Inhibition of IKK activation, through sequestering NEMO, blocks PMMA-induced osteoclastogenesis and calvarial inflammatory osteolysis

Potential of oligonucleotide- and protein/peptide-based therapeutics in the management of toxicant/stressor-induced diseases

Exposure to toxicants/stressors has been linked to the development of many human diseases. They could affect various cellular components, such as DNA, proteins, lipids, and non-coding RNAs (ncRNA), thereby triggering various cellular pathways, particularly oxidative stress, inflammatory responses, and apoptosis, which can contribute to pathophysiological states. Accordingly, modulation of these pathways has been the focus of numerous investigations for managing related diseases. The involvement of various ncRNAs, such as small interfering RNA (siRNA), microRNAs (miRNA), and long non-coding RNAs (lncRNA), as well as various proteins and peptides in mediating these pathways, provides many target sites for pharmaceutical intervention. In this regard, various oligonucleotide- and protein/peptide-based therapies have been developed to treat toxicity-induced diseases, which have shown promising results in vitro and in vivo. This comprehensive review provides information about various aspects of toxicity-related diseases including their causing factors, main underlying mechanisms and intermediates, and their roles in pathophysiological states. Particularly, it highlights the principles and mechanisms of oligonucleotide- and protein/peptide-based therapies in the treatment of toxicity-related diseases. Furthermore, various issues of oligonucleotides and proteins/peptides for clinical usage and potential solutions are discussed.

Molecular Mechanisms of Craniofacial and Dental Abnormalities in Osteopetrosis

Osteopetrosis is a group of genetic bone disorders characterized by increased bone density and defective bone resorption. Osteopetrosis presents a series of clinical manifestations, including craniofacial deformities and dental problems. However, few previous reports have focused on the features of craniofacial and dental problems in osteopetrosis. In this review, we go through the clinical features, types, and related pathogenic genes of osteopetrosis. Then we summarize and describe the characteristics of craniofacial and dental abnormalities in osteopetrosis that have been published in PubMed from 1965 to the present. We found that all 13 types of osteopetrosis have craniomaxillofacial and dental phenotypes. The main pathogenic genes, such as chloride channel 7 gene (CLCN7), T cell immune regulator 1 (TCIRG1), osteopetrosis-associated transmembrane protein 1 (OSTM1), pleckstrin homology domain-containing protein family member 1 (PLEKHM1), and carbonic anhydrase II (CA2), and their molecular mechanisms involved in craniofacial and dental phenotypes, are discussed. We conclude that the telltale craniofacial and dental abnormalities are important for dentists and other clinicians in the diagnosis of osteopetrosis and other genetic bone diseases.

HSP90β promotes osteoclastogenesis by dual-activation of cholesterol synthesis and NF-κB signaling

Heat shock protein 90β (Hsp90β, encoded by Hsp90ab1 gene) is the most abundant proteins in the cells and contributes to variety of biological processes including metabolism, cell growth and neural functions. However, genetic evidences showing Hsp90β in vivo functions using tissue specific knockout mice are still lacking. Here, we showed that Hsp90β exerted paralogue-specific role in osteoclastogenesis. Using myeloid-specific Hsp90ab1 knockout mice, we provided the first genetic evidence showing the in vivo function of Hsp90β. Hsp90β binds to Ikkβ and reduces its ubiquitylation and proteasomal degradation, thus leading to activated NF-κB signaling. Meanwhile, Hsp90β increases cholesterol biosynthesis by activating Srebp2. Both pathways promote osteoclastogenic genes expression. Genetic deletion of Hsp90ab1 in osteoclast or pharmacological inhibition of Hsp90β alleviates bone loss in ovariectomy-induced mice. Therefore, Hsp90β is a promising druggable target for the treatment of osteoporosis.

Involvement of NF-κB/NLRP3 axis in the progression of aseptic loosening of total joint arthroplasties: a review of molecular mechanisms

Particulate wear debris can trigger pro-inflammatory bone resorption and result in aseptic loosening. This complication remains major postoperative discomforts and complications for patients who underwent total joint arthroplasty. Recent studies have indicated that wear debris-induced aseptic loosening is associated with the overproduction of pro-inflammatory cytokines. The activation of osteoclasts as a result of inflammatory responses is associated with osteolysis. Moreover, stimulation of inflammatory signaling pathways such as the NF-κB/NLRP3 axis results in the production of pro-inflammatory cytokines. In this review, we first summarized the potential inflammatory mechanisms of wear particle-induced peri-implant osteolysis. Then, the therapeutic approaches, e.g., biological inhibitors, herbal products, and stem cells or their derivatives, with the ability to suppress the inflammatory responses, mainly NF-κB/NLRP3 signaling pathways, were discussed. Based on the results, activation of macrophages following inflammatory stimuli, overproduction of pro-inflammatory cytokines, and subsequent differentiation of osteoclasts in the presence of wear particles lead to bone resorption. The activation of NF-κB/NLRP3 signaling pathways within the macrophages stimulates the production of pro-inflammatory cytokines, e.g., IL-1β, IL-6, and TNF-α. According to in vitro and in vivo studies, novel therapeutics significantly promoted osteogenesis, suppressed osteoclastogenesis, and diminished particle-mediated bone resorption. Conclusively, these findings offer that suppressing pro-inflammatory cytokines by regulating both NF-κB and NLRP3 inflammasome represents a novel approach to attenuate wear-particle-related osteolytic diseases.

Cell-penetrating peptide-mediated delivery of therapeutic peptides/proteins to manage the diseases involving oxidative stress, inflammatory response and apoptosis

OBJECTIVES

Peptides and proteins represent great potential for modulating various cellular processes including oxidative stress, inflammatory response, apoptosis and consequently the treatment of related diseases. However, their therapeutic effects are limited by their inability to cross cellular barriers. Cell-penetrating peptides (CPPs), which can transport cargoes into the cell, could resolve this issue, as would be discussed in this review.

KEY FINDINGS

CPPs have been successfully exploited in vitro and in vivo for peptide/protein delivery to treat a wide range of diseases involving oxidative stress, inflammatory processes and apoptosis. Their in vivo applications are still limited due to some fundamental issues of CPPs, including nonspecificity, proteolytic instability, potential toxicity and immunogenicity.

SUMMARY

Totally, CPPs could potentially help to manage the diseases involving oxidative stress, inflammatory response and apoptosis by delivering peptides/proteins that could selectively reach proper intracellular targets. More studies to overcome related CPP limitations and confirm the efficacy and safety of this strategy are needed before their clinical usage.

Prophylactic Effects of NFκB Essential Modulator–Binding Domain Peptides on Bone Infection: An Experimental Study in a Rabbit Model

Introduction

Methods

Results

Conclusion

Identifying the Potential Differentially Expressed miRNAs and mRNAs in Osteonecrosis of the Femoral Head Based on Integrated Analysis

Purpose

Osteonecrosis of the femoral head is a common disease of the hip that leads to severe pain or joint disability. We aimed to identify potential differentially expressed miRNAs and mRNAs in osteonecrosis of the femoral head.

Methods

The data of miRNA and mRNA were firstly downloaded from the database. Secondly, the regulatory network of miRNAs-mRNAs was constructed, followed by function annotation of mRNAs. Thirdly, an in vitro experiment was applied to validate the expression of miRNAs and targeted mRNAs. Finally, GSE123568 dataset was used for electronic validation and diagnostic analysis of targeted mRNAs.

Results

Several regulatory interaction pairs between miRNA and mRNAs were identified, such as hsa-miR-378c-WNT3A/DACT1/CSF1, hsa-let-7a-5p-RCAN2/IL9R, hsa-miR-28-5p-RELA, hsa-miR-3200-5p-RELN, and hsa-miR-532-5p-CLDN18/CLDN10. Interestingly, CLDN10, CLDN18, CSF1, DACT1, IL9R, RCAN2, RELN, and WNT3A had the diagnostic value for osteonecrosis of the femoral head. Wnt signaling pathway (involved WNT3A), chemokine signaling pathway (involved RELA), focal adhesion and ECM-receptor interaction (involved RELN), cell adhesion molecules (CAMs) (involved CLDN18 and CLDN10), cytokine-cytokine receptor interaction, and hematopoietic cell lineage (involved CSF1 and IL9R) were identified.

Conclusion

The identified differentially expressed miRNAs and mRNAs may be involved in the pathology of osteonecrosis of the femoral head.

The Role of NLRP3 Inflammasome Activities in Bone Diseases and Vascular Calcification

Continuous stimulation of inflammation is harmful to tissues of an organism. Inflammatory mediators not only have an effect on metabolic and inflammatory bone diseases but also have an adverse effect on certain genetic and periodontal diseases associated with bone destruction. Inflammatory factors promote vascular calcification in various diseases. Vascular calcification is a pathological process similar to bone development, and vascular diseases play an important role in the loss of bone homeostasis. The NLRP3 inflammasome is an essential component of the natural immune system. It can recognize pathogen-related molecular patterns or host-derived dangerous signaling molecules, recruit, and activate the pro-inflammatory protease caspase-1. Activated caspase-1 cleaves the precursors of IL-1β and IL-18 to produce corresponding mature cytokines or recognizes and cleaves GSDMD to mediate cell pyroptosis. In this review, we discuss the role of NLRP3 inflammasome in bone diseases and vascular calcification caused by sterile or non-sterile inflammation and explore potential treatments to prevent bone loss.

Similitude of cement-bone micromechanics in cemented rat and human knee replacement

A preclinical rat knee replacement model was recently developed to explore the biological and mechanobiological changes of trabecular resorption for cement-bone interdigitated regions. The goal here was to evaluate the relevance of this model compared with human knee replacement with regards to functional micromechanics. Eight nonsurvival, cemented knee replacement surgeries were performed, the interdigitated gap morphology was quantified, and interface micromotion between cement and bone was measured for 1 to 5 bodyweight loading. Computational fluid dynamics modeling of unit cell geometries with small gaps between trabeculae and cement was used to estimate fluid flow. Gap width (3.6 μm) was substantially smaller compared with cement-bone gaps reported in human knee replacement (11.8 μm). Micromotion at the cement-bone border was also decreased for the rat knee replacement (0.48 μm), compared with human (1.97 μm), for 1 bodyweight loading. However, the micromotion-to-gap width ratio (0.19 and 0.22 for, rat and human), and estimated fluid shear stress (6.47 and 7.13 Pa, for rat and human) were similar. Replicating the fluid dynamic characteristics of cement-bone interdigitated regions in human knee replacements using preclinical models may be important to recapitulate trabecular resorption mechanisms due to proposed supraphysiologic fluid shear stress. Statement of clinical significance: local cement-bone micromotion due to joint loading may contribute to the process of clinical loosening in total joint replacements. This work shows that while micromotion and gap morphology are diminished for the rat knee model compared to human, the motion-to-gap ratio, and corresponding fluid shear stress are of similar magnitudes.

Diagnosis and management of implant debris-associated inflammation

Introduction: Total joint replacement is one of the most common, safe, and efficacious operations in all of surgery. However, one major long-standing and unresolved issue is the adverse biological reaction to byproducts of wear from the bearing surfaces and modular articulations. These inflammatory reactions are mediated by the innate and adaptive immune systems.Areas covered: We review the etiology and pathophysiology of implant debris-associated inflammation, the clinical presentation and detailed work-up of these cases, and the principles and outcomes of non-operative and operative management. Furthermore, we suggest future strategies for prevention and novel treatments of implant-related adverse biological reactions.Expert opinion: The generation of byproducts from joint replacements is inevitable, due to repetitive loading of the implants. A clear understanding of the relevant biological principles, clinical presentations, investigative measures and treatments for implant-associated inflammatory reactions and periprosthetic osteolysis will help identify and treat patients with this issue earlier and more effectively. Although progressive implant-associated osteolysis is currently a condition that is treated surgically, with further research, it is hoped that non-operative biological interventions could prolong the lifetime of joint replacements that are otherwise functional and still salvageable.

Periprosthetic Osteolysis: Mechanisms, Prevention and Treatment

Clinical studies, as well as in vitro and in vivo experiments have demonstrated that byproducts from joint replacements induce an inflammatory reaction that can result in periprosthetic osteolysis (PPOL) and aseptic loosening (AL). Particle-stimulated macrophages and other cells release cytokines, chemokines, and other pro-inflammatory substances that perpetuate chronic inflammation, induce osteoclastic bone resorption and suppress bone formation. Differentiation, maturation, activation, and survival of osteoclasts at the bone-implant interface are under the control of the receptor activator of nuclear factor kappa-Β ligand (RANKL)-dependent pathways, and the transcription factors like nuclear factor κB (NF-κB) and activator protein-1 (AP-1). Mechanical factors such as prosthetic micromotion and oscillations in fluid pressures also contribute to PPOL. The treatment for progressive PPOL is only surgical. In order to mitigate ongoing loss of host bone, a number of non-operative approaches have been proposed. However, except for the use of bisphosphonates in selected cases, none are evidence based. To date, the most successful and effective approach to preventing PPOL is usage of wear-resistant bearing couples in combination with advanced implant designs, reducing the load of metallic and polymer particles. These innovations have significantly decreased the revision rate due to AL and PPOL in the last decade.

Efficacy of NEMO-binding domain peptide used to treat experimental osteomyelitis caused by methicillin-resistant Staphylococcus aureus: an in-vivo study

Purpose

Treatment of chronic osteomyelitis (bone infection) remains a clinical challenge. Our previous study had demonstrated that NEMO-binding domain (NBD) peptide effectively ameliorates the inhibition of osteoblast differentiation by TNF-α in vitro. In this work, NBD peptide was evaluated in vivo for treating chronic osteomyelitis induced by methicillin-resistant Staphylococcus aureus (MRSA) in a rabbit model.

Methods

Tibial osteomyelitis was induced in 50 New Zealand white rabbits by tibial canal inoculation of MRSA strain. After 3 weeks, 45 rabbits with osteomyelitis were randomly divided into four groups that correspondingly received the following interventions: 1) Control group (9 rabbits, no treatment); 2) Van group (12 rabbits, debridement and parenteral treatment with vancomycin); 3) NBD + Van group (12 rabbits, debridement and local NBD peptide injection, plus parenteral treatment with vancomycin); 4) NBD group (12 rabbits, debridement and local NBD peptide injection). Blood samples were collected weekly for the measurement of leucocyte count, erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP) levels. The rabbits in all four groups were sacrificed 6 weeks after debridement; the anti-infective efficacy was evaluated by radiological, histological, and microbiological examination, and promotion of bone remodeling was quantified by micro-CT using the newly formed bone.

Results

Except two rabbits in the Control group and one in the NBD group that died from severe infection before the end point, the remaining 42 animals (7, 12, 12, 11 in the Control, Van, NBD + Van, and NBD group respectively) were sacrificed 6 weeks after debridement. In general, there was no significant difference in the leucocyte count, and ESR and CRP levels, although there were fluctuations throughout the follow-up period after debridement. MRSA was still detectable in bone tissue samples of all animals. Interestingly, treatment with NBD peptide plus vancomycin significantly reduced radiological and histological severity scores compared to that in other groups. The best therapeutic efficacy in bone defect repair was observed in the NBD peptide + Van group.

Conclusions

In a model of osteomyelitis induced by MRSA, despite the failure in demonstrating antibacterial effectiveness of NBD peptide in vivo, the results suggest antibiotics in conjunction with NBD peptide to possibly have promising therapeutic potential in osteomyelitis.

Early Changes in Cement-Bone Fixation Using a Novel Rat Knee Replacement Model

Trabecular resorption from interdigitated regions between cement and bone has been found in postmortem-retrieved knee replacements, but the viability of interdigitated bone, and the mechanism responsible for this bone loss is not known. In this work, a Sprague-Dawley (age 12 weeks) rat knee replacement model with an interdigitated cement-bone interface was developed. Morphological and cellular changes in the interdigitated region of the knee replacement over time (0, 2, 6, or 12 weeks) were determined for ovariectomy (OVX) and Sham OVX treatment groups. Interdigitated bone volume fraction (BV/TV) increased with time for Sham OVX (0.022 BV/TV/wk) and OVX (0.015 BV/TV/wk) group, but the rate of increase was greater for the Sham OVX group (p = 0.0064). Tissue mineral density followed a similar increase with time in the interdigitated regions. Trabecular resorption, when it did occur, started at the cement border with medullary-adjacent bone in the presence of osteoclasts. There was substantial loss of viable bone (~80% empty osteocyte lacunae) in the interdigitated regions. Pre-surgical fluorochrome labels remained in the interdigitated regions, and did not diminish with time, indicating that the bone was not remodeling. There was also some evidence of continued surface mineralization in the interdigitated region after cementing of the knee, but this diminished over time. Statement of clinical significance: Interdigitated bone with cement provides mechanical stability for success of knee replacements. Improved understanding of the fate of the interdigitated bone over time could lead to a better understanding of the loosening process and interventions to prevent loss of fixation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2163-2171, 2019.

Omnipresence of inflammasome activities in inflammatory bone diseases

The inflammasomes are intracellular protein complexes that are assembled in response to a variety of perturbations including infections and injuries. Failure of the inflammasomes to rapidly clear the insults or restore tissue homeostasis can result in chronic inflammation. Recurring inflammation is also provoked by mutations that cause the constitutive assembly of the components of these protein platforms. Evidence suggests that chronic inflammation is a shared mechanism in bone loss associated with aging, dysregulated metabolism, autoinflammatory, and autoimmune diseases. Mechanistically, inflammatory mediators promote bone resorption while suppressing bone formation, an imbalance which over time leads to bone loss and increased fracture risk. Thus, while acute inflammation is important for the maintenance of bone integrity, its chronic state damages this tissue. In this review, we discuss the role of the inflammasomes in inflammation-induced osteolysis.

Inflammatory Responses Reprogram TREGS Through Impairment of Neuropilin-1

Chronic inflammatory insults compromise immune cell responses and ultimately contribute to pathologic outcomes. Clinically, it has been suggested that bone debris and implant particles, such as polymethylmethacrylate (PMMA), which are persistently released following implant surgery evoke heightened immune, inflammatory, and osteolytic responses that contribute to implant failure. However, the precise mechanism underlying this pathologic response remains vague. T_REGS, the chief immune-suppressive cells, express the transcription factor Foxp3 and are potent inhibitors of osteoclasts. Using an intra-tibial injection model, we show that PMMA particles abrogate the osteoclast suppressive function of T_REGS. Mechanistically, PMMA particles induce T_REG instability evident by reduced expression of Foxp3. Importantly, intra-tibial injection of PMMA initiates an acute innate immune and inflammatory response, yet the negative impact on T_REGS by PMMA remains persistent. We further show that PMMA enhance T_H17 response at the expense of other T effector cells (T_EFF), particularly T_H1. At the molecular level, gene expression analysis showed that PMMA particles negatively regulate Nrp-1/Foxo3a axis to induce T_REG instability, to dampen T_REG activity and to promote phenotypic switch of T_REGS to T_H17 cells. Taken together, inflammatory cues and danger signals, such as bone and implant particles exacerbate inflammatory osteolysis in part through reprogramming T_REGS.

Anthocyanin suppresses CoCrMo particle-induced osteolysis by inhibiting IKKα/β mediated NF-κB signaling in a mouse calvarial model

Wear particle-induced osteolysis and bone resorption have been identified as critical factors of implant failure and total joint revision, in which nuclear factor kappa B (NF-κB) signaling and chronic inflammation have been shown to play key roles. Although anthocyanin is known to have anti-inflammatory function via blocking NF-κB pathway, it is still unclear whether anthocyanin has a protective effect on particle-induced osteolysis. In the present study, we aimed to investigate the detailed effects and the underlying mechanism of anthocyanin on CoCrMo particle-induced osteolysis in a mouse calvavial model. One hundred and twelve male BALB/c mice were divided randomly into four groups: sham group (sham operation and injection with PBS), vehicle group (CoCrMo particle treatment and injection with PBS), low-dose anthocyanin group (CoCrMo particle treatment and injecting anthocyanin with 0.1mg/g/day), and high-dose anthocyanin group (CoCrMo particle treatment and injecting anthocyanin with 0.4mg/g/day). Mice were sacrificed after two weeks, harvesting the calvariae tissue for in depth analysis by micro-CT, histomorphometry, immunohistochemical and molecular biology analysis. As expected, anthocyanin markedly inhibited CoCrMo particle-induced inflammatory infiltration and decreased bone loss in vivo. Anthocyanin also reversed the increase in the ratio of receptor activator of nuclear factor kappa B ligand (RANKL)/osteoproteger (OPG) and suppressed osteoclast formation in CoCrMo particle-stimulated calvaria. Additionally, anthocyanin significantly reduced the expression and secretion of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-6 (IL-6) in the calvaria of CoCrMo-stimulated mice. Furthermore, we confirmed that anthocyanin attenuated osteolysis by blocking NF-κB pathway via inhibiting inhibitor of nuclear factor kappa-B kinase α/β (IKKα/β) phosphorylation. In conclusion, our study demonstrated that anthocyanin can protect against CoCrMo particle-induced inflammatory osteolysis via inhibiting the IKKα/β-NF-κB pathway, and have a potential therapeutic effect on the treatment of wear particle-induced osteolysis.

Myeloid Deletion of Nemo Causes Osteopetrosis in Mice Owing to Upregulation of Transcriptional Repressors

The transcription factor NF-κB is central to numerous physiologic processes including bone development, and its activation is controlled by IKKγ (also called NEMO), the regulatory subunit of IKK complex. NEMO is X-linked, and mutations in this gene result in Incontinentia Pigmenti in human hemizygous females. In mice, global deficiency causes embryonic lethality. In addition, certain point mutations in the NEMO (IKBKG) human gene manifest skeletal defects implicating NEMO in the regulation of bone homeostasis. To specifically investigate such role, we conditionally deleted Nemo from osteoclast and myeloid progenitors. Morphometric, histologic, and molecular analyses demonstrate that myeloid NEMO deletion causes osteopetrosis in mice. Mechanistically, NEMO deficiency hampered activation of IKK complex in osteoclast precursors, causing arrest of osteoclastogenesis and apoptosis. Interestingly, inhibiting apoptosis by genetic ablation of TNFr1 significantly increased cell survival, but failed to rescue osteoclastogenesis or reverse osteopetrosis. Based on this observation, we analyzed the expression of different regulators of osteoclastogenesis and discovered that NEMO deletion leads to increased RBPJ expression, resulting in a decrease of Blimp1 expression. Consequently, expression of IRF8 and Bcl6 which are targets of Blimp1 and potent osteoclastogenic transcriptional repressors, is increased. Thus, NEMO governs survival and osteoclast differentiation programs through serial regulation of multiple transcription factors.

Neonatal High Bone Mass With First Mutation of the NF-κB Complex: Heterozygous De Novo Missense (p.Asp512Ser) RELA (Rela/p65)

Heritable disorders that feature high bone mass (HBM) are rare. The etiology is typically a mutation(s) within a gene that regulates the differentiation and function of osteoblasts (OBs) or osteoclasts (OCs). Nevertheless, the molecular basis is unknown for approximately one-fifth of such entities. NF-κB signaling is a key regulator of bone remodeling and acts by enhancing OC survival while impairing OB maturation and function. The NF-κB transcription complex comprises five subunits. In mice, deletion of the p50 and p52 subunits together causes osteopetrosis (OPT). In humans, however, mutations within the genes that encode the NF-κB complex, including the Rela/p65 subunit, have not been reported. We describe a neonate who died suddenly and unexpectedly and was found at postmortem to have HBM documented radiographically and by skeletal histopathology. Serum was not available for study. Radiographic changes resembled malignant OPT, but histopathological investigation showed morphologically normal OCs and evidence of intact bone resorption excluding OPT. Furthermore, mutation analysis was negative for eight genes associated with OPT or HBM. Instead, accelerated bone formation appeared to account for the HBM. Subsequently, trio-based whole exome sequencing revealed a heterozygous de novo missense mutation (c.1534_1535delinsAG, p.Asp512Ser) in exon 11 of RELA encoding Rela/p65. The mutation was then verified using bidirectional Sanger sequencing. Lipopolysaccharide stimulation of patient fibroblasts elicited impaired NF-κB responses compared with healthy control fibroblasts. Five unrelated patients with unexplained HBM did not show a RELA defect. Ours is apparently the first report of a mutation within the NF-κB complex in humans. The missense change is associated with neonatal osteosclerosis from in utero increased OB function rather than failed OC action. These findings demonstrate the importance of the Rela/p65 subunit within the NF-κB pathway for human skeletal homeostasis and represent a new genetic cause of HBM.

Innate immunity sensors participating in pathophysiology of joint diseases: a brief overview

The innate immune system consists of functionally specialized "modules" that are activated in response to a particular set of stimuli via sensors located on the surface or inside the tissue cells. These cells screen tissues for a wide range of exogenous and endogenous danger/damage-induced signals with the aim to reject or tolerate them and maintain tissue integrity. In this line of thinking, inflammation evolved as an adaptive tool for restoring tissue homeostasis. A number of diseases are mediated by a maladaptation of the innate immune response, perpetuating chronic inflammation and tissue damage. Here, we review recent evidence on the cross talk between innate immune sensors and development of rheumatoid arthritis, osteoarthritis, and aseptic loosening of total joint replacements. In relation to the latter topic, there is a growing body of evidence that aseptic loosening and periprosthetic osteolysis results from long-term maladaptation of periprosthetic tissues to the presence of by-products continuously released from an artificial joint.

Regulation of NF-κB signaling in osteoclasts and myeloid progenitors

The transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is crucial for immune responses and skeletal development. Work in recent years has shown that various members of the NF-κB family are viable targets to regulate activity and survival of bone cells and hence bone metabolism. In this regard, deletion of upstream kinases or distal NF-κB subunits resulted with bone deformities. Thus, it has become increasingly apparent that detailed investigation of NF-κB in bone cells may provide opportunities to design new therapeutic modalities. In this chapter we present modified methodology describing efficient approaches to regulate the NF-κB pathway in vitro and in vivo to assess its function in bone cells and tissues.

Osteopetrosis in TAK1-deficient mice owing to defective NF-κB and NOTCH signaling

The MAP kinase TGFβ-activated kinase (TAK1) plays a crucial role in physiologic and pathologic cellular functions including cell survival, differentiation, apoptosis, inflammation, and oncogenesis. However, the entire repertoire of its mechanism of action has not been elucidated. Here, we found that ablation of Tak1 in myeloid cells causes osteopetrosis in mice as a result of defective osteoclastogenesis. Mechanistically, Tak1 deficiency correlated with increased NUMB-like (NUMBL) levels. Accordingly, forced expression of Numbl abrogated osteoclastogenesis whereas its deletion partially restored osteoclastogenesis and reversed the phenotype of Tak1 deficiency. Tak1 deletion also down-regulated Notch intracellular domain (NICD), but increased the levels of the transcription factor recombinant recognition sequence binding protein at Jκ site (RBPJ), consistent with NUMBL regulating notch signaling through degradation of NICD, a modulator of RBPJ. Accordingly, deletion of Rbpj partially corrected osteopetrosis in Tak1-deficient mice. Furthermore, expression of active IKK2 in RBPJ/TAK1-deficient cells significantly restored osteoclastogenesis, indicating that activation of NF-κB is essential for complete rescue of the pathway. Thus, we propose that TAK1 regulates osteoclastogenesis by integrating activation of NF-κB and derepression of NOTCH/RBPJ in myeloid cells through inhibition of NUMBL.

Novel biological strategies for treatment of wear particle-induced periprosthetic osteolysis of orthopaedic implants for joint replacement

Wear particles and by-products from joint replacements and other orthopaedic implants may result in a local chronic inflammatory and foreign body reaction. This may lead to persistent synovitis resulting in joint pain and swelling, periprosthetic osteolysis, implant loosening and pathologic fracture. Strategies to modulate the adverse effects of wear debris may improve the function and longevity of joint replacements and other orthopaedic implants, potentially delaying or avoiding complex revision surgical procedures. Three novel biological strategies to mitigate the chronic inflammatory reaction to orthopaedic wear particles are reported. These include (i) interference with systemic macrophage trafficking to the local implant site, (ii) modulation of macrophages from an M1 (pro-inflammatory) to an M2 (anti-inflammatory, pro-tissue healing) phenotype in the periprosthetic tissues, and (iii) local inhibition of the transcription factor nuclear factor kappa B (NF-κB) by delivery of an NF-κB decoy oligodeoxynucleotide, thereby interfering with the production of pro-inflammatory mediators. These three approaches have been shown to be viable strategies for mitigating the undesirable effects of wear particles in preclinical studies. Targeted local delivery of specific biologics may potentially extend the lifetime of orthopaedic implants.

Chronic inflammation in biomaterial-induced periprosthetic osteolysis: NF-κB as a therapeutic target

Biomaterial-induced tissue responses in patients with total joint replacement are associated with the generation of wear particles, which may lead to chronic inflammation and local bone destruction (periprosthetic osteolysis). Inflammatory reactions associated with wear particles are mediated by several important signaling pathways, the most important of which involves the transcription factor NF-κB. NF-κB activation is essential for macrophage recruitment and maturation, as well as the production of pro-inflammatory cytokines and chemokines such as TNF-α, IL-1β, IL-6 and MCP1. In addition, NF-κB activation contributes to osteoclast differentiation and maturation via RANK/RANKL signaling, which increases bone destruction and reduces bone formation. Targeting individual downstream cytokines directly (such as TNF-α or IL-1β) may not effectively prevent wear particle induced osteolysis. A more logical upstream therapeutic approach may be provided by direct modulation of the core IκB/IKKα/β/NF-κB signaling pathway in the local environment. However, the timing, dose and strategy for administration should be considered. Suppression of chronic inflammation via inhibition of NF-κB activity in patients with malfunctioning joint replacements may be an effective strategy to mitigate wear particle induced periprosthetic osteolysis.

Anti-oxidation treatment of ultra high molecular weight polyethylene components to decrease periprosthetic osteolysis: evaluation of osteolytic and osteogenic properties of wear debris particles in a murine calvaria model

Wear debris-induced osteolysis remains the greatest limitation of long-term success for total joint replacements with ultra-high molecular weight polyethylene (UHMWPE) bearings. To address oxidative degradation post-gamma irradiation, manufacturers are investigating the incorporation of antioxidants into PE resins. Similarly, larger molecular weight monomers have been developed to increase crosslinking and decrease wear debris, and ultimately osteolysis. However, the effects of modifying monomer size, crosslink density, and antioxidant incorporation on UHMWPE particle-induced osteoclastic bone resorption and coupled osteoblastic bone formation have never been tested. Here, we review the field of antioxidant-containing UHMWPE, and present an illustrative pilot study evaluating the osteolytic and osteogenic potential of wear debris generated from three chemically distinct particles (MARATHON®, XLK, and AOX™) as determined by a novel 3D micro-CT algorithm designed for the murine calvaria model. The results demonstrate an approach by which the potential osteoprotective effects of antioxidants in UHMWPE can be evaluated.

NF-κB signaling and bone resorption

The transcription factor NF-κB is a family of proteins involved in signaling pathways essential for normal cellular functions and development. Deletion of various components of this pathway resulted with abnormal skeletal development. Research in the last decade has established that NF-κB signaling mediates RANK ligand-induced osteoclastogenesis. Consistently, it was shown that inhibition of NF-κB was an effective approach to inhibit osteoclast formation and bone resorptive activity. Identification of the molecular machinery underlying NF-κB activation permitted osteoclast-specific deletion of the major components of this pathway. As a result, it was clear that deletion of members of the proximal IKK kinase complex and the distal NF-κB subunits and downstream regulators affected skeletal development. These studies provided several targets of therapeutic intervention in osteolytic diseases. NF-κB activity has been also described as the centerpiece of inflammatory responses and is considered a potent mediator of inflammatory osteolysis. Indeed, inflammatory insults exacerbate physiologic RANKL-induced NF-κB signals leading to exaggerated responses and to inflammatory osteolysis. These superimposed NF-κB activities appear to underlie several bone pathologies. This review will describe the individual roles of NF-κB molecules in bone resorption and inflammatory osteolysis.

Electroacupuncture Could Regulate the NF-κB Signaling Pathway to Ameliorate the Inflammatory Injury in Focal Cerebral Ischemia/Reperfusion Model Rats

The activated nuclear factor-KappaB signaling pathway plays a critical role in inducing inflammatory injury. It has been reported that electroacupuncture could be an effective anti-inflammatory treatment. We aimed to explore the complex mechanism by which EA inhibits the activation of the NF- κ B signal pathway and ameliorate inflammatory injury in the short term; the effects of NEMO Binding Domain peptide for this purpose were compared. Focal cerebral I/R was induced by middle cerebral artery occlusion for 2 hrs. Total 380 male Sprague-Dawley rats are in the study. The neurobehavioral scores, infarction volumes, and the levels of IL-1 β and IL-13 were detected. NF- κ B p65, I κ B α , IKK α , and IKK β were analyzed and the ability of NF- κ B binding DNA was investigated. The EA treatment and the NBD peptide treatment both reduced infarct size, improved neurological scores, and regulated the levels of IL-1 β and IL-13. The treatment reduced the expression of IKK α and IKK β and altered the expression of NF- κ B p65 and I κ B α in the cytoplasm and nucleus; the activity of NF- κ B was effectively reduced. We conclude that EA treatment might interfere with the process of NF- κ B nuclear translocation. And it also could suppress the activity of NF- κ B signaling pathway to ameliorate the inflammatory injury after focal cerebral ischemia/reperfusion.

Ubiquitin-like domain of IKKβ regulates osteoclastogenesis and osteolysis

The transcription factor NF-κB family is central for osteoclastogenesis and inflammatory osteolysis. Activation of NF-κB dimers is regulated by a kinase complex predominantly containing IKKα (IKK1), IKKβ (IKK2), and a regulatory subunit, IKKγ/NEMO. IKKα and IKKβ catalyze the cytoplasmic liberation and nuclear translocation of various NF-κB subunits. The requirement of IKKα and IKKβ for normal bone homeostasis has been established. Congruently, mice devoid of IKKα or IKKβ exhibit in vitro and in vivo defects in osteoclastogenesis, and IKKβ-null mice are refractory to inflammatory arthritis and osteolysis. To better understand the molecular mechanism underlying IKKβ function in bone homeostasis and bone pathologies, we conducted structure-function analysis to determine IKKβ functional domains in osteoclasts. IKKβ encompasses several domains, of which the ubiquitination-like domain (ULD) has been shown essential for IKKβ activation. In this study, we examined the role of ULD in IKKβ-mediated NF-κB activation in osteoclast precursors and its contribution to osteoclastogenesis and osteolysis. We generated and virally introduced IKKβ in which the ULD domain has been deleted (IKKβ∆ULD) into osteoclast progenitors. The results show that deletion of ULD diminishes IKKβ activity and that IKKβ∆ULD strongly inhibits osteoclastogenesis. In addition, unlike wild type (WT)-IKKβ, IKKβ∆ULD fail to restore RANKL-induced osteoclastogenesis by IKKβ-null precursors. Finally, we provide evidence that IKKβ∆ULD blocks inflammatory osteolysis in a model of murine calvarial osteolysis. Thus, we identified the ULD as crucial for IKKβ activity and osteoclastogenesis and found that ULD-deficient IKKβ is a potent inhibitor of osteoclastogenesis and osteolysis.

Blockade of JNK and NFAT pathways attenuates orthopedic particle-stimulated osteoclastogenesis of human osteoclast precursors and murine calvarial osteolysis

Particles released from orthopedic implants attract immune host defense cells to the bone-implant interface and contribute to development of inflammation. The inflammatory microenvironment supports recruitment and differentiation of osteoclasts, the primary culprit of osteolysis. Therefore, understanding the complex signals that contribute to osteoclastogenesis and osteolysis is a sensible approach to design strategies to inhibit bone loss. The signaling cascades that coordinate osteoclastogenesis have been widely investigated. These include MAP kinases, Akt/PI3K pathway, NF-κB signal transduction pathway, and NFAT pathway. We have recently reported that polymethylmethacrylate (PMMA) particles activate the NFAT pathway in murine osteoclast precursors and that NFAT inhibitors dose-dependently block PMMA-induced osteoclastogenesis. In the current study, we examined the role of JNK and NFATc1 in mice in response to PMMA particles using murine calvaria model. We show that locally administered MAPK/JNK inhibitor SP600125 and calcineurin/NFAT inhibitor cyclosporine-A effectively blocked PMMA-induced osteolysis in murine calvaria. To buttress the clinical relevance of JNK/NFATc1-based regulation of PMMA-induced osteoclastogenesis, we evaluated the effect of PMMA using human macrophages. We demonstrate that SP600125 and cyclosporine-A abolished particle-induced osteoclastogenesis in human osteoclast progenitors retrieved from patients undergoing total hip replacement. Thus JNK and NFATc1 appear to act as significant mediators of orthopedic particle-induced osteolysis in humans.

Lysine392, a K63-linked ubiquitination site in NEMO, mediates inflammatory osteoclastogenesis and osteolysis

PMMA particles released from bone implants are considered major contributor to osteolysis and subsequent implant failure. Although the ensuing inflammatory response has been described, the mechanisms underlying PMMA particulate-induced osteolysis remain enigmatic. In previous studies, we have established that activation of Nuclear factor kappa-B (NF-κB) and MAP kinase pathways plays a central role in the pathogenesis of inflammatory osteolysis. Specifically, we have shown that impeding IKK complex assembly, and thus subsequent NF-κB activation, dampens particle-induced osteolysis. The IKK complex consists of IKKα, IKKβ, and IKKγ, also known as NEMO. NEMO has no catalytic activity and serves as a scaffold protein facilitating assembly and distal activation of NF-κB signaling. In fact, blocking binding of NEMO with IKKα/β abolishes NF-κB activity. In the current study, we identify Lysine 392 residue in NEMO as crucial mediator of PMMA particle-induced inflammatory osteoclastogenesis and osteolysis. Using mice in which NEMO-K392R mutation has been introduced, we provide evidence that PMMA-induced osteoclasts and osteolytic responses are impaired. Furthermore, we show that this impairment is likely due to poor activation of NF-κB and Erk, but not other MAP kinases. Our findings suggest that NEMO Lysine392, a well-established K63-linked polyubiquitination site, is an important mediator of PMMA-induced osteolysis. Therefore, this NEMO motif should be considered as a target to combat PMMA particle-induced osteolysis.

Selective inhibition of nuclear factor-κB by nuclear factor-κB essential modulator-binding domain peptide suppresses the metastasis of highly metastatic oral squamous cell carcinoma

Nuclear factor-κB (NF-κB) activation contributes to the development of metastasis, thus leading to a poor prognosis in many cancers, including OSCC. However, little in vivo experimental data are available about the effects of NF-κB inhibition on OSCC metastasis. OSCC sublines were established from a GFP-expressing parental cell line, GSAS, and designated GSAS/N3 and N5 according to the in vivo passage number after cervical lymph node metastasis by a serial orthotopic transplantation model. In vitro migration and invasion were assessed in these cells, and the NF-κB activities and expression of NF-κB-regulated metastasis-related molecules were also examined. In in vivo experiments, the metastasis and survival of tumor-engrafted mice were monitored. Furthermore, the effects of a selective NF-κB inhibitor, NEMO-binding domain (NBD) peptide, on metastasis in GSAS/N5-engrafted mice were assessed, and engrafted tongue tumors were immunohistochemically examined. Highly metastatic GSAS/N3 and N5 cells showed an enhanced NF-κB activity, thus contributing to increased migration, invasion, and a poor prognosis compared with the parent cells. Furthermore, the expression levels of NF-κB-regulated metastasis-related molecules, such as fibronectin, β1 integrin, MMP-1, -2, -9, and -14, and VEGF-C, were upregulated in the highly metastatic cells. The NBD peptide suppressed metastasis and tongue tumor growth in GSAS/N5-inoculated mice, and was accompanied by the downregulation of the NF-κB-regulated metastasis-related molecules in engrafted tongue tumors. Our results suggest that the selective inhibition of NF-κB activation by NBD peptide may provide an effective approach for the treatment of highly metastatic OSCC.

Evidence that the kinase-truncated c-Src regulates NF-κB signaling by targeting NEMO

The tyrosine kinase c-Src and transcription factor NF-κB are considered crucial components required for normal osteoclastogenesis. Genetic ablation of either pathway leads to detrimental osteopetrotic phenotypes in mice. Similarly, obstruction of either pathway halts osteoclastogenesis and lessens various forms of bone loss. It has been shown previously that mice expressing a kinase domain-truncated c-Src, termed Src251, develop severe osteopetrosis owing to increased osteoclast apoptosis. It was further suggested that this phenomenon is associated with reduced Akt kinase activity. However, the precise mechanism underlying the osteoclast inhibitory effect of Src251 remains obscure. C-Src associates with TRAF6-p62 interacting with receptor activator of NF-κB (RANK) distal region and the complex facilitate activation of RANK down stream signal transduction cascades including NF-κB. Given this proximity between c-Src and NF-κB signaling in osteoclasts, we surmised that inhibition of osteoclastogenesis by Src251 may be achieved through inhibition of NF-κB signaling. We have demonstrated recently that NEMO, the regulatory subunit of the IKK complex, is crucial for osteoclastogenesis and interacts with c-Src in osteoclast progenitors. Transfection studies, in which we employed various forms of c-Src and NEMO, revealed that the dominant negative form of c-Src, namely Src251, mediates degradation of NEMO thus halting NF-κB signaling. Furthermore, degradation of NEMO requires its intact zinc finger domain which is located at the ubiquitination domain. This process also requires appropriate cellular localization of Src251, since deletion of its myristoylation domain ablates its degradation capacity. Buttressing these findings, the expression of NEMO and NF-κB signaling were significantly reduced in monocytes collected from Src251 transgenic mice.

Polyubiquitination events mediate polymethylmethacrylate (PMMA) particle activation of NF-kappaB pathway

The pathologic response to implant wear-debris constitutes a major component of inflammatory osteolysis and remains under intense investigation. Polymethylmethacrylate (PMMA) particles, which are released during implant wear and loosening, constitute a major culprit by virtue of inducing inflammatory and osteolytic responses by macrophages and osteoclasts, respectively. Recent work by several groups has identified important cellular entities and secreted factors that contribute to inflammatory osteolysis. In previous work, we have shown that PMMA particles contribute to inflammatory osteolysis through stimulation of major pathways in monocytes/macrophages, primarily NF-κB and MAP kinases. The former pathway requires assembly of large IKK complex encompassing IKK1, IKK2, and IKKγ/NEMO. We have shown recently that interfering with the NF-κB and MAPK activation pathways, through introduction of inhibitors and decoy molecules, impedes PMMA-induced inflammation and osteolysis in mouse models of experimental calvarial osteolysis and inflammatory arthritis. In this study, we report that PMMA particles activate the upstream transforming growth factor β-activated kinase-1 (TAK1), which is a key regulator of signal transduction cascades leading to activation of NF-κB and AP-1 factors. More importantly, we found that PMMA particles induce TAK1 binding to NEMO and UBC13. In addition, we show that PMMA particles induce TRAF6 and UBC13 binding to NEMO and that lack of TRAF6 significantly attenuates NEMO ubiquitination. Altogether, these observations suggest that PMMA particles induce ubiquitination of NEMO, an event likely mediated by TRAF6, TAK1, and UBC13. Our findings provide important information for better understanding of the mechanisms underlying PMMA particle-induced inflammatory responses.

Peptide-based inhibition of NF-κB rescues diaphragm muscle contractile dysfunction in a murine model of Duchenne muscular dystrophy

Deterioration of diaphragm function is one of the prominent factors that contributes to the susceptibility of serious respiratory infections and development of respiratory failure in patients with Duchenne Muscular Dystrophy (DMD). The NF-κB signaling pathway has been implicated as a contributing factor of dystrophic pathology, making it a potential therapeutic target. Previously, we demonstrated that pharmacological inhibition of NF-κB via a small NEMO Binding Domain (NBD) peptide was beneficial for reducing pathological features of mdx mice. Now, we stringently test the effectiveness and clinical potential of NBD by treating mdx mice with various formulations of NBD and use diaphragm function as our primary outcome criteria. We found that administering DMSO-soluble NBD rescued 78% of the contractile deficit between mdx and wild-type (WT) diaphragm. Interestingly, synthesis of a GLP NBD peptide as an acetate salt permitted its solubility in water, but as a negative consequence, also greatly attenuated functional efficacy. However, replacing the acetic acid counterion of the NBD peptide with trifluoroacetic acid retained the peptide's water solubility and significantly restored mdx diaphragm contractile function and improved histopathological indices of disease in both diaphragm and limb muscle. Together, these results support the feasibility of using a mass-produced, water-soluble NBD peptide for clinical use.

In vitro reactivity to implant metals demonstrates a person-dependent association with both T-cell and B-cell activation

Hypersensitivity to metallic implants remains relatively unpredictable and poorly understood. We initially hypothesized that metal-induced lymphocyte proliferation responses to soluble metal challenge (ions) are mediated exclusively by early T-cell activation (not B-cells), typical of a delayed-type-hypersensitivity response. We tested this by comparing proliferation (6 days) of primary lymphocytes with early T-cell and B-cell activation (48 h) in three groups of subjects likely to demonstrate elevated metal reactivity: group 1 (n = 12) history of metal sensitivity with no implant; group 2a (n = 6) well performing metal-on-metal THRs, and group 2b (n = 20) subjects with poorly performing metal-on-polymer total joint arthroplasties (TJA). Group 1 showed 100% (12/12) metal reactivity (stimulation index > 2) to Ni. Groups 2a and 2b were 83% (5/6) and 75% (15/22) metal reactive (to Co, Cr, or Ni), respectively. Of the n = 32 metal-reactive subjects to Co, Cr, or Ni (SI > 2), n = 22/32 demonstrated >2-fold elevations in % of T-cell or B-cell activation (CD25+, CD69+) to metal challenge when compared with untreated control. 18/22 metal-activated subjects demonstrated an exclusively T-cell or B-cell activation response to metal challenge, where 6/18 demonstrated exclusively B-cell activation and 12/18 demonstrated a T-cell only response, as measured by surface activation markers CD25+ and CD69+. However, there was no direct correlation (R(2) < 0.1) between lymphocyte proliferation and % T-cell or B-cell activation (CD25+:CD69+). Proliferation assays (LTT) showed greater ability to detect metal reactivity than did subject-dependent results of flow-cytometry analysis of T-cell or B-cell activation. The high incidence of lymphocyte reactivity and activation indicate that more complex than initially hypothesized immune responses may contribute to the etiology of debris-induced osteolysis in metal-sensitive individuals.

A review of the biologic effects of spine implant debris: Fact from fiction

BACKGROUND

Biologic-reactivity to implant-debris is the primary determinant of long-term clinical performance. The following reviews: 1) the physical aspects of spinal-implant debris and 2) the local and systemic biologic responses to implant debris.

METHODS

Methods included are: 1) gravimetric wear analysis; 2) SEM and LALLS; 3) metal-ion analysis; 4) ELISA, toxicity testing, patch testing; and 5) metal-lymphocyte transformation testing (metal-LTT).

RESULTS

Wear and corrosion of spine-implants produce particles and ions. Particles (0.01-1000 μm) are generally submicron ( <1 µm). Wear rates of metal-on-polymer and metal-on-metal disc arthroplasties are approximately 2-20 and 1 mm(3)/yr, respectively. Metal-on-metal total disc replacement components have significant increases in circulating metal (less than 10-fold that of controls at 4 ppb-Co and 3 ppb-Cr or ng/mL). Debris reactivity is local and systemic. Local inflammation is caused primarily by ingestion of debris by local macrophages, which produce pro-inflammatory cytokines TNFα, IL-1β, IL-6, and PGE2. Systemic responses associated with implant-debris have been limited to hypersensitivity reactions. Elevated amounts of in the liver, spleen, etc of patients with failed TJA have not been associated with remote toxicological or carcinogenic pathology to date. Implant debris are differentially bioreactive. Greater numbers are pro-inflammatory; the smaller-sized debris are more bioreactive by virtue of their greater numbers (dose) for a given amount of implant mass loss (one 100-μm-diameter particle is equivalent in mass to 1 million 1-μm-diameter particles). Elongated particles are pro-inflammatory (ie, aspect ratio of greater than 3). Metal particles are more proinflammatory than polymers, ceteris paribus.

CONCLUSION

Spinal arthroplasty designs have been in use for more than 20 years internationally; therefore, concerns about neuropathology, toxicity, and carcinogenicity are mitigated. Debris-induced inflammation still depends on the individual and the type of debris. The consequence of debris-induced inflammation is continued; vigilance by physicians is recommended monitoring of spinal implants using physical exams and testing of metal content and bioreactivity, as is planning for the likelihood of revision in younger individuals.

Analysis of bone mineral density and bone turnover in the presence of polymethylmethacrylate particles

Polymethylmethacrylate (PMMA) particles generated from joint arthroplasties appear to contribute to aseptic implant loosening through inflammation-induced periprosthetic osteolysis. However, osteolysis appears to be multifactorial; whether a direct link exists between PMMA particles and osteolysis in vivo is unproven. With the aim to define the relationship between PMMA particles and osteolysis, the authors analyzed the bone mineral density, using microCT scans preoperatively, the first day postoperatively and then every 7-10 days for 32 days, and bone turnover, using (18)F-fluoride positron emission tomography scanner (PET scan) at 8 weeks in four groups of mice that had undergone intramedullary femoral injection. The experimental group of five mice was injected with PMMA particles, and compared with two negative control groups (no injection and injection with the carrier, phosphate-buffered saline) and one positive control group (injection of PMMA particles contaminated with endotoxin). There was no significant change in bone mineral density with addition of PMMA particles, and no evidence of osteolysis. However, bone turnover was increased in the presence of PMMA particles. Even though a direct link between PMMA particles and osteolysis was not found in the short term, PMMA particles appear to influence the regenerative capacity of bone.

NFAT2 is an essential mediator of orthopedic particle-induced osteoclastogenesis

Particle-induced periprosthetic osteolysis is the major cause for orthopedic implant failure. This failure is mediated mainly by the action of osteoclasts, the principal cells responsible for bone resorption and osteolysis. Therapeutic interventions to alleviate osteolysis have been focused on understanding and targeting mechanisms of osteoclastogenesis. The nuclear transcription factor NFAT is an essential terminal differentiation factor of osteoclastogenesis. This transcription factor is known to cooperate with c-jun/AP-1 in mediating RANKL-induced osteoclastogenesis. We have previously determined that RANKL is an essential cytokine mediator of particle-induced osteoclastogenesis, and that PMMA particles activate JNK and c-jun/AP-1 in bone marrow macrophages (osteoclast precursors). In the current study, we investigated the effect of PMMA particles on the NFAT signaling pathway in osteoclast precursor cells. Our findings point out that PMMA particles stimulate nuclear translocation of NFAT2 in wild-type osteoclast precursors, which is associated with increased osteoclastogenesis. More importantly, induction of osteoclastogenesis was selectively blocked in a dose-dependent fashion by the calcineurin inhibitors, Cyclosporine-A and FK506. Further, this activation was also blocked in a time-dependent fashion by the NFAT inhibitor VIVIT. Finally, we provide novel evidence that PMMA particles induce binding of NFAT2 and AP-1 proteins. Thus, our findings demonstrate that activation of the NFAT pathway in conjunction with MAP kinases is essential for basal and PMMA-stimulated osteoclastogenesis.

Cell penetrating peptide inhibitors of nuclear factor-kappa B

The nuclear factor kappa B (NF-kappaB) transcription factors are activated by a range of stimuli including pro-inflammatory cytokines. Active NF-kappaB regulates the expression of genes involved in inflammation and cell survival and aberrant NF-kappaB activity plays pathological roles in certain types of cancer and diseases characterized by chronic inflammation. NF-kappaB signaling is an attractive target for the development of novel anti-inflammatory or anti-cancer drugs and we discuss here how the method of peptide transduction has been used to specifically target NF-kappaB. Peptide transduction relies on the ability of certain small cell-penetrating peptides (CPPs) to enter cells, and a panel of CPP-linked inhibitors (CPP-Is) has been developed to directly inhibit NF-kappaB signaling. Remarkably, several of these NF-kappaB-targeting CPP-Is are effective in vivo and therefore offer exciting potential in the clinical setting.

The IkappaB kinase complex: master regulator of NF-kappaB signaling

The Nuclear Factor-kappa B (NF-kappaB) family of transcription factors regulates the expression of a wide range of genes critical for immune and inflammatory responses, cell survival, immune development, and cell proliferation. Dysregulated NF-kappaB activity occurs in a number of chronic inflammatory diseases and certain types of cancers making NF-kappaB signaling an attractive target for the development of anti-inflammatory and anti-cancer drugs. A pivotal regulator of all inducible NF-kappaB signaling pathways is the IkappaB kinase (IKK) complex that consists of two kinases (IKKalpha and IKKbeta) and a regulatory subunit named NF-kappaB essential modulator (NEMO). Genetic analysis of the IKK complex has identified two separate pathways named the classical and non-canonical mechanisms that are dependent on either NEMO and IKKbeta (classical) or IKKalpha alone (non-canonical). To better understand the mechanisms that regulate IKK complex activity and to address the differential functions of IKKalpha and IKKbeta we have molecularly dissected the IKKs. We describe here how these studies have identified a unique inhibitor of pro-inflammatory NF-kappaB signaling, an unforeseen role for IKKalpha in the classical NF-kappaB pathway, and a novel functional domain in IKKbeta that is not present in IKKalpha.