscles of wild-type mice with 4 weeks of fat feeding. In contrast, GLUT1 overexpressing muscles of transgenic mice fed a high fat diet showed no decreases in their high rates of glucose transport, providing evidence against impaired glucose transporter function. Insulin-stimulated system A amino acid transport, insulin receptor (IR) tyrosine kinase activity, and insulin-stimulated IR and IRS-1 tyrosine phosphorylation were all normal in muscles of rats fed the high fat diet for 8 weeks. However, after 30 weeks on the high fat diet, there was a significant reduction in insulin-stimulated tyrosine phosphorylation in muscle. The increases in GLUT4 at the cell surface induced by insulin or muscle contractions, measured with the 3H-labeled 2-N-4-(1-azi-2,2,2-trifluoroethyl)-benzoyl-1,3-bis-(d-mannose-4-yloxy)-2-propylamine photolabel, were 26–36% smaller in muscles of the 8-week high fat-fed rats as compared with control rats. Our findings provide evidence that (a) impairment of muscle glucose transport by 8 weeks of high fat feeding is not due to plasma membrane composition-related reductions in glucose transporter or insulin receptor function, (b) a defect in insulin receptor signaling is a late event, not a primary cause, of the muscle insulin resistance induced by fat feeding, and (c) impaired GLUT4 translocation to the cell surface plays a major role in the decrease in stimulated glucose transport.

Association Between Specific Adipose Tissue CD4+ T-Cell Populations and Insulin Resistance in Obese Individuals

BACKGROUND & AIMSnAn increased number of macrophages in adipose tissue is associated with insulin resistance and metabolic dysfunction in obese people. However, little is known about other immune cells in adipose tissue from obese people, and whether they contribute to insulin resistance. We investigated the characteristics of T cells in adipose tissue from metabolically abnormal insulin-resistant obese (MAO) subjects, metabolically normal insulin-sensitive obese (MNO) subjects, and lean subjects. Insulin sensitivity was determined by using the hyperinsulinemic euglycemic clamp procedure.nnnMETHODSnWe assessed plasma cytokine concentrations and subcutaneous adipose tissue CD4(+) T-cell populations in 9 lean, 12 MNO, and 13 MAO subjects. Skeletal muscle and liver samples were collected from 19 additional obese patients undergoing bariatric surgery to determine the presence of selected cytokine receptors.nnnRESULTSnAdipose tissue from MAO subjects had 3- to 10-fold increases in numbers of CD4(+) T cells that produce interleukin (IL)-22 and IL-17 (a T-helper [Th] 17 and Th22 phenotype) compared with MNO and lean subjects. MAO subjects also had increased plasma concentrations of IL-22 and IL-6. Receptors for IL-17 and IL-22 were expressed in human liver and skeletal muscle samples. IL-17 and IL-22 inhibited uptake of glucose in skeletal muscle isolated from rats and reduced insulin sensitivity in cultured human hepatocytes.nnnCONCLUSIONSnAdipose tissue from MAO individuals contains increased numbers of Th17 and Th22 cells, which produce cytokines that cause metabolic dysfunction in liver and muscle in vitro. Additional studies are needed to determine whether these alterations in adipose tissue T cells contribute to the pathogenesis of insulin resistance in obese people.

Normal adaptations to exercise despite protection against oxidative stress

It has been reported that supplementation with the antioxidant vitamins C and E prevents the adaptive increases in mitochondrial biogenesis and GLUT4 expression induced by endurance exercise. We reevaluated the effects of these antioxidants on the adaptive responses of rat skeletal muscle to swimming in a short-term study consisting of 9 days of vitamins C and E with exercise during the last 3 days and a longer-term study consisting of 8 wk of antioxidant vitamins with exercise during the last 3 wk. The rats in the antioxidant groups were given 750 mg·kg body wt(-1)·day(-1) vitamin C and 150 mg·kg body wt(-1)·day(-1) vitamin E. In rats euthanized immediately after exercise, plasma TBARs were elevated in the control rats but not in the antioxidant-supplemented rats, providing evidence for an antioxidant effect. In rats euthanized 18 h after exercise there were large increases in insulin responsiveness of glucose transport in epitrochlearis muscles mediated by an approximately twofold increase in GLUT4 expression in both the short- and long-term treatment groups. The protein levels of a number of mitochondrial marker enzymes were also increased about twofold. Superoxide dismutases (SOD) 1 and 2 were increased about twofold in triceps muscle after 3 days of exercise, but only SOD2 was increased after 3 wk of exercise. There were no differences in the magnitudes of any of these adaptive responses between the control and antioxidant groups. These results show that very large doses of antioxidant vitamins do not prevent the exercise-induced adaptive responses of muscle mitochondria, GLUT4, and insulin action to exercise and have no effect on the level of these proteins in sedentary rats.

Effects of resveratrol and SIRT1 on PGC-1α activity and mitochondrial biogenesis: a reevaluation

Feeding resveratrol to rodents has no effect on mitochondrial biogenesis, and deacetylation of PGC-1α results in a decrease, not an increase, in its coactivator activity.

Does calorie restriction induce mitochondrial biogenesis? A reevaluation

It has been reported that 30% calorie restriction (CR) for 3 mo results in large increases in mitochondrial biogenesis in heart, brain, liver, and adipose tissue, with concomitant increases in respiration and ATP synthesis. We found these results surprising, and performed this study to determine whether 30% CR does induce an increase in mitochondria in heart, brain, liver, adipose tissue, and/or skeletal muscle. To this end, we measured the levels of a range of mitochondrial proteins, and mRNAs. With the exception of long‐chain acyl‐CoA dehydrogenase protein level, which was increased ~60% in adipose tissue, none of the mitochondrial proteins or mRNAs that we measured were increased in rats subjected to 30% CR for 14 wk. There was also no increase in citrate synthase activity. Because it is not possible to have an increase in mitochondria without any increase in key mitochondrial proteins, we conclude that 30% CR does not induce an increase in mitochondria in heart, brain, liver, adipose tissue, or skeletal muscle in laboratory rodents.—Hancock, C. R., Han, D.‐H., Higashida, K., Kim, S. H., Holloszy, J. O. Does calorie restriction induce mito‐chondrial biogenesis? A reevaluation. FASEB J. 25, 785–791 (2011). www.fasebj.org

Skeletal muscle overexpression of nuclear respiratory factor 1 increases glucose transport capacity

Nuclear respiratory factor 1 (NRF‐1) is a transcriptional activator of nuclear genes that encode a range of mitochondrial proteins including cytochrome c, various other respiratory chain subunits, and δ‐aminolevulinate synthase. Activation of NRF‐1 in fibroblasts has been shown to induce increases in cytochrome c expression and mitochondrial respiratory capacity. To further evaluate the role of NRF‐1 in the regulation of mitochondrial biogenesis and respiratory capacity, we generated transgenic mice overexpressing NRF‐1 in skeletal muscle. Cytochrome c expression was increased ∼twofold and δ‐aminolevulinate synthase was increased ∼50% in NRF‐1 transgenic muscle. The levels of some mitochondrial proteins were increased 50–60%, while others were unchanged. Muscle respiratory capacity was not increased in the NRF‐1 transgenic mice. A finding that provides new insight regarding the role of NRF‐1 was that expression of MEF2A and GLUT4 was increased in NRF‐1 transgenic muscle. The increase in GLUT4 was associated with a proportional increase in insulin‐stimulated glucose transport. These results show that an isolated increase in NRF‐1 is not sufficient to bring about a coordinated increase in expression of all of the proteins necessary for assembly of functional mitochondria. They also provide the new information that NRF‐1 overexpression results in increased expression of GLUT4.—Baar, K., Song, Z., Semenkovich, C. F., Jones, T. E,. Han, D.‐H., Nolte, L. A., Ojuka, E. O., Chen, M., Holloszy, J. O. Skeletal muscle overexpression of nuclear respiratory factor 1 increases glucose transport capacity. FASEB J. 17, 1666–1673 (2003)

DHEA protects against visceral obesity and muscle insulin resistance in rats fed a high-fat diet

Visceral obesity is frequently associated with muscle insulin resistance. Rats fed a high-fat diet rapidly develop obesity and insulin resistance. Dehydroepiandrosterone (DHEA) has been reported to protect against the development of obesity. This study tested the hypothesis that DHEA protects against the increase in visceral fat and the development of muscle insulin resistance induced by a high-fat diet in rats. Feeding rats a diet providing 50% of the energy as fat for 4 wk resulted in a twofold greater visceral fat mass and a 50% lower rate of maximally insulin-stimulated muscle 2-deoxyglucose (2-DG) uptake compared with controls. Rats fed the high-fat diet plus 0.3% DHEA were largely protected against the increase in visceral fat (+11.3 g in high fat vs. +2.9 g in high fat plus DHEA, compared with controls) and against the decrease in insulin-stimulated muscle 2-DG uptake (0.94 ± 0.15 μmol ⋅ ml-1 ⋅ 20 min-1, controls; 0.46 ± 0.06 μmol ⋅ ml-1 ⋅ 20 min-1, high-fat diet; 0.78 ± 0.07 μmol ⋅ ml-1 ⋅ 20 min-1, high fat + DHEA). DHEA did not affect food intake. These results show that DHEA has a protective effect against accumulation of visceral fat and development of muscle insulin resistance in rats fed a high-fat diet.Visceral obesity is frequently associated with muscle insulin resistance. Rats fed a high-fat diet rapidly develop obesity and insulin resistance. Dehydroepiandrosterone (DHEA) has been reported to protect against the development of obesity. This study tested the hypothesis that DHEA protects against the increase in visceral fat and the development of muscle insulin resistance induced by a high-fat diet in rats. Feeding rats a diet providing 50% of the energy as fat for 4 wk resulted in a twofold greater visceral fat mass and a 50% lower rate of maximally insulin-stimulated muscle 2-deoxyglucose (2-DG) uptake compared with controls. Rats fed the high-fat diet plus 0.3% DHEA were largely protected against the increase in visceral fat (+ 11.3 g in high fat vs. + 2.9 g in high fat plus DHEA, compared with controls) and against the decrease in insulin-stimulated muscle 2-DG uptake (0.94 +/- 0.15 mumol.ml-1.20 min-1, controls; 0.46 +/- 0.06 mumol.ml-1.20 min-1, high-fat diet; 0.78 +/- 0.07 mumol.ml-1.20 min-1, high fat + DHEA). DHEA did not affect food intake. These results show that DHEA has a protective effect against accumulation of visceral fat and development of muscle insulin resistance in rats fed a high-fat diet.

Deficiency of the Mitochondrial Electron Transport Chain in Muscle Does Not Cause Insulin Resistance

Background It has been proposed that muscle insulin resistance in type 2 diabetes is due to a selective decrease in the components of the mitochondrial electron transport chain and results from accumulation of toxic products of incomplete fat oxidation. The purpose of the present study was to test this hypothesis. Methodology/Principal Findings Rats were made severely iron deficient, by means of an iron-deficient diet. Iron deficiency results in decreases of the iron containing mitochondrial respiratory chain proteins without affecting the enzymes of the fatty acid oxidation pathway. Insulin resistance was induced by feeding iron-deficient and control rats a high fat diet. Skeletal muscle insulin resistance was evaluated by measuring glucose transport activity in soleus muscle strips. Mitochondrial proteins were measured by Western blot. Iron deficiency resulted in a decrease in expression of iron containing proteins of the mitochondrial respiratory chain in muscle. Citrate synthase, a non-iron containing citrate cycle enzyme, and long chain acyl-CoA dehydrogenase (LCAD), used as a marker for the fatty acid oxidation pathway, were unaffected by the iron deficiency. Oleate oxidation by muscle homogenates was increased by high fat feeding and decreased by iron deficiency despite high fat feeding. The high fat diet caused severe insulin resistance of muscle glucose transport. Iron deficiency completely protected against the high fat diet-induced muscle insulin resistance. Conclusions/Significance The results of the study argue against the hypothesis that a deficiency of the electron transport chain (ETC), and imbalance between the ETC and β-oxidation pathways, causes muscle insulin resistance.