The IkappaB function of NF-kappaB2 p100 controls stimulated osteoclastogenesis

The noncanonical NFκB pathway: Regulatory mechanisms in health and disease

The noncanonical NFκB signaling pathway mediates the biological functions of diverse cell survival, growth, maturation, and differentiation factors that are important for the development and maintenance of hematopoietic cells and immune organs. Its dysregulation is associated with a number of immune pathologies and malignancies. Originally described as the signaling pathway that controls the NFκB family member RelB, we now know that noncanonical signaling also controls NFκB RelA and cRel. Here, we aim to clarify our understanding of the molecular network that mediates noncanonical NFκB signaling and review the human diseases that result from a deficient or hyper-active noncanonical NFκB pathway. It turns out that dysregulation of RelA and cRel, not RelB, is often implicated in mediating the resulting pathology. This article is categorized under: Immune System Diseases > Molecular and Cellular Physiology Cancer > Molecular and Cellular Physiology Immune System Diseases > Stem Cells and Development.

NF-κB in biology and targeted therapy: new insights and translational implications

NF-κB signaling has been discovered for nearly 40 years. Initially, NF-κB signaling was identified as a pivotal pathway in mediating inflammatory responses. However, with extensive and in-depth investigations, researchers have discovered that its role can be expanded to a variety of signaling mechanisms, biological processes, human diseases, and treatment options. In this review, we first scrutinize the research process of NF-κB signaling, and summarize the composition, activation, and regulatory mechanism of NF-κB signaling. We investigate the interaction of NF-κB signaling with other important pathways, including PI3K/AKT, MAPK, JAK-STAT, TGF-β, Wnt, Notch, Hedgehog, and TLR signaling. The physiological and pathological states of NF-κB signaling, as well as its intricate involvement in inflammation, immune regulation, and tumor microenvironment, are also explicated. Additionally, we illustrate how NF-κB signaling is involved in a variety of human diseases, including cancers, inflammatory and autoimmune diseases, cardiovascular diseases, metabolic diseases, neurological diseases, and COVID-19. Further, we discuss the therapeutic approaches targeting NF-κB signaling, including IKK inhibitors, monoclonal antibodies, proteasome inhibitors, nuclear translocation inhibitors, DNA binding inhibitors, TKIs, non-coding RNAs, immunotherapy, and CAR-T. Finally, we provide an outlook for research in the field of NF-κB signaling. We hope to present a stereoscopic, comprehensive NF-κB signaling that will inform future research and clinical practice.

HTLV-1 infected T cells cause bone loss via small extracellular vesicles

Adult T cell leukemia (ATL), caused by infection with human T cell leukemia virus type 1 (HTLV-1), is often complicated by hypercalcemia and osteolytic lesions. Therefore, we studied the communication between patient-derived ATL cells (ATL-PDX) and HTLV-1 immortalized CD4+ T cell lines (HTLV/T) with osteoclasts and their effects on bone mass in mice. Intratibial inoculation of some HTLV/T lead to a profound local decrease in bone mass similar to marrow-replacing ATL-PDX, despite the fact that few HTLV/T cells persisted in the bone. To study the direct effect of HTLV/T and ATL-PDX on osteoclasts, supernatants were added to murine and human osteoclast precursors. ATL-PDX supernatants from hypercalcemic patients promoted formation of mature osteoclasts, while those from HTLV/T were variably stimulatory, but had largely consistent effects between human and murine cultures. Interestingly, this osteoclastic activity did not correlate with expression of osteoclastogenic cytokine RANKL, suggesting an alternative mechanism. HTLV/T and ATL-PDX produce small extracellular vesicles (sEV), known to facilitate HTLV-1 infection. We hypothesized that these sEV also mediate bone loss by targeting osteoclasts. We isolated sEV from both HTLV/T and ATL-PDX, and found they carried most of the activity found in supernatants. In contrast, sEV from uninfected activated T cells had little effect. Analysis of sEV (both active and inactive) by mass spectrometry and electron microscopy confirmed absence of RANKL and intact virus. Viral proteins Tax and Env were only present in sEV from the active, osteoclast-stimulatory group, along with increased representation of proteins involved in osteoclastogenesis and bone resorption. sEV injected over mouse calvaria in the presence of low dose RANKL caused more osteolysis than RANKL alone. Thus, HTLV-1 infection of T cells can cause release of sEV with strong osteolytic potential, providing a mechanism beyond RANKL production that modifies the bone microenvironment, even in the absence of overt leukemia.

Sex differences of NF-κB-targeted therapy for mitigating osteoporosis associated with chronic inflammation of bone

Aims

Transcription factor nuclear factor kappa B (NF-κB) plays a major role in the pathogenesis of chronic inflammatory diseases in all organ systems. Despite its importance, NF-κB targeted drug therapy to mitigate chronic inflammation has had limited success in preclinical studies. We hypothesized that sex differences affect the response to NF-κB treatment during chronic inflammation in bone. This study investigated the therapeutic effects of NF-κB decoy oligodeoxynucleotides (ODN) during chronic inflammation in male and female mice.

Methods

We used a murine model of chronic inflammation induced by continuous intramedullary delivery of lipopolysaccharide-contaminated polyethylene particles (cPE) using an osmotic pump. Specimens were evaluated using micro-CT and histomorphometric analyses. Sex-specific osteogenic and osteoclastic differentiation potentials were also investigated in vitro, including alkaline phosphatase, Alizarin Red, tartrate-resistant acid phosphatase staining, and gene expression using reverse transcription polymerase chain reaction (RT-PCR).

Results

Local delivery of NF-κB decoy ODN in vivo increased osteogenesis in males, but not females, in the presence of chronic inflammation induced by cPE. Bone resorption activity was decreased in both sexes. In vitro osteogenic and osteoclastic differentiation assays during inflammatory conditions did not reveal differences among the groups. Receptor activator of nuclear factor kappa Β ligand (Rankl) gene expression by osteoblasts was significantly decreased only in males when treated with ODN.

Conclusion

We demonstrated that NF-κB decoy ODN increased osteogenesis in male mice and decreased bone resorption activity in both sexes in preclinical models of chronic inflammation. NF-κB signalling could be a therapeutic target for chronic inflammatory diseases involving bone, especially in males.

FoxO1 knockdown inhibits RANKL-induced osteoclastogenesis by blocking NLRP3 inflammasome activation

OBJECTIVES

This study aimed to elucidate the connection between osteoclastic forkhead transcription factor O1 (FoxO1) and periodontitis and explore the underlying mechanism by which FoxO1 knockdown regulates osteoclast formation.

MATERIALS AND METHODS

A conventional ligature-induced periodontitis model was constructed to reveal the alterations in the proportion of osteoclastic FoxO1 in periodontitis via immunofluorescence staining. Additionally, RNA sequencing (RNA-seq) was performed to explore the underlying mechanisms of FoxO1 knockdown-mediated osteoclastogenesis, followed by western blotting, quantitative polymerase chain reaction, and enzyme-linked immunosorbent assay.

RESULTS

FoxO1+ osteoclasts were enriched in the alveolar bone in experimental periodontitis. Moreover, FoxO1 knockdown led to impaired osteoclastogenesis with low expression of osteoclast differentiation-related genes, accompanied by an insufficient osteoclast maturation phenotype. Mechanistically, RNA-seq revealed that the nuclear factor kappa B (NF-κB) and nucleotide-binding oligomerization domain-like receptor family pyrin domain containing 3 (NLRP3) inflammasome signaling pathways were inhibited in FoxO1-knockdown osteoclasts. Consistent with this, MCC950, an effective inhibitor of the NLRP3 inflammasome, substantially attenuated osteoclast formation.

CONCLUSIONS

FoxO1 knockdown contributed to the inhibition of osteoclastogenesis by effectively suppressing NF-κB signaling and NLRP3 inflammasome activation. This prospective study reveals the role of FoxO1 in mediating osteoclastogenesis and provides a viable therapeutic target for periodontitis treatment.

The effect of Si species released from bioactive glasses on cell behaviour: A quantitative review

Despite over 50 years of silicate bioactive glass (SBG) research, commercial success, and 6000+ published articles, there remains a lack of understanding of how soluble silicate (Si) species released from SBGs influences cellular responses. Using a systematic approach, this article quantitatively compares the in vitro responses of cells to SBG dissolution products reported in the literature and determines if there is a Si concentration ([Si]) dependent effect on cell behaviour. Cell behavioural responses to SBGs [Si] in dissolution products included metabolic activity (reported in 52 % of articles), cell number (24 %), protein production (22 %), gene expression (22 %) and biomineralization (24 %). There was a difference in the [Si] reported to cause increased (desirable) cellular responses (median = 30.2 ppm) compared to the [Si] reported to cause decreased (undesirable) cellular responses (median = 52.0 ppm) (P ≤ 0.001). The frequency of undesirable outcomes increased with increasing [Si], with ∼3 times more negative outcomes reported above 52 ppm. We also investigated the effect of [Si] on specific cellular outcomes (e.g., metabolic activity, angiogenesis, osteogenesis), if cell type/species influenced these responses and the impact of other ions (Ca, P, Na) within the SBG dissolution media on cell behaviour. This review has, for the first time, quantitatively compared the cellular responses to SBGs from the literature, providing a quantitative overview of SBG in vitro practices and presents evidence of a range of [Si] where desirable cellular responses may be more likely (30-52 ppm). This review also demonstrates the need for greater standardisation of in vitro methodological approaches and recommends some minimum reporting standards. STATEMENT OF SIGNIFICANCE: This systematic review investigates the relationship between the concentration of Si released from Si-bioactive glasses (SBG) and in vitro cellular responses. Si releasing materials continue to be of considerable scientific, commercial, and medical interest (with 1500+ articles published in the last 3 years) but there is considerable variation in the reported biologically effective Si concentrations and on the importance of Si on cell behaviour. Despite the variation in methodological approaches, this article demonstrated statistical commonalities in the Si concentrations that cause desirable and undesirable cellular behaviours, suggesting a window where positive cellular outcomes are more likely. This review also provides a quantitative analysis of in vitro practices within the bioactive glass field and highlights the need for greater standardisation.

IL-1β stimulates a novel, IKKα -dependent, NIK -independent activation of non-canonical NFκB signalling

In this study, we examined the activation of non-canonical nuclear factor Kappa B (NFκB) signalling in U2OS cells, a cellular metastatic bone cancer model. Whilst Lymphotoxin α1β2 (LTα₁β₂) stimulated the expected slow, delayed, sustained activation of serine 866/870 p100 phosphorylation and increased cellular expression of p52 NFκB, we found that canonical agonists, Interleukin-1β (IL-1β) and also Tumour necrosis factor-α (TNFα) generated a rapid transient increase in pp100, which was maximal by 15-30 min. This rapid phosphorylation was also observed in other cells types, such as DU145 and HCAECs suggesting the phenomenon is universal. IKKα deletion using CRISPR/Cas9 revealed an IKKα-dependent mechanism for serine 866/870 and additionally serine 872 p100 phosphorylation for both IL-1β and LTα₁β₂. In contrast, knockdown of IKKβ using siRNA or pharmacological inhibition of IKKβ activity was without effect on p100 phosphorylation. Pre-incubation of cells with the NFκB inducing-kinase (NIK) inhibitor, CW15337, had no effect on IL-1β induced phosphorylation of p100 however, the response to LTα₁β₂ was virtually abolished. Surprisingly IL-1β also stimulated p52 nuclear translocation as early as 60 min, this response and the concomitant p65 translocation was partially reduced by IKKα deletion. Furthermore, p52 nuclear translocation was unaffected by CW15337. In contrast, the response to LTα₁β₂ was essentially abolished by both IKKα deletion and CW15337. Taken together, these finding reveal novel forms of NFκB non-canonical signalling stimulated by ligands that activate the canonical NFκB pathway strongly such as IL-1β.

Transcriptional Regulation during Aberrant Activation of NF-κB Signalling in Cancer

The NF-κB signalling pathway is a major signalling cascade involved in the regulation of inflammation and innate immunity. It is also increasingly recognised as a crucial player in many steps of cancer initiation and progression. The five members of the NF-κB family of transcription factors are activated through two major signalling pathways, the canonical and non-canonical pathways. The canonical NF-κB pathway is prevalently activated in various human malignancies as well as inflammation-related disease conditions. Meanwhile, the significance of non-canonical NF-κB pathway in disease pathogenesis is also increasingly recognized in recent studies. In this review, we discuss the double-edged role of the NF-κB pathway in inflammation and cancer, which depends on the severity and extent of the inflammatory response. We also discuss the intrinsic factors, including selected driver mutations, and extrinsic factors, such as tumour microenvironment and epigenetic modifiers, driving aberrant activation of NF-κB in multiple cancer types. We further provide insights into the importance of the interaction of NF-κB pathway components with various macromolecules to its role in transcriptional regulation in cancer. Finally, we provide a perspective on the potential role of aberrant NF-κB activation in altering the chromatin landscape to support oncogenic development.

USP39 Regulates NF-κB-Mediated Inflammatory Responses through Deubiquitinating K48-Linked IκBα

IκBα is a critical protein that inhibits NF-κB nuclear translocation and impairs NF-κB-mediated signaling. The abundance of IκBα determines the activation and restoration of the inflammatory response. However, posttranslational regulation of IκBα remains to be fully understood. In this study, we identified ubiquitin-specific protease 39 (USP39) as a negative regulator in the NF-κB inflammatory response by stabilizing basal IκBα. The expression of USP39 in macrophages was reduced under LPS-induced inflammation. Knockdown or knockout of USP39 in macrophages significantly increased the expression and secretion of proinflammatory cytokines upon exposure to LPS or Escherichia coli, whereas reexpression of exogenous USP39 in USP39-deficient macrophages rescued the effect. Moreover, USP39-defective mice were more sensitive to LPS or E. coli-induced systemic sepsis. Mechanistically, USP39 interacted with and stabilized IκBα by reducing K48-linked polyubiquination of IκBα. Taken together, to our knowledge, our study for the first time revealed the inhibitory function of USP39 in the NF-κB inflammatory response, providing a previously unknown mechanism for control of inflammatory cytokine induction in the cellular anti-inflammatory response.

Osteoclasts, Master Sculptors of Bone

Osteoclasts are multinucleated cells with the unique ability to resorb bone matrix. Excessive production or activation of osteoclasts leads to skeletal pathologies that affect a significant portion of the population. Although therapies that effectively target osteoclasts have been developed, they are associated with sometimes severe side effects, and a fuller understanding of osteoclast biology may lead to more specific treatments. Along those lines, a rich body of work has defined essential signaling pathways required for osteoclast formation, function, and survival. Nonetheless, recent studies have cast new light on long-held views regarding the origin of these cells during development and homeostasis, their life span, and the cellular sources of factors that drive their production and activity during homeostasis and disease. In this review, we discuss these new findings in the context of existing work and highlight areas of ongoing and future investigation.

New insights into the Lck-NF-κB signaling pathway

Lck is essential for the development, activity, and proliferation of T cells, which may contribute to pathological progression and development of human diseases, such as autoimmune disorders and cancers when functioning aberrantly. Nuclear factor-κB (NF-κB) was initially discovered as a factor bound to the κ light-chain immunoglobulin enhancer in the nuclei of activated B lymphocytes. Activation of the nuclear factor-κB pathway controls expression of several genes that are related to cell survival, apoptosis, and inflammation. Abnormal expression of Lck and nuclear factor-κB has been found in autoimmune diseases and malignancies, including rheumatoid arthritis, systemic lupus erythematosus, acute T cell lymphocytic leukemia, and human chronic lymphocytic leukemia, etc. Nuclear factor-κB inhibition is effective against autoimmune diseases and malignancies through blocking inflammatory responses, although it may lead to serious adverse reactions that are unexpected and unwanted. Further investigation of the biochemical and functional interactions between nuclear factor-κB and other signaling pathways may be helpful to prevent side-effects. This review aims to clarify the Lck-nuclear factor-κB signaling pathway, and provide a basis for identification of new targets and therapeutic approaches against autoimmune diseases and malignancies.

The role of secreted osteoclastogenic factor of activated T cells in bone remodeling

The process of bone remodeling is connected with the regulated balance between bone cell populations (including bone-forming osteoblasts, bone-resorbing osteoclasts, and the osteocyte). And the mechanism of bone remodeling activity is related to the major pathway, receptor activator of nuclear factor kappaB (RANK)/RANK ligand (RANKL)/osteoprotegerin (OPG) signaling axis. Recently, researchers have found a novel cytokine secreted by activated T cells, which is related to osteoclastogenesis in the absence of osteoblasts or RANKL, leading to bone destruction. They name it the secreted osteoclastogenic factor of activated T cells (SOFAT). SOFAT has been proven to play an essential role in bone remodeling, like mediating the bone resorption in rheumatoid arthritis (RA) and periodontitis. In this review, we outline the latest research concerning SOFAT and discuss the characteristics, location, and regulation of SOFAT. We also summarize the clinical progress of SOFAT and assume the future therapeutic target in some diseases related to bone remodeling.

CST6 suppresses osteolytic bone disease in multiple myeloma by blocking osteoclast differentiation

Osteolytic bone disease is a hallmark of multiple myeloma (MM). A significant fraction (~20%) of MM patients do not develop osteolytic lesions (OL). The molecular basis for the absence of bone disease in MM is not understood. We combined PET-CT and gene expression profiling (GEP) of purified bone marrow (BM) CD138+ MM cells from 512 newly diagnosed MM patients to reveal that elevated expression of cystatin M/E (CST6) was significantly associated with the absence of OL in MM. An enzyme-linked immunosorbent assay revealed a strong correlation between CST6 levels in BM serum/plasma and CST6 mRNA expression. Both recombinant CST6 protein and BM serum from patients with high CST6 significantly inhibited the activity of the osteoclast-specific protease cathepsin K, and blocked osteoclast differentiation and function. Recombinant CST6 inhibited bone destruction in ex vivo and in vivo myeloma models. Single cell RNA-sequencing identified that CST6 attenuates polarization of monocytes to osteoclast precursors. Furthermore, CST6 protein blocks osteoclast differentiation by suppressing cathepsin-mediated cleavage of NF-κB/p100 and TRAF3 following RANKL stimulation. Secretion by MM cells of CST6, an inhibitor of osteoclast differentiation and function, suppresses osteolytic bone disease in MM and probably other diseases associated with osteoclast-mediated bone loss.

Critical Roles of NF-κB Signaling Molecules in Bone Metabolism Revealed by Genetic Mutations in Osteopetrosis

The nuclear factor-κB (NF-κB) transcription factor family consists of five related proteins, RelA (p65), c-Rel, RelB, p50/p105 (NF-κB1), and p52/p100 (NF-κB2). These proteins are important not only for inflammation and the immune response but also for bone metabolism. Activation of NF-κB occurs via the classic and alternative pathways. Inflammatory cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL)-1β, activate the former, and cytokines involved in lymph node formation, such as receptor activator of NF-κB ligand (RANKL) and CD40L, activate the latter. p50 and p52 double-knockout mice revealed severe osteopetrosis due to the total lack of osteoclasts, which are specialized cells for bone resorption. This finding suggests that the activation of NF-κB is required for osteoclast differentiation. The NF-κB signaling pathway is controlled by various regulators, including NF-κB essential modulator (NEMO), which is encoded by the IKBKG gene. In recent years, mutant forms of the IKBKG gene have been reported as causative genes of osteopetrosis, lymphedema, hypohidrotic ectodermal dysplasia, and immunodeficiency (OL-EDA-ID). In addition, a mutation in the RELA gene, encoding RelA, has been reported for the first time in newborns with high neonatal bone mass. Osteopetrosis is characterized by a diffuse increase in bone mass, ranging from a lethal form observed in newborns to an asymptomatic form that appears in adulthood. This review describes the genetic mutations in NF-κB signaling molecules that have been identified in patients with osteopetrosis.

The Therapeutic Potential of Targeting NIK in B Cell Malignancies

NF-κB-inducing kinase (NIK) is a key player in non-canonical NF-κB signaling, involved in several fundamental cellular processes, and is crucial for B cell function and development. In response to certain signals and ligands, such as CD40, BAFF and lymphotoxin-β activation, NIK protein stabilization and subsequent NF-κB activation is achieved. Overexpression or overactivation of NIK is associated with several malignancies, including activating mutations in multiple myeloma (MM) and gain-of-function in MALT lymphoma as a result of post-translational modifications. Consequently, drug discovery studies are devoted to pharmacologic modulation of NIK and development of specific novel small molecule inhibitors. However, disease-specific in vitro and in vivo studies investigating NIK inhibition are as of yet lacking, and clinical trials with NIK inhibitors remain to be initiated. In order to bridge the gap between bench and bedside, this review first briefly summarizes our current knowledge on NIK activation, functional activity and stability. Secondly, we compare current inhibitors targeting NIK based on efficacy and specificity, and provide a future perspective on the therapeutic potential of NIK inhibition in B cell malignancies.

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.

Systems approaches to investigate the role of NF-κB signaling in aging

The nuclear factor-κB (NF-κB) signaling pathway is one of the most well-studied pathways related to inflammation, and its involvement in aging has attracted considerable attention. As aging is a complex phenomenon and is the result of a multi-step process, the involvement of the NF-κB pathway in aging remains unclear. To elucidate the role of NF-κB in the regulation of aging, different systems biology approaches have been employed. A multi-omics data-driven approach can be used to interpret and clarify unknown mechanisms but cannot generate mechanistic regulatory structures alone. In contrast, combining this approach with a mathematical modeling approach can identify the mechanistics of the phenomena of interest. The development of single-cell technologies has also helped clarify the heterogeneity of the NF-κB response and underlying mechanisms. Here, we review advances in the understanding of the regulation of aging by NF-κB by focusing on omics approaches, single-cell analysis, and mathematical modeling of the NF-κB network.

On the Immunometabolic Role of NF-κB in Adipocytes

Two decades of research have established that Nuclear Factor-κB (NF-κB) signaling plays a critical role in reprogramming the fat cell transcriptome towards inflammation in response to overnutrition and metabolic stress. Several groups have suggested that inhibition of NF-κB signaling could have metabolic benefits for obesity-associated adipose tissue inflammation. However, two significant problems arise with this approach. The first is how to deliver general NF-κB inhibitors into adipocytes without allowing these compounds to disrupt normal functioning in cells of the immune system. The second issue is that general inhibition of canonical NF-κB signaling in adipocytes will likely lead to a massive increase in adipocyte apoptosis under conditions of metabolic stress, leading full circle into a secondary inflammation (However, this problem may not be true for non-canonical NF-κB signaling.). This review will focus on the research that has examined canonical and non-canonical NF-κB signaling in adipocytes, focusing on genetic studies that examine loss-of-function of NF-κB specifically in fat cells. Although the development of general inhibitors of canonical NF-κB signaling seems unlikely to succeed in alleviating adipose tissue inflammation in humans, the door remains open for more targeted therapeutics. In principle, these would include compounds that interrogate NF-κB DNA binding, protein-protein interactions, or post-translational modifications that partition NF-κB activity towards some genes and away from others in adipocytes. I also discuss the possibility for inhibitors of non-canonical NF-κB signaling to realize success in mitigating fat cell dysfunction in obesity. To plant the seeds for such approaches, much biochemical “digging” in adipocytes remains; this includes identifying—in an unbiased manner–NF-κB direct and indirect targets, genomic DNA binding sites for all five NF-κB subunits, NF-κB protein-protein interactions, and post-translational modifications of NF-κB in fat cells.

Regulation of TNF-Induced Osteoclast Differentiation

Increased osteoclast (OC) differentiation and activity is the critical event that results in bone loss and joint destruction in common pathological bone conditions, such as osteoporosis and rheumatoid arthritis (RA). RANKL and its decoy receptor, osteoprotegerin (OPG), control OC differentiation and activity. However, there is a specific concern of a rebound effect of denosumab discontinuation in treating osteoporosis. TNFα can induce OC differentiation that is independent of the RANKL/RANK system. In this review, we discuss the factors that negatively and positively regulate TNFα induction of OC formation, and the mechanisms involved to inform the design of new anti-resorptive agents for the treatment of bone conditions with enhanced OC formation. Similar to, and being independent of, RANKL, TNFα recruits TNF receptor-associated factors (TRAFs) to sequentially activate transcriptional factors NF-κB p50 and p52, followed by c-Fos, and then NFATc1 to induce OC differentiation. However, induction of OC formation by TNFα alone is very limited, since it also induces many inhibitory proteins, such as TRAF3, p100, IRF8, and RBP-j. TNFα induction of OC differentiation is, however, versatile, and Interleukin-1 or TGFβ1 can enhance TNFα-induced OC formation through a mechanism which is independent of RANKL, TRAF6, and/or NF-κB. However, TNFα polarized macrophages also produce anabolic factors, including insulin such as 6 peptide and Jagged1, to slow down bone loss in the pathological conditions. Thus, the development of novel approaches targeting TNFα signaling should focus on its downstream molecules that do not affect its anabolic effect.

Origin of the Functional Distinctiveness of NF-κB/p52

The transcription regulators of the NF-κB family have emerged as a critical factor affecting the function of various adult tissues. The NF-κB family transcription factors are homo- and heterodimers made up of five monomers (p50, p52, RelA, cRel and RelB). The family is distinguished by sequence homology in their DNA binding and dimerization domains, which enables them to bind similar DNA response elements and participate in similar biological programs through transcriptional activation and repression of hundreds of genes. Even though the family members are closely related in terms of sequence and function, they all display distinct activities. In this review, we discuss the sequence characteristics, protein and DNA interactions, and pathogenic involvement of one member of family, NF-κB/p52, relative to that of other members. We pinpoint the small sequence variations within the conserved region that are mostly responsible for its distinct functional properties.

The NF-κB Pathway: Modulation by Entamoeba histolytica and Other Protozoan Parasites

Protozoan parasites have led to worldwide devastation because of their ability to cause infectious diseases. They have evolved as successful pathogens in part because of their remarkable and sophisticated ways to evade innate host defenses. This holds true for both intracellular and extracellular parasites that deploy multiple strategies to circumvent innate host defenses for their survival. The different strategies protozoan parasites use include hijacking the host cellular signaling pathways and transcription factors. In particular, the nuclear factor-κB (NF-κB) pathway seems to be an attractive target for different pathogens owing to their central role in regulating prompt innate immune responses in host defense. NF-κB is a ubiquitous transcription factor that plays an indispensable role not only in regulating immediate immune responses against invading pathogens but is also a critical regulator of cell proliferation and survival. The major immunomodulatory components include parasite surface and secreted proteins/enzymes and stimulation of host cells intracellular pathways and inflammatory caspases that directly or indirectly interfere with the NF-κB pathway to thwart immune responses that are directed for containment and/or elimination of the pathogen. To showcase how protozoan parasites exploits the NF-κB signaling pathway, this review highlights recent advances from Entamoeba histolytica and other protozoan parasites in contact with host cells that induce outside-in and inside-out signaling to modulate NF-κB in disease pathogenesis and survival in the host.

IRF2 enhances RANKL-induced osteoclast differentiation via regulating NF-κB/NFATc1 signaling

Interferon regulatory factors (IRFs) play roles in various biological processes including cytokine signaling, cell growth regulation and hematopoietic development. Although it has been reported that several IRFs are involved in bone metabolism, the role of IRF2 in bone cells has not been elucidated. Here, we investigated the involvement of IRF2 in RANKL-induced osteoclast differentiation. IRF2 overexpression in osteoclast pre-cursor cells enhanced osteoclast differentiation by regulating the expression of NFATc1, a master regulator of osteoclasto-genesis. Conversely, IRF2 knockdown inhibited osteoclast differentiation and decreased the NFATc1 expression. Moreover, IRF2 increased the translocation of NF-κB subunit p65 to the nucleus in response to RANKL and subsequently induced the expression of NFATc1. IRF2 plays an important role in RANKL-induced osteoclast differentiation by regulating NF-κB/NFATc1 signaling pathway. Taken together, we demonstrated the molecular mechanism of IRF2 in osteoclast differentiation, and provide a molecular basis for potential therapeutic targets for the treatment of bone diseases characterized by excessive bone resorption.

Combined immunodeficiency with autoimmunity caused by a homozygous missense mutation in inhibitor of nuclear factor 𝛋B kinase alpha (IKKα)

[Figure: see text].

Inhibitory feedback control of NF-κB signalling in health and disease

Cells must adapt to changes in their environment to maintain cell, tissue and organismal integrity in the face of mechanical, chemical or microbiological stress. Nuclear factor-κB (NF-κB) is one of the most important transcription factors that controls inducible gene expression as cells attempt to restore homeostasis. It plays critical roles in the immune system, from acute inflammation to the development of secondary lymphoid organs, and also has roles in cell survival, proliferation and differentiation. Given its role in such critical processes, NF-κB signalling must be subject to strict spatiotemporal control to ensure measured and context-specific cellular responses. Indeed, deregulation of NF-κB signalling can result in debilitating and even lethal inflammation and also underpins some forms of cancer. In this review, we describe the homeostatic feedback mechanisms that limit and ‘re-set’ inducible activation of NF-κB. We first describe the key components of the signalling pathways leading to activation of NF-κB, including the prominent role of protein phosphorylation and protein ubiquitylation, before briefly introducing the key features of feedback control mechanisms. We then describe the array of negative feedback loops targeting different components of the NF-κB signalling cascade including controls at the receptor level, post-receptor signalosome complexes, direct regulation of the critical ‘inhibitor of κB kinases’ (IKKs) and inhibitory feedforward regulation of NF-κB-dependent transcriptional responses. We also review post-transcriptional feedback controls affecting RNA stability and translation. Finally, we describe the deregulation of these feedback controls in human disease and consider how feedback may be a challenge to the efficacy of inhibitors.

Therapeutic perspective of thiosemicarbazones derivatives in inflammatory pathologies: A summary of in vitro/in vivo studies

Following induction of inflammation, the nuclear factor kappa B (NF-κB) in activated macrophages induces the transcription of pro-inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), and cyclooxygenase (COX), an inflammatory enzyme implicated in the synthesis of prostaglandins (PGs). The latter are involved in the transition and the maintenance of chronic inflammation underling various chronic disorders that require treatment. Concerning this, many anti-inflammatory drugs are available to treat the inflammatory disorders, but their therapeutic use is associated with a variety of side effects. Therefore, the discovery of new safer and potential anti-inflammatory drugs is necessary. In this regard, thiosemicarbazones (TSC) compounds and their metals complexes attracted high interest due to their wide range of biological activities, interestingly, the anti-inflammatory activity. They are formed by the action of thiosemicarbazide on an aldehyde or ketone, and contain a sulfur atom in place of the oxygen atom. Their ability to form a stable complex with transition metal is known to enhances the biological activity and reduces the side effects of the parent compound. Thus, this review article describes the inflammatory response mediated by NF-κB-COX-PGs and summarizes the anti-inflammatory activity of different thiosemicarbazones derivatives synthesized in research area.

BYHWD Alleviates Inflammatory Response by NIK-Mediated Repression of the Noncanonical NF-κB Pathway During ICH Recovery

Intracerebral hemorrhage (ICH) is a life-threatening type of stroke that lacks effective treatments. The inflammatory response following ICH is a vital response that affects brain repair and organism recovery. The nuclear factor κB (NF-κB) signaling pathway is considered one of the most important inflammatory response pathways and one of its response pathways, the noncanonical NF-κB signaling pathway, is known to be associated with persistent effect and chronic inflammation. NF-κB–inducing kinase (NIK) via the noncanonical NF-κB signaling plays a key role in controlling inflammation. Here, we investigated potential effects of the traditional Chinese medicine formula Buyang Huanwu Decoction (BYHWD) on inflammatory response in a rat model of ICH recovery by inhibiting the NIK-mediated the noncanonical NF-κB signaling pathway. In the first part, rats were randomly divided into three groups: the sham group, the ICH group, and the BYHWD group. ICH was induced in rats by injecting collagenase (type VII) into the right globus pallidus of rats' brain. For the BYHWD group, rats were administered BYHWD (4.36 g/kg) once a day by intragastric administration until they were sacrificed. Neurological function was evaluated in rats by a modified neurological severity score (mNSS), the corner turn test, and the foot-fault test. The cerebral edema showed the degree of inflammatory response by sacrificed brain water content. Western blot and real-time quantitative reverse transcription PCR tested the activity of inflammatory response and noncanonical NF-κB signaling. In the second part, siRNA treatment and assessment of inflammation level as well as alterations in the noncanonical NF-κB signaling were performed to determine whether the effect of BYHWD on inflammatory response was mediated by suppression of NIK via the noncanonical NF-κB signaling pathway. We show that BYHWD treated rats exhibited: (i) better health conditions and better neural functional recovery; (ii) decreased inflammatory cytokine and the edema; (iii) reduced expression of NIK, a key protein in unregulated the noncanonical NF-κB signaling pathways; (iv) when compared with pretreated rats with NIK targeting (NIK siRNAs), showed the same effect of inhibiting the pathway and decreased inflammatory cytokine. BYHWD can attenuate the inflammatory response during ICH recovery in rats by inhibiting the NIK-mediated noncanonical NF-κB signaling pathway.

Trim14 promotes osteoclastogenesis and noncanonical NF-κB activation by targeting p100/p52 in chronic periodontitis

BACKGROUND

Periodontitis disease infection initiates host immune response, and alveolar bone damage is a hallmark of periodontitis. Bone damage occurs due to changes in osteoclast activity in response to local inflammation. Nuclear factor κB (NF-κB) signaling is essential for inflammatory responses and plays a pivotal role in osteoclast formation and activation. Tripartite motif 14 (Trim14) is a crucial regulator of the noncanonical NF-κB signaling. Here, we investigated the role of Trim14 in chronic periodontitis.

METHODS

The development of immune cells and osteoclast formation was evaluated with flow cytometry, qRT-PCR, and histochemical staining. Proinflammatory cytokines were checked by ELISA and qRT-PCR. Protein expression was determined by immunoblotting. Also, the cemento-enamel junction-alveolar bone crest distance was evaluated in the mouse model.

RESULTS

Development of innate and adaptive cells was not impaired from the deletion of Trim14. However, the genetic loss of Trim14 remarkably suppressed RANKL-induced osteoclastogenesis, without affecting TLR-induced proinflammatory cytokines except for Il-23a expression. The Trim14 deletion also suppressed the activation of noncanonical NF-κB signaling by targeting p100/p52. Importantly, the deletion of NIK diminished the effects of Trim14 on the inflammatory responses in vivo on chronic periodontitis responses.

CONCLUSION

TRIM14 may be a positive regulator to promote osteoclastogenesis and proinflammatory cytokine secretion.

TNF is a homoeostatic regulator of distinct epigenetically primed human osteoclast precursors

OBJECTIVES

Circulating myeloid precursors are responsible for post-natal osteoclast (OC) differentiation and skeletal health, although the exact human precursors have not been defined. Enhanced osteoclastogenesis contributes to joint destruction in rheumatoid arthritis (RA) and tumour necrosis factor (TNF) is a well-known pro-osteoclastogenic factor. Herein, we investigated the interplay between receptor activator of nuclear factor kappa-Β ligand (RANK-L), indispensable for fusion of myeloid precursors and the normal development of OCs, and TNF in directing the differentiation of diverse pre-OC populations derived from human peripheral blood.

METHODS

Flow cytometric cell sorting and analysis was used to assess the potential of myeloid populations to differentiate into OCs. Transcriptomic, epigenetic analysis, receptor expression and inhibitor experiments were used to unravel RANK-L and TNF signalling hierarchy.

RESULTS

TNF can act as a critical homoeostatic regulator of CD14⁺ monocyte (MO) differentiation into OCs by inhibiting osteoclastogenesis to favour macrophage development. In contrast, a distinct previously unidentified CD14^-CD16^-CD11c⁺ myeloid pre-OC population was exempt from this negative regulation. In healthy CD14⁺ MOs, TNF drove epigenetic modification of the RANK promoter via a TNFR1-IKKβ-dependent pathway and halted osteoclastogenesis. In a subset of patients with RA, CD14⁺ MOs exhibited an altered epigenetic state that resulted in dysregulated TNF-mediated OC homoeostasis.

CONCLUSIONS

These findings fundamentally re-define the relationship between RANK-L and TNF. Moreover, they have identified a novel pool of human circulating non-MO OC precursors that unlike MOs are epigenetically preconditioned to ignore TNF-mediated signalling. In RA, this epigenetic preconditioning occurs in the MO compartment providing a pathological consequence of failure of this pathway.

Stepwise cell fate decision pathways during osteoclastogenesis at single-cell resolution

Osteoclasts are the exclusive bone-resorbing cells, playing a central role in bone metabolism, as well as the bone damage that occurs under pathological conditions^1,2. In postnatal life, haematopoietic stem-cell-derived precursors give rise to osteoclasts in response to stimulation with macrophage colony-stimulating factor and receptor activator of nuclear factor-κB ligand, both of which are produced by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes^1-3. However, the precise mechanisms underlying cell fate specification during osteoclast differentiation remain unclear. Here, we report the transcriptional profiling of 7,228 murine cells undergoing in vitro osteoclastogenesis, describing the stepwise events that take place during the osteoclast fate decision process. Based on our single-cell transcriptomic dataset, we find that osteoclast precursor cells transiently express CD11c, and deletion of receptor activator of nuclear factor-κB specifically in CD11c-expressing cells inhibited osteoclast formation in vivo and in vitro. Furthermore, we identify Cbp/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 2 (Cited2) as the molecular switch triggering terminal differentiation of osteoclasts, and deletion of Cited2 in osteoclast precursors in vivo resulted in a failure to commit to osteoclast fate. Together, the results of this study provide a detailed molecular road map of the osteoclast differentiation process, refining and expanding our understanding of the molecular mechanisms underlying osteoclastogenesis.

Pharmacological significance of the non-canonical NF-κB pathway in tumorigenesis

The understanding of the impact of the non-canonical NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) pathway in several human diseases including autoimmune, inflammatory and cancers has been on the rise. This pathway induces the expression of several important genes involved in diverse biological processes. Though progress has been made in understanding the activation, regulation and biological functions of the non-canonical NF-κB signaling mechanism, no specific drug has been approved to target NF-κB inducing kinase (NIK), the key signaling molecule in this pathway. The inhibition of NIK can serve as a potential therapeutic strategy for various ailments, especially for the treatment of different types of human cancers. There are other targetable downstream molecules in this pathway as well. This review highlights the possible role of the non-canonical NF-κB pathway in normal physiology as well as in different cancers and discusses about various pharmacological strategies to modulate the activation of this pathway.

Grape Seed Proanthocyanidin Extract Prevents Bone Loss via Regulation of Osteoclast Differentiation, Apoptosis, and Proliferation

Dietary procyanidin has been shown to be an important bioactive component that regulates various pharmacological activities to maintain metabolic homeostasis. In particular, grape seed proanthocyanidin extract (GSPE) is a commercially available medicine for the treatment of venous and lymphatic dysfunction. This study aimed to investigate whether GSPE protects against lipopolysaccharide (LPS)-induced bone loss in vivo and the related mechanism of action in vitro. The administration of GSPE restored the inflammatory bone loss phenotype stimulated by acute systemic injection of LPS in vivo. GSPE strongly suppressed receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast differentiation and bone resorption activity of mature osteoclasts by decreasing the RANKL-induced nuclear factor-κB transcription activity. GSPE mediates this effect through decreased phosphorylation and degradation of NF-κB inhibitor (IκB) by IκB kinaseβ, subsequently inhibiting proto-oncogene cellular Fos and nuclear factor of activated T cells. Additionally, GSPE promotes osteoclast proliferation by increasing the phosphorylation of components of the Akt and mitogen-activated protein kinase signaling pathways and it also inhibits apoptosis by decreasing the activity of caspase-8, caspase-9, and caspase-3, as corroborated by a decrease in the Terminal deoxynucleotidyl transferase dUTP nick end labeling -positive cells. Our study suggests a direct effect of GSPE on the proliferation, differentiation, and apoptosis of osteoclasts and reveals the mechanism responsible for the therapeutic potential of GSPE in osteoclast-associated bone metabolism disease.

Both aerobic glycolysis and mitochondrial respiration are required for osteoclast differentiation

Excessive bone resorption over bone formation is the root cause for bone loss leading to osteoporotic fractures. Development of new antiresorptive therapies calls for a holistic understanding of osteoclast differentiation and function. Although much has been learned about the molecular regulation of osteoclast biology, little is known about the metabolic requirement and bioenergetics during osteoclastogenesis. Here, we report that glucose metabolism through oxidative phosphorylation (OXPHOS) is the predominant bioenergetic pathway to support osteoclast differentiation. Meanwhile, increased lactate production from glucose, known as aerobic glycolysis when oxygen is abundant, is also critical for osteoclastogenesis. Genetic deletion of Glut1 in osteoclast progenitors reduces aerobic glycolysis without compromising OXPHOS, but nonetheless diminishes osteoclast differentiation in vitro. Glut1 deficiency in the progenitors leads to osteopetrosis due to fewer osteoclasts specifically in the female mice. Thus, Glut1-mediated glucose metabolism through both lactate production and OXPHOS is necessary for normal osteoclastogenesis.

Small Molecule NF-κB Pathway Inhibitors in Clinic

Nuclear factor kappa B (NF-κB) signaling is implicated in all major human chronic diseases, with its role in transcription of hundreds of gene well established in the literature. This has propelled research into targeting the NF-κB pathways for modulating expression of those genes and the diseases mediated by them. In-spite of the critical, but often promiscuous role played by this pathway and the inhibition causing adverse drug reaction, currently many biologics, macromolecules, and small molecules that modulate this pathway are in the market or in clinical trials. Furthermore, many marketed drugs that were later found to also have NF-κB targeting activity were repurposed for new therapeutic interventions. Despite the rising importance of biologics in drug discovery, small molecules got around 76% of US-FDA (Food and Drug Administration-US) approval in the last decade. This encouraged us to review information regarding clinically relevant small molecule inhibitors of the NF-κB pathway from cell surface receptor stimulation to nuclear signaling. We have also highlighted the underexplored targets in this pathway that have potential to succeed in clinic.

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 tethering function of mitofusin2 controls osteoclast differentiation by modulating the Ca2+-NFATc1 axis

Dynamic regulation of the mitochondrial network by mitofusins (MFNs) modulates energy production, cell survival, and many intracellular signaling events, including calcium handling. However, the relative importance of specific mitochondrial functions and their dependence on MFNs vary greatly among cell types. Osteoclasts have many mitochondria, and increased mitochondrial biogenesis and oxidative phosphorylation enhance bone resorption, but little is known about the mitochondrial network or MFNs in osteoclasts. Because expression of each MFN isoform increases with osteoclastogenesis, we conditionally deleted MFN1 and MFN2 (double conditional KO (dcKO)) in murine osteoclast precursors, finding that this increased bone mass in young female mice and abolished osteoclast precursor differentiation into mature osteoclasts in vitro Defective osteoclastogenesis was reversed by overexpression of MFN2 but not MFN1; therefore, we generated mice lacking only MFN2 in osteoclasts. MFN2-deficient female mice had increased bone mass at 1 year and resistance to Receptor Activator of NF-κB Ligand (RANKL)-induced osteolysis at 8 weeks. To explore whether MFN-mediated tethering or mitophagy is important for osteoclastogenesis, we overexpressed MFN2 variants defective in either function in dcKO precursors and found that, although mitophagy was dispensable for differentiation, tethering was required. Because the master osteoclastogenic transcriptional regulator nuclear factor of activated T cells 1 (NFATc1) is calcium-regulated, we assessed calcium release from the endoplasmic reticulum and store-operated calcium entry and found that the latter was blunted in dcKO cells. Restored osteoclast differentiation by expression of intact MFN2 or the mitophagy-defective variant was associated with normalization of store-operated calcium entry and NFATc1 levels, indicating that MFN2 controls mitochondrion-endoplasmic reticulum tethering in osteoclasts.

Sclareol prevents ovariectomy-induced bone loss in vivo and inhibits osteoclastogenesis in vitro via suppressing NF-κB and MAPK/ERK signaling pathways

Postmenopausal osteoporosis (PMO) is a progressive disease occurring in elderly postmenopausal women that is characterized by low bone mass and impaired bone quality. Sclareol is a natural product (initially isolated from the leaves and flowers of Salvia Sclarea) that possesses immune-regulation and anti-inflammatory effects, but its role in osteoclastic formation and function as well as the PMO remains unknown. In the current study, we investigated the inhibitory effect of sclareol on osteoclastogenesis and progression of PMO. In vitro, sclareol not only inhibited osteoclast formation but also suppressed osteoclast function. The expression of the receptor activator of NF-κB ligand (RANKL)-induced osteoclast marker gene and protein was also reduced by sclareol treatment. Mechanistically, we found that sclareol inhibits RANKL-induced NF-κB and MAPK/ERK pathway activation. Furthermore, sclareol exerted a protective effect against bone loss in an ovariectomy-induced mouse model. Taken together, our findings suggest that sclareol has potential value as a therapeutic agent for PMO.

Persistent Breeding-Induced Endometritis in Mares - a Multifaceted Challenge: From Clinical Aspects to Immunopathogenesis and Pathobiology

Post-breeding endometritis (i.e., inflammation/infection of the endometrium), is a physiological reaction taking place in the endometrium of mares within 48 hours post-breeding, aimed to clear seminal plasma, excess sperm, microorganisms, and debris from the uterine lumen in preparation for the arrival of an embryo. Mares are classified as susceptible or resistant to persistent breeding-induced endometritis (PBIE) based on their ability to clear this inflammation/infection by 48 hours post-breeding. Mares susceptible to PBIE, or those with difficulty clearing infection/inflammation, have a deficient immune response and compromised physical mechanisms of defense against infection. Molecular pathways of the innate immune response known to be involved in PBIE are discussed herein. The role of the adaptive uterine immune response on PBIE remains to be elucidated in horses. Advances in the pathobiology of microbes involved in PBIE are also revised here. Traditional and non-traditional therapeutic modalities for endometritis are contrasted and described in the context of clinical and molecular aspects. In recent years, the lack of efficacy of traditional therapeutic modalities, alongside the ever-increasing incidence of antibiotic-resistant microorganisms, has enforced the development of non-traditional therapies. Novel biological products capable of modulating the endometrial inflammatory response are also discussed here as part of the non-traditional therapies for endometritis.

Intrinsic Restriction of TNF-Mediated Inflammatory Osteoclastogenesis and Bone Resorption

TNF (Tumor necrosis factor) is a pleiotropic cytokine that plays an important role in immunity and inflammatory bone destruction. Homeostatic osteoclastogenesis is effectively induced by RANKL (Receptor activator of nuclear factor kappa-B ligand). In contrast, TNF often acts on cell types other than osteoclasts, or synergically with RANKL to indirectly promote osteoclastogenesis and bone resorption. TNF and RANKL are members of the TNF superfamily. However, the direct osteoclastogenic capacity of TNF is much weaker than that of RANKL. Recent studies have uncovered key intrinsic mechanisms by which TNF acts on osteoclast precursors to restrain osteoclastogenesis, including the mechanisms mediated by RBP-J signaling, RBP-J and ITAM (Immunoreceptor tyrosine-based activation motif) crosstalk, RBP-J mediated regulatory network, NF-κB p100, IRF8, and Def6. Some of these mechanisms, such as RBP-J and its mediated regulatory network, uniquely and predominantly limit osteoclastogenesis mediated by TNF but not by RANKL. As a consequence, targeting RBP-J activities suppresses inflammatory bone destruction but does not significantly impact normal bone remodeling or inflammation. Hence, discovery of these intrinsic inhibitory mechanisms addresses why TNF has a weak osteoclastogenic potential, explains a significant difference between RANKL and TNF signaling, and provides potentially new or complementary therapeutic strategies to selectively treat inflammatory bone resorption, without undesirable effects on normal bone remodeling or immune response in disease settings.

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.

Innate Immunity and Alcohol

The innate immunity has evolved during millions of years, and thus, equivalent or comparable components are found in most vertebrates, invertebrates, and even plants. It constitutes the first line of defense against molecules, which are either pathogen-derived or a danger signal themselves, and not seldom both. These molecular patterns are comprised of highly conserved structures, a common trait in innate immunity, and constitute very potent triggers for inflammation mediated via extracellular or intracellular pattern recognition receptors. Human culture is often interweaved with the consumption of alcohol, in both drinking habits, its acute or chronical misuse. Apart from behavioral effects as often observed in intoxicated individuals, alcohol consumption also leads to immunological modulation on the humoral and cellular levels. In the last 20 years, major advances in this field of research have been made in clinical studies, as well as in vitro and in vivo research. As every physician will experience intoxicated patients, it is important to be aware of the changes that this cohort undergoes. This review will provide a summary of the current knowledge on the influence of alcohol consumption on certain factors of innate immunity after a hit, followed by the current studies that display the effect of alcohol with a description of the model, the mode of alcohol administration, as well as its dose. This will provide a way for the reader to evaluate the findings presented.

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.

Does TNF Promote or Restrain Osteoclastogenesis and Inflammatory Bone Resorption?

Chronic inflammation is one of the most evident and common pathological conditions leading to deregulated osteoclastogenesis and bone remodeling. Tumor necrosis factor (TNF) as a pleiotropic cytokine plays a key role, not only in inflammation, but also in bone erosion in diseases associated with bone loss. TNF can stimulate the proliferation of osteoclast precursors and, in most conditions, act together with other cytokines and growth factors such as receptor activator of nuclear factor (NF)-[kappa]B ligand (RANKL), interleukin-6, and transforming growth factor beta to synergistically promote osteoclast formation and bone resorption in vivo. A longstanding enigma in the field is why TNF alone is not able to induce osteoclast differentiation as effectively as the same superfamily member RANKL, a physiological master osteoclastogenic cytokine. Recent studies have highlighted several lines of evidence showing the intrinsic mechanisms through RBP-J, NF-[kappa]B p100/TNF receptor-associated factor 3, or interferon regulatory factor-8 that restrain TNF-induced osteoclast differentiation and bone resorption. These feedback inhibitory mechanisms driven by TNF shed light into the current paradigm of osteoclastogenesis and would provide novel therapeutic implications on controlling inflammatory bone resorption.

Inflammation time-axis in aseptic loosening of total knee arthroplasty: A preliminary study

OBJECTIVE

Aseptic loosening (AL) is the most frequent long-term reason for revision of total knee arthroplasty (TKA) affecting about 15-20% patients within 20 years after the surgery. Although there is a solid body of evidence about the crucial role of inflammation in the AL pathogenesis, scared information on inflammation signature and its time-axis in tissues around TKA exists.

DESIGN

The inflammation protein signatures in pseudosynovial tissues collected at revision surgery from patients with AL (AL, n = 12) and those with no clinical/radiographic signs of AL (non-AL, n = 9) were investigated by Proximity Extension Assay (PEA)-Immunoassay and immunohistochemistry.

RESULTS

AL tissues had elevated levels of TNF-family members sTNFR2, TNFSF14, sFasL, sBAFF, cytokines/chemokines IL8, CCL2, IL1RA/IL36, sIL6R, and growth factors sAREG, CSF1, comparing to non-AL. High interindividual variability in protein levels was evident particularly in non-AL. Levels of sTNFR2, sBAFF, IL8, sIL6R, and MPO discriminated between AL and non-AL and were associated with the time from index surgery, suggesting the cumulative character of inflammatory osteolytic response to prosthetic byproducts. The source of elevated inflammatory molecules was macrophages and multinucleated osteoclast-like cells in AL and histiocytes and osteoclast-like cells in non-AL tissues, respectively. All proteins were present in higher levels in osteoclast-like cells than in macrophages.

CONCLUSIONS

Our study revealed a differential inflammation signature between AL and non-AL stages of TKA. It also highlighted the unique patient's response to TKA in non-AL stages. Further confirmation of our preliminary results on a larger cohort is needed. Analysis of the time-axis of processes ongoing around TKA implantation may help to understand the mechanisms driving periprosthetic bone resorption needed for diagnostic/preventative strategies.

NF-κB/Rel Transcription Factors in Pancreatic Cancer: Focusing on RelA, c-Rel, and RelB

Regulation of Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB)/Rel transcription factors (TFs) is extremely cell-type-specific owing to their ability to act disparately in the context of cellular homeostasis driven by cellular fate and the microenvironment. This is also valid for tumor cells in which every single component shows heterogenic effects. Whereas many studies highlighted a per se oncogenic function for NF-κB/Rel TFs across cancers, recent advances in the field revealed their additional tumor-suppressive nature. Specifically, pancreatic ductal adenocarcinoma (PDAC), as one of the deadliest malignant diseases, shows aberrant canonical-noncanonical NF-κB signaling activity. Although decades of work suggest a prominent oncogenic activity of NF-κB signaling in PDAC, emerging evidence points to the opposite including anti-tumor effects. Considering the dual nature of NF-κB signaling and how it is closely linked to many other cancer related signaling pathways, it is essential to dissect the roles of individual Rel TFs in pancreatic carcinogenesis and tumor persistency and progression. Here, we discuss recent knowledge highlighting the role of Rel TFs RelA, RelB, and c-Rel in PDAC development and maintenance. Next to providing rationales for therapeutically harnessing Rel TF function in PDAC, we compile strategies currently in (pre-)clinical evaluation.

NIK-IKK complex interaction controls NF-κB-dependent inflammatory activation of endothelium in response to LTβR ligation

NF-κB-inducing kinase (NIK; also known as MAP3K14) is a central regulator of non-canonical NF-κB signaling in response to stimulation of TNF receptor superfamily members, such as the lymphotoxin-β receptor (LTβR), and is implicated in pathological angiogenesis associated with chronic inflammation and cancer. Here, we identify a previously unrecognized role of the LTβR-NIK axis during inflammatory activation of human endothelial cells (ECs). Engagement of LTβR-triggered canonical and non-canonical NF-κB signaling promoted expression of inflammatory mediators and adhesion molecules, and increased immune cell adhesion to ECs. Sustained LTβR-induced inflammatory activation of ECs was NIK dependent, but independent of p100, indicating that the non-canonical arm of NF-κB is not involved. Instead, prolonged activation of canonical NF-κB signaling, through the interaction of NIK with IκB kinase α and β (also known as CHUK and IKBKB, respectively), was required for the inflammatory response. Endothelial inflammatory activation induced by synovial fluid from rheumatoid arthritis patients was significantly reduced by NIK knockdown, suggesting that NIK-mediated alternative activation of canonical NF-κB signaling is a key driver of pathological inflammatory activation of ECs. Targeting NIK could thus provide a novel approach for treating chronic inflammatory diseases.

Constitutive activation of the alternative NF-κB pathway disturbs endochondral ossification

Endochondral ossification is important for skeletal development. Recent findings indicate that the p65 (RelA) subunit, a main subunit of the classical nuclear factor-κB (NF-κB) pathway, plays essential roles in chondrocyte differentiation. Although several groups have reported that the alternative NF-κB pathway also regulates bone homeostasis, the role of the alternative NF-κB pathway in chondrocyte development is still unclear. Here, we analyzed the in vivo function of the alternative pathway on endochondral ossification using p100-deficient (p100^-/-) mice, which carry a homozygous deletion of the COOH-terminal ankyrin repeats of p100 but still express functional p52 protein. The alternative pathway was activated during the periarticular stage in wild-type mice. p100^-/- mice exhibited dwarfism, and histological analysis of the growth plate revealed abnormal arrangement of chondrocyte columns and a narrowed hypertrophic zone. Consistent with these observations, the expression of hypertrophic chondrocyte markers, type X collagen (ColX) or matrix metalloproteinase 13, but not early chondrogenic markers, such as Col II or aggrecan, was suppressed in p100^-/- mice. An in vivo BrdU tracing assay clearly demonstrated less proliferative activity in chondrocytes in p100^-/- mice. These defects were partly rescued when the RelB gene was deleted in p100^-/- mice. Taken together, the alternative NF-κB pathway may regulate chondrocyte proliferation and differentiation to maintain endochondral ossification.

Extracellular vesicles in bone: "dogrobbers" in the "eternal battle field"

Throughout human life, bone is constantly in a delicate dynamic equilibrium of synthesis and resorption, hosting finely-tuned bone mineral metabolic processes for bone homeostasis by collaboration or symphony among several cell types including osteoclasts (OCs), osteoblasts (OBs), osteocytes (OYs), vascular endothelial cells (ECs) and their precursors. Beyond these connections, a substantial level of communication seems to occur between bone and other tissues, and together, they form an organic unit linked to human health and disease. However, the current hypothesis, which includes growth factors, hormones and specific protein secretion, incompletely explains the close connections among bone cells or between bone and other tissues. Extracellular vesicles (EVs) are widely-distributed membrane structures consisting of lipid bilayers, membrane proteins and intravesicular cargo (including proteins and nucleic acids), ranging from 30 nm to 1000 nm in diameter, and their characters have been highly conserved throughout evolution. EVs have targeting abilities and the potential to transmit multidimensional, abundant and complicated information, as powerful and substantial "dogrobbers" mediating intercellular communications. As research has progressed, EVs have gradually become thought of as "dogrobbers" in bone tissue-the "eternal battle field" -in a delicate dynamic balance of destruction and reconstruction. In the current review, we give a brief description of the major constituent cells in bone tissues and explore the progress of current research on bone-derived EVs. In addition, this review also discusses in depth not only potential directions for future research to breakthrough in this area but also problems existing in current research that need to be solved for a better understanding of bone tissues.

NF-κB inducing kinase (NIK) is an essential post-transcriptional regulator of T-cell activation affecting F-actin dynamics and TCR signaling

NF-κB inducing kinase (NIK) is the key protein of the non-canonical NF-κB pathway and is important for the development of lymph nodes and other secondary immune organs. We elucidated the specific role of NIK in T cells using T-cell specific NIK-deficient (NIKΔT) mice. Despite showing normal development of lymphoid organs, NIKΔT mice were resistant to induction of CNS autoimmunity. T cells from NIKΔT mice were deficient in late priming, failed to up-regulate T-bet and to transmigrate into the CNS. Proteomic analysis of activated NIK-/- T cells showed de-regulated expression of proteins involved in the formation of the immunological synapse: in particular, proteins involved in cytoskeleton dynamics. In line with this we found that NIK-deficient T cells were hampered in phosphorylation of Zap70, LAT, AKT, ERK1/2 and PLCγ upon TCR engagement. Hence, our data disclose a hitherto unknown function of NIK in T-cell priming and differentiation.