Direct effects of leptin on brown and white adipose tissue

Atrial natriuretic peptide stimulates lipid mobilization during repeated bouts of endurance exercise

Atrial natriuretic peptide (ANP) controls lipolysis in human adipocytes. Lipid mobilization is increased during repeated bouts of exercise, but the underlying mechanisms involved in this process have not yet been delineated. The relative involvement of catecholamine- and ANP-dependent pathways in the control of lipid mobilization during repeated bouts of exercise was thus investigated in subcutaneous adipose tissue (SCAT) by microdialysis. The study was performed in healthy males. Subjects performed two 45-min exercise bouts (E1 and E2) at 50% of their maximal oxygen uptake separated by a 60-min rest period. Extracellular glycerol concentration (EGC), reflecting SCAT lipolysis, was measured in a control probe perfused with Ringer solution and in two other probes perfused with either Ringer plus phentolamine (alpha(1/2)-AR antagonist) or Ringer plus both phentolamine and propranolol (beta-AR antagonist). Plasma epinephrine, plasma glycerol, and EGC were 1.7-, 1.6-, and 1.2-fold higher in E2 than in E1, respectively. Phentolamine potentiated exercise-induced EGC increase during E2 only. Propranolol reduced the lipolytic rate during both E1 and E2 compared with the probe with phentolamine. Plasma ANP concentration increased more during E2 than during E1 and was correlated with the increase in EGC in the probe containing phentolamine plus propranolol. The results suggest that ANP is involved in the control of lipolysis during exercise and that it contributes to stimulation of lipolysis during repeated bouts of exercise.

Microarray profiling of human white adipose tissue after exogenous leptin injection

BACKGROUND

Leptin is a secreted adipocyte hormone that plays a key role in the regulation of body weight homeostasis. The leptin effect on human white adipose tissue (WAT) is still debated.

OBJECTIVE

The aim of this study was to assess whether the administration of polyethylene glycol-leptin (PEG-OB) in a single supraphysiological dose has transcriptional effects on genes of WAT and to identify its target genes and functional pathways in WAT.

MATERIALS AND METHODS

Blood samples and WAT biopsies were obtained from 10 healthy nonobese men before treatment and 72 h after the PEG-OB injection, leading to an approximate 809-fold increase in circulating leptin. The WAT gene expression profile before and after the PEG-OB injection was compared using pangenomic microarrays. Functional gene annotations based on the gene ontology of the PEG-OB regulated genes were performed using both an 'in house' automated procedure and GenMAPP (Gene Microarray Pathway Profiler), designed for viewing and analyzing gene expression data in the context of biological pathways.

RESULTS

Statistical analysis of microarray data revealed that PEG-OB had a major down-regulated effect on WAT gene expression, as we obtained 1,822 and 100 down- and up-regulated genes, respectively. Microarray data were validated using reverse transcription quantitative PCR. Functional gene annotations of PEG-OB regulated genes revealed that the functional class related to immunity and inflammation was among the most mobilized PEG-OB pathway in WAT. These genes are mainly expressed in the cell of the stroma vascular fraction in comparison with adipocytes.

CONCLUSION

Our observations support the hypothesis that leptin could act on WAT, particularly on genes related to inflammation and immunity, which may suggest a novel leptin target pathway in human WAT.

Direct and indirect effects of leptin on preadipocyte proliferation and differentiation

Leptin has been shown to reduce body fat in vivo. Adipocytes express the leptin receptor; therefore, it is realistic to expect a direct effect of leptin on adipocyte growth and metabolism. In vitro studies examining the effect of leptin on adipocyte metabolism require supraphysiological doses of the protein to see a decrease in lipogenesis or stimulation of lipolysis, implying an indirect action of leptin. It also is possible that leptin reduces adipose mass by inhibiting preadipocyte proliferation (increase in cell number) and/or differentiation (lipid filling). Thus we determined direct and indirect effects of leptin on preadipocyte proliferation and differentiation in vitro. We tested the effect of leptin (0-500 ng/ml), serum from leptin-infused rats (0.25% by volume), and adipose tissue-conditioned medium from leptin-infused rats (0-30% by volume) on preadipocyte proliferation and differentiation in a primary culture of cells from male Sprague-Dawley rat adipose tissue. Leptin (50 ng/ml) stimulated proliferation of preadipocytes (P<0.05), but 250 and 500 ng leptin/ml inhibited proliferation of both preadipocyte and stromal vascular cell fractions (P<0.01), as measured by [3H]thymidine incorporation. Serum from leptin-infused rats inhibited proliferation of the adipose and stromal vascular fractions (P=0.01), but adipose tissue-conditioned medium had no effect on proliferation of either cell fraction. None of the treatments changed preadipocyte differentiation as measured by sn-glycerophosphate dehydrogenase activity. These results suggest that leptin could inhibit preadipocyte proliferation by modifying release of a factor from tissue other than adipose tissue.

Nitric oxide production from rat adipocytes is modulated by beta3-adrenergic receptor agonists and is involved in a cyclic AMP-dependent lipolysis in adipocytes

It is established that the modulation of beta(3)-adrenoceptor function could be associated with impairment of lipolysis in white fat and be responsible for disturbed lipid metabolism. Though two isoforms of nitric oxide synthase (NOS) were reported in adipocytes, the role of nitric oxide (NO) in adipose tissue is still ambiguous. The present work was directed to study the interplay between NO production and beta-adrenoceptor/cyclic AMP (cAMP) pathway on lipid mobilization (glycerol and nonesterified fatty acids, NEFA) in cultures of rat adipocytes isolated from epididymal white adipose tissue. beta-Nonselective (isoprenaline) and beta(3)-selective (BRL-37344) agonists and the postadrenoceptor agents such as dibutyryl-cAMP, forskolin, and 3-isobutyl-1-methylxanthine significantly increased nitrite, glycerol, and NEFA levels with BRL-37344 being the most potent. Conversely, addition of beta-nonselective (propranolol) or beta(3)-selective (bupranolol) antagonist or the adenylyl cyclase inhibitor (SQ 22,536) significantly reduced beta-agonist-induced NO production and lipolysis. For beta-adrenoceptor agonists, antagonists, and their pairs, there was a positive correlation between medium nitrite and glycerol or NEFA with r(2) being 0.90 and 0.84, respectively. The possible relationship between NO and lipolysis was revealed after adipocyte treatment with nonspecific (N(omega)-nitro-l-arginine methyl ester, l-NAME) and specific (aminoguanidine) NOS inhibitors. Both l-NAME and aminoguanidine significantly inhibited the lipolytic effect of BRL-37344. Moreover, NO-donor (S-nitroso-N-acetylpenicillamine) at higher concentration increased basal glycerol and NEFA levels. 8-bromo-cyclic GMP had no effect on adipocyte lipolysis. These data suggest that beta-adrenergic lipolysis, specifically beta(3)-adrenoceptor effect, which is realized via the adenylyl cyclase/cAMP/protein kinase A signaling cascade, involves NO production downstream of beta(3)-adrenoceptor/cAMP pathway.

Fat storage in adipocytes requires inactivation of leptin's paracrine activity: implications for treatment of human obesity

Hyperleptinemia rapidly depletes adipocyte fat in lean rats, whereas comparable hyperleptinemia produced by adipocytes in diet-induced obesity does not, implying a leptinergic blockade in adipocytes during overnutrition. Indeed, activated STAT-3 in white adipose tissue (WAT) of normal rats was less on a 60% high fat diet (HFD) than on 4% fat, despite a 10-fold higher plasma leptin. In 6 days of a HFD, mRNA of the postreceptor leptin inhibitor, suppressor of cytokine signaling-3, increased 22-fold in WAT, while leptin receptor (Lepr-b) mRNA gradually disappeared, implying leptinergic blockade at both postreceptor and receptor levels. Adipocyte-specific Lepr-b overexpression of a Lepr-b transgene completely prevented the adipocyte hypertrophy and hyperplasia and the increase in body fat induced in wild-type mice by HFD. Activated STAT-3 and AMP-activated protein kinase (AMPK), and the mRNA of lipooxidative enzymes, peroxisome proliferator-activated receptor-gamma-coactivator-1alpha, and uncoupling protein-1 and -2 were increased in WAT. Body temperature was elevated in the transgenic mice, suggesting uncoupled fatty acid oxidation of surplus fatty acids. In conclusion, storage of surplus calories in WAT and the development of diet-induced obesity require the blockade of a latent leptin-stimulated caloric sump in white adipocytes.

Glucose intolerance in young TallyHo mice is induced by leptin-mediated inhibition of insulin secretion

The pathophysiology of TallyHo mouse, a recently established animal model for type 2 diabetes mellitus, was analyzed at prediabetic state to examine the inherent defects which contribute to the development of diabetes. At 4 weeks of age, the TallyHo mice already revealed glucose intolerance while their peripheral tissues exhibited normal insulin sensitivity. On the other hand, decreased plasma insulin concentration was observed with little differences in pancreatic insulin contents, indicating the impaired insulin secretion. Such defect, however, was not found in the isolated islets, which suggests a role of endocrine factor in impaired insulin secretion of TallyHo mice. Treatment of leptin inhibited the glucose-stimulated insulin secretion from the isolated islets of TallyHo mice, while in vivo administration of anti-leptin antibody lowered plasma glucose concentration with increased insulin level in TallyHo mice. Expression of glucokinase mRNA was decreased both in whole pancreas and leptin treated islets of TallyHo mice compared with whole pancreas in C57BL/6 mice and untreated islets of TallyHo mice, respectively. These results suggest that elevated plasma leptin can, through the inhibition of insulin secretion, induce glucose intolerance in TallyHo mice.

Adipose tissue: a regulator of inflammation

Adipose tissue is a highly active organ. In addition to storing calories as triglycerides, it also secretes a large variety of proteins, including cytokines, chemokines and hormone-like factors, such as leptin, adiponectin and resistin. Intriguingly, many, if not most, of these adipose-derived proteins have dual actions; cytokines have both immunomodulatory functions and act as systemic or auto-/paracrine regulators of metabolism, while proteins such as leptin and adiponectin are regulators of both metabolism and inflammation. The production of pro-atherogenic chemokines by adipose tissue is of particular interest since their local secretion, e.g. by perivascular adipose depots, may provide a novel mechanistic link between obesity and the associated vascular complications.

Impact of transient correction of increased adrenocortical activity in hypothalamo-damaged, hyperadipose female rats

OBJECTIVE

To explore the effects of transient correction of enhanced corticoadrenal activity in monosodium L-glutamate (MSG)-damaged female rats on peripheral insulin sensitivity and in vitro retroperitoneal (RP) adipocyte function.

DESIGNS

A dose of 4 mg/g body weight (BW) of MSG or vehicle (CTR) was i.p. injected, once every 2 days, between days 2 and 10 of age, in female rats. Intact and 21 day-operated (sham or adrenal enucleation (AE)) rats from both (CTR and MSG) groups were used for experimentation on day 120 of age. Circulating levels of several hormones, in basal and after i.v. high-glucose load conditions, and RP adiposity morphology and function were then evaluated.

RESULTS

MSG rats developed increased adrenocortical function, hyperadiposity, hyperleptinemia, hyperinsulinemia and decreased peripheral insulin sensitivity. These characteristics were fully reversed after transient correction of corticoadrenal hyperactivity induced by AE. In addition, in vitro experimentation with isolated RP adipocytes indicated that cells from intact MSG animals displayed decreased sensitivity to insulin and dexamethasone stimulation of leptin secretion. Interestingly, adipocyte dysfunction in MSG rats was fully abrogated after AE-induced transient correction of insulinemia, leptinemia and adrenocortical activity. Importantly, the reversion of these metabolic abnormalities, induced by AE for 21 days, in MSG animals did occur, despite no significant changes in BW values.

CONCLUSION

Our results support that the changes in adipocyte characteristics and peripheral insulin resistance, developed in this pseudo-obese female rat model, are mainly due to increased glucocorticoid production. Importantly, appropriate correction of the enhanced adrenocortical activity fully reversed these abnormal functions.

Roles for melanocortins and leptin in adipose tissue apoptosis and fat deposition

Leptin has a wide range of effects on physiological functions related to the regulation of body energy balance. Many of leptin's effects are mediated through neuropeptide-containing neurons and neuropeptide receptors in the hypothalamus. The melanocortin system includes both agonist (alpha-melanocyte stimulating hormone, alphaMSH) and antagonist peptides (agouti related peptide, AGRP). Increased melanocortin receptor stimulation following leptin administration plays an important role in leptin-induced hypophagia and increased sympathetic nervous system activity and is partly responsible for leptin-induced weight loss. However, melanocortins do not appear to mediate some of the more striking centrally-mediated effects of leptin on adipose tissue, including adipose tissue apoptosis, that lead to the extensive depletion of fat.

Rat brown adipose tissue thermogenic features are altered during mid-pregnancy

Brown adipose tissue (BAT) thermogenesis is inhibited during late-pregnancy and lactation in the rat. However, scarce information concerning BAT functionality during mid-pregnancy is available. The aim of this work was to investigate uncoupling proteins and leptin expression during placentation in rat BAT as well as other key parameters in the thermogenic function of the tissue. BAT mitochondrial content was found to be reduced 50% in 11 and 13 day pregnant rats as compared to nonpregnant controls, although uncoupling protein 1 (UCP1) content was not modified. Furthermore, UCP3 mRNA levels were found to be highly increased during this period. beta3-adrenergic receptor (beta3-AR) decreased expression resulted in a higher alpha2/beta3 ratio. Finally, leptin mRNA levels in BAT were found to be 3-fold up-regulated in pregnant animals. In conclusion, we show the existence of profound changes in thermogenic features in BAT during gestational days 11 and 13, pointing to the importance of this tissue during mid-pregnancy.

Leptin rapidly activates PPARs in C2C12 muscle cells

Experimental evidence suggests that leptin operates on the tissues, including skeletal muscle, also by modulating gene expression. Using electrophoretic mobility shift assays, we have shown that physiological doses of leptin promptly increase the binding of C2C12 cell nuclear extracts to peroxisome proliferator-activated receptor (PPAR) response elements in oligonucleotide probes and that all three PPAR isoforms participate in DNA-binding complexes. We pre-treated C2C12 cells with AACOCF3, a specific inhibitor of cytosolic phospholipase A2 (cPLA2), an enzyme that supplies ligands to PPARs, and found that it abrogates leptin-induced PPAR DNA-binding activity. Leptin treatment significantly increased cPLA2 activity, evaluated as the release of [3H]arachidonic acid from pre-labelled C2C12 cells, as well as phosphorylation. Further, using MEK1 inhibitor PD-98059 we showed that leptin activates cPLA2 through ERK induction. These results support a direct effect of leptin on skeletal muscle cells, and suggest that the hormone may modulate muscle transcription also by precocious activation of PPARs through ERK-cPLA2 pathway.

High affinity leptin receptors are present in human mesenchymal stem cells (MSCs) derived from control and osteoporotic donors

There are disparate observations on central and peripheral effects of leptin, but several studies consistently support its role as a link between fat and bone. Bone marrow stroma contains mesenchymal stem cells (MSCs), which differentiate into osteoblasts and adipocytes, among others. In this study we assessed the expression of leptin receptors protein in MSCs from control and osteoporotic postmenopausal donors and their change during osteogenic and adipogenic differentiation. Also, we assessed the effects of leptin on osteogenic and adipogenic differentiation of these cells. We demonstrated high affinity leptin binding (KD = 0.36 +/- 0.02 nM) in both types of cells. Binding was very low under basal, but increased significantly (2-3 times) through osteogenic and adipogenic differentiation. Osteoporotic MSCs showed lower leptin binding capacity than control cells at an early osteogenic and adipogenic differentiation time, which could restrict cell sensitivity to the protective action of leptin. In this regard, we observed that leptin significantly inhibited adipocyte differentiation in control but not in osteoporotic MSCs, while it exerted a low stimulatory effect on calcium deposition (10%-20%) in both types of MSCs cells. In summary, we report the presence of high affinity leptin receptors on control and osteoporotic MSCs, which were modified distinctly by osteogenic and adipogenic stimulation and a direct and distinct effect of leptin on both type of cells.

Eicosapentaenoic fatty acid increases leptin secretion from primary cultured rat adipocytes: role of glucose metabolism

Eicosapentaenoic acid (EPA), one of the n-3 polyunsaturated fatty acids, has been shown to stimulate leptin mRNA expression and secretion in 3T3-L1 cells. However, other studies have reported inhibitory effects of EPA on leptin expression and secretion in vivo and in vitro. To determine the direct effects of EPA on basal and insulin-stimulated leptin secretion, isolated rat adipocytes were incubated with EPA in the absence and presence of insulin. EPA (10, 100, and 200 μM) increased basal leptin gene expression and secretion (+43.8%, P < 0.05; +71.1%, P < 0.01; and +73.7%, P < 0.01, respectively). EPA also increased leptin secretion in the presence of 1.6 nM insulin; however, the effect was less pronounced than in the absence of it. Because adipocyte glucose and lipid metabolism are involved in the regulation of leptin production, the metabolic effects of this fatty acid were also examined. EPA (200 μM) increased basal glucose uptake in isolated adipocytes (+50%, P < 0.05). Anaerobic metabolism of glucose, as assessed by lactate production and proportion of glucose metabolized to lactate, has been shown to be inversely correlated to leptin secretion and was decreased by EPA in both the absence and presence of insulin. EPA increased basal glucose oxidation as determined by the proportion of14C-labeled glucose metabolized to CO2. Lipogenesis (14C-labeled glucose incorporation into triglyceride) was decreased by EPA in the absence of insulin, whereas lipolysis (glycerol release) was unaffected. The EPA-induced increase of basal leptin secretion was highly correlated with increased glucose utilization ( r = +0.89, P < 0.01) and inversely related to the anaerobic glucose metabolism to lactate. EPA’s effect on insulin-stimulated leptin secretion was not related to increased glucose utilization but was inversely correlated with anaerobic glucose metabolism to lactate ( r = −0.84, P < 0.01). Together, the results suggest that EPA, like insulin, stimulates leptin production by increasing the nonanaerobic/oxidative metabolism of glucose.

Acute central infusion of leptin modulates fatty acid mobilization by affecting lipolysis and mRNA expression for uncoupling proteins

Chronic administration of leptin has been shown to reduce adiposity through energy intake and expenditure. The present study aims to examine how acute central infusion of leptin regulates peripheral lipid metabolism, as assessed by markers indicative of their mobilization and utilization. A bolus infusion of 1 microg/rat leptin into the third cerebroventricle increased the expression of mRNA for hormone-sensitive lipase (HSL), an indicator of lipolysis, in white adipose tissue (WAT). This was accompanied by elevation of plasma levels of glycerol, but not of free fatty acids, as compared to the saline control (P < 0.03). The same treatment with leptin decreased plasma insulin levels but did not affect the plasma glucose level (P < 0.05 for insulin). Among the major regulators of the transportation or utilization of energy substrates, leptin treatment increased expression of mRNA for uncoupling protein 1 (UCP1) in brown adipose tissue (BAT), UCP2 in WAT, and UCP3 in quadriceps skeletal muscle, but not those for fatty acid-binding protein in WAT, carnitine phosphate transferase-1, a marker for beta oxidation of fatty acids in muscle, nor glucose transporter 4 in WAT and muscle (P < 0.01 for HSL, P < 0.05 for UCP1, and P < 0.005 for UCP2 and UCP3). These results indicate that, even in a single bolus, leptin may regulate the mobilization and/or utilization of energy substrates such as fatty acids by affecting lipolytic activity in WAT and by increasing the expression of UCPs in BAT, WAT, and muscle.

Cross-talk between skeletal muscle and adipose tissue: a link with obesity?

Since the discovery of leptin, the adipocyte and its products have been the subject of intensive research. Thus, it has been demonstrated that adipose tissue plays a central role in energy homeostasis, behaving as an endocrine organ that expresses molecules involved in regulation of metabolism; alterations in the expression or activity of those molecules have a fundamental role in pathologies such as obesity and insulin resistance. However, little is known about the role played by another tissue, skeletal muscle, which may have similar functions regarding metabolism control. Indeed, some molecules expressed in this tissue have recently been shown to modulate adipose metabolism. The present review considers the metabolic interrelationships and cross-talk of signals derived from both skeletal muscle and adipose tissue. It is suggested that cytokines derived from both tissues may have an important role in maintaining an adequate ratio of skeletal muscle to fat and thus may play an important role in the control of body weight. IL-15 (a cytokine highly-expressed in skeletal muscle), TNF-alpha, and leptin could play a decisive role in the suggested "conversation" between adipose tissue and skeletal muscle.

Gene expression profile of isolated rat adipocytes treated with anthocyanins

Adipocyte dysfunction is strongly associated with the development of obesity and insulin resistance. It is accepted that the regulation of adipocytokine secretion or the adipocyte specific gene expression is one of the most important targets for the prevention of obesity and amelioration of insulin sensitivity. Recently, we demonstrated that anthocyanins, which are pigments widespread in the plant kingdom, have the potency of anti-obesity in mice and the enhancement adipocytokine secretion and adipocyte gene expression in adipocytes. In this study, we have shown for the first time the gene expression profile in isolated rat adipocytes treated with anthocyanins (cyanidin 3-glucoside; C3G or cyanidin; Cy). The rat adipocytes were treated with 100 muM C3G, Cy or vehicle for 24 h. The total RNA from the adipocytes was isolated and carried out GeneChip microarray analysis. A total of 633 or 427 genes was up-regulated (>1.5-fold) by the treatment of adipocytes with C3G or Cy, respectively. The up-regulated genes include lipid metabolism and signal transduction-related genes, however, the altered genes were partly different between the C3G- and Cy-treated groups. Based on the gene expression profile, we demonstrated the up-regulation of hormone sensitive lipase and enhancement of the lipolytic activity by the treatment of adipocytes with C3G or Cy. These data have provided an overview of the gene expression profiles in adipocytes treated with anthocyanins and identified new responsive genes with potentially important functions in adipocytes related with obesity and diabetes that merit further investigation.

The relation between serum leptin levels and max VO2 in male patients with type I diabetes and healthy sedentary males

This study aimed at investigating leptin levels in male diabetes type I patients who were on insulin treatment and also healthy sedentary males. The study included 10 male type I diabetes patients and 17 healthy sedentary males. Leptin levels of type I diabetes patients and healthy sedentary males with body mass index (BMI) over 25 kg/m2 were evaluated separately. The relation between serum leptin, max VO2, blood lactic acid levels before and after exercise, and effort durations of participants were investigated. At the end of the tests, no difference was found between leptin levels, max VO2 values, lactic acid values before exercise, and test durations of male type I diabetes patients and healthy sedentary males (p > .05), whereas lactic acid levels after exercise were found to be lower in healthy sedentary males (p < .05). Leptin levels in the group with BMI above 25 kg/m2 were higher than those in the group with BMI below 25 kg/m2 (p < .001). It was also seen that max VO2 values and test durations were higher in the group with BMI below 25 kg/m2 (p < .05). In conclusion, leptin levels of male type I diabetes patients are close to those of healthy sedentary males. The increase in leptin levels in both groups is in proportion to the BMI of individuals.

Hormone-sensitive lipase--new roles for an old enzyme

Although described initially as an intracellular adipocyte-specific triacylglycerol lipase, it is now clear that HSL (hormone-sensitive lipase) is expressed in multiple tissues and plays a number of roles in lipid metabolism, including that of a neutral cholesteryl ester hydrolase. The major isoform is a single polypeptide with a molecular mass of approx. 84 kDa and which comprises three major domains: a catalytic domain, a regulatory domain encoding several phosphorylation sites and an N-terminal domain involved in protein-protein and protein-lipid interactions. The activity of HSL is regulated acutely by several mechanisms, including reversible phosphorylation by a number of different protein kinases, translocation to different sites within the cell and interaction with a number of proteins, some of which may serve to direct the inhibitory products of HSL away from the protein. It is also apparent from work with HSL null mice that more than one enzyme species may be classified as a hormone-sensitive lipase. The possible presence of HSL in macrophages remains controversial, and the role of the protein in pancreatic beta-cells has yet to be fully elucidated. Altered expression of HSL in different cell types may be associated with a number of pathological states, including obesity, atherosclerosis and Type II diabetes.

Impaired leptin expression and abnormal response to fasting in corticotropin-releasing hormone-deficient mice

Leptin has been postulated to comprise part of an adipostat, whereby during states of excessive energy storage, elevated levels of the hormone prevent further weight gain by inhibiting appetite. A physiological role for leptin in this regard remains unclear because the presence of excessive food, and therefore the need to restrain overeating under natural conditions, is doubtful. We have previously shown that CRH-deficient (Crh(-/-)) mice have glucocorticoid insufficiency and lack the fasting-induced increase in glucocorticoid, a hormone important in stimulating leptin synthesis and secretion. We hypothesized that these mice might have low circulating leptin. Indeed, Crh(-/-) mice exhibited no diurnal variation of leptin, whereas normal littermates showed a clear rhythm, and their leptin levels were lower than their counterparts. A continuous peripheral CRH infusion to Crh(-/-) mice not only restored corticosterone levels, but it also increased leptin expression to normal. Surprisingly, 36 h of fasting elevated leptin levels in Crh(-/-) mice, rather than falling as in normal mice. This abnormal leptin change during fasting in Crh(-/-) mice was corrected by corticosterone replacement. Furthermore, Crh(-/-) mice lost less body weight during 24 h of fasting and ate less food during refeeding than normal littermates. Taken together, we conclude that glucocorticoid insufficiency in Crh(-/-) mice results in impaired leptin production as well as an abnormal increase in leptin during fasting, and propose that the fast-induced physiological reduction in leptin may play an important role to stimulate food intake during the recovery from fasting.

Increased leptin and white adipose tissue hypoplasia are sexually dimorphic in Lif null/Igf-I haploinsufficient mice

We previously showed cooperation of leukemia inhibitory factor (LIF) and insulin-like growth factor I (IGF-I) during development. Mice doubly deficient in LIF and IGF-I died at birth. We now analyze the possible combined influence of both factors on postnatal growth. The haploinsufficiency of the Igf-I gene on a Lif null background caused a marked reduction in body mass index and white adipose tissue only in female mice. These animals had increased leptin, increased serum IGF-I and apparent substitution of white adipose tissue by brown adipose tissue. The complex interrelationships between LIF and IGF-I in regulating weight thus involve sexually dimorphic effects on adipose tissue differentiation and circulating leptin.

Porcine leptin inhibits lipogenesis in porcine adipocytes

The present study examined whether recombinant porcine leptin alters lipid synthesis in porcine adipocytes. The stromal-vascular cell fraction of neonatal pig subcutaneous adipose tissue was isolated by collagenase digestion, filtration, and subsequent centrifugation. These cells were seeded on 25-cm2 tissue culture flasks and proliferated to confluency in 10% (vol/vol) fetal bovine serum in Dulbecco's modified Eagle medium/F12 (DMEM/F12, 50:50). Cultures were differentiated using 2.5% pig serum (vol/vol), 10 nM insulin, 100 nM hydrocortisone. After 7 d of lipid filling, cultures were washed free of this medium, incubated overnight in DMEM/F12 containing 2% pig serum (vol/vol), and then used for experiments. Acute experiments assessed U-(14)C-glucose or 1-(14)C-palmitate metabolism in cultures exposed to porcine leptin (0 to 1,000 ng/mL medium) for 4 h. Chronic experiments used cultures incubated with 0 to 1,000 ng porcine leptin/mL medium for 44 h before measurements of U-(14)C-glucose and 1-(14)C-palmitate oxidation and incorporation into lipid. Another experiment examined whether chronic leptin treatment alters insulin responsiveness by including insulin (10 nM) with incubations containing leptin. Leptin had no acute effects on glucose oxidation or conversion to lipid (P > 0.05). Acute leptin treatment decreased palmitate incorporation into lipids up to 45% (P < 0.05). Chronic leptin exposure decreased glucose oxidation (21%), total lipid synthesis (18%), and fatty acid synthesis (23%) at 100 ng/mL medium (P < 0.05). Insulin increased rates of glucose oxidation, total lipid, and fatty acid synthesis (P < 0.05); however, chronic exposure to 10 ng leptin/mL medium decreased the effectiveness of 10 nM insulin to affect these measures of glucose metabolism by approximately 18 to 46% (P < 0.05). Higher concentrations of leptin inhibited all effects of insulin on glucose metabolism (P < 0.05). Chronic exposure to leptin increased palmitate oxidation by 36% (P < 0.05). Chronic leptin exposure decreased palmitate incorporation into total lipids by 40% at 100 ng/mL medium (P < 0.05). Lipoprotein lipase activity was not affected (P > 0.05) by leptin. These data indicate that leptin functions to promote partitioning of energy away from lipid accretion within porcine adipose tissue by inhibiting glucose oxidation and lipogenesis indirectly, by decreasing insulin-mediated stimulation of lipogenesis, and by stimulating fatty acid oxidation while inhibiting fatty acid esterification.

The effect of a genetic variant for obesity and Type 2 Diabetes on the therapeutic potential of exercise and calorie restrictive diets in Zucker rats

Obesity and Type 2 Diabetes are modern pandemics caused by unique genetic-environmental interactions and distinguished by almost universal treatment failures. Relative influences of genome and lifestyle changes on an adult onset Obesity-Type 2 diabetes phenotype were explored. Zucker rats, a recessive model of genetic obesity-Type 2 Diabetes (117 fa/fa and 98 Fa/fa) were used. Dietary induced obesity (DIO) was imposed via a high fat diet on one-half; and one-half were forced to swim daily (EX). After 6 weeks, 78 animals were placed on a calorie (Kcal) restrictive diet for 6 more weeks. Genotype accounted for > 20% additional insulin resistance and obesity and modulated the effects of DIO and EX in adult animals exhibiting obesity-Type 2 diabetes. Only DIO gains were responsive to Kcal restriction. EX effects on insulin resistance were mediated by both Kcal restriction and genotype. Kcal restriction directly reduced hyperglycemia. Genetic variation was the major determinant of obesity and Type 2 Diabetes in Zucker rats. Genetically induced obesity and insulin sensitivity were resistant to EX and Kcal restriction; DIO and hyperglycemia were responsive to both. Successful treatment of Type 2 Diabetes requires understanding of how genotype may continue to modify adult responses to lifestyle change.

The potential beneficial effect of leptin on an experimental model of hyperlipidemia, induced by chronic ethanol treatment

BACKGROUND

Obesity is known to predispose individuals to liver disease by increasing hepatic sensitivity to endotoxin. The aim of the present study was to determine the effect of mouse recombinant leptin on food intake, body weight, hepatic and plasma lipids and lipoproteins in alcohol-induced liver injury.

METHOD

Male Swiss mice weighing 28-32 g were administered ethanol (6.32 g x kg(-1) body weight, p.o.) for the first 30 days. Subsequently, ethanol-fed mice were given intraperitoneal injections of exogenous leptin (230 microg x kg(-1) body weight, i.p.) every alternate day for 15 days. At the end of the total experimental period of 45 days, plasma concentrations of total cholesterol, free fatty acids, triglycerides, lipoprotein lipase and lipoproteins were measured.

RESULTS

Exogenous leptin injections to alcohol-fed mice significantly (P<0.05) inhibited the rise in hepatic and plasma lipid and lipoprotein concentrations as compared with those of the unsupplemented ethanol fed mice. Food intake and average body weight at the end of the experimental period was significantly decreased on leptin administration.

CONCLUSION

Chronic administration of exogenous mouse recombinant leptin prevents the rise in lipids and lipoprotein concentrations significantly in an animal model of alcohol-induced hyperlipidemia.

Neuroendocrinology of insulin resistance: metabolic and endocrine aspects of adiposity

Abdominal obesity is a major risk factor to attract the insulin resistance syndrome. It is proposed that abdominal obesity exposes the liver to elevated levels of free fatty acids, which activate a neuroendocrine reflex, leading to increased circulating levels of glucocorticoids. Besides directly attenuating peripheral insulin signaling, glucocorticoids oppose the activity of central nervous regulatory systems that stimulate insulin action. Among the factors that promote insulin action is leptin. Leptin regulates peripheral fuel partitioning and insulin action mainly through hypothalamic neuronal networks, which in turn, regulate endocrine activity of adipose tissue in a way comparable to thiazolidinediones. These are a class of insulin-sensitizing drugs, which exert their antidiabetic effects through the gamma isoform of peroxisome proliferator-activated receptor (PPAR-gamma). Since glucocorticoids oppose leptin action at several levels of control (including the central nervous system, CNS), it is argued that subjects easily develop obesity and associated metabolic disorders.

Chronic leptin administration increases serum NEFA in the pig and differentially regulates PPAR expression in adipose tissue11Purdue University Agriculture Research Program #16748

Two in vivo studies were conducted with pigs to determine the effects of exogenous leptin on the expression of peroxisome proliferator activated receptors (PPAR), and on serum concentrations of selected metabolites and hormones. Initially, leptin was administered i.m. to young pigs for 15 days at 0 (control), 0.003 (low), 0.01 (medium) and 0.03 (high) mg. kg(-1). day(-1). There was no leptin effect on serum glucose (P > 0.84), triglycerides (P > 0.69), non-esterified fatty acids (NEFA, P > 0.53), or glycerol (P > 0.33). Leptin at the intermediate and high doses depressed adipose expression of both PPARgamma1 (P < 0.06) and PPARgamma2 (P < 0.01). In a second study, we used a paired-feeding experimental design to determine the effects of a higher dose of leptin (0.05 mg. kg(-1). day(-1)) on serum metabolites and PPAR expression in selected tissues. At this dose, leptin increased (P < 0.0001) serum NEFA concentrations relative to both the ad libitum and pair-fed control groups. However, in this study, there was no difference in the expression of PPARgamma1 in adipose tissue, but PPARgamma2 mRNA was upregulated by leptin (P < 0.08). In contrast, leptin had no impact on the expression of PPARalpha in liver, skeletal muscle or adipose tissue. Adipose tissue explants were also incubated with leptin to assess the effect on PPARgamma expression, in vitro. The abundance of PPARgamma1 mRNA (P < 0.05) was increased after 24 hr of exposure, but the effect of leptin on gamma2 was not significant (P > 0.24). The lipolytic effect of leptin was also evaluated in vitro using isolated adipocytes. In keeping with the increase in serum NEFA concentrations in vivo, leptin stimulated lipolysis in vitro, increasing glycerol concentrations in the medium to about 219% of that in basal (non-treated) culture medium after 8 hr of incubation. Collectively, the data presented herein indicate that leptin modulates lipid metabolism in the pig, but that PPARalpha expression is not a parallel target of leptin as it is in rodent models. The regulation of PPARgamma by leptin seems complex in that it varied in relation to dose in vivo, and may be impacted by in vitro vs. in vivo circumstances.

Leptin and insulin modulate nutrient partitioning and weight loss in ob/ob mice through regulation of long-chain fatty acid uptake by adipocytes

Leptin treatment of ob/ob mice leads to weight loss appreciably greater than that in pair-fed mice. To test whether this "extra" weight loss is mediated by leptin-induced alterations in nutrient partitioning, the effects in ob/ob mice of subcutaneous leptin infusion (500 ng/h for <or=21 d) on adipocyte fatty acid uptake and transporter gene expression were examined. Mice were initially hyperinsulinemic (5.25 +/- 1.57 nmol/L). Plasma insulin decreased by 55 +/- 10% within 8 h of leptin infusion, declining progressively to normal by d 14. The V(max) for saturable adipocyte fatty acid uptake fell from 31.1 +/- 5.6 to 25.2 +/- 4.0 pmol/(s. 50000 cells) (P < 0.05) by 24 h, and to a normal rate (8.0 +/- 0.8 pmol/(s. 50000 cells) by d 21 (P > 0.5 vs. normal C57BL/6J controls). Adipocyte mRNA levels for plasma membrane fatty acid binding protein and fatty acid translocase, putative fatty acid transporters that are up-regulated three- to fourfold in adipocytes from ob/ob mice, had also normalized by d 21. The initial changes in V(max) preceded decreases in food intake and body weight by at least 24 h. In pair-fed mice, insulin levels, V(max) and body weight all declined more slowly than in leptin-treated mice, and all remained significantly elevated compared with normal values at d 21. The data suggest that insulin up-regulates and leptin down-regulates adipocyte fatty acid uptake, leading to alterations in fatty acid partitioning that affect adiposity.

Does leptin mediate the effect of photoperiod on immune function in mice?

Seasonal fluctuations in immune status have been documented for avian and mammalian populations. During the late summer and early fall, immune function is bolstered to help animals cope with the more physiologically demanding winter. The environmental cue for these seasonal changes is apparently decreasing photoperiod. In the present study, we determined the potential role of leptin in mediating the effect of photoperiod on cell-mediated immune responses in male mice. Leptin-deficient (ob/ob) and littermate control mice were housed for 10 wk in either a short (8L:16D) or a long (16L:8D) photoperiod beginning at 6 wk of age. After the mice were killed, immune and reproductive organs were weighed and splenocytes isolated. The proliferative and cytokine responses (interleukin [IL]-2 and IL-4) of splenocytes to the T-cell mitogen, concanavalin A (Con A; 0-40 microg/ml), were determined. Body weights were elevated and both testes and seminal vesicle weights subnormal in ob/ob mice (by ANOVA, main effect of leptin deficiency), but thymuses and spleens were of normal size. Serum leptin levels were at minimum detection limits in ob/ob mice, but leptin levels in control mice housed at 8L:16D were higher than in control mice housed at 16L:8D. The proliferative response of splenocytes from ob/ob mice to Con A was subnormal (by ANOVA, main effect of leptin deficiency), but photoperiod had no effect on this response. Production of IL-2 in splenocytes of ob/ob mice was subnormal (by ANOVA, main effect of leptin deficiency) irrespective of photoperiod, but cells from mice housed at 8L:16D (by ANOVA, main effect of photoperiod) produced more IL-2 than cells from animals housed at 16L:8D. In contrast, a leptin deficiency did not alter IL-4 production, but cells from animals (ob/ob and controls) housed at 16L:8D produced less IL-4 than cells from animals housed at 8L:16D (by ANOVA, main effect of photoperiod). The present study suggests that both photoperiod and leptin have mutually independent effects on the proliferation of lymphocytes and cytokine production profiles. The data do not provide definitive support for the hypothesis that photoperiod-induced changes in leptin secretion mediate the effects of season on immune status.

Establishment of a highly sensitive leptin radioimmunoassay and detection of increased leptin levels in hyperlipidemia and pregnancy

The highly effective antibody has been obtained by immunizing rabbits with recombinant leptin many times. The leptin is iodinated with the chloramine-T method and purified with a Sephadex-G25 chromatography column. The reaction between antigen and antibody is carried out by a one-step balance method and cultured at 4 degrees C for 24 h; the binding and free antigen was then separated by PR reagent. The determining range of this method is about 0.5-24 ng/mL; limited detection level is 0.45 ng/mL, relative standard deviation in a group, and among groups, are less than 5.4% and 8%, respectively. The level of blood leptin in 277 samples of normal persons, in 112 samples of overweight persons (weight/hieght m2 > or = 25) and 224 samples of hyperlipidemic patients have been measured by this method. It is demonstrated that the level of blood leptin in males is much lower than that of the females, and becomes elevated with increased age. Serum leptin level in overweight persons and hyperlipidemic patients is also much higher than that of normal groups (P < 0.01). Serum leptin of 21 workers in our lab at 8:00 AM and 4:00 PM has been tested. It was found that there are no differences between the two time points. The same results are obtained within age groups. Leptin levels of pregnant women's serum is higher than those of the control group (P < 0.001). Leptin in newborn's serum is significantly lower than those of mothers (P < 0.01). There is no obvious correlation between leptin level of mother and newborns by correlation analysis (r = 0.19, P > 0.05). The body weight and body weight index of pregnant women are well correlated with their serum leptin levels (r = 0.33 and 0.35, P < 0.05). The body weight and body weight index of newborns are well correlated with their serum leptin levels (r = 0.54 and 0.49, P < 0.001). The serum leptin level of pregnant women is not correlated with newborn's body weight (r = 0.10). These results have shown that the proposed method is stable, simple, and specific, being sensitive enough to determine leptin levels in human serum or plasma.

Leptin alters the structural and functional characteristics of adipose tissue before birth

This study aimed to determine for the first time whether leptin can act to alter the structural and functional characteristics of adipose tissue before birth. Leptin (0.48 mg/kg/day) or saline was infused intravenously into fetal sheep for 4 days from either 136 or 137 days of gestation (term=147+/-3 days). Circulating leptin concentrations were increased approximately four- to fivefold by leptin infusion. Leptin infusion resulted in a significant increase in the proportion of smaller lipid locules present within fetal perirenal adipose tissue (PAT), and this was associated with a significant increase in the proportion of multilocular tissue and a significant decrease in the proportion and relative mass of unilocular tissue in fetal PAT. The relative abundance of leptin mRNA in fetal PAT was significantly lower in the leptin-infused group, and there was a positive correlation between the relative abundance of leptin mRNA and the proportion of unilocular adipose tissue in fetal PAT. The amount of uncoupling protein 1 tended to be higher (P=0.06) in leptin-infused compared with saline-infused fetuses. This is the first demonstration that leptin can act to regulate the lipid storage characteristics, leptin synthetic capacity, and potential thermogenic functions of fat before birth.

Fasting increases gene expressions of uncoupling proteins and peroxisome proliferator-activated receptor-gamma in brown adipose tissue of ventromedial hypothalamus-lesioned rats

Uncoupling proteins (UCPs) are supposed to be involved in diet-induced thermogenesis. Their activities are usually elevated by feeding and reduced by fasting in normal animals. To investigate whether fasting affects the expression of UCPs mRNA in brown adipose tissue (BAT) of bilateral ventromedial hypothalamus (VMH)-lesioned rats, we determined the gene expression of UCP1, UCP2 or UCP3 in BAT of VMH-lesioned rats and examined oxygen consumption in these rats under fed or 48-h fasted conditions. Northern blotting revealed no difference in the expression of UCPs mRNA in BAT between VMH-lesioned and sham-operated rats under the fed condition, however, expressions were increased markedly in BAT of VMH-lesioned rats under the fasted condition. Under the fed condition, no difference in oxygen consumption was observed between VMH-lesioned and sham-operated rats. Under the fasted condition, oxygen consumption decreased in both rats, however, it decreased in VMH-lesioned less than in sham operated rats. To explore the mechanism that fasting elevated BAT UCPs mRNA in VMH-lesioned rats, we measured peroxisome proliferator-activated receptor (PPAR)-gamma mRNA and protein in BAT, because PPAR-gamma agonist can elevate UCPs mRNA levels in BAT. Under the fed condition, no differences in the expression of PPAR-gamma mRNA and protein content were observed between in BAT of VMH-lesioned and sham-operated rats. Under the fasted condition, however, both increased in BAT of VMH-lesioned rats. These results suggest that VMH-lesions enhance the gene expression of UCPs in BAT under long-term fasting as a defensive reaction to inhibit the reduction of body temperature through an increase in PPAR-gamma activity.

Leptin activates the promoter of the interleukin-1 receptor antagonist through p42/44 mitogen-activated protein kinase and a composite nuclear factor kappa B/PU.1 binding site

We have recently shown that leptin strongly induces the expression and secretion of the interleukin-1 receptor antagonist (IL-1Ra) [Gabay, Dreyer, Pellegrinelli, Chicheportiche and Meier (2001) J. Clin. Endocrinol. Metab. 86, 783-791] in monocytes. However, the intracellular signalling mechanisms involved remained unknown. We now demonstrate that the activation of the IL-1Ra promoter by leptin is strictly dependent on the presence of the long form of the leptin receptor (OB-Rb), and that it also requires the activation of the p42/44 mitogen-activated protein kinases (MAPKs) as well as the presence of a nuclear factor kappaB (NF-kappa B)/PU.1 composite site at position -80 of the IL-1Ra promoter. Although leptin is capable of activating a NF-kappa B reporter element in transient transfection experiments, the protein complex binding to the NF-kappa B/PU.1 site of the IL-1Ra promoter is not composed of the p65/p50 subunits of NF-kappa B, as is evident in electrophoretic gel mobility-shift experiments. In contrast, a protein complex which does not contain PU.1 binds to this composite element in a leptin-dependent manner. In summary, we characterize the signalling pathway for leptin and OB-Rb involved in the induction of IL-1Ra, involving p42/44 MAPK, and a yet uncharacterized complex of transcription factor(s) binding to a NF-kappa B/PU.1 composite element of the IL-1Ra promoter.

Blunted lipolytic response to fasting in abdominally obese women: evidence for involvement of hyposomatotropism

BACKGROUND

Abdominal obesity is associated with a blunted lipolytic response to fasting that may contribute to the preservation of adipose tissue mass.

OBJECTIVE

To further explore the pathophysiology of blunted lipolysis during fasting in obesity, we simultaneously measured lipolysis and distinct neuroendocrine regulatory hormones in abdominally obese and normal-weight (NW) women.

DESIGN

Eight abdominally obese [x +/- SD body mass index (BMI; in kg/m(2)): 32.1 +/- 2.6] and 6 NW (BMI: 22.7 +/- 1.5) women were studied during the last 8 h of a 20-h fast. The glycerol appearance rate and the serum and plasma concentrations of insulin, leptin, cortisol, and growth hormone were measured regularly.

RESULTS

At 13 h of fasting, the mean (+/-SD) glycerol appearance rate corrected for fat mass was greater in NW women than in obese women (7.2 +/- 1.0 and 5.1 +/- 0.6 micro mol.kg(-1).min(-1), respectively; P = 0.001). After a 20-h fast, lipolysis increased to 8.9 +/- 1.5 mmol.kg(-1).min(-1) in NW women (23%), whereas it did not change significantly in obese women (-2%). Fasting decreased insulin concentrations by approximately 30% in both groups, but it did not induce significant changes in leptin concentrations. Mean cortisol concentrations and urinary catecholamine excretion were comparable in both groups. However, mean plasma growth hormone concentrations were higher in NW women than in obese women (1.81 +/- 0.98 compared with 0.74 +/- 0.52 mU/L; P = 0.046). The relative change in lipolysis tended to correlate with mean plasma growth hormone concentrations (r = 0.515, P = 0.059).

CONCLUSION

Abdominal obesity-associated hyposomatotropism may be involved in the blunted increase in lipolysis during fasting.

Adipose tissue cellularity and apoptosis after intracerebroventricular injections of leptin and 21 days of recovery in rats

OBJECTIVE

To determine the effect of leptin and post-treatment recovery on adipose tissue cellularity and apoptosis. In addition, to investigate whether Bcl-2 and/or Bax is involved in the mechanism of leptin-induced adipose tissue apoptosis.

DESIGN

A total of 24 adult male Sprague-Dawley rats were injected i.c.v. with either 10 microg mouse leptin or 10 microl vehicle once per day for 4 days. At 24 h after the last injection, one group was killed while the other was monitored for 21 days.

MEASUREMENTS

DNA fragmentation and Bcl-2 and Bax protein levels were determined in inguinal (ING), epididymal (EPI) and retroperitoneal (RP) white adipose tissues and the interscapular brown adipose tissue (BAT). Cellularity was determined in ING and EPI.

RESULTS

Leptin significantly reduced the masses of all white fat pads [RPINGEPI] but not BAT. Cell volume was significantly reduced in EPI and ING. Only ING had a significantly reduced cell number from leptin treatment plus exhibited apoptosis by increased DNA fragmentation and DNA laddering, and upregulation of pro-apoptosis Bax protein. The other fat pads exhibited a general trend to increase the Bcl-2/Bax ratio. Recovery allowed for normalization of white fat pad mass, cell number and cell volume; however, BAT mass increased 42% over control. After recovery, apoptosis was not detected, Bcl-2 protein had increased in ING, and the Bcl-2/Bax ratio had risen overall.

CONCLUSIONS

Central administration of mouse leptin in the rat targets white fat depots individually to reduce mass by a reduction in cell volume plus adipocyte deletion in, at least, the ING fat pad by Bax-mediated apoptosis. Even after a dramatic loss in adipose tissue mass and change in cellularity, the rat demonstrates a resilient return to control levels together with an increase in factors that prevent adipocyte loss.

Interaction between leptin and insulin signaling pathways differentially affects JAK-STAT and PI 3-kinase-mediated signaling in rat liver

Chronic leptin treatment markedly enhances the effect of insulin on hepatic glucose production unproportionally with respect to body weight loss and increased insulin sensitivity. In the present study the cross-talk between insulin and leptin was evaluated in rat liver. Upon stimulation of JAK2 tyrosine phosphorylation, leptin induced JAK2 co-immunoprecipitation with STAT3, STAT5b, IRS-1 and IRS-2. This phenomenon parallels the leptin-induced tyrosine phosphorylation of STAT3, STAT5b, IRS-1 and IRS-2. Acutely injected insulin stimulated a mild increase in tyrosine phosphorylation of JAK2, STAT3 and STAT5b. Leptin was less effective than insulin in stimulating IRS phosphorylation and their association with PI 3-kinase. Simultaneous treatment with both hormones yielded no change in maximal phosphorylation of STAT3, IRS-1, IRS-2 and Akt, but led to a marked increase in tyrosine phosphorylation of JAK2 and STAT5b when compared with isolated administration of insulin or leptin. This indicates that there is a positive cross-talk between insulin and leptin signaling pathways at the level of JAK2 and STAT5b in rat liver.

Peripheral metabolic actions of leptin

The adipocyte-derived hormone, leptin, regulates food intake and systemic fuel metabolism; ob /ob mice, which lack functional leptin, exhibit an obesity syndrome that is similar to morbid obesity in humans. Leptin receptors are expressed most abundantly in the brain but are also present in several peripheral tissues. The role of leptin in controlling energy homeostasis has thus far focused on brain receptors and neuroendocrine pathways that regulate feeding behaviour and sympathetic nervous system activity. This chapter focuses on mounting evidence that leptin's effects on energy balance are also mediated by direct peripheral actions on key metabolic organs such as skeletal muscle, liver, pancreas and adipose tissue. Strong evidence indicates that peripheral leptin receptors regulate cellular lipid balance, favouring beta-oxidation over triacylglycerol storage. There are data to indicate that peripheral leptin also modulates glucose metabolism and insulin action; however, its precise role in controlling gluco-regulatory pathways remains uncertain and requires further investigation.

Leptin: a review of its peripheral actions and interactions

Following the discovery of leptin in 1994, the scientific and clinical communities have held great hope that manipulation of the leptin axis may lead to the successful treatment of obesity. This hope is not yet dashed; however the role of the leptin axis is now being shown to be ever more complex than was first envisaged. It is now well established that leptin interacts with pathways in the central nervous system and through direct peripheral mechanisms. In this review, we consider the tissues in which leptin is synthesized and the mechanisms which mediate leptin synthesis, the structure of leptin and the knowledge gained from cloning leptin genes in aiding our understanding of the role of leptin in the periphery. The discoveries of expression of leptin receptor isotypes in a wide range of tissues in the body have encouraged investigation of leptin interactions in the periphery. Many of these interactions appear to be direct, however many are also centrally mediated. Discovery of the relative importance of the centrally mediated and peripheral interactions of leptin under different physiological states and the variations between species is beginning to show the complexity of the leptin axis. Leptin appears to have a range of roles as a growth factor in a range of cell types: as be a mediator of energy expenditure; as a permissive factor for puberty; as a signal of metabolic status and modulation between the foetus and the maternal metabolism; and perhaps importantly in all of these interactions, to also interact with other hormonal mediators and regulators of energy status and metabolism such as insulin, glucagon, the insulin-like growth factors, growth hormone and glucocorticoids. Surely, more interactions are yet to be discovered. Leptin appears to act as an endocrine and a paracrine factor and perhaps also as an autocrine factor. Although the complexity of the leptin axis indicates that it is unlikely that effective treatments for obesity will be simply derived, our improving knowledge and understanding of these complex interactions may point the way to the underlying physiology which predisposes some individuals to apparently unregulated weight gain.

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.

Is leptin the link between fat and bone mass?

Recently, leptin has emerged as a potential candidate responsible for protective effects of fat on bone tissue. However, it remains difficult to draw a clear picture of leptin effects on bone metabolism because published data are sometimes conflicting or apparently contradictory. Beyond differences in models or experimental procedures, it is tempting to hypothesize that leptin exerts dual effects depending on bone tissue, skeletal maturity, and/or signaling pathway. Early in life, leptin could stimulate bone growth and bone size through direct angiogenic and osteogenic effects on stromal precursor cells. Later, it may decrease bone remodeling in the mature skeleton, when trabecular bone turnover is high, by stimulating osteoprotegerin (OPG) expression. Leptin negative effects on bone formation effected through central nervous system pathway could counterbalance these peripheral and positive effects, the latter being predominant when the blood-brain barrier permeability decreases or the serum leptin level rises above a certain threshold. Thus, the sex-dependent specificity of the relationship between leptin and bone mineral density (BMD) in human studies could be, at least in part, caused by serum leptin levels that are two- to threefold higher in women than in men, independent of adiposity. Although these hypotheses remain highly speculative and require further investigations, existing studies consistently support the role of leptin as a link between fat and bone.

Immunomodulatory actions of leptin

Leptin, the adipocyte-secreted hormone, exerts its main function as regulator of food intake and energy expenditure through central effects at the hypothalamic level. However, it appeared that this cytokine-like peptide has also direct effects on other peripheral tissues and cell types. Remarkable effects have been demonstrated on the immune function in vivo and in vitro. Monocytes are one of the target cells of leptin, and we have demonstrated that secretion of L-1Ra, an IL-1 receptor antagonist, is induced by leptin. In human obesity leptin and IL-1Ra levels are elevated, and these levels are decreased after weight loss. It is discussed that IL-1Ra may contribute to central leptin resistance.

Leptin signalling

The identification of leptin as the product of the obesity (ob) gene has been followed by extensive research identifying a wide spectrum of physiological effects elicited by this adipose-derived hormone. These effects are mediated via a family of cytokine-like receptor isoforms distributed in both the central nervous system and periphery. The signal transduction pathways regulated by leptin are diverse and include those characteristic of both cytokine and growth factor receptor signalling. This review describes the structure and function of leptin receptors and summarizes recent progress that has been made in characterizing the increasing number of signal transduction pathways regulated by leptin.

Accelerated substrate cycling: a new energy-wasting role for leptin in vivo

Simultaneous lipolysis and reesterification form the triacylglycerol/fatty acid (TAG/FA) cycle, a substrate cycle commonly used for thermogenesis. Its rate was measured in vivo by indirect calorimetry and continuous infusion of [2-(3)H]glycerol and [1-(14)C]palmitate, after injection of leptin or vehicle saline in rabbits. Leptin stimulated in vivo lipolysis from 9.66 +/- 0.62 to 14.78 +/- 0.93 micromol x kg(-1) x min(-1), the rate of appearance of FA from 20.69 +/- 2.14 to 29.03 +/- 3.03 micromol x kg(-1) x min(-1), and TAG/FA cycling from 24.82 +/- 1.73 to 37.09 +/- 2.49 micromol FA x kg(-1) x min(-1). This large increase in total cycling was caused by an 85% rise in primary cycling (reesterification without transit in the circulation) and accounted for 14% of the difference in metabolic rate between the controls and the leptin-treated animals. This study shows that leptin causes a strong activation of TAG/FA cycling, lipolysis, and FA oxidation, shifting fuel preference from carbohydrates to lipids. Therefore, the acceleration of substrate cycling is a new mechanism triggered by leptin to increase metabolic rate, besides the known induction of uncoupling proteins.

Cloning, expression, and mapping of a gene that is upregulated in adipose tissue of mice deficient in bombesin receptor subtype-3

To identify novel obesity-related genes in adipose tissue, differential display was performed using bombesin receptor subtype-3 (BRS-3)-deficient mice. These mice exhibit mild late-onset obesity. We report that a gene, Urb, is upregulated in these mice. Full-length Urb cDNA is approximately 3 kb long and comprises an open reading frame of 949 amino acid residues. Interestingly, Urb mRNA expression in brown adipose tissue of BRS-3-deficient mice is fourfold higher than that in wild-type controls. Enhanced Urb mRNA expression was also observed in brain, digestive tissues, kidney, and lung. Within the brain, Urb mRNA is detected in the dorsal endopiriform nucleus and choroid plexus. A T31 radiation hybrid mapping panel revealed that the Urb gene maps to mouse chromosome 16. Collectively, these findings suggest that Urb may have a unique function in the regulation of body weight and energy metabolism.

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.

Developmental regulation of leptin gene expression in rat brain and pituitary

We have previously reported the expression of leptin mRNA and protein in adult rat brain and pituitary gland. We report here the presence of leptin and leptin receptor mRNA in neonatal female rat brain and pituitary using RT-PCR as well as leptin and leptin receptor immunoreactivity in neonatal rat brain. In addition, we describe age-related changes in leptin mRNA expression in female rat brain and pituitary from postnatal day 2 to 28, evaluated using semi-quantitative RT-PCR analysis. Age-related differences in leptin (ob) mRNA levels were tissue-dependent. The most striking developmental changes were noted in the pituitary and cerebral cortex. In the pituitary, ob mRNA levels were maximal during postnatal days 7-14 and fell sharply by postnatal day 22. In cortex, ob mRNA levels were low in neonatal pups (day 2-7) but increased significantly between postnatal days 14 and 28. Leptin mRNA was detectable at postnatal day 2 in hypothalamus and subcutaneous fat. No significant differences in the level of expression were observed between postnatal day 2 and 28. Serum leptin levels were highest at day 7-14 and decreased significantly by day 21-28, coincident with the fall in pituitary leptin expression. The high levels of leptin expression in the neonatal pituitary suggest that this gland may contribute to the circulating leptin levels during early postnatal development, when adipose deposits are minimal. These data indicate that regulation of leptin gene expression in the postnatal period is tissue-dependent, a finding, which suggests that local leptin expression may have important functional significance in the development of the brain-pituitary system.

Modulation of the leptin-induced white adipose tissue lipolysis by nitric oxide

The present study tested the hypothesis that nitric oxide (NO) is involved in the leptin-induced stimulation of lipolysis. The effect of intravenous (iv) administration of leptin (10, 100 and 1000 microg/kg body weight) or vehicle on serum NO concentrations and glycerol release from white adipocytes of Wistar rats was examined. One hour after injection, the three leptin doses tested increased serum NO concentrations 15.1%, 23.4% and 60.0%, respectively (P<.001 vs. baseline). The effect of leptin on NO concentrations was significantly dose dependent on linear trend testing (P=.0001). Simple linear regression analysis showed that the lipolytic rate measured was significantly correlated with serum NO concentrations (P=.0025; r=.52). In order to gain further insight into the potential underlying mechanisms, the effect of leptin on lipolysis was studied in the setting of nitric oxide synthase (NOS) inhibition or acute ganglionic blockade. The stimulatory effect of leptin on lipolysis was significantly decreased (P<.05) under NOS inhibition. On the contrary, the leptin-induced lipolysis was unaltered in pharmacologically induced ganglionic blockade. The lack of effect on isoproterenol-, forskolin- and dibutyryl-cyclic AMP-stimulated lipolysis suggests that leptin does not interfere with the signal transduction pathway at the beta-adrenergic receptor, the adenylate cyclase and the protein kinase A levels. These findings suggest that NO is a potential regulator of leptin-induced lipolysis.

Effects of leptin on the differentiation and metabolism of human adipocytes

BACKGROUND

Leptin is an adipose protein regulating food intake in the hypothalamus. Animal studies have suggested that leptin also acts in an auto-/paracrine fashion on adipose cell function.

OBJECTIVE

The aim of this study was to investigate the effects of leptin on the differentiation and metabolism of cultured human adipocytes.

MATERIAL

Adipose tissue from young healthy, lean women (body mass index (BMI) <27 kg/m(2)) undergoing elective mammary reduction surgery and young obese individuals (BMI>40 kg/m(2)) undergoing laparoscopic gastric banding.

METHODS

Human preadipocytes in primary culture were induced to undergo differentiation by defined adipogenic factors. Mature adipocytes were isolated by collagenase digestion and kept in culture suspension. Glycero-3-phosphate dehydrogenase (GPDH) activity was used as a marker of adipose differentiation; glucose uptake, lipolysis and PAI-1 secretion were measured as parameters of fat cell function.

RESULTS

Human preadipocytes and adipocytes from lean and obese subjects expressed the long leptin receptor isoform and two of the three short forms as assessed by polymerase chain reaction (PCR). Leptin at a supraphysiological concentration induced a transient increase of GPDH activity, but had no effect on glucose uptake and PAI-1 secretion from human adipocytes. In addition, basal and isoproterenol-stimulated lipolysis as well as the antilipolytic action of insulin in human adipocytes was not significantly affected by leptin exposure.

CONCLUSION

In contrast to animal data, the results of our experiments do not demonstrate significant effects of leptin on main metabolic functions of human adipocytes arguing against a local auto-/paracrine action of leptin in human adipose tissue.

The role of leptin in the regulation of energy balance and adiposity

Since its discovery, leptin (a 167-amino acid product of the OB gene) has quickly moved to the forefront as an important hormone for regulation of energy balance. It closes a feedback loop from adipose tissue to hypothalamic neuropeptide-containing neural circuitry involved in regulation of food intake and neuroendocrine/autonomic outflow. While increased central leptin signalling reduces adiposity via a reduction in food intake, it also has remarkable metabolic effects that promote leanness, independent of food intake. These include: (i) increased energy expenditure, (ii) in-place degradation of fat, and (iii) increased thermogenesis. Hypothalamic neurones that synthesize corticotropin releasing hormone and melanocortins (i.e. alpha-melanocyte-stimulating hormone and agouti-related protein) are likely effector pathways that mediate the anorexigenic and metabolic effects of leptin. Activation of sympathetic outflow (via neuropeptidergic effector pathways of central leptin) to a number of tissues that store fat might be an important mechanism through which these peripheral metabolic effects are elicited. It is proposed that these peripheral metabolic effects contribute to the satiating properties of leptin.