SAR Study on Estrogen Receptor α/ß Activity of (Iso)flavonoids: Importance of Prenylation, C-Ring (Un)Saturation, and Hydroxyl Substituents.

Many botanicals used for women's health contain estrogenic (iso)flavonoids. The literature suggests that estrogen receptor beta (ERß) activity can counterbalance estrogen receptor alpha (ERα)-mediated proliferation, thus providing a better safety profile. A structure-activity relationship study of (iso)flavonoids was conducted to identify ERß-preferential structures, overall estrogenic activity, and ER subtype estrogenic activity of botanicals containing these (iso)flavonoids. Results showed that flavonoids with prenylation on C8 position increased estrogenic activity. C8-prenylated flavonoids with C2-C3 unsaturation resulted in increased ERß potency and selectivity [e.g., 8-prenylapigenin (8-PA), EC50 (ERß): 0.0035 ± 0.00040 µM], whereas 4'-methoxy or C3 hydroxy groups reduced activity [e.g., icaritin, EC50 (ERß): 1.7 ± 0.70 µM]. However, nonprenylated and C2-C3 unsaturated isoflavonoids showed increased ERß estrogenic activity [e.g., genistein, EC50 (ERß): 0.0022 ± 0.0004 µM]. Licorice (Glycyrrhiza inflata, [EC50 (ERα): 1.1 ± 0.20; (ERß): 0.60 ± 0.20 µg/mL], containing 8-PA, and red clover [EC50 (ERα): 1.8 ± 0.20; (ERß): 0.45 ± 0.10 µg/mL], with genistein, showed ERß-preferential activity as opposed to hops [EC50 (ERα): 0.030 ± 0.010; (ERß): 0.50 ± 0.050 µg/mL] and Epimedium sagittatum [EC50 (ERα): 3.2 ± 0.20; (ERß): 2.5 ± 0.090 µg/mL], containing 8-prenylnaringenin and icaritin, respectively. Botanicals with ERß-preferential flavonoids could plausibly contribute to ERß-protective benefits in menopausal women.

Estrogen Receptor (ER) Subtype Selectivity Identifies 8-Prenylapigenin as an ERß Agonist from Glycyrrhiza inflata and Highlights the Importance of Chemical and Biological Authentication.

Postmenopausal women are increasingly using botanicals for menopausal symptom relief due to the increased breast cancer risk associated with traditional estrogen therapy. The deleterious effects of estrogens are associated with estrogen receptor (ER)α-dependent proliferation, while ERß activation could enhance safety by opposing ERα effects. Three medicinal licorice species, Glycyrrhiza glabra ( G. glabra), G. uralensis, and G. inflata, were studied for their differential estrogenic efficacy. The data showed higher estrogenic potency for G. inflata in an alkaline phosphatase induction assay in Ishikawa cells (ERα) and an estrogen responsive element (ERE)-luciferase assay in MDA-MB-231/ß41 breast cancer cells (ERß). Bioassay-guided fractionation of G. inflata led to the isolation of 8-prenylapigenin (3). Surprisingly, a commercial batch of 3 was devoid of estrogenic activity. Quality control by MS and qNMR revealed an incorrect compound, 4'- O-methylbroussochalcone B (10), illustrating the importance of both structural and purity verification prior to any biological investigations. Authentic and pure 3 displayed 14-fold preferential ERß agonist activity. Quantitative analyses revealed that 3 was 33 times more concentrated in G. inflata compared to the other medicinal licorice extracts. These data suggest that standardization of G. inflata to 3 might enhance the safety and efficacy of G. inflata supplements used for postmenopausal women's health.