Tumor necrosis factor-alpha cooperates with receptor activator of nuclear factor kappaB ligand in generation of osteoclasts in stromal cell-depleted rat bone marrow cell culture

Characterization and identification of subpopulations of mononuclear preosteoclasts induced by TNF-α in combination with TGF-β in rats

Osteoclasts are unique multinucleated cells formed by fusion of preosteoclasts derived from cells of the monocyte/macrophage lineage, which are induced by RANKL. However, characteristics and subpopulations of osteoclast precursor cells are poorly understood. We show here that a combination of TNF-α, TGF-β, and M-CSF efficiently generates mononuclear preosteoclasts but not multinucleated osteoclasts (MNCs) in rat bone marrow cultures depleted of stromal cells. Using a rat osteoclast-specific mAb, Kat1, we found that TNF-α and TGF-β specifically increased Kat1⁺c-fms⁺ and Kat1⁺c-fms⁻ cells but not Kat1⁻c-fms⁺ cells. Kat1⁻c-fms⁺ cells appeared in early stages of culture, but Kat1⁺c-fms⁺ and Kat1⁺c-fms⁻ cells increased later. Preosteoclasts induced by TNF-α, TGF-β, and M-CSF rapidly differentiated into osteoclasts in the presence of RANKL and hydroxyurea, an inhibitor of DNA synthesis, suggesting that preosteoclasts are terminally differentiated cells. We further analyzed the expression levels of genes encoding surface proteins in bone marrow macrophages (BMM), preosteoclasts, and MNCs. Preosteoclasts expressed itgam (CD11b) and chemokine receptors CCR1 and CCR2; however, in preosteoclasts the expression of chemokine receptors CCR1 and CCR2 was not up-regulated compared to their expression in BMM. However, addition of RANKL to preosteoclasts markedly increased the expression of CCR1. In contrast, expression of macrophage antigen emr-1 (F4/80) and chemokine receptor CCR5 was down-regulated in preosteoclasts. The combination of TNF-α, TGF-β, and M-CSF induced Kat1⁺CD11b⁺ cells, but these cells were also induced by TNF-α alone. In addition, MIP-1α and MCP-1, which are ligands for CCR1 and CCR2, were chemotactic for preosteoclasts, and promoted multinucleation of preosteoclasts. Finally, we found that Kat1⁺c-fms⁺ cells were present in bone tissues of rats with adjuvant arthritis. These data demonstrate that TNF-α in combination with TGF-β efficiently generates preosteoclasts in vitro. We delineated characteristics that are useful for identifying and isolating rat preosteoclasts, and found that CCR1 expression was regulated in the fusion step in osteoclastogenesis.

Tumour necrosis factor blockade for the treatment of erosive osteoarthritis of the interphalangeal finger joints: a double blind, randomised trial on structure modification

Background

Adalimumab blocks the action of tumor necrosis factor-α and reduces disease progression in rheumatoid arthritis and psoriatic arthritis. The effects of adalimumab in controlling progression of structural damage in erosive hand osteoarthritis (HOA) were assessed.

Methods

Sixty patients with erosive HOA on radiology received 40 mg adalimumab or placebo subcutaneously every two weeks during a 12-month randomized double-blind trial. Response was defined as the reduction in progression of structural damage according to the categorical anatomic phase scoring system. Furthermore, subchondral bone, bone plate erosion, and joint-space narrowing were scored according to the continuous Ghent University Score System (GUSSTM).

Results

The disease appeared to be active since 40.0% and 26,7% of patients out of the placebo and adalimumab group, respectively, showed at least one new interphalangeal (IP) joint that became erosive during the 12 months follow-up. These differences were not significant and the overall results showed no effect of adalimumab.

Risk factors for progression were then identified and the presence of palpable soft tissue swelling at baseline was recognized as the strongest predictor for erosive progression. In this subpopulation at risk, statistically significant less erosive evolution on the radiological image (3.7%) was seen in the adalimumab treated group compared to the placebo group (14.5%) (P = 0.009). GUSSTM scoring confirmed a less rapid rate of mean increase in the erosion scores during the first 6 months of treatment in patients in adalimumab-treated patients.

Conclusion

Palpable soft tissue swelling in IP joints in patients with erosive HOA is a strong predictor for erosive progression. In these joints adalimumab significantly halted the progression of joint damage compared to placebo.

Primary human bone marrow adipocytes support TNF-α-induced osteoclast differentiation and function through RANKL expression

PURPOSE

In previous reports, it was demonstrated that bone marrow adipocytes were related to steroid osteoporosis through osteoclastogenesis induced by Receptor Activator of Nuclear factor κ-B Ligand (RANKL) expression. The purpose of this study was to evaluate the effect of Tumor necrosis factor-alpha (TNF-α) on RANKL expression in bone marrow adipocytes, and osteoclast differentiation supported by human bone marrow adipocytes.

METHODS

RANKL, osteoprotegerin (OPG), and macrophage-colony stimulating factor (M-CSF) mRNA expression in bone marrow adipocytes and their regulation by TNF-α treatment were measured by real-time RT-PCR. Co-cultures of bone marrow adipocytes and osteoclast precursors were performed with or without TNF-α, and osteoclast differentiation was evaluated morphologically and functionally.

RESULTS

RANKL expression and an increase in the RANKL/OPG ratio in bone marrow adipocytes were stimulated by TNF-α treatment. In co-culture of bone marrow adipocytes and osteoclast precursors with TNF-α, the number of TRAP-positive multinuclear cells and resorption cavity formations of calcium phosphate film were increased. Osteoclast differentiation was suppressed by anti-RANKL antibody treatment. In co-culture with non-cell-contact conditions, no TRAP-positive cells or resorption cavity formations were observed.

CONCLUSIONS

TNF-α increased RANKL expression in primary human bone marrow adipocytes. TNF-α induced the ability of bone marrow adipocytes to promote osteoclast differentiation and activity in a manner directly related to RANKL expression.

Association of vitamin K deficiency with bone metabolism and clinical disease activity in inflammatory bowel disease

OBJECTIVE

Inflammatory bowel disease (IBD) is a chronic inflammatory process in the digestive tract and patients with IBD develop osteopenia. Although vitamins K and D are important for maintaining bone health and inhibiting inflammation, their roles in patients with IBD are not clear. We investigated the roles of vitamins K and D in the bone health and inflammation in patients with IBD.

METHODS

Bone mineral density (BMD) of patients with IBD (Crohn's disease [CD], n = 47, and ulcerative colitis [UC], n = 40) was measured with dual-energy X-ray absorptiometry. Vitamin K and D levels of patients with IBD and healthy volunteers (n = 41) were evaluated by measuring serum undercarboxylated osteocalcin and 1,25 dihydroxyvitamin D, respectively. Clinical activity index was evaluated in patients with CD and UC.

RESULTS

BMD was low in patients with CD and UC. Serum undercarboxylated osteocalcin levels were significantly higher in patients with CD, but not with UC, compared with healthy subjects, indicating that bone vitamin K is insufficient in patients with CD. The levels of undercarboxylated osteocalcin were significantly correlated with the clinical activity index of CD, although they were not correlated with BMD. The levels of 1,25 dihydroxyvitamin D were significantly lower in patients with CD and UC than in healthy subjects. The levels of 1,25 dihydroxyvitamin D were inversely correlated with BMD in patients with UC and were not correlated with the clinical activity index of CD.

CONCLUSION

Vitamins K and D are insufficient in patients with IBD. Insufficiency of vitamin K is suggested to be associated with inflammatory processes of CD.

Role of NF-κB in the skeleton

Since the discovery that deletion of the NF-κB subunits p50 and p52 causes osteopetrosis in mice, there has been considerable interest in the role of NF-κB signaling in bone. NF-κB controls the differentiation or activity of the major skeletal cell types - osteoclasts, osteoblasts, osteocytes and chondrocytes. However, with five NF-κB subunits and two distinct activation pathways, not all NF-κB signals lead to the same physiologic responses. In this review, we will describe the roles of various NF-κB proteins in basal bone homeostasis and disease states, and explore how NF-κB inhibition might be utilized therapeutically.

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

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

IL-23 promotes osteoclast formation by up-regulation of receptor activator of NF-kappaB (RANK) expression in myeloid precursor cells

Inflammation-mediated bone loss is a major feature of various bone diseases including rheumatoid arthritis, osteoarthritis and advanced periodontitis. Enhanced osteoclast development or activity at the inflammation site results in bone resorption. IL-23 is a heterodimeric cytokine belonging to the IL-6/IL-12 family that has been implicated in the pathogenesis of rheumatoid arthritis and demonstrated to play a role in osteoclastogenesis via stimulation of IL-17 production. In this study we investigated whether IL-23 contributes to the regulation of osteoclast differentiation independent of the IL-17 pathway. We show that IL-23 dose-dependently up-regulates receptor activator of NF-kappaB expression in primary murine bone marrow macrophages and RAW264.7 cells and thereby promotes commitment of myeloid precursor cells to receptor activator of NF-kappaB ligand-mediated osteoclastic differentiation. However, IL-23 by itself is insufficient to induce osteoclastogenesis. Increased osteoclastic differentiation of cells was associated with enhanced cathepsin K expression and dentine resorption indicating enhanced formation of functional osteoclasts. IL-17 was not detectable in culture supernatants and when added to cultures, did not promote differentiation of RAW264.7 cells. These results demonstrate that IL-23 can act directly on myeloid precursor cells in addition to indirectly stimulating receptor activator of NF-kappaB ligand production in osteoblasts and explains its potency in driving osteoclast development in inflammation-mediated bone pathology.

Pathological role of osteoclast costimulation in arthritis-induced bone loss

Abnormal T cell immune responses induce aberrant expression of inflammatory cytokines such as TNF-alpha, leading to osteoclastmediated bone erosion and osteoporosis in autoimmune arthritis. However, the mechanism underlying enhanced osteoclastogenesis in arthritis is not completely understood. Here we show that TNF-alpha contributes to inflammatory bone loss by enhancing the osteoclastogenic potential of osteoclast precursor cells through inducing paired Ig-like receptor-A (PIR-A), a costimulatory receptor for receptor activator of NF-kappaB (RANK). In fact, bone erosion and osteoporosis, but not inflammation, caused by aberrant TNF-alpha expression were ameliorated in mice deficient in Fc receptor common gamma subunit or beta(2)-microglobulin, in which the expression of PIR-As and PIR-A ligands is impaired, respectively. These results establish the pathological role of costimulatory receptors for RANK in bone loss in arthritis and may provide a molecular basis for the future therapy of inflammatory diseases.

Osteochondral repair in synovial joints

PURPOSE OF REVIEW

One of the major challenges in rheumatology remains the induction of osteochondral repair in synovial joints. Remarkable progress has been made in controlling the inflammatory pathways of chronic synovitis and tissue damage in rheumatoid arthritis and spondyloarthropathy. Here, we provide an overview of the current knowledge on the mechanisms involved in osteochondral repair in degenerative joint diseases, as well as in immune mediated inflammatory arthritides, with special emphasis on tumor necrosis factor alpha and IL-1.

RECENT FINDINGS

Homeostasis of articular cartilage and subchondral bone are essential for maintaining the integrity of osteochondral structures within synovial joints. This is achieved by the regulation of a delicate balance between anabolic and catabolic signals. In articular cartilage one cell type, the chondrocyte, is responsible for regulation of homeostasis. In bone, however, two distinct cell types, osteoblasts and osteoclasts, are responsible for anabolic and catabolic pathways, respectively. In inflammatory joint disorders, this tight regulation is profoundly dysregulated, with tumor necrosis factor alpha acting as an important catalyst of a disturbed homeostasis, together with IL-1. Targeting these cytokines may restore the intrinsic repair capacity of osteochondral structures.

SUMMARY

To restore catabolic cytokine balances appears to be a suitable strategy to promote osteochondral repair.

Vertebral fractures and role of low bone mineral density in Crohn's disease

BACKGROUND & AIMS

Vertebral fractures in Crohn's (CD) patients with low bone mineral density (BMD) have been documented as between 14%-22%. Vertebral fractures in CD patients with normal BMD have not been reported. The objectives were to identify the prevalence of vertebral fractures in CD patients and associated predictive factors.

METHODS

Two hundred twenty-four CD patients underwent vertebral BMD analysis and radiographs. Fractures were identified by using quantitative height reduction morphometry, and severity was assessed by spinal fracture index.

RESULTS

Mean age was 40.6 +/- 11.0 years. Sixty percent reported corticosteroid use during the preceding year. Forty-five of 224 (20.0%) patients had 88 vertebral fractures. Sixteen of 45 patients with vertebral fractures had normal BMD (19.0% of all patients with normal BMD). Analysis of patients with or without vertebral fractures did not demonstrate significant differences in BMD or in corticosteroid use. Linear regression analysis demonstrated that elevations in body mass index, C-reactive protein, and parathyroid hormone were significantly predictive of vertebral fractures (r = 0.415, P < .05), and height reduction was >20% (r = 0.417, P < .05).

CONCLUSIONS

This study demonstrates that vertebral fractures in CD patients occur with an equal frequency in those with low and with normal BMD, regardless of corticosteroid use. The mean age of CD patients with vertebral fractures was much lower than that reported in the general population for these fractures. Elevations in body mass index and C-reactive protein and parathyroid hormone levels were predictive of vertebral fractures.

Mature and activated osteoclasts exist in the synovium of rapidly destructive coxarthrosis

We compared histological and functional findings in rapidly destructive coxarthrosis (RDC) and slowly progressive osteoarthritis (OA) to investigate whether osteoclasts contribute to the extensive bone destruction observed in RDC. A histological analysis of tissue specimens from the synovium obtained from 10 cases of RDC and 40 cases with OA of the hip was performed after staining for tartrate-resistant acid phosphatase (TRAP). The cells isolated from these tissue specimens from the synovium were cultured for 24 h, and the numbers of TRAP-positive giant cells were counted. Thereafter, we performed a resorption pit formation assay by isolated cells cultured on dentine slices for 7 days. The number of TRAP-positive multinuclear giant cells present in the synovial membrane obtained from RDC patients was significantly larger than that obtained from OA patients. Large lacunar resorption pits were only seen on the dentin slices in a culture of isolated cells from RDC patients without any stimulators. This is the first report, to our knowledge, to reveal that mature and activated osteoclasts exist only in the synovium of RDC and not in the OA synovium. This result might suggest that the underlying mechanism of RDC is therefore associated with osteoclastogenesis in the synovium.

Increased expression of activating factors in large osteoclasts could explain their excessive activity in osteolytic diseases

Large osteoclasts (>or=10 nuclei) predominate at sites of pathological bone resorption. We hypothesized this was related to increased resorptive activity of large osteoclasts and have demonstrated previously that larger osteoclasts are 8-fold more likely to be resorbing than small osteoclasts (2-5 nuclei). Here we ask whether these differences in resorptive activity can be explained by differences in expression of factors involved in osteoclast signaling, fusion, attachment, and matrix degradation. Authentic rabbit osteoclasts and osteoclasts derived from RAW264.7 cells showed similar increases in c-fms expression (1.7- to 1.8-fold) in large osteoclasts suggesting that RAW cells are a viable system for further analysis. We found 2- to 4.5-fold increases in the expression of the integrins alpha(v) and beta(3), the proteases proMMP9, matMMP9 and pro-cathepsinK, and in activating receptors RANK, IL-1R1, and TNFR1 in large osteoclasts. In contrast, small osteoclasts had higher expression of the fusion protein SIRPalpha1 and the decoy receptor IL-1R2. The higher expression of activation receptors and lower expression of IL-1R2 in large osteoclasts suggest they are hyperresponsive to extracellular factors. This is supported by the observation that the resorptive activity in large osteoclasts was more responsive to IL-1beta, and that this increased activity was inhibited by the IL-1 receptor antagonist, IL-1ra. This increased responsiveness of large osteoclasts to IL-1 may, in part, explain the pathological bone loss noted in inflammatory diseases. The heterogeneity in receptor expression and the differential response to cytokines and their antagonists could prove useful for selective inhibition of large osteoclasts actively engaged in pathological bone loss.

Does TNF have anti-osteoclastogenic actions?

Tumor necrosis factor (TNF) is a pro-resorption agent that leads to bone degradation. Several mechanisms of action have been proposed to account for these effects: TNF directly inhibits osteoblast differentiation; TNF augments osteoclast formation by inducing stromal cells to increase expression of RANKL and macrophage colony-stimulating factor (M-CSF) and decrease that of osteoprotegerin (OPG); and TNF serves to synergize with pathways downstream of RANK to directly increase osteoclast differentiation. All of these actions in sum suggest that TNF dramatically induces osteopenia upon overexpression or injection. However, that the osteopenia seen with TNF is significantly milder than that seen in OPG-/- animals prompted a reevaluation of existing paradigms on TNF action. The hypothesis that TNF directly enhances osteoclast differentiation was tested by examining the effects of TNF on RANKL-induced osteoclast formation and marker expression. The data show that TNF decreased RANKL-induced expression of the osteoclast markers, TRAP and cathepsin K. Furthermore, the addition of 10-60 ng/mL TNF failed to significantly increase RANKL-induced osteoclast differentiation. Instead, data are presented to suggest that the pro-osteoclastogenic actions of TNF are mediated through increases in the number of available osteoclast precursors (macrophages).

The regulation of cathepsin K gene expression

Cathepsin K is essential for normal bone resorption. Osteoclasts synthesize and secrete cathepsin Kinto the extracellular compartment at the attachment site between osteoclasts and the bone surface, wherein the organic matrix is subsequently degraded by cathepsin K. RANKL, NFAT, Mitf, and various components of AP-1 enhance osteoclast formation and bone resorption, whereas IFN-gamma, calcitonin, estradiol, and calcium inhibit it. These agents appear to act correspondingly to alter cathepsin K mRNA and protein expression in order to stimulate and suppress the osteoclast's resorbing potential. RANKL signaling via the calcineurin-calcium-NFAT signaling cascade plays a significant role in the regulation of cathepsin K expression. Activation via p38 and the micropthalmia transcription factor also enhances cathepsin K expression. Future studies will be needed to elucidate the relative roles of various signaling pathways at different stages of osteoclast formation and activation and to determine whether genetically disrupting these pathways can modulate bone resorption with or without impeding other osteoclast functions.

The ratio of circulating osteoprotegerin to RANKL in early rheumatoid arthritis predicts later joint destruction

OBJECTIVE

Rheumatoid arthritis (RA) is a chronic inflammatory disease that may result in debilitating joint deformities with destruction of bone and cartilage. Inflammation is still considered the pivotal inducer of both components of joint damage. Results of recent animal studies suggested a prominent contribution of osteoclastic bone resorption that could be dissociated from inflammation. RANKL and its natural decoy receptor, osteoprotegerin (OPG), play key roles in osteoclast activation. In a group of patients with early RA not treated with disease-modifying drugs, we tested the hypothesis that osteoclast activation, reflected by the serum OPG:RANKL ratio at baseline, is negatively associated with progression of bone damage, independent of inflammation.

METHODS

OPG and RANKL levels, together with a parameter of inflammation (first-year time-averaged erythrocyte sedimentation rate [tESR]), were measured in 92 patients with newly diagnosed early active RA who were participants in a randomized study. The tESR and the OPG:RANKL ratio were evaluated for the ability to predict 5-year radiographic progression of joint damage.

RESULTS

The first-year tESR and the OPG:RANKL ratio, as measured at baseline, independently predicted 5-year radiographic progression of joint damage (both P < or = 0.001). Progression of radiographic damage was greatest in patients with a high tESR and a low OPG:RANKL ratio and was lowest in patients with a low tESR and a high OPG:RANKL ratio.

CONCLUSION

This study in patients with early untreated RA is the first to confirm the findings in animal models of arthritis, that radiographic progression of the bone component of joint destruction is dependent on both inflammation (tESR) and osteoclast activation (the OPG:RANKL ratio).

A role for the immunomodulatory molecules CD200 and CD200R in regulating bone formation

Altered osteoprotogerin (OPG) and OPG ligand (RANKL) ratios are known to regulate bone metabolism. We investigated whether CD200:CD200R interaction would alter OPG:RANKL ratios, and thus modulate bone differentiation in cultures derived from neonatal calvariae, a source of osteoblast precursors (OBp), or bone marrow-derived myeloid cells as a source of osteoclast precursors (OCp). We characterized cells in cultures using real-time PCR to measure expression of a number of mRNAs characteristic of cells differentiating towards the osteoblast or osteoclast lineage, and enumerated bone nodule formation and osteoclasts directly. CD200Fc or anti-CD200 mAbs were included as modulating agents. In addition, calvariae from transgenic mice overexpressing CD200 under control of a doxycycline-inducible promoter were used as a source of OBp endogenously overexpressing CD200. Our data show that increased endogenous expression of CD200 on OBp, or addition of CD200Fc into cultures, led to increased OPG:RANKL ratios and increased bone nodule growth, while anti-CD200 abolished this effect.

Inhibition of p38 mitogen-activated protein kinase prevents inflammatory bone destruction

Mitogen-activated protein kinase (MAPK) pathways are implicated in joint destruction in rheumatoid arthritis (RA) by modulating the production and functions of inflammatory cytokines. Although p38 MAPK (p38) participates in signaling cascades leading to osteolysis in arthritis, the mechanisms of its action in this process remain incompletely understood. Here, we found that the osteoclast (Ocl) precursors expressed p38alpha, but not p38beta, p38delta, and p38gamma isoforms. Treatment of these cells with receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) resulted in p38 activation. Importantly, Ocl development induced by RANKL or RANKL and tumor necrosis factor (TNF)-alpha was blocked with the novel p38 inhibitor 4-(3-(4-chlorophenyl)-5-(1-methylpiperidin-4-yl)-1H-pyrazol-4-yl)pyrimidine (SC-409). To validate in vitro data, p38 role was further investigated in streptococcal cell wall (SCW)-induced arthritis in rats. We found that SCW-induced joint swelling and bone destruction were attenuated by SC-409. Mechanistically, the data show that SCW-stimulated DNA binding activity of the transcription factor myocyte-enhancing factor 2 C, which is downstream of p38, was inhibited by SC-409. In addition, SC-409 inhibited SCW-stimulated expression of numerous factors, including TNF-alpha, interleukin-1beta, and RANKL. Although c-Jun NH2-terminal kinase and NF-kappaB pathways were activated in vitro by RANKL and in vivo by SCW, SC-409 had no significant effect on these pathways. In conclusion, our data show that p38 modulates the production and signaling of cytokines, thus providing a mechanism of the bone-sparing effect of SC-409 in rat arthritis. These data present SC-409 as a novel potent p38 inhibitor and suggest that p38-based therapies may be beneficial in preventing bone loss associated with RA.

Rheumatic diseases: the effects of inflammation on bone

Rheumatoid arthritis, juvenile idiopathic arthritis, the seronegative spondyloarthropathies including psoriatic arthritis, and systemic lupus erythematosus are all examples of rheumatic diseases in which inflammation is associated with skeletal pathology. Although some of the mechanisms of skeletal remodeling are shared among these diseases, each disease has a unique impact on articular bone or on the axial or appendicular skeleton. Studies in human disease and in animal models of arthritis have identified the osteoclast as the predominant cell type mediating bone loss in arthritis. Many of the cytokines and growth factors implicated in the inflammatory processes in rheumatic diseases have also been demonstrated to impact osteoclast differentiation and function either directly, by acting on cells of the osteoclast-lineage, or indirectly, by acting on other cell types to modulate expression of the key osteoclastogenic factor receptor activator of nuclear factor (NF) kappaB ligand (RANKL) and/or its inhibitor osteoprotegerin (OPG). Further elucidation of the mechanisms responsible for inflammation-induced bone loss will potentially lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases. In this review, we provide an overview of the cell types, inflammatory mediators, and mechanisms that are implicated in bone loss and new bone formation in inflammatory joint diseases.

High-turnover osteoporosis is induced by cyclosporin A in rats

Cyclosporin A (CsA) is used widely as an immunosuppressive agent, but it induces osteoporosis as a prominent side effect. To elucidate the mechanisms involved in CsA-induced osteoporosis, the effects of CsA on bone metabolism were investigated in a rat experimental model. Fifteen-day-old rats were fed a powdered diet containing or lacking CsA for 8-30 days. Analysis was performed by micro-computed tomography (muCT) and light microscopy to examine histomorphometric changes in rat tibiae on days 8, 16, and 30. Plasma parathyroid hormone (PTH) and osteocalcin (OCN) levels were determined by enzyme-linked immunosorbent assay (ELISA) on days 8, 16, and 30. The expression of OCN, osteopontin (OPN), and cathepsin K mRNAs in tibial bone marrow was examined by Northern blot analysis on days 8 and 16. Although no significant differences were observed in tibial length during the experimental periods, or in histomorphometric parameters on day 8, an apparent decrease in bone volume was observed in the CsA-treated group after day 16. Histologic analysis showed that the number of osteoblasts and osteoclasts on the surface of trabecular bone in the CsA-treated group had increased significantly on day 16. Plasma PTH and OCN levels in CsA-treated rats were significantly higher than those in control animals on day 8. Northern blot analysis revealed that the CsA-treated group showed an increase in the expression of OCN, OPN, and cathepsin K mRNAs on day 8 compared with the controls. These findings suggest that bone resorption in CsA-treated rats is induced by high-turnover osteoporosis and that bone remodeling activity may be activated by PTH.

Neutral buoyancy and sleep-deprived serum factors alter expression of cytokines regulating osteogenesis

We examined expression of genes associated with cytokine production, and genes implicated in regulating bone metabolism, in bone stromal and osteoblast cells incubated under standard ground conditions and under conditions of neutral buoyancy, and in the presence/absence of serum from normal or sleep-deprived mice. We observed a clear interaction between these two conditions (exposure to neutral buoyancy and serum stimulation) in promoting enhanced osteoclastogenesis. Both conditions independently altered expression of a number of cytokines implicated in the regulation of bone metabolism. However, using stromal cells from IL-1 and TNFx cytokine(r) KO mice, we concluded that the increased bone loss under microgravity conditions was not primarily cytokine mediated.

Interleukin (IL)-17 enhances tumor necrosis factor-alpha-stimulated IL-6 synthesis via p38 mitogen-activated protein kinase in osteoblasts

Inflammatory cytokines are well known to play crucial roles in the pathogenesis of rheumatoid arthritis. Among them, interleukin (IL)-17 is a cytokine that is mainly synthesized by activated T cells and its receptors are present in osteoblasts. The synthesis of IL-6, known to stimulate osteoclastic bone resorption, is reportedly responded to bone resorptive agents such as tumor necrosis factor-alpha (TNF-alpha) in osteoblasts. It has been reported that IL-17 enhances TNF-alpha-stimulated IL-6 synthesis in osteoblast-like MC3T3-E1 cells. We previously showed that sphingosine 1-phosphate (S1-P) mediates TNF-alpha-stimulated IL-6 synthesis in these cells. In the present study, we investigated the mechanism of IL-17 underlying enhancement of IL-6 synthesis in MC3T3-E1 cells. IL-17 induced phosphorylation of p38 mitogen-activated protein (MAP) kinase. SB203580 and PD169316, specific inhibitors of p38 MAP kinase, significantly reduced the enhancement by IL-17 of TNF-alpha-stimulated IL-6 synthesis. IL-17 also amplified S1-P-stimulated IL-6 synthesis, and the amplification by IL-17 was suppressed by SB203580. Anisomycin, an activator of p38 MAP kinase, which alone had no effect on IL-6 level, enhanced the IL-6 synthesis stimulated by TNF-alpha. SB203580 and PD169316 inhibited the amplification by anisomycin of the TNF-alpha-induced IL-6 synthesis. Taken together, our results strongly suggest that IL-17 enhances TNF-alpha-stimulated IL-6 synthesis via p38 MAP kinase activation in osteoblasts.

Alteration of newly induced endochondral bone formation in adult mice without tumour necrosis factor receptor 1

Tumour necrosis factor (TNF)-alpha, a major proinflammatory cytokine, exerts its role on bone cells through two receptors (TNFR1 and TNFR2). TNFR1, but not TNFR2, is expressed by osteoblasts and its function in bone formation in vivo is not fully understood. We compared in vivo new bone formation in TNFR1-deficient (TNFR1(-/-)) mice and wild-type mice, using two models of bone formation: intramembranous ossification following tibial marrow ablation and endochondral ossification induced by bone morphogenetic protein (BMP)-2. Intramembranous osteogenesis in TNFR1(-/-) mice did not differ from the wild-type mice either in histomorphometric parameters or mRNA expression of bone-related markers and inflammatory cytokines. During endochondral osteogenesis, TNFR1(-/-) mice formed more cartilage (at post-implantation day 9), followed by more bone and bone marrow (at day 12). mRNAs for BMP-2, -4 and -7 were increased during the endochondral differentiation sequence in TNFR1(-/-) mice. The expression of receptor activator of NF-kappa B ligand (RANKL) and receptor activator of NF-kappa B (RANK), as assessed by quantitative reverse transcription polymerase chain reaction (RT-PCR), was also increased significantly during endochondral ossification in TNFR1(-/-) mice. In conclusion, signalling through the TNFR1 seems to be a negative regulator of new tissue formation during endochondral but not intramembranous osteogenesis in an adult organism. BMPs and RANKL and its receptor RANK may be involved in the change of local environment in the absence of TNFR1 signalling.

Interleukin-3 and granulocyte-macrophage colony-stimulating factor inhibits tumor necrosis factor (TNF)-alpha-induced osteoclast differentiation by down-regulation of expression of TNF receptors 1 and 2

Osteoclasts, the multinucleated cells that resorb bone, differentiate from hemopoietic precursors of monocyte/macrophage lineage, which also give rise to macrophages or dendritic cells. In this study we investigated the mechanism by which interleukin-3 (IL-3) and granulocyte-macrophage colony-stimulating factor (GM-CSF) inhibit tumor necrosis factor (TNF)-alpha-induced osteoclast differentiation in mouse osteoclast precursors. We show here that both IL-3 and GM-CSF potently inhibits TNF-alpha-induced osteoclast differentiation by direct action on osteoclast precursors. The inhibitory effect of IL-3 and GM-CSF on osteoclast differentiation was completely neutralized by anti-IL-3 and anti-GM-CSF antibodies, respectively. In addition, the inhibitory effect of IL-3 and GM-CSF on TNF-alpha-induced osteoclast differentiation was irreversible. In osteoclast precursors, IL-3 and GM-CSF inhibited c-Fms expression post-transcriptionally. Interestingly, IL-3 and GM-CSF down-regulated both mRNA and surface expression of TNF receptor 1 (TNFR1) and TNFR2. Furthermore, cells in the presence of IL-3 and GM-CSF showed high expression of macrophage antigen CD11b, and low expression of dendritic cells antigen CD11c and prolong exposure of osteoclast precursors to GM-CSF increased the CD11c expression compare with IL-3. In summary, we provide the first evidence that IL-3 and GM-CSF block TNF-alpha-induced osteoclast differentiation by down-regulation of mRNA and surface expression of TNFR1 and TNFR2.

The molecular triad OPG/RANK/RANKL: involvement in the orchestration of pathophysiological bone remodeling

The past decade has seen an explosion in the field of bone biology. The area of bone biology over this period of time has been marked by a number of key discoveries that have opened up entirely new areas for investigation. The recent identification of the receptor activator of nuclear factor kappaB ligand (RANKL), its cognate receptor RANK, and its decoy receptor osteoprotegerin (OPG) has led to a new molecular perspective on osteoclast biology and bone homeostasis. Specifically, the interaction between RANKL and RANK has been shown to be required for osteoclast differentiation. The third protagonist, OPG, acts as a soluble receptor antagonist for RANKL that prevents it from binding to and activating RANK. Any dysregulation of their respective expression leads to pathological conditions such as bone tumor-associated osteolysis, immune disease, or cardiovascular pathology. In this context, the OPG/RANK/RANKL triad opens novel therapeutic areas in diseases characterized by excessive bone resorption. The present article is an update and extension of an earlier review published by Kwan Tat et al. [Kwan Tat S, Padrines M, Theoleyre S, Heymann D, Fortun Y. IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology. Cytokine Growth Factor Rev 2004;15:49-60].

Bone loss in inflammatory arthritis: mechanisms and treatment strategies

PURPOSE OF REVIEW

Focal bone loss in inflammatory arthritis begins early in the disease process and can contribute to patient morbidity. Current treatment strategies primarily target suppression of the inflammatory cascade with varying success in limiting the progression of focal bone destruction. This review outlines the current understanding of the mechanisms mediating inflammation-induced focal bone loss in rheumatoid arthritis and other inflammatory arthritides and highlights recent studies in animal models of arthritis that have contributed to our knowledge of this process.

RECENT FINDINGS

Bone-resorbing osteoclasts have been identified as important effector cells in inflammation-induced bone loss in both experimental animal models and human rheumatoid arthritis and psoriatic arthritis. The RANK/RANKL (receptor activator of nuclear factor-kappaB and RANK ligand) pathway has been shown to be essential for osteoclast differentiation in inflammatory arthritis. In addition, in vitro and in vivo studies have demonstrated that many cytokines and growth factors elaborated by inflamed synovial tissues may contribute to osteoclast differentiation and activation.

SUMMARY

Elucidation of the mechanisms mediating osteoclast differentiation and function has identified new pathways for potential targeted therapeutic intervention for focal bone loss in inflammatory arthritis. Challenges in the application of this approach are that therapies targeting the osteoclast would need to be used in combination with effective anti-inflammatory agents, and that pathways mediating osteoclast differentiation and function would need to remain at least partially functional to allow for continued skeletal remodeling.

(Pre-)osteoclasts induce retraction of osteoblasts before their fusion to osteoclasts

Precursors of osteoclasts seeded on top of a confluent layer of osteoblasts/bone lining cells induced retraction of the latter cells. The (pre)osteoclasts then migrated in the formed cell-free areas and fused to form osteoclast-like cells. Retraction of the osteoblasts/bone lining cells proved to depend on activity of matrix metalloproteinases, and TGF-beta1 prevented the retraction.

INTRODUCTION

It is well known that osteoblasts have a profound effect on (pre)osteoclasts in inducing the formation of bone-resorbing osteoclasts. Whether, on the other hand, (pre)osteoclasts also modulate osteoblast activity is largely unknown. Because osteoblasts/bone lining cells have to retract from the surface before resorption of bone by osteoclasts, we addressed the question of whether (pre)osteoclasts have the capacity to induce such an activity.

MATERIALS AND METHODS

Rabbit calvarial osteoblasts/bone lining cells or periosteal fibroblasts were cultured until confluency, after which rabbit peripheral blood mononuclear cells (PBMCs) were seeded on top of them. The co-cultures were maintained for up to 15 days in the presence or absence of the cytokines transforming growth factor (TGF)-beta1 and TNF-alpha and selective inhibitors of matrix metalloproteinases and serine proteinases. The formation of cell-free areas and the number of TRACP+ multinucleated osteoclast-like cells were analyzed. In addition, formation of cell-free areas was analyzed in co-cultures of osteoblasts with mature osteoclasts.

RESULTS

The seeding of PBMCs on a confluent layer of osteoblasts/bone lining cells resulted in the following sequence of events. (1) A low number of PBMCs strongly attached to osteoblasts. 2) At these sites of contact, the osteoblasts retracted, thus forming cell-free areas. (3) The PBMCs invaded these areas and attached to the surface of the well, after which they fused and formed multinucleated TRACP+ osteoclast-like cells. Retraction was only seen if the cells were in direct contact; conditioned media from cultured PBMCs added to osteoblasts had no effect. Mature osteoclasts seeded on osteoblasts similarly induced retraction, but this retraction occurred at a much faster rate (within 2 days) than the retraction effectuated by the osteoclast precursors (after 8 days in co-culture). Inhibition of matrix metalloproteinase activity, but not of serine proteinases, strongly reduced retraction of the osteoblasts, thus indicating that this type of cell movement depends on the activity of matrix metalloproteinases. A similar inhibitory effect was found with TGF-beta1. TNF-alpha had no effect on osteoblast retraction but enhanced the formation of multinucleated osteoclast-like cells. Addition of PBMCs to confluent layers of periosteal fibroblasts resulted in similar phenomena as observed in co-cultures with osteoblasts. However, the cell-free areas proved to be significantly smaller, and the number of multinucleated cells formed within cell-free areas was three to four times lower.

CONCLUSION

Our results indicate that osteoclast precursors and mature osteoclasts have the capacity to modulate the activity of osteoblasts and that, yet unknown, membrane-bound signaling molecules are essential in inducing retraction of osteoblasts and the subsequent formation of cell-free areas.

IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology

All osteogenic cells (osteoclasts, osteoblasts) contribute individually to bone remodeling. Their cellular interactions control their cellular activities and the bone remodeling intensity. These interactions can be established either through a cell-cell contact, involving molecules of the integrin family, or by the release of many polypeptidic factors and/or their soluble receptor chains. These factors can act directly on osteogenic cells and their precursors to control differentiation, formation and functions (matrix formation, mineralization, resorption...). Here, we present the involvement of three groups of cytokines which seem to be of particular importance in bone physiology: interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha) (TNF-alpha)/IL-1, and the more recently known triad osteoprotegerin (OPG)/receptor activator of NF-kappaB (RANK)/RANK ligand (RANKL). The interactions between these three groups are presented within the framework of bone resorption pathophysiology such as tumor associated osteolysis. The central role of the OPG/RANK/RANKL triad is pointed out.

Evidence that genetic deletion of the TNF receptor p60 or p80 in macrophages modulates RANKL-induced signaling

In the current report, we investigated the possibility of a cross-talk between receptor activator of NF-kappaB ligand (RANKL) and tumor necrosis factor alpha (TNF-alpha) using macrophage cell lines derived from wild-type mice and from mice with genetic deletion of the type 1 TNF receptor (p60(-/-)), the type 2 TNF receptor (p80(-/-)), or both receptors (p60(-/-)p80(-/-)). Deletion of TNF receptors sensitized the cells to RANKL-induced NF-kappaB activation, in order from least to most sensitive of p60(-/-) less than p80(-/-) less than p60(-/-)p80(-/-). The effect on nuclear factor-kappaB (NF-kappaB) activation correlated with RANKL-induced IkappaBalpha kinase activation. Deletion of both TNF receptors also potentiated RANKL-induced c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase 1 and 2 (ERK1/2), and p38 mitogen-activated protein kinase (MAPK) activations in a dose- and time-dependent manner. Nitric oxide (NO) production and expression of inducible NO synthase (iNOS) and cyclooxygenase 2 (COX-2) induced by RANKL was also maximally induced in double knock-out cells. RANKL had no effect on the proliferation of wild-type cells, but deletion of TNF receptors induced growth modulatory effects. We also found that tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), which mediates RANKL signaling, was constitutively bound to RANK in TNF receptor-deleted cells but not in wild-type cells, and this binding was enhanced by RANKL. Overall our results show that RANKL signaling is modulated by the TNF receptors and thus provide evidence of cross-talk between the receptors of 2 cytokines.

Joint erosion in rheumatoid arthritis: interactions between tumour necrosis factor alpha, interleukin 1, and receptor activator of nuclear factor kappaB ligand (RANKL) regulate osteoclasts

BACKGROUND

Osteoclasts, specialised bone resorbing cells regulated by RANKL and M-CSF, are implicated in rheumatoid joint erosion. Lymphocyte-monocyte interactions activate bone resorption, this being attributed to tumour necrosis factor alpha (TNFalpha) and interleukin 1 beta (IL1beta) enhanced osteoblast expression of RANKL. In animal studies, TNF potently increases osteoclast formation in the presence of RANKL. RANKL-independent osteoclastogenesis also occurs, though IL1 is required for resorptive function in most studies. These inflammatory cytokines have a pivotal role in rheumatoid arthritis,

OBJECTIVE

To study the interactions of TNFalpha and IL1beta with RANKL, particularly the time course of the interactions and the role of lymphocytes.

METHOD

Cultures of lymphocytes and monocytes (osteoclast precursors) or of purified CD14(+) cells alone (osteoclast precursors) were exposed to various combinations of TNFalpha, RANKL, and IL1beta or the inhibitors osteoprotegerin, IL1 receptor antagonist, or neutralising antibodies to RANKL or to IL1. Osteoclastogenesis and resorptive activity were assessed on microscopy of dentine slices.

RESULTS

TNFalpha potently increased osteoclast proliferation/differentiation in the presence of RANKL. This effect was greatest when RANKL was present before but not after exposure of osteoclast precursor cells to TNFalpha. The resorptive activity of osteoclasts generated by TNFalpha in the absence of RANKL was critically dependent upon IL1, which was expressed by lymphocyte-monocyte interaction.

CONCLUSION

TNFalpha potently enhances RANKL mediated osteoclast activity. Interactions between TNFalpha and IL1 also result in osteoclastic activity independently of RANKL. These findings will inform therapeutic approaches to the prevention of joint erosion in rheumatoid arthritis.

The TNF receptor superfamily: role in immune inflammation and bone formation

Tumor necrosis factor (TNF) and TNF receptor (TNFR) family proteins play important roles in many biological processes. Recently, the TNF-family molecule, RANKL (also called TRANCE, ODF, and OPGL), and its receptors, RANK and OPG, were found to be regulators of the development and activation of osteoclasts in bone remodeling. TNFalphaalso activates osteoclasts both by themselves and in synergy with RANKL. We used structure-based design to create peptidomimetics and organic therapeutics that inhibit osteoclastogenesis by inhibiting the interaction of ligands and receptors. Here we show for the first time that blocking TNFalpha by these small molecules effectively inhibited osteoclast formation in vitro. These mimetics can be used as a probe to understand the molecular basis of osteoclastogenesis and also as a platform to create useful therapeutic agent.

Tumor necrosis factor-α: molecular and cellular mechanisms in skeletal pathology

Tumor necrosis factor-alpha (TNF) is one member of a large family of inflammatory cytokines that share common signal pathways, including activation of the transcription factor nuclear factor kappa B (Nf-kappa B) and stimulation of the apoptotic pathway. Data derived from early work supported a role for TNF as a skeletal catabolic agent that stimulates osteoclastogenesis while simultaneously inhibiting osteoblast function. The finding that estrogen deficiency was associated with increased production of cytokines led to a barrage of studies and lively debate on the relative contributions of TNF and other cytokines on bone loss, on the potential cell sources of TNF in the bone microenvironment, and on the mechanism of TNF action. TNF has a central role in bone pathophysiology. TNF is necessary for stimulation of osteoclastogenesis along with the receptor activator of Nf-kappa B ligand (RANKL). TNF also stimulates osteoblasts in a manner that hinders their bone-formative action. TNF suppresses recruitment of osteoblasts from progenitor cells, inhibits the expression of matrix protein genes, and stimulates expression of genes that amplify osteoclastogenesis. TNF may also affect skeletal metabolism by inducing resistance to 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) by a mechanism that extends to other members of the steroid hormone nuclear receptor family. Thus, TNF assails bone at many levels. This review will focus on the cellular and molecular mechanisms of TNF action in the skeleton that result in increased bone resorption and impaired formation. TNF and its signal pathway remains an important target for the development of new therapies for bone loss from osteoporosis and inflammatory arthritis.

Molecular mechanisms underlying osteoclast formation and activation

Osteoporosis is one of the leading causes of morbidity in the elderly and is characterized by a progressive loss of total bone mass and bone density. Bone loss in osteoporosis is due to the persistent excess of osteoclastic bone resorption over osteoblastic bone formation. Receptor activator of NFkappaB ligand (RANKL) critically regulates both osteoclast differentiation and activation. TRAFs appear to be central coupling molecules in the signal transduction pathways that regulate osteoclastogenesis, cathepsin K is the major mediator of osteoclastic bone resorption, and sex steroids and aging also affect osteoclastogenesis and osteoclast activity. However, bone homeostasis depends upon the intimate coupling of bone formation and bone resorption, wherein both osteoclasts and osteoblasts exert vital stimulatory and inhibitory effects upon each other via molecules such as RANKL, TGFbeta, PDGF, BMP2, and Mim-1. This review will highlight some of the major features of the complex circuit of cytokines, growth factors, and hormones that underlies the formation and function of osteoclasts and the dynamic equilibrium that marks the interaction between osteoclasts and osteoblasts.

Two histone deacetylase inhibitors, trichostatin A and sodium butyrate, suppress differentiation into osteoclasts but not into macrophages

Histone deacetylase (HDAC) inhibitors are emerging as a new class of anticancer therapeutic agents and have been demonstrated to induce differentiation in some myeloid leukemia cell lines. In this study, we show that HDAC inhibitors have a novel action on osteoclast differentiation. The effect of 2 HDAC inhibitors, trichostatin A (TSA) and sodium butyrate (NaB), on osteoclastogenesis was investigated using rat and mouse bone marrow cultures and a murine macrophage cell line RAW264. Both TSA and NaB inhibited the formation of preosteoclast-like cells (POCs) and multinucleated osteoclast-like cells (MNCs) in rat bone marrow culture. By reverse transcription-polymerase chain reaction analysis, TSA reduced osteoclast-specific mRNA expression of cathepsin K and calcitonin receptor (CTR). In contrast, TSA and NaB did not affect the formation of bone marrow macrophages (BMMs) induced by macrophage colony-stimulating factor as examined by nonspecific esterase staining. Fluorescence-activated cell sorting analysis showed that TSA did not affect the surface expression of macrophage markers for CD11b and F4/80 of BMMs. TSA and NaB also inhibited osteoclast formation and osteoclast-specific mRNA expression in RAW264 cells stimulated with receptor activator of nuclear factor-kappa B (NF-kappa B) ligand (RANKL). Transient transfection assay revealed that TSA and NaB dose dependently reduced the sRANKL-stimulated or tumor necrosis factor alpha (TNF-alpha)-stimulated transactivation of NF-kappa B-dependent reporter genes. The treatment of RAW264 cells with TSA and NaB inhibited TNF-alpha-induced nuclear translocation of NF-kappa B and sRANKL-induced activation of p38 mitogen-activated protein kinase (MAPK) signals. These data suggest that both TSA and NaB exert their inhibitory effects by modulating osteoclast-specific signals and that HDAC activity regulates the process of osteoclastogenesis.

A rat model for testing pharmacologic treatments of pressure-related bone loss

Fluid pressure, instability, or particles have been suggested to initiate the process leading to loosening of prosthetic implants. In a rat model where bone resorption is caused by oscillating fluid pressure, the resorptive response seems much stronger than the response that can be induced by particles or instability. Bone resorption is caused by osteoclasts. It has been suggested that the formation of osteoclasts is influenced by tumor necrosis factor-alpha, which can be blocked by etanercept. Osteoclasts can be inactivated with bisphosphonates, which bind to bone and inactivate osteoclasts when the bisphosphonate-containing bone is resorbed. Bone formation can be increased dramatically by intermittent parathyroid hormone treatment, especially at sites with high bone turnover. This might compensate for increased osteoclastic activity. Forty-two rats received a plate implant, by which fluid pressure was applied to a bone surface by compressing a soft tissue membrane. Eight rats were treated with etanercept 0.75 mg/kg/day, six rats were treated with alendronate 205 microg/kg/day, six rats received saline, and six rats were nonpressurized controls. Nine rats received intermittent parathyroid hormone treatment with nine separate controls. The area of bone resorption under the implant was evaluated by histomorphometry. Alendronate-treated rats showed less bone resorption, but etanercept, intermittent parathyroid hormone treatment, or saline did not reduce the fluid pressure-induced bone resorption. This model is a comparatively simple way of testing pharmacologic reduction of local bone resorption in vivo.

Cytokines synergistically induce osteoclast differentiation: support by immortalized or normal calvarial cells

Conditionally immortalized murine calvarial (CIMC) cells that support differentiation of precursors into mature osteoclasts were isolated. All six CIMC cell lines supported osteoclast differentiation in response to 1,25-dihydroxyvitamin D(3) or interleukin (IL)-11. CIMC-4 cells also supported osteoclast differentiation in response to tumor necrosis factor (TNF)-alpha, IL-1beta, or IL-6. The resultant multinucleated cells expressed tartrate-resistant acid phosphatase and formed resorption lacunae on mineralized surfaces. CIMC-4 cells, therefore, establish an osteoclast differentiation assay that is responsive to many cytokines and does not rely on isolation of primary stromal support cells. Low concentrations of the cytokines synergistically stimulated differentiation when osteoclast precursors were cocultured with either CIMC-4 cells or primary calvarial cells. Osteoclast differentiation induced by all stimuli other than TNF-alpha was completely blocked by osteoprotegerin, whether the stimulators were examined alone or in combination. Moreover, study of precursors that lack TNF-alpha receptors showed that TNF-alpha induces osteoclast differentiation primarily through direct actions on osteoclast precursors, which is a distinct mechanism from that used by the other bone-resorptive agents examined in this study.

TNFalpha potently activates osteoclasts, through a direct action independent of and strongly synergistic with RANKL

TNFalpha is pivotal to the pathogenesis of inflammatory and possibly postmenopausal osteolysis. Much recent work has clarified mechanisms by which TNFalpha promotes osteoclastogenesis, but the means by which it activates osteoclasts to resorb bone remain uncertain. We found that very low concentrations of TNFalpha promoted actin ring formation, which correlates with functional activation in osteoclasts, both in osteoclasts formed in vitro and extracted from newborn rats. TNFalpha was equipotent with RANKL for this action. Activation by TNFalpha was unaffected by blockade of RANKL by OPG, its soluble decoy receptor, suggesting that this was due to a direct action on osteoclasts. Bone resorption was similarly directly and potently stimulated, in a RANKL-independent manner in osteoclasts, whether these were formed in vitro or in vivo. Interestingly, TNFalpha promoted actin ring formation at concentrations an order of magnitude below those required for osteoclastic differentiation. Moreover, TNFalpha strongly synergized with RANKL, such that miniscule concentrations of TNFalpha were sufficient to substantially augment osteoclast activation. The extreme sensitivity of osteoclasts to activation by TNFalpha suggests that the most sensitive osteolytic response of bone to TNFalpha is through activation of existing osteoclasts; and the strong synergy with RANKL provides a mechanism whereby increased osteolysis can be achieved without disturbance to the underlying pattern of osteoclastic localization.

Bone turnover following autologous transplantation in multiple myeloma

Most patients with multiple myeloma have lytic lesions at multiple sites in the axial skeleton. These lesions commonly give pain and are at risk of pathological fracture, and bony disease is therefore a cause of much morbidity in myeloma. Recent data indicates that the Receptor Activator of NF-kappaB ligand (RANKL) and Osteoprotegerin (OPG) may be central to the local pathogenesis of these lytic lesions. Bisphosphonates may ameliorate some of these abnormalities, and clinically these agents improve the skeletal prognosis in myeloma patients. High dose chemotherapy with autologous stem cell rescue is currently under evaluation in the management of myeloma, though little is known of the effect of this therapeutic modality on the skeleton. Studies using biochemical markers of bone turnover suggest that increased osteoclast activity may be present even in apparent plateau phase of myeloma. High dose chemotherapy with autografting may normalise abnormal bone resorption, though the effect may take several weeks to emerge, and may be paralleled by increased osteoblast activity. The findings provide biochemical evidence that autografting may help normalise the abnormal bone turnover characteristic of myeloma.

Involvement of receptor activator of NFkappaB ligand and tumor necrosis factor-alpha in bone destruction in rheumatoid arthritis

Bone loss represents a major unsolved problem in rheumatoid arthritis (RA). The skeletal complications of RA consist of focal bone erosions and periarticular osteoporosis at sites of active inflammation, and generalized bone loss with reduced bone mass. New evidence indicates that osteoclasts are key mediators of all forms of bone loss in RA. TNF-alpha is one of the most potent osteoclastogenic cytokines produced in inflammation and is pivotal in the pathogenesis of RA. Production of tumor necrosis factor-alpha (TNF-alpha) and other proinflammatory cytokines in RA is largely CD4(+) T-cell dependent and mostly a result of interferon-gamma (IFN-gamma) secretion. Synovial T cells contribute to synovitis by secreting IFN-gamma and interleukin (IL)-17 as well as directly interacting with macrophages and fibroblasts through cell-to-cell contact mechanisms. Activated synovial T cells express both membrane-bound and soluble forms of receptor activator of NF-kappaB ligand (RANKL). In rheumatoid synovium, fibroblasts also provide an abundant source of RANKL. Furthermore, TNF-alpha and IL-1 target stromal-osteoblastic cells to increase IL-6, IL-11, and parathyroid hormone-related protein (PTHrP) production as well as expression of RANKL. In the presence of permissive levels of RANKL, TNF-alpha acts directly to stimulate osteoclast differentiation of macrophages and myeloid progenitor cells. In addition, TNF-alpha induces IL-1 release by synovial fibroblasts and macrophages, and IL-1, together with RANKL, is a major survival and activation signal for nascent osteoclasts. Consequently, TNF-alpha and IL-1, acting in concert with RANKL, can powerfully promote osteoclast recruitment, activation, and osteolysis in RA. The most convincing support for this hypothesis has come from in vivo studies of animal models. Protection of bone in the presence of continued inflammation in arthritic rats treated with osteoprotegerin (OPG) supports the concept that osteoclasts mediate bone loss, providing further evidence that OPG protects bone integrity by downregulating osteoclastogenesis and promoting osteoclast apoptosis. Modulation of the RANKL/OPG equilibrium in arthritis may provide additional skeletal benefits, such as chondroprotection. The nexus between T-cell activation, TNF-alpha overproduction, and the RANKL/OPG/RANK ligand-receptor system points to a unifying paradigm for the entire spectrum of skeletal pathology in RA. Strategies that address osteoclastic bone resorption will represent an important new facet of therapy for RA.