Activation of estrogen receptor-β by a special extract of Rheum rhaponticum (ERr 731®), its aglycones and structurally related compounds

The special extract ERr 731® from the roots of Rheum rhaponticum is the major constituent of Phytoestrol® N which has been used for the treatment of climacteric symptoms for decades. However, the molecular mode of action of ERr 731® underlying its proven clinical efficacy is not completely clarified. Therefore, we addressed the question whether ERr 731® and its aglycones trans-rhapontigenin and desoxyrhapontigenin exert their biological action via estrogen receptor-alpha -beta (ERa, ERß). The effects were compared to the structurally related compounds cis-rhapontigenin, resveratrol and piceatannol. As controls, estradiol or selective agonists for ERa (Propylpyrazoltriol) or ERß (Diarylpropionitril) were used. As ERa transactivation systems, ERa-expressing yeasts, alkaline phosphatase induction in Ishikawa cells, and bone derived U2OS cells stably expressing ERa were used. In addition, two independent systems transiently expressing ERß, the human endometrial adenocarcinoma cells HEC1B, and the U2OS cells were investigated. Neither ERr 731® or the hydroxystilbenes activated ERa in the yeast system, or in Ishikawa, or in HEC1B cells. Only in ERa-expressing U2OS cells, a very weak, but statistically significant ERa activity was detectable with ERr 731®, desoxyrhapontigenin and resveratrol which might suggest an osteoprotective effect of ERr 731®. Interestingly, the single test compounds and the total extract ERr 731® significantly induced the reporter gene activity in both ERß test systems with a more pronounced dose-dependency in the endometrial system. The ERß expressing U2OS cells responded to lower doses of the test substances indicating a higher sensitivity of this test system. All effects could be abolished with the pure ER antagonist Faslodex™, indicating an ER-specific effect. This is the first report of ERß specific activities by ERr 731® and related hydroxystilbenes. These findings could be of importance for the clinical use of ERr 731®, as crucial central functions relevant to climacteric complaints are proposed to be mediated via ERß activation.