title: Phosphodiesterase inhibitor-dependent inverse agonism of agouti-related protein on melanocortin 4 receptor in sea bass (Dicentrarchus labrax).
abstract: The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor mainly expressed in the central nervous system of vertebrates. Activation of the MC4R leads to a decrease in food intake, whereas inactivating mutations are a genetic cause of obesity. The binding of agouti-related protein (AGRP) reduces not only agonist-stimulated cAMP production (competitive antagonist) but also the basal activity of the receptor, as an inverse agonist. Transgenic zebrafish overexpressing AGRP display increased food intake and linear growth, indicative of a physiological role for the melanocortin system in the control of the energy balance in fish. We report on the cloning, pharmacological characterization, tissue distribution, and detailed brain mapping of a sea bass (Dicentrarchus labrax) MC4R ortholog. Sea bass MC4R is profusely expressed within food intake-controlling pathways of the fish brain. However, the activity of the melanocortin system during progressive fasting does not depend on the hypothalamic/pituitary proopiomelanocortin (POMC) and MC4R expression, which suggests that sea bass MC4R is constitutively activated and regulated by AGRP binding. We demonstrate that AGRP acts as competitive antagonist and reduces MTII-induced cAMP production. AGRP also decreases the basal activity of the receptor as an inverse agonist. This observation suggests that MC4R is constitutively active and supports the evolutionary conservation of the AGRP/MC4R interactions. The inverse agonism, but not the competitive antagonism, depends on the presence of a phosphodiesterase inhibitor (IBMX). This suggests that inverse agonism and competitive antagonism operate through different intracellular signaling pathways, a view that opens up new targets for the treatment of melanocortin-induced metabolic syndrome.
authors.affiliation: Dept. of Fish Reproductive Physiology, Instituto de Acuicultura de Torre de la Sal, Ribera de Cabanes, Castellón, Spain.
authors.forename: Elisa
authors.initials: E
authors.lastname: Sánchez
authors.forename: Vera Cruz
authors.initials: VC
authors.lastname: Rubio
authors.forename: Darren
authors.initials: D
authors.lastname: Thompson
authors.forename: Juriaan
authors.initials: J
authors.lastname: Metz
authors.forename: Gert
authors.initials: G
authors.lastname: Flik
authors.forename: Glenn L
authors.initials: GL
authors.lastname: Millhauser
authors.forename: José Miguel
authors.initials: JM
authors.lastname: Cerdá-Reverter
mesh_terms: D054366:Agouti Signaling Protein / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001492:Bass / Q000378:metabolism*; D000242:Cyclic AMP / Q000378:metabolism; D004435:Eating / Q000502:physiology; D005215:Fasting / Q000502:physiology; D005260:Female; D008297:Male; D009074:Melanocyte-Stimulating Hormones / Q000378:metabolism; D023421:Models, Animal; D008969:Molecular Sequence Data; D010726:Phosphodiesterase Inhibitors / Q000378:metabolism*; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism*; D011333:Pro-Opiomelanocortin / Q000378:metabolism; D044105:Receptor, Melanocortin, Type 4 / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology
chemical_list: D054366:Agouti Signaling Protein; C515184:Asip protein, rat; D010726:Phosphodiesterase Inhibitors; D044105:Receptor, Melanocortin, Type 4; D011333:Pro-Opiomelanocortin; D009074:Melanocyte-Stimulating Hormones; D000242:Cyclic AMP; D010727:Phosphoric Diester Hydrolases