Dual effects of daidzein on chicken hepatic vitellogenin II expression and estrogen receptor-mediated transactivation in vitro

Two in vitro systems were employed to delineate the estrogenic activity of daidzein (Da), alone or in combination with high or low concentrations of estrogen in two cell types possessing different estrogen-receptor (ER) isoforms, ERα and/or ERβ: (1) vitellogenin II (VTG), the egg yolk precursor protein and the endpoint biomarker for estrogenicity, in chicken primary hepatocytes, and (2) CHO-K1 cells transiently co-transfected with ERα or ERβ and estrogen-response elements (ERE) linked to a luciferase reporter gene. Da (100 μM) alone induced VTG mRNA expression in chicken hepatocytes, albeit with much less potency compared to estradiol (E2). Da exhibited different effects in the presence of 1 μM and 10 μM E2. At a concentration of 100 μM, Da enhanced 1 μM E2-induced VTG transcription by 2.4-fold, but significantly inhibited 10 μM E2-induced VTG mRNA expression in a dose-dependent fashion from 1 to 100 μM. Tamoxifen completely blocked the estrogenic effect of daidzein, alone or in combination with 1 μM of E2, but did not influence its anti-estrogenic effect on 10 μM E2-induced VTG mRNA expression. Furthermore, neither E2 nor daidzein, alone or in combination, affected ERα mRNA expression, yet all the treatments significantly up-regulated ERβ mRNA expression in chicken hepatocytes. E2 effectively triggered estrogen-response elements (ERE)-driven reporter gene transactivation in CHO-K1 cells expressing ERα or ERβ and showed much greater potency with ERα than with ERβ. In contrast, daidzein was 1000 times more powerful in stimulating ERβ- over ERα-mediated transactivation. Daidzein, in concentrations ranging from 5 nM to 50 μM, did not affect ERβ-mediated transactivation induced by 1 nM E2, but it significantly inhibited ERβ-mediated transactivation induced by 10 nM E2 at 500 nM. Despite the tremendous difference in sensitivity between the two in vitro systems, daidzein exhibited greater potency as an estrogen-antagonist for ERβ-mediated activity.