Hypoxia Blocks 11β-Hydroxysteroid Dehydrogenase Type 2 Induction in Human Trophoblast Cells During Differentiation by a Time-dependent Mechanism that Involves both Translation and Transcription

Abstract

The present study was undertaken to determine (1) if hypoxia-induced down-regulation of placental 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2; encoded by HSD11B2 gene) activity and protein in human trophoblast cells during in vitro differentiation was mediated at the level of HSD11B2 gene transcription; and (2) whether the reduced placental 11beta-HSD2 in pregnancies complicated with fetal growth restriction (FGR) was a consequence of intrinsic abnormalities in trophoblast cells. Trophoblast cells were isolated from uncomplicated pregnancies and those complicated with FGR at term, and cultured for up to 72 h under normoxic (20% oxygen) or hypoxic (1% oxygen) conditions. Under normoxia, 11beta-HSD2 activity and protein increased progressively over the 72 h culture period, which was accompanied by a corresponding rise in 11beta-HSD2 mRNA. As demonstrated previously, hypoxia blocked the increase in levels of both 11beta-HSD2 activity and protein within the first 24h. In contrast, although hypoxia also prevented the rise in 11beta-HSD2 mRNA, it did not do so until 48 h. This time-dependent effect of hypoxia on placental 11beta-HSD2 activity/protein and mRNA suggests a dual mechanism of action whereby hypoxia may induce a rapid down-regulation of 11beta-HSD2 protein synthesis, which occurs initially at the level of translation, and later extends to the level of transcription. Indeed, transient transfection studies demonstrated that hypoxia diminished HSD11B2 promoter activity. When trophoblast cells isolated from FGR placentas were cultured and allowed to differentiate under the same conditions, they not only exhibited a similar pattern of 11beta-HSD2 activity and mRNA expression but also responded to hypoxia similarly to those from normal placentas. This suggests that the reduced placental 11beta-HSD2 in FGR is not due to intrinsic abnormalities in trophoblast cells, but likely a result of extrinsic factors associated with FGR.