EGF inhibits secretagogue-induced cAMP production and amylase secretion by Gi proteins in pancreatic acini

In pancreatic acinar cells, the epidermal growth factor (EGF) receptor interacts with both cholera toxin- and pertussis toxin (PTX)-sensitive G proteins. In the present study, isolated rat pancreatic acini were used to investigate the effect of EGF on basal and secretagogue-induced adenosine 3',5'-cyclic monophosphate (cAMP) production and amylase release. EGF increased cAMP production and amylase release in pancreatic acini. However, cAMP accumulation and amylase release elicited by either vasoactive intestinal peptide (VIP) or forskolin were inhibited by EGF (17 nM). EGF inhibited the VIP-induced cAMP production and amylase release with a half-maximal effective concentration of 3 and 2 nM, respectively. EGF had no effect on the N6,2'-O-dibutyryladenosine-3',5'-monophosphate-stimulated amylase release, suggesting that the inhibitory effect of EGF on the VIP- and forskolin-induced cAMP production is due to inhibition of adenylyl cyclase. PTX pretreatment of the acini led to an increase of the basal, EGF-, and VIP-stimulated cAMP accumulation and amylase release, indicating that PTX-sensitive G proteins exert tonic inhibition of adenylyl cyclase even in the absence of agonist. In PTX-pretreated acini, the inhibitory effect of EGF on the VIP-induced cAMP production and amylase release was abolished. In conclusion, these results suggest that EGF inhibits secretagogue-induced cAMP production via activation of PTX-sensitive G proteins in rat pancreatic acini, whereas EGF-induced cAMP production and amylase release occurs via a PTX-insensitive pathway. </jats:p

Tyrphostins inhibit secretagogue-induced 1,4,5-IP3 production and amylase release in pancreatic acini

We examined the role of protein tyrosine kinase inhibitors (tyrphostins) in secretagogue-induced inositol 1,4,5-trisphosphate (1,4,5-IP3) production and amylase secretion in rat pancreatic acinar cells. The data show that various specific cell-permeant tyrphostins (methyl 2,5-dihydroxycinnamate, tyrphostin 25, and genistein) inhibited the cholecystokinin octapeptide-, carbachol-, and bombesin-induced 1,4,5-IP3 production and amylase release. In digitonin-permeabilized cells, tyrphostins decreased 1,4,5-IP3 accumulation and amylase release generated by directly stimulating G proteins with the weakly hydrolyzable GTP analogue guanosine 5'-O-(3-thiotriphosphate). Tyrphostins had no effect on vasoactive intestinal peptide-induced amylase secretion. In isolated pancreatic acinar membranes, cholecystokinin octapeptide caused a rapid increase in tyrosine phosphorylation of a synthetic peptide containing the 12-amino acid sequence around a tyrosine phosphorylation site in pp6osrc. These results provide evidence that tyrosine kinases are involved in the activation of phospholipase C by G protein-coupled receptors in pancreatic acinar cells. </jats:p