Physiological concentrations of leptin do not affect human neutrophils

Leptin and Asthma: What Are the Interactive Correlations?

Leptin is an adipokine directly correlated with the proinflammatory obese-associated phenotype. Leptin has been demonstrated to inhibit adipogenesis, promote fat demarcation, promote a chronic inflammatory state, increase insulin sensitivity, and promote angiogenesis. Leptin, a regulator of the immune response, is implicated in the pathology of asthma. Studies involved in the key cell reaction and animal models of asthma have provided vital insights into the proinflammatory role of leptin in asthma. Many studies described the immune cell and related cellular pathways activated by leptin, which are beneficial in asthma development and increasing exacerbations. Subsequent studies relating to animal models support the role of leptin in increasing inflammatory cell infiltration, airway hyperresponsiveness, and inflammatory responses. However, the conclusive effects of leptin in asthma are not well elaborated. In the present study, we explored the general functions and the clinical cohort study supporting the association between leptin and asthma. The main objective of our review is to address the knowns and unknowns of leptin on asthma. In this perspective, the arguments about the different faces of leptin in asthma are provided to picture the potential directions, thus yielding a better understanding of asthma development.

The complex role of adipokines in obesity, inflammation, and autoimmunity

The global obesity epidemic is a major contributor to chronic disease and disability in the world today. Since the discovery of leptin in 1994, a multitude of studies have characterized the pathological changes that occur within adipose tissue in the obese state. One significant change is the dysregulation of adipokine production. Adipokines are an indispensable link between metabolism and optimal immune system function; however, their dysregulation in obesity contributes to chronic low-grade inflammation and disease pathology. Herein, I will highlight current knowledge on adipokine structure and physiological function, and focus on the known roles of these factors in the modulation of the immune response. I will also discuss adipokines in rheumatic and autoimmune diseases.

An Update on the Role of Leptin in the Immuno-Metabolism of Cartilage

Since its discovery in 1994, leptin has been considered as an adipokine with pleiotropic effects. In this review, we summarize the actual information about the impact of this hormone on cartilage metabolism and pathology. Leptin signalling depends on the interaction with leptin receptor LEPR, being the long isoform of the receptor (LEPRb) the one with more efficient intracellular signalling. Chondrocytes express the long isoform of the leptin receptor and in these cells, leptin signalling, alone or in combination with other molecules, induces the expression of pro-inflammatory molecules and cartilage degenerative enzymes. Leptin has been shown to increase the proliferation and activation of immune cells, increasing the severity of immune degenerative cartilage diseases. Leptin expression in serum and synovial fluid are related to degenerative diseases such as osteoarthritis (OA), rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Inhibition of leptin signalling showed to have protective effects in these diseases showing the key role of leptin in cartilage degeneration.

Scrutinizing Mechanisms of the 'Obesity Paradox in Sepsis': Obesity Is Accompanied by Diminished Formation of Neutrophil Extracellular Traps (NETs) Due to Restricted Neutrophil-Platelet Interactions

Systemic inflammation is a detrimental condition associated with high mortality. However, obese individuals seem to have higher chances of surviving sepsis. To elucidate what immunological differences exist between obese and lean individuals we studied the course of endotoxemia in mice fed high-fat diet (HFD) and ob/ob animals. Intravital microscopy revealed that neutrophil extracellular trap (NET) formation in liver vasculature is negligible in obese mice in sharp contrast to their lean counterparts (ND). Unlike in lean individuals, neutrophil influx is not driven by leptin or interleukin 33 (IL-33), nor occurs via a chemokine receptor CXCR2. In obese mice less platelets interact with neutrophils forming less aggregates. Platelets transfer from ND to HFD mice partially restores NET formation, and even further so upon P-selectin blockage on them. The study reveals that in obesity the overexaggerated inflammation and NET formation are limited during sepsis due to dysfunctional platelets suggesting their targeting as a therapeutic tool in systemic inflammation.

Insulin and obesity transform hypothalamic-pituitary-adrenal axis stemness and function in a hyperactive state

Objective

Metabolic diseases are an increasing problem in society with the brain-metabolic axis as a master regulator of the human body for sustaining homeostasis under metabolic stress. However, metabolic inflammation and disease will trigger sustained activation of the hypothalamic-pituitary-adrenal axis. In this study, we investigated the role of metabolic stress on progenitor cells in the hypothalamic-pituitary-adrenal axis.

Methods

In vitro, we applied insulin and leptin to murine progenitor cells isolated from the pituitary and adrenal cortex and examined the role of these hormones on proliferation and differentiation. In vivo, we investigated two different mouse models of metabolic disease, obesity in leptin-deficient ob/ob mice and obesity achieved via feeding with a high-fat diet.

Results

Insulin was shown to lead to enhanced proliferation and differentiation of both pituitary and adrenocortical progenitors. No alterations in the progenitors were noted in our chronic metabolic stress models. However, hyperactivation of the hypothalamic-pituitary-adrenal axis was observed and the expression of the appetite-regulating genes Npy and Agrp changed in both the hypothalamus and adrenal.

Conclusions

It is well-known that chronic stress and stress hormones such as glucocorticoids can induce metabolic changes including obesity and diabetes. In this article, we show for the first time that this might be based on an early sensitization of stem cells of the hypothalamic-pituitary-adrenal axis. Thus, pituitary and adrenal progenitor cells exposed to high levels of insulin are metabolically primed to a hyper-functional state leading to enhanced hormone production. Likewise, obese animals exhibit a hyperactive hypothalamic-pituitary-adrenal axis leading to adrenal hyperplasia. This might explain how stress in early life can increase the risk for developing metabolic syndrome in adulthood.

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Insulin enhances proliferation and differentiation of adrenocortical and pituitary progenitors.

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Obesity leads to hyperactivation and priming of the HPA axis.

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Obesity leads to overexpression of appetite-regulating genes in the hypothalamus.

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Obesity leads to a decrease in the expression of appetite-regulating genes in the adrenal gland.

Obesity, Fat Mass and Immune System: Role for Leptin

Obesity is an epidemic disease characterized by chronic low-grade inflammation associated with a dysfunctional fat mass. Adipose tissue is now considered an extremely active endocrine organ that secretes cytokine-like hormones, called adipokines, either pro- or anti-inflammatory factors bridging metabolism to the immune system. Leptin is historically one of most relevant adipokines, with important physiological roles in the central control of energy metabolism and in the regulation of metabolism-immune system interplay, being a cornerstone of the emerging field of immunometabolism. Indeed, leptin receptor is expressed throughout the immune system and leptin has been shown to regulate both innate and adaptive immune responses. This review discusses the latest data regarding the role of leptin as a mediator of immune system and metabolism, with particular emphasis on its effects on obesity-associated metabolic disorders and autoimmune and/or inflammatory rheumatic diseases.

Leptin Mediates In Vivo Neutrophil Migration: Involvement of Tumor Necrosis Factor-Alpha and CXCL1

Leptin directly activates macrophages and lymphocytes, but the role of leptin in neutrophil activation and migration is still controversial. Here, we investigate the in vivo mechanisms of neutrophil migration induced by leptin. The intraperitoneal injection of leptin (1 mg/kg) induces a time- and concentration-dependent neutrophil influx. We did not observe the enhancement of lipid bodies/droplets in neutrophils, after leptin treatment, as we had observed previously in peritoneal macrophages. The participation of leukotriene B₄ (LTB₄) in neutrophil recruitment triggered by leptin was investigated using different strategies. Leptin-induced neutrophil recruitment occurs both in the absence of 5-lipoxygenase activity in 5-lipoxygenase (5-LO)^-/- mice and after the administration of either 5-LO inhibitor (Zileuton) or the LTB₄ receptor antagonist (U-75302). Moreover, no direct induction of LTB₄ by leptin could be observed. Neutrophil influx could not be prevented by the mammalian target of rapamycin (mTOR) inhibitor, rapamycin, contrasting with the leptin-induced signaling for lipid body formation in macrophage that is mTOR-dependent. Leptin administration led to tumor necrosis factor-alpha (TNFα) production by the peritoneal cells both in vivo and in vitro. In addition, neutrophil recruitment was inhibited in tumor necrosis factor receptor 1 (TNFR1^-/-) mice, indicating a role for TNF in leptin-induced neutrophil recruitment to the peritoneal cavity. Leptin-induced neutrophil influx was PI3Kγ-dependent, as it was absent in PI3Kγ^-/- mice. Accordingly, leptin induced the peritoneal cells to produce CXCL1, both in vivo and in vitro, and the neutrophil influx was ablated after using an antibody against CXCL1. Our results establish TNFα/TNFR1- and CXCL1-dependent signaling as important pathways for leptin-induced neutrophil migration in vivo.

Leptin in the interplay of inflammation, metabolism and immune system disorders

Leptin is one of the most relevant factors secreted by adipose tissue and the forerunner of a class of molecules collectively called adipokines. Initially discovered in 1994, its crucial role as a central regulator in energy homeostasis has been largely described during the past 20 years. Once secreted into the circulation, leptin reaches the central and peripheral nervous systems and acts by binding and activating the long form of leptin receptor (LEPR), regulating appetite and food intake, bone mass, basal metabolism, reproductive function and insulin secretion, among other processes. Research on the regulation of different adipose tissues has provided important insights into the intricate network that links nutrition, metabolism and immune homeostasis. The neuroendocrine and immune systems communicate bi-directionally through common ligands and receptors during stress responses and inflammation, and control cellular immune responses in several pathological situations including immune-inflammatory rheumatic diseases. This Review discusses the latest findings regarding the role of leptin in the immune system and metabolism, with particular emphasis on its effect on autoimmune and/or inflammatory rheumatic diseases, such as rheumatoid arthritis and osteoarthritis.

Influence of exogenous leptin on redox homeostasis in neutrophils and lymphocytes cultured in synovial fluid isolated from patients with rheumatoid arthritis

Objectives

Leptin is an adipose cells derived hormone that regulates energy homeostasis within the body. Energy metabolism of immune cells influences their activity within numerous pathological states, but the effect of leptin on these cells in unclear. On the one hand, it was observed that leptin induces neutrophils chemotaxis and modulates phagocytosis. On the other hand, neutrophils exposed to leptin did not display detectable Ca²⁺ ions mobilization or β₂-integrin upregulation. In this study, we investigated the effect of leptin on the redox homeostasis in lymphocytes and neutrophils.

Material and methods

Neutrophils and lymphocytes were isolated by density-gradient centrifugation of blood from healthy volunteers. Cells were cultured with or without leptin (100 ng/ml for lymphocytes and 500 ng/ml for neutrophils) or with or without synovial fluid (85%) for 0–72 h. Culture media were not changed during incubation. Cells were homogenized and homogenate was frozen until laboratory measurements. Redox homeostasis was assessed by the reduced glutathione (GSH) vs. oxidized glutathione (GSSG) ratio and membrane lipid peroxidation evaluation.

Results

Lymphocytes cultured with leptin and synovial fluid showed a significant increase of the GSSG level. The GSSG/GSH ratio increased by 184 ±37%. In neutrophils incubated in a similar environment, the GSSG/GSH ratio increased by just 21 ±7%, and the effect was observed irrespectively of whether they were exposed to leptin or synovial fluid or both together. Neither leptin nor synovial fluid influenced lipid peroxidation in neutrophils, but in lymphocytes leptin intensified lipid peroxidation.

Conclusions

Leptin altered the lymphocytes, but not neutrophils redox state. Because firstly neutrophils are anaerobic cells and have just a few mitochondria and secondly lymphocytes have typical aerobic metabolism, the divergence of our data supports the hypothesis that leptin induces oxidative stress by modulation of mitochondria.

Leptin Regulation of Immune Responses

Leptin is a regulatory hormone with multiple roles in the immune system. We favor the concept that leptin signaling 'licenses' various immune cells to engage in immune responses and/or to differentiate. Leptin is an inflammatory molecule that is capable of activating both adaptive and innate immunity. It can also 'enhance' immune functions, including inflammatory cytokine production in macrophages, granulocyte chemotaxis, and increased Th17 proliferation. Leptin can also 'inhibit' cells; CD4(+) T cells are inhibited from differentiating into regulatory T cells in the presence of elevated leptin, while NK cells can exhibit impaired cytotoxicity under the same circumstances. Consequently, understanding the effect of leptin signaling is important to appreciate various aspects of immune dysregulation observed in malnutrition, obesity, and autoimmunity.

Green tea polyphenol extract in vivo attenuates inflammatory features of neutrophils from obese rats

PURPOSE

Our study aimed to evaluate whether obesity induced by cafeteria diet changes the neutrophil effector/inflammatory function and whether treatment with green tea extract (GT) can improve neutrophil function.

METHODS

Male Wistar rats were treated with GT by gavage (12 weeks/5 days/week; 500 mg/kg of body weight), and obesity was induced by cafeteria diet (8 weeks). Neutrophils were obtained from the peritoneal cavity (injection of oyster glycogen). The following analyses were performed: phagocytic capacity, chemotaxis, myeloperoxidase activity (MPO), hypochlorous acid (HOCl), superoxide anion (O 2 (·-) ), hydrogen peroxide (H2O2), IL-1β, IL-6 and TNFα, mRNA levels of inflammatory genes, calcium mobilisation, activities of antioxidant enzymes, hexokinase and G6PDH.

RESULTS

Neutrophils from obese rats showed a significant decrease in migration capacity, H2O2 and HOCl production, MPO activity and O 2 (·-) production. Phagocytosis and CD11b mRNA levels were increased, while inflammatory cytokines release remained unmodified. mRNA levels of TLR4 and IκK were enhanced. Treatment of obese rats with GT increased neutrophil migration, MPO activity, H2O2, HOCl and O 2 (·-) production, whereas TNF-α and IL-6 were decreased (versus obese). Similar reductions in TLR4, IκK and CD11b mRNA were observed. Catalase and hexokinase were increased by obesity, while SOD and G6PDH were decreased. Treatment with GT reduced catalase and increased the GSH/GSSG ratio.

CONCLUSION

In response to a cafeteria diet, we found a decreased chemotaxis, H2O2 release, MPO activity and HOCl production. We also showed a significant immunomodulatory effect of GT on the obese condition recovering some of these factors such H2O2 and HOCl production, also reducing the levels of inflammatory cytokines.

The effect of leptin on the respiratory burst of human neutrophils cultured in synovial fluid

Objectives

Leptin is a hormone responsible for nutritional status and immune competence coordination. In rheumatoid arthritis (RA) increased leptin levels were observed in both serum and synovial fluid. Its influence on development of the disease still remains unclear. So far, research on leptin's influence on the emission of reactive oxygen intermediates (ROI) measured with chemiluminescence (CL) has provided unclear and contradictory results. In this study, we evaluated the influence of leptin on oxidative activity of neutrophils isolated from blood of healthy volunteers and cultured in different amounts of synovial fluid (SF) from patients with RA.

Material and methods

Neutrophils’ oxidative metabolism was measured by two types of CL. The first one, luminol-dependent CL (CL-lum), allows one to determine phagocytic activity and the level of ROI generated in a myeloperoxidase-dependent manner. The second method used was lucigenin-dependent CL (CL-luc), which monitors ROI production dependent on the NADPH oxidase enzyme complex located in the cell membranes of neutrophils and enables one to determine the scope of extracellular ROI emission.

Results

Neutrophils stimulated by opsonized zymosan show a decrease in the level of CL-lum, proportional to the increasing concentration of both SF and serum collected from healthy donors. The observed effect of decreased CL-lum may, therefore, be dependent on the physical conditions (viscosity of fluids used). None of these experiments showed any effect of leptin on the level of CL-lum.

Conclusions

The present study showed that leptin does not affect the level of any of the CL types in inactive neutrophils incubated in normal serum, and it does not affect the level of oxidative activity in resting neutrophils incubated with SF. However, leptin influences extracellular ROI emission (measured by CL-luc). Leptin reduces extracellular emission of ROI, and this effect is dependent on concentration and duration of exposure to leptin.

Obesity is associated with acute inflammation in a sample of adolescents

BACKGROUND

Obesity is associated with a mild chronic inflammatory response, which has been suggested to be pivotal in the development of cardiometabolic alterations of obesity. However, little is known about the involvement of acute inflammation.

OBJECTIVE

To evaluate whether circulating neutrophils, markers of acute inflammation, are associated (quantitatively and qualitatively) with adolescent obesity and whether leptin modulates these associations.

SUBJECTS AND METHODS

We assessed 528 adolescents (16.8 yr old, 47% females), without chronic/acute illness. We measured anthropometry and dual energy X-ray absorptiometry and calculated fat mass percentage (FM%). Fasting serum glucose, high-density lipoprotein (HDL)-cholesterol, and triglycerides were used with blood pressure and waist circumference to compute a metabolic z-score. Leukocyte and neutrophil counts were obtained, together with levels of serum leptin. In a subsample of 23 males, flow cytometry was used to assess degranulation (CD66b expression) of neutrophils.

RESULTS

Female sex and obesity were positively related to mean neutrophil counts (p < 0.05). When accounting for sex and weight status, leptin was associated with neutrophil counts (p < 0.05), partially explaining the association between obesity and neutrophil counts. Neutrophil counts were related to metabolic risk z-scores, controlling for fat mass. Participants with elevated FM% showed more neutrophil degranulation than controls (p < 0.05).

CONCLUSIONS

Participants with increased adiposity had higher circulating neutrophil counts, suggesting acute inflammation. Furthermore, the neutrophils showed more degranulation, indicating inflammation. Obesity-induced alteration of the adipose secretory pattern (i.e., changes in leptin levels) could be involved in acute inflammation.