Osteoprotegerin (OPG) is a recently identified member of the tumor necrosis factor (TNF) receptor superfamily that regulates bone mass through an inhibitory action on osteoclast differentiation and function. To determine its potential roles of OPG in pathological changes in bone metabolism caused by estrogen deficiency, we investigated effects of estrogen on OPG expression by a mouse stromal cell line, ST-2, in vitro. Treatment of ST-2 cells with 17β-E₂ resulted in up-regulation of OPG expression at both the messenger RNA and protein levels. The effect was time and dose dependent and steroid specific. The stimulatory action of 17β-E₂ on OPG expression appeared to be mediated by the estrogen receptor-α (ERα) subtype because stable overexpression of ERα, but not of ERβ, enhanced the OPG induction by 17β-E₂. Moreover, estrogen withdrawal after 5-day pretreatment, mimicking the event occurring in vivo at menopause, dramatically diminished the expression of OPG. These findings suggest that down-regulation of OPG after estrogen withdrawal contributes to the enhanced osteoclastic bone resorption and bone loss after menopause by enhancing RANK ligand-RANK system that lies downstream of a large number of bone-resorbing cytokines.