Resveratrol acts as an estrogen receptor (ER) agonist in breast cancer cells stably transfected with ERα

Resveratrol (Res) is a phytoestrogen found in grapes and present in red wine. Res has been shown to function as an estrogen receptor (ER) agonist, but it remains unclear whether it may also exert antagonist activity. Our aim was to study the effects of Res at both the molecular (TGFα gene activation) and the cellular (cell growth) levels in breast cancer cells stably transfected with wild-type (wt) ER(D351) and mutant (mut) ER (D351Y). TGFα mRNA induction was used as a specific marker of estradiol (E₂) responsiveness. Res caused a concentration-dependent (10^-8-10^-4 M) stimulation of TGFα mRNA, indicating that it acts as an estrogen agonist in these cell lines. The pure antiestrogen ICI 182,780 (ICI) blocked Res-induced activation of TGFα, consistent with action through an ER-mediated pathway. Further studies that combined treatments with E₂ and Res showed that Res does not act as an antagonist in the presence of various (10^-11-10^-8 M) concentrations of E₂. To determine whether Res can be classified as a type I or type II estrogen (Jordan et al., Cancer Res 2001;61:6619-23,), we examined Res with the D351G ER in the TGFα assay and found that Res belongs to the type I estrogens. Both Res and E₂ had concentration-dependent growth inhibitory effects in cells expressing wtER and D351Y ER. Although the pure antiestrogen ICI blocked the growth inhibitory effects of E₂, it did not block the inhibitory effects of Res, suggesting that the antiproliferative effects of Res also involve ER-independent pathways. Interestingly, Res differentially affected the levels of ER protein in these 2 cell lines: Res down-regulated wtER levels while significantly up-regulating the amount of mutD351Y ER. Cotreatment with ICI resulted in strongly reduced ER levels in both cell lines. Gene array studies revealed Res-induced upregulation of more than 80 genes, among them a profound activation of p21^CIP1/WAF1 a gene associated with growth arrest. The p21^CIP1/WAF1 protein levels measured by Western blotting confirmed Res-induced significant up-regulation of this protein in both cell lines. In summary, Res acts as an ER agonist at low doses but also activates ER-independent pathways, some of which inhibit cell growth.