Endogenous bone-resorbing factors in estrogen deficiency: Cooperative effects of IL-1 and IL-6

Abstract

Estrogen deficiency causes a marked bone loss by stimulating osteoclastic bone resorption. To explore the endogenous bone-resorbing factors involved in estrogen deficiency, we examined the bone-resorbing activity present in the supernatant fraction of mouse bone marrow collected from ovariectomized (OVX) mice. Adding bone marrow supernatants at 20–80% to organ cultures of mouse long bones dose-dependently stimulated bone resorption. The endogenous bone-resorbing activity present in bone marrow supernatants from OVX mice was much higher than that from sham-operated mice 2–4 weeks after surgery, and it was significantly diminished by indomethacin in vitro. Anti-IL-1α antibody completely neutralized the bone-resorbing activity present in bone marrow supernatants from OVX mice. Antibodies against IL-1β, IL-6, and IL-6 receptors also neutralized it, but partially. The concentration of IL-1α measured by ELISA was much higher in bone marrow supernatants than in sera, but it was not appreciably changed before or after OVX. The concentration of IL-1β in bone marrow supernatants from OVX mice was less than the detection limit. OVX stimulated IL-1 activity in bone marrow supernatants measured by means of the proliferation of thymocytes. However, the level of IL-1α present in bone marrow supernatants from OVX mice was insufficient to stimulate bone resorption. Compared with the serum concentration, bone marrow supernatants contained a much higher level of IL-6 as well, and it was further increased by OVX. However, IL-6 alone present in bone marrow supernatants from OVX mice again did not stimulate bone resorption. The concurrent addition of IL-1 (50 pg/ml), IL-6 (0.2 ng/ml), sIL-6 receptor (1 ng/ml), and PGE2 (7 ng/ml), which equaled the endogenous concentrations in bone marrow supernatants from OVX mice, co-operatively induced bone resorption. These results suggest that the enhanced bone resorption that occurs during estrogen deficiency is due to multi-factors rather than to a single factor.