Selective Regulation of MAPK Signaling Mediates RANKL-dependent Osteoclast Differentiation

Vitexin suppresses RANKL-induced osteoclastogenesis and prevents lipopolysaccharide (LPS)-induced osteolysis

Osteolytic diseases are characterized by an increase in the number and/or activity of bone-resorbing osteoclasts. Identification of natural compounds that can suppress osteoclast formation and function is crucial for the prevention and treatment of osteolytic diseases. Vitexin, a naturally-derived flavonoid extracted from various medicinal plant species, demonstrates a broad range of pharmacological properties including anticancer and anti-inflammatory effects. Here in this study, we showed that vitexin exerts antiosteoclastogenic effects by directly inhibiting receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast formation and bone resorption in vitro and protected against lipopolysaccharide (LPS)-induced inflammatory osteolysis in vivo. Vitexin suppressed the early activation of ERK and p38 MAPK pathways in response to RANKL thereby attenuating the downstream induction of c-Fos and NFATc1, and abrogating the expression of osteoclast marker genes. Collectively, these results provide evidence for the therapeutic application of vitexin in the treatment of osteoclast-mediated bone lytic diseases.

Hexane Fraction of Turbo brunneus Inhibits Intermediates of RANK-RANKL Signaling Pathway and Prevent Ovariectomy Induced Bone Loss

Osteoporosis is a "silent disease" characterized by fragile and impaired bone quality. Bone fracture results in increased mortality and poor quality of life in aged people particularly in postmenopausal women. Bone is maintained through the delicate balance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation. The imbalance is caused most often by overly active osteoclasts due to estrogen deficiency. Natural products have long been used to prevent and treat osteoporosis since they have fewer side effects. The marine environment is a potential source of biologically and structurally novel biomolecules with promising biological activities but is less explored for the treatment of bone-related diseases. The present study aims to evaluate the antiosteoporotic effect of Hexane fraction of Turbo brunneus methanolic extract (HxTME) and to investigate its role in RANK-RANKL signaling pathway using in vitro osteoclasts cultures and in vivo ovariectomized (OVX) Swiss mice model. The present study demonstrated that the HxTME significantly inhibited RANKL induced osteoclast differentiation and maturation in vitro. HxTME completely downregulated the mRNA expression of key transcription factors such as NFATc1, c-FOS, and osteoclasts related genes involved in osteoclastogenesis. In vivo studies also depicted the effectiveness of HxTME in ovariectomized mice by preserving bone microarchitecture, mineral content, and inhibiting bone loss in treated mice as analyzed by Histomorphometry, MicroCT, and Raman spectroscopy. Oral administration of HxTME fraction resulted in the decreased percentage of F4/80⁺, CD11b⁺, and CD4⁺ RANKL⁺ T cells in OVX mice whereas pro-osteoclastic cytokine, IL6 was markedly reduced upon treatment with HxTME. On stimulation with PMA/Io and PHA, a significant decrease in proliferative response in the splenocytes of HxTME treated OVX mice was observed. Fatty acid profiling revealed that HxTME is rich in ω3 and ω6 polyunsaturated fatty acids (PUFAs), which have high nutraceutical properties and are known to play important role in growth, development and maintenance of health. Therefore, HxTME may be a good source of nutraceutical in the treatment of bone-related diseases particularly in postmenopausal osteoporosis and may be pursued as a potential candidate for treatment and management of osteoporosis.

Th22 Cells Promote Osteoclast Differentiation via Production of IL-22 in Rheumatoid Arthritis

T helper (Th) cells can differentiate into functionally distinct subsets and play a pivotal role in inflammatory and autoimmune diseases such as rheumatoid arthritis (RA). Th22 cells have been identified as a new subset secreting interleukin (IL)-22. Although elevated levels of IL-22 in the synovial fluids of RA patients were reported, its pathological roles remain unclear. Here, we demonstrated that IL-22 was characteristically produced from CD3+CD4+CC-chemokine receptor (CCR)4+CCR6+CCR10+ cells and their ability of the production of IL-22 markedly exceeded that of other Th subsets and the subset, thereby, designated Th22 cells. Th22 cells were efficiently induced by the stimulation with tumor necrosis factor-α, IL-6, and IL-1β. Th22 cells were markedly infiltrated in synovial tissue in patients with active RA, but not in patients with osteoarthritis (OA). CCL17, CCL20, and CCL28, which are chemokine ligands of CCR4, CCR6, and CCR10, respectively, were abundantly expressed in RA synovial tissue compared to OA. By in vitro Trans-well migration assay, Th22 cells efficiently migrated toward CCL28. Co-culture of Th22 cells, which were sorted from peripheral blood, with monocytes in the presence of macrophage colony-stimulating factor and receptor activator of nuclear factor (NF)-κB ligand induced osteoclasts formation more efficiently than that of either Th1 cells or Th17 cells. Furthermore, IL-22 markedly augmented osteoclast differentiation by promoting nuclear factor of activated T cells c1 expression in CD14+ monocytes. Contrarily, the addition of IFN-γ to the culture significantly decreased osteoclasts number, whereas IL-17 had marginal effects. IL-22 neutralizing antibody inhibited osteoclast formation in the co-culture of Th22 cells with CD14+ monocytes. Collectively, the results indicated that Th22 cells, which co-express chemokine receptors CCR4, CCR6, and CCR10, possess strong potency of tissue migration and accumulate into inflamed synovial tissues where the ligands such as CCL28 are highly expressed. Thus, Th22 cells have the capacity to promote osteoclast differentiation through production of IL-22 and thus play a pivotal role in bone destruction in patients with RA.

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

BACKGROUND

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

High Doses of Bupleurum falcatum Partially Prevents Estrogen Deficiency-Induced Bone Loss With Anti-osteoclastogenic Activity Due to Enhanced iNOS/NO Signaling

Background and Objective:Bupleurum falcatum (BF) extract, a natural product with anti-inflammatory properties, has been traditionally used to treat menopausal symptoms, but its role in osteoporosis, another serious health concern of menopausal women, remains unknown. Here we investigated whether and how BF prevents estrogen deficiency-induced bone loss using both in vitro and in vivo models.

Methods: Female Sprague-Dawley rats were ovariectomized (OVX) and subjected to oral BF treatment daily for 8 weeks. Additionally, pre-osteoclastic RAW 264.7 cells were employed to evaluate the effects of BF and its underlying mechanism on receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast formation in vitro.

Results: A high dose of BF partially prevented ovariectomy (OVX)-induced bone loss and reduced the levels of tartrate-resistant acid phosphatase (TRAP) in serum and osteoclast numbers in femurs of OVX rats. Furthermore, BF clearly inhibited RANKL-induced osteoclast differentiation and bone resorption activity in RAW 264.7 cells. BF also inhibited the osteoclastogenic transcription factors c-Fos and nuclear factor of activated T cells c1 (NFATc1) and, consequently, downregulated the expression of osteoclast marker genes. Moreover, BF upregulated interferon-β (IFN-β)/inducible nitric oxide synthase (iNOS)/nitric oxide (NO) signaling, even though it had no impact on mitogen-activated protein kinases (MAPK) or NF-κB. The inhibition of osteoclast formation by BF was abrogated by iNOS-specific inhibitors. Consistent with cellular studies, BF upregulated iNOS protein expression in femurs from OVX rats.

Conclusion: Taken together, our results indicate that BF partially prevented estrogen deficiency-induced bone loss with anti-osteoclastogenic activity potentially due to enhanced iNOS/NO signaling.

Roles of Mitogen-Activated Protein Kinases in Osteoclast Biology

Bone undergoes continuous remodeling, which is homeostatically regulated by concerted communication between bone-forming osteoblasts and bone-degrading osteoclasts. Multinucleated giant osteoclasts are the only specialized cells that degrade or resorb the organic and inorganic bone components. They secrete proteases (e.g., cathepsin K) that degrade the organic collagenous matrix and establish localized acidosis at the bone-resorbing site through proton-pumping to facilitate the dissolution of inorganic mineral. Osteoporosis, the most common bone disease, is caused by excessive bone resorption, highlighting the crucial role of osteoclasts in intact bone remodeling. Signaling mediated by mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38, has been recognized to be critical for normal osteoclast differentiation and activation. Various exogenous (e.g., toll-like receptor agonists) and endogenous (e.g., growth factors and inflammatory cytokines) stimuli contribute to determining whether MAPKs positively or negatively regulate osteoclast adhesion, migration, fusion and survival, and osteoclastic bone resorption. In this review, we delineate the unique roles of MAPKs in osteoclast metabolism and provide an overview of the upstream regulators that activate or inhibit MAPKs and their downstream targets. Furthermore, we discuss the current knowledge about the differential kinetics of ERK, JNK, and p38, and the crosstalk between MAPKs in osteoclast metabolism.

Selective regulation of osteoclast adhesion and spreading by PLCγ/PKCα-PKCδ/RhoA-Rac1 signaling

Bone resorption by multinucleated osteoclasts is a multistep process involving adhesion to the bone matrix, migration to resorption sites, and formation of sealing zones and ruffled borders. Macrophage colony-stimulating factor (M-CSF) and osteopontin (OPN) have been shown to be involved in the bone resorption process by respective activation of integrin αvβ3 via “inside-out” and “outside-in” signaling. In this study, we investigated the link between signal modulators known to M-CSF- and OPN-induced osteoclast adhesion and spreading. M-CSF- and OPN-induced osteoclast adhesion was achieved via activation of stepwise signals, including integrin αvβ3, PLCγ, PKCδ, and Rac1. Osteoclast spreading induced by M-CSF and OPN was shown to be controlled via sequential activation, consistent with the osteoclast adhesion processes. In contrast to osteoclast adhesion, osteoclast spreading induced by M-CSF and OPN was blocked via activation of PLCγ/PKCα/RhoA signaling. The combined results indicate that osteoclast adhesion and spreading are selectively regulated via PLCγ/PKCα-PKCδ/RhoA-Rac1 signaling.

Ameloblastin attenuates RANKL-mediated osteoclastogenesis by suppressing activation of nuclear factor of activated T-cell cytoplasmic 1 (NFATc1)

Ameloblastin (Ambn) is an extracellular matrix protein and member of the family of enamel-related gene products. Like amelogenin, Ambn is mainly associated with tooth development, especially biomineralization of enamel. Previous studies have shown reductions in the skeletal dimensions of Ambn-deficient mice, suggesting that the protein also has effects on the differentiation of osteoblasts and/or osteoclasts. However, the specific pathways used by Ambn to influence osteoclast differentiation have yet to be identified. In the present study, two cellular models, one based on bone marrow cells and another on RAW264.7 cells, were used to examine the effects of Ambn on receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclastogenesis. The results showed that Ambn suppresses osteoclast differentiation, cytoskeletal organization, and osteoclast function by the downregulation of the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclasts, actin ring formation, and areas of pit resorption. The expression of the osteoclast-specific genes TRAP, MMP9, cathepsin K, and osteoclast stimulatory transmembrane protein (OC-STAMP) was abolished in the presence of Ambn, while that of nuclear factor of activated T cells cytoplasmic 1 (NFATc1), the master regulatory factor of osteoclastogenesis, was also attenuated by the downregulation of c-Fos expression. In Ambn-induced RAW264.7 cells, phosphorylation of cAMP-response element-binding protein (CREB), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (p38 MAPK), but not extracellular signal-regulated kinase 1/2 (ERK1/2), was reduced. Calcium oscillation was also decreased in the presence of Ambn, suggesting its involvement in both RANKL-induced osteoclastogenesis and costimulatory signaling. B-lymphocyte-induced maturation protein-1 (Blimp1), a transcriptional repressor of negative regulators of osteoclastogenesis, was also downregulated by Ambn, resulting in the elevated expression of v-maf musculoaponeurotic fibrosarcoma oncogene family, protein B (MafB), B-cell lymphoma 6 (Bcl6), and interferon regulatory factor-8 (Irf8). Taken together, these findings suggest that Ambn suppresses RANKL-induced osteoclastogenesis by modulating the NFATc1 axis.

Niche signals and transcription factors involved in tissue-resident macrophage development

Tissue-resident macrophages form an essential part of the first line of defense in all tissues of the body. Next to their immunological role, they play an important role in maintaining tissue homeostasis. Recently, it was shown that they are primarily of embryonic origin. During embryogenesis, precursors originating in the yolk sac and fetal liver colonize the embryonal tissues where they develop into mature tissue-resident macrophages. Their development is governed by two distinct sets of transcription factors. First, in the pre-macrophage stage, a core macrophage program is established by lineage-determining transcription factors. Under the influence of tissue-specific signals, this core program is refined by signal-dependent transcription factors. This nurturing by the niche allows the macrophages to perform tissue-specific functions. In the last 15 years, some of these niche signals and transcription factors have been identified. However, detailed insight in the exact mechanism of development is still lacking.

Isosteviol Derivative Inhibits Osteoclast Differentiation and Ameliorates Ovariectomy-Induced Osteoporosis

NC-8 (ent-16-oxobeyeran-19-N-methylureido) is an isosteviol-derived analogue with multiple biological effects, including anti-inflammation and anti-bacterial activities and inhibition of HBV viral surface antigen gene expression. In this study, we explored the effects of NC-8 on the formation of osteoclasts from RAW 264.7 cells. We found that NC-8 exerts the novel effect of inhibiting osteoclast-like cell formation. Our experiments showed that RANKL-induced ERK, p38, and JNK phosphorylation were inhibited by NC-8. An ovariectomy-induced osteoporosis animal model was used to examine the protective effects of oral treatment with NC-8. Serum analysis was used to examine markers of osteoblasts, osteoclasts, and renal and hepatic function in rats. Micro CT scanning and histological analysis were used to measure bone loss in ovariectomized rats. Oral administration of NC-8 effectively decreased excess bone resorption and significantly antagonized trabecular bone loss in ovariectomized rats. Serum analysis of C-terminal telopeptide of type-I collagen, an osteoclast marker, also showed that NC-8 administration inhibited excess bone resorption. Furthermore, serum analysis showed that renal and liver function were not affected by these doses of NC-8 during long-term treatment. Our results demonstrate that NC-8 inhibits osteoclast differentiation and effectively ameliorates ovariectomy-induced osteoporosis.

ANGPTL2 deletion inhibits osteoclast generation by modulating NF-κB/MAPKs/Cyclin pathways

Osteoclasts are multinucleated cells essential for bone-resorption. Successful repair of bone defciencies still remains a great challenge worldwide. The signaling factor angiopoietin-like protein 2 (ANGPTL2), one of eight ANGPTL proteins, functions in maintenance of tissue homeostasis partly through regulating inflammation. In the study, ANGPTL2 expression was promoted during osteoclast development and that suppressing ANGPTL2 alleviated osteoclast production regulated by macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL). The results suggested that ANGPTL2 knockdown inhibited M-CSF-caused proliferation of osteoclast precursor cells. Further, ANGPTL2 silence reduced nuclear factor of activated T cell c 1 (NFATC1) and NFATC4 expressions in M-CSF-treated cells, along with decreased Runx2, OPN and Colla1. Moreover, silencing ANGPTL2 down-regulated M-CSF-promoted expressions of pro-inflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-6, and chemoattractant protein-1 (CCL-2). Consistently, ANGPTL2 knockdown reduced M-CSF-enhanced activation of IKKα, IκBα and nuclear factor κB (NF-κB) and mitogen-activated protein kinases (MAPKs) (p38 MAPK, ERK1/2 MAPK and JNK MAPK). Additionally, knockdown of ANGPTL2 inhibited the induction of Cyclin D1, Cyclin D2 and Cyclin E1 due to M-CSF exposure. In vivo, we confirmed that ANGPTL2 knockout (KO) mice were protected against osteoporosis induced by ovariectomy (OVX), as proved by the improved bone loss and bone mineral density (BMD). Decreased expression of NFATCs was also observed in OVX-induced mice in the absence of ANGPTL2. Elevated release of pro-inflammatory cytokines was abrogated by ANGPTL2 knockout in femoral heads of mice with OVX operation, accompanied with a significant reduction of phosphorylated NF-κB and MAPKs signaling pathways. And down-regulated expression of Cyclin D1, Cyclin D2 and Cyclin E1 was observed in OVX-operated mice with ANGPTL2 knockout. Therefore, our study indicated that ANGPTL2 played an essential role in osteoclast generation through regulating the proliferation and inflammation of osteoclast lineage cells, providing new insights into the therapeutic strategy to alleviate bone loss.

Comparative Characterization of Osteoclasts Derived From Murine Bone Marrow Macrophages and RAW 264.7 Cells Using Quantitative Proteomics

Osteoclasts are bone-resorbing cells differentiated from macrophage/monocyte precursors in response to macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL). In vitro models are principally based on primary bone marrow macrophages (BMMs), but RAW 264.7 cells are frequently used because they are widely available, easy to culture, and more amenable to genetic manipulation than primary cells. Increasing evidence, however, has shown that the vastly different origins of these two cell types may have important effects on cell behavior. In particular, M-CSF is a prerequisite for the differentiation of BMMs, by promoting survival and proliferation and priming the cells for RANKL induction. RAW 264.7 cells readily form osteoclasts in the presence of RANKL, but M-CSF is not required. Based on these key differences, we sought to understand their functional implications and how it might affect osteoclast differentiation and related signaling pathways. Using a robust and high-throughput proteomics strategy, we quantified the global protein changes in osteoclasts derived from BMMs and RAW 264.7 cells at 1, 3, and 5 days of differentiation. Data are available via ProteomeXchange with the identifier PXD009610. Correlation analysis of the proteomes demonstrated low concordance between the two cell types (R² ≈ 0.13). Bioinformatics analysis indicate that RANKL-dependent signaling was intact in RAW 264.7 cells, but biological processes known to be dependent on M-CSF were significantly different, including cell cycle control, cytoskeletal organization, and apoptosis. RAW 264.7 cells exhibited constitutive activation of Erk and Akt that was dependent on the activity of Abelson tyrosine kinase, and the timing of Erk and Akt activation was significantly different between BMMs and RAW 264.7 cells. Our findings provide the first evidence for major discrepancies between BMMs and RAW 264.7 cells, indicating that careful consideration is needed when using the RAW 264.7 cell line for studying M-CSF-dependent signaling and functions. © 2018 American Society for Bone and Mineral Research. © 2018 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Dehydrodiconiferyl Alcohol Inhibits Osteoclast Differentiation and Ovariectomy-Induced Bone Loss through Acting as an Estrogen Receptor Agonist

Estrogen deficiency after menopause increases bone loss by activating RANKL-induced osteoclast differentiation. Dehydrodiconiferyl alcohol (DHCA), a lignan originally isolated from Cucurbita moschata, has been thought to be a phytoestrogen based on its structure. In this study, we tested whether DHCA could affect RANKL-induced osteoclastogenesis in vitro and ovariectomy-induced bone loss in vivo. In RAW264.7 cells, DHCA inhibited RANKL-induced differentiation of osteoclasts. Consistently, expression of the six osteoclastogenic genes induced by RANKL was down-regulated. DHCA was also shown to suppress the NF-κB and p38 MAPK signaling pathways by activating AMPK. Data from transient transfection assays suggested that DHCA might activate the estrogen receptor signaling pathway. Effects of DHCA on RANKL-induced osteoclastogenesis were reduced when cells were treated with specific siRNA to ERα, but not to ERβ. Interestingly, DHCA was predicted from molecular docking simulation to bind to both ERα and ERβ. Indeed, data from an estrogen receptor competition assay revealed that DHCA acted as an agonist on both estrogen receptors. In the ovariectomized (Ovx) mouse model, DHCA prevented Ovx-induced bone loss by inhibiting osteoclastogenesis. Taken together, our results suggest that DHCA may be developed as an efficient therapeutic for osteoporosis by regulating osteoclastogenesis through its estrogenic effects.

[Effect of lipopolysaccharide on osteoclasts formation and bone resorption function and its mechanism]

Objective

To study the effect and mechanism of lipopolysaccharide (LPS) on osteoclasts formation and its bone resorption function.

Methods

Bone marrow-derived macrophages (BMMs) were extracted from the marrow of femur and tibia of 4-week-old male C57BL/6 mice. Flow cytometry was used to detect BMMs. The effect of different concentrations of LPS (0, 100, 200, 500, 1 000, 2 000 ng/mL) on BMMs activity was examined by cell counting kit 8 (CCK-8) activity test. In order to investigate the effect of LPS on osteoclastogenesis, BMMs were divided into macrophage colony-stimulating factor (M-CSF) group, M-CSF+receptor activator of nuclear factor κB ligand (RANKL) group, M-CSF+RANKL+50 ng/mL LPS group, M-CSF+RANKL+100 ng/mL LPS group. After the completion of culture, tartrate resistant acid phosphatase (TRAP) staining was used to observe the formation of osteoclasts. In order to investigate the effect of LPS on the expression of Connexin43, BMMs were divided into the control group (M-CSF+RANKL) and the LPS group (M-CSF+RANKL+100 ng/mL LPS); and the control group (M-CSF+RANKL), 50 ng/mL LPS group (M-CSF+RANKL+50 ng/mL LPS), and 100 ng/mL LPS group (M-CSF+RANKL+100 ng/mL LPS). The expressions of Connexin43 mRNA and protein were detected by Western blot and real-time fluorescent quantitative PCR, respectively. In order to investigate the effect of LPS on osteoclast bone resorption, BMMs were divided into M-CSF group, M-CSF+RANKL group, M-CSF+RANKL+50 ng/mL LPS group, and M-CSF+RANKL+100 ng/mL LPS group. Bone absorption test was used to detect the ratio of bone resorption area.

Results

The flow cytometry test confirmed that the cultured cells were BMMs, and CCK-8 activity test proved that the 100 ng/mL LPS could promote the proliferation of BMMs, showing significant differences when compared with the 0, 200, 500, 1 000, and 2 000 ng/mL LPS ( P<0.05). TRAP staining showed no osteoclast formation in M-CSF group. Compared with M-CSF+RANKL group, the osteoclasts in M-CSF+RANKL+50 ng/mL LPS group and M-CSF+RANKL+100 ng/mL LPS group were larger with more nuclei, while the osteoclasts in M-CSF+RANKL+100 ng/mL LPS group were more obvious, and the differences in the ratio of osteoclast area between groups were statistically significant ( P<0.05). Western blot result showed that the relative expression of Connexin43 protein in LPS group was significantly higher than that in control group ( P<0.05). Real-time fluorescent quantitative PCR showed that the relative expression of Connexin43 mRNA in control group, 50 ng/mL LPS group, and 100 ng/mL LPS group increased gradually, and the differences between groups were statistically significant ( P<0.05). Bone resorption test showed that osteoclast bone resorption did not form in M-CSF group, but the ratio of bone resorption area increased gradually in M-CSF+RANKL group, M-CSF+RANKL+50 ng/mL LPS group, and M-CSF+RANKL+100 ng/mL LPS group, and the differences between groups were statistically significant ( P<0.05).

Conclusion

LPS at concentration of 100 ng/mL can promote the expression of Connexin43, resulting in increased osteoclastogenesis and enhanced osteoclastic bone resorption.

Macrolactin F inhibits RANKL-mediated osteoclastogenesis by suppressing Akt, MAPK and NFATc1 pathways and promotes osteoblastogenesis through a BMP-2/smad/Akt/Runx2 signaling pathway

The balance between bone formation and bone resorption is maintained by osteoblasts and osteoclasts. In the current study, macrolactin F (MF) was investigated for novel biological activity on the receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclastogenesis in primary bone marrow-derived macrophages (BMMs). We found that RANKL-induced osteoclast formation and differentiation from BMMs was significantly inhibited by MF in a dose-dependent manner without cytotoxicity. RANKL-induced F-actin ring formation and bone resorption activity in BMMs which was attenuated by MF. In addition, MF suppressed the expression of osteoclast-related genes, including c-myc, RANK, tartrate-resistant acid phosphatase (TRAP), nuclear factor of activated T cells c1 (NFATc1), cathepsin K and matrix metalloproteinase 9 (MMP9). Furthermore, the protein expression NFATc1, c-Fos, MMP9, cathepsin K and phosphorylation of Jun N-terminal kinase (JNK), p38 and Akt were also down-regulated by MF treatment. Interestingly, MF promoted pre-osteoblast cell differentiation on Alizarin Red-mineralization activity, alkaline phosphatase (ALP) activity, and the expression of osteoblastogenic markers including Runx2, Osterix, Smad4, ALP, type I collagen alpha 1 (Col1α), osteopontin (OPN), and osteocalcin (OCN) via activation of the BMP-2/smad/Akt/Runx2 pathway on MC3T3-E1. Taken together, these results indicate that MF may be useful as a therapeutic agent to enhance bone health and treat osteoporosis.

UHMWPE wear particles and dendritic cells promote osteoclastogenesis of RAW264.7 cells through RANK-activated NF-κB/MAPK/AKT pathways

Dendritic cells (DCs) were shown to enhance UHMWPE particle promoted osteoclastogenesis in RAW264.7 macrophages. This study aimed to elucidate the signaling network mediating the osteoclastogenic effects of UHMWPE particles and DCs. RAW264.7 cells were induced with UHMWPE particles and /or DC2.4 cells. The expression of Rank was silenced by shRNA. The activation of p38MAPK, AKT, and NF-κB was inhibited specific inhibitors. The osteoclasts were assessed by TRAP staining. The expression of osteoclastogenic genes and activation of p38MAPK, AKT, and NF-κB were analyzed by qPCR and/or Western blot. UHMWPE particles and DC2.4 cells cooperatively induced RAW264.7 macrophages to differentiate into osteoclasts, activated p38MAPK, AKT, and NF-κB pathways, and upregulated Nfatc1 and osteoclast markers, which was abolished by silencing Rank or inhibited by inhibitors of p38MAPK, AKT, and NF-κB in RAW264.7 cells. These data demonstrated the complexity of signaling network and cell-cell interactions involved in UHMWPE particles-induced osteolysis.

The antimicrobial peptide, human β-defensin-1, potentiates in vitro osteoclastogenesis via activation of the p44/42 mitogen-activated protein kinases

Previous studies have demonstrated increased expression and raised levels of human β-defensin (hBD)-1 in gingival tissue and crevicular fluid of patients with chronic periodontitis and peri-implantitis, oral bone-resorbing diseases caused by enhanced osteoclastogenesis. Therefore, we aimed to investigate the effect of hBD-1 on osteoclast formation and function and to elucidate the involved signaling pathway in vitro. Human peripheral blood mononuclear cells (PBMCs) were first incubated with various doses of hBD-1 and cell viability was assayed by MTT. PBMCs were treated with macrophage-colony stimulating factor and receptor activator of nuclear factor kappa-B ligand (RANKL) in the presence or absence of non-toxic doses of hBD-1. In vitro osteoclastogenesis was analyzed by tartrate-resistant acid phosphatase (TRAP) staining, osteoclast-specific gene expression, and a resorption pit assay. Involvement of mitogen-activated protein kinases (MAPKs) was studied by immunoblotting and specific MAPK inhibitors. HBD-1 potentiated induction of in vitro osteoclastogenesis by RANKL, as shown by significantly increased number of TRAP-positive multinuclear cells and resorption areas on the dentin slices, and further up-regulated expressions of osteoclast-specific genes compared to those by RANKL treatment (p <0.05). However, hBD-1 treatment without RANKL failed to induce formation of osteoclast-like cells. A significant and further increase in transient phosphorylation of the p44/42 MAPKs was demonstrated by hBD-1 co-treatment (p<0.05), consistent with the inhibitory effect by pretreatment with U0126 or PD98059 on hBD-1-enhanced osteoclastogenesis. Collectively, hBD-1 potentiates the induction of in vitro osteoclastogenesis by RANKL via enhanced phosphorylation of the p44/42 MAPKs.

Molecular signaling mechanisms behind polyphenol-induced bone anabolism

For millennia, in the different cultures all over the world, plants have been extensively used as a source of therapeutic agents with wide-ranging medicinal applications, thus becoming part of a rational clinical and pharmacological investigation over the years. As bioactive molecules, plant-derived polyphenols have been demonstrated to exert many effects on human health by acting on different biological systems, thus their therapeutic potential would represent a novel approach on which natural product-based drug discovery and development could be based in the future. Many reports have provided evidence for the benefits derived from the dietary supplementation of polyphenols in the prevention and treatment of osteoporosis. Polyphenols are able to protect the bone, thanks to their antioxidant properties, as well as their anti-inflammatory actions by involving diverse signaling pathways, thus leading to bone anabolic effects and decreased bone resorption. This review is meant to summarize the research works performed so far, by elucidating the molecular mechanisms of action of polyphenols in a bone regeneration context, aiming at a better understanding of a possible application in the development of medical devices for bone tissue regeneration.

Combined treatment with vitamin C and methotrexate inhibits triple-negative breast cancer cell growth by increasing H2O2 accumulation and activating caspase-3 and p38 pathways

Methotrexate (MTX) is widely used as both an anticancer and anti-rheumatoid arthritis drug. Although MTX has been used to inhibit the growth of many cancer cells, it cannot effectively inhibit growth of triple-negative breast cancer cells (TNBC cells). Vitamin C is an antioxidant that can prevent oxidative stress. In addition, vitamin C has been applied as adjunct treatment for growth inhibition of cancer cells. Recent studies indicated that combined treatment with vitamin C and MTX may inhibit MCF-7 and MDA-MB-231 breast cancer cell growth through G2/M elongation. However, the mechanisms remain unknown. The aim of the present study was to determine whether combined treatment with low-dose vitamin C and MTX inhibits TNBC cell growth and to investigate the mechanisms of vitamin C/MTX-induced cytotoxicity. Neither low-dose vitamin C alone nor MTX alone inhibited TNBC cell growth. However, combined low-dose vitamin C and MTX had synergistic anti-proliferative/cytotoxic effects on TNBC cells. In addition, co-treatment increased H2O2 levels and activated both caspase-3 and p38 cell death pathways.

Peroxidase enzymes inhibit osteoclast differentiation and bone resorption

Myeloperoxidase (MPO) and eosinophil peroxidase (EPO) are heme-containing enzymes, well known for their antimicrobial activity, are released in abundance by innate immune infiltrates at sites of inflammation and injury. We have discovered new and previously unrecognised roles for heme peroxidases in extracellular matrix biosynthesis, angiogenesis, and bone mineralisation, all of which play an essential role in skeletal integrity. In this study we used in vitro models of osteoclastogenesis to investigate the effects of heme peroxidase enzymes on osteoclast differentiation and bone resorbing activity, pertinent to skeletal development and remodelling. Receptor activator of nuclear factor kappa B-ligand (RANKL) stimulates the formation of tartate-resistant acid phosphatase (TRAP) positive multinucleated cells and increases bone resorption when cultured with human peripheral blood mononuclear cells (PBMCs) or the RAW264.7 murine monocytic cell line. When RANKL was added in combination with either MPO or EPO, a dose-dependent inhibition of osteoclast differentiation and bone resorption was observed. Notably, peroxidases had no effect on the bone resorbing activity of mature osteoclasts, suggesting that the inhibitory effect of the peroxidases was limited to osteoclast precursor cells. Mechanistically, we observed that osteoclast precursor cells readily internalize peroxidases, and inhibited the phosphorylation of JNK, p38 MAPK and ERK1/2, important signalling molecules central to osteoclastogenesis. Our findings suggest that peroxidase enzymes, like MPO and EPO, may play a fundamental role in inhibiting RANKL-induced osteoclast differentiation at inflammatory sites of bone fracture and injury. Therefore, peroxidase enzymes could be considered as potential therapeutic agents to treat osteolytic bone disease and aberrant bone resorption.

Synchronized Cell Cycle Arrest Promotes Osteoclast Differentiation

Osteoclast progenitors undergo cell cycle arrest before differentiation into osteoclasts, induced by exposure to macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). The role of such cell cycle arrest in osteoclast differentiation has remained unclear, however. We here examined the effect of synchronized cell cycle arrest on osteoclast formation. Osteoclast progenitors deprived of M-CSF in culture adopted a uniform morphology and exhibited cell cycle arrest at the G₀–G₁ phase in association with both down-regulation of cyclins A and D1 as well as up-regulation of the cyclin-dependent kinase inhibitor p27^Kip1. Such M-CSF deprivation also promoted the differentiation of osteoclast progenitors into multinucleated osteoclasts expressing high levels of osteoclast marker proteins such as NFATc1, c-Fos, Atp6v0d2, cathepsin K, and integrin β3 on subsequent exposure to M-CSF and RANKL. Our results suggest that synchronized arrest and reprogramming of osteoclast progenitors renders them poised to respond to inducers of osteoclast formation. Further characterization of such effects may facilitate induction of the differentiation of heterogeneous and multipotent cells into desired cell lineages.

Elevated Gα11 expression in osteoblast lineage cells promotes osteoclastogenesis and leads to enhanced trabecular bone accrual in response to pamidronate

Osteoblastic cells indirectly induce osteoclastogenesis in the bone microenvironment by expressing paracrine factors such as RANKL and M-CSF, leading to increased bone resorption. These cytokines can be regulated by a variety of intracellular pathways, which include G protein-coupled receptor signaling. To explore how enhanced signaling of the Gαq/11 pathway in osteoblast lineage cells may mediate osteoclast formation, we cocultured wild-type (WT) preosteoclasts with BMSCs derived from either WT or transgenic mice with osteoblast-specific overexpression of Gα11 (G11-Tg). G11-Tg cocultures had elevated osteoclast numbers with greater resorptive capacity and increased expression of Rankl, Rankl:Opg (osteoprotegerin), and M-csf compared with cocultures with WT BMSCs. As well, cocultures with G11-Tg BMSCs required a higher concentration of OPG to inhibit osteoclast formation and less angiotensin II to increase osteoclast size. These indicate that G11-Tg osteoblasts drive the increased osteoclast formation and osteopenia seen in G11-Tg mice. Pamidronate treatment of G11-Tg mice restored the trabecular bone loss phenotype, as bone mineral density, bone volume, trabecular number, separation, and expressions of osteoblastic and osteoclastic genes were comparable with WT parameters. These changes were characterized by enhanced accumulation of calcified cartilage in trabecular bone, demonstrating that resorption of the cartilaginous intermediate by osteoclasts is more affected by bisphosphonate treatment in G11-Tg mice. In conclusion, overexpression of Gα11 in osteoblastic cells promotes osteoclastogenesis by upregulation of Rankl and M-csf and bone loss by increased osteoclast resorption of the trabecular bone and cartilaginous matrix.