Insulin determines leptin responses during a glucose challenge in fed and fasted rats

Exogenous leptin promotes reproductive behavior during aphagia in red-sided garter snakes (Thamnophis sirtalis parietalis)

Despite the established dichotomy between investment in either reproduction or self-maintenance, a hormonal mechanism that influences an organism's decision to prioritize these behaviors remains elusive. The protein hormone leptin is a likely candidate because it is secreted from adipocytes in proportion to the amount of stored fat in numerous species. Although the majority of studies suggest that leptin stimulates reproduction, the actions of leptin can be context-dependent. Leptin increases sexual behavior in fed individuals, but inhibits sexual behavior in food-restricted individuals. We investigated if exogenous leptin influences sexual behavior in red-sided garter snakes (Thamnophis sirtalis parietalis) experiencing a predictable bout of aphagia during the mating season. We tested two doses of recombinant murine leptin injected for three days. Males were subjected to three mating trials, one on each day of injections, while females were subjected to one mating trial on the last day of injections. Leptin affects male and female snakes similarly by increasing both appetitive (i.e., mating behavior score) and consummatory (i.e., number of copulations, proportion of individuals copulated) sex behavior. We found no evidence to suggest that leptin influenced latency to copulate or duration of copulation. Because leptin promotes reproductive behavior in non-feeding garter snakes, these findings do not align with research on food-restricted mammals. Further investigations into how leptin affects sexual behavior in snakes exposed to food-restriction manipulations would clarify if the role of leptin is evolutionarily divergent.

Maternal soy protein isolate diet during lactation programmes to higher metabolic risk in adult male offspring

Soy consumption and its components, including its protein, are related to the beneficial effects of the lipid profile, decreased insulin resistance and glycaemia. However, the safety of the consumption of products containing phytoestrogens in critical stages of development has been questioned, since they may be associated with endocrine-metabolic dysfunctions in adult life. The purpose is to evaluate the effects of maternal dietary soy protein isolate (SPI) during lactation on the breast milk composition, body composition, lipid and glycaemic profiles, and thyroid hormones of dams and offspring at weaning (21 days) and in adulthood (150 days). Lactating rats were divided into casein control (C) and SPI diet groups. At 150 days, the SPI offspring presented lower body protein mass and total mineral content, higher serum FT4, insulin, TC and TG. Maternal consumption of SPI during lactation programmes the progeny to higher metabolic risk profile.

Comparative analysis of glucose metabolism responses of large yellow croaker Larimichthys crocea fed diet with fish oil and palm oil

In order to study the effects of dietary fatty acid compositions on glucose metabolism, large yellow croaker juveniles Larimichthys crocea (initial weight, 36.80 ± 0.39 g) were fed with two experiment diets for 12 weeks. The two diets contained 6.5% of fish oil (FO) and palm oil (PO), respectively. Results showed that the contents of saturated fatty acids in liver and muscle, levels of glucose, triglyceride (TG), non-esterified fatty acid (NEFA), and leptin in blood were significantly higher in PO group, while the hepatic glycogen and muscle glycogen significantly decreased (P < 0.05). There were no significant differences in blood insulin and adiponectin levels between the two groups (P > 0.05). Compared with the FO group, the expressions of glucokinase (GK), glucose-6-phosphate dehydrogenase, glycogen synthase (GYS), glucose transporter 2 (GLUT2), insulin receptor 1 (IR1), insulin receptor substrate 1 (IRS1), insulin receptor substrate (IRS2), and protein kinase B (AKT2) were significantly decreased, and the expressions of phosphoenolpyruvate carboxykinase (PEPCK) in liver were significantly increased in the PO group. Meanwhile, the expressions of GK, phosphofructokinase, GYS, GLUT4, and insulin receptor 2 (IR2) were significantly reduced, and the expressions PEPCK, fructose-1 and 6-diphosphatase in muscle were significantly increased in the PO group. In conclusion, palm oil in diet could inhibit the utilization of glucose and promote the endogenous glucose production in large yellow croaker by reducing the sensitivity of insulin, so as to increase the blood glucose level.

Evidence of growth hormone effect on plasma leptin in diet-induced obesity and diet-resistant rats

Background

Plasma leptin is regulated by several factors, including growth hormone (GH), which influences the pathophysiology of obesity.

Objective

To demonstrate the short-term effect of GH on plasma leptin levels in 3 conditions in vivo with the different amount of body fat mass.

Methods

Adult male Wistar rats were fed with standard chow or hypercaloric diet (HC). The HC rats were demonstrated as HC-feeding obese (HC-O) and HC-feeding resistant (HC-R) rats. Then, they were treated with GH or saline for 3 days. Basal plasma leptin levels were measured at 24 and 32 h. For meal-induced condition, all rats were fed for 2 hand plasma leptin was measured. Further 16-h fasting period, plasma leptin, insulin, and insulin sensitivity indexes were determined.

Results

The short-term GH treatment decreased basal plasma leptin at 32 h after the first GH injection in HC-O rats. However, GH treatment had no effect on meal-induced plasma leptin in all rats. Furthermore, GH treatment attenuated fasting effect on plasma leptin in control and HC-R rats. The insulin resistance (IR) induced by the short-term GH treatment was demonstrated by higher fasting plasma insulin and the increased homeostasis model of IR in HC-R rats.

Conclusions

The study demonstrates the important role of greater fat mass in HC-O rats, which results in decreased basal plasma leptin after short-term GH treatment. For meal-induced condition, GH had no effect on plasma leptin in all rats. Interestingly, GH could attenuate fasting effect on plasma leptin in rats that have lower fat mass.

Metal Nanoparticles Released from Dental Implant Surfaces: Potential Contribution to Chronic Inflammation and Peri-Implant Bone Loss

Peri-implantitis is an inflammatory disease affecting tissues surrounding dental implants. Although it represents a common complication of dental implant treatments, the underlying mechanisms have not yet been fully described. The aim of this study is to identify the role of titanium nanoparticles released form the implants on the chronic inflammation and bone lysis in the surrounding tissue. We analyzed the in vitro effect of titanium (Ti) particle exposure on mesenchymal stem cells (MSCs) and fibroblasts (FU), evaluating cell proliferation by MTT test and the generation of reactive oxygen species (ROS). Subsequently, in vivo analysis of peri-implant Ti particle distribution, histological, and molecular analyses were performed. Ti particles led to a time-dependent decrease in cell viability and increase in ROS production in both MSCs and FU. Tissue analyses revealed presence of oxidative stress, high extracellular and intracellular Ti levels and imbalanced bone turnover. High expression of ZFP467 and the presence of adipose-like tissue suggested dysregulation of the MSC population; alterations in vessel morphology were identified. The results suggest that Ti particles may induce the production of high ROS levels, recruiting abnormal quantity of neutrophils able to produce high level of metalloproteinase. This induces the degradation of collagen fibers. These events may influence MSC commitment, with an imbalance of bone regeneration.

Insulin elevates leptin secretion and mRNA levels via cyclic AMP in 3T3-L1 adipocytes deprived of glucose

Aims

Leptin plays an important role in the pathogenesis of obesity and diabetes, yet the regulatory mechanisms of this hormone have not been fully elucidated. In this study, we aimed to clarify the roles of insulin and glucose in leptin secretion and mRNA production using inhibitors of insulin signal transduction in adipocytes cultured under glucose-free or normal conditions.

Methods

Differentiated 3T3-L1 adipocytes were stimulated with insulin in combination with inhibitors for phosphoinositide 3-kinase (PI3K), Akt, and phosphodiesterase 3B (PDE3B), as well as epinephrine and a cyclic AMP (cAMP) analog under glucose-free or normal conditions. After 8 h of stimulation, leptin protein levels in the media and leptin mRNA expression levels in the adipocytes were measured.

Results

Insulin significantly increased the secretion and mRNA levels of leptin under the depletion of glucose. Glucose augmented basal leptin secretion without insulin, while glucose nullified insulin-induced leptin mRNA upregulation. The PI3K inhibitor BEZ-235, the Akt inhibitor MK-2206, and the PDE3B inhibitor cilostazol attenuated the insulin stimulation of leptin secretion, but did not suppress the insulin-induced leptin mRNA upregulation with glucose depletion. In contrast to the glucose-free condition, insulin failed to upregulate leptin mRNA in the presence of glucose. The cAMP analog dibutyryl cAMP and epinephrine decreased both leptin secretion and mRNA regardless of glucose supplementation.

Conclusion

Insulin alone stimulates leptin secretion and elevates leptin mRNA levels via cAMP under the lack of glucose metabolism, while glucose is a significant and ambivalent effector on the insulin effects of leptin.

Effects of glucose, insulin and triiodothyroxine on leptin and leptin receptor expression and the effects of leptin on activities of enzymes related to glucose metabolism in grass carp (Ctenopharyngodon idella) hepatocytes

Leptin is an important regulator of appetite and energy expenditure in mammals, but its role in fish metabolism control is poorly understood. Our previous studies demonstrated that leptin has an effect on the regulation of food intake and energy expenditure as well as lipid metabolism (stimulation of lipolysis and inhibition of adipogenesis) in the grass carp Ctenopharyngodon idella. To further investigate the role of leptin in fish, the effects of glucose, insulin and triiodothyroxine (T3) on the expression levels of leptin and leptin receptor (Lepr) and the effects of leptin on the activities of critical glucose metabolism enzymes in grass carp hepatocytes were evaluated in the present study. Our data indicated that leptin gene expression was induced by glucose in a dose-dependent manner, while Lepr gene expression exhibited a biphasic change. A high dose of insulin (100 ng/mL) significantly up-regulated the expression of leptin and Lepr. Leptin expression was markedly up-regulated by a low concentration of T3 but inhibited by a high concentration of T3. T3 up-regulated Lepr expression in a dose-dependent manner. Together, these data suggest that leptin had a close relationship with three factors (glucose, insulin and T3) and might participate in the regulation of glucose metabolism in grass carp. In addition, we also found that leptin affected the activities of key enzymes that are involved in glucose metabolism, which might be mediated by insulin receptor substrate-phosphoinositol 3-kinase signaling.

Nutrigenomics, beta-cell function and type 2 diabetes

INTRODUCTION

The present investigation was designed to investigate the accuracy and precision of lactate measurement obtained with contemporary biosensors (Chiron Diagnostics, Nova Biomedical) and standard enzymatic photometric procedures (Sigma Diagnostics, Abbott Laboratories, Analyticon).

MATERIALS AND METHODS

Measurements were performed in vitro before and after the stepwise addition of 1 molar sodium lactate solution to samples of fresh frozen plasma to systematically achieve lactate concentrations of up to 20 mmol/l.

RESULTS

Precision of the methods investigated varied between 1% and 7%, accuracy ranged between 2% and -33% with the variability being lowest in the Sigma photometric procedure (6%) and more than 13% in both biosensor methods.

CONCLUSION

Biosensors for lactate measurement provide adequate accuracy in mean with the limitation of highly variable results. A true lactate value of 6 mmol/l was found to be presented between 4.4 and 7.6 mmol/l or even with higher difference. Biosensors and standard enzymatic photometric procedures are only limited comparable because the differences between paired determinations presented to be several mmol. The advantage of biosensors is the complete lack of preanalytical sample preparation which appeared to be the major limitation of standard photometry methods.

Insulin regulates leptin secretion from 3T3-L1 adipocytes by a PI 3 kinase independent mechanism

To better define the molecular mechanisms underlying leptin release from adipocytes, we developed a novel protocol that maximizes leptin production from 3T3-L1 adipocytes. The addition of a PPARgamma agonist to the Isobutylmethylxanthine/Dexamethasone/Insulin differentiation cocktail increased leptin mRNA levels by 5-fold, maintained insulin sensitivity, and yielded mature phenotype in cultured adipocytes. Under these conditions, acute insulin stimulation for 2 h induced a two-fold increase in leptin secretion, which was independent of new protein synthesis, and was not due to alterations in glucose metabolism. Stimulation with insulin for 15 min induced the same level of leptin release and was blocked by Brefeldin A. Inhibiting PI 3-kinase with wortmannin had no effect on insulin stimulation of leptin secretion. These studies show that insulin can stimulate leptin release via a PI3K independent mechanism and provide a cellular system for studying the effect of insulin and potentially other mediators on leptin secretion.

Leptina e exercício físico aeróbio: implicações da adiposidade corporal e insulina

Atualmente, a obesidade pode ser classificada como uma pandemia e suas conseqüências vão desde o diabetes mellitus até a doença cardíaca. Tanto fatores genéticos como ambientais contribuem para isso, porém, em humanos, o componente genético ainda é pouco definido. Com a clonagem do gene ob de ratos e do seu receptor, foi descoberta a leptina, o "hormônio da saciedade". A leptina é secretada, principalmente, pelo tecido adiposo e reflete a quantidade de gordura depositada no tecido adiposo de um indivíduo. Entretanto, diversos fatores influenciam sua expressão e síntese, tais como jejum, atividade simpática, exercício físico e alterações no balanço energético. Os efeitos da atividade física aeróbia sobre esse hormônio ainda não estão muito claros, visto que existem muitas contradições na literatura sobre sua possível ação na regulação da leptina. Estudos transversais sugerem que as concentrações plasmáticas de leptina não são alteradas após uma sessão de exercício aeróbio. Entretanto, se o esforço físico for extremo, como em uma ultramaratona, na qual há um balanço energético negativo, induzido pela atividade física extenuante, ocorre diminuição dessas concentrações. Além disso, exercícios de longa duração (> 60 min) parecem estar associados à diminuição tardia das concentrações de leptina, aproximadamente 48h após a atividade, provavelmente em função de um possível desequilíbrio energético. Em relação aos estudos longitudinais, após o treinamento aeróbio, alguns autores não observam alterações na leptina plasmática, outros encontram alterações em função apenas das alterações da adiposidade e, por fim, alguns estudos observam diminuição da concentração plasmática e/ou expressão de leptina, independentemente de alterações da massa gorda. Tal fato sugere que haja outro, ou outros, fatores, além do conteúdo de gordura corporal, que modulam a diminuição das concentrações plasmáticas de leptina após o treinamento aeróbio, sendo a insulina a principal candidata a tal modulação. Dessa forma, esta revisão aborda os principais aspectos do hormônio leptina, sua ação, função e regulação, associação com a insulina, além dos efeitos do exercício físico agudo e crônico na síntese e secreção da leptina, e possíveis implicações da insulina e adiposidade em função desse estímulo.

Acute and chronic regulation of leptin synthesis, storage, and secretion by insulin and dexamethasone in human adipose tissue

Serum leptin levels are upregulated in proportion to body fat and also increase over the short term in response to meals or insulin. To understand the mechanisms involved, we assessed leptin synthesis and secretion in samples of adipose tissue from subjects with a wide range of BMI. Tissue leptin content and relative rates of leptin biosynthesis, as determined by metabolic labeling, were highly correlated with each other and with BMI and fat cell size. To understand mechanisms regulating leptin synthesis in obesity, we used biosynthetic labeling to directly assess the effects of insulin and glucocorticoids (dexamethasone) on leptin synthesis and secretion in human adipose tissue. Chronic treatment (1-2 days in organ culture) with insulin increased relative rates of leptin biosynthesis without affecting leptin mRNA levels. In contrast, dexamethasone increased leptin mRNA and biosynthesis in parallel. Acute treatment with insulin or dexamethasone (added during 1-h preincubation and 45-min pulse labeling) did not affect relative rates of leptin biosynthesis, but pulse-chase studies showed that addition of insulin nearly doubled the release of [35S]leptin after a 1-h chase. We conclude that the higher leptin stores in adipose tissue of obese humans are maintained by chronic effects of insulin and glucocorticoids acting at pre- and posttranslational levels and that the ability of insulin to increase the release of preformed leptin may contribute to short-term variations in circulating leptin levels.

Diet, obesity and diabetes: a current update

The prevalence of obesity has been increasing at a rapid rate over the last few decades. Although the primary defect can be attributed to an imbalance of energy intake over energy expenditure, the regulation of energy balance is now recognized to be complex. Adipose-tissue factors play a central role in the control of energy balance and whole-body fuel homoeostasis. The regulation of adipose-tissue function, in particular its secretion of adipokines, is impaired by increases in adipose mass associated with obesity, and with the development of insulin resistance and Type 2 diabetes. This review analyses adipose-regulated energy input and expenditure, together with the impact of dietary macronutrient composition on energy balance in relation to susceptibility to the development of obesity and Type 2 diabetes, and how these metabolic conditions may be exacerbated by the consequences of abnormal adipose function. By gaining a greater understanding of how energy balance is controlled in normal, and in obese and diabetic states, a more practical approach can be employed to prevent and better treat obesity and metabolic disorders.

Leptin secretion is related to glucose-derived lipogenesis in isolated adipocytes

OBJECTIVE

Leptin secretion in rats is regulated acutely by nutritional state. Insulin plays an important role in this acute nutritional regulation both directly and indirectly through effects on glucose metabolism. The aim of this study was to investigate if the fasting-induced suppression of leptin secretion was reversed by incubation under conditions mimicking nutritional repletion.

DESIGN

Leptin secretion and glucose metabolism were measured following incubation with glucose and insulin in adipocytes isolated from fed and fasted rats.

RESULTS

Leptin secretion was stimulated by incubation with glucose and insulin in adipocytes isolated from fed but not from fasted rats as was glucose flux through oxidative and lipogenic pathways. Ob expression and intracellular leptin content were decreased in adipocytes isolated from fasted rats throughout the whole incubation period. Suppression of glucose metabolism with cytochalasin B was accompanied by suppression of leptin secretion. The amount of leptin secretion correlated with the glucose incorporated into lipid under insulin-stimulated conditions.

CONCLUSIONS

It is proposed that glucose incorporation into lipid, at least during insulin-stimulated conditions, reflects the metabolic status of the adipocyte and may be a more important regulator of leptin production and secretion than circulating glucose or insulin levels.

Hormonal and metabolic adaptation to fasting: effects on the hypothalamic-pituitary-ovarian axis and reproductive performance of rabbit does

To assess the impact of acute caloric shortage on reproduction, rabbit does were either fed ad libitum (control, AL), or fasted for 24 (STF) or 48 h (LTF) before induction of ovulation with GnRH injection. Blood samples were collected during the last 3 h of fasting, and the following 4 h after GnRH injection, when feed was provided again, to measure plasma concentrations of LH, estradiol-17beta, leptin, insulin, T3, corticosterone, glucose, and NEFA. Before re-feeding, plasma leptin, insulin, and T3 concentrations were lower (P < or = 0.01) in both fasted groups than in controls, but then gradually increased following realimentation to match those of controls. During fasting, corticosterone levels were higher (P < or = 0.01) in LTF than in STF and AL does, but decreased to control values soon after realimentation. During fasting, plasma glucose concentrations did not differ among groups, but upon re-feeding they markedly increased (P < or= 0.01) both in STF and LTF does. NEFA levels were also more elevated (P < or = 0.01) in fasted rabbits than in controls, and rapidly decreased (P < or = 0.01) after re-feeding. Following GnRH injection, LH peak was lower (P < or = 0.01) in LTF than in AL and STF does. Estradiol-17beta showed higher pulse frequency and amplitude in AL than in STF and LTF does. Compared to controls, receptivity rate of STF and LTF artificially inseminated does declined respectively by -20.5% (P < or = 0.05) and -22.7%, and fertility rate by -23.9% (P < or = 0.05) and 21.4%, but no difference was found in ovulation rate. In summary, nutritional status of does, as modified by fasting, greatly influenced fertility, metabolic and reproductive hormones.

Multilevel regulation of leptin storage, turnover, and secretion by feeding and insulin in rat adipose tissue

The mechanisms of the increased serum leptin in response to feeding are poorly understood. Therefore, we used metabolic labeling to directly assess leptin biosynthesis, secretion, and turnover in adipose tissue from 14 h-starved compared with fed 12-14 week old rats. Starvation decreased serum leptin (-47 +/- 7%), adipose tissue leptin content (-32 +/- 5%), and leptin secretion during 3 h of incubation (-65 +/- 12%). Starvation did not affect leptin mRNA levels but decreased rates of leptin biosynthesis by tissue fragments, as determined by [(35)S]methionine/cysteine incorporation into immunoprecipitable leptin. Insulin in vitro did not acutely increase leptin biosynthesis or rates of (125)I-leptin degradation. Pulse-chase studies showed that in adipose tissue from fed but not starved rats, insulin accelerated the secretion of [(35)S]leptin by approximately 2-fold after 30 and 60 min of chase. Degradation of newly synthesized leptin was slower in adipose tissue of starved than fed rats (half-lives of 50 and 150 min, respectively). Inhibitor experiments showed that both lysosomes and proteosomes contributed to leptin degradation. In conclusion, feeding compared with starvation influences leptin production at multiple posttranscriptional levels: synthesis, tissue storage, turnover, and secretion. The insulin-stimulated release of leptin from a preformed intracellular leptin pool may contribute to increases in serum leptin levels after meals.