PROCESSING OF HUMAN PROSOMATOSTATIN IN ATT-20 CELLS - S-28 AND S-14 ARE GENERATED IN DIFFERENT SECRETORY PATHWAYS

Somatostatin-14 (S-14) and somatostatin-28 (S-28) are generated by dif ferential processing of a single precursor at a dibasic (R-K) or monob asic (R) proteolytic cleavage site, respectively. To study the pathway s of processing of prosomatostatin, we have expressed in AtT20 cells c DNA encoding human prosomatostatin and prosomatostatin mutated in one or the other processing site. Analysis of the peptides present in cell extracts or culture media before and after stimulation of the cells w ith 8-BrcAMP indicated that prosomatostatin can enter three distinct s ecretory pathways where it is differently processed: 1) prosomatostati n was secreted through the constitutive pathway; 2) the regulated secr etory pathway generated S-14 which was released upon stimulation of th e cells with 8-BrcAMP; 3) an alternative pathway, insensitive to 8-Brc AMP produced S-28 and S-14. Moreover, our results suggest that the R-K processing site used to produce S-14 is an important structural featu re for targeting the precursor to the regulated secretory pathway. (C) 1994 Academic Press, Inc.