Osteoclasts are bone-resorbing, multinucleated giant cells that are essential for bone remodeling and are formed through cell fusion of mononuclear precursor cells. Although receptor activator of nuclear factor–κB ligand (RANKL) has been demonstrated to be an important osteoclastogenic cytokine, the cell surface molecules involved in osteoclastogenesis are mostly unknown. Here, we report that the seven-transmembrane receptor-like molecule, dendritic cell–specific transmembrane protein (DC-STAMP) is involved in osteoclastogenesis. Expression of DC-STAMP is rapidly induced in osteoclast precursor cells by RANKL and other osteoclastogenic stimulations. Targeted inhibition of DC-STAMP by small interfering RNAs and specific antibody markedly suppressed the formation of multinucleated osteoclast-like cells. Overexpression of DC-STAMP enhanced osteoclastogenesis in the presence of RANKL. Furthermore, DC-STAMP directly induced the expression of the osteoclast marker tartrate-resistant acid phosphatase. These data demonstrate for the first time that DC-STAMP has an essential role in osteoclastogenesis.

It has been reported that inositol hexakisphosphate (InsP ₆ , phytic acid), a natural product, has an anticancer role. However, there is inadequate information regarding the mechanism by which InsP ₆ exerts anticancer actions, and the effect requires relatively high concentration of the agent, both of which hinders the usage of InsP ₆ as an anticancer drug. In the present study, we investigated the mechanism by which InsP ₆ acts as an anticancer agent, and tried to reduce the concentration of effective InsP ₆ . Treatment of HeLa cells with InsP ₆ at 1 mM induced apoptosis, as assessed by counting the cell number, and by Hoechst and TUNEL staining. This is probably mediated by intracellular InsP ₆ itself and/or the dephosphorylated forms of metabolized InsP ₆ , because incubation of HeLa cells with [ ³ H]InsP ₆ produces dephosphorylated forms such as InsP ₄ and InsP ₅ . Induction of apoptosis by InsP ₆ was examined in two ways: inhibition of cell survival signaling and direct induction of apoptosis. Treatment of HeLa cells with tumor necrosis factor (TNF) or insulin stimulated the Akt-nuclear factor κB (NFκB) pathway, a cell survival signal, which involves the phosphorylation of Akt and IκB, nuclear translocation of NFκB and NFκB-luciferase transcription activity. InsP ₆ blocked all these cellular events, but phosphatidylinositol 3-kinase activity was not affected. As well as inhibiting the Akt-NFκB pathway, InsP ₆ itself caused mitochondrial permeabilization, followed by cytochrome c release, which later caused activation of the apoptotic machinery, caspase 9, caspase 3 and poly (ADP-ribose) polymerase. When InsP ₆ was applied together with histone, the effective concentration to induce apoptosis was ~10-fold lower. These results revealed that extracellularly applied InsP ₆ directly activates the apoptotic machinery as well as inhibits the cell survival signaling, probably by the intracellular delivery followed by a dephosphorylation.