Estrogen replacement therapy is effective in the prevention of postmen
opausal osteoporosis, and a direct action of 17-beta-estradiol (17 bet
a E(2)) on osteoblastic and osteoclastic cells has been demonstrated.
The inhibition of bone resorption by ipriflavone (IP), an isoflavone d
erivative devoid of estrogenic properties but active in potentiating t
he effects of estrogen on bone tissue, has been shown in in vitro and
in vivo studies and confirmed by clinical data. To investigate the mol
ecular mechanisms that underlie IP effect, we studied the possible int
eractions 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 fract
ion 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 me
tabolites 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 preincubati
on with metabolites I, III, and V. IP and its metabolites did not indu
ce ER-dependent gene expression in FLG 29.1 and MCF7 cells transfected
with a reporter gene and an estrogen response element (ERE). These re
sults suggest that IP effects on osteoclast precursors are not mediate
d 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.