New understanding in obesity research

A public-health approach considers the relevance of nutritional research in the prevention and management of obesity. Well-defined and internationally-agreed definitions based on BMI allow an assessment of the worldwide prevalence of overweight and obesity. There are about 250 million obese adults in the world, and many more overweight. Obesity is emerging in the Third World, first in urban middle-aged women. With economic developments, obesity then occurs in men and younger women. In the West childhood obesity is rapidly emerging, with concern that early-onset obesity is especially hazardous. In Asians the risks of excess visceral fat occur at lower body weights than in Caucasians. The propensity to visceral obesity in Asians may relate to malnourished mothers and low birth weight. The International Obesity Task Force is considering many issues, including the health economics of obesity. It has developed a strategy to define childhood obesity, which in children over 6 years is likely to predict long-term weight and health problems. While the search for genetic markers of obesity continues, with particular interest in the leptin gene, it is clear that societal change, with the decline in physical activity and the passive overconsumption of high-fat diets are major contributors to the global increase in obesity. The public-health aspects of obesity research are therefore challenging.

Differential effects of leptin in regulation of tissue glucose utilization in vivo

We have recently shown that leptin enhances systemic insulin sensitivity and whole body glucose utilization in the rat. This study examines our hypothesis that leptin has differential effects in regulating glucose utilization among the tissues, i.e. stimulating glucose utilization in brown adipose tissue (BAT) and skeletal muscle but suppressing glucose utilization in white adipose tissue (WAT) in normal male rats (275-350 g BW). The rats were treated with s.c. infusion of recombinant murine leptin (4 mg/kg x day) or vehicle (V) with Alzet osmotic pumps or with vehicle and pair-feeding (PF) for 7 days. Leptin significantly decreased food intake (leptin, 11.5 +/- 0.4 g/day; V, 16.8 +/- 1.5 g/day; P < 0.05) and body weight (maximum change, 5.0 +/- 0.2%; P < 0.05 vs. V) and lowered plasma triglyceride, insulin, and glucose levels, but raised beta-hydroxybutyrate levels. Glucose utilization by individual tissues was determined with an i.v. bolus of [1-(14)C]2-deoxyglucose (2-DG) after a 90-min hyperinsulinemic (2 mU/kg x min) euglycemic clamp. With leptin treatment, the 2-DG-determined glucose utilization in interscapular BAT was almost 3-fold that in V-treated rats and 70% greater than that in PF rats. In contrast, in the epididymal WAT, glucose utilization was reduced by leptin treatment to only 34% that in V-treated rats and 45% that in PF rats. Leptin increased 2-DG uptake by extensor digitorum longus muscle and soleus muscle compared with that in the V and PF groups. With leptin treatment, the GLUT4 glucose transporter mRNA and protein levels were increased in BAT, but decreased in WAT (both P < 0.05). There was no significant change in GLUT4 mRNA and protein expression in extensor digitorum longus muscle and soleus muscle. Oxygen consumption was significantly increased (32.1 +/- 7.4%) in BAT (139.0 +/- 8.2 nmole O2/30 min x 10(6) cells) of leptin-treated rats vs. that in V control rats (105.3 +/- 6.7 nmole O2/30 min x 10(6) cells). In conclusion, leptin has differential, tissue-specific effects on glucose and oxygen utilization, which contribute to the reduction in whole body adiposity by enhancing energy consumption in BAT and muscle while attenuating energy storage in WAT.

Metabolic, gastrointestinal, and CNS neuropeptide effects of brain leptin administration in the rat

To investigate whether brain leptin involves neuropeptidergic pathways influencing ingestion, metabolism, and gastrointestinal functioning, leptin (3.5 micrograms) was infused daily into the third cerebral ventricular of rats for 3 days. To distinguish between direct leptin effects and those secondary to leptin-induced anorexia, we studied vehicle-infused rats with food available ad libitum and those that were pair-fed to leptin-treated animals. Although body weight was comparably reduced (-8%) and plasma glycerol was comparably increased (142 and 17%, respectively) in leptin-treated and pair-fed animals relative to controls, increases in plasma fatty acids and ketones were only detected (132 and 234%, respectively) in pair-fed rats. Resting energy expenditure (-15%) and gastrointestinal fill (-50%) were reduced by pair-feeding relative to the ad libitum group, but they were not reduced by leptin treatment. Relative to controls, leptin increased hypothalamic mRNA for corticotropin-releasing hormone (CRH; 61%) and for proopiomelanocortin (POMC; 31%) but did not reduce mRNA for neuropeptide Y. These results suggest that CNS leptin prevents metabolic/gastrointestinal responses to caloric restriction by activating hypothalamic CRH- and POMC-containing pathways and raise the possibility that these peripheral responses to CNS leptin administration contribute to leptin's anorexigenic action.

Does estradiol mediate leptin's effects on adiposity and body weight?

The role of estradiol in mediating leptin's effects on body weight was assessed in ovariectomized (OVX) mice before and after the onset of obesity. Ovariectomy did not alter leptin levels before the onset of obesity, and estradiol adminstration (0.05-17 microgram/day for 14 days) did not significantly alter leptin levels if they were corrected for the estradiol-induced reduction in body fat. The converse was also true, in that leptin administration (0.4-140 microgram/day) did not alter estradiol levels in intact mice. Furthermore, neither estradiol reduction (via ovariectomy) nor addition (via exogenous administration) significantly altered leptin's ability to reduce fat mass. Leptin was equally effective in reducing body weight in lean or obese OVX mice and intact controls. Finally, estradiol did not change the magnitude of leptin's effect on fat mass reduction when it was given in combination with leptin to lean intact or OVX mice. Estradiol may have indirectly affected leptin efficacy, because leptin did not produce as large a change in fat mass at lower doses in lean OVX mice as it did in intact counterparts. Taken together, these data suggested that 1) estradiol does not directly regulate leptin secretion or its effects on fat mass and 2) leptin does not directly regulate estradiol secretion or its effects on fat mass. Leptin and estradiol, however, may interact in an indirect fashion to affect fat utilization.

Regulation of body weight in humans

The mechanisms involved in body weight regulation in humans include genetic, physiological, and behavioral factors. Stability of body weight and body composition requires that energy intake matches energy expenditure and that nutrient balance is achieved. Human obesity is usually associated with high rates of energy expenditure. In adult individuals, protein and carbohydrate stores vary relatively little, whereas adipose tissue mass may change markedly. A feedback regulatory loop with three distinct steps has been recently identified in rodents: 1) a sensor that monitors the size of adipose tissue mass is represented by the amount of leptin synthesized by adipose cells (a protein encoded by the ob gene) which determines the plasma leptin levels; 2) hypothalamic centers, with specific leptin receptors, which receive and integrate the intensity of the signal; and 3) effector systems that influence the two determinants of energy balance, i.e., energy intake and energy expenditure. With the exception of a few very rare cases, the majority of obese human subjects have high plasma leptin levels that are related to the size of their adipose tissue mass. However, the expected regulatory responses (reduction in food intake and increase in energy expenditure) are not observed in obese individuals. Thus obese humans are resistant to the effect of endogenous leptin, despite unaltered hypothalamic leptin receptors. Whether defects in the leptin signaling cascade play a role in the development of human obesity is a field of great actual interest that needs further research. Present evidences suggest that genetic and environmental factors influence eating behavior of people prone to obesity and that diets that are high in fat or energy dense undermine body weight regulation by promoting an overconsumption of energy relative to need.

Reactivation of pituitary hormone release and metabolic improvement by infusion of growth hormone-releasing peptide and thyrotropin-releasing hormone in patients with protracted critical illness

Protracted critical illness is marked by protein wasting resistant to feeding, by accumulation of fat stores, and by suppressed pulsatile release of GH and TSH. We previously showed that the latter can be reactivated by brief infusion of GH-releasing peptide (GHRP-2) and TRH. Here, we studied combined GHRP-2 and TRH infusion for 5 days, which allowed a limited evaluation of the metabolic effectiveness of this novel trophic endocrine strategy. Fourteen patients (mean +/- SD age, 68 +/- 11 yr), critically ill for 40 +/- 28 days, were compared to a matched group of community-living control subjects at baseline and subsequently received 5 days of placebo and 5 days of GHRP-2 plus TRH (1 + 1 microg/kg x h) infusion in random order. At baseline, impaired anabolism, as indicated by biochemical markers (osteocalcin and leptin), was linked to hyposomatotropism [reduced pulsatile GH secretion, as determined by deconvolution analysis, and low GH-dependent insulin-like growth factor and binding protein (IGFBP) levels]. Biochemical markers of accelerated catabolism (increased protein degradation and bone resorption) were related to tertiary hypothyroidism and the serum concentration of IGFBP-1, but not to hyposomatotropism. Metabolic markers were independent of elevated serum cortisol. After 5 days of GHRP-2 plus TRH infusion, osteocalcin concentrations increased 19% vs. -6% with placebo, and leptin had rose 32% vs. -15% with placebo. These anabolic effects were linked to increased IGF-I and GH-dependent IGFBP, which reached near-normal levels from day 2 onward. In addition, protein degradation was reduced, as indicated by a drop in the urea/creatinine ratio, an effect that was related to the correction of tertiary hypothyroidism, with near-normal thyroid hormone levels reached and maintained from day 2 onward. Concomitantly, a spontaneous tendency of IGFBP-1 to rise and of insulin to decrease was reversed. Cortisol concentrations were not detectably altered. In conclusion, 5-day infusion of GHRP-2 plus TRH in protracted critical illness reactivates blunted GH and TSH secretion, with preserved pulsatility, peripheral responsiveness, and feedback inhibition and without affecting serum cortisol, and induces a shift toward anabolic metabolism. This provides the first evidence of the metabolic effectiveness of short term GHRP-2 plus TRH agonism in this particular wasting condition.

Effects of acute or prolonged exposure to human leptin on isolated human islet function

The adipocyte-derived hormone leptin has been reported to inhibit, have no effect, or potentiate insulin secretion in-vitro; these effects mainly depend on the species considered, the concentrations used, and the length of exposure. We investigated the direct effects of recombinant human leptin (HL) on human pancreatic beta cell function by studying insulin secretion (IS), hexokinase and glucokinase activity and Km, and potassium channel permeability in purified human islets (HI). In acute experiments, no effect of 1, 5, 20, or 50 ng/ml HL on glucose or arginine stimulated insulin release was found, whereas 500 ng/ml HL caused a significant decrease of glucose induced IS. After 24h pre-culture with either 20 or 500 ng/ml HL, a significant reduction of glucose (but not arginine) stimulated IS was observed. Exposure to leptin caused a significant increase of potassium channel permeability, whereas hexokinase and glucokinase activity and Km remained unchanged. These results suggest that physiological human leptin concentration is able to importantly affect glucose (but not arginine) stimulated insulin release from human islets only after prolonged exposure. This effect is probably mediated by changes of potassium channel permeability, and is not accompanied by modifications of glucose phosphorylating enzymes properties.

Regulation of fatty acid homeostasis in cells: novel role of leptin

It is proposed that an important function of leptin is to confine the storage of triglycerides (TG) to the adipocytes, while limiting TG storage in nonadipocytes, thus protecting them from lipotoxicity. The fact that TG content in nonadipocytes normally remains within a narrow range, while that of adipocytes varies enormously with food intake, is consistent with a system of TG homeostasis in normal nonadipocytes. The facts that when leptin receptors are dysfunctional, TG content in nonadipocytes such as islets can increase 100-fold, and that constitutively expressed ectopic hyperleptinemia depletes TG, suggest that leptin controls the homeostatic system for intracellular TG. The fact that the function and viability of nonadipocytes is compromised when their TG content rises above or falls below the normal range suggests that normal homeostasis of their intracellular TG is critical for optimal function and to prevent lipoapoptosis. Thus far, lipotoxic diabetes of fa/fa Zucker diabetic fatty rats is the only proven lipodegenerative disease, but the possibility of lipotoxic disease of skeletal and/or cardiac muscle may require investigation, as does the possible influence of the intracellular TG content on autoimmune and neoplastic processes.

Reversing adipocyte differentiation: implications for treatment of obesity

Conventional treatment of obesity reduces fat in mature adipocytes but leaves them with lipogenic enzymes capable of rapid resynthesis of fat, a likely factor in treatment failure. Adenovirus-induced hyperleptinemia in normal rats results in rapid nonketotic fat loss that persists after hyperleptinemia disappears, whereas pair-fed controls regain their weight in 2 weeks. We report here that the hyperleptinemia depletes adipocyte fat while profoundly down-regulating lipogenic enzymes and their transcription factor, peroxisome proliferator-activated receptor (PPAR)gamma in epididymal fat; enzymes of fatty acid oxidation and their transcription factor, PPARalpha, normally low in adipocytes, are up-regulated, as are uncoupling proteins 1 and 2. This transformation of adipocytes from cells that store triglycerides to fatty acid-oxidizing cells is accompanied by loss of the adipocyte markers, adipocyte fatty acid-binding protein 2, tumor necrosis factor alpha, and leptin, and by the appearance of the preadipocyte marker Pref-1. These findings suggest a strategy for the treatment of obesity by alteration of the adipocyte phenotype.

Effect of leptin deficiency on metabolic rate in ob/ob mice

Reduced metabolic rate may contribute to weight gain in leptin-deficient (ob/ob) mice; however, available studies have been criticized for referencing O2 consumption (VO2) to estimated rather than true lean body mass. To evaluate whether leptin deficiency reduces energy expenditure, four separate experiments were performed: 1) NMR spectroscopy was used to measure fat and nonfat mass, permitting VO2 to be referenced to true nonfat mass; 2) dietary manipulation was used in an attempt to eliminate differences in body weight and composition between ob/ob and C57BL/6J mice; 3) short-term effects of exogenous leptin (0.3 mg. kg-1. day-1) on VO2 were examined; and 4) body weight and composition were compared in leptin-repleted and pair-fed ob/ob animals. ob/ob animals had greater mass, less lean body mass, and a 10% higher metabolic rate when VO2 was referenced to lean mass. Dietary manipulation achieved identical body weight in ob/ob and C57BL/6J animals; however, despite weight gain in C57BL/6J animals, percent fat mass remained higher in ob/ob animals (55 vs. 30%). Exogenous leptin increased VO2 in ob/ob but not control animals. Weight loss in leptin-repleted ob/ob mice was greater than in pair-fed animals (45 vs. 17%). We conclude, on the basis of the observed increase in VO2 and accelerated weight loss seen with leptin repletion, that leptin deficiency causes a reduction in metabolic rate in ob/ob mice. In contrast, these physiological studies suggest that comparison of VO2 in obese and lean animals does not produce useful information on the contribution of leptin to metabolism.

Altered expression of hepatic CYP2E1 and CYP4A in obese, diabetic ob/ob mice, and fa/fa Zucker rats

Hepatic levels of the cytochrome P450 (CYP) proteins 2E1 and 4A are often increased in obesity, diabetes and fasting. In such states of nutritional imbalance, CYPs 2E1 and 4A may play a more significant role in fatty acid oxidation. In order to more fully characterize the regulation of CYP2E1 and CYP4A in obesity and obesity-related (type II) diabetes, we analyzed the hepatic expression of CYP2E1 and CYP4A in ob/ob mice which are leptin deficient, and fa/fa Zucker rats which have defective leptin receptor function. CYP2E1 protein and mRNA were either unchanged or reduced in both models. Conversely, expression of murine Cyp4a10 and 4a14 in the obese mice, and 4A2 in the male fatty Zucker rat, were greatly increased. The levels of other CYP4As were either unchanged or reduced. These results show that CYP2E1 is not inevitably increased by obesity and diabetes and indicate differential regulation of CYP4A subfamily genes in rodent models. Further, they implicate leptin receptor signaling as a factor that may modulate expression of CYP gene products involved in fatty acid oxidation.

Increased high density lipoprotein (HDL), defective hepatic catabolism of ApoA-I and ApoA-II, and decreased ApoA-I mRNA in ob/ob mice. Possible role of leptin in stimulation of HDL turnover

Abnormalities of plasma high density lipoprotein (HDL) levels commonly reflect altered metabolism of the major HDL apolipoproteins, apoA-I and apoA-II, but the regulation of apolipoprotein metabolism is poorly understood. Two mouse models of obesity, ob/ob and db/db, have markedly increased plasma HDL cholesterol levels. The purpose of this study was to evaluate mechanisms responsible for increased HDL in ob/ob mice and to assess potential reversibility by leptin administration. ob/ob mice were found to have increased HDL cholesterol (2-fold), apoA-I (1.3-fold), and apoA-II (4-fold). ApoA-I mRNA was markedly decreased (to 25% of wild-type) and apoA-II mRNA was unchanged, suggesting a defect in HDL catabolism. HDL apoprotein turnover studies using nondegradable radiolabels confirmed a decrease in catabolism of apoA-I and apoA-II and a 4-fold decrease in hepatic uptake in ob/ob mice compared with wild-type, but similar renal uptake. Low dose leptin treatment markedly lowered HDL cholesterol and apoA-II levels in both ob/ob mice and in lean wild-type mice, and it restored apoA-I mRNA to normal levels in ob/ob mice. These changes occurred without significant alteration in body weight. Moreover, ob/ob neuropeptide Y-/- mice, despite marked attenuation of diabetes and obesity phenotypes, showed no change in HDL cholesterol levels relative to ob/ob mice. Thus, increased HDL levels in ob/ob mice reflect a marked hepatic catabolic defect for apoA-I and apoA-II. In the case of apoA-I, this is offset by decreased apoA-I mRNA, resulting in apoA-II-rich HDL particles. The studies reveal a specific HDL particle catabolic pathway that is down-regulated in ob/ob mice and suggest that HDL apolipoprotein turnover may be regulated by obesity and/or leptin signaling.

Control of erythropoietin secretion by doxycycline or mifepristone in mice bearing polymer-encapsulated engineered cells

Cell encapsulation offers a safe and manufacturable method for the systemic delivery of therapeutic proteins from genetically engineered cells. However, control of dose delivery remains a major issue with regard to clinical application. We generated populations of immortalized murine NIH 3T3 fibroblasts that secrete mouse erythropoietin (Epo) in response to stimulation by doxycycline or mifepristone. Engineered cells were introduced into AN69 hollow fibers, which were implanted in the peritoneal cavity or recipient mice. Animals receiving doxycycline or mifepristone showed stable polyhemia and increased serum Epo concentrations over a 6-month observation period, whereas animals not receiving the inducer drug had normal hematocrits. Epo secretion could be switched on and off, depending on the presence of doxycycline in the drinking water. In contrast, polyhemia was hardly reversible after subcutaneous injections of mifepristone. These data show that a permanent and regulated systemic delivery of a therapeutic protein can be obtained by the in vivo implantation of engineered allogeneic cells immunoprotected in membrane polymers.

Interacting appetite-regulating pathways in the hypothalamic regulation of body weight

Various aspects of the complex spatio-temporal patterning of hypothalamic signaling that leads to the development of synchronized nocturnal feeding in the rat are critically examined. Undoubtedly, as depicted in Fig. 7, a distinct ARN in the hypothalamus is involved in the control of nocturnal appetite. At least four basic elements operate within this ARN. These are: 1) A discrete appetite-driving or orexigenic network of NPY, NE, GABA, GAL, EOP, and orexin transduces and releases appetite-stimulating signals. 2) Similarly, anorexigenic signal-producing pathways (e.g., CRH, GLP-1, alpha MSH, and CART) orchestrate neural events for dissipation of appetite and to terminate feeding, possibly by interrupting NPY efflux and action at a postsynaptic level within the hypothalamus. It is possible that some of these may represent the physiologically relevant "off" switches under the influence of GABA alone, or AgrP alone, or in combination with NPY released from the NPY-, GABA-, and AgrP-coproducing neurons. 3) Recent evidence shows that neural elements in the VMN-DMN complex tonically restrain the orexigenic signals during the intermeal interval; the restraint is greatly aided by leptin's action via diminution of orexigenic (NPY) and augmentation of anorexigenic (GLP-1, alpha MSH, and CART) signals. Since interruption of neurotransmission in the VMN resulted in hyperphagia and development of leptin resistance, it seems likely that the VMN is an effector site for the restraint exercised by leptin. The daily rhythms in leptin synthesis and release are temporally dissociable because the onset of daily rise in leptin gene expression in adipocytes precedes that in leptin secretion. Nevertheless, these rhythms are in phase with daily ingestive behavior because the peak in circulating leptin levels occurs during the middle of the feeding period. These observations, coupled with the fact that circulating levels of leptin are directly related to adiposity, pose a new challenge for elucidating the precise role of leptin in daily patterning of feeding in the rat. 4) A neural timing mechanism also operates upstream from the ARN in the daily management of energy homeostasis. Although the precise anatomical boundaries are not clearly defined, this device is likely to be composed of a group of neurons that integrate incoming internal and external information for the timely onset of the drive to eat. Evidently, this network operates independently in primates, but it is entrained to the circadian time keeper in the SCN of rodents. Apart from its role in the onset of drive to eat, the circadian patterns of gene expression of NPY, GAL, and POMC denote independent control of the timing device on the synthesis and availability for release of orexigenic signals. The VMN-DMN-PVN complex is apparently an integrated constituent of the timing mechanism in this context, because lesions in each of these sites result in loss of regulated feeding. The accumulated evidence points to the PVN and surrounding neural sites within this framework as the primary sites of release and action of various orexigenic and anorexigenic signals. A novel finding is the identification of the interconnected wiring of the DMN-mPVN axis that may mediate leptin restraint on NPY-induced feeding. The chemical phenotypes of leptin and NPY target neurons in this axis remain to be identified. These multiple orexigenic and anorexigenic pathways in the hypothalamic ARN appear to represent redundancy, a characteristic of regulated biological systems to provide a "fail-safe" neural mechanism to meet an organism's constant energy needs for growth and maintenance. Within this formulation, the coexisting orexigenic signals (NPY, NE, GAL, GABA, and AgrP) represent either another level of redundancy or it is possible that these signals operate within the ARN as reinforcing agents to varying degrees under different circumstances. (ABSTRACT TRUNCATED)

Association of the serum leptin concentration with weight loss in chronic hemodialysis patients

Circulating leptin, which is partly cleared by the kidney, has been reported to increase with chronic renal failure and thus may play a role in the weight loss of patients with chronic renal failure. We investigated the association of body weight loss with the serum leptin concentration in Japanese hemodialysis patients. The relationship between serum leptin and the body mass index (BMI) or body fat mass was compared among 181 patients undergoing hemodialysis and 185 control subjects. There was no difference in the serum leptin concentration between the hemodialysis patients (HD) and controls (C) for either the men (3.9 +/- 0.2 ng/mL for HD, n=117; 3.9 +/- 0.3 ng/mL for C, n=89; NS) or women (8.9 +/- 1.2 ng/mL for HD, n=64; 7.4 +/- 0.5 ng/mL for C, n=96; NS), whereas BMI of the hemodialysis patients was significantly lower than that of the controls for both the men (20.1 +/- 0.2 kg/m2 for HD, 22.4 +/- 0.3 kg/m2 for C, P < 0.001) and women (19.2 +/- 0.3 kg/m2 for HD, 22.0 +/- 0.4 kg/m2 for C, P < 0.001). The serum leptin/body fat mass ratio was significantly correlated with the weight change of the patients during a follow-up evaluation period of 17 months (r = -0.37, P < 0.05 for men, n=27 and r = -0.53, P < 0.005 for women, n=28), indicating the possibility that a relatively high level of serum leptin had induced weight loss in the hemodialysis patients. The serum leptin/body fat mass ratio also showed a significant inverse correlation with the duration of hemodialysis (r = -0.31, P < 0.05 for men and r = -0.49, P < 0.05 for women). A multiple regression analysis indicated that the body fat mass was significantly correlated with serum leptin concentration, whereas the fat distribution did not have any relationship with leptin. These data indicate that a high level of serum leptin relative to the body fat mass might be associated with weight loss in long-term hemodialysis patients. The serum leptin level relative to the body fat mass also seems to have been affected by the duration of hemodialysis.

From preadipocyte to adipocyte: differentiation-directed signals of insulin from the cell surface to the nucleus

An alarming rise in obesity, and the accompanying threat of type 2 diabetes mellitus and cardiovascular disease, have attracted worldwide attention. The pathogenic mechanism(s) underlying obesity remains obscure. However, new cellular and molecular insights about the development of adipose tissue, with respect to adipocyte number (hyperplasia) and size (hypertrophy), are occurring at a rapid pace. Specialized fibroblasts (preadipocytes) committed to the adipocyte lineage are present throughout life. Primary cell culture systems and immortalized cell line models of preadipocytes have advanced the study of adipocyte differentiation (adipogenesis). Differentiation-inducing cues are able to trigger a complex network of intracellular signaling pathways in the preadipocyte, allowing signals from cell-surface receptors to reach nuclear transcription factors that regulate the genetic program of adipocyte differentiation. The extracellular matrix environment of the preadipocyte, known to modulate adipogenesis, may act by altering some of these signaling events.

Leptin inhibits insulin gene transcription and reverses hyperinsulinemia in leptin-deficient ob/ob mice

Leptin controls feeding behavior and insulin secretion from pancreatic beta-cells. Insulin stimulates the production of leptin, thereby establishing an adipoinsular axis. Earlier we identified leptin receptors on pancreatic beta-cells and showed leptin-mediated inhibition of insulin secretion by activation of ATP-sensitive potassium channels. Here we examine transcriptional effects of leptin on the promoter of the rat insulin I gene in rodent beta-cells. A fall in levels of preproinsulin mRNA is detected in vivo in islets of ob/ob mice 24 h after a single injection of leptin, in isolated ob/ob islets treated with leptin in vitro and in the beta-cell line INS-1 on leptin exposure when preproinsulin mRNA expression is stimulated by 25 mM glucose or 10 nM glucagon-like peptide 1. Under these conditions, transcriptional activity of -410 bp of the rat insulin I promoter is inhibited by leptin, whereas transactivation of a 5'-deleted promoter (-307 bp) is not. The -307 sequence contains the known glucose-responsive control elements (E2:A3/4). Constitutive activation of ATP-sensitive potassium channels by diazoxide does not alter leptin inhibition of preproinsulin mRNA levels. Distinct protein-DNA complexes appear on the rat insulin I promoter sequences located between -307 and -410 with nuclear extracts from ob/ob islets in response to leptin, including a signal transducer and activator of transcription (STAT)5b binding site. These results indicate that leptin inhibits transcription of the preproinsulin gene by altering transcription factor binding to sequences upstream from the elements (307 bp) that confer glucose responsivity to the rat insulin I gene promoter. Thus leptin exerts inhibitory effects on both insulin secretion and insulin gene expression in pancreatic beta-cells, but by different cellular mechanisms.

Disruption in neuropeptide Y and leptin signaling in obese ventromedial hypothalamic-lesioned rats

Electrolytic lesions placed in the ventromedial hypothalamus (VMH) of rats induce instant hyperphagia and excessive weight gain. Since neuropeptide Y (NPY) is a potent hypothalamic orexigenic signal, and leptin secreted by adipocytes regulates NPY output, we tested the hypothesis that altered NPYergic-leptin signaling may underlie hyperphagia in VMH-lesioned rats. VMH-lesioned rats exhibiting hyperphagia and excessive weight gain in a time-related fashion were sacrificed on days 2, 7, and 21 post-surgery. Quite unexpectedly, NPY concentrations in the hypothalamic paraventricular nucleus (PVN), a major site of NPY release for stimulation of feeding, and in other sites, such as the dorsomedial nucleus, lateral hypothalamic area and median eminence-arcuate nucleus decreased, with the earliest diminution occurring on day 2 in the PVN only. In vitro basal and K+-evoked NPY release from the PVN of VMH-lesioned rats was significantly lower than that of controls. Analysis of hypothalamic NPY gene expression showed that although the daily decrease in NPY mRNA from 0800 to 2200 h occurred as in control rats, NPY mRNA concentrations were markedly reduced at these times in the hypothalami of VMH-lesioned rats. Leptin synthesis in adipocytes as indicated by leptin mRNA levels was also profoundly altered in VMH-lesioned rats. The daily pattern of increase in adipocyte leptin mRNA at 2200 h from 0800 h seen in controls was abolished, higher levels of leptin gene expression at 2200 h were maintained at 0800 h. The pattern of increase in serum leptin and insulin levels diverged in VMH-lesioned rats. Serum insulin concentration increased to maximal on day 2 and remained at that level on day 21-post-lesion; serum leptin levels on the other hand, increased slowly in a time-related fashion during this period. These results demonstrate that hyperphagia and excessive weight gain in VMH-lesioned rats are associated with an overall decrease in hypothalamic NPY and augmented leptin signaling to the hypothalamus. The divergent time course of increases in serum leptin and insulin levels suggest independent mechanisms responsible for their augmented secretion, and neither these hormones nor VMH lesions altered the daily rhythm in NPY gene expression. These observations underscore the existence of an independent mechanism controlling the daily rhythm in hypothalamic NPY gene expression and suggest that leptin feedback action requires an intact VMH.

Malonyl-CoA, fuel sensing, and insulin resistance

Malonyl-CoA is an allosteric inhibitor of carnitine palmitoyltransferase (CPT) I, the enzyme that controls the transfer of long-chain fatty acyl (LCFA)-CoAs into the mitochondria where they are oxidized. In rat skeletal muscle, the formation of malonyl-CoA is regulated acutely (in minutes) by changes in the activity of the beta-isoform of acetyl-CoA carboxylase (ACCbeta). This can occur by at least two mechanisms: one involving cytosolic citrate, an allosteric activator of ACCbeta and a precursor of its substrate cytosolic acetyl-CoA, and the other involving changes in ACCbeta phosphorylation. Increases in cytosolic citrate leading to an increase in the concentration of malonyl-CoA occur when muscle is presented with insulin and glucose, or when it is made inactive by denervation, in keeping with a diminished need for fatty acid oxidation in these situations. Conversely, during exercise, when the need of the muscle cell for fatty acid oxidation is increased, decreases in the ATP/AMP and/or creatine phosphate-to-creatine ratios activate an isoform of an AMP-activated protein kinase (AMPK), which phosphorylates ACCbeta and inhibits both its basal activity and activation by citrate. The central role of cytosolic citrate links this malonyl-CoA regulatory mechanism to the glucose-fatty acid cycle concept of Randle et al. (P. J. Randle, P. B. Garland. C. N. Hales, and E. A. Newsholme. Lancet 1: 785-789, 1963) and to a mechanism by which glucose might autoregulate its own use. A similar citrate-mediated malonyl-CoA regulatory mechanism appears to exist in other tissues, including the pancreatic beta-cell, the heart, and probably the central nervous system. It is our hypothesis that by altering the cytosolic concentrations of LCFA-CoA and diacylglycerol, and secondarily the activity of one or more protein kinase C isoforms, changes in malonyl-CoA provide a link between fuel metabolism and signal transduction in these cells. It is also our hypothesis that dysregulation of the malonyl-CoA regulatory mechanism, if it leads to sustained increases in the concentrations of malonyl-CoA and cytosolic LCFA-CoA, could play a key role in the pathogenesis of insulin resistance in muscle. That it may contribute to abnormalities associated with the insulin resistance syndrome in other tissues and the development of obesity has also been suggested. Studies are clearly needed to test these hypotheses and to explore the notion that exercise and some pharmacological agents that increase insulin sensitivity act via effects on malonyl-CoA and/or cytosolic LCFA-CoA.

Metabolic responses to intracerebroventricular leptin and restricted feeding

Leptin is a hormone secreted by adipocytes, which plays an important role in the control of food intake and metabolic processes. In the current study, a dose-dependent relationship was shown between a bolus intracerebroventricular rat recombinant leptin administration and reductions in food intake and body weight in Sprague-Dawley rats. During the 24 h postinjection period, food intake was decreased by 24, 26, and 52% with 0.625, 2.5, and 10 microg of leptin, respectively. Body weight was reduced by 2, 3, and 5% at 24 h after leptin administration at the doses of 0.156, 2.5, and 10 microg, respectively. Furthermore, indirect calorimetry demonstrated that five daily i.c.v. injections of leptin resulted in an increase in heat production per unit of metabolic body size and fat oxidation by approximately 10 and 48%, respectively. In contrast, food-restricted rats that consumed the equivalent amount of food as leptin-treated rats for 5 days decreased their energy expenditure by 10%. Food restriction was found to decrease respiratory quotient in a similar pattern as the leptin administration. When ad lib feeding was resumed, food-restricted rats quickly recovered their normal food intakes, body weights, and metabolism. Conversely, leptin treatment has prolonged effects on body weight resulting from different metabolic responses than food restriction. Leptin not only suppresses food intake, but also enhances energy expenditure to reduce fat depots.

Acute effect of leptin on hepatic glycogenolysis and gluconeogenesis in perfused rat liver

Leptin circulates in blood and is involved in body weight control primarily via hypothalamic receptors. To examine its direct metabolic action, effects of short-term portal leptin infusion: 1) on postprandial basal and epinephrine-stimulated glycogenolysis; and 2) on postabsorptive lactate-stimulated gluconeogenesis were studied in isolated perfused rat livers. Incremental epinephrine (150 pmol x min-1 x g-1 liver)-stimulated glucose release (in micromol/g liver within 30 minutes; control: 28.3 +/- 2.8) was suppressed (P <.05) by 44% (15.8 +/- 1.6), by 48% (14.6 +/- 4.1), and by 53% (13.3 +/- 2.1) during insulin (3 pmol x min-1 x g-1 liver), leptin (30 pmol x min-1 x g-1 liver), and simultaneous leptin + insulin infusion. Perfusate cyclic adenosine monophosphate increased approximately twofold during epinephrine stimulation in all groups. Neither leptin nor insulin affected hepatic lactate production, bile flow, or portal pressure in the fed state. In the postabsorptive state (20-hour fasting), rates of lactate (10 mmol/L)-dependent hepatic glucose release (in micromol. min-1 x g-1 liver; control: 0.12 +/- 0.01) were increased (P <.01) to 0.35 +/- 0.02 and to 0.24 +/- 0.01 by glucagon (3 pmol x min-1 x g-1 liver) and by leptin (15 pmol x min-1 x g-1 liver), respectively. In parallel, lactate uptake rates (in micromol x min-1 x g-1 liver) were higher in the presence of both glucagon (0.90 +/- 0. 03) and leptin (0.84 +/- 0.02) compared with control (0.68 +/- 0.04). In conclusion, leptin modulates hepatic glucose fluxes and may contribute to direct humoral regulation of liver glycogen stores in the fasted as well as in the fed state.

Lipoapoptosis in beta-cells of obese prediabetic fa/fa rats. Role of serine palmitoyltransferase overexpression

We reported that the lipoapoptosis of beta-cells observed in fat-laden islets of obese fa/fa Zucker Diabetic Fatty (ZDF) rats results from overproduction of ceramide, an initiator of the apoptotic cascade and is induced by long-chain fatty acids (FA). Whereas the ceramide of cytokine-induced apoptosis may be derived from sphingomyelin hydrolysis, FA-induced ceramide overproduction seems to be derived from FA. We therefore semiquantified mRNA of serine palmitoyltransferase (SPT), which catalyzes the first step in ceramide synthesis. It was 2-3-fold higher in fa/fa islets than in +/+ controls. [3H]Ceramide formation from [3H]serine was 2.2-4. 5-fold higher in fa/fa islets. Triacsin-C, which blocks palmitoyl-CoA synthesis, and L-cycloserine, which blocks SPT activity, completely blocked [3H]ceramide formation from [3H]serine. Islets of fa/fa rats are unresponsive to the lipopenic action of leptin, which normally depletes fat and prevents FA up-regulation of SPT. To determine the role of leptin unresponsiveness in the SPT overexpression, we transferred wild type OB-Rb cDNA to their islets; now leptin completely blocked the exaggerated FA-induced increase of SPT mRNA while reducing the fat content. Beta-cell lipoapoptosis was partially prevented in vivo by treating prediabetic ZDF rats with L-cycloserine for 2 weeks. Ceramide content and DNA fragmentation both declined 40-50%. We conclude that lipoapoptosis of ZDF rats is mediated by enhanced ceramide synthesis from FA and that blockade by SPT inhibitors prevents lipoapoptosis.

Lipoprotein lipase and the disposition of dietary fatty acids

Lipoprotein lipase (EC 3.1.1.34; LPL) is a key enzyme regulating the disposal of lipid fuels in the body. It is expressed in a number of peripheral tissues including adipose tissue, skeletal and cardiac muscle and mammary gland. Its role is to hydrolyse triacylglycerol (TG) circulating in the TG-rich lipoprotein particles in order to deliver fatty acids to the tissue. It appears to act preferentially on chylomicron-TG, and therefore may play a particularly important role in regulating the disposition of dietary fatty acids. LPL activity is regulated according to nutritional state in a tissue-specific manner according to the needs of the tissue for fatty acids. For instance, it is highly active in lactating mammary gland; in white adipose tissue it is activated in the fed state and suppressed during fasting, whereas the reverse is true in muscle. Such observations have led to the view of LPL as a metabolic gatekeeper, especially for dietary fatty acids. However, closer inspection of its action in white adipose tissue reveals that this picture is only partially true. Normal fat deposition in adipose tissue can occur in the complete absence of LPL, and conversely, if LPL activity is increased by pharmacological means, increased fat storage does not necessarily follow. LPL appears to act as one member of a series of metabolic steps which are regulated in a highly coordinated manner. In white adipose tissue, it is clear that there is a major locus of control of fatty acid disposition downstream from LPL. This involves regulation of the pathway of fatty acid uptake and esterification, and appears to be regulated by a number of factors including insulin, acylation-stimulating protein and possibly leptin.

Leptin expression in porcine adipose tissue is not increased by endotoxin but is reduced by growth hormone

The physiologic response to infection includes reductions in tissue concentrations of anabolic growth factors as a means of reducing growth and conserving nutrients for immunologic processes. This repartitioning of nutrients is accompanied by anorexia, which has been linked to increased leptin expression. Furthermore, leptin and growth hormone (GH) concentrations are inversely related, with leptin being required for normal GH release. The objective of this study was to determine if pretreatment with GH would influence endotoxin-induced changes in leptin expression or attenuate endotoxin-induced reductions in serum insulin-like growth factor-1 (IGF-1) and IGF-1 expression in liver and longissimus muscle. In experiment 1, 40 pigs were assigned to four treatments (n = 10 per treatment) arranged as a 2x2 factorial with GH (s.c. injection, 2 mg 1 h before challenge and 2 mg 2 h after challenge) and endotoxin (single i.m. injection, 25 microg/kg body weight) as main effect variables. Pretreatment with GH resulted in a marked increase (p<0.001) in serum GH within 1 h that was sustained throughout the study. Endotoxin challenge reduced (p<0.003) serum IGF-1 independent of GH (GH x endotoxin, p>0.682), and reduced (p<0.05) IGF-1 expression in longissimus muscle but not liver. Leptin mRNA abundance was reduced 56% (p<0.005) by GH but was not affected by endotoxin (p>0.81). In experiment 2, 36 pigs (n = 12 per treatment) were either allowed ad libitum feed consumption with no injection or deprived of feed and injected twice with either saline or endotoxin 24 h apart. Feed deprivation reduced leptin expression (p<0.05). However, endotoxin did not change leptin expression but markedly increased (p<0.05) serum haptoglobin. These data indicate that changes in IGF-1 status in endotoxin-challenged pigs are independent of serum GH and that leptin expression is not increased by endotoxin challenge in the pig. These data also indicate a regulatory linkage between GH and leptin in vivo.

The obese gene product, leptin: possible role in obesity-related hypertension in adolescents

OBJECTIVE

The mechanism of the association between obesity and hypertension is not clear. The recently discovered obese gene product, leptin, the levels of which increase in obese subjects, has been shown to reduce food intake and increase sympathetic nervous system activity in animal studies. The present study was undertaken to elucidate the relationship between blood pressure and factors related to obesity, including leptin, in different age groups.

METHODS

The subjects were 348 Japanese male adolescents (15-17 years old) and 165 men (40-59 years old) not taking medication for hypertension, diabetes mellitus or hyperlipidaemia. Height, weight, blood pressure, heart rate, plasma glucose, lipid profiles, serum insulin and leptin levels were measured in the morning after an overnight fast

RESULTS

Body mass index (BMI), serum leptin level and the homeostasis model insulin resistance index increased in the order of blood-pressure category (i.e. normotensive < high normal < hypertensive) in both the male adolescents and the middle-aged men. In addition, simple linear regressions revealed that both systolic and diastolic blood pressure correlated significantly with serum leptin and the insulin resistance index in both groups. Even after adjustment for age and BMI, the correlation of mean blood pressure with leptin remained in the obese adolescents (r2 = 0.390, P = 0.02). The heart rate also correlated with leptin in the adolescents (r = 0.18, P< 0.001), but not in the middle-aged subjects (r = 0.04). Even after adjustment for age and BMI in adolescents, serum leptin correlated significantly with heart rate.

CONCLUSION

These results suggest a role for leptin in obesity-related hypertension, especially in adolescents.

Perturbation of fuel homeostasis caused by overexpression of the glucose-6-phosphatase catalytic subunit in liver of normal rats

The terminal step in hepatic gluconeogenesis is catalyzed by glucose-6-phosphatase, an enzyme activity residing in the endoplasmic reticulum and consisting of a catalytic subunit (glucose-6-phosphatase (G6Pase)) and putative accessory transport proteins. We show that Zucker diabetic fatty rats (fa/fa), which are known to exhibit impaired suppression of hepatic glucose output, have 2.4-fold more glucose-6-phosphatase activity in liver than lean controls. To define the potential contribution of increased hepatic G6Pase to development of diabetes, we infused recombinant adenoviruses containing the G6Pase cDNA (AdCMV-G6Pase) or the beta-galactosidase gene into normal rats. Animals were studied by one of three protocols as follows: protocol 1, fed ad libitum for 7 days; protocol 2, fed ad libitum for 5 days, fasted overnight, and subjected to an oral glucose tolerance test; protocol 3, fed ad libitum for 4 days, fasted for 48 h, subjected to oral glucose tolerance test, and then allowed to refeed overnight. Hepatic glucose-6-phosphatase enzymatic activity was increased by 1.6-3-fold in microsomes isolated from AdCMV-G6Pase-treated animals in all three protocols, and the resultant metabolic profile was similar in each case. AdCMV-G6Pase-treated animals exhibited several of the abnormalities associated with early stage non-insulin-dependent diabetes mellitus, including glucose intolerance, hyperinsulinemia, decreased hepatic glycogen content, and increased peripheral (muscle) triglyceride stores. These animals also exhibited significant decreases in circulating free fatty acids and triglycerides, changes not normally associated with the disease. Our studies show that overexpression of G6Pase in liver is sufficient to perturb whole animal glucose and lipid homeostasis, possibly contributing to the development of metabolic abnormalities associated with diabetes.

Intracerebroventricular leptin regulates hepatic but not peripheral glucose fluxes

Acute intravenous infusions of leptin markedly alter hepatic glucose fluxes (Rossetti, L., Massillon, D., Barzilai, N., Vuguin, P., Chen, W., Hawkins, M., Wu, J., and Wang, J. (1997) J. Biol. Chem. 272, 27758-22763). Here we examine whether intracerebroventricular (ICV) leptin administration regulates peripheral and hepatic insulin action. Recombinant mouse leptin (n = 14; 0.02 or 1 microgram/kg.h) or vehicle (n = 9) were administered ICV for 6 h to conscious rats, and insulin action was determined by insulin (3 milliunits/kg.min) clamp and tracer dilution techniques. During physiologic hyperinsulinemia (approximately 65 microunits/ml), the rates of glucose uptake (Rd, 20.1 +/- 0.6 and 23.1 +/- 0.7 versus 21.7 +/- 0.6 mg/kg.min; p = NS), glycolysis and glycogen synthesis were similar in rats receiving low- and high-dose leptin versus vehicle. ICV leptin resulted in a 2-3-fold increase in hepatic phosphoenolpyruvate carboxykinase mRNA levels. Glycogenolysis and PEP-gluconeogenesis (2.1 +/- 0.3 mg/kg. min) contributed similarly to endogenous glucose production (GP) in the vehicle-infused group. However, gluconeogenesis accounted for approximately 80% of GP in both groups receiving ICV leptin, while hepatic glycogenolysis was markedly suppressed (0.7 +/- 0.3 and 1.2 +/- 0.3 versus 2.2 +/- 0.4 mg/kg.min, in rats receiving low- and high-dose leptin versus vehicle, respectively; p < 0.01). In summary, short-term ICV leptin administration: 1) failed to affect peripheral insulin action, but 2) induced a striking re-distribution of intrahepatic glucose fluxes. The latter effect largely reproduced that of leptin given systemically at much higher doses. Thus, the regulation of hepatic glucose fluxes by leptin is largely mediated via its central receptors.

Down-regulation of CCAAT/enhancer binding proteins alpha, beta and delta in adipose tissue by intracerebroventricular leptin in rats

In our previous report, intracerebroventricular (i.c.v.) administration of leptin caused fat depletion by an induced adipocyte apoptosis in addition to influencing lipid metabolism. To uncover the biochemical mechanisms that mediate this response, the present study was designed to determine whether CCAAT/enhancer binding proteins (C/EBP)alpha, -beta and -delta play a role in the leptin-induced fat depletion. Expressions of C/EBPalpha, -beta and -delta in epididymal fat tissues were examined by Western immunoblot and in situ immunocytochemical analysis after 5 days of i.c.v. treatment. Young and old rats (3 and 8 months old) were treated with or without 5 micrograms/day leptin. The expression of C/EBPalpha, -beta and -delta was decreased by i.c.v. leptin treatment in young rats as compared with controls (P<0.05). However, leptin did not influence the expression of C/EBPalpha, -beta and -delta in adipose tissues of 8-month-old rats. The basal level of expression of C/EBPbeta was greater in 8-month-old rats than in 3-month-old rats, (P<0.05) whereas the basal expression of C/EBPalpha and -delta was not different between age groups. These results were confirmed by in situ immunocytochemical analysis. The present study suggests that leptin-induced down-regulation of C/EBPalpha, -beta and -delta might influence adipocyte differentiation and growth in a number of ways.

Serum leptin concentration and insulin sensitivity in men with abdominal obesity

OBJECTIVE

We have examined the association between generalized adiposity, abdominal adiposity, insulin sensitivity, and serum levels of leptin in a cross-sectional study of abdominally obese men.

RESEARCH METHODS AND PROCEDURES

Thirty men, 48 to 66 years of age with a body mass index (BMI) of between 25 kg/m2 and 35 kg/m2 and a waist hip ratio of >0.95, were included in the study. Serum leptin concentration was measured using radioimmunoassay. Total body fat percentage was determined from total body potassium, abdominal adiposity was measured by computed tomography, and the glucose disposal rate (GDR) was measured during an euglycemic, hyperinsulinemic glucose clamp.

RESULTS

Significant correlations were found between serum leptin concentration and BMI, percentage body fat, abdominal subcutaneous adipose tissue, serum insulin, GDR, and 24-hour urinary-free cortisol. In a multiple regression analysis, it was shown that abdominal subcutaneous adipose tissue, GDR, and BMI explained 72% of the variability of serum leptin concentration. GDR demonstrated an independent inverse correlation with serum leptin concentration.

DISCUSSION

In abdominally obese men with insulin resistance, it was demonstrated that most of the individual variability in serum leptin concentration was explained by the amount of subcutaneous abdominal adipose tissue, insulin sensitivity, and BMI.

Leptin and its receptors: regulators of whole-body energy homeostasis

Leptin is the adipocyte-specific product of the ob gene. Expression of leptin in fully fed animals reflects adipocyte size and body-fat mass. Leptin signals the status of body energy stores to the brain, where signals emanate to regulate food intake and whole-body energy expenditure. The leptin gene was identified in the leptin-deficient, obese ob/ob mouse by positional cloning techniques. Recently, leptin has been cloned in domestic species including pigs, cattle, and chickens. The leptin receptor has at least five splice variants; the long form of the receptor is primarily expressed in the hypothalamus and is thought to be the predominant signaling isoform. Leptin receptors are members of the cytokine family of receptors and signal via janus-activated kinases (JAK)/signal transducers and activators of transcription (STAT) and mitogen-activated protein kinase (MAPK) pathways. Mutations in the leptin or leptin receptor genes results in morbid obesity, infertility, and insulin resistance in rodents and humans. Leptin regulates food intake and energy expenditure via central and peripheral mechanisms. Leptin receptors are expressed in most tissues, and in vitro evidence suggests that leptin may have direct effects on some tissues such as adipose tissue, the adrenal cortex, and the pancreatic beta-cell. Leptin is thought to influence whole-body glucose homeostasis and insulin action. Studies are underway to determine the role that leptin plays in the biology of domestic animals.

Leptin and the regulation of body weight in mammals

The assimilation, storage and use of energy from nutrients constitute a homeostatic system that is essential for life. In vertebrates, the ability to store sufficient quantities of energy-dense triglyceride in adipose tissue allows survival during the frequent periods of food deprivation encountered during evolution. However, the presence of excess adipose tissue can be maladaptive. A complex physiological system has evolved to regulate fuel stores and energy balance at an optimum level. Leptin, a hormone secreted by adipose tissue, and its receptor are integral components of this system. Leptin also signals nutritional status to several other physiological systems and modulates their function. Here we review the role of leptin in the control of body weight and its relevance to the pathogenesis of obesity.

Overexpression of protein targeting to glycogen (PTG) in rat hepatocytes causes profound activation of glycogen synthesis independent of normal hormone- and substrate-mediated regulatory mechanisms

Protein targeting to glycogen (PTG), also known as PPP1R5, is a widely expressed member of a growing family of proteins that target protein phosphatase-1 (PP-1) to glycogen particles. Because PTG also binds to glycogen synthase and phosphorylase kinase, it has been suggested that it serves as a "scaffold" for efficient activation of glycogen synthesis. However, very little is known about the metabolic effects of PTG. In this study, we have used recombinant adenovirus to overexpress PTG in primary rat hepatocytes, a cell type with high glycogenic capacity. We find that overexpression of PTG potently activates glycogen synthesis in cultured hepatocytes. Surprisingly, the glycogenic effect of PTG is observed even in the complete absence of carbohydrates or insulin in the culture medium. Furthermore, glycogenolytic agents such as forskolin or glucagon are largely ineffective at activating glycogen degradation in PTG overexpressing hepatocytes, even though large increases in cAMP levels are demonstrated. These metabolic effects of PTG overexpression are accompanied by a 3.6-fold increase in glycogen synthase activation state and a 40% decrease in glycogen phosphorylase activity. Our results are consistent with a model in which PTG overexpression "locks" the hepatocyte in a glycogenic mode, presumably via its ability to promote interaction of enzymes of glycogen metabolism with PP-1.

Leptin is involved in gender-related differences in insulin sensitivity

BACKGROUND

Although the effects of insulin on leptin levels are relatively well characterized, the possible actions of leptin on insulin sensitivity are not so well studied. This study was undertaken to examine whether gender-related differences in insulin sensitivity could be explained partially by leptin levels.

SUBJECTS

The study involved 22 women (13 obese) and 20 (11 obese) fat mass- and age-matched men. All participants were healthy.

MEASUREMENTS

Several anthropometric measures of body fatness were quantified and the percentage of body fat was determined through bioelectric impedance. Oral glucose tolerance test and a frequently sampled intravenous glucose tolerance test was performed in all subjects. Serum leptin was measured by radioimmunoassay.

RESULTS

Nine lean women (BMI 20.4 +/- 2 kg m2 mean +/- SD) showed increased leptin levels (7.8 +/- 2.7 vs. 4.3 +/- 1.3 micrograms/l, P = 0.003), increased insulin sensitivity (5.2 +/- 1.3 vs. 2.9 +/- 0.9 min-1/mU/l, P = 0.001) and similar fat mass (11.1 +/- 3.7 vs. 13.2 +/- 7.8 kg, P = NS) in comparison with 9 age-matched lean men (33.6 +/- 6 vs. 34.5 +/- 6.3 years, P = NS). Thirteen obese women (BMI 32.5 +/- 2.7) kg m2 also showed increased leptin levels (29.6 +/- 8.4 vs. 11.7 +/- 4.8 micrograms/l, P < 0.0001), increased insulin sensitivity (1.7 +/- 0.7 vs. 0.95 +/- 0.9 min-1 mU/l, P = 0.04) and similar fat mass (34.4 +/- 8.0 vs. 30.9 +/- 9.6 kg, P = NS) in comparison with 11 age-matched obese men (34.5 +/- 7.8 vs. 38.7 +/- 8.2 years, P = NS). A strong linear association between leptin levels and insulin sensitivity (Si) was found (r = -0.67, P = 0.001, in men; r = -0.82, P < 0.0001, in women). After controlling for percentage of body fat, this association remained significant only in men (r = -0.56, P = 0.01, in men; r = -0.30, P = NS in women). In stepwise regression analysis models, both gender (P = 0.00001) and leptin (P = 0.00001) contributed to 67% of the variance in Si independently of body fat.

CONCLUSIONS

Leptin levels and gender contribute to the variance of insulin sensitivity, independently of body fat. These results suggest that leptin could affect insulin sensitivity.

Efficacy of exogenous recombinant murine leptin in lean and obese 10- to 12-mo-old female CD-1 mice

Leptin efficacy was compared in obese and lean female CD-1 mice. Body weights in these 10- to 12-mo-old mice ranged from 29.7 to 62.0 g, and leptin levels correlated with body weight. Mice from the lean and obese ends of the weight distribution were treated with daily peripheral leptin injections (1-100 mg/kg) for a 33-day period. The half-maximal effective doses for weight loss and fat reduction were shifted 0.5-0.7 log to the right for obese mice. Leptin was less efficacious at low doses (1-3 mg/kg) in obese mice but equal to or more efficacious in obese than lean mice at high doses (30-100 mg/kg). Leptin's initial effects on weight loss could be explained by appetite suppression in both groups, but its effects on fat reduction were greater in leptin-treated than pair-fed mice, particularly in the lean group. Leptin also prevented the elevations in serum corticosterone and ketones found in pair-fed lean mice. These data allow a quantitative comparison of leptin sensitivity in obese vs. lean CD-1 mice and suggest that in mice where obesity is a function of outbreeding and age, leptin sensitivity is moderately reduced. Furthermore, although appetite suppression has a clear role in leptin's effects on body weight, leptin may also have specific effects on lipid metabolism and mobilization that are different from the metabolic compensations that normally occur with food deprivation.

Overexpression of leptin receptors in pancreatic islets of Zucker diabetic fatty rats restores GLUT-2, glucokinase, and glucose-stimulated insulin secretion

The high-Km glucose transporter, GLUT-2, and the high-Km hexokinase of beta cells, glucokinase (GK), are required for glucose-stimulated insulin secretion (GSIS). GLUT-2 expression in beta cells of Zucker diabetic fatty (ZDF) rats is profoundly reduced at the onset of beta-cell dysfunction of diabetes. Because ZDF rats are homozygous for a mutation in their leptin receptor (OB-R) gene and are therefore leptin-insensitive, we expressed the wild-type OB-R gene in diabetic islets by infusing a recombinant adenovirus (AdCMV-OB-Rb) to determine whether this reversed the abnormalities. Leptin induced a rise in phosphorylated STAT3, indicating that the transferred wild-type OB-R was functional. GLUT-2 protein rose 17-fold in AdCMV-OB-Rb-treated ZDF islets without leptin, and leptin caused no further rise. GK protein rose 7-fold without and 12-fold with leptin. Preproinsulin mRNA increased 64% without leptin and rose no further with leptin, but leptin was required to restore GSIS. Clofibrate and 9-cis-retinoic acid, the partner ligands for binding to peroxisome proliferator-activator receptor alpha (PPARalpha) and retinoid X receptor, up-regulated GLUT-2 expression in islets of normal rats, but not in ZDF rats, in which PPARalpha is very low. Because the fat content of islets of diabetic ZDF rats remains high unless they are treated with leptin, it appears that restoration of GSIS requires normalization of intracellular nutrient homeostasis, whereas up-regulation of GLUT-2 and GK is leptin-independent, requiring only high expression of OB-Rb.

Biological action of leptin as an angiogenic factor

Leptin is a hormone that regulates food intake, and its receptor (OB-Rb) is expressed primarily in the hypothalamus. Here, it is shown that OB-Rb is also expressed in human vasculature and in primary cultures of human endothelial cells. In vitro and in vivo assays revealed that leptin has angiogenic activity. In vivo, leptin induced neovascularization in corneas from normal rats but not in corneas from fa/fa Zucker rats, which lack functional leptin receptors. These observations indicate that the vascular endothelium is a target for leptin and suggest a physiological mechanism whereby leptin-induced angiogenesis may facilitate increased energy expenditure.

Functional properties of leptin receptor isoforms containing the gln-->pro extracellular domain mutation of the fatty rat

Mutations of the leptin receptor have been found to cause obesity in rodents. The fa mutation that is responsible for obesity in Zucker rats is a missense mutation (269 gln-->pro) in the extracellular domain of the leptin receptor. We have characterized the effects of this mutation on the two major isoforms of the leptin receptor, Ob-Rb and Ob-Ra, by studying cell-surface expression, leptin binding affinity, signaling capacity, and receptor-mediated internalization and degradation of leptin in transfected mammalian cell lines. Both Ob-Rb(269 gln-->pro) and Ob-Ra(269 gln-->pro) have decreased cell-surface expression and decreased leptin binding affinity. Ob-Rb(269 gln-->pro) was shown to have defective signaling to the JAK-STAT pathway and markedly diminished ability to activate transcription of the egr-1 promoter. Constitutive ligand-independent activation of Ob-Rb(269 gln-->pro) was observed for activation of egr-1-luc but only under conditions when JAK2 was coexpressed with Ob-Rb(269 gln-->pro), Finally, Ob-Ra(269 gln-->pro) has an increased ability to internalize leptin but is less efficient at degrading leptin, as compared with Ob-Ra. In conclusion, both Ob-Ra(269 gln-->pro) and Ob-Rb(269 gln-->pro) have multiple functional defects.

Monkey leptin receptor mRNA: sequence, tissue distribution, and mRNA expression in the adipose tissue of normal, hyperinsulinemic, and type 2 diabetic rhesus monkeys

OBJECTIVE

We have cloned the rhesus monkey leptin receptor and examined its mRNA expression levels in the adipose tissue of monkeys to investigate the regulation of gene expression of the leptin receptor.

RESEARCH METHODS AND PROCEDURES

Monkey leptin receptor cDNA was cloned by reverse transcriptase-polymerase chain reaction (RT-PCR). Tissue distribution of monkey leptin receptor was examined by Northern blot analysis and RT-PCR. The mRNA levels of monkey leptin receptor in adipose tissue of normal (n=10), hyperinsulinemic obese (n=8), and type 2 diabetic monkeys (n=8) were measured by quantitative RT-PCR.

RESULTS

Monkey leptin receptor cDNA had at least two alternatively spliced isoforms (long and short forms). The long form of the leptin receptor mRNA was expressed relatively highly in liver, adipose tissue, hypothalamus, and choroid plexus, whereas the total leptin receptors were expressed in every tissue examined. The mRNA levels of the long form of the leptin receptor in adipose tissue were not correlated to body weight, fasting plasma insulin, plasma glucose, or plasma leptin levels. The mRNA levels of the long form of the leptin receptor were highly correlated to that of the total leptin receptor (long and short form).

DISCUSSION

The long form of leptin receptor mRNA existed in adipose tissue as well as in liver and hypothalamus, suggesting that the leptin receptor in adipose tissue may be functional in adipose tissue. The expression of the leptin receptor mRNA in adipose tissue is not affected by obesity, hyperinsulinemia, or diabetes.

Leptin is a potent stimulator of spontaneous pulsatile growth hormone (GH) secretion and the GH response to GH-releasing hormone

Pulsatile GH secretion is exquisitely sensitive to perturbations in nutritional status, but the underlying mechanisms are largely unknown. Leptin, a recently discovered adipose cell hormone, is thought to be a sensor of energy stores and to regulate body mass, appetite, and metabolism at the level of the brain. Receptors for leptin are abundantly expressed in hypothalamic nuclei known to be involved in GH regulation, suggesting that leptin may serve as an important hormonal signal to the GH neuroendocrine axis in normal animals. To test this hypothesis, we examined the effects of intracerebroventricular infusion of recombinant murine leptin, at a dose of 1.2 microg/day for 7 days, on both spontaneous and GH-releasing hormone (GHRH)-stimulated GH secretion in free-moving adult male rats. Concomitant with suppressive effects on food intake, body weight, and basal plasma insulin-like growth factor I, insulin, and glucose concentrations, central infusion of leptin resulted in a 2- to 3-fold augmentation of GH pulse amplitude, 5-fold higher GH nadir levels, and a 2- to 3-fold increase in the integrated area under the 6-h GH response curve compared with those in vehicle-infused controls (P < 0.001). The intracerebroventricular infusion of leptin also produced a 3- to 4-fold increase in GHRH-induced GH release at GH trough times (P < 0.01). These studies demonstrate a potent stimulatory action of leptin on both spontaneous pulsatile GH secretion and the GH response to GHRH. The results suggest that the GH-releasing activity of leptin is mediated, at least in part, by an inhibition of hypothalamic somatostatin release. Thus, leptin may be a critical hormonal signal of nutritional status in the neuroendocrine regulation of pulsatile GH secretion.

Resistance to adenovirally induced hyperleptinemia in rats. Comparison of ventromedial hypothalamic lesions and mutated leptin receptors

Leptin regulates appetite and body weight via hypothalamic targets, but it can act directly on cultured pancreatic islets to regulate their fat metabolism. To obtain in vivo evidence that leptin may act peripherally as well as centrally, we compared the effect of adenovirally induced hyperleptinemia on food intake, body weight, and islet fat content in ventromedial hypothalamic-lesioned (VMHL) rats, sham-lesioned (SL) controls, and Zucker Diabetic Fatty (ZDF) rats in which the leptin receptor is mutated. Infusion with recombinant adenovirus containing the rat leptin cDNA increased plasma leptin by approximately 20 ng/ml in VMHL and ZDF rats but had no effect on their food intake, body weight, or fat tissue weight. Caloric matching of hyperphagic VMHL rats to SL controls did not reduce their resistance to hyperleptinemia. Whereas prediabetic ZDF rats had a fourfold elevation in islet fat, in VMHL rats islet fat was normal and none of them became diabetic. Isolated islets from ZDF rats were completely resistant to the lipopenic action of leptin, while VMHL islets exhibited 50% of the normal response; caloric matching of VMHL rats to SL controls increased leptin responsiveness of their islets to 92% of controls. We conclude that leptin regulation of adipocyte fat requires an intact VMH but that islet fat content is regulated independently of the VMH.

Protection against lipoapoptosis of beta cells through leptin-dependent maintenance of Bcl-2 expression

Obesity causes its complications through functional and morphologic damage to remotely situated tissues via undetermined mechanisms. In one rodent model of obesity, the Zucker diabetic fatty fa/fa rat, overaccumulation of triglycerides in the pancreatic islets may be responsible for a gradual depletion of beta cells, leading to the most common complication of obesity, non-insulin-dependent diabetes mellitus. At the onset of non-insulin-dependent diabetes mellitus, the islets from fa/fa rats contain up to 100 times the fat content of islets from normal lean rats. Ultimately, about 75% of the beta cells disappear from these fat-laden islets as a consequence of apoptosis induced by long-chain fatty acids (FA). Here we quantify Bcl-2, the anti-apoptosis factor in these islets, and find that Bcl-2 mRNA and protein are, respectively, 85% and 70% below controls. In normal islets cultured in 1 mM FA, Bcl-2 mRNA declined by 68% and completely disappeared in fa/fa islets cultured in FA. In both groups, suppression was completely blocked by the fatty acyl-CoA synthetase inhibitor, triacsin C, evidence of its mediation by fatty acyl-CoA. To determine whether leptin action blocked FA-induced apoptosis, we cultured normal and fa/fa islets in 1 mM FA with or without leptin. Leptin completely blocked FA-induced Bcl-2 suppression in normal islets but had no effect on islets from fa/fa rats, which are unresponsive to leptin because of a mutation in their leptin receptors (OB-R). However, when wild-type OB-R is overexpressed in fa/fa islets, leptin completely prevented FA-induced Bcl-2 suppression and DNA fragmentation.

Neuropeptides, the hypothalamus and obesity: insights into the central control of body weight

Body weight tends to remain relatively stable for long periods over an adult's lifespan. Dieting can reduce weight by 5-10%, but in most individuals attempts to lose larger amounts of weight are counteracted by a reduction in energy expenditure and an increase in hunger. The fact that body weight appears to be actively defended in this manner suggests that it is homeostatically regulated at a certain "set-point". Such a mechanism is likely to be centrally controlled by the brain since the hypothalamus can sense the amount of adipose tissue stored in the body and can alter both energy intake and expenditure. Over the past four years a number of major advances have reinforced the critical role the brain may play in controlling body weight, and these have greatly enhanced our understanding of this area. Advances have included the identification of several genetic mutations that cause obesity in animal models, examination of the metabolic consequences of such mutations and the development of mice with genetically engineered altered neuropeptide levels. This review summarises what has been recently discovered about the regulation of body weight by the brain and how this may be disrupted in obesity.

Short-term effects of leptin on lipid metabolism in the rat

In this study, we have examined the short-term effects of leptin on lipid metabolism in the rat. Acute leptin administration induced hypertriglyceridaemia (31% increase in plasma triacylglycerols) which was not associated with changes in lipoprotein lipase activity in white adipose tissue. Surprisingly, leptin administration did not induce any changes in the lipogenic rate in either white adipose tissue or liver. Leptin administration caused a decreased tissue uptake of exogenous 14C-triacylglycerols. These data suggest that leptin induces important changes in lipid uptake in adipose tissue and skeletal muscle which could be responsible for the observed hypertriglyceridaemia.

Role of peroxisome proliferator-activated receptor alpha in disease of pancreatic beta cells

Expression of peroxisome proliferator-activated receptor alpha (PPARalpha) and enzymes of fatty acid (FA) oxidation is markedly reduced in the fat-laden, dysfunctional islets of obese, prediabetic Zucker diabetic fatty (fa/fa) rats with mutated leptin receptors (OB-R). Leptin, PPARalpha/retinoid x receptor ligands, and FA all up-regulate PPARalpha and enzymes of FA oxidation and stimulate [3H]-palmitate oxidation in normal islets but not in islets from fa/fa rats. Overexpression of normal OB-R in islets of fa/fa rats corrects all of the foregoing abnormalities and reverses the diabetic phenotype. PPARalpha is a OB-R-dependent factor required for normal fat homeostasis in islet cells.

Interaction of free fatty acids with human leptin

Relatively high concentrations of leptin are present in plasma and it is thought to play a major role in lipid homeostasis. Leptin is reported to lower tissue triglyceride content by increasing intracellular oxidation of free fatty acids (FFA). However very little is known regarding the interaction between leptin and plasma FFA. We studied the interaction of FFA with leptin using a direct radiolabelled fatty acid binding assay, a fluorescence assay, electrophoretic mobility and autoradiobinding. All these data indicate that binding of FFA with leptin is reversible and shows a positive co-operativity. The binding of FFA to leptin produces a change in the pI value of the leptin and also increased the electrophoretic mobility of the protein in native polyacrylamide gels. The change in leptin's electrophoretic mobility depends on the chain length and the number of double bonds of the fatty acid, as stearic acid, 18:0, had no effect whereas oleic acid, 18:1n-9, linoleic acid, 18:2n-6, arachidonic acid, 20:4n-6, and docosahexaneoic acid, 22:6n-3, affected leptin's mobility to different degrees. The physiological implication of leptin-FFA interaction is not known, however the interaction may depend on the plasma FFA composition and concentration which are known to vary in different pathological/physiological conditions.

Molecular or pharmacologic perturbation of the link between glucose and lipid metabolism is without effect on glucose-stimulated insulin secretion. A re-evaluation of the long-chain acyl-CoA hypothesis

The mechanism by which glucose stimulates insulin secretion from the pancreatic islets of Langerhans is incompletely understood. It has been suggested that malonyl-CoA plays a regulatory role by inhibiting fatty acid oxidation and promoting accumulation of cytosolic long-chain acyl-CoA (LC-CoA). In the current study, we have re-evaluated this "long-chain acyl-CoA hypothesis" by using molecular and pharmacologic methods to perturb lipid metabolism in INS-1 insulinoma cells or rat islets during glucose stimulation. First, we constructed a recombinant adenovirus containing the cDNA encoding malonyl-CoA decarboxylase (AdCMV-MCD), an enzyme that decarboxylates malonyl-CoA to acetyl-CoA. INS-1 cells treated with AdCMV-MCD had dramatically lowered intracellular malonyl CoA levels compared with AdCMV-betaGal-treated cells at both 3 and 20 mM glucose. Further, at 20 mM glucose, AdCMV-MCD-treated cells were less effective at suppressing [1-14C]palmitate oxidation and incorporated 43% less labeled palmitate and 50% less labeled glucose into cellular lipids than either AdCMV-betaGAL-treated or untreated INS-1 cells. Despite the large metabolic changes caused by expression of MCD, insulin secretion in response to glucose was unaltered relative to controls. The alternative, pharmacologic approach for perturbing lipid metabolism was to use triacsin C to inhibit long-chain acyl-CoA synthetase. This agent caused potent attenuation of palmitate oxidation and glucose or palmitate incorporation into cellular lipids and also caused a 47% decrease in total LC-CoA. Despite this, the drug had no effect on glucose-stimulated insulin secretion in islets or INS-1 cells. We conclude that significant disruption of the link between glucose and lipid metabolism does not impair glucose-stimulated insulin secretion in pancreatic islets or INS-1 cells.

13C NMR study of the effects of leptin treatment on kinetics of hepatic intermediary metabolism

The recent discovery of leptin receptors in peripheral tissue raises questions about which of leptin's biological actions arise from direct effects of the hormone on extraneural tissues and what intracellular mechanisms are responsible for leptin's effects on carbohydrate and lipid metabolism. The present study is focused on the action of leptin on hepatic metabolism. Nondestructive 13C NMR methodology was used to follow the kinetics of intermediary metabolism by monitoring flux of 13C-labeled substrate through several multistep pathways. In perfused liver from either ob/ob or lean mice, we found that acute treatment with leptin in vitro modulates pathways controlling carbohydrate flux into 13C-labeled glycogen, thereby rapidly enhancing synthesis by an insulin-independent mechanism. Acute treatment of ob/ob liver also caused a rapid stimulation of long-chain fatty acid synthesis from 13C-labeled acetyl-CoA by the de novo synthesis route. Chronic leptin treatment in vivo induced homeostatic changes that resulted in a tripling of the rate of glycogen synthesis via the gluconeogenic pathway from [2-13C]pyruvate in ob/ob mouse liver perfused in the absence of the hormone. Consistent with the 13C NMR results, leptin treatment of the ob/ob mouse in vivo resulted in significantly increased hepatic glycogen synthase activity. Chronic treatment with leptin in vivo exerted the opposite effect of acute treatment in vitro and markedly decreased hepatic de novo synthesis of fatty acids in ob/ob mouse liver. In agreement with the 13C NMR findings, activities of hepatic acetyl-CoA carboxylase and fatty acid synthase were significantly reduced by chronic treatment of the ob/ob mouse with leptin. Our data represent a demonstration of direct effects of leptin in the regulation of metabolism in the intact functioning liver.