Distinct functional properties of native somatostatin receptor subtype 5 compared with subtype 2 in the regulation of ACTH release by corticotroph tumor cells

In a series of human corticotroph adenomas, we recently found predominant mRNA expression of somatostatin (SS) receptor subtype 5 (sst(5)). 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 sst(5) 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 sst(5)-specific analog, on ACTH release. Quantitative PCR analysis showed that DEX lowered sst(2A+2B) mRNA expression significantly after 24 and 48 h, whereas sst(5) mRNA levels were not significantly affected by DEX treatment. Moreover, Scatchard analyses showed that DEX suppressed maximum binding capacity (B-max) by 72% when I-125-Tyr(3)-labeled OCT was used as radioligand, whereas B-max declined only by 17% when At T-20 cells were treated with [I-125-Tyr(11)]SS-14. These data suggest that the sst(5) protein, compared with sst(2), 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.