Abstract
In the present study, Western-blot and radioreceptor analyses have revealed the presence of the epidermal growth factor (EGF) receptor in pancreatic acinar membranes. Isolated pancreatic acinar membranes, which allow access of functional antibodies to individual components of the signal transduction cascade, were used to examine EGF-induced regulation of adenylate cyclase activity. Forskolin, vasoactive intestinal peptide (VIP) and to a smaller extent EGF increased cAMP production in pancreatic acinar membranes. Preincubation of the membranes with anti-GS alpha antibody abolished EGF- and VIP-induced cAMP production, but had no effect on forskolin-induced cAMP accumulation. In the presence of either VIP or forskolin, EGF inhibited the VIP- and forskolin-induced cAMP production with an IC50 of 5 nM. Anti-G alpha i1-2 protein antibody, but not anti-G alpha i3 antibody, increased basal cAMP production, indicating that Gi proteins exert an inhibitory influence on basal adenylate cyclase activity. Anti-G alpha i1-2 antibody, but not anti-G alpha i3 antibody, abolished the inhibitory effect of EGF on the forskolin- and VIP-induced cAMP accumulation. A peptide corresponding to the juxtamembrane region in the cytosolic domain of the rat EGF receptor increased cAMP production in pancreatic acinar membranes in an anti-G alpha s antibody-sensitive fashion, whereas the EGF receptor peptide did not mimic the inhibitory effect of the native EGF receptor. The tyrosine kinase inhibitors genistein and pp60v-src (137-157) inhibited both the stimulatory and the inhibitory effects of EGF on cAMP production. Thus the data of the present study show that EGF regulates adenylate cyclase via activation of Gs and Gi proteins by a tyrosine phosphorylation-dependent mechanism in pancreatic acinar membranes. This leads to stimulation of basal and inhibition of forskolin- and VIP-induced adenylate cyclase activity respectively.
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