Leptin Therapy Research Articles

Leptin and Adiponectin in the HIV Associated Metabolic Syndrome: Physiologic and Therapeutic Implications

Leptin and adiponectin represent two newly discovered adipose tissue derived hormones with important roles in energy homeostasis and insulin resistance. Their interrelations with the manifestations of the HIV associated metabolic syndrome and specific somatomorphic changes i.e. fat redistribution is reviewed. A synopsis of published studies is presented and the potential role of leptin and adiponectin is discussed. We have described an association of the HIV metabolic syndrome with a state of reduced insulin sensitivity due to adiponectin deficiency. The metabolic syndrome is also accompanied by leptin deficiency in lipoatrophic subjects and possibly by a leptin resistance state in lipohypertrophic patients. Adiponectin and / or leptin therapy in a manner similar to other leptin deficiency states may assist in the future management of such patients.

Fat and Energy Partitioning: Longitudinal Observations in Leptin‐treated Adults Homozygous for a Lep Mutation

Partitioning of body energy content in the aleptinemic ob/ob mouse differs from that in wild-type mice, but it is not known whether parallel differences exist between humans with leptin (Lep) gene mutations and healthy adults. The objective of this study was to evaluate body composition in three leptin-treated Turkish adults homozygous for a missense mutation (C-->T substitution at codon 105 resulting in Arg-->Trp) of Lep. Subjects, one male and two female Turkish family members, were evaluated at baseline and treated for 18 months with r-MetHuLeptin. Patient data (fat mass, energy content) were compared with adult sex-specific predicted values (adjusted for weight, height, and age) derived in healthy control subjects. Weight loss with leptin treatment was substantial, ranging from 43.9% to 52.1%. At baseline and with leptin treatment, the two women had a fat mass and energy content similar (+/-12%) to predicted values. Baseline fat and energy content in the male patient was high compared with predicted values (e.g., fat +33%) but approached and reached normal values (e.g., fat, +2%) after 18 months of leptin therapy. Adult women with Lep mutations had body composition similar to normal women at baseline and with leptin treatment. In contrast, the man with a Lep mutation had high body fat in the untreated state but a normal male phenotype with leptin administration, possibly secondary to androgenic fat partitioning effects. Fat and energy partitioning can, thus, be quantitatively assessed and linked with potential hormonal mechanisms in humans with inherited disturbances in energy regulation.

This Week: Getting Lean with Leptin

Treatment of lean rats with the hormone leptin depleted fat from adipocytes, which of course has raised considerable interest in the possibility of using leptin as a treatment for obesity. However, failure of the hormone to reverse obesity has shown that metabolic regulation needs to be understood better in order to take advantage of its potential therapeutic benefits. Wang et al. therefore designed experiments to uncover how white adipocytes are able to store triglycerides at the same time as they secrete leptin at concentrations that, when experimentally administered to lean rats, would block adipogenesis. They identified two mechanisms by which adipocytes from rats fed a 60% fat diet become resistant to leptin. Within 6 days after exposure to the high-fat diet, there was a large increase in the expression of mRNA encoding SOCS3 (suppressor of cytokine signaling 3), an inhibitor of leptin signaling through its receptor (Lepr-b); after several weeks, the level of mRNA encoding Lepr-b decreased. The authors conclude that a high-fat diet causes resistance to leptin signaling in adipocytes and that the hypertrophy and hyperplasia that cause obesity can only occur if such mechanisms allow the adipocytes to ignore the extracellular leptin concentrations to which they are exposed. They further speculate that a period of starvation of patients might reduce such a blockade and allow a beneficial response to leptin therapy in obese patients. — NRG

The Long-term Effect of Recombinant Methionyl Human Leptin Therapy on Hyperandrogenism and Menstrual Function in Female and Pituitary Function in Male and Female Hypoleptinemic Lipodystrophic Patients

The Long-term Effect of Recombinant Methionyl Human Leptin Therapy on Hyperandrogenism and Menstrual Function in Female and Pituitary Function in Male and Female Hypoleptinemic Lipodystrophic Patients

Fat Rats Get Lean with Leptin

Treatment of lean rats with the hormone leptin causes depletion of fat from adipocytes, which of course has raised considerable interest in the possibility of using leptin as a treatment for obesity. However, failure of the hormone to reverse obesity has shown that metabolic regulation by the hormone is complicated and needs to be better understood to take advantage of its potential therapeutic benefits. Wang et al. therefore designed experiments to uncover how it is that white adipocytes are able to store triglycerides while they are secreting concentrations of leptin that, when experimentally administered to lean rats, would block adipogenesis. They identified two mechanisms by which adipocytes from rats fed on a high-fat diet (60% fat) become resistant to the actions of leptin. Within 6 days after exposure to the high-fat diet, the authors detected a large increase in the expression of mRNA encoding SOCS3 (suppressor of cytokine signaling-3), an inhibitor of leptin signaling through its receptor (Lepr-b). Also, after several weeks, the abundance of mRNA encoding Lepr-b was decreased. Transgenic expression of Lepr-b in adipocytes restored sensitivity of adipocytes to leptin [as measured by phosphorylation of the transcription factor STAT3 (signal transducer and activator of transcription 3)]. These mice also remained lean. In fact, the transgenic mice on the diet containing 60% fat remained as lean as control mice on the diet containing only 4% fat. The authors conclude that a high-fat diet causes resistance to leptin signaling in adipocytes and that hypertrophy and hyperplasia that cause obesity can only occur if such a mechanism allows the adipocytes to ignore the extracellular leptin concentrations to which they are exposed. They further speculate that a period of starvation of patients might reduce such a blockade and allow a beneficial response to leptin therapy in obese patients. M.-y. Wang, L. Orci, M. Ravazzola, R. H. Unger, Fat storage in adipocytes requires inactivation of leptin's paracrine activity: Implications for treatment of human obesity. Proc. Natl. Acad. Sci. U.S.A. 102 , 18011-18016 (2005). [Abstract] [Full Text]

Leptin Replacement Therapy Modulates Circulating Lymphocyte Subsets and Cytokine Responsiveness in Severe Lipodystrophy

We conducted this study to understand the role of leptin therapy in immunomodulation. Our objective was to study lymphocyte subpopulations and in vitro peripheral blood mononuclear cell (PBMC) activation during a study evaluating the effects of leptin on metabolic functions in severe lipodystrophy (serum leptin levels < 4 ng/ml). We conducted an open-label study with patients serving as their own control at the Clinical Research Center of the National Institutes of Health. Ten patients (age range, 15-63 yr; one male and nine females) with generalized forms of lipodystrophy were studied. Patients were treated with recombinant human leptin to achieve high normal concentrations for 4 to 8 months. Leptin levels increased from 1.8 +/- 0.4 to 16.5 +/- 3.9 ng/dl (P < 0.001), whereas metabolic control improved [glycosylated hemoglobin (HbA(1c)) fell from 9.3 +/- 0.4 to 7.1 +/- 1.4%, P < 0.001, and triglycerides decreased by 45 +/- 11% from a mean of 1490 +/- 710 mg/dl, P = 0.001]. Lymphocyte subsets were studied by flow cytometry at baseline and at 4 and 8 months of therapy. PBMC responsiveness was evaluated by cytokine release and proliferation after stimulation with phytohemagglutinin, phytohemagglutinin plus IL-12, lipopolysaccharide, and lipopolysaccharide plus interferon-gamma at baseline and 4 months. Various T lymphocyte subsets were significantly lower than age- and sex-matched controls at baseline; however, the CD4/CD8 ratio was normal. The relative percentages of B lymphocytes and monocytes were elevated, although the absolute levels were normal. Leptin therapy induced significant changes in T lymphocyte subsets, which normalized both the absolute number of T lymphocyte subsets and relative percentages of all lineages. Additionally, in vitro TNF-alpha secreted from PBMC of patients was significantly increased to normal after 4 months of leptin therapy compared with baseline. These data support existing evidence that leptin has a modest immunomodulatory effect in hypoleptinemic humans.

Energy adaptation to glucocorticoid-induced hyperleptinemia in human beings

Recombinant leptin therapy potently decreases food intake and body weight in aleptinemic individuals with leptin gene mutations. However, it is unknown whether manipulation of endogenous leptin secretion alters ingestive behavior in otherwise healthy subjects. We therefore assessed energy consumption during administration of hydrocortisone (HC), a known leptin secretagogue. Six healthy adults were admitted overnight on 2 occasions and given HC (12.5 mg/h IV) or saline infusion for 24 hours (8:00 am-8:00 am) in a randomized crossover design. Total energy and macronutrient intake was calculated using a computerized nutrient analysis program. Blood sampling for measurement of leptin, cortisol, glucose, and insulin was performed at baseline and every 1 to 2 hours. A rise in plasma leptin level was noted after ∼5 hours of HC infusion and was sustained throughout the study period. The total energy consumed was 3004 ± 231 kcal for saline and 2486 ± 214 kcal for HC ( P = .005); breakfast energy values on day 1 were similar but energy values consumed at lunch, dinner, and day 2 breakfast were all significantly lower during induced hyperleptinemia. Analysis of macronutrients indicated significant decreases in carbohydrate and fat intake during glucocorticoid-induced hyperleptinemia as compared with placebo. These results indicate that stimulation of native leptin secretion decreases energy consumption, similar to the effect observed with recombinant leptin therapy. To our knowledge, this is the first report documenting anti-ingestive responses to a leptin secretagogue. Because chronic glucocorticoid therapy is fraught with adverse effects, we suggest that nonsteroidal or steroidomimetic leptin secretagogues may be good candidates for anorectic and antiobesity drug development.

Microanalysis of eating behavior of three leptin deficient adults treated with leptin therapy

Leptin deficiency has been associated with extreme obesity and hyperphagia in rodents and humans. A rare genetic disorder in humans yields the absence of the hormone leptin, extreme obesity, and a ravenous appetite. Reports on these rare cases have indicated that therapy using leptin injections can yield significant weight loss and changes in appetite. The aim of this report on acute leptin therapy in three leptin deficient adults was to provide a microanalysis of changes in eating behavior and ratings of hunger and satiety. In addition to substantial weight loss, 15 weeks of leptin therapy was associated with approximately 50% reduction in food intake and substantial changes in ratings of hunger and satiety before most meals. After short-term leptin therapy, the three participants ate until ratings indicated they were satiated, which was comparable to the ratings before leptin therapy. These findings suggest that one of the primary effects of acute leptin therapy may be to reduce the ravenous hunger associated with leptin deficiency, resulting in reduced food intake and significant weight loss. These results are discussed in the context of the scientific literature pertaining to leptin and its effects on appetite and obesity.

Leptin Improves Insulin Resistance and Hyperglycemia in a Mouse Model of Type 2 Diabetes

Leptin has metabolic effects on peripheral tissues including muscle, liver, and pancreas, and it has been successfully used to treat lipodystrophic diabetes, a leptin-deficient state. To study whether leptin therapy can be used for treatment of more common cases of type 2 diabetes, we used a mouse model of type 2 diabetes (MKR mice) that show normal leptin levels and are diabetic due to a primary defect in both IGF-I and insulin receptors signaling in skeletal muscle. Here we show that leptin administration to the MKR mice resulted in improvement of diabetes, an effect that was independent of the reduced food intake. The main effect of leptin therapy was enhanced hepatic insulin responsiveness possibly through decreasing gluconeogenesis. In addition, the reduction of lipid stores in liver and muscle induced by enhancing fatty acid oxidation and inhibiting lipogenesis led to an improvement of the lipotoxic condition. Our data suggest that leptin could be a potent antidiabetic drug in cases of type 2 diabetes that are not leptin resistant.

Authors’ Response: Leptin Therapy Does Not Affect Inflammatory Markers

Authors’ Response: Leptin Therapy Does Not Affect Inflammatory Markers

Perigestational suppression of weight gain with central leptin gene therapy results in lower weight F1 generation

The efficacy of central leptin therapy on weight homeostasis through various phases of reproduction, pregnancy outcome and postnatal, prepubertal and pubertal growth of offspring was assessed. Enhanced leptin transgene expression after a single intracerebroventricular injection of recombinant adeno-associated virus vector encoding the leptin gene (rAAV-lep) decreased calorie intake and weight in adult nulliparous female rats. rAAV-lep treated rats conceived normally, displayed unremarkable pregnancy rate, parturition and delivered normal sized litters. Significantly lower weight was maintained through gestation, lactation, and post-lactation periods. The maintenance of a modest weight reduction was accompanied by voluntarily reduced calorie intake, increased thermogenic energy expenditure, decreased adiposity as reflected by drastically reduced leptin levels, and suppressed insulin and insulin-like growth factor 1 levels through lactation and post-lactation in rAAV-lep treated dams. The offspring at birth weighed significantly less than those of controls and this lower weight range was sustained during postnatal, prepubertal, pubertal and adult (3 months old) periods, contemporaneous with metabolic circulating hormones in the normal range. For the first time we show the persistent efficacy of central leptin gene therapy to suppress weight gain through all phases of reproduction, lactation and post-lactation in dams and reveal the potential imprinting link to producing lower weight in the F1 generation.

The long-term effect of recombinant methionyl human leptin therapy on hyperandrogenism and menstrual function in female and pituitary function in male and female hypoleptinemic lipodystrophic patients

Lipodystrophy patients are hypoleptinemic and insulin resistant. Women have enlarged polycystic ovaries, hyperandrogenism, and amenorrhea. We have determined the role of correction of hypoleptinemia on these metabolic and neuroendocrine parameters. Ten females and 4 males with generalized lipodystrophy were treated with recombinant methionyl human leptin (r-metHuLeptin) in physiologic doses in an open-labeled study for a period of 12 and 8 months, respectively. In the female group, serum free testosterone decreased from 39.6 ± 11 to 18.9 ± 4.5 ng/dL ( P < 0.01) and serum sex hormone binding globulin increased from 14 ± 2.5 to 25 ± 4.8 nmol/L ( P < 0.02). Luteinizing hormone (LH) responses to LH releasing hormone were more robust after therapy and significantly changed in the youngest group of 3 female patients ( P < 0.01). Ovarian ultrasound showed a polycystic ovarian disease pattern in all patients and did not change after therapy. Eight of the 10 patients had amenorrhea prior to therapy and all 8 developed normal menses after therapy. In the male group, serum testosterone tended to increase from 433 ± 110 to 725 ± 184 ng/dL ( P = 0.1) and sex hormone binding globulin also increased from 18.25 ± 2.6 to 27 ± 1.7 nmol/L ( P < 0.04) following r-metHuLeptin therapy. Serum LH response to LH releasing hormone did not show significant changes. Five additional hypoleptinemic male subjects with minimal metabolic abnormalities underwent normal pubertal development without receiving r-metHuLeptin therapy. In both genders, insulin-like growth factor increased significantly and there were no differences in growth hormone, thyroid, or adrenal hormone levels following r-metHuLeptin therapy. Glycemic parameters significantly improved after r-metHuLeptin therapy in both groups. Hypoglycemic medications were discontinued in 7 of 12 patients and dramatically reduced in 5 patients. r-metHuLeptin therapy plays an important role in insulin sensitivity. In females, it plays an additional role in normalizing menstrual function. This is likely to occur both from increasing insulin sensitivity and from restoring LH pulsatility. The persistent hypoleptinemic state in these subjects did not inhibit pubertal development.

Leptin and energy expenditure

A fundamental advance in our understanding of endocrine control of energy balance and body weight came with the discovery of the adipocyte-derived hormone leptin. The leptin pathway appeared to be the long-sought peripheral signal pathway from the adipose tissue to the brain involved in the regulation of feeding and energy balance. Initially, leptin was considered to function as the long-sought antiobesity hormone. According to this hypothesis, rising concentrations of leptin with increasing adiposity would generate a signal to reduce food intake and increase energy expenditure in order to limit further weight gain. However, widespread resistance to the proposed antiobesity action of leptin is observed in humans, which might reflect the fact that the inability to store energy efficiently at times of abundance is evolutionarily disadvantageous. According to this alternative view, falling leptin concentrations observed during fasting act as a peripheral signal of starvation, which serves to conserve energy in the face of limited reserves. However, leptin administration failed to blunt the changes in energy expenditure during severe energy restrictions in several clinical studies. In addition, leptin therapy in several different human low-leptin states failed to affect energy expenditure in recent studies. Increasing evidence from human studies suggests that leptin predominantly influences the human energy balance through appetite but appears not to be involved in regulating energy expenditure. None of the expected factors such as resting metabolic rate, total diurnal energy expenditure or dietary induced thermogenesis was related to blood leptin concentrations.

Changes in body composition in patients with severe lipodystrophy after leptin replacement therapy

Leptin, an adipocyte hormone, when replaced in patients with lipodystrophy, improves insulin resistance, hyperglycemia, dyslipidemia, and hepatic steatosis. Changes in body composition accompany this metabolic improvement. We studied 14 patients (3 men and 11 women); 12 of who had generalized lipodystrophy (7 congenital, 5 acquired), and 2 patients had partial lipodystrophy. Body composition and related parameters were evaluated at baseline and after 4 and 12 months of leptin therapy. Baseline body mass index (BMI) was 21.7 ± 0.8 kg/m 2, the percent body fat was 9.5% ± 1.6%, and the serum leptin level was 1.7 ± 0.3 ng/mL. On treatment, serum leptin levels increased by 10-fold. All patients reported a decrease in appetite on therapy. After 4 months, both daily caloric intake and resting energy expenditure (REE) decreased. The liver volume decreased (baseline = 3,055 ± 281 cm 3; 4 months = 2,433 ± 243 cm 3, P = .006). Dual energy x-ray absorptiometry (DEXA) demonstrated significant decreases in fat mass (5.4 ± 0.8 kg to 5.0 ± 0.8 kg; P = .003) and lean body mass (51.2 ± 3.2 kg to 48.3 ± 3.4 kg; P = .003) at 4 months on therapy. There was no impact of leptin therapy on bone mineral content, mineral density, and metabolism. Changes in body composition occurred during the first 4 months of leptin therapy, but then stabilized and were sustained thereafter.

The clinical uses of leptin.

Leptin is the first of a group of adipocyte-secreted hormones to be used clinically to treat hypoleptinemic states. In children with congenital leptin deficiency and extreme obesity, leptin induces satiety and a dramatic loss of weight. In hypoleptinemic patients with extreme insulin resistance and lipodystrophy, leptin ameliorates insulin resistance, hyperglycemia, hyperinsulinemia, dyslipidemia and hepatic steatosis. In both these leptin-deficient states, leptin therapy restores gonadotrophin secretion, as well as luteinizing hormone and thyroid-stimulating hormone pulsitility.

Keeping Obesity and Metabolic Syndrome at Bay with Central Leptin and Cytokine Gene Therapy

The incidence of obesity and metabolic syndrome along with the annual cost of treating obesity and related metabolic disorders are rising steeply world wide. In this article, the potential of gene therapy application directly into the hypothalamus, a CNS site involved in integration of energy homeostasis, as a therapeutic modality is reviewed. We engineered a non-immunogenic, non-pathogenic recombinant adeno-associated virus vector to encode leptin, leptin receptor, or the cytokines, leukemia inhibitory factor and ciliary neurotrophic factor. A single injection of virus vectors encoding any of these genes either intraventricularly or into selected hypothalamic sites of wild type or genetically obese rodents curtailed the gradual age-related and rapid high fat diet-induced obesity for long periods. Parameters of metabolic syndrome, hyperleptinemia, hyperinsulinemia and hyperlipidemia were normalized along with fat depletion and increased non-shivering thermogenic energy expenditure either alone or in association with voluntary reduction in food intake. Operation of hypothalamic appetite and energy regulating network was correspondingly modified to conform with the anorectic and obesity suppressing effects of central leptin and cytokine gene therapy. These results advocate central gene therapy as a viable alternative to curb the rising tide of obesity and associated metabolic diseases with or without a voluntary reduction in food intake. Keywords: gene therapy, cytokines, leptin, viral vectors, adeno-associated virus, hypothalamus, adiposity, metabolic syndrome, hyperinsulinemia, type 2 diabetes

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

The wide range of phenotypic abnormalities seen in the leptin-deficient ob/ob mouse and their reversibility by leptin administration provide compelling evidence for the existence of multiple physiological functions of this hormone in rodents. In contrast, information regarding the roles of this hormone in humans is limited. Three morbidly obese children, who were congenitally deficient in leptin, were treated with daily subcutaneous injections of recombinant human leptin for up to 4 years with sustained, beneficial effects on appetite, fat mass, hyperinsulinemia, and hyperlipidemia. Leptin therapy resulted in a rapid and sustained increase in plasma thyroid hormone levels and, through its age-dependent effects on gonadotropin secretion, facilitated appropriately timed pubertal development. Leptin deficiency was associated with reduced numbers of circulating CD4+ T cells and impaired T cell proliferation and cytokine release, all of which were reversed by recombinant human leptin administration. The subcutaneous administration of recombinant human leptin has major and sustained beneficial effects on the multiple phenotypic abnormalities associated with congenital human leptin deficiency.

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

The wide range of phenotypic abnormalities seen in the leptin-deficient ob/ob mouse and their reversibility by leptin administration provide compelling evidence for the existence of multiple physiological functions of this hormone in rodents. In contrast, information regarding the roles of this hormone in humans is limited. Three morbidly obese children, who were congenitally deficient in leptin, were treated with daily subcutaneous injections of recombinant human leptin for up to 4 years with sustained, beneficial effects on appetite, fat mass, hyperinsulinemia, and hyperlipidemia. Leptin therapy resulted in a rapid and sustained increase in plasma thyroid hormone levels and, through its age-dependent effects on gonadotropin secretion, facilitated appropriately timed pubertal development. Leptin deficiency was associated with reduced numbers of circulating CD4+ T cells and impaired T cell proliferation and cytokine release, all of which were reversed by recombinant human leptin administration. The subcutaneous administration of recombinant human leptin has major and sustained beneficial effects on the multiple phenotypic abnormalities associated with congenital human leptin deficiency.

Effect of Leptin Replacement on Pituitary Hormone Regulation in Patients with Severe Lipodystrophy

Leptin is important in regulating energy homeostasis. Severe lipodystrophy is associated with leptin deficiency and insulin resistance, hypertriglyceridemia, and hepatic steatosis. Leptin deficiency is also associated with abnormalities of the pituitary hormones in rodent models and patients with congenital absence of leptin. We inquired whether similar abnormalities are seen in patients with lipodystrophy and whether replacement of leptin will make an impact on the regulation of pituitary hormones. Seven female patients (aged 15-42 yr, all diabetic) with lipodystrophy and serum leptin levels less than 4 mg/liter were treated with recombinant methionyl-human leptin (recombinant leptin) in physiological doses in an open-labeled study. The following parameters were evaluated before and at 4 months of leptin treatment: menstrual history, pelvic ultrasonogram, LHRH, TRH, and CRH tests. While on recombinant leptin, mean serum leptin concentration increased from 1.3 +/- 0.3 mg/liter to 11.1 +/- 2.5 mg/liter. Only one of five patients who had intact reproductive systems was cycling normally before leptin therapy, and all five had normal menses by the fourth month of leptin therapy. Serum E2 concentrations increased (110 +/- 44 pmol/liter vs. 546 +/- 247 pmol/liter, P = 0.002), serum T concentrations decreased (3.5 +/- 3.0 nmol/liter vs. 1.3 +/- 0.7 nmol/liter, P = 0.055), and the attenuated LH response to LHRH was corrected with therapy. Serum T(3) and free T(4) were in the normal range before leptin therapy and did not change. However, serum TSH concentrations fell from 2.2 +/- 1.1 microU/ml to 1.2 +/- 0.7 microU/ml (P < 0.001). The percent increase in TSH following TRH administration was similar before (560%) and at 4 months (580%) of leptin therapy. The mean nonstimulated ACTH and cortisol concentrations were, respectively, 6.0 +/- 3.4 pmol/liter and 680 +/- 280 nmol/liter before leptin and did not change after 4 months of therapy (4.2 +/- 1.2 pmol/liter, P = 0.11 and 453 +/- 142 nmol/liter, P = 0.13, respectively). The ACTH and cortisol responses to CRH stimulation were normal both before and after therapy. Leptin replacement improved menstrual abnormalities and low E2 levels and corrected the attenuated LH response to LHRH in a group of young women with lipodystrophy and leptin deficiency. These results add to the growing body of evidence that metabolic signals such as leptin play a role in neuroendocrine regulation.

High-dose leptin activates human leukocytes via receptor expression on monocytes.

Leptin is capable of modulating the immune response. Proinflammatory cytokines induce leptin production, and we now demonstrate that leptin can directly activate the inflammatory response. RNA expression for the leptin receptor (Ob-R) was detectable in human PBMCs. Ob-R expression was examined at the protein level by whole blood flow cytometry using an anti-human Ob-R mAb 9F8. The percentage of cells expressing leptin receptor was 25 +/- 5% for monocytes, 12 +/- 4% for neutrophils, and 5 +/- 1% for lymphocytes (only B lymphocytes). Incubation of resting PBMCs with leptin induced rapid expression of TNF-alpha and IL-6 mRNA and a dose-dependent production of TNF-alpha and IL-6 by monocytes. Incubation of resting PBMCs with high-dose leptin (250 ng/ml, 3-5 days) induced proliferation of resting cultured PBMCs and their secretion of TNF-alpha (5-fold), IL-6 (19-fold), and IFN-gamma (2.5-fold), but had no effect on IL-4 secretion. The effect of leptin was distinct from, and additive to, that seen after exposure to endotoxin or activation by the mixed lymphocyte reaction. In conclusion, Ob-R is expressed on human circulating leukocytes, predominantly on monocytes. At high doses, leptin induces proinflammatory cytokine production by resting human PBMCs and augments the release of these cytokines from activated PBMCs in a pattern compatible with the induction of Th1 cytokines. These results demonstrate that leptin has a direct effect on the generation of an inflammatory response. This is of relevance when considering leptin therapy and may partly explain the relationship among leptin, proinflammatory cytokines, insulin resistance, and obesity.