Novel ipriflavone receptors coupled to calcium influx regulate osteoclast differentiation and function

Ipriflavone (7-isopropoxyisoflavone) is an effective antiresorptive agent used to treat osteoporosis. However, the mechanism of its action on osteoclasts and their precursor cells is not well understood. To determine whether the mechanism involves direct effects on osteoclasts or their precursors, we examined the effects of ipriflavone on cytosolic free calcium ([Ca2+]i) in osteoclasts and their precursors and measured specific binding of 3H-labeled ipriflavone. Highly purified chicken osteoclast precursors, which spontaneously differentiate into multinucleated osteoclasts in 3-6 days, were loaded with fura-2, and the subcellular [Ca2+]i distribution was monitored by videoimaging. Ipriflavone induced a rapid increase in [Ca2+]i followed by a sustained elevation [EC50 = 5 x 10(-7) M, 263 +/- 74 nM (SE) (n = 8) above basal levels, by 10(-6) M ipriflavone, sustained phase]. The responses were the same in differentiated chicken osteoclasts and isolated rabbit osteoclasts. An influx of extracellular Ca2+ is likely to be responsible for the ipriflavone-induced change in [Ca2+]i because the response was abolished by 0.5 mM LaCl3, or by Ca-free medium containing EGTA. Moreover, high [Ca2+]i levels were detected adjacent to the cell membrane after ipriflavone addition. Ipriflavone induced Ca influx mainly through dihydropyridine-insensitive Ca2+ channels, because nicardipine (10(-7)M) and verapamil (10(-7)M) had no effects on ipriflavone-induced [Ca2+]i responses. [3H]Ipriflavone binding studies indicated the presence of specific ipriflavone binding sites (two classes), both in precursor cells [dissociation constant (Kd), 7.60 x 10(-8)M, 2.67 x 10(-6)M] and in mature osteoclasts (Kd, 4.98 x 10(-8)M, 3.70 x 10(-6)M). Specific ipriflavone binding was not displaced by various modulators of avian osteoclast function, such as estradiol (10(-8)M) or retinoic acid (10(-6)M), indicating that ipriflavone receptors differ from the receptors for these Ca-regulating hormones. The fusion of osteoclast precursor cells was significantly inhibited by ipriflavone, which led to dose-dependent inhibition of bone resorption and tartrate-resistant acid phosphatase activity. Novel specific ipriflavone receptors that are coupled to Ca2+ influx were demonstrated in osteoclasts and their precursor cells. These ipriflavone receptors may provide a mechanism to regulate osteoclast differentiation and function.