Bigelow, Maureen L.

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.

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.

DHEA (dehydroepiandrosterone) replacement is not part of the current standard of care in hypoadrenal subjects. Animal studies have shown that DHEA administration prevents diabetes. To determine the physiological effect of DHEA replacement on insulin sensitivity in adrenal-deficient women, we performed a single-center, randomized, double-blind, placebo-controlled, crossover study in 28 hypoadrenal women (mean age 50.2 ± 2.87 years) who received a single 50-mg dose of DHEA daily or placebo. After 12 weeks, insulin sensitivity was assessed using a hyperinsulinemic-euglycemic clamp. DHEA replacement significantly increased DHEA-S (sulfated ester of DHEA), bioavailable testosterone, and androstenedione and reduced sex hormone−binding globulin levels. Fasting plasma insulin and glucagon were lower with DHEA (42 ± 4.94 vs. 53 ± 6.58 pmol/l [P = 0.005] and 178 ± 11.32 vs. 195.04 ± 15 pmol/l [P = 0.02], respectively). The average amount of glucose needed to maintain similar blood glucose levels while infusing the same insulin dosages was higher during DHEA administration (358 ± 24.7 vs. 320 ± 24.6 mg/min; P < 0.05), whereas endogenous glucose production was similar. DHEA also reduced total cholesterol (P < 0.005), triglycerides (P < 0.011), LDL cholesterol (P < 0.05), and HDL cholesterol (P < 0.005). In conclusion, replacement therapy with 50 mg of DHEA for 12 weeks significantly increased insulin sensitivity in hypoadrenal women, thereby suggesting that DHEA replacement could have a potential impact in preventing type 2 diabetes.

Insulin resistance increases and muscle oxidative capacity decreases during aging, but lifestyle changes—especially physical activity—may reverse these trends. Here we report the effect of a 16-week aerobic exercise program (n = 65) or control activity (n = 37) performed by men and women aged 21–87 years on insulin sensitivity and muscle mitochondria. Insulin sensitivity, measured by intravenous glucose tolerance test, decreased with age (r = −0.32) and was related to abdominal fat content (r = −0.65). Exercise increased peak oxygen uptake (Vo2peak; 10%), activity of muscle mitochondrial enzymes (citrate synthase and cytochrome c oxidase, 45–76%) and mRNA levels of mitochondrial genes (COX4, ND4, both 66%) and genes involved in mitochondrial biogenesis (PGC-1α, 55%; NRF-1, 15%; TFAM, 85%). Exercise also increased muscle GLUT4 mRNA and protein (30–52%) and reduced abdominal fat (5%) and plasma triglycerides (25%). None of these changes were affected by age. In contrast, insulin sensitivity improved in younger people but not in middle-aged or older groups. Thus, the muscle mitochondrial response to 4 months of aerobic exercise training was similar in all age-groups, although the older people did not have an improvement in insulin sensitivity.

Improvement of glycemic status by insulin is associated with profound changes in amino acid metabolism in type 1 diabetes. In contrast, a dissociation of insulin effect on glucose and amino acid metabolism has been reported in type 2 diabetes. Type 2 diabetic patients are reported to have reduced muscle oxidative enzymes and Vo2max. We investigated the effect of 11 days of intensive insulin treatment (T2D+) on whole-body amino acid kinetics, muscle protein synthesis rates, and muscle functions in eight type 2 diabetic subjects after withdrawing all treatments for 2 weeks (T2D−) and compared the results with those of weight-matched lean control subjects using stable isotopes of the amino acids. Whole-body leucine, phenylalanine and tyrosine fluxes, leucine oxidation, and plasma amino acid levels were similar in all groups, although plasma glucose levels were significantly higher in T2D−. Insulin treatment reduced leucine nitrogen flux and transamination rates in subjects with type 2 diabetes. Synthesis rates of muscle mitochondrial, sarcoplasmic, and mixed muscle proteins were not affected by glycemic status or insulin treatment in subjects with type 2 diabetes. Muscle strength was also unaffected by diabetes or glycemic status. In contrast, the diabetic patients showed increased tendency for muscle fatigability. Insulin treatment also failed to stimulate muscle cytochrome C oxidase activity in the diabetic patients, although it modestly elevated citrate synthase. In conclusion, improvement of glycemic status by insulin treatment did not alter whole-body amino acid turnover in type 2 diabetic subjects, but leucine nitrogen flux, transamination rates, and plasma ketoisocaproate level were decreased. Insulin treatments in subjects with type 2 diabetes had no effect on muscle mitochondrial protein synthesis and cytochrome C oxidase, a key enzyme for ATP production.

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.

Cumulative mtDNA damage occurs in aging animals, and mtDNA mutations are reported to accelerate aging in mice. We determined whether aging results in increased DNA oxidative damage and reduced mtDNA abundance and mitochondrial function in skeletal muscle of human subjects. Studies performed in 146 healthy men and women aged 18–89 yr demonstrated that mtDNA and mRNA abundance and mitochondrial ATP production all declined with advancing age. Abundance of mtDNA was positively related to mitochondrial ATP production rate, which in turn, was closely associated with aerobic capacity and glucose tolerance. The content of several mitochondrial proteins was reduced in older muscles, whereas the level of the oxidative DNA lesion, 8-oxo-deoxyguanosine, was increased, supporting the oxidative damage theory of aging. These results demonstrate that age-related muscle mitochondrial dysfunction is related to reduced mtDNA and muscle functional changes that are common in the elderly.