Parturition in the pregnant sheep is preceded by an abrupt alteration in placental steroid metabolism causing a shift from progesterone to estrogen production. This change is believed to be a consequence of the prepartum rise in cortisol in the fetal circulation and involves increases in activities of the enzymes steroid 17α-hydroxylase (cytochrome P-45017α), steroid C-17, 20-lyase, and possibly aromatase. We have investigated the activity levels of aromatase and 17α-hydroxylase in placental micro-somes in late pregnancy and dexamethasone-in-duced labor. Over the gestational period of 118-140 days basal levels of placental aromatase were relatively constant [mean value (±sd) of 5.6 ±1.6 pmol min-1 mg microsomal protein-1 (n = 10)]. Steroid 17α-hydroxylase activity was undetectable [<0.5 pmol min-1 mg microsomal protein-1 (n = 7)]. In six animals in labor induced with infusion of dexameth-asone into the fetus, placental aromatase activity had a mean value of 14.0 ± 2.5 pmol min-1 mg protein-1; placental steroid 17α-hydroxylase, measured in four of the animals, had a mean (±sd) activity of 319 ± 58 pmol min-1 mg microsomal protein-1. Immunoblotting of placental microsomal preparations with specific antibodies to cytochrome P-45017αand NADPH-cytochrome P-450-reductase indicated that the glucocorticoid-induced activity of 17α-hy-droxylase was associated with increased content of cytochrome P-45017α. Northern blotting with a cDNA probe for cytochrome P-45017α showed that glucocorticoid increased the levels of mRNA for the enzyme. The data are consistent with a mechanism of glucocorticoid action in which it acts as a stimulus to transcription of the steroid 17α-hydroxylase gene or else enhances the stability of mRNA for cytochrome P-45017α.
ASJC Scopus subject areas
- Molecular Biology