Inhibition of protein phosphatases by okadaic acid and calyculin-A differentially modulates hormonal- and forskolin-stimulated formation of cyclic AMP in AtT-20 corticotrophs: effect of pituitary adenylate activating polypeptide and corticotropin-releasing factor.
The effect of phosphatase inhibitors okadaic acid and calyculin-A on cAMP formation and adrenocorticotropic hormone (ACTH) secretion in AtT-20 corticotrophs was investigated. Both okadaic acid and calyculin-A inhibited dose-dependently the accumulation of cAMP in cells stimulated with pituitary adenylate cyclase activating factor (PACAP) and corticotropin-relating hormone (CRF). While in the case of okadaic acid the half-maximum inhibiting concentration was similar for both peptides (IC50 = 4 x 10(-7) M), it appeared that calyculin-A was about one order of magnitude more efficient in inhibiting the effect of PACAP than that of CRF (IC50 = 3.8 x 10(-9) M vs 2.0 x 10(-8) M, respectively). Importantly, the inhibitors blocked the activation by cholera toxin (which acts on Gs-like proteins) of cAMP formation, but failed to alter the effect of forskolin (which bypasses the receptor-G protein complex and activates adenylyl cyclase directly). Treatment of cells with calyculin-A significantly dampened adenylyl cyclase activity in cell membrane fraction, though to a lesser extent than it blocked cAMP formation in the whole cell. Both okadaic acid and calyculin-A inhibited CRF- and PACAP-induced secretion of ACTH. Our data hint that in AtT-20 corticotrophs, inhibition of phosphatases by modulating the state of phosphorylation of the receptor-G proteins complexes for CRF and PACAP, regulates cAMP formation and ACTH secretion.
Koch
B
B
Institut de Physiologie, CNRS URA 1446, Université Louis Pasteur, Strasbourg, France.
Lutz-Bucher
B
B
D000262:Adenylyl Cyclases / Q000378:metabolism; D000324:Adrenocorticotropic Hormone / Q000378:metabolism; D000818:Animals; D002460:Cell Line; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology*; D003346:Corticotropin-Releasing Hormone / Q000494:pharmacology*; D000242:Cyclic AMP / Q000096:biosynthesis*; D004305:Dose-Response Relationship, Drug; D004988:Ethers, Cyclic / Q000494:pharmacology*; D008387:Marine Toxins; D009479:Neuropeptides / Q000494:pharmacology*; D019319:Okadaic Acid; D010080:Oxazoles / Q000494:pharmacology*; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D051381:Rats
C495403:Adcyap1 protein, rat; D004988:Ethers, Cyclic; D008387:Marine Toxins; D009479:Neuropeptides; D010080:Oxazoles; D051219:Pituitary Adenylate Cyclase-Activating Polypeptide; D005576:Colforsin; D019319:Okadaic Acid; C059041:calyculin A; D000324:Adrenocorticotropic Hormone; D003346:Corticotropin-Releasing Hormone; D000242:Cyclic AMP; D010749:Phosphoprotein Phosphatases; D000262:Adenylyl Cyclases