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

Circulating insulin concentrations, smoking, and alcohol intake are important independent predictors of leptin in young healthy men

OBJECTIVE

Leptin, an adipocyte-secreted hormone, has been shown to signal the status of energy stores to the brain, regulate energy homeostasis, and mediate the neuroendocrine response to food deprivation. Obesity is associated with increased leptin levels, and several hormones, including insulin and glucocorticoids, have been associated with leptin levels and expression in rodents. Although obesity has been strongly associated with increased leptin in humans, a significant percentage of leptin's variability remains unexplained. The role of endogenous hormones, demographic factors, or certain life-style factors in explaining the residual variability of leptin levels has not yet been clarified. We performed this cross-sectional study to document the relative importance of obesity, lifestyle factor, and endogenous hormones in determining serum leptin levels.

RESEARCH METHODS AND PROCEDURES

We measured serum concentrations of insulin, cortisol, testosterone, growth hormone, and dehydroepiandrosterone sulfate; ascertained anthropometric, demographic, and lifestyle characteristics; and studied these variables in relationship to serum leptin concentrations in a sample of young healthy men.

RESULTS

Obesity and alcohol intake were independently and positively associated with circulating leptin concentrations. Additionally, cigarette smoking was negatively and independently associated with leptin concentrations. Finally, serum insulin concentration was an independent hormonal determinant of circulating leptin concentrations, whereas serum testosterone was negatively associated with leptin only by bivariate analysis.

DISCUSSION

We conclude that, in addition to obesity, cigarette smoking, alcohol intake, and serum insulin levels are associated with leptin levels in a population of healthy young men.

Marked and rapid decreases of circulating leptin in streptozotocin diabetic rats: reversal by insulin

Evidence for regulation of circulating leptin by insulin is conflicting. Diabetes was induced in rats with streptozotocin (STZ; 40 mg.kg(-1).day(-1) x 2 days) to examine the effect of insulin-deficient diabetes and insulin treatment on circulating leptin. After 12 wk, plasma leptin concentrations in untreated rats were all < 0.4 ng/ml versus 4.9 +/- 0.9 ng/ml in control animals (P < 0.005). In rats treated with subcutaneous insulin implants for 12 wk, which reduced hyperglycemia by approximately 50%, plasma leptin was 2.1 +/- 0.6 ng/ml, whereas leptin concentrations were 6.0 +/- 1.6 ng/ml in insulin-implanted rats receiving supplemental injections of insulin for 4 days to normalize plasma glucose (P < 0.005 vs. STZ untreated). In a second experiment, plasma leptin was monitored at biweekly intervals during 12 wk of diabetes. In rats treated with insulin implants, plasma leptin concentrations were inversely proportional to glycemia (r = -0.64; P < 0.0001) and unrelated to body weight (P = 0.40). In a third experiment, plasma leptin concentrations were examined very early after the induction of diabetes. Within 24 h after STZ injection, plasma insulin decreased from 480 +/- 30 to 130 +/- 10 pM (P < 0.0001), plasma glucose increased from 7.0 +/- 0.2 to 24.8 +/- 0.5 mM, and plasma leptin decreased from 3.2 +/- 0.2 to 1.2 +/- 0.1 ng/ml (delta = -63 +/- 3%, P < 0.0001). In a subset of diabetic rats treated with insulin for 2 days, glucose decreased to 11.7 +/- 3.9 mM and leptin increased from 0.5 +/- 0.1 to 2.9 +/- 0.6 ng/ml (P < 0.01) without an effect on epididymal fat weight. The change of leptin was correlated with the degree of glucose lowering (r = 0.75, P < 0.05). Thus insulin-deficient diabetes produces rapid and sustained decreases of leptin that are not solely dependent on weight loss, whereas insulin treatment reverses the hypoleptinemia. We hypothesize that decreased glucose transport into adipose tissue may contribute to decreased leptin production in insulin-deficient diabetes.

Leptin depolarizes rat hypothalamic paraventricular nucleus neurons

Leptin, the protein product of the ob/ob gene, is thought to have a central site of action, presumably within the hypothalamus, through which it regulates feeding behavior. THe paraventricular nucleus (PVN) is one structure that has been implicated in regulating feeding behavior. Using patch-clamp recording techniques, this study examines the direct membrane effects of leptin on neurons in a coronal PVN slice. Bath application of the physiologically active leptin fragment (amino acids 22-56) elicited dose-related depolarizations in 82% of the type I cells tested (n = 17) and 67% of the type II cells tested (n = 9). By contrast, the physiologically inactive leptin fragment (amino acids 57-92) had no discernible effect on membrane potential (n = 7). The effects of this peptide were unaffected following synaptic isolation of the cells by bath application of the sodium channel blocker tetrodotoxin (n = 5). Voltage clamp recordings in six cells demonstrated that leptin increased a nonspecific cation conductance with a reversal potential near -30 mV. These findings suggest that neurons in PVN may play an important role in the central neuronal circuitry involved in the physiological response to leptin.

Prader-Willi syndrome: relationship of adiposity to plasma leptin levels

OBJECTIVE

Prader-Willi syndrome (PWS) is an autosomal dominant disorder involving the proximal long arm of chromosome 15, in which obesity is common. However, there is limited information on the underlying physiological mechanisms promoting obesity in this population. We tested whether there was a significant positive association between leptin and total body fat (TBF) in subjects with PWS, and whether this association was stronger among subjects with than without PWS.

RESEARCH METHODS AND PROCEDURES

We studied 21 PWS patients and 64 non-PWS controls on whom we measured serum leptin, total body fat, glucose, insulin, and resting energy expenditure. We tested whether the slope of the regression line between leptin and TBF (in kg), measured by dual energy X-ray absorptiometry, was the same for PWS patients and non-PWS controls.

RESULTS

Regression analyses indicated that the leptin-TBF association was significantly stronger among PWS patients. In contrast, the slope of the leptin-body mass index association did not significantly differ between PWS patients and non-PWS controls. None of the other outcome variables showed associations with leptin.

DISCUSSION

Results suggest that the role of leptin in promoting obesity may be greater among subjects with PWS than among non-PWS controls.

Mapping of serum amylase-1 and quantitative trait loci for milk production traits to cattle chromosome 4

The present study was undertaken to confirm and refine the mapping of a quantitative trait locus in cattle for milk fat percentage that had earlier been reported to be linked to the serum amylase-1 locus, AM1. Five half-sib families from the previous study and 7 new ones were genotyped for nine microsatellite markers spanning chromosome 4. AM1 was mapped between the microsatellite markers BMS648 and BR6303. In a granddaughter design, interval mapping based on multiple-marker regression was utilized for an analysis of five milk production traits: milk yield, fat percentage and yield, and protein percentage and yield. In the families reported on previously, significant effects for fat and protein percentages were detected. In the new families, an effect on milk and fat yields was found. The most likely positions of the quantitative trait locus in both groups of families were in the same area of chromosome 4 in the vicinity of the obese locus. Direct effects of the obese locus were tested for using polymorphism in two closely linked microsatellites located 2.5 and 3.6 top downstream of the coding sequence. No firm evidence was found for an association between the obese locus and the tested traits.

Serum leptin and IGF-I levels in cystic fibrosis

The role of leptin in states of negative energy balance such as cystic fibrosis (CF) has not been explored. We hypothesized that leptin levels in patients with CF would be low due to correlation with body weight. Despite the importance of IGF-I in normal growth and anabolism, there are few data on IGF-I in CF. We studied 27 CF patients (25+/-5 yrs, 57+/-9 kg, 10M/17F) and 12 control subjects (25+/-4 yrs, 57+/-9 kg, 6M/6F). Each subject underwent analysis of lean body mass (LBM) and percent body fat by dual-energy x-ray absorptiometry (DEXA). Serum leptin and IGF-I levels were measured by radioimmunoassay. Serum leptin levels were similar both in CF and in controls (CF=5.3+/-4.1 ng/ml, C=4.4+/-3.6ng/ml; p=0.3), and there was no difference in percent body fat between the two groups (CF=26+/-13%, C=21+/-7%; p=0.3). Leptin levels were significantly lower in CF males than females corresponding to lower fat levels in males in both CF and controls. Leptin levels were positively correlated with percent body fat both in CF and controls (CF: r=0.8; p=0.01, CONTROL: r=0.8; p =0.2). Serum IGF-I levels were significantly lower in CF patients than in controls (CF=1.13+/-0.41 ng/ml, C=6.72+/-3.62 ng/ml; p=<0.01). We conclude that the physiological regulation of leptin is maintained in relation to body fat even in chronic illness and that the negative energy balance in CF is not caused by high leptin levels.

Serum leptin in children and young adults with inflammatory bowel disease

BACKGROUND

Pediatric inflammatory bowel disease is often associated with growth failure and inadequate energy intake. Although several circulating cytokines are known to be elevated in inflammatory bowel disease, the mechanism for the related anorexia has not been described. Leptin is a newly recognized circulating protein that is an important regulator of appetite and energy metabolism; leptin levels are elevated in several animal models of inflammation. This study was conducted to determine whether serum leptin levels are elevated in young patients with inflammatory bowel disease.

METHODS

One hundred twelve children and young adults with Crohn's disease or ulcerative colitis were studied prospectively. Forty-two patients with other gastrointestinal illnesses were used as control subjects. Height, weight, erythrocyte sedimentation rate, serum albumin concentration, and clinical information were collected prospectively, and leptin was measured by radioimmunoassay of stored serum.

RESULTS

No significant differences in leptin levels were found among disease groups or control subjects. Body mass index and gender were the only independent predictors of serum leptin in all groups examined. Disease activity varied inversely with serum leptin in patients with Crohn's disease, but these differences were explained entirely by variations in body mass index.

CONCLUSIONS

The determinants of serum leptin were the same in young patients with inflammatory bowel disease as in normal populations, indicating that alterations in leptin levels are unlikely to mediate the anorexia and growth failure associated with this disease.

Longitudinal changes in maternal serum leptin concentrations, body composition, and resting metabolic rate in pregnancy

OBJECTIVE

We sought to evaluate the longitudinal changes in maternal serum leptin concentrations, body composition, and resting metabolic rate during pregnancy.

STUDY DESIGN

Ten women were evaluated before pregnancy, in early pregnancy (12 to 14 weeks), and in late pregnancy (34 to 36 weeks). Leptin concentrations were measured by radioimmunoassay, body composition with hydrodensitometry with adjustment for total body water, and resting metabolic rate by use of indirect calorimetry.

RESULTS

Using analysis of variance with repeated measures from pregravid to late pregnancy, a 66% increase (mean +/- SD) was found in leptin concentrations (in nanograms per milliliter) (before pregnancy, 25.4 +/- 19.9; in early pregnancy, 37.5 +/- 26.2; and in late pregnancy, 38.4 +/- 27.3, p = 0.003); a 9% increase in body fat (in kilograms) (before pregnancy, 29.4 +/- 15.7; in early pregnancy, 28.7 +/- 14.0; in late pregnancy, 31.4 +/- 14.6; p = 0.04); a 28% increase in oxygen consumption (in milliliters of oxygen per minute) (before pregnancy, 221.2 +/- 29.5; in early pregnancy, 230.4 +/- 42.9; in late pregnancy, 285.3 +/- 51.9; p < 0.0001); and a 9% increase in oxygen consumption (milliliters of oxygen per kilogram per minute) (before pregnancy, 3.02 +/- 0.43; in early pregnancy, 3.05 +/- 0.30; in late pregnancy, 3.31 +/- 0.37, p = 0.002) with advancing gestation. A significant positive correlation was present between leptin and body fat before pregnancy (r = 0.90, p < 0.0001), in early pregnancy (r = 0.91, p < 0.0001), and in late pregnancy (r = 0.87, p = 0.0005) and between leptin and oxygen consumption before pregnancy (r = 0.80, p = 0.004), in early pregnancy (r = 0.92, p < 0.0001), and in late pregnancy (r = 0.62, p = 0.06). When oxygen consumption was adjusted for maternal and fetal tissue mass, a significant negative correlation was found between leptin and oxygen consumption before pregnancy (r = -0.96, p < 0.0001), in early pregnancy (r = -0.80, p = 0.0034), and in late pregnancy (r = -0.70, p = 0.02).

CONCLUSION

We conclude that leptin increases significantly during early pregnancy before any major changes in body fat and resting metabolic rate. These data suggest that pregnancy represents a leptin-resistant state.

Clinical aspects of leptin

Hyperleptinemia is an essential feature of human obesity. Total body fat mass > % body fat > BMI are the best predictors of circulating leptin levels. Although ob gene is differentially expressed in different fat compartments, apart from total body fat, upper or lower body adiposity or visceral fat does not influence basal leptin levels. Similarly, age, basal glucose levels, and ethnicity do not influence circulating leptin levels. Only in insulin-sensitive individuals do basal levels of insulin and leptin correlate positively even after factoring in body fat. Diabetes does not influence leptin secretion in both lean and obese subjects per se. Independent of adiposity, leptin levels are higher in women than in men. This sexual dimorphism is also present in adolescent children. In eating disorders anorexia nervosa and bulimea nervosa, leptin levels are not upregulated but simply reflect BMI and probably body fat. In spite of strong correlation between body fat and leptin levels, there is great heterogeneity in leptin levels at any given index of body fat. About 5% of obese populations can be regarded as "relatively" leptin deficient which could benefit from leptin therapy. Leptin has dual regulation in human physiology. During the periods of weight maintenance, when energy intake and energy output are equal, leptin levels reflect total bodyfat mass. However, in conditions of negative (weight-loss programs) and positive (weight-gain programs) energy balances, the changes in leptin levels function as a sensor of energy imbalance. This latter phenomenon is best illustrated by short-term fasting and overfeeding experiments. Within 24 h of fasting leptin levels decline to approximately 30% of initial basal values. Massive overfeeding over a 12-h period increases leptin levels by approximately 50% of initial basal values. Meal ingestion does not acutely regulate serum leptin levels. A few studies have shown a modest increase in leptin secretion at supraphysiological insulin concentrations 4-6 h following insulin infusion. Under in vitro conditions, insulin stimulates leptin production only after four days in primary cultures of human adipocytes, which is apparently due to its trophic effects and an increased fat-cell size. Similar to other hormones, leptin secretion shows circadian rhythm and oscillatory pattern. The nocturnal rise of leptin secretion is entrained to mealtime probably due to cumulative hyperinsulinemia of the entire day. Like other growth factors and cytokines, leptin binding proteins including soluble leptin receptor are present in human serum. In lean subjects, the majority of leptin circulates in the bound form whereas in obese subjects, the majority of leptin is present in the free form. When free-leptin levels are compared between lean and obese subjects, even more pronounced hyperleptinemia in obesity is observed than that reported by measuring total leptin levels. During short-term fasting, free-leptin levels in lean subjects decrease in much greater proportion than those in obese subjects. In lean subjects with a relatively small energy store and particularly during food deprivation, leptin circulating predominantly in the bound form could be the mechanism to restrict its availability to hypothalamic leptin receptors for inhibiting leptin's effect on food intake and/or energy metabolism. Unlike marked changes in serum leptin, CSF leptin is only modestly increased in obese subjects and the CSF leptin/serum leptin ratio decreases logarithmically with increasing BMI. If CSF leptin levels are any indication of brain interstitial fluid levels, then hypothalami of obese subjects are not exposed to abnormally elevated leptin concentrations. In the presence of normal leptin receptor (functional long form, i.e., OB-Rb) mRNA expression and in the absence of leptin receptor gene mutations, it is logical to assume defective leptin signaling and/or impaired affector system(s) are the likely causes of leptin resistance in

Regional localization of specific [125I]leptin binding sites in rat forebrain

Specific [125I]leptin receptor binding sites have been identified in choroid plexus (CP), but have eluded regional localization within the brain parenchyma. To optimize specific [125I]leptin binding in brain loci, we ran experiments varying the pH of incubation buffers. We found that specific [125I]leptin binding in CP was strikingly pH dependent with the most acidic buffer, pH 5.5, resulting in a greater than 100% increase over the amount of specific binding measured at pH 7.5. While low pH permitted detection of specific binding in parenchymal loci, clear pH dependency was only observed in the CP. In the caudate putamen (CauP), a locus with low specific binding, values for specific binding did not differ significantly across the range of pH conditions tested. Using incubation buffers at pH 6.0 in subsequent binding experiments, we localized specific [125I]leptin binding in several brain loci including thalamus and hypothalamus. In CP and thalamus, where the range of OD permitted analysis of binding parameters, [125I]leptin binding was saturable with increasing concentrations of unlabelled leptin. In all loci, specific [125I]leptin binding was insensitive to competition by high concentrations of other unlabelled compounds. Our results varying pH conditions of the incubation buffer suggest leptin receptors may be divided into subclassifications based on pH sensitivity of the specific binding. Furthermore, our results suggest that although densities are low, high affinity leptin receptors are present in neural loci implicated in food intake and energy balance, and are more widespread in the forebrain than previously determined.

Leptin: a molecule integrating somatic energy stores, energy expenditure and fertility

The signaling of fat mass to central nervous system (CNS) regulators of food intake, energy expenditure and fertility has been inferred by experimental physiologists for over 75 years. The ability to modify such phenotypes based upon the status of body energy stores (fat) has critical survival value and, therefore, has been the object of potent selection pressure in evolution. The recent molecular cloning of the mouse ob mutation and the subsequent elucidation of the fundamentals of its regulatory physiology has identified a protein secreted by adipocytes, leptin, as a plausible candidate for a humoral signal with the requisite endocrinology and neurobiology.

Hypertension: genes and environment

Hypertension can be classified as either Mendelian hypertension or essential hypertension, on the basis of the mode of inheritance. The Mendelian forms of hypertension develop as a result of a single gene defect, and as such are inherited in a simple Mendelian manner. In contrast, essential hypertension occurs as a consequence of a complex interplay of a number of genetic alterations and environmental factors, and therefore does not follow a clear pattern of inheritance, but exhibits familial aggregation of cases. In this review, we discuss recent advances in understanding the pathogenesis of both types of hypertension. We review the causal gene defects identified in several monogenic forms of hypertension, and we discuss their possible relevance to the development of essential hypertension. We describe the current approaches to identifying the genetic determinants of human essential hypertension and rat genetic models of hypertension, and summarise the results obtained to date using these methods. Finally, we discuss the significance of environmental factors, such as stress and diet, in the pathogenesis of hypertension, and we describe their interactions with specific hypertension susceptibility genes.

Obese gene expression in porcine adipose tissue is reduced by food deprivation but not by maintenance or submaintenance intake

The relationship between obese gene expression and energy intake was determined in pigs of various body weights. With ad libitum consumption, expression increased (P < 0.001) with body weight from 55 to 163 kg. Obese mRNA relative abundance was correlated with fat mass (r = 0.74, P < 0.0001) and percentage of fat (r = 0.72, P < 0. 0001). Obese expression was also evaluated at 159 kg (initial weight) and ad libitum, maintenance or 23% of maintenance intake for 28 d. Obese mRNA was independent of treatment (P > 0.78) despite considerable weight differences. Obese mRNA abundance was then compared at 136 kg (initial weight) and ad libitum or maintenance intake for 3 or 28 d. Abundance was not influenced by either duration of treatment or intake, despite a small increase (P < 0.01) in serum nonesterified fatty acids (NEFA) and a reduction (P < 0.02) in insulin attributable to maintenance intake. Finally, mRNA abundance was determined at 60 and 136 kg and conditions of food deprivation or ad libitum intake for 3 d. Food deprivation reduced (P < 0.01) serum insulin and increased (4- to 5-fold) NEFA concentrations. Obese mRNA abundance was greater (P < 0.01) in the heavier pigs and was reduced (P < 0.01) by food deprivation. We conclude that obese mRNA abundance in pigs correlates with fat mass and percentage of body fat under conditions of ad libitum intake. Furthermore, obese mRNA abundance is reduced by food deprivation, whereas lesser degrees of intake restriction do not change obese mRNA abundance, even when accompanied by appreciable weight loss.

The impact of reversible gonadal sex steroid suppression on serum leptin concentrations in children with central precocious puberty

Serum leptin concentrations increase during childhood in both sexes. During sexual maturation, levels rise further in girls, but decrease in boys. These data suggest that testosterone either directly suppresses leptin levels or induces changes in body composition that result in lower leptin concentrations. To examine further the relationship between sex steroids and leptin, we performed a longitudinal study in children with central precocious puberty (28 girls and 12 boys) before, during, and after discontinuation of GnRH agonist-induced pituitary-gonadal suppression. Nighttime and daytime leptin levels were measured to determine whether the activity of the pituitary-gonadal axis affects their diurnal variation. In the boys, suppression of testosterone increased leptin levels, whereas resumption of puberty was associated with decreased leptin levels [3.5 +/- 0.8 vs. 9.5 +/- 3.1 ng/dL (P = 0.005) and 12.2 +/- 4.5 vs. 7.0 +/- 2.6 ng/dL (P = 0.012), respectively]. Serum leptin levels did not change in the girls with alteration of the pituitary-ovarian axis and consistently exceeded those in boys. Nighttime levels were consistently greater than daytime values by an average of 38.3% in the girls and 29.4% in the boys. These serial observations during reversible pituitary-gonadal suppression suggest that testosterone decreases leptin concentrations, but that estrogen, at least in this childhood model, has no discernible effect. In addition, our data indicate that the presence of the diurnal rhythm in leptin concentrations is independent of the state of the reproductive axis.

Plasma leptin and hunger ratings in healthy humans

Leptin, the ob gene product, is considered as a signal involved in the regulation of energy stores through centrally mediated effects on ingestive behavior and metabolism. To investigate the relationships between appetite-related sensations and circulating leptin in humans, 12 healthy male volunteers were served two test lunches (1200 kcal, 38% carbohydrate, 50% fat, 12% protein) with comparable palatability but resulting in different postprandial hunger ratings, and plasma leptin concentrations were monitored during 8 h after meal ingestion. Whereas postprandial hunger ratings (assessed by visual analog scales) and plasma insulin patterns differed significantly between meals, no significant difference in postprandial plasma leptin concentrations was observed between the two meals. A moderate but significant increase in mean plasma leptin was found over time. We conclude that postprandial plasma leptin concentrations do not significantly differ between two meals inducing different effects on hunger subjective feelings. These data are in agreement with a role for leptin in the regulation of energy balance as a long-term adiposity-related signal rather than a short-term meal-related factor.

Ontogeny of leptin in human fetuses and newborns: effect of intrauterine growth retardation on serum leptin concentrations

The aim of this study was to investigate the ontogeny of serum leptin concentrations during the second half of gestation and at birth in small for gestational age and normal fetuses and newborns. Serum leptin concentrations were measured in arterial cord blood of fetuses (n = 79) and newborns (n = 132), with or without intrauterine growth retardation, at 18-42 weeks gestation. Serum leptin was detectable in fetal cord blood in all subjects as early as 18 weeks gestation. Leptin levels dramatically increased after 34 weeks gestation. In newborns, serum leptin concentrations were positively correlated with body weight (P < 0.001) and body mass index (P < 0.001). Newborns with intrauterine growth retardation had significantly lower serum leptin values (P < 0.001) than those with normal growth, and leptin levels were only positively correlated with body mass index (P < 0.001). These results suggest that the development of adipose tissue and the accumulation of fat mass are the major determinants of fetal and neonatal serum leptin levels. In addition, a gender difference, with higher leptin concentrations in female fetuses, was observed during the last weeks of gestation and was confirmed at birth regardless of growth status, suggesting that a sexual dimorphism already exists in utero.

Leptin in relation to prostate cancer and benign prostatic hyperplasia

The aim of our study was to determine whether leptin, a hormone implicated in both energy-balance and reproductive function, is involved in the etiology of prostate cancer or benign prostatic hyperplasia (BPH). We compared the serum leptin levels of 43 cases of incident prostate cancer, 41 patients with BPH, and 48 healthy controls, all recruited in Athens, Greece. Multiple logistic regression modeling was used, with adjustment for age, height, body mass index, education, estradiol, testosterone, dihydrotestosterone, dehydroepiandrosterone sulfate, sex hormone-binding globulin and insulin like growth factor 1. Odds ratios per 4 ng/ml increment of leptin were 0.70 [95% confidence interval (CI) (0.32,1.55)] for prostate cancer and 1.06 [95% CI (0.67,1.67)] for BPH. After adjustment for body mass index, serum leptin levels were not significantly correlated with levels of any of the other hormones under study. Leptin levels are unlikely to affect the risk of either prostate cancer or BPH substantially.

Leptin

Leptin (from the Greek leptos=thin) was identified only 3 years ago. It has attracted huge attention both scientifically, with more than 600 publications, and in the media, where this protein has been portrayed as the way to a cure for obesity. Indeed, leptin was first described as an adipocyte-derived signalling factor, which, after interaction with its receptors, induced a complex response including control of bodyweight and energy expenditure. Leptin seems in addition to its role in metabolic control to have important roles in reproduction and neuroendocrine signalling. Human obesity is a complex disorder, with many factors playing a part; the pathophysiology of leptin is not as simple as it seems to be in rodent models of obesity.

Leptin levels in pregnant women and newborn infants: gender differences and reduction during the neonatal period

OBJECTIVE

Leptin is a highly hydrophilic protein that circulates in plasma as a 16-kDa protein. It is produced in adipose tissue and also recently described to be synthesized by placental tissue. Plasma concentration of leptin is positively correlated to body fat mass, and administration of recombinant leptin to mice indicates that leptin participates in the regulation of food intake and energy expenditure. Leptin may have a role during initiation of human pubertal development. Gender differences have been reported among adults as well as among children, even after correction for body fat content. Little is known about variation in leptin levels during pregnancy or the level or function of leptin in the growing fetus and infants. The aim of the present study was to examine plasma concentration of leptin in pregnant women and their newborn infants during the first 3 months of life, and to relate plasma leptin concentration to body weight and gender during this period.

MATERIALS AND METHODS

Among 609 women recruited to study the effect of very long-chain n-3 fatty acids during pregnancy, 180 women were selected to study leptin as well. The women were all healthy and nulli- or primiparas, and 16% were smokers. The study was randomized and double-blinded, and the participants received either 10 mL of cod liver oil (Peter Moller, avd.av Orkla ASA, Oslo, Norway) daily or the same amount of corn oil. Blood samples were taken from the mothers during pregnancy in weeks 18 and 35, and from the umbilical cord and from 4- and 14-week-old infants. The mothers' body mass index (BMI) at 18 and 35 weeks of pregnancy was calculated by using body weight recorded within 1 week or, if this was missing, by using means from weights at the closest time points before and after the sampling. The infants were weighed and measured at local health care centers. Plasma leptin concentration was measured by radio immunoassay (Linco Research, St Charles, MO) using recombinant 125I-leptin as tracer.

RESULTS

We found no differences between the group receiving cod liver oil and the group receiving corn oil in any of the measured variables; thus, the groups are treated statistically as one. Leptin concentration in maternal plasma increased during pregnancy from 15.5 +/- 9.0 microg/L (n = 175) in week 18 to 17.7 +/- 10.7 microg/L (n = 166) in week 35. Mothers, pregnant with female fetuses (n = 77), had a significant increase in plasma leptin concentration, from 15.5 +/- 8.8 microg/L (n = 83) at 18 weeks to 18.5 +/- 10.9 microg/L (n = 80) at 35 weeks of pregnancy, whereas in mothers pregnant with male fetuses, the increase was insignificant (15.4 +/- 9.3 microg/L (n = 92) to 17.0 +/- 10.5 microg/L (n = 86). BMI increased during the same time period, from 24.2 +/- 3.3 kg/m to 27.8 +/- 3.8 kg/m (n = 174). There was a significant correlation between BMI and plasma leptin concentration at 18 weeks (r = 0.54, n = 169) and at 35 weeks (r = 0.45, n = 160), but we found no change in the relative leptin concentration (plasma leptin concentration/BMI) from week 18 to week 35. We found no significant difference between smokers and nonsmokers in plasma leptin concentration, neither at 18 nor 35 weeks of pregnancy. Gender differences in plasma leptin concentration was present already at birth in umbilical cord plasma (10.8 +/- 9.2 microg/L for girls [n = 65] vs 7.6 +/- 6.6 microg/L for boys [n = 74]). We also observed gender differences in plasma leptin concentration at 4 weeks (3.9 +/- 1.8 microg/L, n = 68 vs 3.2 +/- 1.8 microg/L, n = 71) and 14 weeks of age (4.9 +/- 2.1 microg/L, n = 61 vs 4.1 +/- 3.1 microg/L, n = 73). Plasma leptin levels at 4 and 14 weeks were lower than the level in umbilical cord plasma (n = 101). An increase in plasma leptin concentration was observed from 4 to 14 weeks of age, both for girls (n = 48) and for boys (n = 60). Leptin concentration in umbilical cord plasma correlated with birth weight (r = 0.44, n = 139), and there was significant corre

Leptin and body composition of Nigerians, Jamaicans, and US blacks

The role of leptin in humans remains controversial. Leptin concentrations are highly correlated with body fat stores. We tested whether or not this relation was consistent across the range of body composition encompassing the lean as well as the obese. Individuals participating in community-based comparative research in Nigeria (n = 363), Jamaica (n = 372), and the United States (Maywood, IL; n = 699) had their plasma leptin concentrations and body compositions (with bioelectrical impedance analysis) measured. All participants identified themselves as being black. Body mass index (in kg/m2) ranged from 14 to 62. Large differences in mean plasma leptin were noted across populations for both men and women in Nigeria, Jamaica, and the United States, respectively (men: 2.8, 3.9, and 6.8 microg/L; women: 10.3, 18.6, and 27.7 microg/L). An exponential function fit the relation between percentage body fat or total fat mass and leptin for men and women at each site. For women and men the exponential function with either percentage body fat or total fat mass was of the same shape, but increased by a constant in women, yielding higher leptin concentrations than in men at every level of body fat. On the basis of this broad distribution of body composition, the data suggest an exponential response of leptin to increases in body fat stores, consistent with the development of leptin resistance in individuals developing obesity. These findings likewise confirm that men and women exhibit different set points in terms of leptin production.

Postnatal leptin surge and regulation of circadian rhythm of leptin by feeding. Implications for energy homeostasis and neuroendocrine function

Leptin is thought to regulate energy balance through effects on food intake and thermogenesis. In addition, leptin may serve as a mediator of the neuroendocrine response to starvation, and may modulate the stress response and the timing of puberty. A role for leptin in development is suggested by the presence of neuroendocrine and structural neuronal abnormalities in ob/ob mice with genetic leptin deficiency. Here, we sought to determine the ontogeny of leptin expression and its relationship to the developing neuroendocrine axis. Leptin increased 5-10-fold in female mice during the second postnatal week independent of fat mass, and declined after weaning. The rise in leptin preceded the establishment of adult levels of corticosterone, thyroxine, and estradiol. In contrast to adult mice, leptin was not acutely regulated by food deprivation during the early postnatal period. Circadian rhythms of leptin, corticosterone, and thyroxine were regulated by food intake in adult mice. When ad libitum feeding was restricted to the light cycle, peak corticosterone levels were shifted to the beginning of the light cycle and coincided with the nadir of leptin. The inverse relationship between leptin and corticosterone was maintained such that a rise in leptin after feeding was associated with a decline in corticosterone. To determine whether changes in corticosterone during food restriction are mediated by leptin, we compared the patterns of corticosterone levels among ob/ob, db/db, and lean mice. Despite their higher basal levels of corticosterone, leptin deficiency in ob/ ob mice did not prevent the nocturnal rise in corticosterone. In contrast, the nocturnal surge of corticosterone was blunted in db/db mice. Therefore, it is likely that factors in addition to leptin are involved in the regulation of the circadian rhythm of corticosterone. The temporal relationship between leptin and other hormones in neonatal and adult mice suggests that leptin is involved in the maturation and function of the neuroendocrine axis.

Disordered food intake and obesity in rats lacking cholecystokinin A receptors

Otsuka Long-Evans Tokushima Fatty (OLETF) rats develop obesity, hyperglycemia, and non-insulin-dependent diabetes mellitus and do not express cholecystokinin A (CCK-A) receptors, the receptor subtype mediating the satiety actions of CCK. In short-term feeding tests, male OLETF rats were completely resistant to exogenous CCK, and their response to bombesin was attenuated. Comparisons of liquid meal consumption in OLETF and control Long-Evans Tokushima (LETO) rats demonstrated that 1) OLETF rats had greater intakes during 30-min scheduled daytime meals and significantly larger and fewer spontaneous night-time meals and 2) although the initial rates of licking were the same, OLETF rats maintained the initial rate longer and the rate at which their licking declined was slower. In 24-h solid food access tests, OLETF rats consumed significantly more pellets than LETO controls, and this increase was attributable to significant increases in meal size. Together, these data are consistent with the interpretation that the lack of CCK-A receptors in OLETF rats results in a satiety deficit leading to increases in meal size, overall hyperphagia, and obesity.

Relationship of leptin concentration to gender, menopause, age, diabetes, and fat mass in African Americans

This investigation was designed to determine the relationship of leptin concentration to gender, sex hormones, menopause, age, diabetes, and fat mass in African Americans. Participants included 101 African Americans, 38 men (mean age, 34.2 +/- 7.4 years), 29 age-matched premenopausal women (mean age, 32.6 +/- 3.7 years), and 36 postmenopausal women (mean age, 57.8 +/- 5.9 years). The women were not taking exogenous sex hormones, and 12 subjects were diabetic. Percent body fat was calculated with the Siri formula, fat mass (FM) was calculated as weight x percent body fat, and Fat-free mass (FFM) was calculated as weight minus FM. Fasting plasma was assayed for leptin, estradiol, free testosterone, glucose, and insulin concentrations. The nondiabetics had an oral glucose tolerance test (OGTT). The diabetics compared with the non-diabetics had a higher central fat index (p=0.04) but otherwise were similar to nondiabetics in all parameters measured. Body mass index, percent body fat, and FM were greater in women than men (p<0.001). Leptin concentrations in men, premenopausal, and postmenopausal women were: 7.51 +/- 8.5, 33.9 +/- 17.3, 31.4 +/- 22.3 ng/mL. Leptin/FM x 100 in the three groups were: 28.9 +/- 16.1, 98.65 +/- 44.9, 77.1 +/- 44.5 ng/mL/kg. The gender difference in leptin concentration and leptin/FM was significant (p<0.001), but the difference between premenopausal and postmenopausal women was not. In each group, weight, percent body fat, and FM were highly correlated with leptin concentration. Multiple regression analyses with leptin concentration as the dependent variable and age, diabetic status, percent body fat, weight, FM, FFM, estradiol, and free testosterone concentrations as independent variables demonstrated that the determinants of leptin concentration in men was weight only (R=0.83, p<0.001), in premenopausal women it was FM only (R=0.57, p<0.001), and in postmenopausal women it was weight only (R=0.67, p<0.001). With diabetics excluded, the multiple regression analysis was repeated with fasting insulin concentration and the area under the insulin curve during the OGTT included as independent variables. The results for this multiple regression analyses were the same as the first. Therefore, leptin concentration in African Americans is determined by gender and fat mass. Menopause, age, and diabetes do not affect leptin concentration.

Autosomal genomic scan for loci linked to obesity and energy metabolism in Pima Indians

An autosomal genomic scan to search for linkage to obesity and energy metabolism was completed in Pima Indians, a population prone to obesity. Obesity was assessed by percent body fat (by hydrodensitometry) and fat distribution (the ratio of waist circumference to thigh circumference). Energy metabolism was measured in a respiratory chamber as 24-h metabolic rate, sleeping metabolic rate, and 24-h respiratory quotient (24RQ), an indicator of the ratio of carbohydrate oxidation to fat oxidation. Five hundred sixteen microsatellite markers with a median spacing of 6.4 cM were analyzed, in 362 siblings who had measurements of body composition and in 220 siblings who had measurements of energy metabolism. These comprised 451 sib pairs in 127 nuclear families, for linkage analysis to obesity, and 236 sib pairs in 82 nuclear families, for linkage analysis to energy metabolism. Pointwise and multipoint methods for regression of sib-pair differences in identity by descent, as well as a sibling-based variance-components method, were used to detect linkage. LOD scores >=2 were found at 11q21-q22, for percent body fat (LOD=2.1; P=.001), at 11q23-q24, for 24-h energy expenditure (LOD=2.0; P=.001), and at 1p31-p21 (LOD=2.0) and 20q11.2 (LOD=3.0; P=.0001), for 24RQ, by pointwise and multipoint analyses. With the variance-components method, the highest LOD score (LOD=2.3 P=.0006) was found at 18q21, for percent body fat, and at 1p31-p21 (LOD=2.8; P=.0003), for 24RQ. Possible candidate genes include LEPR (leptin receptor), at 1p31, and ASIP (agouti-signaling protein), at 20q11.2.

Identification of SOCS-3 as a potential mediator of central leptin resistance

Leptin affects food intake and body weight by actions on the hypothalamus. Although leptin resistance is common in obesity, mechanisms have not been identified. We examined the effect of leptin on expression of the suppressors-of-cytokine-signaling (SOCS) family of proteins. Peripheral leptin administration to ob/ob, but not db/db mice, rapidly induced SOCS-3 mRNA in hypothalamus, but had no effect on CIS, SOCS-1, or SOCS-2. A leptin-dependent increase of SOCS-3 mRNA was seen in areas of hypothalamus expressing high levels of the leptin receptor long form. In mammalian cell lines, SOCS-3, but not CIS or SOCS-2, blocked leptin-induced signal transduction. Expression of SOCS-3 mRNA in the arcuate and dorsomedial hypothalamic nuclei is increased in Ay/a mice, a model of leptin-resistant murine obesity. In conclusion, SOCS-3 is a leptin-inducible inhibitor of leptin signaling, and a potential mediator of leptin resistance in obesity.

Leptin receptor gene in a large cohort of massively obese subjects: no indication of the fa/fa rat mutation. Detection of an intronic variant with no association with obesity

The massive obesity caused in rodents by the disruption of the leptin-receptor signal through genetic defects at the level of either leptin (OB) or leptin receptor (OB-R) has raised the question of the relevance of these genes to morbid obesity in humans. In this study, we screened a large population of massively obese subjects for the presence of a leptin receptor mutation homologous to that of fa/fa rats, a single base substitution changing glutamine 269, a highly conserved glutamine found at position 270 in the human sequence. After polymerase chain reaction (PCR) amplification of a DNA region encompassing the end of exon 5, intron 5, and the beginning of exon 6, we performed restriction fragment length polymorphism analysis. Within the limitations of this approach where only mutations introducing restriction sites (5 of 8 possibilities) could be assessed, no evidence of mutation at the codon gln 270 was found in 343 massively obese subjects. However, a new OB-R gene variant in intron 5 was revealed by MaeII digestion of the PCR products. MaeII/hOB-R genotyping revealed no difference in the distribution of the genotypes between obese subjects and a group of 79 unrelated nonobese control subjects. In addition, no significant association between various obesity-related metabolic phenotypes and the presence of MaeII/hOB-R alleles was found. Thus, our results did not support a significant role for the MaeII/hOB-R gene variant in the development of the obese phenotype in the population we studied.

Advances in endocrinology

Molecular genetics will continue to help us to make precise diagnoses. At present, the expertise to achieve this for a specific disease is often exclusive to one unit with a research interest. It will be important to establish a coordinated approach at a supraregional level to provide molecular diagnosis for rare disorders as a fast reliable clinical service. In addition understanding the molecular mechanisms of disease is likely to direct a search for new treatments. For instance, calcium channel blockers have been used in nesidoblastosis to reduce the hypersecretion of insulin, as a result of the recognition of the role that calcium has in the function of the beta-cell ATP sensitive K+ channel. Although the potential benefits of hGH are now being clearly defined in a range of growth disorders, the treatment is invasive and expensive. It is likely that future endocrine therapeutic developments could include slow release growth hormone preparations, orally active growth hormone mimetics, or even hormone production from an ectopic viral cDNA vector. The next "advances in endocrinology" will also reveal whether leptin will have a therapeutic role in appetite control or even the modulation of pubertal development.

Radioimmunoassay of rat leptin: sexual dimorphism reversed from humans

Adipose tissue secretes leptin, which interacts with receptors in the hypothalamus. In rodent models of obesity, leptin increases metabolism and decreases food intake, which helps to maintain normal body composition. Accurate and precise methods to quantitate circulating leptin concentrations are needed for physiological studies. We developed an RIA to measure leptin in rat plasma, serum, or adipocyte culture fluids. The working range of the assay, defined by the detection limit and the highest calibrator, was 0.5–50 μg/L. Recovery of 1.6–11.6 μg/L leptin added to serum was 92–103%. The rat leptin RIA correlated well with a previously developed mouse RIA when rat plasma was assayed with both methods (r = 0.94), but the mouse leptin assay underestimated rat leptin in plasma. Within- and between-run CVs were 2.4% to 5.7%. Plasma leptin concentrations correlated directly with percentage of body fat, and correlation improved when the results were separated by gender (r = 0.796, P &lt;0.001 for males; r = 0.710, P &lt;0.001 for females). Leptin concentrations were generally higher in male rats than in females; plasma leptin increased 0.60 μg/L for each percentage of increase in body fat for males but only 0.22 μg/L for females. We conclude that rat serum/plasma leptin concentrations are accurately and precisely measured with this new RIA.

Serum leptin concentrations in patients with thyroid disorders

OBJECTIVE

Leptin, the obese gene product, is secreted exclusively by adipocytes and is thought to act as a lipostatic signal that regulates body weight homeostasis. We previously reported that thyroid hormone is one of the up-regulating factors of leptin in vitro. T3, at physiological concentrations, stimulates leptin mRNA expression and leptin secretion by 3T3-L1 adipocytes. The aim of this study was to explore the role of thyroid hormone in the regulation of leptin in humans.

DESIGN AND PATIENTS

A total of 59 non-obese women aged 38.4 +/- 1.8 years (mean +/- SEM) were studied: 19 patients with hyperthyroidism, 17 patients with hypothyroidism, and 23 normal control subjects. The correlation between serum leptin concentrations and body mass index (BMI) was analyzed, and serum leptin levels were compared among the three groups.

MEASUREMENTS

Serum leptin concentrations were measured by radioimmunoassay.

RESULTS

Serum leptin concentrations after logarithmic transformation were correlated significantly (P < 0.05) with BMI in the hyperthyroid (r = 0.46), the hypothyroid (r = 0.84), and normal (r = 0.63) groups. Even though age, body weight, and BMI were similar in all groups, serum leptin levels in the hypothyroid patients (5.30 +/- 1.12 micrograms/l) were significantly (P < 0.05) lower than in the hyperthyroid and normal groups (6.87 +/- 0.66 and 6.58 +/- 0.68 micrograms/l, respectively).

CONCLUSIONS

These results indicate that thyroid hormone may play an important role in the appropriate secretion of leptin in humans.

Localization of leptin receptor immunoreactivity in the lean and obese Zucker rat brain

Leptin, a product of the obese (ob) gene, is secreted by adipocytes and appears to act as a hormone to regulate food intake, metabolism and body weight. Subcutaneous administration of leptin causes reductions in food intake and body and fat-depot weights in both lean and genetically obese (ob/ob) mice, and leptin infusion into the lateral cerebral ventricles decreases feeding with short latency, suggesting a central site of action. A gene defect in the Zucker obese rat causes an amino acid substitution in the leptin receptor and reduced leptin binding at the cell surface. An antiserum to a portion of the mouse leptin receptor (AA 877-894) located within the intracellular domain was used to label Zucker lean (Fa/?) and obese (fa/fa) rat brain sections. At optimal dilution (1:8000), only cells in the basal forebrain, preoptic area, hypothalamus and brainstem were moderately or intensely labeled. The most intensely-labeled nuclei, the anterior commissural, magnocellular paraventricular, supraoptic, circularis in the anterior hypothalamus and fornical in the lateral hypothalamus contain large neurons that synthesize and secrete vasopressin or oxytocin and their respective neurophysins. Diminished leptin transport into the central nervous system or defective signal transduction in Zucker obese rats may sufficiently compromise leptin regulation of the HPA axis, NPY-immunoreactive neurons or other hypothalamic elements to cause obesity.

Serum leptin levels and energy expenditure in normal weight women

To investigate whether circulating leptin levels are associated with energy expenditure in healthy humans, doubly labeled water energy measurements and food intake assessment were carried out in 27 women (mean age, 48.6 years; weight, 61.9 kg; body mass index, 23.2). Energy expenditure was determined over 13 days. Food intake was measured by 7-day food records. Leptin was measured by radioimmunoassay. Leptin level was strongly associated with percentage body fat (r = 0.59; p < 0.001), fat mass (r = 0.60; p < 0.001), and body mass index (r = 0.41; p = 0.03), but no correlation was observed with energy expenditure (r = 0.02; p = 0.93). After controlling for percentage body fat, a positive association of leptin level with energy expenditure of marginal significance (p = 0.06) was observed. There were no significant univariate associations of age, physical activity, lean body mass, height, or dietary variables with leptin level. When controlling for body fat, a significant positive correlation was observed for percent energy from carbohydrate and negative correlations with dietary fat and alcohol intake. These findings confirm previous associations between leptin and body fat content and suggest a relationship between serum leptin and energy expenditure level in healthy humans.Key words: leptin, energy expenditure, body composition, diet.

Leptin, leptin receptors, and the control of body weight

The assimilation, storage, and disposition of nutrient energy constitute a complex homeostatic system central to the survival of both prokaryotic and eukaryotic organisms. In vertebrates, and especially among land dwelling mammalian species, the ability to store large quantities of energy-dense fuel in the form of adipose tissue triglyceride permits survival during prolonged periods of food deprivation. In order to maintain such fuel stores during times of dietary scarcity or surfeit, some balance between energy intake and expenditure must be achieved. Lesions of the hypothalamus alter body weight suggesting that this brain region regulates nutritional state. These and other studies led to the hypothesis that body weight was regulated by a feedback loop in which peripheral signals reported nutritional information to an integratory center in the brain. However, the identity of these nutrition signals proved elusive.

A paired sibling analysis of the beta-3 adrenergic receptor and obesity in Mexican Americans

The beta3 adrenergic receptor, located on chromosome 8, is a regulator of energy expenditure and lipolysis. A missense mutation in this gene, characterized by the replacement of tryptophan by arginine at codon 64 (Trp64Arg), is associated with obesity in some studies. We examined the effect of this variant on obesity in Mexican Americans, using a paired sibling design to minimize variability due to genetic background and a previously identified major susceptibility locus for obesity. We identified 45 sib-pairs that were concordant (identical by descent) for a locus on chromosome 2 which we have shown previously to be tightly linked to obesity in this population. The Trp64Arg variant, detected by PCR-restriction fragment length polymorphism analysis, was present in one sibling within each of the 45 sib-pairs. Presence of the variant was associated with significantly higher values in body mass index (P = 0.04), fat mass (P = 0.04), and waist circumference (P = 0.05). We conclude that the Trp64Arg variant is associated with obesity in this Mexican American population. The paired sibling design probably enhanced our ability to detect the effects of this variant by allowing us to account for variation attributable to another obesity susceptibility locus and to background genes.

The effect of exercise training on leptin levels in obese males

The effect of endurance training on plasma leptin levels was investigated in 15 obese male subjects (age 37.3 +/- 5.2 yr, body weight 96.5 +/- 13.6 kg, and body mass index 29.8 +/- 3.0 kg/m2) in a weight loss and exercise program. After 4 mo of treatment consisting of a very low energy diet (VLED) and endurance exercise training (3-4 times weekly, 1 h sessions, moderate intensity), two groups were formed. One group continued the exercise sessions (trained subjects, n = 7) and the other group stopped with the exercise program (control, n = 8). Measurements of anthropometry, aerobic power, and fasted blood samples were executed at fixed time points (0, 2, 4, 10, and 16 mo). With partial regression analysis, keeping the changes in insulin and body fat percentage constant, it was shown that the number of hours of exercise training was significantly correlated with changes in leptin levels, during the 16-mo period (r = 0.56, P < 0.05). Changes in insulin levels were significantly related to the changes in leptin levels (r = 0.47, P < 0.05), which were less for changes in body fat percentage (r = 0.42, P = 0.07). During the VLED, the change in insulin concentration affected leptin levels significantly (r = 0.79) but changes in body fat percentage were not noted. It is concluded that endurance exercise training decreased plasma leptin levels independently of changes in plasma insulin levels and body fat percentage.

Leptin and ciliary neurotropic factor (CNTF) inhibit fasting-induced suppression of luteinizing hormone release in rats: role of neuropeptide Y

Periods of chronic undernutrition and short periods of fasting suppress pituitary luteinizing hormone (LH) secretion and upregulate hypothalamic neuropeptide Y (NPY), the orexigenic peptide. The effect of suppression of NPY upregulation with ciliary neurotropic factor (CNTF), a cytokine, and leptin, an adipocyte hormone, on pituitary LH secretion was evaluated in fasted rats. In the first experiment, daily injection of CNTF (0.2 nmol) intracerebroventricularly (i.c.v.) for 4 days drastically reduced food intake and body weight gain similar to the weight loss seen in pair-fed rats. Food deprivation (FD) also decreased body weight. Despite drastic loss in body weight, plasma LH was reduced in FD and pair-fed rats, but not in CNTF-treated rats. In the second experiment, FD rats received either control vehicle, CNTF (0.2 nmol) or leptin (0.2 nmol) daily for 4 days. FD increased steady state levels of preproNPY mRNA in the hypothalamus over the control freely-fed rats. However, both CNTF and leptin suppressed hypothalamic gene expression and significantly attenuated LH suppression in response to FD. Taken together, these results support the hypothesis that the upregulation of hypothalamic NPY system may underlie diminution in pituitary gonadotropin secretion and that the NPYergic pathway may serve as a communication bridge between the neural processes that regulate reproduction and those that maintain energy balance.

Dysregulation of leptin in response to fasting in insulin-resistant Psammomys obesus (Israeli sand rats)

Leptin is thought to play a significant role in energy balance as an afferent signal to the hypothalamus that reflects body fat content. In addition, leptin may also act as an acute sensor of energy balance independent of body fat mass, since ob gene expression and plasma leptin concentrations are decreased in lean animals and humans in response to short-term caloric deprivation. However, in obese animals and humans, the acute response of leptin to fasting is less clear. We investigated the effects of a 24-hour fast on circulating plasma leptin concentrations in lean and obese Psammomys obesus (Israeli sand rats). In the lean, insulin-sensitive group (n = 25) a 24-hour fast caused a 44% decrease in plasma leptin, whereas in the obese, insulin-resistant group (n = 24) plasma leptin increased by 18% after fasting (P < .003). There was no difference between the two groups regarding the effect of a 24-hour fast on body weight, blood glucose, or plasma insulin. Within the insulin-resistant group, there was no difference in the response of leptin to fasting between hyperglycemic and normoglycemic animals. We conclude that there is a dysregulation of leptin in response to acute caloric deprivation in obese, insulin-resistant but not in lean, insulin-sensitive P obesus.

Rats maintained on high-fat diets exhibit reduced satiety in response to CCK and bombesin

Rats maintained on high-fat diets often exhibit increased food intake and weight gain. We hypothesized that high-fat diets might result in reduced sensitivity to hormonal signals responsible for terminating food intake--satiety signals. The intestinal hormone cholecystokinin (CCK) and the gastrointestinal neuropeptide, bombesin (BBS) both have been proposed as satiety signals. To determine whether maintenance on high-fat diets alters sensitivity to satiating effects of CCK and bombesin (BBS), rats were maintained on a low fat diet (LF), a high-fat diet that was isocaloric with the low-fat diet (HF), or one of two hypercaloric high-fat diets (HF-1, HF-2) that differed from HF and LF in fat, fiber, and total caloric content. CCK and bombesin reduced food intake significantly less in rats maintained on high-fat diets, compared to those on the low fat diet. Neither high caloric intake, which was associated with increased body weight gain on the two hypercaloric diets, nor fiber content of the diet accounted for the reduced response of HF rats to CCK. Rather, reduced sensitivity to CCK was related only to the high proportion of calories taken as fat. We also determined whether reduced CCK sensitivity was due to the maintenance on a particular diet or to the diet eaten during a CCK test. After CCK, rats maintained on LF reduced food intake more (49%) than rats maintained on HF (22%), regardless of whether they ate HF or LF during the CCK test itself. These findings indicate that maintenance of rats on high-fat diets reduces sensitivity to some peptide satiety signals. Reduced sensitivity to satiety signals might contribute to overeating and obesity often observed when rats are maintained on high-fat diets.

A leptin dose-response study in obese (ob/ob) and lean (+/?) mice

This experiment determined the amount of leptin required to correct different abnormalities in leptin-deficient ob/ob mice. Baseline food intakes and body weights of lean (+/?) and obese (ob/ob) C57B1/6J <ob> mice were recorded for 7 days. An Alzet miniosmotic pump was placed in the peritoneal cavity of each mouse and delivered 0, 1, 2, 5, 10, or 42 microg/day human leptin for 7 days. In ob/ob mice, 2 microg leptin/day reduced food intake and body weight, and increased hypothalamic and brain stem serotonin concentrations. All fat pads were reduced 35-40% by 10 microg leptin/day, and liver weight, lipid, and glycogen decreased. Serum insulin and glucose were reduced in all leptin-treated ob/ob mice, and levels were normalized by 10 microg/day leptin. Low rectal temperatures of ob/ob mice were corrected by 10 and 42 microg/day leptin. These doses also increased brown adipose tissue uncoupling protein expression. The only responses in lean mice were a transient reduction in food intake and weight loss with 10 or 42 microg/day leptin. This study shows enhanced leptin sensitivity in ob/ob mice and suggests that increased temperature and sympathetic activity are indirect responses to high concentrations of protein.

IL-1 beta mediates leptin induction during inflammation

Interleukins (IL) are key mediators of the host response to infection and inflammation. Leptin is secreted by adipose tissue and plays an important role in the control of food intake. Administration of lipopolysaccharide (LPS), tumor necrosis factor (TNF), or IL-1 acutely increases leptin mRNA and protein levels. To investigate the role of IL-1 beta and IL-6 in leptin expression during inflammation, we used IL-1 beta-deficient (-/-) and IL-6 -/- mice. Mice were injected intraperitoneally with LPS or subcutaneously with turpentine, as models of systemic or local inflammation, respectively. In IL-1 beta +/+ mice, both LPS and turpentine increased leptin mRNA and circulating leptin. In contrast, neither LPS nor turpentine increased leptin levels in IL-1 beta -/- mice. In IL-6 +/+ or IL-6 -/- mice, turpentine increased leptin protein to comparable levels. We conclude that IL-1 beta is essential for leptin induction by both LPS and turpentine in mice, but IL-6 is not.

Cardiovascular consequences of obesity: role of leptin

1. Several mechanisms have been implicated in the association between obesity and hypertension, including salt-sensitivity, insulin resistance and sympathetic activation. Obese animals and humans exhibit exaggerated blood pressure responses to increases in salt intake. 2. Although insulin resistance is common in obesity, it is clear that abnormal insulin action is not the sole or sufficient cause of hypertension in obesity. Obesity is associated with increased activity of the sympathetic nervous system. Sympathetic blockade has been reported to attenuate sodium retention and hypertension in experimental models of obesity. 3. The mediators responsible for salt sensitivity, insulin resistance and sympathetic activation in obesity remain unclear. 4. The novel protein hormone leptin is produced almost exclusively by adipose tissue and acts in the central nervous system through a specific receptor and multiple neuropeptide pathways to decrease appetite and increase energy expenditure. 5. Increasing evidence suggests that leptin may have wider actions influencing autonomic, cardiovascular, renal and endocrine function. We have shown that leptin increases sympathetic nerve activity to kidney, hindlimb and adrenal gland, in addition to brown adipose tissue. 6. Despite this sympathoexcitatory action, acute systemic administration of leptin does not acutely increase arterial pressure or heart rate in anaesthetized animals. This may reflect opposing antihypertensive actions of leptin. For example, leptin increases renal sodium and water excretion, apparently through a direct tubular action. In addition, leptin increases systemic insulin sensitivity, even in the absence of weight loss. 7. In conclusion, leptin may act as a mediator linking body adiposity with changes in insulin action, sympathetic neural outflow and renal sodium excretion. Alterations in leptin generation or action may, in part, underlie the sympathetic, endocrine and renal consequences of obesity.

Identification of the human leptin 5'-flanking sequences involved in the trophoblast-specific transcription

Leptin is an adipocyte-derived blood-borne satiety factor that is involved in the regulation of energy homeostasis. We have recently demonstrated nonadipose tissue production of leptin; leptin is synthesized in and secreted from placental trophoblasts (Nature Med. 3: 1029-1033, 1997). To understand the transcriptional regulation of the human leptin gene in placental trophoblasts, we examined the promoter activity of various lengths of the human leptin 5'-flanking sequences in BeWo cells, a human trophoblastic cell line. The 2080-bp human leptin gene promoter region (-2080 to +108) showed a high-level transcription activity in BeWo cells. When DNA sequences between -1885 and -1830 were deleted, the promoter activity was reduced dramatically in BeWo cells. No significant changes in the promoter activity were noted when tested in primary cultures of rat mature adipocytes. Electrophoretic mobility shift assays revealed the presence of nuclear protein(s) binding to the sequences in BeWo cells but not in isolated rat mature adipocytes. The present study provides new insight into the trophoblast-specific transcription of the human leptin gene.

Long-term correction of obesity and diabetes in genetically obese mice by a single intramuscular injection of recombinant adeno-associated virus encoding mouse leptin

The ob/ob mouse is genetically deficient in leptin and exhibits a phenotype that includes obesity and non-insulin-dependent diabetes mellitus. This phenotype closely resembles the morbid obesity seen in humans. In this study, we demonstrate that a single intramuscular injection of a recombinant adeno-associated virus (AAV) vector encoding mouse leptin (rAAV-leptin) in ob/ob mice leads to prevention of obesity and diabetes. The treated animals show normalization of metabolic abnormalities including hyperglycemia, insulin resistance, impaired glucose tolerance, and lethargy. The effects of a single injection have lasted through the 6-month course of the study. At all time points measured the circulating levels of leptin in the serum were similar to age-matched control C57 mice. These results demonstrate that maintenance of normal levels of leptin (2-5 ng/ml) in the circulation can prevent both the onset of obesity and associated non-insulin-dependent diabetes. Thus a single injection of a rAAV vector expressing a therapeutic gene can lead to complete and long-term correction of a genetic disorder. Our study demonstrates the long-term correction of a disease caused by a genetic defect and proves the feasibility of using rAAV-based vectors for the treatment of chronic disorders like obesity.

Expression of functional leptin receptors in the human ovary

The size of body fat stores is known to influence fertility, indicating a link between adipose tissue and the reproductive system. Studies in mice have identified the adipocyte-derived hormone, leptin (Ob protein), as a possible mediator of this effect. The aim of this study was to investigate the possibility that leptin may have direct effects on the human ovary. To probe this hypothesis we first analyzed the expression of leptin receptors in the human ovary. Transcripts encoding both the long and short isoforms of the leptin receptor were present in human granulosa cells and thecal cells; however, the short isoforms were expressed at much higher levels. Immunoreactive leptin was present in follicular fluid at levels similar to those found in serum. ob gene expression, however, was undetectable in the ovary, as determined by reverse transcription-PCR, whereas it was easily detected in adipose tissue. To determine whether leptin could induce a biological response in ovarian cells, we examined the effect of leptin on estradiol production in cultured granulosa cells. Leptin (100 ng/mL) inhibited LH (0.1 ng/mL)-stimulated estradiol production. In contrast, leptin had no effect on estradiol production in the absence of LH. In conclusion, this study has demonstrated that the leptin receptor is expressed in the human ovary, that leptin is present in follicular fluid, and that leptin can induce a biological response in ovarian cells. These results suggest that leptin may have a direct effect on the human ovary.

Increased feeding in fatty Zucker rats by the thiazolidinedione BRL 49653 (rosiglitazone) and the possible involvement of leptin and hypothalamic neuropeptide Y

1. The thiazolidinedione BRL 49653 (rosiglitazone) induces hyperphagia and weight gain in obese, insulin-resistant fatty Zucker rats but not in lean insulin-sensitive rats. We investigated whether these responses might involve neuropeptide Y (NPY), leptin and insulin. 2. BRL 49653 (1 mg kg(-1) day(-1), orally) was given for 7 or 20 days to fatty and lean Zucker and Wistar rats. 3. In lean rats of either strain, BRL 49653 had no effect on food intake, body weight, plasma insulin and corticosterone, NPY or NPY mRNA levels. 4. Fatty rats given BRL 49653 showed a 30% increase in food intake and accelerated body weight gain (both P<0.01) after 7 and 20 days, but without significant changes in regional hypothalamic NPY or NPY mRNA levels. 5. Plasma leptin levels were twice as high in untreated fatty Zucker rats as in lean rats (P<0.01), but were unaffected by BRL 49653 given for 20 days. However, BRL 49653 reduced insulin levels by 42% and increased corticosterone levels by 124% in fatty rats (both P<0.01). 6. Hyperphagia induced in fatty Zucker rats by BRL 49653 does not appear to be mediated by either a fall in circulating leptin levels or increased activity of hypothalamic NPYergic neurones. The fall in plasma insulin and/or rise in corticosterone levels during BRL 49653 treatment may be involved, consistent with the postulated role of these hormones in the control of food intake.

Leptin--a regulator of islet function?: its plasma levels correlate with glucagon and insulin secretion in healthy women

It has previously been demonstrated that plasma leptin correlates to body fat content. It has also been demonstrated that in subjects with normal glucose tolerance, circulating leptin correlates to circulating insulin and to insulin secretion and that these relations are independent of body fat. However, whether leptin also covaries with other islet hormones is not known. We therefore studied the relation between plasma levels of leptin and glucagon secretion and circulating pancreatic polypeptide (PP) in healthy humans. Arginine was injected intravenously (5 g) at fasting and at 14 and 28 mmol/L, glucose in 71 postmenopausal women with normal glucose tolerance. In a multivariate analysis controlling for the influence of the body mass index, we found that circulating leptin correlated significantly to fasting insulin (r = .38, P = .002), and to circulating insulin at 14 mmol/L glucose levels (r = .29, P = .0019) and 28 mmol/L glucose (r = .32, P = .009), as well as to the insulin response to arginine at all three glucose levels (r > .30, P < .013). Circulating leptin, independently of the body mass index, also correlated to fasting glucagon (r = .31, P = .012) and to the glucagon response to arginine at all three glucose levels (r > .28, P < .038). In contrast, circulating leptin did not correlate to plasma glucagon at 14 or 28 mmol/L glucose or to plasma levels of PP. We conclude that circulating leptin correlates to the secretory capacity of both glucagon and insulin but not to the reduction of plasma glucagon during hyperglycemia or to PP in a large group of postmenopausal women. This suggests that islet function is related to circulating leptin in humans.