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

Induction by leptin of uncoupling protein-2 and enzymes of fatty acid oxidation

We have studied mechanisms by which leptin overexpression, which reduces body weight via anorexic and thermogenic actions, induces triglyceride depletion in adipocytes and nonadipocytes. Here we show that leptin alters in pancreatic islets the mRNA of the genes encoding enzymes of free fatty acid metabolism and uncoupling protein-2 (UCP-2). In animals infused with a recombinant adenovirus containing the leptin cDNA, the levels of mRNAs encoding enzymes of mitochondrial and peroxisomal oxidation rose 2- to 3-fold, whereas mRNA encoding an enzyme of esterification declined in islets from hyperleptinemic rats. Islet UCP-2 mRNA rose 6-fold. All in vivo changes occurred in vitro in normal islets cultured with recombinant leptin, indicating direct extraneural effects. Leptin overexpression increased UCP-2 mRNA by more than 10-fold in epididymal, retroperitoneal, and subcutaneous fat tissue of normal, but not of leptin-receptor-defective obese rats. By directly regulating the expression of enzymes of free fatty acid metabolism and of UCP-2, leptin controls intracellular triglyceride content of certain nonadipocytes, as well as adipocytes.

Leptin concentrations, sex hormones, and cortisol in nondiabetic men

Leptin, the product of the human ob gene, is increased in obese individuals, suggesting resistance to its effect. However, there is a variability in leptin levels at each level of body mass index, suggesting that genetic and environmental factors other than overall adiposity may regulate leptin concentrations. No data currently exist on the relation of sex hormones to leptin concentrations in men. We examined the relation ofleptin levels to sex hormone-binding globulin, total and free testosterone, dehydroepiandrosterone sulfate, estradiol, and cortisol in 87 normoglycemic men. Leptin levels were significantly correlated with free testosterone (r = -0.14; P < 0.05), sex hormone-binding globulin (r = -0.26; P < 0.05), total testosterone (r = -0.32; P < 0.01), and cortisol (r = -0.09; P = NS). However, after adjustment for body mass index (or, alternatively, waist or hip circumference), leptin concentrations were not significantly related to sex hormones or cortisol. Our data suggest that in men, sex hormones are not important independent modifiers of leptin concentrations.

Plasma leptin is partly cleared by the kidney and is elevated in hemodialysis patients

Leptin, the gene product of the ob gene, is important in the control of appetite in rodents and may have an important role in humans. The clearance of leptin from the circulation is unknown. As the leptin receptor is present in the kidney, we evaluated the role of the kidney in removing circulating leptin in humans. We measured leptin in aortic and renal vein plasma in 8 patients with intact renal function and 6 patients with impaired renal function who were undergoing elective cardiac catheterization. Renal blood flow was measured in all patients to calculate net mass balance across the kidney. In patients with intact renal function there is net renal uptake of 12% of circulating leptin, whereas in patients with renal insufficiency there is no renal uptake of leptin. In a separate cohort of 36 patients with end-stage renal failure on hemodialysis, peripheral leptin levels factored for body mass index was increased by > fourfold as compared to a group of healthy controls (N = 338). In addition, plasma leptin is not cleared by hemodialysis with a modified cellulose membrane. Additional studies are required to evaluate the role of leptin in mediating the anorexia of uremia.

Leptin receptor gene variation and obesity: lack of association in a white British male population

Leptin, a hormone secreted by adipocytes, plays a pivotal role in the control of body weight. Rodents with mutations in the leptin receptor gene develop morbid obesity. It is possible, therefore, that leptin receptor gene mutations contribute to human obesity. To test this possibility, we determined the entire coding sequence of the human leptin receptor cDNA from peripheral blood lymphocytes of 22 morbidly obese patients with body-mass index (BMI) between 35.1 and 60.9 kg/m2. We identified five common DNA sequence variants distributed throughout the coding sequence at codons 109, 223, 343, 656 and 1019, one rare silent mutation at codon 986 and one novel alternatively spliced form of transcript. None of the five common variants, including the three that predict amino acid changes, are null mutations causing morbid obesity, because homozygotes for the variant sequences were also found in lean subjects. Furthermore, the frequency of each variant allele and the distribution of genotypes and haplotypes were similar in 190 obese (BMI >28 kg/m2) and 132 lean (BMI <22 kg/m2) white British males selected from a population-based epidemiological survey. In these subjects, there was no evidence for a significant effect of the common variants on obesity or obesity-related phenotypes. These results suggest that mutations in the leptin receptor gene are not a common cause of human obesity.

The genetics of obesity

Despite the influence of obesity in predisposing to many diseases, and evidence for high heritability, efforts to identify human genes with major effects on bodyweight have not yet been successful. In contrast, remarkable progress has been made in the identification and characterization of the genes mutated in five monogenic mouse models of obesity. These genes have led to new insights into the etiology of obesity and provide promising targets for therapeutic intervention.

Leptin concentrations in the polycystic ovary syndrome

The polycystic ovary syndrome (PCOS) is characterized by menstrual disturbances, chronic anovulation and hyperandrogenism and is associated with insulin resistance and hyperinsulinemia. Leptin, the product of the ob gene, is an adipocyte-secreted molecule that signals the magnitude of energy stores to the brain and has been recently shown to have important effects on the reproductive axis of rodents. To assess the potential contribution of leptin to the pathogenesis of PCOS, we measured leptin levels in 24 obese women with PCOS and 12 weight- and age-matched controls and determined whether alterations in hyperinsulinemia produced by administration of the insulin-sensitizing agent troglitazone had an effect on serum leptin levels. Leptin concentrations at baseline were not different in women with PCOS (38.1 +/- 2.15 ng/mL) and controls (33.12 +/- 2.39 ng/mL). Moreover, leptin concentrations remained unchanged after treatment with troglitazone (38.1 +/- 2.15 vs. 39.21 +/- 2.65 ng/mL). Baseline leptin correlated strongly with body mass index in both controls (r = 0.59; P < 0.05) and women with PCOS (r = 0.70; P = 0.0004). Leptin levels were not associated with baseline insulin, testosterone, non-sex hormone-binding globulin (SHBG)-bound testosterone, dehydroepiandrosterone sulfate, estradiol, or SHBG. Finally, despite significantly reduced insulin, non-SHBG-bound testosterone, and estradiol levels after troglitazone treatment of women with PCOS, their leptin levels remained unchanged. We conclude that circulating leptin levels in patients with PCOS do not differ from those in age- and weight-matched controls. Furthermore, increased circulating insulin due to insulin resistance does not appear to alter circulating leptin levels in women with PCOS.

Serum leptin levels in women with polycystic ovary syndrome: the role of insulin resistance/hyperinsulinemia

Polycystic ovary syndrome (PCOS) is associated with chronic anovulation, hyperandrogenemia, insulin resistance (IR)/hyperinsulinemia, and a high incidence of obesity. Thus, PCOS serves as a useful model to assess the role of IR and chronic endogenous insulin excess on leptin levels. Thirty-three PCOS and 32 normally cycling (NC) women of similar body mass index (BMI) were studied. Insulin sensitivity (S(I)) was assessed by rapid ivGTT in a subset of 28 PCOS and 29 NC subjects; percent body fat was determined by dual-energy x-ray absorptiometry (DEXA) in 14 PCOS and 17 NC. Fasting (0800 h) and 24-h mean hourly insulin levels were 2-fold higher (P < 0.0001), and S(I) was 50% lower (P = 0.005) in PCOS than in NC, while serum androstenedione (A), testosterone (T), 17-alpha hydroxyprogesterone (17OHP), and estrone (E1) levels were elevated (P < 0.0001), and sex hormone-binding globulin (SHBG) levels were decreased (P < 0.01). Twenty-four hour LH pulse frequency, mean pulse amplitude, and mean LH levels were elevated in PCOS (P < 0.001) as compared with NC. Serum leptin levels for PCOS (24.1 +/- 2.6 ng/mL) did not differ from NC (21.5 +/- 3.5 ng/mL) and were positively correlated with BMI (r = 0.81) and percent body fat (r = 0.91) for the two groups (both P < 0.0001). Leptin levels for PCOS and NC correlated positively with fasting and 24-h mean insulin levels (r = 0.81, P < 0.0001 for both PCOS and NC) and negatively with S(I) and SHBG levels. Leptin concentrations for PCOS, but not NC, correlated positively with 24-h mean glucose levels and inversely with 24-h mean LH levels and 24-h mean LH pulse amplitude. Leptin levels were not correlated with estrogen or androgen levels for either PCOS or NC, although leptin levels were positively related to the ratios of E1/SHBG and E2/SHBG for both PCOS and NC and to the ratio of T/SHBG for PCOS only. In stepwise multivariate regression with forward selection, only 24-h mean insulin levels contributed significantly (P < 0.01) to leptin levels independent of BMI and percent body fat for both PCOS and NC. Given this relationship and the presence of 2-fold higher 24-h mean insulin levels in PCOS, the expected elevation of leptin levels in PCOS was not found. This paradox may be explained by the presence of adipocyte IR specific to PCOS, which may negate the stimulatory impact of hyperinsulinemia on leptin secretion, a proposition requiring further study.

Thyroid dysfunction is not associated with alterations in serum leptin levels

To determine if serum leptin levels are affected by thyroid dysfunction, we measured its concentration in serum samples from 25 euthyroid controls and 25 subjects each with hypothyroidism and thyrotoxicosis collected over a 3-month period. Mean leptin levels in the euthyroid (24.1 +/- 8.3 microg/L), hypothyroid (22.7 +/- 7.0 microg/L) and thyrotoxic (23.3 +/- 4.3 microg/L) groups were not significantly different. Data were available to express leptin in terms of body mass index (BMI) in 11 euthyroid, and 6 untreated hypothyroid and thyrotoxic individuals. There was a significant positive correlation between BMI and leptin level (r = 0.60, p = .0002) for this subgroup, irrespective of their thyroid status. These data suggest that leptin levels are not affected by thyroid dysfunction.

Serum leptin concentrations in women with polycystic ovary syndrome

The role of gonadotropins, androgens, and insulin in the regulation of circulating leptin levels is obscure. In order to clarify the relationships of these parameters we studied serum leptin levels in 19 healthy control subjects and in 35 hyperandrogenic and hyperinsulinemic patients with polycystic ovary syndrome (PCOS). Serum leptin concentrations did not differ significantly between PCOS patients and control subjects. When PCOS and control groups were analyzed together by univariate analysis, serum leptin was positively correlated with body mass index (BMI), body weight, serum insulin, serum triglyceride, and serum free testosterone concentrations. Serum leptin was inversely correlated with serum sex hormone binding globulin (SHBG) concentrations. There were no significant correlations between serum leptin and testosterone, androstenedione, or gonadotropin concentrations. Serum insulin, triglyceride, and free testosterone concentrations were positively correlated, and serum SHBG was negatively correlated with BMI. However, when BMI on one hand and serum insulin, triglyceride, free testosterone, or SHBG on other hand were used as independent variables in the partial correlation analysis with leptin, BMI turned out to be the variable primarily responsible for all of the correlations with leptin. In conclusion, the concept that circulating leptin levels would be different in PCOS patients than in regularly menstruating control subjects is not supported by our data.

Leptin and obesity in humans

It now appears that leptin is the peripheral signal, hypothesized in the lipostasis theory, that informs the central nervous system how much adipose tissue there is in the body. The ability of the leptin signal to regulate body composition and the amount of body fat has been demonstrated in animals. Furthermore, defects in the ob gene and the leptin receptor lead to the development of obesity in rodents. No such defects have been found in humans although it appears that obese humans are resistant to the action of their endogenous leptin. Further characterization of the hormone and ultimately, the administration of leptin to humans, will be necessary to determine the role of the leptin signal system in the development of obesity in humans.

Leptin secretion from adipose tissue in women. Relationship to plasma levels and gene expression

The role of expression and secretion of the ob gene product, leptin, for the regulation of plasma leptin levels has been investigated in vitro using abdominal subcutaneous adipose tissue of 20 obese, otherwise healthy, and 11 nonobese women. Body mass index (BMI, mean+/-SEM; kg/m2) in the two groups was 41+/-2 and 23+/-1, respectively. Fat cell volume was 815+/-55 pl in the obese and 320+/-46 pl in the nonobese group. In the obese group, plasma leptin concentrations and adipose leptin mRNA (relative to gamma actin) were increased five and two times, respectively. Moreover, adipose tissue secretion rates per gram lipid weight or per fat cell number were also increased two and seven times, respectively, in the obese group. There were strong linear correlations (r = 0.6-0.8) between plasma leptin, leptin secretion, and leptin mRNA. All of these leptin measurements correlated strongly with BMI and fat cell volume (r = 0.7- 0.9). About 60% of the variation in plasma leptin could be attributed to variations in leptin secretion rate, BMI, or fat cell volume. We conclude that elevated circulating levels of leptin in obese women above all result from accelerated secretion rates of the peptide from adipose tissue because of increased ob gene expression. However, leptin mRNA, leptin secretion, and circulating leptin levels are all more closely related to the stored amount of lipids in the fat cells of adipose tissue than they are to an arbitrary division into obese versus nonobese.

The effect of dietary energy restriction on body weight gain and the development of noninsulin-dependent diabetes mellitus (NIDDM) in Psammomys obesus

Food intake was restricted to 75% of ad libitum levels in 37 male Psammomys obesus (Israeli Sand Rats) from the ages of 4 (weaning) to 10 weeks. Energy restriction reduced the mean bodyweight at 10 weeks by 29% compared with 44 ad libitum fed controls. Hyperglycemia was prevented completely in the food-restricted group, and mean blood glucose concentrations were significantly reduced (3.8 +/- 0.2 vs. 5.5 +/- 0.4 mumol/L; p < 0.05) compared with control animals. Plasma insulin concentrations were also decreased significantly compared with ad libitum fed controls (105 +/- 13 vs. 241 +/- 29 mU/L; p < 0.05). Although energy restriction prevented hyperglycemia from developing in 10-week-old P. obesus, 19% of the food restricted animals still developed hyperinsulinemia. We concluded that hyperphagia between the ages of 4 to 10 weeks may be essential for the development of noninsulin-dependent diabetes mellitus in P. obesus, but that hyperinsulinemia may still occur in the absence of hyperphagia and hyperglycemia, suggesting a significant genetic influence on the development of hyperinsulinemia in this animal model.

Dexamethasone increases leptin expression in humans in vivo

The effect of 2 days of oral dexamethasone administration (0.75 mg twice daily) on leptin expression in healthy volunteers was tested. Dexamethasone increased the relative abundance of leptin messenger RNA in abdominal and gluteal adipose tissues by approximately 70% (P < 0.05). Dexamethasone also significantly increased serum leptin (+ 80%) and insulin concentration (+ 83%) but did not affect serum glucose. We conclude that a hypercortisolemic/hyperinsulinemic state up-regulates leptin expression at the messenger RNA level in humans.

Leptin impairs metabolic actions of insulin in isolated rat adipocytes

Leptin is an adipocyte hormone involved in the regulation of energy homeostasis. Generally accepted biological effects of leptin are inhibition of food intake and stimulation of metabolic rate in ob/ob mice that are defective in the leptin gene. In contrast to these centrally mediated effects of leptin, we are reporting here on leptin effects on isolated rat adipocytes. Leptin impairs several metabolic actions of insulin, i.e. stimulation of glucose transport, glycogen synthase, lipogenesis, inhibition of isoproterenol-induced lipolysis, and protein kinase A activation, as well as stimulation of protein synthesis. Insulin effects were reduced by leptin (2 nM) with a half-life of about 8 h. At low leptin concentrations (<1 nM), the insulin sensitivity was reduced leading to a shift to the right in the dose-response curve. At higher concentrations the responsiveness was diminished, resulting in nearly complete inhibition of insulin effects at >30 nM leptin. The IC50 value of leptin was 3.1 +/- 1 nM after 15 h of preincubation of adipocytes in primary culture. The natural splice variant des-Gln49-leptin exhibited a significantly lower potency. Adipocytes regained full insulin sensitivity within a few hours after leptin removal. The stimulation of glucose transport by vanadate was not affected by leptin. These data show specific and potent impairment of insulin action by leptin in the physiological concentration range of both leptin and insulin, which may be related to the pathophysiology of insulin resistance in both non-insulin-dependent diabetes mellitus and obesity.

Obesity, diabetes and functions for proopiomelanocortin-derived peptides

Melanocortin peptides (adrenocorticotropin (ACTH), alpha-,beta-, and gamma-melanocyte stimulating hormone (MSH), and fragments thereof) derived from proopiomelanocortin (POMC) have a diverse array of biological activities, many of which have yet to be fully elucidated. The recent cloning of a family of five distinct melanocortin receptors through which these peptides act has provided the tools to further our understanding of melanocortin peptide functions. Early work on melanocortin peptides focused on their roles in pigmentation, adrenocortical function, the immune, central and peripheral nervous systems. Although melanocortin peptides have long been known to affect lipolysis, characterisation of the melanocortin receptors has opened up several lines of evidence for important roles in the development of obesity, insulin resistance and type II diabetes. We present here a review of the current evidence for melanocortin peptides playing such a role, and based on this evidence, a model for melanocortin peptides and their receptors in maintaining energy balance.

Pediatric obesity. An overview of etiology and treatment

Pediatric obesity is a chronic and growing problem for which new ideas about the biologic basis of obesity offer hope for effective solutions. Prevalence of pediatric and adult obesity is increasing despite a bewildering array of treatment programs and severe psychosocial and economic costs. The definition of obesity as an increase in fat mass, not just an increase in body weight, has profound influence on the understanding and treatment of obesity. In principle, body weight is determined by a balance between energy expenditure and energy intake, but this observation does not by itself explain obesity. There is surprisingly little evidence that the obese overeat and only some evidence that the obese are more sedentary. Understanding of the biologic basis of obesity has grown rapidly in the last few years, especially with the identification of a novel endocrine pathway involving the adipose tissue secreted hormone leptin and the leptin receptor that is expressed in the hypothalamus. Plasma leptin levels are strongly correlated with body fat mass and are regulated by feeding and fasting, insulin, glucocorticoids, and other factors, consistent with the hypothesis that leptin is involved in body weight regulation and may even be a satiety factor (Fig. 2, Table 1). Leptin injections have been shown to reduce body weight of primates, although human clinical trials will not be reported until summer 1997. So many peptides influencing feeding have been described that one or more may have therapeutic potential (Fig. 2, Table 1). Although the complexity of pathways regulating body weight homeostasis slowed the pace of understanding underlying mechanisms, these complexities now offer many possibilities for novel therapeutic interventions (Fig. 2). Obesity is a major risk factor for insulin resistance and diabetes, hypertension, cancer, gallbladder disease, and atherosclerosis. In particular, adults who were obese as children have increased mortality independent of adult weight. Thus, prevention programs for children and adolescents will have long-term benefits. Treatment programs focus on modification of energy intake and expenditure through decreased calorie intake and exercise programs. Behavior-modification programs have been developed to increase effectiveness of these intake and exercise programs. These programs can produce short-term weight loss. Long-term losses are more modest but achieved more successfully in children than in adults. Several drug therapies for obesity treatment recently have been approved for adults that produce sustained 5% to 10% weight losses but experience with their use in children is limited. Identification of the biochemical pathways causing obesity by genetic approaches could provide the theoretic foundation for novel, safe, and effective obesity treatments. The cloning of leptin in 1994 has already led to testing the efficacy of leptin in clinical trials that are now underway. Although novel treatments of obesity are being developed as a result of the new biology of obesity, prevention of obesity remains an important goal.

In vivo effects of leptin-related synthetic peptides on body weight and food intake in female ob/ob mice: localization of leptin activity to domains between amino acid residues 106-140

In C57BL/6J ob/ob mice, a single base mutation of the ob gene in codon 105 results in the replacement of arginine by a premature stop codon and production of a truncated inactive form of leptin. These observations suggest that leptin activity may be localized, at least in part, to domains distal to amino acid residue 104. To investigate this possibility, we synthesized six overlapping peptide amides corresponding to residues 106-167 of leptin, and examined their effects on body weight and food intake in female C57BL/6J ob/ob mice. When compared with vehicle-injected control mice, weight gain by mice receiving 28 daily 1-mg i.p. injections of LEP-(106-120), LEP-(116-130), or LEP-(126-140) was significantly (P < 0.01) reduced with no apparent toxicity. Weight gain by mice receiving LEP-(136-150), LEP-(146-160), or LEP-(156-167) was not significantly different from that of vehicle-injected control mice. The effects of LEP-(106-120), LEP-(116-130), or LEP-(126-140) were most pronounced during the first week of peptide treatment. Within 7 days, mice receiving these peptides lost 12.3%, 13.8%, and 9.8%, respectively, of their initial body weights. After 28 days, mice given vehicle alone, LEP-(136-150), LEP-(146-160), or LEP-(156-167) were 14.7%, 20.3%, 25.0%, and 24.8% heavier, respectively, than they were at the beginning of the study. Mice given LEP-(106-120) or LEP-(126-140) were only 1.8% and 4.2% heavier, respectively, whereas mice given LEP-(116-130) were 3.4% lighter. Food intake by mice receiving LEP-(106-120), LEP-(116-130), or LEP-(126-140), but not by mice receiving LEP-(136-150), LEP-(146-160), or LEP-(156-167), was reduced by 15%. The results of this study indicate 1) that leptin activity is localized, at least in part, in domains between residues 106-140; 2) that leptin-related peptides have in vivo effects similar to those of native leptin; and 3) offer hope for development of peptide analogs of leptin having potential application in human or veterinary medicine.

The metabolic significance of leptin in humans: gender-based differences in relationship to adiposity, insulin sensitivity, and energy expenditure

Leptin is an adipocyte-derived hormone that interacts with a putative receptor(s) in the hypothalamus to regulate body weight. The relationship of leptin to metabolic abnormalities associated with obesity together with hormonal and substrate regulation of leptin have not been extensively studied. Therefore, 116 subjects (62 men and 54 women) with a wide range of body weight [body mass index (BMI), 17-54 kg/m2] were characterized on a metabolic ward with regard to body composition, glucose intolerance, insulin sensitivity, energy expenditure, substrate utilization, and blood pressure. Eighty-five of the subjects had normal glucose tolerance (50 men and 35 women), and 31 had noninsulin-dependent diabetes mellitus (12 men and 19 women). In both men and women, fasting leptin levels were highly correlated with BMI (r = 0.87 and r = 0.88, respectively) and percent body fat (r = 0.82 and r = 0.88, respectively; all P < 0.0001). However, men exhibited lower leptin levels at any given measure of obesity. Compared with those in men, leptin levels rose 3.4-fold more rapidly as a function of BMI in women [leptin = 1.815 (BMI)-31.103 in women; leptin = 0.534 (BMI)-8.437 in men] and 3.2 times more rapidly as a function of body fat [leptin = 1.293 (% body fat)-24.817 in women; leptin = 0.402 (% body fat)-3.087 in men]. Hyperleptinemia was associated with insulin resistance (r = -0.57; P < 0.0001) and high waist to hip ratio (r = 0.75; P < 0.0001) only in men. On the other hand, during the hyperinsulinemic euglycemic clamp studies, hyperinsulinemia acutely increased leptin concentrations (20%) only in women. There was no correlation noted between fasting leptin levels and either resting energy expenditure or insulin-induced thermogenesis in men or women (P = NS). In stepwise and multiple regression models with leptin as the dependent variable, noninsulin-dependent diabetes mellitus did not enter the equations at a statistically significant level. The data indicate that there are important gender-based differences in the regulation and action of leptin in humans. Serum leptin levels increase with progressive obesity in both men and women. However, for any given measure of obesity, leptin levels are higher in women than in men, consistent with a state of relative leptin resistance. These findings have important implications regarding differences in body composition in men and women. The observation that serum leptin is not related to energy expenditure rates suggests that leptin regulates body fat predominantly by altering eating behavior rather than calorigenesis.

The leptin receptor activates janus kinase 2 and signals for proliferation in a factor-dependent cell line

The antiobesity effects of leptin are mediated by the obese receptor (OB-R), a member of the cytokine receptor superfamily. Several isoforms of OB-R that differ in the length of the cytoplasmic domain have been described. An isoform with a long cytoplasmic domain of 302 amino acids, termed OB-Rb, contains the conserved box 1 and box 2 motifs and is likely to be responsible for leptin-induced signaling. A point mutation in the OB-R gene of diabetes (db) mice generates a new splice donor that interferes with the correct splicing of the OB-Rb mRNA and is predicted to cause absence of the OB-Rb protein in db/db mice. Here we examined the signaling potential of the long isoform, OB-Rb, and of a short isoform, OB-Ra, in BaF3 cells, a factor-dependent hematopoietic cell line. The long isoform was able to generate a proliferative signal and upon leptin binding, activated janus kinase 2 (Jak2). Consistently, antibodies directed against the extracellular domain of OB-R coprecipitated Jak2. The short isoform, OB-Ra, was inactive in both proliferation and Jak activation. These results provide further support for the long isoform, OB-Rb, being the principal mediator of the effects of leptin and help to explain why db/db mice are resistant to leptin, despite the presence of the short OB-R isoforms.

Plasma leptin concentrations and energy expenditure in heart failure patients

Leptin, the protein encoded by the obese gene, is a newly described hormone implicated in the regulation of energy balance. To examine the possible role of leptin in the energy dysregulation that frequently accompanies chronic heart failure, we examined plasma leptin concentrations and energy expenditure in 18 heart failure patients (aged 71 +/- 6 years) and 46 healthy elderly controls (66 +/- 6 years). Plasma leptin concentrations were measured by radioimmunoassay, daily energy expenditure by doubly labeled water, and body composition by dual-energy x-ray absorptiometry. Fat mass was lower (P < .01) in heart failure patients compared with healthy controls, whereas fat-free mass did not differ between groups. Plasma leptin concentrations were not different between heart failure patients and healthy controls (5.1 +/- 4.2 v 6.8 +/- 4.4 pg/mL) and remained similar after statistical control for fat mass (6.0 +/- 3.1 v 7.1 +/- 3.2 pg/mL). Plasma leptin was related to fat mass in heart failure patients (r = .92, P < .01) and healthy controls (r = .69, P < .01). Free-living daily and physical-activity energy expenditures were lower (P < .01) in heart failure patients compared with healthy controls. Plasma leptin concentrations were related to both daily (r = .67, P < .01) and resting (r = .67, P < .01) energy expenditure in heart failure patients, but not in healthy controls (r = .09 and r = .33, respectively). In conclusion, we found an association between plasma leptin concentrations and energy expenditure in heart failure patients, but not in healthy controls. Thus, leptin may participate in the regulation of energy expenditure and body energy stores in heart failure patients.

The arcuate nucleus as a primary site of satiety effect of leptin in rats

The obese (ob) gene encodes a fat cell-derived circulating satiety factor (leptin) that is involved in the regulation of energy homeostasis. In the present study, we examined effects of i.c.v. injection of recombinant human leptin on food intake and body weight gain in rats. We also studied effects of direct microinjections of leptin into the arcuate nucleus (Arc), ventromedial hypothalamus (VMH), and lateral hypothalamus (LH). A single i.c.v. injection of recombinant human leptin (0.25-2.0 micrograms/rat) reduced significantly and dose-dependently food intake and body weight gain in rats. Microinjections (0.125-0.5 microgram/site) into the bilateral Arc, VMH, and LH caused dose-related decreases in food intake and body weight gain as compared with vehicle-treated groups with a rank order of potency; Arc > VMH = LH. The present study provides the first direct evidence that the Arc is a primary site of satiety effect of leptin.

The global epidemiology of non-insulin-dependent diabetes mellitus and the metabolic syndrome

Non-insulin-dependent diabetes mellitus (NIDDM) constitutes about 85% of all cases of diabetes in developed countries and it has now reached epidemic proportions in many developing nations, as well as disadvantaged groups in developed countries, e.g., Mexican- and African-Americans and Australian Aborigines and Torres Strait Islanders. The diagnosis of NIDDM is usually made after the age of 50 years in Europids, but it is seen at much younger age in these high prevalence populations, which also include Pacific Islanders, Native Americans, and migrant Asian Indians and Chinese. There is enormous variation in NIDDM prevalence between populations, and exceptionally high rates have been documented in populations who have changed from a traditional to a modern lifestyle, e.g., American Pima Indians, Micronesians, and other Pacific Islanders, Australian Aborigines, migrant Asian Indians, and Mexican-Americans. Over the next decade, following the initial phase of the NIDDM epidemic, macro- and microvascular complications will emerge as a major threat to future public health throughout the world with huge economic and social costs. The major cause of death in NIDDM is macrovascular disease (coronary artery, peripheral vascular, and cerebrovascular), which accounts for at least two-thirds of NIDDM mortality. A key strategy in reducing macrovascular disease lies in the better understanding of the Deadly Quartet or Metabolic Syndrome. New data suggest that hyperleptinemia rather than hyperinsulinemia may play an important and central role in the genesis of the cardiovascular disease risk factor cluster that constitutes the Metabolic Syndrome.

Suggestive linkages between markers on human 1p32-p22 and body fat and insulin levels in the Quebec Family Study

A single-gene rodent mutation (diabetes) and a quantitative trait locus (dietary obese 1) mapped to the mid portion of mouse chromosome 4 have been related to obesity and/or insulin levels. Synteny relationships place their putative human homologs on 1p31 and 1p35-p31, respectively. In 137 sibships of adult brothers and sisters from the Québec Family Study, genetic linkages between seven microsatellite markers from 1p32-p22 and various obesity- and diabetes-related quantitative phenotypes were examined using single locus sibpair linkage analysis. Suggestive linkages were observed between markers D1S476 and body mass index (p = 0.05), fat mass (p = 0.02), the sum of six skinfolds (p = 0.02), the insulin area after an oral glucose tolerance test (p = 0.02), and between the neighboring marker D1S200 and body mass index (p = 0.03), and fat mass (p = 0.009). Suggestive linkages were also observed between the more telomeric markers D1S193 and body mass index (p = 0.03), and between the neighboring marker D1S197 and fasting insulin level (p = 0.05). No linkage was observed with the trunk to extremity skinfolds ratio. These linkages suggest that human homologs of the mouse diabetes or dietary obese 1 and/or other genes in this interval on chromosome 1 play a role in the regulation of body mass, body composition, and insulin levels, but not of subcutaneous fat distribution.

Increased plasma leptin concentration in end-stage renal disease

Leptin is a 16-kDa protein recently identified as the obese gene product involved in body weight regulation. Administration of recombinant leptin to ob/ob mice, which have a genetic defect in leptin production, reduces food intake and increases energy expenditure. Leptin is synthesized by fat cells, and in normal humans, plasma concentrations are proportional to adiposity. The physiological actions and the degradation pathways of leptin in humans are unknown. We investigated renal elimination of leptin by comparing plasma leptin concentrations in end-stage renal disease (ESRD) patients with normal controls. Our hypothesis was that if renal filtration is a significant route of elimination, the hormone would accumulate in ESRD patients. Mean plasma levels in 141 ESRD patients (26.8 +/- 5.7 and 38.3 +/- 5.6 micrograms/L for males and females, respectively) were significantly higher (P < 0.001) than mean values obtained in normal controls (11.9 +/- 3.1 and 21.2 +/- 3.0 micrograms/L for males and females, respectively). Leptin concentrations in ESRD patients correlated directly with body mass index (BMI; r = 0.77 for men and 0.78 for women). The rate of increase in leptin concentrations with BMI was significantly greater in ESRD patients (5.5 and 6.6 micrograms/L/U BMI for men and women, respectively) than in normal controls (1.4 and 2.6 micrograms/L/U for men and women, respectively). Pre- and postdialysis leptin levels in hemodialysis patients were similar. Western blot of plasma from ESRD patients with high leptin levels showed bands corresponding to the intact protein (16 kDa) with no lesser or greater molecular mass species observed. Leptin concentrations in patients with ESRD did not correlate with measures of residual renal function (serum creatinine, beta 2-microglobulin, PTH, or GH levels). Similarly, we found no correlation between leptin levels and the number of years patients had been on dialysis or with recent weight changes. We conclude that intact leptin is increased in ESRD patients, but does not appear to cause decreased weight. As leptin levels did not correlate with residual renal function, increased production may account for the high levels observed.

A major quantitative trait locus determining serum leptin levels and fat mass is located on human chromosome 2

Obesity is a major predisposing factor for the development of several chronic diseases including non-insulin dependent diabetes mellitus (NIDDM) and coronary heart disease (CHD). Leptin is a serum protein which is secreted by adipocytes and thought to play a role in the regulation of body fat. Leptin levels in humans have been found to be highly correlated with an individual's total adiposity. We performed a genome-wide scan and conducted multipoint linkage analysis using a general pedigree-based variance component approach to identify genes with measurable effects on quantitative variation in leptin levels in Mexican Americans. A microsatellite polymorphism, D2S1788, mapped to chromosome 2p21 (approximately 74 cM from the tip of the short arm) and showed strong evidence of linkage with serum leptin levels with a lod score of 4.95 (P = 9 x 10(-7)). This locus accounted for 47% of the variation in serum leptin levels, with a residual additive genetic component contributing an additional 24%. This region contains several potential candidate genes for obesity, including glucokinase regulatory protein (GCKR) and pro-opiomelanocortin (POMC). Our results show strong evidence of linkage of this region of chromosome 2 with serum leptin levels and indicate that this region could contain an important human obesity gene.

Leptin treatment rescues the sterility of genetically obese ob/ob males

Leptin, a hormone secreted from white adipose tissue, has been shown to normalize the body weight of ob/ob but not db/db mice as postulated by Coleman in his classical parabiosis experiments. The major effect of leptin is therefore to decrease food intake, thus resulting in a breakdown of fat stores. Recently, we have suggested that leptin plays a role in reproductive physiology based on the observation that leptin treatment but not food restriction rescues the sterility of ob/ob females. In the present communication, we treated sterile ob/ob males with leptin and asked whether fertility could be induced, thus selecting their reproductive ability as the endpoint of the experiment. Our results show that all food-restricted ob/ob males are unable to impregnate normal C57BL/6J females. However, all leptin-treated ob/ob males fertilized normal females mice that carried out normal pregnancies and deliveries, demonstrating that the reproductive capacity of ob/ob males was corrected only with leptin treatment. Furthermore, reproductive indices such as testicular weight and histology are normalized in leptin-treated animals. Therefore, as in ob/ob females, leptin plays a significant role in the male mouse reproductive pathways.

Total energy expenditure and the level of physical activity correlate with plasma leptin concentrations in five-year-old children

Leptin, the product of the ob gene, is a hormone secreted by adipocytes that is known to decrease food intake and increase energy expenditure in ob/ob mice. In humans, variants in the OB gene have not been detected and very little is known about the action of leptin on food intake and energy expenditure, although circulating leptin concentrations are positively correlated to body fat stores. The purpose of this study was to assess the relationship between fasting plasma leptin concentrations and energy expenditure in 123 5-yr-old Pima Indian children (67 males/76 females). Body composition was assessed by isotopic water dilution (18O) whereas total energy expenditure (TEE) and resting metabolic rate (RMR) were measured using doubly labeled water and indirect calorimetry, respectively. The physical activity level was calculated as the ratio of TEE:RMR. Plasma leptin concentrations were positively correlated to percent body fat (r = 0.84, P < 0.0001), but were similar in boys and girls after adjusting for percent body fat. Most importantly, we found that, independent of the percentage of body fat, plasma leptin concentrations correlated with TEE (in absolute values, r = 0.37, P < 0.0001, or adjusted for body size r = 0.42; P < 0.0001) and with physical activity level (r = 0.26, P < 0.01), but not RMR. These results suggest that, as in animal models, leptin plays a role in energy expenditure in humans.

A short form of leptin receptor performs signal transduction

The obese (ob) gene product, leptin, a peptide hormone, which is synthesized in adipocytes, is a satiety factor and is involved in the control of body weight via the regulation of energy homeostasis. Several alternate spliced isoforms (a-e, as well as others) of the leptin receptor (OBR) have been cloned, all of which, except for OBRe (soluble form), contain a single transmembrane domain. They share the same extracellular domain, with homology to the class I cytokine receptor family. The OBRb, which has longest cytoplasmic domain, is expressed in high levels in the hypothalamus and is thought to be the only isoform capable of signal transmission. Herein, we report the mRNA expression of immediate early genes, c-fos, c-jun and jun-B, which are induced by leptin addition, not only in CHO cells expressing the OBRb, but also in cells expressing one of the short form receptors, OBRa.

Relatively low plasma leptin concentrations precede weight gain in Pima Indians

Leptin, the product of the ob gene, is a hormone, produced by adipose cells, that inhibits food intake and increases energy expenditure in rodents. In humans, plasma leptin concentrations correlate closely with the size of the adipose tissue depot; however, there is considerable variation in plasma leptin concentrations at any given degree of fatness. To investigate whether individuals prone to weight gain are hypoleptinemic, we measured fasting plasma leptin concentrations in two groups of weight-matched nondiabetic Pima Indians followed for approximately 3 years, 19 of whom subsequently gained weight and 17 of whom maintained their weight. After we adjusted for initial percent body fat, mean plasma leptin concentration was lower in those who gained weight than in those whose weight was stable. These data indicate that relatively low plasma leptin concentrations may play a role in the development of obesity in Pima Indians, a population prone to obesity.

Leptin accelerates the onset of puberty in normal female mice

The fat-derived hormone, leptin, is proposed to serve as an adipostatic signal to the brain to reduce food intake and body weight. In addition to its effects on body weight, chronic leptin treatment restores puberty and fertility to ob/ob mice with total leptin deficiency, and acute treatment substantially corrects hypogonadism in mice starved for 2 d without affecting body weight. Leptin may therefore be a critical signal, linking adiposity and reproduction. Since body weight and adiposity appear to play a critical role in the timing of puberty in humans and rodents, and leptin levels rise with increasing adiposity, we studied the effects of once daily injections of recombinant leptin on the onset of puberty in female mice weaned on day 21 and fed ad libitum. There was a linear increase in body weight during the study period, which was not altered by the dose of leptin used. Mice injected with leptin had an earlier onset of three classic pubertal parameters (i.e., vaginal opening, estrus, and cycling) compared with saline-injected controls. Leptin is the first peripheral molecule demonstrated to accelerate the maturation of the reproductive axis in normal rodents. We propose that leptin is the signal that informs the brain that energy stores are sufficient to support the high energy demands of reproduction, and may be a major determinant of the timing of puberty.

Diet-induced obese mice develop peripheral, but not central, resistance to leptin

Leptin administration reduces obesity in leptin-deficient ob/ob mice; its effects in obese humans, who have high circulating leptin levels, remain to be determined. This longitudinal study was designed to determine whether diet-induced obesity in mice produces resistance to peripheral and/or central leptin treatment. Obesity was induced in two strains of mice by exposure to a 45% fat diet. Serum leptin increased in proportion to body weight (P < 0.00001). Whereas C57BL/6 mice initially responded to peripherally administered leptin with a marked decrease in food intake, leptin resistance developed after 16 d on high fat diet; mice on 10% fat diet retained leptin sensitivity. In AKR mice, peripheral leptin significantly decreased food intake in both 10 and 45% fat-fed mice after 16 d of dietary treatment. However, after 56 d, both groups became resistant to peripherally administered leptin. Central administration of leptin to peripherally leptin-resistant AKR mice on 45% fat diet resulted in a robust response to leptin, with a dose-dependent decrease in food intake (P < 0.00001) and body weight (P < 0.0001) after a single intracerebroventricular infusion. These data demonstrate that, in a diet-induced obesity model, mice exhibit resistance to peripherally administered leptin, while retaining sensitivity to centrally administered leptin.

Structure and function in gene patenting

The United States patent system treats DNA sequences as large chemical compounds in determining their patentability. This approach has been helpful to those who seek to patent previously unidentified DNA sequences, but it may prove less advantageous from the perspective of those who elucidate biological functions and disease relevance of previously identified genes. A current controversy over patent rights for DNA sequences encoding leptin receptors provides a useful case study for illustrating some of the issues that are likely to arise in applying doctrine derived from chemical patent cases in the context of gene discovery.

Developmental and nutritional changes of ob and PPAR gamma 2 gene expression in rat white adipose tissue

The ob gene encodes leptin, a hormone which induces satiety and increases energy expenditure. The peroxisome proliferator-activated receptor gamma 2 isoform (PPAR gamma 2) gene encodes a transcription factor which controls adipocyte differentiation and expression of fat-specific genes. We have studied the regulation of these two genes in white adipose tissue (WAT) during the suckling-weaning transition. Suckling rats ingest a high-fat diet (milk). Fat-pad weight barely varied during the last week of suckling. ob mRNA levels, which were very low in 15-day-old rats, rose approximately 6-fold until weaning at 21 days. When the rats were weaned on to a standard (high-carbohydrate) laboratory chow, epididymal WAT enlarged approximately 7-fold, and ob mRNA kept increasing progressively and doubled between 21 and 30 days. This evolution contrasted with that of fatty acid synthase (FAS) mRNA, which increased sharply, but only after weaning. To distinguish between the influence of developmental and nutritional factors on ob expression, a group of rats was weaned on to a high-fat diet. This prevented the rise in glycaemia and insulinaemia and the decrease in plasma non-esterified fatty acids which otherwise occurred at weaning. This also resulted in a slight (10-15%) decrease in food intake and body weight gain. Under this high-fat diet, the rise of ob mRNA in WAT was augmented (3.7-fold in 30- versus 21-day-old pups), whereas the normal rise in FAS mRNA levels was attenuated. Fat-pad weights and adipocyte cell size and number were roughly similar in high-carbohydrate- and high-fat-weaned pups. mRNA levels of PPAR gamma 2, like those of ob, were low in the WAT of 15-day-old suckling pups, doubled at 21 days, and reached a maximum as soon as 23 days. This evolution further differed from that of ob mRNA in not being influenced by diet composition. In conclusion, ob expression markedly increases during the suckling-weaning transition, and this effect is accentuated by a high-fat diet. Qualitative nutritional changes in ob mRNA were correlated with neither acute changes in adipose-tissue mass, nor cell size/number, nor variations in insulinaemia. PPAR gamma 2 also increased during suckling, but rapidly reached a plateau after weaning and no longer changed thereafter. Unlike ob, PPAR gamma 2 was not influenced by the diet composition.

Targeted disruption of the melanocortin-4 receptor results in obesity in mice

The melanocortin-4 receptor (MC4-R) is a G protein-coupled, seven-transmembrane receptor expressed in the brain. Inactivation of this receptor by gene targeting results in mice that develop a maturity onset obesity syndrome associated with hyperphagia, hyperinsulinemia, and hyperglycemia. This syndrome recapitulates several of the characteristic features of the agouti obesity syndrome, which results from ectopic expression of agouti protein, a pigmentation factor normally expressed in the skin. Our data identify a novel signaling pathway in the mouse for body weight regulation and support a model in which the primary mechanism by which agouti induces obesity is chronic antagonism of the MC4-R.

Heritability of plasma leptin in a population sample of African-American families

The aim of this study was to examine familial patterns of plasma leptin levels and the potential association with cardiovascular risk factors in a population sample of African-American families recruited from metropolitan Chicago. The study included 68 mothers, 31 fathers, 143 daughters, and 119 sons, for a total of 361 individuals from 118 families. Leptin levels were adjusted for the effect of age separately for mothers, fathers, daughters, and sons. Residuals were then standardized before estimating familial correlation using the maximum-likelihood method available in SEGPATH. With the exception of height, plasma leptin level was strongly correlated with all measured anthropometric variables. Familial effect (i.e., heritability) of leptin levels was estimated as 39% in this population at high risk for over weight. A significant sex difference was observed, and most of the estimated familial effect may be attributed to genetic influences since the spouse correlation was not statistically different from zero. A strong nonshared environmental effect is also suggested, however.

Gender differences in the response of plasma leptin concentrations to weight loss in obese older individuals

Plasma leptin concentration is directly related to the degree of obesity and is higher in women than in men of the same body mass index (BMI). We hypothesized that fasting plasma leptin concentrations and the response of leptin to weight loss would differ in older men and women of a similar fat mass. Plasma leptin concentrations (radioimmunoassay) and fat mass (DXA) were measured in 47 older, obese (BMI = 30 +/- 4 kg/m2) women and 23 older, obese (BMI = 31 +/- 3 kg/m2) men after a 2 to 4 week period of weight and dietary stabilization, and then in 22 of the women and 18 of the men after a 6-month weight loss intervention (250-350 kcal/d deficit). Leptin correlated with fat mass in men and women (r = 0.75 and r = 0.77, respectively; p values < 0.0001), but women had 3-fold higher leptin levels for a given fat mass than men (p = 0.01). In response to the 6-month hypocaloric diet, men and women lost a similar percentage of fat mass (-13% and -16%, respectively), but the relative decline in circulating leptin was greater in women than men (-45% and -21%, respectively; p < 0.0001). In addition, when leptin was normalized for fat mass using the ratio method, the decrease in leptin per kilogram of fat mass was greater in women than men (-0.37 +/- 0.34 vs. -0.04 +/- 0.06 ng/mL/kg; p < 0.01). After weight loss, the change in leptin concentrations correlated positively with the change in fat mass in men (r = 0.60; p < 0.01), but not in women (r = 0.31; p = 0.17). Furthermore, the loss in fat mass correlated negatively with baseline leptin levels in women (r = -0.47; p < 0.05), but not in men (r = 0.03, p = NS). These results indicate that the decline in leptin concentration with weight loss correlates with the loss in fat mass in men; but, in women, other factors affect the decrease in leptin concentration. This suggests that the role of leptin in the regulation of obesity is gender-specific and may account for gender differences in response to hypocaloric treatment and maintenance of lost weight.

The new biology of body weight regulation

A growing body of evidence suggests that energy balance (the difference between energy intake and expenditure) and body fuel stores in the form of adipose tissue are maintained by the body within a narrow range. This regulation of adiposity is mediated by the secretion of hormonal signals into the circulation in proportion to body adipose stores and their subsequent actions on brain systems that control caloric intake and energy expenditure. As a result, changes in energy balance sufficient to alter fuel stores elicit compensatory changes in energy intake and expenditure that return fat stores to their regulated level. Recent scientific break-through have identified the key components of this physiologic system. These include the circulating signals, leptin (the hormone encoded by the ob gene that is secreted by fat cells) and the pancreatic hormone insulin; and brain peptides such as neuropeptide Y, which is released from nerve terminals in the hypothalamus to elicit changes in feeding behavior and energy expenditure that mediate adaptive changes in energy balance. This article reviews the discovery of leptin and its receptor and discusses the interaction of leptin and insulin with the hypothalamic neuropeptide Y system. These observations provide a basis for understanding how weight lost during a period of negative energy balance (because of the inability to consume and/or store sufficient energy to meet ongoing energy demands) is eventually recovered. As our understanding of this weight-regulatory system increases, new insights into the causes of human obesity are likely to follow. Such insights may yield improvements in the medical and nutrition management of obese patients.

Obese (ob) gene defects are rare in human obesity

Our knowledge of the role of the recently cloned ob-protein (leptin) in the regulation of body fat stores is largely derived from experiments performed in mice. Different mouse models exhibit abnormalities in ob-gene expression, with extreme overexpression in mice which lack bioactive ob-protein, have nonfunctional ob-receptors or hypothalamic lesions, and undetectable expression in mice with suggested defects in regulatory elements. The aim of this study is to examine if defects, corresponding to those in mice, exist in human obesity. Adipose tissue was obtained from 94 adult obese subjects and from six children who had developed obesity after surgery in the hypothalamic region. Total RNA was isolated and ob-gene expression was examined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Northern blot. The coding region of the ob-gene was sequenced in both directions in the 94 obese adults. No mutations were detected in the coding region of the ob-gene and ob-gene expression was detectable in all subjects and none of the subjects had an extreme overexpression. There was no systematic increase in ob-expression in obese children with hypothalamic disease compared to their healthy brothers and sisters. These results show that severe abnormalities involving the ob-gene, analogous to those described in mouse models, are rare in human obesity. We therefore conclude that the cloning and subsequent analysis of the ob-gene has not provided information that can, by itself, explain the genetic component in the development of human obesity.

Short photoperiod reduces leptin gene expression in white and brown adipose tissue of Djungarian hamsters

Leptin gene expression in white (WAT) and brown adipose tissue (BAT) of the Djungarian hamster (Phodopus sungorus) was analyzed during seasonal acclimatization. Leptin gene expression in WAT was markedly reduced during winter, independent of changes in environmental temperature. Exposure to artificial short photoperiod also decreased leptin gene expression in WATas well as in BAT. Although specific leptin gene expression was lower in BAT, total depot expression was as high as in WAT depots, due to higher RNA content of BAT. Our results indicate that there is significant leptin synthesis in brown fat and that leptin might be involved in photoperiod mediated seasonal adaptations of mammals independent of food deprivation or overfeeding.

Leptin can induce proliferation, differentiation, and functional activation of hemopoietic cells

Many cytokines exert their biological effect through members of the hemopoietin receptor family. Using degenerate oligonucleotides to the common WSXWS motif, we have cloned from human hemopoietic cell cDNA libraries various forms of the receptor that was recently shown to bind the obesity hormone, leptin. mRNAs encoding long and short forms of the human leptin receptor were found to be coexpressed in a range of human and murine hemopoietic organs, and a subset of cells from these tissues bound leptin at the cell surface. Ectopic expression in murine Ba/F3 and M1 cell lines revealed that the long, but not the short, form of the leptin receptor can signal proliferation and differentiation, respectively. In cultures of murine or human marrow cells, human leptin exhibited no capacity to stimulate cell survival or proliferation, but it enhanced cytokine production and phagocytosis of Leishmania parasites by murine peritoneal macrophages. Our data provide evidence that, in addition to its role in fat regulation, leptin may also be able to regulate aspects of hemopoiesis and macrophage function.

Correction of obesity and diabetes in genetically obese mice by leptin gene therapy

The ob/ob mouse is genetically deficient in leptin and exhibits both an obese and a mild non-insulin-dependent diabetic phenotype. To test the hypothesis that correction of the obese phenotype by leptin gene therapy will lead to the spontaneous correction of the diabetic phenotype, the ob/ob mouse was treated with a recombinant adenovirus expressing the mouse leptin cDNA. Treatment resulted in dramatic reductions in both food intake and body weight, as well as the normalization of serum insulin levels and glucose tolerance. The subsequent diminishment in serum leptin levels resulted in the rapid resumption of food intake and a gradual gain of body weight, which correlated with the gradual return of hyperinsulinemia and insulin resistance. These results not only demonstrated that the obese and diabetic phenotypes in the adult ob/ob mice are corrected by leptin gene treatment but also provide confirming evidence that body weight control may be critical in the long-term management of non-insulin-dependent diabetes mellitus in obese patients.

The endocrinology of obesity

Numerous endocrine alterations are associated with obesity (Table 1). The majority of the alterations are secondary to obesity and must be considered simply associated and potentially in the pathogenesis of the complications of obesity. The discovery of new endocrine peptides such as leptin that signal body fat content will increase our understanding of the regulation of body fat content. As a result, therapies will most certainly be developed that are directly targeted at the alterations in endocrine function.

Leptin and OB-R: body weight regulation by a cytokine receptor

There has been intense recent interest in the molecules and pathways governing mammalian body weight regulation. Leptin (OB), an ancestral member of the cytokine family, is an adipocyte-secreted circulating hormone exhibiting weight regulatory properties. Recently, the leptin receptor (OB-R) was identified and shown to exhibit sequence homology and functional similarity to members of the class I cytokine receptor family. The mechanisms governing OB-R triggering and signal transduction have begun to be elucidated, providing new insight into the pathways controlling mammalian body weight homeostasis.