Bovine placental progesterone synthesis: comparison of first and second trimesters of gestation

We have previously reported that dispersed caruncle cells from cows during the first trimester of pregnancy, in comparison to caruncle cells from cows of more than 90 days of gestation, produce little progesterone (P4) and are refractory to agents that enhance steroidogenesis. To explain this refractoriness of the first-trimester cells, we determined (1) the expression of cytochrome P450 side-chain cleavage (P450scc) and its mRNA, (2) the expression of adrenodoxin, and (3) 3 beta-hydroxysteroid dehydrogenase activity. We first determined P4 and pregnenolone (P5) production by dispersed caruncle cells from the two gestation periods using RIA. 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 (5 microgram/ml) to second-trimester maternal cells increased P5 production, but no effect was observed in first-trimester cells. With [3H]P5 used as substrate, analysis of metabolites on thin-layer chromatography indicated that first-trimester maternal cells synthesized a small amount of P4 (3.02% of total radioactivity) compared to second-trimester cells (16.4%). A readily detectable amount of 17 apha-OH-P5 was produced by the second-trimester cells (5.02%) but not by the first-trimester cells (0.6%). No other metabolites could be characterized (< 0.5%). Cytochrome P450scc expression and its mRNA and adrenodoxin content were determined by use of Western blot or dot-blot techniques. Proteins and mRNA were detected in maternal tissues 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 proteins or to the 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.