Immunological functions of leptin and adiponectin

Biochimie 94 (2012) 2082e2088

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Review

Immunological functions of leptin and adiponectin Fortunata Carbone a, Claudia La Rocca a, Giuseppe Matarese a, b, * a

Laboratorio di Immunologia, Istituto di Endocrinologia e Oncologia Sperimentale, Consiglio Nazionale delle Ricerche (IEOS-CNR), Napoli 80131, Italy c/o Dipartimento di Biologia e Patologia Cellulare e Molecolare, Università di Napoli “Federico II”, Napoli 80131, Italy b Dipartimento di Medicina e Chirurgia, Facoltà di Medicina e Chirurgia, Università di Salerno, Baronissi Campus, Baronissi 84081, Salerno, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 March 2012 Accepted 18 May 2012 Available online 26 June 2012

Recent years have seen several advances in our understanding of the functions of adipose tissue regarding not only the energy storage, but also the regulation of complex metabolic and endocrine functions. In this context, leptin and adiponectin, the two most abundant adipocyte products, represent one of the best example of adipocytokines involved in the control of energy expenditure, lipid and carbohydrate metabolism as well as in the regulation of immune responses. Leptin and adiponectin secretion is counter-regulated in vivo, in relation to degree of adiposity, since plasma leptin concentrations are significantly elevated in obese subjects in proportion to body mass index while adiponectin secretion decreases in relation to the amount of adipose tissue. In this review we focus on the main biological activities of leptin and adiponectin on the lipid and carbohydrate metabolism and on their contribute in regulation of innate and adaptive immune responses. Ó 2012 Elsevier Masson SAS. All rights reserved.

Keywords: Innate immunity Adaptive immunity Adipocytokines Autoimmunity

1. Introduction Adipose tissue is a complex organ involved in the control of many processes far beyond the mere energy storage. It can secrete a great number of adipocytokines including leptin and adiponectin, that can act in synergic or antagonistic way in the control of several metabolic and immunological processes. Adipocytokines exert their actions through endocrine, paracrine or autocrine cross talk in a wide variety of physiological or pathophysiological processes. In particular, they are mainly involved in the regulation of food intake and energy metabolism, in both health and disease states, and in the inflammatory response. Leptin, a 16 KDa peptide, secreted in relation to body fat mass index, is mainly involved in the control of food intake by transmitting signals to the brain with the result that it inhibits appetite and stimulates energy expenditure [1]. Leptin mediates its effects by the binding with the leptin receptor (LepR), a member of the class I cytokine receptor family [1]. Also adiponectin controls energy homeostasis and in contrast to leptin, its secretion is often diminished in obesity since its levels inversely correlate with visceral obesity and insulin resistance and weight loss is a potent inducer of adiponectin synthesis. Adiponectin, a 244 amino acid protein exerts its biologic actions by means of two

* Corresponding author. Dipartimento di Medicina e Chirurgia, Facoltà di Medicina, Università di Salerno, Baronissi Campus, Baronissi 84081, Salerno, Italy. Tel.: þ39 0817464580; fax: þ39 0817463252. E-mail address: [email protected] (G. Matarese). 0300-9084/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.biochi.2012.05.018

receptors resulting in the activation of peroxisome-proliferatoractivated receptor-a (PPAR-a), AMP-activated protein kinase (AMPK) and p38 mitogen-activated protein kinase [2]. Adiponectin enhances fatty acid oxidation and insulin sensitivity, in addition decreases circulating glucose levels by suppressing gluconeogenesis in the liver and enhances insulin signalling in the skeletal muscle [3]. Apart from their metabolic functions, both leptin and adiponectin are implicated in the regulation of immune responses. In particular, leptin is a pro-inflammatory cytokine that stimulates the proliferation of naïve T cells and the switch versus a proinflammatory Th1 immune response [4]. On the other hand, adiponectin has anti-inflammatory properties that might be related to its capacity to suppress the synthesis of tumor necrosis factor (TNF) and interferon (IFN)-g and to induce the production of several antiinflammatory cytokines [5]. 2. Leptin Leptin, a cytokine-like hormone, is a circulating non-glycosylated peptide of 16 KDa. It is the product of the obesity (ob) gene and belongs to the family of long-chain helical cytokines (characterized by a four a-helix bundle) [6]. Leptin is mainly produced by adipose tissue and its levels directly correlate with body fat mass and adipocyte size [7]. It is produced, at lower levels, by other tissues such as the stomach, skeletal muscle, placenta and bone marrow [1]. Leptin is secreted with a circadian rhythm (its levels increase by w30% at night) and its production is mainly regulated by food-intake and eating related hormones since insulin increases

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the secretion of leptin and vice versa [1]. Reduction of leptin secretion occurs after fasting [1]. Its synthesis is also regulated by sexual hormones, in particular leptin secretion is inhibited by testosterone and increased by ovarian sex steroids resulting higher in females than in males for any given age and body fat mass [1]. Endotoxin, IL-1 and TNF also increase the secretion of leptin [8]. Once released into the peripheral circulation, leptin has central and peripheral effects. In the hypothalamus, leptin acts on specialized hypothalamic cells inhibiting anabolic and activating catabolic pathways and in addition inhibits appetite and stimulates energy expenditure [9]. In the periphery, leptin increases basal metabolism, influences reproductive function, regulates pancreatic-cell functions and insulin secretion, is pro-angiogenic for endothelial cells, regulates bone-marrow haematopoiesis and affects thymic generation of T cells and the differentiation of T helper 1 (TH1) cells in the lymph nodes [10e15]. Indeed, leptin-deficient (ob/ob) mice and leptin-receptor-deficient (db/db) mice are not only severely obese, but also have a series of marked abnormalities that are secondary to the other effects of leptin including sub-fertility, impaired wound healing and an increase in hormone production from both pituitary and adrenal glands, hyperglycemia, glucose intolerance, elevated plasma insulin, abnormal skeleton development and decreased immune function. Leptin replacement in ob/ob mice corrects all of these abnormalities. 2.1. Leptin receptors and signalling pathways Leptin mediates its effects by the binding with the leptin receptor (LepR), a member of the class I cytokine receptor family (which includes receptors for IL-6, IL-12, OSM and prolactin). Alternative splicing of LepR results in six receptor isoforms with different length of cytoplasmic domains, known as LepRa, LepRb, LepRc, LepRd, LepRe and LepRf [16]. Among all LepR isoforms, only full-length isoform (LepRb) is able to fully transduce the activation signals into the cell since its cytoplasmic region contains several motifs required for signal transduction. The other LepR isoforms lack some or all of these motifs and their function is still unclear. There are several data suggesting that they could be involved in the transport of leptin across the bloodebrain barrier or in its degradation. The LepRs are membrane-spanning glycoproteins with fibronectin type III domains in the extracellular region and with a shared 200-amino-acid module containing four conserved cysteine residues and two membrane proximal cytokine-like binding motifs, Trp-Ser-Xaa-Trp-Ser [17,18]. The short forms of the leptin receptor are expressed by several non-immune tissues and seem to mediate the transport and degradation of leptin. The long form of LepR (LepRb), whose length is 1162 amino acids, is expressed by the hypothalamus, endothelial cells, pancreatic bcells, the ovary, CD34þ haematopoietic bone-marrow precursors, monocytes/macrophages, and T and B cells [17,18]. LepRb is associated with Janus-family tyrosine kinase 2 (JAK2). JAKs are receptor-associated protein tyrosine kinases, which are utilized by leptin receptors to phosphorylate the receptor itself as well as other targets such as STAT proteins. Indeed, after leptin binding, LepRb becomes activated by auto- or cross-phosphorylation and tyrosine phosphorylates the cytoplasmic domain of the receptor. Four of the phosphorylated tyrosine residues function as docking sites for cytoplasmic adaptors such as STAT factors, in particular STAT3. It has been shown that the membrane distal tyrosine (Y1138) become phosphorylated in response to leptin binding and controls subsequent activation of STAT3. Activation of STAT3 results in a subsequent dissociation from the receptor and formation of homo- or heterodimers that translocates to the nucleus and interact with specific DNA elements in the promoters of target genes and regulate the expression of suppressor of cytokine signalling 3 (SOCS3)

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and other genes. SOCS proteins have been shown to negatively regulate cytokine-induced signalling since SOCS proteins contain a central SRC homology 2 (SH2) domain, which may allow these proteins to inhibit signalling by binding to phosphorylated JAK proteins or by direct interaction with tyrosine phosphorylated receptors [19]. SOCS proteins may also play a role in ubiquitination of signalling proteins or complexes and their direction for proteasome digestion [20]. Moreover upon leptin binding SH2 domaincontaining phosphatase 2 (SHP2) is recruited to Tyr985 and Tyr974 and activates the adaptor protein growth factor receptorbound protein 2 (GRB2) with subsequent activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and p38 mitogenactivated protein kinase (MAPK) pathways. These pathways lead to the induction of the expression of FOS and JUN. In addition, JAK2 can induce phosphorylation of the insulin receptor substrate 1/2 (IRS1/2) proteins that are responsible for the activation of phosphatidylinositol 3-kinase (PI3K) [17,18,21e26]. 3. Adiponectin Human adiponectin is encoded by ADIPOQ gene localized on the chromosome locus 3q27. This gene contains three exons with the start and the stop codon in the exon 2 and exon 3 respectively. Adiponectin has a sequence homology with a family of proteins characterized by an amino-terminal collagen-like sequence and a carboxy-terminal complement 1q-like globular region and shares homologies with collagens, complement factors, TNF-a and brain specific factor cerebellin [27,28]. Two different forms of Adiponectin exist: a full length protein and a globular adiponectin consisting of the globular C-terminal domain resulting from a photolytic cleavage although in the plasma is present only the full length adiponectin (at high concentration: 5e10 mg/ml). The cleavage process is mediated by a leukocyte elastase secreted by monocytes and/or neutrophils. After cleavage the globular form can trimerize while the full length can exist as a trimer (low molecular weight adiponectin), as a hexamer, that consist of two trimers bound through a disulphide bond (middle molecular weight adiponectin) and as a 12- to 18-mer (high molecular weight adiponectin). Adiponectin is mainly produced in white adipose tissue (WAT) by mature adipocytes, with increasing expression and secretion during adipocyte differentiation, and by non fat cells, but it also can be found in skeletal muscle cells, cardiac myocytes and endothelial cells. Female have significant higher levels of adiponectin in plasma than males, this sexual dimorphism is due principal to serum androgens since become more evident during puberty [29]. Adiponectin levels inversely correlate with visceral obesity and insulin resistance and weight loss is a potent inducer of adiponectin synthesis. TNF suppress adiponectin secretion in adipocyte [30] and its production is regulated also by other proinflammatory cytokines such as interleukin-6 (IL-6) [31]. 3.1. Adiponectin receptors and signalling pathways Adiponectin acts throughout the interaction with two different receptors: ADIPOR1 and ADIPOR2. Albeit they contain seven transmembrane domains, they differ from G protein-coupled receptors. ADIPOR1 and ADIPOR2 differ both in localization and binding affinity since ADIPOR1 is expressed mainly in skeletal muscle and binds globular adiponectin while ADIPOR2 is expressed mainly in the liver and engages with the full length adiponectin [32]. Expression of ADIPORs was reported on the majority of human monocytes, a substantial number of B cells and NK cells, but only a small percentage of T cells [33]. The binding of adiponectin to ADIPOR1 and/or ADIPOR2 results in the activation of peroxisome-

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proliferator-activated receptor-a (PPAR-a), AMP-activated protein kinase (AMPK) and p38 mitogen-activated protein kinase.

Table 1 Metabolic effects of leptin and adiponectin. Leptin [Ref.]

Adiponectin [Ref.]

Energy consumption in BAT [33e35] Energy storage in WAT [36]

e

Lipid metabolism

Lipolysis [36] Fatty acid oxidation [36] Apoptosis of adipocytes [38] Fatty acid synthesis [37]

Tryglycerides in liver [44] Fatty acid oxidation in muscle [44] Lypogenesis in liver [44]

Carbohydrate metabolism

Levels of GLUT-4 in WAT [40] Levels of GLUT-4 in BAT [40] Glucose uptake in BAT [40] Insulin secretion by pancreatic islets [42] Insulin sensitivity [39] Glycogen synthesis in liver [40] Glycogen synthesis in muscle [41]

Insulin sensitivity in liver [44] Glucose haematic levels [45e47] Glucose production in liver [45e47] Levels of GLUT-4 in muscle [48] e e

4. Leptin and adiponectin in energy and metabolic control Leptin effects are mediated both directly, through actions on specific tissues and indirectly, through central nervous system (CNS) endocrine and neural mechanisms. Leptin receptor is primarily expressed in the in the arcuate, paraventricular, and dorsomedial nuclei and in the lateral hypothalamic area. The abundance of leptin receptors in the ventromedial and lateral hypothalamus supports early observations that these two regions are intimately associated with the regulation of food intake. Leptin receptors have been identified in neuropeptide Y (NPY)/agoutirelated peptide (AgRP)- and proopiomelanocortin (POMC)/cocaineand amphetamine-regulated transcript (CART)-containing neurons of the ventromedial and ventrolateral arcuate nucleus, respectively, and in melanin-concentrating hormone (MCH)-containing neurons of the lateral hypothalamus, suggesting that the above-mentioned messengers are mediators of leptin action in the hypothalamus. Indeed leptin stimulates the expression of anorexigenic factor such as POMC and CART while inhibits the expression of the orexigenic factors, NPY, a strong stimulator of appetite, and AgRP with the result that when leptin levels increase there is an inhibition of food intake. Leptin not only influences energy homeostasis through its actions in the CNS, but also has autocrine or paracrine actions in other tissues, where it influences the rates of synthesis and degradation of lipids and carbohydrate. Adipose tissue is both the primary site of leptin production and a major effector organ for many of leptin actions. Leptin increases mitochondrial uncoupling proteins (UCP) expression in adipose tissue and muscle since chronic peripheral administration of leptin in both wild type and ob/ob mice has been shown to increase UCP1 and 2 expression in BAT and WAT and UCP3 expression in BAT and skeletal muscle thus increasing energy expenditure [34e36]. Several studies have been reported that leptin is a potent stimulator of lipolysis and fatty acid oxidation in adipocytes. In this context leptin stimulates the activity of hormone-sensitive lipase (an enzyme controlled by cellular level of cAMP) in WAT but not in BAT, probably by increasing cAMP concentration in adipocytes [37]. Indeed, hormone sensitive lipase levels are more immediately controlled by cellular levels of cAMP, so it seems that leptin might stimulates lipolysis by increasing cAMP concentrations [38]. On the other hand leptin inhibits the expression of Acetyl-CoA carboxilase (ACC) an enzyme essential for the conversion of carbohydrates to fatty acids and so the caloric storage as triglycerides [39]. Inhibition of ACC results also in increased mitochondrial fatty acid uptake and oxidation. In addition leptin reduces adipose tissue mass also by inducing adipocytes apoptosis via central nervous system [40]. Leptin regulates not only lipid metabolism but also carbohydrate metabolism. Indeed leptin treatment normalizes blood glucose and insulin levels in ob/ob mice moreover, in vivo chronic leptin treatment increases insulin sensitivity suggesting involvement of leptin in the regulation of glucose utilization too [41]. In addition leptin increases the levels of the glucose transporter GLUT4 and glucose uptake in BAT, but decreases GLUT4 and glucose uptake in WAT. Leptin has also been shown to decrease circulating insulin levels, independent of the decrease in food intake. In liver, leptin increases insulin-stimulated glycogen synthesis while in muscle, decreases glycogen synthesis [42,43]. A number of studies have been demonstrated that leptin acts directly on pancreatic islets to inhibit insulin secretion and reduce insulin mRNA levels [44] (Table 1). Adiponectin exerts its function principally acting on the liver that represents its specific target organ [45]. Long-term treatment with adiponectin ameliorates insulin-sensitivity and reduces

Energy metabolism

e

e

triglycerides in the liver [46]. The regulation of metabolic state by adiponectin involves the activation of AMPK. Adiponectin suppresses hepatic glucose production by down-regulation of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6phosphatase (G6Pase) gene expression, thus decreasing plasma glucose levels [47e49]. Adiponectin has no effect on glycogen content and synthesis, glucose uptake or glycolysis in the liver [48,49]. In skeletal muscle adiponectin stimulates fatty acids oxidation and glucose uptake by inhibition of ACC activity. However adiponectin also acts thought PPAR-a and g activation to stimulate fatty acids oxidation and decrease tissue triglyceride content in muscle and liver [46]. In myocytes adiponectin increases glucose uptake stimulating the expression of GLUT-4 and enhances insulinsignals transduction and in this way ameliorates the insulinresistance [50,51]. It has been shown that low adiponectin levels are correlated with development of atherosclerosis and cardiovascular diseases in obese patients. In this context low adiponectin levels have been linked to small dense LDL and high APOB and triglyceride levels [52]. In addition adiponectin protects against cardiovascular diseases acting also on the vascular endothelium by suppressing lipid accumulation in macrophages [38]. In conclusion adiponectin decreases circulating glucose levels by suppressing gluconeogenesis in the liver and enhancing insulin signalling in the skeletal muscle (Table 1). Despite adiponectin may serve as an insulin-sensitizing adipocytokine as adiponectin treatment ameliorates insulin-resistance in lipoatrophic mice, it has been shown that insulin-resistance was completely restored only after the treatment by the combination of adiponectin and leptin [32]. This finding indicates the mutual interplay of both adiponectin and leptin as the two major insulin-sensitizing hormones. 5. Leptin and adiponectin in innate and adaptive immunity 5.1. Effects on innate immunity A series of important studies carried out recently have shown that leptin and adiponectin have a key role not only in the

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regulation of several metabolic processes but also in the control of immune homeostasis exerting different effects on the innate immune system. Indeed recent evidence has shown that leptin has a role as pro-inflammatory cytokine while adiponectin acts mainly as an anti-inflammatory reactant. In particular, leptin stimulates, in innate immune responses, the production of several proinflammatory mediators such as IL-1, IL-6, IL-12, TNF, moreover, it has able to activates neutrophils chemotaxis and stimulates production of reactive oxygen species (ROS), seems to promote activation and phagocytosis by monocytes/macrophages and their secretion of leukotriene B4 (LTB4), cyclo-oxygenase 2 (COX2) and nitric oxide. Another effect of leptin in innate immunity involves activation of NK cells, influencing NK-cell cytotoxicity through activation of signal transducer and activator of transcription 3 (STAT3) and IL-2 [53]. To support these data, it was observed that db/db mice have a deficit in NK cell development. In contrast to leptin, early studies have indicated that adiponectin has an antiinflammatory effects on endothelial cells inhibiting NF-kB activation [54] and TNF-induced adhesion-molecule expression (vascular cell adhesion molecule-1) (VCAM-1), endothelial-leukocyte adhesion molecule-1 (E-selectin), intracellular adhesion molecule-1 (ICAM-1) [55]. Adiponectin induces the secretion of some antiinflammatory cytokines, such as IL-10 and IL-1RA (receptor antagonist), by human monocytes, macrophages and dendritic cells and suppress the production of INF-g [5] and at the same time proinflammatory mediators, such as TNF-a and IL-6 inhibit adiponectin gene expression [55]. Despite the anti-inflammatory proprieties of

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the adiponectin have been well characterized, it has been shown that there might be certain situations in which adiponectin has proinflammatory effects. Indeed high-molecular-weight adiponectin in the presence of LPS increases the production of CXC-chemokine ligand 8, also know as IL-8, by human macrophages and the phagocytes of apoptotic cells [56] (Fig. 1). 5.2. Effects on adaptive immunity The adipocyte-derived hormone leptin has been shown to regulate the adaptive response, both in normal and pathological conditions. The effects of leptin on adaptive immune responses have been extensively investigated on human CD4þ T cells. Leptin has different effects on proliferation and cytokine production by human naive (CD45RAþ) and memory (CD45ROþ) CD4þ T cells (both of which express LepRb). Leptin promotes proliferation and IL-2 secretion by naive T cells, whereas it minimally affects the proliferation of memory cells (on which it promotes a bias towards TH1-cell responses). Another important role of leptin in adaptive immunity is highlighted by the observation that leptin deficiency in ob/ob mice is associated with immunosuppression and thymic atrophy [15]. On the other hand, recently, it has been reported that leptin can act as a negative signal for the expansion of human naturally occurring Foxp3þCD4þCD25high regulatory T cells (Treg) [57] a cellular subset involved in the prevention of autoimmune diseases. De Rosa et al. showed that freshly isolated human Treg cells produce leptin and express high levels of leptin receptor

Fig. 1. Leptin and adiponectin in innate and adaptive immunity. Leptin, in innate immunity, increases cytotoxic ability and the secretion of perforin and interleukin-2 of natural killer (NK) cells. Leptin modulates the activity and function of neutrophils by increasing chemotaxis and the secretion of oxygen radicals. Leptin increases phagocytosis by monocytes/macrophages and enhances the secretion of pro-inflammatory mediators. In adaptive immunity, on naive T-cell responses, leptin increases proliferation and IL-2 secretion, on memory T cells, leptin promotes the switch towards T helper 1 immune responses. Finally leptin acts as a negative signal for the proliferation of regulatory T cells. Adiponectin inhibits NF-kB activation in endothelial cells. Adiponectin induces the secretion of some anti-inflammatory cytokines, such as IL-10 and IL-1RA, by human monocytes, macrophages and dendritic cells and suppress the production of INF-g, TNF-a. Adiponectin-treated dendritic cells show a lower production of IL-12p40 and a lower expression of CD80, CD86 and MHCII. In co-culture experiments of T cells and adiponectin-treated dendritic cells, a reduction in T cells proliferation and IL-2 production and a higher percentage of CD4þCD25þFoxp3þ Treg cells was observed. Adiponectin has also same pro-inflammatory effects. Indeed in the presence of LPS, adiponectin increases the production of IL-8, by human macrophages.

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(LepR). In vitro neutralization with anti-leptin monoclonal antibody (mAb) following anti-CD3/CD28 stimulation resulted in Treg cells proliferation. Leptin neutralization reversed the anergic state of the Treg cells, as indicated by downmodulation of the cyclindependent kinase inhibitor p27 (p27(kip1)) and the phosphorylation of the extracellular-related kinases 1 (ERK1) and ERK2 [57]. Leptin has been shown to inhibit rapamycin-induced proliferation of Tregs, by increasing activation of the mTOR, a 289-kDa serine/ threonine protein kinase that is inhibited by rapamycin. In addition, under normal conditions, Tregs secreted leptin, which activated mTOR in an autocrine manner to maintain their state of hyporesponsiveness. Finally, it has been shown that Treg cells from db/db mice exhibited a decreased mTOR activity and increased proliferation compared with that of wild-type cells [58,59]. Together, these data suggest that the leptinemTOR axis sets the threshold for the responsiveness of Tregs and that this pathway might integrate cellular energy status with metabolic-related signalling in Treg cells that use this information to control immune tolerance. Little is know about the effect of adiponectin on T cell function. Several data suggest that adiponectin is a negative T cell regulator. In particular, although a small percentage of T cells express ADIPOR on their surface, a great amount of T cells store ADIPORs within clathrincoated vesicles and these receptors colocalized with Cytotoxic TLymphocyte Antigen 4 (CTLA4) molecules. After stimulation of T cells, the expression of both ADIPORs and CTLA-4 was upregulated. Interestingly, it was observed that the addition of adiponectin results in a significant diminution of antigen-specific T cell proliferation and cytokines production [60]. A paper by Tsang et al. suggests that the immunomodulatory effect of adiponectin on immune response could be at least partially be mediated by its ability to alter dendritic cell functions [61]. Indeed, adiponectintreated dendritic cells show a lower production of IL-12p40 and a lower expression of CD80, CD86 and histocompatibility complex class II (MHCII). Moreover, in co-culture experiments of T cells and adiponectin-treated dendritic cells, a reduction in T cells proliferation and IL-2 production and a higher percentage of CD4þCD25þFoxp3þ Treg cells was observed [61] suggesting that adiponectin could also control regulatory T cell homeostasis (Fig. 1). 5.3. Role in autoimmune diseases As we previously explained leptin may has a dual effect on the immunity, indeed as is well known, hypoleptinaemia condition causes impaired immune response resulting in increased susceptibility to infections, but on the other hand leptin can be involved in the development of autoimmune disease. The first observation suggesting that leptin could be involved in autoimmunity is the sexual dimorphism of serum leptin concentration (higher in females than in males matched for age and body mass index). In this sense, leptin could add to the list of hormones, such as estradiol and prolactin, that have long been known to have a role in favouring the predisposition of females to the development of autoimmunity [62,63]. In particular, only hyperleptinaemic female mice develop autoimmunity, whereas hypoleptinaemic mice are protected, and treatment of EAE-resistant SJL/J males with recombinant leptin renders them susceptible to EAE [63]. In addition, it was subsequently observed that leptin-deficient mice are resistant to induction of active and adoptively transferred EAE [63e65]. Importantly, a surge of serum leptin anticipates the onset of clinical manifestations of EAE. The peak of serum leptin correlates with inflammatory anorexia, weight loss, and the development of pathogenic T cell responses against myelin. In human, it has been reported that the secretion of leptin is increased in serum and cerebrospinal fluid (CSF) of naive-to-treatment patients with multiple sclerosis, an aspect that positively correlates with the

secretion of IFN-g in the CSF and inversely correlates with the percentage of circulating Treg cells. Of note, the number of peripheral Treg cells in patients with MS inversely correlates with the serum levels of leptin, suggesting a link between the number of Treg cells and leptin secretion [66]. As we previously said a key piece of the puzzle regarding leptin role in autoimmune diseases has been put in place by De Rosa et al. (2007) as the authors observed that leptin can acts as a negative signal for the proliferation of regulatory T cell and in vitro leptin neutralization results in Treg cells proliferation. The question whether adiponectin is protective or detrimental adipokine in autoimmune diseases is still a matter of debate since there are some evidence suggesting that adiponectin has a dual role in course of autoimmune diseases. In this context many data provide evidence that systemic adiponectin delivery significantly decreased clinical disease activity scores of collagen-induced arthritis (CIA). In addition, adiponectin treatment before arthritis progression significantly decreased histological scores of inflammation and cartilage damage, bone erosion, and mRNA levels of pro-inflammatory cytokines in the joints [67]. In addition adiponectin not only significantly mitigated the severity of the arthritis and histopathological findings indicative of CIA mice, but also decreases TNFalpha, IL-1beta, and MMP-3 expression [68]. These results provide novel evidence that systemic adiponectin delivery prevents inflammation and joint destruction in murine arthritis model. In contrast with these data, it was observed that adiponectin levels are elevated in classic chronic inflammatory/autoimmune diseases that are unrelated to increased adipose tissue, such as rheumatoid arthritis (RA) [69], systemic lupus erythematosus (SLE) [70], inflammatory bowel disease [71], type 1 diabetes [72] and cystic fibrosis [73]. Indeed in patients affected by these diseases, adiponectin levels positively correlate with inflammatory markers. Indeed many studies have been reported that patients with RA are characterized by high levels of adiponectin and these levels correlate with severity of disease as adiponectin induces IL-6 production and metalloproteinase-1, the two major mediator of RA [74]. Several data suggest that adiponectin is also implicated in the pathogenesis of osteoarthritis (OA) as in chondrocytes it induces proinflammatory mediators such as nitric oxide, IL-6, MCP-1, MMP-3 and MMP-9 [75]. Thus, adiponectin is regulated in the opposite direction and may exert differential functions in classic versus obesity-associated inflammatory/ autoimmune conditions. 6. Conclusions Leptin and adiponectin, represent the two most abundant products of the adipose tissue with both metabolic and immune functions. Their activities demonstrate the remarkable choreography of molecules at the interface between immune and metabolic regulation. There is increasing evidence that these two mediators crossregulate each other and might be involved in a series of pathological conditions including metabolic and immunemediated disorders. It is evident that adipocytokines are the molecules mainly responsible of the link among obesity, inflammation and autoimmunity. Therefore, understanding of how each adipocytokine interacts with each other in the control of different physiological and pathological processes could be useful for the development of novel therapeutic interventions that could be applied to control both metabolic and autoimmune disorders. In this context a potential therapeutic action could be represented by means of soluble, high-affinity leptin-binding molecule and/or monoclonal humanized antibodies able to bind the leptin receptor without activating it to prevent leptin-induced inflammation. In addition, given the beneficial effects of adiponectin on

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cardiovascular diseases, drugs that stimulate adiponectin production could be used as potential therapeutic intervention.

Acknowledgements G.M. is supported by grants from the EU Ideas Programme, ERCStarting Independent Grant “LeptinMS” n. 202579, Telethon-JDRF Grant n. GJT08004 and FISM Grant n. 2009/R/26. This work is dedicated to the memory of Eugenia Papa and Serafino Zappacosta.

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