Effects of central or peripheral leptin administration on norepinephrine turnover in defined fat depots

Low-dose peripheral leptin infusion produces selective activation of ventromedial hypothalamic and hindbrain STAT3

Previous studies have shown that very low dose, acute, single peripheral leptin injections fully activate arcuate nucleus signal transducer and activator of transcription 3 (STAT3), but ventromedial hypothalamus (VMH) pSTAT3 continues to increase with higher doses of leptin that inhibit food intake. The lowest dose that inhibited intake increased circulating leptin 300-fold whereas food intake is inhibited by chronic peripheral leptin infusions that only double circulating leptin. This study examined whether the pattern of hypothalamic pSTAT3 was the same in leptin-infused rats as in leptin-injected rats. Male Sprague-Dawley rats received intraperitoneal infusions of 0, 5, 10, 20, or 40 µg leptin/day for 9 days. The highest dose of leptin increased serum leptin by 50-100%, inhibited food intake for 5 days, but inhibited weight gain and retroperitoneal fat mass for 9 days. Energy expenditure, respiratory exchange ratio, and brown fat temperature did not change. pSTAT3 was quantified in hypothalamic nuclei and the nucleus of the solitary tract (NTS) when food intake was inhibited and when it had returned to control levels. There was no effect of leptin on pSTAT3 in the medial or lateral arcuate nucleus or in the dorsomedial nucleus of the hypothalamus. VMH pSTAT3 was increased only at day 4 when food intake was inhibited, but NTS pSTAT3 was increased at both 4 and 9 days of infusion. These results suggest that activation of leptin VMH receptors contributes to the suppression of food intake, but that hindbrain receptors contribute to a sustained change in metabolism that maintains a reduced weight and fat mass.NEW & NOTEWORTHY Low-dose, chronic peripheral infusions of leptin produced an initial, transient inhibition of food intake that correlated with signal transducer and activator of transcription 3 (STAT3) activation in the ventromedial hypothalamus (VMH) and nucleus of the solitary tract (NTS). When intake normalized, but weight remained suppressed, the NTS was the only area that remained activated. These data suggest that leptin's primary function is to reduce body fat, that hypophagia is a means of achieving this and that different areas of the brain are responsible for the progressive response.

The emerging role of leptin in obesity-associated cardiac fibrosis: evidence and mechanism

Cardiac fibrosis is a hallmark of various cardiovascular diseases, which is quite commonly found in obesity, and may contribute to the increased incidence of heart failure arrhythmias, and sudden cardiac death in obese populations. As an endogenous regulator of adiposity metabolism, body mass, and energy balance, obesity, characterized by increased circulating levels of the adipocyte-derived hormone leptin, is a critical contributor to the pathogenesis of cardiac fibrosis. Although there are some gaps in our knowledge linking leptin and cardiac fibrosis, this review will focus on the interplay between leptin and major effectors involved in the pathogenesis underlying cardiac fibrosis at both cellular and molecular levels based on the current reports. The profibrotic effect of leptin is predominantly mediated by activated cardiac fibroblasts but may also involve cardiomyocytes, endothelial cells, and immune cells. Moreover, a series of molecular signals with a known profibrotic property is closely involved in leptin-induced fibrotic events. A more comprehensive understanding of the underlying mechanisms through which leptin contributes to the pathogenesis of cardiac fibrosis may open up a new avenue for the rapid emergence of a novel therapy for preventing or even reversing obesity-associated cardiac fibrosis.

Phosphorylation of STAT3 in hypothalamic nuclei is stimulated by lower doses of leptin than are needed to inhibit food intake

This experiment investigated which hypothalamic nuclei were activated by a dose of leptin that inhibited food intake. Foodnot intake, energy expenditure, respiratory exchange ratio (RER), and intrascapular brown adipose tissue (IBAT) temperature were measured in male and female Sprague Dawley rats for 36 h following an intraperitoneal injection of 0, 50, 200, 500, or 1,000 µg leptin/kg with each rat tested with each dose of leptin in random order. In both males and females, RER and 12-h food intake were inhibited only by 1,000 µg leptin/kg, but there was no effect on energy expenditure or IBAT temperature. At the end of the experiment, phosphorylated signal transducer and activator of transcription 3 (pSTAT3) immunoreactivity was measured 1 h after injection of 0, 50, 500, or 1,000 µg leptin/kg. In male rats, the lowest dose of leptin produced a maximal activation of STAT3 in the Arc and nucleus of the solitary tract (NTS). There was no response in the dorsomedial hypothalamus, but there was a progressive increase in ventromedial nucleus of the hypothalamus (VMH) pSTAT3 with increasing doses of leptin. In female rats, there was no significant change in Arc and pSTAT3 NTS activation was maximal with 500 mg leptin/kg, but only the highest dose of leptin increased VMH pSTAT3. These results suggest that the VMH plays an important role in the energetic response to elevations of circulating leptin but do not exclude the possibility that multiple nuclei provide the appropriate integrated response to hyperleptinemia.NEW & NOTEWORTHY The results of this experiment show that doses of leptin too small to inhibit food intake produce a maximal response to leptin in the arcuate nucleus. By contrast the VMH shows a robust response that correlates with inhibition of food intake. This suggests that the VMH plays an important role in the energetic response to hyperleptinemia.

The nutrient sensing pathways FoxO1/3 and mTOR in the heart are coordinately regulated by central leptin through PPARβ/δ. Implications in cardiac remodeling

BACKGROUND

Cardiovascular disease in obese individuals with type 2 diabetes is often associated with hyperleptinemia and leptin resistance, while other studies support that leptin has cardioprotective effects. Besides, the role of leptin in regulating cardiac atrophy or hypertrophy remains to be clearly defined. In fact, in rats with normal leptin sensitivity, the molecular underpinnings of the effects of central leptin regulating cardiac structural pathways remain poorly understood.

OBJECTIVE

Hence, we assessed the effects of intracerebroventricular (icv) leptin infusion on cardiac remodeling analyzing FOXO1/3 and mTORC1 pathways, focusing special attention to PPARβ/δ as mediator of central leptin's effects on cardiac metabolism.

METHODS

Male 3-months-old Wistar rats, infused with icv leptin (0.2 μg/day) for 7 days, were daily co-treated intraperitoneally with the specific PPARβ/δ antagonist GSK0660, at 1 mg/kg per day along leptin treatment.

RESULTS

Central leptin regulated dynamically, in an opposite manner, the network between FOXOs and mTORC1 and induced an atrophy-related gene program in cardiac tissue. Leptin activated the anti-hypertrophic kinase GSK3β and increased the protein levels of muscle-specific ubiquitin ligases, muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx)/Atrogin-1 involved in limiting cardiac hypertrophy. FOXO1 activity and the expression of their target genes, Sod2 and Lpl, were also increased in the heart upon central leptin infusion. Besides, Beclin-1 and LC3B-II, gene products of the autophagic pathway response, were upregulated, while the content and expression levels of phenotypic markers of cardiac hypertrophy as ANP and β-myosin heavy chain, gene product of Myh7 were significantly decreased. On the other hand, mTORC1 activity and OXPHOS protein levels were decreased suggesting a key role of central leptin preventing cardiac oxidative stress. In fact, the content of carbonylated proteins, TBARS and ROS/RSN were not increased in cardiac tissue in response to central leptin infusion. Finally, the pharmacological inhibition of PPARβ/δ, via in vivo administration of the selective antagonist GSK0660, blunted the induction of FOXO1/3, Atrogin-1, MuRF1 and GSK3β in the heart mediated by icv leptin infusion.

CONCLUSIONS

Our results demonstrate that, in lean rats with normal leptin sensitivity, central leptin regulates nutrient sensing pathways in heart contributing to balance cardiac remodeling through the anti- and pro-hypertrophic programs, and in this process is involved PPARβ/δ.

Consuming sucrose solution promotes leptin resistance and site specifically modifies hypothalamic leptin signaling in rats

Rats consuming 30% sucrose solution and a sucrose-free diet (LiqS) become leptin resistant, whereas rats consuming sucrose from a formulated diet (HS) remain leptin responsive. This study tested whether leptin resistance in LiqS rats extended beyond a failure to inhibit food intake and examined leptin responsiveness in the hypothalamus and hindbrain of rats offered HS, LiqS, or a sucrose-free diet (NS). Female LiqS Sprague-Dawley rats initially only partially compensated for the calories consumed as sucrose, but energy intake matched that of HS and NS rats when they were transferred to calorimetry cages. There was no effect of diet on energy expenditure, intrascapular brown fat tissue (IBAT) temperature, or fat pad weight. A peripheral injection of 2 mg of leptin/kg on day 23 or day 26 inhibited energy intake of HS and NS but not LiqS rats. Inhibition occurred earlier in HS rats than in NS rats and was associated with a smaller meal size. Leptin had no effect on energy expenditure but caused a transient rise in IBAT temperature of HS rats. Leptin increased the phosphorylation of signal transducer and activator of transcription 3 (pSTAT3) in the hindbrain and ventromedial hypothalamus of all rats. There was a minimal effect of leptin in the arcuate nucleus, and only the dorsomedial hypothalamus showed a correlation between pSTAT3 and leptin responsiveness. These data suggest that the primary response to leptin is inhibition of food intake and the pattern of sucrose consumption, rather than calories consumed as sucrose, causes leptin resistance associated with site-specific differences in hypothalamic leptin signaling.

There and Back Again: Leptin Actions in White Adipose Tissue

Leptin is a hormone discovered almost 30 years ago with important implications in metabolism. It is primarily produced by white adipose tissue (WAT) in proportion to the amount of fat. The discovery of leptin was a turning point for two principle reasons: on one hand, it generated promising expectations for the treatment of the obesity, and on the other, it changed the classical concept that white adipose tissue was simply an inert storage organ. Thus, adipocytes in WAT produce the majority of leptin and, although its primary role is the regulation of fat stores by controlling lipolysis and lipogenesis, this hormone also has implications in other physiological processes within WAT, such as apoptosis, browning and inflammation. Although a massive number of questions related to leptin actions have been answered, the necessity for further clarification facilitates constantly renewing interest in this hormone and its pathways. In this review, leptin actions in white adipose tissue will be summarized in the context of obesity.

Loss of leptin receptor-expressing cells in the hindbrain decreases forebrain leptin sensitivity

Previous studies indicate that inhibition of food intake by leptin is mediated by an integrated response to activation of hypothalamic and hindbrain receptors. This study tested whether loss of hindbrain leptin receptor signaling changed sensitivity to forebrain leptin. Injections of leptin-conjugated saporin (Lep-Sap) into the medial nucleus of the solitary tract (NTS) were used to destroy hindbrain leptin receptor-expressing neurons of male Sprague-Dawley rats. Controls were injected with saporin conjugated with a nonsense peptide (Blk-Sap). Lep-Sap had no effect on daily food intake or body weight, but expression of phosphorylated signal transducer and activator of transcription 3 (pSTAT3) in the NTS following a peripheral injection of leptin was abolished 26 days after Lep-Sap injections. To test forebrain leptin sensitivity, Lep-Sap and Blk-Sap rats received third-ventricle injections of 0, 0.05, 0.1, 0.25, or 0.5 μg leptin. Food intake was inhibited by 0.25 and 0.5 μg leptin in Blk-Sap rats, but there was no significant effect of leptin on food intake of Lep-Sap rats. There was no difference in hypothalamic pSTAT3 in unstimulated conditions, but pSTAT3 was lower in the dorsomedial region of the ventromedial nucleus of the hypothalamus (VMH) of Lep-Sap rats compared with Blk-Sap rats following a third-ventricle injection of 0.25 μg leptin. These results are consistent with previous data showing that loss of VMH leptin receptor-expressing cells prevents weight loss caused by fourth-ventricle leptin infusion and show that the integrated response between the hindbrain and forebrain is heavily dependent on leptin activity in the VMH.

Apolipoprotein A-IV Enhances Fatty Acid Uptake by Adipose Tissues of Male Mice via Sympathetic Activation

Apolipoprotein A-IV (ApoA-IV) synthesized by the gut regulates lipid metabolism. Sympathetic innervation of adipose tissues also controls lipid metabolism. We hypothesized that ApoA-IV required sympathetic innervation to increase fatty acid (FA) uptake by adipose tissues and brown adipose tissue (BAT) thermogenesis. After 3 weeks feeding of either a standard chow diet or a high-fat diet (HFD), mice with unilateral denervation of adipose tissues received intraperitoneal administration of recombinant ApoA-IV protein and intravenous infusion of lipid mixture with radioactive triolein. In chow-fed mice, ApoA-IV administration increased FA uptake by intact BAT but not the contralateral denervated BAT or intact white adipose tissue (WAT). Immunoblots showed that, in chow-fed mice, ApoA-IV increased expression of lipoprotein lipase and tyrosine hydroxylase in both intact BAT and inguinal WAT (IWAT), while ApoA-IV enhanced protein levels of β3 adrenergic receptor, adipose triglyceride lipase, and uncoupling protein 1 in the intact BAT only. In HFD-fed mice, ApoA-IV elevated FA uptake by intact epididymal WAT (EWAT) but not intact BAT or IWAT. ApoA-IV increased sympathetic activity assessed by norepinephrine turnover (NETO) rate in BAT and EWAT of chow-fed mice, whereas it elevated NETO only in EWAT of HFD-fed mice. These observations suggest that, in chow-fed mice, ApoA-IV activates sympathetic activity of BAT and increases FA uptake by BAT via innervation, while in HFD-fed mice, ApoA-IV stimulates sympathetic activity of EWAT to shunt FAs into the EWAT.

Obesity: a neuroimmunometabolic perspective

Neuroimmunology and immunometabolism are burgeoning topics of study, but the intersection of these two fields is scarcely considered. This interplay is particularly prevalent within adipose tissue, where immune cells and the sympathetic nervous system (SNS) have an important role in metabolic homeostasis and pathology, namely in obesity. In the present Review, we first outline the established reciprocal adipose-SNS relationship comprising the neuroendocrine loop facilitated primarily by adipose tissue-derived leptin and SNS-derived noradrenaline. Next, we review the extensive crosstalk between adipocytes and resident innate immune cells as well as the changes that occur in these secretory and signalling pathways in obesity. Finally, we discuss the effect of SNS adrenergic signalling in immune cells and conclude with exciting new research demonstrating an immutable role for SNS-resident macrophages in modulating SNS-adipose crosstalk. We posit that the latter point constitutes the existence of a new field - neuroimmunometabolism.

Adipose PTEN regulates adult adipose tissue homeostasis and redistribution via a PTEN-leptin-sympathetic loop

OBJECTIVE

Despite the large body of work describing the tumor suppressor functions of Phosphatase and tensin homologue deleted on chromosome ten (PTEN), its roles in adipose homeostasis of adult animals are not yet fully understood. Here, we sought to determine the role of PTEN in whole-body adipose homeostasis.

METHODS

We genetically manipulated PTEN in specific fat depots through recombinant adeno-associated viral vector (rAAV)-based gene transfer of Cre recombinase to adult PTEN^flox mice. Additionally, we used a denervation agent, 6OHDA, to assess the role of sympathetic signaling in PTEN-related adipose remodeling. Furthermore, we chemically manipulated AKT signaling via a pan-AKT inhibitor, MK-2206, to assess the role of AKT in PTEN-related adipose remodeling. Finally, to understand the role of leptin and central signaling on peripheral tissues, we knocked down hypothalamic leptin receptor with a microRNA delivered by a rAAV vector.

RESULTS

Knockdown PTEN in individual fat depot resulted in massive expansion of the affected fat depot through activation of AKT signaling associated with suppression of lipolysis and induction of leptin. This hypertrophic expansion of the affected fat depot led to upregulation of PTEN level, higher lipolysis, and induction of white fat browning in other fat depots, and the compensatory reduced fat mass to maintain a set point of whole-body adiposity. Administration of AKT inhibitor MK-2206 prevented the adipose PTEN knockdown-associated effects. 6OHDA-mediated denervation demonstrated that sympathetic innervation was required for the PTEN knockdown-induced adipose redistribution. Knockdown hypothalamic leptin receptor attenuated the adipose redistribution induced by PTEN deficiency in individual fat depot.

CONCLUSIONS

Our results demonstrate the essential role of PTEN in adipose homeostasis, including mass and distribution in adulthood, and reveal an "adipose PTEN-leptin-sympathetic nervous system" feedback loop to maintain a set point of adipose PTEN and whole-body adiposity.

Differential Sympathetic Activation of Adipose Tissues by Brain-Derived Neurotrophic Factor

Centrally administered brain-derived neurotrophic factor (BDNF) decreases body adiposity beyond what can be accounted for by decreased food intake, implying enhanced lipid metabolism by BDNF. Consistent with this notion, intracerebroventricular (icv) injection of BDNF in rats increased the expression of lipolytic enzymes in white adipose tissues (WAT) and increased circulating concentrations of lipolytic products without changing the levels of adrenal gland hormones. This suggests that central BDNF-induced lipid mobilization is likely due to sympathetic neural activation, rather than activation of the adrenocortical or adrenomedullary system. We hypothesized that BDNF activated sympathetic innervation of adipose tissues to regulate lipolysis. Rats with unilateral denervation of interscapular brown adipose tissue (BAT) and different WAT depots received icv injections of saline or BDNF. Both intact and denervated adipose tissues were exposed to the same circulating factors, but denervated adipose tissues did not receive neural signals. Norepinephrine (NE) turnover (NETO) of BAT and WAT was assessed as a measure of sympathetic activity. Findings revealed that central BDNF treatment induced a change in NETO in some but not all the adipose tissues tested. Specifically, greater NETO rates were found in BAT and gonadal epididymal WAT (EWAT), but not in inguinal WAT (IWAT) or retroperitoneal WAT (RWAT), of BDNF-treated rats compared to saline-treated rats. Furthermore, intact innervation was necessary for BDNF-induced NETO in BAT and EWAT. In addition, BDNF increased the expression of lipolytic enzymes in both intact and denervated EWAT and IWAT, suggesting that BDNF-induced WAT lipolysis was independent of intact innervation. To summarize, centrally administered BDNF selectively provoked sympathetic drives to BAT and EWAT that was dependent on intact innervation, while BDNF also increased lipolysis in a manner independent of intact innervation.

Perivascular Adipose Tissue: the Sixth Man of the Cardiovascular System

Perivascular adipose tissue (PVAT) refers to the local aggregate of adipose tissue surrounding the vascular tree, exhibiting phenotypes from white to brown and beige adipocytes. Although PVAT has long been regarded as simply a structural unit providing mechanical support to vasculature, it is now gaining reputation as an integral endocrine/paracrine component, in addition to the well-established modulator endothelium, in regulating vascular tone. Since the discovery of anti-contractile effect of PVAT in 1991, the use of multiple rodent models of reduced amounts of PVAT has revealed its regulatory role in vascular remodeling and cardiovascular implications, including atherosclerosis. PVAT does not only release PVAT-derived relaxing factors (PVRFs) to activate multiple subsets of endothelial and vascular smooth muscle potassium channels and anti-inflammatory signals in the vasculature, but it does also provide an interface for neuron-adipocyte interactions in the vascular wall to regulate arterial vascular tone. In this review, we outline our current understanding towards PVAT and attempt to provide hints about future studies that can sharpen the therapeutic potential of PVAT against cardiovascular diseases and their complications.

Denervation as a tool for testing sympathetic control of white adipose tissue

This review summarizes the evidence derived from studies utilizing denervation procedures to demonstrate sympathetic control of white adipose tissue metabolism and body fat mass. A majority of the work demonstrating neural control of white fat was performed in the Bartness laboratory with Siberian hamsters as the predominant experimental model. These animals experience dramatic changes in body fat mass in response to changes in photoperiod, however, the mechanisms identified in hamsters have been reproduced or further elucidated by experiments with other animal models. Evidence for the role of sympathetic innervation contributing to the control of white adipocyte lipolysis and preadipocyte proliferation is summarized. In addition, evidence from denervation experiments for neural communication between different white fat depots as well as for a feedback control loop between sensory afferents from individual fat depots and sympathetic efferents to the same or distant white fat depots is discussed.

Leptin and brain-adipose crosstalks

Interactions between the brain and distinct adipose depots have a key role in maintaining energy balance, thereby promoting survival in response to metabolic challenges such as cold exposure and starvation. Recently, there has been renewed interest in the specific central neuronal circuits that regulate adipose depots. Here, we review anatomical, genetic and pharmacological studies on the neural regulation of adipose function, including lipolysis, non-shivering thermogenesis, browning and leptin secretion. In particular, we emphasize the role of leptin-sensitive neurons and the sympathetic nervous system in modulating the activity of brown, white and beige adipose tissues. We provide an overview of advances in the understanding of the heterogeneity of the brain regulation of adipose tissues and offer a perspective on the challenges and paradoxes that the community is facing regarding the actions of leptin on this system.

Sensory denervation of inguinal white fat modifies sympathetic outflow to white and brown fat in Siberian hamsters

White adipose tissue (WAT) and brown adipose tissue (BAT) have sympathetic nervous system (SNS) and sensory innervations. Previous studies from our laboratory revealed central neuroanatomical evidence of WAT sensory and BAT SNS crosstalk with double labeling of inguinal WAT (IWAT) sensory and interscapular BAT (IBAT) SNS neurons. We previously demonstrated that WAT lipolysis increases IBAT temperature, but this effect is absent when IWAT afferents are surgically denervated, which severs both sensory and SNS nerves. It is possible that WAT sensory feedback can regulate SNS drive to itself and other WAT and BAT depots, and thus contribute to the existence of differential SNS outflow to fat during different energy challenges. Here we selectively denervated IWAT sensory nerves in Siberian hamsters using capsaicin and measured norepinephrine turnover (NETO) i.e., SNS drive to WAT and BAT depots, IBAT uncoupling protein 1 (UCP1) expression, body mass, fat mass, blood glucose, and food consumed after a 24-h cold exposure. IWAT sensory denervation decreased both IWAT and IBAT NETO and IBAT UCP1 expression. IWAT sensory denervation, however, increased mesenteric WAT (MWAT) NETO after the 24-h cold exposure and did not modify epididymal WAT (EWAT) and retroperitoneal WAT (RWAT) NETO compared with respective controls. Body mass, fat mass, blood glucose, and food consumed were unchanged across groups. RWAT and EWAT mass decreased in capsaicin-injected hamsters, but did not in the vehicle hamsters. These results functionally demonstrate the existence of IWAT sensory and IBAT SNS crosstalk and that a disruption in this sensory-SNS feedback mechanism modifies SNS drive to IWAT, IBAT, and MWAT, but not EWAT and RWAT.

Central leptin regulates heart lipid content by selectively increasing PPAR β/δ expression

The role of central leptin in regulating the heart from lipid accumulation in lean leptin-sensitive animals has not been fully elucidated. Herein, we investigated the effects of central leptin infusion on the expression of genes involved in cardiac metabolism and its role in the control of myocardial triacylglyceride (TAG) accumulation in adult Wistar rats. Intracerebroventricular (icv) leptin infusion (0.2 µg/day) for 7 days markedly decreased TAG levels in cardiac tissue. Remarkably, the cardiac anti-steatotic effects of central leptin were associated with the selective upregulation of gene and protein expression of peroxisome proliferator-activated receptor β/δ (PPARβ/δ, encoded by Pparb/d) and their target genes, adipose triglyceride lipase (encoded by Pnpla2, herefater referred to as Atgl), hormone sensitive lipase (encoded by Lipe, herefater referred to as Hsl), pyruvate dehydrogenase kinase 4 (Pdk4) and acyl CoA oxidase 1 (Acox1), involved in myocardial intracellular lipolysis and mitochondrial/peroxisomal fatty acid utilization. Besides, central leptin decreased the expression of stearoyl-CoA deaturase 1 (Scd1) and diacylglycerol acyltransferase 1 (Dgat1) involved in TAG synthesis and increased the CPT-1 independent palmitate oxidation, as an index of peroxisomal β-oxidation. Finally, the pharmacological inhibition of PPARβ/δ decreased the effects on gene expression and cardiac TAG content induced by leptin. These results indicate that leptin, acting at central level, regulates selectively the cardiac expression of PPARβ/δ, contributing in this way to regulate the cardiac TAG accumulation in rats, independently of its effects on body weight.

Low-dose leptin infusion in the fourth ventricle of rats enhances the response to third-ventricle leptin injection

We previously reported that low-dose leptin infusions into the third or fourth ventricle that do not affect energy balance when given independently cause rapid weight loss when given simultaneously. Therefore, we tested whether hindbrain leptin enhances the response to forebrain leptin or whether forebrain leptin enhances the response to hindbrain leptin. Rats received fourth-ventricle infusions of saline or 0.01, 0.1, 0.3, or 0.6 μg leptin/day for 13 days. On days 9 and 13, 0.1 μg leptin was injected into the third ventricle. The injection inhibited food intake for 36 h in saline-infused rats but for 60 h in those infused with 0.6 μg leptin/day. Leptin injection increased intrascapular brown fat temperature in leptin-infused, but not saline-infused, rats. In a separate experiment, rats received third-ventricle infusions of saline or 0.005, 0.01, 0.05, or 0.1 μg leptin/day and fourth-ventricle injections of 1.0 μg leptin on days 9 and 13 Leptin injection inhibited food intake, respiratory exchange ratio, and 14-h food intake in rats infused with saline or the two lowest doses of leptin. There was no effect with higher-dose leptin infusions because food intake, body fat, and lean mass were already inhibited. These data suggest that activation of leptin receptors in the hindbrain enhances the response to third-ventricle leptin, whereas activation of forebrain leptin receptors does not enhance the response to fourth-ventricle leptin, consistent with our previous finding that weight loss in rats treated with fourth-ventricle leptin is associated with indirect activation of hypothalamic STAT3.

Differential sympathetic outflow to adipose depots is required for visceral fat loss in response to calorie restriction

The sympathetic nervous system (SNS) regulates energy homeostasis in part by governing fatty acid liberation from adipose tissue. We first examined whether SNS activity toward discrete adipose depots changes in response to a weight loss diet in mice. We found that SNS activity toward each adipose depot is unique in timing, pattern of activation, and habituation with the most dramatic contrast between visceral and subcutaneous adipose depots. Sympathetic drive toward visceral epididymal adipose is more than doubled early in weight loss and then suppressed later in the diet when weight loss plateaued. Coincident with the decline in SNS activity toward visceral adipose is an increase in activity toward subcutaneous depots indicating a switch in lipolytic sources. In response to calorie restriction, SNS activity toward retroperitoneal and brown adipose depots is unaffected. Finally, pharmacological blockage of sympathetic activity on adipose tissue using the β3-adrenergic receptor antagonist, SR59230a, suppressed loss of visceral adipose mass in response to diet. These findings indicate that SNS activity toward discrete adipose depots is dynamic and potentially hierarchical. This pattern of sympathetic activation is required for energy liberation and loss of adipose tissue in response to calorie-restricted diet.

Fourth-ventricle leptin infusions dose-dependently activate hypothalamic signal transducer and activator of transcription 3

Previous studies have shown that very low-dose infusions of leptin into the third or the fourth ventricle alone have little effect on energy balance, but simultaneous low-dose infusions cause rapid weight loss and increased phosphorylation of STAT3 (p-STAT3) in hypothalamic sites that express leptin receptors. Other studies show that injecting high doses of leptin into the fourth ventricle inhibits food intake and weight gain. Therefore, we tested whether fourth-ventricle leptin infusions that cause weight loss are associated with increased leptin signaling in the hypothalamus. In a dose response study 14-day infusions of increasing doses of leptin showed significant hypophagia, weight loss, and increased hypothalamic p-STAT3 in rats receiving at least 0.9 μg leptin/day. In a second study 0.6 μg leptin/day transiently inhibited food intake and reduced carcass fat, but had no significant effect on energy expenditure. In a final study, we identified the localization of STAT3 activation in the hypothalamus of rats receiving 0, 0.3, or 1.2 μg leptin/day. The high dose of leptin, which caused weight loss in the first experiment, increased p-STAT3 in the ventromedial, dorsomedial, and arcuate nuclei of the hypothalamus. The low dose that increased brown fat UCP1 but did not affect body composition in the first experiment had little effect on hypothalamic p-STAT3. We propose that hindbrain leptin increases the precision of control of energy balance by lowering the threshold for leptin signaling in the forebrain. Further studies are needed to directly test this hypothesis.

Role of Hypothalamic VGF in Energy Balance and Metabolic Adaption to Environmental Enrichment in Mice

Environmental enrichment (EE), a housing condition providing complex physical, social, and cognitive stimulation, leads to improved metabolic health and resistance to diet-induced obesity and cancer. One underlying mechanism is the activation of the hypothalamic-sympathoneural-adipocyte axis with hypothalamic brain-derived neurotrophic factor (BDNF) as the key mediator. VGF, a peptide precursor particularly abundant in the hypothalamus, was up-regulated by EE. Overexpressing BDNF or acute injection of BDNF protein to the hypothalamus up-regulated VGF, whereas suppressing BDNF signaling down-regulated VGF expression. Moreover, hypothalamic VGF expression was regulated by leptin, melanocortin receptor agonist, and food deprivation mostly paralleled to BDNF expression. Recombinant adeno-associated virus-mediated gene transfer of Cre recombinase to floxed VGF mice specifically decreased VGF expression in the hypothalamus. In contrast to the lean and hypermetabolic phenotype of homozygous germline VGF knockout mice, specific knockdown of hypothalamic VGF in male adult mice led to increased adiposity, decreased core body temperature, reduced energy expenditure, and impaired glucose tolerance, as well as disturbance of molecular features of brown and white adipose tissues without effects on food intake. However, VGF knockdown failed to block the EE-induced BDNF up-regulation or decrease of adiposity indicating a minor role of VGF in the hypothalamic-sympathoneural-adipocyte axis. Taken together, our results suggest hypothalamic VGF responds to environmental demands and plays an important role in energy balance and glycemic control likely acting in the melanocortin pathway downstream of BDNF.

Chronic central leptin infusion modulates the glycemia response to insulin administration in male rats through regulation of hepatic glucose metabolism

Leptin and insulin use overlapping signaling mechanisms to modify hepatic glucose metabolism, which is critical in maintaining normal glycemia. We examined the effect of an increase in central leptin and insulin on hepatic glucose metabolism and its influence on serum glucose levels. Chronic leptin infusion increased serum leptin and reduced hepatic SH-phosphotyrosine phosphatase 1, the association of suppressor of cytokine signaling 3 to the insulin receptor in liver and the rise in glycemia induced by central insulin. Leptin also decreased hepatic phosphoenolpyruvate carboxykinase levels and increased insulin's ability to phosphorylate insulin receptor substrate-1, Akt and glycogen synthase kinase on Ser9 and to stimulate glucose transporter 2 and glycogen levels. Peripheral leptin treatment reproduced some of these changes, but to a lesser extent. Our data indicate that leptin increases the hepatic response to a rise in insulin, suggesting that pharmacological manipulation of leptin targets may be of interest for controlling glycemia.

Cardiovascular autonomic tone relation to metabolic parameters and hsCRP in normoglycemia and prediabetes

AIMS

The aim of the study was to evaluate cardiovascular autonomic function (CAF) at different stages of obesity and in the presence of metabolic syndrome (MetS), and its association with metabolic parameters and hsCRP in subjects with normal glucose tolerance (NGT) and prediabetes.

METHODS

A total of 259 subjects (mean age 47.1 ± 14.6 years, mean BMI 31.4 ± 8.1 kg/m(2)), divided in 2 groups: NGT and prediabetes, and subdivided according to glucose tolerance, BMI and MetS, were enrolled. Anthropometric indices, glucose tolerance, blood pressure, serum lipids and hsCRP were measured. Body composition was estimated by impedance analysis. CAF was assessed by ANX-3.0 method.

RESULTS

Sympathetic and parasympathetic activity were decreased in severe obesity and MetS as compared to controls in NGT. Negative correlation was observed between sympathetic and parasympathetic tone and BMI, waist circumference, total body fat, visceral fat area (VFA), blood pressure, total and LDL cholesterol, and hsCRP in NGT; and VFA, HbA1c and glycemia in prediabetes.

CONCLUSION

Obesity and MetS seem to be associated with CAF deterioration, and metabolic parameters and hsCRP correlate with CAF and probably increase cardiovascular risk in NGT, whilst VFA, HbA1c and glycemia significantly influence CAF alterations in prediabetes.

Hexosamine biosynthetic pathway activity in leptin resistant sucrose-drinking rats

Rats offered 30% sucrose solution in addition to chow and water become leptin resistant therefore we investigated the effect of sucrose solution consumption on leptin signaling. In Experiment 1 rats were resistant to 3rd ventricle injections of1.5 μg leptin after 36 days of sucrose and western blot indicated that resistance was associated with increased basal levels of signal transducer and activator of transcription 3 phosphorylation (pSTAT3). In Experiment 2 rats were resistant to a peripheral injection of 2mg leptin/kg after 26 days of sucrose. Immunohistochemistry indicated that increased basal pSTAT3 was limited to the medial and lateral arcuate nucleus of the hypothalamus. Increased availability of glucose and fructose can stimulate the hexosamine biosynthetic pathway (HBP) which O-GlcNAc-modifies proteins. This has the potential to change protein bioactivity. We tested whether this pathway could account for the leptin resistance. There was no increase in the expression of HBP enzymes in tissues from sucrose rats in Experiment 1, however, direct activation of the HBP with a 3h intravenous infusion of 30 μmol/kg/min glucosamine significantly increased hypothalamic pSTAT3. Although sucrose consumption and activation of the HBP both increase hypothalamic pSTAT3 experiments described here did not provide evidence of a direct link between sucrose consumption, HBP activity and leptin resistance. Unexpectedly, we found that the HBP enzyme glutamine fructose-6-phosphate amidotransferase (GFAT) in liver and O-GlcNAcase in hypothalamus were increased 30min after leptin injection in leptin responsive animals, implying a complex interaction between activity of the HBP and leptin responsiveness.

Neural innervation of white adipose tissue and the control of lipolysis

White adipose tissue (WAT) is innervated by the sympathetic nervous system (SNS) and its activation is necessary for lipolysis. WAT parasympathetic innervation is not supported. Fully-executed SNS-norepinephrine (NE)-mediated WAT lipolysis is dependent on β-adrenoceptor stimulation ultimately hinging on hormone sensitive lipase and perilipin A phosphorylation. WAT sympathetic drive is appropriately measured electrophysiologically and neurochemically (NE turnover) in non-human animals and this drive is fat pad-specific preventing generalizations among WAT depots and non-WAT organs. Leptin-triggered SNS-mediated lipolysis is weakly supported, whereas insulin or adenosine inhibition of SNS/NE-mediated lipolysis is strongly supported. In addition to lipolysis control, increases or decreases in WAT SNS drive/NE inhibit and stimulate white adipocyte proliferation, respectively. WAT sensory nerves are of spinal-origin and sensitive to local leptin and increases in sympathetic drive, the latter implicating lipolysis. Transsynaptic viral tract tracers revealed WAT central sympathetic and sensory circuits including SNS-sensory feedback loops that may control lipolysis.

Integrated effects of leptin in the forebrain and hindbrain of male rats

Leptin receptors (ObRs) in the forebrain and hindbrain have been independently recognized as important mediators of leptin responses. It is unclear how leptin activity in these areas is integrated. We tested whether both forebrain and hindbrain ObRs have to be activated simultaneously to change energy balance and to maintain metabolic homeostasis. Previous studies used acute leptin injections in either the third ventricle (1-5 μg) or the fourth ventricle (3-10 μg); here we used 12-day infusions of low doses of leptin in one or both ventricles (0.1 μg/24 h in third, 0.6 μg/24 h in fourth). Male Sprague Dawley rats were fitted with third and fourth ventricle cannulas, and saline or leptin was infused from Alzet pumps for 6 or 12 days. Rats that received leptin into only the third or the fourth ventricle were not different from controls that received saline in both ventricles. By contrast, rats with low-dose leptin infusions into both the third and fourth ventricle showed a dramatic 60% reduction in food intake that was reversed on day 6, a 20% weight loss that stabilized on day 6, and a 50% decrease in body fat at day 12 despite the correction of food intake. They displayed normal activity and maintained energy expenditure despite weight loss, indicating inappropriately high thermogenesis that coincided with increased signal transducer and activator of transcription 3 (STAT3) phosphorylation in the brainstem. Altogether, these findings show that with low doses of leptin, chronic activation of both hypothalamic and brainstem ObRs is required to reduce body fat.

Evidence that leptin-induced weight loss requires activation of both forebrain and hindbrain receptors

Previous studies with chronic decerebrate rats and rats infused with leptin into the 4th ventricle suggest that hindbrain leptin receptors attenuate the catabolic effect of forebrain leptin receptor activation. To test this further, rats were fitted with both 3rd and 4th ventricle cannulae. They were infused for 12 days with different combinations of saline, low dose leptin or leptin receptor antagonist (leptin mutein protein). Infusion of 0.1 μg leptin/day into the 3rd ventricle or 0.6 μg leptin/day into the 4th ventricle had no significant effect on food intake, energy expenditure or body composition. Infusion of 2 μg mutein/day into either ventricle caused a small, but significant weight gain. When mutein was infused into one ventricle and leptin into the other, the rats lost weight irrespective of which combination was applied. Surprisingly, rats that received leptin infusions into both ventricles showed an initial hypophagia, no change in energy expenditure, but a 75% loss of carcass fat after 12 days. These data suggest that neuronal pathways activated by leptin receptors in either the forebrain or hindbrain modulate each other's effects. In normal conditions hindbrain leptin may attenuate the catabolic effect of forebrain leptin, but if activity in one area is blocked with mutein, then the catabolic response to leptin in the other ventricle is exaggerated. When receptors in both areas are activated there is an integration of response to produce negative energy balance. This may ensure that leptin causes a loss of fat only when leptin is elevated in both the CSF and periphery.

Leptin-sensitive sensory nerves innervate white fat

Leptin, the primary white adipose tissue (WAT) adipokine, is thought to convey lipid reserve information to the brain via the circulation. Because WAT responds to environmental/internal signals in a fat pad-specific (FPS) manner, systemic signals such as leptin would fail to communicate such distinctive information. Saturation of brain leptin transport systems also would fail to convey increased lipid levels beyond that point. WAT possesses sensory innervation exemplified by proven sensory-associated peptides in nerves within the tissue and by viral sensory nerve-specific transneuronal tract tracer, H129 strain of herpes simplex virus 1 labeling of dorsal root ganglia (DRG) pseudounipolar neurons, spinal cord and central sensory circuits. Leptin as a paracrine factor activating WAT sensory innervation could supply the brain with FPS information. Therefore, we tested for and found the presence of the long form of the leptin receptor (Ob-Rb) on DRG pseudounipolar neurons immunohistochemically labeled after injections of Fluorogold, a retrograde tract tracer, into inguinal WAT (IWAT). Intra-IWAT leptin injections (300 ng) significantly elevated IWAT nerve spike rate within 5 min and persisted for at least 30 min. Intra-IWAT leptin injections also induced significant c-Fos immunoreactivity (ir), indicating neural activation across DRG pseudounipolar sensory neurons labeled with Fluorogold IWAT injections. Intraperitoneal leptin injection did not increase c-Fos-ir in DRG or the arcuate nucleus, nor did it increase arcuate signal transducer and activator of transcription 3 phosphorylation-ir. Collectively, these results strongly suggest that endogenous leptin secreted from white adipocytes functions as a paracrine factor to activate spinal sensory nerves innervating the tissue.

Direct and indirect effects of leptin on adipocyte metabolism

Leptin is hypothesized to function as a negative feedback signal in the regulation of energy balance. It is produced primarily by adipose tissue and circulating concentrations correlate with the size of body fat stores. Administration of exogenous leptin to normal weight, leptin responsive animals inhibits food intake and reduces the size of body fat stores whereas mice that are deficient in either leptin or functional leptin receptors are hyperphagic and obese, consistent with a role for leptin in the control of body weight. This review discusses the effect of leptin on adipocyte metabolism. Because adipocytes express leptin receptors there is the potential for leptin to influence adipocyte metabolism directly. Adipocytes also are insulin responsive and receive sympathetic innervation, therefore leptin can also modify adipocyte metabolism indirectly. Studies published to date suggest that direct activation of adipocyte leptin receptors has little effect on cell metabolism in vivo, but that leptin modifies adipocyte sensitivity to insulin to inhibit lipid accumulation. In vivo administration of leptin leads to a suppression of lipogenesis, an increase in triglyceride hydrolysis and an increase in fatty acid and glucose oxidation. Activation of central leptin receptors also contributes to the development of a catabolic state in adipocytes, but this may vary between different fat depots. Leptin reduces the size of white fat depots by inhibiting cell proliferation both through induction of inhibitory circulating factors and by contributing to sympathetic tone which suppresses adipocyte proliferation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Leptin-induced increase in body fat content of rats

We previously reported that peripheral leptin infusions in chronically decrebrate rats, in which the forebrain is neurally isolated from the hindbrain, increased body fat and decreased energy expenditure. Any central leptin response in decerebrate rats would depend upon the hindbrain. Here, we tested whether selective activation of hindbrain leptin receptors increased body fat. Fourth ventricle infusion of 0.6 μg leptin/day for 12 days increased body fat by 13% with no increase in food intake. Third ventricle leptin infusions decreased food intake, body fat, and lean tissue with a maximal response at 0.3 μg leptin/day. To test whether hindbrain receptors opposed activity of hypothalamic receptors, rats received peripheral infusions of 40 μg leptin/day and increasing 4th ventricle doses of the leptin receptor antagonist mutein protein. Mutein (3.0 μg/day) reduced body fat in PBS-infused rats to the same level as leptin-infused rats and reduced lean tissue in all rats. Leptin, but not mutein, inhibited food intake. By contrast, 3.0 μg/day mutein in the 3rd ventricle increased food intake and body fat in both PBS- and leptin-infused rats. In basal conditions, hindbrain leptin receptors may antagonize activity of forebrain receptors to protect lean and fat tissue, but there is no evidence for an anabolic role for hindbrain receptors when leptin is elevated. In a dietary study, rats increased energy intake when offered lard and 30% sucrose solution in addition to chow. Peripheral leptin infusion exaggerated the gain in body fat without altering energy intake confirming the potential for leptin to increase adiposity.

Neurochemical characterization and sexual dimorphism of projections from the brain to abdominal and subcutaneous white adipose tissue in the rat

Retroperitoneal white adipose tissue (rWAT) and subcutaneous (inguinal) white adipose tissue (iWAT) are both innervated and regulated by sympathetic efferents, but the distribution and identity of the cells in the brain that regulate sympathetic outflow are poorly characterized. Our aim was to use two isogenic strains of a neurotropic virus (pseudorabies, Bartha) tagged with either green or red fluorescent reporters to identify cells in the brain that project to rWAT and/or iWAT. These viruses were injected into separate WAT depots in male and female Sprague Dawley rats. Retrogradely labeled neurons in the CNS were characterized by immunohistochemistry and PCR. For the latter, laser capture of individual virally labeled neurons was used. All virally labeled brain regions contained neurons projecting to either and both WAT depots. Neurons to abdominal fat were the most abundant in males, whereas females contained a greater proportion of neurons to subcutaneous via private lines and collateral branches. Retrogradely labeled neurons directed to WAT expressed estrogen receptor-α (ERα), and fewer neurons to subcutaneous WAT expressed ERα in males. Regardless of sex, projections from the arcuate nucleus were predominantly from pro-opiomelanocortin cells, with a notable lack of projections from agouti-related protein-expressing neurons. Within the lateral hypothalamus, neurons directed to rWAT and iWAT expressed orexin and melanin-concentrating hormone (MCH), but male rats had a predominance of MCH directed to iWAT. In conclusion, the neurochemical substrates that project through polysynaptic pathways to iWAT and rWAT are different in male and female rats, suggesting that metabolic regulation of rWAT and iWAT is sexually dimorphic.

Germline ablation of VGF increases lipolysis in white adipose tissue

Targeted deletion of VGF, a neuronal and endocrine secreted protein and neuropeptide precursor, produces a lean, hypermetabolic mouse that is resistant to diet-, lesion-, and genetically induced obesity and diabetes. We hypothesized that increased sympathetic nervous system activity in Vgf-/Vgf- knockout mice is responsible for increased energy expenditure and decreased fat storage and that increased β-adrenergic receptor stimulation induces lipolysis in white adipose tissue (WAT) of Vgf-/Vgf- mice. We found that fat mass was markedly reduced in Vgf-/Vgf- mice. Within knockout WAT, phosphorylation of protein kinase A substrate increased in males and females, phosphorylation of hormone-sensitive lipase (HSL) (ser563) increased in females, and levels of adipose triglyceride lipase, comparative gene identification-58, and phospho-perilipin were higher in male Vgf-/Vgf- WAT compared with wild-type, consistent with increased lipolysis. The phosphorylation of AMP-activated protein kinase (AMPK) (Thr172) and levels of the AMPK kinase, transforming growth factor β-activated kinase 1, were decreased. This was associated with a decrease in HSL ser565 phosphorylation, the site phosphorylated by AMPK, in both male and female Vgf-/Vgf- WAT. No significant differences in phosphorylation of CREB or the p42/44 MAPK were noted. Despite this evidence supporting increased cAMP signaling and lipolysis, lipogenesis as assessed by fatty acid synthase protein expression and phosphorylated acetyl-CoA carboxylase was not decreased. Our data suggest that the VGF precursor or selected VGF-derived peptides dampen sympathetic outflow pathway activity to WAT to regulate fat storage and lipolysis.

Adipose tissue promotes a serum cytokine profile related to lower insulin sensitivity after chronic central leptin infusion

Obesity is an inflammatory state characterized by an augment in circulating inflammatory factors. Leptin may modulate the synthesis of these factors by white adipose tissue decreasing insulin sensitivity. We have examined the effect of chronic central administration of leptin on circulating levels of cytokines and the possible relationship with cytokine expression and protein content as well as with leptin and insulin signaling in subcutaneous and visceral adipose tissues. In addition, we analyzed the possible correlation between circulating levels of cytokines and peripheral insulin resistance. We studied 18 male Wistar rats divided into controls (C), those treated icv for 14 days with a daily dose of 12 μg of leptin (L) and a pair-fed group (PF) that received the same food amount consumed by the leptin group. Serum leptin and insulin were measured by ELISA, mRNA levels of interferon-γ (IFN-γ), interleukin-2 (IL-2), IL-4, IL-6, IL-10 and tumor necrosis factor-α (TNF-α) by real time PCR and serum and adipose tissue levels of these cytokines by multiplexed bead immunoassay. Serum leptin, IL-2, IL-4, IFN-γ and HOMA-IR were increased in L and TNF-α was decreased in PF and L. Serum leptin and IL-2 levels correlate positively with HOMA-IR index and negatively with serum glucose levels during an ip insulin tolerance test. In L, an increase in mRNA levels of IL-2 was found in both adipose depots and IFN-γ only in visceral tissue. Activation of leptin signaling was increased and insulin signaling decreased in subcutaneous fat of L. In conclusion, leptin mediates the production of inflammatory cytokines by adipose tissue independent of its effects on food intake, decreasing insulin sensitivity.

Evaluating the potential for rostral diffusion in the cerebral ventricles using angiotensin II-induced drinking in rats

In spite of evidence to the contrary, concern that substances injected into the fourth ventricle (4V) reach forebrain structures challenges the validity of using these injections to evaluate the role of hindbrain structures. Injection of AngII into the lateral ventricle (LV) increases water intake, but a similar response is not observed after injection into the 4V. This alone suggests the requirement of forebrain structures, but the potential for a counteracting, anti-dipsogenic pressor response to hindbrain AngII allows for lingering concern that this competing effect of AngII, rather than lack of forebrain access, underlies the negative result. Here, we used a double cannulation approach (LV and 4V) to evaluate the effect of the AngII receptor antagonist, losartan, on the drinking response to AngII injected into the LV. Injections of losartan into the LV blocked the dipsogenic response to AngII given 5min later into the LV. There was no effect, however, when losartan was injected into 4V, even when we used a dose of losartan that was 25 times greater than needed when injected into the LV. Collectively, these experiments suggest that concerns about diffusion from hindbrain ventricles to forebrain structures are overstated and can be circumvented using proper dose and timing of injections. Moreover, these data provide additional support to the existing literature showing that forebrain structures are key sites in the stimulation of drinking behavior by AngII.

Changes in glucose tolerance and leptin responsiveness of rats offered a choice of lard, sucrose, and chow

Rats offered chow, lard, and 30% sucrose solution (choice) rapidly become obese. We tested metabolic disturbances in rats offered choice, chow+lard, or chow+30% sucrose solution [chow+liquid sucrose (LS)] and compared them with rats fed a composite 60% kcal fat, 7% sucrose diet [high-fat diet (HFD)], or a 10% kcal fat, 35% sucrose diet [low-fat diet (LFD)]. Choice rats had the highest energy intake, but HFD rats gained the most weight. After 23 days carcass fat was the same for choice, HFD, chow+lard, and chow+LS groups. Glucose clearance was the same for all groups during an intraperitoneal glucose tolerance test (GTT) on day 12, but fasting insulin was increased in choice, LFD fed, and chow+LS rats. By contrast, only choice and chow+LS rats were resistant to an intraperitoneal injection of 2 mg leptin/kg on day 17. In experiment 2 choice rats were insulin insensitive during an intraperitoneal GTT, but this was corrected in an oral GTT due to GLP-1 release. UCP-1 protein was increased in brown fat and inguinal white fat in choice rats, and this was associated with a significant increase in energy expenditure of choice rats during the dark period whether expenditure was expressed on a per animal or a metabolic body size basis. The increase in expenditure obviously was not great enough to prevent development of obesity. Further studies are required to determine the mechanistic basis of the rapid onset of leptin resistance in choice rats and how consumption of sucrose solution drives this process.

The Relationship between Heart Rate Variability and Adiposity Differs for Central and Overall Adiposity

While frank obesity is associated with reduced HRV, indicative of poorer autonomic nervous system (ANS) function, the association between body mass index (BMI) and HRV is less clear. We hypothesized that effects of adiposity on ANS are mostly mediated by visceral fat and less by subcutaneous fat; therefore, centrally distributed adipose tissue, that is, waist circumference (WC), should be more strongly associated with HRV than overall adiposity (BMI). To examine this hypothesis, we used data collected in a subset of the Baltimore Longitudinal Study of Aging to compare strength of association between HRV and WC to that of HRV and BMI. Time domain HRV variables SDNN (standard deviation of successive differences in normal-to-normal (N-N) intervals) and RMSSD (root mean square of successive differences in N-N intervals) were calculated from 24-hour Holter recordings in 159 participants (29-96 years). Increasing WC was associated with decreasing SDNN and RMSSD in younger but not older participants (P value for WC-by-age interaction = 0.003). BMI was not associated with either SDNN or RMSSD at any age. In conclusion, central adiposity may contribute to sympathetic and parasympathetic ANS declines early in life.

Ablation of LMO4 in glutamatergic neurons impairs leptin control of fat metabolism

The LIM domain only 4 (LMO4) protein is expressed in the hypothalamus, but its function there is not known. Using mice with LMO4 ablated in postnatal glutamatergic neurons, including most neurons of the paraventricular (PVN) and ventromedial (VMH) hypothalamic nuclei where LMO4 is expressed, we asked whether LMO4 is required for metabolic homeostasis. LMO4 mutant mice exhibited early onset adiposity. These mice had reduced energy expenditure and impaired thermogenesis together with reduced sympathetic outflow to adipose tissues. The peptide hormone leptin, produced from adipocytes, activates Jak/Stat3 signaling at the hypothalamus to control food intake, energy expenditure, and fat metabolism. Intracerebroventricular infusion of leptin suppressed feeding similarly in LMO4 mutant and control mice. However, leptin-induced fat loss was impaired and activation of Stat3 in the VMH was blunted in these mice. Thus, our study identifies LMO4 as a novel modulator of leptin function in selective hypothalamic nuclei to regulate fat metabolism.

Sensory and sympathetic nervous system control of white adipose tissue lipolysis

Circulating factors are typically invoked to explain bidirectional communication between the CNS and white adipose tissue (WAT). Thus, initiation of lipolysis has been relegated primarily to adrenal medullary secreted catecholamines and the inhibition of lipolysis primarily to pancreatic insulin, whereas signals of body fat levels to the brain have been ascribed to adipokines such as leptin. By contrast, evidence is given for bidirectional communication between brain and WAT occurring via the sympathetic nervous system (SNS) and sensory innervation of this tissue. Using retrograde transneuronal viral tract tracers, the SNS outflow from brain to WAT has been defined. Functionally, sympathetic denervation of WAT blocks lipolysis to a variety of lipolytic stimuli. Using anterograde transneuronal viral tract tracers, the sensory input from WAT to brain has been defined. Functionally, these WAT sensory nerves respond electrophysiologically to increases in WAT SNS drive suggesting a possible neural negative feedback loop to regulate lipolysis.

Analysis of knockout mice suggests a role for VGF in the control of fat storage and energy expenditure

Background

Previous studies of mixed background mice have demonstrated that targeted deletion of Vgf produces a lean, hypermetabolic mouse that is resistant to diet-, lesion-, and genetically-induced obesity. To investigate potential mechanism(s) and site(s) of action of VGF, a neuronal and endocrine secreted protein and neuropeptide precursor, we further analyzed the metabolic phenotypes of two independent VGF knockout lines on C57Bl6 backgrounds.

Results

Unlike hyperactive VGF knockout mice on a mixed C57Bl6-129/SvJ background, homozygous mutant mice on a C57Bl6 background were hypermetabolic with similar locomotor activity levels to Vgf+/Vgf+ mice, during day and night cycles, indicating that mechanism(s) other than hyperactivity were responsible for their increased energy expenditure. In Vgf-/Vgf- knockout mice, morphological analysis of brown and white adipose tissues (BAT and WAT) indicated decreased fat storage in both tissues, and decreased adipocyte perimeter and area in WAT. Changes in gene expression measured by real-time RT-PCR were consistent with increased fatty acid oxidation and uptake in BAT, and increased lipolysis, decreased lipogenesis, and brown adipocyte differentiation in WAT, suggesting that increased sympathetic nervous system activity in Vgf-/Vgf- mice may be associated with or responsible for alterations in energy expenditure and fat storage. In addition, uncoupling protein 1 (UCP1) and UCP2 protein levels, mitochondrial number, and mitochondrial cristae density were upregulated in Vgf-/Vgf- BAT. Using immunohistochemical and histochemical techniques, we detected VGF in nerve fibers innervating BAT and Vgf promoter-driven reporter expression in cervical and thoracic spinal ganglia that project to and innervate the chest wall and tissues including BAT. Moreover, VGF peptide levels were quantified by radioimmunoassay in BAT, and were found to be down-regulated by a high fat diet. Lastly, despite being hypermetabolic, VGF knockout mice were cold intolerant.

Conclusion

We propose that VGF and/or VGF-derived peptides modulate sympathetic outflow pathways to regulate fat storage and energy expenditure.

Effect of reducing hypothalamic ghrelin receptor gene expression on energy balance

Central and peripheral injections of fghrelin potently stimulates food intake via its receptor, GHSR1a expressed in the brain. In this study, we explored the role of GHSR1a in the paraventricular nucleus of the hypothalamus (PVN) by reducing their gene expression using the RNA interference (RNAi). pSUPER plasmids inserted with sh (short hairpin)-GHSR1a were injected into the PVN to reduce its expression. The transfected rats were monitored daily for their food intake and body weight throughout the experimental period lasting 8 days. We found that knockdown of GHSR1a did not affect daily food intake but significantly reduced body weight and blood ghrelin levels. This suggests that the central ghrelin system could selectively regulate body weight without affecting energy intake.

[Effect of pentoxifylline on the evolution of diabetic nephropathy]

BACKGROUND AND OBJECTIVE

Diabetic nephropathy (DN) is the principal cause of end-chronic kidney disease. Metabolic and hemodynamic components are directly involved. However, convincing data have shown that inflammation participates in the diabetic complications. The aim of this study was to investigate whether the inhibition of the inflammation and pro-inflammatory cytokines with pentoxifylline (PXF) attenuate the progression of the DN.

SUBJECTS AND METHOD

In a prospective, randomized, double-blind, placebo- controlled study, we evaluated the effect of PXF (1200 mg daily) during 12 months, in 34 patients with incipient or established DN. Evaluated parameters were inflammation, pro-inflammatory cytokines and urinary albumin excretion (UAE).

RESULTS

PXF treatment had a reno-protective effect determined by a significant reduction in the UAE in both incipient and established (p<0,01) DN patient. This effect was attributed to a reduction in the C-reactive protein, interleukin-6, tumor necrosis factor-alpha and leptin serum levels (p<0,01).

CONCLUSIONS

PXF treatment caused regression and prevented the progression of renal damage. Thus, PXF should be used in the preventive treatment of DN. These results showed that inflammation and pro-inflammatory cytokines are related to the progression of diabetic nephropathy.

Preadipocyte transplantation: an in vivo study of direct leptin signaling on adipocyte morphogenesis and cell size

Leptin has profound effects on adipose tissue metabolism. However, it remains unclear whether direct leptin signaling in adipocytes is involved. We addressed this question by transplanting inguinal adipose tissue stromal vascular cells (SVCs) from 4- to 5-wk-old wild-type (WT) and leptin receptor-deficient [Lepr(db/db) (db)] mice to inguinal and sternal subcutaneous sites in Ncr nude mice. Both WT and db SVCs gave rise to mature adipocytes with normal morphologies 3 mo after the transplantation. The average adipocyte size (microm(2)/cell) was not significantly different between WT and db transplants at either the inguinal (1,630 +/- 103 vs. 1,491 +/- 74) or the sternal site (1,788 +/- 107 vs. 1,596 +/- 92). Expression levels of beta(3)-adrenergic receptor, a major mediator of lipid mobilization, were indistinguishable between WT and db transplants and similar to those of the hosts. Additionally, adipocyte sizes of inguinal transplants and endogenous inguinal adipose tissues were closely correlated (beta = 0.76, P < 0.001), suggesting that the metabolic milieu of host mice has significant effects on adipocyte size of the transplants. Contrary to the indifference to donor's Lepr genotype, adipocyte size of the transplants was significantly affected by the donor's sex in a leptin receptor-dependent manner. In WT transplants, female SVCs gave rise to smaller adipocytes than male SVCs (1,358 +/- 127 vs. 2,133 +/- 171, P < 0.05). However, this sex difference was not significant in db transplants (1,537 +/- 121 vs. 1,655 +/- 140, P = 0.22). These data suggest that: 1) long-form receptor-mediated direct leptin signaling has no significant cell-autonomous effect on adipocyte differentiation and metabolism in adult mice, 2) sex may affect adipocyte metabolism via genetic and/or epigenetic programming, and 3) leptin may potentiate sexual dimorphism in adipocyte metabolism.

Peroxisome proliferator-activated receptor-gamma-mediated positive energy balance in the rat is associated with reduced sympathetic drive to adipose tissues and thyroid status

Peroxisome proliferator-activated receptor-gamma (PPARgamma) activation up-regulates thermogenesis-related genes in rodent white and brown adipose tissues (WAT and BAT) without increasing whole-body energy expenditure. We tested here whether such dissociation is the result of a negative modulation of sympathetic activity to WAT and BAT and thyroid axis components by PPARgamma activation. Administration of the PPARgamma agonist rosiglitazone (15 mg/kg.d) for 7 d to male Sprague Dawley rats increased food intake (10%), feed efficiency (31%), weight gain (45%), spontaneous motor activity (60%), and BAT and WAT mass and reduced whole-body oxygen consumption. Consistent with an anabolic setting, rosiglitazone markedly reduced sympathetic activity to BAT and WAT (>50%) and thyroid status as evidenced by reduced levels of plasma thyroid hormones (T(4) and T(3)) and mRNA levels of BAT and liver T(3)-generating enzymes iodothyronine type 2 (-40%) and type 1 (-32%) deiodinases, respectively. Rosiglitazone also decreased mRNA levels of the thyroid hormone receptor (THR) isoforms alpha1 (-34%) and beta (-66%) in BAT and isoforms alpha1 (-20%) and alpha2 (-47%) in retroperitoneal WAT. These metabolic effects were associated with a reduction in mRNA levels of the pro-energy expenditure peptides CRH and CART in specific hypothalamic nuclei. A direct central action of rosiglitazone is, however, unlikely based on its low brain uptake and lack of metabolic effects of intracerebroventricular administration. In conclusion, a reduction in BAT sympathetic activity and thyroid status appears to, at least partly, explain the PPARgamma-induced reduction in energy expenditure and the fact that up-regulation of thermogenic gene expression does not translate into functional stimulation of whole-body thermogenesis in vivo.

Differential activation of the sympathetic innervation of adipose tissues by melanocortin receptor stimulation

Melanocortins are implicated in the control of energy intake/expenditure. Centrally administered melanotan II (MTII), a synthetic melanocortin 3/4-receptor agonist, decreases adiposity beyond that accountable by food intake decreases. Melanocortin-4 receptor (MC4-R) mRNA is expressed on sympathetic nervous system (SNS) outflow neurons to white adipose tissue (WAT) in Siberian hamsters, suggesting a role in lipid mobilization. Therefore, we tested whether third ventricular injections of MTII increased sympathetic drive to WAT and interscapular brown adipose tissue (IBAT) using norepinephrine turnover (NETO) as a measure of sympathetic drive. We also tested for MTII-induced changes in lipolysis-related WAT gene expression (beta3-adrenoceptors, hormone sensitive lipase) and IBAT thermogenesis (beta3-adrenoceptor, uncoupling protein-1). Finally, we tested whether third ventricularly injected MTII, a highly selective MC4-R agonist (cyclo[beta-Ala-His-D-Phe-Arg-Trp-Glu]NH2) increased or agouti-related protein decreased IBAT temperature in hamsters implanted with sc IBAT temperature transponders. Centrally administered MTII provoked differential sympathetic drives to WAT and IBAT (increased inguinal WAT, dorsosubcutaneous WAT and IBAT NETO, but not epididymal WAT and retroperitoneal WAT NETO). MTII also increased circulating concentrations of the lipolytic products free fatty acids and glycerol but not plasma catecholamines, suggesting lipid mobilization via WAT SNS innervation and not via adrenal medullary catecholamines. WAT or IBAT gene expression was largely unaffected by acute MTII treatment, but IBAT temperature was increased by MTII and the MC4-R agonist and decreased by agouti-related protein. Collectively, this is the first demonstration of central melanocortin agonist stimulation of WAT lipolysis through the SNS and confirms melanocortin-induced changes in BAT thermogenesis.

Leptin responsiveness in chronically decerebrate rats

Peripheral infusions of physiological doses of leptin decrease body fat mass, but it is not known whether this results from direct effects on peripheral tissue or activation of central leptin receptors. In this study, we infused chronically decerebrate (CD) rats, in which the forebrain was surgically isolated from the caudal brainstem, with 60 microg leptin/d or PBS for 14 d from ip mini-osmotic pumps. The CD rats were tube fed an amount of food equivalent to the intake of ad libitum-fed intact controls or 75% of this amount to account for their reduced energy expenditure. Control rats fed ad libitum or tube fed 75, 100, or 125% of their ad libitum intake also were peripherally infused with leptin or PBS. CD rats had a lower serum testosterone, energy expenditure, and lean body mass compared with controls but had increased levels of adiponectin and leptin and were obese. Leptin increased body fat and decreased energy expenditure during the light period in 100%-fed CD rats, but not 75%-fed CD rats. Leptin decreased body fat of ad libitum- and 100%-fed but not 75%-fed or 125%-fed intact controls. Energy expenditure did not change in any control group. These results show that leptin can change body fat independent of a change in food intake or energy expenditure, that the forebrain normally prevents leptin from inhibiting energy expenditure through mechanisms initiated in the caudal brainstem or peripheral tissues, and that the leptin response in both intact and CD rats is determined by the energy status of the animal.

Brain-adipose tissue neural crosstalk

The preponderance of basic obesity research focuses on its development as affected by diet and other environmental factors, genetics and their interactions. By contrast, we have been studying the reversal of a naturally-occurring seasonal obesity in Siberian hamsters. In the course of this work, we determined that the sympathetic innervation of white adipose tissue (WAT) is the principal initiator of lipid mobilization not only in these animals, but in all mammals including humans. We present irrefutable evidence for the sympathetic nervous system (SNS) innervation of WAT with respect to neuroanatomy (including its central origins as revealed by transneuronal viral tract tracers), neurochemistry (norepinephrine turnover studies) and function (surgical and chemical denervation). A relatively unappreciated role of WAT SNS innervation also is reviewed--the control of fat cell proliferation as shown by selective chemical denervation that triggers adipocyte proliferation, although the precise mechanism by which this occurs presently is unknown. There is no, however, equally strong evidence for the parasympathetic innervation of this tissue; indeed, the data largely are negative severely questioning its existence and importance. Convincing evidence also is given for the sensory innervation of WAT (as shown by tract tracing and by markers for sensory nerves in WAT), with suggestive data supporting a possible role in conveying information on the degree of adiposity to the brain. Collectively, these data offer an additional or alternative view to the predominate one of the control of body fat stores via circulating factors that serve as efferent and afferent communicators.

Changes in environmental temperature influence leptin responsiveness in low- and high-fat-fed mice

Loss of body fat in leptin-treated animals has been attributed to reduced energy intake, increased thermogenesis, and preferential fatty acid oxidation. Leptin does not decrease food intake or body fat in leptin-resistant high-fat (HF)-fed mice, possibly due to a failure of leptin to activate hypothalamic receptors. We measured energy expenditure of male C57BL/6 mice adapted to low-fat (LF) or HF diet and infused them for 13 days with PBS or 10 μg leptin/day from an intraperitoneal miniosmotic pump to test whether leptin resistance prevented leptin-induced increases in energy expenditure and fatty acid oxidation. There was no effect of low-dose leptin infusions on either of these measures in LF-fed or HF-fed mice, even though LF-fed mice lost body fat. Experiment 2 tested leptin responsiveness in LF-fed and HF-fed mice housed at different temperatures (18°C, 23°C, 27°C), assuming that the cold would increase and the hot environment would inhibit food intake and thermogenesis, which could potentially interfere with leptin action. LF-fed mice housed at 23°C were the only mice that lost body fat during leptin infusion, suggesting that an ability to modify energy expenditure is essential to the maintenance of leptin responsiveness. HF-fed mice in cold or warm environments did not respond to leptin. HF-fed mice in the hot environment were fatter than other HF-fed mice, and, surprisingly, leptin caused a further increase in body fat, demonstrating that the mice were not totally leptin resistant and that partial leptin resistance in a hot environment favors positive energy balance and fat deposition.

Thematic review series: adipocyte biology. Sympathetic and sensory innervation of white adipose tissue

During our study of the reversal of seasonal obesity in Siberian hamsters, we found an interaction between receptors for the pineal hormone melatonin and the sympathetic nervous system (SNS) outflow from brain to white adipose tissue (WAT). This ultimately led us and others to conclude that the SNS innervation of WAT is the primary initiator of lipid mobilization in these as well as other animals, including humans. There is strong neurochemical (norepinephrine turnover), neuroanatomical (viral tract tracing), and functional (sympathetic denervation-induced blockade of lipolysis) evidence for the role of the SNS in lipid mobilization. Recent findings suggest the presence of WAT sensory innervation based on strong neuroanatomical (viral tract tracing, immunohistochemical markers of sensory nerves) and suggestive functional (capsaicin sensory denervation-induced WAT growth) evidence, the latter implying a role in conveying adiposity information to the brain. By contrast, parasympathetic nervous system innervation of WAT is characterized by largely negative neuroanatomical evidence (viral tract tracing, immunohistochemical and biochemical markers of parasympathetic nerves). Functional evidence (intraneural stimulation and in situ microdialysis) for the role of the SNS innervation in lipid mobilization in human WAT is convincing, with some controversy regarding the level of sympathetic nerve activity in human obesity.