RANK lacks intrinsic enzymatic activity in its intracellular domain, and it transduces signaling by recruiting adaptor molecules such as the TRAF family of proteins [8]. Genetic experiments

show that TRAF6 is required for osteoclast formation and osteoclast activation [30]. The binding of RANKL to its receptor RANK recruits TRAF6 and subsequently initiates a kinase cascade. RT-PCR analysis shows that kinsenoside did not reduce the RANKL-induced mRNA expression of RANK and TRAF-6, indicating that kinsenoside inhibits NF-κB activation through downstream kinase to TRAF6. The classical NF-κB Selleck Foretinib signaling pathway involves the activation of the IKK complex, which phosphorylates IκBα and targets them for ubiquitin-dependent degradation [8]. In the alternative IκB-independent pathway, direct phosphorylation of NF-κB subunit p65 by IKK also modulates NF-κB transcription activity [31]. In this study, kinsenoside inhibited RANKL-induced NF-κB activation see more in RAW 264.7 cells by inhibiting p-IκBα and p-p65. This indicates that kinsenoside inhibited NF-κB translocation through both IκBα-dependent and IκBα-independent pathways. IKK is the major upstream kinase of IκBα in the NF-κB signaling pathway. In this study, kinsenoside

did not inhibit IKK phosphorylation but suppressed the phosphorylation of IκBα and p65. Therefore, this study also investigates the effects of kinsenoside on IKK activity. Results show that kinsenoside significantly inhibits RANKL induction of IKK activity, suggesting that IKK is a critical target for kinsenoside in inhibiting RANKL-induced osteoclastogenesis. NFATc1 is likely a key regulator of RANKL-induced osteoclast differentiation, fusion, and activation [10].

NF-κB is important for the second initial induction of NFATc1. The binding of NF-κB to the NFATc1 promoter region induces NFATc1 gene expression, allowing NFATc1 to autoamplify its expression by binding to its own promoter. This, in turn, leads to the robust induction of NFATc1 during RANKL-induced osteoclast differentiation [32]. In this study, kinsenoside significantly suppressed RANKL-induced NF-κB translocation and NFATc1 nuclear transport. NFATc1 promotes the expression of osteoclast-specific genes such as TRAP, DC-STAMP, CAK, and MMP-9 [33–35]. In addition to histochemical marker for osteoclasts, TRAP also regulates bone resorption by mediating the degradation of endocytosed matrix products during transcytosis in activated osteoclasts [36]. DC-STAMP, a putative seven-transmembrane spanning protein, is essential for the cell–cell fusion of osteoclasts [37]. Proteinases are necessary for bone resorption. Delaisse et al. showed that CAK and MMP-9 are key proteinases in the bone resorption processes [38]. The RT-PCR analysis in this study shows that kinsenoside dose-dependently suppressed the mRNA expression of TRAP, DC-STAMP, CAK, and MMP-9.