ESTROGEN DEFICIENCY INCREASES THE ABILITY OF STROMAL CELLS TO SUPPORTMURINE OSTEOCLASTOGENESIS VIA AN INTERLEUKIN-1-MEDIATED AND TUMOR NECROSIS FACTOR-MEDIATED STIMULATION OF MACROPHAGE-COLONY-STIMULATING FACTOR PRODUCTION

To analyze how estrogen blocks osteoclastogenesis, we investigated the effects of ovariectomy on osteoclast (OC) formation in co-cultures of purified OC precursors and purified stromal cells (SC). OC formation was higher in co-cultures containing SC from ovariectomized mice than in those containing SC from sham-operated mice, thus suggesting that e strogen regulates osteoclastogenesis by targeting SC. Ovariectomy also increased the mononuclear cell secretion of interleukin (IL)-1) and t umor necrosis factor (TNF) and the SC production of macrophage colony- stimulating factor (M-CSF), Osteoclastogenesis and SC production of M- CSF were not blocked by in vitro estrogen treatment but were decreased by in viva treatment of donor mice with either estrogen or a combinat ion of the IL-1 inhibitor, IL-1 receptor antagonist, and the TNF inhib itor, TNF binding protein. IL-1 and TNF production were also blocked b y in vivo estrogen treatment, demonstrating that the increased bone ma rrow levels of IL-1 and TNF characteristic of ovariectomized mice indu ce the formation of a SC population characterized by a high production of M-CSF and increased pro-osteoclastogenic activity. Since in co-cul tures of SC and OC precursors M-CSF levels correlated with OC producti on (r = 0.7, p < 0.0001), the data also indicate that the pro-osteocla stogenic activity of SC is proportional to their secretion of M-CSF. T he ability of estrogen to decrease SC production of M-CSF and the pro- osteoclastogenic activity of these cells by regulating IL-1 and TNF pr oduction is a previously undescribed mechanism by which estrogen do cv n-regulates osteoclastogenesis.