Inhibition of PC5 expression decreases CCK secretion and increases PC2 expression

Cholecystokinin (CCK) is produced from pro CCK by a series of enzymatic cleavages. One of the enzymes thought to be important for pro CCK cleavage is prohormone convertase 5 (PC5). STC-1 cells, a mouse intestinal tumor cell line that expresses CCK, PC1, PC2, and PC5 were stably transfected with hairpin loop plasmids encoding siRNA targeting PC5 and clones were selected. CCK secretion was reduced significantly. PC5 mRNA and protein expression as measured by quantitative PCR and Western blot analysis was reduced about 50%. CCK and PC1 mRNA expression were not changed. These cells showed a three-fold increase in PC2 mRNA and protein expression. This increase may represent a compensatory mechanism triggered by the loss of PC5. The decrease in CCK in the media was due largely to loss of CCK 22. These results provide the first direct evidence that PC5 is involved in CCK processing.