Ginsenoside Rg1 exerts estrogen-like activities via ligand-independent activation of ERα 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 μM 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 ERα and ERβ in mediating the actions of Rg1, ER-negative human embryonic kidney (HEK293) cells were co-transfected with the ERE-luciferase reporter construct and either ERα or ERβ construct. The results showed that Rg1 could activate ERE-luciferase activity via the ERα-mediated pathway in a dose-dependent manner (10-14to 10-6M); whereas, the activation of ERβ-mediated ERE-luciferase activity by Rg1 only occur at high concentration (10-6M). Furthermore, the results showed that 1 pM Rg1 could rapidly induce phosphorylation of the AF-1 domain of ERα at serine 118 residue within the first 5 min of incubation, suggesting that Rg1 activates ERα in a ligand-independent manner. Taken together, our results indicate that Rg1 preferentially activates ERα 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 ERα pathway.