Fas receptor is required for estrogen deficiency-induced bone loss in mice

Bone mass is determined by bone cell differentiation, activity and death, which mainly occur through apoptosis. Apoptosis can be triggered by death receptor Fas (CD95), expressed on osteoblasts and osteoclasts and may be regulated by estrogen. We have previously shown that signaling through Fas inhibits osteoblast differentiation. We here investigate Fas as a possible mediator of bone loss induced by estrogen withdrawal. Four weeks after ovariectomy (OVX), Fas gene expression was greater in osteoblasts and lower in osteoclasts from ovariectomized C57BL/6J (wild-type, wt) mice compared to sham-operated animals. OVX was unable to induce bone loss in mice with a gene knockout for Fas (Fas −/− mice). The number of osteoclasts increased in wt mice after OVX, while they remained unchanged in Fas −/− mice. OVX induced greater stimulation of osteoblastogenesis in Fas −/− than in wt mice, with higher expression of osteoblast specific genes. Direct effects on bone-cell differentiation and apoptosis in vivo were confirmed in vitro, where addition of estradiol decreased Fas expression and partially abrogated the apoptotic and differentiation-inhibitory effect of Fas in osteoblast lineage cells, while having no effect on Fas-induced apoptosis in osteoclast lineage cells. In conclusion, the Fas receptor has an important role in the pathogenesis of postmenopausal osteoporosis by mediating apoptosis and inhibiting differentiation of osteoblast lineage cells. Modulation of Fas effects on bone cells may be used as a therapeutic target in the treatment of osteoresorptive disorders.