Leptin receptor, NPY, POMC mRNA expression in the diet-induced obese mouse brain

A chronic high fat diet alters the homologous and heterologous control of appetite regulating peptide receptor expression

Leptin, ghrelin and neuropeptide W (NPW) modulate vagal afferent activity, which may underlie their appetite regulatory actions. High fat diet (HFD)-induced obesity induces changes in the plasma levels of these peptides and alters the expression of receptors on vagal afferents. We investigated homologous and heterologous receptor regulation by leptin, ghrelin and NPW. Mice were fed (12 weeks) a standard laboratory diet (SLD) or HFD. Nodose ganglia were cultured overnight in the presence or absence of each peptide. Leptin (LepR), ghrelin (GHS-R), NPW (GPR7) and cholecystokinin type-1 (CCK1R) receptor mRNA, and the plasma leptin, ghrelin and NPW levels were measured. SLD: leptin reduced LepR, GPR7, increased GHS-R and CCK1R mRNA; ghrelin increased LepR, GPR7, CCK1R, and decreased GHS-R. HFD: leptin decreased GHS-R and GPR7, ghrelin increased GHS-R and GPR7. NPW decreased all receptors except GPR7 which increased with HFD. Plasma leptin was higher and NPW lower in HFD. Thus, HFD-induced obesity disrupts inter-regulation of appetite regulatory receptors in vagal afferents.

Whey protein isolate counteracts the effects of a high-fat diet on energy intake and hypothalamic and adipose tissue expression of energy balance-related genes

The intake of whey protein isolate (WPI) is known to reduce high-fat diet (HFD)-induced body-weight gain and adiposity. However, the molecular mechanisms are not fully understood. To this end, we fed C57BL/6J mice for 8 weeks with diets containing 10 % energy as fat (low-fat diet, LFD) or 45 % energy as fat (HFD) enriched with either 20 % energy as casein (LFD and HFD) or WPI (high-fat WPI). Metabolic parameters and the hypothalamic and epididymal adipose tissue expression of energy balance-related genes were investigated. The HFD increased fat mass and plasma leptin levels and decreased the dark-phase energy intake, meal number, RER, and metabolic (VO₂ and heat) and locomotor activities compared with the LFD. The HFD increased the hypothalamic tissue mRNA expression of the leptin receptor, insulin receptor (INSR) and carnitine palmitoyltransferase 1b (CPT1b). The HFD also reduced the adipose tissue mRNA expression of GLUT4 and INSR. In contrast, WPI reduced fat mass, normalised dark-phase energy intake and increased meal size in HFD-fed mice. The dietary protein did not have an impact on plasma leptin, insulin, glucose or glucagon-like peptide 1 levels, but increased plasma TAG levels in HFD-fed mice. At a cellular level, WPI significantly reduced the HFD-associated increase in the hypothalamic tissue mRNA expression of the leptin receptor, INSR and CPT1b. Also, WPI prevented the HFD-induced reduction in the adipose tissue mRNA expression of INSR and GLUT4. In comparison with casein, the effects of WPI on energy intake and hypothalamic and adipose tissue gene expression may thus represent a state of reduced susceptibility to weight gain on a HFD.

The hypothalamic POMC mRNA expression is upregulated in prenatally undernourished male rat offspring under high-fat diet

Epidemiological studies demonstrated that adverse environmental factors leading to intrauterine growth retardation (IUGR) and low birth weight may predispose individuals to increased risk of metabolic syndrome. In rats, we previously demonstrated that adult male IUGR offspring from prenatal 70% food-restricted dams throughout gestation (FR30) were predisposed to energy balance dysfunctions such as impaired glucose intolerance, hyperleptinemia, hyperphagia and adiposity. We investigated whether postweaning moderate high-fat (HF) diet would amplify the phenotype focusing on the hypothalamus gene expression profile. Prenatally undernourished rat offspring were HF-fed from weaning until adulthood while body weight and food intake were measured. Tissue weights, glucose tolerance and plasma endocrine parameters levels were determined in 4-month-old rats. Hypothalamic gene expression profiling of adult FR30 rat was performed using Illumina microarray analysis and the RatRef-12 Expression BeadChip that contains 21,792 rat genes. Under HF diet, contrary to C animals, FR30 rats displayed increased body weight. However, most of the endocrine disorders observed in chow diet-fed adult FR30 were alleviated. We also observed very few gene expression changes in hypothalamus of FR30 rat. Amongst factors involved in hypothalamic energy homeostasis programming system, only the POMC and transthyretin mRNA expression levels were preferentially increased under HF diet. Both elevated gene expression levels may be seen as adaptive mechanisms counteracting against deleterious effects of HF feeding in FR30 animals. This study shows that the POMC gene expression is a key target of long-term developmental programming in prenatally undernourished male rat offspring, specifically within an obesogenic environment.

Neuronal expression of glucosylceramide synthase in central nervous system regulates body weight and energy homeostasis

Body weight and energy homeostasis are regulated by leptin receptor interactions with gangliosides, a class of plasma membrane lipids, in forebrain neurons of mice.

Hypothalamic neurons are main regulators of energy homeostasis. Neuronal function essentially depends on plasma membrane-located gangliosides. The present work demonstrates that hypothalamic integration of metabolic signals requires neuronal expression of glucosylceramide synthase (GCS; UDP-glucose:ceramide glucosyltransferase). As a major mechanism of central nervous system (CNS) metabolic control, we demonstrate that GCS-derived gangliosides interacting with leptin receptors (ObR) in the neuronal membrane modulate leptin-stimulated formation of signaling metabolites in hypothalamic neurons. Furthermore, ganglioside-depleted hypothalamic neurons fail to adapt their activity (c-Fos) in response to alterations in peripheral energy signals. Consequently, mice with inducible forebrain neuron-specific deletion of the UDP-glucose:ceramide glucosyltransferase gene (Ugcg) display obesity, hypothermia, and lower sympathetic activity. Recombinant adeno-associated virus (rAAV)-mediated Ugcg delivery to the arcuate nucleus (Arc) significantly ameliorated obesity, specifying gangliosides as seminal components for hypothalamic regulation of body energy homeostasis.

Obesity is a growing health threat that affects nearly half a billion people worldwide, and its incidence rates in lower income countries are rising dramatically. As obesity is a major risk factor for type II diabetes and cardiovascular disease, significant effort has been put into the exploration of causes, prevention, and potential treatment. Recent research has demonstrated that a region of the brain called the hypothalamus is a major integrator of metabolic and nutrient signals, adapting food intake and energy expenditure to current metabolic needs. Leptin or insulin receptors located in the plasma cell membrane of neurons sense energy signals from the body. They transmit this information inside the cell, which then regulates neuronal function. In this study, we show that leptin receptors interact with gangliosides, a class of plasma membrane lipids. This interaction is a prerequisite for proper receptor activation. Consequently, ganglioside loss in hypothalamic neurons inhibits leptin receptor signal transduction in response to energy metabolites. Furthermore, mice lacking gangliosides in distinct forebrain areas, amongst them the hypothalamus, develop progressive obesity and hypothermia. Our results suggest a previously unknown regulatory mechanism of plasma membrane lipids for hypothalamic control of body weight.

Arcuate NPY controls sympathetic output and BAT function via a relay of tyrosine hydroxylase neurons in the PVN

Neuropepetide Y (NPY) is best known for its powerful stimulation of food intake and its effects on reducing energy expenditure. However, the pathways involved and the regulatory mechanisms behind this are not well understood. Here we demonstrate that NPY derived from the arcuate nucleus (Arc) is critical for the control of sympathetic outflow and brown adipose tissue (BAT) function. Mechanistically, a key change induced by Arc NPY signaling is a marked Y1 receptor-mediated reduction in tyrosine hydroxylase (TH) expression in the hypothalamic paraventricular nucleus (PVN), which is also associated with a reduction in TH expression in the locus coeruleus (LC) and other regions in the brainstem. Consistent with this, Arc NPY signaling decreased sympathetically innervated BAT thermogenesis, involving the downregulation of uncoupling protein 1 (UCP1) expression in BAT. Taken together, these data reveal a powerful Arc-NPY-regulated neuronal circuit that controls BAT thermogenesis and sympathetic output via TH neurons.

Calorie-restricted weight loss reverses high-fat diet-induced ghrelin resistance, which contributes to rebound weight gain in a ghrelin-dependent manner

Twelve weeks of high-fat diet feeding causes ghrelin resistance in arcuate neuropeptide Y (NPY)/agouti-related protein (AgRP) neurons. In the current study, we investigated whether diet-induced weight loss could restore NPY/AgRP neuronal responsiveness to ghrelin and whether ghrelin mediates rebound weight gain after calorie-restricted (CR) weight loss. Diet-induced obese (DIO) mice were allocated to one of two dietary interventions until they reached the weight of age-matched lean controls. DIO mice received chow diet ad libitum or chow diet with 40% CR. Chow-fed and high-fat-fed mice served as controls. Both dietary interventions normalized body weight, glucose tolerance, and plasma insulin. We show that diet-induced weight loss with CR increases total plasma ghrelin, restores ghrelin sensitivity, and increases hypothalamic NPY and AgRP mRNA expression. We propose that long-term DIO creates a higher body weight set-point and that weight loss induced by CR, as seen in the high-fat CR group, provokes the brain to protect the new higher set-point. This adaptation to weight loss likely contributes to rebound weight gain by increasing peripheral ghrelin concentrations and restoring the function of ghrelin-responsive neuronal populations in the hypothalamic arcuate nucleus. Indeed, we also show that DIO ghrelin-knockout mice exhibit reduced body weight regain after CR weight loss compared with ghrelin wild-type mice, suggesting ghrelin mediates rebound weight gain after CR weight loss.

Cigarette smoking and brain regulation of energy homeostasis

Cigarette smoking is an addictive behavior, and is the primary cause of cardiovascular and pulmonary disease, and cancer (among other diseases). Cigarette smoke contains thousands of components that may affect caloric intake and energy expenditure, although nicotine is the major addictive substance present, and has the best described actions. Nicotine exposure from cigarette smoke can change brain feeding regulation to reduce appetite via both energy homeostatic and reward mechanisms, causing a negative energy state which is characterized by reduced energy intake and increased energy expenditure that are linked to low body weight. These findings have led to the public perception that smoking is associated with weight loss. However, its effects at reducing abdominal fat mass (a predisposing factor for glucose intolerance and insulin resistance) are marginal, and its promotion of lean body mass loss in animal studies suggests a limited potential for treatment in obesity. Smoking during pregnancy puts pressure on the mother's metabolic system and is a significant contributor to adverse pregnancy outcomes. Smoking is a predictor of future risk for respiratory dysfunction, social behavioral problems, cardiovascular disease, obesity, and type-2 diabetes. Catch-up growth is normally observed in children exposed to intrauterine smoke, which has been linked to subsequent childhood obesity. Nicotine can have a profound impact on the developing fetal brain, via its ability to rapidly and fully pass the placenta. In animal studies this has been linked with abnormal hypothalamic gene expression of appetite regulators such as downregulation of NPY and POMC in the arcuate nucleus of the hypothalamus. Maternal smoking or nicotine replacement leads to unhealthy eating habits (such as junk food addiction) and other behavioral disorders in the offspring.

Ingestion of Carbohydrate-Rich Supplements during Gestation Programs Insulin and Leptin Resistance but not Body Weight Gain in Adult Rat Offspring

Prenatal nutritional conditions can predispose to development of obesity and metabolic syndrome in adulthood. Gestation with its important modifications in hormonal status is a period of changes in normal feeding habits with pulses of consumption or avoidance of certain categories of food. We tried to mimic in an animal model some changes in food consumption patterns observed in pregnant women. For this purpose, Long–Evans female rats were fed during the dark period, their usual pre-gestational food quantity, and were allowed to complete their daily intake with either a restricted control (Cr), high-fat (HF), or high-carbohydrate (HC) diet available ad libitum during the light period. Dams fed a control diet ad libitum (Ca) served as controls. Body weight and composition, food intake, and metabolic hormones (insulin, leptin) were recorded in male offspring until 20 weeks after birth. Cr and HC females ate less than Ca females (−16%; p < 0.001) and their offspring presented a weight deficit from birth until 6 (HC group) and 10 (Cr group) weeks of age (p < 0.05 or less). Plasma leptin corresponded to low body weight in Cr offspring, but was increased in HC offspring that in addition, had increased plasma insulin, blood glucose, and subcutaneous adipose tissue mass. HF dams ate more than Ca dams (+13%; p < 0.001), but plasma leptin and insulin were similar in their offspring. Hypothalamic Ob-Rb expression was increased in Cr, HC, and HF offspring (+33–100% vs Ca; p < 0.05 or less). HC supplement ingestion during gestation therefore leads to insulin and leptin resistance in adult offspring independently of lower birth weight. These hormonal changes characterize obesity-prone animals. We therefore suggest that attention should be paid to the carbohydrate snacking and overall carbohydrate content in the diet during the last weeks (or months) preceding delivery to limit development of later metabolic disorders in offspring.

Macrophage inhibitory cytokine 1 (MIC-1/GDF15) decreases food intake, body weight and improves glucose tolerance in mice on normal & obesogenic diets

Food intake and body weight are controlled by a variety of central and peripheral factors, but the exact mechanisms behind these processes are still not fully understood. Here we show that that macrophage inhibitory cytokine-1 (MIC-1/GDF15), known to have anorexigenic effects particularly in cancer, provides protection against the development of obesity. Both under a normal chow diet and an obesogenic diet, the transgenic overexpression of MIC-1/GDF15 in mice leads to decreased body weight and fat mass. This lean phenotype was associated with decreased spontaneous but not fasting-induced food intake, on a background of unaltered energy expenditure and reduced physical activity. Importantly, the overexpression of MIC-1/GDF15 improved glucose tolerance, both under normal and high fat-fed conditions. Altogether, this work shows that the molecule MIC-1/GDF15 might be beneficial for the treatment of obesity as well as perturbations in glucose homeostasis.

Alterations to melanocortinergic, GABAergic and cannabinoid neurotransmission associated with olanzapine-induced weight gain

Background/Aim

Second generation antipsychotics (SGAs) are used to treat schizophrenia but can cause serious metabolic side-effects, such as obesity and diabetes. This study examined the effects of low to high doses of olanzapine on appetite/metabolic regulatory signals in the hypothalamus and brainstem to elucidate the mechanisms underlying olanzapine-induced obesity.

Methodology/Results

Levels of pro-opiomelanocortin (POMC), neuropeptide Y (NPY) and glutamic acid decarboxylase (GAD₆₅, enzyme for GABA synthesis) mRNA expression, and cannabinoid CB1 receptor (CB1R) binding density (using [³H]SR-141716A) were examined in the arcuate nucleus (Arc) and dorsal vagal complex (DVC) of female Sprague Dawley rats following 0.25, 0.5, 1.0 or 2.0 mg/kg olanzapine or vehicle (3×/day, 14-days). Consistent with its weight gain liability, olanzapine significantly decreased anorexigenic POMC and increased orexigenic NPY mRNA expression in a dose-sensitive manner in the Arc. GAD₆₅ mRNA expression increased and CB1R binding density decreased in the Arc and DVC. Alterations to neurotransmission signals in the brain significantly correlated with body weight and adiposity. The minimum dosage threshold required to induce weight gain in the rat was 0.5 mg/kg olanzapine.

Conclusions

Olanzapine-induced weight gain is associated with reduced appetite-inhibiting POMC and increased NPY. This study also supports a role for the CB1R and GABA in the mechanisms underlying weight gain side-effects, possibly by altering POMC transmission. Metabolic dysfunction can be modelled in the female rat using low, clinically-comparable olanzapine doses when administered in-line with the half-life of the drug.

Over-expression of leptin receptors in hypothalamic POMC neurons increases susceptibility to diet-induced obesity

Diet-induced obesity (DIO) in rodents is characterized by impaired activation of signal-transducer and activator of transcription 3 (STAT3) by leptin receptors (LepRb) within the hypothalamic arcuate nucleus. This signaling defect likely plays an important role in development of DIO. However, the neuro-chemical identity of the leptin-STAT3 resistant arcuate neurons has not been determined and the underlying mechanisms responsible for development of cellular leptin resistance remain unclear. To investigate this, we first measured arcuate gene expression of known key signaling components of the LepRb signaling pathway and tested whether specifically the critical arcuate pro-opiomelanocortin (POMC) neurons are resistant to LepRb-STAT3 signaling in mice given a high-fat-diet (HFD) compared to mice provided a low-fat control diet (LFD). We found that leptin-dependent STAT3 phosphorylation was decreased within POMC neurons of HFD mice. In addition, Leprb mRNA and suppressor of cytokine signaling 3 (Socs3) mRNA were elevated in the arcuate of HFD mice. To investigate whether increased LepRb expression per se in POMC neurons can influence development of DIO and Socs3 expression, we created mice that over-express LepRb selectively in POMC neurons (POMC-LepRb). No differences in body weight, fat mass or food intake were found between LFD POMC-LepRb mice and LFD controls. Surprisingly, body weight, fat mass and caloric intake of HFD POMC-LepRb mice was markedly higher than HFD control mice. In addition, arcuate Socs3 mRNA was increased in HFD POMC-LepRb mice compared to HFD controls. These data show that specifically POMC neurons of DIO mice are resistant to STAT3 activation by leptin, indicating that those cells might play a role in development of DIO. Furthermore, over-expression of LepRb selectively in POMC neurons increases susceptibility to the development of DIO. We propose a model where over-reactivity of the leptin-LepRb signaling system in arcuate neurons may play causal a role in development of diet-induced obesity.

Arcuate nucleus of hypothalamus is involved in mediating the satiety effect of electroacupuncture in obese rats

Obesity is a major health problem in the world. Since effective remedies are rare, researchers are trying to discover new therapies for obesity, and acupuncture is among the most popular alternative approaches. This study investigated the anti-obesity mechanisms of EA, using a rat model of diet-induced obesity. After feeding with a high-fat diet for 9 weeks, a number of rats who gained weight that surpassed the maximal body weight of rats in the chow-fed group were considered obese and employed in the study. A 2 Hz EA treatment at the acupoints ST36/SP6 with the intensity increasing stepwise from 0.5-1-1.5 mA was given once a day for 30 min. Rats treated with EA showed significantly decreased food intake and reduced body weight compared with the rats in DIO and restraint group. EA treatment increased peptide levels of α-MSH and mRNA levels of its precursor POMC in the arcuate nuclear of hypothalamus (ARH) neurons. In addition, the cerebral spinal fluid (CSF) content of α-MSH was elevated by EA application. ARH lesions by monosodium glutamate abolished the inhibition effect of EA on food intake and body weight. A non-acupoint stimulation did not show the benefit effect on food intake inhibition and body weight reduction compared with restraint and ST36/SP6 EA treatment. We concluded that EA treatment at ST36/SP6 acted through ARH to significantly inhibit food intake and body weight gain when fed a high-fat diet and that the stimulation of α-MSH expression and release might be involved in the mechanism.

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

Background

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

Methods

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

Results

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

Conclusion

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

Moderate long-term modulation of neuropeptide Y in hypothalamic arcuate nucleus induces energy balance alterations in adult rats

Neuropeptide Y (NPY) produced by arcuate nucleus (ARC) neurons has a strong orexigenic effect on target neurons. Hypothalamic NPY levels undergo wide-ranging oscillations during the circadian cycle and in response to fasting and peripheral hormones (from 0.25 to 10-fold change). The aim of the present study was to evaluate the impact of a moderate long-term modulation of NPY within the ARC neurons on food consumption, body weight gain and hypothalamic neuropeptides. We achieved a physiological overexpression (3.6-fold increase) and down-regulation (0.5-fold decrease) of NPY in the rat ARC by injection of AAV vectors expressing NPY and synthetic microRNA that target the NPY, respectively. Our work shows that a moderate overexpression of NPY was sufficient to induce diurnal over-feeding, sustained body weight gain and severe obesity in adult rats. Additionally, the circulating levels of leptin were elevated but the immunoreactivity (ir) of ARC neuropeptides was not in accordance (POMC-ir was unchanged and AGRP-ir increased), suggesting a disruption in the ability of ARC neurons to response to peripheral metabolic alterations. Furthermore, a dysfunction in adipocytes phenotype was observed in these obese rats. In addition, moderate down-regulation of NPY did not affect basal feeding or normal body weight gain but the response to food deprivation was compromised since fasting-induced hyperphagia was inhibited and fasting-induced decrease in locomotor activity was absent.These results highlight the importance of the physiological ARC NPY levels oscillations on feeding regulation, fasting response and body weight preservation, and are important for the design of therapeutic interventions for obesity that include the NPY.

Maternal protein restriction in mice causes adverse metabolic and hypothalamic effects in the F1 and F2 generations

Maternal protein restriction causes metabolic alterations associated with hypothalamic dysfunction. Because the consequences of metabolic programming can be passed transgenerationally, the present study aimed to assess whether maternal protein restriction alters the expression of hypothalamic neuropeptides in offspring and to evaluate hormonal and metabolic changes in male offspring from the F1 and F2 generations. Female Swiss mice (F0) were mated and fed either a normal-protein (NP group; 19 % protein) or a low-protein (LP group; 5 % protein) diet throughout gestation of the F1 generation (NP1 and LP1). At 3 months of age, F1 females were mated to produce the F2 generation (NP2 and LP2). Animals from all groups were evaluated at 16 weeks of age. LP1 offspring had significantly lower weights and shorter lengths than NP1 offspring at birth, but they underwent a phase of rapid catch-up growth. Conversely, the LP2 offspring were not significantly different from the NP2 offspring in either weight or length. At 16 weeks, no differences were found in body mass among any of the groups, although LP1 and LP2 offspring showed hypercholesterolaemia, hypertriacylglycerolaemia, hyperglycaemia, glucose intolerance, insulin resistance, increased levels of insulin, leptin and resistin, decreased endogenous leptin sensitivity, increased adiposity with elevated leptin levels and leptin resistance characterised by altered expression of neuropeptide Y and pro-opiomelanocortin without any changes in the leptin receptor Ob-Rb. We conclude that severe maternal protein restriction promotes metabolic programming in F1 and F2 male offspring due to a dysregulation of the adipoinsular axis and a state of hypothalamic leptin resistance.

Impaired intestinal afferent nerve satiety signalling and vagal afferent excitability in diet induced obesity in the mouse

Gastrointestinal vagal afferents transmit satiety signals to the brain via both chemical and mechanical mechanisms. There is indirect evidence that these signals may be attenuated in obesity. We hypothesized that responses to satiety mediators and distension of the gut would be attenuated after induction of diet induced obesity. Obesity was induced by feeding a high fat diet (60% kcal from fat). Low fat fed mice (10% kcal from fat) served as a control. High fat fed mice were obese, with increased visceral fat, but were not hyperglycaemic. Recordings from jejunal afferents demonstrated attenuated responses to the satiety mediators cholecystokinin (CCK, 100 nm) and 5-hydroxytryptamine (5-HT, 10 μm), as was the response to low intensity jejunal distension, while responses to higher distension pressures were preserved. We performed whole cell patch clamp recordings on nodose ganglion neurons, both unlabelled, and those labelled by fast blue injection into the wall of the jejunum. The cell membrane of both labelled and unlabelled nodose ganglion neurons was less excitable in HFF mice, with an elevated rheobase and decreased number of action potentials at twice rheobase. Input resistance of HFF neurons was also significantly decreased. Calcium imaging experiments revealed reduced proportion of nodose ganglion neurons responding to CCK and 5-HT in obese mice. These results demonstrate a marked reduction in afferent sensitivity to satiety related stimuli after a chronic high fat diet. A major mechanism underlying this change is reduced excitability of the neuronal cell membrane. This may explain the development of hyperphagia when a high fat diet is consumed. Improving sensitivity of gastrointestinal afferent nerves may prove useful to limit food intake in obesity.

An overview on how components of the melanocortin system respond to different high energy diets

High energy diets are used to model the obesity epidemic. Moreover, from a variety of genetic studies, it has become clear that the melanocortin system plays an important role in the regulation of energy metabolism. Since most dietary interventions are not standardized, fat/sugar-induced effects on the melanocortin system vary distinctly among different studies. How components of the melanocortin system are affected by high energy diets remains unclear. Therefore, in this review, we first present an overview of the effects of high energy diets on different elements of the melanocortin system in both mice and rats. The effects of a high energy diet are most consistent for agouti related protein levels which were either not affected or decreased after consumption of a high energy diet, whereas for proopiomelanocortin and the melanocortin receptor expression (and binding) it was difficult to define an overall response to a high energy diet. Because of the complexity of the melanocortin system, it is important to measure more than one element of the system. Only a few studies measured both melanocortin peptide and receptor expression and show that a high fat diet consumed for a longer period of time starting at an early age increases melanocortin signaling, whereas in adulthood a very high fat diet decreases melanocortin signaling. Finally, we review our own data on diet-induced changes in peptide expression and melanocortin binding and show that short term exposure to a free-choice high-fat high-sugar diet also decreases melanocortin signaling which supports hyperphagia observed in this model.

High-fat diet-induced obesity in animal models

Epidemiological studies have shown a positive relationship between dietary fat intake and obesity. Since rats and mice show a similar relationship, they are considered an appropriate model for studying dietary obesity. The present paper describes the history of using high-fat diets to induce obesity in animals, aims to clarify the consequences of changing the amount and type of dietary fats on weight gain, body composition and adipose tissue cellularity, and explores the contribution of genetics and sex, as well as the biochemical basis and the roles of hormones such as leptin, insulin and ghrelin in animal models of dietary obesity. The major factors that contribute to dietary obesity - hyperphagia, energy density and post-ingestive effects of the dietary fat - are discussed. Other factors that affect dietary obesity including feeding rhythmicity, social factors and stress are highlighted. Finally, we comment on the reversibility of high-fat diet-induced obesity.

GLP-1 treatment reduces endogenous insulin resistance via activation of central GLP-1 receptors in mice fed a high-fat diet

Glucagon-like peptide-1 (GLP-1) improves insulin sensitivity in humans and rodents. It is currently unknown to what extent the (metabolic) effects of GLP-1 treatment are mediated by central GLP-1 receptors. We studied the impact of central GLP-1 receptor (GLP-1R) antagonism on the metabolic effects of peripheral GLP-1 administration in mice. High-fat-fed insulin-resistant C57Bl/6 mice were treated with continuous subcutaneous infusion of GLP-1 or saline (PBS) for 2 wk, whereas the GLP-1R antagonist exendin-9 (EX-9) and cerebrospinal fluid (CSF) were simultaneously infused in the left lateral cerebral ventricle (icv). Glucose and glycerol turnover were determined during a hyperinsulinemic euglycemic clamp. VLDL-triglyceride (VLDL-TG) production was determined in hyperinsulinemic conditions. Our data show that the rate of glucose infusion necessary to maintain euglycemia was significantly increased by GLP-1. Simultaneous icv infusion of EX-9 diminished this effect by 62%. The capacities of insulin to stimulate glucose disposal and inhibit glucose production were reinforced by GLP-1. Simultaneous icv infusion of EX-9 significantly diminished the latter effect. Central GLP-1R antagonism alone did not affect glucose metabolism. Also, GLP-1 treatment reinforced the inhibitory action of insulin on VLDL-TG production. In conclusion, peripheral administration of GLP-1 reinforces the ability of insulin to suppress endogenous glucose and VLDL-TG production (but not lipolysis) and boosts its capacity to stimulate glucose disposal in high-fat-fed C57Bl/6 mice. Activation of central GLP-1Rs contributes substantially to the inhibition of endogenous glucose production by GLP-1 treatment in this animal model.

Neuropeptide Y and sex hormone interactions in humoral and neuronal regulation of bone and fat

The hypothalamus regulates the skeleton and adipose tissue via endocrine mechanisms. Changes in sex steroid levels in menopause and aging are central to the associated changes in bone mass and adiposity. Whereas many of these effects occur via direct actions on osteoblasts or adipocytes, sex hormones can also mediate effects on bone and adipose tissue via interaction with neuronal pathways. A key hypothalamic regulator of bone and adipose tissue is neuropeptide Y (NPY), which coordinately influences these tissues via effects on neuroendocrine and sympathetic nervous output. Better understanding of the interaction between NPY and sex steroids in regulating skeletal and energy homeostasis could lead to more effective treatments for osteoporosis and obesity.

Metabolic regulation of ghrelin O-acyl transferase (GOAT) expression in the mouse hypothalamus, pituitary, and stomach

Ghrelin acts as an endocrine link connecting physiological processes regulating food intake, body composition, growth, and energy balance. Ghrelin is the only peptide known to undergo octanoylation. The enzyme mediating this process, ghrelin O-acyltransferase (GOAT), is expressed in the gastrointestinal tract (GI; primary source of circulating ghrelin) as well as other tissues. The present study demonstrates that stomach GOAT mRNA levels correlate with circulating acylated-ghrelin levels in fasted and diet-induced obese mice. In addition, GOAT was found to be expressed in both the pituitary and hypothalamus (two target tissues of ghrelin's actions), and regulated in response to metabolic status. Using primary pituitary cell cultures as a model system to study the regulation of GOAT expression, we found that acylated-ghrelin, but not desacyl-ghrelin, increased GOAT expression. In addition, growth-hormone-releasing hormone (GHRH) and leptin increased, while somatostatin (SST) decreased GOAT expression. The physiologic relevance of these later results is supported by the observation that pituitary GOAT expression in mice lacking GHRH, SST and leptin showed opposite changes to those observed after in vitro treatment with the corresponding peptides. Therefore, it seems plausible that these hormones directly contribute to the regulation of pituitary GOAT. Interestingly, in all the models studied, pituitary GOAT expression paralleled changes in the expression of a dominant spliced-variant of ghrelin (In2-ghrelin) and therefore this transcript may be a primary substrate for pituitary GOAT. Collectively, these observations support the notion that the GI tract is not the only source of acylated-ghrelin, but in fact locally produced des-acylated-ghrelin could be converted to acylated-ghrelin within target tissues by locally active GOAT, to mediate its tissue-specific effects.

Chronic administration of brain-derived neurotrophic factor in the hypothalamic paraventricular nucleus reverses obesity induced by high-fat diet

An acute injection of brain-derived neurotrophic factor (BDNF) in the hypothalamic paraventricular nucleus (PVN) reduces body weight by decreasing feeding and increasing energy expenditure (EE), in animals on standard laboratory chow. Animals have divergent responses to a high-fat diet (HFD) exposure, with some developing obesity and others remaining lean. In the current study, we tested two hypotheses: 1) BDNF in the PVN reverses HFD-induced obesity, and 2) animals with higher body fat have a greater physiological response to BDNF than those with less body fat. Eighty-four 10-wk old rats were allowed HFD ad libitum for 9 wk and then prepared with bilateral PVN cannulas. Animals were then divided into tertiles based on their body fat rank: high, intermediate, and low (H, I, and L). Each group was further divided into 2 subgroups and then PVN injected with BDNF or control (artificial cerebrospinal fluid, aCSF) every other day for 3 wk. Energy intake (EI), body weight, and body composition were measured. At study's end, rats were killed to allow measurement of other metabolic indices. In parallel, another 12 rats were fed control diet (CD), PVN-cannulated and injected with aCSF. HFD exposure induced obesity, particularly in the H body fat group, with a significant increase in EI, body weight, fat mass, liver size, and serum glucose, triglycerides, insulin, and leptin. BDNF significantly reduced EI, body weight, body fat, lean mass, and serum metabolic indices. These BDNF effects were greatest in the H body fat group. These data indicate that BDNF reduced HFD-induced obesity and metabolic syndrome-like measures, and the animals with the most body fat had the most significant response to BDNF.

Mechanisms of amylin/leptin synergy in rodent models

The present studies aimed to identify mechanisms contributing to amylin/leptin synergy in reducing body weight and adiposity. We reasoned that if amylin/leptin harnessed complementary neuronal pathways, then in the leptin-sensitive state, amylin should augment leptin signaling/binding and that in the absence of endogenous amylin, leptin signaling should be diminished. Amylin (50 microg/kg, ip) amplified low-dose leptin-stimulated (15 microg/kg, ip) phosphorylated signal transducer and activator of transcription-3 signaling within the arcuate nucleus (ARC) in lean rats. Amylin (50 microg/kg x d) or leptin (125 microg/kg x d) infusion to lean rats decreased 28-d food intake (14 and 10%, respectively), body weight (amylin by 4.3%, leptin by 4.9%), and epididymal fat (amylin by 19%, leptin by 37%). Amylin/leptin co-infusion additively decreased food intake (by 26%) and reduced body weight (by 15%) and epididymal fat (by 78%; all P < 0.05 vs. all groups) in a greater than mathematically additive manner, consistent with synergy. Amylin increased leptin binding within the ventromedial hypothalamus (VMN) by 35% and dorsomedial hypothalamus by 47% (both P < 0.05 vs. vehicle). Amylin/leptin similarly increased leptin binding in the VMN by 40% and ARC by 70% (P < 0.05 vs. vehicle). In amylin-deficient mice, hypothalamic leptin receptor mRNA expression was reduced by 50%, leptin-stimulated phosphorylated signal transducer and activator of transcription-3 within ARC and VMN was reduced by 40%, and responsiveness to leptin's (1 mg/kg x d for 28 d) weight-reducing effects was attenuated (all P < 0.05 vs. wild-type controls). We suggest that amylin/leptin's marked weight- and fat-reducing effects are due to activation of intrinsic synergistic neuronal signaling pathways and further point to the integrated neurohormonal therapeutic potential of amylin/leptin agonism in obesity.

A free-choice high-fat high-sugar diet induces changes in arcuate neuropeptide expression that support hyperphagia

OBJECTIVES

The mechanisms for how saturated fat and sugar-based beverages contribute to human obesity are poorly understood. This paper describes a series of experiments developed to examine the response of hypothalamic neuropeptides to diets rich in sugar and fat, using three different diets: a high-fat high-sugar (HFHS) choice diet with access to chow, saturated fat and a 30% sugar solution; a high-fat (HF) choice diet with access to chow and saturated fat; or to a high-sugar (HS) choice diet with access to chow and a sugar solution.

METHOD

We first studied caloric intake, body weight gain, hormonal alterations and hypothalamic neuropeptide expression when male Wistar rats were subjected to an HFHS choice, an HF choice or an HS choice diet for 1 week. Next, we studied caloric intake and body weight gain when rats were subjected to the choice diets for 5 weeks. Finally, we measured neuropeptide expression in hepatic vagotomized rats subjected to an HFHS choice, an HF choice or an HS choice diet for 1 week.

RESULTS

In rats on an HF choice diet, plasma leptin concentrations and proopiomelanocortin (POMC) mRNA increased and neuropeptide Y (NPY) mRNA decreased. Rats on an HFHS choice diet showed identical plasma leptin concentrations as rats on an HF choice diet. However, NPY mRNA increased and POMC mRNA decreased. An HS choice diet for 1 week did not alter hypothalamic neuropeptide expression or plasma leptin concentrations. As hormonal changes did not explain the differences in hypothalamic neuropeptide expression between rats on the choice diets, we addressed whether neuronal feedback signals mediated the hypothalamic neuropeptide response. The POMC mRNA response to different diets depended on an intact innervation of liver and upper intestinal tract.

CONCLUSION

Our data suggest that the specific combination of saturated fat and a 30% sugar solution results in hyperphagia-induced obesity and alters hypothalamic neuropeptide expression, and that the response of the melanocortin system is mediated by the hepatic vagus.

Differential effects of restricted versus unlimited high-fat feeding in rats on fat mass, plasma hormones and brain appetite regulators

The rapid rise in obesity has been linked to altered food consumption patterns. There is increasing evidence that, in addition to total energy intake, the macronutrient composition of the diet may influence the development of obesity. The present study aimed to examine the impact of high dietary fat content, under both isocaloric and hypercaloric conditions, compared with a low fat diet, on adiposity, glucose and lipid metabolism, and brain appetite regulators in rats. Male Sprague-Dawley rats were exposed to one of three diets: control (14% fat), ad lib high-fat palatable (HFD, 35% fat) or high-fat palatable restricted (HFD-R, matched to the energy intake of control) and were killed in the fasting state 11 weeks later. Body weight was increased by 28% in unrestricted HFD fed rats, with an almost tripling of caloric intake and fat mass (P < 0.001) and double the plasma triglycerides of controls. Glucose intolerance and increased insulin levels were observed. HFD-R animals calorie matched to control had double their fat mass, plasma insulin and triglycerides (P < 0.05). Only ad lib consumption of the HFD increased the hypothalamic mRNA expression of the appetite-regulating peptides, neuropeptide Y and pro-opiomelanocortin. Although restricted consumption of palatable HFD had no significant impact on hypothalamic appetite regulators or body weight, it increased adiposity and circulating triglycerides, suggesting that the proportion of dietary fat, independent of caloric intake, affects fat deposition and the metabolic profile.

Leptin receptor gene expression and number in the brain are regulated by leptin level and nutritional status

Hormone potency depends on receptor availability, regulated via gene expression and receptor trafficking. To ascertain how central leptin receptors are regulated, the effects of leptin challenge, high-fat diet, fasting and refeeding were measured on leptin receptor number and gene expression. These were measured using quantitative (125)I-labelled leptin in vitro autoradiography and in situ hybridisation, respectively. Ob-R (all forms of leptin receptor) expression in the choroid plexus (CP) was unchanged by high-fat diet or leptin challenge, whereas fasting increased but refeeding failed to decrease expression. (125)I-labelled leptin binding to the CP was increased by fasting and returned to basal levels on refeeding. (125)I-Labelled leptin was reduced by leptin challenge and increased by high-fat feeding. Ob-Rb (signalling form) in the arcuate (ARC) and ventromedial (VMH) nuclei was increased after fasting and decreased by refeeding. Leptin challenge increased Ob-Rb expression in the ARC, but not after high-fat feeding. In general, changes in gene expression in the ARC and VMH appeared to be largely due to changes in area rather than density of labelling, indicating that the number of cells expressing Ob-Rb was the parameter that contributed most to these changes. Leptin stimulation of suppressor of cytokine signalling 3 (SOCS3), a marker of stimulation of the Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathway, was unchanged after high-fat diet. Thus, early loss of leptin sensitivity after high-fat feeding is unrelated to down-regulation of leptin receptor expression or number and does not involve the JAK/STAT pathway. The effect of leptin to decrease (125)I-labelled leptin binding and the loss of ability of leptin to up-regulate Ob-Rb expression in the ARC after high-fat feeding offer potential mechanisms for the development of leptin insensitivity in response to both hyperleptinaemia and high-fat diet.

A functional leptin system is essential for sodium tungstate antiobesity action

Sodium tungstate is a novel agent in the treatment of obesity. In diet-induced obese rats, it is able to reduce body weight gain by increasing energy expenditure. This study evaluated the role of leptin, a key regulator of energy homeostasis, in the tungstate antiobesity effect. Leptin receptor-deficient Zucker fa/fa rats and leptin-deficient ob/ob mice were treated with tungstate. In lean animals, tungstate administration reduced body weight gain and food intake and increased energy expenditure. However, in animals with deficiencies in the leptin system, treatment did not modify these parameters. In ob/ob mice in which leptin deficiency was restored through adipose tissue transplantation, treatment restored the tungstate-induced body weight gain and food intake reduction as well as energy expenditure increase. Furthermore, in animals in which tungstate administration increased energy expenditure, changes in the expression of key genes involved in brown adipose tissue thermogenesis were detected. Finally, the gene expression of the hypothalamic neuropeptides, Npy, Agrp, and Cart, involved in the leptin regulation of energy homeostasis, was also modified by tungstate in a leptin-dependent manner. In summary, the results indicate that the effectiveness of tungstate in reducing body weight gain is completely dependent on a functional leptin system.

Mechanisms mediating renal sympathetic activation to leptin in obesity

Leptin plays a critical role in the control of energy homeostasis. The sympathetic cardiovascular actions of leptin have emerged as a potential link between obesity and hypertension. We previously demonstrated that in mice, modest obesity induced by 10 wk of a high-fat diet is associated with preservation of leptin ability to increase renal sympathetic nerve activity (SNA) despite the resistance to the metabolic effects of leptin. Here, we examined whether selective leptin resistance exists in mice with late-stage diet-induced obesity (DIO) produced by 20 wk of a high-fat diet. The decrease in food intake and body weight induced by intraperitoneal or intracerebroventricular injection of leptin was significantly attenuated in the DIO mice. Regional SNA responses to intravenous leptin were also attenuated in DIO mice. In contrast, intracerebroventricularly administered leptin caused contrasting effects on regional SNA in DIO mice. Renal SNA response to intracerebroventricular leptin was preserved, whereas lumbar and brown adipose tissue SNA responses were attenuated. Intact renal SNA response to leptin combined with the increased cerebrospinal fluid leptin levels in DIO mice represents a potential mechanism for the adverse cardiovascular consequences of obesity. Lastly, we examined the role of phosphoinositol-3 kinase (PI3K) and melanocortin receptors (MCR) in mediating the preserved renal SNA response to leptin in obesity. Presence of PI3K inhibitor (LY294002) or MC3/4R antagonist (SHU9119) significantly attenuated the renal SNA response to leptin in DIO and agouti obese mice. Our results demonstrate the importance of PI3K and melanocortin receptors in the transduction of leptin-induced renal sympathetic activation in obesity.

Fat diet affects leptin receptor levels in the rat cerebellum

OBJECTIVE

The role of leptin receptors (Ob-Rs) within the hypothalamus in the control of energy expenditure has well been established. However, their role and regulation in other brain areas, including the cerebellum, is largely unexplored. In the present study we examined whether Ob-R levels in the rat cerebellum are influenced by a high-fat diet and if these changes are sexually divergent during adolescence.

METHODS

The fat diet (45% energy from fat) was applied from weaning to puberty (postnatal days [P] 22-42), from weaning to adulthood (P22-90), and from puberty to adulthood (P42-90) in female and male Wistar rats. Ob-R levels were detected by western blotting and the data from pubertal and adult rats were analyzed by two-way analysis of variance for the effects of diet and sex.

RESULTS

The fat diet affected Ob-R long isoform levels in a sexually dimorphic manner. In the cerebellum of all fat-fed male groups, Ob-R levels were reduced compared with their chow-fed counterparts (P < 0.05). In contrast, in female rat Ob-R levels were reduced only in the adult P22-90 group.

CONCLUSION

Our data show for the first time that Ob-R levels in the rat cerebellum are subject to diet-induced alterations and that these changes are sexually dimorphic.

Electroacupuncture in the treatment of obesity

Obesity is becoming one of the most common health problems in the world. Many other disorders, such as hypertension and diabetes are considered as the consequences of obesity. Since effective remedies are rare (only two drugs, Orlistat and Sibutramine, were officially approved by the US Food and Drug Administration for long-term obesity treatment so far), researchers are trying to discover new therapies for obesity, and acupuncture is among the most popular alternative approaches. To facilitate weight reduction, one can use manual acupuncture, electroacupuncture (EA) or transcutaneous electrical acupoint stimulation (TEAS). As the parameters of the EA or TEAS can be precisely characterized and the results are more or less reproducible, this review will focus on EA as a treatment modality for obesity. Results obtained in this laboratory in recent five years will be summarized in some detail.

Models and mechanisms of energy balance regulation in the young

The proportion of the child and adolescent population that is in appropriate energy balance is declining throughout the developed world, and childhood obesity is a particular problem in the UK relative to other northern European countries. Assessment of the underlying causes of obesity, and the different routes to its development, may assist in the definition of successful intervention strategies. The network of peripheral and central (brain) regulatory systems that underlie energy balance and body weight and composition can, for the most part, only be approached experimentally through the study of appropriate laboratory animal models. This problem is particularly acute when the target is overweight and obesity in the young. Some of the mechanisms underlying the development of energy imbalance and specifically the onset of overweight and obesity in the young, and the metabolic health consequences of obesity, can be addressed by examination of experimental rodent models in which mutation of a single gene causes early-onset extreme obesity, genetic susceptibility to obesity is revealed in an obesogenic environment or early-life nutritional experience programmes susceptibility to obesity or metabolic problems in later life. These studies highlight genes that are essential to normal body-weight regulation in rodents and man, the impact of diet and diet-induced obesity on regulatory systems in the young and the potential sensitivity of developing regulatory systems to nutritional experiences in utero and during early life.

High-fat diet-induced changes in body mass and hypothalamic gene expression in wild-type and leptin-deficient mice

We tested whether diet-induced obesity results from increased energy consumption, is associated with changes in expression of genes involved in leptin signal transduction, and is altered by hyperleptinemia. C57BL/6 mice were fed a low-fat diet (LFD) or high-fat diet (HFD) for up to 15 weeks. HFD mice weighed significantly more than LFD controls by 3 weeks, despite consuming less energy. HFD mice had significantly greater leptin, insulin, and glucose levels than LFD mice, suggesting leptin and insulin resistance. Adiponectin levels declined with age but were unaffected by diet. HFD was associated with altered hypothalamic expression of genes whose products regulate the activity or nuclear translocation of STAT3, an important mediator of leptin actions. Expression of two isoforms of the leptin receptor decreased at 15 weeks in hypothalami of HFD mice in a tissue-specific manner. The type of fat (saturated versus unsaturated) did not influence weight gain on an HFD, but animals on LFD gained significantly more weight and adiposity if the dietary fat consisted mostly of saturated fats; this occurred despite no difference in energy consumption or absorption. Replacement of leptin to leptin-deficient ob/ob mice decreased hypothalamic leptin receptor expression and did not prevent HFD-induced weight gain. It is concluded that (1) increased energy consumption is not required for HFD-induced obesity in C57BL/6 mice, (2) HFD results in weight gain partly by modulating hypothalamic leptin-signaling pathways, (3) saturated fats induce weight gain even when total fat content of the diet is low, and (4) the effects of HFD are manifest in the presence or absence of circulating leptin.

Effects of high-fat diet and/or body weight on mammary tumor leptin and apoptosis signaling pathways in MMTV-TGF-alpha mice

INTRODUCTION

Obesity is a risk factor for postmenopausal breast cancer and is associated with shortened mammary tumor (MT) latency in MMTV-TGF-alpha mice with dietary-induced obesity. One link between obesity and breast cancer is the adipokine, leptin. Here, the focus is on diet-induced obesity and MT and mammary fat pad (MFP) leptin and apoptotic signaling proteins.

METHODS

MMTV-TGF-alpha mice were fed low-fat or high-fat diets from 10 to 85 weeks of age. High-Fat mice were divided into Obesity-Prone and Obesity-Resistant groups based on final body weights. Mice were followed to assess MT development and obtain serum, MFP, and MT.

RESULTS

Incidence of palpable MTs was significantly different: Obesity-Prone > Obesity-Resistant > Low-Fat. Serum leptin was significantly higher in Obesity-Prone compared with Obesity-Resistant and Low-Fat mice. Low-Fat mice had higher MFP and MT ObRb (leptin receptor) protein and Jak2 (Janus kinase 2) protein and mRNA levels in comparison with High-Fat mice regardless of body weight. Leptin (mRNA) and pSTAT3 (phosphorylated signal transducer and activator of transcription 3) (mRNA and protein) also were higher in MTs from Low-Fat versus High-Fat mice. Expression of MT and MFP pro-apoptotic proteins was higher in Low-Fat versus High-Fat mice.

CONCLUSION

These results confirm a connection between body weight and MT development and between body weight and serum leptin levels. However, diet impacts MT and MFP leptin and apoptosis signaling proteins independently of body weight.

Silencing of OB-RGRP in mouse hypothalamic arcuate nucleus increases leptin receptor signaling and prevents diet-induced obesity

Obesity is a major public health problem and is often associated with type 2 diabetes mellitus, cardiovascular disease, and metabolic syndrome. Leptin is the crucial adipostatic hormone that controls food intake and body weight through the activation of specific leptin receptors (OB-R) in the hypothalamic arcuate nucleus (ARC). However, in most obese patients, high circulating levels of leptin fail to bring about weight loss. The prevention of this "leptin resistance" is a major goal for obesity research. We report here a successful prevention of diet-induced obesity (DIO) by silencing a negative regulator of OB-R function, the OB-R gene-related protein (OB-RGRP), whose transcript is genetically linked to the OB-R transcript. We provide in vitro evidence that OB-RGRP controls OB-R function by negatively regulating its cell surface expression. In the DIO mouse model, obesity was prevented by silencing OB-RGRP through stereotactic injection of a lentiviral vector encoding a shRNA directed against OB-RGRP in the ARC. This work demonstrates that OB-RGRP is a potential target for obesity treatment. Indeed, regulators of the receptor could be more appropriate targets than the receptor itself. This finding could serve as the basis for an approach to identifying potential new therapeutic targets for a variety of diseases, including obesity.

Modulation of central leptin sensitivity and energy balance in a rat model of diet-induced obesity

AIM

The aim of this study was to further explore the time-dependent changes in leptin sensitivity using a rat model of dietary fat-induced obesity and to investigate the potential mechanisms governing these changes.

METHODS

We used male, adult Sprague-Dawley rats that were fed either a standard laboratory chow diet (3% fat) or a high-saturated fat (HF) diet (60% fat) for 2 or 5 weeks. Energy balance (body weight, energy intake and energy expenditure); sensitivity to central leptin and central alpha-melanin stimulating hormone (alpha-MSH) administration and expression levels of hypothalamic ObRb, signal transducers and activators of transcription factor (STAT)-3 phosphorylation, suppressor of cytokine signalling-3 (SOCS-3), proopiomelanocortin (POMC) processing hormones (prohormone convertase-1 and prohormone convertase-2) and neuropeptide Y (NPY) were measured.

RESULTS

After 2 weeks of feeding HF diet, there was an increase in total energy intake (TEI) but a reduction in food intake as measured by the mass of food ingested. Body weight at this time was not significantly different between the two diet groups; however, white adipose tissue (WAT) weight was significantly greater in the HF-fed rats than in the chow-fed rats. In addition, spontaneous physical activity levels were increased, but no changes were observed in resting energy expenditure. Furthermore, chow-fed lean rats responded to central leptin administration by reducing the energy intake by approximately 67 kJ compared with saline treatment (p < 0.05), while the HF-fed diet-induced obese (DIO) rats responded by reducing their energy intake by approximately 197 kJ compared with saline treatment (p < 0.05). After 5 weeks of feeding HF diet, TEI remained significantly higher, body weight was significantly increased by 5% in the HF-fed rats and WAT weight was significantly heavier in HF-fed rats than in the chow-fed lean rats. After leptin treatment, the chow-fed lean rats reduced their energy intake by approximately 97 kJ (p < 0.05); yet, leptin had no significant effect in the HF-fed DIO rats. ObRb protein expression, STAT-3 phosphorylation levels, content and messenger RNA (mRNA) expression of NPY, SOCS-3 mRNA and protein expression and energy intake response to central alpha-MSH administration were not altered after HF diet feeding.

CONCLUSION

These results suggest that early in the course of HF diet-induced weight gain, there was a period of central leptin hypersensitivity, and as the obesity progresses, central leptin insensitivity develops. This insensitivity does not appear to be explained by a downregulation of ObRb protein levels, reduced leptin signalling, an increase in either SOCS-3 or NPY expression or reduced function of the melanocortin system. The effect of an HF diet on other actions of leptin such as its effect on the endocannabinoid system should be investigated.

Association of single-nucleotide polymorphisms in MTMR9 gene with obesity

Genetic factors are clearly involved in the development of obesity, but the genetic background of obesity remains largely unclear. Starting from 62 663 gene-based single-nucleotide polymorphisms (SNPs) in three sequential case-control association studies, we identified a replicated association between the obesity phenotype (BMI > or =30 kg/m(2)) and a SNP (rs2293855) located in the myotublarin-related protein 9 (MTMR9) gene in the chromosomal segment 8p23-p22. P-values (minor allele dominant model) of the first set (93 cases versus 649 controls) and the second set (564 cases versus 562 controls) were 0.008 and 0.0002, respectively. The association was replicated in the third set [394 cases versus 958 controls, P = 0.005, odds ratio (95% CI) =1.40 (1.11-1.78)]. The global P-value was 0.0000005. A multiple regression analysis revealed that gender, age BMI and rs2293855 genotype (minor allele dominant model) were significantly associated with both systolic and diastolic blood pressures. MTMR9 was shown to be the only gene within the haplotype block that contained SNPs associated with obesity. Both the transcript and protein of MTMR9 were detected in the rodent lateral hypothalamic area as well as in the arcuate nucleus, and the protein co-existed with orexin, melanin concentrating hormone, neuropeptide Y and proopiomelanocortin. The levels of MTMR9 transcript in the murine hypothalamic region increased after fasting and were decreased by a high-fat diet. Our data suggested that genetic variations in MTMR9 may confer a predisposition towards obesity and hypertension through regulation of hypothalamic neuropeptides.

Leptin levels and body composition of mice selectively bred for high voluntary locomotor activity

Selective breeding produced four replicate lines of high-runner (HR) mice that run on wheels for approximately 2.7 times more revolutions per day than four unselected control lines. Previous studies found that HR mice of both sexes have lower body fat (isotope dilution at 15 wk of age) and that males (females not studied) have smaller retroperitoneal fat pads (17 wk). HR mice also exhibit elevated plasma corticosterone and insulin-stimulated glucose uptake by some hindlimb muscles but apparently do not differ in circulating insulin or glucose levels (males at 18 wk). Given their lower body fat and higher activity levels, we hypothesized that HR mice would have lower circulating leptin levels than controls. Female mice were given wheel access for 6 d at 7 wk of age, as part of the routine wheel testing for the selective breeding protocol, and then were killed after one additional week without wheels to reduce possible acute effects of activity on leptin. As hypothesized, serum leptin levels were significantly lower in HR mice. ANCOVA indicated that leptin was strongly positively correlated with both total body fat (measured by ether extraction) and body mass change from weaning, but HR mice still had significantly lower adjusted leptin levels (ANCOVA). Within HR lines but not within control lines, individual variation in leptin levels was negatively correlated with amount or speed of wheel running measured a week before being killed. Growth from weaning to euthanasia and body dry mass were lower in HR mice than in controls, but absolute dry masses of the ventricles, liver, gut, and uterus plus ovaries did not significantly differ, nor did percentage of the total dry mass as fat. HR mice offer a novel model for studying the causes and consequences of physiologically relevant variations in serum leptin.

Hormonal induction of leptin resistance during pregnancy

Despite elevated plasma leptin, food intake is increased during pregnancy leading to fat deposition. We have demonstrated that intracerebroventricular (icv) leptin is unable to suppress food intake in pregnant rats, as it does in non-pregnant animals. Hence, central leptin resistance develops during pregnancy. These changes are physiologically appropriate, providing increased energy reserves to help meet the high metabolic demands of fetal development and lactation. To characterise this central leptin resistance, we have measured levels of leptin receptor (Ob-Rb) mRNA in the hypothalamus, and examined leptin-induced phosphorylation of STAT3 (pSTAT3) in specific regions of the hypothalamus. In addition, to investigate the mechanism underlying pregnancy-induced leptin resistance, we have investigated effects of hormone treatments on hypothalamic responses to leptin in a pseudopregnant rat model. We observed a significant reduction of Ob-Rb mRNA levels in the ventromedial hypothalamic nucleus (VMH) during pregnancy, with no changes detected in other hypothalamic nuclei. Levels of leptin-induced pSTAT3 were specifically suppressed in the VMH and arcuate nucleus of pregnant rats compared to non-pregnant rats. Pseudopregnant rats were hyperphagic but did not become leptin resistant, suggesting that fetal or placental factors are required for the induction of leptin resistance. These data implicate the VMH as a key hypothalamic site involved in hormone-induced leptin resistance during pregnancy, and suggest that placental hormone secretion may mediate the hormone-induced loss of response to leptin.

Differential body weight and feeding responses to high-fat diets in rats and mice lacking cholecystokinin 1 receptors

Prior data demonstrated differential roles for cholecystokinin (CCK)1 receptors in maintaining energy balance in rats and mice. CCK1 receptor deficiency results in hyperphagia and obesity of Otsuka Long-Evans Tokushima Fatty (OLETF) rats but not in mice. To ascertain the role of CCK1 receptors in high-fat-diet (HFD)-induced obesity, we compared alterations in food intake, body weight, fat mass, plasma glucose, and leptin levels, and patterns of hypothalamic gene expression in OLETF rats and mice lacking CCK1 receptors in response to a 10-wk exposure to HFD. Compared with Long-Evans Tokushima Otsuka (LETO) control rats, OLETF rats on HFD had sustained overconsumption over the 10-wk period. High fat feeding resulted in greater increases in body weight and plasma leptin levels in OLETF than in LETO rats. In situ hybridization determinations revealed that, while HFD reduced neuropeptide Y (NPY) mRNA expression in both the arcuate nucleus (Arc) and the dorsomedial hypothalamus (DMH) of LETO rats, HFD resulted in decreased NPY expression in the Arc but not in the DMH of OLETF rats. In contrast to these results in OLETF rats, HFD increased food intake and induced obesity to an equal degree in both wild-type and CCK1 receptor(-/-) mice. NPY gene expression was decreased in the Arc in response to HFD, but was not detectable in the DMH in both wild-type and CCK1 receptor(-/-) mice. Together, these data provide further evidence for differential roles of CCK1 receptors in the controls of food intake and body weight in rats and mice.

The intake of physiological doses of leptin during lactation in rats prevents obesity in later life

BACKGROUND

There is epidemiological evidence that perinatal nutritional factors may have long-term effects on obesity. Which nutrients or food components are involved in this programming mechanism are unknown. Breast milk contains leptin, a hormone that regulates food intake and energy expenditure, and previous studies in rats have shown that leptin orally administered during lactation exerts anorexigenic effects.

OBJECTIVE

To evaluate whether supplementation with physiological doses of oral leptin during lactation has long-term effects on body weight regulation.

DESIGN

A daily oral dose of leptin (equivalent to five times the amount of leptin ingested normally from maternal milk during the suckling period) or the vehicle was given to suckling male rats during lactation. Animals were fed after weaning with a normal fat (NF) or a high-fat (HF) diet. We followed body weight and food intake of animals until the age of 6 months, and measured the size of adipose tissue depots, the thermogenic capacity, the expression of leptin in the stomach and adipose tissues and the expression of two appetite-related peptides (neuropeptide Y (NPY) and proopiomelanocortin (POMC)), leptin receptor (OB-Rb) and suppressor of cytokine signalling 3 (SOCS-3) in the hypothalamus at the age of 6 months.

RESULTS

Leptin-treated animals had, in adulthood, lower body weight and fat content and ate fewer calories than their untreated controls. Unlike adipocitary leptin production, adult animals that were leptin-treated during lactation displayed higher gastric leptin production without changes in OB-Rb mRNA levels. In addition, in response to HF diet, leptin-treated animals (contrary to controls) showed lower hypothalamic NPY/POMC mRNA ratio. Hypothalamic OB-Rb mRNA levels decreased in control animals as an effect of HF diet feeding, but remained unchanged in leptin-treated animals; SOCS-3 mRNA levels were lower in leptin-treated animals than in their controls, both under normal or HF diet.

CONCLUSION

The animals that received leptin during lactation become more protected against fat accumulation in adult life and seem to be more sensitive to the short- and long-term regulation of food intake by leptin. Thus, leptin plays an important role in the earlier stages of neonatal life, as a component of breast milk, in the prevention of later obesity.