Regulation of osteoclast activity

Human periodontal ligament cells derived from deciduous teeth induce osteoclastogenesis in vitro

The receptor activator of NF-kappa B ligand (RANKL) and its decoy receptor, osteoprotegerin (OPG), are the important proteins involved in osteoclastogenesis. In this study, we investigated the expressions of RANKL and OPG in cultured human periodontal ligament cells derived from deciduous teeth (DPDL cells) and their roles in osteoclastogenesis. Northern blotting revealed that the OPG mRNA was down-regulated by application of 10(-8) M 1 alpha, 25(OH)2 vitamin D3 [1,25-(OH)2D3] and 10(-7) M dexamethasone (Dex). In contrast, RANKL mRNA was up-regulated by the same treatment. Western blotting demonstrated a decrease in OPG following application of 1, 25-(OH)2D3 and Dex. Tartrate-resistant acid phosphatase (TRAP)-positive multinuclear cells (MNCs) were induced when DPDL cells were co-cultured with mouse bone marrow cells in the presence of 1,25-(OH)2D3 and Dex. TRAP-positive MNCs increased significantly when the DPDL cells were co-cultured with bone marrow cells in the presence of anti-human OPG antibody together with 1, 25-(OH)2D3 and Dex. These results indicate that PDL cells derived from deciduous teeth synthesize both RANKL and OPG and could regulate the differentiation of osteoclasts.

The role of osteoclast differentiation in aseptic loosening

The major cause of orthopaedic implant loosening is thought to be accelerated osteoclastic bone resorption due to the action of cytokines produced in response to phagocytosis of implant-derived wear particles. This accelerated osteoclastic bone resorption could be due to increases in any of the following processes: recruitment of osteoclast precursors to the local microenvironment, differentiation of precursors into mature multinucleated osteoclasts. activation of mature osteoclasts, and/or survival of osteoclasts. Our studies have focused on differentiation and survival to complement work by others who have focused on recruitment of precursors and activation. Taken together, our studies and those of other investigators provide strong evidence that increased recruitment of osteoclast precursors and their subsequent differentiation play major roles in wear particle-induced osteolysis. In contrast, increased osteoclast activation and survival appear to play minor roles. These studies suggest that development of therapeutic interventions that reduce either recruitment or differentiation of osteoclast precursors would improve the performance of orthopaedic implants.

Osteoclast clearance from periodontal tissues during orthodontic tooth movement

The presence of osteoclasts at locations of alveolar bone remodeling is antecedent to orthodontic tooth movement. Cell recruitment and clearance are the mechanisms by which osteoclast populations are regulated. Research in other tissues has revealed that many cells die after their functional lives are past by a process called apoptosis, or programmed cell death. The purpose of this study was to examine the role of apoptosis in osteoclast clearance at orthodontically treated sites as a function of time and location. Orthodontic appliances were placed on 96 rats of the Sprague-Dawley strain. The rats were assigned to either treatment or sham (control) groups and killed 1, 3, 5, and 7 days after appliance placement. Tissue samples were prepared for histochemical evaluation and quantification of morphologic features. Tartrate-resistant acid phosphatase (TRAP) and ApopTag (TdT-mediated dUTP-biotin nick 3' end labeling) stains were used to identify osteoclasts and committed preosteoclasts and to discriminate between apoptotic and nonapoptotic nuclei. Pyknotic nuclei and apoptotic bodies were also counted as a morphologic assessment of apoptosis. The percentages of TRAP/ApopTag-positive nuclei were measured in 4 different periodontal regions. There was a highly significant difference in the overall percentage of TRAP/ApopTag-positive nuclei between the control and the treatment groups at 3, 5, and 7 days (P <.001). Morphologic criteria were also statistically different at days 5 and 7 (P <.05). These data strongly suggested that osteoclasts recruited for orthodontic tooth movement are, at least in part, cleared by apoptosis.

Androgens suppress osteoclast formation induced by RANKL and macrophage-colony stimulating factor

Androgen deficiency in males leads to an increase in osteoclastic bone resorption and a progressive decrease in bone mineral density. In the current studies, we examined the ability of 5 alpha-dihydrotestosterone to suppress osteoclast formation induced by receptor activator of NF-kB ligand (RANKL) and macrophage-colony stimulating factor in vitro. 5 alpha-Dihydrotestosterone suppressed the differentiation of bone marrow monocytes into osteoclasts from both sham-operated and orchidectomized mice. Androgen deficiency also led to an increase in the number of hematopoietic precursors capable of forming osteoclasts and increased the relative responsiveness of these cells to androgens in vitro. Interestingly, E2 was as effective as 5 alpha-dihydrotestosterone in suppressing osteoclast formation in bone marrow monocytes from both sham and orchidectomized mice. As with bone marrow monocytes, 5 alpha-dihydrotestosterone also suppressed RANKL-induced osteoclast formation in the monocyte-macrophagic cell line RAW264.7. In RAW264.7 cells, androgens appear to block RANKL-induced osteoclast formation through selective regulation of c-JUN: Accordingly, 5 alpha-dihydrotestosterone suppressed RANKL-induced c-Jun N-terminal kinase activation and reduced c-Jun expression levels. These effects resulted in a reduction in RANKL-induced activator protein-1 DNA binding activity and a corresponding suppression in activator protein-1-mediated transcriptional activation. These studies indicate that both E and androgens can suppress osteoclast formation via a direct, stromal cell-independent action on osteoclast precursors to block key transcription factors such as c-Jun essential for osteoclast differentiation.

Regulation of interleukin-6 secretion from breast cancer cells and its clinical implications

Interleukin (IL)-6 may play possible roles in the proliferation and metastasis of cancer cells, in the development of osteolysis and humoral hypercalcemia, and in the regulation of estrogen production in breast cancer tissues. IL-6 is also suggested to be a cachectic factor in cancer patients. A decrease in serum IL-6 levels induced by medroxyprogesterone acetate (MPA) has been reported to correlate with a reversion of body weight loss in patients with advanced breast cancer. To elucidate the mechanisms of action of the anti-cachectic effect of MPA its effects on IL-6 secretion from the KPL-4 cell line, the first human breast cancer cell line to secrete IL-6 and to induce cachexia, were explored. It has been suggested that an inhibitory effect of MPA on IL-6 secretion from breast cancer cells causes the anti-cachectic effect of MPA. Our other studies have revealed that 5'-fluorouridine (5'-DFUR) inhibits the growth of KPL-4 tumors and decreases IL-6 levels in both serum and tumor tissues. Decreasing serum IL-6 levels resulted in alleviation of body weight loss. Docetaxel increased IL-6 levels in both serum and KPL-4 tumors, but combined treatment with docetaxel and 5'-DFUR resulted not only in a potent antitumor effect but also in a drastic decrease of serum IL-6 levels. In the present paper the possible roles of IL-6 in the development and progression of breast cancer are reviewed, and the regulatory mechanisms of IL-6 secretion from breast cancer cells and the possible clinical implications of decreasing IL-6 secretion by therapeutic agents are discussed.

Long-term follow-up of a prospective, double-blind, placebo-controlled randomized trial of clodronate in multiple myeloma

Oral clodronate (1600 mg/d) has been shown to significantly reduce the incidence of skeletal complications in multiple myeloma. Preliminary analysis of a double-blind placebo-controlled trial of this treatment indicated that clodronate might prolong survival in patients without vertebral fractures at presentation. This issue was re-examined after further follow-up of the patients recruited into the Medical Research Council (MRC) VIth Myeloma Study. The trial examined the effects of clodronate on the natural history of skeletal disease in multiple myeloma; 619 patients were randomized between June 1986 and May 1992 commencing 15 d after the start of ABCM [adriamycin, BCNU (carmustine), cyclophosphamide, melphalan] chemotherapy or 43 d after ABCMP (ABCM + prenisolone); 535 patients who received clodronate or placebo were included in the analysis. The presence or absence of spinal fractures was assessed centrally from spinal X-rays; long-bone fractures were assessed locally. With a median follow-up of 8.6 years, there was no overall significant difference in survival between the two treatment groups (O/E, chi2 = 0.78, P = 0.38). Among the subgroup of 153 patients with no skeletal fractures at presentation there was a significant survival advantage (O/E, chi2 = 7.52, P = 0.006) in favour of the 73 patients receiving clodronate, with median survivals being, respectively, 59 months (95% CI 43-71 months) and 37 months (95% CI 31-52 months), and 5-year survivals being 46% and 35%. The original analysis of this study shows that there is a benefit in taking 1600 mg clodronate daily for patients with myelomatosis to prevent the development of new skeletal disease. Bearing in mind the limitations of subgroup analysis, the present study indicates that treatment may prolong survival in patients without overt skeletal disease at diagnosis. These observations, however, require confirmation in prospective clinical trials.

Titanium particles stimulate bone resorption by inducing differentiation of murine osteoclasts

BACKGROUND

Loosening of orthopaedic implants is mediated by cytokines that elicit bone resorption and are produced in response to phagocytosis of implant-derived wear particles. This accelerated bone resorption could be due to increased osteoclastic activity, survival, or differentiation. Although a number of in vitro studies have shown that wear particles increase osteoclastic activity, the increase was less than twofold in all cases. The objective of the current study was to test the hypothesis that wear particles stimulate bone resorption by inducing osteoclast differentiation.

METHODS

Conditioned media were prepared from murine marrow cells or human peripheral blood monocytes incubated in the presence or absence of titanium particles. The effects of conditioned media on osteoclast differentiation were examined with use of a recently developed assay in which osteoclast precursors are co-cultured with mesenchymal support cells.

RESULTS

The present study showed that titanium particles induced both murine marrow cells and human peripheral blood monocytes to produce factors that stimulated osteoclast differentiation. The mean increase in osteoclast differentiation was 29.3+/-9.4-fold. The stimulation of osteoclast differentiation led to a parallel increase in bone resorption. The amount of stimulation was regulated in a dose-dependent manner by the concentration of both titanium particles and conditioned media. The stimulation of osteoclast differentiation required interactions between the cells and the particles themselves and, therefore, was not due to metal ions, soluble contaminants released from the particles, or submicrometer particles. In contrast, conditioned media from control cells incubated in the absence of titanium particles had no detectable effect on any of the examined parameters.

CONCLUSIONS

The present study showed that titanium particles stimulate in vitro bone resorption primarily by inducing osteoclast differentiation. In contrast, the titanium particles had only small effects on osteoclast activity or survival.

Efficacy of etanercept for wear debris-induced osteolysis

A major limitation of total joint arthroplasty is that up to 20% of patients require revision surgery to correct prosthetic loosening. Aseptic loosening is believed to result from the phagocytosis of wear debris particles by macrophages, which secrete proinflammatory cytokines that stimulate osteolysis. Tumor necrosis factor alpha (TNF-alpha) has been shown to be one of the prominent cytokines in this cascade and to be involved critically in the generation of particle-induced osteolysis. Etanercept is a soluble inhibitor of TNF-alpha, which is widely used for the treatment of rheumatoid arthritis. Here, we show this agent's ability to prevent wear debris-induced osteolysis. In vitro we show that Etanercept can inhibit directly osteoclastic bone resorption in a bone wafer pit assay, as well as cytokine production from titanium (Ti)-stimulated macrophages. Using a quantitative in vivo model of wear debris-induced osteolysis, we show that Etanercept prevents bone resorption and osteoclastogenesis. In mice treated with Etanercept at the time of osteolysis induction, bone resorption and osteoclast numbers were reduced to background levels in both normal and human TNF-alpha (hTNF-alpha) transgenic mice. In an effort to evaluate its effect on established osteolysis, Etanercept was administered 5 days after Ti implantation, and we observed that further osteolysis was prevented. These data support the concept that TNF-alpha is involved critically in osteoclastogenesis and bone resorption during periprosthetic osteolysis and suggest that soluble TNF-alpha inhibitors may be useful as therapeutic agents for the treatment of prosthetic loosening in humans.

The effects of particulate wear debris, cytokines, and growth factors on the functions of MG-63 osteoblasts

BACKGROUND

Particle-challenged cells release cytokines, chemokines, and eicosanoids, which contribute to periprosthetic osteolysis. The particle-induced activation of macrophages and monocytes has been extensively studied, but only limited information is available on the response of osteoblasts to particulate wear debris. This study examines the effects of particulate wear debris, proinflammatory cytokines, and growth factors on osteoblast functions.

METHODS

MG-63 osteoblasts were treated with metal particles (titanium, titanium alloy, and chromium orthophosphate) or polymeric particles (polyethylene and polystyrene) of phagocytosable sizes or were treated with exogenous cytokines and growth factors. The kinetics of particle phagocytosis and the number of engulfed particles were assessed with use of fluoresceinated particles. Cell proliferation was determined according to [3H]-thymidine incorporation, and cell viability was determined by either fluorescein diacetate uptake or trypan blue exclusion. Expressions of osteoblast-specific genes were quantified with Northern blot hybridization, and the secretions of osteoblast-specific proteins and cytokines were analyzed by enzyme-linked immunosorbent assays.

RESULTS

MG-63 osteoblasts phagocytosed particles and became saturated after twenty-four hours. A maximum of forty to sixty particles per cell were phagocytosed. Each type of particle significantly suppressed procollagen alpha1[I] gene expression (p<0.05), whereas other osteoblast-specific genes (osteonectin, osteocalcin, and alkaline phosphatase) did not show significant changes. Particle-stimulated osteoblasts released interleukin-6 (p<0.05) and a smaller amount of transforming growth factor-beta1. Particles reduced cell proliferation in a dose-dependent manner without affecting cell viability (p<0.05). Exogenous tumor necrosis factor-alpha also enhanced the release of interleukin-6 (p<0.01) and transforming growth factor-beta1 (p<0.05), whereas the secretion of transforming growth factor-beta1 was increased by insulin-like growth factor-I and prostaglandin E2 as well. Insulin-like growth factor-I and transforming growth factor-beta1 significantly increased procollagen alpha1[I] gene expression in osteoblasts (p<0.05), while tumor necrosis factor-alpha and prostaglandin E2 significantly suppressed procollagen alpha1[I] gene expression (p<0.01). In contrast, neither exogenous nor endogenous interleukin-6 had any effect on other cytokine secretion, on proliferation, or on procollagen alpha1[I] gene expression. The transcription inhibitor actinomycin D reduced both procollagen alpha1[I] transcription and interleukin-6 production. Inhibitors of protein synthesis (cyclohexamide) and intracellular protein transport (brefeldin A and monensin) blocked the release of interleukin-6, but none of these compounds influenced the suppressive effect of titanium on procollagen alpha1[I] gene expression.

CONCLUSIONS

MG-63 osteoblasts phagocytose particulate wear debris, and this process induces interleukin-6 production and suppresses type-I collagen synthesis. Osteoblast-derived interleukin-6 may induce osteoclast differentiation and/or activation, but the resorbed bone cannot be replaced by new bone because of diminished osteoblast function (reduced type-I collagen synthesis). Exogenous cytokines (tumor necrosis factor-alpha and interleukin-1beta), growth factors (insulin-like growth factor-I and transforming growth factor-beta1), and prostaglandin E2 can modify particulate-induced alterations of osteoblast functions.

Phosphate ions mediate chondrocyte apoptosis through a plasma membrane transporter mechanism

In a previous investigation we showed that phosphate ions (Pi) induced apoptosis of terminally differentiated hypertrophic chondrocytes. To explore the mechanism by which Pi induces cell death, we asked the following two questions. First, can we prevent Pi-induced apoptosis by inhibiting plasma membrane Na-Pi cotransport? Second, which specific Na-Pi transporters are expressed in chondrocytes and are they developmentally regulated? Terminally differentiated hypertrophic chondrocytes were isolated from chick tibial cartilage and cell death was measured in the presence of 3-7 mmol/L Pi. To ascertain whether apoptosis was linked to a rise in cellular Pi loading, we examined the effect of phosphonoformic acid (PFA), a competitive inhibitor of Na-Pi cotransport on Pi-induced apoptosis in chondrocytes. We found that 1 mmol/L PFA blocked anion-induced cell death and prevented an increase in the cell Pi content. In a parallel study, we determined that the bisphosphonate, alendronate, also protected chondrocytes from death, albeit at a lower concentration than PFA. Using a DNA end-labeling procedure, we showed that the Pi-treated cells were apoptotic and, as might be predicted, the presence of PFA blocked induction of the death sequence. Next, we examined the expression of two Pi transporters in relation to chondrocyte maturation and anion treatment. We noted that there was expression of the constitutive transporter, Glvr-1, and a type II cotransporter in chick growth plate cells. Although these transport systems are active in terminally differentiated cells, it is probable that the initiation of apoptosis may require the induction of other Pi-transport systems. It is concluded that, at the mineralization front, cell death is linked directly to the elevation in environmental anion concentration and the concomitant rise in intracellular Pi levels.

Bone morphogenetic proteins secreted by breast cancer cells upregulate bone sialoprotein expression in preosteoblast cells

It is well established that bone metastases comprise bone; however, the exact factors/mechanisms involved remain unknown. We hypothesized that tumor cells secreted factors capable of altering normal bone metabolism. The aims of the present study were to (1) determine the effects of secretory products isolated from HT-39 cells, a human breast cancer cell line, on osteoprogenitor cell (MC3T3-E1 cells) behavior, and (2) identify tumor-derived factor(s) that alters osteoblast activities. Conditioned media (CM) from HT-39 cells were collected following a 24-h serum-free culture. The ability of CM to alter gene expression in MC3T3-E1 cells was determined by Northern analysis. CM effects on cell proliferation and mineralization ability were determined using a Coulter counter and von Kossa stain, respectively. MC3T3-E1 cells were treated with CM plus noggin, a factor known to block bone morphogenic proteins (BMPs), to determine whether BMPs, shown to be present in CM, were linked with CM effects on MC3T3-E1 cell activity. In addition, inhibitors of MAP kinase kinase (MEK), protein kinase C (PKC), and protein kinase A were used to identify the intracellular signaling pathway(s) by which the active factors in CM regulated osteoblast behavior. CM treatment significantly enhanced BSP mRNA (2.5-fold over control), but had no effect on cell proliferation. Mineralization assay showed that CM enhanced mineral nodule formation compared to controls. Noggin inhibited CM-induced upregulation of BSP mRNA, suggesting that BMPs were responsible for upregulating BSP gene expression in MC3T3-E1 cells. The PKC inhibitor blocked CM-mediated upregulation of BSP, suggesting involvement of the PKC pathway in regulating BSP expression. BMPs secreted by HT-39 cells may be responsible for enhancing BSP expression in MC3T3-E1 cells. Continued studies targeted at determining the role of BMPs in regulating bone metabolism are important for understanding the pathogenesis of bone diseases.

Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis

The adult skeleton regenerates by temporary cellular structures that comprise teams of juxtaposed osteoclasts and osteoblasts and replace periodically old bone with new. A considerable body of evidence accumulated during the last decade has shown that the rate of genesis of these two highly specialized cell types, as well as the prevalence of their apoptosis, is essential for the maintenance of bone homeostasis; and that common metabolic bone disorders such as osteoporosis result largely from a derangement in the birth or death of these cells. The purpose of this article is 3-fold: 1) to review the role and the molecular mechanism of action of regulatory molecules, such as cytokines and hormones, in osteoclast and osteoblast birth and apoptosis; 2) to review the evidence for the contribution of changes in bone cell birth or death to the pathogenesis of the most common forms of osteoporosis; and 3) to highlight the implications of bone cell birth and death for a better understanding of the mechanism of action and efficacy of present and future pharmacotherapeutic agents for osteoporosis.

Nonsteroidal anti-inflammatory drugs and bone metabolism in spinal fusion surgery: a pharmacological quandary

The use of nonsteroidal anti-inflammatory drugs (NSAIDs) is ubiquitous in contemporary medical practice and these agents are efficacious in a number of clinical contexts. In particular, NSAIDs have proven to be highly effective adjuncts in the amelioration of postoperative pain in the subset of patients undergoing spinal surgery requiring fusion. NSAIDs act through inhibition of cyclooxygenase enzymes and therefore diminish prostaglandin production. However, prostaglandins are intimately involved in the modulation of bone metabolism and the balance of data, from both clinical and laboratory contexts, indicate that prostaglandins preferentially favor bone anabolism. Most recently, limited emerging evidence suggests that NSAID administration in patients undergoing spinal fusion surgery may increase nonunion rates, which in turn, has important ramifications to the patient, their family and the entire medical system. Hence, disparate views have evolved regarding the use of NSAIDs in postoperative pain control in patients undergoing spinal surgery requiring fusion. NSAIDs have proven efficacy in the management of postoperative pain in these patients, however, this must be weighed against the risk of nonunion and its associated consequences. In this review, the role of prostaglandins in bone metabolism, the pharmacology of NSAIDs and the modulation of bone metabolism by NSAIDs are discussed. Additionally, the current evidence examining the use of NSAIDs in spinal surgery is presented. As rates of spinal surgery continue to rise, it is imperative that the apparent pharmacological quandary surrounding the administration of NSAIDs in patients undergoing spinal surgery requiring fusion be addressed, both to guide present clinical practice and to outline further directions for investigation.