Jill M. Coenen-Schimke

Asian Indians Have Enhanced Skeletal Muscle Mitochondrial Capacity to Produce ATP in Association with Severe Insulin-Resistance

OBJECTIVE— Type 2 diabetes has become a global epidemic, and Asian Indians have a higher susceptibility to diabetes than Europeans. We investigated whether Indians had any metabolic differences compared with Northern European Americans that may render them more susceptible to diabetes. RESEARCH DESIGN AND METHODS— We studied 13 diabetic Indians, 13 nondiabetic Indians, and 13 nondiabetic Northern European Americans who were matched for age, BMI, and sex. The primary comparisons were insulin sensitivity by hyperinsulinemic-euglycemic clamp and skeletal muscle mitochondrial capacity for oxidative phosphorylation (OXPHOS) by measuring mitochondrial DNA copy number (mtDNA), OXPHOS gene transcripts, citrate synthase activity, and maximal mitochondrial ATP production rate (MAPR). Other factors that may cause insulin resistance were also measured. RESULTS— The glucose infusion rates required to maintain identical glucose levels during the similar insulin infusion rates were substantially lower in diabetic Indians than in the nondiabetic participants (P < 0.001), and they were lower in nondiabetic Indians than in nondiabetic Northern European Americans (P < 0.002). mtDNA (P < 0.02), OXPHOS gene transcripts (P < 0.01), citrate synthase, and MAPR (P < 0.03) were higher in Indians irrespective of their diabetic status. Intramuscular triglyceride, C-reactive protein, interleukin-6, and tumor necrosis factor-α concentrations were higher, whereas adiponectin concentrations were lower in diabetic Indians. CONCLUSIONS— Despite being more insulin resistant, diabetic Indians had similar muscle OXPHOS capacity as nondiabetic Indians, demonstrating that diabetes per se does not cause mitochondrial dysfunction. Indians irrespective of their diabetic status had higher OXPHOS capacity than Northern European Americans, although Indians were substantially more insulin resistant, indicating a dissociation between mitochondrial dysfunction and insulin resistance.

Skeletal muscle mitochondrial functions, mitochondrial DNA copy numbers, and gene transcript profiles in type 2 diabetic and nondiabetic subjects at equal levels of low or high insulin and euglycemia.

We investigated whether previously reported muscle mitochondrial dysfunction and altered gene transcript levels in type 2 diabetes might be secondary to abnormal blood glucose and insulin levels rather than an intrinsic defect of type 2 diabetes. A total of 13 type 2 diabetic and 17 nondiabetic subjects were studied on two separate occasions while maintaining similar insulin and glucose levels in both groups by 7-h infusions of somatostatin, low- or high-dose insulin (0.25 and 1.5 mU/kg of fat-free mass per min, respectively), and glucose. Muscle mitochondrial DNA abundance was not different between type 2 diabetic and nondiabetic subjects at both insulin levels, but the majority of transcripts in muscle that are involved mitochondrial functions were expressed at lower levels in type 2 diabetes at low levels of insulin. However, several gene transcripts that are specifically involved in the electron transport chain were expressed at higher levels in type 2 diabetic patients. After the low-dose insulin infusion, which achieved postabsorptive insulin levels, the muscle mitochondrial ATP production rate (MAPR) was not different between type 2 diabetic and nondiabetic subjects. However, increasing insulin to postprandial levels increased the MAPR in nondiabetic subjects but not in type 2 diabetic patients. The lack of MAPR increment in response to high-dose insulin in type 2 diabetic patients occurred in association with reduced glucose disposal and expression of peroxisome proliferator–activated receptor-γ coactivator 1α, citrate synthase, and cytochrome c oxidase I. In conclusion, the current data supports that muscle mitochondrial dysfunction in type 2 diabetes is not an intrinsic defect, but instead a functional defect related to impaired response to insulin.

Effect of Insulin Deprivation on Muscle Mitochondrial ATP Production and Gene Transcript Levels in Type 1 Diabetic Subjects

OBJECTIVE—Muscle mitochondrial dysfunction occurs in many insulin-resistant states, such as type 2 diabetes, prompting a hypothesis that mitochondrial dysfunction may cause insulin resistance. We determined the impact of insulin deficiency on muscle mitochondrial ATP production by temporarily depriving type 1 diabetic patients of insulin treatment. RESEARCH DESIGN AND METHODS—We withdrew insulin for 8.6 ± 0.6 h in nine C-peptide–negative type 1 diabetic subjects and measured muscle mitochondrial ATP production and gene transcript levels (gene array and real-time quantitative PCR) and compared with insulin-treated state. We also measured oxygen consumption (indirect calorimetry); plasma levels of glucagon, bicarbonate, and other substrates; and urinary nitrogen. RESULTS—Withdrawal of insulin resulted in increased plasma glucose, branched chain amino acids, nonesterified fatty acids, β-hydroxybutyrate, and urinary nitrogen but no change in bicarbonate. Insulin deprivation decreased muscle mitochondrial ATP production rate (MAPR) despite an increase in whole-body oxygen consumption and altered expression of many muscle mitochondrial gene transcripts. Transcript levels of genes involved in oxidative phosphorylation were decreased, whereas those involved in vascular endothelial growth factor (VEGF) signaling, inflammation, cytoskeleton signaling, and integrin signaling pathways were increased. CONCLUSIONS—Insulin deficiency and associated metabolic changes reduce muscle MAPR and expression of oxidative phosphorylation genes in type 1 diabetes despite an increase in whole-body oxygen consumption. Increase in transcript levels of genes involved in VEGF, inflammation, cytoskeleton, and integrin signaling pathways suggest that vascular factors and cell proliferation that may interact with mitochondrial changes occurred.

Insulin fails to enhance mTOR phosphorylation, mitochondrial protein synthesis, and ATP production in human skeletal muscle without amino acid replacement

Systemic insulin administration causes hypoaminoacidemia by inhibiting protein degradation, which may in turn inhibit muscle protein synthesis (PS). Insulin enhances muscle mitochondrial PS and ATP production when hypoaminoacidemia is prevented by exogenous amino acid (AA) replacement. We determined whether insulin would stimulate mitochondrial PS and ATP production in the absence of AA replacement. Using l-[1,2-¹³C]leucine as a tracer, we measured the fractional synthetic rate of mitochondrial as well as sarcoplasmic and mixed muscle proteins in 18 participants during sustained (7-h) insulin or saline infusion (n = 9 each). We also measured muscle ATP production, mitochondrial enzyme activities, mRNA levels of mitochondrial genes, and phosphorylation of signaling proteins regulating protein synthesis. The concentration of circulating essential AA decreased during insulin infusion. Mitochondrial, sarcoplasmic, and mixed muscle PS rates were also lower during insulin (2-7 h) than during saline infusions despite increased mRNA levels of selected mitochondrial genes. Under these conditions, insulin did not alter mitochondrial enzyme activities and ATP production. These effects were associated with enhanced phosphorylation of Akt but not of protein synthesis activators mTOR, p70(S6K), and 4EBP1. In conclusion, sustained physiological hyperinsulinemia without AA replacement did not stimulate PS of mixed muscle or protein subfractions and did not alter muscle mitochondrial ATP production in healthy humans. These results support that insulin and AA act in conjunction to stimulate muscle mitochondrial function and mitochondrial protein synthesis.

Altered regulation of energy homeostasis in older rats in response to thyroid hormone administration

Hyperthyroidism causes increased energy intake and expenditure, although anorexia and higher weight loss have been reported in elderly individuals with hyperthyroidism. To determine the effect of age on energy homeostasis in response to experimental hyperthyroidism, we administered 200 μg triiodothyronine (T3) in 7‐ and 27‐mo‐old rats for 14 d. T3 increased energy expenditure (EE) in both the young and the old rats, although the old rats lost more weight (147 g) than the young rats (58 g) because of the discordant effect of T3 on food intake, with a 40% increase in the young rats, but a 40% decrease in the old ones. The increased food intake in the young rats corresponded with a T3‐mediated increase in the appetite‐regulating proteins agouti‐related peptide, neuropeptide Y, and uncoupling protein 2 in the hypothalamus, but no increase occurred in the old rats. Evidence of mitochondrial biogenesis in response to T3 was similar in the soleus muscle and heart of the young and old animals, but less consistent in old plantaris muscle and liver. Despite the comparable increase in EE, T3s effect on mitochondrial function was modulated by age in a tissue‐specific manner. We conclude that older rats lack compensatory mechanisms to increase caloric intake in response to a T3‐induced increase in EE, demonstrating a detrimental effect of age on energy homeostasis.—Walrand, S., Short, K. R., Heemstra, L. A., Novak, C. M., Levine, J. A., Coenen‐Schimke, J. M., Nair, K. S. Altered regulation of energy homeostasis in older rats in response to thyroid hormone administration. FASEB J. 28, 1499–1510 (2014). www.fasebj.org