Leptin levels in human and rodent: measurement of plasma leptin and ob RNA in obese and weight-reduced subjects

Leptin does not fully account for the satiety activity of adipose tissue-conditioned medium

To determine whether leptin alone accounts for the satiety activity secreted by native adipose tissue, we prepared culture media conditioned by microdissected adipose tissue from overfed Long-Evans rats, fa/fa rats, or db/db mice (media A, B, and C, respectively). Medium A significantly suppressed food intake following intracerebroventricular delivery to Long-Evans rats (2-h chow intake = 68 +/- 5% of baseline, P < 0.001). Media B and C significantly suppressed food intake following intraperitoneal delivery to ob/ob mice (24-h chow intake = 56 +/- 7% of baseline for medium B, P = 0. 001; 4-day chow intake = 78 +/- 3% of baseline for medium C, P = 0. 004). Using a leptin receptor-based bioassay, we determined that the leptin concentration of medium C was 392 +/- 18 ng/ml. This concentration was 20-fold lower than the concentration of recombinant murine leptin required to produce a similar degree of feeding suppression following 5 days of administration to ob/ob mice. Neither medium conditioned by adipose tissue from ob/ob mice nor medium conditioned by adipose tissue from fa/fa rats and subsequently immunodepleted of leptin had significant satiety activity. We conclude that leptin is necessary but not sufficient to account for the satiety activity of native adipose tissue, perhaps due to the production by adipocytes of a cofactor that augments the ability of leptin to suppress feeding.

Efficacy of exogenous recombinant murine leptin in lean and obese 10- to 12-mo-old female CD-1 mice

Leptin efficacy was compared in obese and lean female CD-1 mice. Body weights in these 10- to 12-mo-old mice ranged from 29.7 to 62.0 g, and leptin levels correlated with body weight. Mice from the lean and obese ends of the weight distribution were treated with daily peripheral leptin injections (1-100 mg/kg) for a 33-day period. The half-maximal effective doses for weight loss and fat reduction were shifted 0.5-0.7 log to the right for obese mice. Leptin was less efficacious at low doses (1-3 mg/kg) in obese mice but equal to or more efficacious in obese than lean mice at high doses (30-100 mg/kg). Leptin's initial effects on weight loss could be explained by appetite suppression in both groups, but its effects on fat reduction were greater in leptin-treated than pair-fed mice, particularly in the lean group. Leptin also prevented the elevations in serum corticosterone and ketones found in pair-fed lean mice. These data allow a quantitative comparison of leptin sensitivity in obese vs. lean CD-1 mice and suggest that in mice where obesity is a function of outbreeding and age, leptin sensitivity is moderately reduced. Furthermore, although appetite suppression has a clear role in leptin's effects on body weight, leptin may also have specific effects on lipid metabolism and mobilization that are different from the metabolic compensations that normally occur with food deprivation.

Leptin levels and gene expression during the perinatal phase in the rat

The role of leptin in controlling food intake and adiposity has been the aim of many different investigations in the last 3 years. Pregnancy and lactation are two physiological situations associated with a clear hyperphagia (together with important changes in metabolism and adipose mass) to sustain the different and varying demands for foetal growth and milk production respectively. We therefore focused on the role of leptin in perinatal hyperphagia. The circulating leptin levels and leptin gene expression in adipose tissue of both pregnant and lactating rats were examined. Pregnant rats showed unchanged adipose tissue leptin mRNA levels but increased circulating leptin; this probably reflects the high fat carcass content characteristic of pregnancy. Conversely, lactating rats did not show any change either in circulating leptin or adipose tissue mRNA levels. Litter-removal caused a significant increase in both circulating leptin levels and gene expression. The results obtained permit us to suggest that leptin does not seem to have a role in controlling food intake during the perinatal phase.

Unraveling the central nervous system pathways underlying responses to leptin

Here we summarize recent progress in the biology of leptin, concentrating on its central nervous system (CNS) actions. The product of the ob gene, leptin is a circulating hormone produced by white adipose tissue that has potent effects on feeding behavior, thermogenesis and neuroendocrine responses. Leptin regulates energy homeostasis, as its absence in rodents and humans causes severe obesity. We consider the physiological mechanisms underlying leptin action, along with several novel hypothalamic neuropeptides that affect food intake and body weight. The molecular causes of several other obesity syndromes are discussed to illuminate how the CNS regulates body weight. We describe neural circuits that are downstream of leptin receptors and propose a model linking populations of leptin-responsive neurons with effector neurons underlying leptin's endocrine, autonomic and behavioral effects.

Development of a sensitive ELISA for human leptin, using monoclonal antibodies

A new, sensitive ELISA for human leptin in plasma and cerebrospinal fluid (CSF) was developed, using monoclonal antibodies. The lower limit of detection of this ELISA was 0.78 pg/assay. Both intra- and interassay imprecision values were &lt;7%. The dilution curves of plasma and CSF showed good linearity, and the recovery was 83.2–95.6%. There was good correlation between plasma leptin concentrations by the ELISA and a commercially available RIA (r = 0.99). Our ELISA is advantageous because it does not require radioisotopes, it produces results in hours rather than days, and more importantly, it improves on the detection limit and plasma interference of the RIA kit. The new ELISA enables measurement of low concentrations of leptin, as are seen in CSF and in plasma of patients with anorexia nervosa.

Leptin inhibits directly glucocorticoid secretion by normal human and rat adrenal gland

Different interactions have been described between glucocorticoids and the product of the ob gene leptin. Leptin can inhibit the activation of the hypothalamo-pituitary-adrenal axis by stressful stimuli, whereas adrenal glucocorticoids stimulate leptin production by the adipocyte. The present study was designed to investigate the potential direct effects of leptin to modulate glucocorticoid production by the adrenal. Human adrenal glands from kidney transplant donors were dissociated, and isolated primary cells were studied in vitro. These cells were preincubated with recombinant leptin (10(-10)-10(-7) M) for 6 or 24 h, and basal or ACTH-stimulated cortisol secretion was subsequently measured. Basal cortisol secretion was unaffected by leptin, but a significant and dose-dependent inhibition of ACTH-stimulated cortisol secretion was observed [down by 29 +/- 0.1% of controls with the highest leptin dose, P < 0.01 vs. CT (unrelated positive control)]. This effect of leptin was also observed in rat primary adrenocortical cells, where leptin inhibited stimulated corticosterone secretion in a dose-dependent manner (down by 46 +/- 0.1% of controls with the highest leptin dose, P < 0.001 vs. CT). These effects of leptin in adrenal cells are likely mediated by the long isoform of the leptin receptor (OB-R), because its transcript was found to be expressed in the adrenal tissue and leptin had no inhibitory effect in adrenal glands obtained from db/db mice. Therefore, leptin inhibits directly stimulated cortisol secretion from human and rat adrenal glands, and this may represent an important mechanism to modulate glucocorticoid levels in various metabolic states.

Abnormal regulation of the leptin gene in the pathogenesis of obesity

A subset of obese humans has relatively low plasma levels of leptin. This finding has suggested that in some cases abnormal regulation of the leptin gene in adipose tissue is etiologic in the pathogenesis of the obese state. The possibility that a relative decrease in leptin production can lead to obesity was tested by mating animals carrying a weakly expressed adipocyte specific aP2-human leptin transgene to C57BL/6J ob/ob mice (which do not express leptin). The transgene does not contain the regulatory elements of the leptin gene and is analogous to a circumstance in which the cis elements and/or trans factors regulating leptin RNA production are abnormal. The ob/ob mice carrying the transgene had a plasma leptin level of 1. 78 ng/ml, which is approximately one-half that found in normal, nontransgenic mice (3.72 ng/ml, P < 0.01). The ob/ob animals expressing the leptin transgene were markedly obese though not as obese as ob/ob mice without the transgene. The infertility as well as several of the endocrine abnormalities generally evident in ob/ob mice were normalized in the ob/ob transgenic mice. However, the ob/ob transgenic mice had an abnormal response when placed at an ambient temperature of 4 degreesC, suggesting that different thresholds exist for the different biologic effects of leptin. Leptin treatment of the ob/ob transgenic mice resulted in marked weight loss with efficacy similar to that seen after treatment of wild-type mice. In aggregate these data suggest that dysregulation of leptin gene can result in obesity with relatively normal levels of leptin and that this form of obesity is responsive to leptin treatment.

Brain administration of OB protein (leptin) inhibits neuropeptide-Y-induced feeding in ob/ob mice

OB protein (or leptin) administration causes a long-lasting reduction in food intake and body weight in obese ob/ob mice. Neuropeptide Y, a stimulator of feeding, has been proposed to be a major mediator of the biological actions of OB protein. To test this hypothesis, the interaction of brain administration of exogenous OB protein and NPY on the feeding behavior of ob/ob mice was examined. Human OB protein, in a dose-dependent manner, partially or completely blocked feeding induced by exogenous NPY. These results demonstrate that OB protein can functionally antagonize and dominate the actions of exogenous NPY on feeding.

Monkey leptin receptor mRNA: sequence, tissue distribution, and mRNA expression in the adipose tissue of normal, hyperinsulinemic, and type 2 diabetic rhesus monkeys

OBJECTIVE

We have cloned the rhesus monkey leptin receptor and examined its mRNA expression levels in the adipose tissue of monkeys to investigate the regulation of gene expression of the leptin receptor.

RESEARCH METHODS AND PROCEDURES

Monkey leptin receptor cDNA was cloned by reverse transcriptase-polymerase chain reaction (RT-PCR). Tissue distribution of monkey leptin receptor was examined by Northern blot analysis and RT-PCR. The mRNA levels of monkey leptin receptor in adipose tissue of normal (n=10), hyperinsulinemic obese (n=8), and type 2 diabetic monkeys (n=8) were measured by quantitative RT-PCR.

RESULTS

Monkey leptin receptor cDNA had at least two alternatively spliced isoforms (long and short forms). The long form of the leptin receptor mRNA was expressed relatively highly in liver, adipose tissue, hypothalamus, and choroid plexus, whereas the total leptin receptors were expressed in every tissue examined. The mRNA levels of the long form of the leptin receptor in adipose tissue were not correlated to body weight, fasting plasma insulin, plasma glucose, or plasma leptin levels. The mRNA levels of the long form of the leptin receptor were highly correlated to that of the total leptin receptor (long and short form).

DISCUSSION

The long form of leptin receptor mRNA existed in adipose tissue as well as in liver and hypothalamus, suggesting that the leptin receptor in adipose tissue may be functional in adipose tissue. The expression of the leptin receptor mRNA in adipose tissue is not affected by obesity, hyperinsulinemia, or diabetes.

Increased fat intake during lactation modifies hypothalamic-pituitary-adrenal responsiveness in developing rat pups: a possible role for leptin

High fat feeding reportedly enhances hypothalamus-pituitary-adrenal (HPA) responses to stress in adult rats. The present study tested whether elevated fat intake during suckling could have short and/or long lasting consequences on HPA regulation in the offspring. Mothers were fed either a control (C; 5% fat) or high fat (HF; 20% fat) diet during the last week of gestation and throughout lactation. After weaning (day 21), pups from C and HF mothers were fed a chow diet. Offspring from both C- and HF-fed mothers were tested for ACTH and corticosterone responses to stress on postnatal days 10 and 35. We found that HF feeding produced higher lipid levels in the milk of HF compared with C lactating rat dams and that offspring of these mothers had significantly increased retroperitoneal fat pad weight and relative adipose mass on day 21 as well as elevated plasma leptin levels on days 10 and 21 of age. After weaning, pups from the HF mothers had lower plasma leptin levels than those from C mothers. Maternal dietary fat affected HPA responsiveness in the offspring in an age-related manner. Neonatal pups (day 10) from the HF mothers exhibited a reduction in the ACTH and corticosterone responses to ether stress. However, in 35-day-old offspring from HF-fed dams, stress-induced ACTH secretion was increased compared with that in pups from the C-fed mothers. These results demonstrate that maternal diet and increased fat intake through the milk are important regulators of HPA responsiveness in neonates and prepubertal rats. During neonatal life, the blunted stress responsiveness seen with elevated fat intake and the resulting high leptin levels might protect the pups from excessive HPA activation. After removal of the maternal dietary influence and reduced leptin levels, enhanced ACTH stress responses are observed as in adult rats fed a HF diet. Because of the inverse relationship between plasma levels of leptin and HPA responses in pups, the possibility exists that the effects of the HF diet on stress responsiveness are mediated by changes in leptin exposure during development.

Plasma leptin concentrations are only transiently increased in nephrectomized rats

Leptin is an adipocyte-secreted hormone that has effects on appetite and energy expenditure. Several studies have shown that end-stage renal disease results in elevated plasma leptin concentrations and that the kidney is responsible for most of leptin elimination in rodents. Leptin metabolism was investigated in rats that underwent unilateral nephrectomy to experimentally limit renal elimination function. Within 4 h of nephrectomy, plasma leptin concentrations increased from 2.9 +/- 0.8 to 5.8 +/- 1.0 & microg/l but thereafter rapidly (<24 h) decreased to prenephrectomy concentrations, despite continued elevated plasma creatinine levels. Sham-operated rats maintained presurgical concentrations of leptin and creatinine throughout the experiment. Kinetic studies of 125I-labeled leptin elimination showed that fractional catabolic rates and half-lives of leptin in circulation were similar at 48 h in nephrectomized and sham-operated rats, suggesting that production of leptin was unchanged after nephrectomy. Excretion of 125I derived from leptin in urine of nephrectomized rats was similar to that of sham-operated rats, and residual radioactivity was increased in the remaining kidneys excised from nephrectomized rats. These results demonstrate that 1) leptin concentrations are quickly restored to presurgical levels in nephrectomized rats, and 2) it is leptin elimination, not leptin production, that compensates to maintain leptin concentrations. Rapid metabolic adaptation of remaining renal tissue may explain the restoration of normal leptin elimination in nephrectomized rats.

Serum leptin and insulin concentrations in prepubertal lean, obese and insulin-dependent diabetes mellitus children

OBJECTIVE

To evaluate the relationship between serum levels of leptin and insulin in prepubertal lean, obese and insulin-dependent diabetes mellitus (IDDM) children.

SUBJECTS AND MEASUREMENTS

Prepubertal children, 16 lean, 17 obese and 16 IDDM were included in the study. Fastang serum leptin and insulin concentrations were measured by radioimmunoassays.

RESULTS

The serum level of leptin was significantly higher in obese children than in lean and IDDM children (P < 0.0001 and P < 0.0001, respectively), and showed a positive correlation with body mass index (BMI) for the combined group (lean, obese and IDDM; r = 0.77, P < 0.0001). In addition, the serum leptin level was higher in IDDM children than in lean controls (P < 0.01), whereas no difference was found in BMI between the two groups. The mean fasting serum levels of insulin were significantly elevated in IDDM children as compared with lean controls (P < 0.01). A significant positive correlation was found between serum insulin and leptin levels for the combined group (r = 0.37, P < 0.01). When a multiple regression analysis for all subjects was performed, the total contribution of all parameters, including gender, BMI and log insulin, accounted for 75% of the leptin variation. BMI (57.8%), log insulin (14.0%) and gender (3.2%) contributed significantly to this variation.

CONCLUSIONS

The elevated concentration of leptin in insulin-dependent diabetic children, independent of body mass index, was probably caused by chronically increased serum insulin levels. We demonstrated that not only body mass index but also insulin was a significant independent predictor of serum leptin concentrations. It is therefore suggested that insulin might play an important role in regulating serum leptin concentrations independent of adiposity.

Lifestyle factors fail to explain the variation in plasma leptin concentrations in women

To assess the relationship between circulating leptin concentrations, metabolic parameters, and lifestyle factors such as alcohol intake, physical activity level, smoking habits, and reproductive history, a cohort of 359 women was drawn from a population-based study conducted in Victoria, Australia. The parameters measured included body mass index (BMI); waist and hip circumference; blood pressure; and fasting glucose, insulin, triacylglycerol, cholesterol, and leptin concentrations. In addition, a self-administered questionnaire was used to assess reproductive history, physical activity level, alcohol intake, and smoking habits. Our results demonstrated that BMI, body weight, waist circumference, and hip circumference were all strongly correlated with circulating leptin concentrations in this population (r > 0.56, P < 0.001 in all cases). Waist/hip ratio, triacylglycerols, insulin, glucose, and cholesterol were also associated with leptin (P < 0.05), but there was no association between leptin and age, height, or blood pressure. When these associations were adjusted for BMI, age, glucose, and waist circumference were significantly associated with leptin. The lifestyle factors examined did not help to explain the observed variation in leptin concentrations between individuals when results were adjusted for degree of adiposity and age.

Leptin expression in adipose tissue from obese humans: depot-specific regulation by insulin and dexamethasone

We investigated the in vitro regulation of leptin expression in adipose tissue from severely obese women and men before and after culture with insulin (7 nM) and/or dexamethasone (25 nM). Leptin mRNA and leptin secretion were two- to threefold higher in subcutaneous vs. omental adipose tissue before culture. Dexamethasone transiently increased leptin mRNA approximately twofold in both depots after 1 day of culture [P < 0.01 vs. basal (no hormone control)], but leptin secretion was only increased in omental adipose tissue (P < 0.005 vs. basal). Insulin did not increase leptin mRNA in either depot but increased leptin secretion approximately 1.5- to 3-fold in subcutaneous tissue throughout 7 days of culture (P < 0.05 vs. basal). The combination of insulin and dexamethasone increased leptin mRNA and leptin secretion approximately two- to threefold in both depots at day 1 (P < 0.005 vs. basal or insulin) and maintained leptin expression throughout 7 days of culture. We conclude that insulin and glucocorticoid have depot-specific effects and function synergistically as long-term regulators of leptin expression in omental and subcutaneous adipose tissue from obese subjects.

Do leptin levels predict weight gain?--A 5-year follow-up study in Mauritius. Mauritius Non-communicable Disease Study Group

OBJECTIVE

To investigate whether relative baseline leptin levels predict long-term changes in adiposity and/or its distribution.

RESEARCH METHODS AND PROCEDURES

In a longitudinal study of 2888 nondiabetic Mauritians aged 25 years to 74 years who participated in population-based surveys in 1987 and 1992, changes in body mass index (BMI), waist/hip ratio (WHR), and waist circumference were compared between "hyperleptinemic," "normoleptinemic," and "hypoleptinemic" groups. "Relative leptin levels" were calculated as standardized residuals from the regression of log10 leptin on baseline BMI to provide a leptin measure independent of BMI. Analyses were performed within each sex. A linear regression model was used to assess the effect of standardized residuals on changes in BMI, WHR, and waist circumference, independent of baseline BMI, age, fasting insulin, and ethnicity.

RESULTS

After adjusting for age and baseline BMI by analysis of covariance, there was no difference in changes in BMI, WHR, or waist circumference between men with low, normal, or high relative leptin levels. Among women, there was a significant difference in deltaWHR across leptin groups, such that the largest increase occurred in the "normal" leptin group. For both men and women, the linear regression models explained approximately 10% of variation in dependent variables, and the only significant independent variables were age, BMI, and being of Chinese origin, compared with Indian origin.

DISCUSSION

These findings do not support a role for leptin concentration in predicting weight gain or changes in fat distribution in adults over a 5-year period.

Serum leptin concentrations in patients on hemodialysis

Serum leptin concentrations in normal humans have been reported to correlate with the body mass index (BMI) as well as with the body fat mass. In this study, we measured serum leptin concentrations in 107 patients on hemodialysis, 30 of whom had diabetes mellitus as the cause, and examined the clinical significance. Furthermore, we evaluated the effects of high-flux dialysis membranes on serum leptin levels. Serum leptin concentrations had a linear correlation with BMI as well as with the percentage of body fat in patients on hemodialysis. The serum leptin concentrations showed a positive correlation with the serum concentrations of total cholesterol, low-density lipoprotein cholesterol, and triglyceride, the body weight, the BMI, and the percentage of body fat. The serum leptin levels were not different between the diabetic and the nondiabetic groups. The serum leptin levels in the nondiabetic group were nearly fourfold higher in women than in men. We investigated the differences in the rate of reduction in serum leptin after dialysis with polysulfone membrane dialyzers (PS-N and PS-UW) in comparison with a cellulose membrane dialyzer (AM-SD), and as a result, we found that the polysulfone membrane dialyzers removed serum leptin, while the cellulose membrane dialyzer did not. We conclude that in patients on hemodialysis, the serum leptin concentration is a valuable clinical marker of the body fat content and may also contribute to the evaluation of hyperlipidemia.

Identification of new sequence variants in the leptin gene

The leptin gene (LEP) has been linked to extreme obesity. However, no common obesity-related gene variants have been found to exist in the LEP. The present study was designed to investigate the LEP for variants by screening both the putative promoter and the coding region of this gene in obese Finnish subjects (n = 200; body mass index, > 27 kg/m2). PCR-amplified DNA samples were subjected to single strand conformation analysis. A G144A substitution in codon 48 and a G328A substitution in codon 110 were identified in two obese subjects, both of whom had very low serum leptin levels. A rare silent C538T polymorphism was detected 33 bp downstream of the translation stop codon (TGA). A common polymorphism A19G was identified in the untranslated exon 1. This polymorphism was not associated with traits of obesity; in agreement, the allele frequencies were similar between 64 normal weight and 141 obese Finns. In summary, this study failed to find a common gene variant in the LEP associated with obesity, but introduces 2 rare mutations associated with very low serum leptin concentrations in 2 obese subjects.

Leptin: physiology and pathophysiology

The identification and sequencing of the ob gene and its product, leptin, in late 1994 opened new insights in the study of the mechanisms controlling body weight and led to a surge of research activity. During this time, a considerable body of knowledge regarding leptin's actions has been accumulated and the field continues to expand rapidly. Currently there is particular interest in the interaction of leptin with other peripheral and neural mechanisms to regulate body weight, reproduction and immunological response. In this review, we attempt to place the current state of knowledge about leptin in the broader perspective of physiology, including its structural characteristics, receptors, binding proteins, signalling pathways, regulation of adipose tissue expression and production, secretion patterns, clearance mechanisms and functional effects. In addition, leptin's involvement in the pathophysiology of obesity, anorexia nervosa, diabetes mellitus, polycystic ovary syndrome, acquired immunodeficiency syndrome, cancer, nephropathy, thyroid disease, Cushing's syndrome and growth hormone deficiency will be reviewed.

A rapid, quantitative functional assay for measuring leptin

At present, leptin is quantitated using immuno-assays that measure leptin mass. Leptin biological activity is determined using protocols that measure feed consumption and weight reduction. These in vivo protocols are semi-quantitative and require large quantities of leptin. We describe a rapid, sensitive and quantitative in vitro assay for leptin using HEK-293 cells stably co-transfected with the leptin receptor Ob-Rb isoform and a STAT-inducible promoter regulating the firefly luciferase cDNA. The assay, performed in a 96-well format, has an EC50 of 150 pM and is linear from 3 to 700 pM of leptin. We demonstrate that the assay is capable of measuring leptin in plasma samples. We demonstrate that bacterially-expressed, recombinant leptin and in vivo expressed leptin are equipotent. Furthermore, we demonstrate that a leptin-derived peptide, leptin fragment 22-56, previously shown to be capable of reducing feed intake following ICV injection does not act directly through the leptin receptor.

Reciprocal changes in hypothalamic receptor binding and circulating leptin in anorectic tumor-bearing rats

Although reduced biological activity of the obese gene product, leptin, has been associated with obesity, little information is available concerning leptin alterations during anorexia. Therefore, we measured circulating leptin concentrations and hypothalamic leptin binding in anorectic tumor-bearing and pair-fed control rats. Plasma concentrations of leptin decreased in tumor-bearing rats early in the course of tumor growth, and fell to nearly non-detectable levels during severe anorexia. The pair-fed control rats that ate the same amount of food as did the anorectic tumor-bearing rats exhibited a 50% decrease in plasma leptin concentration. Concentrations of free fatty acids were elevated in both tumor-bearing and pair-fed groups, while circulating levels of triglycerides were increased only in anorectic tumor-bearing rats. Leptin receptor density was doubled in the hypothalamus of tumor bearing rats, while binding affinity was decreased by 50%. These results suggest that peripheral leptin production is down-regulated, perhaps due to increased lipolysis in tumor-bearing rats. It appears that hypothalamic leptin systems up-regulate receptor numbers in response to decreased blood leptin level, however, the decrease in binding affinity may compensate for these alterations. Therefore, the influence of leptin on hypothalamic neuropeptide Y feeding systems may be minimal in anorectic tumor-bearing rats.

[Leptin--new knowledge on the pathogenesis of obesity]

Cloning of the ob-gene and characterization of its gene product leptin has led to the identification of a satiety factor, which signals the amount of peripheral fat stores to the central nervous system and regulates further feeding behaviour, thus playing a central role in the regulation of body weight. Soon after cloning of the ob-gene, a leptin-binding receptor has been identified in the central nervous system as well as in various peripheral organs. A feedback loop between peripheral fat stores and leptin receptors in the central nervous system appears to play an important role in normal body weight regulation. In contrast to human obesity, which associated with leptin resistance of uncertain etiology, the obesity syndromes associated with several animal models are now known to result from the interruption of the feedback loop at different points. Moreover, leptin may play a role in manifestation of insulin resistance and type II diabetes. Since the identification of leptin, a vast number of studies have been conducted to assess the molecular mechanisms and signal transduction pathways that are involved in the development and manifestation of obesity. From the large body of data generated to date, novel concepts of the regulation of energy balance and target strategies to control human obesity should soon be forthcoming.

Lack of associations between serum leptin, a polymorphism in the gene for the beta 3-adrenergic receptor and glucose tolerance in the Dutch population

BACKGROUND

The associations between leptin levels and the prevalence of a polymorphism in the beta 3-adrenergic receptor were studied in a cross-sectional analysis of 600 participants in a population-based study, which were stratified for glucose tolerance by an oral glucose tolerance test.

METHODS

In a random sample of 600 participants in the Rotterdam study, aged 55-75 years at baseline (309 men, 291 women) the relationships were studied between the presence of Trp64 Arg mutation in the beta 3-adrenergic receptor gene and fasting leptin, glucose and insulin (fasting and after an oral glucose load), and other components of the insulin resistance syndrome.

RESULTS

Mean age of the study population was 66.9 years (SD 5.7). Fasting serum leptin levels overall in men and women were 6.1 micrograms/l (SE 0.2) and 21.7 micrograms/l (0.9), respectively, (P < 0.001). These differences were independent of age, body mass index and waist to hip ratio. We identified 73/600 persons who were heterozygotes for the Trp64 Arg polymorphism (allelic frequency 6.1%), but failed to find an association between the presence of this polymorphism and leptin or any measured parameter indicative for obesity, impaired glucose tolerance or type 2 diabetes mellitus.

CONCLUSION

Heterozygosity for the Trp64Arg polymorphism of the beta 3-adrenergic receptor gene is not accompanied by obesity, impaired glucose tolerance and type 2 diabetes mellitus in the general elderly Dutch population, and is also not associated with changes in circulating leptin levels.

Increased disorderliness and decreased mass and daily rate of endogenous growth hormone secretion in adult Turner syndrome: the impact of body composition, maximal oxygen uptake and treatment with sex hormones

The objectives of this study were to (1) quantify pulsatility and orderliness of 24-h growth hormone (GH) secretion in adult Turner syndrome; (2) study the impact of sex steroid replacement therapy in adult Turner syndrome on these measures of GH secretion, and in addition examine the differential effect of oral vs transdermal estrogen administration on GH secretion patterns. To these ends, we used deconvolution analysis and approximate entropy (ApEn) to quantify GH release over 24 h in 21 patients with Turner syndrome before and during sex hormone substitution, compared to an age-matched control group. Deconvolution analysis revealed that the mass of GH secreted per burst and production rate was significantly lower in Turner patients compared to controls, resulting in a significantly lower integrated 24-h GH concentration. However, multiple stepwise regression revealed that lean body mass (LBM) and maximal oxygen uptake were significant discriminative variable, explaining a large part of the variation in mass secreted per burst (r = 0.72, P < 0.0005) and production rate (r = 0.73, P < 0.0005), while group (Turner or control) did not explain any of the difference. There was a significant difference in ApEn between Turner patients and controls, denoting more disorderly GH release in Turner syndrome. During administration of sex hormones, a significant increase was seen in basal secretion and GH secretory burst half-duration, as well as in integrated 24-h GH concentration. No change in ApEn was evident. We conclude that GH secretion in adult Turner syndrome is irregular, reduced in mass and production rate. The reduction in mass and production rate could be explained by differences in body composition and maximal oxygen uptake compared to relevant controls, while the irregularity of GH secretion was unexplained by the measured variables. We hypothesize that the increased irregularity could be attributable to low levels of circulating androgens or an increased biological age in the Turner patients.

Chronic administration of leptin into the lateral ventricle induces sexual maturation in severely food-restricted female rats

In many species, delayed sexual maturation occurs when metabolic conditions are not satisfactory. Recently, leptin was shown to be involved in the regulation of food intake and body mass. Furthermore, leptin administration was shown to advance sexual maturation in mice and to rescue sexual function in adverse metabolic conditions. We examined plasma leptin levels in female rats during development and evaluated the role of leptin on sexual maturation in rats subjected to food restriction. In normal rats, plasma leptin levels were low at day 24 of life, then steadily increased during the juvenile period, reaching 740+/-56 pg/ml at 40 days at time of vaginal opening (VO) and further increasing by day 60 (957+/-73 pg/ml). Food restriction initiated at day 25 strongly impaired this increase, in proportion to the severity of the restriction. With a daily food intake reduced to 7-8 g/day, that permanently prevented VO, plasma leptin levels were very low at day 53 (169+/-67 pg/ml). Following switch to ad libitum feeding, plasma leptin reached high levels within 2 days (1577+/-123 pg/ml), and VO occurred 4 days later. If the severe food restriction was maintained and a central infusion of leptin (10 microg/day) was initiated, a significant decrease in body weight compared with vehicle-infused controls was observed. In these conditions, VO occurred in eight out of the nine leptin-treated rats, representing induction of the process of sexual maturation confirmed by increases in ovarian and uterine weights. This induction of sexual maturation exclusively results from a central effect of leptin because no leak of the i.c.v. administered leptin to the general circulation was observed. These data suggest that the rising plasma levels of leptin in the prepubertal period represent a signal to the brain indicating that the young animal is metabolically ready to go through the process of sexual maturation.

The etiology of obesity: relative contribution of metabolic factors, diet, and physical activity

Three major factors modulate body weight: metabolic factors, diet, and physical activity, each influenced by genetic traits. Despite recent advances in these areas, the prevalence of obesity in Westernized societies has increased. In contrast to monogenic animal models and rare human genetic syndromes, predisposition to common forms of obesity is probably influenced by numerous susceptibility genes, accounting for variations in energy requirements, fuel utilization, muscle metabolic characteristics, and taste preferences. Although recent increases in obesity prevalence cannot be explained by changes in the gene pool, previously "silent" genetic variants may now play important permissive roles in modern societies. Available data suggest that variations in resting energy expenditure, thermic effect of food, and fuel utilization exist but, by themselves, are unlikely to explain the onset of obesity. Regarding diet, the best available trend survey data indicate that fat and energy intake have fallen, in this and other Westernized countries. Diverging trends of decreasing energy intake and increasing body weight suggest that reduced physical activity may be the most important current factor explaining the rising prevalence of obesity. Subsistence in modern societies requires extreme adaptations in previously useful energy-conserving diet and exercise behaviors. Recognizing the difficulties in sustaining energy-restricted diets in the presence of fast foods and social feasts, the current trend toward increasing body weight is not likely to be reversed solely through recommendations for further reductions in energy intake. In all likelihood, activity levels will have to increase in response to an environment engineered to be more physically demanding.

Effect of uteroplacental insufficiency upon brain neuropeptide Y and corticotropin-releasing factor gene expression and concentrations

Various hypothalamic functions such as feeding behavior, energy expenditure, body weight gain, level of anxiety, and sexual maturation are mediated by a balance between the concentrations of neuropeptide Y (NPY) and corticotropin-releasing factor (CRF). To test the hypothesis that maternal uteroplacental insufficiency alters the offspring's brain NPY and/or CRF levels, we examined the effect of maternal uterine artery ligation with intrauterine growth restriction (IUGR) (p < 0.05) upon fetal (20 d) and postnatal (4, 14, and 21 d) brain NPY and CRF synthesis, concentrations, and regional distribution. An age-related increase in NPY (0.8 kb) and CRF (1.4 kb) mRNA levels with peak amounts at the 14-d postnatal age (p < 0.05) was observed. IUGR was associated with a 75% increase in fetal brain NPY mRNA levels (p < 0.05) with no change in NPY peptide, CRF mRNA and peptide amounts. Although the increase in NPY mRNA levels persisted postnatally (p < 0.05) at d 4 and 21, CRF mRNA amounts were 2.5-fold higher only in the 4-d IUGR (p < 0.05). Paralleling the mRNA changes, an age-related increase in RIA of NPY and CRF peptide concentrations was noted (p < 0.05). IUGR caused postnatal brain NPY and CRF peptide changes similar to corresponding mRNA levels (p < 0.05), despite normal postnatal circulating glucose, insulin, corticosterone, and leptin concentrations. The age-specific intergroup differences in the NPY and CRF peptide immunoreactivity appeared predominantly in the hypothalamic region. We conclude that maternal uteroplacental insufficiency causing IUGR leads to a pretranslational imbalance in the immediate (4 d) postnatal brain NPY and CRF peptide concentrations, thereby altering the developmental pattern. This alteration in NPY and CRF peptide concentrations, despite normalization of the metabolic milieu was associated with a persistent diminution in body weight. The IUGR-associated pretranslational increase in NPY and not CRF peptide levels at d 21, may herald changes in feeding behavior during the postsuckling phase.

Serum leptin in obese women with polycystic ovary syndrome is correlated with body weight and fat distribution but not with androgen and insulin levels

Leptin is a hormone produced in the adipose tissue and its concentrations in peripheral blood are significantly correlated with the amount of body fat. Whether other factors, including the pattern of body fat distribution and several hormones (such as insulin, sex steroids, and glucocorticoids), may be involved in the regulation of circulating blood leptin levels is controversial. Women with the polycystic ovary syndrome (PCOS) are hyperandrogenic and most of them are characterized by hyperinsulinemia, insulin resistance, and obesity, particularly the visceral phenotype. To assess the potential contribution of anthropometric factors, androgens, and insulin in determining leptin levels, we examined their relationship with body-mass index (BMI), visceral (VAT) and subcutaneous (SAT) adipose tissue areas, basal androgen levels, and fasting and glucose-stimulated (AUC) insulin in different groups of obese women with PCOS (n = 23) and of age-matched obese (n = 16) and non-obese (n = 10) otherwise healthy controls. The VAT/SAT ratio was measured as a parameter of body fat distribution. Serum leptin levels were significantly higher in obese PCOS women than in obese and normal-weight healthy controls and, within the controls, in the obese than in the non-obese group. In all women considered together, and in each group separately, leptin concentrations were highly significantly correlated with BMI. In addition, after adjusting for BMI, both VAT and the VAT/SAT ratio were positively and significantly correlated with leptin. Partial correlations with the VAT/SAT ratio remained significant in both the obese PCOS group and in controls considered separately, whereas the correlation with the SAT value was significant only in the control group. After adjusting for BMI, no correlation between leptin, androgens and fasting or stimulated (like AUC) insulin was found. These findings indicate that leptin levels in obese women with PCOS are higher than those observed in obese and non-obese controls. Moreover, they suggest that, other than BMI, the pattern of body fat distribution may be an independent factor related to circulating leptin levels, which, on the contrary, do not appear to be related to either androgen or insulin concentrations.

Hormonal and neuroendocrine regulation of energy balance--the role of leptin

A new dimension to the regulation of energy balance has come from the identification of the ob (obese) gene and its protein product, leptin. Leptin is produced primarily in white adipose tissue, but synthesis also occurs in brown fat and the placenta. Several physiological functions have been described for leptin the inhibition of food intake, the stimulation/maintenance of energy expenditure, as a signal of energy reserves to the reproductive system, and as a factor in haematopoiesis. The production of leptin by white fat is influenced by a number of factors, including insulin and glucocorticoids (which are stimulatory), and fasting, cold exposure and beta-adrenoceptor agonists (which are inhibitory). A key role in the regulation of leptin production is envisaged for the sympathetic nervous system, operating through beta 3-adrenoceptors. The leptin receptor gene is expressed in a wide range of tissues, and several splice variants are evident. A long form variant (Ob-Rb) with an intracellular signalling domain is found particularly in the hypothalamus. Leptin exerts its central effects through neuropeptide Y, and through the glucagon-like peptide-1 and melanocortin systems, but it may also interact with other neuroendocrine pathways. The role and function of the leptin system in agricultural animals has not been established, but it offers a potential new target for the manipulation of body fat.

Role of peroxisome proliferator-activated receptor alpha in disease of pancreatic beta cells

Expression of peroxisome proliferator-activated receptor alpha (PPARalpha) and enzymes of fatty acid (FA) oxidation is markedly reduced in the fat-laden, dysfunctional islets of obese, prediabetic Zucker diabetic fatty (fa/fa) rats with mutated leptin receptors (OB-R). Leptin, PPARalpha/retinoid x receptor ligands, and FA all up-regulate PPARalpha and enzymes of FA oxidation and stimulate [3H]-palmitate oxidation in normal islets but not in islets from fa/fa rats. Overexpression of normal OB-R in islets of fa/fa rats corrects all of the foregoing abnormalities and reverses the diabetic phenotype. PPARalpha is a OB-R-dependent factor required for normal fat homeostasis in islet cells.

Decreased leptin levels in normal weight women with hypothalamic amenorrhea: the effects of body composition and nutritional intake

Leptin is a protein encoded by the ob gene and expressed in adipocytes. A sensitive marker of nutritional status, leptin is known to correlate with fat mass and to respond to changes in caloric intake. Leptin may also be an important mediator of reproductive function, as suggested by the effects of leptin infusions to restore ovulatory function in an animal model of starvation. We hypothesized that leptin levels are decreased in women with hypothalamic amenorrhea and that leptin may be a sensitive marker of overall nutritional status in this population. We, therefore, measured leptin levels and caloric intake in 21 women with hypothalamic amenorrhea (HA) and 30 age-, weight-, and body fat-matched eumenorrheic controls. Age (24 +/- 5 vs. 24 +/- 3 yr), body mass index (20.6 +/- 1.3 vs. 21.1 +/- 1.5 kg/m2), percent ideal body weight (94.9 +/- 5% vs. 96.3 +/- 6.3%), and fat mass (14.2 +/- 3.6 vs. 15.5 +/- 2.9 kg, determined by dual energy x-ray absortiometry) did not differ between the groups. Leptin levels were significantly lower in the HA subjects compared with those in the controls (7.1 +/- 3.0 vs. 10.6 +/- 4.9 micrograms/L; P = 0.005). Total caloric intake (1768 +/- 335 vs. 2215 +/- 571 cal/day; P = 0.003), fat intake (333 +/- 144 vs. 639 +/- 261 cal/day; P < 0.0001), and insulin levels (5.6 +/- 1.2 vs. 7.4 +/- 3.2 microU/mL; P = 0.015) were lower in the women with HA than in the eumenorrheic controls. The difference in leptin levels remained significant after controlling for insulin (P = 0.023). These data are the first to demonstrate hypoleptinemia, independent of fat mass, in women with HA. The hypoleptinemia may reflect inadequate calorie intake, fat intake, and/or other subclinical nutritional disturbances in women with HA. The mechanism and reproductive consequences of low leptin in this large population of women remain unknown.

Expression of obese mRNA in genetically lean and fat selection lines of sheep

Genetically separate lines of Coopworth sheep have been bred by selecting for (fat genotype) or against (lean genotype) backfat depth. Typically, the total fat content, adjusted for carcass weight, is 21.2 and 29.3% for the lean and fat lines, respectively. As a homologue of the obese gene, which shows altered expression in several forms of obesity, is also expressed in sheep, it was decided to determine whether the obese gene was differentially expressed in each line of sheep. The relative level of expression of obese mRNA was approximately twofold higher in the fat line compared with the lean line in back, omental and perirenal fat depots of ram lambs fed ad libitum or fasted for 48 h. This elevation in the fat line is most likely a secondary consequence of obesity rather than a cause. Fasting for 48 h decreased obese mRNA levels by 8.9-, 8.5-, and 4.2-fold in back, omental and perirenal fat, respectively, in the lean line, and by 8.3-, 5.7-, and 3.5-fold in back, omental and perirenal fat, respectively, in the fat line. The lean and fat lines of sheep, therefore, responded in a similar way to fasting.

Chronic administration of OB protein decreases food intake by selectively reducing meal size in male rats

The potent hypophagic effect of OB protein (OB) is well established, but the mechanism of this effect is largely unknown. We investigated the effects of chronic administration of a novel modified recombinant human OB (Mod-OB) with a prolonged half-life (>48 h) on ad libitum food intake, spontaneous meal patterns, and body weight in 24 adult, male Sprague-Dawley rats (body weight at study onset: 292 g). Single daily subcutaneous injections of Mod-OB (4 mg/kg daily) for 8 consecutive days significantly reduced ad libitum food intake compared with vehicle injections from injection day 3 through postinjection day 3. Mod-OB-injected rats ate between 4.5 and 7.1 g (or 13-20%) per day less than controls, with the reduction primarily occurring during the dark period. Body weight gain was significantly decreased in response to Mod-OB from injection day 8 until postinjection day 4, with a maximum difference of 24 g on postinjection day 3. The reduction of food intake by Mod-OB was mainly due to a 21-34% decrease in nocturnal spontaneous meal size. There was no significant effect of Mod-OB on nocturnal meal frequency or duration. Mod-OB also did not reliably affect the size, duration, or frequency of diurnal meals. Mod-OB-injected rats displayed no compensatory hyperphagia after the injection period. These results indicate that chronically administered OB selectively affects the mechanisms controlling meal size in male rats.

Interaction of free fatty acids with human leptin

Relatively high concentrations of leptin are present in plasma and it is thought to play a major role in lipid homeostasis. Leptin is reported to lower tissue triglyceride content by increasing intracellular oxidation of free fatty acids (FFA). However very little is known regarding the interaction between leptin and plasma FFA. We studied the interaction of FFA with leptin using a direct radiolabelled fatty acid binding assay, a fluorescence assay, electrophoretic mobility and autoradiobinding. All these data indicate that binding of FFA with leptin is reversible and shows a positive co-operativity. The binding of FFA to leptin produces a change in the pI value of the leptin and also increased the electrophoretic mobility of the protein in native polyacrylamide gels. The change in leptin's electrophoretic mobility depends on the chain length and the number of double bonds of the fatty acid, as stearic acid, 18:0, had no effect whereas oleic acid, 18:1n-9, linoleic acid, 18:2n-6, arachidonic acid, 20:4n-6, and docosahexaneoic acid, 22:6n-3, affected leptin's mobility to different degrees. The physiological implication of leptin-FFA interaction is not known, however the interaction may depend on the plasma FFA composition and concentration which are known to vary in different pathological/physiological conditions.

Leptin produces anorexia and weight loss without inducing an acute phase response or protein wasting

The ob gene product leptin is known to produce anorexia and loss of body fat when chronically administered to both lean and genetically obese mice. The current study was undertaken to examine whether administration of recombinant leptin in quantities sufficient to produce decreases in food intake and body weight and alterations in body composition would elicit either an hepatic acute phase protein response or preferential loss of carcass lean tissue. Mice were administered increasing quantities of recombinant human leptin or human tumor necrosis factor-alpha as a positive control. Although leptin (at 10 mg/kg body wt) produced significant anorexia and weight loss (both P < 0.05), human leptin administration did not appear to induce an hepatic acute phase protein response in either lean or genetically obese mice, as determined by protein synthetic rates in the liver or changes in the plasma concentration of the murine acute phase protein reactants, amyloid A, amyloid P, or seromucoid (alpha1-acid glycoprotein). In addition, human leptin administration did not induce a loss of fat-free dry mass (protein) in lean or obese animals. The findings suggest that at doses adequate to alter food intake and body weight leptin is not a significant inducer of the hepatic acute phase response nor does leptin promote the preferential loss of somatic protein characteristic of a chronic inflammatory process.

Zinc may regulate serum leptin concentrations in humans

OBJECTIVE

Leptin, the product of the ob gene, plays a key role in a feedback loop that maintains energy balance by signaling the state of energy stores to the brain and by influencing the regulation of appetite and energy metabolism. Zinc also plays an important role in appetite regulation. Thus, we evaluated the relationship between zinc status and the leptin system in humans.

METHODS

We studied nine healthy men with marginal zinc deficiency, induced by dietary means, before and after zinc supplementation.

RESULTS

Zinc restriction decreased leptin levels while zinc supplementation of zinc-depleted subjects increased circulating leptin levels. In addition, zinc supplementation increased IL-2 and TNF-alpha production that could be responsible for the observed increase in leptin concentrations.

CONCLUSIONS

Zinc may influence serum leptin levels, possibly by increasing the production of IL-2 and TNF-alpha.

The Ob protein (leptin) and the kidney

Mutation of the Ob gene, which encodes for leptin, or mutation of the leptin receptor leads to obesity in mice. Humans, for the most part, have a positive correlation of leptin with body fat mass suggesting possible defects in leptin effector mechanisms that may contribute to obesity. As patients on hemodialysis have difficulty with appetite, we investigated whether leptin is cleared by the kidney and is elevated in hemodialysis patients. In patients with intact renal function there was a net renal uptake of 12% of circulating leptin, whereas in patients with renal insufficiency there was no renal uptake of leptin. In a separate cohort of 36 patients with end-stage renal disease (ESRD), peripheral leptin levels factored for body mass index was increased by fourfold as compared to a group of healthy controls (N = 338). The leptin receptor exists in a long and short form, with the long form primarily expressed in the hypothalamus but also in the lungs and kidneys of the mouse. Further studies are necessary to clarify the role of leptin in regulating appetite in patients with ESRD and the role of leptin in directly affecting kidney function via its receptors.

The effect of exercise on leptin concentration in healthy men and in type 1 diabetic patients

PURPOSE

Leptin is a recently discovered hormone that appears as a regulator of energy balance. It is important to know whether leptin concentrations are changed under conditions of altered energy homeostasis. Consequently, we examined the effects of exercise with fasting and exercise with feeding on circulating leptin concentrations in healthy men and in type 1 diabetic patients with normal body weight and well controlled diabetes.

METHODS

Leptin concentrations were determined with radioimmunoassay.

RESULTS

During a 3-h cycle ergometer exercise with fasting, leptin decreased by 42% (P < 0.01) in nine healthy men and by 23% (P = 0.05) in eight male type 1 diabetic patients. Leptin fell equally by 12% (P < 0.03) both in nine healthy men and in eight male type 1 diabetic patients who were studied as a resting control group. The absolute fall in leptin in healthy men was similar in the exercise and resting control groups (0.8 +/- 0.1 microgram.L-1 vs 0.8 +/- 0.2 microgram.L-1). However, due to lower leptin concentration before the exercise, the relative decrease (42%) was greater than during the resting control study (12%, P < 0.005). This difference was not seen in the diabetic patients. Fasting leptin concentration correlated positively with BMI (r = 0.75, P < 0.001) and fasting insulin (r = 0.71, P < 0.01) in healthy men as well as with insulin level (r = 0.54, p < 0.05) in type 1 diabetic patients. When exercise was performed with feeding, and this was associated with a significant rise in serum cortisol level (marathon run, 14 healthy men and 7 type 1 diabetic patients), leptin concentration did not change significantly.

CONCLUSIONS

1) During morning hours, leptin decreases both in healthy men and in type 1 diabetic patients, reflecting a diurnal variation of leptin concentration and the effect of fasting on leptin concentration. 2) The fall in leptin during morning hours is augmented by physical exercise in healthy men. 3) If exercise is performed with feeding and associated with a rise in serum cortisol level, leptin concentration remains unchanged. These data suggest that although exercise may reduce circulating leptin levels, the effect is small and can be counterbalanced by feeding or a rise in serum cortisol concentration.

Diurnal variation of the serum leptin concentration in patients with anorexia nervosa

OBJECTIVE

In rodents, leptin is involved in regulating eating behaviour, fat storage, and reproductive function. In humans, the serum leptin concentration in obese and normal weight subjects correlates with body mass index, reflecting the body fat store. The serum leptin exhibit diurnal variation, however, this has been reported to be absent in normal weighted amenorrheic athletes. Anorexia nervosa is associated with multiple endocrine abnormalities. Hypothalamic amenorrhoea often precedes the weight loss and may persist after weight recovery. We hypothesized that leptin could be involved in the regulation of eating behaviour and gonadal function in anorexia nervosa.

DESIGN

We measured the concentration of leptin in serum samples taken after an overnight fast in 18 female anorexia nervosa patients and 11 controls. To study diurnal variation, eight patients and 11 controls were hospitalized for 24 h and had a standardized diet at regular times. Seven blood samples were obtained at 4 h intervals from each subject.

PATIENTS

The patients fulfilled the DSM-IV criteria for anorexia nervosa. The mean body mass index for the patients was 14.2 +/- 2.3 kg/m2 and for controls 20.3 +/- 1.7 kg/m2.

RESULTS

The mean fasting leptin concentration as well as the 24 h mean concentration were significantly lower in the anorectic group than in the control group (2.5 +/- 0.9 vs 10.1 +/- 6.1 micrograms/l, P < 0.01 and 2.7 +/- 1.5 vs 10.6 +/- 7.1 micrograms/l, P < 0.01 respectively). In the whole group of subjects (n = 28) a significant positive correlation between the leptin level and body mass index was found (r = 0.63, P < 0.001). In the anorectic group it was found that the leptin level correlated better with body fat percentage than with body mass index. In normalized data the time course of the mean leptin levels showed a monophasic variation with nadir and zenith at about 0900 and 0100 h respectively. However, the individual coefficients of variance were significantly lower in the anorectic group compared to the group of healthy women.

CONCLUSION

In patients with anorexia nervosa the leptin level is low, reflecting the low body fat mass, and the relative diurnal variation is strikingly reduced. The similarity to that of normal weighted women with hypothalamic amenorrhoea suggest that altered leptin oscillations may be of particular significance in the hypothalamic regulation of reproductive function.

Plasma leptin in relation to regional body fat in older New Zealand women

BACKGROUND

Leptin is the protein product of the obesity (ob) gene and is produced in adipose tissue. Plasma leptin values are highly correlated with total body fat mass but less is known concerning relationships of leptin with regional adiposity.

AIM

To investigate associations between leptin and dual energy X-ray absorptiometry (DEXA) measures of regional body fat deposition in older New Zealand women.

METHODS

Body composition was measured in 80 women aged 40 to 79 years (20 in each decade) using DEXA. Height and weight were measured by conventional anthropometry. Plasma leptin concentrations were assayed by radioimmunoassay.

RESULTS

Leptin was strongly correlated (p < 0.001) with weight (r = 0.751), body mass index (r = 0.782), total fat mass (r = 0.854) and % fat mass (r = 0.813). A statistical model with trunk fat (kg) and leg fat (kg) as the independent variables showed that trunk fat accounted for 72.1% of the variance in plasma leptin with leg fat explaining only a further 1.6% of the variance. No relationships were found between leptin and age, height, lean tissue mass or menopausal status (p > 0.05).

CONCLUSIONS

We conclude that trunk fat explains more of the variance in circulating leptin concentrations than leg fat, suggesting that the propensity to leptin resistance may be increased in women with higher central adiposity.

Genetic loci controlling body fat, lipoprotein metabolism, and insulin levels in a multifactorial mouse model

We analyzed the inheritance of body fat, leptin levels, plasma lipoprotein levels, insulin levels, and related traits in an intercross between inbred mouse strains CAST/Ei and C57BL/6J. CAST/Ei mice are unusually lean, with only approximately 8% of body weight as fat, whereas C57BL/6J mice have approximately 18% body fat. Quantitative trait locus analysis using > 200 F2 mice revealed highly significant loci (lod scores > 4.3) on chromosomes 2 (three separate loci) and 9 that contribute to mouse fat-pad mass for mice on a high-fat diet. Some loci also influenced plasma lipoprotein levels and insulin levels either on chow or high-fat diets. Two loci for body fat and lipoprotein levels (on central and distal chromosome 2) coincided with a locus having strong effects on hepatic lipase activity, an activity associated with visceral obesity and lipoprotein levels in humans. A locus contributing to plasma leptin levels (lod score 5.3) but not obesity was identified on chromosome 4, near the leptin receptor gene. These data identify candidate regions and candidate genes for studies of human obesity and diabetes, and suggest obesity is highly complex in terms of the number of genetic factors involved. Finally, they support the existence of specific genetic interactions between body fat, insulin metabolism, and lipoprotein metabolism.

Strategies and potential molecular targets for obesity treatment

Obesity is an increasingly prevalent and important health problem. Although treatment is available, the long-term maintenance of medically significant weight loss (5 to 10 percent of initial body weight) is rare. Since 1995 there has been an explosion of research focused on the regulation of energy balance and fat mass. Characterization of obesity-associated gene products has revealed new biochemical pathways and molecular targets for pharmacological intervention that will likely lead to new treatments. Ideally, these treatments will be viewed as adjuncts to behavioral and lifestyle changes aimed at maintenance of weight loss and improved health.

Enhancing leptin response by preventing SH2-containing phosphatase 2 interaction with Ob receptor

Leptin is an adipocyte-derived cytokine that regulates food intake and body weight via interaction with its Ob receptor (ObR). Serum leptin levels are chronically elevated in obese humans, suggesting that obesity may be associated with leptin resistance and the inability to generate an adequate ObR response. Evidence suggests that transcriptional activation of target genes by STAT3 (signal transducer and activator of transcription) in the hypothalamus is a critical pathway that mediates leptin's action. Herein we report that activation of ObR induces the tyrosine phosphorylation of the tyrosine phosphatase SH2-containing phosphatase 2 (SHP-2) and demonstrate that Tyr986 within the ObR cytoplasmic domain is essential to mediate phosphorylation of SHP-2 and binding of SHP-2 to ObR. Surprisingly, mutation of Tyr986 to Phe, which abrogates SHP-2 phosphorylation and binding to the receptor, dramatically increases gene induction mediated by STAT3. Our findings indicate that SHP-2 is a negative regulator of STAT3-mediated gene induction after activation of ObR and raise the possibility that blocking the interaction of SHP-2 with ObR could overcome leptin resistance by boosting leptin's weight-reducing effects in obese individuals.

A biphasic developmental pattern of circulating leptin in the male rhesus macaque (Macaca mulatta)

To help elucidate the physiological role of leptin during somatic and sexual maturation, circulating concentrations of leptin were measured in 36 male rhesus monkeys of ages ranging from 0-20 yr. The body weight of these animals showed a steady increase of approximately 1 kg/yr during the first decade of life and reached a plateau at approximately 13 yr. In contrast, serum leptin concentrations showed a biphasic developmental pattern, which was highlighted by a strong negative correlation with body weight (r = -0.74, P < 0.001) before the onset of puberty (at approximately 3.5 yr) and by a strong positive correlation afterward (r = 0.77, P < 0.001). Overall, the developmental changes in serum leptin concentrations closely mimicked the expected developmental changes in serum testosterone concentrations (r = 0.62, P < 0.001), which were highly elevated at birth, fell to basal levels during the juvenile phase of development, and gradually rose again after the initiation of puberty. However, mean serum leptin concentrations during the peripubertal period itself (3-5 yr) were significantly lower (P < 0.01) than those observed during the first year of life or those observed in fully mature adults (i.e. > 7 yr) (3.5 +/- 0.3, 1.4 +/- 0.2, and 3.3 +/- 0.6 ng/ml, respectively). These data demonstrate that the role of leptin in energy homeostasis of primates is more than a simple linear relationship, being highly dependent upon the developmental age. Furthermore, the data do not support the hypothesis that leptin plays a major role in triggering the onset of puberty in primates, although the strong correlation between serum concentrations of leptin and testosterone suggests that the secretion of these two hormones may be causally linked.