Leptin Signaling in the Arcuate Nucleus Reduces Insulin’s Capacity to Suppress Hepatic Glucose Production in Obese Mice

Protein tyrosine phosphatase 1B in metabolic diseases and drug development

Protein tyrosine phosphatase 1B (PTP1B), a non-transmembrane phosphatase, has a major role in a variety of signalling pathways, including direct negative regulation of classic insulin and leptin signalling pathways, and is implicated in the pathogenesis of several cardiometabolic diseases and cancers. As such, PTP1B has been a therapeutic target for over two decades, with PTP1B inhibitors identified either from natural sources or developed throughout the years. Some of these inhibitors have reached phase I and/or II clinical trials in humans for the treatment of type 2 diabetes mellitus, obesity and/or metastatic breast cancer. In this Review, we summarize the cellular processes and regulation of PTP1B, discuss evidence from in vivo preclinical and human studies of the association between PTP1B and different disorders, and discuss outcomes of clinical trials. We outline challenges associated with the targeting of this phosphatase (which was, until the past few years, viewed as difficult to target), the current state of the field of PTP1B inhibitors (and dual phosphatase inhibitors) and future directions for manipulating the activity of this key metabolic enzyme.

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

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

NPFFR2-deficient mice fed a high-fat diet develop strong intolerance to glucose

A previous study on neuropeptide FF receptor 2 (NPFFR2)-deficient mice has demonstrated that NPFFR2 is involved in the control of energy balance and thermogenesis. Here, we report on the metabolic impact of NPFFR2 deficiency in male and female mice that were fed either a standard diet (STD) or a high-fat diet (HFD) and each experimental group consisted of ten individuals. Both male and female NPFFR2 knockout (KO) mice exhibited severe glucose intolerance that was exacerbated by a HFD diet. In addition, reduced insulin pathway signaling proteins in NPFFR2 KO mice fed a HFD resulted in the development of hypothalamic insulin resistance. HFD feeding did not cause liver steatosis in NPFFR2 KO mice of either sex, but NPFFR2 KO male mice fed a HFD had lower body weights, white adipose tissues, and liver and lower plasma leptin levels compared with their wild-type (WT) controls. Lower liver weight in NPFFR2 KO male mice compensated for HFD-induced metabolic stress by increased liver PPARα and plasma FGF21 hepatokine, which supported fatty acid β-oxidation in the liver and white adipose tissue. Conversely, NPFFR2 deletion in female mice attenuated the expression of Adra3β and Pparγ, which inhibited lipolysis in adipose tissue.

Trends in Gliosis in Obesity, and the Role of Antioxidants as a Therapeutic Alternative

Obesity remains a global health problem. Chronic low-grade inflammation in this pathology has been related to comorbidities such as cognitive alterations that, in the long term, can lead to neurodegenerative diseases. Neuroinflammation or gliosis in patients with obesity and type 2 diabetes mellitus has been related to the effect of adipokines, high lipid levels and glucose, which increase the production of free radicals. Cerebral gliosis can be a risk factor for developing neurodegenerative diseases, and antioxidants could be an alternative for the prevention and treatment of neural comorbidities in obese patients.

AIM

Identify the immunological and oxidative stress mechanisms that produce gliosis in patients with obesity and propose antioxidants as an alternative to reducing neuroinflammation.

METHOD

Advanced searches were performed in scientific databases: PubMed, ProQuest, EBSCO, and the Science Citation index for research on the physiopathology of gliosis in obese patients and for the possible role of antioxidants in its management.

CONCLUSION

Patients with obesity can develop neuroinflammation, conditioned by various adipokines, excess lipids and glucose, which results in an increase in free radicals that must be neutralized with antioxidants to reduce gliosis and the risk of long-term neurodegeneration.

Hypothalamic GABAergic neurocircuitry in the regulation of energy homeostasis and sleep/wake control

Gamma-aminobutyric acid (GABAergic) neuron, as one of important cell types in synaptic transmission, has been widely involved in central nervous system (CNS) regulation of organismal physiologies including cognition, emotion, arousal and reward. However, upon their distribution in various brain regions, effects of GABAergic neurons in the brain are very diverse. In current report, we will present an overview of the role of GABAergic mediated inhibitory neurocircuitry in the hypothalamus, underlying mechanism of feeding and sleep homeostasis as well as the characteristics of latest transcriptome profile in order to call attention to the GABAergic system as potentially a promising pharmaceutical intervention or a deep brain stimulation target in eating and sleep disorders.

Protein Tyrosine Phosphatase 1B Deficiency Improves Glucose Homeostasis in Type 1 Diabetes Treated With Leptin

Leptin, a hormone secreted by adipocytes, exhibits therapeutic potential for the treatment of type 1 diabetes (T1D). Protein tyrosine phosphatase 1B (PTP1B) is a key enzyme that negatively regulates leptin receptor signaling. Here, the role of PTP1B in the treatment of T1D was investigated using PTP1B-deficient (knockout [KO]) mice and a PTP1B inhibitor. T1D wild-type (WT) mice induced by streptozotocin showed marked hyperglycemia compared with non-T1D WT mice. KO mice displayed significantly improved glucose metabolism equivalent to non-T1D WT mice, whereas peripheral or central administration of leptin partially improved glucose metabolism in T1D WT mice. Peripheral combination therapy of leptin and a PTP1B inhibitor in T1D WT mice improved glucose metabolism to the same level as non-T1D WT mice. Leptin was shown to act on the arcuate nucleus in the hypothalamus to suppress gluconeogenesis in liver and enhance glucose uptake in both brown adipose tissue and soleus muscle through the sympathetic nervous system. These effects were enhanced by PTP1B deficiency. Thus, treatment of T1D with leptin, PTP1B deficiency, or a PTP1B inhibitor was shown to enhance leptin activity in the hypothalamus to improve glucose metabolism. These findings suggest a potential alternative therapy for T1D.

The Head-to-Toe Hormone: Leptin as an Extensive Modulator of Physiologic Systems

Leptin is a well-known hunger-sensing peptide hormone. The role of leptin in weight gain and metabolic homeostasis has been explored for the past two decades. In this review, we have tried to shed light upon the impact of leptin signaling on health and diseases. At low or moderate levels, this peptide hormone supports physiological roles, but at chronically higher doses exhibits detrimental effects on various systems. The untoward effects we observe with chronically higher levels of leptin are due to their receptor-mediated effect or due to leptin resistance and are not well studied. This review will help us in understanding the non-anorexic roles of leptin, including their contribution to the metabolism of various systems and inflammation. We will be able to get an alternative perspective regarding the physiological and pathological roles of this mysterious peptide hormone.

The timeline of neuronal and glial alterations in experimental obesity

In experimental models, hypothalamic dysfunction is a key component of the pathophysiology of diet-induced obesity. Early after the introduction of a high-fat diet, neurons, microglia, astrocytes and tanycytes of the mediobasal hypothalamus undergo structural and functional changes that impact caloric intake, energy expenditure and systemic glucose tolerance. Inflammation has emerged as a central component of this response, and as in other inflammatory conditions, there is a time course of events that determine the fate of distinct cells involved in the central regulation of whole-body energy homeostasis. Here, we review the work that identified key mechanisms, cellular players and temporal features of diet-induced hypothalamic abnormalities.

Central nervous system regulation of organismal energy and glucose homeostasis

Growing evidence implicates the brain in the regulation of both immediate fuel availability (for example, circulating glucose) and long-term energy stores (that is, adipose tissue mass). Rather than viewing the adipose tissue and glucose control systems separately, we suggest that the brain systems that control them are components of a larger, highly integrated, 'fuel homeostasis' control system. This conceptual framework, along with new insights into the organization and function of distinct neuronal systems, provides a context within which to understand how metabolic homeostasis is achieved in both basal and postprandial states. We also review evidence that dysfunction of the central fuel homeostasis system contributes to the close association between obesity and type 2 diabetes, with the goal of identifying more effective treatment options for these common metabolic disorders.

Insulin on the brain: The role of central insulin signalling in energy and glucose homeostasis

Insulin signals to the brain where it coordinates multiple physiological processes underlying energy and glucose homeostasis. This review explores where and how insulin interacts within the brain parenchyma, how brain insulin signalling functions to coordinate energy and glucose homeostasis and how this contributes to the pathogenesis of metabolic disease.

Leptin Modulates the Response of Brown Adipose Tissue to Negative Energy Balance: Implication of the GH/IGF-I Axis

The growth hormone (GH)/insulin-like growth factor I (IGF-I) axis is involved in metabolic control. Malnutrition reduces IGF-I and modifies the thermogenic capacity of brown adipose tissue (BAT). Leptin has effects on the GH/IGF-I axis and the function of BAT, but its interaction with IGF-I and the mechanisms involved in the regulation of thermogenesis remains unknown. We studied the GH/IGF-I axis and activation of IGF-I-related signaling and metabolism related to BAT thermogenesis in chronic central leptin infused (L), pair-fed (PF), and control rats. Hypothalamic somatostatin mRNA levels were increased in PF and decreased in L, while pituitary GH mRNA was reduced in PF. Serum GH and IGF-I concentrations were decreased only in PF. In BAT, the association between suppressor of cytokine signaling 3 and the IGF-I receptor was reduced, and phosphorylation of the IGF-I receptor increased in the L group. Phosphorylation of Akt and cyclic AMP response element binding protein and glucose transporter 4 mRNA levels were increased in L and mRNA levels of uncoupling protein-1 (UCP-1) and enzymes involved in lipid anabolism reduced in PF. These results suggest that modifications in UCP-1 in BAT and changes in the GH/IGF-I axis induced by negative energy balance are dependent upon leptin levels.

Insulin signaling in AgRP neurons regulates meal size to limit glucose excursions and insulin resistance

The importance of hypothalamic insulin signaling on feeding and glucose metabolism remains unclear. We report that insulin acts on AgRP neurons to acutely decrease meal size and thereby limit postprandial glucose and insulin excursions. The promotion of insulin signaling in AgRP neurons decreased meal size without altering total caloric intake, whereas the genetic ablation of the insulin receptor had the opposite effect. The promotion of insulin signaling also decreased the intake of sucrose-sweetened water or high-fat food over standard chow, without influencing food-seeking and hedonic behaviors. The ability of heightened insulin signaling to override the hedonistic consumption of highly palatable high-fat food attenuated the development of systemic insulin resistance, without affecting body weight. Our findings define an unprecedented mechanism by which insulin acutely influences glucose metabolism. Approaches that enhance insulin signaling in AgRP neurons may provide a means for altering feeding behavior in a nutrient-dense environment to combat the metabolic syndrome.

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Calcitonin Gene-Related Peptide-Induced Phosphorylation of STAT3 in Arcuate Neurons Is a Link in the Metabolic Benefits of Portal Glucose

INTRODUCTION

Intestinal gluconeogenesis (IGN) exerts metabolic benefits in energy homeostasis via the neural sensing of portal glucose.

OBJECTIVE

The aim of this work was to determine central mechanisms involved in the effects of IGN on the control of energy homeostasis.

METHODS

We investigated the effects of glucose infusion into the portal vein, at a rate that mimics IGN, in conscious wild-type, leptin-deficient Ob/Ob and calcitonin gene-related peptide (CGRP)-deficient mice.

RESULTS

We report that portal glucose infusion decreases food intake and plasma glucose and induces in the hypothalamic arcuate nucleus (ARC) the phosphorylation of STAT3, the classic intracellular messenger of leptin signaling. This notably takes place in POMC-expressing neurons. STAT3 phosphorylation does not require leptin, since portal glucose effects are observed in leptin-deficient Ob/Ob mice. We hypothesized that the portal glucose effects could require CGRP, a neuromediator previously suggested to suppress hunger. In line with this hypothesis, neither the metabolic benefits nor the phosphorylation of STAT3 in the ARC take place upon portal glucose infusion in CGRP-deficient mice. Moreover, intracerebroventricular injection of CGRP activates hypothalamic phosphorylation of STAT3 in mice, and CGRP does the same in hypothalamic cells. Finally, no metabolic benefit of dietary fibers (known to depend on the induction of IGN), takes place in CGRP-deficient mice.

CONCLUSIONS

CGRP-induced phosphorylation of STAT3 in the ARC is part of the neural chain determining the hunger-modulating and glucose-lowering effects of IGN/portal glucose.

Overexpression of Smad7 in hypothalamic POMC neurons disrupts glucose balance by attenuating central insulin signaling

OBJECTIVE

Although the hypothalamus is crucial for peripheral metabolism control, the signals in specific neurons involved remain poorly understood. The aim of our current study was to explore the role of the hypothalamic gene mothers against decapentaplegic homolog 7 (Smad7) in peripheral glucose disorders.

METHODS

We studied glucose metabolism in high-fat diet (HFD)-fed mice and middle-aged mice with Cre-mediated recombination causing 1) overexpression of Smad7 in hypothalamic proopiomelanocortin (POMC) neurons, 2) deletion of Smad7 in POMC neurons, and 3) overexpression of protein kinase B (AKT) in arcuate nucleus (ARC) in Smad7 overexpressed mice. Intracerebroventricular (ICV) cannulation of insulin was used to test the hypothalamic insulin sensitivity in the mice. Hypothalamic primary neurons were used to investigate the mechanism of Smad7 regulating hypothalamic insulin signaling.

RESULTS

We found that Smad7 expression was increased in POMC neurons in the hypothalamic ARC of HFD-fed or middle-aged mice. Furthermore, overexpression of Smad7 in POMC neurons disrupted the glucose balance, and deletion of Smad7 in POMC neurons prevented diet- or age-induced glucose disorders, which was likely to be independent of changes in body weight or food intake. Moreover, the effect of Smad7 was reversed by overexpression of AKT in the ARC. Finally, Smad7 decreased AKT phosphorylation by activating protein phosphatase 1c in hypothalamic primary neurons.

CONCLUSIONS

Our results demonstrated that an excess of central Smad7 in POMC neurons disrupts glucose balance by attenuating hypothalamic insulin signaling. In addition, we found that this regulation was mediated by the activity of protein phosphatase 1c.

Hyperleptinemia as a contributing factor for the impairment of glucose intolerance in obesity

Obesity has emerged as a major risk factor for insulin resistance leading to the development of type 2 diabetes (T2D). The condition is characterized by high circulating levels of the adipose-derived hormone leptin and a state of chronic low-grade inflammation. Pro-inflammatory signaling in the hypothalamus is associated with a decrease of central leptin- and insulin action leading to impaired systemic glucose tolerance. Intriguingly, leptin not only regulates body weight and glucose homeostasis but also acts as a pro-inflammatory cytokine. Here we demonstrate that increasing leptin levels (62,5 µg/kg/d, PEGylated leptin) in mice fed a high-fat diet (HFD) exacerbated body weight gain and aggravated hypothalamic micro- as well as astrogliosis. In contrast, administration of a predetermined dose of a long-acting leptin antagonist (100 µg/kg/d, PESLAN) chosen to block excessive leptin signaling during diet-induced obesity (DIO) showed the opposite effect and significantly improved glucose tolerance as well as decreased the total number of microglia and astrocytes in the hypothalamus of mice fed HFD. These results suggest that high levels of leptin, such as in obesity, worsen HFD-induced micro-and astrogliosis, whereas the partial reduction of hyperleptinemia in DIO mice may have beneficial metabolic effects and improves hypothalamic gliosis.

Intranasal Targeting of Hypothalamic PTP1B and TCPTP Reinstates Leptin and Insulin Sensitivity and Promotes Weight Loss in Obesity

The importance of hypothalamic leptin and insulin resistance in the development and maintenance of obesity remains unclear. The tyrosine phosphatases protein tyrosine phosphatase 1B (PTP1B) and T cell protein tyrosine phosphatase (TCPTP) attenuate leptin and insulin signaling and are elevated in the hypothalami of obese mice. We report that elevated PTP1B and TCPTP antagonize hypothalamic leptin and insulin signaling and contribute to the maintenance of obesity. Deletion of PTP1B and TCPTP in the hypothalami of obese mice enhances CNS leptin and insulin sensitivity, represses feeding, and increases browning, to decrease adiposity and improve glucose metabolism. The daily intranasal administration of a PTP1B inhibitor, plus the glucocorticoid antagonist RU486 that decreases TCPTP expression, represses feeding, increases browning, promotes weight loss, and improves glucose metabolism in obese mice. Our findings causally link heightened hypothalamic PTP1B and TCPTP with leptin and insulin resistance and the maintenance of obesity and define a viable pharmacological approach by which to promote weight loss in obesity.

Appetite control: hormones or diet strategies?

PURPOSE OF REVIEW

Appetite control results from metabolic, behavioral, and environmental factors that influence hunger and the desire to eat. We summarize the latest advances in the hormonal and nutritional strategies to control appetite and reduce hunger.

RECENT FINDINGS

The fed-hunger-state is regulated by central and peripheric hormones, which modulate energy balance. Leptin, insulin, ghrelin, peptide YY (PYY), and other gut-derived peptides represent the main appetite controllers. The role of orexins, obestatin, and liver-expressed antimicrobial peptide 2 has been uncovered recently. New insights have demonstrated the role of hippocampal activity as a possible mechanism of action. Glucagon-like peptide 1 (GLP1) receptor agonists are well known agents controlling appetite. Association of GLP1 receptor agonist, PYY, or glucose-dependent insulinotropic polypeptide agonists have been tested as new approaches. Appetite-control hormones have also risen as factors involved in the efficacy of bariatric procedures. High-protein, ketogenic diet, and intermittent fasting have been described as nutritional strategies to reduce appetite, although the physiological mechanism and long-term safety remains unclear.

SUMMARY

Appetite control has been an important target for the treatment of obesity and associated disorders. New studies have demonstrated promising adoption of dietary approaches, hormone-based drugs, and bariatric surgery to control energy intake. Further research will establish a significant association, benefits, and safety of these new therapies.

Roles of Gangliosides in Hypothalamic Control of Energy Balance: New Insights

Gangliosides are essential components of cell membranes and are involved in a variety of physiological processes, including cell growth, differentiation, and receptor-mediated signal transduction. They regulate functions of proteins in membrane microdomains, notably receptor tyrosine kinases such as insulin receptor (InsR) and epidermal growth factor receptor (EGFR), through lateral association. Studies during the past two decades using knockout (KO) or pharmacologically inhibited cells, or KO mouse models for glucosylceramide synthase (GCS; Ugcg), GM3 synthase (GM3S; St3gal5), and GD3 synthase (GD3S; St8sia1) have revealed essential roles of gangliosides in hypothalamic control of energy balance. The a-series gangliosides GM1 and GD1a interact with leptin receptor (LepR) and promote LepR signaling through activation of the JAK2/STAT3 pathway. Studies of GM3S KO cells have shown that the extracellular signal-regulated kinase (ERK) pathway, downstream of the LepR signaling pathway, is also modulated by gangliosides. Recent studies have revealed crosstalk between the LepR signaling pathway and other receptor signaling pathways (e.g., InsR and EGFR pathways). Gangliosides thus have the ability to modulate the effects of leptin by regulating functions of such receptors, and by direct interaction with LepR to control signaling.

High Glucose Induces Lipid Accumulation via 25-Hydroxycholesterol DNA-CpG Methylation

This work investigates the relationship between high-glucose (HG) culture, CpG methylation of genes involved in cell signaling pathways, and the regulation of carbohydrate and lipid metabolism in hepatocytes. The results indicate that HG leads to an increase in nuclear 25-hydroxycholesterol (25HC), which specifically activates DNA methyltransferase-1 (DNMT1), and regulates gene expression involved in intracellular lipid metabolism. The results show significant increases in ^5mCpG levels in at least 2,225 genes involved in 57 signaling pathways. The hypermethylated genes directly involved in carbohydrate and lipid metabolism are of PI3K, cAMP, insulin, insulin secretion, diabetic, and NAFLD signaling pathways. The studies indicate a close relationship between the increase in nuclear 25HC levels and activation of DNMT1, which may regulate lipid metabolism via DNA CpG methylation. Our results indicate an epigenetic regulation of hepatic cell metabolism that has relevance to some common diseases such as non-alcoholic fatty liver disease and metabolic syndrome.

Is "Leptin Resistance" Another Key Resistance to Manage Type 2 Diabetes?

Although novel pharmacological options for the treatment of type 2 diabetes mellitus (DM2) have been observed to modulate the functionality of several key organs in glucose homeostasis, successful regulation of insulin resistance (IR), body weight management, and pharmacological treatment of obesity remain notable problems in endocrinology. Leptin may be a pivotal player in this scenario, as an adipokine which centrally regulates appetite and energy balance. In obesity, excessive caloric intake promotes a low-grade inflammatory response, which leads to dysregulations in lipid storage and adipokine secretion. In turn, these entail alterations in leptin sensitivity, leptin transport across the blood-brain barrier and defects in post-receptor signaling. Furthermore, hypothalamic inflammation and endoplasmic reticulum stress may increase the expression of molecules which may disrupt leptin signaling. Abundant evidence has linked obesity and leptin resistance, which may precede or occur simultaneously to IR and DM2. Thus, leptin sensitivity may be a potential early therapeutic target that demands further preclinical and clinical research. Modulators of insulin sensitivity have been tested in animal models and small clinical trials with promising results, especially in combination with agents such as amylin and GLP-1 analogs, in particular, due to their central activity in the hypothalamus.

Role of Leptin in Cardiovascular Diseases

The adipocyte-derived adipokine leptin exerts pleiotropic effects, which are essential for the regulation of energy balance and cell metabolism, for controlling inflammatory and immune responses, and for the maintenance of homeostasis of the cardiovascular system. Leptin resistance in obese or type 2 diabetes mellitus (T2DM) patients is defined as a decrease in tissue response to leptin. In the cardiovascular system, leptin resistance exhibits the adverse effect on the heart's response to stress conditions and promoting cardiac remodeling due to impaired cardiac metabolism, increased fibrosis, vascular dysfunction, and enhanced inflammation. Leptin resistance or leptin signaling deficiency results in the risk increase of cardiac dysfunction and heart failure, which is a leading cause of obesity- and T2DM-related morbidity and mortality. Animal studies using leptin- and leptin receptor- (Lepr) deficient rodents have provided many useful insights into the underlying molecular and pathophysiological mechanisms of obese- and T2DM-associated metabolic and cardiovascular diseases. However, none of the animal models used so far can fully recapitulate the phenotypes of patients with obese or T2DM. Therefore, the role of leptin in the human cardiovascular system, and whether leptin affects cardiac function directly or acts through a leptin-regulated neurohumoral pathway, remain elusive. As the prevalence of obesity and diabetes is continuously increasing, strategies are needed to develop and apply human cell-based models to better understand the precise role of leptin directly in different cardiac cell types and to overcome the existing translational barriers. The purpose of this review is to discuss the mechanisms associated with leptin signaling deficiency or leptin resistance in the development of metabolic and cardiovascular diseases. We analyzed and comprehensively addressed substantial findings in pathophysiological mechanisms in commonly used leptin- or Lepr-deficient rodent models and highlighted the differences between rodents and humans. This may open up new strategies to develop directly and reliably applicable models, which resemble the human pathophysiology in order to advance health care management of obesity- and T2DM-related cardiovascular complications.

Hypothalamic neuronal cellular and subcellular abnormalities in experimental obesity

The characterization of the hypothalamic neuronal network, that controls food intake and energy expenditure, has provided great advances in the understanding of the pathophysiology of obesity. Most of the advances in this field were obtained thanks to the development of a number of genetic and nongenetic animal models that, at least in part, overtook the anatomical constraints that impair the study of the human hypothalamus. Despite the undisputed differences between human and rodent physiology, most seminal studies undertaken in rodents that have unveiled details of the neural regulation of energy homeostasis were eventually confirmed in humans; thus, placing experimental studies in the forefront of obesity research. During the last 15 years, researchers have provided extensive experimental proof that supports the existence of hypothalamic dysfunction, which leads to a progressive whole-body positive energy balance, and thus, to obesity. Here, we review the experimental work that unveiled the mechanisms behind hypothalamic dysfunction in obesity.

Auto-Regulation of Leptin Neurobiology

The clinical use of the hormone leptin, a key regulator of food intake, to treat the most common instances of obesity has so far failed. In this issue, Zhao et al. (2019) report that, paradoxically, reducing leptin levels in obese mice increases their sensitivity to the concentrations that remain, and leads to reductions in weight gain, thus suggesting why these earlier trials may have failed and possibly a new approach to treating obesity.

Partial Leptin Reduction as an Insulin Sensitization and Weight Loss Strategy

The physiological role of leptin is thought to be a driving force to reduce food intake and increase energy expenditure. However, leptin therapies in the clinic have failed to effectively treat obesity, predominantly due to a phenomenon referred to as leptin resistance. The mechanisms linking obesity and the associated leptin resistance remain largely unclear. With various mouse models and a leptin neutralizing antibody, we demonstrated that hyperleptinemia is a driving force for metabolic disorders. A partial reduction of plasma leptin levels in the context of obesity restores hypothalamic leptin sensitivity and effectively reduces weight gain and enhances insulin sensitivity. These results highlight that a partial reduction in plasma leptin levels leads to improved leptin sensitivity, while pointing to a new avenue for therapeutic interventions in the treatment of obesity and its associated comorbidities.

Inter-organ communication: a gatekeeper for metabolic health

Multidirectional interactions between metabolic organs in the periphery and the central nervous system have evolved concomitantly with multicellular organisms to maintain whole-body energy homeostasis and ensure the organism's adaptation to external cues. These interactions are altered in pathological conditions such as obesity and type 2 diabetes. Bioactive peptides and proteins, such as hormones and cytokines, produced by both peripheral organs and the central nervous system, are key messengers in this inter-organ communication. Despite the early discovery of the first hormones more than 100 years ago, recent studies taking advantage of novel technologies have shed light on the multiple ways used by cells in the body to communicate and maintain energy balance. This review briefly summarizes well-established concepts and focuses on recent advances describing how specific proteins and peptides mediate the crosstalk between gut, brain, and other peripheral metabolic organs in order to maintain energy homeostasis. Additionally, this review outlines how the improved knowledge about these inter-organ networks is helping us to redefine therapeutic strategies in an effort to promote healthy living and fight metabolic disorders and other diseases.

Estradiol Protects Neuropeptide Y/Agouti-Related Peptide Neurons against Insulin Resistance in Females

When it comes to obesity, men exhibit a higher incidence of metabolic syndrome than women in early adult life, but this sex advantage wanes in postmenopausal women. A key diagnostic of the metabolic syndrome is insulin resistance in both peripheral tissues and brain, especially in the hypothalamus. Since the anorexigenic hormone 17β-estradiol (E2) regulates food intake in part by inhibiting the excitability of the hypothalamic neuropeptide Y/agouti-related peptide (NPY/AgRP) neurons, we hypothesized that E2 would protect against insulin resistance in NPY/AgRP neurons with diet-induced obesity (DIO). Therefore, we did whole-cell recordings and single cell quantitative polymerase chain reaction in arcuate NPYGFP neurons from both female and male mice to test the efficacy of insulin with DIO. The resting membrane potential and input resistance of NPY/AgRP neurons were significantly increased in DIO versus control-diet fed males. Most notably, the efficacy of insulin to activate KATP channels in NPY/AgRP neurons was significantly attenuated, although the KATP channel opener diazoxide was fully effective in NPY/AgRP neurons from DIO males, indicating that the KATP channels were expressed and functional. In contrast, insulin was fully efficacious to activate KATP channels in DIO females, and the response was reversed by the KATP channel blocker tolbutamide. However, the ability of insulin to activate KATP channels was abrogated with ovariectomy but fully restored with E2 replacement. Insulin resistance in obese males was likely mediated by an increase in suppressor of cytokine signaling-3 (SOCS-3), protein tyrosine phosphatase B (PTP1B) and T-cell protein tyrosine phosphatase (TCPTP) activity, since the expression of all 3 mRNAs were upregulated in the obese males but not in females. As proof of principle, pre-incubation of hypothalamic slices from DIO males with the PTP1B/TCPTP inhibitor CX08005 completely rescued the effects of insulin. Therefore, E2 protects NPY/AgRP neurons in females against insulin resistance through, at least in part, attenuating phosphatase activity. The neuroprotective effects of E2 may explain sex differences in the expression of metabolic syndrome that disappears with the loss of E2 in aging.

'Et Tu, Leptin?'

Leptin promotes adequate caloric intake and glycemia in healthy lean individuals, harnessing the benefits of the ideal therapy against metabolic syndrome. Yet, new evidence demonstrates an unexpected causal role for leptin in obesity-associated hyperglycemia. Like the betrayal of Julius Caesar by Brutus, insulin did not see that coming from leptin.

Persistent Leptin Signaling in the Arcuate Nucleus Impairs Hypothalamic Insulin Signaling and Glucose Homeostasis in Obese Mice

BACKGROUND

Obesity is associated with reduced physiological responses to leptin and insulin, leading to the concept of obesity-associated hormonal resistance.

OBJECTIVES

Here, we demonstrate that contrary to expectations, leptin signaling not only remains functional but also is constantly activated in the arcuate nucleus of the hypothalamus (ARH) neurons of obese mice. This state of persistent response to endogenous leptin underpins the lack of response to exogenous leptin.

METHODS AND RESULTS

The study of combined leptin and insulin signaling demonstrates that there is a common pool of ARH neurons responding to both hormones. More importantly, we show that the constant activation of leptin receptor neurons in the ARH prevents insulin signaling in these neurons, leading to impaired glucose tolerance. Accordingly, antagonising leptin signaling in diet-induced obese (DIO) mice restores insulin signaling in the ARH and improves glucose homeostasis. Direct inhibition of PTP1B in the CNS restores arcuate insulin signaling similarly to leptin inhibition; this effect is likely to be mediated by AgRP neurons since PTP1B deletion specifically in AgRP neurons restores glucose and insulin tolerance in DIO mice.

CONCLUSIONS

Finally, our results suggest that the constant activation of arcuate leptin signaling in DIO mice increases PTP1B expression, which exerts an inhibitory effect on insulin signaling leading to impaired glucose homeostasis.