MIP-1 gamma promotes receptor-activator-of-NF-kappa-B-ligand-induced osteoclast formation and survival

Chemokines play an important role in immune and inflammatory responses by inducing migration and adhesion of leukocytes, and have also been reported to modulate osteoclast differentiation from hemopoietic precursor cells of the monocyte-macrophage lineage. In this study, we examined the effect of MIP-1 gamma, a C-C chemokine family member, on receptor activator of NF-kappa B ligand (RANKL)-stimulated osteoclast differentiation, survival, and activation. RANKL induced osteoclasts to dramatically increase production of MIP-1 gamma and to also express the MIP-1 gamma receptor CCR1, but had only minor effects on the related C-C chemokines MIP-1 alpha and RANTES. Neutralization of MIP-1 gamma with specific Ab reduced RANKL-stimulated osteoclast differentiation by 60-70%. Mature osteoclasts underwent apoptosis within 24 h after removal of RANKL, as shown by increased caspase 3 activity and DNA fragmentation. Apoptosis was reduced by the addition of exogenous MIP-1 gamma or RANKL, both of which increased NF-kappa B activation in osteoclasts. Neutralization studies showed that the prosurvival effect of RANKL was in part dependent on its ability to induce MIP-1 gamma. Finally, osteoclast activation for bone resorption was stimulated by MIP-1 gamma. Taken together, these results demonstrate that MIP-1 gamma plays an important role in the differentiation and survival of osteoclasts, most likely via an autocrine pathway.     

Mechanisms by which cells of the osteoblast lineage control osteoclast formation and activity

The cells of bone are of two lineages, the osteoblasts arising from pluripotential mesenchymal cells and osteoclasts from hemopoietic precursors of the monocyte-macrophage series. Resorption of bone by the multinucleate osteoclast requires the generation of new osteoclastsw and their activation. Many hormones and cytokines are able to promote bone resorption by influencing these processes, but they achieve this without acting directly on osteoclastws. Most evidence indicates that their actions are mediated by cells of the osteoblast lineage. Evidence for hormone-and cytokine-induced activation of osteoclasts requiring the mediation of osteoblasts comes from studies of rsorption by isolated osteoclasts. However, consistent evidence for a spiceific “activating factor” is lacking, and the argument is presented that the isolated osteoclast resorption assays have not been shown convincingly to be assays of osteoclast activation. The view is presented that osteoblast-mediated osteoclast activation is the result of several events in the microenvironment without necessarily requiring the existence of a spicific, essential osteoclast activator. On the other hand, a specific promoter of osteoclast differentiation does seem likely to be a product of cells of the stromal/osteoblast series. Evidence in facour of this comes from studies of osteoclast generation in co-cultures of osteoblast/stromal cells with hemopoietic cells. Conflicting view, maintaining that osteoclasts can develop from hemooietic cells without stromal intervention, might be explaind by varying criteria used in identification of osteoclasts. Osteoblastic and osteoclastic renewal, and the interactions of these lineages, are central to the process of bone remodeling.     

Dasatinib as a bone-modifying agent: anabolic and anti-resorptive effects

Background

Bone loss, in malignant or non-malignant diseases, is caused by increased osteoclast resorption and/or reduced osteoblast bone formation, and is commonly associated with skeletal complications. Thus, there is a need to identify new agents capable of influencing bone remodeling. We aimed to further pre-clinically evaluate the effects of dasatinib (BMS-354825), a multitargeted tyrosine kinase inhibitor, on osteoblast and osteoclast differentiation and function.

Methods

For studies on osteoblasts, primary human bone marrow mensenchymal stem cells (hMSCs) together with the hMSC-TERT and the MG-63 cell lines were employed. Osteoclasts were generated from peripheral blood mononuclear cells (PBMC) of healthy volunteers. Skeletally-immature CD1 mice were used in the in vivo model.

Results

Dasatinib inhibited the platelet derived growth factor receptor-β (PDGFR-β), c-Src and c-Kit phosphorylation in hMSC-TERT and MG-63 cell lines, which was associated with decreased cell proliferation and activation of canonical Wnt signaling. Treatment of MSCs from healthy donors, but also from multiple myeloma patients with low doses of dasatinib (2–5 nM), promoted its osteogenic differentiation and matrix mineralization. The bone anabolic effect of dasatinib was also observed in vivo by targeting endogenous osteoprogenitors, as assessed by elevated serum levels of bone formation markers, and increased trabecular microarchitecture and number of osteoblast-like cells. By in vitro exposure of hemopoietic progenitors to a similar range of dasatinib concentrations (1–2 nM), novel biological sequelae relative to inhibition of osteoclast formation and resorptive function were identified, including F-actin ring disruption, reduced levels of c-Fos and of nuclear factor of activated T cells 1 (NFATc1) in the nucleus, together with lowered cathepsin K, αVβ3 integrin and CCR1 expression.

Conclusions

Low dasatinib concentrations show convergent bone anabolic and reduced bone resorption effects, which suggests its potential use for the treatment of bone diseases such as osteoporosis, osteolytic bone metastasis and myeloma bone disease.     

Role of osteoblast suppression in multiple myeloma

Multiple myeloma is the most common form of plasma cell dyscrasia and virtually all cases of myeloma exhibit osteolytic lesions, which result in bone pain, pathological fractures, spinal cord compression, and hypercalcaemia. Malignant plasma cells disrupt the delicate balance between bone formation and bone resorption, which ultimately leads to the debilitating osteolytic lesions. This review focuses principally on mechanisms of osteoblast inhibition by malignant plasma cells with emphasis placed on our experimental findings, which support a model for abnormal Wnt signaling in osteoblast suppression. We describe how excessive amounts of soluble Wnt inhibitors secreted by malignant plasma cells in multiple myeloma could promote osteolytic lesions, tumor growth, suppress hematopoiesis, prevent proper engraftment, and expansion of transplanted stem cells. Finally, we detail current therapies shown to disrupt the interaction between the myeloma cell and the microenvironment, leading to activation of osteoblasts.     

High level secretory expression of murine OCIL by CHO cells and action of OCIL on osteoclast differentiation

Receptor activator of nuclear factor-kB ligand (RANKL), a well-known membrane-bound molecule expressed on osteoblasts and bone marrow stromal cells, is believed to induce osteoclast differentiation and activation by binding to the receptor activator of nuclear factor-kB (RANK), which is expressed on the surface of osteoclast lineage cells. This induction is inhibited by osteoprotegerin (OPG) that is secreted by osteoblast lineage and acts as a decoy receptor of RANKL. Currently the essential role of the OPG/RANKL/RANK system in the process of osteoclast maturation, as well as activation, has been well established, and the majority of bone resorption regulators control osteoclast formation and activation through their effects on this system and especially on the relative expression levels of RANKL and OPG [1].     

MIP-1δ activates NFATc1 and enhances osteoclastogenesis: involvement of both PLCγ2 and NFκB signaling

Pathological bone resorption is a source of significant morbidity in diseases affecting the skeleton such as rheumatoid arthritis, periodontitis, and cancer metastasis to bone. Evidence indicates that elevated levels of inflammatory mediators such as IL-1, IL-6, and TNF-α play a role in this process by promoting the formation of bone-resorbing osteoclasts. Additionally, current studies have identified inflammatory chemokines of the macrophage inflammatory protein (MIP) family as potential mediators of pathological bone resorption, where both MIP-1α and -3α have been shown to enhance osteoclast (OCL) development. In this study we provide evidence that MIP-1δ, whose expression is associated with renal cell carcinoma bone metastasis and rheumatoid arthritis, enhances OCL formation in vitro via a direct effect on OCL precursors. Consistent with this ability, exposure of OCL precursors to MIP-1δ resulted in the activation of PLCγ2 and NF-κB, two signaling pathways known to regulate OCL differentiation. Moreover, MIP-1δ induced expression and nuclear translocation of NFATc1, a master regulator of osteoclastogenesis, which was dependent on activation of both the PLCγ2 and NFκB signaling pathways. Lastly, consistent with in vitro studies, in vivo administration of MIP-1δ significantly increased OCL number and resorption area as determined using a murine calvarial bone resorption model. Taken together, these data highlight the potential of MIP-1δ as a mediator of pathological bone resorption and provide insight into the molecular mechanism through which MIP-1δ enhances osteoclastogenesis.     

Multiple myeloma/hypercalcemia

Multiple myeloma, a cancer of plasma cells, is associated with excessive tumor-induced, osteoclast-mediated bone destruction. Hypercalcemia remains the most frequent metabolic complication of myeloma in patients, and excessive osteolysis plays a major contributory role in its pathogenesis. The clinical presentation of hypercalcemia in patients varies depending on the level of ionized calcium; it can be life threatening, as in the case of hypercalcemic crisis, requiring immediate medical treatment to prevent death. During the past few years there have been exciting developments in our understanding of the pathogenesis of myeloma bone disease; in particular, key mediators of the osteoclastic bone resorption in myeloma have been identified, including receptor activator of nuclear factor-kappaB ligand (RANKL) and macrophage inflammatory protein-1alpha. There is also increasing evidence that Dickkopf 1, which has been shown to be over-expressed in myeloma patients, is also a potent stimulator of osteoclast formation and activity. Importantly, the available data suggest that RANKL is the final common mediator of osteoclastic bone resorption, irrespective of the upstream initiator molecule. This brief review presents an overview of the roles played by these mediators in inducing osteolysis in myeloma bone disease, and it discusses targeting RANKL as a potential new treatment strategy in myeloma bone disease and myeloma-associated hypercalcemia.     

Reciprocal role of ERK and NF-kappaB pathways in survival and activation of osteoclasts

To examine the role of mitogen-activated protein kinase and nuclear factor kappa B (NF-kappaB) pathways on osteoclast survival and activation, we constructed adenovirus vectors carrying various mutants of signaling molecules: dominant negative Ras (Ras(DN)), constitutively active MEK1 (MEK(CA)), dominant negative IkappaB kinase 2 (IKK(DN)), and constitutively active IKK2 (IKK(CA)). Inhibiting ERK activity by Ras(DN) overexpression rapidly induced the apoptosis of osteoclast-like cells (OCLs) formed in vitro, whereas ERK activation after the introduction of MEK(CA) remarkably lengthened their survival by preventing spontaneous apoptosis. Neither inhibition nor activation of ERK affected the bone-resorbing activity of OCLs. Inhibition of NF-kappaB pathway with IKK(DN) virus suppressed the pit-forming activity of OCLs and NF-kappaB activation by IKK(CA) expression upregulated it without affecting their survival. Interleukin 1alpha (IL-1alpha) strongly induced ERK activation as well as NF-kappaB activation. Ras(DN) virus partially inhibited ERK activation, and OCL survival promoted by IL-1alpha. Inhibiting NF-kappaB activation by IKK(DN) virus significantly suppressed the pit-forming activity enhanced by IL-1alpha. These results indicate that ERK and NF-kappaB regulate different aspects of osteoclast activation: ERK is responsible for osteoclast survival, whereas NF-kappaB regulates osteoclast activation for bone resorption.     

Toddaculin,Isolated from of Toddalia asiatica (L.) Lam., Inhibited Osteoclastogenesis in RAW 264 Cells and Enhanced Osteoblastogenesis in MC3T3-E1 Cells

Osteoporosis with bone loss is widely recognized as a major health problem. Bone homeostasis is maintained by balancing bone formation and bone resorption. The imbalance caused by increased bone resorption over bone formation can lead to various bone-related diseases such as osteoporosis and rheumatoid arthritis. Osteoclasts are the principal cells responsible for bone resorption and the main targets of anti-resorptive therapies. However, excessive inhibition of osteoclast differentiation may lead to inhibition of osteoblast differentiation. Therefore, it is important to screen for new compounds capable of inhibiting bone resorption and enhancing bone formation. Toddalia asiatica (L.) Lam. has been utilized traditionally for medicinal purposes such as the treatment of rheumatism. Currently, the extract is considered to be a good source of pharmacological agents for the treatment of bone-related diseases, but the active compounds have yet to be identified. We investigated whether toddaculin, derived from Toddalia asiatica (L.) Lam., affects both processes by inhibiting bone resorption and enhancing bone formation. Towards this end, we used pre-osteoclastic RAW 264 cells and pre-osteoblastic MC3T3-E1 cells. We found that toddaculin not only inhibited the differentiation of osteoclasts via activation of the NF-κB, ERK 1/2, and p38 MAPK signaling pathways, but it also induced differentiation and mineralization of osteoblasts by regulating differentiation factors. Thus, toddaculin might be beneficial for the prevention and treatment of osteoporosis.     

Effect of oestradiol on cytokine production in immortalized human marrow stromal cell lines.

Oestrogen deficiency enhances bone osteoclastogenesis and bone resorption. Evidence of cooperation between stromal cells and osteoclast precursors in mice suggests that oestradiol acts by regulating cytokine release from stromal cells. Bone marrow stroma contains multipotent progenitors that give rise to many mesenchymal lineages, including osteoblasts that may regulate osteoclast differentiation. We immortalized and characterized six human bone marrow stromal cell lines (presence of Stro1, secretion of alkaline phosphatase, osteocalcin, formation of lipid droplets, and presence of alpha and beta oestrogen receptors). The response of cytokines to oestradiol was then evaluated in vitro, as were the phorbol myristate acetate (PMA)-stimulated cytokine levels. Cells had the characteristics of undifferentiated stromal cells (Stro1+, RANK-L+), and expressed alpha-oestrogen receptors. The osteoblast phenotype (amounts of alkaline phosphatase and osteocalcin) was weak and there was a poor capacity to differentiate into adipocytes. These cell lines did not respond to oestradiol by producing interleukin 6 (IL-6), IL-1 or tumour necrosis factor alpha (TNF-alpha) either constitutively or after stimulation with PMA. Moreover, RANK-L and osteoprotegerin expressions were not regulated by oestradiol in vitro. Thus, modulation of these cytokines by stromal cells do not appear to be the mechanism by which oestradiol regulates bone resorption in humans.     

Role of vitamin D in bone resorption.

The idea that vitamin D must function at the bone site to promote bone mineralization has long existed since its discovery as an anti-rachitic agent. However, the definite evidence for this is still lacking. In contrast, much evidence has accumulated that 1 alpha,25(OH)2D3 in involved in bone resorption. 1 alpha,25(OH)2D3 tightly regulates differentiation of osteoclast progenitors into osteoclasts. Osteoclast progenitors have been thought to belong to the monocyte-macrophage lineage. 1 alpha,25(OH)2D3 greatly stimulates differentiation and activation of mononuclear phagocytes. Recent reports have indicated that differentiation of mononuclear phagocytes into osteoclasts is strictly regulated by osteoblastic cells, the process of which is also stimulated by 1 alpha,25(OH)2D3. In the differentiation of mononuclear phagocytes into osteoclasts, the target cells for 1 alpha,25(OH)2D3 appear to be osteoblastic stromal cells. Osteoblastic cells produce several proteins such as BGP, MGP, osteopontin and the third component of complement (C3) in response to the vitamin. They appear to be somehow involved in osteoclast differentiation and functions. Thus, 1 alpha,25(OH)2D3 seems to be involved in the differentiation of osteoclast progenitors into osteoclasts directly and also by an indirect mechanism involving osteoblastic cells. The precise role of osteoblastic cells in osteoclast development has to be elucidated in the future.     

CKbeta-8 [CCL23], a novel CC chemokine, is chemotactic for human osteoclast precursors and is expressed in bone tissues

We have previously demonstrated that a tartrate-resistant acid phosphatase (TRAP)-positive subpopulation of mononuclear cells isolated from collagenase digests of human osteoclastoma tissue exhibits an osteoclast phenotype and can be induced to resorb bone. Using these osteoclast precursors as a model system, we have assessed the chemotactic potential of 16 chemokines. Three CC chemokines, the recently described CKbeta-8, RANTES, and MIP-1alpha elicited significant chemotactic responses. In contrast, 10 other CC chemokines (MIP-1beta, MCP-1, MCP-2, MCP-3, MCP-4, HCC-1, eotaxin-2, PARC, SLC, ELC) and 3 CXC chemokines (IL-8, GROalpha, SDF-1) were inactive. None of these chemokines showed any chemotactic activity for either primary osteoblasts derived from human bone explants or the osteoblastic MG-63 cell line. The identity of the osteoclast receptor that mediates the chemotactic response remains to be established. However, all three active chemokines have been reported to bind to CCR1 and cross-desensitization studies demonstrate that RANTES and MIP-1alpha can partially inhibit the chemotactic response elicited by CKbeta-8. CKbeta-8, the most potent of the active CC chemokines (EC(max) 0.1-0.3 nM), was further characterized with regard to expression in human bone and cartilage. Although expression is not restricted to these tissues, CKbeta-8 mRNA was shown to be highly expressed in osteoblasts and chondrocytes in human fetal bone by in situ hybridization. In addition, CKbeta-8 protein was shown to be present in human osteophytic tissue by immunolocalization. These observations suggest that CKbeta-8, and perhaps other chemokines, may play a role in the recruitment of osteoclast precursors to sites of bone resorption.     

NOTCH1 regulates osteoclastogenesis directly in osteoclast precursors and indirectly via osteoblast lineage cells

NOTCH signaling is a key regulator of cell fate decisions in prenatal skeletal development and is active during adult tissue renewal. In addition, its association with neoplasia suggests that it is a candidate therapeutic target. We find that attenuated NOTCH signaling enhances osteoclastogenesis and bone resorption in vitro and in vivo by a combination of molecular mechanisms. First, deletion of Notch1-3 in bone marrow macrophages directly promotes their commitment to the osteoclast phenotype. These osteoclast precursors proliferate more rapidly than the wild type in response to macrophage colony-stimulating factor and are sensitized to RANKL and macrophage colony-stimulating factor, undergoing enhanced differentiation in response to low doses of either cytokine. Conforming with a role for NOTCH in this process, presentation of the NOTCH ligand JAGGED1 blunts the capacity of wild-type bone marrow macrophages to become osteoclasts. Combined, these data establish that NOTCH suppresses osteoclastogenesis via ligand-mediated receptor activation. Although NOTCH1 and NOTCH3 collaborate in regulating osteoclast formation, NOTCH1 is the dominant paralog. In addition, NOTCH1 deficiency promotes osteoclastogenesis indirectly by enhancing the ability of osteoblast lineage cells to stimulate osteoclastogenesis. This is achieved by decreasing the osteoprotegerin/RANKL expression ratio. Thus, NOTCH1 acts as a net inhibitor of bone resorption, exerting its effect both directly in osteoclast precursors and indirectly via osteoblast lineage cells. These observations raise caution that therapeutic inhibition of NOTCH signaling may adversely accelerate bone loss in humans.     

The cytoskeletal regulatory scaffold protein GIT2 modulates mesenchymal stem cell differentiation and osteoblastogenesis

G protein-coupled receptor kinase interacting protein 2 (GIT2) is a signaling scaffold protein involved in the regulation of cytoskeletal structure, membrane trafficking, and G protein-coupled receptor internalization. Since dynamic cytoskeletal reorganization plays key roles both in osteoblast differentiation and in the maintenance of osteoclast polarity during bone resorption, we hypothesized that skeletal physiology would be altered in GIT2(-/-) mice. We found that adult GIT2(-/-) mice have decreased bone mineral density and bone volume in both the trabecular and cortical compartments. This osteopenia was associated with decreased numbers of mature osteoblasts, diminished osteoblastic activity, and increased marrow adiposity, suggesting a defect in osteoblast maturation. In vitro, mesenchymal stem cells derived from GIT2(-/-) mice exhibited impaired differentiation into osteoblasts and increased adipocyte differentiation, consistent with a role for GIT2 in mesenchymal stem cell fate determination. Despite elevated osteoclast inducing cytokines and osteoclast numbers, GIT2(-/-) mice also exhibit impaired bone resorption, consistent with a further role for GIT2 in regulating osteoclast function. Collectively, these findings underscore the importance of the cytoskeleton in both osteoblast and osteoclast function and demonstrate that GIT2 plays essential roles in skeletal metabolism, affecting both bone formation and bone resorption in vivo.     

Rho-A is critical for osteoclast podosome organization, motility, and bone resorption

Rho plays a regulatory role in the formation of actin stress fibers and focal adhesions, and it is also involved in integrin-mediated signaling events. To study the role of Rho in alpha(v)beta(3)/gelsolin-dependent signaling, the HIV-Tat peptide, hemagglutinin (HA)-tagged Rho(Val-14) (constitutively active) and Rho(Asn-19) (dominant negative) were transduced into avian osteoclasts. Protein transduction by HA-Tat was highly efficient, and 90-100% of the cells were transduced with HA-tagged proteins. We demonstrate here that Rho(Val-14) transduction (100 nM) stimulated gelsolin-associated phosphatidylinositol 3-kinase activity, podosome assembly, stress fiber formation, osteoclast motility, and bone resorption, mimicking osteoclast stimulation by osteopontin/alpha(v)beta(3.) The effects of Rho(Val-14) transduction stimulation was time-dependent. C3 exoenzyme blocked the effects of Rho(Val-14) and induced podosome disassembly, loss of motility, and inhibition of bone resorption. Transduction of Rho(Asn-19) produced podosome disassembly, and blocked osteopontin stimulation. These data demonstrate that integrin-dependent activation of phosphoinositide synthesis, actin stress fiber formation, podosome reorganization for osteoclast motility, and bone resorption require Rho stimulation.     

TNAP, TrAP, ecto-purinergic signaling, and bone remodeling

Bone remodeling is a process of continuous resorption and formation/mineralization carried out by osteoclasts and osteoblasts, which, along with osteocytes, comprise the bone multicellular unit (BMU). A key component of the BMU is the bone remodeling compartment (BRC), isolated from the marrow by a canopy of osteoblast-like lining cells. Although much progress has been made regarding the cytokine-dependent and hormonal regulation of bone remodeling, less attention has been placed on the role of extracellular pH (pH(e)). Osteoclastic bone resorption occurs at acidic pH(e). Furthermore, osteoclasts can be regarded as epithelial-like cells, due to their polarized structure and ability to form a seal against bone, isolating the lacunar space. The major ecto-phosphatases of osteoclasts and osteoblasts, acid and alkaline phosphatases, both have ATPase activity with pH optima several units different from neutrality. Furthermore, osteoclasts and osteoblasts express plasma membrane purinergic P2 receptors that, upon activation by ATP, accelerate bone osteoclast resorption and impair osteoblast mineralization. We hypothesize that these ecto-phosphatases help regulate [ATP](e) and localized pH(e) at the sites of bone resorption and mineralization by pH-dependent ATP hydrolysis coupled with P2Y-dependent regulation of osteoclast and osteoblast function. Furthermore, osteoclast cellular HCO3(-), formed as a product of lacunar V-ATPase H(+) secretion, is secreted into the BRC, which could elevate BRC pH(e), in turn affecting osteoblast function. We will review the existing data addressing regulation of BRC pH(e), present a hypothesis regarding its regulation, and discuss the hypothesis in the context of the function of proteins that regulate pH(e).