Leptin administration improves skeletal muscle insulin responsiveness in diet-induced insulin-resistant rats

Pancreatic Islets as a Target of Adipokines

Rising rates of obesity are intricately tied to the type 2 diabetes epidemic. The adipose tissues can play a central role in protection against or triggering metabolic diseases through the secretion of adipokines. Many adipokines may improve peripheral insulin sensitivity through a variety of mechanisms, thereby indirectly reducing the strain on beta cells and thus improving their viability and functionality. Such effects will not be the focus of this article. Rather, we will focus on adipocyte-secreted molecules that have a direct effect on pancreatic islets. By their nature, adipokines represent potential druggable targets that can reach the islets and improve beta-cell function or preserve beta cells in the face of metabolic stress. © 2022 American Physiological Society. Compr Physiol 12:1-27, 2022.

CNS Regulation of Glucose Homeostasis: Role of the Leptin-Melanocortin System

PURPOSE OF REVIEW

In this brief review, we highlight studies that have contributed to our current understanding of glucose homeostasis by the central nervous system (CNS) leptin-melanocortin system, particularly proopiomelanocortin neurons and melanocortin-4 receptors (MC4R).

RECENT FINDINGS

Leptin deficiency is associated with insulin resistance and impaired glucose metabolism whereas leptin administration improves tissue glucose uptake/oxidation and reduces hepatic glucose output. These antidiabetic effects of leptin have been demonstrated in experimental animals and humans, even when circulating insulin levels are barely detectable. Recent evidence suggests that these antidiabetic actions of leptin are mediated, in large part, by stimulation of leptin receptors (LRs) in the CNS and require activation of proopiomelanocortin (POMC) neurons and MC4R. These chronic antidiabetic effects of the CNS leptin-melanocortin system appear to be independent of autonomic nervous system and pituitary-thyroid-adrenal (PTA) axis mechanisms. The powerful antidiabetic actions of the CNS leptin-melanocortin system are capable of normalizing plasma glucose even in the absence of insulin and involve interactions of multiple neuronal populations and intracellular signaling pathways. Although the links between the CNS leptin-melanocortin system and its chronic effects on peripheral tissue glucose metabolism are still uncertain, they are independent of insulin action, activation of the autonomic nervous system, or the PTA axis. Unraveling the pathways that contribute to the powerful antidiabetic effects of the CNS leptin-melanocortin system may provide novel therapeutic approaches for diabetes mellitus.

New Frontiers for the Use of IP6 and Inositol Combination in Treating Diabetes Mellitus: A Review

Inositol, or myo-inositol, and associated analog molecules, including myo-inositol hexakisphosphate, are known to possess beneficial biomedical properties and are now being widely studied. The impact of these compounds in improving diabetic indices is significant, especially in light of the high cost of treating diabetes mellitus and associated disorders globally. It is theorized that, within ten years, the global population of people with the disease will reach 578 million individuals, with the cost of care projected to be approximately 2.5 trillion dollars. Natural alternatives to pharmaceuticals are being sought, and this has led to studies involving inositol, and myo-inositol-hexakisphosphate, also referred to as IP6. It has been reported that IP6 can improve diabetic indices and regulate the activities of some metabolic enzymes involved in lipid and carbohydrate metabolism. Current research activities have been focusing on the mechanisms of action of inositol and IP6 in the amelioration of the indices of diabetes mellitus. We demonstrated that an IP6 and inositol combination supplement may regulate insulin secretion, modulate serum leptin concentrations, food intake, and associated weight gain, which may be beneficial in both prediabetic and diabetic states. The supplement attenuates vascular damage by reducing red cell distribution width. Serum HDL is increased while serum triglycerides tend to decrease with consumption of the combination supplement, perhaps due to the modulation of lipogenesis involving reduced serum lipase activity. We also noted increased fecal lipid output following combination supplement consumption. Importantly, liver function was found to be preserved. Concurrently, serum reactive oxygen species production was reduced, indicating that inositol and IP6 supplement consumption may reduce free radical damage to tissues and organs as well as serum lipids and blood glucose by preserving liver function. This review provides an overview of the findings associated with inositol and IP6 supplementation in the effective treatment of diabetes with a view to proposing the potential mechanisms of action.

Augmented insulin secretory response in early pregnancy

AIMS/HYPOTHESIS

This study aimed to examine changes in the insulin secretory response in early pregnancy, while accounting for changes in insulin sensitivity.

METHODS

This is a secondary analysis of a previously conducted longitudinal physiological study. In 34 women, insulin secretory response (by IVGTT) and insulin sensitivity (by euglycaemic clamp) were assessed prior to pregnancy, in early pregnancy (12-14 weeks gestation) and in late pregnancy (34-36 weeks gestation). Using mixed-effects models, we compared insulin secretory response and sensitivity in early pregnancy to the same variables prior to pregnancy and in late pregnancy, with adjustment for age, obesity status and gestational diabetes mellitus (GDM). We examined changes in insulin secretory response after adjustment for insulin sensitivity using both multivariate modelling and the disposition index (DI). We explored the relationship between insulin secretory response and circulating hormones.

RESULTS

The insulin secretory response increased from prior to pregnancy to early pregnancy (unadjusted mean [SD] first-phase insulin response 465.1 [268.5] to 720 [358.2], p < 0.0001) and from early pregnancy to late pregnancy (to 924 [494.6], p = 0.01). Insulin sensitivity increased from prior to pregnancy to early pregnancy (insulin sensitivity index 0.10 [0.04] to 0.12 [0.05], p = 0.001) and decreased in late pregnancy (to 0.06 [0.03], p < 0.0001). Accounting for changes in insulin sensitivity, using either multivariate modelling or the DI, did not attenuate the early-pregnancy augmentation of insulin secretory response. Leptin was positively associated with insulin secretory response, independent of insulin sensitivity and adiposity (p = 0.004). Adjustment for leptin attenuated the observed augmentation of insulin secretory response in early pregnancy (adjusted mean change 121.5, p = 0.13).

CONCLUSIONS/INTERPRETATION

The insulin secretory response increases markedly in early pregnancy, prior to and independent of changes in insulin sensitivity. Circulating hormones may mediate this metabolic adaptation. Identifying mediators of this physiological effect could have therapeutic implications for treating hyperglycaemia during and outside of pregnancy.

The Impact of OMEGA-3 Fatty Acids Supplementation on Insulin Resistance and Content of Adipocytokines and Biologically Active Lipids in Adipose Tissue of High-Fat Diet Fed Rats

It has been established that OMEGA-3 polyunsaturated fatty acids (PUFAs) may improve lipid and glucose homeostasis and prevent the "low-grade" state of inflammation in animals. Little is known about the effect of PUFAs on adipocytokines expression and biologically active lipids accumulation under the influence of high-fat diet-induced obesity. The aim of the study was to examine the effect of fish oil supplementation on adipocytokines expression and ceramide (Cer) and diacylglycerols (DAG) content in visceral and subcutaneous adipose tissue of high-fat fed animals. The experiments were carried out on Wistar rats divided into three groups: standard diet-control (SD), high-fat diet (HFD), and high-fat diet + fish oil (HFD+FO). The fasting plasma glucose and insulin concentrations were examined. Expression of carnitine palmitoyltransferase 1 (CPT1) protein was determined using the Western blot method. Plasma adipocytokines concentration was measured using ELISA kits and mRNA expression was determined by qRT-PCR reaction. Cer, DAG, and acyl-carnitine (A-CAR) content was analyzed by UHPLC/MS/MS. The fish oil supplementation significantly decreased plasma insulin concentration and Homeostatic Model Assesment for Insulin Resistance (HOMA-IR) index and reduced content of adipose tissue biologically active lipids in comparison with HFD-fed subjects. The expression of CPT1 protein in HFD+FO in both adipose tissues was elevated, whereas the content of A-CAR was lower in both HFD groups. There was an increase of adiponectin concentration and expression in HFD+FO as compared to HFD group. OMEGA-3 fatty acids supplementation improved insulin sensitivity and decreased content of Cer and DAG in both fat depots. Our results also demonstrate that PUFAs may prevent the development of insulin resistance in response to high-fat feeding and may regulate the expression and secretion of adipocytokines in this animal model.

Differential Effects of High Sugar, High Lard or a Combination of Both on Nutritional, Hormonal and Cardiovascular Metabolic Profiles of Rodents

BACKGROUND

Dietary interventions in rodents can induce an excess of adipose tissue and metabolic disorders that resemble human obesity. Nevertheless, these approaches are not standardized, and the phenotypes may vary distinctly among studies. The aim of this study was to investigate the effects of different dietary interventions on nutritional, metabolic, biochemical, hormonal, and cardiovascular profiles, as well as to add to development and characterization of an experimental model of obesity.

METHODS

Male Wistar rats were randomized into four groups: control diet (C), high-sugar (HS), high-fat (HF), or high-sugar and high-fat (HFHS). Weekly measurements of body weight, adiposity, area under the curve (AUC) for glucose, blood pressure (BP) and serum triglycerides, total cholesterol level, and leptin were performed.

RESULTS

HF and HFHS models were led to obesity by increases in adipose tissue deposition and the adiposity index. All hypercaloric diets presented systolic BP increases. In addition, the AUC for glucose was greater in HF and HFHS than in C, and only the HF group presented hyperleptinemia.

CONCLUSIONS

HF and HFHS diet approaches promote obesity and comorbidities, and thus represent a useful tool for studying human obesity-related disorders. By contrast, the HS model did not prove to be a good model of obesity.

The role of adipokines in skeletal muscle inflammation and insulin sensitivity

Background

There is currently an unmet clinical need to develop better pharmacological treatments to improve glucose handling in Type II Diabetes patients with obesity. To this end, determining the effect of obesity-associated adipokines on skeletal muscle insulin sensitivity has emerged as an important area of drug discovery research. This review draws together the data on the functional role of adipokines on skeletal muscle insulin signalling, highlights several understudied novel adipokines and provides a perspective on the direction of future research.

Main body

The adipokines leptin, resistin, visfatin and adiponectin have all been shown to affect skeletal muscle insulin sensitivity by impacting on the activity of components within insulin signalling pathways, affecting GLUT4 translocation and modulating insulin-mediated skeletal muscle glucose uptake. Furthermore, proteomic analysis of the adipose tissue secretome has recently identified several novel adipokines including vaspin, chemerin and pref-1 that are associated with obesity and insulin resistance in humans and functionally impact on insulin signalling pathways. However, predominantly, these functional findings are the result of studies in rodents, with in vitro studies utilising either rat L6 or murine C2C12 myoblasts and/or myotubes. Despite the methodology to isolate and culture human myoblasts and to differentiate them into myotubes being established, the use of human muscle in vitro models for the functional validation of adipokines on skeletal muscle insulin sensitivity is limited.

Conclusion

Understanding the mechanism of action and function of adipokines in mediating insulin sensitivity in skeletal muscle may lead to the development of novel therapeutics for patients with type 2 diabetes. However, to date, studies conducted in human skeletal muscle cells and tissues are limited. Such human in vitro studies should be prioritised in order to reduce the risk of candidate drugs failing in the clinic due to the assumption that rodent skeletal muscle target validation studies will to translate to human.

The effect of combined inositol hexakisphosphate and inositol supplement in streptozotocin-induced type 2 diabetic rats

Inositol hexakisphosphate (IP6) and inositol both regulate insulin secretion, but their combined use in the management of diabetes deserves investigation. The combined effects of IP6 and inositol supplementation were investigated in streptozotocin-induced type 2 diabetic rats. The following groups of rats were studied for 8 weeks: non-diabetic control, non-diabetic high-fat diet control, diabetic untreated, diabetic rats treated with the combination of IP6 and inositol (650 mg/kg bw) and diabetic rats treated with glibenclamide (10 mg/kg bw). High-fat diet and streptozotocin were used to induce type 2 diabetes mellitus in Sprague-Dawley rats. Body weight, blood glucose, glycated haemoglobin, insulin, serum leptin, HOMA-insulin resistance scores, intestinal amylase activity, serum and faecal lipids and food and fluid consumption were measured. Treatment with the combination significantly reduced blood glucose (306 ± 53 mg/dl) and insulin resistance score (1.93 ± 0.45) compared with diabetic controls (522 ± 24 mg/dl and 5.1 ± 0.69 respectively). Serum leptin (2.8 ± 0.6 ng/dl) and faecal triglycerides (108 ± 8 mg/dl) were significantly increased in rats treated with the combination compared with the diabetic control (1.8 ± 0.06 ng/dl and 86 ± 4 mg/dl). Serum triglyceride (47 ± 5.1 mg/dl), total cholesterol (98 ± 3.2 mg/dl) and food intake (26 ± 0.3 g) were significantly reduced by 45%, 25% and 25%, respectively, in rats treated with the combination compared with the diabetic control. Inositol and IP6 combined supplementation may be effective in the management of type 2 diabetes mellitus and related metabolic disorders by regulating some aspects of lipid and carbohydrate metabolism.

The role of leptin in glucose homeostasis

The fat‐derived hormone, leptin, is well known to regulate body weight. However, there is now substantial evidence that leptin also plays a primary role in the regulation of glucose homeostasis, independent of actions on food intake, energy expenditure or body weight. As such, leptin might have clinical utility in treating hyperglycemia, particularly in conditions of leptin deficiency, such as lipodystrophy and diabetes mellitus. The mechanisms through which leptin modulates glucose metabolism have not been fully elucidated. Leptin receptors are widely expressed in peripheral tissues, including the endocrine pancreas, liver, skeletal muscle and adipose, and both direct and indirect leptin action on these tissues contributes to the control of glucose homeostasis. Here we review the role of leptin in glucose homeostasis, along with our present understanding of the mechanisms involved. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2012.00203.x, 2012)

The role of leptin in the control of insulin-glucose axis

Obesity and diabetes mellitus are great public health concerns throughout the world because of their increasing incidence and prevalence. Leptin, the adipocyte hormone, is well known for its role in the regulation of food intake and energy expenditure. In addition to the regulation of appetite and satiety that recently has attracted much attentions, insight has also been gained into the critical role of leptin in the control of the insulin-glucose axis, peripheral glucose and insulin responsiveness. Since the discovery of leptin, leptin has been taken for its therapeutic potential to obesity and diabetes. Recently, the therapeutic effects of central leptin gene therapy have been reported in insulin-deficient diabetes in obesity animal models such as ob/ob mise, diet-induced obese mice, and insulin-deficient type 1 diabetes mice, and also in patients with inactivating mutations in the leptin gene. Herein, we review the role of leptin in regulating feeding behavior and glucose metabolism and also the therapeutic potential of leptin in obesity and diabetes mellitus.

Skeletal muscle mitochondrial energetics in obesity and type 2 diabetes mellitus: endocrine aspects

During the development of type 2 diabetes mellitus, skeletal muscle is a major site of insulin resistance. The latter has been linked to mitochondrial dysfunction and impaired fatty acid oxidation. Some hormones like insulin, thyroid hormones and adipokines (e.g., leptin, adiponectin) have positive effects on muscle mitochondrial bioenergetics through their direct or indirect effects on mitochondrial biogenesis, mitochondrial protein expression, mitochondrial enzyme activities and/or AMPK pathway activation--all of which can improve fatty acid oxidation. It is therefore not surprising that treatment with these hormones has been proposed to improve muscle and whole body insulin sensitivity. However, treatment of diabetic patients with leptin and adiponectin has no effect on muscle mitochondrial bioenergetics showing resistance to these hormones during type 2 diabetes. Furthermore, treatment with most thyroid hormones has unexpectedly revealed negative effects on muscle insulin sensitivity. Future research should focus on development of agents that improve metabolic dysfunction downstream of hormone receptors.

Exercise restores insulin, but not adiponectin, response in skeletal muscle of high-fat fed rodents

High-fat (HF) diets impair skeletal muscle response to the insulin-sensitizing adipokine adiponectin (Ad) in rodents, preceding the development of insulin resistance. Skeletal muscle insulin response in HF-fed rats can be restored with chronic exercise; whether recovery of skeletal muscle Ad response is necessary for the exercise-induced recovery of insulin-stimulated glucose transport is not known. In the current study, insulin and Ad resistance were induced in rodents with 4 wk of HF feeding (HF4; low-fat fed animals used as control). Rats were then treadmill-exercised (HF5EX1, HF6EX2) or supplemented orally with the pharmacological agent β-guadinoproprionic acid (GPA; HF5GPA1, HF6GPA2) for 1 or 2 wk with continued HF feeding. Insulin and Ad responses (glucose transport and palmitate oxidation, respectively) were assessed 48 h after the last exercise bout ex vivo in isolated solei. Insulin response was impaired following 4 wk of HF feeding and improved with 1 and 2 wk of exercise and β-GPA supplementation (HF5EX1, HF6EX2, HF5GPA1, and HF6GPA2). The recovery of insulin response generally coincided with improved Akt Thr308 phosphorylation in HF5GPA1, HF6EX2, and HF6GPA2, although not in HF5EX1. Ad-stimulated palmitate oxidation was not restored with either treatment. Total protein contents of AdipoR1, AdipoR2, APPL1, and APPL2, as well as total and phosphorylated AMPK and ACC were unaltered by diet, exercise, and β-GPA at the assessed time points. We conclude that the exercise and pharmacologically (β-GPA)-induced recovery of skeletal muscle insulin response after HF feeding is not dependent on the restoration of Ad response, as assessed ex vivo.

Insulin receptor and IRS-1 co-immunoprecipitation with SOCS-3, and IKKα/β phosphorylation are increased in obese Zucker rat skeletal muscle

AIMS

We evaluated if selected pro-inflammatory cytokines and/or the protein suppressor of cytokine signaling 3 (SOCS-3) could account for decreased insulin-stimulated phosphatidylinositol 3-kinase (PI3-K) activity in the skeletal muscle of the obese Zucker rat.

MAIN METHODS

Eight lean and eight obese Zucker rats ~4weeks of age were obtained and allowed to feed ad libitum for 4weeks before undergoing hind limb perfusion in the presence of 500μU/ml insulin.

KEY FINDINGS

Insulin-stimulated skeletal muscle PI3-K activity and 3-O-methylglucose transport rates were reduced (P<0.05) in obese compared to lean animals. IRS-1 concentration remained unchanged although IRS-1 tyrosine phosphorylation was decreased (P<0.05), and IRS-1 serine phosphorylation (pS) was increased (P<0.05) in obese animals compared to lean animals. IKKα/β pS and JNK theronine/tyrosine phosphorylation was increased (P<0.05) in the obese animals. IκBα concentration was decreased (P<0.05) and IκBα pS was increased (P<0.05) in the obese compared to lean Zucker animals. SOCS-3 concentration and SOCS-3 co-immunoprecipitation with both insulin receptor β-subunit (IR-β) and IRS-1 were elevated (P<0.05) in obese compared to lean animals. IRS-1 co-immunoprecipitation with IR-β was reduced 56% in the obese animals.

SIGNIFICANCE

Increased IKKα/β and JNK serine phosphorylation may contribute to increasing IRS-1 serine phosphorylation, while concurrent co-localization of SOCS-3 with both IR-β and IRS-1 may prevent IRS-1 from interacting with IR-β. These two mechanisms thusly may independently contribute to impairing insulin-stimulated PI3-K activation in the skeletal muscle of the obese Zucker rat.

Impact of an obesogenic diet program on bone densitometry, micro architecture and metabolism in male rat

BACKGROUND

The relationships between fat mass and bone tissue are complex and not fully elucidated. A high-fat/high-sucrose diet has been shown to induce harmful effects on bone micro architecture and bone biomechanics of rat. When such diet leads to obesity, it may induce an improvement of biomechanical bone parameters in rodent.Here, we examined the impact of a high-fat/high-sucrose diet on the body composition and its resulting effects on bone density and structure in male rats. Forty three Wistar rats aged 7 months were split into 3 groups: 1 sacrificed before diet (BD, n = 14); 1 subjected to 16 weeks of high-fat/high-sucrose diet (HF/HS, n = 14); 1 subjected to standard diet (Control, n = 15). Abdominal circumference and insulin sensitivity were measured and visceral fat mass was weighed. The bone mineral density (BMD) was analyzed at the whole body and tibia by densitometry. Microcomputed tomography and histomorphometric analysis were performed at L2 vertebrae and tibia to study the trabecular and cortical bone structures and the bone cell activities. Osteocalcin and CTX levels were performed to assess the relative balance of the bone formation and resorption. Differences between groups have been tested with an ANOVA with subsequent Scheffe post-hoc test. An ANCOVA with global mass and global fat as covariates was used to determine the potential implication of the resulting mechanical loading on bone.

RESULTS

The HF/HS group had higher body mass, fat masses and abdominal circumference and developed an impaired glucose tolerance (p < 0.001). Whole body bone mass (p < 0.001) and BMD (p < 0.05) were higher in HF/HS group vs. Control group. The trabecular thickness at vertebrae and the cortical porosity of tibia were improved (p < 0.05) in HF/HS group. Bone formation was predominant in HF/HS group while an unbalance bone favoring bone resorption was observed in the controls. The HF/HS and Control groups had higher total and abdominal fat masses and altered bone parameters vs. BD group.

CONCLUSIONS

The HF/HS diet had induced obesity and impaired glucose tolerance. These changes resulted in an improvement of quantitative, qualitative and metabolic bone parameters. The fat mass increase partly explained these observations.

Leptin in human physiology and pathophysiology

Leptin, discovered through positional cloning 15 years ago, is an adipocyte-secreted hormone with pleiotropic effects in the physiology and pathophysiology of energy homeostasis, endocrinology, and metabolism. Studies in vitro and in animal models highlight the potential for leptin to regulate a number of physiological functions. Available evidence from human studies indicates that leptin has a mainly permissive role, with leptin administration being effective in states of leptin deficiency, less effective in states of leptin adequacy, and largely ineffective in states of leptin excess. Results from interventional studies in humans demonstrate that leptin administration in subjects with congenital complete leptin deficiency or subjects with partial leptin deficiency (subjects with lipoatrophy, congenital or related to HIV infection, and women with hypothalamic amenorrhea) reverses the energy homeostasis and neuroendocrine and metabolic abnormalities associated with these conditions. More specifically, in women with hypothalamic amenorrhea, leptin helps restore abnormalities in hypothalamic-pituitary-peripheral axes including the gonadal, thyroid, growth hormone, and to a lesser extent adrenal axes. Furthermore, leptin results in resumption of menses in the majority of these subjects and, in the long term, may increase bone mineral content and density, especially at the lumbar spine. In patients with congenital or HIV-related lipoatrophy, leptin treatment is also associated with improvements in insulin sensitivity and lipid profile, concomitant with reduced visceral and ectopic fat deposition. In contrast, leptin's effects are largely absent in the obese hyperleptinemic state, probably due to leptin resistance or tolerance. Hence, another emerging area of research pertains to the discovery and/or usefulness of leptin sensitizers. Results from ongoing studies are expected to further increase our understanding of the role of leptin and the potential clinical applications of leptin or its analogs in human therapeutics.

A palatable hyperlipidic diet causes obesity and affects brain glucose metabolism in rats

Background

We have previously shown that either the continuous intake of a palatable hyperlipidic diet (H) or the alternation of chow (C) and an H diet (CH regimen) induced obesity in rats. Here, we investigated whether the time of the start and duration of these feeding regimens are relevant and whether they affect brain glucose metabolism.

Methods

Male Wistar rats received C, H, or CH diets during various periods of their life spans: days 30-60, days 30-90, or days 60-90. Experiments were performed the 60^th or the 90^th day of life. Rats were killed by decapitation. The glucose, insulin, leptin plasma concentration, and lipid content of the carcasses were determined. The brain was sliced and incubated with or without insulin for the analysis of glucose uptake, oxidation, and the conversion of [1-¹⁴C]-glucose to lipids.

Results

The relative carcass lipid content increased in all of the H and CH groups, and the H30-60 and H30-90 groups had the highest levels. Groups H30-60, H30-90, CH30-60, and CH30-90 exhibited a higher serum glucose level. Serum leptin increased in all H groups and in the CH60-90 and CH30-90 groups. Serum insulin was elevated in the H30-60, H60-90, CH60-90, CH30-90 groups. Basal brain glucose consumption and hypothalamic insulin receptor density were lower only in the CH30-60 group. The rate of brain lipogenesis was increased in the H30-90 and CH30-90 groups.

Conclusion

These findings indicate that both H and CH diet regimens increased body adiposity independent treatment and the age at which treatment was started, whereas these diets caused hyperglycemia and affected brain metabolism when started at an early age.

Recombinant human leptin treatment does not improve insulin action in obese subjects with type 2 diabetes

OBJECTIVE

Leptin therapy improves insulin sensitivity in people with leptin deficiency, but it is not known whether it improves insulin action in people who are not leptin deficient. The purpose of the current study was to determine whether leptin treatment has weight loss-independent effects on insulin action in obese subjects with type 2 diabetes.

RESEARCH DESIGN AND METHODS

We conducted a randomized, placebo-controlled trial in obese subjects (BMI: 35.4 ± 0.6 kg/m(2); mean ± SE) with newly diagnosed type 2 diabetes. Subjects were randomized to treatment with placebo (saline), low-dose (30 mg/day), or high-dose (80 mg/day) recombinant methionyl human (r-Met hu) leptin for 14 days. Multiorgan insulin sensitivity before and after treatment was evaluated by using the hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled tracer infusions to measure glucose, glycerol, and fatty acid kinetics.

RESULTS

Low-dose and high-dose leptin treatment resulted in a threefold (P < 0.01) and 150-fold (P < 0.001) increase in basal plasma leptin concentrations, respectively. However, neither low-dose nor high-dose therapy had an effect on insulin-mediated suppression of glucose, glycerol, or palmitate rates of appearance into plasma compared with placebo. In addition, leptin treatment did not increase insulin-mediated stimulation of glucose disposal compared with placebo (14.3 ± 3.1, 18.4 ± 3.6, 16.7 ± 2.4 vs. 17.5 ± 2.5, 20.7 ± 3.0, 19.1 ± 3.3 μmol/kg body wt/min before vs. after treatment in the placebo, low-dose, and high-dose leptin groups, respectively).

CONCLUSIONS

r-Met hu leptin does not have weight loss-independent, clinically important effects on insulin sensitivity in obese people with type 2 diabetes.

Chronic central leptin infusion differently modulates brain and liver insulin signaling

Recent studies reported the impact of leptin on peripheral insulin sensitivity and glucose utilization. However, little is known concerning the effect of central leptin on hypothalamic and hepatic insulin efficiency. This study aimed to determine the consequence of chronic intra-cerebroventricular (ICV) leptin or murine leptin antagonist (MLA) infusion on hypothalamic and hepatic insulin signaling pathways, in rats. A 2-week central leptin infusion enhanced insulin-dependent Akt phosphorylation in the liver without changing PTP-1B protein expression, associated to insulin receptor (IR) upregulation and reduced IRS-1 phosphorylation on Ser302 residue. In the hypothalamus, a chronic ICV leptin infusion induced PTP-1B associated with a specific decrease in insulin-dependent Akt phosphorylation. In contrast, a chronic MLA infusion did not alter IR and PTP-1B expressions in hypothalamus and liver. Our results underline a brain leptin-dependent increase in hepatic insulin efficiency as mirrored by IR up-regulation, increased insulin-dependent Akt phosphorylation and reduced IRS-1 phosphorylation on Ser302 residue.

Recovered insulin response by 2 weeks of leptin administration in high-fat fed rats is associated with restored AS160 activation and decreased reactive lipid accumulation

Leptin is an adipokine that increases fatty acid (FA) oxidation, decreases intramuscular lipid stores, and improves insulin response in skeletal muscle. In an attempt to elucidate the underlying mechanisms by which these metabolic changes occur, we administered leptin (Lep) or saline (Sal) by miniosmotic pumps to rats during the final 2 wk of a 6-wk low-fat (LF) or high-fat (HF) diet. Insulin-stimulated glucose transport was impaired by the HF diet (HF-Sal) but was restored with leptin administration (HF-Lep). This improvement was associated with restored phosphorylation of Akt and AS160 and decreased in reactive lipid species (ceramide, diacylglycerol), known inhibitors of the insulin-signaling cascade. Total muscle citrate synthase (CS) activity was increased by both leptin and HF diet, but was not additive. Leptin increased subsarcolemmal (SS) and intramyofibrillar (IMF) mitochondria CS activity. Total muscle, sarcolemmal, and mitochondrial (SS and IMF) FA transporter (FAT/CD36) protein content was significantly increased with the HF diet, but not altered by leptin. Therefore, the decrease in reactive lipid stores and subsequent improvement in insulin response, secondary to leptin administration in rats fed a HF diet was not due to a decrease in FA transport protein content or altered cellular distribution.

Aerobic training reverses high-fat diet-induced pro-inflammatory signalling in rat skeletal muscle

High-fat feeding activates components of the pro-inflammatory pathway and increases co-immunoprecipitation of suppressor of cytokine signalling (SOCS)-3 with both the insulin receptor (IR)-β subunit and IRS-1, which together contribute to keeping PI-3 kinase from being fully activated. However, whether aerobic training reverses these impairments is unknown. Sprague-Dawley rats were fed a chow (CON, n = 8) or saturated high-fat (n = 16) diets for 4 weeks. High-fat-fed rats were then allocated (n = 8/group) to either sedentary (HF) or aerobic exercise training (HFX) for an additional 4 weeks after which all animals underwent hind limb perfusions. Insulin-stimulated red quadriceps 3-O-methylglucose transport rates and PI-3 kinase activity were greater (p < 0.05) in CON and HFX compared to HF. IRS-1 tyrosine phosphorylation was increased (p < 0.05) and IRS-1 serine 307 phosphorylation was decreased (p < 0.05) in HFX compared to HF. IR-β subunit co-immunoprecipitation with IRS-1 was increased in HFX compared to HF. SOCS-3 co-immunoprecipitation with both the IR-β subunit and IRS-1 was decreased (p < 0.05) in HFX compared to HF. IKKα/β serine phosphorylation, and IκBα serine phosphorylation were decreased (p < 0.05) while IκBα protein concentration was increased in HFX compared to HF. By decreasing the association of SOCS-3 with both the IR-β subunit and IRS-1 the interaction between IRS-1 and the IR-β subunit was normalized in the HFX, and may have contributed to skeletal muscle PI-3 kinase being fully activated by insulin. Additionally, the reduction in IKKα/β serine phosphorylation in HFX may have contributed to decreasing IRS-1 serine phosphorylation, and in turn, promoted the normalization of insulin-stimulated activation of PI-3 kinase.

Restoring AS160 phosphorylation rescues skeletal muscle insulin resistance and fatty acid oxidation while not reducing intramuscular lipids

We examined whether AICAR or leptin rapidly rescued skeletal muscle insulin resistance via increased palmitate oxidation, reductions in intramuscular lipids, and/or restoration of insulin-stimulated AS60 phosphorylation. Incubation with palmitate (2 mM, 0-18 h) induced insulin resistance in soleus muscle. From 12-18 h, palmitate was removed or AICAR or leptin was provided while 2 mM palmitate was maintained. Palmitate oxidation, intramuscular triacylglycerol, diacylglycerol, ceramide, AMPK phosphorylation, basal and insulin-stimulated glucose transport, plasmalemmal GLUT4, and Akt and AS160 phosphorylation were examined at 0, 6, 12, and 18 h. Palmitate treatment (12 h) increased intramuscular lipids (triacylglycerol +54%, diacylglycerol +11%, total ceramide +18%, C16:0 ceramide +60%) and AMPK phosphorylation (+118%), whereas it reduced fatty acid oxidation (-60%) and insulin-stimulated glucose transport (-70%), GLUT4 translocation (-50%), and AS160 phosphorylation (-40%). Palmitate removal did not rescue insulin resistance or associated parameters. The AICAR and leptin treatments did not consistently reduce intramuscular lipids, but they did rescue palmitate oxidation and insulin-stimulated glucose transport, GLUT4 translocation, and AS160 phosphorylation. Increased AMPK phosphorylation was associated with these improvements only when AICAR and leptin were present. Hence, across all experiments, AMPK phosphorylation did not correlate with any parameters. In contrast, palmitate oxidation and insulin-stimulated AS160 phosphorylation were highly correlated (r = 0.83). We speculate that AICAR and leptin activate both of these processes concomitantly, involving activation of unknown kinases in addition to AMPK. In conclusion, despite the maintenance of high concentrations of palmitate (2 mM), as well as increased concentrations of intramuscular lipids (triacylglycerol, diacylglycerol, and ceramide), the rapid AICAR- and leptin-mediated rescue of palmitate-induced insulin resistance is attributable to the restoration of insulin-stimulated AS160 phosphorylation and GLUT4 translocation.

Impairment of mitochondrial beta-oxidation in rats under cold-hypoxic environment

Mitochondrial beta-oxidation of fatty acid provides a major source of energy in mammals. High altitude (HA), characterized by hypobaric hypoxia and low ambient temperatures, causes alteration in metabolic homeostasis. Several studies have depicted that hypoxic exposure in small mammals causes hypothermia due to hypometabolic state. Moreover, cold exposure along with hypoxia reduces hypoxia tolerance in animals. The present study investigated the rate of beta-oxidation and key enzymes, carnitine palmitoyl transferase-I (CPT-I) and hydroxyacyl CoA dehydrogenase (HAD), in rats exposed to cold-hypobaric hypoxic environment. Male Sprague Dawley rats (190-220 g) were randomly divided into eight groups (n = 6 rats in each group): 1 day hypoxia (H1); 7 days hypoxia (H7); 1 day cold (C1); 7 days cold (C7); 1 day cold-hypoxia (CH1); 7 days cold-hypoxia (CH7) exposed; and unexposed control for 1 and 7 days (UC1 and UC7). After exposure, animals were anaesthetized with ketamine (50 mg/kg body weight) and xylazine (10 mg/kg body weight) intraperitonialy and sacrificed. Mitochondrial CPT-I, HAD, (14)C-palmitate oxidation in gastrocnemius muscle and liver, and plasma leptin were measured. Mitochondrial CPT-I was significantly reduced in muscle and liver in CH1 and CH7 as compared to respective controls. HAD activity was significantly reduced in H1 and CH7, and in H1, H7, CH1, and CH7 as compared to unexposed controls in muscle and liver, respectively. A concomitant decrease in (14)C-palmitate oxidation was found. Significant reduction in plasma leptin in hypoxia and cold-hypoxia suggested hypometabolic state. It can be concluded that ss-oxidation of fatty acids is reduced in rats exposed to cold-hypoxic environment due to the persisting hypometabolic state in cold-hypoxia exposure.

High-fat feeding increases insulin receptor and IRS-1 coimmunoprecipitation with SOCS-3, IKKalpha/beta phosphorylation and decreases PI-3 kinase activity in muscle

Suppressor of cytokine signaling (SOCS) proteins and/or activation of the proinflammatory pathway have been postulated as possible mechanisms that may contribute to skeletal muscle insulin resistance. Thus, the aims of the present investigation were to determine in high-fat-fed skeletal muscle: 1) whether SOCS-3 protein concentration is increased, 2) whether coimmunoprecipitation of SOCS-3 with the insulin receptor-beta subunit and/or IRS-1 is increased, and 3) whether select components of the proinflammatory pathway are altered. Thirty-two male Sprague-Dawley rats were assigned to either control (CON, n = 16) or high-fat-fed (HF, n = 16) dietary groups for 12 wk and then subjected to hind limb perfusions in the presence (n = 8/group) or absence (n = 8/group) of insulin. Insulin-stimulated skeletal muscle 3-MG transport rates and PI-3 kinase activity were greater (P < 0.05) in CON. IRS-1 tyrosine phosphorylation was decreased (P < 0.05), and IRS-1 serine 307 phosphorylation was increased (P < 0.05) in HF. Insulin receptor-beta (IR-beta) subunit coimmunoprecipitation with IRS-1 was reduced in HF. SOCS-3 protein concentration and SOCS-3 coimmunoprecipitation with both the IR-beta subunit and IRS-1 was increased (P < 0.05) in HF. IKKalpha/beta serine phosphorylation was increased (P < 0.05), IkappaBalpha protein concentration was decreased (P < 0.05) and IkappaBalpha serine phosphorylation was increased (P < 0.05) in HF. Increased colocalization of SOCS-3 with both the IR-beta subunit and IRS-1 may provide steric hindrance that prevents IRS-1 from interacting with IR-beta, while increased IKKbeta serine phosphorylation may contribute to increasing IRS-1 serine phosphorylation, both of which independently can have deleterious effects on insulin-stimulated PI-3 kinase activation in high-fat-fed rodent skeletal muscle.

Resistin acutely impairs insulin-stimulated glucose transport in rodent muscle in the presence, but not absence, of palmitate

Resistin is a cytokine implicated in the development of insulin resistance. However, there has been little investigation of the effects of resistin on fatty acid (FA) metabolism and insulin response in skeletal muscle, a key tissue for glucose disposal. The purpose of the present study was to examine the role of altered FA metabolism as a cause of resistin's inhibition of insulin-stimulated glucose transport in muscle. Isolated rat soleus muscles were incubated acutely (2 h) in the presence or absence of 600 ng/ml resistin, with or without 2 mM palmitate. Resistin acutely impaired insulin-stimulated glucose transport and Akt phosphorylation, but only in the presence of palmitate, implicating a role for altered FA metabolism. This impairment of glucose transport induced by resistin plus palmitate could be pharmacologically rescued by the inclusion of aimidazole carboxamide ribonucleotide, a stimulator of AMP-activated protein kinase and FA oxidation, as well as inhibitors of ceramide synthesis (myriocin, fumonisin). However, to our surprise, resistin actually blunted the palmitate-induced increase in muscle ceramide content; as expected, ceramide content was significantly lowered by fumonisin. In summary, the acute impairment of insulin response by resistin was manifested only in the presence of high palmitate and was alleviated when FA metabolism was manipulated (increased oxidation, inhibited ceramide synthesis). Resistin's acute impairment of insulin response does not appear to require an absolute increase in ceramide content; however, reducing ceramide content alleviated the impairment in glucose transport and insulin signaling.

Activation of atypical protein kinase Czeta toward TC10 is regulated by high-fat diet and aerobic exercise in skeletal muscle

We determined whether sustained aerobic exercise reverses high-fat diet-induced impairments in the c-Cbl associated protein (CAP)/Casitas b-lineage lymphoma (c-Cbl) signaling cascade in rodent skeletal muscle. Sprague-Dawley rats were placed into either control (n = 16) or high-fat-fed (n = 32) diet groups for 4 weeks. During a subsequent 4-week experimental period, 16 high-fat-fed rats remained sedentary, 16 high-fat-fed rats completed 4 weeks of exercise training, and control animals were sedentary and remained on the control diet. After the intervention period, animals were subjected to hind limb perfusions in the presence (n = 8 per group) or absence (n = 8 per group) of insulin. In the plasma membrane fractions, neither high-fat feeding nor exercise training altered adaptor protein with PH and SH2 domains, (APS), c-Cbl, or TC10 protein concentrations. In contrast, CAP protein concentration and insulin-stimulated plasma membrane c-Cbl tyrosine phosphorylation were reduced by high-fat feeding; but exercise training reversed these impairments. Of note was that insulin-stimulated atypical protein kinase Czeta kinase activity toward TC10 was reduced by high-fat feeding but normalized by exercise training. We conclude that sustained (4 weeks) exercise training can reverse high-fat diet-induced impairments on the CAP/c-Cbl pathway in high-fat-fed rodent skeletal muscle. We also provide the first evidence that the CAP/c-Cbl insulin signaling cascade in skeletal muscle may directly interact with components of the classic (phosphoinositide 3-kinase dependent) insulin signaling cascade.

Effects of leptin on energy metabolism in beta-less mice

OBJECTIVE

To investigate the impact of beta-adrenoceptor deficiency on the metabolic effects of leptin.

MEASUREMENTS

Leptin was infused subcutaneously through an osmotic minipump in wild-type (WT) and beta(1)/beta(2)/beta(3)-adrenoceptor knockout (beta-less) mice and its effects on food intake, energy expenditure, carbohydrate and lipid utilization as well as on the levels of expression of the brown adipose tissue (BAT), thermogenic marker uncoupling protein-1 (UCP1) and type II deiodinase (D2) mRNAs were compared.

RESULTS

Leptin treatment decreased food intake by 23% in both the WT and the beta-less mice. In pair-fed animals being used as controls, leptin treatment was found to increase energy expenditure in WT, but not in beta-less mice. No difference was observed in carbohydrate or fat utilization between leptin-treated WT and beta-less mice. Leptin increased UCP1 and D2 mRNA levels in WT mouse BAT 1.7- and 3-fold, respectively, but had no effect on the expression of these genes in beta-less mouse BAT.

CONCLUSION

The stimulatory effects of leptin on oxygen consumption, BAT UCP1 and D2 expression require functional beta-adrenoceptors, but its inhibitory effect on food intake and its stimulatory effect on fat utilization is independent of the beta-adrenoceptor signalling.

Exercise reverses high-fat diet-induced impairments on compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle

The aims of this investigation were 1) to determine whether endurance exercise training could reverse impairments in insulin-stimulated compartmentalization and/or activation of aPKCzeta/lambda and Akt2 in skeletal muscle from high-fat-fed rodents and 2) to assess whether the PPARgamma agonist rosiglitazone could reverse impairments in skeletal muscle insulin signaling typically observed after high-fat feeding. Sprague-Dawley rats were placed on chow (NORCON, n = 16) or high-fat (n = 64) diets for 4 wk. During a subsequent 4-wk experimental period, high-fat-fed rats were allocated (n = 16/group) to either sedentary control (HFC), exercise training (HFX), rosiglitazone treatment (HFRSG), or a combination of both exercise training and rosiglitazone (HFRX). Following the 4-wk experimental period, animals underwent hindlimb perfusions. Insulin-stimulated plasma membrane-associated aPKCzeta and -lambda protein concentration, aPKCzeta/lambda activity, GLUT4 protein concentration, cytosolic Akt2, and aPKCzeta/lambda activities were reduced (P < 0.05) in HFC compared with NORCON. Cytosolic Akt2, aPKCzeta, and aPKClambda protein concentrations were not affected in HFC compared with NORCON. Exercise training reversed the deleterious effects of the high-fat diet such that insulin-stimulated compartmentalization and activation of components of the insulin-signaling cascade in HFX were normalized to NORCON. High-fat diet-induced impairments to skeletal muscle glucose metabolism were not reversed by rosiglitazone administration, nor did rosiglitazone augment the effect of exercise. Our findings indicate that chronic exercise training, but not rosiglitazone, reverses high-fat diet induced impairments in compartmentalization and activation of components of the insulin-signaling cascade in skeletal muscle.

Leptin, skeletal muscle lipids, and lipid-induced insulin resistance

Leptin-induced increases in insulin sensitivity are well established and may be related to the effects of leptin on lipid metabolism. However, the effects of leptin on the levels of lipid metabolites implicated in pathogenesis of insulin resistance and the effects of leptin on lipid-induced insulin resistance are unknown. The current study addressed in rats the effects of hyperleptinemia (HL) on insulin action and markers of skeletal muscle (SkM) lipid metabolism in the absence or presence of acute hyperlipidemia induced by an infusion of a lipid emulsion. Compared with controls (CONT), HL increased insulin sensitivity, as assessed by hyperinsulinemic-euglycemic clamp ( approximately 15%), and increased SkM Akt ( approximately 30%) and glycogen synthase kinase 3 alpha ( approximately 52%) phosphorylation. These improvements in insulin action were associated with decreased SkM triglycerides (TG; approximately 61%), elevated ceramides ( approximately 50%), and similar diacylglycerol (DAG) levels in HL compared with CONT. Acute hyperlipidemia in CONT decreased insulin sensitivity ( approximately 25%) and increased SkM DAG ( approximately 33%) and ceramide ( approximately 60%) levels. However, hyperlipidemia did not induce insulin resistance or SkM DAG and ceramide accumulation in HL. SkM total fatty acid transporter CD36, plasma membrane fatty acid binding protein, acetyl Co-A carboxylase phosphorylation, and fatty acid oxidation were similar in HL compared with CONT. However, HL decreased SkM protein kinase C theta (PKC theta), a kinase implicated in mediating the detrimental effects of lipids on insulin action. We conclude that increases in insulin sensitivity induced by HL are associated with decreased levels of SkM TG and PKC theta and increased SkM insulin signaling, but not with decreases in other lipid metabolites implicated in altering SkM insulin sensitivity (DAG and ceramide). Furthermore, insulin resistance induced by an acute lipid infusion is prevented by HL.

High-fat diets: modeling the metabolic disorders of human obesity in rodents

RESEARCH METHODS AND PROCEDURES

High-fat (HF) diet feeding can induce obesity and metabolic disorders in rodents that resemble the human metabolic syndrome. However, this dietary intervention is not standardized, and the HF-induced phenotype varies distinctly among different studies. The question which HF diet type is best to model the metabolic deterioration seen in human obesity remains unclear. Therefore, in this review, metabolic data obtained with different HF diet approaches are compiled. Both whole-body and organ-specific diet effects are analyzed.

RESULTS

On the basis of these results, we conclude that animal fats and omega-6/omega-9-containing plant oils can be used to generate an obese and insulin-resistant phenotype in rodents, whereas fish oil-fed animals do not develop these disorders.

DISCUSSION

Looking at the present data, it does not seem possible to define an ideal HF diet, and an exact definition of diet composition and a thorough metabolic characterization of the HF diet effects in a researcher's specific laboratory setting remains essential for metabolic studies with this model.

Phosphatidylinositol 3-kinase p85alpha regulatory subunit gene PIK3R1 haplotype is associated with body fat and serum leptin in a female twin population

AIMS/HYPOTHESIS

Phosphatidylinositol 3-kinase (PI3K) couples the leptin and insulin signalling pathways via the insulin receptor substrates IRS1 and IRS2. Hence, defective activation of PI3K could be a novel mechanism of peripheral leptin or insulin resistance. We investigated associations of tagging single-nucleotide polymorphisms (tSNPs) in the PI3K p85alpha regulatory subunit gene PIK3R1 with anthropometry, leptin, body fat and insulin sensitivity in a female twin population of European extraction.

MATERIALS AND METHODS

Eight tSNPs were genotyped in 2,778 women (mean age 47.4+/-12.5 years) from the St Thomas' UK Adult Twin Registry (Twins UK).

RESULTS

SNP rs1550805 was associated with serum leptin (p=0.028), BMI (p=0.025), weight (p=0.019), total fat (p=0.004), total fat percentage (p=0.002), waist circumference (p=0.025), central fat (p=0.005) and central fat percentage (p=0.005). SNPs rs7713645 and rs7709243 were associated with BMI (p=0.020 and p=0.029, respectively), rs7709243 with weight, total and central fat (p=0.026, p=0.031 and p=0.023, respectively) and both SNPs with fasting glucose (p=0.003 and p=0.001, respectively) and glucose 2-h post OGTT (p=0.023 and p=0.007, respectively). Subjects with haplotype 222 (frequency 7.2%) showed higher serum leptin concentration (p=0.007) and body fat measures (p< or =0.001 for all), and those with haplotype 221 (frequency 38.7%) showed higher fasting and 2-h glucose (p=0.035 and p=0.021, respectively) compared with subjects with the most common haplotype, 111 (frequency 45.5%).

CONCLUSIONS/INTERPRETATION

Association of the PIK3R1 SNP rs1550805 with serum leptin and body fat may reflect a diminished ability of PI3K to signal via IRS1 or IRS2 in response to leptin.

Effect of Lipids from Erabu Sea Snake, Laticauda semifasciata, on Plasma Glucose, Insulin, and Adipocytokine Concentrations of Normal and Streptozotocin-Diabetic Mice

AIMS

To clarify the influence of Erabu sea snake lipid on levels of plasma insulin and adipocyte-derived hormones.

METHODS

Normal male mice (6 months) and streptozotocin-diabetic mice (a single administration, 150 mg/kg i.p.) were fed diets containing 10% fat as either lard or a mixture of 5% lard and 5% sea snake lipid for 4 months.

RESULTS

The normal mice on the sea snake lipid diet had a significantly lower plasma glucose concentration (198 +/- 16 mg/dl; 148 +/- 11 mg/dl) than those mice on the lard diet. Although plasma insulin concentrations were not influenced by the dietary fat type, leptin (10 +/- 1 ng/ml; 16 +/- 2 ng/ml) and adiponectin (18 +/- 1 mug/ml; 21 +/- 1 mug/ml) concentrations were significantly higher in the sea snake lipid group than in the lard group. However, these effects of a sea snake lipid intake were not evident in the streptozotocin-diabetic mice.

CONCLUSION

The results suggest that the decrease in plasma glucose following the intake of sea snake lipids could be related to a corresponding increase in leptin and adiponectin level.

High-fat feeding effects on components of the CAP/Cbl signaling cascade in Sprague-Dawley rat skeletal muscle

The aim of this investigation was to determine whether the CAP (Cbl-associated protein)/Cbl signaling cascade is present and responsive to insulin in skeletal muscle and if high-fat feeding impairs insulin-stimulated activation of this signaling cascade. Sprague-Dawley rats were assigned to either control (n = 16) or high fat-fed (n = 16) dietary groups. After a 12-week dietary period, animals were subjected to hind limb perfusions in the presence (n = 8 per group) or absence (n = 8 per group) of insulin. High-fat feeding reduced rates of insulin-stimulated skeletal muscle phosphatidylinositol 3-kinase activity and 3-O-methylglucose transport. In plasma membrane fractions, neither the high-fat diet nor insulin altered the insulin receptor beta subunit (IR-beta), APS (adaptor protein containing PH and SH2 domains), c-Cbl, or TC10 protein concentration, but high-fat feeding did decrease CAP protein concentration. APS, c-Cbl, CAP, and TC10 messenger RNA were present in the skeletal muscle and reflected the protein concentration of experimental groups. Despite insulin-stimulated plasma membrane IR-beta tyrosine phosphorylation being unaffected by high-fat feeding, c-Cbl tyrosine phosphorylation, the kinase activity of IR-beta toward APS, and glucose transporter 4 protein concentration were all significantly reduced in insulin-stimulated plasma membrane prepared from the skeletal muscle of high fat-fed animals. These findings suggest that the CAP/Cbl signaling cascade is present in skeletal muscle, activated by insulin, and impaired by high-fat feeding.

The role of adipokines as regulators of skeletal muscle fatty acid metabolism and insulin sensitivity

Several adipose-derived cytokines (adipokines) have been suggested to act as a link between accumulated fat mass and altered insulin sensitivity. Resistin and tumour necrosis factor-alpha (TNF-alpha) have been implicated in impairing insulin sensitivity in rodents; conversely, two other adipokines, leptin and adiponectin, increase insulin sensitivity in lean and obese rodents. Currently, there is considerable focus on the concept that lipid accumulation in skeletal muscle leads to the development of insulin resistance. Adiponectin and leptin have each been demonstrated to increase rates of fatty acid (FA) oxidation and decrease muscle lipid content, which may in part be the underlying mechanism to their insulin sensitizing effect. These effects on FA metabolism appear to be mediated in part through the activation of AMP-activated protein kinase. Evidence derived from animal and human studies suggests that the ability of leptin and adiponectin to stimulate FA oxidation in muscle is impaired in the obese condition. Thus, leptin and adiponectin resistance may be an initiating factor in the accumulation of intramuscular lipids, such as diacylglycerol and ceramide, and the ensuing development of insulin resistance. Lifestyle factors such as diet and exercise are able to restore the sensitivity of muscle to leptin. The actual physiological roles of resistin and TNF-alpha in altering muscle lipid metabolism are more controversial, but each has been shown to directly impair insulin signalling and consequently, insulin stimulated glucose uptake in muscle. However, the possibility that resistin and TNF-alpha reduces insulin sensitivity in muscle by directly impairing FA metabolism in this tissue leading to lipid accumulation, has been virtually unexamined. Thus, the contribution of various adipokines to the development of insulin resistance is complex and not fully understood. Finally, the effects of these adipokines on metabolism and insulin sensitivity are generally studied in isolation, making it difficult to predict the interactive effects and the net impact on insulin sensitivity.

Resistance Training Improves Insulin Signaling and Action in Skeletal Muscle

Resistance training can improve glucose transport in both normal and insulin-resistant skeletal muscle by enhancing the activation of the insulin signaling cascade and increasing GLUT-4 protein concentration. These training-induced alterations improve the quality of the skeletal muscle and can occur independent of significant increases in skeletal muscle mass.

Short-term leptin treatment increases fatty acids uptake and oxidation in muscle of high fat-fed rats

The purpose of this study was to measure the effects of short-term (10 days) leptin treatment on insulin sensitivity as it pertains to fatty acid (FA) uptake, oxidation, and muscle triglyceride (mTG) synthesis in animals that have been administered a high-fat (HF) diet for 3 months. Male Wistar rats were randomly assigned to 1 of 4 groups. One group was fed a control diet (CON) and 3 groups were fed a HF diet. The HF and HF-leptin (HF-LEP) groups were fed the HF diet ad libitum and the amount of food eaten by the HF-pair fed (HF-P) group was equal to that of the HF-LEP group. At the end of the dietary period, rats were injected daily either with saline (CON, HF, HF-P) or with leptin (HF-LEP; 10 mg.kg-1.d-1) for 10 days before hindlimb perfusion. The perfusate contained 600 micromol/L palmitate traced with [14C]palmitate, 9 mmol/L glucose, and 100 microU/mL insulin. As dictated by the protocol, energy expenditure was not significantly different (P>.05) between HF-LEP and HF-P. Palmitate uptake and oxidation as well as mTG synthesis were greater (P<.05) in HF (9.8+/-0.3, 2.0+/-0.1, and 1.9+/-0.2 nmol.min-1.g-1) than in CON (8.0+/-0.4, 1.4+/-0.1, and 1.1+/-0.1 nmol.min-1.g-1) and this was associated with higher levels of mTG in HF. Palmitate uptake and oxidation were higher (P<.05) in HF-LEP (10.3+/-0.6 and 2.0+/-0.1 nmol.min-1.g-1) than in HF-P (8.3+/-0.5 and 1.5+/-0.2 nmol.min-1.g-1, P<.05), but mTG synthesis and mTG levels were not changed significantly by leptin treatment (P>.05). High-fat feeding decreased glucose uptake by 41% when compared with CON (2.4+/-0.4 vs 4.1+/-0.4 micromol.h-1.g-1; P<.05) but pair feeding alone (4.7+/-0.4 micromol.h-1.g-1) or leptin treatment (3.8+/-0.3 micromol.h-1.g-1) similarly prevented the HF diet-induced decrease in glucose uptake. These data indicate that short-term leptin treatment in HF-fed rats alters muscle FA metabolism by increasing FA uptake and oxidation relative to pair feeding alone. This results in a decrease in the FA esterification-oxidation ratio.

Insulin-stimulated plasma membrane association and activation of Akt2, aPKC zeta and aPKC lambda in high fat fed rodent skeletal muscle

Several recent reports using cell lines have suggested that both Akt and atypical protein kinase C (aPKC) zeta/lambda are translocated to the plasma membrane (PM) in response to insulin. However, it has yet to be determined in skeletal muscle whether: (1) insulin increases PM-associated Akt2, aPKC zeta and/or lambda protein concentration, (2) the activity of these kinases is altered by insulin at the PM, and (3) high fat feeding alters the insulin-stimulated PM concentration and/or activity of Akt2 and aPKC zeta/lambda. Sprague-Dawley rats were randomly assigned to either normal (n=16) or high fat (n=16) dietary groups. Following a 12 week dietary period, animals were subjected to hind limb perfusions in the presence (n=8 per group) or absence (n=8 per group) of insulin. In normal skeletal muscle, total PI3-kinase, Akt2 and aPKC zeta/lambda activities were increased by insulin. PM-associated aPKC zeta and lambda, and aPKC zeta/lambda activity, but not Akt2 or Akt2 activity, were increased by insulin in normal muscle. High fat feeding did not alter total skeletal muscle Akt2, aPKC zeta or aPKC lambda protein concentration. Insulin-stimulated total PI3-kinase, Akt2 and aPKC zeta/lambda activities were reduced in the high fat fed animals. Insulin-stimulated PM aPKC zeta, aPKC lambda, aPKC zeta/lambda activity and GLUT4 protein concentration were also reduced in high fat fed animals. These findings suggest that in skeletal muscle, insulin stimulates translocation of aPKC zeta and lambda, but not Akt2, to the PM. In addition, high fat feeding impairs insulin-stimulated activation of total aPKC zeta/lambda and Akt2, as well as PM association and activation of aPKC zeta and lambda.

Chronic leptin treatment enhances insulin-stimulated glucose disposal in skeletal muscle of high-fat fed rodents

The aim of this investigation was to evaluate if chronic leptin administration corrects high fat diet-induced skeletal muscle insulin resistance, in part, by enhancing rates of glucose disposal and if the improvements are accounted for by alterations in components of the insulin-signaling cascade. Sprague-Dawley rats consumed normal (CON) or high fat diets for three months. After the dietary lead in, the high fat diet group was further subdivided into high fat (HF) and high fat, leptin treated (HF-LEP) animals. HF-LEP animals were injected twice daily with leptin (5 mg/100 g body weight) for 10 days, while the CON and HF animals were injected with vehicle. Following the treatment periods, all animals were prepared for and subjected to hind limb perfusion. The high fat diet decreased rates of insulin-stimulated skeletal muscle glucose uptake and glycogen synthesis in the red gastrocnemius (RG), but did not affect glycogen synthase activity, rates of glucose oxidation or nonoxidative disposal of glucose. Of interest, IRS-1-associated PI3-K activity and total GLUT4 protein concentration were reduced in the RG of the high fat-fed animals. Leptin treatment increased rates of insulin-stimulated glucose uptake and glucose oxidation, and normalized rates of glycogen synthesis. Leptin appeared to mediate these effects by normalizing insulin-stimulated PI3-K activation and GLUT4 protein concentration in the RG. Collectively, these data suggest that chronic leptin treatment reverses the effects of a high fat diet thereby allowing the insulin signaling cascade and glucose transport effector system to be fully activated which in turn affects the amount of glucose that is transported across the plasma membrane and made available for glycogen synthesis.

Resistance training enhances components of the insulin signaling cascade in normal and high-fat-fed rodent skeletal muscle

Our laboratory recently reported that chronic resistance training (RT) improved insulin-stimulated glucose transport in normal rodent skeletal muscle, owing, in part, to increased GLUT-4 protein concentration (Yaspelkis BB III, Singh MK, Trevino B, Krisan AD, and Collins DE. Acta Physiol Scand 175: 315-323, 2002). However, it remained to be determined whether these improvements resulted from alterations in the insulin signaling cascade as well. In addition, the possibility existed that RT might improve skeletal muscle insulin resistance. Thirty-two male Sprague-Dawley rats were assigned to four groups: control diet (Con)-sedentary (Sed); Con-RT; high-fat diet (HF)-Sed; and HF-RT. Animals consumed their respective diets for 9 wk; then RT animals performed 12 wk of training (3 sets, 10 repetitions at 75% one-repetition maximum, 3x/wk). Animals remained on their dietary treatments over the 12-wk period. After the training period, animals were subjected to hindlimb perfusions. Insulin-stimulated insulin receptor substrate-1-associated phosphatidylinositol-3 kinase activity was enhanced in the red gastrocnemius and quadriceps of Con-RT and HF-RT animals. Atypical PKC-zeta/lambda and Akt activities were reduced in HF-Sed and normalized in HF-RT animals. Resistance training increased GLUT-4 protein concentration in red gastrocnemius and quadriceps of Con-RT and HF-RT animals. No differences were observed in total protein concentrations of insulin receptor substrate-1, Akt, atypical PKC-zeta/lambda, or phosphorylation of Akt. Collectively, these findings suggest that resistance training increases insulin-stimulated carbohydrate metabolism in normal skeletal muscle and reverses high-fat diet-induced skeletal muscle insulin resistance by altering components of both the insulin signaling cascade and glucose transporter effector system.

Resistance training increases glucose uptake and transport in rat skeletal muscle

The aim of this investigation was to determine if resistance training exercise improved glucose uptake and transport in rodent skeletal muscle. Sprague-Dawley rats were assigned to one of the three groups: control (CON), resistance trained (RT) and aerobic exercise trained (AT). Resistance trained rats were placed in a rodent squat apparatus and performed three sets of 10 repetitions at 75% of their one repetition maximum 3 days week-1 for 12 weeks. Aerobic exercise training consisted of running the rats 3 days week-1 for 45 min over a 12-week period on a motor-driven treadmill (32 m min-1, 15% grade). Following the training period, all animals were subjected to hind limb perfusion in the presence of 500 microU mL-1 insulin. Hind limb glucose uptake was similar in the RT (9.91 +/- 0.7 micromol g-1 h-1) and AT (10.23 +/- 1.0 micromol g-1 h-1) animals and significantly greater than control (CON) (6.40 +/- 0.6 micromol g-1 h-1). Rates of 3-O-methyl-d-glucose transport in the RT animals were elevated in the muscles utilized for RT while in the AT animals rates of 3-O-methyl-d-glucose transport were increased in those muscles recruited for running. The increased rates of 3-O-methyl-d-glucose transport in the skeletal muscles of the resistance trained and aerobic exercise trained animals appeared to be, in part, because of an increased GLUT4 protein concentration. These findings suggest that both resistance or aerobic training exercise can improve insulin-stimulated skeletal muscle glucose uptake and transport, but the training adaptations are restricted to the muscles recruited for the exercise performance.

Leptin increases FA oxidation in lean but not obese human skeletal muscle: evidence of peripheral leptin resistance

The adipocyte-derived hormone leptin has been shown to acutely increase fatty acid (FA) oxidation and decrease esterification in resting rodent skeletal muscle. However, the effects of leptin on human skeletal muscle FA metabolism are completely unknown. In these studies, we have utilized an isolated human skeletal muscle preparation combined with the pulse-chase technique to measure FA metabolism with and without leptin in lean and obese human skeletal muscle. Under basal conditions (in the absence of leptin), muscle from the obese demonstrated significantly elevated levels of total FA uptake (+72%, P = 0.038) and enhanced rates of FA esterification into triacylglycerol (+102%, P = 0.042) compared with lean subjects. In the presence of leptin, lean muscle had elevated rates of endogenous (+103%, P = 0.01) and exogenous (+150%, P = 0.03) palmitate oxidation. When the ratio of esterification to exogenous oxidation was examined, leptin reduced this ratio (-47%, P = 0.032), demonstrating the increased partitioning of FA toward oxidation and away from storage. Contrary to these findings in lean muscle, leptin had no effect on FA metabolism in skeletal muscle of the obese. This study provides the first evidence that leptin increases FA oxidation in skeletal muscle of lean, but not obese humans, thus demonstrating the development of leptin resistance in obese human skeletal muscle.

Adipose tissue IL-6 content correlates with resistance to insulin activation of glucose uptake both in vivo and in vitro

Obesity and type 2 diabetes are associated with insulin resistance, the mechanisms of which remain poorly understood. A significant correlation between circulating IL-6 level and insulin sensitivity has recently been found in humans. Because adipose tissue could be a significant source of IL-6, we analyzed the relationship between the levels of adipose tissue IL-6 and insulin action in vivo, during a hyperinsulinemic normoglycemic clamp, and in vitro by measuring glucose transport in adipocytes from 12 obese subjects with (n = 7) or without (n = 5) diabetes. We observed an inverse correlation between adipose tissue IL-6 content and maximal insulin-responsiveness measured in vivo (P < 0.02) and in vitro (P < 0.02). Conversely, there was no significant correlation between these two later parameters and adipose tissue leptin or tumor necrosis factor-alpha protein contents. Furthermore, we showed, for the first time, the presence of immunoreactive IL-6 receptors in the plasma membrane of human abdominal sc adipocytes. This suggests that locally secreted IL-6 could act on adipocytes by an autocrine/paracrine mechanism. In conclusion, increased IL-6 production by sc adipose cells might participate to the insulin-resistant state observed in human obesity.

Fatty acid oxidation and triacylglycerol hydrolysis are enhanced after chronic leptin treatment in rats

Leptin acutely increases fatty acid (FA) oxidation and triacylglycerol (TG) hydrolysis and decreases TG esterification in oxidative rodent muscle. However, the effects of chronic leptin administration on FA metabolism in skeletal muscle have not been examined. We hypothesized that chronic leptin treatment would enhance TG hydrolysis as well as the capacity to oxidize FA in soleus (SOL) muscle. Female Sprague-Dawley rats were infused for 2 wk with leptin (LEPT; 0.5 mg x kg(-1) x day(-1)) by use of subcutaneously implanted miniosmotic pumps. Control (AD-S) and pair-fed (PF-S) animals received saline-filled implants. Subsequently, FA metabolism was monitored for 45 min in isolated, resting, and contracting (20 tetani/min) SOL muscles by means of pulse-chase procedures. Food intake (-33 +/- 2%, P < 0.01) and body mass (-12.5 +/- 4%, P = 0.01) were reduced in both LEPT and PF-S animals. Leptin levels were elevated (+418 +/- 7%, P < 0.001) in treated animals but reduced in PF-S animals (-73 +/- 8%, P < 0.05) relative to controls. At rest, TG hydrolysis was increased in leptin-treated rats (1.8 +/- 2.2, AD-S vs. 23.5 +/- 8.1 nmol/g wet wt, LEPT; P < 0.001). In contracting SOL muscles, TG hydrolysis (1.5 +/- 0.6, AD-S vs. 3.6 +/- 1.0 micromol/g wet wt, LEPT; P = 0.02) and palmitate oxidation (18.3 +/- 6.7, AD-S vs. 45.7 +/- 9.9 nmol/g wet wt, LEPT; P < 0.05) were both significantly increased by leptin treatment. Chronic leptin treatment had no effect on TG esterification either at rest or during contraction. Markers of overall (citrate synthase) and FA (hydroxyacyl-CoA dehydrogenase) oxidative capacity were unchanged with leptin treatment. Protein expression of hormone-sensitive lipase (HSL) was also unaltered following leptin treatment. Thus leptin-induced increases in lipolysis are likely due to HSL activation (i.e., phosphorylation). Increased FA oxidation secondary to chronic leptin treatment is not due to an enhanced oxidative capacity and may be a result of enhanced flux into the mitochondrion (i.e., carnitine palmitoyltransferase I regulation) or electron transport uncoupling (i.e., uncoupling protein-3 expression).