Fibroblasts from the inner granulation tissue of the pseudocapsule in hips at revision arthroplasty induce osteoclast differentiation, as do stromal cells

Nano wear particles and the periprosthetic microenvironment in aseptic loosening induced osteolysis following joint arthroplasty

Joint arthroplasty is an option for end-stage septic arthritis due to joint infection after effective control of infection. However, complications such as osteolysis and aseptic loosening can arise afterwards due to wear and tear caused by high joint activity after surgery, necessitating joint revision. Some studies on tissue pathology after prosthesis implantation have identified various cell populations involved in the process. However, these studies have often overlooked the complexity of the altered periprosthetic microenvironment, especially the role of nano wear particles in the etiology of osteolysis and aseptic loosening. To address this gap, we propose the concept of the "prosthetic microenvironment". In this perspective, we first summarize the histological changes in the periprosthetic tissue from prosthetic implantation to aseptic loosening, then analyze the cellular components in the periprosthetic microenvironment post prosthetic implantation. We further elucidate the interactions among cells within periprosthetic tissues, and display the impact of wear particles on the disturbed periprosthetic microenvironments. Moreover, we explore the origins of disease states arising from imbalances in the homeostasis of the periprosthetic microenvironment. The aim of this review is to summarize the role of relevant factors in the microenvironment of the periprosthetic tissues, in an attempt to contribute to the development of innovative treatments to manage this common complication of joint replacement surgery.

Is titanium alloy Ti-6Al-4 V cytotoxic to gingival fibroblasts-A systematic review

OBJECTIVES

Grade V titanium alloy (Ti-6Al-4 V) is a well-recognized metallic biomaterial for medical implants. There has been some controversy regarding the use of this alloy in medical devices in relation to the toxicity of vanadium. In Dentistry, Ti-6Al-4 V remains prevalent. This systematic review aims to evaluate the effects of Ti-6Al-4 V on cells relevant to oral environments such as gingival fibroblasts.

MATERIALS AND METHODS

A literature search was undertaken for relevant English language publications in the following databases: Dental and Oral Science, Medline and Web of Science. The electronic search was supplemented with a search of references.

RESULTS

After application of inclusion and exclusion criteria. A total of eight papers are included in this review. These papers were all in vitro studies and were categorized into whole implant, discs, or implant particles based on the type of test materials used in the studies.

CONCLUSION

Based on the analyses of the eight included studies in this review, if Ti-6Al-4 V as a material is unchallenged, i.e., as a whole implant in pH neutral environments, there appears to be little effect on fibroblasts. If Ti-6Al-4 V is challenged through corrosion or wear (particle release), the subsequent release of vanadium and aluminium particles has an increased cytotoxic effect in vitro in comparison to commercially pure titanium, hence concerns should be raised in the clinical setting.

Effect of denosumab on osteolytic lesion activity after total hip arthroplasty: a single-centre, randomised, double-blind, placebo-controlled, proof of concept trial

BACKGROUND

Osteolysis causes recurrent pain and disability after total hip arthroplasty. We investigated the effect of the human monoclonal antibody denosumab on osteolytic lesion activity in patients undergoing revision total hip arthroplasty surgery to show the biological proof of concept for a non-surgical treatment for the disease.

METHODS

We did a phase 2, randomised, double-blind, placebo-controlled, proof of concept superiority trial at Sheffield Teaching Hospitals, Sheffield, UK. Eligible patients aged 30 years or older and scheduled for revision surgery for symptomatic, radiographically confirmed osteolysis were randomly allocated (1:1) to subcutaneous denosumab (60 mg single-dose) or placebo by an independent pharmacist using a random number table. The primary outcome was the between-group difference in osteoclast number per mm of bone surface of biopsies taken from the osteolytic membrane-bone interface at surgery 8 weeks later, measured by quantitative histomorphometry in all patients who underwent revision surgery. Adverse events were analysed in all randomly assigned participants. This trial is registered with the EU Clinical Trials Register (EudraCT 2011-000541-20).

FINDINGS

Between Dec 12, 2012, and June 24, 2018, 51 patients were assessed for eligibility, of whom 24 were randomly assigned to study treatment. Two patients had their revision surgery cancelled for unrelated reasons, leaving 22 patients (ten in the denosumab group) for analysis of the primary outcome. There were 83% fewer osteoclasts at the osteolysis membrane-bone interface in the denosumab versus the placebo group (median 0·05 per mm [IQR 0·11] vs 0·30 mm [0·40], p=0·011). No deaths or treatment-related serious adverse events occurred. Seven adverse events, including one severe adverse event, occurred in four (36%) of 11 patients in the denosumab group. In the placebo group ten adverse events, including three severe adverse events, occurred in five (38%) of 13 patients.

INTERPRETATION

To our knowledge, this is the first clinical trial of an investigational drug for osteolysis that shows tissue-specific biological efficacy. These results justify the need for future trials that target earlier-stage disease to test for clinical efficacy in reducing the need for revision surgery.

FUNDING

Amgen.

Naringin increases osteoprotegerin expression in fibroblasts from periprosthetic membrane by the Wnt/β-catenin signaling pathway

Background

The osteoclast bone resorption is critical in aseptic loosening after joint replacement. The balance between activator of nuclear factor kappa B ligand (RANKL) and osteoprotegerin (OPG) is considered to play a central role in osteoclast maturation. Fibroblasts from the periprosthetic membrane express RANKL and promote osteoclast formation. Studies have demonstrated that naringin inhibited osteoclastogenesis and wear particle-induced osteolysis. In this study, the naringin-induced OPG/RANKL effects and its underlying mechanism were studied in fibroblasts from periprosthetic membrane.

Methods

Fibroblasts were isolated from the periprosthetic membrane during hip arthroplasty for revision due to aseptic loosening. Fibroblasts were cultured and treated with or without naringin and DKK-1 (the classical inhibitor of Wnt/β-catenin signaling pathway). OPG and RANKL mRNA and protein levels, gene expression of β-catenin, and cyclin D1, which participate in the Wnt signaling pathway, were examined by real-time polymerase chain reaction and enzyme-linked immunosorbent assay.

Results

The mRNA and protein levels of OPG were enhanced by naringin in a dose-dependent manner compared to that of the non-treated control. In contrast, naringin did not affect the expression of RANKL. Importantly, DKK-1 attenuated OPG expression in fibroblasts under naringin treatment. Moreover, naringin stimulated the gene expression of β-catenin and cyclin D1 in fibroblasts, and the effect could be inhibited by DKK-1.

Conclusion

The results indicated that naringin enhanced OPG expression through Wnt/β-catenin signaling pathway in fibroblasts from periprosthetic membrane, which may be useful to inhibit periprosthetic osteolysis during aseptic loosening after total joint arthroplasty.

Fibroblast growth factor receptor 2 expression in apical periodontitis in mice

AIM

To investigate the presence, localization and the possible correlation of the fibroblast growth factor receptor-2 (FGFR2) with inflammatory resorption of cementum, periodontal ligament and alveolar bone during development of apical periodontitis in mice.

METHODOLOGY

Apical periodontitis was experimentally induced in mandibular first molars of mice by pulp exposure to the oral environment. Healthy teeth without pulp exposure were used as controls. At 7, 21 and 42 days following pulp exposure, the animals were euthanized and the jaws were prepared for analysis under conventional and fluorescence microscopy, immunohistochemistry (FGFR2), RT-PCR (RNAm levels of RANK, RANKL, OPG, Runx2 and cathepsin K) and enzyme histochemistry (cementoclasts and osteoclasts). Statistical analysis was performed by Kruskal-Wallis tests and Dunn's post hoc tests for multiple comparisons (α = 0.05) using SAS 9.4 software.

RESULTS

FGFR2-positive cells were not observed in the tissues surrounding healthy teeth but were observed in teeth with periapical lesions from seven days after root canal contamination. At days 21 and 42 after endodontic infection, the increase in periapical lesion size was accompanied by significantly enhanced expression of FGFR2 (P < 0.0001), significantly increased intensity of inflammatory cells, number of osteoclasts (P < 0.0001) and cementoclasts (P < 0.0001), and significantly enhanced RNAm levels of RANK/RANKL/OPG, Runx2 and cathepsin K compared to day 0 (P < 0.0001). At 21 and 42 days, FGFR2 was also expressed on osteoblasts, fibroblasts and inside enlarged lacunae of cementocytes along with acute and chronic inflammatory cells (macrophages, plasma cells and neutrophils). At all periods and cells, FGFR2 expression was observed in the cell membrane and cytoplasm, but not in the nucleus.

CONCLUSION

In mice, FGFR2 was not expressed in tissues surrounding healthy teeth but was expressed in apical periodontitis, specifically in the membrane and cytoplasm of osteoblasts, fibroblasts, lacunae of cementocytes, and acute and chronic inflammatory cells (macrophages, plasma cells and neutrophils). Its expression was correlated with the size of the periapical lesions.

Nano-sized Al2O3 particle-induced autophagy reduces osteolysis in aseptic loosening of total hip arthroplasty by negative feedback regulation of RANKL expression in fibroblasts

Aseptic loosening is mainly caused by wear debris generated by friction that can increase the expression of receptor activation of nuclear factor (NF)-κB (RANKL). RANKL has been shown to support the differentiation and maturation of osteoclasts. Although autophagy is a key metabolic pathway for maintaining the metabolic homeostasis of cells, no study has determined whether autophagy induced by Al₂O₃ particles is involved in the pathogenesis of aseptic loosening. The aim of this study was to evaluate RANKL levels in patients experiencing aseptic loosening after total hip arthroplasty (THA) and hip osteoarthritis (hOA) and to consequently clarify the relationship between RANKL and LC3II expression. We determined the levels of RANKL and autophagy in fibroblasts treated with Al₂O₃ particles in vitro while using shBECN-1 interference lentivirus vectors to block the autophagy pathway and BECN-1 overexpression lentivirus vectors to promote autophagy. We established a novel rat model of femoral head replacement and analyzed the effects of Al₂O₃ particles on autophagy levels and RANKL expression in synovial tissues in vivo. The RANKL levels in the revision total hip arthroplasty (rTHA) group were higher than those in the hOA group. In patients with rTHA with a ceramic interface, LC3II expression was high, whereas RANKL expression was low. The in vitro results showed that Al₂O₃ particles promoted fibroblast autophagy in a time- and dose-dependent manner and that RANKL expression was negatively correlated with autophagy. The in vivo results further confirmed these findings. Al₂O₃ particles induced fibroblast autophagy, which reduced RANKL expression. Decreasing the autophagy level promoted osteolysis and aseptic prosthetic loosening, whereas increasing the autophagy level reversed this trend.

Expression of XBP1s in fibroblasts is critical for TiAl6 V4 particle-induced RANKL expression and osteolysis

Wear particle-induced osteolysis is a major cause of aseptic loosening, which is one of the most common reasons for total hip arthroplasty (THA) failure. Previous studies have shown that the expression of Receptor activation of nuclear factor (NF)-kB (RANKL) by fibroblasts in periprosthetic membrane played a crucial role in wear particle-induced osteolysis. However, the underlying mechanism of RANKL expression remains largely unknown. In the present study, we investigated the effect of TiAl₆ V₄ particle (TiPs)-induced XBP1s (spliced form of X-box binding protein 1) on RANKL expression and osteoclastogenesis both in vitro and in vivo. The levels of XBP1s in peri-implant membrane, animal models, and TiPs-stimulated fibroblasts were determined by western blots. To assess the effect of XBP1s on RANKL expression, fibroblasts were treated with both a small interfering RNA (siRNA) and an inhibitor of XBP1 prior to exposure to TiPs. The effect of XBP1s on osteoclasts formation was determined by tartrate-resistant acid phosphatase (TRAP) staining in vitro osteoclastogenesis assay and in animal models. The resorption of bone was assessed by micro-computed tomography (micro-CT) with three-dimensional reconstruction. Our results demonstrated that XBP1s was activated in periprosthetic membrane, mouse calvaria models, and TiPs-stimulated human synovial fibroblasts. Further, inhibition of XBP1s decreased the expression of RANKL and osteoclasts formation in vitro. In mouse calvaria models, both of the osteoclastogenesis and osteolysis were inhibited XBP1s inhibitor. Our results suggested that XBP1s mediated TiPs-induced of RANKL expression in fibroblasts, and down regulating XBP1s may represent a potential therapy for wear particle-induced osteolysis. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:752-759, 2017.

Alendronate stimulates osteoprotegerin expression in fibroblasts from periprosthetic membrane

PURPOSE

Aseptic loosening of an implant after total joint arthroplasty is still a major complication that results from periprosthetic osteolysis. Fibroblasts in the interface membrane express receptor activator of nuclear factor kappa B ligand (RANKL) and stimulate osteoclast formation. Studies demonstrate that through the control of osteoclastic bone loss bisphosphonates inhibit wear particle-induced bone resorption around total hip arthroplasty. The majority of bisphosphonates studies have focused on their effects on osteoblasts and osteoclasts. Little attention has been paid to their action on fibroblasts.

METHODS

We isolated fibroblasts from the interface membrane that was obtained when revision hip arthroplasty was performed because of aseptic loosening. Fibroblasts were stimulated with alendronate. RANKL and osteoprotegerin (OPG) assays were performed using enzyme-linked immunosorbent assay kit and real time Reverse Transcription- Polymerase Chain Reaction (RT-PCR).

RESULTS

We found that alendronate stimulated OPG mRNA and protein expression in a time and dose dependent manner. By contrast, alendronate did not affect RANKL expression.

CONCLUSIONS

The results indicate that alendronate modulated OPG production by fibroblasts from periprosthetic membrane, which may prove helpful for the inhibition of bone loss during aseptic loosening following total joint arthroplasty.

A review of UHMWPE wear-induced osteolysis: the role for early detection of the immune response

In a world where increasing joint arthroplasties are being performed on increasingly younger patients, osteolysis as the leading cause of failure after total joint arthroplasty (TJA) has gained considerable attention. Ultra-high molecular weight polyethylene wear-induced osteolysis is the process by which prosthetic debris mechanically released from the surface of prosthetic joints induces an immune response that favors bone catabolism, resulting in loosening of prostheses with eventual failure or fracture. The immune response initiated is innate in that it is nonspecific and self-propagating, with monocytic cells and osteoclasts being the main effectors. To date, detecting disease early enough to implement effective intervention without unwanted systemic side effects has been a major barrier. These barriers can be overcome using newer in vivo imaging techniques and modules linked with fluorescence and/or chemotherapies. We discuss the pathogenesis of osteolysis, and provide discussion of the challenges with imaging and therapeutics. We describe a positron emission tomography imaging cinnamoyl-Phe-(D)-Leu-Phe-(D)-Leu-Phe-Lys module, specific to macrophages, which holds promise in early detection of disease and localization of treatment. Further research and increased collaboration among therapeutic and three-dimensional imaging researchers are essential in realizing a solution to clinical osteolysis in TJA.

Wear debris stimulates bone-resorbing factor expression in the fibroblasts and osteoblasts

Wear debris is believed to cause periprosthetic osteolysis and loosening of total joint arthroplasties. We investigated the wear debris-mediated osteolysis in wild-type mice and macrophage-deficient Csf1op/Csf1op (op/op) mice using high density polyethylene (HDP) particles transplanted on the parietal bone surface. Four weeks after surgery, phagocytosis of the HDP particles by F4/80-positive macrophages and tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts was observed in the normal mice, but not in the macrophage-deficient op/op mice. These results suggest that macrophages are implicated in wear debris-dependent osteoclast formation. However, HDP particles were phagocytosed not only by macrophages but also by F4/80-negative cells in both genotypes of mice. Electron microscopic observation identified these cells as fibroblasts. Cell culture studies demonstrated that fibroblasts cultured with HDP-particles showed upregulation of interleukin-6 (IL-6) expression compared with non-treated fibroblasts. When we examined the gene expression of osteoblasts that belong to the mesenchymal cell lineage as fibroblasts, we found that the expression of not only IL-6 but also interleukin-1 beta (IL-1ß), tumor necrosis factor-alpha (TNF-α) and cyclooxygenase2 (Cox2) were up-regulated by HDP particle-stimulation. These findings suggest the possibility that fibroblasts and osteoblasts are involved in wear debris-mediated osteolysis within the tissue surrounding artificial joints through the production of bone resorbing factors IL-6, IL-1ß, TNF-α, and Cox2.

ER Stress Mediates TiAl6V4 Particle-Induced Peri-Implant Osteolysis by Promoting RANKL Expression in Fibroblasts

Wear particle-induced osteolysis is a major cause of aseptic loosening, which is one of the most common reasons for total hip arthroplasty (THA) failure. Previous studies have shown that the synovial fibroblasts present in the periprosthetic membrane are important targets of wear debris during osteolysis. However, the interaction mechanisms between the wear debris and fibroblasts remain largely unknown. In the present study, we investigated the effect of ER (endoplasmic reticulum) stress induced by TiAl₆V₄ particles (TiPs) in human synovial fibroblasts and calvarial resorption animal models. The expression of ER stress markers, including IRE1-α, GRP78/Bip and CHOP, were determined by western blot in fibroblasts that had been treated with TiPs for various times and concentration. To address whether ER stress was involved in the expression of RANKL, the effects of ER stress blockers (including 4-PBA and TUDCA) on the expression of RANKL in TiPs-treated fibroblasts were examined by real-time PCR, western blot and ELISA. Osteoclastogenesis was assessed by tartrate resistant acid phosphatase (TRAP) staining. Our study demonstrated that ER stress markers were markedly upregulated in TiPs-treated fibroblasts. Blocking ER stress significantly reduced the TiPs-induced expression of RANKL both in vitro and in vivo. Moreover, the inhibition of ER stress ameliorated wear particle-induced osteolysis in animal models. Taken together, these results suggested that the expression of RANKL induced by TiPs was mediated by ER stress in fibroblasts. Therefore, down regulating the ER stress of fibroblasts represents a potential therapeutic approach for wear particle-induced periprosthetic osteolysis.

The fibroblast expression of RANKL in CoCrMo-particle-induced osteolysis is mediated by ER stress and XBP1s

Particle-induced osteolysis is a major cause of aseptic loosening, which is the most common reason for total hip arthroplasty (THA) failure and revision surgery. Although existing studies suggest that synovial fibroblasts present in the interfacial membrane are important targets of wear particles during bone resorption, the interaction mechanisms between the particles and fibroblasts remains elusive. In the present study, we investigated the effect of ER stress induced by CoCrMo particles (CoPs) in fibroblasts, calvarial resorption animal models and aseptic loosening clinical samples and its role in the stimulation of the RANKL expression. Our study further demonstrated that CoPs could induce significant ER stress in fibroblasts. Blocking ER stress with a specific inhibitor dramatically reduced the particle-induced expression of RANKL in vitro and in vivo. Furthermore, in fibroblasts, downregulation of the expression of XBP1s, a signaling molecule of ER stress, significantly reduced the expression of RANKL induced by wear particles. Moreover, inhibition of ER stress or XBP1s both ameliorated the CoPs-induced osteolysis in animal models. Collectively, these results suggested that in particle-induced osteolysis, CoPs could stimulate fibroblasts to secret RANKL through ER stress and the signaling molecule XBP1s. Therefore, downregulating ER stress or the signaling molecule XBP1s of fibroblasts represents a potential therapeutic approach for treating particle-induced peri-implant osteolysis.

STATEMENT OF SIGNIFICANCE

For the first time, our study demonstrated that ER stress mediated the induction of RANKL expression by CoPs in fibroblasts and promoted particle-induced osteolysis. Furthermore, the upregulation of RANKL by CoPs in fibroblasts was mediated by the ER stress signaling molecule XBP1s. Both blocking ER stress and inhibiting the protein XBP1s by specific inhibitors resulted in downregulation of the expression of RANKL and amelioration of osteolysis induced by the implanted particles. Collectively, these findings suggest a possible mechanism underlying the RANKL expression induced by wear particles in fibroblasts, and downregulating ER stress and the XBP1s expression of fibroblasts represents a potential therapeutic approach for treating aseptic loosening.

Membrane nanotube formation in osteoclastogenesis

Membrane tunneling nanotubes (TNTs) are unique intercellular structures, which enable rapid transport of various materials and rapid communication between cells present in a long distance. During osteoclastogenesis, mononuclear osteoclast precursors form abundant TNTs in prior to cell-cell fusion. Here we introduce a protocol for detecting TNTs during osteoclastogenesis by use of live cell imaging utilizing a confocal laser microscopy. We also demonstrate a standard protocol for observation of TNTs by scanning electron microscope.

Regulation of osteoclastogenesis through Tim-3: possible involvement of the Tim-3/galectin-9 system in the modulation of inflammatory bone destruction

Galectins are a unique family of lectins bearing one or two carbohydrate recognition domains (CRDs) that have the ability to bind molecules with β-galactoside-containing carbohydrates. It has been shown that galectins regulate not only cell growth and differentiation but also immune responses, as well as inflammation. Galectin-9, a tandem repeat type of galectin, was originally identified as a chemotactic factor for eosinophils, and is also involved in the regulatory process of inflammation. Here, we examined the involvement of galectin-9 and its receptor, T-cell immunoglobulin- and mucin-domain-containing molecule 3 (Tim-3), in the control of osteoclastogenesis and inflammatory bone destruction. Expression of Tim-3 was detected in osteoclasts and its mononuclear precursors in vivo and in vitro. Galectin-9 markedly inhibited osteoclastogenesis as evaluated in osteoclast precursor cell line RAW-D cells and primary bone marrow cells of mice and rats. The inhibitory effects of galectin-9 on osteoclastogenesis was negated by the addition of β-lactose, an antagonist for galectin binding, suggesting that the inhibitory effect of galectin-9 was mediated through CRD. When galectin-9 was injected into rats with adjuvant-induced arthritis, marked suppression of bone destruction was observed. Inflammatory bone destruction could be efficiently ameliorated by controlling the Tim-3/galectin-9 system in rheumatoid arthritis.

The role of oxidative stress in aseptic loosening of total hip arthroplasties

This study investigated the hypothesis that wear particle-induced oxidative stress initiates osteolysis after total hip arthroplasty (THA). Patient radiographs were scored for osteolysis and periprosthetic tissues were immunostained and imaged to quantify polyethylene wear, inflammation, and five osteoinflammatory and oxidative stress-responsive factors. These included high mobility group protein-B1 (HMGB1), cyclooxygenase-2 (COX2), inducible nitric oxide synthase (iNOS), 4-hydroxynonenal (4-HNE), and nitrotyrosine (NT). The results show wear debris correlated with inflammation, 4-HNE, NT and HMGB1, whereas inflammation only correlated with NT and HMGB1. Similar to wear debris and inflammation, osteolysis correlated with HMGB1. Additionally, osteolysis correlated with COX2 and 4-HNE, but not iNOS or NT. Understanding the involvement of oxidative stress in wear-induced osteolysis will help identify diagnostic biomarkers and therapeutic targets to prevent osteolysis after THA.

Substance P enhanced titanium particles-induced RANKL expression in fibroblasts from periprosthetic membrane

Aseptic loosening remains the primary cause of failure in total joint arthroplasty. Implant-derived particles are thought to be a main cause of osteolysis that leads to the failure. Substance P (SP) immunoreactive nerve fibers have been detected in the periprosthetic membrane of aseptic loose hip prostheses. We isolated fibroblasts from periprosthetic membrane. Fibroblasts were examined by real-time RT-PCR and enzyme-linked immunosorbent assay for expression of the receptor activator of nuclear factor kappa B ligand (RANKL), osteoprotegerin (OPG), cyclooxygenase (COX)-1, and COX-2. Experiments were performed in the presence and absence of titanium particles, SP and NS-398 (a selective COX-2 inhibitor). Titanium particles or SP stimulated RANKL and COX-2 expression in fibroblasts, whereas NS-398 inhibited RANKL production, suggesting a COX-2-mediated event. Moreover, SP enhanced COX-2 and RANKL expression by titanium particles-stimulated fibroblasts. Thus, SP and titanium particles acted synergistically to increase RANKL expression.

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

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

Hypoxia-like effect of cobalt chromium alloy micro particles on fibroblasts in vitro

Periprosthetic osteolysis leading to asceptic loosening remains the primary cause of failure of joint replacement. Although many inflammatory cell types have been implicated, the exact pathomechanisms of asceptic loosening have not been delineated. In the present study we have adopted a proteomic approach to elucidate the initial signals that are expressed to particulate material, using an in vitro cell culture system. Human lung fibroblasts MRC-5 were cultured with Cobalt Chromium (CoCr ASTM F-75, 1-7 µm) particles. Cells were harvested after 72 h incubation and total cellular proteins extracted for downstream analysis via 2D Gel Electrophoresis and tandem mass spectrometry using MALDI-TOF-TOF-MS. Thirteen protein spots showed greater than twofold increase, following 72 h incubation of fibroblast with CoCr particles. Four of these proteins were identified by tandem mass spectrometry. These were Annexin II, Pyruvate kinase, Triose phosphate isomerase, and N-myc downstream regulated gene 1 protein. Cobalt is a hypoxia mimicking agent and N-myc downstream regulated gene 1 protein, Triose phosphate isomerase, Pyruvate kinase, and Annexin II are important hypoxia regulated gene products that are found to be over expressed in cellular oxidative stress response. Our data indicates that exposure of fibroblast to CoCr alloy induces the transition of these cells into a hypoxia like state and oxidative stress even in normoxic culture conditions. The study reflects the possibility of the presence of a hypoxic environment in the periprosthetic tissue surrounding metallic implants.

PGE2 signaling through the EP4 receptor on fibroblasts upregulates RANKL and stimulates osteolysis

Periprosthetic osteolysis is the most common cause of aseptic loosening in total joint arthroplasty. The role of inflammatory mediators such as prostaglandin E2 (PGE2) and osteoclast promoting factors including RANKL in the pathogenesis of osteolysis has been well characterized. However, the PGE2 receptor (EP1, EP2, or EP4), and cell type in which it is expressed, which is responsible for PGE2 induction of RANKL during wear debris-induced osteolysis, has yet to be elucidated. To address this, we used mice genetically deficient in these EP receptors to assess PGE2 and wear debris responses in vitro and in vivo. Wear debris-induced osteolysis and RANKL expression were observed at similar levels in WT, EP1(-/-), and EP2(-/-) mice, indicating that these receptors do not mediate PGE2 signals in this process. A conditional knockout approach was used to eliminate EP4 expression in FSP1(+) fibroblasts that are the predominant source of RANKL. In the absence of EP4, fibroblasts do not express RANKL after stimulation with particles or PGE2, nor do they exhibit high levels of osteoclasts and osteolysis. These results show that periprosthetic fibroblasts are important mediators of osteolysis through the expression of RANKL, which is induced after PGE2 signaling through the EP4 receptor.

Receptor activator of nuclear factor κB ligand is expressed in resident and inflammatory cells in aseptic and septic prosthesis loosening

OBJECTIVE

The pathogenesis of periprosthetic bone loss in aseptic and septic prosthesis loosening is unclear. There is considerable evidence that macrophages and osteoclasts play a key role in focal bone erosion and osteolysis around the prosthesis. RANKL (receptor activator of nuclear factor kappaB ligand) was shown to be a potent osteoclastogenic factor, and to be involved in bone destruction of myeloma and rheumatoid arthritis patients. Osteoprotegerin (OPG) is the natural RANKL inhibitor and may prevent periprosthetic bone loss.

METHODS

The presence and distribution of RANKL, its receptor RANK and OPG in the periprosthetic interface of septically (n = 5) and aseptically (n = 6) loosened prostheses was examined by immunohistochemistry and immunoblotting. Additionally, the immunophenotype of the inflammatory infiltrate was determined [CD3, CD68, Ki-67, tartrate-resistant acid posphatase (TRAP)].

RESULTS

Aseptic and septic cases revealed a different histopathologic pattern. However, in all cases RANKL and RANK could be demonstrated in macrophages and giant cells. In addition, RANKL detected by immunoblot analysis proved to have the same molecular weight as a recombinant RANKL used as a control (31 kD and approximately 48 kD). OPG was detected in aseptic loosening, where macrophages showed a strong staining, but multinucleated giant cells were only weakly stained. A weak OPG staining was also observed in septic loosening.

CONCLUSION

The pathogenesis of bone loss in septic loosening remains unclear, because the septic membrane bears few macrophages and giant cells, and half of them express OPG. In aseptic loosening, macrophages might not be stimulated by RANKL as a result of OPG expression. But multinucleated giant cells may be activated, as they hardly express OPG. They might be responsible for periprosthetic bone loss in aseptic loosening as a result of their RANKL and RANK expression.

Severe periprosthetic osteolytic lesions after the Ankle Evolutive System total ankle replacement

Between 2002 and 2008, 130 consecutive ankles were replaced with an hydroxyapatite (HA) and titanium-HA-coated Ankle Evolutive System total ankle prosthesis. Plain radiographs were analysed by two independent observers. Osteolytic lesions were classified by their size and location, with cavities > 10 mm in diameter considered to be ‘marked’. CT scanning was undertaken in all patients with marked osteolysis seen on the plain radiographs.

Osteolytic lesions were seen on the plain films in 48 (37%) and marked lesions in 27 (21%) ankles. The risk for osteolysis was found to be 3.1 (95% confidence interval 1.6 to 5.9) times higher with implants with Ti-HA porous coating.

Care should be taken with ankle arthroplasty until more is known about the reasons for these severe osteolyses.

Titanium particles stimulate COX-2 expression in synovial fibroblasts through an oxidative stress-induced, calpain-dependent, NF-kappaB pathway

In prosthetic loosening, bone resorption is induced by wear debris particles generated from the artificial joint articulation. Our prior work showed that synovial-like fibroblasts respond to titanium particles by producing receptor activator of NF-kappaB ligand (RANKL), a critical activator of osteoclastogenesis. While this effect occurs through a cyclooxygenase-2 (COX-2)-dependent pathway, the mechanism of COX-2 stimulation by titanium particles is not clear. Here we show that titanium particles induce COX-2 gene expression by activating NF-kappaB signaling. Inhibitor of NF-kappaB (IkappaBalpha) is degraded following particle treatment, permitting active NF-kappaB to translocate to the nucleus where it interacts with the COX-2 promoter and drives transcription. NF-kappaB activation is dependent on reactive oxygen species since antioxidants block the NF-kappaB signaling induced by particles. Surprisingly, IkappaBalpha degradation is independent of IKK (IkappaB kinase) and the 26S proteasome. Instead, calpain inhibitor can block the IkappaBalpha degradation induced by particles. Furthermore, the calpain-targeted COOH-terminal PEST sequence of IkappaBalpha is necessary for phosphorylation and degradation, consistent with a proteasome-independent mechanism of catabolism. Altogether, the data demonstrate a signaling pathway by which titanium particles induce oxidative stress, stimulate calpain-mediated NF-kappaB activation, and activate target gene expression, including COX-2. These findings define important targets for osteolysis but may also have importance in other diseases where fibroblasts respond to environmental particles, including pulmonary diseases.

Role of fibroblasts and fibroblast-derived growth factors in periprosthetic angiogenesis

The periprosthetic granulomatous soft tissue [designated iterfacial membrane (IFM) in this study] exhibits heterogeneous histopathological features, in which highly vascularized areas with dense cellularity alternate with fibrotic and pseudocapsule-like tissue structures. Although macrophage/monocyte activation is a prominent event in the periprosthetic environment, fibroblasts also phagocytose particulate wear debris both in vivo and in vitro. Particulate wear debris and/or cytokines/growth factors alone or in combination (e.g., in conditioned media of explant cultures of IFMs) stimulated normal synovial and IFM fibroblasts to express inflammatory mediators and growth factors such as interleukin (IL)-1beta, IL-6, IL-8, three isoforms of vascular endothelial growth factor (VEGF), monocyte/macrophage chemoattractant protein-1 (MCP-1), macrophage-colony-stimulating factor (M-CSF), cycloxygenases (Cox-1 and Cox-2), acid- and basic-fibroblast growth factors (FGF-1 and FGF-2), leukemia inhibitory factor-1 (LIF-1), transforming growth factor beta-1 (TGF-beta1), receptor activator of nuclear factor-kappa B ligand (RANKL), and osteoprotegerin (OPG). Thus, the fibroblast is capable of expressing a wide array of angiogenic and osteoclastogenic factors which are involved in the detrimental processes of the periprosthetic osteolysis.

The role of fibroblasts and fibroblast-derived factors in periprosthetic osteolysis

OBJECTIVE

This study was undertaken to investigate how fibroblasts respond to stimulation with particulate wear debris and/or conditioned media obtained from pathologic tissue, and whether these activated fibroblasts express compounds that are involved in bone resorption.

METHODS

Conditioned media from explant cultures of synovial tissue, periprosthetic soft tissue (interface membranes), titanium particles, and proinflammatory cytokines were used to stimulate fibroblasts. RNase protection assay was used to measure altered gene expression, and enzyme-linked immunosorbent assay, Western blot hybridization, and flow cytometry were used to determine fibroblast protein expression. Tartrate-resistant acid phosphatase staining was used to identify multinucleated osteoclast-like cells.

RESULTS

The most dominant compounds measured in the conditioned media from interface membranes were tumor necrosis factor alpha (TNFalpha), monocyte chemoattractant protein 1 (MCP-1), interleukin-1beta (IL-1beta), IL-6, IL-8, and vascular endothelial growth factor. Fibroblasts phagocytosed particulate wear debris and responded to cytokine/chemokine stimulation. The most prominent up-regulated genes and proteins secreted by fibroblasts in response to stimulation were matrix metalloproteinase 1, MCP-1, IL-1beta, IL-6, IL-8, cyclooxygenase 1 (COX-1), COX-2, leukemia inhibitory factor 1, transforming growth factor beta1 (TGFbeta1), and TGFbeta receptor type I. In addition, interface membrane fibroblasts expressed RANKL and osteoprotegerin in response to stimulation with conditioned media, TNFalpha, or IL-1beta. Stimulated fibroblasts cocultured with bone marrow cells in the presence of macrophage colony-stimulating factor induced osteoclastogenesis.

CONCLUSION

Interface membrane fibroblasts respond directly to particulate wear debris, possibly via phagocytosis, expressing proinflammatory cytokines and RANKL. Thus, these cells may be actively involved in osteoclastogenesis and pathologic (periprosthetic) bone resorption.

Periprosthetic osteolysis: an immunologist's update

PURPOSE OF REVIEW

Inflammation-induced osteolysis is a problem in both inflammatory arthritis and total joint arthroplasty. New drug therapies have been shown to slow, halt, or even reverse the osteolysis associated with inflammatory arthritis. Unfortunately, similar advances in the medical treatment of periprosthetic osteolysis have not occurred. This review will update the state of periprosthetic osteolysis.

RECENT FINDINGS

Preliminary results with phase I and II clinical trials with AMG-162, a human IgG2 that binds receptor activator of nuclear factor kappaB (RANK) ligand, have been reported. Based on these results AMG-162 appears to be safe and to have a potent effect on osteoclast function. Based on animal studies, it is expected that regents such as AMG-162 that block RANK-ligand/RANK interaction will have activity in inflammation-induced osteolysis. Volumetric three-dimensional and magnetic resonance imaging scans for detecting and quantifying periprosthetic osteolysis have been validated in cadaver studies. Lymphocytic infiltrates and positive skin tests to cobalt have been found in patients with periprosthetic osteolysis after second generation metal-on-metal prostheses. These findings again raise the question of whether metal allergy may contribute to implant failure in these patients. A new subset of T helper cells that are neither Th1 nor Th2, but secrete a unique pattern of cytokines including IL-17, has recently been discovered. The importance of these cells in modifying particle-induced osteolysis remains to be determined.

SUMMARY

There have been significant advances in our understanding of periprosthetic osteolysis, imaging technology to quantify osteolysis, and drug development. The time now seems ripe to translate these advances in clinical trials.

Effects of bone cement particles on the function of pseudocapsule-derived fibroblasts

BACKGROUND

Despite the wide clinical use of bone cement, little is known about cellular responses to the debris from this material. We thus investigated the effects of bone cement particles on the secretion of soluble osteotropic factors in prosthetic pseudomembrane-derived fibroblasts.

METHODS

Bone cement particles were added to fibroblasts maintained in tissue culture. The secretions of soluble receptor activator for nuclear factor kappa B ligand and osteoprotegerin together with interleukin-6 and tumor necrosis factor-alpha were assessed by enzyme-linked immunosorbent assays. The fibroblasts were also co-cultured with osteoclast precursors in the presence and absence of particles, and we assessed osteoclast formation and bone resorption.

RESULTS

The particles produced an increase in the secretion of soluble receptor activator for nuclear factor kappa B ligand, interleukin-6 and tumor necrosis factor-alpha, but not osteoprotegerin. At a concentration of 88 particles/cell, bone cement particles yielded a 2-fold increase (327 pg/mL) in soluble receptor activator for nuclear factor kappa B ligand secretion, a 5-fold (239 pg/mL) increase in interleukin-6 secretion and 4-fold (129 pg/mL) increase in tumor necrosis factor-alpha secretion. The particles also enhanced bone resorption in the co-culture group. Both the increase in soluble receptor activator for nuclear factor kappa B ligand secretion and the increase in bone resorption were inhibited by the addition of neutralizing antibodies to the proinflammatory cytokines.

INTERPRETATION

Our findings show that bone cement particles are capable of stimulating the secretion of soluble receptor activator for nuclear factor kappa B ligand in pseudocapsule-derived fibroblasts by increasing the secretion of proinflammatory cytokines, and may also promote implant loosening.

Hard work never hurt anyone: or did it? A review of occupational associations with soft tissue musculoskeletal disorders of the neck and upper limb

Pain in the neck and upper limb is common and contributes considerably to absence from work due to sickness. Evidence suggest that prolonged abnormal posture and repetition contribute to such conditions. Psychosocial risk factors may also play a part in the aetiology of upper limb disorders.

Fibroblasts express RANKL and support osteoclastogenesis in a COX-2-dependent manner after stimulation with titanium particles

Synovial fibroblasts are possible mediators of osteolysis. Fibroblasts respond directly to titanium particles and increase RANKL expression through a COX-2/PGE2/EP4/PKA signaling pathway. Fibroblasts pretreated with titanium or PGE2 stimulated osteoclast formation, showing the functional importance of RANKL induction. Synovial fibroblasts and their activation pathways are potential targets to prevent osteolysis.

INTRODUCTION

Bone loss adjacent to the implant is a major cause of joint arthroplasty failure. Although the cellular and molecular response to microscopic wear debris particles is recognized as causative, little is known concerning role of synovial fibroblasts in these events.

MATERIALS AND METHODS

Murine embryonic fibroblasts and knee synovial fibroblasts in culture stimulated with titanium particles were examined by FACS, real time RT-PCR, Northern blot, and Western blot for expressions of vascular cell adhesion molecule (VCAM)1, RANKL, cyclooxygenase (COX)-1, and COX-2, and the four prostaglandin E2 (PGE2) receptor isoforms. Experiments were performed in the presence and absence of COX inhibitors, protein kinase A (PKA) and protein kinase C (PKC) inhibitors, and various EP receptor agonists. Osteoclast formation was examined in co-cultures of pretreated glutaraldehyde-fixed fibroblasts and primary murine spleen cells treated with macrophage-colony stimulating factor (M-CSF) for 7-days.

RESULTS

TNF-alpha stimulated VCAM1 expression, consistent with a synovial fibroblast phenotype. Titanium particles stimulated RANKL gene and protein expressions in fibroblasts in a dose-dependent manner. Gene expression was increased 5-fold by 4 h, and protein levels reached a maximum after 48 h. Within 1 h, titanium particles also induced COX-2 mRNA and protein levels, whereas both indomethacin and celecoxib blocked the stimulation of RANKL, suggesting a COX-2-mediated event. Furthermore, PGE2 induced RANKL gene and protein expression and rescued RANKL expression in titanium-treated cultures containing COX-2 inhibitors. Fibroblast cultures pretreated with either PGE2 or titanium particles enhanced osteoclast formation, indicating the functional importance of RANKL induction. EP4 was the most abundant PGE2 receptor isoform, EP1 and EP2 were expressed at low levels, and EP3 was absent. The EP1 selective agonist iloprost and the EP2 selective agonist butaprost minimally stimulated RANKL. In contrast, the EP2 and EP4 agonist misoprostol induced RANKL to a magnitude similar to PGE2. Finally, PKA antagonism strongly repressed RANKL stimulation by PGE2.

CONCLUSION

Fibroblasts respond directly to titanium particles and increase RANKL expression through a COX-2/PGE2/EP4/PKA signaling pathway. Thus, the synovial fibroblast is important mediator of osteolysis and target for therapeutic strategies.

Arthroplasty membrane-derived fibroblasts directly induce osteoclast formation and osteolysis in aseptic loosening

PURPOSE

Both macrophages and fibroblasts are the main cell types found in periprosthetic tissues surrounding failed joint arthroplasties. These fibroblasts are known to express RANKL and to produce TNFalpha, factors which promote osteoclast formation and bone resorption. In this study we have analysed the role that arthroplasty membrane-derived fibroblasts (AFb) play in inducing the generation of bone resorbing osteoclasts.

METHODS

Fibroblasts were isolated from periprosthetic tissues and co-cultured with human monocytes in an osteoclast differentiation assay in the presence or absence of M-CSF and inhibitors of RANKL (OPG) and/or TNFalpha. RANKL expression by AFbs was determined by RT-PCR and the extent of osteoclast differentiation by the expression of TRAP, VNR and evidence of lacunar resorption.

RESULTS

In the presence of M-CSF, large numbers of TRAP(+) and VNR(+) multinucleated cells capable of lacunar resorption, were noted in co-cultures of monocytes and RANKL-expressing AFbs. Cell-cell contact was required for osteoclast formation. The addition of OPG and anti-TNFalpha alone significantly reduced but did not abolish the extent of osteoclast formation, whereas the addition of both together abolished osteoclast formation and lacunar resorption.

CONCLUSION

Our results indicate that fibroblasts in periprosthetic tissues are capable of inducing the differentiation of normal human peripheral blood mononuclear cells to mature osteoclasts by a mechanism that involves both RANKL and TNFalpha. Suppression of both RANKL and inflammatory cytokines is likely to be required to control periprosthetic osteolysis.

Hard work never hurt anyone--or did it? A review of occupational associations with soft tissue musculoskeletal disorders of the neck and upper limb

Pain in the neck and upper limb is common and contributes greatly to absence due to sickness. Evidence is accumulating that factors such as prolonged abnormal posture and repetition contribute to these conditions. Psychosocial factors may also play a part and the relative importance of these is not yet known. Primary and secondary prevention trials are needed.

Alendronate inhibits bone resorption at the bone-screw interface

In the current study, we investigated whether the systemic administration of alendronate, a third-generation bisphosphonate, suppressed the loosening of screws at the bone-screw interface. We systemically administered alendronate to rats fitted with external fixators. External fixators with two half pins were applied to the right femurs of rats, and alendronate was administrated once a week during a 5-week postoperative period. Radiographic, histologic, and immunohistochemical findings subsequently were analyzed. Treatment with alendronate reduced the width of the fibrous loosening membrane and the number of osteoclasts at the bone-screw interface. These findings indicate that systemic treatment with alendronate exerts an inhibitory effect on local bone resorption at the bone-screw interface.

Regulation of osteoclast activity in peri-implant tissues

Implants, particularly joint replacement prostheses, are one of the great success stories of modern medicine. However, too many implants fail prematurely, mainly due to aseptic bone loss around the implant. This paper reviews our current understanding of the role of osteoclasts in this peri-implant bone lysis. Prosthetic particles, often produced by articulating prostheses, are one of the major causes of elevated osteoclast lysis of peri-implant bone. Over the past decade there have been major advances in our understanding of the factors that regulate osteoclast activity, many of which were found to be important in osteoclast formation and activity in the peri-implant tissues. These factors are targets of a number of recently developed drugs that have been used successfully to prevent and treat peri-implant bone lysis in experimental models. Treatments such as these are being used in a number of bone loss pathologies in humans and have the potential for successful treatment of peri-implant osteolysis. In addition, understanding how different biomaterials influence the expression of key osteoclastogenic factors may allow us to select biomaterials for implantation that will last the lifetime of the recipient.

Regulation of osteoclast activity in peri-implant tissues

Implants, particularly joint replacement prostheses, are one of the great success stories of modern medicine. However, too many implants fail prematurely, mainly due to aseptic bone loss around the implant. This paper reviews our current understanding of the role of osteoclasts in this peri-implant bone lysis. Prosthetic particles, often produced by articulating prostheses, are one of the major causes of elevated osteoclast lysis of peri-implant bone. Over the past decade there have been major advances in our understanding of the factors that regulate osteoclast activity, many of which were found to be important in osteoclast formation and activity in the peri-implant tissues. These factors are targets of a number of recently developed drugs that have been used successfully to prevent and treat peri-implant bone lysis in experimental models. Treatments such as these are being used in a number of bone loss pathologies in humans and have the potential for successful treatment of peri-implant osteolysis. In addition, understanding how different biomaterials influence the expression of key osteoclastogenic factors may allow us to select biomaterials for implantation that will last the lifetime of the recipient.

[Culture of human osteoclasts]

OBJECTIVE

To establish a stable, useful culture system for human osteoclasts and to investigate the effect of osteoblasts on the differentiation, proliferation and activation of osteoclasts so as to provide a base for the studies on prevention and treatment of osteolysis and osteoporosis.

METHODS

In the presence of 1,25-(OH)2D3, monocytes abstracted from human bone marrow were cultured in three groups: co-culture of monocytes and osteoblasts, monocytes alone, monocytes with conditional media (CM) of osteoblasts. Differentiation process of the cultured cells was observed under biological microscope. HE staining and tartrate-resistant acid phosphatase (Trap) staining were employed to assay the cultured cells. The resorption pits on bone slices, on which cells were cultured, were observed under scanning electronic microscope (SEM).

RESULTS

In the group of co-culture of monocytes and osteoblasts, monocytes gradually fused to form multinucleated cells (MNCs), and the MNCs were also indicated in HE staining and Trap staining. The SEM showed a number of resorption pits on bone slices. In the other two groups, Trap-positive MNCs were not obtained, and resorption pits were not observed on bone slices.

CONCLUSION

In this culture, monocytes obtained from human marrow fused to form multinucleated cells (MNCs) that express the main characteristics of the osteoclast phenotype, such as Trap-positive and the ability to form resorption lacunae when cultured on bone slices. Cell-to-cell contact with osteoblasts was necessary for the differentiation, proliferation and activation of osteoclasts.

Factors regulating osteoclast formation in human tissues adjacent to peri-implant bone loss: expression of receptor activator NFkappaB, RANK ligand and osteoprotegerin

Aseptic bone loss adjacent to orthopedic joint implants is a common cause of joint implant failure in humans. This study investigates the expression of key regulators of osteoclast formation, receptor activator NFkappaB (RANK), Receptor activator of NFkappaB ligand (RANKL) and osteoprotegerin (OPG), in the peri-implant tissues of patients with osteolysis compared with levels in synovial tissues from osteoarthritic and healthy subjects. Immunohistochemical studies demonstrated that significantly higher levels of RANKL protein (p<0.05) were found in the peri-implant tissues of patients with implant failure than in similar tissues from osteoarthritic and healthy subjects. In contrast, OPG protein levels were similar in all tissues. RANKL, expressed as mRNA and protein, was predominantly associated with cells containing wear particles. Dual labeling studies showed that the cells expressing RANKL protein were macrophages. In situ hybridization studies confirmed that mRNA encoding for these proteins is also expressed by cells in the peri-implant tissues. In addition, RANK mRNA was expressed in cells that contained wear particles. These findings show that abnormally high levels of RANKL are expressed in peri-implant tissues of patients with prosthetic loosening and that these abnormal levels of RANKL may significantly contribute to aseptic implant loosening.

Metabolic acidosis stimulates RANKL RNA expression in bone through a cyclo-oxygenase-dependent mechanism

Metabolic acidosis inhibits osteoblastic bone formation and stimulates osteoclastic resorption. To determine whether acidosis alters expression of RNA for the osteoclastic differentiation factor RANKL, mouse calvariae were incubated in neutral or physiologically acidic media. Acidosis resulted in a significant cyclo-oxygenase-dependent increase in RANKL RNA levels, which would be expected to induce the associated increase in bone resorption.

INTRODUCTION

Metabolic acidosis increases net calcium efflux from bone, initially through physicochemical mechanisms and later through predominantly cell-mediated mechanisms. Acidosis decreases osteoblastic bone formation and increases osteoclastic resorption. The growth and maturation of osteoclasts, derived from hematopoietic precursors in the monocyte/macrophage lineage, are dependent on the interplay of a number of factors. Commitment of pre-osteoclasts to osteoclasts is induced by the interaction of the osteoclastic cell-surface receptor RANK with a ligand expressed by osteoblasts, RANKL. The RANK/RANKL interaction not only initiates a differentiation cascade that culminates in mature bone-resorbing osteoclasts but also increases osteoclastic resorptive capacity and survival.

METHODS

To test the hypothesis that metabolic acidosis increases expression of RANKL, we cultured neonatal mouse calvariae in acidic (initial medium pH approximately 7.1 and [HCO3-] approximately 11 mM) or neutral (initial medium pH approximately 7.5 and [HCO3-] approximately 25 mM) medium for 24 and 48 h. We determined the relative expression of RANKL RNA by reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitated the expression by Northern analysis.

RESULTS

In this model of metabolic acidosis, there was significantly increased expression of RANKL RNA at both 24 (2-fold) and 48 h (5-fold) compared with respective controls. Net calcium efflux from bone was also increased in acidic medium compared with control medium. At 48 h, net calcium efflux correlated directly with RANKL expression (r = 0.77, n = 15, p < 0.001). Inhibition of prostaglandin synthesis with indomethacin blocked the acid-induced increase in RANKL RNA as well as the increased calcium efflux.

CONCLUSIONS

Metabolic acidosis induces osteoblastic prostaglandin synthesis, followed by autocrine or paracrine induction of RANKL. This increase in RANKL would be expected to augment osteoclastic bone resorption and help explain the increase in cell-mediated net calcium efflux.