Steroidogenic enzymes in placentas shape steroid hormone profiles in the maternal circulation of each mammalian species. These include 3ß-hydroxysteroid dehydrogenase/Δ5-4 isomerase (3ßHSD) and 17a-hydroxylase/17,20-lyase cytochrome P450 (P450c17) crucial for progesterone and androgen synthesis, respectively, as well as aromatase cytochrome P450 (P450arom) that converts 4-androgens to estrogens. 5a-reductase is another important enzyme in equine placentas because 5a-dihydroprogesterone (DHP) sustains pregnancy in the absence of progesterone in the second half of equine pregnancy. DHP and its metabolites decline dramatically days before foaling, but few studies have investigated placental enzyme activity before or at parturition in mares. Thus, key enzyme activities and transcript abundance were investigated in equine placentas at 300 days of gestation (GD300) and post-partum (term). Equine testis was used as a positive control for P450c17 activity. Substrates were incubated with microsomal preparations, together with enzyme inhibitors, and products were measured by liquid chromatography tandem mass spectrometry or radiometric methods (aromatase). Equine placenta expressed high levels of 3ßHSD, 5a-reductase and aromatase, and minimal P450c17 activity at GD300 compared with testis (600-fold higher). At foaling, 3ßHSD and aromatase activities and transcript abundance were unchanged but 5a-reductase (and P450c17) was no longer detectable (P < 0.05) and transcript was decreased. Trilostane inhibited 3ßHSD significantly more in testis than placenta, suggesting possible existence of different 3ßHSD isoforms. Equine placentas have significant capacity for steroid metabolism by 5a-reductase, 3ßHSD and aromatase but little for androgen synthesis lacking P450c17. Declining pre-partum 5a-reduced pregnane concentrations coincide with selective loss of placental 5a-reductase activity and expression at parturition in horses.
|Number of pages||9|
|State||Published - 2018|
Bibliographical noteFunding Information:
The authors acknowledge the generous support of the John P Hughes and Albert G Clay Endowments that funded these studies. The sampling of mares at the University of California, Davis was conducted by Joel Viloria and undergraduate students from the Department of Animal Science without whose help these studies could not have been completed. The authors are extremely grateful for the help and guidance of the technical staff at the Equine Analytical Chemistry Laboratory, School of Veterinary Medicine, University of California, Davis, especially the efforts and support of Daniel McKemie, Teresa Bowers, Dr Go Sugiarto and Sandy Yim, providing expertise, training and technical support for analysis of samples by liquid chromatography tandem mass spectroscopy. The authors also wish to thank Dr Alex Esteller-Vico, Dr Shavahn Loux, and Michelle Wynn for their assistance with tissue collection.
© 2018 Society for Reproduction and Fertility.
ASJC Scopus subject areas
- Reproductive Medicine
- Obstetrics and Gynecology
- Cell Biology