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A Select Combination of Clinically Relevant Phytoestrogens Enhances Estrogen Receptor β-Binding Selectivity and Neuroprotective Activities in Vitro and in Vivo

Department of Pharmacology and Pharmaceutical Sciences (L.Z., Z.M., R.D.B.), School of Pharmacy, and Neuroscience Program (R.D.B.), University of Southern California, Los Angeles, California 90089

Address all correspondence and requests for reprints to: Liqin Zhao, Department of Pharmacology and Pharmaceutical Sciences, School of Pharmacy, University of Southern California, 1985 Zonal Avenue, Los Angeles, California 90089. E-mail: liqinz{at}usc.edu.

We have previously shown that a number of naturally occurring phytoestrogens and derivatives were effective to induce some measures of neuroprotective responses but at a much lower magnitude than those induced by the female gonadal estrogen 17β-estradiol. In the present study, we sought to investigate whether a combination of select phytoestrogens could enhance neural responses without affecting the reproductive system. We performed a range of comparative analyses of the estrogen receptor (ER) /β binding profile, and in vitro to in vivo estrogenic activities in neural and uterine tissues induced by clinically relevant phytoestrogens: genistein, daidzein, equol, and IBSO03569, when used alone or in combination. Our analyses revealed that both the ER/β binding profile and neural activities associated with individual phytoestrogens are modifiable when used in combination. Specifically, the combination of genistein plus daidzein plus equol resulted in the greatest binding selectivity for ERβ and an overall improved efficacy/safety profile when compared with single or other combined formulations, including: 1) an approximate 30% increase in ERβ-binding selectivity (83-fold over ER); 2) a greater effect on neuronal survival against toxic insults in primary neurons; 3) an enhanced activity in promoting neural proactive defense mechanisms against neurodegeneration, including mitochondrial function and β-amyloid degradation; and 4) no effect on uterine growth. These observations suggest that select phytoestrogens in combination have the therapeutic potential of an alternative approach to conventional estrogen therapy for long-term safe use to reduce the increased risk of cognitive decline and neurodegenerative disease associated with menopause in women.