Reciprocal control of expression of mRNAs for osteoclast differentiation factor and OPG in osteogenic stromal cells by genistein: Evidence for the involvement of topoisomerase II in osteoclastogenesis

Osteoclast-like cells, in cocultures with mouse spleen cells and clonal ost eogenic stromal ST2 cells, are formed from spleen cells with monocyte/macro phage lineage in response to a combination of osteoclast differentiation fa ctor (RANKL) and OPG, a decoy receptor for RANKL, produced by ST2 cells in response to 1 alpha ,25-dihydroxyvitamin D-3. Treatment of ST2 cells with t he natural isoflavonoid genistein for 6 h before coculture with spleen cell s inhibited the formation of tartrate-resistant acid phosphatase-positive o steoclast-like cells. When we measured levels of RANKL mRNA in ST2 cells, w e found that genistein decreased the level of this mRNA. By contrast, the l evel of OPG mRNA was enhanced by genistein. Genistein is a specific inhibit or of topoisomerase II (topo II) and an inhibitor of protein tyrosine kinas e, as well as being a potent phytoestrogen. To characterize the mode of act ion of genistein, we examined the effects of an inactive form of genistein (daidzein), 17 beta -estradiol, inhibitors of topo II, and inhibitors of ty rosine kinases on the formation of tartrate-resistant acid phosphatase-posi tive osteoclast-like cells. Among the compounds tested, two inhibitors of t opo II, amsacrine and etoposide, attenuated the formation of osteoclast-lik e cells via reciprocal regulation of the expression of mRNAs for RANKL and OPG in ST2 cells, acting similarly to genistein. Our findings indicate that genistein might inhibit the formation of osteoclast-like cells via inhibit ion of the activity of topo II, suggesting the novel possibility that topo II might play an important role in osteoclastogenesis.