PRESENCE AND CHARACTERIZATION OF THE SOMATOSTATIN PRECURSOR IN NORMALHUMAN 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 filt ration of normal pituitary and adenoma acid extracts revealed the pres ence of three immunoreactive SRIH peaks, distinct from SRIH 1-28 and S RIH 1-14. Western blot analysis performed under both nonreducing and r educing conditions confirmed the presence of the three different immun oreactive forms with respective molecular masses estimated as 21, 17, and 12 kilodaltons. When analyzed in reverse phase high performance li quid chromatography, the three immunoreactive SRIH material contained in the three peaks coeluted with the proSRIH extracted from rat hypoth alamus and from a neuroblastoma cell line (N2A) transfected with the h uman prepro-SRIH complementary DNA. None of these three forms could bi nd concanavalin A. Digestion of the three forms with a specific endope ptidase resulted in the generation of SRIH 1-14, indicating that they can be considered as SRIH precursor or related forms. Estimation of pr oSRIH amount after molecular sieve filtration indicated that normal hu man pituitaries (n = 4) contained proSRIH in variable amounts (from 26 .5-303 pg/mg proteins), the 21-, 17-, and 12-kilodalton forms being al ways present, in addition to SRIH 1-28. In contrast, among the 21 aden omas tested, only 11 contained proSRIH material (from 9-4480 pg/mg pro teins). 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 m aterial. 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 alter ation in messenger RNA translation or in proSRIH processing, or both, resulting in the absence of mature SRIH in these adenomas.