Tumor necrosis factor receptor-associated factor 6 is an intranuclear transcriptional coactivator in osteoclasts

Involvement of tumor necrosis factor receptor-associated factor 6 (TRAF6) in NF-κB activation and antiviral immunity: Molecular and functional characterization of TRAF6 in red-spotted grouper (Epinephelus akaara)

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a member of the TRAF family of adaptor proteins involved in the signal transduction pathways of both TNF receptor and interleukin-1 receptor/Toll-like receptor superfamilies. In this study, red-spotted grouper (Epinephelus akaara) TRAF6 (EaTraf6) was identified and characterized. The open reading frame of EaTraf6, 1713 bp in length, encodes a putative protein of 570 amino acids and has a predicted molecular weight and theoretical isoelectric point of 64.11 kDa and 6.07, respectively. EaTraf6 protein contains an N-terminal RING-type zinc finger domain, two TRAF-type zinc finger domains, a coiled-coil region (zf-TRAF), and a conserved C-terminal meprin and TRAF homology (MATH) domain. EaTraf6 shared the highest amino acid sequence identity with its ortholog from Epinephelus coioides, and phylogenetic analysis showed all fish TRAF6s clustered together and apart from other species. qRT-PCR results revealed that EaTraf6 was ubiquitously expressed in all examined tissues, with the highest level detected in the blood. In the immune challenge, EaTraf6 exhibited modulated mRNA expression levels in the blood and spleen. The subcellular localization analysis revealed that the EaTraf6 protein was predominantly present in the cytoplasm; however, it could translocate into the nucleus following poly (I:C) stimulation. The antiviral function of EaTraf6 was confirmed by analyzing the expression of host antiviral genes and viral genomic RNA during viral hemorrhagic septicemia virus infection. Additionally, luciferase reporter assay results indicated that EaTraf6 is involved in the activation of the NF-κB signaling pathway upon poly (I:C) stimulation. Finally, the effect of EaTraf6 on cytokine gene expression and its role in regulating macrophage M1 polarization were demonstrated. Collectively, these findings suggest that EaTraf6 is a crucial immune-related gene that significantly contributes to antiviral functions and regulation of NF-κB activity in the red-spotted grouper.

Structural Characterization of TRAF6 N-Terminal for Therapeutic Uses and Computational Studies on New Derivatives

Tumor necrosis factor receptor-associated factors (TRAFs) are a protein family with a wide variety of roles and binding partners. Among them, TRAF6, a ubiquitin ligase, possesses unique receptor binding specificity and shows diverse functions in immune system regulation, cellular signaling, central nervous system, and tumor formation. TRAF6 consists of an N-terminal Really Interesting New Gene (RING) domain, multiple zinc fingers, and a C-terminal TRAF domain. TRAF6 is an important therapeutic target for various disorders and structural studies of this protein are crucial for the development of next-generation therapeutics. Here, we presented a TRAF6 N-terminal structure determined at the Turkish light source "Turkish DeLight" to be 3.2 Å resolution at cryogenic temperature (PDB ID: 8HZ2). This structure offers insight into the domain organization and zinc-binding, which are critical for protein function. Since the RING domain and the zinc fingers are key targets for TRAF6 therapeutics, structural insights are crucial for future research. Separately, we rationally designed numerous new compounds and performed molecular docking studies using this template (PDB ID:8HZ2). According to the results, 10 new compounds formed key interactions with essential residues and zinc ion in the N-terminal region of TRAF6. Molecular dynamic (MD) simulations were performed for 300 ns to evaluate the stability of three docked complexes (compounds 256, 322, and 489). Compounds 256 and 489 was found to possess favorable bindings with TRAF6. These new compounds also showed moderate to good pharmacokinetic profiles, making them potential future drug candidates as TRAF6 inhibitors.

Sexually Dimorphic Increases in Bone Mass Following Tissue-specific Overexpression of Runx1 in Osteoclast Precursors

Many metabolic bone diseases arise as a result excessive osteoclastic bone resorption, which has motivated efforts to identify new molecular targets that can inhibit the formation or activity of these bone-resorbing cells. Mounting evidence indicates that the transcription factor Runx1 acts as a transcriptional repressor of osteoclast formation. Prior studies using a conditional knockout approach suggested that Runx1 in osteoclast precursors acts as an inhibitor of osteoclastogenesis; however, the effects of upregulation of Runx1 on osteoclast formation remain unknown. In this study, we investigated the skeletal effects of conditional overexpression of Runx1 in preosteoclasts by crossing novel Runx1 gain-of-function mice (Rosa26-LSL-Runx1) with LysM-Cre transgenic mice. We observed a sex-dependent effect whereby overexpression of Runx1 in female mice increased trabecular bone microarchitectural indices and improved torsion biomechanical properties. These effects were likely mediated by delayed osteoclastogenesis and decreased bone resorption. Transcriptomics analyses during osteoclastogenesis revealed a distinct transcriptomic profile in the Runx1-overexpressing cells, with enrichment of genes related to redox signaling, apoptosis, osteoclast differentiation, and bone remodeling. These data further confirm the antiosteoclastogenic activities of Runx1 and provide new insight into the molecular targets that may mediate these effects.

How (Epi)Genetic Regulation of the LIM-Domain Protein FHL2 Impacts Multifactorial Disease

Susceptibility to complex pathological conditions such as obesity, type 2 diabetes and cardiovascular disease is highly variable among individuals and arises from specific changes in gene expression in combination with external factors. The regulation of gene expression is determined by genetic variation (SNPs) and epigenetic marks that are influenced by environmental factors. Aging is a major risk factor for many multifactorial diseases and is increasingly associated with changes in DNA methylation, leading to differences in gene expression. Four and a half LIM domains 2 (FHL2) is a key regulator of intracellular signal transduction pathways and the FHL2 gene is consistently found as one of the top hyper-methylated genes upon aging. Remarkably, FHL2 expression increases with methylation. This was demonstrated in relevant metabolic tissues: white adipose tissue, pancreatic β-cells, and skeletal muscle. In this review, we provide an overview of the current knowledge on regulation of FHL2 by genetic variation and epigenetic DNA modification, and the potential consequences for age-related complex multifactorial diseases.

Four and a Half LIM Domains Protein 2 Mediates Bortezomib-Induced Osteogenic Differentiation of Mesenchymal Stem Cells in Multiple Myeloma Through p53 Signaling and β-Catenin Nuclear Enrichment

Myeloma bone disease (MBD), caused by the inhibition of osteoblast activity and the activation of osteoclast in the bone marrow environment, is the most frequent and life-threatening complication in multiple myeloma (MM) patients. Bortezomib (Bzb) was shown to promote MM-derived mesenchymal stem cells (MM-MSCs) differentiation to osteoblast in vitro and in animal models, promoting the bone formation and regeneration, may be mediated via β-catenin/T-cell factor (TCF) pathway. Further defining molecular mechanism of Bzb-enhanced bone formation in MM will be beneficial for the treatment of myeloma patients. The present study has identified for the first time four and a half LIM domains protein 2 (FHL2), a tissue-specific coregulator that interacts with many osteogenic marker molecules, as a therapeutic target to ameliorate MM bone disease. First, increased messenger RNA (mRNA) and protein levels of FHL2, and the mRNA level of main osteoblast markers (including Runx2, ALP, and Col1A1), were found in MM-patients-derived MSCs after Bzb treatment. FHL2 KD with short hairpin RNA (shRNA) reduced the expression of osteoblast marker genes and blocked the osteogenic differentiation of MM-MSCs regardless of the presence or absence of Bzb, implying that FHL2 is an important activator of the osteogenic differentiation of human MSCs under a proteasome inhibition condition. Molecular analysis showed that the enhanced expression of FHL2 was associated with the Bzb-induced upregulation of p53. No significant change at protein level of total β-catenin was observed with or without Bzb treatment. However, it was mostly enriched to nuclei in MSCs after Bzb treatment. Moreover, β-catenin was restricted to the perinuclear region in FHL2 KD cells. These data provide evidence that FHL2 is essential for promoting β-catenin nuclear enrichment in MM-MSCs. In conclusion, FHL2 is critical for Bzb-induced osteoblast differentiation of MM-MSCs and promotes the osteogenesis, through p53 signaling and β-catenin activation. Targeting FHL2 in MM may provide a new therapeutic strategy for treating MBD.

Deletion of Runx1 in osteoclasts impairs murine fracture healing through progressive woven bone loss and delayed cartilage remodeling

Conditional deletion of the transcription factor Runt-related transcription factor 1 (Runx1) in myeloid osteoclast precursors promotes osteoclastogenesis and subsequent bone loss. This study posits whether Runx1 regulates clastic cell-mediated bone and cartilage resorption in the fracture callus. We first generated mice, in which Runx1 was conditionally abrogated in osteoclast precursors (LysM-Cre;Runx1^F/F ; Runx1 cKO). Runx1 cKO and control mice were then subjected to experimental mid-diaphyseal femoral fractures. Our study found differential resorption of bony and calcified cartilage callus matrix by osteoclasts and chondroclasts within Runx1 cKO calluses, with increased early bony callus resorption and delayed calcified cartilage resorption. There was an increased number of osteoclasts and chondroclasts in the chondro-osseous junction of Runx1 cKO calluses starting at day 11 post-fracture, with minimal woven bone occupying the callus at day 18 post-fracture. LysM-Cre;Runx1^F/F mutant mice had increased bone compliance at day 28, but their strength and work to failure were comparable with controls. Taken together, these results indicate that Runx1 is a critical transcription factor in controlling osteoclastogenesis that negatively regulates bone and cartilage resorption in the fracture callus. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:1007-1015, 2020.

LIM-only protein FHL2 attenuates inflammation in vascular smooth muscle cells through inhibition of the NFκB pathway

Despite the advent of new-generation drug-eluting stents, in-stent restenosis remains a significant problem in patients with coronary artery disease. In- stent restenosis is defined as the gradual re-narrowing of a stented coronary artery lesion due to arterial damage with subsequent local inflammation of the vessel wall and excessive growth of the vascular smooth muscle cells (vSMCs). Four-and-a-half LIM-domain protein 2 (FHL2) is a scaffold protein involved in regulating vSMC function and inflammation. Previously we have demonstrated that FHL2 prevents vSMC proliferation in a murine carotid artery ligation model. However, the effect of FHL2 on the inflammatory response of the vSMCs is not investigated. Therefore, we studied the inflammatory response in the vessel wall of FHL2-deficient (-KO) mice after carotid artery ligation. We found that circulating cytokines and local macrophage infiltration in the ligated carotid vessels were increased in FHL2-KO mice after carotid artery ligation. Moreover, FHL2-KO vSMCs showed increased secretion of cytokines such as SDF-1α and RANTES, and enhanced activation of the NFκB pathway. Finally, we found that blocking the NFκB signalling pathway abrogated this pro-inflammatory state in FHL2-KO vSMCs. Taken together, our results demonstrate that FHL2 decreases the inflammatory response of vSMCs through inhibition of the NFkB-signalling pathway.

Beclin1 Modulates Bone Homeostasis by Regulating Osteoclast and Chondrocyte Differentiation

Autophagy (ATG), an important cellular recycling process whereby macromolecules or organelles are encapsulated by autophagosome and degraded upon merging with lysosome, has recently been shown to play an essential role in bone biology. However, the involvement of ATG in bone and bone-related cells remains unclear. Here, we show that Beclin1, an ATG-related protein involved in ATG initiation, plays a pivotal role in osteoclasts. ATG was activated during osteoclast differentiation in vitro. Beclin1 was enhanced and required for osteoclast differentiation. Mechanistically, we found that TRAF6-mediated ubiquitination of Beclin1 at K117, but not ULK1-mediated phosphorylation, is required for RANKL-stimulated osteoclast differentiation. In vivo, mice lacking Beclin1 in CstK-expressing cells exhibited an increased cortical bone thickness caused by impaired osteoclasts' function. Interestingly, these mice also exhibited diminished trabecular bone mass, which was associated with a defect in cartilage formation and chondrocyte differentiation. Collectively, our study highlights the functional importance of ATG in osteoclasts and chondrocytes, and identifies ATG as a potential therapeutic target for managing bone-related diseases. © 2019 American Society for Bone and Mineral Research.

Dehydrocostus lactone attenuates osteoclastogenesis and osteoclast-induced bone loss by modulating NF-κB signalling pathway

Osteolysis is characterized by overactivated osteoclast formation and potent bone resorption. It is enhanced in many osteoclast‐related diseases including osteoporosis and periprosthetic osteolysis. The shortage of effective treatments for these pathological processes emphasizes the importance of screening and identifying potential regimens that could attenuate the formation and function of osteoclasts. Dehydrocostus lactone (DHE) is a natural sesquiterpene lactone containing anti‐inflammatory properties. Here, we showed that DHE suppressed receptor activator of nuclear factor‐κB ligand (RANKL)‐induced osteoclast formation and osteoclast marker gene expression. It also inhibited F‐actin ring formation and bone resorption in a dose‐dependent manner in vitro. Moreover, DHE inhibited the RANKL‐induced phosphorylation of NF‐κB, mitigated bone erosion in vivo in lipopolysaccharide‐induced inflammatory bone loss model and particle‐induced calvarial osteolysis model. Together, these results suggest that DHE reduces osteoclast‐related bone loss via the modulation of NF‐κB activation during osteoclastogenesis indicating that it might be a useful treatment for osteoclast‐related skeletal disorders.

The role of FHL2 in wound healing and inflammation

The 4-and-a-half LIM domain protein 2 (FHL2) is a multifunctional adaptor protein that can interact with cell surface receptors, cytosolic adaptor and structural proteins, kinases, and nuclear transcription factors. It is involved in numerous functional activities, including the epithelial-mesenchymal transition, cell proliferation, apoptosis, adhesion, migration, structural stability, and gene expression. Despite this, FHL2-knockout (KO) mice are viable and fertile with no obvious abnormalities, rather suggesting a high capacity for fine-tuning adjustment and functional redundancy of FHL2. Indeed, challenging FHL2-KO cells or mice provided numerous evidences for the great functional significance of FHL2. In recent years, several reviews have been published describing the high capacity of FHL2 to bind diverse proteins as well as the versatile functions of FHL2, emphasizing in particular its role in cardiovascular diseases and carcinogenesis. Here, we view the function of FHL2 from a different perspective. We summarize the published data demonstrating the impact of FHL2 on wound healing and inflammation. FHL2 seems to be involved in numerous steps of these extremely complex and multidirectional but tightly regulated tissue remodeling processes, supporting tissue repair and coordinating inflammation. Deficiency of FHL2 not only slows down ongoing wound healing but also often turns it into a chronic condition.-Wixler, V. The role of FHL2 in wound healing and inflammation.

TRAIL inhibits RANK signaling and suppresses osteoclast activation via inhibiting lipid raft assembly and TRAF6 recruitment

Human osteoclast formation from mononuclear phagocyte precursors involves interactions between members of the tumor necrosis factor (TNF) ligand superfamily and their receptors. Recent evidence indicated that TNF-α-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation via a TRAF6-dependent signaling pathway; but paradoxically, it inhibits RANK ligand (RANKL)-induced osteoclast differentiation. Although a number of signaling pathways were linked to the RANK and osteoclastogenesis, it is not known how TRAIL regulates RANK signaling. In this study, we demonstrate that TRAIL regulates RANK-induced osteoclastogenesis in terms of the assembly of lipid raft-associated signaling complexes. RANKL stimulation induced recruitment of TRAF6, c-Src, and DAP-12 into lipid rafts. However, the RANKL-induced assembly of lipid raft-associated signaling complexes and TRAF6 recruitment was abolished in the presence of TRAIL. TRAIL-induced dissociation of RANKL-induced lipid raft signaling complexes was reversed by treatment with TRAIL receptor (TRAIL-R) siRNA or an anti-TRAIL-R blocking antibody, indicating that TRAIL mediates suppression of RANKL-induced lipid raft signaling via interactions with TRAIL-R. Finally, we demonstrated that TRAIL suppressed inflammation-induced bone resorption and osteoclastogenesis in a collagen-induced arthritis (CIA) rat animal model. Our results provide a novel apoptosis-independent role of TRAIL in regulating RANK signaling and suppresses osteoclast activation via inhibiting lipid raft assembly and TRAF6 recruitment.

Tomatidine suppresses osteoclastogenesis and mitigates estrogen deficiency-induced bone mass loss by modulating TRAF6-mediated signaling

Postmenopausal osteoporosis is initiated by estrogen withdrawal and is characterized mainly by overactivated osteoclastic bone resorption. Targeting TNF receptor-associated factor 6 (TRAF6) or its downstream signaling pathways to modulate osteoclast formation and function is an appealing strategy for osteoclast-related disorders. In the present study, we determined the effect of tomatidine, a steroidal alkaloid derived from Solanaceae, on the formation and function of receptor activator of NF-κB (RANK) ligand-induced osteoclasts and the underlying mechanism. Tomatidine inhibited osteoclast formation in a dose-dependent manner and decreased the expression of osteoclast marker genes. Actin ring formation and osteoclastic bone resorption were attenuated in the presence of tomatidine in vitro. Eight weeks after ovariectomy, tomatidine prevented estrogen deficiency-induced bone loss and restored the mechanical properties of the femur. At the molecular level, tomatidine abrogated phosphorylation of c-Jun N-terminal kinase (JNK)/p38, NF-κB, and protein kinase B (Akt) pathway proteins by suppressing RANK expression, inhibiting the binding of TRAF6 to RANK, and downregulating the osteoclastogenesis marker-related protein expression. In summary, these data demonstrated that tomatidine attenuated osteoclast formation and function by modulating multiple TRAF6-mediated pathways. Therefore, tomatidine could be a novel candidate for the treatment of osteoclast-related disorders, including osteoporosis.-Hu, B., Sun, X., Yang, Y., Ying, Z., Meng, J., Zhou, C., Jiang, G., Li, S., Wu, F., Zhao, X., Zhu, H., Wu, H., Cai, X., Shi, Z., Yan, S. Tomatidine suppresses osteoclastogenesis and mitigates estrogen deficiency-induced bone mass loss by modulating TRAF6-mediated signaling.

PARP12 (ARTD12) suppresses hepatocellular carcinoma metastasis through interacting with FHL2 and regulating its stability

PARP12 is a mono-ADP-ribosyltransferase, but its function remains largely unknown. Here, we identified four-and-a-half LIM-only protein 2 (FHL2) as a functional partner of PARP12 through protein affinity purification. Although PARP12 did not mono-ADP-ribosylate FHL2 in vitro and in vivo, PARP12 deficiency decreased the protein level of FHL2 by promoting its ubiquitination and increased the expression level of transforming growth factor beta1 (TGF-β1), which is independent of PARP12 enzymatic activity. We also provided evidence that PARP12 deficiency increased the migration and invasion of hepatocellular carcinoma (HCC) cells and promoted HCC metastasis in vivo by regulating the epithelial–mesenchymal transition process. These results indicated that PARP12 is a tumor suppressor that plays an important role in HCC metastasis through the regulation of FHL2 stability and TGF-β1 expression.

The Four-and-a-Half LIM Domain Protein 2 Supports Influenza A Virus-Induced Lung Inflammation by Restricting the Host Adaptive Immune Response

Four-and-a-half LIM domain protein 2 (FHL2) is a multifunctional adaptor protein with fine-tuning adjustment properties. It acts as a regulator of signaling cascades but also as a cofactor of transcription and controls several anti-inflammatory immune responses. Recently, we described FHL2 as a novel regulator of influenza A virus propagation. We have shown that in vitro FHL2 restricts viral replication by accelerating the interferon regulatory factor 3-dependent transcription of the Ifnb1 gene. In this work, we unraveled an ambiguous role of FHL2 during influenza A virus infection in vivo. Although FHL2 restrained viral replication during the first 24 hours of infection, it significantly delayed viral clearance afterward. Comparison of lung immune status of wild-type and FHL2 knockout mice during influenza virus infection did not acknowledge significant differences in the innate host immune response but revealed an improved migration of dendritic cells from infected lungs into draining lymph nodes as well as increased levels of activated CD8⁺ T lymphocytes accumulated in the lungs of FHL2 knockout mice.

Sophocarpine attenuates wear particle-induced implant loosening by inhibiting osteoclastogenesis and bone resorption via suppression of the NF-κB signalling pathway in a rat model

BACKGROUND AND PURPOSE

Aseptic prosthesis loosening, caused by wear particles, is one of the most common causes of arthroplasty failure. Extensive and over-activated osteoclast formation and physiological functioning are regarded as the mechanism of prosthesis loosening. Therapeutic modalities based on inhibiting osteoclast formation and bone resorption have been confirmed to be an effective way of preventing aseptic prosthesis loosening. In this study, we have investigated the effects of sophocarpine (SPC, derived from Sophora flavescens) on preventing implant loosening and further explored the underlying mechanisms.

EXPERIMENTAL APPROACH

The effects of SPC in inhibiting osteoclastogenesis and bone resorption were evaluated in osteoclast formation, induced in vitro by the receptor activator of NF-κB ligand (RANKL). A rat femoral particle-induced peri-implant osteolysis model was established. Subsequently, micro-CT, histology, mechanical testing and bone turnover were used to assess the effects of SPC in preventing implant loosening.

KEY RESULTS

In vitro, we found that SPC suppressed osteoclast formation, bone resorption, F-actin ring formation and osteoclast-associated gene expression by inhibiting NF-κB signalling, specifically by targeting IκB kinases. Our in vivo study showed that SPC prevented particle-induced prosthesis loosening by inhibiting osteoclast formation, resulting in reduced periprosthetic bone loss, diminished pseudomembrane formation, improved bone-implant contact, reduced bone resorption-related turnover and enhanced stability of implants. Inhibition of NF-κB signalling by SPC was confirmed in vivo.

CONCLUSION AND IMPLICATIONS

SPC can prevent implant loosening through inhibiting osteoclast formation and bone resorption. Thus, SPC might be a novel therapeutic agent to prevent prosthesis loosening and for osteolytic diseases.

FHL2 Regulates Natural Killer Cell Development and Activation during Streptococcus pneumoniae Infection

Recent in silico studies suggested that the transcription cofactor LIM-only protein FHL2 is a major transcriptional regulator of mouse natural killer (NK) cells. However, the expression and role of FHL2 in NK cell biology are unknown. Here, we confirm that FHL2 is expressed in both mouse and human NK cells. Using FHL2^−/− mice, we found that FHL2 controls NK cell development in the bone marrow and maturation in peripheral organs. To evaluate the importance of FHL2 in NK cell activation, FHL2^−/− mice were infected with Streptococcus pneumoniae. FHL2^−/− mice are highly susceptible to this infection. The activation of lung NK cells is altered in FHL2^−/− mice, leading to decreased IFNγ production and a loss of control of bacterial burden. Collectively, our data reveal that FHL2 is a new transcription cofactor implicated in NK cell development and activation during pulmonary bacterial infection.

Runx1 Regulates Myeloid Precursor Differentiation Into Osteoclasts Without Affecting Differentiation Into Antigen Presenting or Phagocytic Cells in Both Males and Females

Runt-related transcription factor 1 (Runx1), a master regulator of hematopoiesis, is expressed in preosteoclasts. Previously we evaluated the bone phenotype of CD11b-Cre Runx1(fl/fl) mice and demonstrated enhanced osteoclasts and decreased bone mass in males. However, an assessment of the effects of Runx1 deletion in female osteoclast precursors was impossible with this model. Moreover, the role of Runx1 in myeloid cell differentiation into other lineages is unknown. Therefore, we generated LysM-Cre Runx1(fl/fl) mice, which delete Runx1 equally (∼80% deletion) in myeloid precursor cells from both sexes and examined the capacity of these cells to differentiate into osteoclasts and phagocytic and antigen-presenting cells. Both female and male LysM-Cre Runx1(fl/fl) mice had decreased trabecular bone mass (72% decrease in bone volume fraction) and increased osteoclast number (2-3 times) (P < .05) without alteration of osteoblast histomorphometric indices. We also demonstrated that loss of Runx1 in pluripotential myeloid precursors with LysM-Cre did not alter the number of myeloid precursor cells in bone marrow or their ability to differentiate into phagocytizing or antigen-presenting cells. This study demonstrates that abrogation of Runx1 in multipotential myeloid precursor cells significantly and specifically enhanced the ability of receptor activator of nuclear factor-κB ligand to stimulate osteoclast formation and fusion in female and male mice without affecting other myeloid cell fates. In turn, increased osteoclast activity in LysM-Cre Runx1(fl/fl) mice likely contributed to a decrease in bone mass. These dramatic effects were not due to increased osteoclast precursors in the deleted mutants and argue that inhibition of Runx1 in multipotential myeloid precursor cells is important for osteoclast formation and function.

Tumor necrosis factor receptor- associated factor 6 (TRAF6) regulation of development, function, and homeostasis of the immune system

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is an adapter protein that mediates a wide array of protein-protein interactions via its TRAF domain and a RING finger domain that possesses non-conventional E3 ubiquitin ligase activity. First identified nearly two decades ago as a mediator of interleukin-1 receptor (IL-1R)-mediated activation of NFκB, TRAF6 has since been identified as an actor downstream of multiple receptor families with immunoregulatory functions, including members of the TNFR superfamily, the Toll-like receptor (TLR) family, tumor growth factor-β receptors (TGFβR), and T-cell receptor (TCR). In addition to NFκB, TRAF6 may also direct activation of mitogen-activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), and interferon regulatory factor pathways. In the context of the immune system, TRAF6-mediated signals have proven critical for the development, homeostasis, and/or activation of B cells, T cells, and myeloid cells, including macrophages, dendritic cells, and osteoclasts, as well as for organogenesis of thymic and secondary lymphoid tissues. In multiple cellular contexts, TRAF6 function is essential not only for proper activation of the immune system but also for maintaining immune tolerance, and more recent work has begun to identify mechanisms of contextual specificity for TRAF6, involving both regulatory protein interactions, and messenger RNA regulation by microRNAs.

Protein-protein interactions of the LIM-only protein FHL2 and functional implication of the interactions relevant in cardiovascular disease

FHL2 belongs to the LIM-domain only proteins and contains four and a half LIM domains, each of which are composed of two zinc finger structures. FHL2 exhibits specific interaction with proteins exhibiting diverse functions, including transmembrane receptors, transcription factors and transcription co-regulators, enzymes, and structural proteins. The function of these proteins is regulated by FHL2, which modulates intracellular signal transduction pathways involved in a plethora of cellular tasks. The present review summarizes the current knowledge on the protein interactome of FHL2 and provides an overview of the functional implication of these interactions in apoptosis, migration, and regulation of nuclear receptor function. FHL2 was originally identified in the heart and there is extensive literature available on the role of FHL2 in the cardiovascular system, which is also summarized in this review.

Total saponin from Anemone flaccida Fr. Schmidt abrogates osteoclast differentiation and bone resorption via the inhibition of RANKL-induced NF-κB, JNK and p38 MAPKs activation

Osteoclasts, bone-specialized multinucleated cells, are responsible for bone destructive diseases such as rheumatoid arthritis and osteoporosis. Natural plant-derived products have received substantial attention given their potential therapeutic and preventive activities against bone destructive diseases. In the present study, we investigated the effects of total saponin (TS) from Anemone flaccida Fr. Schmidt, on receptor activator of nuclear factor-κB ligand (RANKL)-induced in vitro osteoclast differentiation. We observed that TS concentration-dependently inhibited RANKL-induced osteoclast formation from RAW 264.7 cell and bone marrow-derived macrophages (BMMs), as well as decreased extent of actin ring formation and lacunar resorption. The RANKL-stimulated expression of osteoclast-related transcription factors were also diminished by TS. Moreover, TS blocked the RANKL-triggered TRAF6 expression, phosphorylation of mitogen-activated protein kinases (MAPKs) and IκB-α, and inhibited NF-κB p65 DNA binding activity. Furthermore, TS almost abrogated the nuclear factor of activated T cells (NFATc1) and c-Fos expression. Taken together, our results demonstrated that TS suppresses RANKL-induced osteoclast differentiation and inflammatory bone loss via the down-regulation of TRAF6 level, suppression of JNK and p38 MAPKs and NF-κB activation, and subsequent decreased expression of c-Fos and NFATc1. Therefore, TS may be a potential agent and needs to be more evaluated in vivo or in clinical trials to become a therapeutic for lytic bone diseases.

TRAF6 is a novel regulator of Notch signaling in Drosophila melanogaster

Notch signaling pathway unravels a fundamental cellular communication system that plays an elemental role in development. It is evident from different studies that the outcome of Notch signaling depends on signal strength, timing, cell type, and cellular context. Since Notch signaling affects a spectrum of cellular activity at various developmental stages by reorganizing itself in more than one way to produce different intensities in the signaling output, it is important to understand the context dependent complexity of Notch signaling and different routes of its regulation. We identified, TRAF6 (Drosophila homolog of mammalian TRAF6) as an interacting partner of Notch intracellular domain (Notch-ICD). TRAF6 genetically interacts with Notch pathway components in trans-heterozygous combinations. Immunocytochemical analysis shows that TRAF6 co-localizes with Notch in Drosophila third instar larval tissues. Our genetic interaction data suggests that the loss-of-function of TRAF6 leads to the rescue of previously identified Kurtz-Deltex mediated wing notching phenotype and enhances Notch protein survival. Co-expression of TRAF6 and Deltex results in depletion of Notch in the larval wing discs and down-regulates Notch targets, Wingless and Cut. Taken together, our results suggest that TRAF6 may function as a negative regulator of Notch signaling.

Wnt4 signaling prevents skeletal aging and inflammation by inhibiting nuclear factor-κB

Aging-related bone loss and osteoporosis affect millions of patients worldwide. Chronic inflammation associated with aging and arthritis promotes bone resorption and impairs bone formation. Here we show that Wnt4 attenuated bone loss in osteoporosis and skeletal aging by inhibiting nuclear factor-kappa B (NF-κB) via non-canonical Wnt signaling. Transgenic mice expressing Wnt4 from osteoblasts were significantly protected from bone loss and chronic inflammation induced by ovariectomy, tumor necrosis factor or natural aging. In addition to promoting bone formation, Wnt4 could inhibit osteoclast formation and bone resorption. Mechanistically, Wnt4 inhibited transforming growth factor beta-activated kinase 1-mediated NF-κB activation in macrophages and osteoclast precursors independent of β-catenin. Moreover, recombinant Wnt4 proteins were able to alleviate osteoporotic bone loss and inflammation by inhibiting NF-κB in vivo. Taken together, our results suggest that Wnt4 might be used as a therapeutic agent for treating osteoporosis by attenuating NF-κB.

TRP14 inhibits osteoclast differentiation via its catalytic activity

We previously reported the inhibitory role of thioredoxin-related protein of 14 kDa (TRP14), a novel disulfide reductase, in nuclear factor-κB (NF-κB) activation, but its biological function has remained to be explored. Here, we evaluated the role of TRP14 in the differentiation and function of osteoclasts (OCs), for which NF-κB and cellular redox regulation have been known to be crucial, using RAW 264.7 macrophage cells expressing wild-type TRP14 or a catalytically inactive mutant, as well as its small interfering RNA. TRP14 depletion enhanced OC differentiation, actin ring formation, and bone resorption, as well as the accumulation of reactive oxygen species (ROS). TRP14 depletion promoted the activation of NF-κB, c-Jun NH2-terminal kinase, and p38, the expression of c-Fos, and the consequent induction of nuclear factor of activated T cell, cytoplasmic 1 (NFATc1), a key determinant of osteoclastogenesis. However, pretreatment with N-acetylcysteine or diphenylene iodonium significantly reduced the OC differentiation, as well as the ROS accumulation and NF-κB activation, that were enhanced by TRP14 depletion. Furthermore, receptor activator of NF-κB ligand (RANKL)-induced ROS accumulation, NF-κB activation, and OC differentiation were inhibited by the ectopic expression of wild-type TRP14 but not by its catalytically inactive mutant. These results suggest that TRP14 regulates OC differentiation and bone resorption through its catalytic activity and that enhancing TRP14 may present a new strategy for preventing bone resorption diseases.

Isolation and characterization of tumor necrosis factor receptor-associated factor 6 (TRAF6) from grouper, Epinephelus tauvina

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is one of the key adapter molecules in Toll-like receptor signal transduction that triggers downstream cascades involved in innate immunity. In the present study, a TRAF6 (named as Et-TRAF6) was identified from the marine fish grouper, Epinephelus tauvina by RACE PCR. The full-length cDNA of Et-TRAF6 comprised 1949 bp with a 1713 bp open reading frame (ORF) that encodes a putative protein of 570 amino acids. Similar to most TRAF6s, Et-TRAF6 includes one N-terminal RING domain (78aa-116aa), two zinc fingers of TRAF-type (159aa-210aa and 212aa-269aa), one coiled-coil region (370aa-394aa), and one conserved C-terminal meprin and TRAF homology (MATH) domain (401aa-526aa). Quantitative real-time PCR analysis revealed that Et-TRAF6 mRNA is expressed in all tested tissues, with the predominant expression in the stomach and intestine. The expression of Et-TRAF6 was up-regulated in the liver after challenge with Lipoteichoic acid (LTA), Peptidoglycan (PGN), Zymosan, polyinosine-polycytidylic acid [Poly(I:C)] and Polydeoxyadenylic acid · Polythymidylic acid sodium salt [Poly(dA:dT)]. The expression of Et-TRAF6 was also up-regulated in the liver after infection with Vibrio alginolyticus, Singapore grouper iridovirus (SGIV) and grouper nervous necrosis virus (GNNV). Recombinant Et-TRAF6 (rEt-TRAF6) was expressed in Escherichia BL21 (DE3) and purified for mouse anti-Et-TRAF6 serum preparation. Intracellular localization revealed that Et-TRAF6 is distributed in both cytoplasm and nucleus, and predominantly in the cytoplasm. These results together indicated that Et-TRAF6 might be involved in immune responses toward bacterial and virus challenges.

Runx1-mediated regulation of osteoclast differentiation and function

Excessive bone resorption is the cause of several metabolic bone diseases including osteoporosis. Thus, identifying factors that can inhibit osteoclast formation and/or activity may define new drug targets that can be used to develop novel therapies for these conditions. Emerging evidence demonstrates that the master regulator of hematopoiesis, Runx1, is expressed in preosteoclasts and may influence skeletal health. To examine the potential role of Runx1 in osteoclast formation and function, we deleted its expression in myeloid osteoclast precursors by crossing Runx1 floxed mice (Runx1(F/F)) with CD11b-Cre transgenic mice. Mice lacking Runx1 in preosteoclasts (CD11b-Cre;Runx1(F/F)) exhibited significant loss of femoral trabecular and cortical bone mass compared with that in Cre-negative mice. In addition, serum levels of collagen type 1 cross-linked C-telopeptide, a biomarker of osteoclast-mediated bone resorption, were significantly elevated in CD11b-Cre;Runx1(F/F) mice compared with those in Runx1(F/F) mice. Tartrate-resistant acid phosphatase-positive osteoclasts that differentiated from bone marrow cells of CD11b-Cre;Runx1(F/F) mice in vitro were larger, were found in greater numbers, and had increased bone resorbing activity than similarly cultured cells from Runx1(F/F) mice. CD11b-Cre;Runx1(F/F) bone marrow cells that were differentiated into osteoclasts in vitro also had elevated mRNA levels of osteoclast-related genes including vacuolar ATPase D2, cathepsin K, matrix metalloproteinase 9, calcitonin receptor, osteoclast-associated receptor, nuclear factor of activated T cells cytoplasmic 1, and cFos. These data indicate that Runx1 expression in preosteoclasts negatively regulates osteoclast formation and activity and contributes to overall bone mass.

Runx3-mediated transcriptional program in cytotoxic lymphocytes

The transcription factor Runx3 is highly expressed in CD8⁺ T and NK cytotoxic lymphocytes and is required for their effective activation and proliferation but molecular insights into the transcription program regulated by Runx3 in these cells are still missing. Using Runx3-ChIP-seq and transcriptome analysis of wild type vs. Runx3^-/- primary cells we have now identified Runx3-regulated genes in the two cell types at both resting and IL-2-activated states. Runx3-bound genomic regions in both cell types were distantly located relative to gene transcription start sites and were enriched for RUNX and ETS motifs. Bound genomic regions significantly overlapped T-bet and p300-bound enhancer regions in Runx3-expressing Th1 helper cells. Compared to resting cells, IL-2-activated CD8⁺ T and NK cells contain three times more Runx3-regulated genes that are common to both cell types. Functional annotation of shared CD8⁺ T and NK Runx3-regulated genes revealed enrichment for immune-associated terms including lymphocyte activation, proliferation, cytotoxicity, migration and cytokine production, highlighting the role of Runx3 in CD8⁺ T and NK activated cells.

LIM-only protein FHL2 activates NF-κB signaling in the control of liver regeneration and hepatocarcinogenesis

Four-and-a-half LIM-only protein 2 (FHL2) is an important mediator in many signaling pathways. In this study, we analyzed the functions of FHL2 in nuclear factor κB (NF-κB) signaling in the liver. We show that FHL2 enhanced tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) activity in transcriptional activation of NF-κB targets by stabilizing the protein. TRAF6 is a binding partner of FHL2 and an important component of the Toll-like receptor-NF-κB pathway. Knockdown of FHL2 in 293-hTLR4/MD2-CD14 cells impaired lipopolysaccharide (LPS)-induced NF-κB activity, which regulates expression of inflammatory cytokines. Indeed, FHL2(-/-) macrophages showed significantly reduced production of TNF and interleukin 6 (IL-6) following LPS stimulation. TNF and IL-6 are the key cytokines that prime liver regeneration after hepatic injury. Following partial hepatectomy, FHL2(-/-) mice exhibited diminished induction of TNF and IL-6 and delayed hepatocyte regeneration. In the liver, NF-κB signaling orchestrates inflammatory cross talk between hepatocytes and hepatic immune cells that promote chemical hepatocarcinogenesis. We found that deficiency of FHL2 reduced susceptibility to diethylnitrosamine-induced hepatocarcinogenesis, correlating with the activator function of FHL2 in NF-κB signaling. Our findings demonstrate FHL2 as a positive regulator of NF-κB activity in liver regeneration and carcinogenesis and highlight the importance of FHL2 in both hepatocytes and hepatic immune cells.

RUNX family: Regulation and diversification of roles through interacting proteins

The Runt-related transcription factors (RUNX) belong to an ancient family of metazoan genes involved in developmental processes. Through multiple protein-interacting partners, RUNX proteins have been implicated in diverse signaling pathways and cellular processes. The frequent inactivation of RUNX genes in cancer indicates crucial roles for RUNX in tumor suppression. This review discusses the abilities of RUNX proteins, in particular RUNX3, to integrate oncogenic signals or environmental cues and to initiate appropriate tumor suppressive responses.

The four and a half LIM-only protein 2 (FHL2) activates transforming growth factor β (TGF-β) signaling by regulating ubiquitination of the E3 ligase Arkadia

Arkadia is a RING-based ubiquitin ligase that positively regulates TGF-β signaling by targeting several pathway components for ubiquitination and degradation. However, little is known about the mechanisms controlling Arkadia activity. Here we show that the LIM-only protein FHL2 binds and synergistically cooperates with Arkadia to activate Smad3/Smad4-dependent transcription. Knockdown of FHL2 by RNA interference decreases Arkadia level and restricts the amplitude of Arkadia-induced TGF-β target gene responses. We found that Arkadia is ubiquitinated via K63- and K27-linked polyubiquitination. A single mutation at the RING domain that abolishes the E3 activity diminishes Arkadia ubiquitination, indicating that this modification partly involves autocatalytic process. Mutation of seven lysines at the C-terminal region of Arkadia severely impairs ubiquitination through the K27 but not the K63 linkage and slows down the turnover of Arkadia, suggesting that K27-linked polyubiquitination might promote proteolysis-dependent regulation of Arkadia. We show that FHL2 increases the half-life of Arkadia through inhibition of ubiquitin chain assembly on the protein, which provides a molecular basis for functional cooperation between Arkadia and FHL2 in enhancing TGF-β signaling. Our study uncovers a novel regulatory mechanism of Arkadia by ubiquitination and identifies FHL2 as important regulator of Arkadia ubiquitination and TGF-β signal transduction.

TRAF-6 dependent signaling pathway is essential for TNF-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation

Human osteoclast formation from mononuclear phagocyte precursors involves interactions between tumor necrosis factor (TNF) ligand superfamily members and their receptors. Recent evidence indicates that in addition to triggering apoptosis, the TNF-related apoptosis-inducing ligand (TRAIL) induces osteoclast differentiation. To understand TRAIL-mediated signal transduction mechanism in osteoclastogenesis, we demonstrated that TRAIL induces osteoclast differentiation via a Tumor necrosis factor receptor-associated factor 6 (TRAF-6)-dependent signaling pathway. TRAIL-induced osteoclast differentiation was significantly inhibited by treatment with TRAF-6 siRNA and TRAF6 decoy peptides in both human monocytes and murine RAW264.7 macrophage cell lines, as evaluated in terms of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells and bone resorption activity. Moreover, TRAIL-induced osteoclast differentiation was also abolished in TRAF6 knockout bone marrow macrophages. In addition to induction of NFATc1, treatment of TRAIL also induced ubiquitination of TRAF6 in osteoclast differentiation. Thus, our data demonstrate that TRAIL induces osteoclastic differentiation via a TRAF-6 dependent signaling pathway. This study suggests TRAF6-dependent signaling may be a central pathway in osteoclast differentiation, and that TNF superfamily molecules other than RANKL may modify RANK signaling by interaction with TRAF6-associated signaling.

Microarray expression analysis of genes and pathways involved in growth plate cartilage injury responses and bony repair

The injured growth plate cartilage is often repaired by a bone bridge which causes bone growth deformities. Whilst previous studies have identified sequential inflammatory, fibrogenic, osteogenic and bone remodelling responses involved in the repair process, the molecular pathways which regulated these cellular events remain unknown. In a rat growth plate injury model, tissue from the injury site was collected across the time-course of bone bridge formation using laser capture microdissection and was subjected to Affymetrix microarray gene expression analysis. Real Time PCR and immunohistochemical analyses were used to confirm changes in levels of expression of some genes identified in microarray. Four major functional groupings of differentially expressed genes with known roles in skeletal development were identified across the time-course of bone bridge formation, including Wnt signalling (SFRP1, SFRP4, β-catenin, Csnk2a1, Tcf7, Lef1, Fzd1, Fzd2, Wisp1 and Cpz), BMP signalling (BMP-2, BMP-6, BMP-7, Chrd, Chrdl2 and Id1), osteoblast differentiation (BMP-2, BMP-6, Chrd, Hgn, Spp1, Axin2, β-catenin, Bglap2) and skeletal development (Chrd, Mmp9, BMP-1, BMP-6, Spp1, Fgfr1 and Traf6). These studies provide insight into the molecular pathways which act cooperatively to regulate bone formation following growth plate cartilage injury and highlight potential therapeutic targets to limit bone bridge formation.

A comprehensive manually curated reaction map of RANKL/RANK-signaling pathway

Receptor activator of nuclear factor-kappa B ligand (RANKL) is a member of tumor necrosis factor (TNF) superfamily that plays a key role in the regulation of differentiation, activation and survival of osteoclasts and also in tumor cell migration and bone metastasis. Osteoclast activation induced by RANKL regulates hematopoietic stem cell mobilization as part of homeostasis and host defense mechanisms thereby linking regulation of hematopoiesis with bone remodeling. Binding of RANKL to its receptor, Receptor activator of nuclear factor-kappa B (RANK) activates molecules such as NF-kappa B, mitogen activated protein kinase (MAPK), nuclear factor of activated T cells (NFAT) and phosphatidyl 3-kinase (PI3K). Although the molecular and cellular roles of these molecules have been reported previously, a systematic cataloging of the molecular events induced by RANKL/RANK interaction has not been attempted. Here, we present a comprehensive reaction map of the RANKL/RANK-signaling pathway based on an extensive manual curation of the published literature. We hope that the curated RANKL/RANK-signaling pathway model would enable new biomedical discoveries, which can provide novel insights into disease processes and development of novel therapeutic interventions.

Database URL:http://www.netpath.org/pathways?path_id=NetPath_21

Litopenaeus vannamei tumor necrosis factor receptor-associated factor 6 (TRAF6) responds to Vibrio alginolyticus and white spot syndrome virus (WSSV) infection and activates antimicrobial peptide genes

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is a key signaling adaptor protein not only for the TNFR superfamily but also for the Interleukin-1 receptor/Toll-like receptor (IL-1/TLR) superfamily. To investigate TRAF6 function in invertebrate innate immune responses, Litopenaeus vannamei TRAF6 (LvTRAF6) was identified and characterized. The full-length cDNA of LvTRAF6 is 2823bp long, with an open reading frame (ORF) encoding a putative protein of 594 amino acids, including a RING-type Zinc finger, two TRAF-type Zinc fingers, a coiled-coil region, and a meprin and TRAF homology (MATH) domain. The overall amino acid sequence identity between LvTRAF6 and other known TRAF6s is 22.2-33.3%. Dual luciferase reporter assays in Drosophila S2 cells revealed that LvTRAF6 could activate the promoters of antimicrobial peptide genes (AMPs), including Drosophila Attacin A and Drosomycin, and shrimp Penaeidins. Real-time quantitative PCR (qPCR) indicated that LvTRAF6 was constitutively expressed in various tissues of L. vannamei. After Vibrio alginolyticus and white spot syndrome virus (WSSV) challenge, LvTRAF6 was down-regulated, though with different expression patterns in the intestine compared to other tissues. After WSSV challenge, LvTRAF6 was up-regulated 2.7- and 2.3-fold over the control at 3h in gills and hepatopancreas, respectively. These results indicated that LvTRAF6 may play a crucial role in antibacterial and antiviral responses via regulation of AMP gene expression.

Genomic and functional uniqueness of the TNF receptor-associated factor gene family in amphioxus, the basal chordate

The TNF-associated factor (TRAF) family, the crucial adaptor group in innate immune signaling, increased to 24 in amphioxus, the oldest lineage of the Chordata. To address how these expanded molecules evolved to adapt to the changing TRAF mediated signaling pathways, here we conducted genomic and functional comparisons of four distinct amphioxus TRAF groups with their human counterparts. We showed that lineage-specific duplication and rearrangement were responsible for the expansion of amphioxus TRAF1/2 and 3 lineages, whereas TRAF4 and 6 maintained a relatively stable genome and protein structure. Amphioxus TRAF1/2 and 3 molecules displayed various expression patterns in response to microbial infection, and some of them can attenuate the NF-kappaB activation mediated by human TRAF2 and 6. Amphioxus TRAF4 presented two unique functions: activation of the NF-kappaB pathway and involvement in somite formation. Although amphioxus TRAF6 was conserved in activating NF-kappaB pathway for antibacterial defense, the mechanism was not the same as that observed in humans. In summary, our findings reveal the evolutionary uniqueness of the TRAF family in this basal chordate, and suggest that genomic duplication and functional divergence of the TRAF family are important for the current form of the TRAF-mediated signaling pathways in humans.