Modulation the cleavage of the gastrin precursor by prohormone phosphorylation

Background & Aims: Amidated gastrins are acid secretagogues and growth factors. Their precursor, progastrin, is a growth factor but not a secretagogue. Cleavage of progastrin at Arg94/95 determines the expression of these two alternative patterns of biological activity. We examined the hypothesis that cleavage at Arg94/95 is regulated by phosphorylation of the adjacent Ser96 residue. Methods: Hamster insulinoma cells were stably transfected with wild-type rat preprogastrin and phosphorylation site mutants; biosynthesis was studied by a pulse-chase protocol. Results: Rates of cleavage at Arg94/95 were increased in Ser96-->Ala compared with wild-type progastrin. Mutation of Glu98 to Ala inhibited incorporation of [P-32]phosphate into progastrin and increased the rate of cleavage at Arg94/95. Conversely, mutation of Ser96 to Asp reduced rates of cleavage at Arg94/95. Depletion of calcium stores decreased phosphorylation of Ser96 and increased cleavage at Arg94/95. Modulation of Ser96 phosphorylation also directly influenced the ratio of progastrin-cleavage products (progastrin/CFP; G17Gly/ G34Gly) secreted into the medium. Conclusions: Phosphorylation of progastrin is dependent on calcium stores, determines prohormone cleavage rates, and thereby controls the production of the alternative active products of preprogastrin translation.