Regulation of steroidogenesis in the bovine placenta

As pregnancy progresses in the cow, the secretory activity of the corpus luteum is markedly diminished. This reduced secretion is due to a decline in the number of viable luteal cells as well as reduction in the secretory activity and responsiveness of the cells to trophic agents. The principal extra-ovarian source of progesterone (P4) by mid-gestation therefore appears to be the placenta. Uniquely this P4 biosynthesis is cyclic-nucleotide independent, but the Ca+2 dependent. It therefore appears that the Ca+2 second messenger and protein kinase C systems are responsible for regulation of sterol biosynthesis in the cow placenta. Dispersed bovine caruncle cells from the first trimester of pregnancy in comparison to caruncle cells of older than 90 days of gestation produce little P4 and are refractory to agents which enhance placental steroidogenesis. In order to explain this refractoriness of the first trimester cells, we determine (1) the expression of P450scc and its mRNA and (2) the expression of adrenodoxin. It was found that P4 synthesis by bovine maternal caruncle cells was low or undetectable in the first trimester but increased more than 10-fold in the second trimester of gestation. Addition of 25-OH-cholesterol to second trimester maternal cells increased P5 production but no effect was observed in first trimester cells. Cytochrome P450scc and its mRNA and adrenodoxin content were determined using Western blot or dot-blot techniques. Both proteins and the mRNA were detected in maternal tissue of first and second trimesters of gestation. In conclusion low P4 levels synthesized by first trimester maternal cells are not due to the absence of either cytochrome P450scc or adrenodoxin protein or production of P450scc mRNA. The data suggest that the refractoriness of the maternal caruncle cells during the first trimester is the result of post-translational regulation.