PRESENCE AND CHARACTERIZATION OF THE SOMATOSTATIN PRECURSOR IN NORMAL HUMAN PITUITARIES AND IN GROWTH-HORMONE SECRETING ADENOMAS

Normal and tumoral human pituitaries release in vitro SRIH and contain messenger RNAs encoding preproSRIH. In the present study, we document the presence and characterization of the SRIH precursor in both human normal pituitaries and in GH-secreting adenomas. Molecular sieve filtration of normal pituitary and adenoma acid extracts revealed the presence of three immunoreactive SRIH peaks, distinct from SRIH 1-28 and SRIH 1-14. Western blot analysis performed under both nonreducing and reducing conditions confirmed the presence of the three different immunoreactive forms with respective molecular masses estimated as 21, 17, and 12 kilodaltons. When analyzed in reverse phase high performance liquid chromatography, the three immunoreactive SRIH material contained in the three peaks coeluted with the proSRIH extracted from rat hypothalamus and from a neuroblastoma cell line (N2A) transfected with the human prepro-SRIH complementary DNA. None of these three forms could bind concanavalin A. Digestion of the three forms with a specific endopeptidase resulted in the generation of SRIH 1-14, indicating that they can be considered as SRIH precursor or related forms. Estimation of proSRIH amount after molecular sieve filtration indicated that normal human pituitaries (n = 4) contained proSRIH in variable amounts (from 26.5-303 pg/mg proteins), the 21-, 17-, and 12-kilodalton forms being always present, in addition to SRIH 1-28. In contrast, among the 21 adenomas tested, only 11 contained proSRIH material (from 9-4480 pg/mg proteins). Moreover, neither SRIH 1-28 nor SRIH 1-14 could be detected in these adenomas. These data demonstrate the presence of high molecular mass SRIH in normal pituitaries which represent unprocessed proSRIH material. This provides an additional argument for a local synthesis of SRIH. Additionally, the fact that only 50% of GH-secreting pituitary adenomas here studied contain exclusively proSRIH may reflect an alteration in messenger RNA translation or in proSRIH processing, or both, resulting in the absence of mature SRIH in these adenomas.