Saurolactam inhibits osteoclast differentiation and stimulates apoptosis of mature osteoclasts

Cinacalcet Improves Bone Parameters Through Regulation of Osteoclast Endoplasmic Reticulum Stress, Autophagy, and Apoptotic Pathways in Chronic Kidney Disease-Mineral and Bone Disorder

The possible mechanisms underlying the quantitative and qualitative effects of cinacalcet on bone were explored in a chronic kidney disease-mineral and bone disorder (CKD-MBD) mouse model in relation to the influence of the interactions among the osteoclast (OC) endoplasmic reticulum (ER) stress, autophagy and apoptosis pathways on OC differentiation. Body weight and biochemical parameters improved significantly in the CKD + cinacalcet groups compared to the CKD group. Micro-computed tomography (μCT) revealed both cortical and trabecular parameters deteriorated significantly in the CKD group and were reversed by cinacalcet in a dose-dependent manner. Nanoindentation analysis of bone quality proved that both cortical hardness and elastic modulus improved significantly with high dose cinacalcet treatment. In vitro studies revealed that cinacalcet inhibited receptor activator of NF-κB ligand (RANKL)/receptor activator of NF-κB (RANK)-induced OC differentiation in a concentration-dependent manner through a close interaction between activation of caspase-related apoptosis, reversal of OC autophagy through the protein kinase B (Akt)/mammalian target of rapamycin (mTOR) and adenosine monophosphate-activated protein kinase (AMPK) pathways, and attenuation of the OC ER stress/CREBH/NFATc1 signaling pathway. Cinacalcet improves both bone quantity and bone quality in CKD mouse model and inhibits OC differentiation through regulation of the interactions among the apoptosis, ER stress, and autophagy pathways within OCs. © 2021 American Society for Bone and Mineral Research (ASBMR).

N-[2-(4-benzoyl-1-piperazinyl)phenyl]-2-(4-chlorophenoxy) acetamide is a novel inhibitor of resorptive bone loss in mice

The dynamic balance between bone formation and bone resorption is vital for the retention of bone mass. The abnormal activation of osteoclasts, unique cells that degrade the bone matrix, may result in many bone diseases such as osteoporosis. Osteoporosis, a bone metabolism disease, occurs when extreme osteoclast‐mediated bone resorption outstrips osteoblast‐related bone synthesis. Therefore, it is of great interest to identify agents that can regulate the activity of osteoclasts and prevent bone loss‐induced bone diseases. In this study, we found that N‐[2‐(4‐benzoyl‐1‐piperazinyl)phenyl]‐2‐(4‐chlorophenoxy) acetamide (PPOAC‐Bz) exerted a strong inhibitory effect on osteoclastogenesis. PPOAC‐Bz altered the mRNA expressions of several osteoclast‐specific marker genes and blocked the formation of mature osteoclasts, suppressing F‐actin belt formation and bone resorption activity in vitro. In addition, PPOAC‐Bz prevented OVX‐induced bone loss in vivo. These findings highlighted the potential of PPOAC‐Bz as a prospective drug for the treatment of osteolytic disorders.

N-[2-(4-Acetyl-1-Piperazinyl)Phenyl]-2-(3-Methylphenoxy)Acetamide (NAPMA) Inhibits Osteoclast Differentiation and Protects against Ovariectomy-Induced Osteoporosis

Osteoclasts are large, multinucleated cells responsible for bone resorption and are induced in response to the regulatory activity of receptor activator of nuclear factor-kappa B ligand (RANKL). Excessive osteoclast activity causes pathological bone loss and destruction. Many studies have investigated molecules that specifically inhibit osteoclast activity by blocking RANKL signaling or bone resorption. In recent years, we screened compounds from commercial libraries to identify molecules capable of inhibiting RANKL-induced osteoclast differentiation. Consequently, we reported some compounds that are effective at attenuating osteoclast activity. In this study, we found that N-[2-(4-acetyl-1-piperazinyl)phenyl]-2-(3-methylphenoxy)acetamide (NAPMA) significantly inhibited the formation of multinucleated tartrate-resistant acid phosphatase (TRAP)-positive cells from bone marrow-derived macrophages in a dose-dependent manner, without cytotoxic effects. NAPMA downregulated the expression of osteoclast-specific markers, such as c-Fos, NFATc1, DC-STAMP, cathepsin K, and MMP-9, at the transcript and protein levels. Accordingly, bone resorption and actin ring formation were decreased in response to NAPMA treatment. Furthermore, we demonstrated the protective effect of NAPMA against ovariectomy-induced bone loss using micro-CT and histological analysis. Collectively, the results showed that NAPMA inhibited osteoclast differentiation and attenuated bone resorption. It is thus a potential drug candidate for the treatment of osteoporosis and other bone diseases associated with excessive bone resorption.

Polyphyllin VII attenuated RANKL-induced osteoclast differentiation via inhibiting of TRAF6/c-Src/PI3K pathway and ROS production

Background

Osteoporosis is a worldwide severe bone disease. This study aimed to evaluate the effect of polyphyllin VII on the genesis of osteoclasts from bone marrow macrophages (BMMs) and its potentiality as a therapeutic drug for osteoporosis.

Methods

BMMs were induced to differentiate into osteoclasts by RANKL and M-CSF. The cells were then treated with various concentrations of polyphyllin VII. Intracellular reactive oxygen species (ROS) measurement assay, resorption pit formation assay, tartrate-resistant acid phosphatase (TRAP) staining and TRAP activity assessment, cell viability assay, active GTPase pull-down assay, immunofluorescent staining, immunoblotting, and RT-PCR were performed.

Results

RANKL + M-CSF significantly increased TRAP activity, number of osteoclasts, number and area of lacunae, intracellular content of ROS, protein levels of Nox1, TRAF6, c-Src and p-PI3K, as well as the content of activated GTP-Rac1, which were significantly blocked by polyphyllin VII in a concentration-dependent manner.

Conclusion

These findings suggested that polyphyllin VII inhibited differentiation of BMMs into osteoclasts through suppressing ROS synthesis, which was modulated by TRAF6–cSrc–PI3k signal transduction pathway including GTP-Rac1 and Nox1. Polyphyllin VII could be a therapeutic drug for osteoporosis.

MicroRNA‑20a negatively regulates the growth and osteoclastogenesis of THP‑1 cells by downregulating PPARγ

The present study aimed to explore the mechanisms through which microRNA (miR)‑20a may be involved in the differentiation of THP‑1 human acute monocytic leukemia cells into osteoclasts. THP‑1 cells were differentiated into macrophages (osteoclast precursors) and subsequently into osteoclast cells. The expression levels of miR‑20a in THP‑1 cells were significantly reduced in a time‑dependent manner during phorbol‑12‑myristate‑13‑acetate (PMA), macrophage colony‑stimulating factor (M‑CSF) and receptor activator of nuclear factor‑κB ligand RANKL‑induced osteoclastogenesis. Following transfection with a miR‑20a mimics, the levels of miR‑20a in PMA‑treated THP‑1 cells increased more than 40‑fold as compared with expression in the control cells. In addition, the overexpression of miR‑20a inhibited proliferation, initiated S phase cell cycle arrest and induced apoptosis of PMA‑treated THP‑1 cells. Additionally, miR‑20a mimics treatment notably decreased the levels of tartrate‑resistant acid phosphatase, nuclear factor of activated T‑cells, cytoplasmic 1 and peroxisome proliferator‑activated receptor γ (PPARγ) during THP‑1 cell further differentiation progress. In summary, miR‑20a may negatively regulate the proliferation and osteoclastogenesis of THP‑1 cells during its osteoclast differentiation progress by downregulating PPARγ.

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.

Inhibitory Effects of 2N1HIA (2-(3-(2-Fluoro-4-Methoxyphenyl)-6-Oxo-1(6H)-Pyridazinyl)-N-1H-Indol-5-Ylacetamide) on Osteoclast Differentiation via Suppressing Cathepsin K Expression

Osteoclasts are large multinucleated cells which are induced by the regulation of the receptor activator of nuclear factor kappa-Β ligand (RANKL), which is important in bone resorption. Excessive osteoclast differentiation can cause pathologic bone loss and destruction. Numerous studies have targeted molecules inhibiting RANKL signaling or bone resorption activity. In this study, 11 compounds from commercial libraries were examined for their effect on RANKL-induced osteoclast differentiation. Of these compounds, only 2-(3-(2-fluoro-4-methoxyphenyl)-6-oxo-1(6H)-pyridazinyl)-N-1H-indol-5-ylacetamide (2N1HIA) caused a significant decrease in multinucleated tartrate-resistant acid phosphatase (TRAP)-positive cell formation in a dose-dependent manner, without inducing cytotoxicity. The 2N1HIA compound neither affected the expression of osteoclast-specific gene markers such as TRAF6, NFATc1, RANK, OC-STAMP, and DC-STAMP, nor the RANKL signaling pathways, including p38, ERK, JNK, and NF-κB. However, 2N1HIA exhibited a significant impact on the expression levels of CD47 and cathepsin K, the early fusion marker and critical protease for bone resorption, respectively. The activity of matrix metalloprotease-9 (MMP-9) decreased due to 2N1HIA treatment. Accordingly, bone resorption activity and actin ring formation decreased in the presence of 2N1HIA. Taken together, 2N1HIA acts as an inhibitor of osteoclast differentiation by attenuating bone resorption activity and may serve as a potential candidate in preventing and/or treating osteoporosis, or other bone diseases associated with excessive bone resorption.

A paeonol derivative, YPH-PA3 promotes the differentiation of monocyte/macrophage lineage precursor cells into osteoblasts and enhances their autophagy

Previous studies have indicated that paeonol inhibits RANKL-induced osteoclastogenesis by inhibiting the ERK, p38, and NF-κB pathway. We modified paeonol to form a new compound, YPH-PA3, and found that it promoted osteoclastogenesis rather than inhibited it the way paeonol does. The aim of this study is to investigate the mechanisms involved in YPH-PA3-promoted osteoclastogenesis. YPH-PA3-promoted differentiation of RAW264.7 cells (human monocytes) into osteoclasts is activated through ERK/p38/JNK phosphorylation, affecting c-FOS, NF-κB, and NFATc2. Real-time quantitative PCR and western blot revealed an increased expression of autophagy-related markers during YPH-PA3-induced osteoclastogenesis. We also demonstrated the relationship between p62/LC3 localization and F-actin ring formation by double-labeling immunofluorescence. Knockdown of p62 small-interfering RNA (siRNA) attenuated YPH-PA3-induced expression of autophagy-related genes. Our study results indicated that p62 may play a role in YPH-PA3-induced autophagy and osteoclastogenesis, which may help to develop a novel therapeutic strategy against osteoclastogenesis-related diseases.

Death effects of reveromycin A in normal and disease-associated cells of the joint

Earlier work in our laboratory demonstrated that naturally occurring reveromycin A (Rev A) causes apoptosis in osteoclasts without accompanying necrosis. Rev A death effects in both normal and diseased joint cells were investigated in this study. A dose of 10 μM Rev A did not cause apoptosis nor necrosis in monolayer chondrocytes, even at pH 6.8, a pH mimicking that of an inflamed joint. In contrast, at the acidic pH Rev A did induce significant apoptosis (fourfold increase at 48 h of treatment, P < 0.005) in normal synoviocytes without accompanying necrosis. Western blot of the normal synoviocyte proteins revealed that cytochrome c levels were not significantly changed over the time course of treatment nor did caspase 8 activity increase; therefore, Rev A appears to exert this apoptotic effect through a mechanism independent of the classical intrinsic and extrinsic pathways. Fibroblast-like synoviocytes isolated from rheumatoid arthritis patients (RAFLS) as well as normal human fibroblast-like synoviocytes (NHFLS), cells known to play key roles in arthritic joint pathology, were also subjected to Rev A treatment at both physiologic and acidic pH's. Neither apoptosis nor necrosis was induced in either RAFLS or NHFLS. Parallel mitomycin C treatment of NHFLS induced both apoptosis and necrosis. Comparative structure-activity analyses of Rev A and mitomycin C revealed that Rev A is less likely to cross the cell membrane at near neutral pH. Collectively the data reveal that a physiological dose of Rev A under acidic conditions induces normal synoviocytes to undergo apoptosis while pathologic fibroblast-like synoviocytes are resistant to apoptosis and necrosis.

Saurolactam Inhibits Proliferation, Migration, and Invasion of Human Osteosarcoma Cells

Osteosarcoma is a common type of malignant bone tumor with features of osteoid formation or osteolytic lesions of bone. New therapeutic approaches are urgently needed since it lacks response to chemotherapeutic treatments. Saurolactam, a natural compound isolated from the aerial portions of Saururus chinensis, was reported to have an anti-inflammatory activity. Here, we demonstrate that saurolactam shows anti-cancer activity against human osteosarcoma cells. Saurolactam treatment inhibited proliferation of human osteosarcoma cell lines MG-63 and HOS and decreased colony formation in soft agar in a dose-dependent manner. Intraperitoneal administration of saurolactam at 25 mg/kg of body weight for 21 days dramatically inhibited the growth of MG-63 xenografts in nude mice. Flow cytometric analysis indicated that saurolactam treatment (20 μM) led to G1 cell cycle arrest and induced apoptosis in these two cell lines. Western analysis suggested that saurolactam treatment resulted in a reduction of Akt/PKB, phospho-Ser473-Akt, c-Myc, and S-phase kinase-associated protein 2 (Skp2) in MG-63 and HOS osteosarcoma cells. Akt overexpression significantly abolished saurolactam-induced decrease in protein and phosphorylation levels of Akt, c-Myc, and Skp2 protein levels, implying that Akt inactivation was a causal mediator of saurolactam-induced inhibition of c-Myc and Skp2. Moreover, Skp2 overexpression in MG-63 cells partly abolished the growth inhibition induced by saurolactam. Saurolactam treatment repressed migration and invasion ability, and Skp2 overexpression significantly blocked these inhibitory effects of saurolactam in MG-63 Cells. The present study indicates that saurolactam might represent a new promising agent to improve osteosarcoma treatment.

Aconine inhibits RANKL-induced osteoclast differentiation in RAW264.7 cells by suppressing NF-κB and NFATc1 activation and DC-STAMP expression

AIM

Aconiti Lateralis Radix Preparata is a traditional Chinese medicine used to treat chronic arthritis and is highly effective against rheumatoid arthritis. However, the effects of aconine, a derivative of aconitum alkaloids, on osteoclasts, which can absorb bone, remain unknown. Here, we investigated the effects of aconine on osteoclast differentiation and bone resorption in vitro.

METHODS

The viability of mouse leukemic monocyte/macrophage cell line RAW264.7 was measured using CCK-8 assays. Osteoclast differentiation was induced by incubation of RAW264.7 cells in the presence of RANKL, and assessed with TRAP staining assay. Bone resorption was examined with bone resorption pits assay. The expression of relevant genes and proteins was analyzed using RT-PCR and Western blots. The activation of NF-κB and nuclear factor of activated T-cells (NFAT) was examined using stable NF-κB and NFATc1 luciferase reporter gene systems, RT-PCR and Western blot analysis.

RESULTS

Aconine (0.125, 0.25 μmol/L) did not affect the viability of RAW264.7 cells, but dose-dependently inhibited RANKL-induced osteoclast formation and bone resorptive activity. Furthermore, aconine dose-dependently inhibited the RANKL-induced activation of NF-κB and NFATc1 in RAW264.7 cells, and subsequently reduced the expression of osteoclast-specific genes (c-Src, β3-Integrin, cathepsin K and MMP-9) and the expression of dendritic cell-specific transmembrane protein (DC-STAMP), which played an important role in cell-cell fusion.

CONCLUSION

These findings suggest that aconine inhibits RANKL-induced osteoclast differentiation in RAW264.7 cells by suppressing the activation of NF-κB and NFATc1 and the expression of the cell-cell fusion molecule DC-STAMP.

Decursin inhibits osteoclastogenesis by downregulating NFATc1 and blocking fusion of pre-osteoclasts

Bone sustains its structure through dynamic interaction between osteoblastic cells and osteoclastic cells. But imbalance may lead to osteoporosis caused by overactivated osteoclast cells that have bone-resorbing function. Recently, herbs have been researched as major sources of medicines in many countries. In vitro and in vivo anti-osteoclastogenic activity of Angelica gigas NAKAI have been reported, but the biological activity of decursin, its major component in osteoclast differentiation is still unknown. Therefore, in this study, we explored whether decursin could affect RANKL-mediated osteoclastogenesis. The results showed that decursin efficiently inhibited RANKL-activated osteoclast differentiation by inhibiting transcriptional and translational expression of NFATc1, a major factor in RANKL-mediated osteoclastogenesis. Furthermore, decursin decreased fusion and migration of pre-osteoclasts by downregulating mRNA expression levels of DC-STAMP and β3 integrin, respectively. In addition, decursin prevents lipopolysaccharide (LPS)-induced bone erosion in vivo. In summary, decursin could prevent osteoclastogenesis and inflammatory bone loss via blockage of NFATc1 activity and fusion and migration of pre-osteoclasts, and it could be developed as a potent phytochemical candidate for treating pathologies of bone diseases.

CA-074Me compound inhibits osteoclastogenesis via suppression of the NFATc1 and c-FOS signaling pathways

The osteoclast is an integral cell of bone resorption. Since osteolytic disorders hinge on the function and dysfunction of the osteoclast, understanding osteoclast biology is fundamental to designing new therapies that curb osteolytic disorders. The identification and study of lysosomal proteases, such as cathepsins, have shed light on mechanisms of bone resorption. For example, Cathepsin K has already been identified as a collagen degradation protease produced by mature osteoclasts with high activity in the acidic osteoclast resorption pits. Delving into the mechanisms of cathepsins and other osteoclast related compounds provides new targets to explore in osteoclast biology. Through our anti-osteoclastogenic compound screening experiments we encountered a modified version of the Cathepsin B inhibitor CA-074: the cell membrane-permeable CA-074Me (L-3-trans-(Propylcarbamoyl) oxirane-2-carbonyl]-L-isoleucyl-L-proline Methyl Ester). Here we confirm that CA-074Me inhibits osteoclastogenesis in vivo and in vitro in a dose-dependent manner. However, Cathepsin B knockout mice exhibited unaltered osteoclastogenesis, suggesting a more complicated mechanism of action than Cathepsin B inhibition. We found that CA-074Me exerts its osteoclastogenic effect within 24 h of osteoclastogenesis stimulation by suppression of c-FOS and NFATc1 pathways.

miR-218 is involved in the negative regulation of osteoclastogenesis and bone resorption by partial suppression of p38MAPK-c-Fos-NFATc1 signaling: Potential role for osteopenic diseases

The increased osteoclastic activity accounts for pathological bone loss in diseases including osteoporosis. MicroRNAs are widely accepted to be involved in the regulation of osteopenic diseases. Recently, the low expression of miR-218 was demonstrated in CD14(+) peripheral blood mononuclear cells (PBMCs) from patients with postmenopausal osteoporosis. However, its role and the underlying mechanism in osteoporosis are still undefined. Here, an obvious decrease in miR-218 expression was observed during osteoclastogenesis under receptor activator of nuclear factor κB ligand (RANKL) stimulation, in both osteoclast precursors of bone marrow macrophages (BMMs) and RAW 264.7. Further analysis confirmed that overexpression of miR-218 obviously attenuated the formation of multinuclear mature osteoclasts, concomitant with the decrease in Trap and Cathepsin K levels, both the master regulators of osteoclastogenesis. Moreover, miR-218 up-regulation dramatically inhibited osteoclast precursor migration, actin ring formation and bone resorption. Mechanism assay demonstrated that miR-218 overexpression attenuated the expression of p38MAPK, c-Fos and NFATc1 signaling molecules. Following preconditioning with P79350, an agonist of p38MAPK, the inhibitor effect of miR-218 on osteoclastogenesis and bone-resorbing activity was strikingly ameliorated. Together, this study revealed a crucial role of miR-218 as a negative regulator for osteoclastogenesis and bone resorption by suppressing the p38MAPK-c-Fos-NFATc1 pathway. Accordingly, this research will provide a promising therapeutic agent against osteopenic diseases including osteoporosis.

Reactive oxygen species and oxidative stress in osteoclastogenesis, skeletal aging and bone diseases

Osteoclasts are cells derived from bone marrow macrophages and are important in regulating bone resorption during bone homeostasis. Understanding what drives osteoclast differentiation and activity is important when studying diseases characterized by heightened bone resorption relative to formation, such as osteoporosis. In the last decade, studies have indicated that reactive oxygen species (ROS), including superoxide and hydrogen peroxide, are crucial components that regulate the differentiation process of osteoclasts. However, there are still many unanswered questions that remain. This review will examine the mechanisms by which ROS can be produced in osteoclasts as well as how it may affect osteoclast differentiation and activity through its actions on osteoclastogenesis signaling pathways. In addition, the contribution of ROS to the aging-associated disease of osteoporosis will be addressed and how targeting ROS may lead to the development of novel therapeutic treatment options.

In vitro and in vivo Bone-Forming Activity of Saururus chinensis Extract

Bone is maintained by osteoclast-mediated resorption and osteoblast-mediated formation. Recently, anti-osteoporotic activity of Saururus chinensis extract (SCE) and anti-osteoclastogenic activity of its components have been reported, but the effect of SCE on bone formation has not been studied well. Therefore, in this study, we investigated whether Saururus chinensis SCE exhibits in vitro osteogenic and in vivo bone-forming activity. extract strongly enhanced the bone morphogenetic protein (BMP)-2-stimulated induction of alkaline phosphatase, an early phase biomarker of osteoblast differentiation, in bi-potential mesenchymal progenitor C2C12 cells. In vitro osteogenic activity of SCE was accompanied by enhanced expression of BMP-2, BMP-4, BMP-7 and BMP-9 mRNA. In addition, a pharmacological inhibition study suggested the involvement of p38 activation in the osteogenic action of SCE. Moreover, the BMP dependency and the involvement of p38 activation in the osteogenic action of SCE were confirmed by the treatment of noggin, an antagonist of BMP. Saururus chinensis extract also exhibited to induce runt-related transcription factor 2 activation at the high concentration. Furthermore, the in vivo osteogenic activity of SCE was confirmed in zebrafish and mouse calvarial bone formation models, suggesting the possibility of its use for bone formation. In conclusion, we suggested that in vivo anti-osteoporotic activity of SCE could be because of its dual action in bone, anti-osteoclastogenic and anabolic activity.

The adaptor protein p62 is involved in RANKL-induced autophagy and osteoclastogenesis

Previous studies have implicated autophagy in osteoclast differentiation. The aim of this study was to investigate the potential role of p62, a characterized adaptor protein for autophagy, in RANKL-induced osteoclastogenesis. Real-time quantitative PCR and western blot analyses were used to evaluate the expression levels of autophagy-related markers during RANKL-induced osteoclastogenesis in mouse macrophage-like RAW264.7 cells. Meanwhile, the potential relationship between p62/LC3 localization and F-actin ring formation was tested using double-labeling immunofluorescence. Then, the expression of p62 in RAW264.7 cells was knocked down using small-interfering RNA (siRNA), followed by detecting its influence on RANKL-induced autophagy activation, osteoclast differentiation, and F-actin ring formation. The data showed that several key autophagy-related markers including p62 were significantly altered during RANKL-induced osteoclast differentiation. In addition, the expression and localization of p62 showed negative correlation with LC3 accumulation and F-actin ring formation, as demonstrated by western blot and immunofluorescence analyses, respectively. Importantly, the knockdown of p62 obviously attenuated RANKL-induced expression of autophagy- and osteoclastogenesis-related genes, formation of TRAP-positive multinuclear cells, accumulation of LC3, as well as formation of F-actin ring. Our study indicates that p62 may play essential roles in RANKL-induced autophagy and osteoclastogenesis, which may help to develop a novel therapeutic strategy against osteoclastogenesis-related diseases.

The role of small molecules in bone regeneration

Although several methods have been used in bone regeneration medicine, current methods still have many limitations. The tissue used for autogenous bone graft is limited and allograft has weak osteoinductive activity. Tissue engineering provides a good choice for bone regeneration. However, the growth factors needed have a high price and short half-life. Recently, a number of small molecules have been confirmed to have osteoinductive activity and some have been clinically used. Natural small molecules including decalpenic acid, flavonoids, quinones can be extracted from plants and others can be synthesized according to the structure designed or mimicking the structure of natural small molecules. Small molecules can act as co-activator of BMP2 pathway or activate Wnt pathway; others can be the inhibitors of NF-κB signaling pathway. This review gives an overview on the small molecules with osteoinductive activity and discusses the mechanism of the small molecules.

Anti-osteoclastogenic activity of praeruptorin A via inhibition of p38/Akt-c-Fos-NFATc1 signaling and PLCγ-independent Ca2+ oscillation

Background

A decrease of bone mass is a major risk factor for fracture. Several natural products have traditionally been used as herbal medicines to prevent and/or treat bone disorders including osteoporosis. Praeruptorin A is isolated from the dry root extract of Peucedanum praeruptorum Dunn and has several biological activities, but its anti-osteoporotic activity has not been studied yet.

Materials and Methods

The effect of praeruptorin A on the differentiation of bone marrow–derived macrophages into osteoclasts was examined by phenotype assay and confirmed by real-time PCR and immunoblotting. The involvement of NFATc1 in the anti-osteoclastogenic action of praeruptorin A was evaluated by its lentiviral ectopic expression. Intracellular Ca²⁺ levels were also measured.

Results

Praeruptorin A inhibited the RANKL-stimulated osteoclast differentiation accompanied by inhibition of p38 and Akt signaling, which could be the reason for praeruptorin A-downregulated expression levels of c-Fos and NFATc1, transcription factors that regulate osteoclast-specific genes, as well as osteoclast fusion-related molecules. The anti-osteoclastogenic effect of praeruptorin A was rescued by overexpression of NFATc1. Praeruptorin A strongly prevented the RANKL-induced Ca²⁺ oscillation without any changes in the phosphorylation of PLCγ.

Conclusion

Praeruptorin A could exhibit its anti-osteoclastogenic activity by inhibiting p38/Akt-c-Fos-NFATc1 signaling and PLCγ-independent Ca²⁺ oscillation.

Sinomenine induces apoptosis in RAW 264.7 cell-derived osteoclasts in vitro via caspase-3 activation

AIM

Sinomenine (SIN) is an alkaloid found in the roots and stems of Sinomenium acutum, which has been used to treat rheumatic arthritis in China and Japan. In this study we investigated the effects of SIN on osteoclast survival in vitro and the mechanisms of the actions.

METHODS

Mature osteoclasts were differentiated from murine monocyte/macrophage cell line RAW264.7 through incubation in the presence of receptor activator of NF-κB ligand (RANKL, 100 ng/mL) for 4 d. The cell viability was detected using the CCK-8 method. The survival and actin ring construction of the osteoclasts were scored using TRACP staining and phalloidin-FITC staining, respectively. The apoptosis of the osteoclasts was detected by DNA fragmentation and Hoechst 33258 staining, and the cell necrosis was indicated by LDH activity. The activation of caspase-3 in osteoclasts was measured using Western blotting and the caspase-3 activity colorimetric method.

RESULTS

SIN (0.25-2 mmol/L) inhibited the viability of mature osteoclasts in dose-dependent and time-dependent manners, but did not affect that of RAW264.7 cells. Consistently, SIN dose-dependently suppressed the survival of mature osteoclasts. The formation of actin ring, a marker associated with actively resorbing osteoclasts, was also impaired by the alkaloid. SIN (0.5 mmol/L) induced the apoptosis of mature osteoclasts, which was significantly attenuated in the presence of the caspase-3 inhibitor Ac-DEVD-CHO. SIN increased the cleavage of caspase-3 in mature osteoclasts in dose-dependent and time-dependent manners. Furthermore, SIN dose-dependently enhanced caspase-3 activity, which was blocked in the presence of Ac-DEVD-CHO.

CONCLUSION

Sinomenine inhibits osteoclast survival in vitro through caspase-3-mediated apoptosis, thus it is a potential agent for treating excessive bone resorption diseases.

Anti-osteoclastogenic activity of matairesinol via suppression of p38/ERK-NFATc1 signaling axis

Background

Matairesinol is a plant lignan present in a wide variety of foodstuffs such as seeds, vegetables and fruits. It has various biological functions including anti-angiogenic, anti-cancer and anti-fungal activities, but its anti-osteoporotic activity, if any, is unknown.

Methods

For osteoclast differentiation, primary mouse bone marrow-derived macrophage cells (BMMs) were cultured for 4 days in the presence of RANKL and M-CSF with the vehicle (DMSO) or matairesinol. Cell cytotoxicity was examined by CCK-8 assay. Gene expression of NFATc1, TRAP, OSCAR, v-ATPasev0d2 were observed in the presence or absence of matairesinol (10 μM) for the indicated times. For evaluating the involvement of NFATc1 in the anti-osteoclastogenic action of matairesinol, BMMs were infected with pMX-IRES-GFP or pMX-IRES-CA-NFATc1-GFP for 8 h with polybrene, and then infected BMMs were cultured with M-CSF and RANKL for 4 days in the presence or absence of matairesinol (10 μM). MAPK signaling activation was examined by immunoblotting. For measuring the resorptive activity of mature osteoclasts, osteoclasts and osteoblasts were co-cultured on BioCoat Osteologic MultiTest slides, and treated with matairesinol for 24 h.

Result

Here we show that matairesinol dose-dependently inhibited the RANKL-induced differentiation of BMMs into osteoclasts by downregulating RANKL-induced expression and activity of NFATc1. Ectopic overexpression of NFATc1 blunted the anti-osteoclastogenic effect of matairesinol implicating NFATc1 in the action of matairesinol. Additionally, matairesinol blocked the RANKL-induced activation of p38 and ERK in BMMs, but had no effect on bone resorption activity in mature osteoclasts.

Conclusion

Taken together, our results suggest that the anti-osteoporotic activity of matairesinol could arise from its anti-osteoclastogenic potential via p38/ERK-NFATc1 signaling, but not by way of anti-resorptive action.

Sinomenine suppresses osteoclast formation and Mycobacterium tuberculosis H37Ra-induced bone loss by modulating RANKL signaling pathways

Receptor activator of NF-κB ligand (RANKL) is essential for osteoclastogenesis. Targeting RANKL signaling pathways has been an encouraging strategy for treating lytic bone diseases such as osteoporosis and rheumatoid arthritis (RA). Sinomenine (SIN), derived from Chinese medicinal plant Sinomenioumacutum, is an active compound to treat RA, but its effect on osteoclasts has been hitherto unknown. In the present study, SIN was found to ameliorate M. tuberculosis H37Ra (Mt)-induced bone loss in rats with a decreased serum level of TRACP5b and RANKL, and an increased level of osteoprotegerin (OPG). In vitro study also showed that SIN could inhibit RANKL-induced osteoclast formation and bone resorption. The osteoclastic specific marker genes induced by RANKL including c-Src, MMP-9, TRACP were inhibited by SIN in a dose dependent manner. Signal transduction studies showed that SIN could obviously reduce the expression of RANK adaptor molecule TRAF6 and down-regulate RANKL-induced NF-κB activation. It decreased the RANKL-induced p38, JNK posphorylation but not ERK1/2 posphorylation. SIN could also reduce RANKL-mediated calcium influx which is associated with TRAF6/c-Src complex. Finally, SIN suppressed RANKL induced AP-1 and NFAT transcription, as well as the gene expression of NFATc1 and AP-1 components (Fra-1, Fra-2, c-Fos). The protein expression of c-Fos and TRAF6 were also inhibited by SIN after RANKL stimulation. Taken together, SIN could attenuate osteoclast formation and Mt-induced bone loss by mediating RANKL signaling pathways.

6,4'-Dihydroxy-7-methoxyflavanone inhibits osteoclast differentiation and function

6,4'-Dihydroxy-7-methoxyflavanone (DMF) is a flavonoid isolated from Heartwood Dalbergia odorifera. It has been known that DMF has antioxidant, anti-inflammatory and neuroprotective effects. DMF, however, the efficacy of bone related diseases has not been reported. In this study, we determined DMF's efficacy on osteoclasts differentiation and function using in vitro bone marrow macrophage osteoclast differentiation culture system. DMF inhibited receptor activators of nuclear factor kappa-B ligand (RANKL) induced osteoclastogenesis dose dependently. In addition, DMF decreased osteoclast function through disruption of actin ring formation and consequently suppression of the pit-forming activity of mature osteoclasts. Mechanistically, DMF inhibited RANKL-induced expression of nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1) and c-Fos via inhibition of mitogen activated protein kinases (MAPKs) pathway. Collectively, the inhibition of osteoclasts differentiation and function by DMF suggests that DMF can be a potential therapeutic molecule for osteoclastogenic bone diseases such osteoporosis, rheumatoid arthritis and periodontal diseases.

The role of Src kinase in macrophage-mediated inflammatory responses

Src kinase (Src) is a tyrosine protein kinase that regulates cellular metabolism, survival, and proliferation. Many studies have shown that Src plays multiple roles in macrophage-mediated innate immunity, such as phagocytosis, the production of inflammatory cytokines/mediators, and the induction of cellular migration, which strongly implies that Src plays a pivotal role in the functional activation of macrophages. Macrophages are involved in a variety of immune responses and in inflammatory diseases including rheumatoid arthritis, atherosclerosis, diabetes, obesity, cancer, and osteoporosis. Previous studies have suggested roles for Src in macrophage-mediated inflammatory responses; however, recently, new functions for Src have been reported, implying that Src functions in macrophage-mediated inflammatory responses that have not been described. In this paper, we discuss recent studies regarding a number of these newly defined functions of Src in macrophage-mediated inflammatory responses. Moreover, we discuss the feasibility of Src as a target for the development of new pharmaceutical drugs to treat macrophage-mediated inflammatory diseases. We provide insights into recent reports regarding new functions for Src that are related to macrophage-related inflammatory responses and the development of novel Src inhibitors with strong immunosuppressive and anti-inflammatory properties, which could be applied to various macrophage-mediated inflammatory diseases.

Fisetin Inhibits Osteoclast Differentiation via Downregulation of p38 and c-Fos-NFATc1 Signaling Pathways

The prevention or therapeutic treatment of loss of bone mass is an important means of improving the quality of life for patients with disorders related to osteoclast-mediated bone loss. Fisetin, a flavonoid dietary ingredient found in the smoke tree (Continus coggygria), exhibits various biological activities, but its effect on osteoclast differentiation is unknown. In this study, fisetin dose-dependently inhibited the RANKL-induced osteoclast differentiation with downregulation of the activity or expression of p38, c-Fos, and NFATc1 signaling molecules. The p38/c-Fos/NFATc1-regulated expression of genes required for cell fusion and bone resorption, such as DC-STAMP and cathepsin K, was also inhibited by fisetin. Considering the rescue of fisetin's inhibitory action by NFATc1 over-expression, the cascade of p38-c-Fos-NFATc1 could be strongly involved in the inhibitory effect of fisetin on osteoclast differentiation. Furthermore, fisetin inhibited the bone-resorbing activity of mature osteoclasts. In conclusion, fisetin may be of use in the treatment of osteoclast-related disorders, including osteoporosis.

Ormeloxifene inhibits osteoclast differentiation in parallel to downregulating RANKL-induced ROS generation and suppressing the activation of ERK and JNK in murine RAW264.7 cells

Ormeloxifene (Orm), a triphenylethylene compound, has been established as a selective estrogen receptor modulator (SERM) that suppresses the ovariectomy-induced bone resorption in rats. However, the precise mechanism underlying the bone-preserving action of Orm remains unclear. In this study, we evaluated the effect of Orm on osteoclast formation induced by receptor activator of nuclear factor κB ligand (RANKL) in the murine macrophage cell line RAW264.7. We also explored the mechanism of action of Orm by studying the RANKL-induced signaling pathways required for osteoclast differentiation. We found that Orm inhibited osteoclast formation from murine macrophage RAW264.7 cells induced by RANKL in a dose-dependent manner. Orm was able to abolish RANKL-induced reactive oxygen species (ROS) elevation and inhibited the transcriptional activation of two key RANKL-induced transcription factors namely activator protein-1 (AP-1) and NF-κB through mechanisms involving MAPKs. Activation of two MAPKs, i.e. ERK (MAPK1) and JNK (MAPK8), was alleviated by Orm effectively, which subsequently affected the activation of c-Jun and c-Fos, which are the essential components of the AP-1 transcription complex. Taken together, our results demonstrate that Orm potentially inhibits osteoclastogenesis by inhibiting ROS generation and thereby suppressing the activation of ERK1/2 (MAPK3/MAPK1) and JNK (MAPK8) and transcription factors (NF-κB and AP-1), which subsequently affect the regulation of osteoclastogenesis. These results provide a possible mechanism of action of Orm in regulating osteoclastogenesis, thereby supporting the beneficial bone-protective effects of this compound.

Quinoxaline derivative of oleanolic acid inhibits osteoclastic bone resorption and prevents ovariectomy-induced bone loss

OBJECTIVE

Through bioassay-guided natural product research, it has been discovered that oleanolic acid and its glycosides possess an antibone resorption activity. Quinoxaline derivative of oleanolic acid (QOA-8a), a novel compound, is sourced from a structural modification of oleanolic acid. The aim of the present study was to evaluate the activities of QOA-8a on bone resorption in vitro and its osteoprotective effect in vivo.

METHODS

Osteoclast precursors and mature osteoclasts were used to assay antibone resorption activities in vitro. RAW264.7 cells cultured for 2 days in the presence of the receptor activator for nuclear factor κB ligand were used as osteoclast precursors. Mature osteoclasts were generated from either primary cultures of mouse bone marrow-derived macrophages or RAW264.7 cells. Eight-week-old female mice that underwent either ovariectomy or sham surgical operation were used for the evaluation of the osteoprotective effect of QOA-8a at doses of 0.1, 1, and 10 mg kg(-1) day(-1).

RESULTS

QOA-8a significantly inhibited the differentiation, formation, and bone resorptive activity of mature osteoclasts without cytotoxicity. QOA-8a selectively induced apoptosis at an early stage of mature osteoclasts at least via increasing the caspase-3 activity, but not osteoclast precursors. Furthermore, QOA-8a significantly prevented bone loss in ovariectomized mice without any hormone-like adverse effects, whereas the mice treated with 1 mg kg(-1) day(-1) kept the same bone mineral density level as that of the sham mice.

CONCLUSIONS

QOA-8a inhibits bone resorption without cytotoxicity and prevents bone loss without any hormone-like adverse effects. Although further investigations are necessary to elucidate the detailed molecular mechanisms, QOA-8a demonstrates great potential as a novel agent for the treatment of osteoporosis.

Anti-resorptive activity and pharmacokinetic study of N(1),N(1)-diisopropyl-N(2)-(diphenylphosphoryl)-2-(4-nitrophenyl)acetamidine

In vitro anti-resorptive activity, mechanism of action, pharmacokinetic profile and in vivo anti-resorptive activity of N(1),N(1)-diisopropyl-N(2)-(diphenylphosphoryl)-2-(4-nitrophenyl)acetamidine (1) were evaluated.