Enhanced leptin sensitivity and attenuation of diet-induced obesity in mice with haploinsufficiency of Socs3

Role of inflammatory mediators in the suppression of insulin receptor phosphorylation in circulating mononuclear cells of obese subjects

AIMS/HYPOTHESIS

Obesity is associated with insulin resistance and inflammation. The circulating human mononuclear cell (MNC) has been shown to respond to low-dose insulin infusion. We have now investigated whether in obesity: (1) phosphorylated insulin receptor beta subunit (p-INSR-beta) is reduced in the MNC; (2) pro-inflammatory mediators including inhibitor of kappa light polypeptide gene enhancer in B cells-kinase beta (IKBKB), suppressor of cytokine signalling-3 (SOCS) and protein kinase C-beta 2 (PRKCB2) are increased and related to p-INSR-beta; and (3) the reduction in MNC p-INSR-beta is related to the reduction in insulin sensitivity.

MATERIALS AND METHODS

MNCs were prepared from fasting blood samples of 16 normal weight and 16 obese female subjects.

RESULTS

Our data show that p-INSR-beta is reduced significantly in MNCs from obese subjects compared with that of normal controls. MNCs from obese subjects have higher IKBKB expression, increased nuclear factor kappa B (NFkappaB) binding and higher mRNA expression of TNFAIP1 and IL6 genes. NFkappaB binding, TNFAIP1 mRNA and plasma C-reactive protein are inversely related to p-INSR-beta. PRKCB2 mRNA and protein expression were significantly higher in the obese subjects and were related significantly to pro-inflammatory mediators but not to p-INSR-beta. SOCS3 mRNA expression was markedly elevated and positively related to pro-inflammatory mediators including IKBKB and PRKCB2 on the one hand and inversely related to p-INSR-beta on the other.

CONCLUSIONS/INTERPRETATION

We conclude that in obesity the MNC is characterised by reduced p-INSR-beta and increased inflammatory mediators including IKBKB, PRKCB2 and SOCS3. The increase in SOCS3 but not IKBKB or PRKCB2 is related inversely to p-INSR-beta and might mediate the inhibition of p-INSR-beta. These data elucidate the relationship between inflammation and insulin resistance using the MNC as a model.

The C-terminus of CIS defines its interaction pattern

Proteins of the SOCS (suppressors of cytokine signalling) family are characterized by a conserved modular structure with pre-SH2 (Src homology 2), SH2 and SOCS-box domains. Several members, including CIS (cytokine-inducible SH2 protein), SOCS1 and SOCS3, are induced rapidly upon cytokine receptor activation and function in a negative-feedback loop, attenuating signalling at the receptor level. We used a recently developed mammalian two-hybrid system [MAPPIT (mammalian protein-protein interaction trap)] to analyse SOCS protein-interaction patterns in intact cells, allowing direct comparison with biological function. We find that, besides the SH2 domain, the C-terminal part of the CIS SOCS-box is required for functional interaction with the cytokine receptor motifs examined, but not with the N-terminal death domain of the TLR (Toll-like receptor) adaptor MyD88. Mutagenesis revealed that one single tyrosine residue at position 253 is a critical binding determinant. In contrast, substrate binding by the highly related SOCS2 protein, and also by SOCS1 and SOCS3, does not require their SOCS-box.

Neuronal SH2B1 is essential for controlling energy and glucose homeostasis

SH2B1 (previously named SH2-B), a cytoplasmic adaptor protein, binds via its Src homology 2 (SH2) domain to a variety of protein tyrosine kinases, including JAK2 and the insulin receptor. SH2B1-deficient mice are obese and diabetic. Here we demonstrated that multiple isoforms of SH2B1 (alpha, beta, gamma, and/or delta) were expressed in numerous tissues, including the brain, hypothalamus, liver, muscle, adipose tissue, heart, and pancreas. Rat SH2B1beta was specifically expressed in neural tissue in SH2B1-transgenic (SH2B1(Tg)) mice. SH2B1(Tg) mice were crossed with SH2B1-knockout (SH2B1(KO)) mice to generate SH2B1(TgKO) mice expressing SH2B1 only in neural tissue but not in other tissues. Systemic deletion of the SH2B1 gene resulted in metabolic disorders in SH2B1(KO) mice, including hyperlipidemia, leptin resistance, hyperphagia, obesity, hyperglycemia, insulin resistance, and glucose intolerance. Neuron-specific restoration of SH2B1beta not only corrected the metabolic disorders in SH2B1(TgKO) mice, but also improved JAK2-mediated leptin signaling and leptin regulation of orexigenic neuropeptide expression in the hypothalamus. Moreover, neuron-specific overexpression of SH2B1 dose-dependently protected against high-fat diet-induced leptin resistance and obesity. These observations suggest that neuronal SH2B1 regulates energy balance, body weight, peripheral insulin sensitivity, and glucose homeostasis at least in part by enhancing hypothalamic leptin sensitivity.

Inflammation and metabolic disorders

Metabolic and immune systems are among the most fundamental requirements for survival. Many metabolic and immune response pathways or nutrient- and pathogen-sensing systems have been evolutionarily conserved throughout species. As a result, immune response and metabolic regulation are highly integrated and the proper function of each is dependent on the other. This interface can be viewed as a central homeostatic mechanism, dysfunction of which can lead to a cluster of chronic metabolic disorders, particularly obesity, type 2 diabetes and cardiovascular disease. Collectively, these diseases constitute the greatest current threat to global human health and welfare.

Adipokine gene expression in a novel hypothalamic neuronal cell line: resistin-dependent regulation of fasting-induced adipose factor and SOCS-3

Adipokines such as leptin, resistin, and fasting-induced adipose factor (FIAF) are secreted by adipocytes, but their expression is also detectable in the brain and pituitary. The role of central adipokines remains elusive, but we speculate that they may modulate those hypothalamic signaling pathways that control energy homeostasis. Here we describe experiments to test this in which we exploited a novel hypothalamic neuronal cell line (N-1) that expresses a variety of neuropeptides and receptors that are known to be implicated in appetite regulation. Using real-time RT-PCR, we confirmed that N-1 neurons express resistin (rstn) and fiaf, as well as suppressor of cytokine signaling-3 (socs-3), a feedback inhibitor of leptin signaling. Treating N-1 cells with recombinant resistin (200 ng/ml, 30 min) reduced both fiaf (25%, p < 0.005) and socs-3 (29%, p < 0.005) mRNA levels, and similar reductions in fiaf (40%, p < 0.001) and socs-3 (25%, p < 0.001) resulted following the overexpression of resistin. Conversely, when RNA interference (RNAi) was used to reduce endogenous rstn levels (-60%, p < 0.005), fiaf and socs-3 expression was increased (46 and 65% respectively, p < 0.005). A similar reduction in rstn mRNA was achieved using RNAi in differentiated 3T3-L1 adipocytes, and this manipulation also reduced fiaf and socs-3 expression (-53, -21 and -20% respectively, p < 0.005). In contrast, although RNAi successfully reduced fiaf mRNA by 50% (p < 0.001) in N-1 cells and 40% (p < 0.001) in 3T3-L1 cells, there was no effect on rstn or socs-3 mRNA. These data suggest that resistin exerts a novel autocrine/paracrine control over fiaf and socs-3 expression in both 3T3-L1 adipocytes and N-1 neurons. Such a mechanism could be part of the central feedback system that modulates the effects of adipokines, and other adiposity signals, implicated in hypothalamic energy homeostasis. However, it remains to be determined whether these in vitro results can be translated to the control of adipokine expression in brain and adipose tissue.

The role of leptin and ghrelin in the regulation of food intake and body weight in humans: a review

Leptin and ghrelin are two hormones that have been recognized to have a major influence on energy balance. Leptin is a mediator of long-term regulation of energy balance, suppressing food intake and thereby inducing weight loss. Ghrelin on the other hand is a fast-acting hormone, seemingly playing a role in meal initiation. As a growing number of people suffer from obesity, understanding the mechanisms by which various hormones and neurotransmitters have influence on energy balance has been a subject of intensive research. In obese subjects the circulating level of the anorexigenic hormone leptin is increased, whereas surprisingly, the level of the orexigenic hormone ghrelin is decreased. It is now established that obese patients are leptin-resistant. However, the manner in which both the leptin and ghrelin systems contribute to the development or maintenance of obesity is as yet not clear. The purpose of this review is to provide background information on the leptin and ghrelin hormones, their role in food intake and body weight in humans, and their mechanism of action. Possible abnormalities in the leptin and ghrelin systems that may contribute to the development of obesity will be mentioned. In addition, the potentials of leptin and ghrelin as drug targets will be discussed. Finally, the influence of the diet on leptin and ghrelin secretion and functioning will be described.

Developmental programming of obesity in mammals

Converging lines of evidence from epidemiological studies and animal models now indicate that the origins of obesity and related metabolic disorders lie not only in the interaction between genes and traditional adult risk factors, such as unbalanced diet and physical inactivity, but also in the interplay between genes and the embryonic, fetal and early postnatal environment. Whilst studies in man initially focused on the relationship between low birth weight and risk of adult obesity and metabolic syndrome, evidence is also growing to suggest that increased birth weight and/or adiposity at birth can also lead to increased risk for childhood and adult obesity. Hence, there appears to be increased risk of obesity at both ends of the birth weight spectrum. Animal models, including both under- and overnutrition in pregnancy and lactation lend increasing support to the developmental origins of obesity. This review focuses upon the influence of the maternal nutritional and hormonal environment in pregnancy in permanently programming appetite and energy expenditure and the hormonal, neuronal and autocrine mechanisms that contribute to the maintenance of energy balance in the offspring. We discuss the potential maternal programming 'vectors' and the molecular mechanisms that may lead to persistent pathophysiological changes resulting in subsequent disease. The perinatal environment, which appears to programme subsequent obesity, provides a potential therapeutic target, and work in this field will readily translate into improved interventional strategies to stem the growing epidemic of obesity, a disease which, once manifest, has proven particularly resistant to treatment.

Loss of SOCS3 Gene Expression Converts STAT3 Function from Anti-apoptotic to Pro-apoptotic

The transcription factor STAT3 is activated by interleukin-6-related cytokines and has been implicated as an oncogene; it promotes cell proliferation and is anti-apoptotic. However, in some cases, STAT3 has been shown to be pro-apoptotic, especially in mammary epithelial cells. In this report, we generated SOCS3-deficient murine embryonic fibroblasts (MEFs), in which STAT3 activation is extremely enhanced and prolonged. We found that LIF induces caspase-3 activation and apoptosis of SOCS3(-/-) MEFs. Exogenous expression of the dominant negative form of STAT3 but not STAT1 suppressed LIF-induced apoptosis of SOCS3(-/-) MEFs, indicating that STAT3 plays a critical role in apoptosis induction. As shown in mammary gland epithelial cells, expression of the phosphatidylinositol 3-kinase regulatory subunits p50alpha and p55alpha was induced in response to LIF in SOCS3(-/-) MEFs but not in wild-type MEFs, and Akt/protein kinase B activity was substantially reduced in SOCS3(-/-) MEFs. Furthermore, we found that some of the STAT3 target genes related to apoptosis and proliferation, such as Bcl-2 and cyclin D1, were repressed upon LIF treatment in SOCS3(-/-) cells. Not only the up-regulation of p50alpha and p55alpha but also the repression of cyclin D1 and Bcl-2 in SOCS3(-/-) MEFs was inhibited by dominant negative STAT3. These data suggest that prolonged activation of STAT3 could induce apoptosis/growth arrest rather than anti-apoptosis and proliferation in certain cases, and SOCS3 is a critical regulator of this balance.

Emerging therapeutic strategies for obesity

The rising tide of obesity is one of the most pressing health issues of our time, yet existing medicines to combat the problem are disappointingly limited in number and effectiveness. Fortunately, a recent burgeoning of mechanistic insights into the neuroendocrine regulation of body weight provides an expanding list of molecular targets for novel, rationally designed antiobesity pharmaceuticals. In this review, we articulate a set of conceptual principles that we feel could help prioritize among these molecules in the development of obesity therapeutics, based on an understanding of energy homeostasis. We focus primarily on central targets, highlighting selected strategies to stimulate endogenous catabolic signals or inhibit anabolic signals. Examples of the former approach include methods to enhance central leptin signaling through intranasal leptin delivery, use of superpotent leptin-receptor agonists, and mechanisms to increase leptin sensitivity by manipulating SOCS-3, PTP-1B, ciliary neurotrophic factor, or simply by first losing weight with traditional interventions. Techniques to augment signaling by neurochemical mediators of leptin action that lie downstream of at least some levels of obesity-associated leptin resistance include activation of melanocortin receptors or 5-HT2C and 5-HT1B receptors. We also describe strategies to inhibit anabolic molecules, such as neuropeptide Y, melanin-concentrating hormone, ghrelin, and endocannabinoids. Modulation of gastrointestinal satiation and hunger signals is discussed as well. As scientists continue to provide fundamental insights into the mechanisms governing body weight, the future looks bright for development of new and better antiobesity medications to be used with diet and exercise to facilitate substantial weight loss.

Brain adipocytokine action and metabolic regulation

Adipose tissue secretes factors that control various physiological systems. The fall in leptin during fasting mediates hyperphagia and suppresses thermogenesis, thyroid and reproductive hormones, and immune system. On the other hand, rising leptin levels in the fed state stimulate fatty acid oxidation, decrease appetite, and limit weight gain. These divergent effects of leptin occur through neuronal circuits in the hypothalamus and other brain areas. Leptin also regulates the activities of enzymes involved in lipid metabolism, e.g., AMP-activated protein kinase and stearoyl-CoA desaturase-1, and also interacts with insulin signaling in the brain. Adiponectin enhances fatty acid oxidation and insulin sensitivity, in part by stimulating AMP-activated protein kinase phosphorylation and activity in liver and muscle. Moreover, adiponectin decreases body fat by increasing energy expenditure and lipid catabolism. These effects involve peripheral and possibly central mechanisms. Adipose tissue mediates interconversion of steroid hormones and secretes proinflammatory cytokines, vasoactive peptides, and coagulation and complement proteins. Understanding the actions of these "adipocytokines" will provide insight into the pathogenesis and treatment of obesity and related diseases.

Functional Cross-modulation between SOCS Proteins Can Stimulate Cytokine Signaling

SOCS (suppressors of cytokine signaling) proteins are negative regulators of cytokine signaling that function primarily at the receptor level. Remarkably, in vitro and in vivo observations revealed both inhibitory and stimulatory effects of SOCS2 on growth hormone signaling, suggesting an additional regulatory level. In this study, we examined the possibility of direct cross-modulation between SOCS proteins and found that SOCS2 could interfere with the inhibitory actions of other SOCS proteins in growth hormone, interferon, and leptin signaling. This SOCS2 effect was SOCS box-dependent, required recruitment of the elongin BC complex, and coincided with degradation of target SOCS proteins. Detailed mammalian protein-protein interaction trap (MAPPIT) analysis indicated that SOCS2 can interact with all members of the SOCS family. SOCS2 may thus function as a molecular bridge between a ubiquitin-protein isopeptide ligase complex and SOCS proteins, targeting them for proteasomal turnover. We furthermore extended these observations to SOCS6 and SOCS7. Our findings point to a unique regulatory role for SOCS2, SOCS6, and SOCS7 within the SOCS family and provide an explanation for the unexpected phenotypes observed in SOCS2 and SOCS6 transgenic mice.

Attenuation of leptin and insulin signaling by SOCS proteins

Leptin and insulin are key hormones involved in the regulation of energy balance and glucose homeostasis. Development of resistance to the action of these hormones, which can occur with age, obesity and inflammation, appears to have a prime role in the pathogenesis of obesity and type 2 diabetes. Specific members of the suppressor of cytokine signaling (SOCS) family of proteins are now thought to have a role in the development of leptin and insulin resistance owing to their ability to inhibit leptin and insulin signaling pathways. In the case of leptin, current evidence suggests that SOCS3 appears to be of particular importance in the development of leptin resistance, whereas the ability to diminish insulin action has been described for several SOCS proteins (SOCS1, SOCS3, SOCS6 and SOCS7).

Obesity

There is a widespread epidemic of obesity in the United States, which has been associated with an increased risk of diabetes mellitus, cancer, and cardiovascular diseases. Although lifestyle modifications and long-term dietary vigilance remain cornerstones of weight reduction treatment, the continued availability of U.S. Food and Drug Administration-approved pharmacotherapies has expanded the options available for the management of obesity. These agents include anorexiants, thermogenic drugs, and lipid-partitioning drugs. As knowledge regarding the possible causes of obesity increases, there are new drugs under investigation, which include beta3-adrenergic receptor agonists, modifiers of leptin, and cannabinoid receptor-1 antagonists (rimonabant). Also under investigation are antidiabetic agents (metformin, exenatide), anticonvulsant drugs (topiramate, zonisamide), antidepressants (bupropion, fluoxetine), and growth hormones. New targets for pharmacotherapy include uncoupling proteins, fatty acid synthase, neuropeptide Y, melanocortin, ghrelin, various regulatory gut peptides, and ciliary neurotropic factor. Pharmacologic agents are in clinical development that target these substances.

The suppressor of cytokine signaling 3 inhibits leptin activation of AMP-kinase in cultured skeletal muscle of obese humans

CONTEXT

Leptin is thought to regulate whole-body adiposity and insulin sensitivity, at least in part, by stimulating fatty acid metabolism via activation of AMP-kinase (AMPK) in skeletal muscle. Human obesity is associated with leptin resistance, and recent studies have demonstrated that hypothalamic expression of the suppressors of cytokine signaling 3 (SOCS3) regulates leptin sensitivity in rodents.

OBJECTIVE

The objective of the study was to investigate the effects of leptin on fatty acid oxidation and AMPK signaling in primary myotubes derived from lean and obese skeletal muscle and evaluate the contribution of SOCS3 to leptin resistance and AMPK signaling in obese humans.

RESULTS

We demonstrate that leptin stimulates AMPK activity and increases AMPK Thr172 and acetyl-CoA carboxylase-beta Ser222 phosphorylation and fatty acid oxidation in lean myotubes but that in obese subjects leptin-dependent AMPK signaling and fatty acid oxidation are suppressed. Reduced activation of AMPK was associated with elevated expression of IL-6 ( approximately 3.5-fold) and SOCS3 mRNA ( approximately 2.5-fold) in myotubes of obese subjects. Overexpression of SOCS3 via adenovirus-mediated infection in lean myotubes to a similar degree as observed in obese myotubes prevented leptin but not AICAR (5-amino-imidazole-4-carboxamide-1-beta-d-ribofuranoside) activation of AMPK signaling.

CONCLUSIONS

These data demonstrate that SOCS3 inhibits leptin activation of AMPK. These data suggest that this impairment of leptin signaling in skeletal muscle may contribute to the aberrant regulation of fatty acid metabolism observed in obesity and that pharmacological activation of AMPK may be an effective therapy to bypass SOCS3-mediated skeletal muscle leptin resistance for the treatment of obesity-related disorders.

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.

Enhanced leptin sensitivity and improved glucose homeostasis in mice lacking suppressor of cytokine signaling-3 in POMC-expressing cells

Suppressor of cytokine signaling-3 (Socs-3) negatively regulates the action of various cytokines, as well as the metabolic hormones leptin and insulin. Mice with haploinsufficiency of Socs-3, or those with neuronal deletion of Socs-3, are lean and more leptin and insulin sensitive. To examine the role of Socs-3 within specific neurons critical to energy balance, we created mice with selective deletion of Socs-3 within pro-opiomelanocortin (POMC)-expressing cells. These mice had enhanced leptin sensitivity, measured by weight loss and food intake after leptin infusion. On chow diet, glucose homeostasis was improved despite normal weight gain. On a high-fat diet, the rate of weight gain was reduced, due to increased energy expenditure rather than decreased food intake; glucose homeostasis and insulin sensitivity were substantially improved. These studies demonstrate that Socs-3 within POMC neurons regulates leptin sensitivity and glucose homeostasis, and plays a key role in linking high-fat diet to disordered metabolism.

Adipose tissue as an endocrine organ

Adipose tissue plays a critical role in energy homeostasis, not only in storing triglycerides, but also responding to nutrient, neural, and hormonal signals and secreting adipokines that control feeding, thermogenesis, immunity, and neuroendocrine function. A rise in leptin signals satiety to the brain through receptors in hypothalamic and brainstem neurons. Leptin activates tyrosine kinase, Janus kinase 2, and signal transducer and activator of transcription 3, leading to increased levels of anorexigenic peptides, e.g., alpha-melanocyte stimulating hormone and cocaine- and amphetamine-regulated transcript, and inhibition of orexigenic peptides, e.g., neuropeptide Y and agouti-related peptide. Obesity is characterized by hyperleptinemia and hypothalamic leptin resistance, partly caused by induction of suppressor of cytokine signaling-3. Leptin falls rapidly during fasting and potently stimulates appetite, reduces thermogenesis, and mediates the inhibition of thyroid and reproductive hormones and activation of the hypothalamic-pituitary-adrenal axis. These actions are integrated by the paraventicular hypothalamic nucleus. Leptin also decreases glucose and stimulates lipolysis through central and peripheral pathways involving AMP-activated protein kinase (AMPK). Adiponectin is secreted exclusively by adipocytes and has been linked to glucose, lipid, and cardiovascular regulation. Obesity, diabetes, and atherosclerosis have been associated with reduced adiponectin levels, whereas adiponectin treatment reverses these abnormalities partly through activation of AMPK in liver and muscle. Administration of adiponectin in the brain recapitulates the peripheral actions to increase fatty acid oxidation and insulin sensitivity and reduce glucose. Although putative adiponectin receptors are widespread in peripheral organs and brain, it is uncertain whether adiponectin acts exclusively through these targets. As with leptin, adiponectin requires the central melanocortin pathway. Furthermore, adiponectin stimulates fatty acid oxidation and reduces glucose and lipids, at least in part, by activating AMPK in muscle and liver.

Inducible-NOS but not neuronal-NOS participate in the acute effect of TNF-α on hypothalamic insulin-dependent inhibition of food intake

TNF-alpha acts on the hypothalamus modulating food intake and energy expenditure through mechanisms incompletely elucidated. Here, we explore the hypothesis that, to modulate insulin-induced anorexigenic signaling in hypothalamus, TNF-alpha requires the synthesis of NO. TNF-alpha activates signal transduction through JNK and p38 in hypothalamus, peaking at 10(-8) M. This is accompanied by the induction of expression of the inducible and neuronal forms of NOS, in both cases peaking at 10(-12) M. In addition, TNF-alpha stimulates NOS catalytic activity. Pre-treatment with TNF-alpha at a low dose (10(-12) M) inhibits insulin-dependent anorexigenic signaling, and this effect is abolished in iNOS but not in nNOS knockout mice.

Diet-induced Obesity Alters AMP Kinase Activity in Hypothalamus and Skeletal Muscle

AMP-activated protein kinase (AMPK) is a key regulator of cellular energy balance and of the effects of leptin on food intake and fatty acid oxidation. Obesity is usually associated with resistance to the effects of leptin on food intake and body weight. To determine whether diet-induced obesity (DIO) impairs the AMPK response to leptin in muscle and/or hypothalamus, we fed FVB mice a high fat (55%) diet for 10-12 weeks. Leptin acutely decreased food intake by approximately 30% in chow-fed mice. DIO mice tended to eat less, and leptin had no effect on food intake. Leptin decreased respiratory exchange ratio in chow-fed mice indicating increased fatty acid oxidation. Respiratory exchange ratio was low basally in high fat-fed mice, and leptin had no further effect. Leptin (3 mg/kg intraperitoneally) increased alpha2-AMPK activity 2-fold in muscle in chow-fed mice but not in DIO mice. Leptin decreased acetyl-CoA carboxylase activity 40% in muscle from chow-fed mice. In muscle from DIO mice, acetyl-CoA carboxylase activity was basally low, and leptin had no further effect. In paraventricular, arcuate, and medial hypothalamus of chow-fed mice, leptin inhibited alpha2-AMPK activity but not in DIO mice. In addition, leptin increased STAT3 phosphorylation 2-fold in arcuate of chow-fed mice, but this effect was attenuated because of elevated basal STAT3 phosphorylation in DIO mice. Thus, DIO in FVB mice alters alpha2-AMPK in muscle and hypothalamus and STAT3 in hypothalamus and impairs further effects of leptin on these signaling pathways. Defective responses of AMPK to leptin may contribute to resistance to leptin action on food intake and energy expenditure in obese states.

Deletion of the SOCS3 gene in liver parenchymal cells promotes hepatitis-induced hepatocarcinogenesis

BACKGROUND & AIMS

A recent study has suggested that the methylation silencing of the suppressor of cytokine signaling-3 (SOCS3), a negative regulator of interleukin-6-related cytokines, could be involved in hepatocellular carcinoma (HCC). However, the roles of SOCS3 in hepatocellular carcinogenesis and hepatitis have not been established. We investigated the effect of deleting the SOCS3 gene on the development of hepatitis and HCC in hepatitis C virus-infected patients and mouse models.

METHODS

The expression of SOCS genes in HCC and non-HCC regions of patient samples was determined by real-time reverse-transcription polymerase chain reaction and immunoblotting. The conditional knockout approach in mice was used to determine the hepatocyte-specific roles of SOCS3. To generate a liver-specific deletion, floxed SOCS3 (SOCS3(fl/fl)) mice were crossed with albumin-Cre transgenic mice. Hepatitis and HCC were induced by administering concanavalin A and diethylnitrosamine, respectively.

RESULTS

SOCS3 expression was reduced in the HCC regions compared with the non-HCC regions. Carcinogen-induced hepatic tumor development was enhanced by deletion of the SOCS3 gene, which was associated with higher levels of the targets of signal transducers and activators of transcription (ie, B-cell lymphoma-XL, B-cell lymphoma-2, C-myelocytomatosis, cyclin D1, and vascular endothelial growth factor). In the concanavalin A-mediated hepatitis model, deletion of the SOCS3 gene in the hepatocytes protected against liver injury through suppression of interferon-gamma signaling and induction of the antiapoptotic protein Bcl-XL.

CONCLUSIONS

Deletion of the SOCS3 gene in hepatocytes promotes the activation of STAT3, resistance to apoptosis, and an acceleration of proliferation, resulting in enhanced hepatitis-induced hepatocarcinogenesis.

Tumor necrosis factor-alpha activates signal transduction in hypothalamus and modulates the expression of pro-inflammatory proteins and orexigenic/anorexigenic neurotransmitters

Tumor necrosis factor-alpha (TNF-alpha) is known to participate in the wastage syndrome that accompanies cancer and severe infectious diseases. More recently, a role for TNF-alpha in the pathogenesis of type 2 diabetes mellitus and obesity has been shown. Much of the regulatory action exerted by TNF-alpha upon the control of energy stores depends on its action on the hypothalamus. In this study, we show that TNF-alpha activates canonical pro-inflammatory signal transduction pathways in the hypothalamus of rats. These signaling events lead to the transcriptional activation of an early responsive gene and to the induction of expression of cytokines and a cytokine responsive protein such as interleukin-1beta, interleukin-6, interleukin-10 and suppressor of cytokine signalling-3, respectively. In addition, TNF-alpha induces the expression of neurotransmitters involved in the control of feeding and thermogenesis. Thus, TNF-alpha may act directly in the hypothalamus inducing a pro-inflammatory response and the modulation of expression of neurotransmitters involved in energy homeostasis.

Perinatal overfeeding in rats results in increased levels of plasma leptin but unchanged cerebrospinal leptin in adulthood

OBJECTIVE

To study the effect of perinatal programming and overfeeding on the hypothalamic control mechanisms of food intake in adult rats.

DESIGN

Neonatal programming effects on body weight, food intake, central and peripheral leptin levels, hypothalamic neuropeptides, leptin receptors and central leptin responsiveness in adult rats.

MEASUREMENTS

Plasma and cerebrospinal fluid (CSF) leptin levels were analyzed using radioimmunoassay. Neuropeptide mRNA levels were analyzed using in situ hybridization. Leptin receptor mRNA levels were analyzed using reverse transcriptase-polymerase chain reaction.

RESULTS

Perinatally overfed rats growing up in small litters (SL) maintain their obese and hyperleptinemic phenotype in adulthood. However, leptin levels in CSF are abnormally low considering the plasmatic hyperleptinemia. In contrast to the already reported changes in perinatally overfed juvenile rats, perinatally overfed adult rats did not show any alteration in the expression of leptin receptor isoforms and evaluated neuropeptides. Moreover, SL adult rats showed a normal sensitivity regarding the inhibitory effect of intracerebroventricular leptin administration on food intake.

CONCLUSION

Perinatal overfeeding does not induce alterations in either the anorectic response to central leptin administration or expression of leptin receptors and neuropeptides in adulthood. The leptin resistance to peripheral leptin in SL adult rats may be related to impaired leptin transport across the blood-brain barrier.

SOCS-2 interferes with myotube formation and potentiates osteoblast differentiation through upregulation of JunB in C2C12 cells

Suppressor of cytokine signaling (SOCS)-2 regulates normal postnatal growth and its deficiency in mice causes gigantism with increased bone length and proportional enlargement in skeletal muscles. Using C2C12 mesenchymal precursor cell line as a model, we investigated a possible role of SOCS-2 in the differentiation process of mesenchymal precursors. Stable transfection of SOCS-2 into C2C12 cells resulted in the acceleration of proliferation and survival, and inhibition of spontaneous myotube formation. In addition, SOCS-2 potentiated bone morphogenic protein (BMP)-induced transdifferentiation of C2C12 cells into osteoblast phenotypes. These effects of SOCS-2 on C2C12 cells differed strikingly from that of SOCS-1, another member of SOCS family, and its mechanisms were evaluated. SOCS-2 did not alter BMP-induced phosphorylation and nuclear accumulation of Smad1, nor the expression of inhibitory-Smads mRNA. However, SOCS-2 enhanced BMP-induced transcriptional activation of the Smad-responsive reporter gene, suggesting that the action of SOCS-2 is exerted at the transcriptional level. Interestingly, SOCS-2 overexpression in C2C12 cells increased the endogenous JunB protein, one of the key transcriptional factors in the control of BMP/Smad signaling responsiveness. In addition, the proteasome inhibitor enhanced JunB protein expression in C2C12 cells. Moreover, we found that SOCS-2 reduced JunB ubiquitination in COS-7 cells. Although SOCS-2 is a modulator of growth hormone (GH) signaling, the upregulation of JunB by SOCS-2 did not require GH signaling. Taken together, these results suggest that SOCS-2 positively regulates endogenous JunB protein expression in C2C12 cells through inhibition of JunB destabilization by the ubiquitin-proteasome pathway, and thereby regulates the cell fate of mesenchymal precursors.

A complex interaction pattern of CIS and SOCS2 with the leptin receptor

Hypothalamic leptin receptor signalling plays a central role in weight regulation by controlling fat storage and energy expenditure. In addition, leptin also has direct effects on peripheral cell types involved in regulation of diverse body functions including immune response, bone formation and reproduction. Previous studies have demonstrated the important role of SOCS3 (suppressor of cytokine signalling 3) in leptin physiology. Here, we show that CIS (cytokine-inducible SH2 protein) and SOCS2 can also interact with the leptin receptor. Using MAPPIT (mammalian protein-protein interaction trap), a cytokine receptor-based two-hybrid method operating in intact cells, we show specific binding of CIS with the conserved Y985 and Y1077 motifs in the cytosolic domain of the leptin receptor. SOCS2 only interacts with the Y1077 motif, but with higher binding affinity and can interfere with CIS and STAT5a prey recruitment at this site. Furthermore, although SOCS2 does not associate with Y985 of the leptin receptor, we find that SOCS2 can block interaction of CIS with this position. This unexpected interference can be explained by the direct binding of SOCS2 on the CIS SOCS box, whereby elongin B/C recruitment is crucial to suppress CIS activity.

Role of leptin in regulating appetite, neuroendocrine function, and bone remodeling

Leptin, a hormone secreted by adipocytes, acts on the hypothalamus to regulate appetite and neuroendocrine function. In the hypothalamus, both the arcuate nucleus and the ventromedial nucleus express leptin receptors. Specific neurons in the arcuate nucleus regulate appetite and reproduction. In contrast, neurons in the ventromedial nucleus regulate bone mass. The melanocortin system is the downstream pathway for regulating appetite and neuroendocrine function. In contrast, the sympathetic nervous system is the downstream pathway for regulating bone mass. Leptin, in regulating food intake and body weight, acts, in part, by inhibiting the synthesis of neuropeptide Y and its release from the hypothalamus. The leptin and insulin pathways may interact and may be important in the pathogenesis of the metabolic syndrome.

[The hypothalamic control of feeding and thermogenesis: implications on the development of obesity]

The worldwide increase in the prevalence of obesity is becoming one of the most important clinical-epidemiological phenomena of the present days. Environmental factors such as changes in life-style and feeding behavior associated with poorly characterized genetic determinants are though to play the most important roles in the pathogenesis of this disease. During the last ten years, since the discovery of leptin, great advances were obtained in the characterization of the hypothalamic mechanisms involved in the control of food intake and thermogenesis. Such advances are unveiling a complex and integrated system and are opening a wide perspective for the finding of novel therapeutic targets for the treatment of this harming condition. This review will present some of the most recent findings in this field. It will be focused on the actions of leptin and insulin in the hypothalamus and will explore the hypothesis that hypothalamic resistance to the action of these hormones may play a role in the development of obesity and may act as a molecular link between obesity, type 2 diabetes mellitus and other clinical conditions on which insulin resistance plays an important pathogenetic role.

CNTF reverses obesity-induced insulin resistance by activating skeletal muscle AMPK

Ciliary neurotrophic factor (CNTF) induces weight loss and improves glucose tolerance in humans and rodents. CNTF is thought to act centrally by inducing hypothalamic neurogenesis to modulate food intake and peripherally by altering hepatic gene expression, in a manner similar to that of leptin. Here, we show that CNTF signals through the CNTFRalpha-IL-6R-gp130beta receptor complex to increase fatty-acid oxidation and reduce insulin resistance in skeletal muscle by activating AMP-activated protein kinase (AMPK), independent of signaling through the brain. Thus, our findings further show that the antiobesogenic effects of CNTF in the periphery result from direct effects on skeletal muscle, and that these peripheral effects are not suppressed by diet-induced or genetic models of obesity, an essential requirement for the therapeutic treatment of obesity-related diseases.

Techniques: new pharmacological perspectives for the leptin receptor

The function of leptin, initially confined to its role in energy homeostasis and obesity, has now expanded to the regulation of reproduction, glucose homeostasis, bone formation, wound healing and the immune system. Both stimulation and inhibition of the molecular target of leptin, the leptin receptor (LR), might find applications in disease treatment. Recent advances in the understanding of LR activation mechanisms have led to the design of LR antagonists. Several assays have been developed for the screening and evaluation of LR ligands. Both the extracellular and the intracellular domains of the LR are potential drug targets. The bioluminescence resonance energy transfer technique can be used to screen for compounds that target the extracellular part of the LR, and we propose that the novel reverse mammalian protein-protein interaction trap technique can be used to screen compounds that affect intracellular aspects of LR signalling. These assays can be easily adapted to other pharmacologically relevant receptors.

Overexpression of suppressor of cytokine signaling 3 in adipose tissue causes local but not systemic insulin resistance

In adipocytes, suppressor of cytokine signaling (SOCS)3 deficiency increases insulin-stimulated insulin receptor substrate (IRS)-1 and -2 phosphorylation, IRS-associated phosphatidylinositol 3 kinase activity, and insulin-stimulated glucose uptake. Moreover, SOCS3 is required for tumor necrosis factor-alpha full inhibition of insulin-stimulated IRS-1 and -2 phosphorylation, phosphatidylinositol 3 kinase activity, and glucose uptake. Whether SOCS3 also inhibits adipocyte insulin signaling in vivo and whether this action further affects systemic insulin sensitivity is not clear. We therefore generated a transgenic mouse (aP2-SOCS3 mouse) overexpressing SOCS3 in adipose tissue. Overexpression of SOCS3 in adipocytes decreases IRS1 protein levels and subsequent insulin-stimulated IRS-1 and -2 phosphorylation, decreases p85 binding to IRS-1, and leads to decreased insulin-stimulated glucose uptake in adipocytes. This impaired insulin signaling in adipose tissue of aP2-SOCS3 mice causes decreased lipogenesis and blocks insulin's antilipolytic action. However, because of decreased energy partitioning in adipose tissue, aP2-SOCS3 mice are resistant to diet-induced obesity and are protected against systemic insulin resistance caused by a high-fat diet. Therefore, overexpression of SOCS3 in adipocytes causes local adipocyte insulin resistance, but it is not sufficient to cause systemic insulin resistance.

Leptin and its emerging role in children and adolescents

Leptin is an adipocyte-secreted hormone which plays a key role in energy homeostasis. Recent “proof of concept” studies involving leptin administration to humans support its critical role in regulating energy homeostasis, neuroendocrine and immune function as well as insulin resistance in states of energy/ caloric deprivation. Moreover, interventional studies in leptin deficient children and observational studies in normal girls and boys support a role for leptin as a permissive factor for the initiation of puberty in children. The potential clinical usefulness of leptin in several disease states in children and adolescents, including hypothalamic amenorrhea, eating disorders and syndromes of insulin resistance is still under investigation.

Circumventing central leptin resistance: lessons from central leptin and POMC gene delivery

We identified that leptin resistance in aged-obese rats has both peripheral and central components. The central resistance is characterized by diminished hypothalamic leptin receptors and impaired leptin signal transduction. We developed a new model of leptin-induced leptin resistance in which application of the central leptin gene delivery produces unabated hypothalamic leptin over-expression. The chronic central elevation of leptin precipitates leptin resistance in young animals devoid of obesity and exacerbates it in mature or aged animals with obesity. Despite leptin resistance, our aged obese, DIO, and leptin-induced leptin resistant rats were fully responsive to central pharmacological melanocortin activation. We propose that the central leptin resistance resides between leptin receptor and melanocortin receptor activation. Our central POMC gene therapy overcame leptin resistance, producing weight and fat loss and improved insulin sensitivity in obese Zucker and aged rats. This success highlights the central melanocortin system as a useful drug target for combating obesity.

The suppressor of cytokine signalling 3, SOCS3, may be one critical modulator of seasonal body weight changes in the Siberian hamster, Phodopus sungorus

The Siberian hamster, Phodopus sungorus, exhibits a remarkable cycle of body weight, reproduction and leptin sensitivity in response to a seasonal change in photoperiod. In the present study, we investigated the hypothesis that the suppressor of cytokine signalling 3 (SOCS3) plays a critical role in the regulation of the seasonal body weight cycle. We analysed arcuate nucleus SOCS3 gene expression in short day length (SD; 8 : 16 h light/dark) acclimated Siberian hamsters that were transferred back to long day length (LD; 16 : 8 h light/dark) and in hamsters that spontaneously became photorefractory to SD induced by prolonged exposure. SD acclimated hamsters that were transferred back to LD for 1, 2, 3, 4 or 6 weeks, increased arcuate nucleus SOCS3 gene expression to the LD level within 2 weeks, and maintained this higher level thereafter. The early increase of SOCS3 gene expression preceded the LD-induced rise in body weight by approximately 3 weeks. Hamsters kept in SD for an extended period (25 weeks), began to become refractory to SD and to increase body weight. By this time, there was no difference in level of SOCS3 gene expression between LD and SD photoperiods, although body weight was still suppressed in SD hamsters. Finally, we addressed whether SOCS3 gene expression is related to SD-induced gonadal regression or to body weight decrease by comparing Siberian hamsters with Syrian hamsters. The latter exhibited substantial SD-induced gonadal regression but only limited seasonal changes in body weight. Acclimation to either LD or SD for 14 weeks had no effect on SOCS3 gene expression. This implies that arcuate nucleus SOCS3 gene expression is unlikely to be related to seasonal cycles in reproductive activity. Taken together, the findings further strengthen our hypothesis that SOCS3 may be one molecular trigger of seasonal cycles in body weight.

Neuroendocrine and metabolic effects of adipocyte-derived hormones

Obesity is characterized by an increase in adipose tissue mass. Contrary to the previous view of adipose tissue as simply an inert tissue devoted to energy storage, studies over the past decade have shown that adipose tissue is actively involved in regulating physiological processes and participates in disease. Adipose tissue secretes factors that exert local and systemic effects. Leptin, pro-inflammatory cytokines, resistin and proteins involved in haemodynamic regulation and coagulation are increased in obesity while adiponectin is reduced. The production of active corticosteroids is also increased in obesity. There is now growing evidence that adipocyte secretory factors regulate energy homoeostasis, as well as cardiovascular and immune systems. Some adipocyte hormones, most notably leptin, act in the brain to influence the neuroendocrine axis and energy balance, whereas adiponectin and resistin exert opposing effects on glucose and lipids. Understanding the actions of adipocyte hormones will provide novel insights into the pathophysiology and treatment of obesity.

Adipokines that link obesity and diabetes to the hypothalamus

Adipose tissue plays a crucial role in energy homeostasis not only in storing triglyceride, but also responding to nutrient, neural, and hormonal signals, and producing factors which control feeding, thermogenesis, immune and neuroendocrine function, and glucose and lipid metabolism. Adipose tissue secretes leptin, steroid hormones, adiponectin, inflammatory cytokines, resistin, complement factors, and vasoactive peptides. The endocrine function of adipose tissue is typified by leptin. An increase in leptin signals satiety to neuronal targets in the hypothalamus. Leptin activates Janus-activating kinase2 (Jak2) and STAT 3, resulting in stimulation of anorexigenic peptides, e.g., alpha-MSH and CART, and inhibition of orexigenic peptides, e.g., NPY and AGRP. The reduction in leptin levels during fasting stimulates appetite, decreases thermogenesis, thyroid and reproductive hormones, and increases glucocorticoids. Leptin also stimulates fatty acid oxidation, insulin release, and peripheral insulin action. These effects involve regulation of PI-3 kinase, PTP-1B, suppressor of cytokine signaling-3 (SOCS-3), and AMP-activated protein kinase in the brain and peripheral organs. There is emerging evidence that leptin, adiponectin, and resistin act through overlapping pathways. Understanding the signal transduction of adipocyte hormones will provide novel insights on the pathogenesis and treatment of obesity, diabetes, and various metabolic disorders.

Intracellular signalling pathways activated by leptin

Leptin is a versatile 16 kDa peptide hormone, with a tertiary structure resembling that of members of the long-chain helical cytokine family. It is mainly produced by adipocytes in proportion to fat size stores, and was originally thought to act only as a satiety factor. However, the ubiquitous distribution of OB-R leptin receptors in almost all tissues underlies the pleiotropism of leptin. OB-Rs belong to the class I cytokine receptor family, which is known to act through JAKs (Janus kinases) and STATs (signal transducers and activators of transcription). The OB-R gene is alternatively spliced to produce at least five isoforms. The full-length isoform, OB-Rb, contains intracellular motifs required for activation of the JAK/STAT signal transduction pathway, and is considered to be the functional receptor. Considerable evidence for systemic effects of leptin on body mass control, reproduction, angiogenesis, immunity, wound healing, bone remodelling and cardiovascular function, as well as on specific metabolic pathways, indicates that leptin operates both directly and indirectly to orchestrate complex pathophysiological processes. Consistent with leptin's pleiotropic role, its participation in and crosstalk with some of the main signalling pathways, including those involving insulin receptor substrates, phosphoinositide 3-kinase, protein kinase B, protein kinase C, extracellular-signal-regulated kinase, mitogen-activated protein kinases, phosphodiesterase, phospholipase C and nitric oxide, has been observed. The impact of leptin on several equally relevant signalling pathways extends also to Rho family GTPases in relation to the actin cytoskeleton, production of reactive oxygen species, stimulation of prostaglandins, binding to diacylglycerol kinase and catecholamine secretion, among others.

Inhibition of preproinsulin gene expression by leptin induction of suppressor of cytokine signaling 3 in pancreatic beta-cells

Leptin inhibits insulin secretion and preproinsulin gene expression in pancreatic beta-cells, but signal transduction pathways and molecular mechanisms underlying this effect are poorly characterized. In this study, we analyzed leptin-mediated signal transduction and preproinsulin gene regulation at the molecular level in pancreatic beta-cells. Leptin stimulation led to janus kinase (JAK)2-dependent phosphorylation and nuclear translocation of the transcription factors signal transducer and activator of transcription (STAT)3 and STAT5b in INS-1 beta-cells. Leptin also induced mRNA expression of the JAK-STAT inhibitor suppressor of cytokine signaling (SOCS)3 in INS-1 beta-cells and human pancreatic islets in vitro and in pancreatic islets of ob/ob mice in vivo. Transcriptional activation of the rat SOCS3 promoter by leptin was observed with concomitant leptin-induced STAT3 and STAT5b DNA binding to specific promoter regions. Unexpectedly, SOCS3 inhibited both basal and STAT3/5b-dependent rat preproinsulin 1 gene promoter activity in INS-1 cells. These results suggest that SOCS3 represents a transcriptional inhibitor of preproinsulin gene expression, which is induced by leptin through JAK-STAT3/5b signaling in pancreatic beta-cells. In conclusion, although SOCS3 is believed to be a negative feedback regulator of JAK-STAT signaling, our findings suggest involvement of SOCS3 in a direct gene regulatory pathway downstream of leptin-activated JAK-STAT signaling in pancreatic beta-cells.

Hypothalamic phosphatidylinositol 3-kinase-phosphodiesterase 3B-cyclic AMP pathway of leptin signalling is impaired following chronic central leptin infusion

Leptin signalling in the hypothalamus is critical for the maintenance of normal body weight. Although hyperleptinaemia in obese people suggests a state of leptin resistance, and diet-induced obesity in rodents is associated with central leptin resistance, the underlying mechanisms remain unclear. Recent evidence suggests that, in addition to the signal transducer and activator of the transcription-3 (STAT3) pathway, leptin action is critical for energy homeostasis through an insulin-like signalling pathway involving an increase in phosphatidylinositol 3-kinase (PI3K) and phosphodiesterase 3B (PDE3B) activities and reduction in cyclic AMP (cAMP) levels in the hypothalamus. Here, we show that chronic central leptin (160 ng/h) infusion, which resulted in the development of resistance to the satiety action of leptin, impaired the PI3K-PDE3B-cAMP pathway of leptin signalling in the hypothalamus in that PI3K and PDE3B activities were increased and cAMP levels were decreased in the hypothalamus on day 2 of leptin infusion but remained unchanged on day 16. Additionally, induction of tyrosyl phosphorylation of insulin receptor substrate-1 observed on day 2 was not evident on day 16 of leptin infusion. By contrast, signalling through the STAT3-pathway remained activated in the hypothalamus throughout 16 days of leptin infusion. These findings show a differential response in PI3K-PDE3B-cAMP (impaired) and STAT3 (up-regulated) pathways to chronic central leptin infusion, and suggest a selective resistance in the PI3K-PDE3B-cAMP pathway of leptin signalling following a chronic increase in hypothalamic leptin tone attained by central infusion of this peptide hormone.

Hormonal regulation of food intake

Our knowledge of the physiological systems controlling energy homeostasis has increased dramatically over the last decade. The roles of peripheral signals from adipose tissue, pancreas, and the gastrointestinal tract reflecting short- and long-term nutritional status are now being described. Such signals influence central circuits in the hypothalamus, brain stem, and limbic system to modulate neuropeptide release and hence food intake and energy expenditure. This review discusses the peripheral hormones and central neuronal pathways that contribute to control of appetite.

Central actions of adipocyte hormones

Adipose tissue secretes a variety of proteins with important roles in metabolism, reproduction, immunity and cardiovascular function. The endocrine function of adipose tissue, especially that of leptin, is linked to energy storage and thus might provide insights into obesity and other diseases associated with energy imbalance. This review highlights the current understanding of the actions of leptin in the brain, with particular emphasis on transport across the blood-brain barrier, signal transduction, neuropeptide targets and roles during fasting and obesity. Moreover, data pertaining to the potential central effects of adiponectin, cytokines and resistin on energy homeostasis, glucose and lipid metabolism are discussed.

Direct metabolic regulation in skeletal muscle and fat tissue by leptin: implications for glucose and fatty acids homeostasis

In recent years, the adipose tissue has emerged as an important endocrine organ. It is now recognized that besides storing energy the adipocytes also secrete several bioactive peptides, collectively called adipocytokines. Among these adipocytokines, leptin, the product of the ob gene, has been extensively investigated over the last decade. Skeletal muscle and adipose tissue, two major tissues involved in the regulation of glucose and fatty acids metabolism, have been consistently demonstrated to be directly affected by leptin. By binding to its receptors located in skeletal muscle and fat cells, leptin promotes energy dissipation and prevents fatty acid accumulation and 'lipotoxicity' in these tissues. On the other hand, under conditions of peripheral leptin resistance, such as observed in obese humans, the activation of pathways involved in fatty acid oxidation may be impaired. This leads to intracellular accumulation of lipid intermediates and causes insulin resistance. This review examines the metabolic pathways that are directly activated by leptin and how it regulates glucose and fatty acids metabolism in skeletal muscle and fat tissue. Furthermore, the impact of peripheral leptin resistance in these tissues leading to dysfunctional metabolic adaptations is also discussed.

Inflammation, stress, and diabetes

Over the last decade, an abundance of evidence has emerged demonstrating a close link between metabolism and immunity. It is now clear that obesity is associated with a state of chronic low-level inflammation. In this article, we discuss the molecular and cellular underpinnings of obesity-induced inflammation and the signaling pathways at the intersection of metabolism and inflammation that contribute to diabetes. We also consider mechanisms through which the inflammatory response may be initiated and discuss the reasons for the inflammatory response in obesity. We put forth for consideration some hypotheses regarding important unanswered questions in the field and suggest a model for the integration of inflammatory and metabolic pathways in metabolic disease.

Inflammation, stress, and diabetes

Over the last decade, an abundance of evidence has emerged demonstrating a close link between metabolism and immunity. It is now clear that obesity is associated with a state of chronic low-level inflammation. In this article, we discuss the molecular and cellular underpinnings of obesity-induced inflammation and the signaling pathways at the intersection of metabolism and inflammation that contribute to diabetes. We also consider mechanisms through which the inflammatory response may be initiated and discuss the reasons for the inflammatory response in obesity. We put forth for consideration some hypotheses regarding important unanswered questions in the field and suggest a model for the integration of inflammatory and metabolic pathways in metabolic disease.

Inflammation, stress, and diabetes

Over the last decade, an abundance of evidence has emerged demonstrating a close link between metabolism and immunity. It is now clear that obesity is associated with a state of chronic low-level inflammation. In this article, we discuss the molecular and cellular underpinnings of obesity-induced inflammation and the signaling pathways at the intersection of metabolism and inflammation that contribute to diabetes. We also consider mechanisms through which the inflammatory response may be initiated and discuss the reasons for the inflammatory response in obesity. We put forth for consideration some hypotheses regarding important unanswered questions in the field and suggest a model for the integration of inflammatory and metabolic pathways in metabolic disease.

Molecular and anatomical determinants of central leptin resistance

The increasing incidence of obesity in developed nations is an ever-growing challenge to health care, promoting diabetes and other diseases. The hormone leptin, which is derived from adipose tissue, regulates feeding and energy expenditure. Most forms of obesity are associated with diminished responsiveness to the appetite-suppressing effects of leptin. Here we review the mechanisms by which leptin activates intracellular signals, the roles of these signals in leptin action in vivo, and mechanisms that may attenuate leptin signaling, limiting its action in obese individuals. We highlight data regarding the expression of SOCS3 (a potential mediator of leptin resistance) in the arcuate nucleus of the hypothalamus.

Negative regulation of cytokine signaling and immune responses by SOCS proteins

Immune and inflammatory systems are controlled by multiple cytokines, including interleukins and interferons. Many of these cytokines exert their biological functions through JAKs (Janus tyrosine kinases) and STAT (signal transduction and activators of transcription) transcription factors. CIS (cytokine-inducible SH2 (Src homology 2) protein) and SOCS (suppressor of cytokine signaling) are a family of intracellular proteins, several of which have emerged as key physiological regulators of cytokine-mediated homeostasis, including innate and adaptive immunity. In this review we focus on the molecular mechanism of the action of CIS/SOCS family proteins and their roles in immune regulation and inflammatory diseases including rheumatoid arthritis.