Han Bok Kwak

α-Lipoic Acid Inhibits Inflammatory Bone Resorption by Suppressing Prostaglandin E2 Synthesis

α-Lipoic acid (LA) has been intensely investigated as a therapeutic agent for several pathological conditions, including diabetic polyneuropathy. In the present study, we examined the effects of LA on osteoclastic bone loss associated with inflammation. LA significantly inhibited IL-1-induced osteoclast formation in cocultures of mouse osteoblasts and bone marrow cells, but LA had only a marginal effect on osteoclastogenesis from bone marrow macrophages induced by receptor activator of NF-κB ligand (RANKL). LA inhibited both the sustained up-regulation of RANKL expression and the production of PGE2 induced by IL-1 in osteoblasts. In addition, treatment with either prostaglandin E2 (PGE2) or RANKL rescued IL-1-induced osteoclast formation inhibited by LA or NS398, a specific cyclooxygenase-2 (COX-2) inhibitor, in cocultures. LA blocked IL-1-induced PGE2 production even in the presence of arachidonic acid, without affecting the expression of COX-2 and membrane-bound PGE2 synthase. Dihydrolipoic acid (the reduced form of LA), but not LA, attenuated recombinant COX-2 activity in vitro. LA also inhibited osteoclast formation and bone loss induced by IL-1 and LPS in mice. Our results suggest that the reduced form of LA inhibits COX-2 activity, PGE2 production, and sustained RANKL expression, thereby inhibiting osteoclast formation and bone loss in inflammatory conditions.

Tanshinone IIA inhibits osteoclast differentiation through down-regulation of c-Fos and NFATc1

Bone is a dynamic tissue that is regulated by the activity of bone-resorbing osteoclasts and bone-forming osteoblasts. Excessive osteoclast formation causes diseases such as osteoporosis and rheumatoid arthritis. Natural substances may be useful as therapeutic drugs to prevent many diseases in humans because they avoid the many side effects of treatment with chemical compounds. Here we show that tanshinone IIA isolated from Salvia miltiorrhiza Bunge inhibits the receptor activator of NF-κB ligand (RANKL)-mediated osteoclast differentiation of osteoclast precursors. Tanshinone IIA suppressed the expression levels of c-Fos and NFATc1 induced by RANKL. However, retrovirus-mediated overexpression of c-Fos induced the expression of NFATc1 despite the presence of tanshinone IIA and reversed the inhibitory effect of tanshinone IIA on osteoclast differentiation. Also, the introduction of osteoclast precursors with the NFATc1 retrovirus led to osteoclast differentiation in the presence of tanshinone IIA. Our results suggest that tanshinone IIA may have a role as a therapeutic drug in the treatment of bone disease such as osteoporosis.

Stimulation by TLR5 Modulates Osteoclast Differentiation through STAT1/IFN-β

Osteoclasts are bone-resorbing cells that are differentiated from hemopoietic precursors of the monocyte-macrophage lineage. Stimulation of TLRs has been shown to positively or negatively modulate osteoclast differentiation, depending on the experimental condition. However, the molecular mechanism by which this modulation takes place remains unclear. In the present study, we examined the effects of flagellin, a specific microbial ligand of TLR5, on the receptor activator of NF-κB ligand (RANKL)-stimulated osteoclastogenesis. Flagellin suppressed RANKL induction of c-Fos protein expression in bone marrow-derived macrophages without affecting c-Fos mRNA expression. Ectopic overexpression of c-Fos and a constitutively active form of NFATc1 reversed the flagellin-induced anti-osteoclastogenic effect. The inhibitory effect of flagellin was mediated by IFN-β production. Flagellin stimulated IFN-β expression and release in bone marrow-derived macrophages, and IFN-β-neutralizing Ab prevented the flagellin-induced c-Fos down-regulation and the anti-osteoclastogenic effect. IFN-β gene induction by flagellin, LPS, or RANKL was dependent on STAT1 activation. Treatment with flagellin or RANKL stimulated STAT1 activation, and STAT1 deficiency or the JAK2 inhibitor AG490 dramatically prevented IFN-β induction in response to flagellin or RANKL. In addition, STAT1 deficiency abolished the anti-osteoclastogenic effect induced by flagellin or LPS. In contrast, flagellin stimulated osteoclast differentiation in cocultures of osteoblasts and bone marrow cells without inducing IFN-β. Thus, IFN-β acts as a critical modulator of osteoclastogenesis in response to TLR5 activation.

Lipid rafts are important for the association of RANK and TRAF6.

Rafts, cholesterol- and sphingolipid-rich membrane microdomains, have been shown to play an important role in immune cell activation. More recently rafts were implicated in the signal transduction by members of the TNF receptor (TNFR) family. In this study, we provide evidences that the raft microdomain has a crucial role in RANK (receptor activator of NF-κB) signaling. We found that the majority of the ectopically expressed RANK and substantial portion of endogenous TRAF2 and TRAF6 were detected in the low-density raft fractions. In addition, TRAF6 association with rafts was increased by RANKL stimulation. The disruption of rafts blocked the TRAF6 translocation by RANK ligand and impeded the interaction between RANK and TRAF6. Our observations demonstrate that proper RANK signaling requires the function of raft membrane microdomains.