Potential role of cbfa1, an essential transcriptional factor for osteoblast differentiation, in osteoclastogenesis: regulation of mRNA expression of osteoclast differentiation factor (ODF)

Osteoprotegerin reduces osteoclast resorption activity without affecting osteogenesis on nanoparticulate mineralized collagen scaffolds

The instructive capabilities of extracellular matrix-inspired materials for osteoprogenitor differentiation have sparked interest in understanding modulation of other cell types within the bone regenerative microenvironment. We previously demonstrated that nanoparticulate mineralized collagen glycosaminoglycan (MC-GAG) scaffolds efficiently induced osteoprogenitor differentiation and bone healing. In this work, we combined adenovirus-mediated delivery of osteoprotegerin (AdOPG), an endogenous anti-osteoclastogenic decoy receptor, in primary human mesenchymal stem cells (hMSCs) with MC-GAG to understand the role of osteoclast inactivation in augmentation of bone regeneration. Simultaneous differentiation of osteoprogenitors on MC-GAG and osteoclast progenitors resulted in bidirectional positive regulation. AdOPG expression did not affect osteogenic differentiation alone. In the presence of both cell types, AdOPG-transduced hMSCs on MC-GAG diminished osteoclast-mediated resorption in direct contact; however, osteoclast-mediated augmentation of osteogenic differentiation was unaffected. Thus, the combination of OPG with MC-GAG may represent a method for uncoupling osteogenic and osteoclastogenic differentiation to augment bone regeneration.

Overview of RAW264.7 for osteoclastogensis study: Phenotype and stimuli

Bone homeostasis is preserved by the balance of maintaining between the activity of osteogenesis and osteoclastogenesis. However, investigations for the osteoclastogenesis were hampered by considerable difficulties associated with isolating and culturing osteoclast in vivo. As the alternative, stimuli‐induced osteoclasts formation from RAW264.7 cells (RAW‐OCs) have gain its importance for extensively osteoclastogenic study of bone diseases, such as rheumatoid arthritis, osteoporosis, osteolysis and periodontitis. However, considering the RAW‐OCs have not yet been well‐characterized and RAW264.7 cells are polymorphic because of a diverse phenotype of the individual cells comprising this cell linage, and different fate associated with various stimuli contributions. Thus, in present study, we provide an overview for current knowledge of the phenotype of RAW264.7 cells, as well as the current understanding of the complicated interactions between various stimuli and RAW‐OCs in the light of the recent progress.

Starch-derived absorbable polysaccharide hemostat enhances bone healing via BMP-2 protein

Surgical hemostasis is critical in reducing the likelihood of excessive bleeding and blood transfusion. In treating some cases, commonly used hemostatic agent showed limited efficacy and prolonged degradation and clearance, causing an inhibition of bone healing. Starch absorbable polysaccharide (SAPH) is a novel hemostatic agent made from a plant starch, which can be completely absorbed and achieve better hemostatic effects than many commonly used hemostatic agents. However, whether SAPH can induce a promotion of bone healing remains unknown. In this study, we used a model of rabbit parietal bone defect and a mouse osteoblast cell line MC3T3-E1 to evaluate the effects of SAPH on bone healing. We found that SAPH significantly decreased bone healing scores, reduced defective area of parietal bone, and increased the areas of bone trabeculae and cavitas medullaris. In addition, SAPH enhanced MC3T3-E1 osteoblasts proliferation, up-regulated the expressions of alkaline phosphatase (ALP) and osteocalcin and increased the level of bone morphogenetic protein 2 (BMP-2) in MC3T3-E1 osteoblasts. These SAPH-induced benefits in MC3T3-E1 osteoblasts were significantly abolished by the application of BMP-2-siRNA. These findings suggested that SAPH enhances bone healing, promotes the proliferation, differentiation and maturation of osteoblast by up-regulating BMP-2 expression in osteoblastic cells.

Osteogenic differentiation of placenta-derived multipotent cells in vitro

OBJECTIVE

Stem cells offer great potential for clinical therapeutic use because of their ability to rejuvenate and to differentiate into numerous types of cells. We isolated multipotent cells from the human term placenta that were capable of differentiation into cells of all three germ layers.

MATERIALS AND METHODS

We examined the ability of these placenta-derived multipotent cells (PDMCs) to differentiate into osteoblasts (OBs) or OB-like cells. The PDMCs were treated with osteogenic medium (OM) consisting of dexamethasone, β-glycerol phosphate, and ascorbic acid. At sequential time intervals (0 day, 3 days, 6 days, 9 days, and 12 days) we measured several parameters. These included alkaline phosphatase (ALP) activity, alizarin red staining (ARS) to measure calcium deposition, messenger RNA (mRNA) expressions of osteogenesis-related transcription factor (Cbfa1), and calcium coordination protein (osteocalcin). These variables were used as indicators of PDMC osteodifferentiation.

RESULTS

We showed that ALP activity in the early stage of differentiation and calcium deposition were both significantly increased in PDMCs after OM induction. Moreover, the Cbfa1 and osteocalcin gene expressions were upregulated. The results suggested that OM induced an osteodifferentiation potential in PDMCs.

CONCLUSION

PDMC-derived osteocytes provide a useful model to evaluate the mechanisms of key biomolecules and bioengineering processes.

The relevance of leukotrienes for bone resorption induced by mechanical loading

5-Lipoxygenase (5-LO) metabolites are important pro-inflammatory lipid mediators. However, much still remains to be understood about the role of such mediators in bone remodeling. This study aimed to investigate the effect of 5-LO metabolites, LTB4 and CysLTs, in a model of mechanical loading-induced bone remodeling. Strain-induced tooth movement and consequently alveolar bone resorption/apposition was achieved by using a coil spring placed on molar and attached to incisors of C57BL6 (wild-type-WT), 5-LO deficient mice (5-LO(-/-)) and mice treated with 5-LO inhibitor (zileuton-ZN) or with antagonist of CysLTs receptor (montelukast-MT). The amount of bone resorption and the number of osteoclasts were determined morphometrically. The expression of inflammatory and bone remodeling markers in periodontium was analyzed by qPCR. Osteoclast differentiation and TNF-α production were evaluated in vitro using RAW 264.7 cells treated with LTB4 or LTD4. Bone resorption, TRAP(+) cells and expression of Tnfa, Il10 and Runx2 were significantly diminished in 5-LO(-/-), ZN- and MT-treated mice. The expression of Rank was also reduced in 5-LO(-/-) and MT-treated mice. Accordingly, LTB4 and LTD4 in association with RANKL promoted osteoclast differentiation and increased TNF-α release in vitro. These data demonstrate that the absence of 5-LO metabolites, LTB4 and CysLTs reduces osteoclast recruitment and differentiation, consequently diminishing bone resorption induced by mechanical loading. Thus, 5-LO might be a potential target for controlling bone resorption in physiological and pathological conditions.

Biology of the RANKL-RANK-OPG System in Immunity, Bone, and Beyond

Discovery and characterization of the cytokine receptor-cytokine-decoy receptor triad formed by receptor activator of nuclear factor kappa-B ligand (RANKL)–receptor activator of NF-κB (RANK)–osteoprotegerin (OPG) have led not only to immense advances in understanding the biology of bone homeostasis, but have also crystalized appreciation of the critical regulatory relationship that exists between bone and immunity, resulting in the emergence of the burgeoning field of osteoimmunology. RANKL–RANK–OPG are members of the tumor necrosis factor (TNF) and TNF receptor superfamilies, and share signaling characteristics common to many members of each. Developmentally regulated and cell-type specific expression patterns of each of these factors have revealed key regulatory functions for RANKL–RANK–OPG in bone homeostasis, organogenesis, immune tolerance, and cancer. Successful efforts at designing and developing therapeutic agents targeting RANKL–RANK–OPG have been undertaken for osteoporosis, and additional efforts are underway for other conditions. In this review, we will summarize the basic biology of the RANKL–RANK–OPG system, relate its cell-type specific functions to system-wide mechanisms of development and homeostasis, and highlight emerging areas of interest for this cytokine group.

Kruppel-like factor 4 expression in osteoblasts represses osteoblast-dependent osteoclast maturation

Kruppel-like factor 4 (KLF4) is a zinc-finger-type transcription factor with a restricted expression pattern during skeletal development. We have previously shown that KLF4 represses osteoblast mineralization concomitant with a down-regulation in the expression of a number of osteoblastic genes, both in vivo and in vitro. In addition to the cell-autonomous effects of KLF4 in osteoblasts, transgenic osteoblastic-KLF4 mice show severe defects in osteoclast maturation. Wild-type bone-marrow-derived macrophages co-cultured with KLF4-expressing osteoblasts exhibit reduced formation of multinuclear osteoclasts as compared with control cultures overexpressing green fluorescent protein. Significantly, the transduction of Runx2, a master regulator of osteoblastogenesis, together with KLF4 into osteoblasts restores the reduction in osteoclastogenesis induced by KLF4 alone. Various extracellular matrix molecules are down-regulated by KLF4 overexpression but this down-regulation can be partially restored by the co-transduction of Runx2. These results suggest that osteoblastic-KLF4 affects osteoclast maturation by regulating cell-matrix interactions and reinforce the importance of the regional down-regulation of KLF4 expression in the subset of osteoblasts for normal skeletal modeling and remodeling.

Osteoclastogenic potential of peripheral blood mononuclear cells in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is an autosomal dominant skeletal dysplasia characterized by hypoplastic or aplastic clavicles, dental abnormalities, and delayed closure of the cranial sutures. In addition, mid-face hypoplasia, short stature, skeletal anomalies and osteoporosis are common. We aimed to evaluate osteoclastogenesis in a child (4 years old), who presented with clinical signs of CCD and who have been diagnosed as affected by deletion of RUNX2, master gene in osteoblast differentiation, but also affecting T cell development and indirectly osteoclastogenesis. The results of this study may help to understand whether in this disease is present an alteration in the bone-resorptive cells, the osteoclasts (OCs). Unfractionated and T cell-depleted Peripheral Blood Mononuclear Cells (PBMCs) from patient were cultured in presence/absence of recombinant human M-CSF and RANKL. At the end of the culture period, OCs only developed following the addition of M-CSF and RANKL. Moreover, real-time PCR experiment showed that freshly isolated T cells expressed the osteoclastogenic cytokines (RANKL and TNFα) at very low level, as in controls. This is in accordance with results arising from flow cytometry experiments demonstrating an high percentage of circulating CD4(+)CD28(+) and CD4(+)CD27(+) T cells, not able to produce osteoclastogenic cytokines. Also RANKL, OPG and CTX serum levels in CCD patient are similar to controls, whereas QUS measurements showed an osteoporotic status (BTT-Z score -3.09) in the patient. In conclusions, our findings suggest that the heterozygous deletion of RUNX2 in this CCD patient did not alter the osteoclastogenic potential of PBMCs in vitro.

Runx2 promotes both osteoblastogenesis and novel osteoclastogenic signals in ST2 mesenchymal progenitor cells

We profiled the global gene expression of a bone marrow-derived mesenchymal pluripotent cell line in response to Runx2 expression. Besides osteoblast differentiation, Runx2 promoted the osteoclastogenesis of co-cultured splenocytes. This was attributable to the upregulation of many novel osteoclastogenic genes and the downregulation of anti-osteoclastogenic genes.

INTRODUCTION

In addition to being a master regulator for osteoblast differentiation, Runx2 controls osteoblast-driven osteoclastogenesis. Previous studies profiling gene expression during osteoblast differentiation had limited focus on Runx2 or paid little attention to its role in mediating osteoblast-driven osteoclastogenesis.

METHODS

ST2/Rx2(dox), a bone marrow-derived mesenchymal pluripotent cell line that expresses Runx2 in response to Doxycycline (Dox), was used to profile Runx2-induced gene expression changes. Runx2-induced osteoblast differentiation was assessed based on alkaline phosphatase staining and expression of classical marker genes. Osteoclastogenic potential was evaluated by TRAP staining of osteoclasts that differentiated from primary murine splenocytes co-cultured with the ST2/Rx2(dox) cells. The BeadChip™ platform (Illumina) was used to interrogate genome-wide expression changes in ST2/Rx2(dox) cultures after treatment with Dox or vehicle for 24 or 48 h. Expression of selected genes was also measured by RT-qPCR.

RESULTS

Dox-mediated Runx2 induction in ST2 cells stimulated their own differentiation along the osteoblast lineage and the differentiation of co-cultured splenocytes into osteoclasts. The latter was attributable to the stimulation of osteoclastogenic genes such as Sema7a, Ltc4s, Efnb1, Apcdd1, and Tnc as well as the inhibition of anti-osteoclastogenic genes such as Tnfrsf11b (OPG), Sema3a, Slco2b1, Ogn, Clec2d (Ocil), Il1rn, and Rspo2.

CONCLUSION

Direct control of osteoblast differentiation and concomitant indirect control of osteoclast differentiation, both through the activity of Runx2 in pre-osteoblasts, constitute a novel mechanism of coordination with a potential crucial role in coupling bone formation and resorption.

Trabecular bone microstructure and local gene expression in iliac crest biopsies of men with idiopathic osteoporosis

Male idiopathic osteoporosis (MIO) is a metabolic bone disease that is characterized by low bone mass, microstructural alterations, and increased fracture risk in otherwise healthy men. Although the detailed pathophysiology of MIO has yet to be clarified, evidence increasingly suggests an osteoblastic defect as the underlying cause. In this study we tested the hypothesis that the expression profile of certain osteoblastic or osteoblast-related genes (ie, WNT10B, RUNX2, Osterix, Osteocalcin, SOST, RANKL, and OPG) is different in iliac crest biopsies of MIO patients when compared with healthy controls. Furthermore, we investigated the relation of local gene expression characteristics with histomorphometric, microstructural, and clinical features. Following written informed consent and diligent clinical patient characterization, iliac crest biopsies were performed in nine men. While RNA extraction, reverse-transcription, and real-time polymerase chain reactions (PCRs) were performed on one biopsy, a second biopsy of each patient was submitted for histomorphometry and micro-computed tomography (µCT). Age-matched bone samples from forensic autopsies served as controls. MIO patients displayed significantly reduced WNT10B, RUNX2, RANKL, and SOST expression. Performing µCT for the first time in MIO biopsies, we found significant decreases in trabecular number and connectivity density. Trabecular separation was increased significantly, but trabecular thickness was similar in both groups. Histomorphometry revealed decreased BV/TV and osteoid volume and fewer osteoclasts in MIO. By providing evidence for reduced local WNT10B, RUNX2, and RANKL gene expression and histomorphometric low turnover, our data support the osteoblast dysfunction model discussed for MIO. Further, MIO seems to lead to a different microstructural pathology than age-related bone loss.

TIEG1/KLF10 modulates Runx2 expression and activity in osteoblasts

Deletion of TIEG1/KLF10 in mice results in a gender specific osteopenic skeletal phenotype with significant defects in both cortical and trabecular bone, which are observed only in female animals. Calvarial osteoblasts isolated from TIEG1 knockout (KO) mice display reduced expression levels of multiple bone related genes, including Runx2, and exhibit significant delays in their mineralization rates relative to wildtype controls. These data suggest that TIEG1 plays an important role in regulating Runx2 expression in bone and that decreased Runx2 expression in TIEG1 KO mice is in part responsible for the observed osteopenic phenotype. In this manuscript, data is presented demonstrating that over-expression of TIEG1 results in increased expression of Runx2 while repression of TIEG1 results in suppression of Runx2. Transient transfection and chromatin immunoprecipitation assays reveal that TIEG1 directly binds to and activates the Runx2 promoter. The zinc finger containing domain of TIEG1 is necessary for this regulation supporting that activation occurs through direct DNA binding. A role for the ubiquitin/proteasome pathway in fine tuning the regulation of Runx2 expression by TIEG1 is also implicated in this study. Additionally, the regulation of Runx2 expression by cytokines such as TGFβ1 and BMP2 is shown to be inhibited in the absence of TIEG1. Co-immunoprecipitation and co-localization assays indicate that TIEG1 protein associates with Runx2 protein resulting in co-activation of Runx2 transcriptional activity. Lastly, Runx2 adenoviral infection of TIEG1 KO calvarial osteoblasts leads to increased expression of Runx2 and enhancement of their ability to differentiate and mineralize in culture. Taken together, these data implicate an important role for TIEG1 in regulating the expression and activity of Runx2 in osteoblasts and suggest that decreased expression of Runx2 in TIEG1 KO mice contributes to the observed osteopenic bone phenotype.

Embryonic stem cells for osteo-degenerative diseases

Current orthopedic practice to treat osteo-degenerative diseases, such as osteoporosis, calls for antiresorptive therapies and anabolic bone medications. In some cases, surgery, in which metal rods are inserted into the bones, brings symptomatic relief. As these treatments may ameliorate the symptoms, but cannot cure the underlying dysregulation of the bone, the orthopedic field seems ripe for regenerative therapies using transplantation of stem cells. Stem cells bring with them the promise of completely curing a disease state, as these are the cells that normally regenerate tissues in a healthy organism. This chapter assembles reports that have successfully used stem cells to generate osteoblasts, osteoclasts, and chondrocytes - the cells that can be found in healthy bone tissue - in culture, and review and collate studies about animal models that were employed to test the function of these in vitro "made" cells. A particular emphasis is placed on embryonic stem cells, the most versatile of all stem cells. Due to their pluripotency, embryonic stem cells represent the probably most challenging stem cells to bring into the clinic, and therefore, the associated problems are discussed to put into perspective where the field currently is and what we can expect for the future.

The clinical significance of OPG/sRANKL ratio in thalassemia patients suffering from osteopenia or osteoporosis in Egyptian patients

Osteopenia and osteoporosis are considered major health problems in patients suffering from thalassemia due to increased life expectancy of those patients. Osteoprotegerin (OPG) and receptor activator of NF-kappa-B ligand (RANKL) have been recently implicated in the pathogenesis of various types of osteoporosis. The aim of this study is to evaluate the role of serum OPG/RANKL ratio in patients suffering from thalassemia complicated with osteoporosis. Serum OPG and RANKL were measured in thalassemia patients and 20 healthy control subjects, using ELISA methods. Stastistically, the results demonstrate lower OPG and OPG/RANKL ratio in patients suffering from thalassemia with documented osteopenia or osteoporosis in comparison with control group and patients suffering from thalassemia without osteopenia or osteoporosis. OPG/RANKL ratio could become a promising rapid and cheap screening marker for osteopenia or osteoporosis in patients suffering from thalassemia. Furthermore, OPG may become a therapeutic option in treatment of osteoporosis of various etiologies including thalassemia.

Immature and mature megakaryocytes enhance osteoblast proliferation and inhibit osteoclast formation

Recent data suggest that megakaryocytes (MKs) play a role in skeletal homeostasis. In vitro and in vivo data show that MKs stimulate osteoblast (OB) proliferation and inhibit osteoclast (OC) formation, thus favoring net bone deposition. There are several mouse models with dysregulated megakaryopoiesis and resultant high bone mass phenotypes. One such model that our group has extensively studied is GATA-1 deficient mice. GATA-1 is a transcription factor required for normal megakaryopoiesis, and mice deficient in GATA-1 have increases in immature MK number and a striking increase in bone mass. While the increased bone mass could simply be a result of increased MK number, here we take a more in depth look at the MKs of these mice to see if there is a unique factor inherent to GATA-1 deficient MKs that favors increased bone deposition. We show that increased MK number does correspond with increased OB proliferation and decreased OC formation that stage of maturation does not alter the effect of MKs on bone cell lineages beyond the megakaryoblast stage, and that GATA-1 deficient MKs survive longer than wild-type controls. So while increased MK number in GATA-1 deficient mice likely contributes to the high bone mass phenotype, we propose that the increased longevity of this lineage also plays a role. Since GATA-1 deficient MKs live longer they are able to exert both more proliferative influence on OBs and more inhibitory influence on OCs.

Expression of Runx2 and type X collagen in vertebral growth plate of patients with adolescent idiopathic scoliosis

The different expression of type X collagen and Runx2 between the convex and concave side of vertebral growth plate in scoliosis may help to improve our understanding of the role that growth plate tissue play in the development or progression of idiopathic scoliosis. In this investigation, there were significant differences of the total expression of type X collagen, Runx2 protein, and Runx2 mRNA between convex side and concave side growth plates of the apex vertebrae (p < 0.05). The total expression of type X collagen in the concave side growth plates of the lower end vertebrae was higher than that in the same side growth plates of apex (p < 0.05). The total expression of Runx2 in the concave side growth plates in the upper and lower end vertebrae were higher than that in the concave side growth plates of apex (p < 0.05). The expression of type X collagen, Runx2, and Runx2 mRNA, the cell density of type X collagen and Runx2 positive chondrocytes, and histological changes between convex side and concave side of the vertebral growth plate indicated that the vertebral growth plate was affected by mechanical forces, which was a secondary change and could contribute to progression of adolescent idiopathic scoliosis.

Bone tissue engineering: a review in bone biomimetics and drug delivery strategies

Critical-sized defects in bone, whether induced by primary tumor resection, trauma, or selective surgery have in many cases presented insurmountable challenges to the current gold standard treatment for bone repair. The primary purpose of a tissue-engineered scaffold is to use engineering principles to incite and promote the natural healing process of bone which does not occur in critical-sized defects. A synthetic bone scaffold must be biocompatible, biodegradable to allow native tissue integration, and mimic the multidimensional hierarchical structure of native bone. In addition to being physically and chemically biomimetic, an ideal scaffold is capable of eluting bioactive molecules (e.g., BMPs, TGF-betas, etc., to accelerate extracellular matrix production and tissue integration) or drugs (e.g., antibiotics, cisplatin, etc., to prevent undesired biological response such as sepsis or cancer recurrence) in a temporally and spatially controlled manner. Various biomaterials including ceramics, metals, polymers, and composites have been investigated for their potential as bone scaffold materials. However, due to their tunable physiochemical properties, biocompatibility, and controllable biodegradability, polymers have emerged as the principal material in bone tissue engineering. This article briefly reviews the physiological and anatomical characteristics of native bone, describes key technologies in mimicking the physical and chemical environment of bone using synthetic materials, and provides an overview of local drug delivery as it pertains to bone tissue engineering is included.

Dual transcriptional regulation by runx2 of matrix Gla protein in Xenopus laevis

Matrix Gla protein (MGP) is an extracellular mineral-binding protein expressed in several tissues but it only accumulates in bone and calcified cartilage under physiological conditions. Available evidence indicates that it acts as a physiological inhibitor of mineralization. Runx2 is a transcription factor essential for bone formation in mammals, affecting osteoblast and chondrocyte differentiation by regulating key genes crucial for bone and cartilage development. Being an important cartilage-associated gene, MGP is a potential target for Runx2, and thus we have investigated the possible functional interactions between them. In A6 cells, Runx2 was found to modulate MGP transcription and deletion analysis of MGP distal and proximal promoter-luciferase constructs identified cis-regulatory regions. Interestingly, we have also identified a runx2-binding site that mediates transcriptional repression of XlMGP. Mutation of this element, located between -54 and +33 bp, results in 18-fold up-regulation of transcription. Furthermore, and in addition to the previously reported Xlrunx2 types I and II, we have identified three transcripts encoding novel, truncated Xlrunx2 isoforms. Although only type I and type II could transactivate XlMGP, the truncated isoforms identified in this study, which result from alternative splicing, could be involved in negative regulation of MGP expression, as described for other RUNX2 truncated isoforms acting in other target genes. In vivo microinjection of XlMGP promoter constructs and runx2 mRNA confirmed that those promoters are targets for this transcription factor. These data also indicate that MGP is under dual regulation by runx2 through the use of various isoforms and context-dependent formation of transcriptional complexes.

The role of periodontal ligament cells in delayed tooth eruption in patients with cleidocranial dysostosis

OBJECTIVE

The clinical appearance of patients with cleidocranial dysplasia (CCD), which is caused by mutations in the RUNX2 gene, is characterized by anomalies of the clavicles, thorax, spine, pelvis and extremities and by disturbances of the skull and tooth development. Of orthodontic relevance are multiple supernumerary teeth associated with delayed tooth eruption. The present investigation is based on the hypothesis that an altered phenotypic expression of periodontal ligament (PDL) cells from CCD patients and a reduced ability of those cells to support the differentiation of bone-resorbing osteoclasts might contribute to delayed tooth eruption.

MATERIALS AND METHODS

To test this hypothesis, PDL cells from healthy donors and from two patients with clinically and molecular biologically diagnosed CCD were characterized for the basal and induced mRNA expression of osteoblast marker genes. The physiological relevance of the findings for the differentiation of osteoclasts was examined in an osteoclast assay, as well as in a co-culture model of PDL cells and osteoclast precursors.

RESULTS

Both CCD patients displayed missense mutations of the RUNX2 gene. The in vitro experiments revealed an unaltered expression of RUNX2 mRNA, however especially in CCD patient 2 there was a reduced basal expression of mRNA for the key regulatory gene for bone remodeling RANKL. Furthermore, compared to the control cells from healthy donors, these factors were less inducible by stimulation of the cultures with 1alpha,25(OH)(2)D(3). In the osteoclast assays as well as in the co-culture experiments, PDL cells from the CCD patients showed a reduced capacity to induce the differentiation of active osteoclasts.

CONCLUSIONS

These data indicate that PDL cells from CCD patients express a less distinctive osteoblastic phenotype resulting in an impaired ability to support osteoclastogenesis which might, in part, account for the delayed tooth eruption that can be observed clinically.

New perspective in osteoarthritis: the OPG and RANKL system as a potential therapeutic target?

Bone remodelling is tightly regulated by a molecular triad composed of OPG/RANK/RANKL. The receptor activator of NF-kappaB ligand (RANKL) (localized on osteoblasts) enhances osteoclastogenesis via interaction with its receptor RANK (localized on osteoclasts), whereas osteoprotegerin (OPG) (produced by osteoblasts) inhibits this osteoclastogenesis by binding to RANKL. The equilibrium between OPG and RANKL plays a crucial role in the pathophysiology of bone. Although some studies have shown the efficacy of OPG as a therapeutic agent against bone resorption, its bioavailability and mechanism of action after binding to RANKL have only recently been studied. A mechanistic investigation based on what becomes of OPG after binding to cells expressing membranous RANKL demonstrated an internalization process of OPG through the clathrin pathway prior to proteasomal and/or lysosomal degradation. Interestingly, the OPG internalization process reduced the half-life of RANKL. Recent evidence has shown that subchondral bone alterations in osteoarthritis (OA) are intimately involved in cartilage degradation, and that OPG/RANKL may be implicated. Data show that human OA subchondral bone osteoblasts have abnormal OPG and RANKL levels and consequently an altered OPG and RANKL ratio. Further data also reveal the involvement of some osteotropic factors in these altered levels and that some of these factors generally target RANKL with a differential modulation of the RANKL isoforms. Altogether, data suggest that this system could be targeted as a new strategy for the treatment of OA.

LTbetaR signaling induces cytokine expression and up-regulates lymphangiogenic factors in lymph node anlagen

The formation of lymph nodes is a complex process crucially controlled through triggering of LTbetaR on mesenchymal cells by LTalpha(1)beta(2) expressing lymphoid tissue inducer (LTi) cells. This leads to the induction of chemokines to attract more hematopoietic cells and adhesion molecules to retain them. In this study, we show that the extravasation of the first hematopoietic cells at future lymph node locations occurs independently of LTalpha and that these cells, expressing TNF-related activation-induced cytokine (TRANCE), are the earliest LTi cells. By paracrine signaling the first expression of LTalpha(1)beta(2) is induced. Subsequent LTbetaR triggering on mesenchymal cells leads to their differentiation to stromal organizers, which now also start to express TRANCE, IL-7, as well as VEGF-C, in addition to the induced adhesion molecules and chemokines. Both TRANCE and IL-7 will further induce the expression of LTalpha(1)beta(2) on newly arrived immature LTi cells, resulting in more LTbetaR triggering, generating a positive feedback loop. Thus, LTbetaR triggering by LTi cells during lymph node development creates a local environment to which hematopoietic precursors are attracted and where they locally differentiate into fully mature, LTalpha(1)beta(2) expressing, LTi cells. Furthermore, the same signals may regulate lymphangiogenesis to the lymph node through induction of VEGF-C.

Commitment to the osteoblast lineage is not required for RANKL gene expression

Differentiation of bone-resorbing osteoclasts from hematopoietic precursors depends upon expression of the cytokine receptor activator of NFkappaB ligand (RANKL) by fibroblastic stromal cells, which some evidence suggests are of the osteoblast lineage. We have shown previously that hormonal-responsiveness of the murine RANKL gene is mediated in part by a distal enhancer that binds Runx2, a transcription factor required for commitment to the osteoblast lineage, supporting the idea that osteoclast-supporting stromal cells may be osteoblasts or their progenitors. However, in this study we demonstrate that parathyroid hormone (PTH) stimulation of RANKL in mice is not affected by a significant reduction in the number of osteoblasts. Consistent with this, neither Runx2, nor Cbfb, a binding partner essential for Runx activity, are required for basal or PTH-stimulated RANKL expression in fibroblastic stromal cell models. Nonetheless, RANKL responsiveness to PTH was elevated in cultured calvaria cells expressing high levels of osterix, another transcription factor required for osteoblast differentiation, and this was associated with elevated PTH receptor expression. The responsiveness of RANKL to 1,25-dihydroxyvitamin D(3) was not elevated in the osterix-expressing cells. Together, these results suggest that commitment to the osteoblast lineage is not a requirement for RANKL gene transcription in fibroblastic stromal cells but may enhance responsiveness of this gene to specific hormones via control of their receptors.

Pressure-loaded MSCs during early osteodifferentiation promote osteoclastogenesis by increase of RANKL/OPG ratio

Mechanical stress plays an important role in bone remodeling. However, it is still unclear whether mechanical stress regulates osteoclastogenesis mediated by mesenchymal stem cells (MSCs) during initial osteodifferentiation. We investigated the effects of static and dynamic pressures on osteoclast-inducing potential of MSCs during early osteodifferentiation. The osteoclastogenesis was examined using TRAP staining. The mRNA levels of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) genes were analyzed using real-time RT-PCR. It was shown that MSCs exposed to either pressure during initial osteodifferentiation promoted osteoclastogenesis with the up-regulation of RANKL/OPG ratio. MSCs displayed diverse responses to pressures at different points of initial osteodifferentiation. The RANKL/OPG ratio was significantly increased after osteoinduction in the primary MSCs without pressures exposure, which contradicted the previous report. These results suggest novel mechanisms of the initial biological responses of bone remodeling upon mechanical stimuli.

Enhanced chondrogenesis of mesenchymal stem cells in collagen mimetic peptide-mediated microenvironment

A new type of synthetic hydrogel scaffold that mimics certain aspects of structure and function of natural extracellular matrix (ECM) has been developed. We previously reported the conjugation of collagen mimetic peptide (CMP) to poly(ethylene oxide) diacrylate (PEODA) to create a polymer-peptide hybrid scaffold for a suitable cell microenvironment. In this study, we showed that the CMP-mediated microenvironment enhances the chondrogenic differentiation of mesenchymal stem cells (MSCs). MSCs were harvested and photo-encapsulated in CMP-conjugated PEODA (CMP/PEODA). After 3 weeks, the histological and biochemical analysis of the CMP/PEODA gel revealed twice as much glycosaminoglycan and collagen contents as in control PEODA hydrogels. Moreover, MSCs cultured in CMP/PEODA hydrogel exhibited a lower level of hypertrophic markers, core binding factor alpha 1, and type X collagen than MSCs in PEODA hydrogel as revealed by gene expression and immunohistochemisty. These results indicate that CMP/PEODA hydrogel provides a favorable microenvironment for encapsulated MSCs and regulates their downstream chondrogenic differentiation.

Megakaryocyte-bone cell interactions

Emerging data show that megakaryocytes (MKs) play a role in the replication and development of bone cells. Both in vivo and in vitro evidence now show that MKs can have significant effects on cells of the osteoclast (OC) and osteoblast (OB) lineage, with obvious manifestations on bone phenotype, and probable significance for human pathology.There are currently four mouse models in which increases in MK number lead to a specific bone pathology of markedly increased bone volume. While these models all achieve megakaryocytosis by different mechanisms, the resultant osteosclerotic phenotype observed is consistent across all models.In vitro data suggest that MKs play a role in OC and OB proliferation and differentiation. While MKs express receptor activator of nuclear factor kappa B ligand (RANKL), a prerequisite for osteoclastogenesis, they also express many factors known to inhibit OC development, and co-cultures of MKs with OCs show a significant decrease in osteoclastogenesis. In contrast, MKs express several proteins with a known critical role in osteoblastogenesis and bone formation, and co-cultures of these two lineages result in up to a six-fold increase in OB proliferation and alterations in OB differentiation.This research demonstrates the complex regulatory interactions at play between MKs and bone cells, and opens up potential targets for therapeutic intervention.

Enhanced chondrogenic differentiation of embryonic stem cells by coculture with hepatic cells

Enhancing the specific differentiation of pluripotent embryonic stem (ES) cells has been a challenge in the field of tissue engineering. Previously, hepatic cells have been shown to secrete various soluble morphogenic factors to direct mesodermal differentiation of ES cells. In this study, we hypothesized that factors secreted by hepatic cells possess chondrogenic-differentiating effects, and, therefore, the co-culture of hepatic cells would enhance chondrogenesis of ES cells. ES-derived cells(ESDCs) were co-cultured with hepatic cells (HEPA-1C1c7) in three-dimensional bilayered hydrogels. After 3 weeks culture, the histological and biochemical analysis of the HEPA-co-cultured ESDCs revealed a four-fold increase in glycosaminoglycan (GAG) compared to ESDCs cultured alone. This result was supported by real-time PCR analysis, which demonstrated an 80-fold increase in aggrecan expression in co-cultured ESDCs. Additionally, type IIB collagen expression was observed only with co-cultured ESDCs, and immunohistochemical analysis resulted in significantly more positive type II collagen staining with co-cultured ESDCs. Moreover, at day 21, gene expression of other lineages in HEPA-co-cultured ESDCs was either comparable to or lower than those of ESDCs cultured alone. These results indicated that co-culture of ESDCs with hepatic cells significantly enhanced specific chondrogenic differentiation of ESDCs.

Pharmacological topics of bone metabolism: the physiological function of the sympathetic nervous system in modulating bone resorption

The vertebrate skeleton is richly innervated with adrenergic and peptidergic nerve terminals, and these play important roles in bone remodeling. Recent studies have generally shown that increased sympathetic nervous activity causes bone loss via an increase in bone resorption and a decrease in bone formation. Increased bone resorption is based on the stimulation of both osteoclast formation and osteoclast activity. These effects are associated with beta(2)-adrenergic activity toward both osteoblastic and osteoclastic cells. Such findings indicate that beta-blockers may be effective against osteoporosis, in which case there is increased sympathetic activity. This review summarizes evidence obtained both in vitro and in vivo implicating sympathetic neuron action in bone resorption.

Factors Regulating Endochondral Ossification in the Spheno-occipital Synchondrosis

Objectives: To identify the temporal pattern of core-binding factor α1 (Cbfa1) and vascular endothelial growth factor (VEGF) expressions in the spheno-occipital synchondrosis in vitro with and without tensile stress.

Materials and Methods: Sixty male BALB/c mice were randomly divided into an experimental group (with tensile stress) and a control group (without tensile stress) at each of five time points. Animals were sacrificed and the cranial base synchondroses were aseptically removed. In the experimental groups, mechanical stress was applied on the surgical explants with helical springs and incubated as organ culture for 6, 24, 48, 72, and 168 hours. In the control group, the springs were kept at zero stress. Tissue sections were subjected to immunohistochemical staining for quantitative analysis of Cbfa1 and VEGF expression.

Results: Quantitative analysis revealed that Cbfa1 and VEGF expressions reached a peak increase at 24 and 48 hours, respectively. Compared with the control groups, both Cbfa1 and VEGF were expressed consistently higher in the experimental groups at all time points.

Conclusion: Mechanical stress applied to the spheno-occipital synchondrosis elicits Cbfa1 expression and subsequently up-regulates the expression of VEGF. Increased levels of expression of both factors could play a role in the growth of the spheno-occipital synchondrosis.

A new approach to control condylar growth by regulating angiogenesis

OBJECTIVE

To provide a comprehensive review of the mechanisms of growth of mandibular condyle, the roles of angiogenesis enhancers and inhibitors during endochondral ossification in mandibular condyle and newly developed delivery methods for local gene delivery that may represent strategies to regulate condylar growth.

DESIGN

Narrative review.

RESULTS

Angiogenesis is the crucial step in mandibular condylar growth for it regulates the transformation from cartilage to bone. Angiognesis enhancers, especially VEGF and FGF, play important roles in the process of new blood lumen formation and invasion. On the other hand, angiostatin and endostatin inhibit angiogenesis by targeting endothelial cells and several signal cascades. Delivery methods such as liposomes, stem cells and virus vectors have been studied. Recombinant AAV-mediated gene therapy is considered as one of the most promising strategies of condylar growth management.

CONCLUSION

AAV-mediated gene therapy using VEGF or angiogenesis inhibitor will be a promising way to regulate condylar growth at an early stage.

The role of the sympathetic nervous system in controlling bone metabolism

Experimental studies have generally shown that increased sympathetic nervous activity causes bone loss via an increase in bone resorption and a decrease in bone formation. Increased bone resorption is based on the stimulation of both osteoclast formation and osteoclast activity. These effects are associated with beta2-adrenergic activity towards both osteoblastic and osteoclastic cells. Decreased bone formation is based on the inhibition of osteoblastic activity through beta2-adrenergic receptors on osteoblasts. Such findings indicate that beta-blockers may be effective against osteoporosis, in which case there is increased sympathetic activity. In fact, in a population-based, case-control study, the current use of beta-blockers has been demonstrated to be associated with a reduced risk of fractures. These clinical studies suggest that pharmacological blockade of the beta-adrenergic system is beneficial to the human skeleton. In another prospective study, however, no association between beta-blocker use and fracture risk was shown in perimenopausal and older women. To confirm this important new therapeutic avenue to prevent bone loss, the relationship between the pharmacological effectiveness of beta-blockers and the pathogenesis of osteoporosis must be explored in detail.

RANKL and cathepsin K in diffuse sclerosing osteomyelitis of the mandible

BACKGROUND

Diffuse sclerosing osteomyelitis (DSO) of the mandible is characterized by mixed bone resorption and formation.

METHODS

Immunohistopathology of DSO in the clinically acute and subacute phases was compared with healthy bone.

RESULTS

Receptor activator of nuclear factor kappaB ligand (RANKL) was found in DSO lesions. When it was used in vitro to stimulate monocytes, cathepsin K expression was observed in mononuclear prefusion precursors and in multinuclear giant cells. Similarly, exacerbations of DSO were characterized by RANKL and induction of cathepsin K in mononuclear precursor cells, which subsequently seem to differentiate into osteoclasts or foreign body giant cells. The proportion of bone to soft tissue increased with the duration of disease.

CONCLUSIONS

RANKL-driven osteoclastogenesis and acidic cysteine endoproteinase cathepsin K seem to play important roles in DSO as osteoclast-mediated bone resorption may represent the primary disease process later followed by new bone formation.

IFN-gamma stimulates osteoclast formation and bone loss in vivo via antigen-driven T cell activation

T cell-produced cytokines play a pivotal role in the bone loss caused by inflammation, infection, and estrogen deficiency. IFN-gamma is a major product of activated T helper cells that can function as a pro- or antiresorptive cytokine, but the reason why IFN-gamma has variable effects in bone is unknown. Here we show that IFN-gamma blunts osteoclast formation through direct targeting of osteoclast precursors but indirectly stimulates osteoclast formation and promotes bone resorption by stimulating antigen-dependent T cell activation and T cell secretion of the osteoclastogenic factors RANKL and TNF-alpha. Analysis of the in vivo effects of IFN-gamma in 3 mouse models of bone loss - ovariectomy, LPS injection, and inflammation via silencing of TGF-beta signaling in T cells - reveals that the net effect of IFN-gamma in these conditions is that of stimulating bone resorption and bone loss. In summary, IFN-gamma has both direct anti-osteoclastogenic and indirect pro-osteoclastogenic properties in vivo. Under conditions of estrogen deficiency, infection, and inflammation, the net balance of these 2 opposing forces is biased toward bone resorption. Inhibition of IFN-gamma signaling may thus represent a novel strategy to simultaneously reduce inflammation and bone loss in common forms of osteoporosis.

Runx2 and dental development

The Runx2 gene is a master transcription factor of bone and plays a role in all stages of bone formation. It is essential for the initial commitment of mesenchymal cells to the osteoblastic lineage and also controls the proliferation, differentiation, and maintenance of these cells. Control is complex, with involvement of a multitude of factors, thereby regulating the expression and activity of this gene both temporally and spatially. The use of multiple promoters and alternative splicing of exons further extends its diversity of actions. RUNX2 is also essential for the later stages of tooth formation, is intimately involved in the development of calcified tooth tissue, and exerts an influence on proliferation of the dental lamina. Furthermore, RUNX2 regulates the alveolar remodelling process essential for tooth eruption and may play a role in the maintenance of the periodontal ligament. In this article, the structure of Runx2 is described. The control and function of the gene and its product are discussed, with special reference to developing tooth tissues, in an attempt to elucidate the role of this gene in the development of the teeth and supporting structures.

Parathyroid hormone controls receptor activator of NF-kappaB ligand gene expression via a distant transcriptional enhancer

RANKL, a protein essential for osteoclast development and survival, is stimulated by parathyroid hormone (PTH) via a PTH receptor 1/cyclic AMP (cAMP)/protein kinase A (PKA)/CREB cascade, exclusively in osteoblastic cells. We report that a bacterial artificial chromosome-based transcriptional reporter construct containing 120 kb of RANKL 5'-flanking region was stimulated by dibutyryl-cAMP in stromal/osteoblastic cells, but not other cell types. Full cAMP responsiveness was dependent upon a conserved 715-bp region located 76 kb upstream from the transcription start site, which we identified by sequential deletion analysis and by comparison of human and mouse genomic sequences in silico. This region contained conserved consensus sequences which bound CREB and the osteoblast-specific transcription factor Runx2, and when mutated blunted cAMP responsiveness. Overexpression of Runx2 potentiated cAMP responsiveness of the endogenous RANKL gene in a cell-type-specific manner. Lastly, PTH responsiveness of the endogenous RANKL gene was abrogated in mice from which we deleted this conserved upstream region. Thus, PTH responsiveness of the RANKL gene is determined by a distant regulatory region that responds to cAMP in a cell-type-specific manner and Runx2 may contribute to such cell-type specificity.

On allosteric control model of bone turnover cycle containing osteocyte regulation loop

One approach to developing a mathematical model that predicts osteoactivity both in bio-scaffolds, as well as the in bone tissue in vivo, is based on a bio-cybernetic vision of basic multicellular unit (BMU) action. In the case of the model presented in this paper, some of the loops of regulation have been modified to reflect the range of allosteric control mechanisms: Michaelis-Menten, Hill, Adair, Koshland-Nemethy-Filmer (KNF), Monod-Wyman-Changeux (MWC). This approach has resulted in a four-dimensional system that shows steady cyclic behaviour using a range of constants with clear biological meaning. The initial findings suggesting that a steady state appears as a cycle in multidimensional phase space and this is discussed in this paper. The existence of this cycle in the osteoclasts-osteoblasts-osteocytes-bone subspace indicates that there is a conservative value along steady trajectories for this dynamic system. Biophysical interpretation of this conservative value has been proposed as a kind of substrate-energy regenerative potential of the bone remodelling system with a similarity to the classical physical value-energy. Such a recovery "potential" is directed against both mechanical and biomechanical damage to the bone. The current model has credibility when compared to the normal bone remodelling process. In the framework of widely recognised Michaelis-Menten mechanisms of allosteric regulation the cyclic attractor, described formerly for a pure cellular model, prevails for different forms of feedback control. This finding demonstrates the viability of the suggestion of the subsistence of conservative value (analogous to energy) that characterises the recovery potential of the bone remodelling cycle. The results indicate that the robust behaviour of the model is maintained from the simple cellular level to the molecular biochemical level of regulation.

The role of megakaryocytes in skeletal homeostasis and rheumatoid arthritis

PURPOSE OF REVIEW

This review provides an update on the role of megakaryocytes in skeletal homeostasis, and discusses these findings in the context of rheumatoid arthritis.

RECENT FINDINGS

Thrombocytosis is a common complication of rheumatoid arthritis, and is presumably caused by an up-regulation in megakaryocytopoiesis. In general, patients with rheumatoid arthritis exhibit localized joint bone erosion with systemic bone loss, and rheumatoid arthritis patients with thrombocytosis tend to have more severe disease. Interestingly, in addition to their role in rheumatoid arthritis with thrombocytosis, it has been demonstrated recently that megakaryocytes play a dual role in regulating skeletal mass by inhibiting bone resorption while simultaneously stimulating bone formation. This seeming contradiction in the putative role of megakaryocytes in skeletal regulation and rheumatoid arthritis is the focus of this review.

SUMMARY

In rheumatoid arthritis there are substantial increases in the levels of several pro-inflammatory pleiotropic cytokines. As would be expected, in addition to their role in inflammation, these cytokines play a critical role in the megakaryocytopoiesis seen in patients who develop reactive thrombocytosis, and these cytokines also are known to regulate osteoclastogenesis. Thus, it appears that in rheumatoid arthritis with reactive thrombocytosis, the ability of the cytokines to enhance osteoclastogenesis outweighs the ability of megakaryocytes to inhibit osteoclastogenesis.

Prostate-specific antigen modulates genes involved in bone remodeling and induces osteoblast differentiation of human osteosarcoma cell line SaOS-2

PURPOSE

The high prevalence of osteoblastic bone metastases in prostate cancer involves the production of osteoblast-stimulating factors by prostate cancer cells. Prostate-specific antigen (PSA) is a serine protease uniquely produced by prostate cancer cells and is an important serologic marker for prostate cancer. In this study, we examined the role of PSA in the induction of osteoblast differentiation.

EXPERIMENTAL DESIGN

Human cDNA containing a coding region for PSA was transfected into human osteosarcoma SaOS-2 cells. SaOS-2 cells were also treated with exogenously added PSA. We evaluated changes in global gene expression using cDNA arrays and Northern blot analysis resulting from expression of PSA in human osteosarcoma SaOS-2 cells.

RESULTS

SaOS-2 cells expressing PSA had markedly up-regulated expression of genes associated with osteoblast differentiation including runx-2 and osteocalcin compared with the controls. Consistent with these results, the stable clones expressing PSA showed increased mineralization and increased activity of alkaline phosphatase in vitro compared with controls, suggesting that these cells undergo osteoblast differentiation. We also found that osteoprotegerin expression was down-regulated and that the receptor activator of NF-kappaB ligand expression was up-regulated in cells expressing PSA compared with controls.

CONCLUSIONS

Modulation of the expression of osteogenic genes and alteration of the balance between osteoprotegerin-receptor activator of NF-kappaB ligand by PSA suggests that PSA produced by metastatic prostate cancer cells may participate in bone remodeling in favor of the development of osteoblastic metastases in the heterogeneous mixture of osteolytic and osteoblastic lesions. These findings provide a molecular basis for understanding the high prevalence of osteoblastic bone metastases in prostate cancer.

Identification of a new pebp2alphaA2 isoform from zebrafish runx2 capable of inducing osteocalcin gene expression in vitro

The zebrafish runx2b transcription factor is an ortholog of RUNX2 and is highly conserved at the structural level. The runx2b pebp2alphaA2 isoform induces osteocalcin gene expression by binding to a specific region of the promoter and seems to have been selectively conserved in the teleost lineage.

INTRODUCTION

RUNX2 (also known as CBFA1/Osf2/AML3/PEBP2alphaA) is a transcription factor essential for bone formation in mammals, as well as for osteoblast and chondrocyte differentiation, through regulation of expression of several bone- and cartilage-related genes. Since its discovery, Runx2 has been the subject of intense studies, mainly focused in unveiling regulatory targets of this transcription factor in high vertebrates. However, no single study has been published addressing the role of Runx2 in bone metabolism of low vertebrates. While analyzing the zebrafish (Danio rerio) runx2 gene, we identified the presence of two orthologs of RUNX2, which we named runx2a and runx2b and cloned a pebp2alphaA-like transcript of the runx2b gene, which we named pebp2alphaA2.

MATERIALS AND METHODS

Zebrafish runx2b gene and cDNA were isolated by RT-PCR and sequence data mining. The 3D structure of runx2b runt domain was modeled using mouse Runx1 runt as template. The regulatory effect of pebp2alphaA2 on osteocalcin expression was analyzed by transient co-transfection experiments using a luciferase reporter gene. Phylogenetic analysis of available Runx sequences was performed with TREE_PUZZLE 5.2. and MrBayes.

RESULTS AND CONCLUSIONS

We showed that the runx2b gene structure is highly conserved between mammals and fish. Zebrafish runx2b has two promoter regions separated by a large intron. Sequence analysis suggested that the runx2b gene encodes three distinct isoforms, by a combination of alternative splicing and differential promoter activation, as described for the human gene. We have cloned a pebp2alphaA-like transcript of the runx2b gene, which we named pebp2alphaA2, and showed its high degree of sequence similarity with the mammalian pebp2alphaA. The cloned zebrafish osteocalcin promoter was found to contain three putative runx2-binding elements, and one of them, located at -221 from the ATG, was capable of mediating pebp2alphaA2 transactivation. In addition, cross-species transactivation was also confirmed because the mouse Cbfa1 was able to induce the zebrafish osteocalcin promoter, whereas the zebrafish pebp2alphaA2 activated the murine osteocalcin promoter. These results are consistent with the high degree of evolutionary conservation of these proteins. The 3D structure of the runx2b runt domain was modeled based on the runt domain of mouse Runx1. Results show a high degree of similarity in the 3D configuration of the DNA binding regions from both domains, with significant differences only observed in non-DNA binding regions or in DNA-binding regions known to accommodate considerable structure flexibility. Phylogenetic analysis was used to clarify the relationship between the isoforms of each of the two zebrafish Runx2 orthologs and other Runx proteins. Both zebrafish runx2 genes clustered with other Runx2 sequences. The duplication event seemed, however, to be so old that, whereas Runx2b clearly clusters with the other fish sequences, it is unclear whether Runx2a clusters with Runx2 from higher vertebrates or from other fish.

The RUNX genes: gain or loss of function in cancer

The RUNX genes have come to prominence recently because of their roles as essential regulators of cell fate in development and their paradoxical effects in cancer, in which they can function either as tumour-suppressor genes or dominant oncogenes according to context. How can this family of transcription factors have such an ambiguous role in cancer? How and where do these genes impinge on the pathways that regulate growth control and differentiation? And what is the evidence for a wider role for the RUNX genes in non-haematopoietic cancers?

Runx-2 is not essential for the vitamin D-regulated expression of RANKL and osteoprotegerin in osteoblastic cells

The differentiation and activity of osteoclasts are positively and negatively controlled by receptor activator of nuclear factor-kappaB ligand (RANKL), which is expressed on the surface of osteoblasts and stromal cells, and its decoy receptor osteoprotegerin (OPG), which is secreted by osteoblasts and stromal cells, respectively. The expression of the genes for RANKL and OPG is regulated by 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)]. Runt-related gene-2 (Runx-2) is essential for osteoblast differentiation and there are several reports that Runx-2 is involved in osteoclast formation. Therefore, to clarify the role of Runx-2 in osteoclastogenesis, we designed a series of experiments using C2 cells and C6 cells, which are derived from calvariae of runx2-deficient mice. Treatment of C2 cells and C6 cells with 1alpha,25(OH)(2)D(3) for 2-4 days increased and decreased the levels of expression of the mRNAs for RANKL and OPG, respectively, and the effects were dose-dependent. However, by day 8, the level of RANKL mRNA had fallen and that of OPG mRNA had risen. Furthermore, C6 cells induced the differentiation of mouse spleen cells into tartrate-resistant acid phosphatase-positive (TRAP-positive) multinucleated cells (osteoclast-like cells) in the presence of 10(-7)M 1alpha,25(OH)(2)D(3). Such formation of osteoclast-like cells was inhibited by exogenous OPG in a dose-dependent manner. Thus, our findings indicate that Runx-2 is not essential for the expression of RANKL and OPG, and the formation of osteoclast-like cells.

The molecular triad OPG/RANK/RANKL: involvement in the orchestration of pathophysiological bone remodeling

The past decade has seen an explosion in the field of bone biology. The area of bone biology over this period of time has been marked by a number of key discoveries that have opened up entirely new areas for investigation. The recent identification of the receptor activator of nuclear factor kappaB ligand (RANKL), its cognate receptor RANK, and its decoy receptor osteoprotegerin (OPG) has led to a new molecular perspective on osteoclast biology and bone homeostasis. Specifically, the interaction between RANKL and RANK has been shown to be required for osteoclast differentiation. The third protagonist, OPG, acts as a soluble receptor antagonist for RANKL that prevents it from binding to and activating RANK. Any dysregulation of their respective expression leads to pathological conditions such as bone tumor-associated osteolysis, immune disease, or cardiovascular pathology. In this context, the OPG/RANK/RANKL triad opens novel therapeutic areas in diseases characterized by excessive bone resorption. The present article is an update and extension of an earlier review published by Kwan Tat et al. [Kwan Tat S, Padrines M, Theoleyre S, Heymann D, Fortun Y. IL-6, RANKL, TNF-alpha/IL-1: interrelations in bone resorption pathophysiology. Cytokine Growth Factor Rev 2004;15:49-60].

[Culture of human osteoclasts]

OBJECTIVE

To establish a stable, useful culture system for human osteoclasts and to investigate the effect of osteoblasts on the differentiation, proliferation and activation of osteoclasts so as to provide a base for the studies on prevention and treatment of osteolysis and osteoporosis.

METHODS

In the presence of 1,25-(OH)2D3, monocytes abstracted from human bone marrow were cultured in three groups: co-culture of monocytes and osteoblasts, monocytes alone, monocytes with conditional media (CM) of osteoblasts. Differentiation process of the cultured cells was observed under biological microscope. HE staining and tartrate-resistant acid phosphatase (Trap) staining were employed to assay the cultured cells. The resorption pits on bone slices, on which cells were cultured, were observed under scanning electronic microscope (SEM).

RESULTS

In the group of co-culture of monocytes and osteoblasts, monocytes gradually fused to form multinucleated cells (MNCs), and the MNCs were also indicated in HE staining and Trap staining. The SEM showed a number of resorption pits on bone slices. In the other two groups, Trap-positive MNCs were not obtained, and resorption pits were not observed on bone slices.

CONCLUSION

In this culture, monocytes obtained from human marrow fused to form multinucleated cells (MNCs) that express the main characteristics of the osteoclast phenotype, such as Trap-positive and the ability to form resorption lacunae when cultured on bone slices. Cell-to-cell contact with osteoblasts was necessary for the differentiation, proliferation and activation of osteoclasts.

Fidelity of Runx2 Activity in Breast Cancer Cells Is Required for the Generation of Metastases-Associated Osteolytic Disease

The osteolytic bone destruction associated with breast cancer skeletal metastases represents a serious and incurable clinical condition. However, the molecular mechanisms regulating tumor cell expression of factors involved in the generation of osteolytic disease remain elusive. We demonstrated recently that breast cancer cells express the Runx2 transcription factor, essential for bone formation and a regulator of skeletal homeostasis. Our experimental results demonstrate that perturbation of Runx2 regulatory function in tumor cells abolishes their ability to form osteolytic lesions in vivo. In vitro, we show that breast cancer cells inhibit osteoblast differentiation while concurrently enhancing osteoclast differentiation in marrow stromal cell cultures. Disruption of Runx2 activity abrogates both of these cancer cell-mediated effects on bone cells. These results demonstrate that Runx2 expression in breast cancer cells provides a molecular phenotype that enables the interactions between tumor cells and the bone microenvironment that lead to osteolytic disease.