Hyperinsulinemia precedes insulin resistance in mice lacking pancreatic beta-cell leptin signaling

The Role of Cdc42 in the Insulin and Leptin Pathways Contributing to the Development of Age-Related Obesity

Age-related obesity significantly increases the risk of chronic diseases such as type 2 diabetes, cardiovascular diseases, hypertension, and certain cancers. The insulin-leptin axis is crucial in understanding metabolic disturbances associated with age-related obesity. Rho GTPase Cdc42 is a member of the Rho family of GTPases that participates in many cellular processes including, but not limited to, regulation of actin cytoskeleton, vesicle trafficking, cell polarity, morphology, proliferation, motility, and migration. Cdc42 functions as an integral part of regulating insulin secretion and aging. Some novel roles for Cdc42 have also been recently identified in maintaining glucose metabolism, where Cdc42 is involved in controlling blood glucose levels in metabolically active tissues, including skeletal muscle, adipose tissue, pancreas, etc., which puts this protein in line with other critical regulators of glucose metabolism. Importantly, Cdc42 plays a vital role in cellular processes associated with the insulin and leptin signaling pathways, which are integral elements involved in obesity development if misregulated. Additionally, a change in Cdc42 activity may affect senescence, thus contributing to disorders associated with aging. This review explores the complex relationships among age-associated obesity, the insulin-leptin axis, and the Cdc42 signaling pathway. This article sheds light on the vast molecular web that supports metabolic dysregulation in aging people. In addition, it also discusses the potential therapeutic implications of the Cdc42 pathway to mitigate obesity since some new data suggest that inhibition of Cdc42 using antidiabetic drugs or antioxidants may promote weight loss in overweight or obese patients.

Mitochondrial Reactive Oxygen Species, Insulin Resistance, and Nrf2-Mediated Oxidative Stress Response-Toward an Actionable Strategy for Anti-Aging

Reactive oxygen species (ROS) are produced mainly by mitochondrial respiration and function as signaling molecules in the physiological range. However, ROS production is also associated with the pathogenesis of various diseases, including insulin resistance (IR) and type 2 diabetes (T2D). This review focuses on the etiology of IR and early events, especially mitochondrial ROS (mtROS) production in insulin-sensitive tissues. Importantly, IR and/or defective adipogenesis in the white adipose tissues (WAT) is thought to increase free fatty acid and ectopic lipid deposition to develop into systemic IR. Fatty acid and ceramide accumulation mediate coenzyme Q reduction and mtROS production in IR in the skeletal muscle, while coenzyme Q synthesis downregulation is also involved in mtROS production in the WAT. Obesity-related IR is associated with the downregulation of mitochondrial catabolism of branched-chain amino acids (BCAAs) in the WAT, and the accumulation of BCAA and its metabolites as biomarkers in the blood could reliably indicate future T2D. Transcription factor NF-E2-related factor 2 (Nrf2), which regulates antioxidant enzyme expression in response to oxidative stress, is downregulated in insulin-resistant tissues. However, Nrf2 inducers, such as sulforaphane, could restore Nrf2 and target gene expression and attenuate IR in multiple tissues, including the WAT.

Obesity Duration and Cardiometabolic Disease

Cardiovascular disease risk is known to be influenced by both the severity of a risk factor and the duration of exposure (eg, LDL [low-density lipoprotein] cholesterol, tobacco smoke). However, this concept has been largely neglected within the obesity literature. While obesity severity has been closely linked with cardiometabolic diseases, the risk of developing these conditions among those with obesity may be augmented by greater obesity duration over the life span. Few longitudinal or contemporary studies have investigated the influence of both factors in combination-cumulative obesity exposure-instead generally focusing on obesity severity, often at a single time point, given ease of use and lack of established methods to encapsulate duration. Our review focuses on what is known about the influence of the duration of exposure to excess adiposity within the obesity-associated cardiometabolic disease risk equation by means of summarizing the hypothesized mechanisms for and evidence surrounding the relationships of obesity duration with diverse cardiovascular and metabolic disease. Through the synthesis of the currently available data, we aim to highlight the importance of a better understanding of the influence of obesity duration in cardiovascular and metabolic disease pathogenesis. We underscore the clinical importance of aggressive early attention to obesity identification and intervention to prevent the development of chronic diseases that arise from exposure to excess body weight.

Both early and late maternal age at childbirth is associated with increasing odds of central obesity in offspring

OBJECTIVE

Despite studies on offspring obesity and delayed parenthood, little attention has been paid to the central obesity of offspring. The purpose of this study was to test the hypothesis that maternal age at childbirth (MAC) was associated with central obesity in offspring among the adult population, and fasting insulin may play a role in this association as a mediating factor.

METHODS

A total of 423 adults (mean age 37.9 years, 37.1% female) were included. Information about maternal variables and other confounders was collected by face-to-face interview. Waist circumference and insulin were determined through physical measurements and biochemical examinations. Logistic regression model and restricted cubic spline model were used to analyze the relationship between MAC and central obesity of offspring. The mediating effect of fasting insulin levels on association between MAC and offspring waist circumference was also analyzed.

RESULTS

There was a nonlinear relationship between MAC and central obesity in offspring. Compared with subjects with MAC 27-32 years, those with MAC 21-26 years (OR = 1.814, 95% CI: 1.129-2.915) and MAC ≥33 years (OR = 3.337, 95% CI: 1.638-6.798) had higher odds to develop central obesity. Offspring fasting insulin was also higher in MAC 21-26 years and MAC ≥33 years compared with those with MAC 27-32 years. Taking the group MAC 27-32 years as reference, the mediating effect of fasting insulin levels on the waist circumference was 20.6% and 12.4% for MAC 21-26 years and ≥ 33 years, respectively.

CONCLUSION

MAC 27-32 years has the lowest odds of central obesity in offspring. Fasting insulin levels may have a partial mediating effect on the association between MAC and central obesity.

Pathophysiology of Prediabetes, Diabetes, and Diabetic Remission in Cats

Diabetes mellitus (DM) has a heterogenous cause, and the exact pathogenesis differs between patients. Most diabetic cats have a cause similar to human type 2 DM but, in some, DM is associated with underlying conditions, such as hypersomatotropism, hyperadrenocorticism, or administration of diabetogenic drugs. Predisposing factors for feline DM include obesity, reduced physical activity, male sex, and increasing age. Gluco(lipo)toxicity and genetic predisposition also likely play roles in pathogenesis. Prediabetes cannot be accurately diagnosed in cats at the current time. Diabetic cats can enter remission, but relapses are common, as these cats might have ongoing, abnormal glucose homeostasis.

Proposal for standardizing normal insulin ranges in Brazilian patients and a new classification of metabolic syndrome

Background

Insulin resistance and/or hyperinsulinemia are closely linked to adiposity, metabolic syndrome (MetS) and prolonged inflammatory processes.

Methods

We retrospectively analyzed 1,018 adult individuals with a mean age of 46 years (74% male) and classified them as: Metabolically normal: without any of the five criteria of the International Diabetes Federation (IDF) used for the diagnosis of MetS, plus normal fasting insulin (Men < 8 mU/L, Women < 10 mU/L); Level 1 MetS: with one or two IDF criteria, plus hyperinsulinemia (Men: ≥ 8 mU/L), and Women: ≥ 10 mU/L); Level 2 MetS: with three or more IDF criteria, plus hyperinsulinemia.

Results

The mean values for fasting insulinemia in metabolically normal individuals was 4.6 ± 1.8 mU/L and 5.6 ± 2.3 mU/L, while their means for the Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) were 1.0 and 1.2 for men and women, respectively. In addition, the mean values for insulin (and HOMA-IR) for individuals with two normal anthropometric parameters (body mass index and waist girth), or two normal anthropometric parameters plus no IDF criteria, were similar to the metabolically normal group. Based on the obtained mean + 2 SD, we established the following insulin (and HOMA-IR) values as diagnostic cut-offs for hyperinsulinemia: Men: ≥ 8 mU/L (≥ 1.5), and Women: ≥ 10 mU/L (≥ 2.0). The mean serum insulin was significantly higher for individuals with Level 1 MetS (approx. 9 mU/L for both genders) compared with metabolically normal individuals, as was the prevalence of hepatic steatosis, which was more evident in men. Thus, the presence of one or two abnormal IDF criteria, combined with hyperinsulinemia and/or raised HOMA-IR, suggests the presence of MetS and insulin resistance. Patients of both genders with Level 2 MetS had higher serum insulin and/or HOMA-IR values than Level 1, as well as a higher prevalence of hypertension and hepatic steatosis, being more pronounced among men. The process was progressive and proportional to the degree of hyperinsulinemia.

Conclusion

It is proposed that intervention against MetS progression should be started in individuals with Level 1 MetS, rather than waiting for more criteria for diagnostic confirmation, which this should help to reduce the occurrence of known complications such as type 2 diabetes, atherosclerosis, hypertension, and chronic kidney disease, among others.

Non-Alcoholic Fatty Liver Disease (NAFLD) Is an Independent Risk Factor for Developing New-Onset Diabetes After Acute Pancreatitis: A Multicenter Retrospective Cohort Study in Chinese Population

BACKGROUND

Numerous studies validated frequent glucose dysfunction in patients with acute pancreatitis (AP). However, the prevalence of new-onset diabetes in individuals after a first episode of AP varies widely among previous studies. This study aims to determine the incidence of post-acute pancreatitis diabetes mellitus (PPDM-A) in Chinese people and further identify potential risk factors that influence diabetes development in patients with AP.

METHODS

This was a multi-center retrospective cohort study including 6009 inpatients with a first attack of AP. A total of 1804 patients with AP without known endocrine pancreatic disorders or other pancreatic exocrine diseases were eligible for analysis. Data was collected from medical records by hospital information system and telephone follow-ups after discharge. The multiple logistic regression analysis was established to evaluate the potential influencing factors of PPDM-A.

RESULTS

The prevalence of newly diagnosed diabetes after a first episode of AP in China was 6.2%. Data showed that patients who developed PPDM-A were more likely to be younger (X² = 6.329, P = 0.012), experienced longer hospital stays (X² = 6.949, P = 0.008) and had a higher frequency of overweight or obesity (X² = 11.559, P = 0.003) compared to those with normal glycemia. The frequency of stress hyperglycemia on admission (X² = 53.815, P < 0.001), hyperlipidemia (X² = 33.594, P < 0.001) and non-alcoholic fatty liver disease (NAFLD) (X² = 36.335, P < 0.001) were significantly higher among individuals with PPDM-A compared with control group. Also, patients with PPDM-A were more likely to be hyperlipidemic AP (X² = 16.304, P = 0.001) and show a higher degree of severity (X² = 7.834, P = 0.020) and recurrence rate (X² = 26.908, P < 0.001) of AP compared to those without diabetes. In addition, multiple logistic regression analysis indicated that stress hyperglycemia, hyperlipidemia, NAFLD and repeated attacks of AP were the independent influence factors for developing PPDM-A.

CONCLUSION

Our study first demonstrated the prevalence of secondary diabetes in Chinese patients after AP. The disorder of glucose metabolism in individuals with AP should be regularly evaluated in clinical practice. Further studies are needed to verify the relationship between liver and pancreas in keeping glucose homeostasis under AP condition.

The aetiology and molecular landscape of insulin resistance

Insulin resistance, defined as a defect in insulin-mediated control of glucose metabolism in tissues - prominently in muscle, fat and liver - is one of the earliest manifestations of a constellation of human diseases that includes type 2 diabetes and cardiovascular disease. These diseases are typically associated with intertwined metabolic abnormalities, including obesity, hyperinsulinaemia, hyperglycaemia and hyperlipidaemia. Insulin resistance is caused by a combination of genetic and environmental factors. Recent genetic and biochemical studies suggest a key role for adipose tissue in the development of insulin resistance, potentially by releasing lipids and other circulating factors that promote insulin resistance in other organs. These extracellular factors perturb the intracellular concentration of a range of intermediates, including ceramide and other lipids, leading to defects in responsiveness of cells to insulin. Such intermediates may cause insulin resistance by inhibiting one or more of the proximal components in the signalling cascade downstream of insulin (insulin receptor, insulin receptor substrate (IRS) proteins or AKT). However, there is now evidence to support the view that insulin resistance is a heterogeneous disorder that may variably arise in a range of metabolic tissues and that the mechanism for this effect likely involves a unified insulin resistance pathway that affects a distal step in the insulin action pathway that is more closely linked to the terminal biological response. Identifying these targets is of major importance, as it will reveal potential new targets for treatments of diseases associated with insulin resistance.

Mechanism of insulin resistance in obesity: a role of ATP

Obesity increases the risk of type 2 diabetes through the induction of insulin resistance. The mechanism of insulin resistance has been extensively investigated for more than 60 years, but the essential pathogenic signal remains missing. Existing hypotheses include inflammation, mitochondrial dysfunction, hyperinsulinemia, hyperglucagonemia, glucotoxicity, and lipotoxicity. Drug discoveries based on these hypotheses are unsuccessful in the development of new medicines. In this review, multidisciplinary literature is integrated to evaluate ATP as a primary signal for insulin resistance. The ATP production is elevated in insulin-sensitive cells under obese conditions independent of energy demand, which we have named "mitochondrial overheating." Overheating occurs because of substrate oversupply to mitochondria, leading to extra ATP production. The ATP overproduction contributes to the systemic insulin resistance through several mechanisms, such as inhibition of AMPK, induction of mTOR, hyperinsulinemia, hyperglucagonemia, and mitochondrial dysfunction. Insulin resistance represents a feedback regulation of energy oversupply in cells to control mitochondrial overloading by substrates. Insulin resistance cuts down the substrate uptake to attenuate mitochondrial overloading. The downregulation of the mitochondrial overloading by medicines, bypass surgeries, calorie restriction, and physical exercise leads to insulin sensitization in patients. Therefore, ATP may represent the primary signal of insulin resistance in the cellular protective response to the substrate oversupply. The prevention of ATP overproduction represents a key strategy for insulin sensitization.

Hyperinsulinemia in Obesity, Inflammation, and Cancer

The relative insufficiency of insulin secretion and/or insulin action causes diabetes. However, obesity and type 2 diabetes mellitus can be associated with an absolute increase in circulating insulin, a state known as hyperinsulinemia. Studies are beginning to elucidate the cause-effect relationships between hyperinsulinemia and numerous consequences of metabolic dysfunctions. Here, we review recent evidence demonstrating that hyperinsulinemia may play a role in inflammation, aging and development of cancers. In this review, we will focus on the consequences and mechanisms of excess insulin production and action, placing recent findings that have challenged dogma in the context of the existing body of literature. Where relevant, we elaborate on the role of specific signal transduction components in the actions of insulin and consequences of chronic hyperinsulinemia. By discussing the involvement of hyperinsulinemia in various metabolic and other chronic diseases, we may identify more effective therapeutics or lifestyle interventions for preventing or treating obesity, diabetes and cancer. We also seek to identify pertinent questions that are ripe for future investigation.

IP6-assisted CSN-COP1 competition regulates a CRL4-ETV5 proteolytic checkpoint to safeguard glucose-induced insulin secretion

COP1 and COP9 signalosome (CSN) are the substrate receptor and deneddylase of CRL4 E3 ligase, respectively. How they functionally interact remains unclear. Here, we uncover COP1–CSN antagonism during glucose-induced insulin secretion. Heterozygous Csn2^WT/K70E mice with partially disrupted binding of IP₆, a CSN cofactor, display congenital hyperinsulinism and insulin resistance. This is due to increased Cul4 neddylation, CRL4^COP1 E3 assembly, and ubiquitylation of ETV5, an obesity-associated transcriptional suppressor of insulin secretion. Hyperglycemia reciprocally regulates CRL4-CSN versus CRL4^COP1 assembly to promote ETV5 degradation. Excessive ETV5 degradation is a hallmark of Csn2^WT/K70E, high-fat diet-treated, and ob/ob mice. The CRL neddylation inhibitor Pevonedistat/MLN4924 stabilizes ETV5 and remediates the hyperinsulinemia and obesity/diabetes phenotypes of these mice. These observations were extended to human islets and EndoC-βH1 cells. Thus, a CRL4^COP1-ETV5 proteolytic checkpoint licensing GSIS is safeguarded by IP₆-assisted CSN-COP1 competition. Deregulation of the IP₆-CSN-CRL4^COP1-ETV5 axis underlies hyperinsulinemia and can be intervened to reduce obesity and diabetic risk.

Mediators of insulin signalling are targets of cullin-RING ubiquitin ligases (CRL) that mediate protein degradation, but the role of protein degradation in insulin signalling is incompletely understood. Here, the authors identified a glucose-responsive CRL4-COP1-ETV5 proteolytic axis that promotes insulin secretion, and is inhibited under hypoglycemia.

Sphingosine 1-phosphate metabolism and insulin signaling

Insulin is the main anabolic hormone secreted by β-cells of the pancreas stimulating the assimilation and storage of glucose in muscle and fat cells. It modulates the postprandial balance of carbohydrates, lipids and proteins via enhancing lipogenesis, glycogen and protein synthesis and suppressing glucose generation and its release from the liver. Resistance to insulin is a severe metabolic disorder related to a diminished response of peripheral tissues to the insulin action and signaling. This leads to a disturbed glucose homeostasis that precedes the onset of type 2 diabetes (T2D), a disease reaching epidemic proportions. A large number of studies reported an association between elevated circulating fatty acids and the development of insulin resistance. The increased fatty acid lipid flux results in the accumulation of lipid droplets in a variety of tissues. However, lipid intermediates such as diacylglycerols and ceramides are also formed in response to elevated fatty acid levels. These bioactive lipids have been associated with the pathogenesis of insulin resistance. More recently, sphingosine 1-phosphate (S1P), another bioactive sphingolipid derivative, has also been shown to increase in T2D and obesity. Although many studies propose a protective role of S1P metabolism on insulin signaling in peripheral tissues, other studies suggest a causal role of S1P on insulin resistance. In this review, we critically summarize the current state of knowledge of S1P metabolism and its modulating role on insulin resistance. A particular emphasis is placed on S1P and insulin signaling in hepatocytes, skeletal muscle cells, adipocytes and pancreatic β-cells. In particular, modulation of receptors and enzymes that regulate S1P metabolism can be considered as a new therapeutic option for the treatment of insulin resistance and T2D.

Mechanisms of Post-Pancreatitis Diabetes Mellitus and Cystic Fibrosis-Related Diabetes: A Review of Preclinical Studies

Anatomical proximity and functional correlations between the exocrine and endocrine pancreas warrant reciprocal effects between the two parts. Inflammatory diseases of the exocrine pancreas, such as acute or chronic pancreatitis, or the presence of cystic fibrosis disrupt endocrine function, resulting in diabetes of the exocrine pancreas. Although novel mechanisms are being increasingly identified, the intra- and intercellular pathways regulating exocrine-endocrine interactions are still not fully understood, making the development of new and more effective therapies difficult. Therefore, this review sought to accumulate current knowledge regarding the pathogenesis of diabetes in acute and chronic pancreatitis, as well as cystic fibrosis.

A preliminary evaluation of the circulating leptin/adiponectin ratio in dogs with pituitary-dependent hyperadrenocorticism and concurrent diabetes mellitus

Leptin and adiponectin are thought to modulate insulin sensitivity and pancreatic β-cell function, but there is limited information regarding the adipokine status of hyperglycemic dogs with hyperadrenocorticism. This study aimed to determine whether alterations in the leptin/adiponectin ratio, insulin sensitivity, and/or pancreatic β-cell function are associated with diabetes mellitus (DM) in dogs with pituitary-dependent hyperadrenocorticism (PDH). A total of 48 client-owned dogs were included in this prospective observational study: 20 dogs with PDH (10 normoglycemic and 10 with DM), 15 dogs with DM, and 13 healthy dogs. The serum concentrations of leptin, adiponectin, resistin, interleukin (IL)-1β, IL-6, IL-10, IL-18, and tumor necrosis factor (TNF)-α were measured, and homeostatic model assessment indices (HOMAs) were calculated and compared among the groups. Serum leptin was significantly higher in PDH dogs with and without DM than in healthy and DM dogs, and it was lower in DM dogs than in PDH dogs without DM. Serum adiponectin was significantly lower in PDH dogs with DM than in healthy and PDH dogs, and it was significantly lower in DM dogs than in healthy dogs. Serum IL-10 was significantly higher in PDH dogs with DM than in healthy and PDH dogs without DM. The leptin/adiponectin ratio was significantly higher in PDH dogs with DM than in normoglycemic PDH dogs. Serum IL-6 concentrations were significantly higher in DM dogs than in healthy dogs. Serum IL-1β concentration was significantly higher in DM dogs than in healthy dogs and PDH dogs with DM and without DM. Serum TNF-α and IL-18 concentrations were not different among groups. The HOMA_{β-cell function} was significantly lower in PDH dogs with DM than in normoglycemic PDH dogs, while HOMA_{insulin sensitivity} was significantly lower in PDH dogs with DM than in healthy dogs. These results suggest that adipokine dysregulation, a reduction in insulin sensitivity, and a further impairment in pancreatic β-cell function might predispose PDH dogs to DM. Further longitudinal study will be necessary to confirm this result.

Hyperinsulinemia or Insulin Resistance: What Impacts the Progression of Alzheimer's Disease?

Type 2 diabetes mellitus (T2D), which is often accompanied by hyperinsulinemia and insulin resistance, is associated with an increased risk for developing mild cognitive impairment and Alzheimer's disease (AD); however, the underlying mechanisms for this association are still unclear. Recent findings have shown that hyperinsulinemia and insulin resistance can coexist or be independent events. This makes it imperative to determine the contribution of these individual conditions in impacting AD. This literature review highlights the recent developments of hyperinsulinemia and insulin resistance involvement in the progression and pathogenesis of AD.

Obesity, Systemic Hypertension, and Pulmonary Hypertension: A Tale of Three Diseases

Cardiovascular disease (CVD), especially ischemic heart disease and stroke, is the major cause of death worldwide, accounting for more than one-third of all deaths annually. Hypertension is the most prevalent and modifiable risk factor of CVD-related deaths. The same is true for obesity, which is currently being recognized as a major global epidemic. The prevalence of obesity in the United States has increased dramatically, from 13.4% in 1960 to 36.5% in 2014, with as much as 70.7% of the American adult population being overweight or obese (CDC). Epidemiological studies have shown that obesity predisposes to hypertension and CVD - with the relationship between markers of obesity and blood pressure being almost linear across different populations. In this review, we discuss systemic and pulmonary hypertension in the context of obesity.

Vanadium and insulin: Partners in metabolic regulation

Since the 1970s, the biological role of vanadium compounds has been discussed as insulin-mimetic or insulin-enhancer agents. The action of vanadium compounds has been investigated to determine how they influence the insulin signaling pathway. Khan and coworkers proposed key proteins for the insulin pathway study, introducing the concept "critical nodes". In this review, we also considered critical kinases and phosphatases that participate in this pathway, which will permit a better comprehension of a critical node, where vanadium can act: a) insulin receptor, insulin receptor substrates, and protein tyrosine phosphatases; b) phosphatidylinositol 3'-kinase, 3-phosphoinositide-dependent protein kinase and mammalian target of rapamycin complex, protein kinase B, and phosphatase and tensin homolog; and c) insulin receptor substrates and mitogen-activated protein kinases, each node having specific negative modulators. Additionally, leptin signaling was considered because together with insulin, it modulates glucose and lipid homeostasis. Even in recent literature, the possibility of vanadium acting against metabolic diseases or cancer is confirmed although the mechanisms of action are not well understood because these critical nodes have not been systematically investigated. Through this review, we establish that vanadium compounds mainly act as phosphatase inhibitors and hypothesize on their capacity to affect kinases, which are critical to other hormones that also act on common parts of the insulin pathway.

Partial impairment of insulin receptor expression mimics fasting to prevent diet-induced fatty liver disease

Excessive insulin signaling through the insulin receptor (IR) may play a role in the pathogenesis of diet-induced metabolic disease, including obesity and type 2 diabetes. Here we investigate whether heterozygous impairment of insulin receptor (IR) expression limited to peripheral, i.e. non-CNS, tissues of adult mice impacts the development of high-fat diet-induced metabolic deterioration. While exhibiting some features of insulin resistance, PerIRKO^+/− mice display a hepatic energy deficit accompanied by induction of energy-sensing AMPK, mitochondrial biogenesis, PPARα, unexpectedly leading to protection from, and reversal of hepatic lipid accumulation (steatosis hepatis, NAFLD). Consistently, and unlike in control mice, the PPARα activator fenofibrate fails to further affect hepatic lipid accumulation in PerIRKO^+/− mice. Taken together, and opposing previously established diabetogenic features of insulin resistance, incomplete impairment of insulin signaling may mimic central aspects of calorie restriction to limit hepatic lipid accumulation during conditions of metabolic stress.

Hyper-insulinemia associated with excess calorie intake may cause metabolic dysfunction. Here the authors report that mice with partially reduced insulin receptor expression in peripheral tissues are protected from and experience reversal of fatty liver disease.

The Potential Role of Activating the ATP-Sensitive Potassium Channel in the Treatment of Hyperphagic Obesity

To evaluate the potential role of ATP-sensitive potassium (K_ATP) channel activation in the treatment of hyperphagic obesity, a PubMed search was conducted focused on the expression of genes encoding the K_ATP channel, the response to activating the K_ATP channel in tissues regulating appetite and the establishment and maintenance of obesity, the evaluation of K_ATP activators in obese hyperphagic animal models, and clinical studies on syndromic obesity. K_ATP channel activation is mechanistically involved in the regulation of appetite in the arcuate nucleus; the regulation of hyperinsulinemia, glycemic control, appetite and satiety in the dorsal motor nucleus of vagus; insulin secretion by β-cells; and the synthesis and β-oxidation of fatty acids in adipocytes. K_ATP channel activators have been evaluated in hyperphagic obese animal models and were shown to reduce hyperphagia, induce fat loss and weight loss in older animals, reduce the accumulation of excess body fat in growing animals, reduce circulating and hepatic lipids, and improve glycemic control. Recent experience with a K_ATP channel activator in Prader-Willi syndrome is consistent with the therapeutic responses observed in animal models. K_ATP channel activation, given the breadth of impact and animal model and clinical results, is a viable target in hyperphagic obesity.

Intermittent fasting benefits on alpha- and beta-cell arrangement in diet-induced obese mice pancreatic islet

AIMS

There is a pancreatic islet adaptation in obese subjects, resulting in insulin resistance and diabetes type 2. We studied the effect of intermittent fasting (IntF) on the islet structure of diet-induced obese (DIO) mice.

METHODS

Three-month-old male mice fed a control diet (C, 10% Kcal fat) or a high-fat diet (HF, 50% Kcal fat) for two months (n = 20 each group). Then, half of each group did IntF (alternating 24 h fed/24 h fast), continuing in their diets four more weeks: C, C-IntF, HF, HF-IntF. Islets were prepared to microscopy or isolated for molecular analysis.

RESULTS

HF group (vs. C group) showed hyperglycemia, hyperinsulinemia, hyperleptinemia, hypoadiponectinemia, glucose intolerance, insulin resistance, and islet hypertrophy with a consequent higher both the alpha-cell and beta-cell masses. In the HF group (vs. C), there was low PDX1 (pancreatic and duodenal homeobox 1), and IntF did not alter PDX1. There was a low p-AKT/AKT ratio (protein kinase B), and IntF enhanced it. Also, tumor suppressor p53 was increased, and IntF decreased it. IL (interleukin) -6 was higher in the HF group (vs. C), and HF-IntF (vs. C-IntF). Any significant change in NFkB was seen among groups.

CONCLUSIONS

IntF improves pancreatic islet structure in DIO mice, even with continued HF diet intake, primarily considering on the alpha- and beta-cell masses regulation, then improving insulin signaling and decreasing cell apoptosis. Future research should explore whether the shortening of the IntF extend could maintain the benefits observed in the long term.

Reducing Glucokinase Activity to Enhance Insulin Secretion: A Counterintuitive Theory to Preserve Cellular Function and Glucose Homeostasis

Pancreatic beta-cells are the only cells in the body that can synthesize and secrete insulin. Through the process of glucose-stimulated insulin secretion, beta-cells release insulin into circulation, stimulating GLUT4-dependent glucose uptake into peripheral tissue. Insulin is normally secreted in pulses that promote signaling at the liver. Long before type 2 diabetes is diagnosed, beta-cells become oversensitive to glucose, causing impaired pulsatility and overstimulation in fasting levels of glucose. The resulting hypersecretion of insulin can cause poor insulin signaling and clearance at the liver, leading to hyperinsulinemia and insulin resistance. Continued overactivity can eventually lead to beta-cell exhaustion and failure at which point type 2 diabetes begins. To prevent or reverse the negative effects of overstimulation, beta-cell activity can be reduced. Clinical studies have revealed the potential of beta-cell rest to reverse new cases of diabetes, but treatments lack durable benefits. In this perspective, we propose an intervention that reduces overactive glucokinase activity in the beta-cell. Glucokinase is known as the glucose sensor of the beta-cell due to its high control over insulin secretion. Therefore, glycolytic overactivity may be responsible for hyperinsulinemia early in the disease and can be reduced to restore normal stimulus-secretion coupling. We have previously reported that reducing glucokinase activity in prediabetic mouse islets can restore pulsatility and enhance insulin secretion. Building on this counterintuitive finding, we review the importance of pulsatile insulin secretion and highlight how normalizing glucose sensing in the beta cell during prediabetic hyperinsulinemia may restore pulsatility and improve glucose homeostasis.

Novel aspects of the role of the liver in carbohydrate metabolism

Malfunction of the liver is a central factor in metabolic disease. Glucose production by liver is complex and controlled via indirect mechanisms; insulin regulates adipose tissue lipolysis, and free fatty acids in turn regulate liver glucose output. This latter concept is confirmed by studies in L-Akt-Foxo1 knockout mice. The adipocyte is a likely locus of hepatic insulin resistance. Also, kidneys play a role in regulating glucose production; denervated kidneys abrogate the effect of fat feeding to cause insulin resistance. Glucose itself is an important regulator of liver metabolism ("glucose effectiveness"); after entering liver, glucose is phosphorylated and can be exported as lactate. Using the dynamic glucose/lactate relationship, we have been able to estimate glucose effectiveness in intact animals and human subjects. Families have been identified with a glucokinase regulatory protein defect; modeling demonstrates elevated glucokinase activity. Insulin clearance by liver is highly variable among normal individuals, and is under environmental control: high fat diet reduces clearance by 30%. Liver insulin clearance is significantly lower in African American (AA) adults and children compared to European American participants, accounting for fasting hyperinsulinemia in AA. We hypothesize that reduced hepatic insulin clearance causes peripheral insulin resistance and increased Type 2 diabetes in AA.

Leptin-induced Trafficking of KATP Channels: A Mechanism to Regulate Pancreatic β-cell Excitability and Insulin Secretion

The adipocyte hormone leptin was first recognized for its actions in the central nervous system to regulate energy homeostasis but has since been shown to have direct actions on peripheral tissues. In pancreatic β-cells leptin suppresses insulin secretion by increasing K_ATP channel conductance, which causes membrane hyperpolarization and renders β-cells electrically silent. However, the mechanism by which leptin increases K_ATP channel conductance had remained unresolved for many years following the initial observation. Recent studies have revealed that leptin increases surface abundance of K_ATP channels by promoting channel trafficking to the β-cell membrane. Thus, K_ATP channel trafficking regulation has emerged as a mechanism by which leptin increases K_ATP channel conductance to regulate β-cell electrical activity and insulin secretion. This review will discuss the leptin signaling pathway that underlies K_ATP channel trafficking regulation in β-cells.

The renin-angiotensin system as a target to solve the riddle of endocrine pancreas homeostasis

Local renin-angiotensin system (RAS) in the pancreas is linked to the modulation of glucose-stimulated insulin secretion (GSIS) in beta cells and insulin sensitivity in target tissues, emerging as a promising tool in the prevention and/or treatment of obesity, diabetes, and systemic arterial hypertension. Insulin resistance alters pancreatic islet cell distribution and morphology and hypertrophied islets exhibit upregulated angiotensin II type 1 receptor, which drives oxidative stress, apoptosis, and fibrosis, configuring beta cell dysfunction and diminishing islet lifespan. Pharmacological modulation of RAS has shown beneficial effects in diet-induced obesity model, mainly related to the translational potential that angiotensin receptor blockers and ECA2/ANG (1-7)/MAS receptor axis modulation have when it comes to islet preservation and type 2 diabetes prevention and/or treatment. This review describes the existing evidence for different approaches to blocking RAS elements in the management of insulin resistance and diabetes and focuses on islet remodeling and GSIS in rodents and humans.

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.

Mild Suppression of Hyperinsulinemia to Treat Obesity and Insulin Resistance

Insulin plays roles in lipid uptake, lipolysis, and lipogenesis, in addition to controlling blood glucose levels. Excessive circulating insulin is associated with adipose tissue expansion and obesity, yet a causal role for hyperinsulinemia in the development of mammalian obesity has proven controversial, with many researchers suggesting it as a consequence of insulin resistance. Recently, evidence that specifically reducing hyperinsulinemia can prevent and reverse obesity in animal models has been presented. Our experiments, and others in this field, question the current dogma that hyperinsulinemia is a response to obesity and/or insulin resistance. In this review, we discuss preclinical evidence in the context of the broader literature and speculate on the possibility of clinical translation of alternative approaches for treating obesity.

Dendrobium mixture regulates hepatic gluconeogenesis in diabetic rats via the phosphoinositide-3-kinase/protein kinase B signaling pathway

The present study aimed to evaluate the impact of dendrobium mixture (DMix) on the gene and protein expression of insulin signaling pathway-associated factors in the livers of diabetic rats. The molecular mechanisms by which DMix inhibits gluconeogenesis were also investigated. A total of 47 female Wistar rats were used in the present study. Of these, 11 rats were randomly selected as healthy controls and diabetes was induced in the remaining 36 rats by administering a high-fat and high-sugar diet for 6 weeks, followed by two intraperitoneal injections of streptozotocin. The 36 rats were screened for diabetes and then randomly divided into three groups: Model, metformin and DMix groups. Following 12 weeks of treatment, the fasting blood glucose (FBG), glycosylated serum protein (GSP), serum insulin, blood lipids [total cholesterol (Tch) and triglycerides (TG)], alanine transaminase (ALT) and aspartate transaminase (AST) were assessed. In addition, hematoxylin and eosin staining was used for histomorphological examination of the liver tissues. The mRNA expression of insulin receptor (InsR), forkhead box protein O1 (FoxO1), phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase) in the liver was measured with reverse transcription-quantitative polymerase chain reaction and the protein expression of InsR, phosphoinositide-3-kinase (PI3K), phosphorylated (p)-PI3K, protein kinase B (Akt), p-Akt, FoxO1, PEPCK and G6Pase in the liver was measured by western blot analysis. The FBG, GSP, InsR, Tch, TG, ALT and AST levels were significantly lower in the DMix-treated group compared with the model group (P<0.05). In addition, DMix treatment notably improved liver histopathology and significantly increased the gene and protein expression of InsR, PI3K and Akt (P<0.05). DMix treatment also significantly reduced the gene and protein expression of FoxO1, PEPCK and G6Pase (P<0.05). DMix effectively reduced FBG and blood lipids and significantly improved liver function and insulin resistance in diabetic rats, possibly by regulating the gene and protein expression of molecules associated with the PI3K/Akt signaling pathway.

Reduced Circulating Insulin Enhances Insulin Sensitivity in Old Mice and Extends Lifespan

The causal relationships between insulin levels, insulin resistance, and longevity are not fully elucidated. Genetic downregulation of insulin/insulin-like growth factor 1 (Igf1) signaling components can extend invertebrate and mammalian lifespan, but insulin resistance, a natural form of decreased insulin signaling, is associated with greater risk of age-related disease in mammals. We compared Ins2^+/- mice to Ins2^+/+ littermate controls, on a genetically stable Ins1 null background. Proteomic and transcriptomic analyses of livers from 25-week-old mice suggested potential for healthier aging and altered insulin sensitivity in Ins2^+/- mice. Halving Ins2 lowered circulating insulin by 25%-34% in aged female mice, without altering Igf1 or circulating Igf1. Remarkably, decreased insulin led to lower fasting glucose and improved insulin sensitivity in aged mice. Moreover, lowered insulin caused significant lifespan extension, observed across two diverse diets. Our study indicates that elevated insulin contributes to age-dependent insulin resistance and that limiting basal insulin levels can extend lifespan.

Differential actions of PPAR-α and PPAR-β/δ on beige adipocyte formation: A study in the subcutaneous white adipose tissue of obese male mice

Background and aims

Obesity compromises adipocyte physiology. PPARs are essential to adipocyte plasticity, but its isolated role in the browning phenomenon is not clear. This study aimed to examine whether activation of PPAR-α or PPAR-β/δ could induce beige cell depots in the subcutaneous white adipose tissue of diet-induced obese mice.

Material and methods

Sixty animals were randomly assigned to receive a control diet (C, 10% lipids) or a high-fat diet (HF, 50% lipids) for ten weeks. Then each group was re-divided to begin the treatments that lasted 4 weeks, totalizing six groups: C, C-α (C plus PPAR-α agonist, 2.5 mg/kg BM), C-β (C plus PPAR-β/δ agonist, 1 mg/kg BM), HF, HF-α (HF plus PPAR-α agonist), HF-β (HF plus PPAR-β/δ agonist).

Results

HF animals presented with overweight, glucose intolerance and subcutaneous white adipocyte hypertrophy. Both treatments significantly attenuated these parameters. Browning, verified by UCP1 positive beige cells and enhanced body temperature, was just observed in PPAR-α treated groups. PPAR-α agonism also elicited an enhanced gene expression of the thermogenesis effector UCP1, the beige-selective gene TMEM26 and the PRDM16, an essential gene for brown-like phenotype maintenance in the beige adipocytes when compared to their counterparts. The enhanced CIDEA and the reduced UCP1 gene levels might justify the white phenotype predominance after the treatment with the PPAR-β/δ agonist.

Conclusions

This work provides evidence that the PPAR-β/δ agonist ameliorated metabolic disorders through enhanced beta-oxidation and better tolerance to glucose, whereas the PPAR-α agonism was confirmed as a promising therapeutic target for treating metabolic diseases via beige cell induction and enhanced thermogenesis.

Reducing insulin via conditional partial gene ablation in adults reverses diet-induced weight gain

Excess circulating insulin is associated with obesity in humans and in animal models. However, the physiologic causality of hyperinsulinemia in adult obesity has rightfully been questioned because of the absence of clear evidence that weight loss can be induced by acutely reversing diet-induced hyperinsulinemia. Herein, we describe the consequences of inducible, partial insulin gene deletion in a mouse model in which animals have already been made obese by consuming a high-fat diet. A modest reduction in insulin production/secretion was sufficient to cause significant weight loss within 5 wk, with a specific effect on visceral adipose tissue. This result was associated with a reduction in the protein abundance of the lipodystrophy gene polymerase I and transcript release factor ( Ptrf; Cavin) in gonadal adipose tissue. RNAseq analysis showed that reduced insulin and weight loss also associated with a signature of reduced innate immunity. This study demonstrates that changes in circulating insulin that are too fine to adversely affect glucose homeostasis nonetheless exert control over adiposity.-Page, M. M., Skovsø, S., Cen, H., Chiu, A. P., Dionne, D. A., Hutchinson, D. F., Lim, G. E., Szabat, M., Flibotte, S., Sinha, S., Nislow, C., Rodrigues, B., Johnson, J. D. Reducing insulin via conditional partial gene ablation in adults reverses diet-induced weight gain.

Loss of sirtuin 4 leads to elevated glucose- and leucine-stimulated insulin levels and accelerated age-induced insulin resistance in multiple murine genetic backgrounds

Several inherited metabolic disorders are associated with an accumulation of reactive acyl-CoA metabolites that can non-enzymatically react with lysine residues to modify proteins. While the role of acetylation is well-studied, the pathophysiological relevance of more recently discovered acyl modifications, including those found in inherited metabolic disorders, warrants further investigation. We recently showed that sirtuin 4 (SIRT4) removes glutaryl, 3-hydroxy-3-methylglutaryl, 3-methylglutaryl, and 3-methylglutaconyl modifications from lysine residues. Thus, we used SIRT4 knockout mice, which can accumulate these novel post-translational modifications, as a model to investigate their physiological relevance. Since SIRT4 is localized to mitochondria and previous reports have shown SIRT4 influences metabolism, we thoroughly characterized glucose and lipid metabolism in male and female SIRT4KO mice across different genetic backgrounds. While only minor perturbations in overall lipid metabolism were observed, we found SIRT4KO mice consistently had elevated glucose- and leucine-stimulated insulin levels in vivo and developed accelerated age-induced insulin resistance. Importantly, elevated leucine-stimulated insulin levels in SIRT4KO mice were dependent upon genetic background since SIRT4KO mice on a C57BL/6NJ genetic background had elevated leucine-stimulated insulin levels but not SIRT4KO mice on the C57BL/6J background. Taken together, the data suggest that accumulation of acyl modifications on proteins in inherited metabolic disorders may contribute to the overall metabolic dysfunction seen in these patients.

Hyperinsulinemia: a Cause of Obesity?

PURPOSE OF REVIEW

This perspective is motivated by the need to question dogma that does not work: that the problem is insulin resistance (IR). We highlight the need to investigate potential environmental obesogens and toxins.

RECENT FINDINGS

The prequel to severe metabolic disease includes three interacting components that are abnormal: (a) IR, (b) elevated lipids and (c) elevated basal insulin (HI). HI is more common than IR and is a significant independent predictor of diabetes. We hypothesize that (1) the initiating defect is HI that increases nutrient consumption and hyperlipidemia (HL); (2) the cause of HI may include food additives, environmental obesogens or toxins that have entered our food supply since 1980; and (3) HI is sustained by HL derived from increased adipose mass and leads to IR. We suggest that HI and HL are early indicators of metabolic dysfunction and treating and reversing these abnormalities may prevent the development of more serious metabolic disease.

Plasma soluble leptin receptor levels are associated with pancreatic β-cell dysfunction in patients with type 2 diabetes

Aims/Introduction

A soluble form of the leptin receptor (soluble Ob‐R) in the circulation regulates leptin's bioactivity, and is inversely associated with body adiposity and circulating leptin levels. However, no study has examined the clinical impact of soluble Ob‐R on glucose metabolism in diabetes. The present study aimed to investigate the association of plasma soluble Ob‐R levels with insulin resistance and pancreatic β‐cell function in patients with type 2 diabetes.

Materials and Methods

A total of 289 Japanese patients with type 2 diabetes were included in the present study. Fasting plasma soluble Ob‐R levels and plasma leptin levels were measured by enzyme‐linked immunosorbent assay. Insulin resistance and pancreatic β‐cell function were estimated by homeostasis model assessment of insulin resistance, homeostasis model assessment of β‐cell function and fasting C‐peptide index.

Results

The median plasma soluble Ob‐R level and plasma leptin level were 3.4 ng/mL and 23.6 ng/mL, respectively. Plasma soluble Ob‐R levels were negatively correlated with homeostasis model assessment of insulin resistance, homeostasis model assessment of β‐cell function and the C‐peptide index, whereas plasma leptin levels were positively correlated with each index in univariate analyses. Multivariate analyses including plasma soluble Ob‐R levels, plasma leptin levels and use of sulfonylureas, along with age, sex, body mass index and other covariates, showed that soluble Ob‐R levels were independently and negatively associated with homeostasis model assessment of β‐cell function and the C‐peptide index, but not significantly associated with homeostasis model assessment of insulin resistance.

Conclusions

Plasma soluble Ob‐R levels are independently associated with pancreatic β‐cell function, but not with insulin resistance, in patients with type 2 diabetes. The present study implicates the role of soluble Ob‐R in pancreatic β‐cell dysfunction in type 2 diabetes.

SIRT4 Is a Lysine Deacylase that Controls Leucine Metabolism and Insulin Secretion

Sirtuins are NAD⁺-dependent protein deacylases that regulate several aspects of metabolism and aging. In contrast to the other mammalian sirtuins, the primary enzymatic activity of mitochondrial sirtuin 4 (SIRT4) and its overall role in metabolic control have remained enigmatic. Using a combination of phylogenetics, structural biology, and enzymology, we show that SIRT4 removes three acyl moieties from lysine residues: methylglutaryl (MG)-, hydroxymethylglutaryl (HMG)-, and 3-methylglutaconyl (MGc)-lysine. The metabolites leading to these post-translational modifications are intermediates in leucine oxidation, and we show a primary role for SIRT4 in controlling this pathway in mice. Furthermore, we find that dysregulated leucine metabolism in SIRT4KO mice leads to elevated basal and stimulated insulin secretion, which progressively develops into glucose intolerance and insulin resistance. These findings identify a robust enzymatic activity for SIRT4, uncover a mechanism controlling branched-chain amino acid flux, and position SIRT4 as a crucial player maintaining insulin secretion and glucose homeostasis during aging.

A causal role for hyperinsulinemia in obesity

Insulin modulates the biochemical pathways controlling lipid uptake, lipolysis and lipogenesis at multiple levels. Elevated insulin levels are associated with obesity, and conversely, dietary and pharmacological manipulations that reduce insulin have occasionally been reported to cause weight loss. However, the causal role of insulin hypersecretion in the development of mammalian obesity remained controversial in the absence of direct loss-of-function experiments. Here, we discuss theoretical considerations around the causal role of excess insulin for obesity, as well as recent studies employing mice that are genetically incapable of the rapid and sustained hyperinsulinemia that normally accompanies a high-fat diet. We also discuss new evidence demonstrating that modest reductions in circulating insulin prevent weight gain, with sustained effects that can persist after insulin levels normalize. Importantly, evidence from long-term studies reveals that a modest reduction in circulating insulin is not associated with impaired glucose homeostasis, meaning that body weight and lipid homeostasis are actually more sensitive to small changes in circulating insulin than glucose homeostasis in these models. Collectively, the evidence from new studies on genetic loss-of-function models forces a re-evaluation of current paradigms related to obesity, insulin resistance and diabetes. The potential for translation of these findings to humans is briefly discussed.

Hypothalamic Insulin Resistance in Obesity: Effects on Glucose Homeostasis

The central link between obesity and type 2 diabetes is the development of insulin resistance. To date, it is still not clear whether hyperinsulinemia causes insulin resistance, which underlies the pathogenesis of obesity-associated type 2 diabetes, owing to the sophisticated regulatory mechanisms that exist in the periphery and in the brain. In recent years, accumulating evidence has demonstrated the existence of insulin resistance within the hypothalamus. In this review, we have integrated the recent discoveries surrounding both central and peripheral insulin resistance to provide a comprehensive overview of insulin resistance in obesity and the regulation of systemic glucose homeostasis. In particular, this review will discuss how hyperinsulinemia and hyperleptinemia in obesity impair insulin sensitivity in tissues such as the liver, skeletal muscle, adipose tissue, and the brain. In addition, this review highlights insulin transport into the brain, signaling pathways associated with hypothalamic insulin receptor expression in the regulation of hepatic glucose production, and finally the perturbation of systemic glucose homeostasis as a consequence of central insulin resistance. We also suggest future approaches to overcome both central and peripheral insulin resistance to treat obesity and type 2 diabetes.

PPP2R5B, a regulatory subunit of PP2A, contributes to adipocyte insulin resistance

Insulin resistance is associated with deregulation of insulin signaling owing to the chronic exposure of insulin (hyperinsulinemia) to the tissues. Phosphorylation and dephosphorylation events in insulin signaling pathway play an essential role in signal transduction and glucose uptake. Amongst all, Akt protein is considered to be central to the overall insulin signaling proteins. In glucose responsive tissues like adipose and muscles, activation of Akt is responsible for triggering GLUT4 translocation and glucose transport. Several phosphatases such as PTEN, PP2A have been reported to be involved in dephosphorylation and inactivation of Akt protein. We have identified increased PP2A activity during state of chronic hyperinsulinemia exposure along-with development of adipocyte insulin resistance. This increased phosphatase activity leads activation of cAMP/PKA axis, which in turn increased cAMP levels in insulin resistant (IR) adipocytes. Okadaic acid, an inhibitor of PP2A restored and increased insulin stimulated glucose uptake in insulin resistant (IR) and insulin sensitive (IS) adipocytes respectively. In IS adipocyte, chemical activation of PP2A through MG132 and FTY720 showed decreased insulin sensitivity corroborated with decreased Akt phosphorylation and glucose uptake. We also observed an increased expression of PP2A-B (regulatory) subunit in IR adipocytes. We found PPP2R5B, a regulatory subunit of PP2A is responsible for the dephosphorylation and inactivation of Akt protein. Increased expression of PPP2R5B was also confirmed in white adipose tissue of high fat diet induced IR mice model. Overexpression and suppression strategies confirmed the role of PPP2R5B in regulating insulin signaling. Thus, we conclude that PPP2R5B, a B subunit of PP2A is a negative regulator of Akt phosphorylation contributing partly to the chronic hyperinsulinemia induced insulin resistance in adipocytes.

Hepatocyte glutathione peroxidase-1 deficiency improves hepatic glucose metabolism and decreases steatohepatitis in mice

AIMS/HYPOTHESIS

In obesity oxidative stress is thought to contribute to the development of insulin resistance, non-alcoholic fatty liver disease and the progression to non-alcoholic steatohepatitis. Our aim was to examine the precise contributions of hepatocyte-derived H₂O₂ to liver pathophysiology.

METHODS

Glutathione peroxidase (GPX) 1 is an antioxidant enzyme that is abundant in the liver and converts H₂O₂ to water. We generated Gpx1 ^lox/lox mice to conditionally delete Gpx1 in hepatocytes (Alb-Cre;Gpx1 ^lox/lox) and characterised mice fed chow, high-fat or choline-deficient amino-acid-defined (CDAA) diets.

RESULTS

Chow-fed Alb-Cre;Gpx1 ^lox/lox mice did not exhibit any alterations in body composition or energy expenditure, but had improved insulin sensitivity and reduced fasting blood glucose. This was accompanied by decreased gluconeogenic and increased glycolytic gene expression as well as increased hepatic glycogen. Hepatic insulin receptor Y1163/Y1163 phosphorylation and Akt Ser-473 phosphorylation were increased in fasted chow-fed Alb-Cre;Gpx1 ^lox/lox mice, associated with increased H₂O₂ production and insulin signalling in isolated hepatocytes. The enhanced insulin signalling was accompanied by the increased oxidation of hepatic protein tyrosine phosphatases previously implicated in the attenuation of insulin signalling. High-fat-fed Alb-Cre;Gpx1 ^lox/lox mice did not exhibit alterations in weight gain or hepatosteatosis, but exhibited decreased hepatic inflammation, decreased gluconeogenic gene expression and increased insulin signalling in the liver. Alb-Cre;Gpx1 ^lox/lox mice fed a CDAA diet that promotes non-alcoholic steatohepatitis exhibited decreased hepatic lymphocytic infiltrates, inflammation and liver fibrosis.

CONCLUSIONS/INTERPRETATION

Increased hepatocyte-derived H₂O₂ enhances hepatic insulin signalling, improves glucose control and protects mice from the development of non-alcoholic steatohepatitis.

Chronic insulin infusion induces reversible glucose intolerance in lean rats yet ameliorates glucose intolerance in obese rats

BACKGROUND

Although insulin resistance (IR) is a key factor in the pathogenesis of type 2 diabetes (T2D), the precise role of insulin in the development of IR remains unclear. Therefore, we investigated whether chronic basal insulin infusion is causative in the development of glucose intolerance.

METHODS

Normoglycemic lean rats surgically instrumented with i.v. catheters were infused with insulin (3mU/kg/min) or physiological saline for 6weeks. At infusion-end, plasma insulin levels along with glucose tolerance were assessed.

RESULTS

Six weeks of insulin infusion induced glucose intolerance and impaired insulin response in healthy rats. Interestingly, the effects of chronic insulin infusion were completely normalized following 24h withdrawal of exogenous insulin and plasma insulin response to glucose challenge was enhanced, suggesting improved insulin secretory capacity. As a result of this finding, we assessed whether the effects of insulin therapy followed by a washout could ameliorate established glucose intolerance in obese rats. Obese rats were similarly instrumented and infused with insulin or physiological saline for 7days followed by 24h washout. Seven day-insulin therapy in obese rats significantly improved glucose tolerance, which was attributed to improved insulin secretory capacity and improved insulin signaling in liver and skeletal muscle.

CONCLUSION

Moderate infusion of insulin alone is sufficient to cause glucose intolerance and impair endogenous insulin secretory capacity, whereas short-term, intensive insulin therapy followed by insulin removal effectively improves glucose tolerance, insulin response and peripheral insulin sensitivity in obese rats.

GENERAL SIGNIFICANCE

New insight into the link between insulin and glucose intolerance may optimize T2D management.

Suppressing hyperinsulinemia prevents obesity but causes rapid onset of diabetes in leptin-deficient Lepob/ob mice

Objective

Hyperinsulinemia is commonly associated with obesity. Mice deficient in the adipose-derived hormone leptin (Lep^ob/ob) develop hyperinsulinemia prior to onset of obesity and glucose intolerance. Whether the excess of circulating insulin is a major contributor to obesity and impaired glucose homeostasis in Lep^ob/ob mice is unclear. It has been reported previously that diet-induced obesity in mice can be prevented by reducing insulin gene dosage. In the present study, we examined the effects of genetic insulin reduction in Lep^ob/ob mice on circulating insulin, body composition, and glucose homeostasis.

Methods

Leptin expressing (Lep^wt/wt) mice lacking 3 insulin alleles were crossed with Lep^ob/ob mice to generate Lep^ob/ob and Lep^wt/wt littermates lacking 1 (Ins1^+/+;Ins2^+/−), 2 (Ins1^+/+;Ins2^−/−) or 3 (Ins1^+/−;Ins2^−/−) insulin alleles. Animals were assessed for body weight gain, body composition, glucose homeostasis, and islet morphology.

Results

We found that in young Lep^ob/ob mice, loss of 2 or 3 insulin alleles reduced plasma insulin levels by 75–95% and attenuated body weight gain by 50–90% compared to Ins1^+/+;Ins2^+/−;Lep^ob/ob mice. This corresponded with ∼30% and ∼50% reduced total body fat in Ins1^+/+;Ins2^−/−;Lep^ob/ob and Ins1^+/−;Ins2^−/−;Lep^ob/ob mice, respectively. Loss of 2 or 3 insulin alleles in young Lep^ob/ob mice resulted in onset of fasting hyperglycemia by 4 weeks of age, exacerbated glucose intolerance, and abnormal islet morphology. In contrast, loss of 1,2 or 3 insulin alleles in Lep^wt/wt mice did not significantly alter plasma insulin levels, body weight, fat mass, fasting glycemia, or glucose tolerance.

Conclusion

Taken together, our findings indicate that hyperinsulinemia is required for excess adiposity in Lep^ob/ob mice and sufficient insulin production is necessary to maintain euglycemia in the absence of leptin.

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Loss of 2 or 3 insulin alleles results in a dose dependent decrease of circulating insulin levels and body fat in Lep^ob/ob mice.

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Loss of 2 or 3 insulin alleles produced a greater reduction in body weight in male as compared to female Lep^ob/ob mice.

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Attenuation of hyperinsulinemia accelerates onset of hyperglycemia in Lep^ob/ob mice.

Chronic hyperinsulinemia reduces insulin sensitivity and metabolic functions of brown adipocyte

The growing pandemics of diabetes have become a real threat to world economy. Hyperinsulinemia and insulin resistance are closely associated with the pathophysiology of type 2 diabetes. In pretext of brown adipocytes being considered as the therapeutic strategy for the treatment of obesity and insulin resistance, we have tried to understand the effect of hyperinsulinemia on brown adipocyte function. We here with for the first time report that hyperinsulinemia-induced insulin resistance in brown adipocyte is also accompanied with reduced insulin sensitivity and brown adipocyte characteristics. CI treatment decreased expression of brown adipocyte-specific markers (such as PRDM16, PGC1α, and UCP1) and mitochondrial content as well as activity. CI-treated brown adipocytes showed drastic decrease in oxygen consumption rate (OCR) and spare respiratory capacity. Morphological study indicates increased accumulation of lipid droplets in CI-treated brown adipocytes. We have further validated these findings in vivo in C57BL/6 mice implanted with mini-osmotic insulin pump for 8weeks. CI treatment in mice leads to increased body weight gain, fat mass and impaired glucose intolerance with reduced energy expenditure and insulin sensitivity. CI-treated mice showed decreased BAT characteristics and function. We also observed increased inflammation and ER stress markers in BAT of CI-treated animals. The above results conclude that hyperinsulinemia has deleterious effect on brown adipocyte function, making it susceptible to insulin resistance. Thus, the above findings have greater implication in designing approaches for the treatment of insulin resistance and diabetes via recruitment of brown adipocytes.

Disrupted Leptin Signaling in the Lateral Hypothalamus and Ventral Premammillary Nucleus Alters Insulin and Glucagon Secretion and Protects Against Diet-Induced Obesity

Leptin signaling in the central nervous system, and particularly the arcuate hypothalamic nucleus, is important for regulating energy and glucose homeostasis. However, the roles of extra-arcuate leptin responsive neurons are less defined. In the current study, we generated mice with widespread inactivation of the long leptin receptor isoform in the central nervous system via Synapsin promoter-driven Cre (Lepr(flox/flox) Syn-cre mice). Within the hypothalamus, leptin signaling was disrupted in the lateral hypothalamic area (LHA) and ventral premammillary nucleus (PMV) but remained intact in the arcuate hypothalamic nucleus and ventromedial hypothalamic nucleus, dorsomedial hypothalamic nucleus, and nucleus of the tractus solitarius. To investigate the role of LHA/PMV neuronal leptin signaling, we examined glucose and energy homeostasis in Lepr(flox/flox) Syn-cre mice and Lepr(flox/flox) littermates under basal and diet-induced obese conditions and tested the role of LHA/PMV neurons in leptin-mediated glucose lowering in streptozotocin-induced diabetes. Lepr(flox/flox) Syn-cre mice did not have altered body weight or blood glucose levels but were hyperinsulinemic and had enhanced glucagon secretion in response to experimental hypoglycemia. Surprisingly, when placed on a high-fat diet, Lepr(flox/flox) Syn-cre mice were protected from weight gain, glucose intolerance, and diet-induced hyperinsulinemia. Peripheral leptin administration lowered blood glucose in streptozotocin-induced diabetic Lepr(flox/flox) Syn-cre mice as effectively as in Lepr(flox/flox) littermate controls. Collectively these findings suggest that leptin signaling in LHA/PMV neurons is not critical for regulating glucose levels but has an indispensable role in the regulation of insulin and glucagon levels and, may promote the development of diet-induced hyperinsulinemia and weight gain.

Islet Hypersensitivity to Glucose Is Associated With Disrupted Oscillations and Increased Impact of Proinflammatory Cytokines in Islets From Diabetes-Prone Male Mice

Pulsatile insulin release is the primary means of blood glucose regulation. The loss of pulsatility is thought to be an early marker and possible factor in developing type 2 diabetes. Another early adaptation in islet function to compensate for obesity is increased glucose sensitivity (left shift) associated with increased basal insulin release. We provide evidence that oscillatory disruptions may be linked with overcompensation (glucose hypersensitivity) in islets from diabetes-prone mice. We isolated islets from male 4- to 5-week-old (prediabetic) and 10- to 12-week-old (diabetic) leptin-receptor-deficient (db/db) mice and age-matched heterozygous controls. After an overnight incubation in media with 11 mM glucose, we measured islet intracellular calcium in 5, 8, 11, or 15 mM glucose. Islets from heterozygous 10- to 12-week-old mice were quiescent in 5 mM glucose and displayed oscillations with increasing amplitude and/or duration in 8, 11, and 15 mM glucose, respectively. Islets from diabetic 10- to 12-week-old mice, in contrast, showed robust oscillations in 5 mM glucose that declined with increasing glucose. Similar trends were observed at 4-5-weeks of age. A progressive left shift in maximal insulin release was also observed in islets as db/db mice aged. Reducing glucokinase activity with 1 mM D-mannoheptulose restored oscillations in 11 mM glucose. Finally, overnight low-dose cytokine exposure negatively impacted oscillations preferentially in high glucose in diabetic islets compared with heterozygous controls. Our findings suggest the following: 1) islets from frankly diabetic mice can produce oscillations, 2) elevated sensitivity to glucose prevents diabetic mouse islets from producing oscillations in normal postprandial (11-15 mM glucose) conditions, and 3) hypersensitivity to glucose may magnify stress effects from inflammation or other sources.

A major role of insulin in promoting obesity-associated adipose tissue inflammation

Objective

Adipose tissue (AT) inflammation is associated with systemic insulin resistance and hyperinsulinemia in obese rodents and humans. A longstanding concept is that hyperinsulinemia may promote systemic insulin resistance through downregulation of its receptor on target tissues. Here we tested the novel hypothesis that insulin also impairs systemic insulin sensitivity by specifically enhancing adipose inflammation.

Methods

Circulating insulin levels were reduced by about 50% in diet-induced and genetically obese mice by treatments with diazoxide or streptozotocin, respectively. We then examined AT crown-like structures, macrophage markers and pro-inflammatory cytokine expression in AT. AT lipogenesis and systemic insulin sensitivity was also monitored. Conversely, insulin was infused into lean mice to determine its affects on the above parameters.

Results

Lowering circulating insulin levels in obese mice by streptozotocin treatment decreased macrophage content in AT, enhancing insulin stimulated Akt phosphorylation and de novo lipogenesis (DNL). Moreover, responsiveness of blood glucose levels to injected insulin was improved by streptozotocin and diazoxide treatments of obese mice without changes in body weight. Remarkably, even in lean mice, infusion of insulin under constant euglycemic conditions stimulated expression of cytokines in AT. Consistent with these findings, insulin treatment of 3T3-L1 adipocytes caused a 10-fold increase in CCL2 mRNA levels within 6 h, which was blocked by the ERK inhibitor PD98059.

Conclusion

Taken together, these results indicate that obesity-associated hyperinsulinemia unexpectedly drives AT inflammation in obese mice, which in turn contributes to factors that suppress insulin-stimulated adipocyte DNL and systemic insulin sensitivity.

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Adipose tissue inflammation correlates with hyperinsulinemia in obese mice and humans independent of BMI.

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Reduction of hyperinsulinemia ameliorates adipose tissue inflammation and enhances systemic insulin sensitivity.

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Insulin increases adipose inflammation in vivo and enhances adipocyte MCP-1 expression in vitro through ERK activation.

Fetal hyperglycemia and a high-fat diet contribute to aberrant glucose tolerance and hematopoiesis in adult rats

BACKGROUND

Children exposed to gestational diabetes mellitus (GDM) during pregnancy are at increased risk of obesity, diabetes, and hypertension. Our goal was to identify metabolic and hematopoietic alterations after intrauterine exposure to maternal hyperglycemia that may contribute to the pathogenesis of chronic morbidities.

METHODS

Streptozotocin treatment induced maternal hyperglycemia during the last third of gestation in rat dams. Offspring of control mothers (OCM) and diabetic mothers (ODM) were evaluated for weight, glucose tolerance, insulin tolerance, and hematopoiesis defects. The effects of aging were examined in normal and high-fat diet (HFD)-fed young (8-wk-old) and aged (11-mo-old) OCM and ODM rats.

RESULTS

Young adult ODM males on a normal diet, but not females, displayed improved glucose tolerance due to increased insulin levels. Aged ODM males and females gained more weight than OCM on a HFD and had worse glucose tolerance. Aged ODM males fed a HFD were also neutrophilic. Increases in bone marrow cellularity and myeloid progenitors preceded neutrophilia in ODM males fed a HFD.

CONCLUSION

When combined with other risk factors like HFD and aging, changes in glucose metabolism and hematopoiesis may contribute to the increased risk of obesity, type 2 diabetes, and hypertension observed in children of GDM mothers.

Pulsatile insulin secretion, impaired glucose tolerance and type 2 diabetes

Type 2 diabetes (T2DM) results when increases in beta cell function and/or mass cannot compensate for rising insulin resistance. Numerous studies have documented the longitudinal changes in metabolism that occur during the development of glucose intolerance and lead to T2DM. However, the role of changes in insulin secretion, both amount and temporal pattern, has been understudied. Most of the insulin secreted from pancreatic beta cells of the pancreas is released in a pulsatile pattern, which is disrupted in T2DM. Here we review the evidence that changes in beta cell pulsatility occur during the progression from glucose intolerance to T2DM in humans, and contribute significantly to the etiology of the disease. We review the evidence that insulin pulsatility improves the efficacy of secreted insulin on its targets, particularly hepatic glucose production, but also examine evidence that pulsatility alters or is altered by changes in peripheral glucose uptake. Finally, we summarize our current understanding of the biophysical mechanisms responsible for oscillatory insulin secretion. Understanding how insulin pulsatility contributes to normal glucose homeostasis and is altered in metabolic disease states may help improve the treatment of T2DM.

Counterregulation of insulin by leptin as key component of autonomic regulation of body weight

A re-examination of the mechanism controlling eating, locomotion, and metabolism prompts formulation of a new explanatory model containing five features: a coordinating joint role of the (1) autonomic nervous system (ANS); (2) the suprachiasmatic (SCN) master clock in counterbalancing parasympathetic digestive and absorptive functions and feeding with sympathetic locomotor and thermogenic energy expenditure within a circadian framework; (3) interaction of the ANS/SCN command with brain substrates of reward encompassing dopaminergic projections to ventral striatum and limbic and cortical forebrain. These drive the nonhomeostatic feeding and locomotor motivated behaviors in interaction with circulating ghrelin and lateral hypothalamic neurons signaling through melanin concentrating hormone and orexin-hypocretin peptides; (4) counterregulation of insulin by leptin of both gastric and adipose tissue origin through: potentiation by leptin of cholecystokinin-mediated satiation, inhibition of insulin secretion, suppression of insulin lipogenesis by leptin lipolysis, and modulation of peripheral tissue and brain sensitivity to insulin action. Thus weight-loss induced hypoleptimia raises insulin sensitivity and promotes its parasympathetic anabolic actions while obesity-induced hyperleptinemia supresses insulin lipogenic action; and (5) inhibition by leptin of bone mineral accrual suggesting that leptin may contribute to the maintenance of stability of skeletal, lean-body, as well as adipose tissue masses.