The estrogen 17beta-estradiol and phytoestrogen genistein mediate differential effects on osteoblastic NF-kappaB activity.

Estrogen (17beta-estradiol) and genistein, a phytoestrogen, are both endowed with anabolic activities on bone in vivo and stimulate osteoblastic differentiation and mineralization in vitro. However, the mechanisms by which these agents promote osteoblastic differentiation and bone anabolic responses are multifactorial and only partly understood. Recently, the NF-kappaB signal transduction pathway was implicated as a negative regulator of osteoblastic differentiation and suppression of this pathway leads to osteoblastic differentiation and mineralization in vitro. To examine whether estrogen and/or genistein regulate osteoblast differentiation by modulating the NF-kappaB pathway, we examined the effect of 17beta-estradiol and genistein on basal and TNFalpha-stimulated NF-kappaB activity in the preosteoblastic cell line MC3T3. MC3T3 cells were transiently transfected with an NF-kappaB responsive luciferase reporter and cultured for 24 h with either vehicle, or physiological doses of 17beta-estradiol (10(-9) to 10(-7) M), or genistein (10(-6) to 10(-5) M). Our data reveal that while 17beta-estradiol had no effect on basal NF-kappaB activity in MC3T3 cells, it significantly antagonized NF-kappaB activity induced by TNFalpha (1 or 10 ng/ml). By contrast, genistein (10(-6) or 10(-5) M) significantly increased NF-kappaB activity, and showed no antagonistic effects on TNFalpha-induced NF-kappaB promoter activity. These studies suggest that the estrogenic compounds, 17beta-estradiol and genistein, mediate very different actions on osteoblastic cells. While 17beta-estradiol may stimulate bone anabolism, in part, by antagonizing TNFalpha-induced NF-kappaB activation, genistein not only fails to prevent cytokine-induced NF-kappaB activation, but directly promotes NF-kappaB activation in MC3T3 cells. These data suggest important mechanistic differences in the mechanisms by which 17beta-estradiol and genistein promote osteoblast differentiation.