AMP-activated protein kinase plays a role in the control of food intake

AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that acts as an intracellular energy sensor maintaining the energy balance within the cell. The finding that leptin and adiponectin activate AMPK to alter metabolic pathways in muscle and liver provides direct evidence for this role in peripheral tissues. The hypothalamus is a key regulator of food intake and energy balance, coordinating body adiposity and nutritional state in response to peripheral hormones, such as leptin, peptide YY-(3-36), and ghrelin. To date the hormonal regulation of AMPK in the hypothalamus, or its potential role in the control of food intake, have not been reported. Here we demonstrate that counter-regulatory hormones involved in appetite control regulate AMPK activity and that pharmacological activation of AMPK in the hypothalamus increases food intake. In vivo administration of leptin, which leads to a reduction in food intake, decreases hypothalamic AMPK activity. By contrast, injection of ghrelin in vivo, which increases food intake, stimulates AMPK activity in the hypothalamus. Consistent with the effect of ghrelin, injection of 5-amino-4-imidazole carboxamide riboside, a pharmacological activator of AMPK, into either the third cerebral ventricle or directly into the paraventricular nucleus of the hypothalamus significantly increased food intake. These results suggest that AMPK is regulated in the hypothalamus by hormones which regulate food intake. Furthermore, direct pharmacological activation of AMPK in the hypothalamus is sufficient to increase food intake. These findings demonstrate that AMPK plays a role in the regulation of feeding and identify AMPK as a novel target for anti-obesity drugs.     

AMP-activated protein kinase--the key role in metabolic regulation

AMP-activated protein kinase (AMPK) is the central component of a protein kinase cascade that acts as an energy sensor maintaining the energy balance at the cellular as well as at the whole body level. Within the healthy cell, metabolic stress leading to an increase in AMP concentration results in AMPK activation. Once activated, AMPK "switches off" many anabolic pathways e.g. fatty acid and protein synthesis while "switches on" catabolic pathways such as fatty acid oxidation or glycolysis which serve to restore intracellular ATP level. Adipocyte derived hormones leptin and adiponectin activate AMPK in peripheral tissues increasing energy expenditure. AMPK also regulates food intake due to response to hormonal and nutrient signals in hypothalamus. Antidiabetic drugs that mimic the action of insulin activate the AMPK signaling pathways. Further studies are needed to clarify the importance of the AMPK activation for therapeutic effects of this drugs.     

AMPK, an active player in the control of metabolism

Impairment in the regulation of energy homeostasis and imbalance between energy intake and energy expenditure lead to many metabolic disorders and diseases such as obesity and type 2 diabetes. AMP-activated protein kinase (AMPK) is considered as a "fuel-gauge" in the cell and plays a key role in the regulation of energy metabolism. Activated by an increase in the AMP/ATP ratio, AMPK switches on catabolic pathways such as fatty acid oxidation and switches off anabolic pathways such as lipogenesis or gluconeogenesis. Insulin-sensitizing adipokines (leptin and adiponectin) and anti-diabetic drugs (thiazolidinediones and biguanides) are acting in part through the activation of AMPK. More recent findings indicate that AMPK plays also a major role in the control of whole body energy homeostasis by integrating, at the hypothalamus level, nutrient and hormonal signals that regulate food intake and energy expenditure. AMPK provides therefore a potential target for the treatment of metabolic diseases such as obesity and type II diabetes.     

Leptin-induced signal transduction pathways

Leptin is a multifunctional cytokine and hormone that primarily acts in the hypothalamus and plays a key role in the regulation of food intake and energy expenditure. In addition, it has direct effects on many cell types on the periphery. Leptin acts through its receptor, the product of the db gene, which has six isoforms. Only one of them (OB-Rb) has full signalling capabilities and is able to activate the Jak/STAT pathway, the major pathway used by leptin to exert its effects. However, some signalling events can be initiated by the short isoforms. Besides Jak/STAT, other pathways, such as MAPK and the 5'-AMP-activated protein kinase (AMPK) pathway, are also involved in leptin signalling. Leptin also interacts with insulin signalling. In this paper, we give an overview of the signal transduction mechanisms that are related to the actions of leptin.     

AMP-activated protein kinase: ancient energy gauge provides clues to modern understanding of metabolism

The AMP-activated protein kinase (AMPK) is an evolutionarily conserved sensor of cellular energy status, and recent data demonstrate that it also plays a critical role in systemic energy balance. AMPK integrates nutritional and hormonal signals in peripheral tissues and the hypothalamus. It mediates effects of adipokines (leptin, adiponectin, and possibly resistin) in regulating food intake, body weight, and glucose and lipid homeostasis. AMPK is regulated by upstream kinases of which the tumor suppressor, LKB1, is the first to be identified. Complex signaling networks suggest that AMPK may prevent insulin resistance, in part by inhibiting pathways that antagonize insulin signaling. Through signaling, metabolic, and gene expression effects, AMPK enhances insulin sensitivity and fosters a metabolic milieu that may reduce the risk for obesity and type 2 diabetes.     

Muscarinic receptor activation of AMP-activated protein kinase inhibits orexigenic neuropeptide mRNA expression

AMP-activated protein kinase (AMPK) plays a crucial role in both cellular and whole body energy homeostasis. Here we demonstrate that the muscarinic receptor agonist carbachol activates AMPKalpha1-containing complexes in the human SH-SY5Y cell line via a mechanism specific for the AMPK upstream kinase, Ca(2+)/calmodulin-dependent protein kinase kinase beta. Activation of AMPK inhibits mRNA expression of the orexigenic neuropeptides Agouti-related peptide and melanin-concentrating hormone but surprisingly has no effect on neuropeptide Y mRNA, a neuropeptide previously shown to be regulated by AMPK. Rather than restoring mRNA levels to baseline, pharmacological inhibition of Ca(2+)/calmodulin-dependent protein kinase kinase beta or AMPK greatly increases Agouti-related peptide and melanin-concentrating hormone mRNA expression. These data support a hypothesis that modulating basal AMPK activity in the hypothalamus is essential for maintaining tight regulation of pathways contributing to food intake.     

Diet-induced obesity alters AMP kinase activity in hypothalamus and skeletal muscle

AMP-activated protein kinase (AMPK) is a key regulator of cellular energy balance and of the effects of leptin on food intake and fatty acid oxidation. Obesity is usually associated with resistance to the effects of leptin on food intake and body weight. To determine whether diet-induced obesity (DIO) impairs the AMPK response to leptin in muscle and/or hypothalamus, we fed FVB mice a high fat (55%) diet for 10-12 weeks. Leptin acutely decreased food intake by approximately 30% in chow-fed mice. DIO mice tended to eat less, and leptin had no effect on food intake. Leptin decreased respiratory exchange ratio in chow-fed mice indicating increased fatty acid oxidation. Respiratory exchange ratio was low basally in high fat-fed mice, and leptin had no further effect. Leptin (3 mg/kg intraperitoneally) increased alpha2-AMPK activity 2-fold in muscle in chow-fed mice but not in DIO mice. Leptin decreased acetyl-CoA carboxylase activity 40% in muscle from chow-fed mice. In muscle from DIO mice, acetyl-CoA carboxylase activity was basally low, and leptin had no further effect. In paraventricular, arcuate, and medial hypothalamus of chow-fed mice, leptin inhibited alpha2-AMPK activity but not in DIO mice. In addition, leptin increased STAT3 phosphorylation 2-fold in arcuate of chow-fed mice, but this effect was attenuated because of elevated basal STAT3 phosphorylation in DIO mice. Thus, DIO in FVB mice alters alpha2-AMPK in muscle and hypothalamus and STAT3 in hypothalamus and impairs further effects of leptin on these signaling pathways. Defective responses of AMPK to leptin may contribute to resistance to leptin action on food intake and energy expenditure in obese states.     

p70S6 kinase phosphorylates AMPK on serine 491 to mediate leptin's effect on food intake

The PI3K-AKT, mTOR-p70S6 kinase and AMPK pathways play distinct and critical roles in metabolic regulation. Each pathway is necessary for leptin's anorexigenic effects in the hypothalamus. Here we show that these pathways converge in an integrated phosphorylation cascade to mediate leptin action in the hypothalamus. We identify serine(491) on α2AMPK as the site of convergence and show that p70S6 kinase forms a complex with α2AMPK, resulting in phosphorylation on serine(491). Blocking α2AMPK-serine(491) phosphorylation increases hypothalamic AMPK activity, food intake, and body weight. Serine(491) phosphorylation is necessary for leptin's effects on hypothalamic α2AMPK activity, neuropeptide expression, food intake, and body weight. These results identify an inhibitory AMPK kinase, p70S6 kinase, and demonstrate that AMPK is a substrate for mTOR-p70S6 kinase. This discovery has broad biologic implications since mTOR-p70S6 kinase and AMPK have multiple, fundamental and generally opposing cellular effects that regulate metabolism, cell growth, and development.     

Ca2+/calmodulin-dependent protein kinase kinase-beta acts upstream of AMP-activated protein kinase in mammalian cells

AMP-activated protein kinase (AMPK) is the downstream component of a kinase cascade that plays a pivotal role in energy homeostasis. Activation of AMPK requires phosphorylation of threonine 172 (T172) within the T loop region of the catalytic alpha subunit. Recently, LKB1 was shown to activate AMPK. Here we show that AMPK is also activated by Ca(2+)/calmodulin-dependent protein kinase kinase (CaMKK). Overexpression of CaMKKbeta in mammalian cells increases AMPK activity, whereas pharmacological inhibition of CaMKK, or downregulation of CaMKKbeta using RNA interference, almost completely abolishes AMPK activation. CaMKKbeta isolated from rat brain or expressed in E. coli phosphorylates and activates AMPK in vitro. In yeast, CaMKKbeta expression rescues a mutant strain lacking the three kinases upstream of Snf1, the yeast homolog of AMPK. These results demonstrate that AMPK is regulated by at least two upstream kinases and suggest that AMPK may play a role in Ca(2+)-mediated signal transduction pathways.     

C75, a fatty acid synthase inhibitor, reduces food intake via hypothalamic AMP-activated protein kinase

Energy homeostasis and feeding are regulated by the central nervous system. C75, a fatty acid synthase (FAS) inhibitor, causes weight loss and anorexia, implying a novel central nervous system pathway(s) for sensing energy balance. AMP-activated protein kinase (AMPK), a sensor of peripheral energy balance, is phosphorylated and activated when energy sources are low. Here, we identify a role for hypothalamic AMPK in the regulation of feeding behavior and in mediating the anorexic effects of C75. 5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR), an activator of AMPK, increased food intake, whereas compound C, an inhibitor of AMPK, decreased food intake. C75 rapidly reduced the level of the phosphorylated AMPK alpha subunit (pAMPKalpha) in the hypothalamus, even in fasted mice that had elevated hypothalamic pAMPKalpha levels. Furthermore, AICAR reversed both the C75-induced anorexia and the decrease in hypothalamic pAMPKalpha levels. C75 elevated hypothalamic neuronal ATP levels, which may contribute to the mechanism by which C75 decreased AMPK activity. C75 reduced the levels of pAMPKalpha and phosphorylated cAMP response element-binding protein (pCREB) in the arcuate nucleus neurons of the hypothalamus, suggesting a mechanism for the reduction in NPY expression seen with C75 treatment. These data indicate that modulation of FAS activity in the hypothalamus can alter energy perception via AMPK, which functions as a physiological energy sensor in the hypothalamus.     

5'-AMP-activated protein kinase signaling in Caenorhabditis elegans

5'-AMP-activated protein kinase (AMPK) has been called "the metabolic master switch" because of its central role in regulating fuel homeostasis. AMPK, a heterotrimeric serine/threonine protein kinase composed of alpha, beta, and gamma subunits, is activated by upstream kinases and by 5'-AMP in response to various nutritional and stress signals. Downstream effects include regulation of metabolism, protein synthesis, cell growth, and mediation of the actions of a number of hormones, including leptin. However, AMPK research represents a young and growing field; hence, there are many unanswered questions regarding the control and action of AMPK. This review presents evidence for the existence of AMPK signaling pathways in Caenorhabditis elegans, a genetically tractable model organism that has yet to be fully exploited to elucidate AMPK signaling mechanisms.     

Adiponectin stimulates AMP-activated protein kinase in the hypothalamus and increases food intake

Adiponectin has been shown to stimulate fatty acid oxidation and enhance insulin sensitivity through the activation of AMP-activated protein kinase (AMPK) in the peripheral tissues. The effects of adiponectin in the central nervous system, however, are still poorly understood. Here, we show that adiponectin enhances AMPK activity in the arcuate hypothalamus (ARH) via its receptor AdipoR1 to stimulate food intake; this stimulation of food intake by adiponectin was attenuated by dominant-negative AMPK expression in the ARH. Moreover, adiponectin also decreased energy expenditure. Adiponectin-deficient mice showed decreased AMPK phosphorylation in the ARH, decreased food intake, and increased energy expenditure, exhibiting resistance to high-fat-diet-induced obesity. Serum and cerebrospinal fluid levels of adiponectin and expression of AdipoR1 in the ARH were increased during fasting and decreased after refeeding. We conclude that adiponectin stimulates food intake and decreases energy expenditure during fasting through its effects in the central nervous system.     

腺苷-磷酸激活的蛋白激酶在脂类营养代谢中的研究及应用

腺苷-磷酸激活的蛋白激酶（AMP-activated protein kinase,AMPK）是公认的重要能量感受酶。其作用与多个代谢途径有关,尤其在脂类营养代谢过程中发挥着关键的调控作用。AMPK对脂质代谢的调控通过多个信号通路进行,涉及到骨骼肌、肝脏、乳腺等多个组织。对AMPK调控脂类营养代谢机理的研究为2型糖尿病、脂肪肝、肥胖症、癌症等多种疾病的治疗提供了靶点,但AMPK在奶牛乳腺组织的研究较少,其在提高奶牛生产性能方面潜能巨大。     

An Obligate Role of Oxytocin Neurons in Diet Induced Energy Expenditure

Oxytocin neurons represent one of the major subsets of neurons in the paraventricular hypothalamus (PVH), a critical brain region for energy homeostasis. Despite substantial evidence supporting a role of oxytocin in body weight regulation, it remains controversial whether oxytocin neurons directly regulate body weight homeostasis, feeding or energy expenditure. Pharmacologic doses of oxytocin suppress feeding through a proposed melanocortin responsive projection from the PVH to the hindbrain. In contrast, deficiency in oxytocin or its receptor leads to reduced energy expenditure without feeding abnormalities. To test the physiological function of oxytocin neurons, we specifically ablated oxytocin neurons in adult mice. Our results show that oxytocin neuron ablation in adult animals has no effect on body weight, food intake or energy expenditure on a regular diet. Interestingly, male mice lacking oxytocin neurons are more sensitive to high fat diet-induced obesity due solely to reduced energy expenditure. In addition, despite a normal food intake, these mice exhibit a blunted food intake response to leptin administration. Thus, our study suggests that oxytocin neurons are required to resist the obesity associated with a high fat diet; but their role in feeding is permissive and can be compensated for by redundant pathways.     

The AMP-activated protein kinase cascade--a unifying system for energy control

AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that acts as an intracellular energy sensor maintaining the energy balance within the cell. This pivotal role of AMPK places it in an ideal position for regulating whole-body energy metabolism, and AMPK might play a part in protecting the body from metabolic diseases such as type 2 diabetes and obesity. Mutations in AMPK cause cardiac hypertrophy and arrhythmia. Recent findings have identified LKB1--a protein kinase that is mutated in a hereditary form of cancer--as a candidate for the upstream kinase in the AMPK cascade. AMPK could provide a link in human diseases of which the underlying cause is due to defects in energy metabolism.     

Obesity: the role of hypothalamic AMP-activated protein kinase in body weight regulation

Obesity is rapidly increasing and is of great public health concern worldwide. Although there have been remarkable developments in obesity research over the past 10 years, the molecular mechanism of obesity is still not completely understood. Body weight results from the balance between food intake and energy expenditure. Recent studies have found that hypothalamic AMP-activated protein kinase plays a key role in regulating these processes. Leptin, insulin, glucose and alpha-lipoic acid have been shown to reduce food intake by lowering hypothalamic AMP-activated protein kinase activity, whereas ghrelin and glucose depletion increase food intake by increasing hypothalamic AMP-activated protein kinase activity. In addition, this enzyme plays a role in the central regulation of energy expenditure. These findings indicate that hypothalamic AMP-activated protein kinase is an important signal molecule, which integrates nutritional and hormonal signals and modulates feeding behavior and energy expenditure.     

The elusive short gene--an ensemble method for recognition for prokaryotic genome

Glucagon-like peptide-1 (GLP-1) and leptin are anorectic hormones produced in the small intestine and white adipose tissue, respectively. Investigating how these hormones act together as an integrated anorectic signal is important to elucidate a mechanism to maintain energy balance. In the present study, coadministration of subthreshold GLP-1 and leptin dramatically reduced feeding in rats. Although coadministration of GLP-1 with leptin did not enhance leptin signal transduction in the hypothalamus, it significantly decreased phosphorylation of AMP-activated protein kinase (AMPK). In addition, coadministration of GLP-1 with leptin significantly increased proopiomelanocortin (POMC) mRNA levels. Considering that α-melanocortin stimulating hormone (α-MSH) is derived from POMC and functions through the melanocortin-4-receptor (MC4-R) as a key molecule involved in feeding reduction, the interaction of GLP-1 and leptin on feeding reduction may be mediated through the α-MSH/MC4-R system. As expected, the interaction of GLP-1 and leptin was abolished by intracerebroventricular preadministration of the MC4-R antagonists agouti-related peptide and SHU9119. Taken together, GLP-1 and leptin cooperatively reduce feeding at least in part via inhibition of AMPK following binding of α-MSH to MC4-R.     

Synthesis and biological evaluation of benzimidazole derivatives as potent AMP-activated protein kinase activators

Design, synthesis and structure-activity relationships of benzimidazole derivatives as activators of the AMP-activated protein kinase (AMPK) are presented in this paper. AMPK is the central component of a protein kinase cascade that plays a key role in the regulation of energy balance. Once activated, AMPK initiates a series of responses that are aimed at restoring the energy balance of the cell and recent studies have indicated that AMPK plays an important role in regulation of the whole-body energy metabolism. The following study based on the lead compound S27847 involved modification of three regions of this compound. Preliminary structure-activity relationships are being described.     

Leptin activates AMP-activated protein kinase in hepatic cells via a JAK2-dependent pathway

AMP-activated protein kinase (AMPK) plays a key role in the regulation of energy homeostasis within the individual cell. Recent reports have suggested that leptin, an adipocyte-secreted hormone, phosphorylates AMPK in skeletal muscle directly. However, little is known about the interaction between leptin signaling and AMPK activation. Here, we report that the leptin-induced phosphorylation of AMPK was detected in Huh7 cells expressing long form leptin receptor (OBRb) as well as short form leptin receptor (OBRa). In addition, we demonstrate that AMPK activation does not require the phosphorylation of either Tyr-985 or Tyr-1138 within the OBRb and may occur via a STAT3-independent signaling pathway. We also show that Huh7 cells expressing OBRb and SOCS3 (inhibitor of JAK2) resulted in a marked reduction of AMPK activation in response to leptin. These findings suggest that the activation of JAK2, but not STAT3, may play a critical role in leptin-induced AMPK activation in Huh7 cells.     

Adipokines and the peripheral and neural control of energy balance

Adipokines are secreted by adipose tissue and control various physiological systems. Low leptin levels during fasting stimulate feeding, reduce energy expenditure, and modulate neuroendocrine and immune function to conserve energy stores. On the other hand, rising leptin levels in the overfed state prevent weight gain by inhibiting food intake and increasing energy expenditure. These actions are mediated by neuronal circuits in the hypothalamus and brainstem. Leptin also controls glucose and lipid metabolism by targeting enzymes such as AMP-activated protein kinase and stearoyl-coenzyme A desaturase-1 in liver and muscle. Likewise, adiponectin and resistin control energy balance and insulin sensitivity via central and peripheral targets. As highlighted in this review, there are distinct as well as common signaling pathways for adipokines. Understanding adipokine signaling in the brain and other organs will provide insights into the pathogenesis and treatment of obesity, diabetes and various metabolic disorders.