Elucidation of roles of serine/threonine phosphatases PP1 and PP2A in mediating CCK-stimulated growth and enzyme secretion in pancreatic acinar cells.

Serine/threonine phosphatases, protein phosphatases 1 and 2A (PP1 and PP2A), play important roles in mediating cellular signaling in different tissues to different stimuli, including glycogen metabolism, protein synthesis/growth, and secretion. However, the roles of PP1/PP2A in pancreatic acinar cell secretion/growth are both unclear and controversial. To address this issue, in the present study, we examined the ability of gastrointestinal hormones/growth factors (GFs) to activate PP1 and PP2A and the signaling cascades involved in rat pancreatic acini and the pancreatic acinar tumor cell line, AR42J cells. PP1 and PP2A were both detected in pancreatic acini and AR42J cells. In acini, PP1 and PP2A were activated by pancreatic secretagogues-stimulating phospholipase C (bombesin, CCK-8, and carbachol) and endothelin and by pancreatic GFs (insulin, hepatocyte growth factor, epidermal growth factor, basic fibroblast growth factor, platelet-derived growth factor, and insulin-like growth factor 1). Full CCK-8 activation of PP1/PP2A required activation of both high- and low-affinity CCK1-receptor states. Using specific PP1 and PP2 assays, in both acini and AR42J cells, experimental conditions were established, where calyculin A, a known nonselective PP1/PP2A inhibitor, inhibited activation of both, whereas okadaic acid and fostriecin inhibited only PP2A activation and tautomycetin inhibited only PP1 activation. Under these conditions, CCK-stimulated enzyme secretion and stimulation of p44/42, a key mediator of growth, required PP2A activation, without activation of PP1. Using specific siRNA for PP1/PP2A in AR42J cells, similar results were found. These results establish that only PP2A activation is essential for CCK-mediated stimulation of growth and enzyme secretion in pancreatic acinar cells and pancreatic acinar AR42J tumor cells.