Leptin induces proliferation of pancreatic beta cell line MIN6 through activation of mitogen-activated protein kinase

The leptin receptor in human osteoblasts and the direct effect of leptin on bone metabolism

It is important to elucidate whether the leptin receptor, especially the long signal-transducing form of the leptin receptor (OB-Rb) is expressed in human osteoblasts. We detected the expression of human OB-Rb in cultured commercially available human osteoblasts (NHOst cells) using real-time quantitative reverse transcriptase polymerase chain reaction (RT-PCR). After confirming the expression of OB-Rb, we investigated the effect of leptin on NHOst cells. Leptin enhanced cell proliferation of the cells shown by the MTT assay. Furthermore, leptin changed the copy numbers of Bax and Bcl-2 mRNAs in the cultured cells as shown by real-time quantitative RT-PCR, although the effect was not consistent. Leptin did not change the production of osteocalcin and osteopontin by the cells. Leptin did not change the expression of OB-Rb mRNA in the cells. In conclusion, OB-Rb mRNA is expressed in cultured commercially available human osteoblasts. Leptin may have some effects on bone metabolism by directly modulating cell proliferation and apoptosis of osteoblasts in humans.

Leptin modulates beta cell expression of IL-1 receptor antagonist and release of IL-1beta in human islets

High concentrations of glucose induce beta cell production of IL-1beta, leading to impaired beta cell function and apoptosis in human pancreatic islets. IL-1 receptor antagonist (IL-1Ra) is a naturally occurring antagonist of IL-1beta and protects cultured human islets from glucotoxicity. Therefore, the balance of IL-1beta and IL-1Ra may play a crucial role in the pathogenesis of diabetes. In the present study, we observed expression of IL-1Ra in human pancreatic beta cells of nondiabetic individuals, which was decreased in tissue sections of type 2 diabetic patients. In vitro, chronic exposure of human islets to leptin, a hormone secreted by adipocytes, decreased beta cell production of IL-1Ra and induced IL-1beta release from the islet preparation, leading to impaired beta cell function, caspase-3 activation, and apoptosis. Exogenous addition of IL-1Ra protected cultured human islets from the deleterious effects of leptin. Antagonizing IL-1Ra by introduction of small interfering RNA to IL-1Ra into human islets led to caspase-3 activation, DNA fragmentation, and impaired beta cell function. Moreover, siIL-1Ra enhanced glucose-induced beta cell apoptosis. These findings demonstrate expression of IL-1Ra in the human beta cell, providing localized protection against leptin- and glucose-induced islet IL-1beta.

Role of Leptin in Farm Animals: a Review

The discovery of hormone leptin has led to better understanding of the energy balance control. In addition to its effects on food intake and energy expenditure, leptin has now been implicated as a mediator of diverse physiological functions. Recently, leptin has been cloned in several domestic species. The sequence similarity suggests a common function or mechanism of this peptide hormone across species. Leptin receptors are expressed in most of tissues, which is consistent with the multiplicity of leptin functions. The main goal of this review was to summarize knowledge about effect of leptin on physiology of farm animals. Experiments point to a stimulatory action of leptin on growth hormone (GH) secretion, normal growth and development of the brain. Surprisingly, leptin is synthesized at a high rate in placenta and may function as a growth factor for fetus, signalling the nutritional status from the mother to her offspring. Maturation of reproductive system can be stimulated by leptin administration. Morphological and hormonal changes, consistent with a major role of leptin in the reproductive system, have also been described, including the stimulation of the release of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin. Leptin has a substantial effect on food intake and feeding behaviour in animals. Administration of leptin reduces food intake. Its level decrease within hours after initiation of fasting. Leptin also serves as a mediator of the adaptation to fasting, and this role may be the primary function for which was the molecule evolved.

Leptin increases cardiomyocyte hyperplasia via extracellular signal-regulated kinase- and phosphatidylinositol 3-kinase-dependent signaling pathways

Obesity is a major risk factor for the development of heart failure. Importantly, it is now appreciated that a change in the number of myocytes is one of multiple structural and functional alterations (remodeling) leading to heart failure. Here we investigate the effect of leptin, the product of the obese (ob) gene, on proliferation of human and murine cardiomyocytes. Leptin caused a time- and dose-dependent significant increase in proliferation of HL-1 cells that was inhibited by preincubation with PD98059 and LY294002, suggesting that leptin mediated proliferation via extracellular signal-regulated kinase-1/2- and phosphatidylinositol-3-kinase-dependent signaling pathways. We confirmed that leptin activates both extracellular signal-regulated kinase-1/2 phosphorylation and association of phosphatidylinositol-3-kinase (regulatory p85 subunit) with phosphotyrosine immunoprecipitates. We also examined bromodeoxyuridine incorporation as a measure of new DNA synthesis and demonstrated a stimulatory effect of leptin in both HL-1 cells and human cardiomyocytes. Bromodeoxyuridine incorporation in HL-1 cells was inhibited by PD98059 and LY294002. Our results establish a mitogenic effect of leptin in cardiomyocytes and provide additional evidence for a potential direct link between leptin and cardiac remodeling in obesity.

Leptin: a multifaceted hormone in the central nervous system

It is well established that the adipocyte-derived hormone leptin is an important circulating satiety factor that regulates body weight and food intake via its actions on specific hypothalamic nuclei. However, there is growing evidence that leptin and its receptors are widely expressed throughout the brain, in regions not generally associated with energy homeostasis, such as cortex, cerebellum, brainstem, basal ganglia, and hippocampus. In this review the author discusses recent advances made in leptin neurobiology, with particular emphasis on the role of this endocrine peptide in normal and pathophysiological hippocampal function.

Leptin Stimulates Tissue Inhibitor of Metalloproteinase-1 in Human Hepatic Stellate Cells

Leptin is recognized as a profibrogenic hormone in the liver, but the mechanisms involved have not been clarified. The tissue inhibitor of metalloproteinase (TIMP)-1, which acts through inhibition of collagen degradation, is synthesized by activated hepatic stellate cells (HSC) in response to fibrogenic substances. The capacity of leptin to induce TIMP-1 and its signaling molecules were investigated in a human HSC cell line, LX-2. Leptin stimulated TIMP-1 protein, mRNA, and promoter activity. JAK1 and -2, as well as STAT3 and -5, were activated. After leptin, there was increased expression of tyrosine 1141-phosphorylated leptin receptor, which may contribute to STAT3 activation. AG 490, a JAK inhibitor, blocked JAK phosphorylation with concomitant inhibition of STAT activation, TIMP-1 mRNA expression, and promoter activity. Leptin also induced an oxidative stress, which was inhibited by AG 490, indicating a JAK mediation process. ERK1/2 MAPK and p38 were activated, which was prevented by catalase, indicating an H2O2-dependent mechanism. Catalase treatment resulted in total suppression of TIMP-1 mRNA expression and promoter activity. SB203580, a p38 inhibitor, prevented p38 activation and reduced TIMP-1 message half-life with down-regulation of TIMP-1 mRNA. These changes were reproduced by overexpression of the dominant negative p38alpha and p38beta mutants. PD098059, an ERK1/2 inhibitor, opposed ERK1/2 activation and TIMP-1 promoter activity, leading to TIMP-1 mRNA down-regulation. Thus, leptin has a direct action on liver fibrogenesis by stimulating TIMP-1 production in activated HSC. This process appears to be mediated by the JAK/STAT pathway via the leptin receptor long form and the H2O2-dependent p38 and ERK1/2 pathways via activated JAK.

Acute and chronic leptin treatment mediate contrasting effects on signaling, glucose uptake, and GLUT4 translocation in L6-GLUT4myc myotubes

We have previously shown that in L6-GLUT4myc rat skeletal muscle cells, acute treatment with leptin reduced insulin-stimulated glucose uptake without altering insulin-stimulated GLUT4 translocation. In contrast, we show here that the ability of leptin to increase phosphorylation of its receptor and to reduce insulin-stimulated glucose uptake was lost in cells that were continuously exposed to leptin for 24 h. This desensitization correlated with an increase in expression of suppressor of cytokine signaling-3 (SOCS-3). Time course analysis demonstrated that the transition from acute to chronic effects of leptin occurs after 2 h. The desensitization of leptin action at 24 h was not reversed by 30 min washout period prior to re-exposing cells to leptin. However, despite insulin-stimulated glucose uptake being unaffected upon 24 h preincubation with leptin, a small but significant decrease (37%) in insulin-stimulated GLUT4 translocation and phosphorylation of Akt on T308 was detected. Insulin-stimulated phosphorylation of Akt on S473 or of p38 MAPK were unaffected. These results suggest that the chronic leptin treatment leads to desensitization of leptin signaling yet can simultaneously decrease the ability of insulin to phosphorylate Akt on T308 and translocate GLUT4. However, this does not manifest as a reduction in total glucose uptake into L6 myotubes.

The obesity-associated peptide leptin induces hypertrophy in neonatal rat ventricular myocytes

One of the major manifestations of obesity is increased production of the adipocyte-derived 16-kDa peptide leptin, which is also elevated in heart disease, including congestive heart failure. However, whether leptin can directly alter the cardiac phenotype is not known. We therefore studied the effect of leptin as a potential hypertrophic factor in cultured myocytes from 1- to 4-day-old neonatal rat heart ventricles. Using RT-PCR, we demonstrate that these cells express the short-form (OB-Ra) leptin receptor. Twenty-four hours of exposure to leptin (0.31 to 31.3 nmol/L) produces a significantly increased cell surface area that peaked at 0.63 nmol/L. Subsequent experiments were done with 3.1 nmol/L leptin, which significantly increased cell area by 42%, protein synthesis by 32%, and alpha-skeletal actin and myosin light chain-2 expression by 250% and 300%, respectively. These events occurred in the absence of any increased cell death. Hypertrophy was preceded by rapid activation of the mitogen-activated protein kinase system including p38 and p44/42 as early as 5 minutes after leptin addition, whereas hypertrophy was inhibited by the p38 inhibitor SB203580 but not by the p44/42 inhibitor PD98059. Our results demonstrate a direct hypertrophic effect of leptin and may offer a biological link between hypertrophy and hyperleptinemic conditions such as obesity.

Inflammatory mediators and islet beta-cell failure: a link between type 1 and type 2 diabetes

Pancreatic islet beta-cell death occurs in type 1 and 2 diabetes mellitus, leading to absolute or relative insulin deficiency. beta-cell death in type 1 diabetes is due predominantly to autoimmunity. In type 2 diabetes beta-cell death occurs as the combined consequence of increased circulating glucose and saturated fatty acids together with adipocyte secreted factors and chronic activation of the innate immune system. In both diabetes types intra-islet inflammatory mediators seem to trigger a final common pathway leading to beta-cell apoptosis. Therefore anti-inflammatory therapeutic approaches designed to block beta-cell apoptosis could be a significant new development in type 1 and 2 diabetes.

Role of leptin as an immunomodulator of blood mononuclear cells: mechanisms of action

Leptin is a an adipocyte-secreted hormone that regulates weight centrally. However, the leptin receptor is expressed not only in the central nervous system, but also in peripheral tissues, such as haematopoietic and immune systems. Therefore, the physiological role of leptin should not be limited to the regulation of food intake and energy expenditure. Moreover, the leptin receptor bears homology to members of the class I cytokine family, and recent data have demonstrated that leptin is able to modulate the immune response. Thus, the leptin receptor is expressed in human peripheral blood mononuclear cells, mediating the leptin effect on proliferation and activation. In vitro activation and HIV infection in vivo induce the expression of the long isoform of the leptin receptor in mononuclear cells. Also, leptin stimulates the production of proinflammatory cytokines from cultured monocytes and enhances the production of Th1 type cytokines from stimulated lymphocytes. Moreover, leptin has a trophic effect on monocytes, preventing apoptosis induced by serum deprivation. Leptin stimulation activates JAK-STAT, IRS-1-PI3K and MAPK signalling pathways. Leptin also stimulates Tyr-phosphorylation of the RNA-binding protein Sam68 mediating the dissociation from RNA. In this way, leptin signalling could modulate RNA metabolism. These signal transduction pathways provide possible mechanisms whereby leptin may modulate activation of peripheral blood mononuclear cells. Therefore, these data support the hypothesis regarding leptin as a proinflammatory cytokine with a possible role as a link between the nutritional status and the immune response. Moreover, these immunoregulatory functions of leptin could have some relevance in the pathophysiology of obesity.

Leptin in the CNS: much more than a satiety signal

The discovery of the obese gene product, leptin has generated enormous interest in how the periphery signals the status of nutritional stores to specific hypothalamic nuclei involved in regulating feeding and energy balance. However it is emerging that leptin, in addition to its role as a circulating satiety factor, is a multi-faceted hormone that plays a key role in a variety of CNS functions. In this review, we summarise recent progress in leptin biology, with particular focus on its diversity of actions within the CNS, ranging from satiety signal, to regulator of bone formation and inhibitor of neuronal excitability.

Transduction of leptin growth signals in placental cells is independent of JAK-STAT activation

We have characterized the transduction pathways induced by leptin in the placenta, using human BeWo cells that express endogenous leptin receptors and synthesize leptin in a regulated manner. We first examined if the JAK-STAT phosphorylation cascade was functional in these cells. Phosphorylated JAK2 was primarily bound to a short 106kDa leptin receptor isoform and to a lesser extent to a 210kDa molecule. Leptin neither enhanced JAK2 phosphorylation nor activated STAT3 and STAT1 proteins indicating that JAK2 is constitutively activated and that the JAK-STAT transduction pathway is not recruited by leptin in BeWo cells. By contrast, leptin stimulated the transcription of the c-fos gene (3-fold) and cell proliferation (2-fold) as measured by DNA synthesis. Both effects were dependent on the rapid phosphorylation of p42-44 MAPK but not p38 MAPK. We conclude that a functional JAK-STAT pathway is not required for leptin to transduce proliferative signals in human placental cells. These findings extend the physiological action of leptin beyond its central effects, to the control of placental gene transcription and cell proliferation.

Expression and activation of STAT proteins during mouse retina development

Cytokines and growth factors play important roles in mammalian ocular development and maintenance. Recent studies have indicated that some of these ligands can activate signal transducer and activator of transcription factors (STATs) and modulate gene transcription. The purpose of this study was to investigate the expression and activation of STAT proteins in the developing mouse retina. Anti-STAT and anti-phosphorylated STAT antibodies were used to detect the expression and activation of STATs in embryonic and postnatal neuronal retina, ciliary margin, and retinal pigment epithelium (RPE). In situ hybridization and Western blot were also employed. In embryonic stages, all STAT proteins were expressed in the neuronal retina in distinct cell populations at different embryonic stages. For example, Stat3 expression and activation gradually increased in the inner neuroblast layer and ciliary margin during development. In adult retina, Stat3 was detected in the inner nuclear layer and ganglion cells layers. Stat1 was strongly expressed in both outer and inner plexiform layers. Stat5a was clearly expressed in the outer/inner nuclear layer, the ganglion cell layer, and the inner plexiform layer. Strong expression of Stat3, Stat5a, and Stat6 was observed in the RPE. Activated Stat3 and Stat5a were found in the neural retina and the RPE. Distinct STAT proteins were present in different cell populations in neuronal retina and RPE suggesting multiple functions of STATs in mammalian eye development. Studies of STAT signal pathways in the eye may contribute to the understanding of molecular mechanisms in control of ocular development and pathogenesis.

Novel actions of leptin in the hippocampus

It is well established that the obese gene product, leptin is an important circulating satiety factor that signals nutritional status to specific hypothalamic nuclei involved in body weight regulation. However, evidence is accumulating that, in addition to its pivotal role as an adiposity signal, leptin is a multi-faceted hormone that plays an important role in a plethora of CNS functions. In this review we summarize the recent advances made in leptin biology, with particular focus on its potential role as a cognitive enhancer and antiepileptic agent in the hippocampus.

Leptin inhibits epileptiform-like activity in rat hippocampal neurones via PI 3-kinase-driven activation of BK channels

The obese gene product, leptin is an important circulating satiety factor that regulates energy balance via its actions in the hypothalamus. However, leptin receptors are also expressed in brain regions not directly associated with energy homeostasis, such as the hippocampus. Here, leptin inhibits hippocampal neurones via activation of large conductance Ca(2+)-activated K(+) (BK) channels, a process that may be important in regulating neuronal excitability. We now show that leptin receptor labelling is expressed on somata, dendrites and axons, and is also concentrated at synapses in hippocampal cultures. In functional studies, leptin potently and reversibly reduces epileptiform-like activity evoked in lean, but not leptin-resistant Zucker fa/fa rats. Furthermore, leptin also depresses enhanced Ca(2+) levels evoked following Mg(2+) removal in hippocampal cultures. The ability of leptin to modulate this activity requires activation of BK, but not K(ATP), channels as the effects of leptin were mimicked by the BK channel activator NS-1619, and inhibited by the BK channel inhibitors, iberiotoxin and charybdotoxin. The signalling mechanisms underlying this process involve stimulation of phosphoinositide 3-kinase (PI 3-kinase), but not mitogen-activated protein kinase (MAPK), as two structurally unrelated inhibitors of PI 3-kinase, LY294002 and wortmannin, blocked the actions of leptin. These data indicate that leptin, via PI 3-kinase-driven activation of BK channels, elicits a novel mechanism for controlling neuronal excitability. As uncontrolled excitability in the hippocampus is one underlying cause of temporal lobe epilepsy, this novel action of leptin could provide an alternative therapeutic target in the management of epilepsy.

Appearance of leptin in wound fluid as a response to injury

The adiposity hormone leptin regulates food intake, body weight, reproduction and other metabolic and endocrine functions mainly through signaling to the hypothalamus. Leptin signaling to peripheral tissues other than the hypothalamus has been suggested for a number of processes such as immunity, bone metabolism, hematopoiesis, angiogenesis, and wound healing. It was previously shown that exogenously applied leptin accelerated wound healing and that leptin mRNA is expressed at the wound site, but there is no published evidence showing that it is translated into leptin protein that is available at the site of repair. To address this question we analyzed pig wound fluids collected from partial-thickness excisional wounds during the first 9 days after injury. Leptin was measured using a modified culture of HEK-293 cells, expressing both the human leptin receptor gene and the firefly luciferase gene driven by a STAT-inducible promoter. Relatively high levels of leptin activity (50-250 ng/ml) were detected in wound fluids using the leptin receptor expressing HEK-293 cells. Our results suggest that leptin is normally induced (4.8- to 10.2-fold) in wound tissue during the first few days following injury and may operate in a paracrine or autocrine circuit during the wound repair process.

Pancreatic beta-cell growth and survival--a role in obesity-linked type 2 diabetes?

Obesity-linked type 2 diabetes is a disease of insulin resistance combined with pancreatic beta-cell dysfunction. Although a role for beta-cell mass in the pathogenesis of obesity-linked type 2 diabetes has recently gained prominence, the idea is still being developed. It is proposed that in early obesity an increase in beta-cell mass and function might compensate for peripheral insulin resistance. However, as time and/or the severity of the obesity continue, there is decay in such adaptation and the beta-cell mass becomes inadequate. This, together with beta-cell dysfunction, leads to the onset of type 2 diabetes. It is becoming evident that elements in insulin and insulin growth factor (IGF)-1 signal-transduction pathways are key to regulating beta-cell growth. Current evidence indicates that interference of insulin signaling in obesity contributes to peripheral insulin resistance. This article examines whether a similar interference of IGF-1 signaling in the beta-cell could hinder upregulation of beta-cell mass and/or function, resulting in a failure to compensate for insulin resistance.

Leptin promotes the development of mouse preimplantation embryos in vitro

Leptin acts as a modulator of diverse reproductive functions, and recent studies have implicated involvement of leptin in the early embryo development in mammal. The aim of this study was to investigate the expression of leptin and its receptor (OB-R) in mouse oocyte and preimplantation embryo, and to examine whether leptin influenced the early embryo development. Leptin mRNA was detected in blastocyst and hatched blastocyst, and two splice variants of OB-R (OB-Ra and OB-Rb) mRNAs were detected in oocytes, 1-cell, 2-cell, morula, blastocyst, and hatched blastocyst. As for the origin of leptin, both leptin mRNA and protein were identified in the oviduct epithelium and endometrium of pregnant mouse. In the pregnant mouse, the levels of leptin in uterine fluid were higher than those in nonpregnant mouse. Addition of leptin to embryo culture media promotes the development from 2-cell stage embryos to the blastocysts, fully expanded blastocysts and hatched blastocysts. This effect was neutralized by an antibody against the extracellular domain of OB-R. Leptin significantly increased the total cell number of blastocysts, and the effect was preferentially observed in the trophectoderm. These findings raise the possibility of a paracrine/autocrine leptin signaling system regulating the development of mouse preimplantation embryo.

Steatosis is not sufficient to cause an impaired regenerative response after partial hepatectomy in rats

BACKGROUND/AIMS

Fatty liver is known to be associated with increased mortality and morbidity after liver resection. The ability of fatty liver to regenerate after two-thirds partial hepatectomy was studied in three different models of steatosis in rats: obese Zucker rats, orotic acid-fed Wistar rats and Wistar rats fed a methionine-low, choline-deficient diet.

METHODS

Liver regeneration was assessed 24 h after partial hepatectomy by bromodeoxyuridine incorporation (immunohistochemistry), proliferating cell nuclear antigen, cyclin E and cyclin-dependent kinase 2 protein expression (Western blot analysis) and cyclin-dependent kinase 2 activity (kinase assays using histone H1 as a substrate).

RESULTS

No significant difference of proliferative response was found between orotic acid or methionine-low, choline-deficient diet-fed and control Wistar rats 24 h after partial hepatectomy. In contrast, hepatocyte proliferation in obese Zucker rats after partial hepatectomy was significantly reduced when compared with their lean controls.

CONCLUSIONS

Steatosis per se does not impair liver regeneration. The reduced liver regeneration observed in obese Zucker rats may not be due to fatty infiltration itself but to other factors such as leptin receptor dysfunction.

Effect of interlukin-1beta on proliferation of gastric epithelial cells in culture

BACKGROUND

Helicobacter pylori is the main risk factor for the development of non-cardia gastric cancer. Increased proliferation of the gastric mucosa is a feature of H. pylori infection. Mucosal interkeukin-1beta production is increased in H. pylori infection and IL-1beta genotypes associated with increased pro-inflammatory activity are risk factors for the development of gastric cancer. The effect of IL-1beta on gastric epithelial cell proliferation has been examined in this study.

METHODS

AGS cells were cultured with IL-1beta. DNA synthesis was assed by [3H]thymidine incorporation and total viable cell numbers by MTT assay.

RESULTS

IL-1beta dose dependently increased DNA synthesis and cell numbers. The enhanced proliferation was blocked by interleukin-1 receptor antagonist. Addition of neutralising antibody to GM-CSF reduced IL-1beta-stimulated proliferation by 31 +/- 4 %. GM-CSF alone significantly stimulated proliferation. Addition or neutralisation of IL-8 had no effect on basal or IL-1beta-stimulated proliferation. The tyrosine kinase inhibitor genistein completely blocked IL-1beta-stimulated proliferation and inhibition of the extracellular signal related kinase pathway with PD 98059 inhibited IL-1beta stimulated proliferation by 58 +/- 5 %.

CONCLUSIONS

IL-1beta stimulates proliferation in gastric epithelial cells. Autocrine stimulation by GM-CSF contributes to this proliferative response. Signalling via tyrosine kinase activity is essential to the mitogenic response to IL-1beta. The extracellular signal related kinase pathway is involved in, but not essential to downstream signalling. IL-1beta may contribute to the hyperproliferation seen in H. pylori- infected gastric mucosa, and be involved in the carcinogenic process.

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.

Human follicular papilla cells carry out nonadipose tissue production of leptin

Leptin, a satiety-regulating cytokine, is predominantly expressed by adipocytes, although recently the nonadipose tissue production of leptin has been reported. To investigate the possibility of leptin production by human scalp hair follicles, we examined leptin production and its mRNA expression by cultured human follicular papilla cells. We isolated 12 human follicular papilla cell lines from different individuals. They were identified by their morphology, their high alpha-smooth-muscle actin expression, their inability to differentiate into adipocytes, and by the lack of mRNA for adipose-specific fatty acid binding protein. All the human follicular papilla cell lines, but not neonatal human dermal fibroblasts, produced significant amounts of leptin demonstrable by enzyme-linked immunosorbent assay. We demonstrated leptin mRNA expression by human follicular papilla cell lines, but not by neonatal human dermal fibroblasts, by reverse transcription polymerase chain reaction. By immunohistochemistry and in situ hybridization, we detected both leptin protein and mRNA at the lower portion of the hair follicle, i.e., hair matrix, inner root sheath of the hair bulb, and human follicular papilla cells. In contrast, the leptin receptor with intracytoplasmic signal sequence was detected in the follicular papilla cells immunohistochemically, and the long isoform of the leptin receptor mRNA was demonstrated in the human follicular papilla cell lines by reverse transcription polymerase chain reaction. Finally, by using these human follicular papilla cell lines, we showed that cytokines such as interleukin-1 beta, tumor necrosis factor alpha, interferon-gamma, and interleukin-4, and growth factors such as epidermal growth factor, basic fibroblast growth factor, and transforming growth factor beta1, but not vascular endothelial growth factor, hepatocyte growth factor, keratinocyte growth factor, and insulin-like growth factor 1, significantly downregulated the production of leptin. These data demonstrated that human follicular papilla cells produce leptin and express the functional leptin receptor in vivo and in vitro, suggesting its autocrine function. Moreover, the regulation pattern of its production by various factors suggests a pivotal role of leptin in hair biology.

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 increases the viability of isolated rat pancreatic islets by suppressing apoptosis

To test the hypothesis that leptin secreted from adipose tissue is a mediator linking obesity and pancreatic islet hypertrophy, we examined the effects of leptin on proliferative and apoptotic responses in rat islet cells. Rat pancreatic islets were isolated and incubated with 0, 1, 5, or 75 nM leptin for 24 h under serum-deprived conditions. Cell viability was assessed with 2,5-diphenyltetrazolium bromide and trypan blue dye exclusion tests. Cell proliferation and apoptosis were evaluated with 5-bromo-2'-deoxyuridine incorporation into DNA and DNA ladder formation, respectively. Incubation for 24 h with 1 and 5 nM leptin, the concentrations observed in obese subjects, increased the viability of isolated pancreatic islet cells. Five nanomolar concentrations of leptin did not stimulate 5-bromo-2'-deoxyuridine incorporation into incubated islet cells, indicating no influence on cell proliferation, but did inhibit DNA ladder formation, a hallmark of cell apoptosis. Moreover, 5 nM leptin reduced the triglyceride content and suppressed inducible nitric oxide synthase mRNA expression in incubated islets. These results suggest that leptin increased viable cell numbers via suppression of apoptosis in isolated pancreatic islet cells under these experimental conditions. This mechanism might account at least in part for an obesity-induced increase in pancreatic beta-cell mass.

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.

Human articular chondrocytes express functional leptin receptors

The effects of leptin hormone are mediated by interactions with several physiological regulatory systems and the cytokine network, and by targeting cells directly. The leptin receptor is a member of the class I cytokine receptor family, and its signal transduction resembles that induced by many cytokines. We demonstrated that serially cultured human articular chondrocytes possess the leptin receptor (Ob-R), and that this receptor was present on chondrocytes in native human cartilage. In cultured chondrocytes we detected mRNA for the functional isoform of leptin receptor (Ob-Rb or Ob-R(L)), and it was revealed that ligand binding resulted in phosphorylation of signal transducers and activators of transcription, namely STAT1 and STAT5. Chondrocytes stimulated with leptin exhibited an increased proliferation and an enhanced synthesis of extracellular matrix (proteoglycans and collagen). These results indicate that leptin affects cartilage generation directly, which is a novel role for leptin in skeletal growth and development.

A novel keratinocyte mitogen: regulation of leptin and its functional receptor in skin repair

Wound re-epithelialization represents a tissue movement that crucially participates in wound closure. Recently, we demonstrated that supplemented leptin improved re-epithelialization processes in leptin-deficient ob/ob mice. In this study we investigated regulation of the leptin system during normal repair in healthy animals. We found leptin to be present at the wound site during healing, although leptin levels were clearly reduced upon injury compared with uninvolved control skin. The functional leptin receptor subtype obRb was observed to be constitutively expressed in nonwounded skin. During early healing, the leptin receptor obRb was downregulated, but re-increased again from day 5 postwounding. Immunohistochemistry revealed that highly proliferative keratinocytes of the wound margin epithelia strongly expressed the functional leptin receptor subtype obRb. In vitro studies demonstrated that murine and human primary epidermal keratinocytes responded to exogenously added leptin with a proliferative response. Moreover, specificity of leptin-mediated mitogenic effects on primary keratinocytes could be shown by completely blocking leptin actions by a soluble, nonfunctional chimeric leptin receptor. Finally, we report that leptin, besides the recently described activation of the janus tyrosine kinase signal transducers, also activated extracellular signal-regulated kinase-controlled signaling pathways in primary keratinocytes.

Evidence for a circulating islet cell growth factor in insulin-resistant states

Insulin resistance is a feature of many common disorders including obesity and type 2 diabetes mellitus. In these disorders, the beta-cells compensate for the insulin resistance for long periods of time with an increase in secretory capacity, an increase in beta-cell mass, or both. To determine whether the beta-cell response might relate to a circulating growth factor, we have transplanted normal islets under the kidney capsule of normoglycemic insulin-resistant mice with two different models of insulin resistance: lean mice that have a double heterozygous deletion of the insulin receptor and insulin receptor substrate-1 (DH) or the obese, hyperglycemic ob/ob mice. In the grafts transplanted into both hosts, there was a marked increase in beta-cell mitotic activity and islet mass that was comparable with that observed in the endogenous pancreas. By contrast, islets of the DH mouse transplanted into normal mice showed reduced mitotic index. These data suggest the insulin resistance is associated with a circulating islet cell growth factor that is independent of glucose and obesity.

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 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.

Fetal pancreatic islets express functional leptin receptors and leptin stimulates proliferation of fetal islet cells

OBJECTIVE

Previous studies have demonstrated that leptin can stimulate proliferation of insulin-secreting tumor cell lines. The objective of this study was to characterize whether leptin could stimulate proliferation of primary beta-cells too. Since adult beta-cells have very limited capacity for replication, we examined the effect of leptin on islets of Langerhans obtained from fetal rats, in a tissue culture system.

METHODS

Leptin receptor mRNA and c-fos mRNA were measured by RT-PCR. Proliferation of fetal rat islet cells was measured by a WST-1 colorimetric assay and [3H]-thymidine incorporation assay.

RESULTS

Leptin stimulated proliferation of serum-deprived fetal rat islet cells, as indicated by increased formation of formazan dye from a tetrazolium salt WST-1. Leptin stimulated DNA synthesis in islet cells, as indicated by increased [3H]-thymidine incorporation into DNA. The effect of leptin on islet cell proliferation was on average 39-50% of the effect obtained with 10% fetal bovine serum. Leptin increased c-fos mRNA expression by 2.8-fold in isolated fetal islets after 30 min treatment. In fetal pancreatic islets, both the common extracellular portion (OB-R) and the intact long form (OB-Rb) of the leptin receptor were readily detected by reverse transcriptase polymerase chain reaction.

CONCLUSION

Functional leptin receptors are expressed in pancreatic islet cells, as early as during the fetal stage of development of these microorgans. Leptin stimulates proliferation of fetal islet cells and might play a role in determining islet cell mass at birth.

Leptin signaling in human peripheral blood mononuclear cells, activation of p38 and p42/44 mitogen-activated protein (MAP) kinase and p70 S6 kinase

The adipocyte-derived hormone leptin plays an important role as a relayer of nutritional status to several organ systems. Evidence is accumulating that leptin plays an important role in the adequate functioning and maintenance of the immune system. Here we show that leptin induces sustained phosphorylation of p38 MAP kinase in human peripheral blood mononuclear cells (PBMCs). We show furthermore that leptin induces two routes to phosphorylation of the 40S ribosomal protein S6, one is activation of the p90 ribosomal S6 kinase (RSK) via the MEK/p42/p44 MAP kinase pathway, the other is via activation of p70 S6 kinase. Thus, these results give new insight in the mechanism that underlies the immunomodulatory effects of leptin.

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.

Essential role of phosphoinositide 3-kinase in leptin-induced K(ATP) channel activation in the rat CRI-G1 insulinoma cell line

The mechanism by which leptin increases ATP-sensitive K(+) (K(ATP)) channel activity was investigated using the insulin-secreting cell line, CRI-G1. Wortmannin and LY 294002, inhibitors of phosphoinositide 3-kinase (PI3-kinase), prevented activation of K(ATP) channels by leptin. The inositol phospholipids phosphatidylinositol bisphosphate and phosphatidylinositol trisphosphate (PtdIns(3,4,5)P(3)) mimicked the effect of leptin by increasing K(ATP) channel activity in whole-cell and inside-out current recordings. LY 294002 prevented phosphatidylinositol bisphosphate, but not PtdIns(3,4,5)P(3), from increasing K(ATP) channel activity, consistent with the latter lipid acting as a membrane-associated messenger linking leptin receptor activation and K(ATP) channels. Signaling cascades, activated downstream from PI 3-kinase, utilizing PtdIns(3,4,5)P(3) as a second messenger and commonly associated with insulin and cytokine action (MAPK, p70 ribosomal protein-S6 kinase, stress-activated protein kinase 2, p38 MAPK, and protein kinase B), do not appear to be involved in leptin-mediated activation of K(ATP) channels in this cell line. Although PtdIns(3,4,5)P(3) appears a plausible and attractive candidate for the messenger that couples K(ATP) channels to leptin receptor activation, direct measurement of PtdIns(3,4,5)P(3) demonstrated that insulin, but not leptin, increased global cellular levels of PtdIns(3,4,5)P(3). Possible mechanisms to explain the involvement of PI 3-kinases in K(ATP) channel regulation are discussed.

Signaling through the p38 and p42/44 mitogen-activated families of protein kinases in pancreatic beta-cell proliferation

The present study has focused on the role of the 42- and 44-kDa mitogen-activated protein kinases (p42/44 MAPKs) and the 38-kDa mitogen-activated protein kinase (p38 MAPK) in the proliferation of the pancreatic beta-cell line MIN6. MIN6 beta-cell proliferation was assessed by measuring 5-bromo-2'-deoxyuridine (BrdU) incorporation into cellular DNA. Inhibition of both the p42/44 MAPK pathway using the MEK inhibitor PD098059 (PD) and the p38 MAPK pathway using the p38 inhibitor SB203580 (SB) caused a marked, concentration-dependent reduction in the BrdU immunostaining observed in the presence of 15% FCS when assessed using fluorescence immunocytochemistry. These data provide direct evidence of a role for p42/44 MAPKs in the mitogenic response of MIN6 beta-cells to FCS. Furthermore, these data also suggest a novel role for the p38 MAPK pathway in MIN6 beta-cell proliferation.

The adipoinsular axis: effects of leptin on pancreatic beta-cells

The prevalence of obesity and related diabetes mellitus is increasing worldwide. Here we review evidence for the existence of an adipoinsular axis, a dual hormonal feedback loop involving the hormones insulin and leptin produced by pancreatic beta-cells and adipose tissue, respectively. Insulin is adipogenic, increases body fat mass, and stimulates the production and secretion of leptin, the satiety hormone that acts centrally to reduce food intake and increase energy expenditure. Leptin in turn suppresses insulin secretion by both central actions and direct actions on beta-cells. Because plasma levels of leptin are directly proportional to body fat mass, an increase of adiposity increases plasma leptin, thereby curtailing insulin production and further increasing fat mass. We propose that the adipoinsular axis is designed to maintain nutrient balance and that dysregulation of this axis may contribute to obesity and the development of hyperinsulinemia associated with diabetes.

DEF-1, a novel Src SH3 binding protein that promotes adipogenesis in fibroblastic cell lines

The Src homology 3 (SH3) motif is found in numerous signal transduction proteins involved in cellular growth and differentiation. We have purified and cloned a novel protein, DEF-1 (differentiation-enhancing factor), from bovine brain by using a Src SH3 affinity column. Ectopic expression of DEF-1 in fibroblasts resulted in the differentiation of a significant fraction of the culture into adipocytes. This phenotype appears to be related to the induction of the transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma), since DEF-1 NIH 3T3 cells demonstrated augmented levels of PPARgamma mRNA and, when treated with activating PPARgamma ligands, efficient induction of differentiation. Further evidence for a role for DEF-1 in adipogenesis was provided by heightened expression of DEF-1 mRNA in adipose tissue isolated from obese and diabetes mice compared to that in tissue isolated from wild-type mice. However, DEF-1 mRNA was detected in multiple tissues, suggesting that the signal transduction pathway(s) in which DEF-1 is involved is not limited to adipogenesis. These results suggest that DEF-1 is an important component of a signal transduction process that is involved in the differentiation of fibroblasts and possibly of other types of cells.

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.