Activation of p38 mitogen-activated protein kinase is crucial in osteoclastogenesis induced by tumor necrosis factor

RANKL-RANK-OPG Pathway in Charcot Diabetic Foot: Pathophysiology and Clinical-Therapeutic Implications

Charcot Foot (CF), part of a broader condition known as Charcot Neuro-Osteoarthropathy (CNO), is characterized by neuropathic arthropathy with a progressive alteration of the foot. CNO is one of the most devastating complications in patients with diabetes mellitus and peripheral neuropathy but can also be caused by neurological or infectious diseases. The pathogenesis is multifactorial; many studies have demonstrated the central role of inflammation and the Receptor Activator of NF-κB ligand (RANKL)-Receptor Activator of NF-κB (RANK)-Osteoprotegerin (OPG) pathway in the acute phase of the disease, resulting in the serum overexpression of RANKL. This overexpression and activation of this signal lead to increased osteoclast activity and osteolysis, which is a prelude to bone destruction. The aim of this narrative review is to analyze this signaling pathway in bone remodeling, and in CF in particular, to highlight its clinical aspects and possible therapeutic implications of targeting drugs at different levels of the pathway. Drugs that act at different levels in this pathway are anti-RANKL monoclonal antibodies (Denosumab), bisphosphonates (BP), and calcitonin. The literature review showed encouraging data on treatment with Denosumab, although in a few studies and in small sample sizes. In contrast, BPs have been re-evaluated in recent years in relation to the high possibility of side effects, while calcitonin has shown little efficacy on CNO.

Effect of Enterococcus faecalis on osteoclastogenesis under cobalt-mimicked hypoxia in vitro

OBJECTIVE

The bone destruction in persistent apical periodontitis associated with infection and a periapical hypoxic microenvironment is not well known. Thus, we aimed to investigate the effects of Enterococcus faecalis on osteoclastogenesis under cobalt-mimicked hypoxia.

MATERIALS AND METHODS

Mouse bone marrow-derived macrophages (BMMs) were isolated as osteoclast precursors and stimulated by heat-killed E. faecalis in an environment of cobalt-mimicked hypoxia environment. The cell proliferation and apoptosis were detected using CCK-8 and flow cytometry, respectively. Osteoclast differentiation was determined via tartrate-resistant acid phosphatase staining (TRAP) and immunofluorescence staining. The osteoclastogenic protein and gene expressions were measured by western blotting and real-time PCR.

RESULTS

Under cobalt-mimicked hypoxia, E. faecalis markedly inhibited the proliferation of the BMMs and significantly promoted the apoptosis of the BMMs. The differentiation of the BMMs into osteoclasts was enhanced in the presence of the E. faecalis under hypoxia, and the expression of Blimp, c-Fos, and NFATc1 was up-regulated, while the expression of RBP-J was inhibited.

CONCLUSIONS

E. faecalis markedly promotes osteoclast differentiation under cobalt-mimicked hypoxia in vitro.

Melatonin Inhibits Osteoclastogenesis and Bone Loss in Ovariectomized Mice by Regulating PRMT1-Mediated Signaling

Melatonin, a pineal gland hormone, has been suggested to treat postmenopausal osteoporosis due to its inhibitory effect on osteoclast differentiation. We previously reported that protein arginine methyltransferase 1 (PRMT1) was an important mediator of receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclastogenesis. However, the relationship between melatonin and PRMT1 in osteoclast differentiation and estrogen deficiency-induced osteoporosis is unclear. In this study, we investigated the inhibitory mechanisms of melatonin in vitro and in vivo by focusing on PRMT1. Melatonin treatment effectively blocked RANKL-induced osteoclastogenesis by inhibiting PRMT1 and asymmetric dimethylarginine (ADMA) expression. RANKL-induced tumor necrosis factor receptor-associated factor 6 (TRAF6) and the phosphorylation of JNK were also suppressed by melatonin, and TRAF6 siRNA attenuated RANKL-induced p-JNK and PRMT1 production. Melatonin inhibited the transcriptional activity of NF-κB by interfering with the binding of PRMT1 and NF-κB subunit p65 in RANKL-treated bone marrow-derived macrophages. Our results also revealed that melatonin inhibits RANKL-induced PRMT1 expression through receptors-independent pathway. Thus, the anti-osteoclastogenic effect of melatonin was mediated by a cascade of inhibition of RANKL-induced TRAF6, JNK, PRMT1, and NF-κB signaling in melatonin receptors-independent pathway. In vivo, ovariectomy caused significant decreases in bone mineral density, but melatonin treatment alleviated the ovariectomized (OVX)-induced bone loss by inhibiting bone resorption. Furthermore, the expression PRMT1 and TRAP mRNA was upregulated in OVX-femurs, but effectively suppressed by melatonin injection. These findings suggest that melatonin inhibited osteoclast differentiation and estrogen deficiency-induced osteoporosis by suppressing RANKL-induced TRAF6, JNK, PRMT1, and NF-κB signaling cascades in melatonin receptors-independent pathway.

Osteoblast role in the pathogenesis of rheumatoid arthritis

In the pathogenesis of several rheumatic diseases, such as rheumatoid arthritis, spondyloarthritis, osteoarthritis, osteoporosis, alterations in osteoblast growth, differentiation and activity play a role. In particular, in rheumatoid arthritis bone homeostasis is perturbed: in addition to stimulating the pathologic bone resorption process performed by osteoclasts in course of rheumatoid arthritis, proinflammatory cytokines (such as Tumor Necrosis factor-α, Interleukin-1) can also inhibit osteoblast differentiation and function, resulting in net bone loss. Mouse models of rheumatoid arthritis showed that complete resolution of inflammation (with maximal reduction in the expression of pro-inflammatory factors) is crucial for bone healing, performed by osteoblasts activity. In fact, abnormal activity of factors and systems involved in osteoblast function in these patients has been described. A better understanding of the pathogenic mechanisms involved in osteoblast dysregulation could contribute to explain the generalized and focal articular bone loss found in rheumatoid arthritis. Nevertheless, these aspects have not been frequently and directly evaluated in studies. This review article is focused on analysis of the current knowledge about the role of osteoblast dysregulation occurring in rheumatoid arthritis: a better knowledge of these mechanisms could contribute to the realization of new therapeutic strategies.

Roles of Mitogen-Activated Protein Kinases in Osteoclast Biology

Bone undergoes continuous remodeling, which is homeostatically regulated by concerted communication between bone-forming osteoblasts and bone-degrading osteoclasts. Multinucleated giant osteoclasts are the only specialized cells that degrade or resorb the organic and inorganic bone components. They secrete proteases (e.g., cathepsin K) that degrade the organic collagenous matrix and establish localized acidosis at the bone-resorbing site through proton-pumping to facilitate the dissolution of inorganic mineral. Osteoporosis, the most common bone disease, is caused by excessive bone resorption, highlighting the crucial role of osteoclasts in intact bone remodeling. Signaling mediated by mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, has been recognized to be critical for normal osteoclast differentiation and activation. Various exogenous (e.g., toll-like receptor agonists) and endogenous (e.g., growth factors and inflammatory cytokines) stimuli contribute to determining whether MAPKs positively or negatively regulate osteoclast adhesion, migration, fusion and survival, and osteoclastic bone resorption. In this review, we delineate the unique roles of MAPKs in osteoclast metabolism and provide an overview of the upstream regulators that activate or inhibit MAPKs and their downstream targets. Furthermore, we discuss the current knowledge about the differential kinetics of ERK, JNK, and p38, and the crosstalk between MAPKs in osteoclast metabolism.

PRMT1 mediates RANKL-induced osteoclastogenesis and contributes to bone loss in ovariectomized mice

Protein arginine methylation is a novel form of posttranslational modification mediated by protein arginine methyltransferase (PRMTs). PRMT1, a major isoform of the PRMT family, is responsible for various biological functions, including cellular differentiation. Although the important function that PRMT1 plays in various tissues is being increasingly recognized, its role in receptor activation of NF-κB ligand (RANKL)-induced osteoclastogenesis or osteoporosis has not yet been described. Here, we show that PRMT1 is essential for RANKL-induced osteoclastogenesis in vitro and for bone loss in vivo. RANKL treatment increased the expression of PRMT1 and its nuclear localization in bone marrow-derived macrophages (BMDMs) in a c-Jun N-terminal kinase (JNK)-dependent manner. Silencing PRMT1 attenuated RANKL-induced osteoclastogenesis by decreasing tartrate-resistant acid phosphatase (TRAP)-positive cells and inhibiting F-actin ring formation and bone resorption, which was confirmed in a separate experiment using haploinsufficient cells from PRMT1^+/- mice. Our results also revealed that PRMT1 regulates the transcription activity of NF-κB by directly interacting with it in RANKL-treated BMDMs. An in vivo study showed that the haploinsufficiency of PRMT1 reduced the enzyme activity of TRAP and increased the bone mineral density in the metaphysis of ovariectomized (OVX) mice. Finally, treatment with estrogen (E2) downregulated the RANKL-induced expression of PRMT1, suggesting that estrogen may exert an inhibitory effect on osteoclastogenesis by suppressing PRMT1 expression. Our results suggest that PRMT1 plays an important role in the progression of osteoporosis and that it might be a good therapeutic target for postmenopausal osteoporosis.

A protein that helps trigger bone loss in postmenopausal osteoporosis could be a potential therapeutic target. After the menopause, decreases in estrogen hormone levels can lead to bone diseases including osteoporosis. Osteoporosis occurs when the bone remodeling process breaks down, and bone resorption by cells called osteoclasts outweighs bone formation. In a mouse model of postmenopausal osteoporosis, Jong-Hwan Park at Chonnam National University, Gwangju, South Korea and co-workers identified key players in the progression of the disease. The team focused on factors influencing the RANKL protein, a known controller of bone remodeling. They found that RANKL triggers the formation of osteoclasts via interaction with another protein, PRMT1. Suppression of PRMT1 by estrogen appears to inhibit excessive osteoclast formation, suggesting it could be a potential therapeutic target for treating osteoporosis.

Optimized Extract from Corylopsis coreana Uyeki (Hamamelidaceae) Flos Inhibits Osteoclast Differentiation

Osteoporosis is a metabolic disorder that decreases the stability against fractures of the spine, femur, and radius by weakening the strength and integrity of bones. Receptor activator of nuclear factor-kappa B ligand signaling ultimately activated nuclear factor-activated T cells c1, a major transcription factor for osteoclast formation. This study researched the effects of Corylopsis coreana (C. coreana) Uyeki flos extracts on the antiosteoclastic potential of macrophages and the phytochemicals contained therein. The alcoholic extract of C. coreana Uyeki flos inhibited the differentiation of osteoclast. We carried out the experiments of the pattern of differentiation of osteoclasts based on the alcoholic percentage of extracts. Among them, 80% alcoholic extract showed the highest inhibitory effect. The alcoholic extract was composed of phytochemicals such as bergenin, quercetin, and quercitrin. This extract inhibited not only mRNA expression levels of NFATc1, osteoclast-associated receptor (OSCAR), cathepsin K, and tartrate-resistant acid phosphatase (TRAP), but also the translational expression of NFATc1. The inhibitory effect for osteoclast differentiation of the alcoholic extract was confirmed using the resorption pit assay. This is the first scientific report of the antiosteoclastic effects of C. coreana Uyeki flos extract, which can be applied therapeutically for the treatment of osteoporosis.

Cyanidin Chloride inhibits ovariectomy-induced osteoporosis by suppressing RANKL-mediated osteoclastogenesis and associated signaling pathways

Over-production and activation of osteoclasts is a common feature of osteolytic conditions such as osteoporosis, tumor-associated osteolysis, and inflammatory bone erosion. Cyanidin Chloride, a subclass of anthocyanin, displays antioxidant and anti-carcinogenesis properties, but its role in osteoclastic bone resorption and osteoporosis is not well understood. In this study, we showed that Cyanidin Chloride inhibits osteoclast formation, hydroxyapatite resorption, and receptor activator of NF-κB ligand (RANKL)-induced osteoclast marker gene expression; including ctr, ctsk, and trap. Further investigation revealed that Cyanidin Chloride inhibits RANKL-induced NF-κB activation, suppresses the degradation of IκB-α and attenuates the phosphorylation of extracellular signal-regulated kinases (ERK). In addition, Cyanidin Chloride abrogated RANKL-induced calcium oscillations, the activation of nuclear factor of activated T cells calcineurin-dependent 1 (NFATc1), and the expression of c-Fos. Further, we showed that Cyanidin Chloride protects against ovariectomy-induced bone loss in vivo. Together our findings suggest that Cyanidin Chloride is capable of inhibiting osteoclast formation, hydroxyapatite resorption and RANKL-induced signal pathways in vitro and OVX-induced bone loss in vivo, and thus might have therapeutic potential for osteolytic diseases.

Granzyme A Contributes to Inflammatory Arthritis in Mice Through Stimulation of Osteoclastogenesis

OBJECTIVE

Granzyme A (GzmA) levels are elevated in the plasma and synovium of patients with rheumatoid arthritis (RA), suggesting involvement of this protease in the pathogenesis of the disease. GzmA contributes to sepsis by regulating the production of proinflammatory cytokines. The purpose of this study was to evaluate the contribution of GzmA to the pathogenesis of RA in vivo and to examine the possibility that GzmA acting via tumor necrosis factor (TNF) stimulates osteoclastogenesis.

METHODS

Inflammatory arthritis induced by type II collagen was evaluated in wild-type, GzmA-deficient, and perforin-deficient mice. The osteoclastogenic potential of GzmA was examined in vitro using bone marrow cells and colony-forming unit-granulocyte-macrophage (CFU-GM) cells and in vivo using GzmA-deficient mice.

RESULTS

Gene deletion of GzmA attenuated collagen-induced arthritis, including serum levels of proinflammatory cytokines, joint damage, and bone erosion in affected mice, suggesting that osteoclast activity is reduced in the absence of GzmA. Accordingly, GzmA-treated bone marrow cells produced multinucleated cells that fulfilled the criteria for mature osteoclasts: tartrate-resistant acid phosphatase (TRAP) activity, β integrin expression, calcitonin receptor expression, and resorptive activity on dentin slices. GzmA appeared to act without accessory cells, and its activity was not affected by osteoprotegerin, suggesting a minor contribution of RANKL. It also induced the expression and secretion of TNF. Neutralization of TNF or stimulation of CFU-GM cells from TNF^-/- mice prevented GzmA-induced osteoclastogenesis. GzmA-deficient mice had reduced osteoclastogenesis in vivo (fewer calcitonin receptor-positive multinucleated cells and fewer transcripts for cathepsin K, matrix metalloproteinase 9, and TRAP in joints) and reduced serum levels of C-terminal telopeptide of type I collagen.

CONCLUSION

GzmA contributes to the joint destruction of RA partly by promoting osteoclast differentiation.

Rheumatoid arthritis: Recent advances on its etiology, role of cytokines and pharmacotherapy

An autoimmune disease is defined as a clinical syndrome resulted from an instigation of both T cell and B cell or individually, in the absence of any present infection or any sort of distinguishable cause. Clonal deletion of auto reactive cells remains the central canon of immunology for decades, keeping the role of T cell and B cell aside, which are actually the guards to recognize the entry of foreign body. According to NIH, 23.5 million Americans are all together affected by these diseases. They are rare, but with the exception of RA. Rheumatoid arthritis is chronic and systemic autoimmune response to the multiple joints with unknown ethology, progressive disability, systemic complications, early death and high socioeconomic costs. Its ancient disease with an old history found in North American tribes since 1500 BCE, but its etiology is yet to be explored. Current conventional and biological therapies used for RA are not fulfilling the need of the patients but give only partial responses. There is a lack of consistent and liable biomarkers of prognosis therapeutic response, and toxicity. Rheumatoid arthritis is characterized by hyperplasic synovium, production of cytokines, chemokines, autoantibodies like rheumatoid factor (RF) and anticitrullinated protein antibody (ACPA), osteoclastogensis, angiogenesis and systemic consequences like cardiovascular, pulmonary, psychological, and skeletal disorders. Cytokines, a diverse group of polypeptides, play critical role in the pathogenesis of RA. Their involvement in autoimmune diseases is a rapidly growing area of biological and clinical research. Among the proinflammatory cytokines, IL-1α/β and TNF-α trigger the intracellular molecular signalling pathway responsible for the pathogenesis of RA that leads to the activation of mesenchymal cell, recruitment of innate and adaptive immune system cells, activation of synoviocytes which in term activates various mediators including tumour necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), interleukin-6 (IL-6) and interleukin-8 (IL-8), resulting in inflamed synovium, increase angiogenesis and decrease lymphangiogensis. Their current pharmacotherapy should focus on their three phases of progression i.e. prearthritis phase, transition phase and clinical phase. In this way we will be able to find a way to keep the balance between the pro and anti-inflammatory cytokines that is believe to be the dogma of pathogenesis of RA. For this we need to explore new agents, whether from synthetic or natural source to find the answers for unresolved etiology of autoimmune diseases and to provide a quality of life to the patients suffering from these diseases specifically RA.

PINTnet: construction of condition-specific pathway interaction network by computing shortest paths on weighted PPI

Background

Identifying perturbed pathways in a given condition is crucial in understanding biological phenomena. In addition to identifying perturbed pathways individually, pathway analysis should consider interactions among pathways. Currently available pathway interaction prediction methods are based on the existence of overlapping genes between pathways, protein-protein interaction (PPI) or functional similarities. However, these approaches just consider the pathways as a set of genes, thus they do not take account of topological features. In addition, most of the existing approaches do not handle the explicit gene expression quantity information that is routinely measured by RNA-sequecing.

Results

To overcome these technical issues, we developed a new pathway interaction network construction method using PPI, closeness centrality and shortest paths. We tested our approach on three different high-throughput RNA-seq data sets: pregnant mice data to reveal the role of serotonin on beta cell mass, bone-metastatic breast cancer data and autoimmune thyroiditis data to study the role of IFN- α. Our approach successfully identified the pathways reported in the original papers. For the pathways that are not directly mentioned in the original papers, we were able to find evidences of pathway interactions by the literature search. Our method outperformed two existing approaches, overlapping gene-based approach (OGB) and protein-protein interaction-based approach (PB), in experiments with the three data sets.

Conclusion

Our results show that PINTnet successfully identified condition-specific perturbed pathways and the interactions between the pathways. We believe that our method will be very useful in characterizing biological mechanisms at the pathway level. PINTnet is available at http://biohealth.snu.ac.kr/software/PINTnet/.

Ellagic acid inhibits RANKL-induced osteoclast differentiation by suppressing the p38 MAP kinase pathway

Bone undergoes continuous remodeling by a coupled action between osteoblasts and osteoclasts. During osteoporosis, osteoclast activity is often elevated leading to increased bone destruction. Hence, osteoclasts are deemed as potential therapeutic targets to alleviate bone loss. Ellagic acid (EA) is a polyphenol reported to possess anticancer, antioxidant and anti-inflammatory properties. However, its effects on osteoclast formation and function have not yet been examined. Here, we explored the effects of EA on RANKL-induced osteoclast differentiation in RAW264.7 murine macrophages (in vitro) and human CD14⁺monocytes (ex vivo). EA dose-dependently attenuated RANKL-induced TRAP⁺ osteoclast formation in osteoclast progenitors with maximal inhibition seen at 1 µM concentration without cytotoxicity. Moreover, owing to perturbed osteoclastogenesis, EA disrupted actin ring formation and bone resorptive function of osteoclasts. Analysis of the underlying molecular mechanisms revealed that EA suppressed the phosphorylation and activation of the p38 MAP kinase pathway which subsequently impaired the RANKL-induced differentiation of osteoclast progenitors. Taken together, these novel results indicate that EA alleviates osteoclastogenesis by suppressing the p38 signaling pathway downstream of RANKL and exerts inhibitory effects on bone resorption and actin ring formation.

Calycosin Suppresses RANKL-Mediated Osteoclastogenesis through Inhibition of MAPKs and NF-κB

Calycosin, an isoflavonoid phytoestrogen, isolated from Radix Astragali, was reported to possess anti-tumor, anti-inflammation, and osteogenic properties, but its impact on osteoclast differentiation remains unclear. In this study, we examined the effects of calycosin on osteoclastogenesis induced by RANKL. The results showed that calycosin significantly inhibited RANKL-induced osteoclast formation from primary bone marrow macrophages (BMMs). Calycosin also dose-dependently suppressed the formation of bone resorption pits by mature osteoclasts. In addition, the expression of osteoclatogenesis-related genes, including cathepsin K (CtsK), tartrate-resistant acid phosphatase (TRAP), and MMP-9, was significantly inhibited by calycosin. Furthermore, the results indicated that calycosin down-regulated the expression levels of NFATc1 and c-Fos through suppressing the activation of NF-κB and MAPKs. Our results indicate that calycosin has an inhibitory role in the bone loss by preventing osteoclast formation, as well as its bone resorptive activity. Therefore, calycosin may be useful as a therapeutic reagent for bone loss-associated diseases.

A systems pharmacology approach to investigate the mechanisms of action of Semen Strychni and Tripterygium wilfordii Hook F for treatment of rheumatoid arthritis

ETHNOPHARMACOLOGY RELEVANCE

The angiogenesis control at the initiation of rheumatoid arthritis (RA) that mainly blocks the inflammatory cascades expects to attenuate the action of angiogenic mediators, synovial angiogenesis, and to partially reverse the erosive bone damage. Two typical Chinese herbs, Semen Strychni and Tripterygium wilfordii Hook F (TwHF) have been used as a remedy to treat RA since ancient time. However, their functioning mechanisms are still unknown. Thus it is necessary to exploit their underlying mechanism for the treatment of RA.

METHODS

This study was undertaken to analyze their underlying mechanism based on a systems biology platform. Firstly, active components of the two herbs were screened out from TcmSP database based on their OB and DL values. Then their potential targets were predicted by using Random Forest, Support Vector Machine, and validated via docking process. Finally, a network of compound-target was constructed.

RESULTS

In this work, 27 and 33 active compounds were screened out from Semen Strychni and TwHF, targeting 28 and 32 potential proteins, respectively. The results show that the two herbs modulate the angiogenesis mediators through both direct and indirect pathways, and 21 common targets shared by Semen Strychni and TwHF bear major responsibility for treating RA.

CONCLUSIONS

The underlying mechanism of Semen Strychni and TwHF in treatment of RA is through multiple targets interaction by their blocking of the angiogenesis mediator cascades. This may provide us a better understanding of the function of the two herbs for the treatment of RA, as well as a clue to unveil their possible treatment effects of other systemic diseases, and in this way, hopefully the screening models may facilitate the discovery of novel combined drugs.

Enterococcus faecalis promotes osteoclastogenesis and semaphorin 4D expression

Enterococcus faecalis is considered a major bacterial pathogen implicated in endodontic infections and contributes considerably to periapical periodontitis. This study aimed to investigate the potential mechanisms by which E. faecalis accounts for the bone destruction in periapical periodontitis in vitro. Osteoclast precursor RAW264.7 cells were treated with E. faecalis ATCC 29212 and a wild strain of E. faecalis derived clinically from an infected root canal. The results showed that, to some extent, E. faecalis induced the RAW264.7 cells to form tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclast-like cells. This pathogen markedly stimulated RAW264.7 cells to express semaphorin 4D (Sema4D), which inhibits bone formation. Once RAW264.7 cells were primed by low-dose receptor activator of nuclear factor-kappa B ligand (RANKL), E. faecalis could significantly increase the production of TRAP-positive multinucleated cells and up-regulate the expression of osteoclast-specific markers, including NFATc1, TRAP and cathepsin K. Both p38 and ERK1/2 MAPK signaling pathways were activated by E. faecalis in RANKL-primed RAW264.7 cells, and meanwhile the expression of Sema4D was highly increased. In conclusion, E. faecalis may greatly contribute to the bone resorption in periapical periodontitis by promoting RANKL-dependent osteoclastogenesis and expression of Sema4D through activation of p38 and ERK1/2 MAPK signaling pathways.

Focus on the p38 MAPK signaling pathway in bone development and maintenance

The p38 mitogen-activated protein kinase (MAPK) signaling pathway can be activated in response to a wide range of extracellular signals. As a consequence, it can generate many different biological effects that depend on the stimulus and on the activated cell type. Therefore, this pathway has been found to regulate many aspects of tissue development and homeostasis. Recent work with the aid of genetically modified mice has highlighted the physiological functions of this pathway in skeletogenesis and postnatal bone maintenance. In this review, emphasis is given to the roles of the p38 MAPK pathway in chondrocyte, osteoblast and osteoclast biology. In particular, we describe the molecular mechanisms of p38 MAPK activation and downstream targets. The requirement of this pathway in physiological bone development and homeostasis is demonstrated by the ability of p38 MAPK to regulate master transcription factors controlling geneses and functions of chondrocytes, osteoblasts and osteoclasts.

Parthenolide inhibits osteoclast differentiation and bone resorbing activity by down-regulation of NFATc1 induction and c-Fos stability, during RANKL-mediated osteoclastogenesis

Parthenolide, a natural product derived from Feverfew, prevents septic shock and inflammation. We aimed to identify the effects of parthenolide on the RANKL (receptor activator of NF-κB ligand)-induced differentiation and bone resorbing activity of osteoclasts. In this study, parthenolide dose-dependently inhibited RANKL-mediated osteoclast differentiation in BMMs, without any evidence of cytotoxicity and the phosphorylation of p38, ERK, and IκB, as well as IκB degradation by RANKL treatment. Parthenolide suppressed the expression of NFATc1, OSCAR, TRAP, DC-STAMP, and cathepsin K in RANKL-treated BMMs. Furthermore, parthenolide down-regulated the stability of c-Fos protein, but could not suppress the expression of c-Fos. Overexpression of NFATc1 and c-Fos in BMMs reversed the inhibitory effect of parthenolide on RANKL-mediated osteoclast differentiation. Parthenolide also inhibited the bone resorbing activity of mature osteoclasts. Parthenolide inhibits the differentiation and bone-resolving activity of osteoclast by RANKL, suggesting its potential therapeutic value for bone destructive disorders associated with osteoclast-mediated bone resorption. [BMB Reports 2014; 47(8): 451-456]

Sexual Dimorphism in MAPK-Activated Protein Kinase-2 (MK2) Regulation of RANKL-Induced Osteoclastogenesis in Osteoclast Progenitor Subpopulations

Osteoclasts (OCs) are bone-resorptive cells critical for maintaining skeletal integrity through coupled bone turnover. OC differentiation and activation requires receptor activator of NF-kB ligand (RANKL) signaling through the p38 MAPK pathway. However the role of the p38 MAPK substrate, MAPK-activated protein kinase 2 (MK2), is not clearly delineated. Within the bone marrow exists a specific subpopulation of defined osteoclast progenitor cells (dOCPs) with surface expression of B220^-Gr1^-CD11b^lo/-CD115⁺ (dOCP^lo/-). In this study, we isolated dOCPs from male and female mice to determine sex-specific effects of MK2 signaling in osteoclastogenesis (OCgen). Male Mk2^-/- mice display an increase in the dOCP^lo cell population when compared to Mk2^+/+ mice, while female Mk2^-/- and Mk2^+/+ mice exhibit no difference. Defined OCPs from male and female Mk2^+/+ and Mk2^-/- bone marrow were treated with macrophage colony stimulation factor (M-CSF) and RANKL cytokines to promote OCgen. RANKL treatment of dOCP^lo cells stimulated p38 and MK2 phosphorylation. Tartrate-resistant acid phosphatase (TRAP) assays were used to quantify OC number, size, and TRAP enzyme activity post-RANKL stimulation. MK2 signaling was critical for male dOCP^lo OCgen, yet MK2 signaling regulated OCgen from female dOCP^- and CD11b^hi subpopulations as well. The functional gene, Ctsk, was attenuated in both male and female Mk2^-/- dOCP^lo-derived OCs. Conversely, MK2 signaling was only critical for gene expression of pre-OC fusion genes, Oc-stamp andTm7sf4, in male OCgen. Therefore, these data suggest there is a sexual dimorphism in MK2 signaling of OCP subpopulations.

The RANKL-RANK Story

Receptor activator of nuclear factor x03BA;B (RANK) and its ligand (RANKL) have originally been described for their key roles in bone metabolism and the immune system. Subsequently, it has been shown that the RANKL-RANK system is critical in the formation of mammary epithelia in lactating females and the thermoregulation of the central nervous system. RANKL and RANK are under the tight control of the female sex hormones estradiol and progesterone. A reduction of the circulating female sex hormones leading to an increase in RANKL-RANK signaling is the leading cause of osteoporosis in postmenopausal women. Denosumab, a human monoclonal anti-RANKL antibody, has been approved for the treatment of postmenopausal osteoporosis, where it is showing great promise. In addition, RANKL-RANK signaling also plays a critical role in other bone pathologies, bone metastasis or hormone-driven breast cancer. This review will highlight some of the functions of RANKL-RANK in bone turnover, the immune system and brain with a focus on the regulatory role of the female sex hormones.

Ampelopsis brevipedunculata extract prevents bone loss by inhibiting osteoclastogenesis in vitro and in vivo

Osteoclasts play a critical role in bone resorbing disorders such as osteoporosis, periodontitis, and rheumatoid arthritis. Therefore, discovery of agents capable of suppressing osteoclast differentiation may aid the development of a therapeutic access for the treatment of pathological bone loss. Ampelopsis brevipedunculata has been used as herbal folk medicine to treat liver diseases and inflammation in Asia. However, its effects on osteoclast differentiation are unknown. We were aimed to investigate the anti-osteoclastogenic activity in vitro and in vivo and to elucidate the underlying mechanism of Ampelopsis brevipedunculata extract (ABE). In this study, ABE inhibited receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation, the formation of filamentous actin rings and the bone resorbing activity of mature osteoclasts. ABE inhibited RANKL-induced p38 and IκB phosphorylation and IκB degradation. Also, ABE suppressed the mRNA and protein expression of nuclear factor of activated T cells c1 (NFATc1) and c-Fos, and the mRNA expression of genes required for cell fusion and bone resorption, such as osteoclast-associatedreceptor (OSCAR), tartrate resistant acid phosphatase (TRAP), cathepsinK, dendritic cell-specific transmembrane protein(DC-STAMP), β3-integrin and osteoclast stimulatory transmembrane protein(OC-STAMP). Furthermore, results of micro-CT and histologic analysis indicated that ABE remarkably prevented lipopolysaccharide (LPS)-induced bone erosion. These results demonstrate that ABE prevents LPS-induced bone erosion through inhibition of osteoclast differentiation and function, suggesting the promise of ABE as a potential cure for various osteoclast-associated bone diseases.

The prevention of titanium-particle-induced osteolysis by OA-14 through the suppression of the p38 signaling pathway and inhibition of osteoclastogenesis

Wear-particle-induced osteolysis leads to prosthesis loosening, which is one of the most common causes of joint-implant failure, a problem that must be fixed using revision surgery. Thus, a potential treatment for prosthetic loosening is focused on inhibiting osteoclastic bone resorption, which prevents wear-particle-induced osteolysis. In this study, we synthesized a compound named OA-14 (N-(3- (dodecylcarbamoyl)phenyl)-1H-indole-2-carboxamide) and examined how OA-14 affects titanium (Ti)-particle-induced osteolysis and osteoclastogenesis. We report that OA-14 treatment protected against Ti-particle-induced osteolysis in a mouse calvarial model. Interestingly, the number of tartrate-resistant acid phosphatase-positive osteoclasts decreased after treatment with OA-14 in vivo, which suggested that OA-14 inhibits osteoclast formation. To test this hypothesis, we conducted in vitro studies, and our results revealed that OA-14 markedly diminished osteoclast differentiation and osteoclast-specific gene expression in a dose- and time-dependent manner. Moreover, OA-14 suppressed osteoclastic bone resorption and F-actin ring formation. Furthermore, we determined that OA-14 inhibited osteoclastogenesis by specifically blocking the p38-Mitf-c-fos-NFATc1 signaling cascade induced by RANKL (ligand of receptor activator of nuclear factor κB). Collectively, our results suggest that the compound OA-14 can be safely used for treating particle-induced peri-implant osteolysis and other diseases caused by excessive osteoclast formation and function.

Differential roles of MAPK kinases MKK3 and MKK6 in osteoclastogenesis and bone loss

Bone mass is maintained by osteoclasts that resorb bone and osteoblasts that promote matrix deposition and mineralization. Bone homeostasis is altered in chronic inflammation as well as in post-menopausal loss of estrogen, which favors osteoclast activity that leads to osteoporosis. The MAPK p38α is a key regulator of bone loss and p38 inhibitors preserve bone mass by inhibiting osteoclastogenesis. p38 function is regulated by two upstream MAPK kinases, namely MKK3 and MKK6. The goal of this study was to assess the effect of MKK3- or MKK6-deficiency on osteoclastogenesis in vitro and on bone loss in ovariectomy-induced osteoporosis in mice. We demonstrated that MKK3 but not MKK6, regulates osteoclast differentiation from bone marrow cells in vitro. Expression of NFATc1, a master transcription factor in osteoclastogenesis, is decreased in cells lacking MKK3 but not MKK6. Expression of osteoclast-specific genes Cathepsin K, osteoclast-associated receptor and MMP9, was inhibited in MKK3−/− cells. The effect of MKK-deficiency on ovariectomy-induced bone loss was then evaluated in female WT, MKK3−/− and MKK6−/− mice by micro-CT analysis. Bone loss was partially inhibited in MKK3−/− as well as MKK6−/− mice, despite normal osteoclastogenesis in MKK6−/− cells. This correlated with the lower osteoclast numbers in the MKK-deficient ovariectomized mice. These studies suggest that MKK3 and MKK6 differentially regulate bone loss due to estrogen withdrawal. MKK3 directly mediates osteoclastogenesis while MKK6 likely contributes to pro-inflammatory cytokine production that promotes osteoclast formation.

The paired-box homeodomain transcription factor Pax6 binds to the upstream region of the TRAP gene promoter and suppresses receptor activator of NF-κB ligand (RANKL)-induced osteoclast differentiation

Osteoclast formation is regulated by balancing between the receptor activator of nuclear factor-κB ligand (RANKL) expressed in osteoblasts and extracellular negative regulatory cytokines such as interferon-γ (IFN-γ) and interferon-β (IFN-β), which can suppress excessive bone destruction. However, relatively little is known about intrinsic negative regulatory factors in RANKL-mediated osteoclast differentiation. Here, we show the paired-box homeodomain transcription factor Pax6 acts as a negative regulator of RANKL-mediated osteoclast differentiation. Electrophoretic mobility shift and reporter assays found that Pax6 binds endogenously to the proximal region of the tartrate acid phosphatase (TRAP) gene promoter and suppresses nuclear factor of activated T cells c1 (NFATc1)-induced TRAP gene expression. Introduction of Pax6 retrovirally into bone marrow macrophages attenuates RANKL-induced osteoclast formation. Moreover, we found that the Groucho family member co-repressor Grg6 contributes to Pax6-mediated suppression of the TRAP gene expression induced by NFATc1. These results suggest that Pax6 interferes with RANKL-mediated osteoclast differentiation together with Grg6. Our results demonstrate that the Pax6 pathway constitutes a new aspect of the negative regulatory circuit of RANKL-RANK signaling in osteoclastogenesis and that the augmentation of Pax6 might therefore represent a novel target to block pathological bone resorption.

New insights into the role of sequestosome 1/p62 mutant proteins in the pathogenesis of Paget's disease of bone

Paget's disease of bone (PDB) is characterized by focal areas of aberrant and excessive bone turnover, specifically increased bone resorption and disorganized bone formation. Germline mutations in the sequestosome 1/p62 (SQSTM1/p62) gene are common in PDB patients, with most mutations affecting the ubiquitin-associated domain of the protein. In vitro, osteoclast precursor cells expressing PDB-mutant SQSTM1/p62 protein are associated with increases in nuclear factor κB activation, osteoclast differentiation, and bone resorption. Although the precise mechanisms by which SQSTM1/p62 mutations contribute to disease pathogenesis and progression are not well defined, it is apparent that as well as affecting nuclear factor κB signaling, SQSTM1/p62 is a master regulator of ubiquitinated protein turnover via autophagy and the ubiquitin-proteasome system. Additional roles for SQSTM1/p62 in the oxidative stress-induced Keap1/Nrf2 pathway and in caspase-mediated apoptosis that were recently reported are potentially relevant to the pathogenesis of PDB. Thus, SQSTM1/p62 may serve as a molecular link or switch between autophagy, apoptosis, and cell survival signaling. The purpose of this review is to outline recent advances in understanding of the multiple pathophysiological roles of SQSTM1/p62 protein, with particular emphasis on their relationship to PDB, including challenges associated with translating SQSTM1/p62 research into clinical diagnosis and treatment.

Inhibition of p38-MAPK signaling pathway attenuates breast cancer induced bone pain and disease progression in a murine model of cancer-induced bone pain

BACKGROUND

Mechanisms driving cancer-induced bone pain are poorly understood. A central factor implicated to be a key player in the process of tumorigenesis, osteoclastogenesis and nociception is p38 MAPK. We determined the role of p38 MAPK in a mouse model of breast cancer induced bone pain in which mixed osteolytic and osteoblastic remodeling occurs.

RESULTS

In cancer-treated mice, acute as well as chronic inhibition of p38 MAPK with SB203580 blocked flinching and guarding behaviors in a dose-dependent manner whereas no effect on thresholds to tactile stimuli was observed. Radiographic analyses of bones demonstrated that chronic inhibition of p38 MAPK reduced bone loss and incidence of spontaneous fracture in cancer-treated mice. Histological analysis of bones collected from mice treated with the p38 MAPK inhibitor showed complete absence of osteoblastic growth in the intramedullary space as well as significantly reduced tumor burden.

CONCLUSIONS

Blockade of non-evoked pain behaviors but not hypersensitivity suggests differences in the underlying mechanisms of specific components of the pain syndrome and a possibility to individualize aspects of pain management. While it is not known whether the role of p38 MAPK signaling can be expanded to other cancers, the data suggest a need for understanding molecular mechanisms and cellular events that initiate and maintain cancer-induced bone pain for effective management for both ongoing pain as well as breakthrough pain.

P38 mitogen-activated protein kinase inhibitor, FR167653, inhibits parathyroid hormone related protein-induced osteoclastogenesis and bone resorption

p38 mitogen-activated protein kinase (MAPK) acts downstream in the signaling pathway that includes receptor activator of NF-κB (RANK), a powerful inducer of osteoclast formation and activation. We investigated the role of p38 MAPK in parathyroid hormone related protein (PTHrP)-induced osteoclastogenesis in vitro and PTHrP-induced bone resorption in vivo. The ability of FR167653 to inhibit osteoclast formation was evaluated by counting the number of tartrate-resistant acid phosphatase positive multinucleated cells (TRAP-positive MNCs) in in vitro osteoclastgenesis assays. Its mechanisms were evaluated by detecting the expression level of c-Fos and nuclear factor of activated T cells c1 (NFATc1) in bone marrow macrophages (BMMs) stimulated with sRANKL and M-CSF, and by detecting the expression level of osteoprotegerin (OPG) and RANKL in bone marrow stromal cells stimulated with PTHrP in the presence of FR167653. The function of FR167653 on bone resorption was assessed by measuring the bone resorption area radiographically and by counting osteoclast number per unit bone tissue area in calvaria in a mouse model of bone resorption by injecting PTHrP subcutaneously onto calvaria. Whole blood ionized calcium levels were also recorded. FR167653 inhibited PTHrP-induced osteoclast formation and PTHrP-induced c-Fos and NFATc1 expression in bone marrow macrophages, but not the expression levels of RANKL and OPG in primary bone marrow stromal cells treated by PTHrP. Furthermore, bone resorption area and osteoclast number in vivo were significantly decreased by the treatment of FR167653. Systemic hypercalcemia was also partially inhibited. Inhibition of p38 MAPK by FR167653 blocks PTHrP-induced osteoclastogenesis in vitro and PTHrP-induced bone resorption in vivo, suggesting that the p38 MAPK signaling pathway plays a fundamental role in PTHrP-induced osteoclastic bone resorption.

Naringin abrogates osteoclastogenesis and bone resorption via the inhibition of RANKL-induced NF-κB and ERK activation

Osteolytic bone diseases including osteoporosis are commonly accompanied with enhanced osteoclast formation and bone resorption. Naringin, a natural occurring flavonoid has been found to protect against retinoic acid-induced osteoporosis and improve bone quality in rats. Here, we showed that naringin perturbs osteoclast formation and bone resorption by inhibiting RANK-mediated NF-κB and ERK signaling. Naringin suppressed gene expression of key osteoclast marker genes. Naringin was found to inhibit RANKL-induced activation of NF-κB by suppressing RANKL-mediated IκB-α degradation. In addition, naringin inhibited RANKL-induced phosphorylation of ERK. This study identifies naringin as an inhibitor for osteoclast formation and bone resorption, and provides evidence that natural compounds such as naringin might be beneficial as an alternative medicine for the prevention and treatment of osteolysis.

Advances in osteoclast biology: old findings and new insights from mouse models

The maintenance of adequate bone mass is dependent upon the controlled and timely removal of old, damaged bone. This complex process is performed by the highly specialized, multinucleated osteoclast. Over the past 15 years, a detailed picture has emerged describing the origins, differentiation pathways and activation stages that contribute to normal osteoclast function. This information has primarily been obtained by the development and skeletal analysis of genetically modified mouse models. Mice harboring mutations in specific genetic loci exhibit bone defects as a direct result of aberrations in normal osteoclast recruitment, formation or function. These findings include the identification of the RANK-RANKL-OPG system as a primary mediator of osteoclastogenesis, the characterization of ion transport and cellular attachment mechanisms and the recognition that matrix-degrading enzymes are essential components of resorptive activity. This Review focuses on the principal observations in osteoclast biology derived from genetic mouse models, and highlights emerging concepts that describe how the osteoclast is thought to contribute to the maintenance of adequate bone mass and integrity throughout life.

Tumor necrosis factor-α signaling in macrophages

Tumor necrosis factor-α (TNFα) was cloned over 2 decades ago and its identification in part led to the discovery of a super family of tumor necrosis factors (TNFs) and their receptors. TNFα signals through two transmembrane receptors, TNFR1 and TNFR2, and regulates a number of critical cell functions including cell proliferation, survival, differentiation, and apoptosis. Macrophages are the major producers of TNFα and interestingly are also highly responsive to TNFα. Aberrant TNFα production and TNF receptor signaling have been associated with the pathogenesis of several diseases, including rheumatoid arthritis, Crohn's disease, atherosclerosis, psoriasis, sepsis, diabetes, and obesity. TNFα has been shown to play a pivotal role in orchestrating the cytokine cascade in many inflammatory diseases and because of this role as a "master-regulator" of inflammatory cytokine production, it has been proposed as a therapeutic target for a number of diseases. Indeed anti-TNFα drugs are now licensed for treating certain inflammatory diseases including rheumatoid arthritis and inflammatory bowel disease. In this review we discuss the discovery of TNFα and its actions especially in regulating macrophage biology. Given its importance in several human diseases, we also briefly discuss the role of anti-TNFα therapeutics in the treatment of inflammatory diseases.

The alpha-isoform of p38 MAPK specifically regulates arthritic bone loss

Pharmacological inhibitors have provided evidence for the key role of p38 MAPK in osteoclast differentiation and in inflammation-induced bone loss. However, these inhibitors block more than one of the four p38 isoforms, usually p38alpha and p38beta, and sometimes also other kinases such as JNK3. We show in this study that p38alpha is the main p38 isoenzyme expressed in the osteoclast precursors and in the mature osteoclasts. p38alpha as well as its downstream substrates were phosphorylated in osteoclast progenitors stimulated by TNF-alpha. Using Mx-cre-mediated conditional gene inactivation we demonstrated that mice lacking p38alpha were protected against TNF-alpha-induced bone destruction at the site of inflammation as well as against TNF-alpha-mediated systemic bone loss. The bone protection was associated to decreased osteoclast numbers in vivo as well as a decreased IL-1beta expression in the inflamed tissue and in the isolated monocytes. The phenotype was cell autonomous because, similarly to p38alpha-deficient cells, knockdown of p38alpha in monocytes resulted in a decreased osteoclast differentiation in vitro. It was not caused by major changes in RANKL-mediated ERK or JNK activation but rather associated to an increased NF-kappaB activation caused by a decrease in IkappaBalpha recovery. Thus, our data show that developing specific inhibitors of the alpha-isoenzyme of p38 would be beneficial for the treatment of inflammation-induced bone destruction as observed in rheumatoid arthritis.

High extracellular inorganic phosphate concentration inhibits RANK-RANKL signaling in osteoclast-like cells

In this work, we investigated the effect of inorganic phosphate (Pi) on the differentiation of monocyte/macrophage precursors into an "osteoclastic" phenotype, and we delineated the molecular mechanisms which could be involved in this phenomenon. This was achieved by stimulating human peripheral blood monocytic cells and RAW 264.7 monocyte-macrophage precursor cells to differentiate into osteoclast-like cells in the presence of receptor activator of NF-kappaB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). RANKL has been previously reported to stimulate the signaling kinases ERK 1/2, p38, Akt, JNK, and the DNA-binding activity of the transcription factors AP-1 and NF-kappaB. Increase in extracellular Pi concentration (1.5-4.5 mM) dose-dependently inhibits both osteoclastic differentiation and bone resorption activity induced by RANKL and M-CSF. Pi was found to specifically inhibit the RANKL-induced JNK and Akt activation, while RANKL-induced p38 and ERK 1/2 phosphorylation were not significantly affected. Moreover, we found that Pi significantly reduced the RANKL-stimulated DNA-binding activity of NF-kappaB. The effects of Pi on osteoclast differentiation and DNA-binding activity of NF-kappaB were prevented by Foscarnet, a sodium-phosphate cotransport inhibitor, suggesting that the effects of Pi occur subsequently to its intake. These results demonstrate that Pi downregulates the differentiation of osteoclasts via a negative feedback exerted on RANK-RANKL signaling.

[p38 MAP Kinase inhibitor]

FR167653 is a potent inhibitor of p38 MAP Kinase and inhibits TNF-alpha and IL-1beta production in inflammatory cells. In this study we investigated the effect of FR167653 on CIA. CIA rats were subcutaneously injected with FR167653 (32 mg/kg/day) starting on the day of the booster injection and after the onset of arthritis in the prophylactic and therapeutic treatment groups, respectively. The hind paw swelling, radiolographic and histologic scores, and osteoclast number were evaluated. Serum and tissue cytokine levels were assessed by ELISA. Flow cytometric analysis of T-lymphocytes from bone marrow was also performed. The effect of FR167653 on in vitro osteoclast formation induced by sRANKL and TNF-alpha was examined. Hind paw swelling occurred in CIA rats but not in the prophylactic treatment group. Therapeutic treatment also significantly reduced the paw swelling. The mean radiographic, histologic score, and osteoclast number of the treatment group were significantly lower than those of CIA rats without treatment. FR167653 treatment reduced serum TNF-alpha and IL-1beta levels, ankle IL-1beta concentration, and CD4-CD8a+ T-cell population in bone marrow. Furthermore, FR167653 inhibited the osteoclast-like cell differentiation induced by both sRANKL and TNF-alpha in vitro. FR167653 prevented the onset of arthritis in a prophylactic treatment model and suppresses the progression of joint destruction in a therapeutic treatment model, suggesting that p38 MAP Kinase is a potential therapeutic target for rheumatoid arthritis.

The p38 mitogen-activated protein kinase (MAPK) pathway in rheumatoid arthritis

Chronic inflammatory processes are based on a sustained and tightly regulated communication network among different cells types. This network comprises extracellular mediators such as cytokines, chemokines and matrix-degrading proteases, which orchestrate the participation of cells in the chronic inflammatory process. The mirrors of this outside communication world are intracellular transcription factor pathways, which shuttle information about inflammatory stimuli to the cell nucleus. This review examines the function of one key signal transduction pathway of inflammation--the p38 mitogen-activated protein kinases (p38MAPK). The signalling pathway is considered as crucial for the induction and maintenance of chronic inflammation, and its components thus emerge as interesting molecular targets of small molecule inhibitors for controlling inflammation. This review not only summarises the current knowledge of activation, regulation and function of the p38MAPK pathway but also examines the role of this pathway in clinical disease. It gives an overview of current evidence of p38MAPK activation in inflammatory arthritis and elaborates the key molecular determinants which contribute to p38MAPK activation in joint disease.

A Triterpene Glycoside from Black Cohosh that Inhibits Osteoclastogenesis by Modulating RANKL and TNFα Signaling Pathways

Osteoporosis is a major age-related source of morbidity and mortality. Increased bone resorption mediated by osteoclasts is central to its pathogenesis. Cytokines, particularly RANKL and TNFalpha, are often increased under pathologic conditions, leading to enhanced osteoclastogenesis. Black cohosh (Actaea/Cimicifuga racemosa L), a popular herbal supplement for the treatment of menopausal symptoms, was recently shown to have the beneficial effect of preventing bone loss. Here, we demonstrate that 25-acetylcimigenol xylopyranoside (ACCX), a triterpenoid glycoside isolated from black cohosh, potently blocks in vitro osteoclastogenesis induced by either RANKL or TNFalpha. This blockage of osteoclastogenesis elicited by ACCX results from abrogation of the NF-kappaB and ERK pathways induced by either RANKL or TNFalpha, respectively. Importantly, this compound attenuates TNFalpha-induced bone loss in vivo. Therefore, ACCX represents a potential lead for the development of a new class of antiosteoporosis agents.

Cells of the synovium in rheumatoid arthritis. Osteoclasts

Osteoclasts are multinucleated cells of hematopoietic origin and are the primary bone resorbing cells. Numerous osteoclasts are found within the synovial tissue at sites adjacent to bone, creating resorption pits and local bone destruction. They are equipped with specific enzymes and a proton pump that enable them to degrade bone matrix and solubilize calcium, respectively. The synovial tissue of inflamed joints has a particularly high potential to accumulate osteoclasts because it harbors monocytes/macrophages, which function as osteoclast precursors, as well as cells that provide the specific molecular signals that drive osteoclast formation. Osteoclasts thus represent a link between joint inflammation and structural damage since they resorb mineralized tissue adjacent to the joint and destroy the joint architecture.

Effects of Mitogen-Activated Protein Kinase Inhibitors on Tumor Necrosis Factor-α Gene Expression and Apoptosis Induction in Cultured Human Fetal Membrane Chorion Cells Infected with Influenza Virus

OBJECTIVES

We investigated the involvement of p38 mitogen-activated protein (MAP) kinase in tumor necrosis factor (TNF)-alpha gene expression, apoptosis induction and virus replication in cultured human fetal membrane chorion cells infected with influenza virus.

METHODS

Influenza virus-infected chorion cells were incubated in the absence or presence of inhibitors of p38 MAP kinase, SB203580 and SB202190. TNF-alpha mRNA and hemagglutinin viral RNA (HA vRNA) were amplified with reverse transcriptase-polymerase chain reaction techniques. TNF-alpha protein concentrations were determined by enzyme-liked immunosorbent assay. The extent of apoptosis induction was estimated by DNA agarose gel electrophoresis. Pyrrolidine dithiocarbamate (PDTC) and ribavirin, which have been shown to inhibit apoptosis induction via the inhibition of viral gene replication, were used as positive control reagents.

RESULTS

PDTC and ribavirin inhibited the accumulation of TNF-alpha mRNA and HA vRNA in the virus-infected chorion cells, resulting in the suppression of TNF-alpha protein secretion. Both SB203580 and SB202190 suppressed TNF-alpha protein secretion, but not the accumulation of TNF-alpha mRNA as well as HA vRNA and the induction of apoptosis.

CONCLUSIONS

These results suggest that p38 MAP kinase pathway is critical in TNF-alpha gene expression at a post-transcriptional level but not in the apoptosis induction and influenza virus replication in cultured chorion cells.

Activation of p38 MAPK is a key step in tumor necrosis factor-mediated inflammatory bone destruction

OBJECTIVE

To investigate whether activation of p38 MAPK is a crucial signaling factor in inflammatory bone destruction mediated by tumor necrosis factor (TNF). Mice overexpressing TNF were treated with 2 different inhibitors of p38 MAPK, and the effect of this treatment on joint inflammation and structural damage was assessed.

METHODS

Human TNF-transgenic mice received systemic treatment with 2 different p38 MAPK inhibitors (RO4399247 and AVE8677). Treatment was started at the time of symptom onset and lasted for 6 weeks. Mice were assessed for clinical signs of arthritis, bone erosion, and cartilage damage. In addition, the effect of these inhibitors on osteoclast generation in vitro and in vivo was assessed.

RESULTS

Both p38 MAPK inhibitors significantly reduced clinical signs of TNF-mediated arthritis. This was attributable to reducing synovial inflammation by 50% without affecting the cellular composition of the infiltrate. Synovial expression of interleukin-1 and RANKL was reduced upon p38 MAPK blockade, and activation of the molecular target MAPK-activated protein kinase 2 (MAPKAP-2) was also inhibited. Proteoglycan loss of articular cartilage was reduced by 50%, although p38 MAPK inhibition did not change matrix molecule synthesis by cultivated chondrocytes. Importantly, bone loss was almost completely prevented by p38 MAPK inhibition. The numbers of synovial osteoclasts and precursors were dramatically reduced, and both p38 MAPK inhibitors also inhibited in vitro osteoclastogenesis at micromolar concentrations and blocked activation of MAPKAP-2 as well as differentiation markers in cultured osteoclast precursors.

CONCLUSION

These results suggest the major importance of p38 MAPK for TNF-mediated inflammatory bone destruction in arthritis and suggest that inhibition of p38 MAPK might be an important tool for reducing structural damage in rheumatoid arthritis.

Thapsigargin modulates osteoclastogenesis through the regulation of RANKL-induced signaling pathways and reactive oxygen species production

The mechanism by which TG modulates osteoclast formation and apoptosis is not clear. In this study, we showed a biphasic effect of TG on osteoclast formation and apoptosis through the regulation of ROS production, caspase-3 activity, cytosolic Ca2+, and RANKL-induced activation of NF-kappaB and AP-1 activities.

INTRODUCTION

Apoptosis and differentiation are among the consequences of changes in intracellular Ca2+ levels. In this study, we investigated the effects of the endoplasmic reticular Ca2+-ATPase inhibitor, thapsigargin (TG), on osteoclast apoptosis and differentiation.

MATERIALS AND METHODS

Both RAW264.7 cells and primary spleen cells were used to examine the effect of TG on RANKL-induced osteoclastogenesis. To determine the action of TG on signaling pathways, we used reporter gene assays for NF-kappaB and activator protein-1 (AP-1) activity, Western blotting for phospho-extracellular signal-related kinase (ERK), and fluorescent probes to measure changes in levels of intracellular calcium and reactive oxygen species (ROS). To assess rates of apoptosis, we measured changes in annexin staining, caspase-3 activity, and chromatin and F-actin microfilament structure.

RESULTS

At concentrations that caused a rapid rise in intracellular Ca2+, TG increased caspase-3 activity and promoted apoptosis in osteoclast-like cells (OLCs). Low concentrations of TG, which were insufficient to measurably alter intracellular Ca2+, unexpectedly suppressed caspase-3 activity and enhanced RANKL-induced osteoclastogenesis. At these lower concentrations, TG potentiated ROS production and RANKL-induced NF-kappaB activity, but suppressed RANKL-induced AP-1 activity and had little effect on ERK phosphorylation.

CONCLUSION

Our novel findings of a biphasic effect of TG are incompletely explained by our current understanding of TG action, but raise the possibility that low intensity or local changes in subcellular Ca2+ levels may regulate intracellular differentiation signaling. The extent of cross-talk between Ca2+ and RANKL-mediated intracellular signaling pathways might be important in determining whether cells undergo apoptosis or differentiate into OLCs.

Regulatory roles and molecular signaling of TNF family members in osteoclasts

The tumor necrosis factor (TNF) family has been one of the most intensively studied families of proteins in the past two decades. The TNF family constitutes 19 members that mediate diverse biological functions in a variety of cellular systems. The TNF family members regulate cellular functions through binding to membrane-bound receptors belonging to the TNF receptor (TNFR) family. Members of the TNFR family lack intrinsic kinase activity and thus they initiate signaling by interacting intracellular signaling molecules such as TNFR associated factor (TRAF), TNFR associated death domain (TRADD) and Fas-associated death domain (FADD). In bone metabolism, it has been shown that numerous TNF family members including receptor activator of nuclear factor kappaB ligand (RANKL), TNF-alpha, Fas ligand (FasL) and TNF-related apoptosis-inducing ligand (TRAIL) play pivotal roles in the differentiation, function, survival and/or apoptosis of osteoclasts, the principal bone-resorbing cells. These TNF family members not only regulate physiological bone remodeling but they are also implicated in the pathogenesis of various bone diseases such as osteoporosis and bone loss in inflammatory conditions. This review will focus on our current understanding of the regulatory roles and molecular signaling of these TNF family members in osteoclasts.

IL-1 mediates TNF-induced osteoclastogenesis

TNF-induced receptor activator NF-kappaB ligand (RANKL) synthesis by bone marrow stromal cells is a fundamental component of inflammatory osteolysis. We found that this process was abolished by IL-1 receptor antagonist (IL-1Ra) or in stromal cells derived from type I IL-1 receptor-deficient (IL-1RI-deficient) mice. Reflecting sequential signaling of the cytokines TNF and IL-1, TNF induces stromal cell expression of IL-1 and IL-1RI. These data suggest that TNF regulates RANKL expression via IL-1, and, therefore, IL-1 plays a role in TNF-induced periarticular osteolysis. Consistent with this posture, TNF-stimulated osteoclastogenesis in cultures consisting of WT marrow macrophages and stromal cells exposed to IL-1Ra or in cocultures established with IL-1RI-deficient stromal cells was reduced approximately 50%. The same magnitude of osteoclast inhibition occurred in IL-1RI-deficient mice following TNF administration in vivo. Like TNF, IL-1 directly targeted osteoclast precursors and promoted the osteoclast phenotype in a TNF-independent manner in the presence of permissive levels of RANKL. IL-1 is able to induce RANKL expression by stromal cells and directly stimulate osteoclast precursor differentiation under the aegis of p38 MAPK. Thus, IL-1 mediates the osteoclastogenic effect of TNF by enhancing stromal cell expression of RANKL and directly stimulating differentiation of osteoclast precursors.