Obesity is an epidemic in Western society, and causes rapidly accelerating rates of type 2 diabetes and cardiovascular disease. The evolutionarily conserved serine/threonine kinase, AMP-activated protein kinase (AMPK), functions as a 'fuel gauge' to monitor cellular energy status. We investigated the potential role of AMPK in the hypothalamus in the regulation of food intake. Here we report that AMPK activity is inhibited in arcuate and paraventricular hypothalamus (PVH) by the anorexigenic hormone leptin, and in multiple hypothalamic regions by insulin, high glucose and refeeding. A melanocortin receptor agonist, a potent anorexigen, decreases AMPK activity in PVH, whereas agouti-related protein, an orexigen, increases AMPK activity. Melanocortin receptor signalling is required for leptin and refeeding effects on AMPK in PVH. Dominant negative AMPK expression in the hypothalamus is sufficient to reduce food intake and body weight, whereas constitutively active AMPK increases both. Alterations of hypothalamic AMPK activity augment changes in arcuate neuropeptide expression induced by fasting and feeding. Furthermore, inhibition of hypothalamic AMPK is necessary for leptin's effects on food intake and body weight, as constitutively active AMPK blocks these effects. Thus, hypothalamic AMPK plays a critical role in hormonal and nutrient-derived anorexigenic and orexigenic signals and in energy balance.

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.

A high-fat, ketogenic diet induces a unique metabolic state in mice. Am J Physiol Endocrinol Metab 292: E1724 -E1739, 2007. First published February 13, 2007; doi:10.1152/ajpendo.00717.2006.-Ketogenic diets have been used as an approach to weight loss on the basis of the theoretical advantage of a low-carbohydrate, high-fat diet. To evaluate the physiological and metabolic effects of such diets on weight we studied mice consuming a very-low-carbohydrate, ketogenic diet (KD). This diet had profound effects on energy balance and gene expression. C57BL/6 mice animals were fed one of four diets: KD; a commonly used obesogenic high-fat, high-sucrose diet (HF); 66% caloric restriction (CR); and control chow (C). Mice on KD ate the same calories as mice on C and HF, but weight dropped and stabilized at 85% initial weight, similar to CR. This was consistent with increased energy expenditure seen in animals fed KD vs. those on C and CR. Microarray analysis of liver showed a unique pattern of gene expression in KD, with increased expression of genes in fatty acid oxidation pathways and reduction in lipid synthesis pathways. Animals made obese on HF and transitioned to KD lost all excess body weight, improved glucose tolerance, and increased energy expenditure. Analysis of key genes showed similar changes as those seen in lean animals placed directly on KD. Additionally, AMP kinase activity was increased, with a corresponding decrease in ACC activity. These data indicate that KD induces a unique metabolic state congruous with weight loss.liver; gene expression OVER THE PAST FEW DECADES the rates of obesity have risen substantially worldwide. Paradoxically, the increases in body weight, particularly in Western countries, occurred during a period of emphasis on diets low in fat as a means for avoiding weight gain. These dietary recommendations were based largely on the concept that high-fat diets were less satiating (39) and that reducing dietary fat reduced risk for cardiovascular disease by lowering circulating fat and cholesterol (5). As a result of the perceived failure of traditional dietary advice, attention shifted to alternative dietary regimes, including lowglycemic-index diets and very-low-carbohydrate ketogenic diets. Interest in these diets derives in part from the theoretical effects of dietary composition on energy expenditure. Although a small number of human studies have found such diets to be more effective in short-term weight loss and without adverse effects on glucose, insulin, lipids, or blood pressure (7,14,42,49,51), reports on metabolic effects remain inconclusive (8,38). Thus the precise effects of macronutrient diet composition on energy balance remain controversial.Studies of the physiological effects of dietary composition are intrinsically difficult in human populations because of problems achieving both compliance and accurate dietary reporting. Therefore, we developed a mouse model to examine the effect of diet composition on physiology, with particular reference to energy expenditure and metab...

In this study, we aim to determine cellular mechanisms linking nutrient metabolism to the regulation of inflammation and insulin resistance. The nutrient sensors AMP-activated protein kinase (AMPK) and SIRT1 show striking similarities in nutrient sensing and regulation of metabolic pathways. We find that the expression, activity, and signaling of the major isoform ␣1AMPK in adipose tissue and macrophages are substantially down-regulated by inflammatory stimuli and in nutrient-rich conditions, such as exposure to lipopolysaccharide (LPS), free fatty acids (FFAs), and diet-induced obesity. Activating AMPK signaling in macrophages by 5-aminoimidazole-4-carboxamide-1-␤4-ribofuranoside or constitutively active ␣1AMPK (CA-␣1) significantly inhibits; although inhibiting ␣1AMPK by short hairpin RNA knock-down or dominant-negative ␣1AMPK (DN-␣1) increases LPS-and FFA-induced tumor necrosis factor ␣ expression. Chromatin immunoprecipitation and luciferase reporter assays show that activation of AMPK by CA-␣1 in macrophages significantly inhibits LPS-or FFA-induced NF-B signaling. More importantly, in a macrophage-adipocyte co-culture system, we find that inactivation of macrophage AMPK signaling inhibits adipocyte insulin signaling and glucose uptake. Activation of AMPK by CA-␣1 increases the SIRT1 activator NAD ؉ content and SIRT1 expression in macrophages. Furthermore, ␣1AMPK activation mimics the effect of SIRT1 on deacetylating NF-B, and the full capacity of AMPK to deacetylate NF-B and inhibit its signaling requires SIRT1. In conclusion, AMPK negatively regulates lipid-induced inflammation, which acts through SIRT1, thereby contributing to the protection against obesity, inflammation, and insulin resistance. Our study defines a novel role for AMPK in bridging the signaling between nutrient metabolism and inflammation.Obesity is causally associated with insulin resistance and type 2 diabetes (1, 2). The underlying mechanisms linking obesity to insulin resistance/type 2 diabetes have received intense investigation during the past decade. Obesity is characterized by a state of chronic inflammation featuring increased production of pro-inflammatory cytokines and chemokines, such as tumor necrosis factor-␣ (TNF-␣), 3 interleukin-6 (IL-6), and monocyte chemoattractant protein 1, and activation of the inflammatory signaling networks, including inhibitor of NF-B (IB) kinase ␤ (IKK-␤)/NF-B and the c-Jun NH 2 -terminal kinase (JNK) pathways in key metabolic tissues as well as macrophages (1-4).Loss of function studies have demonstrated pivotal roles of these inflammatory pathways in the development of obesityinduced insulin resistance (3,4).Adipose tissue plays a key role in the generation of inflammatory responses and mediators in obesity (1, 2). Obese adipose tissue exhibits increased infiltration of macrophages, and acrophages may be a significant source of inflammation (5, 6). The importance of macrophages in obesity-induced inflammation and insulin resistance has been well documented in genetic studies (7-10). Genetic dis...

The evolutionarily conserved serine/threonine kinase, AMP-activated protein kinase (AMPK), functions as a cellular fuel gauge that regulates metabolic pathways in glucose and fatty acid metabolism and protein synthesis. Recent data strongly implicate the AMPK-acetyl CoA carboxylase (ACC)-malonyl CoA pathway in the hypothalamus in the regulation of food intake, body weight and hepatic glucose production. Furthermore, data indicate that AMPK is a mediator of the effects of adipocyte-derived and gut-derived hormones and peptides on fatty acid oxidation and glucose uptake in peripheral tissues. Studies are now elucidating the potential role of kinases upstream of AMPK in these metabolic effects. In addition, recently, several novel downstream effectors of AMPK have been identified. The AMPK pathway in the hypothalamus and peripheral tissues coordinately integrates inputs from multiple hormones, peptides and nutrients to maintain energy homeostasis. AMPK is a serine/threonine kinase that is evolutionarily conserved from yeast to mammals. It is a heterotrimeric protein consisting of an α catalytic subunit and two regulatory subunits, β and γ . AMPK is a fuel-sensing enzyme activated by physiological and pathological stresses that deplete cellular ATP, including hypoxia, ischaemia, glucose deprivation, uncouplers of oxidative phosphorylation, exercise and muscle contraction. Activation of AMPK represses ATP-consuming anabolic pathways and induces ATP-producing catabolic pathways. This is accomplished, at least in part, by regulating gene expression and the activities of key metabolic enzymes in fatty acid, cholesterol and glucose metabolism, and protein synthesis, as well as other metabolic pathways (Kahn et al. 2005). In addition to its role in the periphery, AMPK also regulates energy intake and body weight by mediating opposing effects of anorexigenic and orexigenic signals in the hypothalamus (Andersson et al. 2004;MS Kim et al. 2004;Minokoshi et al. 2004;Han et al. 2005;Kola et al. 2005). Thus, the coordinated regulation of the AMPK pathway in the hypothalamus and peripheral tissues plays a critical role in integrating hormonal and nutrient-derived signals that impact energy homeostasis. This review will focus on recent findings pertaining to the regulation of energy balance by AMPK in both the central nervous system (Fig. 1) and peripheral tissues (Fig. 2), and shed light on the upstream kinases and novel downstream targets for these actions of AMPK.Hypothalamic AMPK activity regulates food intake and body weight (Fig. 1) A number of recent studies demonstrate that hypothalamic AMPK activity regulates food intake. Hypothalamic AMPK is regulated under physiological conditions. Fasting results in increased AMPK activity in multiple hypothalamic regions, whereas refeeding inhibits it (Minokoshi et al. 2004). Activation of AMPK in the hypothalamus is sufficient to increase food intake, and body weight and suppression of hypothalamic AMPK activity is sufficient to decrease these parameters as demonstrated by experimen...

Cold exposure induces brown adipocytes in retroperitoneal fat (RP) of adult A/J mice but not in C57BL/6J (B6) mice. In contrast, induction of the mitochondrial uncoupling protein 1 gene (Ucp1) in interscapular brown adipose tissue (iBAT) shows no strain dependence. We now show that unlike iBAT, in which Ucp1 was expressed in the fetus and continued throughout life, in RP, Ucp1 was transiently expressed between 10 and 30 days of age and then disappeared. Similar to the lack of genetic variation in the expression of Ucp1 in iBAT during cold induction of adult mice, no genetic variation in Ucp1 expression in iBAT was detected during development. In contrast, UCP1-positive multilocular adipocytes, together with corresponding increases in Ucp1 expression, appeared in RP at 10 days of age in A/J and B6 mice, but with much higher expression in A/J mice. At 20 days of age, brown adipocytes represent the major adipocyte present in RP of A/J mice. The disappearance of brown adipocytes by 30 days of age suggested that tissue remodeling occurred in RP. Genetic variability in Ucp1 expression could not be explained by variation in the expression of selective transcription factors and signaling molecules of adipogenesis. In summary, the existence of genetic variability between A/J and B6 mice during the development of brown adipocyte expression in RP, but not in iBAT, suggests that developmental mechanisms for the brown adipocyte differentiation program are different in these adipose tissues.-Xue, B., J-S. Rim, J. C. Hogan, A. A. Coulter, R. A. Koza, and L. P. Kozak. Genetic variability affects the development of brown adipocytes in white fat but not in interscapular brown fat. J. Lipid Res. 2007. 48: 41-51.

Summary Lipid droplet (LD) lipolysis in brown adipose tissue (BAT) is generally considered to be required for cold-induced nonshivering thermogenesis. Here we show that mice lacking BAT Comparative Gene Identification-58 (CGI-58), a lipolytic activator essential for the stimulated LD lipolysis, have normal thermogenic capacity and are not cold sensitive. Relative to littermate controls, these animals had higher body temperatures when they were provided food during cold exposure. The increase in body temperature in the fed, cold-exposed knockout mice was associated with increased energy expenditure and increased sympathetic innervation and browning of white adipose tissue (WAT). Mice lacking CGI-58 in both BAT and WAT were cold sensitive, but only in the fasted state. Thus, LD lipolysis in BAT is not essential for cold-induced nonshivering thermogenesis in vivo. Rather CGI-58-dependent LD lipolysis in BAT regulates WAT thermogenesis, and our data uncover an essential role of WAT lipolysis in fueling thermogenesis during fasting.

In chow-fed mice, ASO-mediated depletion of CGI-58 did not alter weight gain, plasma TG, or plasma glucose, yet raised hepatic TG levels ‫ف‬ 4-fold. When challenged with a high-fat diet (HFD), CGI-58 ASO-treated mice were protected against diet-induced obesity, but their hepatic contents of TG, diacylglycerols, and ceramides were all elevated, and intriguingly, their hepatic phosphatidylglycerol content was increased by 10-fold. These hepatic lipid alterations were associated with signifi cant decreases in hepatic TG hydrolase activity, hepatic lipoprotein-TG secretion, and plasma concentrations of ketones, nonesterifi ed fatty acids, and insulin. Additionally, HFD-fed CGI-58 ASO-treated mice were more glucose tolerant and insulin sensitive.