BMP2 differentially regulates the expression of Gremlin1 and Gremlin2, the negative regulators of BMP function, during osteoblast differentiation

Bone morphogenetic proteins (BMPs) control the expressions of many genes involved in bone formation. On the basis of our hypothesis that BMP2 stimulation-regulated gene expression plays a critical role in osteoblast differentiation, we performed genome-wide screening of messenger RNA from BMP2-treated and -untreated C2C12 cells using a DNA microarray technique. We found that the expressions of Gremlin1 and Gremlin2, which are known BMP antagonists, were bidirectionally regulated by BMP2. Gremlin1 was down-regulated by BMP2, while Gremlin2 was up-regulated in both time- and dose-dependent manners. Ablation of Gremlin1 or Gremlin2 enhanced osteoblast differentiation induced by BMP2. On the other hand, treatment with recombinant Gremlin1 inhibited BMP2-induced osteoblast differentiation. Furthermore, treatment with Smad4 siRNA and the p38 MAPK inhibitor SB203580 suppressed BMP2-induced Gremlin2 gene expression. The differential regulation of Gremlin1 and Gremlin2 gene expressions by BMP2 may explain the critical function of these genes during osteoblast differentiation.

D-chiro-inositol negatively regulates the formation of multinucleated osteoclasts by down-regulating NFATc1

PURPOSE

Osteoclasts (OCs) are multinucleated giant cells that resorb bone matrix. Accelerated bone destruction by OCs might cause several metabolic bone-related diseases, such as osteoporosis and inflammatory bone loss. D-pinitol (3-O-methyl-D-chiro-inositol) is a prominent component of dietary legumes and is actively converted to D-chiro-inositol, which is a putative insulin-like mediator. In this study, we analyzed the effect of D-chiro-inositol on OC differentiation.

METHODS

To analyze the role of D-chiro-inositol on OC differentiation, we examined OC differentiation by the three types of osteoclastogenesis cultures with tartrate-resistant acid phosphatase (TRAP) staining and solution assay. Then, we carried out cell fusion assay with purified TRAP(+) mononuclear OC precursors. Finally, we analyzed the effect of D-chiro-inositol on OC maker expression in response to the regulation of nuclear factor of activated T cells c1 (NFATc1).

RESULTS

We demonstrated that D-chiro-inositol acts as an inhibitor of receptor activator of NF-κB ligand-induced OC differentiation. The formation of multinucleated OCs by cell-cell fusion is reduced by treatment with D-chiro-inositol in a dose-dependent manner. In addition, we demonstrated that D-chiro-inositol inhibits the expression of several osteoclastogenic genes by down-regulating NFATc1.

CONCLUSIONS

We have shown that D-chiro-inositol is negatively involved in osteoclastogenesis through the inhibition of multinucleated OC formation by cell-cell fusion. The expression of NFATc1 was significantly down-regulated by D-chiro-inositol in OCs and consequently, the expression of OC marker genes was significantly reduced. Hence, these results show that D-chiro-inositol might be a good candidate to treat inflammatory bone-related diseases or secondary osteoporosis in diabetes mellitus.

R848, a toll-like receptor 7 agonist, inhibits osteoclast differentiation but not survival or bone-resorbing function of mature osteoclasts

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 D(3) [1,25(OH)(2)D(3)]. 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.

Splenic extramedullary hemopoiesis caused by a dysfunctional mutation in the NF-κB-inducing kinase gene

NF-κB-inducing kinase (NIK) plays critical roles in the development of lymph nodes and Peyer's patches, and microarchitecture of the thymus and spleen via NF-κB activation. Alymphoplasia (aly/aly) mice have a point mutation in the NIK gene that causes a defect in the activation of an NF-κB member RelB. Here, we developed a novel method to determine the aly mutation by genetic typing using PCR. This method facilitated the easy establishment of a congeneic aly/aly mouse line. Indeed, we generated a mouse line with aly mutation on a BALB/cA background (BALB/cA-aly/aly). BALB/cA-aly/aly mice showed significant splenomegaly with extramedullary hemopoiesis, which was not significant in aly/aly mice on a C57BL/6 background. Interestingly, the splenomegaly and extramedullary hemopoiesis caused by the aly mutation was gender-dependent. These data together with previous reports on extramedullary hemopoiesis in RelB-deficient mice suggest that NIK-RelB signaling may be involved in the suppression of extramedullary hemopoiesis in adult mice.

Feedback inhibition of osteoclastogenesis during inflammation by IL-10, M-CSF receptor shedding, and induction of IRF8

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.

Shared and distinct functions of the transcription factors IRF4 and IRF8 in myeloid cell development

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.

Monocarboxylate transporter-1 is required for cell death in mouse chondrocytic ATDC5 cells exposed to interleukin-1beta via late phase activation of nuclear factor kappaB and expression of phagocyte-type NADPH oxidase

Interleukin-1β (IL-1β) induces cell death in chondrocytes in a nitric oxide (NO)- and reactive oxygen species (ROS)-dependent manner. In this study, increased production of lactate was observed in IL-1β-treated mouse chondrocytic ATDC5 cells prior to the onset of their death. IL-1β-induced cell death in ATDC5 cells was suppressed by introducing an siRNA for monocarboxylate transporter-1 (MCT-1), a lactate transporter distributed in plasma and mitochondrial inner membranes. Mct-1 knockdown also prevented IL-1β-induced expression of phagocyte-type NADPH oxidase (NOX-2), an enzyme specialized for production of ROS, whereas it did not have an effect on inducible NO synthase. Suppression of IL-1β-induced cell death by Nox-2 siRNA indicated that NOX-2 is involved in cell death. Phosphorylation and degradation of inhibitor of κBα (IκBα) from 5 to 20 min after the addition of IL-1β was not affected by Mct-1 siRNA. In addition, IκBα was slightly decreased after 12 h of incubation with IL-1β, and the decrease was prominent after 36 h, whereas activation of p65/RelA was observed from 12 to 48 h after exposure to IL-1β. These changes were not seen in Mct-1-silenced cells. Forced expression of IκBα super repressor as well as treatment with the IκB kinase inhibitor BAY 11-7082 suppressed NOX-2 expression. Furthermore, Mct-1 siRNA lowered the level of ROS generated after 15-h exposure to IL-1β, whereas a ROS scavenger, N-acetylcysteine, suppressed both late phase degradation of IκBα and Nox-2 expression. These results suggest that MCT-1 contributes to NOX-2 expression via late phase activation of NF-κB in a ROS-dependent manner in ATDC5 cells exposed to IL-1β.

Enhancement of bone morphogenetic protein-2-induced ectopic bone formation by transforming growth factor-β1

Bone morphogenetic proteins (BMPs) possess osteoinductive activities and are useful for clinical treatments, including bone regeneration. We found that transforming growth factor (TGF)-β1 strongly enhances the osteoinductive activity of BMP-2. Collagen sponges containing 5 μg of BMP-2 were implanted into mouse muscle tissues, after which lump-like masses appeared and grew until day 7. Subsequently, calcification occurred in the lump-like masses by day 14. Addition of 50 ng of TGF-β1 to the BMP-2-containing sponges markedly accelerated the growth of the lump-like masses and resulted in a fivefold increase in total bone volume as compared with BMP-2 alone. The number of osteoblasts in ectopic bone tissues at 14 days after implantation induced by BMP-2+TGF-β1 was twofold greater than that with BMP-2 alone, whereas the number of osteoclasts was decreased by half. On the other hand, TGF-β1 accelerated the differentiation of both osteoblasts and osteoclasts in the early stage (2-7 days after implantation) of ectopic bone formation. We also implanted collagen sponges into bone defects surgically created in mouse calvaria. Sponges containing 2.5 μg of BMP-2 and 25 ng of TGF-β1 caused complete filling of the defects with orthotopic bone, whereas those containing 2.5 μg of BMP-2 alone caused only partial filling. These results suggest that TGF-β1 enhances BMP-2-induced ectopic bone formation by accelerating the growth of lump-like masses, and regulates osteoblast and osteoclast generation. Our findings may contribute to the development of a new treatment method for skeletal disorders.

Bone morphogenetic protein 2 enhances mouse osteoclast differentiation via increased levels of receptor activator of NF-κB ligand expression in osteoblasts

1α,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] induces osteoclast formation via induction of receptor activator of NF-κB ligand (RANKL, also called TNF-related activation-induced cytokine: TRANCE) in osteoblasts. In cocultures of mouse bone marrow cells and osteoblasts, 1,25(OH)(2)D(3) induced osteoclast formation in a dose-dependent manner, with maximum osteoclast formation observed at concentrations greater than 10(-9) M of 1,25(OH)(2)D(3). In the presence of bone morphogenetic protein 2 (BMP-2), the maximum formation of osteoclasts was seen with lower concentrations of 1,25(OH)(2)D(3) (greater than 10(-11) M), suggesting that BMP-2 enhances osteoclast formation induced by 1,25(OH)(2)D(3). In addition, the expressions of RANKL mRNA and proteins were induced by 1,25(OH)(2)D(3) in osteoblasts, and further upregulated by BMP-2. In mouse bone marrow cell cultures without 1,25(OH)(2)D(3), BMP-2 did not enhance osteoclast differentiation induced by recombinant RANKL and macrophage colony-stimulating factor (M-CSF), indicating that BMP-2 does not target osteoclast precursors. Furthermore, BMP-2 up-regulated the expression level of vitamin D receptor (VDR) in osteoblasts. These results suggest that BMP-2 regulates mouse osteoclast differentiation via upregulation of RANKL in osteoblasts induced by 1,25(OH)(2)D(3).

IFN-gamma down-regulates Secretoglobin 3A1 gene expression

STAT1 mediates Interferon (IFN)-dependent positive and negative regulation of inflammatory gene expression in lung. In this study, we examined the effect of IFN-gamma 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-gamma in a time- and dose-dependent manner in the murine transformed Clara Cells (mtCC) line. IFN-gamma 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.

TGF-beta suppresses POEM expression through ERK1/2 and JNK in osteoblasts

POEM, also called nephronectin, is an extracellular matrix protein that is considered to play a critical role as an adhesion molecule in the development and functioning of various tissues, such as kidneys and bones. In the present study, we examined the molecular mechanism of POEM gene expression, and found that transforming growth factor-beta (TGF-beta) strongly inhibited POEM expression in the mouse osteoblastic cell line, MC3T3-E1. TGF-beta-induced decrease of POEM expression occurred in both time- and dose-dependent manners through the activation of TGF-beta receptor I and extracellular signal-regulated kinase/c-Jun N-terminal kinase pathways.

Low affinity IgE receptors: regulation and functional roles in cell activation

The low affinity IgE receptors (Fc epsilon RII/CD23) homologous to animal lectins have the unique property of cleaving-off the extracytoplasmic portion as the soluble form (IgE binding factor; IgE-BF). Molecular analysis using Fc epsilon RII/CD23 cDNA proved that Fc epsilon RII is not unique to B lymphocytes but is expressed on a variety of cell lineages including T lymphocytes, macrophages and eosinophils. In these cell types, IL-4 is a general inducer of this molecule while IFN-gamma down-regulates B cell Fc epsilon RII/CD23 and up-regulates Fc epsilon RII/CD23 on macrophage and eosinophil cell lines. As predicted by the expression of Fc epsilon RII/CD23 in some HTLV-1(+) T cell lines, Fc epsilon RII/CD23 proved to be induced on normal peripheral T lymphocytes by IL-4 or IL-2 in the presence of additional permissive signals. As indicated by IL-2-dependent Fc epsilon RII/CD23 induction, there is an interesting bilateral co-regulation between Fc epsilon RII/CD23 and the 55 kDa chain of the IL-2 receptor complex with Tac antigen (IL-2R/p55(Tac]. Triggering of Fc epsilon RII/CD23 resulted in the enhanced expression of IL-2R/p55(Tac), whereas IL-2 enhanced the expression of Fc epsilon RII/CD23 in some systems. It is suggested that the triggering of cell surface Fc epsilon RII/CD23 by natural ligands is effectively buffered by soluble Fc epsilon RII/CD23 (IgE-BF).

Heparin enhances osteoclastic bone resorption by inhibiting osteoprotegerin activity

Heparin is a highly sulfated glycosaminoglycan and has been shown to activate osteoclastic bone resorption though how is not yet clear. Here we investigate the molecule involved in heparin-induced activation of osteoclasts using an in vitro osteoclast culture assay. The formation and activation of osteoclasts are induced by receptor activator of NFkappaB ligand (RANKL) on osteoblasts, and inhibited by osteoprotegerin (OPG), a decoy receptor of RANKL, which is secreted from osteoblasts. In a coculture of mouse bone marrow cells and osteoblasts treated with 1,25-dihydroxyvitamin D(3) and prostaglandin E(2) on dentin slices, the bone marrow cells differentiate into osteoclasts, and resorption pits are formed on the dentin slices. Addition of heparin, various glycosaminoglycans, and chemically modified heparins to the coculture reveals that heparin enhances the pit-forming activity of osteoclasts, and this effect of heparin on the activation of osteoclasts is dependent on its sugar chain structure. By contrast, mRNA expression levels of RANKL, RANK, and OPG in the coculture are not altered by heparin treatment. Furthermore, neither RANK nor RANKL binds to heparin, suggesting that heparin does not directly interact with these proteins. Instead, heparin specifically binds to OPG and prevents OPG-mediated inhibition of osteoclastic bone resorption in the coculture. Heparin treatment does not enhance osteoclastic bone resorption in a monoculture of osteoclasts derived from bone marrow cells, and in the coculture using osteoblasts from OPG-deficient mice. A (125)I-OPG binding assay showed that OPG binds to osteoblasts and that this binding is inhibited by the addition of heparin, suggesting that OPG binds to RANKL on the osteoblast membrane and that heparin blocks this interaction. These results demonstrate that heparin enhances osteoclastic bone resorption by inhibiting OPG activity.

Interleukin-4 inhibition of osteoclast differentiation is stronger than that of interleukin-13 and they are equivalent for induction of osteoprotegerin production from osteoblasts

Interleukin (IL)-4 and IL-13 are closely related cytokines known to inhibit osteoclast formation by targeting osteoblasts to produce an inhibitor, osteoprotegerin (OPG), as well as by directly targeting osteoclast precursors. However, whether their inhibitory actions are the same remains unclear. The inhibitory effect of IL-4 was stronger than that of IL-13 in an osteoclast-differentiation culture system containing mouse osteoblasts and osteoclast precursors. Both cytokines induced OPG production by osteoblasts in similar time- and dose-dependent manners. However, IL-4 was stronger in direct inhibition that targeted osteoclast precursors. Furthermore, IL-4 induced phosphorylation of signal transducer and activator of transcription-6 (STAT6) at lower concentrations than those of IL-13 in osteoclast precursors. IL-4 but not IL-13 strongly inhibited the expression of nuclear factor of activated T-cells, cytoplasmic 1 (nuclear factor-ATc1), a key factor of osteoclast differentiation, by those precursors. Thus, the activities of IL-4 and IL-13 toward osteoclast precursors were shown to be different in regards to inhibition of osteoclast differentiation, whereas those toward osteoblasts for inducing OPG expression were equivalent.

Nitric oxide in pulp cell growth, differentiation, and mineralization

Dental preparation sometimes causes transient congestion, edema, and necrosis of the pulp. We hypothesized that nitric oxide (NO) is involved in the pathophysiological changes in pulp after preparation. The mRNA and protein expression of the inducible isoform of NO synthase (iNOS) was examined in murine pulp after dental preparation. The effects of NO on the proliferation, mineralization, and apoptosis of pulp cells were also studied in vitro. We found that not only iNOS, but also mRNAs for alkaline phosphatase and plasma membrane glycoprotein-1, were expressed in the pulp after preparation. NOC-18, an NO donor, suppressed the proliferation of pulp cells without inducing cell death, whereas it promoted the mineralization of cells cultured in the presence of beta-glycerophosphate, ascorbic acid, dexamethasone, and KH(2)PO(4). Under these conditions, NOC-18 induced the apoptosis of pulp cells. These results suggest that NO regulates the growth, apoptosis, and mineralization of pulp cells.

Identification of two biologically crucial hydroxyl groups of (−)-epigallocatechin gallate in osteoclast culture

(-)-Epigallocatechin gallate (EGCG) induces cell death of osteoclasts in an Fe(2+)- and H(2)O(2)-dependent manner. In the present study, we further explore the cytotoxic mechanism of EGCG using four EGCG analogues. Molecules methylated at position 4' in the B ring (EGCG-4'-O-Me) or at position 4'' in the D-ring (EGCG-4''-O-Me) showed markedly decreased cytotoxicity to osteoclasts, indicating that hydroxyl groups at these two positions of EGCG are crucial for inducing cell death of osteoclasts. EGCG-4'-O-Me also showed the lowest Fe(3+)-reducing activity among five EGCGs. The Fe(3+)-reducing activity of EGCG was enhanced under conditions whereby protonated EGCG levels were increased, indicating that the protonated status of EGCG was involved in the Fe(3+)-reducing activity. The hydroxyl group at position 4'' in the D-ring was shown by quantum chemical calculation to be preferentially deprotonated among all of the hydroxyl groups in EGCGs. It was also shown that the highest occupied molecular orbital (HOMO) was localized to the B-ring of EGCGs, except for EGCG-4'-O-Me. We report here that the HOMO on the B-ring plays crucial roles in both the Fe(3+)-reducing activity of EGCG and the cytotoxicity of EGCG to osteoclasts, while deprotonation of the hydroxyl group at position 4'' in the D-ring plays a supplementary role.

Identification and Characterization of the Precursors Committed to Osteoclasts Induced by TNF-Related Activation-Induced Cytokine/Receptor Activator of NF-κB Ligand

Osteoclasts are terminally differentiated from cells of monocyte/macrophage lineage by stimulation with TNF-related activation-induced cytokine (TRANCE) (receptor activator of NF-kappaB ligand/osteoprotegerin ligand/osteoclast differentiation factor/TNFSF11/CD254). In the present study, we attempted to determine when and how the cell fate of precursors becomes committed to osteoclasts following TRANCE stimulation. Although mouse bone marrow-derived macrophages (BMMs) were able to differentiate into either osteoclasts or dendritic cells, the cells no longer differentiated into dendritic cells after treatment with TRANCE for 24 h, indicating that their cell fate was committed to osteoclasts. Committed cells as well as BMMs were still quite weak in tartrate-resistant acid phosphatase activity, an osteoclast marker, and incorporated zymosan particles by phagocytosis. Interestingly, committed cells, but not BMMs, could still differentiate into osteoclasts even after incorporation of the zymosan particles. Furthermore, IL-4 and IFN-gamma, potent inhibitors of osteoclast differentiation, failed to inhibit osteoclast differentiation from committed cells, and blocking of TRANCE stimulation by osteoprotegerin resulted in cell death. Adhesion to culture plates was believed to be essential for osteoclast differentiation; however, committed cells, but not BMMs, differentiated into multinucleated osteoclasts without adhesion to culture plates. Although LPS activated the NF-kappaB-mediated pathway in BMMs as well as in committed cells, the mRNA expression level of TNF-alpha in the committed cells was significantly lower than that in BMMs. These results suggest that characteristics of the committed cells induced by TRANCE are distinctively different from that of BMMs and osteoclasts.

Analogs of 1α,25-dihydroxyvitamin D3 with high potency in induction of osteoclastogenesis and prevention of dendritic cell differentiation: Synthesis and biological evaluation of 2-substituted 19-norvitamin D analogs

In our previous papers, we found that introduction of a substituent at C(2) into 1alpha,25-dihydroxy-19-norvitamin D(3) (2a) caused dramatic changes in binding affinity for the vitamin D receptor (VDR) and in transcriptional activity compared with the parent compound. To investigate the broad biological activity of 2-substituted 19-norvitamin D analogs, we synthesized two new (20S)-2-hydroxyethylidene-19-norvitamin D derivatives (3b and 4b) and a total of 16 A-ring-modified analogs including 3b and 4b were tested for the following in vitro and in vivo biological activities: (1) affinity for the VDR, (2) transcriptional activity, (3) osteoclast formation, (4) bone calcium mobilization in rats, and (5) effects on differentiation of dendritic cells (DCs). The biological effects of the analogs were compared with those of 1alpha,25-dihydroxyvitamin D(3) (1a) and 2MD, which is being developed for the treatment of osteoporosis. The efficacy of the (20S)-19-norvitamin D analogs with 2-hydroxyethylidene, 2-hydroxyethoxy, and 2-methyl moieties (3b, 5b, 6b, and 9b) was more than 10-fold stronger than that of 1a with respect to transcriptional activity, ability to induce osteoclast formation, and ability to inhibit CD86 expression, a marker of mature DCs, and was similar to that of 2MD. The (20S)-2beta-hydroxyethoxy derivative 6b was 2 orders of magnitude more active than 1a and approximately twice as potent as 2MD in preventing CD86 production. The 2-epoxy derivatives 7 and 8 were relatively poor ligands for the VDR and exhibited activity lower than that of the natural hormone 1a.

Reveromycin A, an agent for osteoporosis, inhibits bone resorption by inducing apoptosis specifically in osteoclasts

Mature bone-resorbing osteoclasts (OCs) mediate excessive bone loss seen in several bone disorders, including osteoporosis. Here, we showed that reveromycin A (RM-A), a small natural product with three carboxylic groups in its structure, induced apoptosis specifically in OCs, but not in OC progenitors, nonfunctional osteoclasts, or osteoblasts. RM-A inhibited protein synthesis in OCs by selectively blocking enzymatic activity of isoleucyl-tRNA synthetase. The proapoptotic effect of RM-A was inhibited by neutralization or disruption of the acidic microenvironment, a prominent characteristic of OCs. RM-A was incorporated in OCs but not in nonfunctional osteoclasts and OC progenitors in neutral culture medium. Effects of RM-A on OC apoptosis increased under acidic culture conditions. RM-A not only was incorporated, but also induced apoptosis in OC progenitors in acidic culture medium. RM-A inhibited osteoclastic pit formation, decreased prelabeled (45)Ca release in organ cultures, and antagonized increased bone resorption in ovariectomized mice. These results suggested that preventive effects of RM-A on bone resorption in vitro and in vivo were caused by apoptosis through inhibition of isoleucyl-tRNA synthetase in OCs and that specific sensitivity of OCs to RM-A was due to the acidic microenvironment, which increased cell permeability of RM-A by suppressing dissociation of protons from carboxylic acid moieties, making them less polar. This unique mechanism suggested that RM-A might represent a type of therapeutic agent for treating bone disorders associated with increased bone loss.

Osteoclast differentiation independent of the TRANCE-RANK-TRAF6 axis

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.

Interleukin-1beta induces death in chondrocyte-like ATDC5 cells through mitochondrial dysfunction and energy depletion in a reactive nitrogen and oxygen species-dependent manner

IL-1 (interleukin-1) acts as a key mediator of the degeneration of articular cartilage in RA (rheumatoid arthritis) and OA (osteoarthritis),where chondrocyte death is observed. It is still controversial, however, whether IL-1 induces chondrocyte death. In the present study, the viability of mouse chondrocyte-like ATDC5 cells was reduced by the treatment with IL-1beta for 48 h or longer. IL-1beta augmented the expression of the catalytic gp91 subunit of NADPH oxidase, gp91phox, as well as inducible NO synthase in ATDC5 cells. Generation of nitrated guanosine and tyrosine suggested the formation of reactive nitrogen species including ONOO- (peroxynitrite), a reaction product of NO and O2-, in ATDC5 cells and rat primary chondrocytes treated with IL-1beta. Death of ATDC5 cells after IL-1beta treatment was prevented by an NADPH-oxidase inhibitor, AEBSF[4-(2-aminoethyl)benzene-sulphonyl fluoride], an NO synthase inhibitor, L-NAME (NG-nitro-L-arginine methyl ester), and a ONOO- scavenger, uric acid. The viability of ATDC5 cells was reduced by the ONOO(-)-generator 3-(4-morpholinyl)sydnonimine hydrochloride, but not by either the NO-donor 1-hydroxy-2-oxo-3-(N-methyl-2-aminopropyl)-3-methyl-1-triazene or S-nitrosoglutathione. Disruption of mitochondrial membrane potential and ATP deprivation were observed in IL-1beta-treated ATDC5 cells, both of which were restored by L-NAME, AEBSF or uric acid. On the other hand, no morphological or biochemical signs indicating apoptosis were observed in these cells. These results suggest that the death of chondrocyte-like ATDC5 cells was mediated at least in part by mitochondrial dysfunction and energy depletion through ONOO- formation after IL-1beta treatment.

Differentiation and function of osteoclasts cultured on bone and cartilage

We examined the differentiation and resorptive function of osteoclasts (OC) cultured on the slices of calcified bone, decalcified bone and hyaline cartilage, and found that OC differentiation depends on the co-cultured substratum, as well as osteoblast-derived factors. Bone marrow-derived macrophages (BMM) were formed from marrow cells of 5 week old ddY mice and cultured for 3 days on freeze-dried slices of calcified bone, decalcified bone or cartilage, all prepared from rabbit costal bone. BMM cultured on calcified bone slices exhibited tartrate-resistant acid phosphatase (TRAP) activity and were structurally characterized by multinucleation and ruffled border development. However, on decalcified bone slices, BMM seldom became multinucleated and exhibited weak TRAP activity. BMM cultured on cartilage slices were mononuclear, devoid of TRAP activity and structurally resembled mononuclear phagocytes. In SEM observations of co-cultured slices, resorption lacunae were formed only on calcified bone slices, and not on slices of decalcified bone and cartilage. Our results, therefore, indicated that BMM could differentiate into functional OC only on calcified bone slices, suggesting a key role of calcified components in the bone matrix for the terminal OC differentiation. Then, we cultured BMM on the same slices with yeast particles. In cultures with yeast particles, BMM exhibited intense TRAP activity, developed a ruffled border-like structure and formed resorption lacunae even on decalcified bone and cartilage slices. Vacuolar-type H+-ATPase was strongly expressed along the ruffled border membranes of these OC. Only the BMM that had not incorporated yeast particles developed a ruffled border, whereas the BMM that had incorporated yeast particles did not become multinucleated and lacked a ruffled border structure. Thus, our results further suggest that, even on uncalcified substrata, the terminal differentiation of BMM into functional OC is induced by an unidentified external stimulus, which may be contained in the cell membrane of yeast particles.

Phosphodiesterase inhibitors stimulate osteoclast formation via TRANCE/RANKL expression in osteoblasts: possible involvement of ERK and p38 MAPK pathways

Phosphodiesterases (PDEs) are enzymes that degrade intracellular cAMP. In the present study, 3-isobutyl-1-methylxanthine (IBMX) and pentoxifylline, PDE inhibitors, induced osteoclast formation in cocultures of mouse bone marrow cells and calvarial osteoblasts. These inhibitors induced the expression of the osteoclast differentiation factor, TNF-related activation induced cytokine (TRANCE, identical to RANKL, ODF, and OPGL), in calvarial osteoblasts. IBMX induced phosphorylation of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) in osteoblasts. Induction of TRANCE expression by IBMX was partially suppressed by the inhibitors of protein kinase A (PKA), ERK, and p38 MAPK, suggesting that activation of ERK and p38 MAPK, as well as PKA, is involved in TRANCE expression by IBMX. Osteoblasts expressed PDE4, a PDE subtype, and rolipram, a selective inhibitor of PDE4, induced TRANCE expression. These results suggest that PDE4 is a key regulator of TRANCE expression in osteoblasts, which in turn controls osteoclast formation.

Dimer formation of receptor activator of nuclear factor κB induces incomplete osteoclast formation

Receptor activator of nuclear factor kappaB-ligand (RANKL) transduces a differentiation signal appropriate to osteoclasts likely through induction a receptor homotrimer; however, biological importance of RANK-trimerizarion is unknown. To address the signaling mechanism of the RANK receptor, we analyzed the effect of two different types of homodimer inducers RANK-TM-FKBP36v and hEpoR-RANK-TM on osteoclastogenesis. Dimerizing component FKBP36v or extracellular portion of human erythropoietin receptor (hEpoR) was fused to RANK lacking the extracellular domain, and the dimerization of this fusion protein was induced by addition of the chemical inducer of dimerization AP20187 or erythropoietin, respectively. Such treatment resulted in induction of TRAP-activity, a marker of osteoclast in a dose dependent manner, with an efficiency equivalent to that of induction by RANKL. However, dimerized-RANK-induced osteoclasts showed relatively low levels of multinucleation, pit forming activity, and expression of calcitonin receptor and cathepsin K, compared with osteoclasts which were induced in the presence of RANKL. As expression of nuclear factor of activated T cells 1 (NFATc1) was also reduced in dimerized-RANK-induced osteoclasts, RANK oligomerization by RANKL is a critical event to generate fully matured osteoclasts through upregulation of NFATc1.

Osteoimmunology: interactions of the immune and skeletal systems

Bone is a dynamic tissue that provides mechanical support, physical protection, and enables movement. Bone also serves as a storage site for minerals and is where blood cells are produced. Bone homeostasis is regulated by the balance between bone formation and resorption, and involves the coordinated action of osteoblasts and osteoclasts. Osteoblasts are bone-forming cells that secrete organic matrix molecules, while osteoclasts are derived from hematopoietic precursors and resorb bone matrix. Although osteoblasts and osteoclasts are the major regulators of bone metabolism and are regulated by the local microenvironment, it has recently come to be appreciated that skeletal system homeostasis is greatly influenced by components of the immune system. For example, some pathological bone resorption observed under inflammatory conditions has been shown to be due, in part, to direct and indirect effects of activated T cells on osteoclasts. In this regard, we would like to review current progress and perspectives in "osteoimmunology", an interdisciplinary research principle governing the cross-talk between the bone and immune systems. Better understanding of how the osteoimmune system operates in normal and pathological situations is likely to lay the groundwork for future therapies for the variety of diseases that affect both bone and the immune system.

Suppression of Osteoprotegerin Expression by Prostaglandin E2Is Crucially Involved in Lipopolysaccharide-Induced Osteoclast Formation

LPS is a potent stimulator of bone resorption in inflammatory diseases. The mechanism by which LPS induces osteoclastogenesis was studied in cocultures of mouse osteoblasts and bone marrow cells. LPS stimulated osteoclast formation and PGE(2) production in cocultures of mouse osteoblasts and bone marrow cells, and the stimulation was completely inhibited by NS398, a cyclooxygenase-2 inhibitor. Osteoblasts, but not bone marrow cells, produced PGE(2) in response to LPS. LPS-induced osteoclast formation was also inhibited by osteoprotegerin (OPG), a decoy receptor of receptor activator of NF-kappaB ligand (RANKL), but not by anti-mouse TNFR1 Ab or IL-1 receptor antagonist. LPS induced both stimulation of RANKL mRNA expression and inhibition of OPG mRNA expression in osteoblasts. NS398 blocked LPS-induced down-regulation of OPG mRNA expression, but not LPS-induced up-regulation of RANKL mRNA expression, suggesting that down-regulation of OPG expression by PGE(2) is involved in LPS-induced osteoclast formation in the cocultures. NS398 failed to inhibit LPS-induced osteoclastogenesis in cocultures containing OPG knockout mouse-derived osteoblasts. IL-1 also stimulated PGE(2) production in osteoblasts and osteoclast formation in the cocultures, and the stimulation was inhibited by NS398. As seen with LPS, NS398 failed to inhibit IL-1-induced osteoclast formation in cocultures with OPG-deficient osteoblasts. These results suggest that IL-1 as well as LPS stimulates osteoclastogenesis through two parallel events: direct enhancement of RANKL expression and suppression of OPG expression, which is mediated by PGE(2) production.

Disabling of Receptor Activator of Nuclear Factor-κB (RANK) Receptor Complex by Novel Osteoprotegerin-like Peptidomimetics Restores Bone Loss in Vivo

The tumor necrosis factor family ligand, tumor necrosis factor-related activation-induced cytokine (TRANCE), and its receptors, receptor activator of nuclear factor-kappaB (RANK) and osteoprotegerin (OPG), are known to be regulators of development and activation of osteoclasts in bone remodeling. Sustained osteoclast activation that occurs through TRANCE-RANK causes osteopenic disorders such as osteoporosis and contributes to osteolytic metastases. Here, we report a rationally designed small molecule mimic of osteoprotegerin to inhibit osteoclast formation in vitro and limit bone loss in an animal model of osteoporosis. One of the mimetics, OP3-4, significantly inhibited osteoclast formation in vitro (IC(50) = 10 microm) and effectively inhibited total bone loss in ovariectomized mice at a dosage of 2 mg/kg/day. Unlike soluble OPG receptors, which preclude TRANCE binding to RANK, OP3-4 shows the ability to modulate RANK-TRANCE signaling pathways and alters the biological functions of the RANK-TRANCE receptor complex by facilitating a defective receptor complex. These features suggest that OPG-derived small molecules can be used as a probe to understand complex biological functions of RANK-TRANCE-OPG receptors and also can be used as a platform to develop more useful therapeutic agents for inflammation and bone disease.

p38 Mitogen-activated protein kinase is crucially involved in osteoclast differentiation but not in cytokine production, phagocytosis, or dendritic cell differentiation of bone marrow macrophages

We previously reported that p38 MAPK signaling is required for osteoclast differentiation but not osteoclast function. Here we further investigated the role of p38 MAPK in the function and differentiation of mouse bone marrow macrophages (BMM phi), common precursors of osteoclasts and dendritic cells. Lipopolysaccharide (LPS) activated the p38 MAPK signaling pathway in BMM phi by sequential phosphorylation of MAPK kinase 3/6, p38 MAPK, and activating transcription factor-2. Treatment of BMM phi with SB203580, a p38 MAPK inhibitor, suppressed LPS-induced phosphorylation of activating transcription factor-2. LPS stimulated production of IL-1 beta, TNF alpha, and IL-6 in BMM phi, and SB203580 failed to inhibit the LPS-induced cytokine production. BMM phi incorporated latex beads via phagocytosis, and SB203580 had no effect on this phagocytosis. BMM phi differentiated into dendritic cells when treated with granulocyte macrophage colony-stimulating factor together with CD40 ligand, TNF alpha, or LPS, and SB203580 failed to inhibit this differentiation. Thus, p38 MAPK-mediated signals are not involved in either BMM phi function or BMM phi differentiation into dendritic cells. The differentiation of BMM phi into osteoclasts in response to receptor activator of nuclear factor-kappa B ligand or TNF alpha was strongly inhibited by SB203580. These findings emphasize the crucial roles of p38 MAPK-mediated signaling in osteoclast differentiation.

Destruxins, cyclodepsipeptides, block the formation of actin rings and prominent clear zones and ruffled borders in osteoclasts

Bone-resorbing osteoclasts exhibit polarized morphological structures such as actin rings, clear zones, and ruffled borders. To gain insight into the mechanism of bone-resorbing activity of osteoclast and to discover new types of anti-resorptive agents, we have screened for natural compounds that inhibit the bone-resorbing activity of osteoclast-like multinucleated cells (OCLs). Destruxin B (DestB) and E (DestE), cyclodepsipeptides, were found to inhibit pit formation without affecting osteoclast differentiation and survival. Destruxins reversibly induced morphological changes in OCLs in a dose-dependent manner (DestB, 0.2-1 microM; DestE, 0.01-0.05 microM) and inhibited pit formation. Destruxin-induced morphological changes were accompanied by disruption of the actin rings in OCLs. The formation of actin rings in OCLs after adhesion was also inhibited by destruxins. Electron microscopical analysis revealed that destruxin-treated OCLs on dentine slices have no prominent clear zones and ruffled borders. The effective concentrations of destruxins on the morphological changes were almost the same as those that inhibited bone resorption in organ culture system. These results suggest that the anti-resorptive effects of destruxins result from induction of a disorder of the morphological structures in polarized OCLs.

Involvement of vacuolar H+ -ATPase in incorporation of risedronate into osteoclasts

Although osteoclasts incorporate bisphosphonates during bone resorption, the mechanism of this incorporation by osteoclasts is not known. We previously reported that bisphosphonates disrupt the actin rings (clear zones) formed in normal osteoclasts, but did not disrupt actin rings in osteoclasts derived from osteosclerotic oc/oc mice, which have a defect in the gene encoding vacuolar H(+)-ATPase (V-ATPase). The present study showed that V-ATPase is directly involved in the incorporation of risedronate, a nitrogen containing bisphosphonate, into osteoclasts. Treatment of osteoclasts with risedronate disrupted actin rings and inhibited pit formation by osteoclasts on dentine slices. Bafilomycin A(1), a V-ATPase inhibitor, inhibited the pit-forming activity of osteoclasts but did not disrupt actin rings. Risedronate failed to disrupt actin rings in the presence of bafilomycin A(1). E-64, a lysosomal cysteine proteinase inhibitor, showed no inhibitory effect on the demineralization of dentine by osteoclasts but inhibited the digestion of dentine matrix proteins without disrupting actin rings. Risedronate disrupted actin rings even in the presence of E-64. Treatment of osteoclasts placed on plastic plates with risedronate also disrupted actin rings. Bafilomycin A(1) but not E64 prevented the disruption of actin rings in osteoclasts treated with risedronate on plastic plates. Inhibition of V-ATPase with bafilomycin A(1) also prevented disruption of actin rings by etidronate, a non-nitrogen-containing bisphosphonate. These results suggest that V-ATPase induced acidification beneath the ruffled borders of osteoclasts and subsequent bone demineralization triggers the incorporation of both nitrogen-containing and non-nitrogen-containing bisphosphonates into osteoclasts.

Lipopolysaccharide promotes the survival of osteoclasts via Toll-like receptor 4, but cytokine production of osteoclasts in response to lipopolysaccharide is different from that of macrophages

Lipopolysaccharide is a pathogen that causes inflammatory bone loss. Monocytes and macrophages produce proinflammatory cytokines such as IL-1, TNF-alpha, and IL-6 in response to LPS. We examined the effects of LPS on the function of osteoclasts formed in vitro in comparison with its effect on bone marrow macrophages, osteoclast precursors. Both osteoclasts and bone marrow macrophages expressed mRNA of Toll-like receptor 4 (TLR4) and CD14, components of the LPS receptor system. LPS induced rapid degradation of I-kappaB in osteoclasts, and stimulated the survival of osteoclasts. LPS failed to support the survival of osteoclasts derived from C3H/HeJ mice, which possess a missense mutation in the TLR4 gene. The LPS-promoted survival of osteoclasts was not mediated by any of the cytokines known to prolong the survival of osteoclasts, such as IL-1beta, TNF-alpha, and receptor activator of NF-kappaB ligand. LPS stimulated the production of proinflammatory cytokines such as IL-1beta, TNF-alpha, and IL-6 in bone marrow macrophages and peritoneal macrophages, but not in osteoclasts. These results indicate that osteoclasts respond to LPS through TLR4, but the characteristics of osteoclasts are quite different from those of their precursors, macrophages, in terms of proinflammatory cytokine production in response to LPS.

Simple decompression with small skin incision for cubital tunnel syndrome

Eighteen elbows in 17 patients with cubital tunnel syndrome were treated by simple decompression using only a 1.5-2.5 cm skin incision with no endoscopic assistance. According to McGowan's criteria, three elbows were classified preoperatively as grade I, six as grade II and nine as grade III. The mean follow-up period was 14 months (range 3-25). Clinical results were evaluated as excellent for four elbows, good for ten and fair for four. Improvement of symptoms occurred in all patients and dislocation of the ulnar nerve was not observed. Simple decompression through a small skin incision can be recommended for the treatment of cubital tunnel syndrome, if the indication is appropriate.

Regulation of osteoclast differentiation and function by receptor activator of NFkB ligand and osteoprotegerin

The differentiation and functions of osteoclasts (OCs) are regulated by osteoblast-derived factors. Receptor activator of NFkB ligand (RANKL) is one of the key regulatory molecules in OC formation. Osteoprotegerin (OPG) is a novel secreted member of the TNF receptor superfamily that negatively regulates osteoclastogenesis and binds to RANKL. We examined the biological actions of macrophage-colony-stimulating factor (M-CSF), RANKL, and OPG on the differentiation of OCs isolated from cocultures of mouse osteoblastic cells and bone marrow cells. Preosteoclasts (pOCs) and OCs were characterized by their ultrastructure and the expression of OC markers such as tartrate-resistant acid phosphatase (TRAP) and vacuolar-type H(+)-ATPase. pOCs formed without any additives expressed TRAP, but showed little resorptive activity on cocultured dentine slices. TRAP-positive pOCs treated with M-CSF began to fuse with each other, but lacked a ruffled border (RB) and showed almost no resorptive activity. pOCs treated with RANKL became TRAP-positive multinucleated cells, which expressed intense vacuolar-type H(+)-ATPase along the RB membranes and exhibited prominent resorptive activity. Such effects of RANKL on pOCs were completely inhibited by the addition of OPG. OPG inhibited RB formation in mature OCs and reduced their resorptive activity, and also induced apoptosis of some OCs. These results suggest that 1) RANKL induces differentiation of functional OCs from pOCs, 2) M-CSF induces macrophage-like multinucleated cells, but not OCs, 3) OPG inhibits RB formation and resorptive activity in mature OCs, 4) OPG also induces apoptosis of OCs, and 5) RANKL and OPG are, therefore, important regulators of not only the terminal differentiation of OCs but also their resorptive function.

The bovine fibroblast growth factor receptor 3 (FGFR3) gene is not the locus responsible for bovine chondrodysplastic dwarfism in Japanese brown cattle

Fibroblast growth factor receptor 3 (FGFR3) is one of the four distinct membrane-spanning tyrosine kinase receptors for fibroblast growth factors. The FGFR3 is a negative regulator of endochondral ossification and mutations in the FGFR3 gene have been found in patients of human hereditary diseases with chondrodysplastic phenotypes. Recently, we mapped the locus responsible for hereditary chondrodysplastic dwarfism in Japanese brown cattle to the distal region of bovine chromosome 6 close to the FGFR3 gene, suggesting that FGFR3 was a positional candidate gene for this disorder. In the present study, we isolated complementary DNA (cDNA) clones containing the entire coding region of the bovine FGFR3 gene. Comparison of the nucleotide sequence between affected and normal animals revealed no disease-specific differences in the deduced amino acid sequences. We further refined the localization of FGFR3 by radiation hybrid mapping, which is distinct from that of the disease locus. Therefore we conclude that bovine chondrodysplastic dwarfism in Japanese brown cattle is not caused by mutation in the FGFR3 gene.

Stimulation by toll-like receptors inhibits osteoclast differentiation

Osteoclasts, the cells capable of resorbing bone, are derived from hemopoietic precursor cells of monocyte-macrophage lineage. The same precursor cells can also give rise to macrophages and dendritic cells, which are essential for proper immune responses to various pathogens. Immune responses to microbial pathogens are often triggered because various microbial components induce the maturation and activation of immunoregulatory cells such as macrophages or dendritic cells by stimulating Toll-like receptors (TLRs). Since osteoclasts arise from the same precursors as macrophages, we tested whether TLRs play any role during osteoclast differentiation. We showed here that osteoclast precursors prepared from mouse bone marrow cells expressed all known murine TLRs (TLR1-TLR9). Moreover, various TLR ligands (e.g., peptidoglycan, poly(I:C) dsRNA, LPS, and CpG motif of unmethylated DNA, which act as ligands for TLR2, 3, 4, and 9, respectively) induced NF-kappa B activation and up-regulated TNF-alpha production in osteoclast precursor cells. Unexpectedly, however, TLR stimulation of osteoclast precursors by these microbial products strongly inhibited their differentiation into multinucleated, mature osteoclasts induced by TNF-related activation-induced cytokine. Rather, TLR stimulation maintained the phagocytic activity of osteoclast precursors in the presence of osteoclastogenic stimuli M-CSF and TNF-related activation-induced cytokine. Taken together, these results suggest that TLR stimulation of osteoclast precursors inhibits their differentiation into noninflammatory mature osteoclasts during microbial infection. This process favors immune responses and may be critical to prevent pathogenic effects of microbial invasion on bone.