Heat shock protein 60 causes osteoclastic bone resorption via toll-like receptor-2 in estrogen deficiency

Estrogen deficiency leads to marked increases in osteoclastic bone resorption, but the exact mechanism is unclear. Proteomic analysis was performed on the femur and tibia of ovariectomy (OVX) and sham-operated Sprague–Dawley rats using two-dimensional gel electrophoresis (2-DE) and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectrometer (MS). Among the nine proteins differentially expressed between OVX and sham-operated rats, heat shock protein 60 (HSP60) was upregulated by 2.6-fold in the bones of OVX rats, and the plasma concentration of HSP60 was also significantly increased in OVX rats. Estrogen deficiency increases in secretions of interleukin (IL)-1β and tumor necrosis factor (TNF)-α in T cell and osteoclasts (OCs) lineages, IL-1β and TNF-α stimulated the production and secretion of HSP60 from OCs lineages. IL-1 receptor antagonist (ra), TNF-blocking antibody (Ab), and estradiol (E2) significantly suppressed the OVX-induced increase in plasma concentrations of HSP60 in mice. HSP60 potentiated OC formation and bone resorption, and pretreatment with HSP60-blocking Ab markedly reduced the potentiation of OC formation and bone resorption by IL-1β- and TNF-α. HSP60 upregulated the expression levels of toll-like receptor (TLR)-2 in bone marrow macrophage (BMMϕ), and pretreatment with a TLR-2-blocking Ab almost completely inhibited HSP60- or cytokine-induced potentiation of OC formation and/or bone resorption. In conclusion, HSP60 and TLR-2 are novel mediators of estrogen-deficiency-induced bone loss.