A ginsenoside-Rh1, a component of ginseng saponin, activates estrogen receptor in human breast carcinoma MCF-7 cells

Abstract

We have examined the possibility that a component of Panax ginseng, ginsenoside-Rh1, acts by binding to steroid hormone receptors such as receptors for estrogen, glucocorticoid, androgen, and retinoic acid. Ginsenoside-Rh1 activated the transcription of the estrogen-responsive luciferase reporter gene in MCF-7 breast cancer cells at a concentration of 50 μM. Activation was inhibited by the specific estrogen receptor antagonist ICI 182,780, indicating that the estrogenic effect of ginsenoside-Rh1 is estrogen receptor dependent. Ginsenoside-Rh1 induction of luciferase activity was dose-dependent in CV-1 cells transiently transfected with estrogen receptor and reporter plasmids. Next, we evaluated the ability of ginsenoside-Rh1 to induce the estrogen-responsive genes in MCF-7 cells. Ginsenoside-Rh1 increased c-fos and pS2 at the mRNA levels at 24 h after treatment, although the effects were not as prominent as 17β-estradiol. Western blot analysis showed that progesterone receptor protein was induced at 24 h of treatment of ginsenoside-Rh1. However, ginsenoside-Rh1 failed to activate the glucocorticoid receptor, the androgen receptor, or the retinoic acid receptor in CV-1 cells transiently transfected with the corresponding steroid hormone receptors and hormone responsive reporter plasmids. These data support our hypothesis that ginsenoside-Rh1 acts as a weak phytoestrogen, presumably by binding and activating the estrogen receptor.

Introduction

Ginseng is a popular herbal medicine that has been used for over 2000 years in oriental countries. It has been reported that ginseng has a wide range of pharmacological activities in cardiovascular, endocrine, immune, and central nervous systems [1]. Ginseng and its constituents have been used for their tonic, immunomodulatory adaptogenic, anticancer, and anti-aging activities [2]. These beneficial effects of ginseng have prompted tremendous effort to discover the pharmacology of its action through biochemical and molecular biology techniques.

A number of components have been isolated and characterized from ginseng including ginsenosides, polysaccharides, peptides, polyacetylenic alcohols, and fatty acids [3]. However, how each of these components contributes to the pharmacological properties of ginseng is not very well known. Among these ingredients, a group of saponin glycosides known as ginsenosides, comprising a four trans-ring rigid steroid skeleton with a modified side chain at C-20, have received considerable attention because of their biological activity [4].

Ginsenosides could exercise their activities at either the cellular membrane, inside the cell or even in the nucleus [5]. Ginsenosides may initiate their mode of action by binding to transmembrane membrane receptors and subsequently activating the associated downstream signaling pathways. Due to the amphiphilic nature of ginsenosides, ginsenosides might even intercalate into the cell membrane, thereby influencing membrane fluidity [5]. It is also possible that the hydrophobic properties of ginsenosides favor their binding to intracellular steroid hormone receptors, such as receptors for glucocorticoids, progesterone, androgens, mineralocorticoid, and estrogen. Like steroids, ginsenosides are lipophilic [6], which enhances their ability to traverse cell membranes by simple diffusion and acquire access to the nucleus to control gene transcription by binding to specific intracellular receptors. A few studies showed several biological effects resulting from steroid hormone receptor activation [5]. Lee et al. [7] showed that ginsenoside-Rg1 is a functional ligand of the glucocorticoid receptor (GR). Ginsenoside-Rg1 competitively inhibited the binding of the synthetic glucocorticoid dexamethasone to the GR, although the affinity of ginsenoside-Rg1 for GR was lower than for dexamethasone [8]. Ginseng has been recommended for the alleviation of the symptoms of menopause, indicating that some components of ginseng act as phytoestrogens [9] and/or involve activation of the estrogen receptor (ER). Ginseng extracts are able to stimulate the growth of ER-positive cells [10]. However, there has been no conclusive scientific data to show that ginseng contains phytoestrogens [11].

The experiments presented in this report show that ginsenoside-Rh1 (G-Rh1) is an active estrogenic ligand as determined by measuring the transcription activation of an estrogen-responsive reporter plasmid and endogenous estrogen responsive genes.