BOMBESIN STIMULATES CHOLECYSTOKININ SECRETION THROUGH MITOGEN-ACTIVATED PROTEIN-KINASE-DEPENDENT AND PROTEIN-KINASE-INDEPENDENT MECHANISMS IN THE ENTEROENDOCRINE STC-1-CELL-LINE

Bombesin has been reported to stimulate cholecystokinin (CCK) secretio n from rat duodeno-jejunal I-cells. Bombesin was shown to activate mit ogen-activated protein kinases (MAPKs) in cell types such as Swiss 3T3 fibroblasts and rat pancreatic acinar cells. No information is availa ble on whether MAPK is activated in intestinal endocrine cells upon bo mbesin stimulation. This was studied by using the CCK-producing entero endocrine cell line STC-1. Bombesin stimulated markedly and transientl y both p42(MAPK) and p44(MAPK), With a maximum at 2 min, and a decreas e to basal levels within 10 min. As expected, bombesin stimulated MAPK kinase : (MEK-1) activity. Activation of protein kinase C (PKC) with PMA also stimulated p42(MAPK), p44(MAPK) and MEK-1. Treatment of cells with PD 098059 (at 10 mu M or 30 mu M), which selectively inhibits ME K phosphorylation, blocked bombesin-induced p42(MAPK) and p44(MAPK) ac tivation for at least 90 min. However, PD 098059 inhibited bombesin-an d PMA-stimulated CCK secretion during the first 15 min, but failed to significantly reduce CCK release at later times. Inhibition of PKC wit h staurosporine, or PKC down-regulation by prolonged treatment with PM A, both drastically decreased MEK-1, p42(MAPK) and p44(MAPK) activatio n upon bombesin stimulation. Additionally, PKC activation appeared to be required for both MAPK-dependent (early) and -independent (late) CC K responses to bombesin. It is concluded that the early CCK secretory response of STC-1 cells to bombesin involves MAPK pathway activation t hrough a PKC-dependent mechanism, whereas the late phase of bombesin-i nduced CCK secretion, that also requires PKC, appears to result from a MAPK-independent process.