Overfeeding rapidly induces leptin and insulin resistance

Energy expenditure in obesity-prone and obesity-resistant rats before and after the introduction of a high-fat diet

While most rats gain weight when placed on a high-fat diet (HFD), some strains resist HFD-induced weight gain. To maintain weight, obesity-resistant (OR) rats must either eat less than obesity-prone (OP) rats or increase total energy expenditure (TEE). To determine if changes in TEE predispose to or protect from weight gain, energy expenditure, energy intake, and weight gain were measured in male and female OP and OR rats consuming a low-fat diet (LFD) and for 5 days after switching to a HFD. After 5 days on a HFD, OP rats gained significantly more weight (male: 42.8 ± 6.9 g, female: 25.5 ± 3.0 g) than their OR counterparts (male: 24.0 ± 7.5 g, female: 13.7 ± 1.4 g). Both male and female rats significantly increased their energy intake when transitioned to the HFD, and TEE increased modestly in all groups. Compared with female OP rats, female OR rats had a significantly greater increase in TEE on the HFD. This was due to an increase in both resting and nonresting energy expenditure. In contrast, the effect of the HFD in males was minor. TEE was also measured in female rats consuming a HFD, pair fed to LFD calories. The increase in TEE of pair-fed female OR rats was substantially less than what was seen in the HFD ad libitum condition. Physical activity was also measured in female rats. There was no evidence that increases in physical activity were the cause of the increased TEE seen in female OR rats consuming a HFD. These results suggest that resistance to HFD-induced weight gain in female OR rats may be due in part to an increase in TEE and a greater reliance on lipid as an energy source. Changes in TEE appear to be triggered by overconsumption of the HFD and not simply the diet composition.

Targeting intermediary metabolism in the hypothalamus as a mechanism to regulate appetite

The central nervous system mediates energy balance (energy intake and energy expenditure) in the body; the hypothalamus has a key role in this process. Recent evidence has demonstrated an important role for hypothalamic malonyl CoA in mediating energy balance. Malonyl CoA is generated by the carboxylation of acetyl CoA by acetyl CoA carboxylase and is then either incorporated into long-chain fatty acids by fatty acid synthase, or converted back to acetyl-CoA by malonyl CoA decarboxylase. Increased hypothalamic malonyl CoA is an indicator of energy surplus, resulting in a decrease in food intake and an increase in energy expenditure. In contrast, a decrease in hypothalamic malonyl CoA signals an energy deficit, resulting in an increased appetite and a decrease in body energy expenditure. A number of hormonal and neural orexigenic and anorexigenic signaling pathways have now been shown to be associated with changes in malonyl CoA levels in the arcuate nucleus (ARC) of the hypothalamus. Despite compelling evidence that malonyl CoA is an important mediator in the hypothalamic ARC control of food intake and regulation of energy balance, the mechanism(s) by which this occurs has not been established. Malonyl CoA inhibits carnitine palmitoyltransferase-1 (CPT-1), and it has been proposed that the substrate of CPT-1, long-chain acyl CoA(s), may act as a mediator(s) of appetite and energy balance. However, recent evidence has challenged the role of long-chain acyl CoA(s) in this process, as well as the involvement of CPT-1 in hypothalamic malonyl CoA signaling. A better understanding of how malonyl CoA regulates energy balance should provide novel approaches to targeting intermediary metabolism in the hypothalamus as a mechanism to control appetite and body weight. Here, we review the data supporting an important role for malonyl CoA in mediating hypothalamic control of energy balance, and recent evidence suggesting that targeting malonyl CoA synthesis or degradation may be a novel approach to favorably modify appetite and weight gain.

Energy regulatory signals and food reward

The hormones insulin, leptin, and ghrelin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis, acting at medial hypothalamic sites. Here, we summarize research demonstrating that, in addition to direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and is also a direct and indirect target for the action of these endocrine regulators of energy homeostasis. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, the midbrain dopamine (DA) and opioidergic pathways. Ghrelin can increase food reward behaviors, and support midbrain DA neuronal function. We summarize discussion of behavioral, systems, and cellular evidence in support of the contributions of reward circuitry to the homeostatic roles of these hormones in the CNS. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Obesity and nonalcoholic fatty liver disease: biochemical, metabolic, and clinical implications

Obesity is associated with an increased risk of nonalcoholic fatty liver disease (NAFLD). Steatosis, the hallmark feature of NAFLD, occurs when the rate of hepatic fatty acid uptake from plasma and de novo fatty acid synthesis is greater than the rate of fatty acid oxidation and export (as triglyceride within very low-density lipoprotein). Therefore, an excessive amount of intrahepatic triglyceride (IHTG) represents an imbalance between complex interactions of metabolic events. The presence of steatosis is associated with a constellation of adverse alterations in glucose, fatty acid, and lipoprotein metabolism. It is likely that abnormalities in fatty acid metabolism, in conjunction with adipose tissue, hepatic, and systemic inflammation, are key factors involved in the development of insulin resistance, dyslipidemia, and other cardiometabolic risk factors associated with NAFLD. However, it is not clear whether NAFLD causes metabolic dysfunction or whether metabolic dysfunction is responsible for IHTG accumulation, or possibly both. Understanding the precise factors involved in the pathogenesis and pathophysiology of NAFLD will provide important insights into the mechanisms responsible for the cardiometabolic complications of obesity.

Mechanism of attenuation of leptin signaling under chronic ligand stimulation

Background

Leptin is an adipocyte-derived hormone that acts via its hypothalamic receptor (LEPRb) to regulate energy balance. A downstream effect essential for the weight-regulatory action of leptin is the phosphorylation and activation of the latent transcription factor STAT3 by LEPRb-associated Janus kinases (JAKs). Obesity is typically associated with chronically elevated leptin levels and a decreased ability of LEPRb to activate intracellular signal transduction pathways (leptin resistance). Here we have studied the roles of the intracellular tyrosine residues in the negative feedback regulation of LEPRb-signaling under chronic leptin stimulation.

Results

Mutational analysis showed that the presence of either Tyr985 and Tyr1077 in the intracellular domain of LEPRb was sufficient for the attenuation of STAT3 phosphorylation, whereas mutation of both tyrosines rendered LEPRb resistant to feedback regulation. Overexpression and RNA interference-mediated downregulation of suppressor of cytokine signaling 3 (SOCS3) revealed that both Tyr985 and Tyr1077 were capable of supporting the negative modulatory effect of SOCS3 in reporter gene assays. In contrast, the inhibitory effect of SOCS1 was enhanced by the presence of Tyr985 but not Tyr1077. Finally, the reduction of the STAT-phosphorylating activity of the LEPRb complex after 2 h of leptin stimulation was not accompanied by the dephosphorylation or degradation of LEPRb or the receptor-associated JAK molecule, but depended on Tyr985 and/or Tyr1077.

Conclusions

Both Tyr985 and Tyr1077 contribute to the negative regulation of LEPRb signaling. The inhibitory effects of SOCS1 and SOCS3 differ in the dependence on the tyrosine residues in the intracellular domain of LEPRb.

Early responses of insulin signaling to high-carbohydrate and high-fat overfeeding

Background

Early molecular changes of nutritionally-induced insulin resistance are still enigmatic. It is also unclear if acute overnutrition alone can alter insulin signaling in humans or if the macronutrient composition of the diet can modulate such effects.

Methods

To investigate the molecular correlates of metabolic adaptation to either high-carbohydrate (HC) or high-fat (HF) overfeeding, we conducted overfeeding studies in 21 healthy lean (BMI < 25) individuals (10 women, 11 men), age 20-45, with normal glucose metabolism and no family history of diabetes. Subjects were studied first following a 5-day eucaloric (EC) diet (30% fat, 50% CHO, 20% protein) and then in a counter balanced manner after 5 days of 40% overfeeding of both a HC (20% fat, 60% CHO) diet and a HF (50% fat, 30% CHO) diet. At the end of each diet phase, in vivo insulin sensitivity was assessed using the hyperinsulinemic-euglycemic clamp technique. Ex vivo insulin action was measured from skeletal muscle tissue samples obtained 15 minutes after insulin infusion was initiated.

Results

Overall there was no change in whole-body insulin sensitivity as measured by glucose disposal rate (GDR, EC: 12.1 ± 4.7; HC: 10.9 ± 2.7; HF: 10.8 ± 3.4). Assessment of skeletal muscle insulin signaling demonstrated increased tyrosine phosphorylation of IRS-1 (p < 0.001) and increased IRS-1-associated phosphatidylinositol 3 (PI 3)-kinase activity (p < 0.001) following HC overfeeding. In contrast, HF overfeeding increased skeletal muscle serine phosophorylation of IRS-1 (p < 0.001) and increased total expression of p85α (P < 0.001).

Conclusion

We conclude that acute bouts of overnutrition lead to changes at the cellular level before whole-body insulin sensitivity is altered. On a signaling level, HC overfeeding resulted in changes compatible with increased insulin sensitivity. In contrast, molecular changes in HF overfeeding were compatible with a reduced insulin sensitivity.

Adipokines as regulators of muscle metabolism and insulin sensitivity

Skeletal muscle is the largest tissue responsible for the insulin-stimulated disposal of glucose. However, identifying the link between excess body fat and impaired insulin sensitivity in skeletal muscle has been difficult. Several adipose-derived cytokines (adipokines) have been implicated in the impairment of insulin sensitivity, while adipokines such as leptin and adiponectin exert an insulin-sensitizing effect. Leptin and adiponectin have each been shown to increase fatty acid (FA) oxidation and decrease triglyceride storage in muscle, which may explain, in part, the insulin-sensitizing effect of these cytokines. Recent evidence strongly implicates an increased localization of the FA transporters to the plasma membrane (PM) as an important factor in the accumulation of intramuscular lipids with high-fat diets and obesity. Perhaps surprisingly, relatively little attention has been paid to the ability of insulin-sensitizing compounds, such as leptin and adiponectin, to decrease the abundance of FA transporters in the PM, thereby decreasing lipid accumulation. In the case of both adipokines, there is also evidence that a resistance to their ability to stimulate FA oxidation in skeletal muscle develops during obesity. One of our recent studies indicates that this development can be very rapid (i.e., within days), and precedes the increase in lipid uptake and accumulation that leads to insulin resistance. It is noteworthy that leptin resistance can be modulated by both diet and training in rodents. Further studies examining the underlying mechanisms of the development of leptin and adiponectin resistance are warranted.

Implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity

Insulin and leptin play complementary roles in regulating the consumption, uptake, oxidation and storage of nutrients. Chronic consumption of diets that contain a high proportion of calories from saturated fat induces a progressive deterioration in function of both hormones. Certain rat lines and strains of mice are particularly sensitive to the obesogenic and diabetogenic effects of high fat diets, and have been used extensively to study the developmental progression of insulin and leptin resistance in relation to the increasing adiposity that is characteristic of their response to these diets. Some aspects of the diminished efficacy of each hormone are secondary to increased adiposity but a consensus is emerging to support the view that direct effects of dietary components or their metabolites, independent of the resulting obesity, play important roles in development of insulin and leptin resistance. In this minireview, we will examine the implications of crosstalk between leptin and insulin signaling during the development of diet-induced obesity, emphasizing potential interactions between pathways that occur among target sites, and exploring how these interactions may influence the progression of obesity and diabetes.

States of serum leptin and insulin in children with epilepsy: risk predictors of weight gain

BACKGROUND AND OBJECTIVES

Weight gain is an adverse metabolic effect in some children with epilepsy. The studies done to detect the effect of antiepileptic drugs and weight homeostatic hormones, insulin and leptin, were limited and controversial.

MATERIALS AND METHODS

We evaluated the serum leptin and insulin as predictors of weight gain in children receiving long-term treatment with valproate (VPA), carbamazepine (CBZ), lamotrigine (LTG). This study included 90 patients (treated: 70; untreated: 20). Serum lipid profile, insulin and leptin were measured.

RESULTS

BMI, serum leptin and insulin were significantly elevated in VPA compared with controls, untreated patients and those treated with CBZ, LTG and combined therapy with LTG. Girls on VPA had higher BMI and leptin levels than boys. With VPA, serum insulin was correlated with BMI (r=0.625, p<0.01), leptin (r=0.823, p<0.001), treatment duration (r=0.775, p<0.01) and VPA dose (r=0.975, p<0.0001). Serum leptin was correlated with age (r=0.980, p<0.0001), BMI (r=0.704, p<0.01), serum insulin (r=0.823, p<0.001), LDL-c (r=0.630, p<0.01), HDL-c (r=-0.880, p<0.001), treatment duration (r=0.770, p<0.01) and VPA dose (r=0.970, p<0.001). BMI is correlated with serum insulin, leptin, LDL-c (r=0.835, p<0.001) and HDL-c (r=-0.955, p<0.0001).

CONCLUSION

Hyperinsulinemia and hyperleptinemia are common with VPA and marked among epileptic children who gained weight suggesting states of insulin and leptin resistances. These alterations were not demonstrated with CBZ or LTG. The relationship between VPA, leptin and weight seems to be gender specific. Serum leptin may serve as a sensitive parameter for weight gain and reduction with intervention programs during follow-up of girls with epilepsy.

Oleic acid directly regulates POMC neuron excitability in the hypothalamus

The mammalian CNS relies on a constant supply of external glucose for its undisturbed operation. However, neurons can readily switch to using fatty acids and ketones as alternative fuels. Here, we show that oleic acid (OA) excites pro-opiomelanocortin (POMC) neurons by inhibition of ATP-activated potassium (K(ATP)) channels. The involvement of K(ATP) channels is further supported by experiments in SUR1 KO animals. Inhibition of beta-oxidation using carnitine palmitoyltransferase-1 inhibitors blocks OA-induced depolarization. The depolarizing effect of OA is specific because it is not mimicked by octanoic acid. Furthermore, OA does not regulate the excitability of agouti-related peptide neurons. High-fat feeding alters POMC neuron excitability, but not its response to OA. Thus beta-oxidation in POMC neurons may mediate the appetite-suppressing (anorexigenic) effects of OA.

Polymorphism of human leptin receptor gene is associated with type 2 diabetic patients complicated with non-alcoholic fatty liver disease in China

BACKGROUND AND AIM

To investigate the relationship between human leptin receptor (LEPR) gene G3057A polymorphism and type 2 diabetes mellitus (T2DM) patients complicated with or without non-alcoholic fatty liver disease (NAFLD).

METHODS

Two hundred and sixteen cases of newly diagnosed T2DM patients (104 cases complicated with NAFLD) and 108 cases of normal glucose tolerances (NGT) were recruited. Hemi-nested polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) and PCR-direct sequence analysis were conducted to detect the polymorphism of LEPR G3057A variation. Plasma leptin levels were measured by enzyme-linked immunosorbent assay kit. Plasma lipid and glucose metabolic parameters were measured routinely. Liver ultrasound was carried out for all subjects.

RESULTS

T2DM patients complicated with NAFLD had higher plasma insulin, leptin, triglycerides (TG) and low-density lipoprotein cholesterol (LDL-C) levels than those without NAFLD and NGT subjects. The variant frequency at nucleotide 3057 G-->A transversion was 76.0% in type 2 diabetic patients complicated with NAFLD, which was also significantly higher than those without NAFLD (62.1%) and NGT cases (53.2%). There was also significant difference in genotype distribution between the three groups (chi(2) = 14.63, P < 0.01).

CONCLUSION

The polymorphism of LEPR gene 3057 probably contributes to the onset of NAFLD by regulating lipid metabolism and affecting insulin sensitivity.

A functional leptin system is essential for sodium tungstate antiobesity action

Sodium tungstate is a novel agent in the treatment of obesity. In diet-induced obese rats, it is able to reduce body weight gain by increasing energy expenditure. This study evaluated the role of leptin, a key regulator of energy homeostasis, in the tungstate antiobesity effect. Leptin receptor-deficient Zucker fa/fa rats and leptin-deficient ob/ob mice were treated with tungstate. In lean animals, tungstate administration reduced body weight gain and food intake and increased energy expenditure. However, in animals with deficiencies in the leptin system, treatment did not modify these parameters. In ob/ob mice in which leptin deficiency was restored through adipose tissue transplantation, treatment restored the tungstate-induced body weight gain and food intake reduction as well as energy expenditure increase. Furthermore, in animals in which tungstate administration increased energy expenditure, changes in the expression of key genes involved in brown adipose tissue thermogenesis were detected. Finally, the gene expression of the hypothalamic neuropeptides, Npy, Agrp, and Cart, involved in the leptin regulation of energy homeostasis, was also modified by tungstate in a leptin-dependent manner. In summary, the results indicate that the effectiveness of tungstate in reducing body weight gain is completely dependent on a functional leptin system.

Monitoring FoxO1 localization in chemically identified neurons

The PI3K-Akt-FoxO1 pathway contributes to the actions of insulin and leptin in several cell types, including neurons in the CNS. However, identifying these actions in chemically identified neurons has proven difficult. To address this problem, we have developed a reporter mouse for monitoring PI3K-Akt signaling in specific populations of neurons, based on FoxO1 nucleocytoplasmic shuttling. The reporter, FoxO1 fused to green fluorescent protein (FoxO1GFP), is expressed under the control of a ubiquitous promoter that is silenced by a loxP flanked transcriptional blocker. Thus, the expression of the reporter in selected cells is dependent on the action of Cre recombinase. Using this model, we found that insulin treatment resulted in the nuclear exclusion of FoxO1GFP within POMC and AgRP neurons in a dose- and time-dependent manner. FoxO1GFP nuclear exclusion was also observed in POMC neurons following in vivo administration of insulin. In addition, leptin induced transient nuclear export of FoxO1GFP in POMC neurons in a dose dependent manner. Finally, insulin-induced nuclear export was impaired in POMC neurons by pretreatment with free fatty acids, a paradigm known to induce insulin resistance in peripheral insulin target tissues. Thus, our FoxO1GFP mouse provides a tool for monitoring the status of PI3K-Akt signaling in a cell-specific manner under physiological and pathophysiological conditions.

Hypothalamic control of hepatic glucose production and its potential role in insulin resistance

The liver plays a pivotal role in the regulation of glucose metabolism because it is the key organ that maintains glucose levels during fasting. An emerging body of literature has demonstrated the important role of the hypothalamus in controlling hepatic glucose production (HGP). The hypothalamus senses circulating nutrients and hormones, conveying the energy status to the central nervous system, which, in turn, controls HGP in part by way of the autonomic nervous system. Overfeeding results in the failure of the hypothalamus to sense circulating nutrients and hormones, and in a loss of the central control of HGP.

Should peripheral CB(1) cannabinoid receptors be selectively targeted for therapeutic gain?

Endocannabinoids, endogenous lipid ligands of cannabinoid receptors, mediate a variety of effects similar to those of marijuana. Cannabinoid CB(1) receptors are highly abundant in the brain and mediate psychotropic effects, which limits their value as a potential therapeutic target. There is growing evidence for CB(1) receptors in peripheral tissues that modulate a variety of functions, including pain sensitivity and obesity-related hormonal and metabolic abnormalities. In this review we propose that selective targeting of peripheral CB(1) receptors has potential therapeutic value because it would help to minimize addictive, psychoactive effects in the case of CB(1) agonists used as analgesics, or depression and anxiety in the case of CB(1) antagonists used in the management of cardiometabolic risk factors associated with the metabolic syndrome.

Maternal and postnatal overnutrition differentially impact appetite regulators and fuel metabolism

Maternal obesity is increasing, and it is known that the intrauterine experience programs fetal and newborn metabolism. However, the relative contributions of pre- or postnatal factors are unknown. We hypothesized that maternal overnutrition caused by long-term maternal obesity would exert a stronger detrimental impact than postnatal overnutrition on offspring metabolic homeostasis, with additional postnatal overnutrition exaggerating these alterations. Female Sprague Dawley rats were exposed to chow or high-fat cafeteria diet for 5 wk before mating and throughout gestation and lactation. On postnatal d 1, litters were adjusted to three per litter to induce postnatal overnutrition (vs. 12 in control). Hypothalamic appetite regulators neuropeptide Y and proopiomelanocortin, glucose transporter 4, and lipid metabolic markers were measured. At postnatal d 20, male pups born of obese dams, or those overnourished postnatally, were 42% heavier than controls; combining both interventions led to 80% greater body weight. Maternal obesity increased pup adiposity and led to glucose intolerance in offspring; these were exaggerated by additional postnatal overnutrition during lactation. Maternal obesity was also linked to hyperlipidemia in offspring and reduced hypothalamic neuropeptide Y and increased proopiomelanocortin mRNA expression. Postnatal overnutrition of offspring from obese dams amplified these hypothalamic changes. Both maternal and postnatal overnutrition reduced muscle glucose transporter 4. Adipose carnitine palmitoyl-transferase-1 and adipose triglyceride lipase mRNA was up-regulated only by postnatal overnutrition. Maternal overnutrition appears to alter central appetite circuits and promotes early-onset obesity; postnatal overnutrition interacted to cause peripheral lipid and glucose metabolic disorders, supporting the critical message to reduce early-life adverse nutritional impact.

Hypothalamic IKKbeta/NF-kappaB and ER stress link overnutrition to energy imbalance and obesity

Overnutrition is associated with chronic inflammation in metabolic tissues. Whether metabolic inflammation compromises the neural regulatory systems and therefore promotes overnutrition-associated diseases remains unexplored. Here we show that a mediator of metabolic inflammation, IKKbeta/NF-kappaB, normally remains inactive although enriched in hypothalamic neurons. Overnutrition atypically activates hypothalamic IKKbeta/NF-kappaB at least in part through elevated endoplasmic reticulum stress in the hypothalamus. While forced activation of hypothalamic IKKbeta/NF-kappaB interrupts central insulin/leptin signaling and actions, site- or cell-specific suppression of IKKbeta either broadly across the brain or locally within the mediobasal hypothalamus, or specifically in hypothalamic AGRP neurons significantly protects against obesity and glucose intolerance. The molecular mechanisms involved include regulation by IKKbeta/NF-kappaB of SOCS3, a core inhibitor of insulin and leptin signaling. Our results show that the hypothalamic IKKbeta/NF-kappaB program is a general neural mechanism for energy imbalance underlying obesity and suggest that suppressing hypothalamic IKKbeta/NF-kappaB may represent a strategy to combat obesity and related diseases.

Insulin, leptin, and food reward: update 2008

The hormones insulin and leptin have been demonstrated to act in the central nervous system (CNS) as regulators of energy homeostasis at medial hypothalamic sites. In a previous review, we described new research demonstrating that, in addition to these direct homeostatic actions at the hypothalamus, CNS circuitry that subserves reward and motivation is also a direct and an indirect target for insulin and leptin action. Specifically, insulin and leptin can decrease food reward behaviors and modulate the function of neurotransmitter systems and neural circuitry that mediate food reward, i.e., midbrain dopamine and opioidergic pathways. Here we summarize new behavioral, systems, and cellular evidence in support of this hypothesis and in the context of research into the homeostatic roles of both hormones in the CNS. We discuss some current issues in the field that should provide additional insight into this hypothetical model. The understanding of neuroendocrine modulation of food reward, as well as food reward modulation by diet and obesity, may point to new directions for therapeutic approaches to overeating or eating disorders.

Regulation of food intake through hypothalamic signaling networks involving mTOR

To maintain normal activity, single cells must assure that their energy needs and utilization are continuously matched. Likewise, multicellular organisms must constantly coordinate energy intake and expenditure to maintain energy homeostasis. The brain, and the hypothalamus in particular, plays a critical role in integrating and coordinating several types of signals, including hormones and nutrients, to guarantee such homeostasis. Like single cells, the hypothalamus also profits from intracellular pathways known to work as fuel sensors to maintain energy balance. One such pathway is the mammalian target of rapamycin (mTOR). mTOR integrates different sensory inputs to regulate protein synthesis rates in individual cells, and it has recently been implicated in the central nervous system to regulate food intake and body weight as well. This review provides an overview of the role of hypothalamic intracellular fuel sensors in the overall control of energy balance and discusses the potential contribution of these fuel-sensing mechanisms to the metabolic dysregulation associated with obesity.

Hepatic CB1 receptor is required for development of diet-induced steatosis, dyslipidemia, and insulin and leptin resistance in mice

Diet-induced obesity is associated with fatty liver, insulin resistance, leptin resistance, and changes in plasma lipid profile. Endocannabinoids have been implicated in the development of these associated phenotypes, because mice deficient for the cannabinoid receptor CB1 (CB1-/-) do not display these changes in association with diet-induced obesity. The target tissues that mediate these effects, however, remain unknown. We therefore investigated the relative role of hepatic versus extrahepatic CB1 receptors in the metabolic consequences of a high-fat diet, using liver-specific CB1 knockout (LCB1-/-) mice. LCB1(-/-) mice fed a high-fat diet developed a similar degree of obesity as that of wild-type mice, but, similar to CB1(-/-) mice, had less steatosis, hyperglycemia, dyslipidemia, and insulin and leptin resistance than did wild-type mice fed a high-fat diet. CB1 agonist-induced increase in de novo hepatic lipogenesis and decrease in the activity of carnitine palmitoyltransferase-1 and total energy expenditure were absent in both CB1(-/-) and LCB1(-/-) mice. We conclude that endocannabinoid activation of hepatic CB1 receptors contributes to the diet-induced steatosis and associated hormonal and metabolic changes, but not to the increase in adiposity, observed with high-fat diet feeding. Theses studies suggest that peripheral CB1 receptors could be selectively targeted for the treatment of fatty liver, impaired glucose homeostasis, and dyslipidemia in order to minimize the neuropsychiatric side effects of nonselective CB1 blockade during treatment of obesity-associated conditions.

Insulin sensitivity: modulation by nutrients and inflammation

Insulin resistance is a major metabolic feature of obesity and is a key factor in the etiology of a number of diseases, including type 2 diabetes. In this review, we discuss potential mechanisms by which brief nutrient excess and obesity lead to insulin resistance and propose that these mechanisms of action are different but interrelated. We discuss how pathways that "sense" nutrients within skeletal muscle are readily able to regulate insulin action. We then discuss how obesity leads to insulin resistance via a complex interplay among systemic fatty acid excess, microhypoxia in adipose tissue, ER stress, and inflammation. In particular, we focus on the hypothesis that the macrophage is an important cell type in the propagation of inflammation and induction of insulin resistance in obesity. Overall, we provide our integrative perspective regarding how nutrients and obesity interact to regulate insulin sensitivity.

Activation of hypothalamic S6 kinase mediates diet-induced hepatic insulin resistance in rats

Prolonged activation of p70 S6 kinase (S6K) by insulin and nutrients leads to inhibition of insulin signaling via negative feedback input to the signaling factor IRS-1. Systemic deletion of S6K protects against diet-induced obesity and enhances insulin sensitivity in mice. Herein, we present evidence suggesting that hypothalamic S6K activation is involved in the pathogenesis of diet-induced hepatic insulin resistance. Extending previous findings that insulin suppresses hepatic glucose production (HGP) partly via its effect in the hypothalamus, we report that this effect was blunted by short-term high-fat diet (HFD) feeding, with concomitant suppression of insulin signaling and activation of S6K in the mediobasal hypothalamus (MBH). Constitutive activation of S6K in the MBH mimicked the effect of the HFD in normal chow-fed animals, while suppression of S6K by overexpression of dominant-negative S6K or dominant-negative raptor in the MBH restored the ability of MBH insulin to suppress HGP after HFD feeding. These results suggest that activation of hypothalamic S6K contributes to hepatic insulin resistance in response to short-term nutrient excess.

Intracerebroventricular administration of neuropeptide Y induces hepatic insulin resistance via sympathetic innervation

OBJECTIVE

We recently showed that intracerebroventricular infusion of neuropeptide Y (NPY) hampers inhibition of endogenous glucose production (EGP) by insulin in mice. The downstream mechanisms responsible for these effects of NPY remain to be elucidated. Therefore, the aim of this study was to establish whether intracerebroventricular NPY administration modulates the suppressive action of insulin on EGP via hepatic sympathetic or parasympathetic innervation.

RESEARCH DESIGN AND METHODS

The effects of a continuous intracerebroventricular infusion of NPY on glucose turnover were determined in rats during a hyperinsulinemic-euglycemic clamp. Either rats were sham operated, or the liver was sympathetically (hepatic sympathectomy) or parasympathetically (hepatic parasympathectomy) denervated.

RESULTS

Sympathectomy or parasympathectomy did not affect the capacity of insulin to suppress EGP in intracerebroventricular vehicle-infused animals (50 +/- 8 vs. 49 +/- 6 vs. 55 +/- 6%, in hepatic sympathectomy vs. hepatic parasympathectomy vs. sham, respectively). Intracerebroventricular infusion of NPY significantly hampered the suppression of EGP by insulin in sham-denervated animals (29 +/- 9 vs. 55 +/- 6% for NPY/sham vs. vehicle/sham, respectively, P = 0.038). Selective sympathetic denervation of the liver completely blocked the effect of intracerebroventricular NPY administration on insulin action to suppress EGP (NPY/hepatic sympathectomy, 57 +/- 7%), whereas selective parasympathetic denervation had no effect (NPY/hepatic parasympathectomy, 29 +/- 7%).

CONCLUSIONS

Intracerebroventricular administration of NPY acutely induces insulin resistance of EGP via activation of sympathetic output to the liver.

Increased hepatic steatosis and insulin resistance in mice lacking hepatic androgen receptor

Early studies demonstrated that whole-body androgen receptor (AR)-knockout mice with hypogonadism exhibit insulin resistance. However, details about the mechanisms underlying how androgen/AR signaling regulates insulin sensitivity in individual organs remain unclear. We therefore generated hepatic AR-knockout (H-AR(-/y)) mice and found that male H-AR(-/y) mice, but not female H-AR(-/-) mice, fed a high-fat diet developed hepatic steatosis and insulin resistance, and aging male H-AR(-/y) mice fed chow exhibited moderate hepatic steatosis. We hypothesized that increased hepatic steatosis in obese male H-AR(-/y) mice resulted from decreased fatty acid beta-oxidation, increased de novo lipid synthesis arising from decreased PPARalpha, increased sterol regulatory element binding protein 1c, and associated changes in target gene expression. Reduced insulin sensitivity in fat-fed H-AR(-/y) mice was associated with decreased phosphoinositide-3 kinase activity and increased phosphenolpyruvate carboxykinase expression and correlated with increased protein-tyrosine phosphatase 1B expression.

CONCLUSION

Together, our results suggest that hepatic AR may play a vital role in preventing the development of insulin resistance and hepatic steatosis. AR agonists that specifically target hepatic AR might be developed to provide a better strategy for treatment of metabolic syndrome in men.

The integrative role of CNS fuel-sensing mechanisms in energy balance and glucose regulation

The incidences of both obesity and type 2 diabetes mellitus are rising at epidemic proportions. Despite this, the balance between caloric intake and expenditure is tremendously accurate under most circumstances. Growing evidence suggests that nutrient and hormonal signals converge and directly act on brain centers, leading to changes in fuel metabolism and, thus, stable body weight over time. Growing evidence also suggests that these same signals act on the central nervous system (CNS) to regulate glucose metabolism independently. We propose that this is not coincidental and that the CNS responds to peripheral signals to orchestrate changes in both energy and glucose homeostasis. In this way the CNS ensures that the nutrient demands of peripheral tissues (and likely of the brain itself) are being met. Consequently, dysfunction of the ability of the CNS to integrate fuel-sensing signals may underlie the etiology of metabolic diseases such as obesity and diabetes.

Activation of central lactate metabolism lowers glucose production in uncontrolled diabetes and diet-induced insulin resistance

OBJECTIVE

Hypothalamic lactate metabolism lowers hepatic glucose production and plasma glucose levels in normal rodents. However, it remains unknown whether activation of hypothalamic lactate metabolism lowers glucose production and plasma glucose levels in rodents with diabetes and obesity.

RESEARCH DESIGN AND METHODS

We performed intracerebroventricular (ICV) administration of lactate to enhance central lactate metabolism in 1) early-onset streptozotocin-induced uncontrolled diabetic rodents, 2) experimentally induced hypoinsulinemic normal rodents, and 3) early-onset diet-induced insulin-resistant rodents. Tracer-dilution methodology was used to assess the impact of ICV lactate on the rate of glucose production in all three models.

RESULTS

We first report that in the absence of insulin treatment, ICV lactate administration lowered glucose production and glucose levels in rodents with uncontrolled diabetes. Second, ICV lactate administration lowered glucose production and glucose levels in normal rodents with experimentally induced hypoinsulinemia. Third, and finally, ICV lactate administration lowered glucose production in normal rodents with diet-induced insulin resistance.

CONCLUSIONS

Central lactate metabolism lowered glucose production in uncontrolled diabetic and normal rodents with hypoinsulinemia and in rodents with diet-induced insulin resistance. These data suggest that insulin signaling is not required for central lactate to lower glucose production and that the activation of hypothalamic lactate metabolism could consequently bypass insulin resistance and lower glucose levels in early-onset diabetes and obesity.

Prevention and treatment of type 2 diabetes: current role of lifestyle, natural product, and pharmacological interventions

Common complications of type 2 diabetes (T2D) are eye, kidney and nerve diseases, as well as an increased risk for the development of cardiovascular disease and cancer. The overwhelming influence of these conditions contributes to a decreased quality of life and life span, as well as significant economic consequences. Although obesity once served as a surrogate marker for the risk of T2D, we know now that excess adipose tissue secretes inflammatory cytokines that left unchecked, accelerate the progression to insulin resistance and T2D. In addition, excess alcohol consumption may also increase the risk of T2D. From a therapeutic standpoint, lifestyle interventions such as dietary modification and/or exercise training have been shown to improve glucose homeostasis but may not normalize the disease process unless weight loss is achieved and increased physical activity patterns are established. Furthermore, utilization of natural products may serve as a significant adjunct in the fight against insulin resistance but further research is needed to ascertain their validity. Since it is clear that pharmaceutical therapy plays a significant role in the treatment of insulin resistance, this review will also discuss some of the newly developed pharmaceutical therapies that may work in conjunction with lifestyle interventions, and lessen the burden of behavioral change as the only strategy against the development of T2D.

Short-term overfeeding induces insulin resistance in weight-stable patients after bariatric surgery

BACKGROUND

Short time overfeeding of rats rapidly leads to insulin resistance (IR). A study with healthy human volunteers, which we suggest are less susceptible for developing IR after short time overfeeding, did not show these effects on IR. Therefore a study population of weight-stable, former morbidly obese subjects (BMI 31.3 kg/m2), which were treated with bariatric surgery approximately 3 years ago was selected.

METHODS

Eleven subjects were submitted to a 7-day overfeeding study, resulting in a 53% increase in caloric intake (1,227 +/- 394.4 to 1,879.2 +/- 298.4 kcal/day). During normal diet and after overfeeding, insulin sensitivity was measured using steady state plasma glucose (SSPG) levels. At these time points, BMI and waist/hip ratio together with plasma levels of inflammatory markers (CRP, AGP, LBP, and TNF-alpha receptors) and plasma leptin values were also measured.

RESULTS

SSPG levels after overfeeding increased from 8.2 +/- 3.2 to 10.6 +/- 2.6 mmol/l (P < 0.05), indicating decreased insulin sensitivity after overfeeding. Fasting plasma insulin, glucose, circulating levels of inflammatory markers, BMI, and waist/hip ratio remained unchanged.

CONCLUSIONS

This study shows that overfeeding in a group of weight-stable, former morbidly obese subjects 3 years after bariatric surgery results in decreased insulin sensitivity. The mechanisms behind decreased insulin sensitivity induced by overfeeding are poorly understood, but the present results reveal that a unique human model is available to study these mechanisms, leading to a better understanding of the pathophysiology of IR.

Lack of starvation-induced activation of AMP-activated protein kinase in the hypothalamus of the Lou/C rats resistant to obesity

OBJECTIVE

The AMP-activated protein kinase (AMPK) is involved in the control of food intake by the hypothalamus. The aim of this work was to investigate if modification of hypothalamic AMPK regulation could be related to the spontaneous food restriction of Lou/C rats, a strain resistant to obesity exhibiting a 40% reduction in caloric intake compared with their lean Wistar counterparts.

DESIGN

Three-month-old male Lou/C rats were compared with age-matched male Wistar rats in both fed ad libitum and 24-h food deprivation state.

MEASUREMENTS AND RESULTS

We first confirmed that starvation activated both isoforms of AMPK catalytic alpha subunits and enhanced the phosphorylation state of its downstream targets acetyl-CoA carboxylase and elongation factor 2 in the hypothalamus of Wistar rats. These changes were not observed in the hypothalamus of Lou/C rats. Interestingly, the starvation-induced changes in hypothalamic mRNA levels of the main orexigenic and anorexigenic neuropeptides were also blunted in the Lou/C rats. Analysis of the concentrations of circulating substrates and hormones known to regulate hypothalamic AMPK indicated that the starvation-induced changes in ghrelin, adiponectin and leptin were not observed in Lou/C rats. Furthermore, an increased phosphorylation state of signal transducer and activator of transcription 3 (STAT3), which admittedly mediates leptin signaling, was evidenced in the hypothalamus of the starved Lou/C rats, as well as modifications of expression of the leptin-sensitive genes suppressor of cytokine signaling-3 and stearoyl-coenzyme A desaturase 1. In addition, despite reduced leptin level in fed Lou/C rats, the phosphorylation state of hypothalamic STAT3 remained similar to that found in fed Wistar rats, an adaptation that could be explained by the concomitant increase in ObRb leptin receptor mRNA expression.

CONCLUSION

Activation of hypothalamic AMPK by starvation, which stimulates food intake through changes in (an)orexigenic neuropeptides in the normal rats, was not observed in the spontaneously hypophagic Lou/C rats.

Central neural and endocrine mechanisms of non-exercise activity thermogenesis and their potential impact on obesity

The rise in obesity is associated with a decline in the amount of physical activity in which people engage. The energy expended through everyday non-exercise activity, called non-exercise activity thermogenesis (NEAT), has a considerable potential impact on energy balance and weight gain. Comparatively little attention has been paid to the central mechanisms of energy expenditure and how decreases in NEAT might contribute to obesity. In this review, we first examine the sensory and endocrine mechanisms through which energy availability and energy balance are detected that may influence NEAT. Second, we describe the neural pathways that integrate these signals. Lastly, we consider the effector mechanisms that modulate NEAT through the alteration of activity levels as well as through changes in the energy efficiency of movement. Systems that regulate NEAT according to energy balance may be linked to neural circuits that modulate sleep, addiction and the stress response. The neural and endocrine systems that control NEAT are potential targets for the treatment of obesity.

Monounsaturated fat-rich diet prevents central body fat distribution and decreases postprandial adiponectin expression induced by a carbohydrate-rich diet in insulin-resistant subjects

OBJECTIVE

Central obesity is associated with insulin resistance through factors that are not fully understood. We studied the effects of three different isocaloric diets on body fat distribution, insulin sensitivity, and peripheral adiponectin gene expression.

RESEARCH DESIGN AND METHODS

Eleven volunteers, offspring of obese type 2 diabetic patients with abdominal fat deposition, were studied. These subjects were considered insulin resistant as indicated by Matsuda index values <4 after an oral glucose tolerance test, and they maintained A1C <6.5% without therapeutic intervention. All subjects underwent three dietary periods of 28 days each in a crossover design: 1) diet enriched in saturated fat (SAT), 2) diet rich in monounsaturated fat (MUFA) (Mediterranean diet), and 3) diet rich in carbohydrates (CHOs).

RESULTS

Weight, body composition, and resting energy expenditure remained unchanged during the three sequential dietary periods. Using dual-energy X-ray absorptiometry we observed that when patients were fed a CHO-enriched diet, their fat mass was redistributed toward the abdominal depot, whereas periphery fat accumulation decreased compared with isocaloric MUFA-rich and high-SAT diets (ANOVA P < 0.05). Changes in fat deposition were associated with decreased postprandial mRNA adiponectin levels in peripheral adipose tissue and lower insulin sensitivity index values from a frequently sampled insulin-assisted intravenous glucose tolerance test in patients fed a CHO-rich diet compared with a MUFA-rich diet (ANOVA P < 0.05).

CONCLUSIONS

An isocaloric MUFA-rich diet prevents central fat redistribution and the postprandial decrease in peripheral adiponectin gene expression and insulin resistance induced by a CHO-rich diet in insulin-resistant subjects.

Pediatric nonalcoholic fatty liver disease (NAFLD): a "growing" problem?

Nonalcoholic fatty liver disease (NAFLD) can affect both adults and children. With the current worldwide epidemic of pediatric obesity, pediatric NAFLD is increasingly being diagnosed. It is not exactly identical to NAFLD in adults, and these differences may be due in part to the occurrence of hepatic metabolic derangements typical of NAFLD during periods of active growth (infancy, mid-childhood and puberty). The natural history of pediatric NAFLD is not yet known; however, children with pediatric NAFLD can develop cirrhosis. Although details of disease mechanism in pediatric NAFLD remain unclear, hyperinsulinemia with insulin resistance appears to be critical. Determining the pathogenesis of pediatric NAFLD is likely to enhance our understanding of NAFLD in all age groups and may identify new treatment opportunities. Finding effective ways to prevent pediatric NAFLD is an important issue for children's health.

Leptin and insulin homeostasis in epilepsy: relation to weight adverse conditions

During managing patients with epilepsy, there is a great risk of weight changes, particularly weight gain with some antiepileptic medications. Weight gain is not only a cosmetic problem that leads to non-compliance to medications but also increases the risk for atherosclerosis and its related complications. The mechanisms underlying weight gain in epilepsy are multiple and controversial and have been attributed to the effect of epilepsy and more commonly the effect of antiepileptic medications on the central and peripheral mechanisms regulating weight homeostasis including the two main homeostatic hormones, leptin, a protein product of obesity gene secreted by adipocytes and insulin, a protein product of pancreatic beta-cells. Increased blood levels of leptin and insulin due to leptin and insulin resistance is observed in patients with epilepsy. Leptin forms an important link between weight gain, insulin resistance, epilepsy and atherosclerosis. The knowledge of the novel roles of leptin in patients with epilepsy will help identification of early markers for the related adverse weight changes, thus allowing proper characterization of suitable antiepileptic medication as initial step during management and follow up of patients.

The role of CNS fuel sensing in energy and glucose regulation

Individual cells must carefully regulate their energy flux to ensure nutrient levels are adequate to maintain normal cellular activity. The same principle holds in multicellular organisms. Thus, for mammals to perform necessary physiological functions, sufficient nutrients need to be available. It is more complex, however, to understand how the energy status of different cells impacts on the overall energy balance of the entire organism. We propose that the central nervous system is the critical organ for the coordination of intracellular metabolic processes that are essential to guarantee energy homeostasis at the organismal level. In particular, we suggest that in specific hypothalamic neurons, evolutionarily conserved fuel sensors, such as adenosine monophosphate-activated protein kinase and mammalian target of rapamycin (mTOR), integrate sensory input from nutrients, including those derived from recently ingested food or those that are stored in adipose tissue, to regulate effector pathways responsible for fuel intake and utilization. The corollary to this hypothesis is that dysregulation of these fuel-sensing mechanisms in the brain may contribute to metabolic dysregulation underlying diseases, such as obesity and type 2 diabetes.

Enhanced muscle mixed and mitochondrial protein synthesis rates after a high-fat or high-sucrose diet

OBJECTIVE

Obesity and insulin resistance are associated with muscle mitochondrial dysfunction, which might be related to impairment of mitochondrial protein synthesis. This study aimed at investigating mixed and mitochondrial protein synthesis in skeletal muscle in response to dietary manipulations.

RESEARCH METHODS AND PROCEDURES

High-sucrose (SU) and high-fat, high-sucrose (F) diets were provided for 6 weeks to Wistar rats at standard (N) and high (H) energy intakes and compared with controls. Fractional synthesis rates of mixed (FSRPT) and mitochondrial (FSRm) proteins within the oxidative (soleus) and glycolytic (tibialis) muscles were measured using stable isotope flooding dose technique using L-[13C]-valine. Carbonyl content, citrate synthase, and cytochrome c oxidase activities were assayed spectrophotometrically on isolated mitochondria.

RESULTS

In the soleus, FSRPT was increased by 40% in the NSU and NF groups and by 65% in the HSU and HF groups (p<0.001 vs. control). FSRm was increased with high-fat diets (NF, +16%; HF, +32%; p<0.01). In the tibialis, FSR(PT) was enhanced in all experimental groups (+31% to 37%, p<0.05 vs. control). FSRm was augmented in the NSU, NF, and HF groups (+28% to 32%, p<0.01). Cytochrome c oxidase activity was significantly decreased in all experimental groups in the soleus (p<0.001).

DISCUSSION

Muscle mixed and mitochondrial protein FSR are enhanced after short-term dietary intervention known to induce insulin resistance and obesity. Adaptations are muscle type specific and may not explain alterations in mitochondrial oxidative capacity but might contribute to maintain mitochondrial functioning.

Inflammatory process in type 2 diabetes: The role of cytokines

Population-based studies have shown strong relationship between inflammatory markers and metabolic disturbances, obesity, and atherosclerosis, whereas inflammation has been considered as a "common soil" between these clinical entities and type 2 diabetes (T2D). The accumulation of macrophages in adipose tissue (AT), the common origin of macrophages and adipocytes, the prevalent presence of peripheral mononuclear cells, and apoptotic beta cells by themselves seem to be the sources of inflammation present in T2D, since they generate the mediators of the inflammatory processes, namely cytokines. The main cytokines involved in the pathogenesis of T2D are interleukin-1beta (IL-1beta), with an action similar to the one present in type 1 diabetes, tumor necrosis factor-alpha (TNF-alpha), and IL-6, considered as the main regulators of inflammation, leptin, more recently introduced, and several others, such as monocyte chemoattractant protein-1, resistin, adiponectin, with either deleterious or beneficial effects in diabetic pathogenesis. The characterization of these molecules targeted diabetes treatment beyond the classical interventions with lifestyle changes and pharmaceutical agents, and toward the determination of specific molecular pathways that lead to low grade chronic inflammatory state mainly due to an immune system's unbalance.

Hepatic steatosis and plasma dyslipidemia induced by a high-sucrose diet are corrected by an acute leptin infusion

High sucrose (HS) feeding in rats induces hepatic steatosis and plasma dyslipidemia. In previous reports (Huang W, Dedousis N, Bhatt BA, O'Doherty RM. J Biol Chem 279: 21695-21700, 2004; and Huang W, Dedousis N, Bandi A, Lopaschuk GD, O'Doherty RM. Endocrinology 147: 1480-1487, 2006), our laboratory demonstrated a rapid ( approximately 100 min) leptin-induced decrease in liver and plasma VLDL triglycerides (TG) in lean rats, effects that were abolished in obese rats fed a high-fat diet, a model that also presents with hepatic steatosis and plasma dyslipidemia. To further examine the capacity of acute leptin treatment to improve metabolic abnormalities induced by nutrient excess, hepatic leptin action was studied in rats after 5 wk of HS feeding. HS feeding induced hepatic steatosis (TG+80+/-8%; P=0.001), plasma hyperlipidemia (VLDL-TG+102+/-14%; P=0.001), hyperinsulinemia (plasma insulin +67+/-12%; P=0.04), and insulin resistance as measured by homeostasis model assessment (+125+/-20%; P=0.02), without increases in adiposity or plasma leptin concentration compared with standard chow-fed controls. A 120-min infusion of leptin (plasma leptin 13.6+/-0.7 ng/ml) corrected hepatic steatosis (liver TG-29+/-3%; P=0.003) and plasma hyperlipidemia in HS (VLDL-TG-42+/-4%; P=0.001) and increased plasma ketones (+45+/-3%; P=0.006), without altering plasma glucose, insulin, or homeostasis model assessment compared with saline-infused HS controls. In addition, leptin activated liver phosphatidylinositol 3-kinase (+70+/-18%; P=0.01) and protein kinase B (Akt; +90+/-29%; P=0.02), and inhibited acetyl-CoA carboxylase (40+/-7%; P=0.04) in HS, further demonstrating that hepatic leptin action was intact in these animals. We conclude that 1) leptin action on hepatic lipid metabolism remains intact in HS-fed rats, 2) leptin rapidly reverses hepatic steatosis and plasma dyslipidemia induced by sucrose, and 3) the preservation of hepatic leptin action after a HS diet is associated with the maintenance of low adiposity and plasma leptin concentrations.

Diet, obesity and diabetes: a current update

The prevalence of obesity has been increasing at a rapid rate over the last few decades. Although the primary defect can be attributed to an imbalance of energy intake over energy expenditure, the regulation of energy balance is now recognized to be complex. Adipose-tissue factors play a central role in the control of energy balance and whole-body fuel homoeostasis. The regulation of adipose-tissue function, in particular its secretion of adipokines, is impaired by increases in adipose mass associated with obesity, and with the development of insulin resistance and Type 2 diabetes. This review analyses adipose-regulated energy input and expenditure, together with the impact of dietary macronutrient composition on energy balance in relation to susceptibility to the development of obesity and Type 2 diabetes, and how these metabolic conditions may be exacerbated by the consequences of abnormal adipose function. By gaining a greater understanding of how energy balance is controlled in normal, and in obese and diabetic states, a more practical approach can be employed to prevent and better treat obesity and metabolic disorders.

Serum leptin levels, hepatic leptin receptor transcription, and clinical predictors of non-alcoholic steatohepatitis in obese bariatric surgery patients

BACKGROUND

Non-alcoholic steatohepatitis (NASH) is a major cause of liver disease in morbidly obese patients. Clinical predictors of NASH remain elusive, as do molecular mechanisms of pathogenesis.

METHODS

A series of 35 morbidly obese patients undergoing bariatric surgery had a liver biopsy performed for standard histologic analysis. In addition, RNA was obtained from liver tissue and analyzed for leptin receptor gene expression. Regression analysis was used to correlate clinical variables, including serum leptin levels and hepatic leptin receptor gene expression, with the presence of histologically confirmed NASH.

RESULTS

Of the 35 subjects enrolled, 29% had steatosis only, 60% had NASH, and 11% had normal liver histology. Among the clinical variables studied, only diabetes mellitus was an independent predictor of NASH. There was a trend toward lower levels of mRNA encoding the long form of the leptin receptor in hepatic tissue from patients with NASH compared to those with steatosis only.

CONCLUSIONS

Diabetes mellitus is associated with an increased risk of NASH in obese patients. Downregulation of hepatic leptin receptor may play a role in the pathogenesis of NASH.

Resistance to leptin action is the major determinant of hepatic triglyceride accumulation in vivo

Impairment of both insulin and leptin action has been implicated in the pathogenesis of nonalcoholic fatty liver disease. By assessing hepatic triglyceride (TG) stores in response to modulation of leptin action (by leptin infusion), we attempted to determine whether leptin has the major role in hepatic TG accumulation. TG were markedly decreased (by 63%, P<0.05) in young animals treated with leptin. However, this was also associated with improvement in hepatic insulin action (2-fold decrease in HGP during clamp, P<0.05). These effects on hepatic TG stores and insulin action were abolished in old rats who demonstrate leptin resistance. Since these experiments could not discern the role of leptin from the role of hepatic insulin action on hepatic TG stores, we further examined the effect of improvement of hepatic insulin action by visceral fat removal (VF-). Enhancement of hepatic insulin action in old VF-rats was associated with reduced hepatic TG stores (by 64% P<0.01). Because this manipulation may have induced an improvement in leptin action as well, we studied VF removal in a genetically leptin-resistant model (Zucker Diabetic Fatty rats, ZDF). Only in this mode was exclusive improvement of hepatic insulin action by VF removal not associated with reduced hepatic TG stores, suggesting that improved hepatic insulin action is not necessary for modulation of hepatic TG stores. By dissociating action of leptin from that of insulin, we suggest that the failure of leptin action is the major physiological mechanism for hepatic steatosis.

Trafficking of dietary fat in obesity-prone and obesity-resistant rats

The trafficking of dietary fat was assessed in obesity-prone (OP) and obesity-resistant (OR) male and female rats. Test meals containing [1-(14)C]palmitate were delivered through gastric feeding tubes while rats consumed a high-carbohydrate diet (HCD) or after 5 days of a high-fat diet (HFD). Over the subsequent 24 h, the appearance of (14)C was followed in the GI tract, skeletal muscles (SM), liver, adipose tissues (AT), and expired CO(2). There was no difference in the production of (14)CO(2) between OP and OR rats consuming a HCD. However, after 5 days on HFD, OR rats produced significantly more (14)CO(2) after the test meal than OP rats (P < 0.001 females, P = 0.03 males). The differential oxidation of dietary fat between OP and OR rats on HFD was not due to differences in absorption but rather was associated with preferential disposition of tracer to AT in OP rats. Measurements of lipoprotein lipase in part explained increased tracer uptake by AT in OP rats but were not consistent with increased SM tracer uptake in OR rats. Surprisingly, female rats oxidized more tracer than male rats irrespective of phenotype or diet. These results are consistent with the notion that differences in the partitioning of dietary fat between storage in AT and oxidation in SM and liver that develop shortly after the introduction of a HFD may in part underlie the differential tendency for OR and OP rats to gain weight on this diet.

Relationship of serum leptin concentration with age, gender, and biomedical parameters in healthy, non-obese subjects

It is known that the circulating levels of leptin, the adipocyte hormone implicated in the control of energy balance, are correlated with fat body mass (FBM), although the influences of other physiological conditions are not fully understood. We investigated the relationships of serum leptin concentration with age, gender, and 36 hormone-metabolic parameters in a sample of a well defined healthy population (n=246; age range 20-93 years), and in subgroups of lean individuals according to their body mass index (BMI), within similar age range and gender distribution. Only insulin secretion (positively) and testosteronemia (negatively, in males) show direct correlations. The other relationships are not significant but throughout collaborating variables, such as serum lipids, especially through FBM, lean body mass (LBM) through insulin secretion, and gender through FBM. In males, LBM correlates with insulin secretions, s-IGF-1 and with s-testosterone. The relationship between insulin secretion and LBM persists up to advanced age. From the present study it may be concluded that the positive relationship of leptin with insulin secretion and the negative one with testosterone, indicate direct implications of leptin in insulin signaling, as well as in male sexual development. Finally, the fact that the amount of secreted insulin depends on LBM and the latter on testosterone and IGF-1, indicates the importance of muscle mass in the control of insulin secretion.

How the hypothalamus controls glucose production: an update

Recent evidence highlights a crucial role of the brain in the control of glucose homeostasis. The hypothalamus senses and integrates signals of fuel abundance, such as circulating macronutrients (glucose and fatty acids) and nutrient-induced hormones (insulin and leptin). This, in turn, results in the activation of neural pathways that return circulating nutrients to baseline by reducing hepatic glucose production and food intake. In Type 2 diabetes and obesity, the ability of the brain to sense and respond to circulating signals is impaired. In this review, the neuroendocrine circuits that have recently been involved in the regulation of endogenous glucose production in rodents will be described. The study of these neural pathways promises to unveil new targets for the therapy of Type 2 diabetes and obesity.

Critical role of stearoyl-CoA desaturase-1 (SCD1) in the onset of diet-induced hepatic insulin resistance

Stearoyl-CoA desaturase-1 (SCD1) catalyzes the synthesis of monounsaturated fatty acids from saturated fatty acids. Mice with a targeted disruption of Scd1 gene locus are lean and display increased insulin sensitivity. To examine whether Scd1 activity is required for the development of diet-induced hepatic insulin resistance, we used a sequence-specific antisense oligodeoxynucleotide (ASO) to lower hepatic Scd1 expression in rats and mice with diet-induced insulin resistance. Treatment of rats with Scd1 ASO markedly decreased liver Scd1 expression (approximately 80%) and total Scd activity (approximately 50%) compared with that in rats treated with scrambled ASO (control). Insulin clamp studies revealed severe hepatic insulin resistance in high-fat-fed rats and mice that was completely reversed by 5 days of treatment with Scd1 ASO. The latter treatment decreased glucose production (by approximately 75%), gluconeogenesis, and glycogenolysis. Downregulation of Scd1 also led to increased Akt phosphorylation and marked decreases in the expression of glucose-6-phosphatase (Glc-6-Pase) and phosphoenolpyruvate carboxykinase (PEPCK). Thus, Scd1 is required for the onset of diet-induced hepatic insulin resistance.

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.