INTERACTIONS BETWEEN IPRIFLAVONE AND THE ESTROGEN-RECEPTOR

Estrogen replacement therapy is effective in the prevention of postmenopausal osteoporosis, and a direct action of 17-beta-estradiol (17 beta E(2)) on osteoblastic and osteoclastic cells has been demonstrated. The inhibition of bone resorption by ipriflavone (IP), an isoflavone derivative devoid of estrogenic properties but active in potentiating the effects of estrogen on bone tissue, has been shown in in vitro and in vivo studies and confirmed by clinical data. To investigate the molecular mechanisms that underlie IP effect, we studied the possible interactions of IP and its four main in vivo metabolites (I, II, III, and V) with the estrogen receptor (ER) in the human preosteoclastic cell line FLG 29.1, whose growth and function are modulated by the compound. In parallel experiments, the human breast cancer cell line MCF7 was also analyzed. IP binding sites were demonstrated in the nuclear fraction of FLG 29.1 cells. 17 beta E(2) and other steroid compounds failed to displace IP binding to intact FLG 29.1 cells. Similarly, IP and metabolites I, III, and V were not able to displace 17 beta E(2) binding to intact MCF7 cells, whereas metabolite II showed an IC50 of 61 nM. 17 beta E(2) binding to FLG 29.1 cells was increased after preincubation with metabolites I, III, and V. IP and its metabolites did not induce ER-dependent gene expression in FLG 29.1 and MCF7 cells transfected with a reporter gene and an estrogen response element (ERE). These results suggest that IP effects on osteoclast precursors are not mediated by a direct interaction with the ER, even if a crosstalk between the mechanisms of action of IP and 17 beta E(2) cannot be excluded.