Estrogenic activity of isoflavonoids from Onobrychis ebenoides

Fractionation of the neutral extract of Onobrychis ebenoides (Leguminosae) yielded a new isoflavone, named ebenosin (1), in addition to the known ones, afrormosin (2), formononetin (3) and daidzein (4). Although the relative binding affinities of 1-4 for estrogen receptor alpha(ER alpha) were nearly comparable and matched those of 1-3 for ER beta that of 4 for the latter receptor was significantly higher than any of the other. Compounds 1-4 induced cell proliferation and gene expression in breast and endometrial cancer cells in an ER-dependent manner. Nonetheless, the rank order of induction potencies (4 > 3 2 1) matched better that of affinities for ER beta (4 > 3 > 2 > 1) rather than ERa (4 >= 3 >= 2 >= 1). While the antiestrogen ICI 182,780 could inhibit the induction of proliferation of ER-positive breast cancer cells by 1-4, it could not prevent 1 from exhibiting significant ER-independent cytoOF toxicity at 10 mu M. By contrast, 1 was much less cytotoxic and only weakly estrogenic for ER-positive endometrial adenocarcinoma cells. In conclusion, our data suggest that the C-8 isoprenyl substituent of 1 renders it cytotoxic and/or estrogenic in a cell-dependent manner.