title: Distinct functional properties of native somatostatin receptor subtype 5 compared with subtype 2 in the regulation of ACTH release by corticotroph tumor cells.
abstract: In a series of human corticotroph adenomas, we recently found predominant mRNA expression of somatostatin (SS) receptor subtype 5 (sst5). After 72 h, the multiligand SS analog SOM230, which has a very high sst5 binding affinity, but not Octreotide (OCT), significantly inhibited basal ACTH release. To further explore the role of sst5 in the regulation of ACTH release, we conducted additional studies with mouse AtT-20 cells. SOM230 showed a 7-fold higher ligand binding affinity and a 19-fold higher potency in stimulating guanosine 5'-O-(3-thiotriphosphate) binding in AtT-20 cell membranes compared with OCT. SOM230 potently suppressed CRH-induced ACTH release, which was not affected by 48-h dexamethasone (DEX) pretreatment. However, DEX attenuated the inhibitory effects of OCT on ACTH release, whereas it increased the inhibitory potency of BIM-23268, an sst5-specific analog, on ACTH release. Quantitative PCR analysis showed that DEX lowered sst(2A+2B) mRNA expression significantly after 24 and 48 h, whereas sst5 mRNA levels were not significantly affected by DEX treatment. Moreover, Scatchard analyses showed that DEX suppressed maximum binding capacity (B(max)) by 72% when 125I-Tyr3-labeled OCT was used as radioligand, whereas B(max) declined only by 17% when AtT-20 cells were treated with [125I-Tyr11]SS-14. These data suggest that the sst5 protein, compared with sst2, is more resistant to glucocorticoids. Finally, after SS analog preincubation, compared with OCT both SOM230 and BIM-23268 showed a significantly higher inhibitory effect on CRH-induced ACTH release. In conclusion, our data support the concept that the sst5 receptor might be a target for new therapeutic agents to treat Cushing's disease.
authors.affiliation: Dept. of Internal Medicine, section Endocrinology, Erasmus MC, 3015 GE Rotterdam, The Netherlands. j.vanderhoek@erasmusmc.nl
authors.forename: Joost
authors.initials: J
authors.lastname: van der Hoek
authors.forename: Marlijn
authors.initials: M
authors.lastname: Waaijers
authors.forename: Peter M
authors.initials: PM
authors.lastname: van Koetsveld
authors.forename: Diana
authors.initials: D
authors.lastname: Sprij-Mooij
authors.forename: Richard A
authors.initials: RA
authors.lastname: Feelders
authors.forename: Herbert A
authors.initials: HA
authors.lastname: Schmid
authors.forename: Philippe
authors.initials: P
authors.lastname: Schoeffter
authors.forename: Daniel
authors.initials: D
authors.lastname: Hoyer
authors.forename: Davide
authors.initials: D
authors.lastname: Cervia
authors.forename: John E
authors.initials: JE
authors.lastname: Taylor
authors.forename: Michael D
authors.initials: MD
authors.lastname: Culler
authors.forename: Steven W J
authors.initials: SW
authors.lastname: Lamberts
authors.forename: Leo J
authors.initials: LJ
authors.lastname: Hofland
mesh_terms: D000324:Adrenocorticotropic Hormone / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D003346:Corticotropin-Releasing Hormone / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D005938:Glucocorticoids / Q000502:physiology; D051379:Mice; D015282:Octreotide / Q000494:pharmacology; D010902:Pituitary Gland / Q000166:cytology / Q000378:metabolism*; D010911:Pituitary Neoplasms; D012333:RNA, Messenger / Q000032:analysis; D017481:Receptors, Somatostatin / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D013004:Somatostatin / Q000031:analogs & derivatives / Q000494:pharmacology; D013268:Stimulation, Chemical; D014407:Tumor Cells, Cultured
chemical_list: C121506:BIM 23268; D005938:Glucocorticoids; D012333:RNA, Messenger; D017481:Receptors, Somatostatin; D013004:Somatostatin; C094573:somatostatin receptor 5; D000324:Adrenocorticotropic Hormone; D003346:Corticotropin-Releasing Hormone; C517782:pasireotide; C093733:somatostatin receptor 2; D015282:Octreotide