Applicability of the OECD 455 in-vitro assay for determination of hERa agonistic activity of isoflavonoids

The Organisation for Economic Co-operation and Development (OECD)-validated transactivation assay using the human estrogen receptor alpha (hER alpha) Hela9903 cell line is used for activity evaluation of hERa agonists and antagonists. Due to many advantages, this assay is broadly used as an initial screening process. However, response significantly higher from that of 17-beta estradiol (E2) was observed with phytoestrogens for concentrations commonly above 1 mu M in previous studies. The main aim of this study was thus to ascertain the applicability of OECD protocol 455 for evaluation of estrogenic activity of natural flavonoids, including known phytoestrogens. The estrogenic activities of aglycones as well as of O-methylated and glycosylated flavonoids were evaluated. Supra-maximal luciferase activity was seen for most of the flavonoids tested at concentrations even below 1 mu M. hER alpha-mediated luciferase expression was confirmed with the competition assay specified in OECD protocol 455. However, at concentrations above 1 mu M, non-specific interactions were also observed. Instead of EC50 values, which could not be determined for most of the isoflavonoids tested, the concentrations corresponding to 10% (PC10) and 50% (PC50) of the maximum activity of the positive control, E2, were used for quantitative determination of estrogenic activities. Appropriate evaluation of the data obtained with the current OECD protocol 455 validated assay represents a valuable tool for initial screening of natural flavonoids for estrogenic activity.