Neutrophil extracellular traps mediate bone erosion in rheumatoid arthritis by enhancing RANKL-induced osteoclastogenesis

BACKGROUND AND PURPOSE

Rheumatoid arthritis (RA) is a chronic autoimmune disease that can cause bone erosion due to increased osteoclastogenesis. Neutrophils involvement in osteoclastogenesis remains uncertain. Given that neutrophil extracellular traps (NETs) can act as inflammatory mediators in rheumatoid arthritis, we investigated the role of NETs in stimulating bone loss by potentiating osteoclastogenesis during arthritis.

EXPERIMENTAL APPROACH

The level of NETs in synovial fluid from arthritis patients was assessed. Bone loss was evaluated by histology and micro-CT in antigen-induced arthritis (AIA)-induced WT mice treated with DNase or in Padi4-deficient mice (Padi4flox/flox LysMCRE ). The size and function of osteoclasts and the levels of RANKL and osteoprotegerin (OPG) released by osteoblasts that were incubated with NETs were measured. The expression of osteoclastogenic marker genes and protein levels were evaluated by qPCR and western blotting. To assess the participation of TLR4 and TLR9 in osteoclastogenesis, cells from Tlr4-/- and Tlr9-/- mice were cultured with NETs.

KEY RESULTS

Rheumatoid arthritis patients had higher levels of NETs in synovial fluid than osteoarthritis patients, which correlated with increased levels of RANKL/OPG. Moreover, patients with bone erosion had higher levels of NETs. Inhibiting NETs with DNase or Padi4 deletion alleviated bone loss in arthritic mice. Consistently, NETs enhanced RANKL-induced osteoclastogenesis that was dependent on TLR4 and TLR9 and increased osteoclast resorptive functions in vitro. In addition, NETs stimulated the release of RANKL and inhibited osteoprotegerin in osteoblasts, favouring osteoclastogenesis.

CONCLUSIONS AND IMPLICATIONS

Inhibiting NETs could be an alternative strategy to reduce bone erosion in arthritis patients.

[Research Progress in the Molecular Regulatory Mechanisms of Alveolar Bone Restoration]

Alveolar bone, the protruding portion of the maxilla and the mandible that surrounds the roots of teeth, plays an important role in tooth development, eruption, and masticatory performance. In oral inflammatory diseases, including apical periodontitis, periodontitis, and peri-implantitis, alveolar bone defects cause the loosening or loss of teeth, impair the masticatory function, and endanger the physical and mental health of patients. However, alveolar bone restoration is confronted with great clinical challenges due to the the complicated effect of the biological, mechanical, and chemical factors in the oral microenvironment. An in-depth understanding of the underlying molecular regulatory mechanisms will contribute to the exploration of new targets for alveolar bone restoration. Recent studies have shown that Notch, Wnt, Toll-like receptor (TLR), and nuclear factor-κB (NF-κB) signaling pathways regulate the proliferation, differentiation, apoptosis, and autophagy of osteoclasts, osteoblasts, osteocytes, periodontal ligament cells, macrophages, and adaptive immune cells, modulate the expression of inflammatory mediators, affect the balance of the receptor activator for nuclear factor-κB ligand/receptor activator for nuclear factor-κB/osteoprotegerin (RANKL/RANK/OPG) system, and ultimately participate in alveolar bone restoration. Additionally, alveolar bone restoration involves AMP-activated protein kinase (AMPK), phosphatidyl inositol 3-kinase/protein kinase B (PI3K/AKT), Hippo/YAP, Janus kinase/signal transducer and activator of transcription (JAK/STAT), and transforming growth factor β (TGF-β) signaling pathways. However, current studies have failed to construct mature molecular regulatory networks for alveolar bone restoration. There is an urgent need for further research on the molecular regulatory mechanisms of alveolar bone restoration by using new technologies such as single-cell transcriptome sequencing and spatial transcriptome sequencing.

Isoorientin ameliorates osteoporosis and oxidative stress in postmenopausal rats

CONTEXT

Isoorientin has many biological activities, including antioxidant, anti-inflammatory, antitumor. However, the effect of isoorientin on postmenopausal osteoporosis remains unclear.

OBJECTIVE

To evaluate the effect of isoorientin on postmenopausal osteoporosis.

MATERIALS AND METHODS

Sprague-Dawley rats were divided into five groups (n = 5): sham, model, 17-β-oestradiol (E2, 10 μg/kg/day), low-dose isoorientin (L-Iso, 50 mg/kg), and high-dose isoorientin (H-Iso, 100 mg/kg). The rats were ovariectomized, treated by gavage daily for 12 weeks, and serum and femur samples were collected. Bone mineral density, bone metabolism, and oxidative stress were assessed. H&E staining, immunohistochemistry, and western blotting were employed.

RESULTS

Isoorientin improved the bone mineral density of the lumbar vertebrae (2.01 ± 0.05 g/cm³ in H-Iso group vs. 1.74 ± 0.07 g/cm³ in model group) and femur (1.46 ± 0.06 g/cm³ vs. 1.19 ± 0.03 g/cm³), increased the trabecular bone number (1.97 ± 0.03 vs. 1.18 ± 0.13) and thickness (0.27 ± 0.02 vs. 0.16 ± 0.03 mm). Isoorientin decreased the separation degree of trabecular bone, ameliorated bone histomorphology changes, and significantly improved the mechanical properties. Isoorientin diminished MDA (by 60%) and increased SOD (by 49.2%), and GSH-Px (by 159%) activity. Furthermore, osteoprotegerin (OPG), nuclear factor erythroid 2-like 2 (Nrf2), haem oxygenase (HO-1), NAD(P)H quinone dehydrogenase 1(NQO1), and oestrogen receptor 1(ESR1) protein expression increased, while receptor activator of nuclear factor-κB ligand (RANKL) protein expression decreased after treatment.

CONCLUSIONS

Isoorientin ameliorates osteoporosis via upregulating OPG and Nrf2/ARE signalling, suggesting isoorientin maybe a potential therapeutic drug for PMOP.

Targeted blockade of immune mechanisms inhibit B precursor acute lymphoblastic leukemia cell invasion of the central nervous system

Acute lymphoblastic leukemia (ALL) dissemination to the central nervous system (CNS) is a challenging clinical problem whose underlying mechanisms are poorly understood. Here, we show that primary human ALL samples injected into the femora of immunodeficient mice migrate to the skull and vertebral bone marrow and provoke bone lesions that enable passage into the subarachnoid space. Treatment of leukemia xenografted mice with a biologic antagonist of receptor activator of nuclear factor κB ligand (RANKL) blocks this entry route. In addition to erosion of cranial and vertebral bone, samples from individuals with B-ALL also penetrate the blood-cerebrospinal fluid barrier of recipient mice. Co-administration of C-X-C chemokine receptor 4 (CXCR4) and RANKL antagonists attenuate both identified routes of entry. Our findings suggest that targeted RANKL and CXCR4 pathway inhibitors could attenuate routes of leukemia blast CNS invasion and provide benefit for B-ALL-affected individuals.

New Generation of Meso and Antiprogestins (SPRMs) into the Osteoporosis Approach

Receptor activator of nuclear factor κB (RANK) and its ligand (RANKL) play key roles in bone metabolism and the immune system. The RANK/RANKL complex has also been shown to be critical in the formation of mammary epithelia cells. The female hormones estradiol and progesterone closely control the action of RANKL with RANK. Blood concentration of these sex hormones in the postmenopausal period leads to an increase in RANK/RANKL signaling and are a major cause of women's osteoporosis, characterized by altered bone mineralization. Knowledge of the biochemical relationships between hormones and RANK/RANKL signaling provides the opportunity to design novel therapeutic agents to inhibit bone loss, based on the anti-RANKL treatment and inhibition of its interaction with the RANK receptor. The new generation of both anti- and mesoprogestins that inhibit the NF-κB-cyclin D1 axis and blocks the binding of RANKL to RANK can be considered as a potential source of new RANK receptor ligands with anti-RANKL function, which may provide a new perspective into osteoporosis treatment itself as well as limit the osteoporosis rise during breast cancer metastasis to the bone.

RANKL regulates male reproductive function

Infertile men have few treatment options. Here, we demonstrate that the transmembrane receptor activator of NF-kB ligand (RANKL) signaling system is active in mouse and human testis. RANKL is highly expressed in Sertoli cells and signals through RANK, expressed in most germ cells, whereas the RANKL-inhibitor osteoprotegerin (OPG) is expressed in germ and peritubular cells. OPG treatment increases wild-type mouse sperm counts, and mice with global or Sertoli-specific genetic suppression of Rankl have increased male fertility and sperm counts. Moreover, RANKL levels in seminal fluid are high and distinguishes normal from infertile men with higher specificity than total sperm count. In infertile men, one dose of Denosumab decreases RANKL seminal fluid concentration and increases serum Inhibin-B and anti-Müllerian-hormone levels, but semen quality only in a subgroup. This translational study suggests that RANKL is a regulator of male reproductive function, however, predictive biomarkers for treatment-outcome requires further investigation in placebo-controlled studies.

Comparative study of the effects of osteoprotegerin and testosterone on bone quality in male orchidectomised rats

Aim: The aim of this work is to compare the effects of osteoprotegerin (OPG) and testosterone on bone quality in a model of orchidectomised (ORX) rats.Methods: Three-month-old ORX or SHAM operated groups (n = 15 each group) were used. The SHAM and ORX groups received saline. There were two ORX groups, receiving OPG-Fc (10 mg/kg twice weekly) (ORX + OPG-Fc) or testosterone cypionate (1.7 mg/kg/weekly) for 8 weeks. After sacrifice, bone analysis by femoral and lumbar dual-energy X-ray absorptiometry and micro-computed tomography in femora were performed. Histological sections of vertebrae were dyed with hematoxylin-eosin or safranin. Serum osteocalcin (BGP), total alkaline phosphatase (ALP), and C-terminal telopeptide of type I collagen (CTX) were analyzed.Results: ORX resulted in femoral and vertebral bone loss and in microarchitectural deterioration. Treatment with OPG-Fc and testosterone recovered lumbar (L) and femoral (F) bone mineral densitometry bone mineral density (BMD) to SHAM levels. Femoral BMD was significantly higher after treatment with OPG-Fc than after testosterone treatment due to the presence of osteopetrotic changes in the metaphyseal region of long bones. Serum levels of ALP and CTX increased, while OPG levels were unchanged in ORX rats. Treatment with OPG-Fc decreased the levels of BGP, ALP, and CTX. Treatment with testosterone maintained biochemical markers of bone turnover at levels similar to or higher than those of ORX rats.

Osteoprotegerin/bone morphogenetic protein 2 combining with collagen sponges on tendon-bone healing in rabbits

INTRODUCTION

The aim was to investigate the effect of collagen sponges (CS) as a delivery device for osteoprotegerin (OPG)/bone morphogenetic protein 2 (BMP-2) and support matrix on the tendon-bone healing after anterior crusicate ligament (ACL) reconstruction in modeled rabbits.

MATERIALS AND METHODS

Sixty New Zealand white rabbits were randomly divided into four groups based on treatments they received at the tendon-bone interface after left knee ACL reconstruction: the control group, OPG/BMP-2, CS, and OPG/BMP-2/CS combination. At 4, 8 and 12 weeks post-surgery, five rabbits from each group were euthanized to examine the tendon-bone healing. Levels of OPG and BMP-2 in synovial fluid, the bone tunnel enlargement value, the histomorphological typing of tendon-bone interface, and the bone tunnel area of the tendon-bone interface were compared among different treatments.

RESULTS

The OPG/BMP-2/CS combination treatment group had the highest levels of OPG and BMP-2 in synovial fluid (both P < 0.05), the greatest number of Sharpey-like collagen fibers at all test points (P < 0.05), the most fibrocartilage enthesis on week 12, the greatest bone tunnel area (P < 0.05), and the greatest decrease in bone tunnel enlargement on week 12 (P < 0.05). Histomorphological typing of tendon-bone interface of all groups showed changes varying from tendon-bone separation to firm healing, and the change was most significant in the OPG/BMP-2/CS combination treatment group.

CONCLUSION

CS treatment alone serves as a fixing support, and CS combining with growth factors OPG/BMP-2 ensures slow and stable release of OPG/BMP-2, significantly improves the tendon-bone healing in the rabbit ACL model.

Serum calcium concentration is maintained when bone resorption is suppressed by osteoprotegerin in young growing male rats

Serum calcium (Ca) is maintained in a narrow range through regulation of Ca metabolism in the intestine, kidney, and bone. Calcium is incorporated and resorbed from bone during bone remodeling via cellular processes as well as by exchange. Both routes contribute to calcium homeostasis. To assess the magnitude of bone turnover contribution to calcium homeostasis we labeled bone with a Ca tracer and measured Ca release following stimulation or suppression of bone resorption. Young growing male rats (n = 162) were dosed with ⁴⁵Ca to label skeletal Ca. After a one-month period to allow the label to incorporate into the skeleton, rats were treated with a bone resorption antagonist (OPG), a bone resorption agonist (RANKL), or vehicle control (PBS). Serum and urine ⁴⁵Ca and total Ca, and serum TRACP5b (a bone resorption biomarker), were monitored for 45 days following treatment. Tracer data were analyzed by a compartmental model using WinSAAM to quantify dynamic changes in Ca metabolism and identify sites of change following treatment. In RANKL treated rats, both serum ⁴⁵Ca and serum TRACP5b were increased by >70% due to a 25-fold increase in bone resorption. In OPG treated rats, both serum ⁴⁵Ca and serum TRACP5b were suppressed by >70% due to a 75% decrease in bone resorption, a 3-fold increase in bone formation, and a 50% increase in absorption. Because TRACP5b and ⁴⁵Ca responded similarly, we conclude that Ca release from bone into serum occurs mostly via osteoclast-mediated bone resorption. However, because serum Ca concentration did not change with altered resorption in response to either RANKL or OPG treatment, we also conclude that serum Ca concentration under normal dietary conditions in young growing male rats is maintained by processes in addition to cellular bone resorption.

Therapeutic and Preventive Effects of Osteoclastogenesis Inhibitory Factor on Osteolysis, Proliferation of Mammary Tumor Cell and Induction of Cancer Stem Cells in the Bone Microenvironment

Background: We examined the effects of recombinant human osteoclastogenesis inhibitory factor (hOCIF) on osteolysis, proliferation of mammary tumor cells, and induction of cancer stem cells (CSCs) in the tumor-bone and tumor-subcutaneous microenvironments (TB- and TS-microE). Methods: Mouse mammary tumor cells were transplanted onto the calvaria or into a subcutaneous lesion of female mice, creating a TB-microE and a TS-microE, and the mice were then treated with hOCIF. To investigate the preventive effects of hOCIF, mice were treated with hOCIF before tumor cell implantation onto the calvaria (Pre), after (Post), and both before and after (Whole). The number of CSCs and cytokine levels were evaluated by IHC and ELISA assay, respectively. Results: hOCIF suppressed osteolysis, and growth of mammary tumors in the TB-microE, but not in the TS-microE. In the Pre, Post, and Whole groups, hOCIF suppressed osteolysis, and cell proliferation. hOCIF increased mouse osteoprotegrin (mOPG) levels in vivo, which suppressed mammary tumor cell proliferation in vitro. These preventive effects were observed in the dose-dependent. hOCIF did not affect the induction of CSCs in either microenvironment. Conclusion: While receptor activator of NF-κB ligand (RANKL) targeting therapy may not affect the induction of CSCs, RANKL is a potential target for prevention as well as treatment of breast cancer bone metastasis.

Regulation of osteoclasts by osteoblast lineage cells depends on titanium implant surface properties

A critical stage during osseointegration of a titanium (Ti) implant is primary bone remodeling, which involves cross talk among osteoclast precursors, osteoclasts, mesenchymal stem cells (MSCs), and osteoblasts. This phase couples the processes of bone formation and resorption. During remodeling, osteoclasts produce factors capable of regulating MSC migration and osteogenesis. Furthermore, they degrade primary bone, creating a foundation with a specific chemistry, stiffness, and morphology for osteoblasts to synthesize and calcify their matrix. MSCs and osteoblasts receiving cues from the implant surface produce factors capable of regulating osteoclasts in order to promote net new bone formation. The purpose of this study was to determine the effects Ti implant surfaces have on bone remodeling. Human MSCs and normal human osteoblasts (NHOsts) were cultured separately on 15 mm grade 2 smooth PT, hydrophobic-microrough SLA, hydrophilic-microrough Ti (mSLA) (Institut Straumann AG, Basel, Switzerland), or tissue culture polystyrene (TCPS). After 7d, conditioned media from surface cultures were used to treat human osteoclasts for 2d. Activity was measured by fluorescence of released collagen followed by mRNA quantification. This study demonstrates that MSC and NHOst cultures are able to suppress osteoclast activity in a surface dependent manner and osteoclast mRNA levels are selectively regulated by surface treatments. The substrate-dependent regulatory effect was mitigated when MSCs were silenced for integrin subunits and when conditioned media were denatured. These results indicate that MSCs and NHOsts regulate at least two aspects of remodeling: reduced fusion of new osteoclasts and reduced activity of existing osteoclasts.

STATEMENT OF SIGNIFICANCE

In this study, we developed a novel in vitro model to study how microstructured and hydrophilic titanium implants impact bone remodeling for dental and orthopaedic applications. Our approach intersects biomaterials and systems physiology, revealing for the first time that implant surface properties are capable of regulating the communication among the cells involved in remodeling of primary bone during osseointegration. We believe that the basic research presented in our manuscript will provide important knowledge in our understanding of factors that impact implant success. Furthermore, it provides a solid foundation for the development of materials that enable rapid osseointegration and earlier loading times for implants in bone that has been compromised by trauma or disease.

Novel factors modulating human β-cell proliferation

β-Cell dysfunction in type 1 and type 2 diabetes is accompanied by a progressive loss of β-cells, and an understanding of the cellular mechanism(s) that regulate β-cell mass will enable approaches to enhance hormone secretion. It is becoming increasingly recognized that enhancement of human β-cell proliferation is one potential approach to restore β-cell mass to prevent and/or cure type 1 and type 2 diabetes. While several reports describe the factor(s) that enhance β-cell replication in animal models or cell lines, promoting effective human β-cell proliferation continues to be a challenge in the field. In this review, we discuss recent studies reporting successful human β-cell proliferation including WS6, an IkB kinase and EBP1 inhibitor; harmine and 5-IT, both DYRK1A inhibitors; GNF7156 and GNF4877, GSK-3β and DYRK1A inhibitors; osteoprotegrin and Denosmab, receptor activator of NF-kB (RANK) inhibitors; and SerpinB1, a protease inhibitor. These studies provide important examples of proteins and pathways that may prove useful for designing therapeutic strategies to counter the different forms of human diabetes.

Elevated Gα11 expression in osteoblast lineage cells promotes osteoclastogenesis and leads to enhanced trabecular bone accrual in response to pamidronate

Osteoblastic cells indirectly induce osteoclastogenesis in the bone microenvironment by expressing paracrine factors such as RANKL and M-CSF, leading to increased bone resorption. These cytokines can be regulated by a variety of intracellular pathways, which include G protein-coupled receptor signaling. To explore how enhanced signaling of the Gαq/11 pathway in osteoblast lineage cells may mediate osteoclast formation, we cocultured wild-type (WT) preosteoclasts with BMSCs derived from either WT or transgenic mice with osteoblast-specific overexpression of Gα11 (G11-Tg). G11-Tg cocultures had elevated osteoclast numbers with greater resorptive capacity and increased expression of Rankl, Rankl:Opg (osteoprotegerin), and M-csf compared with cocultures with WT BMSCs. As well, cocultures with G11-Tg BMSCs required a higher concentration of OPG to inhibit osteoclast formation and less angiotensin II to increase osteoclast size. These indicate that G11-Tg osteoblasts drive the increased osteoclast formation and osteopenia seen in G11-Tg mice. Pamidronate treatment of G11-Tg mice restored the trabecular bone loss phenotype, as bone mineral density, bone volume, trabecular number, separation, and expressions of osteoblastic and osteoclastic genes were comparable with WT parameters. These changes were characterized by enhanced accumulation of calcified cartilage in trabecular bone, demonstrating that resorption of the cartilaginous intermediate by osteoclasts is more affected by bisphosphonate treatment in G11-Tg mice. In conclusion, overexpression of Gα11 in osteoblastic cells promotes osteoclastogenesis by upregulation of Rankl and M-csf and bone loss by increased osteoclast resorption of the trabecular bone and cartilaginous matrix.

Combination therapy with BMP-2 and a systemic RANKL inhibitor enhances bone healing in a mouse critical-sized femoral defect

Recombinant human BMP-2 (rhBMP-2) is a potent osteoinductive agent, but has been associated not only with bone formation, but also osteoclastogenesis and bone resorption. Osteoprotegerin (OPG) is a RANKL inhibitor that blocks differentiation and function of osteoclasts. We hypothesized that the combination of local BMP-2 (recombinant protein or a product of gene therapy) plus systemic OPG-Fc is more effective than BMP-2 alone in promoting bone repair. To test this hypothesis we used a mouse critical-sized femoral defect model. Col2.3eGFP (osteoblastic marker) male mice were treated with rhBMP-2 (group I), rhBMP-2 and systemic OPG (group II), rhBMP-2 and delayed administration of OPG (group III), mouse BM cells transduced with a lentiviral vector containing the BMP-2 gene (LV-BMP-2; group IV), LV-BMP-2 and systemic OPG (group V), a carrier alone (group VI) and administration of OPG alone (group VII). All bone defects treated with BMP-2 (alone or combined with OPG) healed, whereas minimal bone formation was noted in animals treated with the carrier alone or OPG alone. MicroCT analysis showed that bone volume (BV) in rhBMP-2+OPG and LV-BMP-2+OPG groups was significantly higher compared to rhBMP-2 alone (p<0.01) and LV-BMP-2 alone (p<0.001). Similar results were observed in histomorphometry, with rhBMP-2 alone defects exhibiting significantly lower bone area (B.Ar) compared to rhBMP-2+OPG defects (p<0.005) and LV-BMP-2 defects having a significantly lower B.Ar compared to all BMP-2+OPG treated groups (p≤0.01). TRAP staining demonstrated a major osteoclast response in the groups that did not receive OPG (rhBMP-2, LV-BMP-2 and sponge alone) beginning as early as 7days post-operatively. In conclusion, we demonstrated that locally delivered BMP-2 (recombinant protein or gene therapy) in combination with systemically administered OPG improved bone healing compared to BMP-2 alone in a mouse critical-sized bone defect. These data indicate that osteoclasts can diminish healing responses to BMP-2 and that RANKL inhibition may thus accentuate BMP-2 efficacy.

Osteoprotegerin Regulates Pancreatic β-Cell Homeostasis upon Microbial Invasion

Osteoprotegerin (OPG), a decoy receptor for receptor activator of NF-κB ligand (RANKL), antagonizes RANKL’s osteoclastogenic function in bone. We previously demonstrated that systemic administration of lipopolysaccharide (LPS) to mice elevates OPG levels and reduces RANKL levels in peripheral blood. Here, we show that mice infected with Salmonella, Staphylococcus, Mycobacteria or influenza virus also show elevated serum OPG levels. We then asked whether OPG upregulation following microbial invasion had an effect outside of bone. To do so, we treated mice with LPS and observed OPG production in pancreas, especially in β-cells of pancreatic islets. Insulin release following LPS administration was enhanced in mice lacking OPG, suggesting that OPG inhibits insulin secretion under acute inflammatory conditions. Consistently, treatment of MIN6 pancreatic β-cells with OPG decreased their insulin secretion following glucose stimulation in the presence of LPS. Finally, our findings suggest that LPS-induced OPG upregulation is mediated in part by activator protein (AP)-1 family transcription factors, particularly Fos proteins. Overall, we report that acute microbial infection elevates serum OPG, which maintains β-cell homeostasis by restricting glucose-stimulated insulin secretion, possibly preventing microbe-induced exhaustion of β-cell secretory capacity.

Osteoprotegerin is an effective countermeasure for spaceflight-induced bone loss in mice

Bone loss associated with microgravity exposure poses a significant barrier to long-duration spaceflight. Osteoprotegerin-Fc (OPG-Fc) is a receptor activator of nuclear factor kappa-B ligand (RANKL) inhibitor that causes sustained inhibition of bone resorption after a single subcutaneous injection. We tested the ability of OPG-Fc to preserve bone mass during 12 days of spaceflight (SF). 64-day-old female C57BL/6J mice (n=12/group) were injected subcutaneously with OPG-Fc (20mg/kg) or an inert vehicle (VEH), 24h prior to launch. Ground control (GC) mice (VEH or OPG-Fc) were maintained under environmental conditions that mimicked those in the space shuttle middeck. Age-matched baseline (BL) controls were sacrificed at launch. GC/VEH, but not SF/VEH mice, gained tibia BMD and trabecular volume fraction (BV/TV) during the mission (P<0.05 vs. BL). SF/VEH mice had lower BV/TV vs. GC/VEH mice, while SF/OPG-Fc mice had greater BV/TV than SF/VEH or GC/VEH. SF reduced femur elastic and maximum strength in VEH mice, with OPG-Fc increasing elastic strength in SF mice. Serum TRAP5b was elevated in SF/VEH mice vs. GC/VEH mice. Conversely, SF/OPG-Fc mice had lower TRAP5b levels, suggesting that OPG-Fc preserved bone during spaceflight via inhibition of osteoclast-mediated bone resorption. Decreased bone formation also contributed to the observed osteopenia, based on the reduced femur periosteal bone formation rate and serum osteocalcin level. Overall, these observations suggest that the beneficial effects of OPG-Fc during SF are primarily due to dramatic and sustained suppression of bone resorption. In growing mice, this effect appears to compensate for the SF-related inhibition of bone formation, while preventing any SF-related increase in bone resorption. We have demonstrated that the young mouse is an appropriate new model for SF-induced osteopenia, and that a single pre-flight treatment with OPG-Fc can effectively prevent the deleterious effects of SF on mouse bone.

Bioinspired Collagen/Glycosaminoglycan-Based Cellular Microenvironments for Tuning Osteoclastogenesis

Replicating the biocomplexity of native extracellular matrices (ECM) is critical for a deeper understanding of biochemical signals influencing bone homeostasis. This will foster the development of bioinspired biomaterials with adjustable bone-inducing properties. Collagen-based coatings containing single HA derivatives have previously been reported to promote osteogenic differentiation and modulate osteoclastogenesis and resorption depending on their sulfation degree. However, the potential impact of different GAG concentrations as well as the interplay of multiple GAGs in these coatings is not characterized in detail to date. These aspects were addressed in the current study by integrating HA and different sulfate-modified HA derivatives (sHA) during collagen in vitro fibrillogenesis. Besides cellular microenvironments with systematically altered single-GAG concentrations, matrices containing both low and high sHA (sHA1, sHA4) were characterized by biochemical analysis such as agarose gel electrophoresis, performed for the first time with sHA derivatives. The morphology and composition of the collagen coatings were altered in a GAG sulfation- and concentration-dependent manner. In multi-GAG microenvironments, atomic force microscopy revealed intermediate collagen fibril structures with thin fibrils and microfibrils. GAG sulfation altered the surface charge of the coatings as demonstrated by ζ-potential measurements revealed for the first time as well. This highlights the prospect of GAG-containing matrices to adjust defined surface charge properties. The sHA4- and the multi-GAG coatings alike significantly enhanced the viability of murine osteoclast-precursor-like RAW264.7 cells. Although in single-GAG matrices there was no dose-dependent effect on cell viability, osteoclastogenesis was significantly suppressed only on sHA4-coatings in a dose-dependent fashion. The multi-GAG coatings led to an antiosteoclastogenic effect in-between those with single-GAGs which cannot simply be attributed to the overall content of sulfate groups. These data suggest that the interplay of sGAGs influences bone cell behavior. Whether these findings translate into favorable biomaterial properties needs to be validated in vivo.

Muscadine grape skin extract can antagonize Snail-cathepsin L-mediated invasion, migration and osteoclastogenesis in prostate and breast cancer cells

To develop new and effective chemopreventive agents against bone metastasis, we assessed the effects of muscadine grape skin extract (MSKE), whose main bioactive component is anthocyanin, on bone turnover, using prostate and breast cancer cell models overexpressing Snail transcription factor. MSKE has been shown previously to promote apoptosis in prostate cancer cells without affecting normal prostate epithelial cells. Snail is overexpressed in prostate and breast cancer, and is associated with increased invasion, migration and bone turnover/osteoclastogenesis. Cathepsin L (CatL) is a cysteine cathepsin protease that is overexpressed in cancer and involved in bone turnover. Snail overexpression in prostate (LNCaP, ARCaP-E) and breast (MCF-7) cancer cells led to increased CatL expression/activity and phosphorylated STAT-3 (pSTAT-3), compared to Neo vector controls, while the reverse was observed in C4-2 (the aggressive subline of LNCaP) cells with Snail knockdown. Moreover, CatL expression was higher in prostate and breast tumor tissue compared to normal tissue. MSKE decreased Snail and pSTAT3 expression, and abrogated Snail-mediated CatL activity, migration and invasion. Additionally, Snail overexpression promoted osteoclastogenesis, which was significantly inhibited by the MSKE as effectively as Z-FY-CHO, a CatL-specific inhibitor, or osteoprotegerin, a receptor activator of nuclear factor kappa B ligand (RANKL) antagonist. Overall, these novel findings suggest that Snail regulation of CatL may occur via STAT-3 signaling and can be antagonized by MSKE, leading to decreased cell invasion, migration and bone turnover. Therefore, inhibition using a natural product such as MSKE could potentially be a promising bioactive compound for bone metastatic cancer.

OPG Treatment Prevents Bone Loss During Lactation But Does Not Affect Milk Production or Maternal Calcium Metabolism

Lactation is associated with increased bone turnover and rapid bone loss, which liberates skeletal calcium used for milk production. Previous studies suggested that an increase in the skeletal expression of receptor activator of nuclear factor kappa-light-chain-enhancer of activated B cells ligand (RANKL) coupled with a decrease in osteoprotegerin (OPG) levels likely triggered bone loss during lactation. In this study, we treated lactating mice with recombinant OPG to determine whether bone loss during lactation was dependent on RANKL signaling and whether resorption of the maternal skeleton was required to support milk production. OPG treatment lowered bone resorption rates and completely prevented bone loss during lactation but, surprisingly, did not decrease osteoclast numbers. In contrast, OPG was quite effective at lowering osteoblast numbers and inhibiting bone formation in lactating mice. Furthermore, treatment with OPG during lactation prevented the usual anabolic response associated with reversal of lactational bone loss after weaning. Preventing bone loss had no appreciable effect on milk production, milk calcium levels, or maternal calcium homeostasis when mice were on a standard diet. However, when dietary calcium was restricted, treatment with OPG caused maternal hypocalcemia, maternal death, and decreased milk production. These studies demonstrate that RANKL signaling is a requirement for bone loss during lactation, and suggest that osteoclast activity may be required to increase osteoblast numbers during lactation in preparation for the recovery of bone mass after weaning. These data also demonstrate that maternal bone loss is not absolutely required to supply calcium for milk production unless dietary calcium intake is inadequate.

Osteoprotegerin promotes the proliferation of chondrocytes and affects the expression of ADAMTS-5 and TIMP-4 through MEK/ERK signaling

The involvement of osteoprotegerin (OPG) in bone metabolism has previously been established; however, whether OPG regulates chondrocytes directly and exerts precise cellular and molecular effects on chondrocytes remains to be determined. Thus, the present study aimed to investigate the direct effect of OPG on the viability, proliferation and functional consequences of chondrocytes. Primary chondrocytes were isolated from the knee of Sprague-Dawley rats. Passage 1 chondrocytes were identified by toluidine blue staining and used in the experiments. The cell proliferation induced by OPG at various concentrations was measured by a Cell Counting kit-8 (CCK-8) assay. Following pretreatment with mitogen-activated/extracellular signal-regulated kinase kinase (MEK) inhibitor U0126, extracellular signal-regulated kinase (ERK) inhibitor PD098059, and P38 mitogen-activated protein kinase (P38MAPK) inhibitor SB203580 for 30 min, chondrocytes were treated with OPG, and CCK-8 was performed. The cellular signals of MAPKs, including ERK, P38MAPK and c-Jun N-terminal protein kinase (JNK), were investigated by western blot analysis following treatment with OPG. The functional consequences following treatment with soluble OPG were analyzed by qPCR and western blot analysis. OPG increased chondrocyte proliferation with maximal effect at 10 ng/ml, and induced the phosphorylation of MEK and ERK but not P38MAPK or JNK. Suppression of ERK activity via PD098095 inhibited OPG-induced chondrocyte proliferation. Administration of OPG significantly downregulated ADAMTS‑5 and upregulated tissue inhibitor of metalloproteinase (TIMP)-4 production, but had no effect on the expression of TIMP-1, -2 and -3, insulin-like growth factor I, transforming growth factor-β, basic fibroblast growth factor, bone morphogenetic protein-2, collagen II, aggrecan and ADAMTS-4. Suppression of ERK activity via PD098095 inhibited the alteration of ADAMTS-5 and TIMP-4 expression induced by OPG. OPG therefore regulated the proliferation of chondrocytes via MEK/ERK signaling, and directly affected chondrocytes by influencing the expression profile of ADAMTS-5 and TIMP-4.

Osteoprotegerin inhibits calcification of vascular smooth muscle cell via down regulation of the Notch1-RBP-Jκ/Msx2 signaling pathway

Objective

Vascular calcification is a common pathobiological process which occurs among the elder population and in patients with diabetes and chronic kidney disease. Osteoprotegerin, a secreted glycoprotein that regulates bone mass, has recently emerged as an important regulator of the development of vascular calcification. However, the mechanism is not fully understood. The purpose of this study is to explore novel signaling mechanisms of osteoprotegerin in the osteoblastic differentiation in rat aortic vascular smooth muscle cells (VSMCs).

Methods and Results

VSMCs were isolated from thoracic aorta of Sprague Dawley rats. Osteoblastic differentiation of VSMCs was induced by an osteogenic medium. We confirmed by Von Kossa staining and direct cellular calcium measurement that mineralization was significantly increased in VSMCs cultured in osteogenic medium; consistent with an enhanced alkaline phosphatase activity. This osteoblastic differentiation in VSMCs was significantly reduced by the addition of osteoprotegerin in a dose responsive manner. Moreover, we identified, by real-time qPCR and western blotting, that expression of Notch1 and RBP-Jκ were significantly up-regulated in VSMCs cultured in osteogenic medium at both the mRNA and protein levels, these effects were dose-dependently abolished by the treatment of osteoprotegerin. Furthermore, we identified that Msx2, a downstream target of the Notch1/RBP-Jκ signaling, was markedly down-regulated by the treatment of osteoprotegerin.

Conclusion

Osteoprotegerin inhibits vascular calcification through the down regulation of the Notch1-RBP-Jκ signaling pathway.

Osteoprotegerin inhibits aortic valve calcification and preserves valve function in hypercholesterolemic mice

BACKGROUND

There are no rigorously confirmed effective medical therapies for calcific aortic stenosis. Hypercholesterolemic Ldlr (-/-) Apob (100/100) mice develop calcific aortic stenosis and valvular cardiomyopathy in old age. Osteoprotegerin (OPG) modulates calcification in bone and blood vessels, but its effect on valve calcification and valve function is not known.

OBJECTIVES

To determine the impact of pharmacologic treatment with OPG upon aortic valve calcification and valve function in aortic stenosis-prone hypercholesterolemic Ldlr (-/-) Apob (100/100) mice.

METHODS

Young Ldlr (-/-) Apob (100/100) mice (age 2 months) were fed a Western diet and received exogenous OPG or vehicle (N = 12 each) 3 times per week, until age 8 months. After echocardiographic evaluation of valve function, the aortic valve was evaluated histologically. Older Ldlr (-/-) Apob (100/100) mice were fed a Western diet beginning at age 2 months. OPG or vehicle (N = 12 each) was administered from 6 to 12 months of age, followed by echocardiographic evaluation of valve function, followed by histologic evaluation.

RESULTS

In Young Ldlr (-/-) Apob (100/100) mice, OPG significantly attenuated osteogenic transformation in the aortic valve, but did not affect lipid accumulation. In Older Ldlr (-/-) Apob (100/100) mice, OPG attenuated accumulation of the osteoblast-specific matrix protein osteocalcin by ∼80%, and attenuated aortic valve calcification by ∼ 70%. OPG also attenuated impairment of aortic valve function.

CONCLUSIONS

OPG attenuates pro-calcific processes in the aortic valve, and protects against impairment of aortic valve function in hypercholesterolemic aortic stenosis-prone Ldlr (-/-) Apob (100/100) mice.

[Effects of recombinant human osteoprotegerin and recombinant RANK protein on the differentiation of osteoclast precursors]

OBJECTIVE

To compare the effect of recombinant OPG-Fc and recombinant RANK protein on the differentiation of osteoclast precursors.

METHODS

Mouse osteoblasts cell lines were incubated with osteoclast precursors cell lines RAW 264.7 for 9 days with 10(-5) g/L rhRANK or rhOPG-Fc or PBS added to the coculture system. TRAP stain positive cells counting and cortical bone pit formation counting were performed in the 9th day.

RESULTS

Multinuleated TRAP stain positive cells were observed in the cocluture systems after 6 days incubation,and plenty of mature osteoclasts could be observed in the 9th day. With the addition of 10(-5) g/L rhOPG-Fc or rhRANK, multinucleated giant cells and cortical bone pit formation couting decreased significantly compared with the control group, and the rhRANK group decreased more significantly than the rhOPG-Fc group.

CONCLUSIONS

Both rhOPG-Fc and rhRANK can inhabit the differentiation of osteoclast precursors and prevent them forming mature osteoclasts,moreover,the rhRANK shows the significant inhabition effect than the rhOPG-Fc.

Integrin α(v)β₃ as a PET imaging biomarker for osteoclast number in mouse models of negative and positive osteoclast regulation

PURPOSE

The goal of this study was to determine the specificity of ⁶⁴Cu-CB-TE2A-c(RGDyK) (⁶⁴Cu-RGD) for osteoclast-related diseases, such as Paget's disease or rheumatoid arthritis.

PROCEDURES

C57BL/6 mice were treated systemically with osteoprotegerin (OPG) for 15 days or RANKL for 11 days to suppress and stimulate osteoclastogenesis, respectively. The mice were then imaged by positron emission tomography/computed tomography using ⁶⁴Cu-RGD, followed by determination of serum TRAP5b and bone histology. Standard uptake values were determined to quantify ⁶⁴Cu-RGD in bones and other tissues.

RESULTS

Mice treated with OPG showed decreased bone uptake of ⁶⁴Cu-RGD at 1, 2, and 24 h post-injection of the tracer (p < 0.01 for all time points) compared to vehicle controls, which correlated with a post-treatment decrease in serum TRAP5b. In contrast, mice treated with RANKL showed significantly increased bone uptake at 2 h post-injection of (⁶⁴Cu-RGD (p < 0.05) compared to the vehicle control group, corresponding to increased serum TRAP5b and OC numbers as determined by bone histology.

CONCLUSIONS

These data demonstrate that ⁶⁴Cu-RGD localizes to areas in bone with increased osteoclast numbers and support the use of ⁶⁴Cu-RGD as an imaging biomarker for osteoclast number that could be used to monitor osteoclast-related pathologies and their treatments.

A pharmacodynamic investigation into the efficacy of osteoprotegerin during aseptic inflammation

BACKGROUND

Osteoprotegerin (OPG), as an osteoclast antagonist, limits mineralised tissue resorption under physiological conditions. Previous work investigating OPG in a rat periodontal ligament (PDL) ankylosis model found no inhibitory effect on osteoclasts when OPG was administered at a dosage of 2.5mg/kg.

AIMS

The object of this study was to determine whether dosages higher than 2.5 mg/kg of OPG were required to limit osteoclastic activity in an aseptic inflammatory model in rats.

MATERIALS AND METHODS

Dry ice was applied for 15 minutes to the upper right first molar crown of eighteen, 8-week-old, male Sprague-Dawley rats. Three groups of 3 were injected with OPG at dosages of 2.5, 5.0 and 7.5 mg/kg of body weight immediately following the thermal insult. After 7 days, the rats were sacrificed and each maxilla processed for histological examination and stained for osteoclastic activity using tartrate-resistant acid phosphatase (TRAP). Osteoclast population numbers were estimated via light microscopy and results were analysed using a comparative mixed model statistical analysis.

RESULTS

Results showed OPG inhibited osteoclastic activity in a dose-dependent manner. From 2.5 mg/kg to 7.5 mg/kg, osteoclast populations were linearly reduced by 39.78% (p < 0.05). OPG did not appear to affect the inflammatory process and had varied efficacy in different regions of individual teeth.

CONCLUSION

Although osteoclastic activity reduced, it was not completely eliminated, perhaps because dosages were still inadequate, or additional factors might influence OPG and osteoclast activation in the aseptic inflammatory model.

OPG-Fc treatment in growing pigs leads to rapid reductions in bone resorption markers, serum calcium, and bone formation markers

Inhibition of the receptor activator of NF-κB ligand (RANKL) is a novel therapeutic option in the treatment of osteoporosis and related diseases. The aim of this study was to evaluate bone metabolism and structure in pigs after RANKL inhibition. 12 growing pigs were assigned to 2 groups with 6 animals each. The OPG group received recombinant human OPG-Fc (5 mg/kg IV) at day 0, the control group was given 0.9% NaCl solution. Serum levels of OPG-Fc, calcium (Ca), phosphorus (P), and bone turnover markers were evaluated every 5 days, and pigs were euthanized on day 20. Serum OPG-Fc concentration peaked at day 5 and coincided with significantly decreased Ca, P, and bone turnover markers. By day 15, measureable OPG-Fc serum levels could only be detected in 2/6 animals. With OPG-Fc clearance starting at day 10, serum Ca and P concentrations were not different between the 2 groups. TRACP5b, P1CP, and BAP levels significantly decreased by 40-70% relative to vehicle controls in the OPG-Fc group between days 5 and 10, indicating that pharmacologic concentration of OPG-Fc led to systemic concomitant inhibition of bone formation and resorption in young growing pigs. Dual X-ray absorptiometry data derived from the proximal femur did not differ between the 2 groups. μCT analysis of selected bone sites demonstrated an OPG-Fc-induced improvement of specific bone architectural indices and bone mineralization.

The presence of TNF-alpha and TNFR1 in aseptic root resorption. A preliminary study

BACKGROUND

It is hypothesised that osteoprotegerin (OPG), as an osteoclast antagonist, may offer molecular control over the process of orthodontic root resorption. Previous work investigating OPG in a rat periodontal ligament (PDL) ankylosis model found no inhibitory effect on osteoclasts and odontoclasts when given at a recommended dosage of 2.5 mg/kg. It was considered that traumatically-induced PDL inflammation produces mediators and cytokines with the ability to stimulate clast cell differentiation and counter the effects of OPG.

AIMS

The present study investigated the presence of Tumour Necrosis Factor Alpha (TNF-alpha) and its receptor Tumour Necrosis Factor Receptor 1 (TNFR1) in a PDL sterile inflammatory model.

METHODS

Dry ice was applied for 15 minutes to the upper right first molar crown of eighteen, 8-week-old, male Sprague-Dawley rats of which 9 were injected with OPG at a dose of 2.5 mg/kg of body weight at the time of freezing. After 7 days, the rats were sacrificed and each maxilla processed for immunohistochemical identification of TNF-alpha and TNFR1.

RESULTS

Results showed the presence of root resorption in varying amounts and locations in both experimental and control rats. Reparative processes appeared greater in the OPG-treated rats, often with the presence of an ankylotic union. Immunolabelling showed the presence of TNF-alpha and TNFR1 in the sterile inflammation located mainly in the interradicular PDL area. More definitive labelling appeared in OPG-treated rats.

CONCLUSION

The results indicated that TNF-alpha, and its receptor TNFR1, by their presence, may modify OPG effectiveness by offering an alternative pathway for osteoclast formation, which counters the anti-resorptive effects of OPG.

[Interleukin-4 and osteoprotegerin suppress polyethylene wear debris-induced osteolysis in a murine air pouch model]

OBJECTIVE

To test the effect of recombinant interleukin-4 (IL-4) and recombinant osteoprotegerin (OPG) in suppressing bone resorption induced by polyethylene wear particles..

METHODS

A cranial bone allograft was introduced into the air pouches induced on the back of BALB/c mice, followed by injection of 1 ml suspension of polyethylene particles into the pouches. The mouse models were then divided into 3 groups to receive injections of saline (control), IL-4 alone, or IL-4 and OPG into the pouches. The tissues were harvested 21 days after bone implantation for molecular and histological analyses.

RESULTS

Polyethylene wear particles-stimulated inflammatory responses (increased cellular infiltration and IL-1 and TNF production) were markedly reduced by IL-4 treatment either alone or combined with OPG (P<0.05). Polyethylene particles significantly increased tartrate-resistant acid phosphatase (TRAP) staining and bone absorption of the implanted bone graft, and IL-4 treatment, either alone or combined with OPG, obviously reduced the osteolysis induced by polyethylene particles (P<0.05).

CONCLUSION

IL-4 offers protection against polyethylene wear debris-induced inflammation and bone resorption in this mouse model. IL-4 combined with OPG can be a feasible and effective therapeutic approach to the treatment and prevention of polyethylene wear debris-associated osteolysis and aseptic loosening of the prosthetic components.

Osteoprotegerin prevents glucocorticoid-induced osteocyte apoptosis in mice

The adverse skeletal effects of glucocorticoid excess are due to increased osteoclast survival, decreased production of osteoblasts, and increased apoptosis of osteoblasts and osteocytes, but it remains unknown which of these is the principle cause of the decrease in bone strength. Previous studies suggested that osteocytes contribute to bone strength independently of changes in bone mass. Administration of the receptor activator for nuclear factor κB ligand (RANKL) antagonist osteoprotegerin (OPG) rapidly decreases osteoclasts followed by a decrease in osteoblasts but should not affect the long-lived osteocytes. Therefore, to distinguish between glucocorticoid effects on osteoclasts, osteoblasts, or osteocytes, we administered glucocorticoids, alone or in combination with OPG with the fragment crystallizable region of Ig heavy chains (OPG-Fc), to mice. The suppressive effect of glucocorticoids on spinal bone mineral density, cortical thickness, and strength was prevented by OPG-Fc. OPG-Fc, with or without glucocorticoids, profoundly reduced osteoclasts, osteoblasts, and bone formation. Unexpectedly, OPG-Fc prevented the glucocorticoid-induced increase in osteocyte apoptosis and reduction in solute transport from the systemic circulation to the osteocyte-lacunar-canalicular network. The fluid in the osteocyte-lacunar-canalicular network was inversely related to osteocyte apoptosis and directly related to bone mineral density. Consistent with the in vivo findings, Both OPG-Fc and OPG decreased glucocorticoid-induced apoptosis of MLO-Y4 osteocytic cells. OPG can also bind and antagonizes the activity of the TNF-related apoptosis-inducing ligand (TRAIL), but glucocorticoids did not change TRAIL expression, and knockdown of TRAIL did not prevent OPG-Fc from reducing glucocorticoid-induced osteocyte apoptosis. Based on these results, we conclude that at least part of the OPG-induced preservation of bone strength is due to the maintenance of osteocyte viability and the lacunar-canalicular network.

TSG-6 inhibits osteoclast activity via an autocrine mechanism and is functionally synergistic with osteoprotegerin

OBJECTIVE

TSG-6 (the product of tumor necrosis factor [TNF]-stimulated gene 6) has a potent inhibitory effect on RANKL-mediated bone erosion. The aim of this study was to compare the activity of TSG-6 with that of osteoprotegerin (OPG) and to investigate its role as an autocrine modulator of cytokine-mediated osteoclast formation/activation. We also determined TSG-6 expression in inflammatory joint disease.

METHODS

The effects of TSG-6, OPG, and the inflammation mediators TNFα, interleukin-1 (IL-1), and IL-6 on the formation of osteoclasts from peripheral blood mononuclear cells and synovial fluid (SF) macrophages were determined by tartrate-resistant acid phosphatase staining. Lacunar resorption and filamentous actin ring formation were measured as indicators of osteoclast activity. The amount of TSG-6 in culture media or SF was quantified by enzyme-linked immunosorbent assay, and expression of TSG-6 in synovial tissue was assessed by immunohistochemistry.

RESULTS

TSG-6 acted in synergy with OPG to inhibit RANKL-mediated bone resorption and was produced by osteoclast precursors and mature osteoclasts in response to TNFα, IL-1, and IL-6. Expression of TSG-6 correlated with inhibition of lacunar resorption; this effect was ameliorated by an anti-TSG-6 antibody. The level of TSG-6 protein was determined in SF from patients with various arthritides; it was highest in patients with inflammatory conditions such as rheumatoid arthritis, in which it correlated with the amount of TSG-6 immunostaining in the synovium. TSG-6 inhibited the activation but not the formation of osteoclasts from SF macrophages.

CONCLUSION

In the presence of inflammatory cytokines, osteoclasts produced TSG-6 at concentrations that are sufficient to inhibit lacunar resorption. This may represent an autocrine mechanism to limit the degree of bone erosion during joint inflammation.

Do RANKL inhibitors (denosumab) affect inflammation and immunity?

Receptor activator of nuclear factor kappa B ligand (RANKL) and its natural antagonist, osteoprotegerin (OPG), are, respectively, an indispensable factor and a potent inhibitor for osteoclast differentiation, activity, and survival. The development of a human monoclonal antibody to RANKL, denosumab, constitutes a novel approach to prevent fragility fractures in osteoporosis, skeletal complications of malignancy, and potentially bone erosions in rheumatoid arthritis (RA). In addition to being expressed by osteoblasts, RANKL is abundantly produced by activated T cells, and synoviocytes in RA, whereas its receptor, RANK, is also expressed by monocytes/macrophages and dendritic cells. However, in preclinical and clinical studies of RA-including patients with some degree of immunosuppression-RANKL inhibitors did not significantly alter inflammatory processes. RANKL, RANK, and OPG deficiency in murine models highlights the important role of this pathway in the development and maturation of the immune system in rodents, including functions of T and/or B cells, whereas OPG overexpression in mice and rats seems innocuous with regard to immunity. In contrast, loss-of-function mutations in humans have more limited effects on immune cells. In clinical studies, the overall rate of infections, cancer, and death was similar with denosumab and placebo. Nevertheless, the risk of severe infections and cancer in some specific tissues remains to be carefully scrutinized.

Osteoprotegerin induces morphological and functional alterations in mouse pancreatic islets

Although serum osteoprotegerin (OPG) is significantly increased in diabetic subjects, its potential role in beta cell dysfunction has not been investigated. This study aimed to assess the effect of full-length OPG administered in vivo in mice on pancreatic islet structure and function and its interaction with the renin-angiotensin system (RAS). OPG-treated mice showed increased islet monocyte/macrophage infiltration, fibrosis and apoptosis with reduction of islet function. The remodeling of islet architecture was associated with increased pancreatic expression of components of the RAS, growth factor genes (transforming growth factor β and connective tissue growth factor) and inflammatory molecules (monocyte chemotactic protein-1 and vascular adhesion molecule type 1). Prevention of these changes with improvement of insulin secretion was observed in ramipril treated animals. Our data suggest that OPG might play an important role in promoting beta cell dysfunction and that the upregulation of the local RAS represents one possible mechanism responsible for the OPG-induced beta cell dysfunction.

Inhibition of RANKL increases the anti-tumor effect of the EGFR inhibitor panitumumab in a murine model of bone metastasis

Bone metastases cause severe skeletal complications and are associated with osteoclast-mediated bone destruction. RANKL is essential for osteoclast formation, function, and survival, and is the primary effector of tumor-induced osteoclastogenesis and osteolysis. RANKL inhibition by its soluble decoy receptor osteoprotegerin (OPG) prevents tumor-induced osteolysis and decreases skeletal tumor burden. Because osteoclast-mediated bone resorption releases growth factors from the bone matrix, the host bone microenvironment induces a vicious cycle of bone destruction and tumor proliferation and survival. A prediction of this vicious cycle hypothesis is that targeting the host bone microenvironment by osteoclast inhibition would reduce tumor growth and survival and may enhance the anti-tumor effects of targeted therapies. The epidermal growth factor receptor (EGFR) pathway regulates critical processes such as cell growth and survival, and anti-EGFR therapies can cause tumor cell arrest and apoptosis. We evaluated whether reduction of osteolysis by RANKL inhibition could enhance the anti-tumor effects of an anti-EGFR antibody (panitumumab) in a novel murine model of human A431 epidermoid carcinoma bone metastasis. Skeletal tumor progression was assessed longitudinally by bioluminescence imaging. RANKL inhibition by OPG-Fc treatment resulted in a reduction in tumor progression in bony sites. OPG-Fc treatment also caused a dose-dependent reduction in tumor-induced osteolysis, supporting the essential role of RANKL in this process. In combination, RANKL inhibition increased the anti-tumor efficacy of an anti-EGFR antibody, and completely blocked tumor-induced bone breakdown, demonstrating that addition of the indirect anti-tumor effect of RANKL inhibition increases the anti-tumor efficacy of panitumumab, a targeted anti-EGFR antibody.

Immobilized-OPG-Fc on a titanium surface inhibits RANKL-dependent osteoclast differentiation in vitro

The purpose of the present study was to examine the effect of osteoprotegerin (OPG)-Fc fusion protein immobilized on a titanium surface on the initial differentiation of osteoclast precursor RAW264.7 cells. These cells were cultured on titanium specimens over which OPG-Fc was immobilized. The enhancement of tartrate-resistant acid phosphatase (TRAP) and cathepsin K mRNA expression in RAW264.7 cells exposed to receptor activator of NF-kappaB ligand (RANKL) stimulation on OPG-Fc-coated titanium was significantly lower than that in RAW264.7 cells exposed to RANKL on titanium specimens without immobilized OPG-Fc (ANOVA, P < 0.01). Preincubation of OPG-Fc-coated titanium, in a medium supplemented with 10% fetal bovine serum at 37 degrees C for two days before the cells were seeded, had no significant effect on the decrease in mRNA expression (ANOVA, P < 0.01). Taken together, these results indicate that OPG-Fc immobilized on a titanium surface blocks the differentiation of RAW264.7 cells induced by RANKL stimulation.

A novel T cell cytokine, secreted osteoclastogenic factor of activated T cells, induces osteoclast formation in a RANKL-independent manner

OBJECTIVE

Chronic T cell activation is central to the etiology of rheumatoid arthritis (RA), an inflammatory autoimmune disease that leads to severe focal bone erosions and generalized systemic osteoporosis. Previous studies have shown novel cytokine-like activities in medium containing activated T cells, characterized by potent induction of the osteoblastic production of interleukin-6 (IL-6), an inflammatory cytokine and stimulator of osteoclastogenesis, as well as induction of an activity that directly stimulates osteoclast formation in a manner independent of the key osteoclastogenic cytokine RANKL. This study was undertaken to identify the factors secreted by T cells that are responsible for these activities.

METHODS

Human T cells were activated using anti-human CD3 and anti-human CD28 antibodies for 72 hours in AIM V serum-free medium to obtain T cell-conditioned medium, followed by concentration and fractionation of the medium by fast-protein liquid chromatography. Biologically active fractions were resolved using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Major bands were analyzed by mass spectrometry, and a major candidate protein was identified. This novel cytokine was cloned, and its expression was analyzed using recombinant DNA technologies.

RESULTS

A single novel cytokine that could induce both osteoblastic IL-6 production and functional osteoclast formation in the absence of osteoblasts or RANKL and that was insensitive to the effects of the RANKL inhibitor osteoprotegerin was identified in the activated T cell-conditioned medium; this cytokine was designated secreted osteoclastogenic factor of activated T cells (SOFAT). Further analysis of SOFAT revealed that it was derived from an unusual messenger RNA splice variant coded by the threonine synthase-like 2 gene homolog, which is a conserved gene remnant coding for threonine synthase, an enzyme that functions only in microorganisms and plants.

CONCLUSION

SOFAT may act to exacerbate inflammation and/or bone turnover under inflammatory conditions such as RA or periodontitis and in conditions of estrogen deficiency.

Increased RANK ligand in bone marrow of orchiectomized rats and prevention of their bone loss by the RANK ligand inhibitor osteoprotegerin

Orchiectomized (ORX) rats were used to examine the extent to which their increased bone resorption and decreased bone density might relate to increases in RANKL, an essential cytokine for bone resorption. Serum testosterone declined by >95% in ORX rats 1 and 2 weeks after surgery (p<0.05 versus sham controls), with no observed changes in serum RANKL. In contrast, RANKL in bone marrow plasma and bone marrow cell extracts was significantly increased (by approximately 100%) 1 and 2 weeks after ORX. Regression analyses of ORX and sham controls revealed a significant inverse correlation between testosterone and RANKL levels measured in marrow cell extracts (R=-0.58), while marrow plasma RANKL correlated positively with marrow plasma TRACP-5b, an osteoclast marker (R=0.63). The effects of RANKL inhibition were then studied by treating ORX rats for 6 weeks with OPG-Fc (10 mg/kg, twice/week SC) or with PBS, beginning immediately after surgery. Sham controls were treated with PBS. Vehicle-treated ORX rats showed significant deficits in BMD of the femur/tibia and lower trabecular bone volume in the distal femur (p<0.05 versus sham). OPG-Fc treatment of ORX rats increased femur/tibia BMD and trabecular bone volume to levels that significantly exceeded values for ORX or sham controls. OPG-Fc reduced trabecular osteoclast surfaces in ORX rats by 99%, and OPG-Fc also prevented ORX-related increases in endocortical eroded surface and ORX-related reductions in periosteal bone formation rate. Micro-CT of lumbar vertebrae from OPG-Fc-treated ORX rats demonstrated significantly greater cortical and trabecular bone volume and density versus ORX-vehicle controls. In summary, ORX rats exhibited increased RANKL protein in bone marrow plasma and in bone marrow cells, with no changes in serum RANKL. Data from regression analyses were consistent with a potential role for testosterone in suppressing RANKL production in bone marrow, and also suggested that soluble RANKL in bone marrow might promote bone resorption. RANKL inhibition prevented ORX-related deficits in trabecular BMD, trabecular architecture, and periosteal bone formation while increasing cortical and trabecular bone volume and density. These results support the investigation of RANKL inhibition as a strategy for preventing bone loss associated with androgen ablation or deficiency.

[Prevention of joint destruction by osteoclast-targeting therapy in search of new tools, such as OPG or cathepsin K inhibitor]

We should consider not only controlling disease activity using DMARDs and biologics as a matter of course, but also preventing against joint destruction, in the treatment for rheumatoid arthritis. Although we can indirectly regulate bone erosion via controlling disease activity, the osteoclast-targeting therapy might be more effective to stop joint destruction. We are waiting for new drugs directly targeting osteoclasts, such as OPG which is the natural inhibitor of RANKL, or cathepsin K inhibitor which reduces degeneration of bone matrix.

Inhibition of bone resorption and growth of breast cancer in the bone microenvironment

Breast cancer frequently metastasizes to bone, where tumor cells induce osteoclasts to locally destroy bone. During bone resorption, growth factors are locally released that may support bone metastatic growth. Differently from most other tissues, drugs that can limit local turnover, such as bisphosphonates and osteoprotegerin (OPG), are available for bone. We examined the hypothesis that inhibition of bone resorption by two different mechanisms may also affect the growth of cancer cells in bone. For this, we tested the effects of high doses of OPG and zoledronic acid (ZOL) on progression of MDA-231-B/Luc+ breast cancer cells in the bone microenvironment using whole body bioluminescent reporter imaging (BLI). Both treatments significantly inhibited the development of radiographically detectable osteolytic lesions. Histologic examination corroborated the radiographic findings, showing that both treatments preserved the integrity of bone trabeculae and prevented bone destruction (significantly higher trabecular bone volumes vs. vehicle). However, whereas practically no TRAcP-positive osteoclasts were observed in tibiae preparations of animals treated with Fc-OPG, TRAcP-positive osteoclasts were still present in the animals treated with ZOL. Intra-bone tumor burden was reduced with ZOL and Fc-OPG treatment. Although there appeared to be a trend for less overall total tumor burden upon treatment with both compounds, this was not significant as assessed by BLI and histomorphometric analysis due to the extramedullary growth of cancer cells which was not affected by these treatments. Collectively, anti-resorptive agents with different mechanisms of action - ZOL and OPG - significantly reduced cancer-induced osteolysis and intra-osseous tumor burden, but failed to restrain local tumor growth. However, interference with the bone micro-environmental growth support could still be of therapeutic relevance when given to patients early in the course of bone metastatic disease.

[Effect of recombinant human osteoprotegerin on osteoclasts stimulated by polyethylene particles]

OBJECTIVE

To observe the effect of recombinant human osteoprotegerin (rhOPG) on osteoclasts which were stimulated by polyethylene particles and to investigate the feasibility of applying rhOPG for the prosthetic aseptic looseness.

METHODS

The osteoclasts were isolated from the long bones of 5 New Zealand rabbits born within 24 hours, weighing 80-100 g, male or female, and were plated into the 24-well coverslips (10 mm x 10 mm) and bone slices (8 mm x 8 mm x 50 microm) at the density of 1 x 10(5)/mL. Based on the different concentration and density of rhOPG and polyethylene particles, the plates of culture were divided into 3 groups: the group with polyethylene particles of 1 x 109/mL (polyethylene group), the group co-cultured with polyethylene particles of 1 x 10(9)/mL and rhOPG of 100 ng/mL (polyethylene/rhOPG group) and the control group. The glass cover slips and bone slices were exposed to HE, toluidine blue and tartrate-resistant acid phosphatase (TRAP) staining at 1, 3, 5 and 7 days, and TRAP positive multinucleated cells and bone resorption tips were counted. Scanning electron microscope was used to observe the pits of bone resorption.

RESULTS

The osteoclast was rose-red when exposed to TRAP staining. For the number of the TRAP-positive osteoclasts, the polyethylene group witnessed an obvious increase compared with the control group and the polyethylene/rhOPG group after 5 and 7 days of culture (P < 0.05). And no significant difference between the control group and the polyethylene/rhOPG group was evident (P > 0.05). The pits of bone resorption was blue-purple when exposed to toluidine blue staining. For the number of bone resorption pits in the bone slice, significant difference was evident between the polyethylene group and the control group after 5 and 7 days of culture (P < 0.05), and there was significant different between the polyethylene/rhOPG group and the polyethylene group 1, 3, 5 and 7 days after culture (P < 0.05).

CONCLUSION

rhOPG could inhibit the stimulated effect of polyethylene particles on osteoclasts, and might be used to prevent the prosthetic aseptic looseness after artificial joint substitution.

RANKL acts directly on RANK-expressing prostate tumor cells and mediates migration and expression of tumor metastasis genes

BACKGROUND

Metastases to bone are a frequent complication of human prostate cancer and result in the development of osteoblastic lesions that include an underlying osteoclastic component. Previous studies in rodent models of breast and prostate cancer have established that receptor activator of NF-kappaB ligand (RANKL) inhibition decreases bone lesion development and tumor growth in bone. RANK is essential for osteoclast differentiation, activation, and survival via its expression on osteoclasts and their precursors. RANK expression has also been observed in some tumor cell types such as breast and colon, suggesting that RANKL may play a direct role on tumor cells.

METHODS

Male CB17 severe combined immunodeficient (SCID) mice were injected with PC3 cells intratibially and treated with either PBS or human osteprotegerin (OPG)-Fc, a RANKL antagonist. The formation of osteolytic lesions was analyzed by X-ray, and local and systemic levels of RANKL and OPG were analyzed. RANK mRNA and protein expression were assessed on multiple prostate cancer cell lines, and events downstream of RANK activation were studied in PC3 cells in vitro.

RESULTS

OPG-Fc treatment of PC3 tumor-bearing mice decreased lesion formation and tumor burden. Systemic and local levels of RANKL expression were increased in PC3 tumor bearing mice. PC3 cells responded to RANKL by activating multiple signaling pathways which resulted in significant changes in expression of genes involved in osteolysis and migration. RANK activation via RANKL resulted in increased invasion of PC3 cells through a collagen matrix.

CONCLUSION

These data demonstrate that host stromal RANKL is induced systemically and locally as a result of PC3 prostate tumor growth within the skeleton. RANK is expressed on prostate cancer cells and promotes invasion in a RANKL-dependent manner.

Prevention of cartilage destruction with intraarticular osteoclastogenesis inhibitory factor/osteoprotegerin in a murine model of osteoarthritis

OBJECTIVE

To investigate the effect of osteoclastogenesis inhibitory factor/osteoprotegerin (OPG) on chondrocytes in the development of osteoarthritis (OA) in vivo.

METHODS

To determine the role of endogenous OPG in the progression of OA, OA was surgically induced in OPG+/- mice and their wild-type (WT) littermates. To determine the role of exogenous OPG, knee joints of C57BL/6J mice with surgically induced OA were injected intraarticularly with recombinant human OPG (rHuOPG) or vehicle 5 times a week. All mice were euthanized 4 weeks after OA induction; joints were harvested and evaluated immunohistochemically.

RESULTS

Although OA changes were induced in both WT and OPG+/- mice, the degenerative changes in the articular cartilage were significantly enhanced in OPG+/- mice. In C57BL/6J mice with surgically induced OA, intraarticular OPG administration protected the articular cartilage from the progression of OA. The Mankin and cartilage destruction scores in OPG-treated animals were approximately 50% of those seen in the control group. Furthermore, OPG administration significantly protected articular cartilage thickness. Findings of the TUNEL assay indicated that rHuOPG prevented chondrocyte apoptosis in joints with surgically induced OA. Results of immunostaining indicated that OPG protein was detected in the synovium and in resident chondrocytes at higher levels in the OPG-treated group than in the control group.

CONCLUSION

These data indicate that endogenous OPG had a protective effect against the cartilage destruction that occurs during OA progression. Furthermore, direct administration of rHuOPG to articular chondrocytes prevented cartilage destruction in an experimental murine model of OA via prevention of chondrocyte apoptosis.

Accelerated bone resorption, due to dietary calcium deficiency, promotes breast cancer tumor growth in bone

The skeleton is a major site of breast cancer metastases. High bone turnover increases risk of disease progression and death. However, there is no direct evidence that high bone turnover is causally associated with the establishment and progression of metastases. In this study, we investigate the effects of high bone turnover in a model of breast cancer growth in bone. Female nude mice commenced a diet containing normal (0.6%; 'Normal-Ca') or low (0.1%; 'Low-Ca') calcium content. Mice were concurrently treated with vehicle or osteoprotegerin (1 mg/kg/d s.c; n = 16 per group). Three days later (day 0), 50,000 Tx-SA cells (variant of MDA-MB-231 cells) were implanted by intratibial injection. On day 0, mice receiving Low-Ca had increased serum parathyroid hormone (PTH) and tartrate-resistant acid phosphatase 5b levels, indicating secondary hyperparathyroidism and high bone turnover, which was maintained until day 17. Osteoprotegerin increased serum PTH but profoundly reduced bone resorption. On day 17, in mice receiving Low-Ca alone, lytic lesion area, tumor area, and cancer cell proliferation increased by 43%, 24%, and 24%, respectively, compared with mice receiving Normal Ca (P < 0.01). Osteoprotegerin treatment completely inhibited lytic lesions, reduced tumor area, decreased cancer cell proliferation, and increased cancer cell apoptosis. Increased bone turnover, due to dietary calcium deficiency, promotes tumor growth in bone, independent of the action of PTH. Breast cancer patients frequently have low dietary calcium intake and high bone turnover. Treatment to correct calcium insufficiency and/or treatment with antiresorptive agents, such as osteoprotegerin, may be of benefit in the adjuvant as well as palliative setting.

The effect of osteoclastic activity on tendon-to-bone healing: an experimental study in rabbits

BACKGROUND

Healing of a tendon graft in a bone tunnel depends on bone ingrowth into the interface between tendon and bone. Excessive osteoclastic activity may contribute to bone resorption, tunnel widening, and impaired healing. We hypothesized that inhibition of osteoclastic activity by osteoprotegerin (OPG) would increase bone formation around a tendon graft in anterior cruciate ligament reconstruction in a rabbit model, while increased osteoclastic activity due to the application of receptor activator of nuclear factor-kappa B ligand (RANKL) would impair bone ingrowth.

METHODS

Sixty skeletally mature, male New Zealand White rabbits underwent bilateral anterior cruciate ligament reconstruction. OPG (100 microg per tunnel) or RANKL (10 microg per tunnel) was delivered to the tendon-bone interface with use of a synthetic calcium phosphate carrier vehicle. Twenty animals were killed at two, four, and eight weeks after surgery. Two rabbits from each group were prepared for histological evaluation, and the other rabbits were used for biomechanical testing.

RESULTS

A significantly greater amount of bone surrounded the tendon at the healing tendon-bone interface in the OPG-treated limbs compared with the controls and the RANKL-treated limbs at all time-points (p < 0.05). There were significantly fewer osteoclasts in the OPG-treated limbs compared with the controls and the RANKL-treated limbs (p < 0.05). The average tunnel area in the OPG group was significantly smaller than that in the RANKL group (p = 0.003 at two weeks and p = 0.004 at four weeks). The femur-anterior cruciate ligament-tibia complex of the OPG-treated limbs had significantly increased stiffness compared with RANKL-treated limbs at eight weeks (p = 0.04).

CONCLUSIONS

Osteoprotegerin significantly improves bone formation around the grafted tendon and improves the stiffness at the healing tendon-bone junction in a rabbit model.

Further insights in the mechanisms of interleukin-1beta stimulation of osteoprotegerin in osteoblast-like cells

The mechanisms of IL-1beta stimulation of OPG were studied in more detail. Whereas p38 and ERK activation was confirmed to be needed, NF-kappaB was not necessary for this regulation. We also found that OPG production after IL-1beta stimulation was not sufficient to block TRAIL-induced apoptosis in MG-63 cells.

INTRODUCTION

Osteoprotegerin (OPG) plays a key role in the regulation of bone resorption and is stimulated by interleukin (IL)-1beta. Herein, we defined the mechanisms of IL-1beta stimulation of OPG focusing on the potential involvement of MAPK and NF-kappaB. We also examined whether OPG production in response to IL-1beta influences TRAIL-induced apoptosis in MG-63 cells.

MATERIALS AND METHODS

OPG mRNA levels in MG-63 cells were quantified by real-time RT-PCR and protein levels of OPG and IL-6 by ELISA. Cell viability was assessed using the methyltetrazidium salt (MTS) reduction assay. The role of the MAPK pathway was studied by both Western blotting and the use of specific chemical inhibitors. NF-kappaB function was studied using BAY 11-7085 and by siRNA transfection to inhibit p65 synthesis. Transcription mechanisms were analyzed by transiently transfecting MG-63 cells with OPG promoter constructs. Post-transcriptional effects were examined by using cycloheximide and actinomycin D.

RESULTS

MG-63 cells treatment with IL-1beta resulted in the phosphorylation of c-Jun NH(2)-terminal kinase (JNK), p38, and extracellular signal-regulated kinase (ERK). The use of the specific inhibitors showed that p38 and ERK but not JNK were needed for IL-1beta-induced OPG production. In contrast, NF-kappaB was not essential for IL-1beta induction of OPG. We also showed a small transcriptional and a possible post-transcriptional or translational regulation of OPG by IL-1beta. Exogenous OPG blocked TRAIL-induced apoptosis, but IL-1beta induction of OPG did not influence TRAIL-induced cell death.

CONCLUSIONS

IL-1beta stimulates OPG production by mechanisms dependent on p38 and ERK. In contrast, NF-kappaB was not essential for this regulation. Although the relevance of IL-1beta stimulation of OPG is still not fully understood, our data showed that IL-1beta stimulation of OPG does not modify TRAIL-induced cell death.

Local delivery of osteoprotegerin inhibits mechanically mediated bone modeling in orthodontic tooth movement

INTRODUCTION

The RANKL-OPG axis is a key regulator of osteoclastogenesis and bone turnover activity. Its contribution to bone resorption under altered mechanical states, however, has not been fully elucidated. Here we examined the role of OPG in regulating mechanically induced bone modeling in a rat model of orthodontic tooth movement.

METHODS

The maxillary first molars of male Sprague-Dawley rats were moved mesially using a calibrated nickel-titanium spring attached to the maxillary incisor teeth. Two different doses (0.5 mg/kg, 5.0 mg/kg) of a recombinant fusion protein (OPG-Fc), were injected twice weekly mesial to the first molars. Tooth movement was measured using stone casts that were scanned and magnified. Changes in bone quantity were measured using micro-computed tomography and histomorphometric analysis was used to quantify osteoclasts and volumetric parameters. Finally, circulating levels of TRAP-5b (a bone resorption marker) was measured using enzyme-linked immunosorbent assay.

RESULTS

The 5.0 mg/kg OPG-Fc dose showed a potent reduction in mesial molar movement and osteoclast numbers compared to controls (p<0.01). The molar movement was inhibited by 45.7%, 70.6%, and 78.7% compared to controls at days 7, 14, and 21 respectively, with the high dose of OPG. The 0.5 mg dose also significantly (p<0.05) inhibited molar movement at days 7 (43.8%) and 14 (31.8%). While incisor retraction was also decreased by OPG-Fc, the ratio of incisor to molar tooth movement was markedly better in the high-dose OPG group (5.2:1, p<0.001) compared to the control group (2.3:1) and the low-dose OPG group (2.0:1).

CONCLUSIONS

Local delivery of OPG-Fc inhibits osteoclastogenesis and tooth movement at targeted dental sites.

Rutile and titanium particles differentially affect the production of osteoblastic local factors

Titanium and its alloys are widely used as implant materials for dental and orthopaedic applications. To improve their wear and corrosion resistance, several surface modifications that give rise to an outer ceramic layer of rutile have been developed. It is expected that after a long period of functional loading, rutile debris will arise from these modified surfaces. We have compared the in vitro biocompatibility of subcytotoxic doses of rutile and titanium particles of phagocytosable size in primary cultures of human osteoblasts. Particles were visualized using a spectral confocal microscope by reflection. Both types of particles aggregated in the culture media and were efficiently internalized by osteoblasts as agglomerates. Treatment of isolated cultures of osteoblasts with rutile particles stimulated the release of IL-6, PGE2, and GM-CSF to a lesser extent than titanium. The influence of macrophages on the particle-induced stimulation of those local factors was analyzed by coculturing TPA-differentiated THP-1 cells with osteoblasts. Under these conditions, levels of IL-6 and PGE2 after treatment of cocultured osteoblasts with rutile particles were lower than after exposure to titanium. These results indicate that rutile debris shows a lower bioreactivity than titanium when tested in cultures of human osteoblasts and support the improved biocompatibility of titanium-based implants modified to create an outer layer of rutile on their surfaces.

Cross-reactive adaptive immune response to oral commensal bacteria results in an induction of receptor activator of nuclear factor-?B ligand (RANKL)-dependent periodontal bone resorption in a mouse model

INTRODUCTION

The present study examined whether induction of an adaptive immune response to orally colonizing non-pathogenic Pasteurella pneumotropica by immunization with the phylogenetically closely related bacterium, Actinobacillus actinomycetemcomitans, can result in periodontal bone loss in mice.

METHODS

BALB/c mice harboring P. pneumotropica (P. pneumotropica(+) mice) in the oral cavity or control P. pneumotropica-free mice were immunized with fixed A. actinomycetemcomitans. The animals were sacrificed on day 30, and the following measurements were carried out: (i) serum immunoglobulin G and gingival T-cell responses to A. actinomycetemcomitans and P. pneumotropica; (ii) periodontal bone loss; and (iii) identification of receptor activator of nuclear factor-kappaB ligand (RANKL) -positive T cells in gingival tissue.

RESULTS

Immunization with A. actinomycetemcomitans induced a significantly elevated serum immunoglobulin G response to the 29-kDa A. actinomycetemcomitans outer membrane protein (Omp29), which showed strong cross-reactivity with P. pneumotropica OmpA compared to results in the control non-immunized mice. The A. actinomycetemcomitans-immunized P. pneumotropica(+) mice developed remarkable periodontal bone loss in a RANKL-dependent manner, as determined by the abrogation of bone loss by treatment with osteoprotegerin-Fc. The T cells isolated from the gingival tissue of A. actinomycetemcomitans-immunized P. pneumotropica(+) mice showed an in vitro proliferative response to both A. actinomycetemcomitans and P. pneumotropica antigen presentation, as well as production of soluble(s)RANKL in the culture supernatant. Double-color confocal microscopy demonstrated that the frequency of RANKL(+) T cells in the gingival tissue of A. actinomycetemcomitans-immunized P. pneumotropica(+) mice was remarkably elevated compared to control mice.

CONCLUSION

The induction of an adaptive immune response to orally colonizing non-pathogenic P. pneumotropica results in RANKL-dependent periodontal bone loss in mice.

Cellular and humoral mechanisms of osteoclast formation in Ewing's sarcoma

Cellular mechanisms that account for tumour osteolysis associated with Ewing's sarcoma are uncertain. Osteoclasts are marrow-derived multinucleated cells (MNCs) that effect tumour osteolysis. Osteoclasts are known to form from macrophages by both receptor activator for nuclear factor-κB (RANK) ligand (RANKL)-dependent and -independent mechanisms. In this study, our aim has been to determine whether tumour-associated macrophages (TAMs) isolated from Ewing's sarcoma are capable of differentiating into osteoclasts and to characterise the cellular and humoral mechanisms whereby this occurs. Tumour-associated macrophages were isolated from two Ewing's sarcomas and cultured on both coverslips and dentine slices for up to 21 days with soluble RANKL and macrophage colony stimulating factor (M-CSF). Osteoclast formation from TAMs (CD14⁺) was evidenced by the formation of tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor (VNR)-positive MNCs, which were capable of carrying out lacunar resorption. This osteoclast formation was inhibited by the addition of bisphosphonates. Both Ewing's sarcoma-derived fibroblasts and some bone stromal cells expressed RANKL and supported osteoclast formation by a contact-dependent mechanism. We also found that osteoclast differentiation occurred when Ewing's TAMs were cultured with tumour necrosis factor (TNF)-α in the presence of M-CSF and that TC71 Ewing's sarcoma cells stimulated osteoclast formation through the release of a soluble factor, the action of which was abolished by an antibody to TNF-α. These results indicate that TAMs in Ewing's sarcoma are capable of osteoclast differentiation by both RANKL-dependent and TNF-α-dependent mechanisms and that Ewing's sarcoma cells produce osteoclastogenic factor(s). Our findings suggest that anti-resorptive and anti-osteoclastogenic therapies may be useful in inhibiting the osteolysis of Ewing's sarcoma.