Biol. Pharm. Bull. 28(10) 1948—1953 (2005)

most common cause of age-related bone loss. Hormone replacement therapy (HRT) can resolve most postmenopausal problems. However, compliance with HRT is poor because of risks of breast and endometrial cancers associated with longterm use of HRT. In an effort to search for an alternative treatment, potential health benefits of phytoestrogens have been suggested. Phytoestrogens are naturally occurring plant-derived nonsteroidal estrogens which are present in the human diet. Their chemical structure is similar to that of estrogen, what enables them to bind the estrogen receptor thus acting as estrogen agonists or antagonists. Isoflavonoids are a group of natural plant substances that share structural similarity to natural animal estrogens, such as estradiol, and exhibit affinity for the estrogen receptor (ER). Dietary consumption of isoflavones has been linked to lower risks for breast cancer and prostate cancer, as well as protection from osteoporosis and post-menopausal “hot flashes”. Biochanin A, one of the major isoflavonoids in pasture legumes is a methoxylated isoflavone not present in soy foods, but rather is the major isoflavone constituent in red clover and in commercially available extracts of this plant. The presence of biochanin A in mammalian cell cultures reduced the metabolism of benzo[a]pyrene (B[a]P) to mutagenic intermediates by 54% and the binding of metabolites to DNA by 30% to 40%. In vivo carcinogenic studies in a mouse B[a]P-induced lung tumor model revealed that biochanin A significantly reduced the incidence of tumor as well as the mean number of tumors. Biochanin A showed transactivation potentials at 10 6 M concentrations and had efficiencies of 51% compared to 17b-estradiol (efficiency 100%) in yeast system containing human ER. In the HepG2 cell based transactivation assay, biochanin A had efficiencies of 150% compared to 17b-estradiol (efficiency 100%). Also, biochanin A that activate the estrogen receptor in both yeast and mammalian cells bind efficiently to hER. Normal bone remodeling is achieved by a balance of bone formation and bone resorption. These processes are closely regulated and are under the control of both systemic hormones as well as locally derived cytokines. The inflammatory cytokine tumor necrosis factor (TNF) have been shown to exert complex effects on bone remodeling. As the bone cell responsible for bone formation, the osteoblast appears to be the bone cell targeted by TNF-a , in mediating their inhibitory effect on bone formation. However, it is now clearly established that the process of osteoclastic bone resorption mediated by TNF-a is also dependent on the coexistence of osteoblasts, suggesting that TNF-a stimulate the release of soluble factors from osteoblasts, which, in turn, stimulate osteoclasts to resorb bone. MC3T3-E1 cells, an osteoblast-like cell line, have been reported to retain the capacity to differentiate into osteoblasts and to have both estrogen receptors (ER) a and b . Those cells may provide very useful information about the effects of phytoestrogens on the differentiation of osteoblasts. In the present study, the in vitro effect of biochanin A on the activity of osteoblasts and the production of bone-resorbing mediators in osteoblasic MC3T3-E1 cells was investigated in order to determine the possible bioactivities of biochanin A on bone metabolism.