Leptin: structure, function and biology

Effect of leptin on activation and cytokine synthesis in peripheral blood lymphocytes of malnourished infected children

Malnutrition compromises immune function, resulting in reduced resistance to infection. Recent animal and human studies have suggested that leptin is capable of modulating the immune response and that its levels, which are regulated by nutritional status, fall rapidly during starvation. Leptin deficiency is associated with impaired cell-mediated immunity, an increased incidence of infectious disease and an associated increase in mortality. The purpose of this study was to examine the effect of leptin on activation and cytokine production in peripheral blood T cells from malnourished children. The data obtained in the present study demonstrate that leptin produced an increase in the percentage of CD4(+) and CD8(+) cells producing interleukin (IL)-2 and interferon (IFN)-gamma in 24-h cultures. Moreover, leptin decreased the percentage of CD4(+) and CD8(+) cells producing IL-4 and IL-10, and enhanced activation of circulating T cells when co-stimulated by phorbol 12-myristate 13 acetate (PMA)-ionomycin. Leptin enhanced the expression of activation markers CD69 and CD25 in both CD4(+) and CD8(+) cells after 5 h of stimulation. In conclusion, the results obtained show that leptin modulates CD4(+) and CD8(+) cell activation towards a T helper 1 (Th1) phenotype by stimulating the synthesis of IL-2 and IFN-gamma. In contrast, leptin decreases IL-4 and IL-10 production. Moreover, leptin enhanced the expression of CD69 and CD25 on CD4(+) and CD8(+) cells after stimulation with PMA-ionomycin.

Obesity and eye diseases

The prevalence of obesity has reached epidemic proportions in many countries. Although its impact on overall health is well documented, less is known about the ocular manifestations of obesity. Among different eye diseases, obesity has been linked with age-related cataract, glaucoma, age-related maculopathy, and diabetic retinopathy. Numerous population-based and prospective studies support an association between obesity and risk of age-related cataract. However, the nature and strength of these associations, particularly with the different cataract subtypes, remains to be determined. There is strong evidence that obesity is associated with elevated intraocular pressure, but there is no convincing data to support a more direct association between obesity and glaucomatous optic neuropathy. Studies to date have not found a consistent pattern of association between obesity and risk of age-related maculopathy or diabetic retinopathy. Thus, although obesity may be a risk factor for many ocular conditions, the present literature is inadequate to establish any convincing associations. Furthermore, whether weight loss reduces the risk of eye diseases remains unresolved. Because of the potential public health impact of obesity, there is a greater need to understand its ocular effects.

Inflammatory process in type 2 diabetes: The role of cytokines

Population-based studies have shown strong relationship between inflammatory markers and metabolic disturbances, obesity, and atherosclerosis, whereas inflammation has been considered as a "common soil" between these clinical entities and type 2 diabetes (T2D). The accumulation of macrophages in adipose tissue (AT), the common origin of macrophages and adipocytes, the prevalent presence of peripheral mononuclear cells, and apoptotic beta cells by themselves seem to be the sources of inflammation present in T2D, since they generate the mediators of the inflammatory processes, namely cytokines. The main cytokines involved in the pathogenesis of T2D are interleukin-1beta (IL-1beta), with an action similar to the one present in type 1 diabetes, tumor necrosis factor-alpha (TNF-alpha), and IL-6, considered as the main regulators of inflammation, leptin, more recently introduced, and several others, such as monocyte chemoattractant protein-1, resistin, adiponectin, with either deleterious or beneficial effects in diabetic pathogenesis. The characterization of these molecules targeted diabetes treatment beyond the classical interventions with lifestyle changes and pharmaceutical agents, and toward the determination of specific molecular pathways that lead to low grade chronic inflammatory state mainly due to an immune system's unbalance.

Leptin and the regulation of the hypothalamic-pituitary-adrenal axis

Leptin, the product of the obesity gene (ob) predominantly secreted from adipocytes, plays a major role in the negative control of feeding and acts via a specific receptor (Ob-R), six isoforms of which are known at present. Evidence has been accumulated that leptin, like other peptides involved in the central regulation of food intake, controls the function of the hypothalamic-pituitary-adrenal (HPA) axis, acting on both its central and peripheral branches. Leptin, along with Ob-R, is expressed in the hypothalamus and pituitary gland, where it modulates corticotropin-releasing hormone and ACTH secretion, probably acting in an autocrine-paracrine manner. Only Ob-R is expressed in the adrenal gland, thereby making it likely that leptin affects it by acting as a circulating hormone. Although in vitro and in vivo findings could suggest a glucocorticoid secretagogue action in the rat, the bulk of evidence indicates that leptin inhibits steroid-hormone secretion from the adrenal cortex. In keeping with this, leptin was found to dampen the HPA axis response to many kinds of stress. In contrast, leptin enhances catecolamine release from the adrenal medulla. This observation suggests that leptin activates the sympathoadrenal axis and does not appear to agree with its above-mentioned antistress action. Leptin and/or Ob-R are also expressed in pituitary and adrenal tumors, but little is known about the role of this cytokine in the pathophysiology.

The adipocyte-myocyte axis in insulin resistance

Insulin resistance in skeletal muscle is linked to an elevated adipose tissue mass, as is found in obesity, but can also be observed in lipodystrophy, in which adipose tissue is greatly reduced. Adipose tissue releases endocrine and metabolic mediators and is actively involved in crosstalk with skeletal muscle, a process that precedes and underlies the development of insulin resistance in muscles. Adipokines including tumor necrosis factor alpha, interleukin-6, leptin and adiponectin influence insulin signaling in skeletal muscle. Free fatty acids, their metabolites and ectopic fat in muscle also contribute to insulin resistance. Recent research indicates inflammation, endoplasmic reticulum stress and oxidative stress could be underlying mechanisms at the center of the development of insulin resistance. Insights into the role of macrophages in adipose tissue add to the complicated interplay between adipose tissue and skeletal muscle.

Neuronal macroautophagy: from development to degeneration

Macroautophagy, a lysosomal pathway responsible for the turnover of organelles and long-lived proteins, has been regarded mainly as an inducible process in neurons, which is mobilized in states of stress and injury. New studies show, however, that macroautophagy is also constitutively active in healthy neurons and is vital to cell survival. Neurons in the brain, unlike cells in the periphery, are protected from large-scale autophagy induction because they can use several different energy sources optimally, receive additional nutrients and neurotrophin support from glial cells, and benefit from hypothalamic regulation of peripheral nutrient supplies. Due to its exceptional efficiency, constitutive autophagy in healthy neurons proceeds in the absence of easily detectable autophagic vacuole intermediates. These intermediates can accumulate rapidly, however, when late steps in the autophagic process are blocked. Autophagic vacuoles also accumulate abnormally in affected neurons of several major neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease, where they have been linked to various aspects of disease pathogenesis including neuronal cell death. The build-up of autophagic vacuoles in these neurological disorders and others may reflect either heightened autophagy induction, impairment in later digestive steps in the autophagy pathway, or both. Determining the basis for AV accumulation is critical for understanding the pathogenic significance of autophagy in a given pathologic state and for designing possible therapies based on modulating autophagy. In this review, we discuss the special features of autophagy regulation in the brain, its suspected roles in neurodevelopment and plasticity, and recent progress toward understanding how dysfunctional autophagy contributes to neurodegenerative disease.

Leptin resistance and obesity

The prevalence of obesity, and the human and economic costs of the disease, creates a need for better therapeutics and better understanding of the physiological processes that balance energy intake and energy expenditure. Leptin is the primary signal from energy stores and exerts negative feedback effects on energy intake. In common obesity, leptin loses the ability to inhibit energy intake and increase energy expenditure; this is termed leptin resistance. This review discusses the evidence in support of leptin resistance in mouse models and humans and the possible mechanisms of leptin resistance.

Towards a pro-inflammatory and immunomodulatory emerging role of leptin

Leptin is a 16 kDa adipocyte-secreted hormone that regulates weight centrally and links nutritional status with neuroendocrine and immune function. Since its cloning in 1994, leptin's role in regulating immune and inflammatory response has become increasingly evident. Actually, the increase of leptin production that occurs during infection and inflammation strongly suggests that leptin is a part of the cytokines loop which governs the inflammatory-immune response and the host defence mechanism. Indeed, leptin stimulates the production of pro-inflammatory cytokines from cultured monocytes and enhances the production of Th1 type cytokines from stimulated lymphocytes. Several studies have implicated leptin in the pathogenesis of autoimmune inflammatory conditions such as type 1 diabetes, rheumatoid arthritis and chronic bowel disease. Obesity is characterized by elevated circulating leptin levels which might contribute significantly to the so called low-grade systemic inflammation, making obese individuals more susceptible to the increased risk of developing cardiovascular diseases, type II diabetes or inflammatory articular degenerative disease such as osteorathritis (OA). As a matter of fact, a key role for leptin in OA has been recently demonstrated since leptin exhibits, in synergy with other pro-inflammatory cytokines, a detrimental effect on articular cartilage cells by promoting nitric oxide synthesis. This review will focus prevalently on the complex relationships existing among leptin, inflammatory response and immunity, trying to provide surprising insights into leptin's role and to discuss challenges and prospects for the future.

Adipose tissue as a secretory organ: from adipogenesis to the metabolic syndrome

Adipose tissue contains various types of cells that include preadipocytes and adipocytes. Studies have emphasized that (i) preadipocytes secrete factors involved in their own differentiation and (ii) adipocytes acquire the ability to communicate systemically with other organs (brain, liver, skeletal muscle) and locally with other cells (preadipocytes, endothelial cells and monocytes/macrophages). Adipocytes secrete proteins exhibiting either beneficial (leptin, adiponectin) or deleterious effects (angiotensinogen). Associated to the effect of secretory products from macrophages (cytokines), a disturbance in the balance between these various secreted factors leads to the development of a metabolic syndrome.

Lipid-lowering drugs

Although a change in life-style is often the method of first choice for lipid lowering, lipid-lowering drugs, in general, help to control elevated levels of different forms of lipids in patients with hyperlipidemia. While one group of drugs, statins, lowers cholesterol, the other group, fibrates, is known to take care of fatty acids and triglycerides. In addition, other drugs, such as ezetimibe, colesevelam, torcetrapib, avasimibe, implitapide, and niacin are also being considered to manage hyperlipidemia. As lipids are very critical for cardiovascular diseases, these drugs reduce fatal and nonfatal cardiovascular abnormalities in the general population. However, a number of recent studies indicate that apart from their lipid-lowering activities, statins and fibrates exhibit multiple functions to modulate intracellular signaling pathways, inhibit inflammation, suppress the production of reactive oxygen species, and modulate T cell activity. Therefore, nowadays, these drugs are being considered as possible therapeutics for several forms of human disorders including cancer, autoimmunity, inflammation, and neurodegeneration. Here I discuss these applications in the light of newly discovered modes of action of these drugs.

Somatotropic and gonadotropic axes linkages in infancy, childhood, and the puberty-adult transition

Integrative neuroendocrine control of the gonadotropic and somatotropic axes in childhood, puberty, and young adulthood proceeds via multiple convergent and divergent pathways in the human and experimental animal. Emerging ensemble concepts are required to embody independent, parallel, and interacting mechanisms that subserve physiological adaptations and pathological disruption of reproduction and growth. Significant advances in systems biology will be needed to address these challenges.

Gingival Crevicular Fluid Leptin Levels in Periodontitis Patients With Long-Term and Heavy Smoking

BACKGROUND

The aim of the present study was to evaluate gingival crevicular fluid (GCF) leptin levels and the influence of long-term and heavy smoking on GCF leptin levels in patients with chronic periodontitis.

METHODS

In this study, 143 individuals were divided into three groups: non-smokers (NS), smokers (S), and control (C). Three subgroups of NS and S were grouped as follows: a) probing depth (PD) <or=3 mm; b) PD = 4 to 5 mm; and c) PD >5 mm. For each patient, PD, gingival index (GI), plaque index (PI), gingival bleeding time index (GBTI), and clinical attachment level (CAL) values were recorded. The GCF leptin levels obtained from sampling sites were determined by using enzyme-linked immunosorbent assay kits.

RESULTS

The GCF leptin levels were found significantly lower in the a and b subgroups in the S group than those in the NS group (P <0.05). The inflammatory markers GI and GBTI showed significant correlations with leptin in NS (P <0.05).

CONCLUSIONS

Our results suggest that higher leptin GCF levels in healthy sites in periodontitis patients may play a protective role in periodontal disease. Further studies are needed to determine the cellular origin of the leptin in the gingiva and the effect of plasma leptin levels on GCF leptin concentrations.

Leptin and inflammation-associated cachexia in chronic kidney disease

Leptin is an adipocyte-derived hormone that acts as a major regulator of food intake and energy homeostasis. It circulates both as a free and as a protein-bound entity. Leptin is released into the blood in proportion to the amount of body fat and exerts sustained inhibitory effects on food intake while increasing energy expenditure. The leptin receptor belongs to the class I cytokine receptor superfamily and possesses strong homology to the signal-transducing subunits of the IL-6 receptor. The hypothalamic melanocortin system, and specifically the melanocortin-4 receptor (MC-4R), is critical in mediating leptin's effect on appetite and metabolism. Serum leptin concentrations are elevated in patients with chronic kidney disease (CKD) and correlate with C-reactive protein levels suggesting that inflammation is an important factor that contributes to hyperleptinemia in CKD. Hyperleptinemia may be important in the pathogenesis of inflammation-associated cachexia in CKD. We showed that experimental uremic cachexia was attenuated in db/db mice, a model of leptin receptor deficiency. Nephrectomy in these animals did not result in any change in weight gain, body composition, resting metabolic rate, and efficiency of food consumption. Furthermore, experimental uremic cachexia could be ameliorated by blocking leptin signaling through the hypothalamic MC-4R. MC-4R knockout mice or mice administered the MC-4R and MC-3R antagonist, agouti-related peptide, resisted uremia-induced loss of lean body mass and maintained normal basal metabolic rates. Thus, melanocortin receptor antagonism may provide a novel therapeutic strategy for inflammation-associated cachexia in CKD.

Globular adiponectin decreases leptin-induced tumor necrosis factor-alpha expression by murine macrophages: involvement of cAMP-PKA and MAPK pathways

Several lines of evidence have supported a link between obesity and inflammation. The present study investigated the capacity of leptin and globular adiponectin to affect tumor necrosis factor alpha (TNF-alpha) production in murine peritoneal macrophages. Leptin stimulated TNF-alpha production at mRNA as well as protein levels in a dose- and time-dependent manner. Intracellular cAMP concentration was increased and protein kinase A (PKA) was activated with the treatment of leptin, subsequently downstream MAPK signal proteins, ERK1/2 and p38, were phosphorylated. Specific inhibitors for the signal proteins, Rp cAMPS, H89, PD98059, and U0126, or SB203580, suppressed the signaling pathway and TNF-alpha expression. Although gAd partially increased cAMP concentration and PKA activity, it directly reduced leptin-induced ERK1/2 and p38 MAPK phosphorylation thus inhibiting TNF-alpha production. In conclusion, leptin promotes inflammation by stimulating TNF-alpha production, which is mediated by cAMP-PKA-ERK1/2 and p38 MAPK pathways. gAd inhibited leptin-induced TNF-alpha production through suppressing phosphorylation of ERK1/2 and p38 pathways.