Central administration of resistin promotes short-term satiety in rats

Central resistin regulates hypothalamic and peripheral lipid metabolism in a nutritional-dependent fashion

Evidence suggests that the adipocyte-derived hormone resistin (RSTN) directly regulates both feeding and peripheral metabolism through, so far, undefined hypothalamic-mediated mechanisms. Here, we demonstrate that the anorectic effect of RSTN is associated with inappropriately decreased mRNA expression of orexigenic (agouti-related protein and neuropeptide Y) and increased mRNA expression of anorexigenic (cocaine and amphetamine-regulated transcript) neuropeptides in the arcuate nucleus of the hypothalamus. Of interest, RSTN also exerts a profound nutrition-dependent inhibitory effect on hypothalamic fatty acid metabolism, as indicated by increased phosphorylation levels of both AMP-activated protein kinase and its downstream target acetyl-coenzyme A carboxylase, associated with decreased expression of fatty acid synthase in the ventromedial nucleus of the hypothalamus. In addition, we also demonstrate that chronic central RSTN infusion results in decreased body weight and major changes in peripheral expression of lipogenic enzymes, in a tissue-specific and nutrition-dependent manner. Thus, in the fed state central RSTN is associated with induced expression of fatty acid synthesis enzymes and proinflammatory cytokines in liver, whereas its administration in the fasted state does so in white adipose tissue. Overall, our results indicate that RSTN controls feeding and peripheral lipid metabolism and suggest that hepatic RSTN-induced insulin resistance may be mediated by central activation of de novo lipogenesis in liver.

Hypothalamic regulation of appetite

The prevalence of obesity is steadily rising and has huge health and financial implications for society. Weight gain is due to an imbalance between dietary intake and energy expenditure and research has focused on trying to understand the complex pathways involved in controlling these aspects. This review highlights the key areas of research in the hypothalamic control of appetite. The hypothalamus consists of several nuclei that integrate peripheral signals, such as adiposity and caloric intake, to regulate important pathways within the CNS controlling food intake. The best characterized pathways are the orexigenic neuropeptide Y/Agouti-related protein and the anorexigenic pro-opiomelanocortin/cocaine- and amphetamine-related transcript neurons in the arcuate nucleus of the hypothalamus. These project from the arcuate nucleus to other key hypothalamic nuclei, such as the paraventricular, dorsomedial, ventromedial and lateral hypothalamic nuclei. There are also projections to and from the brainstem, cortical areas and reward pathways, all of which influence food intake. The challenge at present is to understand the complexity of these pathways and try to find ways of modulating them in order to find potential therapeutic targets.

How cytokines can influence the brain: a role for chemokines?

Following inflammation or infection, cytokines are released in the blood. Besides their effect on the immune system, cytokines can also act in the brain to modulate our behaviors, inducing for example anorexia when produced in large amount. This review focuses on our current knowledge on how cytokines can influence the brain and the behaviors through several possible pathways: modulating peripheral neurons which project to the brain through the vagus nerve, modulating the levels of hormones such as leptin which can act to the brain through the humoral pathway and possibly acting directly in the brain, through the local production of cytokines and chemokines such as SDF-1alpha/CXCL12.

Adipose tissue hormones and the regulation of food intake

Over the past decade, adipose tissue has been shown to produce numerous factors that act as hormones. Many of these act on the brain to regulate energy balance via dual effects on food intake and energy expenditure. These include well-characterised hormones such as leptin, oestrogen and glucocorticoids and novel factors such as adiponectin and resistin. This review provides a perspective on the role of these factors as lipostats.

Long-term effects of central leptin and resistin on body weight, insulin resistance, and beta-cell function and mass by the modulation of hypothalamic leptin and insulin signaling

To determine the long-term effect of central leptin and resistin on energy homeostasis, peripheral insulin resistance, and beta-cell function and mass, intracerebroventricular (ICV) infusion of leptin (3 ng/h), resistin (80 ng/h), leptin plus resistin, and cerebrospinal fluid (control) was conducted by means of an osmotic pump for 4 wk on normal rats and 90% pancreatectomized diabetic rats fed 40% fat-energy diets. Overall, the effects were greater in diabetic rats than normal rats. Leptin infusion, causing a significant reduction in food intake, decreased body weight and epididymal fat. However, resistin and leptin plus resistin reduced epididymal fat with decreased serum leptin levels in comparison with the control. Unlike serum leptin, only resistin infusion lowered serum resistin levels. Central leptin increased glucose infusion rates during euglycemic hyperinsulinemic clamp and suppressed hepatic glucose production in the hyperinsulinemic state in comparison with the control. However, central leptin did not affect glucose-stimulated insulin secretion and beta-cell mass. Central resistin infusion also increased peripheral insulin sensitivity, but not as much as leptin. Unlike leptin, resistin significantly increased first-phase insulin secretion during hyperglycemic clamp and beta-cell mass by augmenting beta-cell proliferation. These metabolic changes were associated with hypothalamic leptin and insulin signaling. ICV infusion of leptin potentiated signal transducer and activator of transcription 3 phosphorylation and attenuated AMP kinase in the hypothalamus, but resistin had less potent effects than leptin. Leptin enhanced insulin signaling by potentiating IRS2-->Akt pathways, whereas resistin activated Akt without augmenting insulin receptor substrate 2 phosphorylation. In conclusion, long-term ICV infusion of leptin and resistin independently improved energy and glucose homeostasis by modulating in different ways hypothalamic leptin and insulin signaling.

Metabolic and hormonal changes during the refeeding period of prolonged fasting

OBJECTIVE

The discovery of leptin, a hormone primarily involved in adaptation to fasting, led to an increased interest in appetite regulation and appetite-modulating hormones. Here, we present unique data from a case of extreme starvation and refeeding, showing changes in plasma concentrations of appetite-modulating and metabolic hormones as well as biochemical changes, and draw attention to the dangers of the refeeding syndrome.

PATIENTS AND METHODS

We studied the refeeding period of a 44-day voluntary fast uncomplicated by underlying disease. Biochemical and hormonal variables were compared with 16 matched subjects such that the BMI range of the controls covered the entire spectrum for the index subject's recovering BMI.

RESULTS

Lack of calorie intake with free access to water resulted in 25% loss of body weight. Haemoconcentration was observed and feeding was started with a low sodium, hypocaloric liquid formulation. During early refeeding, marked hypophosphataemia, haemodilution and slight oedema developed. Vitamins B1, B12 and B6 were depleted while serum free fatty acids, ketone bodies and zinc levels were abnormally high; abnormal liver function developed over the first week. The hormonal profile showed low IGF-I and insulin levels, and elevated IGF-binding protein-1 concentrations. Appetite-regulating hormones were either very low (leptin and ghrelin) or showed no marked difference from the control group (peptide YY, agouti-related peptide, alpha-melanocyte-stimulating hormone, neuropeptide Y and pro-opiomelanocortin). Appetite was low at the beginning of refeeding and a transient increase in orexin and resistin was observed coincidently with an increase in subjective hunger.

CONCLUSIONS

Our study illustrates the potential dangers of refeeding and provides a comprehensive insight into the endocrinology of prolonged fasting and the refeeding process.

Peripheral tissue-brain interactions in the regulation of food intake

More than 70 years ago the glucostatic, lipostatic and aminostatic hypotheses proposed that the central nervous system sensed circulating levels of different metabolites, changing feeding behaviour in response to the levels of those molecules. In the last 20 years the rapid increase in obesity and associated pathologies in developed countries has involved a substantial increase in the knowledge of the physiological and molecular mechanism regulating body mass. This effort has resulted in the recent discovery of new peripheral signals, such as leptin and ghrelin, as well as new neuropeptides, such as orexins, involved in body-weight homeostasis. The present review summarises research into energy balance, starting from the original classical hypotheses proposing metabolite sensing, through peripheral tissue-brain interactions and coming full circle to the recently-discovered role of hypothalamic fatty acid synthase in feeding regulation. Understanding these molecular mechanisms will provide new pharmacological targets for the treatment of obesity and appetite disorders.

Adipokine gene expression in a novel hypothalamic neuronal cell line: resistin-dependent regulation of fasting-induced adipose factor and SOCS-3

Adipokines such as leptin, resistin, and fasting-induced adipose factor (FIAF) are secreted by adipocytes, but their expression is also detectable in the brain and pituitary. The role of central adipokines remains elusive, but we speculate that they may modulate those hypothalamic signaling pathways that control energy homeostasis. Here we describe experiments to test this in which we exploited a novel hypothalamic neuronal cell line (N-1) that expresses a variety of neuropeptides and receptors that are known to be implicated in appetite regulation. Using real-time RT-PCR, we confirmed that N-1 neurons express resistin (rstn) and fiaf, as well as suppressor of cytokine signaling-3 (socs-3), a feedback inhibitor of leptin signaling. Treating N-1 cells with recombinant resistin (200 ng/ml, 30 min) reduced both fiaf (25%, p < 0.005) and socs-3 (29%, p < 0.005) mRNA levels, and similar reductions in fiaf (40%, p < 0.001) and socs-3 (25%, p < 0.001) resulted following the overexpression of resistin. Conversely, when RNA interference (RNAi) was used to reduce endogenous rstn levels (-60%, p < 0.005), fiaf and socs-3 expression was increased (46 and 65% respectively, p < 0.005). A similar reduction in rstn mRNA was achieved using RNAi in differentiated 3T3-L1 adipocytes, and this manipulation also reduced fiaf and socs-3 expression (-53, -21 and -20% respectively, p < 0.005). In contrast, although RNAi successfully reduced fiaf mRNA by 50% (p < 0.001) in N-1 cells and 40% (p < 0.001) in 3T3-L1 cells, there was no effect on rstn or socs-3 mRNA. These data suggest that resistin exerts a novel autocrine/paracrine control over fiaf and socs-3 expression in both 3T3-L1 adipocytes and N-1 neurons. Such a mechanism could be part of the central feedback system that modulates the effects of adipokines, and other adiposity signals, implicated in hypothalamic energy homeostasis. However, it remains to be determined whether these in vitro results can be translated to the control of adipokine expression in brain and adipose tissue.

Adipokine gene expression in brain and pituitary gland

The brain has been recognized as a prominent site of peptide biosynthesis for more than 30 years, and many neuropeptides are now known to be common to gut and brain. With these precedents in mind it is remarkable that adipose-derived peptides like leptin have attracted minimal attention as brain-derived putative neuromodulators of energy balance. This review outlines the evidence that several adipose-specific genes are also expressed in the central nervous system and pituitary gland. We, and others, confirmed that the genes for leptin, resistin, adiponectin, FIAF (fasting-induced adipose factor) and adiponutrin are expressed and regulated in these tissues. For example, leptin mRNA was readily detectable in human, rat, sheep and pig brain, but not in the mouse. Leptin expression in rat brain and pituitary was regulated through development, by food restriction, and following traumatic brain injury. In contrast, hypothalamic resistin mRNA was unaffected by age or by fasting, but was significantly depleted by food restriction in mouse pituitary gland. Similar results were seen in the ob/ob mouse, and we noted a marked reduction in resistin-positive hypothalamic nerve fibres. Resistin and fiaf mRNA were also upregulated in hypoxic/ischaemic mouse brain. Our studies on the regulation of neuronal adipokines were greatly aided by the availability of clonal hypothalamic neuronal cell lines. One of these, N-1, expresses both rstn and fiaf together with several other neuropeptides and receptors involved in energy homeostasis. Selective silencing of rstn revealed an autocrine/paracrine regulatory system, mediated through socs-3 expression that may influence the feedback effects of insulin and leptin in vivo. A similar convergence of signals in the pituitary gland could also influence anterior pituitary hormone secretion. In conclusion, the evidence is suggestive that brain and pituitary-derived adipokines represent a local regulatory circuit that may fine tune the feedback effects of adipose hormones in the control of energy balance.

Type 2 diabetes and cardiovascular disease: getting to the fat of the matterThis paper is one of a selection of papers published in this Special Issue, entitled Young Investigators' Forum

The increasing national prevalence of obesity is a major public health concern and a substantial burden on the health care resources of Canada. In addition to the direct health impact of obesity, this condition is a well-established risk factor for the development of various prevalent comorbidities including type 2 diabetes, hypertension, and cardiovascular disease. Historically, adipose tissue has been regarded primarily as an organ for energy storage. However, the discovery of leptin in the mid 1990’s revolutionized our understanding of this tissue and has focused attention on the endocrine function of adipose tissue as a source of secreted bioactive peptides. These compounds, collectively termed adipokines, regulate a number of biological functions including appetite and energy balance, insulin sensitivity, lipid metabolism, blood pressure, and inflammation. The physiological importance of adipokines has led to the hypothesis that changes in the synthesis and secretion of these compounds in the obese are a causative factor contributing to the development of obesity and obesity-related diseases in these individuals. Following from this it has been proposed that pharmacologic manipulation of adipokine levels may provide novel effective therapeutic strategies to treat and prevent obesity, type 2 diabetes, and cardiovascular disease.

Brain adipocytokine action and metabolic regulation

Adipose tissue secretes factors that control various physiological systems. The fall in leptin during fasting mediates hyperphagia and suppresses thermogenesis, thyroid and reproductive hormones, and immune system. On the other hand, rising leptin levels in the fed state stimulate fatty acid oxidation, decrease appetite, and limit weight gain. These divergent effects of leptin occur through neuronal circuits in the hypothalamus and other brain areas. Leptin also regulates the activities of enzymes involved in lipid metabolism, e.g., AMP-activated protein kinase and stearoyl-CoA desaturase-1, and also interacts with insulin signaling in the brain. Adiponectin enhances fatty acid oxidation and insulin sensitivity, in part by stimulating AMP-activated protein kinase phosphorylation and activity in liver and muscle. Moreover, adiponectin decreases body fat by increasing energy expenditure and lipid catabolism. These effects involve peripheral and possibly central mechanisms. Adipose tissue mediates interconversion of steroid hormones and secretes proinflammatory cytokines, vasoactive peptides, and coagulation and complement proteins. Understanding the actions of these "adipocytokines" will provide insight into the pathogenesis and treatment of obesity and related diseases.

Serum resistin increases in a postprandial state during liquid meal challenge test in healthy human subjects

The role of resistin in humans is controversial although resistin has been linked to atherosclerotic and inflammatory processes. In rodents, resistin expression is suppressed after food restriction while central administration of resistin promotes short-term satiety. However, the nature of postprandial responses in circulating resistin in humans is unknown. Therefore, we investigated postprandial resistin concentrations in a pilot study in 19 healthy subjects and 19 controls matched for age and body mass index (BMI). Serum resistin, insulin and non-esterified fatty acids (NEFA) concentrations as well as plasma glucose and triglycerides were repeatedly assessed before and after ingestion of an isocaloric standardized liquid meal during a 300 min period.After consumption of liquid meal, serum resistin levels increased compared to fasting control (p=0.037). Postprandial plasma glucose and serum insulin increased (p<0.001) with lower glucose responses in females (p=0.001) and lower insulin responses in males (p=0.012). Plasma triglycerides increased and serum NEFA decreased with similar gender responses (p=0.025 and p<0.001, respectively). Serum resistin was not correlated to glucose, insulin, triglyceride, and NEFA responses to liquid meal challenge tests. The present data suggest that serum resistin increases postprandially in healthy humans. Additional studies are needed to elucidate normal 24-h daytime profiles in humans and differential response of serum resistin to macronutrient composition of meals.

Loss of resistin improves glucose homeostasis in leptin deficiency

Resistin levels are increased in obesity, and hyperresistinemia impairs glucose homeostasis in rodents. Here, we have determined the role of resistin in ob/ob mice that are obese and insulin resistant because of genetic deficiency of leptin. Loss of resistin increased obesity in ob/ob mice by further lowering the metabolic rate without affecting food intake. Nevertheless, resistin deficiency improved glucose tolerance and insulin sensitivity in these severely obese mice, largely by enhancing insulin-mediated glucose disposal in muscle and adipose tissue. In contrast, in C57BL/6J mice with diet-induced obesity but wild-type leptin alleles, resistin deficiency reduced hepatic glucose production and increased peripheral glucose uptake. Resistin deficiency enhanced Akt phosphorylation in muscle and liver and decreased suppressor of cytokine signaling-3 level in muscle, and these changes were reversed by resistin replacement. Together, these results provide strong support for an important role of resistin in insulin resistance and diabetes associated with genetic or diet-induced obesity.

Differential hypothalamic neuronal activation following peripheral injection of GLP-1 and oxyntomodulin in mice detected by manganese-enhanced magnetic resonance imaging

The anorexigenic gut hormones oxyntomodulin (OXM) and glucagon-like peptide-1 (GLP-1) are thought to physiologically regulate appetite and food intake. Using manganese-enhanced magnetic resonance imaging, we have shown distinct patterns of neuronal activation in the hypothalamus in response to intraperitoneal injections into fasted mice of 900 and 5400 nmol/kg OXM or 900 nmol/kg GLP-1. Administration of OXM at either dose resulted in a reduced rate of signal enhancement, reflecting a reduction in neuronal activity, in the arcuate, paraventricular, and supraoptic nuclei of the hypothalamus. Conversely, GLP-1 caused a reduction in signal enhancement in the paraventricular nucleus only and an increase in the ventromedial hypothalamic nucleus. Our data show that these two apparently similar peptides generate distinct patterns of activation within the hypothalamus, suggesting that GLP-1 and OXM may act via different hypothalamic pathways.

Increased expression of the adipokine genes resistin and fasting-induced adipose factor in hypoxic/ischaemic mouse brain

Adipose tissue is the primary source of the adipokines resistin and fasting-induced adipose factor (FIAF). We reported that the brain is also a site of adipokine expression, although their function there is unknown. Peripheral resistin and fasting-induced adipose factor are reported to be inflammatory markers, and we hypothesized that they would be induced in the brain by hypoxia/ischaemia. We show that neonatal hypoxia/ischaemia rapidly increased fiaf mRNA in the injured cortex and hippocampus at 2 and 7 days after hypoxia/ischaemia. In contrast, resistin (retn) mRNA was increased in the cortex only at 21 days after hypoxia/ischaemia. As a lipopolysaccharide-induced inflammatory response did not increase brain fiaf and retn mRNA levels, we conclude that brain injury may be responsible for the novel hypoxia/ischaemia-induced changes in adipokine gene expression. In summary, our results indicate that brain injury, or an inflammatory stimulus, regulates the central expression of two genes normally considered to be adipose tissue-specific. These observations add to our previous evidence that the brain is an important site of adipokine gene expression.

Adipokines that link obesity and diabetes to the hypothalamus

Adipose tissue plays a crucial role in energy homeostasis not only in storing triglyceride, but also responding to nutrient, neural, and hormonal signals, and producing factors which control feeding, thermogenesis, immune and neuroendocrine function, and glucose and lipid metabolism. Adipose tissue secretes leptin, steroid hormones, adiponectin, inflammatory cytokines, resistin, complement factors, and vasoactive peptides. The endocrine function of adipose tissue is typified by leptin. An increase in leptin signals satiety to neuronal targets in the hypothalamus. Leptin activates Janus-activating kinase2 (Jak2) and STAT 3, resulting in stimulation of anorexigenic peptides, e.g., alpha-MSH and CART, and inhibition of orexigenic peptides, e.g., NPY and AGRP. The reduction in leptin levels during fasting stimulates appetite, decreases thermogenesis, thyroid and reproductive hormones, and increases glucocorticoids. Leptin also stimulates fatty acid oxidation, insulin release, and peripheral insulin action. These effects involve regulation of PI-3 kinase, PTP-1B, suppressor of cytokine signaling-3 (SOCS-3), and AMP-activated protein kinase in the brain and peripheral organs. There is emerging evidence that leptin, adiponectin, and resistin act through overlapping pathways. Understanding the signal transduction of adipocyte hormones will provide novel insights on the pathogenesis and treatment of obesity, diabetes, and various metabolic disorders.