Tumor-promoting phorbol esters affect production of prolactin and growth hormone by rat pituitary cells.

GH4C1 cells are strain of rat pituitary cells which synthesize and secrete both PRL and GH. Phorbol esters, which are potent tumor promoters in mouse skin and have diverse actions in a variety of cell and tissue systems, were found to alter hormone production by GH4C1 cells. 12-O-Tetradecanoylphorbol-13-acetate (TPA), the most potent phorbol ester, stimulated both PRL synthesis and release. The synthesis of PRL was stimulated 2.3- to 6.2-fold over control values after 3 days of treatment; the ED50 for TPA was 8.9 +/- 2.1 nM (5.5 +/- 1.3 ng/ml). The release of stored PRL was stimulated within 10 min of the addition of 100 ng/ml TPA to the culture medium. Maximum stimulation of release was 1.3- to 2.3-fold over control, and the ED50 for TPA was 13.2 +/- 2.3 nM (8.1 +/- 1.4 ng/ml). Another tumor-promoting phorbol ester, phorbol 12,13-didecanoate (PDD), stimulated PRL synthesis and release to the same maximum extent at TPA, but two biologically inactive compounds, 4 alpha-PDD and phorbol, had no effect. The stimulation of PRL release by TPA was a Ca++-dependent process; Co++, a competitive antagonist of Ca++, at 2.0 mM blocked completely the stimulation of PRL release by 100 ng/ml TPA, and this block was overcome by 2.0 mM Ca++, but not by 2.0 mM Mg++. Although phorbol esters stimulate prostaglandin (PG) synthesis in certain other cells types, PGs were not required for TPA action in GH4C1 cells. Indomethacin, a PG cyclooxygenase inhibitor, at 200 ng/ml did not inhibit the stimulation of PRL synthesis and release by TPA. Furthermore, there was no detectable (less than 50 pg/ml) PGE2 or PGF2 alpha in the medium of control or TPA-treated cells. In GH3 cells, a related cell strain, TPA (100 ng/ml) decreased GH production in both control and hydrocortisone-stimulated cells to 35% of untreated control values after 3 days of incubation. TPA also altered cell morphology, but had little or no effect on cell growth. We conclude that GH cells provide a useful system to study the mechanisms of phorbol ester action on differentiated functions.