A new mechanism of bone destruction in rheumatoid arthritis: synovial fibroblasts induce osteoclastogenesis

Possible involvement of glutamatergic signaling machineries in pathophysiology of rheumatoid arthritis

The prevailing view is that L-glutamate (Glu) functions as an excitatory amino acid neurotransmitter through a number of molecular machineries required for the neurocrine signaling at synapses in the brain. These include Glu receptors for signal input, Glu transporters for signal termination, and vesicular Glu transporters for signal output through exocytotic release. Although relatively little attention has been paid to the functional expression of these molecules required for glutamatergic signaling in peripheral tissues, recent molecular biological analyses including ours give rise to a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system in several peripheral and non-neuronal tissues, including bone and cartilage. In particular, a drastic increase is demonstrated in the endogenous levels of both Glu and aspartate in the synovial fluid with intimate relevance to increased edema and sensitization to thermal hyperalgesia in experimental arthritis models. However, to date, there is only limited information about the physiological and pathological significance of glutamatergic signaling machineries expressed by articular synovial tissues. In this review, we have outlined the role of Glu in synovial fibroblasts in addition to the possible involvement of glutamatergic signaling machineries in the pathogenesis of joint diseases such as rheumatoid arthritis.

Regulation of bone by the adaptive immune system in arthritis

Studies on the immune regulation of osteoclasts in rheumatoid arthritis have promoted the new research field of 'osteoimmunology', which investigates the interplay between the skeletal and immune systems at the molecular level. Accumulating evidence lends support to the theory that bone destruction associated with rheumatoid arthritis is caused by the enhanced activity of osteoclasts, resulting from the activation of a unique helper T cell subset, 'Th17 cells'. Understanding the interaction between osteoclasts and the adaptive immune system in rheumatoid arthritis and the molecular mechanisms of Th17 development will lead to the development of potentially effective therapeutic strategies.

Osteoclastogenesis and arthritis

There is emerging interest for osteoclasts as key players in the erosive and inflammatory events leading to joint destruction in chronic arthritis. In fact, chronic inflammatory joint diseases such as psoriatic arthritis and rheumatoid arthritis are often characterized by destruction of juxta-articular bone and erosions due to the elevated activity of osteoclasts, which are involved in bone resorption. The main step in inflammatory bone erosion is an imbalance between bone resorption and bone formation: osteoclast formation is enhanced by proinflammatory cytokines such as TNF-α, IL-1β, and IL-17 and is not balanced by increased activity of bone-forming osteoblasts. T-cells, stromal cells, and synoviocytes enhance osteoclast formation via expression of RANKL and, under pathologic conditions, of proinflammatory cytokines. In rheumatoid arthritis, accumulation of osteoclasts in synovial tissues and their activation associated with osteoclastogenic cytokines and chemokines at cartilage erosion sites suggest that they could be usefully selected as therapeutic target. In particular, in consideration of the primary role of RANKL and TNF-α in osteoclastogenesis, the control of the production of RANKL and the inhibition of TNF-α represent important strategies for reducing bone damage in this disease.

LTB4 can stimulate human osteoclast differentiation dependent of RANKL

Our previous study showed that Leukotriene B4 can directly stimulate osteoclast differentiation independent of RANKL. In order to determine whether Leukotriene B4 could indirectly stimulate human osteoclast differentiation through increasing RANKL expression of rheumatoid arthritis fibroblast-like synoviocytes, we utilize the coculture model of rheumatoid arthritis fibroblast-like synoviocytes and monocyte, which were stimulated in the presence of 2.5 ng/ml M-CSF in the control group, 2.5 ng/ml M-CSF+10(-8)M LTB4 in the experimental group a, and 2.5 ng/ml M-CSF+10(-8)M LTB4+100 ng/ml OPG in the experimental group b. After culture for 3 weeks, the number of multinucleated TRAP staining positive osteoclast-like cells stained with TRAP was counted to evaluate the differentiation effect in each group. There was almost no osteoclast-like cell in the control group and the experimental group b. There were many osteoclast-like cells in the experimental group a. These results indicated that Leukotriene B4 is capable of inducing osteoclast differentiation by a RANKL-dependent mechanism.

Surgery on the rheumatoid ankle joint: efficacy versus effectiveness

This article examines synovectomy and ankle arthrodesis for the rheumatoid ankle joint. Reviews of osteoimmunology and gait analyses specific to rheumatoid arthritis are included. Comparison studies including ankle arthrodesis and total ankle arthroplasty are reviewed.

Prednisolone phosphate-containing TRX-20 liposomes inhibit cytokine and chemokine production in human fibroblast-like synovial cells: a novel approach to rheumatoid arthritis therapy

To evaluate the potential of using prednisolone phosphate (PSLP)-containing 3,5-dipentadecyloxybenzamidine hydrochloride (TRX-20) liposomes to treat rheumatoid arthritis (RA), we examined their ability to bind human fibroblast-like synovial (HFLS) cells and their effects in these cells. To test for binding, Lissamine rhodamine B-1, 2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (rhodamine)-labelled PSLP-containing TRX-20 liposomes were added to HFLS cells, and the fluorescence intensity of the rhodamine bound to the cells was evaluated. Rhodamine-labelled PSLP-containing liposomes without TRX-20 were used as a negative control. To evaluate the uptake of liposomes by the HFLS cells, we used TRX-20 liposomes containing 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) and p-xylene-bis-pyridinium bromide (DPX), and observed the cells by fluorescence microscopy. The effects of the PSLP in TRX-20 liposomes on HFLS cells were assessed by the inhibition of the production of two inflammatory cytokines (interleukin 6 and granulocyte macrophage colony-stimulating factor) and one inflammatory chemokine (interleukin 8). The interaction of the PSLP-containing TRX-20 liposomes with HFLS cells was approximately 40 times greater than that of PSLP-containing liposomes without TRX-20. PSLP-containing TRX-20 liposomes bound to HFLS cells primarily via chondroitin sulfate. TRX-20 liposomes taken up by the cell were localized to acidic compartments. Furthermore, the PSLP-containing TRX-20 liposomes inhibited the production of the inflammatory cytokines and the chemokine more effectively than did the PSLP-containing liposomes without TRX-20. These results indicate that PSLP-containing TRX-20 liposomes show promise as a novel drug delivery system that could enhance the clinical use of glucocorticoids for treating RA.

Interleukin-32gamma enhances the production of IL-6 and IL-8 in fibroblast-like synoviocytes via Erk1/2 activation

INTRODUCTION

In the present study, we examined the effect of the pro-inflammatory cytokine IL-32gamma, the most biologically active isoform, and its related molecules in fibroblast-like synoviocytes (FLS).

MATERIALS AND METHODS

FLS were isolated from synovial tissues of rheumatoid arthritis (RA) patients. The secretion and expression of IL-6 and IL-8 were examined by ELISA and real-time PCR, and the activation of signaling molecules was evaluated by Western blot, electrophoretic mobility shift assay (EMSA), real-time PCR, and siRNA transfection.

RESULTS

By IL-32gamma stimulation in RA FLS, the expressions of IL-6 and IL-8 were increased significantly, and the phosphorylated Erk1/2 and AP-1 were expressed prominently in Western blot and EMSA. In the Erk1/2 inhibited cells, IL-32gamma stimulation did not increase the mRNA expression of IL-6 and IL-8.

CONCLUSION

Our results suggest that IL-32gamma stimulation can induce the production of IL-6 and IL-8 from RA FLS via Erk1/2 activation.

Oral administration of curcumin suppresses production of matrix metalloproteinase (MMP)-1 and MMP-3 to ameliorate collagen-induced arthritis: inhibition of the PKCdelta/JNK/c-Jun pathway

We investigated whether oral administration of curcumin suppressed type II collagen-induced arthritis (CIA) in mice and its effect and mechanism on matrix metalloproteinase (MMP)-1 and MMP-3 production in CIA mice, RA fibroblast-like synoviocytes (FLS), and chondrocytes. CIA in mice was suppressed by oral administration of curcumin in a dose-dependent manner. Macroscopic observations were confirmed by histological examinations. Histological changes including infiltration of immune cells, synovial hyperplasia, cartilage destruction, and bone erosion in the hind paw sections were extensively suppressed by curcumin. The histological scores were consistent with clinical arthritis indexes. Production of MMP-1 and MMP-3 were inhibited by curcumin in CIA hind paw sections and tumor necrosis factor (TNF)-alpha-stimulated FLS and chondrocytes in a dose-dependent manner. As for the mechanism, curcumin inhibited activating phosphorylation of protein kinase Cdelta (PKCdelta) in CIA, FLS, and chondrocytes. Curcumin also suppressed the JNK and c-Jun activation in those cells. This study suggests that the suppression of MMP-1 and MMP-3 production by curcumin in CIA is mediated through the inhibition of PKCdelta and the JNK/c-Jun signaling pathway.

IL-23 induces receptor activator of NF-kappaB ligand expression in fibroblast-like synoviocytes via STAT3 and NF-kappaB signal pathways

Interleukin (IL)-23 stimulates T lymphocytes to produce inflammatory molecules, which can cause inflammatory arthritis. This study was undertaken to explore the role of IL-23 in stimulating the expression of the receptor activator of the nuclear factor kappa B (NF-kappaB) ligand (RANKL) and osteoclastogenic activity in human fibroblast-like synoviocytes (FLS). These cells were separated from the synovium of patients with rheumatoid arthritis (RA-FLS) and osteoarthritis (OA-FLS) and stimulated with IL-23. RANKL expression was measured by real-time polymerase chain reaction (PCR) amplification and immunostaining. Osteoclast precursor cells were cocultured with IL-23-stimulated RA-FLS and OA-FLS and subsequently stained for tartrate-resistant acid phosphatase (TRAP) activity. IL-23 upregulated RANKL expression in RA-FLS. The expression of RANKL mRNA and protein was blocked completely by inhibitors of NF-kappaB (parthenolide) or of the JAK II-STAT3 pathway (AG490), showing that the RANKL expression pathway is mediated by NF-kappaB and STAT3. TRAP-positive osteoclastogenesis was enhanced in IL-23-stimulated FLS. RA-FLS were more responsive to IL-23 in terms of their RANKL expression than OA-FLS or normal FLS. Thus, IL-23 appears to induce joint inflammation and bone destruction by stimulating RANKL expression in RA-FLS. These interactions between IL-23 and FLS indicate possible new therapeutic approaches for treating bone destruction in patients with inflammatory diseases.

Osteoimmunology: crosstalk between the immune and bone systems

INTRODUCTION

The interaction between the immune and skeletal systems has long been acknowledged, but investigation into rheumatoid arthritis (RA) as well as the various bone phenotypes found in immunocompromised gene-deficient mice has highlighted the importance of the dynamic interplay between the two systems. This has led to the recent emergence and subsequent rapid evolution of the field of osteoimmunology. BONE DESTRUCTION WITH ARTHRITIS AS A RANKL DISEASE: In the bone destruction associated with RA, IL-17-producing helper T cells (T(H)17) play a major role by inducing receptor activator of nuclear factor-kappaB ligand (RANKL). RANKL stimulates osteoclastogenesis through nuclear factor of activated T cells cytoplasmic 1 (NFATc1), which is well known as a crucial regulator of immunity.

NEW PLAYERS IN OSTEOIMMUNOLOGY

In addition to cellular interactions via cytokines, the immune and skeletal systems share various molecules, including transcription factors, signaling molecules, and membrane receptors.

CONCLUSION

The scope of osteoimmunology has grown to encompass a wide range of molecular and cellular interactions, the elucidation of which will provide a scientific basis for future therapeutic approaches to diseases of both the immune and skeletal systems.

Tumor necrosis factor alpha-induced interleukin-32 is positively regulated via the Syk/protein kinase Cdelta/JNK pathway in rheumatoid synovial fibroblasts

OBJECTIVE

Interleukin-32 (IL-32) is a recently discovered cytokine that appears to play a critical role in human rheumatoid arthritis (RA). It is highly expressed in synovium and fibroblast-like synoviocytes (FLS) from RA patients, but not in patients with osteoarthritis (OA). This study was undertaken to assess IL-32 levels in RA synovial fluid (SF) and to investigate the secretion and regulation of IL-32 in RA FLS.

METHODS

FLS and SF were obtained from the joints of RA patients. The secretion and expression of IL-32 and activation of signaling molecules were examined by enzyme-linked immunosorbent assay, immunoblotting, immunoprecipitation, reverse transcriptase-polymerase chain reaction, and small interfering RNA (siRNA) transfection.

RESULTS

IL-32 levels were high in RA SF compared with OA SF. Furthermore, RA FLS expressed and secreted IL-32 when stimulated with tumor necrosis factor alpha (TNFalpha). TNFalpha-induced expression of IL-32 was significantly suppressed, in a dose-dependent manner, by inhibitors of Syk, protein kinase Cdelta (PKCdelta), and JNK and by knockdown of these kinases and c-Jun with siRNA. We also observed that PKCdelta mediated the activation of JNK and c-Jun, and experiments using specific inhibitors and siRNA demonstrated that Syk was the upstream kinase for the activation of PKCdelta.

CONCLUSION

The present findings suggest that IL-32 may be a newly identified prognostic biomarker in RA, thereby adding valuable knowledge to the understanding of this disease. The results also demonstrate that the production of IL-32 in RA FLS is regulated by Syk/PKCdelta-mediated signaling events.

RANK/RANKL: regulators of immune responses and bone physiology

Bone-related diseases, such as osteoporosis and rheumatoid arthritis, affect hundreds of millions of people worldwide and pose a tremendous burden to health care. By deepening our understanding of the molecular mechanisms of bone metabolism and bone turnover, it became possible over the past years to devise new and promising strategies for treating such diseases. In particular, three tumor necrosis factor (TNF) family molecules, the receptor activator of NF-kappaB (RANK), its ligand RANKL, and the decoy receptor of RANKL, osteoprotegerin (OPG), have attracted the attention of scientists and pharmaceutical companies alike. Genetic experiments revolving around these molecules established their pivotal role as central regulators of osteoclast development and osteoclast function. RANK-RANKL signaling not only activates a variety of downstream signaling pathways required for osteoclast development, but crosstalk with other signaling pathways also fine-tunes bone homeostasis both in normal physiology and disease. In addition, RANKL and RANK have essential roles in lymph node formation, establishment of the thymic microenvironment, and development of a lactating mammary gland during pregnancy. Consequently, novel drugs specifically targeting RANK, RANKL, and their signaling pathways in osteoclasts are expected to revolutionize the treatment of various ailments associated with bone loss, such as arthritis, periodontal disease, cancer metastases, and osteoporosis.

Osteoclasts and the immune system

Investigation into arthritis as well as the numerous bone phenotypes found in mice lacking immune-related genes has highlighted the importance of the dynamic interplay between the bone and immune systems. It has recently led to both the emergence and subsequent rapid evolution of the field of osteoimmunology. Receptor activator of nuclear factor-kappaB ligand (RANKL) stimulates osteoclastogenesis through the nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), which is well known as a crucial regulator of immunity. Studies on RANKL signaling revealed various immune-related genes which are involved in the regulation of osteoclastogenesis. Bone destruction in rheumatoid arthritis is caused by the enhanced activity of osteoclasts resulting from the activation of T cells. Here we describe our efforts to address the challenging question as to how abnormal T-cell activation mechanistically induces bone destruction. The scope of osteoimmunology has been extended to encompass a wide range of molecular and cellular interactions, the elucidation of which will provide a scientific basis for future therapeutic approaches to diseases related to both the bone and immune systems.

Pathophysiological roles of osteoprotegerin (OPG)

Osteoprotegerin (OPG) is a secreted glycoprotein central to bone turnover via its role as a decoy receptor for the receptor activator of nuclear factor kappaB ligand (RANKL) and has traditionally been linked to a number of bone-related diseases. However, there is additional evidence that OPG can promote cell survival by inhibiting TNF-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. As a result, a number of in vitro, in vivo and clinical studies have been performed assessing the role of OPG in tumourigenesis. Similar studies have been performed regarding vascular pathologies, resulting from observations of expression and regulation of OPG in the vasculature. This review aims to provide an update on this area and assess the potential protective or detrimental role of OPG in both vascular pathologies and tumourigenesis.

The dynamic interplay between osteoclasts and the immune system

Investigation into arthritis, as well as numerous bone phenotypes found in mice lacking immune-related genes, has highlighted the importance of the interplay between the bone and immune systems, which has led to the emergence and evolution of the field of osteoimmunology. RANKL stimulates osteoclastogenesis through nuclear factor of activated T cells (NFAT) c1, which is also a crucial regulator of immunity. In rheumatoid arthritis, bone destruction is caused by the enhanced activity of osteoclasts, which is mainly dependent on interleukin-17-producing helper T cells (T(H)17). The scope of osteoimmunology has been extended to encompass a wide range of molecular and cellular interactions. The framework of osteoimmunology will provide a scientific basis for future therapeutic approaches to diseases related to both of these systems.

Treatment with anti-gamma-glutamyl transpeptidase antibody attenuates osteolysis in collagen-induced arthritis mice

The effectiveness of a new antibody treatment on arthritis-associated osteolysis was studied by using CIA mice. GGT, a newly identified bone-resorbing factor, was upregulated in arthritic joints. We generated monoclonal antibodies against GGT and injected them into CIA mice. Mice treated with antibodies showed a reduction in osteoclast number and bone erosion.

INTRODUCTION

Gamma-glutamyl transpeptidase (GGT) acts as a bone-resorbing factor that stimulates osteoclast formation. GGT expression has been detected in active lymphocytes that accumulate at inflammation sites, such as rheumatoid arthritis (RA). We hypothesize that GGT is an effective target for suppression of arthritis-related osteoclastogenesis and joint destruction. Here, we describe the therapeutic effect of neutralizing antibodies against GGT on joint destruction using a collagen-induced arthritis (CIA) mouse model.

MATERIALS AND METHODS

GGT expression in the synovium of RA patients and CIA mice was determined by immunohistochemistry and RT-PCR. Monoclonal antibodies were generated against recombinant human GGT (GGT-mAbs) using BALB/c mice. Antibody treatment was performed by intraperitoneal injections of GGT-mAbs into CIA mice. Effects of antibody treatment on arthritis and bone erosion were evaluated by incidence score, arthritis score, and histopathological observations. The role of GGT in osteoclast development was examined by using the established osteoclastogenic culture system.

RESULTS

GGT expression was significantly upregulated in inflamed synovium. Immunohistochemistry revealed that GGT was present in lymphocytes, plasma cells, and macrophages, as well as capillaries. Injection of GGT-mAbs significantly decreased the number of osteoclasts and attenuated the severity of joint destruction in CIA mice. In vitro examination showed that GGT enhanced RANKL-dependent osteoclast formation. GGT stimulated the expression of RANKL in osteoblasts and its receptor RANK in osteoclast precursors, respectively.

CONCLUSIONS

This study indicates that inflamed synovial tissue-derived GGT acts as a risk factor for joint destruction and that the antibody-mediated inhibition of GGT significantly decreases osteoclast number and bone erosion in CIA mice. GGT antagonists might be novel therapeutic agents for attenuating joint destruction in RA patients.

Development of an ex vivo cellular model of rheumatoid arthritis: critical role of CD14-positive monocyte/macrophages in the development of pannus tissue

OBJECTIVE

To establish an ex vivo cellular model of pannus, the aberrant overgrowth of human synovial tissue (ST).

METHODS

Inflammatory cells that infiltrated pannus tissue from patients with rheumatoid arthritis (RA) were collected without enzyme digestion, and designated as ST-derived inflammatory cells. Single-cell suspensions of ST-derived inflammatory cells were cultured in medium alone. Levels of cytokines produced in culture supernatants were measured using enzyme-linked immunosorbent assay kits. ST-derived inflammatory cells were transferred into the joints of immunodeficient mice to explore whether these cells could develop pannus. CD14 and CD2 cells were depleted by negative selection.

RESULTS

Culture of ST-derived inflammatory cells from 92 of 111 patients with RA resulted in spontaneous reconstruction of inflammatory tissue in vitro within 4 weeks. Ex vivo tissue contained fibroblasts, macrophages, T cells, and tartrate-resistant acid phosphatase-positive multinucleated cells. On calcium phosphate-coated slides, ST-derived inflammatory cell cultures showed numerous resorption pits. ST-derived inflammatory cell cultures continuously produced matrix metalloproteinase 9 and proinflammatory cytokines associated with osteoclastogenesis, such as tumor necrosis factor alpha, interleukin-8, and macrophage colony-stimulating factor. More importantly, transferring ST-derived inflammatory cells into the joints of immunodeficient mice resulted in the development of pannus tissue and erosive joint lesions. Both in vitro development and in vivo development of pannus tissue by ST-derived inflammatory cells were inhibited by depleting CD14-positive, but not CD2-positive, cells from ST-derived inflammatory cells.

CONCLUSION

These findings suggest that overgrowth of inflammatory cells from human rheumatoid synovium simulates the development of pannus. This may prove informative in the screening of potential antirheumatic drugs.

Signaling axis in osteoclast biology and therapeutic targeting in the RANKL/RANK/OPG system

Bone integrity is maintained through a balance between bone formation and bone resorption, and osteoclasts are primary cells involved in bone resorption. Recent studies have revealed an essential role of macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa-B ligand (RANKL) in the development of osteoclasts, and detailed molecular cascades that induce osteoclast differentiation, activation and apoptosis have been clarified. Osteoclasts are involved in various pathologic conditions, such as osteoporosis, rheumatoid arthritis and tumor-induced bone disease, which are characterized by abnormal bone resorption, and the finding of RANKL has provided us a good therapeutic target for such pathologic conditions.

Osteoimmunological insight into bone damage in rheumatoid arthritis

Research into the bone destruction associated with rheumatoid arthritis has highlighted the importance of the interplay of the immune and skeletal systems. Arthritic bone destruction is attributable to the defective control of osteoclastogenesis by T cells. We revealed that excessive expression of receptor activator of NF-kappaB ligand (RANKL) and a paucity of interferon-gamma underlie the enhanced osteoclastogenesis in arthritis. The interdisciplinary research field called osteoimmunology has attracted further attention after identification of a number of unexpected bone phenotypes in mice lacking immunomodulatory molecules. Accumulating evidence suggests that the immune and skeletal systems share not only cytokines but also various signaling molecules, transcription factors, and membrane receptors. Thus, bone turns out to be a dynamic tissue that is constantly renewed, where the immune system participates to a hitherto unexpected extent. This emerging field will be of great importance for a better understanding and treatment of rheumatic diseases.

Interaction between the immune system and bone metabolism: an emerging field of osteoimmunology

The interaction between the immune and bone systems has long been appreciated, but recent research into arthritis as well as various bone phenotypes found in immune-related knockout mice has highlighted the importance of the interplay and the interdisciplinary field called osteoimmunology. In rheumatoid arthritis, IL-17-producing helper T cells (TH17) induces receptor activator of NF-κB ligand (RANKL), which stimulates osteoclast differentiation through nuclear factor of activated T cells (NFAT)c1. Accumulating evidence suggests that the immune and skeletal systems share cytokines, signaling molecules, transcription factors and membrane receptors. In addition, the immune cells are maintained in the bone marrow, which provides a space for mutual interaction. Thus, bone turns out to be a dynamic tissue that is constantly renewed, where the immune system participates to a hitherto unexpected extent. This emerging field of osteoimmunology will be of great importance not only to the better understanding of the two systems but also to the development of new treatment for rheumatic diseases.

A tumor necrosis factor receptor loop peptide mimic inhibits bone destruction to the same extent as anti-tumor necrosis factor monoclonal antibody in murine collagen-induced arthritis

OBJECTIVE

The cyclic peptide WP9QY (YCWSQYLCY) was designed to mimic the most critical tumor necrosis factor alpha (TNFalpha) recognition loop on TNF receptor I, and it prevents interactions of TNFalpha with its receptor. We undertook this study to compare the effects of the WP9QY peptide on collagen-induced arthritis (CIA) in mice with those of anti-TNFalpha monoclonal antibody.

METHODS

CIA was induced by primary and secondary immunizations. Osmotic minipumps were implanted in the backs of all mice on the day of the booster injection (day 21), and vehicle, anti-TNF antibody (4 mg/kg/day), or WP9QY peptide (2 mg/kg/day or 4 mg/kg/day) was continuously infused until the mice were killed (day 40). Thereafter, clinical, radiographic, and histologic assessments were performed.

RESULTS

WP9QY treatment inhibited CIA-induced increases in the arthritis score, but onset of disease was not delayed by the peptide. The inhibitory effect of WP9QY on inflammation was definitely weaker than that of anti-TNF antibody. Microfocal computed tomography analyses, however, revealed that WP9QY blocked CIA-induced bone destruction at the knee joints to the same extent as did anti-TNF antibody. In addition, WP9QY inhibited synovial pannus infiltration and reduced osteoclast number. Furthermore, inhibition of CIA-induced systemic bone loss by WP9QY was more apparent than that by anti-TNF antibody.

CONCLUSION

The TNFalpha antagonist WP9QY would be a useful template for the development of small molecular inhibitors to prevent both inflammatory bone destruction and systemic bone loss in rheumatoid arthritis.

Human rheumatoid synovial fibroblasts promote osteoclastogenic activity by activating RANKL via TLR-2 and TLR-4 activation

The interplay between the innate immune system and inflammatory bone destruction in the joints of individuals with rheumatoid arthritis (RA) remains unclear. This study was undertaken to explore the effect of toll-like receptor (TLR) signaling in fibroblast-like synoviocytes (FLS) on the expression of RANKL and induction of osteoclastogenic activity. The levels of RANKL mRNA and protein were measured using RT-PCR, real-time PCR, and immunostaining. Monocytes were cocultured with RA -FLS that had been stimulated with TLR ligands in fresh media and subsequently stained for tartrate-resistant acid phosphatase (TRAP) activity. Osteoclast molecule markers were measured using real-time PCR. Expression of TLR-2 and TLR-4 was higher in RA-FLS than in OA-FLS and normal skin fibroblasts. TLR-2 and TLR-4 ligands induced RANKL expression in RA-FLS. TLR stimulation of RA-FLS also induced the production of IL-1beta and TNF-alpha to a lesser extent; however, it had no effect on IL-17 production. Inhibition of TLR induced IL-1beta production, which partially reversed the upregulation of RANKL induced by TLR ligands. RA-FLS stimulated by TLR-2 and TLR-4 ligands and cocultured with human monocytes induced high levels of expression of TRAP, RANK, cathepsin K, calcitonin receptor, and matrix metalloproteinase-9, suggesting that RA-FLS promote osteoclast differentiation. Our results suggest that the TLR signaling pathway, through TLR-2 and TLR-4, induces RANKL expression in RA-FLS and the expression of RANKL promotes the differentiation of osteoclasts in RA synovium. Targeting specific TLRs may be a promising approach to prevent inflammatory bone destruction in the pathogenesis of RA.

Human CD4+CD25+ regulatory T cells inhibit the differentiation of osteoclasts from peripheral blood mononuclear cells

Regulatory T cell (Treg) is a subset of CD4+ T lymphocytes expressing CD25 with immunosuppressive activity. However the function of Tregs onto osteoclastogenesis remains unknown. We investigated the effect and regulatory mechanism of Treg focusing on osteoclastogenesis from PBMCs. Tregs were isolated from PBMCs by magnetic cell sorting-column and analyzed by flow cytometry. RT-PCR was performed to identify Foxp3 mRNA. Using PBMCs and Tregs coculture system, we could find that Tregs inhibited osteoclasts differentiation from PBMCs and reduced the resorbed areas on pit assay (p <0.01). This suppression of osteoclast differentiation was cytokine-dependent, not cell-to-cell direct contact proved by Transwell system. Tregs-induced osteoclast differentiation was blocked by anti-TGF-beta or anti-IL-4 antibody treatment. These results suggest that Tregs inhibit osteoclast differentiation from PBMCs in a cytokine-dependent manner, not by cell-to-cell contact manner and that TGF-beta and IL-4 may be the key cytokines for this suppressive function of Tregs.

Scientific basis for the efficacy of combined use of antirheumatic drugs against bone destruction in rheumatoid arthritis

Finding a means to ameliorate and prevent bone destruction is one of the urgent issues in the treatment of rheumatoid arthritis. Recent studies revealed bone-resorbing osteoclasts to be essential for arthritic bone destruction, but to date there has been scarce experimental evidence for the underlying mechanism of the bone-protective effect of antirheumatic drugs. Here we examined the effects of one or a combination of disease-modifying antirheumatic drugs (DMARDs) on osteoclast differentiation to provide a cellular and molecular basis for their efficacy against bone destruction. The effects on osteoclast precursor cells and osteoclastogenesis-supporting cells were distinguished by two in vitro osteoclast culture systems. Methotrexate (MTX), bucillamine (Buc) and salazosulphapyridine (SASP) inhibited osteoclastogenesis by acting on osteoclast precursor cells and interfering with receptor activator of NF-kappaB ligand (RANKL)-mediated induction of the nuclear factor of activated T cells (NFAT) c1. MTX and SASP also suppressed RANKL expression on osteoclastogenesis-supporting mesenchymal cells. Interestingly, the combination of three antirheumatic drugs exerted a marked inhibitory effect on osteoclastogenesis even at a low dose at which there was much less of an effect when administered individually. These results are consistent with the reported efficacy of combined DMARDs therapy in humans and suggest that osteoclast culture systems are useful tools to provide an experimental basis for the bone-protective effects of antirheumatic drugs.

Inhibitory effect of ribbon-type NF-kappaB decoy oligodeoxynucleotides on osteoclast induction and activity in vitro and in vivo

In this study we examined the effect of ribbon-type (circular-type) NF-κB decoy oligodeoxynucleotides (RNODN) on osteoclast induction and activity. We extracted bone marrow cells from the femurs of rats and incubated non-adherent cells with receptor activator of nuclear factor κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). First, transfer efficiency into osteoclasts and their precursors, resistance to exonuclease, and binding activity of decoy to NF-κB were examined. Next, to examine the effect of RNODN on osteoclast induction and activity, osteoclast differentiation and pit formation assays were performed. RNODN were injected into the ankle joints of rats with collagen-induced arthritis. Joint destruction and osteoclast activity were examined by histological study. The resistance of RNODN to exonuclease and their binding activity on NF-κB were both greater than those of phosphorothionated NF-κB decoy oligodeoxynucleotides. The absolute number of multinucleate cells scoring positive for tartrate-resistant acid phosphatase was significantly decreased in the RNODN-treated group. The average calcified matrix resorbed area was significantly decreased in the RNODN-treated group. Histological study showed marked suppression of joint destruction and osteoclast activity by intra-articular injection of RNODN. These results suggest the inhibitory effect of RNODN on the induction and activity of osteoclasts. Direct intra-articular injection of RNODN into the joints may be an effective strategy for the treatment of arthritis.

Expression and possible role of PVR/CD155/Necl-5 in osteoclastogenesis

Osteoclasts, the bone-resorbing cells, are differentiated from hematopoietic precursors via two-step cell-cell interactions. One is the interaction between the osteoclast precursor and the stromal cell to initiate differentiation. The other is the interaction among osteoclast precursors to form multinucleated osteoclasts. Recently, the poliovirus receptor (PVR, CD155, Necl-5) was reported to play important roles in cell adhesion and migration. However, there are no reports of PVR in osteoclastogenesis. In this paper, we examined the expression of PVR and its ligand, DNAX accessory molecule-1 (DNAM-1, CD226), in osteoclast precursors, mature osteoclasts, and stromal cells. We found that the PVR was constitutively expressed in both osteoclast cells and stromal cells. The expression of PVR was not changed at various stages of osteoclast formation. In contrast, the expression of DNAM-1 was observed in mononuclear cells and was down-regulated during osteoclastogenesis. Moreover, multinucleated osteoclast formation was inhibited by treatment with the extracellular domain of DNAM-1 (ED-DNAM-1) as a soluble ligand for PVR, but mononuclear preosteoclast formation was not affected. Especially, during the 7-day cultivation, osteoclast formation was suppressed by the treatment with ED-DNAM-1 on days 6 and 7, when the mononuclear preosteoclasts fused into multinucleated osteoclasts. This suppression was abrogated partially by a small interfering RNA specific for PVR. These results suggest that, at least in part, the binding of PVR with DNAM-1 negatively regulates osteoclast formation. Furthermore, our results indicate that the cellular fusion process may be inhibited by the PVR-mediated signaling.

Interplay between the immune and skeletal cells in the regulation of inflammatory bone destruction

The immune and skeletal systems share a number of regulatory molecules including cytokines, signaling molecules, transcription factors and membrane receptors, in common. Consequently, the physiology and pathology of one system may very well affect the other. Research into the cartilage and bone destruction associated with rheumatoid arthritis (RA) has highlighted the importance of the interplay between the immune and skeletal systems. This interdisciplinary field called osteoimmunology has attracted much attention in recent years. Recently, animal models deficient in immunomodulatory molecules have been found frequently to develop an unexpected skeletal phenotype. Receptor activator of NF-kappaB ligand (RANKL) is an essential factor for the induction of osteoclastogenesis that links the immune and skeletal systems. Thus, osteoimmunology is becoming increasingly important for understanding the pathogenesis of bone destruction in RA and for developing new therapeutic strategies for diseases affecting both systems. Here we summarize recent advances on the study of the regulation of cartilage and bone destruction by the immune system.

Osteoclasts, rheumatoid arthritis, and osteoimmunology

PURPOSE OF REVIEW

Osteoclasts are terminally differentiated cells of the monocyte/macrophage lineage that resorb bone matrix. Bone destruction in rheumatoid arthritis is mainly attributable to the abnormal activation of osteoclasts, and studies on activation of osteoclasts by the immune system have led to the new research field called osteoimmunology. This interdisciplinary field is very important to biologic research and to the treatment of diseases associated with the bone and immune systems.

RECENT FINDINGS

The T-cell-mediated regulation of osteoclast differentiation is dependent on cytokines and membrane-bound factors expressed by T cells. The cross-talk between receptor activator of nuclear factor-kappaB ligand and interferon-gamma has been shown to be crucial for the regulation of osteoclast formation in arthritic joints. Recent studies indicate that an increasing number of immunomodulatory factors are associated with the regulation of bone metabolism: nuclear factor of activated T cells c1 has been shown to be the key transcription factor for osteoclastogenesis, the activation of which requires calcium signaling induced by the immunoglobulin-like receptors.

SUMMARY

New findings in osteoimmunology will be instrumental in the development of strategies for research into the treatment of various diseases afflicting the skeletal and immune systems.

Compressive mechanical stress promotes osteoclast formation through RANKL expression on synovial cells

OBJECTIVES

We investigated the effects of compressive mechanical stress on osteoclastogenesis of synovial cells to clarify the mechanism of osteoclast formation by those cells in temporomandibular joint (TMJ) disorders.

STUDY DESIGN

Synovial cells were isolated from rat knee joints and continuously compressed using a conventional method. The expression of receptor activator nuclear factor kappaB ligand (RANKL) mRNA and protein in synovial cells was analyzed by reverse transcriptase-polymerase chain reaction, immunoblotting, and immunofluorescence staining. Mouse bone marrow cells were cultured with synovial cells for 7 days to detect osteoclasts.

RESULTS

The expressions of RANKL mRNA and protein in synovial cells were increased with compressive force. When mouse bone marrow cells were cultured with continuously compressed synovial cells, tartrate-resistant acid phosphatase-positive multinucleated cells were formed. Osteoprotegerin completely inhibited osteoclast formation induced by culturing with compressed synovial cells.

CONCLUSION

Our results indicated that the expression of RANKL in compressed synovial cells enhanced osteoclast formation, whereas continuous compressive force may induce osteoclastic bone destruction in the TMJ.

Alpha-lipoic acid suppresses the development of collagen-induced arthritis and protects against bone destruction in mice

OBJECTIVE

To test the ability of alpha-lipoic acid (LA) to attenuate the development of collagen-induced arthritis (CIA) in mice.

METHODS

Mice were divided into three groups and treated with intraperitoneal administration of LA (10 or 100 mg/kg) or placebo. Clinical, histologic, and biochemical parameters were assessed. Human synovial fibroblasts and peripheral blood mononuclear cells were cocultured in various concentrations of LA to evaluate the effects on osteoclastogenesis.

RESULTS

LA was associated with a dose-dependent reduction of CIA, as well as preventing bone erosion and destructive changes. Intracellular reactive oxygen species in lymphocytes obtained from inguinal lymph nodes, which was significantly higher in CIA than control mice, was significantly reduced in CIA by LA. The concentrations of TNF-alpha, IL-1beta, and IL-6 in the paws, and synovial NF-kappaB binding, all of which were markedly higher in CIA than control mice, were reduced by treatment with LA. In addition, LA inhibited the formation of human osteoclasts in vitro.

CONCLUSION

Amelioration of joint disease by LA was associated with reduction in oxidative stress, as well as inhibition of inflammatory cytokine activation and NF-kappaB DNA binding activity. Moreover, LA inhibited bone destruction in vivo and osteoclastogenesis in vitro. Collectively, these results indicate that LA may be a new adjunctive therapy for rheumatoid arthritis.

Nurse-like cells from patients with rheumatoid arthritis support the survival of osteoclast precursors via macrophage colony-stimulating factor production

OBJECTIVE

To elucidate the role of nurse-like cells (NLCs) obtained from rheumatoid arthritis (RA) patients in bone loss during progressive synovial expansion.

METHODS

CD14+ monocytes were cocultured with NLCs for 4 weeks and collected as NLC-supported CD14+ (NCD14+) monocytes. To determine their ability to differentiate into osteoclasts, NCD14+ monocytes were further cultured with macrophage colony-stimulating factor (M-CSF) together with RANKL or tumor necrosis factor alpha (TNFalpha). NCD14+ monocytes were also cocultured with SaOS-4/3 cells, which were shown to support osteoclastogenesis in response to parathyroid hormone (PTH). CD14+ monocytes were cocultured with SaOS-4/3 cells to elucidate how SaOS-4/3 cells and NLCs supported CD14+ monocytes for a long period. Synovial expansion adjacent to bone in RA patients was examined immunohistochemically to detect osteoclast precursors such as NCD14+ monocytes.

RESULTS

NLCs supported the survival of CD14+ monocytes for 4 weeks. NCD14+ as well as CD14+ monocytes differentiated into osteoclasts in the presence of M-CSF together with RANKL or TNFalpha. NCD14+ monocytes also differentiated into osteoclasts in PTH-treated cocultures with SaOS-4/3 cells. SaOS-4/3 cells supported the survival of CD14+ monocytes for 4 weeks in the presence, but not absence, of PTH. Treatment of SaOS-4/3 cells with PTH up-regulated the expression of M-CSF messenger RNA. Neutralizing antibodies against M-CSF inhibited the NLC-supported survival of CD14+ monocytes. CD68+ monocytes and M-CSF+ fibroblast-like synoviocytes were colocalized in regions adjacent to the destroyed bone of RA patients.

CONCLUSION

Our findings suggest that NLCs are involved in RA-induced bone destruction by maintaining osteoclast precursors via production of M-CSF.

Role of RANKL in physiological and pathological bone resorption and therapeutics targeting the RANKL-RANK signaling system

Osteoclasts are primary cells for physiological and pathological bone resorption, and receptor activator of nuclear factor-kappaB ligand (RANKL) is critically involved in the differentiation, activation, and survival of these cells. Recently, therapeutics for pathological bone destruction targeting RANKL pathways has attracted a great deal of attention. Herein, we review the recent advances in the research on osteoclast biology and discuss the advantages and disadvantages of anti-RANKL therapies.

CD40 ligation of rheumatoid synovial fibroblasts regulates RANKL-medicated osteoclastogenesis: Evidence of NF-κB–dependent, CD40-mediated bone destruction in rheumatoid arthritis

OBJECTIVE

To determine whether CD40 ligation of rheumatoid arthritis synovial fibroblasts (RASFs) is able to induce RANKL expression and osteoclastogenesis in RASFs, and to identify its mechanism of action in patients with RA.

METHODS

CD40 of RASFs was ligated with CD40 ligand (CD40L)-transfected L cells or activated T cells. The formation of osteoclasts in cocultures of CD40-ligated RASFs and T lymphocyte-depleted peripheral blood mononuclear cells was evaluated by tartrate-resistant acid phosphatase staining, detection of calcitonin receptor, and resorption pit formation assay. The expression of NF-kappaB, IkappaB alpha, ERK-1/2, phospho-ERK-1/2, p38, phospho-p38, and RANKL was examined by immunoblotting and/or semiquantitative reverse transcription-polymerase chain reaction.

RESULTS

CD40 ligation of RASFs by CD40L-transfected L cells or activated T cells induced RANKL expression and enhanced osteoclastogenesis. CD40 ligation of RASFs also induced activation of ERK-1/2, p38 MAPK, and NF-kappaB and up-regulation of CD40 ligation-induced RANKL expression, whereas osteoclastogenesis was reduced in RASFs transfected with a dominant-negative mutant of IkappaB alpha or by an NF-kappaB inhibitor. However, specific inhibitors of MAPK/ERK-1/2 and p38 MAPK partially blocked the induction of RANKL expression and osteoclastogenesis. Monoclonal antibodies against interleukin-1 and tumor necrosis factor alpha partially inhibited CD40 ligation-mediated osteoclastogenesis.

CONCLUSION

These results indicate that CD40 ligation of RASFs induces RANKL expression mainly via NF-kappaB activation and also results in enhanced osteoclast formation, both of which might play important roles in bone and cartilage destruction in RA. Inhibition of the CD40-CD40L interaction is a potential strategy for the prevention of bone damage in RA.

Peripheral blood T lymphocytes from patients with early rheumatoid arthritis express RANKL and interleukin-15 on the cell surface and promote osteoclastogenesis in autologous monocytes

OBJECTIVE

To investigate the osteoclastogenic potential of T cells from the peripheral blood (PB) and synovial fluid (SF) of patients with rheumatoid arthritis (RA) on autologous monocytes, and to study the cytokines implicated in this process.

METHODS

T cells and monocytes were isolated from the PB of 20 healthy subjects and 20 patients with early RA, and from the SF of 20 patients with established RA. Autologous T cell/monocyte cocultures were established in the absence of exogenous cytokines or growth factors in order to examine spontaneous ex vivo osteoclast differentiation by tartrate-resistant acid phosphatase staining and calcified matrix resorption activity.

RESULTS

Surface RANKL was expressed on freshly isolated T cells from the PB of patients with early RA and the SF of patients with established RA. In addition, surface interleukin-15 (IL-15) was detected on freshly isolated T cells and monocytes from the PB of patients with early RA and the SF of patients with established RA. Autologous T cell/monocyte cocultures derived from the SF of patients with established RA and from the PB of patients with early RA, but not from the PB of healthy controls, resulted in osteoclast differentiation that was significantly inhibited by osteoprotegerin (OPG) and by neutralizing monoclonal antibodies to IL-15, IL-17, tumor necrosis factor alpha (TNFalpha), and IL-1beta. OPG, anti-TNFalpha, and anti-IL-1beta demonstrated a cooperative inhibitory effect. At 1-year followup, surface RANKL and IL-15 and ex vivo osteoclastogenesis were no longer observed on PB T cells or monocytes from patients with early RA in whom clinical remission had been achieved with treatment.

CONCLUSION

T cells are important contributors to the pathogenesis of bone erosions in RA through interaction with osteoclast precursors of the monocyte/macrophage lineage.

Interplay between interferon and other cytokine systems in bone metabolism

Interferons (IFNs) play crucial roles in the regulation of a wide variety of innate and adaptive immune responses. Type I interferons (IFN-alpha/beta) are central to the host defense against pathogens such as viruses, whereas type II interferon (IFN-gamma) mainly contributes to the T-cell-mediated regulation of the immune responses. Studies of bone destruction associated with rheumatoid arthritis have highlighted the importance of the interaction between the immune and skeletal systems. Recently, a new research area, termed osteoimmunology, has been spawned by a series of studies focusing on the signaling networks between IFN and other cytokines in bone metabolisms. It has been revealed that IFN-gamma interferes with the osteoclast differentiation induced by receptor activator of nuclear factor-kappaB ligand (RANKL), and this mechanism is critical for the suppression of pathological bone resorption associated with inflammation. In addition, RANKL induces the IFN-beta gene in osteoclast precursor cells, and this induction constitutes a critical aspect of the negative feedback regulation mechanisms of RANKL signaling to suppress excessive osteoclastogenesis. Furthermore, a novel function of signal transducer and activator of transcription 1 (Stat1), the essential transcription factor for both type I and type II IFN responses, was revealed in the regulation of osteoblast differentiation. Collectively, these studies unveil novel aspects of the IFN system and indicate the operation of the intricate signaling network among IFN and other cytokine systems in bone remodeling, which might offer a molecular basis for the treatment of bone diseases.

Butyrate suppresses tumor necrosis factor alpha production by regulating specific messenger RNA degradation mediated through a cis-acting AU-rich element

OBJECTIVE

To study the capacity of butyrate to inhibit production of tumor necrosis factor alpha (TNFalpha) in macrophage-like synoviocytes (MLS) from patients with rheumatoid arthritis (RA), in human peripheral monocytes, and in murine RAW264.7 macrophages.

METHODS

The concentrations of TNFalpha in culture supernatants of these cells were measured using enzyme-linked immunosorbent assay. The expression levels of various messenger RNAs (mRNA), such as those for TNFalpha, the mRNA-binding protein TIS11B, and luciferase, were measured using real-time quantitative polymerase chain reaction. The in vitro effects of butyrate on transcriptional regulation were evaluated by transfection with various reporter plasmids in RAW264.7 macrophages. The effects of TIS11B on TNFalpha expression were examined using an overexpression model of TIS11B in RAW264.7 cells.

RESULTS

Butyrate suppressed TNFalpha protein and mRNA production in MLS and monocytes, but paradoxically enhanced transactivation of the TNFalpha promoter. Expression of the AU-rich element (ARE)-binding protein TIS11B was up-regulated by butyrate. Induction of TNFalpha mRNA by lipopolysaccharide was significantly inhibited when TIS11B was overexpressed. Butyrate facilitated the degradation of luciferase transcripts containing the 3'-untranslated region (3'-UTR) of TNFalpha, and this effect was dependent on the ARE in the 3'-UTR that is known to be involved in the regulation of mRNA degradation.

CONCLUSION

These results indicate that butyrate suppresses TNFalpha expression by facilitating mRNA degradation mediated through a cis-acting ARE. Butyrate has the ability to regulate TNFalpha at the mRNA level and is therefore a potential therapeutic drug for RA patients.

Excitatory amino acid transporters expressed by synovial fibroblasts in rats with collagen-induced arthritis

Although previous studies have demonstrated increased levels of the brain neurotransmitter glutamate (Glu) in the synovial fluid from patients with arthritis, not much attention has been paid to the possible role of Glu in joint synovial tissues to date. Constitutive expression of mRNA was for the first time shown with glutamate aspartate transporter, glutamate transporter-1 and excitatory amino acid carrier-1 (EAAC1), in addition to with particular ionotropic and metabotropic Glu receptors, in cultured synovial fibroblasts prepared from knee joints of male Lewis rats. Immunohistochemical analysis revealed high localization of immunoreactive EAAC1 at synovial tissues. The accumulation of [3H]Glu occurred in a temperature- and sodium-dependent manner in cultured synovial fibroblasts, with a Km of 23.1+/-1.1 microM and a Vmax of 237.1+/-31.1 pmol/(mg protein min), respectively. In rats with arthritis induced by immunization to type-II collagen, marked increases were seen in hind paw volume, cytokine mRNA expression and Glu levels in synovial tissues, in addition to histological erosion. In cultured synovial fibroblasts prepared from these arthritic rats, [3H]Glu accumulation was drastically increased with biochemical and pharmacological profiles similar to those seen in normal synovial fibroblasts. The exposure to Glu at 500 microM doubled the incorporation of 5-bromo-2'-deoxyuridine in cultured synovial fibroblasts of arthritic but not normal rats, without significantly affecting mRNA expression of different cytokines in both synovial fibroblasts. These results suggest that Glu may at least in part play a role in mechanisms associated with cellular proliferation through particular transporters functionally expressed by synovium in rheumatoid arthritis.

RANKL is a marker and mediator of local and systemic bone loss in two rat models of inflammatory arthritis

RANKL is an essential mediator of bone erosions, but the role of RANKL in systemic bone loss had not been studied in arthritis. RANKL protein was increased in rat joint extracts and serum at the earliest stages of arthritis. Osteoprotegerin (OPG) treatment reversed local and systemic bone loss, suggesting that RANKL is both a marker and mediator of bone loss in arthritis.

INTRODUCTION

RANKL is well established as an essential mediator of bone erosions in inflammatory arthritis, but the role of RANKL in systemic bone loss in arthritis had not been studied. We hypothesized that serum RANKL could serve as both a mediator and as a novel biomarker for local and systemic bone loss in arthritis. We challenged this hypothesis in two established rat models of inflammatory arthritis. We sought to determine whether serum RANKL was elevated early in disease progression and whether RANKL suppression could prevent both local and systemic bone loss in these models.

MATERIALS AND METHODS

Detailed time-course studies were conducted in animals with collagen-induced (CIA) or adjuvant-induced (AIA) arthritis to evaluate the onset and progression of inflammation (paw swelling), bone erosions, osteoclast numbers, and RANKL protein levels in arthritic joints and in serum. Additional CIA and AIA rats (n=8/group) received placebo (PBS) or recombinant OPG (3 mg/kg three times weekly) for 10 days beginning 4 days after disease onset (first macroscopic evidence of hind paw erythema and edema) to assess the role of RANKL in local and systemic bone loss.

RESULTS

RANKL protein was significantly elevated in the joints and serum of CIA and AIA rats within 1-2 days of disease onset. Increased RANKL levels were associated with local (hind paw) and systemic (vertebral) osteopenia in both models. The RANKL inhibitor OPG prevented local and systemic osteopenia in both models of established disease.

CONCLUSIONS

RANKL protein is significantly increased both locally and systemically during the earliest stages of inflammatory arthritis in rats, suggesting that serum RANKL might have prognostic value for bone erosions and systemic osteopenia in this condition. RANKL inhibition through OPG prevented local and systemic bone loss in these arthritis models, suggesting that RANKL inhibition is a promising new approach for treating bone loss in arthritis.