Hyperinsulinemia drives diet-induced obesity independently of brain insulin production

Chromium propionate increases insulin sensitivity in horses following oral and intravenous carbohydrate administration

Forty-eight Quarter Horse geldings (3 to 8 yr of age) were used to determine the effects of dietary chromium (Cr), in the form of Cr propionate (Cr Prop) on insulin sensitivity. Horses were blocked by age, body condition score, and glucose response to concentrate feeding on day 0 and randomly assigned to treatments. Treatments consisted of 0, 2, 4, or 8 mg Cr/d from Cr Prop. Horses were fed daily a concentrate mix at a rate of 0.2 kg/100 kg body weight (BW) and grass hay at 1.75 to 2.0 kg/100 kg BW. All horses were fed the control diet for 7 d prior to the initiation of the study. After an overnight fast, blood samples from the jugular vein were obtained at 0, 2, and 4 h after concentrate feeding on days 0 and 28 for the determination of glucose, nonesterified fatty acids, and insulin. A glucose tolerance test (GTT) was conducted on day 42. Glucose was infused via jugular vein catheters, and blood samples were collected at various times relative to dosing for glucose and insulin determination. Plasma glucose on day 28 was affected (P < 0.05) by treatment, time, and treatment × time. Horses fed 4 mg Cr/d had lesser (P < 0.05) plasma glucose concentrations than those in the other treatments at 0 h. At 2 h post-feeding glucose concentrations were greater (P < 0.05) in horses fed 0 or 8 mg Cr/d than in those given 4 mg Cr. Horses fed 2 mg Cr/d had lesser (P < 0.05) plasma glucose at 4 h post feeding compared with those fed 0 or 8 mg Cr. Plasma glucose did not differ among horses receiving 2 or 4 mg Cr/d at 2 or 4 h. Serum insulin was affected (P < 0.05) by treatment, time, and treatment × time. Insulin concentrations were greater (P < 0.05) in horses fed 0 or 2 mg Cr/d than in those given 4 or 8 mg Cr at 0 h. At 4 h post-feeding insulin concentrations were greater (P < 0.05) in horses given 0 or 8 mg Cr than in those fed 2 or 4 mg Cr/d. Plasma glucose was affected (P < 0.05) by treatment and time, but not by treatment × time following the GTT. Mean plasma glucose (across sampling times) concentrations were greater (P < 0.05) in controls than in horses fed 2 or 4 mg Cr/d. Glucose concentrations following the GTT did not differ among controls and horses given 8 mg Cr/d. Following glucose infusion, serum insulin concentrations were greater (P < 0.05) in horses fed 2 or 4 mg Cr and tended to be greater in those fed 8 mg Cr/d compared with controls. The results of this study indicate that 2 or 4 mg Cr/d from Cr Prop increased insulin sensitivity in adult horses following oral carbohydrate consumption.

Pancreatic β cell microRNA-26a alleviates type 2 diabetes by improving peripheral insulin sensitivity and preserving β cell function

Type 2 diabetes (T2D) is characterized by insulin resistance along with pancreatic β cell failure. β cell factors are traditionally thought to control glucose homeostasis by modulating insulin levels, not insulin sensitivity. Exosomes are emerging as new regulators of intercellular communication. However, the role of β-cell–derived exosomes in metabolic homeostasis is poorly understood. Here, we report that microRNA-26a (miR-26a) in β cells not only modulates insulin secretion and β cell replication in an autocrine manner but also regulates peripheral insulin sensitivity in a paracrine manner through circulating exosomes. MiR-26a is reduced in serum exosomes of overweight humans and is inversely correlated with clinical features of T2D. Moreover, miR-26a is down-regulated in serum exosomes and islets of obese mice. Using miR-26a knockin and knockout mouse models, we showed that miR-26a in β cells alleviates obesity-induced insulin resistance and hyperinsulinemia. Mechanistically, miR-26a in β cells enhances peripheral insulin sensitivity via exosomes. Meanwhile, miR-26a prevents hyperinsulinemia through targeting several critical regulators of insulin secretion and β cell proliferation. These findings provide a new paradigm for the far-reaching systemic functions of β cells and offer opportunities for the treatment of T2D.

A study using mouse models and human samples reveals a previously unknown role for pancreatic β-cell regulators in glucose homeostasis, in which β cell miR-26a not only modulates insulin secretion and β cell replication in an autocrine manner but also regulates peripheral insulin sensitivity in a paracrine manner through circulating exosomes.

DNA Methylation Changes are Associated with the Programming of White Adipose Tissue Browning Features by Resveratrol and Nicotinamide Riboside Neonatal Supplementations in Mice

Neonatal supplementation with resveratrol (RSV) or nicotinamide riboside (NR) programs in male mice brown adipocyte-like features in white adipose tissue (WAT browning) together with improved metabolism in adulthood. We tested the involvement in this programming of long-term epigenetic changes in two browning-related genes that are overexpressed in WAT of supplemented mice, Slc27a1 and Prdm16. Suckling mice received orally the vehicle, RSV or NR from postnatal days 2-to-20. After weaning (d21) onto a chow diet, male mice were habituated to a normal-fat diet (NFD) starting d75, and split on d90 into continuation on the NFD or switching to a high-fat diet (HFD) until euthanization on d164. CpG methylation by bisulfite-sequencing was analyzed on inguinal WAT. Both treatments modified methylation marks in Slc27a1 and Prdm16 and the HFD-dependent dynamics of these marks in the adult WAT, with distinct and common effects. The treatments also affected gene expression of de novo DNA methylases in WAT of young animals (euthanized at d35 in independent experiments). Studies in 3T3-L1 adipocytes indicated the direct effects of RSV and NR on the DNA methylation machinery and favoring browning features. The results support epigenetic effects being involved in WAT programming by neonatal RSV or NR supplementation in male mice.

Mitochondrial Proton Leak Regulated by Cyclophilin D Elevates Insulin Secretion in Islets at Nonstimulatory Glucose Levels

Fasting hyperinsulinemia precedes the development of type 2 diabetes. However, it is unclear whether fasting insulin hypersecretion is a primary driver of insulin resistance or a consequence of the progressive increase in fasting glycemia induced by insulin resistance in the prediabetic state. Herein, we have discovered a mechanism that specifically regulates non-glucose-stimulated insulin secretion (NGSIS) in pancreatic islets that is activated by nonesterified free fatty acids, the major fuel used by β-cells during fasting. We show that the mitochondrial permeability transition pore regulator cyclophilin D (CypD) promotes NGSIS, but not glucose-stimulated insulin secretion, by increasing mitochondrial proton leak. Islets from prediabetic obese mice show significantly higher CypD-dependent proton leak and NGSIS compared with lean mice. Proton leak-mediated NGSIS is conserved in human islets and is stimulated by exposure to nonesterified free fatty acids at concentrations observed in obese subjects. Mechanistically, proton leak activates islet NGSIS independently of mitochondrial ATP synthesis but ultimately requires closure of the KATP channel. In summary, we have described a novel nonesterified free fatty acid-stimulated pathway that selectively drives pancreatic islet NGSIS, which may be therapeutically exploited as an alternative way to halt fasting hyperinsulinemia and the progression of type 2 diabetes.

Spatial dynamics of the insulin receptor in living neurons

Insulin signaling through the insulin receptor has long been studied in classic target organs, such as adipose tissue and skeletal muscle, where one of its effects is to increase glucose uptake. Insulin and insulin receptor are present in many areas of the brain, but the functions of brain insulin signaling outside feeding circuits are not well defined. It has been proposed that hippocampal insulin signaling is important for memory, that brain insulin signaling is deficient in Alzheimer's disease, and that intranasal insulin treatment improves cognition, but the mechanisms remain unclear and do not seem to involve increased glucose uptake by neurons. The molecular behavior of the insulin receptor itself is not well known in living neurons; therefore, we investigated the spatial dynamics of the insulin receptor on somatodendritic membranes of live rat hippocampal neurons in culture. Using single-molecule tracking of quantum dot-tagged insulin receptors and single-particle tracking photoactivation localization microscopy, we show that the insulin receptor is distributed over both dendritic shafts and spines. Using colocalization with synaptic markers, we also show that in contrast to the glutamate receptor subunit glutamate receptor subunit A1, the dynamics of the insulin receptor are not affected by association with excitatory synapses; however, the insulin receptor is immobilized by components of inhibitory synapses. The mobility of the insulin receptor is reduced both by low concentrations of the pro-inflammatory cytokine tumor necrosis factor α and by cholesterol depletion, suggesting an association with sphingolipid-rich membrane domains. On the other hand, the insulin receptor dynamics in hippocampal neurons are not affected by increased excitatory signaling. Finally, using real-time single-event quantification, we find evidence of strong insulin receptor exocytosis on dendritic shafts. Our results suggest an association of the neuronal insulin receptor with specific elements of the dendritic shaft, rather than excitatory synapses.

Endothelial-specific deficiency of megalin in the brain protects mice against high-fat diet challenge

BACKGROUND

The increasing risk of obesity and diabetes among other metabolic disorders are the consequence of shifts in dietary patterns with high caloric-content food intake. We previously reported that megalin regulates energy homeostasis using blood-brain barrier (BBB) endothelial megalin-deficient (EMD) mice, since these animals developed obesity and metabolic syndrome upon normal chow diet administration. Obesity in mid-life appears to be related to greater dementia risk and represents an increasing global health issue. We demonstrated that EMD phenotype induced impaired learning ability and recognition memory, neurodegeneration, neuroinflammation, reduced neurogenesis, and mitochondrial deregulation associated with higher mitochondrial mass in cortical tissues.

METHODS

EMD mice were subjected to normal chow and high-fat diet (HFD) for 14 weeks and metabolic changes were evaluated.

RESULTS

Surprisingly, BBB megalin deficiency protected against HFD-induced obesity improving glucose tolerance and preventing hepatic steatosis. Compared to wild type (wt), the brain cortex in EMD mice showed increased levels of the mitochondrial biogenesis regulator, peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), and uncoupling protein 2 (UCP2), a thermogenic protein involved in the regulation of energy metabolism. This agreed with the previously found increased mitochondrial mass in the transgenic mice. Upon HFD challenge, we demonstrated these two proteins were found elevated in wt mice but reported no changes over the already increased levels in EMD animals.

CONCLUSION

We propose a protective role for megalin on diet-induce obesity, suggesting this could be related to metabolic disturbances found in dementia through brain endocrine system communications.

Development of Obesity: The Driver and the Passenger

Obesity has reached epidemic proportions and is one of the greatest challenges for public health in the twenty-first century. The macronutrient composition of diets, in particular the amount and ratio of carbohydrates, fat and protein, have received considerable attention in recent decades due to its potential relevance to the development of obesity and weight loss. The effects of various macronutrients on body weight regulation are still under debate. High-carbohydrate diets, and particularly high-fat diets, have been blamed for the increase in the prevalence of obesity. This paper shows that neither fat nor carbohydrates are fattening per se. Mixed diets with substantial amounts of fat and high-glycemic carbohydrates, like current WDs, are required to promote weight gain and obesity. High-glycemic carbohydrates are the active partner (the "driver"), which promotes fat storage through its insulinogenic effect, while fat is the passive partner (the "passenger") on the way to obesity. Elevated insulin levels (postprandial, but more importantly due to hypersecretion and hyperinsulinemia) promote fat storage and play a key role in obesogenesis and the obesity epidemic. Furthermore, mixed diets high in high-glycemic carbohydrates and fat promote fetal programming, with long-term adverse impacts on the offspring, including insulin hypersecretion, (childhood) obesity and metabolic diseases. Maternal obesity and high weight gain during pregnancy have also been linked to deleterious effects on fetal programming. As the global obesity epidemic increasingly affects women of reproductive age, a significant percentage of fetuses will experience fetal programming with a tendency towards obesity - a self-reinforcing process that further fuels the epidemic. A change in lifestyle and diet composition is needed to prevent or limit the development of obesity and related diseases.

New Insight into Diabetes Management: From Glycemic Index to Dietary Insulin Index

BACKGROUND

Despite efforts to control hyperglycemia, diabetes management is still challenging. This may be due to focusing on reducing hyperglycemia and neglecting the importance of hyperinsulinemia; while insulin resistance and resultant hyperinsulinemia preceded diabetes onset and may contribute to disease pathogenesis.

OBJECTIVE

The present narrative review attempts to provide a new insight into the management of diabetes by exploring different aspects of glycemic index and dietary insulin index.

RESULTS

The current data available on this topic is limited and heterogeneous. Conventional diet therapy for diabetes management is based on reducing postprandial glycemia through carbohydrate counting, choosing foods with low-glycemic index and low-glycemic load. Since these indicators are only reliant on the carbohydrate content of foods and do not consider the effects of protein and fat on the stimulation of insulin secretion, they cannot provide a comprehensive approach to determine the insulin requirements.

CONCLUSION

Selecting foods based on carbohydrate counting, glycemic index or glycemic load are common guides to control glycemia in diabetic patients, but neglect the insulin response, thus leading to failure in diabetes management. Therefore, paying attention to insulinemic response along with glycemic response seems to be more effective in managing diabetes.

Release of insulin produced by the choroid plexis is regulated by serotonergic signaling

The choroid plexus (ChP) is a highly vascularized tissue found in the brain ventricles, with an apical epithelial cell layer surrounding fenestrated capillaries. It is responsible for the production of most of the cerebrospinal fluid (CSF) in the ventricular system, subarachnoid space, and central canal of the spinal cord, while also constituting the blood-CSF barrier (BCSFB). In addition, epithelial cells of the ChP (EChP) synthesize neurotrophic factors and other signaling molecules that are released into the CSF. Here, we show that insulin is produced in EChP of mice and humans, and its expression and release are regulated by serotonin. Insulin mRNA and immune-reactive protein, including C-peptide, are present in EChP, as detected by several experimental approaches, and appear in much higher levels than any other brain region. Moreover, insulin is produced in primary cultured mouse EChP, and its release, albeit Ca2+ sensitive, is not regulated by glucose. Instead, activation of the 5HT2C receptor by serotonin treatment led to activation of IP3-sensitive channels and Ca2+ mobilization from intracellular storage, leading to insulin secretion. In vivo depletion of brain serotonin in the dorsal raphe nucleus negatively affected insulin expression in the ChP, suggesting an endogenous modulation of ChP insulin by serotonin. Here, we show for the first time to our knowledge that insulin is produced by EChP in the brain, and its release is modulated at least by serotonin but not glucose.

The carbohydrate-insulin model does not explain the impact of varying dietary macronutrients on the body weight and adiposity of mice

Objectives

The carbohydrate-insulin model (CIM) predicts that increases in fasting and post-prandial insulin in response to dietary carbohydrates stimulate energy intake and lower energy expenditures, leading to positive energy balance and weight gain. The objective of the present study was to directly test the CIM's predictions using C57BL/6 mice.

Methods

Diets were designed by altering dietary carbohydrates with either fixed protein or fat content and were fed to C57BL/6 mice acutely or chronically for 12 weeks. The body weight, body composition, food intake, and energy expenditures of the mice were measured. Their fasting and post-prandial glucose and insulin levels were also measured. RNA-seq was performed on RNA from the hypothalamus and subcutaneous white adipose tissue. Pathway analysis was conducted using IPA.

Results

Only the post-prandial insulin and fasting glucose levels followed the CIM's predictions. The lipolysis and leptin signaling pathways in the sWAT were inhibited in relation to the elevated fasting insulin, supporting the CIM's predicted impact of high insulin. However, because higher fasting insulin was unrelated to carbohydrate intake, the overall pattern did not support the model. Moreover, the hypothalamic hunger pathways were inhibited in relation to the increased fasting insulin, and the energy intake was not increased. The browning pathway in the sWAT was inhibited at higher insulin levels, but the daily energy expenditure was not altered.

Conclusions

Two of the predictions were partially supported (and hence also partially not supported) and the other three predictions were not supported. We conclude that the CIM does not explain the impact of dietary macronutrients on adiposity in mice.

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Higher fasting insulin related to inhibited lipolysis and leptin pathways in sWAT, supporting CIM.

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Higher fasting insulin related to inhibited hypothalamic hunger pathway, contrasting CIM.

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Fasting insulin decreased with higher dietary carbohydrate, overall contrasting CIM.

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Higher dietary carbohydrate did not lead to greater EI/adiposity, or lowered EE.

Diabetes, a Contemporary Risk for Parkinson's Disease: Epidemiological and Cellular Evidences

Diabetes mellitus (DM), a group of diseases characterized by defective glucose metabolism, is the most widespread metabolic disorder affecting over 400 million adults worldwide. This pathological condition has been implicated in the pathogenesis of a number of central encephalopathies and peripheral neuropathies. In further support of this notion, recent epidemiological evidence suggests a link between DM and Parkinson’s disease (PD), with hyperglycemia emerging as one of the culprits in neurodegeneration involving the nigrostriatal pathway, the neuroanatomical substrate of the motor symptoms affecting parkinsonian patients. Indeed, dopaminergic neurons located in the mesencephalic substantia nigra appear to be particularly vulnerable to oxidative stress and degeneration, likely because of their intrinsic susceptibility to mitochondrial dysfunction, which may represent a direct consequence of hyperglycemia and hyperglycemia-induced oxidative stress. Other pathological pathways induced by increased intracellular glucose levels, including the polyol and the hexosamine pathway as well as the formation of advanced glycation end-products, may all play a pivotal role in mediating the detrimental effects of hyperglycemia on nigral dopaminergic neurons. In this review article, we will examine the epidemiological as well as the molecular and cellular clues supporting the potential susceptibility of nigrostriatal dopaminergic neurons to hyperglycemia.

Disordered Eating Behaviours and Eating Disorders in Women in Australia with and without Polycystic Ovary Syndrome: A Cross-Sectional Study

Psychological co-morbidities common in polycystic ovary syndrome (PCOS) may contribute to disordered eating and subsequent weight gain. This cross-sectional study aimed to determine the prevalence of disordered eating and a range of eating disorders and demographic risk factors associated with these behaviours within an Australian group of women with and without PCOS. Data from 899 women with (n = 501) and without (n = 398) PCOS were analysed as possibly indicative of disordered eating or eating disorders using the Eating Disorder Examination Questionnaire (EDE-Q) and The Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) criteria. Disordered eating (p = 0.012) but not eating disorders (p = 0.076) were more prevalent in women with PCOS compared to controls. Increased body mass index (BMI) [Odds Ratio (OR): 1.03; 95%; Confidence Interval (CI): 1.01, 1.05, p = 0.012] and older age [OR: 1.05; 95%CI: 1.02, 1.08, p = 0.002] but not PCOS diagnosis [OR: 1.43; 95%CI: 0.96, 2.13 p = 0.078] increased the odds of disordered eating. Increased BMI [OR: 1.04; 95%CI: 1.02, 1.06, p < 0.001] and younger age [OR: -0.95; 95%CI: 0.93–0.95, p < 0.001] but not PCOS diagnosis [OR: 1.38; 95%CI: 0.97, 1.95, p = 0.076] increased the odds of an eating disorder. Clinicians are recommended to screen all women with PCOS for possible disordered eating behaviours, with particular attention to women with elevated BMI.

Altered metabolic gene expression in the brain of a triprolyl-human amylin transgenic mouse model of type 2 diabetes

Type 2 diabetes mellitus is a major health concern worldwide; however, the molecular mechanism underlying its development is poorly understood. The hormone amylin is postulated to be involved, as human amylin forms amyloid in the pancreases of diabetic patients, and oligomers have been shown to be cytotoxic to β-cells. As rodent amylin is non-amyloidogenic, mice expressing human amylin have been developed to investigate this hypothesis. However, it is not possible to differentiate the effects of amylin overexpression from β-cell loss in these models. We have developed transgenic mice that overexpress [^{25, 28, 29} triprolyl]human amylin, a non-amyloidogenic variant of amylin, designated the Line 44 model. This model allows us to investigate the effects of chronic overexpression of non-cytotoxic amylin. We characterised this model and found it developed obesity, hyperglycaemia and hyperinsulinaemia. This phenotype was associated with alterations in the expression of genes involved in the amylin, insulin and leptin signalling pathways within the brain. This included genes such as c-Fos (a marker of amylin activation); Socs3 (a leptin inhibitor); and Cart, Pomc and Npy (neuropeptides that control appetite). We also examined Socs3 protein expression and phosphorylated Stat3 to determine if changes at the mRNA level would be reflected at the protein level.

Sex-specific Mendelian randomization study of genetically predicted insulin and cardiovascular events in the UK Biobank

Insulin drives growth and reproduction which trade-off against longevity. Genetically predicted insulin, i.e., insulin proxied by genetic variants, is positively associated with ischemic heart disease, but sex differences are unclear, despite different disease rates and reproductive strategies by sex. We used Mendelian randomization in 392,010 white British from the UK Biobank to assess the sex-specific role of genetically predicted insulin in myocardial infarction (MI) (14,442 cases, 77% men), angina (21,939 cases, 65% men) and heart failure (5537 cases, 71% men). Genetically predicted insulin was associated with MI (odds ratio (OR) 4.27 per pmol/L higher insulin, 95% confidence interval (CI) 1.60 to 11.3) and angina (OR 2.93, 1.27 to 6.73) in men, but not women (MI OR 0.80, 95% CI 0.23 to 2.84, angina OR 1.10, 95% CI 0.38 to 3.18). Patterns were similar for insulin resistance and heart failure. Mitigating the effects of insulin might address sexual disparities in health.

Origins of Brain Insulin and Its Function

The brain or central nervous system (CNS) utilizes a vast amount of energy to sustain its basic functions, and most of the energy in the brain is derived from glucose. Whole-body energy and glucose homeostasis in the periphery of the human body are regulated by insulin, while the brain had been considered as an "insulin-insensitive" organ, because bulk brain glucose uptake is not affected by insulin in either rodents and humans. However, recently it has become clear that the actions of insulin are more widespread in the CNS and are a critical part of normal development, food intake, and energy balance, as well as plasticity throughout adulthood. Moreover, there are substantial evidence demonstrating that brain insulin is derived from pancreas, neurons, and astrocytes. In this chapter, I reviewed recent progress in roles of insulin in the brain, expression of insulin genes, and multiple origins of the brain insulin.

Androgens sensitise mice to glucocorticoid-induced insulin resistance and fat accumulation

AIMS/HYPOTHESIS

Chronic glucocorticoid therapy causes insulin resistance, dyslipidaemia, abnormal fat accumulation, loss of muscle mass and osteoporosis. Here we describe a hitherto unknown sexual dimorphism in the metabolic response to chronic glucocorticoid exposure in mice. This led us to investigate whether glucocorticoid-induced insulin resistance and obesity were dependent on sex hormones.

METHODS

Male and female CD1 mice were treated for 4 weeks with supraphysiological doses (~250 μg/day) of corticosterone, the main glucocorticoid in rodents, or equivalent volume of vehicle (drinking water without corticosterone). To investigate the effects of sex hormones, a separate group of mice were either orchidectomised or ovariectomised prior to corticosterone treatment, with or without dihydrotestosterone replacement. Body composition was determined before and after corticosterone treatment, and insulin tolerance was assessed after 7 and 28 days of treatment. Adipocyte morphology was assessed in white and brown adipose tissues by immunohistochemistry, and fasting serum concentrations of NEFA, triacylglycerols, total cholesterol and free glycerol were measured using colorimetric assays. Obesity- and diabetes-related hormones were measured using multiplex assays, and RNA and protein expression in adipose tissues were measured by RT-PCR and immunoblotting, respectively.

RESULTS

Chronic corticosterone treatment led to insulin resistance, fasting hyperinsulinaemia, increased adiposity and dyslipidaemia in male, but not female mice. In males, orchidectomy improved baseline insulin sensitivity and attenuated corticosterone-induced insulin resistance, but did not prevent fat accumulation. In androgen-deficient mice (orchidectomised males, and intact and ovariectomised females) treated with dihydrotestosterone, corticosterone treatment led to insulin resistance and dyslipidaemia. In brown adipose tissue, androgens were required for corticosterone-induced intracellular lipid accumulation ('whitening'), and dihydrotestosterone specifically exacerbated corticosterone-induced accumulation of white adipose tissue by increasing adipocyte hypertrophy. Androgens also suppressed circulating adiponectin concentrations, but corticosterone-induced insulin resistance did not involve additional suppression of adiponectin levels. In white adipose tissue, androgens were required for induction of the glucocorticoid target gene Gilz (also known as Tsc22d3) by corticosterone.

CONCLUSIONS/INTERPRETATION

In mice, androgens potentiate the development of insulin resistance, fat accumulation and brown adipose tissue whitening following chronic glucocorticoid treatment.

Sex differences in metabolic effects of angiotensin-(1-7) treatment in obese mice

Background

Angiotensin-(1-7) is a beneficial hormone of the renin-angiotensin system known to play a positive role in regulation of blood pressure and glucose homeostasis. Previous studies have shown that in high-fat diet (HFD)-induced obese male mice, circulating angiotensin-(1-7) levels are reduced and chronic restoration of this hormone reverses diet-induced insulin resistance; however, this has yet to be examined in female mice. We hypothesized angiotensin-(1-7) would improve insulin sensitivity and glucose tolerance in obese female mice, to a similar extent as previously observed in male mice.

Methods

Five-week-old male and female C57BL/6J mice (8–12/group) were placed on control diet or HFD (16% or 59% kcal from fat, respectively) for 11 weeks. After 8 weeks of diet, mice were implanted with an osmotic pump for 3-week subcutaneous delivery of angiotensin-(1-7) (400 ng/kg/min) or saline vehicle. During the last week of treatment, body mass and composition were measured and intraperitoneal insulin and glucose tolerance tests were performed to assess insulin sensitivity and glucose tolerance, respectively. Mice were euthanized at the end of the study for blood and tissue collection.

Results

HFD increased body mass and adiposity in both sexes. Chronic angiotensin-(1-7) infusion significantly decreased body mass and adiposity and increased lean mass in obese mice of both sexes. While both sexes tended to develop mild hyperglycemia in response to HFD, female mice developed less marked hyperinsulinemia. There was no effect of angiotensin-(1-7) on fasting glucose or insulin levels among diet and sex groups. Male and female mice similarly developed insulin resistance and glucose intolerance in response to HFD feeding. Angiotensin-(1-7) improved insulin sensitivity in both sexes but corrected glucose intolerance only in obese female mice. There were no effects of sex or angiotensin-(1-7) treatment on any of the study outcomes in control diet-fed mice.

Conclusions

This study provides new evidence for sex differences in the impact of chronic angiotensin-(1-7) in obese mice, with females having greater changes in glucose tolerance with treatment. These findings improve understanding of sex differences in renin-angiotensin mechanisms in obesity and illustrate the potential for targeting angiotensin-(1-7) for treatment of this condition.

Hepatic Insulin Clearance: Mechanism and Physiology

Upon its secretion from pancreatic β-cells, insulin reaches the liver through the portal circulation to exert its action and eventually undergo clearance in the hepatocytes. In addition to insulin secretion, hepatic insulin clearance regulates the homeostatic level of insulin that is required to reach peripheral insulin target tissues to elicit proper insulin action. Receptor-mediated insulin uptake followed by its degradation constitutes the basic mechanism of insulin clearance. Upon its phosphorylation by the insulin receptor tyrosine kinase, carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) takes part in the insulin-insulin receptor complex to increase the rate of its endocytosis and targeting to the degradation pathways. This review summarizes how this process is regulated and how it is associated with insulin-degrading enzyme in the liver. It also discusses the physiological implications of impaired hepatic insulin clearance: Whereas reduced insulin clearance cooperates with increased insulin secretion to compensate for insulin resistance, it can also cause hepatic insulin resistance. Because chronic hyperinsulinemia stimulates hepatic de novo lipogenesis, impaired insulin clearance also causes hepatic steatosis. Thus impaired insulin clearance can underlie the link between hepatic insulin resistance and hepatic steatosis. Delineating these regulatory pathways should lead to building more effective therapeutic strategies against metabolic syndrome.

Brain insulin action: Implications for the treatment of schizophrenia

Insulin action in the central nervous system is a major regulator of energy balance and cognitive processes. The development of central insulin resistance is associated with alterations in dopaminergic reward systems and homeostatic signals affecting food intake, glucose metabolism, body weight and cognitive performance. Emerging evidence has highlighted a role for antipsychotics (APs) to modulate central insulin-mediated pathways. Although APs remain the cornerstone treatment for schizophrenia they are associated with severe metabolic complications and fail to address premorbid cognitive deficits, which characterize the disorder of schizophrenia. In this review, we first explore how the hypothesized association between schizophrenia and CNS insulin dysregulation aligns with the use of APs. We then investigate the proposed relationship between CNS insulin action and AP-mediated effects on metabolic homeostasis, and different domains of psychopathology, including cognition. We briefly discuss a potential role of CNS insulin signaling to explain the hypothesized, but somewhat controversial association between therapeutic efficacy and metabolic side effects of APs. Finally, we propose how this knowledge might inform novel treatment strategies to target difficult to treat domains of schizophrenia. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Insulin and Autophagy in Neurodegeneration

Crosstalk in the pathophysiological processes underpinning metabolic diseases and neurodegenerative disorders have been the subject of extensive investigation, in which insulin signaling and autophagy impairment demonstrate to be a common factor in both conditions. Although it is still somewhat conflicting, pharmacological and genetic strategies that regulate these pathways may be a promising approach for aggregate protein clearancing and consequently the delaying of onset or progression of the disease. However, as the response due to this modulation seems to be time-dependent, finding the right regulation of autophagy may be a potential target for drug development for neurodegenerative diseases. In this way, this review focuses on the role of insulin signaling/resistance and autophagy in some neurodegenerative diseases, discussing pharmacological and non-pharmacological interventions in these diseases.

Appetite regulating hormones in first-episode psychosis: A systematic review and meta-analysis

We aimed to perform a systematic review and meta-analysis of appetite regulating hormones in patients with first-episode psychosis (FEP). Meta-analyses were conducted using random-effects models with Hedges' g as the effect size estimate. We identified 31 eligible studies, investigating the levels of 7 appetite regulating hormones (adiponectin, insulin, leptin, ghrelin, orexin, resistin and visfatin) in 1792 FEP patients and 1364 controls. The insulin levels in FEP patients were higher than in controls (g = 0.34, 95%CI: 0.19 - 0.49, p < 0.001), even considering only antipsychotic-naïve patients (g = 0.39, 95%CI: 0.12 - 0.66, p = 0.005). The severity of negative symptoms was positively associated with the effect size estimates (β = 0.08, 95%CI: 0.01 - 0.16, p = 0.030). Moreover, we found lower levels of leptin in antipsychotic-naïve FEP patients (g = -0.62, 95%CI: -1.11 - 0.12, p = 0.015). Impaired appetite regulation, in terms of elevated insulin levels and decreased leptin levels, occurs in early psychosis, before antipsychotic treatment. Hyperinsulinemia might be related to negative symptoms.

Short-Term High-Starch, Low-Protein Diet Induces Reversible Increase in β-cell Mass Independent of Body Weight Gain in Mice

Long-term exposure to a high starch, low-protein diet (HSTD) induces body weight gain and hyperinsulinemia concomitantly with an increase in β-cell mass (BCM) and pancreatic islets number in mice; however, the effect of short-term exposure to HSTD on BCM and islet number has not been elucidated. In the present study, we investigated changes in body weight, plasma insulin levels, BCM and islet number in mice fed HSTD for 5 weeks followed by normal chow (NC) for 2 weeks. BCM and islet number were increased in mice fed HSTD for 5 weeks compared with those in mice fed NC. On the other hand, mice fed HSTD for 5 weeks followed by NC for 2 weeks (SN) showed decreased BCM and insulin levels, compared to mice fed HSTD for 7 weeks, and no significant differences in these parameters were observed between SN and the control NC at 7 weeks. No significant difference in body weight was observed among HSTD, NC and SN fed groups. These results suggest that a high-starch diet induces an increase in BCM in a manner independent of body weight gain, and that 2 weeks of NC feeding is sufficient for the reversal of the morphological changes induced in islets by HSTD feeding.

Involvement of neuropeptide CART in the central effects of insulin on feeding and body weight

While insulin secreted from pancreas plays a pivotal role in the control of glucose homeostasis, it also interacts with hypothalamic sites and negatively influences the energy balance. The present study was undertaken to reveal the functional interaction between cocaine- and amphetamine-regulated transcript (CART), a well-known anorexic peptide, and insulin within the framework of hypothalamus in the regulation of feeding behavior and body weight. Insulin was administered daily by intracerebroventricular (icv) route, alone or in combination with CART (icv) for a period of seven days. Immediately thereafter, preweighed food was offered to the animals at the commencement of the dark phase. The food intake and body weight were measured daily just prior to next injection. Furthermore, brains of insulin-treated rats were processed for the immunohistochemical analysis of CART-containing elements in the hypothalamus. Treatment with insulin (6 mU, icv) for a period of 7 days caused a significant decrease in food intake and body weight as compared to control. Concomitant administration of CART (0.5 μg, icv) potentiated insulin-induced anorexia and weight loss. Insulin administration resulted in a significant increase in CART immunoreactivity in the hypothalamic arcuate, paraventricular, dorsomedial and ventromedial nuclei. We suggest that increased CART contents in the hypothalamus may be causally linked with anorexia and weight loss induced by insulin.

Elevated fasting insulin is associated with cardiovascular and metabolic risk in women with polycystic ovary syndrome

AIMS

PCOS is associated with various immediate and long term health complications. The aim of this study was to investigate the association of serum fasting insulin concentration with cardiovascular and metabolic risk factors in women with polycystic ovary syndrome.

METHODS

A total of 349 women, 249 women with polycystic ovary syndrome and 100 age-matched healthy controls, were recruited in this case-control study. Fasting insulin and various other biochemical, hormonal and clinical parameters were measured in all participants. The correlation of insulin with cardiometabolic risk factors was evaluated in PCOS women with normal and high serum insulin concentration.

RESULTS

Fasting Insulin, BMI, WHR, FAI, LH: FSH, HOMA, QUICKI were significantly higher in PCOS women compared with healthy controls (p < 0.01). Fasting insulin showed a positive correlation with more cardiovascular and metabolic risk factors in PCOS compared to controls. The BMI, BAI, LAP, HOMA IR, QUICKI and FAI were significantly higher (all p < 0.05) in PCOS patients with higher insulin levels than with PCOS women with normal levels.

CONCLUSION

Fasting insulin is an important determinant in the pathogenesis of obesity and hyperandrogenism in PCOS. It is associated with an increased risk of cardiovascular and metabolic disorders in women with PCOS.

The short-chain fatty acid propionate increases glucagon and FABP4 production, impairing insulin action in mice and humans

The short-chain fatty acid propionate is a potent inhibitor of molds that is widely used as a food preservative and endogenously produced by gut microbiota. Although generally recognized as safe by the U.S. Food and Drug Administration, the metabolic effects of propionate consumption in humans are unclear. Here, we report that propionate stimulates glycogenolysis and hyperglycemia in mice by increasing plasma concentrations of glucagon and fatty acid-binding protein 4 (FABP4). Fabp4-deficient mice and mice lacking liver glucagon receptor were protected from the effects of propionate. Although propionate did not directly promote glucagon or FABP4 secretion in ex vivo rodent pancreatic islets and adipose tissue models, respectively, it activated the sympathetic nervous system in mice, leading to secretion of these hormones in vivo. This effect could be blocked by the pharmacological inhibition of norepinephrine, which prevented propionate-induced hyperglycemia in mice. In a randomized, double-blind, placebo-controlled study in humans, consumption of a propionate-containing mixed meal resulted in a postprandial increase in plasma glucagon, FABP4, and norepinephrine, leading to insulin resistance and compensatory hyperinsulinemia. Chronic exposure of mice to a propionate dose equivalent to that used for food preservation resulted in gradual weight gain. In humans, plasma propionate decreased with weight loss in the Dietary Intervention Randomized Controlled Trial (DIRECT) and served as an independent predictor of improved insulin sensitivity. Thus, propionate may activate a catecholamine-mediated increase in insulin counter-regulatory signals, leading to insulin resistance and hyperinsulinemia, which, over time, may promote adiposity and metabolic abnormalities. Further evaluation of the metabolic consequences of propionate consumption is warranted.

NCS-1 Deficiency Is Associated With Obesity and Diabetes Type 2 in Mice

Neuronal calcium sensor-1 (NCS-1) knockout (KO) in mice (NCS-1^−/− mice) evokes behavioral phenotypes ranging from learning deficits to avolition and depressive-like behaviors. Here, we showed that with the onset of adulthood NCS-1^−/− mice gain considerable weight. Adult NCS-1^−/− mice are obese, especially when fed a high-fat diet (HFD), are hyperglycemic and hyperinsulinemic and thus develop a diabetes type 2 phenotype. In comparison to wild type (WT) NCS-1^−/− mice display a significant increase in adipose tissue mass. NCS-1^−/− adipocytes produce insufficient serum concentrations of resistin and adiponectin. In contrast to WT littermates, adipocytes of NCS-1^−/− mice are incapable of up-regulating insulin receptor (IR) concentration in response to HFD. Thus, HFD-fed NCS-1^−/− mice exhibit in comparison to WT littermates a significantly reduced IR expression, which may explain the pronounced insulin resistance observed especially with HFD-fed NCS-1^−/− mice. We observed a direct correlation between NCS-1 and IR concentrations in the adipocyte membrane and that NCS-1 can be co-immunoprecipitated with IR indicating a direct interplay between NCS-1 and IR. We propose that NCS-1 plays an important role in adipocyte function and that NCS-1 deficiency gives rise to obesity and diabetes type 2 in adult mice. Given the association of altered NCS-1 expression with behaviorial abnormalities, NCS-1^−/− mice may offer an interesting perspective for studying in a mouse model a potential genetic link between some psychiatric disorders and the risk of being obese.

Molecular pathways linking adipose innervation to insulin action in obesity and diabetes mellitus

Adipose tissue comprises adipocytes and many other cell types that engage in dynamic crosstalk in a highly innervated and vascularized tissue matrix. Although adipose tissue has been studied for decades, it has been appreciated only in the past 5 years that extensive arborization of nerve fibres has a dominant role in regulating the function of adipose tissue. This Review summarizes the latest literature, which suggests that adipocytes signal to local sensory nerve fibres in response to perturbations in lipolysis and lipogenesis. Such adipocyte signalling to the central nervous system causes sympathetic output to distant adipose depots and potentially other metabolic tissues to regulate systemic glucose homeostasis. Paracrine factors identified in the past few years that mediate such adipocyte-neuron crosstalk are also reviewed. Similarly, immune cells and endothelial cells within adipose tissue communicate with local nerve fibres to modulate neurotransmitter tone, blood flow, adipocyte differentiation and energy expenditure, including adipose browning to produce heat. This understudied field of neurometabolism related to adipose tissue biology has great potential to reveal new mechanistic insights and potential therapeutic strategies for obesity and type 2 diabetes mellitus.

Transgenic substitution with Greater Amberjack Seriola dumerili fish insulin 2 in NOD mice reduces beta cell immunogenicity

Type I diabetes (T1D) is caused by immune-mediated destruction of pancreatic beta cells. This process is triggered, in part, by specific (aa 9–23) epitopes of the insulin Β chain. Previously, fish insulins were used clinically in patients allergic to bovine or porcine insulin. Fish and human insulin differ by two amino acids in the critical immunogenic region (aa 9–23) of the B chain. We hypothesized that β cells synthesizing fish insulin would be less immunogenic in a mouse model of T1D. Transgenic NOD mice in which Greater Amberjack fish (Seriola dumerili) insulin was substituted for the insulin 2 gene were generated (mouse Ins1^−/− mouse Ins2^−/− fish Ins2^+/+). In these mice, pancreatic islets remained free of autoimmune attack. To determine whether such reduction in immunogenicity is sufficient to protect β cells from autoimmunity upon transplantation, we transplanted fish Ins2 transgenic (expressing solely Seriola dumerili Ins2), NOD, or B16:A-dKO islets under the kidney capsules of 5 weeks old female NOD wildtype mice. The B:Y16A Β chain substitution has been previously shown to be protective of T1D in NOD mice. NOD mice receiving Seriola dumerili transgenic islet transplants showed a significant (p = 0.004) prolongation of their euglycemic period (by 6 weeks; up to 18 weeks of age) compared to un-manipulated female NOD (diabetes onset at 12 weeks of age) and those receiving B16:A-dKO islet transplants (diabetes onset at 12 weeks of age). These data support the concept that specific amino acid sequence modifications can reduce insulin immunogenicity. Additionally, our study shows that alteration of a single epitope is not sufficient to halt an ongoing autoimmune response. Which, and how many, T cell epitopes are required and suffice to perpetuate autoimmunity is currently unknown. Such studies may be useful to achieve host tolerance to β cells by inactivating key immunogenic epitopes of stem cell-derived β cells intended for transplantation.

Insulin deficiency and intranasal insulin alter brain mitochondrial function: a potential factor for dementia in diabetes

Despite the strong association between diabetes and dementia, it remains to be fully elucidated how insulin deficiency adversely affects brain functions. We show that insulin deficiency in streptozotocin-induced diabetic mice decreased mitochondrial ATP production and/or citrate synthase and cytochrome oxidase activities in the cerebrum, hypothalamus, and hippocampus. Concomitant decrease in mitochondrial fusion proteins and increased fission proteins in these brain regions likely contributed to altered mitochondrial function. Although insulin deficiency did not cause any detectable increase in reactive oxygen species (ROS) emission, inhibition of monocarboxylate transporters increased ROS emission and further reduced ATP production, indicating the causative roles of elevated ketones and lactate in counteracting oxidative stress and as a fuel source for ATP production during insulin deficiency. Moreover, in healthy mice, intranasal insulin administration increased mitochondrial ATP production, demonstrating a direct regulatory role of insulin on brain mitochondrial function. Proteomics analysis of the cerebrum showed that although insulin deficiency led to oxidative post-translational modification of several proteins that cause tau phosphorylation and neurofibrillary degeneration, insulin administration enhanced neuronal development and neurotransmission pathways. Together these results render support for the critical role of insulin to maintain brain mitochondrial homeostasis and provide mechanistic insight into the potential therapeutic benefits of intranasal insulin.-Ruegsegger, G. N., Manjunatha, S., Summer, P., Gopala, S., Zabeilski, P., Dasari, S., Vanderboom, P. M., Lanza, I. R., Klaus, K. A., Nair, K. S. Insulin deficiency and intranasal insulin alter brain mitochondrial function: a potential factor for dementia in diabetes.

Could the high consumption of high glycaemic index carbohydrates and sugars, associated with the nutritional transition to the Western type of diet, be the common cause of the obesity epidemic and the worldwide increasing incidences of Type 1 and Type 2 diabetes?

The globally increasing incidences of Type 1 diabetes (T1DM) and Type 2 diabetes (T2DM) can have a common background. If challenged by the contemporary high level of nutritional glucose stimulation, the β-cells in genetically predisposed individuals are at risk for damage which can lead to the diseases. The fat to carbohydrate dietary shift can also contribute to the associated obesity epidemic.

Ectopic fat, insulin resistance and metabolic disease in non-obese Asians: investigating metabolic gradation

Although metabolic abnormalities commonly occur in non-obese Asians, their pathogenesis is not fully understood. Proton magnetic resonance spectroscopy has been used to analyze intracellular lipids in humans, and results suggest that ectopic fat accumulation in muscle and liver may induce insulin resistance in each tissue independently of obesity. Thus, measurement of ectopic fat currently plays an important role in the study of insulin resistance in non-obese Asians. In addition, studies using 2-step hyperinsulinemic euglycemic clamp with a glucose tracer may clarify how tissue-specific insulin resistance in muscle, liver, and adipose tissue contributes to the development of metabolic disease in non-obese Japanese. Although numerous studies have elucidated the pathophysiology of insulin resistance in obese subjects, research on "metabolic gradation," defined as the gradual transition from an insulin-sensitive to an insulin-resistant state, is less common, especially in terms of early metabolic changes. This review addresses a simple question: when and how is insulin resistance induced in non-obese East Asians? Several studies revealed that impaired insulin clearance and hyperinsulinemia not only compensated for insulin resistance, but also secondarily facilitated insulin resistance and weight gain. In this regard, we recently found that impaired insulin clearance and hyperinsulinemia could occur in apparently healthy subjects without significant insulin resistance, suggesting that this change may be an initial trigger that drives subsequent insulin resistance and weight gain. Further research is required to clarify the pathogenesis of metabolic gradation in non-obese Asians.

How Western Diet And Lifestyle Drive The Pandemic Of Obesity And Civilization Diseases

Westernized populations are plagued by a plethora of chronic non-infectious degenerative diseases, termed as "civilization diseases", like obesity, diabetes, cardiovascular diseases, cancer, autoimmune diseases, Alzheimer's disease and many more, diseases which are rare or virtually absent in hunter-gatherers and other non-westernized populations. There is a growing awareness that the cause of this amazing discrepancy lies in the profound changes in diet and lifestyle during recent human history. This paper shows that the transition from Paleolithic nutrition to Western diets, along with lack of corresponding genetic adaptations, cause significant distortions of the fine-tuned metabolism that has evolved over millions of years of human evolution in adaptation to Paleolithic diets. With the increasing spread of Western diet and lifestyle worldwide, overweight and civilization diseases are also rapidly increasing in developing countries. It is suggested that the diet-related key changes in the developmental process include an increased production of reactive oxygen species and oxidative stress, development of hyperinsulinemia and insulin resistance, low-grade inflammation and an abnormal activation of the sympathetic nervous system and the renin-angiotensin system, all of which play pivotal roles in the development of diseases of civilization. In addition, diet-related epigenetic changes and fetal programming play an important role. The suggested pathomechanism is also able to explain the well-known but not completely understood close relationship between obesity and the wide range of comorbidities, like type 2 diabetes mellitus, cardiovascular disease, etc., as diseases of the same etiopathology. Changing our lifestyle in accordance with our genetic makeup, including diet and physical activity, may help prevent or limit the development of these diseases.

Ginsenoside Rb2 Alleviates Obesity by Activation of Brown Fat and Induction of Browning of White Fat

Ginsenoside Rb2 (Rb2), the most abundant saponin contained in Panax ginseng, has been used to treat variety of metabolic diseases. However, its effects in obesity and potential mechanisms are not well-understood. In the present study, we investigated metabolic performance with a Rb2 supplement in diet-induced obese (DIO) mice, focusing on the effects and mechanisms of Rb2 on brown and beige fat functions. Our results demonstrated that Rb2 effectively reduced body weight, improved insulin sensitivity, as well as induced energy expenditure in DIO mice. Histological and gene analysis revealed that Rb2 induced activation of brown fat and browning of white fat by reducing lipid droplets, stimulating uncoupling protein 1 (UCP1) staining, and increasing expression of thermogenic and mitochondrial genes, which could be recapitulated in 3T3-L1, C3H10T1/2, and primary adipocytes. In addition, Rb2 induced phosphorylation of AMP-activated protein kinase (AMPK) both in vitro and in vivo. These effects were shown to be dependent on AMPK since its inhibitor blocked Rb2 from inducing expressions of Pgc1α and Ucp1. Overall, the present study revealed that Rb2 activated brown fat and induced browning of white fat, which increased energy expenditure and thermogenesis, and consequently ameliorated obesity and metabolic disorders. These suggest that Rb2 holds promise in treating obesity.

Deficiency of T cell CD40L has minor beneficial effects on obesity-induced metabolic dysfunction

OBJECTIVE

Obesity-associated metabolic dysfunction increases the risk of multiple diseases such as type 2 diabetes and cardiovascular disease. The importance of the co-stimulatory CD40-CD40L dyad in diet-induced obesity (DIO), with opposing phenotypes arising when either the receptor (aggravating) or the ligand (protective) is deleted, has been described previously. The functions of CD40 and CD40L are cell type dependent. As co-stimulation via T cell-mediated CD40L is essential for driving inflammation, we here investigate the role of T cell CD40L in DIO.

RESEARCH DESIGN AND METHODS

CD4CreCD40L^fl/fl mice on a C57BL/6 background were generated and subjected to DIO by administration of 15 weeks of high fat diet (HFD).

RESULTS

HFD-fed CD4CreCD40L^fl/fl mice had similar weight gain, adipocyte sizes, plasma cholesterol and triglyceride levels as their wild-type (WT) counterparts. Insulin and glucose tolerance were comparable, although CD4CreCD40L^fl/fl mice did have a decreased plasma insulin concentration, suggesting a minor improvement of insulin resistance. Furthermore, although the degree of hepatosteatosis was similar in both genotypes, the gene expression of fatty acid synthase 1 and ATP-citrate lyase had decreased, whereas expression of peroxisome proliferator-activated receptor-α had increased in livers of CD4CreCD40L^fl/fl mice, suggesting decreased hepatic lipid uptake in absence of T cell CD40L.Moreover, CD4CreCD40L^fl/fl mice displayed significantly lower numbers of effector memory CD4⁺ T cells and regulatory T cells in blood and lymphoid organs compared with WT. However, immune cell composition and inflammatory status of the adipose tissue was similar in CD4CreCD40L^fl/fl and WT mice.

CONCLUSIONS

T cell CD40L deficiency results in a minor improvement of insulin sensitivity and hepatic steatosis in DIO, despite the strong decrease in effector T cells and regulatory T cells in blood and lymphoid organs. Our data indicate that other CD40L-expressing cell types are more relevant in the pathogenesis of obesity-associated metabolic dysfunction.

Identification, pathophysiology, and clinical implications of primary insulin hypersecretion in nondiabetic adults and adolescents

BACKGROUND

Excessive insulin secretion may lead to glucose dysregulation. Our aim was to identify primary (independent of insulin resistance) insulin hypersecretion in subjects with normal glucose tolerance and its role in the progression of dysglycemia.

METHODS

In 1,168 adults, insulin secretion rate (ISR) and β cell function were estimated by C-peptide modeling during an oral glucose tolerance test (OGTT) and an i.v. glucose tolerance test. Whole-body insulin sensitivity was measured by a hyperinsulinemic-euglycemic clamp. After regressing ISR on insulin sensitivity, subjects in the upper tertile of the distribution of residuals were defined as primary hypersecretors. This approach was applied to a biethnic cohort of 182 obese adolescents, who received an OGTT, a hyperglycemic, and a euglycemic clamp.

RESULTS

Adult hypersecretors showed older age, more familial diabetes, sedentary lifestyle, increased fat mass, and worse lipid profile compared with the rest of the cohort, despite virtually identical BMI and insulin sensitivity. Insulin secretion was increased by 53% due to enhanced (+23%) β cell glucose sensitivity. Despite the resulting hyperinsulinemia, glucose tolerance was worse in hypersecretors among both adults and adolescents, coupled with higher indices of liver insulin resistance and increased availability of gluconeogenic substrates. At the 3-year follow-up, adult hypersecretors had increased incidence of impaired glucose tolerance/type 2 diabetes.

CONCLUSION

Primary insulin hypersecretion, independent of insulin resistance, is associated with a worse clinical and metabolic phenotype in adults and adolescents and predicts deterioration of glucose control over time.

FUNDING

The relationship between insulin sensitivity and cardiovascular disease (RISC) Study was partly supported by EU grant QLG1-CT-2001-01252.

Insulin translates unfavourable lifestyle into obesity

Lifestyle factors conferring increased diabetes risk are associated with elevated basal insulin levels (hyperinsulinaemia). The latter predicts later obesity in children and adolescents.A causal role of hyperinsulinaemia for adipose tissue growth is probable because pharmacological reduction of insulin secretion lowers body weight in people who are obese. Genetic inactivation of insulin gene alleles in mice also lowers their systemic insulin levels and prevents or ameliorates high-fat diet-induced obesity. Hyperinsulinaemia causes weight gain because of a physiological property of insulin. Insulin levels that are on the high side of normal, or which are slightly elevated, are sufficient to suppress lipolysis and promote lipogenesis in adipocytes. The effect of insulin on glucose transport or hepatic glucose production requires six or two times higher hormone levels, respectively.It seems justified to suggest a lifestyle that avoids high insulin levels in order to limit anabolic fat tissue activity.

Dietary Proteins, Brown Fat, and Adiposity

High protein diets have become popular for body weight maintenance and weight loss despite controversies regarding efficacy and safety. Although both weight gain and weight loss are determined by energy consumption and expenditure, data from rodent trials consistently demonstrate that the protein:carbohydrate ratio in high fat diets strongly influences body and fat mass gain per calorie eaten. Here, we review data from rodent trials examining how high protein diets may modulate energy metabolism and the mechanisms by which energy may be dissipated. We discuss the possible role of activating brown and so-called beige/BRITE adipocytes including non-canonical UCP1-independent thermogenesis and futile cycles, where two opposing metabolic pathways are operating simultaneously. We further review data on how the gut microbiota may affect energy expenditure. Results from human and rodent trials demonstrate that human trials are less consistent than rodent trials, where casein is used almost exclusively as the protein source. The lack of consistency in results from human trials may relate to the specific design of human trials, the possible distinct impact of different protein sources, and/or the differences in the efficiency of high protein diets to attenuate obesity development in lean subjects vs. promoting weight loss in obese subjects.

Screening for insulin-independent pathways that modulate glucose homeostasis identifies androgen receptor antagonists

Pathways modulating glucose homeostasis independently of insulin would open new avenues to combat insulin resistance and diabetes. Here, we report the establishment, characterization, and use of a vertebrate ‘insulin-free’ model to identify insulin-independent modulators of glucose metabolism. insulin knockout zebrafish recapitulate core characteristics of diabetes and survive only up to larval stages. Utilizing a highly efficient endoderm transplant technique, we generated viable chimeric adults that provide the large numbers of insulin mutant larvae required for our screening platform. Using glucose as a disease-relevant readout, we screened 2233 molecules and identified three that consistently reduced glucose levels in insulin mutants. Most significantly, we uncovered an insulin-independent beneficial role for androgen receptor antagonism in hyperglycemia, mostly by reducing fasting glucose levels. Our study proposes therapeutic roles for androgen signaling in diabetes and, more broadly, offers a novel in vivo model for rapid screening and decoupling of insulin-dependent and -independent mechanisms.

Diabetes is a disease that affects the ability of the body to control the level of sugar in the blood. Individuals with diabetes are unable to make a hormone called insulin – which normally stimulates certain cells to absorb sugar from the blood – or their cells are less able to respond to this hormone. Most treatments for diabetes involve replacing the lost insulin or boosting the hormone’s activity in the body. However, these treatments can also cause individuals to gain weight or become more resistant to insulin, making it harder to control blood sugar levels.

In addition to insulin, several other factors regulate the levels of sugar in the blood and some of them may operate independently of insulin. However, little is known about such factors because it is impractical to carry out large-scale screens to identify drugs that target them in humans or mice, which are often used as experimental models for human biology.

To overcome this challenge, Mullapudi et al. turned to another animal known as the zebrafish and generated mutant fish that lack insulin. The mutant zebrafish had similar problems with regulating sugar levels as those observed in humans and mice with diabetes. This observation suggests that insulin is just as important in zebrafish as it is in humans and other mammals.

The mutant zebrafish did not survive into adulthood, and so Mullapudi et al. transplanted healthy tissue into the zebrafish to allow them to produce enough insulin to survive. These adult zebrafish produced many offspring that still carried the insulin mutation. Mullapudi et al. used these mutant offspring to screen over 2,000 drugs for their ability to decrease blood sugar levels in the absence of insulin. The screen identified three promising candidate drugs, including a molecule that interferes with a receptor for a signal known as androgen.

These findings will help researchers investigate new ways to treat diabetes. In the future, the screening approach developed by Mullapudi et al. could be adapted to search for new drugs to treat other human metabolic conditions.

Chronic hyperinsulinemia promotes meta-inflammation and extracellular matrix deposition in adipose tissue: Implications of nitric oxide

Various imperative studies support the notion that hyperinsulinemia (HI) itself serves as the common link between adipose tissue inflammation (ATI) and metabolic syndrome. However, the contribution of HI mediated ATI and its metabolic consequences are yet to be explored. We induced chronic HI per se in mice by administration of exogenous insulin for 8 weeks through mini-osmotic pumps. For the reduction of circulating insulin in response to excess calorie intake, we have partially ablated β-cells by using streptozotocin (STZ) in the diet-induced obesity (DIO) and genetic mice models (db/db). Flow cytometry analysis was performed for the quantification of immune cells in stromal vascular fraction (SVF) isolated from epididymal white adipose tissue (eWAT). Our studies demonstrated that chronic HI augmented ATI in terms of elevated pro-inflammatory cells (M1 macrophages and NK-cells) and suppressed anti-inflammatory cells (M2 macrophages, eosinophils and regulatory T-cells). These results were correlated with altered obesity-associated metabolic phenotype. Partial reduction of circulating insulin level attenuated excess calorie-induced ATI and improved insulin sensitivity. Mechanistically, an imbalance in M1 and M2 macrophage proportions in eWAT promoted iNOS (inducible nitric oxide synthase): arginase-1 imbalance that resulted into extracellular matrix (ECM) deposition and insulin resistance (IR) development. However, iNOS^-/- mice were protected from HI-induced M1:M2 macrophage imbalance, ECM deposition and IR in adipose tissue. Overall, we conclude that chronic HI per se contributed in ATI and iNOS corroborated ECM deposition.

TNF, but not hyperinsulinemia or hyperglycemia, is a key driver of obesity-induced monocytosis revealing that inflammatory monocytes correlate with insulin in obese male mice

Inflammation contributes to obesity-related hyperinsulinemia and insulin resistance, which often precede type 2 diabetes. Inflammation is one way that obesity can promote insulin resistance. It is not clear if the extent of obesity, hyperinsulinemia, or hyperglycemia, underpins changes in cellular immunity during diet-induced obesity. In particular, the requirement for obesity or directionality in the relationship between insulin resistance and monocyte characteristics is poorly defined. Inflammatory cytokines such as tumor necrosis factor (TNF) can contribute to insulin resistance. It is unclear if TNF alters monocytosis or specific markers of cellular immunity in the context of obesity. We measured bone marrow and blood monocyte characteristics in WT and TNF-/- mice that were fed obesogenic, high fat (HF) diets. We also used hyperglycemic Akita mice and mice implanted with insulin pellets in order to determine if glucose or insulin were sufficient to alter monocyte characteristics. We found that diet-induced obesity in male mice increased the total number of monocytes in blood, but not in bone marrow. Immature, inflammatory (Ly6Chigh ) monocytes decreased within the bone marrow and increased within peripheral blood of HF-fed mice. We found that neither hyperinsulinemia nor hyperglycemia was sufficient to induce the observed changes in circulating monocytes in the absence of diet-induced obesity. In obese HF-fed mice, antibiotic treatment lowered insulin and insulin resistance, but did not alter circulating monocyte characteristics. Fewer Ly6Chigh monocytes were present within the blood of HF-fed TNF-/- mice in comparison to HF-fed wild-type (WT) mice. The prevalence of immature Ly6Chigh monocytes in the blood correlated with serum insulin and insulin resistance irrespective of the magnitude of adipocyte or adipose tissue hypertrophy in obese mice. These data suggest that diet-induced obesity instigates a TNF-dependent increase in circulating inflammatory monocytes, which predicts increased blood insulin and insulin resistance independently from markers of adiposity or adipose tissue expansion.

Polysaccharides from Portulaca oleracea L. regulated insulin secretion in INS-1 cells through voltage-gated Na+ channel

The present study was undertaken to determine the involvement of voltage-gated Na⁺ channel (VGSC) and other mechanism related to insulin secretion in polysaccharides from Portulaca oleracea L. (POP)-induced secretion of insulin from insulin-secreting β-cell line cells (INS-1) cells. Our results showed that the concentration of insulin both in culture medium and inside INS-1 cells were increased under the existing of different concentration of glucose by POP or TTX, respectively. However, the effect POP on insulin secretion and production were blocked by TTX, a VGSC blocker. Meanwhile, POP improved the mitochondrial membrane potential (Δψm), increased adenosine triphosphate (ATP) production, depolarized cell membrane potential (MP) and increased intracellular Ca²⁺ levels ([Ca²⁺]_i). Furthermore, POP treatment increased the expression level of Nav_1.3 and decreased the expression level of Nav_1.7. TTX treatment decreased the expression level of Nav_1.3 and Nav_1.7. On the other hand, POP also elevated the survival of INS-1 cells. These results suggested that POP induced-secretion/production of insulin in INS-1 cells were mediated by VGSC through its change of function and subunits expression and subsequent VGSC- dependent events such as change of intracellular Ca²⁺ releasing, ATP metabolism, cell membrane and mitochondrial membrane potential, and also improvement of INS-1 cell survival. Meanwhile, our data indicated the potentiality of developing POP to be a drug for diabetes treatment and VGSC as a therapeutic target in diabetes treatment is valuable to be investigated further.

β-Cell Receptor Tyrosine Kinases in Controlling Insulin Secretion and Exocytotic Machinery: c-Kit and Insulin Receptor

Insulin secretion from pancreatic β-cells is initiated through channel-mediated depolarization, cytoskeletal remodeling, and vesicle tethering at the cell membrane, all of which can be regulated through cell surface receptors. Receptor tyrosine kinases (RTKs) promote β-cell development and postnatal signaling to improve β-cell mass and function, yet their activation has also been shown to initiate exocytotic events in β-cells. This review examines the role of RTK signaling in insulin secretion, with a focus on RTKs c-Kit and insulin receptor (IR). Pathways that control insulin release and the potential interplay between c-Kit and IR signaling are discussed, along with clinical implications of RTK therapy on insulin secretion.

Reducing Glucokinase Activity Restores Endogenous Pulsatility and Enhances Insulin Secretion in Islets From db/db Mice

An early sign of islet failure in type 2 diabetes (T2D) is the loss of normal patterns of pulsatile insulin release. Disruptions in pulsatility are associated with a left shift in glucose sensing that can cause excessive insulin release in low glucose (relative hyperinsulinemia, a hallmark of early T2D) and β-cell exhaustion, leading to inadequate insulin release during hyperglycemia. Our hypothesis was that reducing excessive glucokinase activity in diabetic islets would improve their function. Isolated mouse islets were exposed to glucose and varying concentrations of the glucokinase inhibitor d-mannoheptulose (MH) to examine changes in intracellular calcium ([Ca2+]i) and insulin secretion. Acutely exposing islets from control CD-1 mice to MH in high glucose (20 mM) dose dependently reduced the size of [Ca2+]i oscillations detected by fura-2 acetoxymethyl. Glucokinase activation in low glucose (3 mM) had the opposite effect. We then treated islets from male and female db/db mice (age, 4 to 8 weeks) and heterozygous controls overnight with 0 to 10 mM MH to determine that 1 mM MH produced optimal oscillations. We then used 1 mM MH overnight to measure [Ca2+]i and insulin simultaneously in db/db islets. MH restored oscillations and increased insulin secretion. Insulin secretion rates correlated with MH-induced increases in amplitude of [Ca2+]i oscillations (R2 = 0.57, P < 0.01, n = 10) but not with mean [Ca2+]i levels in islets (R2 = 0.05, not significant). Our findings show that correcting glucose sensing can restore proper pulsatility to diabetic islets and improved pulsatility correlates with enhanced insulin secretion.

High-fat-diet-induced remission of diabetes in a subset of KATP -GOF insulin-secretory-deficient mice

AIMS

To examine the effects of a high-fat-diet (HFD) on monogenic neonatal diabetes, without the confounding effects of compensatory hyperinsulinaemia.

METHODS

Mice expressing KATP channel gain-of-function (KATP -GOF) mutations, which models human neonatal diabetes, were fed an HFD.

RESULTS

Surprisingly, KATP -GOF mice exhibited resistance to HFD-induced obesity, accompanied by markedly divergent blood glucose control, with some KATP -GOF mice showing persistent diabetes (KATP -GOF-non-remitter [NR] mice) and others showing remission of diabetes (KATP -GOF-remitter [R] mice). Compared with the severely diabetic and insulin-resistant KATP -GOF-NR mice, HFD-fed KATP -GOF-R mice had lower blood glucose, improved insulin sensitivity, and increased circulating plasma insulin and glucagon-like peptide-1 concentrations. Strikingly, while HFD-fed KATP -GOF-NR mice showed increased food intake and decreased physical activity, reduced whole body fat mass and increased plasma lipids, KATP -GOF-R mice showed similar features to those of control littermates. Importantly, KATP -GOF-R mice had restored insulin content and β-cell mass compared with the marked loss observed in both HFD-fed KATP -GOF-NR and chow-fed KATP -GOF mice.

CONCLUSION

Together, our results suggest that restriction of dietary carbohydrates and caloric replacement by fat can induce metabolic changes that are beneficial in reducing glucotoxicity and secondary consequences of diabetes in a mouse model of insulin-secretory deficiency.

The sliding set-point: how insulin and diet interact to explain the obesity epidemic (and how to fix it)

PURPOSE OF REVIEW

The current approach to weight loss (intentional energy deficit) is difficult to implement and sustain, and rarely leads to successful long-term weight loss maintenance. The aim of this article is to review recent literature on the role of insulin in obesity propensity, and by extension, the effectiveness of carbohydrate restriction in facilitating weight loss, with particular attention to individual variability in patient response.

RECENT FINDINGS

A genetic signature for insulin secretion predisposes to elevated BMI. A genetic signature for insulin resistance is a marker for impaired fat storage, is associated with relative leanness, and predisposes to cardiometabolic disease. The largest randomized weight-loss trial ever conducted to examine insulin/diet interactions revealed no interactive effect of insulin phenotype with diet composition on body weight in the context of energy restriction. However, smaller studies revealed unique effects of carbohydrate restriction on energy partitioning that are not reflected in body weight; that is, preferential loss of total and ectopic adipose tissue. Carbohydrate-restricted diets are associated with greater adherence, and with greater total and resting energy expenditure.

SUMMARY

For patients with a predisposition to high insulin secretion, carbohydrate restriction may facilitate long-term reductions in body fat, perhaps by reducing hunger, maintaining energy expenditure, and promoting adherence.

Targeting the Incretin/Glucagon System With Triagonists to Treat Diabetes

Glucagonlike peptide 1 (GLP-1) receptor agonists have been efficacious for the treatment of type 2 diabetes due to their ability to reduce weight and attenuate hyperglycemia. However, the activity of glucagonlike peptide 1 receptor-directed strategies is submaximal, and the only potent, sustainable treatment of metabolic dysfunction is bariatric surgery, necessitating the development of unique therapeutics. GLP-1 is structurally related to glucagon and glucose-dependent insulinotropic peptide (GIP), allowing for the development of intermixed, unimolecular peptides with activity at each of their respective receptors. In this review, we discuss the range of tissue targets and added benefits afforded by the inclusion of each of GIP and glucagon. We discuss considerations for the development of sequence-intermixed dual agonists and triagonists, highlighting the importance of evaluating balanced signaling at the targeted receptors. Several multireceptor agonist peptides have been developed and evaluated, and the key preclinical and clinical findings are reviewed in detail. The biological activity of these multireceptor agonists are founded in the success of GLP-1-directed strategies; by including GIP and glucagon components, these multireceptor agonists are thought to enhance GLP-1's activities by broadening the tissue targets and synergizing at tissues that express multiple receptors, such at the brain and pancreatic islet β cells. The development and utility of balanced, unimolecular multireceptor agonists provide both a useful tool for querying the actions of incretins and glucagon during metabolic disease and a unique drug class to treat type 2 diabetes with unprecedented efficacy.