Comparison of Placental HSD17B1 Expression and Its Regulation in Various Mammalian Species

SIMPLE SUMMARY: 17β-Hydroxysteroid dehydrogenase type1 (HSD17B1) is responsible for converting estrone, a weak estrogen, into the more potent estradiol in the ovaries of vertebrates ranging from teleosts to mammals. We evaluated its expression in the placentae of various eutherian species and demonstrated that it was only expressed in the human placenta. The ubiquitous transcription factor SP1-binding site was conserved in the promoter region of the HSD17B1/Hsd17b1 gene across the species. This region conferred the promoter activities both in human and ovine HSD17B1. However, in the ovine placenta, cytosine residues of this SP1 site in the HSD17B1 promoter region were completely methylated. Human HSD17B1 had high activity for testosterone production, similar to its homologs in other species. On the basis of this fact, the significance of placental HSD17B1 expression is discussed. ABSTRACT: During mammalian gestation, large amounts of progesterone are produced by the placenta and circulate for the maintenance of pregnancy. In contrast, primary plasma estrogens are different between species. To account for this difference, we compared the expression of ovarian and placental steroidogenic genes in various mammalian species (mouse, guinea pig, porcine, ovine, bovine, and human). Consistent with the ability to synthesize progesterone, CYP11A1/Cyp11a1, and bi-functional HSD3B/Hsd3b genes were expressed in all species. CYP17A1/Cyp17a1 was expressed in the placenta of all species, excluding humans. CYP19A/Cyp19a1 was expressed in all placental estrogen-producing species, whereas estradiol-producing HSD17B1 was only strongly expressed in the human placenta. The promoter region of HSD17B1 in various species possesses a well-conserved SP1 site that was activated in human placental cell line JEG-3 cells. However, DNA methylation analyses in the ovine placenta showed that the SP1-site in the promoter region of HSD17B1 was completely methylated. These results indicate that epigenetic regulation of HSD17B1 expression is important for species-specific placental sex steroid production. Because human HSD17B1 showed strong activity for the conversion of androstenedione into testosterone, similar to HSD17B1/Hsd17b1 in other species, we also discuss the biological significance of human placental HSD17B1 based on the symptoms of aromatase-deficient patients.