title: Impaired desensitization of a mutant adrenocorticotropin receptor associated with apparent constitutive activity.
abstract: A naturally occurring ACTH receptor [melanocortin 2 receptor (MC2R)] mutation (F278C) has been identified in a subject with ACTH-independent Cushing's syndrome. Functional characterization of this mutant receptor reveals that it is associated with elevated basal cAMP accumulation when compared with wild-type receptor-expressing cell lines. Dose responsiveness is similar between wild-type and mutant receptors in cell lines expressing similar numbers of binding sites. In view of the location of this mutation in the C-terminal tail of the MC2R, desensitization and internalization were investigated and found to be impaired. Inhibition of protein kinase A by H89 blocks wild-type MC2R desensitization and also results in increased basal activity, as does alanine substitution of Ser 280 in the C-terminal tail. Alanine substitution of Ser 208, the consensus protein kinase A phosphorylation target in the third cytoplasmic loop also results in a reduction in desensitization without significant change in basal activity or internalization. These findings suggest a novel mechanism is involved in the apparently constitutive activation of the MC2R in which failure of desensitization appears to be associated with enhanced basal receptor activity.
authors.affiliation: Department of Endocrinology, Barts & the London, Queen Mary School of Medicine, West Smithfield, London EC1A 7BE, United Kingdom.
authors.forename: Francesca M
authors.initials: FM
authors.lastname: Swords
authors.forename: Asma
authors.initials: A
authors.lastname: Baig
authors.forename: Diana M
authors.initials: DM
authors.lastname: Malchoff
authors.forename: Carl D
authors.initials: CD
authors.lastname: Malchoff
authors.forename: Michael O
authors.initials: MO
authors.lastname: Thorner
authors.forename: Peter J
authors.initials: PJ
authors.lastname: King
authors.forename: László
authors.initials: L
authors.lastname: Hunyady
authors.forename: Adrian J L
authors.initials: AJ
authors.lastname: Clark
mesh_terms: D000302:Adrenal Cortex; D000324:Adrenocorticotropic Hormone / Q000378:metabolism / Q000494:pharmacology; D000409:Alanine; D000818:Animals; D002460:Cell Line; D003480:Cushing Syndrome / Q000235:genetics; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D015870:Gene Expression; D006801:Humans; D007457:Iodine Radioisotopes; D007700:Kinetics; D051379:Mice; D009154:Mutation; D010766:Phosphorylation; D044103:Receptor, Melanocortin, Type 2; D018043:Receptors, Corticotropin / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism*; D012694:Serine; D015398:Signal Transduction; D013329:Structure-Activity Relationship; D014162:Transfection
chemical_list: D004791:Enzyme Inhibitors; D007457:Iodine Radioisotopes; D044103:Receptor, Melanocortin, Type 2; D018043:Receptors, Corticotropin; D012694:Serine; D000324:Adrenocorticotropic Hormone; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D000409:Alanine