Diminished prohormone convertase 3 expression (PC1/PC3) inhibits progastrin post-translational processing

Gastrin is initially synthesized as a large precursor that requires endoproteolytic cleavage by a prohormone convertase (PC) for bioactivation. Gastric antral G-cells process progastrin at Arg(94)Arg(95) and Lys(74)Lys(75) residues generating gastrin heptadecapeptide (G17-NH(2)). Conversely, duodenal G-cells process progastrin to gastrin tetratriacontapeptide (G34-NH(2)) with little processing at Lys(74)Lys(75). Both tissues express PC1/PC3 and PC2. Previously, we demonstrated that heterologous expression of progastrin in an endocrine cell line that expresses PC1/PC3 and little PC2 (AtT-20) resulted in the formation of G34-NH(2). To confirm that PC1/PC3 was responsible for progastrin processing in AtT-20 cells and capable of processing progastrin in vivo we coexpressed either human wild-type (Lys(74)Lys(75)) or mutant (Arg(74)Arg(75), Lys(74)Arg(75), and Arg(74)Lys(75)) progastrins in AtT-20 cells with two different antisense PC1/PC3 constructs. Coexpression of either antisense construct resulted in a consistent decrease in G34-NH(2) formation. Gastrin mRNA expression and progastrin synthesis were equivalent in each cell line. Although mutation of the Lys(74)Lys(75) site within G34-NH(2) to Lys(74)Arg(75) resulted in the production of primarily G17-NH(2) rather than G34-NH(2), inhibition of PC1/PC3 did not significantly inhibit processing at the Lys(74)Arg(75) site. We conclude that PC1/PC3 is a progastrin processing enzyme, suggesting a role for PC1/PC3 progastrin processing in G-cells.