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

Expression of leptin and leptin receptor isoforms in the human stomach

BACKGROUND

Leptin is an important regulator of food intake and energy expenditure. Initially it was thought to be expressed exclusively in and secreted by adipocytes. Recently, leptin expression was also noted in other tissues, including rat gastric mucosa. Information on leptin and leptin receptor expression in the human stomach is lacking.

AIM

To investigate expression of leptin and its corresponding receptors in human gastric epithelial cells.

METHODS

Fundic and antral gastric mucosal biopsies, primary cultures of human gastric epithelial cells, and the human gastric cancer cell line AGS were screened for expression of leptin and different leptin receptor isoform mRNA by reverse transcriptase-polymerase chain reaction. Immunohistochemistry was performed for localisation of leptin and leptin receptor proteins in gastric mucosa.

RESULTS

mRNA of leptin and its four receptor isoforms (huOB-R, long receptor isoform; huB219.1-3, short receptor isoforms) was detected in gastric mucosal biopsies, cultured human gastric epithelial cells, and gastric cancer cells. Immunohistochemistry demonstrated that chief as well as parietal cells were reactive to leptin and leptin receptors.

CONCLUSIONS

Leptin and leptin receptors are expressed in human gastric mucosa. These findings suggest a paracrine and/or autocrine effect of leptin on gastric epithelial cell function.

Leptin

The discovery of the adipose-derived hormone leptin has generated enormous interest in the interaction between peripheral signals and brain targets involved in the regulation of feeding and energy balance. Plasma leptin levels correlate with fat stores and respond to changes in energy balance. It was initially proposed that leptin serves a primary role as an anti-obesity hormone, but this role is commonly thwarted by leptin resistance. Leptin also serves as a mediator of the adaptation to fasting, and this role may be the primary function for which the molecule evolved. There is increasing evidence that leptin has systemic effects apart from those related to energy homeostasis, including regulation of neuroendocrine and immune function and a role in development.

High concentration of leptin stimulates myeloid differentiation from human bone marrow CD34+ progenitors: potential involvement in leukocytosis of obese subjects

OBJECTIVE AND DESIGN

As well as its involvement in control of adipose mass and body energy balance, several reports suggest a link between leptin and hemopoiesis. To test its putative role in human hemopoiesis, we developed a homologous system, ie recombinant human leptin treatment of purified CD34+ progenitors from adult human bone marrow.

RESULTS

Leptin (50-100 ng/ml) significantly stimulated the appearance of granulocyte-macrophage colonies in the presence or absence of erythropoietin. The concentration of leptin required for this effect was rather high but within the range of plasma leptin levels observed in obesity. Two results further support the hypothesis that leptin may be involved in the leukocytosis associated with obesity: (i) leptin concentrations in bone marrow and plasma of subjects studied were highly correlated; (ii) leptin and leukocyte count were correlated only in obese subjects. Paracrine effects of locally released leptin from bone marrow adipocytes could also be involved in the regulation of hemopoiesis, a hypothesis supported by marrow immunocytochemistry revealing the close association of CD34+ cells with adipocytes and by previous demonstration that leptin is secreted at a high level by these cells.

CONCLUSION

These results indicate that leptin acts on human multilineage CD34+ cells and that high plasma leptin levels associated with obesity could participate in the differentiation of granulocytes from hemopoietic progenitors.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Leptin is a potent stimulator of bone growth in ob/ob mice

Leptin, the product of the obese gene, is a circulating hormone secreted primarily from adipocytes. The lack of leptin in ob/ob mice, who are homozygous for the obese gene, results in hyperglycemia, hyperinsulinemia, hyperphagia, obesity, infertility, decreased brain size and decreased stature. To this end, we investigated the role of leptin as a hormonal regulator of bone growth. Leptin administration led to a significant increase in femoral length, total body bone area, bone mineral content and bone density in ob/ob mice as compared to vehicle treated controls. The increase in total body bone mass was a result of an increase in both trabecular and cortical bone mass. These results suggest that the decreased stature of the ob/ob mouse is due to a developmental defect that is readily reversible upon leptin administration. Our demonstration that the signalling or long form (Ob-Rb) of the leptin receptor is present in both primary adult osteoblasts and chondrocytes suggests that the growth promoting effects of leptin could be direct. In summary, these results indicate a novel role for leptin in skeletal bone growth and development.

Leptin values in placental cord blood of human newborns with normal intrauterine growth after 30-42 weeks of gestation

OBJECTIVE

To evaluate leptin values in placental cord blood of newborns with normal intrauterine growth after 30-42 weeks of gestation.

DESIGN

Leptin, a protein encoded by the ob gene, plays an important role in the regulation of feeding behaviour and energy balance in rodents, primates and humans. The presence of leptin in human amniotic fluid and cord blood has recently been reported in human gestations at term and the possible role of leptin in human fetal growth suggested. However, little is known of leptin synthesis during human foetal development. Thus, the aim of our work was to measure leptin (RIA, Linco Research, Inc.) in placental cord blood of human newborns at different fetal ages.

PATIENTS

One hundred and twenty-six healthy newborns with normal intrauterine growth were studied. Twenty-nine were preterm (15 males and 14 females; gestational age: 30-36 weeks) and 99 were at term (49 males and 48 females; gestational age: 37-42 weeks).

RESULTS

Leptin values increase progressively throughout gestation from 1.30 +/- 0.53 ng/ml at 30 weeks of gestation to 7.98 +/- 4.96 ng/ml (mean +/- SD) at term, and correlate positively with birth weight (r = 0.56, p < 0. 005, n = 126), length (r = 0.37, p < 0.005, n = 126), BMI (r = 0.57, p < 0.005, n = 126), head circumference (r = 0.37, p < 0.005, n = 126), gestational age (r = 0.48, p < 0.005, n = 126) and placental weight (r = 0.38, p < 0.003, n = 59). Leptin values are statistically significantly lower (p < 0.005) preterm (median: 2.05 ng/ml; range: 0.7-8.3 ng/ml) than at term (median: 7.0 ng/ml; range: 1.1-28.1 ng/ml). Leptin values are also significantly (p < 0.005) higher in females (median: 7.2 ng/ml; range: 0.9-23.6 ng/ml, n = 62) than in males (median: 4.8 ng/ml; range: 0.7-28.1 ng/ml, n = 64), although there are no differences in weight (2,864 +/- 536 g in females vs. 2,937 +/- 744 g in males). Multiple regression analysis shows weight to be a positive sex-independent predictor of serum leptin values (p < 0.0005). Sex also proves to be a predictor of leptin, independently of weight and is higher in females than in males (p < 0.003).

CONCLUSION

Leptin is present in placental human cord blood after 30-42 weeks of gestation. Newborn weight and sex are independent predictors of leptin values.

Leptin enhances wound re-epithelialization and constitutes a direct function of leptin in skin repair

Wound-healing disorders are a therapeutic problem of extensive clinical importance. Leptin-deficient ob/ob mice are characterized by a severely delayed wound healing that has been explained by the mild diabetic phenotype of these animals. Here we demonstrate that systemically and topically supplemented leptin improved re-epithelialization of wounds in ob/ob mice. Leptin completely reversed the atrophied morphology of the migrating epithelial tongue observed at the wound margins of leptin-deficient animals into a well-organized hyperproliferative epithelium. Moreover, topically supplemented leptin accelerated normal wound-healing conditions in wild-type mice. As assessed by immunohistochemistry, proliferating keratinocytes located at the wound margins specifically expressed the leptin-receptor subtype ObRb during repair. Additionally, leptin mediated a mitogenic stimulus to the human keratinocyte cell line HaCaT and human primary keratinocytes in vitro. Therefore, leptin might represent an effective novel therapeutic factor to improve impaired wound-healing conditions.

Subcellular localization of leptin in non-tumorous and adenomatous human pituitaries: an immuno-ultrastructural study

Leptin is a key mediator in the maintenance of neuroendocrine homeostasis. Recently, leptin and leptin receptor expression were demonstrated in non-tumorous and adenomatous human pituitaries. This study was performed to determine the subcellular localization of leptin in human adenohypophyses (n = 3) and in various types of pituitary adenoma (n = 16). Immunoelectron microscopy showed leptin immunolabeling in most hormone-producing cells of the human non-tumorous adenohypophysis, but not in stellate cells. Labeling was noted over secretory granules. Immunocytochemistry using double labeling revealed leptin expression in GH-, ACTH-, TSH-, and FSH/LH-containing cells but not in PRL cells. The percentage of immunopositive cells and the intensity of immunostaining varied considerably among the various cell types. Immunoelectron microscopy with double gold labeling showed co-localization of leptin and adenohypophysial hormones in the same secretory granules. Among pituitary tumors, leptin immunolabeling was evident only in corticotroph adenomas. Compared to non-tumorous corticotrophs, leptin immunoexpression was less abundant in corticotroph adenomas. The presence of leptin and adenohypophysial hormones in the same secretory granules suggests that leptin is secreted concomitantly with various adenohypophysial hormones and that its release is under the control of hypothalamic stimulating and inhibiting hormones.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Role of adipose tissue in body-weight regulation: mechanisms regulating leptin production and energy balance

Adipose tissue performs complex metabolic and endocrine functions. Among the endocrine products produced by adipose tissue are tumour necrosis factor alpha, interleukin 6, acylation-stimulating protein and leptin. The present review will focus primarily on mechanisms regulating leptin production and leptin action, and the implications of this regulation in the control of energy balance. Leptin acts in the central nervous system where it interacts with a number of hypothalamic neuropeptide systems to regulate feeding behaviour and energy expenditure. The presence of extreme obesity in animals and human subjects with mutations of the leptin gene or the leptin receptor demonstrates that normal leptin production and action are critical for maintaining energy balance. Insulin is the major regulator of leptin production by adipose tissue. Insulin infusions increase circulating leptin concentrations in human subjects. Plasma leptin levels are markedly decreased in insulin-deficient diabetic rodents, and the low leptin levels contribute to diabetic hyperphagia. Based on in vitro studies, the effect of insulin to stimulate leptin production appears to involve increased glucose metabolism. Blockade of glucose transport or glycolysis inhibits leptin expression and secretion in isolated adipocytes. Evidence suggests that anaerobic metabolism of glucose to lactate does not stimulate leptin production. Alterations in insulin-mediated glucose metabolism in adipose tissue are likely to mediate the effects of energy restriction to decrease, and refeeding to increase, circulating leptin levels. Changes in glucose metabolism may also explain the observation that high-fat meals lower 24h circulating leptin levels relative to high-carbohydrate meals in human subjects, suggesting a mechanism that may contribute to the effects that high-fat diets have in promoting increased energy intake, weight gain and obesity. The decreased circulating leptin observed during energy restriction is related to increased sensations of hunger in human subjects. Thus, decreases in leptin during energy-restricted weight-loss regimens may contribute to the strong propensity for weight regain. A better understanding of the precise mechanisms regulating leptin production and leptin action may lead to new approaches for managing obesity.

Long form leptin receptor mRNA expression in the brain, pituitary, and other tissues in the pig

Much effort has focused recently on understanding the role of leptin, the obese gene product secreted by adipocytes, in regulating growth and reproduction in rodents, humans and domestic animals. We previously demonstrated that leptin inhibited feed intake and stimulated growth hormone (GH) and luteinizing hormone (LH) secretion in the pig. This study was conducted to determine the location of long form leptin receptor (Ob-Rl) mRNA in various tissues of the pig. The leptin receptor has several splice variants in the human and mouse, but Ob-Rl is the major form capable of signal transduction. The Ob-Rl is expressed primarily in the hypothalamus of the human and rodents, but has been located in other tissues as well. In the present study, a partial porcine Ob-Rl cDNA, cloned in our laboratory and specific to the intracellular domain, was used to evaluate the Ob-Rl mRNA expression by RT-PCR in the brain and other tissues in three 105 d-old prepuberal gilts and in a 50 d-old fetus. In 105 d-old gilts, Ob-Rl mRNA was expressed in the hypothalamus, cerebral cortex, amygdala, thalamus, cerebellum, area postrema and anterior pituitary. In addition, Ob-Rl mRNA was expressed in ovary, uterine body, liver, kidney, pancreas, adrenal gland, heart, spleen, lung, intestine, bone marrow, muscle and adipose tissue. However, expression was absent in the thyroid, thymus, superior vena cava, aorta, spinal cord, uterine horn and oviduct. In the 50 d-old fetus, Ob-Rl mRNA was expressed in brain, intestine, muscle, fat, heart, liver and umbilical cord. These results support the idea that leptin might play a role in regulating numerous physiological functions.

Expression of the leptin receptor during germ cell development in the mouse testis

Leptin, a recently identified hormonal product of the ob gene, is known to regulate appetite, body metabolism, and reproductive functions. We investigated the expression of the leptin receptor (Ob-R) in testes from different age groups. The messenger RNA for Ob-R was found in testes from all age groups using RT-PCR. Using immunohistochemistry, we observed age- and stage-dependent distribution of the Ob-R in mouse testis. In testis of 5-day-old mice, its expression was mainly in type A spermatogonia. In the 20- and 30-day-old testis, Ob-R expression was in the spermatocytes; in the adult testis, it was specific to spermatocytes in stages IX and X of the cycle of the seminiferous epithelium. Five main immunoreactive proteins were detected using Western blot (220, 120, 90, 66, and 46 kDa). The 120-kDa protein was evident only in 20-day-old and older testes, whereas the 90-kDa band was present only in the 5- and 10-day-old testis. Leptin treatment induced phosphorylation of signal transducer and activator of transcription-3 in cultured seminiferous tubules from adult and 5-day-old testes. Our results show for the first time age- and stage-specific localization of a functional Ob-R in testicular germ cells. We hypothesize a direct role for leptin, through phosphorylation of signal transducer and activator of transcription-3, in proliferation and differentiation of germ cells, which may partially explain the infertility observed in leptin-deficient mice.

Identification and expression analysis of leptin-regulated immediate early response and late target genes

Using PC12 cells as an in vitro model system, we have identified a series of transcripts induced through activation of the leptin receptor. On the basis of kinetic studies, two distinct gene sets could be discerned: signal transducer and activator of transciption-3 (STAT-3), suppressor of cytokine signalling-3 (SOCS-3), MT-II (metallothionein-II), the serine/threonine kinase fibroblast-growth-factor-inducible kinase (Fnk) and modulator recognition factor (MRF-1), which are immediate early response genes, and pancreatitis-associated protein I (PAP I), squalene epoxidase, uridine diphosphate glucuronosyltransferase and annexin VIII, which are late induced target genes. At late time points a strong co-stimulation with beta-nerve growth factor or with the adenylate cyclase activator forskolin was observed. To assess the validity of the PC12-cell model system, we examined the effect of leptin administration on the gene transcription of STAT-3, MT-II, Fnk and PAP I in vivo. Leptin treatment of leptin-deficient ob/ob mice increased the STAT-3, SOCS-3, MT-II and Fnk mRNA, and MT-I protein levels in liver, whereas, in jejunum, expression of PAP I mRNA was down-regulated. Furthermore, administration of leptin to starved wild-type mice enhanced the expression of MT-II and Fnk mRNA in liver, but decreased MT-II and PAP I mRNA expression in jejunum. These findings may help to explain the obese phenotype observed in some colonies of MT-I- and MT-II-null mice and/or the observation that leptin protects against tumour-necrosis-factor toxicity in vivo.

Binding of a pure 125I-monoiodoleptin analog to mouse tissues: a developmental study

The preparation of a pure 125I-labeled monoiododerivative of mouse leptin is described. This radiolabeled analog has been used to characterize and localize central and peripheral leptin binding sites (Ob-R) of the mouse at different stages of its development. The affinity values found in membrane homogenates of various mouse tissues are similar and range between 0.1 and 0.3 nM, indicating that all the Ob-R isoforms have a similar affinity. Leptin binding sites are highly expressed at the membrane level in lung, intestine, kidney, liver, and skin and to a lesser degree in stomach, heart, and spleen. Brain, thymus, and pancreas homogenates are devoid of any specific binding. The distribution of mouse Ob-R has also been explored by autoradiography and dipping techniques on whole mouse sections. In lung, leptin binding sites are located at the pulmonary parenchyma and at the bronchiolar epithelial level. Binding sites are expressed all along the digestive tract from the tongue to the rectum (esophagus, stomach, intestine, colon, and rectum). In muscular visceral structures (stomach, intestine, and bladder) the binding is mainly present in the lamina propria. During development, leptin receptors are early expressed in the liver, kidney, and bone. In the lung, the Ob-R level increased gradually from birth to adulthood where the expression is maximal. By contrast, leptin receptors located in the medulla of the kidney remain remarkably constant all along the development. A broad signal is present in cartilage and bone particularly in vertebrae, limb, and ribs. Interestingly, leptin receptors are barely detectable in the mouse brain except in the choroid plexus and leptomeninges, whereas in the rat brain leptin binding sites are located in the thalamus, the piriform cortex, the cerebellum (at the granular and molecular cell layer), and the pineal gland.

Ontogeny of the expression of leptin and its receptor in the murine fetus and placenta

Leptin is a 167-amino acid protein that is secreted from adipose cells and expressed in placental tissues. It is important nutritionally in the regulation of energy balance, but also has other functions such as a role in reproduction. To investigate the function of the leptin system in fetal development we examined, primarily by in-situ hybridization and immunohistochemistry, the expression (both mRNA and protein) of leptin and its receptor (including the signalling splice variant) in tissues from 11.5, 13.5, 16.5 and 18.5 d postcoitus murine fetuses and associated placentas. We detected leptin mRNA (at low levels) and protein predominantly in the cytotrophoblasts of the labyrinth part of the placenta, an area of nutrient exchange between the developing fetus and the placenta, and in the trophoblast giant cells situated in the junctional zone at the maternal interface. In addition, leptin was strongly expressed in the fetal cartilage-bone and at a lower level in the hair follicles, heart, and liver of the murine fetus at differing stages of development. The leptin receptor, including the signalling splice variant, was also identified in specific fetal tissues. The physiological importance of expression of both leptin and the leptin receptor (OB-R and OB-Rb) in the placenta remains to be determined. In addition, the high levels of expression of leptin and its receptor in discrete areas of the murine fetus suggest that leptin has a critical role in fetal development.

Altered hepatic lymphocyte subpopulations in obesity-related murine fatty livers: potential mechanism for sensitization to liver damage

Although obesity-related fatty livers are vulnerable to damage from endotoxin, the mechanisms involved remain obscure. The purpose of this study was to determine if immunologic priming might be involved by determining if fatty livers resemble normal livers that have been sensitized to endotoxin damage by Propionibacterium acnes infection. The latter induces interleukin (IL)-12 and -18, causing a selective reduction of CD4+NK T cells, diminished IL-4 production, deficient production of T-helper type 2 (Th-2) cytokines (e.g., IL-10), and excessive production of Th-1 cytokines (e.g., interferon gamma [IFN-gamma]). Liver and spleen lymphocyte populations and hepatic cytokine production were compared in genetically obese, ob/ob mice (a model for obesity-related fatty liver) and lean mice. Obese mice have a selective reduction of hepatic CD4+NK T cells. Serum IL-18 is also increased basally, and the hepatic mRNA levels of IL-18 and -12 are greater after endotoxin challenge. Thus, up-regulation of IL-18 and IL-12 in fatty livers may reduce hepatic CD4+NK T cells. In addition, mononuclear cells from fatty livers have decreased expression of the adhesion molecule, leukocyte factor antigen-1 (LFA-1), which is necessary for the hepatic accumulation of CD4+NK T cells. Consistent with reduced numbers of hepatic CD4+NK T cells, mononuclear cells from fatty livers produce less IL-4. Furthermore, after endotoxin treatment, hepatic induction of IL-10 is inhibited, while that of IFN-gamma is enhanced. Thus, fatty livers have inherent immunologic alterations that may predispose them to damage from endotoxin and other insults that induce a proinflammatory cytokine response.

Leptin-deficient (ob/ob) mice are protected from T cell-mediated hepatotoxicity: role of tumor necrosis factor alpha and IL-18

The role of leptin was investigated in two models of T cell-mediated hepatitis: the administration of Con A or of Pseudomonas aeruginosa exotoxin A (PEA). In both models, leptin-deficient (ob/ob) mice were protected from liver damage and showed lower induction of tumor necrosis factor (TNF) alpha and IL-18 compared with their lean littermates. Neutralization of TNF-alpha reduced induction of IL-18 by either Con A (70% reduction) or PEA (40% reduction). Pretreatment of lean mice with either soluble TNF receptors or with an anti-IL-18 antiserum significantly reduced Con A- and PEA-induced liver damage. The simultaneous neutralization of TNF-alpha and IL-18 fully protected the mice against liver toxicity. However, neutralization of either IL-18 or TNF-alpha did not inhibit Con A-induced production of IFN-gamma. Thymus atrophy and alterations in the number of circulating lymphocytes and monocytes were observed in ob/ob mice. Exogenous leptin replacement restored the responsiveness of ob/ob mice to Con A and normalized their lymphocyte and monocyte populations. These results demonstrate that leptin deficiency leads to reduced production of TNF-alpha and IL-18 associated with reduced T cell-mediated hepatotoxicity. In addition, both TNF-alpha and IL-18 appear to be essential mediators of T cell-mediated liver injury.

Central leptin suppresses splenic lymphocyte functions through activation of the corticotropin-releasing hormone-sympathetic nervous system

Leptin is one of the key afferent signals that regulate food intake and energy expenditure by acting on specific receptors in the hypothalamus. Recently, leptin was reported to activate the peripheral immune system by acting directly on lymphocytes. To elucidate the brain-mediated participation of leptin in the modulation of peripheral immune functions, we examined the effects of intracerebroventricular (icv) injection of murine recombinant leptin on the proliferative response to Concanavalin A (ConA response) of splenic lymphocytes in rats. The ConA response of splenic lymphocytes was markedly reduced 30 min after icv injection of leptin. The suppressive effect of leptin was abolished completely either by surgical severing of the splenic nerves or by icv injection of an antibody against corticotropin-releasing hormone (CRH), but only partially by an anti-urocortin antibody. Icv injection of CRH and urocortin also suppressed the ConA response of splenic lymphocytes, and the effect of urocortin was prevented by the anti-CRH antibody, while that of CRH was not prevented by the anti-urocortin antibody. These results suggest that leptin suppresses peripheral lymphocyte functions, in contrast to the direct activating effects, indirectly through the activation of the CRH (urocortin)-sympathetic nervous system.

Leptin receptor and leukemia

The receptor for leptin, the gene product of the obese gene, is expressed in hematopoietic stem cells. Leptin stimulates normal myeloid and erythroid development, and is secreted from bone marrow adipocytes, which occupy most of the marrow cavity in humans. Leptin might thus play an important role in the control of the expansion and differentiation of primitive hematopoietic cells through paracrine interaction in the bone marrow microenvironment. Leukemic cells of some patients with acute myeloblastic leukemia, acute lymphoblastic leukemia, and chronic myeloid leukemia (CML) also express the leptin receptor. In cases of CML, higher expression of leptin receptor is observed during blast crisis than in chronic phase. Leptin alone and in combination with other cytokines has stimulative effects on proliferation of leukemia cells as well as anti-apoptotic effects. These findings suggest the possibility that leptin plays roles in the pathophysiology of leukemia.

Human leptin enhances activation and proliferation of human circulating T lymphocytes

Leptin is an adipocyte-secreted hormone that centrally regulates weight control. However, leptin receptor is expressed not only in the central nervous system, but also in other systems such as reproductive and hematopoietic tissues. Human leptin has previously been shown to enhance cytokine production by murine peritoneal macrophages and human circulating monocytes. In this paper we have assessed the presence of leptin receptors in peripheral human T lymphocytes and we have studied their functional role. Both CD4(+) and CD8(+) T lymphocytes express leptin receptors. Moreover, we show that human leptin dose-dependently enhances proliferation and activation of human circulating T lymphocytes when they are costimulated by PHA or Con A. Leptin alone was not able to activate T lymphocytes. To confirm a direct effect of leptin on T lymphocytes, monocytes were extracted by adhesion to culture flasks. The early activation surface marker CD69 was then induced in both CD4(+) and CD8(+) T lymphocytes after 8 h stimulation with PHA or Con A. Leptin dose-dependently enhanced stimulated CD69 expression. Moreover, leptin dose-dependently enhanced the expression of the late activation markers CD25 and CD71 in both CD4(+) and CD8(+) T lymphocytes after 48 h stimulation with PHA or Con A. Finally, we have found that leptin modulates CD4(+) T lymphocyte activation toward Th1 phenotype by stimulating the synthesis of IL-2 and IFN-gamma. These results demonstrate the presence of the leptin receptor in human circulating CD4(+) and CD8(+) T lymphocytes and a functional role of leptin as a modulator (enhancer) of lymphocyte stimulation with a shift toward Th1 cytokine-production profile. This function of leptin may have some relevance in the pathophysiology of immunologic alterations related to obesity.

Leptin expression is reduced with acute endotoxemia in the pig: correlation with glucose, insulin, and insulin-like growth factor-1 (IGF-1)

Leptin has been implicated in the regulation of anorexia associated with cachexia in rodents and humans. Regulation of leptin expression is under complex endocrine and metabolic control. To determine if leptin expression is regulated by acute inflammation and to define the endocrine and metabolic factor(s) that regulates leptin expression during acute inflammation, castrate male pigs (ad libitum fed, used as their own controls) were treated with saline (control period) and endotoxin (lipopolysaccharide [LPS] period). Frequent blood samples were collected to identify dynamic changes in hormones and metabolites that are known to regulate leptin expression. LPS caused fever and elevated plasma cortisol (p < 0.0004), tumor necrosis factor-alpha (TNF-alpha) (p < 0.0001), and plasma nonesterified fatty acids (NEFA) (p < 0.001) compared with control. Circulating insulin (p < 0.01), glucose (p < 0.003), and insulin-like growth factor-1 (IGF-1) (p < 0.0001), as well as adipose leptin mRNA abundance (p < 0.01), were profoundly reduced following LPS treatment compared with control. Our data indicate that during acute endotoxemia (1-10 h after injection), leptin gene expression is decreased compared with ad libitum fed animals and is more closely related to energy homeostasis than cytokine profiles in plasma.

Detection and regulation of leptin receptor mRNA in ovine mammary epithelial cells during pregnancy and lactation

Adipocyte-epithelial cell interactions and their secretions are critical determinants of mammary gland development. In this present study, we examined the possible involvement of leptin and its receptors in the process of mammogenesis/lactogenesis. We demonstrated by reverse transcription and polymerase chain reaction analysis that long and short forms of leptin receptors were expressed in the ovine mammary gland during pregnancy and lactation. Furthermore, quantitative determinations, via ribonuclease protection assays, provided evidence that the level of leptin receptor expression was greatest during mid-pregnancy when active growth of the mammary gland is initiated. Location of the leptin receptors, as determined by in situ hybridization analysis, revealed that leptin receptor transcripts were expressed specifically in mammary epithelial cells. These data provide evidence that leptin, with its receptors, could be an important mediator in regulating mammary gland growth and development.

Physiological roles of the leptin endocrine system: differences between mice and humans

Leptin is a 16-kDa cytokine secreted in humans primarily but not exclusively by adipose tissues. Its concentration in blood is usually proportional to body fat mass, but is higher in women than in men not only because of a different distribution of and greater fat mass in women, but also because testosterone reduces its level in men. Leptin features in different ways during the life span. It is synthesized in the ovary, transported in the oocyte, and made by both fetus and placenta, particularly during the last month of gestation. It is made by the lactating mammary gland and ingested by the newborn infant in its milk. The prime importance of leptin is realized at puberty when it is necessary for progression to a normal adult reproductive status in females. Fasting and chronic undernutrition result in a lower level of leptin in the blood. Lack of leptin results in hunger, ensuring that the individual eat to survive, and also inhibition of reproduction, until such time as food and fat stores are adequate to supply energy for pregnancy and lactation. Thus, leptin is important for survival of the individual and survival of the species. Although an extremely rare genetic absence of leptin induces hyperphagia and obesity in humans, as it does in mice, there appears to be little role for leptin in humans in ensuring that fat stores are not in excess of adequate, that is, in preventing obesity. The mouse differs from humans in many respects, in particular in the far more drastic ways it conserves energy when it very rapidly adapts to lack of food. These include not only suppression of reproduction but also lowering of its body temperature (torpor), suppressing its thyroid function, suppressing its growth, and increasing secretion of stress hormones (from the adrenal). This review concentrates on roles of leptin in human physiology and pathophysiology but also discusses why some observations on actions of leptin in mice are not applicable to humans.

Leptin stimulates glucose uptake in C2C12 muscle cells by activation of ERK2

Leptin regulates energy homeostasis via binding to receptors in the hypothalamus and peripheral tissues. We have investigated the signaling pathways and effects of leptin on glucose transport in C2C12 muscle cells. Long and short forms of leptin receptor are expressed in differentiated C2C12 myotubes. Leptin enhanced the DNA-binding activity of the transcription factor STAT3 and extracellular signal-regulated kinase 2 (ERK2) activity was stimulated by leptin after 15 min. Leptin increased glucose uptake and GLUT4 recruitment to the cell surface after 30 min, whereas no changes in GLUT1 was observed. PD98059, an ERK2 kinase-1 inhibitor, and wortmannin, an inhibitor of phosphatidylinositol 3-kinase blocked the leptin-induced increase in glucose uptake and GLUT4 recruitment to the cell surface. In contrast, insulin-stimulated glucose transport and GLUT4 translocation was inhibited by wortmannin, but not by PD98059. Our results suggest that leptin may regulate glucose metabolism by acting directly on skeletal muscle and that the signaling pathways involved may be different from that activated by insulin.

Expression of functional leptin receptors in rodent Leydig cells

Several studies indicate that the size of body fat stores and the circulating levels of the adipocyte-derived hormone leptin are able to influence the activity of the hypothalamic-pituitary-gonadal axis. The leptin-hypothalamic-pituitary-gonadal interactions have been mainly studied at the level of the central nervous system. In this study, we investigated the possibility that leptin may have direct effects on the rodent Leydig cell function. To probe this hypothesis, we first analyzed the expression of leptin receptors (OB-R) in rodent Leydig cells in culture. RT-PCR studies showed that rat Leydig cells express both the long (OB-Rb) and short isoform (OB-Ra) of leptin receptor, whereas MLTC-1 cells (a murine Leydig tumor cell line) express only the long isoform. Short-term (30-90 min) incubation of rat Leydig cells with increasing concentrations ofleptin (2-500 ng/ml) led to a significant and dose-dependent inhibition of human (h)CG-stimulated testosterone (T) production (approximately 60% reduction, IC50 = 20 ng/ml) but no change in basal androgen release. Also, leptin (150 ng/ml) amplified hCG-induced intracellular cAMP formation (1- to 2-fold) without modifying basal cAMP levels. Subsequent experiments showed that leptin inhibited 8Br-cAMP-stimulated T production, indicating that leptin's effect is exerted beyond cAMP. The inhibitory effect of leptin on hCG-induced T secretion was accompanied by a significant reduction of androstenedione and a concomitant rise of the precursor metabolites pregnenolone, progesterone, and 17-OH-progesterone, conceivable with a leptin-induced lesion of 17,20 lyase activity. Separate experiments performed with the MLTC-1 cells (not expressing cytochrome P450-17alpha) showed that leptin, though amplifying hCG-stimulated cAMP production, did not modify hCG-stimulated pregnenolone and progesterone release. These results further indicate that leptin action on steroidogenesis occurs downstream of progesterone synthesis. Northern Blot experiments showed no acute effect of leptin on cytochrome P450-17alpha messenger RNA accumulation in rat Leydig cells in basal and hCG-stimulated conditions, excluding that the rapid changes observed were caused by messenger RNA degradation. In conclusion, these findings, for the first time, show that leptin has direct, receptor-mediated actions on rodent Leydig cells in culture, at concentrations within the range of obese men.

The effect of restricted nutrition on uterine macrophage populations in mice

The abundant macrophage populations present in the endometrium are implicated in the tissue remodelling events and immunological changes necessary for pregnancy. Using two regimens of restricted nutrition (95 and 88% of ad libitum intake for 19 days), we have shown that moderately reduced food consumption can dramatically alter the number of endometrial macrophages and their immunoaccessory function in mice. Restricted nutrition also interfered with the estrous cycle, but the effects on endometrial macrophages were more extensive and qualitatively different than could be explained by diminished ovarian steroid hormone activity. Significantly less F4/80+ and Ia+ cells were found in the endometrium of food restricted mice than in ad libitum mice at the same estrous cycle stage. In the more severely restricted mice the losses were even greater than those seen after ovariectomy. In ad libitum fed animals, uterine but not peritoneal macrophages showed an ovarian hormone-dependent inhibitory phenotype in a splenocyte mitogenesis assay. Macrophages derived from both locations exhibited greater immunostimulatory activity following restricted nutrition. We conclude that endometrial macrophage populations are influenced by nutritional status and this may be mediated through both steroid hormone-dependent and -independent mechanisms. Nutritionally induced aberrations in the number or behaviour of endometrial macrophages during the estrous cycle or in early pregnancy could have important implications for the quality of the pre- and peri-implantation environment and the maternal immune response to pregnancy.

Plasma leptin level is associated with myocardial wall thickness in hypertensive insulin-resistant men

Leptin, the product of the ob gene, has been shown to increase heart rate and blood pressure through a stimulation of cardiac sympathetic nervous system activity, a phenomenon also involved in the pathogenesis of left ventricular hypertrophy in hypertensives. Thus, we hypothesize that plasma leptin concentration is associated with left ventricular hypertrophy. Forty hypertensive males and 15 healthy male subjects underwent anthropometric and echocardiographic evaluations, assessment of insulin sensitivity through euglycemic glucose clamp combined with indirect calorimetry, and determination of fasting plasma leptin concentration. Fasting plasma leptin levels were higher in hypertensives than in controls (6.48+/-2.9 versus 4. 62+/-1.5 ng/mL, P<0.05); these results were unchanged after adjustment for body mass index (P<0.05). In the whole group of patients (n=55), fasting plasma leptin concentration was correlated with body mass index (r=0.46, P<0.001) and waist/hip ratio (r=0.50, P<0.001); independent of body mass index and waist/hip ratio, fasting plasma leptin concentration was correlated (n=55) with whole-body glucose disposal (r=-0.27, P<0.04), interventricular septum thickness (r=0.34, P<0.001), posterior wall thickness (r=0.38, P<0.003), and the sum of wall thicknesses (r=0.68, P<0.001). In a multivariate analysis (n=55), age, body mass index, fasting plasma leptin concentration, plasma Na(+) concentration, whole-body glucose disposal, and diastolic blood pressure explained 68% of the variability of the sum of wall thicknesses with fasting plasma leptin concentration (P<0.03), whole body glucose disposal (P<0.002), and diastolic blood pressure (P<0.001), which were significantly and independently associated with the sum of wall thicknesses. In conclusion, our study demonstrates that fasting plasma leptin levels are associated with increased myocardial wall thickness independent of body composition and blood pressure levels in hypertensives.

Leptin inhibits insulin secretion induced by cellular cAMP in a pancreatic B cell line (INS-1 cells)

The effect of leptin on insulin secretion is controversial due to conflicting results in the literature. In the present study, we incubated insulin-producing rat insulinoma INS-1 cells for 60 min and examined the effects of recombinant murine leptin (20 nmol/l). We found that leptin (0.1-100 nmol/l) did not affect the insulin response to glucose (1-20 mmol/l). However, when cells were incubated with agents that increase the intracellular content of cAMP, i.e., glucagon-like peptide-1 (100 nmol/l), pituitary adenylate cyclase activating polypeptide (100 nmol/l), forskolin (2.5 micromol/l), dibutyryl-cAMP (1 mmol/l), or 3-isobutyl-1-methylxanthine (100 micromol/l), leptin significantly reduced insulin secretion (by 34-58%, P < 0.05-0.001). In contrast, when insulin secretion was stimulated by the cholinergic agonist carbachol (100 micromol/l) or the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (1 micromol/l), both of which activate protein kinase C, leptin was without effect. We conclude that leptin inhibits insulin secretion from INS-1 cells under conditions in which intracellular cAMP is increased. This suggests that the cAMP-protein kinase A signal transduction pathway is a target for leptin to inhibit insulin secretion in insulin-producing cells.

Neuroendocrine regulation and actions of leptin

The discovery of the adipocyte-produced hormone leptin has greatly changed the field of obesity research and our understanding of energy homeostasis. It is now accepted that leptin is the afferent loop informing the hypothalamus about the state of fat stores, with hypothalamic efferents regulating appetite and energy expenditure. In addition, leptin has a role as a metabolic adaptator in overweight and fasting states. New and previously unsuspected neuroendocrine roles have emerged for leptin. In reproduction, leptin is implicated in fertility regulation, and it is a permissive factor for puberty. Relevant gender-based differences in leptin levels exist, with higher levels in women at birth, which persist throughout life. In adult life, there is experimental evidence that leptin is a permissive factor for the ovarian cycle, with a regulatory role exerted at the hypothalamic, pituitary, and gonadal levels, and with unexplained changes in pregnancy and postpartum. Leptin is present in human milk and may play a role in the adaptive responses of the newborn. Leptin plays a role in the neuroendocrine control of GH secretion, through a complex interaction at hypothalamic levels with GHRH and somatostatin. Leptin participates in the expression of CRH in the hypothalamus, interacts at the adrenal level with ACTH, and is regulated by glucocorticoids. Since leptin and cortisol show an inverse circadian rhythm, it has been suggested that a regulatory feedback is present. Finally, regulatory actions on TRH-TSH and PRL secretion have been found. Thus leptin reports the state of fat stores to the hypothalamus and other neuroendocrine areas, and the neuroendocrine systems adapt their function to the current status of energy homeostasis and fat stores.

Leptin stimulates proliferation and TGF-beta expression in renal glomerular endothelial cells: potential role in glomerulosclerosis [seecomments]

BACKGROUND

Leptin inhibits food intake and increases energy expenditure. Although the kidney expresses abundant transcripts of the short form of the leptin receptor (Ob-Ra), a role for this hormone in renal function remains unclear. Because individuals with massive obesity who may exhibit increased leptin serum concentrations develop renal glomerulosclerosis, we studied whether leptin can influence renal growth and profibrogenic processes.

METHODS

The effects of recombinant leptin on proliferation and synthesis of transforming growth factor-beta1 (TGF-beta1) was investigated in cultured glomerular endothelial cells of the rat (GERs) and syngeneic mesangial cells. Furthermore, leptin receptor expression and potential signal transduction pathways were evaluated in GERs. In addition, leptin was also infused for different time periods (72 hr and 3 weeks) into naive rats.

RESULTS

Recombinant mouse leptin induced proliferation of GERs, but not of syngeneic mesangial cells. Coincubation with angiotensin II and leptin exerts additive proliferative effects in GERs. An antileptin-receptor antibody totally abolished this proliferation but did not influence serum-induced proliferation. GER expressed high affinity receptors of the Ob-Ra type (Kd, 4 nM; Bmax, 9700 receptors/cell). Leptin also stimulated phosphorylation of STAT1alpha, and kinase inhibitors attenuated proliferation, suggesting a pivotal role of phosphorylation in this process. Incubation of GERs with leptin also induced mRNA expression of TGF-beta1 and enhanced secretion of this profibrogenic cytokine. Short-term leptin infusion (72 hr) into naive rats induced a significant proliferation, mainly restricted to glomerular endothelial cells, and enhanced glomerular TGF-beta1 mRNA levels. In rats continuously infused for three weeks with leptin, glomerular TGF-beta1 expression was still enhanced, and an additional increase in glomerular collagen type IV mRNA and protein expression was detected. These animals revealed an increase in proteinuria compared with control-infused rats.

CONCLUSION

Our findings are the first in vitro and in vivo demonstration that leptin is a renal growth and profibrogenic factor. These results may be an important contribution to our understanding of how leptin can contribute to renal damage, characterized by endocapillary proliferation and subsequent development of glomerulosclerosis, in pathophysiological situations with high circulating levels such as in diabetics or obese individuals. Although the effects of leptin itself are moderate, growth-promoting and profibrogenic effects may be enhanced in concert with other factors such as angiotensin II.

The human apM-1, an adipocyte-specific gene linked to the family of TNF's and to genes expressed in activated T cells, is mapped to chromosome 1q21.3-q23, a susceptibility locus identified for familial combined hyperlipidaemia (FCH)

The human adipocyte-specific apM-1 gene encodes a secretory protein of the adipose tissue that has been suggested to play a role in the pathogenesis of obesity. The regulation of apM-1 was studied along adipocyte differentiation. While apM-1-mRNA and apM-1 protein were absent in preadipocytes and in 48 h differentiated adipocytes, they were found upregulated from day 4 to day 9 of adipocyte differentiation as shown by RNase protection assay and Western blot analysis. These data indicate that apM-1 may be a late marker of adipocyte differentiation. In human sera apM-1 protein is also detectable by Western blots using a polyclonal antibody raised against a synthetic peptide sequence of the human apM-1. The genomic structure of the human apM-1 gene together with a total of 2.7 kb of the 5'-flanking region with putative transcription factor binding sites is presented. Interestingly, sequence comparisons link the apM-1 gene to the family of TNF's and to genes expressed in activated T-cells. The chromosomal localization of apM-1 was investigated by FISH and mapped to human chromosome 1q21.3-1q23, a region that was identified as a susceptibility locus for Familial Combined Hyperlipidaemia (FCH) and polygenic NIDDM. These data and the chromosomal localization on chromosome 1q21.3-q23 raises the possibility that apM-1 as an adipocyte-specific secretory protein may play a role in the pathogenesis of FCH and associated insulin resistance. Exon- and intron-specific primer sequences are presented as a basis for mutation screening of patients affected with FCH.

Leptin and its clinical implications in chronic renal failure

Leptin, the recently identified ob gene product, regulates food intake and energy expenditure in animal models. Leptin reaches the brain by a saturable transport mechanism and, via direct effects on the hypothalamus, decreases appetite and increases metabolism. Several recent studies have demonstrated markedly elevated serum leptin levels in patients with chronic renal failure (CRF) and it has been speculated that hyperleptinemia may contribute to uremic anorexia and malnutrition. Several factors may influence serum leptin levels in uremia and apart from decreased glomerular filtration rate also body fat mass and plasma insulin levels are important factors that determine serum leptin levels. The possible influence of chronic inflammation on serum leptin levels in CRF need further studies. Patients treated by peritoneal dialysis seem to have higher leptin levels compared to patients treated by hemodialysis. This could be the effect of a marked increase in body fat mass as a consequence of the continuous carbohydrate load. Leptin receptors have by now been identified in several peripheral organs which suggests that leptin besides having central effects also has a pleiotropic action. Indeed, recent findings indicate that besides regulating appetite leptin may play a role in sympathico-activation, insulin metabolism, renal sodium handling and hematopoiesis.

Human leptin stimulates proliferation and activation of human circulating monocytes

The Ob gene product, leptin, is an adipocyte-secreted hormone that centrally regulates weight control. However, leptin receptor is expressed not only in the central nervous system, but also in other systems such as reproductive and hematopoietic tissues. Human leptin has previously been shown to enhance cytokine production by murine peritoneal macrophages. In this paper we show that human leptin stimulates proliferation in a dose-dependent manner and functionally activates human circulating monocytes in vitro, by inducing the production of cytokines such as TNF-alpha and IL-6. Proliferation was assessed both by [3H]thymidine and bromodeoxyuridine incorporation at 48 h. We also checked the leptin stimulated monocyte expression of activation markers by flow cytometry: CD25, HLA-DR, CD38, CD71, CD11b, and CD11c expression increased after 72 h. Moreover, leptin increases the expression of the early activation marker CD69 in monocytes but not in lymphocytes. The stimulation produced by leptin is comparable to that produced by endotoxin [lipopolysaccharide (LPS)]. In addition, leptin can potentiate the stimulatory effect of LPS or PMA. Furthermore, we studied cytokine production (TNF-alpha and IL-6) simultaneously by flow cytometric detection of intracellular cytokines in the activated monocytes. Leptin produced a dose-dependent increase in the number of activated monocytes producing cytokines. These data indicate that leptin is a potent stimulatory hormone on human peripheral blood monocytes and suggest that it may have a role as a proinflammatory cytokine.

A role for leptin in hemopoieses?

The recent discovery of leptin as a major controller of appetite has led to a detailed analysis of its specific actions in this process as well as any potential role in the etiology of obesity. It has also emerged that leptin has a wider spectrum of biological activities and has been strongly implicated in fertility and reproduction. The structural similarity between leptin and its receptor and cytokine-receptor systems that control hemopoiesis has also prompted investigation of the potential for this hormone to influence blood cell formation. Recent studies have shown that the leptin receptor is expressed on a diverse range of hemopoietic cells. Leptin itself appears to enhance proliferation of hemopoietic cells in vitro, particularly in combination with other cytokines and may augment some mature hemopoietic cell functions. Although only relatively minor hemopoietic deficiencies have been reported in mice lacking leptin or its receptor, these emerging studies suggest that further analysis of leptin actions in hemopoiesis may be warranted.

Mechanism of adipose tissue iNOS induction in endotoxemia

The aim of the present study was to investigate the mechanism of adipose tissue inducible nitric oxide synthase (iNOS) induction in endotoxemia. Systemic administration of the bacterial endotoxin lipopolysaccharide (LPS) to rats for </=8 h markedly increased iNOS mRNA and protein levels in white and brown adipose tissues. This effect was comparable to or greater than the induction of iNOS in liver, kidney, or skeletal muscle. iNOS activity was also found to be greatly enhanced in both white and brown adipose tissues of LPS-treated rats (an approximately 12- to 20-fold increase). Treatment of cultured 3T3-L1 adipocytes with LPS, tumor necrosis factor-alpha (TNF-alpha), or interferon-gamma (IFN-gamma) alone failed to induce iNOS activity. However, when used in combination, TNF-alpha, IFN-gamma, and LPS markedly and synergistically increased iNOS activity in these cells. In conclusion, these results suggest that adipose tissue is a major site of iNOS expression in endotoxemia. Our data further indicate that iNOS induction can be reproduced in vitro in cultured adipocytes and that a concerted action of cytokines and endotoxin is needed for maximal activation of the enzyme.

Effects of acute or prolonged exposure to human leptin on isolated human islet function

The adipocyte-derived hormone leptin has been reported to inhibit, have no effect, or potentiate insulin secretion in-vitro; these effects mainly depend on the species considered, the concentrations used, and the length of exposure. We investigated the direct effects of recombinant human leptin (HL) on human pancreatic beta cell function by studying insulin secretion (IS), hexokinase and glucokinase activity and Km, and potassium channel permeability in purified human islets (HI). In acute experiments, no effect of 1, 5, 20, or 50 ng/ml HL on glucose or arginine stimulated insulin release was found, whereas 500 ng/ml HL caused a significant decrease of glucose induced IS. After 24h pre-culture with either 20 or 500 ng/ml HL, a significant reduction of glucose (but not arginine) stimulated IS was observed. Exposure to leptin caused a significant increase of potassium channel permeability, whereas hexokinase and glucokinase activity and Km remained unchanged. These results suggest that physiological human leptin concentration is able to importantly affect glucose (but not arginine) stimulated insulin release from human islets only after prolonged exposure. This effect is probably mediated by changes of potassium channel permeability, and is not accompanied by modifications of glucose phosphorylating enzymes properties.

Central administration of leptin to ovariectomized ewes inhibits food intake without affecting the secretion of hormones from the pituitary gland: evidence for a dissociation of effects on appetite and neuroendocrine function

We have studied the effect of leptin on food intake and neuroendocrine function in ovariectomized ewes. Groups (n = 5) received intracerebroventricular infusions of either vehicle or leptin (20 microg/h) for 3 days and were blood sampled over 6 h on days -1, 2, and for 3 h on day 3 relative to the onset of the infusion. The animals were then killed to measure hypothalamic neuropeptide Y expression by in situ hybridization. Plasma samples were assayed for metabolic parameters and pituitary hormones. Food intake was reduced by leptin, but did not change in controls. Leptin treatment elevated plasma lactate and nonesterified fatty acids, but did not affect glucose or insulin levels, indicating a state of negative energy balance that was met by the mobilization of body stores. Pulse analysis showed that the secretion of LH and GH was not affected by leptin treatment, nor were the mean plasma concentrations of FSH, PRL, or cortisol. Expression of messenger RNA for neuropeptide Y in the arcuate nucleus was reduced by the infusion of leptin, primarily due to reduced expression per cell rather than a reduction in the number of cells observed. Thus, the action of leptin to inhibit food intake is dissociated from neuroendocrine function. These results suggest that the metabolic effects of leptin are mediated via neuronal systems that possess leptin receptors rather than via endocrine effects.

Expression and Function of Leptin Receptor Isoforms in Myeloid Leukemia and Myelodysplastic Syndromes: Proliferative and Anti-Apoptotic Activities

The receptor for the gene product of the obesity gene, leptin, was recently reported to be expressed on murine and human hematopoietic progenitor cells. Therefore, we studied the expression of the leptin receptor, OB-R, in normal myeloid precursors, human leukemia cell lines, and primary leukemic cells using reverse-transcriptase polymerase chain reaction. In normal hematopoiesis, OB-R was expressed in CD34+ cells. Normal promyelocytes (CD34−33+ and CD34−13+) expressed only very low levels of the short, presumably nonsignaling isoform. Both the long and short isoforms of OB-R were expressed in 10 of 22 samples from patients with newly diagnosed primary or secondary acute myeloid leukemia (AML), with a higher incidence of the long isoform in primary AML (87.6% v28.6%; P = .01). The incidence of OB-R expression was higher in recurrent than in newly diagnosed AML (P < .001), and samples from four patients with refractory AML showed strong expression of both isoforms. Both OB-R isoforms were also expressed in newly diagnosed and recurrent acute promyelocytic leukemia cells but were essentially absent in samples of chronic or acute lymphocytic leukemia. In vitro growth of myeloid leukemic cell lines and of blasts from 14 primary AMLs demonstrated that recombinant human leptin alone induced low level proliferation, significantly (P < .05) increased proliferation induced by recombinant human granulocyte colony-stimulating factor, interleukin 3, and stem cell factor in a subset of AML and increased colony formation (P < .005). Also, leptin reduced apoptosis induced by cytokine withdrawal in MO7E and TF-1 cells. Serum leptin levels correlated only with body mass index (P < .001) and gender (P = .03). Results confirm the reported expression of leptin receptor in normal CD34+ cells and demonstrate the frequent expression of leptin receptors in AML blasts. While normal promyelocytes lack receptor expression, leukemic promyelocytes express both isoforms. We also demonstrate proliferative effects of leptin alone and in combination with other physiologic cytokines, and anti-apoptotic properties of leptin. These findings could have implications for the pathophysiology of AML.

Expression and Function of Leptin Receptor Isoforms in Myeloid Leukemia and Myelodysplastic Syndromes: Proliferative and Anti-Apoptotic Activities

The receptor for the gene product of the obesity gene, leptin, was recently reported to be expressed on murine and human hematopoietic progenitor cells. Therefore, we studied the expression of the leptin receptor, OB-R, in normal myeloid precursors, human leukemia cell lines, and primary leukemic cells using reverse-transcriptase polymerase chain reaction. In normal hematopoiesis, OB-R was expressed in CD34+ cells. Normal promyelocytes (CD34−33+ and CD34−13+) expressed only very low levels of the short, presumably nonsignaling isoform. Both the long and short isoforms of OB-R were expressed in 10 of 22 samples from patients with newly diagnosed primary or secondary acute myeloid leukemia (AML), with a higher incidence of the long isoform in primary AML (87.6% v28.6%; P = .01). The incidence of OB-R expression was higher in recurrent than in newly diagnosed AML (P &lt; .001), and samples from four patients with refractory AML showed strong expression of both isoforms. Both OB-R isoforms were also expressed in newly diagnosed and recurrent acute promyelocytic leukemia cells but were essentially absent in samples of chronic or acute lymphocytic leukemia. In vitro growth of myeloid leukemic cell lines and of blasts from 14 primary AMLs demonstrated that recombinant human leptin alone induced low level proliferation, significantly (P &lt; .05) increased proliferation induced by recombinant human granulocyte colony-stimulating factor, interleukin 3, and stem cell factor in a subset of AML and increased colony formation (P &lt; .005). Also, leptin reduced apoptosis induced by cytokine withdrawal in MO7E and TF-1 cells. Serum leptin levels correlated only with body mass index (P &lt; .001) and gender (P = .03). Results confirm the reported expression of leptin receptor in normal CD34+ cells and demonstrate the frequent expression of leptin receptors in AML blasts. While normal promyelocytes lack receptor expression, leukemic promyelocytes express both isoforms. We also demonstrate proliferative effects of leptin alone and in combination with other physiologic cytokines, and anti-apoptotic properties of leptin. These findings could have implications for the pathophysiology of AML.

Phenotypic abnormalities in macrophages from leptin-deficient, obese mice

Obesity is a complex syndrome that involves defective signaling by a number of different factors that regulate appetite and energy homeostasis. Treatment with exogenous leptin reverses hyperphagia and obesity in ob/ob mice, which have a mutation that causes leptin deficiency, proving the importance of this factor and its receptors in the obesity syndrome. Cells with leptin receptors have been identified outside of the appetite regulatory centers in the brain. Thus leptin has peripheral targets. Because macrophages express signaling-competent leptin receptors, these cells may be altered during chronic leptin deficiency. Consistent with this concept, the present study identifies several phenotypic abnormalities in macrophages from ob/ob mice, including decreased steady-state levels of uncoupling protein-2 mRNA, increased mitochondrial production of superoxide and hydrogen peroxide, constitutive activation of CCAAT enhancer binding protein (C/EBP)-beta, an oxidant-sensitive transcription factor, increased expression of interleukin-6 and cyclooxygenase (COX)-2, two C/EBP-beta target genes, and increased COX-2-dependent production of PGE2. Given the importance of macrophages in the general regulation of inflammation and immunity, these alterations in macrophage function may contribute to obesity-related pathophysiology.

Expression of leptin receptor in lung: leptin as a growth factor

Leptin receptors are expressed in various tissues in rodents but their function is not clear. The present studies were undertaken to investigate the function of the leptin receptor in mouse and human lungs. Cell proliferation, assessed with [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] (MTT), was significantly less in primary cultures of tracheal epithelial cells of db/db mice than in those of their lean littermates. Mouse recombinant leptin significantly increased cell proliferation only in lean mice, but not in db/db mice. Reverse transcription-polymerase chain reaction (RT-PCR) study demonstrated the existence of a long form, OB-Rb type leptin receptor in both human lung tissue and lung squamous cell line (SQ-5). Cell proliferation, assessed with MTT, was dose-dependently increased in SQ-5 cells incubated with 10-1000 ng/ml human recombinant leptin for 6 h. The 5-bromo-2'-deoxyuridine (BrdU) uptake into SQ-5 cells was also increased by the addition of 10-100 ng/ml human recombinant leptin. Mitogen-activated protein (MAP) kinase activity was significantly increased by 10 and 100 ng/ml human recombinant leptin in SQ-5 cells. MAP kinase kinase (MEK)-1-specific inhibitor, (2-[2-amino-3-methoxyphenyl]-4H-1-benzopyran-4-one) (PD98059), blocked the increase in BrdU uptake into SQ-5 cells caused by human recombinant leptin. In conclusion, leptin (OB-Rb) receptors exist in human lung tissue and leptin may have stimulatory effects on the proliferation of cells of a human cell line and mouse tracheal epithelial cells through its specific leptin receptor.

Parabiosis between db/db and ob/ob or db/+ mice

Genetically obese C57B1/6J-m db/db mice were parabiosed with either lean male db/+ or obese female ob/ob mice. Male db/db mice had lower serum leptin than females, and this was reflected in the amount of protein that crossed the parabiotic union into their partners. Eighteen days post operation, ob/ob partners of db/db mice had increased body temperature, lost 50% body weight and 60% body fat, but maintained carcass protein. The db/+ partners of db/db mice had a normal gut content and (by implication) food intake, did not raise their body temperature, but lost significant amounts of both lean and fat tissue during 25 days of parabiosis. The differences between the db/+ and ob/ob partners of db/db mice may be caused by leptin inhibiting growth of male mice, but not of female mice that are on a slower growth curve, or by the excess lipid in ob/ob mice sparing body protein. The db/db partners of ob/ob mice lost a small amount of body fat, but carcass protein was increased by 30%, compared with their controls. These results imply that leptin stimulated release of a circulating growth factor, possibly through activation of the long-form leptin receptor, in ob/ob partners of db/db mice.

Leptin deficiency enhances sensitivity to endotoxin-induced lethality

Leptin is induced by lipopolysaccharide (LPS) and cytokines. We investigated the role of leptin in LPS-induced toxicity using leptin-deficient (ob/ob) and leptin receptor-deficient (db/db) mice. Sensitivity to LPS-induced mortality is significantly greater in ob/ob mice compared with their own lean littermates but not in db/db mice. LPS reduced serum glucose in both ob/ob and db/db mice but induced corticosterone only in db/db mice. Despite the very high basal levels of serum leptin in db/db mice, a twofold increase in serum leptin levels was observed after LPS in both db/db mice and their lean littermates. No differences were detected in LPS-induced serum levels of interleukin (IL)-1beta, tumor necrosis factor, macrophage inflammatory protein-1alpha, and interferon-gamma in ob/ob mice compared with their own littermates. In contrast, a blunted induction of IL-10 and IL-1 receptor antagonist (IL-1Ra) was observed in ob/ob mice compared with their littermates. In vitro, leptin induced IL-1Ra production and upregulated the IL-1Ra induction by LPS in macrophages. Moreover, treatment with leptin reversed the increased sensitivity to LPS-induced lethality found in ob/ob mice. These results suggest that leptin participates in the host response to inflammation by modulating the host immune and cytokine responses after LPS.