Obese gene expression: reduction by fasting and stimulation by insulin and glucose in lean mice, and persistent elevation in acquired (diet-induced) and genetic (yellow agouti) obesity

The effects of physical activity on adipokines in individuals with overweight/obesity across the lifespan: A narrative review

This narrative review summarizes current knowledge on the effects of physical activity (PA) on adipokine levels in individuals with overweight and obesity. Approximately 90 investigations including randomized control, cross-sectional and longitudinal studies that reported on the effects of a single session of PA (acute) or long-term PA (chronic) on adipokine levels in individuals with overweight/obesity were reviewed. The findings support the notion that there is consensus on the benefits of chronic exercise training-regardless of the mode (resistance vs. aerobic), intensity and cohort (healthy vs. diabetes)-on adipokine levels (such as tumour necrosis factor-alpha, interleukin-6, adiponectin, visfatin, omentin-1 and leptin). However, several confounding factors (frequency, intensity, time and type of exercise) can alter the magnitude of the effects of an acute exercise session. Available evidence suggests that PA, as a part of routine lifestyle behaviour, improves obesity complications by modulating adipokine levels. However, additional research is needed to help identify the most effective interventions to elicit the most beneficial changes in adipokine levels in individuals with overweight/obesity.

Early overnutrition alters synaptic signaling and induces leptin resistance in arcuate proopiomelanocortin neurons

Early overnutrition disrupts leptin sensitivity and the development of hypothalamic pathways involved in the regulation of metabolism and feeding behavior. While previous studies have largely focused on the development of neuronal projections, few studies have examined the impact of early nutrition on hypothalamic synaptic physiology. In this study we characterized the synaptic development of proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH), their sensitivity to leptin, and the impact of early overnutrition on the development of these neurons. Electrophysiology recordings were performed in mouse ARH brain slices containing POMC-EGFP neurons from postnatal age (P) 7-9 through adulthood. We determined that pre- and postsynaptic components of inhibitory inputs increased throughout the first 3 weeks of the postnatal period, which coincided with a decreased membrane potential in POMC neurons. We then examined whether chronic postnatal overnutrition (CPO) altered these synaptic connections. CPO mice exhibited increased body weight and circulating leptin levels, as described previously. POMC neurons in CPO mice had an increase in post-synaptic inhibitory currents compared to controls at 2 weeks of age, but this effect reversed by the third week. In control mice we observed heterogenous effects of leptin on POMC neurons in early life that transitioned to predominantly stimulatory actions in adulthood. However, postnatal overfeeding resulted in POMC neurons becoming leptin-resistant which persisted into adulthood. These studies suggest that postnatal overfeeding alters the postsynaptic development of POMC neurons and induces long-lasting leptin resistance in ARH-POMC neurons.

Insulin as a hormone regulator of the synthesis and release of leptin by white adipose tissue

Leptin and its receptor are widely distributed in several tissues, mainly in white adipose tissue. The serum leptin is highly correlated with body mass index in rodents and humans, being documented that leptin levels reduces in the fasting state and increase during refeeding, similarly to insulin release by pancreatic islets. Insulin appears to increase leptin mRNA and protein expression and its release by adipocytes. Some studies have suggested that insulin acts through the activation of the transcription factors: sterol regulatory element binding protein 1 (SREBP1), CCAAT enhancer binding protein-α (C/EBP-α) and specificity protein 1 (Sp1). Insulin stimulates the release of preformed and newly synthesized leptin by adipocytes through its signaling cascade. Its effects are blocked by inhibitors of the insulin signaling pathway, as well as by inhibitors of protein synthesis and agents that increase the intracellular cAMP. The literature data suggest that chronic hyperinsulinemia increases serum leptin levels in humans and rodents. In this review, we summarized the most updated knowledge on the effects of insulin on serum leptin levels, presenting the cell mechanisms that control leptin synthesis and release by the white adipose tissue.

The Acute-Phase Protein Orosomucoid Regulates Food Intake and Energy Homeostasis via Leptin Receptor Signaling Pathway

The acute-phase protein orosomucoid (ORM) exhibits a variety of activities in vitro and in vivo, notably modulation of immunity and transportation of drugs. We found in this study that mice lacking ORM1 displayed aberrant energy homeostasis characterized by increased body weight and fat mass. Further investigation found that ORM, predominantly ORM1, is significantly elevated in sera, liver, and adipose tissues from the mice with high-fat diet (HFD)-induced obesity and db/db mice that develop obesity spontaneously due to mutation in the leptin receptor (LepR). Intravenous or intraperitoneal administration of exogenous ORM decreased food intake in C57BL/6, HFD, and leptin-deficient ob/ob mice, which was absent in db/db mice and was significantly reduced in mice with arcuate nucleus (ARC) LepR knockdown, whereas enforced expression of ORM1 in ARC significantly decreased food intake, body weight, and serum insulin level. Furthermore, we found that ORM is able to bind directly to LepR and activate the receptor-mediated JAK2-STAT3 signaling in hypothalamus tissue and GT1-7 cells, which was derived from hypothalamic tumor. These data indicated that ORM could function through LepR to regulate food intake and energy homeostasis in response to nutrition status. Modulating the expression of ORM is a novel strategy for the management of obesity and related metabolic disorders.

Leptin signaling as a therapeutic target of obesity

INTRODUCTION

Leptin is a hormone with a key role in food intake and body weight homeostasis. Congenital leptin deficiency (CLD) is a rare disease that causes hyperphagia and early severe obesity. However, common obesity conditions are associated with hyperleptinemia and leptin resistance.

AREAS COVERED

The main signaling pathways activated by leptin as well as the mechanisms underlying the regulatory actions of leptin on food intake and on lipid and glucose metabolism are reviewed. The potential mechanisms involving leptin resistance and the main regulatory hormonal and nutritional factors controlling leptin production/functions are also analyzed. The pathophysiology of leptin in human obesity, and especially the trials analyzing effects of leptin replacement therapy in patients with CLD or in subjects with common obesity and in post-obese weight-reduced subjects are also summarized.

EXPERT OPINION

The use of drugs or specific bioactive food components with anti-inflammatory properties to reduce the inflammatory state associated with obesity, especially at the hypothalamus, may help to overcome leptin resistance. Research should also be focused on investigating dietary strategies, food supplements or drugs capable of avoiding or reversing the leptin fall during weight management, in order to promote sustained body weight lowering and weight loss maintenance.

A new biology of diabetes revealed by leptin

A variety of leptin actions require a re-examination of classic concepts of metabolic diseases. Here we present evidence for two physiologic pathways: a pathway that protects nonadipose tissues from overaccumulation of potentially toxic lipids and unrecognized paracrine interactions between α and β cells revealed by leptin's ability to suppress diabetic hyperglucagonemia. These observations strongly point to new therapeutic possibilities for both type 1 and type 2 diabetes.

Sweet tea leaves extract improves leptin resistance in diet-induced obese rats

AIM OF THE STUDY

Dietary obesity is usually characterized by leptin resistance and abnormal lipid metabolism. Lithocarpus polystachyus Rehd.(Sweet Tea) leaf is a kind of Chinese folkloric medicine, and it has been widely used for obesity, diabetes, and hypertension in South China. The present study is aimed at investigating the pharmacological mechanism of the anti-hyperleptinaemia effects of Sweet Tea leaves extract in high fat diet-induced obese rats.

MATERIALS AND METHODS

We induced high fat diet obesity for 14 weeks to test the corrective effects of three ST doses (75, 150 and 300 mg/kg per day) for 8 weeks. At the end of the experiment, body weight, fasting blood glucose and serum lipids, superoxide dismutase (SOD), malondialdehyde (MDA), fasting serum insulin and leptin, C-reactive protein, adiponectin and resistin levels were measured, Homeostasis Model Assessment for Insulin Resistance (HOMA-IR) was also calculated. mRNA gene expression of PPARγ (peroxisome proliferator-activated receptor γ) and C/EBPα(CCAAT/enhancer-binding protein α) in epididymal adipose tissue of DIO control and experimental groups were evaluated.

RESULTS

Sweet Tea leaves extract could significantly decrease the levels of serum lipids, attenuate body weight gain and lower circulating leptin and insulin levels, ameliorate the state of oxidative stress, raise serum adiponectin, reduce circulating CRP and resistin levels, and depress the expression of PPARγ and C/EBPα in epididymal adipose tissue of obese rats.

CONCLUSION

The present findings suggest that ST can effectively attenuate the leptin resistance at least through anti-hyperlipidemic activity and thus has the therapeutic potential in treating hyperlipidemia and hyperleptinaemia related to dietary obesity.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

The somatotrope as a metabolic sensor: deletion of leptin receptors causes obesity

Leptin, the product of the Lep gene, reports levels of adiposity to the hypothalamus and other regulatory cells, including pituitary somatotropes, which secrete GH. Leptin deficiency is associated with a decline in somatotrope numbers and function, suggesting that leptin may be important in their maintenance. This hypothesis was tested in a new animal model in which exon 17 of the leptin receptor (Lepr) protein was selectively deleted in somatotropes by Cre-loxP technology. Organ genotyping confirmed the recombination of the floxed LepR allele only in the pituitary. Deletion mutant mice showed a 72% reduction in pituitary cells bearing leptin receptor (LEPR)-b, a 43% reduction in LEPR proteins and a 60% reduction in percentages of immunopositive GH cells, which correlated with reduced serum GH. In mutants, LEPR expression by other pituitary cells was like that of normal animals. Leptin stimulated phosphorylated Signal transducer and activator of transcription 3 expression in somatotropes from normal animals but not from mutants. Pituitary weights, cell numbers, IGF-I, and the timing of puberty were not different from control values. Growth curves were normal during the first 3 months. Deletion mutant mice became approximately 30-46% heavier than controls with age, which was attributed to an increase in fat mass. Serum leptin levels were either normal in younger animals or reflected the level of obesity in older animals. The specific ablation of the Lepr exon 17 gene in somatotropes resulted in GH deficiency with a consequential reduction in lipolytic activity normally maintained by GH and increased adiposity.

Inhibitory effects of ursolic acid on hepatic polyol pathway and glucose production in streptozotocin-induced diabetic mice

The effects of ursolic acid on the polyol pathway and glucose homeostasis-related metabolism were examined in the livers of streptozotocin (STZ)-induced diabetic mice fed a high-fat (37% calories from fat) diet for 4 weeks. Male mice were divided into nondiabetic, diabetic control, and diabetic-ursolic acid (0.05% wt/wt) groups. Diabetes was induced by the injection of STZ (200 mg/kg body weight, intraperitoneally). Although an ursolic acid supplement lowered the blood glucose level, it did not affect the plasma leptin and adiponectin levels. The present study shows that the blood glucose levels have a positive correlation with the hepatic sorbitol dehydrogenase activities (r = 0.39, P < .05). Ursolic acid significantly inhibited sorbitol dehydrogenase activity as well as aldose reductase activity in the liver. The supplementation of ursolic acid significantly increased glucokinase activity, while decreasing glucose-6-phosphatase activity in the livers of STZ-induced diabetic mice. Ursolic acid significantly elevated the hepatic glycogen content compared with the diabetic control group. Supplementation with ursolic acid significantly lowered the plasma total cholesterol, free fatty acid, and triglyceride concentrations compared with the diabetic control group, whereas it normalized hepatic triglyceride concentration. A negative correlation was found between the hepatic triglyceride concentration and blood glucose levels (r = -0.50, P < .01) in regard to insulin-dependent diabetic mice. The hepatic fatty acid synthase activity was significantly lower in the ursolic acid group than in the diabetic control group, whereas hepatic fatty acid beta-oxidation and carnitine palmitoyltransferase activities were significantly higher. These results indicate that ursolic acid may be beneficial in preventing diabetic complications by improving the polyol pathway as well as the lipid metabolism and that it can function as a potential modulator of hepatic glucose production, which is partly mediated by up-regulating glucose utilization and glycogen storage and down-regulating glyconeogenesis in the liver.

Higher habitual intake of dietary fat and carbohydrates are associated with lower leptin and higher ghrelin concentrations in overweight and obese postmenopausal women with elevated insulin levels

A highly regulated homeostatic system governs body weight; however, it is possible that this system might be impaired by the sustained intake of highly palatable foods. Short-term feeding studies suggest that the appetite-stimulating hormone ghrelin is suppressed less effectively by dietary fat intake, and diets high in sucrose decrease levels of the adipose hormone leptin. We hypothesized that higher habitual intake of dietary fat and carbohydrate (CHO) would be associated with elevated concentrations of circulating plasma ghrelin and lower circulating leptin in humans, a hormonal profile that could promote weight gain. To test our hypothesis, we examined the cross-sectional associations of ghrelin and leptin with the habitual macronutrient intake of 165 healthy overweight and obese sedentary women and tested the modifying role of insulin in these associations. We observed a significant inverse association between leptin concentrations and percentage energy from CHO independent of body mass index, percentage body fat, age, and intraabdominal fat (beta = -0.11 P = .04). No significant associations were observed between ghrelin and macronutrients or their subtypes among the total cohort. Among women with insulin concentrations at or greater than the median, we found a statistically significant positive association between intake of saturated fat and ghrelin concentrations, as well as additional statistically significant associations between leptin concentrations and macronutrients not observed among the total cohort. Our results provide some evidence that diets higher in fat and CHO are associated with a hormonal profile (ie, lower leptin and higher ghrelin concentrations), which could enhance weight gain, particularly among individuals with higher circulating insulin concentrations.

The relationship between leptin and norepinephrine levels during OGTT in normotensive and hypertensive obese adolescents

BACKGROUND/AIMS

Studies in adult population have suggested that leptin might play a role in inducing obesity related hypertension mediated by the sympathetic nervous system. This association has not been established for adolescents. Our study is designed to explore the relationship between leptin and norepinephrine levels in pediatric patients and to identify any contributors to hypertension for this population.

METHODS

Thirty-nine obese adolescents, divided into four groups by gender and hypertension status were included in the study. Leptin and norepinephrine levels were measured during oral glucose tolerance tests (OGTT) to optimize hormonal secretion. T tests were used to compare baseline levels of glucose, insulin, leptin and norepinephrine at 0 hour point of OGTT between the hypertensive and normotensive patients for both genders. Analysis of covariance (ANCOVA) was used for comparison of subsequent levels between the hypertensive and normotensive groups for in both genders, with the corresponding baseline level as the covariance. Models with and without BMI adjustment were created and their results were found to be consistent. Correlation between leptin and norepinephrine was examined at each time point and through analysis of area under the curve (AUC).

RESULTS

Contrary to the previous findings obtained in adult patients, our results did not show any direct relationships between levels of leptin and norepinephrine. A slight decrease in norepinephrine level at 1 hour in the normotensive male group and a significant increase in leptin level at 1 hour in the hypertensive female group was observed.

CONCLUSION

Our preliminary data suggest that norepinephrine and leptin levels at 0 and 1 hour during routine OGTT, for males and females, respectively, may help identify a subgroup of obese adolescents who have higher risk for hypertension and cardiovascular complications.

Fasting and glucose effects on pituitary leptin expression: is leptin a local signal for nutrient status?

Leptin, a potent anorexigenic hormone, is found in the anterior pituitary (AP). The aim of this study was to determine whether and how pituitary leptin-bearing cells are regulated by nutritional status. Male rats showed 64% reductions in pituitary leptin mRNA 24 hr after fasting, accompanied by significant (30-50%) reductions in growth hormone (GH), prolactin, and luteinizing hormone (LH), and 70-80% reductions in target cells for gonadotropin-releasing hormone or growth hormone-releasing hormone. There was a 2-fold increase in corticotropes. Subsets (22%) of pituitary cells coexpressed leptin and GH, and <5% coexpressed leptin and LH, prolactin, thyroid-stimulating hormone, or adrenocorticotropic hormone. Fasting resulted in significant (55-75%) losses in cells with leptin proteins or mRNA, and GH or LH. To determine whether restoration of serum glucose could rescue leptin, LH, and GH, additional fasted rats were given 10% glucose water for 24 hr. Restoring serum glucose in fasted rats resulted in pituitary cell populations with normal levels of leptin and GH and LH cells. Similarly, LH and GH cells were restored in vitro after populations from fasted rats were treated for as little as 1 hr in 10-100 pg/ml leptin. These correlative changes in pituitary leptin, LH, and GH, coupled with leptin's rapid restoration of GH and LH in vitro, suggest that pituitary leptin may signal nutritional changes. Collectively, the findings suggest that pituitary leptin expression could be coupled to glucose sensors like glucokinase to facilitate rapid responses by the neuroendocrine system to nutritional cues.

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.

Pulmonary responses to acute ozone exposure in fasted mice: effect of leptin administration

Leptin is a satiety hormone that also has proinflammatory effects, including augmentation of ozone-induced pulmonary inflammation. The purpose of this study was to determine whether reductions in endogenous levels of leptin can attenuate pulmonary responses to ozone. To reduce serum leptin, we fasted mice overnight before ozone exposure. Fasting caused a marked reduction in serum leptin to approximately one-sixth the levels observed in fed mice, and continuous infusion of leptin via Alzet micro-osmotic pumps restored serum leptin to, but not above, fed levels. Ozone exposure (2 ppm for 3 h) caused a significant, ∼40% increase in pulmonary resistance ( P < 0.01) and increased airway responsiveness in fasted but not in fed mice. The increased effect of ozone on pulmonary mechanics and airway responsiveness in fasted mice was not observed when leptin was restored via continuous infusion. Ozone exposure caused pulmonary inflammation, as evident by increases in bronchoalveolar lavage cells, protein, and soluble tumor necrosis factor receptors. There was no effect of fasting status on ozone-induced changes in the bronchoalveolar lavage inflammatory profile, and leptin treatment did not alter these responses. Our results indicate that fasting augments ozone-induced changes in pulmonary mechanics and airway responsiveness in mice. These effects of fasting are the result of declines in serum leptin. The mechanistic basis for this protective effect of leptin in fasted mice remains to be determined but is not related to effects on ozone-induced inflammation.

Involvement of Insulin-Like Growth Factor-1 in the Effect of Caloric Restriction: Regulation of Plasma Adiponectin and Leptin

Insulin-like growth factor (IGF)-1 signaling might partly mediate effects of caloric restriction (CR), an experimental intervention for increasing longevity in mammals. The present study evaluated effects of recombinant human (rh)IGF-1 infusion on adipokine levels in CR and transgenic (Tg) dwarf rats with the reduced growth hormone-IGF-1 axis, which shared similar body weight and food intake. At 9 months of age, each rat received a continuous infusion of rhIGF-1 for 14 days, and rats received an injection of glucose after overnight fasting. Infusion of rhIGF-1 had metabolic effects in all rat groups although it did not affect insulin sensitivity in any of the groups. In addition, plasma adiponectin was decreased to the control group levels and plasma leptin was further reduced in CR and Tg rats. The similarity of phenotypes and adipokine responses to rhIGF-1 between CR and Tg rats supports a role for reduced IGF-1 signaling in the CR effect.

Randomly interesterified triacylglycerol containing medium- and long-chain fatty acids stimulates fatty acid metabolism in white adipose tissue of rats

We have reported previously that randomly interesterified triacylglycerol containing medium- and long-chain fatty acids in the same glycerol molecule (MLCT) resulted in significantly lower body fat accumulation and higher hepatic fatty acid oxidation than from long-chain triacylglycerol (LCT) in rats. To understand the metabolic changes occurring in white adipose tissue, the fatty acid oxidation and synthesis, and the adipocytokine level were measured in rats fed with MLCT or LCT for 2 weeks. In comparison with LCT, MLCT lowered not only the fatty acid synthase and glycerol-3-phosphate dehydrogenase activities in perirenal adipose tissue, but also the serum insulin and leptin levels, in addition to significantly reducing the body fat accumulation. In contrast, fatty acid oxidation measured as the carnitine palmitoyltransferase activity in the tissue was significantly higher in the MLCT-fed rats than in the LCT-fed rats. It seems that the altered fatty acid metabolism in adipose tissue per se was also responsible for the lower adiposity by dietary MLCT.

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.

Model for predicting and phenotyping at normal weight the long-term propensity for obesity in Sprague–Dawley rats

Tests were conducted to determine whether weight gain or nutrient intake measures during the first week of exposure to a macronutrient diet can accurately predict an animal's long-term propensity towards obesity. In multiple groups of normal-weight Sprague-Dawley rats (n=35-70/group), daily weight gain during the first 5 days on a high-fat diet (45-60% fat) was found to be strongly, positively correlated (r=+0.71 to r=+0.82) with accumulated body fat in 4 dissected depots after 4-6 weeks on the diet. This measure consistently identified obesity-prone (OP) rats which, relative to the obesity-resistant (OR) rats, were only slightly heavier (+15 g, 4%) and hyperphagic (+9 kcal, 8%) after 5 days but markedly heavier (+70g) with up to 2-fold greater fat mass after several weeks on the diet. Other dietary conditions and measures revealed weaker relationships to ultimate body fat accrual. The OP rats identified by their 5-day weight-gain score exhibited at this early stage clear disturbances characteristic of markedly obese rats. These included elevated leptin, insulin, triglycerides and glucose, along with increased lipoprotein lipase activity (LPL) in adipose tissue and galanin expression in the paraventricular nucleus. Most notable were significant reductions in muscle of LPL activity and ratio of beta-hydroxyacyl-CoA dehydrogenase to citrate synthase activity, indicating a decline in lipid transport and capacity of muscle to metabolize lipids. By occurring early with initial weight gain, these hypothalamic and metabolic disturbances in OP rats, favoring fat storage in adipose tissue over fat oxidation in muscle, may have causal relationships to long-term accumulation of body fat.

Possible role of triacylglycerol-rich lipoproteins in the down-regulation of adipose obese mRNA expression in rats re-fed a high-fat diet

The large amount of absorbed dietary lipid after feeding a high-fat diet is mainly transported as triacylglycerol (TG)-rich lipoproteins (TRL) in the post-prandial blood and is subsequently distributed to peripheral tissues including adipose and muscle tissues. An in vivo and an in vitro study were conducted to investigate the possible role of post-prandial TRL after high fat feeding in the regulation of obese (ob) gene expression. Adult male Wistar rats were fasted for 48 h and re-fed either a fat-free/high-carbohydrate diet or a high-fat diet for 2, 4, or 8 h and plasma glucose, insulin, TG, and leptin as well as ob mRNA expression in epididymal fat pads were examined. Rats re-fed the high-fat diet had significantly higher plasma TG (p < 0.05) and lower plasma leptin and adipose ob mRNA (p < 0.05) than those fed the fat-free/high-carbohydrate diet; however, plasma glucose and insulin concentrations were not significantly different between the two groups. Plasma lipid analysis found large amount of TRL in rats fed with high-fat diet; however, only very small amount of the TRL was found in rats fed with fat-free/high-carbohydrate diet. We speculated that TRL might involve in regulation of ob gene expression. To further examine the regulation of TRL on ob mRNA expression, differentiated 3T3-L1 adipocytes were treated with TRL collected from rats fed 5 ml soybean oil by gastric intubations. TRL down-regulated ob mRNA not only in a dose and time dependent manner but also in the presence of insulin in 3T3-L1 adipocytes. These results suggest a possible role of TRL in the down-regulation of adipose ob mRNA expression and may account, at least in part, for the previous observations that short-term high fat feeding resulted in lower plasma leptin.

Acute insulin-induced elevations of circulating leptin and feeding inhibition in lean but not obese rats

Insulin has been shown to stimulate leptin mRNA expression acutely in rat adipose tissue, but its short-term effects on circulating leptin levels, and subsequent feeding behavior, have not been well described. We used 11-mo-old female selectively bred obesity-resistant (OR) and obesity-prone (OP) Sprague-Dawley rats maintained on laboratory chow to investigate this question. At testing, body weights and basal leptin levels of the OP rats were significantly elevated compared with the OR rats. In the 3-h fasted state, injection of 2.0 U insulin/kg ip resulted in significant elevations of plasma leptin at 4 h postinjection in both OP and OR groups ( hour 4, +2.50 and +5.98 ng/ml, respectively). In separate feeding tests with the same groups, intake of laboratory chow pellets was significantly inhibited during hours 2–4 after 2.0 U/kg of insulin in the OR (−80.1%, P < 0.05), but not in the OP group, compared with intake after saline injections. In feeding tests with palatable moderately high-fat pellets after 2.0 and 3.0 U insulin/kg ip, significant decreases between hours 2 and 4 in intake were seen in the OR group only (−41.0 and −68.3%, respectively). Thus feeding inhibition coincides with insulin-induced elevations of plasma leptin in lean but not obese Sprague-Dawley rats. Our data suggest that elevations of leptin within the physiological range may contribute to short-term inhibition of food intake in rats and that this process may be stimulated by feeding-related insulin release.

Leptin pulsatility in formerly obese women

Plasma leptin and growth hormone (GH) profile and pulsatility have been studied in morbidly obese subjects before and 14 months after bilio-pancreatic diversion (BPD), a bariatric technique producing massive lipid malabsorption. The maximum leptin diurnal variation (acrophase) decreased (10.27+/-1.70 vs. 22.60+/-2.79 ng x ml(-1); P=0.001), while its pulsatility index (PI) increased (1.084+/-0.005 vs. 1.050+/-0.004 ng x ml(-1) x min(-1); P=0.02) after BPD. Plasma GH acrophase increased (P=0.0001) from 0.91+/-0.20 to 4.58+/-0.80 microg x l(-1) x min(-1) after BPD as well as GH PI (1.70+/-0.13 vs. 1.20+/-0.04 microg x l(-1) x min(-1); P=0.024). Whole-body glucose uptake (M), assessed by euglycemic-hyperinsulinemic clamp, almost doubled after BPD (from 0.274+/-0.022 to 0.573+/-0.027 mmol x kgFFM(-1) x min(-1); P<0.0001), while 24 h lipid oxidation was significantly (P<0.0001) reduced (131.94+/-35.58 vs. 44.56+/-15.10 g). However, the average lipid oxidation was 97.2+/-3.1% (P<0.01) of the metabolizable lipid intake after the bariatric operation, while it was 69.2+/-8.5% before. After the operation, skeletal muscle ACC2 mRNA decreased (P<0.0001) from 452.82+/-76.35 to 182.45+/-40.69% of cyclophilin mRNA as did the malonyl-CoA (from 0.28+/-0.02 to 0.16+/-0.01 nmol x g(-1); P<0.0001). Leptin changes negatively correlated with M changes (R2=0.69, P<0.001). In a stepwise regression (R2=0.87, P=0.0055), only changes in 24 h free fatty acids (B=0.105+/-0.018, P=0.002) and glucose/insulin ratio (B=0.247+/-0.081, P=0.029) were the best predictors of leptin variations. In conclusion, the reversion of insulin resistance after BPD might allow reversal of leptin resistance, restoration of leptin pulsatility, and consequent inhibition of ACC2 mRNA expression, translating to a reduced synthesis of malonyl-CoA, which, in turn, results in increased fatty acid oxidation. Finally, since leptin inhibits GH secretion, a reduction of circulating leptin levels might have produced an increase in GH secretion, as observed in our series.

Diurnal variations of serum leptin in dogs: effects of fasting and re-feeding

Leptin is a protein synthesized and secreted primarily by adipocytes, and plays a key role in the regulation of energy balance. We have reported that serum leptin is elevated in obese dogs. In the present study, we examined diurnal variations of serum leptin in the dog, with special references to feeding and fasting cycles. Four male beagles were accustomed to feed once a day at 10:00 h, and blood samples were taken every 3 h for 24-36 h. Serum leptin concentration showed clear diurnal variations, being lowest before food intake (2.3+/-0.5 ng/mL) at 09:00 h, and highest (10.5+/-2.4 ng/mL) at 18:00 h. Such diurnal variations disappeared when the dogs were fasted. Serum insulin also showed diurnal variation with higher levels at 12:00-15:00 h. When insulin or glucose was injected in the fasted dogs to mimic the post-prandial insulin rise, serum leptin concentration was significantly increased in 4-8 h, but in both cases to a lesser extents than those after food intake. The results indicate that serum leptin concentrations change diurnally in association with feeding-fasting cycles in the dog, partially due to changes in insulin secretion.

Effect of retinoic acid on leptin, glycerol, and glucose levels in mature rat adipocytes in vitro

To elucidate the effects of retinoic acids (RAs) on adipogenesis and insulin sensitivity, we treated mature adipocytes with two different kinds of RA, 9-cis-RA and all-trans-RA. Both 9-cis- and all-trans-RA inhibited the secretion of leptin. However, the inhibition was significantly decreased at a higher dose of each RA. The inhibitory effect of 9-cis-RA was synergistically enhanced by the addition of rosiglitazone, a synthetic ligand for peroxisome proliferator-activated receptor (PPAR) gamma. 9-cis-RA also leads to adipogenesis in a dose-dependent manner. On the contrary, all-trans-RA does not increase adipogenesis in a dose-dependent manner. To clarify the antidiabetic effects of RA, glucose uptake was assessed by estimating glucose concentrations in the medium. 9-cis-RA reduced glucose levels in the culture media, but all-trans-RA did not. In conclusion, all-trans-RA does not alter adipogenesis and glucose uptake but does inhibit leptin secretion. 9-cis-RA, however, seems to increase both adipogenesis and glucose uptake through activation of the retinoid X receptor/PPARgamma heterodimer.

Effect of starvation on hepatic acyl-CoA synthetase, carnitine palmitoyltransferase-I, and acetyl-CoA carboxylase mRNA levels in rats

OBJECTIVES

This study investigated the effect of starvation on mRNA levels of hepatic acyl coenzyme A synthetase (ACS), carnitine palmitoyltransferase-I (CPT-I), and acetyl coenzyme A carboxylase (ACC) and on serum concentrations of leptin, insulin, and glucose in male Sprague-Dawley rats.

METHODS

Rats were fed an AIN-76 diet for 5 wk and then assigned to a normal group (NG) and a starvation group (SG). The SG was starved for 48 h and the NG was fasted for 12 h before being killed. Serum and hepatic lipids and serum levels of leptin, insulin, and glucose were determined. Expressions of ACS, CPT-1, and ACC mRNA were assessed in liver.

RESULTS

Serum concentrations of triacylglycerol and high-density lipoprotein cholesterol in the SG were lower than those in the NG. Serum concentrations of low-density lipoprotein cholesterol in the SG were significantly higher than in the NG. Hepatic concentrations of total lipid in the SG were significantly higher than those in the NG, and triacylglycerol concentrations in the SG were significantly lower than those in the NG. Serum concentrations of leptin and glucose in the SG were significantly lower than those in the NG. The ratio of abdominal fat to total body weight in the SG was lower than that in the NG. Hepatic ACS and CPT-I mRNA levels in the SG were significantly higher than those in the NG, but hepatic ACC mRNA levels were lower in the SG than in the NG.

CONCLUSIONS

We demonstrated that starvation increases hepatic levels of ACS and CPT-I and decreases transcription levels of ACC, implicating increases in fatty acid oxidation. This research demonstrates a coordinated regulation of ACS, CPT-I, and ACC mRNA levels and serves to enhance our understanding of the molecular mechanisms underlying fatty acid metabolism during starvation.

Differential regulation of hepatic gene expression by starvation versus refeeding following a high-sucrose or high-fat diet

OBJECTIVES

The objective of this work was to determine the effects of starvation versus refeeding following a high-sucrose diet (HS) or high-fat diet (HF) on fatty acid metabolism in mice.

METHODS

The mice were fed an AIN-76 control diet (CD), a modified HS, or an HF. The three dietary groups were subdivided into three groups each: those fed experimental diets for 12 wk, mice starved for 48 h after 12 wk on an experimental diet, and those with the same starvation treatment but with 72 h of refeeding after starvation, respectively.

RESULTS

Serum total cholesterol levels of CD and HF groups decreased and then increased under starvation and refeeding states, respectively. Refeeding HS and HF increased serum levels of low-density lipoprotein (LDL) cholesterol compared with refeeding of the CD group. Starvation significantly increased hepatic levels of total cholesterol in the HS and HF groups compared with the CD group. Hepatic acyl coenzyme A (CoA) synthetase (ACS) levels in the CD and HS groups but not the HF group increased and then decreased under starved and refed states, respectively; an opposite regulation was observed in the HF group. Levels of hepatic acetyl-CoA carboxylase (ACC) in the HS and HF groups were significantly increased by refeeding. Hepatic levels of carnitine palmitoyltransferase-I mRNA were significantly enhanced by starvation and refeeding in the HS group but decreased in CD and then increased in the HF group.

CONCLUSIONS

Changes in dietary energy nutrients, fasting, and refeeding affect hepatic ACS, CPT-I, and ACC mRNA expression, and these results will serve to enhance our understanding of the molecular mechanisms underlying regulation of fatty acid metabolism.

Reproduction and plasma concentrations of leptin, insulin and insulin-like growth factor 1 in growth-hormone-transgenic female sheep before and after artificial insemination

The transgenic sheep used in this study expressed an additional copy of the gene for ovine growth hormone (GH), so they had continuously high plasma concentrations of GH. They were used to test whether the GH transgene affected plasma concentrations of the metabolic hormones leptin, insulin-like growth factor 1 (IGF-1) and insulin, and whether these effects were associated with changes in conception, pregnancy or parturition following artificial insemination. Compared with control animals, the GH-transgenic sheep had higher bodyweight, lower body condition score and less subcutaneous fat (P < 0.05). These sheep also had lower plasma concentrations of leptin, higher plasma concentrations of insulin, and higher plasma concentrations of IGF-1 (P < 0.001). A similar proportion of GH-transgenic and control ewes came into oestrus, but the conception rate to artificial insemination was lower in GH-transgenic ewes than in the controls. Only four live lambs were recovered from 12 GH-transgenic ewes (33%) compared with 38 lambs from 43 controls (88%). This outcome was not associated with any difference in plasma progesterone profile in the period leading up to artificial insemination (Day 0). The GH-transgenic ewes had lower concentrations of FSH at all times measured (Day -19, Day -2 and Day 19). These results indicate that appropriate regulation of GH secretion from pituitary or peripheral tissues is necessary for normal reproduction and normal levels of metabolic hormones. Chronically high concentrations of GH were associated with increased levels of IGF-1 and insulin, and decreased levels of leptin.

Influence of nutrient intake and body fat on concentrations of insulin-like growth factor-I, insulin, thyroxine, and leptin in plasma of gestating beef cows1

Pregnant Angus x Hereford cows (n = 73) were used to determine the effects of amount of nutrient intake and BCS on concentrations of IGF-I, insulin, leptin, and thyroxine in plasma. At 2 to 4 mo of gestation, cows were blocked by BCS and assigned to one of four nutritional treatments: high (H = a 50% concentrate diet fed ad libitum in a drylot) or adequate native grass pastures and one of three amounts of a 40% CP supplement each day (M = moderate, 1.6 kg; L = low, 1.1 kg; or VL = very low, 0.5 kg; as-fed basis). After 110 d of treatment, all cows grazed dormant native grass pasture and received 1.6 kg/d of a 40% CP supplement. At 68, 109, and 123 d of treatment, cows were gathered, and plasma samples were collected by tail venipuncture (fed sample). After 18 h without feed and water, a second plasma sample was collected (fasted sample). At 109 d of treatment, BCS was greatest (P < 0.05) for H cows, similar for M and L cows, and least for VL cows. Concentrations of insulin and leptin were greater (P < 0.05) for H cows than for M and VL cows at 68 and 109 d, but similar for all groups at 123 d. Thyroxine in plasma was greatest (P < 0.05) for H cows at 68 d and similar for cows on all treatments at 123 d. Concentrations of IGF-I, insulin, and leptin in fed and fasted cows were positively correlated with BCS at 109 d. Body condition was predictive of concentrations of IGF-I, insulin, and leptin when cows had different nutrient intakes, but BCS accounted for less than 12% of the variation in plasma concentrations of IGF-I, insulin, and leptin when nutrient intake was the same for all cows. We conclude that amount of nutrient intake has a greater influence than body energy reserves on IGF-I, insulin, and leptin concentrations in the plasma of gestating beef cows.

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

OBJECTIVE

Leptin secretion has been shown to respond acutely to changes in blood glucose and insulin. Nutritional state also has a marked effect on both the level of circulating leptin protein and leptin gene expression. The aim of this study was to assess whether the prior nutritional state altered the leptin secretory response to an acute glucose challenge, and to determine potential mechanisms.

DESIGN

Male fed or fasted rats (200-250 g) were administered a single intravenous glucose bolus (1, 4 or 7 g/kg). The serum leptin, glucose, insulin and free fatty acid responses were studied over the following 5 h. The level of leptin gene expression and leptin protein was then determined in the epididymal fat pads, and in fed and fasted untreated rats for basal comparison.

RESULTS

Leptin secretion in response to glucose was suppressed in fasted rats following all glucose doses. The total leptin response was correlated with the total insulin response in all conditions (r = 0.85) and with the glucose response in fed rats (r = 0.69). Both leptin gene expression and leptin protein content were lower in basal fasted rats. Leptin gene expression and leptin protein content still remained lower 5 h following a glucose bolus but there was partial reversal of the effects of fasting following the 7 g/kg glucose dose.

CONCLUSIONS

Leptin secretion in response to an intravenous glucose bolus was determined by the insulin response and was significantly suppressed in fasted compared to fed rats. In addition to differences in the total insulin response of the animals, lower leptin responses may be facilitated by lower levels of both leptin gene mRNA and pre-existing leptin protein in epididymal adipose tissue of fasted rats.

Obesity over the life course

Obesity in middle-aged humans is a risk factor for many age-related diseases and decreases life expectancy by about 7 years, which is roughly comparable to the combined effect of all cardiovascular disease and cancer on life span. The prevalence of obesity increases up until late middle age and decreases thereafter. Mechanisms that lead to increased obesity with age are not yet well understood, but current evidence implicates impairments in hypothalamic function, especially impairments in the ability of hypothalamic pro-opiomelanocortin neurons to sense nutritional signals. The rapid increase in the prevalence of obesity at all ages in the past decade suggests that, in the next two or three decades, diseases associated with obesity, especially diabetes, will begin to rise rapidly. Indeed, these trends suggest that for the first time in modern history, the life expectancy of people in developed societies will begin to decrease, unless the rapid increase in the prevalence of obesity can be reversed.

Specific preservation of biosynthetic responses to insulin in adipose tissue may contribute to hyperleptinemia in insulin-resistant obese mice

Obesity is characterized by whole-body insulin resistance, yet the expression of many insulin-stimulated genes, including leptin, is elevated in obesity. These observations suggest that insulin resistance may depend on tissue type and gene. To address this hypothesis, we examined the regulation of immediate-early gene expression in liver and adipose tissue after injection of insulin and glucose, in lean insulin-sensitive, and in A(y)/a obese insulin-sensitive and obese insulin-resistant mice. Expression of hepatic jun-B mRNA was robustly increased after insulin injection in lean insulin-sensitive a/a mice and insulin-sensitive A(y)/a mice. In contrast, induction of hepatic jun-B and c-fos gene expression by insulin was markedly attenuated in obese insulin-resistant mice. Surprisingly, induction of adipose jun-B and c-fos gene expression by insulin was markedly enhanced in obese insulin-resistant mice. Furthermore, the expressions of jun-B and leptin were also enhanced in insulin-resistant mice after injection of glucose. Leptin mRNA was positively correlated with blood glucose levels and jun-B mRNA in lean but not insulin-resistant mice. Multiple regression analysis indicated that the correlation between leptin mRNA and jun-B mRNA was significant even after removing the effect of blood glucose, but the correlation between leptin mRNA and glucose was no longer significant after removing the effect of jun-B mRNA. These data suggest that some impairments in biosynthetic responses to insulin are manifest primarily in the liver, leading to hyperinsulinemia and stimulating the expression of some adipose insulin-stimulated genes, including leptin. These studies demonstrate the utility of immediate-early gene expression in the analysis of biosynthetic mechanisms of insulin resistance.

Transgenic neuronal expression of proopiomelanocortin attenuates hyperphagic response to fasting and reverses metabolic impairments in leptin-deficient obese mice

Hypothalamic proopiomelanocortin (POMC) gene expression is reduced in many forms of obesity and diabetes, particularly in those attributable to deficiencies in leptin or its receptor. To assess the functional significance of POMC in mediating metabolic phenotypes associated with leptin deficiency, leptin-deficient mice bearing a transgene expressing the POMC gene under control of the neuron-specific enolase promoter were produced. The POMC transgene attenuated fasting-induced hyperphagia in wild-type mice. Furthermore, the POMC transgene partially reversed obesity, hyperphagia, and hypothermia and effectively normalized hyperglycemia, glucosuria, glucose intolerance, and insulin resistance in leptin-deficient mice. Effects of the POMC transgene on glucose homeostasis were independent of the partial correction of hyperphagia and obesity. Furthermore, the POMC transgene normalized the profile of hepatic and adipose gene expression associated with gluconeogenesis, glucose output, and insulin sensitivity. These results indicate that central POMC is a key modulator of glucose homeostasis and that agonists of POMC products may provide effective therapy in treating impairments in glucose homeostasis when hypothalamic POMC expression is reduced, as occurs with leptin deficiency, hypothalamic damage, and aging.

Role of glucocorticoids in mediating effects of fasting and diabetes on hypothalamic gene expression

BACKGROUND

Fasting and diabetes are characterized by elevated glucocorticoids and reduced insulin, leptin, elevated hypothalamic AGRP and NPY mRNA, and reduced hypothalamic POMC mRNA. Although leptin replacement can reverse changes in hypothalamic gene expression associated with fasting and diabetes, leptin also normalizes corticosterone; therefore the extent to which the elevated corticosterone contributes to the regulation of hypothalamic gene expression in fasting and diabetes remains unclear. To address if elevated corticosterone is necessary for hypothalamic responses to fasting and diabetes, we assessed the effects of adrenalectomy on hypothalamic gene expression in 48-hour-fasted or diabetic mice. To assess if elevated corticosterone is sufficient for the hypothalamic responses to fasting and diabetes, we assessed the effect of corticosterone pellets implanted for 48 hours on hypothalamic gene expression.

RESULTS

Fasting and streptozotocin-induced diabetes elevated plasma glucocorticoid levels and reduced serum insulin and leptin levels. Adrenalectomy prevented the rise in plasma glucocorticoids associated with fasting and diabetes, but not the associated reductions in insulin or leptin. Adrenalectomy blocked the effects of fasting and diabetes on hypothalamic AGRP, NPY, and POMC expression. Conversely, corticosterone implants induced both AGRP and POMC mRNA (with a non-significant trend toward induction of NPY mRNA), accompanied by elevated insulin and leptin (with no change in food intake or body weight).

CONCLUSION

These data suggest that elevated plasma corticosterone mediate some effects of fasting and diabetes on hypothalamic gene expression. Specifically, elevated plasma corticosterone is necessary for the induction of NPY mRNA with fasting and diabetes; since corticosterone implants only produced a non-significant trend in NPY mRNA, it remains uncertain if a rise in corticosterone may be sufficient to induce NPY mRNA. A rise in corticosterone is necessary to reduce hypothalamic POMC mRNA with fasting and diabetes, but not sufficient for the reduction of hypothalamic POMC mRNA. Finally, elevated plasma corticosterone is both necessary and sufficient for the induction of hypothalamic AGRP mRNA with fasting and diabetes.

Impact of dietary FA and energy restriction on plasma leptin and ob gene expression in mice

The aim of the present study was to elucidate whether the qualitative composition of dietary fat influences plasma leptin and adipose tissue ob gene expression differentially. Two high-fat diets and a diet rich in carbohydrate were each administered both ad libitum and with a 25% energy restriction. The high-fat diets contained 58 energy percent as either monounsaturated FA (MUFA) or saturated FA (SAFA), whereas the carbohydrate-rich diet (CH) contained 7 energy percent as fat. We aimed at obtaining the same final weight for the animals in the ad libitum group as in the energy-restricted groups. This goal was reached at the same time (days 22-24) for all groups except for the ad libitum animals fed on saturated fat (day 36). The plasma leptin concentrations on ad libitum CH and MUFA diets did not differ significantly (24.3 +/- 2.1 and 34.7 +/- 6.7 ng/mL, respectively) whereas the saturated fat diet caused a lower concentration (13.9 +/- 1.9 ng/mL; P < 0.05). Interestingly, no differences in plasma leptin levels between groups were seen in the energy-restricted groups (mean 8.0 +/- 1.0 ng/mL). The type of diet did not alter the ob gene expression in intraabdominal white adipose tissue; however, a lower expression level was found in the energy-restricted groups. The percentage of body fat in the three ad libitum fed groups did not differ (23 +/- 1%). Thus, short-term administration of a diet rich in SAFA suppresses circulating leptin levels without altering the adipose tissue ob gene expression. This indicates that saturated fat may alter protein handling by adipose tissue or the whole body clearance of leptin.

Relationships between plasma concentrations of leptin and other metabolic hormones in GH-transgenic sheep infused with glucose

To study the regulation of leptin secretion in sheep, we infused glucose (0.32 g/h/kg for 12 h) into GH-transgenic animals (n = 8) that have chronically high plasma concentrations of ovine GH and insulin, but low body condition and low plasma leptin concentrations, and compared the responses with those in controls (n = 8). In both groups, the infusion increased plasma concentrations of glucose and insulin within 1 h and maintained high levels throughout the infusion period (P < 0.0001). Compared with controls, GH-transgenics had higher concentrations of insulin, IGF-1, GH (all P < 0.0001) and cortisol (P < 0.05), but lower GH pulse frequency (P < 0.0001). Overall, leptin concentrations were lower in GH-transgenics than in controls (P < 0.01). A postprandial increase in leptin concentrations was observed in both groups, independently of glucose treatment, after which the values remained elevated in animals infused with glucose, but returned to basal levels in those infused with saline, independently of transgene status. In both GH-transgenics and controls, glucose infusion did not affect the concentrations of GH, IGF-1, or cortisol. In conclusion, GH-transgenic and control sheep show similar responses to glucose infusion for leptin and other metabolic hormones, despite differences between them in body condition and basal levels of these hormones. Glucose, insulin, GH, IGF-1 and cortisol are probably not major factors in the acute control of leptin secretion in sheep, although sustained high concentrations of GH and IGF-1 might reduce adipose tissue mass or inhibit leptin gene expression.

Effect of short-term fasting on bone elongation rates: an analysis of catch-up growth in young male rats

Bone elongation in the postnatal animal is a result of cellular activity during endochondral ossification. Growth plate chondrocytes undergo a differentiation cascade involving stem cell clonal expansion and cellular enlargement during hypertrophy. Nutritional status has a significant effect on rates of bone growth, and a period of accelerated growth will occur if nutritional stunting of growth in early childhood can be corrected. This study focuses on changes in rates of increase in bone length in a model of catch-up growth in 4-wk-old male rats. Animals fasted for 3 d reached a weight approximately 60% of the control littermates. By 28 d postfasting, fasted animals had regained weight to 95% of control levels. A 3-d fast caused an immediate and profound decrease in rate of growth in the proximal tibial growth plate to only 30% of that of control animals, while stopping growth in the distal tibial growth plate. During the rapid initial rate acceleration of bone elongation, growth rate in both growth plates reached that of the control littermates by 7 d postfasting. The proximal tibial growth plate then maintained rates that were 10-15% higher than control over the rest of the experimental period. By 10 d postfasting, the previously fasted animals were on the same weight/rate trajectory as the control littermates. Changes in elongation rates were reflected by dramatic changes in growth plate morphology in all cellular zones. This is the first study to directly correlate weight recovery during catch-up with growth rate responses at the level of the growth plate.

Adiponectin is stimulated by adrenalectomy in ob/ob mice and is highly correlated with resistin mRNA

Plasma levels of the adipocyte product adiponectin, a putative insulin-sensitizing agent, are reduced in obesity, whereas plasma levels of resistin, an agent that some believe to confer insulin resistance, are thought to increase with obesity. Because adrenalectomy can increase insulin sensitivity, we hypothesized that adrenalectomy would increase expression of adiponectin and decrease expression of resistin. Therefore, we measured adiponectin mRNA, adiponectin peptide, and resistin mRNA in adrenalectomized ob/ob mice. Adrenalectomy restored adiponectin expression in ob/ob mice to wild-type levels and stimulated adiponectin peptide to above wild-type levels. Surprisingly, expression of adiponectin and resistin was highly positively correlated even after statistical removal of effects of insulin, glucose, and adiposity. In addition, adiponectin and resistin expression were also highly correlated in diet-induced obese mice. The data support a role for adiponectin in mediating some effects of adrenalectomy on insulin sensitivity.

Reducing hypothalamic AGRP by RNA interference increases metabolic rate and decreases body weight without influencing food intake

BACKGROUND

Several lines of evidence strongly suggest that agouti-related peptide (AGRP) plays a key role in the regulation of metabolic function but ablation of the AGRP gene has no apparent effect on metabolic function. Since specific pharmacological antagonists of AGRP do not presently exist, we assessed if reduction of hypothalamic AGRP mRNA by RNA interference (RNAI) would influence metabolic function, an outcome suggesting that pharmacological antagonists might constitute useful reagents to treat obesity.

RESULTS

The RNAI protocol specifically reduced hypothalamic expression of AGRP mRNA by 50% and resulted in reduction of AGRP peptide immunoreactivity. Physiologically, the reduction in AGRP levels was associated with increased metabolic rate and reduced body weight without changes in food intake.

CONCLUSION

AGRP can function to increase body weight and reduce metabolic rate without influencing food intake. The present study demonstrates that RNAI protocols can be used to assess physiological function of neuronal genes in vivo.

Hypothalamic NPY, AGRP, and POMC mRNA responses to leptin and refeeding in mice

Food deprivation (FD) increases hypothalamic neuropeptide Y (NPY) and agouti-related protein (AGRP) mRNA levels and decreases proopiomelanocortin (POMC) mRNA levels; refeeding restores these levels. We determined the time course of changes in hypothalamic NPY, AGRP, and POMC mRNA levels on refeeding after 24 h FD in C57BL mice by in situ hybridization. After 24 h deprivation, mice were refed with either chow or a palatable mash containing no calories or were injected with murine leptin (100 microg) without food. Mice were perfused 2 or 6 h after treatment. Food deprivation increased hypothalamic NPY mRNA (108 +/- 6%) and AGRP mRNA (78 +/- 7%) and decreased hypothalamic POMC mRNA (-15 +/- 1%). Refeeding for 6 h, but not 2 h, was sufficient to reduce (but not restore) NPY mRNA, did not affect AGRP mRNA, and restored POMC mRNA levels to ad libitum control levels. Intake of the noncaloric mash had no effect on mRNA levels, and leptin administration after deprivation (at a dose sufficient to reduce refeeding in FD mice) was not sufficient to affect mRNA levels. These results suggest that gradual postabsorptive events subsequent to refeeding are required for the restoration of peptide mRNA to baseline levels after food deprivation in mice.

VGF is required for obesity induced by diet, gold thioglucose treatment, and agouti and is differentially regulated in pro-opiomelanocortin- and neuropeptide Y-containing arcuate neurons in response to fasting

Targeted deletion of the gene encoding the neuronal and neuroendocrine secreted polypeptide VGF (nonacronymic) produces a lean, hypermetabolic mouse. Consistent with this phenotype, VGF mRNA levels are regulated in the hypothalamic arcuate nucleus in response to fasting. To gain insight into the site(s) and mechanism(s) of action of VGF, we further characterized VGF expression in the hypothalamus. Double-label studies indicated that VGF and pro-opiomelanocortin were coexpressed in lateral arcuate neurons in the fed state, and that VGF expression was induced after fasting in medial arcuate neurons that synthesize neuropeptide Y (NPY). Like NPY, VGF mRNA induction in this region of the hypothalamus in fasted mice was inhibited by exogenous leptin. In leptin-deficient ob/ob and receptor-mutant db/db mice, VGF mRNA levels in the medial arcuate were elevated. To identify neural pathways that are functionally compromised by Vgf ablation, VGF mutant mice were crossed with obese A(y)/a (agouti) and ob/ob mice. VGF deficiency completely blocked the development of obesity in A(y)/a mice, whereas deletion of Vgf in ob/ob mice attenuated weight gain but had no impact on adiposity. Hypothalamic levels of NPY and agouti-related polypeptide mRNAs in both double-mutant lines were dramatically elevated 10- to 15-fold above those of wild-type mice. VGF-deficient mice were also found to resist diet- and gold thioglucose-induced obesity. These data and the susceptibility of VGF mutant mice to monosodium glutamate-induced obesity are consistent with a role for VGF in outflow pathways, downstream of hypothalamic and/or brainstem melanocortin 4 receptors, that project via the autonomic nervous system to peripheral metabolic tissues and regulate energy homeostasis.

Leptin production by the stomach is up-regulated in obese (fa/fa) Zucker rats

OBJECTIVE

Genetically obese (fa/fa) Zucker rats display markedly elevated circulating leptin levels compared with their lean counterparts; this is expected because of the lack of a LepR-mediated feedback inhibition. The aim of this study was to determine the effect of the leptin receptor mutation in the Zucker rat on gastric leptin production and on the response to 14 hours of starvation. The response to a short-term period of food intake (20 minutes) on gastric leptin release was also analyzed.

RESEARCH METHODS AND PROCEDURES

Leptin mRNA expression in the gastric mucosa and in adipose tissue depots (epididymal, retroperitoneal, mesenteric, and inguinal) was assessed by reverse transcriptase-polymerase chain reaction and serum and stomach leptin content by enzyme-linked immunosorbent assay.

RESULTS

Obese Zucker rats overexpressed leptin in the stomach. They overexpress leptin in the inguinal adipose tissue but not in visceral adipose tissue depots, indicating tissue-specific obesity-dependent differences. Gastric leptin expression is regulated by feeding conditions in lean but not in obese (fa/fa) rats. In lean animals, leptin mRNA levels decrease in fasting conditions and increase rapidly with a short period of food intake. Obese Zucker rats also overdisplay stomach leptin levels. Feeding acutely stimulates leptin secretion by the stomach in lean, and to a lesser extent, in obese rats.

DISCUSSION

These results indicate impaired regulation of leptin expression in the stomach of obese (fa/fa) Zucker rats. However, there is still an effect of the nutritional status on gastric leptin levels despite the lack of a functional leptin receptor.

Glucose or essential amino acid infusions in late pregnant and early lactating Simmenthal cows failed to induce a leptin response

To assess the leptin response to metabolic challenges, three Italian Simmental cows were infused for 6 h: with (a) saline (control); (b) glucose; and (c) amino acid solutions according to a 3 x 3 Latin square experiment. The infusions were carried out at the 36th week of pregnancy, and the second and 12th week of the following lactation. At each of the three infusion periods, blood samples were collected from the jugular vein before and 15, 30, 120, 180, 240, 300 and 360 min after the beginning of each infusion. All samples were analysed for leptin, insulin, glucagon, growth hormone (GH), glucose, non-esterified fatty acids (NEFA) and urea. The physiological phase of the cows significantly affected the basal concentrations of insulin, glucagon, urea and NEFA. The infusion of both glucose and the amino acid solutions did not affect leptin concentrations. Insulin response was significantly increased when animals were infused with the glucose solution and, within treatment, the greatest response was observed at the 12th week of lactation. The greatest glucagon response was observed when infusing the amino acid solution. Urea response to all treatments increased from the dry period to the 12th week of lactation. The GH and NEFA responses were not affected by treatments. The Multi Species radio-immunoassay used in this study showed a lower sensitivity for ruminant leptin which may partially explain the lack of significant leptin variations. However, it can be hypothesized that leptin variations around parturition can be affected by the negative energy balance, and leptin release is not acutely affected by glucose and amino acid availability. In addition, no short-term relationship were found between insulin, glucagon and GH and leptin release in Italian Simmental cows during the dry period and early lactation.

Enhanced glucose uptake into adipose tissue induced by early growth restriction augments excursions in plasma leptin response evoked by changes in insulin status

OBJECTIVE

The study used a rat model of moderate protein restriction exclusively during fetal and early neonatal life, which has been established to cause intrauterine early growth retardation, to investigate possible association between adipocyte glucose utilisation and leptin secretion in vivo.

DESIGN

These rats, termed early protein restricted, were transferred to a diet containing the standard amount of protein at weaning and remained on this diet til adulthood, at which time adipocyte glucose utilisation and leptin secretion was compared with that of age-matched controls. Insulin status was modulated by acute (2 h) insulin infusion at a constant rate (4.2 mU/min per kg) to elevate insulin to the high physiological range. Euglycaemia was maintained by variable glucose infusion.

MEASUREMENTS

Glucose utilisation was measured in vivo in conscious unrestrained rats using 2-deoxy[1-3H] glucose. Leptin concentrations (measured by radioimmunoassay) and whole-body glucose kinetics (measured using [3-3H] glucose) were studied in the postabsorptive state and after acute insulin stimulation.

RESULTS

Adipose-tissue glucose utilisation rates in vivo tended to be higher in the post-absorptive state and were consistently 1.8-3.0-fold higher after insulin stimulation in the early-protein-restricted group compared with the control group. Both the absolute increase in leptin concentration elicited by hyperinsulinaemia and the magnitude of the effect of insulin to elevate plasma leptin levels were greater in the early-protein-restricted group compared with the control group (by 2.2-fold and 1.6-fold, respectively). The effect of insulin to stimulate R(d) was much greater in the early-protein-restricted group (4.1-fold) than in the control group (2.2-fold) and the absolute increase in R(d) elicited by insulin was 43% higher in the early-protein-restricted group than in the control group.

CONCLUSIONS

It is concluded that poor early growth enhances the acute leptin response to changes in insulin status through programmed changes in adipocyte glucose handling.

Chronic central infusion of ghrelin increases hypothalamic neuropeptide Y and Agouti-related protein mRNA levels and body weight in rats

Ghrelin, an endogenous ligand for the growth hormone secretagogue receptor (GHS-R), was originally purified from the rat stomach. Like the synthetic growth hormone secretagogues (GHSs), ghrelin specifically releases growth hormone (GH) after intravenous administration. Also consistent with the central actions of GHSs, ghrelin-immunoreactive cells were shown to be located in the hypothalamic arcuate nucleus as well as the stomach. Recently, we showed that a single central administration of ghrelin increased food intake and hypothalamic agouti-related protein (AGRP) gene expression in rodents, and the orexigenic effect of this peptide seems to be independent of its GH-releasing activity. However, the effect of chronic infusion of ghrelin on food consumption and body weight and their possible mechanisms have not been elucidated. In this study, we determined the effects of chronic intracerebroventricular treatment with ghrelin on metabolic factors and on neuropeptide genes that are expressed in hypothalamic neurons that have been previously shown to express the GHS-R and to regulate food consumption. Chronic central administration of rat ghrelin (1 microg/rat every 12 h for 72 h) significantly increased food intake and body weight. However, it did not affect plasma insulin, glucose, leptin, or GH concentrations. We also found that chronic central administration of ghrelin increased both neuropeptide Y (NPY) mRNA levels (151.0 +/- 10.1% of saline-treated controls; P < 0.05) and AGRP mRNA levels (160.0 +/- 22.5% of saline-treated controls; P < 0.05) in the arcuate nucleus. Thus, the primary hypothalamic targets of ghrelin are NPY/AGRP-containing neurons, and ghrelin is a newly discovered orexigenic peptide in the brain and stomach.

Hormonal modulation of food intake in response to low leptin levels induced by hypergravity

A loss in fat mass is a common response to centrifugation and it results in low circulating leptin concentrations. However, rats adapted to hypergravity are euphagic. The focus of this study was to examine leptin and other peripheral signals of energy balance in the presence of a hypergravity-induced loss of fat mass and euphagia. Male Sprague-Dawley rats were centrifuged for 14 days at gravity levels of 1.25, 1.5, or 2 G, or they remained stationary at 1 G. Urinary catecholamines, urinary corticosterone, food intake, and body mass were measured on Days 11 to 14. Plasma hormones and epididymal fat pad mass were measured on Day 14. Mean body mass of the 1.25, 1.5, and 2 G groups were significantly (P < 0.05) lower than controls, and no differences were found in food intake (g/day/100 g body mass) between the hypergravity groups and controls. Epididymal fat mass was 14%, 14%, and 21% lower than controls in the 1.25, 1.5, and 2.0 G groups, respectively. Plasma leptin was significantly reduced from controls by 46%, 45%, and 65% in the 1.25, 1.5, and 2 G groups, respectively. Plasma insulin was significantly lower in the 1.25, 1.5, and 2.0 G groups than controls by 35%, 38%, and 33%. No differences were found between controls and hypergravity groups in urinary corticosterone. Mean urinary epinephrine was significantly higher in the 1.5 and 2.0 G groups than in controls. Mean urinary norepinephrine was significantly higher in the 1.25, 1.5 and 2.0 G groups than in controls. Significant correlations were found between G load and body mass, fat mass, leptin, urinary epinephrine, and norepinephrine. During hypergravity exposure, maintenance of food intake is the result of a complex relationship between multiple pathways, which abates the importance of leptin as a primary signal.

The effects of insulin, glucose, and pyruvate on the kinetics of leptin secretion

Leptin is a hormone that is secreted by fat cells and has roles in body weight regulation, glucose metabolism, reproduction, and other neuroendocrine functions. The purpose of this study was to determine whether the secretagogues, insulin, glucose, and pyruvate, enhance leptin secretion by increasing leptin synthesis, or whether these secretagogues stimulate the quantal release of a stored cytosolic pool of leptin. We found that in the absence of secretagogues, the rate of leptin secretion from isolated rat adipocytes approximately equals the rate of leptin synthesis. For 60 min after the addition of secretagogues, leptin synthesis rapidly increases, with little or no leptin secretion; leptin increases intracellularly by approximately 60% (P < 0.05). After 60 min, leptin is significantly released from cells. At 120 and 240 min, secretagogues enhance leptin secretion into the medium by 35% (P < 0.05) and 40% (P < 0.01), respectively. Cycloheximide prevents the synthesis and the secretagogue-mediated secretion of leptin. Monensin, an inhibitor of protein translocation, has no effect on leptin synthesis, but it blocks the secretagogue-mediated secretion of leptin. These findings suggest that secretagogues enhance leptin release by increasing leptin synthesis, rather than by enhancing the release of a preexisting cytosolic pool of leptin.

Long-term leptin treatment of ob/ob mice improves glucose-induced insulin secretion

OBJECTIVE

Previous studies have demonstrated that leptin inhibits glucose-stimulated insulin secretion from isolated islets, although a lack of leptin effect on insulin secretion has also been reported. The effect of long term in vivo leptin treatment of insulin secretion has, however, not been established. Therefore, in the present study, we have evaluated the effect of long term in vivo treatment of leptin on glucose-induced insulin secretion in ob/ob mice.

METHODS

After 7 days' treatment of leptin (100 microg daily s.c.), insulin release was measured in isolated islets by batch incubation followed by radioimmunoassay. Glucose utilization and oxidation were measured by measuring the formation of (3)H(2)O and (14)CO(2) from [5-(3)H] and [U-(14)C] glucose, respectively. Glucose-6-phosphatase activity was measured by measuring the conversion of (14)C-glucose-6-P to (14)C-glucose. In addition, immunohistochemistry of pancreatic specimens was undertaken for study of expression of insulin, GLUT-2 and hormone-sensitive lipase (HSL).

RESULTS

Leptin treatment significantly improved insulin secretion both at 5.5 mM (by 15%; P<0.05) and 16.7 mM (by 85%; P<0.001) glucose, compared to vehicle-treated controls. Furthermore, whereas leptin treatment did not affect islet insulin or DNA contents, a significant decrease in islet triglyceride content and glucose-6-phosphatase activity was observed. Moreover, the immunocytochemical data revealed an increased immunostaining for insulin, GLUT-2 and hormone-sensitive lipase (HSL) in islets from leptin-treated ob/ob mice.

CONCLUSION

The results suggest that long-term leptin treatment of ob/ob mice improves glucose-stimulated insulin secretion in parallel with reduced glucose-6-phosphatase activity, increased HSL and decreased triglyceride levels in islets. These perturbations may explain the improvement of glucose-stimulated insulin secretion induced by leptin.

Pretreatment with glucose increases entry of urocortin into mouse brain

Although urocortin is a potent inhibitor of food ingestion after peripheral administration, it was recently shown that under normal conditions this peptide crosses the blood-brain barrier (BBB) at a very slow rate. We examined whether hyperglycemia could stimulate the rate of entry (K(i)) of (125)I-urocortin into the mouse brain. In euglycemic mice, (125)I-urocortin injected iv entered the brain at a rate similar to that of the vascular marker (99m)Tc-albumin. However, injection of glucose (3 g/kg, ip) 0.5, 1, or 2 h before the (125)I-urocortin greatly increased the influx of urocortin. Without the glucose, the self-inhibition characteristic of a saturable transport system was not apparent. Self-inhibition could be demonstrated after the glucose injection, indicating activation of a transport system for urocortin that was saturable. Injection of insulin (10 U/kg, ip) 1 or 2 h before the (125)I-urocortin decreased the K(i). Thus, the entry of urocortin into brain can be activated by changes in the concentration of blood glucose, illustrating the responsiveness of the BBB to regulatory influences.