Hormonal activation of the cAMP-dependent protein kinases in AtT20 cells. Preferential activation of protein kinase I by corticotropin releasing factor, isoproterenol, and forskolin

The hormonal regulation of adenylate cyclase, cAMP-dependent protein kinase activation, and adrenocorticotropic hormone (ACTH) secretion was studied in AtT20 mouse pituitary tumor cells. Corticotropin releasing factor (CRF) stimulated cAMP accumulation and ACTH release in these cells. Maximal ACTH release was seen with 30 nM CRF and was accompanied by a 2-fold rise in intracellular cAMP. When cells were incubated with both 30 nM CRF and 0.5 mM 3-methylisobutylxanthine (MIX) cAMP levels were increased 20-fold, however, ACTH release was not substantially increased beyond release seen with CRF alone. The activation profiles of cAMP-dependent protein kinases I and II were studied by measuring residual cAMP-dependent phosphotransferase activity associated with immunoprecipitated regulatory subunits of the kinases. Cells incubated with CRF in the absence of MIX showed concentration-dependent activation of protein kinase I which paralleled stimulation of ACTH release. Protein kinase II was minimally activated. When cells were exposed to CRF in the presence of 0.5 mM MIX there was still a preferential activation of protein kinase I, although 50% of the cytosolic protein kinase II was activated. Complete activation of both protein kinases I and II was seen when cells were incubated with 0.5 mM MIX and 10 microM forskolin. Under these conditions cAMP levels were elevated 80-fold. CRF, isoproterenol, and forskolin stimulated adenylate cyclase activity in isolated membranes prepared from AtT20 cells. CRF and isoproterenol stimulated cyclase activity up to 5-fold while forskolin stimulated cyclase activity up to 15-fold. Our data demonstrate that ACTH secretion from AtT20 cells is mediated by small changes in intracellular levels of cAMP and activation of only a small fraction of the total cytosolic cAMP-dependent protein kinase in these cells is required for maximal ACTH secretion.