Inhibition of RANKL-mediated osteoclast differentiation by selective TRAF6 decoy peptides

Exploring the Role of Ubiquitin-Proteasome System in the Pathogenesis of Parkinson's Disease

Parkinson's disease (PD) is a prevalent neurodegenerative disorder among the elderly population. The pathogenesis of PD encompasses genetic alterations, environmental factors, and age-related neurodegenerative processes. Numerous studies have demonstrated that aberrant functioning of the ubiquitin-proteasome system (UPS) plays a crucial role in the initiation and progression of PD. Notably, E3 ubiquitin ligases serve as pivotal components determining substrate specificity within UPS and are intimately associated with the regulation of various proteins implicated in PD pathology. This review comprehensively summarizes the mechanisms by which E3 ubiquitin ligases and deubiquitinating enzymes modulate PD-associated proteins and signaling pathways, while exploring the intricate relationship between UPS dysfunctions and PD etiology. Furthermore, this article discusses recent research advancements regarding inhibitors targeting PD-related E3 ubiquitin ligases.

Ethanol extract of Cyathulae Radix inhibits osteoclast differentiation and bone loss

Cyathulae Radix, a traditional Chinese medicine and a common vegetable, boasts a history spanning millennia. It enhances bone density, boosts metabolism, and effectively alleviates osteoporosis-induced pain. Despite its historical use, the molecular mechanisms behind Cyathulae Radix's impact on osteoporosis remain unexplored. In this study, we investigated the effects and mechanisms of Cyathulae Radix ethanol extract (CEE) in inhibiting osteoporosis and osteoclastogenesis. Eight-week-old female mice underwent ovariectomy and were treated with CEE for eight weeks. Micro-computed tomography (micro-CT) assessed histomorphometric parameters, bone tissue staining observed distal femur histomorphology, and three-point bending tests evaluated tibia mechanical properties. Enzyme-linked immunosorbent assay (ELISA) measured serum estradiol (E2), receptor activator for nuclear factor B ligand (RANKL), and osteoprotegerin (OPG) levels. Osteoclastogenesis-related markers were analyzed via Western blotting (WB) and quantitative real-time polymerase chain reaction (qRT-PCR). Additionally, CEE effects on RANKL-induced osteoclast formation and bone resorption were investigated in vitro using tartrate-resistant acid phosphatase (TRAP) staining, qRT-PCR, and WB assay. Compared with the ovariectomy (OVX) group, CEE treatment enhanced trabecular bone density, maximal load-bearing capacity, and various histomorphometric parameters. Serum E2 and OPG levels significantly increased, while Receptor activator of nuclear factor-κB (RANK) decreased in the CEE group. CEE downregulated matrix metallopeptidase 9 (MMP-9), Cathepsin K (CTSK), and TRAP gene and protein expression. In bone marrow macrophages (BMMs), CEE reduced mature osteoclasts, bone resorption pit areas, and MMP-9, CTSK, and TRAP expression during osteoclast differentiation. Compared with DMSO treatment, CEE markedly inhibited RANK, TNF receptor associated factor 6 (TRAF6), Proto-oncogene c-Fos (c-Fos), Nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) expressions, and Extracellular regulated protein kinases (ERK), c-Jun N-terminal kinase (JNK), NF-kappa B-p65 (p65) phosphorylation in osteoclasts. In conclusion, CEE significantly inhibits OVX-induced osteoporosis and RANKL-induced osteoclastogenesis, potentially through modulating the Estrogen Receptor (ER)/RANK/NFATc1 signaling pathway.

Epstein-Barr virus-driven B cell lymphoma mediated by a direct LMP1-TRAF6 complex

Epstein-Barr virus (EBV) latent membrane protein 1 (LMP1) drives viral B cell transformation and oncogenesis. LMP1's transforming activity depends on its C-terminal activation region 2 (CTAR2), which induces NF-κB and JNK by engaging TNF receptor-associated factor 6 (TRAF6). The mechanism of TRAF6 recruitment to LMP1 and its role in LMP1 signalling remains elusive. Here we demonstrate that TRAF6 interacts directly with a viral TRAF6 binding motif within CTAR2. Functional and NMR studies supported by molecular modeling provide insight into the architecture of the LMP1-TRAF6 complex, which differs from that of CD40-TRAF6. The direct recruitment of TRAF6 to LMP1 is essential for NF-κB activation by CTAR2 and the survival of LMP1-driven lymphoma. Disruption of the LMP1-TRAF6 complex by inhibitory peptides interferes with the survival of EBV-transformed B cells. In this work, we identify LMP1-TRAF6 as a critical virus-host interface and validate this interaction as a potential therapeutic target in EBV-associated cancer.

Icariin: A Potential Alternative Against Osteoporosis

Osteoporosis is a metabolic skeletal disorder characterized by increased fragility and fracture risk as s result of reduced bone mineral density and microstructural destruction and caused a heavy burden on families and society. Current medicines, on the other hand, have some limitations, with side effects and doubts regarding long-term efficacy being highlighted. Studies seeking for natural constituents as potential treatment options therefore come into focus. Icariin is a phytochemical derived from a traditional Chinese medicine, Herba epimedium, that has been used to treat orthopedic disorders in ancient China for thousands of years, including osteoporosis, osteoarthritis, and fracture. Icariin belongs to a category of prenylated flavonoids and has been shown to help reduce osteoporosis bone loss while having relatively low side effects. Icariin's anti-osteoporosis properties manifest in a variety of ways, like promoting osteogenesis, suppressing osteoclastogenesis and bone resorption, regulating migration, proliferation, and differentiation of mesenchymal stem cells, enhancing angiogenesis, anti-inflammation, and antioxidation. These procedures entail a slew of critical signaling pathways, such as PPARγ, ERα/AKT/β-catenin, and MAPK. Therefore, icariin can be an applicable alternative to improve osteoporosis although the underlying mechanisms have yet to be fully understood. In this study, we searched using the terms “icariin” and “osteoporosis,” and included 64 articles meeting the inclusion criteria and reviewed the research of icariin in anti-osteoporosis over the last 10 years, and discussed new prospects for future study. Therefore, this review may provide some references for further studies.

Molecular determinants of TRAF6 binding specificity suggest that native interaction partners are not optimized for affinity

TRAF6 is an adaptor protein involved in signaling pathways that are essential for development and the immune system. It participates in many protein-protein interactions, some of which are mediated by the C-terminal MATH domain, which binds to short peptide segments containing the motif PxExx[FYWHDE], where x is any amino acid. Blocking MATH domain interactions is associated with favorable effects in various disease models. To better define TRAF6 MATH domain binding preferences, we screened a combinatorial library using bacterial cell-surface peptide display. We identified 236 of the best TRAF6-interacting peptides and a set of 1,200 peptides that match the sequence PxE but do not bind TRAF6 MATH. The peptides that were most enriched in the screen bound TRAF6 tighter than previously measured native peptides. To better understand the structural basis for TRAF6 interaction preferences, we built all-atom structural models of the MATH domain in complex with high-affinity binders and nonbinders identified in the screen. We identified favorable interactions for motif features in binders as well as negative design elements distributed across the motif that can disfavor or preclude binding. Searching the human proteome revealed that the most biologically relevant TRAF6 motif matches occupy a different sequence space from the best hits discovered in combinatorial library screening, suggesting that native interactions are not optimized for affinity. Our experimentally determined binding preferences and structural models support the design of peptide-based interaction inhibitors with higher affinities than endogenous TRAF6 ligands.

Ultrasonication Improves Solid Phase Synthesis of Peptides Specific for Fibroblast Growth Factor Receptor and for the Protein-Protein Interface RANK-TRAF6

Considering our interest in the use of peptides as potential target-specific drugs or as delivery vectors of metallodrugs for various biomedical applications, it is crucial to explore improved synthetic methodologies to accomplish the highest peptide crude purity in the shortest time possible. Therefore, we compared “classical” fluorenylmethoxycarbonyl (Fmoc)-solid phase peptide synthesis (SPPS) with ultrasound(US)-assisted SPPS based on the preparation of three peptides, namely the fibroblast growth factor receptor 3(FGFR3)-specific peptide Pep1 (VSPPLTLGQLLS-NH₂) and the novel peptides Pep2 (RQMATADEA-NH₂) and Pep3 (AAVALLPAVLLALLAPRQMATADEA-NH₂), which are being developed aimed at interfering with the intracellular protein-protein interaction(PPI) RANK-TRAF6. Our results demonstrated that US-assisted SPPS led to a 14-fold (Pep1) and 4-fold time reduction (Pep2) in peptide assembly compared to the “classical” method. Interestingly, US-assisted SPPS yielded Pep1 in higher purity (82%) than the “classical” SPPS (73%). The significant time reduction combined with high crude peptide purity attained prompted use to apply US-assisted SPPS to the large peptide Pep3, which displays a high number of hydrophobic amino acids and homooligo-sequences. Remarkably, the synthesis of this 25-mer peptide was attained during a “working day” (347 min) in moderate purity (approx. 49%). In conclusion, we have reinforced the importance of using US-SPPS towards facilitating the production of peptides in shorter time with increased efficacy in moderate to high crude purity. This is of special importance for long peptides such as the case of Pep3.

TRAF6 mediates human DNA2 polyubiquitination and nuclear localization to maintain nuclear genome integrity

The multifunctional human DNA2 (hDNA2) nuclease/helicase is required to process DNA ends for homology-directed recombination repair (HDR) and to counteract replication stress. To participate in these processes, hDNA2 must localize to the nucleus and be recruited to the replication or repair sites. However, because hDNA2 lacks the nuclear localization signal that is found in its yeast homolog, it is unclear how its migration into the nucleus is regulated during replication or in response to DNA damage. Here, we report that the E3 ligase TRAF6 binds to and mediates the K63-linked polyubiquitination of hDNA2, increasing the stability of hDNA2 and promoting its nuclear localization. Inhibiting TRAF6-mediated polyubiquitination abolishes the nuclear localization of hDNA2, consequently impairing DNA end resection and HDR. Thus, the current study reveals a mechanism for the regulation of hDNA2 localization and establishes that TRAF6-mediated hDNA2 ubiquitination activates DNA repair pathways to maintain nuclear genome integrity.

Icariin abrogates osteoclast formation through the regulation of the RANKL-mediated TRAF6/NF-κB/ERK signaling pathway in Raw264.7 cells

BACKGROUND

Icariin is pharmacologically active prenylated flavonoid glycoside that has various biologic effects such as antioxidant, anticancer, and anti-inflammatory activities. In addition, icariin has been used in Chinese medicine for thousands of years to treat osteoporosis and it is still being used today. However, direct mechanism of icariin in the treatment of bone disease is not understood.

PURPOSE

The purpose of this study is to investigate whether icariin influences RANKL-induced osteoclast formation in murine macrophages.

METHODS

Osteoclastogenesis was determined by TRAP staining and activity assay. Inhibition of signaling pathways and marker protein expression were evaluated by western blot analysis. The NF-κB (p65) nuclear localization was detected by immunofluorescence assay, and NF-κB/DNA-binding activity was detected by electrophoretic mobility shift assay (EMSA).

RESULTS

In our study, icariin inhibited the differentiation of pre-osteoclast cells into osteoclasts and suppressed expression of various genes involved in osteoclast formation and bone resorption. Also, icariin blocked the osteoclastogenesis induced by MCF7 and MDA-MB-231 breast cancer cells through inhibition of NF-κB activation. We found that icariin inhibited RANKL-stimulated TRAF-6 expression, and subsequently suppressed the phosphorylation of ERK, but icariin did not show an effect on p38, JNK, and Akt activation.

CONCLUSION

These results indicate that icariin is likely to be a candidate for bone-related disease treatment and that icariin provides insights into the molecular mechanisms that influence RANKL-induced osteoclast differentiation.

Syringin prevents bone loss in ovariectomized mice via TRAF6 mediated inhibition of NF-κB and stimulation of PI3K/AKT

BACKGROUND

Syringin, also called eleutheroside B, is a main bioactive phenolic glycoside in Acanthopanax senticosus (Rupr. et Maxim.) Harms. Based on the "kidney dominates bone" theory of TCM, A. senticosus can strengthen bone and Syringin may be one of the responsibilities.

PURPOSE

The objectives of this study were to estimate the osteoporotic activity of Syringin and reveal the possible molecular mechanisms in vivo.

METHODS

Sixty female ICR mice were randomly assigned into sham operated group (SHAM, treated with vehicle) and five ovariectomized subgroups (n = 10 each), treated with vehicle as OVX group, estradiol valerate (EV, 1 mg/kg/day) as positive group, and Syringin (10, 20 and 40 mg/kg/day) as low, moderate and high dosage groups. The therapeutic effect of Syringin against osteoporosis was systematically analyzed by determining the bone mineral density (BMD), bone biomechanical properties, bone microarchitecture and serum biochemical parameters, and the molecular mechanism was also evaluated.

RESULTS

After three months of orally administrated intervention, Syringin (10, 20 and 40 mg/kg/day) significantly improved the BMD, bone maximum load and trabecular bone microarchitecture in ovariectomized mice, evidenced by the increased bone mineral content, tissue mineral content, tissue mineral density, trabecular thickness and trabecular number, as well as the decreased trabecular separation in OVX mice. Meanwhile, the activities of tartrate-resistant acid phosphatase, deoxypyridinoline and cathepsin K in OVX mice were also inhibited by Syringin, while the increased body weight and decreased uterus weight seemed not influenced by Syringin administration. Concerning the underlying molecular mechanisms, Syringin significantly downregulated the expression of tumor-necrosis factor receptor-associated factor 6 (TRAF6), nuclear factor kappa B (NF-κB) and receptor activator of nuclear factor kappa B ligand (RANKL) proteins levels, upregulated the expression of osteoprotegerin (OPG), phosphoinositide 3-kinase (PI3K) and protein kinase B (AKT) levels, suggesting that Syringin prevented bone lost by TRAF6-mediated inhibition of NF-κB and stimulation of PI3K/AKT, and subsequently increasing the OPG/RANKL ratio and inhibiting the osteoclastogenesis, finally promoting bone formation.

CONCLUSIONS

All of the data implied Syringin possessed the potent anti-osteoporosis activity on ovariectomized mice, and the underlying molecular mechanism may be related to the NF-κB and PI3K/AKT signaling pathways.

Possible targets to treat myeloma-related osteoclastogenesis

INTRODUCTION

Bone destruction is the hallmark of multiple myeloma (MM). About 80% of MM patients at diagnosis presents myeloma bone disease (MBD) leading to bone pain and pathological fractures, significantly affecting patients' quality of life. Bisphosphonates are the treatment of choice for MBD, but osteolytic lesions remain a critical issue in the current management of MM patients. Several studies clarified the mechanisms involved in MM-induced osteoclast formation and activation, leading to the identification of new possible targets and the development of better bone-directed therapies, that are discussed in this review. Areas covered: This review summarizes the latest advances in the knowledge of the pathophysiology of the osteoclast formation and activation induced by MM cells, and the new therapeutic targets identified. Recently, neutralizing antibodies (i.e. denosumab, siltuximab, daratumumab), as well as recombinant fusion proteins, and receptor molecular inhibitors, have been developed to block these targets. Clinical trials testing their anti-MBD potential are ongoing. The emerging role of exosomes and microRNAs in the regulation of osteoclast differentiation has been also discussed. Expert commentary: Although further studies are needed to arrive at a clinical approving, the basis for the development of better bone-directed therapies has been established.

The Simultaneous Inhibitory Effect of Niclosamide on RANKL-Induced Osteoclast Formation and Osteoblast Differentiation

The bone destruction disease including osteoporosis and rheumatoid arthritis are caused by the imbalance between osteoblastogenesis and osteoclastogenesis. Inhibition of the NF-κB pathway was responsible for decreased osteoclastogenesis. Recently many studies indicated that niclosamide, the FDA approved an antihelminth drug, inhibits prostate and breast cancer cells growth by targeting NF-κB signaling pathways. This study evaluated the effects of niclosamide on osteoclast and osteoblast differentiation and function in vitro. In RANKL-induced murine osteoclast precursor cell RAW264.7 and M-CSF/RANKL-stimulated primary murine bone marrow-derived macrophages (BMM), niclosamide dose-dependently inhibited the formation of TRAP-positive multinucleated osteoclasts and resorption pits formation between 0.5uM and 1uM. In addition, niclosamide suppressed the expression of nuclear factor of activated T cells c1 (NFATc1) and osteoclast differentiated-related genes in M-CSF/ RANKL-stimulated BMM by interference with TRAF-6, Erk1/2, JNK and NF-κB activation pathways. However, the cytotoxic effects of niclosamide obviously appeared at the effective concentrations for inhibiting osteoclastogenesis (0.5-1uM) with increase of apoptosis through caspase-3 activation in osteoblast precursor cell line, MC3T3-E1. Niclosamide also inhibited ALP activity, bone mineralization and osteoblast differentiation-related genes expression in MC3T3-E1. Therefore, our findings suggest the new standpoint that niclosamide's effects on bones must be considered before applying it in any therapeutic treatment.

The osteoimmunology of alveolar bone loss

The mineralized structure of bone undergoes constant remodeling by the balanced actions of bone-producing osteoblasts and bone-resorbing osteoclasts (OCLs). Physiologic bone remodeling occurs in response to the body's need to respond to changes in electrolyte levels, or mechanical forces on bone. There are many pathological conditions, however, that cause an imbalance between bone production and resorption due to excessive OCL action that results in net bone loss. Situations involving chronic or acute inflammation are often associated with net bone loss, and research into understanding the mechanisms regulating this bone loss has led to the development of the field of osteoimmunology. It is now evident that the skeletal and immune systems are functionally linked and share common cells and signaling molecules. This review discusses the signaling system of immune cells and cytokines regulating aberrant OCL differentiation and activity. The role of these cells and cytokines in the bone loss occurring in periodontal disease (PD) (chronic inflammation) and orthodontic tooth movement (OTM) (acute inflammation) is then described. The review finishes with an exploration of the emerging role of Notch signaling in the development of the immune cells and OCLs that are involved in osteoimmunological bone loss and the research into Notch signaling in OTM and PD.

Lansoprazole Upregulates Polyubiquitination of the TNF Receptor-Associated Factor 6 and Facilitates Runx2-mediated Osteoblastogenesis

The transcription factor, runt-related transcription factor 2 (Runx2), plays a pivotal role in the differentiation of the mesenchymal stem cells to the osteochondroblast lineages. We found by the drug repositioning strategy that a proton pump inhibitor, lansoprazole, enhances nuclear accumulation of Runx2 and induces osteoblastogenesis of human mesenchymal stromal cells. Systemic administration of lansoprazole to a rat femoral fracture model increased osteoblastogenesis. Dissection of signaling pathways revealed that lansoprazole activates a noncanonical bone morphogenic protein (BMP)-transforming growth factor-beta (TGF-β) activated kinase-1 (TAK1)-p38 mitogen-activated protein kinase (MAPK) pathway. We found by in cellulo ubiquitination studies that lansoprazole enhances polyubiquitination of the TNF receptor-associated factor 6 (TRAF6) and by in vitro ubiquitination studies that the enhanced polyubiquitination of TRAF6 is attributed to the blocking of a deubiquitination enzyme, cylindromatosis (CYLD). Structural modeling and site-directed mutagenesis of CYLD demonstrated that lansoprazole tightly fits in a pocket of CYLD where the C-terminal tail of ubiquitin lies. Lansoprazole is a potential therapeutic agent for enhancing osteoblastic differentiation.

The critical role of toll-like receptors--From microbial recognition to autoimmunity: A comprehensive review

Toll-like receptors (TLRs) constitute an important mechanism in the activation of innate immune cells including monocytes, macrophages and dendritic cells. Macrophage activation by TLRs is pivotal in the initiation of the rapid expression of pro-inflammatory cytokines TNF, IL-1β and IL-6 while promoting Th17 responses, all of which play critical roles in autoimmunity. Surprisingly, in inflammatory arthritis, activation of specific TLRs can not only induce but also inhibit cellular processes associated with bone destruction. The intercellular and intracellular orchestration of signals from different TLRs, their endogenous or microbial ligands and accessory molecules determine the activating or inhibitory responses. Herein, we review the TLR-mediated activation of innate immune cells in their activation and differentiation to osteoclasts and the capacity of these signals to contribute to bone destruction in arthritis. Detailed understanding of the opposing mechanisms of TLRs in the induction and suppression of cellular processes in arthritis may pave the way to develop novel therapies to treat autoimmunity.

RANKL pretreatment plays an important role in the differentiation of pit-forming osteoclasts induced by TNF-α on murine bone marrow macrophages

BACKGROUND

Osteoclasts differentiated from bone marrow macrophages (BMMs) induced by TNF-α alone do not have resorbing activity. When BMMs are stimulated with receptor activator of NF-κB ligand (RANKL) before TNF-α stimulation, pit-forming osteoclasts are differentiated. However, the details of the effect of RANKL pretreatment on the pit-forming osteoclast differentiation by TNF-α have not been established. The aim of this study is to examine the condition of RANKL pretreatment for differentiation of pit-forming osteoclasts induced by TNF-α. Murine BMMs were stimulated with various concentrations of RANKL for 24h in the presence of M-CSF, then the medium was changed and TNF-α was added. Osteoclasts and pits formation were examined. Osteoprotegerin (OPG), decoy receptor of RANKL, was added to the culture to examine the necessity of co-existing RANKL with TNF-α on the formation of pit-forming osteoclasts. To investigate the influence of RANKL of sufficient concentration as pretreatment for pit-forming osteoclast formation by TNF-α, dose- and time-dependent changes of osteoclast formation were checked.

RESULTS

The pit formation by osteoclasts in response to TNF-α required 10ng/mL RANKL pretreatment. Stimulation with this concentration of RANKL led to the differentiation of mature osteoclasts in the 72h culture. The pit formation was not inhibited by the OPG.

CONCLUSION

These results suggested that the concentration of RANKL pretreatment, which also alone can differentiate BMMs into osteoclasts, may be important in the differentiation of pit-forming osteoclasts by TNF-α. In addition, the effects of TNF-α after RANKL treatment might be independent of RANKL.

Lipopolysaccharide (LPS) promotes osteoclast differentiation and activation by enhancing the MAPK pathway and COX-2 expression in RAW264.7 cells

Bone degradation is a serious complication of chronic inflammatory diseases such as septic arthritis, osteomyelitis and infected orthopedic implant failure. At present, effective therapeutic treatments for lipopolysaccharide (LPS)-induced bone destruction are limited to antibiotics and surgical repair in chronic inflammatory diseases. The present study aimed to evaluate the mechanism of LPS on osteoclast differentiation and activation. RAW264.7 cells were non-induced, or induced by the receptor activator of nuclear factor-κB (RANK) ligand (RANKL) and macrophage-colony stimulating factor (M-CSF), and then treated with LPS. Following treatment, the number of osteoclasts and cell viability were measured. The expression of osteoclast-related genes including tartrate-resistant acid phosphatase (TRAP), matrix metalloproteinase-9 (MMP-9), cathepsin K (CK), carbonic anhydrase II (CAII) and cyclooxygenase-2 (COX-2) was determined by RT-PCR. Protein levels of RANK, tumor necrosis factor receptor-associated factor 6 (TRAF6), COX-2 and mitogen-activated protein kinases (MAPK) were measured using western blotting assays. LPS promoted osteoclast differentiation of RAW264.7 cells and differentiated osteoclasts. LPS significantly increased mRNA expression of osteoclast-related genes in RAW264.7 cells. Differentiated osteoclasts were treated with LPS (100 ng/ml) and the results showed a significantly increased mRNA expression of osteoclast-related genes and protein levels of RANK, TRAF6 and COX-2. Furthermore, LPS at 100 ng/ml significantly promoted the MAPK pathway including increasing the phosphorylation of c-Jun N-terminal kinases (JNK) and the phosphorylation of the extracellular signal-regulated kinase (ERK1/2). In conclusion, LPS promoted osteoclast differentiation and activation by enhancing RANK signaling and COX-2 expression. LPS also promoted osteoclast differentiation via activation of the JNK and ERK1/2 cell proliferation pathways.

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.

TRAF6 regulates proliferation, apoptosis, and invasion of osteosarcoma cell

TRAF6, a unique tumor necrosis factor receptor-associated factor (TRAF) family member, possesses a unique receptor-binding specificity that results in its crucial role as the signaling mediator for TNF receptor superfamily and interleukin-1 receptor/Toll-like receptor superfamily. TRAF6 plays an important role in tumorigenesis, invasion and metastasis. This study aimed to explore the expression of TRAF6 in osteosarcoma tissues and its correlation to the clinical pathology of osteosarcoma and to discuss the relationship between TRAF6 expression and osteosarcoma invasion. These data will provide the experimental base for the biological treatment of osteosarcoma in the future. Using RT-PCR and Western blot, the results showed that the expression rate of TRAF6 mRNA in osteosarcoma tissues was significantly higher than that in normal bone tissue (p < 0.05), that the expression rate of TRAF6 mRNA in the carcinoma tissues from patients with lung metastasis was significantly higher than that from patients without lung metastasis (p < 0.05), and that the expression rate of TRAF6 mRNA also increased with increasing Enneking stage (p < 0.05). However, the mRNA expression of TRAF6 in osteosarcoma was independent of the patient's gender, age, and tumor size (p > 0.05). The TRAF6 protein displayed an up-regulation in osteosarcoma tissues compared to normal bone tissue (p < 0.05), displayed an up-regulation in osteosarcoma tissues from patients with lung metastasis compared to from patients without lung metastasis (p < 0.05), and displayed a gradual increase with increasing Enneking stage (p < 0.05). By the technique of RNA interference, the expression of TRAF6 in the human osteosarcoma MG-63 cell line was down-regulated, and the invasive ability of MG-63 cells was examined. The results showed that TRAF6 protein expression was significantly decreased in the MG-63 cells from TRAF6 siRNA-transfected group (p < 0.05), and the proliferation ability of MG-63 cells and the number of MG-63 cells that passed through the Transwell chamber were significantly lower than that in the non-transfected control group as well as the transfected control group (p < 0.05). In addition, the percentage of MG-63 cells undergoing apoptosis was significantly higher in the TRAF6 siRNA-transfected group compared with the non-transfected control group as well as the transfected control group (p < 0.05). The expression of p-p65, cyclin D1, MMP-9 was down-regulated in the MG-63 cells from TRAF6 siRNA-transfected group. The expression of caspase 3 was up-regulated in the MG-63 cells from TRAF6 siRNA-transfected group compared to the non-transfected control group as well as the transfected control group (p < 0.05). To make a long story short, the overexpression of TRAF6 in osteosarcoma might be related to the tumorigenesis, invasion of osteosarcoma.

TRAF6 activation in multiple myeloma: a potential therapeutic target

Multiple myeloma (MM) is an incurable B-lymphocyte malignancy. New therapeutic options have become available during the past several years; however nearly all patients acquire resistance to currently available therapeutic agents. Mechanisms contributing to the pathogenesis and chemoresistance of MM include genetic abnormalities, chromosomal translocations, gene mutations, the interaction between MM cells and the bone marrow microenvironment, and defects in the apoptotic signaling pathways. Survival signaling pathways associated with the pathogenesis of MM and bone marrow stromal cells play crucial roles in promoting growth, survival, adhesion, immortalization, angiogenesis, and drug resistance. The receptor activator of nuclear factor-kappa B/receptor activator of nuclear factor-kappa B ligand/tumor necrosis factor receptor-associated factor (RANK/RANKL-TRAF6) signal pathway mediates osteolytic bone lesions through the activation of the NF-κB and Janus kinase/signal transducer and activator of transcription (JNK) pathways in osteoclast precursor cells and thus contributes to the main clinical manifestations of bone disease. TRAF6 has also been identified as a ligase for Akt ubiquitination and membrane recruitment and its phosphorylation on growth factor stimulation. The inhibition of TRAF6 by silencing RNA or by decoy peptides decreases MM tumor cell proliferation and increases apoptosis as well as bone resorption. Some proteasome inhibitors and benzoxadiazole derivatives showed inhibitory effects on the activity and function of TRAF6. Overall, we propose that TRAF6 may be considered as a potential therapeutic target for the treatment of MM.

A RANKL G278R mutation causing osteopetrosis identifies a functional amino acid essential for trimer assembly in RANKL and TNF

Receptor activator of nuclear factor-κB ligand (RANKL), a trimeric tumor necrosis factor (TNF) superfamily member, is the central mediator of osteoclast formation and bone resorption. Functional mutations in RANKL lead to human autosomal recessive osteopetrosis (ARO), whereas RANKL overexpression has been implicated in the pathogenesis of bone degenerative diseases such as osteoporosis. Following a forward genetics approach using N-ethyl-N-nitrosourea (ENU)-mediated random mutagenesis, we generated a novel mouse model of ARO caused by a new loss-of-function allele of Rankl with a glycine-to-arginine mutation at codon 278 (G278R) at the extracellular inner hydrophobic F β-strand of RANKL. Mutant mice develop severe osteopetrosis similar to Rankl-deficient mice, whereas exogenous administration of recombinant RANKL restores osteoclast formation in vivo. We show that RANKL(G278R) monomers fail to assemble into homotrimers, are unable to bind and activate the RANK receptor and interact with wild-type RANKL exerting a dominant-negative effect on its trimerization and function in vitro. Since G278 is highly conserved within the TNF superfamily, we identified that a similar substitution in TNF, G122R, also abrogated trimerization, binding to TNF receptor and consequently impaired TNF biological activity. Notably, SPD304, a potent small-molecule inhibitor of TNF trimerization that interacts with G122, also inhibited RANKL activity, suggesting analogous inhibitory mechanisms. Our results provide a new disease model for ARO and identify a functional amino acid in the TNF-like core domain essential for trimer formation both in RANKL and in TNF that could be considered a novel potential target for inhibiting their biological activities.

The inhibitory effect of microRNA-146a expression on bone destruction in collagen-induced arthritis

OBJECTIVE

MicroRNA, a class of noncoding RNA, play a role in human diseases. MicroRNA-146a (miR-146a) is a negative regulator of immune and inflammatory responses, and is strongly expressed in rheumatoid arthritis (RA) synovium and peripheral blood mononuclear cells (PBMCs). This study was undertaken to examine whether miR-146a expression inhibits osteoclastogenesis, and whether administration of miR-146a prevents joint destruction in mice with collagen-induced arthritis (CIA).

METHODS

PBMCs from healthy volunteers were isolated and seeded in culture plates. The following day, double-stranded miR-146a was transfected and cultured in the presence of macrophage colony-stimulating factor and either tumor necrosis factor α or RANKL. After 3 weeks, tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells were counted. Three days after miR-146a culture, the expression of c-Jun, nuclear factor of activated T cells c1 (NF-ATc1), PU.1, and TRAP was evaluated by quantitative reverse transcriptase-polymerase chain reaction. After the onset of distinct arthritis in mice with CIA, double-stranded miR-146a or nonspecific double-stranded RNA was administered twice by intravenous injection. Radiographic and histologic examinations were performed at 4 weeks.

RESULTS

The number of TRAP-positive multinucleated cells in human PBMCs was significantly reduced by miR-146a in a dose-dependent manner. The expression of c-Jun, NF-ATc1, PU.1, and TRAP in PBMCs was significantly down-regulated by miR-146a. Administration of miR-146a prevented joint destruction in mice with CIA, although it did not completely ameliorate inflammation.

CONCLUSION

Our findings indicate that expression of miR-146a inhibits osteoclastogenesis and that administration of double-stranded miR-146a prevents joint destruction in arthritic mice. Administration of miR-146a has potential as a novel therapeutic target for bone destruction in RA.