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

Osteoporosis: interferon-gamma-mediated bone remodeling in osteoimmunology

As the world population ages, osteoporosis, the most common disease of bone metabolism, affects more than 200 million people worldwide. The etiology is an imbalance in bone remodeling process resulting in more significant bone resorption than bone remodeling. With the advent of the osteoimmunology field, the immune system's role in skeletal pathologies is gradually being discovered. The cytokine interferon-gamma (IFN-γ), a member of the interferon family, is an important factor in the etiology and treatment of osteoporosis because it mediates bone remodeling. This review starts with bone remodeling process and includes the cellular and key signaling pathways of bone remodeling. The effects of IFN-γ on osteoblasts, osteoclasts, and bone mass are discussed separately, while the overall effects of IFN-γ on primary and secondary osteoporosis are summarized. The net effect of IFN-γ on bone appears to be highly dependent on the environment, dose, concentration, and stage of cellular differentiation. This review focuses on the mechanisms of bone remodeling and bone immunology, with a comprehensive discussion of the relationship between IFN-γ and osteoporosis. Finding the paradoxical balance of IFN-γ in bone immunology and exploring the potential of its clinical application provide new ideas for the clinical treatment of osteoporosis and drug development.

Mechanistic insight of interleukin-9 induced osteoclastogenesis

Interleukin (IL)-9 is an emerging player in the pathogenesis of various chronic inflammatory diseases including bone disorders like rheumatoid arthritis (RA) and psoriatic arthritis. Recently, IL-9 was shown to enhance the osteoclast formation and their function in RA. However, the mechanisms by which IL-9 influences osteoclastogenesis are not known. Therefore, in this study we aimed to unravel the direct and indirect ways by which IL-9 can influence osteoclast formation. We used mouse bone marrow precursor cells for checking the effect of IL-9 on osteoclast differentiation and its function. Next, IL-9 induced signalling pathway were checked in the process of osteoclastogenesis. T cells play an important role in enhancing osteoclastogenesis in inflammatory conditions. We used splenic T cells to understand the impact of IL-9 on the functions of T effector (Teff) and regulatory T (Treg) cells. Furthermore, the effect of IL-9 mediated modulation of the T cell response on osteoclasts was checked using a coculture model of T cells with osteoclast precursors. We showed that IL-9 enhanced osteoclast formation and its function. We found that IL-9 activates STAT3, P38 MAPK, ERK1/2, NFκB and we hypothesize that it mediates the effect on osteoclastogenesis by accelerating mitochondrial biogenesis. Additionally, IL-9 was observed to facilitate the functions of pro-osteoclastogenic IL-17 producing T cells, but inhibits the function of anti-osteoclastogenic Treg cells. Our observations suggest that IL-9 can influence osteoclastogenesis directly by modulating the signalling cascade in the precursor cells; indirectly by enhancing IL-17 producing T cells and by reducing the functions of Treg cells.

Isopsoralen suppresses receptor activator of nuclear factor kappa-β ligand-induced osteoclastogenesis by inhibiting the NF-κB signaling

Osteoporosis is a serious systemic metabolic bone system disease.This study aimed to identify the target genes of isopsoralen and the signaling pathways involved in the differential expression of the genes involved in osteoclast differentiation. We hypothesized that isopsoralen may inhibit osteoclast differentiation by blocking the nuclear factor kappa-B (NF-κB) signaling pathway and verified our hypothesis through basic experiments. The 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) assay was used to detect the effect of isopsoralen on the proliferation and viability of primary mouse bone marrow monocytes (BMMCs). The effect of isopsoralen on receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast differentiation was determined by using tartrate-resistant acid phosphatase (TRAP) staining. Quantitative real-time PCR (qRT-PCR) and Western blot were used to detect the expression of the related genes and proteins. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway of isopsoralen target genes were obtained through comprehensive analysis using the STITCH database, Cytoscape 3.8.2, and R-Studio software. Differentially expressed genes (DEGs) were found in osteoclasts induced by RANKL before and after 3 days using R-Studio, following which KEGG analysis was performed. Next, enrichment analysis was performed on the KEGG pathway shared by the target genes of isopsoralen and the differentially expressed genes during osteoclast differentiation to predict the signaling pathway underlying the inhibition of osteoclast differentiation by isopsoralen. Finally, Western blot was used to detect the effect of isopsoralen on the activation of signaling pathways to verify the results of our bioinformatics analysis. Based on the enrichment analysis of isopsoralen target genes and differentially expressed genes during osteoclastogenesis, we believe that isopsoralen can inhibit RANKL-induced osteoclastogenesis by inhibiting the NF-κB signaling pathway.

Molecular characterization and elucidation of the function of Hap38 MAPK in the response of Helicoverpa armigera (Hübner) to UV-A stress

The cotton bollworm Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), an important pest of cotton, is detrimental to cotton production. Light from UV-A ultraviolet lamps is regarded as a form of environmental stress for insects. In order to investigate the response of H. armigera exposed to UV-A, we explored Hap38 MAPK expression and functions. We hope that the findings of this study will lay the foundation for future investigations into the insect's phototaxis mechanism. A p38 MAPK was cloned and named Hap38 MAPK. A phylogenetic tree showed that Hap38 MAPK was highly conserved. The gene was highly expressed in the thorax and females. Under UV-A stress, the expression of the gene decreased significantly. After silencing Hap38 MAPK, the activity of the antioxidant enzymes SOD, POD, CAT, and GR decreased. This study suggested that Hap38 MAPK responds to UV-A irradiation and plays critical roles in the defense response to environmental stresses.

A Nitrobenzoyl Sesquiterpenoid Insulicolide A Prevents Osteoclast Formation via Suppressing c-Fos-NFATc1 Signaling Pathway

It is a viable strategy to inhibit osteoclast differentiation for the treatment of osteolytic diseases such as osteoporosis, rheumatoid arthritis and tumor bone metastases. Here we assessed the effects of insulicolide A, a natural nitrobenzoyl sesquiterpenoid derived from marine fungus, on receptor activator of nuclear factor-κB ligand (RANKL)-stimulated osteoclastogenesis in vitro and its protective effects on LPS-induced osteolysis mice model in vivo. The results demonstrated that insulicolide A inhibited osteoclastogenesis from 1 μM in vitro. Insulicolide A could prevent c-Fos and nuclear factor of activated T-cell cytoplasmic 1 (NFATc1) nuclear translocation and attenuate the expression levels of osteoclast-related genes and DC-STAMP during RANKL-stimulated osteoclastogenesis but have no effects on NF-κB and MAPKs. Insulicolide A can also protect the mice from LPS-induced osteolysis. Our research provides the first evidence that insulicolide A may inhibit osteoclastogenesis both in vitro and in vivo, and indicates that it may have potential for the treatment of osteoclast-related diseases.

Fumitremorgin C Attenuates Osteoclast Formation and Function via Suppressing RANKL-Induced Signaling Pathways

Excessive bone resorption conducted by osteoclasts is considered as the main cause of osteoclast-related bone diseases such as osteoporosis. Therefore, the suppression of excessive osteoclast formation and function is one of the strategies to treat osteoclast-related bone diseases. Fumitremorgin C (Fum) is a mycotoxin extracted from Aspergillus fumigatus. It has been shown to have extensive pharmacological properties, but its role in the treatment of osteoclast-related bone diseases remains unclear. In this study, we aim to find out whether Fum can inhibit the receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation and function. The results showed that Fum could significantly attenuate osteoclast formation and function at concentrations from 2.5 to 10 µM. The protein expression of bone resorption factors such as NFATc1, cathepsin K, V-ATPase-d2, and c-Fos was suppressed with the treatment of Fum at a concentration of 10 µM. In addition, Fum was also shown to suppress the activity of NF-κB, intracellular reactive oxygen species level, and MAPK pathway. Taken together, the present study showed that Fum could attenuate the formation and function of osteoclast via suppressing RANKL-induced signaling pathways, suggesting that Fum might be a potential novel drug in the treatment of osteoclast-related bone diseases.

Molecular Characterization and Expression of OfJNK and Ofp38 in Ostrinia furnacalis (Guenée) Under Different Environmental Stressors

Ostrinia furnacalis, an important pest of corn, has substantial detrimental effects on corn production. The mitogen-activated protein kinase (MAPK) signaling pathway plays a pivotal role in an insect’s resistance to environmental stress. The expression levels of JNK and p38 have been well recorded in several insects under different environmental stressors, at different developmental stages, and in various tissue types; however, there is limited information on JNK and p38 in agricultural insects. To clarify the mechanism whereby O. furnacalis responds to environmental stress, we cloned JNK and p38 from O. furnacalis and subsequently named them OfJNK and Ofp38, respectively. Further, we examined the expression levels of OfJNK and Ofp38 under different environmental stressors. In this study, we obtained full-length sequences of OfJNK and Ofp38, and RT-qPCR results showed that these genes were expressed at all developmental stages, in various tissues (head, chest, abdomen, leg, wing, antennae, compound eye, midgut, and ovary) and under different environmental stressors (4°C and ultraviolet A treatment for 0, 30, 60, 90, and 120 min). The expression levels of OfJNK and Ofp38 were relatively higher in eggs and 3-day-old adult females than in other developmental stages. Moreover, the expression level of OfJNK was higher in the wings than in other tissues, whereas that of Ofp38 was significantly higher in the ovaries than in other tissues. OfJNK and Ofp38 showed high expression 90 min after being subjected to treatment at 4°C and ultraviolet A irradiation; the expression of Ofp38 peaked at 30 min, whereas that of OfJNK peaked at 60 min. These results indicate that O. furnacalis differs in terms of its response under different environmental stressors. In summary, our results will provide a foundation for additional research needed to determine the role of the MAPK signaling pathway and the underlying mechanisms by which it shows resistance to environmental stresses in insects.

Formononetin Regulates Multiple Oncogenic Signaling Cascades and Enhances Sensitivity to Bortezomib in a Multiple Myeloma Mouse Model

Here, we determined the anti-neoplastic actions of formononetin (FT) against multiple myeloma (MM) and elucidated its possible mode of action. It was observed that FT enhanced the apoptosis caused by bortezomib (Bor) and mitigated proliferation in MM cells, and these events are regulated by nuclear factor-κB (NF-κB), phosphatidylinositol 3-kinase (PI3K)/AKT, and activator protein-1 (AP-1) activation. We further noted that FT treatment reduced the levels of diverse tumorigenic proteins involved in myeloma progression and survival. Interestingly, we observed that FT also blocked persistent NF-κB, PI3K/AKT, and AP-1 activation in myeloma cells. FT suppressed the activation of these oncogenic cascades by affecting a number of signaling molecules involved in their cellular regulation. In addition, FT augmented tumor growth-inhibitory potential of Bor in MM preclinical mouse model. Thus, FT can be employed with proteasomal inhibitors for myeloma therapy by regulating the activation of diverse oncogenic transcription factors involved in myeloma growth.

Mannan-Binding Lectin Attenuates Inflammatory Arthritis Through the Suppression of Osteoclastogenesis

Mannan-binding lectin (MBL) is a vital element in the host innate immune system, which is primarily produced by the liver and secreted into the circulation. Low serum level of MBL is reported to be associated with an increased risk of arthritis. However, the underlying mechanism by which MBL contributes to the pathogenesis of arthritis is poorly understood. In this study, we investigated the precise role of MBL on the course of experimental murine adjuvant-induced arthritis (AIA). MBL-deficient (MBL^−/−) AIA mice showed significantly increased inflammatory responses compared with wild-type C57BL/6 AIA mice, including exacerbated cartilage damage, enhanced histopathological features and high level of tartrate-resistant acid phosphatase (TRAP)-positive cells. MBL protein markedly inhibited the osteoclast formation from human blood monocytes induced by receptor activator of nuclear factor-κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) in vitro. Mechanistic studies established that MBL inhibited osteoclast differentiation via down-regulation of p38 signaling pathway and subsequent nuclear translocation of c-fos as well as activation of nuclear factor of activated T-cells c1 (NFATc1) pathway. Importantly, we have provided the evidence that concentrations of MBL correlated negatively with the serum levels of amino-terminal propeptide of type I procollagen (PINP) and C-terminal telopeptide of type I collagen (β-CTX), serum markers of bone turnover, in patients with arthritis. Our study revealed an unexpected function of MBL in osteoclastogenesis, thus providing new insight into inflammatory arthritis and other bone-related diseases in patients with MBL deficiency.

Differential Effects of p38, MAPK, PI3K or Rho Kinase Inhibitors on Bacterial Phagocytosis and Efferocytosis by Macrophages in COPD

Pulmonary inflammation and bacterial colonization are central to the pathogenesis of chronic obstructive pulmonary disease (COPD). Defects in macrophage phagocytosis of both bacteria and apoptotic cells contribute to the COPD phenotype. Small molecule inhibitors with anti-inflammatory activity against p38 mitogen activated protein kinases (MAPKs), phosphatidyl-inositol-3 kinase (PI3K) and Rho kinase (ROCK) are being investigated as novel therapeutics in COPD. Concerns exist, however, about off-target effects. We investigated the effect of p38 MAPK inhibitors (VX745 and SCIO469), specific inhibitors of PI3K α (NVS-P13K-2), δ (NVS-P13K-3) or γ (NVS-P13K-5) and a ROCK inhibitor PF4950834 on macrophage phagocytosis, early intracellular killing of bacteria and efferocytosis of apoptotic neutrophils. Alveolar macrophages (AM) obtained from broncho-alveolar lavage (BAL) or monocyte-derived macrophages (MDM) from COPD patients (GOLD stage II/III) enrolled from a well characterized clinical cohort (MRC COPD-MAP consortium) or from healthy ex-smoker controls were studied. Both COPD AM and MDM exhibited lower levels of bacterial phagocytosis (using Streptococcus pneumoniae and non-typeable Haemophilus influenzae) and efferocytosis than healthy controls. None of the inhibitors altered bacterial internalization or early intracellular bacterial killing in AM or MDM. Conversely PF4950834, but not other inhibitors, enhanced efferocytosis in COPD AM and MDM. These results suggest none of these inhibitors are likely to exacerbate phagocytosis-related defects in COPD, while confirming ROCK inhibitors can enhance efferocytosis in COPD.

Role of osteoclasts in heterotopic ossification enhanced by fibrodysplasia ossificans progressiva-related activin-like kinase 2 mutation in mice

Fibrodysplasia ossificans progressiva (FOP) is a disorder of skeletal malformations and progressive heterotopic ossification. The constitutively activating mutation (R206H) of the bone morphogenetic protein type 1 receptor, activin-like kinase 2 (ALK2), is responsible for the pathogenesis of FOP. Although transfection of the causal mutation of FOP into myoblasts enhances osteoclast formation by transforming growth factor-β (TGF-β), the role of osteoclasts in heterotopic ossification is unknown. We therefore examined the effects of alendronate, SB431542 and SB203580 on heterotopic ossification induced by the causal mutation of FOP. Total bone mineral content as well as numbers of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated and alkaline phosphatase (ALP)-positive cells in heterotopic bone were significantly higher in muscle tissues implanted with ALK2 (R206H)-transfected mouse myoblastic C2C12 cells than in the tissues implanted with empty vector-transfected cells in nude mice. Alendronate, an aminobisphosphonate, did not affect total mineral content or numbers of TRAP-positive multinucleated and ALP-positive cells in heterotopic bone, which were enhanced by the implantation of ALK2 (R206H)-transfected C2C12 cells, although it significantly decreased serum levels of cross-linked C-telopeptide of type I collagen, a bone resorption index. Moreover, neither SB431542, an inhibitor of TGF-β receptor type I kinase, nor SB203580, an inhibitor of p38 mitogen-activated protein kinase, affected the increase in heterotopic ossification due to the implantation of ALK2 (R206H)-transfected C2C12 cells. In conclusion, the present study indicates that osteoclast inhibition does not affect heterotopic ossification enhanced by FOP-related mutation.

The MET/Vascular Endothelial Growth Factor Receptor (VEGFR)-targeted Tyrosine Kinase Inhibitor Also Attenuates FMS-dependent Osteoclast Differentiation and Bone Destruction Induced by Prostate Cancer

The tyrosine kinase inhibitor TAS-115 that blocks VEGF receptor and hepatocyte growth factor receptor MET signaling exhibits antitumor properties in xenografts of human gastric carcinoma. In this study, we have evaluated the efficacy of TAS-115 in preventing prostate cancer metastasis to the bone and bone destruction using the PC3 cell line. When PC3 cells were injected into proximal tibiae in nude mouse, severe trabecular and cortical bone destruction and subsequent tumor growths were detected. Oral administration of TAS-115 almost completely inhibited both PC3-induced bone loss and PC3 cell proliferation by suppressing osteoclastic bone resorption. In an ex vivo bone organ culture, PC3 cells induced osteoclastic bone resorption when co-cultured with calvarial bone, but TAS-115 effectively suppressed the PC3-induced bone destruction. We found that macrophage colony-stimulating factor-dependent macrophage differentiation and subsequent receptor activator of NF-κB ligand-induced osteoclast formation were largely suppressed by adding TAS-115. The phosphorylation of the macrophage colony-stimulating factor receptor FMS and osteoclast related kinases such as ERK and Akt were also suppressed by the presence of TAS-115. Gene expression profiling showed that FMS expression was only seen in macrophage and in the osteoclast cell lineage. Our study indicates that tyrosine kinase signaling in host pre-osteoclasts/osteoclasts is critical for bone destruction induced by tumor cells and that targeting of MET/VEGF receptor/FMS activity makes it a promising therapeutic candidate for the treatment of prostate cancer patients with bone metastasis.

Piperine alleviates osteoclast formation through the p38/c-Fos/NFATc1 signaling axis

Increased bone fracture is one of the health risk factors in patients with bone loss related disorders such as osteoporosis and breast cancer metastasis to bone. Over activity of osteoclasts leads to uncoupling of bone remodeling favoring bone loss over bone formation. Receptor activator of nuclear factor-κβ ligand (RANKL) triggers the differentiation pathway leading to multinucleated osteoclast formation. Modulation of RANKL or its downstream signaling pathways involved in osteoclast formation is of significant interest in the development of anti-resorptive agents. In this study, the effects of piperine, an alkaloid present in Piper nigrum L. on osteoclast formation was investigated. Piperine inhibited tartrate-resistant acid phosphatase-positive multinucleated osteoclast formation in murine RAW264.7 macrophages and human CD14+ monocytes induced by RANKL and breast cancer cells. Piperine attenuated the p38-mitogen activated protein kinase pathway activation, while the extracellular-signal-regulated kinase, c-Jun N-terminal kinase, or NF-κβ pathways downstream of RANKL remained unaffected. Concomitantly, expression of c-Fos and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), the key transcription factors involved in osteoclastogenesis were remarkably inhibited by piperine. Furthermore, piperine disrupted the actin ring structure and bone resorption, a characteristic hallmark of osteoclasts. Collectively, these results suggested that piperine inhibited osteoclast differentiation by suppressing the p38/NFATc1/c-Fos signaling axis..

The p38 MAPK inhibitor SB203580 differentially modulates LPS-induced interleukin 6 expression in macrophages

The p38 mitogen-activated protein kinase (MAPK) plays a key role in lipopolysaccharide (LPS)-induced signal transduction pathways that lead to inflammatory cytokine synthesis in macrophages; however, whether the inhibition of p38 MAPK regulates LPS-induced inflammatory cytokine expression in different types of macrophages remains the subject of debate. Herein, we assessed whether the inhibition of p38 MAPK by SB203580 regulates LPS-induced expression of the inflammatory cytokines tumor necrosis factor α (TNF-α) and interleukin 6 (IL-6) in RAW264.7 and resident peritoneal macrophages. Lipopolysaccharide stimulation of RAW264.7 macrophages or mouse resident peritoneal macrophages significantly increased TNF-α and IL-6 production. The addition of SB203580 to cultures dramatically blocked LPS-induced TNF-α production in RAW264.7 and mouse resident peritoneal macrophages, and dramatically blocked LPS-induced IL-6 production in RAW264.7 macrophages, but not in mouse resident peritoneal macrophages. Additionally, high concentrations of SB203580 resulted in increased IL-6 production. However, LPS-stimulation significantly up-regulated the mRNA transcript levels of TNF-α and IL-6 in RAW264.7 and mouse resident peritoneal macrophages, whereas pretreatment with SB203580 dramatically down-regulated LPS-induced mRNA transcript levels of TNF-α and IL-6 in these cells. Our data show that SB203580 differentially modulates LPS-induced production of the inflammatory cytokine IL-6 in two different sources of macrophages, and that this course of regulation occurs at the IL-6 mRNA post-transcriptional stage.

In vitro anti-osteoclastogenic activity of p38 inhibitor doramapimod via inhibiting migration of pre-osteoclasts and NFATc1 activity

The mitogen activated protein kinase p38 plays a role in the receptor activator of NF-ĸB ligand (RANKL)-induced osteoclast differentiation. In this study, we investigated the effect of p38 inhibitor doramapimod on the osteoclast differentiation. Doramapimod significantly inhibited the osteoclastogenesis of bone marrow macrophages (BMMs) via attenuating the activation of p38 induced by M-CSF and RANKL. Importantly, doramapimod blocked the migration and fusion in pre-osteoclasts via the down-regulating NFATc1. The inhibitory effect of doramapimod on the migration/fusion of pre-osteoclasts via inhibiting NFATc1 activity were confirmed by measuring NFATc1 luciferase activity and evaluating the mRNA expression of NFATc1-responsive genes related to the osteoclastic migration/fusion. These results suggested anti-osteoclastogenic activity of doramapimod via inhibiting migration/fusion of pre-osteoclasts and NFATc1 activity.

Spatholobus suberectus inhibits osteoclastogenesis and stimulates chondrogenesis

This study was carried out to investigate the effect of Spatholobus suberectus Dunn (SS) on the protection of chondral defect and inhibition of osteoclastogenesis. To examine these effects, we measured the matrix metalloproteinase (MMP) levels in SW1353 chondrosarcoma cells and performed tartrate-resistant acid phosphatase (TRAP) staining in bone marrow macrophage (BMM)-derived osteoclasts. To investigate the anti-osteoarthritis (OA) effects, we assessed TNF-α-induced MMP-1, -3, -9 and tissue inhibitors of matrix metalloproteinase (TIMP) expression levels in SW1353 cells. We observed that SS extract significantly inhibited MMP and TIMP expression in SW1353 cells. Also, SS extract inhibited the receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation. These results suggest that SS extract may have a potential in the treatment of bone loss and chondral defect by suppressing osteoclast differentiation and decreasing the expression of OA factors. Therefore, clarification of the mechanism of the action of SS extract and its active components is needed.

Rosmarinic acid exerts an antiosteoporotic effect in the RANKL-induced mouse model of bone loss by promotion of osteoblastic differentiation and inhibition of osteoclastic differentiation

SCOPE

Bone homeostasis is ensured by the balance between bone formation and resorption. Thus, control of the recruitment, proliferation, and differentiation of bone cells is essential to maintain bone mass. The aim of this study was to elucidate the effects of rosmarinic acid as a potential therapeutic agent on bone metabolism using bone cells and a mouse model.

METHODS AND RESULTS

Rosmarinic acid increased alkaline phosphatase activity and induced mineralization in osteoblasts. Addition of rosmarinic acid to cultures of calvarial osteoblastic cells prepared from T-cell factor/β-catenin TOP-GAL mutant mice strongly induced the expression of LacZ and promoted stabilization of β-catenin in the cytoplasm of ST2 cells, suggesting that rosmarinic acid affects the canonical Wnt signaling pathway. Moreover, rosmarinic acid inhibited not only osteoclast formation in cocultures of mouse bone marrow cells and osteoblasts, but also receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastic differentiation in bone marrow-derived macrophages. RANKL-induced p38 mitogen-activated protein kinase and the expression of nuclear factor of activated T cell, c-Jun, and c-Fos were inhibited by rosmarinic acid in bone marrow macrophages. Finally, we confirmed that rosmarinic acid improved bone mass in a soluble RANKL-induced bone loss mouse model.

CONCLUSION

Rosmarinic acid has dual regulatory effects on bone metabolism and may control the bone functions by controlling osteoblastic and osteoclastic differentiation.

Establishment of optimized in vitro assay methods for evaluating osteocyte functions

Recent studies have revealed that osteocytes play multiple important physiological roles. To analyze osteocyte functions in detail, an in vitro experimental system for primary osteocytes would be useful. Unfortunately, osteocytes tend to dedifferentiate and acquire osteoblast-like features even when the cells are cultured in three-dimensional (3D) collagen gel. Therefore, it is desirable to establish osteocyte culture conditions that prevent dedifferentiation over longer periods. In this study, we obtained systematic information about the influence of culture conditions on osteocyte differentiation states. Fetal bovine serum (FBS) concentrations from 0.1 to 0.5 % in 3D culture matrix did not significantly influence the expression of osteocyte markers. On the other hand, addition of Matrigel to the culture matrix significantly enhanced the expression of Rankl and late osteocyte markers such as Sost and Fgf23. Matrigel addition also inhibited upregulation of Opg and early osteocyte markers such as Dmp1 and Gp38. These effects on osteocyte properties were maximal at a Matrigel culture matrix content of 50 %. Matrigel addition to the matrix also increased dendritic process extension by osteocytes. In addition, Matrigel addition significantly stimulated tartrate-resistant acid phosphatase activity in co-culture with bone marrow macrophages. Among the conditions tested, 50 % Matrigel and 0.2 % FBS in type I collagen matrix were optimal for culture of primary osteocytes.

Blockade of CD26 signaling inhibits human osteoclast development

Bone remodeling is maintained by the delicate balance between osteoblasts (OBs) and osteoclasts (OCs). However, the role of CD26 in regulating bone remodeling has not yet been characterized. We herein show that CD26 is preferentially expressed on normal human OCs and is intensely expressed on activated human OCs in osteolytic bone alterations. Macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of NF-κB ligand (sRANKL) induced human OC differentiation, in association with CD26 expression on monocyte-macrophage lineage cells. CD26 expression was accompanied by increased phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK), which is crucial for early human OC differentiation. The humanized anti-CD26 monoclonal antibody, huCD26mAb, impaired the formation and function of tartrate-resistant acid phosphatase (TRAP)/CD26 positive multi-nucleated (nuclei > 3) OCs with maturation in the manner of dose-dependency. It was revealed that huCD26mAb inhibits early OC differentiation via the inactivation of MKK3/6, p38 MAPK and subsequent dephosphorylation of microphthalmia-associated transcription factor (mi/Mitf). These inhibitions occur immediately after RANKL binds to RANK on the human OC precursor cells and were demonstrated using the OC functional assays. huCD26mAb subsequently impaired OC maturation and bone resorption by suppressing the expression of TRAP and OC fusion proteins. In addition, p38 MAPK inhibitor also strongly inhibited OC formation and function. Our results suggest that the blockade of CD26 signaling impairs the development of human functional OCs by inhibiting p38 MAPK-mi/Mitf phosphorylation pathway and that targeting human OCs with huCD26mAb may have therapeutic potential for the treatment of osteolytic lesions following metastasis to alleviate bone destruction and reduce total skeletal-related events (SREs).

Fibrodysplasia ossificans progressiva-related activated activin-like kinase signaling enhances osteoclast formation during heterotopic ossification in muscle tissues

Fibrodysplasia ossificans progressiva is characterized by extensive ossification within muscle tissues, and its molecular pathogenesis is responsible for the constitutively activating mutation (R206H) of the bone morphogenetic protein type 1 receptor, activin-like kinase 2 (ALK2). In this study, we investigated the effects of implanting ALK2 (R206H)-transfected myoblastic C2C12 cells into nude mice on osteoclast formation during heterotopic ossification in muscle and subcutaneous tissues. The implantation of ALK2 (R206H)-transfected C2C12 cells with BMP-2 in nude mice induced robust heterotopic ossification with an increase in the formation of osteoclasts in muscle tissues but not in subcutaneous tissues. The implantation of ALK2 (R206H)-transfected C2C12 cells in muscle induced heterotopic ossification more effectively than that of empty vector-transfected cells. A co-culture of ALK2 (R206H)-transfected C2C12 cells as well as the conditioned medium from ALK2 (R206H)-transfected C2C12 cells enhanced osteoclast formation in Raw264.7 cells more effectively than those with empty vector-transfected cells. The transfection of ALK2 (R206H) into C2C12 cells elevated the expression of transforming growth factor (TGF)-β, whereas the inhibition of TGF-β signaling suppressed the enhanced formation of osteoclasts in the co-culture with ALK2 (R206H)-transfected C2C12 cells and their conditioned medium. In conclusion, this study demonstrated that the causal mutation transfection of fibrodysplasia ossificans progressiva in myoblasts enhanced the formation of osteoclasts from its precursor through TGF-β in muscle tissues.

Electromagnetically controllable osteoclast activity

The time-varying electromagnetic field (EMF) has been widely studied as one of the exogenous stimulation methods for improving bone healing. Our previous study showed that osteogenic differentiation of adipose-derived stem cells was accelerated by a 45-Hz EMF, whereas a 7.5-Hz EMF inhibited osteogenic marker expression. Accordingly, we hypothesized that each negative and positive condition for the osteogenic differentiation could inversely influence osteoclast formation and differentiation. Here, we demonstrated that osteoclast formation, differentiation, and activity can be regulated by altering the frequency of the electromagnetic stimulation, such as 7.5 (negative for osteogenic differentiation) and 45 Hz (positive for osteogenic differentiation). A 45 Hz EMF inhibited osteoclast formation whereas a 7.5-Hz EMF induced differentiation and activity. Osteoclastogenic markers, such as NFATc1, TRAP, CTSK, MMP9, and DC-STAMP were highly expressed under the 7.5-Hz EMF, while they were decreased at 45 Hz. We found that the 7.5-Hz EMF directly regulated osteoclast differentiation through ERK and p38 MAPK activation, whereas the EMF at 45 Hz suppressed RANKL-induced phosphorylation of IκB. Additionally, actin ring formation with tubules and bone resorptive activity were enhanced at 7.5 Hz through increased integrin β3 expression. However, these were inhibited at 45 Hz. Although many questions remain unanswered, our study indicates that osteoclast formation and differentiation were controllable using physical tools, such as an EMF. It will now be of great interest to study the ill-defined correlation between electromagnetic conditions and osteoclast activities, which eventually could lead to determining the therapeutic characteristics of an EMF that will treat bone-related diseases.

RANKL subcellular trafficking and regulatory mechanisms in osteocytes

The receptor activator of the NF-κB ligand (RANKL) is the central player in the regulation of osteoclastogenesis, and the quantity of RANKL presented to osteoclast precursors is an important factor determining the magnitude of osteoclast formation. Because osteoblastic cells are thought to be a major source of RANKL, the regulatory mechanisms of RANKL subcellular trafficking have been studied in osteoblastic cells. However, recent reports showed that osteocytes are a major source of RANKL presentation to osteoclast precursors, prompting a need to reinvestigate RANKL subcellular trafficking in osteocytes. Investigation of molecular mechanisms in detail needs well-designed in vitro experimental systems. Thus, we developed a novel co-culture system of osteoclast precursors and osteocytes embedded in collagen gel. Experiments using this model revealed that osteocytic RANKL is provided as a membrane-bound form to osteoclast precursors through osteocyte dendritic processes and that the contribution of soluble RANKL to the osteoclastogenesis supported by osteocytes is minor. Moreover, the regulation of RANKL subcellular trafficking, such as OPG-mediated transport of newly synthesized RANKL molecules to lysosomal storage compartments, and the release of RANKL to the cell surface upon stimulation with RANK are confirmed to be functional in osteocytes. These results provide a novel understanding of the regulation of osteoclastogenesis.

Lipopolysaccharide of Aggregatibacter actinomycetemcomitans up-regulates inflammatory cytokines, prostaglandin E2 synthesis and osteoclast formation in interleukin-1 receptor antagonist-deficient mice

BACKGROUND AND OBJECTIVE

The interleukin (IL)-1 receptor antagonist (Ra) binds to IL-1 receptors and inhibits IL-1 activity. However, it is unclear whether the IL-1Ra plays a protective role in periodontal disease. The purpose of this study was to compare IL-1Ra knockout (KO) and wild-type (WT) mice in regard to proinflammatory cytokine production, osteoclast formation and bone resorption in response to periodontal bacterial lipopolysaccharide (LPS).

MATERIAL AND METHODS

Peritoneal macrophages (Mφs) were obtained from 13-wk-old IL-1Ra KO and WT mice. Peritoneal Mφs were cultured with or without 10 μg/mL of Aggregatibacter actinomycetemcomitans LPS for 24 h. The levels of IL-1alpha (IL-1α), IL-1beta (IL-1β), tumor necrosis factor-α (TNF-α) and IL-6 were measured in periotoneal Mφs supernatant fluid (PM-SF) using an ELISA. Bone marrow cells were obtained from the mice and stimulated with PM-SF for 9 d, then stained with TRAP. The frequency of TRAP-positive multinucleated giant cell formation was calculated based on a fusion index. PM-SF-stimulated calvarial bone resorption was analyzed using micro-computed tomography, and calvarial histological analysis was performed using hematoxylin and eosin and TRAP staining. The expression of cyclooxygenase-2 (Cox2), prostanoid receptor EP4 (Ep4) and Rank mRNAs in bone marrow cells were measured using real-time quantitative PCR, while prostaglandin E2 (PGE2 ) production was determined by ELISA.

RESULTS

The levels of IL-1α, IL-1β, TNF-α and IL-6 in IL-1Ra KO mice PM-SF stimulated with A. actinomycetemcomitans LPS were significantly increased by approximately 4- (p < 0.05), 5- (p < 0.05), 1.3- (p < 0.05) and 6- (p < 0.05) fold, respectively, compared with the levels in WT mice. Moreover, osteoclast formation, expression of Rank, Ep4 and Cox2 mRNAs and production of PGE2 were significantly increased by approximately 2- (p < 0.05), 1.6- (p < 0.05), 2.5- (p < 0.05), 1.6- (p < 0.05) and 1.9- (p < 0.05) fold, respectively, in IL-1Ra KO mice stimulated with A. actinomycetemcomitans LPS compared with WT mice.

CONCLUSION

IL-1Ra regulates IL-1 activity and appears to reduce the levels of other inflammatory cytokines, including TNF-α and IL-6, while it also reduces expression of the EP4 receptor related to prostanoid sensitivity and osteoclast formation. These results suggest that IL-1Ra is an important molecule for inhibition of inflammatory periodontal bone resorption.

6,4'-Dihydroxy-7-methoxyflavanone inhibits osteoclast differentiation and function

6,4'-Dihydroxy-7-methoxyflavanone (DMF) is a flavonoid isolated from Heartwood Dalbergia odorifera. It has been known that DMF has antioxidant, anti-inflammatory and neuroprotective effects. DMF, however, the efficacy of bone related diseases has not been reported. In this study, we determined DMF's efficacy on osteoclasts differentiation and function using in vitro bone marrow macrophage osteoclast differentiation culture system. DMF inhibited receptor activators of nuclear factor kappa-B ligand (RANKL) induced osteoclastogenesis dose dependently. In addition, DMF decreased osteoclast function through disruption of actin ring formation and consequently suppression of the pit-forming activity of mature osteoclasts. Mechanistically, DMF inhibited RANKL-induced expression of nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1) and c-Fos via inhibition of mitogen activated protein kinases (MAPKs) pathway. Collectively, the inhibition of osteoclasts differentiation and function by DMF suggests that DMF can be a potential therapeutic molecule for osteoclastogenic bone diseases such osteoporosis, rheumatoid arthritis and periodontal diseases.

The p38 MAPK Pathway in Rheumatoid Arthritis: A Sideways Look

The p38 mitogen-activated protein kinase (MAPK) signaling pathway has been strongly implicated in many of the processes that underlie the pathology of rheumatoid arthritis (RA). For many years it has been considered a promising target for development of new anti-inflammatory drugs with which to treat RA and other chronic immune-mediated inflammatory diseases. However, several recent clinical trials have concluded in a disappointing manner. Why is this so, if p38 MAPK clearly contributes to the excessive production of inflammatory mediators, the destruction of bone and cartilage? We argue that, to explain the apparent failure of p38 inhibitors in the rheumatology clinic, we need to understand better the complexities of the p38 pathway and its many levels of communication with other cellular signaling pathways. In this review we look at the p38 MAPK pathway from a slightly different perspective, emphasising its role in post-transcriptional rather than transcriptional control of gene expression, and its contribution to the off-phase rather than the on-phase of the inflammatory response.

TRAF-6 dependent signaling pathway is essential for TNF-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation

Human osteoclast formation from mononuclear phagocyte precursors involves interactions between tumor necrosis factor (TNF) ligand superfamily members and their receptors. Recent evidence indicates that in addition to triggering apoptosis, the TNF-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation. To understand TRAIL-mediated signal transduction mechanism in osteoclastogenesis, we demonstrated that TRAIL induces osteoclast differentiation via a Tumor necrosis factor receptor-associated factor 6 (TRAF-6)-dependent signaling pathway. TRAIL-induced osteoclast differentiation was significantly inhibited by treatment with TRAF-6 siRNA and TRAF6 decoy peptides in both human monocytes and murine RAW264.7 macrophage cell lines, as evaluated in terms of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells and bone resorption activity. Moreover, TRAIL-induced osteoclast differentiation was also abolished in TRAF6 knockout bone marrow macrophages. In addition to induction of NFATc1, treatment of TRAIL also induced ubiquitination of TRAF6 in osteoclast differentiation. Thus, our data demonstrate that TRAIL induces osteoclastic differentiation via a TRAF-6 dependent signaling pathway. This study suggests TRAF6-dependent signaling may be a central pathway in osteoclast differentiation, and that TNF superfamily molecules other than RANKL may modify RANK signaling by interaction with TRAF6-associated signaling.

Tetracyclines convert the osteoclastic-differentiation pathway of progenitor cells to produce dendritic cell-like cells

Tetracyclines, such as doxycycline and minocycline, are used to suppress the growth of bacteria in patients with inflammatory diseases. Tetracyclines have been shown to prevent bone loss, but the mechanism involved is unknown. Osteoclasts and dendritic cells (DCs) are derived from common progenitors, such as bone marrow-derived macrophages (BMMs). In this article, we show that tetracyclines convert the differentiation pathway, resulting in DC-like cells not osteoclasts. Doxycycline and minocycline inhibited the receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis of BMMs, but they had no effects on cell growth and phagocytic activity. They influenced neither the proliferation nor the differentiation of bone-forming osteoblasts. Surprisingly, doxycycline and minocycline induced the expression of DC markers, CD11c and CD86, in BMMs in the presence of RANKL. STAT5 is involved in DC differentiation induced by GM-CSF. Midostaurin, a STAT5-signaling inhibitor, and an anti-GM-CSF-neutralizing Ab suppressed the differentiation induced by GM-CSF but not by tetracyclines. In vivo, the injection of tetracyclines into RANKL-injected mice and RANKL-transgenic mice suppressed RANKL-induced osteoclastogenesis and promoted the concomitant appearance of CD11c(+) cells. These results suggested that tetracyclines prevent bone loss induced by local inflammation, including rheumatoid arthritis and periodontitis, through osteoclast-DC-like cell conversion.

Critical roles of the WASP N-terminal domain and Btk in LPS-induced inflammatory response in macrophages

While Wiskott-Aldrich syndrome protein (WASP) plays critical roles in TCR signaling as an adaptor molecule, how it transduces innate immune signals remains to be elucidated. To investigate the roles of WASP in innate immune cells, we established bone marrow-derived macrophage (BMDM) cell lines from WASP15 transgenic (Tg) mice overexpressing the WASP N-terminal region (exons 1–5). Upon LPS stimulation, WASP15 Tg BMDM cell lines produce lower levels of inflammatory cytokines, such as TNF-α, IL-6, and IL-12p40 than the wild-type BMDM cell line. In addition, the production of nitric oxide by WASP15 Tg BMDM cells in response to LPS and IFN-γ was significantly impaired. Furthermore, we uncovered that the WASP N-terminal domain associates with the Src homology (SH) 3 domain of Bruton's tyrosine kinase (Btk). Overexpression of the WASP N-terminal domain diminishes the extent of tyrosine phosphorylation of endogenous WASP in WASP15 Tg BMDM cells, possibly by interfering with the specific binding between endogenous WASP and Btk during LPS signaling. These observations strongly suggest that the interaction between WASP N-terminal domain and Btk plays important roles in the LPS signaling cascade in innate immunity.

Lineage-committed osteoclast precursors circulate in blood and settle down into bone

Osteoclasts are derived from the monocyte/macrophage lineage, but little is known about osteoclast precursors in circulation. We previously showed that cell cycle-arrested quiescent osteoclast precursors (QOPs) were detected along bone surfaces as direct osteoclast precursors. Here we show that receptor activator of NF-κB (RANK)-positive cells isolated from bone marrow and peripheral blood possess characteristics of QOPs in mice. RANK-positive cells expressed c-Fms (receptors of macrophage colony-stimulating factor) at various levels, but scarcely expressed other monocyte/granulocyte markers. RANK-positive cells failed to exert phagocytic and proliferating activities, and differentiated into osteoclasts but not into dendritic cells. To identify circulating QOPs, collagen disks containing bone morphogenetic protein-2 (BMP disks) were implanted into mice, which were administered bromodeoxyuridine daily. Most nuclei of osteoclasts detected in BMP-2-induced ectopic bone were bromodeoxyuridine-negative. RANK-positive cells in peripheral blood proliferated more slowly and had a much longer lifespan than F4/80 (a macrophage marker)-positive macrophages. When BMP disks and control disks were implanted in RANK ligand-deficient mice, RANK-positive cells were observed in the BMP disks but not in the controls. F4/80-positive cells were distributed in both disks. Administration of FYT720, a sphingosine 1-phosphate agonist, promoted the egress of RANK-positive cells from hematopoietic tissues into bloodstream. These results suggest that lineage-determined QOPs circulate in the blood and settle in the bone.

Over-expression of Sox2 in C3H10T1/2 cells inhibits osteoblast differentiation through Wnt and MAPK signalling pathways

PURPOSE

Many Sox proteins play important roles both in mesoderm and ectoderm development. It is reported that Sox2, a member of this family, is essential for the maintenance of the self-renewal of embryonic stem cells (ES) and neural stem cells (NSCs). To investigate whether Sox2 participates in mesoderm development besides ectoderm, Sox2 was introduced into C3H10T1/2 cells.

METHODS

We produced recombinant retrovirus expressing Sox2 in GP2-293t cells and infected the virus into C3H10T1/2 cells. Growth property, alkaline phosphatase (ALP) staining, mineralized nodules, osteogenic gene expression and related signal pathways were analysed and compared between Sox2-expressing cells and control cells.

RESULTS

Sox2 over-expression led to increased proliferation of C3H10T1/2 cells, activation of Wnt/β-catenin and p38MAPK pathways. When cultured in osteogenic differentiation medium, ALP and mineralized nodules formation were inhibited in Sox2 over-expressing cells with down-regulation of osteogenic gene expression as well as inhibition of Wnt/β-catenin and p38MAPK pathways.

CONCLUSIONS

All these data suggested that over-expression of Sox2 promoted proliferation and inhibited osteoblast differentiation of C3H10T1/2 cells.

Ribosome inactivating protein B-chain induces osteoclast differentiation from monocyte/macrophage lineage precursor cells

Human osteoclast formation from mononuclear phagocyte precursors involves interactions between lectins and their receptors. A type-2 ribosome inactivating protein consists of an A chain and a B chain. The glycosylated B chain binds specifically to galactose moieties of sugar molecules. In this study we showed that the recombinant ribosome inactivating protein B-chain (rRBC) could induce osteoclast formation from human monocytes and murine RAW264.7 macrophages. Tartrate-resistant acid phosphatase (TRAP) staining and bone resorption assays demonstrated that differentiation of osteoclast-like cells was induced in the presence of rRBC in a dose-dependent manner. The rRBC-induced osteoclast differentiation was independent of caspase activation and apoptosis induction activity; however, rRBC-induced osteoclastogenesis was dependent on activation of NF-κB, ERK1/2, and p38 MAP kinase. Thus, our data demonstrated that rRBC induced osteoclast differentiation through a non-apoptotic signaling pathway. In addition to triggering apoptosis, the rRBC also induced osteoclast differentiation. According to this study, a novel role is proposed for rRBC in regulating osteoclast differentiation and in osteoimmunology.

Acidosis environment promotes osteoclast formation by acting on the last phase of preosteoclast differentiation: a study to elucidate the action points of acidosis and search for putative target molecules

Acidosis promoted tartaric acid-resistant acid phosphatase-positive multinuclear cell (TRAP+MNC) or osteoclast formation. Large osteoclast or TRAP+LMNC formation was observed far more in an acidosis environment than in a physiologically neutral environment. One of the major action points of acidosis was determined to be located in the last phase of preosteoclast differentiation using a co-culture system and a soluble RANKL-dependent bone marrow cell culture system. On-going osteoclast formation in an acidosis environment markedly deteriorated when the medium was replaced with physiologically neutral medium within the first 6h; however, bone marrow cells previously stimulated in an acidosis environment for 9h differentiated into TRAP+LMNC in pH 7.4 medium. Messenger RNA (mRNA) expression levels of DC-STAMP, a key molecule in cell fusion, and NFATc1 did not increase in the acidosis environment compared with those under physiologically neutral conditions. Ruthenium red, a general TRP antagonist, deteriorated acidosis-promoted TRAP+LMNC formation. 4-Alpha-PDD, a TRPV4-specific agonist, added in the last 21 h of preosteoclast differentiation, potentiated TRAP+LMNC formation in a mild acidosis environment, showing synergism between TRPV4 activation and acidosis. RN1734, a TRPV4-specific antagonist, partly inhibited acidosis-promoted TRAP+LMNC formation. We thus narrowed down the major action points of acidosis in osteoclast formation and elucidated the characteristics of this system in detail. Our results show that acidosis effectively uses TRPV4 to drive large-scale cell fusion and also utilizes systems independently of TRPV4.

Rab27a and Rab27b are involved in stimulation-dependent RANKL release from secretory lysosomes in osteoblastic cells

The quantity of the receptor activator of NF-κB ligand (RANKL) expressed at the cell surface of osteoblastic cells is an important factor regulating osteoclast activation. Previously, RANKL was found to be localized to secretory lysosomes in osteoblastic cells and to translocate to the cell surface in response to stimulation with RANK-Fc-conjugated beads. However, the in vivo significance of stimulation-dependent RANKL release has not been elucidated. In this study we show that small GTPases Rab27a and Rab27b are involved in the stimulation-dependent RANKL release pathway in osteoblastic cells. Suppression of either Rab27a or Rab27b resulted in a marked reduction in RANKL release after stimulation. Slp4-a, Slp5, and Munc13-4 acted as effector molecules that coordinated Rab27a/b activity in this pathway. Suppression of Rab27a/b or these effector molecules did not inhibit accumulation of RANKL in lysosomal vesicles around the stimulated sites but did inhibit the fusion of these vesicles to the plasma membrane. In osteoblastic cells, suppression of the effector molecules resulted in reduced osteoclastogenic ability. Furthermore, Jinx mice, which lack a functional Munc13-4 gene, exhibited a phenotype characterized by increased bone volume near the tibial metaphysis caused by low bone resorptive activity. In conclusion, stimulation-dependent RANKL release is mediated by Rab27a/b and their effector molecules, and this mechanism may be important for osteoclast activation in vivo.

Locally administered interferon-γ accelerates lipopolysaccharide-induced osteoclastogenesis independent of immunohistological RANKL upregulation

BACKGROUND AND OBJECTIVE

Interferon-γ (IFN-γ) potently inhibits RANKL-induced osteoclastogenesis in vitro. In contrast, previous studies have shown that an increase in IFN-γ expression is correlated with an increase in lipopolysaccharide (LPS)-induced bone loss in vivo. However, it is not clear whether local IFN-γ accelerates osteoclastogenesis or not in vivo. Therefore, the aim of this study was to clarify the role of local IFN-γ in LPS-induced osteoclastogenesis.

MATERIALS AND METHODS

We induced bone loss in calvaria by injecting LPS. One group of mice received an IFN-γ injection together with LPS injection, while another group received IFN-γ 2 d after LPS injection. Bone resorption was observed histologically. Next, we stimulated murine bone marrow macrophages with macrophage-colony stimulating factor and RANKL in vitro. We added different doses of IFN-γ and/or LPS at 0 or 48 h time points. Cells were stained with tartrate-resistant acid phosphatase at 72 h.

RESULTS

Local administration of IFN-γ together with LPS injection did not affect osteoclast formation. However, IFN-γ injected after LPS injection accelerated osteoclast formation. Also, we confirmed that IFN-γ added at 0 h inhibited RANKL-induced osteoclastogenesis in vitro. However, inhibition by IFN-γ added at 48 h was reduced compared with that by IFN-γ added at 0 h. Interestingly, IFN-γ together with a low concentration of LPS accelerated osteoclast formation when both were added at 48 h compared with no addition of IFN-γ.

CONCLUSION

The results suggest that local IFN-γ accelerates osteoclastogenesis in certain conditions of LPS-induced inflammatory bone loss.

Function of OPG as a traffic regulator for RANKL is crucial for controlled osteoclastogenesis

The amount of the receptor activator of NF-κB ligand (RANKL) on the osteoblastic cell surface is considered to determine the magnitude of the signal input to osteoclast precursors and the degree of osteoclastogenesis. Previously, we have shown that RANKL is localized predominantly in lysosomal organelles, but little is found on the osteoblastic cell surface, and consequently, the regulated subcellular trafficking of RANKL in osteoblastic cells is important for controlled osteoclastogenesis. Here we have examined the involvement of osteoprotegerin (OPG), which is currently recognized as a decoy receptor for RANKL, in the regulation of RANKL behavior. It was suggested that OPG already makes a complex with RANKL in the Golgi apparatus and that the complex formation is necessary for RANKL sorting to the secretory lysosomes. It was also shown that each structural domain of OPG is indispensable for exerting OPG function as a traffic regulator. In particular, the latter domains of OPG, whose physiologic functions have been unclear, were indicated to sort RANKL molecules to lysosomes from the Golgi apparatus. In addition, the overexpression of RANK-OPG chimeric protein, which retained OPG function as a decoy receptor but lost the function as a traffic regulator, inhibited endogenous OPG function as a traffic regulator selectively in osteoblastic cells and resulted in the upregulation of osteoclastogenic ability despite the increased number of decoy receptor molecules. Conclusively, OPG function as a traffic regulator for RANKL is crucial for regulating osteoclastogenesis at least as well as that as a decoy receptor.

IL-17 induces osteoclastogenesis from human monocytes alone in the absence of osteoblasts, which is potently inhibited by anti-TNF-alpha antibody: a novel mechanism of osteoclastogenesis by IL-17

IL-17 is a proinflammatory cytokine crucial for osteoclastic bone resorption in the presence of osteoblasts or synoviocytes in rheumatoid arthritis. However, the role of IL-17 in osteoclastogenesis from human monocytes alone remains unclear. Here, we investigated the role of IL-17 in osteoclastogenesis from human monocytes alone and the direct effect of infliximab on the osteoclastogenesis induced by IL-17. Human peripheral blood mononuclear cells (PBMC) were cultured for 3 days with M-CSF. After non-adherent cells were removed, IL-17 was added with either infliximab or osteoprotegerin (OPG). Seven days later, adherent cells were stained for vitronectin receptor. On the other hand, CD11b-positive monocytes purified from PBMC were also cultured and stained as described above. CD11b-positive cells were cultured with TNF-alpha and receptor activator of NF-kappaB ligand (RANKL). In the cultures of both adherent cells and CD11b-positive cells, IL-17 dose-dependently induced osteoclastogenesis in the absence of soluble-RANKL. OPG or infliximab inhibited IL-17-induced osteoclastogenesis. Interestingly, in the culture of CD11b-positive cells, the osteoclastogenesis was more potently inhibited by infliximab than by OPG. TNF-alpha and RANKL synergistically induced osteoclastogenesis. The present study clearly demonstrated the novel mechanism by which IL-17 directly induces osteoclastogenesis from human monocytes alone. In addition, infliximab potently inhibits the osteoclastogenesis directly induced by IL-17.

Role of CX3CL1/fractalkine in osteoclast differentiation and bone resorption

The recruitment of osteoclast precursors toward osteoblasts and subsequent cell-cell interactions are critical for osteoclast differentiation. Chemokines are known to regulate cell migration and adhesion. CX3CL1 (also called fractalkine) is a unique membrane-bound chemokine that has dual functions for cells expressing its receptor CX3CR1: a potent chemotactic factor in its soluble form and a type of efficient cell adhesion molecule in its membrane-bound form. In this paper, we demonstrate a novel role of CX3CL1 in osteoblast-induced osteoclast differentiation. We found that osteoclast precursors selectively expressed CX3CR1, whereas CX3CL1 is expressed by osteoblasts. We confirmed that soluble CX3CL1 induced migration of bone marrow cells containing osteoclast precursors, whereas immobilized CX3CL1 mediated firm adhesion of osteoclast precursors. Furthermore, a blocking mAb against CX3CL1 efficiently inhibited osteoclast differentiation in mouse bone marrow cells cocultured with osteoblasts. Anti-CX3CL1 also significantly suppressed bone resorption in neonatal mice by reducing the number of bone-resorbing mature osteoclasts. Collectively, CX3CL1 expressed by osteoblasts plays an important role in osteoclast differentiation, possibly through its dual functions as a chemotactic factor and adhesion molecule for osteoclast precursors expressing CX3CR1. The CX3CL1-CX3CR1 axis may be a novel target for the therapeutic intervention of bone resorbing diseases such as rheumatoid arthritis, osteoporosis, and cancer bone metastasis.

GM-CSF and IL-4 induce dendritic cell differentiation and disrupt osteoclastogenesis through M-CSF receptor shedding by up-regulation of TNF-alpha converting enzyme (TACE)

Monocytes give rise to macrophages, osteoclasts (OCs), and dendritic cells (DCs). Macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-kappaB (RANK) ligand induce OC differentiation from monocytes, whereas granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4) trigger monocytic differentiation into DCs. However, regulatory mechanisms for the polarization of monocytic differentiation are still unclear. The present study was undertaken to clarify the mechanism of triggering the deflection of OC and DC differentiation from monocytes. GM-CSF and IL-4 abolished monocytic differentiation into OCs while inducing DC differentiation even in the presence of M-CSF and RANK ligand. GM-CSF and IL-4 in combination potently up-regulate tumor necrosis factor-alpha (TNF-alpha) converting enzyme (TACE) and activity in monocytes, causing ectodomain shedding of M-CSF receptor, resulting in the disruption of its phosphorylation by M-CSF as well as the induction of osteoclastogenesis from monocytes by M-CSF and RANK ligand. Interestingly, TACE inhibition robustly causes the resumption of the surface expression of M-CSF receptor on monocytes, facilitating M-CSF-mediated phosphorylation of M-CSF receptor and macrophage/OC differentiation while impairing GM-CSF- and IL-4-mediated DC differentiation from monocytes. These results reveal a novel proteolytic regulation of M-CSF receptor expression in monocytes to control M-CSF signaling and monocytic differentiation into macrophage/OC-lineage cells or DCs.

Inhibitor of DASH proteases affects expression of adhesion molecules in osteoclasts and reduces myeloma growth and bone disease

Dipeptidyl peptidase (DPP) IV activity and/or structure homologues (DASH) are serine proteases implicated in tumourigenesis. We previously found that a DASH protease, fibroblast activation protein (FAP), was involved in osteoclast-induced myeloma growth. Here we further demonstrated expression of various adhesion molecules in osteoclasts cultured alone or cocultured with myeloma cells, and tested the effects of DASH inhibitor, PT-100, on myeloma cell growth, bone disease, osteoclast differentiation and activity, and expression of adhesion molecules in osteoclasts. PT-100 had no direct effects on viability of myeloma cells or mature osteoclasts, but significantly reduced survival of myeloma cells cocultured with osteoclasts. Real-time PCR array for 85 adhesion molecules revealed upregulation of 17 genes in osteoclasts after coculture with myeloma cells. Treatment of myeloma/osteoclast cocultures with PT-100 significantly downregulated 18 of 85 tested genes in osteoclasts, some of which are known to play roles in tumourigenesis and osteoclastogenesis. PT-100 also inhibited osteoclast differentiation and subsequent pit formation. Resorption activity of mature osteoclasts and differentiation of osteoblasts were not affected by PT-100. In primary myelomatous severe combined immunodeficient (SCID)-hu mice PT-100 reduced osteoclast activity, bone resorption and tumour burden. These data demonstrated that DASH proteases are involved in myeloma bone disease and tumour growth.

Involvement of c-Abl, p53 and the MAP kinase JNK in the cell death program initiated in A2E-laden ARPE-19 cells by exposure to blue light

The lipofuscin fluorophore A2E has been shown to mediate blue light-induced damage to retinal pigmented epithelial (RPE) cells. To understand the events that lead to RPE cell apoptosis under these conditions, we explored signaling pathways upstream of the cell death program. Human RPE cells (ARPE-19) that had accumulated A2E were exposed to blue light to induce apoptosis and the involvement of the transcription factors p53 and c-Abl and the mitogen activated protein kinases p38 and JNK were examined. We found that A2E/blue light caused upregulation and phosphorylation of c-Abl, and upregulation of p53. Pretreatment with the c-Abl inhibitor STI571 and transfection with siRNA specific to c-Abl and p53 prior to irradiation reduced A2E/blue light-induced cell death. Gene and protein expression of JNK and p38 was upregulated in response to A2E/blue light. Treatment with the JNK inhibitor SP600125 before irradiation resulted in increase in cell death whereas inhibition of p38 with SB203580 had no effect. This study indicates that c-Abl and p53 are important for execution of the cell death program initiated in A2E-laden RPE cells exposed to blue light, while JNK might play an anti-apoptotic role.

Docetaxel inhibits bone resorption through suppression of osteoclast formation and function in different manners

Osteoclasts are formed from the monocyte-macrophage lineage in response to receptor activator of nuclear factor kappaB ligand (RANKL) expressed by osteoblasts. Bone is the most common site of breast cancer metastasis, and osteoclasts play roles in the metastasis. The taxane-derived compounds paclitaxel and docetaxel are used for the treatment of malignant diseases, including breast cancer. Here we explored the effects of docetaxel on osteoclastic bone resorption in mouse culture systems. Osteoclasts were formed within 6 days in cocultures of osteoblasts and bone marrow cells treated with 1,25-dihydroxyvitamin D(3) plus prostaglandin E(2). Docetaxel at 10(-8) M inhibited osteoclast formation in the coculture when added for the entire culture period or for the first 3 days. Docetaxel, even at 10(-6) M added for the final 3 days, failed to inhibit osteoclast formation. Osteoprotegerin, a decoy receptor of RANKL, completely inhibited osteoclast formation when added for the final 3 days. Docetaxel at 10(-8) M inhibited the proliferation of osteoblasts and bone marrow cells. RANKL mRNA expression induced by 1,25-dihydroxyvitamin D(3) plus prostaglandin E(2) in osteoblasts was not affected by docetaxel even at 10(-6) M. Docetaxel at 10(-6) M, but not at 10(-8) M, inhibited pit-forming activity of osteoclasts cultured on dentine. Actin ring formation and L: -glutamate secretion by osteoclasts were also inhibited by docetaxel at 10(-6) M. Thus, docetaxel inhibits bone resorption in two different manners: inhibition of osteoclast formation at 10(-8) M and of osteoclast function at 10(-6) M. These results suggest that taxanes have beneficial effects in the treatment of bone metastatic cancers.

Stimulation of macrophage TNFalpha production by orthopaedic wear particles requires activation of the ERK1/2/Egr-1 and NF-kappaB pathways but is independent of p38 and JNK

Bone loss that causes aseptic loosening of orthopedic implants is initiated by pro-inflammatory cytokines produced by macrophages in response to implant-derived wear particles. MAPK and NF-kappaB signaling pathways are activated by the particles; however, it is not clear which of the signaling pathways are important for the initial response to the wear particles and which are only involved at later steps in the process, such as osteoclast differentiation. Here, we show that the ERK1/2, p38, JNK, and NF-kappaB pathways are rapidly activated by the wear particles but that only the ERK1/2 and NF-kappaB pathways are required for the initial response to the wear particles, which include increases in TNFalpha promoter activity, TNFalpha mRNA expression, and secretion of TNFalpha protein. Moreover, ERK1/2 activation by wear particles is also required for increased expression of the transcription factor Egr-1 as well as Egr-1's ability to bind to and activate the TNFalpha promoter. These results, together with our previous studies of the PI3K/Akt pathway, demonstrate that wear particles coordinately activate multiple signaling pathways and multiple transcription factors to stimulate production of pro-inflammatory cytokines, such as TNFalpha. The current study also demonstrates that the signaling pathways are activated to a much greater extent by wear particles with adherent endotoxin than by "endotoxin-free" wear particles. These results, together with those demonstrating the requirement for ERK1/2/Egr-1 and NF-kappaB, show that activation of these signaling pathways is responsible for the ability of adherent endotoxin to potentiate cytokine production, osteoclast differentiation, and bone loss induced by wear particles.

Efficacy of a p38 mitogen activated protein kinase inhibitor in mitigating an established inflammatory reaction to polyethylene particles in vivo

The inhibitor of p38 mitogen-activated protein kinase (MAPK) is of interest in the nonoperative treatment of periprosthetic osteolysis due to wear particles. Previous studies demonstrated that an oral p38 MAPK inhibitor did not suppress bone formation when given during the initial phase of tissue differentiation. However, the oral p38 MAPK inhibitor also did not curtail the foreign body and chronic inflammatory response to particles when given simultaneously. The purpose of the current study was to examine the efficacy of a p38 MAPK inhibitor, SCIO-323, on mitigating an established inflammatory reaction that parallels the clinical situation more closely. The Bone Harvest Chamber was implanted in rabbits and submicron polyethylene particles were placed in the chamber for 6 weeks. The contents of the chambers were harvested every 6 weeks. Oral treatment with the SCIO-323 included delivery for 3 weeks and stopping for 3 weeks, delivery for 3 weeks after an initial 3-week delay, and delivery for 6 weeks continuously. Administration of the SCIO-323 continuously for 6 weeks with/without the presence of particles, or for the initial 3 of 6 weeks had minor effects on bone ingrowth. After establishing a particle-induced chronic inflammatory reaction for 3 weeks, administration of SCIO-323 for a subsequent 3 weeks suppressed net bone formation. The activity of osteoclast-like cells remained low among all treatments when compared with the first control. Using the present model, the oral p38 MAPK inhibitor was ineffective in improving bone ingrowth in the presence of polyethylene particles.

MKK6-p38 MAPK signaling pathway enhances survival but not bone-resorbing activity of osteoclasts

Phosphorylated p38 mitogen-activating kinase (MAPK) is observed in osteoclasts under in vivo inflammatory situations. However, the role of p38 MAPK in osteoclast function has not been elucidated, because all external stimuli tested hitherto failed to induce the phosphorylation of p38 MAPK in osteoclasts in culture. In this study, a constitutively active form of MKK6 (MKK6CA) was expressed in osteoclasts using adenoviral gene transfer in vitro. MKK6CA expressed in osteoclasts phosphorylated p38 MAPK and enhanced the survival of osteoclasts. Dentine-resorbing activity of osteoclasts was not enhanced by the MKK6CA expression. These results suggest that p38 MAPK signaling plays a critical role in the survival of osteoclasts in inflammatory diseases.

Icaritin and its glycosides enhance osteoblastic, but suppress osteoclastic, differentiation and activity in vitro

Icariin, a principal flavonoid glycoside in Herba Epimedii, is hypothesized to possess beneficial effects on bone mass. Icariin is metabolized to icariside II and then to icaritin in vivo. In the present study, we investigated the in vitro effects of icariin, icariside II and icaritin on both osteoblasts and osteoclasts. After treatment with these compounds at concentrations 10(-5)-10(-8) mol/l, osteoblasts were examined for proliferation, alkaline phosphatase activity, osteocalcin secretion and matrix mineralization, as well as expression levels of bone-related proteins. The formation of osteoclasts was assessed by counting the number of multinucleated TRAP-positive cells. The activity of isolated rat osteoclasts was evaluated by measuring pit area, actin rings and superoxide generation. Icariside II and icaritin increased the mRNA expression of ALP, OC, COL-1 and OPG, but suppressed that of RANKL. In addition, these compounds reduced the number of multinucleated TRAP-positive cells and the osteoclastic resorption area. Also decreases were observed in superoxide generation and actin ring formation that are required for osteoclast survival and bone resorption activity. These findings suggest that icaritin, which was more potent than icariin and icariside II, enhanced the differentiation and proliferation of osteoblasts, and facilitated matrix calcification; meanwhile it inhibited osteoclastic differentiation in both osteoblast-preosteoclast coculture and osteoclast progenitor cell culture, and reduced the motility and bone resorption activity of isolated osteoclasts.