Expression Levels of Genes Likely Involved in Glucose-sensing in the Obese Zucker Rat Brain

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.

An Insulin-Like Modular Basis for the Evolution of Glucose Transporters (GLUT) with Implications for Diabetes

Glucose transporters (GLUT) are twelve-transmembrane spanning proteins that contain two pores capable of transporting glucose and dehydroascorbate in and out of cells. The mechanism by which transport is effected is unknown. An evolutionarily-based hypothesis for the mechanism of glucose transport is presented here based on reports that insulin has multiple binding sites for glucose. It is proposed that insulin-like peptides were incorporated as modular elements into transmembrane proteins during evolution, resulting in glucose transporting capacity. Homology searching reveals that all GLUT contain multiple copies of insulin-like regions. These regions map onto a model of GLUT in positions that define the glucose transport cores. This observation provides a mechanism for glucose transport involving the diffusion of glucose from one insulin-like glucose-binding region to another. It also suggests a mechanism by which glucose disregulation may occur in both type 1 and type 2 diabetes: insulin rapidly self-glycates under hyperglycemic conditions. Insulin-like regions of GLUT may also self-glycate rapidly, thereby interfering with transport of glucose into cells and disabling GLUT sensing of blood glucose levels. All aspects of the hypothesis are experimentally testable.

A Prospective Investigation of the Association Between Urinary Excretion of Dietary Lignan Metabolites and Weight Change in US Women

Results from animal studies have consistently suggested that lignans play a role in the regulation of in body weight, but evidence from human studies has been limited. We examined the associations between urinary excretion of enterolactone and enterodiol, the major intestinal microbial metabolites of dietary lignans, and 10-year prospective weight change using data from 2 well-characterized cohort studies of US women: the Nurses' Health Study (2000-2010) and Nurses' Health Study II (1997-2007). Urinary excretion levels of enterolactone and enterodiol were measured at baseline. Associations with prospective weight change were analyzed using a multivariable-adjusted linear mixed-effects model. We observed that women in the highest quartile of urinary excretion of total lignans had significantly lower baseline body mass indices (weight in kilograms divided by height in meters squared) (mean, 24.6, 95% confidence interval (CI): 23.9, 25.2) than did those in the lowest quartile (mean, 27.7, 95% CI: 27.0, 28.4; P for trend < 0.01). Compared with women in the lowest quartile of enterodiol excretion, those in the highest quartile gained 0.27 kg/year less weight (95% CI: 0.12, 0.41; P for trend < 0.01) during the 10-year follow-up. The association was borderline significant for enterolactone (for the fourth vs. first quartile, least square mean of weight change rate = -0.14 kg/year, 95% CI: -0.29, 0.00). Our data suggest that higher urinary excretion of lignan metabolites, especially enterodiol, is associated with modestly slower weight gain.

Modulation of olfactory sensitivity and glucose-sensing by the feeding state in obese Zucker rats

The Zucker fa/fa rat has been widely used as an animal model to study obesity, since it recapitulates most of its behavioral and metabolic dysfunctions, such as hyperphagia, hyperglycemia and insulin resistance. Although it is well established that olfaction is under nutritional and hormonal influences, little is known about the impact of metabolic dysfunctions on olfactory performances and glucose-sensing in the olfactory system of the obese Zucker rat. In the present study, using a behavioral paradigm based on a conditioned olfactory aversion, we have shown that both obese and lean Zucker rats have a better olfactory sensitivity when they are fasted than when they are satiated. Interestingly, the obese Zucker rats displayed a higher olfactory sensitivity than their lean controls. By investigating the molecular mechanisms involved in glucose-sensing in the olfactory system, we demonstrated that sodium-coupled glucose transporters 1 (SGLT1) and insulin dependent glucose transporters 4 (GLUT4) are both expressed in the olfactory bulb (OB). By comparing the expression of GLUT4 and SGLT1 in OB of obese and lean Zucker rats, we found that only SGLT1 is regulated in genotype-dependent manner. Next, we used glucose oxidase biosensors to simultaneously measure in vivo the extracellular fluid glucose concentrations ([Gluc]ECF) in the OB and the cortex. Under metabolic steady state, we have determined that the OB contained twice the amount of glucose found in the cortex. In both regions, the [Gluc]ECF was 2 fold higher in obese rats compared to their lean controls. Under induced dynamic glycemia conditions, insulin injection produced a greater decrease of [Gluc]ECF in the OB than in the cortex. Glucose injection did not affect OB [Gluc]ECF in Zucker fa/fa rats. In conclusion, these results emphasize the importance of glucose for the OB network function and provide strong arguments towards establishing the OB glucose-sensing as a key factor for sensory olfactory processing.

Sex differences in glucose and insulin trajectories prior to diabetes diagnosis: the Whitehall II study

To examine sex differences in trajectories of glucose and insulin prior to the development of type 2 diabetes. Glucose and insulin levels from oral glucose tolerance tests performed every 5 years in the prospective Whitehall II cohort were analysed using mixed-effects models. Out of 6,712 White participants, 475 men and 177 women developed diabetes during a median follow-up time of 14 years. Men who developed diabetes had higher fasting glucose levels than women over the entire follow-up period (P < 0.001), whereas trajectories of 2-h glucose did not differ between sexes (P = 0.34). Two-hour insulin levels tended to be higher among women than men developing diabetes (P = 0.05), but not after adjustment for height (P = 0.45). Men have higher fasting glucose levels than women many years before the diagnosis of diabetes and at time of diagnosis, indicating that the pathogenesis of diabetes may be different between sexes.

Estrogen Deficiency and the Origin of Obesity during Menopause

Sex hormones strongly influence body fat distribution and adipocyte differentiation. Estrogens and testosterone differentially affect adipocyte physiology, but the importance of estrogens in the development of metabolic diseases during menopause is disputed. Estrogens and estrogen receptors regulate various aspects of glucose and lipid metabolism. Disturbances of this metabolic signal lead to the development of metabolic syndrome and a higher cardiovascular risk in women. The absence of estrogens is a clue factor in the onset of cardiovascular disease during the menopausal period, which is characterized by lipid profile variations and predominant abdominal fat accumulation. However, influence of the absence of these hormones and its relationship to higher obesity in women during menopause are not clear. This systematic review discusses of the role of estrogens and estrogen receptors in adipocyte differentiation, and its control by the central nervous systemn and the possible role of estrogen-like compounds and endocrine disruptors chemicals are discussed. Finally, the interaction between the decrease in estrogen secretion and the prevalence of obesity in menopausal women is examined. We will consider if the absence of estrogens have a significant effect of obesity in menopausal women.

Dietary resistant starch improves selected brain and behavioral functions in adult and aged rodents

Resistant starch (RS) is a dietary fiber that exerts multiple beneficial effects. The current study explored the effects of dietary RS on selected brain and behavioral functions in adult and aged rodents. Because glucokinase (GK) expression in hypothalamic arcuate nucleus and area postrema of the brainstem is important for brain glucose sensing, GK mRNA was measured by brain nuclei microdissection and PCR. Adult RS-fed rats had a higher GK mRNA than controls in both brain nuclei, an indicator of improved brain glucose sensing. Next, we tested whether dietary RS improve selected behaviors in aged mice. RS-fed aged mice exhibited (i) an increased eating responses to fasting, a behavioral indicator of improvement in aged brain glucose sensing; (ii) a longer latency to fall from an accelerating rotarod, a behavioral indicator of improved motor coordination; and (iii) a higher serum active glucagon-like peptide-1 (GLP-1). Then, GLP-1 receptor null (GLP-1RKO) mice were used to test the role of GLP-1 in brain glucose sensing, and they exhibited impaired eating responses to fasting. We conclude that in rodents (i) dietary RS improves two important indicators of brain function: glucose sensing and motor coordination, and (ii) GLP-1 is important in the optimal feeding response to a fast.

Direct and indirect effects of leptin on adipocyte metabolism

Leptin is hypothesized to function as a negative feedback signal in the regulation of energy balance. It is produced primarily by adipose tissue and circulating concentrations correlate with the size of body fat stores. Administration of exogenous leptin to normal weight, leptin responsive animals inhibits food intake and reduces the size of body fat stores whereas mice that are deficient in either leptin or functional leptin receptors are hyperphagic and obese, consistent with a role for leptin in the control of body weight. This review discusses the effect of leptin on adipocyte metabolism. Because adipocytes express leptin receptors there is the potential for leptin to influence adipocyte metabolism directly. Adipocytes also are insulin responsive and receive sympathetic innervation, therefore leptin can also modify adipocyte metabolism indirectly. Studies published to date suggest that direct activation of adipocyte leptin receptors has little effect on cell metabolism in vivo, but that leptin modifies adipocyte sensitivity to insulin to inhibit lipid accumulation. In vivo administration of leptin leads to a suppression of lipogenesis, an increase in triglyceride hydrolysis and an increase in fatty acid and glucose oxidation. Activation of central leptin receptors also contributes to the development of a catabolic state in adipocytes, but this may vary between different fat depots. Leptin reduces the size of white fat depots by inhibiting cell proliferation both through induction of inhibitory circulating factors and by contributing to sympathetic tone which suppresses adipocyte proliferation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Is leptin the parabiotic "satiety" factor? Past and present interpretations

In 1959 Hervey hypothesized that a circulating feedback signal informed the hypothalamus of the size of fat stores and initiated appropriate corrections to energy balance. The hypothesis resulted from a parabiosis study in which one animal became obese following lesioning of the ventromedial hypothalamus. The partner of the lesioned rat was hypophagic and lost a large amount of body fat. Similar results came from parabiosis studies with obese Zucker rats and rats that overate due to stimulation of the lateral hypothalamus. In studies in which one parabiont was made obese by overfeeding the non-overfed partners lost substantial amounts of fat with a minimal reduction in food intake and no loss of lean tissue. The loss of fat was due to inhibition of adipose lipogenesis and other metabolic adjustments typical of food restriction. Parabiosis with genetically obese mice implied that ob/ob mice did not produce the feedback signal and subsequently the mutant ob protein, leptin, was identified. This paper provides a review and interpretation of parabiosis work that preceded the discovery of leptin, an evaluation of leptin in relation to its function as the circulating feedback signal and evidence for additional circulating factors involved in the control of adipose tissue mass.

Regulation of energy metabolism pathways by estrogens and estrogenic chemicals and potential implications in obesity associated with increased exposure to endocrine disruptors

The prevalence of obesity among children, adolescents and adults has been dramatically increasing worldwide during the last several decades. The obesity epidemic has been recognized as one of the major global health problems, because its health hazard is linked to a number of common diseases including breast and prostate cancers. Obesity is caused by combination of genetic and environmental factors. While genetic contribution to obesity has been known to be significant, the genetic factors remain relatively unchanged. Recent studies have highlighted the involvement of environmental "obesogens", i.e. the xenobiotic chemicals that can disrupt the normal development and homeostatic control over adipogenesis and energy balance. Several lines of evidence suggest that increasing exposure to chemicals with endocrine-disrupting activities (endocrine-disrupting chemicals, EDCs) contributes to the increased obesity. The cellular and molecular mechanisms underlying obesogen-associated obesity are just now being appreciated. In this paper, we comprehensively reviewed current knowledge about the role of estrogen receptors alpha and beta (ERalpha and ERbeta) in regulation of energy metabolism pathways, including glucose transport, glycolysis, tricarboxylic acid (TCA) cycle, mitochondrial respiratory chain (MRC), adenosine nucleotide translocator (ANT) and fatty acid beta-oxidation and synthesis, by estrogens; and then examined the disturbance of E(2)/ER-mediated energy metabolism pathways by environmental obesogens; and finally, we discussed the potential implications of disturbance of energy metabolism pathways by obesogens in obesity and pointed out several key aspects of this area that need to be further explored. A better understanding of the cellular and molecular mechanisms underlying obesogen-associated obesity will lead to new approaches for slow down and/or prevention of the increased trend of obesity associated with exposure to obesogens.

Leptin transport in the central nervous system

Synthesized and released by the adipose tissue, leptin is the widely studied 167-amino acid hormonal protein product of the obesity gene. Originally leptin was defined in association with satiety and energy balance and claimed to be an anti-obesity factor that functioned via a feedback effect from adipocytes to hypothalamus. There is a growing body of evidence that emphasizes the importance of leptin in the regulation of food intake and body weight in animals and humans, alike. Other research findings point out that it plays a role in the regulation of the metabolism, sexual development, reproduction, hematopoiesis, immunity, gastrointestinal functions, sympathetic activation, and angiogenesis. The aim of this review is to evaluate the relation between leptin and the central nervous system (CNS).

Protective effects of antioxidant-fortified diet on renal function and metabolic profile in obese Zucker rat

Oxidative stress contributes to the pathophysiology of type 2 diabetes mellitus and its complications, including nephropathy. The current study was designed to test the hypothesis that a diet fortified with antioxidants would be beneficial to delay or prevent the progression of this disease. Male and female Zucker fa/fa rats were fed a control or an antioxidant (AO)-fortified diet starting at 4 weeks of age. Metabolic parameters, renal function, and renal histopathology were analyzed at 6, 13, and 20 weeks of age. Females on the AO diet had significantly lower blood glucose at 6 and 13 weeks, less severe renal pathology at 20 weeks, and higher glomerular filtration rates (GFR) at 20 weeks than age-matched females on the regular diet (P < 0.05). Metabolic parameters including blood glucose, insulin resistance, and serum cholesterol, and mean arterial pressure (MAP), worsened with age in both males and females, as expected. GFR decreased and renal pathology also became more severe with age. Finally, females on the AO diet had higher GFRs and lower MAP at 20 weeks than males on the same diet. This may denote a protective effect of the AO diet in females, but not in males. These findings may have implications for the role of antioxidants as therapy in humans with T2DM.

Rat models of caloric intake and activity: relationships to animal physiology and human health

Every rodent experiment is based on important parameters concerning the levels of caloric intake and physical activity. In many cases, these decisions are not made consciously, but are based on traditional models. For experimental models directed at the study of caloric intake and activity, the selection of parameters is usually aimed at modeling human conditions, the ultimate goal of which is to gain insight into the pathophysiology of the disease process in man. In each model, it is important to understand the influence of diet, exercise, and genetic background on physiology and the development of disease states. Along the continuum of energy intake from caloric restriction to high-fat feeding, and of energy output from total inactivity to forced exercise, a number of models are used to study different disease states. In this paper, we will evaluate the influence of the quantity and composition of diet and exercise in several animal models, and will discuss how each model can be applied to various human conditions. This review will be limited to traditional models using the rat as the experimental animal, and although it is not an exhaustive list, the models presented are those most commonly represented in the literature. We will also review the mechanisms by which each affects rat physiology, and will compare these to the analogous mechanisms in the modeled human disease state. We hope that the information presented here will help researchers make choices among the available models and will encourage discussion on the interpretation and extrapolation of results obtained from traditional and novel rodent experiments on diet, exercise, and chronic disease.

Rapid inhibition of neurons in the dorsal motor nucleus of the vagus by leptin

The peptide leptin conveys the availability of adipose energy stores to the brain. Increasing evidence implicates a significant role for extrahypothalamic sites of leptin action, including the dorsal vagal complex, a region critical for regulating visceral parasympathetic function. The hypothesis that leptin suppresses cellular activity in the dorsal motor nucleus of the vagus nerve (DMV) was tested using whole-cell patch-clamp recordings in brainstem slices. Leptin caused a rapid membrane hyperpolarization in 50% of rat DMV neurons. Leptin also hyperpolarized a subset of gastric-related neurons (62%), identified after gastric inoculation with a transneuronal retrograde viral tracer. The hyperpolarization was associated with a decrease in input resistance and cellular responsiveness and displayed characteristics consistent with an increased K+ conductance. Perfusion of tolbutamide (200 microM) reversed the leptin-induced hyperpolarization, and tolbutamide or wortmannin (10-100 nM) prevented the hyperpolarization, indicating that leptin activated an ATP-sensitive K+ channel via a phosphoinositide-3-kinase-dependent mechanism. Leptin reduced the frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs), whereas inhibitory postsynaptic currents (IPSCs) were largely unaffected. Electrical stimulation of the nucleus tractus solitarii (NTS) resulted in constant-latency EPSCs, which were decreased in amplitude by leptin. The paired-pulse ratio was increased, suggesting leptin effects involved activation of receptors presynaptic to the recorded neuron. A leptin-induced suppression of EPSCs, but not IPSCs, evoked by focal photolytic uncaging of glutamate within the NTS was also observed, supportive of leptin effects on the glutamatergic NTS projection to the DMV. Therefore, leptin directly hyperpolarized and indirectly suppressed excitatory synaptic activity to DMV neurons involved in visceral regulation, including gastric-related neurons.

Molecular connexions between dementia and diabetes

Recent evidence suggests that the molecular defects associated with the development of diabetes also contribute to an increased risk of all types of dementia, including Alzheimer's disease, vascular dementia and Pick's disease. Indeed, the presence of type II diabetes mellitus results in a two to three fold higher risk of developing dementia [Fontbonne et al., 2001. Changes in cognitive abilities over a 4-year period are unfavourably affected in elderly diabetic subjects: results of the Epidemiology of Vascular Aging Study. Diabetes Care 24, 366-370; Gregg et al., 2000. Is diabetes associated with cognitive impairment and cognitive decline among older women? Study of Osteoporotic Fractures Research Group. Archives of Internal Medicine 160, 174-180; Peila et al., 2002. Type 2 diabetes, APOE gene, and the risk for dementia and related pathologies: The Honolulu-Asia Aging Study. Diabetes 51, 1256-1262]. There are currently 250 million people worldwide (>2 million in the UK) diagnosed with diabetes, and this number is predicted to double within the next 20 years, therefore the associated risk translates into a potential explosion in the appearance of dementia in the population. This review primarily focuses on the proposed molecular links between insulin action, Diabetes and Alzheimer's disease, while discussing the potential for therapeutic intervention to alleviate these disorders. In particular, we will review the regulation of glycogen synthase kinase-3 (GSK-3) and its neuronal substrates.

Effects of resistant starch, a non-digestible fermentable fiber, on reducing body fat

OBJECTIVE

To assess the effects of energy dilution with non-fermentable and fermentable fibers on abdominal fat and gut peptide YY (PYY) and glucagon-like peptide (GLP)-1 expressions, three rat studies were conducted to: determine the effects of energy dilution with a non-fermentable fiber, compare similar fiber levels of fermentable and non-fermentable fibers, and compare similar metabolizable energy dilutions with fermentable and non-fermentable fibers.

RESEARCH METHODS AND PROCEDURES

In Study 1, rats were fed one of three diets with different metabolizable energy densities. In Study 2, rats were fed diets with similar fiber levels using high amylose-resistant cornstarch (RS) or methylcellulose. In Study 3, rats were fed diets with a similar dilution of metabolizable energy using cellulose or RS. Measurements included food intake, body weight, abdominal fat, plasma PYY and GLP-1, gastrointestinal tract weights, and gene transcription of PYY and proglucagon.

RESULTS

Energy dilution resulted in decreased abdominal fat in all studies. In Study 2, rats fed fermentable RS had increased cecal weights and plasma PYY and GLP-1, and increased gene transcription of PYY and proglucagon. In Study 3, RS-fed rats had increased short-chain fatty acids in cecal contents, plasma PYY (GLP-1 not measured), and gene transcription for PYY and proglucagon.

DISCUSSION

Inclusion of RS in the diet may affect energy balance through its effect as a fiber or a stimulator of PYY and GLP-1 expression. Increasing gut hormone signaling with a bioactive functional food such as RS may be an effective natural approach to the treatment of obesity.

Brain glucose sensing: a subtle mechanism

PURPOSE OF REVIEW

Brain nutrient sensing allows a fine regulation of different physiological functions, such as food intake and blood glucose, related to energy homeostasis. Glucose sensing is the most studied function and a parallel has been made between the cellular mechanisms involved in pancreatic beta cells and neurons.

RECENT FINDINGS

Two types of glucosensing neurons have been characterized--those for which the activity is proportional to changes in glucose concentration and those for which the activity is inversely proportional to these changes. A new level of complexity has recently been demonstrated, as the response and the mechanism appear to vary in function according to the level of the glucose change. For some of the responses, the detection is probably not at the level of the neuron itself, but astrocytes also appear to be involved, indicating a coupling between the two types of cells. Finally, numerous data have demonstrated the modulation of glucose sensing by other nutrients, in particular fatty acids, hormones (insulin, leptin and ghrelin) and peptides (neuropeptide Y). This implies a common pathway in which AMPkinase may play a crucial role.

SUMMARY

Recent observations in brain nutrient sensing indicate subtle mechanisms, with different cellular and molecular mechanisms involved. This fact would explain the discrepancies reported in the expression of different proteins (glucose transporters, hexokinases, channels). Astrocytes may be involved in one type of response, thus adding a new level of complexity.