Leptin does not affect adipocyte glucose metabolism: studies in fresh and cultured adipocytes

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Leptin inhibits glycogen catabolism but does not modify acutely the suppressive effect of insulin on glucose production and glycogenolysis stimulated by 8-Br-cAMP in rat liver perfused in situ

Leptin, a hormone secreted by the adipocytes, plays a central role in glucose metabolism and the action of insulin. Here we assessed, by means of rat-liver perfusion, the direct influence of physiological (10 ng/ml) and supraphysiological (50 or 100 ng/ml) concentrations of leptin on the suppressive effect of insulin on the glucose production and glycogenolysis stimulated by 8-bromoadenosine-3':5'-monophosphate (8-Br-cAMP). Portal infusion of insulin (20 microU/ml) or leptin (10 ng/ml) reduced (p<0.05) the glucose production and glycogenolysis induced by 8-Br-cAMP (0.3 microM). However, portal infusion of physiological (10 ng/ml) and supraphysiological (50 or 100 ng/ml) concentrations of leptin together with the insulin did not modify the suppressive effect of the latter on the glucose production and glycogenolysis stimulated by 8-Br-cAMP. Moreover, prolonging the period of leptin infusion from 20 to 40 min also failed to influence the liver response to insulin. Thus, we conclude that: (a) leptin, at physiological levels, has a direct and acute effect, inhibiting the glucose production and glycogenolysis stimulated by 8-Br-cAMP; (b) leptin, at either physiological or supraphysiological concentrations, has no short-term influence on the suppressive effect of insulin on glycogen catabolism stimulated by 8-Br-cAMP.

Potential role of leptin in increase of fatty acid synthase gene expression in chicken liver

Leptin is reported to have direct effects on lipogenesis in peripheral tissues that are independent of its central effects on food intake and body weight. These experiments have been mainly carried out in rodents (different models of obesity) in which lipogenesis occurs in both adipose tissue and liver. Such effects are unknown in birds in which lipogenesis occurs essentially in the liver. In this study, leptin gene expression and circulating leptin levels were examined in two chicken lines, selected for high (FL) or low (LL) abdominal fat pad size, at different nutritional states (fasted and fed state). In addition, effects of recombinant chicken leptin on liver metabolism were investigated. Hepatic leptin and fatty acid synthase (FAS) gene expression and plasma leptin levels were significantly higher in FL than in LL chickens (P < 0.05). In both lines, fasting significantly reduced hepatic leptin and FAS mRNA levels (P < 0.05). Continuous administration of recombinant chicken leptin (8 microg/kg/h) during 6 h significantly inhibited food intake (51%) and increased leptinemia (23-fold) compared to untreated group. Despite the decrease of food intake, leptin significantly induced the expression of FAS in chicken liver. These changes were accompanied by a significant down-regulation of leptin receptor gene expression, however SREBP-1, the main transcription factor of lipogenic genes, remained unchanged. This result suggests a local potential role of leptin in the regulation of avian hepatic lipogenesis, and explain, at least partly, the metabolic changes evolved during the divergent selection of FL and LL chickens.

Porcine leptin alters isolated adipocyte glucose and fatty acid metabolism

This study examined if leptin can acutely affect glucose or fatty acid metabolism in pig adipocytes and whether leptin's actions on lipogenesis are manifested through interaction with insulin or growth hormone. Subcutaneous adipose tissue was obtained from approximately 55 kg crossbred barrows at the USDA abattoir. Isolated adipocytes were prepared using a collagenase procedure. Experiments assessed U-14C-glucose or 1-14C-palmitate metabolism in isolated adipocytes exposed to: basal medium (control), 100 nM insulin, 100 ng/ml porcine growth hormone, 100 ng/ml recombinant porcine leptin, and combinations of these hormones. Treatments were performed in triplicate and the experiment was repeated with adipocytes isolated from five different animals. Cell aliquots (250 microl) were added to 1 ml of incubation medium, then incubated for 2h at 37 degrees C for measurement of glucose and palmitate oxidation or incorporation into lipid. Incubation of isolated adipocytes with insulin increased glucose oxidation rate by 18% (P<0.05), while neither growth hormone nor leptin affected glucose oxidation (P>0.5). Total lipid synthesis from glucose was increased by approximately 25% by 100 nM insulin or insulin+growth hormone (P<0.05). Insulin+leptin reduced the insulin response by 37% (P<0.05). The combination of all three hormones increased total lipid synthesis by 35%, relative to controls (P<0.05), a rate similar to insulin alone. Fatty acid synthesis was elevated by insulin (32%, P<0.05) or growth hormone (13%, P<0.05). Leptin had no effect on fatty acid synthesis (P>0.05). Leptin reduced the esterification rate by 10% (P<0.05). Growth hormone and insulin could overcome leptin's inhibition of palmitate esterification (P>0.05).

Leptin-induced changes in body composition in high fat-fed mice

Female C57BL/6J mice were adapted to 10% or 45% kcal fat diets for 8 weeks. Continuous intraperitoneal infusion of 10 micro g of leptin/day from a miniosmotic pump transiently inhibited food intake in low fat-fed but not high fat-fed mice. In contrast, both low and high fat-fed leptin-infused mice were less fat than their phosphate-buffered saline (PBS) controls after 13 days. Leptin infusion inhibited insulin release but did not change glucose clearance in low fat-fed mice during a glucose tolerance test. A single intraperitoneal injection of 30 micro g of leptin inhibited 24-hr energy intake and inhibited weight gain in both low and high fat-fed mice. Insulin responsiveness was improved in high fat-fed mice during an insulin sensitivity test due to an exaggerated elevation of circulating insulin concentrations. Thus, leptin infusion reduced adiposity independently of energy intake in high fat-fed mice and improved insulin sensitivity in low fat-fed mice, whereas leptin injections, which produced much greater, but transient, increases in serum leptin concentration, inhibited energy intake in both low and high fat-fed mice.

Analysis of paradoxical observations on the association between leptin and insulin resistance

Obesity is commonly associated with the development of insulin resistance and diabetes in humans and rodents. Insulin resistance and diabetes are observed in lipoatrophic individuals or rodent models of lipoatrophy. Here we focus on the role of leptin, the product of the obesity (ob) gene, in the development of insulin resistance and diabetes associated with obesity and lipoatrophy. We review the reported effects of leptin on whole body glucose metabolism and compare and contrast these with direct effects on skeletal muscle, fat and liver. This summary of paradoxical observations on the effects of leptin on glucose homeostasis and the ability of leptin to induce or improve insulin resistance suggests that a complex interplay exists between direct peripheral and centrally mediated effects of the hormone. Evidence suggesting that leptin acts as a mediator of insulin release from pancreatic beta cells is reviewed. Finally, intracellular signaling mechanisms stimulated by both leptin and insulin are discussed, with potential points of cross-talk suggested.

White adipocyte vascular endothelial growth factor: regulation by insulin

White adipose tissue from rats was examined for insulin- responsive vascular endothelial growth factor 165 (VEGF) secretion and mRNA expression. When separated into it constituent fat vs. stromal-vascular cells using collagenase digestion methods, only the adipocytes (or whole fat tissue) responded to physiological insulin concentrations by doubling VEGF release over 4 and 24 h in culture. Adipocyte VEGF mRNA expression increased similarly. Several adipose depots were tested. Although omental fat cells had the highest rates of VEGF release, the differences were not significant. Insulin-stimulated VEGF release was mediated in part via PI3K, but not PKC. Additional hormones/agents were tested, including steroids, leptin, an adenosine analog, and norepinephrine. Only the latter compound increased VEGF production, and this effect was mediated by adenylate cyclase. Adjusting the incubation glucose concentration between 0-20 mM did not alter adipocyte VEGF release. An experimental mimic of hypoxia, CoCl(2), also increased adipocyte VEGF, and this effect was additive with 100 nM insulin. These studies demonstrate that physiological insulin concentrations stimulate VEGF formation and expression in cultured rodent white adipocytes. Although the biological significance of this observation remains to be determined, if white adipocyte-derived VEGF has paracrine or systemic endocrine actions, these might hypothetically impact on adipose expansion or the vascular comorbidities of obesity.

High leptin levels acutely inhibit insulin-stimulated glucose uptake without affecting glucose transporter 4 translocation in l6 rat skeletal muscle cells

Obesity is a major risk factor for the development of insulin resistance, characterized by impaired stimulation of glucose disposal into muscle. The mechanisms underlying insulin resistance are unknown. Here we examine the direct effect of leptin, the product of the obesity gene, on insulin-stimulated glucose uptake in cultured rat skeletal muscle cells. Preincubation of L6 myotubes with leptin (2 or 100 nM, 30 min) had no effect on basal glucose uptake but reduced insulin-stimulated glucose uptake. However, leptin had no effect on the insulin-induced gain in myc-tagged glucose transporter 4 (GLUT4) appearance at the cell surface of L6 myotubes. Preincubation of cells with leptin also had no effect on insulin-stimulated tyrosine phosphorylation of insulin receptor, IRS-1 and IRS-2, phosphatidylinositol 3-kinase activity, or Akt phosphorylation. We have previously shown that insulin regulates glucose uptake via a signaling pathway sensitive to inhibitors of p38 MAP kinase. Here, leptin pretreatment reduced the extent of insulin-stimulated p38 MAP kinase phosphorylation and phosphorylation of cAMP response element binder, a downstream effector of p38 MAP kinase. These results show that high leptin levels can directly reduce insulin-stimulated glucose uptake in L6 muscle cells despite normal GLUT4 translocation. The mechanism of this effect could involve inhibition of insulin-stimulated p38 MAP kinase and GLUT4 activation.

Leptin in ruminants. Gene expression in adipose tissue and mammary gland, and regulation of plasma concentration

This paper reviews data on leptin gene expression in adipose tissue (AT) and mammary gland of adult ruminants, as well as on plasma leptin variations, according to genetic, physiological, nutritional and environmental factors. AT leptin mRNA level was higher in sheep and goat subcutaneous than visceral tissues, and the opposite was observed in cattle; it was higher in fat than in lean selection line in sheep; it was decreased by undernutrition and increased by refeeding in cattle and sheep, and not changed by adding soybeans to the diet of lactating goats; it was increased by injection of NPY to sheep, and by GH treatment of growing sheep and cattle. Insulin and glucocorticoids in vitro increased AT leptin mRNA in cattle, and leptin production in sheep. Long daylength increased AT lipogenic activities and leptin mRNA, as well as plasma leptin in sheep. Mammary tissue leptin mRNA level was high during early pregnancy and was lower but still expressed during late pregnancy and lactation in sheep. Leptin was present in sheep mammary adipocytes, epithelial and myoepithelial cells during early pregnancy, late pregnancy and lactation, respectively. Plasma leptin in cattle and sheep was first studied thanks to a commercial "multi-species" kit. It was positively related to body fatness and energy balance or feeding level, and decreased by beta-agonist injection. The recent development of specific RIA for ruminant leptin enabled more quantitative study of changes in plasma leptin concentration, which were explained for 35--50% by body fatness and for 15--20% by feeding level. The response of plasma leptin to meal intake was related positively to glycemia, and negatively to plasma 3-hydroxybutyrate. The putative physiological roles of changes in leptin gene expression are discussed in relation with published data on leptin receptors in several body tissues, and on in vivo or in vitro effects of leptin treatment.

Signals of adiposity

Adipose tissue, a reserve of energy, has played an essential role in mammalian evolution. Adipose tissue differs from other tissues in that its mass has considerable capacity to expand, which while beneficial in decreasing the risk of starvation, increases the risk of predation. Adipose tissue mass is thus under tight control in nondomestic species. Adipose tissue secretes a variety of factors, some of which (leptin, tumor necrosis factor (TNF) alpha, resistin) are thought to be involved in modulation of adipose mass. Leptin has a variety of functions, primarily targetting the hypothalamus where it acts to decrease appetite and increase energy expenditure. Leptin is also involved in the adaptations to fasting, and leptin is also required for normal reproductive and immune function. TNF alpha and resistin appear to have key paracrine roles, attenuating the anabolic effects of insulin on adipose tissue metabolism.

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.

Acylation stimulating protein (ASP) acute effects on postprandial lipemia and food intake in rodents

BACKGROUND

In vitro studies have shown that acylation stimulating protein (ASP) stimulates triglyceride (TG) synthesis and storage in adipocytes. We have previously demonstrated that intraperitoneal (i.p.) injection of ASP in C57BL/6J mice accelerated TG clearance following an orally-administered fat load as well as reducing postprandial glucose levels.

RESULTS

In the present study, we first examined the effect of i.p. and intracerebroventricular (i.c.v.) injection of ASP on food intake in Sprague-Dawley rats. Intraperitoneal injection resulted in a short-term increase in food intake (maximum increase 29.3% within the first hour, P<0.025) decreasing thereafter as compared to vehicle alone. i.c.v. Administration of a comparable dose of ASP resulted in a similar but delayed increase in food intake with a maximum at 2-4 h, suggesting that the actions of ASP are peripherally mediated. However, there was no significant difference in 24 h food intake with either i.p. or i.c.v. injection. We also examined the effects of ASP on TG clearance in two obese mouse strains with different metabolic profiles: ob/ob (C57BL/6J-Lep(ob)) and db/db (C57BLKS/J-Lepr(db)). In a crossover design, the response to an oral fat load was determined with and without i.p. injection of exogenous ASP. In ob/ob mice, there was a 44% greater clearance of postprandial TG (area under the curve (AUC)=245+/-49 control vs 138+/-43 mg/dl h with ASP; P<0.05 by RM ANOVA). The db/db mice showed a greater response, with a 62% decrease in postprandial TG (AUC=4080+/-1489 control vs 1540+/-719 mg/dl h with ASP; P=0.004 by RM ANOVA). In addition there were decreases in postprandial glucose and non-esterified fatty acid (NEFA) levels in response to ASP.

CONCLUSION

These results are the first to report that ASP can increase food intake in rats and also enhance postprandial TG clearance in obese animals. These data therefore support previous in vitro evidence pointing to ASP as a regulator of lipid metabolism.

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.

Transgenic complementation of leptin-receptor deficiency. I. Rescue of the obesity/diabetes phenotype of LEPR-null mice expressing a LEPR-B transgene

Mice homozygous for the Leprdb3J (db3J) mutation are null for all known isoforms of the leptin receptor (LEPR). These animals are obese, hyperphagic, cold intolerant, insulin resistant, and infertile. Mice homozygous for the Leprdb (db) mutation (lacking the B isoform only) have the same phenotype as db3J animals. To better understand the function(s) of the LEPR isoforms in vivo, we generated db3J/db3J and db/db mice bearing a transgene (neuron-specific enolase [NSE]-Rb) expressing the B isoform of LEPR, the isoform capable of activating the signal transducer and activator of transcription (STAT) pathway, under the control of the neuron-specific enolase enhancer/promoter. The NSE-Rb transgene was expressed in the brain, with low levels of expression in adrenals, testis, and white adipose tissue. LEPR-B transgene expression in NSE-Rb db3J/db3J mice partially corrected the increased fat mass, hyperphagia, and glucose intolerance while restoring fertility in males and rescuing the cold intolerance in both sexes. The body weights of NSE-Rb transgenic mice that possessed the full complement of short LEPR isoforms (NSE-Rb db/db mice) were similar to those of NSE-Rb db3J/db3J mice, suggesting that the short LEPR isoforms play little role in body weight regulation. Based on quantitative analysis of hypothalamic neuropeptide gene expression in the transgenic animals, we infer full restoration of leptin sensitivity to proopiomelanocortin (POMC) neurons, partial correction of leptin sensitivity in agouti gene-related protein (AGRP)/neuropeptide Y (NPY) neurons, and a lack of effect on leptin sensitivity of melanin concentrating hormone neurons. Thus, hypothalamic POMC and AGRP/NPY neurons are primary candidates as the mediators of the effects of the NSE-Rb transgene on energy homeostasis, ingestive behavior, the neuroendocrine system, and glucose metabolism.

Inhibition of leptin secretion by insulin and metformin in cultured rat adipose tissue

Leptin's role in the regulation of food intake, energy expenditure and weight control are widely recognized, especially in rodents. Likewise, the potential regulation of leptin secretion by insulin (and vice versa) has been of particular interest insofar as these nutrient signals may have meaningful, even adverse (inter)actions, in diabetes. We used a freshly isolated rat adipose tissue culture model to examine the effect of insulin, metformin and glibenclamide on basal and steroid-stimulated leptin secretion. This model was selected because of its physiologic rates of leptin formation and preservation of potentially significant cell-cell interactions compared to isolated cells. The basal rate of leptin secretion was 3. 4+/-1.2 ng/100 mg tissue/24 h. The addition of 100 nM dexamethasone or 400 nM hydrocortisone stimulated leptin secretion by 3-4 fold over basal (no steroid). Insulin inhibited both basal and steroid-activated leptin secretion by 35-50%. This inhibition was present with either 1 mM pyruvate or 5 mM glucose as a substrate suggesting that glycolysis was not required. Metformin inhibited basal and dexamethasone-stimulated leptin secretion in a dose dependent manner (50% inhibition occurred at 1 mM metformin) while glibenclamide was ineffective. The effect of insulin on isolated fat cells versus fat tissue was tested in parallel. After 24 h in culture, insulin inhibited leptin secretion similarly in both adipose preparations. The addition of 200 nM (-)N6-(2-phenylisopropyl)-adenosine did not alter the results.

Selective interaction between leptin and insulin signaling pathways in a hepatic cell line

Leptin is a 16-kDa hormone secreted by adipocytes and plays an important role in control of feeding behavior and energy expenditure. In obesity, circulating levels of leptin and insulin are high because of the presence of increased body fat mass and insulin resistance. Recent reports have suggested that leptin can act through some of the components of the insulin signaling cascade, such as insulin receptor substrates (IRS-1 and IRS-2), phosphatidylinositol 3-kinase (PI 3-kinase), and mitogen-activated protein kinase, and can modify insulin-induced changes in gene expression in vitro and in vivo. Well differentiated hepatoma cells (Fao) possess both the long and short forms of the leptin receptor and respond to leptin with a stimulation of c-fos gene expression. In Fao cells, leptin alone had no effects on the insulin signaling pathway, but leptin pretreatment transiently enhanced insulin-induced tyrosine phosphorylation and PI 3-kinase binding to IRS-1, while producing an inhibition of tyrosine phosphorylation and PI 3-kinase binding to IRS-2. Leptin alone also induced serine phosphorylation of Akt and glycogen synthase kinase 3 but to a lesser extent than insulin, and the combination of these hormones was not additive. These results suggest complex interactions between the leptin and insulin signaling pathways that can potentially lead to differential modification of the metabolic and mitotic effects of insulin exerted through IRS-1 and IRS-2 and the downstream kinases that they activate.

Neuroendocrine regulation of nutrient partitioning

Leptin is a satiety factor which acts within the hypothalamus to decrease the levels of several neuropeptides stimulating food intake (among them, neuropeptide Y [NPY]), while increasing those that inhibit food intake. These effects of leptin bring about decreased body weight. In vivo, leptin potentiates basal and insulin-stimulated glucose utilization, presumably its oxidation, and decreases fat storage. Leptin increases sympathetic-mediated energy dissipation, and the expression of uncoupling proteins-1, -2, and -3. In peripheral tissues (muscles, adipose, others), leptin decreases triglyceride content by increasing fatty acid oxidation, decreasing the activity/expression of esterification and lipogenic enzymes, and favoring lipolysis. It decreases the lipogenic activity of insulin. Ultimately, leptin depletes fat stores and promotes leanness. NPY, taken as one example of what an orexigenic agent may produce, increases food intake and body weight. It favors fat storage in adipose tissue by stimulating lipogenic activity. It decreases glucose utilization by muscles, making more glucose carbon available for lipogenesis. Effects of NPY result from vagus nerve-mediated hyperinsulinemia and overactivity of the hypothalamo-pituitary-adrenal axis. Thus, NPY favors fat stores, and ultimately obesity. Glucocorticoids are necessary for NPY effects to occur, because central administration of the neuropeptide in adrenalectomized animals is ineffective. Glucocorticoids also have genuine effects when administered centrally to normal rats. They increase the hypothalamic content of NPY and decrease that of CRH. This double neuro-peptidic change stimulates food intake, insulin output, adipose tissue storage ability, decreases the expression of uncoupling proteins-1 and -3, and increases body weight. Body weight homeostasis appears to require a finely tuned regulation of both leptin and glucocorticoids, with their respective opposite effects.