Agonist-specific coupling of growth hormone secretagogue receptor type 1a to different intracellular signaling systems. Role of adenosine.
The growth hormone secretagogue receptor subtype 1a (GHSR-1a) is involved in biological actions of ghrelin by triggering intracellular second messengers coupled to heterotrimeric G-protein complex involving Galpha(q/11). Adenosine is a partial agonist of the GHSR-1a, binding to a binding pocket distinct from the one described for ghrelin. This suggests a variety of functions for the poorly understood GHSR1a receptor. In this work, a sequential analysis of the pathways involved in the regulation of GHSR-1a signaling was undertaken to characterize the intracellular calcium mobilization that is observed following adenosine binding. The results showed that adenosine induced, in a dose-dependent manner, a calcium mobilization from IP(3)-sensitive intracellular stores since the IP(3) receptor blocker 2-APB was able to suppress the calcium response. However, adenosine did not show any effect in the formation of inositol phosphates. The calcium-mobilizing activity was blocked after preincubation of cells with CTX, the inhibitor of adenylate cyclase MDL-12,330A and the protein kinase A blocker H-89. Furthermore, the administration of adenosine stimulated cAMP production. Based on the experimental data, a signaling pathway is proposed involving adenylate cyclase and protein kinase A, which causes phosphorylation of the IP(3) receptor, with a cross-talk between the signaling pathways activated by ghrelin and adenosine. The data described in this report suggest that GHSR-1a is able to activate different intracellular second-messenger systems depending on the agonist that activates it. The regulation of the ghrelin-activated earliest signaling pathways by adenosine may have unexpected implications in the GHSR-1a actions.
Carreira
Marcos C
MC
Department of Medicine, Research Area, Molecular Endocrinology Laboratory, Complejo Hospitalario Universitario de Santiago (CHUS) and University of Santiago de Compostela, Santiago de Compostela, Spain.
Camiña
Jesus P
JP
Smith
Roy G
RG
Casanueva
Felipe F
FF
D000241:Adenosine / Q000494:pharmacology*; D000276:Adjuvants, Immunologic / Q000494:pharmacology; D000704:Analysis of Variance; D001896:Boron Compounds / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002460:Cell Line; D002772:Cholera Toxin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004622:Embryo, Mammalian; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D054439:Ghrelin; D006801:Humans; D007295:Inositol Phosphates / Q000378:metabolism; D007456:Iodine Isotopes / Q000378:metabolism; D007668:Kidney / Q000166:cytology; D019216:Metals, Heavy / Q000494:pharmacology; D036361:Peptide Hormones / Q000378:metabolism / Q000494:pharmacology; D037342:Pertussis Toxin / Q000494:pharmacology; D011869:Radioligand Assay / Q000379:methods; D043562:Receptors, G-Protein-Coupled / Q000819:agonists* / Q000378:metabolism; D054440:Receptors, Ghrelin; D015398:Signal Transduction / Q000187:drug effects* / Q000502:physiology; D019284:Thapsigargin / Q000494:pharmacology; D014162:Transfection; D010738:Type C Phospholipases / Q000494:pharmacology; D014665:Vasodilator Agents / Q000494:pharmacology
D000276:Adjuvants, Immunologic; D001896:Boron Compounds; D004791:Enzyme Inhibitors; D054439:Ghrelin; D007295:Inositol Phosphates; D007456:Iodine Isotopes; D019216:Metals, Heavy; D036361:Peptide Hormones; D043562:Receptors, G-Protein-Coupled; D054440:Receptors, Ghrelin; D014665:Vasodilator Agents; D019284:Thapsigargin; D002772:Cholera Toxin; D000242:Cyclic AMP; C109986:2-aminoethoxydiphenyl borate; D037342:Pertussis Toxin; D010738:Type C Phospholipases; D000241:Adenosine; D002118:Calcium