title: Role of calcium and EPAC in norepinephrine-induced ghrelin secretion.
abstract: Ghrelin is an orexigenic hormone secreted principally from a distinct population of gastric endocrine cells. Molecular mechanisms regulating ghrelin secretion are mostly unknown. Recently, norepinephrine (NE) was shown to enhance ghrelin release by binding to β1-adrenergic receptors on ghrelin cells. Here, we use an immortalized stomach-derived ghrelin cell line to further characterize the intracellular signaling pathways involved in NE-induced ghrelin secretion, with a focus on the roles of Ca(2+) and cAMP. Several voltage-gated Ca(2+) channel (VGCC) family members were found by quantitative PCR to be expressed by ghrelin cells. Nifedipine, a selective L-type VGCC blocker, suppressed both basal and NE-stimulated ghrelin secretion. NE induced elevation of cytosolic Ca(2+) levels both in the presence and absence of extracellular Ca(2+). Ca(2+)-sensing synaptotagmins Syt7 and Syt9 were also highly expressed in ghrelin cell lines, suggesting that they too help mediate ghrelin secretion. Raising cAMP with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine also stimulated ghrelin secretion, although such a cAMP-mediated effect likely does not involve protein kinase A, given the absence of a modulatory response to a highly selective protein kinase A inhibitor. However, pharmacological inhibition of another target of cAMP, exchange protein-activated by cAMP (EPAC), did attenuate both basal and NE-induced ghrelin secretion, whereas an EPAC agonist enhanced basal ghrelin secretion. We conclude that constitutive ghrelin secretion is primarily regulated by Ca(2+) influx through L-type VGCCs and that NE stimulates ghrelin secretion predominantly through release of intracellular Ca(2+). Furthermore, cAMP and its downstream activation of EPAC are required for the normal ghrelin secretory response to NE.
authors.affiliation: Division of Endocrinology and Metabolism (B.K.M., J.-C.C., I.S., A.K.W., A.K., S.O.-L., J.M.Z.), Department of Internal Medicine; Division of Hypothalamic research (B.K.M., J.-C.C., L.K., I.S., A.K.W., A.K., S.O.-L., J.J.R., J.M.Z.), Department of Internal Medicine; and Departments of Psychiatry (B.K.M., J.-C.C., I.S., A.K.W., A.K., S.O.-L., J.M.Z.) and Physiology (L.K., J.J.R.), University of Texas Southwestern Medical Center, Dallas, Texas 75390.
authors.forename: Bharath K
authors.initials: BK
authors.lastname: Mani
authors.forename: Jen-Chieh
authors.initials: JC
authors.lastname: Chuang
authors.forename: Lilja
authors.initials: L
authors.lastname: Kjalarsdottir
authors.forename: Ichiro
authors.initials: I
authors.lastname: Sakata
authors.forename: Angela K
authors.initials: AK
authors.lastname: Walker
authors.forename: Anna
authors.initials: A
authors.lastname: Kuperman
authors.forename: Sherri
authors.initials: S
authors.lastname: Osborne-Lawrence
authors.forename: Joyce J
authors.initials: JJ
authors.lastname: Repa
authors.forename: Jeffrey M
authors.initials: JM
authors.lastname: Zigman
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000737:chemistry; D000818:Animals; D019256:Cadmium Chloride / Q000737:chemistry; D002118:Calcium / Q000378:metabolism*; D002460:Cell Line; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004533:Egtazic Acid / Q000737:chemistry; D054439:Ghrelin / Q000378:metabolism*; D020662:Guanine Nucleotide Exchange Factors / Q000502:physiology*; D007150:Immunohistochemistry; D051379:Mice; D009543:Nifedipine / Q000737:chemistry; D009638:Norepinephrine / Q000378:metabolism*; D015398:Signal Transduction; D050857:Synaptotagmins / Q000378:metabolism
chemical_list: D000316:Adrenergic alpha-Agonists; C482601:Epac protein, mouse; D054439:Ghrelin; D020662:Guanine Nucleotide Exchange Factors; D050857:Synaptotagmins; D004533:Egtazic Acid; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D009543:Nifedipine; D019256:Cadmium Chloride; D002118:Calcium; D009638:Norepinephrine