Molecular mechanisms of corticotropin-releasing factor stimulation of calcium mobilization and adrenocorticotropin release from anterior pituitary tumor cells.

In a tumor cell line of the mouse anterior pituitary (AtT-20/D16-16) consisting of a homogeneous population of corticotrophs, corticotropin-releasing factor (CRF) activates adenylate cyclase and cAMP-dependent protein kinase. In addition, CRF induces a rise in cytosolic calcium levels in AtT-20/D16-16 cells and stimulates adrenocorticotropin hormone release. To determine whether activation of cAMP-dependent protein kinase is essential for CRF to stimulate calcium mobilization and trigger adrenocorticotropin hormone release, an inhibitor of cAMP-dependent protein kinase was inserted into AtT-20/D16-16 cells using a liposome technique. In control cells, CRF, forskolin (a direct activator of adenylate cyclase) and potassium increased cytosolic calcium levels. Insertion of the protein kinase inhibitor into AtT-20/D16-16 cells greatly attenuated CRF and forskolin-stimulated calcium mobilization although it did not alter the rise in cytosolic calcium induced by potassium. Treatment of the cells with liposomes lacking protein kinase inhibitor (but containing an equivalent amount of bovine serum albumin) had no effect upon the calcium mobilization elicited by any of the agents tested. These results reveal an essential role for cAMP-dependent protein kinase in mediating CRF-stimulated calcium mobilization and suggest that its activation may be an essential molecular event for CRF to evoke adrenocorticotropin hormone secretion.