A brain-liver circuit regulates glucose homeostasis

The meter of metabolism

The circadian system orchestrates the temporal organization of many aspects of physiology, including metabolism, in synchrony with the 24 hr rotation of the Earth. Like the metabolic system, the circadian system is a complex feedback network that involves interactions between the central nervous system and peripheral tissues. Emerging evidence suggests that circadian regulation is intimately linked to metabolic homeostasis and that dysregulation of circadian rhythms can contribute to disease. Conversely, metabolic signals also feed back into the circadian system, modulating circadian gene expression and behavior. Here, we review the relationship between the circadian and metabolic systems and the implications for cardiovascular disease, obesity, and diabetes.

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.

Insulin and the constituent branches of the hepatic vagus interact to modulate hypothalamic and limbic neuropeptide mRNA expression differentially

Insulin and signalling through the vagus nerve act in concert to regulate metabolic homeostasis and ingestive behaviour. Our previous studies using streptozotocin (STZ)-diabetic rats have shown that hepatic branch vagotomy (HV), gastroduodenal branch vagotomy (GV) and capsaicin treatment of the common hepatic branch that selectively destroys afferent fibres (CapV), all promote lard, but not total, caloric intake to levels similar to those achieved with insulin treatment. Because hypothalamic and limbic mRNA expression of neuropeptides linked to energy balance is altered by STZ-diabetes and HV, we examined the role(s) of insulin and the common hepatic and gastroduodenal branches of the vagus nerve and hepatic afferent fibres in the regulation of these neuropeptides in rats with high, steady-state corticosterone levels. STZ-diabetic rats were prepared with osmotic minipumps containing either saline or insulin and were compared with nondiabetic counterparts: half of each group received a vagal manipulation, the other half were sham operated. Five days after surgery, rats were offered the choice of lard and chow to consume for another 5 days, when brains were collected and processed for in situ hybridisation. Paraventricular nucleus corticotrophin-releasing factor (CRF) mRNA was elevated by STZ treatment, an effect prevented by either insulin treatment or GV. By contrast, CRF mRNA expression in the central nucleus of the amygdala and bed nuclei of the stria terminalis was unaffected by STZ treatment, but HV and CapV manipulations elevated expression in the nondiabetic, but not STZ-diabetic groups. Arcuate nucleus neuropeptide Y, but not pro-opiomelanocortin, mRNA expression was elevated by STZ treatment and all vagal manipulations; however, exogenous insulin treatment failed to prevent this, in keeping with their previously documented elevated caloric intake. These results strongly suggest that the gastroduodenal branch and hepatic branch proper, which merge to form the common hepatic branch, differentially interact with prevailing insulin levels to regulate hypothalamic and limbic neuropeptide mRNA expression.

The vagus nerve, food intake and obesity

Food interacts with sensors all along the alimentary canal to provide the brain with information regarding its composition, energy content, and beneficial effect. Vagal afferents innervating the gastrointestinal tract, pancreas, and liver provide a rapid and discrete account of digestible food in the alimentary canal, as well as circulating and stored fuels, while vagal efferents, together with the sympathetic nervous system and hormonal mechanisms, codetermine the rate of nutrient absorption, partitioning, storage, and mobilization. Although vagal sensory mechanisms play a crucial role in the neural mechanism of satiation, there is little evidence suggesting a significant role in long-term energy homeostasis. However, increasing recognition of vagal involvement in the putative mechanisms making bariatric surgeries the most effective treatment for obesity should greatly stimulate future research to uncover the many details regarding the specific transduction mechanisms in the periphery and the inter- and intra-neuronal signaling cascades disseminating vagal information across the neuraxis.

Molecular and neural mediators of leptin action

The adipose tissue-derived hormone, leptin, acts via its receptor (LepRb) in the brain to regulate energy balance and neuroendocrine function. Parsing the biology of leptin requires understanding LepRb signaling and the roles for specific signaling pathways in neural and physiological leptin action. Since the leptin acts via a broadly distributed network of LepRb-expressing neurons, understanding the function of each of these LepRb neural populations will also be crucial. Here, we review the status of knowledge regarding the molecular mediators of leptin action and the neural substrate via which leptin acts to regulate physiologic processes.

Reduction of hypothalamic protein tyrosine phosphatase improves insulin and leptin resistance in diet-induced obese rats

Protein tyrosine phosphatase (PTP1B) has been implicated in the negative regulation of insulin and leptin signaling. PTP1B knockout mice are hypersensitive to insulin and leptin and resistant to obesity when fed a high-fat diet. We investigated the role of hypothalamic PTP1B in the regulation of food intake, insulin and leptin actions and signaling in rats through selective decreases in PTP1B expression in discrete hypothalamic nuclei. We generated a selective, transient reduction in PTP1B by infusion of an antisense oligonucleotide designed to blunt the expression of PTP1B in rat hypothalamic areas surrounding the third ventricle in control and obese rats. The selective decrease in hypothalamic PTP1B resulted in decreased food intake, reduced body weight, reduced adiposity after high-fat feeding, improved leptin and insulin action and signaling in hypothalamus, and may also have a role in the improvement in glucose metabolism in diabetes-induced obese rats.

Pck1 gene silencing in the liver improves glycemia control, insulin sensitivity, and dyslipidemia in db/db mice

OBJECTIVE

Cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C; encoded by Pck1) catalyzes the first committed step in gluconeogenesis. Extensive evidence demonstrates a direct correlation between PEPCK-C activity and glycemia control. Therefore, we aimed to evaluate the metabolic impact and their underlying mechanisms of knocking down hepatic PEPCK-C in a type 2 diabetic model.

RESEARCH DESIGN AND METHODS

PEPCK-C gene targeting was achieved using adenovirus-transduced RNAi. The study assessed several clinical symptoms of diabetes and insulin signaling in peripheral tissues, in addition to changes in gene expression, protein, and metabolites in the liver. Liver bioenergetics was also evaluated.

RESULTS

Treatment resulted in reduced PEPCK-C mRNA and protein. After treatment, improved glycemia and insulinemia, lower triglyceride, and higher total and HDL cholesterol were measured. Unsterified fatty acid accumulation was observed in the liver, in the absence of de novo lipogenesis. Despite hepatic lipidosis, treatment resulted in improved insulin signaling in the liver, muscle, and adipose tissue. O(2) consumption measurements in isolated hepatocytes demonstrated unaltered mitochondrial function and a consequent increased cellular energy charge. Key regulatory factors (FOXO1, hepatocyte nuclear factor-4alpha, and peroxisome proliferator-activated receptor-gamma coactivator [PGC]-1alpha) and enzymes (G6Pase) implicated in gluconeogenesis were downregulated after treatment. Finally, the levels of Sirt1, a redox-state sensor that modulates gluconeogenesis through PGC-1alpha, were diminished.

CONCLUSIONS

Our observations indicate that silencing PEPCK-C has direct impact on glycemia control and energy metabolism and provides new insights into the potential significance of the enzyme as a therapeutic target for the treatment of diabetes.

Overproduction of very low-density lipoproteins is the hallmark of the dyslipidemia in the metabolic syndrome

Insulin resistance is a key feature of the metabolic syndrome and often progresses to type 2 diabetes. Both insulin resistance and type 2 diabetes are characterized by dyslipidemia, which is an important and common risk factor for cardiovascular disease. Diabetic dyslipidemia is a cluster of potentially atherogenic lipid and lipoprotein abnormalities that are metabolically interrelated. Recent evidence suggests that a fundamental defect is an overproduction of large very low-density lipoprotein (VLDL) particles, which initiates a sequence of lipoprotein changes, resulting in higher levels of remnant particles, smaller LDL, and lower levels of high-density liporotein (HDL) cholesterol. These atherogenic lipid abnormalities precede the diagnosis of type 2 diabetes by several years, and it is thus important to elucidate the mechanisms involved in the overproduction of large VLDL particles. Here, we review the pathophysiology of VLDL biosynthesis and metabolism in the metabolic syndrome. We also review recent research investigating the relation between hepatic accumulation of lipids and insulin resistance, and sources of fatty acids for liver fat and VLDL biosynthesis. Finally, we briefly discuss current treatments for lipid management of dyslipidemia and potential future therapeutic targets.

The hepatic vagus nerve attenuates Fas-induced apoptosis in the mouse liver via alpha7 nicotinic acetylcholine receptor

BACKGROUND & AIMS

Although accumulating evidence has recently shown that the efferent vagus nerve attenuates systemic inflammation, it remains unclear whether or not the vagus nerve can affect Fas-induced liver apoptosis. We investigated the effect of the vagus nerve by using a selective hepatic vagotomy.

METHODS

We assessed the mortality and apoptosis in Fas-induced fulminant hepatitis in sham-operated and vagotomized male C57BL/6 mice. To determine how the nerve influences hepatocyte apoptosis, hepatitis was preceded by pretreatment with nicotine; PNU-282987, an alpha7 nicotinic acetylcholine receptor (AChR) agonist; liposome-encapsulated dichloromethylene diphosphonate (lipo-Cl(2)MDP), a macrophage eliminator; and Mn (III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP), an oxidative inhibitor.

RESULTS

Mortality in the vagotomized mice was significantly higher than that in the sham-operated mice following intravenous administration with the anti-Fas antibody Jo-2. This result was also supported by the data from both terminal deoxynucleotidyl-transferase mediated dUTP nick-end labeling and caspase-3 assay, in which vagotomized livers showed a significant elevation in the number of apoptotic hepatocytes and increased caspase-3 activity following Jo-2 treatment compared with the sham-operated livers. Supplementation with nicotine and PNU-282987 dose dependently inhibited this detrimental effect of the vagotomy. Moreover, the vagotomy-triggered exacerbation of Fas-induced hepatitis was completely blocked by lipo-Cl(2)MDP. Similarly, pretreatment with MnTBAP also completely suppressed the vagotomy-triggered exacerbation.

CONCLUSIONS

The hepatic vagus nerve appears to play an important role in attenuating Fas-induced hepatocyte apoptosis through alpha7 nicotinic AChR, perhaps by causing the Kupffer cells to reduce their generation of an excessive amount of reactive oxygen species.

Paracrine activation of hepatic CB1 receptors by stellate cell-derived endocannabinoids mediates alcoholic fatty liver

Alcohol-induced fatty liver, a major cause of morbidity, has been attributed to enhanced hepatic lipogenesis and decreased fat clearance of unknown mechanism. Here we report that the steatosis induced in mice by a low-fat, liquid ethanol diet is attenuated by concurrent blockade of cannabinoid CB1 receptors. Global or hepatocyte-specific CB1 knockout mice are resistant to ethanol-induced steatosis and increases in lipogenic gene expression and have increased carnitine palmitoyltransferase 1 activity, which, unlike in controls, is not reduced by ethanol treatment. Ethanol feeding increases the hepatic expression of CB1 receptors and upregulates the endocannabinoid 2-arachidonoylglycerol (2-AG) and its biosynthetic enzyme diacylglycerol lipase beta selectively in hepatic stellate cells. In control but not CB1 receptor-deficient hepatocytes, coculture with stellate cells from ethanol-fed mice results in upregulation of CB1 receptors and lipogenic gene expression. We conclude that paracrine activation of hepatic CB1 receptors by stellate cell-derived 2-AG mediates ethanol-induced steatosis through increasing lipogenesis and decreasing fatty acid oxidation.

Free fatty acids as mediators of adaptive compensatory responses to insulin resistance in dexamethasone-treated rats

BACKGROUND

Chronic low-dose dexamethasone (DEX) treatment in rats is associated to insulin resistance with compensatory hyperinsulinaemia and reduction in food intake. We tested the hypothesis that the elevation in circulating free fatty acids (FFAs) induced by DEX is the common mediator of both insulin resistance and insulin hyperproduction.

METHODS

For this purpose, an anti-lipolytic agent was administered during DEX treatment to lower lipacidaemia for several hours prior to glucose and insulin tolerance tests. Leptin expression in adipose tissue (by Northern blot) and plasma leptin levels (by radioimmunoassay) were also investigated to verify whether a rise in circulating leptin could be responsible for the anorectic effect of DEX.

RESULTS

Our data show that a transient pharmacological reduction of elevated plasma FFA levels abates the post-loading hyperinsulinaemia and counteracts the insulin resistance induced by DEX, supporting the hypothesis that the chronic elevation in FFAs is the common mediator of DEX-induced changes. Despite enhanced leptin expression in white adipose tissue, DEX-treated rats show no significant increase in plasma leptin levels. This suggests that the anorectic effect of DEX should be mediated, at least partially, by other factors, possibly related to the influence of concomitantly elevated plasma FFA and insulin levels on the hypothalamic centers regulating feeding.

CONCLUSIONS

Our results sustain the idea that a prolonged increase in plasma FFA levels plays an important role in the adaptive regulation of glucose and energy homeostasis, not only by potentiating insulin secretion but also by providing a signal of 'nutrient abundance' capable of restraining food intake.

Central resistin induces hepatic insulin resistance via neuropeptide Y

Sensing of peripheral hormones and nutrients by the hypothalamus plays an important role in maintaining peripheral glucose homeostasis. The hormone resistin impairs the response to insulin in liver and other peripheral tissues. Here we demonstrate that in normal mice resistin delivered in the lateral cerebral ventricle increased endogenous glucose production during hyperinsulinemic-euglycemic clamp, consistent with induction of hepatic insulin resistance. In agreement, central resistin inhibited Akt phosphorylation and increased the expression of glucose-6-phosphatase, the enzyme regulating glucose output in the liver. Central resistin induced expression of proinflammatory cytokines as well as suppressor of cytokine signaling-3, a negative regulator of insulin action in liver. Central infusion of resistin was associated with neuronal activation in the arcuate, paraventricular and dorsomedial nuclei, and increased neuropeptide Y (NPY) expression in the hypothalamus. The effects of central resistin on glucose production as well as hepatic expression of proinflammatory cytokines were abrogated in mice lacking NPY. Pretreatment of wild-type mice with antagonists of the NPY Y1 receptor, but not the Y5 receptor, also prevented the effects of central resistin. Together, these results suggest that resistin action on NPY neurons is an important regulator of hepatic insulin sensitivity.

The endocannabinoid system in the regulation of cardiometabolic risk factors

Obesity has increased at a striking rate over the last 3 decades in the Western world. This negative trend dramatically affects physical health and, ultimately, cardiovascular risks. In fact, particularly at the visceral level, obesity is strongly associated with an increased risk for life-threatening conditions, such as type 2 diabetes mellitus, hypertension, dyslipidemia, and cardiovascular disease. Although nutritional changes and physical activity are commonly thought of as the core treatments for obesity, it is necessary to further support obese patients with a pharmacologic approach for 2 reasons: to reduce the metabolic risk profile, and to avoid the regaining of weight. Among the various pharmacologic targets explored in recent years, the endocannabinoid (EC) system now constitutes the most promising proposal so far. In this review, after focusing on the central and peripheral signaling pathways that preserve energy homeostasis, we review the role of the EC system in regulating food's rewarding properties, controlling caloric intake by acting in hypothalamic pathways, and in modulating metabolic functions of several peripheral organs. In addition, we provide evidence that supports the recently proposed hypothesis that a close association exists between obesity and overactivation of the EC system.

Minireview: Circadian control of metabolism by the suprachiasmatic nuclei

In the present review, first we present the anatomical connections used by the mammalian biological clock to enforce its endogenous rhythmicity on the rest of the body, especially the energy homeostatic systems. Subsequently, we present a number of physiological experiments investigating the functional significance of this neuroanatomical substrate. Together, this overview of experimental data, for a major part derived from our own experiments, reveals a highly specialized organization of connections between the endogenous pacemaker and both the presympathetic and pre-parasympathetic hypothalamic systems, providing the biological clock with a unique opportunity to modulate the balance of sympathetic/parasympathetic inputs to peripheral organs. We hypothesize that a well-balanced autonomic nervous input, differentiated according to the time of day and the body compartment, is an important companion to withstand the progressive burden of the current 24/7 society on our health and well-being.

Serotonin 2C receptor agonists improve type 2 diabetes via melanocortin-4 receptor signaling pathways

The burden of type 2 diabetes and its associated premature morbidity and mortality is rapidly growing, and the need for novel efficacious treatments is pressing. We report here that serotonin 2C receptor (5-HT(2C)R) agonists, typically investigated for their anorectic properties, significantly improve glucose tolerance and reduce plasma insulin in murine models of obesity and type 2 diabetes. Importantly, 5-HT(2C)R agonist-induced improvements in glucose homeostasis occurred at concentrations of agonist that had no effect on ingestive behavior, energy expenditure, locomotor activity, body weight, or fat mass. We determined that this primary effect on glucose homeostasis requires downstream activation of melanocortin-4 receptors (MC4Rs), but not MC3Rs. These findings suggest that pharmacological targeting of 5-HT(2C)Rs may enhance glucose tolerance independently of alterations in body weight and that this may prove an effective and mechanistically novel strategy in the treatment of type 2 diabetes.

Pro-opiomelanocortin modulates the thermogenic and physical activity responses to high-fat feeding and markedly influences dietary fat preference

Complete proopiomelanocortin (POMC) deficiency causes a human syndrome of hypoadrenalism, altered skin and hair pigmentation, and severe hyperphagic obesity. Heterozygote carriers of nonsense mutations are strongly predisposed to obesity. Pomc(+/-) mice have normal body weight on a chow diet but increase food intake and become more obese than wild-type littermates when placed on a high-fat diet. To further explore the mechanisms whereby dietary fat interacts with Pomc genotype to produce obesity, we examined Pomc-null, Pomc(+/-), and wild-type mice for changes in the components of energy balance in response to provision of a high-fat diet and macronutrient preference when presented with a selection of dietary choices. In contrast to wild-type mice, Pomc null mice did not increase their resting energy expenditure or their spontaneous physical activity when given a high-fat diet. Pomc(+/-) mice increased resting energy expenditure similarly to wild types, but their increase in physical activity was significantly less than that seen in wild-type mice. In two independent experimental tests of macronutrient preference, Pomc genotype was a strong predictor of dietary fat preference with Pomc null animals choosing to eat approximately twice as much fat, but similar amounts of carbohydrate and protein, as wild-type animals. Pomc(+/-) mice showed an intermediate response. In summary, POMC-derived peptides have influences on multiple aspects of the organism's response to the presentation of high-fat diet. This includes a major influence, readily discernible even in heterozygote animals, on the dietary preference for fat.

Dorsal motor nucleus of the vagus: a site for evoking simultaneous changes in crural diaphragm activity, lower esophageal sphincter pressure, and fundus tone

The sphincter mechanism at the esophagogastric junction includes smooth muscle of the lower esophagus and skeletal muscle of the crural diaphragm (CD). Smooth muscle is known to be under the control of the dorsal motor nucleus of the vagus (DMV), while central nervous system (CNS) control of the CD is unknown. The main purposes of our study were to determine the CNS site that controls the CD and whether simultaneous changes in lower esophageal sphincter (LES) pressure and CD activity occur when this site is activated. Experiments were performed on anesthetized male ferrets whose LES pressure, CD activity, and fundus tone were monitored. To activate DMV neurons, L-glutamate was microinjected unilaterally into the DMV at three areas: intermediate, rostral, and caudal. Stimulation of the intermediate DMV decreased CD activity (-4.8 +/- 0.1 bursts/min and -0.3 +/- 0.01 mV) and LES pressure (-13.2 +/- 2.0 mmHg; n = 9). Stimulation of this brain site also produced an increase in fundus tone. Stimulation of the rostral DMV elicited increases in the activity of all three target organs (n = 5). Stimulation of the caudal DMV had no effect on the CD but did decrease both LES pressure and fundus tone (n = 5). All changes in LES pressure, fundus tone, and some DMV-induced changes in CD activity (i.e., bursts/min) were prevented by ipsilateral vagotomy. Our data indicate that simultaneous changes in activity of esophagogastric sphincters and fundus tone occur from rostral and intermediate areas of the DMV and that these changes are largely mediated by efferent vagus nerves.

Obesity-associated improvements in metabolic profile through expansion of adipose tissue

Excess caloric intake can lead to insulin resistance. The underlying reasons are complex but likely related to ectopic lipid deposition in nonadipose tissue. We hypothesized that the inability to appropriately expand subcutaneous adipose tissue may be an underlying reason for insulin resistance and beta cell failure. Mice lacking leptin while overexpressing adiponectin showed normalized glucose and insulin levels and dramatically improved glucose as well as positively affected serum triglyceride levels. Therefore, modestly increasing the levels of circulating full-length adiponectin completely rescued the diabetic phenotype in ob/ob mice. They displayed increased expression of PPARgamma target genes and a reduction in macrophage infiltration in adipose tissue and systemic inflammation. As a result, the transgenic mice were morbidly obese, with significantly higher levels of adipose tissue than their ob/ob littermates, leading to an interesting dichotomy of increased fat mass associated with improvement in insulin sensitivity. Based on these data, we propose that adiponectin acts as a peripheral "starvation" signal promoting the storage of triglycerides preferentially in adipose tissue. As a consequence, reduced triglyceride levels in the liver and muscle convey improved systemic insulin sensitivity. These mice therefore represent what we believe is a novel model of morbid obesity associated with an improved metabolic profile.

Inter-organ metabolic communication involved in energy homeostasis: potential therapeutic targets for obesity and metabolic syndrome

The global rate of obesity is rising alarmingly, exerting a major adverse impact on human health by increasing the prevalences of disorders, such as diabetes, hypertension and heart disease. To maintain systemic energy homeostasis, metabolic information must be communicated among organs/tissues. Obesity-related disorders can be thought of as resulting from dysregulation of this vital inter-tissue communication. Remarkable advances in obesity research during this decade have shown humoral factors manufactured and secreted by adipose tissue (adipocytokines) to be of great importance. In addition to these humoral factors, such as nutrients (glucose, fatty acids and amino acids) and hormones (insulin, adipocytokines and so on), the functional significance of the autonomic nervous system has recently attracted research attention. Autonomic nerves are essential components of the endogenous system for maintaining energy homeostasis, making them potential therapeutic targets for obesity-related disorders. This review focuses on the therapeutic possibilities of targeting inter-organ communication systems.

Understanding metabolic homeostasis and imbalance: what is the role of the endocannabinoid system?

Endogenous endocannabinoids (ECs) (anandamide and 2-arachidonoyl glycerol) are part of the leptin-regulated neural circuitry involved in appetite regulation. One of the sites of the orexigenic action of ECs involves activation of cannabinoid-1 (CB1) receptors in the lateral hypothalamus, from which neurons involved in mediating food reward project into the limbic system. In animal models of obesity, pharmacologic blockade or genetic ablation of CB1 receptors causes a transient reduction in food intake accompanied by sustained weight loss, reduced adiposity, and reversal of hormonal/metabolic changes, such as elevated levels of plasma leptin, insulin, glucose, and triglyceride, and reduced levels of plasma adiponectin (Acrp30). However, the beneficial effects of CB1 blockade on weight and metabolism cannot be explained by appetite suppression alone. Animal studies suggest that CB1 blockade exerts a direct peripheral as well as a central effect on fat metabolism. CB1 receptor blockade with rimonabant has been shown to not only reduce weight and adiposity but also to directly modulate fat metabolism at peripheral sites in skeletal muscle, adipose tissue, and the liver. Preclinical animal studies suggest that CB1 blockade acts on adipocytes to increase Acrp30 expression, on hepatocytes to decrease de novo lipogenesis and increase fatty acid oxidation, and on skeletal muscle to reduce blood glucose and insulin levels. Extrapolating from animal studies to the clinic, CB1 receptor blockade offers a promising strategy not only for reducing weight and abdominal adiposity but also for preventing and reversing its metabolic consequences.

Mechanisms of obesity-associated insulin resistance: many choices on the menu

Obesity-associated insulin resistance is a major risk factor for type 2 diabetes and cardiovascular disease. In the past decade, a large number of endocrine, inflammatory, neural, and cell-intrinsic pathways have been shown to be dysregulated in obesity. Although it is possible that one of these factors plays a dominant role, many of these factors are interdependent, and it is likely that their dynamic interplay underlies the pathophysiology of insulin resistance. Understanding the biology of these systems will inform the search for interventions that specifically prevent or treat insulin resistance and its associated pathologies.

A balance of lipid-sensing mechanisms in the brain and liver

Recent work has cast a spotlight on the brain as a nutrient-sensing organ that regulates the body's metabolic processes. Here we discuss the physiological and molecular mechanisms of brain lipid sensing and compare these mechanisms to liver lipid sensing. A direct comparison between the lipid-sensing mechanisms in the brain and liver reveals similar biochemical/molecular but opposing physiological mechanisms in operation. We propose that an imbalance between the lipid-sensing mechanisms in the brain and liver may contribute to obesity-associated type 2 diabetes.

Avenues of communication between the brain and tissues/organs involved in energy homeostasis

Obesity is a rapidly increasing public health concern worldwide as a major risk factor for numerous disorders, including diabetes, hypertension and heart disease. Despite remarkable advances in obesity research over the past 10 years, the molecular mechanisms underlying obesity are still not completely understood. To maintain systemic energy homeostasis, it is important that organs/tissues communicate metabolic information among each other. Obesity-related disorders can be thought of as resulting from dysregulation of this inter-tissue communication. This system has both afferent sensing components and efferent effecter limbs. The afferent signals consist of not only humoral factors, such as nutrients (glucose, fatty acids and amino acids) and adipocytokines (leptin, adiponectin and so on), but also autonomic afferent nerve systems. Both converge on brain centers, most importantly within the hypothalamus, where the signals are integrated, and the direction and magnitude of efferent responses are determined. The efferent elements of this physiological system include those regulating energy inputs and outputs, i.e. food intake and metabolic rates. In this review, we will summarize recent advances in research on metabolic information avenues to the brain, which are important for energy homeostasis.

Actions of insulin beyond glycemic control: a perspective on insulin detemir

The physiologic effects of insulin on carbohydrate metabolism in health in general and in diabetes are well known. Less understood, but far more intriguing, are the extrapancreatic effects of insulin that go beyond glycemic control to help sense, integrate, and maintain energy balance. Virtually every organ, including the brain, is a target for insulin action. When exogenous insulin is administered directly into the brains of experimental animals, the net effect is anorectic; however, patients with type 2 diabetes who transition to insulin therapy often gain weight--a tendency that opposes good glycemic control and overall therapeutic goals. After the brief review of extrapancreatic insulin--signaling pathways presented here, the physiologic impact of developing insulin resistance in relation to body weight is considered. Attention is then focused on insulin detemir, a longacting insulin analog that has consistently been associated with less weight gain than conventional formulations such as neutral protamine Hagedorn insulin. Mechanisms offered to explain this effect include the lower incidence of hypoglycemia and less within-patient variability associated with insulin detemir; however, recent observations and considerations of insulin-signaling pathways have shed light on other important properties of insulin detemir that may impart these weight-neutral effects. Namely, albumin binding, faster transport across the bloodbrain barrier, and preferential activity in brain and liver are characteristics of insulin detemir that potentially explain the observed weight benefit seen in clinical trials, as well as in the real-world practice setting.

Hepatic branch vagotomy, like insulin replacement, promotes voluntary lard intake in streptozotocin-diabetic rats

Although high insulin concentrations reduce food intake, low insulin concentrations promote lard intake over chow, possibly via an insulin-derived, liver-mediated signal. To investigate the role of the hepatic vagus in voluntary lard intake, streptozotocin-diabetic rats with insulin or vehicle replaced into either the superior mesenteric or jugular veins received a hepatic branch vagotomy (HV) or a sham operation. All rats received a pellet of corticosterone that clamped the circulating steroid at moderately high concentrations to enhance lard intake. After 5 d of recovery, rats were offered the choice of lard and chow for 5 d. In streptozotocin-diabetic rats, HV, like insulin replacement, restored lard intake to nondiabetic levels. Consequently, this reduced chow intake without affecting total caloric intake, and insulin site-specifically increased white adipose tissue weight. HV also ablated the effects of insulin on reducing circulating glucose levels and attenuated the streptozotocin-induced weight loss in most groups. Collectively, these data suggest that the hepatic vagus normally inhibits lard intake and can influence glucose homeostasis and the pattern of white adipose tissue deposition. These actions may be modulated by insulin acting both centrally and peripherally.

Transgenic MSH overexpression attenuates the metabolic effects of a high-fat diet

To determine whether long-term melanocortinergic activation can attenuate the metabolic effects of a high fat diet, mice overexpressing an NH(2)-terminal POMC transgene that includes alpha- and gamma(3)-MSH were studied on either a 10% low-fat diet (LFD) or 45% high-fat diet (HFD). Weight gain was modestly reduced in transgenic (Tg-MSH) male and female mice vs. wild type (WT) on HFD (P < 0.05) but not LFD. Substantial reductions in body fat percentage were found in both male and female Tg-MSH mice on LFD (P < 0.05) and were more pronounced on HFD (P < 0.001). These changes occurred in the absence of significant feeding differences in most groups, consistent with effects of Tg-MSH on energy expenditure and partitioning. This is supported by indirect calorimetry studies demonstrating higher resting oxygen consumption and lower RQ in Tg-MSH mice on the HFD. Tg-MSH mice had lower fasting insulin levels and improved glucose tolerance on both diets. Histological and biochemical analyses revealed that hepatic fat accumulation was markedly reduced in Tg-MSH mice on the HFD. Tg-MSH also attenuated the increase in corticosterone induced by the HFD. Higher levels of Agrp mRNA, which might counteract effects of the transgene, were measured in Tg-MSH mice on LFD (P = 0.02) but not HFD. These data show that long-term melanocortin activation reduces body weight, adiposity, and hepatic fat accumulation and improves glucose metabolism, particularly in the setting of diet-induced obesity. Our results suggest that long-term melanocortinergic activation could serve as a potential strategy for the treatment of obesity and its deleterious metabolic consequences.

Afferent signalling through the common hepatic branch of the vagus inhibits voluntary lard intake and modifies plasma metabolite levels in rats

The common hepatic branch of the vagus nerve is a two-way highway of communication between the brain and the liver, duodenum, stomach and pancreas that regulates many aspects of food intake and metabolism. In this study, we utilized the afferent-specific neurotoxin capsaicin to examine if common hepatic vagal sensory afferents regulate lard intake. Rats implanted with a corticosterone pellet were made diabetic using streptozotocin (STZ) and a subset received steady-state exogenous insulin replacement into the superior mesenteric vein. These were compared with non-diabetic counterparts. Each group was then subdivided into those whose common hepatic branch of the vagus was treated with vehicle or capsaicin. Five days after surgery, the rats were offered the choice of chow and lard to consume for a further 5 days. The STZ-diabetic rats ate significantly less lard than the non-diabetic rats. Capsaicin treatment restored lard intake to that of the insulin-replaced, STZ-diabetic rats, but modified neither chow nor total caloric intake. This increased lard intake led to selective fat deposition into the mesenteric white adipose tissue depot, as opposed to an increase in all visceral fat pad depots evident after insulin replacement-induced lard intake. Capsaicin treatment also increased the levels of circulating glucose and triglycerides and negated the actions of insulin on these and free fatty acids and ketone bodies. Collectively, these data suggest that afferent signalling through the common hepatic branch of the vagus inhibits lard, but not chow, intake, directs fat deposition and regulates plasma metabolite levels.

Hypothalamic leptin regulation of energy homeostasis and glucose metabolism

Growing evidence suggests that food intake, energy expenditure and endogenous glucose production are regulated by hypothalamic areas that respond to a variety of peripheral signals. Therefore, in response to a reduction in energy stores or circulating nutrients, the brain initiates responses in order to promote positive energy balance to restore and maintain energy and glucose homeostasis. In contrast, in times of nutrient abundance and excess energy storage, key hypothalamic areas activate responses to promote negative energy balance (i.e. reduced food intake and increased energy expenditure) and decreased nutrient availability (reduced endogenous glucose production). Accordingly, impaired responses or 'resistance' to afferent input from these hormonal or nutrient-related signals would be predicted to favour weight gain and insulin resistance and may contribute to the development of obesity and type 2 diabetes.

Changes in the expression of hypothalamic lipid sensing genes in rat model of intrauterine growth retardation (IUGR)

Intrauterine growth retardation (IUGR) has been linked to the development of type 2 diabetes in later life. The mechanisms underlying this phenomenon are unknown. Recent data suggest that some of the molecular defects underlying type 2 diabetes reside in the CNS. The enzyme carnitine palmitoyltransferase-1 (CPT1) regulates long-chain fatty acid (LCFA) entry into mitochondria, where LCFA undergo beta-oxidation. Hypothalamic inhibition of CPT1 decreases food intake and suppresses endogenous glucose production. Our aim was to investigate the effects of uterine artery ligation, a procedure that mimics uteroplacental insufficiency, on the CNS expression of CPT1 and other key enzymes of LCFA metabolism. Bilateral uterine artery ligation was performed on d 19 of gestation in the pregnant rat; sham-operated pregnant rats served as controls. Hypothalamus, cerebellum, hippocampus, and cortex were dissected and analyzed at birth by real-time PCR. Nonesterified fatty acid (NEFA) serum levels were significantly higher in IUGR pups (p<0.0001). In IUGR rats, the hypothalamic expression of CPT1 isoform C (p=0.005) and acetyl-CoA carboxylase (ACC) isoforms alpha (p<0.05) and beta (p=0.005) were significantly decreased. The data presented here support the hypothesis that an abnormal intrauterine milieu can induce changes in hypothalamic lipid sensing.

Hypothalamic fatty acid metabolism: a housekeeping pathway that regulates food intake

The hypothalamus is a specialized area in the brain that integrates the control of energy homeostasis. More than 70 years ago, it was proposed that the central nervous system sensed circulating levels of metabolites such as glucose, lipids and amino acids and modified feeding according to the levels of those molecules. This led to the formulation of the Glucostatic, Lipostatic and Aminostatic Hypotheses. It has taken almost that much time to demonstrate that circulating long-chain fatty acids act as signals of nutrient surplus in the hypothalamus. Moreover, pharmacological and/or genetic inhibition of fatty acid synthase, AMP-activated protein kinase and carnitine palmitoyltransferase 1 results in profound decrease in feeding and body weight in rodents. The molecular mechanism behind these actions depends on changes in the cellular pool of malonyl-CoA and fatty acyl-CoAs. Current evidence also suggests that this pathway may play a major role in the physiological regulation of feeding, by integrating hormonal and nutrient-derived signals in the hypothalamus. Here, we summarize what is known about hypothalamic fatty acid metabolism and feeding control and provide future directions for research. Understanding these molecular mechanisms could provide new targets for the treatment of obesity and related disorders.

A sense of time: how molecular clocks organize metabolism

The discovery of an internal temporal clockwork that coordinates behavior and metabolism according to the rising and setting of the sun was first revealed in flies and plants. However, in the past decade, a molecular transcription-translation feedback loop with similar properties has also been identified in mammals. In mammals, this transcriptional oscillator programs 24-hour cycles in sleep, activity and feeding within the master pacemaker neurons of the suprachiasmatic nucleus of the hypothalamus. More recent studies have shown that the core transcription mechanism is also present in other locations within the brain, in addition to many peripheral tissues. Processes ranging from glucose transport to gluconeogenesis, lipolysis, adipogenesis and mitochondrial oxidative phosphorylation are controlled through overlapping transcription networks that are tied to the clock and are thus time sensitive. Because disruption of tissue timing occurs when food intake, activity and sleep are altered, understanding how these many tissue clocks are synchronized to tick at the same time each day, and determining how each tissue 'senses time' set by these molecular clocks might open new insight into human disease, including disorders of sleep, circadian disruption, diabetes and obesity.

Transforming growth factor-beta in the brain regulates fat metabolism during endurance exercise

We have previously reported that the concentration of transforming growth factor-beta (TGF-beta) increases in the cerebrospinal fluid of rats during exercise and that there is an increase in whole body fat oxidation following the intracisternal administration of TGF-beta. These results led us to postulate that TGF-beta in the brain regulates the enhancement of fatty acid oxidation during exercise. To test this hypothesis, we carried out respiratory gas analysis during treadmill running following the inhibition of TGF-beta activity in rat brain by intracisternal administration of anti-TGF-beta antibody or SB-431542, an inhibitor of the type 1 TGF-beta receptor. We found that each reagent partially blocked the increase in the fatty acid oxidation. We also compared the plasma concentrations of energy substrates in the group administered anti-TGF-beta antibody and the control group during running. We found that the plasma concentrations of nonesterified fatty acids and ketone bodies in the group administered anti-TGF-beta antibody were lower than in the control group at the end of running. In the same way, we carried out respiratory gas analysis during treadmill running after depressing corticotropin-releasing factor activity in the brain using intracisternal administration of astressin, an inhibitor of the corticotropin-releasing factor receptor. However, there were no significant differences in respiratory exchange ratio or oxygen consumption in moderate running (60% maximum oxygen consumption). These results suggest that brain TGF-beta has a role in enhancing fatty acid oxidation during endurance exercise and that this regulation is executed at least partly via the type 1 TGF-beta receptor signal transduction system.

Impact of in vivo fatty acid oxidation blockade on glucose turnover and muscle glucose metabolism during low-dose AICAR infusion

AMPK plays a central role in influencing fuel usage and selection. The aim of this study was to analyze the impact of low-dose AMP analog 5-aminoimidazole-4-carboxamide-1-beta-d-ribosyl monophosphate (ZMP) on whole body glucose turnover and skeletal muscle (SkM) glucose metabolism. Dogs were restudied after prior 48-h fatty acid oxidation (FA(OX)) blockade by methylpalmoxirate (MP; 5 x 12 hourly 10 mg/kg doses). During the basal equilibrium period (0-150 min), fasting dogs (n = 8) were infused with [3-(3)H]glucose followed by either 2-h saline or AICAR (1.5-2.0 mg x kg(-1) x min(-1)) infusions. SkM was biopsied at completion of each study. On a separate day, the same protocol was undertaken after 48-h in vivo FA(OX) blockade. The AICAR and AICAR + MP studies were repeated in three chronic alloxan-diabetic dogs. AICAR produced a transient fall in plasma glucose and increase in insulin and a small decline in free fatty acid (FFA). Parallel increases in hepatic glucose production (HGP), glucose disappearance (R(d tissue)), and glycolytic flux (GF) occurred, whereas metabolic clearance rate of glucose (MCR(g)) did not change significantly. Intracellular SkM glucose, glucose 6-phosphate, and glycogen were unchanged. Acetyl-CoA carboxylase (ACC approximately pSer(221)) increased by 50%. In the AICAR + MP studies, the metabolic responses were modified: the glucose was lower over 120 min, only minor changes occurred with insulin and FFA, and HGP and R(d tissue) responses were markedly attenuated, but MCR(g) and GF increased significantly. SkM substrates were unchanged, but ACC approximately pSer(221) rose by 80%. Thus low-dose AICAR leads to increases in HGP and SkM glucose uptake, which are modified by prior FA(ox) blockade.

Differential effects of a perioperative hyperinsulinemic normoglycemic clamp on the neurohumoral stress response during coronary artery surgery

BACKGROUND

Hyperglycemia in patients undergoing coronary artery bypass grafting (CABG) is associated with adverse outcome. Although insulin infusion strategies are increasingly used to improve outcome, a pathophysiological rationale is currently lacking. The present study was designed to quantify the effects of a perioperative hyperinsulinemic normoglycemic clamp on the neurohumoral stress response during CABG.

METHODS

Forty-four nondiabetic patients, scheduled for elective CABG, were randomized to either a control group (n = 22) receiving standard care or to a clamp group (n = 22) receiving additionally a perioperative hyperinsulinemic (regular insulin at a fixed rate of 0.1 IU.kg(-1).h(-1)) normoglycemic (plasma glucose between 3.0 and 6.0 mmol.liter(-1)) clamp during 26 h. We measured the endocrine response of the hypothalamus-pituitary-adrenal (HPA) axis, the sympathoadrenal axis, and glucagon, as well as plasma glucose and insulin at regular intervals from the induction of anesthesia at baseline through the end of the second postoperative day (POD).

RESULTS

There were no differences in clinical outcome between the groups. In the control group, hyperglycemia developed at the end of surgery and remained present until the final measurement point on POD2, whereas plasma insulin levels remained unchanged until the morning of POD1. In the intervention group, normoglycemia was well maintained during the clamp, whereas insulin levels ranged between 600 and 800 pmol.liter(-1). In both groups, plasma ACTH and cortisol increased from 6 h after discontinuation of cardiopulmonary bypass onward. However, during the clamp period, a marked reduction in the HPA axis response was found in the intervention group, as reflected by a 47% smaller increase in area under the curve in plasma ACTH (P = 0.035) and a 27% smaller increase in plasma cortisol (P = 0.002) compared with the control group. Compared with baseline, epinephrine and norepinephrine increased by the end of the clamp interval until POD2 in both groups. Surprisingly, the area under the curve of epinephrine levels was 47% higher (P = 0.026) after the clamp interval in the intervention group as compared with the control group.

CONCLUSION

A hyperinsulinemic normoglycemic clamp during CABG delays and attenuates the HPA axis response during the first 18 h of the myocardial reperfusion period, whereas after the clamp, plasma epinephrine is higher. The impact of delaying cortisol responses on clinical outcome of CABG remains to be elucidated.

Perturbation of glucose flux in the liver by decreasing F26P2 levels causes hepatic insulin resistance and hyperglycemia

Hepatic insulin resistance is one of the characteristics of type 2 diabetes and contributes to the development of hyperglycemia. How changes in hepatic glucose flux lead to insulin resistance is not clearly defined. We determined the effects of decreasing the levels of hepatic fructose 2,6-bisphosphate (F26P(2)), a key regulator of glucose metabolism, on hepatic glucose flux in the normal 129J mice. Upon adenoviral overexpression of a kinase activity-deficient 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, the enzyme that determines F26P(2) level, hepatic F26P(2) levels were decreased twofold compared with those of control virus-treated mice in basal state. In addition, under hyperinsulinemic conditions, hepatic F26P(2) levels were much lower than those of the control. The decrease in F26P(2) leads to the elevation of basal and insulin-suppressed hepatic glucose production. Also, the efficiency of insulin to suppress hepatic glucose production was decreased (63.3 vs. 95.5% suppression of the control). At the molecular level, a decrease in insulin-stimulated Akt phosphorylation was consistent with hepatic insulin resistance. In the low hepatic F26P(2) states, increases in both gluconeogenesis and glycogenolysis in the liver are responsible for elevations of hepatic glucose production and thereby contribute to the development of hyperglycemia. Additionally, the increased hepatic gluconeogenesis was associated with the elevated mRNA levels of peroxisome proliferator-activated receptor-gamma coactivator-1alpha and phosphoenolpyruvate carboxykinase. This study provides the first in vivo demonstration showing that decreasing hepatic F26P(2) levels leads to increased gluconeogenesis in the liver. Taken together, the present study demonstrates that perturbation of glucose flux in the liver plays a predominant role in the development of a diabetic phenotype, as characterized by hepatic insulin resistance.

Dirty dealing: hepatic vagal afferents reshuffle fat distribution

To evaluate the role of hepatic peroxisome-proliferator-activated receptor (PPAR) gamma 2 in the control of energy balance, Uno and colleagues examined the metabolic effects of overexpression of PPAR gamma 2 selectively in mouse and rat liver. Mice demonstrated a surprising degree of hepatic steatosis accompanied by significant reductions in peripheral adiposity. This crosstalk between liver and adipose tissue appears to be mediated by both the sensory component of the hepatic vagus nerve and sympathetic efferents. These data suggest a novel hepatic-adipose neuraxis that regulates the distribution of stored fat.

Signalling mechanisms linking hepatic glucose and lipid metabolism

Fatty liver and hepatic triglyceride accumulation are strongly associated with obesity, insulin resistance and type 2 diabetes, and are subject to nutritional influences. Hepatic regulation of glucose and lipid homeostasis is influenced by a complex system of hormones, hormonally regulated signalling pathways and transcription factors. Recently, considerable progress has been made in elucidating molecular pathways and potential factors that are affected in insulin-resistant states. In this review we discuss some of the key factors that are involved in both the regulation of glucose and lipid metabolism in the liver. Understanding the molecular network that links hepatic lipid accumulation and impaired glucose metabolism may provide targets for dietary or pharmacological interventions.

Essential role of Pten in body size determination and pancreatic beta-cell homeostasis in vivo

PTEN (phosphatase with tensin homology) is a potent negative regulator of phosphoinositide 3-kinase (PI3K)/Akt signaling, an evolutionarily conserved pathway that signals downstream of growth factors, including insulin and insulin-like growth factor 1. In lower organisms, this pathway participates in fuel metabolism and body size regulation and insulin-like proteins are produced primarily by neuronal structures, whereas in mammals, the major source of insulin is the pancreatic beta cells. Recently, rodent insulin transcription was also shown in the brain, particularly the hypothalamus. The specific regulatory elements of the PI3K pathway in these insulin-expressing tissues that contribute to growth and metabolism in higher organisms are unknown. Here, we report PTEN as a critical determinant of body size and glucose metabolism when targeting is driven by the rat insulin promoter in mice. The partial deletion of PTEN in the hypothalamus resulted in significant whole-body growth restriction and increased insulin sensitivity. Efficient PTEN deletion in beta cells led to increased islet mass without compromise of beta-cell function. Parallel enhancement in PI3K signaling was found in PTEN-deficient hypothalamus and beta cells. Together, we have shown that PTEN in insulin-transcribing cells may play an integrative role in regulating growth and metabolism in vivo.

Neuronal PTP1B regulates body weight, adiposity and leptin action

Obesity is a major health problem and a risk factor for type 2 diabetes. Leptin, an adipocyte-secreted hormone, acts on the hypothalamus to inhibit food intake and increase energy expenditure. Most obese individuals develop hyperleptinemia and leptin resistance, limiting the therapeutic efficacy of exogenously administered leptin. Mice lacking the tyrosine phosphatase PTP1B are protected from diet-induced obesity and are hypersensitive to leptin, but the site and mechanism for these effects remain controversial. We generated tissue-specific PTP1B knockout (Ptpn1(-/-)) mice. Neuronal Ptpn1(-/-) mice have reduced weight and adiposity, and increased activity and energy expenditure. In contrast, adipose PTP1B deficiency increases body weight, whereas PTP1B deletion in muscle or liver does not affect weight. Neuronal Ptpn1(-/-) mice are hypersensitive to leptin, despite paradoxically elevated leptin levels, and show improved glucose homeostasis. Thus, PTP1B regulates body mass and adiposity primarily through actions in the brain. Furthermore, neuronal PTP1B regulates adipocyte leptin production and probably is essential for the development of leptin resistance.

Neuronal pathway from the liver modulates energy expenditure and systemic insulin sensitivity

Coordinated control of energy metabolism and glucose homeostasis requires communication between organs and tissues. We identified a neuronal pathway that participates in the cross talk between the liver and adipose tissue. By studying a mouse model, we showed that adenovirus-mediated expression of peroxisome proliferator-activated receptor (PPAR)-g2 in the liver induces acute hepatic steatosis while markedly decreasing peripheral adiposity. These changes were accompanied by increased energy expenditure and improved systemic insulin sensitivity. Hepatic vagotomy and selective afferent blockage of the hepatic vagus revealed that the effects on peripheral tissues involve the afferent vagal nerve. Furthermore, an antidiabetic thiazolidinedione, a PPARg agonist, enhanced this pathway. This neuronal pathway from the liver may function to protect against metabolic perturbation induced by excessive energy storage.

Critical role of STAT3 in leptin's metabolic actions

Leptin has pleiotropic effects on glucose homeostasis and feeding behavior. Here, we validate the use of a cell-permeable phosphopeptide that blocks STAT3 activation in vivo. The combination of this biochemical approach with stereotaxic surgical techniques allowed us to pinpoint the contribution of hypothalamic STAT3 to the acute effects of leptin on food intake and glucose homeostasis. Leptin's ability to acutely reduce food intake critically depends on intact STAT3 signaling. Likewise, hypothalamic signaling of leptin through STAT3 is required for the acute effects of leptin on liver glucose fluxes. Lifelong obliteration of STAT3 signaling via the leptin receptor in mice (s/s mice) results in severe hepatic insulin resistance that is comparable to that observed in db/db mice, devoid of leptin receptor signaling. Our results demonstrate that the activation of the hypothalamic STAT3 pathway is an absolute requirement for the effects of leptin on food intake and hepatic glucose metabolism.

Brain glucose sensing: a subtle mechanism

PURPOSE OF REVIEW

Brain nutrient sensing allows a fine regulation of different physiological functions, such as food intake and blood glucose, related to energy homeostasis. Glucose sensing is the most studied function and a parallel has been made between the cellular mechanisms involved in pancreatic beta cells and neurons.

RECENT FINDINGS

Two types of glucosensing neurons have been characterized--those for which the activity is proportional to changes in glucose concentration and those for which the activity is inversely proportional to these changes. A new level of complexity has recently been demonstrated, as the response and the mechanism appear to vary in function according to the level of the glucose change. For some of the responses, the detection is probably not at the level of the neuron itself, but astrocytes also appear to be involved, indicating a coupling between the two types of cells. Finally, numerous data have demonstrated the modulation of glucose sensing by other nutrients, in particular fatty acids, hormones (insulin, leptin and ghrelin) and peptides (neuropeptide Y). This implies a common pathway in which AMPkinase may play a crucial role.

SUMMARY

Recent observations in brain nutrient sensing indicate subtle mechanisms, with different cellular and molecular mechanisms involved. This fact would explain the discrepancies reported in the expression of different proteins (glucose transporters, hexokinases, channels). Astrocytes may be involved in one type of response, thus adding a new level of complexity.

Role of hepatic STAT3 in brain-insulin action on hepatic glucose production

STAT3 regulates glucose homeostasis by suppressing the expression of gluconeogenic genes in the liver. The mechanism by which hepatic STAT3 is regulated by nutritional or hormonal status has remained unknown, however. Here, we show that an increase in the plasma insulin concentration, achieved either by glucose administration or by intravenous insulin infusion, stimulates tyrosine phosphorylation of STAT3 in the liver. This effect of insulin was mediated by the hormone's effects in the brain, and the increase in hepatic IL-6 induced by the brain-insulin action is essential for the activation of STAT3. The inhibition of hepatic glucose production and of expression of gluconeogenic genes induced by intracerebral ventricular insulin infusion was impaired in mice with liver-specific STAT3 deficiency or in mice with IL-6 deficiency. These results thus indicate that IL-6-STAT3 signaling in the liver contributes to insulin action in the brain, leading to the suppression of hepatic glucose production.

Insulin's effect on the liver: "direct or indirect?" continues to be the question

Previous studies suggest that insulin can inhibit hepatic glucose production (HGP) by both direct and indirect actions. The indirect effects include inhibition of glucagon secretion, reduction in plasma nonesterified fatty acid levels, reduction of the amount of gluconeogenic precursor supplied to the liver, and change in neural input to the liver. A study in this issue of the JCI demonstrates that, in overnight-fasted dogs, an acute, selective increase of portal insulin induces a rapid inhibition of HGP, and a 4-fold rise in head insulin level does not enhance the inhibition of HGP in response to portal insulin infusion. This study demonstrates that insulin's direct effects on the liver dominate the control of HGP. These data balance previous studies in mice that suggested that indirect effects of insulin via the hypothalamus are the primary determinant of HGP.

Restoration of hypothalamic lipid sensing normalizes energy and glucose homeostasis in overfed rats

Short-term overfeeding blunts the central effects of fatty acids on food intake and glucose production. This acquired defect in nutrient sensing could contribute to the rapid onset of hyperphagia and insulin resistance in this model. Here we examined whether central inhibition of lipid oxidation is sufficient to restore the hypothalamic levels of long-chain fatty acyl-CoAs (LCFA-CoAs) and to normalize food intake and glucose homeostasis in overfed rats. To this end, we targeted the liver isoform of carnitine palmitoyltransferase-1 (encoded by the CPT1A gene) by infusing either a sequence-specific ribozyme against CPT1A or an isoform-selective inhibitor of CPT1A activity in the third cerebral ventricle or in the mediobasal hypothalamus (MBH). Inhibition of CPT1A activity normalized the hypothalamic levels of LCFA-CoAs and markedly inhibited feeding behavior and hepatic glucose fluxes in overfed rats. Thus central inhibition of lipid oxidation is sufficient to restore hypothalamic lipid sensing as well as glucose and energy homeostasis in this model and may be an effective approach to the treatment of diet-induced obesity and insulin resistance.