Molecular mechanisms of insulin resistance and associated diseases

An update on preventive and regenerative therapies in diabetes mellitus

Type 1A (immune-mediated) and type 2 diabetes mellitus are two of the most common severe chronic illnesses, affecting over 230 million people worldwide with an estimated global prevalence of 5.1%. Although type 1 and type 2 diabetes differ greatly in modes of pathogenesis, these illnesses share a common pathology and consequences characterized by loss of functional beta-cell mass and subsequent dysregulation of carbohydrate and lipid metabolism. Since therapy for diabetes and the associated complications poses enormous public health and economic burdens, novel preventive and regenerative therapies have emerged in the past decade with the aim to preserve beta-cell mass and delay the onset of diabetes. The goal of this review is to provide a comprehensive overview of current efforts in the fight against diabetes, and attempts to document all strategies that have emerged in clinical studies within the past 25 years. First, strategies to identify individuals at risk, ranging from whole-genome scans to autoantibody screening, will be discussed. Second, novel approaches to prevent or delay the onset of disease will be covered. Particular focus is given on emerging strategies for individuals at risk for type 1 diabetes that target T-cell regulation and induction of tolerance, while new pharmaceutical concepts in combination with lifestyle interventions are discussed within the scope of type 2 diabetes prevention. Lastly, important efforts to halt disease progression with emphasis on beta-cell regeneration are presented.

Apelin stimulates glucose utilization in normal and obese insulin-resistant mice

Adipose tissue (AT) secretes several adipokines that influence insulin sensitivity and potentially link obesity to insulin resistance. Apelin, a peptide present in different tissues, is also secreted by adipocytes. Apelin is upregulated in obese and hyperinsulinemic humans and mice. Although a tight relation exists between the regulation of apelin and insulin, it remains largely unknown whether apelin affects whole-body glucose utilization. Herein, we show that in chow-fed mice, acute intravenous injection of apelin has a powerful glucose-lowering effect associated with enhanced glucose utilization in skeletal muscle and AT. Through in vivo and in vitro pharmacological and genetic approaches, we demonstrate the involvement of endothelial NO synthase, AMP-activated protein kinase, and Akt in apelin-stimulated glucose uptake in soleus muscle. Remarkably, in obese and insulin-resistant mice, apelin restored glucose tolerance and increased glucose utilization. Apelin could thus represent a promising target in the management of insulin resistance.

The effect of metformin on measurements of insulin sensitivity and beta cell response in 18 horses and ponies with insulin resistance

REASONS FOR PERFORMING STUDY

Laminitis in equids is a very common debilitating disease, and insulin resistance (IR) and hyperinsulinaemia are increasingly recognised as important predisposing factors. Pharmacological modification of IR and hyperinsulinaemia might reduce the risk of laminitis.

HYPOTHESIS

Metformin, a drug commonly prescribed for treatment of human IR, may also decrease IR in equids.

METHODS

Eighteen horses and ponies with IR and recurrent laminitis were treated with 15 mg/kg bwt metformin per os q. 12 h. Each animal served as its own control by comparing pre- and post treatment proxies for IR, insulin sensitivity (IS) and pancreatic beta cell function while controlling for possible dietary and managemental influences on IR.

RESULTS

Evidence of significantly improved IS and decreased pancreatic beta cell secretion was found following metformin treatment. The magnitude of effect was greater at earlier resampling (6-14 days) than at later times (23-220 days). Apparent subjective clinical benefits were good but less favourable than effects on IR.

CONCLUSIONS

Metformin is safe and appears to increase IS in equids.

POTENTIAL RELEVANCE

Metformin may be indicated as a treatment for IR in equids. Further studies are required to define appropriate selection of subjects warranting therapy, dosing schedule and pharmacokinetics.

The major green tea polyphenol, (-)-epigallocatechin-3-gallate, inhibits obesity, metabolic syndrome, and fatty liver disease in high-fat-fed mice

In this study, we investigated the effects of the major green tea polyphenol, (-)-epigallocatechin-3-gallate (EGCG), on high-fat-induced obesity, symptoms of the metabolic syndrome, and fatty liver in mice. In mice fed a high-fat diet (60% energy as fat), supplementation with dietary EGCG treatment (3.2 g/kg diet) for 16 wk reduced body weight (BW) gain, percent body fat, and visceral fat weight (P < 0.05) compared with mice without EGCG treatment. The BW decrease was associated with increased fecal lipids in the high-fat-fed groups (r(2) = 0.521; P < 0.05). EGCG treatment attenuated insulin resistance, plasma cholesterol, and monocyte chemoattractant protein concentrations in high-fat-fed mice (P < 0.05). EGCG treatment also decreased liver weight, liver triglycerides, and plasma alanine aminotransferase concentrations in high-fat-fed mice (P < 0.05). Histological analyses of liver samples revealed decreased lipid accumulation in hepatocytes in mice treated with EGCG compared with high-fat diet-fed mice without EGCG treatment. In another experiment, 3-mo-old high-fat-induced obese mice receiving short-term EGCG treatment (3.2 g/kg diet, 4 wk) had decreased mesenteric fat weight and blood glucose compared with high-fat-fed control mice (P < 0.05). Our results indicate that long-term EGCG treatment attenuated the development of obesity, symptoms associated with the metabolic syndrome, and fatty liver. Short-term EGCG treatment appeared to reverse preexisting high-fat-induced metabolic pathologies in obese mice. These effects may be mediated by decreased lipid absorption, decreased inflammation, and other mechanisms.

A Hypercaloric pellet-diet cycle induces obesity and co-morbidities in wistar rats

The present study was carried to develop and analyze the consequences of hypercaloric pellet-diet cycle that promotes obesity in rats. Male Wistar rats were randomly distributed into two groups that received either normal diet (ND; n =32; 3,5 Kcal/g) or a hypercaloric diet (HD; n =32; 4,6 Kcal/g). The ND group received commercial Labina rat feeding while the HD animals received a cycle of five hypercaloric diets over a 14-week period. The effects of the diets were analyzed in terms of body weight, body composition, hormone-metabolite levels, systolic arterial pressure and glucose tolerance at the 5% significance level. The hypercaloric pellet diet cycle promoted an increase in body weight and fat, systolic arterial pressure and a high serum level of glucose, triacylglycerol, insulin and leptin. The HD group also presented an impaired glucose tolerance. In conclusion, the results of this study show that the hypercaloric pellet-diet cycle promoted obesity in Wistar rats and displayed several characteristics that are commonly associated with human obesity, such as high arterial pressure, insulin resistance, hyperglycaemia, hyperinsulinaemia, hyperleptinaemia and dyslipidaemia.

Effects of green tea on insulin sensitivity, lipid profile and expression of PPARα and PPARγ and their target genes in obese dogs

As in man, canine obesity is associated with insulin resistance, dyslipidaemia and other chronic diseases. This study was designed to examine the effects of a nutritional supplement (green tea) on insulin sensitivity and plasma lipid concentrations in an obese insulin-resistant dog model. We also determined mRNA expression of two transcription factors, PPARγ and PPARα, and some of their target genes, including GLUT4, lipoprotein lipase (LPL) and adiponectin. Obese dogs were divided into two groups: a green tea group (n 6); a control group (n 4). Dogs in the green tea group were given green tea extract (80 mg/kg per d) orally, just before their single daily meal, for 12 weeks. Insulin sensitivity (using a euglycaemic–hyperinsulinaemic clamp) and concentrations of plasma TAG, total cholesterol and NEFA were assessed in each group. Gene expression was measured in visceral and subcutaneous adipose tissues and in liver and skeletal muscle, by real-time PCR. At 12 weeks in the green tea group, mean insulin sensitivity index was 60 (sem 11) % higher (P < 0·05) and TAG concentration 50 (sem 10) % lower (P < 0·001), than baseline. PPARγ, GLUT4, LPL and adiponectin expression were significantly higher in both adipose tissues, whilst PPARα and LPL expression were significantly higher in skeletal muscle, compared with baseline. These findings show that nutritional doses of green tea extract may improve insulin sensitivity and lipid profile and alter the expression of genes involved in glucose and lipid homeostasis.

Fat feeding potentiates the diabetogenic effect of dexamethasone in Wistar rats

Background

The role of cortisol and its increased action/availability is implicated in the pathogenesis of insulin resistance associated with obesity and metabolic syndrome but the mechanism of increased action/availability is not known. Availability of several other lipophilic hormones, drugs and pollutants are also reported to be increased in obesity. Increased lipids in the circulation are reported to alter the fluidity and permeability of membranes. Hyperlipidemia is also reported to alter the pharmacokinetics and pharmacodynamics of lipophilic molecules and also membrane fluidity and permeability. In this context we assumed that the hyperlipidemia associated with human obesity might play a role in the altered action/availability of cortisol and this in turn might have initiated the metabolic complications. To evaluate our assumption we have administered dexamethasone [low [50 μg/kg/day] or high [250 μg/kg/day] dose] to high-fat [coconut oil & vanaspati] fed rats and the results were compared with rats administered with either dexamethasone or high-fat.

Results and Discussion

Within two weeks, the rats co-administered with high-fat and dexamethasone developed severe hyperglycemia, hyperlipidemia and insulin resistance compared to rats treated either of them alone. High-fat fed rats treated with higher dose of dexamethasone were presented with severe hyperglycemia, insulin resistance and also severe glycosuria. The hyperlipidemia caused by high-fat feeding might have altered the transport and distribution of dexamethasone, probably by altering the physical state of membranes and transport proteins.

Conclusion

From the results obtained, it can be speculated that the altered lipid and cortisol metabolism could affect one another, forming a vicious cycle.

Relationship between Leptin, Insulin Resistance, Insulin-like Growth Factor-1 and Insulin-like Growth Factor Binding Protein-3 in Patients with Chronic Kidney Disease

This study examined the relationship between leptin, insulin-like growth factor-1 (IGF-1), IGF binding protein-3 (IGFBP-3) and insulin resistance in patients with chronic kidney disease (CKD). Levels of leptin, insulin, IGF-1, IGFBP-3 and common routine parameters were measured in 45 patients (23 males and 22 females) with CKD and 45 healthy controls matched for age, gender and body mass index. IGF-1 and IGFBP-3 levels were measured using a two-site immunoradiometric assay. Leptin levels were measured using an enzyme-linked immunosorbent assay. A homeostasis model assessment computer-solved model was used to assess insulin resistance (HOMA-IR). Levels of serum leptin, insulin, IGF-1, IGFBP-3 and HOMA-IR were significantly increased in patients with CKD compared with healthy subjects, whereas fasting blood glucose was not significantly different between the two groups. In patients with CKD, the serum leptin level was significantly correlated with IGF-1, IGFBP-3 and HOMA-IR. In conclusion, this study suggests that there is an interaction between leptin, IGF-1, IGFBP-3 and insulin resistance in patients with CKD.

Old biliverdin reductase: links to insulin resistance and may be a novel therapeutic target

Insulin resistance has been assigned a pivotal role in the pathological progression associated with type 2 diabetes and other chronic metabolic diseases. However, the molecular mechanism involved in this progression is still incompletely understood, and there are still no effective approaches to scavenge it. Many biological molecules, such as ROS, IRS-1, PI3K, have been identified involving in the causes of insulin resistance. Restoring these molecules could ameliorate the phenomenon of insulin resistance. BVR was known for a long time solely as an enzyme reducing biliverdin to bilirubin in the heme metabolic pathway. Presently, accumulative research data showed that BVR was a strong antioxidant enzyme, which could scavenge the excess ROS, and the characteristics of kinase activity and binding with p85 could modulate the biological function of IRS-1 and PI3K. We hypothesize that BVR has a significant role in the progression of insulin resistance, and it will be a promising therapeutic target for treating insulin resistance.

Reduced susceptibility of muscle-specific insulin receptor knockout mice to colon carcinogenesis

Insulin resistance is a risk factor for colon cancer, but it is not clear which of its metabolic sequelae are involved. The objective of this study was to determine whether increased adiposity and elevated circulating lipids commonly seen in insulin resistance promote colon carcinogenesis independent of changes in insulin. We made use of muscle-specific insulin receptor knockout (MIRKO) mice that exhibit elevated serum triglycerides (TG), free fatty acids (FFA), and fat mass but have similar body weights, circulating glucose, and insulin and insulin sensitivity to their wild-type littermates used as controls. Seven-week-old male MIRKO mice and controls received four weekly intraperitoneal injections of either 5 mg/kg azoxymethane (AOM) to induce aberrant crypt foci (ACF) or 10 mg/kg AOM to induce tumors and were killed at 24 or 40 wk of age, respectively. The MIRKO mice displayed hyperinsulinemia at 7 wk of age and reduced insulin sensitivity at 16 wk of age compared with controls. The previously reported MIRKO phenotype developed between 16 and 24 wk of age. By 40 wk of age, however, MIRKO mice were again insulin resistant. ACF development did not differ between MIRKO mice and controls, but MIRKO mice developed significantly fewer colon tumors. Our results suggest that circulating TG and FFA are not promoters of colon tumor development. Indeed, we show that the cumulative effects of the metabolic changes that occur with knockout of the insulin receptor in muscle are associated with reduced susceptibility to colon tumorigenesis.

Polycystic ovarian syndrome: pathophysiology, molecular aspects and clinical implications

Polycystic ovarian syndrome (PCOS) is universally recognised as the commonest endocrinopathy of women. The definition and the aetiological hypotheses of PCOS are continuously evolving to accommodate expanding knowledge on the syndrome, which is now known to be more complex than purely a reproductive disorder. Increased androgen synthesis, disrupted folliculogenesis and insulin resistance lie at the pathophysiological core of PCOS. An intriguing concept involves the perpetuation of a vicious circle with endocrine/reproductive and metabolic components. An unfavourable metabolic environment may unmask genetic traits of ovarian dysfunction, and the unfolding endocrine derangement could further aggravate the metabolic disarray. This article reviews the molecular mechanisms known to underlie the ovarian and metabolic abnormalities characterising PCOS. The putative interdependence between reproductive and metabolic aspects of PCOS, and therapeutic implications for the management of PCOS, are also discussed.

[Influence of body weight excess and central adiposity in glycemic and lipid profile in patients with type 2 diabetes mellitus]

The purpose of the present study was to evaluate, in diabetic type 2 patients, the influence of body weight excess assessed by BMI and the central adiposity assessed by the waist circumference in the lipid and glycemic profile. One hundred and forty five individuals assisted in a unit of health, aged >or= 20 years old, were appraised. The female frequency was 61.4%. Among men, the weight excess was correlated with HDL (r = -0.34, p < 0.05), with the total cholesterol (r = 0.35, p < 0.01), and with the triglycerides (r = 49, p < 0.0001), and among women there was correlation just with the triglycerides (r = 0.24, p < 0,05). Waist circumference was correlated with the fast glycemia women's group (r = 0.3, p < 0.01), with the total cholesterol among men (r = 0.33, p < 0.05) and with triglycerides in both genders (F: r = 0.22, p < 0.05; M: r = 0.49, p < 0.001). Highest glycemic and triglycerides levels were present in patients with excess body weight and central adiposity and lower HDL levels were present in patients with body weight excess. In conclusion, there is association between inadequate metabolic profile and the excess of body weight and/or central obesity, evidencing the need for a nutritional and clinical intervention in the diabetic type 2 patients, in order to reduce the risk of future chronic complications.

The development of insulin resistance in Type 2 diabetes: insights from knockout studies

Diabetes is a common endocrine disorder, primarily characterised by elevated plasma glucose levels. The disease affects all age groups worldwide. Most patients suffer from Type 2 diabetes, which is mainly due to insulin resistance. It is thought that changes in insulin signalling pathways underlie the development of insulin resistance. This article aims to review recent studies that have elucidated the role of individual proteins in these insulin signalling pathways. These studies have been undertaken using two strategies, one employing mice carrying a global null mutation of particular gene-encoding proteins by the homologous recombination method and another strategy using mice with tissue-specific insulin receptor and/or GLUT4 knockout by the Cre-loxP system. The various phenotypes of these knockout mice, and the light they shed on the etiology of insulin resistance, are discussed. By advancing our understanding of the complex molecular mechanisms underlying insulin resistance, these knock-out models may help us to develop more effective treatments for Type 2 diabetes.

Serum retinol-binding protein 4 concentrations in response to short-term overfeeding in normal-weight, overweight, and obese men

BACKGROUND

Retinol-binding protein 4 (RBP4) is a novel adipokine that induces insulin resistance in mice. Studies in humans have shown a correlation between serum RBP4 and insulin resistance in obese subjects and in subjects with type 2 diabetes. Few data are available regarding the nutritional regulation of RBP4.

OBJECTIVE

The study investigated the relation of RBP4 with phenotypes of glucose and lipid metabolism at baseline and in response to a 7-d overfeeding protocol in young men.

DESIGN

Sixty-five men participated in the study. Subjects were classified on the basis of body mass index (BMI; kg/m(2)) as normal-weight (</=24.9) or as overweight or obese (>/=25.0). Serum RBP4, interleukin-6, visfatin, glucose, insulin, total cholesterol, HDL cholesterol, and LDL cholesterol (calculated), and triacylglycerols were measured. Insulin resistance and beta cell function were assessed by using the homeostasis model. Percentage body fat was measured by using dual-energy X-ray absorptiometry.

RESULTS

No significant differences were found in serum RBP4 between the 2 groups at baseline. Likewise, no significant differences were observed in fasting serum RBP4 in response to overfeeding. Baseline RBP4 was negatively correlated with the change in insulin resistance in normal-weight subjects, independent of age and BMI. No significant correlation was found between serum RBP4 and visfatin, interleukin-6, or any other variables measured.

CONCLUSIONS

Short-term overfeeding did not induce significant changes in RBP4. Baseline RBP4 concentrations may predict insulin resistance when exposed to a positive energy challenge in normal-weight men.

Congenital and environmental factors associated with adipocyte dysregulation as defects of insulin resistance

The metabolic syndrome refers to insulin resistance and its associated cluster of related cardiovascular metabolic risk factors including type 2 diabetes, hypertension, dyslipidemia and central obesity. Although many hypotheses and facts have been proposed to explain the interaction between genetic and environmental causes of this syndrome, the primary etiology of the metabolic syndrome is adipose tissue dysregulation. Firstly, the thrifty genotype and phenotype hypothesis may explain the endemic increase in type 2 diabetes and cardiovascular disease in developing countries and elucidates the congenital susceptibility and environmental triggering of the metabolic syndrome. Secondly, over-nutrition leads to fatty acid (FA) accumulation in adipocytes and to an overflow to ectopic fat storage organs. This causes functional changes in adipocytes shifting the intra-cellular metabolic pathway toward insulin resistance. Thirdly, obese subjects exhibit increased fat cell size and over-secretion of biologic adipocytokines. Fourthly, failure to adequately develop adipose tissue mass, as seen in lipodystrophy cases, causes severe insulin resistance and diabetes. Lastly, similar to human type 2 diabetes, Psammonys obesus, a desert rat which feeds mainly on low-calorie vegetation, develops the metabolic syndrome when given a diet of calorie rich food. The above evidence indicates adipocyte dysregulation and secretion of FA as well as certain molecules from overloaded adipocytes/adipokines contribute to the pathogenesis of impaired insulin secretion and insulin resistance, endothelial dysfunction, a pro-inflammatory state and promote progression of atherosclerosis. The metabolic syndrome is a modern disease resulting adipocyte dysmetabolism resulting from the paradox of the slow human evolution combined with rapid environmental changes.

Hypoglycemic effects of exopolysaccharides produced by mycelial cultures of two different mushrooms Tremella fuciformis and Phellinus baumii in ob/ob mice

The anti-diabetic activities of the exopolysaccharides (EPS) produced by submerged mycelial culture of two different mushrooms, Tremella fuciformis and Phellinus baumii, in ob/ob mice were investigated. All the animals were randomly divided into three groups with seven animals in each group: The control group received 0.9% NaCl solution; the diabetic groups were treated with EPS from T. fuciformis (Tf EPS) and P. baumii (Pb EPS) at the level of 200 mg/kg body weight using an oral zoned daily for 52 days. The plasma glucose levels in the EPS-fed mice were substantially reduced by about 52% (Tf EPS) and 32% (Pb EPS), respectively, as compared to control mice. The results of oral glucose tolerance test (OGTT) revealed that both EPS-fed groups significantly increased the glucose disposal after 52 days of EPS treatments. Furthermore, higher food efficiency ratios and reduced blood triglyceride levels were observed in the EPS-treated groups. Because peroxisome proliferator-activated receptor gamma (PPAR-gamma) is indeed a key regulator of insulin action, we investigated the expression pattern of adipose tissue PPAR-gamma messenger RNA (mRNA) and plasma levels of PPAR-gamma. It was revealed that PPAR-gamma was significantly activated in response to EPS treatments. The results suggested that both EPS exhibited considerable hypoglycemic effect and improved insulin sensitivity possibly through regulating PPAR-gamma-mediated lipid metabolism. Our results indicated that two mushroom-derived EPS might be developed as potential oral hypoglycemic agents or functional foods for the management of non-insulin-dependent diabetes mellitus.