Leptin (L4146) is the product of the obese (ob) gene and is produced predominantly in white adipose tissue . The leptin receptor (Ob-R), isolated by expression cloning, is encoded by the diabetes (db) gene and alternate splicing of the db gene generates six leptin receptor (Ob-Ra-f) isoforms . The Ob-Rb isoform, the long isoform, plays a vital role in regulating obesity and high levels of the isoform exist in the hypothalamus, the main site of leptin action . The absence of this isoform results in the obese phenotype in db/db mice . The leptin receptor is a member of the class I cytokine receptor superfamily. Like other family members, leptin receptor activation stimulates STAT1, STAT3 and STAT5 tyrosine phosphorylation in vitro and STAT3 tyrosine phosphorylation in vivo . Indeed, the Ob-Rb isoform acts as a potent JAK/STAT signaling activator and the ability of leptin to activate STAT3 is decreased in the hypothalamus when mice are subjected to high fat diets . Leptin deficiency occurs through two mechanisms, either by a defect in leptin transport across the blood brain barrier or by suppression of leptin signaling . Leptin signaling is blocked by suppressors of cytokine signaling-3 (SOCS-3) and by dephosphorylation by protein tyrosine phosphatase 1B (PTP-1B, Product Code P6244). SOCS-3 and PTP-1B deficient mice exhibit increased sensitivity to leptin and resistance to obesity . In addition, leptin receptor activation stimulates both the phosphotidylinositol 3-kinase (PI3K) and Ras-mitogen activated protein kinase (MAPK) signaling pathways, both of which are downstream of JAK .
Figure 1: Leptin receptor signaling. Adapted from Harvey and Ashford, Neuropharmacology, 44, 947-849 (2003).
The arcuate nucleus (ARC) of the hypothalamus serves as the leptin signaling center. Leptin targets two adjacent pathways within the ARC, the orexigenic (appetite-stimulating) pathway mediated by neuropeptide Y (NPY) and agouti-related protein (AgRP), and the anorexigenic (appetite-suppressing) pathway mediated by proopiomelanocortin (POMC) and cocaine- and amphetamine regulated transcript (CART) via the Ob-Rb form of the leptin receptor [1, 2]. When leptin binds to its receptor, a signaling cascade is initiated, activating PI3K and several lipid intermediates. This causes a conversion of F-actin to G-actin, which subsequently opens KATP channels . This cascade is defective in fatty Zucker (fa/fa) rats, which exhibit resistance to both leptin and insulin (I1507).
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The melanocortin pathway is involved in controlling food intake and autonomic activity. Neurons within the ARC express POMC, from which α-melanocyte stimulating hormone (α-MSH) is cleaved. POMC neurons are activated by both leptin and insulin and are suppressed in states of negative energy balance or genetically defective leptin signaling . α-MSH acts as a melanocortin 4 receptor (MC4R) agonist and is induced by leptin . Interestingly, AgRP, whose expression is activated by leptin, acts as an endogenous MC4R antagonist. Although MC4R (H2396) pathways are regulated by leptin, MC4R regulates 5-HT2C serotonin receptors, activation of which induces weight loss. The MC4R also activates brain-derived neurotrophic factor (BDNF) (B3795) through TrkB receptors in the ventromedial (VMH) region of the hypothalamus. Central administration of BDNF in db/db mice decreases food intake and increases energy expenditure, demonstrating that BDNF plays a role in regulating food balance, possibly mediated through MC4R [4, 5].
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Neuropeptide Y (NPY) (N5017), a neuronal signaling peptide that selectively binds to Y1 and Y5 neuropeptide Y receptors in the hypothalamus, induces orexigenic signals, thereby increasing food intake and decreasing energy expenditure . NPY/AgRP neurons exist as neuronal targets of leptin in the ARC [1, 7]. where they are concentrated in the ventromedial ARC and are located adjacent to POMC neurons. NPY/AgRP neurons are suppressed by leptin and insulin and are activated in states of negative energy balance . NPY/AgRP neurons antagonize melanocortin signaling while γ-aminobutyric acid (GABA) (A5835) released from NPY/AgRP neurons suppress POMC neurons . Thus, NPY/AgRP neurons activate an anabolic pathway and inactivate a catabolic pathway at the same time .
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Ghrelin (G3902), recently discovered as an endogenous ligand for the growth hormone ecretagogue receptor, acts as an appetite-stimulating hormone and is implicated in both short- and long-term appetite and body weight regulation . It is secreted from the stomach and circulates in the blood stream where its levels rise and fall before and after meals, respectively, in response to hunger levels. Ghrelin stimulates food intake by activating NPY/AgRP neurons, which express ghrelin receptors, thereby balancing inhibitory signals received from insulin, leptin and peptide YY (P1306) . Individuals suffering from Prader-Willi syndrome, a genetic disorder characterized by a chronic feeling of hunger that leads to obesity, experience high plasma ghrelin levels . One therapeutic target for obesity treatment involves developing ghrelin receptor antagonists to block orexigenic signals from the gut to the brain. Another target involves developing ghrelin receptor inverse agonists that would block constitutive activity of the ghrelin receptor, thereby increasing the response to inhibitory signals to block “between-meals" food intake .
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Adiponectin is another adipocyte-derived hormone that plays an important role in regulating energy homeostasis and insulin sensitivity. Adiponectin has been shown to suppress hepatic glucose production and is reported to have anti-atherogenic properties in vivo . Recent studies have demonstrated that treating obese mice with recombinant adiponectin increases fatty acid metabolism thereby ameliorating insulin resistance . In addition, physiological doses of both leptin and adiponectin completely reversed insulin resistance in lipoatrophic mice, indicating that both leptin and adiponectin are the two major insulin-sensitizing hormones secreted from adipose tissue. Adiponectin appears to act by activating and phosphorylating AMP-activated protein kinase (AMPK), which then activates and phosphorylates acetyl CoA carboxylase, an enzyme downstream of AMPK in adipose tissue .
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Melanin-concentrating hormone (MCH) (M2567), a 19-amino acid neuropeptide derived from a larger prohormone precursor of MCH (Pmch), is expressed predominantly in the lateral hypothalamus and zona incerta. The MCH receptor was initially discovered as an orphan G protein-coupled receptor and two subtypes, referred to as MCHR1 and MCHR2, have been identified. MCH binds with high affinity to MCHR1 and over-expression of MCH produces obesity in mice. Conversely, MCHR1- deficient mice are resistant to the orexigenic actions of MCH and maintain leanness and increased energy expenditure . Interestingly, MCH administration reduces adrenocorticotropic hormone (ACTH) (02275)levels, which regulate glucocorticoid production, while MCH-deficient mice exhibit elevated glucocorticoid levels, indiciating a role for MCHR1 in adrenal function . Compared to leptin deficient ob/ob mice that over-express NPY, AgRP and MCH, mice lacking both leptin and MCH (double null) show improved glucose tolerance, lower body fat and lower plasma glucocorticoid levels .
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Orexin-A (O6012) and Orexin-B (O6137), recently isolated from rat hypothalamus and also known as hypocretins, are two peptides derived from prepro-orexin, a 130 amino acid precursor. These ligands act at two G protein-coupled receptors referred to as OX1R and OX2R. The orexins regulate wakefulness and energy balance, and disruption of their function causes narcolepsy and obesity. Orexins act through the ventral posterior ARC where activation of OX2R orexin receptors stimulates a Na+/Ca2+ exchange current in GABAergic neurons, thereby depolarizing the cells and increasing their firing rate . As GABA acts as a potent stimulus for feeding, orexins may utilize this mechanism to control appetite.
The endocannabinoids anandamide and 2-arachidonyl glycerol (A8973) act through CB1 cannabinoid receptors present in the hypothalamus to stimulate appetite, thereby mimicking the effects of marijuana . Leptin suppresses these endocannabinoids and thus, they are implicated in regulating energy balance . The CB1 cannabinoid receptor and the OX1R orexin receptor are closely positioned in the lateral hypothalamus so that hetero-oligomers could be formed. Co-expression of the CB1 cannabinoid receptor and the OX1R orexin receptor in CHO cells caused a 100-fold increase in orexin-mediated MAPK activation which was blocked by the CB1 cannabinoid receptor antagonist SR141716 . Thus, cannabinoid receptor antagonists may prove useful as potential drugs for the treatment of obesity.
Figure 2: Leptin actions and downstream effects in the arcuate nucleus, ventromedial hypothalamus (VH) and lateral hypothalamus. Adapted from Flier, Cell, 116, 342 (2004).
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Glucagon-like peptide (GLP) (G3265) is a peptide hormone that stimulates the secretion of insulin. Along with peptide YY and cholecystokinins (CCKs), GLP is released from the lower gastrointestinal tract following food intake. Obese subjects exhibit significantly lower plasma levels of endogenous PYY . PYY acts as a metabolic signal indicating satiety by regulating POMC neurons and inhibiting NPY/AgRP neurons in the ARC, possibly through presynaptic Y2 neuropeptide Y receptors. CCKs act as physiological satiety factors by signaling to reduce meal size . CCKA cholecystokinin receptors (C6980) slow gastric emptying and transmit signals to the hypothalamus to deter feeding . Bombesin-like peptides are widely distributed in the central nervous system and gastrointestinal tract. Mice lacking the BB3 bombesin receptor develop obesity, hyperphagia and impaired glucose metabolism, indicating a role for this receptor in energy balance .
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