Id helix-loop-helix proteins negatively regulate TRANCE-mediated osteoclast differentiation

Inhibitory regulation of osteoclast differentiation by interleukin-3 via regulation of c-Fos and Id protein expression

Interleukin-3 (IL-3) is produced under various pathological conditions and is thought to be involved in the pathogenesis of inflammatory diseases; however, its function in bone homeostasis under normal conditions or nature of the downstream molecular targets remains unknown. Here we examined the effect of IL-3 on osteoclast differentiation from mouse and human bone marrow-derived macrophages (BMMs). Although IL-3 can induce osteoclast differentiation of multiple myeloma bone marrow cells, IL-3 greatly inhibited osteoclast differentiation of human BMMs isolated from healthy donors. These inhibitory effects of IL-3 were only observed at early time points (days 0 and 1). IL-3 inhibited the expression of c-Fos and NFATc1 in BMMs treated with RANKL. However, IL-3-mediated inhibition of osteoclast differentiation was not completely reversed by ectopic expression of c-Fos or NFATc1. Importantly, IL-3 induced inhibitor of DNA binding/differentiation (Id)1 in hBMMs, while Id2 were sustained during osteoclast differentiation of mBMMs treated with IL-3. Ectopic expression of NFATc1 in Id2-deficient BMMs completely reversed the inhibitory effect of IL-3 on osteoclast differentiation. Furthermore, inflammation-induced bone erosion was markedly inhibited by IL-3 administration. Taken together, our results suggest that IL-3 plays an inhibitory role in osteoclast differentiation by regulating c-Fos and Ids, and also exerts anti-bone erosion effects.

The transmembrane adaptor protein, linker for activation of T cells (LAT), regulates RANKL-induced osteoclast differentiation

RANKL induces the formation of osteoclasts, which are responsible for bone resorption. Herein we investigate the role of the transmembrane adaptor proteins in RANKL-induced osteoclastogenesis. LAT positively regulates osteoclast differentiation and is up-regulated by RANKL via c-Fos and NFATc1, whereas LAB and LIME act as negative modulators of osteoclastogenesis. In addition, silencing of LAT by RNA interference or overexpression of a LAT dominant negative in bone marrow-derived macrophage cells attenuates RANKL-induced osteoclast formation. Furthermore, LAT is involved in RANKL-induced PLC(γ) activation and NFATc1 induction. Thus, our data suggest that LAT acts as a positive regulator of RANKL-induced osteoclastogenesis.

TNF-induced osteoclastogenesis and inflammatory bone resorption are inhibited by transcription factor RBP-J

Tumor necrosis factor (TNF) plays a key role in the pathogenesis of inflammatory bone resorption and associated morbidity in diseases such as rheumatoid arthritis and periodontitis. Mechanisms that regulate the direct osteoclastogenic properties of TNF to limit pathological bone resorption in inflammatory settings are mostly unknown. Here, we show that the transcription factor recombinant recognition sequence binding protein at the J(κ) site (RBP-J) strongly suppresses TNF-induced osteoclastogenesis and inflammatory bone resorption, but has minimal effects on physiological bone remodeling. Myeloid-specific deletion of RBP-J converted TNF into a potent osteoclastogenic factor that could function independently of receptor activator of NF-κB (RANK) signaling. In the absence of RBP-J, TNF effectively induced osteoclastogenesis and bone resorption in RANK-deficient mice. Activation of RBP-J selectively in osteoclast precursors suppressed inflammatory osteoclastogenesis and arthritic bone resorption. Mechanistically, RBP-J suppressed induction of the master regulator of osteoclastogenesis (nuclear factor of activated T cells, cytoplasmic 1) by attenuating c-Fos activation and suppressing induction of B lymphocyte-induced maturation protein-1, thereby preventing the down-regulation of transcriptional repressors such as IRF-8 that block osteoclast differentiation. Thus, RBP-J regulates the balance between activating and repressive signals that regulate osteoclastogenesis. These findings identify RBP-J as a key upstream negative regulator of osteoclastogenesis that restrains excessive bone resorption in inflammatory settings.

Involvement of PRIP, phospholipase C-related, but catalytically inactive protein, in bone formation

PRIP (phospholipase C-related, but catalytically inactive protein) is a novel protein isolated in this laboratory. PRIP-deficient mice showed increased serum gonadotropins, but decreased gonadal steroid hormones. This imbalance was similar to that for the cause of bone disease, such as osteoporosis. In the present study, therefore, we analyzed mutant mice with special reference to the bone property. We first performed three-dimensional analysis of the femur of female mice. The bone mineral density and trabecular bone volume were higher in mutant mice. We further performed histomorphometrical assay of bone formation parameters: bone formation rate, mineral apposition rate, osteoid thickness, and osteoblast number were up-regulated in the mutant, indicating that increased bone mass is caused by the enhancement of bone formation ability. We then cultured primary cells isolated from calvaria prepared from both genotypes. In mutant mice, osteoblast differentiation, as assessed by alkaline phosphatase activity and the expression of osteoblast differentiation marker genes, was enhanced. Moreover, we analyzed the phosphorylation of Smad1/5/8 in response to bone morphogenetic protein, with longer phosphorylation in the mutant. These results indicate that PRIP is implicated in the negative regulation of bone formation.

The small molecule harmine regulates NFATc1 and Id2 expression in osteoclast progenitor cells

Small molecule compounds that potently affect osteoclastogenesis could be useful as chemical probes for elucidating the mechanisms of various biological phenomena and as effective therapeutic strategies against bone resorption. An osteoclast progenitor cell-based high-throughput screening system was designed to target activation of NFAT, which is a key event for osteoclastogenesis. Orphan ligand library screening using this system identified the β-carboline derivative harmine, which is a highly potent inhibitor of dual-specificity tyrosine-phosphorylation regulated kinase 1A (DYRK1A), to be an NFAT regulator in osteoclasts. RAW264.7 cells highly expressed DYRK1A protein, and in vitro phosphorylation assay demonstrated that harmine directly inhibited the DYRK1A-mediated phosphorylation (in-activation) of NFATc1. Harmine promoted the dephosphorylation (activation) of NFATc1 in RAW264.7 cells within 24h, and it significantly increased the expression of NFATc1 in RAW264.7 cells and mouse primary bone marrow macrophages (BMMs) both in the presence and absence of RANKL stimulation. Although harmine promoted NFATc1 expression and stimulated target genes for osteoclastogenesis, cell-cell fusion and the formation of TRAP-positive multinucleated osteoclasts from RAW264.7 cells and BMMs was significantly inhibited by harmine treatment. Meanwhile, harmine remarkably promoted the expression of inhibitor of DNA binding/differentiation-2 (Id2), which is a negative regulator for osteoclastogenesis, in RAW264.7 cells and BMMs. An Id2-null-mutant showed slightly increased osteoclast formation from BMMs, and the harmine-mediated inhibition of osteoclast formation was abolished in the BMMs of Id2-null-mutant mice. These results suggest that harmine is a potent activator of NFATc1 that interferes with the function of DYRK1A in osteoclast precursors and also up-regulates Id2 protein, which may dominantly inhibit expression pathways associated with cell-cell fusion, thereby leading to the disruption of the fusion events mediating osteoclastogenesis. The small molecule harmine is therefore expected to provide an experimental tool for investigating signaling cascades in osteoclastogenesis, especially those centered on DYRK1A-mediated NFATc1 and Id2 regulation.

Negative regulation of osteoclastogenesis and bone resorption by cytokines and transcriptional repressors

Bone remodeling in physiological and pathological conditions represents a balance between bone resorption mediated by osteoclasts and bone formation by osteoblasts. Bone resorption is tightly and dynamically regulated by multiple mediators, including cytokines that act directly on osteoclasts and their precursors, or indirectly by modulating osteoblast lineage cells that in turn regulate osteoclast differentiation. The critical role of cytokines in inducing and promoting osteoclast differentiation, function and survival is covered by the accompanying review by Zwerina and colleagues. Recently, it has become clear that negative regulation of osteoclastogenesis and bone resorption by inflammatory factors and cytokines, downstream signaling pathways, and a newly described network of transcriptional repressors plays a key role in bone homeostasis by fine tuning bone remodeling and restraining excessive bone resorption in inflammatory settings. In this review we discuss negative regulators of osteoclastogenesis and mechanisms by which these factors suppress bone resorption.

Mouse genome-wide association and systems genetics identify Asxl2 as a regulator of bone mineral density and osteoclastogenesis

Significant advances have been made in the discovery of genes affecting bone mineral density (BMD); however, our understanding of its genetic basis remains incomplete. In the current study, genome-wide association (GWA) and co-expression network analysis were used in the recently described Hybrid Mouse Diversity Panel (HMDP) to identify and functionally characterize novel BMD genes. In the HMDP, a GWA of total body, spinal, and femoral BMD revealed four significant associations (-log10P>5.39) affecting at least one BMD trait on chromosomes (Chrs.) 7, 11, 12, and 17. The associations implicated a total of 163 genes with each association harboring between 14 and 112 genes. This list was reduced to 26 functional candidates by identifying those genes that were regulated by local eQTL in bone or harbored potentially functional non-synonymous (NS) SNPs. This analysis revealed that the most significant BMD SNP on Chr. 12 was a NS SNP in the additional sex combs like-2 (Asxl2) gene that was predicted to be functional. The involvement of Asxl2 in the regulation of bone mass was confirmed by the observation that Asxl2 knockout mice had reduced BMD. To begin to unravel the mechanism through which Asxl2 influenced BMD, a gene co-expression network was created using cortical bone gene expression microarray data from the HMDP strains. Asxl2 was identified as a member of a co-expression module enriched for genes involved in the differentiation of myeloid cells. In bone, osteoclasts are bone-resorbing cells of myeloid origin, suggesting that Asxl2 may play a role in osteoclast differentiation. In agreement, the knockdown of Asxl2 in bone marrow macrophages impaired their ability to form osteoclasts. This study identifies a new regulator of BMD and osteoclastogenesis and highlights the power of GWA and systems genetics in the mouse for dissecting complex genetic traits.

The role of osteoclast-associated receptor in osteoimmunology

The term osteoimmunology is coined for molecular and cellular cross talk between the skeletal and immune system. Immunomodulatory signals have long been implicated as key regulators of bone metabolism. Recently, osteoclast-associated receptor (OSCAR), an IgG-like receptor, has been identified as an important osteoimmunological mediator. OSCAR expression in bone is highly conserved across different species, and the molecule is an important costimulatory receptor for osteoclast differentiation through activation of NFATc1. In humans, OSCAR is expressed by macrophages, monocytes, and monocyte-derived dendritic cells and modulates the response of the innate and adaptive immune systems by promoting cell activation and maturation, Ag presentation, and proinflammatory circuits. Human studies indicate that OSCAR may contribute to the pathogenesis and severity of osteoporosis and rheumatoid arthritis. In this paper, we review the structure-function relationship, expression pattern, and physiological role of OSCAR in osteoimmunology and summarize its potential implications for human diseases.

Osteoclastogenesis and arthritis

There is emerging interest for osteoclasts as key players in the erosive and inflammatory events leading to joint destruction in chronic arthritis. In fact, chronic inflammatory joint diseases such as psoriatic arthritis and rheumatoid arthritis are often characterized by destruction of juxta-articular bone and erosions due to the elevated activity of osteoclasts, which are involved in bone resorption. The main step in inflammatory bone erosion is an imbalance between bone resorption and bone formation: osteoclast formation is enhanced by proinflammatory cytokines such as TNF-α, IL-1β, and IL-17 and is not balanced by increased activity of bone-forming osteoblasts. T-cells, stromal cells, and synoviocytes enhance osteoclast formation via expression of RANKL and, under pathologic conditions, of proinflammatory cytokines. In rheumatoid arthritis, accumulation of osteoclasts in synovial tissues and their activation associated with osteoclastogenic cytokines and chemokines at cartilage erosion sites suggest that they could be usefully selected as therapeutic target. In particular, in consideration of the primary role of RANKL and TNF-α in osteoclastogenesis, the control of the production of RANKL and the inhibition of TNF-α represent important strategies for reducing bone damage in this disease.

Osteoclast differentiation factor RANKL controls development of progestin-driven mammary cancer

Breast cancer is one of the most common cancers in humans and will on average affect up to one in eight women in their lifetime in the United States and Europe. The Women's Health Initiative and the Million Women Study have shown that hormone replacement therapy is associated with an increased risk of incident and fatal breast cancer. In particular, synthetic progesterone derivatives (progestins) such as medroxyprogesterone acetate (MPA), used in millions of women for hormone replacement therapy and contraceptives, markedly increase the risk of developing breast cancer. Here we show that the in vivo administration of MPA triggers massive induction of the key osteoclast differentiation factor RANKL (receptor activator of NF-κB ligand) in mammary-gland epithelial cells. Genetic inactivation of the RANKL receptor RANK in mammary-gland epithelial cells prevents MPA-induced epithelial proliferation, impairs expansion of the CD49f(hi) stem-cell-enriched population, and sensitizes these cells to DNA-damage-induced cell death. Deletion of RANK from the mammary epithelium results in a markedly decreased incidence and delayed onset of MPA-driven mammary cancer. These data show that the RANKL/RANK system controls the incidence and onset of progestin-driven breast cancer.

Bone morphogenetic protein 2 acts via inhibitor of DNA binding proteins to synergistically regulate follicle-stimulating hormone beta transcription with activin A

We recently reported that bone morphogenetic proteins (BMPs) 2 and 4 can stimulate FSHbeta-subunit (Fshb) transcription alone and in synergy with activins. We further showed that BMP2 signals via the BMP type IA receptor (or activin receptor-like kinase 3) to mediate its effects. However, the intracellular mechanisms through which BMP2 regulates Fshb are unknown. In the current study, we used cDNA microarray analyses (and validation by real-time quantitative RT-PCR) to identify BMP2 target genes in the murine gonadotrope cell line, LbetaT2. Short-interfering RNA-mediated knockdown, overexpression, and coimmunoprecipitation experiments were used to examine the potential functional roles of selected gene products. Quantitative RT-PCR analysis largely confirmed the results of the array analyses, and inhibitors of DNA binding 1, 2, and 3 (Id1, Id2, and Id3) were selected for functional analyses. Knockdown of endogenous Id2 or Id3, but not Id1, diminished the synergistic effects of BMP2 and activin A on Fshb transcription. Overexpression of Id1, Id2, or Id3 alone had no effect, but all three potentiated activin A or mothers against decapentaplegic homolog (SMAD)3 induction of Fshb transcription. Though the precise mechanism through which Ids produce their effects are not yet known, we observed physical interactions between Id1, Id2, or Id3 and SMAD3. Collectively, the data suggest that BMP2 synergistically regulates Fshb transcription with activins, at least in part, through the combined actions of Ids 2 or 3 and SMAD3.

Osteosarcoma is characterised by reduced expression of markers of osteoclastogenesis and antigen presentation compared with normal bone

Background:

Osteosarcoma (OS) is the most common primary bone tumour in children and adolescents. Patients who respond poorly to chemotherapy have a higher risk of metastatic disease and 5-year survival rates of only 10–20%. Therefore, identifying molecular targets that are specific for OS, or more specifically, metastatic OS, will be critical to the development of new treatment strategies to improve patient outcomes.

Methods:

We performed a transcriptomic analysis of chemo-naive OS biopsies and non-malignant bone biopsies to identify differentially expressed genes specific to OS, which could provide insight into OS biology and chemoresistance.

Results:

Statistical analysis of the OS transcriptomes found differential expression of several metallothionein family members, as well as deregulation of genes involved in antigen presentation. Tumours also exhibited significantly increased expression of ID1 and profound down-regulation of S100A8, highlighting their potential as therapeutic targets for OS. Finally, we found a significant correlation between OS and impaired osteoclastogenesis and antigen-presenting activity. The reduced osteoclastogenesis and antigen-presenting activity were more profound in the chemoresistant OS samples.

Conclusion:

Our results indicate that OS displays gene signatures consistent with decreased antigen-presenting activity, enhanced chemoresistance, and impaired osteoclastogenesis. Moreover, these alterations are more pronounced in chemoresistant OS tumour samples.

The Blimp1-Bcl6 axis is critical to regulate osteoclast differentiation and bone homeostasis

Controlling osteoclastogenesis is critical to maintain physiological bone homeostasis and prevent skeletal disorders. Although signaling activating nuclear factor of activated T cells 1 (NFATc1), a transcription factor essential for osteoclastogenesis, has been intensively investigated, factors antagonistic to NFATc1 in osteoclasts have not been characterized. Here, we describe a novel pathway that maintains bone homeostasis via two transcriptional repressors, B cell lymphoma 6 (Bcl6) and B lymphocyte–induced maturation protein-1 (Blimp1). We show that Bcl6 directly targets ‘osteoclastic’ molecules such as NFATc1, cathepsin K, and dendritic cell-specific transmembrane protein (DC-STAMP), all of which are targets of NFATc1. Bcl6-overexpression inhibited osteoclastogenesis in vitro, whereas Bcl6-deficient mice showed accelerated osteoclast differentiation and severe osteoporosis. We report that Bcl6 is a direct target of Blimp1 and that mice lacking Blimp1 in osteoclasts exhibit osteopetrosis caused by impaired osteoclastogenesis resulting from Bcl6 up-regulation. Indeed, mice doubly mutant in Blimp1 and Bcl6 in osteoclasts exhibited decreased bone mass with increased osteoclastogenesis relative to osteoclast-specific Blimp1-deficient mice. These results reveal a Blimp1–Bcl6–osteoclastic molecule axis, which critically regulates bone homeostasis by controlling osteoclastogenesis and may provide a molecular basis for novel therapeutic strategies.

Inhibitor of DNA binding/differentiation 2 induced by hypoxia promotes synovial fibroblast-dependent osteoclastogenesis

OBJECTIVE

To map hypoxic areas in arthritic synovium and to establish the relevance of low oxygen levels to the phenotype of synovial fibroblasts, with special focus on bone degradation.

METHODS

To analyze the distribution of hypoxia in arthritic joints, the hypoxia marker EF5 was administered to mice with collagen-induced arthritis (CIA). To evaluate the effect of hypoxia on rheumatoid arthritis synovial fibroblasts (RASFs), reverse suppression subtractive hybridization and complementary DNA array were used. Real-time polymerase chain reaction, Western blotting, and immunohistochemistry were used to evaluate the expression of inhibitor of DNA binding/differentiation 2 (ID-2). To investigate the function of ID-2 in RASFs, cells were transfected either with ID-2 vector or with ID-2-specific small interfering RNA.

RESULTS

EF5 staining showed the presence of hypoxia in arthritic joints, particularly at sites of synovial invasion into bone. Differential expression analysis revealed that ID-2 was strongly induced by hypoxia in RASFs. Immunohistochemical analysis of CIA mouse synovium and human RA synovium showed a strong expression of ID-2 by RASFs at sites of synovial invasion into bone. Overexpression of ID-2 in RASFs significantly induced the expression of several factors promoting osteoclastogenesis. The biologic relevance of the potent osteoclastogenesis-promoting effects was shown by coculture assays of ID-2-overexpressing RASFs with bone marrow cells, leading to an increased differentiation of osteoclasts from bone marrow precursors.

CONCLUSION

The data show that hypoxic conditions are present at sites of inflammation and synovial invasion into bone in arthritic synovium. Hypoxia-induced ID-2 may contribute to joint destruction in RA patients by promoting synovial fibroblast-dependent osteoclastogenesis.

Class A scavenger receptor promotes osteoclast differentiation via the enhanced expression of receptor activator of NF-kappaB (RANK)

Osteoclasts originate from bone marrow monocyte/macrophage lineage cells, and their differentiation depends on macrophage colony-stimulating factor (M-CSF) and receptor activator nuclear factor kappa B (RANK) ligand. Class A scavenger receptor (SR-A) is one of the principal functional molecules of macrophages, and its level of expression declines during osteoclast differentiation. To investigate the role of SR-A in osteoclastogenesis, we examined pathological changes in femoral bone and the expression levels of osteoclastogenesis-related molecules in SR-A(-/-) mice. The femoral osseous density of SR-A(-/-) mice was higher than that of SR-A(+/+) mice, and the number of multinucleated osteoclasts was significantly decreased. An in vitro differentiation assay revealed that the differentiation of multinucleated osteoclasts from bone marrow-derived progenitor cells is impaired in SR-A(-/-) mice. Elimination of SR-A did not alter the expression level of the M-CSF receptor, c-fms; however, the expression levels of RANK and RANK-related osteoclast-differentiation molecules such as nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1) and microphthalmia-associated transcription factor (MITF) significantly decreased. Furthermore, acetylated low-density lipoprotein (AcLDL), an SR-A ligand, significantly increased the expression level of RANK and MITF during osteoclast differentiation. These data indicate that SR-A promotes osteoclastogenesis via augmentation of the expression level of RANK and its related molecules.

Negative feedback control of osteoclast formation through ubiquitin-mediated down-regulation of NFATc1

The regulation of NFATc1 expression is important for osteoclast differentiation and function. Herein, we demonstrate that macrophage-colony-stimulating factor induces NFATc1 degradation via Cbl proteins in a Src kinase-dependent manner. NFATc1 proteins are ubiquitinated and rapidly degraded during late stage osteoclastogenesis, and this degradation is mediated by Cbl-b and c-Cbl ubiquitin ligases in a Src-dependent manner. In addition, NFATc1 interacts endogenously with c-Src, c-Cbl, and Cbl-b in osteoclasts. Overexpression of c-Src induces down-regulation of NFATc1, and depletion of Cbl proteins blocks NFATc1 degradation during late stage osteoclastogenesis. Taken together, our data provide a negative regulatory mechanism by which macrophage-colony-stimulating factor activates Src family kinases and Cbl proteins, and subsequently, induces NFATc1 degradation during osteoclast differentiation.

Id1 represses osteoclast-dependent transcription and affects bone formation and hematopoiesis

BACKGROUND

The bone-bone marrow interface is an area of the bone marrow microenvironment in which both bone remodeling cells, osteoblasts and osteoclasts, and hematopoietic cells are anatomically juxtaposed. The close proximity of these cells naturally suggests that they interact with one another, but these interactions are just beginning to be characterized.

METHODOLOGY/PRINCIPAL FINDINGS

An Id1(-/-) mouse model was used to assess the role of Id1 in the bone marrow microenvironment. Micro-computed tomography and fracture tests showed that Id1(-/-) mice have reduced bone mass and increased bone fragility, consistent with an osteoporotic phenotype. Osteoclastogenesis and pit formation assays revealed that loss of Id1 increased osteoclast differentiation and resorption activity, both in vivo and in vitro, suggesting a cell autonomous role for Id1 as a negative regulator of osteoclast differentiation. Examination by flow cytometry of the hematopoietic compartment of Id1(-/-) mice showed an increase in myeloid differentiation. Additionally, we found increased expression of osteoclast genes, TRAP, Oscar, and CTSK in the Id1(-/-) bone marrow microenvironment. Lastly, transplantation of wild-type bone marrow into Id1(-/-) mice repressed TRAP, Oscar, and CTSK expression and activity and rescued the hematopoietic and bone phenotype in these mice.

CONCLUSIONS/SIGNIFICANCE

In conclusion, we demonstrate an osteoporotic phenotype in Id1(-/-) mice and a mechanism for Id1 transcriptional control of osteoclast-associated genes. Our results identify Id1 as a principal player responsible for the dynamic cross-talk between bone and bone marrow hematopoietic cells.

Silibinin inhibits osteoclast differentiation mediated by TNF family members

Silibinin is a polyphenolic flavonoid compound isolated from milk thistle (Silybum marianum), with known hepatoprotective, anticarcinogenic, and antioxidant effects. Herein, we show that silibinin inhibits receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis from RAW264.7 cells as well as from bone marrow-derived monocyte/macrophage cells in a dose-dependent manner. Silibinin has no effect on the expression of RANKL or the soluble RANKL decoy receptor osteoprotegerin (OPG) in osteoblasts. However, we demonstrate that silibinin can block the activation of NF-κB, c-Jun N-terminal kinase (JNK), p38 mitogen-activated protein (MAP) kinase, and extracellular signal-regulated kinase (ERK) in osteoclast precursors in response to RANKL. Furthermore, silibinin attenuates the induction of nuclear factor of activated T cells (NFAT) c1 and osteoclast-associated receptor (OSCAR) expression during RANKL-induced osteoclastogenesis. We demonstrate that silibinin can inhibit TNF-α-induced osteoclastogenesis as well as the expression of NFATc1 and OSCAR. Taken together, our results indicate that silibinin has the potential to inhibit osteoclast formation by attenuating the downstream signaling cascades associated with RANKL and TNF-α.

The mechanism of osteoclast differentiation induced by IL-1

IL-1 is a potent cytokine that can induce bone erosion in inflammatory sites such as rheumatoid joint regions via activation of osteoclasts. Not only is IL-1 capable of activating osteoclasts, but it is also a key cytokine involved in the differentiation, multinucleation, and survival of osteoclasts. Herein, we show that IL-1 has the potential to drive osteoclast differentiation via a receptor activator of NF-kappaB ligand (RANKL)/RANK-independent mechanism. Although IL-1 has a synergistic effect on RANKL-induced osteoclast formation, IL-1 alone cannot induce osteoclast differentiation from osteoclast precursors (bone marrow-derived macrophages (BMMs)) due to a lack of IL-1 signaling potential in these cells. However, we demonstrate that overexpression of the IL-1RI receptor in BMMs or induction of IL-1RI by c-Fos overexpression enables IL-1 alone to induce the formation of authentic osteoclasts by a RANKL/RANK-independent mechanism. The expression of IL-1RI is up-regulated by RANKL via c-Fos and NFATc1. Furthermore, the addition of IL-1 to IL-1RI overexpressing BMMs (IL-1/IL-1RI) strongly activates NF-kappaB, JNK, p38, and ERK which is a hallmark gene activation profile of osteoclastogenesis. Interestingly, IL-1/IL-1RI does not induce expression of c-Fos or NFATc1 during osteoclast differentiation, although basal levels of c-Fos and NFATc1 seem to be required. Rather, IL-1/IL-1RI strongly activates MITF, which subsequently induces osteoclast-specific genes such as osteoclast-associated receptor and tartrate-resistant acid phosphatase. Together, these results reveal that IL-1 has the potential to induce osteoclast differentiation via activation of microphthalmia transcription factor under specific microenvironmental conditions.

Cell-nonautonomous function of Id1 in the hematopoietic progenitor cell niche

Development of hematopoietic stem cells (HSCs) and their immediate progeny is maintained by the interaction with cells in the microenvironment. We found that hematopoiesis was dysregulated in Id1(-/-) mice. Although the frequency of HSCs in Id1(-/-) bone marrow was increased, their total numbers remained unchanged as the result of decreased bone marrow cellularity. In addition, the ability of Id1(-/-) HSCs to self-renew was normal, suggesting Id1 does not affect HSC function. Id1(-/-) progenitors showed increased cycling in vivo but not in vitro, suggesting cell nonautonomous mechanisms for the increased cycling. Id1(-/-) HSCs developed normally when transplanted into Id1(+/+) mice, whereas the development of Id1(+/+) HSCs was impaired in Id1(-/-) recipients undergoing transplantation and reproduced the hematologic features of Id1(-/-) mice, indicating that the Id1(-/-) microenvironment cannot support normal hematopoietic development. Id1(-/-) stromal cells showed altered production of cytokines in vitro, and cytokine levels were deregulated in vivo, which could account for the Id1(-/-) hematopoietic phenotypes. Thus, Id1 is required for regulating the hematopoietic progenitor cell niche but is dispensable for maintaining HSCs.

Identification of LRRc17 as a negative regulator of receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast differentiation

Osteoblasts are the primary cells responsible for bone formation. They also support osteoclast formation from bone marrow precursors in response to osteotropic factors by inducing receptor activator of NF-kappaB ligand (RANKL) expression and down-regulating osteoprotegerin (OPG) production. In addition to the RANKL-RANK-OPG signaling axis, other factors produced by osteoblasts/stromal cells are involved in osteoclastogenesis. Here, we describe the identification and characterization of leucine-rich repeat-containing 17 (LRRc17), a member of the LRR superfamily that acts as a negative regulator of RANKL-induced osteoclast differentiation. Osteoblasts showed high levels of LRRc17 expression, which was down-regulated in response to the pro-osteoclastogenic factor 1,25-dihydroxyvitamin D(3). Recombinant LRRc17 protein inhibited RANKL-induced osteoclast differentiation from bone marrow precursors, whereas it did not affect the differentiation or activation of macrophages and dendritic cells. These results suggest that among the cell types derived from common myeloid precursors, LRRc17 specifically regulates osteoclasts. Further analysis revealed that LRRc17 attenuated RANKL-induced expression of NFATc1 by blocking phospholipase C-gamma signaling, which, in turn, inhibited RANKL-mediated osteoclast differentiation. Taken together, our results demonstrated a novel inhibitory activity of LRRc17 in RANKL-induced osteoclastogenesis.

Induction of osteoclast-associated receptor, a key osteoclast costimulation molecule, in rheumatoid arthritis

OBJECTIVE

Osteoclast-associated receptor (OSCAR) is a newly identified osteoclast-specific receptor and is of key importance in the process of osteoclast costimulation. This study was undertaken to define the role of costimulation in osteoclast differentiation during inflammatory arthritis.

METHODS

OSCAR expression was assessed in the synovium and peripheral blood monocytes of patients with rheumatoid arthritis (RA), and associations with disease activity were assessed. Serum levels of OSCAR were determined by enzyme-linked immunosorbent assay. In vitro osteoclast assays were performed to characterize the role of OSCAR in human osteoclastogenesis. Cytokine regulation of OSCAR was investigated by reverse transcriptase-polymerase chain reaction.

RESULTS

OSCAR was expressed by osteoclasts at the erosion front and by mononuclear cells around synovial microvessels. Flow cytometry revealed enhanced expression of OSCAR in peripheral blood monocytes of RA patients as compared with healthy controls. OSCAR expression was correlated with disease activity and acute-phase reactant concentrations. Serum levels of soluble OSCAR were lower in RA patients than in healthy controls. Monocytes with high OSCAR expression exhibited an enhanced potential to differentiate into osteoclasts. Tumor necrosis factor alpha was identified as the main inducer of OSCAR expression in monocytes.

CONCLUSION

These data suggest that the osteoclast costimulation pathway is activated in RA. OSCAR is induced in monocytes of RA patients, facilitating their differentiation into osteoclasts and bone resorption.

NFATc1 induces osteoclast fusion via up-regulation of Atp6v0d2 and the dendritic cell-specific transmembrane protein (DC-STAMP)

NFATc1 has been characterized as a master regulator of nuclear factor kappaB ligand-induced osteoclast differentiation. Herein, we demonstrate a novel role for NFATc1 as a positive regulator of nuclear factor kappaB ligand-mediated osteoclast fusion as well as other fusion-inducing factors such as TNF-alpha. Exogenous overexpression of a constitutively active form of NFATc1 in bone marrow-derived monocyte/macrophage cells (BMMs) induces formation of multinucleated osteoclasts as well as the expression of fusion-mediating molecules such as the d2 isoform of vacuolar ATPase V(o) domain (Atp6v0d2) and the dendritic cell-specific transmembrane protein (DC-STAMP). Moreover, inactivation of NFATc1 by cyclosporin A treatment attenuates expression of Atp6v0d2 and DC-STAMP and subsequent fusion process of osteoclasts. We show that NFATc1 binds to the promoter regions of Atp6v0d2 and DC-STAMP in osteoclasts and directly induces their expression. Furthermore, overexpression of Atp6v0d2 and DC-STAMP rescues cell-cell fusion of preosteoclasts despite reduced NFATc1 activity. Our data indicate for the first time that the NFATc1/Atp6v0d2 and DC-STAMP signaling axis plays a key role in the osteoclast multinucleation process, which is essential for efficient bone resorption.

Transcription factors in the control of dendritic cell life cycle

Dendritic cells (DCs) are potent antigen-presenting cells that guard all parts of the body. They have the unique ability to prime T cells and generate primary immune responses. Their journey beginning with the development from precursor cells and ending with their death is controlled by a group of transcription factors. Some of the transcription factors like PU.1 are involved in more than one stage of DC life. Other transcription factors including Ikaros and JAK3 are involved in the development of more than one cell type. For a long time, the cellular and molecular mechanisms underlying the development, differentiation, maturation, and other stages of DC life were not well understood. However, in recent years novel information has been published by many researchers to better understand the molecular mechanisms of the development and function of DCs in immunological diseases such as asthma, cancer, autoimmunity, and transplantation. This review will discuss the various transcription factors and signaling pathways involved in each stage of the life cycle of DCs.

Interleukin-1 beta-induced Id2 gene expression is mediated by Egr-1 in vascular smooth muscle cells

OBJECTIVE

Id2 (inhibitor of DNA-binding 2), a member of the helix-loop-helix family of transcription regulators, plays important roles in cell proliferation and differentiation. Recent reports have documented that Id2 is up-regulated during vascular lesion formation and overexpression of Id2 promotes vascular smooth muscle cell (VSMC) proliferation. However, the transcriptional regulation of Id2 gene expression in VSMC remains unexplored.

METHODS AND RESULTS

Using Northern- and Western-blot analyses, we documented that interleukin-1beta (IL-1beta) induced Id2 gene expression in VSMC in a time- and dose-dependent manner. Overexpression of early growth response-1 (Egr-1) in VSMC induced Id2 expression while IL-1beta-induced Id2 expression was abrogated in VSMC by the Egr-1 repressor, NGFI-A binding protein 2 (NAB2), expressed from an adenovirus. Overexpression of Egr-1 transactivated the Id2 promoter in reporter assays dependent on the presence of intact putative Egr-1 binding sites as determined by mutagenesis. Finally, electrophoretic mobility shift assays (EMSA) demonstrated that the Egr-1 protein can bind the Egr-1 sites derived from the human Id2 promoter in vitro and chromatin immunoprecipitation identified the putative Egr-1 site between -723 to -712 as the functional Egr-1 binding site in vivo.

CONCLUSIONS

Our data demonstrate that IL-1beta-induced Id2 expression in VSMC is mediated by the transcription factor Egr-1 in VSMC.

Protein inhibitor of activated STAT 3 modulates osteoclastogenesis by down-regulation of NFATc1 and osteoclast-associated receptor

Protein inhibitor of activated STAT3 (PIAS3) has been shown to regulate the activity of various transcription factors. In this study, we show that the overexpression of PIAS3 in bone marrow-derived monocyte/macrophage lineage cells attenuates osteoclast formation and down-regulates the expression of NFATc1 and osteoclast-associated receptor (OSCAR), which are important modulators in osteoclastogenesis. PIAS3 has been shown to associate with histone deacetylase 1 as well as with transcription factors, including the microphthalmia transcription factor, NFATc1, and c-Fos. Moreover, overexpression of PIAS3 inhibits the transactivation of target genes such as NFATc1 and OSCAR. This inhibitory effect of PIAS3 is possibly mediated by histone deacetylase 1 recruitment to the promoter regions of NFATc1 and OSCAR. Furthermore, silencing of PIAS3 by RNA interference in osteoclast precursors enhances osteoclast formation as well as gene expression of NFATc1 and OSCAR. Taken together, our results reveal that PIAS3 acts as a modulator in osteoclastogenesis.

Hypoxic stress underlies defects in erythroblast islands in the Rb-null mouse

Definitive erythropoiesis occurs in islands composed of a central macrophage in contact with differentiating erythroblasts. Erythroid maturation including enucleation can also occur in the absence of macrophages both in vivo and in vitro. We reported previously that loss of Rb induces cell-autonomous defects in red cell maturation under stress conditions, while other reports have suggested that the failure of Rb-null erythroblasts to enucleate is due to defects in associated macrophages. Here we show that erythropoietic islands are disrupted by hypoxic stress, such as occurs in the Rb-null fetal liver, that Rb(-/-) macrophages are competent for erythropoietic island formation in the absence of exogenous stress and that enucleation defects persist in Rb-null erythroblasts irrespective of macrophage function.

Osteoimmunology: shared mechanisms and crosstalk between the immune and bone systems

Osteoimmunology is an interdisciplinary research field focused on the molecular understanding of the interplay between the immune and skeletal systems. Although osteoimmunology started with the study of the immune regulation of osteoclasts, its scope has been extended to encompass a wide range of molecular and cellular interactions, including those between osteoblasts and osteoclasts, lymphocytes and osteoclasts, and osteoblasts and haematopoietic cells. Therefore, the two systems should be understood to be integrated and operating in the context of the 'osteoimmune' system, a heuristic concept that provides not only a framework for obtaining new insights by basic research, but also a scientific basis for the discovery of novel treatments for diseases related to both systems.

MafB negatively regulates RANKL-mediated osteoclast differentiation

Receptor activator of nuclear factor kappaB ligand (RANKL) induces osteoclast formation from hematopoietic cells via regulation of various transcription factors. Here, we show that MafB negatively regulates RANKL-induced osteoclast differentiation. Expression levels of MafB are significantly reduced by RANKL during osteoclastogenesis. Overexpression of MafB in bone marrow-derived monocyte/macrophage lineage cells (BMMs) inhibits the formation of TRAP(+) multinuclear osteoclasts, but phagocytic activity of BMMs is retained. Furthermore, overexpression of MafB in BMMs attenuates the gene induction of NFATc1 and osteoclast-associated receptor (OSCAR) during RANKL-mediated osteoclastogenesis. In addition, MafB proteins interfere with the DNA-binding ability of c-Fos, Mitf, and NFATc1, inhibiting their transactivation of NFATc1 and OSCAR. Furthermore, reduced expression of MafB by RNAi enhances osteoclastogenesis and increases expression of NFATc1 and OSCAR. Taken together, our results suggest that MafB can act as an important modulator of RANKL-mediated osteoclastogenesis.