EXPRESSION OF 3 SOMATOSTATIN RECEPTOR SUBTYPES IN PITUITARY-ADENOMAS - EVIDENCE FOR PREFERENTIAL SSTR5 EXPRESSION IN THE MAMMOSOMATOTROPH LINEAGE

The expression of three somatostatin receptor subtypes, SSTR3, SSTR4, and SSTR5, was evaluated in 33 pituitary tumor specimens. SSTR3 expres sion was studied by reverse transcription coupled to polymerase chain reaction, whereas SSTR4 and SSTR5 expression was determined by ribonuc lease protection assay. SSTR3 was expressed in 6 of 7 GH-secreting tum ors, all 8 clinically nonfunctioning tumors, all 3 prolactinomas, and 1 of 2 ACTH-secreting tumors tested. Eight nonfunctioning adenomas had undetectable messenger ribonucleic acid levels of SSTR4, and only 1 o f them expressed SSTR5. SSTR4 expression was also undetectable in 11 G H-secreting tumors, 3 prolactinomas, and 1 ACTH-secreting tumor tested . In contrast, SSTR5 was highly expressed in 10 of 11 GH-secreting ade nomas and 1 prolactinoma. Two prolactinomas and 1 ACTH-secreting tumor had low levels of expression of SSTR5. The widespread pituitary adeno ma expression of SSTR3, regardless of hormonal secretory type, suggest s that SSTR3 might be involved in a somatostatin action(s) other than GH or TSH regulation. SSTR5 is expressed predominantly in mammosomatot roph-derived tumors, suggesting that this receptor subtype may be an i mportant determinant of GH secretion in acromegaly.