David H. Wasserman

Mitochondrial H2O2 emission and cellular redox state link excess fat intake to insulin resistance in both rodents and humans

High dietary fat intake leads to insulin resistance in skeletal muscle, and this represents a major risk factor for type 2 diabetes and cardiovascular disease. Mitochondrial dysfunction and oxidative stress have been implicated in the disease process, but the underlying mechanisms are still unknown. Here we show that in skeletal muscle of both rodents and humans, a diet high in fat increases the H(2)O(2)-emitting potential of mitochondria, shifts the cellular redox environment to a more oxidized state, and decreases the redox-buffering capacity in the absence of any change in mitochondrial respiratory function. Furthermore, we show that attenuating mitochondrial H(2)O(2) emission, either by treating rats with a mitochondrial-targeted antioxidant or by genetically engineering the overexpression of catalase in mitochondria of muscle in mice, completely preserves insulin sensitivity despite a high-fat diet. These findings place the etiology of insulin resistance in the context of mitochondrial bioenergetics by demonstrating that mitochondrial H(2)O(2) emission serves as both a gauge of energy balance and a regulator of cellular redox environment, linking intracellular metabolic balance to the control of insulin sensitivity.

Physical Activity/Exercise and Type 2 Diabetes A consensus statement from the American Diabetes Association

For decades, exercise has been considered a cornerstone of diabetes management, along with diet and medication. However, high-quality evidence on the importance of exercise and fitness in diabetes was lacking until recent years. The present document summarizes the most clinically relevant recent advances related to people with type 2 diabetes and the recommendations that follow from these. Our recently published technical review on physical activity/exercise and type 2 diabetes (1) includes greater detail on individual studies, on prevention of diabetes, and on the physiology of exercise. The present statement focuses on type 2 diabetes. Issues primarily germane to type 1 diabetes will be covered in a subsequent technical review and ADA Statement. The levels of evidence used are defined by the ADA in ref. 2. Two randomized trials each found that lifestyle interventions including ∼150 min/week of physical activity and diet-induced weight loss of 5–7% reduced the risk of progression from impaired glucose tolerance (IGT) to type 2 diabetes by 58% (3,4). A cluster-randomized trial found that diet alone, exercise alone, and combined diet and exercise were equally effective in reducting the progression from IGT to diabetes (5). Therefore, there is firm and consistent evidence that programs of increased physical activity and modest weight loss reduce the incidence of type 2 diabetes in individuals with IGT. Boule et al. (6) undertook a systematic review and meta-analysis on the effects of structured exercise interventions in clinical trials of ≥8 weeks duration on HbA1c (A1C) and body mass in people with type 2 diabetes. Postintervention A1C was significantly lower in exercise than control groups (7.65 vs. 8.31%, weighted mean difference −0.66%; P < 0.001). In contrast, postintervention body weight did not differ between the exercise and control groups. Meta-regression confirmed that the beneficial effect of exercise on A1C …

Standard operating procedures for describing and performing metabolic tests of glucose homeostasis in mice

The Mouse Metabolic Phenotyping Center (MMPC) Consortium was established to address the need to characterize the growing number of mouse models of metabolic diseases, particularly diabetes and obesity. A goal of the MMPC Consortium is to propose standard methods for assessing metabolic phenotypes in mice. In this article, we discuss issues pertaining to the design and performance of various tests of glucose metabolism. We also propose guidelines for the description of methods, presentation of data and interpretation of results. The recommendations presented in this article are based on the experience of the MMPC Consortium and other investigators.

Chronic Treatment With Sildenafil Improves Energy Balance and Insulin Action in High Fat–Fed Conscious Mice

Stimulation of nitric oxide–cGMP signaling results in vascular relaxation and increased muscle glucose uptake. We show that chronically inhibiting cGMP hydrolysis with the phosphodiesterase-5 inhibitor sildenafil improves energy balance and enhances in vivo insulin action in a mouse model of diet-induced insulin resistance. High-fat–fed mice treated with sildenafil plus l-arginine or sildenafil alone for 12 weeks had reduced weight and fat mass due to increased energy expenditure. However, uncoupling protein-1 levels were not increased in sildenafil-treated mice. Chronic treatment with sildenafil plus l-arginine or sildenafil alone increased arterial cGMP levels but did not adversely affect blood pressure or cardiac morphology. Sildenafil treatment, with or without l-arginine, resulted in lower fasting insulin and glucose levels and enhanced rates of glucose infusion, disappearance, and muscle glucose uptake during a hyperinsulinemic (4 mU · kg−1 · min−1)–euglycemic clamp in conscious mice. These effects occurred without an increase in activation of muscle insulin signaling. An acute treatment of high fat–fed mice with sildenafil plus l-arginine did not improve insulin action. These results show that phosphodiesterase-5 is a potential target for therapies aimed at preventing diet-induced energy imbalance and insulin resistance.

Fibroblast Growth Factor 21 Controls Glycemia via Regulation of Hepatic Glucose Flux and Insulin Sensitivity

Fibroblast growth factor 21 (FGF21) is a novel metabolic regulator shown to improve glycemic control. However, the molecular and functional mechanisms underlying FGF21-mediated improvements in glycemic control are not completely understood. We examined FGF21 effects on insulin sensitivity and glucose fluxes upon chronic (daily injection for 8 d) and acute (6 h infusion) administration in ob/+ and ob/ob mice. Results show that chronic FGF21 ameliorated fasting hyperglycemia in ob/ob mice via increased glucose disposal and improved hepatic insulin sensitivity. Acute FGF21 suppressed hepatic glucose production, increased liver glycogen, lowered glucagon, and improved glucose clearance in ob/+ mice. These effects were blunted in ob/ob mice. Neither chronic nor acute FGF21 altered skeletal muscle or adipose tissue glucose uptake in either genotype. In conclusion, FGF21 has potent glycemic effects caused by hepatic changes in glucose flux and improved insulin sensitivity. Thus, these studies define mechanisms underlying anti-hyperglycemic actions of FGF21 and support its therapeutic potential.

Circadian disruption leads to insulin resistance and obesity

BACKGROUND Disruption of circadian (daily) timekeeping enhances the risk of metabolic syndrome, obesity, and type 2 diabetes. While clinical observations have suggested that insulin action is not constant throughout the 24 hr cycle, its magnitude and periodicity have not been assessed. Moreover, when circadian rhythmicity is absent or severely disrupted, it is not known whether insulin action will lock to the peak, nadir, or mean of the normal periodicity of insulin action. RESULTS We used hyperinsulinemic-euglycemic clamps to show a bona fide circadian rhythm of insulin action; mice are most resistant to insulin during their daily phase of relative inactivity. Moreover, clock-disrupted Bmal1-knockout mice are locked into the trough of insulin action and lack rhythmicity in insulin action and activity patterns. When rhythmicity is rescued in the Bmal1-knockout mice by expression of the paralogous gene Bmal2, insulin action and activity patterns are restored. When challenged with a high-fat diet, arhythmic mice (either Bmal1-knockout mice or wild-type mice made arhythmic by exposure to constant light) were obese prone. Adipose tissue explants obtained from high-fat-fed mice have their own periodicity that was longer than animals on a chow diet. CONCLUSIONS This study provides rigorous documentation for a circadian rhythm of insulin action and demonstrates that disturbing the natural rhythmicity of insulin action will disrupt the rhythmic internal environment of insulin sensitive tissue, thereby predisposing the animals to insulin resistance and obesity.

Four grams of glucose

Four grams of glucose circulates in the blood of a person weighing 70 kg. This glucose is critical for normal function in many cell types. In accordance with the importance of these 4 g of glucose, a sophisticated control system is in place to maintain blood glucose constant. Our focus has been on the mechanisms by which the flux of glucose from liver to blood and from blood to skeletal muscle is regulated. The body has a remarkable capacity to satisfy the nutritional need for glucose, while still maintaining blood glucose homeostasis. The essential role of glucagon and insulin and the importance of distributed control of glucose fluxes are highlighted in this review. With regard to the latter, studies are presented that show how regulation of muscle glucose uptake is regulated by glucose delivery to muscle, glucose transport into muscle, and glucose phosphorylation within muscle.

Exercise in Individuals With IDDM

The greater energy requirements that accompany exercise necessitate an increase in fuel mobilization from sites of storage and a subsequent increase in the oxidation of thesefuels within the working muscle. A precise endocrine response controls the needed increment in fuel metabolism. The importance of the endocrine system is readily apparent in individuals with insulin-dependent diabetes mellitus (IDDM) in whom the normal endocrine response to exercise is lost. When a person with IDDM exercises with too little insulin in the circulation, an excessive counterregulatory hormone response may ensue, and the alreadyelevated blood glucose and ketone body levels can become even greater. On the other hand,if too much insulin is present, the exercise-induced increase in substrate mobilization can be attenuated or completely blocked and, as a result, hypoglycemia may ensue. Insulin therapy must be modified in anticipation of exercise to avoid states of under- or overinsulinization, and/or carbohydrate ingestion must be adjusted to compensate for the potential glycemic effects of inappropriate insulin levels. The presence of advanced diabetic complications can further accentuate the difficulty of exercise for those with IDDM. The purpose of this review is to provide the scientific information necessary to assess therationale for recommending regular exercise to individuals with IDDM and to develop guidelines for integrating daily exercise with insulin treatment and dietary management.

Glucose Metabolism In Vivo in Four Commonly Used Inbred Mouse Strains

OBJECTIVE—To characterize differences in whole-body glucose metabolism between commonly used inbred mouse strains. RESEARCH DESIGN AND METHODS—Hyperinsulinemic-euglycemic (∼8.5 mmol/l) and -hypoglycemic (∼3.0 mmol/l) clamps were done in catheterized, 5-h-fasted mice to assess insulin action and hypoglycemic counter-regulatory responsiveness. Hyperglycemic clamps (∼15 mmol/l) were done to assess insulin secretion and compared with results in perifused islets. RESULTS—Insulin action and hypoglycemic counter-regulatory and insulin secretory phenotypes varied considerably in four inbred mouse strains. In vivo insulin secretion was greatest in 129X1/Sv mice, but the counter-regulatory response to hypoglycemia was blunted. FVB/N mice in vivo showed no increase in glucose-stimulated insulin secretion, relative hepatic insulin resistance, and the highest counter-regulatory response to hypoglycemia. In DBA/2 mice, insulin action was lowest among the strains, and islets isolated had the greatest glucose-stimulated insulin secretion in vitro. In C57BL/6 mice, in vivo physiological responses to hyperinsulinemia at euglycemia and hypoglycemia were intermediate relative to other strains. Insulin secretion by C57BL/6 mice was similar to that in other strains in contrast to the blunted glucose-stimulated insulin secretion from isolated islets. CONCLUSIONS—Strain-dependent differences exist in four inbred mouse strains frequently used for genetic manipulation and study of glucose metabolism. These results are important for selecting inbred mice to study glucose metabolism and for interpreting and designing experiments.

Skeletal Muscle AMP-activated Protein Kinase Is Essential for the Metabolic Response to Exercise in Vivo

AMP-activated protein kinase (AMPK) has been postulated as a super-metabolic regulator, thought to exert numerous effects on skeletal muscle function, metabolism, and enzymatic signaling. Despite these assertions, little is known regarding the direct role(s) of AMPK in vivo, and results obtained in vitro or in situ are conflicting. Using a chronically catheterized mouse model (carotid artery and jugular vein), we show that AMPK regulates skeletal muscle metabolism in vivo at several levels, with the result that a deficit in AMPK activity markedly impairs exercise tolerance. Compared with wild-type littermates at the same relative exercise capacity, vascular glucose delivery and skeletal muscle glucose uptake were impaired; skeletal muscle ATP degradation was accelerated, and arterial lactate concentrations were increased in mice expressing a kinase-dead AMPKα2 subunit (α2-KD) in skeletal muscle. Nitric-oxide synthase (NOS) activity was significantly impaired at rest and in response to exercise in α2-KD mice; expression of neuronal NOS (NOSμ) was also reduced. Moreover, complex I and IV activities of the electron transport chain were impaired 32 ± 8 and 50 ± 7%, respectively, in skeletal muscle of α2-KD mice (p < 0.05 versus wild type), indicative of impaired mitochondrial function. Thus, AMPK regulates neuronal NOSμ expression, NOS activity, and mitochondrial function in skeletal muscle. In addition, these results clarify the role of AMPK in the control of muscle glucose uptake during exercise. Collectively, these findings demonstrate that AMPK is central to substrate metabolism in vivo, which has important implications for exercise tolerance in health and certain disease states characterized by impaired AMPK activation in skeletal muscle.