Osteoblastic cells induce fusion and activation of osteoclasts through a mechanism independent of macrophage-colony-stimulating factor production

Effects of anti-RANKL antibodies administered to pregnant mice on bone and tooth development in neonates

OBJECTIVES

This study examined how the anti-bone resorptive agent denosumab, which comprises anti-receptor activator of nuclear factor kappa B ligand (anti-RANKL) monoclonal antibodies, administered during pregnancy affected neonatal development. Anti-RANKL antibodies, which are known to bind to mouse RANKL and inhibit osteoclast formation, were administered to pregnant mice. Following this, the survival, growth, bone mineralization, and tooth development of their neonates were analyzed.

METHODS

Anti-RANKL antibodies (5 mg/kg) were injected into pregnant mice on day 17 of gestation. After parturition, their neonatal offspring underwent microcomputed tomography at 24 h and at 2, 4, and 6 weeks after birth. Three-dimensional bone and teeth images were subjected to histological analysis.

RESULTS

Approximately 70% of the neonatal mice born to mice who received anti-RANKL antibodies died within 6 weeks after birth. These mice had a significantly lower body weight and significantly higher bone mass compared with the control group. Furthermore, delayed tooth eruption and abnormal tooth morphology (eruption length, enamel surface, and cusps) were observed. Conversely, while the tooth germ shape and mothers against decapentaplegic homolog 1/5/8 expression remained unchanged at 24 h after birth in the neonatal mice born to mice that received anti-RANKL antibodies, osteoclasts were not formed.

CONCLUSIONS

These results suggest that anti-RANKL antibodies administered to mice in the late stage of pregnancy results in adverse events in their neonatal offspring. Thus, it is speculated that administering denosumab to pregnant humans will affect fetal development and growth after birth.

Rac1 Inhibition Via Srgap2 Restrains Inflammatory Osteoclastogenesis and Limits the Clastokine, SLIT3

The Rac1-specific guanosine triphosphatase (GTPase)-activating protein Slit-Robo GAP2 (Srgap2) is dramatically upregulated during RANKL-induced osteoclastogenesis. Srgap2 interacts with the cell membrane to locally inhibit activity of Rac1. In this study, we determined the role of Srgap2 in the myeloid lineage on bone homeostasis and the osteoclastic response to TNFα treatment. The bone phenotype of mice specifically lacking Srgap2 in the myeloid lineage (Srgap2 ^f/f :LysM-Cre; Srgap2 conditional knockout [cKO]) was investigated using histomorphometric analysis, in vitro cultures and Western blot analysis. Similar methods were used to determine the impact of TNFα challenge on osteoclast formation in Srgap2 cKO mice. Bone parameters in male Srgap2 cKO mice were unaffected. However, female cKO mice displayed higher trabecular bone volume due to increased osteoblast surface and bone formation rate, whereas osteoclastic parameters were unaltered. In vitro, cells from Srgap2 cKO had strongly enhanced Rac1 activation, but RANKL-induced osteoclast formation was unaffected. In contrast, conditioned medium from Srgap2 cKO osteoclasts promoted osteoblast differentiation and had increased levels of the bone anabolic clastokine SLIT3, providing a possible mechanism for increased bone formation in vivo. Rac1 is rapidly activated by the inflammatory cytokine TNFα. Supracalvarial injection of TNFα caused an augmented osteoclastic response in Srgap2 cKO mice. In vitro, cells from Srgap2 cKO mice displayed increased osteoclast formation in response to TNFα. We conclude that Srgap2 plays a prominent role in limiting osteoclastogenesis during inflammation through Rac1, and restricts expression of the paracrine clastokine SLIT3, a positive regulator of bone formation. © 2019 American Society for Bone and Mineral Research.

The Use of Denosumab in Osteoblastoma of the Metacarpal

Osteoblastoma is a benign aggressive primary bone tumor that occasionally presents in the hand. Because surgical treatment remains the mainstay of treatment, there are no established nonsurgical pharmacological options for patients in whom resection is not feasible. Novel therapies, such as denosumab, are currently being investigated in primary bone tumors. We report a case of osteoblastoma of the first metacarpal that was successfully treated with denosumab. The patient showed a rapid and dramatic response to treatment that led to the transformation of a locally destructive tumor into an ossified painless mass, restoring function while avoiding surgery.

Substance P modulates bone remodeling properties of murine osteoblasts and osteoclasts

Clinical observations suggest neuronal control of bone remodeling. Sensory nerve fibers innervating bone, bone marrow and periosteum signal via neurotransmitters including substance P (SP). In previous studies we observed impaired biomechanical and structural bone parameters in tachykinin (Tac) 1-deficient mice lacking SP. Here, we aim to specify effects of SP on metabolic parameters of bone marrow macrophage (BMM)/osteoclast cultures and osteoblasts isolated from Tac1-deficient and wildtype (WT) mice. We demonstrated endogenous SP production and secretion in WT bone cells. Absence of SP reduced bone resorption rate, as we found reduced numbers of precursor cells (BMM) and multinucleated osteoclasts and measured reduced cathepsin K activity in Tac1-/- BMM/osteoclast cultures. However, this might partly be compensated by reduced apoptosis rate and increased fusion potential of Tac1-/- precursor cells to enlarged "super" osteoclasts. Contrarily, increased ALP enzyme activity and apoptosis rate during early osteoblast differentiation accelerated osteogenesis and cell death in the absence of SP together with reduced ALP activity of Tac1-/- osteoblasts during late osteogenic differentiation resulting in reduced bone formation at later stages. Therefore, we suggest that absence of SP presumably results in a slight reduction of bone resorption rate but concomitantly in a critical reduction of bone formation and mineralization rate.

Neoadjuvant denosumab for the treatment of a sacral osteoblastoma

PURPOSE

To present a case of aggressive sacral osteoblastoma (OB) treated with neoadjuvant denosumab therapy and en bloc resection.

METHODS

Case report of a 14-year-old male with an aggressive OB affecting the superior articular process of the left first sacral segment. The lesion was lytic and metabolically active and involved the left-sided posterior elements of S1-S3 with extension into the spinal canal, affecting the left S1, S2, S3, S4 and S5 nerve roots. He was treated for 1 month with neoadjuvant denosumab followed by en bloc resection.

RESULTS

Denosumab therapy caused regression of the tumour and converted the diffuse infiltrative mass into a well-defined solid (osteoma-like) structure, aiding surgical resection and preserving the S1, S4 and S5 nerve roots. Histologically, the treated lesion showed abundant sclerotic woven bone and osteoblasts with absence of osteoclasts.

CONCLUSIONS

A short course of denosumab caused tumour regression, ossification and conversion of an aggressive OB into a sclerotic, well-defined lesion thus aiding surgical resection and preservation of neural structures. Neoadjuvant therapy reduced osteoclast numbers but PET showed that the lesion remained FDG avid post-therapy.

Myristoleic acid inhibits osteoclast formation and bone resorption by suppressing the RANKL activation of Src and Pyk2

Cytoskeletal changes in osteoclasts such as formation of actin ring is required for bone-resorbing activity. The tyrosine kinase Src is a key player in massive cytoskeletal change of osteoclasts, thereby in bone destruction. In order for Src to be activated, trafficking to the inner plasma membrane via myristoylation is of importance. A previous study reported that myristoleic acid derived from myristic acid, inhibited N-myristoyl-transferase, an essential enzyme for myristoylation process. This prompted us to investigate whether myristoleic acid could affect osteoclastogenesis. Indeed, we observed that myristoleic acid inhibited RANKL-induced osteoclast formation in vitro, especially, at later stages of differentiation. Myristoleic acid attenuated the tyrosine phosphorylation of c-Src and Pyk2, which associates with Src, by RANKL. When myristoleic acid was co-administered with soluble RANKL into mice, RANKL-induced bone loss was substantially prevented. Bone dissection clearly revealed that the number of multinucleated osteoclasts was significantly diminished by myristoleic acid. On the other hand, myristoleic acid treatment had little or no influence on early osteoclast differentiation markers, such as c-Fos and NFATc1, and proteins related to cytoskeletal rearrangement, including DC-STAMP, integrin αv and integrin β3 in vitro. Taken together, our data suggest that myristoleic acid is capable of blocking the formation of large multinucleated osteoclasts and bone resorption likely through suppressing activation of Src and Pyk2.

Enterococcus faecalis promotes osteoclastogenesis and semaphorin 4D expression

Enterococcus faecalis is considered a major bacterial pathogen implicated in endodontic infections and contributes considerably to periapical periodontitis. This study aimed to investigate the potential mechanisms by which E. faecalis accounts for the bone destruction in periapical periodontitis in vitro. Osteoclast precursor RAW264.7 cells were treated with E. faecalis ATCC 29212 and a wild strain of E. faecalis derived clinically from an infected root canal. The results showed that, to some extent, E. faecalis induced the RAW264.7 cells to form tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclast-like cells. This pathogen markedly stimulated RAW264.7 cells to express semaphorin 4D (Sema4D), which inhibits bone formation. Once RAW264.7 cells were primed by low-dose receptor activator of nuclear factor-kappa B ligand (RANKL), E. faecalis could significantly increase the production of TRAP-positive multinucleated cells and up-regulate the expression of osteoclast-specific markers, including NFATc1, TRAP and cathepsin K. Both p38 and ERK1/2 MAPK signaling pathways were activated by E. faecalis in RANKL-primed RAW264.7 cells, and meanwhile the expression of Sema4D was highly increased. In conclusion, E. faecalis may greatly contribute to the bone resorption in periapical periodontitis by promoting RANKL-dependent osteoclastogenesis and expression of Sema4D through activation of p38 and ERK1/2 MAPK signaling pathways.

Natural polyamines inhibit the migration of preosteoclasts by attenuating Ca2+-PYK2-Src-NFATc1 signaling pathways

Natural polyamines have numerous biological activities. Several studies have reported their beneficial role in bone metabolism, but their mode of action is not fully understood. Bone diseases such as osteoporosis, which is characterized by impaired bone structure and low bone mass, are caused by an increased number of osteoclasts and/or overactivation of osteoclastogenesis. Osteoclast differentiation is a multi-complex procedure involving the following sequential steps: differentiation-migration-fusion-resorption. In this study, we found that putrescine, spermidine or spermine inhibited the RANKL-mediated migration of preosteoclasts. Furthermore, the RANKL-mediated activation of the Src-PYK2 signaling axis and of transcription factors such as NF-κB and NFATc1 was prevented by each polyamine. Anti-osteoclastogenic and anti-migration activities of polyamines were confirmed by evaluating their potential to downregulate the mRNA expression levels of osteoclastogenesis-related genes such as OSCAR, TRAP, cathepsin K and c-Src, and genes related to fusion and/or migration of preosteoclasts. Moreover, ATP-mediated elevation of cytosolic free Ca(2+) concentration ([Ca(2+)]i) was strongly inhibited by each polyamine, indicating the involvement of [Ca(2+)]i in the anti-fusion activities of polyamines. In conclusion, polyamines could exhibit anti-osteoclastogenic activity by inhibiting the migration of preosteoclasts via the Ca(2+)-PYK2-Src-NFATc1 signaling axis.

Osteoclast-Forming Suppressive Compounds from Makomotake,Zizania latifoliaInfected withUstilago esculenta

A novel compound (1) and a known one (2) were isolated from Makomotake, Zizania latifolia infected with Ustilago esculenta, as osteoclast-forming suppressive substances.

Suppressive effects of the leaf of Terminalia catappa L. on osteoclast differentiation in vitro and bone weight loss in vivo

Oral administration of Terminalia catappa extract (TCE; 1,000 mg/kg) for 5 wk suppressed bone weight loss and trabecular bone loss in ovariectomized mice. An in vitro experiment showed that TCE (1.3-20 µg/mL) did not increase alkaline phosphatase activity, which would indicate osteoclast formation, in osteoblast-like 3T3-L1 cells. On the other hand, TCE (12.5 µg/mL) markedly decreased the number of tartrate resistant acid phosphatase (TRAP)-positive multinucleated cells, which would indicate osteoclast formation, in a co-culture system (bone marrow cells/osteoblastic UAMS-32 cells). A detailed analysis of the stages of osteoclast differentiation revealed that TCE mainly suppressed the differentiation of bone marrow mononuclear cells into osteoclast progenitor cells in the presence of M-CSF and TGF-β. An additional experiment using fractionated TCE revealed that the water-soluble fraction suppressed the bone weight loss in OVX-mice and osteoclast differentiation in vitro. Therefore, the suppressive effects of TCE on bone weight loss in mice might be due to the suppressive effects of highly polar components on the early stage of osteoclast differentiation.

Osteoclast activated FoxP3+ CD8+ T-cells suppress bone resorption in vitro

Background

Osteoclasts are the body’s sole bone resorbing cells. Cytokines produced by pro-inflammatory effector T-cells (T_EFF) increase bone resorption by osteoclasts. Prolonged exposure to the T_EFF produced cytokines leads to bone erosion diseases such as osteoporosis and rheumatoid arthritis. The crosstalk between T-cells and osteoclasts has been termed osteoimmunology. We have previously shown that under non-inflammatory conditions, murine osteoclasts can recruit naïve CD8 T-cells and activate these T-cells to induce CD25 and FoxP3 (Tc_REG). The activation of CD8 T-cells by osteoclasts also induced the cytokines IL-2, IL-6, IL-10 and IFN-γ. Individually, these cytokines can activate or suppress osteoclast resorption.

Principal Findings

To determine the net effect of Tc_REG on osteoclast activity we used a number of in vitro assays. We found that Tc_REG can potently and directly suppress bone resorption by osteoclasts. Tc_REG could suppress osteoclast differentiation and resorption by mature osteoclasts, but did not affect their survival. Additionally, we showed that Tc_REG suppress cytoskeletal reorganization in mature osteoclasts. Whereas induction of Tc_REG by osteoclasts is antigen-dependent, suppression of osteoclasts by Tc_REG does not require antigen or re-stimulation. We demonstrated that antibody blockade of IL-6, IL-10 or IFN-γ relieved suppression. The suppression did not require direct contact between the Tc_REG and osteoclasts.

Significance

We have determined that osteoclast-induced Tc_REG can suppress osteoclast activity, forming a negative feedback system. As the CD8 T-cells are activated in the absence of inflammatory signals, these observations suggest that this regulatory loop may play a role in regulating skeletal homeostasis. Our results provide the first documentation of suppression of osteoclast activity by CD8 regulatory T-cells and thus, extend the purview of osteoimmunology.

R848, a toll-like receptor 7 agonist, inhibits osteoclast differentiation but not survival or bone-resorbing function of mature osteoclasts

R848, also known as resiquimod, acts as a ligand for toll-like receptor 7 (TLR7) and activates immune cells. In this study, we examined the effects of R848 on differentiation, survival, and bone-resorbing function of osteoclasts. R848 inhibited osteoclast differentiation of mouse bone marrow-derived macrophages (BMMs) and human peripheral blood-derived monocytes induced by receptor activator of NF-κB ligand in a dose-dependent manner. In addition, it inhibited mouse osteoclast differentiation induced in cocultures of bone marrow cells and osteoblasts in the presence of dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]. However, R848 did not affect the survival or bone-resorbing activity of mouse mature osteoclasts. R848 also upregulated the mRNA expression levels of interleukin (IL)-6, IL-12, interferon (IFN)-γ, and inducible nitric oxide synthase in mouse BMMs expressing TLR7. IFN-β was consistently expressed in the BMMs and addition of neutralizing antibodies against IFN-β to the cultures partially recovered osteoclast differentiation inhibited by R848. These results suggest that R848 targets osteoclast precursors and inhibits their differentiation into osteoclasts via TLR7.

Green tea and bone metabolism

Osteoporosis is a major health problem in both elderly women and men. Epidemiological evidence has shown an association between tea consumption and the prevention of age-related bone loss in elderly women and men. Ingestion of green tea and green tea bioactive compounds may be beneficial in mitigating bone loss of this population and decreasing their risk of osteoporotic fractures. This review describes the effect of green tea or its bioactive components on bone health, with an emphasis on (i) the prevalence and etiology of osteoporosis; (ii) the role of oxidative stress and antioxidants in osteoporosis; (iii) green tea composition and bioavailability; (iv) the effects of green tea and its active components on osteogenesis, osteoblastogenesis, and osteoclastogenesis from human epidemiological, animal, as well as cell culture studies; (v) possible mechanisms explaining the osteoprotective effects of green tea bioactive compounds; (vi) other bioactive components in tea that benefit bone health; and (vii) a summary and future direction of green tea and bone health research and the translational aspects. In general, tea and its bioactive components might decrease the risk of fracture by improving bone mineral density and supporting osteoblastic activities while suppressing osteoclastic activities.

Selective inhibition of RANK blocks osteoclast maturation and function and prevents bone loss in mice

Regulation of the formation and function of bone-resorbing osteoclasts (OCs) is a key to understanding the pathogenesis of skeletal disorders. Gene-targeting studies have shown that the RANK signaling pathway plays a critical role in OC differentiation and function. Although pharmaceutical blockade of RANK may be a viable strategy for preventing bone destruction, RANK is implicated in multiple biological processes. Recently, a cytoplasmic motif of RANK was identified that may be specifically involved in OC differentiation. Here, we developed a cell-permeable inhibitor termed the RANK receptor inhibitor (RRI), which targets this motif. The RRI peptide blocked RANKL-induced OC formation from murine bone marrow-derived macrophages. Furthermore, RRI inhibited the resorptive function of OCs and induced OC apoptosis. Treatment with the peptide impaired downstream signaling of RANK linked to Vav3, Rac1, and Cdc42 and resulted in disruptions of the actin cytoskeleton in differentiated OCs. In addition, RRI blocked inflammation-induced bone destruction and protected against ovariectomy-induced bone loss in mice. These data may be useful in the development of selective therapeutic agents for the treatment of osteoporosis and other bone diseases.

v-ATPase V0 subunit d2-deficient mice exhibit impaired osteoclast fusion and increased bone formation

Matrix-producing osteoblasts and bone-resorbing osteoclasts maintain bone homeostasis. Osteoclasts are multinucleated, giant cells of hematopoietic origin formed by the fusion of mononuclear pre-osteoclasts derived from myeloid cells. Fusion-mediated giant cell formation is critical for osteoclast maturation; without it, bone resorption is inefficient. To understand how osteoclasts differ from other myeloid lineage cells, we previously compared global mRNA expression patterns in these cells and identified genes of unknown function predominantly expressed in osteoclasts, one of which is the d2 isoform of vacuolar (H(+)) ATPase (v-ATPase) V(0) domain (Atp6v0d2). Here we show that inactivation of Atp6v0d2 in mice results in markedly increased bone mass due to defective osteoclasts and enhanced bone formation. Atp6v0d2 deficiency did not affect differentiation or the v-ATPase activity of osteoclasts. Rather, Atp6v0d2 was required for efficient pre-osteoclast fusion. Increased bone formation was probably due to osteoblast-extrinsic factors, as Atp6v02 was not expressed in osteoblasts and their differentiation ex vivo was not altered in the absence of Atp6v02. Our results identify Atp6v0d2 as a regulator of osteoclast fusion and bone formation, and provide genetic data showing that it is possible to simultaneously inhibit osteoclast maturation and stimulate bone formation by therapeutically targeting the function of a single gene.

RANKL expression in rat periodontal ligament subjected to a continuous orthodontic force

OBJECTIVES

This study investigated longitudinal changes in receptor activator NF kappa B ligand (RANKL) expression in periodontal ligament (PDL) cells subjected to a continuous orthodontic force.

DESIGN

Fifty-five-day-old male Wistar rats were divided into experimental and control groups. The experimental group had the first molars laterally expanded by a continuous orthodontic force. In each group, the horizontal section specimens were embedded in OTC compound and frozen at 0, 1, 3 and 7 days after the expansion. Sections were observed by immunostaining with anti-RANKL and the tartrate-resistant acid phosphatase (TRAP) staining.

RESULT

Immunoreaction of RANKL and TRAP-positive cells were observed in the distal region of the controls and on the compressed side of the expansion group in the 3 and 7 days. Immunoreaction of RANKL was also observed after 1 day on the compression side of the expansion group, but here TRAP-positive cells were few.

CONCLUSIONS

The experiments have showed that PDL cells are continuously producing RANKL on the PDL pressure side of rats subjected to mechanical stress with a continuous orthodontic force, there was no noticeable the excessive appearance of osteoclasts however. Considering this, it is expected that not only RANKL production but also other cytokines play an important role in the balancing adjustment in the alveolar bone remodeling.

Reveromycin A, an agent for osteoporosis, inhibits bone resorption by inducing apoptosis specifically in osteoclasts

Mature bone-resorbing osteoclasts (OCs) mediate excessive bone loss seen in several bone disorders, including osteoporosis. Here, we showed that reveromycin A (RM-A), a small natural product with three carboxylic groups in its structure, induced apoptosis specifically in OCs, but not in OC progenitors, nonfunctional osteoclasts, or osteoblasts. RM-A inhibited protein synthesis in OCs by selectively blocking enzymatic activity of isoleucyl-tRNA synthetase. The proapoptotic effect of RM-A was inhibited by neutralization or disruption of the acidic microenvironment, a prominent characteristic of OCs. RM-A was incorporated in OCs but not in nonfunctional osteoclasts and OC progenitors in neutral culture medium. Effects of RM-A on OC apoptosis increased under acidic culture conditions. RM-A not only was incorporated, but also induced apoptosis in OC progenitors in acidic culture medium. RM-A inhibited osteoclastic pit formation, decreased prelabeled (45)Ca release in organ cultures, and antagonized increased bone resorption in ovariectomized mice. These results suggested that preventive effects of RM-A on bone resorption in vitro and in vivo were caused by apoptosis through inhibition of isoleucyl-tRNA synthetase in OCs and that specific sensitivity of OCs to RM-A was due to the acidic microenvironment, which increased cell permeability of RM-A by suppressing dissociation of protons from carboxylic acid moieties, making them less polar. This unique mechanism suggested that RM-A might represent a type of therapeutic agent for treating bone disorders associated with increased bone loss.

Effects of osteoprotegerin administration on osteoclast differentiation and trabecular bone structure in osteoprotegerin-deficient mice

Osteoprotegerin (OPG)-deficient mice exhibit severe bone loss including the destruction of growth plate cartilage. Using OPG-deficient mice, we attempted to clarify the differentiation and ultrastructure of osteoclasts located on the destroyed growth plate cartilage and trabecular bone matrix in long bones. In (-/-) homozygous OPG knockout mice, adjacent to the growth plate cartilage, the formation of bone trabeculae without a calcified cartilaginous core resulted in an irregular chondrocyte distribution in the growth plate cartilage. At the metaphyseal ossification center, TRAP-positive osteoclasts showed unusual localization on both type-II collagen-positive cartilage and type-I collagen-positive bone matrix. Osteoclasts located on cartilage matrix lacked a typical ruffled border structure, but formed resorption lacunae. During growth plate cartilage destruction, osteoclasts formed ruffled border structures on bone matrix deposited on the remaining cartilage surfaces. These findings suggest that, in OPG (-/-) mice, osteoclast structure differs, depending on the matrix of either cartilage or bone. Then, we examined the effects of OPG administration on the internal trabecular bone structure and osteoclast differentiation in OPG (-/-) mice. OPG administration to OPG (-/-) mice significantly inhibited trabecular bone loss and maintained the internal trabecular bone structure, but did not reduce the osteoclast number on bone trabeculae. For most osteoclasts, OPG administration caused disappearance or reduction of the ruffled border, but induced neither necrotic nor apoptotic damages. These results suggest that OPG administration is an effective means of maintaining the internal structure and volume of trabecular bone in metabolic bone diseases by inhibition of osteoclastic bone resorption.

Actinobacillus actinomycetemcomitansLipopolysaccharide Activates Matrix Metalloproteinase-2 and Increases Receptor Activator of Nuclear Factor-κB Ligand Expression in Human Periodontal Ligament Cells

BACKGROUND

The lipopolysaccharide (LPS) of A. actinomycetemcomitans is one of the major pathogenic factors in periodontal disease. It induces secretion of proinflammatory cytokines and is involved in alveolar bone destruction. We hypothesized that the LPS of A. actinomycetemcomitans could affect the activation of matrix metalloproteinase (MMP)-2 and the expression of receptor activator of nuclear factor kappa B ligand (RANKL) and osteoprotegerin in human periodontal ligament (HPDL) cells leading to the destruction of periodontium.

METHODS

HPDL cells were cultured in serum-free medium with or without the LPS of A. actinomycetemcomitans for 36 hours. The activation of MMP-2 was analyzed by zymography. Changes of the expression of RANKL and osteoprotegerin (OPG) were examined by reverse transcription-polymerase chain reaction and supported by Western blot analysis.

RESULTS

The activation of MMP-2 could be induced by the LPS of A. actinomycetemcomitans in HPDL cells and could be inhibited by a serine protease inhibitor. This result suggested that the LPS might activate MMP-2 through a serine protease-dependent pathway. This activation was also blocked by NF-kappaB inhibitor, which indicated the involvement of NF-kappaB. The upregulation of RANKL but not OPG by the LPS was found in both transcription and translation and could be reduced by indomethacin. In addition, serine protease inhibitor also inhibited the upregulation of RANKL, suggesting the activity of serine protease.

CONCLUSIONS

The effect of the LPS of A. actinomycetemcomitans on HPDL cells is serum-independent and the induction of the activation of MMP-2 and the expression of RANKL are serine protease-dependent pathways. The results suggest the role of HPDL cells in the pathogenesis of periodontitis.

Destruxins, cyclodepsipeptides, block the formation of actin rings and prominent clear zones and ruffled borders in osteoclasts

Bone-resorbing osteoclasts exhibit polarized morphological structures such as actin rings, clear zones, and ruffled borders. To gain insight into the mechanism of bone-resorbing activity of osteoclast and to discover new types of anti-resorptive agents, we have screened for natural compounds that inhibit the bone-resorbing activity of osteoclast-like multinucleated cells (OCLs). Destruxin B (DestB) and E (DestE), cyclodepsipeptides, were found to inhibit pit formation without affecting osteoclast differentiation and survival. Destruxins reversibly induced morphological changes in OCLs in a dose-dependent manner (DestB, 0.2-1 microM; DestE, 0.01-0.05 microM) and inhibited pit formation. Destruxin-induced morphological changes were accompanied by disruption of the actin rings in OCLs. The formation of actin rings in OCLs after adhesion was also inhibited by destruxins. Electron microscopical analysis revealed that destruxin-treated OCLs on dentine slices have no prominent clear zones and ruffled borders. The effective concentrations of destruxins on the morphological changes were almost the same as those that inhibited bone resorption in organ culture system. These results suggest that the anti-resorptive effects of destruxins result from induction of a disorder of the morphological structures in polarized OCLs.

The receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) is a new chemotactic factor for human monocytes

Bone resorption is regulated by the immune system, where receptor activator of nuclear factor (NF)kappaB ligand (RANKL), a new member of the tumor-necrosis factor family, may contribute to pathological conditions. Due to the role of RANKL in the maturation of monocyte-derived osteoclasts, we hypothesized that RANKL could exert chemotactic properties toward monocytic cells. Our results demonstrate that RANKL induces the migration of MonoMac-6 monocytic cells as well as human freshly isolated total peripheral blood mononuclear cells (PBMC) and CD14+ purified PBMC. RANKL induces the migration of MonoMac-6 cells in a dose-dependent manner and with an efficacy similar to MCP-1. After an 8-h incubation, the soluble form of RANKL (sRANKL) started to exhibit a chemoattractive effect on MonoMac-6 cells, with an increased effect observed up to 24 h. RANKL elicits an additive chemotactic effect to MCP-1. Furthermore, addition of the RANKL decoy receptor osteoprotegerin in the lower well or RANKL in the upper well abrogates the RANKL-induced migration of MonoMac-6 cells, hallmarking a true specific activity. RNase protection assay experiments indicate that exposure of MonoMac-6 cells to RANKL had no significant effect on the expression of a variety of chemokines, known to attract monocytes. This study provides evidence that RANKL behaves as a chemotactic factor for monocytic cells, emphazing the cross-talk between bone and immune systems.

Differentiation and functions of osteoclasts and odontoclasts in mineralized tissue resorption

The differentiation and functions of osteoclasts (OC) are regulated by osteoblast-derived factors such as receptor activator of NFKB ligand (RANKL) that stimulates OC formation, and a novel secreted member of the TNF receptor superfamily, osteoprotegerin (OPG), that negatively regulates osteoclastogenesis. In examination of the preosteoclast (pOC) culture, pOCs formed without any additives expressed tartrate-resistant acid phosphatase (TRAP), but showed little resorptive activity. pOC treated with RANKL became TRAP-positive OC, which expressed intense vacuolar-type H(+)-ATPase and exhibited prominent resorptive activity. Such effects of RANKL on pOC were completely inhibited by addition of OPG. OPG inhibited ruffled border formation in mature OC and reduced their resorptive activity, and also induced apoptosis of some OC. Although OPG administration significantly reduced trabecular bone loss in the femurs of ovariectomized (OVX) mice, the number of TRAP-positive OC in OPG-administered OVX mice was not significantly decreased. Rather, OPG administration caused the disappearance of ruffled borders and decreased H(+)-ATPase expression in most OC. OPG deficiency causes severe osteoporosis. We also examined RANKL localization and OC induction in periodontal ligament (PDL) during experimental movement of incisors in OPG-deficient mice. Compared to wild-type OPG (+/+) littermates, after force application, TRAP-positive OC were markedly increased in the PDL and alveolar bone was severely destroyed in OPG-deficient mice. In both wild-type and OPG-deficient mice, RANKL expression in osteoblasts and fibroblasts became stronger by force application. These in vitro and in vivo studies suggest that RANKL and OPG are important regulators of not only the terminal differentiation of OC but also their resorptive function. To determine resorptive functions of OC, we further examined the effects of specific inhibitors of H(+)-ATPase, bafilomycin A1, and lysosomal cysteine proteinases (cathepsins), E-64, on the ultrastructure, expression of these enzymes and resorptive functions of cultured OC. In bafilomycin A1-treated cultures, OC lacked ruffled borders, and H(+)-ATPase expression and resorptive activity were significantly diminished. E-64 treatment did not affect the ultrastructure and the expression of enzyme molecules in OC, but significantly reduced resorption lacuna formation, by inhibition of cathepsin activity. Lastly, we examined the expression of H(+)-ATPase, cathepsin K, and matrix metalloproteinase-9 in odontoclasts (OdC) during physiological root resorption in human deciduous teeth, and found that there were no differences in the expression of these molecules between OC and OdC. RANKL was also detected in stromal cells located on resorbing dentine surfaces. This suggests that there is a common mechanism in cellular resorption of mineralized tissues such as bone and teeth.

Regulation of osteoclast differentiation and function by receptor activator of NFkB ligand and osteoprotegerin

The differentiation and functions of osteoclasts (OCs) are regulated by osteoblast-derived factors. Receptor activator of NFkB ligand (RANKL) is one of the key regulatory molecules in OC formation. Osteoprotegerin (OPG) is a novel secreted member of the TNF receptor superfamily that negatively regulates osteoclastogenesis and binds to RANKL. We examined the biological actions of macrophage-colony-stimulating factor (M-CSF), RANKL, and OPG on the differentiation of OCs isolated from cocultures of mouse osteoblastic cells and bone marrow cells. Preosteoclasts (pOCs) and OCs were characterized by their ultrastructure and the expression of OC markers such as tartrate-resistant acid phosphatase (TRAP) and vacuolar-type H(+)-ATPase. pOCs formed without any additives expressed TRAP, but showed little resorptive activity on cocultured dentine slices. TRAP-positive pOCs treated with M-CSF began to fuse with each other, but lacked a ruffled border (RB) and showed almost no resorptive activity. pOCs treated with RANKL became TRAP-positive multinucleated cells, which expressed intense vacuolar-type H(+)-ATPase along the RB membranes and exhibited prominent resorptive activity. Such effects of RANKL on pOCs were completely inhibited by the addition of OPG. OPG inhibited RB formation in mature OCs and reduced their resorptive activity, and also induced apoptosis of some OCs. These results suggest that 1) RANKL induces differentiation of functional OCs from pOCs, 2) M-CSF induces macrophage-like multinucleated cells, but not OCs, 3) OPG inhibits RB formation and resorptive activity in mature OCs, 4) OPG also induces apoptosis of OCs, and 5) RANKL and OPG are, therefore, important regulators of not only the terminal differentiation of OCs but also their resorptive function.

Osteoclast induction in periodontal tissue during experimental movement of incisors in osteoprotegerin-deficient mice

Osteoprotegerin (OPG) is a novel secreted member of the tumor necrosis factor (TNF) receptor superfamily that negatively regulates osteoclastogenesis. The receptor activator of the NFKB ligand (RANKL) is one of the key regulatory molecules in osteoclast formation and binds to OPG. In this study, it was suggested that OPG and RANKL are involved in alveolar bone remodeling during orthodontic tooth movement. We examined RANKL localization and osteoclast induction in periodontal tissues during experimental movement of incisors in OPG-deficient mice. To produce orthodontic force, an elastic band was inserted between the upper right and left incisors for 2 or 5 days, and the dissected maxillae were examined for cytochemical and immunocytochemical localization of tartrate-resistant acid phosphatase (TRAP), vacuolar-type H(+)-ATPase, and RANKL. Compared to wild-type OPG (+/+) littermates, TRAP-positive multinucleated cells were markedly induced in the periodontal ligament (PDL) on the compressed side and in the adjacent alveolar bone of OPG-deficient mice. These multinucleated cells exhibited intense vacuolar-type H(+)-ATPase along the ruffled border membranes. Because of accelerated osteoclastic resorption in OPG-deficient mice, alveolar bone was severely destroyed and partially perforated at 2 and 5 days after force application. In both wild-type and OPG-deficient mice, RANKL expression became stronger at 2 and 5 days after force application than before force application. There was no apparent difference in intensity of RANKL expression between OPG (+/+) littermates and OPG-deficient mice. In both wild-type and OPG-deficient mice, expression of RANKL protein was detected in osteoblasts, fibroblasts, and osteoclasts mostly located in resorption lacunae. These results suggest that during orthodontic tooth movement, RANKL and OPG in the periodontal tissues are important determinants regulating balanced alveolar bone resorption.

Fenton reaction is primarily involved in a mechanism of (-)-epigallocatechin-3-gallate to induce osteoclastic cell death

To propose candidates for the prevention or treatment of osteoporosis, we have screened compounds naturally in food for their ability to regulate the differentiation and function of osteoclasts. One of the major green tea flavonoids, (-)-epigallocatechin-3-gallate (EGCG), was found to induce apoptotic cell death of osteoclast-like multinucleated cells after 24 h treatment in a dose-dependent manner (25-100 microM), whereas osteoblasts were not affected. In the present study, we report for the first time a novel cell-death-inducing mechanism triggered by EGCG. The induction of apoptosis by EGCG was suppressed by pretreatment of catalase or calcitonin. It was also suppressed by Fe(III) and Fe(II) chelators. Furthermore, EGCG promoted the reduction of Fe(III) into Fe(II), and the combination of EGCG/Fe(III)/H(2)O(2) induced single-strand DNA breakage in a cell free system. These results indicate that the Fenton reaction is primarily involved in EGCG-induced osteoclastic cell death.

Lipopolysaccharide supports survival and fusion of preosteoclasts independent of TNF-alpha, IL-1, and RANKL

Lipopolysaccharide (LPS), a cell component of Gram-negative bacteria, is a pathogen of inflammatory bone loss. To examine the effects of LPS on the survival and fusion of osteoclasts, mononuclear osteoclasts (preosteoclasts, pOCs) were collected from a mouse co-culture system and cultured in the presence or absence of LPS. Most pOCs died within 24 h in the absence of any stimulus. LPS as well as receptor activator of NF-kappaB ligand (RANKL) supported the survival of pOCs, and induced their fusion to form multinucleated cells (MNCs). Like authentic osteoclasts, MNCs induced by LPS expressed calcitonin receptors, and formed actin rings on culture plates. LPS-induced MNC formation in pOC cultures was observed even in the presence of osteoprotegerin and interleukin (IL)-1-receptor antagonists. MNC formation was also stimulated by LPS in pOC cultures prepared from tumor necrosis factor (TNF)-receptor-I or TNF-receptor-II deficient mice. LPS induced the degradation of IkappaB in pOCs within 20 min. Lactacystin, an inhibitor of NF-kappaB activation, and wortmannin, an inhibitor of phosphatidylinositol-3 kinase, strongly inhibited LPS-induced MNC formation in pOC cultures. LPS induced pit-forming activity of pOCs in the presence of macrophage-colony stimulating factor (M-CSF). These findings suggest that LPS stimulates the survival and fusion of pOCs, independent of RANKL, IL-1 or TNF-alpha action. Activation of NF-kappaB and phosphatidylinositol-3 kinase appeared to be involved in LPS-induced effects on pOCs. These observations suggest that LPS is involved directly in inflammatory bone loss, and also indirectly through the production of LPS-induced host factors such as IL-1 and TNF-alpha.

Immunolocalization of vacuolar-type H+-ATPase, cathepsin K, matrix metalloproteinase-9, and receptor activator of NFkappaB ligand in odontoclasts during physiological root resorption of human deciduous teeth

To investigate the cellular mechanisms of physiological root resorption in human deciduous teeth, the authors examined the immunocytochemical localization of vacuolar-type H+-ATPase, a lysosomal cysteine proteinase, cathepsin K, matrix metalloproteinase-9 (MMP-9), and receptor activator of NFKB ligand (RANKL) in odontoclasts. H+-ATPase, cathepsin K, and MMP-9 are the most important enzymes for decalcification of apatite crystals and degradation of type-I collagen. In addition, RANKL is one of the key regulatory molecules in osteoclast formation and functions. Odontoclasts developed extensive ruffled borders and clear zones apposed to the resorbing root dentine surfaces. On immunoelectron microscopy, the expression of vacuolar-type H+-ATPase was detected along the limiting membranes of pale vacuoles and the ruffled border membranes of odontoclasts. Cathepsin K in odontoclasts was localized within pale vacuoles, lysosomes, the extracellular canals of ruffled borders, and the underlying resorbing dentine surfaces. MMP-9 localization in odontoclasts was similar to those of cathepsin K. RANKL was detected in both mononuclear stromal cells and odontoclasts located on resorbing dentine surfaces. These results suggest that (1) odontoclasts are directly involved in decalcification of apatite crystals by active extrusion of proton ions mediated by H+-ATPase and (2) extracellular degradation of dentine type-I collagen by both cathepsin K and MMP-9, and (3) odontoclast differentiation and activity are regulated, at least in part, by RANKL, possibly produced by mononuclear stromal cells and odontoclasts themselves in the resorbing tissues. Thus, the cellular mechanisms of physiological root resorption appear to be quite similar to those of osteoclastic bone resorption.

Postmenopausal osteoporosis as a failure of bone's adaptation to functional loading: a hypothesis

There is substantial evidence that bones' ability to withstand functional loading without damage depends on the processes of bone modeling and remodeling, which are responsible for establishing and maintaining bone architecture, being influenced by a feedback mechanism related to the control of functional strains. It is probably useful to consider the diminished ability to maintain bone strength in postmenopausal osteoporosis as a failure of this mechanism. Acceptance of this approach would not only increase understanding of the etiology of postmenopausal osteoporosis but also significantly influence the ways in which it is investigated and treated. This would not mean that the many other factors affecting bone mass and bone cell activity will be ignored, but rather these factors will be put in perspective. Research to prevent or treat osteoporosis could be directed usefully to understanding how osteoblasts, lining cells, and osteocytes respond to mechanically derived information and how these responses are converted into stimuli controlling structurally appropriate modeling and remodeling. Evidence suggesting that early strain-related responses of bone cells in males and females involve the estrogen receptor (ER) could explain decreased effectiveness of this pathway when ER levels are low.