NIK stabilization in osteoclasts results in osteoporosis and enhanced inflammatory osteolysis

From Stem to Sternum: The Role of Shp2 in the Skeleton

Src homology-2 domain-containing phosphatase 2 (SHP2) is a ubiquitously expressed phosphatase that is vital for skeletal development and maintenance of chondrocytes, osteoblasts, and osteoclasts. Study of SHP2 function in small animal models has led to insights in phenotypes observed in SHP2-mutant human disease, such as Noonan syndrome. In recent years, allosteric SHP2 inhibitors have been developed to specifically target the protein in neoplastic processes. These inhibitors are highly specific and have great potential for disease modulation in cancer and other pathologies, including bone disorders. In this review, we discuss the importance of SHP2 and related signaling pathways (e.g., Ras/MEK/ERK, JAK/STAT, PI3K/Akt) in skeletal development. We review rodent models of pathologic processes caused by germline mutations that activate SHP2 enzymatic activity, with a focus on the skeletal phenotype seen in these patients. Finally, we discuss SHP2 inhibitors in development and their potential for disease modulation in these genetic diseases, particularly as it relates to the skeleton.

HDAC5-Mediated Acetylation of p100 Suppresses Its Processing

INTRODUCTION

Periodontitis is a condition involving chronic inflammation in the gums, periodontal ligaments, cementum, and alveolar bone. Nuclear factor-κB (NF-κB) activation is the prominent mediator of inflammation and osteoclast differentiation. The role of histone deacetylase 5 (HDAC5) in periodontitis development and NF-κB regulation is not fully understood.

METHODS

We used primary mouse bone marrow-derived osteoclast cultures in vitro and a mouse model of chronic periodontists (CPD) treated with the HDAC4/5 inhibitor LMK-235. Real-time polymerase chain reaction, micro computed tomography, flow cytometry, western blot, and immunoprecipitation were used to study proinflammatory cytokines, NF-κB activation, HDAC5 activity, and the interaction of HDAC5 with NF-κB p100.

RESULTS

LMK-235, a selective inhibitor of HDAC4 and HDAC5, reduced osteoclast marker gene expression (Cstk, Acp5, and Calcr) and tartrate-resistant acid phosphatase activity in primary osteoclast cultures. LMK-235 reduced the increase in cementoenamel junction-alveolar bone crest distance, inflammatory cell infiltration of gingival tissues, and expression levels of interleukin (IL)-1β, tumor necrosis factor alpha, IL-6, and IL-23a, indicating an ameliorative effect on CPD. Immunoprecipitation experiments have further confirmed p100-HDAC5 interaction, acetylation levels of p100, and NF-κB activation.

CONCLUSIONS

These results indicate that HDAC5 binds and deacetylates p100, leading to its activation, increased proinflammatory cytokine production, gingival infiltration, and osteoclast differentiation, thus promoting alveolar bone resorption. HDAC5 inhibition is therefore a potentially promising therapeutic strategy for the treatment of periodontitis.

Regulation of osteoclast-mediated bone resorption by microRNA

Osteoclast-mediated bone resorption is responsible for bone metabolic diseases, negatively impacting people's health and life. It has been demonstrated that microRNA influences the differentiation of osteoclasts by regulating the signaling pathways during osteoclast-mediated bone resorption. So far, the involved mechanisms have not been fully elucidated. This review introduced the pathways involved in osteoclastogenesis and summarized the related microRNAs binding to their specific targets to mediate the downstream pathways in osteoclast-mediated bone resorption. We also discuss the clinical potential of targeting microRNAs to treat osteoclast-mediated bone resorption as well as the challenges of avoiding potential side effects and producing efficient delivery methods.

Plumbagin is a NF-κB-inducing kinase inhibitor with dual anabolic and antiresorptive effects that prevents menopausal-related osteoporosis in mice

Osteoporosis is caused by enhanced bone resorption and relatively reduced bone formation. There is an unmet need to develop new agents with both antiresorptive and anabolic effects to treat osteoporosis, although drugs with either effect alone are available. A small molecular compound, plumbagin, was reported to inhibit receptor activator of nuclear factor kappa-B ligand-induced osteoclast (OC) differentiation by inhibiting IκBα phosphorylation-mediated canonical NF-κB activation. However, the key transcriptional factor RelA/p65 in canonical NF-κB pathway functions to promote OC precursor survival but not terminal OC differentiation. Here, we found that plumbagin inhibited the activity of NF-κB inducing kinase, the key molecule that controls noncanonical NF-κB signaling, in an ATP/ADP-based kinase assay. Consistent with this, plumbagin inhibited processing of NF-κB2 p100 to p52 in the progenitor cells of both OCs and osteoblasts (OBs). Interestingly, plumbagin not only inhibited OC but also stimulated OB differentiation in vitro. Importantly, plumbagin prevented trabecular bone loss in ovariectomized mice. This was associated with decreased OC surfaces on trabecular surface and increased parameters of OBs, including OB surface on trabecular surface, bone formation rate, and level of serum osteocalcin, compared to vehicle-treated mice. In summary, we conclude that plumbagin is a NF-κB-inducing kinase inhibitor with dual anabolic and antiresorptive effects on bone and could represent a new class of agent for the prevention and treatment of osteoporosis.

Conditional loss of IKKα in Osterix + cells has no effect on bone but leads to age-related loss of peripheral fat

NF-κB has been reported to both promote and inhibit bone formation. To explore its role in osteolineage cells, we conditionally deleted IKKα, an upstream kinase required for non-canonical NF-κB activation, using Osterix (Osx)-Cre. Surprisingly, we found no effect on either cancellous or cortical bone, even following mechanical loading. However, we noted that IKKα conditional knockout (cKO) mice began to lose body weight after 6 months of age with severe reductions in fat mass and lower adipocyte size in geriatric animals. qPCR analysis of adipogenic markers in fat pads of cKO mice indicated no difference in early differentiation, but instead markedly lower leptin with age. We challenged young mice with a high fat diet finding that cKO mice gained less weight and showed improved glucose metabolism. Low levels of recombination at the IKKα locus were detected in fat pads isolated from old cKO mice. To determine whether recombination occurs in adipocytes, we examined fat pads in Osx-Cre;TdT reporter mice; these showed increasing Osx-Cre-mediated expression in peripheral adipocytes from 6 weeks to 18 months. Since Osx-Cre drives recombination in peripheral adipocytes with age, we conclude that fat loss in cKO mice is most likely caused by progressive deficits of IKKα in adipocytes.

NF-κB signaling in inflammation and cancer

Since nuclear factor of κ-light chain of enhancer-activated B cells (NF-κB) was discovered in 1986, extraordinary efforts have been made to understand the function and regulating mechanism of NF-κB for 35 years, which lead to significant progress. Meanwhile, the molecular mechanisms regulating NF-κB activation have also been illuminated, the cascades of signaling events leading to NF-κB activity and key components of the NF-κB pathway are also identified. It has been suggested NF-κB plays an important role in human diseases, especially inflammation-related diseases. These studies make the NF-κB an attractive target for disease treatment. This review aims to summarize the knowledge of the family members of NF-κB, as well as the basic mechanisms of NF-κB signaling pathway activation. We will also review the effects of dysregulated NF-κB on inflammation, tumorigenesis, and tumor microenvironment. The progression of the translational study and drug development targeting NF-κB for inflammatory diseases and cancer treatment and the potential obstacles will be discussed. Further investigations on the precise functions of NF-κB in the physiological and pathological settings and underlying mechanisms are in the urgent need to develop drugs targeting NF-κB for inflammatory diseases and cancer treatment, with minimal side effects.

Constitutive activation of NF-κB inducing kinase (NIK) in the mesenchymal lineage using Osterix (Sp7)- or Fibroblast-specific protein 1 (S100a4)-Cre drives spontaneous soft tissue sarcoma

Aberrant NF-κB signaling fuels tumor growth in multiple human cancer types including both hematologic and solid malignancies. Chronic elevated alternative NF-κB signaling can be modeled in transgenic mice upon activation of a conditional NF-κB-inducing kinase (NIK) allele lacking the regulatory TRAF3 binding domain (NT3). Here, we report that expression of NT3 in the mesenchymal lineage with Osterix (Osx/Sp7)-Cre or Fibroblast-Specific Protein 1 (FSP1)-Cre caused subcutaneous, soft tissue tumors. These tumors displayed significantly shorter latency and a greater multiple incidence rate in Fsp1-Cre;NT3 compared to Osx-Cre;NT3 mice, regardless of sex. Histological assessment revealed poorly differentiated solid tumors with some spindled patterns, as well as robust RelB immunostaining, confirming activation of alternative NF-κB. Even though NT3 expression also occurs in the osteolineage in Osx-Cre;NT3 mice, we observed no bony lesions. The staining profiles and pattern of Cre expression in the two lines pointed to a mesenchymal tumor origin. Immunohistochemistry revealed that these tumors stain strongly for alpha-smooth muscle actin (αSMA), although vimentin staining was uniform only in Osx-Cre;NT3 tumors. Negative CD45 and S100 immunostains precluded hematopoietic and melanocytic origins, respectively, while positive staining for cytokeratin 19 (CK19), typically associated with epithelia, was found in subpopulations of both tumors. Principal component, differential expression, and gene ontology analyses revealed that NT3 tumors are distinct from normal mesenchymal tissues and are enriched for NF-κB related biological processes. We conclude that constitutive activation of the alternative NF-κB pathway in the mesenchymal lineage drives spontaneous sarcoma and provides a novel mouse model for NF-κB related sarcomas.

Targeting NF-κB-Inducing Kinase (NIK) in Immunity, Inflammation, and Cancer

NF-κB-inducing kinase (NIK), the essential upstream kinase, which regulates activation of the noncanonical NF-κB pathway, has important roles in regulating immunity and inflammation. In addition, NIK is vital for maintaining cellular health through its control of fundamental cellular processes, including differentiation, growth, and cell survival. As such aberrant expression or regulation of NIK is associated with several disease states. For example, loss of NIK leads to severe immune defects, while the overexpression of NIK is observed in inflammatory diseases, metabolic disorders, and the development and progression of cancer. This review discusses recent studies investigating the therapeutic potential of NIK inhibitors in various diseases.

miR-346-3p promotes osteoclastogenesis via inhibiting TRAF3 gene

MicroRNAs (miRNAs) modulate gene expression and regulate many physiological and pathological conditions. However, their modulation and effect in osteoclastogenesis remain unknown. In this study, we investigated the role of miR-346-3p in regulating the osteoclast differentiation from RAW264.7 cells. We used the miRNA microarray assay, miR-346-3p mimic transfection, tartrate resistant acid phosphatase (TRAP) staining, bone resorption assay, qRT-PCR, and western blot. Our results showed that the expression of miR-346-3p was significantly upregulated during osteoclast differentiation. Further, by transfecting cells with miR-346-3p mimic, we observed an increased number of TRAP-positive multinucleated cells, increased pit area caused by bone resorption, and enhanced expression of osteoclast-specific genes and proteins. Conversely, miR-346-3p inhibition attenuated the osteoclast differentiation and function. Software-mediated prediction and validation using luciferase reporter assay showed that TRAF3, a negative regulator of osteoclast differentiation, was inhibited by miR-346-3p overexpression. Our results showed that miR-346-3p directly targeted TRAF3 mRNA via binding to its 3'-UTR and inhibited the expression of TRAF3 protein. Taken together, our results revealed that miR-346-3p promotes the regulation of osteoclastogenesis by suppressing the TRAF3 gene. In conclusion, miR-346-3p could be a novel therapeutic target for bone loss-related pathogenesis.

A novel inhibitor of NF-κB-inducing kinase prevents bone loss by inhibiting osteoclastic bone resorption in ovariectomized mice

Musculoskeletal diseases and disorders, including osteoporosis and rheumatoid arthritis are diseases that threaten a healthy life expectancy, and in order to extend the healthy life expectancy of elderly people, it is important to prevent bone and joint diseases and disorders. We previously reported that alymphoplasia (aly/aly) mice, which have a loss-of-function mutation in the Nik gene involved in the processing of p100 to p52 in the alternative NF-κB pathway, show mild osteopetrosis with a decrease in the osteoclast number, suggesting that the alternative NF-κB pathway is a potential drug target for ameliorating bone diseases. Recently, the novel NF-κB-inducing kinase (NIK)-specific inhibitor compound 33 (Cpd33) was developed, and we examined its effect on osteoclastic bone resorption in vitro and in vivo. Cpd33 inhibited the receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis accompanied by a decrease in the expression of nfatc1, dc-stamp, and cathepsin K, markers of osteoclast differentiation, without affecting the cell viability, in a dose-dependent manner. Cdp33 specifically suppressed the RANKL-induced processing of p100 to p52 but not the phosphorylation of p65 or the degradation or resynthesis of IκBα in osteoclast precursors. Cpd33 also suppressed the bone-resorbing activity in mature osteoclasts. Furthermore, Cdp33 treatment prevented bone loss by suppressing the osteoclast formation without affecting the osteoblastic bone formation in ovariectomized mice. Taken together, NIK inhibitors may be a new option for patients with a reduced response to conventional pharmacotherapy or who have serious side effects.

The Role of NF-κB in Physiological Bone Development and Inflammatory Bone Diseases: Is NF-κB Inhibition "Killing Two Birds with One Stone"?

Nuclear factor-κB (NF-κB) is a transcription factor that regulates the expression of various genes involved in inflammation and the immune response. The activation of NF-κB occurs via two pathways: inflammatory cytokines, such as TNF-α and IL-1β, activate the "classical pathway", and cytokines involved in lymph node formation, such as CD40L, activate the "alternative pathway". NF-κB1 (p50) and NF-κB2 (p52) double-knockout mice exhibited severe osteopetrosis due to the total lack of osteoclasts, suggesting that NF-κB activation is required for osteoclast differentiation. These results indicate that NF-κB may be a therapeutic target for inflammatory bone diseases, such as rheumatoid arthritis and periodontal disease. On the other hand, mice that express the dominant negative form of IκB kinase (IKK)-β specifically in osteoblasts exhibited increased bone mass, but there was no change in osteoclast numbers. Therefore, inhibition of NF-κB is thought to promote bone formation. Taken together, the inhibition of NF-κB leads to "killing two birds with one stone": it suppresses bone resorption and promotes bone formation. This review describes the role of NF-κB in physiological bone metabolism, pathologic bone destruction, and bone regeneration.

NF-κB Signaling Regulates Physiological and Pathological Chondrogenesis

The nuclear factor-κB (NF-κB) is a transcription factor that regulates the expression of genes that control cell proliferation and apoptosis, as well as genes that respond to inflammation and immune responses. There are two means of NF-κB activation: the classical pathway, which involves the degradation of the inhibitor of κBα (IκBα), and the alternative pathway, which involves the NF-κB-inducing kinase (NIK, also known as MAP3K14). The mouse growth plate consists of the resting zone, proliferative zone, prehypertrophic zone, and hypertrophic zone. The p65 (RelA), which plays a central role in the classical pathway, is expressed throughout the cartilage layer, from the resting zone to the hypertrophic zone. Inhibiting the classical NF-κB signaling pathway blocks growth hormone (GH) or insulin-like growth factor (IGF-1) signaling, suppresses cell proliferation, and suppresses bone morphogenetic protein 2 (BMP2) expression, thereby promoting apoptosis. Since the production of autoantibodies and inflammatory cytokines, such as tumor necrosis factor-α (TNFα), interleukin (IL)-1β, IL-6, and IL-17, are regulated by the classical pathways and are increased in rheumatoid arthritis (RA), NF-κB inhibitors are used to suppress inflammation and joint destruction in RA models. In osteoarthritis (OA) models, the strength of NF-κB-activation is found to regulate the facilitation or suppression of OA. On the other hand, RelB is involved in the alternative pathway, and is expressed in the periarticular zone during the embryonic period of development. The alternative pathway is involved in the generation of chondrocytes in the proliferative zone during physiological conditions, and in the development of RA and OA during pathological conditions. Thus, NF-κB is an important molecule that controls normal development and the pathological destruction of cartilage.

Conditional Activation of NF-κB Inducing Kinase (NIK) in the Osteolineage Enhances Both Basal and Loading-Induced Bone Formation

Studies from global loss-of-function mutants suggest that alternative NF-κB downstream of NF-κB inducing kinase (NIK) is a cell-intrinsic negative regulator of osteogenesis. However, the interpretation of the osteoblast and/or osteocyte contribution to the bone phenotype is complicated by simultaneous osteoclast defects in these models. Therefore, we turned to a transgenic mouse model to investigate the direct role of NIK in the osteolineage. Osx-Cre;NT3 animals (NT3-Cre +), which bear a constitutively active NIK allele (NT3) driven by Osx-Cre, were compared with their Cre-negative, Control (Ctrl) littermates. NT3-Cre + mice had elevated serum P1NP and CTX levels. Despite this high turnover state, µCT showed that constitutive activation of NIK resulted in a net increase in basal bone mass in both cortical and cancellous compartments. Furthermore, NT3-Cre + mice exhibited a greater anabolic response following mechanical loading compared with controls. We next performed RNA-Seq on nonloaded and loaded tibias to elucidate possible mechanisms underlying the increased bone anabolism seen in NT3-Cre + mice. Hierarchical clustering revealed two main transcriptional programs: one loading-responsive and the other NT3 transgene-driven. Gene ontology (GO) analysis indicated a distinct upregulation of receptor, kinase, and growth factor activities including Wnts, as well as a calcium-response signature in NT3-Cre + limbs. The promoters of these GO-term associated genes, including many known to be bone-anabolic, were highly enriched for multiple κB recognition elements (κB-RE) relative to the background frequency in the genome. The loading response in NT3-Cre + mice substantially overlapped (>90%) with Ctrl. Surprisingly, control animals had 10-fold more DEGs in response to loading. However, most top DEGs shared between genotypes had a high incidence of multiple κB-RE in their promoters. Therefore, both transcriptional programs (loading-responsive and NT3 transgene-driven) are modulated by NF-κB. Our studies uncover a previously unrecognized role for NF-κB in the promotion of both basal and mechanically stimulated bone formation. © 2019 American Society for Bone and Mineral Research.

Staphylococcus aureus Infects Osteoclasts and Replicates Intracellularly

The inflammation of bone tissue is called osteomyelitis, and most cases are caused by an infection with the bacterium Staphylococcus aureus. To date, the bone-building cells, osteoblasts, have been implicated in the progression of these infections, but not much is known about how the bone-resorbing cells, osteoclasts, participate. In this study, we show that S. aureus can infect osteoclasts and proliferate inside these cells, whereas bone-residing macrophages, immune cells related to osteoclasts, destroy the bacteria. These findings elucidate a unique role for osteoclasts to harbor bacteria during infection, providing a possible mechanism by which bacteria could evade destruction by the immune system.

Osteomyelitis (OM), or inflammation of bone tissue, occurs most frequently as a result of bacterial infection and severely perturbs bone structure. OM is predominantly caused by Staphylococcus aureus, and even with proper treatment, OM has a high rate of recurrence and chronicity. While S. aureus has been shown to infect osteoblasts, it remains unclear whether osteoclasts (OCs) are also a target of intracellular infection. Here, we demonstrate the ability of S. aureus to intracellularly infect and divide within OCs. OCs were differentiated from bone marrow macrophages (BMMs) by exposure to receptor activator of nuclear factor kappa-B ligand (RANKL). By utilizing an intracellular survival assay and flow cytometry, we found that at 18 h postinfection the intracellular burden of S. aureus increased dramatically in cells with at least 2 days of RANKL exposure, while the bacterial burden decreased in BMMs. To further explore the signals downstream of RANKL, we manipulated factors controlling OC differentiation, NFATc1 and alternative NF-κB, and found that intracellular bacterial growth correlates with NFATc1 levels in RANKL-treated cells. Confocal and time-lapse microscopy in mature OCs showed a range of intracellular infection that correlated inversely with S. aureus-phagolysosome colocalization. The propensity of OCs to become infected, paired with their diminished bactericidal capacity compared to BMMs, could promote OM progression by allowing S. aureus to evade initial immune regulation and proliferate at the periphery of lesions where OCs are most abundant.

Bone Remodeling and the Role of TRAF3 in Osteoclastic Bone Resorption

Skeletal health is maintained by bone remodeling, a process in which microscopic sites of effete or damaged bone are degraded on bone surfaces by osteoclasts and subsequently replaced by new bone, which is laid down by osteoblasts. This normal process can be disturbed in a variety of pathologic processes, including localized or generalized inflammation, metabolic and endocrine disorders, primary and metastatic cancers, and during aging as a result of low-grade chronic inflammation. Osteoclast formation and activity are promoted by factors, including cytokines, hormones, growth factors, and free radicals, and require expression of macrophage-colony stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) by accessory cells in the bone marrow, including osteoblastic and immune cells. Expression of TNF receptor-associated factor 6 (TRAF6) is required in osteoclast precursors to mediate RANKL-induced activation of NF-κB, which is also necessary for osteoclast formation and activity. TRAF3, in contrast is not required for osteoclast formation, but it limits RANKL-induced osteoclast formation by promoting proteasomal degradation of NF-κB-inducing kinase in a complex with TRAF2 and cellular inhibitor of apoptosis proteins (cIAP). TRAF3 also limits osteoclast formation induced by TNF, which mediates inflammation and joint destruction in inflammatory diseases, including rheumatoid arthritis. Chloroquine and hydroxychloroquine, anti-inflammatory drugs used to treat rheumatoid arthritis, prevent TRAF3 degradation in osteoclast precursors and inhibit osteoclast formation in vitro. Chloroquine also inhibits bone destruction induced by ovariectomy and parathyroid hormone in mice in vivo. Mice genetically engineered to have TRAF3 deleted in osteoclast precursors and macrophages develop early onset osteoporosis, inflammation in multiple tissues, infections, and tumors, indicating that TRAF3 suppresses inflammation and tumors in myeloid cells. Mice with TRAF3 conditionally deleted in mesenchymal cells also develop early onset osteoporosis due to a combination of increased osteoclast formation and reduced osteoblast formation. TRAF3 protein levels decrease in bone and bone marrow during aging in mice and humans. Development of drugs to prevent TRAF3 degradation in immune and bone cells could be a novel therapeutic approach to prevent or reduce bone loss and the incidence of several common diseases associated with aging.

Manipulation of the Alternative NF-κB Pathway in Mice Has Sexually Dimorphic Effects on Bone

Alternative NF‐κB signaling promotes osteoclastogenesis and pathological bone loss, but the effect of sex on phenotype has not been explored. We disrupted alternative NF‐κB signaling by deletion of upstream kinase NF‐κB‐inducing kinase (NIK) or NF‐κB subunit RelB and found that both NIK‐deficient and RelB‐deficient female mice possessed more than twofold higher trabecular bone mass compared to controls, whereas no differences were observed in males. In vitro, RelB‐deficient precursors from female mice showed a more severe osteoclast (OC) differentiation defect than male, while WT had no sex bias. Next, we asked whether pharmacologic activation of alternative NF‐κB by inhibitor of apoptosis (IAP) antagonist BV6 has sex‐dependent effects on bone. Unlike male mice that lost bone, female mice on BV6 for 4 weeks showed no changes in either trabecular bone mass or OC number. Because estrogen generally suppresses NF‐κB, we hypothesized that estrogen protects bone from BV6 effects in vivo. Thus, we performed ovariectomy or sham surgery in female mice, then treated with BV6 or vehicle for 4 weeks. Although ovariectomy caused bone loss, BV6 did not have any additional impact, suggesting that direct estrogen effects do not cause resistance to BV6 in vivo. The osteopenic effects of IAP antagonists in males may have implications for their use in cancer therapy. © 2018 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

TRAF molecules in inflammation and inflammatory diseases

Purpose of Review

This review presents an overview of the current knowledge of TRAF molecules in inflammation with an emphasis on available human evidence and direct in vivo evidence of mouse models that demonstrate the contribution of TRAF molecules in the pathogenesis of inflammatory diseases.

Recent Findings

The tumor necrosis factor receptor (TNF-R)-associated factor (TRAF) family of cytoplasmic proteins was initially identified as signaling adaptors that bind directly to the intracellular domains of receptors of the TNF-R superfamily. It is now appreciated that TRAF molecules are widely employed in signaling by a variety of adaptive and innate immune receptors as well as cytokine receptors. TRAF-dependent signaling pathways typically lead to the activation of nuclear factor-κBs (NF-κBs), mitogen-activated protein kinases (MAPKs), or interferon-regulatory factors (IRFs). Most of these signaling pathways have been linked to inflammation, and therefore TRAF molecules were expected to regulate inflammation and inflammatory responses since their discovery in 1990s. However, direct in vivo evidence of TRAFs in inflammation and especially in inflammatory diseases had been lacking for many years, partly due to the difficulty imposed by early lethality of TRAF2^-/-, TRAF3^-/-, and TRAF6^-/- mice. With the creation of conditional knockout and lineage-specific transgenic mice of different TRAF molecules, our understanding about TRAFs in inflammation and inflammatory responses has rapidly advanced during the past decade.

Summary

Increasing evidence indicates that TRAF molecules are versatile and indispensable regulators of inflammation and inflammatory responses and that aberrant expression or function of TRAFs contributes to the pathogenesis of inflammatory diseases.

The non-canonical NF-κB pathway in immunity and inflammation

The nuclear factor-κB (NF-κB) family of transcription factors is activated by canonical and non-canonical signalling pathways, which differ in both signalling components and biological functions. Recent studies have revealed important roles for the non-canonical NF-κB pathway in regulating different aspects of immune functions. Defects in non-canonical NF-κB signalling are associated with severe immune deficiencies, whereas dysregulated activation of this pathway contributes to the pathogenesis of various autoimmune and inflammatory diseases. Here we review the signalling mechanisms and the biological function of the non-canonical NF-κB pathway. We also discuss recent progress in elucidating the molecular mechanisms regulating non-canonical NF-κB pathway activation, which may provide new opportunities for therapeutic strategies.

MYC-dependent oxidative metabolism regulates osteoclastogenesis via nuclear receptor ERRα

Osteoporosis is a metabolic bone disorder associated with compromised bone strength and an increased risk of fracture. Inhibition of the differentiation of bone-resorbing osteoclasts is an effective strategy for the treatment of osteoporosis. Prior work by our laboratory and others has shown that MYC promotes osteoclastogenesis in vitro, but the underlying mechanisms are not well understood. In addition, the in vivo importance of osteoclast-expressed MYC in physiological and pathological bone loss is not known. Here, we have demonstrated that deletion of Myc in osteoclasts increases bone mass and protects mice from ovariectomy-induced (OVX-induced) osteoporosis. Transcriptomic analysis revealed that MYC drives metabolic reprogramming during osteoclast differentiation and functions as a metabolic switch to an oxidative state. We identified a role for MYC action in the transcriptional induction of estrogen receptor-related receptor α (ERRα), a nuclear receptor that cooperates with the transcription factor nuclear factor of activated T cells, c1 (NFATc1) to drive osteoclastogenesis. Accordingly, pharmacological inhibition of ERRα attenuated OVX-induced bone loss in mice. Our findings highlight a MYC/ERRα pathway that contributes to physiological and pathological bone loss by integrating the MYC/ERRα axis to drive metabolic reprogramming during osteoclast differentiation.

The non-canonical NF-κB pathway in immunity and inflammation

The nuclear factor-κB (NF-κB) family of transcription factors is activated by canonical and non-canonical signalling pathways, which differ in both signalling components and biological functions. Recent studies have revealed important roles for the non-canonical NF-κB pathway in regulating different aspects of immune functions. Defects in non-canonical NF-κB signalling are associated with severe immune deficiencies, whereas dysregulated activation of this pathway contributes to the pathogenesis of various autoimmune and inflammatory diseases. Here we review the signalling mechanisms and the biological function of the non-canonical NF-κB pathway. We also discuss recent progress in elucidating the molecular mechanisms regulating non-canonical NF-κB pathway activation, which may provide new opportunities for therapeutic strategies.

RANKL cytokine enhances TNF-induced osteoclastogenesis independently of TNF receptor associated factor (TRAF) 6 by degrading TRAF3 in osteoclast precursors

Cytokines, including receptor activator of nuclear factor κB ligand (RANKL) and TNF, induce increased osteoclast (OC) formation and bone loss in postmenopausal osteoporosis and inflammatory arthritides. RANKL and TNF can independently induce OC formation in vitro from WT OC precursors via TNF receptor-associated factor (TRAF) adaptor proteins, which bind to their receptors. Of these, only TRAF6 is required for RANKL-induced osteoclastogenesis in vitro However, the molecular mechanisms involved remain incompletely understood. Here we report that RANKL induced the formation of bone-resorbing OCs from TRAF6^-/- OC precursors when cultured on bone slices but not on plastic. The mechanisms involved increased TNF production by TRAF6^-/- OC precursors resulting from their interaction with bone matrix and release of active TGFβ from the resorbed bone, coupled with RANKL-induced autophagolysosomal degradation of TRAF3, a known inhibitor of OC formation. Consistent with these findings, RANKL enhanced TNF-induced OC formation from TRAF6^-/- OC precursors. Moreover, TNF induced significantly more OCs from mice with TRAF3 conditionally deleted in myeloid lineage cells, and it did not inhibit RANKL-induced OC formation from these cells. TRAF6^-/- OC precursors that overexpressed TRAF3 or were treated with the autophagolysosome inhibitor chloroquine formed significantly fewer OCs in response to TNF alone or in combination with RANKL. We conclude that RANKL can enhance TNF-induced OC formation independently of TRAF6 by degrading TRAF3. These findings suggest that preventing TRAF3 degradation with drugs like chloroquine could reduce excessive OC formation in diseases in which bone resorption is increased in response to elevated production of these cytokines.

Constitutive Activation of NIK Impairs the Self-Renewal of Hematopoietic Stem/Progenitor Cells and Induces Bone Marrow Failure

Previously we have shown that loss of non-canonical NF-κB signaling impairs self-renewal of hematopoietic stem/progenitor cells (HSPCs). This prompted us to investigate whether persistent activation of the non-canonical NF-κB signaling will have supportive effects on HSPC self-renewal. NF-κB-inducing kinase (NIK) is an important kinase that mainly activates the non-canonical pathway through directly phosphorylating IKKα. In contrast to our expectations, constitutive activation of NIK in the hematopoietic system leads to bone marrow (BM) failure and postnatal lethality due to intrinsic impairment of HSPC self-renewal and extrinsic disruption of BM microenvironment through enhancing osteoclastogenesis. The impaired HSPC function is associated with reduced cell proliferation and increased apoptosis and inflammatory cytokine responses. RNAseq analysis of control and NIK-activated HSPCs reveals that these effects are through non-canonical NF-κB signaling without significant changes in the canonical pathway. Gene set expression analysis of RNAseq data reveals globally decreased stem cell signature, increased maturation signature, and increased inflammatory responses. Many genes (Mpl, Tifab, Emcn, Flt3, Bcl2, and others) that regulate HSPC self-renewal, lineage commitment, and apoptosis are significantly downregulated-and those genes that regulate inflammatory responses and cell cycle inhibition (Cdkn2a and Cdkn2b) are significantly upregulated-by activation of NIK. Importantly, our data demonstrate that activation of NIK-non-canonical signaling has distinct phenotypes-smaller spleen size, decreased white blood cell counts, and reduced HSPC proliferation-compared to activation of canonical signaling. Collectively, these data indicate that the balanced non-canonical NF-κB signaling is essential for maintaining normal hematopoiesis and NIK-non-canonical signaling contributes to the development of BM failure. Stem Cells 2017;35:777-786.

The incidence and risk of osteoporosis in patients with anxiety disorder: A Population-based retrospective cohort study

The purpose of this study was to investigate the relationship between anxiety disorder (AD) and the subsequent development of osteoporosis.We conducted a population-based retrospective cohort analysis according to the data in the Longitudinal Health Insurance Database 2000 of Taiwan. We included 7098 patients in both the AD and no-anxiety cohort who were matched according to age and sex between January 1, 2000, and December 31, 2013. The incidence rate and the risk ratios (RRs) of subsequent new-onset osteoporosis were calculated for both cohorts. We used Cox proportional hazards models to assess the effect of AD. The Kaplan-Meier method was applied to estimate the cumulative osteoporosis incidence curves.The AD cohort consisted of 7098 patients, and the comparison cohort comprised the same matched control patients without anxiety. The risk of osteoporosis was higher in the AD cohort than in the comparison cohort. In addition, the incidence of newly diagnosed osteoporosis remained significantly increased in all of the stratified follow-up durations (0-1, 1-5, 5-10, ≥10years). Patients with AD were 1.79 times more likely to get osteoporosis than those without AD. We also observed a significant increase in osteoporotic risk in AD patients who are comorbid with hypertension, diabetes mellitus, and chronic liver disease.The incidence of osteoporosis in Taiwan is associated with an a priori AD history. The risk ratios are the highest for osteoporosis within 1 year of AD diagnosis, but the risk remains statistically significant for >1 year. Clinicians should pay particular attention to osteoporotic comorbidities in AD patients.

Animal Models of Bone Loss in Inflammatory Arthritis: from Cytokines in the Bench to Novel Treatments for Bone Loss in the Bedside-a Comprehensive Review

Throughout life, bone is continuously remodelled. Bone is formed by osteoblasts, from mesenchymal origin, while osteoclasts induce bone resorption. This process is tightly regulated. During inflammation, several growth factors and cytokines are increased inducing osteoclast differentiation and activation, and chronic inflammation is a condition that initiates systemic bone loss. Rheumatoid arthritis (RA) is a chronic inflammatory auto-immune disease that is characterised by active synovitis and is associated with early peri-articular bone loss. Peri-articular bone loss precedes focal bone erosions, which may progress to bone destruction and disability. The incidence of generalised osteoporosis is associated with the severity of arthritis in RA and increased osteoporotic vertebral and hip fracture risk. In this review, we will give an overview of different animal models of inflammatory arthritis related to RA with focus on bone erosion and involvement of pro-inflammatory cytokines. In addition, a humanised endochondral ossification model will be discussed, which can be used in a translational approach to answer osteoimmunological questions.

Osteoclasts-Key Players in Skeletal Health and Disease

The differentiation of osteoclasts (OCs) from early myeloid progenitors is a tightly regulated process that is modulated by a variety of mediators present in the bone microenvironment. Once generated, the function of mature OCs depends on cytoskeletal features controlled by an αvβ3-containing complex at the bone-apposed membrane and the secretion of protons and acid-protease cathepsin K. OCs also have important interactions with other cells in the bone microenvironment, including osteoblasts and immune cells. Dysregulation of OC differentiation and/or function can cause bone pathology. In fact, many components of OC differentiation and activation have been targeted therapeutically with great success. However, questions remain about the identity and plasticity of OC precursors and the interplay between essential networks that control OC fate. In this review, we summarize the key principles of OC biology and highlight recently uncovered mechanisms regulating OC development and function in homeostatic and disease states.

Alternative NF-κB Regulates RANKL-Induced Osteoclast Differentiation and Mitochondrial Biogenesis via Independent Mechanisms

Mitochondrial biogenesis, the generation of new mitochondrial DNA and proteins, has been linked to osteoclast (OC) differentiation and function. In this study we used mice with mutations in key alternative NF-κB pathway proteins, RelB and NF-κB-inducing kinase (NIK), to dissect the complex relationship between mitochondrial biogenesis and osteoclastogenesis. In OC precursors lacking either NIK or RelB, receptor activator of NF-κB ligand (RANKL) was unable to increase mitochondrial DNA or oxidative phosphorylation (OxPhos) protein expression, which was associated with lower oxygen consumption rates. Transgenic OC precursors expressing constitutively active NIK showed normal RANKL-induced mitochondrial biogenesis (OxPhos expression and mitochondria copy number) compared to controls, but larger mitochondrial dimensions and increased oxygen consumption rates, suggesting increased mitochondrial function. To deduce the mechanism for mitochondrial biogenesis defects in NIK-deficient and RelB-deficient precursors, we examined expression of genes known to control this process. PGC-1β (Ppargc1b) expression, but not PGC-1α, PPRC1, or ERRα, was significantly reduced in RelB(-/-) and NIK(-/-) OCs. Because PGC-1β has been reported to positively regulate both mitochondrial biogenesis and differentiation in OCs, we retrovirally overexpressed PGC-1β in RelB(-/-) cells, but surprisingly found that it did not affect differentiation, nor did it restore RANKL-induced mitochondrial biogenesis. To determine whether the blockade in osteoclastogenesis in RelB-deficient cells precludes mitochondrial biogenesis, we rescued RelB(-/-) differentiation via overexpression of NFATc1. Mitochondrial parameters in neither WT nor RelB-deficient cultures were affected by NFATc1 overexpression, and bone resorption in RelB(-/-) was not restored. Furthermore, NFATc1 co-overexpression with PGC-1β, although allowing OC differentiation, did not rescue mitochondrial biogenesis or bone resorption in RelB(-/-) OCs, by CTX-I levels. Thus, our results indicate that the alternative NF-κB pathway plays dual, but distinct, roles in controlling the independent processes of OC differentiation and OC mitochondrial biogenesis. Furthermore, the inability of PGC-1β to drive mitochondrial biogenesis in OCs without RelB indicates a cell-type specificity in mitochondria regulation.

NLRP12 provides a critical checkpoint for osteoclast differentiation

The alternative or noncanonical nuclear factor kappa B (NF-κB) pathway regulates the osteoclast (OC) response to receptor activator of nuclear factor kappa B ligand (RANKL) and thus bone metabolism. Although several lines of evidence support the emerging concept that nucleotide-binding leucine-rich repeat and pyrin domain-containing receptor 12 (NLRP12) impedes alternative NF-κB activation in innate immune cells, a functional role for NLRP12 outside an inflammatory disease model has yet to be reported. Our study demonstrates that NLRP12 has a protective role in bone via suppression of alternative NF-κB-induced osteoclastogenesis and is down-modulated in response to osteoclastogenic stimuli. Here, we show that retroviral overexpression of NLRP12 suppressed RelB nuclear translocation and OC formation. Conversely, genetic ablation of NLRP12 promoted NIK stabilization, RelB nuclear translocation, and increased osteoclastogenesis in vitro. Using radiation chimeras, we demonstrated these in vitro observations dovetail with our in vivo findings that NLRP12 deficiency leads to enhanced OC numbers accompanied by a significant decline in bone mass under physiological conditions. Consistent with the basal bone phenotype, we also observed an enhanced osteolytic response following RANKL injection over the calvaria of NLRP12-deficient chimeric mice compared with wild-type control mice. Thus, modulation of NLRP12 levels controls alternative NF-κB signaling in OC precursors, altering bone homeostasis and osteolytic responses.

Non-canonical NF-κB signaling in rheumatoid arthritis: Dr Jekyll and Mr Hyde?

The nuclear factor-κB (NF-κB) family of transcription factors is essential for the expression of pro-inflammatory cytokines, but can also induce regulatory pathways. NF-κB can be activated via two distinct pathways: the classical or canonical pathway, and the alternative or non-canonical pathway. It is well established that the canonical NF-κB pathway is essential both in acute inflammatory responses and in chronic inflammatory diseases, including rheumatoid arthritis (RA). Although less extensively studied, the non-canonical NF-κB pathway is not only central in lymphoid organ development and adaptive immune responses, but is also thought to play an important role in the pathogenesis of RA. Importantly, this pathway appears to have cell type-specific functions and, since many different cell types are involved in the pathogenesis of RA, it is difficult to predict the net overall contribution of the non-canonical NF-κB pathway to synovial inflammation. In this review, we describe the current understanding of non-canonical NF-κB signaling in various important cell types in the context of RA and consider the relevance to the pathogenesis of the disease. In addition, we discuss current drugs targeting this pathway, as well as future therapeutic prospects.

Cellular and molecular bases of skeletal regeneration: what can we learn from genetic mouse models?

Although bone repairs through a very efficient regenerative process in 90% of the patients, many factors can cause delayed or impaired healing. To date, there are no reliable biological parameters to predict or diagnose bone repair defects. Orthopedic surgeons mostly base their diagnoses on radiographic analyses. With the recent progress in our understanding of the bone repair process, new methods may be envisioned. Animal models have allowed us to define the key steps of bone regeneration and the biological and mechanical factors that may influence bone healing in positive or negative ways. Most importantly, small animal models such as mice have provided powerful tools to apprehend the genetic bases of normal and impaired bone healing. The current review presents a state of the art of the genetically modified mouse models that have advanced our understanding of the cellular and molecular components of bone regeneration and repair. The review illustrates the use of these models to define the role of inflammation, skeletal cell lineages, signaling pathways, the extracellular matrix, osteoclasts and angiogenesis. These genetic mouse models promise to change the field of orthopedic surgery to help establish genetic predispositions for delayed repair, develop models of non-union that mimic the human conditions and elaborate new therapeutic approaches to enhance bone regeneration.

Chloroquine reduces osteoclastogenesis in murine osteoporosis by preventing TRAF3 degradation

The cytokines RANKL and TNF activate NF-κB signaling in osteoclast precursors (OCPs) to induce osteoclast (OC) formation. Conversely, TNF can limit OC formation through NF-κB p100, which acts as an inhibitor, and TNF receptor-associated receptor 3 (TRAF3); however, a role for TRAF3 in RANKL-mediated OC formation is unknown. We found that TRAF3 limits RANKL-induced osteoclastogenesis by suppressing canonical and noncanonical NF-κB signaling. Conditional OC-specific Traf3-KO (cKO) mice had mild osteoporosis and increased OC formation. RANKL induced TRAF3 degradation via the lysosome/autophagy system. The autophagy/lysosome inhibitor chloroquine reduced RANKL-induced OC formation and function by increasing TRAF3 expression in OCPs in vitro and in vivo. Although chloroquine had no effect on basal bone resorption, it inhibited parathyroid hormone- and ovariectomy-induced OC activation in WT, but not cKO, mice. Deletion of the transcription factor gene Relb resulted in increased TRAF3 expression in OCPs, which was associated with decreased RANKL-induced TRAF3 degradation. RelB directly increased expression of BECN1, a key autophagy regulator, by binding to its promoter. These data indicate that autophagic/lysosomal degradation of TRAF3 is an important step in RANKL-induced NF-κB activation in OCPs. Furthermore, treatments that increase TRAF3 levels in OCPs, including pharmacological inhibition of its degradation with compounds such as chloroquine, may limit bone destruction in common bone diseases.

Anti-cancer IAP antagonists promote bone metastasis: a cautionary tale

The bone microenvironment is complex, containing bone-forming osteoblasts, bone-resorbing osteoclasts, bone-maintaining osteocytes, hematopoietic lineage cells, as well as blood vessels, nerves, and stromal cells. Release of embedded growth factors from the bone matrix via osteoclast resorption has been shown to participate in the alteration of bone microenvironment to facilitate tumor metastasis to this organ. Many types of malignancies including solid tumors and leukemias are associated with elevated levels of inhibitor of apoptosis (IAP) proteins, and IAP antagonists represent an important emerging class of anti-cancer agents. IAPs exert anti-apoptotic roles by inhibiting caspases and upregulating pro-survival proteins, at least in part by activating classical NF-κB signaling. In addition, IAPs act as negative regulators in the alternative NF-κB pathway, so that IAP antagonists stimulate this pathway. The role of the classical NF-κB pathway in IAP antagonist-induced apoptosis has been extensively studied, whereas much less attention has been paid to the role of these agents in the alternative pathway. Thus far, several IAP antagonists have been tested in preclinical and early stage clinical trials, and have shown promise in sensitizing tumor cells to apoptosis without significant side effects. However, recent preclinical evidence suggests an increased risk of bone metastasis caused by IAP antagonists, along with potential for promoting osteoporosis. In this review, the connection between IAP antagonists, the alternative NF-κB pathway, osteoclasts, and bone metastasis are discussed. In light of these effects of IAP antagonists on the bone microenvironment, more attention should be paid to this and other host tissues as these drugs are developed further.

Antagonism of inhibitor of apoptosis proteins increases bone metastasis via unexpected osteoclast activation

Inhibitor of apoptosis (IAP) proteins play a central role in many types of cancer, and IAP antagonists are in development as anticancer agents. IAP antagonists cause apoptosis in many cells, but they also activate alternative NF-κB signaling through NF-κB-inducing kinase (NIK), which regulates osteoclasts. In bone metastasis, a positive feedback loop between tumors and osteoclasts promotes tumor growth and osteolysis. We therefore tested the effect of IAP antagonists on the bone microenvironment for metastasis. In both drug-sensitive and drug-resistant tumors, growth in bone was favored, as compared with other sites during IAP antagonist treatment. These drugs also caused osteoporosis and increased osteoclastogenesis, mediated by NIK, and enhanced tumor-associated osteolysis. Cotreatment with zoledronic acid, a potent osteoclast inhibitor, reduced IAP antagonist-enhanced tumor growth in bone and osteolysis. Thus, IAP antagonist-based cancer treatment may be compromised by osteoporosis and enhanced skeletal metastasis, which may be prevented by antiresorptive agents.

SIGNIFICANCE

Although IAP antagonists are a class of anticancer agents with proven efficacy in multiple cancers, we show that these agents can paradoxically increase tumor growth and metastasis in the bone by stabilizing NIK and activating the alternative NF-κB pathway in osteoclasts. Future clinical trials of IAP antagonist-based therapy may require detailed examination of this potential for enhanced bone metastasis and osteoporosis, as well as possible combination with antiresorptive agents.

Osteoclast fusion and regulation by RANKL-dependent and independent factors

Osteoclasts are the bone resorbing cells essential for bone remodeling. Osteoclasts are formed from hematopoietic progenitors in the monocyte/macrophage lineage. Osteoclastogenesis is composed of several steps including progenitor survival, differentiation to mono-nuclear pre-osteoclasts, fusion to multi-nuclear mature osteoclasts, and activation to bone resorbing osteoclasts. The regulation of osteoclastogenesis has been extensively studied, in which the receptor activator of NF-κB ligand (RANKL)-mediated signaling pathway and downstream transcription factors play essential roles. However, less is known about osteoclast fusion, which is a property of mature osteoclasts and is required for osteoclasts to resorb bone. Several proteins that affect cell fusion have been identified. Among them, dendritic cell-specific transmembrane protein (DC-STAMP) is directly associated to osteoclast fusion in vivo. Cytokines and factors influence osteoclast fusion through regulation of DC-STAMP. Here we review the recently discovered new factors that regulate osteoclast fusion with specific focus on DC-STAMP. A better understanding of the mechanistic basis of osteoclast fusion will lead to the development of a new therapeutic strategy for bone disorders due to elevated osteoclast bone resorption. Cell-cell fusion is essential for a variety of cellular biological processes. In mammals, there is a limited number of cell types that fuse to form multinucleated cells, such as the fusion of myoblasts for the formation of skeletal muscle and the fusion of cells of the monocyte/macrophage lineage for the formation of multinucleated osteoclasts and giant cells. In most cases, cell-cell fusion is beneficial for cells by enhancing function. Myoblast fusion increases myofiber size and diameter and thereby increases contractile strength. Multinucleated osteoclasts have far more bone resorbing activity than their mono-nuclear counterparts. Multinucleated giant cells are much more efficient in the removal of implanted materials and bacteria due to chronic infection than macrophages. Therefore, they are also called foreign-body giant cells. Cell fusion is a complicated process involving cell migration, chemotaxis, cell-cell recognition and attachment, as well as changes into a fusion-competent status. All of these steps are regulated by multiple factors. In this review, we will discuss osteoclast fusion and regulation.

TAK1 is essential for osteoclast differentiation and is an important modulator of cell death by apoptosis and necroptosis

Transforming growth factor β (TGF-β)-activated kinase 1 (TAK1), a mitogen-activated protein 3 (MAP3) kinase, plays an essential role in inflammation by activating the IκB kinase (IKK)/nuclear factor κB (NF-κB) and stress kinase (p38 and c-Jun N-terminal kinase [JNK]) pathways in response to many stimuli. The tumor necrosis factor (TNF) superfamily member receptor activator of NF-κB ligand (RANKL) regulates osteoclastogenesis through its receptor, RANK, and the signaling adaptor TRAF6. Because TAK1 activation is mediated through TRAF6 in the interleukin 1 receptor (IL-1R) and toll-like receptor (TLR) pathways, we sought to investigate the consequence of TAK1 deletion in RANKL-mediated osteoclastogenesis. We generated macrophage colony-stimulating factor (M-CSF)-derived monocytes from the bone marrow of mice with TAK1 deletion in the myeloid lineage. Unexpectedly, TAK1-deficient monocytes in culture died rapidly but could be rescued by retroviral expression of TAK1, inhibition of receptor-interacting protein 1 (RIP1) kinase activity with necrostatin-1, or simultaneous genetic deletion of TNF receptor 1 (TNFR1). Further investigation using TAK1-deficient mouse embryonic fibroblasts revealed that TNF-α-induced cell death was abrogated by the simultaneous inhibition of caspases and knockdown of RIP3, suggesting that TAK1 is an important modulator of both apoptosis and necroptosis. Moreover, TAK1-deficient monocytes rescued from programmed cell death did not form mature osteoclasts in response to RANKL, indicating that TAK1 is indispensable to RANKL-induced osteoclastogenesis. To our knowledge, we are the first to report that mice in which TAK1 has been conditionally deleted in osteoclasts develop osteopetrosis.

In situ fatty acid profile of femoral cancellous subchondral bone in osteoarthritic and fragility fracture females: implications for bone remodelling

We report here differences in the fatty acid profile of cancellous bone matrix, including n-3, n-6, mono- and poly-unsaturated, as well as saturated fats, between femoral heads from female OA (n=8, aged 68-88years), fractured neck of femur (#NOF) (n=19, 67-88years) and autopsy controls (CTRL) (n=4, 85-97years). Femoral heads were collected from individuals undergoing orthopaedic surgery for OA or #NOF; the fatty acid profile of sub-samples from the superior principal compressive and superior principal tensile regions were determined by gas chromatography. A total of 42 individual fatty acids were detected at varying concentrations with significant differences between subchondral bone from OA subjects, subchondral bone from #NOF subjects and subchondral bone from CTRL subjects, as well as between the superior principal compressive and superior principal tensile regions (for saturated fats only). Subchondral bone from OA subjects had higher total n-6 (OA=10.89±3.17, #NOF=11.11±1.83, CTRL=8.32±2.05, p=0.008) and total n-3 (OA=1.34±0.38, #NOF=1.19±0.18, CTRL=1.15±0.48, p=0.011) percentages than subchondral bone from #NOF subjects and subchondral bone from CTRL subjects, and there was no difference in the n-6:n-3 ratio, nor within the percentage of n-9 fatty acids. Arachidonic acid (OA=0.42±0.16, #NOF=0.26±0.06, CTRL=0.28±0.06, p=0.01), and γ-linolenic acid (OA=0.11±0.03, #NOF=0.05±0.02, CTRL=0.04±0.02, p<0.001) were higher in subchondral bone from OA subjects than subchondral bone from #NOF subjects and subchondral bone from CTRL subjects. In conclusion, there is a wide diversity of fatty acids in cancellous bone matrix from the femoral heads of OA and #NOF, suggesting they may have regulatory effects on inflammatory processes, and their metabolites. This article is part of a Special Issue entitled "Osteoarthritis".

The noncanonical NF-κB pathway

The noncanonical nuclear factor-κB (NF-κB) signaling pathway mediates activation of the p52/RelB NF-κB complex and, thereby, regulates specific immunological processes. This NF-κB pathway relies on the inducible processing of NF-κB2 precursor protein, p100, as opposed to the degradation of IκBα in the canonical NF-κB pathway. A central signaling component of the noncanonical NF-κB pathway is NF-κB-inducing kinase (NIK), which functions together with a downstream kinase, IKKα (inhibitor of NF-κB kinase α), to induce phosphorylation-dependent ubiquitination and processing of p100. Under normal conditions, NIK is targeted for continuous degradation by a tumor necrosis factor (TNF) receptor-associated factor-3 (TRAF3)-dependent E3 ubiquitin ligase. In response to signals mediated by a subset of TNF receptor superfamily members, NIK becomes stabilized as a result of TRAF3 degradation, leading to the activation of noncanonical NF-κB. This review discusses both the historical perspectives and the recent progress in the regulation and biological function of the noncanonical NF-κB pathway.