Hypothalamic phosphatidylinositol 3-kinase pathway of leptin signaling is impaired during the development of diet-induced obesity in FVB/N mice

Oligonol enhances brain cognitive function in high-fat diet-fed mice

Oligonol, a low-molecular-weight polyphenol derived from lychee fruit, is well recognized for its antioxidant properties, blood glucose regulation, and fat mass reduction capability. However, its effect on the central nervous system remains unclear. Here, we investigated the effects of oligonol on brain in a high-fat diet (HFD) fed mouse model, and SH-SY5Y neuronal cells and primary cultured cortical neuron under insulin resistance conditions. HFD mice were orally administered oligonol (20 mg/kg) daily, and SH-SY5Y cells and primary cortical neurons were pretreated with 500 ng/mL oligonol under in vitro insulin resistance conditions. Our findings revealed that oligonol administration reduced blood glucose levels and improved spatial memory function in HFD mice. In vitro data demonstrated that oligonol protected neuronal cells and enhanced neural structure against insulin resistance. We confirmed RNA sequencing in the oligonol-pretreated insulin-resistant SH-SY5Y neuronal cells. Our RNA-sequencing data indicated that oligonol contributes to metabolic signaling and neurite outgrowth. In conclusion, our study provides insights into therapeutic potential of oligonol with respect to preventing neuronal cell damage and improving neural structure and cognitive function in HFD mice.

Hypothalamic-Ovarian axis and Adiposity Relationship in Polycystic Ovary Syndrome: Physiopathology and Therapeutic Options for the Management of Metabolic and Inflammatory Aspects

PURPOSE OF REVIEW

The goal of the present review is to address the main adiposity-related alterations in Polycystic Ovary Syndrome (PCOS) focusing on hypothalamic-pituitary-ovarian (H-P-O) axis and to provide an overview of nutraceutical and pharmacological therapeutic strategies.

RECENT FINDINGS

Female reproduction is a complex and delicate interplay between neuroendocrine signals involving the H-P-O axis. Elements that disrupt the balance of these interactions can lead to metabolic and reproductive disorders, such as PCOS. This disorder includes menstrual, metabolic, and biochemical abnormalities as well as hyperandrogenism, oligo-anovulatory menstrual cycles, insulin resistance, and hyperleptinemia which share an inflammatory state with other chronic diseases. Moreover, as in a self-feeding cycle, high androgen levels in PCOS lead to visceral fat deposition, resulting in insulin resistance and hyperinsulinemia, further stimulating ovarian and adrenal androgen production. In fact, regardless of age and BMI, women with PCOS have more adipose tissue and less lean mass than healthy women. Excessive adiposity, especially visceral adiposity, is capable of affecting female reproduction through direct mechanisms compromising the luteal phase, and indirect mechanisms as metabolic alterations able to affect the function of the H-P-O axis. The intricate crosstalk between adiposity, inflammatory status and H-P-O axis function contributes to the main adiposity-related alterations in PCOS, and alongside currently available hormonal treatments, nutraceutical and pharmacological therapeutic strategies can be exploited to treat these alterations, in order to enable a more comprehensive synergistic and tailored treatment.

The Mechanism of Leptin Resistance in Obesity and Therapeutic Perspective

Leptin resistance is induced via leptin signaling blockade by chronic overstimulation of the leptin receptor and intracellular signaling defect or increased hypothalamic inflammation and suppressor of cytokine signaling (SOCS)-3 expression. High-fat diet triggers leptin resistance induced by at least two independent causes: first, the limited ability of peripheral leptin to activate hypothalamic signaling transducers and activators of transcription (STAT) signaling and secondly a signaling defect in leptin-responsive hypothalamic neurons. Central leptin resistance is dependent on decreased leptin transport efficiency across the blood brain barrier (BBB) rather than hypothalamic leptin insensitivity. Since the hypothalamic phosphorylated STAT3 (pSTAT3) represents a sensitive and specific readout of leptin receptor-B signaling, the assessment of pSTAT3 levels is the gold standard. Hypertriglyceridemia is one of important factors to inhibit the transport of leptin across BBB in obesity. Mismatch between high leptin and the amount of leptin receptor expression in obesity triggers brain leptin resistance via increasing hypothalamic inflammation and SOCS-3 expression. Therapeutic strategies that regulate the passage of leptin to the brain include the development of modifications in the structure of leptin analogues as well as the synthesis of new leptin receptor agonists with increased BBB permeability. In the hyperleptinemic state, polyethylene glycol (PEG)-modified leptin is unable to pass through the BBB. Peripheral histone deacetylase (HDAC) 6 inhibitor, tubastatin, and metformin increase central leptin sensitization. While add-on therapy with anagliptin, metformin and miglitol reduce leptin concentrations, the use of long-acting leptin analogs, and exendin-4 lead to the recovery of leptin sensitivity. Contouring surgery with fat removal, and bariatric surgery independently of the type of surgery performed provide significant improvement in leptin concentrations. Although approaches to correcting leptin resistance have shown some success, no clinically effective application has been developed to date. Due to the impairment of central and peripheral leptin signaling, as well as the extensive integration of leptin-sensitive metabolic pathways with other neurons, the effectiveness of methods used to eliminate leptin resistance is extremely limited.

The role of obesity and bariatric surgery-induced weight loss in breast cancer

Obesity is a complex metabolic condition considered a worldwide public health crisis, and a deeper mechanistic understanding of obesity-associated diseases is urgently needed. Obesity comorbidities include many associated cancers and are estimated to account for 20% of female cancer deaths in the USA. Breast cancer, in particular, is associated with obesity and is the focus of this review. The exact causal links between obesity and breast cancer remain unclear. Still, interactions have emerged between body mass index, tumor molecular subtype, genetic background, and environmental factors that strongly suggest obesity influences the risk and progression of certain breast cancers. Supportive preclinical research uses various diet-induced obesity models to demonstrate that weight loss, via dietary interventions or changes in energy expenditure, reduces the onset or progression of breast cancers. Ongoing and future studies are now aimed at elucidating the underpinning mechanisms behind weight-loss-driven observations to improve therapy and outcomes in patients with breast cancer and reduce risk. This review aims to summarize the rapidly emerging literature on obesity and weight loss strategies with a focused discussion of bariatric surgery in both clinical and preclinical studies detailing the complex interactions between metabolism, immune response, and immunotherapy in the setting of obesity and breast cancer.

An orally active plant Rubisco-derived peptide increases neuronal leptin responsiveness

Nutrient excess, such as the intake of a high-fat diet, reduces hypothalamic responses to exogenously administered leptin and induces dietary obesity; however, orally active components that attenuate neural leptin dysregulation have yet to be identified. We herein demonstrated that YHIEPV, derived from the pepsin-pancreatin digestion of the green leaf protein Rubisco, increased the leptin-induced phosphorylation of STAT3 in ex vivo hypothalamic slice cultures. We also showed that YHIEPV mitigated palmitic acid-induced decreases in leptin responsiveness. Furthermore, orally administered YHIEPV promoted leptin-induced reductions in body weight and food intake in obese mice. In addition, dietary-induced body weight gain was significantly less in mice orally or centrally administered YHIEPV daily than in saline-control mice. Cellular leptin sensitivity and the levels of proinflammatory-related factors, such as IL1β and Socs-3, in the hypothalamus of obese mice were also restored by YHIEPV. YHIEPV blocked cellular leptin resistance induced by forskolin, which activates Epac-Rap1 signaling, and reduced the level of the GTP-bound active form of Rap1 in the brains of obese mice. Collectively, the present results demonstrated that the orally active peptide YHIEPV derived from a major green leaf protein increased neural leptin responsiveness and reduced body weight gain in mice with dietary obesity.

Increased Variation in Body Weight and Food Intake Is Related to Increased Dietary Fat but Not Increased Carbohydrate or Protein in Mice

A variety of inbred mouse strains have been used for research in metabolic disorders. Despite being inbred, they display large inter-individual variability for many traits like food intake and body weight. However, the relationship between dietary macronutrients and inter-individual variation in body weight and food intake of different mouse strains is still unclear. We investigated the association between macronutrient content of the diet and variations in food intake, body composition, and glucose tolerance by exposing five different mouse strains (C57BL/6, BALB/c, C3H, DBA/2, and FVB) to 24 different diets with variable protein, fat, and carbohydrate contents. We found only increasing dietary fat, but not protein or carbohydrate had a significant association (positive) with variation in both food intake and body weight. The highest variation in both body weight and food intake occurred with 50% dietary fat. However, there were no significant relationships between the variation in fat and lean mass with dietary protein, fat, or carbohydrate levels. In addition, none of the dietary macronutrients had significant impacts on the variation in glucose tolerance ability in C57BL/6 mice. In conclusion, the variations in food intake and body weight changes increased with the elevation of dietary fat levels.

Impaired leptin responsiveness in the nucleus accumbens of leptin-overexpressing transgenic mice with dysregulated sucrose and lipid preference independent of obesity

While hypothalamic leptin resistance can occur prior to establishment of obesity, clarification is needed as to whether the impaired response to leptin in the reward-related nuclei occurs independently of obesity. To answer this question, we attempted to dissociate the normally coexisting leptin resistance from obesity. We investigated phenotypes of leptin-overexpressing transgenic mice fed for 1 week with 60 % high-fat diet (HFD) (LepTg-HFD1W mice). After 1 week, we observed that LepTg-HFD1W mice weighed as same as wild type (WT) mice fed standard chow diet (CD) for 1 week (WT-CD1W mice). However, compared to WT-CD1W mice, LepTg-HFD1W mice exhibited attenuated leptin-induced anorexia, decreased leptin-induced c-fos immunostaining in nucleus accumbens (NAc), one of important site of reward system, decreased leptin-stimulated pSTAT3 immunostaining in hypothalamus. Furthermore, neither sucrose nor lipid preference was suppressed by leptin in LepTg-HFD1W mice. On the contrary, leptin significantly suppressed both preferences in WT mice fed HFD (WT-HFD1 W mice). These results indicate that leptin responsiveness decreases in NAc independently of obesity. Additionally, in this situation, suppressive effect of leptin on the hedonic feeding results in impaired regulation. Such findings suggest the impaired leptin responsiveness in NAc partially contributes to dysregulated hedonic feeding behavior independently of obesity.

Does Modern Lifestyle Favor Neuroimmunometabolic Changes? A Path to Obesity

Factors linked to modern lifestyles, such as physical inactivity, Western diet, and poor sleep quality have been identified as key contributors to the positive energy balance (PEB). PEB rises adipose tissue hypertrophy and dysfunction over the years, affecting cells and tissues that are metabolically critical for energy homeostasis regulation, especially skeletal muscle, hypothalamic-pituitary-adrenal axis, and gut microbiota. It is known that the interaction among lifestyle factors and tissue metabolic dysfunction increases low-grade chronic systemic inflammation, leading to insulin resistance and other adverse metabolic disorders. Although immunometabolic mechanisms are widely discussed in obesity, neuroimmunoendocrine pathways have gained notoriety, as a link to neuroinflammation and central nervous system disorders. Hypothalamic inflammation has been associated with food intake dysregulation, which comprises homeostatic and non-homeostatic mechanisms, promoting eating behavior changes related to the obesity prevalence. The purpose of this review is to provide an updated and integrated perspective on the effects of Western diet, sleep debt, and physical exercise on the regulation of energy homeostasis and low-grade chronic systemic inflammation. Subsequently, we discuss the intersection between systemic inflammation and neuroinflammation and how it can contribute to energy imbalance, favoring obesity. Finally, we propose a model of interactions between systemic inflammation and neuroinflammation, providing new insights into preventive and therapeutic targets for obesity.

Mice held at an environmental photic cycle oscillating at their tau-like period length do not show the high-fat diet-induced obesity that develops under the 24-hour photic cycle

Circadian disruptions precede high-fat diet (HFD)-induced obesity (DIO). Deviation of the endogenous circadian rhythm period length (tau) from 24 hours correlates with mice inter-strain DIO under the 24-hour light-dark cycle (T-cycle). Additionally, entrainment to a tau-resembling T-cycle attenuates DIO, to some extent, in muted mice. These phenomena suggest that entrainment to a 24-hour T-cycle promotes DIO beyond that expected from the HFD-induced metabolic disruptions. However, the hypothesis that entrainment to a tau-resembling T-cycle attenuates DIO has not been tested in wild-type mice. Therefore, we examined, in newborn female FVB/N mice, whether DIO found under their 'regular' 24-hour T-cycle is attenuated under a T-cycle oscillating at their tau-resembling period of 23.7 h, which is diverted from 24 hours by only 0.3 h. Compared to mice fed a low-fat diet, those fed an HFD under the 24-hour T-cycle showed a disrupted pattern of circadian locomotor activity prior to DIO onset. Both these phenomena were absent under the tau-like T-cycle. DIO developed under the 24-hour T-cycle despite similar caloric intake, and was associated with the lower locomotor activity of HFD-fed mice compared to the other mouse groups. These results demonstrated that DIO is secondary to HFD-induced circadian disruptions that are not harmonized by the strongest Zeitgeber (light-dark cycle) when oscillating at a period that diverts by as little as ca. 0.3-h from tau. More importantly, imposing a light-dark cycle oscillating at a tau-like period length, which enhances entrainment and presumably reinforces endogenous circadian rhythms, prevented HFD-induced circadian disruptions and enabled tighter control of energy homeostasis, as manifested by the absence of DIO, even under ad-lib HFD feeding. These results support the identification of tau-related biomarkers, which may be considered as risk-factors for DIO. Moreover, these findings may promote the development of clock-related pharmaceutical interventions that will reduce the gap between tau and 24 hours, and increase the robustness of the endogenous and entrained circadian rhythms. This will enable reducing DIO, even without caloric restriction or time-restricted feeding.

Nutritional modulation of leptin expression and leptin action in obesity and obesity-associated complications

In obesity, an elevated accumulation and dysregulation of adipose tissue, due to an imbalance between energy intake and energy expenditure, usually coexists with the loss of responsiveness to leptin in central nervous system, and subsequently with hyperleptinemia. Leptin, a peptide hormone mainly produced by white adipose tissue, regulates energy homeostasis by stimulating energy expenditure and inhibiting food intake. Human obesity is characterized by increased plasma leptin levels, which have been related with different obesity-associated complications, such as chronic inflammatory state (risk factor for diabetes, cardiovascular and autoimmune diseases), as well as infertility and different types of cancer. Besides, leptin is also produced by placenta, and high leptin levels during pregnancy may be related with some pathological conditions such as gestational diabetes. This review focuses on the current insights and emerging concepts on potentially valuable nutrients and food components that may modulate leptin metabolism. Notably, several dietary food components, such as phenols, peptides, and vitamins, are able to decrease inflammation and improve leptin sensitivity by up- or down-regulation of leptin signaling molecules. On the other hand, some food components, such as saturated fatty acids may worsen chronic inflammation increasing the risk for pathological complications. Future research into nutritional mechanisms that restore leptin metabolism and signals of energy homeostasis may inspire new treatment options for obesity-related disorders.

DHA reduces hypothalamic inflammation and improves central leptin signaling in mice

AIMS

Anti-obesity effects and improved leptin sensitivity from n-3 polyunsaturated fatty acids (n-3 PUFAs) have been reported in diet-induced obese animals. This study sought to determine the beneficial central effects and mechanism of docosahexaenoic acid (DHA, 22:6 n-3) in high-fat (HF) diet fed mice.

MAIN METHODS

Male C57BL/6J mice were given HF diet with or without intracerebroventricular (icv) injection of docosahexaenoic acid (DHA, 22:6 n-3) for two days. Central leptin sensitivity, hypothalamic inflammation, leptin signaling molecules and tyrosine hydroxylase (TH) were examined by central leptin sensitivity test and Western blot. Furthermore, the expression of hepatic genes involved in lipid metabolism was examined by RT-PCR.

KEY FINDINGS

We found that icv administration of DHA not only reduced energy intake and body weight gain but also corrected the HF diet-induced hypothalamic inflammation. DHA decreased leptin signaling inhibitor SOCS3 and improved the leptin JAK2-Akt signaling pathways in the hypothalamus. Furthermore, icv administration of DHA improved the effects of leptin in the regulation of mRNA expression of enzymes related to lipogenesis, fatty acid β-oxidation, and cholesterol synthesis in the liver. DHA increased leptin-induced activation of TH in the hypothalamus.

SIGNIFICANCE

Therefore, increasing central DHA concentration may prevent the deficit of hypothalamic regulation, which is associated with disorders of energy homeostasis in the liver as a result of a high-fat diet.

The Phlorotannin-Rich Fraction of Ecklonia cava Extract Attenuated the Expressions of the Markers Related with Inflammation and Leptin Resistance in Adipose Tissue

Obesity is associated with systemic chronic inflammation, and it induces central leptin resistance which blocks the appetite-suppressing effect of leptin and leptin resistance in adipocytes. In the present study, we evaluated the effects of Ecklonia cava extract (ECE), which contained rich phlorotannins, on inflammation and leptin resistance in the adipose tissue of a diet-induced obese model. Effects of ECE on fat deposition, inflammation, M1/M2 macrophage, and T-cell infiltrations were investigated, and leptin resistance and SOCS3 were also measured in adipose tissue. Furthermore, ECE attenuated the expression of inflammation-related receptors such as TLR4 and RAGE and leptin resistance by reducing SOCS3 expression, increasing expression of leptin receptor in adipose tissue, and increasing lipolysis. ECE showed antiadiposity and anti-inflammatory effects, attenuated leptin resistance, and increased lipolysis in the diet-induced obese model. This study shows that ECE is a suitable dietary supplement candidate for the prevention or treatment of obesity or obesity-associated diseases, especially inflammation-related diseases.

Postnatal administration of leptin antagonist mitigates susceptibility to obesity under high-fat diet in male αMUPA mice

Perturbations in postnatal leptin signaling have been associated with altered susceptibility to diet-induced obesity (DIO) under high-fat-diet (HFD), albeit with contradicting evidence. Previous studies have shown that alpha murine urokinase-type plasminogen activator (αMUPA) mice have a higher and longer postnatal leptin surge compared with their wild types (WTs) as well as lower body weight and food intake under regular diet (RD). Here we explored αMUPA's propensity for DIO and the effect of attenuating postnatal leptin signaling with leptin antagonist (LA) on energy homeostasis under both RD and HFD. Four-day-old αMUPA pups were treated on alternate days until postnatal day 18 with either vehicle or LA (10 or 20 mg·day^-1·kg^-1) and weaned into RD or HFD. Compared with RD-fed αMUPA males, HFD-fed αMUPA males showed higher energy intake, even when corrected for body weight difference, and became hyperinsulinemic and obese. Additionally, HFD-fed αMUPA males gained body weight at a higher rate than their WTs mainly because of strain differences in energy expenditure. LA administration did not affect strain differences under RD but attenuated αMUPA's hyperinsulinemia and DIO under HFD, most likely by mediating energy expenditure. Together with our previous findings, these results suggest that αMUPA's leptin surge underlies its higher susceptibility to obesity under HFD, highlighting the role of leptin-related developmental processes in inducing obesity in a postweaning obesogenic environment, at least in αMUPA males. This study therefore supports the use of αMUPA mice for elucidating developmental mechanisms of obesity and the efficacy of early-life manipulations via leptin surge axis in attenuating DIO.

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

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

Neuronal Rap1 Regulates Energy Balance, Glucose Homeostasis, and Leptin Actions

The CNS contributes to obesity and metabolic disease; however, the underlying neurobiological pathways remain to be fully established. Here, we show that the small GTPase Rap1 is expressed in multiple hypothalamic nuclei that control whole-body metabolism and is activated in high-fat diet (HFD)-induced obesity. Genetic ablation of CNS Rap1 protects mice from dietary obesity, glucose imbalance, and insulin resistance in the periphery and from HFD-induced neuropathological changes in the hypothalamus, including diminished cellular leptin sensitivity and increased endoplasmic reticulum (ER) stress and inflammation. Furthermore, pharmacological inhibition of CNS Rap1 signaling normalizes hypothalamic ER stress and inflammation, improves cellular leptin sensitivity, and reduces body weight in mice with dietary obesity. We also demonstrate that Rap1 mediates leptin resistance via interplay with ER stress. Thus, neuronal Rap1 critically regulates leptin sensitivity and mediates HFD-induced obesity and hypothalamic pathology and may represent a potential therapeutic target for obesity treatment.

Pharmacological Inhibition of c-Jun N-terminal Kinase Reduces Food Intake and Sensitizes Leptin's Anorectic Signaling Actions

The role for c-Jun N-terminal Kinase (JNK) in the control of feeding and energy balance is not well understood. Here, by use of novel and highly selective JNK inhibitors, we investigated the actions of JNK in the control of feeding and body weight homeostasis. In lean mice, intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) administration of SR-3306, a brain-penetrant and selective pan-JNK (JNK1/2/3) inhibitor, reduced food intake and body weight. Moreover, i.p. and i.c.v. administrations of SR11935, a brain-penetrant and JNK2/3 isoform-selective inhibitor, exerted similar anorectic effects as SR3306, which suggests JNK2 or JNK3 mediates aspect of the anorectic effect by pan-JNK inhibition. Furthermore, daily i.p. injection of SR3306 (7 days) prevented the increases in food intake and weight gain in lean mice upon high-fat diet feeding, and this injection paradigm reduced high-fat intake and obesity in diet-induced obese (DIO) mice. In the DIO mice, JNK inhibition sensitized leptin’s anorectic effect, and enhanced leptin-induced STAT3 activation in the hypothalamus. The underlying mechanisms likely involve the downregulation of SOCS3 by JNK inhibition. Collectively, our data suggest that JNK activity promotes positive energy balance, and the therapeutic intervention inhibiting JNK activities represents a promising approach to ameliorate diet-induced obesity and leptin resistance.

Diet-Induced Obesity and the Mechanism of Leptin Resistance

Leptin signaling blockade by chronic overstimulation of the leptin receptor or hypothalamic pro-inflammatory responses due to elevated levels of saturated fatty acid can induce leptin resistance by activating negative feedback pathways. Although, long form leptin receptor (Ob-Rb) initiates leptin signaling through more than seven different signal transduction pathways, excessive suppressor of cytokine signaling-3 (SOCS-3) activity is a potential mechanism for the leptin resistance that characterizes human obesity. Because the leptin-responsive metabolic pathways broadly integrate with other neurons to control energy balance, the methods used to counteract the leptin resistance has extremely limited effect. In this chapter, besides the impairment of central and peripheral leptin signaling pathways, limited access of leptin to central nervous system (CNS) through blood-brain barrier, mismatch between high leptin and the amount of leptin receptor expression, contradictory effects of cellular and circulating molecules on leptin signaling, the connection between leptin signaling and endoplasmic reticulum (ER) stress and self-regulation of leptin signaling has been discussed in terms of leptin resistance.

The role of leptin in central nervous system diseases

Leptin is a peptide hormone produced by adipose tissue and acts in brain centers to control critical physiological functions. Leptin receptors are especially abundant in the hypothalamus and trigger specific neuronal subpopulations, and activate several intracellular signaling events, including the JAK/STAT, MAPK, PI3K, and mTOR pathway. Although most studies focus on its role in energy intake and expenditure, leptin also plays a critical role in many central nervous system diseases.

Bardoxolone methyl prevents obesity and hypothalamic dysfunction

High-fat (HF) diet-induced obesity is associated with hypothalamic leptin resistance and low grade chronic inflammation, which largely impairs the neuroregulation of negative energy balance. Neuroregulation of negative energy balance is largely controlled by the mediobasal and paraventricular nuclei regions of the hypothalamus via leptin signal transduction. Recently, a derivative of oleanolic acid, bardoxolone methyl (BM), has been shown to have anti-inflammatory effects. We tested the hypothesis that BM would prevent HF diet-induced obesity, hypothalamic leptin resistance, and inflammation in mice fed a HF diet. Oral administration of BM via drinking water (10 mg/kg daily) for 21 weeks significantly prevented an increase in body weight, energy intake, hyperleptinemia, and peripheral fat accumulation in mice fed a HF diet. Furthermore, BM treatment prevented HF diet-induced decreases in the anorexigenic effects of peripheral leptin administration. In the mediobasal and paraventricular nuclei regions of the hypothalamus, BM administration prevented HF diet-induced impairments of the downstream protein kinase b (Akt) pathway of hypothalamic leptin signalling. BM treatment also prevented an increase in inflammatory cytokines, tumour necrosis factor alpha (TNFα) and interleukin 6 (IL-6) in these two hypothalamic regions. These results identify a potential novel neuropharmacological application for BM in preventing HF diet-induced obesity, hypothalamic leptin resistance, and inflammation.

Hypothalamic Phosphodiesterase 3B Pathway Mediates Anorectic and Body Weight-Reducing Effects of Insulin in Male Mice

BACKGROUND

Insulin action in the hypothalamus plays a critical role in the regulation of energy homeostasis, yet the intracellular signaling mechanisms mediating insulin action are incompletely understood. Although phosphodiesterase 3B (PDE3B) mediates insulin action in the adipose tissue and it is highly expressed in the hypothalamic areas implicated in energy homeostasis, its role, if any, in mediating insulin action in the hypothalamus is unknown. We tested the hypothesis that insulin action in the hypothalamus is mediated by PDE3B.

METHODS

Using enzymatic assay, we examined the effects of peripheral or central administration of insulin on hypothalamic PDE3B activity in adult mice. Western blotting and immunohistochemistry also examined p-Akt and p-STAT3 levels in the hypothalamus. Effects of leptin on these parameters were also compared. We injected cilostamide, a PDE3 inhibitor, prior to central injection of insulin and examined the 12- to 24-hour food intake and 24-hour body weight. Finally, we examined the effect of cilostamide on insulin-induced proopiomelanocortin (Pomc), neurotensin (Nt), neuropeptide Y (Npy) and agouti-related peptide (Agrp) gene expression in the hypothalamus by qPCR.

RESULTS

Peripheral or central injection of insulin significantly increased PDE3B activity in the hypothalamus in association with increased p-Akt levels but without any change in p-STAT3 levels. However, leptin-induced increase in PDE3B activity was associated with an increase in both p-Akt and p-STAT3 levels in the hypothalamus. Prior administration of cilostamide reversed the anorectic and body weight-reducing effects as well as stimulatory effect of insulin on hypothalamic Pomc mRNA levels. Insulin did not alter Nt, Npy and Agrp mRNA levels.

CONCLUSION

Insulin induction of hypothalamic PDE3B activity and the reversal of the anorectic and body weight-reducing effects and stimulatory effect of insulin on hypothalamic Pomc gene expression by cilostamide suggest that activation of PDE3B is a novel mechanism of insulin signaling in the hypothalamus.

Metformin Reduces Desmoplasia in Pancreatic Cancer by Reprogramming Stellate Cells and Tumor-Associated Macrophages

Background

Pancreatic ductal adenocarcinoma (PDAC) is a highly desmoplastic tumor with a dismal prognosis for most patients. Fibrosis and inflammation are hallmarks of tumor desmoplasia. We have previously demonstrated that preventing the activation of pancreatic stellate cells (PSCs) and alleviating desmoplasia are beneficial strategies in treating PDAC. Metformin is a widely used glucose-lowering drug. It is also frequently prescribed to diabetic pancreatic cancer patients and has been shown to associate with a better outcome. However, the underlying mechanisms of this benefit remain unclear. Metformin has been found to modulate the activity of stellate cells in other disease settings. In this study, we examine the effect of metformin on PSC activity, fibrosis and inflammation in PDACs.

Methods/Results

In overweight, diabetic PDAC patients and pre-clinical mouse models, treatment with metformin reduced levels of tumor extracellular matrix (ECM) components, in particular hyaluronan (HA). In vitro, we found that metformin reduced TGF-ß signaling and the production of HA and collagen-I in cultured PSCs. Furthermore, we found that metformin alleviates tumor inflammation by reducing the expression of inflammatory cytokines including IL-1β as well as infiltration and M2 polarization of tumor-associated macrophages (TAMs) in vitro and in vivo. These effects on macrophages in vitro appear to be associated with a modulation of the AMPK/STAT3 pathway by metformin. Finally, we found in our preclinical models that the alleviation of desmoplasia by metformin was associated with a reduction in ECM remodeling, epithelial-to-mesenchymal transition (EMT) and ultimately systemic metastasis.

Conclusion

Metformin alleviates the fibro-inflammatory microenvironment in obese/diabetic individuals with pancreatic cancer by reprogramming PSCs and TAMs, which correlates with reduced disease progression. Metformin should be tested/explored as part of the treatment strategy in overweight diabetic PDAC patients.

Role of MCP-1 on inflammatory processes and metabolic dysfunction following high-fat feedings in the FVB/N strain

Background

MCP-1 is known to be an important chemokine for macrophage recruitment. Thus, targeting MCP-1 may prevent the perturbations associated with macrophage-induced inflammation in adipose tissue. However, inconsistencies in the available animal literature have questioned the role of this chemokine in this process. The purpose of this study was to examine the role of MCP-1 on obesity-related pathologies.

Methods

Wild-type (WT) and MCP-1 deficient mice on an FVB/N background were assigned to either low-fat-diet (LFD) or high-fat-diet (HFD) treatment for a period of 16 weeks. Body weight and body composition were measured weekly and monthly, respectively. Fasting blood glucose and insulin, and glucose tolerance were measured at 16 weeks. Macrophages, T cell markers, inflammatory mediators, and markers of fibrosis were examined in the adipose tissue at sacrifice.

Results

As expected, HFD increased adiposity (body weight, fat mass, fat percent, and adipocyte size), metabolic dysfunction (impaired glucose metabolism and insulin resistance) macrophage number (CD11b⁺F480⁺ cells, and gene expression of EMR1 and CD11c), T cell markers (gene expression of CD4 and CD8), inflammatory mediators (pNFκB and pJNK, and mRNA expression of MCP-1, CCL5, CXCL14, TNF-α, and IL-6), and fibrosis (expression of IL-10, IL-13, TGF-β, and MMP2) (P<0.05). However, contrary to our hypothesis, MCP-1 deficiency exacerbated many of these responses resulting in a further increase in adiposity (body weight, fat mass, fat percent and adipocyte size), metabolic dysregulation, macrophage markers (EMR1), inflammatory cell infiltration, and fibrosis (formation of type I and III collagens, mRNA expression of IL-10 and MMP2) (P<0.05).

Conclusions

These data suggest that MCP-1 may be a necessary component of the inflammatory response required for adipose tissue protection, remodeling, and healthy expansion in the FVB/N strain in response to HFD feedings.

Leptin receptor expressing neurons express phosphodiesterase-3B (PDE3B) and leptin induces STAT3 activation in PDE3B neurons in the mouse hypothalamus

Leptin signaling in the hypothalamus is critical for normal food intake and body weight regulation. Cumulative evidence suggests that besides the signal transducer and activator of transcription-3 (STAT3) pathway, several non-STAT3 pathways including the phosphodiesterase-3B (PDE3B) pathway mediate leptin signaling in the hypothalamus. We have shown that PDE3B is localized in various hypothalamic sites implicated in the regulation of energy homeostasis and that the anorectic and body weight reducing effects of leptin are mediated by the activation of PDE3B. It is still unknown if PDE3B is expressed in the long form of the leptin-receptor (ObRb)-expressing neurons in the hypothalamus and whether leptin induces STAT3 activation in PDE3B-expressing neurons. In this study, we examined co-localization of PDE3B with ObRb neurons in various hypothalamic nuclei in ObRb-GFP mice that were treated with leptin (5mg/kg, ip) for 2h. Results showed that most of the ObRb neurons in the arcuate nucleus (ARC, 93%), ventromedial nucleus (VMN, 94%), dorsomedial nucleus (DMN, 95%), ventral premammillary nucleus (PMv, 97%) and lateral hypothalamus (LH, 97%) co-expressed PDE3B. We next examined co-localization of p-STAT3 and PDE3B in the hypothalamus in C57BL6 mice that were treated with leptin (5mg/kg, ip) for 1h. The results showed that almost all p-STAT3 positive neurons in different hypothalamic nuclei including ARC, VMN, DMN, LH and PMv areas expressed PDE3B. These results suggest the possibility for a direct role for the PDE3B pathway in mediating leptin action in the hypothalamus.

New insights in leptin resistance mechanisms in mice

Leptin resistance is one of the main challenges of obesity. To date, two levels of resistance have been identified, first a decreased rate of leptin uptake into the brain and secondly a diminished central response to leptin. New findings have identified the mechanisms of leptin transport and demonstrated that it can be rescued in obesity, but it did not overcome the problem of central resistance. Alteration in the actions of leptin following diet-induced obesity (DIO) appears to be a multifactorial condition. Several phosphatases are inhibiting leptin signaling pathways in a pathological way. Besides, hypothalamic inflammation alters the neuronal circuits that control metabolism. Recent studies describing both mechanisms (inhibition of leptin signaling and inflammation), have provided key insights to potential new targets for treatment. However, recent data showing that DIO mice may conserve a cellular and physiological response to endogenous leptin, highlights the need to redefine the concept of "leptin resistance".

Arachidonic acid impairs hypothalamic leptin signaling and hepatic energy homeostasis in mice

Epidemiological evidence suggests that the consumption of a diet high in n-6 polyunsaturated fatty acids (PUFA) is associated with the development of leptin resistance and obesity. We aim to examine the central effect of n-6 PUFA, arachidonic acid (ARA) on leptin sensitivity and leptin-regulated hepatic glucose and lipid metabolism. We found that intracerebroventricular injection of ARA (25 nmol/day) for 2.5 days reversed the effect of central leptin on hypothalamic JAK2, pSTAT3, pAkt, and pFOXO1 protein levels, which was concomitant with a pro-inflammatory response in the hypothalamus. ARA also attenuated the effect of central leptin on hepatic glucose and lipid metabolism by reversing the mRNA expression of the genes involved in gluconeogenesis (G6Pase, PEPCK), glucose transportation (GLUT2), lipogenesis (FAS, SCD1), and cholesterol synthesis (HMG-CoA reductase). These results indicate that an increased exposure to central n-6 PUFA induces central cellular leptin resistance with concomitant defective JAK2-STAT3 and PI3K-Akt signaling.

Phosphodiesterase-3B-cAMP pathway of leptin signalling in the hypothalamus is impaired during the development of diet-induced obesity in FVB/N mice

The phosphodiesterase-3B (PDE3B)-cAMP pathway plays an important role in transducing the action of leptin in the hypothalamus. Obesity is usually associated with hyperleptinaemia and resistance to anorectic and body weight-reducing effects of leptin. To determine whether the hypothalamic PDE3B-cAMP pathway of leptin signalling is impaired during the development of diet-induced obesity (DIO), we fed male FVB/N mice a high-fat diet (HFD: 58% kcal as fat) or low-fat diet (LFD: 6% kcal as fat) for 4 weeks. HFD fed mice developed DIO in association with hyperphagia, hyperleptinaemia and hyperinsulinaemia. Leptin (i.p.) significantly increased hypothalamic PDE3B activity and phosphorylated (p)-Akt levels in LFD-fed but not in HFD-fed mice. However, basal p-Akt levels in hypothalamus were increased in DIO mice. Additionally, amongst six-microdissected brain nuclei examined, leptin selectively decreased cAMP levels in the arcuate nucleus (ARC) of LFD-fed mice but failed to do so in HFD-fed mice. We next tested whether both the PBE3B and Akt pathways of leptin signalling remained impaired in DIO mice on the HFD for 12 weeks (long-term). DIO mice were hyperinsulinaemic and hyperleptinaemic in association with impaired glucose and insulin tolerance. Although, in LFD-fed mice, leptin significantly increased PDE3B activity and p-Akt levels in the hypothalamus, it failed to do so in HFD-fed mice. Also, basal p-Akt levels in the hypothalamus were increased in DIO mice and leptin had no further effect. Similarly, immunocytochemistry showed that leptin increased the number of p-Akt-positive cells in the ARC of LFD-fed but not in HFD-fed mice, and there was an increased basal number of p-Akt positive cells in the ARC of DIO mice. These results suggest that the PDE3B-cAMP- and Akt-pathways of leptin signalling in the hypothalamus are impaired during the development of DIO. Thus, a defect in the regulation by leptin of the hypothalamic PDE3B-cAMP pathway and Akt signalling may be one of the mechanisms of central leptin resistance and the development of DIO.

Palmitic acid induces central leptin resistance and impairs hepatic glucose and lipid metabolism in male mice

The consumption of diets rich in saturated fat largely contributes to the development of obesity in modern societies. A diet high in saturated fats can induce inflammation and impair leptin signaling in the hypothalamus. However, the role of saturated fatty acids on hypothalamic leptin signaling, and hepatic glucose and lipid metabolism remains largely undiscovered. In this study, we investigated the effects of intracerebroventricular (icv) administration of a saturated fatty acid, palmitic acid (PA, C16:0), on central leptin sensitivity, hypothalamic leptin signaling, inflammatory molecules and hepatic energy metabolism in C57BL/6J male mice. We found that the icv administration of PA led to central leptin resistance, evidenced by the inhibition of central leptin's suppression of food intake. Central leptin resistance was concomitant with impaired hypothalamic leptin signaling (JAK2-STAT3, PKB/Akt-FOXO1) and a pro-inflammatory response (TNF-α, IL1-β, IL-6 and pIκBa) in the mediobasal hypothalamus and paraventricular hypothalamic nuclei. Furthermore, the pre-administration of icv PA blunted the effect of leptin-induced decreases in mRNA expression related to gluconeogenesis (G6Pase and PEPCK), glucose transportation (GLUT2) and lipogenesis (FAS and SCD1) in the liver of mice. Therefore, elevated central PA concentrations can induce pro-inflammatory responses and leptin resistance, which are associated with disorders of energy homeostasis in the liver as a result of diet-induced obesity.

Temporal relationship between diet-induced steatosis and onset of insulin/leptin resistance in male Wistar rats

Rats fed with high-fat-high-sucrose (HFHS) diet are known to manifest metabolic syndrome including hyperinsulinemia, hyperleptinemia, hyperglycemia, diabetic dyslipidemia, and hepatic steatosis. The aim of the current study is to determine the temporal relationships between the development of hepatic steatosis and the onset of insulin and leptin resistance in hypothalamus and liver in male Wistar rats (six weeks of age) fed chow or HFHS diet for up to 8 weeks. Fasting plasma glucose, lipids/lipoproteins, insulin and leptin levels were quantified, histopathologic score of hepatic steatosis and inflammation were assessed, and the responses of common checkpoints of insulin and leptin signalling responsible for lipogenesis and gluconeogenesis were analyzed. In addition, acute insulin or leptin administration was performed at different stages of HFHS dieting to determine the responsiveness of the respective signalling pathways. Hyperinsulinemia, hyperglycemia, dyslipidemia, and increased homeostasis model assessment of basal insulin resistance occurred 1-week after HFHS dieting, coinciding with upregulation of suppressor of cytokine signalling 3 in both hypothalamus and liver. However, hepatosteatosis, accompanied with increased expression of sterol regulatory element binding protein 1c and phosphoenolpyruvate carboxykinase, did not manifest until 4- to 8-week after HFHS dieting. Lowered insulin sensitivity (shown by decreased insulin receptor substrate 1 and protein kinase B phosphorylation) occurred approximately 2 weeks prior to leptin resistance (shown by impaired signal transducer and activator of transcription 3 activation) in both the liver and hypothalamus. Acute insulin/leptin administration also demonstrated the impaired insulin or leptin signalling transduction. These data suggest that lowered insulin sensitivity and leptin resistance occurred at least 2–3 weeks earlier than the manifestation of hepatosteatosis in rats fed HFHS diet.

Inflammatory Signaling Involved in High-Fat Diet Induced Prostate Diseases

High-Fat Diet (HFD) has emerged as an important risk factor not only for obesity and diabetes but also for urological disorders. Recent research provides ample evidence that HFD is a putative cause for prostatic diseases including prostate cancer. The mechanisms whereby these diseases develop in the prostate have not been fully elucidated. In this review we discuss signaling pathways intricately involved in HFD-induced prostate disease. We performed a search through PUBMED using key words "high fat diet" and "prostate". Our data and perspectives are included in this review along with research performed by various other groups. HFD is positively associated with an increased risk of benign prostatic hyperplasia (BPH) and prostate cancer. HFD induces oxidative stress and inflammation in the prostate gland, and these adverse influences transform it from a normal to a diseased state. Studies demonstrate that HFD accelerates the generation of reactive oxygen species by driving the NADPH oxidase system, exacerbating oxidative stress in the prostate. HFD also causes a significant increase in the levels of pro-inflammatory cytokines and gene products through activation of two important signaling pathways: the Signal Transducer and Activator of Transcription (STAT)-3 and Nuclear Factor-kappa B (NF-κB). Both these pathways function as transcription factors required for regulating genes involved in proliferation, survival, angiogenesis, invasion and inflammation. The crosstalk between these two pathways enhances their regulatory function. Through its influences on the NF-κB and Stat-3 signaling pathways, it appears likely that HFD increases the risk of development of BPH and prostate cancer.

You are what you eat, or are you? The challenges of translating high-fat-fed rodents to human obesity and diabetes

Obesity and type 2 diabetes mellitus (T2DM) are rapidly growing worldwide epidemics with major health consequences. Various human-based studies have confirmed that both genetic and environmental factors (particularly high-caloric diets and sedentary lifestyle) greatly contribute to human T2DM. Interactions between obesity, insulin resistance and β-cell dysfunction result in human T2DM, but the mechanisms regulating the interplay among these impairments remain unclear. Rodent models of high-fat diet (HFD)-induced obesity have been used widely to study human obesity and T2DM. With >9000 publications on PubMed over the past decade alone, many aspects of rodent T2DM have been elucidated; however, correlation to human obesity/diabetes remains poor. This review investigates the reasons for this translational discrepancy by critically evaluating rodent HFD models. Dietary modification in rodents appears to have limited translatable benefit for understanding and treating human obesity and diabetes due—at least in part—to divergent dietary compositions, species/strain and gender variability, inconsistent disease penetrance, severity and duration and lack of resemblance to human obesogenic pathophysiology. Therefore future research efforts dedicated to acquiring translationally relevant data—specifically human data, rather than findings based on rodent studies—would accelerate our understanding of disease mechanisms and development of therapeutics for human obesity/T2DM.

Adipose tissue insulin resistance due to loss of PI3K p110α leads to decreased energy expenditure and obesity

Adipose tissue is a highly insulin-responsive organ that contributes to metabolic regulation. Insulin resistance in the adipose tissue affects systemic lipid and glucose homeostasis. Phosphoinositide 3-kinase (PI3K) mediates downstream insulin signaling in adipose tissue, but its physiological role in vivo remains unclear. Using Cre recombinase driven by the aP2 promoter, we created mice that lack the class 1A PI3K catalytic subunit p110α or p110β specifically in the white and brown adipose tissue. The loss of p110α, not p110β, resulted in increased adiposity, glucose intolerance and liver steatosis. Mice lacking p110α in adipose tissue exhibited a decrease in energy expenditure but no change in food intake or activity compared with control animals. This low energy expenditure is a consequence of low cellular respiration in the brown adipocytes caused by a decrease in expression of key mitochondrial genes including uncoupling protein-1. These results illustrate a critical role of p110α in the regulation of energy expenditure through modulation of cellular respiration in the brown adipose tissue and suggest that compromised insulin signaling in adipose tissue might be involved in the onset of obesity.

Advances in understanding the interrelations between leptin resistance and obesity

Obesity, which has developed into a global epidemic, is a risk factor in most chronic diseases and some forms of malignancy. The discovery of leptin in 1994 has opened a new field in obesity research. Currently, we know that leptin is the primary signal from energy stores and exerts negative feedback effects on energy intake. However, most individuals with diet-induced obesity (DIO) develop leptin resistance, which is characterized by elevated circulating leptin levels and decreased leptin sensitivity. To date, though various mechanisms have been proposed to explain leptin resistance, the exact mechanisms of leptin resistance in obesity are poorly understood. Consequently, it's an important issue worth discussing regarding what the exact interrelations between leptin resistance and obesity are. Here, we review the latest advancements in the molecular mechanisms of leptin resistance and the exact interrelations between leptin resistance, obesity, and obesity-related diseases, in order to supply new ideas for the study of obesity.

High fat diet produces brain insulin resistance, synaptodendritic abnormalities and altered behavior in mice

Insulin resistance and other features of the metabolic syndrome are increasingly recognized for their effects on cognitive health. To ascertain mechanisms by which this occurs, we fed mice a very high fat diet (60% kcal by fat) for 17days or a moderate high fat diet (HFD, 45% kcal by fat) for 8weeks and examined changes in brain insulin signaling responses, hippocampal synaptodendritic protein expression, and spatial working memory. Compared to normal control diet mice, cerebral cortex tissues of HFD mice were insulin-resistant as evidenced by failed activation of Akt, S6 and GSK3β with ex-vivo insulin stimulation. Importantly, we found that expression of brain IPMK, which is necessary for mTOR/Akt signaling, remained decreased in HFD mice upon activation of AMPK. HFD mouse hippocampus exhibited increased expression of serine-phosphorylated insulin receptor substrate 1 (IRS1-pS(616)), a marker of insulin resistance, as well as decreased expression of PSD-95, a scaffolding protein enriched in post-synaptic densities, and synaptopodin, an actin-associated protein enriched in spine apparatuses. Spatial working memory was impaired as assessed by decreased spontaneous alternation in a T-maze. These findings indicate that HFD is associated with telencephalic insulin resistance and deleterious effects on synaptic integrity and cognitive behaviors.

Selective leptin resistance revisited

In addition to effects on appetite and metabolism, leptin influences many neuroendocrine and physiological systems, including the sympathetic nervous system. Building on my Carl Ludwig Lecture of the American Physiological Society, I review the sympathetic and cardiovascular actions of leptin. The review focuses on a critical analysis of the concept of selective leptin resistance (SLR) and the role of leptin in the pathogenesis of obesity-induced hypertension in both experimental animals and humans. We introduced the concept of SLR in 2002 to explain how leptin might increase blood pressure (BP) in obese states, such as diet-induced obesity (DIO), that are accompanied by partial leptin resistance. This concept, analogous to selective insulin resistance in the metabolic syndrome, holds that in several genetic and acquired models of obesity, there is preservation of the renal sympathetic and pressor actions of leptin despite attenuation of the appetite and weight-reducing actions. Two potential overlapping mechanisms of SLR are reviewed: 1) differential leptin molecular signaling pathways that mediate selective as opposed to universal leptin action and 2) brain site-specific leptin action and resistance. Although the phenomenon of SLR in DIO has so far focused on preservation of sympathetic and BP actions of leptin, consideration should be given to the possibility that this concept may extend to preservation of other actions of leptin. Finally, I review perplexing data on the effects of leptin on sympathetic activity and BP in humans and its role in human obesity-induced hypertension.

Timing of maternal exposure to a high fat diet and development of obesity and hyperinsulinemia in male rat offspring: same metabolic phenotype, different developmental pathways?

Objective. Offspring born to mothers either fed an obesogenic diet throughout their life or restricted to pregnancy and lactation demonstrate obesity, hyperinsulinemia, and hyperleptinemia, irrespective of their postweaning diet. We examined whether timing of a maternal obesogenic diet results in differential regulation of pancreatic adipoinsular and inflammatory signaling pathways in offspring. Methods. Female Wistar rats were randomized into 3 groups: (1) control (CONT): fed a control diet preconceptionally and during pregnancy and lactation; (2) maternal high fat (MHF): fed an HF diet throughout their life and during pregnancy and lactation; (3) pregnancy and lactation HF (PLHF): fed a control diet throughout life until mating, then HF diet during pregnancy and lactation. Male offspring were fed the control diet postweaning. Plasma and pancreatic tissue were collected, and mRNA concentrations of key factors regulating adipoinsular axis signaling were determined. Results. MHF and PLHF offspring exhibited increased adiposity and were hyperinsulinemic and hyperleptinemic compared to CONT. Despite a similar anthropometric phenotype, MHF and PLHF offspring exhibited distinctly different expression for key pancreatic genes, dependent upon maternal preconceptional nutritional background. Conclusions. These data suggest that despite using differential signaling pathways, obesity in offspring may be an adaptive outcome of early life exposure to HF during critical developmental windows.

Mouse strain-dependent variation in obesity and glucose homeostasis in response to high-fat feeding

AIMS/HYPOTHESIS

Metabolic disorders are commonly investigated using knockout and transgenic mouse models. A variety of mouse strains have been used for this purpose. However, mouse strains can differ in their inherent propensities to develop metabolic disease, which may affect the experimental outcomes of metabolic studies. We have investigated strain-dependent differences in the susceptibility to diet-induced obesity and insulin resistance in five commonly used inbred mouse strains (C57BL/6J, 129X1/SvJ, BALB/c, DBA/2 and FVB/N).

METHODS

Mice were fed either a low-fat or a high-fat diet (HFD) for 8 weeks. Whole-body energy expenditure and body composition were then determined. Tissues were used to measure markers of mitochondrial metabolism, inflammation, oxidative stress and lipid accumulation.

RESULTS

BL6, 129X1, DBA/2 and FVB/N mice were all susceptible to varying degrees to HFD-induced obesity, glucose intolerance and insulin resistance, but BALB/c mice exhibited some protection from these detrimental effects. This protection could not be explained by differences in mitochondrial metabolism or oxidative stress in liver or muscle, or inflammation in adipose tissue. Interestingly, in contrast with the other strains, BALB/c mice did not accumulate excess lipid (triacylglycerols and diacylglycerols) in the liver; this is potentially related to lower fatty acid uptake rather than differences in lipogenesis or lipid oxidation.

CONCLUSIONS/INTERPRETATION

Collectively, our findings indicate that most mouse strains develop metabolic defects on an HFD. However, there are inherent differences between strains, and thus the genetic background needs to be considered carefully in metabolic studies.

SH2-containing inositol 5'-phosphatase 2 selectively impairs hypothalamic insulin signalling and regulation of food intake in mice

SH2-containing inositol 5'-phosphatase 2 (SHIP2) is a lipid phosphatase that negatively regulates the metabolic signalling of insulin in peripheral tissues; however, the expression of SHIP2 in the hypothalamus and its functional roles are largely unknown. In the present study, immunohistochemical analysis demonstrated that SHIP2 protein exists in neuronal cells expressing neuropeptide Y and pro-opiomelanocortin in the arcuate nucleus of the hypothalamus in C57BL/6J mice. Interestingly, the expression levels of SHIP2 in the hypothalamus were elevated in aged C57BL/6J mice and diabetic db/db mice. To clarify the significance of the increased expression of SHIP2 in the hypothalamus, we examined the central effects of insulin and leptin in transgenic mice overexpressing SHIP2 (SHIP2-Tg). Accumulation of phosphatidylinositol (3,4,5)-trisphosphate and phosphorylation of Akt in the hypothalamus, induced by i.c.v. injection of insulin, were attenuated in SHIP2-Tg compared to wild-type mice, whereas leptin-induced phosphorylation of signal transducer and activator of transcription 3 in the hypothalamus was comparable between them. The suppression of food intake after i.c.v. administration of insulin (but not leptin) was attenuated consistently in SHIP2-Tg. In addition, SHIP2-Tg showed increased food consumption after starvation and become heavier with visceral fat accumulation than wild-type mice, despite normal levels of oxygen consumption and spontaneous movement. These results suggest that SHIP2 contributes to the regulation of food intake mainly via the attenuation of insulin signalling in the hypothalamus of mice.

Phosphatidylinositol 3-kinase is an upstream regulator of the phosphodiesterase 3B pathway of leptin signalling that may not involve activation of Akt in the rat hypothalamus

Leptin, the product of the obese gene, regulates energy homeostasis by acting primarily at the level of the hypothalamus. Leptin action through its receptor involves various pathways, including the signal transducer and activator of transcription (STAT)3, phosphatidylinositol 3-kinase (PI3K), and phosphodiesterase 3B (PDE3B)-cAMP signalling in the central nervous system and peripheral tissues. In the hypothalamus, leptin stimulates STAT3 activation, and induces PI3K and PDE3B activities, among others. We have previously demonstrated that PDE3B activation in the hypothalamus is critical for transducing the anorectic and body weight reducing effects of leptin. Similarly, PI3K has been implicated to play a critical role in leptin signalling in the hypothalamus. Although, in the insulin signalling pathway, PI3K is known to be an upstream regulator of PDE3B in non-neuronal tissues, it is still unknown whether this is also the case for leptin signalling in the hypothalamus. To address this possibility, the effect of wortmannin, a specific PI3K inhibitor, was examined on leptin-induced PDE3B activity in the hypothalamus of male rats. Intracerebroventricular injection of leptin (4 μg) significantly increased PDE3B activity by two-fold in the hypothalamus as expected. However, previous administration of wortmannin completely reversed the stimulatory effect of leptin on PDE3B activity in the hypothalamus. To investigate whether leptin stimulates phospho (p)-Akt levels and that there might be a possible upstream regulator of PDE3B, we examined the effects of i.c.v. leptin on p-Akt levels in the hypothalamus and compared them with the known stimulatory effect of insulin on p-Akt. We observed that insulin increased p-Akt levels but leptin failed to do so, although it increased p-STAT3 levels, in the rat hypothalamus. Immunocytochemistry confirmed the biochemical findings in that leptin failed but insulin increased the number of p-Akt positive cells in various hypothalamic nuclei. Taken together, these results implicate PI3K but not Akt as an upstream regulator of the PDE3B pathway of leptin signalling in the rat hypothalamus.

Postnatal growth after intrauterine growth restriction alters central leptin signal and energy homeostasis

Intrauterine growth restriction (IUGR) is closely linked with metabolic diseases, appetite disorders and obesity at adulthood. Leptin, a major adipokine secreted by adipose tissue, circulates in direct proportion to body fat stores, enters the brain and regulates food intake and energy expenditure. Deficient leptin neuronal signalling favours weight gain by affecting central homeostatic circuitry. The aim of this study was to determine if leptin resistance was programmed by perinatal nutritional environment and to decipher potential cellular mechanisms underneath.We clearly demonstrated that 5 months old IUGR rats develop a decrease of leptin sentivity, characterized by no significant reduction of food intake following an intraperitoneal injection of leptin. Apart from the resistance to leptin injection, results obtained from IUGR rats submitted to rapid catch-up growth differed from those of IUGR rats with no catch-up since we observed, for the first group only, fat accumulation, increased appetite for food rich in fat and increased leptin synthesis. Centrally, the leptin resistant state of both groups was associated with a complex and not always similar changes in leptin receptor signalling steps. Leptin resistance in IUGR rats submitted to rapid catch-up was associated with alteration in AKT and mTOR pathways. Alternatively, in IUGR rats with no catch-up, leptin resistance was associated with low hypothalamic expression of LepRa and LepRb. This study reveals leptin resistance as an early marker of metabolic disorders that appears before any evidence of body weight increase in IUGR rats but whose mechanisms could depend of nutritional environment of the perinatal period.

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.

Impairment of central leptin-mediated PI3K signaling manifested as hepatic steatosis independent of hyperphagia and obesity

Hepatic steatosis is generally thought to develop via peripheral mechanisms associated with obesity. We show that chronic central infusion of leptin suppresses hepatic lipogenic gene expression and reduces triglyceride content via stimulation of hepatic sympathetic activity. This leptin function is independent of feeding and body weight but requires phosphatidylinositol 3-kinase (PI3K) signaling. Attenuation of leptin-induced PI3K signaling, brought about by transgenic expression of phosphatase and tensin homolog (PTEN) in leptin receptor neurons, leads to decreased hepatic sympathetic tone and increased triglyceride levels without affecting adiposity or hepatic insulin signaling. Central leptin's effects on hepatic norepinephrine levels and triglyceride content are blunted in these mutant mice. Simultaneous downregulation of PI3K and signal transducer and activator of transcription-3 (Stat3) in leptin receptor neurons does not exacerbate obesity but causes more severe hepatic steatosis. Together, our results indicate that central cellular leptin resistance in PI3K signaling manifests as hepatic steatosis without causing obesity.

Central nervous control of energy and glucose balance: focus on the central melanocortin system

Studies have suggested that manipulations of the central melanocortin circuitry by pharmacological agents produce robust effects on the regulation of body weight and glucose homeostasis. In this review, we discuss recent findings from genetic mouse models that have further established the physiological relevance of this circuitry in the context of glucose and energy balance. In addition, we will discuss distinct neuronal populations that respond to central melanocortins to regulate food intake, energy expenditure, insulin sensitivity, and insulin secretion, respectively. Finally, multiple hormonal and neural cues (e.g., leptin, estrogen, and serotonin) that use the melanocortin systems to regulate energy and glucose homeostasis will be reviewed. These findings suggest that targeting the specific branches of melanocortin circuits may be potential avenues to combat the current obesity and diabetes epidemics.

Cellular leptin resistance impairs the leptin-mediated suppression of neuropeptide Y secretion in hypothalamic neurons

Evidence shows that neuropeptide Y (NPY) neurons are involved in mediating the anorexigenic action of leptin via neuronal circuits in the hypothalamus. However, studies have produced limited data on the cellular processes involved and whether hypothalamic NPY neurons are susceptible to cellular leptin resistance. To investigate the direct regulation of NPY secretion by leptin, we used novel NPY-synthesizing, immortalized mHypoA-NPY/green fluorescent protein and mHypoA-59 hypothalamic cell lines derived from adult hypothalamic primary cultures. We report that leptin treatment significantly suppressed NPY secretion in the cells by approximately 20%. We found a decrease in c-fos expression upon leptin exposure, indicating deactivation or hyperpolarization of the neurons. Protein analysis indicated that leptin inhibits AMP-activated protein kinase (AMPK) activity and activates acetyl-coenzyme A carboxylase in NPY neurons, supporting the hypothesis of an AMPK-dependent mechanism. Inhibiting both AMPK with Compound C or phosphatidylinositol 3 kinase (PI3K) with 2-(4-morpholinyl)-8-phenyl-1(4H)-1-benzopyran-4-one hydrochloride prevented the leptin-mediated decrease in NPY secretion, indicating both AMPK- and PI3K-mediated mechanisms. Further, NPY secretion was stimulated by 30% by the AMPK activator, aminoimidazole carboxamide ribonucleotide. Importantly, prolonged leptin exposure in the mHypoA-NPY/green fluorescent protein cells prevented leptin-induced changes in AMPK phosphorylation and suppression of NPY secretion, indicating that NPY neurons are susceptible to leptin resistance. Our studies indicate that AMPK and PI3K pathways are involved in leptin action in NPY neurons and that leptin resistance blocks the feedback response likely required to maintain energy homeostasis.

Hypothalamic control of energy and glucose metabolism

The central nervous system (CNS), generally accepted to regulate energy homeostasis, has been implicated in the metabolic perturbations that either cause or are associated with obesity. Normally, the CNS receives hormonal, metabolic, and neuronal input to assure adequate energy levels and maintain stable energy homeostasis. Recent evidence also supports that the CNS uses these same inputs to regulate glucose homeostasis and this aspect of CNS regulation also becomes impaired in the face of dietary-induced obesity. This review focuses on the literature surrounding hypothalamic regulation of energy and glucose homeostasis and discusses how dysregulation of this system may contribute to obesity and T2DM.