RANKing c-Jun in osteoclast development

Ginsenoside Rg2 inhibits osteoclastogenesis by downregulating the NFATc1, c-Fos, and MAPK pathways

Ginsenosides, among the most active components of ginseng, exhibit several therapeutic effects against cancer, diabetes, and other metabolic diseases. However, the molecular mechanism underlying the anti-osteoporotic activity of ginsenoside Rg2, a major ginsenoside, has not been clearly elucidated. This study aimed to determine the effects of ginsenoside Rg2 on receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation. Results indicate that ginsenoside Rg2 inhibits RANKLinduced osteoclast differentiation of bone marrow macrophages (BMMs) without cytotoxicity. Pretreatment with ginsenoside Rg2 significantly reduced the RANKL-induced gene expression of c-fos and nuclear factor of activated T-cells (Nfatc1), as well as osteoclast-specific markers tartrate-resistant acid phosphatase (TRAP, Acp5) and osteoclast-associated receptor (Oscar). Moreover, RANKL-induced phosphorylation of mitogen-activated protein kinases (MAPKs) was decreased by ginsenoside Rg2 in BMM. Therefore, we suggest that ginsenoside Rg2 suppresses RANKLinduced osteoclast differentiation through the regulation of MAPK signaling-mediated osteoclast markers and could be developed as a therapeutic drug for the prevention and treatment of osteoporosis. [BMB Reports 2023; 56(10): 551-556].

Biological mechanism of surgery-mediated acceleration of orthodontic tooth movement: A narrative review

Surgery-mediated acceleration of orthodontic tooth movement (SAOTM) has been proven effective for decades. Research has confirmed that surgical approaches play an important role in adult patients with a short orthodontic treatment time. The mechanism of SAOTM involves short-term acceleration of localized hard and soft tissue remodeling, known as the regional acceleratory phenomenon. However, no relevant review on the biological mechanism of SAOTM has been performed to date. The proposed biological mechanism of acceleration of OTM involves the participation of various cells, cytokines, and signaling pathways. We herein review the relevant literature and summarize the biological mechanism of SAOTM to provide new insights for further research on acceleration of OTM.

6-Shogaol promotes bone resorption and accelerates orthodontic tooth movement through the JNK-NFATc1 signaling axis

INTRODUCTION

Corticotomy is widely used in clinical practice to accelerate tooth movement and shorten the duration of orthodontic treatment. It is effective, but an invasive surgery is needed to induce alveolar bone osteopenia that enable rapid tooth movement. In this study, we discovered the potential of 6-shogaol as a more patient-friendly non-invasive alternative to induce transient osteopenia and accelerate tooth movement.

MATERIALS AND METHODS

The effects of 6-shogaol on the bone marrow macrophages (BMM) proliferation and osteoclast differentiation, and bone resorption were determined in vitro. Sprague-Dawley rats were distributed into three groups: CON, IPinj or Localinj and euthanized at day 28. Micro-CT, histology, immunohistological, and TUNEL analysis were performed to evaluate the tooth movement acceleration effect of 6-shogaol.

RESULTS

In vitro, 6-shogaol promotes osteoclast differentiation and functional demineralization of alveolar bone. RANKL-induced mRNA expression of osteoclastic-specific genes was significantly higher in the presence of 6-shogaol. A dose-dependent increase in the area of TRAP-positive cells was observed with 6-shogaol treatment. F-actin ring formation and increased bone resorption confirmed that osteoclasts treated with 6-shogaol were mature and functional. 6-shogaol stimulated JNK activation and NFATc1 expression during osteoclast differentiation. In vivo, 6-shogaol promotes alveolar bone transient osteopenia and accelerates orthodontic tooth movement. Alveolar bone mass was reduced, more osteoclasts were observed in bone resorption lacunae on the compression side, and the expression of RANKL and sclerostin were higher than the control group. In conclusion, our results suggest that 6-shogoal accelerates tooth movement by inducing osteopenia by a mechanism similar to surgically induced bone injury.

Protein phosphatase 1 regulatory subunit 18 suppresses the transcriptional activity of NFATc1 via regulation of c-fos

The transcription factor NFATc1 and its binding partner AP-1 (a complex containing c-fos and c-Jun) play a central role in osteoclast differentiation. NFATc1 and AP-1 promote the expression of target genes such as Acp5, Ctsk and also auto-regulate NFATc1 expression as well. We previously reported that protein phosphatase 1 regulatory subunit 18 (PPP1r18) is a negative regulator of osteoclast bone resorption by inhibiting cell attachment to bone matrix. We also reported that PPP1r18 potentially regulates NFATc1 expression during osteoclast differentiation. To further explore this, in this study we have examined the effect of PPP1r18 on NFATc1 expression and activity by overexpressing PPP1r18 during the early stage of osteoclast differentiation. We found that PPP1r18 suppressed NFATc1 expression through inhibition of the transcriptional activity of NFATc1. Since PPP1r18 does not regulate NFATc1 directly, we next explored the involvement of AP-1. Our data showed that c-fos phosphorylation and nuclear localization were reduced by PPP1r18 overexpression. Further experiments showed that overexpression of c-fos together with PPP1r18 rescued NFATc1 expression and transcriptional activity. Moreover, c-fos activity inhibition by PPP1r18 was canceled by mutation of the phosphatase binding site of PPP1r18. Taken together, PPP1r18-regulated phosphatase activity targets c-fos phosphorylation and suppresses subsequent NFATc1 expression and activity.

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PPP1r18 suppresses osteoclast differentiation.

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PPP1r18 suppresses c-fos phosphorylation and nuclear localization.

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PPP1r18 suppresses NFAT via c-fos.

Water Extracts of Hull-less Waxy Barley (Hordeum vulgare L.) Cultivar 'Boseokchal' Inhibit RANKL-induced Osteoclastogenesis

Osteoporosis is a disease that leads to reduced bone mineral density. The increase in patient and medical costs because of global aging is recognized as a problem. Decreased bone mass is a common symptom of bone diseases such as Paget’s disease, rheumatoid arthritis, and multiple myeloma. Osteoclasts, which directly affect bone mass, show a marked increase in differentiation and activation in the aforementioned diseases. Moreover, these multinucleated cells made from monocytes/macrophages under the influence of RANKL and M-CSF, are the only cells capable of resorbing bones. In this study, we found that the water extracts of Boseokchal (BSC-W) inhibited osteoclast differentiation in vitro and investigated its inhibitory mechanism. BSC-W was obtained by extracting flour of Boseokchal using hexane and water. To osteoclast differentiation, bone marrow-derived macrophage cells (BMMs) were cultured with the vehicle (0.1% DMSO) or BSC-W in the presence of M-CSF and RANKL for 4 days. Cytotoxicity was measured by CCK-8. Gene expression of cells was confirmed by real-time PCR. Protein expression of cells was observed by western blot assay. Bone resorption activity of osteoclast evaluated by bone pit formation assay using an Osteo Assay Plate. BSC-W inhibited RANKL-induced osteoclastogenesis in a dose-dependent manner without exerting a cytotoxic effect on BMMs. BSC-W decreased the transcriptional and translational expression of c-Fos and NFATc1, which are regulators of osteoclastogenesis and reduced the mRNA expression level of TRAP, DC-STAMP, and cathepsin K, which are osteoclast differentiation marker. Furthermore, BSC-W reduced the resorption activity of osteoclasts. Taken together, our results indicate that BSC-W is a useful candidate for health functional foods or therapeutic agents that can help treat bone diseases such as osteoporosis.

Puerarin Exerts Protective Effects on Wear Particle-Induced Inflammatory Osteolysis

Wear particle-stimulated inflammatory bone destruction and the consequent aseptic loosening remain major postoperative problems for artificial joints. Studies have indicated that puerarin promotes osteogenesis and alleviates lipopolysaccharide-induced osteoclastogenesis in vitro. However, the underlying molecular mechanism by which puerarin interacts with receptor activator of nuclear factor kappa-B ligand (RANKL)-mediated osteoclast formation in vitro and wear particle-stimulated osteolysis in vivo has not been reported. In this work, the protective effects exerted by puerarin on titanium particle-stimulated bone destruction in vivo and on RANKL-induced osteoclast activation in osteoclastic precursor cells in vitro were investigated. As expected, puerarin significantly inhibited wear particle-mediated bone resorption and proinflammatory cytokine productions in a calvarial resorption model. Additionally, puerarin inhibited RANKL-induced osteoclast activation, bone resorption ability, and F-actin ring formation in vitro as puerarin concentration increased. Furthermore, mechanistic investigation indicated that reduced RANKL-stimulated MEK/ERK/NFATc1 signaling cascades might regulate the protective effect of puerarin. Conclusively, these results indicate that puerarin, a type of polyphenol, might serve as a protective agent to prevent osteoclast-related osteolytic diseases.

Egg White Ovotransferrin Attenuates RANKL-Induced Osteoclastogenesis and Bone Resorption

Ovotransferrin, a member of the transferrin family, is the second main protein found in egg white. Ovotransferrin was reported to have antimicrobial, antioxidant, and immunomodulating activities. The aim of this work was to characterize the cellular and molecular functions of egg white ovotransferrin on osteoclasts differentiation and function. Osteoclasts were prepared from mouse macrophage RAW 264.7 cells stimulated with receptor activator of nuclear factor κB ligand (RANKL). Ovotransferrin inhibited osteoclasts differentiation and the calcium–phosphate resorptive ability via the suppression of RANKL-induced nuclear factor κ-light chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways. Ovotransferrin induced apoptosis of matured osteoclasts, accompanied by increased expression of Bcl-2-like protein 11 (Bim) and Bcl-2-assoicated death promoter (Bad), but decreased expression of B-cell lymphoma 2 (Bcl-2) and B-cell lymphoma-extra-large (Bcl-xl). We established a novel role of egg white ovotransferrin as an inhibitor of osteoclastogenesis, which may be used for the prevention of osteoporosis.

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.

I-BET151 suppresses osteoclast formation and inflammatory cytokines secretion by targetting BRD4 in multiple myeloma

Background: Multiple myeloma (MM) is an incurable hematologic cancer, accompanied by excessive osteoclast formation and inflammatory cytokine secretion. The mechanisms by which bromodomain and extra-terminal domain (BET) protein inhibitor I-BET151 regulates osteoclast differentiation and inflammatory cytokine secretion in MM are largely unknown. Methods: The isolated peripheral blood mononuclear cells from normal or patients with MM were treated with receptor activator of NF-κB ligand (RANKL) and M-CSF to induce osteoclast differentiation. RAW 264.7 cells were treated with RANKL. I-BET151 was applied to investigate the effects of BRD4 inhibition on osteoclast formation and inflammatory cytokine secretion. Osteoclast formation was determined by tartrate-resistant acid phosphatase (TRACP) staining. The expression of osteoclast-specific genes TRACP, matrix metalloproteinase-9 (MMP-9), cathepsin K (Ctsk), and c-Src was tested using quantitative real-time PCR. And the level of inflammatory cytokines TNF-α, IL-1β, and IL-6 was assessed by ELISA. Tumor necrosis factor receptor-associated factor 6 (TRAF6), BRD4, nuclear and cytoplasm p65, IκB-α, nuclear factor of activated T cells cytoplasmic (NFATc1), and osteoprotegerin (OPG) expression were measured by Western blotting. RNAi technology was applied to knock down BET family member BRD4. Results: I-BET151 dose-dependently suppressed osteoclast formation, inhibited the levels of osteoclast-specific genes TRACP, MMP-9, Ctsk, and c-Src and inflammatory cytokines TNF-α, IL-1β, and IL-6 secretion in peripheral blood mononuclear cells and RAW 264.7. I-BET151 inhibited the protein levels of BRD4 and NFATc1, increased OPG expression, and suppressed IκB-α degradation and p65 nuclear translocation. Further, the effects of I-BET151 on osteoclast formation, osteoclast-specific genes expression, inflammatory cytokine secretion, and NF-κB inhibition were promoted by BRD4 knockdown. Conclusion: I-BET151 inhibits osteoclast formation and inflammatory cytokine secretion by targetting BRD4-mediated RANKL-NF-κB signal pathway and BRD4 inhibition might be beneficial for MM treatment.

Propofol promotes osteoclastic bone resorption by increasing DC-STAMP expression

Background

Propofol is an intravenous anesthetic which has antioxidant effects due to its similarity in molecular structure to α-tocopherol. It has been reported that α-tocopherol increases osteoclast fusion and bone resorption. Here, we investigated the effects of propofol on signaling pathways of osteoclastogenic gene expression, as well as osteoclastogenesis and bone resorption using bone marrow-derived macrophages (BMMs).

Methods

BMMs were cultured with macrophage colony-stimulating factor (M-CSF) alone or M-CSF plus receptor activator of nuclear factor kappa B ligand (RANKL) in the presence of propofol (0-50 µM) for 4 days. Mature osteoclasts were stained for tartrate-resistant acid phosphatase (TRAP) and the numbers of TRAP-positive multinucleated osteoclasts were counted. To examine the resorption activities of osteoclasts, a bone resorption assay was performed. To identify the mechanism of action of propofol on the formation of multinucleated osteoclasts, we focused on dendritic cell-specific transmembrane protein (DC-STAMP), a protein essential for pre-osteoclastic cell fusion.

Results

Propofol increased the formation of TRAP-positive multinucleated osteoclasts. In addition, the bone resorption assay revealed that propofol increased the bone resorption area on dentin discs. The mRNA expression of DC-STAMP was upregulated most strongly in the presence of both RANKL and propofol. However, SB203580, a p38 inhibitor, significantly suppressed the propofol/RANKL-induced increase in mRNA expression of DC-STAMP.

Conclusion

We have demonstrated that propofol enhances osteoclast differentiation and maturation, and subsequently increases bone resorption. Additionally, we identified the regulatory pathway underlying osteoclast cell-cell fusion, which was enhanced by propofol through p38-mediated DC-STAMP expression.

Carnosic acid attenuates RANKL-induced oxidative stress and osteoclastogenesis via induction of Nrf2 and suppression of NF-κB and MAPK signalling

Nuclear factor-erythroid 2-related factor 2 (Nrf2) is a redox-sensitive transcription factor, which plays an important role in the cellular defense against oxidative stress by induction of anti-oxidant and cytoprotective enzymes. In the current study, we sought to investigate the osteoprotective effect of carnosic acid (CA), a phenolic (catecholic) diterpene. It is widely identified for its electrophilic nature under oxidative stress conditions and thus anticipated to counter osteoporosis by facilitation of Nrf2 signalling. Osteoclast differentiation was induced by incubation of RAW 264.7 (mouse macrophage) cells and mouse bone marrow macrophages (BMMs) in the presence of receptor activator of NF-κB ligand (RANKL) (100 ng/ml). After treatment, osteoclastogenesis was assessed using tartrate-resistant acid phosphatase (TRAP) assay. We observed that 6 h pretreatment with CA (1.25, 2.5, 5 μM) decreased RANKL-induced osteoclast formation and abolished RANKL-induced ROS generation by activating Nrf2 and its transcriptional targets. Further, CA also inhibited RANKL-induced activation of NF-κB and MAPK signalling. RANKL-induced mRNA expression of osteoclast related genes and transcription factors was also diminished by CA. In vivo osteolysis was developed in C57BL/6 male mice using lipopolysaccharide (LPS). Consistent with in vitro results, in vivo μ-CT analysis of femurs showed that bone mineral density (BMD), bone mineral content (BMC), and bone architecture parameters such as trabecular thickness (Tb.Th) and trabecular space (Tb.Sp) were positively modulated by CA in LPS-injected mice. The results obtained in this study support that CA inhibits RANKL-induced osteoclastogenesis by maintaining redox homeostasis through modulation of Nrf2 and NF-κB pathways.

KEY MESSAGES

Carnosic acid (CA) inhibits RANKL-induced osteoclastogenesis. CA inhibits RANKL-induced oxidative stress by upregulating Nrf2 transcriptional targets. CA attenuates RANKL-induced NF-κB and MAPK signalling activation. CA decreases NFATc1 and c-Fos expression. In vivo μ-CT analysis reveals that CA prevents bone loss in LPS-injected mice.

The Simultaneous Inhibitory Effect of Niclosamide on RANKL-Induced Osteoclast Formation and Osteoblast Differentiation

The bone destruction disease including osteoporosis and rheumatoid arthritis are caused by the imbalance between osteoblastogenesis and osteoclastogenesis. Inhibition of the NF-κB pathway was responsible for decreased osteoclastogenesis. Recently many studies indicated that niclosamide, the FDA approved an antihelminth drug, inhibits prostate and breast cancer cells growth by targeting NF-κB signaling pathways. This study evaluated the effects of niclosamide on osteoclast and osteoblast differentiation and function in vitro. In RANKL-induced murine osteoclast precursor cell RAW264.7 and M-CSF/RANKL-stimulated primary murine bone marrow-derived macrophages (BMM), niclosamide dose-dependently inhibited the formation of TRAP-positive multinucleated osteoclasts and resorption pits formation between 0.5uM and 1uM. In addition, niclosamide suppressed the expression of nuclear factor of activated T cells c1 (NFATc1) and osteoclast differentiated-related genes in M-CSF/ RANKL-stimulated BMM by interference with TRAF-6, Erk1/2, JNK and NF-κB activation pathways. However, the cytotoxic effects of niclosamide obviously appeared at the effective concentrations for inhibiting osteoclastogenesis (0.5-1uM) with increase of apoptosis through caspase-3 activation in osteoblast precursor cell line, MC3T3-E1. Niclosamide also inhibited ALP activity, bone mineralization and osteoblast differentiation-related genes expression in MC3T3-E1. Therefore, our findings suggest the new standpoint that niclosamide's effects on bones must be considered before applying it in any therapeutic treatment.

Fucoidan, a sulfated polysaccharide, inhibits osteoclast differentiation and function by modulating RANKL signaling

Multinucleated osteoclasts differentiate from hematopoietic progenitors of the monocyte/macrophage lineage. Because of its pivotal role in bone resorption, regulation of osteoclast differentiation is a potential therapeutic approach to the treatment of erosive bone disease. In this study, we have found that fucoidan, a sulfated polysaccharide extracted from brown seaweed, inhibited osteoclast differentiation. In particular, addition of fucoidan into the early stage osteoclast cultures significantly inhibited receptor activator of nuclear factor kappa B (NF-κB) ligand (RANKL)-induced osteoclast formation, thus suggesting that fucoidan affects osteoclast progenitors. Furthermore, fucoidan significantly inhibited the activation of RANKL-dependent mitogen-activated protein kinases (MAPKs) such as JNK, ERK, and p38, and also c-Fos and NFATc1, which are crucial transcription factors for osteoclastogenesis. In addition, the activation of NF-κB, which is an upstream transcription factor modulating NFATc1 expression, was alleviated in the fucoidan-treated cells. These results collectively suggest that fucoidan inhibits osteoclastogenesis from bone marrow macrophages by inhibiting RANKL-induced p38, JNK, ERK and NF-κB activation, and by downregulating the expression of genes that partake in both osteoclast differentiation and resorption.

Characterization of functional reprogramming during osteoclast development using quantitative proteomics and mRNA profiling

In addition to forming macrophages and dendritic cells, monocytes in adult peripheral blood retain the ability to develop into osteoclasts, mature bone-resorbing cells. The extensive morphological and functional transformations that occur during osteoclast differentiation require substantial reprogramming of gene and protein expression. Here we employ -omic-scale technologies to examine in detail the molecular changes at discrete developmental stages in this process (precursor cells, intermediate osteoclasts, and multinuclear osteoclasts), quantitatively comparing their transcriptomes and proteomes. The data have been deposited to the ProteomeXchange with identifier PXD000471.

Our analysis identified mitochondrial changes, along with several alterations in signaling pathways, as central to the development of mature osteoclasts, while also confirming changes in pathways previously implicated in osteoclast biology. In particular, changes in the expression of proteins involved in metabolism and redirection of energy flow from basic cellular function toward bone resorption appeared to play a key role in the switch from monocytic immune system function to specialized bone-turnover function. These findings provide new insight into the differentiation program involved in the generation of functional osteoclasts.

Genome-wide Association Studies for Osteoporosis: A 2013 Update

In the past few years, the bone field has witnessed great advances in genome-wide association studies (GWASs) of osteoporosis, with a number of promising genes identified. In particular, meta-analysis of GWASs, aimed at increasing the power of studies by combining the results from different study populations, have led to the identification of novel associations that would not otherwise have been identified in individual GWASs. Recently, the first whole genome sequencing study for osteoporosis and fractures was published, reporting a novel rare nonsense mutation. This review summarizes the important and representative findings published by December 2013. Comments are made on the notable findings and representative studies for their potential influence and implications on our present understanding of the genetics of osteoporosis. Potential limitations of GWASs and their meta-analyses are evaluated, with an emphasis on understanding the reasons for inconsistent results between different studies and clarification of misinterpretation of GWAS meta-analysis results. Implications and challenges of GWAS are also discussed, including the need for multi- and inter-disciplinary studies.

MKP-1 signaling events are required for early osteoclastogenesis in lineage defined progenitor populations by disrupting RANKL-induced NFATc1 nuclear translocation

Cytokine-directed osteoclastogenesis is initiated in response to macrophage colony stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) to drive formation of osteoclasts (OC), large bone resorptive cells of hematopoietic origin. RANKL-induced signaling activates the MAPK pathways, which initiates nuclear translocation of the master regulator of osteoclast formation, transcription factor NFATc1. Proper control over these signaling events is essential to normal OC formation response to stimuli. MAPK phosphatase 1 (MKP-1), a serine and tyrosine phosphatase encoded by the gene Dusp1, functions to dephosphorylate and subsequently inactivate MAPK (p38 and JNK) signaling essential in osteoclastogenesis. Here, we explored the role of MKP-1 during RANKL-driven osteoclastogenesis from defined (B220/CD45(-)GR1(-)CD11b(lo/-)CD115(+)) OC progenitor (dOCP) populations using WT and Dusp1(-/-) global knockout mice. Sorted cells were driven to OC by M-CSF pre-treatment followed by RANKL stimulation for 3days. OC formation and qPCR products were analyzed for maturation. Results indicate that Dusp1(-/-) dOCP form less numerous, significantly smaller and less functional OC compared to WT controls. These data were corroborated by mRNA expression of the key OC genes, Nfatc1 and Tm7sf4 (DC-STAMP), which were significantly reduced in early osteoclastogenesis in OC progenitor from Dusp1(-/-) mice. Intriguingly, our data reveals that MKP-1 may positively control OC formation in response to RANKL by regulating NFATc1 nuclear translocation. Collectively, this report supports the idea that MKP-1 signaling is essential in early osteoclastogenesis in response to RANKL-induced signaling.

Acteoside suppresses RANKL-mediated osteoclastogenesis by inhibiting c-Fos induction and NF-κB pathway and attenuating ROS production

Numerous studies have reported that inflammatory cytokines are important mediators for osteoclastogenesis, thereby causing excessive bone resorption and osteoporosis. Acteoside, the main active compound of Rehmannia glutinosa, which is used widely in traditional Oriental medicine, has anti-inflammatory and antioxidant potentials. In this study, we found that acteoside markedly inhibited osteoclast differentiation and formation from bone marrow macrophages (BMMs) and RAW264.7 macrophages stimulated by the receptor activator of nuclear factor-kappaB (NF-κB) ligand (RANKL). Acteoside pretreatment also prevented bone resorption by mature osteoclasts in a dose-dependent manner. Acteoside (10 µM) attenuated RANKL-stimulated activation of p38 kinase, extracellular signal-regulated kinases, and c-Jun N-terminal kinase, and also suppressed NF-κB activation by inhibiting phosphorylation of the p65 subunit and the inhibitor κBα. In addition, RANKL-mediated increases in the expression of c-Fos and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) and in the production of tumor necrosis factor-α, interleukin (IL)-1β, and IL-6 were apparently inhibited by acteoside pretreatment. Further, oral acteoside reduced ovariectomy-induced bone loss and inflammatory cytokine production to control levels. Our data suggest that acteoside inhibits osteoclast differentiation and maturation from osteoclastic precursors by suppressing RANKL-induced activation of mitogen-activated protein kinases and transcription factors such as NF-κB, c-Fos, and NFATc1. Collectively, these results suggest that acteoside may act as an anti-resorptive agent to reduce bone loss by blocking osteoclast activation.

Patient specific proteolytic activity of monocyte-derived macrophages and osteoclasts predicted with temporal kinase activation states during differentiation

Patient-to-patient variability in disease progression continues to complicate clinical decisions of treatment regimens for cardiovascular diseases, metastatic cancers and osteoporosis. Here, we investigated if monocytes, circulating white blood cells that enter tissues and contribute to disease progression by differentiating into macrophages or osteoclasts, could be useful in understanding this variability. Monocyte-derived macrophages and osteoclasts produce cysteine cathepsins, powerful extracellular matrix proteases which have been mechanistically linked to accelerated atherosclerotic, osteoporotic, and tumor progression. We hypothesized that multivariate analysis of temporal kinase activation states during monocyte differentiation could predict cathepsin proteolytic responses of monocyte-derived macrophages and osteoclasts in a patient-specific manner. Freshly isolated primary monocytes were differentiated with M-CSF or RANKL into macrophages or osteoclasts, respectively, and phosphorylation of ERK1/2, Akt, p38 MAPK, JNK, c-jun, and IκB-α were measured at days 1, 3, 6, and 9. In parallel, cell diameters and numbers of nuclei were measured, and multiplex cathepsin zymography was used to quantify cathepsins K, L, S, and V activity from cell extracts and conditioned media. There was extensive patient-to-patient variability in temporal kinase activation states, cell morphologies, and cathepsin K, L, S, and V proteolytic activity. Partial least squares regression models trained with temporal kinase activation states successfully predicted patient-specific morphological characteristics (mean cell diameter and number of nuclei) and patient-specific cathepsin proteolytic activity with predictability as high as 95%, even with the challenge of incorporating the complex, unknown cues from individual patients' unique genetic and biochemical backgrounds. This personalized medicine approach considers patient variability in kinase signals to predict cathepsin activity. Such analyses may provide beneficial tools for personalized kinase and protease inhibitor therapies for tissue destructive diseases.

The natural polyamines spermidine and spermine prevent bone loss through preferential disruption of osteoclastic activation in ovariectomized mice

BACKGROUND AND PURPOSE

Although naturally occurring polyamines are indispensable for a variety of cellular events in eukaryotic cells, little attention has been paid to their physiological and pathological significance in bone remodelling to date. In this study, we evaluated the pharmacological properties of several natural polyamines on the functionality and integrity of bone in both in vitro and in vivo experiments.

EXPERIMENTAL APPROACH

Mice were subjected to ovariectomy (OVX) and subsequent oral supplementation with either spermidine or spermine for determination of the bone volume together with different parameters regarding bone formation and resorption by histomorphometric analyses in vivo. Pre-osteoclasts were cultured with receptor activator of NF-κB ligand (RANKL), with or without spermidine and spermine to determine cellular maturation by tartrate-resistant acid phosphatase (TRAP) staining and cellular viability by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide reduction in vitro.

KEY RESULTS

Spermidine or spermine, given in drinking water for 28 days, significantly prevented the increased osteoclast surface/bone surface ratio and the reduced bone volume following OVX in mice. Either spermidine or spermine significantly inhibited the increased number of multinucleated TRAP-positive cells in osteoclasts cultured with RANKL in a concentration-dependent manner without affecting cell survival.

CONCLUSIONS AND IMPLICATIONS

The natural polyamines spermidine and spermine prevented OVX-induced bone loss through the disruption of differentiation and maturation of osteoclasts, rather than affecting osteoblasts. The supplementation with these natural polyamines could be beneficial for the prophylaxis as well as therapy of metabolic bone diseases such as post-menopausal osteoporosis.

Coptisine inhibits RANKL-induced NF-κB phosphorylation in osteoclast precursors and suppresses function through the regulation of RANKL and OPG gene expression in osteoblastic cells

Excessive receptor activator of NF-κB ligand (RANKL) signaling causes enhanced osteoclast formation and bone resorption. The downregulation of RANKL expression and its downstream signals may be an effective therapeutic approach to the treatment of bone loss diseases such as osteoporosis. Here, we found that coptisine, one of the isoquinoline alkaloids from Coptidis Rhizoma, exhibited inhibitory effects on osteoclastogenesis in vitro. Although coptisine has been studied for its antipyretic, antiphotooxidative, dampness dispelling, antidote, antinociceptive, and anti-inflammatory activities in vitro and in vivo, its effects on osteoclastogenesis have not been investigated. Therefore, we evaluated the effects of coptisine on osteoblastic cells as well as osteoclast precursors for osteoclastogenesis in vitro. The addition of coptisine to cocultures of mouse bone marrow cells and primary osteoblastic cells with 10(-8) M 1α,25(OH)(2)D(3) caused significant inhibition of osteoclast formation in a dose-dependent manner. Reverse transcriptase polymerase chain reaction (RT-PCR) analyses revealed that coptisine inhibited RANKL gene expression and stimulated the osteoprotegerin gene expression induced by 1α,25(OH)(2)D(3) in osteoblastic cells. Coptisine strongly inhibited RANKL-induced osteoclast formation when added during the early stage of bone marrow macrophage (BMM) cultures, suggesting that it acts on osteoclast precursors to inhibit RANKL/RANK signaling. Among the RANK signaling pathways, coptisine inhibited NF-κB p65 phosphorylations, which are regulated in response to RANKL in BMMs. Coptisine also inhibited the RANKL-induced expression of NFATc1, which is a key transcription factor. In addition, 10 μM coptisine significantly inhibited both the survival of mature osteoclasts and their pit-forming activity in cocultures. Thus, coptisine has potential for the treatment or prevention of several bone diseases characterized by excessive bone destruction.

Inhibitory effect of luteolin on osteoclast differentiation and function

Osteoclasts are multinucleated cells that play a crucial role in bone resorption, and are formed by the fusion of mononuclear osteoclasts derived from osteoclast precursors of the macrophage lineage. Compounds that specifically target functional osteoclasts would be ideal candidates for anti-resorptive agents for clinical applications. In the present study, we investigated the effects of luteolin, a flavonoid, on the regulation of receptor activator of nuclear factor-kappaB ligand (RANKL)-induced osteoclastogenesis, functions and signaling pathway. Addition of luteolin to a coculture system of mouse bone marrow cells and ST2 cells in the presence of 10(-8) M 1alpha,25(OH)(2)D(3) caused significant inhibition of osteoclastogenesis. Luteolin had no effects on the 1alpha,25(OH)(2)D(3)-induced expressions of RANKL, osteoprotegerin and macrophage colony-stimulating factor mRNAs. Next, we examined the direct effects of luteolin on osteoclast precursors using bone marrow macrophages and RAW264.7 cells. Luteolin completely inhibited RANKL-induced osteoclast formation. Moreover, luteolin inhibited the bone resorption by mature osteoclasts accompanied by the disruption of their actin rings, and these effects were reversely induced by the disruption of the actin rings in mature osteoclasts. Finally, we found that luteolin inhibited RANKL-induced osteoclastogenesis through the suppression of ATF2, downstream of p38 MAPK and nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1) expression, respectively. Taken together, the present results indicate that naturally occurring luteolin has inhibitory activities toward both osteoclast differentiation and functions through inhibition of RANKL-induced signaling pathway as well as actin ring disruption, respectively.

Trolox prevents osteoclastogenesis by suppressing RANKL expression and signaling

Excessive receptor activator of NF-kappaB ligand (RANKL) signaling causes enhanced osteoclast formation and bone resorption. Thus, down-regulation of RANKL expression or its downstream signals may be a therapeutic approach to the treatment of pathological bone loss. In this study, we investigated the effects of Trolox, a water-soluble vitamin E analogue, on osteoclastogenesis and RANKL signaling. Trolox potently inhibited interleukin-1-induced osteoclast formation in bone marrow cell-osteoblast coculture by abrogating RANKL induction in osteoblasts. This RANKL reduction was attributed to the reduced production of prostaglandin E(2) via a down-regulation of cyclooxygenase-2 activity. We also found that Trolox inhibited osteoclast formation from bone marrow macrophages induced by macrophage colony-stimulating factor plus RANKL in a reversible manner. Trolox was effective only when present during the early stage of culture, which implies that it targets early osteoclast precursors. Pretreatment with Trolox did not affect RANKL-induced early signaling pathways, including MAPKs, NF-kappaB, and Akt. We found that Trolox down-regulated the induction by RANKL of c-Fos protein by suppressing its translation. Ectopic overexpression of c-Fos rescued the inhibition of osteoclastogenesis by Trolox in bone marrow macrophages. Trolox also suppressed interleukin-1-induced osteoclast formation and bone loss in mouse calvarial bone. Taken together, our findings indicate that Trolox prevents osteoclast formation and bone loss by inhibiting both RANKL induction in osteoblasts and c-Fos expression in osteoclast precursors.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

Defective osteoclastogenesis by IKKbeta-null precursors is a result of receptor activator of NF-kappaB ligand (RANKL)-induced JNK-dependent apoptosis and impaired differentiation

It has been reported previously that inhibitory kappaB kinase (IKK) supports osteoclastogenesis through NF-kappaB-mediated prevention of apoptosis. This finding suggests that the ligand for receptor activator of NF-kappaB (RANKL), the master osteoclastogenic cytokine, induces apoptosis of osteoclast precursors (OCPs) in the absence of IKKbeta/NF-kappaB competency. To validate this hypothesis, we sought to determine the pro-apoptotic signaling factors induced by RANKL in IKKbeta-null osteoclast OCPs and to rescue osteoclast differentiation in the absence of IKKbeta through their inhibition. To accomplish this, we generated mice that lack IKKbeta in multiple hematopoietic lineages, including OCPs. We found that these mice possess both in vitro and in vivo defects in osteoclast generation, in concurrence with previous reports, and that this defect is a result of susceptibility to RANKL-mediated apoptosis as a result of gain-of-function of JNK activation. We demonstrate that differentiation of OCPs depends on IKKbeta because reduced IKKbeta mRNA expression correlates with impaired induction of osteoclast differentiation markers in response to RANKL stimulation. We further show that fine-tuned inhibition of JNK activation in these cells inhibits RANKL-induced apoptosis and restores the ability of IKKbeta-null OCPs to become mature osteoclasts. Our data highlight the pro-osteoclastogenic and anti-apoptotic roles of IKKbeta in OCPs and identify a pro-apoptotic mechanism activated within the RANK signalosome.

Contributions to osteoclast biology from Japan

Bone is a dynamic tissue, in which bone formation by osteoblasts and bone resorption by osteoclasts continue throughout life. In 1998, we molecularly cloned osteoclast differentiation factor (ODF), a long-thought factor responsible for osteoclast formation. This review article describes how Japanese scientists contributed to osteoclast biology before and after the discovery of ODF. This review article is based on the Louis V. Avioli Memorial Lecture of the American Society for Bone and Mineral Research (ASBMR) held in Honolulu in September, 2007.

RGS12 is essential for RANKL-evoked signaling for terminal differentiation of osteoclasts in vitro

How RANKL evokes [Ca(2+)](i) oscillations and leads to osteoclast differentiation is unclear. We identified a new signaling protein, RGS12, and found that RGS12 is essential for [Ca(2+)](i) oscillations and osteoclast differentiation induced by RANKL. RGS12 may play a critical role in the RANKL-evoked PLCgamma-calcium channels-[Ca(2+)](i) oscillation-NFAT2 pathway.

INTRODUCTION

RANKL-induced [Ca(2+)](i) oscillations play a switch-on role in NFAT2 expression and osteoclast differentiation. However, RANKL evokes [Ca(2+)](i) oscillations and leads to osteoclast differentiation by an unknown mechanism. In this study, we identified a new RANKL-induced signaling protein, regulator of G signaling protein 12 (RGS12), and investigated its effect on osteoclast differentiation in vitro.

MATERIALS AND METHODS

We used a genome-wide screening approach to identify genes that are specifically or prominently expressed in osteoclasts. To study the role of the RGS12 in osteoclast differentiation, we used vector and lentivirus-based RNAi gene silencing technology to silence the RGS12 gene in the monocyte progenitor cell lines and primary bone marrow-derived monocytes (BMMs). The interaction between RGS12 and N-type calcium channels was elucidated using co-immunoprecipitation and immunoblotting.

RESULTS

We found that RGS12 was prominently expressed in osteoclast-like cells (OLCs) induced by RANKL. This result was further confirmed at both the mRNA and protein level in human osteoclasts and mouse OLCs. Silence of RGS12 expression using vector and lentivirus based RNA interference (RNAi) impaired phosphorylation of phospholipase C (PLC)gamma and blocked [Ca(2+)](i) oscillations, NFAT2 expression, and osteoclast differentiation in RANKL-induced RAW264.7 cells and BMMs. We further found that N-type calcium channels were expressed in OLCs after RANKL stimulation and that RGS12 directly interacted with the N-type calcium channels.

CONCLUSIONS

These results reveal that RGS12 is essential for the terminal differentiation of osteoclasts induced by RANKL. It is possible that RGS12 regulates osteoclast differentiation through a PLC gamma-calcium channel-[Ca(2+)](i) oscillation-NFAT2 pathway.