PROSTAGLANDINS STIMULATE THE SECRETION OF INHIBIN FROM HUMAN PLACENTAL CELLS

In this study, the effects of prostaglandins (PGs) on inhibin secretion were investigated in primary culture of human placental cells. Isolated trophoblast cells were cultured for 2-5 days with PGE1, PGE2, 6-keto-PGF1alpha, PGF2alpha, and thromboxane B2. Inhibin levels in the culture medium were measured by immunoenzymatic assay. PGs significantly stimulated inhibin secretion in the cell culture. The addition of PGs (1 nm to 10 mum) to the culture resulted in dose-dependent increases in inhibin levels in the medium. At a dose of 10 mum, inhibin levels in the medium were increased by 35.2-172.5% compared to the control value. Compared with the efficacies of these PGs, PGE2 and PGF2alpha enhanced inhibin secretion more potently than other tested PGs. A close correlation between the effects of PGs and the number of trophoblasts seeded in the culture was observed, with an optimal response of the cells to PGs at 1.0-2.0 X 10(6) cells/well. Time-course studies showed that a significant increment in inhibin levels in the PG-treated culture occurred after 48-120 h, but not during the first 24 h, of the culture, indicating a possible involvement of PGs in inhibin biosynthesis in the cells. Epidermal growth factor (EGF) could enhance PG-stimulated inhibin releases in the cell culture. Treatment with EGF (100 ng/mL) increased inhibin in the medium by 83.3% (ED50) in the presence of PGE2 or by 70.3% (ED50) in the presence of PGF2alpha compared to the control values. The addition of 8-bromo-cAMP (100 muM) and cholera toxin (1000 ng/mL) to the culture also enhanced basal and PG-induced inhibin secretion, but the addition of the 4-bromo-calcium ionophore A23187 (1 muM) did not alter inhibin releases, suggesting that the stimulatory effects of PGs on inhibin secretion in placental cells may be mediated through the cAMP pathway. In the cell cultures treated with PGE and PGF2alpha, there was no change in cell growth or intracellular DNA content, as measured by 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide assay and DNA determination with fluorescence spectrophotometry. Morphological studies showed that PGE2alpha (1 muM), alone or combined with EGF (100 ng/mL), significantly accelerated the process of differentiation from cytotrophoblasts to syncytiotrophoblasts, indicating that the actions of PGs may be related to their effects on syncytial transformation, rather than cellular proliferation. Our observations indicate that PGs may be important stimulators of inhibin production in the placenta and, consequently, play a role in the regulation of circulating inhibin levels in the maternal serum during human pregnancy.