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 sst₅binding affinity, but not Octreotide (OCT), significantly inhibited basal ACTH release. To further explore the role of sst₅in 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+2BmRNA expression significantly after 24 and 48 h, whereas sst₅mRNA levels were not significantly affected by DEX treatment. Moreover, Scatchard analyses showed that DEX suppressed maximum binding capacity (B_max) by 72% when¹²⁵I-Tyr³-labeled OCT was used as radioligand, whereas B_maxdeclined only by 17% when AtT-20 cells were treated with [¹²⁵I-Tyr¹¹]SS-14. These data suggest that the sst₅protein, 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 sst₅receptor might be a target for new therapeutic agents to treat Cushing’s disease.