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

Increased uncoupling protein-2 and -3 mRNA expression during fasting in obese and lean humans

Uncoupling protein-2 and -3 (UCP2 and UCP3) are mitochondrial proteins that show high sequence homology with the brown adipocyte-specific UCP1. UCP1 induces heat production by uncoupling respiration from ATP synthesis. UCP2 is widely expressed in human tissues, whereas UCP3 expression seems restricted to skeletal muscle, an important site of thermogenesis in humans. We have investigated the regulation of UCP2 and UCP3 gene expression in skeletal muscle and adipose tissue from lean and obese humans. UCP2 and -3 mRNA levels were not correlated with body mass index (BMI) in skeletal muscle, but a positive correlation (r = 0.55, P < 0.01, n = 22) was found between UCP2 mRNA level in adipose tissue and BMI. The effect of fasting was investigated in eight lean and six obese subjects maintained on a hypocaloric diet (1,045 kJ/d) for 5 d. Calorie restriction induced a similar 2-2.5-fold increase in UCP2 and -3 mRNA levels in lean and obese subjects. To study the effect of insulin on UCP gene expression, six lean and five obese subjects underwent a 3-h euglycemic hyperinsulinemic clamp. Insulin infusion did not modify UCP2 and -3 mRNA levels. In conclusion, the similar induction of gene expression observed during fasting in lean and obese subjects shows that there is no major alteration of UCP2 and -3 gene regulation in adipose tissue and skeletal muscle of obese subjects. The increase in UCP2 and -3 mRNA levels suggests a role for these proteins in the metabolic adaptation to fasting.

Whole body leptin kinetics and renal metabolism in vivo

Leptin metabolism was investigated in male Sprague-Dawley rats by use of 125I-labeled leptin plasma kinetic and arteriovenous balance studies. When conscious rats received bolus venous injections of 125I-leptin, intact (precipitable) leptin quickly disappeared from circulation in a biexponential manner during the 2-h experimental period. After substantial delay, most of the injected radioactivity appeared in the urine. The data were described by a two-compartment model, which postulated that plasma leptin exchanged with a nonplasma pool and that all of the tracer cleared from plasma appeared in urine or in a degraded form in plasma. The half-life of leptin was 9.4 +/- 3.0 min, and the leptin production rate was 3.6 +/- 1.2 ng 100 g fat-1.min-1. The left kidney extracted 21 +/- 1.5% of intact arterial 125I-leptin 5 min after femoral venous injection. Endogenous arterial leptin was reduced 21 +/- 8 and 18 +/- 12%, respectively, in simultaneously sampled left and right renal veins. Renal elimination appears to be the major elimination mechanism for leptin in normal rats, and the kinetic studies suggest that uptake of leptin by renal tissue rather than glomerular filtration is the predominant elimination mechanism.

Leptin rapidly suppresses insulin release from insulinoma cells, rat and human islets and, in vivo, in mice

Obesity is associated with diabetes, and leptin is known to be elevated in obesity. To investigate whether leptin has a direct effect on insulin secretion, isolated rat and human islets and cultured insulinoma cells were studied. In all cases, mouse leptin inhibited insulin secretion at concentrations within the plasma range reported in humans. Insulin mRNA expression was also suppressed in the cultured cells and rat islets. The long form of the leptin receptor (OB-Rb) mRNA was present in the islets and insulinoma cell lines. To determine the significance of these findings in vivo, normal fed mice were injected with two doses of leptin. A significant decrease in plasma insulin and associated rise in glucose concentration were observed. Fasted normal and leptin receptor-deficient db/db mice showed no response to leptin. A dose of leptin, which mimicked that found in normal mice, was administered to leptin-deficient, hyperinsulinemic ob/ob mice. This caused a marked lowering of plasma insulin concentration and a doubling of plasma glucose. Thus, leptin has a powerful acute inhibitory effect on insulin secretion. These results suggest that the action of leptin may be one mechanism by which excess adipose tissue could acutely impair carbohydrate metabolism.

Identification of a placental enhancer for the human leptin gene

Leptin is a hormone that regulates metabolic efficiency, energy expenditure, and food intake. Leptin is produced chiefly in adipose cells, but in humans, mRNA encoding leptin is also present in the placenta. Here we elucidate the basis for placental leptin production. The same promoter is used for adipose and placental transcription. An upstream enhancer functions in the JEG-3 and JAR choriocarcinoma cell lines but not in adipocytes or HeLa cells. The minimal positive acting region is 60 base pairs in length. This region is within a MER11 repetitive element, suggesting that human placental expression of leptin is the result of insertion of this element. Binding analyses demonstrated three protein binding sites, designated placental leptin enhancer elements (PLE)1, PLE2, and PLE3. PLE2 binds Sp1. Enhancer activity was reduced by mutation of the PLE1 or PLE3 sites but was unaffected by alteration of PLE2. Proteins binding to PLE3 were present in JEG-3 and human placental nuclear extracts but not in extracts from non-placental sources. Upon triplication, the PLE3 element was a strong enhancer in choriocarcinoma cells but not in HeLa cells. The protein binding to the PLE3 motif appears to be a novel, placenta-specific transcription factor.

Hyperleptinemia of pregnancy associated with the appearance of a circulating form of the leptin receptor

Leptin is a hormone produced in adipose cells that regulates energy expenditure, food intake, and adiposity. In mice, we observed that circulating leptin levels increase 20-40-fold during pregnancy. Pregnant ob/ob females had no detectable serum leptin, demonstrating that the heterozygous conceptus was not the source of the leptin. However, leptin RNA and protein levels in maternal adipose tissue were not elevated. The circulating leptin was in a high molecular weight complex, suggesting that the rise in leptin was due to expression of a binding protein. Indeed, quantitative assays of serum leptin binding capacity revealed a 40-fold increase, coincident with the rise in serum leptin. Leptin binding activity reached a capacity of 207 +/- 15 nmol/liter of serum at day 18 of gestation, and half-maximal binding was observed with approximately 3 nM leptin. The binding protein was purified and partially sequenced, revealing sequence identity to the extracellular domain of the leptin receptor. We found that the placenta produces large amounts of the OB-Re isoform of leptin receptor mRNA, which encodes a soluble binding protein. Thus, the extreme hyperleptinemia of late pregnancy is attributable to binding of the leptin by a secreted form of the leptin receptor made by the placenta.

Independent and additive effects of central POMC and leptin pathways on murine obesity

The lethal yellow (AY/a) mouse has a defect in proopiomelanocortin (POMC) signaling in the brain that leads to obesity, and is resistant to the anorexigenic effects of the hormone leptin. It has been proposed that the weight-reducing effects of leptin are thus transmitted primarily by way of POMC neurons. However, the central effects of defective POMC signaling, and the absence of leptin, on weight gain in double-mutant lethal yellow (AY/a) leptin-deficient (lepob/lepob) mice were shown to be independent and additive. Furthermore, deletion of the leptin gene restored leptin sensitivity to AY/a mice. This result implies that in the AY/a mouse, obesity is independent of leptin action, and resistance to leptin results from desensitization of leptin signaling.

Ob (obese) gene expression and leptin levels in Psammomys obesus

In this study we investigated ob gene expression and plasma leptin levels in Psammomys obesus (the Israeli Sand Rat), a polygenic animal model of obesity and non-insulin-dependent diabetes mellitus. The ob gene was expressed exclusively in adipocytes of Psammomys obesus. DNA sequencing revealed a high degree of homology with other species (90% with mouse, 88% with rat and 79% with human). No ob gene sequence differences were found between lean and obese Psammomys obesus, and the codon 105 mutation found in ob/ob mice was not detected. Ob gene expression in Psammomys obesus correlated with body weight (r = 0.436, p < 0.001), percent body fat (r = 0.645, p < 0.001) and plasma insulin concentration (r = 0.651, p < 0.001). This is the first time that ob gene expression has been shown to increase steadily over a continuous wide range of body weight or plasma insulin in an animal model of obesity. Ob gene expression was significantly elevated in obese compared with lean Psammomys obesus (p < 0.05). No significant difference in ob gene expression was found between the four adipose tissue depots tested. Psammomys obesus plasma leptin levels correlated with body weight (r = 0.36, p < 0.05), percent body fat (r = 0.702, p < 0.01) and plasma insulin concentration (r = 0.735, p < 0.001). Plasma leptin concentrations were significantly increased in insulin-resistant animals independent of body weight. These results show that Psammomys obesus is an excellent animal model in which to study the ob gene and leptin, and confirm the importance of insulin as a significant factor in the regulation of leptin and ob gene expression.

The leptin receptor mediates apparent autocrine regulation of leptin gene expression

The possibility that the leptin receptor (LEPR) mediates autocrine regulation of leptin expression in adipose tissue was examined in 10-day-old Zucker rat pups with different copy numbers of the leptin receptor mutation (Lepr(fa)). Plasma leptin concentrations and adipose tissue mRNA levels for leptin were related to copy number of the mutation (fa/fa > fa/+ > +/+). These relationships were independent of plasma insulin concentration. Reduced copy number for the functional leptin receptor apparently results in a diminished negative feedback signal to the leptin gene in adipose tissue. Thus, leptin appears to close a short regulatory loop controlling its own synthesis in adipose tissue.

Mice lacking bombesin receptor subtype-3 develop metabolic defects and obesity

Mammalian bombesin-like peptides are widely distributed in the central nervous system as well as in the gastrointestinal tract, where they modulate smooth-muscle contraction, exocrine and endocrine processes, metabolism and behaviour. They bind to G-protein-coupled receptors on the cell surface to elicit their effects. Bombesin-like peptide receptors cloned so far include, gastrin-releasing peptide receptor (GRP-R), neuromedin B receptor (NMB-R), and bombesin receptor subtype-3 (BRS-3). However, despite the molecular characterization of BRS-3, determination of its function has been difficult as a result of its low affinity for bombesin and its lack of an identified natural ligand. We have generated BRS-3-deficient mice in an attempt to determine the in vivo function of the receptor. Mice lacking functional BRS-3 developed a mild obesity, associated with hypertension and impairment of glucose metabolism. They also exhibited reduced metabolic rate, increased feeding efficiency and subsequent hyperphagia. Our data suggest that BRS-3 is required for the regulation of endocrine processes and metabolism responsible for energy balance and adiposity. BRS-3-deficient mice provide a useful new model for the investigation of human obesity and associated diseases.

Leptin and the regulation of body weight

Leptin has received considerable attention as a newly recognized metabolic hormone and for its potential for therapeutic use in the treatment of human obesity. Furthermore, defects in the leptin signal pathway that result in obesity in animal models have raised the possibility of a similar etiology for obesity in humans. This review will summarize the current findings on leptin in both humans and rodents. These findings will be discussed with respect to our view of the physiology and potential for pathophysiology in leptin-mediated regulation of body weight and composition.

Role of the kidney in human leptin metabolism

To assess the role of the human kidney in leptin metabolism, we measured renal leptin net balance and urinary leptin excretion in 16 normal postabsorptive volunteers with varying degrees of obesity. Arterial leptin concentrations (11.6 +/- 2.7 ng/ml) significantly exceeded renal vein concentrations (10.3 +/- 2.5 ng/ml, P < 0.001). Renal leptin fractional extraction averaged 13.1 +/- 1.1%, and renal leptin net balance (uptake) averaged 1,070 +/- 253 ng/min. Lineweaver-Burk analysis indicated that renal leptin uptake followed saturation kinetics with an apparent Michaelis-Menten constant of 10.9 ng/ml and maximal velocity of 1,730 ng/min. Leptin was generally undetectable in urine. Using literature values for systemic leptin clearance, we calculated that renal leptin uptake could account for approximately 80% of all leptin removal from plasma. These data indicate that the human kidney plays a substantial role in leptin removal from plasma by taking up and degrading the peptide.

Serum leptin concentrations in relation to pubertal development

OBJECTIVES

The amount of adipose tissue influences pubertal development and fertility in girls. A candidate for mediating this is the hormone leptin, derived from adipocytes. This work was carried out to determine whether the leptin concentration in serum is regulated during pubertal development.

SUBJECTS AND METHODS

Serum concentrations of leptin were determined by radioimmunoassay in a sample of 252 healthy children representing all pubertal stages.

RESULTS

Serum leptin concentrations correlated directly with age (r = 0.53), body mass index (BMI) (r = 0.71), and weight for height SD score (r = 0.44) in girls and with BMI (r = 0.33) and weight for height SD score in boys (r = 0.36). Leptin concentrations increased with pubertal development in girls, resulting in significantly higher concentrations at pubertal stages 4 and 5 than at the prepubertal stage, whereas there was no change in the boys.

CONCLUSIONS

Serum leptin concentrations increased during pubertal development in the girls, but remained constant in the boys. Whether the increase in serum leptin concentrations in girls is of importance for, or a consequence of, pubertal development is still to be determined.

Increased leptin messenger RNA and serum leptin levels in children with Prader-Willi syndrome and nonsyndromal obesity

To study the potential role of the ob gene pathway in childhood obesity, we have investigated leptin mRNA levels in s.c. adipose tissue obtained from nonobese prepubertal children (n = 20), obese nonsyndromal children (n = 6), and children with Prader-Willi syndrome (n = 6) by in situ hybridization histochemistry. We have also investigated the fasting serum leptin levels in such children. Compared with nonobese children, leptin mRNA expression was higher both in children with Prader-Willi syndrome and in children with nonsyndromal obesity (p < 0.01). Furthermore, the serum leptin levels were also significantly higher in both children with Prader-Willi syndrome and nonsyndromal obesity compared with the nonobese children (p < 0.001). However, no significant differences in adipose tissue leptin mRNA or serum leptin levels were observed between children with Prader-Willi syndrome and nonsyndromal obese children. As expected both fasting serum leptin levels and leptin mRNA expression levels correlated to body mass index (rs = 0.80 and 0.73, respectively, p < 0.005). No difference in leptin expression between Prader-Willi syndrome and nonsyndromal childhood obesity could be revealed in the present study. However, differences in the hypothalamic response to leptin between the two forms of obesity cannot be excluded.

Fine structure of the murine leptin receptor gene: splice site suppression is required to form two alternatively spliced transcripts

The fine structure of the murine leptin receptor gene (Lepr) is described. Duplicated ligand binding domains (conserved among cytokine receptors) are found in eight exons (coding exons 3 to 6 and 8 to 11). Thus, it is possible that a single leptin receptor molecule could have two functional ligand binding domains. The transmembrane region of Lepr is in coding exon 16 while the juxtamembrane JAK docking site is in coding exon 17. For all membrane-bound forms, the transcript must include 17 invariant exons and 1 alternatively spliced 3' terminal exon. The transcript encoding the soluble receptor (Re) includes 14 coding exons and an alternatively spliced 3' terminal exon. We have identified two splice variants (Rc and Re) for which there are no intervening sequences between the two final exons. This unusual juxtaposition of exons requires that splice donor sites at the 5' end of the respective terminal exons be ignored in the production of these splice variants. We suggest that splice site suppression is responsible for the formation of two of the alternatively spliced forms of the mouse Lepr gene. The juxtaposition of two coding exons separated by a consensus splice donor sequence is the structural substrate for this mode of alternative splicing. We present evidence that the Rc form is expressed in human tissues while the Re form, the soluble receptor, is not expressed.

Leptin normalizes the impaired response of proinsulin mRNA to long chain fatty acids in heterozygous Zucker diabetic fatty rats

To determine if underleptinization of islets of Zucker diabetic fatty (ZDF) rats is the proximal cause of their inability to compensate for obesity, we compared the proinsulin/beta-actin mRNA ratio in heterozygous (fa/+) ZDF rats with that of wild-type (+/+) and homozygous (fa/fa) ZDF rats. In +/+ islets cultured with 2 mM free fatty acids (FFA) the proinsulin mRNA ratio rose 2.4-fold at 12 h. In fa/+ islets, the ratio rose only 65% above normal. There was no change in fa/fa islets. The presence of leptin (20 ng/ml) in the culture medium increased the FFA-induced response of proinsulin mRNA of fa/+ islets to that of +/+ islets while reducing FFA incorporation into triglycerides. The leptin-induced improvement in the proinsulin mRNA response was independent of any changes in glucose usage. These findings support a causal relationship between diminished leptin action on islets and the impaired beta-cell response to FFA in ZDF rats.

Reversal of the sex difference in serum leptin levels upon cross-sex hormone administration in transsexuals

Women have higher circulating leptin levels than men. This sex difference is not simply explained by differences in the amount of body fat and is possibly influenced by their different sex steroid milieus. This prompted us to study prospectively the effects of cross-sex steroid hormones on serum leptin levels in 17 male to female transsexuals and 15 female to male transsexuals. Male to female transsexuals were treated with 100 micrograms ethinyl estradiol and 100 mg cyproterone acetate (antiandrogen) daily, and female to male transsexuals received testosterone esters (250 mg/2 weeks, im). Before and after 4 and 12 months of cross-sex hormone treatment, serum leptin levels and measures of body fatness were assessed. Before treatment, female subjects had higher serum leptin levels than male subjects independently of the amount of body fat (P < 0.01). Cross-sex hormone administration induced a reversal of the sex difference in serum leptin levels. Estrogen treatment in combination with antiandrogens in male subjects increased median serum leptin levels from 1.9 ng/mL before treatment to 4.8 ng/mL after 4 months and 5.5 ng/mL after 12 months of treatment (P < 0.0001). Testosterone administration in female subjects decreased median serum leptin levels from 5.6 to 2.6 ng/mL after 4 months and to 2.5 ng/mL after 12 months (P < 0.0001). Analysis of covariance revealed that the changes in serum leptin levels were independent of changes in body fatness in both groups (P < 0.01). In conclusion, these results indicate that sex steroid hormones, in particular testosterone, play an important role in the regulation of serum leptin levels. The prevailing sex steroid milieu, not genetic sex, is a significant determinant of the sex difference in serum leptin levels.

Leptin levels and insulin sensitivity in obese and non-obese patients with polycystic ovary syndrome

The study was conducted to assess leptin levels and insulin sensitivity in obese and non-obese patients with polycystic ovary syndrome (PCOS). Twenty-two women with PCOS and 19 control healthy women were included in the study, divided into obese and non-obese groups. Leptin was determined using Linco Research radio-immunoassay while insulin sensitivity was calculated from intravenous glucose tolerance tests with frequent blood sampling using MINMOD analysis. Significantly higher basal leptin levels were found in obese compared to non-obese PCOS (31.76 +/- 3.06 vs. 10.42 +/- 2.31 ng/ml; p < 0.05) as well as in obese in comparison to non-obese controls (29.16 +/- 5.06 vs 8.51 +/- 0.88 ng/ml; p < 0.05). A negative correlation was found between insulin sensitivity and leptin levels in both obese (r = -0.2480; p > 0.05) and non-obese PCOS groups (r = -0.4620; p > 0.05). In conclusion, high serum leptin, insulin and testosterone levels together with reduced insulin sensitivity were found in obese PCOS women, suggesting that high leptin levels could be a characteristic of the obese PCOS phenotype.

Prolonged exercise decreases serum leptin concentrations

Serum leptin and free fatty acid concentrations were determined in two groups of subjects undergoing strenuous exercise: 12 men who fasted overnight and then pedaled a stationary ergometer for 2 hours, and 14 nonfasting ultramarathon runners. Blood samples were collected before exercise, immediately after cessation of exercise, and 6 to 24 hours after the end of the exercise period. Two hours of strenuous pedaling following an overnight fast significantly reduced mean leptin levels by 8.3%; free fatty acids were highly increased and correlated well with the decrease in serum leptin (r = .737, P = .01). After 6 hours of rest and refeeding, leptin concentrations recovered to preexercise levels and free fatty acid concentrations were decreased to less than preexercise levels. A similar decrease in serum leptin levels (12.3%) occurred in subjects who fasted overnight and then for a period corresponding to the cycle exercise period. The prolonged exercise of an ultramarathon significantly reduced leptin concentrations by 32% in comparison to prerace levels; free fatty acid concentrations were highly increased, but did not correlate with the change in serum leptin concentrations (r = .366, P = .20). Leptin and free fatty acid concentrations all trended toward prerace levels in blood samples collected 18 to 24 hours after cessation of racing. The results suggest that the negative energy balance of exercise can reduce serum leptin concentrations, but that the significant decrease occurs only at extremes of severity/duration of the exercise-induced negative balance. The possible physiological role of reduced leptin concentrations in response to energy balance and the role of free fatty acids in mediating the response are discussed.

Constitutive and impaired signaling of leptin receptors containing the Gln --> Pro extracellular domain fatty mutation

Leptin (OB), an adipocyte-secreted circulating hormone, and its receptor (OB-R) are key components of an endocrine loop that regulates mammalian body weight. In this report we have analyzed signal transduction activities of OB-R containing the fatty mutation [OB-R(fa)], a single amino acid substitution at position 269 (Gln --> Pro) in the OB-R extracellular domain that results in the obese phenotype of the fatty rat. We find that this mutant receptor exhibits both ligand-independent transcriptional activation via interleukin 6 and hematopoietin receptor response elements and ligand-independent activation of signal transducer and activator of transcription (STAT) proteins 1 and 3. However, OB-R(fa) is unable to constitutively activate STAT5B and is highly impaired for ligand induced activation of STAT5B compared with OB-R(wt). Introduction of the fatty mutation into a OB-R/G-CSF-R chimera generates a receptor with constitutive character that is similar but distinct from that of OB-R(fa). Constitutive mutant OB-R(fa) receptor signaling is repressed by coexpression of OB-R(wt). The implications of an extracellular domain amino acid substitution generating a cytokine receptor with a partially constitutive phenotype are discussed both in terms of the mechanism of OB-R triggering and the biology of the fatty rat.

Development of obesity and insulin resistance in the Israeli sand rat (Psammomys obesus). Does leptin play a role?

The Israeli Sand Rat (Psammomys obesus) is an excellent polygenic model for the study of obesity and diabetes. The metabolic characteristics and the heterogeneous development of these defects, including elevated leptin levels, mimic those found in susceptible human populations. Interestingly, only animals that develop metabolic abnormalities demonstrate hyperleptinemia and, in these animals, leptin administration at the same dose that is effective in ob/ob mice is ineffective in reducing food intake or body weight. Perhaps leptin resistance needs to develop in Israeli Sand Rats to allow the development of obesity and, in fact, leptin resistance may be the "thrifty gene" that predisposes individuals to the development of obesity and subsequent metabolic abnormalities. However, there remain many unanswered questions about the physiological actions of leptin. The widespread tissue location of receptors and the actions of leptin independent of food intake highlight the need for further research aimed at determining the major physiological action of this newly discovered and exciting hormone.

Developmental changes in ob gene expression and circulating leptin peptide concentrations

We examined the developmental changes in murine white and brown adipose tissue leptin and circulating immunoreactive total leptin concentrations. The approximately 4.4 kb leptin mRNA levels were higher at 2 and 7d postnatal ages, but declined to adult levels by the 14d stage and remained so until 160d. Paralleling the mRNA concentrations, leptin peptide levels also were higher at 2d, 7d, and 14d, declining to adult values by the 21d weaning stage. No difference in mRNA levels was observed between brown-enriched and white adipose tissue. No sexual dimorphism was observed in the leptin mRNA or peptide levels between 14 and 160d; however, at 2 and 7d, while no sex related differences were observed in the peptide levels, adipose mRNA concentrations were mildly higher in males than in the females. We conclude that leptin mRNA and peptide levels are higher during consumption of a high fat milk diet. High levels of leptin with increasing food intake and body weight gain signify hypothalamic leptin receptor resistance during the immediate postnatal period.

Plasma leptin and acute serotoninergic stimulation of the corticotropic axis in women who are normal weight or obese

In some recent studies, glucocorticoid treatment was associated with rapid induction of obese (ob) gene expression in adipose tissue of normal rats and in isolated adipocytes. We studied the effect of acute stimulation of the corticotropic axis on plasma leptin, the ob gene product, in 7 women of normal weight and 12 women with obesity. Under double-blind, placebo-controlled conditions, a single 12.5-mg dose of clomipramine, a serotonin uptake inhibitor, was administered intravenously in 15 minutes. Mean basal plasma leptin was increased more than 3-fold in subjects with obesity compared with subjects of normal weight (35.1 +/- 4.9 ng/mL vs. 8.9 +/- 1.4 ng/mL, p = 0.001). Whereas corticotropin (ACTH) and cortisol responses were increased in women who were obese compared with women who were lean, no significant effect of clomipramine infusion was found on plasma leptin concentrations measured during the following 150 minutes in both groups. There was a strong positive correlation between basal plasma leptin concentrations and body mass index (r = 0.92, p < 0.0001). In six subjects with obesity studied after a moderate weight loss, mean basal plasma leptin was significantly decreased (43.7 +/- 6.4 ng/mL before vs. 28.0 +/- 8.1 ng/mL after, p = 0.04), but the hormonal response pattern to clomipramine administration was unchanged. We conclude that, at least in the short term, an acute stimulation of the corticotropic axis does not seem to increase leptin secretion in humans, as shown by the response to the serotoninergic agent clomipramine.

Relationship between circulating leptin and energy expenditure in adult men and women aged 18 years to 81 years

Recent studies suggest that leptin may be an important metabolic signal for energy regulation in rodents, but the role of leptin in human energy regulation remains uncertain. Because adaptive variations in energy expenditure play an important role in human energy regulation, we investigated the relationship between leptin and energy expenditure parameters in 61 weight-stable men and women aged 18 years to 81 years who were not obese. Measurements were made of circulating leptin in the fasting state, body fat and fat free mass, resting metabolic rate (n = 61), free-living total energy expenditure (n = 52), and the thermic effect of feeding (n = 33). After statistically accounting for age, body fat, and fat free mass, there was no association between leptin and any measured energy expenditure parameter. In addition, there was no effect of age on the relationship between circulating leptin and body fat mass. These results indicate that physiological variations in circulating leptin are not linked with adaptive variations in energy expenditure in humans, in contrast to indications of this phenomenon in the ob/ob mouse.

Effects of prolonged hyperinsulinemia on serum leptin in normal human subjects

We have studied the effect of prolonged hyperinsulinemia and hyperglycemia on serum leptin levels in young nonobese males during 72-h euglycemic-hyperinsulinemic and hyperglycemic ( approximately 8.5 and 12.6 mM) clamps. Hyperinsulinemia increased serum leptin concentrations (by RIA) dose-dependently. An increase in serum insulin concentration of > 200 pM for > 24 h was needed to significantly increase serum leptin. An increase of approximately 800 pM increased serum leptin by approximately 70% over 72 h. Changes in plasma glucose concentrations (from approximately 5.0 to approximately 12.6 mM) or changes in plasma FFA concentrations (from < 100 to > 1,000 microM) had no effect on serum leptin. Serum leptin concentrations changed with circadian rhythmicity. The cycle length was approximately 24 h, and the cycle amplitude (peak to trough) was approximately 50%. The circadian leptin cycles and the circadian cycles of total body insulin sensitivity (i.e., GIR, the glucose infusion rates needed to maintain euglycemia during hyperinsulinemic clamping) changed in a mirror image fashion. Moreover, GIR decreased between Days 2 and 3 (from 11.4+/-0.2 to 9. 8+/-0.2 mg/kg min, P< 0.05) when mean 24-h leptin levels reached a peak. In summary, we found (a) that 72 h of hyperinsulinemia increased serum leptin levels dose-dependently; (b) that hyperglycemia or high plasma FFA levels did not affect leptin release; (c) that leptin was released with circadian rhythmicity, and (d) that 24-h leptin cycles correlated inversely with 24-h cycles of insulin sensitivity. We speculate that the close positive correlation between body fat and leptin is mediated, at least in part, by insulin.

Sympathetic and cardiorenal actions of leptin

Body weight is tightly regulated physiologically. The recent discovery of the peptide hormone leptin has permitted more detailed evaluation of the mechanisms responsible for control of body fat. Leptin is almost exclusively produced by adipose tissue and acts in the CNS through a specific receptor and multiple neuropeptide pathways to decrease appetite and increase energy expenditure. Leptin thus functions as the afferent component of a negative feedback mechanism to control adipose tissue mass. Increasing evidence suggests that leptin may have wider actions influencing autonomic, cardiovascular, and endocrine function. Intravenous leptin increases norepinephrine turnover and sympathetic nerve activity to thermogenic brown adipose tissue. Studies from our laboratory suggest that leptin also increases sympathetic nerve activity to kidney, hindlimb, and adrenal gland. However, systemic administration of leptin does not acutely increase arterial pressure or heart rate in anesthetized animals. Thus, longer-term exposure to hyperleptinemia may be necessary for full expression of the expected pressor effect of renal sympathoexcitation. Alternatively, leptin may have additional cardiovascular actions to oppose sympathetically mediated vasoconstriction. Leptin in high doses increases renal sodium and water excretion, apparently through a direct tubular action. In addition, leptin appears to increase systemic insulin sensitivity, even in the absence of weight loss. Although we are at an early stage of understanding, we speculate that abnormalities in the actions of leptin may have implications for the sympathetic, cardiovascular, and renal changes associated with obesity.

Effects of intracerebroventricular infusion of leptin in obese Zucker rats

The obese Zucker rat (OZR) exhibits a missense mutation in the cDNA for the leptin receptor, producing a single amino acid substitution in the extracellular domain of the receptor. A mutation in the leptin receptor gene of the db/db mouse prevents the synthesis of the long splice variant of the receptor. The possibility that the OZR, like the db/db mouse, is refractory to the actions of murine leptin was tested by infusing the protein intracerebroventricularly via a minipump for 7 days. Lean Zucker rats (LZR) infused with leptin acted as positive controls, and other groups of OZR and LZR were infused with vehicle. In LZR, leptin reduced bodyweight and food intake and increased brown adipose tissue (BAT) temperature. Plasma corticosterone increased (61%) in these rats, and plasma triglycerides fell (78%). Leptin treatment improved tolerance to an oral glucose load (16% reduction in the area under the blood glucose curve) while lowering plasma insulin. In OZR, the actions of leptin were blunted. Food intake was slightly, but not significantly, reduced. Although there was a reduction in the rate of increase in body mass, the effect of leptin was about half that seen in LZR. BAT temperature and glucose tolerance were unchanged. In contrast to the elevated plasma corticosterone seen in LZR, leptin reduced the level of this hormone (27%) in OZR. In OZR and LZR treated with leptin, the plasma leptin levels were increased 24-fold and 47-fold, respectively. The results suggest that leptin retains some efficacy in OZR, although these rats are less responsive than LZR.

Plasma leptin responses to fasting in Pima Indians

Leptin is believed to play a role in the regulation of energy balance, but little is known about factors influencing plasma leptin concentrations. To determine the effect of short-term changes in energy balance, we measured plasma leptin concentrations as well as plasma glucose, insulin, triglyceride, nonesterified fatty acid concentrations, and metabolic rate in response to a standard test meal followed by a 24-h fast in 21 healthy Pima Indians. Plasma leptin concentrations decreased by 8% (P < 0.05) 2-4 h after the test meal. They returned to baseline 6-12 h after the subjects ate, then subsequently decreased, and, by the end of the fast, were an average of 37% below baseline (P < 0.0001). Changes in plasma leptin concentrations did not correlate with changes in plasma glucose, insulin, triglyceride, or nonesterified fatty acid concentrations or with changes in metabolic rate. The results of this study indicate that plasma leptin concentrations decrease in response to short-term energy restriction. These changes were not due to changes in glucose, insulin, triglycerides, or nonesterified fatty acids, nor did they relate to changes in metabolic rate. The decrease in plasma leptin concentrations with fasting may be an important homeostatic response to an energy deficit, stimulating food intake and thus restoring energy balance.

Nonadipose tissue production of leptin: leptin as a novel placenta-derived hormone in humans

Leptin is a circulating hormone that is expressed abundantly and specifically in the adipose tissue. It is involved in the regulation of energy homeostasis, as well as the neuroendocrine and reproductive systems. Here, we demonstrate production of leptin by nonadipose tissue, namely, placental trophoblasts and amnion cells from uteri of pregnant women. We show that pregnant women secrete a considerable amount of leptin from the placenta into the maternal circulation as compared with nonpregnant obese women. Leptin production was also detected in a cultured human choriocarcinoma cell line, BeWo cells, and was augmented during the course of forskolin-induced differentiation of cytotrophoblasts into syncytiotrophoblasts. Plasma leptin levels were markedly elevated in patients with hydatidiform mole or choriocarcinoma and were reduced after surgical treatment or chemotherapy. Leptin is also produced by primary cultured human amnion cells and is secreted into the amniotic fluid. The present study provides evidence for leptin as a novel placenta-derived hormone in humans and suggests the physiologic and pathophysiologic significance of leptin in normal pregnancy and gestational trophoblastic neoplasms.

The effect of a desogestrel-containing oral contraceptive on glucose tolerance and leptin concentrations in hyperandrogenic women

Ovarian hyperandrogenism can be associated with insulin resistance, hyperinsulinemia, glucose intolerance, and obesity. High levels of the lipostatic hormone, leptin, have also been reported in this condition. The purpose of the present study was to examine the effect of an oral contraceptive (OC) of low androgenicity containing desogestrel on glucose tolerance in hyperandrogenic women and the impact of changes in androgenic/estrogenic status on leptin concentrations. Sixteen nondiabetic hyperandrogenic women, aged 29 +/- 1 yr with a body mass index (BMI) of 36.8 +/- 1.8 kg/m2, underwent an oral glucose tolerance test before and after 6 months of therapy with the OC. Free testosterone decreased and sex hormone-binding globulin increased after therapy (P < 0.001). Glucose tolerance deteriorated significantly, and two women developed diabetes. Body weight, BMI, and leptin did not change significantly. Leptin correlated with BMI before (r = 0.56; P = 0.02) and after (r = 0.51; P = 0.04) treatment, but not with glucose, insulin, total and free testosterone, or sex hormone-binding globulin before or after treatment. In conclusion, 1) glucose tolerance should be monitored in hyperandrogenic women using OC, even those of low androgenicity; and 2) changes in androgenic/estrogenic status had no effect on the leptin concentration, suggesting that its sexual dimorphism is not related to sex steroids.

Plasma leptin levels in healthy children and adolescents: dependence on body mass index, body fat mass, gender, pubertal stage, and testosterone

Leptin, the product of the ob gene, is thought to play a key role in the regulation of body fat mass. Beyond this function, it appears to be an integral component of various hypothalamo-pituitary-endocrine feedback loops. Because childhood and puberty are periods of major metabolic and endocrine changes, leptin levels and various hormonal parameters were investigated in a large cohort of healthy children and adolescents (312 males, 401 females, age 5.8-19.9 yr). For this purpose, a specific and sensitive RIA was developed that allowed the accurate measurement of low leptin levels in young lean children. With this assay, leptin proved to be a comparatively stable protein under common conditions of blood sampling and storage. Leptin levels increased in girls with age (r = 0.47, P < 0.0001), but decreased in boys (r = -0.34, P < 0.0001). An analysis according to pubertal stage showed a steady increase in girls between 2.51 micrograms/L (median) at Tanner stage 1 to 6.24 micrograms/L at Tanner stage 5. In boys, leptin levels were highest at Tanner stage 2 (2.19 micrograms/L) and declined thereafter to 0.71 microgram/L at Tanner stage 5. A strong exponential relationship was observed for leptin levels with body mass index (BMI) and percentage body fat as determined by bioelectric impedance measurements in a subgroup of subjects. This relationship was similar between boys and girls at Tanner stages 1 and 2. In boys, there was a significant decline of leptin at a given BMI with further progression of puberty that was much less pronounced in girls. Although the relative increase of leptin with BMI and percent body fat was the same in both genders, the absolute values at a given BMI or percent body fat were significantly lower in boys in late puberty and in adolescents. In boys, but not in girls, there was an inverse correlation with testosterone concentrations (r = -0.43, P < 0.0001), which explained 10.5% of the variation of leptin levels in a multiple regression model. Since BMI proved to be the major influencing variable, reference ranges were constructed using a best-fit regression line of the form leptin = a*e(b*BMI) and stratifying ranges according to gender and pubertal stage. In conclusion, these data suggest that 1) plasma leptin levels increase in girls and decrease in boys after Tanner stage 2 as the pubertal development proceeds; 2) they show a significant gender difference especially in late puberty and adolescence, even after adjustment for BMI or percent body fat; 3) the lower levels in males may be explained at least in part by a suppressive effect of androgens; 4) reference ranges with BMI as the independent variable should be stratified according to gender and pubertal stage.

Leptin constrains acetylcholine-induced insulin secretion from pancreatic islets of ob/ob mice

Hypersecretion of insulin from the pancreas is among the earliest detectable metabolic alterations in some genetically obese animals including the ob/ob mouse and in some obesity-prone humans. Since the primary cause of obesity in the ob/ob mouse is a lack of leptin due to a mutation in the ob gene, we tested the hypothesis that leptin targets a regulatory pathway in pancreatic islets to prevent hypersecretion of insulin. Insulin secretion is regulated by changes in blood glucose, as well as by peptides from the gastrointestinal tract and neurotransmitters that activate the pancreatic islet adenylyl cyclase (e.g., glucagon-like peptide-1) and phospholipase C (PLC) (e.g., acetylcholine) signaling pathways to further potentiate glucose-induced insulin secretion. Effects of leptin on each of these regulatory pathways were thus examined. Leptin did not influence glucose or glucagon-like peptide-1-induced insulin secretion from islets of either ob/ob or lean mice, consistent with earlier findings that these regulatory pathways do not contribute to the early-onset hypersecretion of insulin from islets of ob/ob mice. However, leptin did constrain the enhanced PLC- mediated insulin secretion characteristic of islets from ob/ob mice, without influencing release from islets of lean mice. A specific enhancement in PLC-mediated insulin secretion is the earliest reported developmental alteration in insulin secretion from islets of ob/ob mice, and thus a logical target for leptin action. This action of leptin on PLC-mediated insulin secretion was dose-dependent, rapid-onset (i.e., within 3 min), and reversible. Leptin was equally effective in constraining the enhanced insulin release from islets of ob/ob mice caused by protein kinase C (PKC) activation, a downstream mediator of the PLC signal pathway. One function of leptin in control of body composition is thus to target a PKC-regulated component of the PLC-PKC signaling system within islets to prevent hypersecretion of insulin.

Interleukin 1 alpha increases serum leptin concentrations in humans

Leptin, the protein product of the ob gene, regulates appetite and body weight in animals. Endotoxin and cytokines, induced by endotoxin, interleukin (IL) 1 and tumor necrosis factor, increase expression of leptin in mice and hamsters. We measured serum leptin concentrations in patients with cancer before and after administration of recombinant human IL-1 alpha. Fourteen patients received IL-1 alpha at one of three dose levels (0.03, 0.1, or 0.3 microgram/kg.day) for 5 days. Serum leptin concentrations increased in all but two patients within 24 h after the first dose. The increase in leptin was correlated directly with IL-1 alpha dose (P = 0.0030). Despite continued administration of IL-1 alpha, serum leptin concentrations returned to pretreatment levels by day 5 of therapy. An increase in serum leptin concentrations may be one mechanism by which anorexia is induced by IL-1 alpha. However, tachyphylaxis of the leptin response suggests that other mechanisms also are involved.

Contribution of androgens to the gender difference in leptin production in obese children and adolescents

Recent studies demonstrated significantly higher serum leptin concentrations in females as compared with males, even after correction for differences in body fat mass. The aim of our study was to measure serum leptin concentrations in a large group of obese children and adolescents to determine the possible role of sex steroid hormones on both leptin serum concentrations and production in human adipocytes. Obese girls were found to have significantly higher leptin concentrations than boys at the same degree of adiposity (25.2+/-14.1 vs. 17.2+/-12.6 ng/ml, P < 0.001). In a multiple regression analysis with age and body mass index (percent body fat) as fixed variables, it turned out that testosterone had a potent negative effect on serum leptin in boys, but not in girls. In vitro experiments using newly developed human adipocytes in primary culture showed that both testosterone and its biologically active metabolite dihydrotestosterone are able to reduce leptin secretion into the culture medium by up to 62%. Using a semiquantitative reverse transcriptase-PCR method, testosterone was found to suppress leptin mRNA to a similar extent. These results suggest that, apart from differences in body fat mass, the higher androgen concentrations in obese boys are responsible for the lower leptin serum concentrations compared with obese girls.

Physiological response to long-term peripheral and central leptin infusion in lean and obese mice

Recent data have identified leptin as an afferent signal in a negative-feedback loop regulating the mass of the adipose tissue. High leptin levels are observed in obese humans and rodents, suggesting that, in some cases, obesity is the result of leptin insensitivity. This hypothesis was tested by comparing the response to peripherally and centrally administered leptin among lean and three obese strains of mice: diet-induced obese AKR/J, New Zealand Obese (NZO), and Ay. Subcutaneous leptin infusion to lean mice resulted in a dose-dependent loss of body weight at physiologic plasma levels. Chronic infusions of leptin intracerebroventricularly (i.c.v.) at doses of 3 ng/hr or greater resulted in complete depletion of visible adipose tissue, which was maintained throughout 30 days of continuous i.c.v. infusion. Direct measurement of energy balance indicated that leptin treatment did not increase total energy expenditure but prevented the decrease that follows reduced food intake. Diet-induced obese mice lost weight in response to peripheral leptin but were less sensitive than lean mice. NZO mice were unresponsive to peripheral leptin but were responsive to i.c.v. leptin. Ay mice did not respond to subcutaneous leptin and were 1/100 as sensitive to i.c.v. leptin. The decreased response to leptin in diet-induced obese, NZO, and Ay mice suggests that obesity in these strains is the result of leptin resistance. In NZO mice, leptin resistance may be the result of decreased transport of leptin into the cerebrospinal fluid, whereas in Ay mice, leptin resistance probably results from defects downstream of the leptin receptor in the hypothalamus.

Deficits in reproductive behavior in diabetic female rats are due to hypoinsulinemia rather than hyperglycemia

These studies determined whether deficits in reproductive behavior observed in streptozotocin (STZ)-induced diabetic female rats are caused by hyperglycemia or loss of insulin. Female Sprague-Dawley rats were ovariectomized and made diabetic by a single ip injection of STZ (75 mg/kg). Reproductive behavior was measured 12 days after the onset of hyperglycemia following the injection of estrogen and progesterone in doses known to restore reproductive behavior in nondiabetic rats. Rats in which STZ produced diabetes showed significantly reduced receptive and proceptive sexual behaviors. Normalization of blood glucose levels either by restricting diet or by phlorizin treatment failed to restore reproductive behavior in diabetic animals. However, even doses of insulin which were not fully effective in correcting peripheral hyperglycemia were able to prevent the STZ-induced behavioral deficit. No changes in general activity were observed in any experimental group as assessed by open field activity. The density of the norepinephrine transporter, as measured by [3H]nisoxetine binding, was reduced in the cortex but not in the brain stem, hypothalamus, or hippocampus of diabetic animals. Insulin treatment prevented the loss of cortical [3H]nisoxetine binding, and even partial normalization of blood glucose restored cortical [3H]nisoxetine binding to control levels. These findings suggest that diabetes-induced reproductive deficits are due to hypoinsulinemia and cannot be corrected simply by the normalization of blood glucose, whereas reductions in the density of cortical norepinephrine transporter result from hyperglycemia.

Glucocorticoid regulation of leptin synthesis and secretion in humans: elevated plasma leptin levels in Cushing's syndrome

Leptin, the obese (ob) gene product, is an adipocyte-derived satiety factor that is involved in the regulation of food ingestion and body weight. To investigate glucocorticoid regulation of leptin synthesis and secretion in humans, we measured plasma leptin levels in patients with Cushing's syndrome with adrenal or pituitary adenoma and in patients with iatrogenic Cushing's syndrome. Plasma leptin levels in patients with Cushing's syndrome were significantly elevated compared to those in nonobese healthy subjects and obese subjects without any metabolic or endocrine diseases at a given percentage of body fat by analysis of covariance. In patients with adrenal or pituitary adenoma, after the tumor resection, plasma leptin levels were reduced, with a concurrent decrease in plasma cortisol levels. With no significant changes in body weight, plasma leptin levels were also elevated significantly in lean healthy volunteers 24 h after the administration of 1 mg dexamethasone. Dexamethasone potently induced ob gene expression and leptin secretion in the organ culture of human adipose tissue. The data demonstrate that glucocorticoids act, at least in part, directly on the adipose tissue and increase leptin synthesis and secretion in humans.

Serum leptin levels are independently correlated with two measures of HDL

Leptin is the peptide product of the OB gene, which is associated with obesity in some strains of mice. Because dyslipidemias are frequently associated with obesity, we have begun to characterize the pathways connecting these related traits. In this investigation we tested for correlation of HDL phenotype measures with leptin concentrations using data from 1159 participants in the San Antonio Family Heart Study, a study of risk factors for cardiovascular disease in Mexican Americans living in and around San Antonio, Texas. In a subset of 288 unrelated individuals, we tested for correlation of leptin with nine different measures of HDL phenotype and found that only three were significantly related. However, stepwise regression analysis suggested that only two measures, HDL triglyceride concentrations (HDL-TG) and the proportion of apo A-I on HDL particles larger than HDL3 (Large HDL-apo A-I), were independently correlated with leptin. Because obesity and HDL phenotypes are both under strong genetic control, we conducted a trivariate genetic analysis, using the entire data set, to test the hypothesis that the phenotypic correlations were due to the effects of shared genes (i.e., pleiotropy). Heritabilities for the three traits were estimated to be 0.47 for leptin, 0.46 for HDL-TG, and 0.46 for Large HDL-apo A-I. Results from the genetic analyses revealed that the phenotypic correlation of leptin with HDL-TG was nongenetic (i.e., shared environment), while the phenotypic correlation with Large HDL-apo A-I was due to pleiotropy (i.e., shared genes). These results confirmed the result derived from the subset of unrelated individuals that the two measures of HDL are independently correlated with leptin. To our knowledge, this is the first report of a relationship between leptin and any aspect of lipoprotein phenotype. A better understanding of the genes responsible for this relationship may provide a molecular explanation for the aggregation of atherogenic phenotypes, such as diabetes, obesity, and dyslipoproteinemia.

Leptin inhibits glycogen synthesis in the isolated soleus muscle of obese (ob/ob) mice

The ob gene product, leptin, causes significant and dose-dependent inhibition of basal and insulin-stimulated glycogen synthesis in isolated soleus muscle from ob/ob mice, and a smaller, non-significant inhibition in muscle from wild-type mice. Leptin had no inhibitory effect on glycogen synthesis in soleus muscle from the diabetic (db/db) mice, which lack the functional leptin receptor. The full-length leptin receptor (Ob-Rb), is expressed in soleus muscle of both ob/ob and wild-type mice, however with no detectable differences in expression level. These results suggest that hyperleptinaemia may attenuate insulin action on glucose storage in skeletal muscle.

Relationship between muscle sympathetic nerve activity and plasma leptin concentration

In humans, basal muscle sympathetic nerve activity (MSNA), a direct measure of sympathetic nervous outflow, is correlated with percentage of body fat. The underlying physiological mechanism is unknown. On the basis of the observation that leptin increases sympathetic nervous outflow in the ob/ob mouse, we hypothesized that leptin, a hormone secreted by the adipose tissue, may act as a peripheral signal to increase sympathetic nervous outflow from the central nervous system. We therefore tested whether basal MSNA is correlated with plasma leptin concentration. Fasting plasma samples and recordings of basal MSNA in the peroneal nerve were obtained from 37 healthy, nondiabetic men (35 whites and 2 Mexican-Americans; 29 +/- 7 years, 86 +/- 14 kg, 24 +/- 10% body fat; mean +/- SD) who were fed a weight-maintenance diet on a metabolic ward. As expected, plasma leptin concentration (geometric mean, 6.4 ng/mL; 95% confidence interval, 4.6 ng/mL to 9.0 ng/mL) correlated with % body fat (r = 0.93, p < 0.001). Basal MSNA was 31.6 +/- 10.0 bursts/min and correlated with % body fat (r = 0.53, p < 0.001) and with plasma leptin concentration (r = 0.44, p < 0.01). In conclusion, the results demonstrate a correlation between MSNA and plasma leptin concentration of a magnitude similar to that between MSNA and % body fat. Leptin may therefore be the peripheral signal explaining the correlation between MSNA and % body fat. A full understanding of the relationship between leptin and the activity of the sympathetic nervous system requires further studies, including the administration of leptin in humans.

Response of serum leptin concentrations to 7 d of energy restriction in centrally obese African Americans with impaired or diabetic glucose tolerance

The aim of this study was to determine whether serum leptin concentrations are reduced in response to short-term energy restriction in centrally obese individuals with impaired glucose tolerance or non-insulin-dependent diabetes mellitus. Twenty African Americans [16 females and 4 males, 44 +/- 7 y (x +/- SD), 107.2 +/- 23.8 kg, 39 +/- 7% body fat] consumed a 7-d energy-restricted diet (4.03 +/- 0.72 MJ/d) of whole foods. Oral-glucose-tolerance tests (OGTTs) were performed before and immediately after the diet to assess changes in serum leptin, glucose, and insulin concentrations. Baseline leptin concentration correlated significantly with percentage body fat (r = 0.80), body mass index (r = 0.72), fat mass (4 = 0.64), waist-height ratio (r = 0.6), body weight (r = 0.59, all P < 0.01), waist circumference (r = 0.49), and basal insulin concentration (r = 0.48, both P < 0.05). Seven days of energy restriction resulted in significant reductions (P < 0.005) in leptin (-6.1 +/- 8.4 micrograms/L), basal glucose (-0.9 +/- 0.8 mmol/L), OGTT glucose area under the curve (-158 +/- 164 mmol/L), and basal insulin concentration (-34 +/- 69 pmol/L, P < 0.05). In addition, there was a trend for a reduction in OGTT insulin area under the curve (-15,567 +/- 3,658 pmol/L, P = 0.05), and a tendency for basal insulin and leptin to change together (r = 0.41, P = 0.07). Despite the weight loss of 3.1 +/- 1.3 kg (P < 0.0001), the loss of fat mass was calculated to be only -1.0 +/- 0.1 kg. These results suggest that negative energy balance or improved insulin action was responsible for the changes in leptin, glucose, and insulin concentrations. In summary, short-term energy restriction effectively reduced serum leptin concentrations and improved glucose tolerance and insulin action in obese individuals with impaired or diabetic glucose tolerance.

Acute and chronic effects of exercise on leptin levels in humans

The acute (single bout of exercise) and chronic (exercise training) effects of exercise on plasma leptin were investigated in 97 sedentary adult men (n = 51) and women (n = 46) participating in the HERITAGE Family Study. Exercise training consisted of a standardized 20-wk endurance training program performed in the laboratory on a computer-controlled cycle ergometer. Maximal oxygen uptake, body composition assessed by hydrostatic weighing, and fasting insulin level were also measured before and after training. Pre- and posttraining blood samples were obtained before and after completion of a maximal exercise test on the cycle ergometer. Exercise training resulted in significant changes in maximal oxygen uptake (increase in both genders) and body composition (reduction of fat mass in men and increase in fat-free mass in women). There were considerable interindividual differences in the leptin response to acute and chronic effects of exercise, some individuals showing either increase or reduction in leptin, others showing almost no change. On average, leptin levels were not acutely affected by exercise. After endurance training was completed, leptin levels decreased significantly in men (from 4.6 to 3.9 ng/ml; P = 0.004) but not in women. However, after the training-induced changes in body fat mass were accounted for, the effects of exercise training were no longer significant. Most of the variation observed in leptin levels after acute exercise or endurance training appears to be within the confidence intervals of the leptin assay. We conclude that there are no meaningful acute or chronic effects of exercise, independent of the amount of body fat, on leptin levels in humans.

Serum leptin is increased in growth hormone-deficient adults: relationship to body composition and effects of placebo-controlled growth hormone therapy for 1 year

The gene product from the ob gene, leptin, has recently been characterized in humans. The circulating level of leptin is related to body mass index (BMI) and more closely to estimates of total body fat, whereas visceral fat has been reported to be of minor importance. However, it is unknown if leptin is directly regulated by hormones that influence substrate metabolism and body composition. We studied leptin in adult growth hormone (GH)-deficient (GHD) patients substituted with GH treatment for 12 months in a parallel double-blind, placebo-controlled study. Twenty-seven GHD adults aged 44.9 +/- 1.9 years underwent anthropometric measurements for determination of regional and total body fat (BMI, waist to hip ratio [WHR], computed tomographic [CT] scan, dual-energy x-ray absorptiometry [DEXA] scan, and bioimpedance analysis [BIA]) before and after 12 months of placebo-controlled GH substitution (2 IU/m2) in a parallel design. The same measurements were performed in 42 healthy adults aged 39.1 +/- 1.7 years. The logarithm of serum leptin levels correlated positively with abdominal subcutaneous fat and total body fat (BIA and DEXA) in untreated GHD patients and healthy subjects. Fasting insulin did not correlate with leptin levels in either of the groups. After 12 months of GH administration, the body composition of GHD patients was significantly changed with respect to a marked decrease in body fat. The relations of leptin to the estimates of body fat were maintained, and leptin was furthermore related to BMI and fasting insulin. In multiple linear regression analyses, additional estimates of visceral adiposity (intraabdominal fat and maximal anterior-posterior diameter determined by CT scan) were significant determinants of leptin in the healthy subjects. The increase in fasting insulin levels during GH substitution correlated negatively with the reduction in leptin levels (r = -.823, P = .003). At baseline, leptin levels were increased in the patients compared with controls in both sexes (women, 21.8 +/- 3.3 v 11.3 +/- 1.4 ng/mL, P = .002; men, 8.1 +/- 1.2 v 4.7 +/- 0.7 ng/mL, P = .008). Leptin levels were similar in GHD patients treated for 12 months compared with healthy controls for both women and men (women, 15.9 +/- 2.3 and 11.3 +/- 1.4 ng/mL, P = .163; men, 7.1 +/- 2.8 and 4.7 +/- 0.7 ng/mL, P = .759). In healthy adults and in GHD patients, leptin levels were significantly higher in women than in men (11.3 +/- 1.4 v 4.7 +/- 0.7 ng/mL, P < .001; 21.8 +/- 3.3 v 8.1 +/- 1.2 ng/mL, P < .001). Gender remained a significant determinant of leptin levels in several models of multiple linear regression analysis also including age, estradiol levels, insulin, and estimates of body fat. We conclude that leptin is increased but not differently regulated in GHD patients compared with normal subjects, and that leptin levels are closely related to estimates of body fat. This relationship is maintained during a decrease in body fat due to GH substitution.

Congenital leptin deficiency is associated with severe early-onset obesity in humans

The extreme obesity of the obese (ob/ob) mouse is attributable to mutations in the gene encoding leptin, an adipocyte-specific secreted protein which has profound effects on appetite and energy expenditure. We know of no equivalent evidence regarding leptin's role in the control of fat mass in humans. We have examined two severely obese children who are members of the same highly consanguineous pedigree. Their serum leptin levels were very low despite their markedly elevated fat mass and, in both, a homozygous frame-shift mutation involving the deletion of a single guanine nucleotide in codon 133 of the gene for leptin was found. The severe obesity found in these congenitally leptin-deficient subjects provides the first genetic evidence that leptin is an important regulator of energy balance in humans.

Ciliary neurotrophic factor corrects obesity and diabetes associated with leptin deficiency and resistance

Receptor subunits for the neurocytokine ciliary neurotrophic factor (CNTF) share sequence similarity with the receptor for leptin, an adipocyte-derived cytokine involved in body weight homeostasis. We report here that CNTF and leptin activate a similar pattern of STAT factors in neuronal cells, and that mRNAs for CNTF receptor subunits, similarly to the mRNA of leptin receptor, are localized in mouse hypothalamic nuclei involved in the regulation of energy balance. Systemic administration of CNTF or leptin led to rapid induction of the tis-11 primary response gene in the arcuate nucleus, suggesting that both cytokines can signal to hypothalamic satiety centers. Consistent with this idea, CNTF treatment of ob/ob mice, which lack functional leptin, was found to reduce the adiposity, hyperphagia, and hyperinsulinemia associated with leptin deficiency. Unlike leptin, CNTF also reduced obesity-related phenotypes in db/db mice, which lack functional leptin receptor, and in mice with diet-induced obesity, which are partially resistant to the actions of leptin. The identification of a cytokine-mediated anti-obesity mechanism that acts independently of the leptin system may help to develop strategies for the treatment of obesity associated with leptin resistance.