Somatostatin is a tetradecapeptide that can modulate a host of important neuroendocrine functions. We have established a permanent rat C-cell line that maintains very high levels of somatostatin production. We have used this thyroid C-cell line to identify and characterize somatostatin's 92-amino-acid biosynthetic precursor, prosomatostatin. The structures of somatostatin and other peptides derived from prosomatostatin's carboxyl-terminal 28 residues are proven. Those putatively derived from the amino-terminal 64 residues of prosomatostatin are not proven. We propose to ascertain the structure of biosynthetic intermediates and secretory peptides generated as a consequence of prosomatostatin processing, particularly processing within the amino-terminal two thirds of the precursor and to identify factors that regulate the synthesis and cleavage/degradation of prosomatostatin and its derived intermediate and final products. To achieve these aims cellular and secretory peptides that cross-react with antibodies directed at specific sequences in the amino-terminal 64-residue portion of prosomatostatin will be purified from the cell cultures and their structures ascertained. Then pulse-chase studies will be conducted in the somatostatin-producing cell line to establish the origin of these peptides from prosomatostatin and derived biosynthetic intermediates. Some studies will be directed at identifying factors that regulate this biosynthetic process and comparing regulatory effects on the various prosomatostatin-derived secretory products characterized by our structural analyses. Such work is a critical step toward the long-term objectives of this research program which include: a) elucidation of the somatostatin precursor and its co- and post-translational processing in normal and neoplastic tissues; b) the determination of the factors that regulate the biosynthesis and secretion of prosomatostatin-derived peptides and elucidation of the mechanisms underlying that regulation; and c) understanding the biological importance of the multiple peptides that result from somatostatin gene expression.
