Inhibition of osteoclast differentiation by gold nanoparticles functionalized with cyclodextrin curcumin complexes. 
Gold nanoparticles (GNPs) have been previously reported to inhibit osteoclast (OC) formation.  However, previous research only confirmed the osteoclastogenesis inhibitory effect under in vitro conditions.  The aim of this study was to develop a therapeutic agent for osteoporosis based on the utilization of GNPs and confirm their effect both in vitro and in vivo.  We prepared β-cyclodextrin (CD) conjugated GNPs (CGNPs), which can form inclusion complexes with curcumin (CUR-CGNPs), and used these to investigate their inhibitory effects on receptor activator of nuclear factor-κb ligand (RANKL)-induced osteoclastogenesis in bone marrow-derived macrophages (BMMs).  The CUR-CGNPs significantly inhibited the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells in BMMs without inducing cytotoxicity.  The mRNA expressions of genetic markers of OC differentiation including c-Fos, nuclear factor of activated T cells 1 (NFATc1), TRAP, and osteoclast associated receptor (OSCAR) were significantly decreased in the presence of CUR-CGNPs.  In addition, the CUR-CGNPs inhibited OC differentiation of BMMs through suppression of the RANKL-induced signaling pathway.  Additionally, CUR-CGNPs caused a decrease in RANKL-induced actin ring formation, which is an essential morphological characteristic of OC formation allowing them to carry out bone resorption activity.  Furthermore, the in vivo results of an ovariectomy (OVX)-induced osteoporosis model showed that CUR-CGNPs significantly improved bone density and prevented bone loss.  Therefore, CUR-CGNPs may prove to be useful as therapeutic agents for preventing and treating osteoporosis.