Involvement of Protein Kinase C in the Regulation of Adrenocorticotropin Release from Rat Anterior Pituitary Cells

The involvement of protein kinase C in normal corticotroph function was studied by analysis of the effects of the phorbol ester derivative phorbol 12-myristate-13-acetate (PMA) and the synthetic diacylglycerol dioctanoylglycerol (DOG) on basal and stimulated ACTH release in cultured rat anterior pituitary cells. Incubation of rat pituitary cells with increasing concentrations of PMA or DOG caused dose-related increases in ACTH release up to 13.4 +/- 2.1- and 10.1 +/- 0.9-fold, respectively, similar to that caused by CRF (9.8 +/- 1.6-fold). Also, stimulation of endogenous diglyceride formation by phospholipase C (100 mU/ml) stimulated ACTH release by 2.5 +/- 0.1-fold. In cells incubated with maximum stimulatory concentrations of CRF (10 nM) or 8-bromo-cAMP (8-Br-cAMP; 5 mM), addition of either 100 microM DOG or 100 nM PMA caused significantly higher ACTH responses than those obtained with CRF, 8-Br-cAMP, DOG, or PMA alone. 8-Br-cAMP (5 mM) and 10 nM CRF significantly increased the effect of 100 nM PMA by 1.4 +/- 0.2- and 1.5 +/- 0.1-fold, respectively. Combinations of 10 nM CRF with either vasopressin (VP) or angiotensin II (AII) increased ACTH secretion to values higher than those produced by CRF, VP, or AII alone. However, addition of maximal stimulatory concentrations of VP or AII (10 nM) did not further increase the effects of either PMA alone or PMA/CRF combinations, indicating that their mechanisms of action may be similar to that of PMA. These results indicate that in addition to the established cAMP-dependent mechanism, stimulation of ACTH release in normal pituitary cells may be elicited by activation of protein kinase C. The evidence also suggests that protein kinase C is involved during stimulation of ACTH release by the cAMP-independent regulators VP and AII and in the synergistic effects of VP and AII with CRF on the corticotroph.