Beneficial effects of leptin on obesity, T cell hyporesponsiveness, and neuroendocrine/metabolic dysfunction of human congenital leptin deficiency

In search of fat profits

A pill to prevent obesity is proving as elusive to the drug industry as weight loss is to a growing proportion of the population. Geoff Watts assesses the latest candidates

The hormonal control of food intake

Numerous circulating peptides and steroids produced in the body influence appetite through their actions on the hypothalamus, the brain stem, and the autonomic nervous system. These hormones come from three major sites—fat cells, the gastrointestinal tract, and the pancreas. In this Review we provide a synthesis of recent evidence concerning the actions of these hormones on food intake.

Circulating leptin levels are not associated with cardiovascular morbidity and mortality in women with diabetes: a prospective cohort study

AIMS/HYPOTHESIS

Leptin, an adipocyte-secreted hormone, plays an important role in regulating neuroendocrine and immune function as well as insulin resistance and metabolism. Our objective was to examine the relationship between leptin levels and cardiovascular morbidity and overall mortality in women with type 2 diabetes.

SUBJECTS AND METHODS

This prospective cohort study included 1,194 women with a confirmed diagnosis of type 2 diabetes, who provided a blood sample at baseline in 1989-1990. Participants were followed for 12 years for the development of health outcomes including cardiovascular disease (CVD) events as well as total mortality.

RESULTS

There were 218 new CVD events and 228 deaths from all causes. Cox proportional hazards analysis was used to estimate the relative risks (RRs) for each quintile level of leptin compared with the lowest quintile. Leptin levels were positively associated with several CVD risk factors including BMI and inflammatory markers, but were not independently associated with the incidence of CVD or total mortality in women with diabetes. The multivariate RRs (95% CIs) for CVD across the quintiles of leptin were 0.96 (0.61-1.53), 0.99 (0.61-1.61), 1.04 (0.63-1.71), 1.02 (0.59-1.75) (p for trend = 0.83).

CONCLUSIONS/INTERPRETATION

Although circulating leptin levels are associated with obesity and inflammatory markers, they are not significantly related to the risk of CVD or mortality in women with diabetes.

Relationship of obesity with osteoporosis

CONTEXT

The relationship between obesity and osteoporosis has been widely studied, and epidemiological evidence shows that obesity is correlated with increased bone mass. Previous analyses, however, did not control for the mechanical loading effects of total body weight on bone mass and may have generated a confounded or even biased relationship between obesity and osteoporosis.

OBJECTIVE

The objective of this study was to reevaluate the relationship between obesity and osteoporosis by accounting for the mechanical loading effects of total body weight on bone mass.

METHODS

We measured whole body fat mass, lean mass, percentage fat mass, body mass index, and bone mass in two large samples of different ethnicity: 1988 unrelated Chinese subjects and 4489 Caucasian subjects from 512 pedigrees. We first evaluated the Pearson correlations among different phenotypes. We then dissected the phenotypic correlations into genetic and environmental components with bone mass unadjusted or adjusted for body weight. This allowed us to compare the results with and without controlling for mechanical loading effects of body weight on bone mass.

RESULTS

In both Chinese and Caucasian subjects, when the mechanical loading effect of body weight on bone mass was adjusted for, the phenotypic correlation (including its genetic and environmental components) between fat mass (or percentage fat mass) and bone mass was negative. Further multivariate analyses in subjects stratified by body weight confirmed the inverse relationship between bone mass and fat mass, after mechanical loading effects due to total body weight were controlled.

CONCLUSIONS

Increasing fat mass may not have a beneficial effect on bone mass.

Analysis of sequence variations in the suppressor of cytokine signaling (SOCS)-3 gene in extremely obese children and adolescents

Background

The suppressor of cytokine signaling (SOCS)-3 is a negative feedback regulator of cytokine signaling and also influences leptin signaling. We investigated association of variations in the coding sequence and promoter region of SOCS3 with extreme obesity in German children and adolescents.

Methods

An initial screen for sequence variations in 181 extremely obese children and adolescents and 188 healthy underweight adults revealed two previously reported single nucleotide polymorphisms (SNPs) in the SOCS3 5' region: -1044 C>A (numbering refers to bases upstream of ATG in exon 2) within a predicted STAT3 binding element and -920 C>A (rs12953258, for numbering, see above).

Results

We did not detect significant differences in allele or genotype frequencies for any of these SNPs between the analysed study groups (all nominal p > 0.2). In addition, we performed a pedigree transmission disequilibrium test (PDT) for the SNP -1044 C>A in families comprising 703 obese children and adolescents, 281 of their obese siblings and both biological parents. The PDT revealed no transmission disequilibrium (nominal p > 0.05).

Conclusion

In conclusion, our data do not suggest evidence for a major role of the respective SNPs in SOCS3 in the pathogenesis of extreme obesity in our study groups.

Effects of leptin on intake of specific micro- and macronutrients in a woman with leptin gene deficiency studied off and on leptin at stable body weight

In this report, we examine the effects of leptin on the intake of specific macro- and micronutrients in a female patient with leptin gene deficiency. The patient was studied off and on leptin at stable body weight, within the normal to mildly overweight range. The data were obtained by detailed dietary assessments, measuring dietary intakes by weighed food and fluid consumption records, and analyzed using nutrition analysis software. Overall, significant differences were found in the off versus on leptin treatment periods in the following categories: (i) macronutrients: kilocalories, protein, carbohydrates, monounsaturated fats, MFA 18:1 oleic and total fiber; (ii) vitamins: vitamin C, pyridoxine and pantothenic acid; (iii) minerals: potassium, magnesium, copper and chromium; and amino acids: threonine, lysine and histidine. The nutritional data from this study indicates a direct link between the effects of leptin and ingestion of several specific micronutrients. The mechanisms underlying these effects warrant further investigation and study.

gp130 receptor ligands as potential therapeutic targets for obesity

Obesity and its related cluster of pathophysiologic conditions including insulin resistance, glucose intolerance, dyslipidemia, and hypertension are recognized as growing threats to world health. It is now estimated that 10% of the world's population is overweight or obese. As a result, new therapeutic options for the treatment of obesity are clearly warranted. Recent research has focused on the role that gp130 receptor ligands may play as potential therapeutic targets in obesity. One cytokine in particular, ciliary neurotrophic factor (CNTF), acts both centrally and peripherally and mimics the biologic actions of the appetite control hormone leptin, but unlike leptin, CNTF appears to be effective in obesity and as such may have therapeutic potential. In addition, CNTF suppresses inflammatory signaling cascades associated with lipid accumulation in liver and skeletal muscle. This review examines the potential role of gp130 receptor ligands as part of a therapeutic strategy to treat obesity.

Genetics of obesity

Considerable attention is currently being paid to the secular changes in food intake and physical activity that underlie the increase in the prevalence of obesity that is apparent in many societies. While this is laudable it would be unwise to view these environmental factors in isolation from the biological factors that normally control body weight and composition and the compelling evidence that inter-individual differences in susceptibility to obesity have strong genetic determinants. This is particularly important, as it is only in the past decade that we have begun to obtain substantive information regarding the molecular constituents of pathways controlling mammalian energy balance and therefore, for the first time, are in a position to achieve a better mechanistic understanding of this disease. Population-based association and linkage studies have highlighted a number of loci at which genetic variation is associated with obesity and related phenotypes and the identification and characterization of monogenic obesity syndromes has been particularly fruitful. While there is widespread acceptance that hereditary factors might predispose to human obesity, it is frequently assumed that such factors would influence metabolic rate or the selective partitioning of excess calories into fat. However, it is notable that, thus far, all monogenic defects causing human obesity actually disrupt hypothalamic pathways and have a profound effect on satiety and food intake. To conclude, the evidence we have to date suggests that the major impact of genes on human obesity is just as likely (or perhaps more likely) to directly impact on hunger, satiety and food intake rather than metabolic rate or nutrient partitioning. At the risk of oversimplification, it seems that from an aetiological/genetic standpoint, human obesity appears less a metabolic than a neuro-behavioural disease.

Appetite and energy balance signals from adipocytes

Interest in the biology of white adipose tissue has risen markedly with the recent surge in obesity and its associated disorders. The tissue is no longer viewed simply as a vehicle for lipid storage; instead, it is recognized as a major endocrine and secretory organ. White adipocytes release a multiplicity of protein hormones, signals and factors, termed adipokines, with an extensive range of physiological actions. Foremost among these various adipokines is the cytokine-like hormone, leptin, which is synthesized predominantly in white fat. Leptin plays a critical role in the control of appetite and energy balance, with mutations in the genes encoding the hormone or its receptor leading to profound obesity in both rodents and man. Leptin regulates appetite primarily through an interaction with hypothalamic neuroendocrine pathways, inhibiting orexigenic peptides such as neuropeptide Y and orexin A, and stimulating anorexigenic peptides such as proopiomelanocortin. White fat also secretes several putative appetite-related adipokines, which include interleukin-6 and adiponectin, but whether these are indeed significant signals in the regulation of food intake has not been established. Through leptin and the other adipokines it is evident that adipose tissue communicates extensively with other organs and plays a pervasive role in metabolic homeostasis.

Unraveling the genetics of human obesity

The use of modern molecular biology tools in deciphering the perturbed biochemistry and physiology underlying the obese state has proven invaluable. Identifying the hypothalamic leptin/melanocortin pathway as critical in many cases of monogenic obesity has permitted targeted, hypothesis-driven experiments to be performed, and has implicated new candidates as causative for previously uncharacterized clinical cases of obesity. Meanwhile, the effects of mutations in the melanocortin-4 receptor gene, for which the obese phenotype varies in the degree of severity among individuals, are now thought to be influenced by one's environmental surroundings. Molecular approaches have revealed that syndromes (Prader-Willi and Bardet-Biedl) previously assumed to be controlled by a single gene are, conversely, regulated by multiple elements. Finally, the application of comprehensive profiling technologies coupled with creative statistical analyses has revealed that interactions between genetic and environmental factors are responsible for the common obesity currently challenging many Westernized societies. As such, an improved understanding of the different “types” of obesity not only permits the development of potential therapies, but also proposes novel and often unexpected directions in deciphering the dysfunctional state of obesity.

Plasma concentrations of free triiodothyronine predict weight change in euthyroid persons

BACKGROUND

Factors that influence energy metabolism and substrate oxidation, such as thyroid hormones (THs), may be important regulators of body weight.

OBJECTIVE

We investigated associations of THs cross-sectionally with obesity, energy expenditure, and substrate oxidation and prospectively with weight change.

DESIGN

Euthyroid, nondiabetic, healthy, adult Pima Indians (n = 89; 47 M, 42 F) were studied. Percentage body fat (%BF) was measured by using dual-energy X-ray absorptiometry; sleeping metabolic rate (SMR), respiratory quotient, and substrate oxidation rates were measured in a respiratory chamber. Thyroid-stimulating hormone (TSH), free thyroxine (T(4)), free triiodothyronine (T(3)), and leptin concentrations were measured in fasting plasma samples.

RESULTS

TSH, but neither free T(3) nor free T(4), was associated with %BF and leptin concentrations (r = 0.27 and 0.29, respectively; both: P <or= 0.01). In multiple regression analyses adjusted for age, sex, fat mass, and fat-free mass, free T(3) was a positive predictor of SMR (P = 0.02). After adjustment for age, sex, %BF, and energy balance, free T(3) was a negative predictor of 24-h respiratory quotient (P < 0.05) and a positive predictor of 24-h lipid oxidation rate (P = 0.006). Prospectively, after an average follow-up of 4 +/- 2 y, the mean increase in weight was 3 +/- 9 kg. Baseline T(3) concentrations were associated with absolute and annual percentage of changes in weight (r = -0.27, P = 0.02, and r = -0.28, P = 0.009, for the age- and sex-adjusted associations, respectively).

CONCLUSIONS

In euthyroid Pima Indians, lower free T(3) but not free T(4) concentrations were an independent predictor of SMR and lipid oxidation and a predictor of weight gain. This finding indicates that control of T(4)-to-T(3) conversion may play a role in body weight regulation.

Clinical and molecular genetic spectrum of congenital deficiency of the leptin receptor

BACKGROUND

A single family has been described in which obesity results from a mutation in the leptin-receptor gene (LEPR), but the prevalence of such mutations in severe, early-onset obesity has not been systematically examined.

METHODS

We sequenced LEPR in 300 subjects with hyperphagia and severe early-onset obesity, including 90 probands from consanguineous families, and investigated the extent to which mutations cosegregated with obesity and affected receptor function. We evaluated metabolic, endocrine, and immune function in probands and affected relatives.

RESULTS

Of the 300 subjects, 8 (3%) had nonsense or missense LEPR mutations--7 were homozygotes, and 1 was a compound heterozygote. All missense mutations resulted in impaired receptor signaling. Affected subjects were characterized by hyperphagia, severe obesity, alterations in immune function, and delayed puberty due to hypogonadotropic hypogonadism. Serum leptin levels were within the range predicted by the elevated fat mass in these subjects. Their clinical features were less severe than those of subjects with congenital leptin deficiency.

CONCLUSIONS

The prevalence of pathogenic LEPR mutations in a cohort of subjects with severe, early-onset obesity was 3%. Circulating levels of leptin were not disproportionately elevated, suggesting that serum leptin cannot be used as a marker for leptin-receptor deficiency. Congenital leptin-receptor deficiency should be considered in the differential diagnosis in any child with hyperphagia and severe obesity in the absence of developmental delay or dysmorphism.

Adipocytes as regulators of energy balance and glucose homeostasis

Adipocytes have been studied with increasing intensity as a result of the emergence of obesity as a serious public health problem and the realization that adipose tissue serves as an integrator of various physiological pathways. In particular, their role in calorie storage makes adipocytes well suited to the regulation of energy balance. Adipose tissue also serves as a crucial integrator of glucose homeostasis. Knowledge of adipocyte biology is therefore crucial for understanding the pathophysiological basis of obesity and metabolic diseases such as type 2 diabetes. Furthermore, the rational manipulation of adipose physiology is a promising avenue for therapy of these conditions.

Leptin levels in various manifestations of pulmonary tuberculosis

BACKGROUND

Proinflammatory cytokines are prime candidates as causative agents of the metabolic changes that eventually result in tuberculosis-associated weight loss. Microbial products and cytokines such as TNF and IL-1 increase leptin expression dose dependently in adipose tissue. Leptin plays an important role in cellular immunity.

OBJECTIVES

In this study, we investigated serum leptin and TNF-alpha levels before and after antituberculosis therapy in patients with active pulmonary tuberculosis (TB).

METHODS

Twenty five in patients with active pulmonary TB and 18 healthy controls participated in the study. Leptin and TNF-alpha levels were measured before treatment and six months after the treatment and they were compared with the control group. Body mass index (BMI) and chest X-rays before and after the treatment were also evaluated.

RESULTS

The leptin levels before and after the treatment were 1.66+/-1.68 ng/mL and 3.26+/-3.81 ng/mL, respectively. The leptin levels of tuberculous patients were significant than in healthy patients (P < .05). The BMI was 19.36+/-2.55 kg/m2 before the treatment and 22.87+/-3.13 kg/m2 after the treatment. The TNF-alpha level was 23.19+/-12.78 pg/mL before the treatment and 15.95+/-6.58 pg/mL after the treatment. There was no correlation between leptin and TNF-alpha levels. Leptin levels were low in patients who had sequela lesion on chest radiographs.

CONCLUSION

Leptin levels are suppressed in tuberculous patients and low leptin levels may contribute to increased susceptibility to infection and recovery with sequela lesions.

Leptin: at the crossroads of energy balance and systemic inflammation

In addition to playing a central role in energy homeostasis, leptin is also an important player in the inflammatory response. Systemic inflammation is accompanied by fever (less severe cases) or hypothermia (more severe cases). In leptin-irresponsive mutants, the hypothermia of systemic inflammation is exaggerated, presumably due to the enhanced production and cryogenic action of tumor necrosis factor (TNF)-alpha. Mechanisms that exaggerate hypothermia can also attenuate fever, particularly in a cool environment. Another common manifestation of systemic inflammation is behavioral depression. Along with the production of interleukin (IL)-1beta, this manifestation is exaggerated in leptin-irresponsive mutants. The enhanced production of TNF-alpha and IL-1beta may be due, at least in part, to insufficient activation of the anti-inflammatory hypothalamo-pituitary-adrenal axis by immune stimuli in the absence of leptin signaling. In experimental animals and humans that are responsive to leptin, suppression of leptin production under conditions of negative energy balance (e.g., fasting) can exaggerate both hypothermia and behavioral depression. Since these manifestations aid energy conservation, exaggeration of these manifestations under conditions of negative energy balance is likely to be beneficial.

Genes involved in obesity: Adipocytes, brain and microflora

The incidence of obesity and related metabolic disorders such as cardiovascular diseases and type 2 diabetes, are reaching worldwide epidemic proportions. It results from an imbalance between caloric intake and energy expenditure leading to excess energy storage, mostly due to genetic and environmental factors such as diet, food components and/or way of life. It is known since long that this balance is maintained to equilibrium by multiple mechanisms allowing the brain to sense the nutritional status of the body and adapt behavioral and metabolic responses to changes in fuel availability. In this review, we summarize selected aspects of the regulation of energy homeostasis, prevalently highlighting the complex relationships existing between the white adipose tissue, the central nervous system, the endogenous microbiota, and nutrition. We first describe how both the formation and functionality of adipose cells are strongly modulated by the diet before summarizing where and how the central nervous system integrates peripheral signals from the adipose tissue and/or the gastro-intestinal tract. Finally, after a short description of the intestinal commensal flora, rangingfrom its composition to its importance in immune surveillance, we enlarge the discussion on how nutrition modified this perfectly well-balanced ecosystem.

Obesity and its therapy: from genes to community action

Like many diseases, the causes of obesity are complex, and their investigation requires novel approaches. Given the many contributors to our weight status, as well as the dynamic nature, genomic tools must be applied in an ecological model. Evaluating disparate factors can be difficult, such as feeding behavior, nutritional genomics, and gene-environment interaction. Many of these behaviors are being evaluated in animal models and hold great promise for targeted interventions in the future.

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.

Differential regulation of metabolic, neuroendocrine, and immune function by leptin in humans

To elucidate whether the role of leptin in regulating neuroendocrine and immune function during short-term starvation in healthy humans is permissive, i.e., occurs only when circulating leptin levels are below a critical threshold level, we studied seven normal-weight women during a normoleptinemic-fed state and two states of relative hypoleptinemia induced by 72-h fasting during which we administered either placebo or recombinant methionyl human leptin (r-metHuLeptin) in replacement doses. Fasting for 72 h decreased leptin levels by approximately = 80% from a midphysiologic (14.7 +/- 2.6 ng/ml) to a low-physiologic (2.8 +/- 0.3 ng/ml) level. Administration of r-metHuLeptin during fasting fully restored leptin to physiologic levels (28.8 +/- 2.0 ng/ml) and reversed the fasting-associated decrease in overnight luteinizing hormone pulse frequency but had no effect on fasting-induced changes in thyroid-stimulating hormone pulsatility, thyroid and IGF-1 hormone levels, hypothalamic-pituitary-adrenal and renin-aldosterone activity. FSH and sex steroid levels were not altered. Short-term reduction of leptin levels decreased the number of circulating cells of the adaptive immune response, but r-metHuLeptin did not have major effects on their number or in vitro function. Thus, changes of leptin levels within the physiologic range have no major physiologic effects in leptin-replete humans. Studies involving more severe and/or chronic leptin deficiency are needed to precisely define the lower limit of normal leptin levels for each of leptin's physiologic targets.

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.

Low-dose leptin reverses skeletal muscle, autonomic, and neuroendocrine adaptations to maintenance of reduced weight

Maintenance of a reduced body weight is accompanied by decreased energy expenditure that is due largely to increased skeletal muscle work efficiency. In addition, decreased sympathetic nervous system tone and circulating concentrations of leptin, thyroxine, and triiodothyronine act coordinately to favor weight regain. These "weight-reduced" phenotypes are similar to those of leptin-deficient humans and rodents. We examined metabolic, autonomic, and neuroendocrine phenotypes in 10 inpatient subjects (5 males, 5 females [3 never-obese, 7 obese]) under 3 sets of experimental conditions: (a) maintaining usual weight by ingesting a liquid formula diet; (b) maintaining a 10% reduced weight by ingesting a liquid formula diet; and (c) receiving twice-daily subcutaneous doses of leptin sufficient to restore 8 am circulating leptin concentrations to pre-weight-loss levels and remaining on the same liquid formula diet required to maintain a 10% reduced weight. During leptin administration, energy expenditure, skeletal muscle work efficiency, sympathetic nervous system tone, and circulating concentrations of thyroxine and triiodothyronine returned to pre-weight-loss levels. These responses suggest that the weight-reduced state may be regarded as a condition of relative leptin insufficiency. Prevention of weight regain might be achievable by strategies relevant to reversing this leptin-insufficient state.

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.

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.

Correlation of serum leptin levels with anthropometric and metabolic parameters and biochemical liver function in Chinese patients with chronic hepatitis C virus infection

AIM: To determine serum leptin levels and investigate their correlations with anthropometric and metabolic parameters and biochemical liver function in patients with chronic hepatitis C virus (HCV) infection and their potential clinical implications.

METHODS: Forty-two chronic HCV-infected patients without anti-viral treatment were enrolled in this study, 30 patients had chronic hepatitis C, 10 had cirrhosis, and 2 had hepatocellular carcinoma (HCC). Thirty age- and sex-matched healthy individuals served as controls. Serum leptin levels were determined by ELISA. The biochemical liver function and serum lipids were determined at the same time. The height and body weight of patients and controls were measured, and body mass index (BMI) and body fat were calculated simultaneously. The correlations of serum leptin levels with anthropometric and metabolic parameters and biochemical liver function were assessed statistically.

RESULTS: The mean of serum leptin levels in patients with chronic hepatitis C, HCV-associated cirrhosis, HCV-associated HCC and control groups was (6.13±3.94), (5.25±4.21), (4.17±0.28), and (3.59±3.44) ng/mL, respectively. The serum leptin level in patients with chronic hepatitis C was significantly higher than that in controls. The serum leptin levels between cirrhotic patients and controls and between male and female cirrhotic patients had no significant difference. Serum leptin levels were positively-correlated with body fat, BMI, and apolipoprotein B (Apo B) in patients with chronic HCV infection. The serum alanine aminotransferase (ALT) levels were closely-correlated with BMI in patients with chronic hepatitis C.

CONCLUSION: HCV infection interferes with fat and lipid metabolism in patients with chronic HCV infection and leptin may play a role in hepatosteatosis.

Neuronal Shp2 tyrosine phosphatase controls energy balance and metabolism

Shp2, a Src homology 2-containing tyrosine phosphatase, has been implicated in a variety of growth factor or cytokine signaling pathways. However, it is conceivable that this enzyme acts predominantly in one pathway versus the others in a cell, depending on the cellular context. To determine the putative functions of Shp2 in the adult brain, we selectively deleted Shp2 in postmitotic forebrain neurons by crossing CaMKIIalpha-Cre transgenic mice with a conditional Shp2 mutant (Shp2(flox)) strain. Surprisingly, a prominent phenotype of the mutant (CaMKIIalpha-Cre:Shp2(flox/flox) or CaSKO) mice was the development of early-onset obesity, with increased serum levels of leptin, insulin, glucose, and triglycerides. The mutant mice were not hyperphagic but developed enlarged and steatotic liver. Consistent with previous in vitro data, we found that Shp2 down-regulates Jak2/Stat3 (signal transducer and activator of transcription 3) activation by leptin in the hypothalamus. However, Jak2/Stat3 down-regulation is offset by a dominant Shp2 promotion of the leptin-stimulated Erk pathway, leading to induction rather than suppression of leptin resistance upon Shp2 deletion in the brain. Collectively, these results suggest that a primary function of Shp2 in postmitotic forebrain neurons is to control energy balance and metabolism, and that this phosphatase is a critical signaling component of leptin receptor ObRb in the hypothalamus. Shp2 shows potential as a neuronal target for pharmaceutical sensitization of obese patients to leptin action.

Hyperleptinemia prevents lipotoxic cardiomyopathy in acyl CoA synthase transgenic mice

The physiologic function of the progressive hyperleptinemia of diet-induced obesity is unknown. However, that lipotoxicity in nonadipose tissues of congenitally unleptinized obese rodents is far greater than in hyperleptinemic diet-induced obesity rodents has suggested an antilipotoxic role. To test this hypothesis, mice with severe lipotoxic cardiomyopathy, induced transgenically by cardiomyocyte-specific overexpression of the acyl CoA synthase (ACS) gene, were made hyperleptinemic by treatment with recombinant adenovirus containing the leptin cDNA. Normoleptinemic control ACS-transgenic mice developed severe dilated cardiomyopathy with thickened left ventricular walls and profound impairment of systolic function on echocardiogram; histologically, there was severe myofiber disorganization and interstitial fibrosis, with intracytoplasmic lipid vacuoles identifiable by electron microscope. By contrast, the hearts of hyperleptinemic ACS-transgenic mice appeared normal, with normal echocardiograms and cardiac triglyceride (TG) contents. Their lower myocardial TG content was ascribed primarily to profound lowering of plasma TG and free fatty acids; free fatty acids were 17% of normal at 8 weeks. Additionally, enhanced myocardial AMP-activated protein kinase phosphorylation may have increased fatty acid oxidation, thereby contributing to the lowering of lipid stores. We conclude that obesity-level hyperleptinemia protects the heart from lipotoxicity.

Recombinant human leptin in women with hypothalamic amenorrhea

BACKGROUND

Disruptions in hypothalamic-gonadal and other endocrine axes due to energy deficits are associated with low levels of the adipocyte-secreted hormone leptin and may result in hypothalamic amenorrhea. We hypothesized that exogenous recombinant leptin replacement would improve reproductive and neuroendocrine function in women with hypothalamic amenorrhea.

METHODS

Eight women with hypothalamic amenorrhea due to strenuous exercise or low weight were studied for one month before receiving recombinant human leptin and then while receiving treatment for up to three months. Six control subjects with hypothalamic amenorrhea received no treatment and were studied for a mean (+/-SD) of 8.5+/-8.1 months.

RESULTS

Luteinizing hormone (LH) pulsatility, body weight, ovarian variables, and hormone levels did not change significantly over time in the controls and during a one-month control period before recombinant leptin therapy in the treated subjects. In contrast, recombinant leptin treatment increased mean LH levels and LH pulse frequency after two weeks and increased maximal follicular diameter, the number of dominant follicles, ovarian volume, and estradiol levels over a period of three months. Three patients had an ovulatory menstrual cycle (P<0.05 for the comparison with an expected rate of spontaneous ovulation of 10 percent); two others had preovulatory follicular development and withdrawal bleeding during treatment (P<0.05). Recombinant leptin significantly increased levels of free triiodothyronine, free thyroxine, insulin-like growth factor 1, insulin-like growth factor-binding protein 3, bone alkaline phosphatase, and osteocalcin but not cortisol, corticotropin, or urinary N-telopeptide.

CONCLUSIONS

Leptin administration for the relative leptin deficiency in women with hypothalamic amenorrhea appears to improve reproductive, thyroid, and growth hormone axes and markers of bone formation, suggesting that leptin, a peripheral signal reflecting the adequacy of energy stores, is required for normal reproductive and neuroendocrine function.

Effects of exogenous leptin on satiety and satiation in patients with lipodystrophy and leptin insufficiency

To examine leptin's role in human appetite regulation, we studied recombinant methionyl human leptin's effects on satiation and satiety in a model of leptin insufficiency, lipodystrophy. Eight females with hypoleptinemia and lipodystrophy were given sc injections of A-100 (maximal dose, 200% of that predicted to normalize serum leptin) for 4 months. Satiation and satiety were determined before and again during leptin treatment. Satiation was measured as the time to voluntary cessation of eating from a standardized food array after a 12-h fast. Satiety was determined as the time to hunger sufficient to consume a full meal after consumption of a standardized preload. During leptin treatment, satiation time decreased (41.2 +/- 18.2 to 19.5 +/- 10.6 min; P = 0.01), satiety time increased (62.9 +/- 64.8 to 137.8 +/- 91.6 min; P = 0.04), energy consumed to produce satiation decreased (2034 +/- 405 to 1135 +/- 432 kcal or 8.5 +/- 1.7 to 4.7 +/- 1.8 MJ; P < 0.01), and the amount of food desired in the postabsorptive state decreased (P < 0.02). Ghrelin concentrations also decreased during leptin administration (284.3 +/- 127.9 to 140.6 +/- 104.5 pmol/liter; P < 0.002). We conclude that increased leptin in patients with lipodystrophy results in less caloric, shorter, more satiating meals and longer-lived satiety. These data support the hypothesis that leptin plays an important, permissive role in human appetite regulation.

Delayed resolution of acute inflammation during zymosan-induced arthritis in leptin-deficient mice

The severity of antigen-induced arthritis (AIA) is decreased in leptin-deficient ob/ob mice. However, joint inflammation in AIA depends on the immune response, which is impaired in ob/ob mice. In the present study we investigated the effects of leptin deficiency on zymosan-induced arthritis (ZIA), which is independent of adaptive immunity. Arthritis was induced by injection of zymosan into the knee joint. Joint swelling was similar after 6 and 24 hours in ob/ob and control mice. However, it remained elevated in ob/ob animals on day 3 whereas values normalized in controls. Histology revealed similar articular lesions in all animals on day 3, but on days 14 and 21 arthritis tended to be more severe in ob/ob mice. The acute phase response, reflected by circulating levels of IL-6 and serum amyloid A, was also more pronounced in ob/ob mice, although corticosterone was significantly elevated in these animals. Similar results were obtained in leptin receptor-deficient db/db mice. Thus, in contrast to AIA, ZIA is not impaired in leptin-deficient animals. On the contrary, resolution of acute inflammation appears to be delayed in the absence of leptin or leptin signalling, suggesting that chronic leptin deficiency interferes with adequate control of the inflammatory response in ZIA.

Phenotypic effects of leptin replacement on morbid obesity, diabetes mellitus, hypogonadism, and behavior in leptin-deficient adults

Genetic mutations in the leptin pathway can be a cause of human obesity. It is still unknown whether leptin can be effective in the treatment of fully established morbid obesity and its endocrine and metabolic consequences in adults. To test the hypothesis that leptin has a key role in metabolic and endocrine regulation in adults, we examined the effects of human leptin replacement in the only three adults identified to date who have genetically based leptin deficiency. We treated these three morbidly obese homozygous leptin-deficient adult patients with recombinant human leptin at low, physiological replacement doses in the range of 0.01-0.04 mg/kg for 18 months. Patients were hypogonadal, and one of them also had type 2 diabetes mellitus. We chose the doses of recombinant methionyl human leptin that would achieve normal leptin concentrations and administered them daily in the evening to model the normal circadian variation in endogenous leptin. The mean body mass index dropped from 51.2 +/- 2.5 (mean +/- SEM) at baseline to 26.9 +/- 2.1 kg/m2 after 18 months of treatment, mainly because of loss of fat mass. We document here that leptin replacement therapy in leptin-deficient adults with established morbid obesity results in profound weight loss, increased physical activity, changes in endocrine function and metabolism, including resolution of type 2 diabetes mellitus and hypogonadism, and beneficial effects on ingestive and noningestive behavior. These results highlight the role of the leptin pathway in adults with key effects on the regulation of body weight, gonadal function, and behavior.

Leptin stimulates the oxidative burst in control monocytes but attenuates the oxidative burst in monocytes from HIV-infected patients

Leptin, the 16 kDa product of the ob gene, is a an adipocyte-secreted hormone that centrally regulates weight. However, the physiological role of leptin is not limited to the regulation of food intake and energy expenditure, and leptin has a variety of effects in peripheral tissues, such as a regulatory role modulating the immune system. Thus, leptin receptor is expressed in human peripheral blood mononuclear cells, mediating the leptin stimulation of proliferation and activation, the production of proinflammatory cytokines from cultured monocytes, and the prevention of apoptotic death in serum-deprived monocytes. Because leptin can stimulate monocytes and the production of reactive oxygen species (ROS) are the result of monocyte activation, we investigated the effect of leptin on ROS production by human monocytes in vitro. Oxidative burst was measured by oxidation of the redox-sensitive dye 2',7'-dichlorofluorescein diacetate, and analysed by flow cytometry. We have found that stimulation with leptin produces oxygen radical formation by monocytes. This effect is dependent on the dose and maximal response is achieved at 10 nM leptin. Because HIV infection induces the production of ROS, we next investigated the effect of leptin on ROS production in monocytes from HIV-positive (HIV+) subjects. We have also found that monocytes from HIV+ subjects spontaneously produced increased amounts of free radicals. In contrast, leptin stimulation of monocytes from these patients partially inhibited the production of ROS. This effect of leptin was also dependent on the dose and maximal effect was achieved at 10 nM. The effect of leptin stimulating the production of ROS is consistent with the proinflammatory role in the immune system. On the other hand, the inhibitory effect on monocytes from HIV+ subjects may be explained by the attenuation of the oxidative burst by a delayed activation of monocytes in a hyperinflammatory state.

Unraveling the multiple roles of leptin in inflammation and autoimmunity

Initially described as an antiobesity hormone, leptin has subsequently been shown also to influence hematopoiesis, thermogenesis, reproduction, angiogenesis, and immune homeostasis. Leptin links nutritional status and proinflammatory T helper 1 immune responses, and the decrease in leptin plasma concentration during food deprivation leads to impaired immune function. This review focuses on the multiple roles of leptin in chronic inflammation and autoimmunity and suggests new possible therapeutic implications for leptin modulators.

The role of leptin-->STAT3 signaling in neuroendocrine function: an integrative perspective

The hormone leptin is secreted by adipose tissue in proportion to fat mass to signal the repletion of body energy stores to the neuroendocrine system. Leptin acts on neurons in the hypothalamus and elsewhere in the brain to decrease appetite and regulate the activity of the thyroid, adrenal, growth, gonadal, and lactational axes. Conversely, absence of leptin signaling initiates the neuroendocrine starvation response. Leptin mediates these effects by activating the long form (LRb) of its receptor. One LRb signal, STAT3, has recently been shown to play a critical role in the regulation of body weight and some elements of neuroendocrine function (thyroid, adrenal, lactation), although the participation of STAT3 in the gonadal and growth axes is negligible. We discuss these findings in the context of the hypothalamic neuroendocrine system as it is presently understood.

Role of leptin as an immunomodulator of blood mononuclear cells: mechanisms of action

Leptin is a an adipocyte-secreted hormone that regulates weight centrally. However, the leptin receptor is expressed not only in the central nervous system, but also in peripheral tissues, such as haematopoietic and immune systems. Therefore, the physiological role of leptin should not be limited to the regulation of food intake and energy expenditure. Moreover, the leptin receptor bears homology to members of the class I cytokine family, and recent data have demonstrated that leptin is able to modulate the immune response. Thus, the leptin receptor is expressed in human peripheral blood mononuclear cells, mediating the leptin effect on proliferation and activation. In vitro activation and HIV infection in vivo induce the expression of the long isoform of the leptin receptor in mononuclear cells. Also, leptin stimulates the production of proinflammatory cytokines from cultured monocytes and enhances the production of Th1 type cytokines from stimulated lymphocytes. Moreover, leptin has a trophic effect on monocytes, preventing apoptosis induced by serum deprivation. Leptin stimulation activates JAK-STAT, IRS-1-PI3K and MAPK signalling pathways. Leptin also stimulates Tyr-phosphorylation of the RNA-binding protein Sam68 mediating the dissociation from RNA. In this way, leptin signalling could modulate RNA metabolism. These signal transduction pathways provide possible mechanisms whereby leptin may modulate activation of peripheral blood mononuclear cells. Therefore, these data support the hypothesis regarding leptin as a proinflammatory cytokine with a possible role as a link between the nutritional status and the immune response. Moreover, these immunoregulatory functions of leptin could have some relevance in the pathophysiology of obesity.

The role of falling leptin levels in the neuroendocrine and metabolic adaptation to short-term starvation in healthy men

To elucidate the role of leptin in regulating neuroendocrine and metabolic function during an acute fast, six to eight healthy, lean men were studied under four separate conditions: a baseline fed state and three 72-hour fasting studies with administration of either placebo, low-dose recombinant-methionyl human leptin (r-metHuLeptin), or replacement-dose r-metHuLeptin designed to maintain serum leptin at levels similar to those in the fed state. Replacement-dose r-metHuLeptin administered during fasting prevents the starvation-induced changes in the hypothalamic-pituitary-gonadal axis and, in part, the hypothalamic-pituitary-thyroid axis and IGF-1 binding capacity in serum. Thus, in normal men, the fall in leptin with fasting may be both necessary and sufficient for the physiologic adaptations of these axes, which require leptin levels above a certain threshold for activation. In contrast to findings in mice, fasting-induced changes in the hypothalamic-pituitary-adrenal, renin-aldosterone, and growth hormone-IGF-1 axes as well as fuel utilization may be independent of leptin in humans. The role of leptin in normalizing several starvation-induced neuroendocrine changes may have important implications for the pathophysiology and treatment of eating disorders and obesity.

Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene

BACKGROUND

Melanocortin 4 receptor (MC4R) deficiency is the commonest monogenic form of obesity. However, the clinical spectrum and mode of inheritance have not been defined, pathophysiological mechanisms leading to obesity are poorly understood, and there is little information regarding genotype-phenotype correlations.

METHODS

We determined the nucleotide sequence of the MC4R gene in 500 probands with severe childhood obesity. Family studies were undertaken to examine cosegregation of identified mutations with obesity. Subjects with MC4R deficiency underwent metabolic and endocrine evaluation; the results were correlated with the signaling properties of mutant receptors.

RESULTS

Twenty-nine probands (5.8 percent) had mutations in MC4R; 23 were heterozygous, and 6 were homozygous. Mutation carriers had severe obesity, increased lean mass, increased linear growth, hyperphagia, and severe hyperinsulinemia; homozygotes were more severely affected than heterozygotes. Subjects with mutations retaining residual signaling capacity had a less severe phenotype.

CONCLUSIONS

Mutations in MC4R result in a distinct obesity syndrome that is inherited in a codominant manner. Mutations leading to complete loss of function are associated with a more severe phenotype. The correlation between the signaling properties of these mutant receptors and energy intake emphasizes the key role of this receptor in the control of eating behavior in humans.