CANINE PROSOMATOSTATIN - ISOLATION OF A CDNA, REGULATION OF GENE-EXPRESSION, AND CHARACTERIZATION OF POSTTRANSLATIONAL PROCESSING INTERMEDIATES

Somatostatin is a tetradecapeptide (SS-14) initially isolated from the hypothalamus that is also found in D cells of the stomach and pancrea s where it exerts an inhibitory action on a variety of gastrointestina l functions. Since many of concepts important to an understanding of g astrointestinal physiology are derived from experiments in the dog we examined somatostatin gene expression and post-translational processin g in the canine fundus, antrum and pancreas. The canine somatostatin c DNA which is highly homologous to other known mammalian somatostatins was used to examine somatostatin expression in isolated canine fundic D-cells. Somatostatin expression induced by cholecystokinin (10(-8) M) was inhibited by the somatostatin analog, octreotide (10(-7) M). To e xamine somatostatin processing in the canine gut we noted that synthes is of SS-14 and somatostatin octacosapeptide (SS-28) involves endoprot eolytic cleavage of prosomatostatin (proSS) at both paired and single basic amino-acid residues, respectively. Antisera capable of recognizi ng the amino-terminal residues of SS-28, SS-28((1-14)) and SS-28((1-12 )) were characterized and identified concentrations of SS-28((1-12)) b ut not SS-28((1-14)) in the fundus, antrum and pancreas equivalent to those of SS-14. Since previous biosynthetic studies in canine fundic D -cells showed that SS-14 was synthesized without the appearance of a S S-28 intermediate, we hypothesize that proSS is sequentially cleaved a t a dibasic site to produce SS-14 followed by monobasic cleavage that results in the formation of SS-28((1-12)). Furthermore, equivalent amo unts of SS-14 and SS-28((1-12)) were co-released from canine fundic D- cells by CCK (10(-8) M) suggesting that the generation of these produc ts occurs within the same regulated pathway of secretion.