A phorbol ester, phorbol 12-myristate 13-acetate, and a calcium ionophore, A23187, can mimic the luteolytic effect of prostaglandin F2 alpha in isolated rat luteal cells.

Abstract

To explore the possible role of protein kinase C and calcium in the luteolytic process, we treated luteal cells with a protein kinase C activator, the phorbol ester, phorbol 12-myristate 13-acetate (PMA), and with the calcium ionophore, A23187. Lower concentrations of PMA could clearly mimic the inhibitory, luteolytic effects of prostaglandin F2 alpha (PGF2 alpha) on LH-induced cAMP and progesterone production. A nontumor promoting phorbol ester, 4 alpha-phorbol 12,13-didecanoate, had no inhibitory effect, indicating a specific PMA effect. The calcium ionophore, A23187, also gave a marked inhibition of LH-induced cAMP and progesterone production. Hormone-stimulated adenylate cyclase activity was markedly impaired after preincubation of the cells with PGF2 alpha, PMA, or A23187, but no effect was seen when the substances were added to isolated membranes. In addition, the stimulation of progesterone production in the luteal cells with the cAMP analogs, 8-bromo-cAMP and (Bu)2cAMP, was almost totally abolished when PMA or A23187 was present. We conclude that PMA and A23187 in many ways mimic the effect of PGF2 alpha in luteal cells. The inhibition of steroidogenesis is partly dependent on depressed activity of the hormone-sensitive adenylate cyclase, but also obtained by inhibiting steps distal to cAMP formation. Both points of action seem to be calcium and/or protein kinase C dependent. In contrast, higher concentrations of PMA markedly stimulated steroidogenesis without affecting the cAMP level, a stimulation not seen after incubation with 4 alpha-phorbol 12,13-didecanoate, suggesting again a specific PMA effect. The stimulation of steroidogenesis by higher concentrations of PMA seems to be specific, but the interpretation of this finding is unclear at present. In conclusion, PMA and A23187 mimic some of the luteolytic properties of PGF2 alpha, not only inhibiting the luteal cAMP system, but also by inducing lesions in the steroidogenic steps beyond the cAMP system.