title: Corticotropin-releasing factor-induced adrenocorticotropin hormone release and synthesis is blocked by incorporation of the inhibitor of cyclic AMP-dependent protein kinase into anterior pituitary tumor cells by liposomes.
abstract: Corticotropin-releasing factor (CRF) is the most potent and effective natural stimulant of corticotropin (ACTH) secretion. In a tumor cell line of the mouse anterior pituitary (AtT-20/D16-16) consisting of a homogeneous population of corticotrophs, CRF is known to increase adenylate cyclase and cAMP-dependent protein kinase activities as well as to release ACTH. To determine whether activation of cAMP-dependent protein kinase is essential for CRF to evoke the secretion of ACTH, an inhibitor (PKI) of this kinase was inserted into AtT-20 cells. This was accomplished by first encapsulating PKI into liposomes and then covalently coupling them to protein A for binding to antibodies directed against an AtT-20 cell surface antigen, N-CAM (neural cell adhesion molecule). The binding of the liposomes to the anti-N-CAM antibodies led to the internalization of the PKI into the tumor cells. The PKI treatment greatly attenuated CRF-stimulated ACTH release as well as the secretory response to beta-adrenergic agonists. However, ACTH release in response to caerulein, an agonist of cholecystokinin 8 receptors, was not altered by the PKI treatment. CRF treatment also increased the levels of mRNA for proopiomelanocortin (POMC), the precursor for ACTH in AtT-20 cells. Application of liposomes containing PKI to AtT-20 cells blocked the ability of CRF and 8-bromo-cAMP, but not phorbol ester, to increase POMC mRNA levels. The results revealed an essential role for cAMP in mediating the effect of CRF on ACTH release and POMC gene expression.
mesh_terms: D000324:Adrenocorticotropic Hormone / Q000378:metabolism*; D000818:Animals; D002108:Ceruletide / Q000494:pharmacology; D005576:Colforsin / Q000037:antagonists & inhibitors; D003346:Corticotropin-Releasing Hormone / Q000037:antagonists & inhibitors*; D000242:Cyclic AMP / Q000502:physiology*; D007545:Isoproterenol / Q000037:antagonists & inhibitors; D008081:Liposomes; D051379:Mice; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D011333:Pro-Opiomelanocortin / Q000096:biosynthesis / Q000235:genetics; D047428:Protein Kinase Inhibitors; D011494:Protein Kinases / Q000008:administration & dosage; D012333:RNA, Messenger / Q000378:metabolism; D012636:Secretory Rate / Q000187:drug effects
chemical_list: D008081:Liposomes; D047428:Protein Kinase Inhibitors; D012333:RNA, Messenger; D005576:Colforsin; D011333:Pro-Opiomelanocortin; D002108:Ceruletide; D000324:Adrenocorticotropic Hormone; D003346:Corticotropin-Releasing Hormone; D000242:Cyclic AMP; D011494:Protein Kinases; D007545:Isoproterenol