Defining the role of T cell-derived leptin in the modulation of hepatic or intestinal inflammation in mice

Lactiplantibacillus plantarum-Nomad and Ideal Probiotic

Probiotics are increasingly recognized as capable of positively modulating several aspects of human health. There are numerous attributes that make an ideal probiotic. Lactiplantibacillus plantarum (Lp) exhibits an ecological and metabolic flexibility that allows it to thrive in a variety of environments. The present review will highlight the genetic and functional characteristics of Lp that make it an ideal probiotic and summarizes the current knowledge about its potential application as a prophylactic or therapeutic intervention.

5-ALA/SFC Attenuated Binge Alcohol-Induced Gut Leakiness and Inflammatory Liver Disease in HIV Transgenic Rats

BACKGROUND

This study aimed to investigate the protective effect of 5-aminolevulinic acid (5-ALA) and sodium ferrous citrate (SFC) against binge alcohol-induced gut leakiness and inflammatory liver disease in HIV transgenic (TG) rats.

METHODS

TG rats were treated with 3 consecutive doses of binge ethanol (EtOH) with or without 5-ALA/SFC. Blood and liver tissue samples were collected at 6 hours following the last dose of EtOH.

RESULTS

Compared with the wild-type (WT) rats, the TG rats showed increased sensitivity to alcohol-mediated inflammation, as evidenced by the significantly elevated levels of serum endotoxin, AST, ALT, ED1, and ED2 staining in liver. In contrast, 5-ALA/SFC improved the above biochemical and histochemical profiles. 5-ALA/SFC also attenuated the up-regulated mRNA expression of leptin and CCL2. Furthermore, down-regulated intestinal ZO-1 protein expression was also inhibited by 5-ALA/SFC. Moreover, the expressions of HO-1, HO-2, Sirt1, and related signal transduction molecules in liver were increased by 5-ALA/SFC. These results demonstrated that 5-ALA/SFC treatment ameliorated binge alcohol exposure liver injury in a rat model of HIV-infected patients by reducing macrophage activation and expression of inflammatory cytokines/chemokines, and by inducing HO-1, HO-2, and Sirt1 expression.

CONCLUSIONS

Taken together, these findings suggested that treatment with 5-ALA/SFC has a potential therapeutic effect for binge alcohol exposure liver injury in HIV-infected patients.

Adipokines and Their Role in Intestinal Inflammation

Fat tissue was initially described for its endocrine and metabolic function. Over the last two decades increasing evidence indicated a close interaction with the immune system. Partly responsible for this immune modulatory function are soluble factors released by the fat tissue, most prominently the so-called adipokines. These discoveries led to the question how adipokines influence inflammatory diseases. Linking inflammation and adipose tissue, Crohn's disease, a chronic inflammatory bowel disease, is of particular interest for studying the immune modulatory properties of adipokines since it is characterized by a hyperplasia of the mesenteric fat that subsequently is creeping around the inflamed segments of the small intestine. Thus, the role of several adipokines in the creeping fat as well as in intestinal inflammation was recently explored. The present review selected the four adipokines adiponectin, apelin, chemerin, and leptin and provides a working model based on the available literature how these factors participate in the maintenance of intestinal immune homeostasis.

Leptin Aggravates Reflux Esophagitis by Increasing Tissue Levels of Macrophage Migration Inhibitory Factor in Rats

Leptin, produced primarily by the adipose tissue, acts as a pro-inflammatory modulator, thereby contributing to the development of obesity-related disease. Although high levels of leptin in the obese are closely related to gastroesophageal reflux disease, the mechanism by which leptin influences esophageal inflammation remains unknown. Macrophage migration inhibitory factor (MIF) is produced by immune cells, such as T lymphocytes and macrophages, and MIF is known to induce the production of tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β) and interleukin 6 (IL-6). We therefore investigated the mechanism whereby leptin aggravates reflux esophagitis, by focusing on esophageal tissue levels of MIF and CD3+ T lymphocytes, both of which are crucial for the reflux-induced epithelial damage. Esophageal inflammation was surgically induced in male Wistar rats by ligating the forestomach and narrowing the duodenum to facilitate gastroesophageal reflux, followed by administration of leptin or vehicle with an osmotic pump system for 1 week. We demonstrated that the administration of leptin exacerbated the reflux esophagitis with the apparent infiltration of CD3+ T lymphocytes and caused the significant increase in the esophageal tissue levels of MIF. Moreover, the leptin caused increases in the esophageal tissue levels of TNF-α, IL-1β and IL-6, downstream targets of MIF. Importantly, the increases in these pro-inflammatory cytokines were accompanied by increased protein levels of phospho-STAT3 and phospho-AKT, pivotal molecules of leptin signaling pathways. In conclusion, through enhancing the MIF-induced inflammatory signaling, leptin could contribute to the development of gastroesophageal reflux disease.

Adipokines and the role of visceral adipose tissue in inflammatory bowel disease

Recently, adipocytes have been recognized as actively participating in local and systemic immune responses via the secretion of peptides detectable in relevant levels in the systemic circulation, the so-called "adipo(cyto)kines". Multiple studies appearing within the last 10-15 years have focused on the possible impact of adipose tissue depots on inflammatory bowel disease (IBD). Consequently, various hypotheses regarding the role of different adipokines in inflammatory diseases in general and in intestinal inflammatory processes in particular have been developed and have been further refined in recent years. After a focused summary of the data reported concerning the impact of visceral adipose tissue on IBD, such as Crohn's disease and ulcerative colitis, our review focuses on recent developments indicating that adipocytes as part of the innate immune system actively participate in antimicrobial host defenses in the context of intestinal bacterial translocation, which are of utmost importance for the homeostasis of the whole organism. Modulators of adipose tissue function and regulators of adipokine secretion, as well as modifiers of adipocytic pattern recognition molecules, might represent future potential drug targets in IBD.

Increased Sensitivity to Binge Alcohol-Induced Gut Leakiness and Inflammatory Liver Disease in HIV Transgenic Rats

The mechanisms of alcohol-mediated advanced liver injury in HIV-infected individuals are poorly understood. Thus, this study was aimed to investigate the effect of binge alcohol on the inflammatory liver disease in HIV transgenic rats as a model for simulating human conditions. Female wild-type (WT) or HIV transgenic rats were treated with three consecutive doses of binge ethanol (EtOH) (3.5 g/kg/dose oral gavages at 12-h intervals) or dextrose (Control). Blood and liver tissues were collected at 1 or 6-h following the last dose of ethanol or dextrose for the measurements of serum endotoxin and liver pathology, respectively. Compared to the WT, the HIV rats showed increased sensitivity to alcohol-mediated gut leakiness, hepatic steatosis and inflammation, as evidenced with the significantly elevated levels of serum endotoxin, hepatic triglycerides, histological fat accumulation and F4/80 staining. Real-time PCR analysis revealed that hepatic levels of toll-like receptor-4 (TLR4), leptin and the downstream target monocyte chemoattractant protein-1 (MCP-1) were significantly up-regulated in the HIV-EtOH rats, compared to all other groups. Subsequent experiments with primary cultured cells showed that both hepatocytes and hepatic Kupffer cells were the sources of the elevated MCP-1 in HIV-EtOH rats. Further, TLR4 and MCP-1 were found to be upregulated by leptin. Collectively, these results show that HIV rats, similar to HIV-infected people being treated with the highly active anti-retroviral therapy (HAART), are more susceptible to binge alcohol-induced gut leakiness and inflammatory liver disease than the corresponding WT, possibly due to additive or synergistic interaction between binge alcohol exposure and HIV infection. Based on these results, HIV transgenic rats can be used as a surrogate model to study the molecular mechanisms of many disease states caused by heavy alcohol intake in HIV-infected people on HAART.

Plasma Leptin Levels in Children Hospitalized with Cholera in Bangladesh

Vibrio cholerae, the cause of cholera, induces both innate and adaptive immune responses in infected humans. Leptin is a hormone that plays a role in both metabolism and mediating immune responses. We characterized leptin levels in 11 children with cholera in Bangladesh, assessing leptin levels on days 2, 7, 30, and 180 following cholera. We found that patients at the acute stage of cholera had significantly lower plasma leptin levels than matched controls, and compared with levels in late convalescence. We then assessed immune responses to V. cholerae antigens in 74 children with cholera, correlating these responses to plasma leptin levels on day 2 of illness. In multivariate analysis, we found an association between day 2 leptin levels and development of later anti-cholera toxin B subunit (CtxB) responses. This finding appeared to be limited to children with better nutritional status. Interestingly, we found no association between leptin levels and antibody responses to V. cholerae lipopolysaccharide, a T cell-independent antigen. Our results suggest that leptin levels may be associated with cholera, including the development of immune responses to T cell-dependent antigens.

Modulation of the leptin receptor mediates tumor growth and migration of pancreatic cancer cells

Obesity has been implicated as a significant risk factor for development of pancreatic cancer. In the setting of obesity, a systemic chronic inflammatory response is characterized by alterations in the production and secretion of a wide variety of growth factors. Leptin is a hormone whose level increases drastically in the serum of obese patients. High fat diet induced obesity in mice leads to an overall increased body weight, pancreatic weight, serum leptin, and pancreatic tissue leptin levels. Here we report the contribution of obesity and leptin to pancreatic cancer growth utilizing an in vivo orthotopic murine pancreatic cancer model, which resulted in increased tumor proliferation with concomitant increased tumor burden in the diet induced obese mice compared to lean mice. Human and murine pancreatic cancer cell lines were found to express the short as well as the long form of the leptin receptor and functionally responded to leptin induced activation through an increased phosphorylation of AKT473. In vitro, leptin stimulation increased cellular migration which was blocked by addition of a PI3K inhibitor. In vivo, depletion of the leptin receptor through shRNA knockdown partially abrogated increased orthotopic tumor growth in obese mice. These findings suggest that leptin contributes to pancreatic tumor growth through activation of the PI3K/AKT pathway, which promotes pancreatic tumor cell migration.

Metabolic inflammation in inflammatory bowel disease: crosstalk between adipose tissue and bowel

Epidemiological studies show that both the incidence of inflammatory bowel disease (IBD) and the proportion of people with obesity and/or obesity-associated metabolic syndrome increased markedly in developed countries during the past half century. Obesity is also associated with the development of more active IBD and requirement for hospitalization and with a decrease in the time span between diagnosis and surgery. Patients with IBD, especially Crohn's disease, present fat-wrapping or "creeping fat," which corresponds to ectopic adipose tissue extending from the mesenteric attachment and covering the majority of the small and large intestinal surface. Mesenteric adipose tissue in patients with IBD presents several morphological and functional alterations, e.g., it is more infiltrated with immune cells such as macrophages and T cells. All these lines of evidence clearly show an association between obesity, adipose tissue, and functional bowel disorders. In this review, we will show that the mesenteric adipose tissue and creeping fat are not innocent by standers but actively contribute to the intestinal and systemic inflammatory responses in patients with IBD. More specifically, we will review evidence showing that adipose tissue in IBD is associated with major alterations in the secretion of cytokines and adipokines involved in inflammatory process, in adipose tissue mesenchymal stem cells and adipogenesis, and in the interaction between adipose tissue and other intestinal components (immune, lymphatic, neuroendocrine, and intestinal epithelial systems). Collectively, these studies underline the importance of adipose tissue for the identification of novel therapeutic approaches for IBD.

Leptin differentially regulate STAT3 activation in ob/ob mouse adipose mesenchymal stem cells

Background

Leptin-deficient ob/ob mice exhibit adipocyte hypertrophy and hyperplasia as well as elevated adipose tissue and systemic inflammation. Multipotent stem cells isolated from adult adipose tissue can differentiate into adipocytes ex vivo and thereby contribute toward increased adipocyte cell numbers, obesity, and inflamm ation. Currently, information is lacking regarding regulation of adipose stem cell numbers as well as leptin-induced inflammation and its signaling pathway in ob/ob mice.

Methods

Using leptin deficient ob/ob mice, we investigated whether leptin injection into ob/ob mice increases adipose stem cell numbers and adipose tissue inflammatory marker MCP-1 mRNA and secretion levels. We also determined leptin mediated signaling pathways in the adipose stem cells.

Results

We report here that adipose stem cell number is significantly increased following leptin injection in ob/ob mice and with treatment of isolated stem cells with leptin in vitro. Leptin also up-regulated MCP-1 secretion in a dose- and time-dependent manner. We further showed that increased MCP-1 mRNA levels were due to increased phosphorylation of Signal Transducer and Activator of Transcription 3 (STAT3) Ser727 but not STAT3 Tyr705 phosphorylation, suggesting differential regulation of MCP-1 gene expression under basal and leptin-stimulated conditions in adipose stem cells.

Conclusions

Taken together, these studies demonstrate that leptin increases adipose stem cell number and differentially activates STAT3 protein resulting in up-regulation of MCP-1 gene expression. Further studies of mechanisms mediating adipose stem cell hyperplasia and leptin signaling in obesity are warranted and may help identify novel anti-obesity target strategies.

Leptin, CD4(+) T(reg) and the prospects for vaccination against H. pylori infection

Helicobacter pylori infection induces chronic inflammation which is characterized not only by infiltrations of inflammatory cells such as neutrophils and CD4⁺ T cells, but also significant populations of regulatory T cells (T_reg). These cells are important for disease pathogenesis because they are believed to contribute to the persistence of the infection. Despite encouraging results in animal models, the prospects for an effective H. pylori vaccine are currently poor because of generally disappointing results in preclinical and phase 1 trials. As a result, a current major focus of basic research on vaccination is to better understand the mechanisms regulating the inflammatory response with the view it can inform future vaccine design. Our studies in this area have focused on gastric CD4⁺ T_reg in vaccinated mice, and raised the hypothesis that adipokines in particular leptin are involved the establishment of a protective gastric immune response. Here we discuss the hypothesis that vaccination deregulates T_reg responses in the gastric mucosa, and that this process is mediated by leptin. We propose that reduced suppression permits a protective sub population of H. pylori-specific CD4⁺ T cells to exert protective effects, presumably via the gastric epithelium. Evidence from the literature and experimental approaches will be discussed.

Mesenteric fat - control site for bacterial translocation in colitis?

In Crohn's disease bacteria could be detected in the adjacent mesenteric fat characterized by hypertrophy of unknown function. This study aimed to define effector responses of this compartment induced by bacterial translocation during intestinal inflammation. Dextran sulfate sodium-induced colitis served as a model of intestinal inflammation. Translocation of peptides and bacteria into mesenteric fat was evaluated. Innate functions of mesenteric fat and epithelium were characterized at whole tissue, cellular, and effector molecule levels. Orally applied peptides translocated in healthy wild-type (WT) mice. Bacterial translocation was not detected in healthy and acute but increased in chronic colitis. Mesenteric fat from colitic mice released elevated levels of cytokines and was infiltrated by immune cells. In MyD88(-/-) mice bacterial translocation occurred in health and increased in colitis. The exaggerated cytokine production in mesenteric fat accompanying colonic inflammation in WT mice was less distinct in MyD88(-/-) mice. In vitro studies revealed that fat not only increases cytokine production following contact with bacterial products, but also that preadipocytes are potent phagocytes. Colonic inflammation is accompanied by massive cytokine production and immune cell infiltration in adjacent adipose tissue. These effects can be considered as protective mechanisms of the mesenteric fat in the defense of bacterial translocation.

The role of visceral fat

Until a decade ago, fat tissue had been exclusively considered as an endocrine organ. The emerging functional characterization of adipokines as well as adipocytes and preadipocytes suggested for the first time a close link between the endocrine and the immune system. This is emphasized by the changes of the expression pattern of adipokines when the fat tissue is adjacent to inflamed sites. In addition, adipokines are capable of regulating adaptive and acquired immune responses. Remarkably, adipocytes express functional pattern recognition receptors and can consequently respond to bacterial and viral antigens. This seems to be highly relevant for intestinal inflammation and here in particular transmural inflammation where bacteria or bacterial antigens translocalize into the mesenteric fat tissue. Besides phagocytosis of these antigens, adipocytes as well as preadipocytes can be activated resulting in a release of adipokines and chemokines mediating the infiltration of immune cells thus allowing for an immune response. Recent data suggest that the adipokine milieu of the fat tissue closely regulates the polarization of infiltrating immune cells. This is of increasing interest since the pattern of infiltrating cells allows for a characterization of the underlying disease. Thus, in obesity pro-inflammatory M1 macrophages dominate this site. Remarkably, in colorectal carcinoma the presence of M1 and M2 macrophages represents a prognostic marker for the disease course. In conclusion, the visceral fat tissue represents a complex organ with multifaceted function linking the endocrine and the immune system.

Leptin in immuno-rheumatological diseases

Leptin is one of the most important hormones secreted by adipocytes, with a variety of physiological roles related to the control of metabolism and energy homeostasis. Since its discovery in 1994, leptin has attracted increasing interest in the scientific community for its pleiotropic actions. One of these functions is the relationship between nutritional status and immune competence. It structurally resembles proinflammatory cytokines, such as IL-6 and IL-12. The cytokine-like structural characteristic of leptin is implicative of its function in regulating immune responses. The role of leptin in regulating immune responses has been assessed in vitro as well as in clinical studies. It has been shown that disease conditions of reduced leptin production are associated with increased infection susceptibility. Conversely, immune-mediated disorders, such as autoimmune diseases, are associated with the increased secretion of leptin and the production of proinflammatory pathogenic cytokines. In this paper, we review the most recent advances of the role of leptin in immune-rheumatological diseases, and we discuss whether strategies aimed at modifying leptin levels could represent innovative and therapeutic tools for autoimmune disorders.

At the crossroad between immunity and metabolism: focus on leptin

White adipose tissue is currently considered to be an active endocrine organ that secretes a plethora of factors named adipokines, most of them proinflammatory in nature, which probably contribute to low-level systemic inflammation; a state that is often present in metabolic syndrome-associated chronic pathologies such as obesity and atherosclerosis. Leptin is historically and indisputably one of the most important adipokines secreted by fat cells, with a variety of physiological roles related to the control of metabolism, energy homeostasis and inflammatory response. One of these functions is the connection between nutritional status and immune competence. Indeed, leptin has been shown to modulate both the innate and adaptive immune responses in both normal and pathological conditions. It has been shown that conditions characterized by low leptin levels are associated with increased susceptibility to infection. Conversely, immune-mediated disorders, such as autoimmune diseases, are associated with increased secretion of leptin and the production of proinflammatory pathogenic cytokines. Thus, leptin can easily be considered a frank mediator of the inflammatory/immune response.

Unraveling the link between leptin, ghrelin and different types of colitis

Leptin and ghrelin are hormones with a tight inverse functional connection. Their inverse association is observed not only in the modulation of metabolism but also in the interaction with the immune system. A large number of studies have been launched regarding their association with various disorders, including different types of colitis. The majority of the available literature, however, focuses on inflammatory bowel disease. The role of leptin and ghrelin appears to be aggravating in most of these studies. Concerning intestinal infections, their levels seem to depend on the presence of certain species of micro-biota. As for models of ischemic and miscellaneous colitis, both hormones seem to act protectively, although evidence deriving from human studies is needed before any safe conclusions can be made. Conclusively, it seems that available data, from in vitro, animal and human studies, suggest of a multifarious role for leptin and ghrelin, in the face of different triggers, which in turn cause diverse types of colitis. Bearing this in mind, gaps and loose ends are detected in the associated literature to encourage further research through which the association of leptin and ghrelin with intestinal inflammation could be clarified and expanded so that other types of colitis could also be included.

Metabolic imprinting, programming and epigenetics – a review of present priorities and future opportunities

Metabolic programming and metabolic imprinting describe early life events, which impact upon on later physiological outcomes. Despite the increasing numbers of papers and studies, the distinction between metabolic programming and metabolic imprinting remains confusing. The former can be defined as a dynamic process whose effects are dependent upon a critical window(s) while the latter can be more strictly associated with imprinting at the genomic level. The clinical end points associated with these phenomena can sometimes be mechanistically explicable in terms of gene expression mediated by epigenetics. The predictivity of outcomes depends on determining if there is causality or association in the context of both early dietary exposure and future health parameters. The use of biomarkers is a key aspect of determining the predictability of later outcome, and the strengths of particular types of biomarkers need to be determined. It has become clear that several important health endpoints are impacted upon by metabolic programming/imprinting. These include the link between perinatal nutrition, nutritional epigenetics and programming at an early developmental stage and its link to a range of future health risks such as CVD and diabetes. In some cases, the evidence base remains patchy and associative, while in others, a more direct causality between early nutrition and later health is clear. In addition, it is also essential to acknowledge the communication to consumers, industry, health care providers, policy-making bodies as well as to the scientific community. In this way, both programming and, eventually, reprogramming can become effective tools to improve health through dietary intervention at specific developmental points.

Leptin: a critical regulator of CD4+ T-cell polarization in vitro and in vivo

Besides being mandatory in the metabolic system, adipokines like leptin directly affect immunity. Leptin was found to be necessary in T helper 1 (Th1)-dependent inflammatory processes, whereas effects on Th2 cells are rarely understood. Here, we focused on leptin in T-helper cell polarization and in Th2-mediated intestinal inflammation in vivo. The induction of cytokine-producing Th1 or Th2 cells from naive CD4(+) T cells under polarizing conditions in vitro was generally decreased in cells from leptin-deficient ob/ob mice compared with wild-type mice. To explore the in vivo relevance of leptin in Th2-mediated inflammation, the model of oxazolone-induced colitis was employed in wild-type, ob/ob, and leptin-reconstituted ob/ob mice. Ob/ob mice were protected, whereas wild-type and leptin-reconstituted ob/ob mice developed colitis. The disease severity went in parallel with local production of the Th2 cytokine IL-13. A possible explanation for the protection of ob/ob mice in Th1- as well as in Th2-dependent inflammation is provided by a decreased expression of the key transcription factors for Th1 and Th2 polarization, T-bet and GATA-3, in naive ob/ob T cells. In conclusion, these results support the regulatory function of the adipokine leptin within T-cell polarization and thus in the acquired immune system and support the concept that there is a close interaction with the endocrine system.

Three questions about leptin and immunity

Leptin is a protein produced by adipocytes (and other cell types) that acts in the brain to regulate appetite and energy expenditure according to the amount of energy stored in adipose tissue. Leptin also exerts a variety of other functions, including important roles as a regulator of immune and inflammatory reactions. The present article is not meant to be a comprehensive review on leptin and immunity, but rather highlights a few controversial issues about leptin's place in the complex network of mediators regulating immune and inflammatory responses. Three issues are discussed: (1) Where am I going, or What is the cellular target of leptin for modulation of immune responses?; (2) Where am I coming from, or Is the cellular source important in determining leptin's effects on immune responses? and (3) What am I doing, or What are leptin's effects on immune and inflammatory responses?

Role of leptin receptor-induced STAT3 signaling in modulation of intestinal and hepatic inflammation in mice

Leptin-deficient ob/ob mice are resistant to dextran sulfate sodium (DSS)-induced colitis and Concanavalin A (Con A)-induced hepatitis. However, the signal transduction pathways involved have not been identified. The present study investigated the effect of leptin-induced STAT3 signaling in the DSS and Con A models. Mice carrying a leptin receptor (LEPR) gene mutant for Y1138 (s/s mice), with abrogated leptin-induced STAT3 signaling, were compared with wild-type (WT) and LEPR-deficient db/db mice. Administration of DSS to s/s mice resulted in a clinical score and colon shortening of intermediate severity compared with disease induced in WT and db/db mice-the latter group having the lowest disease severity. A comparable degree of inflammatory infiltrate and epithelial damage was observed in the colon of WT and s/s mice, and these parameters were reduced in db/db mice. Levels of IFN-gamma, IL-6, IL-10, and TNF-alpha were comparable in the colon of s/s and db/db mice, and a similar trend was observed for CXCL2. s/s and WT mice developed severe liver disease in response to Con A, whereas db/db mice were protected. However, Con A-induced serum IL-6 and TNF-alpha levels in s/s mice mimicked levels observed in db/db rather than WT mice. In conclusion, lack of leptin-induced STAT3 signaling is associated with reduced cytokine production following DSS and Con A administration, but it appears to sensitize mice to the effects of proinflammatory mediators.

Pharmacogenetic contribution of leptin gene polymorphism in cutaneous T-cell lymphoma

Leptin has recently attracted more attention due to its specific effects in the pathogenesis of malignancies. The aim of this study was to investigate the possible association between variants of -2548 G/A polymorphism in leptin (LEP) gene and cutaneous T-cell lymphomas (CTCL), with respect to the treatment responsiveness. A sample of 91 patients with CTCL was compared to 198 control individuals. The CTCL men with AG and/or GG genotype were more likely to receive the topical steroids treatment (odds ratio 7.88, 95% confidential interval 1.51-41.04) when compared to AA patients. Our data supports the possible involvement of LEP-2548G/A polymorphism in CTCL treatment responsiveness and thus might provide important information for individual therapy tailoring.

Leptin beyond body weight regulation--current concepts concerning its role in immune function and inflammation

Leptin, a 16 kDa non-glycosylated polypeptide produced primarily by adipocytes and released into the systemic circulation, exerts a multitude of regulatory functions including energy utilization and storage, regulation of various endocrine axes, bone metabolism, and thermoregulation. In addition to leptin's best known role as regulator of energy homeostasis, several studies indicate that leptin plays a pivotal role in immune and inflammatory response. Because of its dual nature as a hormone and cytokine, leptin can be nowadays considered the link between neuroendocrine and immune system. The increase in leptin production that occurs during infections and inflammatory processes strongly suggests that this adipokine is a part of the cytokines network which governs inflammatory/immune response and host defence mechanisms. Indeed, leptin plays a relevant role in inflammatory processes involving either innate or adaptive immune responses. Several studies have implicated leptin in the pathogenesis of autoimmune inflammatory conditions such as encephalomyelitis, type I diabetes, bowel inflammation and also articular degenerative diseases such as rheumatoid arthritis and osteoarthritis. Although the mechanisms by which leptin exerts its action as modulator of inflammatory/immune response are likely to be more complex than predicted and far to be completely depicted, there is a general consensus about its pivotal role as pro-inflammatory and immune-modulating agent. Here, we review the most recent advances on leptin biology with a particular attention to its adipokine facet, even though its role as metabolic hormone will be also addressed.

Leptin-dependent toll-like receptor expression and responsiveness in preadipocytes and adipocytes

Leptin, an adipokine mainly produced by adipocytes, has been well characterized with regard to its regulatory function on immune cells. Thus the question occurred of how adipocytes and preadipocytes interact with the immune system and whether or not this communication is regulated by leptin. With the present study we evaluated the Toll-like receptor (TLR) expression and TLR ligand-specific activation of murine preadipocytes and adipocytes in the presence [wild type (WT), 3T3L1] or absence of leptin (ob/ob) or leptin signaling (db/db). The ob/ob as well as db/db adipocytes and preadipocytes were characterized by a significant up-regulation of TLR1 to -9 expression when compared with WT cells. In WT preadipocytes the TLR responsiveness increased during maturation to adipocytes; however, stimulation of ob/ob and db/db cells resulted in a 10- to 20-fold higher interleukin-6 production. Signaling studies revealed, in addition to the increased TLR expression, alterations in the phosphoinositide 3 kinase signaling cascade in ob/ob and db/db cells as an explanation for this increased responsiveness. In conclusion, the present study indicates the expression and responsiveness of TLR1 to -9 in murine preadipocytes as well as adipocytes, both of which are strongly regulated by the adipokine leptin. In summary, these data further emphasize the role of fat tissue in the immune system.

The emerging role of adipokines as mediators of inflammation and immune responses

Interest in the biology of white adipose tissue (WAT) has increased dramatically since the discovery of leptin in 1994. The identification of the product of the gene obese (ob) threw light on the role of adipose tissue in the physiopathology of obesity-related diseases, and spurred the identification of numerous other adipokines, many of a pro-inflammatory nature. It has become increasingly evident that WAT-derived cytokines mediate between obesity-related exogenous factors (nutrition and lifestyle) and the molecular events that lead to metabolic syndrome and inflammatory and/or autoimmune conditions. Here, we review recent adipokine research, with particular attention to the roles of leptin, adiponectin, resistin, visfatin, apelin, vaspin and hepcidin in such conditions.

Transplantation of wild-type white adipose tissue normalizes metabolic, immune and inflammatory alterations in leptin-deficient ob/ob mice

Leptin-deficient ob/ob mice exhibit several metabolic and immune abnormalities, including thymus atrophy and markedly reduced inflammatory responses. We evaluated whether transplantation of wild-type (WT) white adipose tissue (WAT) into ob/ob mice could mimic the effect of recombinant leptin administration in normalizing metabolic, immune and inflammatory abnormalities. Female ob/ob mice received a subcutaneous transplantation of WAT obtained from WT littermates. A separate group of ob/ob mice was sham-operated. Despite raising leptin levels to only 15% of those observed in WT mice, WAT transplantation normalized metabolic abnormalities (glycemia, ALT, liver weight) in ob/ob mice and prevented further body weight gain. The transplanted group demonstrated normalization of thymus and spleen cellularity, thymocyte subpopulations and rates of thymocyte apoptosis. In the model of dextran sulfate sodium-induced colitis, WAT transplantation restored inflammation to levels equivalent to those of WT mice. Colonic production of IL-6 and MIP-2 was markedly reduced in the non-transplanted ob/ob group compared to transplanted ob/ob and WT mice. Our data indicate that WAT transplantation is an effective way to normalize metabolic as well as immune and inflammatory parameters in ob/ob mice. The threshold of leptin sufficient to normalize metabolic, immune and inflammatory function is significantly lower than levels present in lean WT mice. Finally, leptin derived exclusively from WAT is sufficient to normalize metabolic, immune and inflammatory parameters in ob/ob mice.

Brief report: plasma leptin levels are elevated in autism: association with early onset phenotype?

There is evidence of both immune dysregulation and autoimmune phenomena in children with autism spectrum disorders (ASD). We examined the hormone/cytokine leptin in 70 children diagnosed with autism (including 37 with regression) compared with 99 age-matched controls including 50 typically developing (TD) controls, 26 siblings without autism, and 23 children with developmental disabilities (DD). Children with autism had significantly higher plasma leptin levels compared with TD controls (p<.006). When further sub-classified into regression or early onset autism, children with early onset autism had significantly higher plasma leptin levels compared with children with regressive autism (p<.042), TD controls (p<.0015), and DD controls (p<.004). We demonstrated an increase in leptin levels in autism, a finding driven by the early onset group.

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.

The role of leptin in innate and adaptive immune responses

Leptin is produced primarily by adipocytes and functions in a feedback loop regulating body weight. Leptin deficiency results in severe obesity and a variety of endocrine abnormalities in animals and humans. Several studies indicated that leptin plays an important role in immune responses. It exerts protective anti-inflammatory effects in models of acute inflammation and during activation of innate immune responses. In contrast, leptin stimulates T lymphocyte responses, thus having rather a proinflammatory role in experimental models of autoimmune diseases. Clinical studies have so far yielded inconsistent results, suggesting a rather complex role for leptin in immune-mediated inflammatory conditions in humans.

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