Disruption of progesterone synthesis by PBDEs: Potent inhibition of human and rat placental 3β-hydroxysteroid dehydrogenase by the metabolite 4-Bromodiphenyl ether.

Polybrominated diphenyl ethers (PBDEs) and their metabolite 4-bromodiphenyl ether (BDE3) are pervasive environmental contaminants suspected of endocrine-disrupting effects. This study investigates their inhibitory potential and mechanism of action against human (h3β-HSD1) and rat (r3β-HSD4) placental 3β-hydroxysteroid dehydrogenase, a pivotal enzyme in progesterone biosynthesis. Comprehensive enzymatic analysis revealed that among eight tested PBDE congeners and the structural analog 4-bromobiphenyl (BDP), only BDE3 and BDP exhibited significant inhibition. BDE3 was a potent, mixed/competitive inhibitor with IC50 values of 5.86 μM (h3β-HSD1) and 12.35 μM (r3β-HSD4), significantly more potent than BDP (IC50 ~ 100 μM). Mechanistic studies using pregnenolone as substrate confirmed mixed/competitive inhibition for both compounds, indicating binding at the steroid substrate site, while analysis with NAD⁺ suggested mixed/noncompetitive inhibition. Molecular docking and dynamics simulations corroborated these findings, indicating BDE3 and BDP binding at the steroid/NAD⁺ interface with favorable binding energies, stabilizing the active-site conformation. Network toxicology analysis identified HSD3B1 (encoding h3β-HSD1) as a central hub gene linking PBDE exposure to preeclampsia, enriched in steroidogenesis and estrogen signaling pathways. Single-cell RNA-seq analysis revealed predominant HSD3B1 expression in extravillous and syncytiotrophoblasts, with reduced expression in preeclampsia. In silico virtual knockout of HSD3B1 predicted downstream disruption of cell cycle regulation and steroidogenic pathways. Pharmacokinetic predictions indicated both compounds have high intestinal absorption but potential hepatotoxicity. These results suggest that BDE3 may act as a potent inhibitor of placental 3β-HSD, potentially linking its disruption of progesterone synthesis to adverse pregnancy outcomes.