The soy phytoestrogen daidzein has biphasic dose responses, but the underlying mechanisms are not yet clear. Transcriptional and biochemical data show that PPARs, in addition to ERs, are molecular targets of daidzein, which divergently regulates osteogenesis and adipogenesis. Dose responses are the result of a balance among PPARs and between ERs and PPARs.
Introduction: Soy phytoestrogens have been used for the purposes of treatment and prevention of osteoporosis. Biphasic dose responses of daidzein, one of the main soy phytoestrogens, have long been recognized, but the underlying molecular mechanisms of action are not yet clear.
Materials and methods: Mouse bone marrow cells and mouse osteoprogenitor KS483 cells that concurrently differentiate into osteoblasts and adipocytes were cultured. Biochemical measurement of alkaline phosphatase (ALP) activity, RT-PCR, and gene reporter assays were used in this study.
Results: Daidzein, one of the major soy phytoestrogens, had biphasic effects on osteogenesis and adipogenesis. Daidzein stimulated osteogenesis (ALP activity and nodule formation) and decreased adipogenesis (the number of adipocytes) at concentrations below 20 microM, whereas it inhibited osteogenesis and stimulated adipogenesis at concentrations higher than 30 microM. When estrogen receptors (ERs) were blocked by ICI182,780, daidzein-induced effects were not biphasic. A decrease in osteogenesis and an increase in adipogenesis were observed at the concentrations higher than 20 and 10 microM, respectively. In addition to ERs, daidzein transactivated not only peroxisome proliferator-activate receptor gamma (PPARgamma), but also PPARalpha and PPARdelta at micromolar concentrations. Activation of PPARalpha had no direct effects on osteogenesis and adipogenesis. In contrast, activation of PPARdelta stimulated osteogenesis but had no effects on adipogenesis, whereas PPARgamma inhibited osteogenesis and stimulated adipogenesis. Transfection experiments show that an activation of PPARalpha or PPARgamma by daidzein downregulated its estrogenic transcriptional activity, whereas activation of PPARdelta upregulated its estrogenic transcriptional activity. Activation of ERalpha or ERbeta by daidzein downregulated PPARgamma transcriptional activity but had no influence on PPARalpha or PPARdelta transcriptional activity.
Conclusions: Daidzein at micromolar concentrations concurrently activates different amounts of ERs and PPARs, and the balance of the divergent actions of ERs and PPARs determines daidzein-induced osteogenesis and adipogenesis.