Ginsenoside Rg1 exerts estrogen-like activities via ligand-independent activation of ER alpha pathway

Ginsenoside Rg1, an active ingredient commonly found in ginseng root, was previously demonstrated to be a phytoestrogen that exerted estrogen-like activity without direct interaction with estrogen receptors (ERs) in human breast cancer (MCF-7) cells. The present study was designed to determine the molecular mechanism by which Rg1 exerted estrogenic effects. Co-incubation of MCF-7 cells with 1 mu M of ER antagonist ICI182780 abolished the inductive effects of Rg1 on pS2 expression as well as ERE-luciferase activity, suggesting that the estrogenic effects of Rg1 were mediated through the endogenous ERs. To evaluate the relative involvement of ER alpha and ER beta in mediating the actions of Rg1, ER-negative human embryonic kidney (HEK293) cells were co-transfected with the ERE-luciferase reporter construct and either ER alpha or ER beta construct. The results showed that Rg1 could activate ERE-luciferase activity via the ER alpha-mediated pathway in a dose-dependent manner (10(-14) to 10(-6) M); whereas, the activation of ER beta-mediated ERE-luciferase activity by Rg1 only occur at high concentration (10-6 M). Furthermore, the results showed that 1 pM Rg1 could rapidly induce phosphorylation of the AF-1 domain of ER alpha at serine 118 residue within the first 5 min of incubation, suggesting that Rg1 activates ER alpha in a ligand-independent manner. Taken together, our results indicate that Rg1 preferentially activates ER alpha via phosphorylation of AF-1 domain in the absence of receptor binding. This study is the first to provide evidence that ginsenoside Rg1 exerts estrogen-like actions via ligand-independent activation of ER alpha pathway. (c) 2007 Elsevier Ltd. All rights reserved.