Isoproterenol decreases leptin expression in adipose tissue of obese humans

Effects of the central melanocortin system on feed intake, metabolic hormones and insulin action in the sheep

Voluntary feed intake is insufficient to meet the nutrient demands associated with late pregnancy in prolific ewes and early lactation in high-yielding dairy cows. Under these conditions, peripheral signals such as growth hormone and ceramides trigger adaptations aimed at preserving metabolic well-being. Recent work in rodents has shown that the central nervous system-melanocortin (CNS-MC) system, consisting of alpha-melanocyte-stimulating hormone (α-MSH) and agouti-related peptide (AGRP) acting respectively as agonist and antagonist on central MC receptors, contributes to the regulation of some of the same adaptations. To assess the effects of the CNC-MC on peripheral adaptations in ruminants, ewes were implanted with an intracerebroventricular cannula in the third ventricle and infused over days with artificial cerebrospinal fluid (aCSF), the α-MSH analog melanotan-I (MTI), or AGRP. Infusion of MTI at 0.03 nmol/h reduced intake, expressed as a fold of maintenance energy requirement (M), from 1.8 to 1.1 M (P < 0.0001), whereas AGRP at 0.3 nmol/h increased intake from 1.8 to 2.0 M (P < 0.01); these doses were used in all subsequent experiments. To assess the effect of MTI on plasma variables, sheep were fed ad libitum and infused with aCSF or MTI or pair-fed to MTI-treated sheep and infused with aCSF (aCSFPF). Feed intake of the MTI and aCSFPF groups was 40% lower than the aCSF group (P < 0.0001). MTI increased plasma triiodothyronine and thyroxine in an intake-independent manner (P < 0.05 or less) but was devoid of effects on plasma glucose, insulin, and cortisol. None of these variables were altered by AGRP infusion in sheep fed at a fixed intake of 1.6 M. To assess the effect of CNS-MC activation on insulin action, ewes were infused with aCSF or MTI over the last 3 d of a 14-d period when energy intake was limited to 0.3 M and studied under basal conditions and during hyperinsulinemic-euglycemic clamps. MTI had no effect on plasma glucose, plasma insulin, or glucose entry rate under basal conditions but blunted the ability of insulin to inhibit endogenous glucose production during hyperinsulinemic-euglycemic clamps (P < 0.0001). Finally, MTI tended to reduce plasma leptin in sheep fed at 0.3 M (P < 0.08), and this effect became significant at 0.6 M (P < 0.05); MTI had no effect on plasma adiponectin irrespective of feeding level. These data suggest a role for the CNC-MC in regulating metabolic efficiency and peripheral insulin action.

Immunometabolic adaptation and immune plasticity in pregnancy and the bi-directional effects of obesity

Mandatory maternal metabolic and immunological changes are essential to pregnancy success. Parallel changes in metabolism and immune function make immunometabolism an attractive mechanism to enable dynamic immune adaptation during pregnancy. Immunometabolism is a burgeoning field with the underlying principle being that cellular metabolism underpins immune cell function. With whole body changes to the metabolism of carbohydrates, protein and lipids well recognised to occur in pregnancy and our growing understanding of immunometabolism as a determinant of immunoinflammatory effector responses, it would seem reasonable to expect immune plasticity during pregnancy to be linked to changes in the availability and handling of multiple nutrient energy sources by immune cells. While studies of immunometabolism in pregnancy are only just beginning, the recognised bi-directional interaction between metabolism and immune function in the metabolic disorder obesity might provide some of the earliest insights into the role of immunometabolism in immune plasticity in pregnancy. Characterised by chronic low-grade inflammation including in pregnant women, obesity is associated with numerous adverse outcomes during pregnancy and beyond for both mother and child. Concurrent changes in metabolism and immunoinflammation are consistently described but any causative link is not well established. Here we provide an overview of the metabolic and immunological changes that occur in pregnancy and how these might contribute to healthy versus adverse pregnancy outcomes with special consideration of possible interactions with obesity.

The effects of moderate alterations in adrenergic activity on acute appetite regulation in obese women: A randomised crossover trial

BACKGROUND

Previous evidence has demonstrated that serum leptin is correlated with appetite in combination with, but not without, modest exercise.

AIM

The present experiments investigated the effects of exogenous adrenaline and α/β adrenoceptor blockade in combination with moderate exercise on serum leptin concentrations, appetite/satiety sensations and subsequent food intake in obese women.

METHODS

A total of 10 obese women ((mean ± SEM), age: 50 (1.9) years, body mass index 36 (4.1) kg/m2, waist 104.8 (4.1) cm) participated in two separate, double-blind randomised experimental trials. Experiment 1: moderate exercise after α/β adrenergic blocker (labetalol, 100 mg orally) versus moderate exercise plus placebo; experiment 2: adrenaline infusion for 20 minutes versus saline infusion. Appetite/satiety and biochemistry were measured at baseline, pre- and immediately post-intervention, then 1 hour post-intervention (i.e., before dinner). Food intake was assessed via ad libitum buffet-style dinner.

RESULTS

No differences were found in appetite/satiety, subsequent food intake or serum leptin in any of the studies (experiment 1 or experiment 2). In experiment 1, blood glucose was higher (p < 0.01) and plasma free fatty acids lower (p = 0.04) versus placebo. In experiment 2, plasma free fatty acids (p < 0.05) increased after adrenaline versus saline infusion.

CONCLUSIONS

Neither inhibition of exercise-induced adrenergic activity by combined α/β adrenergic blockade nor moderate increases in adrenergic activity induced by intravenous adrenaline infusion affected acute appetite regulation.

Leptin resensitisation: a reversion of leptin-resistant states

Leptin resistance refers to states in which leptin fails to promote its anticipated effects, frequently coexisting with hyperleptinaemia. Leptin resistance is closely associated with obesity and also observed in physiological situations such as pregnancy and in seasonal animals. Leptin resensitisation refers to the reversion of leptin-resistant states and is associated with improvement in endocrine and metabolic disturbances commonly observed in obesity and a sustained decrease of plasma leptin levels, possibly below a critical threshold level. In obesity, leptin resensitisation can be achieved with treatments that reduce body adiposity and leptinaemia, or with some pharmacological compounds, while physiological leptin resistance reverts spontaneously. The restoration of leptin sensitivity could be a useful strategy to treat obesity, maintain weight loss and/or reduce the recidivism rate for weight regain after dieting. This review provides an update and discussion about reversion of leptin-resistant states and modulation of the molecular mechanisms involved in each situation.

Investigations into the involvement of leptin in responses to stress

Stress is defined as a state that can threaten homeostasis in an organism to initiate the adaptive process. Stress mediators, which include the classic neuroendocrine hormones and a number of neurotransmitters, cytokines, and growth factors, regulate both basal and threatened homeostasis to help control the stress. Severity of stress, as well as malfunctioning of stress pathways, may impair its controllability, leading to the pathogenesis of psychiatric illnesses including depression. Leptin was initially identified as an antiobesity hormone, acting as a negative feedback adiposity signal to control energy homeostasis by binding to its receptors in the hypothalamus. Accumulating evidence has expanded the function of leptin from the control of energy balance to the regulation of other physiological and psychological processes. The aim of this paper is to evaluate the potential role of leptin in stress controllability. To this end, studies on the role of leptin in stress-induced activation of the hypothalamus-pituitary-adrenocortical axis, feeding behavior, learned helplessness, and other depression models have been accumulated. The knowledge accumulated in this article may facilitate the development of alternative treatment strategies, beyond serotonin and noradrenaline reuptake inhibition, for psychiatric care and stress-related disorders.

Enhanced leptin sensitivity, reduced adiposity, and improved glucose homeostasis in mice lacking exchange protein directly activated by cyclic AMP isoform 1

The prototypic second messenger cyclic AMP (cAMP) is essential for controlling cellular metabolism, including glucose and lipid homeostasis. In mammals, the majority of cAMP functions are mediated by cAMP-dependent protein kinase (PKA) and exchange proteins directly activated by cAMP (Epacs). To explore the physiological functions of Epac1, we generated Epac1 knockout mice. Here we report that Epac1 null mutants have reduced white adipose tissue and reduced plasma leptin levels but display heightened leptin sensitivity. Epac1-deficient mice are more resistant to high-fat diet-induced obesity, hyperleptinemia, and glucose intolerance. Furthermore, pharmacological inhibition of Epac by use of an Epac-specific inhibitor reduces plasma leptin levels in vivo and enhances leptin signaling in organotypic hypothalamic slices. Taken together, our results demonstrate that Epac1 plays an important role in regulating adiposity and energy balance.

Increased cardiovascular reactivity to acute stress and salt-loading in adult male offspring of fat fed non-obese rats

Diet-induced obesity in rat pregnancy has been shown previously to be associated with consistently raised blood pressure in the offspring, attributed to sympathetic over-activation, but the relative contributions to this phenotype of maternal obesity versus raised dietary fat is unknown. Sprague-Dawley female rats were fed either a control (4.3% fat, n = 11) or lard-enriched (23.6% fat, n = 16) chow 10 days prior to mating, throughout pregnancy and lactation. In conscious adult (9-month-old) offspring cardiovascular parameters were measured (radiotelemetry). The short period of fat-feeding did not increase maternal weight versus controls and the baseline blood pressure was similar in offspring of fat fed dams (OF) and controls (OC). However, adult male OF showed heightened cardiovascular reactivity to acute restraint stress (p<0.01; Δ systolic blood pressure (SBP) and Δheart rate (HR)) with a prolonged recovery time compared to male OC. α1/β-adrenergic receptor blockade normalised the response. Also, after dietary salt-loading (8%-NaCl ad libitum for 1 week) male OF demonstrated higher SBP (p<0.05) in the awake phase (night-time) and increased low/high frequency ratio of power spectral density of HR variability versus OC. Baroreflex gain and basal power spectral density components of the heart rate or blood pressure were similar in male OF and OC. Minor abnormalities were evident in female OF. Fat feeding in the absence of maternal obesity in pregnant rats leads to altered sympathetic control of cardiovascular function in adult male offspring, and hypertension in response to stressor stimuli.

Adipokines as novel modulators of lipid metabolism

In the mid-1990s, interest in adipose tissue - until then generally regarded as a mere energy reserve - was revived by the discovery of leptin. Since then numerous other cytokine-like hormones have been isolated from white adipose tissue. These adipokines have been investigated in relation to obesity, metabolic syndrome, insulin resistance and other pathological conditions and processes. In addition, it is now established that adipokines play a role in the maintenance of an inflammatory state in adipose tissue and in the development of obesity and comorbidities. The contributions of individual adipokines in the pathophysiological features of obesity have yet to be determined in full, but recent data highlight important roles for adipokines in lipid metabolism.

Integration of hormonal and nutrient signals that regulate leptin synthesis and secretion

This review summarizes recent advances in our understanding of the pre- and posttranscriptional mechanisms that regulate leptin production and secretion in adipocytes. Basal leptin production is proportional to the status of energy stores, i.e., fat cell size, and this is mainly regulated by alterations in leptin mRNA levels. Leptin mRNA levels are regulated by hormones, including glucocorticoids and catecholamines, but little is known about the transcriptional mechanisms involved. Leptin synthesis and secretion is also acutely modulated in response to hormones such as insulin and the availability of metabolic fuels. Acute variations in leptin production over a time course of minutes to hours are mediated at the levels of both translation and secretion. Increases in amino acids and insulin after a meal activate the mammalian target of rapamycin (mTOR) pathway, leading to an increase in specific rates of leptin biosynthesis. Cross-talk among mTOR, PKA, and AMP-activated protein kinase pathways appears to integrate hormonal and nutrient signals that regulate leptin mRNA translation, at least in part through mechanisms involving its 5'- and 3'-untranslated regions. In addition, the rate of leptin secretion from preformed stores in response to hormonal cues is also regulated. Insulin stimulates, and adrenergic agonists inhibit, leptin secretion, and this likely contributes to variations in the magnitude of nutrition-related leptin excursions and oscillations. Overall, the study of leptin production has contributed to a deepening understanding of leptin biology and, more broadly, to our understanding of the cellular and molecular mechanisms by which the adipocyte integrates hormonal and nutrient signals to regulate adipokine production.

Acute cold- and chronic heat-exposure upregulate hepatic leptin and muscle uncoupling protein (UCP) gene expression in broiler chickens

Emerging evidence showed that variations in environmental temperature affect both leptin and uncoupling protein (UCP) gene expression in mammals, whereas a little is known about such interactions in birds. Thus, we conducted the present study to investigate the influence of acute (2 hours) cold (4 degrees C) and chronic (10 days) heat (32 degrees C) exposure on hepatic leptin and muscle UCP gene expression in 5-wk-old broiler chickens. Both cold- and heat-exposure significantly (P < 0.05 to P < 0.001) upregulated hepatic leptin (by 35 and 46%, respectively) and muscle UCP mRNA levels (by 71 and 71%, respectively) compared to the thermoneutrality (22 degrees C). This result suggests that leptin and UCP may be involved in the thermoregulation response of chickens to extreme climate (cold and hot temperatures). The upregulation of hepatic leptin gene expression was accompanied by an increase in plasma leptin levels, indicating that leptin may be regulated at transcriptional level. The increase of leptin and UCP mRNA abundance, and leptinemia we report here were not related to plasma glucose or insulin levels. In conclusion, the exposure of broiler chickens to extreme ambient temperatures (cold and heat) increases hepatic leptin and muscle UCP gene expression.

Leptin and perioperative neuroendocrine stress response with two different anaesthetic techniques

BACKGROUND

Stress response to surgery is modulated by several factors, including magnitude of the injury, pain, type of procedure and choice of anaesthesia. Our purpose was to compare intra- and post-operative hormonal changes during total intravenous anaesthesia (TIVA) using propofol and remifentanil vs. sevoflurane anaesthesia in a low stress level surgical model (laparoscopy).

METHODS

We randomly allocated 18 patients undergoing laparoscopic surgery for benign ovarian cysts in two groups to receive either TIVA (group A=9) or sevoflurane anaesthesia (group B=9). Perioperative plasma levels of norepinephrine (NE), epinephrine (E), adrenocorticotropic hormone (ACTH), cortisol and leptin were measured. Blood samples were collected pre-operatively (time 0), 30 min after the beginning of surgery (time 1), after extubation (time 2), and 2 h (time 3) and 4 h after surgery (time 4).

RESULTS

The comparative analysis between the groups shows significantly higher values of NE (P<0.001 at time 1 and P<0.01 at time 3), E (P<0.001 at times 1 and 2; P<0.01 at time 3 and P<0.05 at time 4), ACTH (P<0.001 at times 1 and 2; P<0.05 at time 3) and cortisol (P<0.001 at times 1 and 2; P<0.01 at time 3; P<0.05 at time 4) in group B. The serum values of leptin were not significantly different between the two groups.

CONCLUSION

The choice of anaesthesia does not seem to affect the leptin serum levels but influences the release of stress response markers: ACTH, cortisol, NE and E.

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.

Why some of us get fat and what we can do about it

There is a widespread obesity epidemic in the developed world which is having an adverse impact on the health of affected individuals. Many of the afflicted have a genetic predisposition to become obese. These individuals become obese when they over consume highly palatable, calorically dense foods which are readily available at low cost. Once obesity occurs, fewer than 10% of affected individuals can sustain significant weight loss permanently. The hypothesis of this review is that some obesity-prone individuals have an inborn reduction in their ability to sense and respond to inhibitory signals from adipose stores and other organs which should limit their intake of energy when it exceeds their metabolic needs. Furthermore, the physiological processes which drive all of us to seek and ingest food and limit energy expenditure during periods of negative energy balance provide an irresistible drive to regain lost adipose stores in weight-reduced obese individuals. For this reason, prevention of obesity and the identification factors which promote the development of neural pathways which enhance the negative feedback signals from the periphery should be a major focus of ongoing research in this field.

A real-time PCR approach to evaluate adipogenic potential of amniotic fluid-derived human mesenchymal stem cells

Regulation of adipocyte differentiation is an important process in the control of adipose tissue development. So far, adipogenesis has been investigated through the use of various experimental models. In this work, we used human mesenchymal stem cells (hMSCs) obtained from amniotic fluid (AF) as an alternative model more representative of what naturally happens in vivo. In our opinion, these hMSCs are still not influenced by differentiation stimuli and could act in a way more correspondent to the physiological process of adipogenesis, representing also an ethically acceptable alternative to totipotent human embryonic stem cells (ES). Adipocyte differentiation was monitorated following the expressions of key genes. We measured the expression levels of PPARgamma2, PPARgamma-C1alpha, UCP-1, adipsin, and leptin genes using quantitative real-time PCR. We tested our experimental model with two different media. Understanding in vivo adipogenesis mechanisms will shed light on the pathophysiology of many diseases.

Central regulation of energy homeostasis intelligent design: how to build the perfect survivor

The perfect survivor must be able to eat and store as many calories as possible when food is readily available as a buffer against periods of scarcity. He must also reduce energy expenditure when food is scarce and efficiently and accurately restore lost adipose stores when food is again available. These processes are dependent on information relayed to a distributed central network of metabolic sensing neurons through hard-wired neural, metabolic, and hormonal signals from the periphery. These sensing neurons engage neuroendocrine, autonomic, and motor processes involved in arousal, motor activity, and the ingestion, absorption, assimilation, storage, and expenditure of calories. A raised threshold in these metabolic sensors for detecting inhibitory signals from increasing adipose stores allows continued intake of excess calories when they are readily available. Unfortunately, this mechanism for surviving periods of feast and famine predisposes the perfect survivor to become obese when highly palatable, energy dense foods are readily available at low energetic cost. It further assures that raised adipose stores are metabolically defended against attempts to lower them. Thus, effective treatment of obesity will only come with a better understanding of the physiological, metabolic, and neurochemical processes that ensure this defense of an elevated body weight.

Intermittent and rhythmic exposure to melatonin in primary cultured adipocytes enhances the insulin and dexamethasone effects on leptin expression

Considering the cyclic characteristic of production and secretion of pineal melatonin, it is reasonable to assume that this oscillation might be important in determining the variety of its circadian and seasonal effects. To simulate this physiological condition in vitro, isolated adipocytes were exposed to melatonin in a circadian-like pattern by adding the hormone to the incubating medium during 12 hr (mimicking the night), followed by an equal period without melatonin (mimicking the day). This intermittent procedure was interrupted when three cycles with melatonin were fulfilled (60-hr incubation). Here, we report the effects of melatonin (1 nM) added intermittently or continuously to the incubating medium alone or in combination with insulin (5 nM) and/or dexamethasone (7 nM) on leptin release and expression by rat adipocytes. After acute 12-hr incubation neither melatonin nor insulin alone affected leptin expression, but together they increased it by 105%. Dexamethasone increased leptin mRNA content and release (70%) but this effect was not enhanced by melatonin. Nevertheless, after 60 hr under intermittent melatonin, we observed a synergism between melatonin and dexamethasone. This interaction promoted an increment (75% compared with dexamethasone alone) in leptin release and expression. Our results suggest that circadian-like exposure to melatonin potentiates the dexamethasone action and is important to the effects promoted by insulin on leptin expression. Based on an in vitro approach, this work helps to clarify the physiological relevance and the repercussions of the in vivo circadian pattern of melatonin secretion.

Thyrotropin-releasing hormone in cardiovascular pathophysiology

Thyrotropin (TSH)-releasing hormone (TRH) also known as thyroliberin was the first of a number of peptides exerting several roles as a hormone and as a neuropeptide. Its ubiquitous distribution in the hypothalamus and in the extrahypothalamic regions and its diverse pharmacological and physiological effects are all features of its dual functions. For this reason, TRH has been the subject of much research throughout the past 20 years, work that has examined the structure, function, distribution, and regulation of the tripeptide and it has been extensively reviewed elsewhere [O'Leary R., O'Connor B. Thyrotropin-releasing hormone. J Neurochem. 1995;65:953-963.; Nillni E., Sevarino K. The biology of pro-thyrotropin-releasing hormone-derived peptides. Endocrine Reviews, 1999;20:599-664.]. After a brief overview of its distribution, hypothalamic and extrahypothalamic functions, and receptors involved, this review discusses efforts devoted to support TRH role in cardiovascular regulation with a main focus on hypertension pathophysiology in experimental models and humans.

Isoproterenol decreases leptin release from rat and human adipose tissue through posttranscriptional mechanisms

In vivo and in vitro studies indicate that beta-adrenergic receptor agonists decrease leptin release from fat cells in as little as 30 min. Our objective was to determine whether alterations in leptin biosynthesis or secretion were involved in the short-term adrenergic regulation of leptin in human and rat adipose tissue. Isoproterenol (Iso) decreased leptin release from incubated adipose tissue of both nonobese and obese subjects to similar extent (-28 vs. -21% after 3 h). Inhibition of protein synthesis with cycloheximide did not block the effect of Iso on leptin release from human adipose tissue, suggesting that the Iso effect is independent of leptin synthesis. Iso also tended to increase tissue leptin content at the end of the 3-h incubation, as expected from the observed inhibition of release. Consistent with a posttranslational mechanism, Iso treatment did not affect leptin mRNA levels or relative rate of leptin biosynthesis as directly assessed by [35S]methionine incorporation into immunoprecipitable leptin. In contrast to these results in human adipose tissues, Iso did not decrease basal leptin release from rat adipose tissue. However, Iso did decrease insulin-stimulated leptin release by inhibiting the ability of insulin to increase leptin biosynthesis without detectably affecting leptin mRNA levels. Thus, in both human and rat, adrenergic regulation of posttranscriptional events (secretion in humans, translation in rats) may contribute to the rapid decline in circulating leptin that occurs when the sympathetic nervous system is activated, such as during fasting and cold exposure. Furthermore, the rat does not provide an ideal model to study mechanisms of cellular leptin regulation in humans.

Melatonin enhances leptin expression by rat adipocytes in the presence of insulin

Leptin and melatonin play an important role in the regulation of body mass and energy balance. Both hormones show a circadian rhythm, with increasing values at night. In addition, melatonin receptors were recently described in adipocytes, where leptin is synthesized. Here, we investigated the influence of melatonin and its interaction with insulin and dexamethasone on leptin expression. Isolated rat adipocytes were incubated with melatonin (1 nM) alone or in combination with insulin (5 nM) and/or dexamethasone (7 nM) for 6 h. Melatonin or insulin alone did not affect leptin expression, but together they increased it by 120%. Dexamethasone increased leptin mRNA content (105%), and this effect was not enhanced by melatonin. Simultaneous treatment with the three hormones provoked a further increase in leptin release (250%) and leptin mRNA (100%). Melatonin prevented the forskolin-induced inhibition (95%) of leptin expression. In addition, melatonin's ability to stimulate leptin release (in the presence of insulin) was completely blocked by pertussis toxin and luzindole. To gain further insight into the molecular basis of melatonin and insulin synergism, the insulin-signaling pathway was investigated. Melatonin increased the insulin-induced insulin receptor-beta tyrosine phosphorylation, which led to an increased serine phosphorylation of the downstream convergent protein Akt. We concluded that melatonin interacts with insulin and upregulates insulin-stimulated leptin expression. These effects are caused by melatonin binding to the pertussis toxin-sensitive G(i) protein-coupled membrane receptor (MT1 subtype) and the cross talk with insulin, since insulin receptor and its convergent target Akt are coactivated by melatonin.

Regulation of leptin secretion from white adipocytes by free fatty acids

Norepinephrine stimulates lipolysis and concurrently inhibits insulin-stimulated leptin secretion from white adipocytes. To assess whether there is a cause-effect relationship between these two metabolic events, the effects of fatty acids were investigated in isolated rat adipocytes incubated in buffer containing low (0.1%) and high (4%) albumin concentrations. Palmitic acid (1 mM) mimicked the inhibitory effects of norepinephrine (1 microM) on insulin (10 nM)-stimulated leptin secretion, but only at low albumin concentrations. Studies investigating the effects of the chain length of saturated fatty acids [from butyric (C4) to stearic (C18) acids] revealed that only fatty acids with a chain length superior or equal to eight carbons effectively inhibited insulin-stimulated leptin secretion. Long-chain mono- and polyunsaturated fatty acids constitutively present in adipocyte triglyceride stores (oleic, linoleic, gamma-linolenic, palmitoleic, eicosapentanoic, and docosahexanoic acids) also completely suppressed leptin secretion. Saturated and unsaturated fatty acids inhibited insulin-stimulated leptin secretion with the same potency and without any significant effect on basal secretion. On the other hand, inhibitors of mitochondrial fatty acid oxidation (palmoxirate, 2-bromopalmitate, 2-bromocaproate) attenuated the stimulatory effects of insulin on leptin release without reversing the effects of fatty acids or norepinephrine, suggesting that fatty acids do not need to be oxidized by the mitochondria to inhibit leptin release. These results demonstrate that long-chain fatty acids mimic the effects of norepinephrine on leptin secretion and suggest that they may play a regulatory role as messengers between stimulation of lipolysis by norepinephrine and inhibition of leptin secretion.

Regulation of leptin release by insulin, glucocorticoids, G(i)-coupled receptor agonists, and pertussis toxin in adipocytes and adipose tissue explants from obese humans in primary culture

The basal release of leptin by adipocytes from massively obese human subjects incubated for 48 hours in serum-free suspension culture was comparable to that by explants of subcutaneous adipose tissue from the same obese individuals. There was no stimulation due to dexamethasone or insulin alone of leptin release by adipocytes. However, the combination of insulin and dexamethasone doubled leptin release by adipocytes. The release of leptin was also stimulated by agonists of G(i)-coupled receptors (prostaglandin E(2) [PGE(2)], brimonidine [an alpha(2) catecholamine agonist] and cyclopentyladenosine [CPA]) in the presence of dexamethasone. Leptin release by these agents was further enhanced by insulin in both adipocytes and adipose tissue. Pertussis toxin, which irreversibly inactivates G(i) heterotrimers, inhibited leptin release and abolished the stimulatory effects of G(i)-coupled receptor agonists. However, pertussis toxin did not block the stimulation of leptin release by insulin in either adipose tissue or adipocytes. These data indicate that the release of leptin by human adipocytes cultured for 48 hours in a serum-free medium is comparable to that by explants of adipose tissue except that dexamethasone stimulation of leptin release requires the presence of insulin.

Mechanisms of leptin secretion from white adipocytes

The mechanisms regulating leptin secretion were investigated in isolated rat white adipocytes. Insulin (1-100 nM) linearly stimulated leptin secretion from incubated adipocytes for at least 2 h. The adrenergic agonists norepinephrine, isoproterenol (two nonselective beta-agonists), or CL-316243 (potent beta3) all inhibited insulin (10 nM)-stimulated leptin release. The inhibitory effects of norepinephrine and isoproterenol could be reversed not only by the nonselective antagonist propranolol but also by the selective antagonists ICI-89406 (beta1) or ICI-118551 (beta2), the beta2-antagonist being less effective than the beta1. Insulin-stimulated leptin secretion could also be inhibited by a series of agents increasing intracellular cAMP levels, such as lipolytic hormones (ACTH and thyrotropin-stimulating hormone), various nonhydrolyzable cAMP analogs, pertussis toxin, forskolin, methylxanthines (caffeine, theophylline, IBMX), and specific inhibitors of phosphodiesterase III (imazodan, milrinone, and amrinone). Significantly, antilipolytic agents other than insulin (adenosine, nicotinic acid, acipimox, and orthovanadate) did not mimic the acute stimulatory effects of insulin on leptin secretion under these conditions. We conclude that norepinephrine specifically inhibits insulin-stimulated leptin secretion not only via the low-affinity beta3-adrenoceptors but also via the high-affinity beta1/beta2-adrenoceptors. Moreover, it is suggested that 1) activation of phosphodiesterase III by insulin represents an important metabolic step in stimulation of leptin secretion, and 2) lipolytic hormones competitively counterregulate the stimulatory effects of insulin by activating the adenylate cyclase system.

Thyrotropin-releasing hormone decreases leptin and mediates the leptin-induced pressor effect

Leptin, an adipocyte-released hormone, modifies food intake and energy expenditure regulating hypothalamic-pituitary-thyroid axis function. We previously reported that thyrotropin-releasing hormone (TRH) precursor gene overexpression induces hypertension in the normal rat and that spontaneously hypertensive rats have central TRH hyperactivity with increased TRH synthesis and release and an elevated TRH receptor number. In both models, intracerebroventricular antisense (AS) treatment against the TRH precursor produced a dose-dependent reduction of the increased diencephalic TRH content while normalizing high arterial blood pressure. In this article, we report that male Wistar rats that were made hypertensive by intracerebroventricular injection of a eucaryotic expression plasmid containing the pre-TRH cDNA showed decreased leptin plasma levels and that pre-TRH AS treatment reversed this phenomenon. In addition, male and female spontaneously hypertensive rats showed lower levels of circulating leptin than did sex-matched Wistar-Kyoto control rats. This difference also was abated by the pre-TRH AS treatment. Conversely, 20 microg ICV leptin induced a long-lasting pressor effect (18 +/- 5 mm Hg, n=6, P<0.01, >60 minutes) that was not observed in pre-TRH AS pretreated rats (2 +/- 3 mm Hg, n=6) but persisted in rats used as controls that were treated with inverted oligonucleotide (20 +/- 6 mm Hg, n=4, P<0.01). These data suggest that in rats with TRH-induced hypertension, leptin is decreased, inducing compensatory adiposity. We propose that because leptin produces central TRH synthesis and release, obesity may induce hypertension through TRH system activation and that the TRH-leptin interaction may thus contribute to the strong association between hypertension and obesity.

Plasma leptin response to an epinephrine infusion in lean and obese women

OBJECTIVE

Because leptin production by adipose tissue is under hormonal control, we examined the impact of epinephrine administration on plasma leptin concentrations.

RESEARCH METHODS AND PROCEDURES

We measured plasma leptin, insulin, and free fatty acid (FFA) responses after a 60-minute epinephrine infusion (0.010 microg/kg fat free mass/min) followed by a 30-minute recovery period (no infusion) in a group of 11 lean (mean body mass index +/- SD: 22.6 +/- 1.1 kg/m(2)) and 15 obese (30.0 +/- 1.3 kg/m(2)) premenopausal women. Leptin, insulin, and FFA levels were measured in plasma before (-15 and 0 minutes) and at every 30 minutes over the 90-minute period.

RESULTS

In both lean and obese individuals, plasma leptin was significantly reduced by epinephrine (p < 0.0001). Body fat mass was associated with fasting leptin levels (r = 0.64, p < 0.0005) as well as with the decrease in leptinemia (r = -0.51, p < 0.01) produced by epinephrine administration. Furthermore, we noted a large range of leptin response to epinephrine among our subjects, especially in obese women (from -12 to -570 ng/mL per 60 minutes). However, there was no association between postepinephrine leptin and FFA levels (r = -0.14, p = 0.55).

DISCUSSION

Results of this study indicate that leptin levels decrease after epinephrine administration in both lean and obese premenopausal women. However, the heterogeneity in the response of leptin to catecholamines suggests potential alterations of the leptin axis that may contribute to generate a positive energy balance and, thus, may favor weight gain in some obese individuals.

Insulin and glucocorticoids differentially regulate leptin transcription and secretion in brown adipocytes

Leptin, the ob gene product, is produced by adipose tissue and is submitted to a complex hormonal and metabolic regulation. Leptin plays a critical role in the balance of body weight. Here we report on secretion and hormonal regulation of leptin by brown adipocytes. Using the recently established T37i cell line, we show that leptin expression and secretion occurred as a function of cell differentiation. In differentiated T37i cells, insulin induced leptin release ( approximately 0.25 ng/10(6) cells/h) in a concentration-dependent manner (EC50=0.1 nM), and this was totally suppressed by beta3-adrenergic ligand, thiazolidinedione, cycloheximide, or actinomycin D. Insulin induced a strong, rapid (within 2 h) but transient fivefold increase in leptin mRNA levels. This transcriptional control of ob gene expression by insulin involved both phosphatidylinositol 3-kinase- and MAP kinase-dependent pathways. Glucocorticoids inhibited both insulin-stimulated leptin secretion and ob gene expression without affecting leptin mRNA stability (t(1/2)=3h05). Altogether, our results demonstrate that brown adipocytes express and secrete leptin, whose hormonal regulation clearly differs from that described in white adipose tissue. These findings point to tissue-specific molecular mechanisms and suggest that leptin might exert direct effects on energy homeostasis through an autocrine mechanism.

Regulation of the leptin content of obese human adipose tissue

The objective of this study was to determine whether obese human adipose tissue contains preformed stores of leptin and their relationship to secreted leptin. Detergent increased detectable leptin by about twofold, suggesting that leptin is stored in a membrane-bound location. Subcutaneous tissue leptin was approximately 1.6-fold higher than omental, paralleling known differences in leptin secretion and expression. The amount of leptin secreted during a 3-h incubation was similar to that of extractable tissue leptin. Tissue leptin levels were maintained over the incubation. Inhibition of protein synthesis decreased tissue leptin content but did not decrease leptin secretion until after 3 h of incubation. Culture of adipose tissue for 2 days with the combination of insulin and dexamethasone, but not with either hormone alone, increased tissue leptin content about twofold in both depots. Although insulin did not affect tissue leptin content, it potentiated leptin secretion (as a % of tissue stores). These data suggest that adipose tissue leptin storage and secretion per se are regulated. Regulation of the release of preformed leptin may modulate serum leptin levels in obese humans.

Regulation of leptin production in humans

Serum levels of the adipocyte hormone leptin are increased in proportion to body fat stores as a result of increased production in enlarged fat cells from obese subjects. In vitro studies indicate that insulin and glucocorticoids work directly on adipose tissue to upregulate in a synergistic manner leptin mRNA levels and rates of leptin secretion in human adipose tissue over the long term. Thus, the increased leptin expression observed in obesity could result from the chronic hyperinsulinemia and increased cortisol turnover. Superimposed upon the long-term regulation, nutritional status can influence serum leptin over the short term, independent of adiposity. Fasting leads to a gradual decline in serum leptin that is probably attributable to the decline in insulin and the ability of catecholamines to decrease leptin expression, as observed in both in vivo and in vitro studies. In addition, increases in serum leptin occur approximately 4-7 h after meals. Increasing evidence indicates that insulin, in concert with permissive effects of cortisol, can increase serum leptin over this time frame and likely contributes to meal-induced increases in serum leptin. Further research is required to elucidate the cellular and molecular mechanisms underlying short- and long-term nutritional and hormonal regulation of leptin production and secretion.

Secretory, endocrine and autocrine/paracrine function of the adipocyte

Obesity is a major public health problem in Western countries, and >55% of adult Americans are overweight or obese. A major contributor to the epidemic of obesity is the current environment, which is characterized by increased availability of high energy foods and decreased physical activity. Several studies also demonstrated that genetic susceptibility contributes to obesity in some populations. Obesity research has focused primarily on the role of the hypothalamus in neuroendocrine regulation of food intake. However, a growing number of studies support a potential contribution of adipose tissue, via its newly discovered secretory function, to the pathogenesis of obesity and co-morbid conditions including cardiovascular disease, diabetes and hypertension. This paper will review the role of four factors secreted by adipose tissue (leptin, agouti, angiotensin II and prostaglandins) and their functions in the regulation of energy balance and whole-body homeostasis. Several other peptide and nonpeptide substances are secreted from adipose tissue; their function and regulation have been documented extensively.

Steroid-dependent up-regulation of adipose leptin secretion in vitro during pregnancy in mice

Circulating leptin levels are elevated during the later stages of pregnancy in mammals, suggesting that maternal leptin may play a role in maintenance of pregnancy and/or preparation for parturition and lactation. The regulation and source of circulating leptin during pregnancy remains undetermined, but leptin mRNA levels increase in adipose tissue during this time in some species. Considerable controversy exists whether placenta is also a leptin-secreting tissue during pregnancy. Here, we directly demonstrate that leptin secretion rates from mouse adipose tissue in vitro are decreased during early pregnancy and up-regulated during late pregnancy and lactation. Changes in leptin secretion rates in vitro paralleled those of circulating leptin in vivo during gestation. Subcutaneous implants of estradiol or corticosterone into lactating mice for 48 h stimulated adipose leptin secretion rates in vitro to the level of that in pregnant mice. However, corticosterone, but not estradiol, increased leptin secretion when added to isolated adipose tissue in vitro. Placentae obtained at two stages of pregnancy did not secrete leptin in vitro, either when acutely isolated or when dissociated into cells for long-term cultures. Placental tissue (or cells) secreted progesterone, however, demonstrating placental viability. We conclude that hyperleptinemia during late pregnancy in mice primarily results from corticosterone-dependent up-regulation of leptin secretion from adipose tissue, and that the placenta does not contribute to leptin secretion. The initial decrease in leptin secretory rates from adipose tissue during early pregnancy may facilitate energy storage for the subsequent, increased metabolic demands of later pregnancy and lactation.

Synergism between insulin and low concentrations of isoproterenol in the stimulation of leptin release by cultured human adipose tissue

The release of leptin by pieces of human adipose tissue incubated in primary culture for 24 or 48 hours in the presence of dexamethasone was reduced by isoproterenol. An inhibition of leptin release was observed at 24 hours in the presence of isoproterenol and was mediated by beta1-adrenergic receptors, since it was blocked by the specific beta1-adrenoceptor antagonist CGP-20712A. The inhibitory effect of 33 nmol/L isoproterenol on leptin release was reversed in the presence of 0.1 nmol/L insulin to a 2-fold stimulation of leptin release. These data suggest that the primary mechanism by which insulin stimulates leptin release is to blunt the inhibitory effects of beta1-adrenergic receptor agonists, and low concentrations of catecholamines actually enhance the stimulation of leptin release by insulin.

Acute cold exposure decreases plasma leptin in women

We investigated whether cold exposure affects circulating leptin in humans. Five women (age, 32+/-4 years; body mass index, 23.1+/-1.7 kg/m2) participated in two separate trials. Subjects sat at room temperature ([RT] 24.8 degrees+/-0.3 degrees C) or in the cold (6.3 degrees+/-0.5 degrees C) for 90 minutes. During RT exposure, plasma leptin and norepinephrine were unchanged over time. Cold exposure significantly decreased plasma leptin by 14%, 17%, and 22% at 30, 60, and 90 minutes, respectively (temperature x time interaction, P < .04). Plasma norepinephrine increased by 400% to 500% (P < .001) and plasma glycerol increased by 110% over baseline during cold exposure (temperature effect, P < .005). We conclude that circulating leptin decreases during cold exposure, probably as a result of activation of the sympathetic nervous system (SNS).

Physiological roles of the leptin endocrine system: differences between mice and humans

Leptin is a 16-kDa cytokine secreted in humans primarily but not exclusively by adipose tissues. Its concentration in blood is usually proportional to body fat mass, but is higher in women than in men not only because of a different distribution of and greater fat mass in women, but also because testosterone reduces its level in men. Leptin features in different ways during the life span. It is synthesized in the ovary, transported in the oocyte, and made by both fetus and placenta, particularly during the last month of gestation. It is made by the lactating mammary gland and ingested by the newborn infant in its milk. The prime importance of leptin is realized at puberty when it is necessary for progression to a normal adult reproductive status in females. Fasting and chronic undernutrition result in a lower level of leptin in the blood. Lack of leptin results in hunger, ensuring that the individual eat to survive, and also inhibition of reproduction, until such time as food and fat stores are adequate to supply energy for pregnancy and lactation. Thus, leptin is important for survival of the individual and survival of the species. Although an extremely rare genetic absence of leptin induces hyperphagia and obesity in humans, as it does in mice, there appears to be little role for leptin in humans in ensuring that fat stores are not in excess of adequate, that is, in preventing obesity. The mouse differs from humans in many respects, in particular in the far more drastic ways it conserves energy when it very rapidly adapts to lack of food. These include not only suppression of reproduction but also lowering of its body temperature (torpor), suppressing its thyroid function, suppressing its growth, and increasing secretion of stress hormones (from the adrenal). This review concentrates on roles of leptin in human physiology and pathophysiology but also discusses why some observations on actions of leptin in mice are not applicable to humans.