Porphyromonas gingivalis is one of the major pathogens of periodontitis, a condition characterized by excessive alveolar bone resorption by osteoclasts. The bacterium produces cysteine proteases called gingipains, which are classified according to their cleavage-site specificity into lysine-specific (Kgp) and arginine-specific gingipains (Rgps). In this study, we examined the effects of gingipains on osteoclast differentiation. In co-cultures of mouse bone marrow cells and osteoblasts, formation of multinucleated osteoclasts induced by 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] was augmented by Kgp but not by RgpB. A physiological concentration (0.1 nM) of 1α,25(OH)2D3 induced the osteoclast formation in the presence of 100 nM Kgp to the extent comparable to that induced by 10 nM 1α,25(OH)2D3. Kgp also enhanced osteoclastogenesis induced by various microbial components including lipopolysaccharide. Combined use of Kgp and 1α,25(OH)2D3 or lipopolysaccharide also increased the number of resorption pits developed on dentin slices, indicating the osteoclasts formed in the presence of Kgp possess bone-resorbing activity. The enhanced osteoclastogenesis by Kgp was correlated with a depletion of osteoprotegerin in co-culture media and proteolytic activity-dependent, since benzyloxycarbonyl-phenylalanyl-lysyl-acycloxyketone, an inhibitor of Kgp, completely abolished osteoclastogenesis induced by Kgp. Kgp digested osteoprotegerin, since its recombinant protein was susceptible to degradation by Kgp in the presence of serum. As a result, Kgp did not augment osteoclastogenesis in co-cultures of osteoprotegerin-deficient osteoblasts and bone marrow cells. In addition, enhanced osteoclastogenesis by Kgp was abolished by excess amount of recombinant osteoprotegerin. These findings suggest that degradation of osteoprotegerin is one of the mechanisms underlying promotion of osteoclastogenesis by Kgp.
periodontitis; bacterial protease; osteoclastogenesis; bone resorption
R848, also known as resiquimod, acts as a ligand for toll-like receptor 7 (TLR7) and activates immune cells. In this study, we examined the effects of R848 on differentiation, survival, and bone-resorbing function of osteoclasts. R848 inhibited osteoclast differentiation of mouse bone marrow-derived macrophages (BMMs) and human peripheral blood-derived monocytes induced by receptor activator of NF-κB ligand in a dose-dependent manner. In addition, it inhibited mouse osteoclast differentiation induced in cocultures of bone marrow cells and osteoblasts in the presence of dihydroxyvitamin D3 [1,25(OH)2D3]. However, R848 did not affect the survival or bone-resorbing activity of mouse mature osteoclasts. R848 also upregulated the mRNA expression levels of interleukin (IL)-6, IL-12, interferon (IFN)-γ, and inducible nitric oxide synthase in mouse BMMs expressing TLR7. IFN-β was consistently expressed in the BMMs and addition of neutralizing antibodies against IFN-β to the cultures partially recovered osteoclast differentiation inhibited by R848. These results suggest that R848 targets osteoclast precursors and inhibits their differentiation into osteoclasts via TLR7.
R848; Resiquimod; Osteoclasts; Toll-like receptor; Cellular differentiation; Bone
Carbonic anhydrase (CA) IX is a transmembrane isozyme of CAs that catalyzes reversible hydration of CO2. While it is known that CA IX is distributed in human embryonic chondrocytes, its role in chondrocyte differentiation has not been reported. In the present study, we found that Car9 mRNA and CA IX were expressed in proliferating but not hypertrophic chondrocytes. Next, we examined the role of CA IX in the expression of marker genes of chondrocyte differentiation in vitro. Introduction of Car9 siRNA to mouse primary chondrocytes obtained from costal cartilage induced the mRNA expressions of Col10a1, the gene for type X collagen α-1 chain, and Epas1, the gene for hypoxia-responsible factor-2α (HIF-2α), both of which are known to be characteristically expressed in hypertrophic chondrocytes. On the other hand, forced expression of CA IX had no effect of the proliferation of chondrocytes or the transcription of Col10a1 and Epas1, while the transcription of Col2a1 and Acan were up-regulated. Although HIF-2α has been reported to be a potent activator of Col10a1 transcription, Epas1 siRNA did not suppress Car9 siRNA-induced increment in Col10a1 expression, indicating that down-regulation of CA IX induces the expression of Col10a1 in chondrocytes in a HIF-2α-independent manner. On the other hand, cellular cAMP content was lowered by Car9 siRNA. Furthermore, the expression of Col10a1 mRNA after Car9 silencing was augmented by an inhibitor of protein kinase A, and suppressed by an inhibitor for phosphodiesterase as well as a brominated analog of cAMP. While these results suggest a possible involvement of cAMP-dependent pathway, at least in part, in induction of Col10a1 expression by down-regulation of Car9, more detailed study is required to clarify the role of CA IX in regulation of Col10a1 expression in chondrocytes.
Cells with monocyte/macrophage lineage expressing receptor activator of NF-κB (RANK) differentiate into osteoclasts following stimulation with the RANK ligand (RANKL). Cell adhesion signaling is also required for osteoclast differentiation from precursors. However, details of the mechanism by which cell adhesion signals induce osteoclast differentiation have not been fully elucidated. To investigate the participation of cell adhesion signaling in osteoclast differentiation, mouse bone marrow-derived macrophages (BMMs) were used as osteoclast precursors, and cultured on either plastic cell culture dishes (adherent condition) or the top surface of semisolid methylcellulose gel loaded in culture tubes (non-adherent condition). BMMs cultured under the adherent condition differentiated into osteoclasts in response to RANKL stimulation. However, under the non-adherent condition, the efficiency of osteoclast differentiation was markedly reduced even in the presence of RANKL. These BMMs retained macrophage characteristics including phagocytic function and gene expression profile. Lipopolysaccharide (LPS) and tumor necrosis factor –αTNF-α activated the NF-κB-mediated signaling pathways under both the adherent and non-adherent conditions, while RANKL activated the pathways only under the adherent condition. BMMs highly expressed RANK mRNA and protein under the adherent condition as compared to the non-adherent condition. Also, BMMs transferred from the adherent to non-adherent condition showed downregulated RANK expression within 24 hours. In contrast, transferring those from the non-adherent to adherent condition significantly increased the level of RANK expression. Moreover, interruption of cell adhesion signaling by echistatin, an RGD-containing disintegrin, decreased RANK expression in BMMs, while forced expression of either RANK or TNFR-associated factor 6 (TRAF6) in BMMs induced their differentiation into osteoclasts even under the non-adherent condition. These results suggest that cell adhesion signaling regulates RANK expression in osteoclast precursors.
Inflammation plays a key role in excessive bone loss in conditions such as rheumatoid arthritis and periodontitis. An important paradigm in immunology is that inflammatory factors activate feedback inhibition mechanisms to restrain inflammation and limit associated tissue damage. We hypothesized that inflammatory factors would activate similar feedback mechanisms to restrain bone loss in inflammatory settings. We have identified three mechanisms that inhibit osteoclastogenesis and are induced by inflammatory factors, such as toll-like receptor ligands and cytokines: downregulation of expression of costimulatory molecules such as TREM-2; induction of shedding and thereby inactivation of the M-CSF receptor c-Fms, leading to decreased RANK transcription; and induction of transcriptional repressors such as interferon regulatory factor 8. It is likely that these mechanisms work in a complementary and cooperative manner to fine tune the extent of osteoclastogenesis in inflammatory settings, and their augmentation may represent an alternative therapeutic approach to suppress bone resorption.
inflammation; osteoclasts; toll-like receptors; M-CSF; c-Fms; IRF8
Interferon regulatory factor (IRF) 8 and IRF4 are structurally-related, hematopoietic cell-specific transcription factors that cooperatively regulate the differentiation of dendritic cells and B cells. Whilst in myeloid cells IRF8 is known to modulate growth and differentiation, the role of IRF4 is poorly understood. In this study, we show that IRF4 has activities similar to IRF8 in regulating myeloid cell development. The ectopic expression of IRF4 in myeloid progenitor cells in vitro inhibits cell growth, promotes macrophages, but hinders granulocytic cell differentiation. We also show that IRF4 binds to and activates transcription through the IRF-Ets composite sequence (IECS). Furthermore, we demonstrate that Irf8-/-Irf4-/- mice exhibit a more severe chronic myeloid leukemia (CML)-like disease than Irf8-/- mice, involving a disproportionate expansion of granulocytes at the expense of monocytes/macrophages. Irf4-/- mice, however, display no obvious abnormality in myeloid cell development, presumably because IRF4 is expressed at a much lower level than IRF8 in granulocyte-macrophage progenitors. Our results also suggest that IRF8 and IRF4 have not only common but also specific activities in myeloid cells. Since the expression of both the IRF8 and IRF4 genes is downregulated in CML patients, these results may add to our understanding of CML pathogenesis.
Bone metabolism results from a balance between osteoclast-driven bone resorption and osteoblast-mediated bone formation. Diseases such as periodontitis and rheumatoid arthritis are characterized by increased bone destruction due to enhanced osteoclastogenesis1,2. Here we report that interferon regulatory factor 8 (IRF8), a transcription factor expressed in immune cells, is a key regulatory molecule for osteoclastogenesis. IRF8 expression in osteoclast precursors was downregulated during the initial phase of osteoclast differentiation induced by receptor activator of nuclear factor κB ligand (RANKL, also called TRANCE, ODF, and OPGL), which is encoded by the Tnfsf11 gene. Mice deficient in IRF8 exhibited severe osteoporosis due to increased numbers of osteoclasts, and enhanced bone destruction following lipopolysaccharide (LPS) administration. Irf8–/– osteoclast precursors underwent increased osteoclastogenesis in response to RANKL and tumor necrosis factor α (TNFα). IRF8 suppressed osteoclastogenesis by inhibiting the function and expression of nuclear factor of activated T cells c1 (NFATc1). Our results show that IRF8 inhibits osteoclast formation under physiological and pathological conditions, and suggest a model where downregulation of inhibitory factors like IRF8 contributes to RANKL-mediated osteoclastogenesis.
STAT1 mediates Interferon (IFN)-dependent positive and negative regulation of inflammatory gene expression in lung. In this study, we examined the effect of IFN-γ on the expression of SCGB3A1 which is thought to play crucial roles in inflammation and epithelial cell differentiation in lung. We found that expression of SCGB3A1 was down-regulated by IFN-γ in a time- and dose-dependent manner in the murine transformed Clara Cells (mtCC) line. IFN-γ induced the phosphorylation of STAT1, which binds to a STAT-binding element (SBE) in the SCGB3A1 gene promoter, leading to decreased transcriptional activation of this gene.
SCGB3A1; Uteroglobin-related protein 2; IFN-γ
Osteoclasts are derived from myeloid lineage cells, and their differentiation is supported by various osteotropic factors, including the tumor necrosis factor (TNF) family member TNF-related activation-induced cytokine (TRANCE). Genetic deletion of TRANCE or its receptor, receptor activator of nuclear factor κB (RANK), results in severely osteopetrotic mice with no osteoclasts in their bones. TNF receptor-associated factor (TRAF) 6 is a key signaling adaptor for RANK, and its deficiency leads to similar osteopetrosis. Hence, the current paradigm holds that TRANCE–RANK interaction and subsequent signaling via TRAF6 are essential for the generation of functional osteoclasts. Surprisingly, we show that hematopoietic precursors from TRANCE-, RANK-, or TRAF6-null mice can become osteoclasts in vitro when they are stimulated with TNF-α in the presence of cofactors such as TGF-β. We provide direct evidence against the current paradigm that the TRANCE–RANK–TRAF6 pathway is essential for osteoclast differentiation and suggest the potential existence of alternative routes for osteoclast differentiation.
Osteoclasts (OCs) are multinucleated cells that resorb bone and are essential for bone homeostasis. They develop from hematopoietic cells of the myelomonocytic lineage. OC formation requires cell-to-cell interactions with osteoblasts and can be achieved by coculturing bone marrow precursor cells with osteoblasts/stromal cells. Two of the key factors mediating the osteoblast-induced osteoclastogenesis are macrophage–colony stimulating factor (M-CSF) and the tumor necrosis factor (TNF) family member TNF–related activation-induced cytokine (TRANCE) that are produced by osteoblasts/stromal cells in response to various bone resorbing hormones. In addition, other factors produced by osteoblasts/stromal cells further influence osteoclastogenesis. Here we report the identification and characterization of OC-associated receptor (OSCAR), a novel member of the leukocyte receptor complex (LRC)-encoded family expressed specifically in OCs. Genes in the LRC produce immunoglobulin (Ig)-like surface receptors and play critical roles in the regulation of both innate and adaptive immune responses. Different from the previously characterized members of the LRC complex, OSCAR expression is detected specifically in preosteoclasts or mature OCs. Its putative–ligand (OSCAR-L) is expressed primarily in osteoblasts/stromal cells. Moreover, addition of a soluble form of OSCAR in coculture with osteoblasts inhibits the formation of OCs from bone marrow precursor cells in the presence of bone-resorbing factors, indicating that OSCAR may be an important bone-specific regulator of OC differentiation. In addition, this study suggests that LRC-encoded genes may have evolved to regulate the physiology of cells beyond those of the immune system.
osteoclast; Ig-like receptor; TRANCE; differentiation; costimulation
The osteoclasts are bone-resorbing multinucleatedcells formed by the fusion of mononuclearpreosteoclasts (pOCs) of hematopoietic origin.Although receptor activator of NF-κBligand (RANKL) has been shown to regulate osteoclastdifferentiation and function, its effect on the fusionof pOCs into multinucleated osteoclast-like cells(OCLs) has not been known. Using our fusion assaysystem, that is not contaminated with multinucleatedcells (MNCs) and osteoblastic cells, we determined theeffect of RANKL on the fusion of pOCs into MNCs. WhenpOCs were cultured on the plates, most of pOCs diedand disappeared from the plates within 24 h in theabsence of additives, but pOCs were fused to MNCswithin 6 h in the presence of RANKL. RANKL-inducedMNCs showed typical properties of OCL such astartrate-resistant acid phosphatase (TRAP) activity,actin ring formation, and bone-resorbing activity. Thefusion of pOCs into OCLs induced by osteoblastic cellsor RANKL was inhibited by OPG/OCIF, but that inducedby IL-1β was not. Both RANKL- andIL-1β-induced OCL formation from pOCs wasinhibited by ZLLL-H, a peptide inhibitor ofproteasome. These findings indicate that RANKLsupports the survival of pOCs and induces the fusionof pOCs into OCLs and suggest that NF-κBactivation is involved in these processes induced byRANKL and IL-1β.
fusion; IL-1β; NF-κB; osteoclast; RANKL
Osteoclast differentiation factor (ODF, also called RANKL/TRANCE/OPGL) stimulates the differentiation of osteoclast progenitors of the monocyte/macrophage lineage into osteoclasts in the presence of macrophage colony-stimulating factor (M-CSF, also called CSF-1). When mouse bone marrow cells were cultured with M-CSF, M-CSF–dependent bone marrow macrophages (M-BMMφ) appeared within 3 d. Tartrate-resistant acid phosphatase–positive osteoclasts were also formed when M-BMMφ were further cultured for 3 d with mouse tumor necrosis factor α (TNF-α) in the presence of M-CSF. Osteoclast formation induced by TNF-α was inhibited by the addition of respective antibodies against TNF receptor 1 (TNFR1) or TNFR2, but not by osteoclastogenesis inhibitory factor (OCIF, also called OPG, a decoy receptor of ODF/RANKL), nor the Fab fragment of anti–RANK (ODF/RANKL receptor) antibody. Experiments using M-BMMφ prepared from TNFR1- or TNFR2-deficient mice showed that both TNFR1- and TNFR2-induced signals were important for osteoclast formation induced by TNF-α. Osteoclasts induced by TNF-α formed resorption pits on dentine slices only in the presence of IL-1α. These results demonstrate that TNF-α stimulates osteoclast differentiation in the presence of M-CSF through a mechanism independent of the ODF/RANKL–RANK system. TNF-α together with IL-1α may play an important role in bone resorption of inflammatory bone diseases.
bone resorption; tumor necrosis factor receptor; nuclear factor-κB; macrophage colony-stimulating factor; interleukin-1