Reduced plasma adiponectin concentrations may contribute to impaired insulin activation of glycogen synthase in skeletal muscle of patients with type 2 diabetes

Effect of Different Intensities of Aerobic Exercise Combined with Resistance Exercise on Body Fat, Lipid Profiles, and Adipokines in Middle-Aged Women with Obesity

We aimed to investigate the effect of different intensities of aerobic exercise (VO₂max: 50% vs. 80%) on body weight, body fat percentage, lipid profiles, and adipokines in obese middle-aged women after 8 weeks of combined aerobic and resistance exercise. The participants included 16 women aged >40 years with a body fat percentage of ≥30%; they were randomly assigned to the resistance and either moderate (RME, 50% VO₂max, 200 kcal [n = 8]) or vigorous aerobic exercise groups (RVE, 80% VO₂max, 200 kcal [n = 8]), respectively. After 8 weeks of exercise, we observed that body weight and body fat percentage decreased significantly in both groups (p < 0.01). The total cholesterol (p < 0.01) and LDL (p < 0.05) levels decreased significantly in the RME group, while triglyceride levels decreased significantly in both groups (p < 0.01). The HDL levels tended to increase only slightly in both groups. The adiponectin levels decreased significantly in the RVE group (p < 0.05), and the leptin levels decreased significantly in both groups (p < 0.05). To prevent and treat obesity in middle-aged women, combined exercise (aerobic and resistance) is deemed effective; additionally, aerobic exercise of moderate intensity during combined exercise could be more effective than that of vigorous intensity.

Effects of green tea catechins and exercise training on body composition parameters

The impact of phytochemicals, as green tea catechins, on body composition measures has become a relevant topic as ongoing epidemiological evidence suggests their potential role in weight loss. Although catechins have been shown to modulate fat and energy metabolism, clinical effects of green tea consumption still remain controversial. Given the role played by physical exercise in weight management, it is important to determine whether the association of catechins and exercise is able to improve outcomes over and above the beneficial effects of exercise alone. Considering that scientific findings on this topic are not entirely consistent, aim of the present review was to assess the current scientific literature regarding the interplay between green tea catechins and exercise in overweight and obese populations. In particular, it was evaluated whether the addition of green tea supplementation to exercise training was able to further improve the exercise-induced changes in body composition parameters.

Altered intramuscular network of lipid droplets and mitochondria in type 2 diabetes

Excessive storage of lipid droplets (LDs) in skeletal muscles is a hallmark of type 2 diabetes. However, LD morphology displays a high degree of subcellular heterogeneity and varies between single muscle fibers, which impedes the current understanding of lipid-induced insulin resistance. Using quantitative transmission electron microscopy (TEM), we conducted a comprehensive single-fiber morphological analysis to investigate the intramuscular network of LDs and mitochondria, and the effects of 8 wk of high-intensity interval training (HIIT) targeting major muscle groups, in patients with type 2 diabetes and nondiabetic obese and lean controls. We found that excessive storage of intramuscular lipids in patients with type 2 diabetes was exclusively explained by extremely large LDs situated in distinct muscle fibers with a location-specific deficiency in subsarcolemmal mitochondria. After HIIT, this intramuscular deficiency was improved by a remodeling of LD size and subcellular distribution and mitochondrial content. Analysis of LD morphology further revealed that individual organelles were better described as ellipsoids than spheres. Moreover, physical contact between LD and mitochondrial membranes indicated a dysfunctional interplay between organelles in the diabetic state. Taken together, type 2 diabetes should be recognized as a metabolic disease with high cellular heterogeneity in intramuscular lipid storage, underlining the relevance of single-cell technologies in clinical research. Furthermore, HIIT changed intramuscular LD storage toward nondiabetic characteristics.

A Signature of Exaggerated Adipose Tissue Dysfunction in Type 2 Diabetes Is Linked to Low Plasma Adiponectin and Increased Transcriptional Activation of Proteasomal Degradation in Muscle

Insulin resistance in skeletal muscle in type 2 diabetes (T2D) is characterized by more pronounced metabolic and molecular defects than in obesity per se. There is increasing evidence that adipose tissue dysfunction contributes to obesity-induced insulin resistance in skeletal muscle. Here, we used an unbiased approach to examine if adipose tissue dysfunction is exaggerated in T2D and linked to diabetes-related mechanisms of insulin resistance in skeletal muscle. Transcriptional profiling and biological pathways analysis were performed in subcutaneous adipose tissue (SAT) and skeletal muscle biopsies from 17 patients with T2D and 19 glucose-tolerant, age and weight-matched obese controls. Findings were validated by qRT-PCR and western blotting of selected genes and proteins. Patients with T2D were more insulin resistant and had lower plasma adiponectin than obese controls. Transcriptional profiling showed downregulation of genes involved in mitochondrial oxidative phosphorylation and the tricarboxylic-acid cycle and increased expression of extracellular matrix (ECM) genes in SAT in T2D, whereas genes involved in proteasomal degradation were upregulated in the skeletal muscle in T2D. qRT-PCR confirmed most of these findings and showed lower expression of adiponectin in SAT and higher expression of myostatin in muscle in T2D. Interestingly, muscle expression of proteasomal genes correlated positively with SAT expression of ECM genes but inversely with the expression of ADIPOQ in SAT and plasma adiponectin. Protein content of proteasomal subunits and major ubiquitin ligases were unaltered in the skeletal muscle of patients with T2D. A transcriptional signature of exaggerated adipose tissue dysfunction in T2D, compared with obesity alone, is linked to low plasma adiponectin and increased transcriptional activation of proteasomal degradation in skeletal muscle.

High-intensity interval training combining rowing and cycling efficiently improves insulin sensitivity, body composition and VO2max in men with obesity and type 2 diabetes

AIMS

Non-weight-bearing high-intensity interval training (HIIT) involving several muscle groups may efficiently improve metabolic health without compromising adherence in obesity and type 2 diabetes. In a non-randomized intervention study, we examined the effect of a novel HIIT-protocol, recruiting both lower and upper body muscles, on insulin sensitivity, measures of metabolic health and adherence in obesity and type 2 diabetes.

METHODS

In 15 obese men with type 2 diabetes and age-matched obese (n=15) and lean (n=18) glucose-tolerant men, the effects of 8-weeks supervised HIIT combining rowing and cycling on ergometers (3 sessions/week) were examined by DXA-scan, incremental exercise test and hyperinsulinemic-euglycemic clamp combined with indirect calorimetry.

RESULTS

At baseline, insulin-stimulated glucose disposal rate (GDR) was ~40% reduced in the diabetic vs the non-diabetic groups (all p<0.01). In response to HIIT, insulin-stimulated GDR increased ~30-40% in all groups (all p<0.01) entirely explained by increased glucose storage. These changes were accompanied by ~8-15% increases in VO₂max, (all p<0.01), decreased total fat mass and increased lean body mass in all groups (all p<0.05). There were no correlations between these training adaptations and no group-differences in these responses. HbA1c showed a clinically relevant decrease in men with type 2 diabetes (4±2 mmol/mol; p<0.05). Importantly, adherence was high (>95%) in all groups and no injuries were reported.

CONCLUSIONS

A novel HIIT-protocol recruiting lower and upper body muscles efficiently improves insulin sensitivity, VO₂max and body composition with intact responses in obesity and type 2 diabetes. The high adherence and lack of injuries show that non-weight-bearing HIIT involving several muscle groups is a promising mode of exercise training in obesity and type 2 diabetes.

Physical activity and adipokine levels in individuals with type 2 diabetes: A literature review and practical applications

We review the effects of acute and long-term physical activity on adipokine levels in individuals with type 2 diabetes (T2D). Three electronic databases were searched. Studies made in animal models were excluded, while studies based on participants with and without T2D, and also studies with type 1 diabetes were included. Of the 2,450 citations, 63 trials, including randomised control trials, cross-sectional and longitudinal studies, met our inclusion criteria. Seventy and five percent of studies reported the effects of physical activity on tumor necrosis factor-alpha (TNFα), interleukin 6 (IL-6), adiponectin, visfatin, omentin-1, and leptin levels. There are no robust results due to variations in exercise modality, intensity, duration, and also differences in cohort characteristics in the literature. Only four studies described the effects of an acute session of physical activity on adipokine levels. Overall, physical activity improves diabetes status by regulating adipokine levels. However, long-term aerobic + resistance training combined with dietary modifications is likely to be a more effective strategy for improving adipokines profiles in patients with type 2 diabetes.

The effects of physical activity on adipokines in individuals with overweight/obesity across the lifespan: A narrative review

This narrative review summarizes current knowledge on the effects of physical activity (PA) on adipokine levels in individuals with overweight and obesity. Approximately 90 investigations including randomized control, cross-sectional and longitudinal studies that reported on the effects of a single session of PA (acute) or long-term PA (chronic) on adipokine levels in individuals with overweight/obesity were reviewed. The findings support the notion that there is consensus on the benefits of chronic exercise training-regardless of the mode (resistance vs. aerobic), intensity and cohort (healthy vs. diabetes)-on adipokine levels (such as tumour necrosis factor-alpha, interleukin-6, adiponectin, visfatin, omentin-1 and leptin). However, several confounding factors (frequency, intensity, time and type of exercise) can alter the magnitude of the effects of an acute exercise session. Available evidence suggests that PA, as a part of routine lifestyle behaviour, improves obesity complications by modulating adipokine levels. However, additional research is needed to help identify the most effective interventions to elicit the most beneficial changes in adipokine levels in individuals with overweight/obesity.

Sex differences in the expression of lipid oxidation and glucose uptake genes in muscles of fasted mice

Fasting has become increasingly popular for treatment and prevention of obesity. Sex differences in the mechanisms of adaptation to fasting may contribute to choosing a therapeutic strategy for correction of metabolic disorders. Hepatokine fibroblast growth factor 21 (FGF21) is involved in the adaptation to fasting. Muscles are assumed to be the main energy-consuming tissue in the body, as muscle metabolism plays an important role in the adaptation to nutritional deficit. However, there is still little information on sex differences in muscle and FGF21 physiological response to fasting. Our aim was to find out whether there were sex differences in hormonal regulation and the expression of genes controlling glucose and lipid metabolism in skeletal muscles in response to fasting. We estimated the effect of 24-hour fasting on the expression of genes involved in lipid (Ucp3, Cpt1) and carbohydrate (Slc2a4) metabolism in muscles and evaluated changes in body weight and blood plasma levels of glucose, insulin, free fatty acids (FFA), adiponectin, and FGF21 in male and female C57BL/6J mice. None of the genes studied (Ucp3, Cpt1 and Slc2a4) showed sex-related changes at mRNA levels in control groups, but females exposed to fasting demonstrated a significant increase in the expression of all genes as compared to control. Fasting significantly decreased body weight and glucose blood plasma levels in animals of both sexes but exerted no effect on the levels of insulin or FFA. The adiponectin and FGF21 levels were increased in response to fasting, the increase in females being significant. We were first to show sex dimorphism in muscle gene expression and FGF21 blood level in response to fasting. In females, the greater increase in FGF21 and adiponectin blood levels was positively associated with the greater upregulation of lipid oxidation and glucose uptake gene expression.

Metabolic Syndrome Exacerbates the Recognition Memory Impairment and Oxidative-Inflammatory Response in Rats with an Intrahippocampal Injection of Amyloid Beta 1-42

An important worldwide health problem as the result of current lifestyle is metabolic syndrome (MS). It has been shown that MS induced by a high-calorie diet (HCD) in rats produces cognitive deterioration in the novel object recognition test (NORt) and decreases synaptic connections and dendritic order in the hippocampus and temporal cortex. However, it is unknown whether MS induced by an HCD participates in the cognitive process observed with the injection of Aβ_1–42 into the hippocampus of rats as a model of Alzheimer disease (AD). The induction of MS in rats produces a deterioration in NORt; however, rats with MS injected with Aβ_1–42 show a major deterioration in the cognitive process. This event could be explained by the increment in the oxidative stress in both cases studied (MS and Aβ_1–42): together, the hippocampus and temporal cortex produce an enhancer effect. In the same way, we observed an increment in interleukin-1β, TNF-α, and GFAP, indicative of exacerbated inflammatory processes by the combination of MS and Aβ_1–42. We can conclude that MS might play a key role in the apparition and development of cognitive disorders, including AD. We propose that metabolic theory is important to explain the apparition of cognitive diseases.

Mitochondria, glycogen, and lipid droplets in skeletal muscle during testosterone treatment and strength training: a randomized, double-blinded, placebo-controlled trial

Low testosterone levels in aging men are associated with insulin resistance. Mitochondrial dysfunction, changes in glycogen metabolism, and lipid accumulation are linked to insulin resistance in skeletal muscle. In this randomized, double-blinded, placebo-controlled study, we investigated the effects of six-month testosterone replacement therapy (TRT) and strength training (ST) on mitochondrial, glycogen, and lipid droplet (LD) content in skeletal muscle of aging men with subnormal bioavailable testosterone (BioT) levels. Mitochondrial, glycogen, and LD volume fractions in muscle biopsies were estimated by transmission electron microscopy. Insulin sensitivity (insulin-stimulated Rd) and body composition were assessed by euglycemic-hyperinsulinemic clamp and dual X-ray absorptiometry, respectively. TRT significantly increased total testosterone levels, BioT, and lean body mass (LBM) (p < 0.05), whereas percent body fat decreased (p < 0.05), and insulin sensitivity was unchanged. Baseline mitochondrial volume fraction correlated inversely with percent body fat (ρ = -0.43; p = 0.003). Δ-mitochondrial fraction correlated positively with Δ-total testosterone (ρ = 0.70; p = 0.02), and Δ-glycogen fraction correlated inversely with Δ-LBM (ρ = -0.83; p = 0.002) during six-month TRT, but no significant changes were observed in mitochondrial, glycogen, and LD volume fractions during TRT and ST. In conclusion, in this exploratory small-scale study, the beneficial effects of six-month TRT on total testosterone, LBM, and percent body fat were not followed by significant changes in fractions of mitochondria, glycogen, or lipid in skeletal muscle of aging men with lowered testosterone levels. Six-month ST or combined three-month ST+TRT did not change intramyocellular mitochondria, glycogen, and LD fractions compared to placebo. However, further studies with a larger sample size are needed.

A potential determinant role of adiponectin and receptors for the early embryo development in PCOS patients with obesity hinted by quantitative profiling

OBJECTIVE

To identify the quantitative profiling of adiponectin and its receptors (AdipoR1, AdipoR2, and T-cadherin) in cumulus cells (CCs) and to evaluate their roles in the early embryo development of polycystic ovary syndrome (PCOS) patients, in part, with obesity.

METHODS

Fifty-five subjects were divided into two groups according to the body mass index. Oocytes were further inseminated and only mature and normal fertilized oocytes (2PN) were included in this research. Real-time PCR and western blot were performed to identify adiponectin and its receptors in CCs.

RESULTS

Adiponectin and receptors were ubiquitously expressed in CCs of PCOS and non-PCOS patients. The level of AdipoR2 in CCs from the oocytes yielding blastocyst after 5/6 days in vitro culture was markedly higher than in those from oocytes could not develop to blastocyst stage after Day 6, for non-obese or obese PCOS patients (0.1647 ± 0.0161 versus 0.0783 ± 0.0385, 0.1948 ± 0.0307 versus 0.1057 ± 0.0236, respectively, p < 0.05). In addition, only in patients with PCOS and concurrent obesity the AdipoR1 in CCs was considerably increased in CC-B⁺ compared with CC-B^- subgroup (0.5162 ± 0.0371 versus 0.2448 ± 0.0333, p < 0.01).

CONCLUSION

The development of early embryo was associated with the up-regulation of AdipoR1 and AdipoR2 in PCOS patients. Our results suggested that adiponectin could positively modulate embryo development in humans. Further investigations should be carried out to unlock the crucial role that adiponectin plays in embryo development.

Endogenous and exogenous factors influencing the concentrations of adiponectin in body fluids and tissues in the bovine

Adiponectin, one of the messenger molecules secreted from adipose tissue that are collectively termed adipokines, has been demonstrated to play a central role in lipid and glucose metabolism in humans and laboratory rodents; it improves insulin sensitivity and exerts antidiabetic and antiinflammatory actions. Adiponectin is synthesized as a 28 kDa monomer but is not secreted as such; instead, it is glycosylated and undergoes multimerization to form different molecular weight multimers before secretion. Adiponectin is one of the most abundant adipokines (μg/mL range) in the circulation. The concentrations are negatively correlated with adipose depot size, in particular with visceral fat mass in humans. Adiponectin exerts its effects by activating a range of different signaling molecules via binding to 2 transmembrane receptors, adiponectin receptor 1 and adiponectin receptor 2. The adiponectin receptor 1 is expressed primarily in the skeletal muscle, whereas adiponectin receptor 2 is predominantly expressed in the liver. Many of the functions of adiponectin are relevant to growth, lactation, and health and are thus of interest in both beef and dairy production systems. Studies on the role of the adiponectin protein in cattle have been impeded by the lack of reliable assays for bovine adiponectin. Although there are species-specific bovine adiponectin assays commercially available, they suffer from a lack of scientific peer-review of validity. Quantitative data about the adiponectin protein in cattle available in the literature emerged only during the last 3 yr and were largely based on Western blotting using either antibodies against human adiponectin or partial peptides from the bovine sequence. Using native bovine high-molecular-weight adiponectin purified from serum, we were able to generate a polyclonal antiserum that can be used for Western blot but also in an ELISA system, which was recently validated. The objective of this review is to provide an overview of the literature about the adiponectin protein in cattle addressing the following aspects: (1) the course of the adiponectin serum concentrations during development in both sexes, during inflammation, nutritional energy deficit and energy surplus, and lactation-induced changes including the response to supplementation with conjugated linoleic acids and with niacin, (2) the concentrations of adiponectin in subcutaneous vs visceral fat depots of dairy cows, (3) the protein expression of adiponectin in tissues other than adipose, and (4) the concentrations in different body fluids including milk.

Markers of autophagy are adapted to hyperglycaemia in skeletal muscle in type 2 diabetes

AIMS/HYPOTHESIS

Autophagy is a catabolic process that maintains cellular homeostasis by degradation of protein aggregates and selective removal of damaged organelles, e.g. mitochondria (mitophagy). Insulin resistance in skeletal muscle has been linked to mitochondrial dysfunction and altered protein metabolism. Here, we investigated whether abnormalities in autophagy are present in human muscle in obesity and type 2 diabetes.

METHODS

Using a case-control design, skeletal muscle biopsies obtained in the basal and insulin-stimulated states from patients with type 2 diabetes during both euglycaemia and hyperglycaemia, and from glucose-tolerant lean and obese individuals during euglycaemia, were used for analysis of mRNA levels, protein abundance and phosphorylation of autophagy-related proteins.

RESULTS

Muscle transcript levels of autophagy-related genes (ULK1, BECN1, PIK3C3, ATG5, ATG7, ATG12, GABARAPL1, MAP1LC3B, SQSTM1, TP53INP2 and FOXO3A [also known as FOXO3]), including some specific for mitophagy (BNIP3, BNIP3L and MUL1), and protein abundance of autophagy-related gene (ATG)7 and Bcl-2/adenovirus E1B 19-kDa-interacting protein 3 (BNIP3), as well as content and phosphorylation of forkhead box O3A (FOXO3A) were similar among the groups. Insulin reduced lipidation of microtubule-associated protein light chain 3 (LC3)B-I to LC3B-II, a marker of autophagosome formation, with no effect on p62/sequestosome 1 (SQSTM1) content in muscle of lean and obese individuals. In diabetic patients, insulin action on LC3B was absent and p62/SQSTM1 content increased when studied under euglycaemia, whereas the responses of LC3B and p62/SQSTM1 to insulin were normalised during hyperglycaemia.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that the levels of autophagy-related genes and proteins in muscle are normal in obesity and type 2 diabetes. This suggests that muscle autophagy in type 2 diabetes has adapted to hyperglycaemia, which may contribute to preserve muscle mass.

Chronic activity-based therapy does not improve body composition, insulin-like growth factor-I, adiponectin, or myostatin in persons with spinal cord injury

Spinal cord injury (SCI) induces dramatic changes in body composition including reductions in fat-free mass (FFM) and increases in fat mass (FM).

OBJECTIVE

To examine changes in body composition in response to chronic activity-based therapy (ABT) in persons with SCI.

DESIGN

Longitudinal exercise intervention.

METHODS

Seventeen men and women with SCI (mean age=36.1±11.5 years) completed 6 months of supervised ABT consisting of load bearing, resistance training, locomotor training, and functional electrical stimulation. At baseline and after 3 and 6 months of ABT, body weight, body fat, and FFM were assessed using dual-energy X-ray absorptiometry, and fasting blood samples were obtained to assess changes in insulin-like growth factor-I (IGF-I), adiponectin, and myostatin.

RESULTS

Across all subjects, there was no change (P>0.05) in body weight, percent body fat, or FFM of the leg, arm, or trunk, whereas whole-body FFM declined (P=0.02, 50.4±8.4 to 49.2±7.4 kg). No changes (P=0.21-0.41) were demonstrated in IGF-I, adiponectin, or myostatin during the study.

CONCLUSIONS

Chronic ABT focusing on the lower extremity does not slow muscle atrophy or alter body fat, body mass, or regional depots of FFM in persons with SCI. Further, it does not induce beneficial changes in adiponectin, myostatin, or IGF-I. Alternative exercise-based therapies are needed in SCI to reverse muscle atrophy and minimize the onset of related health risks.

The rapid increase of circulating adiponectin in neonatal calves depends on colostrum intake

Adiponectin, an adipokine, regulates metabolism and insulin sensitivity. Considering that the transplacental transfer of maternal proteins of high molecular weight is hindered in ruminants, this study tested the hypothesis that the blood concentration of adiponectin in neonatal calves largely reflects their endogenous synthesis whereby the intake of colostrum might modify the circulating concentrations. We thus characterized the adiponectin concentrations in neonatal and young calves that were fed either colostrum or formula. Three trials were performed: in trial 1, 20 calves were all fed colostrum for 3 d, and then formula until weaning. Blood samples were collected on d 0 (before colostrum feeding), and on d 1, 3, 11, 22, 34, 43, 52, 70, 90, and 108 postnatum. In trial 2, 14 calves were studied for the first 4 d of life. They were fed colostrum (n=7) or formula (n=7), and blood samples were taken right after birth and before each morning feeding on d 2, 3, and 4. In trial 3, calves born preterm (n=7) or at term received colostrum only at 24 h postnatum. Blood was sampled at birth, and before and 2 h after feeding. Additionally, allantoic fluid and blood from 4 Holstein cows undergoing cesarean section were sampled. Adiponectin was quantified by ELISA. In trial 1, the serum adiponectin concentrations recorded on d 3 were 4.7-fold higher than before colostrum intake. The distribution of the molecular weight forms of adiponectin differed before and after colostrum consumption. In trial 2, the colostrum group had consistently greater plasma adiponectin concentrations than the formula group after the first meal. In trial 3, the preterm calves tended to have lower concentrations of plasma adiponectin than the term calves at birth and before and 2 h after feeding. Furthermore, the adiponectin concentrations were substantially lower in allantoic fluid than in the sera from neonatal calves and from cows at parturition. Our results show that calves are born with very low blood concentrations of adiponectin and placental transfer of adiponectin to the bovine fetus is unlikely. In conclusion, colostrum intake is essential for the postnatal increase of circulating adiponectin in newborn calves.

Integrative neurobiology of metabolic diseases, neuroinflammation, and neurodegeneration

Alzheimer's disease (AD) is a complex, multifactorial disease with a number of leading mechanisms, including neuroinflammation, processing of amyloid precursor protein (APP) to amyloid β peptide, tau protein hyperphosphorylation, relocalization, and deposition. These mechanisms are propagated by obesity, the metabolic syndrome and type-2 diabetes mellitus. Stress, sedentariness, dietary overconsumption of saturated fat and refined sugars, and circadian derangements/disturbed sleep contribute to obesity and related metabolic diseases, but also accelerate age-related damage and senescence that all feed the risk of developing AD too. The complex and interacting mechanisms are not yet completely understood and will require further analysis. Instead of investigating AD as a mono- or oligocausal disease we should address the disease by understanding the multiple underlying mechanisms and how these interact. Future research therefore might concentrate on integrating these by “systems biology” approaches, but also to regard them from an evolutionary medicine point of view. The current review addresses several of these interacting mechanisms in animal models and compares them with clinical data giving an overview about our current knowledge and puts them into an integrated framework.

Effect of testosterone on markers of mitochondrial oxidative phosphorylation and lipid metabolism in muscle of aging men with subnormal bioavailable testosterone

OBJECTIVE

Recent studies have indicated that serum testosterone in aging men is associated with insulin sensitivity and expression of genes involved in oxidative phosphorylation (OxPhos), and that testosterone treatment increases lipid oxidation. Herein, we investigated the effect of testosterone therapy on regulators of mitochondrial biogenesis and markers of OxPhos and lipid metabolism in the skeletal muscle of aging men with subnormal bioavailable testosterone levels.

METHODS

Skeletal muscle biopsies were obtained before and after treatment with either testosterone gel (n=12) or placebo (n=13) for 6 months. Insulin sensitivity and substrate oxidation were assessed by euglycemic-hyperinsulinemic clamp and indirect calorimetry. Muscle mRNA levels and protein abundance and phosphorylation of enzymes involved in mitochondrial biogenesis, OxPhos, and lipid metabolism were examined by quantitative real-time PCR and western blotting.

RESULTS

Despite an increase in lipid oxidation (P<0.05), testosterone therapy had no effect on insulin sensitivity or mRNA levels of genes involved in mitochondrial biogenesis (PPARGC1A, PRKAA2, and PRKAG3), OxPhos (NDUFS1, ETFA, SDHA, UQCRC1, and COX5B), or lipid metabolism (ACADVL, CD36, CPT1B, HADH, and PDK4). Consistently, protein abundance of OxPhos subunits encoded by both nuclear (SDHA and UQCRC1) and mitochondrial DNA (ND6) and protein abundance and phosphorylation of AMP-activated protein kinase and p38 MAPK were unaffected by testosterone therapy.

CONCLUSION

The beneficial effect of testosterone treatment on lipid oxidation is not explained by increased abundance or phosphorylation-dependent activity of enzymes known to regulate mitochondrial biogenesis or markers of OxPhos and lipid metabolism in the skeletal muscle of aging men with subnormal bioavailable testosterone levels.

Relationships of serum soluble E-selectin concentration with insulin sensitivity and metabolic flexibility in lean and obese women

The markers of endothelial dysfunction, including soluble E-selectin (sE-selectin), are related to insulin resistance, which is associated with metabolic inflexibility, i.e., impaired stimulation of carbohydrate oxidation and impaired inhibition of lipid oxidation by insulin. Endothelial dysfunction may also be important in the metabolic syndrome. The aim of our study was to analyze the association of sE-selectin with insulin sensitivity and metabolic flexibility in lean and obese women. We examined 22 lean women (BMI < 25 kg m(-2)) and 26 overweight or obese women (BMI > 25 kg m(-2)) with normal glucose tolerance. A hyperinsulinemic euglycemic clamp and indirect calorimetry were performed. An increase in the respiratory exchange ratio in response to insulin was used as a measure of metabolic flexibility. Obese women had lower insulin sensitivity (P < 0.01), higher plasma sE-selectin (P = 0.007), and higher the metabolic syndrome total Z-score (MS Z-score) (P < 0.0001). Insulin sensitivity was negatively correlated with sE-selectin level (r = -0.24, P = 0.04). sE-selectin was associated with the rate of carbohydrate oxidation at the baseline state (r = 0.31, P = 0.007) and was negatively correlated with metabolic flexibility (r = -0.34, P = 0.003). MS Z-score correlated positively with sE-selectin level and negatively with metabolic flexibility and insulin sensitivity (r = 0.49, P < 0.0001, r = -0.29, P = 0.04, r = -0.51, P < 0.0001, respectively). In multiple regression analysis we observed that the relationship between metabolic flexibility and sE-selectin (β = -0.36; P = 0.004) was independent of the other evaluated factors. Our data suggest that endothelial dysfunction as assessed by plasma sE-selectin is associated with metabolic flexibility, inversely and independently of the other estimated factors.

A Comparison of the Effects of Aerobic and Intense Exercise on the Type 2 Diabetes Mellitus Risk Marker Adipokines, Adiponectin and Retinol Binding Protein-4

With a more sedentary population comes growing rates of obesity and increased type 2 diabetes mellitus (T2DM) risk. Exercise generally induces positive changes in traditional T2DM risk markers such as lipids, glucose tolerance, and insulin sensitivity; however alterations in concentrations of many circulating cytokines and their respective receptors are also becoming apparent. These cytokines may be early-response health risk factors otherwise overlooked in traditional T2DM risk marker analysis. Plasma levels of two adipocyte-originating cytokines, adiponectin and retinol binding protein 4 (RBP-4), alter following exercise. Adiponectin has anti-inflammatory, anti-atherosclerotic, and anti-insulin resistance roles and its secretion increases with physical activity, whilst elevated RBP-4 leads to increased insulin resistance, and secretion decreases with increasing physical activity; thus these plasma adipokine levels alter favourably following exercise. Although current data are limited, they do suggest that the more intense the exercise, the greater the positive effect on plasma RBP-4 levels, whilst lower intensity aerobic exercise may positively improve adiponectin concentrations. Therefore short-duration, high intensity training may provide a time-efficient alternative to the recommended 150 min moderate aerobic exercise per week in providing positive changes in RBP-4 and other traditional T2DM risk markers and due to increased compliance give greater health benefits over the longer term.

Gender differences in skeletal muscle substrate metabolism - molecular mechanisms and insulin sensitivity

It has become increasingly apparent that substrate metabolism is subject to gender-specific regulation, and the aim of this review is to outline the available evidence of molecular gender differences in glucose and lipid metabolism of skeletal muscle. Female sex has been suggested to have a favorable effect on glucose homeostasis, and the available evidence from hyperinsulinemic-euglycemic clamp studies is summarized to delineate whether there is a gender difference in whole-body insulin sensitivity and in particular insulin-stimulated glucose uptake of skeletal muscle. Whether an eventual higher insulin sensitivity of female skeletal muscle can be related to gender-specific regulation of molecular metabolism will be topic for discussion. Gender differences in muscle fiber type distribution and substrate availability to and in skeletal muscle are highly relevant for substrate metabolism in men and women. In particular, the molecular machinery for glucose and fatty acid oxidative and storage capacities in skeletal muscle and its implications for substrate utilization during metabolic situations of daily living are discussed, emphasizing their relevance for substrate choice in the fed and fasted state, and during periods of physical activity and recovery. Together, handling of carbohydrate and lipids and regulation of their utilization in skeletal muscle have implications for whole-body glucose homeostasis in men and women. 17-β estradiol is the most important female sex hormone, and the identification of estradiol receptors in skeletal muscle has opened for a role in regulation of substrate metabolism. Also, higher levels of circulating adipokines as adiponectin and leptin in women and their implications for muscle metabolism will be considered.

Adiponectin concentration is associated with muscle insulin sensitivity, AMPK phosphorylation, and ceramide content in skeletal muscles of men but not women

Adiponectin is an adipokine that regulates metabolism and increases insulin sensitivity. Mechanisms behind this insulin-sensitizing effect have been investigated in rodents, but little is known in humans, especially in skeletal muscle. Women have higher serum concentrations of adiponectin than men and are generally more insulin sensitive in skeletal muscle than men. We show here that large differences exist between men and women with regard to apparent adiponectin regulation of insulin-stimulated glucose uptake in skeletal muscle. Serum adiponectin was significantly associated with leg glucose uptake in healthy, young, lean men, but the association was absent in women. In addition, serum adiponectin was significantly associated with AMP-activated protein kinase (AMPK) phosphorylation in skeletal muscles of men but not in women. Serum adiponectin was also significantly, negatively associated with skeletal muscle ceramide content in men only, and interestingly, ceramide content was negatively associated with adiponectin receptor 1 (AdipoR1) expression in skeletal muscles of men. Women had lower AdipoR1 expression in skeletal muscle and a lower percentage of glycolytic adiponectin-sensitive type 2 muscle fibers than men. These associations suggest that the insulin-sensitizing effect of adiponectin on human male skeletal muscles may be mediated via AdipoR1 to activation of AMPK, leading to lowering of ceramide content. The lower skeletal muscle AdipoR1 protein expression and lower expression of adiponectin-sensitive type 2 muscle fibers in women than in men may explain the apparent lesser sensitivity to adiponectin in women.

Increased serum concentrations of persistent organic pollutants among prediabetic individuals: potential role of altered substrate oxidation patterns

CONTEXT

There is a need for a better understanding of the potential role of persistent organic pollutants (POPs) in the pathogenesis of type 2 diabetes.

OBJECTIVE

Our objective was to determine the association of serum concentrations of POPs with early signs of type 2 diabetes in regard to glucose and lipid metabolism.

RESEARCH DESIGN AND METHODS

In this cross-sectional study, we used recent studies of 148 Danish middle-aged normoglycemic, prediabetic, and diabetic individuals examined by the euglycemic-hyperinsulinemic clamp technique with indirect calorimetry; 66 of these individuals also had an i.v. glucose tolerance test. Concentrations of POPs were analyzed in banked serum from the participants. Associations with basal and insulin-stimulated glucose and lipid metabolism were assessed after adjustment for age, sex, and body fat percentage.

RESULTS

Individuals with prediabetes and diabetes had higher serum concentrations of several POPs compared with normoglycemic individuals. In the nondiabetic population, higher POPs levels were associated with elevated fasting plasma glucose concentrations as well as reduced glucose oxidation, elevated lipid oxidation, and elevated serum concentrations of free fatty acids (P < 0.05). We found no associations of POPs with first-phase insulin secretion, hepatic or peripheral insulin sensitivity, or nonoxidative glucose metabolism.

CONCLUSIONS

Diabetic and prediabetic individuals have elevated serum concentrations of POPs. In nondiabetic individuals, POPs exposure is related to altered substrate oxidation patterns with lower glucose oxidation and higher lipid oxidation rates. These findings indicate that POPs may affect peripheral glucose metabolism by modifying pathways involved in substrate partitioning rather than decreasing insulin-dependent glucose uptake.

Hyperglycaemia normalises insulin action on glucose metabolism but not the impaired activation of AKT and glycogen synthase in the skeletal muscle of patients with type 2 diabetes

AIMS/HYPOTHESIS

In type 2 diabetes, reduced insulin-stimulated glucose disposal, primarily glycogen synthesis, is associated with defective insulin activation of glycogen synthase (GS) in skeletal muscle. Hyperglycaemia may compensate for these defects, but to what extent it involves improved insulin signalling to glycogen synthesis remains to be clarified.

METHODS

Whole-body glucose metabolism was studied in 12 patients with type 2 diabetes, and 10 lean and 10 obese non-diabetic controls by means of indirect calorimetry and tracers during a euglycaemic-hyperinsulinaemic clamp. The diabetic patients underwent a second isoglycaemic-hyperinsulinaemic clamp maintaining fasting hyperglycaemia. Muscle biopsies from m. vastus lateralis were obtained before and after the clamp for examination of GS and relevant insulin signalling components.

RESULTS

During euglycaemia, insulin-stimulated glucose disposal, glucose oxidation and non-oxidative glucose metabolism were reduced in the diabetic group compared with both control groups (p < 0.05). This was associated with impaired insulin-stimulated GS and AKT2 activity, deficient dephosphorylation at GS sites 2 + 2a, and reduced Thr308 and Ser473 phosphorylation of AKT. When studied under hyperglycaemia, all variables of insulin-stimulated glucose metabolism were normalised compared with the weight-matched controls. However, insulin activation and dephosphorylation (site 2 + 2a) of GS as well as activation of AKT2 and phosphorylation at Thr308 and Ser473 remained impaired (p < 0.05).

CONCLUSIONS/INTERPRETATIONS

These data confirm that hyperglycaemia compensates for decreased whole-body glucose disposal in type 2 diabetes. In contrast to previous less well-controlled studies, we provide evidence that the compensatory effect of hyperglycaemia in patients with type 2 diabetes does not involve normalisation of insulin action on GS or upstream signalling in skeletal muscle.

The proteomic signature of insulin-resistant human skeletal muscle reveals increased glycolytic and decreased mitochondrial enzymes

AIMS/HYPOTHESIS

The molecular mechanisms underlying insulin resistance in skeletal muscle are incompletely understood. Here, we aimed to obtain a global picture of changes in protein abundance in skeletal muscle in obesity and type 2 diabetes, and those associated with whole-body measures of insulin action.

METHODS

Skeletal muscle biopsies were obtained from ten healthy lean (LE), 11 obese non-diabetic (OB), and ten obese type 2 diabetic participants before and after hyperinsulinaemic-euglycaemic clamps. Quantitative proteome analysis was performed by two-dimensional differential-gel electrophoresis and tandem-mass-spectrometry-based protein identification.

RESULTS

Forty-four protein spots displayed significant (p < 0.05) changes in abundance by at least a factor of 1.5 between groups. Several proteins were identified in multiple spots, suggesting post-translational modifications. Multiple spots containing glycolytic and fast-muscle proteins showed increased abundance, whereas spots with mitochondrial and slow-muscle proteins were downregulated in the OB and obese type 2 diabetic groups compared with the LE group. No differences in basal levels of myosin heavy chains were observed. The abundance of multiple spots representing glycolytic and fast-muscle proteins correlated negatively with insulin action on glucose disposal, glucose oxidation and lipid oxidation, while several spots with proteins involved in oxidative metabolism and mitochondrial function correlated positively with these whole-body measures of insulin action.

CONCLUSIONS/INTERPRETATION

Our data suggest that increased glycolytic and decreased mitochondrial protein abundance together with a shift in muscle properties towards a fast-twitch pattern in the absence of marked changes in fibre-type distribution contribute to insulin resistance in obesity with and without type 2 diabetes. The roles of several differentially expressed or post-translationally modified proteins remain to be elucidated.

Characterisation of adiponectin multimers and the IGF axis in humans with a heterozygote mutation in the tyrosine kinase domain of the insulin receptor gene

OBJECTIVE

Low levels of adiponectin, IGF-binding protein 1 (IGFBP1) and IGFBP2 and high levels of leptin correlate with several indices of insulin resistance and risk of type 2 diabetes. However, in insulin receptoropathies, plasma adiponectin is paradoxically increased despite severe insulin resistance, whereas the IGF axis is sparsely described. Here, we aimed to characterise the multimeric distribution of adiponectin and the IGF axis in humans with a heterozygous INSR mutation (Arg1174Gln).

METHODS

Blood samples obtained from six Arg1174Gln carriers and ten lean, healthy controls before and after a euglycaemic-hyperinsulinaemic clamp were examined for plasma adiponectin multimers, leptin, total IGF1, IGF2, free IGF1, IGFBP1 and IGFBP2.

RESULTS

Despite tenfold elevated fasting insulin and marked insulin resistance in Arg1174Gln carriers, the levels of total adiponectin, leptin, IGFBP1 and IGFBP2 were similar to those observed in controls, while total IGF1, IGF2 and free IGF1 levels were increased. The relative fraction of high-molecular weight adiponectin was increased, whereas both the absolute concentration and the fraction of low-molecular weight adiponectin were decreased in Arg1174Gln carriers. Interestingly, exogenous insulin failed to suppress total adiponectin in Arg1174Gln carriers, but reduced IGFBP1 and increased IGFBP2 as in controls.

CONCLUSION

The normal levels of adiponectin, IGFBP1 and IGFBP2 in the face of highly elevated insulin levels suggest an impaired ability of insulin to suppress markers of common insulin resistance in carriers of a dominant-negative INSR mutation. However, together with the adaptive increases in IGF1 and IGF2 and a potentially improved distribution of adiponectin multimers, these changes may contribute to rescue insulin action in insulin receptor-deficient individuals.

Relationships between serum adiponectin and soluble TNF-α receptors and glucose and lipid oxidation in lean and obese subjects

Insulin resistance might be associated with an impaired ability of insulin to stimulate glucose oxidation and inhibit lipid oxidation. Insulin action is also inversely associated with TNF-α system and positively related to adiponectin. The aim of the present study was to analyze the associations between serum adiponectin, soluble TNF-α receptors concentrations and the whole-body insulin sensitivity, lipid and glucose oxidation, non-oxidative glucose metabolism (NOGM) and metabolic flexibility in lean and obese subjects. We examined 53 subjects: 25 lean (BMI < 25 kg × m(-2)) and 28 with overweight or obesity (BMI > 25 kg × m(-2)) with normal glucose tolerance. Hyperinsulinemic euglycemic clamp and indirect calorimetry were performed. An increase in respiratory exchange ratio in response to insulin was used as a measure of metabolic flexibility. Obese subjects had lower insulin sensitivity, adiponectin and higher sTNFR1 (all P < 0.001) and sTNFR2 (P = 0.001). Insulin sensitivity was positively related to adiponectin (r = 0.49, P < 0.001) and negatively related to sTNFR1 (r = -0.40, P = 0.004) and sTNFR2 (r = -0.52, P < 0.001). Adiponectin was related to the rate of glucose (r = 0.47, P < 0.001) and lipid (r = -0.40, P = 0.003) oxidation during the clamp, NOGM (r = 0.41, P = 0.002) and metabolic flexibility (r = 0.36, P = 0.007). Serum sTNFR1 and sTNFR2 were associated with the rate of glucose (r = -0.45, P = 0.001; r = -0.51, P < 0.001, respectively) and lipid (r = 0.52, P < 0.001; r = 0.46, P = 0.001, respectively) oxidation during hyperinsulinemia, NOGM (r = -0.31, P = 0.02; r = -0.43, P = 0.002, respectively) and metabolic flexibility (r = -0.47 and r = -0.51, respectively, both P < 0.001) in an opposite manner than adiponectin. Our data suggest that soluble TNF-α receptors and adiponectin have multiple effects on glucose and lipid metabolism in obesity.

Testosterone therapy increased muscle mass and lipid oxidation in aging men

The indication for testosterone therapy in aging hypogonadal men without hypothalamic, pituitary, or testicular disease remains to be elucidated. The aim of this study was to investigate the effect of testosterone therapy on insulin sensitivity, substrate metabolism, body composition, and lipids in aging men with low normal bioavailable testosterone levels using a predefined cutoff level for bioavailable testosterone. A randomized, double-blinded, placebo-controlled study of testosterone treatment (gel) was done on 38 men, aged 60-78 years, with bioavailable testosterone <7.3 nmol/l and a waist circumference >94 cm. Insulin-stimulated glucose disposal (Rd) and substrate oxidation were assessed by euglycemic hyperinsulinemic clamps combined with indirect calorimetry. Lean body mass (LBM) and total fat mass (TFM) were measured by dual x-ray absorptiometry, and serum total testosterone was measured by tandem mass spectrometry. Bioavailable testosterone was calculated. Coefficients (b) represent the placebo-controlled mean effect of intervention. LBM (b = 1.9 kg, p = 0.003) increased while HDL-cholesterol (b = -0.12 mmol/l, p = 0.043) and TFM decreased (b = -1.2 kg, p = 0.038) in the testosterone group compared to placebo. Basal lipid oxidation (b = 5.65 mg/min/m(2), p = 0.045) increased and basal glucose oxidation (b = -9.71 mg/min/m(2), p = 0.046) decreased in response to testosterone therapy even when corrected for changes in LBM. No significant changes in insulin-stimulated Rd was observed (b = -0.01mg/min/m(2), p = 0.92). Testosterone therapy increased muscle mass and lipid oxidation in aging men with low normal bioavailable testosterone levels; however, our data did not support an effect of testosterone on whole-body insulin sensitivity using the euglycemic hyperinsulinemic clamp technique.

Is adiponectin a marker of preclinical atherosclerosis in kidney transplantation?

The aim of this study was to analyze the relationship between pre-transplant adiponectin (pre-ADP), abnormalities in glucose homeostasis (AGH) at three months post-transplantation, and preclinical atherosclerosis in non-diabetic patients prior to kidney transplantation (KT).

METHODS

We carried out a multicenter study in 157 non-diabetic KT patients (66.5% men; age: 50±13 yr). Pre-ADP levels were analyzed using radioimmunoassay. Carotid ultrasound was performed to determine carotid intima-media thickness (c-IMT). Oral glucose tolerance test was carried out to classify patients according ADA criteria.

RESULTS

Of the patients, 52.8% had AGH. Median pre-ADP was 19.5 (14-27) μg/mL. An inverse correlation was found between ADP and HOMA index (r=-0.432; p<0.001). Median c-IMT was 0.6 (0.48-0.71) mm. Significant inverse correlation existed between ADP and c-IMT on both sides (p<0.05). Patients with c-IMT >0.6 mm had more AGH (p=0.012) and lower ADP levels (p=0.02). We performed a logistic regression analysis using preclinical atherosclerosis (c-IMT ≥0.6 mm) as dependent variable and sex, age, BMI, ADP, AGH, and HOMA index as independent variables of altered c-IMT. Age, pre-ADP, and AGH were independent risk factors for elevated c-IMT.

CONCLUSIONS

Patients with AGH have a greater presence of preclinical atherosclerosis. ADP has an inverse relationship with AGH and is an independent marker of preclinical atherosclerosis.

Improvement in insulin sensitivity by weight loss does not affect hyperinsulinemia-mediated reduction in total and high molecular weight adiponectin: a MONET study

Acute hyperinsulinemia reduces total and high molecular weight (HMW) adiponectin levels in humans. Whether an increase in insulin sensitivity (IS) is accompanied by a greater suppressive effect of hyperinsulinemia on adiponectin levels is unknown, however. To clarify the inhibitory role of insulin on adiponectin, total and HMW adiponectin levels were measured during acute hyperinsulinemia before and after an improvement in insulin sensitivity in response to weight loss. Forty-six overweight and obese postmenopausal women were randomized to either 6-month caloric restriction (CR) alone (n = 22), or CR with resistance training (CR+RT, n = 24). IS (hyperinsulinemic-euglycemic clamp) was assessed before and after weight loss. Total and HMW adiponectin levels were measured by ELISA at baseline, 90, 160, and 180 min of each clamp. Relative mean body weight loss was -8.0% ± 4.4% for both groups (CR: -7.7% ± 3.8%; CR+RT: -8.2% ± 5.0%). IS increased significantly, by 18.4% ± 25.3% (CR: 19.3% ± 29.7%; CR+RT: 17.7% ± 21.0%). Before each intervention, total and HMW adiponectin levels in both groups significantly decreased in response to hyperinsulinemia (total: -8.4% ± 19.4%; HMW: -3.2% ± 13.2%). Despite the improvement in IS seen after each intervention, a similar pattern of reduction to that before weight loss was observed in total and HMW adiponectin levels during hyperinsulinemia. These results establish that total and HMW adiponectin levels decline during a hyperinsulinemic-euglycemic clamp. Also, the insulin-sensitizing effect of weight loss via caloric restriction alone or with resistance training does not amplify the reduction in adiponectin levels observed during hyperinsulinemia in healthy postmenopausal women.

Adipocytokines, gut hormones and growth factors in anorexia nervosa

Anorexia nervosa is a complex eating disorder of unknown etiology which affects adolescent girls and young women and leads to chronic malnutrition. Clinical manifestations of prolonged semistarvation include a variety of physical features and psychiatric disorders. The study of different biological factors involved in the pathophysiology of anorexia nervosa is an area of active interest. In this review we have described the role of adipocytokines, neurotrophins, peptides of the gastrointestinal system and growth factors in appetite regulation, energy balance and insulin sensitivity in anorexia nervosa patients.

Impaired insulin-induced site-specific phosphorylation of TBC1 domain family, member 4 (TBC1D4) in skeletal muscle of type 2 diabetes patients is restored by endurance exercise-training

AIMS/HYPOTHESIS

Insulin-mediated glucose disposal rates (R(d)) are reduced in type 2 diabetic patients, a process in which intrinsic signalling defects are thought to be involved. Phosphorylation of TBC1 domain family, member 4 (TBC1D4) is at present the most distal insulin receptor signalling event linked to glucose transport. In this study, we examined insulin action on site-specific phosphorylation of TBC1D4 and the effect of exercise training on insulin action and signalling to TBC1D4 in skeletal muscle from type 2 diabetic patients.

METHODS

During a 3 h euglycaemic-hyperinsulinaemic (80 mU min⁻¹ m⁻²) clamp, we obtained M. vastus lateralis biopsies from 13 obese type 2 diabetic and 13 obese, non-diabetic control individuals before and after 10 weeks of endurance exercise-training.

RESULTS

Before training, reductions in insulin-stimulated R (d), together with impaired insulin-stimulated glycogen synthase fractional velocity, Akt Thr³⁰⁸ phosphorylation and phosphorylation of TBC1D4 at Ser³¹⁸, Ser⁵⁸⁸ and Ser⁷⁵¹ were observed in skeletal muscle from diabetic patients. Interestingly, exercise-training normalised insulin-induced TBC1D4 phosphorylation in diabetic patients. This happened independently of increased TBC1D4 protein content, but exercise-training did not normalise Akt phosphorylation in diabetic patients. In both groups, training-induced improvements in insulin-stimulated R(d) (~20%) were associated with increased muscle protein content of Akt, TBC1D4, α2-AMP-activated kinase (AMPK), glycogen synthase, hexokinase II and GLUT4 (20-75%).

CONCLUSIONS/INTERPRETATION

Impaired insulin-induced site-specific TBC1D4 phosphorylation may contribute to skeletal muscle insulin resistance in type 2 diabetes. The mechanisms by which exercise-training improves insulin sensitivity in type 2 diabetes may involve augmented signalling of TBC1D4 and increased skeletal muscle content of key insulin signalling and effector proteins, e.g., Akt, TBC1D4, AMPK, glycogen synthase, GLUT4 and hexokinase II.

The SNARE protein SNAP23 and the SNARE-interacting protein Munc18c in human skeletal muscle are implicated in insulin resistance/type 2 diabetes

OBJECTIVE

Our previous studies suggest that the SNARE protein synaptosomal-associated protein of 23 kDa (SNAP23) is involved in the link between increased lipid levels and insulin resistance in cardiomyocytes. The objective was to determine whether SNAP23 may also be involved in the known association between lipid accumulation in skeletal muscle and insulin resistance/type 2 diabetes in humans, as well as to identify a potential regulator of SNAP23.

RESEARCH DESIGN AND METHODS

We analyzed skeletal muscle biopsies from patients with type 2 diabetes and healthy, insulin-sensitive control subjects for expression (mRNA and protein) and intracellular localization (subcellular fractionation and immunohistochemistry) of SNAP23, and for expression of proteins known to interact with SNARE proteins. Insulin resistance was determined by a euglycemic hyperinsulinemic clamp. Potential mechanisms for regulation of SNAP23 were also investigated in the skeletal muscle cell line L6.

RESULTS

We showed increased SNAP23 levels in skeletal muscle from patients with type 2 diabetes compared with that from lean control subjects. Moreover, SNAP23 was redistributed from the plasma membrane to the microsomal/cytosolic compartment in the patients with the type 2 diabetes. Expression of the SNARE-interacting protein Munc18c was higher in skeletal muscle from patients with type 2 diabetes. Studies in L6 cells showed that Munc18c promoted the expression of SNAP23.

CONCLUSIONS

We have translated our previous in vitro results into humans by showing that there is a change in the distribution of SNAP23 to the interior of the cell in skeletal muscle from patients with type 2 diabetes. We also showed that Munc18c is a potential regulator of SNAP23.

Oxidation of intramyocellular lipids is dependent on mitochondrial function and the availability of extracellular fatty acids

Obesity and insulin resistance are related to both enlarged intramyocellular triacylglycerol stores and accumulation of lipid intermediates. We investigated how lipid overflow can change the oxidation of intramyocellular lipids (ICL(OX)) and intramyocellular lipid storage (ICL). These experiments were extended by comparing these processes in primary cultured myotubes established from healthy lean and obese type 2 diabetic (T2D) individuals, two extremes in a range of metabolic phenotypes. ICLs were prelabeled for 2 days with 100 microM [(14)C]oleic acid (OA). ICL(OX) was studied using a (14)CO(2) trapping system and measured under various conditions of extracellular OA (5 or 100 microM) and glucose (0.1 or 5.0 mM) and the absence or presence of mitochondrial uncoupling [carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP)]. First, increased extracellular OA availability (5 vs. 100 microM) reduced ICL(OX) by 37%. No differences in total lipolysis were observed between low and high OA availability. Uncoupling with FCCP restored ICL(OX) to basal levels during high OA availability. Mitochondrial mass was positively related to ICL(OX), but only in myotubes from lean individuals. In all, a lower mitochondrial mass and lower ICL(OX) were related to a higher cell-associated OA accumulation. Second, myotubes established from obese T2D individuals showed reduced ICL(OX). ICL(OX) remained lower during uncoupling (P < 0.001), even with comparable mitochondrial mass, suggesting decreased mitochondrial function. Furthermore, the variation in ICL(OX) in vitro was significantly related to the in vivo fasting respiratory quotient of all subjects (P < 0.02). In conclusion, the rate of ICL(OX) is dependent on the availability of extracellular fatty acids and mitochondrial function rather than mitochondrial mass.

Circulating levels of insulin-like growth factor-II/mannose-6-phosphate receptor in obesity and type 2 diabetes

OBJECTIVE

The extracellular domain of the insulin-like growth factor II/mannose-6-phosphate receptor (IGF-II/M6P-R) is present in the circulation, but its relationship with plasma IGF-II is largely unknown. As IGF-II appears to be nutritionally regulated, we studied the impact of obesity, type 2 diabetes (T2D) and weight loss on circulating levels of IGF-II and its soluble receptor.

METHODS

Twenty-three morbidly obese non-diabetic subjects were studied before and after gastric banding (GB), reducing their BMI from 59.3+/-1.8 to 52.7+/-1.6 kg/m(2). Lean controls (n=10, BMI 24.2+/-0.5 kg/m(2)), moderately obese controls (n=21, BMI 31.8+/-1.0 kg/m(2)) and obese T2D patients (n=20, BMI 32.3+/-0.8 kg/m(2)) were studied before and after a hyperinsulinaemic euglycaemic clamp.

RESULTS

Morbidly obese subjects had elevated IGF-II/M6P-R and IGF-II levels, which both decreased following GB (IGF-II/M6P-R: from 0.97+/-0.038 to 0.87+/-0.030 nmol/l, P=0.001; IGF-II: from 134+/-7 to 125+/-6 nmol/l, P=0.01), as did fasting plasma glucose and insulin (P<0.05). However, the metabolic parameters correlated with neither IGF-II nor IGF-II/M6P-R. Obese diabetics had increased IGF-II/M6P-R as compared with lean and obese controls (0.82+/-0.031 vs. 0.70+/-0.033 vs. 0.74+/-0.026 nmol/l; P<0.03) and levels were unaffected by clamp. In the latter cohort, IGF-II/M6P-R but not IGF-II correlated with HbA1c, and fasting plasma C-peptide, insulin and glucose (0.34<r<0.45; P<0.05). In all subjects, BMI correlated with IGF-II/M6P-R (r=0.57; P<0.001) and IGF-II (r=0.39; P<0.005). IGF-II/M6P-R and IGF-II were not associated.

CONCLUSION

Serum IGF-II/M6P-R is up-regulated in morbid obesity, down-regulated by weight loss and elevated in moderately obese T2D. However, although plasma IGF-II was also reduced following GB, the two peptides were not statistically correlated. No acute effect of insulin was seen. These findings indicate that the IGF-II/M6P-R is nutritionally regulated, independently of IGF-II.

Human ATP synthase beta is phosphorylated at multiple sites and shows abnormal phosphorylation at specific sites in insulin-resistant muscle

AIMS/HYPOTHESIS

Insulin resistance in skeletal muscle is linked to mitochondrial dysfunction in obesity and type 2 diabetes. Emerging evidence indicates that reversible phosphorylation regulates oxidative phosphorylation (OxPhos) proteins. The aim of this study was to identify and quantify site-specific phosphorylation of the catalytic beta subunit of ATP synthase (ATPsyn-beta) and determine protein abundance of ATPsyn-beta and other OxPhos components in skeletal muscle from healthy and insulin-resistant individuals.

METHODS

Skeletal muscle biopsies were obtained from lean, healthy, obese, non-diabetic and type 2 diabetic volunteers (each group n = 10) for immunoblotting of proteins, and hypothesis-driven identification and quantification of phosphorylation sites on ATPsyn-beta using targeted nanospray tandem mass spectrometry. Volunteers were metabolically characterised by euglycaemic-hyperinsulinaemic clamps.

RESULTS

Seven phosphorylation sites were identified on ATPsyn-beta purified from human skeletal muscle. Obese individuals with and without type 2 diabetes were characterised by impaired insulin-stimulated glucose disposal rates, and showed a approximately 30% higher phosphorylation of ATPsyn-beta at Tyr361 and Thr213 (within the nucleotide-binding region of ATP synthase) as well as a coordinated downregulation of ATPsyn-beta protein and other OxPhos components. Insulin increased Tyr361 phosphorylation of ATPsyn-beta by approximately 50% in lean and healthy, but not insulin-resistant, individuals.

CONCLUSIONS/INTERPRETATION

These data demonstrate that ATPsyn-beta is phosphorylated at multiple sites in human skeletal muscle, and suggest that abnormal site-specific phosphorylation of ATPsyn-beta together with reduced content of OxPhos proteins contributes to mitochondrial dysfunction in insulin resistance. Further characterisation of phosphorylation of ATPsyn-beta may offer novel targets of treatment in human diseases with mitochondrial dysfunction, such as diabetes.

Dysregulation of glycogen synthase COOH- and NH2-terminal phosphorylation by insulin in obesity and type 2 diabetes mellitus

CONTEXT

Insulin-stimulated glucose disposal is impaired in obesity and type 2 diabetes mellitus (T2DM) and is tightly linked to impaired skeletal muscle glucose uptake and storage. Impaired activation of glycogen synthase (GS) by insulin is a well-established defect in both obesity and T2DM, but the underlying mechanisms remain unclear.

DESIGN AND PARTICIPANTS

Insulin action was investigated in a matched cohort of lean healthy, obese nondiabetic, and obese type 2 diabetic subjects by the euglycemic-hyperinsulinemic clamp technique combined with muscle biopsies. Activity, site-specific phosphorylation, and upstream signaling of GS were evaluated in skeletal muscle.

RESULTS

GS activity correlated inversely with phosphorylation of GS site 2+2a and 3a. Insulin significantly decreased 2+2a phosphorylation in lean subjects only and induced a larger dephosphorylation at site 3 in lean compared with obese subjects. The exaggerated insulin resistance in T2DM compared with obese subjects was not reflected by differences in site 3 phosphorylation but was accompanied by a significantly higher site 1b phosphorylation during insulin stimulation. Hyperphosphorylation of another Ca(2+)/calmodulin-dependent kinase-II target, phospholamban-Thr17, was also evident in T2DM. Dephosphorylation of GS by phosphatase treatment fully restored GS activity in all groups.

CONCLUSIONS

Dysregulation of GS phosphorylation plays a major role in impaired insulin regulation of GS in obesity and T2DM. In obesity, independent of T2DM, this is associated with impaired regulation of site 2+2a and likely site 3, whereas the exaggerated insulin resistance to activate GS in T2DM is linked to hyperphosphorylation of at least site 1b. Thus, T2DM per se seems unrelated to defects in the glycogen synthase kinase-3 regulation of GS.

Adiponectin and its response to thiazolidinediones are associated with insulin-mediated glucose metabolism in type 2 diabetic patients and their first-degree relatives

Patients with type 2 diabetes (T2D) and their first-degree relatives (FDRs) are characterized by hypoadiponectinaema and insulin resistance. In T2D patients, plasma adiponectin and insulin sensitivity (SI) increase in response to thiazolidinediones (TZDs). These findings suggest a role for adiponectin in the regulation of SI. We studied the relationship between plasma adiponectin and glucose and lipid metabolism and the effect of troglitazone (200 mg/day) for 12 weeks in 19 normoglycaemic, obese FDR and 20 obese T2D patients, using euglycaemic-hyperinsulinaemic clamps, glycolytic flux calculations and indirect calorimetry. Plasma adiponectin was similar in both groups, despite higher glucose disposal (Rd), glucose oxidation and glycolytic flux and lower lipid oxidation during insulin stimulation in FDR compared with T2D patients. Plasma adiponectin correlated with insulin-stimulated Rd, non-oxidative glucose disposal (NOGD), glucose storage and SI in both groups after adjustment for sex and body fat. The troglitazone-mediated upregulation of plasma adiponectin was associated with increased insulin-stimulated Rd, NOGD and glucose storage in both groups. No effect on endogenous glucose production was observed. In FDR, plasma adiponectin correlated with insulin-stimulated glycogen synthase activity and the troglitazone-induced increase in plasma adiponectin correlated with the improvement in insulin-stimulated Rd and SI after adjustment for sex and body fat. In conclusion, plasma adiponectin in weight-matched FDR and T2D patients is comparably low and correlates with insulin-mediated glucose uptake and storage. Moreover, these data provide evidence for an adiponectin-dependent insulin-sensitizing effect of TZDs at an early stage before development of T2D and that this effect is exerted mainly on insulin-mediated glucose metabolism.

Mitochondrial dysfunction in type 2 diabetes and obesity

Insulin resistance in skeletal muscle is a major hallmark of type 2 diabetes mellitus (T2D) and obesity that is characterized by impaired insulin-mediated glucose transport and glycogen synthesis and by increased intramyocellular content of lipid metabolites. Several studies have provided evidence for mitochondrial dysfunction in skeletal muscle of type 2 diabetic and prediabetic subjects, primarily due to a lower content of mitochondria (mitochondrial biogenesis) and possibly to a reduced functional capacity per mitochondrion. This article discusses the latest advances in the understanding of the molecular mechanisms underlying insulin resistance in human skeletal muscle in T2D and obesity, with a focus on possible links between insulin resistance and mitochondrial dysfunction.

Impaired insulin-stimulated phosphorylation of Akt and AS160 in skeletal muscle of women with polycystic ovary syndrome is reversed by pioglitazone treatment

OBJECTIVE

Insulin resistance in skeletal muscle is a major risk factor for type 2 diabetes in women with polycystic ovary syndrome (PCOS). However, the molecular mechanisms underlying skeletal muscle insulin resistance and the insulin-sensitizing effect of thiazolidinediones in PCOS in vivo are less well characterized.

RESEARCH DESIGN AND METHODS

We determined molecular mediators of insulin signaling to glucose transport in skeletal muscle biopsies of 24 PCOS patients and 14 matched control subjects metabolically characterized by euglycemic-hyperinsulinemic clamps and indirect calorimetry, and we examined the effect of 16 weeks of treatment with pioglitazone in PCOS patients.

RESULTS

Impaired insulin-mediated total (R(d)) oxidative and nonoxidative glucose disposal (NOGD) was paralleled by reduced insulin-stimulated Akt phosphorylation at Ser473 and Thr308 and AS160 phosphorylation in muscle of PCOS patients. Akt phosphorylation at Ser473 and Thr308 correlated positively with R(d) and NOGD in the insulin-stimulated state. Serum free testosterone was inversely related to insulin-stimulated R(d) and NOGD in PCOS. Importantly, the pioglitazone-mediated improvement in insulin-stimulated glucose metabolism, which did not fully reach normal levels, was accompanied by normalization of insulin-mediated Akt phosphorylation at Ser473 and Thr308 and AS160 phosphorylation. AMPK activity and phosphorylation were similar in the two groups and did not respond to pioglitazone in PCOS patients.

CONCLUSIONS

Impaired insulin signaling through Akt and AS160 in part explains insulin resistance at the molecular level in skeletal muscle in PCOS, and the ability of pioglitazone to enhance insulin sensitivity involves improved signaling through Akt and AS160. Moreover, our data provide correlative evidence that hyperandrogenism in PCOS may contribute to insulin resistance.

Total and high molecular weight (HMW) adiponectin levels and measures of glucose and lipid metabolism following pioglitazone treatment in a randomized placebo-controlled study in polycystic ovary syndrome

OBJECTIVE

Recent studies suggested that the effect of adiponectin on insulin-stimulated glucose metabolism is mediated primarily by the high molecular weight (HMW) form of adiponectin. In the present study we evaluated total and HMW adiponectin in polycystic ovary syndrome (PCOS) patients and controls and examined possible mechanisms for increased insulin sensitivity during pioglitazone treatment.

STUDY SUBJECTS

Thirty PCOS patients randomized to pioglitazone, 30 mg/day, or placebo for 16 weeks and 14 weight-matched healthy females were studied.

DESIGN

Total and HMW adiponectin levels were measured, and euglycaemic hyperinsulinaemic clamps and indirect calorimetry were performed. Delta-values denoted changes during pioglitazone treatment (16 weeks--basal).

RESULTS

Pretreatment adiponectin levels were decreased in PCOS patients vs. controls (P < 0.05), whereas no significant differences were found in HMW adiponectin levels. Following pioglitazone treatment, total and HMW adiponectin increased (all P < 0.05), whereas no significant changes were observed with placebo. Delta-total adiponectin levels correlated positively with the rate of Delta-insulin-stimulated glucose disposal (R(d)) (r = 0.89) and Delta-oxidative glucose metabolism (r = 0.71) and inversely with Delta-fasting free fatty acid (FFA) levels (r = -0.69) and Delta-lipid oxidation (r = -0.73) during insulin stimulation (all P < 0.01). Weaker correlations were found between Delta-HMW adiponectin levels and Delta-measures of glucose and lipid metabolism during insulin stimulation than with Delta-total adiponectin.

CONCLUSION

A close correlation between increased total adiponectin levels and increased insulin-stimulated glucose metabolism during pioglitzone treatment supports the hypothesis that the insulin-sensitizing effect of pioglitazone in PCOS is, at least in part, mediated by adiponectin. Measures of changes in HMW adiponectin did not add further information to this relationship.

Chronological approach of diet-induced alterations in muscle mitochondrial functions in rats

OBJECTIVE

Mitochondrial dysfunction might predispose individuals to develop insulin resistance. Our objective was to determine whether mitochondrial dysfunction or insulin resistance was the primary event during high-fat (HF) diet.

RESEARCH METHODS AND PROCEDURES

Rats were fed an HF diet for 0, 3, 6, 9, 14, 20, or 40 days and compared with control. Soleus and tibialis muscle mitochondrial activity were assessed using permeabilized fiber technique. Insulin [area under the curve for insulin (AUC(I))] and glucose [area under the curve for glucose (AUC(G))] responses to intraperitoneal glucose tolerance test as well as fasting plasma non-esterified fatty acids (NEFAs), triglyceride, and glycerol concentrations were determined.

RESULTS

AUC(I) and AUC(G) were altered from Day 6 (p < 0.01 vs. Day 0). In soleus, oxidative phosphorylation (OXPHOS) activity was transiently enhanced by 26% after 14 days of HF diet (p < 0.05 vs. Day 0) conjointly with 62% increase in NEFA concentration (p < 0.05 vs. Day 0). This was associated with normalized AUC(G) at Day 14 and with a decline of plasma NEFA concentration together with stabilization of intra-abdominal adiposity at Day 20. Prolongation of HF diet again caused an increase in plasma NEFA concentration, intra-abdominal adiposity, AUC(I), and AUC(G). At Day 40, significant decrease in OXPHOS activity was observed in soleus.

DISCUSSION

Mitochondria first adapt to overfeeding in oxidative muscle limiting excess fat deposition. This potentially contributes to maintain glucose homeostasis. Persistent overfeeding causes insulin resistance and results in a slow decline in oxidative muscle OXPHOS activity. This shows that the involvement of mitochondria in the predisposition to insulin resistance is mainly due to an inability to face prolonged excess fat delivery.

Plasma Adiponectin Levels in Relation to Carotid Intima Media Thickness and Markers of Insulin Resistance

BACKGROUND

Circulating adiponectin is a marker for insulin sensitivity, derived from fat cells. It is largely unknown if adiponectin is also an independent marker for early atherosclerosis.

METHODS AND RESULTS

Plasma adiponectin levels were measured in 373 men and 514 women of middle-age by a time-resolved immunofluorometric assay. The subjects were sampled stratified for degree of insulin sensitivity (HOMA-IR). An ultrasound measurement of the right common carotid artery intima media thickness (IMT) was made. When the distribution of adiponectin was stratified into sex-specific quartiles (Q1 to Q4), men in Q4 differed from Q1 in higher mean age and high-density lipoprotein (HDL) cholesterol, but lower blood pressure, HbA1c, HOMA-index, and body mass index. Women showed similar associations. Mean IMT for men was significantly lower (P=0.03) in adiponectin Q4 as compared with Q1 when adjusted for age, waist, smoking, HDL cholesterol, and diastolic blood pressure. When adding HbA1c and HOMA to the model, the association was no longer significant (P=0.15). In women no difference in IMT was noticed across adiponectin quartiles.

CONCLUSIONS

Plasma adiponectin is a marker of glucose metabolism and obesity and shows an inverse age-adjusted association with carotid ultrasound IMT in men, but not in women. This association is attenuated after adjustments for other risk factors.