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A key enzyme in the regulation of prostaglandin (PG) synthesis is PG synthase (PGS; cyclooxygenase), which converts arachidonic acid to PGs. Since both PGs and glucocorticoids are elevated before parturition, we studied the regulation of dexamethasone (DEX; 150 nM) on PGF2 alpha synthesis and PGS expression in human placental cells in vitro. Both first trimester and term placental cells were used. DEX reduced PGF2 alpha synthesis in human term placental cells, in contrast to first trimester cells which were unaffected by the same treatment. DEX inhibition of PGF2 alpha production by term placental cells was time and dose dependent. PGS expression was analyzed by [35S]methionine metabolic labeling and immunoprecipitation using polyclonal antibodies developed in rabbits against ram seminal vesicle PGS. DEX reduced PGS expression in term placental cells, but not in first trimester cells. In contrast to the effect of DEX on PGF2 alpha, progesterone and estradiol production by cells were unaffected at any stage of gestation examined. DEX inhibition of PGF2 alpha synthesis required de novo biosynthesis of RNA and proteins. These results suggest 1) corticosteroids play a role in the regulation of placental PG synthesis during parturition; 2) the inhibition of PG synthesis and PGS expression by glucocorticoids is RNA and protein biosynthesis dependent; and 3) induction of labor by glucocorticoids is not directly related to changes in placental progesterone or estradiol biosynthesis.

It was previously reported that dispersed bovine placentome secretes progesterone and that the steroidogenic activity of these cells is stimulated by a calcium-mediated, cyclic nucleotide independent mechanism. In the present study, the influence of substrate availability was explored and the roles of calmodulin and protein kinase C in progestin production examined. Incubation of dispersed fetal cotyledon cells with 25-hydroxycholesterol (25-OH-C), a soluble sterol which readily enters cells and is metabolized to steroid hormones, increased progesterone secretion in a dose-dependent manner. The response to 25-OH-C was dependent on the extracellular calcium concentration. Methyl isobutyl xanthine (MIX) alone also increased pregnenolone as well as progesterone secretion, and the combination of 25-OH-C and MIX stimulated progesterone secretion was inhibited by trifluoperazine. The phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), caused no major effects on steroidogenesis but the stimulatory effects of MIX or the ionophore A23187 were enhanced in its presence. These findings suggest that (1) basal progesterone secretion by fetal cotyledon cells is limited by cholesterol availability; (2) MIX increases steroidogenesis in part by increasing the synthesis of pregnenolone, but its actions are expressed independently of cholesterol availability; (3) both calmodulin and protein kinase C may participate in the modulation of bovine placental steroidogenesis.

We have previously reported that progesterone synthesis in the bovine placenta is regulated by Ca2+ dependent and cyclic nucleotide independent mechanism. In studies conducted to further define the role of Ca2+ in the synthesis of progestins in bovine placental tissue, it was found that both protein kinase C (PKC), as determined by phosphorylation, and cytochrome P-450 side chain cleavage, as determined by Western blot analysis, were detectable in the steroidogenetically active portion of the placentome. To determine the site of action of PKC, fetal cotyledon cells were incubated in media containing 25-hydroxycholesterol in the absence or or presence of 10 ng/ml 12-O-tetradecanoyl-phorbol-13-acetate (TPA). It was found that TPA significantly (P less than 0.05) increased the conversion of the exogenous cholesterol analog to progesterone. To determine if the TPA could act synergistically with calcium activators, fetal cotyledon cells were incubated with either methyl isobutyl xanthine (MIX), an activator of intracellular calcium, or the calcium ionophore, A23187, which increases extracellular calcium influx, or both of these agents, in the presence or absence of TPA. It was found that TPA synergistically increased the conversion of sterol to progestins induced by submaximal concentrations of either MIX or A23187. In the presence of both compounds, TPA induced an even more dramatic increase in progestin synthesis. In experiments in which cyanoketone, an agent that inhibits the conversion of pregnenolone to progesterone, was added, TPA addition resulted in increased pregnenolone production, indicating that side chain cleavage of cholesterol is the site of action. The data, therefore, suggest that: (a) Ca2+ affects mechanisms regulating placental steroidogenesis; (2) one locus of Ca2+ is the cholesterol side chain cleavage reaction; and (3) PKC found in this tissue has a role in the Ca activated progestin production.