Activated T cells regulate bone loss and joint destruction in adjuvant arthritis through osteoprotegerin ligand

A novel in vivo role for osteoprotegerin ligand in activation of monocyte effector function and inflammatory response

Osteoprotegerin Ligand (OPGL) is a member of the tumor necrosis factor ligand superfamily and has been shown to be involved in interactions between T cells and dendritic cells. Its role in monocyte effector function, however, has not been defined. In the present study a role for OPGL in activating monocytes/macrophages has been characterized. OPGL was found to up-regulate receptor activator of NF-kappaB (RANK) receptor expression on monocytes, regulate their effector function by inducing cytokine and chemokine secretion, activate antigen presentation through up-regulation of co-stimulatory molecule expression, and promote survival. This activation is mediated through the MAPK pathway as evidenced by activation of p38 and p42/44 MAPK and up-regulation of BCL-XL protein levels. A physiological role for OPGL in monocyte activation and effector function was tested in a model of lipopolysaccharide-induced endotoxic shock. Administration of receptor activator of NF-kappaB (RANK)-Fc to block OPGL activity in vivo was able to protect mice from death induced by sepsis, indicating a hitherto undescribed role for OPGL in monocyte function and in mediating inflammatory response. This was further tested in an animal model of inflammation-mediated arthritis. Treatment with RANK-Fc significantly ameliorated disease development and attenuated bone destruction. Thus, our study strongly suggests that administration of receptor fusion proteins to specifically block OPGL activity in vivo may result in blocking development of monocyte/macrophage-mediated diseases.

The role of TNF-related activation-induced cytokine-receptor activating NF-kappa B interaction in acute allograft rejection and CD40L-independent chronic allograft rejection

We analyzed the role of TNF-related activation-induced cytokine (TRANCE), a member of the TNF family expressed on activated T cells that shares functional properties with CD40L, and its receptor-activating NF-kappaB (RANK) which is mostly expressed on mature dendritic cells, during allogenic responses in vivo using a rodent heart allograft model. TRANCE mRNA was strongly up-regulated in acutely rejected allografts on days 4 and 5 posttransplantation whereas RANK was detected as early as day 1 but did not show further up-regulation during the first week. Immunofluoresence analyses of heart allografts showed that 80 and 100% of TRANCE and RANK-expressing cells were T cells and APCs, respectively. We show for the first time that short-term TRANCE blockade using a mouse RANKIg fusion molecule can significantly prolong heart allograft survival in both rat and mouse models. Similarly, rat heart allografts transduced with a RANKIg encoding recombinant adenovirus exhibited a significant prolongation of survival (14.3 vs 7.6 days, p < 0.0001). However, TRANCE blockade using RANKIg did not appear to inhibit allogeneic T and B cell priming humoral responses against RANKIg. Interestingly, TRANCE blockade induced strong up-regulation of CD40 ligand (CD40L) mRNA in allografts. Combined CD40L and TRANCE blockade resulted in significantly decreased chronic allograft rejection lesions as well as allogeneic humoral responses compared with CD40L blockade alone. We conclude that TRANCE-RANK interactions play an important role during acute allograft rejection and that CD40L-independent allogeneic immune responses can be, at least in part, dependent on the TRANCE pathway of costimulation.

Estrogen deficiency accelerates murine autoimmune arthritis associated with receptor activator of nuclear factor-kappa B ligand-mediated osteoclastogenesis

The aims of this study were to evaluate the in vivo effects of estrogen deficiency in MRL/lpr mice as a model for rheumatoid arthritis and to analyze the possible relationship between immune dysregulation and receptor activator of nuclear factor-kappaB ligand (RANKL)-mediated osteoclastogenesis. Experimental studies were performed in ovariectomized (Ovx)-MRL/lpr, Ovx-MRL+/+, sham-operated-MRL/lpr, and sham-operated-MRL+/+ mice. Severe autoimmune arthritis developed in younger Ovx-MRL/lpr mice until 24 wk of age, whereas these lesions were entirely recovered by pharmacological levels of estrogen administration. A significant elevation in serum rheumatoid factor, anti-double-stranded DNA, and anti-type II collagen was found in Ovx-MRL/lpr mice and recovered in mice that underwent estrogen administration. A high proportion of CD4(+) T cells bearing RANKL was found, and an enhanced expression of RANKL mRNA and an impaired osteoprotegerin mRNA was detected in the synovium. An increase in both osteoclast formation and bone resorption pits was found. These results indicate that estrogen deficiency may play a crucial role in acceleration of autoimmune arthritis associated with RANKL-mediated osteoclastogenesis in a murine model for rheumatoid arthritis.

Porphyromonas gingivalis induces receptor activator of NF-kappaB ligand expression in osteoblasts through the activator protein 1 pathway

Porphyromonas gingivalis, an important periodontal pathogen, is closely associated with inflammatory alveolar bone resorption, and several components of the organism such as lipopolysaccharides have been reported to stimulate production of cytokines that promote inflammatory bone destruction. We investigated the effect of infection with viable P. gingivalis on cytokine production by osteoblasts. Reverse transcription-PCR and real-time PCR analyses revealed that infection with P. gingivalis induced receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL) mRNA expression in mouse primary osteoblasts. Production of interleukin-6 was also stimulated; however, osteoprotegerin was not. SB20350 (an inhibitor of p38 mitogen-activated protein kinase), PD98059 (an inhibitor of classic mitogen-activated protein kinase kinase, MEK1/2), wortmannin (an inhibitor of phosphatidylinositol 3 kinase), and carbobenzoxyl-leucinyl-leucinyl-leucinal (an inhibitor of NF-kappaB) did not prevent the RANKL expression induced by P. gingivalis. Degradation of inhibitor of NF-kappaB-alpha was not detectable; however, curcumin, an inhibitor of activator protein 1 (AP-1), prevented the RANKL production induced by P. gingivalis infection. Western blot analysis revealed that phosphorylation of c-Jun, a component of AP-1, occurred in the infected cells, and an analysis of c-Fos binding to an oligonucleotide containing an AP-1 consensus site also demonstrated AP-1 activation in infected osteoblasts. Infection with P. gingivalis KDP136, an isogenic deficient mutant of arginine- and lysine-specific cysteine proteinases, did not stimulate RANKL production. These results suggest that P. gingivalis infection induces RANKL expression in osteoblasts through AP-1 signaling pathways and cysteine proteases of the organism are involved in RANKL production.

Novel and selective small molecule stimulators of osteoprotegerin expression inhibit bone resorption

Osteoprotegerin (OPG), a secreted member of the tumor necrosis factor receptor superfamily, is a potent inhibitor of osteoclast formation and bone resorption. Because OPG functions physiologically as a locally generated (paracrine) factor, we used high-throughput screening to identify small molecules that enhance the activity of the promoter of the human OPG gene. We found three structurally unrelated compounds that selectively increased OPG gene transcription, OPG mRNA levels, and OPG protein production and release by osteoblastic cells. Structural analysis of one compound, a benzamide derivative, led to the identification of four related molecules, which are also OPG inducers. The most potent of these compounds, Cmpd 5 inhibited osteoclast formation and parathyroid hormone-induced calvarial bone resorption. In vivo, Cmpd 5 completely blocked resorptive activity (serum calcium, osteoclast number) in parathyroid hormone-treated rats. Furthermore, Cmpd 5 reduced the ability of a rat breast cancer to metastasize to bone. Finally, the compound also prevented bone loss in a rat adjuvant arthritis model. These results provide proof of the concept that low molecular weight compounds can enhance OPG production in ways that can result in effective therapies.

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.

Megakaryocyte-osteoblast interaction revealed in mice deficient in transcription factors GATA-1 and NF-E2

Mice deficient in GATA-1 or NF-E2 have a 200-300% increase in bone volume and formation parameters. Osteoblasts and osteoclasts generated in vitro from mutant and control animals were similar in number and function. Osteoblast proliferation increased up to 6-fold when cultured with megakaryocytes. A megakaryocyte-osteoblast interaction plays a role in the increased bone formation in these mice.

INTRODUCTION

GATA-1 and NF-E2 are transcription factors required for the differentiation of megakaryocytes. Mice deficient in these factors have phenotypes characterized by markedly increased numbers of immature megakaryocytes, a concomitant drastic reduction of platelets, and a striking increased bone mass. The similar bone phenotype in both animal models led us to explore the interaction between osteoblasts and megakaryocytes.

MATERIALS AND METHODS

Histomorphometry, microCT, and serum and urine biochemistries were used to assess the bone phenotype in these mice. Wildtype and mutant osteoblasts were examined for differences in proliferation, alkaline phosphatase activity, and osteocalcin secretion. In vitro osteoclast numbers and resorption were measured. Because mutant osteoblasts and osteoclasts were similar to control cells, and because of the similar bone phenotype, we explored the interaction between cells of the osteoblast lineage and megakaryocytes.

RESULTS

A marked 2- to 3-fold increase in trabecular bone volume and bone formation indices were observed in these mice. A 20- to 150-fold increase in trabecular bone volume was measured for the entire femoral medullary canal. The increased bone mass phenotype in these animals was not caused by osteoclast defects, because osteoclast number and function were not compromised in vitro or in vivo. In contrast, in vivo osteoblast number and bone formation parameters were significantly elevated. When wildtype or mutant osteoblasts were cultured with megakaryocytes from GATA-1- or NF-E2-deficient mice, osteoblast proliferation increased over 3- to 6-fold by a mechanism that required cell-to-cell contact.

CONCLUSIONS

These observations show an interaction between megakaryocytes and osteoblasts, which results in osteoblast proliferation and increased bone mass, and may represent heretofore unrecognized anabolic pathways in bone.

Enhanced osteoclast development in collagen-induced arthritis in interferon-gamma receptor knock-out mice as related to increased splenic CD11b+ myelopoiesis

Collagen-induced arthritis (CIA) in mice is accompanied by splenomegaly due to the selective expansion of immature CD11b+ myeloblasts. Both disease manifestations are more pronounced in interferon-gamma receptor knock-out (IFN-gammaR KO) mice. We have taken advantage of this difference to test the hypothesis that the expanding CD11b+ splenic cell population constitutes a source from which osteoclast precursors are recruited to the joint synovia. We found larger numbers of osteoclasts and more severe bone destruction in joints of IFN-gammaR KO mice than in joints of wild-type mice. Osteoclast-like multinucleated cells appeared in splenocyte cultures established in the presence of macrophage colony-stimulating factor (M-CSF) and stimulated with the osteoclast-differentiating factor receptor activator of NF-kappaB ligand (RANKL) or with tumour necrosis factor-alpha (TNF-alpha). Significantly larger numbers of such cells could be generated from splenocytes of IFN-gammaR KO mice than from those of wild-type mice. This was not accompanied, as might have been expected, by increased concentrations of the intracellular adaptor protein TRAF6, known to be involved in signalling of RANKL- and TNF-alpha-induced osteoclast formation. Splenocyte cultures of IFN-gammaR KO mice also produced more TNF-alpha and more RANKL than those of wild-type mice. Finally, splenocytes isolated from immunised IFN-gammaR KO mice contained comparatively low levels of pro-interleukin-1beta (pro-IL-1beta) and pro-caspase-1, indicating more extensive conversion of pro-IL-1beta into secreted active IL-1beta. These observations provide evidence that all conditions are fulfilled for the expanding CD11b+ splenocytes to act as a source of osteoclasts and to be indirectly responsible for bone destruction in CIA. They also provide a plausible explanation for the higher susceptibility of IFN-gammaR KO mice to CIA.

Repair of local bone erosions and reversal of systemic bone loss upon therapy with anti-tumor necrosis factor in combination with osteoprotegerin or parathyroid hormone in tumor necrosis factor-mediated arthritis

Local bone erosion and systemic bone loss are hallmarks of rheumatoid arthritis and cause progressive disability. Tumor necrosis factor (TNF) is a key mediator of arthritis and acts catabolically on bone by stimulating bone resorption and inhibiting bone formation. We hypothesized that the concerted action of anti-TNF, which reduces inflammation and parathyroid hormone (PTH), which stimulates bone formation, or osteoprotegerin (OPG), which blocks bone resorption and could lead to repair of local bone erosions and reversal of systemic bone loss. To test this, human TNF-transgenic mice with established erosive arthritis and systemic bone loss were treated with PTH, OPG, and anti-TNF, alone or in combination. Local bone erosions almost fully regressed, on combined treatment with anti-TNF and PTH and/or OPG, suggesting repair of inflammatory skeletal lesions. In contrast, OPG and anti-TNF alone led to arrest of bone erosions but did not achieve repair. Treatment with PTH alone had no influence on the progression of bone erosions. Local bone erosions all showed signs of new bone formation such as the presence of osteoblasts, osteoid formation, and mineralization. Furthermore, systemic bone loss was completely reversed on combined treatment and this effect was mediated by osteoblast stimulation and osteoclast blockade. In summary, we conclude that local joint destruction and systemic inflammatory bone loss because of TNF can regress and that repair requires a combined approach by reducing inflammation, blocking bone resorption, or stimulating bone formation.

The antirheumatic drug leflunomide inhibits osteoclastogenesis by interfering with receptor activator of NF-κB ligand-stimulated induction of nuclear factor of activated T cells c1

OBJECTIVE

Suppression of bone destruction is required as part of an effective therapeutic strategy for autoimmune arthritis. Although numerous antirheumatic drugs are in clinical use, little is known about whether they inhibit bone destruction by acting on activated T cells or other cell types, such as bone-resorbing osteoclasts. This study was undertaken to determine whether leflunomide has a direct action on the osteoclast lineage and to gain insights into the molecular basis for the bone-protective effect of leflunomide.

METHODS

The direct effect of leflunomide on osteoclast differentiation was investigated using an in vitro culture system of bone marrow monocyte/macrophages stimulated with receptor activator of NF-kappa B ligand (RANKL) and macrophage colony-stimulating factor. The molecular mechanism of the inhibition was analyzed by genome-wide screening. The T cell-independent effect of leflunomide was examined in rag-2(-/-) mice.

RESULTS

Leflunomide blocked de novo pyrimidine synthesis and RANKL-induced calcium signaling in osteoclast precursor cells in vitro; hence, the induction of nuclear factor of activated T cells c1 (NF-ATc1) was strongly inhibited. The inhibition of this pathway is central to the action of leflunomide, since the inhibition was overcome by ectopic expression of NF-ATc1 in the precursor cells. Leflunomide suppressed endotoxin-induced inflammatory bone destruction even in rag-2(-/-) mice.

CONCLUSION

Leflunomide has a direct inhibitory effect on RANKL-mediated osteoclast differentiation by inhibiting the induction of NF-ATc1, the master switch regulator for osteoclast differentiation. Our study suggests that the direct inhibitory action of leflunomide on osteoclast differentiation constitutes an important aspect in the amelioration of bone destruction, and that the RANKL-dependent NF-ATc1 induction pathway is a promising target for pharmacologic intervention in arthritic bone destruction.

Relevance of an in vitro osteoclastogenesis system to study receptor activator of NF-kB ligand and osteoprotegerin biological activities

Receptor activator of NF-kB Ligand (RANKL) is an essential requirement for osteoclastogenesis and its activity is neutralized by binding to the soluble decoy receptor osteoprotegerin (OPG). The purpose of this work was to study the effects of RANKL and OPG during osteoclastogenesis using the murine monocytic cell line RAW 264.7 that can differentiate into osteoclasts in vitro. RAW 264.7 cells plated at 10(4) cells/cm(2) and cultured for 4 days in the presence of RANKL represent the optimal culture conditions for osteoclast differentiation, with an up-regulation of all parameters related to bone resorption: tartrate resistant acid phosphatase (TRAP), calcitonin receptor (CTR), RANK, cathepsin K, matrix metalloproteinase (MMP)-9 mRNA expressions. RANKL and OPG biological effects vary according to the differentiation state of the cells: in undifferentiated RAW 264.7 cells, TRAP expression was decreased by OPG and RANKL, RANK expression was inhibited by OPG, while MMP-9 and cathepsin K mRNA expressions were not modulated. In differentiated RAW 264.7 cells, RANKL and OPG both exert an overall inhibitory effect on the expression of all the parameters studied. In these experimental conditions, OPG-induced MMP-9 inhibition was abrogated in the presence of a blocking anti-RANKL antibody, suggesting that part of OPG effects are RANKL-dependent.

Rheumatoid arthritis: regulation of synovial inflammation

Rheumatoid arthritis (RA) is a systemic, inflammatory autoimmune disorder that presents as a symmetric polyarthritis associated with swelling and pain in multiple joints, often initially occurring in the joints of the hands and feet. Articular inflammation causes activation and proliferation of the synovial lining, expression of inflammatory cytokines, chemokine-mediated recruitment of additional inflammatory cells, as well as B cell activation with autoantibody production. A vicious cycle of altered cytokine and signal transduction pathways and inhibition of programmed cell death contribute to synoviocyte and osteoclast mediated cartilage and bone destruction. A combination of targeted interventions at various stages in the pathogenesis of RA will likely be required to control symptoms in certain patients with this complex and potentially disabling disease. The regulation of rheumatoid synovial inflammation will be reviewed, followed by a brief summary of the therapeutic implications of these advances, including strategies targeting key cytokines, signal transduction molecules, co-stimulatory molecules, B cells, chemokines, and adhesion molecules.

Decoy receptor 3 (DcR3) induces osteoclast formation from monocyte/macrophage lineage precursor cells

Recent evidence indicates that the decoy receptor 3 (DcR3) of the TNF receptor superfamily, which initially though prevents cytokine responses of FasL, LIGHT and TL1A by binding and neutralization, can modulate monocyte function through reverse signaling. We show in this work that DcR3 can induce osteoclast formation from human monocytes, murine RAW264.7 macrophages, and bone marrow cells. DcR3-differentiated cells exhibit characteristics unique for osteoclasts, including polynuclear giant morphology, bone resorption, TRAP, CD51/61, and MMP-9 expression. Consistent with the abrogation of osteoclastogenic effect of DcR3 by TNFR-Fc, DcR3 treatment can induce osteoclastogenic cytokine TNF-alpha release through ERK and p38 MAPK signaling pathways. We conclude that DcR3 via coupling reverse signaling of ERK and p38 MAPK and stimulating TNF-alpha synthesis is a critical regulator of osteoclast formation. This action of DcR3 might play an important role in significant osteoclastic activity in osteolytic bone metastases.

Molecular pathogenicity of the oral opportunistic pathogen Actinobacillus actinomycetemcomitans

Periodontitis is mankind's most common chronic inflammatory disease. One severe form of periodontitis is localized aggressive periodontitis (LAP), a condition to which individuals of African origin demonstrate an increased susceptibility. The main causative organism of this disease is Actinobacillus actinomycetemcomitans. A member of the Pasteurellaceae, A. actinomycetemcomitans produces a number of interesting putative virulence factors including (a) an RTX leukotoxin that targets only neutrophils and monocytes and whose action is influenced by a novel type IV secretion system involved in bacterial adhesion; (b) the newly discovered toxin, cytolethal distending toxin (CDT); and (c) a secreted chaperonin 60 with potent leukocyte-activating and bone resorbing activities. This organism also produces a plethora of proteins able to inhibit eukaryotic cell cycle progression and proteins and peptides that can induce distinct forms of proinflammatory cytokine networks. A range of other proteins interacting with the host is currently being uncovered. In addition to these secreted factors, A. actinomycetemcomitans is invasive with an unusual mechanism for entering, and traveling within, eukaryotic cells. This review focuses on recent advances in our understanding of the molecular and cellular pathogenicity of this fascinating oral bacterium.

RANK signaling is not required for TNFalpha-mediated increase in CD11(hi) osteoclast precursors but is essential for mature osteoclast formation in TNFalpha-mediated inflammatory arthritis

To address the controversy of whether TNFalpha can compensate for RANKL in osteoclastogenesis in vivo, we used a TNFalpha-induced animal model of inflammatory arthritis and blocked RANKL/RANK signaling. TNFalpha increased osteoclast precursors available for RANK-dependent osteoclastogenesis. RANK signaling is not required for the TNFalpha-stimulated increase in CD11b(hi) osteoclast precursors but is essential for mature osteoclast formation.

INTRODUCTION

Although critical roles of TNFalpha in inflammatory arthritis and RANKL in bone resorption have been firmly established, a central controversy remains about the extent to which TNFalpha can compensate for RANKL during osteoclastogenesis and the stage at which RANK signaling is required for osteoclastogenesis. Here, we used the human TNFalpha transgenic mouse model (TNF-Tg) of erosive arthritis to determine if there are both RANK-dependent and -independent stages of osteoclastogenesis in TNFalpha-induced erosive arthritis.

MATERIALS AND METHODS

Osteoclastogenesis and osteoclast precursor (OCP) frequency were analyzed using histology, fluorescence-activated cell sorting (FACS), and cell culture from (1) TNF-Tg mice treated with the RANKL antagonist, RANK:Fc, or (2) TNF-Tg X RANK -/- mice generated by crossing TNF-Tg mice with RANK-/- mice.

RESULTS

Treatment of TNF-Tg mice, which have increased OCPs in their spleens, with RANK:Fc dramatically reduced osteoclast numbers on the surface of their arthritic joints and within their bones, but did not decrease CD11b(hi) OCP numbers in their spleens. Long-term RANK:Fc administration alleviated joint erosion. Furthermore, TNF-Tg x RANK -/- mice had severe osteopetrosis, no osteoclasts, and no joint erosion, but increased CD11b(hi) precursor numbers that failed to form mature osteoclasts in vitro.

CONCLUSION

RANK signaling is essential for mature osteoclast formation in TNFalpha-mediated inflammatory arthritis but not for the TNFalpha-induced increase in CD11b(hi) OCP that subsequently can differentiate into osteoclasts in inflamed joints.

Disabling of Receptor Activator of Nuclear Factor-κB (RANK) Receptor Complex by Novel Osteoprotegerin-like Peptidomimetics Restores Bone Loss in Vivo

The tumor necrosis factor family ligand, tumor necrosis factor-related activation-induced cytokine (TRANCE), and its receptors, receptor activator of nuclear factor-kappaB (RANK) and osteoprotegerin (OPG), are known to be regulators of development and activation of osteoclasts in bone remodeling. Sustained osteoclast activation that occurs through TRANCE-RANK causes osteopenic disorders such as osteoporosis and contributes to osteolytic metastases. Here, we report a rationally designed small molecule mimic of osteoprotegerin to inhibit osteoclast formation in vitro and limit bone loss in an animal model of osteoporosis. One of the mimetics, OP3-4, significantly inhibited osteoclast formation in vitro (IC(50) = 10 microm) and effectively inhibited total bone loss in ovariectomized mice at a dosage of 2 mg/kg/day. Unlike soluble OPG receptors, which preclude TRANCE binding to RANK, OP3-4 shows the ability to modulate RANK-TRANCE signaling pathways and alters the biological functions of the RANK-TRANCE receptor complex by facilitating a defective receptor complex. These features suggest that OPG-derived small molecules can be used as a probe to understand complex biological functions of RANK-TRANCE-OPG receptors and also can be used as a platform to develop more useful therapeutic agents for inflammation and bone disease.

Colonic dendritic cells, intestinal inflammation, and T cell-mediated bone destruction are modulated by recombinant osteoprotegerin

Autoimmune associated bone disease and intestinal inflammation are closely linked with deregulation and hyperactivation of autoreactive CD4 T cells. How these T cells are activated and mediate disease is not clear. Here we show that in the Interleukin 2-deficient mouse model of autoimmunity spontaneous osteopenia and colitis are caused by increased production of the ligand for receptor activator of NFkappaB (RANKL). RANKL acting via its receptor, receptor activator of NFkappaB (RANK), increases bone turnover and promotes intestinal dendritic cell (DC) survival in vivo. Modulation of RANKL-RANK interactions with exogenous recombinant osteoprotegerin (Fc-OPG) reverses skeletal abnormalities and reduces colitis by decreasing colonic DC numbers. This study identifies a common causal link between bone disease and intestinal inflammation and establishes the importance of DC in mediating colonic inflammation in vivo.

Aseptic loosening

Although total joint replacement surgery is one of the most successful clinical procedures performed today, bone loss around knee and hip implants (osteolysis), resulting in aseptic loosening of the prosthesis, remains a major problem for many patients. Over the last decade much has been learned about this process, which is caused by wear debris particles that simulate a local inflammatory response and osteoclastic bone resorption. Aseptic loosening cannot be prevented or treated by existing nonsurgical methods. Gene transfer, however, offers novel possibilities. Here, we review the current state of the field and the experimental gene therapy approaches that have been investigated toward a solution to aseptic loosening of prosthetic implants.

Protective effects of IL-1Ra or vIL-10 gene transfer on a murine model of wear debris-induced osteolysis

The current study evaluated the protective effects of anti-inflammatory cytokine gene transfer on osteolysis provoked by orthopedic biomaterial particles using a murine model of inflammatory bone loss. A section of bone was surgically implanted into an air pouch established on a syngeneic recipient mouse. Inflammation was provoked by introduction of ultra-high-molecular-weight polyethylene (UHMWPE) particles into the pouch, and retroviruses encoding for interleukin-1 receptor antagonist (hIL-1Ra), viral interleukin-10 (vIL-10), or LacZ genes were injected. Pouch fluid and tissue were harvested 7 days later for histological and molecular analyses. The results indicated that IL-1Ra or vIL-10 gene transfer significantly inhibited IL-1beta and tumor necrosis factor (TNF) expression at both mRNA and protein levels. There were significantly lower mRNA expressions of calcitonin receptor and cathepsin K in RNA isolated from hIL-1Ra- or vIL-10-transduced pouches than LacZ-transduced and virus-free controls. Both anti-inflammatory cytokine gene transfers significantly reduced the mRNA expression of M-CSF (70-90%) and RANK (>65%) in comparison with LacZ- and virus-free controls. Histological examination showed that hIL-1Ra or vIL-10 gene transfer dramatically abolished UHMWPE-induced inflammatory cellular infiltration and bone pit erosion compared to LacZ-transduced and virus-free controls. Histochemical staining revealed significantly fewer osteoclast-like cells in samples treated with IL-1Ra or vIL-10 gene transfer. In addition, bone collagen content was markedly preserved in the groups with anti-inflammatory cytokine gene transfers compared with the other two groups. Overall, retrovirus-mediated hIL-1Ra or vIL-10 gene transfer effectively protected against UHMWPE-particle-induced bone resorption, probably due to the inhibition of IL-1/TNF-induced M-CSF production and the consequent osteoclast recruitment and maturation.

Osteoclasts, mononuclear phagocytes, and c-Fos: new insight into osteoimmunology

Osteoimmunology is the emerging concept that certain molecules link the skeletal and immune systems. The transcription factor c-Fos, a component of activator protein-1 (AP-1), is essential for osteoclast differentiation. Mice lacking c-Fos are osteopetrotic owing to impaired osteoclast development. Recent studies suggest that in contrast to this positive role in osteoclastogenesis, c-Fos expression inhibits differentiation and activation of mononuclear phagocytes. Here, we focus on the contrasting roles of c-Fos in the bone and immune lineages. Both osteoclasts and mononuclear phagocytes are derived from common myeloid precursors. Osteoclasts resorb bone, whereas macrophages and myeloid dendritic cells phagocytose microbial pathogens, initiating innate and adaptive immunity. Differentiation of the common precursors into either bone or immune lineage is determined by ligand binding to cell-surface receptors, particularly receptor activator of NF-kappa B (RANK) for osteoclasts, or Toll-like receptors (TLRs) for mononuclear phagocytes. Both RANK and TLRs activate the dimeric transcription factors NF-kappa B and AP-1. Yet, c-Fos/AP-1 plays a positive role in osteoclasts but a negative role in macrophages and dendritic cells. Further study is necessary to clarify this dual role of c-Fos.

Selective blockade of voltage-gated potassium channels reduces inflammatory bone resorption in experimental periodontal disease

The effects of the potassium channel (Kv1.3) blocker kaliotoxin on T-cell-mediated periodontal bone resorption were examined in rats. Systemic administration of kaliotoxin abrogated the bone resorption in conjunction with decreased RANKL mRNA expression by T-cells in gingival tissue. This study suggests a plausible therapeutic approach for inflammatory bone resorption by targeting Kv1.3.

INTRODUCTION

Kv1.3 is a critical potassium channel to counterbalance calcium influx at T-cell receptor activation. It is not known if Kv1.3 also regulates RANKL expression by antigen-activated T-cells, and consequently affects in vivo bone resorption mediated by activated T-cells.

MATERIALS AND METHODS

Actinobacillus actinomycetemcomitans 29-kDa outer membrane protein-specific Th1-clone cells were used to evaluate the expression of Kv1.3 (using reverse transcriptase-polymerase chain reaction [RT-PCR] and Western blot analyses) and the effects of the potassium channel blocker kaliotoxin (0-100 nM) on T-cell activation parameters ([3H]thymidine incorporation assays and ELISA) and expression of RANKL and osteoprotegerin (OPG; flow cytometry, Western blot, and RT-PCR analyses). A rat periodontal disease model based on the adoptive transfer of activated 29-kDa outer membrane protein-specific Th1 clone cells was used to analyze the effects of kaliotoxin in T-cell-mediated alveolar bone resorption and RANKL and OPG mRNA expression by gingival T-cells. Stimulated 29-kDa outer membrane protein-specific Th1 clone cells were transferred intravenously on day 0 to all animals used in the study (n = 7 animals per group). Ten micrograms of kaliotoxin were injected subcutaneously twice per day on days 0, 1, 2, and 3, after adoptive transfer of the T-cells. The control group of rats was injected with saline as placebo on the same days as injections for the kaliotoxin-treated group. The MOCP-5 osteoclast precursor cell line was used in co-culture studies with fixed 29-kDa outer membrane protein-specific Th1-clone cells to measure T-cell-derived RANKL-mediated effects on osteoclastogenesis and resorption pit formation assays in vitro. Statistical significance was evaluated by Student's t-test.

RESULTS

Kaliotoxin decreased T-cell activation parameters of 29-kDa outer membrane protein-specific Th1 clone cells in vitro and in vivo. Most importantly, kaliotoxin administration resulted in an 84% decrease of the bone resorption induced in the saline-treated control group. T-cells recovered from the gingival tissue of kaliotoxin-treated rats displayed lower ratios of RANKL and OPG mRNA expression than those recovered from the control group. The ratio of RANKL and osteoprotegerin protein expression and induction of RANKL-dependent osteoclastogenesis by the activated T-cells were also markedly decreased after kaliotoxin treatments in vitro.

CONCLUSION

The use of kaliotoxin or other means to block Kv1.3 may constitute a potential intervention therapy to prevent alveolar bone loss in periodontal disease.

An Increased Risk of Osteoporosis during Acquired Immunodeficiency Syndrome

Osteoporosis is characterized by decreased bone mineral density and mechanistic imbalances of bone tissue that may result in reduced skeletal strength and an enhanced susceptibility to fractures. Osteoporosis in its most common form affects the elderly (both sexes) and all racial groups of human beings. Multiple environmental risk factors like acquired immune deficiency syndrome (AIDS) are believed to be one of the causes of osteoporosis. Recently a high incidence of osteoporosis has been observed in human immunodeficiency virus (HIV) infected individuals. The etiology of this occurrence in HIV infections is controversial. This problem seems to be more frequent in patients receiving potent antiretroviral therapy. In AIDS, the main suggested risk factors for the development of osteoporosis are use of protease inhibitors, longer duration of HIV infection, lower body weight before antiretroviral therapy, high viral load. Variations in serum parameters like osteocalcin, c-telopeptide, levels of elements like Calcium, Magnesium, Phosphorus, concentration of vitamin-D metabolites, lactate levels, bicarbonate concentrations, amount of alkaline phosphatase are demonstrated in the course of development of osteoporosis. OPG/RANKL/RANK system is final mediator of bone remodeling. Bone mineral density (BMD) test is of added value to assess the risk of osteoporosis in patients infected with AIDS. The biochemical markers also aid in this assessment. Clinical management mostly follows the lines of treatment of osteoporosis and osteopenia.

Bone loss in unclassified polyarthritis and early rheumatoid arthritis is better detected by digital x ray radiogrammetry than dual x ray absorptiometry: relationship with disease activity and radiographic outcome

OBJECTIVE

To compare changes in regional bone mineral density (BMD) of the metacarpal joints measured by dual x ray absorptiometry (DXA) and digital x ray radiogrammetry (DXR) in relation to disease activity and radiographic outcome in a two year follow up study of patients with early RA and unclassified polyarthritis.

PATIENTS AND METHODS

72 patients with symmetrically swollen and tender second and third metacarpophalangeal or proximal interphalangeal joints for at least four weeks and less than two years were included. 51 patients fulfilled the ACR criteria for RA. 21 patients had unclassified polyarthritis. The patients with RA were divided into groups according to mean disease activity, average glucocorticoid dose, and MRI and x ray detected bone erosions in the hands. Clinical and biochemical measurements were made every month and an x ray examination of the hands and BMD of the metacarpal joints every six months.

RESULTS

DXR BMD decreased significantly only in patients with RA from month 6 and was associated with the mean disease activity. Patients with RA and erosive as well as non-erosive disease showed a significant decrease in the rate of bone loss, greatest in those with erosive disease. No changes in BMD measured by DXA were seen in any patient group.

CONCLUSION

DXR is a useful measure of the destructive disease activity in patients with RA and unclassified polyarthritis, providing valuable information about bone changes associated with disease activity and erosive disease in early RA. DXR is better than DXA for detecting and monitoring periarticular osteoporosis of the metacarpal bone.

Suppressive Effect of Kanzo-bushi-to, a Kampo Medicine, on Collagen-Induced Arthritis

Kanzo-bushi-to (KBT) is a traditional Japanese herbal medicine (Kampo medicine), which is used in Japan to treat rheumatoid arthritis. In the present study, we investigated the suppressive effect of KBT on collagen-induced arthritis (CIA) and further studied the underlying mechanism. CIA was induced in male DBA/1J mice by immunization with bovine type II collagen, followed by a booster injection 21 d later. KBT was given at a dose of 430 mg/kg/d from three days before the first immunization to the end of the experiment. KBT suppressed CIA development effectively and further protected focal bone erosion and bone destruction as evidenced by the reduced histological score. Histochemical examination revealed that KBT decreased TRAP-positive cells at the synovium-bone interface and at the sites of focal bone erosion, coincident with the findings that RANKL/OPG mRNA ratio was significantly reduced by KBT treatment. KBT also decreased mRNA levels of M-CSF and iNOS in joints and of iNOS in peritoneal macrophages. In conclusion, KBT prevented osteoclast generation by decreasing RANKL/OPG ratio and M-CSF mRNA levels, resulting in reduction in bone erosion and destruction. In addition, KBT has anti-inflammatory effect such as the suppression of iNOS expression in peritoneal macrophages and joints of CIA mice. These finding suggests that KBT is a potential new therapeutic agent for the treatment of RA.

NEW MOLECULES IN THE TUMOR NECROSIS FACTOR LIGAND AND RECEPTOR SUPERFAMILIES WITH IMPORTANCE FOR PHYSIOLOGICAL AND PATHOLOGICAL BONE RESORPTION

Osteoclasts are tissue-specific polykaryon bone-resorbing cells derived from the monocyte/macrophage hematopoietic lineage with specialized functions required for the adhesion of the cells to bone and the subsequent polarization of the cell membrane, secretion of acid to dissolve mineral crystals, and release of proteolytic enzymes to degrade the extracellular matrix proteins. Most pathological conditions in the skeleton lead to loss of bone due to excess osteoclastic bone resorption, including periodontal disease, rheumatoid arthritis, and osteoporosis. In rare cases, most of them genetic, patients with osteopetrosis exhibit sclerotic bone due either to a lack of osteoclasts or to non-functional osteoclasts. Mainly because of phenotypic findings in genetically manipulated mice or due to spontaneous mutations in humans, mice, and rats, several genes have been discovered as being crucial for osteoclast formation and activation. Recent breakthroughs in our understanding of osteoclast biology have revealed the critical roles in osteoclast differentiation played by RANKL, RANK, and OPG, three novel members of the tumor necrosis factor ligand and receptor superfamilies. The further study of these molecules and downstream signaling events are likely to provide a molecular basis for the development of new drugs for the treatment of diseases with excess or deficient osteoclastic bone resorption.

Bone marrow metastatic myeloma cells promote osteoclastogenesis through RANKL on endothelial cells

We have been using the B9/BM1 murine bone marrow metastasis model to study the function of adhesion molecules in the cell-cell interactions and transendothelial migration, necessary for tumor metastasis. The cell surface phenotype of these cells, which colonize vertebral and femoral marrow after intravenous injection, shows great similarity to that of human myeloma cells. In the present study, we investigated the interaction between B9/BM1 cells and osteoclasts, which likely support tumor metastasis in bone marrow. We found that co-culturing B9/BM1 cells and bone marrow-derived endothelial cells (BMECs) in the presence of vitamin D3 and M-CSF promoted differentiation of primary osteoclast progenitors to osteoclasts (detected by TRAP staining), and that this effect was blocked when BMECs were separated from the other cells by a porous polycarbonate membrane. Flow cytometry analysis showed that BMECs expressed RANKL (receptor activator of NF-kappaB ligand) protein on their surface, and that this expression was up-regulated by co-culture with B9/BM1 cells. Accordingly, RT-PCR showed expression of RANKL mRNA also to be up-regulated in BMECs co-cultured with B9/BM1 cells. Addition of OPG (osteoprotegerin, a decoy RANKL receptor) to the co-culture system completely blocked osteoclast induction, as did addition of anti-CD44 antibody. Furthermore, intravenous injection of B9/BM1 cells substantially increased the numbers of TRAP-positive osteoclasts detected in mice in vivo. Taken together, these findings suggest that B9/BM1 myeloma cells act via CD44 to stimulate RANKL expression on BMECs, which in turn physically interact with osteoclast progenitors to promote their differentiation to osteoclasts and metastasis in bone marrow.

Receptor activator of nuclear factor kappaB ligand (RANKL)/osteoprotegerin (OPG) ratio is increased in severe osteolysis

Pathological osteolyses are considered a consequence of a disturbance in the mechanisms that govern the bone remodeling, mainly the communication between osteoclasts and osteoblasts. Osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL) are newly discovered molecules that play a key role in these communications. RANKL is essential for osteoclast differentiation via its receptor RANK located on the osteoclast membrane. OPG is a soluble decoy receptor that inhibits osteoclast differentiation through its binding to RANKL. The aim of this study is the analysis of the RANKL/OPG balance by complementary methods (semiquantitative reverse transcription-polymerase chain reaction, immunohistochemistry, and enzyme-linked immunosorbent assay) in human osteolysis associated to various bone etiologies (n = 60), tumoral (primitive, secondary) or not, compared to healthy tissues (n = 16). Results demonstrated that RANKL/OPG ratio was significantly increased in patients suffering from severe osteolysis compared to the control group and that this imbalance is involved in bone resorption mechanisms. In this study, OPG expression appears to reflect a protective mechanism of the skeleton to compensate increased bone resorption by inhibiting osteoclast formation and bone resorbing activity. Moreover, as revealed by immunohistochemistry, RANKL and OPG were colocalized in all of the tissues analyzed. To define the veracity of RANKL/OPG index in assessing and managing patients with severe osteolysis, an extended population of patients suffering from severe osteolysis must be now monitored.

Regulation of osteoclast protease expression by RANKL

Receptor activator of NF-kappaB ligand (RANKL) is essential for osteoclast (OC) differentiation/activation and functions through its receptor RANK at the surface of the osteoclastic cells. This study investigated for the first time the direct effects of hRANKL on protease/protease inhibitor expressions and protease activities in purified rabbit osteoclast cultures, using semi-quantitative RT-PCR, gelatin zymography, and enzymatic assays. RANKL was shown to exert in vitro pro-resorptive effects by increasing osteoclast marker expressions (Tartrate resistant acid phosphatase (TRAP) and cathepsin K), MMP-9 expression, and pro-MMP-9 activity and by diminishing TIMP-1 expression, leading to an up-regulation of the MMP-9/TIMP-1 ratio.

Regulation of bone lysis in inflammatory diseases

Focal bone erosion is a major pathological feature of several common inflammatory diseases. Over the past decade there have been major advances in our understanding of the factors that regulate osteoclast formation and activity. It is now apparent that receptor activator for NFkappaB (RANK), its ligand RANKL (also known as TRANCE, osteoclast differentiation factor and osteoprotegerin (OPG) ligand) and the RANKL inhibitor OPG, are the major factors regulating osteoclast formation. These molecules influence normal bone physiology and now there is growing evidence that RANK-RANKL interactions also regulate osteoclast formation in disease. This paper reviews recent findings showing expression of RANK, RANKL and OPG in inflammatory diseases including rheumatoid arthritis, periodontal disease and peri-implant loosening. It is emerging that OPG and RANKL are key molecules regulating bone loss in disease and therapeutic intervention that targets these molecules may be helpful in treating a wide range of diseases.

Dendritic cell-induced autoimmune heart failure requires cooperation between adaptive and innate immunity

Genetic susceptibility and autoimmunity triggered by microbial infections are factors implicated in the pathogenesis of dilated cardiomyopathy, the most common cause of heart failure in young patients. Here we show that dendritic cells (DCs) loaded with a heart-specific self peptide induce CD4+ T-cell-mediated myocarditis in nontransgenic mice. Toll-like receptor (TLR) stimulation, in concert with CD40 triggering of self peptide-loaded dendritic cells, was shown to be required for disease induction. After resolution of acute myocarditis, DC-immunized mice developed heart failure, and TLR stimulation of these mice resulted in relapse of inflammatory infiltrates. Injection of damaged, syngeneic cardiomyocytes also induced myocarditis in mice if TLRs were activated in vivo. DC-induced myocarditis provides a unifying theory as to how tissue damage and activation of TLRs during infection can induce autoimmunity, relapses and cardiomyopathy.

Streptococcus pyogenes infection induces septic arthritis with increased production of the receptor activator of the NF-kappaB ligand

Bacterial arthritis is a rapidly progressive and highly destructive joint disease in humans, with Staphylococcus aureus and Neisseria gonorrhoeae the major causative agents, although beta-hemolytic streptococci as well often induce the disease. We demonstrate here that intravenous inoculation of CD-1 mice with the group A streptococcus (GAS) species Streptococcus pyogenes resulted in a high incidence of septic arthritis. Signs of arthritis emerged within the first few days after injection, and bacterial examinations revealed that colonization of the inoculated GAS in the arthritic joints persisted for 21 days. Induction of persistent septic arthritis was dependent on the number of microorganisms inoculated. Immunohistochemical staining of GAS with anti-GAS antibodies revealed colonization in the joints of infected mice. Cytokine levels were quantified in the joints and sera of infected mice by using an enzyme-linked immunosorbent assay. High levels of interleukin-1beta (IL-1beta) and IL-6 were detected in the joints from 3 to 20 days after infection. We noted that an increase in the amount of receptor activator of NF-kappaB ligand (RANKL), which is a key cytokine in osteoclastogenesis, was also evident in the joints of the infected mice. RANKL was not detected in sera, indicating local production of RANKL in the infected joints. Blocking of RANKL by osteoprotegerin, a decoy receptor of RANKL, prevented bone destruction in the infected joints. These results suggest that GAS can colonize in the joints and induce bacterial arthritis. Local RANKL production in the infected joints may be involved in bone destruction.

Bacterial infection induces expression of functional MHC class II molecules in murine and human osteoblasts

A growing body of evidence has shown that bacterially challenged osteoblasts can play a significant role in the initiation of inflammatory immune responses at sites of bone disease. We have recently demonstrated the surprising ability of osteoblasts exposed to bacteria to express CD40, a molecule that plays a critical costimulatory role in the activation of T lymphocytes. In the present study, we have extended our investigations into the ability of osteoblasts to interact with CD4+ T lymphocytes by determining the expression of antigen-presenting major histocompatibility complex (MHC) class II molecules in murine and human osteoblasts following exposure to two common pathogens of bone, Staphylococcus aureus and Salmonella. Cultured osteoblasts were found to respond rapidly to bacterial challenge by induction of mRNA encoding MHC class II molecules or its transcriptional regulator. Increased mRNA expression translated into expression of MHC class II proteins in murine and human osteoblasts as determined by Western blot analysis and by immunohistochemical and immunofluorescent microscopy. Furthermore, the increased surface expression of these molecules on osteoblasts exposed to bacteria was confirmed by FACS analysis. Finally, we show that bacterial challenge results in the elevated functional expression of MHC class II molecules on osteoblasts by demonstrating the enhanced ability of these cells to interact with T lymphocytes and to initiate antigen-specific T cell activation. Taken together, these data suggest a previously unappreciated role for osteoblasts in the initiation of T lymphocyte activation at sites of bacterial infection in bone tissue.

Expression profiling of lymphocyte plasma membrane proteins

The physicochemical properties of plasma membrane proteins of mammalian cells render them refractory to systematic analysis by two-dimensional electrophoresis. We have therefore used in vivo cell surface labeling with a water-soluble biotinylation reagent, followed by cell lysis and membrane purification, prior to affinity capture of biotinylated proteins. Purified membrane proteins were then separated by solution-phase isoelectric focusing and SDS-PAGE and identified by high-pressure liquid chromatography electrospray/tandem mass spectrometry. Using this approach, we identified 42 plasma membrane proteins from a murine T cell hybridoma and 46 from unfractionated primary murine splenocytes. These included three unexpected proteins; nicastrin, osteoclast inhibitory lectin, and a transmembrane domain-containing hypothetical protein of 11.4 kDa. Following stimulation of murine splenocytes with phorbol ester and calcium ionophore, we observed differences in expression of CD69, major histocompatibility complex class II molecules, the glucocorticoid-induced TNF receptor family-related gene product, and surface immunoglobulin M and D that were subsequently confirmed by Western blot or flow cytometric analysis. This approach offers a generic and powerful strategy for investigating differential expression of surface proteins in many cell types under varying environmental and pathophysiological conditions.

Vascular endothelial growth factor up-regulates expression of receptor activator of NF-kappa B (RANK) in endothelial cells. Concomitant increase of angiogenic responses to RANK ligand

Vascular endothelial growth factor (VEGF) is known as a key regulator of angiogenesis during endochondral bone formation. Recently, we demonstrated that TNF-related activation-induced cytokine (TRANCE or RANKL), which is essential for bone remodeling, also had an angiogenic activity. Here we report that VEGF up-regulates expression of receptor activator of NF-kappa B (RANK) and increases angiogenic responses of endothelial cells to TRANCE. Treatment of human umbilical vein endothelial cells (HUVECs) with VEGF increased both RANK mRNA and surface protein expression. Although placenta growth factor specific to VEGF receptor-1 had no significant effect on RANK expression, inhibition of downstream signaling molecules of the VEGF receptor-2 (Flk-1/KDR) such as Src, phospholipase C, protein kinase C, and phosphatidylinositol 3'-kinase suppressed VEGF-stimulated RANK expression in HUVECs. Moreover, the MEK inhibitor PD98059 or expression of dominant negative MEK1 inhibited induction of RANK by VEGF but not the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM). VEGF potentiated TRANCE-induced ERK activation and tube formation via RANK up-regulation in HUVECs. Together, these results show that VEGF enhances RANK expression in endothelial cells through Flk-1/KDR-protein kinase C-ERK signaling pathway, suggesting that VEGF plays an important role in modulating the angiogenic action of TRANCE under physiological or pathological conditions.

The role of calmodulin in the regulation of osteoclastogenesis

Calmodulin plays an important role in regulating the function of mature osteoclasts. However, its role in osteoclastogenesis has not been investigated. In the present study, we examined the role of calmodulin in osteoclastogenesis using in vivo and in vitro systems. Calmodulin antagonists, trifluoperazine (TFP), W7, and tamoxifen, dose-dependently inhibited osteoclast formation, which occurred only in the last 24 h of a 4-d osteoclastogenesis culture using mouse bone marrow macrophages. Inhibitory effects were quantitated by measuring tartrate-resistant acid phosphatase activity and counting osteoclast numbers. In contrast, bis indolylmaleimide, a protein kinase C inhibitor, showed no such inhibitory effect even when applied at a concentration that was 10-fold greater than its IC50. Overexpressing calmodulin by recombinant retrovirus reversed the inhibitory effect of TFP on osteoclast-like differentiation in RAW264.7 cells. Furthermore, administration of TFP to mice was as effective as estrogen in abolishing the ovariectomy-induced increment of osteoclastogenesis as determined by quantitative assessment of tartrate-resistant acid phosphatase activity in tibias, which led to the recovery of the ovariectomy-induced decrement in trabecular bone volume. To investigate potential cellular and molecular mechanisms by which calmodulin antagonists inhibit osteoclastogenesis, Z-VAD-FMK, a broad caspase inhibitor, failed to block the inhibitory effect of TFP on mouse osteoclast formation, indicating that apoptosis is not the underlying mechanism. Pretreatment of RAW264.7 cells with different concentrations of TFP dose-dependently inhibited receptor activator of nuclear factor kappaB ligand-stimulated phosphorylation of c-Jun N-terminal kinase and inhibitory kappaBalpha but not that of p38. Taken together, our data indicate that calmodulin mediates osteoclast differentiation, possibly via modulating specific receptor activator of NF-kappaB-signaling pathways.

Resting T cells negatively regulate osteoclast generation from peripheral blood monocytes

There is accumulating evidence that T cells may be involved in osteoclastogenesis in a variety of murine systems. However, the precise role of human T cells in the regulation of osteoclast generation is still unclear. To address this issue, we investigated the effect of resting peripheral T cells on receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast generation from human peripheral monocytes. Although osteoclasts were not generated in the culture of human peripheral blood mononuclear cells (PBMC) in the presence of RANKL and macrophage colony-stimulating factor (M-CSF), the addition of cyclosporine A (CsA), a potent inhibitor of T-cell function, resulted in the formation of an increasing number of lacunae resorption on dentine, suggesting T cells may inhibit osteoclast formation. In a coculture of T cells and monocytes, which were isolated from PBMC, T cells inhibited the osteoclast generation from monocytes, as determined by tartrate-resistant acid phosphatase (TRAP) staining and a pit assay using dentine. This inhibition of osteoclast generation by T cells was also observed in a culture of the parathyroid hormone-stimulated SaOS4/3 osteoblast cell line and monocytes. The culture in Transwell plates revealed that the cell-to-cell interaction was not required for the inhibition, suggesting that T-cell cytokines may be responsible for the inhibition. Among inhibitory T-cell cytokines on osteoclastogenesis, granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon-gamma (IFN-gamma) were actively produced by CD4 T cells but not CD8 T cells in the coculture of T cells with monocytes, and the neutralizing antibodies to these cytokines partially rescued the T-cell-induced inhibition of osteoclast formation. Although CsA did not affect RANKL-induced osteoclast generation in the culture of monocytes alone, it completely rescued the T-cell-induced inhibition of osteoclast formation and strongly inhibited the production of GM-CSF and IFN-gamma. Thus, we demonstrate that resting T cells negatively regulate the osteoclast generation via production of GM-CSF and IFN-gamma by CD4 T cells and that CsA stimulates the osteoclast generation through the inhibition of the production of these cytokines. These findings provide new insight into therapeutic strategies for immunosuppression-induced bone loss in transplant and other diseases.

Prevention of the onset and progression of collagen-induced arthritis in rats by the potent p38 mitogen-activated protein kinase inhibitor FR167653

OBJECTIVE

FR167653 is a potent inhibitor of p38 mitogen-activated protein kinase (MAPK) and inhibits tumor necrosis factor alpha (TNFalpha) and interleukin-1 beta (IL-1 beta) production in inflammatory cells. In this study we investigated the effect of FR167653 on collagen-induced arthritis (CIA).

METHODS

Rats with CIA were subcutaneously injected with FR167653 (32 mg/kg/day) starting on the day of the booster injection (day 7) in the prophylactic treatment group and after the onset of arthritis (day 21) in the therapeutic treatment group. Hind-paw swelling, body weight, radiographic and histologic scores, and osteoclast number were evaluated. Cytokine levels in the serum and tissue were assessed by enzyme-linked immunosorbent assays. Flow cytometric analysis of T lymphocytes from bone marrow was performed. The effect of FR167653 on in vitro osteoclast formation induced by soluble receptor activator of nuclear factor kappa B ligand (sRANKL) and TNFalpha was examined.

RESULTS

Swelling of hind paws and loss of weight occurred in the CIA rats, but this was not evident in the prophylactic treatment group. Therapeutic treatment also significantly reduced paw swelling. The mean radiographic and histologic scores as well as the osteoclast numbers were significantly lower in the treatment group than in the CIA rats without treatment. FR167653 treatment reduced the serum levels of TNFalpha and IL-1 beta, lowered the IL-1 beta concentration in the ankle joints, and decreased the CD4-,CD8a+ T cell population in bone marrow. Furthermore, FR167653 inhibited the osteoclast-like cell differentiation induced by both sRANKL and TNFalpha in vitro.

CONCLUSION

FR167653 prevents the onset of arthritis in a prophylactic treatment model and suppresses the progression of joint destruction in a therapeutic treatment model, suggesting that p38 MAPK is a potential therapeutic target for rheumatoid arthritis.

The receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) is a new chemotactic factor for human monocytes

Bone resorption is regulated by the immune system, where receptor activator of nuclear factor (NF)kappaB ligand (RANKL), a new member of the tumor-necrosis factor family, may contribute to pathological conditions. Due to the role of RANKL in the maturation of monocyte-derived osteoclasts, we hypothesized that RANKL could exert chemotactic properties toward monocytic cells. Our results demonstrate that RANKL induces the migration of MonoMac-6 monocytic cells as well as human freshly isolated total peripheral blood mononuclear cells (PBMC) and CD14+ purified PBMC. RANKL induces the migration of MonoMac-6 cells in a dose-dependent manner and with an efficacy similar to MCP-1. After an 8-h incubation, the soluble form of RANKL (sRANKL) started to exhibit a chemoattractive effect on MonoMac-6 cells, with an increased effect observed up to 24 h. RANKL elicits an additive chemotactic effect to MCP-1. Furthermore, addition of the RANKL decoy receptor osteoprotegerin in the lower well or RANKL in the upper well abrogates the RANKL-induced migration of MonoMac-6 cells, hallmarking a true specific activity. RNase protection assay experiments indicate that exposure of MonoMac-6 cells to RANKL had no significant effect on the expression of a variety of chemokines, known to attract monocytes. This study provides evidence that RANKL behaves as a chemotactic factor for monocytic cells, emphazing the cross-talk between bone and immune systems.

The role of acquired immunity and periodontal disease progression

Our understanding of the pathogenesis in human periodontal diseases is limited by the lack of specific and sensitive tools or models to study the complex microbial challenges and their interactions with the host's immune system. Recent advances in cellular and molecular biology research have demonstrated the importance of the acquired immune system not only in fighting the virulent periodontal pathogens but also in protecting the host from developing further devastating conditions in periodontal infections. The use of genetic knockout and immunodeficient mouse strains has shown that the acquired immune response-in particular, CD4+ T-cells-plays a pivotal role in controlling the ongoing infection, the immune/inflammatory responses, and the subsequent host's tissue destruction. In particular, studies of the pathogen-specific CD4+ T-cell-mediated immunity have clarified the roles of: (i) the relative diverse immune repertoire involved in periodontal pathogenesis, (ii) the contribution of pathogen-associated Th1-Th2 cytokine expressions in periodontal disease progression, and (iii) micro-organism-triggered periodontal CD4+ T-cell-mediated osteoclastogenic factor, 'RANK-L', which is linked to the induction of alveolar bone destruction in situ. The present review will focus on some recent advances in the acquired immune responses involving B-cells, CD8+ T-cells, and CD4+ T-cells in the context of periodontal disease progression. New approaches will further facilitate our understanding of their underlying molecular mechanisms that may lead to the development of new treatment modalities for periodontal diseases and their associated complications.

Stat1 functions as a cytoplasmic attenuator of Runx2 in the transcriptional program of osteoblast differentiation

Bone remodeling is central to maintaining the integrity of the skeletal system, wherein the developed bone is constantly renewed by the balanced action of osteoblastic bone formation and osteoclastic bone resorption. In the present study, we demonstrate a novel function of the Stat1 transcription factor in the regulation of bone remodeling. In the bone of the Stat1-deficient mice, excessive osteoclastogenesis is observed, presumably caused by a loss of negative regulation of osteoclast differentiation by interferon (IFN)-beta. However, the bone mass is unexpectedly increased in these mice. This increase is caused by excessive osteoblast differentiation, wherein Stat1 function is independent of IFN signaling. Actually, Stat1 interacts with Runx2 in its latent form in the cytoplasm, thereby inhibiting the nuclear localization of Runx2, an essential transcription factor for osteoblast differentiation. The new function of Stat1 does not require the Tyr 701 that is phosphorylated when Stat1 becomes a transcriptional activator. Our study provides a unique example in which a latent transcription factor attenuates the activity of another transcription factor in the cytoplasm, and reveals a new regulatory mechanism in bone remodeling.

Human T-cell leukemia virus type I and adult T-cell leukemia

Human T-cell leukemia virus type I (HTLV-I) causes adult T-cell leukemia (ATL) in about 5% of carriers after a long latent period. After its infection, HTLV-I promotes the clonal proliferation of HTLV-I infected cells in vivo by actions of encoded viral proteins, including Tax. However, leukemic cells frequently lack the expression of Tax by the genetic and epigenetic changes of HTLV-I provirus, suggesting that Tax is not always necessary after transformation. Alternatively, ATL cells without Tax protein could escape from the host immune system since Tax is the major target of cytotoxic lymphocytes. During the latent period, alterations of host genome accumulate, finally leading to onset of ATL.

Stromal cell-derived factor-1 (SDF-1) recruits osteoclast precursors by inducing chemotaxis, matrix metalloproteinase-9 (MMP-9) activity, and collagen transmigration

Signals targeting OCs to bone and resorption sites are not well characterized. A chemoattractant receptor (CXCR4), highly expressed in murine OC precursors, mediated their chemokine (SDF-1)-induced chemoattraction, collagen transmigration, and MMP-9 expression. Thus, bone vascular and stromal SDF-1 may direct OC precursors into bone and marrow sites for development and bone resorption.

INTRODUCTION

Although chemokines are essential for trafficking and homing of circulating hematopoietic cells under normal and pathological conditions, their potential roles in osteoclast (OC) recruitment or function are generally unknown. CXCR4 and its unique ligand, stromal cell-derived factor-1 (SDF-1), critically control the matrix metalloproteinase (MMP)-dependent targeting of hematopoietic cells into bone and within the marrow microenvironment. Therefore, SDF-1/CXCR4 may regulate OC precursor recruitment to sites for development and activation.

METHODS

Chemokine receptor mRNA expression was analyzed during OC formation induced by RANKL in murine RAW 264.7 cells. SDF-1 versus RANKL effects on chemotaxis, transcollagen migration, MMP-9 expression and activity, OC development, and bone resorption were evaluated in RAW cells or RAW-OCs.

RESULTS

CXCR4 was highly expressed in RAW cells and downregulated during their RANKL development into bone-resorptive RAW-OCs. SDF-1, but not RANKL, elicited RAW cell chemotaxis. Conversely, RANKL, but not SDF-1, promoted RAW-OC development, TRAP activity, cathepsin K expression, and bone pit resorption, and SDF-1 did not modify these RANKL responses. Both SDF-1 and RANKL increased MMP-9, a matrix-degrading enzyme essential for OC precursor migration into developing bone marrow cavities, and increased transcollagen migration of RAW cells in a MMP-dependent manner. SDF-1 also upregulated MMP-9 in various primary murine OC precursor cells. Because RANKL induced a higher, more sustained expression of MMP-9 in RAW cells than did SDF-1, MMP-9 may have an additional role in mature OCs. Consistent with this, MMP-9 upregulation during RANKL-induced RAW-OC development was necessary for initiation of bone pit resorption.

CONCLUSIONS

SDF-1, a chemokine highly expressed by bone vascular endothelial and marrow stromal cells, may be a key signal for the selective attraction of circulating OC precursors into bone and their migration within marrow to appropriate perivascular stromal sites for RANKL differentiation into resorptive OCs. Thus, SDF-1 and RANKL likely serve complementary physiological functions, partly mediated through increases in MMP-9, to coordinate stages of OC precursor recruitment, development, and function.

Involvement of p38MAP kinase in bone morphogenetic protein-4-induced osteoprotegerin in mouse bone-marrow-derived stromal cells

Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor is a recently identified cytokine that belongs to the tumour necrosis factor receptor superfamily and regulates bone mass by inhibiting osteoclastic bone resorption. This study found that bone morphogenetic protein (BMP)-4 markedly increased the level of soluble OPG in the mouse bone-marrow-derived stromal cell line, ST2. In contrast, BMP-4 showed no effect on OPG ligand production in ST2 cells under similar conditions. Using an in vitro immunocomplex kinase assay, BMP-4 was found to activate p38 mitogen-activated protein (MAP) kinase. Pre-treatment of ST2 cells with SB203580 (a specific inhibitor of p38MAP kinase) inhibited BMP-4-induced increase in OPG, although PD98059 (a specific inhibitor of classic MAP kinase cascade) showed no effect on OPG production. These results clearly suggest that activation of the p38MAP kinase pathway is necessary for BMP-4-induced OPG induction in ST2 cells.

The pathophysiology of bone disease in gastrointestinal disease

Reduced bone mass and the increased risk of fracture in gastrointestinal diseases have a multifactorial pathogenesis. Undoubtedly, genetics play an important role, but other factors such as systemic inflammation, malnutrition, hypogonadism, glucocorticoid therapy in inflammatory bowel disease (IBD) and other lifestyle factors, such as smoking or being sedentary, may contribute to reduced bone mass. At a molecular level the proinflammatory cytokines that contribute to the intestinal immune response in IBD and probably also in coeliac disease are also known to enhance bone resorption. The discovery of the role of the receptor to activated NFkappaB (RANK) interaction with its ligand RANKL in orchestrating the balance between bone resorption and formation may link mucosal and systemic inflammation with bone remodelling, since RANK-RANKL are also involved in lymphopoiesis and T-cell apoptosis. Low circulating leptin in response to weight loss in any gastrointestinal disease may be an important factor in reducing bone mass. This report will summarize current concepts regarding gastrointestinal diseases (primarily IBD, coeliac disease and postgastrectomy states) and low bone mass and fracture.

Essential role of the cryptic epitope SLAYGLR within osteopontin in a murine model of rheumatoid arthritis

It has been shown that osteopontin (OPN) plays a pivotal role in the pathogenesis of rheumatoid arthritis (RA). However, the molecular mechanism of OPN action is yet to be elucidated. Splenic monocytes obtained from arthritic mice exhibited a significant capacity for cell migration toward thrombin-cleaved OPN but not toward full-length OPN. Migratory monocytes expressed alpha9 and alpha4 integrins. Since cleavage of OPN by thrombin exposes the cryptic epitope recognized by alpha9 and alpha4 integrins, we investigated the role of the cryptic epitope SLAYGLR in a murine RA model by using a specific antibody (M5) reacting to SLAYGLR sequence. The M5 antibody could abrogate monocyte migration toward the thrombin-cleaved form of OPN. Importantly, M5 antibody could inhibit the proliferation of synovium, bone erosion, and inflammatory cell infiltration in arthritic joints. Thus, we demonstrated that a cryptic epitope, the SLAYGLR sequence of murine OPN, is critically involved in the pathogenesis of a murine model of RA.