Partial Leptin Reduction as an Insulin Sensitization and Weight Loss Strategy

Adiponectin, Leptin and Cardiovascular Disorders

The landmark discoveries of leptin and adiponectin firmly established adipose tissue as a sophisticated and highly active endocrine organ, opening a new era of investigating adipose-mediated tissue crosstalk. Both obesity-associated hyperleptinemia and hypoadiponectinemia are important biomarkers to predict cardiovascular outcomes, suggesting a crucial role for adiponectin and leptin in obesity-associated cardiovascular disorders. Normal physiological levels of adiponectin and leptin are indeed essential to maintain proper cardiovascular function. Insufficient adiponectin and leptin signaling results in cardiovascular dysfunction. However, a paradox of high levels of both leptin and adiponectin is emerging in the pathogenesis of cardiovascular disorders. Here, we (1) summarize the recent progress in the field of adiponectin and leptin and its association with cardiovascular disorders, (2) further discuss the underlying mechanisms for this new paradox of leptin and adiponectin action, and (3) explore the possible application of partial leptin reduction, in addition to increasing the adiponectin/leptin ratio as a means to prevent or reverse cardiovascular disorders.

Amyloid Beta Peptide Is an Endogenous Negative Allosteric Modulator of Leptin Receptor

INTRODUCTION

Metabolic dysfunction is now recognized as a pivotal component of Alzheimer's disease (AD), the most common dementia worldwide. However, the precise molecular mechanisms linking metabolic dysfunction to AD remain elusive.

OBJECTIVE

Here, we investigated the direct impact of soluble oligomeric amyloid beta (Aβ) peptides, the main molecular hallmark of AD, on the leptin system, a major component of central energy metabolism regulation.

METHODS

We developed a new time-resolved fluorescence resonance energy transfer-based Aβ binding assay for the leptin receptor (LepR) and studied the effect of Aβ on LepR function in several in vitro assays. The in vivo effect of Aβ on LepR function was studied in an Aβ-specific AD mouse model and in pro-opiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus.

RESULTS

We revealed specific and high-affinity (Ki = 0.1 nM) binding of Aβ to LepR. Pharmacological characterization of this interaction showed that Aβ binds allosterically to the extracellular domain of LepR and negatively affects receptor function. Negative allosteric modulation of LepR by Aβ was detected at the level of signaling pathways (STAT-3, AKT, and ERK) in vitroand in vivo. Importantly, the leptin-induced response of POMC neurons, key players in the regulation of metabolic function, was completely abolished in the presence of Aβ.

CONCLUSION

Our data indicate that Aβ is a negative allosteric modulator of LepR, resulting in impaired leptin action, and qualify LepR as a new and direct target of Aβ oligomers. Preventing the interaction of Aβ with LepR might improve both the metabolic and cognitive dysfunctions in AD condition.

The Leptin System and Diet: A Mini Review of the Current Evidence

Leptin promotes satiety and modulates energy balance and weight. Diet-induced obesity leads to leptin resistance, exacerbating overeating. We reviewed the literature on the relationship between diet and leptin, which suggests that addressing leptin resistance through dietary interventions can contribute counteracting obesity. Albeit some limitations (e.g., limited rigor, small samples sizes), studies in animals and humans show that diets high in fat, carbohydrates, fructose, and sucrose, and low in protein are drivers of leptin resistance. Despite methodological heterogeneity pertaining to this body of literature, experimental studies show that energy-restricted diets can reduce leptinemia both in the short and long term and potentially reverse leptin resistance in humans. We also discuss limitations of this evidence, future lines of research, and implications for clinical and public health translations. Main limitations include the lack of a single universally-accepted definition of leptin resistance, and of adequate ways to accurately measure it in humans. The use of leptin sensitizers (drugs) and genetically individualized diets are alternatives against leptin resistance that should be further researched in humans. The tested very-low-energy intervention diets are challenging to translate into wide clinical or population recommendations. In conclusion, the link between nutritional components and leptin resistance, as well as research indicating that this condition is reversible, emphasizes the potential of diet to recover sensitivity to this hormone. A harmonized definition of leptin resistance, reliable methods to measure it, and large-scale, translational, clinical, and precision nutrition research involving rigorous methods are needed to benefit populations through these approaches.

The Association Between Serum/Plasma Leptin Levels and Obstructive Sleep Apnea Syndrome: A Meta-Analysis and Meta-Regression

BACKGROUND

Obstructive sleep apnea syndrome (OSAS) is associated with various adipokines. Leptin, a common adipokine, has attracted considerable attention of many researchers in recent years. So far, there has been little agreement on whether blood leptin levels differ in patients with OSAS. Thus, this meta-analysis examined the relationship between serum/plasma leptin levels and the occurrence of OSAS.

METHOD

WanFang, Embase, CNKI, Medline, SinoMed, Web of Science, and PubMed were searched for articles before March 30, 2021, with no language limitations. STATA version 11.0 and R software version 3.6.1 were used to analyze the obtained data. The weighted mean difference and correlation coefficients were used as the main effect sizes with a random-effects model and a fixed-effects model, respectively. Trial sequential analysis was conducted using dedicated software.

RESULT

Screening of 34 publications identified 45 studies that met the inclusion criteria of this meta-analysis and meta-regression. Our results suggested that plasma/serum leptin levels were remarkably higher in individuals with OSAS than in healthy individuals. Subgroup analyses were performed based on OSAS severity, ethnicity, age, body mass index, assay type, and sample source. The serum and plasma leptin levels were increased in nearly all OSAS subgroups compared to those in the corresponding control groups. Meta-regression analysis indicated that age, BMI, severity, assay approaches, study design, PSG type and ethnicity did not have independent effect on leptin levels. Furthermore, a positive relationship between the serum/plasma leptin level and apnea-hypopnea index (AHI) was found in the meta-analysis. The results of the trial sequential analysis suggested that the enrolled studies surpassed the required information size, confirming that our study findings were reliable.

CONCLUSION

Our study results demonstrate that OSAS patients have higher leptin levels in serum/plasma compared to controls, and the serum/plasma leptin level is positively correlated with AHI, especially in adults.

Central Regulation of PCOS: Abnormal Neuronal-Reproductive-Metabolic Circuits in PCOS Pathophysiology

Polycystic ovary syndrome (PCOS) is a common reproductive endocrine disease. PCOS patients are characterized by hyperandrogenemia, anovulation, and metabolic dysfunction. Hypothalamus-pituitary-ovary axis imbalance is considered as an important pathophysiology underlying PCOS, indicating that central modulation, especially the abnormal activation of hypothalamic GnRH neurons plays a vital role in PCOS development. Increased GnRH pulse frequency can promote LH secretion, leading to ovarian dysfunction and abnormal sex steroids synthesis. By contrast, peripheral sex steroids can modulate the action of GnRH neurons through a feedback effect, which is impaired in PCOS, thus forming a vicious cycle. Additionally, hypothalamic GnRH neurons not only serve as the final output pathway of central control of reproductive axis, but also as the central connection point where reproductive function and metabolic state inter-regulate with each other. Metabolic factors, such as insulin resistance and obesity in PCOS patients can regulate GnRH neurons activity, and ultimately regulate reproductive function. Besides, gut hormones act on both brain and peripheral organs to modify metabolic state. Gut microbiota disturbance is also related to many metabolic diseases and has been reported to play an essential part in PCOS development. This review concludes with the mechanism of central modulation and the interaction between neuroendocrine factors and reproductive or metabolic disorders in PCOS development. Furthermore, the role of the gut microenvironment as an important part involved in the abnormal neuronal-reproductive-metabolic circuits that contribute to PCOS is discussed, thus offering possible central and peripheral therapeutic targets for PCOS patients.

Leptin and obesity

An imbalance between calorie intake and energy expenditure produces obesity. It has been a major problem in societies of the developing and developed world. In obesity an excessive amount of fat accumulates in adipose tissue cells as well as in other vital organs like liver, muscles, and pancreas. The adipocytes contain ob genes and express leptin, a 16 kDa protein. In the present communication, we reviewed the molecular basis of the etiopathophysiology of leptin in obesity. Special emphasis has been given to the use of leptin as a drug target for obesity treatment, the role of diet in the modulation of leptin secretion, and reduction of obesity at diminished level of blood leptin induced by physical exercise.

Unraveling the Role of Leptin in Liver Function and Its Relationship with Liver Diseases

Since its discovery twenty-five years ago, the fat-derived hormone leptin has provided a revolutionary framework for studying the physiological role of adipose tissue as an endocrine organ. Leptin exerts pleiotropic effects on many metabolic pathways and is tightly connected with the liver, the major player in systemic metabolism. As a consequence, understanding the metabolic and hormonal interplay between the liver and adipose tissue could provide us with new therapeutic targets for some chronic liver diseases, an increasing problem worldwide. In this review, we assess relevant literature regarding the main metabolic effects of leptin on the liver, by direct regulation or through the central nervous system (CNS). We draw special attention to the contribution of leptin to the non-alcoholic fatty liver disease (NAFLD) pathogenesis and its progression to more advanced stages of the disease as non-alcoholic steatohepatitis (NASH). Likewise, we describe the contribution of leptin to the liver regeneration process after partial hepatectomy, the mainstay of treatment for certain hepatic malignant tumors.

Factors affecting weight loss variability in obesity

Current obesity treatment strategies include diet, exercise, bariatric surgery, and a limited but growing repertoire of medications. Individual weight loss in response to each of these strategies is highly variable. Here we review research into factors potentially contributing to inter-individual variability in response to treatments for obesity, with a focus on studies in humans. Well-recognized factors associated with weight loss capacity include diet adherence, physical activity, sex, age, and specific medications. However, following control for each of these, differences in weight loss appear to persist in response to behavioral, pharmacological and surgical interventions. Adaptation to energy deficit involves complex feedback mechanisms, and inter-individual differences likely to arise from a host of poorly defined genetic factors, as well as differential responses in neurohormonal mechanisms (including gastrointestinal peptides), metabolic efficiency and capacity of tissues, non-exercise activity thermogenesis, thermogenic response to food, and in gut microbiome. A better understanding of the factors involved in inter-individual variability in response to therapies will guide more personalized approaches to the treatment of obesity.

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.

Immunotherapy against angiotensin II receptor ameliorated insulin resistance in a leptin receptor-dependent manner

The angiotensin II type 1 receptor (AT1R) signaling pathway is reported to modulate glucose metabolism. Targeting AT1R, our group invented ATRQβ-001 vaccine, a novel immunotherapeutic strategy to block the activation of AT1R. Here, we evaluated the therapeutic efficacy of ATRQβ-001 vaccine in insulin resistance, and investigated the mechanism. Our results showed that ATRQβ-001 vaccine and specific monoclonal antibody against epitope ATR-001 (McAb-ATR) decreased fasting serum insulin concentration and improved glucose and insulin tolerance in ob/ob mice. These beneficial effects were verified in high-fat diet-induced obese mice. McAb-ATR activated insulin signaling in skeletal muscle and insulin-resistant C2C12 myotubes without affecting liver or white adipose tissue of ob/ob mice. Mechanistically, the favorable impact of McAb-ATR on insulin resistance was abolished in db/db mice and in C2C12 myotubes with leptin receptor knockdown. AT1R knockdown also eradicated the effects of McAb-ATR in C2C12 myotubes. Furthermore, McAb-ATR treatment was able to activate the leptin receptor-mediated JAK2/STAT3 signaling in skeletal muscle of ob/ob mice and C2C12 myotubes. Additionally, angiotensin II downregulated the leptin signaling in skeletal muscle of ob/ob and diet-induced obese mice. We demonstrated that ATRQβ-001 vaccine and McAb-ATR improved whole-body insulin resistance and regulated glucose metabolism in skeletal muscle in a leptin receptor-dependent manner. Our data suggest that immunotherapy targeting AT1R is a novel strategy for treating insulin resistance.

Brown Adipose Tissue: A Metabolic Regulator in a Hypothalamic Cross Talk?

Since the discovery of functionally competent, energy-consuming brown adipose tissue (BAT) in adult humans, much effort has been devoted to exploring this tissue as a means for increasing energy expenditure to counteract obesity. However, despite promising effects on metabolic rate and insulin sensitivity, no convincing evidence for weight-loss effects of cold-activated human BAT exists to date. Indeed, increasing energy expenditure would naturally induce compensatory feedback mechanisms to defend body weight. Interestingly, BAT is regulated by multiple interactions with the hypothalamus from regions overlapping with centers for feeding behavior and metabolic control. Therefore, in the further exploration of BAT as a potential source of novel drug targets, we discuss the hypothalamic orchestration of BAT activity and the relatively unexplored BAT feedback mechanisms on neuronal regulation. With a holistic view on hypothalamic-BAT interactions, we aim to raise ideas and provide a new perspective on this circuit and highlight its clinical relevance.

Associations of short sleep duration with appetite-regulating hormones and adipokines: A systematic review and meta-analysis

In the current study, a systematic review and meta-analysis were conducted to summarize and assess whether short sleep duration is associated with appetite-regulating hormones and adipokine levels. Reference databases were searched for studies related to sleep and appetite-regulating hormones and adipokines. Qualitative and quantitative syntheses were conducted to evaluate the relationship between sleep duration and the level of appetite-regulating hormones and adipokines, including leptin, ghrelin, adiponectin, resistin, and orexin. Twenty-one of 3536 studies, covering a total of 2250 participants, met the inclusion criteria. Leptin, ghrelin, and adiponectin were included in the meta-analysis. Ghrelin levels were higher in the short sleep group (standard mean difference [SMD] = 0.14, 95% CI [0.03, 0.25], p = 0.01). Significant differences between the short sleep group and recommended sleep group were also noted in leptin level experimental subgroup studies (SMD = 0.19, 95% CI [0.03, 0.35], p = 0.02) and ghrelin level cross-sectional subgroup studies (SMD = 0.14, 95% CI [0.02, 0.27], p = 0.03). A rise in leptin and ghrelin levels were also observed in sleep deprivation groups (SMD = 0.24, 95% CI [0.10, 0.39], p = 0.001 and SMD = 0.18, 95% CI [0.04, 0.33], p = 0.01, respectively). In conclusion, short sleep duration is associated with an increased ghrelin level, while sleep deprivation had a significant effect on the levels of both leptin and ghrelin.

The Role of Ceramides in Diabetes and Cardiovascular Disease Regulation of Ceramides by Adipokines

Metabolic dysfunction is intertwined with the pathophysiology of both diabetes and cardiovascular disease. Recently, one particular lipid class has been shown to influence the development and sustainment of these diseases: ceramides. As a subtype of sphingolipids, these species are particularly central to many sphingolipid pathways. Increased levels of ceramides are known to correlate with impaired cardiovascular and metabolic health. Furthermore, the interaction between ceramides and adipokines, most notably adiponectin and leptin, appears to play a role in the pathophysiology of these conditions. Adiponectin appears to counteract the detrimental effects of elevated ceramides, largely through activation of the ceramidase activity of its receptors. Elevated ceramides appear to worsen leptin resistance, which is an important phenomenon in the pathophysiology of obesity and metabolic syndrome.

Preoperative Level of Leptin Can Be a Predictor of Glycemic Control for Patients with Diabetes Undergoing Bariatric Surgery

INTRODUCTION

Bariatric surgery is a first-line treatment for patients with obesity and diabetes. It is uncertain whether leptin has an influence on glycemia in the postoperative period.

METHODS

A cohort study of thirty-eight individuals with obesity and diabetes who underwent laparoscopic Roux-en-Y gastric bypass was undertaken. The levels of leptin, glucose, and glycosylated hemoglobin were verified in the preoperative period and in the first and third postoperative months.

RESULTS

The majority of patients were women (81.6%), and the mean age was 47.3 years (SD 8.8). The mean BMI was reduced by 17.75% (from 47.3 to 38.9 kg/m²) and the mean glycemia by 26.76%. Preoperative leptin had a moderate positive correlation with glucose level at the third month (Pearson r = 0.46, P = 0.02), but not with HbA1c. Patients with leptin above 27.34 ng/mL had a higher glucose level at the end of observation (101.9 versus 88.9, t test, P = 0.042).

CONCLUSION

The search for factors that influence diabetes control after bariatric surgery is of major importance in clinical practice. Our study reported a level of leptin that can predict the prognosis of glycemic control after the intervention. This finding still needs to be validated and confirmed in other populations.

CNS control of the endocrine pancreas

Increasing evidence suggests that, although pancreatic islets can function autonomously to detect and respond to changes in the circulating glucose level, the brain cooperates with the islet to maintain glycaemic control. Here, we review the role of the central and autonomic nervous systems in the control of the endocrine pancreas, including mechanisms whereby the brain senses circulating blood glucose levels. We also examine whether dysfunction in these systems might contribute to complications of type 1 diabetes and the pathogenesis of type 2 diabetes. Graphical abstract.

Circadian rhythms and obesity: Timekeeping governs lipid metabolism

Almost all living organisms have evolved autoregulatory transcriptional-translational feedback loops that produce oscillations with a period of approximately 24-h. These endogenous time keeping mechanisms are called circadian clocks. The main function of these circadian clocks is to drive overt circadian rhythms in the physiology of the organisms to ensure that main physiological functions are in synchrony with the external environment. Disruption of circadian rhythms caused by genetic or environmental factors has long-term consequences for metabolic health. Of relevance, host circadian rhythmicity and lipid metabolism are increasingly recognized to cross-regulate and the circadian clock-lipid metabolism interplay may involve in the development of obesity. Multiple systemic and molecular mechanisms, such as hormones (ie, melatonin, leptin, and glucocorticoid), the gut microbiome, and energy metabolism, link the circadian clock and lipid metabolism, and predictably, the deregulation of circadian clock-lipid metabolism interplay can increase the risk of obesity, which in turn may exacerbate circadian disorganization. Feeding time and dietary nutrients are two of key environmental Zeitgebers affecting the circadian rhythm-lipid metabolism interplay, and the influencing mechanisms in obesity development are highlighted in this review. Together, the characterization of the clock machinery in lipid metabolism aimed at producing a healthy circadian lifestyle may improve obesity care.

Metformin effectively restores the HPA axis function in diet-induced obese rats

INTRODUCTION

The hypothalamo-pituitary-adrenal (HPA) axis is perturbed in obesity. We previously reported presence of leptin resistance in the brainstem and uncoupling between central noradrenergic tone and the HPA axis in obesity-prone (DIO) rats. Metformin is shown to lower body weight and adiposity, but the underlying mechanism is unclear. We hypothesized that this is associated with restored HPA axis function.

METHODS

Adult male DIO rats were placed on either a regular chow or HF diet for 7 weeks. Starting week 4, the animals were given either a low dose (60 mg/kg) or high dose (300 mg/kg) of metformin in drinking water. In addition to body weight and feeding, we examined different arms of the HPA axis to test if metformin can reinstate its function and coupling. To understand potential mechanisms, leptin signaling in the brainstem and circulating free fatty acid levels were also assessed.

RESULTS

Metformin treatment lowered weight gain, fat mass, caloric intake, and serum leptin levels. HPA axis activity as determined by corticotropin-releasing hormone in the median eminence and serum corticosterone was decreased by metformin in a dose-dependent manner, and so was norepinephrine (NE) in the paraventricular nucleus. Importantly, metformin completely normalized the NE-HPA axis uncoupling. While brainstem pSTAT-3 and SOCS-3, key markers of leptin signaling, were not different between groups, circulating saturated and unsaturated free fatty acids were reduced in HF-fed, metformin-treated animals.

CONCLUSIONS

These findings suggest that oral metformin can successfully correct HPA axis dysfunction that is associated with lowered circulating free fatty acids in DIO rats, thereby uncovering a novel effect of metformin in the treatment of 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.

A Combination of Apple Vinegar Drink with Bacillus coagulans Ameliorates High Fat Diet-Induced Body Weight Gain, Insulin Resistance and Hepatic Steatosis

Obesity is a worldwide epidemic characterized by excessive fat accumulation, associated with multiple comorbidities and complications. Emerging evidence points to gut microbiome as a driving force in the pathogenesis of obesity. Vinegar intake, a traditional remedy source of exogenous acetate, has been shown to improve glycemic control and to have anti-obesity effects. New functional foods may be developed by supplementing traditional food with probiotics. B. coagulans is a suitable choice because of its resistance to high temperatures. To analyze the possible synergic effect of Vinegar and B. coagulans against the metabolic alterations induced by a high fat diet (HFD), we fed twelve-week-old C57BL/6 mice with HFD for 5 weeks after 2 weeks of acclimation on a normal diet. Then, food intake, body weight, blood biochemical parameters, histology and liver inflammatory markers were analyzed. Although vinegar drink, either alone or supplemented with B. coagulans, reduced food intake, attenuated body weight gain and enhanced glucose tolerance, only the supplemented drink improved the lipid serum profile and prevented hepatic HFD-induced overexpression of CD36, IL-1β, IL-6, LXR and SREBP, thus reducing lipid deposition in the liver. The beneficial properties of the B. coagulans-supplemented vinegar appear to be mediated by a reduction in insulin and leptin circulating levels.

JAK Inhibitors Suppress Cancer Cachexia-Associated Anorexia and Adipose Wasting in Mice

BACKGROUND

Cachexia, a syndrome of muscle atrophy, adipose loss, and anorexia, is associated with reduced survival in cancer patients. The colon adenocarcinoma C26c20 cell line secretes the cytokine leukemia inhibitory factor (LIF) which induces cachexia. We characterized how LIF promotes cachexia-associated weight loss and anorexia in mice through JAK-dependent changes in adipose and hypothalamic tissues.

METHODS

Cachexia was induced in vivo with the heterotopic allotransplanted administration of C26c20 colon adenocarcinoma cells or the intraperitoneal administration of recombinant LIF in the absence or presence of JAK inhibitors. Blood, adipose, and hypothalamic tissues were collected and processed for cyto/adipokine ELISAs, immunoblot analysis, and quantitative RT-PCR. Cachexia-associated lipolysis was induced in vitro by stimulating differentiated adipocytes with recombinant LIF or IL-6 in the absence or presence of lipase or JAK inhibitors. These adipocytes were processed for glycerol release into the media, immunoblot analysis, and RT-PCR.

RESULTS

Tumor-secreted LIF induced changes in adipose tissue expression and serum levels of IL-6 and leptin in a JAK-dependent manner influencing cachexia-associated adipose wasting and anorexia. We identified two JAK inhibitors that block IL-6 family-mediated adipocyte lipolysis and IL-6 induction using an in vitro cachexia lipolysis assay. JAK inhibitors administered to the in vivo C26c20 cancer cachexia mouse models led to 1) a decrease in STAT3 phosphorylation in hypothalamic and adipose tissues, 2) a reverse in the cachexia serum cyto/adipokine signature, 3) a delay in cancer cachexia-associated anorexia and adipose loss, and 4) an improvement in overall survival.

CONCLUSIONS

JAK inhibitors suppress LIF-associated adipose loss and anorexia in both in vitro and in vivo models of cancer cachexia.

NLRP3 Inflammasome Activation in Adipose Tissues and Its Implications on Metabolic Diseases

Adipose tissue is an active endocrine and immune organ that controls systemic immunometabolism via multiple pathways. Diverse immune cell populations reside in adipose tissue, and their composition and immune responses vary with nutritional and environmental conditions. Adipose tissue dysfunction, characterized by sterile low-grade chronic inflammation and excessive immune cell infiltration, is a hallmark of obesity, as well as an important link to cardiometabolic diseases. Amongst the pro-inflammatory factors secreted by the dysfunctional adipose tissue, interleukin (IL)-1β, induced by the NLR family pyrin domain-containing 3 (NLRP3) inflammasome, not only impairs peripheral insulin sensitivity, but it also interferes with the endocrine and immune functions of adipose tissue in a paracrine manner. Human studies indicated that NLRP3 activity in adipose tissues positively correlates with obesity and its metabolic complications, and treatment with the IL-1β antibody improves glycaemia control in type 2 diabetic patients. In mouse models, genetic or pharmacological inhibition of NLRP3 activation pathways or IL-1β prevents adipose tissue dysfunction, including inflammation, fibrosis, defective lipid handling and adipogenesis, which in turn alleviates obesity and its related metabolic disorders. In this review, we summarize both the negative and positive regulators of NLRP3 inflammasome activation, and its pathophysiological consequences on immunometabolism. We also discuss the potential therapeutic approaches to targeting adipose tissue inflammasome for the treatment of obesity and its related metabolic disorders.

Genetic variation, adipokines, and cardiometabolic disease

Adipokines are adipocyte-secreted cell signalling proteins that travel to distant target organs and tissues, where they regulate a variety of biological actions implicated in cardiometabolic health. In the past decade, genome-wide association studies have identified multiple genetic variants associated with circulating levels of adipokines, providing new instruments for examining the role of adipokines in cardiometabolic pathologies. Currently, there is limited genetic evidence of causal relationships between adipokines and cardiometabolic disease, which is consistent with findings from randomized clinical trials that have thus far shown limited success for adipokine-based treatments in improving cardiometabolic health. Incorporating human genetic data in early phases of target selection is essential for enhancing the success of adipokine-based therapies for cardiometabolic disease.

Partial Leptin Reduction: An Emerging Weight Loss Paradigm

Leptin-based obesity pharmacotherapies were originally developed according to the lipostatic view that elevated circulating leptin levels promote a negative energy balance. A series of independent preclinical findings suggest, however, that a partial reduction in circulating leptin levels (either by immunoneutralization, a peripherally restricted CB1 receptor inverse agonist, or bariatric surgery) can paradoxically lead to weight loss.

CNS Regulation of Glucose Homeostasis: Role of the Leptin-Melanocortin System

PURPOSE OF REVIEW

In this brief review, we highlight studies that have contributed to our current understanding of glucose homeostasis by the central nervous system (CNS) leptin-melanocortin system, particularly proopiomelanocortin neurons and melanocortin-4 receptors (MC4R).

RECENT FINDINGS

Leptin deficiency is associated with insulin resistance and impaired glucose metabolism whereas leptin administration improves tissue glucose uptake/oxidation and reduces hepatic glucose output. These antidiabetic effects of leptin have been demonstrated in experimental animals and humans, even when circulating insulin levels are barely detectable. Recent evidence suggests that these antidiabetic actions of leptin are mediated, in large part, by stimulation of leptin receptors (LRs) in the CNS and require activation of proopiomelanocortin (POMC) neurons and MC4R. These chronic antidiabetic effects of the CNS leptin-melanocortin system appear to be independent of autonomic nervous system and pituitary-thyroid-adrenal (PTA) axis mechanisms. The powerful antidiabetic actions of the CNS leptin-melanocortin system are capable of normalizing plasma glucose even in the absence of insulin and involve interactions of multiple neuronal populations and intracellular signaling pathways. Although the links between the CNS leptin-melanocortin system and its chronic effects on peripheral tissue glucose metabolism are still uncertain, they are independent of insulin action, activation of the autonomic nervous system, or the PTA axis. Unraveling the pathways that contribute to the powerful antidiabetic effects of the CNS leptin-melanocortin system may provide novel therapeutic approaches for diabetes mellitus.

Interaction of glucose sensing and leptin action in the brain

Background

In response to energy abundant or deprived conditions, nutrients and hormones activate hypothalamic pathways to maintain energy and glucose homeostasis. The underlying CNS mechanisms, however, remain elusive in rodents and humans.

Scope of review

Here, we first discuss brain glucose sensing mechanisms in the presence of a rise or fall of plasma glucose levels, and highlight defects in hypothalamic glucose sensing disrupt in vivo glucose homeostasis in high-fat fed, obese, and/or diabetic conditions. Second, we discuss brain leptin signalling pathways that impact glucose homeostasis in glucose-deprived and excessed conditions, and propose that leptin enhances hypothalamic glucose sensing and restores glucose homeostasis in short-term high-fat fed and/or uncontrolled diabetic conditions.

Major conclusions

In conclusion, we believe basic studies that investigate the interaction of glucose sensing and leptin action in the brain will address the translational impact of hypothalamic glucose sensing in diabetes and obesity.

Suppressing adipocyte inflammation promotes insulin resistance in mice

OBJECTIVE

Obesity-induced insulin resistance is closely associated with chronic subclinical inflammation in white adipose tissue. However, the mechanistic involvement of adipocyte-derived inflammation under these disease conditions remains unclear. Our aim was to investigate the relative inflammation-related contributions of adipocytes and macrophages to insulin sensitivity.

METHODS

RIDα/β is an adenoviral protein complex that inhibits several inflammatory pathways, including TLR4, TNFα, and IL1β signaling. We generated novel mouse models with adipocyte-specific and macrophage-specific doxycycline (dox)-inducible RIDα/β-transgenic mice (RID^ad and RID^mac mice, respectively).

RESULTS

RIDα/β induction significantly reduced LPS-stimulated inflammatory markers, such as Tnf, Il1b, and Saa3 in adipose tissues. Surprisingly, RID^ad mice had elevated levels of postprandial glucose and insulin and exhibited glucose intolerance and insulin resistance, even under chow-fed conditions. Moreover, the RID^ad mice displayed further insulin resistance under obesogenic (high-fat diet, HFD) conditions despite reduced weight gain. In addition, under pre-existing obese and inflamed conditions on an HFD, subsequent induction of RIDα/β in RID^ad mice reduced body weight gain, further exacerbating glucose tolerance, enhancing insulin resistance and fatty liver, and reducing adiponectin levels. This occurred despite effective suppression of the inflammatory pathways (including TNFα and IL1β). In contrast, RID^mac mice, upon HFD feeding, displayed similar weight gain, comparable adiponectin levels, and insulin sensitivity, suggesting that the inflammatory properties of macrophages did not exert a negative impact on metabolic readouts. RIDα/β expression and the ensuing suppression of inflammation in adipocytes enhanced adipose tissue fibrosis and reduced vascularization.

CONCLUSION

Our novel findings further corroborate our previous observations suggesting that suppressing adipocyte inflammation impairs adipose tissue function and promotes insulin resistance, despite beneficial effects on weight gain.

Leptin: Less Is More

The successful use of leptin for the treatment of individuals with lipodystrophy and leptin deficiency is well established. However, pharmacological approaches of leptin therapy for the treatment of diet-induced obesity have been ineffective. There is ample room for a better understanding of the much famed "leptin resistance" phenomenon. Our recent data in this area prompt us to call for a conceptual shift. This shift entails a model in which a reduction of bioactive leptin levels in the context of obesity triggers a high degree of leptin sensitization and improved leptin action, both centrally and peripherally. Put another way, hyperleptinemia per se causes leptin resistance and associated metabolic disorders. In this perspective, we briefly discuss the underlying conceptual steps that led us to explore partial leptin reduction as a viable therapeutic avenue. We hope this discussion will contribute to potential future applications of partial leptin reduction therapy for the treatment of obesity and type 2 diabetes.

Adipose Tissue-Liver Cross Talk in the Control of Whole-Body Metabolism: Implications in Nonalcoholic Fatty Liver Disease

Adipose tissue and the liver play significant roles in the regulation of whole-body energy homeostasis, but they have not evolved to cope with the continuous, chronic, nutrient surplus seen in obesity. In this review, we detail how prolonged metabolic stress leads to adipose tissue dysfunction, inflammation, and adipokine release that results in increased lipid flux to the liver. Overall, the upshot of hepatic fat accumulation alongside an insulin-resistant state is that hepatic lipid enzymatic pathways are modulated and overwhelmed, resulting in the selective buildup of toxic lipid species, which worsens the pro-inflammatory and pro-fibrotic shift observed in nonalcoholic steatohepatitis.

Anti-obesity therapy with peripheral CB1 blockers: from promise to safe(?) practice

Pharmacological blockers of the cannabinoid receptor type-1 (CB1) have been considered for a long time as the holy grail of obesity pharmacotherapy. These agents were hastily released in the clinical setting, due to their clear-cut therapeutic efficacy. However, the first generation of these drugs, which were able to target both the brain and peripheral tissues, had serious neuropsychiatric effects, leading authorities to ban their clinical use. New peripherally restricted CB1 blockers, characterized by low brain penetrance, have been developed over the past 10 years. In preclinical studies, these molecules seem to overcome the neuropsychiatric negative effects previously observed with brain-penetrant CB1 inhibitors, while retaining or even outperforming their efficacy. The mechanisms of action of these peripherally restricted compounds are only beginning to emerge, and a balanced discussion of the risk/benefits ratio associated to their possible clinical use is urgently needed, in order to avoid repeating past mistakes. Here, we will critically discuss the advantages and the possible hidden threats associated with the use of peripheral CB1 blockers for the pharmacotherapy of obesity and its associated metabolic complications. We will address whether this novel pharmacological approach might 'compete' with current pharmacotherapies for obesity and diabetes, while also conceptualizing future CB1-based pharmacological trends that may significantly lower the risk/benefits ratio associated with the use of these drugs.

Partial leptin deficiency confers resistance to diet-induced obesity in mice

Objective

Hyperleptinemia per se is sufficient to promote leptin resistance in the obese state. Leptin sensitivity can be restored by reducing circulating leptin levels within a physiologically healthy range and is a viable antiobesity and antidiabetic strategy. However, a previous study suggests that partial leptin deficiency favors diet-induced obesity and related metabolic disorders in mice, arguing that a lower leptin level may indeed promote diet-induced obesity and its associated metabolic disorders. Here, we aim to elucidate what the impact of partial leptin deficiency is on fat mass and insulin sensitivity.

Methods

We used two different mouse models of partial leptin deficiency: an adipocyte-specific congenital heterozygous leptin knockout mouse line (LepHZ) and the well-established whole body heterozygous leptin knockout mouse (OBHZ). The metabolic studies of OBHZ and LepHZ mice were performed both on normal carbohydrate-rich chow diet and on a high-fat diet (HFD). Male and female mice were included in the study to account for sex-specific differences. Body weight, food intake, glucose tolerance, and insulin tolerance were tested. Histology of adipose tissue and liver tissue allowed insights into adipose tissue inflammation and hepatic triglyceride content. Immunohistochemistry was paired with RT-PCR analysis for expression levels of inflammatory markers.

Results

Both OBHZ and LepHZ mice displayed reduced circulating leptin levels on the chow diet and HFD. On chow diet, male OBHZ and LepHZ mice showed elevated fat mass and body weight, while their glucose tolerance and insulin sensitivity remained unchanged. However, the inability in partially leptin-deficient mice to fully induce circulating leptin during the development of diet-induced obesity results in reduced food intake and leaner mice with lower body weight compared to their littermate controls. Importantly, a strong reduction of adipose tissue inflammation is observed along with improvements in insulin sensitivity and enhanced glucose tolerance. Additionally, partial leptin deficiency protects the mice from fatty liver and liver fibrosis. Chronically HFD-fed OBHZ and LepHZ mice remain more sensitive to exogenous leptin injection, as reflected by their reduced food intake upon an acute leptin treatment.

Conclusion

In response to HFD feeding, the inability to upregulate leptin levels due to partial leptin deficiency protects mice from diet-induced obesity and metabolic dysregulation. Thus, in an obesogenic environment, maintaining lower leptin levels is highly beneficial for both obesity and diabetes management. Chronic leptin reduction represents a viable preventive strategy whose efficacy awaits clinical testing.

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Partial leptin deficiency protects from diet-induced obesity.

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Reduced leptin protects from diet-induced obesity independent of sex.

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Reduction of circulating leptin levels inhibits HFD-induced adipose tissue inflammation.

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Partial leptin deficiency confers resistance to HFD-induced liver fibrosis.

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Partial leptin deficiency enhances leptin sensitivity.

Neuronal control of peripheral nutrient partitioning

The appropriate utilisation, storage and conversion of nutrients in peripheral tissues, referred to as nutrient partitioning, is a fundamental process to adapt to nutritional and metabolic challenges and is thus critical for the maintenance of a healthy energy balance. Alterations in this process during nutrient excess can have deleterious effects on glucose and lipid homeostasis and contribute to the development of obesity and type 2 diabetes. Nutrient partitioning is a complex integrated process under the control of hormonal and neural signals. Neural control relies on the capacity of the brain to sense circulating metabolic signals and mount adaptive neuroendocrine and autonomic responses. This review aims to discuss the hypothalamic neurocircuits and molecular mechanisms controlling nutrient partitioning and their potential contribution to metabolic maladaptation and disease.

Leptin: Is It Thermogenic?

Animals that lack the hormone leptin become grossly obese, purportedly for 2 reasons: increased food intake and decreased energy expenditure (thermogenesis). This review examines the experimental evidence for the thermogenesis component. Analysis of the data available led us to conclude that the reports indicating hypometabolism in the leptin-deficient ob/ob mice (as well as in the leptin-receptor-deficient db/db mice and fa/fa rats) derive from a misleading calculation artefact resulting from expression of energy expenditure per gram of body weight and not per intact organism. Correspondingly, the body weight-reducing effects of leptin are not augmented by enhanced thermogenesis. Congruent with this, there is no evidence that the ob/ob mouse demonstrates atrophied brown adipose tissue or diminished levels of total UCP1 mRNA or protein when the ob mutation is studied on the inbred C57BL/6 mouse background, but a reduced sympathetic nerve activity is observed. On the outbred "Aston" mouse background, brown adipose tissue atrophy is seen, but whether this is of quantitative significance for the development of obesity has not been demonstrated. We conclude that leptin is not a thermogenic hormone. Rather, leptin has effects on body temperature regulation, by opposing torpor bouts and by shifting thermoregulatory thresholds. The central pathways behind these effects are largely unexplored.

A role for leptin and ghrelin in the augmentation of heroin seeking induced by chronic food restriction

RATIONAL

Caloric restriction increases the risk of relapse in abstinent drug users. Hormones involved in the regulation of energy balance and food intake, such as leptin and ghrelin, are implicated in drug-related behaviors.

OBJECTIVES

We investigated the role of leptin and ghrelin in the augmentation of heroin seeking induced by chronic food restriction.

METHODS

Rats self-administered heroin (0.1 mg/kg/infusion) for 10 days followed by 14 days of drug withdrawal. During withdrawal, rats were food restricted to 90% of their original body weight or were given free access to food. In experiment 1, we measured the plasma concentrations of leptin and ghrelin following heroin self-administration and withdrawal. In experiment 2, leptin was administered centrally (2.0 or 4.0 μg; i.c.v.) prior to a heroin-seeking test under extinction conditions. High density of both leptin and ghrelin receptors was previously identified in the ventral tegmental area (VTA), suggesting a direct effect on reward and motivation. Hence, we administered leptin (experiment 3; 0.125 or 0.250 μg/side), or ghrelin receptor antagonist JMV 2959 (experiment 4; 2.0 or 10.0 μg/side) directly into the VTA prior to the heroin-seeking test.

RESULTS

Chronic food restriction significantly decreased plasma levels of leptin and elevated plasma levels of ghrelin. Central administration of leptin had no statistically significant effect on heroin seeking. Intra-VTA administration of either leptin or JMV 2959 dose-dependently and selectively decreased heroin seeking in the food-restricted rats.

CONCLUSIONS

Leptin and ghrelin transmission in the VTA can modulate the augmentation of heroin seeking induced by chronic food restriction.

Alliin, an Allium sativum Nutraceutical, ReducesMetaflammation Markers in DIO Mice

Obesity generates a chronic low-grade inflammatory state which promotes oxidative stress and triggers comorbidities. Alliin is the main organosulfur compound in garlic and has been shown to induce a decrease in the expression of proinflammatory cytokines; its systemic effect on metabolic parameters and adipose tissue is not yet known, however. After nine weeks of HFD and with obesity established in C57BL/6 mice, we observed that a daily treatment with alliin for 3.5 weeks (15 mg/kg) did not affect body weight, but significantly improved insulin sensitivity and glucose tolerance, both evaluated through a blood glucose monitoring system. Once alliin treatment was completed, serum, adipose tissue, and organs of interest related to metabolism were removed for further analysis. We observed that alliin significantly decreased the size of adipocytes from epididymal adipose tissue, evaluated via microscopy. A decrease in gene expression and serum protein levels of the adipocytokines leptin and resistin, as well as decreased serum IL-6 concentration, were detected by qRT-PCR and ELISA, respectively. It did not, however, affect mRNA expression of antioxidant enzymes in the liver. Taken altogether, these results indicate that treatment with alliin reduces metaflammation markers in DIO mice and improves some metabolic parameters without affecting others.

Salvianolic acid B prevents body weight gain and regulates gut microbiota and LPS/TLR4 signaling pathway in high-fat diet-induced obese mice

Salvianolic acid B prevents body weight gain and improves insulin sensitivity in obese mice. The underlying mechanism behind these effects may be associated with the regulation of metabolic endotoxemia, gut microbiota homeostasis and LPS/TLR4 pathway.

Anti-Lipolysis Induced by Insulin in Diverse Pathophysiologic Conditions of Adipose Tissue

As an important energy reservoir, adipose tissue maintains lipid balance and regulates energy metabolism. When the body requires energy, adipocytes provide fatty acids to peripheral tissues through lipolysis. Insulin plays an important role in regulating normal fatty acid levels by inhibiting lipolysis. When the morphology of adipose tissue is abnormal, its microenvironment changes and the lipid metabolic balance is disrupted, which seriously impairs insulin sensitivity. As the most sensitive organ to respond to insulin, lipolysis levels in adipose tissue are affected by impaired insulin function, which results in serious metabolic diseases. However, the specific underlying mechanisms of this process have not yet been fully elucidated, and further study is required. The purpose of this review is to discuss the effects of adipose tissue on the anti-lipolysis process triggered by insulin under different conditions. In particular, the functional changes of this process respond to inconsonantly morphological changes of adipose tissue.

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.

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.