Phytoestrogens are plant derived polyphenolic non steroidal

Phytoestrogens are plant-derived polyphenolic non-steroidal compounds that have estrogenic properties [18]. The main classes of phytoestrogens are lignans and isoflavones. Lignans, found in cereals, fruits, and vegetables, are metabolized by intestinal bacteria to enterolactone and enterodiol. Soybeans are a rich source of isoflavones. The principal isoflavones are genistein and daidzein. Daidzein is further converted to equol by gut microflora but this conversion has a high inter-individual variability [19]. Equol has been shown to have higher estrogenic activity than the precursor compound daidzein [20]. The interest in phytoestrogens has been increasing due to their suggested beneficial health effects. Phytoestrogen rich diet appears to reduce the risk of cardiovascular disease, osteoporosis as well as breast and prostate cancer [18]. ERRγ is expressed in several adult and embryonic tissues but its biological role is still largely unknown [21]. Overexpression of ERRγ in breast cancer correlates with a favorable outcome [22]. In addition, ERRγ has been reported to inhibit the growth of prostate tumor Bcl Family Set I [23]. In this study we investigated whether phytoestrogens modulated the transcriptional activity of ERRγ. We analyzed a selection of phytoestrogens for their potential agonistic or antagonistic activities on ERRγ. Using reporter gene assays in transiently transfected U2-OS and PC-3 cells we identified equol as an ERRγ agonist. Based on structural models of the ERRγ LBD we were able to introduce mutations that modulated the agonistic effect of equol. Finally, the growth inhibitory effect of ERRγ on the prostate cancer cell line PC-3 was enhanced by equol. In conclusion, our data suggest that ERRγ is involved in mediating the biological effects of equol.
Materials and methods
Results
Discussion A major step in defining the biological functions of an orphan nuclear receptor is the identification of ligands that can be used to modulate its activity in cell cultures and in vivo. Identification of physiological orphan receptor ligands provides also information about the signaling pathways the receptor is involved in. Recently, several synthetic ERR ligands have been identified. Studies utilizing the synthetic ERRα inverse agonist XCT790 facilitated the elucidation of the role of ERRα as a regulator of oxidative phosphorylation [42], [43]. Synthetic ligands modulating the activities of ERRβ and ERRγ have been recently identified but the biological functions of these receptors are not as well characterized. The identified ERRγ ligands such as 4-OHT and DES imply, however, that ERRγ potentially impinges on estrogen signaling pathways. In addition, the endocrine disruptor bisphenol A (BPA) which is an estrogenic chemical with only a weak binding ability to ERs was recently reported to bind to ERRγ and prevent 4-OHT from antagonizing the intrinsic transcriptional activity of ERRγ [44]. Most of the identified ERR ligands act as inverse agonists. However, the activities of ERRs can also be stimulated by ligand binding as indicated by the identification of the synthetic ERRβ/γ agonists GSK4716 and DY131 [14], [16], [45].