Downregulated fat mass and obesity-associated protein inhibits bone resorption and osteoclastogenesis by nuclear factor-kappa B inactivation

During osteoporosis, fat mass and obesity-associated protein (FTO) promotes the shift of bone marrow mesenchymal stem cells to adipocytes and represses osteoblast activity. However, the role and mechanisms of FTO on osteoclast formation and bone resorption remain unknown. In this study, we investigated the effect of FTO on RAW264.7 cells and bone marrow monocytes (BMMs)-derived osteoclasts in vitro and observed the influence of FTO on ovariectomized (OVX) mice model to mimic postmenopausal osteoporosis in vivo. Results found that FTO was up-regulated in BMMs from OVX mice. Double immunofluorescence assay showed co-localization of FTO with tartrate-resistant acid phosphatase (TRAP) in femurs of OVX mice. FTO overexpression enhanced TRAP-positive osteoclasts and F-actin ring formation in RAW264.7 cells upon RANKL stimulation. The expression of osteoclast differentiation-related genes, including nuclear factor of activated T cells c1 (NFATc1) and c-FOS, was upregulated in BMMs and RAW264.7 cells after FTO overexpression. FTO overexpression induced the phosphorylation and nuclear translocation of factor-kappa B (NF-κB) p65 in BMMs and RAW264.7 cells exposed to RANKL. ChIP and dual-luciferase assays revealed that FTO overexpression contributed to RANKL-induced binding of NF-κB to NFATc1 promoter. Rescue experiments suggested that FTO overexpression-mediated osteoclast differentiation was suppressed after intervention with a NF-κB inhibitor pyrrolidine dithiocarbamate. Further in vivo evidence revealed that FTO knockdown increased bone trabecula and bone mineral density, inhibited bone resorption and osteoclastogenesis in osteoporotic mice. Collectively, our research demonstrates that downregulated FTO inhibits bone resorption and osteoclastogenesis through NF-κB inactivation, which provides a novel reference for osteoporosis treatment.     

Osteoporosis regulation by salubrinal through eIF2α mediated differentiation of osteoclast and osteoblast

Nuclear factor-κB (NF-κB) ligand (RANKL) was shown to induce osteoclast differentiation by increasing the expression of c-Fos, NFATc1 and TRAP. Salubrinal treatment to bone marrow macrophage (BMM) cells, however, significantly blocked NFATc1 expression and osteoclast differentiation by RANKL. Overexpression of NFATc1 further confirmed that NFATc1 is a key factor affected by salubrinal in osteoclast differentiation by RANKL. Unexpectedly, NFATc1 and c-Fos mRNA expressions were not affected by salubrinal, implicating that NFATc1 expression is regulated at a translational stage. In support of this, salubrinal increased the phosphorylation of a translation factor eIF2α, decreasing the global protein synthesis including NFATc1. In contrast, a phosphorylation mutant plasmid pLenti-eIF2α-S51A restored RANKL-induced NFATc1 expression and osteoclast differentiation even in the presence of salubrinal. Furthermore, knockdown of ATF4 significantly reduced salubrinal-induced osteoblast differentiation as evidenced by decreased calcium accumulation and lowered expressions of the osteoblast differentiation markers, alkaline phosphatase and RANKL in MC3T3-E1 osteoblast cells. Salubrinal treatment to co-cultured BMM and MC3T3-E1 cells also showed reduction of osteoclast differentiation. Finally, salubrinal efficiently blocked osteoporosis in mice model treated with RANKL as evidenced by elevated bone mineral density (BMD) and other osteoporosis factors. Collectively, our data indicate that salubrinal could affect the differentiation of both osteoblast and osteoclast, and be developed as an excellent anti-osteoporosis drug. In addition, modulation of ATF4 and NFATc1 expressions through eIF2α phosphorylation could be a valuable target for the treatment of osteoporosis.     

绝经后骨质疏松症TNF-α通过激活NF-κB促进RANKL诱导的破骨细胞形成

本研究检测了绝经后骨质疏松症妇女的肿瘤坏死因子-α(TNF-α)和雌激素水平,并探讨了TNF-α对破骨前体细胞RAW264.7中破骨细胞标志物核因子κB受体激活因子(nuclear factor kappa-B, RANK)、组织蛋白酶K (Cathepsin K, CTSK)和凝血酶受体激活肽(thrombin receptor activating peptide, TRAP)以及核因子-κB (NF-κB)亚基(p65)和NF-κB抑制蛋白(IκBα)的影响。研究结果表明,绝经后骨质疏松症患者的TNF-α水平显著升高,而雌二醇水平显著降低。核因子κB受体激活因子配体(receptor activator for NF-κBligand, RANKL)处理1周后,破骨前体细胞RAW264.7中破骨细胞标志物RANK、CTSK和TRAP的mRNA和蛋白高度表达。与RANKL对照组相比,TNF-α处理可上调RANK、CTSK和TRAP m RNA的表达。但是,仅TNF-α不能诱导培养的RAW264.7细胞分化为破骨细胞成。TNF-α以剂量依赖性方式诱导NF-κB亚基p65和IκBα磷酸化,而NF-κB抑制剂处理则有效降低了RANK和TRAP的表达。本研究结论表明,绝经后骨质疏松症中TNF-α通过激活NF-κB来促进RANKL诱导的破骨细胞形成。     

Ellagic acid protects ovariectomy-induced bone loss in mice by inhibiting osteoclast differentiation and bone resorption

Osteoporosis is a devastating disease that features reduced bone quantity and microstructure, which causes fragility fracture and increases mortality, especially in the aged population. Due to the long-term side-effects of current drugs for osteoporosis, it is of importance to find other safe and effective medications. Ellagic acid (EA) is a phenolic compound found in nut galls, plant extracts, and fruits, and exhibits antioxidant and antineoplastic effects. Here, we showed that EA attenuated the formation and function of osteoclast dose-dependently. The underlying mechanism was further discovered by western blot, immunofluorescence assay, and luciferase assay, which elucidated that EA suppressed osteoclastogenesis and bone resorption mainly through attenuating receptor activator of nuclear factor-κB (NF-κB) ligand-induced NF-κB activation and extracellular signal-regulated kinase signaling pathways, accompanied by decreased protein expression of nuclear factor of activated T-cells calcineurin-dependent 1 and c-Fos. Moreover, EA inhibits osteoclast marker genes expression including Dc-stamp, Ctsk, Atp6v0d2, and Acp5. Intriguingly, we also found that EA treatment could significantly protect ovariectomy-induced bone loss in vivo. Conclusively, this study suggested that EA might have the therapeutic potentiality for preventing or treating osteoporosis.     

Hesperetin suppresses RANKL-induced osteoclastogenesis and ameliorates lipopolysaccharide-induced bone loss

Destructive bone diseases caused by osteolysis are increasing in incidence. They are characterized by an excessive imbalance of osteoclast formation and activation. During osteolysis, the activation of nuclear factor-κB (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways are triggered by receptor activator of NF-κB ligand (RANKL), inflammatory factors, and oxidative stress. Previous studies have indicated that the common flavanone glycoside compound hesperetin exhibits anti-inflammatory and antioxidant activity by inhibition of NF-κB and MAPK signaling pathways. However, the direct relationship between hesperetin and osteolysis remain unclear. In the present study, we investigated the effects of hesperetin on lipopolysaccharide (LPS)-induced osteoporosis and elucidated the related mechanisms. Hesperetin effectively suppressed RANKL-induced osteoclastogenesis, osteoclastic bone resorption, and F-actin ring formation in a dose-dependent manner. It also significantly suppressed the expression of osteoclast-specific markers including tartrate-resistant acid phosphatase, matrix metalloproteinase-9, cathepsin K, c-Fos, and nuclear factor of activated T-cells cytoplasmic 1. Furthermore, it inhibited osteoclastogenesis by inhibiting activation of NF-κB and MAPK signaling, scavenging reactive oxygen species, and activating the nuclear factor E2 p45-related factor 2/heme oxygenase 1 (Nrf2/HO-1) signaling pathway. Consistent with in vitro results, hesperetin effectively ameliorated LPS-induced bone loss, reduced osteoclast numbers, and decreased the RANKL/OPG ratio in vivo. As such, our results suggest that hesperetin may be a great candidate for developing a novel drug for destructive bone diseases such as periodontal disease, tumor bone metastasis, rheumatoid arthritis, and osteoporosis.     

Pseudogene PTENP1 sponges miR-214 to regulate the expression of PTEN to modulate osteoclast differentiation and attenuate osteoporosis

Background aimsOsteoporosis (OP) is a common bone metabolic disease with a high incidence. Our study aimed to explore the pseudogene PTENP1/miR-214/PTEN axis to modulate the osteoclast differentiation in osteoporosis.MethodsPatients with osteoporosis were recruited in our study, and RANKL-induced osteoclast differentiation and ovariectomy-induced osteoporosis mouse model were established in vitro and in vivo, respectively.ResultsPseudogene PTENP1 and PTEN were significantly down-regulated and miR-214 was up-regulated in osteoporosis patients. In addition, overexpression of PTENP1 or silence of miR-214 inhibited the expression levels of osteoclast specific markers and osteoclast differentiation induced by RANKL. Overexpression of PTENP1 or silence of miR-214 also inhibited the levels of phosphorylation of PI3K and AKT, p65 nuclear translocation, IκBα degradation and the expression level of NFATc1. AlsoSilence of PTENP1 or overexpression of miR-214 induced the osteoclast differentiation under normal physiological condition. Pseudogene PTENP1 sponged miR-214 to regulate the expression of PTEN.ConclusionsIn an ovariectomy-induced osteoporosis mouse model, obvious pathological changes in bone tissues were found, and bone marrow mononuclear cells in this group were more likely to differentiate into osteoclasts. Therefore, pseudogene PTENP1 sponged miR-214 to regulate the expression of PTEN to inhibit osteoclast differentiation and attenuate osteoporosis by suppressing the PI3K/AKT/NF-κB signaling pathway.     

Jolkinolide B inhibits RANKL-induced osteoclastogenesis by suppressing the activation NF-κB and MAPK signaling pathways

Osteoclasts together with osteoblasts play pivotal roles in bone remodeling. The unique function and ability of osteoclasts to resorb bone makes them critical in both normal bone homeostasis and pathologic bone diseases such as osteoporosis and rheumatoid arthritis. Thus, new compounds that may inhibit osteoclastogenesis and osteoclast function may be of great value in the treatment of osteoclast-related diseases. In the present study, we examined the effect of jolkinolide B (JB), isolated from the root of Euphorbia fischeriana Steud on receptor activator of NF-κB ligand (RANKL)-induced osteoclast formation. We found that JB inhibited RANKL-induced osteoclast differentiation from bone marrow macrophages (BMMs) without cytotoxicity. Furthermore, the expression of osteoclastic marker genes, such as tartrate-resistant acid phosphatase (TRAP), cathepsin K (CtsK), and calcitonin receptor (CTR), was significantly inhibited. JB inhibited RANKL-induced activation of NF-κB by suppressing RANKL-mediated IκBα degradation. Moreover, JB inhibited RANKL-induced phosphorylation of mitogen-activated protein kinases (p38, JNK, and ERK). This study thus identifies JB as an inhibitor of osteoclast formation and provides evidence that JB might be an alternative medicine for preventing and treating osteolysis.     

Cepharanthine suppresses osteoclast formation by modulating the nuclear factor-κB and nuclear factor of activated T-cell signaling pathways

The increased activation of osteoclasts is the major manifestation of several lytic bone diseases, including osteoporosis, rheumatoid arthritis, aseptic loosening of orthopedic implants, Paget disease and malignant bone diseases. One important bone-protective therapy in these diseases focuses on the inhibition of osteoclast differentiation and resorptive function. Given that the deleterious side-effects of currently available drugs, it is beneficial to search for effective and safe medications from natural compounds. Cepharanthine (CEP) is a compound extracted from Stephania japonica and has been found to have antioxidant and anti-inflammatory effects. In this study, we found that CEP inhibited receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclast formation and bone-resorbing activities using osteoclastogenesis and bone resorption assay. By polymerase chain reaction, we also found that CEP inhibited the expression of osteoclast-differentiation marker genes including Ctsk, Calcr, Atp6v0d2, Mmp9 and Nfatc1. Mechanistic analyses including Western blot and luciferase reporter assay revealed that CEP inhibited RANKL-induced activation of NF-κB and nuclear factor of activated T-cell, which are essential for the formation of osteoclast. Collectively, these data suggested that CEP can potentially be used as an alternative therapy for preventing or treating osteolytic diseases.     

A relation between osteoclastogenesis inhibition and membrane-type estrogen receptor GPR30

Disruption of the cooperative balance between osteoblasts and osteoclasts causes various bone disorders, some of which are because of abnormal osteoclast recruitment. Osteoporosis, one of the bone disorders, is not effectively treated by currently available medicines. In addition to the development of novel drugs for palliative treatment, the exploitation of novel compounds for preventive treatment is important in an aging society. Quercetin, a major flavonoid found in many fruits and vegetables, has been expected to inhibit cancer and prevent several diseases because of its anti-inflammatory and estrogenic functions. It has been reported that quercetin has the potential to reduce bone resorption, but the mechanism by which this compound affects the differentiation of osteoclasts remains unknown. Here, using a bone marrow cell-based in vitro osteoclast differentiation system from bone marrow cells, we found that the ability of quercetin to inhibit osteoclastogenesis was related to its estrogenic activity. The inhibition was partially blocked by a specific antagonist for the nuclear receptor estrogen receptor α, but a specific antagonist of the membrane-type receptor GPR30 completely ablated this inhibition. Furthermore, quercetin suppressed the transient increase of Akt phosphorylation induced by the stimulation of macrophage colony-stimulating factor and receptor activator of NF-κB ligand with no effect on MAPK phosphorylation, suggesting exquisite crosstalk between cytokine receptor and G-protein coupled receptor signaling. These results indicate the important role of GPR30 in osteoclast differentiation and provide new insights to the development of new treatments for osteoporosis.     

TRAF Family Member-associated NF-κB Activator (TANK) Is a Negative Regulator of Osteoclastogenesis and Bone Formation

The differentiation of bone-resorbing osteoclasts is induced by RANKL signaling, and leads to the activation of NF-κB via TRAF6 activation. TRAF family member-associated NF-κB activator (TANK) acts as a negative regulator of Toll-like receptors (TLRs) and B-cell receptor (BCR) signaling by inhibiting TRAF6 activation. Tank(-/-) mice spontaneously develop autoimmune glomerular nephritis in an IL-6-dependent manner. Despite its importance in the TCRs and BCR-activated TRAF6 inhibition, the involvement of TANK in RANKL signaling is poorly understood. Here, we report that TANK is a negative regulator of osteoclast differentiation. The expression levels of TANK mRNA and protein were up-regulated during RANKL-induced osteoclastogenesis, and overexpression of TANK in vitro led to a decrease in osteoclast formation. The in vitro osteoclastogenesis of Tank(-/-) cells was significantly increased, accompanied by increased ubiquitination of TRAF6 and enhanced canonical NF-κB activation in response to RANKL stimulation. Tank(-/-) mice showed severe trabecular bone loss, but increased cortical bone mineral density, because of enhanced bone erosion and formation. TANK mRNA expression was induced during osteoblast differentiation and Tank(-/-) osteoblasts exhibited enhaced NF-κB activation, IL-11 expression, and bone nodule formation than wild-type control cells. Finally, wild-type mice transplanted with bone marrow cells from Tank(-/-) mice showed trabecular bone loss analogous to that in Tank(-/-) mice. These findings demonstrate that TANK is critical for osteoclastogenesis by regulating NF-κB, and is also important for proper bone remodeling.     

NF-κB signaling participates in both RANKL- and IL-4-induced macrophage fusion: receptor cross-talk leads to alterations in NF-κB pathways

NF-κB activation is essential for receptor activator for NF-κB ligand (RANKL)-induced osteoclast formation. IL-4 is known to inhibit the RANKL-induced osteoclast differentiation while at the same time promoting macrophage fusion to form multinucleated giant cells (MNG). Several groups have proposed that IL-4 inhibition of osteoclastogenesis is mediated by suppressing the RANKL-induced activation of NF-κB. However, we found that IL-4 did not block proximal, canonical NF-κB signaling. Instead, we found that IL-4 inhibited alternative NF-κB signaling and induced p105/50 expression. Interestingly, in nfκb1(-/-) bone marrow-derived macrophages (BMM), the formation of both multinucleated osteoclast and MNG induced by RANKL or IL-4, respectively, was impaired. This suggests that NF-κB signaling also plays an important role in IL-4-induced macrophage fusion. Indeed, we found that the RANKL-induced and IL-4-induced macrophage fusion were both inhibited by the NF-κB inhibitors IκB kinase 2 inhibitor and NF-κB essential modulator inhibitory peptide. Furthermore, overexpression of p50, p65, p52, and RelB individually in nfκb1(-/-) or nfκb1(+/+) BMM enhanced both giant osteoclast and MNG formation. Interestingly, knockdown of nfκb2 in wild-type BMM dramatically enhanced both osteoclast and MNG formation. In addition, both RANKL- and IL-4-induced macrophage fusion were impaired in NF-κB-inducing kinase(-/-) BMM. These results suggest IL-4 influences NF-κB pathways by increasing p105/p50 and suppressing RANKL-induced p52 translocation and that NF-κB pathways participate in both RANKL- and IL-4-induced giant cell formation.     

Naringin abrogates osteoclastogenesis and bone resorption via the inhibition of RANKL-induced NF-κB and ERK activation

Osteolytic bone diseases including osteoporosis are commonly accompanied with enhanced osteoclast formation and bone resorption. Naringin, a natural occurring flavonoid has been found to protect against retinoic acid-induced osteoporosis and improve bone quality in rats. Here, we showed that naringin perturbs osteoclast formation and bone resorption by inhibiting RANK-mediated NF-κB and ERK signaling. Naringin suppressed gene expression of key osteoclast marker genes. Naringin was found to inhibit RANKL-induced activation of NF-κB by suppressing RANKL-mediated IκB-α degradation. In addition, naringin inhibited RANKL-induced phosphorylation of ERK. This study identifies naringin as an inhibitor for osteoclast formation and bone resorption, and provides evidence that natural compounds such as naringin might be beneficial as an alternative medicine for the prevention and treatment of osteolysis.     

A novel PPARγ agonist,KR62776, suppresses RANKL-induced osteoclast differentiation and activity by inhibiting MAP kinase pathways

We investigated the effects of a novel peroxisome proliferator-activated receptor γ (PPARγ) agonist, KR62776, on osteoclast differentiation and function, and on the underlying signaling pathways. KR62776 markedly suppressed differentiation into osteoclasts in various osteoclast model systems, including bone marrow mononuclear (BMM) cells and a co-culture of calvarial osteoblasts and BMM cells. KR62776 suppressed the activation of tartrate-resistant acid phosphatase (TRAP) and the expression of genes associated with osteoclast differentiation, such as TRAP, dendritic cell-specific transmembrane protein (DC-STAMP), and osteoclast-associated receptor (OSCAR). Furthermore, KR62776 reduced resorption pit formation in osteoclasts, and down-regulated genes essential for osteoclast activity, such as Src and αvβ3 integrin. An analysis of a signaling pathway showed that KR62776 inhibited the receptor activator of nuclear factor-κB ligand (RANKL)-induced activation of p38 mitogen-activated protein kinase (p38MAPK), extracellular regulated kinase (ERK), c-Jun N-terminal kinase (JNK) and nuclear factor-κB (NF-κB). Together, these results demonstrate that KR62776 negatively affects osteoclast differentiation and activity by inhibiting the RANKL-induced activation of MAP kinases and NF-κB.     

Ly49Q, an ITIM-bearing NK receptor, positively regulates osteoclast differentiation

Osteoclasts, multinucleated cells that resorb bone, play a key role in bone remodeling. Although immunoreceptor tyrosine-based activation motif (ITAM)-mediated signaling is critical for osteoclast differentiation, the significance of immunoreceptor tyrosine-based inhibitory motif (ITIM) has not been well understood. Here we report the function of Ly49Q, an Ly49 family member possessing an ITIM motif, in osteoclastogenesis. Ly49Q is selectively induced by receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL) stimulation in bone marrow-derived monocyte/macrophage precursor cells (BMMs) among the Ly49 family of NK receptors. The knockdown of Ly49Q resulted in a significant reduction in the RANKL-induced formation of tartrate-resistance acid phosphatase (TRAP)-positive multinucleated cells, accompanied by a decreased expression of osteoclast-specific genes such as Nfatc1, Tm7sf4, Oscar, Ctsk, and Acp5. Osteoclastogenesis was also significantly impaired in Ly49Q-deficient cells in vitro. The inhibitory effect of Ly49Q-deficiency may be explained by the finding that Ly49Q competed for the association of Src-homology domain-2 phosphatase-1 (SHP-1) with paired immunoglobulin-like receptor-B (PIR-B), an ITIM-bearing receptor which negatively regulates osteoclast differentiation. Unexpectedly, Ly49Q deficiency did not lead to impaired osteoclast formation in vivo, suggesting the existence of a compensatory mechanism. This study provides an example in which an ITIM-bearing receptor functions as a positive regulator of osteoclast differentiation.     

TLR2-dependent modulation of osteoclastogenesis by Porphyromonas gingivalis through differential induction of NFATc1 and NF-kappaB

Osteolytic diseases, including rheumatoid arthritis, osteomyelitis, and periodontitis, are usually associated with bacterial infections. However, the precise mechanisms by which bacteria induce bone loss still remain unclear. Evidence exists that Toll-like receptor (TLR) signaling regulates both inflammation and bone metabolism and that the receptor activator of NF-κB ligand (RANKL) and its receptor RANK are the key regulators for bone remodeling and for the activation of osteoclasts. Here, we investigate the direct effects of the periodontal pathogen Porphyromonas gingivalis on osteoclast differentiation and show that P. gingivalis differentially modulates RANKL-induced osteoclast formation contingent on the state of differentiation of osteoclast precursors. In addition, although an optimal induction of cytokines by P. gingivalis is dependent on TLR2 and TLR4, as well as myeloid differentiation factor 88 and Toll/IL-1R domain-containing adaptor-inducing IFN-β, P. gingivalis utilizes TLR2/ myeloid differentiation factor 88 in modulating osteoclast differentiation. P. gingivalis modulates RANKL-induced osteoclast formation by differential induction of NFATc1 and c-Fos. More importantly, RANKL-mediated lineage commitment also has an impact on P. gingivalis-induced cytokine production. RANKL inhibits P. gingivalis-induced cytokine production by down-regulation of TLR/NF-κB and up-regulation of NFATc1. Our findings reveal novel aspects of the interactions between TLR and RANK signaling and provide a new model for understanding the mechanism underlying the pathogenesis of bacteria-mediated bone loss.