THE GASTRIN-RELEASING PEPTIDE RECEPTOR IS DIFFERENTIALLY COUPLED TO ADENYLATE-CYCLASE AND PHOSPHOLIPASE-C IN DIFFERENT TISSUES

Recent studies suggest that in some tissues GRP receptor activation ca n both stimulate phospholipase C and the adenylate cyclase pathway and that activation of the latter pathway may be important in mediating s ome of its well-described growth effects. However, other studies sugge st GRP-R may not be coupled to adenylate cyclase. To investigate this possibility, in the present study we determined the coupling of the GR P receptors to each pathway in mouse, rat, and guinea pig pancreatic a cini and compared it to that in mouse Swiss 3T3 cells and human SCLC c ells, all of which possess well-characterized GRP receptors. Moreover, we tested the effect of PKC activation on the ability of GRP-related peptides to increase cAMP accumulation in these tissues. Changes in cA MP levels were determined with or without IBMX present, with or withou t forskolin, or both to amplify small increases in cAMP. In mouse, rat and guinea pig pancreatic acini, murine Swiss 3T3 cells and human SCL C cells, GRP-related peptides caused a 600%, 500%, 250%, 300% and 60% increase, respectively, in [H-3]IP with 1-3 nM causing a half-maximal effect. In murine Swiss 3T3 cells, IBMX, forskolin, and IBMX plus fors kolin caused a 300%, 3500% and 10500% increase in cAMP, respectively. GRP-related peptides and VIP caused an additional 70% increase in cAMP with GRP causing a half-maximal (EC50) increase in cAMP at 2.1 +/- 0. 5 nM, which was not significantly different from the EC50 of 3.1 +/- 0 .9 nM for increasing [H-3]IP in these cells. GRP-related peptides did not stimulate increases in cAMP in mouse, rat or guinea pig pancreatic acini or in SCLC cells either alone, with IBMX or forskolin or both. However, in pancreatic acini IBMX, forskolin or both increased cAMP 3 to 8-, IO to 500-, and 100 to 1000-fold increase and the addition of V IP caused an additional 20-, 2- and 3-fold increase in cAMP in the dif ferent species. In mouse pancreatic acini with TPA alone or IBMX plus TPA, neither bombesin nor GRP increased cAMP. Furthermore, in mouse pa ncreatic acini, neither TPA nor TPA plus IBMX altered basal or VIP-sti mulated increases in cAMP. In mouse Swiss 3T3 cells TPA significantly increased cAMP stimulated by Bn, GRP or VIP. These results demonstrate d that GRP receptor activation in normal tissues from three different species and a human tumoral cell line do not result in adenylate cycla se activation, whereas in Swiss 3T3 cells it causes such activation. T he results suggest that the difference in coupling to adenylate cyclas e is likely at least partially due to a difference in coupling to an a denylate cyclase subtype whose activation is regulated by PKC. Therefo re, the possible growth effects mediated by this receptor in different embryonic or tumoral cells through activation of adenylate cyclase ar e not likely to he an important intracellular pathway for these effect s in normal tissues. (C) 1997 Elsevier Science B.V.