Differential Expression of Somatostatin and Dopamine Receptor Subtype Genes in Adrenocorticotropin (ACTH)-secreting Pituitary Tumors and Silent Corticotroph Adenomas

Somatostatin analogs and dopamine agonists are clinically used for medical therapy of functioning pituitary tumors, such as growth hormone- and prolactin-secreting tumors, however, their effects on ACTH-secreting tumors are controversial. This study was aimed to determine whether somatostatin receptor (SSTR) subtype (1-5) and dopamine receptor type 2 (D2R) are differentially expressed in pituitary tumors causing Cushing's disease (CD), silent corticotroph adenoma (SCA), and non-functioning pituitary tumor (NET). Tissue specimens were obtained from 35 pituitary tumors during transsphenoidal surgery. The steady-state mRNA levels of SSTR1-5 and D2R genes were determined by real-time reverse-transcription polymerase chain reaction. Both SSTR1 and 2 mRNA levels in SCA were greater than CD, while SSTR1 mRNA levels, but not SSTR2, in SCA were also greater than NFT. SSTR5 mRNA levels in CD were greater than SCA, but did not differ between NET and SCA. SSTR4 mRNA expression was undetectable. D2R mRNA levels were markedly lower in CD and SCA than in NFT. The present study suggests that somatostatin analogs more selective for SSTR5 and for SSTR1 and/or 2may have the therapeutic potential for medical treatment of CD and SCA, respectively, whereas clinical application of dopamine agonists selective for D2R is very limited in either CD or SCA.