Estimation of safety/risk profile of the phytoestrogens 8-prenylnaringenin, 6-(1.1-dimethylallyl)naringenin and naringenin in MCF-7 cells and the rat mammary gland

Evaluation of stimulation of proliferation in female reproductive organs by phytoestrogens of reproductive tract organs is important to estimate their safety/risk profile if used as an alternative to hormone therapy. To assess the proliferative capacity of the flavanones 8-Prenylnaringenin (8-PN), 6-(1.1-dimethylallyl)naringenin (6-DMAN) and Naringenin (Nar) on the mammary gland we compared their estrogenic properties to 17β-Estradiol (E2) in three independent assays. We performed competitive estrogen receptor (ER) ligand binding assays, MCF-7 proliferation studies and a three day exposure experiment in young adult ovariectomized female rats. 8-PN displayed a high relative binding affinity for both ERs with a preference for ERα (19.46%) compared to ERβ (6.5%). Among the tested flavanones 8-PN had the strongest mitotic effect on MCF-7 cells. In vivo 8-PN increased the number of terminal end buds in the mammary gland and the protein expression of the proliferation markers Ki-67 and PCNA in epithelial ductal cells and further led to an increase of Amphiregulin (Areg) mRNA expression, endpoints correlated to mammary gland proliferation. 6-DMAN also possesses a higher relative binding affinity for the ERα than for the ERβ and stimulated the cell proliferation of MCF-7 cells. It enhanced Ki-67 protein expression and β-Casein mRNA expression in vivo. Nar displayed the weakest estrogenic properties and was the only tested compound with a higher affinity for the ERβ than the ERα. It displayed proliferative effects on MCF-7 cells and slightly induced Ki-67 protein expression in vivo.