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

Proadipogenic effect of leptin on rat preadipocytes in vitro: activation of MAPK and STAT3 signaling pathways

Because leptin has recently been shown to induce proliferation and/or differentiation of different cell types through different pathways, the aim of the present study was to investigate, in vitro, the influence of leptin on adipogenesis in rat preadipocytes. A prerequisite to this study was to identify leptin receptors (Ob-Ra and Ob-Rb) in preadipocytes from femoral subcutaneous fat. We observed that expressions of Ob-Ra and Ob-Rb increase during adipogenesis. Furthermore, leptin induces an increase of p42/p44 mitogen-activated protein kinase phosphorylated isoforms in both confluent and differentiated preadipocytes and of STAT3 phosphorylation only in confluent preadipocytes. Moreover, exposure to leptin promoted activator protein-1 complex DNA binding activity in confluent preadipocytes. Finally, exposure of primary cultured preadipocytes from the subcutaneous area to leptin (10 nM) resulted in an increased proliferation ([(3)H]thymidine incorporation and cell counting) and differentiation (glycerol-3-phosphate dehydrogenase activity and mRNA levels of lipoprotein lipase, peroxisome proliferator-activated receptor-gamma2, and c-fos). Altogether, these results indicate that, in vitro at least, leptin through its functional receptors exerts a proadipogenic action in subcutaneous preadipocytes.

The effect of pegylated recombinant human leptin (PEG-OB) on weight loss and inflammatory status in obese subjects

OBJECTIVE

To investigate whether weekly subcutaneous administration of 60 mg of long-acting pegylated human leptin (PEG-OB) for 8 weeks was able to influence weight loss, metabolic profile and inflammatory status of obese subjects on a mildly hypoenergetic diet (deficit: 3.2 MJ/day).

DESIGN

A prospective, randomized, double-blind and placebo-controlled single-center trial.

SUBJECTS

Twenty-eight healthy, obese subjects (16 women, 12 men; age 22-65 y; body mass index 27.7-38.7 kg/m2).

MEASUREMENTS

Bodyweight, metabolic profile (including lipids), C-reactive protein (CRP) and soluble TNF alpha-receptor (sTNF-R) 55 and 75 levels.

RESULTS

At the end of the study no significant differences in the delta or percentage weight loss between the placebo (n = 14) and PEG-OB (n = 14) groups was observed. Also the changes in metabolic profile, CRP, sTNF-R55 and R75 concentrations between the two groups after 8 weeks of treatment did not differ.

CONCLUSION

Weekly injection of 60 mg PEG-OB did not lead to additional weight loss after 8 weeks of treatment. Furthermore, PEG-OB administration did not affect the changes in metabolic profile and the inflammatory status of obese subjects.

Identification of leptin receptors in human breast cancer: functional activity in the T47-D breast cancer cell line

To evaluate whether leptin plays a putative role in breast tumorigenesis, we studied the expression of its long and short receptor isoforms in various tumoral breast tissues. We applied semiquantitative RT-PCR method to RNA extracted from 20 breast cancer biopsies and two human breast cancer cell lines (T47-D and MCF-7). Our results showed the expression of both leptin receptor transcripts in all tumoral tissues examined. By in situ hybridization experiments, we localized leptin receptors in proliferating epithelial cells. Study of leptin effects on human breast cancer cells growth was performed by [3H]-thymidine incorporation method and colorimetric MTT assay. We demonstrated that leptin (50-100 ng/ml) significantly stimulates proliferation of the human breast cancer cell line T47-D (P<0.05). Western blot analysis indicated that leptin induces a time-dependent activation of mitogen-activated protein kinases (MAPKinase) 1 and 2 in T47-D cell line. Moreover, the specific MAPK-inhibitor PD 98059 blocked cell proliferation induced by leptin. In conclusion, we demonstrate that leptin receptors are expressed in breast cancer and that leptin induces proliferation in the T47-D cell line via activation of the MAPKinases pathway. These data suggest that leptin and its receptors may be implicated in mammary cell proliferation and in breast cancer pathogenesis.

Potential role of leptin in endochondral ossification

Leptin, a 16-kD circulating hormone secreted mainly by white adipose tissue, is a product of the obese (ob) gene. Leptin acts on human marrow stromal cells to enhance differentiation into osteoblasts and inhibit differentiation into adipocytes. Leptin also inhibits bone formation through a hypothalamic relay. To obtain a better understanding of the potential role of leptin in bone formation, the localization of leptin in endochondral ossification was examined immunohistochemically. High expression of leptin was identified in hypertrophic chondrocytes in the vicinity of capillary blood vessels invading hypertrophic cartilage and in a number of osteoblasts of the primary spongiosa beneath the growth plate. The hypertrophic chondrocytes far from the blood vessels were negative for leptin. Moreover, we detected the production and secretion of leptin by a mouse osteoblast cell line (MC3T3-E1) and a mouse chondrocyte cell line (MCC-5) by RT-PCR, immunocytochemistry, and Western blotting. Leptin enhanced the proliferation, migration, tube formation, and matrix metalloproteinase-2 (MMP-2) activity of human endothelial cells (HUVECs) in vitro. These findings suggest the possibility that leptin exerts its influence on endochondral ossification by regulating angiogenesis.

Embryonic expression of the leptin receptor gene in mesoderm-derived tissues

Leptin acts on the hypothalamus to reduce food intake and on a number of non-neuronal tissues via specific receptors (Lepr). The use of in situ hybridisation to map the Lepr gene in pre-natal mice revealed transcripts in the yolk sac in various structures of the central nervous system and in mesoderm-derived tissues, such as cartilage/bone primordia and musculoaponeurotic laminae. At later stages, significant amounts of Lepr were expressed in the region surrounding the developing eye of the embryo. Lepr was also found to be expressed in the choroid, sclera and connective tissues of the limbus in the adult eye. In conclusion, we have identified new targets for leptin action during embryogenesis and adulthood.

Decreased plasma leptin concentrations in tuberculosis patients are associated with wasting and inflammation

Tuberculosis patients often suffer from severe weight loss, which is considered to be immunosuppressive and a major determinant of severity and outcome of disease. Because leptin is involved in weight regulation and cellular immunity, its possible role in tuberculosis-associated wasting was investigated. In an urban clinic in Indonesia, plasma leptin concentrations, indicators of adipocyte mass, appetite, C-reactive protein (CRP), tuberculin reactivity, and cytokine response were measured in tuberculosis patients and healthy controls. Plasma leptin concentrations were lower in patients than in controls (615 vs. 2,550 ng/liter; P < 0.001). Multivariate regression analysis showed that body fat mass and inflammation were two independent factors determining plasma leptin concentrations; there was a positive correlation between fat and leptin, whereas, unexpectedly, leptin was inversely associated with CRP and tumor necrosis factor-alpha production. Concentrations of both CRP and leptin were independently associated with loss of appetite. Our results do not support the concept that weight loss in tuberculosis is caused by enhanced production of leptin. Rather, loss of body fat leads to low plasma leptin concentrations, and prolonged inflammation may further suppress leptin production. Because leptin is important for cell-mediated immunity, low leptin production during active tuberculosis may contribute to increased disease severity, especially in cachectic patients.

Obesity modulates the expression of haptoglobin in the white adipose tissue via TNFalpha

Increase in adipose mass results in obesity and modulation of several factors in white adipose tissue (WAT). Two important examples are tumor necrosis factor alpha (TNFalpha) and leptin, both of which are upregulated in adipose tissue in obesity. In order to isolate genes differentially expressed in the WAT of genetically obese db/db mice compared to their lean littermates, we performed RNA fingerprinting and identified haptoglobin (Hp), which is significantly upregulated in the obese animals. Hp is a glycoprotein induced by a number of cytokines, LPS (Lipopolysaccharide), and more generally by inflammation. A significant upregulation of WAT Hp expression was also evident in several experimental obese models including the yellow agouti (/) A(y), ob/ob and goldthioglucose-treated mice (10-, 8-, and 7-fold, respectively). To identify the potential signals for an increase in Hp expression in obesity, we examined leptin and TNFalpha in vivo. Wild type animals treated with recombinant leptin did not show any alteration in WAT Hp expression compared to controls that were food restricted to the level of intake of the treated animals. On the other hand, Hp expression was induced in mice transgenically expressing TNFalpha in adipose tissue. Finally, a significant downregulation of WAT Hp mRNA was observed in ob/ob mice deficient in TNFalpha function, when compared to the ob/ob controls. These results demonstrate that haptoglobin expression in WAT is increased in obesity in rodents and TNFalpha is an important signal for this regulation.

Prospective function of placental leptin at maternal-fetal interface

Leptin is an endocrine and a growth factor which is important for regulation of body fat, feeding, and energy homeostasis. The anti-obesity function of leptin has been recently extended to reproduction, puberty and pregnancy as an endocrine signal to the hypothalamus. Leptin controls the functional integrity of the feto-placental unit thereby maintaining pregnancy by virtue of its immunomodulatory property via T lymphocytes or other proto-oncogenes. Dysregulation of autocrine/paracrine function of leptin at feto-placento-maternal interface may be implicated in the pathogenesis of recurrent miscarriage gestational diabetes, pre-eclampsia and intra-uterine fetal growth retardation including disturbance of fetal bone turnover. This review will focus on the role of leptin in normal and abnormal pregnancy and fetal growth.

Leptin inhibits osteoclast generation

Originally, leptin was described as a product of adipocytes that acts on the hypothalamus to regulate appetite. However, subsequently, it has been shown that leptin receptors are distributed widely and that leptin has diverse functions, including promotion of hemopoietic and osteoblastic differentiation. It has been recognized for some time that both serum leptin and bone mass are correlated positively to body fat mass and, recently, we have shown a direct positive relationship between serum leptin and bone mass in nonobese women. We now report that leptin inhibits osteoclast generation in cultures of human peripheral blood mononuclear cells (PBMCs) and murine spleen cells incubated on bone in the presence of human macrophage colony-stimulating factor (hM-CSF) and human soluble receptor activator of NF-kappaB ligand (sRANKL). The half-maximal concentration inhibitory of leptin was approximately 20 nM in the presence of sRANKL at 40 ng/ml but decreased to approximately 2 nM when sRANKL was used at 5 ng/ml. The majority of the inhibitory effect occurred in the first week of the 3-week cultures. Inhibition did not occur when the PBMC cultures were washed vigorously to remove nonadherent cells or when purified CD14+ monocytes were used to generate osteoclasts, indicating an indirect or permissive effect via CD14- PBMC. Leptin increased osteoprotegerin (OPG) messenger RNA (mRNA) and protein expression in PBMC but not in CD14+ cells, suggesting that the inhibitory effect may be mediated by the RANKL/RANK/OPG system. Leptin may act locally to increase bone mass and may contribute to linkage of bone formation and resorption.

Leptin signaling deficiency impairs humoral and cellular immune responses and attenuates experimental arthritis

Leptin is produced almost exclusively by adipocytes and regulates body weight at the hypothalamic level. In addition, recent studies showed that leptin plays an important role in T lymphocyte responses. To examine the role of leptin in Ag-induced arthritis, the development of joint inflammation was assessed in immunized leptin-deficient mice (ob/ob), +/?, and wild-type mice (+/+) following the administration of methylated BSA into the knees. The results showed that ob/ob mice developed less severe arthritis compared with control mice. The levels of IL-1beta and TNF-alpha mRNA in the synovium of arthritic knees were lower in ob/ob than in +/? mice. In vitro Ag-specific T cell proliferative responses were significantly decreased in ob/ob mice with lower IFN-gamma and higher IL-10 production, suggesting a shift toward a Th2-type response in ob/ob mice. The serum levels of anti-methylated BSA Abs of any isotype were significantly decreased in arthritic ob/ob mice compared with controls. Essentially identical results were obtained in db/db mice, which lack the expression of the long isoform of leptin receptor. By RT-PCR, we observed that B lymphocytes express leptin receptor mRNA, indicating that in addition to its effect on the cellular response, leptin may exert a direct effect on B cell function. In conclusion, leptin contributes to the mechanisms of joint inflammation in Ag-induced arthritis by regulating both humoral and cell-mediated immune responses.

IL-6-like cytokines and cancer cachexia: consequences of chronic inflammation

An estimated 30% of cancer deaths are attributed to cachexia and its consequences. Cachexia (wasting syndrome) is the hypercatabolism of the body's carbon sources, proteins and lipids, for conversion into energy. It is induced by a variety of pathological conditions, including cancer. Among the inflammatory responses to cancer is the synthesis of cytokines, including IL-6 and related cytokines. These cytokines have been found to induce cachexia by altering metabolism of lipids and proteins. IL-6-like cytokines have been found to inhibit lipid biosynthesis by adipocytes, which increased the rate of lipid catabolism. Others have described the atrophy and increased catabolism of muscle protein due to IL-6. A cytokine closely-related to IL-6 is leptin, which plays a major role in lipid metabolism under normal conditions. The role of leptin in pathological conditions such as cancer cachexia has not yet been fully elucidated. Detailed mechanistic information about the induction of cancer cachexia by IL-6-like cytokines requires more research.

Leptin and renal disease

Leptin is a small peptide hormone that is mainly, but not exclusively, produced in adipose tissue. The circulating leptin concentration therefore directly reflects the amount of body fat. Leptin was identified through positional cloning of the obese (ob) gene, which is mutated in the massively obese ob/ob mouse, and it has a pivotal role in regulating food intake and energy expenditure. It binds to the so-called long receptor (Ob-Rb) in the hypothalamus and regulates food intake through the release of other neurotransmitters. Moreover, leptin exerts several other important metabolic effects on peripheral tissue, including modification of insulin action, induction of angiogenesis, and modulation of the immune system. As a small peptide, leptin is cleared principally by the kidney. Not surprisingly, serum leptin concentrations are increased in patients with chronic renal failure and those undergoing maintenance dialysis. Whether the hyperleptinemia of chronic renal failure contributes to some uremic manifestations, such as anorexia and weight loss, requires additional investigation. The kidney expresses abundant concentrations of the truncated isoform of the leptin receptor Ob-Ra, but only a small amount of the full-length receptor Ob-Rb. We recently discovered that leptin has direct effects on renal pathophysiological characteristics. Both cultured glomerular endothelial cells and mesangial cells obtained from the diabetic db/db mouse possess the Ob-Ra receptor, but whether biological effects of leptin are transduced through this receptor remains unknown. In glomerular endothelial cells, leptin stimulates cellular proliferation, transforming growth factor-beta1 (TGF-beta1) synthesis, and type IV collagen production. Conversely, in mesangial cells, leptin upregulates synthesis of the TGF-beta type II receptor, but not TGF-beta1, and stimulates glucose transport and type I collagen production through signal transduction pathways involving phosphatidylinositol-3-kinase. These data suggest that leptin triggers a paracrine interaction in which glomerular endothelial cells secrete TGF-beta, to which sensitized mesangial cells may respond. Both cell types increase their expression of extracellular matrix in response to leptin. Infusion of leptin into normal rats for 3 weeks fosters the development of focal glomerulosclerosis and proteinuria. Additional previously described direct and indirect effects of leptin on the kidney include natriuresis, increased sympathetic nervous activity, and stimulation of reactive oxygen species. These findings collectively suggest that the kidney is not only a site of leptin metabolism, but also a target organ for leptin action in pathophysiological states.

Leptin regulation of the immune response and the immunodeficiency of malnutrition

Leptin is a 16 kDa protein mainly produced by adipose tissue in proportion to adipose tissue mass. Originally thought to be a satiety factor, leptin is a pleiotropic molecule. In addition to playing a role in energy regulation, leptin also regulates endocrine and immune functions. Both the structure of leptin and that of its receptor suggest that leptin might be classified as a cytokine. The secondary structure of leptin has similarities to the long-chain helical cytokines family, which includes interleukin 6 (IL-6), IL-11, CNTF, and LIF, and the leptin receptor is homologous to the gp-130 signal-transducing subunit of the IL-6-type cytokine receptors. Leptin plays a role in innate and acquired immunity. Leptin levels increase acutely during infection and inflammation, and may represent a protective component of the host response to inflammation. More important, leptin deficiency increases susceptibility to infectious and inflammatory stimuli and is associated with dysregulation of cytokine production. Leptin deficiency also causes a defect in hematopoiesis. Leptin regulates T cells responses, polarizing Th cells toward a Th1 phenotype. Low leptin levels occurring during starvation mediate the neuroendocrine and immune dysfunction of starvation.

Antral mucosa expresses functional leptin receptors coupled to STAT-3 signaling, which is involved in the control of gastric secretions in the rat

BACKGROUND & AIMS

Leptin is a circulating hormone that communicates the peripheral nutritional status to the hypothalamus, which controls food intake, energy expenditure, and body weight. This study characterizes leptin receptors and leptin-sensitive STAT proteins in the antrum and investigates the effects of leptin on gastric secretions.

METHODS

The effects of leptin on gastrin messenger RNA (mRNA), plasma gastrin, gastric acid in vivo in the rat, and on somatostatin and gastrin secretions by isolated antral cells were determined in vitro. Leptin receptors were investigated in isolated rat antral cells by reverse transcription-polymerase chain reaction and binding of [(125)I]-leptin studies. The effects of in vivo and in vitro leptin on transduction signal STAT proteins were investigated by immunoblotting antral extracts.

RESULTS

Peripheral injection of leptin inhibited in a dose-dependent manner, basal gastric secretion, gastrinemia, and mucosal gastrin mRNA in vivo. mRNAs encoding the long (Ob-Rb) and short (Ob-Ra) receptor forms were detected in rat antral mucosa, as were STAT-1, -3, and -5b immunoreactive proteins. Isolated antral cells specifically bound [(125)I]-leptin, and addition of leptin to these cells inhibited the release of somatostatin and increased the release of gastrin. These effects were associated with an increase in nuclear STAT-3 proteins in vitro and in vivo.

CONCLUSIONS

This study provides the first molecular evidence for the coexpression of leptin receptors and STAT-3 in antral mucosa. It provides further evidence for the involvement of leptin in the control of gastric secretions.

Leptin and the regulation of food intake, energy homeostasis and immunity with special focus on periparturient ruminants

The biology of leptin has been studied most extensively in rodents and in humans. Leptin is involved in the regulation of food intake, energy homeostasis and immunity. Leptin is primarily produced in white adipose tissue and acts via a family of membrane bound receptors, including an isoform with a long intracellular domain (OB-Rb), and many isoforms with short intracellular domains (Ob-Rs). OB-Rb is predominantly expressed in the hypothalamic regions involved in the regulation of food intake and energy homeostasis. The other isoforms are distributed ubiquitously and are found in most peripheral tissues in far greater abundance than OB-Rb. The effects of leptin on food intake and energy homeostasis are central and are mediated via a network of orexigenic neuropeptides (neuropeptide Y, galanin, galanin-like peptide, melanin-concentrating hormone, orexins, agouti-related peptide) and anorexigenic neuropeptides (corticotropin-releasing hormone, pro-opiomelanocortin, alpha-melanocyte stimulating hormone and cocaine- and amphetamine-regulated transcript). In addition, leptin acts directly on immune cells to stimulate hematopoesis, T-cell immunity, phagocytosis, cytokine production, and to attenuate susceptibility to infectious insults. Emerging data in ruminants suggest that leptin is dynamically regulated by many factors and physiological states. Thus, leptin is secreted in a pulsatile fashion, but without a marked diurnal rhythm. A positive relationship between adiposity and plasma leptin concentration exists in growing and lactating ruminants. The concentration of plasma leptin increases during pregnancy, starts to decline 1--2 wk before parturition, and reaches a nadir in early lactation. The reduction of plasma leptin at parturition is likely to promote centrally mediated adaptations required in periods of energy deficit, but could have negative effects on immune cell function. Future research is needed in ruminants to address the roles played by leptin and the central nervous system in orchestrating metabolism during the periparturient period and during infectious diseases.

Leptin does not play a major role in platelet aggregation in obesity and leptin deficiency

OBJECTIVE

A recent study suggested that high concentrations of leptin enhance platelet aggregations. Therefore, the aim of this study was to investigate whether platelet aggregation is altered in patients with leptin gene mutations compared with obese subjects or controls.

RESEARCH METHODS AND PROCEDURES

Four men (one homozygous man and his three heterozygous brothers) carrying a leptin gene mutation; 20 age-matched, healthy, unrelated men; and 18 age-matched obese men were enrolled in the study. Adenosine diphosphate (ADP)-, collagen-, and epinephrine-induced platelet aggregation were evaluated in all individuals.

RESULTS

Our results show that patients with the leptin gene mutation (both the homozygous and heterozygous patients) had significantly higher ADP-induced (78.3 +/- 3.4% vs. 57.9 +/- 9.3%, p = 0.001), collagen-induced (78.1 +/- 2.9% vs. 56.7 +/- 9.3%, p = 0.007), and epinephrine-induced (76.5 +/- 9.2% vs. 59.5 +/- 7.70%, p = 0.003) platelet aggregation compared with controls. However, ADP-, collagen-, or epinephrine-induced platelet aggregations were similar to those in obese patients. Platelet aggregation responses to a combination of pretreatment with leptin at concentrations of 20, 50, 100, or 500 ng/mL for 5 minutes and ADP at concentrations of 2 micromol/liter also were evaluated. However, we did not find significant increases in platelet aggregation even at high concentrations of leptin (100 or 500 ng/mL) in leptin-deficient patients, obese subjects, or controls.

DISCUSSION

Our data show that similar to findings in obese humans, homozygous or heterozygous leptin deficiency is associated with increased platelet aggregation compared with controls, and that higher concentrations of leptin do not increase platelet aggregation.

Human leptin activates PI3K and MAPK pathways in human peripheral blood mononuclear cells: possible role of Sam68

Leptin, the adipocyte-secreted hormone that centrally regulates weight control, is known to function as an immunomodulatory regulator. Thus, we have recently found that human leptin promotes stimulation and proliferation of human peripheral blood mononuclear cells. In the present work, we sought to study the mechanisms underlying these effects. First, we have assessed the presence of the long isoform of the human leptin receptor by RT-PCR. Next, we have studied tyrosine phosphorylation of cell proteins in response to leptin stimulation. We have found that leptin receptor, IRS-1 and the RNA-binding protein Sam68 are tyrosine phosphorylated upon leptin challenge in a dose-dependent manner. Moreover, tyrosine phosphorylation of IRS-1 and Sam68 promotes their association with p85, the regulatory subunit of PI3K, and this association leads to the stimulation of PI3K activity. On the other hand, the leptin-stimulated tyrosine phosphorylation of Sam68 mediates the dissociation from RNA as assessed by Sepharose-conjugated poly(U) binding. Finally, leptin receptor activation also triggers MAPK signaling pathway. Thus, leptin dose-dependently stimulates tyrosine and threonine phosphorylation of MAPK in mononuclear cells. In summary, the present work demonstrates the presence of the long isoform of the human leptin receptor in peripheral blood mononuclear cells and the activation of two signaling pathways, PI3K and MAPK. The effects on Sam68 phosphorylation may modulate its binding to RNA, although the physiological implications remain to be studied. These signal transduction pathways may mediate the described effects of human leptin on human peripheral blood mononuclear cells.

Nonalcoholic fatty liver disease: implications for alcoholic liver disease pathogenesis

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The presentation was Nonalcoholic fatty liver disease: Implications for alcoholic liver disease pathogenesis, by Anna Mae Diehl.

Leptin within healthy and diseased human gingiva

BACKGROUND

Plasma leptin concentrations are reported to be elevated in patients with inflammatory diseases. There is no consensus concerning the biological mechanism for this phenomenon. To date, tissue leptin concentrations have not been assessed within normal or inflamed gingiva. The purpose of this study was to assess concentrations of human leptin within healthy and diseased gingiva to define its possible role in periodontal disease progression.

METHODS

Healthy (non-hemorrhagic gingiva adjacent to a < or =3 mm gingival sulcus) and inflamed gingiva (hemorrhagic gingiva adjacent to a > or =3 mm periodontal pocket) were studied. Leptin, vascular endothelial growth factor (VEGF; to assess potential vascular expansion), and interleukin-6 (IL-6; to assess periodontal disease activity and severity) concentrations were assessed within solubilized gingival biopsies by enzyme-linked immunosorbent assay. Data were grouped by sulcular depth and compared by factorial analysis of variance, regression analysis, and Scheffé comparisons.

RESULTS

Leptin concentrations were highest within gingiva adjacent to a < or =3 mm sulcus and progressively declined within gingiva adjacent to a > or =3 mm sulcus. VEGF concentrations were highest within gingiva adjacent to 4 to 6 mm pockets and nearly equivalent in healthy (< or =3 mm sulcus) and severely diseased gingiva (>6 mm sulcus). IL-6 was positively correlated and leptin negatively correlated with adjacent probing depth; IL-6 concentration was significantly higher and leptin significantly lower in gingiva adjacent to >6 mm pockets compared to sites adjacent to <6 mm pockets (P<0.001).

CONCLUSIONS

Human leptin is present within healthy and marginally inflamed gingiva and decreases in concentration as the adjacent probing depth increases. When leptin concentrations decreased (> or =3 mm sulcus), VEGF concentrations increased, suggesting that leptin could be released from gingiva coincident to vascular expansion. Thus, gingiva, in addition to adipose tissue, could be a source of circulating leptin in patients with periodontal disease. This possibility requires further investigation.

Expression of leptin (Ob) and leptin receptor (Ob-R) in human fibroblasts: regulation of leptin secretion by insulin

Leptin, a hormone of the cytokine family, is mainly synthesized by white adipocytes. As fibroblasts and adipocytes share a common stem cell origin, we hypothesized that connective tissue may be another candidate for leptin synthesis. We demonstrated leptin receptors, inclusive of all isoforms, on cultured fibroblasts (n = 13) by RT-PCR and immunohistochemistry. In contrast to its receptor, basal leptin mRNA expression and protein secretion were found in 8 of 13 cultures, reaching 1.4 ng/350,000 cells.24 h. Incubation with physiological insulin concentrations (1 nmol/liter) increased leptin secretion in fibroblast culture supernatants to 152% of basal levels. A maximal stimulation of the basal level up to 192% was found with 10 nmol/liter insulin after 24 h. Actinomycin D and cycloheximide abolished this effect, providing evidence that active RNA and protein synthesis are involved in insulin's action. Completing these in vitro results, we could show protein expression for leptin and leptin receptors in fibroblasts by immunostaining of human skin biopsies in situ. In conclusion, we provide evidence of leptin synthesis and secretion by human fibroblasts that are regulated by insulin. Leptin produced by fibroblasts may thus exert important local autocrine and paracrine actions and contribute to the total plasma pool. Hence it might in part account for variations in body mass index-dependent reference ranges of leptin as well as disruptions in the relationship between fat content and leptin.

A heliocentric view of leptin

Leptin is significantly broadening our understanding of the mechanisms underlying neuroendocrine function. Initially, based on a rather static view of the hormone, most investigations focused on the effects of leptin on food intake control and body-weight homeostasis, with attention primarily focused on the implications of leptin as a lipostatic factor and central satiety agent. However, the almost ubiquitous distribution of leptin receptors in peripheral tissues provided a fertile area for investigation and a more dynamic view of leptin started to unfold. This adipocyte-derived circulating peptidic hormone, with a tertiary structure resembling that of members of the long-chain helical cytokine family, has generated an enormous interest in the interaction as well as integration between brain targets and peripheral signals. Considerable evidence for systemic effects of leptin on specific tissues and metabolic pathways indicates that leptin operates both directly and indirectly to orchestrate complex pathophysiological processes. Disentangling the biochemical and molecular mechanisms in which leptin is involved represents one of the major challenges ahead.

Human leptin signaling in human peripheral blood mononuclear cells: activation of the JAK-STAT pathway

Leptin is an adipocyte-secreted hormone that centrally regulates weight control. However, the leptin receptor is expressed not only in the central nervous system, but also in other systems, such as reproductive, hematopoietic, and immune tissues, suggesting various roles in addition to the regulation of food intake and energy expenditure. The leptin receptor bears homology to members of the class I cytokine receptor family. Leptin has previously been shown to enhance cytokine production by murine peritoneal macrophages and human circulating monocytes, where human leptin promotes activation and proliferation. We have recently found that the leptin receptor is expressed not only in monocytes but also in both CD4(+) and CD8(+) T lymphocytes. Besides, leptin enhances proliferation and activation of T lymphocytes when they are costimulated by PHA or Con A. In this paper, we have studied the signal transduction of the leptin receptor in peripheral blood mononuclear cells. We found that leptin stimulation activates the JAK-STAT signaling pathway. More specifically, we found that JAK-2/3 and STAT-3 are activated by tyrosine phosphorylation upon leptin stimulation. Moreover, leptin stimulated tyrosine phosphorylation of the RNA binding protein Sam68 and its association with STAT-3. These effects were dose-dependent (0.1-10 nM) and transient (5-30 min). We also observed the leptin stimulated translocation of activated STAT-3 from the cytoplasm to the nucleus. These results indicate that human leptin receptor in circulating mononuclear cells has the signaling capacity to activate JAK-STAT cascade. This pathway may mediate, at least in part, the action of human leptin in human peripheral blood mononuclear cells.

Leptin levels in preterm human breast milk and infant formula

OBJECTIVE

Leptin, a hormone present in breast milk, is involved in energy regulation and metabolism. The purpose of this investigation was to determine whether leptin is present in either preterm breast milk (PBM) or preterm formula (PF). The effects of delivery methods and pasteurization on leptin levels also were evaluated.

METHODS

PBM samples were obtained from 29 mothers who delivered infants at between 23 and 34 weeks' gestation. Leptin levels were measured in PBM and PF with the use of a radioimmunoassay specific for human and bovine leptin, respectively. Milk samples were pasteurized by fast- and slow-heating methods. PBM and PF spiked with human leptin were delivered through catheters by bolus and continuous administration to determine the effects of delivery method on recoverable leptin levels.

RESULTS

Median PBM leptin concentration was 5.28 ng/mL (intraquartile range: 24.79). Birth gestational age, birth weight, and gender of the infant did not significantly influence PBM leptin levels. Neither bolus nor continuous feeding practices affected leptin levels in PBM or spiked PF. However, pasteurization significantly reduced the amount of detectable leptin in PBM.

CONCLUSIONS

PBM leptin levels were highly variable and similar to levels reported for term breast milk. There was no effect of postnatal age on PBM leptin concentrations. Sterilization decreased detectable leptin levels, whereas feeding practices had no adverse effect on the quantity of leptin delivered. Although no infant formula contained leptin, leptin could be added to formula and delivered through various feeding methods without loss.

The release of leptin and its effect on hormone release from human pituitary adenomas

BACKGROUND

Leptin is the protein product of the obese gene, known to play an important role in body energy balance. The leptin receptor exists in numerous isoforms, the long isoform being the major form involved in signal transduction. Leptin expression has recently been demonstrated in the human pituitary, both in normal tissue and in pituitary adenomas. The long isoform of the leptin receptor has also been shown to be present in pituitary adenomas; however, contrasting results have been obtained regarding its expression in the normal human pituitary.

AIM

The aim of this study was (i) to investigate the presence and pattern of distribution of leptin mRNA and the long isoform of its receptor mRNA in the normal pituitary and in different types of pituitary adenomas with RT-PCR; (ii) to study leptin secretion from human pituitary tumours in culture and (iii) to assess in vitro pituitary hormone release following stimulation with human leptin.

RESULTS

Leptin receptor long isoform expression was detected in 2/4 GH-secreting adenomas, 12/17 non-functioning adenomas, 5/9 ACTH-secreting adenomas, 1/2 prolactinomas, 2/2 FSH-secreting adenomas and 5/5 normal pituitaries. The receptor long isoform did not segregate with any particular tumour type, and varying levels of expression were detected between the tissues studied. Leptin mRNA was detected at a low level of expression in 2/7 GH-secreting adenomas, 9/14 non-functioning adenomas, 2/3 ACTH-secreting adenomas, 1/3 prolactinomas and 1/3 FSH-secreting adenomas. We were unable to detect leptin mRNA in any of the five normal pituitaries removed at autopsy; however, immunostaining of a non-tumorous pituitary adjacent to an adenoma removed at transsphenoidal surgery showed scattered leptin positive cells. Culture of pituitary adenomas showed that 16/47 released leptin into the incubation media. Leptin release did not correlate with tumour type or with any of the other pituitary hormones released. In vitro leptin stimulation of pituitary tumours caused stimulation of FSH and alpha-subunit secretion from a non-functioning adenoma and TSH secretion from a somatotroph adenoma.

CONCLUSION

We conclude that not only is leptin stored within the pituitary, but it may also be released from pituitary cells and modulate other pituitary hormone secretion. Pituitary leptin may therefore be a novel paracrine regulator of pituitary function.

The adipocyte: a model for integration of endocrine and metabolic signaling in energy metabolism regulation

The ability to ensure continuous availability of energy despite highly variable supplies in the environment is a major determinant of the survival of all species. In higher organisms, including mammals, the capacity to efficiently store excess energy as triglycerides in adipocytes, from which stored energy could be rapidly released for use at other sites, was developed. To orchestrate the processes of energy storage and release, highly integrated systems operating on several physiological levels have evolved. The adipocyte is no longer considered a passive bystander, because fat cells actively secrete many members of the cytokine family, such as leptin, tumor necrosis factor-alpha, and interleukin-6, among other cytokine signals, which influence peripheral fuel storage, mobilization, and combustion, as well as energy homeostasis. The existence of a network of adipose tissue signaling pathways, arranged in a hierarchical fashion, constitutes a metabolic repertoire that enables the organism to adapt to a wide range of different metabolic challenges, such as starvation, stress, infection, and short periods of gross energy excess.

Endothelin-1 stimulates leptin production in adipocytes

Leptin is an adipocyte-derived hormone that regulates body fat stores and feeding behavior. In an effort to identify endogenous diffusible modulators of leptin production, we found that endothelin-1 (ET-1) up-regulates leptin expression in adipocytes. ET-1 is as potent and efficacious as insulin in stimulating leptin production in two different adipocyte cell lines. Endothelins stimulate leptin production via the endothelin-A receptor (ET(A)), as judged by a potency rank order of ET-1 ET-3. We detected expression of ET(A) but not ET(B) in both cell lines by Northern blot analysis. In addition, the ET(A)-selective antagonist FR139317 inhibited ET-1-induced leptin expression more potently than did the ET(B)-selective antagonist BQ788. ET-1 and insulin positively interact with each other in increasing leptin production in adipocytes. In primary mouse white fat cells, we detected expression of both ET(A) and ET(B) by Northern blot and in situ hybridization analyses. We conclude that ET-1 stimulates leptin production via the ET(A) receptor in cultured adipocytes.

Insulin and leptin acutely regulate cholesterol ester metabolism in macrophages by novel signaling pathways

Leptin is produced in adipose tissue and acts in the hypothalamus to regulate food intake. However, recent evidence also indicates a potential for direct roles for leptin in peripheral tissues, including those of the immune system. In this study, we provide direct evidence that macrophages are a target tissue for leptin. We found that J774.2 macrophages express the functional long form of the leptin receptor (ObRb) and that this becomes tyrosine-phosphorylated after stimulation with low doses of leptin. Leptin also stimulates both phosphoinositide 3-kinase (PI 3-kinase) activity and tyrosine phosphorylation of JAK2 and STAT3 in these cells. We investigated the effects of leptin on hormone-sensitive lipase (HSL), which acts as a neutral cholesterol esterase in macrophages and is a rate-limiting step in cholesterol ester breakdown. Leptin significantly increased HSL activity in J774.2 macrophages, and these effects were additive with the effects of cAMP and were blocked by PI 3-kinase inhibitors. Conversely, insulin inhibited HSL in macrophages, but unlike adipocytes, this effect did not require PI 3-kinase. These results indicate that leptin and insulin regulate cholesterol-ester homeostasis in macrophages and, therefore, defects in this process caused by leptin and/or insulin resistance could contribute to the increased incidence of atherosclerosis found associated with obesity and type 2 diabetes.

Obesity and immune function relationships

The immunological processes involved in the collaborative defence of organisms are affected by nutritional status. Thus, a positive chronic imbalance between energy intake and expenditure leads to situations of obesity, which may influence unspecific and specific immune responses mediated by humoral and cell mediated mechanisms. Furthermore, several lines of evidence have supported a link between adipose tissue and immunocompetent cells. This interaction is illustrated in obesity, where excess adiposity and impaired immune function have been described in both humans and genetically obese rodents. However, limited and often controversial information exist comparing immunity in obese and non-obese subjects as well as about the cellular and molecular mechanisms implicated. In general terms, clinical and epidemiological data support the evidence that the incidence and severity of specific types of infectious illnesses are higher in obese persons as compared to lean individuals together with the occurrence of poor antibody responses to antigens in overweight subjects. Leptin might play a key role in linking nutritional status with T-cell function. The complexities and heterogeneity of the host defences concerning the immune response in different nutritional circumstances affecting the energy balance require an integral study of the immunocompetent cells, their subsets and products as well as specific and unspecific inducer/regulator systems. In this context, more research is needed to clarify the clinical implications of the alterations induced by obesity on the immune function.

Chemokines control fat accumulation and leptin secretion by cultured human adipocytes

In addition to their role in inflammation, cytokines like TNFalpha have been reported to regulate the adipose tissue function suggesting a role for these soluble mediators in metabolism. However, it is not known whether adipocytes have the capacity to secrete chemokines, a group of low molecular weight inflammatory mediators that control leukocyte migration into tissues. Here we show that primary cultures of human preadipocytes constitutively produce three chemokines, interleukin-8 (IL-8), macrophage inflammatory protein-1alpha (MIP-1alpha) and monocyte chemotactic protein-1 (MCP-1), while their level of expression is low in mature adipocytes. Upon TNFalpha treatment, the expression of all the three chemokines is upregulated in adipocytes differentiated in vitro. In addition, we describe the presence of seven different chemokine receptors, mainly in mature adipocytes, both in vitro and in human fat tissue sections. Prolonged stimulation of cultured human adipocytes with exogenous chemokines leads to a decrease in lipid content in association with the downregulation of PPARgamma mRNA expression. Moreover, chemokines positively control the secretion of leptin, a hormone that regulates appetite, by a post-transcriptional mechanism. These findings reveal a new role for chemokines in the regulation of adipose tissue and suggest a novel therapeutic basis for the treatment of obesity, diabetes and cachexia.

Leptin in obstetrics and gynecology: a review

Leptin, a recently described type-1 cytokine, is involved in cellular maturation and growth and appears to have a relationship to some obstetrical and gynecologic diseases. The MEDLINE database was accessed, and leptin-related articles published during the past 6 years were reviewed for their relevance to gynecologic and obstetrical diseases. The relationships between this cytokine and obesity, puberty, polycystic ovary syndrome, endometriosis, assisted fertility, and menopause are discussed. The role of leptin in fetal physiology and in normal and abnormal fetal growth as well as its role in diabetes, pregnancy, and pregnancy-induced hypertension are reviewed.

Altered macrophage-like functions of preadipocytes in inflammation and genetic obesity

We recently demonstrated that preadipocytes exhibit functional features of macrophages, such as phagocytosis and anti-microbial activity, suggesting that preadipose cells could play a role in the inflammatory process or immune response. The aim of this study was to compare these functions of both macrophages and cells from stroma-vascular fraction (SVF) of the adipose tissue in two different situations, obesity and inflammation, characterized by alterations in immune responsiveness. We demonstrated that ob/ob mice exhibited strong decrease in antimicrobial activity of both macrophages and SVF. This defect is compensated in SVF, at least in part, by an enhancement of phagocytosis that does not seem to be due to an increased macrophage number. In vitro leptin treatment of SVF and macrophages from obese mice did not restore their immune defects. Thioglycollate treatment of lean and obese mice induced an inflammatory process that led to an increase in macrophage activity in both strains. This stimulation also observed in SVF from lean mice is not present in obese ones. This work demonstrated that SVF immune functions could be modified in different pathological situations such as inflammation and obesity and sustained the new physiological role of preadipocytes in these processes.

Leptin--much more than a satiety signal

Much attention has focused on the effects of leptin as a central satiety agent. There is now a significant amount of evidence that leptin is active in the periphery. This review focuses on the ability of leptin to modify insulin sensitivity, tissue metabolism, stress responses, and reproductive function. Leptin's effect on several of these systems is mediated via the hypothalamic-pituitary axis. Therefore, although in vitro studies provide evidence for direct effects on specific tissues and metabolic pathways, it is essential to consider the interactions between leptin and other regulatory factors in vivo. Little is known about the regulation of peripheral receptor expression or the production of binding proteins. Both of these factors determine the bioactivity of circulating leptin and have the potential to induce a peripheral resistance to leptin, similar to the central "leptin resistance" observed in obese subjects. Future research will clarify which of the endocrine and metabolic actions of peripheral leptin are of physiological relevance and which should be considered a pharmacological manipulation.

Stimulation of catecholamine synthesis in cultured bovine adrenal medullary cells by leptin

Recently, we characterized leptin receptors in bovine adrenal medullary cells (Yanagihara et al. 2000). Here we report the stimulatory effect of leptin on catecholamine synthesis in the cells. Incubating cells with leptin (10 nM) for 20 min increased the synthesis of 14C-catecholamines from [14C]tyrosine, but not from L-3,4-dihydroxyphenyl [3-14C]alanine. The stimulation of catecholamine synthesis in the cells by leptin was associated with the phosphorylation and activation of tyrosine hydroxylase, the rate-limiting enzyme of catecholamine biosynthesis. The incubation of cells with leptin resulted in a rapid activation of the mitogen-activated protein kinases (MAPKs). An inhibitor of MAPK kinase, U0126, nullified the stimulatory effect of leptin on the synthesis of 14C-catecholamines. Leptin potentiated the stimulatory effect of acetylcholine on 14C-catecholamine synthesis, whereas leptin failed to enhance the phosphorylation and activation of tyrosine hydroxylase induced by acetylcholine. These findings suggest that leptin stimulates catecholamine synthesis via the activation of tyrosine hydroxylase by two different mechanisms, i.e., one is dependent on tyrosine hydroxylase phosphorylation mediated through the MAPK pathway and the second is independent of enzyme phosphorylation.

Leptin in septic arthritis: decreased levels during infection and amelioration of disease activity upon its administration

Weight loss is typically found during severe infections, e.g. septic arthritis. The aim of our study was to evaluate the role of leptin, regulator of food intake and energy expenditure, for the development of Staphylococcus aureus-triggered arthritis. Leptin production was found to be decreased during murine S. aureus-induced arthritis. Treatment with recombinant leptin neither restored the basal leptin levels nor affected the weight loss during the disease, but it significantly decreased the severity of septic arthritis. Exogenous leptin did not affect the staphylococcal load as measured in blood, joints and kidneys. Preceding the effects on joint manifestations, serum levels of interleukin-6 decreased in leptin-treated mice. In conclusion, the treatment with recombinant leptin reduced both the severity of joint manifestations in S.aureus-induced arthritis and the inflammatory response, as measured by serum IL-6 levels, without affecting the survival of bacteria in vivo.

Leptin in pituitary adenomas--a novel paracrine regulatory system

A growing number of physiological and pathophysiological processes have been shown to be influenced by leptin apart from its first recognised role as a modulator of hypothalamic appetite and weight control centers. We investigated the presence and pattern of distribution of leptin mRNA and the mRNA of the long isoform of the leptin receptor in the normal pituitary and in different types of pituitary adenomas. We also studied leptin secretion from human pituitary tumors in culture, and the in vitro pituitary hormone release following stimulation with human leptin. Leptin mRNA expression was detected at a low level of expression in 50% of tumors but in none of the normal pituitaries. By immunohistochemistry, leptin was present in occasional scattered cells in the normal pituitary and in pituitary tumors. The leptin receptor long isoform was detected in the majority (65%) of pituitary tumors and in all normal pituitaries. It did not segregate with any particular tumor type, and varying levels of expression were detected between the tissues studied. 34% of pituitary adenomas showed leptin release into the incubation media during in vitro culture. Leptin mRNA, the mRNA of the long isoform of the receptor, or in vitro leptin release, did not correlate with tumor type or with any of the other pituitary hormones released. In vitro leptin stimulation of pituitary tumors caused stimulation of FSH and a-subunit secretion from a non-functioning adenoma and TSH secretion from a somatotroph adenoma. As the co-localisation of ACTH and leptin in corticotroph cells was previously suggested, we investigated whether in vivo ACTH release is accompanied by a simultaneous plasma leptin level rise (i) in peripheral plasma samples after food intake-induced ACTH rise in healthy obese and nonobese individuals and (ii) in petrosal sinus samples after CRH injection in Cushing's disease patients. Our data suggest that a rise in ACTH levels is not accompanied by detectable rise in leptin levels in peripheral and in petrosal sinus blood samples. In summary, leptin is synthesized and stored within the pituitary and may modulate other pituitary hormone secretion, although probably it does not contribute to plasma leptin level changes. Pituitary leptin may therefore be a novel paracrine regulator of pituitary function.

Effects of vascular endothelial and platelet-derived growth factor receptor inhibitors on long-term cultures from normal human bone marrow

Endothelial cells and fibroblasts are important constituents of the haemopoietic microenvironment. Growth and function of these cells are controlled by a variety of cytokines, including vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF). We analysed the effects of novel tyrosine kinase inhibitors targeting the VEGF and PDGF receptors (compounds SU5614 and SU5768) on the performance of long-term cultures from normal human bone marrow. In developing cultures, the inhibitors induced a dose-dependent reduction in stromal fibroblasts, macrophages and endothelial cells with a concomitant decrease in blood cell production and an increase in fat cells. For SU5614, the concentration inhibiting stroma formation by 50% (IC50) was 123nM, and the IC50 for haemopoietic colony forming cell output was 186 nM. For SU5768, the respective values were 871 nM and 331 nM. Changes in stroma composition and inhibition of haemopoietic cell production were also demonstrable after delayed addition of the inhibitors to established cultures. By contrast, haemopoietic colony formation in clonogenic agar cultures was unimpaired (IC50 not reached at 100 microM). Immunofluorescence studies and time course analyses suggested that the primary effect of the inhibitors was interference with the proliferation and function of fibroblasts and endothelial cells which in turn resulted in decreased haemopoiesis and increased adipogenesis. This was associated with decreased levels in conditioned media of granulocyte-macrophage colony-stimulating factor, interleukin-6 and leptin. VEGF and PDGF may play a hitherto underestimated role in the control of blood cell formation. VEGF/PDGF receptor inhibitors may have therapeutic potential in stroma diseases such as myelofibrosis. Since they weaken the stimulatory signals provided by the microenvironment, they may also be of value in the treatment of leukaemia and other neoplastic bone marrow diseases.

SOCS3 mediates feedback inhibition of the leptin receptor via Tyr985

During leptin signaling, each of the phosphorylated tyrosine residues on the long form of the leptin receptor (LRb) mediates distinct signals. Phosphorylated Tyr(1138) binds STAT3 to mediate its tyrosine phosphorylation and transcriptional activation, while phosphorylated Tyr(985) binds the tyrosine phosphatase SHP-2 and reportedly mediates both activation of ERK kinases and inhibition of LRb-mediated STAT3 activation. We show here that although mutation of Tyr(985) does not alter STAT3 signaling by erythropoietin receptor-LRb (ELR) chimeras in transfected 293 cells at short times of stimulation, this mutation enhances STAT3 signaling at longer times of stimulation (>6 h). These data suggest that Tyr(985) may mediate feedback inhibition of LRb signaling by an LRb-induced LRb inhibitor, such as SOCS3. Indeed, SOCS3 binds specifically to phosphorylated Tyr(985) of LRb, and SOCS3 fails to inhibit transcription by ELR following mutation of Tyr(985), suggesting that SOCS3 inhibits LRb signaling by binding to phosphorylated Tyr(985). Additionally, overexpression of SOCS3, but not SHP-2, impairs ELR signaling, and the overexpression of SHP-2 blunts SOCS3-mediated inhibition of ELR signaling. Thus, our data suggest that in addition to mediating SHP-2 binding and ERK activation during acute stimulation, Tyr(985) of LRb mediates feedback inhibition of LRb signaling by binding to LRb-induced SOCS3.

Expression and intracellular localization of leptin receptor long isoform-GFP chimera

The leptin receptor (OBR) and its ligand leptin (OB) are key players in the regulation of body weight. The OBR is a member of the class I cytokine receptor family and is alternatively spliced into at least six different isoforms. The multiple forms are identical in their extracellular and transmembrane regions but differ in lengths. The two predominant isoforms include a long form (OBR(l)) with an intracellular domain of 303 amino acids and a shorter form (OBR(s)) with an intracellular domain of 34 amino acids. We have constructed a recombinant OBR(l) chimera with the green fluorescent protein (GFP) by fusing GFP to the C-terminus of the OBR(l). The OBR(l)-GFP chimera was transiently transfected and expressed in SHSY5Y and HEK293 cells. In a STAT-Luciferase assay we show that the GFP moiety in this chimera did not affect the signalling capacity of OBR(l)-GFP. In both SHSY5Y and HEK293 cells transfected with OBR(l)-GFP, a predominant intracellular green OBR(l)-GFP fluorescence was detected in vesicles also positive for internalized fluorophore conjugated leptin. We also found that treatment with the lysosomotropic reagent monensin did not relocalize OBR(l)-GFP together with the human transferrin receptor in recycling endosomes, indicating OBR(l)-GFP not to participate in this pathway. In biotinylation-streptavidin pulse chase experiments, using antibodies raised against GFP and OBR, we observed that the rate of early appearance of OBR(s) at the cell surface, upon leptin stimulation, was faster than that found for OBR(l)-GFP. Taken together, our results provide novel data concerning the intracellular trafficking of the two different isoforms of the leptin receptor.

Leptin and the adrenal gland

BACKGROUND

Leptin is involved in the maintenance of energy balance acting on food intake, thermogenesis and energy expenditure. Via its receptor in the hypothalamus, leptin modulates the functioning of the hypothalamic-pituitary-adrenal axis and the systemic sympathetic/adrenomedullary system, which are closely linked to the regulation of energy balance and body weight. In regard of potential interactions of leptin and adrenal hormones this study intended to characterize the role of leptin in the human adrenal gland.

MATERIALS AND METHODS

A novel technique of laser capture microdissection was used to separate cortical and chromaffin cells for mRNA expression studies of leptin receptor isoforms and leptin mRNA in adrenal tissue and cell line NCI-H295. Immunostaining was used to localize leptin receptor in human adrenal slices. The influence of leptin on basal and ACTH-stimulated steroid hormone secretion and enzyme expression was assessed. The effect of leptin on proliferation and viability of adrenal cells in primary culture and of the NCI-H295 cell line was studied by the WST-1 assay and by 3H-thymidine test.

RESULTS

Our data demonstrate that leptin can regulate the human adrenal function directly, via its receptors on adrenocortical cells. Leptin decreased the corticotropin-stimulated release of steroid hormones in vitro without any effect on cell proliferation. Leptin did not significantly affect the expression of cytochrome P450 scc m RNA in humans, but decreased the ACTH stimulated expression of the cytochrome P450 17alpha mRNA [corrected].

CONCLUSIONS

The adipo-adrenal interaction mediated by leptin further underscores the close link of metabolism and stress regulation in humans.

Leptin promotes invasiveness of kidney and colonic epithelial cells via phosphoinositide 3-kinase-, rho-, and rac-dependent signaling pathways

Leptin plays a key role regulating food intake, body weight and fat mass. These critical parameters are associated with an increased risk for digestive and mammary gland cancer in the Western population. Here we determined whether leptin contributes to the invasive phenotype of colonic and kidney epithelial cells at various stages of the neoplastic progression. First, leptin potently (EC50 = 10-30 ng/ml) induces invasion of collagen gels by premalignant familial adenomatous colonic cells PC/AA/C1 and nontumorigenic MDCK kidney epithelial cells, their src-transformed counterparts, and the human adenocarcinoma colonic cells LoVo and HCT-8/S11. Leptin and its Ob-Rb receptors were consistently identified by RT-PCR and immunoblotting in these cell lines, as well as in human colonic epithelial crypts, polyps, colonic tumor resections, and adjacent mucosa. Leptin-induced invasion was effectively blocked by pharmacological inhibitors of several downstream signaling pathways involved in cell transformation, namely, JAK2 tyrosine kinase (AG490), phosphoinositide PI3'-kinase (wortmannin and LY294002), mTOR kinase (rapamycin), and protein kinases C (GF109203X, Gö6976). Accordingly, leptin induces transient elevation of the PI3'-kinase lipid products in JAK2 immunoprecipitates prepared from parental MDCK cells. The leptin effect on invasion was potentiated by the activated form of the small GTPase RhoA and was abrogated by dominant negative mutants of RhoA, Rac1, and the p110alpha of PI3'-K. Our data indicate that leptin may exert a local and beneficial effect on migration of normal colonic epithelial cells and reparation of the inflamed or wounded digestive mucosa. We also emphasize a new role for leptin, linking the nutritional and body fat status to digestive cancer susceptibility by stimulating the invasive capacity of colonic epithelial cells at early stages of neoplasia. This finding has potential clinical implications for colon cancer progression and management of obesity.

Regulatory mechanisms controlling hematopoiesis: principles and problems

Hematopoiesis is regulated by the combined action of specialized stromal cells and a consortium of hematopoietic regulatory factors. The multiplicity of these regulatory controls does result in overlapping regulator action, but multiple regulators are required to stimulate stem cell proliferation and are more efficient than single regulators when stimulation of progenitor cells is required. Gene inactivation studies have indicated that despite overlapping actions each hematopoietic regulator does have unique functions. Delayed elevations of stem and progenitor cells in the blood are a feature of enhanced hematopoiesis induced by the injection of regulators. These cells are not a random sample of marrow cells in such situations and may well be selected to rapidly amplify hematopoiesis by seeding previously inactive hematopoietic regions.

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.