The molecular basis of osteoclast differentiation and activation

Artemisinic acid attenuates osteoclast formation and titanium particle-induced osteolysis via inhibition of RANKL-induced ROS accumulation and MAPK and NF-κB signaling pathways

Periprosthetic osteolysis (PPO) is the most common cause of joint arthroplasty failure. Its progression involves both biological and mechanical factors. Osteoclastogenesis induced by wear from debris-cell interactions, ultimately leading to excessive bone erosion, is considered the primary cause of PPO; therefore, targeting osteoclasts is a promising treatment approach. Currently available drugs have various side effects and limitations. Artemisinic acid (ArA) is a sesquiterpene isolated from the traditional herb Artemisia annua L. that has various pharmacological effects, such as antimalarial, anti-inflammatory, and antioxidant activities. Therefore, this study was aimed at investigating the effect of ArA on osteoclast formation and bone resorption function in vitro, as well as wear particle-induced osteolysis in vivo, and to explore its molecular mechanism of action. Here, we report that ArA inhibits RANKL-stimulated osteoclast formation and function. Mechanistically, ArA suppresses intracellular reactive oxygen species levels by activating the antioxidant response via nuclear factor erythroid-2-related factor 2 (Nrf2) pathway upregulation. It also inhibits the mitogen-activated kinases (MAPK) and nuclear factor-κB (NF-κB) pathways, as well as the transcription and expression of NFATc1 and c-Fos. In vivo experiments demonstrated that ArA reduces osteoclast formation and alleviates titanium particle-induced calvarial osteolysis. Collectively, our study highlights that ArA, with its osteoprotective and antioxidant effects, is a promising therapeutic agent for preventing and treating PPO and other osteoclast-mediated osteolytic diseases.

The effect of cytokines on osteoblasts and osteoclasts in bone remodeling in osteoporosis: a review

The complicated connections and cross talk between the skeletal system and the immune system are attracting more attention, which is developing into the field of Osteoimmunology. In this field, cytokines that are among osteoblasts and osteoclasts play a critical role in bone remodeling, which is a pathological process in the pathogenesis and development of osteoporosis. Those cytokines include the tumor necrosis factor (TNF) family, the interleukin (IL) family, interferon (IFN), chemokines, and so on, most of which influence the bone microenvironment, osteoblasts, and osteoclasts. This review summarizes the effect of cytokines on osteoblasts and osteoclasts in bone remodeling in osteoporosis, aiming to providing the latest reference to the role of immunology in osteoporosis.

The Diverse Roles of 17β-Estradiol in Non-Gonadal Tissues and Its Consequential Impact on Reproduction in Laying and Broiler Breeder Hens

Estradiol-17β (E2) has long been studied as the primary estrogen involved in sexual maturation of hens. Due to the oviparous nature of avian species, ovarian production of E2 has been indicated as the key steroid responsible for activating the formation of the eggshell and internal egg components in hens. This involves the integration and coordination between ovarian follicular development, liver metabolism and bone physiology to produce the follicle, yolk and albumen, and shell, respectively. However, the ability of E2 to be synthesized by non-gonadal tissues such as the skin, heart, muscle, liver, brain, adipose tissue, pancreas, and adrenal glands demonstrates the capability of this hormone to influence a variety of physiological processes. Thus, in this review, we intend to re-establish the role of E2 within these tissues and identify direct and indirect integration between the control of reproduction, metabolism, and bone physiology. Specifically, the sources of E2 and its activity in these tissues via the estrogen receptors (ERα, ERβ, GPR30) is described. This is followed by an update on the role of E2 during sexual differentiation of the embryo and maturation of the hen. We then also consider the implications of the recent discovery of additional E2 elevations during an extended laying cycle. Next, the specific roles of E2 in yolk formation and skeletal development are outlined. Finally, the consequences of altered E2 production in mature hens and the associated disorders are discussed. While these areas of study have been previously independently considered, this comprehensive review intends to highlight the critical roles played by E2 to alter and coordinate physiological processes in preparation for the laying cycle.

Transcriptomic Analysis Provides Insights to Reveal the bmp6 Function Related to the Development of Intermuscular Bones in Zebrafish

Intermuscular bones (IBs) are small, hard-boned spicules located in the muscle tissue that mainly exist in the myosepta of lower teleosts, which hurt the edibleness and economic value of fish. The study of the development of IBs is very important for freshwater aquaculture fish, but the molecular mechanism of its formation and the key regulatory genes remain unclear. In this study, we first constructed two types of zebrafish mutants (the mutants losing IBs and the mutants with partial deletion of IBs) by knocking out bmp6. We then carried out a transcriptomic analysis to reveal the role of bmp6 in the developmental mechanism of IBs; we used the caudal musculoskeletal tissues of these mutants and wild-type zebrafish at three development stages (20, 45, and 60 dph) to perform transcriptomic analysis. The results showed that the deficiency of bmp6 upregulated sik1 and activated the TNF-A signaling via the NF-KB pathway, which inhibited the development of osteoblasts and promoted osteoclast formation, thereby inhibiting the formation of IBs. These results provided insights to understand the role of bmp6 in the development of IBs in zebrafish and are useful for selective breeding of IBs in cyprinids.

Proinflammatory Microenvironment During Kingella kingae Infection Modulates Osteoclastogenesis

Kingella kingae is an emerging pathogen that causes septic arthritis, osteomyelitis, and bacteremia in children from 6 to 48 months of age. The presence of bacteria within or near the bone is associated with an inflammatory process that results in osteolysis, but the underlying pathogenic mechanisms involved are largely unknown. To determine the link between K. kingae and bone loss, we have assessed whether infection per se or through the genesis of a pro-inflammatory microenvironment can promote osteoclastogenesis. For that purpose, we examined both the direct effect of K. kingae and the immune-mediated mechanism involved in K. kingae-infected macrophage-induced osteoclastogenesis. Our results indicate that osteoclastogenesis is stimulated by K. kingae infection directly and indirectly by fueling a potent pro-inflammatory response that drives macrophages to undergo functional osteoclasts via TNF-α and IL-1β induction. Such osteoclastogenic capability of K. kingae is counteracted by their outer membrane vesicles (OMV) in a concentration-dependent manner. In conclusion, this model allowed elucidating the interplay between the K. kingae and their OMV to modulate osteoclastogenesis from exposed macrophages, thus contributing to the modulation in joint and bone damage.

'Educated' Osteoblasts Reduce Osteoclastogenesis in a Bone-Tumor Mimetic Microenvironment

Breast cancer (BC) metastases to bone disrupt the balance between osteoblasts and osteoclasts, leading to excessive bone resorption. We identified a novel subpopulation of osteoblasts with tumor-inhibitory properties, called educated osteoblasts (EOs). Here we sought to examine the effect of EOs on osteoclastogenesis during tumor progression. We hypothesized that EOs affect osteoclast development in the bone-tumor niche, leading to suppressed pre-osteoclast fusion and bone resorption. Conditioned media (CM) was analyzed for protein expression of osteoclast factors receptor activator of nuclear factor kappa-β ligand (RANKL), osteoprotegerin (OPG), and tumor necrosis factor alpha (TNFα) via ELISA. EOs were co-cultured with pre-osteoclasts on a bone mimetic matrix to assess osteoclast resorption. Pre-osteoclasts were tri-cultured with EOs plus metastatic BC cells and assessed for tartrate-resistance acid phosphatase (TRAP)-positive, multinucleated (≥3 nuclei), mature osteoclasts. Tumor-bearing murine tibias were stained for TRAP to determine osteoclast number in-vivo. EO CM expressed reduced amounts of soluble TNFα and OPG compared to naïve osteoblast CM. Osteoclasts formed in the presence of EOs were smaller and less in number. Upon co-culture on a mimetic bone matrix, a 50% reduction in the number of TRAP-positive osteoclasts formed in the presence of EOs was observed. The tibia of mice inoculated with BC cells had less osteoclasts per bone surface in bones with increased numbers of EO cells. These data suggest EOs reduce osteoclastogenesis and bone resorption. The data imply EOs provide a protective effect against bone resorption in bone metastatic BC.

Inhibitory effects of biochanin A on titanium particle-induced osteoclast activation and inflammatory bone resorption via NF-κB and MAPK pathways

Revision operations have become a new issue after successful artificial joint replacements, and periprosthetic osteolysis leading to prosthetic loosening is the main cause of why the overactivation of osteoclasts (OCs) plays an important role. The effect of biochanin A (BCA) has been examined in osteoporosis, but no study on the role of BCA in prosthetic loosening osteolysis has been conducted yet. In this study, we utilised enzyme-linked immunosorbent assay, computed tomography imaging, and histological analysis. Results showed that BCA downregulated the secretion levels of tumor necrosis factor-α, interleukin-1α (IL-1α), and IL-1β to suppress inflammatory responses. The secretion levels of receptor-activated nuclear factor-κB ligand, CTX-1, and osteoclast-associated receptor as well as Ti-induced osteolysis were also reduced. BCA effectively inhibited osteoclastogenesis and suppressed hydroxyapatite resorption by downregulating OC-related genes in vitro. Analysis of mechanisms indicated that BCA inhibited the signalling pathways of mitogen-activated protein kinase (P38, extracellular signal-regulated kinase, and c-JUN N-terminal kinase) and nuclear factor-κB (inhibitor κB-α and P65), thereby downregulating the expression of nuclear factor of activated T cell 1 and c-Fos. In conclusion, BCA may be an alternative choice for the prevention of prosthetic loosening caused by OCs.

The protective effect of WKYMVm peptide on inflammatory osteolysis through regulating NF-κB and CD9/gp130/STAT3 signalling pathway

The balance between bone formation and bone resorption is closely related to bone homeostasis. Osteoclasts, originating from the monocyte/macrophage lineage, are the only cell type possessing bone resorption ability. Osteoclast overactivity is thought to be the major reason underlying osteoclast‐related osteolytic problems, such as Paget's disease, aseptic loosening of prostheses and inflammatory osteolysis; therefore, disruption of osteoclastogenesis is considered a crucial treatment option for these issues. WKYMVm, a synthetic peptide, which is a potent FPR2 agonist, exerts an immunoregulatory effect. This peptide inhibits the production of inflammatory cytokines, such as (IL)‐1β and TNF‐α, thus regulating inflammation. However, there are only few reports on the role of WKYMVm and FPR2 in osteoclast cytology. In the current study, we found that WKYMVm negatively regulates RANKL‐ and lipopolysaccharide (LPS)‐induced osteoclast differentiation and maturation in vitro and alleviates LPS‐induced osteolysis in animal models. WKYMVm down‐regulated the expression of osteoclast marker genes and resorption activity. Furthermore, WKYMVm inhibited osteoclastogenesis directly through reducing the phosphorylation of STAT3 and NF‐kB and indirectly through the CD9/gp130/STAT3 pathway. In conclusion, our findings demonstrated the potential medicinal value of WKYMVm for the treatment of inflammatory osteolysis.

Zoledronic acid inhibits osteoclast differentiation and function through the regulation of NF-κB and JNK signalling pathways

It is well known that extensive osteoclast formation plays a key role in osteoporosis in post‑menopausal women and the elderly. The suppression of extensive osteoclastogenesis and bone resorption may be an effective preventive strategy for osteoporosis. Zoledronic acid (ZOL) has been indicated to play an essential role in regulating bone mineral density and has already been used in large clinical trials. However, the effects of ZOL on osteoclastogenesis remain to be fully elucidated. Therefore, the present study aimed to determine the effects of ZOL on osteoclastogenesis, and to explore the corresponding signalling pathways. By using a cell viability assay, as well as in vitro osteoclastogenesis, immunofluorescence and resorption pit assays, we demonstrated that ZOL (0.1‑5 µM) suppressed receptor activator of nuclear factor‑κB ligand (RANKL)‑induced osteoclast differentiation and bone resorptive activity. Furthermore, western blot analysis and reverse transcription‑quantitative PCR indicated that ZOL inhibited the RANKL‑induced activation of NF‑κB and the phosphorylation of JNK in RAW264.7 cells, and subsequently decreased the expression of osteoclastogenesis‑associated genes, including calcitonin receptor, tartrate‑resistant acid phosphatase and dendritic cell‑specific transmembrane protein. ZOL inhibited osteoclast formation and resorption in vitro by specifically suppressing NF‑κB and JNK signalling. On the whole, the findings of this study indicate that ZOL may serve as a potential agent for the treatment of osteoclast‑associated diseases, including osteoporosis.

Rhoifolin ameliorates titanium particle-stimulated osteolysis and attenuates osteoclastogenesis via RANKL-induced NF-κB and MAPK pathways

Prosthesis loosening is a highly troublesome clinical problem following total joint arthroplasty. Wear-particle-induced osteoclastogenesis has been shown to be the primary cause of periprosthetic osteolysis that eventually leads to aseptic prosthesis loosening. Therefore, inhibiting osteoclastogenesis is a promising strategy to control periprosthetic osteolysis. The possible mechanism of action of rhoifolin on osteoclastogenesis and titanium particle-induced calvarial osteolysis was examined in this study. The in vitro study showed that rhoifolin could strongly suppress the receptor activators of nuclear factor-κB (NF-κB) ligand-stimulated osteoclastogenesis, hydroxyapatite resorption, F-actin formation, and the gene expression of osteoclast-related genes. Western blot analysis illustrated that rhoifolin could attenuate the NF-κB and mitogen-activated protein kinase pathways, and the expression of transcriptional factors nuclear factor of activated T cells 1 (NFATc1) and c-Fos. Further studies indicated that rhoifolin inhibited p65 translocation to the nucleus and the activity of NFATc1 and NF-κB rhoifolin could decrease the number of tartrate-resistant acid phosphate-positive osteoclasts and titanium particle-induced C57 mouse calvarial bone loss in vivo. In conclusion, our results suggest that rhoifolin can ameliorate the osteoclasts-stimulated osteolysis, and may be a potential agent for the treatment of prosthesis loosening.

B. Abortus Modulates Osteoblast Function Through the Induction of Autophagy

Osteoarticular brucellosis is the most common localization of human active disease. Osteoblasts are specialized mesenchymal-derived cells involved in bone formation and are considered as professional mineralizing cells. Autophagy has been involved in osteoblast metabolism. The present study demonstrates that Brucella abortus infection induces the activation of the autophagic pathway in osteoblast cells. Autophagy was revealed by upregulation of LC3II/LC3I ratio and Beclin-1 expression as well as inhibition of p62 expression in infected cells. Induction of autophagy was also corroborated by using the pharmacological inhibitors wortmannin, a PI 3-kinase inhibitor, and leupeptin plus E64 (inhibitors of lysosomal proteases). Autophagy induction create a microenvironment that modifies osteoblast metabolism by the inhibition of the deposition of organic and mineral matrix, the induction of matrix metalloproteinase (MMP)-2, osteopontin, and RANKL secretion leading to bone loss. Accordingly, autophagy is also involved in the down-modulation of the master transcription factor in bone formation osterix during B. abortus infection. Taking together our results indicate that B. abortus induces the activation of autophagy pathway in osteoblast cells and this activation is involved in the modulation of osteoblast function and bone formation.

Creating diversity in mammalian facial morphology: a review of potential developmental mechanisms

Mammals (class Mammalia) have evolved diverse craniofacial morphology to adapt to a wide range of ecological niches. However, the genetic and developmental mechanisms underlying the diversification of mammalian craniofacial morphology remain largely unknown. In this paper, we focus on the facial length and orofacial clefts of mammals and deduce potential mechanisms that produced diversity in mammalian facial morphology. Small-scale changes in facial morphology from the common ancestor, such as slight changes in facial length and the evolution of the midline cleft in some lineages of bats, could be attributed to heterochrony in facial bone ossification. In contrast, large-scale changes of facial morphology from the common ancestor, such as a truncated, widened face as well as the evolution of the bilateral cleft possessed by some bat species, could be brought about by changes in growth and patterning of the facial primordium (the facial processes) at the early stages of embryogenesis.

CCL4 enhances preosteoclast migration and its receptor CCR5 downregulation by RANKL promotes osteoclastogenesis

Chemokine CCL4 (MIP-1β) is released from osteoblast cells to restore the homeostasis of hematopoietic stem cells during the activation of bone marrow. In this study, we investigated the function of CCL4 and its receptor CCR5 during osteoclastogenesis. CCL4 promoted the migration and viability of preosteoclast cells. However, CCL4 had no direct effect on the receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation in mouse preosteoclast cells. In addition, CCR5 expression was rapidly reduced by RANKL treatment, which was recovered by IFN-γ during osteoclastogenesis. CCR5 downregulation by RANKL was mediated by MEK and JNK in preosteoclast cells and promoted osteoclastogenesis. These results suggest that CCL4 can enhance the recruitment of preosteoclasts to bone in the early stage, and the reduction of CCR5 promotes osteoclastogenesis when RANKL is prevalent.

Cathepsin K Inhibitors for Osteoporosis: Biology, Potential Clinical Utility, and Lessons Learned

Cathepsin K is a cysteine protease member of the cathepsin lysosomal protease family. Although cathepsin K is highly expressed in osteoclasts, lower levels of cathepsin K are also found in a variety of other tissues. Secretion of cathepsin K from the osteoclast into the sealed osteoclast-bone cell interface results in efficient degradation of type I collagen. The absence of cathepsin K activity in humans results in pycnodysostosis, characterized by increased bone mineral density and fractures. Pharmacologic cathepsin K inhibition leads to continuous increases in bone mineral density for ≤5 years of treatment and improves bone strength at the spine and hip. Compared with other antiresorptive agents, cathepsin K inhibition is nearly equally efficacious for reducing biochemical markers of bone resorption but comparatively less active for reducing bone formation markers. Despite multiple efforts to develop cathepsin K inhibitors, potential concerns related to off-target effects of the inhibitors against other cathepsins and cathepsin K inhibition at nonbone sites, including skin and perhaps cardiovascular and cerebrovascular sites, prolonged the regulatory approval process. A large multinational randomized, double-blind phase III study of odanacatib in postmenopausal women with osteoporosis was recently completed. Although that study demonstrated clinically relevant reductions in fractures at multiple sites, odanacatib was ultimately withdrawn from the regulatory approval process after it was found to be associated with an increased risk of cerebrovascular accidents. Nonetheless, the underlying biology and clinical effects of cathepsin K inhibition remain of considerable interest and could guide future therapeutic approaches for osteoporosis.

Theaflavin-3,3'-digallate represses osteoclastogenesis and prevents wear debris-induced osteolysis via suppression of ERK pathway

Peri-implant osteolysis (PIO) and the following aseptic loosening is the leading cause of implant failure. Emerging evidence suggests that receptor activator of nuclear factor kappa-B ligand (RANKL)-induced osteoclast formation and osteoclastic bone resorption are responsible for particle-stimulated PIO. Here, we explored the effect of theaflavin-3,3'-digallate (TF3) on titanium particle-induced osteolysis in vivo and in vitro. Twenty-eight male C57BL/6 mice were randomly separated into four groups: sham control (sham), titanium particles only (titanium), titanium particles with low TF3 concentration (low-TF3, 1mg/kg TF3), and titanium particles with high TF3 concentration (high-TF3, 10mg/kg TF3). Two weeks later, micro-computed tomography and histological analysis were performed. Bone-marrow-derived macrophages and RAW264.7 murine macrophages were applied to examine osteoclast formation and differentiation. TF3 significantly inhibited titanium particle-induced osteolysis and prevented bone destruction compared with titanium group. Interestingly, the number of mature osteoclasts reduced after treatment with TF3 in vivo, suggesting osteoclast formation might be inhibited by TF3. In vitro, TF3 suppressed osteoclast formation, polarization and osteoclastic bone resorption by specifically targeting the RANKL-induced ERK signal pathway. Collectively, these results suggest that TF3, a natural active compound derived from black tea, is a promising candidate for the treatment of osteoclast-related osteolytic diseases, such as wear debris-induced PIO.

STATEMENT OF SIGNIFICANCE

Total joint arthroplasty is widely accepted for the treatment of end-stage joint diseases. However, it is reported that aseptic loosening, initiated by peri-implant osteolysis, is the major reason for prosthesis failure. Although the pathophysiology of PIO remains unclear, increasing evidence indicates that osteoclasts are excessively activated at the implant site by wear debris from materials. Here, we demonstrated that theaflavin-3,3'-digallate, a natural active compound derived from black tea, inhibited osteoclast formation and osteoclastic bone resorption mainly via suppressing the ERK pathway. Moreover, the findings of this study have confirmed for the first time that theaflavin-3,3'-digallate has a protective effect on particle-induced osteolysis in a mouse calvarial model, thus preventing bone loss. These results indicate that theaflavin-3,3'-digallate may be a suitable therapeutic agent to treat wear debris-induced peri-implant osteolysis.

Ethanolic extract of Schizonepeta tenuifolia attenuates osteoclast formation and activation in vitro and protects against lipopolysaccharide-induced bone loss in vivo

Background

Excessive osteoclast activity is a major cause of metabolic bone disorders, such as osteopenia, rheumatoid arthritis, and osteoporosis. Thus, discovery of agents targeting osteoclast differentiation and bone resorption is important for development of novel treatments for bone diseases. It has been demonstrated that ethanolic extract of schizonepeta tenuifolia (EEST) has potent anti-oxidant and anti-inflammatory activities. However, the beneficial effects of EEST on bone metabolism have not been studied. Therefore, we intend to investigate the effects of EEST on osteoclast differentiation.

Methods

We examined the effects and mechanisms of action of the EEST on osteoclastogenesis in vitro in bone marrow macrophages (BMMs) stimulated with receptor activator of nuclear factor kappa-B ligand (RANKL) and in vivo using a mouse model of lipopolysaccharide (LPS)-induced bone destruction.

Results

We found that EEST inhibited phosphorylation of Akt and IkB at early stages of RANKL-induced osteoclastogenesis. Furthermore, EEST negatively controlled the transcription and translation levels of nuclear factor of activated T cells c1 (NFATc1) and the translation level of c-Fos at the final stage of osteoclast differentiation. Reflecting these effects, EEST blocked both filamentous actin (F-actin) ring formation and bone resorbing activity of mature osteoclasts in vitro. The inhibitory effects of EEST on osteoclast formation and activity were observed in an LPS-mediated bone erosion mouse model using micro-CT and histological analysis.

Conclusions

EEST is a potential agent that is able to treat osteoclast-related bone diseases, such as osteoporosis.

Recombinant Human Endostatin Suppresses Mouse Osteoclast Formation by Inhibiting the NF-κB and MAPKs Signaling Pathways

Rheumatoid arthritis is an autoimmune disease characterized by synovial hyperplasia and progressive joint destruction. As reported previously, recombinant human endostatin (rhEndostatin) is associated with inhibition of joint bone destruction present in rat adjuvant-induced arthritis; however, the effect of rhEndostatin on bone destruction is not known. This study was designed to assess the inhibitory effect and mechanisms of rhEndostatin on formation and function of osteoclasts in vitro, and to gain insight into the mechanism underlying the inhibitory effect of bone destruction. Bone marrow-derived macrophages isolated from BALB/c mice were stimulated with receptor activator of NF-κB ligand (RANKL) and macrophage colony-stimulating factor to establish osteoclast formation. Osteoclast formation was determined by TRAP staining. Cell viability of BMMs affected by rhEndostatin was determined using a MTT assay. Bone resorption was examined with a bone resorption pits assay. The expression of osteoclast-specific markers was analyzed using quantitative real-time PCR. The related signaling pathways were examined using a Luciferase reporter assay and western blot analysis. Indeed, rhEndostatin showed a significant reduction in the number of osteoclast-like cells and early-stage bone resorption. Moreover, molecular analysis demonstrated that rhEndostatin attenuated RANKL-induced NF-κB signaling by inhibiting the phosphorylation of IκBα and NF-κB p65 nuclear translocation. Furthermore, rhEndostatin significantly inhibited the activation of RANKL-dependent mitogen-activated protein kinases, such as ERK1/2, JNK, and p38. Hence, we demonstrated for the first time that preventing the formation and function of osteoclasts is an important anti-bone destruction mechanism of rhEndostatin, which might be useful in the prevention and treatment of bone destruction in RA.

Brucella abortus-infected B cells induce osteoclastogenesis

Brucella abortus is an intracellular bacterium that establishes lifelong infections in livestock and humans although the mechanisms of its chronicity are poorly understood. Activated B cells have long lifespan and B. abortus infection activates B cells. Our results indicate that the direct infection of B cells with B. abortus induced matrix metalloproteinase-9 (MMP-9), receptor activator for NF κB ligand (RANKL), tumor necrosis factor (TNF)-α and interleukin (IL)-6 secretion. In addition, supernatants from B. abortus-infected B cells induced bone marrow-derived monocytes to undergo osteoclastogenesis. Using osteoprotegerin, RANKL's decoy receptor, we determined that RANKL is involved in osteoclastogenesis induced by supernatants from B. abortus-infected B cells. The results presented here shed light on how the interactions of B. abortus with B cells may have a role in the pathogenesis of brucellar osteoarticular disease.

Brucella abortus Invasion of Osteocytes Modulates Connexin 43 and Integrin Expression and Induces Osteoclastogenesis via Receptor Activator of NF-κB Ligand and Tumor Necrosis Factor Alpha Secretion

Osteoarticular brucellosis is the most common localization of human active disease. Osteocytes are the most abundant cells of bone. They secrete factors that regulate the differentiation of both osteoblasts and osteoclasts during bone remodeling. The aim of this study is to determine if Brucella abortus infection modifies osteocyte function. Our results indicate that B. abortus infection induced matrix metalloproteinase 2 (MMP-2), receptor activator for NF-κB ligand (RANKL), proinflammatory cytokines, and keratinocyte chemoattractant (KC) secretion by osteocytes. In addition, supernatants from B. abortus-infected osteocytes induced bone marrow-derived monocytes (BMM) to undergo osteoclastogenesis. Using neutralizing antibodies against tumor necrosis factor alpha (TNF-α) or osteoprotegerin (OPG), RANKL's decoy receptor, we determined that TNF-α and RANKL are involved in osteoclastogenesis induced by supernatants from B. abortus-infected osteocytes. Connexin 43 (Cx43) and the integrins E11/gp38, integrin-α, integrin-β, and CD44 are involved in cell-cell interactions necessary for osteocyte survival. B. abortus infection inhibited the expression of Cx43 but did not modify the expression of integrins. Yet the expression of both Cx43 and integrins was inhibited by supernatants from B. abortus-infected macrophages. B. abortus infection was not capable of inducing osteocyte apoptosis. However, supernatants from B. abortus-infected macrophages induced osteocyte apoptosis in a dose-dependent manner. Taken together, our results indicate that B. abortus infection could alter osteocyte function, contributing to bone damage.

Leonurine hydrochloride inhibits osteoclastogenesis and prevents osteoporosis associated with estrogen deficiency by inhibiting the NF-κB and PI3K/Akt signaling pathways

Osteoclasts, the primary bone resorbing cells, are responsible for destructive bone diseases such as postmenopausal osteoporosis, rheumatoid arthritis, and periodontitis. Many plant-derived traditional medicines that might suppress the formation and/or function of osteoclasts are promising treatments for osteoclast-related diseases. In this study, we investigated the effects of leonurine hydrochloride (LH) on receptor activator NF-κB ligand (RANKL)-induced osteoclastogenesis and ovariectomy-induced bone loss. LH is a synthetic chemical compound based on the structure of leonurine, which is found in motherwort and has been reported to exhibit phytoestrogenic activity. In RAW 264.7 cells and mouse bone marrow monocytes (BMMs), LH suppressed RANKL-induced osteoclastogenesis and actin ring formation in a dose-dependent manner. LH targeted RANKL-induced osteoclastogenesis and bone resorption at an early stage. Molecular analysis demonstrated that LH attenuated RANKL-induced NF-κB signaling by inhibiting the phosphorylation and degradation of IκBα and NF-κB p65 nuclear translocation. LH inhibited the RANK-TRAF6 association triggered by RANKL binding and the phosphatidylinositol 3-kinase (PI3K)/Akt axis, without significantly affecting the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) and AP-1 signaling pathways. LH attenuated the RANKL-stimulated expression of osteoclast-related genes including NFATc1, tartrate resistant acid phosphatase (TRAP), cathepsin K, and osteoclast-associated receptor (OSCAR). Consistent with the in vitro results, LH administration attenuated osteoclast activity, thus preventing bone loss caused by estrogen deficiency in mice. In this study, LH suppressed RANKL-induced osteoclastogenesis via RANK-TRAF6, NF-κB, and PI3K/Akt signaling. These data provide the first evidence that LH might be a promising therapeutic compound to treat osteoclast-related diseases, such as osteoporosis.

Quetiapine inhibits osteoclastogenesis and prevents human breast cancer-induced bone loss through suppression of the RANKL-mediated MAPK and NF-κB signaling pathways

Bone loss is one of the major complications of advanced cancers such as breast cancer, prostate cancer, and lung cancer. Extensive research has revealed that the receptor activator of NF-κB ligand (RANKL), which is considered to be a key factor in osteoclast differentiation, plays an important role in cancer-associated bone resorption. Therefore, agents that can suppress this bone loss have therapeutic potential. In this study, we detected whether quetiapine (QUE), a commonly used atypical antipsychotic drug, can inhibit RANKL-induced osteoclast differentiation in vitro and prevent human breast cancer-induced bone loss in vivo. RAW 264.7 cells and bone marrow-derived macrophages (BMMs) were used to detect inhibitory effect of QUE on osteoclastogenesis in vitro. Mouse model of breast cancer metastasis to bone was used to test suppressive effect of QUE on breast cancer-induced bone loss in vivo. Our results show that QUE can inhibit RANKL-induced osteoclast differentiation from RAW 264.7 cells and BMMs without signs of cytotoxicity. Moreover, QUE reduced the occurrence of MDA-MB-231 cell-induced osteolytic bone loss by suppressing the differentiation of osteoclasts. Finally, molecular analysis revealed that it is by inhibiting RANKL-mediated MAPK and NF-κB signaling pathways that QUE suppressed the osteoclast differentiation. We demonstrate, for the first time, the novel suppressive effects of QUE on RANKL-induced osteoclast differentiation in vitro and human breast cancer-induced bone loss in vivo, suggesting that QUE may be a potential therapeutic drug for osteolysis treatment.

Control of CSF-1 induced inflammation in teleost fish by a soluble form of the CSF-1 receptor

The colony-stimulating factor-1 (CSF-1) is the principal regulator of the survival, proliferation, differentiation, and function of macrophages and their precursors, and has been shown to play a role in the etiology of inflammation. We recently identified a novel mechanism for the control of CSF-1 activity in teleost fish, through the production of an inhibitory soluble form of the CSF-1 receptor (sCSF-1R). Primary goldfish kidney macrophages selectively expressed sCSF-1R during the senescence phase, which corresponds to a defined stage of in vitro culture development where inhibition of macrophage proliferation and apoptotic cell death are prominent. In contrast, primary macrophage cultures undergoing active proliferation displayed low levels of sCSF-1R expression. Addition of purified recombinant sCSF-1R to developing primary macrophage cultures leads to a dose-dependent decrease in macrophage proliferation and inhibits macrophage antimicrobial functions including chemotaxis, phagocytosis, and production of reactive oxygen intermediates. Using a goldfish in vivo model of self-resolving peritonitis, we found that sCSF-1R plays a role in the inhibition of inflammation, following an initial acute phase of antimicrobial responses within an inflammatory site. Soluble CSF-1R inhibits pro-inflammatory cytokine production, inhibits leukocyte recruitment to the inflammatory site and decreases ROS production in a dose-dependent manner. This sCSF-1R-dependent regulation of inflammation appears to be an elegant mechanism for the control of macrophage numbers at inflammatory sites of lower vertebrates. Overall, our results provide new insights into the evolutionary origins of the CSF-1 immune regulatory axis.

Dual Effect of Chrysanthemum indicum Extract to Stimulate Osteoblast Differentiation and Inhibit Osteoclast Formation and Resorption In Vitro

The risk of bone-related diseases increases due to the imbalance between bone resorption and bone formation by osteoclasts and osteoblasts, respectively. The goal in the development of antiosteoporotic treatments is an agent that will improve bone through simultaneous osteoblast stimulation and osteoclast inhibition without undesirable side effects. To achieve this goal, numerous studies have been performed to identify novel approaches using natural oriental herbs to treat bone metabolic diseases. In the present study, we investigated the effect of Chrysanthemum indicum extract (CIE) on the differentiation of osteoclastic and osteoblastic cells. CIE inhibited the formation of TRAP-positive mature osteoclasts and of filamentous-actin rings and disrupted the bone-resorbing activity of mature osteoclasts in a dose-dependent manner. CIE strongly inhibited Akt, GSK3β, and IκB phosphorylation in RANKL-stimulated bone marrow macrophages and did not show any effects on MAP kinases, including p38, ERK, and JNK. Interestingly, CIE also enhanced primary osteoblast differentiation via upregulation of the expression of alkaline phosphatase and the level of extracellular calcium concentrations during the early and terminal stages of differentiation, respectively. Our results revealed that CIE could have a potential therapeutic role in bone-related disorders through its dual effects on osteoclast and osteoblast differentiation.

Effects of extracellular magnesium on the differentiation and function of human osteoclasts

Magnesium-based implants have been shown to influence the surrounding bone structure. In an attempt to partially reveal the cellular mechanisms involved in the remodelling of magnesium-based implants, the influence of increased extracellular magnesium content on human osteoclasts was studied. Peripheral blood mononuclear cells were driven towards an osteoclastogenesis pathway via stimulation with receptor activator of nuclear factor kappa-B ligand and macrophage colony-stimulating factor for 28 days. Concomitantly, the cultures were exposed to variable magnesium concentrations (from either magnesium chloride or magnesium extracts). Osteoclast proliferation and differentiation were evaluated based on cell metabolic activity, total protein content, tartrate-resistant acid phosphatase activity, cathepsin K and calcitonin receptor immunocytochemistry, and cellular ability to form resorption pits. While magnesium chloride first enhanced and then opposed cell proliferation and differentiation in a concentration-dependent manner (peaking between 10 and 15mM magnesium chloride), magnesium extracts (with lower magnesium contents) appeared to decrease cell metabolic activity (≈50% decrease at day 28) while increasing osteoclast activity at a lower concentration (twofold higher). Together, the results indicated that (i) variations in the in vitro extracellular magnesium concentration affect osteoclast metabolism and (ii) magnesium extracts should be used preferentially in vitro to more closely mimic the in vivo environment.

Salivary biomarker analysis complementing regular clinical examination

AIM

Tools for the diagnosis and evaluation of the severity and prognosis of periodontitis are currently insufficient. The aim of the current study was to find an array of salivary biomarkers that would be both sensitive and specific enough to be used as a complement to regular clinical examination. Furthermore, we investigated salivary markers of successful periodontal treatment to elucidate the underlying mechanism of disease.

PATIENTS & METHODS

Saliva was collected from periodontitis patients during illness and following successful disease termination. Parameters measured included sodium, potassium, chloride, calcium, phosphate, magnesium, total protein, Alb, LDH, amylase, IL-1α, uric acid and carbonyls.

RESULTS

Seven of the parameters analyzed were significantly altered when the disease is active and may be considered biomarkers.

CONCLUSION

Salivary composition reflects disease severity and response to therapy, suggesting the efficacy of saliva monitoring for periodontal disease status and care.

Lysophosphatidic acid stimulates osteoclast fusion through OC-STAMP and P2X7 receptor signaling

Bone is continuously remodeled by bone formation and resorption, and cooperative bone metabolism is precisely regulated to maintain homeostasis. Osteoclasts, which are responsible for bone resorption, are differentiated through multiple steps that include cell fusion at the last step of differentiation, yielding multinuclear cells. However, the factors involved in and the precise mechanism of cell fusion are still unknown. To determine the molecules involved in osteoclast fusion, we examined the effect of lysophosphatidic acid (LPA), which has been reported to participate in the progression of cancer bone metastasis. LPA had no effect on osteoclast formation and bone resorption under receptor activator of nuclear factor kappa B ligand (RANKL) conditions, whereas LPA stimulated osteoclast fusion, thereby causing increased osteoclast diameter and bone resorptive capacity under a RANKL-limited condition. This result encouraged us to assess what molecules are needed for LPA-stimulated osteoclast fusion. Interestingly, LPA stimulated osteoclast stimulatory transmembrane protein (OC-STAMP) and P2X7 receptor mRNA expression during osteoclast fusion under a RANKL limiting condition. siRNA-induced OC-STAMP or P2X7 receptor knockdown significantly suppressed the LPA-stimulated increase in osteoclast diameter and bone resorptive capacity in differentiating cultures. Using cyclosporin A as an inhibitor, we revealed that NF-ATc1 directly regulates OC-STAMP and P2X7 receptor expression during LPA-stimulated osteoclast fusion. These results suggest that LPA is a critical regulator of osteoclast fusion by inducing the OC-STAMP and P2X7 receptor. Therefore, LPA signaling might be useful to help understand their effects on osteoclast formation and as a therapeutic target for patients with pathologically increased osteoclast formation.

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.

Silicate modulates the cross-talk between osteoblasts (SaOS-2) and osteoclasts (RAW 264.7 cells): inhibition of osteoclast growth and differentiation

It has been shown that inorganic monomeric and polymeric silica/silicate, in the presence of the biomineralization cocktail, increases the expression of osteoprotegerin (OPG) in osteogenic SaOS-2 sarcoma cells in vitro. In contrast, silicate does not affect the steady-state gene expression level of the osteoclastogenic ligand receptor activator of NF-κB ligand (RANKL). In turn it can be expected that the concentration ratio of the mediators OPG/RANKL increases in the presence of silicate. In addition, silicate enhances the growth potential of SaOS-2 cells in vitro, while it causes no effect on RAW 264.7 cells within a concentration range of 10-100 µM. Applying a co-cultivation assay system, using SaOS-2 cells and RAW 264.7 cells, it is shown that in the presence of 10 µM silicate the number of RAW 264.7 cells in general, and the number of TRAP(+) RAW 264.7 cells in particular markedly decreases. The SaOS-2 cells retain their capacity of differential gene expression of OPG and RANKL in favor of OPG after exposure to silicate. It is concluded that after exposure of the cells to silicate a factor(s) is released from SaOS-2 cells that causes a significant inhibition of osteoclastogenesis of RAW 264.7 cells. It is assumed that it is an increased secretion of the cytokine OPG that is primarily involved in the reduction of the osteoclastogenesis of the RAW 264.7 cells. It is proposed that silicate might have the potential to stimulate osteogenesis in vivo and perhaps to ameliorate osteoporotic disorders.

Myricetin suppresses LPS-induced MMP expression in human gingival fibroblasts and inhibits osteoclastogenesis by downregulating NFATc1 in RANKL-induced RAW 264.7 cells

OBJECTIVE

Periodontitis is an inflammatory disease that affects connective tissue attachments and the supporting bone that surrounds the teeth. Gingival fibroblasts induce the overexpression of matrix metalloproteinase (MMP), which is involved in inflammatory progression in periodontitis. Osteoclasts are responsible for skeletal modeling and remodeling but may also destroy bone in several bone diseases, including osteoporosis and periodontitis. This study examined the anti-destructive effects of myricetin on human gingival fibroblasts (HGF) under lipopolysaccharide- (LPS-) induced inflammatory conditions, and the anti-osteoclastogenetic effect of myricetin on the receptor activator of NF-κB ligand (RANKL) induced RAW264.7 cells was also investigated.

DESIGN

The effects of myricetin on HGF were determined by measuring the cell viability and mRNA expression and enzyme activity of tissue-destructive proteins, including MMP-1, MMP-2 and MMP-8. The effects of myricetin on osteoclasts were examined by measuring the following: (1) the cell viability, (2) the formation of tartrate-resistant acid phosphatase (TRAP)(+) multinucleated cells, (3) MAPK signalling pathways (4) mRNA expression of osteoclast-associated genes and (5) tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) secretion.

RESULTS

The myricetin had no effects on the cell viability of the HGF and decreased the mRNA expression and enzyme activity of MMP-1, MMP-2 and MMP-8 in the HGF. Myricetin inhibited the formation of RANKL-stimulated TRAP(+) multinucleated cells. Myricetin also inhibited the RANKL-stimulated activation of p-38, ERK and cSrc signaling, and inhibited the RANKL-stimulated degradation of I(k)B in the RAW264.7 cells. In addition, the RANKL-stimulated induction of NFATc1 transcription factors was abrogated by myricetin. Myricetin decreased the mRNA expression of osteoclast-associated genes, including cFOS, TRAP and cathepsin K in the RAW264.7 cells. Myricetin inhibited the secretion of LPS-induced TNF-α and IL-1β in the RAW264.7 cells.

CONCLUSIONS

These findings suggest that myricetin has therapeutic effects on bone-destructive processes, such as those that occur in periodontal diseases.

Capric acid inhibits NO production and STAT3 activation during LPS-induced osteoclastogenesis

Capric acid is a second medium-chain fatty acid, and recent studies have shown that fatty acids are associated with bone density and reduce bone turnover. In this study, we investigated the effects of capric acid on lipopolysaccharide (LPS)-induced osteoclastogenesis in RAW264.7 cells. After treatment with capric acid (1 mM), the number of tartrate resistant acid phosphatase (TRAP)-positive cells decreased significantly. Capric acid reduced LPS-induced TRAP expression, an osteoclast differentiation marker, without inhibiting cell viability. LPS strongly upregulated inducible nitric oxide synthase (iNOS) mRNA levels and nitric oxide (NO) production, whereas capric acid inhibited them. Furthermore, capric acid also inhibited monocyte chemoattractant protein-1 (MCP-1) mRNA expression. Subsequently, we investigated various intracellular signaling proteins, including nuclear factor-κB (NF-κB), c-Jun-N-terminal kinase (JNK), extracellular signal regulated kinase 1/2 (ERK1/2), and signal transducer and activator of transcription 1 (STAT1) and STAT3 associated with osteoclastogenesis. Capric acid had no effects on LPS-induced activation of the NF-κB, JNK, ERK1/2, and STAT1 pathways. However, capric acid inhibited LPS-induced phosphorylation of Ser⁷²⁷ in STAT3. Additionally, stattic (a STAT3 inhibitor) inhibited LPS-induced iNOS and MCP-1 gene expression. In conclusion, we demonstrated that capric acid inhibited LPS-induced osteoclastogenesis by suppressing NO production via the STAT3 pathway. These results suggest that capric acid has important therapeutic implications for treating bone diseases associated with excessive osteoclastogenesis.

Actions of fibroblast growth factor-8 in bone cells in vitro

The fibroblast growth factors (FGFs) are a group of at least 25 structurally related peptides that are involved in many biological processes. Some FGFs are active in bone, including FGF-1, FGF-2, and FGF-18, and recent evidence indicates that FGF-8 is osteogenic, particularly in mesenchymal stem cells. In the current study, we found that FGF-8 was expressed in rat primary osteoblasts and in osteoblastic UMR-106 and MC3T3-E1 cells. Both FGF-8a and FGF-8b potently stimulated the proliferation of osteoblastic cells, whereas they inhibited the formation of mineralized bone nodules in long-term cultures of osteoblasts and reduced the levels of osteoblast differentiation markers, osteocalcin, and bone sialoprotein. FGF-8a induced the phosphorylation of p42/p44 mitogen-activated protein kinase (MAPK) in osteoblastic cells; however, its mitogenic actions were not blocked by either the MAPK kinase (MEK) inhibitor U-0126 or the PI 3-kinase (PI3K) inhibitor LY-294002. Interestingly, FGF-8a, unlike FGF-8b and other members of the family, inhibited osteoclastogenesis in mouse bone marrow cultures, and this was via a receptor activator of NF-kappaB ligand (RANKL)/osteoprotegerin (OPG)-independent manner. However, FGF-8a did not affect osteoclastogenesis in RAW 264.7 cells (a macrophage cell line devoid of stromal cells) exogenously stimulated by RANKL, nor did it affect mature osteoclast function as assessed in rat calvarial organ cultures and isolated mature osteoclasts. In summary, we have demonstrated that FGF-8 is active in bone cells, stimulating osteoblast proliferation in a MAPK-independent pathway and inhibiting osteoclastogenesis via a RANKL/OPG-independent mechanism. These data suggest that FGF-8 may have a physiological role in bone acting in an autocrine/paracrine manner.

High oxygen tension prolongs the survival of osteoclast precursors via macrophage colony-stimulating factor

The oxygen tension affects the function, differentiation, and transformation of various cells, including bone cells. In pathological conditions such as rheumatoid arthritis (RA), rapidly destructive arthropathy, and primary or metastatic tumors, severe bone destruction or osteolysis occurs. Abundant blood vessels are often observed around these destructive lesions. At such sites, we have confirmed the increased production of reactive oxygen species (ROS) induced by a high oxygen tension and/or oxidative stress, as well as numerous osteoclasts detectable by immunohistochemistry. These findings suggest that osteoclasts are influenced by the high oxygen tension in pathological bone lesions because the zone around blood vessels has a relatively high oxygen tension. In this study, we investigated the effects of oxygen tension on osteoclastogenesis by culturing human CD14-positive cells (osteoclast precursors) with or without osteoblast-like supporting cells (Saos-4/3 cells) under a normal oxygen tension (20% O(2)) or a high oxygen tension (40% O(2)). A high oxygen tension markedly prolonged the duration of osteoclast precursor formation in the presence of supporting cells, and also markedly and persistently increased the production of macrophage colony stimulating factor (M-CSF) by supporting cells. Furthermore, we found an increase of cells expressing M-CSF and cells positive for tartrate-resistant acid phosphatase (TRAP) in hypervascular destructive bone lesions of RA patients where ROS were also abundant.

Cannabinoids and the skeleton: from marijuana to reversal of bone loss

The active component of marijuana, Delta(9)-tetrahydrocannabinol, activates the CB1 and CB2 cannabinoid receptors, thus mimicking the action of endogenous cannabinoids. CB1 is predominantly neuronal and mediates the cannabinoid psychotropic effects. CB2 is predominantly expressed in peripheral tissues, mainly in pathological conditions. So far the main endocannabinoids, anandamide and 2-arachidonoylglycerol, have been found in bone at 'brain' levels. The CB1 receptor is present mainly in skeletal sympathetic nerve terminals, thus regulating the adrenergic tonic restrain of bone formation. CB2 is expressed in osteoblasts and osteoclasts, stimulates bone formation, and inhibits bone resorption. Because low bone mass is the only spontaneous phenotype so far reported in CB2 mutant mice, it appears that the main physiologic involvement of CB2 is associated with maintaining bone remodeling at balance, thus protecting the skeleton against age-related bone loss. Indeed, in humans, polymorphisms in CNR2, the gene encoding CB2, are strongly associated with postmenopausal osteoporosis. Preclinical studies have shown that a synthetic CB2-specific agonist rescues ovariectomy-induced bone loss. Taken together, the reports on cannabinoid receptors in mice and humans pave the way for the development of 1) diagnostic measures to identify osteoporosis-susceptible polymorphisms in CNR2, and 2) cannabinoid drugs to combat osteoporosis.

The immune response is involved in atherosclerotic plaque calcification: could the RANKL/RANK/OPG system be a marker of plaque instability?

Atherogenesis is characterized by an intense inflammatory process, involving immune and vascular cells. These cells play a crucial role in all phases of atherosclerotic plaque formation and complication through cytokine, protease, and prothrombotic factor secretion. The accumulation of inflammatory cells and thus high amounts of soluble mediators are responsible for the evolution of some plaques to instable phenotype which may lead to rupture. One condition strongly associated with plaque rupture is calcification, a physiopathological process orchestrated by several soluble factors, including the receptor activator of nuclear factor (NF)κB ligand (RANKL)/receptor activator of nuclear factor (NF)κB (RANK)/osteoprotegerin (OPG) system. Although some studies showed some interesting correlations with acute ischemic events, at present, more evidences are needed to evaluate the predictive and diagnostic value of serum sRANKL and OPG levels for clinical use. The major limitation is probably the poor specificity of these factors for cardiovascular disease. The identification of tissue-specific isoforms could increase the importance of sRANKL and OPG in predicting calcified plaque rupture and the dramatic ischemic consequences in the brain and the heart.

Building bone to reverse osteoporosis and repair fractures

An important, unfilled clinical need is the development of new approaches to improve fracture healing and to treat osteoporosis by increasing bone mass. Recombinant forms of bone morphogenetic protein 2 (BMP2) and BMP7 are FDA approved to promote spinal fusion and fracture healing, respectively, and the first FDA-approved anabolic drug for osteoporosis, parathyroid hormone, increases bone mass when administered intermittently but can only be given to patients in the US for two years. As we discuss here, the tremendous explosion over the last two decades in our fundamental understanding of the mechanisms of bone remodeling has led to the prospect of mechanism-based anabolic therapies for bone disorders.

A p38 mitogen-activated protein kinase inhibitor arrests active alveolar bone loss in a rat periodontitis model

BACKGROUND

Gram-negative bacterial species, such as Actinobacillus actinomycetemcomitans, contain lipopolysaccharide (LPS) that initiates the innate immune system, resulting in inflammatory alveolar bone loss. LPS activates Toll-like receptors on membrane surfaces, stimulating many intracellular signaling cascades, including the p38 mitogen-activated protein kinase (MAPK). Activation of p38 signaling mediates inflammatory cytokine expression, contributing toward osteoclastogenesis and bone loss. The aim of this study was to determine whether the novel, orally active p38 MAPK inhibitor SD282 could arrest progression of LPS-induced alveolar bone destruction in rats.

METHODS

Three groups of female Sprague-Dawley rats received LPS injections to the palatal molar gingiva three times per week for 4 weeks to establish periodontitis. From weeks 5 through 8, two groups received the drug SD282 (N = 14) or 1% polyethylene glycol drug vehicle (N = 14) via oral gavage in addition to LPS injections. The third group continued to receive only LPS injections (N = 8). Microcomputed tomography was used to measure volumetric alveolar bone loss, expressed as bone volume fraction (BVF). Expression of interleukin (IL)-1 and -6 and tumor necrosis factor-alpha (TNF-alpha) was assessed by immunohistochemistry, and osteoclasts were enumerated by tartrate-resistant acid phosphatase staining.

RESULTS

By 4 weeks, severe alveolar bone resorption was seen in LPS-injected animals. Administration of SD282 significantly blocked additional volumetric bone loss in the LPS-only versus LPS + SD282 groups (0.37 +/- 0.01 BVF versus 0.43 +/- 0.01 BVF; P < 0.01). Significant reductions in IL-1beta (P < 0.01 ), TNF-alpha (P < 0.05), and osteoclast formation (P < 0.01) occurred in the presence of SD282.

CONCLUSIONS

An orally active p38 MAPK inhibitor reduced LPS-induced inflammatory cytokine expression, osteoclastogenesis, and alveolar bone loss in rats. Within the limits of the current study, SD282 arrested periodontal disease progression, thus highlighting the therapeutic potential of this novel class of inhibitors.

Cannabinoid receptors and the regulation of bone mass

A functional endocannabinoid system is present in several mammalian organs and tissues. Recently, endocannabinoids and their receptors have been reported in the skeleton. Osteoblasts, the bone forming cells, and osteoclasts, the bone resorbing cells, produce the endocannabinoids anandamide and 2-arachidonoylglycerol and express CB2 cannabinoid receptors. Although CB2 has been implicated in pathological processes in the central nervous system and peripheral tissues, the skeleton appears as the main system physiologically regulated by CB2. CB2-deficient mice show a markedly accelerated age-related bone loss and the CNR2 gene (encoding CB2) in women is associated with low bone mineral density. The activation of CB2 attenuates ovariectomy-induced bone loss in mice by restraining bone resorption and enhancing bone formation. Hence synthetic CB2 ligands, which are stable and orally available, provide a basis for developing novel anti-osteoporotic therapies. Activation of CB1 in sympathetic nerve terminals in bone inhibits norepinephrine release, thus balancing the tonic sympathetic restrain of bone formation. Low levels of CB1 were also reported in osteoclasts. CB1-null mice display a skeletal phenotype that is dependent on the mouse strain, gender and specific mutation of the CB1 encoding gene, CNR1.

Bacterial lipopolysaccharide induces osteoclast formation in RAW 264.7 macrophage cells

Lipopolysaccharide (LPS) is a potent bone resorbing factor. The effect of LPS on osteoclast formation was examined by using murine RAW 264.7 macrophage cells. LPS-induced the formation of multinucleated giant cells (MGC) in RAW 264.7 cells 3 days after the exposure. MGCs were positive for tartrate-resistant acid phosphatase (TRAP) activity. Further, MGC formed resorption pits on calcium-phosphate thin film that is a substrate for osteoclasts. Therefore, LPS was suggested to induce osteoclast formation in RAW 264.7 cells. LPS-induced osteoclast formation was abolished by anti-tumor necrosis factor (TNF)-alpha antibody, but not antibodies to macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor (NF)-kappaB ligand (RANKL). TNF-alpha might play a critical role in LPS-induced osteoclast formation in RAW 264.7 cells. Inhibitors of NF-kappaB and stress activated protein kinase (SAPK/JNK) prevented the LPS-induced osteoclast formation. The detailed mechanism of LPS-induced osteoclast formation is discussed.

Effect of YM529 on a model of mandibular invasion by oral squamous cell carcinoma in mice

PURPOSE

This study examined the mechanisms of osteoclast-mediated bone invasion in a model of oral squamous cell carcinoma (OSCC). C3H/HeN mice were inoculated with SCC VII cells into the masseter region to establish an animal model of mandibular invasion by OSCC.

EXPERIMENTAL DESIGN

The mice were divided into three groups: a control group, given daily s.c. injections of saline; group 1, given 2 microg per mouse per day of the bisphosphonate YM529; and group 2, given 10 microg per mouse per day of YM529. After 3 weeks of treatment, the lesions were studied by micro-computed tomography. After tartrate-resistant acid phosphatase (TRAP) staining, the osteoclasts were easily identified, and the percentages of the area occupied by osteoclasts were calculated by computer for each sample. The tumors were analyzed by RT-PCR to determine the mRNA expression of interleukin-6 (IL-6), parathyroid hormone-related protein (PTHrP), tumor necrosis factor-alpha (TNF-alpha), receptor activator of nuclear factor-kappaB (RANK), RANK ligand (RANKL), and osteoprotegerin.

RESULTS

SCC VII cells rapidly multiplied in the masseter muscle of the mice. Bone invasion was evident only in the control group on micro-computed tomography. On TRAP-stained slices, the percentages of osteoclasts in groups 1 and 2 were significantly lower than that in the control group. The mRNA expressions of IL-6, PTHrP, THF-alpha, and RANK decreased as the concentration of YM529 increased.

CONCLUSIONS

We conclude that various cancer-derived cytokines play important roles in the invasion of bone by OSCC. YM529, a third-generation bisphosphonate, can suppress osteoclast-mediated bone invasion by OSCC. The mechanism of this effect might involve inhibition of cytokines such as IL-6, PTHrP, TNF-alpha, and RANK by YM529.

RANK ligand and TNF-alpha mediate acid-induced bone calcium efflux in vitro

Chronic metabolic acidosis stimulates net calcium efflux from bone due to increased osteoclastic bone resorption and decreased osteoblastic collagen synthesis. Previously, we determined that incubation of neonatal mouse calvariae in medium simulating physiological metabolic acidosis leads to a significant, cyclooxygenase-dependent, increase in RNA for bone cell receptor activator of NF-kappaB ligand (RANKL) compared with incubation in neutral pH medium. In this study, we tested the hypothesis that the acid-mediated increase in RANKL expression is a primary mechanism for the stimulated osteoclastic resorption. Acid medium increased the medium concentration of sRANKL without altering the concentration of the decoy receptor osteoprotegerin (OPG). Inhibition of the RANKL pathway with concentrations of OPG up to 25 ng/ml, far greater than physiological, did not significantly decrease the robust acid-induced Ca efflux from bone nor did incubation of the calvariae with a different inhibitor, RANK/Fc (up to 50 ng/ml). Thus acid-induced net Ca efflux appears to involve mechanisms in addition to the RANK/RANKL pathway. Osteoblasts also produce TNF-alpha, another cytokine that stimulates the maturation and activity of osteoclasts. Incubation of calvariae in acid medium caused a significant increase in TNF-alpha levels. Incubation of calvariae with anti-TNF (up to 250 ng/ml) did not significantly decrease acid-induced net Ca efflux. However, the combination of RANK/Fc plus anti-TNF caused a significant but subtotal reduction in acid-induced Ca efflux, whereas the combination of RANK/Fc plus an isotype-matched control for the anti-TNF had no effect on Ca release. Thus simultaneous inhibition of RANKL and TNF-alpha is necessary to reduce acid-induced, cell-mediated net Ca efflux from bone; however, additional osteoblast-produced factors must also be involved in acid-induced, cell-mediated bone resorption.

Roles of stromal cell RANKL, OPG, and M-CSF expression in biphasic TGF-beta regulation of osteoclast differentiation

To better understand the complex roles of transforming growth factor-beta (TGF-beta) in bone metabolism, we examined the impact of a range of TGF-beta concentrations on osteoclast differentiation. In co-cultures of support cells and spleen or marrow osteoclast precursors, low TGF-beta concentrations stimulated while high concentrations inhibited differentiation. We investigated the influences of TGF-beta on macrophage colony stimulating factor (M-CSF), receptor activator of NF-kappaB ligand (RANKL), and osteoprotegerin (OPG) expression and found a dose dependent inhibition of M-CSF expression. RANKL expression was elevated at low TGF-beta concentrations with a less dramatic increase in OPG. Addition of OPG blocked differentiation at the stimulatory TGF-beta dose. Thus, low TGF-beta concentrations elevated the RANKL/OPG ratio while high concentrations did not, supporting that, at low TGF-beta concentrations, there is sufficient M-CSF and a high RANKL/OPG ratio to stimulate differentiation. At high TGF-beta concentrations, the RANKL/OPG ratio and M-CSF expression were both repressed and there was no differentiation. We examined whether TGF-beta-mediated repression of osteoclasts differentiation is due to these changes by adding M-CSF and/or RANKL and did not observe any impact on differentiation repression. We studied direct TGF-beta impacts on osteoclast precursors by culturing spleen or marrow cells with M-CSF and RANKL. TGF-beta treatment dose-dependently stimulated osteoclast differentiation. These data indicate that low TGF-beta levels stimulate osteoclast differentiation by impacting the RANKL/OPG ratio while high TGF-beta levels repress osteoclast differentiation by multiple avenues including mechanisms independent of the RANKL/OPG ratio or M-CSF expression regulation.

IL-6, LIF, and TNF-α regulation of GM-CSF inhibition of osteoclastogenesis in vitro

During pathological bone loss, factors that are both stimulatory and inhibitory for osteoclast differentiation are over-expressed. Despite the presence of inhibitory factors, osteoclast differentiation is significantly enhanced to bring about bone loss. To examine the hypothesis that stimulatory growth factors overcome the effects of inhibitory factors, we have examined the ability of IGF-I, IGF-II, IL-6, LIF, and TNF-alpha to overcome osteoclast differentiation inhibition by GM-CSF in vitro. Osteoclast numbers were significantly elevated by treatment with IGF-I, IGF-II, IL-6, LIF, or TNF-alpha alone whereas GM-CSF treatment of stromal cell and osteoclast co-cultures inhibited osteoclast formation. IL-6, LIF, or TNF-alpha, individually overcame GM-CSF inhibition whereas neither IGF-I nor IGF-II treatment overcame GM-CSF inhibition. Interestingly, GM-CSF addition with either IL-6 or TNF-alpha increased osteoclast numbers beyond that seen with either IL-6 or TNF-alpha alone. Combined treatment with TNF-alpha and IL-6 showed a significant increase in osteoclast numbers with GM-CSF addition. Examination of the impacts of these growth factors individually or in combinations on stromal cell M-CSF, RANKL, and OPG expression revealed a complex pattern involving alterations in the ratio of RANKL to OPG and/or M-CSF expression as candidate mechanisms of action.