Activation of estrogen receptor-beta 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 is used for the treatment of climacteric symptoms in menopausal women. However, the molecular mode of action of ERr 731^® was unknown. For the first time, ERr 731^® and its aglycones trans-rhapontigenin and desoxyrhapontigenin were investigated with regard to the activation of the estrogen receptor- or estrogen receptor-β (ER, ERβ). The related hydroxystilbenes cis-rhapontigenin, resveratrol and piceatannol were studied as comparators. As controls, 17β-estradiol or the selective ER-(propylpyrazoltriol) or ERβ-agonists (diarylpropionitril) were used. Neither in ER-expressing yeast cells, in the ER-responsive Ishikawa cells, nor in human endometrial HEC-1B cells transiently transfected with the ER an activation of ER by ERr 731^® or the other single compounds was detected. Furthermore, an antiestrogenic effect was not observed. In contrast in human endometrial HEC-1B cells transiently transfected with the ERβ, 100 ng/ml ERr 731^® and the single compounds significantly induced the ERβ-coupled luciferase activity in a range comparable to 10⁻⁸ M 17β-estradiol. All effects were abolished with the pure ER antagonist ICI 182780, indicating an ER-specific effect. The ERβ agonistic activity by ERr 731^® could be of importance for its clinical use, as central functions relevant to climacteric complaints are proposed to be mediated via ERβ activation.     

Subtype-specific activation of estrogen receptors by a special extract of Rheum rhaponticum (ERr 731), its aglycones and structurally related compounds in U2OS human osteosarcoma cells

The special extract ERr 731 from the roots of Rheum rhaponticum is the major constituent of Phytoestrol N which is used for the alleviation of menopausal symptoms. Recently, we demonstrated that ERr 731 and its aglycones trans-rhapontigenin and desoxyrhapontigenin as single test substances do not activate the estrogen receptors-alpha (ERalpha) in human endometrial adenoarcinoma cells. However, these substances together with the structurally related hydroxystilbenes cis-rhapontigenin, resveratrol and piceatannol activated the ERbeta-dependent reporter gene activity. To investigate if these substance are tissue selective ER activators, ERr 731 and the single test substances were examined in bone-derived U2OS cells stably expressing ERalpha or transiently expressing ERbeta. In the ERalpha expressing U2OS cells, a weak, but statistically significant ERalpha-coupled luciferase activity was detected with ERr 731 and desoxyrhapontigenin which was 10-times lower than with 10(8) M 17 beta-estradiol. In the ERbeta test system, all test substances significantly induced the luciferase activity in a magnitude comparable to 17beta-estradiol. All effects were abolished with the pure ER antagonist ICI 182 780, indicating an ER-specific effect. Intracellular actions were also examined with the glycosylated ERr 731 constituents rhaponticin and desoxyrhaponticin. Treatment of U2OS cells with defined mixtures of both glycosides resulted in a reporter gene activity comparable to that of ERr 731, thereby providing evidence for the existence of cellular uptake mechanisms for glycosylated hydroxystilbenes. This report confirms the strong ERbeta-dependent activity of ERr 731 and provides evidence for a tissue selective ER agonistic activity by ERr 731 and its aglycones, as demonstrated by the activation of ERalpha in bone cells but not in endometrial cells.