HMGB1 expression and release by bone cells

Immune and bone cells are functionally coupled by pro-inflammatory cytokine intercellular signaling networks common to both tissues and their crosstalk may contribute to the etiologies of some immune-associated bone pathologies. For example, the receptor activator of NF-kappaB ligand (RANKL)/osteoprotegerin (OPG)/receptor activator of NF-kappaB (RANK) signaling axis plays a critical role in dendritic cell (DC) function as well as bone remodeling. The expression of RANKL by immune cells may contribute to bone loss in periodontitis, arthritis, and multiple myeloma. A recent discovery reveals that DCs release the chromatin protein high mobility group box 1 (HMGB1) as a potent immunomodulatory cytokine mediating the interaction between DCs and T-cells, via HMGB1 binding to the membrane receptor for advanced glycation end products (RAGE). To determine whether osteoblasts or osteoclasts express and/or release HMGB1 into the bone microenvironment, we analyzed tissue, cells, and culture media for the presence of this molecule. Our immunohistochemical and immunocytochemical analyses demonstrate HMGB1 expression in primary osteoblasts and osteoclasts and that both cells express RAGE. HMGB1 is recoverable in the media of primary osteoblast cultures and cultures of isolated osteoclast precursors and osteoclasts. Parathyroid hormone (PTH), a regulator of bone remodeling, attenuates HMGB1 release in cultures of primary osteoblasts and MC3T3-E1 osteoblast-like cells but augments this release in the rat osteosarcoma cell line UMR 106-01, both responses primarily via activation of adenylyl cyclase. PTH-induced HMGB1 discharge by UMR cells exhibits similar release kinetics as reported for activated macrophages. These data confirm the presence of the HMGB1/RAGE signaling axis in bone.     

Stage-specific secretion of HMGB1 in cartilage regulates endochondral ossification

High mobility group box 1 protein (HMGB1) is a chromatin protein that has a dual function as a nuclear factor and as an extracellular factor. Extracellular HMGB1 released by damaged cells acts as a chemoattractant, as well as a proinflammatory cytokine, suggesting that HMGB1 is tightly connected to the process of tissue organization. However, the role of HMGB1 in bone and cartilage that undergo remodeling during embryogenesis, tissue repair, and disease is largely unknown. We show here that the stage-specific secretion of HMGB1 in cartilage regulates endochondral ossification. We analyzed the skeletal development of Hmgb1(-/-) mice during embryogenesis and found that endochondral ossification is significantly impaired due to the delay of cartilage invasion by osteoclasts, osteoblasts, and blood vessels. Immunohistochemical analysis revealed that HMGB1 protein accumulated in the cytosol of hypertrophic chondrocytes at growth plates, and its extracellular release from the chondrocytes was verified by organ culture. Furthermore, we demonstrated that the chondrocyte-secreted HMGB1 functions as a chemoattractant for osteoclasts and osteoblasts, as well as for endothelial cells, further supporting the conclusion that Hmgb1(-/-) mice are defective in cell invasion. Collectively, these findings suggest that HMGB1 released from differentiating chondrocytes acts, at least in part, as a regulator of endochondral ossification during osteogenesis.     

High mobility group box 1 protein regulates osteoclastogenesis through direct actions on osteocytes and osteoclasts in vitro

Old age and Cx43 deletion in osteocytes are associated with increased osteocyte apoptosis and osteoclastogenesis. We previously demonstrated that apoptotic osteocytes release elevated concentrations of the proinflammatory cytokine, high mobility group box 1 protein (HMGB1) and apoptotic osteocyte conditioned media (CM) promotes osteoclast differentiation. Further, prevention of osteocyte apoptosis blocks osteoclast differentiation and attenuates the extracellular release of HMGB1 and RANKL. Moreover, sequestration of HMGB1, in turn, reduces RANKL production/release by MLO-Y4 osteocytic cells silenced for Cx43 (Cx43^def), highlighting the possibility that HMGB1 promotes apoptotic osteocyte-induced osteoclastogenesis. However, the role of HMGB1 signaling in osteocytes has not been well studied. Further, the mechanisms underlying its release and the receptor(s) responsible for its actions is not clear. We now report that a neutralizing HMGB1 antibody reduces osteoclast formation in RANKL/M-CSF treated bone marrow cells. In bone marrow macrophages (BMMs), toll-like receptor 4 (TLR4) inhibition with LPS-RS, but not receptor for advanced glycation end products (RAGE) inhibition with Azeliragon attenuated osteoclast differentiation. Further, inhibition of RAGE but not of TLR4 in osteoclast precursors reduced osteoclast number, suggesting that HGMB1 produced by osteoclasts directly affects differentiation by activating TLR4 in BMMs and RAGE in preosteoclasts. Our findings also suggest that increased osteoclastogenesis induced by apoptotic osteocytes CM is not mediated through HMGB1/RAGE activation and that direct HMGB1 actions in osteocytes stimulate pro-osteoclastogenic signal release from Cx43^def osteocytes. Based on these findings, we propose that HMGB1 exerts dual effects on osteoclasts, directly by inducing differentiation through TLR4 and RAGE activation and indirectly by increasing pro-osteoclastogenic cytokine secretion from osteocytes.     

Intercellular signaling between ameloblastoma and osteoblasts

Ameloblastoma is an odontogenic tumor located in the bone jaw with clinical characteristics of extensive bone resorption. It is a locally invasive tumor with a high recurrence rate despite adequate surgical removal. In bone disease, tumors and other cells including osteoblasts, osteoclasts, and osteocytes in the bone microenvironment contribute to the pathogenesis of tumor growth. However, the effect of osteoblasts on ameloblastoma cells is not well-understood, and there has been limited research on interactions between them.This study investigated interactions between ameloblastoma cells and osteoblasts using a human ameloblastoma cell line (AM-3 ameloblastoma cells) and a murine pre-osteoblast cell line (MC3T3-E1 cells). We treated each cell type with the conditioned medium by the other cell type. We analyzed the effect on cytokine production by MC3T3-E1 cells and the production of MMPs by AM-3 cells. Treatment with AM-3-conditioned medium induced inflammatory cytokine production of IL-6, MCP-1, and RANTES from MC3T3-E1 cells. The use of an IL-1 receptor antagonist suppressed the production of these inflammatory cytokines by MC3T3-E1 cells stimulated with AM-3-conditioned medium. The MC3T3-E1-conditioned medium triggered the expression of MMP-2 from AM-3 cells. Furthermore, we have shown that the proliferation and migration activity of AM-3 cells were accelerated by MC3T3-E1 conditioned media.In conclusion, these intercellular signalings between ameloblastoma cells and osteoblasts may play multiple roles in the pathogenesis of ameloblastoma.     

Is HMGB1 an osteocyte alarmin?

The death of osteocytes, the terminally differentiated cells of the osteoblast lineage that are embedded in bone and regulate remodeling, is significant to both normal and pathological bone resorption. Apoptotic osteocytes putatively release a clarion signal that enhances the development of the bone-resorbing osteoclasts and targets their migration to the breach in the osteocyte network. This phenomenon is thought to underlie normal repair of bone microdamage and contribute to the etiologies of inflammatory bone loss. The chromatin protein high mobility group box 1 protein (HMGB1) has been identified as an "alarmin" in other tissues. An alarmin is an endogenous molecule released by dead and dying cells that alert the innate immune system to damage and the need for tissue repair. Wang and colleagues presented evidence in a landmark 1999 study showing that released HMGB1 is a lethal mediator of sepsis. Extracellular HMGB1 is a ligand for the toll-like receptors (TLRs) and for the receptor for advanced glycation end products (RAGE) all of which amplify inflammation. Recent studies by our lab and others have shown that HMGB1 is a bone-active cytokine. It is released by apoptotic osteoblasts in vitro, including the MLO-Y4 osteocyte-like cells. Extracellular HMGB1 enhances the expression of RANKL, TNFalpha, and IL6 in osteoblastogenic bone marrow stromal cell cultures, and it is chemotactic to osteoclasts. In this prospectus we will review HMGB1 activity at the immune-bone interface and propose a role for HMGB1 as an osteocyte alarmin and mediator of normal remodeling and inflammatory bone loss.     

Conditioned medium from osteocytes stimulates the proliferation of bone marrow mesenchymal stem cells and their differentiation into osteoblasts

Osteocytes are the most abundant cells in bone and there is increasing evidence that they control bone remodeling via direct cell-to-cell contacts and by soluble factors. In the present study, we have used the MLO-Y4 cell line to study the effect of osteocytes on the proliferation, differentiation and bone-forming capacity of bone marrow mesenchymal stem cells (MSC). Conditioned media (CM) from osteocytic MLO-Y4 and osteoblastic MC3T3-E1 cell lines were collected and added on mouse bone marrow cultures, in which MSC were induced to osteoblasts. There was a significant increase in alkaline phosphatase activity and osteocalcin expression in the presence of MLO-Y4 CM. No such stimulus could be observed with MC3T3-E1 CM. There was almost 4-fold increase in bone formation and up to 2-fold increase in the proliferation of MSC with MLO-Y4 CM. The highly proliferating bone marrow cells were negative for ALP and OCN, suggesting that they could represent early osteoblast precursors. MLO-Y4 CM did not enhance the viability of mature osteoblasts nor protected them of apoptosis. This is the first study to describe soluble signals between osteocytes and osteoblasts and there most likely are several still unidentified or unknown factors in osteocyte CM. We conclude that osteocytes have an active stimulatory role in controlling bone formation.     

Activation of the Cpx phosphorelay signal transduction system in acidic phospholipid-deficient pgsA mutant cells of Escherichia coli

Low-intensity pulsed ultrasound (LIPUS) has been used as a safe and effective modality to enhance fracture healing. As the most abundant cells in bone, osteocytes orchestrate biological activities of effector cells via direct cell-to-cell contacts and by soluble factors. In this study, we have used the osteocytic MLO-Y4 cells to study the effects of conditioned medium from LIPUS-stimulated MLO-Y4 cells on proliferation and differentiation of osteoblastic MC3T3-E1 cells. Conditioned media from LIPUS-stimulated MLO-Y4 cells (LIPUS-Osteocyte-CM) were collected and added on MC3T3-E1 cell cultures. MC3T3-E1 cells cultured in LIPUS-Osteocyte-CM demonstrated a significant inhibition of proliferation and an increased alkaline phosphatase activity. The results of PGE(2) and NO assay showed that LIPUS could enhance PGE(2) and NO secretion from MLO-Y4 cells at all time points within 24h after LIPUS stimulation. We conclude that LIPUS regulates proliferation and differentiation of osteoblasts through osteocytes in vitro. Increased secretion of PGE(2) from osteocytes may play a role in this effect.     

Oscillating fluid flow activation of gap junction hemichannels induces ATP release from MLO-Y4 osteocytes

Mechanical loads are required for optimal bone mass. One mechanism whereby mechanical loads are transduced into localized cellular signals is strain-induced fluid flow through lacunae and canaliculi of bone. Gap junctions (GJs) between osteocytes and osteoblasts provides a mechanism whereby flow-induced signals are detected by osteocytes and transduced to osteoblasts. We have demonstrated the importance of GJ and gap junctional intercellular communication (GJIC) in intracellular calcium and prostaglandin E(2) (PGE(2)) increases in response to flow. Unapposed connexons, or hemichannels, are themselves functional and may constitute a novel mechanotransduction mechanism. Using MC3T3-E1 osteoblasts and MLO-Y4 osteocytes, we examined the time course and mechanism of hemichannel activation in response to fluid flow, the composition of the hemichannels, and the role of hemichannels in flow-induced ATP release. We demonstrate that fluid flow activates hemichannels in MLO-Y4, but not MC3T3-E1, through a mechanism involving protein kinase C, which induces ATP and PGE(2) release.     

Parathyroid hormone-responsive Smad3-related factor, Tmem119, promotes osteoblast differentiation and interacts with the bone morphogenetic protein-Runx2 pathway

The mechanisms whereby the parathyroid hormone (PTH) exerts its anabolic action on bone are incompletely understood. We previously showed that inhibition of ERK1/2 enhanced Smad3-induced bone anabolic action in osteoblasts. These findings suggested the hypothesis that changes in gene expression associated with the altered Smad3-induced signaling brought about by an ERK1/2 inhibitor would identify novel bone anabolic factors in osteoblasts. We therefore performed a comparative DNA microarray analysis between empty vector-transfected mouse osteoblastic MC3T3-E1 cells and PD98059-treated stable Smad3-overexpressing MC3T3-E1 cells. Among the novel factors, Tmem119 was selected on the basis of its rapid induction by PTH independent of later increases in endogenous TGF-β. The levels of Tmem119 increased with time in cultures of MC3T3-E1 cells and mouse mesenchymal ST-2 cells committed to the osteoblast lineage by BMP-2. PTH stimulated Tmem119 levels within 1 h as determined by Western blot analysis and immunocytochemistry in MC3T3-E1 cells. MC3T3-E1 cells stably overexpressing Tmem119 exhibited elevated levels of Runx2, osteocalcin, alkaline phosphatase, and β-catenin, whereas Tmem119 augmented BMP-2-induced Runx2 levels in mesenchymal cells. Tmem119 interacted with Runx2, Smad1, and Smad5 in C2C12 cells. In conclusion, we identified a Smad3-related factor, Tmem119, that is induced by PTH and promotes differentiation in mouse osteoblastic cells. Tmem119 is an important molecule in the pathway downstream of PTH and Smad3 signaling in osteoblasts.     

Expression and regulation of resistin in osteoblasts and osteoclasts indicate a role in bone metabolism

The adipose tissue is the site of expression and secretion of a range of biologically active proteins, called adipokines, for example, leptin, adiponectin, and resistin. Leptin has previously been shown to be expressed in osteoblasts and to promote bone mineralization, whereas adiponectin expression is enhanced during osteoblast differentiation. In the present study we explored the possible role of resistin in bone metabolism. We found that resistin is expressed in murine preosteoclasts and preosteoblasts (RAW 264.7, MC3T3-E1), in primary human bone marrow stem cells and in mature human osteoblasts. The expression of resistin mRNA in RAW 264.7 was increased during differentiation and seemed to be regulated through PKC- and PKA-dependent mechanisms. Recombinant resistin increased the number of differentiated osteoclasts and stimulated NFkappaB promoter activity, indicating a role in osteoclastogenesis. Resistin also enhanced the proliferation of MC3T3-E1 cells in a PKA and PKC-dependent manner, but only weakly interfered with genes known to be upregulated during differentiation of MC3T3-E1 into osteoblasts. All together, our results indicate that resistin may play a role in bone remodeling.     

Effects of arachidonic acid, docosahexaenoic acid, prostaglandin E(2) and parathyroid hormone on osteoprotegerin and RANKL secretion by MC3T3-E1 osteoblast-like cells

Bone is continuously remodeled through resorption by osteoclasts and the subsequent synthesis of the bone matrix by osteoblasts. Cell-to-cell contact between osteoblasts and osteoclast precursors is required for osteoclast formation. RANKL (receptor activator of nuclear factor-kappaB ligand) expressed on osteoblastic cell membranes stimulates osteoclastogenesis, while osteoprotegerin (OPG) secreted by osteoblasts inhibits osteoclastogenesis. Although polyunsaturated fatty acids (PUFAs) have been implicated in bone homeostasis, the effects thereof on OPG and RANKL secretion have not been investigated. MC3T3-E1 osteoblasts were exposed to the n-6 PUFA arachidonic acid (AA) and the n-3 PUFA docosahexaenoic acid (DHA); furthermore, the bone-active hormone parathyroid hormone (PTH) and the effects thereof were tested on OPG and RANKL secretion. Prostaglandin E(2) (PGE(2)), a product of AA metabolism that was previously implicated in bone homeostasis, was included in the study. AA (5.0-20 microg/ml) inhibited OPG secretion by 25-30%, which was attenuated by pretreatment with the cyclooxygenase blocker indomethacin, suggesting that the inhibitory effect of AA on OPG could possibly be PGE(2)-mediated. MC3T3-E1 cells secreted very low basal levels of RANKL, but AA stimulated RANKL secretion, thereby decreasing the OPG/RANKL ratio. DHA suppressed OPG secretion to a smaller extent than AA. This could, however, be due to endogenous PGE(2) production. No RANKL could be detected after exposing the MC3T3-E1 cells to DHA. PTH did not affect OPG secretion, but stimulated RANKL secretion. This study demonstrates that AA and PTH reduce the OPG/RANKL ratio and may increase osteoclastogenesis. DHA, however, had no significant effect on OPG or RANKL in this model.     

Interleukin-17A induces cathepsin K and MMP-9 expression in osteoclasts via celecoxib-blocked prostaglandin E2 in osteoblasts

Interleukin-17 (IL-17) is a cytokine secreted primarily by T_H-17 cells that can stimulate the development of osteoclasts (osteoclastogenesis) in the presence of osteoblasts. IL-17, through osteoblasts, has indirect effects on the expression of bone resorption-related enzymes in osteoclasts, which have not been well clarified. Here, using MC3T3-E1 cells and RAW264.7 cells as osteoblasts and osteoclast precursors, we aimed to clarify these effects of IL-17A. MC3T3-E1 cells were cultured in the presence or absence of IL-17A for 72 h and the conditioned media collected (in the presence of soluble receptor activator of NF-кB ligand) and used to culture RAW264.7 cells. To assess osteoclast differentiation, adherent cells were fixed and stained for tartrate-resistant acid phosphatase (TRAP). Our analyses demonstrated that the number of TRAP-positive multinucleated cells increases after 3 days of culture in conditioned medium from IL-17A-treated cells compared to untreated controls. In addition, we observed that the levels of cathepsin K and MMP-9 increase in the conditioned medium from IL-17A-treated cells, whereas CA II expression levels remain unaffected. PGE₂ production from MC3T3-E1 cells increased in the presence of IL-17A. Celecoxib, a specific inhibitor of cyclooxygenase-2 (COX-2), blocked both the IL-17A-stimulated increase in TRAP-positive multinucleated cells and the expression of cathepsin K and MMP-9. Furthermore, when MC3T3-E1 cells were transformed with small interfering RNA to silence COX-2 expression before IL-17A treatment, the resulting conditioned medium was less effective at inducing cathepsin K and MMP-9 expression in RAW264.7 cells. These results suggest that IL-17A induces the differentiation and function of osteoclasts via celecoxib-blocked prostaglandin, mainly PGE₂, in osteoblasts.     

IL-6 is produced by osteoblasts and induces bone resorption

To examine the possible involvement of IL-6 in bone metabolism, a mouse osteoblastic cell line (MC3T3-E1) and primary osteoblast-like cells from fetal mouse calvaria were cultured with several systemic and local bone-resorbing agents and their expression of IL-6 mRNA was determined. Local bone-resorbing agents such as IL-1 alpha, IL-1 beta, TNF-alpha, and LPS greatly induced IL-6 mRNA expression in both MC3T3-E1 cells and primary osteoblast-like cells. Parathyroid hormone slightly increased expression of IL-6 mRNA in primary osteoblast-like cells but not in MC3T3-E1 cells. Neither IL-6 nor 1 alpha,25-dihydroxyvitamin D3 increased expression of IL-6 mRNA in either of the osteoblast-like cells. In agreement with the expression of IL-6 mRNA, biologically active IL-6 was produced in response to the treatment with IL-1 alpha, TNF-alpha, and LPS in MC3T3-E1 cells. Adding IL-6 dose dependently stimulated the release of 45Ca from prelabeled fetal mouse calvaria. Simultaneously adding suboptimal concentrations of IL-6 and IL-1 alpha induced bone resorption cooperatively. In accord with the increase in the release of 45Ca by IL-6, there were three times as many osteoclasts in the bone sections of calvaria cultured with IL-6 for 5 days as in the controls. IL-6 slightly suppressed alkaline phosphatase activity and collagen synthesis in MC3T3-E1 cells. These results indicate that IL-6 is also produced by osteoblasts, preferentially in response to local bone-resorbing agents, and it induces bone resorption both alone and in concert with other bone-resorbing agents.     

Round versus flat: bone cell morphology, elasticity, and mechanosensing

There is increasing evidence that cell function and mechanical properties are closely related to morphology. However, most in vitro studies investigate flat adherent cells, which might not reflect physiological geometries in vivo. Osteocytes, the mechanosensors in bone, reside within ellipsoid containment, while osteoblasts adhere to flatter bone surfaces. It is unknown whether morphology difference, dictated by the geometry of attachment is important for cell rheology and mechanosensing. We developed a novel methodology for investigating the rheology and mechanosensitivity of bone cells under different morphologies using atomic force microscopy and our two-particle assay for optical tweezers. We found that the elastic constant of MLO-Y4 osteocytes when flat and adherent (>1 kPa) largely differed when round but partially adherent (<1 kPa). The elastic constant of round suspended MLO-Y4 osteocytes, MC3T3-E1 osteoblasts, and primary osteoblasts were similarly <1 kPa. The mechanosensitivity of round suspended MLO-Y4 osteocytes was investigated by monitoring nitric oxide (NO) release, an essential signaling molecule in bone. A preliminary observation of high NO release from round suspended MLO-Y4 osteocytes in response to 5 pN force is reported here, in contrast with previous studies where flat cells routinely release lesser NO while being stimulated with higher force. Our results suggest that a round cellular morphology supports a less stiff cytoskeleton configuration compared with flat cellular morphology. This implies that osteocytes take advantage of their ellipsoid morphology in vivo to sense small strains benefiting bone health. Our assay provides novel opportunities for in vitro studies under a controlled suspended morphology versus commonly studied adherent morphologies.     

Stimulation of prostaglandin E2 (PGE2) production by arachidonic acid, oestrogen and parathyroid hormone in MG-63 and MC3T3-E1 osteoblast-like cells

Polyunsaturated fatty acids (PUFAs) as well as oestrogen (E2) and parathyroid hormone (PTH) affect bone cells. The aim of the study was to determine whether arachidonic acid (AA), E2, and PTH increase prostaglandin E₂ (PGE₂) synthesis in MG-63 and MC3T3-E1 osteoblastic cells and the level of mediation by COX-1 and COX-2. PGE₂ levels were determined in the conditioned culture media of MG-63 and MC3T3-E1 osteoblasts after exposure to AA, PTH and E2. Cells were pre-incubated in some experiments with the unselective COX inhibitor indomethacin or the COX-2 specific blocker NS-398. Indirect immunofluorescence was performed on MG-63 cells to detect the presence and location of the two enzymes involved. AA increased PGE₂ secretion in both cell lines; production by MC3T3-E1 cells, however, was significantly higher than that of MG-63 cells. This could be due to autoamplification via the EP₁ subtype of PGE receptors in mouse MC3T3-E1 osteoblasts. Both COX-1 and COX-2 affected the regulation of PGE₂ synthesis in MG-63 cells. E2 had no effect on PGE₂ secretion in both cell lines, while PTH caused a slight increase in PGE₂ synthesis in the MG-63 cell line.     

Carboxyl-terminal propeptide of type I collagen (c-propeptide) modulates the action of TGF-beta on MC3T3-E1 osteoblastic cells

Previously we found that the carboxyl-terminal propeptide of type I collagen (c-propeptide) is a major secretory protein of MC3T3-E1 osteoblastic cells. In this study, we found that c-propeptide suppresses collagen synthesis and alkaline phosphatase activity of MC3T3-E1 osteoblastic cells at the early-differentiated stage in a dose dependent manner. Mature osteoblasts did not respond to c-propeptide. These findings imply that c-propeptide modulates the function of osteoblasts at an early differentiation stage. Transforming growth factor-beta (TGF-beta) is stored in bone and released from bone matrix after the resorption by osteoclasts. We investigated the effect of c-propeptide on the action of TGF-beta, and found that it enhanced the effect of TGF-beta. We conclude that c-propeptide is a physiological modulator of TGF-beta in bone metabolism.