Comparative study on estrogen receptor alpha dimerization and transcriptional activity of parabens

Parabens are used as preservatives in various household products, including oral products, cosmetics, and hair/body washes. In recent years, the widespread use of parabens has raised concerns due to the potential health risks associated with their estrogenic effects. In the present study, we evaluated and compared the estrogenic activity of parabens using two cell-based in vitro tests: (1) bioluminescence resonance energy transfer (BRET)-based estrogen receptor alpha (ERα) dimerization using HEK293 cells that were stably transfected with ERα-fused NanoLuc luciferase (Nluc) and HaloTag (HT) expression vector, and (2) stably transfected transcriptional activation (STTA) assays using ERα-HeLa9903 cells. The following parabens were tested using the BRET-based ERα dimerization assay and showed estrogenic activity (PC₂₀ values): methyl paraben (MP, 5.98 × 10^-5 M), ethyl paraben (EP, 3.29 × 10^-5 M), propylparaben (PP, 3.09 × 10^-5 M), butyl paraben (BP, 2.58 × 10^-5 M), isopropyl paraben (IsoPP, 1.37 × 10^-5 M), and isobutyl paraben (IsoBP, 1.43 × 10^-5 M). Except MP, all other parabens tested using the STTA assay also showed estrogenic activity: EP, 7.57 × 10^-6 M; PP, 1.18 × 10^-6 M; BP, 3.02 × 10^-7 M; IsoPP, 3.58 × 10^-7 M; and IsoBP, 1.80 × 10^-7 M. Overall, EP, PP, BP, IsoPP, and IsoBP tested positive for estrogenic activity using both assays. These findings demonstrate that most parabens, albeit not all, induce ERα dimerization and possess estrogenic activity.