Coupling of pancreatic gastrin/cholecystokinin-B (G/CCKB) receptors to phospholipase C and protein kinase C in AR4-2J tumoral cells.

Gastrin and cholecystokinin (CCK) have proven trophic effects on the gut. We have previously demonstrated that these peptides stimulate an early event in cellular proliferation, namely ornithine decarboxylase activity (ODC), in a rat exocrine pancreatic cell line AR4-2J. Furthermore, this effect is mediated through a G/CCKB receptor. Thus, in the present study we sought to examine the signal transduction mechanisms linked to the G/CCKB receptor occupancy. Both gastrin and CCK induced a rapid (maximum at 40 s) increase in inositol triphosphates (InsP3) and diacylglycerol (DAG) formation in a dose-dependent manner (EC50 = 5.6 nM) that quickly returned to baseline. Although InsP3 levels remained at baseline, DAG levels demonstrated a second gradual increase that was maximal at 15 min. CCK/gastrin efficiency to stimulate DAG and InsP3 formation (EC50 = 5.6 nM) could be correlated to the G/CCKB receptor occupancy, suggesting a coupling of this receptor to phospholipase C. To examine the involvement of protein kinase C (PKC) activation in the increase in ODC activity, we stimulated the AR4-2J cells with the phorbol ester TPA and observed an increase in ODC activity with a maximal effect at 100 nM. TPA stimulation of ODC activity was completely abolished by the PKC inhibitor staurosporine (50 nM). However, 50 nM staurosporine inhibited only 65% of the gastrin and CCK induced increase in ODC activity suggesting that a portion of the G/CCKB receptor-mediated increase in ODC activity is PKC independent.